Benzannulated tris(2-mercapto-1-imidazolyl)-hydroborato ligands: tetradentate κ4-S3H binding and access to monomeric monovalent thallium in an [S3] coordination environment

The benzannulated tris(mercaptoimidazolyl) borohydride sodium complex, [Tm-ButBenz]Na, has been synthesized via the reaction of NaBH4 with 1-tert-butyl-1,3-dihydro-2H-benzimidazole-2-thione, while [Tm-MeBenz]K has been synthesized via the reaction of KBH4 with 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione. The molecular structures of the solvated adducts, {[Tm-ButBenz]Na(THF)}(2)(mu-THF)(2) and [Tm-MeBenz]K(OCMe2)(3), have been determined by X-ray diffraction, which demonstrates that the [Tm-R] ligands in these complexes adopt different coordination modes to that in {[Tm-MeBenz]Na}(2)(mu-THF)(3). Specifically, while the [Tm-MeBenz] ligand of the sodium complex {[Tm-MeBenz]Na}(2)(mu-THF)(3) adopts a kappa(3)-S-3 coordination mode, the potassium complex [Tm-MeBenz]K(OCMe2)(3) adopts a most uncommon inverted kappa(4)-S3H coordination mode in which the potassium binds to all three sulfur donors and the hydrogen of the B-H group in a linear K...H-B manner. Furthermore, the [Tm-ButBenz] ligand of {[Tm-ButBenz]Na(THF)}(2)(mu-THF)(2) adopts kappa(3)-S2H coordination mode, thereby demonstrating the flexibility of this ligand system. The monovalent thallium compounds, [Tm-MeBenz]Tl and [Tm-ButBenz]Tl, have been obtained via the corresponding reactions of [Tm-MeBenz] Na and [Tm-ButBenz] Na with TlOAc. X-ray diffraction demonstrates that the three sulfur donors of the [Tm-RBenz] ligands of both [Tm-MeBenz]Tl and [Tm-ButBenz]Tl chelate to thallium. This coordination mode is in marked contrast to that in other [Tm-R]Tl compounds, which exist as dinuclear molecules wherein two of the sulfur donors coordinate to different thallium centers. As such, this observation provides further evidence that benzannulation promotes kappa(3)-S-3 coordination in this system.