Stepwise synthesis of the heterotrimetallic chains

Volume 47 Number 5 7 February 2018 Pages 1357–1740

Dalton Transactions An international journal of inorganic chemistry

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ISSN 1477-9226

PAPER Shie-Ming Peng et al. Stepwise synthesis of the heterotrimetallic chains [MRu2(dpa)4X2]0/1+ using group 7 to group 12 transition metal ions and [Ru2(dpa)4Cl]

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Stepwise synthesis of the heterotrimetallic chains [MRu2(dpa)4X2]0/1+ using group 7 to group 12 transition metal ions and [Ru2(dpa)4Cl]† Ming-Chuan Cheng, a,b Shao-An Hua,a Qiying Lv,a,b Marc Sigrist,a,b Gene-Hsiang Lee, a Yu-Chiao Liu,b Ming-Hsi Chiang b and Shie-Ming Peng

*a,b

The CoRu2(dpa)4Cl2 (1) (dpa: 2,2’-dipyridylamide) is synthesized by the reaction of Ru2(OAc)4Cl and Co3(dpa)4Cl2. By mixing 1 with NH3, Co2+ can be removed and result in the formation of unique binuclear complex 4,0-Ru2(dpa)4Cl (2) featuring one coordination pocket supported by free pyridine groups. Hence, this complex can act as an outstanding precursor for the formation of heterotrimetallic chains with MRu2 cores. A series of M–Ru25+ complexes (M = Co2+ (3), Ag+ (4), Mn2+ (5), Fe2+ (6), Zn2+ (7), Cd2+ (8), Pd2+ (9), Rh2+ (10), and Ir2+ (11)) were prepared and isolated, representing the most complete series of heterotrimetallic chains to date. All these metal string complexes are in a linear trimetallic framework helically wrapped by four dpa− ligands, characterized by X-ray diffraction measurements. The bending of the trinuclear metal cores in RhRu2 (10) and IrRu2 (11) (∠Ru–Ru–Rh: 167.58° and ∠Ru–Ru–Ir: 167.61°) indicates Received 1st November 2017, Accepted 18th December 2017

that a heterometallic metal–metal bonds (Ru–Rh; Ru–Ir) are generated. The studies from DFT calculation of

DOI: 10.1039/c7dt04114d

10 and 11 coincide with the experimental results. Furthermore, the M⋯Ru25+ distances are regulated by the factors including the bonding force of M–pyridyl and the static repulsion between M and Ru25+ unit.

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Interestingly, the trend for these distances is in line with that observed in trans-M(py)4Cl2 complexes.

Introduction The study of heterometallic complexes continually leads researchers to create new compounds with diverse structural, electronic and magnetic properties as well as substantial catalytic activity.1–8 Additionally, extended metal-atom chains (EMACs) are proved conductors and have therefore possible applications as metal wires in the field of nanotechnology. The combination of different metal centers within atom chains can generate an asymmetric current–voltage response, a potentially advantageous performance feature of molecular-electronic devices.9,10 Many tri-nuclear heterometallic chains have been investigated. The complexes can be divided into three classes, the symmetric MA–MB–MA type, as well as the asymmetric MA– MB–MC and MA–MA–MB types.11 The major representatives of the latter class of complexes are formed from a multiply bonded MA–MA dimer and a third metal MB, such as MCr2, MMo2, MW2, and MRu2.12–15 In the case of MRu2, only the Cu

and Ni variants are reported, where the M+/2+ ions are coupled to the Ru25+ moiety to form the (MRu2)6+/7+ core. The interactions between Ru2 and other metals are still largely unexplored and need to be studied further. Generally, there are two methods to construct heterometallic metal chains with dpa− ligands. One is a regioselective reaction by mixing different kinds of metal ions with the ligands in a one-step reaction. In this case, the metal ions freely take the most stable positions to establish the metal atom chain. The other method is stepwise reaction, where an additional metal ion is introduced into a dimetallic complex which is wound by four dpa− ligands (Scheme 1, the definition of “2,2-trans” and “4,0-form” is from previous report16). Heterotrimetallic chain complexes by reaction of 2,2-trans dinuclear complexes (Cr, Mo, W) with other metals have been

a

Department of Chemistry, National Taiwan University, Taipei, Taiwan. E-mail: [email protected] b Institute of Chemistry, Academia Sinica, Taipei, Taiwan † Electronic supplementary information (ESI) available. CCDC 1582131–1582142. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt04114d

1422 | Dalton Trans., 2018, 47, 1422–1434

Scheme 1

(A) 2,2-trans form, (B) 4,0-form.

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established by Berry group.14,15,17–20 For the development of a MRu2 series and the assessment of their properties, the 4,0-form of the diruthenium complex, which is similar to the well-known 4,0-Ru2(ap)4Cl (ap: phenylamidopyridyl), is designed to serve as an excellent precursor. The (4,0) conformation provides a coordination pocket consisting of four uncoordinated pyridyl groups at the same side. There is no requirement for 2 dpa− ligands shift to produce a heterotrinuclear chain like it is the case for the 2,2-trans form. Herein, we synthesized a series of heterotrimetallic chains [MRu2(dpa)4X2]0/1+ by a stepwise synthesis using Groups 7 to 12 transition metal ions and [Ru2(dpa)4Cl].

Results and discussion Synthetic procedures Ru2(ap)4Cl and its derivatives are known for years. Many trials to synthesize Ru2(dpa)4Cl by the reaction of Ru2(OAc)4Cl with the Hdpa ligand according to the Ru2(ap)4Cl methods were unsuccessful.21,22 Similarly, neither did the reaction of Hdpa, Ru2(OAc)2Cl, and CoCl2 with base yield the expected Ru/Co heterotrimetallic complex.13 However, an alternative approach was found by using Ru2(OAc)2Cl and [Co3(dpa)4Cl2] as reactants. The [Co3(dpa)4Cl2] is thermally unstable with a decomposed species containing the dpa− and cobalt ions used for the construction of the heterometallic metal chain (Scheme 2). This method shortened the reaction time and prevented side products from the reaction of diruthenium acetate with Hdpa.23 [CoRu2(dpa)4Cl2] (1) is fairly stable in benzene and in

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solid form but it is air-sensitive in CH2Cl2 solution as it gets oxidized to [Ru2(dpa)4Cl] (2) and [CoRu2(dpa)4Cl2]+ (3) in a yield ratio of 2 : 1, as determined through the 1H NMR integrals (see Fig. S1†). However, only compound 2 was observed during early stages of the oxidation, while compound 3 was only detectable after several hours. This suggests that the formation of compound 3 was not directly from the oxidization of 1 by air but rather from the reaction of Co2+ with compound 2 (eqn (1)). Compound 3 is stable in CH2Cl2 solution and did not show any decomposition or loss of the cobalt ion. If compound 1 was dissolved in non-polar solvent (benzene), it prevented the charge separation of the complex, the disintegration to the [Ru2(dpa)4Cl]− and [CoCl]+ species. Previously, the Co2+ metal ion was considered stable in terminal position of dpa− supported metal-atom chains based on several reports.14,24,25 As the loss of cobalt ions in EMACs is uncommon, we do not have an explanation for this fact yet. For the improved synthesis of the targeted diruthenium compound 2 in its 4,0-form, NH3 was introduced to a dichloromethane solution of compound 1 to capture the cobalt ions and prevent the formation of compound 3. Compound 2 was isolated as air-stable dark red crystals. However, it decomposes into an unknown green component when in contact with water for several hours. Compound 3 could be obtained via oxidation of compound 1 using FcPF6 or by the reaction of CoCl2 with compound 2. Unlike in compound 1, the Co2+ metal ion in compound 3 did not dissociate to generate compound 2 when dissolved in CH2Cl2.

ð1Þ

Scheme 2 Various types of synthetic route of [MRu2(dpa)4Cl2]0/1+ complexes.


The four free pyridyl groups of compound 2 provide a cavity for coordination and is therefore undoubtedly a suitable candidate for two-step metalation to construct of heterometallic chains of the form [MRu2(dpa)4Cl2]+ under mild conditions. Therefore, several new elements could be incorporated into heterometallic chains with high yields, such as Ag, Cd, and Ir. The isolated compounds are the first representatives of those metal ions in dpa− ligand supported metal strings. To obtain [AgRu2(dpa)4Cl(OAc)] (4), compound 2 was stirred with an excess amount of Ag(OAc) in benzene. According to X-ray structure analysis, the acetate group is coordinated to the terminal ruthenium ion rather than the silver ion. Due to chlorides higher affinity for silver, Cl− from the ruthenium site gets captured by the silver ion. In this paper, we furthermore present the synthesis of new heterometallic metal chains [MRu(dpa)4Cl2]+ with the following metals, Mn2+ (5), Fe2+ (6), Zn2+ (7), and Cd2+ (8) using the corresponding chloride salts as metal sources. Furthermore, the previously reported metal chains of Ni2+, Cu2+, and Cu+ with diruthenium Ru25+ and [Ru3(dpa)4Cl2]+ could also be obtained by treating NiCl2, CuCl2, CuCl, and Ru(CH3CN)4Cl2, respectively, with compound (2). In the case of the Pd2+ analog 9, [Pd(CH3CN)4](BF4)2 was

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used in order to avoid having to break the inert Pd–Cl polymer when using the usual precursor PdCl2. Those complexes could be simply obtained by mixing the related metal ions with 2 in CH2Cl2 at room temperature. All above complexes are stable in solution and do not show any detachment of the terminal metal ion. Rh(I) and Ir(I) were chosen to be the precursors from the Co family (group 9) due to their higher reactivity compared to RhCl3 and IrCl3. The reaction of [Rh(COD)2](BF4) with compound (2) was carried out under mild condition and resulted in a highly air sensitive dark blue solution. The crystal growth of the proposed product [RhRu2(dpa)4Cl](BF4) was not successful, therefore, LiCl was added to the solution as an additional source of Cl− axial ligands and the stable [RhRu2(dpa)4Cl2]+ was isolated after oxidation by air. As for the iridium metal ion, the reaction was carried out in refluxing naphthalene and the isolated compound was similar to the Rh case, as only the oxidized form could be obtained. With the above compounds and the previous reports by our group,13,26 we mostly completed the heterometallic M–Ru25+ chain framework, where M are the first and second row transition metal ions from group 7 to group 12. Crystallography Diruthenium complex. The crystallographic data for all compounds are listed in ESI Table S1.† The ORTEP view of 2 is displayed in Fig. 1 and its selected bond distances are listed in Table 1. The diruthenium unit Ru25+ is surrounded by four dpa− ligands in a typical paddlewheel structure with a (4, 0) form. The Ru–Ru distance is 2.2932(5) Å, which is among the ordinary distance range of Ru25+ with N,N′-donor bridging ligands.27 Compound 2 is a structural analogue to Ru2(ap)4Cl (ap: phenylamidopyridyl),21 though the difference is that one of the additional pyridines in the four dpa− ligands is weakly

coordinated to the Ru metal with a Ru2–N3 distance of 2.490(4) Å. Due to this weak coordination, the Ru–Ru metal bond distance in 2 is slightly elongated compared to Ru2(ap)4Cl (Ru–Ru: 2.275 Å). In order to enable this coordination, this dpa− ligand requires to be slightly oblique. Consequently, the distance of Ru2–N2 2.032(3) Å is shorter than the Ru2–N average (2.051 Å) and the Ru1–N1 distance (2.132(4) Å) is longer than the Ru1–N average (2.111 Å). However, this Ru2–N3 bond is breakable in solution. The 1H NMR spectrum of 2 (ESI, Fig. S5(b)†) shows only 8 observable peaks, which indicates that the four pyridyl groups are in fast interchange coordinating to the Ru ion with an exchange rate beyond the 1H NMR timescale. The Ru–Cl distance in 2 is rather long (2.5349(12) Å), compared to the value of 2.437 Å observed for Ru2(ap)4Cl. Neutral heterotrimetallic compounds. Compound 1 crystallizes in the orthorhombic space group Pnn2, having a disordered structure of the trimetallic framework as shown in Fig. 2(a). The terminal metal positions are each occupied by ruthenium and cobalt in equal ratio and the Ru–Ru bond distance 2.324 Å is similar to the Ni analogue (2.337 Å) having longer distance compared usual Ru25+ distances (2.25–2.29 Å). This may be explained by the diruthenium unit being Ru24+ (double bond) rather than Ru25+ (two and half bonds). The Ruinner–Co distance 2.396 Å is shorter than Co⋯Pd (2.518 Å) in [CoPdCo(dpa)4Cl2]24 and Co⋯Co2 (2.459–2.472 Å) in usym[Co3(dpa)4Cl2].28,29 With a such extremely short distance (FSR (Formal Shortness Ratio): 1.02; calculated by single bond of covalent radii),30 a metal–metal interaction or bonding is

Fig. 1 ORTEP view of the molecular structure of 2. All atoms are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.

Table 1

Selected bond lengths (Å) for Ru2(dpa)4Cl2 and Ru2(ap)4Cl21

Ru2(dpa)4Cl Ru2(ap)4Cl

Ru1–Ru2

Ru1–Nav

Ru2–Nav

Ru1–Cl1

2.2932(5) 2.275(3)

2.111(4) 2.104(12)

2.051(4) 2.026(12)

2.5349(12) 2.437(7)

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Fig. 2 ORTEP view of the molecular structure of 1 (a) and 4 (b). All atoms are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.


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expected. Owing to the structural disorder and the indistinguishable axial chlorides, exact bond distances of Ru–Cl and M–Cl cannot be obtained and can therefore not be further discussed here. [AgRu2(dpa)4Cl(OAc)] (4) crystallizes in the monoclinic space group C2/c and has a non-disordered linear metal atom chain structure, which is aided by its unequal axial ligands Cl− and OAc− as shown in Fig. 2(b). The silver ion is in a square pyramidal geometry with a Cl− axial ligand. It is a rare example of an Ag+ ion in square pyramidal geometry similar to the reported complex [Ag(Hdpa)4NO3]31,32 whose structure was described as four pyridyl groups coordinated in square planar geometry with two weakly interacting axial NO3− ligands (Ag⋯ONO2 > 2.711 Å). Compound 4 is furthermore the first example of an EMAC containing a silver ion. The Ru–Ru bond length of 2.2801 Å in 4 is slightly shorter than in compound 2 and longer than in [CuRu2(dpa)4Cl2] 2.246 Å, but is still within the expected range of Ru–Ru bond distances of structurally similar Ru25+ complexes. The complex features a long Ru⋯Ag distance of 2.7858 Å. This means that the silver ion is shift out from the basal plane of the pyramid groups by about 0.59 Å. Both 4 and [CuRu2(dpa)4Cl2] contain a d10 ion, Ag+ and Cu+, respectively, but they have significantly different M⋯Ru distances. As the Ru25+ units have the same charge in both compounds and there is also no charge difference between the terminal ions Cu+ and Ag+, differences in Coulomb static repulsion can be neglected as reason for the difference. The Ru⋯M distance is mainly governed by the M–Npyridine distances. The silver requires a longer M+–Npyridine distance compared to Cu+. Hence, it is pushed away from the basal plane to achieve that distance. The distance of Ru–OAc 2.159 Å is close to the Ru–O axial (2.172 Å) in [Ru2(O2CCH2CH3)5].33 Reasonably, this shorter distance is due to the small atom size of oxygen compared to chloride. Interestingly, this complex 4 has the shortest Ru–Nouter distance 2.086 Å and the longest Ru– Ninner among all the Ru25+ compounds in this article. An analysis of the bond distances of the coordinated dpa− ligands in this compound (ESI, Fig. S2†) reveals that the Cpyridyl–N2 and Cpyridyl–N5 distances at the Ru25+ unit coordination site is longer than the one on the Ag+ site (1.370 vs. 1.396 Å). It reveals that the more negative charge delocalized among the N–C–N in the diruthenium site rather than silver. A similar distribution of the bond lengths is observed in compound 2. Cationic heterotrimetallic compounds. The ORTEP view of 3 is displayed in Fig. 3(a). Compound 3 crystalizes in the monoclinic space group P21/n with structural disorder as it is expected for a trimetallic framework with identical Cl− ligands at the axial position. The occupancy of Ru/Co at the terminal positions is about 0.55/0.45. A comparison of the distances in the crystal structures of 2 and 3 (Tables 2 and 3) reveals that the Ru–Ru bond (2.265 Å) and Ru–Ninner (2.018 Å) in 3 are shorter than that in 2 (2.324 and 2.040 Å, respectively). Those discrepancies are due to the oxidation of diruthenium from Ru24+ to Ru25+. The Ru⋯Co distance (2.608 Å) is similar to the Mo⋯Co distance (2.617 Å) in [CoMo2(dpa)4Cl2]17 which does not show metal–metal bonding. The Co–N distance (2.160 Å) indicates that the Co2+ ion is in a high spin state.


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Fig. 3 ORTEP view of the molecular structure of 3 (a) and 9 (b). All atoms are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.

Table 2 Selected bond lengths (Å) for neutral compounds: [MRu2(dpa)4Cl2] (M = Co (1), Ni, Cu) and [AgRu2(dpa)4Cl(OAc)] (4)

Ru–Ru Ru⋯M Ru–Cl M–Cl Ru–Nouter Ru–Ninner M–N a

Co

Ag

Nib

Cub

2.324(6) 2.396(10) 2.497(6) 2.433(10) 2.119(7) 2.040(5) 2.126(11)

2.2801(4) 2.7858(4) 2.159(3)a 2.5403(10) 2.086(2) 2.053(2) 2.433(2)

2.341(4) 2.349(5) 2.513(9) 2.462(10) 2.106(3) 2.012(3) 2.102(3)

2.246(3) 2.575(3) 2.624(3) 2.296(3) 2.136(4) 2.046(2) 2.173(6)

Ru–OAc distance. b Data from ref. 13.

The ORTEP diagrams of 5 (Mn), 6 (Fe), 7 (Zn), and 8 (Cd) are displayed in ESI Fig. S3.† The structures of all four compounds are analogous to 3 (Co) and all of them form isomorphous crystals with the space group P21/n and similar cell parameters. Likewise, all of them have disordered structures with different occupancies of M/Ru at both terminal positions as described above for compound 3. Compound 9 (Pd) also shows structural disorder of the metal ions, its space group is P2/n (Fig. 3(b)). Due to the similar electron density of Pd2+ and Ru2+, they are difficult to differentiate by X-ray diffraction. The atom positions of Pd and Ru are furthermore too close to each other to be resolved. As a consequence, the only metal ligand distance that can reliably be determined for this crystal is the

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Paper Table 3

Cd (8) Mn (5) Zn (7) Fe (6) Co (3) Nia Cua Pd (9) Pdb


a

Dalton Transactions List of selected distances (Å) for cationic compounds with a M2+⋯Ru25+ motif

Ru⋯M

Ru–Ru

M–N

Ru–Ninner

Ru–Nouter

Ru–Cl

M–Cl

Covalent radii30

trans-M(py)4Cl2 M–Npy

2.7873(17) 2.716(2) 2.678(2) 2.610(3) 2.608(12) 2.513(12) 2.510(14) 2.512(3) 2.4910(13)

2.2695(19) 2.2568(15) 2.272(2) 2.267(2) 2.265(8) 2.263(7) 2.313(12) 2.241(3) 2.2584(13)

2.276(6) 2.224(5) 2.184(5) 2.169(7) 2.160(20) 2.135(18) 2.115(19) 2.082(2) 2.045(12)

2.029(5) 2.027(4) 2.022(4) 2.016(4) 2.018(8) 2.018(4) 2.012(6) 2.030(5) 2.040(12)

2.191(6) 2.155(4) 2.126(5) 2.126(5) 2.120(17) 2.135(18) 2.115(16) 2.082(2) 2.126(12)

2.686(3) 2.610(2) 2.546(3) 2.578(3) 2.532(8) 2.496(7) 2.530(12) 2.6757(7) 2.517(3)

2.168(3) 2.151(3) 2.141(3) 2.222(3) 2.190(12) 2.248(13) 2.333(14) 2.6757(7) 2.718(10)b

1.36 1.19 1.18 1.16 1.11 1.10 1.12 1.20 —

2.411(9)c 34 2.331(6)35 — 2.229(6)36 2.183(4)36 2.133(4)36 2.046(4)37 2.032(3)d 38 —

Data from ref. 13. b [PdRu2(dpa)4Cl(OC(CH3)2)](PF6)2; Pd⋯OC(CH3)2. c Cd(py)4I2. d Square planar and Pd⋯Cl = 3.422 Å.

Ru2–Ninner distance (2.030 Å) in the middle position. The Ru2 atom slightly shifts about 0.14 Å from the middle position resulting in a shorter Ru–Ru distance (2.241 Å). In order to obtain a more accurate result of the Ru–Pd distance, [PdRu2(dpa)4Cl(OC(CH3)2)](PF6)2 was obtained by introducing 1 equivalent TlPF6 with compound 9 in acetone, which allowed for an axial Cl− ligand to be replaced by acetone. As the bonding of the Cl− ligand with Pd is weaker than with Ru25+, the acetone substitution is selective on the Pd site. Nevertheless, the coordination geometry of the Pd is close to perfect square planar (ESI Fig. S3(e)†). Some of the crucial distances of all compounds are listed in Table 3. The distance of Ru⋯Cd (2.7873 Å) is the longest metal– metal distance found in the trimetallic framework supported by dpa− ligands up to now. The Cd2+ ion is located above the basal plane of the pyramid by 0.63 Å. The Ru⋯Pd distance (2.491 Å) is the shortest distance between ruthenium and palladium to date to the best of our knowledge.39–41 With this short distance (Ru–Pd, FSR 1.02), a strong metal–metal interaction between Pd and Ru has to be presumed. Overview of the distances of M⋯Ru25+ series. Berry et al. concluded that the distances of the M⋯Cr in [MCr2(dpa)4Cl2] series indeed follow the trend of the covalent radii of the elements Mn > Zn > Fe > Co (high-spin) > Ni > Co (low-spin) > Cr.14 These findings, however, cannot simply be applied to the M⋯Ru25+ series (Fig. 4, dark blue line). The first point of intrigue is that Cu2+ and Pd2+ display extremely short M⋯Ru25+ distances despite having different covalent radii (1.12 Å for Cu2+ and 1.2 Å for Pd2+, respectively).30 Secondly, although the covalent radius of silver (1.28 Å) is somewhat different from Cd (1.36 Å), Cd2+ and Ag+ have very similar M⋯Ru25+ distances. Instead of covalent radii trend, we observe that the M⋯Ru25+ distances in truth follow the trend of the metal–pyridyl distances observed for trans-M( py)4Cl2 (Table 3). The two major forces at play are on one side the static repulsion force between the two metals that pushes Mn+ away from Ru25+, while on the other side the bonding force of the metal–pyridyl coordination is pulling the metal ion back. Hence, the observed M⋯Ru25+ distances are regulated by these two controlling forces (Fig. 4(b)). This explains the similar metal– metal distances found in 4 and 8, in which the lesser charged

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Fig. 4 (a) The diagram of essential distances containing M+/2+⋯Ru25+ motif and arranged by the trend of Ru⋯M distances. The distances of complexes containing Cu+, Ni2+ and Cu2+ are from ref. 13. (b) The illustration of factors regulating in Ru⋯M distance.

Ag+ experiences a smaller static repulsion than Cd2+, and the difference gets offset by the weaker coordination bond Ag–Npy (2.50 Å).32 From this, we can also conclude that Pd2+ has the shortest distance in the M⋯Ru25+ series due to the strong Pd2+–N interactions. The length of the metal–metal bond between the two Ru ions only varies slightly around 2.26 Å (Fig. 4, green line). Typically, the Ru25+ distance is significantly sensitive to axial ligands.27,42 The terminal metal ions assumed acting as an axial “ligand”, however, have less influence in Ru–Ru distances. It indicates that sharing electron between M and Ru is less possibility. A similar result was also observed in [MCr2(dpa)4Cl2].14 It is noteworthy that the Ru–Ninner and Ru– Nouter distances are also in a narrow range (cyan blue and purple line in Fig. 4), except for the Ru–Nouter distance in [AgRu2(dpa)4Cl(OAc)] (4). The only variance of 4 is that the Cl−


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is replaced by OAc− on the ruthenium side. The weaker donor ligand OAc− may require compensation from stronger Ru– Nouter bonding, which was observed in [Ru2(ap)4(H2O)](BF4) with a similar structure.43 Heterometallic metal–metal bond. Fig. 5 shows the crystal structures of 10 and 11 (also, refer to Fig. S4 in ESI†). Some selected bond lengths and angles are listed in Table 4. In the crystal structures of compounds 10 and 11, the RhRuRu and IrRuRu units are helically wrapped by four dpa− ligands with two chloride ions as the axial ligands. The positive charge of the molecule is balanced by one PF6− counter ion. The most intriguing feature is that the metal framework MRuRu is notably bent (∠MRuRu ≈ 168°), which lowers the symmetry to C2. McGrady et al. have revealed that, through DFT calculations, due to the second-order Jahn–Teller distortion, there is a mixing of the σnb and π* orbital of the triruthenium core in [Ru3(dpa)4Cl2] leading to the bend.44 According to the above theory, the bending of the RhRuRu and the IrRuRu metal cores is evidence that both Rh and Ir indeed share electrons with the diruthenium core and that a heterometallic metal– metal bond is created. Through a literature research using the Cambridge Structural Database and a recent review,45 the Rh– Ru and Ir–Ru distances of compounds 10 and 11 have been determined to be the shortest to date. Although the structure of compound 10 is isoelectronic to [Ru3(dpa)4Cl2], all distances are close to its oxidized form [Ru3(dpa)4Cl2](BF4). Especially,

the bending angle of the metal core (171.15°), the Ru–Cl (2.592 Å) and Ru–Ninner (2.066 Å) distances in [Ru3(dpa)4Cl2] are not similar with 10 and 11. DFT calculation The magnetic susceptibility of 10 and 11 were measured by Evans method.46 The effective magnetic moments are 2.93 and 2.72 B.M. for 10 and 11, respectively. Therefore, the triplet ground state was implemented in the C2 symmetric DFT calculation44 in order to get further insight into their electronic structures and M–M bonding. The geometric optimizations in bond distances and angles are reproduced well with the experimental values, and the bending of trimetallic backbones are also comparable with those observed via structural determination (Table S3 in ESI†). Fig. 6a–c display the delocalized σ and π bonds along Rh–Ru–Ru metal chain, in accordance with previous report in the complex of RuMo2(dpa)4Cl2.15 Moreover, two unpaired electrons are accommodated on the δ* orbital of diruthenium unit and the π* hybridized orbital of trinuclear chain, respectively, as shown in 6d and 6e (similar results are observed for 11, as reported in ESI†). These results are in line with the calculated Mayer bond order (Table 5), indicative of three-center, delocalized M–M bonding between M (Rh or Ir) and diruthenium units in 10 and 11. Finally, the bending of metal chains can be ascribed to the second-order Jahn–Teller distortion, resulting in the mixing of M(dz2) and (Ru–Ru) π* orbitals (Fig. 6c), which is analogy to the Ru3(dpa)4Cl2.44

Fig. 5 ORTEP view of the molecular structure of 10 and 11. Both compounds have similar structures. All atoms are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.

Table 4 Selected bond lengths (Å) of complexes having metal–metal bonds. ([MRu2(dpa)4Cl2]+; M1/M2 = Rh (10), Ir (11) and Ru)26

Ru2–M1 Ru2–M2 M1–Cl M2–Cl M1–Nouter M2–Nouter Ru2–Ninner ∠M1–Ru–M2

(RhRu2)7+

(IrRu2)7+

Ru37+

2.3254(4) 2.3352(4) 2.4869(10) 2.4888 (10) 2.103(3) 2.099(3) 2.025(3) 167.48(2)

2.3472(4) 2.3312(4) 2.4782(11) 2.4746(11) 2.100(4) 2.103(4) 2.027(4) 167.61(2)

2.2882(6) 2.2939(6) 2.4656(13) 2.4656(13) 2.108(4) 2.118(5) 2.041(5) 166.66(3)


Fig. 6 The molecular orbital diagram of 10. (a), (b) and (c): selected doubly occupied orbitals. (d) and (e): singly occupied orbitals.

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Paper Table 5

Dalton Transactions Calculated Mayer bond orders for 10 and 11

10 (M = Rh) 11 (M = Ir)

Table 6 The redox potential [MRu2(dpa)4Cl2]+. (V vs. Ag/AgCl)

Ru–Ru

Ruinner–M

Ruouter–M

1.09 1.19

0.56 0.53

0.22 0.12


Electrochemistry Cyclic voltammograms of [MRu2(dpa)4Cl2]+ are displayed in Fig. 7 and the redox-active peaks are listed in Table 6. The electrochemical performances of all compounds were performed in CH2Cl2 solution with TBAP as the electrolyte. From the CV data of compounds 7 and 8 with the redox-inactive metal ions Cd2+ and Zn2+, the peaks of reversible reduction between 0 and −0.21 V can be assigned to the reduction process of Ru25+/Ru24+ and the peaks around 1.0 V to the oxidation of Ru26+/Ru25+. Interestingly, for first row transition metals, the absolute E1/2(Red. 1) values of compound 5 (Mn), 7 (Zn), 6 (Fe), and 3 (Co) follow the same slight decrease as the M–Npy distance. Hence, the terminal metal ions may indirectly cause the shift of the reduction potential through structural interference. Here, it should also be mentioned that the E1/2(Red. 1) of CuRu2 and NiRu2 were previously assigned to the reduction of Cu and Ni centres.13 However, it is observed that the E1/2(Red. 1) of compound 3 is −0.11 V, which is a negative shift in comparison to CuRu2 (+0.19 V) and NiRu2 (+0.02 V). Owing to the Zn2+ and Cd2+ are inactive in redox couple, the reductive potential E1/2(Red. 1) in 7 and 8 are mainly contributed by the Ru24+ ↔ Ru25+ couple. The reduction of compound 3 is close to 7 and 8. It can be

Fig. 7 The diagram of cyclic voltammetry measurements of compounds 3 and 5–11 in CH2Cl2 with 0.1 M tetra-n-butylammonium perchlorate (TBAP) at a scan rate of 100 mV s−1.

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of

cyclic

voltammetry

Metal core

E1/2(Oxid. 1)

E1/2(Red. 1)

(MnRu2)7+ (5) (ZnRu2)7+ (7) (FeRu2)7+ (6) (CoRu2)7+ (3) (NiRu2)7+ (CuRu2)7+ (CdRu2)7+ (8) (PdRu2)7+ (9) (RhRu2)7+ (10) (IrRu2)7+ (11) Ru37+

+0.97 (irr.) +1.02 (irr.) +0.96 +1.02 +1.06 +1.07 +1.06 +1.06 +0.88 +0.76 +0.89

−0.20 −0.14 −0.13 −0.11 +0.02 +0.19 −0.17 −0.17 0.00 −0.21 +0.07

for

E1/2(Red. 2)

−1.0 (irr.) −0.73 −0.85 (irr.)

The data of Ru37+, (CuRu2)7+ and (NiRu2)7+ are from ref. 13 and 26.

explained by the reduction of compound 3 occurring on the Ru25+ unit to form the CoII–RuII–RuII (type I) compound 1 (Scheme 3). This also matches the crystallographic results in compounds 1 and 3. The oxidation of diruthenium at E1/2(Oxid. 1) forming Ru26+, of 5 (Mn2+) and 7 (Zn2+) are irreversible, which is attributed to the weaker metal–pyridyl bonds.14 The oxidized species M2+Ru26+ of these two compounds are unstable due to higher static repulsion than M2+Ru25+. While 8 (Cd2+) features the longest M2+⋯Ru25+ distance, it still shows a reversible E1/2(Oxid. 1) peak at 1.06 V. This reversibility is due to the higher bonding strength of pyridine with the second row transition metal (compared to Zn2+) countering the static repulsion from Ru26+. It is unclear yet why E1/2(Oxid. 1) and E1/2(Red. 1) of Pd2+ are the same as for Cd2+. Generally, for metals which have covalent metal–metal bonds, more valance states can be observed by reduction during cyclic voltammetry leading to a redox potential shift.8 Both RhRu2 and IrRu2 have three redox-active peaks. The redox potentials of (RhRu2)7+ are quite similar to Ru37+ and the E1/2(Oxid. 1) shifts from ∼1.0 V to +0.88 V in comparison to these compounds that do not show M–Ru2 metal bonds. Furthermore, E1/2(Red. 1) of 10 is located at 0.00 V rather than −0.17 V in Cd2+ and Pd2+. An additional reversible reduction peak is observable at −0.73 V. This highly negative potential was not observed in the triruthenium metal chain and can be ascribed to the reduction of Rh ion. As to (IrRu2)7+, the whole cyclic voltammogram shifts to a more negative potential compared to (RhRu2)7+. This indicates that the heterometallic chain is oxidized more easily due to the participation of the third row transition metal. Likewise, the irreversibility of E1/2(Red. 2) is attributed to the air-sensitive nature of Ir(I) coordinated by four pyridyl and one Cl− ligands.

Scheme 3

The charge assignment of compound (1).


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1

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H and 2D COSY NMR spectra

The 1H and 2D COSY NMR spectra of all heterotrimetallic chains are shown in Fig. S5 and S6,† the chemical shifts are summarized in Table S3,† except for the 2D COSY of compound 4 due to its low solubility in CD2Cl2. All 1H NMR spectra feature eight peaks, confirming that the metal core ions are in an asymmetric arrangement. In aspect of crystallographic characterization, compound 9 (Pd), 10 (Rh), and 11 (Ir) are mixed metal at the same site. Although the Rh and Pd atoms are undistinguishable with Ru atom by X-ray structural method. The 8 peaks in 1H NMR spectra, however, confirm that the metal cores in “MRuRu” are asymmetric arrangement. In addition, the atomic isotope pattern in the high resolution mass spectra (Fig. S7 in ESI†) further verify these asymmetric structures. Recently, the paramagnetic NMR of Ru complexes had been investigated via experiments and DFT calculations.47–50 The 1H NMR spectra of ruthenium complexes exhibit unique patterns with extreme upfield by contact shifts. Spin density from the ruthenium metal is distributed into the ligands’ π space and shields the protons leading to the negative chemical shifts. The 8 proton peaks of the different ‘M’Ru2 complexes can be divided into 2 sets through 2D COSY NMR spectroscopy. With the above described specific NMR shift associated with ruthenium, the set of signals belonging to protons of Ru or ‘M’ coordinated pyridyl rings respectively, can be identified (ESI, Table S2†). All the protons of pyridyl groups coordinated to Co2+, Fe2+, and Mn2+ metals are located in the positive chemical shift area. However, the pyridyl protons coordinated to diamagnetic metal ions (M: Zn2+, Cd2+, and Pd2+) retain the contact shift nature. The influence of the dipole interaction (via space) from the paramagnetic “M2+” metal is stronger than the contact shift by diruthenium due to protons near the paramagnetic “M2+” center. The NMR spectra not only provides the geometric assignment and a measure of sample purity but also provide information on the spin state. From the

Fig. 9 The 2D COSY 1H-NMR spectrum of compound 10 in CD2Cl2 and illustration of protons’ position assignment.

chemical shifts of [CoRu2(dpa)4Cl2] (1), the protons of the pyridine coordinated to the Ru24+ unit are still contact shifted, which shows that the spin state of Ru24+ is S = 1 rather than the diamagnetic S = 0. For the diamagnetic metal ions (Ag(I), Zn(II), Cd(II), and Pd(II)), the chemical shifts are mainly influenced by the paramagnetic Ru25+ unit. All of them, with the exception of Pd(II) (9), have very similar chemical shifts (Fig. 8). The chemical shifts of HRu and HPd are enormous in comparison to the peaks for Zn2+ and Cd2+. The very short Pd–Ru distance (2.49 Å), deduced from the crystallographic data, suggests that these peak’s movements could originate from a metal–metal interaction between Ru and Pd. The 1H NMR chemical shifts of the heterometallic bonds (Rh and Ir) are very similar. The 4d–4d and 4d–5d metal–metal bonds do not have any obvious influence on the chemical shifts in these cases. The spectra also show a preservation of the contact shift, having some upfield peaks, although with smaller values. In the 1H NMR of 10, the doublet peaks at −26.43 and −26.07 ppm are observed providing further information for the peaks assignment (Fig. 9 and Fig. S6(h) in ESI†).

Conclusions

Fig. 8 The 1H NMR spectrums of compound 7 (Zn, green curve), 8 (Cd, red curve) and 9 (Pd, blue curve) in CD2Cl2.


The 4,0-Ru2(dpa)4Cl having four uncoordinated pyridyl groups located at the same side has proven to be a powerful precursor for the preparation of heterometallic chains with MRu2 cores. With the advantage of chelating pyridyl groups, it was possible to include three new elements (Cd, Ir, and Ag) in the family of EMACs using dpa− ligands. The Cd2+ and Ag+ ions are located at terminal positions in square pyramidal geometry. The Ir2+ is connected to the Ru25+ unit by an Ir–Ru heterometallic metal– metal bond. From previous work and this article, the MRu2 with group 7 to group 12 metals are now completed. By summarizing the structures of the heterotrimetallic chains

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[MRu2(dpa)4X2]0/1+, it can be concluded that the trend of the M1+/2+⋯Ru25+ distances in the core follows the sequence Cd2+ > Ag+ > Mn2+ > Zn2+ > Fe2+ > Co2+ > Cu1+ > Ni2+ > Cu2+ > Pd2+. This trend mainly follows the metal–pyridine distances. The two exceptions Ag+ and Cu+ while having longer M–N distances compensate with less static repulsion slightly shortening the M⋯Ru distance. This trend is a consequence of a balance of static repulsion and the metal pyridine bond. In the Ir and Rh cases, heterometallic metal–metal bonds are observed. The electrochemical measurements show that a redox occurred in the Ru2 dimer of CoRu2 and other chain compounds lacking heterometallic bonds, which is different from the previously observed CuRu2 and NiRu2 cases. All 1H NMR spectra show eight peaks, demonstrating that the metal core ions are in an asymmetric arrangement. [CoRu2(dpa)4Cl2] features Ru24+ is in a paramagnetic state (S = 1) as it can be concluded from the 1H NMR spectra. The early transition metals are interesting as examples in EMACs are rare. They could be studied in a future investigation using this 4,0Ru2(dpa)4Cl as precursor.

Experimental section Materials and methods All manipulations were carried out under argon or nitrogen atmosphere by using standard Schlenk techniques unless indicated otherwise. Solvents were purified by standard methods and freshly distilled under nitrogen atmosphere prior to use. The ligand bis(2-pyridyl)amine (Hdpa),51 Ru2(OAc)4Cl,52 [Rh(COD)2](BF4),53 and [Ir(COD)Cl]2 54 were synthesized following published procedures. The Ru2(OAc)4Cl was ground into fine powder before use. Physical measurements Crystal data was collected on a Bruker SMART Apex and Bruker D8 VENTURE diffractometer with monochromatized Mo-Kα radiation (lambda = 0.71073 Å) at T = 150(2) K. Cell parameters were retrieved and refined using DENZOSMN software on all observed reflections. Data reduction was performed with the DENZO-SMN software. Empirical absorption was based on the symmetry-equivalent reflections and absorption corrections were applied with the SORTAV program. All the structures were solved and refined with SHELXL-2014 by full-matrix least squares on F2 values with the Olex2 interface.55 The hydrogen atoms were included in calculated positions and refined with a riding model. IR spectra were obtained with a Nicolet FourierTransform in the range 450–4000 cm−1. FAB mass spectra were obtained with a JEOL HX-110 HF double-focusing spectrometer operating in the positive ion detection mode. The high resolution mass spectra were measured by Bruker New ultrafleXtreme™ MALDI-TOF mass spectrometer. 1H NMR spectra were recorded with a Bruker AMX 300 MHz spectrometer. Electrochemistry measurements were carried out with CH Instruments equipment (Model 750A) with the use of CH2Cl2 as the solvent with 0.1 M TBAP. Cyclic voltammetry

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was recorded with a homemade three-electrode cell equipped with glassy carbon (0.07 cm2) disk as the working electrode, a platinum wire as the auxiliary electrode, and a homemade Ag/ AgCl (saturated) reference electrode. Potentials are reported vs. Ag/AgCl (saturated). Computational methods DFT calculations were performed with the ORCA program package, version 3.0.3.56 The initial coordinates were taken from the crystallographic data of 10 and 11. Geometry optimization was computed using BP86 functional57–60 with the resolution of the identity approximation RI.61 An additional refined B3LYP62 single-point energy calculation with RIJCOSX approximation62 was performed employing the geometries obtained with the BP functional. Atom-pairwise dispersion correction with the Becke–Johnson damping scheme (D3BJ),63,64 and the scalar relativistically recontracted version of the Aldrichs triple-ζ basis set, def2-TZVP and the def2-TZVP/J auxiliary basis set,65–67 were used on all atoms. For Ru, Rh, and Ir atoms, an effective core potential approach was applied68 and the scalar relativistic zero-order regular approximation ZORA approximation was included.69 Tight self-consistent field convergence (TightSCF) with unrestricted spin (UKS) was employed for all calculations. Synthesis [Co3(dpa)4Cl2]. The previously reported70 procedure was modified for the synthesis on the gram scale. Anhydrous CoCl2 (1.440 g, 11 mmol) was heated to 100 °C under vacuum for 1 hour while stirring to remove moisture. CH3Li was replaced with n-butyllithium (1.6 M in hexane, 7.8 mL, 12.5 mmol) for the deprotonation of Hdpa. The n-butyllithium was slowly added to a solution of Hdpa (2.052 g, 12 mmol) in THF solution (200 mL) at −60 °C. After addition of n-butyllithium, the bottle was moved to the −25 °C bath for 10 min. At −25 °C, the solution was transferred into a flask containing the CoCl2. The mixed solution was stirred for approximately 10 min at room temperature before heating to reflux (8 h). After the reaction, the THF solvent was removed by rotatory evaporation. The product was extracted from the solid using ∼250 mL CH2Cl2. The resulting solution was condensed to ∼100 mL after a filtration and then large amount of ether was added in order to obtain micro-crystals. The collected crystals were dissolved in ∼150 mL CH2Cl2 and filtered. The final product was obtained after layering with ether (2.36 g, yield: 85%). [CoRu2(dpa)4Cl2] (1). A mixture of [Co3(dpa)4Cl2] (430 mg, 0.46 mmol), Ru2(OAc)4Cl (200 mg, 0.42 mmol) and naphthalene (28 g) in a 125 mL Erlenmeyer flask was heated to 160 °C for 20 min and subsequently heated to 220 °C for 1 min. The solution turned to deep red-orange gradually. The mixture was then cooled to room temperature and treated with warm deaerated n-hexane (∼100 mL) to dissolved naphthalene. The precipitate was collected and washed with hexane to remove naphthalene residues. The red brown precipitate was dissolved in 100 mL benzene. After filtration, the red-orange solution was condensed to ∼30 mL in vacuum and a large amount of


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ether and hexane were added to precipitate the product, subsequently. The dark orange micro-crystals were collected and dried in vacuum. Yield: 171 mg, 40%. IR (KBr, cm−1): 1600 m, 1593 m, 1548 w, 1457 s, 1421 s, 1354 s, 1310 m, 1152 m, 1020 m, 861 m, 764 m. MS(FAB) m/z: 1013 [M]+, 978 [M − Cl]+; 1 H NMR (300 MHz, CD2Cl2): δ ( ppm) 126.04 (b, 4H), 78.4 (4H), 57.24 (4H), 41.67 (4H), 14.54 (4H), 1.87 (4H), −18.56 (4H), −49.06 (4H). Elemental analysis (%) [CoRu2(dpa)4Cl2]: calcd C 47.44, H 3.19, N 16.60; found: C 47.31, H 3.25, N 16.49. (4,0)-[Ru2(dpa)4Cl] (2). The compound 2 was synthesised using the same reagents and reaction conditions as compound 1. The brown precipitate was obtained after removing naphthalene by hexane. 100 mL CH2Cl2, then, was rigidly stirred with 4 mL 29% NH3(aq) and the CH2Cl2 fraction was collected. The above brown precipitate was dissolved in this ammonia containing CH2Cl2 and the resulting solution was stirred for 20 min at room temperature under air. After filtration and evaporation of the solvent, the dark brown solid was treated with 50 mL benzene and successively 30 mL ether was added. The orange solution was obtained by filtration and the solvent was removed by rotatory evaporation. After that, the solid was dissolved in 30 mL CH2Cl2 and 30 mL hexane was added subsequently. The solution was slowly evaporated by rotatory evaporation without water bath until dark-red crystal precipitated. The crystals were collected by filtration and washed by hexane and H2O. The final product was dried under vacuum immediately (124 mg, 32% yield). Single crystals suitable for X-ray diffraction studies can be obtained by layering CH2Cl2 solution of the complex with hexane. IR (KBr, cm−1): 1604 s, 1548 w, 1464 s, 1425 s, 1355 m, 1339 m, 1316 m, 1161 m, 1022 m, 1084 m, 1025 m, 872 m, 766 m. MS(FAB) m/z: 919 [M]+, 884 [M − Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 36.7 (4H), 13.4 (4H), 13.22 (4H), 2.46 (4H), −13.16 (4H), −20.16 (4H), −23.72 (4H), −64.7 (4H). Elemental analysis (%) [Ru2(dpa)4Cl]: calcd C 52.31, H 3.51, N 18.30; found: C 52.52, H 3.65, N 18.21. [CoRu2(dpa)4Cl2](PF6) (3). Compound 1 [CoRu2(dpa)4Cl2] (100 mg, 0.1 mmol) was dissolved in CH2Cl2 (30 mL) and solid FcPF6 (ferrocenium hexafluorophosphate) (50 mg, 0.15 mmol) was added. After stirring for 5 min, the mixture was filtered and an excess of ether was added to the filtrate to precipitate an orange brown solid. Single crystals were obtained after 24 hours by layering the CH2Cl2 solution with ether. Yield: 105 mg, 92%, IR (KBr, cm−1): 1603 s, 1551 w, 1463 s, 1425 s, 1352 m, 1339 m, 1160 m, 1023 m, 842 s, 769 m, 558 m. MS (FAB) m/z: 1013 [M]+, 978 [M − Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 122.01 (b, 4H), 47.72 (4H), 42.02 (4H), 36.27 (4H), 1.63 (4H), −11.05 (4H), −27.80 (4H), and −59.35 (4H). Elemental analysis (%) [CoRu2(dpa)4Cl2](PF6): calcd C 41.50, H 2.79, N 14.52; found: C 41.58, H 2.90, N 14.35. [AgRu2(dpa)4Cl(OAc)] (4). Compound 2 (80 mg, 0.087 mmol) was dissolved in benzene (80 mL) and Ag(OAc) (44 mg, 0.26 mmol) was added into the solution. After 10 min, the mixture was filtered to remove excess Ag(OAc). The red-orange solution was condensed to ∼10 mL by rotatory evaporation and a large amount of ether was added to precipitate the


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product. The polycrystalline product was obtained after several days by layering the CH2Cl2 solution with ether. Yield: 81 mg, 86%, IR (KBr, cm−1): 1606 m, 1593 m, 1546 m, 1469 s, 1427 s, 1359 s, 1316 m, 1152 m, 1028 m, 881 m, 759 m, 735 m. MS (MALDI) m/z: 990 [M − OAc − Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 35.2 (4H), 30.5 (4H), 14.40 (4H), 0.53 (3H), −2.80 (4H), −15.19 (4H), −26.68 (4H), −27.33 (4H), −57.3 (4H). Elemental analysis (%) [AgRu2(dpa)4Cl(OAc)]: calcd C 46.48, H 3.25, N 15.49; found: C 46.17, H 3.49, N 15.60. General procedures for the preparation of [MRu2(dpa)4Cl2] (Anion), (M = Co (3), Mn (5), Fe (6), Zn (7), Cd (8); Anion = ClO4− or PF6−) The diruthenium compound 2 (80 mg, 0.087 mmol), 3 equivalents of MCl2 and KPF6 (or LiClO4) were mixed in 50 mL deaerated anhydrous CH2Cl2 and the mixture was stirred overnight under nitrogen at room temperature. After filtration, the resulting orange solution was condensed to ∼20 mL. The dark orange crystal was collected after 3 days by ether diffusion. While anhydrous metal chloride salts were used for 3, 5 and 6, ZnCl2·4H2O and CdCl2·2H2O were used for the synthesis of 7 and 8 respectively. [MnRu2(dpa)4Cl2](ClO4) (5). Yield: 88 mg, 91%, IR (KBr, cm−1): 1602 m, 1471 s, 1462 s, 1423 s, 1352 m, 1312 m, 1286 m, 1158 m, 1121 m, 1089 s, 881 m, 872 m, 769 m, 741 m. MS(FAB) m/z: 1009 [M]+, 974 [M − Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 98.87 (b, 4H), 33.58 (4H), 24.37 (4H), 8.14 (4H), 4.14 (4H), 1.41 (4H), −15.82 (4H), −28.9 (4H). Elemental analysis (%) [MnRu2(dpa)4Cl2](ClO4): calcd C 43.35, H 2.91, N 15.17; found: C 43.52, H 3.12, N 15.26. [FeRu2(dpa)4Cl2](PF6) (6). Yield: 93 mg, 93%, IR (KBr, cm−1): 1631 w, 1603 s, 1593 s, 1551 w, 1460 s, 1424 s, 1384 m, 1312 m, 1288 m, 1028 m, 1239 m, 1159 m, 872 m, 841 s, 768 m, 740 m. MS(FAB) m/z: 1010 [M]+, 975 [M − Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 123.43 (b, 4H), 41.48 (4H), 30.72 (4H), 29 (4H), 9.15 (4H), −4.95 (4H), −20.2 (4H), and −42.42 (4H). Elemental analysis (%) [FeRu2(dpa)4Cl2](PF6): calcd C 41.61, H 2.79, N 14.56; found: C 41.35, H 2.88, N 14.39. [ZnRu2(dpa)4Cl2](ClO4) (7). Yield: 85 mg, 87%, IR (KBr, cm−1): 1602 s, 1562 w, 1549 w, 1463 s, 1424 s, 1355 m, 1313 m, 1286 m, 1267 m, 1239 m, 1158 m, 1121 m, 1089 s, 888 m, 872 m, 768 m, 740 m. MS(FAB) m/z: 1020 [M]+, 985 [M − Cl]+; 1 H NMR (300 MHz, CD2Cl2): δ ( ppm) 35.84 (4H), 29.26 (4H), 15.59 (4H), −3.49 (4H), −22.8 5(4H), −30.08 (4H), −36.6 (4H), −55.07 (4H). Elemental analysis (%) [ZnRu2(dpa)4Cl2](ClO4): calcd C 42.95, H 2.88, N 15.03; found: C 42.79, H 3.04, N 14.99. [CdRu2(dpa)4Cl2](ClO4) (8). Yield: 88 mg, 87%, IR (KBr, cm−1): 1602 s, 1593 m, 1549 w, 1462 s, 1423 s, 1350 m, 1331 m, 1287 m, 1240 m, 1158 m, 1121 m, 1089 s, 887 m, 871 m, 767 m, 740 m. MS(FAB) m/z: 1066 [M]+, 1031 [M − Cl]+; 1 H NMR (300 MHz, CD2Cl2): δ ( ppm) 35.23 (4H), 31.12 (4H), 15.85 (4H), −3.67 (4H), −21.5 (4H), −30.35 (4H), −34.48 (4H), −54.37 (4H). Elemental analysis (%) [CdRu2(dpa)4Cl2](ClO4): calcd C 41.22, H 2.77, N 14.42; found: C 41.22, H 2.87, N 14.12.

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[PdRu2(dpa)4Cl2](PF6) (9). A 10 mL acetone solution of [Pd (CH3CN)4](BF4)2 (39 mg, 0.086 mmol) was slowly added to compound 2 [Ru2(dpa)4Cl] (80 mg, 0.086 mmol) in 50 mL CH2Cl2 and the orange solution was stirred overnight. An excess LiCl was added and stirred for 30 min. After evaporating the solvent, the solid was dissolved again in CH2Cl2 and stirred with an excess of KPF6 for 4 hours. The solution was filtered and the CH2Cl2 solvent was removed. The solid was dissolved in CH2Cl2, diffusion with ether afforded orange crystals after one weak. Yield: 72 mg, 69%, IR (KBr, cm−1): 1604 s, 1548 w, 1478 m, 1464 s, 1425 s, 1355 s, 1339 m, 1316 m, 1289 m, 1161 m, 1122 m, 1084 m, 1025 m, 891 w, 872 m, 766 m. MS (FAB) m/z: 1025 [M − Cl]+, 989 [M − 2Cl]+, 901[M − 2Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 40.14 (4H), 24.11 (4H), 1.21 (4H), −1.25 (4H), −16.36 (4H), −20.75 (4H), −38.31 (4H), and −43.67 (4H). Elemental analysis (%) [PdRu2(dpa)4Cl2](PF6): calcd C 39.86, H 2.68, N 13.95; found: C 39.67, H 2.90, N 13.72. [RhRu2(dpa)4Cl2](PF6) (10). [Ru2(dpa)4Cl] (2) (80 mg, 0.087 mmol) and [Rh(COD)2](BF4) (36 mg, 0.089 mmol) were mixed in deaerated 50 mL CH2Cl2 and 1 mL acetone under nitrogen at room temperature. After 24 hours, the solution was transferred into a flask containing excess LiCl and stirred for 30 min. After evaporating the solvent, an excess of solid KPF6 and 30 mL CH2Cl2 were added and the solution was stirred overnight under air. The resulted solution was filtered and the crude compound was purified by silica-gel column with a mixture of acetone and CH2Cl2 (ratio 15/100) as eluent. The dark olive-green crystals were formed by layering a CH2Cl2 solution of the compound with ether. Yield: 71 mg, 68%. IR (KBr, cm−1): 1603 s, 1549 w, 1460 s, 1424 s, 1350 m, 1313 m, 1287 m, 1237 m, 1160 m, 1029 w, 880 m, 841 s, 767 m, 740 m. MS(MALDI) m/z: 1056 [M]+, 1022 [M − Cl]+, 987 [M − 2Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 28.17 (4H), 25.26 (4H), 15.31 (4H), 11.28 (4H), −26.07 (d, 4H), −26.43 (d, 4H), −30.33 (4H), −33.5 (4H). Elemental analysis (%) [RhRu2(dpa)4Cl2](PF6): calcd C 39.98, H 2.68, N 13.99; found: C 39.71, H 2.89, N 13.69. [IrRu2(dpa)4Cl](PF6) (11). In a 150 mL Erlenmeyer flask, [Ru2(dpa)4Cl] (1) (80 mg, 0.087 mmol) and [Ir(COD)Cl]2 (58 mg, 0.086 mmol) were refluxed in naphthalene (24 g) under Argon for 10 min. The mixture was then cooled to room temperature, and treated with warm deaerated n-hexane (∼100 mL) to dissolve naphthalene. The precipitate was collected and washed with hexane to remove naphthalene residues. The orange solid was dissolved in 30 mL CH2Cl2 and mixed with excess KPF6 overnight. After filtration, the crude compound was purified by silica-gel column with a mixture of acetone and CH2Cl2 (ratio 15/100) as eluent. The dark olivegreen crystals were formed by layering a solution of the compound in CH2Cl2 with ether. Yield: 58 mg, 52%. IR (KBr, cm−1): 1605 m, 1547 w, 1459 s, 1423 s, 1350 m, 1313 m, 1288 m, 1237 m, 1160 m, 1032 w, 880 m, 841 s, 766 m, 741 m. MS(FAB) m/z: 1147 [M]+, 1112 [M − Cl]+, 1076 [M − 2Cl]+; 1H NMR (300 MHz, CD2Cl2): δ ( ppm) 27.68 (4H), 26.04 (4H), 17.34 (4H), 11.68 (4H), −21.96 (4H), −23.22 (4H), −29.59 (8H).

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Elemental analysis (%) [IrRu2(dpa)4Cl2](PF6): calcd C 37.21, H 2.50, N 13.02; found: C 37.03, H 2.80, N 13.00.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The authors acknowledge the National Science Council and the Ministry of Education of Taiwan for financial support.

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