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

Abstract

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(μ-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(μ-THF)₃. Specifically, while the [Tm^MeBenz] ligand of the sodium complex {[Tm^MeBenz]Na}₂(μ-THF)₃ adopts a κ³-S₃ coordination mode, the potassium complex [Tm^MeBenz]K(OCMe₂)₃ adopts a most uncommon inverted κ⁴-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(μ-THF)₂ adopts a κ³-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 κ³-S₃ coordination in this system.