Disilicon Complexes with Two Hexacoordinate Si Atoms: Paddlewheel‐Shaped Isomers with (ClN₄)SiSi(S₄Cl) and (ClN₂S₂)SiSi(S₂N₂Cl) Skeletons

Abstract

The reaction of 1-methyl-3-trimethylsilylimidazoline-2-thione with hexachlorodisilane proceeds toward substitution of four of the disilane Cl atoms during the formation of disilicon complexes with two neighboring hexacoordinate Si atoms. The N,S-bidentate methimazolide moieties adopt a buttressing role, thus forming paddlewheel-shaped complexes of the type ClSi(μ-mt)4 SiCl (mt=methimazolyl). Most interestingly, three isomers (i.e., with (ClN4 )SiSi(S4 Cl), (ClN3 S)SiSi(S3 NCl), and (ClN2 S2 )SiSi(S2 N2 Cl) skeletons as so-called (4,0), (3,1), and cis-(2,2) paddlewheels) were detected in solution by using (29) Si NMR spectroscopic analysis. Two of these isomers could be isolated as crystalline solids, thus allowing their molecular structures to be analyzed by using X-ray diffraction studies. In accord with time-dependent NMR spectroscopy, computational analyses proved the cis-(2,2) isomer with a (ClN2 S2 )SiSi(S2 N2 Cl) skeleton to be the most stable. The compounds presented herein are the first examples of crystallographically evidenced disilicon complexes with two SiSi-bonded octahedrally coordinated Si atoms and representatives of the still scarcely explored class of Si coordination compounds with sulfur donor atoms.

The First Structurally Characterised Example of Silicon in an S6 Coordination Sphere

“…Higher coordination numbers involving mixed donor sets including sulfur are rare. Amongst these, however, are species based on amine‐thiones which are the precursors of the ligand system discussed here . Indeed, the silylene compounds reported by Wagler et al could be described as inverse scorpionates or silatranes.…”

“…Few structurally characterised sulfur donor adducts of silicon are known, but the mono‐ and bis‐thiolates adopt a simple S 4 tetrahedral motif . However, a number of structures and reports have appeared which suggest that under certain circumstances the coordination number at silicon centres containing sulfur donor atoms can rise to six, although no species with S 6 ‐donor sets are known . Among these six coordinate species are the halo‐silylene adducts of methimazole which are reminiscent of our scorpionate compounds .…”

“…However, a number of structures and reports have appeared which suggest that under certain circumstances the coordination number at silicon centres containing sulfur donor atoms can rise to six, although no species with S 6 ‐donor sets are known . Among these six coordinate species are the halo‐silylene adducts of methimazole which are reminiscent of our scorpionate compounds . These observations, coupled with our general interest in the group 14 adducts of hydrotris(methimazolyl)borate anion, encouraged us to explore the reaction of Tm R with silicon tetraiodide …”

Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand

“…Whereas the former preferentially bind to Si atoms that carry strongly electron withdrawing groups (e.g., formation of pyridine adducts of halosilanes, Scheme 1, A, [1][2][3]), the latter offer greater opportunities of creating five-and six-coordinate silicon compounds even in case of the absence of halides from the silicon coordination sphere (e.g., pentacoordinate silicon with SiC 5 coordination sphere, B [4][5][6]; and hexacoordinate silicon with a tetradentate chelator and two Si-CH 3 groups, C [7]). For various reasons, such as activation of Si-X bonds by silicon hypercoordination [8][9][10][11][12][13][14][15][16], exploring special electronic/optical properties arising from the higher coordination number of silicon in combination with selected ligands [17][18][19][20] or the aim of creating and exploring hitherto unusual Si coordination compounds, e.g., with transition metals [21][22][23][24][25][26][27][28][29] or very soft Lewis bases in their ligand sphere [30][31][32][33][34][35][36][37][38], silicon coordination chemistry continues to be an attractive research field, reflected by frequently published research articles [39][40]…”

2‐Acylpyrroles as Mono‐anionic O,N‐Chelating Ligands in Silicon Coordination Chemistry

“…In the course of our investigations, we have studied silicon complexes with bi‐, tri‐, and tetradentate O , N ‐donor ligands,3b,c,f, 4a,b, 6 silicon complexes with five‐membered N ‐heterocyclic donor moieties,2g, h, 7 and other silicon compounds of pyrrole‐derived ligand systems 8. In the work reported herein, we now address silicon complexes with pyrrole‐functionalized bidentate O , N ‐chelating ligands.…”