Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ3-coordination

Abstract

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [Bm^MeBenz]Na and [Tm^MeBenz]Na, have been synthesized via the reactions of NaBH₄ with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[Bm^MeBenz]Na(THF)₂}₂ and {[Tm^MeBenz]Na}₂(μ-THF)₃, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[Bm^MeBenz]Na(THF)₂}₂ possesses bridging [Bm^MeBenz] ligands and terminal THF ligands, while {[Tm^MeBenz]Na}₂(μ-THF)₃ possesses terminal [Tm^MeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[Tm^MeBenz]Na}₂(μ-THF)₃ coordinates in a κ³-manner, which is in marked contrast to the κ²-, κ¹- and κ⁰-modes that have been reported for various [Tm^Me]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [Tm^MeBenz]⁻ and [Tm^Me]⁻ in the gas phase indicate that the conformation required for κ³-S₃ coordination, i.e. one in which the three sulfur donors point away from the B–H group, is relatively more stable for [Tm^MeBenz]⁻ than for [Tm^Me]⁻, and thus provides a rationalization for the observation that benzannulation enables κ³-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[Tm^MeBenz]Na}₂(μ-THF)₃. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [Tm^MeBenz]Re(CO)₃ with those of [Tm^Me]Re(CO)₃ indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[Bm^MeBenz]Na(THF)₂}₂ and {[Tm^MeBenz]Na}₂(μ-THF)₃ react with [Me₃PCuCl]₄ to give [Bm^MeBenz]CuPMe₃ and [Tm^MeBenz]CuPMe₃, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.