Synthesis, Structure, and Reactivity of Ga‐Substituted

DOI: 10.1002/chem.201701248

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Synthesis, Structure, and Reactivity of Ga-Substituted Distibenes and Sb-Analogues of Bicyclo[1.1.0]butane Lars Tuscher,[a] Christoph Helling,[a] Chelladurai Ganesamoorthy,[a] Julia Kreger,[a] Christoph Wçlper,[a] Walter Frank,[b] Anton S. Nizovtsev,[c, d] and Stephan Schulz*[a] Dedicated to Professor Dieter Fenske on the occasion of his 75th birthday

Abstract: Monovalent gallanediyl LGa {L = HC[C(Me)N(2,6iPr2C6H3)]2} reacts with SbX3 to form the Ga-substituted distibenes [(LGaX)2Sb2] (X = NMeEt 1, Cl 2). Upon heating, 2 reacts to the bicyclo[1.1.0]butane analogue [(LGaCl)2(m,h1:1Sb4)] 3 containing a [Sb4]2@ dianion. Moreover, 2 reacts with Li amides LiNR2 in salt elimination reactions that form the corresponding amido-substituted compounds 1 and [(LGaN-

Introduction Polyanionic Zintl-type anions [Ey]x@ of Group 15 elements have been intensively studied since their first discovery in the early years of the last century.[1] They are typically prepared by reactions of the elements with alkaline metals in liquid NH3 or other amines or by solid state reactions of the elements. Moreover, reduction reactions of P4 and As4 with organometallic complexes have also given access to a large variety of such polyanions including realgar-type P8 tetraanions [(Cp*2Sm)4P8] (Cp* = C5Me5), [{(NNfc)Sc}4P8] (NNfc = 1,1’-fc(NSitBuMe2)2, fc = fer[a] L. Tuscher, C. Helling, Dr. C. Ganesamoorthy, J. Kreger, Dr. C. Wçlper, Prof. Dr. S. Schulz Faculty of Chemistry and Center for NanoIntegration (CENIDE) University of Duisburg-Essen Universit-tsstr. 5–7, S07 S03 C30, 45117 Essen (Germany) Fax: (+ 49) 201-183-3830 E-mail: [email protected] Homepage: https://www.uni-due.de/ak schulz/index en.php [b] Prof. Dr. W. Frank Institute for Inorganic Chemistry and Structural Chemistry University of Desseldorf Universit-tsstrasse 1, 40225, Desseldorf (Germany) [c] Dr. A. S. Nizovtsev Nikolaev Institute of Inorganic Chemistry Siberian Branch of the Russian Academy of Sciences Academician Lavrentiev Avenue 3 630090, Novosibirsk (Russian Federation) [d] Dr. A. S. Nizovtsev Novosibirsk State University Pirogova Street 2, 630090, Novosibirsk (Russian Federation) Supporting information and the ORCID number(s) for the author(s) of this article can be found under https://doi.org/10.1002/chem.201701248. Part of a Special Issue to celebrate the 150th anniversary of the German Chemical Society (GDCh). To view the complete issue, visit https://doi.org/ chem.v23.50. Chem. Eur. J. 2017, 23, 12297 – 12304

Me2)2Sb2] 4, whereas reactions of 4 and [(LGaNMe2)2(m,h1:1Sb4)] 5 with two equivalents of GaCl3 resulted in the formation of 2 and 3, respectively. 1, 2 and 3 were characterized by 1H and 13C NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. In addition, their bonding situation was analyzed by quantum chemical calculations.

rocenylene),[2] transition-metal-coordinated E4–8 cages,[3] and Lewis acid/base-stabilized P4 and P8 polyanions.[4] In addition, transition-metal complexes containing neutral E4 (E = P, As) tetrahedra have been synthesized.[5] In remarkable contrast, analogous metal complexes containing Sb polyanions are rare and the most likely reason for this finding is that they cannot be synthesized by activation reactions of the neutral Sb4 tetrahedron, which is not a stable species under ambient conditions. To the best of our knowledge, an intact Sb4 tetrahedron was only reported in vapor phase deposited Sb thin films and identified by scanning tunneling microscopy (STM).[6] Therefore, the development of bottom-up strategies for the synthesis of Sb polyanions using molecular precursors is of high interest.[7] Wright et al. reported on the synthesis of the Zintl compound [Sb7Li3·6 HNMe2] by controlled thermolysis reaction of {[Sb(PCy)3]2Li6·6 HNMe2} in toluene at low temperature (40 8C),[7a] whereas in recent years Kloo, Burford, and Weigand et al. established reductive catenation reactions for the synthesis of Group 15 polycations.[8] Our long term general interest in the reactivity of low-valent main group metal complexes and in Group 15 metal chemistry recently resulted in the synthesis of several polyatomic antimony clusters. They were synthesized by controlled reduction reactions of molecular Sb complexes, in which the Sb atoms are either bonded to different heteroatoms X with different Sb@X bonding energies (X = C, N) or adopt different formal oxidation states (+ I, + II, + III). Although antimony trialkyls SbR3 in contrast to BiEt3[9] failed to react with gallanediyl LGa {L = HC[C(Me)N(2,6-iPr2C6H3)]2}, the reaction between Sb(NMe2)3 and LGa proceeded under mild reaction conditions to form the Ga-substituted distibene [(LGaNMe2)2Sb2] 4, which contains a central Sb=Sb double bond and formally a [Sb2]2@ dianion.[10] Upon heating, 4 was converted into [(LGaNMe2)2(m,h1:1-Sb4)] 5,

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Full Paper the first Sb analogue of bicyclo[1.1.0]butane, which formally contains a [Sb4]2@ dianion.[10] In addition, reduction reactions of distibines R4Sb2 (R = Me, Et) with LM (M = Al, Ga) proceeded with insertion of LM into the weak Sb@Sb bond and subsequent formation of LM(SbEt2)2,[11] whereas analogous reactions with stronger reducing MgI reagents formed the Zintl-type [Sb8]4@ anions [(LMg)4(m4,h2:2:2:2-Sb8)] and [(L’Mg)4(m4,h2:2:2:2-Sb8)] {L’ = HC[C(Me)N(2,4,6-Me3C6H2)]2}.[12] Very recently, we showed that reduction reactions of cyclo-tetrastibine [Cp*Sb]4, in which the central Sb atoms adopt the formal oxidation state + I, with LGa and [L’’Mg]2 (L’’ = iPr2NC[N(2,6-iPr2C6H3)]2), yielded [(LGa)2(m,h2:2-Sb4)] and [(L’’Mg)4(m4,h1:2:2:2-Sb4)].[13] Although both compounds formally contain [Sb4]4@ tetraanions as the central structural motifs, their connectivities and bonding natures strongly differ. We now systematically expanded our studies and report here on the synthesis, solid state structures, thermal stability and chemical reactivity of several new complexes containing [Sb2]2@ and [Sb4]2@ dianions. In addition, their bonding situation was analyzed in detail by quantum chemical calculations.

Results and Discussion To probe if the reduction reaction of Sb(NR2)3 with LGa opens up a general route for the synthesis of Ga-coordinated distibenes, we investigated this reaction in more detail. Unfortunately, only the reaction of LGa with Sb(NMeEt)3 proceeded with formation of the distibene [(LGaNMeEt)2Sb2] 1, whereas those with sterically more hindered Sb trisamides including Sb(NEt2)3, Sb[N(iPr)2]3 and Sb[N(SiMe3)2]3 completely failed, even at elevated temperatures (up to 100 8C) and elongated reaction times (up to 7 days). These findings show the subtle influence of small changes of the steric size of the organic substituents R on the reactivity of Sb(NR2)3. This is further underlined by the observation that the reaction of LGa with Sb(NMe2)3 proceeded already slowly at room temperature upon formation of [(LGaNMe2)2Sb2] 4, whereas that with Sb(NMeEt)3 occurred only under neat reaction conditions in the absence of any solvents at temperatures of at least 60 8C (Scheme 1). We were interested in reactions of LGa with other SbIII reagents to evaluate the reduction potential of LGa in detail. The reaction with SbCl3 in 2:1 molar ratio in toluene at ambient temperature proceeded with formation of a yellow-greenish solution, from which yellow-green crystals of [(LGaCl)2Sb2] 2 were isolated upon storage at room temperature (Scheme 2). The thermal stability of 1, 2, and [(LGaNMe2)2Sb2] 4 significantly differed. Heating a toluene solution of 4 to 120 8C for

Scheme 1. Synthesis of 1. Chem. Eur. J. 2017, 23, 12297 – 12304

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Scheme 2. Synthesis of 2.

24 h yielded [(LGaNMe2)2(m,h1:1-Sb4)] 5,[10] 2 required 7 days of heating at 130 8C for the formation of [(LGaCl)2(m,h1:1-Sb4)] 3 (Scheme 3), and thermolysis of a toluene solution of 1 at

Scheme 3. Synthesis of 3.

120 8C for 3 days yielded several products according to 1H NMR studies. Unfortunately, any attempts to separate the different species through fractional crystallization failed. However, our studies clearly demonstrate the high synthetic potential of the insertion-reduction method using LGa as compared to the coordination-reduction approach using N-heterocyclic carbenes (NHCs); the attempted synthesis of an NHC-stabilized Sb4 complex by reduction of a NHC-SbCl3 adduct failed.[14a,b] However, the three-electron reduction of the adduct CAAC-SbCl3 (CAAC = cyclic alkyl(amino)carbene) with 3 equiv. of KC8 yielded the CAAC-stabilized diatomic Sb2 species [{(CAAC)Sb}2], whereas one- and two-electron reduction reactions proceeded with formation of the CAAC-stabilized neutral antimony-centered radical (CAAC)SbCl2 and the chloro-stibinidene (CAAC)SbCl, respectively.[14c] Compounds 1 and 2 dissolve moderately in benzene, toluene, and hexane, whereas 3 dissolves poorly in benzene and toluene and is insoluble in hexane. Crystals of 1–3 can be stored in a glovebox under Ar atmosphere for several months. The 1H NMR spectrum of 1 in [D8]toluene shows broad and overlapping signals for the organic substituents. It consists of broad multiplets (3.95–3.43 ppm) and two doublets (1.29, 1.02 ppm) for the iPr groups of the b-diketiminate ligand, whereas the g-CH and two methyl groups of the C3N2M ring exhibit only single resonances (4.78, 1.61 ppm). Furthermore, the amide ligand (NMeEt) shows two singlets (3.10, 2.67 ppm, Me) and rather broad multiplets (2.91, 1.34 ppm, Et). Unfortunately, no satisfactory 13C NMR spectrum was obtained from 1 due to its poor solubility. The 1H NMR spectra of 2 and 3 are similar and show two septets (2: 3.91, 3.05; 3: 3.63, 3.05 ppm) and four doublets (2: 1.38, 1.20, 1.11, 1.01; 3: 1.52, 1.41, 1.17, 0.97 ppm) for the magnetically inequivalent iPr groups of the b-diketiminate ligand, whereas the g-CH and two methyl groups of the C3N2M ring exhibit only single resonances (2: 4.97, 1.63; 3: 4.79, 1.52 ppm). The 13C NMR spectra of 2 and 3 each show 14 signals of the b-diketiminate groups and the

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Full Paper data are consistent with the molecular structures determined by X-ray diffraction. Compounds 1–5 are promising starting reagents for salt elimination and substitution reactions due to the presence of reactive Ga@Cl and Ga@N groups. We therefore studied NMRscale reactions of 2 and 3 with Li amides (LiNMe2, LiNMeEt) as well as reactions of 4 and 5 with GaCl3 (Scheme 4). The reac-

Figure 1. Molecular structure of [(LGaNMeEt)2Sb2] 1. H-atoms and minor components of the disordered NMeEt residues have been omitted for clarity. Displacement ellipsoids are drawn at the 50 % probability level and the symmetry generated part of the molecule is depicted in pale colors.

Scheme 4. Reactivity studies of 1 to 5 with Li-amides and GaCl3.

tion of 2 at room temperature with 2 equiv. of LiNMe2 and LiNMeEt in [D8]toluene resulted in LiCl elimination and formation of the corresponding amide-substituted complexes [(LGaNR2)2Sb2] (NR2 = NMeEt 1, NMe2 4), which were unambiguously identified by their 1H NMR spectra (Figure S12 and S13 in the Supporting Information). In contrast, the reaction of 3 with 2 equiv. of LiNMe2 only showed the formation of LGa and LGa(NMe2)2 and an unidentified product, whereas resonances due to the formation of 5 were not observed (Figure S14). Interestingly, the previously reported NMe2-substituted complexes [(LGaNMe2)2Sb2] 4 and [(LGaNMe2)2(m,h1:1-Sb4)] 5 underwent Cl/amide exchange reactions in reactions with GaCl3 at room temperature and afforded 2 and 3, respectively (Figures S15 and S16). Use of an excess of GaCl3 in the reaction with 4 resulted in the formation of LGaCl2 and a grey precipitate, respectively.

Figure 2. Molecular structure of [(LGaCl)2Sb2] in the crystal of 2. H-atoms have been omitted for clarity. Displacement ellipsoids are drawn at the 50 % probability level and the symmetry generated part of the molecule is depicted in pale colors.

Solid state structures The molecular structures of 1, 2, and 3 were determined by single-crystal X-ray diffraction. The reaction of LGa with Sb(NMeEt)3 yielded crystalline materials, from which suitable red crystals of 1 were hand-picked under the microscope. Single crystals of 2·C7H8 and 3·0.5C6H6 were obtained from saturated solutions in toluene (2) and benzene (3), respectively, upon storage at room temperature for 2 days. 1 and 3 crystallize in the monoclinic system with space group P21/n and P21/c, respectively, whereas 2 crystallizes in the triclinic space group P1¯ (Figures 1–3). The molecules of 1 and 2 are located on centers of inversion. The asymmetric unit of 2 contains an additional toluene molecule. In 3, the molecule occupies a general position, whereas a benzene molecule is placed on a center of inversion.[15] The centrosymmetric distibene (Sb2) units within 1 and 2 are coordinated by two Chem. Eur. J. 2017, 23, 12297 – 12304

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Figure 3. Molecular structure of [(LGaCl)2(m,h1:1-Sb4)] in the crystal of 3. Hatoms have been omitted for clarity. Displacement ellipsoids are drawn at the 50 % probability level.

LGaX groups (X = NMeEt 1, Cl 2), resulting in a trans-bent orientation of the planar Ga@Sb=Sb@Ga units (Figures 1 and 2). The Ga atoms, which typically adopt distorted tetrahedral coor-

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Full Paper dination spheres, are slightly displaced from the best plane of the C3N2 units of the ligand [1: 0.679(4) a, 2: 0.6121(14) a] as was previously observed for LGa as well as [(LGaNMe2)2Sb2] 4 and [(LGaNMe2)2(m,h1:1-Sb4] 5. The Sb@Sb bond lengths in 1 [2.6433(6) a] and 2 [2.6461(2) a] are almost identical and comparable to those observed in 4 [2.6477(3) a][10] and other distibenes of the general type RSb=SbR, which were found to range from 2.64–2.70 a.[16] These bonds are substantially shorter when compared to Sb@Sb bond lengths in distibines of the type R2Sb@SbR2, which instead contain Sb@Sb single bonds.[17] The Ga@Sb bond length in 1 [2.6169(5) a] and 4 (2.6200(4) a)[10] are comparable, whereas that of 2 [2.5818(19) a] is slightly shorter (Table 1). This finding most

Table 2. Comparison of bond lengths (a) and angles (8) in the Ga2Sb4 cores of [(LGa)2(m,h2:2-Sb4)],[13] [(LGaNMe2)2(m,h1:1-Sb4)] 5,[10] and [(LGaCl)2(m,h1:1-Sb4)] 3.[a]

Ga1@Sb1 Ga1@Sb2 Ga2@Sb3 Ga2@Sb4 Sb1@Sb2 Sb1@Sb3 Sb1@Sb4 Sb2@Sb3 Sb2@Sb4 Sb3@Sb4 Ga1-Sb1-Sb2 Ga1-Sb1-Sb3 Ga1-Sb2-Sb3 Ga1-Sb2-Sb4 Ga2-Sb4-Sb2 Ga2-Sb4-Sb3

Table 1. Comparison of bond lengths (a) and angles (8) in Ga-substituted distibenes 1, 2, and [(LGaNMe2)2Sb2] 4.[10]

Sb1@Sb1a Ga1@Sb1 Ga1@Cl1 Ga1@N1 Ga1@N2 Ga1@N3 Ga1-Sb1-Sb1a N1-Ga1-N2 Cl1-Ga1-Sb1 N3-Ga1-Sb1

1

2

4

2.6433(6) 2.6169(5) – 2.005(3) 1.991(3) 1.864(3) 95.839(16) 93.28(12) – 115.28(11)

2.6461(2) 2.58178(19) 2.2319(4) 1.9439(11) 1.9564(10) – 89.476(6) 96.40(5) 117.383(13) –

2.6477(3) 2.6200(4) – 1.9894(13) 1.9826(13) 1.8558(13) 94.710(8) 93.16(5) – 116.37(4)

Ga2-Sb3-Sb1 Ga2-Sb3-Sb2 Sb2-Sb1-Sb3 Sb1-Sb2-Sb3 Sb1-Sb3-Sb2 Sb1-Sb3-Sb4 Sb2-Sb3-Sb4 Sb3-Sb2-Sb4 Sb2-Sb4-Sb3 Sb1-Sb2-Sb4

likely results from the electron-withdrawing effect of the Clsubstituent, which enhances the Lewis acidity of the Ga atom. The Ga@Sb bonds are significantly shorter than Ga@Sb single bonds as observed in Lewis acid-base adducts of the general type R3Sb-GaR’3,[18] the base-stabilized monomer dmapGa(Et)2Sb(SiMe3)2 (dmap = 4-NMe2-pyridine; 2.648(1) a)[19] and in heterocyclic compounds such as [R2GaSbR’2]x, in which the Ga@Sb bond lengths were found to range from 2.666– 2.772 a.[20] They are also slightly shorter than the sum of the covalent radii (Ga = 1.24; Sb = 1.40 a).[21] The Sb-Sb-Ga bond angle (94.71(1)8) agrees with the Sb-Sb-H angle calculated for HSb=SbH (93.08)[22] and those reported for ArSb=SbAr.[23] The central Sb4 unit in 3 adopts a “butterfly-type” conformation and is coordinated by two LGaCl fragments as was observed for [(LGaNMe2)2(m,h1:1-Sb4)] 5.[10] Within the Sb4 core, two approximately equilateral Sb3 triangles share a mutual edge (Sb2@Sb3) and the angle between the planes of the Sb3 triangles is 94.398. The endocyclic Sb-Sb-Sb bond angles within the triangles are close to the ideal value of 608, whereas the extratriangular ones are enlarged ( & 808), as was previously observed in 5 (758).[10] Alternatively, the Sb4 unit can be described as tetrahedron with one missing edge. The shortest Sb@Sb bond in 3 and 5 occurs between Sb2 and Sb3, whereas the bonds to Sb1 and Sb4, which are each further coordinated to one Ga atom, are slightly elongated (Table 2). Comparable Sb@Sb bond lengths were observed in phosphine-stabilized, cationic Sb4 rings of the type [(R3P)4Sb4][OTf]4 [R = Me 2.8354(6)-2.8797(5) a; R = Et 2.838(2)-2.884(2) a], Chem. Eur. J. 2017, 23, 12297 – 12304

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[(LGa)2(m,h2:2-Sb4)]

5

3

2.6637(11) 2.6748(11) 2.6676(11) 2.6779(11) – 2.8500(9) 2.8675(7) 2.8683(8) 2.8722(8) – 80.30(3) (Ga1-Sb1-Sb4) 79.08(3) 78.57(3) 80.03(3) 80.33(3) 79.35(3) (Ga2-Sb4-Sb2) 79.84(3) 80.58(3) 81.19(2) (Sb3-Sb1-Sb4) – 81.86(2) 81.49(2) (Sb1-Sb4-Sb2) – 80.79(2) – –

2.5975(5) – – – 2.8298(4) 2.8139(4) – 2.7920(5) 2.8299(4)[b] 2.8139(4)[b] 101.704(14)

2.6008(13) – – 2.6044(14) 2.8368(11) 2.8480(11) 3.6277(10) 2.7850(11) 2.8215(11) 2.8434(11) 89.86(4)

105.770(14) – – – –

96.40(4) – – 91.80(4) 93.58(4)

– – 59.300(13)

– – 58.67(3)

60.065(11) 60.635(11) 75.585(14)[b]

60.87(3) 60.47(3) 79.20(3)

60.636(11)[b] 60.065(11)[b] 59.300(13)[b] 75.084(14)[b]

60.16(3) 60.95(3) 58.89(3) 79.75(3)

[a] Ideal Sb4 tetrahedron: Sb@Sb 2.80 a, Sb-Sb-Sb 608.[21] [b] Sb4=Sb1a.

[(Me3P)3Sb4R2] [2.8209(5)-2.8612(5) a][8d,g] and in the homoatomic Sb42@ (av. 2.750 a) and Sb73@ polyanions (av. 2.797 a).[24] The Sb1···Sb4 distance of 3.6277(10) a is far too large to be considered as a covalent bond. The Ga atoms in 3 adopt distorted tetrahedral geometries. The Sb@Ga bonds are comparable to those of 1, 3, 5,[10] and [(LGa)2(m,h2:2-Sb4)] [2.6637(11)2.6779(11) a],[13] but slightly shorter than the sum of the covalent radii. The offsets from the best plane of the C3N2 units of the ligands are 0.60(2) a (Ga1) and 0.51(2) a (Ga2). Quantum chemical calculations The bonding situations in 1, 2, and 3 were analyzed by using a number of quantum chemical techniques to gain further insight into the chemical bonding within the Ga2Sb2 and Ga2Sb4 skeletons and the results were also compared to those observed in [(LGaNMe2)2(m,h1:1-Sb4)] 5 and [(LGa)2(m,h2:2-Sb4)],[13] respectively. All calculated bond lengths within the Ga2Sb2 (1, 2) and Ga2Sb4 (3) cores (BP86-D3/def2-SVP level of theory,[25–29] Tables S2–S4 in the Supporting Information) agree well with the corresponding experimental values (Dr = 0.02–0.08 a). As predicted by atoms in molecules (AIM), electron localization function (ELF), and natural bond orbital (NBO) analyses,[30–33] all Sb@ Sb bonds in 1, 2, and 3 are covalent in nature (Tables S2–S4,

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Figure 4. (a) Atomic labeling for the Ga2Sb2 skeleton and (b) ELF distribution in [(LGaNMeEt)2Sb2] 1 in the Ga1-Sb1-Sb2 plane. V(Ga,Sb), V(Sb), and V(Sb,Sb) basins are indicated by white, yellow, and black arrows, respectively.

Figure 5. (a) Atomic labeling for the Ga2Sb2 skeleton and (b) ELF distribution in [(LGaCl)2Sb2] 2 in the Ga1-Sb1-Sb2 plane. V(Ga,Sb), V(Sb), and V(Sb,Sb) basins are indicated by white, yellow, and black arrows, respectively.

formation) in 1 and 2, whereas 3 is characterized by only sSb-Sb bonds (ON = 1.95–1.97 j e j , Table S4). ELF distribution reveals one V(Sb,Sb) valence disynaptic basin populated by 1.2–1.3 j e j between each pair of connected Sb atoms in 3 (Figure 6), and ¯ [V(Sb,Sb)] = 1.1–1.3 j e j ; Figtwo V(Sb,Sb) basins in 1 and 2 (N ures 4 and 5). Thus, 3 contains five Sb@Sb single bonds, whereas 1 and 2 possess Sb=Sb double bonds. In addition, each Sb atom from 1, 2, and 3 has one electron lone pair (Tables S2– S4), as calculated by NBO analysis (ON = 1.9–2.0 j e j) and ELF ¯ [V(Sb)] = 2.8–3.2 j e j ; Figures 4–6).The Ga@Sb bonds are also (N covalent, which is supported by the presence of sGa-Sb bonds (ON = 1.96–1.97 j e j) with notable values of polarization coefficients (j cGa j 2 = 41–45 %), the high contribution of the electrons of Ga into the V(Ga,Sb) basins according to ELF/AIM intersec¯ [V(Ga,Sb) j Ga] = 1.2–1.3 j e j),[35] and sharedtion procedure (N type Ga···Sb interactions (r21(rb) < 0; j V(rb) j /G(rb) > 2, H(rb) < 0). Substitution of the NMeEt group by Cl atom leads to decreasing of natural population analysis (NPA) partial charges on Ga from + 1.23/ + 1.26 j e j (1) to + 1.03/ + 1.07 j e j (2) and + 1.06 j e j (3). The chemical bonding pattern in the Ga2Sb4 core of 3 is very similar to that of 5. Despite the slightly shorter Ga@Sb distances (2.633/2.634 a) compared to 5 (2.647/2.652 a), the calculated parameters of the bonds within the cores are almost the same.[13]

Conclusion In summary, Ga-substituted distibenes of the general type [(LGaX)2Sb2] (X = NMeEt 1, Cl 2, NMe2 4) were obtained from reactions of LGa with SbX3 [X = Cl, NMeEt, NMe2]. Upon thermal treatment, only 2 and 4 rearranged into the corresponding tetrastibines [(LGaX)2(m,h1:1-Sb4)] (X = Cl 3, NMe2 5), demonstrating the subtle influence of the substituent X on the reactivity of the distibenes. In addition, salt elimination reactions of 2 with Li amides resulted in the formation of amido-substituted compounds 1 and 4. Compounds 4 and 5 smoothly reacted with GaCl3 in an amide/Cl exchange reaction with subsequent formation of 2 and 3, respectively. 1–3 were structurally characterized by single-crystal X-ray diffraction and the bonding situation within these new complexes was investigated by quantum chemical calculations. Figure 6. (a) Atomic labeling for the Ga2Sb4 skeleton and ELF distribution in [(LGaCl)2(m,h1:1-Sb4)] 3 in the (b) Sb1-Sb2-Sb3, (c) Sb2-Sb3-Sb4, (d) Ga1-Sb1Sb3, and (e) Ga2-Sb4-Sb3 planes. V(Ga,Sb), V(Sb), and V(Sb,Sb) basins are indicated by white, yellow, and black arrows, respectively.

Figures 4–6), in accordance with recently reported computational data for these types of complexes.[12, 13, 34] However, the Sb@Sb bonds in 1 (2.677 a) and 2 (2.682 a) are considerably shorter compared to those in 3 (2.845–2.906 a), clearly indicating multiple bonding character between the Sb atoms. Indeed, NBO analysis finds two-center two-electron sSbSb and pSb-Sb bonds with occupation numbers (ON) ranging between 1.89 and 1.95 j e j (Tables S2 and S3 in the Supporting InChem. Eur. J. 2017, 23, 12297 – 12304

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Experimental Section General procedures: Standard Schlenk and glovebox techniques were used to carry out all reactions and the following work-ups were performed in purified argon atmosphere. Prior to use, toluene, hexane, pentane, and diethyl ether were passed through activated alumina columns on an MBraun Solvent Purification System. Deuterated NMR solvents were stored over activated molecular sieves (4 a) and degassed prior to use. Karl Fischer titration of the dry solvents showed values less than 3 ppm. Exact molar ratios of the reactions and reaction conditions were optimized using 1 H NMR by performing several small-scale reactions in J-Young NMR tubes using deuterated solvents under different reaction conditions. LGa,[36] [(LGaNMe2)2Sb2] 4,[10] and Sb(NRR’)3 (R,R’ = Me, Et)[37] were prepared according to literature methods, Li amides Li(NRR’)

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Full Paper were synthesized by reaction of the corresponding amine with nBuLi in hexane and isolated as colorless crystalline solids, whereas GaCl3 and solvents were obtained from commercial sources and purified prior to use. Instrumentation: 1H (300 and 500 MHz) and 13C{1H} (75.5 and 150 MHz) NMR (d in ppm) spectra were recorded using a Bruker Avance DPX-300 or Bruker Avance III HD spectrometer and the spectra were referenced to internal C6D5H (1H: d = 7.154; 13C: d = 128.39) and C6D5CHD2 (1H: d = 2.09; 13C: d = 20.40). Microanalyses were performed at the elemental analysis laboratory of the University of Duisburg-Essen. IR spectra were measured with an ALPHA-T FT-IR spectrometer equipped with a single reflection ATR sampling module. Melting points were measured using a Thermo Scientific 9300 apparatus. Synthesis of [(LGaNMeEt)2Sb2] 1: A solution of LGa (0.20 g, 0.40 mmol) and Sb(NMeEt)3 (0.12 g, 0.10 mL, 0.40 mmol) was heated at 60 8C without any solvent for 7 days in a J-Young NMR tube. During this time, the color of the reaction solution changed from yellow to red. The solution was then slowly cooled to room temperature to yield a mixture of red and colorless crystals. The colorless crystals were washed out with toluene (2 V 0.5 mL) to afford the pure form of 1 as an insoluble residue. M.p. 188 8C (dec.); Yield 0.063 g (47 %; Anal. Calcd. for C64H98N6Ga2Sb2 : C, 57.60; H, 7.40; N, 6.30. Found: C, 57.71; H, 7.43; N, 6.27 %; IR (neat): n˜ 3061, 2962, 2925, 2865, 2807, 2757, 1543, 1519, 1460, 1437, 1384, 1314, 1257, 1211, 1184, 1097, 1016, 992, 935, 880, 856, 792, 757, 720, 626, 565, 530, 440 cm@1; 1H NMR ([D8]toluene, 300 MHz, 25 8C): d 7.12–6.89 (m, 6 H, C6H3-2,6-iPr2), 4.78 (s, 1 H, g-CH), 3.95 and 3.82 (br m, 2 H, CH(CH3)2), 3.43 (br m, 2 H, CH(CH3)2), 3.10 and 2.67 (2 V br s, 3 H, NMeEt), 2.91 (br m, 2 H, NMeCH2CH3), 1.61 (s, 6 H, ArNCCH3), 1.34 (br m, 3 H, NMeCH2CH3), 1.29 (br d, 12 H, 3JH-H = 6.6 Hz, CH(CH3)2), 1.02 ppm (br d, 6 H, 3JH-H = 6.6 Hz, CH(CH3)2); 13 C NMR ([D8]toluene, 75 MHz, 25 8C): No clear data because of poor solubility of 1. Synthesis of [(LGaCl)2Sb2] 2. Method A: LGa (100 mg, 0.205 mmol) and SbCl3 (23.4 mg, 0.103 mmol) were dissolved in 0.5 mL of [D8]toluene in a J-Young NMR tube and stirred at room temperature for 2 days. The solution was kept at room temperature for further 2 days to yield green crystals of 2. Crystals were separated from the solution and recrystallized again from toluene to afford the pure form of 2. Method B: In a J-Young NMR tube, [(LGaNMe2)2Sb2] 4 (30 mg, 0.023 mmol) and GaCl3 (8.1 mg, 0.046 mmol) were dissolved in 0.5 mL of [D8]toluene and the reaction progress was monitored periodically using 1H NMR spectroscopy. The color changed from deep red into a greenish solution. After 24 h at ambient temperature, all resonances corresponding to [(LGaNMe2)2Sb2] 4 vanished and 2 was isolated as a green crystalline solid after removal of the solvent in vacuo. M.p. 240 8C (dec.); Yield 15.2 mg (51.3 %); Anal. Calcd. for C58H82N4Cl2Ga2Sb2 : C, 54.04; H, 6.41; N, 4.35. Found: C, 54.50; H, 6.43; N, 4.28 %. IR (neat): n˜ 3059, 2960, 2925, 2867, 1551, 1520, 1463, 1436, 1385, 1314, 1261, 1176, 1099, 1017, 939, 863, 793, 770, 711, 637, 525 cm@1; 1H NMR (C6D6, 300 MHz, 25 8C): d 7.11–6.91 (m, 6 H, C6H3-2,6-iPr2), 4.97 (s, 1 H, g-CH), 3.91 (sept, 3JH-H = 6.7 Hz, 2 H, CH(CH3)2), 3.05 (sept, 3JH-H = 6.7 Hz, 2 H, CH(CH3)2), 1.63 (s, 6 H, ArNCCH3), 1.38 (d, 6 H, 3JH-H = 6.7 Hz, CH(CH3)2), 1.20 (d, 6 H, 3JH-H = 6.7 Hz, CH(CH3)2), 1.11 (d, 6 H, 3 JH-H = 6.7 Hz, CH(CH3)2), 1.01 ppm (d, 6 H, 3JH-H = 6.7 Hz, CH(CH3)2); 13 C NMR (C6D6, 75 MHz, 25 8C): d 169.46 (ArNCCH3), 146.52 (C6H3), 142.92 (C6H3), 141.58 (C6H3), 125.82 (C6H3), 124.49 (C6H3), 98.39 (gCH), 30.03 (CH(CH3)2), 28.40 (CH(CH3)2), 27.80 (CH(CH3)2), 25.17 (CH(CH3)2), 24.89 (CH(CH3)2), 24.84(CH(CH3)2), 24.28 ppm (ArNCCH3). Synthesis of [(LGaCl)2(m,h1:1-Sb4)] 3: A solution of 2 (30 mg, 0.0233 mmol) in 0.5 mL of [D8]toluene was heated at 130 8C for Chem. Eur. J. 2017, 23, 12297 – 12304

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7 days in a J-Young NMR tube. The solution was slowly cooled to room temperature and kept at this temperature for 1 day to afford yellow crystals of 3. The crystals were separated from the mother liquor, washed with hexane and dried in vacuo. Yield: 10 mg (0.0065 mmol, 56 %); M.p. 194 8C (dec.); Anal. Calcd. for C58H82N4Cl2Ga2Sb4 : C, 45.45; H, 5.39; N, 3.66. Found: C, 45.40; H, 5.34; N, 3.71 %.; IR (neat): n˜ 2958, 2917, 2860, 1521, 1441, 1383, 1320, 1257, 1171, 1096, 1021, 935, 866, 798, 757, 677, 631, 528, 441 cm@1; 1H NMR (C6D6, 500 MHz): d 7.27–7.09 (m, 6 H, C6H3-2,6iPr2), 4.79 (s, 1 H, g-CH), 3.63 (sept, 2 H, CH(CH3)2), 3.05 (sept, 2 H, CH(CH3)2), 1.52 (s, 6 H, ArNCCH3), 1.52 (d, 6 H, J = 6.5 Hz, CH(CH3)2), 1.41 (d, 6 H, J = 6.5 Hz, CH(CH3)2), 1.17 (d, 6 H, J = 7 Hz, CH(CH3)2), 0.97 ppm (d, 6 H, J = 7 Hz, CH(CH3)2); 13C NMR (C6D6, 150 MHz): d 168.60 (ArNCCH3), 146.16 (C6H3), 142.92 (C6H3), 141.00 (C6H3), 126.00 (C6H3), 124.19 (C6H3), 97.39 (g-CH-), 29.94 (CH(CH3)2), 28.05 (CH(CH3)2), 27.81 (CH(CH3)2), 24.76 (CH(CH3)2), 24.67 (CH(CH3)2), 24.63 (CH(CH3)2), 23.65 ppm (ArNCCH3). Computational details: The geometric parameters of the species under study were fully optimized in the gas phase at the BP86-D3/ def2-SVP theoretical level[25@28] with a corresponding small-core relativistic effective core potential for Sb[29] employing ultrafine grid. The stationary points were characterized as minima on the potential energy surface by vibrational analysis (the number of imaginary frequencies (NImag) was equal to zero) and the structures obtained were used for the subsequent calculations. AIM[30] and ELF[31, 32] computations were performed with DGrid program[38] using densities from the all-electron scalar relativistic (SR) ZORABP86-D3/TZP computations.[25@27, 39] ELF basin populations were calculated for a rectangular parallelepipedic grid with a mesh size of 0.1 bohr. The NBO[33] was performed at the BP86-D3/def2-SVP theoretical level as implemented in Gaussian09.[40] SR-ZORA-BP86-D3/ TZP computations were performed using ADF2013 suite of programs (core potentials were not used, and quality of the Becke numerical integration grid was set to the keyword good),[41–43] whereas the remaining computations were carried out in Gaussian09 code.[40] Detailed information about AIM, ELF, and NBO can be found elsewhere.[30–33] Single-crystal X-ray diffraction: The crystals were mounted on nylon loops (1, 2) and a glass fiber (3) in inert oil. Data of 1 and 2 were collected on a Bruker AXS D8 Kappa diffractometer with APEX2 detector (mono-chromated MoKa radiation, l = 0.71073 a) and those of 3 on a Stoe IPDS II (mono-chromated MoKa radiation, l = 0.71073 a). The structures were solved by Direct Methods (SHELXS-97)[44] and refined anisotropically by full-matrix leastsquares on F2 (SHELXL-2014).[45] Absorption corrections were performed semi-empirically from equivalent reflections on the basis of multi-scans (SADABS, XPREP). Hydrogen atoms were refined using a riding model or rigid methyl groups. The NMeEt ligand in 1 is disordered over two positions. Not all atoms of the alkyl residues could be refined anisotropically. Where refinement with anisotropic displacement parameters was possible, appropiate ISOR and RIGU restraints had to be applied.

Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft (SCHU 1069/22-1) is acknowledged. A.S.N. is also thankful to the Siberian Supercomputer Center SB RAS for providing computational resources. We thank E. Hammes for technical support.

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Full Paper Conflict of interest The authors declare no conflict of interest. Keywords: cluster compounds · main-group elements · polystibide · subvalent compounds

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Manuscript received: March 20, 2017 Accepted manuscript online: May 12, 2017 Version of record online: June 26, 2017

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