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

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, {mu-[Bm(MeBenz)]Na(THF)(2)}(2) and {[Tm-MeBenz]Na}(2)(mu-THF)(3), indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {mu-[Bm(MeBenz)]Na(THF)(2)}(2) possesses bridging [Bm(MeBenz)] ligands and terminal THF ligands, while {[Tm-MeBenz]Na}(2)(mu-THF)(3) possesses terminal [Tm-MeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl) borohydride ligand of {[Tm-MeBenz]Na}(2)(mu-THF)(3) coordinates in a kappa(3)-manner, which is in marked contrast to the kappa(2)-, kappa(1)- and kappa(0)- 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 kappa(3)-S-3 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 kappa(3)-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[Tm-MeBenz]Na}(2)(mu-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. {mu-[Bm(MeBenz)]Na(THF)(2)}(2) and {[Tm-MeBenz]Na}(2)(mu-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.