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Substitution Effect on the Charge Transfer Processes in Organo-Imido Lindqvist-Polyoxomolybdate Patricio Hermosilla-Ibáñez 1,2 , Kerry Wrighton-Araneda 1,2 , Walter Cañón-Mancisidor 1,2 , Marlen Gutiérrez-Cutiño 1,2 , Verónica Paredes-García 2,3 and Diego Venegas-Yazigi 1,2, * 1

2 3

*

Facultad de Química y Biología, Universidad de Santiago de Chile, USACH, Av. Libertador Bernardo O’Higgins 3363, 9170022 Santiago, Chile; [email protected] (P.H.-I.); [email protected] (K.W.-A.); [email protected] (W.C.-M.); [email protected] (M.G.-C.) Centro para el Desarrollo de la Nanociencia y Nanotecnología, CEDENNA, 9170022 Santiago, Chile Departamento de Ciencias Químicas, Universidad Andres Bello, Republica 275, 8370146 Santiago, Chile; [email protected] Correspondence: [email protected]; Tel.: +56-22-718-1079

Received: 1 December 2018; Accepted: 21 December 2018; Published: 22 December 2018

Abstract: Two new aromatic organo-imido polyoxometalates with an electron donor triazole group ([n-Bu4 N]2 [Mo6 O18 NC6 H4 N3 C2 H2 ]) (1) and a highly conjugated fluorene ([n-Bu4 N]2 [Mo6 O18 NC13 H9 ]) (2) have been obtained. The electrochemical and spectroscopic properties of several organo-imido systems were studied. These properties were analysed by the theoretical study of the redox potentials and by means of the excitation analysis, in order to understand the effect on the substitution of the organo-imido fragment and the effect of the interaction to a metal centre. Our results show a bathochromic shift related to the charge transfer processes induced by the increase of the conjugated character of the organic fragment. The cathodic shift obtained from the electrochemical studies reflects that the electronic communication and conjugation between the organic and inorganic fragments is the main reason of this phenomenon. Keywords: organoimido; polyoxometalate; charge transfer; electronic properties; TD-DFT; Rhenium

1. Introduction Polyoxometalates (POMs) represent a large family of anionic clusters formed by oxygen and transition metal atoms presenting a rich structural and electronic versatility [1–3]. Among the family of POMs, the Lindqvist-type hexamolybdate [Mo6 O19 ]2− is one of the most studied [4–6] due to its simplicity and the high symmetry between the six centres. The functionalization of this compound with organo-imido fragments has a general formula [Mo6 O(19−x) (N-R)x ]2− , where x = 1–6 and N-R is an organic unit derived from a isocyanate or a primary amine molecule bonded to a molybdenum centre [7–9]. This functionalization permits to obtain and design new organic-inorganic hybrid systems where the electronic properties can be modulated depending on the nature of the incorporated organic fragment [10]. The charge transfer phenomenon from an electron-rich fragment to an acceptor fragment, both covalently bonded, is an interesting feature that these molecular systems can present. The ligand to metal charge transfer (LMCT) transition in the hexamolybdate, i.e., from the oxygen to the molybdenum centres, is in the region of ≈325 nm. Upon functionalization with organic fragments, an intense band with a maximum at lower energy is observed. For long time it was assigned to a shift of the LMCT (oxygen to molybdenum) [11]. However, according to the results published by our group the band that appears at lower energies is a new band that corresponds to LMCT from the organic fragment to the metal centres [12]. The use of theoretical calculations based on TD-DFT (time-dependent

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dependent density functional theory) has permitted the corroboration of experimental assignments density functional theory) has the[13]. corroboration of experimental assignments of these bands, of these bands, as reported by permitted Ravelli et al. as reported by Ravelli et al. [13]. The electrochemical processes have also been studied, both theoretically and experimentally. havework also published been studied, both theoretically and experimentally. From The the electrochemical theoretical pointprocesses of view, the by López et al. recommends the use of a From the theoretical point of view, the work published by López et al. recommends use of a different approach to calculate redox potentials of polyoxometalates [14] with respect to the method different approach to calculate redox potentials of polyoxometalates [14] with respect to the method published by Truhlar et al. [15]. published Truhlarorbitals et al. [15]. As theby frontier are involved in the electrochemical and electronic properties of these As the frontier orbitals in the electrochemical and electronic of these hybrid materials, the nature ofare theinvolved organic fragment will affect both of them. The properties electronic features hybrid materials, the nature organic fragment will affect both them. The electronicand/or features of of the organic fragment can of bethe tuned through the introduction ofof organic substituents the the organic can becompounds. tuned throughItthe organic substituents thekind bonding bonding of fragment coordination is introduction possible tooffind some examplesand/or of this of of coordination compounds. It is possible to find examples of complexes this kind of[12,16–20]. functionalization with functionalization with first, second, and third rowsome transition metal first,Itsecond, andshown third row transition metal complexes [12,16–20]. has been that the presence of an isolated and well-defined charge transfer (CT) It has been shown that the presence of an isolated andexample, well-defined transferthat (CT) transition is necessary to have good charge transfer process. For it has charge been reported a transition is necessary to have good charge transfer process. For example, it has been reported ferrocenyl unit, which is an electron-rich system, coordinated to an imido-functionalized that a ferrocenyl unit, which an electron-rich to anfrom imido-functionalized hexamolybdate through a highlyisconjugated bridge,system, presentscoordinated a charge transfer the ferrocenyl to hexamolybdate through a highly conjugated bridge, presents a charge transfer from the ferrocenyl to the hexamolybdate fragment more than 200 nm lower in energy than the oxygen to molybdenum CT the hexamolybdate fragment more than 200 nm lower in energy than the oxygen to molybdenum CT of the hexamolybdate [17]. of the [17]. aromatic organo-imido polyoxometalates with an electron donor triazole In hexamolybdate this work, two new this work, new organo-imido an electron groupIn ([n-Bu 4N]2[Motwo 6O18NC 6H4Naromatic 3C2H2]) (1) and a polyoxometalates highly conjugatedwith fluorene ([ndonor triazole group conjugated fluorene Bu 4N]2[Mo 6O18NC 13H9]) ([n-Bu (2) are presented. Five6 Haliphatically-functionalized hexamolybdates were 4 N] 2 [Mo6 O18 NC 4 N3 C2 H2 ]) (1) and a highly ([n-Bufrom ]) (2) Seven are presented. aliphatically-functionalized taken the6 Oliterature aromatic Five systems with different electronhexamolybdates donating and 4 N]2 [Mo 18 NC13 H9[21]. were taken from the were literature [21]. from Seventhe aromatic systems with different electron donating and withdrawing abilities also taken literature [10,22,23]. For the spectroscopic study, some withdrawing abilities wererepresentative also taken from the literature [10,22,23]. For the spectroscopic study, some molecules being the most systems were used. The electrochemical and spectroscopic molecules of being the mostwere representative systems were used. electrochemical and spectroscopic properties all systems studied. These properties wereThe analysed and discussed by means of properties of all systems were studied. These properties analysed andfragment discussedand by means of the the excitation analysis depending on the substitution of were the organo-imido the effect of excitation analysis depending the interaction to a metal centre.on the substitution of the organo-imido fragment and the effect of the interaction to a metal centre. 2. Results and Discussions 2. Results and Discussions 2.1. Synthesis and X-ray Structures 2.1. Synthesis and X-ray Structures Two new organo-imido polyoxometalates have been synthesized as described in Section 3. The Two new organo-imido polyoxometalates have been synthesized as described in Section 3. compounds were crystalized, and the X-ray data are shown in the supporting information (Figures The compounds were crystalized, and the X-ray data are shown in the supporting information S1 and S2, and Table S1), given a formula of [n-Bu4N]2[Mo6O18NC6H4N3C2H2] for (1) and [n(Figures S1 and S2, and Table S1), given a formula of [n-Bu4 N]2 [Mo6 O18 NC6 H4 N3 C2 H2 ] for (1) Bu4N]2[Mo6O18NC13H9] for (2) (Figure 1). The analysis of the bond angle Mo-N-R gives an idea of the and [n-Bu4 N]2 [Mo6 O18 NC13 H9 ] for (2) (Figure 1). The analysis of the bond angle Mo-N-R gives an double/triple character of the Mo-N bond. The values observed in this work for 1 and 2, as well as idea of the double/triple character of the Mo-N bond. The values observed in this work for 1 and the literature values for the triazolylmethyl analogue of 1 (compound 3) and four other compounds 2, as well as the literature values for the triazolylmethyl analogue of 1 (compound 3) and four other with aliphatic substitution (ethyl, n-propyl, n-butyl, n-hexyl) are summarized in Table 1. compounds with aliphatic substitution (ethyl, n-propyl, n-butyl, n-hexyl) are summarized in Table 1.

Figure 1. Ball-and-stick representation of 1 and 2 organo-imido-polyoxometalate. [n-Bu4 N]+ ions are Figure 1. for Ball-and-stick representation of 1 and 2 organo-imido-polyoxometalate. [n-Bu4N]+ ions are omitted clarity. omitted for clarity.

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Table 1. Summary of the Mo–N–C angle that connects the organic fragment and the polyoxometalate Table of 1. the Mo–N–C Summaryangle of the Mo–N–Cthe angle that connects organic fragment and the Table 1. Summary that connects organic fragment andthe the polyoxometalate unit. polyoxometalate unit. unit. Mo6O18N-R O18 18N-R N-R Mo66O Mo

Ref. Mo–N–C Angle (°) Ref. Mo–N–C (◦ ) Mo–N–C Angle (°) Angle

Ref.

159.8(5) 159.8(5)

This work This work 159.8(5)

This work

158.1(2) 158.1(2)

This work 158.1(2) This work

This work

1 11

222

163.5(2)/164.1(1) [12] 163.5(2)/164.1(1) 163.5(2)/164.1(1) [12] 33 3 –CH –CH22CH CH33 –CH 2CH 3 –CH CH CH –CH22CH22CH33 –CH 2 CH 2 CH 3 –CH CH CH CH –CH22CH22CH22CH33 –CH2 –CH CH2 CH 2CH CH22CH CH23 CH3 22CH –CH2CH2CH2CH2CH2CH3 –CH2CH2CH2CH2CH2CH3

178.1(1) 178.1(1) 177.9(5) 177.9(5) 178.3(8) 178.3(8) 176.7(1) 176.7(1)

178.1(1) 177.9(5) 178.3(8) 176.7(1)

[21] [21] [21] [21] [21] [21] [21] [21]

[12]

[21] [21] [21] [21]

It can be observed that all aliphatically-substituted values higher It can be observed that all aliphatically-substituted compoundscompounds show anglesshow withangles valueswith higher It can be observed that all aliphatically-substituted compounds show angles with values higher than 176 the more linearthus structure, having the nitrogen than 176 degrees, i.e.degrees, showingi.e., theshowing more linear structure, havingthus the nitrogen with a morewith sp a more sp than 176 degrees, i.e. showing the more linear structure, thus having the nitrogen with a more sp character. On the other hand, compounds with aromatic substituents have narrower Mo-N-C angles, character. On the other hand, compounds with aromatic substituents have narrower Mo-N-C angles, character. On the other hand, compounds with aromatic substituents have narrower Mo-N-C angles, hence nitrogen with less triple-bond character toward molybdenum. Summarizing, hence nitrogen atoms with atoms less triple-bond character toward molybdenum. Summarizing, in the case in the case hence nitrogen atoms with less triple-bond character toward molybdenum. Summarizing, in the case of aliphatic substituents, a more sp nitrogen atom is observed, therefore it interacts a triple bond to of aliphatic substituents, a more sp nitrogen atom is observed, therefore it interacts as a tripleasbond of aliphatic substituents, a more sp nitrogen atom is observed, therefore it interacts as a triple bond the molybdenum and single bond to the substituent. In the case of the aromatic substitution, the to the molybdenum atom andatom single bond to the substituent. In the case of the aromatic substitution, to the molybdenum atom and single bond to the substituent. In the case of the aromatic substitution, interaction hasofa less of a triple-bond character the molybdenum, an increased double-bond the interaction has a less a triple-bond character to the to molybdenum, and anand increased doublethe interaction has a less of a triple-bond character to the molybdenum, and an increased doubleinteraction to the substituent. bond interaction to the substituent. bond interaction to the substituent. 2.2. Electrochemical 2.2. Electrochemical Properties Properties 2.2. Electrochemical Properties Cyclic voltammetry studies of of [n-Bu [Mo NC6 H46N C23C H22H ] (1) and 4 N]24N] 6 O18 4 N]2 [Mo 6 O18 NC13 H9 ] Cyclic voltammetry studies [n-Bu 2[Mo 6O18NC H34N 2] (1)[n-Buand [nCyclic (2) were voltammetry studies of solution [n-Bu4N] 2[Mo6O18NC6H4N3C2H2] (1) electrolyte and [n- (Figure S3). carried out in acetonitrile using n-Bu NClO as a supporting Bu4N]2[Mo6O18NC13H9] (2) were carried out in acetonitrile solution4 using 4n-Bu4NClO4 as a supporting Bu4N]2[Mo6O18order NC13Hto9] compare (2) were carried out in(1acetonitrile solution using n-Bufrom 4NClO4 as a supporting values andvalues 2) with obtained the literature, electrolyte In (Figure S3). In order our to compare our (1those and 2) with those obtained from the the concept of electrolyte cathodic (Figure S3). In order to compare our values (1 and 2) with those obtained from the shift (E ) is used. This shift is the difference between the E from the hexamolybdate and 1/2 1/2 literature, the concept of cathodic shift (E1/2) is used. This shift is the difference between the E1/2 from literature, the concept of cathodic shift (E 1/2) is used. This shift is the difference between the E1/2 from the E1/2 and of each the same in conditions. calculated shift (Ec ) was the hexamolybdate the compound E1/2 of each measured compoundinmeasured the same The conditions. Thecathodic calculated the hexamolybdate and thesame E1/2 of each compound measured in thethe same conditions. The calculated obtained in the way, i.e., as the difference between calculated E for each molecule and the 1/2 cathodic shift (Ec) was obtained in the same way, i.e., as the difference between the calculated E1/2 for cathodic shift (Efor c) was obtained in the same way, i.e., as the difference between the calculated E1/2 for E the naked hexamolybdate. The use of ∆E and ∆E values allowed us to analyse the data 1/2and the E1/2 for the naked hexamolybdate. The Cuse of ∆E1/2 each molecule C and ∆E1/2 values allowed us each molecule and the E 1/2 for the naked hexamolybdate. The use of ∆EC and ∆E1/2 values allowed us independently of the reference electrode. The E1/2-Teo the hexamolybdate used toused evaluate all the to analyse the data independently of the reference electrode. The of E1/2-Teo of the hexamolybdate to analyse the data independently of is the reference electrode. The Ecalculate 1/2-Teo of the hexamolybdate used theoretical cathodic shifts − 0.687 V. The E used to the ∆E for 1 and 2 was −0.836 V. 1/2 ∆E1/2 for 1 and to evaluate all the theoretical cathodic shifts is −0.687 V.1/2 The E1/2 used to calculate the to evaluate Table all the2 theoretical cathodic shifts is −0.687 V. The E1/2cathodic used to calculate thestudied ∆E1/2 for 1 and summarizes the experimental and theoretical shift for all compounds. 2 was −0.836 V. Table 2 summarizes the experimental and theoretical cathodic shift for all studied 2 was −0.836 V. Table 2 summarizes the experimental and theoretical cathodic shift for all studied compounds. compounds.

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1/21/2 ) )and C)C)cathodic Table 2.2.Summary experimental andtheoretical theoretical(ΔE (ΔE cathodicshifts shiftsfor forthe thefirst firstreduction reduction Table Summaryofof experimental(ΔE (ΔE 1/2 Table 1/2) ) and and theoretical theoretical (ΔE (ΔECCC)) cathodic cathodic shifts shifts for for the the first first reduction reduction Table 2. 2. Summary Summary of of experimental experimental (ΔE (ΔE1/2 1/2) and theoretical (ΔE C) cathodic shifts thefirst firstAlso, reduction Table 2.Summary Summary ofexperimental experimental (ΔE1/2 Table 2. hexamolybdate Summary of experimental (∆E )different and theoretical (∆E )for cathodic shifts for the first reduction potential ofof functionalized with organo-imido groups. the potential of hexamolybdate functionalized with different organo-imido groups. Also, the C 1/2 ) and theoretical (ΔE C ) cathodic shifts for the reduction Table 2. of (ΔE potential hexamolybdate functionalized with different organo-imido groups. Also, potential of hexamolybdate functionalized with different organo-imido Also, the the 1/2) and theoretical (ΔECC) cathodic shifts forgroups. the first reduction Table 2. Summary of experimental (ΔE1/2 potential ofEhexamolybdate hexamolybdate functionalized with with different organo-imido groups. Also, Also, the the experimental experimental 1/2 ofof 1,1, 2,2, 3,3, and 4functionalized presented. experimental E 1/2 and 44are are presented. 1/2 potential of hexamolybdate functionalized different organo-imido groups. potential of with different organo-imido groups. Also, the experimental E 1/2 of 1, 2, 3, and are presented. experimental E 1/2 of 1, 2, 3, and 4 are presented. potential of hexamolybdate functionalized with different organo-imido groups. Also, the experimentalEE1/2 1/2 of 1, 2, 3, and 4 are presented. experimental of63, 1, 2,18 3, and 4 are presented. E1/2 of 1, 2, and 4 are presented. Mo 18 N–R ΔE 1/21/2 (V) CC (V) 1/21/2 (V) 1/2-Teo (V) Ref. Mo 66O ΔE (V) ΔE ΔE (V) EE (V) EE 1/2-Teo (V) Ref. 18N–R 1/2 1/2 1/2-Teo 1/2 Mo 66O 18 ΔE Ref. experimental E1/2 of O 1, 3, and 4 are presented. Mo O2, 18N–R N–R ΔE1/2 1/2 (V) (V) ΔE ΔECCC (V) (V) E E1/2 1/2 (V) (V) E E1/2-Teo 1/2-Teo (V) (V) Ref. Mo 6O18N–R ΔE1/2 1/2 (V) ΔECC(V) (V) EE1/2 1/2 (V) 1/2-Teo (V) Ref. Mo6O18N–R ΔE (V) ΔE (V) EE1/2-Teo (V) Ref. 6O18 18N–R N–R 1/21/2 1/2 1/2-Teo Mo ∆E (V) E1/2 Ref. Mo6 6O 18 ΔE 1/2 (V)(V) ΔECC (V)∆EE (V) E1/2-Teo (V)(V) Ref. E1/2-Teo (V) C1/2 0.190 0.286 -0.973 This 0.190 0.286 −1.026 −1.026 -0.973 Thiswork work 0.190 0.286 −1.026 -0.973 This work 0.190 0.286 −1.026 -0.973 This work 0.190 0.286 −1.026 -0.973 Thiswork work 0.190 0.286 −1.026 -0.973 This 1 11 0.190 0.286 − 1.026 This work 0.190 0.286 −1.026 -0.973 This work -0.973 1 1 1 11

2 22 2 2 222

3 33 3 333 3

4 44 4 44 3 33 –CH –CH –CH 4 33 –CH 4 –CH –CH 2CH –CH 3 3 33 –CH 223 CH –CH 22CH 33 –CH 3 –CH CH –CH 3 3 2 CH –CH 2–CH CH 23CH –CH 2CH 3 3 33 –CH 22CH 22CH –CH 2 CH 22CH 3 –CH 2 CH 3 –CH 2CHCH CH –CH CH 2 22CH 2 33 3 3 –CH 2–CH CH 2CH 2CH –CH 2CH 2CH 3 3 33 –CH 22CH 22CH 22CH –CH 2 CH 2 CH 2 CH 3 –CH 2 CH 2 CH 3 –CH 2 CH 2 CH 2 CH –CH2 CH 2 CH 2 CH 33 –CH 22CH 22CH 33 2CH –CH 2–CH CH 2CH 2CH 2CH 3 3 222CH 222CH 222CH 322CH –CH 22CH CH CH CH –CH 22CH 22CH 22CH 22CH 22CH CH 2CH CH 2CH CH 3 CH –CH CH –CH CH CH CH CH CH3333 2–CH 2 2CH 2 2 2 2 –CH–CH 2CH22CH CH222CH CH222CH CH332CH3 –CH 2CH2CH2CH2CH2CH3 –CH22CH22CH22CH22CH22CH33

0.183 0.183 0.183 0.183 0.183 0.183 0.183 0.183

0.308 −1.019 0.308 −1.019 0.308 −1.019 0.308 −1.019 0.308 −1.019 0.308 −1.019 0.308 0.308 −1.019

−0.995 This work −0.995 This work −0.995 This −0.995 This work work −0.995 Thiswork work −0.995 This −1.019 This work −0.995 −0.995

0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175

0.252 −1.011 0.252 −1.011 0.252 −1.011 0.252 −1.011 0.252 0.252 −1.011 0.252 −1.011 0.252 −1.011

−0.939 −0.939 −0.939 −0.939 −0.939 −0.939 −1.011 −0.939

[12] [12] [12] [12] [12] [12] [12]

0.130 0.130 0.130 0.130 0.130 0.130 0.130 0.130

0.224 −0.966 0.224 −0.966 0.224 −0.966 0.224 −0.966 0.224 0.224 −0.966 0.224 −0.966 0.224 −0.966

−0.910 −0.910 −0.910 −0.910 −0.966 −0.910 −0.910 −0.910

[12] [12] [12] [12] [12] [12] [12]

0.270 0.270 0.270 0.270 0.270 0.320 0.270 0.320 0.320 0.270 0.320 0.320 0.270 0.258 0.320 0.258 0.258 0.320 0.258 0.258 0.320 0.288 0.258 0.288 0.288 0.258 0.288 0.288 0.258 0.270 0.288 0.270 0.270 0.288 0.270 0.270 0.288 0.270 0.270 0.270 0.190 0.190 0.190 0.190 0.190 0.190 0.190 0.190

0.341 0.341 0.341 0.341 0.340 0.341 0.340 0.340 0.341 0.340 0.341 0.278 0.340 0.278 0.278 0.340 0.278 0.340 0.324 0.278 0.324 0.324 0.278 0.324 0.278 0.306 0.324 0.306 0.306 0.324 0.306 0.324 0.306 0.306 0.306 0.286 0.286 0.286 0.286 0.286 0.286 0.286

−1.028 −1.028 −1.028 −1.028 −1.020 −1.028 −1.020 −1.020 −1.028 −1.020 −1.028 −0.964 −1.020 −0.964 −0.964 −1.020 −0.964 −1.020 −1.011 −0.964 −1.011 −1.011 −0.964 −1.011 −0.964 −0.993 −1.011 −0.993 −0.993 −1.011 −0.993 −1.011 −0.993 −0.993 −0.993 −0.973 −0.973 −0.973 −0.973 −0.973 −0.973 −0.973

[21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [23] [23] [23] [23] [23] [23] [23]

0.191 0.191 0.191 0.191 0.191 0.191 0.191 0.191

0.237 0.237 0.237 0.237 0.237 0.237 0.237 0.237

−0.924 −0.924 −0.924 −0.924 −0.924 −0.924 −0.924

[22] [22] [22] [22] [22] [22] [22]

0.197 0.197 0.197 0.197 0.197 0.197 0.197 0.197

0.307 0.307 0.307 0.307 0.307 0.307 0.307 0.307

−0.994 −0.994 −0.994 −0.994 −0.994 −0.994 −0.994

[22] [22] [22] [22] [22] [22] [22]

0.161 0.161 0.161 0.161 0.161 0.161 0.161 0.161 0.119 0.119 0.119 0.119 0.119 0.119 0.119 0.119

0.219 0.219 0.219 0.219 0.219 0.219 0.219 0.219 0.169 0.169 0.169 0.169 0.169 0.169 0.169 0.169

−0.906 −0.906 −0.906 −0.906 −0.906 −0.906 −0.906 −0.856 −0.856 −0.856 −0.856 −0.856 −0.856 −0.856

[10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10]

0.341 0.340 0.278 0.324 0.306 0.286

This work

−0.939

[12]

−0.910

[12]

−1.028 −1.020 −0.964 −1.011 −0.993

[21] [21] [21] [21] [21]

−0.973

[23]

−0.924

[22]

−0.994

[22]

−0.906

[10]

−0.856

[10]

AA good correlation between the experimental and theoretical cathodic shift values, ∆E 1/21/2 and ∆E C and A good correlation between the experimental and theoretical cathodic shift values, ∆E 1/2 ∆E A good good correlation correlation between between the the experimental experimental and and theoretical theoretical cathodic cathodic shift shift values, values, ∆E ∆E1/2 1/2 and and ∆E ∆ECCCC 2 shift 2 A good correlation between the experimental and theoretical cathodic shift Agood good correlation between the experimental andtheoretical theoretical cathodic values, ∆E1/2 1/2 and ∆E Cvalues, ∆E1/2 and respectively, have been obtained, giving aa coefficient of determination, RR 0.9728 (Figure 2). The respectively, have been obtained, giving coefficient of determination, 0.9728 (Figure 2). The 222== A correlation between the experimental and cathodic shift values, ∆E and ∆E C respectively, have been obtained, giving a coefficient of determination, R = 0.9728 (Figure 2). The respectively, have beenbetween obtained, giving a coefficient of determination, Rshift = 0.9728 (Figure 2).∆E The 1/2 and C A show good that correlation thegiving experimental and theoretical cathodic values, ∆E1/2 C 2relation 2 = 0.9728 respectively, have been obtained, a coefficient of determination, R (Figure 2). The 2 results the systems can be classified in two groups, aliphatic and aromatic. No clear relation ∆E respectively, have been obtained, giving a coefficient of determination, R = 0.9728 (Figure 2). The results show that the systems can be classified in two groups, aliphatic and aromatic. No clear respectively, been obtained, giving a coefficient of determination, = 0.9728 No (Figure The results that systems can classified in aliphatic aromatic. clear relation resultsCshow showhave that the the systems can be be classified in two two groups, groups, aliphatic and andR aromatic. clear 2). relation 22 = 0.9728 No respectively, have been obtained, giving a coefficient of determination, Raromatic. (Figure 2). The results show that the systems can be classified in two groups, aliphatic and No clear relation between the length of the aliphatic chain and the redox potential is observed. On the other hand, the between the length of the aliphatic chain and the redox potential is observed. On the other hand, the results show that the systems can be classified two groups, aliphatic and aromatic. No clear relation results that thealiphatic systems canand be in classified in two groups, aliphatic and aromatic. between the length of the chain the redox potential is On other hand, the between theshow length of the aliphatic chain and the redox potential is observed. observed. On the the other hand, the No clear relation results show that theof systems can be classified in groups, aliphatic and aromatic. No clear relation between the length the aliphatic chain and thetwo redox potential observed. Onthe the other hand, the aromatic substitution the redox potential ofof the POM ranging from 0.119 toto 0.197 V for the aromatic substitution affected the redox potential the POM ranging from 0.119 0.197 V for the between the length of affected the aliphatic chain and the redox potential isisobserved. On other hand, the aromatic substitution affected the redox potential of the POM ranging from 0.119 to 0.197 V for the between the length of the aliphatic chain and the redox potential is observed. On the other hand, the aromatic substitution affected the redox potential of the POM ranging from 0.119 to 0.197 V for the between the length ofand the aliphatic chain and the redox potential is observed. On the other hand, the aromatic substitution affected the redox potential of the POM ranging from 0.119 to 0.197 V for the p-nitrophenyl group the m-chlorophenyl group, respectively. Thus the ∆E CC depended on the p-nitrophenyl group and the m-chlorophenyl group, respectively. Thus the ∆E depended on the C aromatic substitution affected the redox potential of the POM ranging from 0.119 to 0.197 V for the p-nitrophenyl group and m-chlorophenyl group, Thus the ∆E depended on the p-nitrophenyl group affected and the theaffected m-chlorophenyl group, respectively. Thus the0.119 ∆ECC to depended on the theto 0.197 V for the aromatic substitution the redoxthe potential ofpotential therespectively. POM ranging from 0.197 V for aromatic substitution redox of the POM ranging from 0.119 p-nitrophenyl group and and the m-chlorophenyl m-chlorophenyl group, respectively. Thus the ∆EThe CThe depended on the the ability ofof the organic fragment toto increase the electron density into the POM. most electronability of the organic fragment to increase the electron density into the POM. most electronp-nitrophenyl the respectively. Thus ∆E CThe depended on ability organic fragment the electron density into the POM. most electronability of the the group organic fragment to increase increase thegroup, electron density into thethe POM. most electronp-nitrophenyl groupgroup and the m-chlorophenyl group, respectively. Thus the ∆E CThe depended on the C p-nitrophenyl and the m-chlorophenyl group, respectively. Thus the ∆E C depended on the ability ability of the thegroup, organic fragment tosubstitution, increase theallowed electron density intofragment the POM. POM. The most electronwithdrawing i.e., the nitro for the POM toto be easily reduced. withdrawing group, i.e., the nitro substitution, allowed for the POM fragment to be easily reduced. ability of organic fragment to increase the electron density into the The most electronwithdrawing group, i.e., the nitro substitution, allowed for the POM fragment be easily reduced. withdrawing group, i.e., the nitro substitution, allowed for the POM fragment to be most easilyelectronreduced. ability of the organic fragment increase the electron density into the POM. of the organic fragment totoincrease the electron density into the POM. most electron-withdrawing withdrawing group, i.e., the nitro nitro substitution, allowed for the POM fragment toThe be easily reduced. Moreover, the halogen substitution, which isisallowed an electron donor group, caused aa more difficult Moreover, the halogen substitution, which an electron donor group, caused aaThe more difficult withdrawing group, i.e., the substitution, for the POM fragment to be easily Moreover, halogen substitution, which an donor group, caused more difficult Moreover, the the halogen substitution, which is isallowed an electron electron donor group, caused morereduced. difficult withdrawing group, i.e., the nitro substitution, for the POM fragment to be easily reduced. Moreover, the halogen substitution, which is an electron donor group, caused a more difficult group,the i.e.,halogen the nitro substitution, for the POM fragment be easily reduced. Moreover, Moreover, substitution, which allowed is an electron donor group, caused to a more difficult Moreover, the halogen substitution, which is an electron donor group, caused a more difficult

the halogen substitution, which is an electron donor group, caused a more difficult reduction of the POM. By comparing 1 and 3, it was possible to observe that compound 1 had a higher cathodic shift, probably due to the absence of the methylene group between the aromatic rings interrupting the electronic communication from the electron donating triazole group to the phenyl group (Figure S4). Finally, the cathodic shift of the ReI complex of 3 is lower than that of compound 3 alone, meaning that this coordination compound was acting as a withdrawing group. These results suggest that

reduction of the POM. By comparing 1 and 3, it was possible to observe that compound 1 had a higher cathodic shift, probably due to the absence of the methylene group between the aromatic rings interrupting the electronic communication from the electron donating triazole group to the phenyl group (Figure S4). Finally, the cathodic shift of the ReI complex of 3 is lower than that of compound Molecules 2019, 24, 44 5 of 14 3 alone, meaning that this coordination compound was acting as a withdrawing group. These results suggest that organic functionalization affected the virtual orbitals, therefore shifting the redox organic functionalization affectedItthe orbitals, therefore shifting the redox of these potentials of these systems. is virtual interesting to point out that, as shown in potentials Table 1, all aliphatic systems. It is interesting to point out that, as shown in Table 1, all aliphatic substituted systems show substituted systems show the Mo–N–C angle nearer 180°contrary to the aromatic ones, i.e. the the aromatic Mo–N–Csubstituents angle nearershowed 180◦ contrary aromaticcharacter ones, i.e.,between the aromatic substituents showed less oftoa the triple-bond Mo and N, and an increased lessdouble-bond of a triple-bond character between and increasedsystem double-bond character between N character between N Mo andand C, N, thus theanaromatic could receive electron density andfrom C, thus the aromatic system could receive electron density from the POM, lowering the reduction the POM, lowering the reduction potential of the inorganic fragment. potential of the inorganic fragment.

Figure 2. Relation between the experimental ∆E1/2 and theoretical ∆EC cathodic shift potentials. Figure Relation betweentothe ∆E1/2 and theoretical C cathodic shift potentials. In In black the2.points correspond theexperimental aromatic organo-imido POM and in∆E blue the aliphatic ones. black the points correspond to the aromatic organo-imido POM and in blue the aliphatic ones.

2.3. Spectroscopic Characterization 2.3. Spectroscopic Characterization UV-vis absorption spectra in acetonitrile solution of compounds 1 and 2 showed the lowest energy maximum at 356 nmspectra and 386in nm, respectively (Figureof3 compounds and Figure S5). This2 absorption UV-vis absorption acetonitrile solution 1 and showed theband lowest Molecules 2018,maximum 23,to x FOR 9 of 15 corresponded the PEER overlap theand oxygen molybdenum transfer transition and the organic energy at REVIEW 356ofnm 386 to nm, respectivelycharge (Figures 3 and S5). This absorption band fragment to the molybdenum CT. corresponded to the overlap of the oxygen to molybdenum charge transfer transition and the organic fragment to the molybdenum CT. Theoretical calculations were done in order to understand the electronic spectra of this organoimido systems using eight representative compounds. We have considered four aromatic systems [Mo6O18N-R]2−, where R = –C6H5 (8), –C6H4–N3C2H2 (1), –C13H9 (2) and –C6H4–CH2–N3C2H2 (3), two ReI complexes, [Mo6O18NC6H4-CH2-N3C2H2-phenRe(CO)3](4) and [Mo6O18NC6H4-N3C2H2− phenRe(CO)3] (5), and finally, two aliphatic systems [Mo6O18N–CH2CH3]2− (6), and [Mo6O18N– CH2CH2CH3]2− (7) in order to show how these groups can affect the charge transfer process. Using TD-DFT calculations, we were able to identify the oxygen (O) to molybdenum CT and the organic fragment (O.F.) to molybdenum CT. The hole–electron formalism applied to molecular systems was employed to offer an efficient tool to assign each band, because this scheme takes into account the local and non-local contribution of the set of molecular orbitals involved in each excited state [12,24]. The hole-density surfaces represent electron density be promoted to an excited state (a) allocated in the ground state before it is to (b) (“hole”, light-cyan). The electron-density surface represents the electron density promoted in the Figure 3. Overlap plot of the experimental spectra and oscillator strength calculated using TD-DFT vertical Franck–Condon excited state (“electron”, yellow), thus describing the excited state. Figure 3. Overlap plotCH of 3the spectra and oscillator strength calculated using TD-DFT methods, employing CNexperimental as a continuum solvent. Hole–electron density surface for the most methods, employingof:CH as a continuum solvent. Hole–electron the most intensive excitation (a)3CN compound 1 and (b) compound 3. Light cyan density and lightsurface brown for represent the intensive excitation of: (a) compound 1 and (b) compound 3. Light cyan and light brown represent hole and electron surfaces, respectively. the hole and electron surfaces, respectively.

Theoretical calculations were done in order to understand the electronic spectra of this organo-imido usinghow eight representative compounds. We have fourprocesses aromatic In order tosystems understand a coordination compound can affect the considered charge transfer between the organic fragment to the POM, we have considered two coordination compounds based on ReI. The first corresponds to [Mo6O18NC6H4–CH2–N3C2H2–phenRe(CO)3]−(4), and the second to [Mo6O18NC6H4–N3C2H2–phenRe(CO)3]− (5). Complex 4 is an organometallic system based on 3 that has been previously reported by us [12], whereas compound 5 is a proposed complex that is based on 1 and has been calculated by DFT methods as described in Section 3. The major difference between

Molecules 2019, 24, 44

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systems [Mo6 O18 N-R]2− , where R = –C6 H5 (8), –C6 H4 –N3 C2 H2 (1), –C13 H9 (2) and –C6 H4 –CH2 –N3 C2 H2 (3), two ReI complexes, [Mo6 O18 NC6 H4 -CH2 -N3 C2 H2 -phenRe(CO)3 ]− (4) and [Mo6 O18 NC6 H4 -N3 C2 H2 phenRe(CO)3 ]− (5), and finally, two aliphatic systems [Mo6 O18 N–CH2 CH3 ]2− (6), and [Mo6 O18 N– CH2 CH2 CH3 ]2− (7) in order to show how these groups can affect the charge transfer process. Using TD-DFT calculations, we were able to identify the oxygen (O) to molybdenum CT and the organic fragment (O.F.) to molybdenum CT. The hole–electron formalism applied to molecular systems was employed to offer an efficient tool to assign each band, because this scheme takes into account the local and non-local contribution of the set of molecular orbitals involved in each excited state [12,24]. The hole-density surfaces represent electron allocated state (“hole”, Molecules 2018, 23, density x FOR PEER REVIEW in the ground state before it is to be promoted to an6excited of 15 Molecules 2018, 23, x FOR PEER REVIEW 6 of 15 light-cyan). The electron-density surface represents the electron density promoted in the vertical Molecules 2018, 23, FOR PEERvia REVIEW 6 of 15 A band isxformed several transitions, andyellow), the transition presenting the oscillator Franck–Condon excited state (“electron”, thus describing thehigher excited state. A band is formed via several transitions, and the transition presenting the higher oscillator strength value was taken as the representative. The most intense O.F. to Mo CT bands are presented A band is formed via several transitions, and O.F. the to transition presenting the higher oscillator strength value was taken as the representative. The most intense Mo CT bands are presented A band formed in Table 3 andisTable S2. via several transitions, and the transition presenting the higher oscillator strength value wasastaken as the representative. The most O.F. to Mo bands are presented in in Table 3value and Table S2. strength was taken the representative. The most intense O.F.intense to Mo CT bands areCT presented Table 3 and Table S2. in Table 3 and Table S2. Table 3. Hole-electron surfaces and summary of calculated wavelengths (O.F. → Mo) and oscillator factors

Table 3. Hole-electron surfaces and summary of calculated wavelengths (O.F. → Mo) and oscillator factors (f) of organic fragment to metal charge transfers”. (f) of organic fragment to metal charge transfers”. Table 3. Hole-electron surfaces summary of calculated (O.F. → Mo) and oscillator Table 3. Hole-electron surfaces and summary ofand calculated wavelengths (O.F. →wavelengths Mo) and oscillator factors Mo6O 18N–R λ O.F. charge → Mo (nm) f References (f) of organic fragment to metal charge transfers”. factors (f) of organic fragment to metal transfers”. Mo6O18N–R λ O.F. → Mo (nm) f References Mo6O O18N–R Mo 6 18 N–R

λ O.F.λ → Mo O.F. →(nm) Mo (nm) f

375 375

References f

References

0.2951 0.2951

This work This work

375 375

0.2951

This0.2951 work

This work

389 389 389 389

0.5770 0.5770

This work

0.5770

This work This work 0.5770 This work

346 346 346

0.3772 0.3772

0.3772 [12] [12]

1 1 1 1

2 22 2

346

3 3 3

3

0.3772

[12]

[12]


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Molecules 2018, 23, x FOR PEER REVIEW Molecules 2018, 23, x FOR PEER REVIEW Molecules 2018, 23, x FOR PEER REVIEW Molecules 2018, 23, x FOR PEER REVIEW

Mo6 O18 N–R

7 of 15 7 of 15 7 of 15 7 of 15

Table 3. Cont. λ O.F. → Mo (nm)

f

References

345 345 345 345 345

0.3976 0.3976 0.3976 0.3976

[12] [12] [12] 0.3976 [12]

366 366 366 366 366

0.5993 0.5993 0.5993 0.5993

This work This work This 0.5993 work This work

328 328 328 328 328

0.0100 0.0100 0.0100 0.0100

[21] 0.0100 [21] [21] [21]

328 328 328 328 328

0.0119 0.0119 0.0119 0.0119

[21] 0.0119 [21] [21] [21]

8 of 15

342 342

0.3901

0.3901 [23]

[23]

[12]

4 4 4 4 4

This work

5 5 55 5

[21]

6 66 6 6

Molecules 2018, 23, x FOR PEER REVIEW

[21]

77 7 7 7

8 8

The calculation results show that the band related to the oxygen to molybdenum CT for all studied compounds was practically unaffected, appearing at the same energy of ≈311 nm. The molecular orbitals related to the charge transfer transition from the oxygen to molybdenum centres involved the dxy orbitals. Therefore, these orbitals were not affected by the formation of the imido compound. In the case of the aliphatic systems, the oscillator strength related to the oxygen to molybdenum CT is more intense than the of organic fragment to molybdenum CT, as can be observed in Table 3 and S2. In the case of the oxygen to molybdenum CT for aromatic systems the value of the


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The calculation results show that the band related to the oxygen to molybdenum CT for all studied compounds was practically unaffected, appearing at the same energy of ≈311 nm. The molecular orbitals related to the charge transfer transition from the oxygen to molybdenum centres involved the dxy orbitals. Therefore, these orbitals were not affected by the formation of the imido compound. In the case of the aliphatic systems, the oscillator strength related to the oxygen to molybdenum CT is more intense than the of organic fragment to molybdenum CT, as can be observed in Table 3 and Table S2. In the case of the oxygen to molybdenum CT for aromatic systems the value of the oscillator strength was around ten times lower than the of organic fragment to molybdenum charge transfer. On the other hand, when we compared the CT energy from the organic fragments to the molybdenum, it could be observed that fluorene was the one with the lowest energy absorption, whereas the aliphatic-substituted ones were those with highest energy absorption. Therefore, it can be inferred that a bathochromic shift exists, with this being related to the electron density given by the π-donor character of the organic fragment to the POM, decreasing the energy difference between the ground and excited states. All these results suggest that when the organic fragment is more conjugated, the O.F. to molybdenum CT band shifted to lower energies. The molecular structure of compound 3 differed from 1 only in the existence of the methylene group between the phenyl and triazole fragments [12]. In 1, the CT band appeared at lower energies (375 nm) compared to 3, in which the CT energy absorption was at 346 nm. The existence of the methylene group in 3 disrupts the conjugation between phenyl-imido-POM and the triazole ring. The orbital structure based on frontier molecular orbitals was studied. For both systems, the LUMO (Lowest Unoccupied Molecular Orbital) was located mainly on the Mo centres, resembling the dxy orbitals, whereas the HOMO (Highest Occupied Molecular Orbital) was located over the organic fragment (Figure S6). In 1, the distribution of the HOMO was allocated all over the aromatic fragment (phenyl and triazole units), which does not happen in 3. The hole-electron surfaces show that the electronic communication between both aromatic rings is highly conjugated in 1, confirming that the electron transfer process occurs from the whole aromatic fragment to the molybdenum centres. When a comparison was made between this system and 3, it was possible to observe that the electron transfer occurred from the phenyl group to the POM, meaning that the methylene group (–CH2 –) cuts the electronic communication between the POM and 1,2,4-triazole ring (Figure 3). In order to understand how a coordination compound can affect the charge transfer processes between the organic fragment to the POM, we have considered two coordination compounds based on ReI . The first corresponds to [Mo6 O18 NC6 H4 –CH2 –N3 C2 H2 –phenRe(CO)3 ]− (4), and the second to [Mo6 O18 NC6 H4 –N3 C2 H2 –phenRe(CO)3 ]− (5). Complex 4 is an organometallic system based on 3 that has been previously reported by us [12], whereas compound 5 is a proposed complex that is based on 1 and has been calculated by DFT methods as described in Section 3. The major difference between 4 and 5 is the presence of the methylene group between the phenyl and triazole rings. Both systems allowed us to compare how the coordination compound could affect the charge transfer processes (O.F. → Mo and O → Mo) in the compounds. The calculation results showed that the band related to the oxygen to molybdenum CT was unaffected by the presence of the ReI complex, since the energy of this absorption for 4 and 5 were at ≈311 nm, which is the same energy observed for 1 and 3, the corresponding non-metalated systems (See Table S2). The energy of the O.F. to Mo CT for 4, compared to 3, was only 1 nm shifted to higher energies (See Table 3). However, in the case of 1 and 5, this energy difference was 9 nm, shifted to higher energies (See Table 3). These results suggest that the coordination compound fragment had a small effect in modifying the CT processes of organo-imido systems. For 4 and 5, the LUMO orbitals were located mainly in the Mo centres, resembling the dxy orbitals, as observed for 1 and 3. The HOMO orbitals were located over the organic fragment, the ReI centre and a small part in the CO ligands of the complex. The comparison between these frontier orbitals of 4 and 5 shows that there was a major distribution of these orbitals over the whole organic fragment, even


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over the ReI centre in 5 (Figure S6). The hole–electron surface for 5 showed that the electron surface morphology was distributed all over the organic fragment and in the metal centres of the POM, which is quite similar to the dxy orbital of the Mo ions. This characteristic could be due to the presence of the Molecules 2018, 23, x in FOR REVIEW 10 of methylene group 4, PEER meaning that in the case of 5, the extension of the conjugation was between all15 the organic fragment of the organo-imido POM to the ReI complex (Figure 4).

(a)

(b)

Figure 4.4.Hole-electron surfaces forfor complex 4 (a) and 5 (b) are Light cyan and light brown Figure Hole-electron surfaces complex 4 (a) and 5 (b) arepresented. presented. Light cyan and light brown represent the hole and electron surfaces, respectively. represent the hole and electron surfaces, respectively.

All Allthese theseresults resultssuggested suggestedthat thatwhen whenthe theorganic organicfragment fragmentofofananorgano-imido organo-imidoPOM POMsystem systemwas was more conjugated, the CT band moved to lower energies. The functionalization with a coordination more conjugated, the CT band moved to lower energies. The functionalization with a coordination compound shift ofof this band toto higher energies. compoundproduced produceda slight a slight shift this band higher energies. 3. Materials and Methods 3. Materials and Methods 3.1. Reagents and Instruments 3.1. Reagents and Instruments All reagents were used without further purification. [n-Bu4 N]2 [Mo6 O19 ] was synthesised All reagents were used without further purification. [n-Bu4N]2[Mo6O19] was synthesised according to a previously reported method in the literature [25]. The UV-vis spectra were obtained according to a previously reported method in the literature [25]. The UV-vis spectra were obtained in acetonitrile at room temperature using a Perkin-Elmer Lambda 1050 Wideband UV-vis-NIR in acetonitrile at room temperature using a Perkin-Elmer Lambda 1050 Wideband UV-vis-NIR spectrophotometer (Perkin-Elmer, Waltham, MA, USA). FTIR-ATR (Fourier Transform Infraredspectrophotometer (Perkin-Elmer, Waltham, MA, USA). FTIR-ATR (Fourier Transform InfraredAttenuated Total Reflectance) spectra (4000–400 cm−1 )−1of the compounds were measured using a Jasco Attenuated Total Reflectance) spectra (4000–400 cm ) of the compounds were measured using a Jasco FTIR-4600 spectrophotometer equipped with an ATR PRO ONE (Jasco, Easton, MD, USA). Elemental FTIR-4600 spectrophotometer equipped with an ATR PRO ONE (Jasco, Easton, MD, USA). Elemental analyses (C, N, H) of bulk samples were performed using a Thermo elemental analyser Flash 2000. analyses (C, N, H) of bulk samples were performed using a Thermo elemental analyser Flash 2000. Electrospray ionization mass spectrometry (ESI-MS) studies were done for compounds 1 and 2 in Electrospray ionization mass spectrometry (ESI-MS) studies were done for compounds 1 and 2 in a a Thermo scientific Linear Ion Trap Mass Spectrometer LTQ XL. The mass detection was carried Thermo scientific Linear Ion Trap Mass Spectrometer LTQ XL. The mass detection was carried out out using electrospray ionization (ESI), with the spray voltage set at 3 kV (at 250 ◦ C). Detection was using electrospray ionization (ESI), with the spray voltage set at 3 kV (at 250 °C). Detection was performed in full scan mode in the 100–1000 m/z range in negative mode. The spectra were obtained performed in full scan mode in the 100–1000 m/z range in negative mode. The spectra were obtained using He for collision-induced fragmentation (CID) with a normalized collision energy of 35 units using He for collision-induced fragmentation (CID) with a normalized collision energy of 35 units and detection of fragments in full scan mode. Cyclic voltammetry (CV) studies were performed on a and detection of fragments in full scan mode. Cyclic voltammetry (CV) studies were performed on a CH-Instruments 650E potentiostat. All electrochemical data were obtained in acetonitrile (SeccoSolv® CH-Instruments 650E potentiostat. All electrochemical data were obtained in acetonitrile (SeccoSolv® Merck, Darmstadt, Germany), with sample concentration of 1.0 mM and 0.1 M tetrabutylammonium Merck, Darmstadt, Germany), with sample concentration of 1.0 mM and 0.1 M tetrabutylammonium perchlorate (n-Bu4 NClO4 ) as the supporting electrolyte. A standard three-electrode cell was used, with perchlorate (n-Bu4NClO4) as the supporting electrolyte. A standard three-electrode cell was used, a 3 mm diameter glassy carbon working electrode, a platinum wire counter electrode and an Ag/AgCl with a 3 mm diameter glassy carbon working electrode, a platinum wire counter electrode and an Ag/AgCl coupled with a Luggin capillary reference electrode. All potentials were referred to the redox potential of ferrocene (Fc) / ferrocenium ion (Fc+) as an internal reference (Figures S3 and S4). 3.2. Synthesis of [n-Bu4N]2[Mo6O18NC6H4N3C2H2] (1) A mixture of [n-Bu4N]2[Mo6O19] (1.3636 g, 1.0 mmol), 4-(1H-1,2,4-Triazol-1-yl)aniline (0.1762 g,


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coupled with a Luggin capillary reference electrode. All potentials were referred to the redox potential of ferrocene (Fc)/ferrocenium ion (Fc+ ) as an internal reference (Figures S3 and S4). 3.2. Synthesis of [n-Bu4 N]2 [Mo6 O18 NC6 H4 N3 C2 H2 ] (1) A mixture of [n-Bu4 N]2 [Mo6 O19 ] (1.3636 g, 1.0 mmol), 4-(1H-1,2,4-Triazol-1-yl)aniline (0.1762 g, 1.1 mmol), and N,N 0 -dicyclohexyl carbodiimide (DCC) (0.2682 g, 1.3 mmol) was added to 20 mL anhydrous acetonitrile under a N2 gas flow and refluxed for 24 h. During the course of the reaction, the colour of the solution changed from yellow to red, and some white precipitate (N,N 0 -dicyclohexylurea obtained via the decomposition of the DCC) was formed. After being cooled down to room temperature, the red solution was filtered to remove the white precipitate. The filtrate was poured into 150:50 mL of diethyl ether:ethanol obtaining a red precipitate. The solid was washed with ethanol, ethyl acetate, and diethyl ether several times and then dissolved in acetone. The red solution was filtered and concentrated with a rotary evaporator. Single crystals were obtained within five days in a tube by slow diffusion of diethyl ether into an acetone solution of the crude product. (See Supporting Information, Figure S1) Yield: 0.1250 g, ≈8.3% based on Mo. Elemental Analysis: Calculated (experimental) for C40 H78 Mo6 N6 O18 : C: 31.89% (32.03); N: 5.58% (5.39); H: 5.22% (6.91) FTIR-ATR (cm−1 ): 2959(w), 2933(w), 2871(w), 1507(m), 1474(m), 1462(w), 1379(w), 1331(w), 1277(w), 1213(w), 1140(w), 1104(vw), 1046(w), 1025(vw), 974 (m, shoulder: characteristic of mono-organoimido-substituted hexamolybdate), 948(s), 882(w), 845(w), 771(vs) (Figure S7). UV-vis (CH3 CN, nm): λmax = 356. ESI-MS (CH3 CN, m/z): 510.6 [Mo6 O18 NC6 H4 -N3 C2 H2 ]2− (Figure S8). 3.3. Synthesis of [n-Bu4 N]2 [Mo6 O18 NC13 H9 ] (2) A mixture of [n-Bu4 N]2 [Mo6 O19 ] (1.3636 g, 1.0 mmol), 2-Aminofluorene (0.1994 g, 1.1 mmol), and N,N 0 -dicyclohexyl carbodiimide (DCC) (0.2682 g, 1.3 mmol) was added to 20 mL anhydrous acetonitrile under a N2 gas flow and refluxed for 24 hours. During the course of the reaction, the colour of the solution changed from yellow to red, and some white precipitate (N,N 0 -dicyclohexylurea was obtained via the decomposition of the DCC) was formed. After being cooled down to room temperature, the red solution was filtered to remove the white precipitate. The filtrate was poured into 150:50 mL of diethyl ether:ethanol obtaining a red precipitate. The solid was washed with ethanol, ethyl acetate, and diethyl ether for several times and then dissolved in acetone. The red solution was filtered and concentrated with a rotary evaporator. Single crystals were obtained within seven days in a tube by slow diffusion of diethyl ether into an acetonitrile solution of the crude product. (See Supporting Information, Figure S2) Yield: Yield: 0.1200 g, ≈7.8% based on Mo. Elemental Analysis: Calculated (experimental) for C45 H81 Mo6 N3 O18 : C: 35.38% (35.50); N: 2.75% (2.63); H: 5.34% (5.93) FTIR-ATR (cm−1 ): 2959(m), 2931(m), 2871(m), 1712(w) 1631(w), 1599(w), 1479(m), 1448(m), 1378(w), 1322(w), 1295(w), 1223(w), 1154(w), 1107(w), 1065(vw), 1026(vw), 975 (w, shoulder: characteristic of mono-organoimido-substituted hexamolybdate), 945(s), 880(w), 765(vs) (Figure S7). UV-vis (CH3 CN, nm): λmax = 386. ESI-MS (CH3 CN, m/z): 522.1 [Mo6 O18 NC13 H9 ]2− (Figure S8). 3.4. Crystal Structure Determination Single-crystal X-ray diffraction analyses of 1 and 2 were done on a Bruker Smart APEX2 system at 297 K and SAINT [26] was used for data reduction. The crystal structures were solved by direct methods using XS in SHELXTL [27]. Hydrogen atom positions were calculated after each cycle of refinement with SHELXL [28] using a riding model for each structure, with a C–H distance of 0.93 or 0.97 Å. Uiso (H) values were set equal to 1.2Ueq of the parent carbon atom. For 1, during the last stages of refinement, it was clear that the position of the nitrogen atoms in the 1,2,4-triazole ring of 1 ([Mo6 O18 NC6 H4 N3 C2 H2 ]2− ) may have two possible conformations, A and B. Occupations were then refined (with the constraint of adding 1) and finally held constant at 50%/50% for A/B, respectively (Scheme S1). Additional crystallographic and refinement details are given in Table S1. Structural drawings were carried out with DIAMOND-3.2k, supplied by Crystal Impact [29]. Crystallographic


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data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC-1577980 (1) and CCDC-1881932 (2). 3.5. Computational Details 3.5.1. Prediction of UV-Vis Spectra Geometry optimization for the all the systems studied in this work were done using DFT calculations. The hybrid PBE0 functional [30–32] and a triple−ζ basis set for H, C, N, and O atoms including a polarization function for non-hydrogen atoms were employed [33]. Prediction of the electronic spectrum was developed using the time-dependent density functional theory included in the Gaussian09 Code [34]. The hybrid functional of Perdew Burk and Ernzerhof, PBE0, was used to represent the electron density. We have chosen a hybrid functional for the calculations instead of a GGA (Generalized Gradient Approximation) BP86 (Becke-Perdew 86) [35] because the HOMO–LUMO gap will be under-estimated due to the lack of a Hartree-Fock exchange. Therefore, the calculated electronic spectra will not be representative. For the Mo centres, the Stuttgart–Dresden core effective potential was employed to represent the orbital basis of these centres [36]. The continuum SMD (Solvation Model based on Density) model was employed to simulate the solvation effect including geometry relaxation [37]. All relaxed structures obtained were validated by vibrational analysis. In all calculations, 150 singlet excitations were considered. The more intense excitations were treated using the hole–electron framework [24] by means of the MULTIWFN3.6 package [38]. Orbital and electronic density surfaces were plotted using isovalues of 0.05 and 0.001 atomic units, respectively. 3.5.2. Theoretical Redox Potentials All calculations were carried out using the conceptual framework of density functional theory included in the Gaussian09 Code [34]. An open-shell spin-unrestricted approach was considered to describe the electron density, using the hybrid functional B3LYP (Becke, three-parameter, Lee-Yang-Parr) [39]. To represent H, C, N, and O atoms, a tripleε−ζ orbital basis set including a polarization function for non-hydrogen atoms was employed [33]. In the case of Mo centres, the relativistic pseudo-potential proposed by Hay and Wadt LANL2DZ (Los Alamos National Laboratory 2-double-z) was used [40]. The continuum universal solvation model developed by Thrular (SMD) was employed to represent the acetonitrile solvation effect, and was considered for all geometry optimization processes and single point calculations [37]. All structures were validated by means of their vibrational analyses. Adiabatic redox potentials were calculated from solvated geometries considering the direct step as recommended by Poblet et al. [14]. Ox(solv) + e(solv) → Red(solv)

(1)

The Redox potentials were obtained from Equation (2) considering an only one–electron reaction: ∆Gredox = nFEredox

(2)

where ∆Gredox is the Gibbs free energy change of the redox reaction, n is the number of electrons transferred in the reaction, F is Faraday´s constant, and Eredox is the redox potential. To refer the potential to the ferrocene/ferrocenium process, these redox potentials were calculated with the same method, i.e., as an adiabatic redox potential considering relaxed geometries in solution. 4. Conclusions For aliphatic substituents a more sp nitrogen atom is observed, therefore it interacted as a triple bond to the molybdenum atom and single bond to the substituent. On the other hand, for the aromatic substitution, the interaction presented less of a triple-bond character to the molybdenum, and an increased double-bond interaction to the substituent.


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For all the organo-imido studied systems, the aliphatic fragments compared to the aromatic ones had a bigger influence in the electrochemical properties as observed using the cathodic shift parameter. The spectroscopic properties of all studied organo-imido compounds showed that the aromatic substitution with a higher conjugation could shift to higher energies through the O.F. to Mo CT compared to the aliphatic substituents. The CT band was almost not affected by the coordination of a metal complex connected to organo-imido POM. No relation between the wavelength of both CT transitions and the theoretical and experimental cathodic shift was observed, suggesting that the orbitals involved in each process were not equally affected upon functionalization (Figure S9). Supplementary Materials: The following are available online. X-Ray structures of 1 and 2 Figure S1. (a) Ball-and-stick representation of organic-inorganic hybrid ligand 1. (b) Hydrogen bond interactions, showing the connectivity between each organoimido-POM units. [n-Bu4 N]+ ions are omitted for clarity. Scheme S1. Nitrogen location possibilities for the 1,2,4-triazole ring in 1. Figure S2. (a) Ball-and-stick representation of 2. (b) Crystalline lattice. [n-Bu4 N]+ ions are omitted for clarity. Table S1. Crystal parameters and refinement details for 1 and 2. Figure S3. Cyclic voltamograms of 1 and 2, measured in CH3 CN and n-NBu4 ClO4 as the supporting electrolyte at a scan rate of 100 mV s−1 . Figure S4. Comparison of the cyclic voltamograms of Mo6 , [n-NBu4 ]2 [Mo6 O19 ] (black), 1 (blue), and 3 (red) measured in CH3 CN and n-NBu4 ClO4 as the supporting electrolyte at a scan rate of 100 mV s−1 . All measurements are referred to the Fc/Fc+ potential. Figure S5. Overlap plot of the experimental spectra and oscillator strength calculated using TD-DFT methods, employing CH3 CN as the continuum solvent for 2. Figure S6. Frontier orbitals surfaces (HOMO–LUMO) of systems 1, 2, 3, 4 and of the calculated structure of 5. Figure S7. FTIR-ATR spectrum in the 400 to 4000 cm−1 region of compound 1 (blue), 2 (red), and Mo6 (black). Figure S8. Experimental ESI-MS in CH3 CN and theoretical isotopic patterns of 1 and 2. Figure S9. Relation between the charge transfer processes and the electrochemical processes. (a) Organic fragment to molybdenum charge transfer band vs. the experimental cathodic shift. (b) Oxygen to molybdenum charge transfer band vs. the experimental cathodic shift. (c) Organic fragment to molybdenum charge transfer band vs. the theoretical cathodic shift. (d) Oxygen to molybdenum charge transfer band vs. the theoretical cathodic shift. Author Contributions: All authors have contributed equally in different part of the measurements, analysed and redaction of the article. Acknowledgments: The authors thank ACT-1404 (IPMaG), FONDECYT 1161255 and Financiamiento Basal FB0807 (CEDENNA). Powered@NLHPC: This research was partially supported by the super-computing infrastructure of the National Laboratory for High Performance Computing, NLHPC (ECM-02), CMM, U de Chile. The authors thank CONICYT FONDEQUIP/UHPLC MS/MS EQM 120065. This work was done under the LIA-M3 1027 CNRS Collaborative Program. WCM and DVY thank DICYT, Universidad de Santiago de Chile (USACH) POSTDOC_DICYT, 021640VY_POSTDOC, Vicerrectoria de Investigación, Desarrollo e Innovación. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds 1 and 2 are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).