Chemistry of group 5 metallaboranes with heterocyclic thiol ligands: a combined experimental and theoretical study.

Thermolysis of [(Cp*Nb)2(B2H6)2], 1b (Cp* = η5-C5Me5), with 2-mercaptobenzothiazole, C6H4NSCSH (MBT), and 2-mercaptobenzoxazole, C6H4NOCSH (MBO), yielded hydrogen substituted compounds 2 and 3 with a general formula [(Cp*Nb)2(B2H6)(B2H5L)] (2: L = C6H4NSCS and 3: L = C6H4NOCS). A similar reaction of 1b with Ph2Se2 yielded the monosubstituted derivative [(Cp*Nb)2(B2H6){B2H5(PhSe)}], 4. All further efforts towards persubstitution of 1b under various drastic conditions were unfruitful. In parallel, in an effort to find a better synthetic route to the known Ta-aziridine complex [Cp*TaBH(C7H4NS2)CH2S2NC6H4], Cp*TaCl4 was treated with a 2-mercaptobenzothiazolyl-based borate ligand Na[H2B(C6H4NSCS)2]. Surprisingly, the reaction led to the formation of the half-sandwich trichloroaryltantalum(v) complex [Cp*TaCl3{κ2-N,S-C6H4NSCS}], 5, containing a heterocyclic thiol ligand. Using an alternative method complex 5 was isolated in good yield when Cp*TaCl4 was treated with the potassium salt of 2-mercaptobenzothiazole K[C6H4NSCS]. All the compounds were characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy, and their structures were unequivocally established by crystallographic analysis.