Borate-based ligands with soft heterocycles and their ruthenium complexes

In a quest for effective synthetic precursors for the preparation of B-agostic complexes of ruthenium, we have shown that the reaction of [Cp∗RuCl<inf>2</inf>]<inf>2</inf> (Cp∗ = η5-C<inf>5</inf>Me<inf>5</inf>) with [NaBt] or [NaBo] (Bt = dihydrobis(2-mercaptobenzthiazolyl)borate; Bo = dihydrobis(2-mercaptobenzoxazolyl)borate) led to the formation of B-agostic complexes [Cp∗RuBH<inf>2</inf>L<inf>2</inf>], 1a,b (1a: L = 2-mercaptobenzthiazol, 1b: L = 2-mercaptobenzoxazol) and [Cp∗RuBH<inf>3</inf>L], 2a,b (2a: L = 2-mercaptobenzthiazol, 2b: L = 2-mercaptobenzoxazol) in good yields. In parallel to the formation of 1a,b and 2a,b, this method also allowed the formation of ruthenium hydrotrisborate complexes [Cp∗RuBYL<inf>3</inf>], 3a-c (3a: L = 2-mercaptobenzthiazol, Y = H; 3b: L = 2-mercaptobenzoxazol, Y = H; 3c: L = 2-mercaptobenzoxazol, Y = Cl). The key feature of complexes 3a-c is the coordination of one of the 2-mercaptobenzothiazole ligand that connects to the metal and the boron centre through a common sulfur atom. Upon heating, compounds 3a,b change into their corresponding S→N linkage isomers, in which the boron atom is bonded to three nitrogen atoms. The cyclic voltammetric studies on compounds 3a-c and 4a,b suggest that a deviation in coordination of the ligand change the oxidation potential of the metal centre. All the new compounds have been characterized in solution by ¹H, ¹¹B and ¹³C NMR spectroscopy, mass spectrometry and the structural types of 3a-c and 4b were unequivocally established by crystallographic analysis.