Lewis-base adducts of the diborane(4) compounds B-2(1,2-E(2)C(6)H(4))(2) (E=O or S)

The reactivity of the diborane(4) compounds B-2(1,2-E(2)C(6)H(4))(2) (E = O or S) have been studied with respect to their ability to co-ordinate Lewis bases, specifically 4-methylpyridine (mpy) and the phosphines PMe(2)Ph and PEt(3). Four monoadducts have been characterised by X-ray crystallography, namely B-2(1,2-O2C6H4)(2)(mpy), B-2(1,2-S2C6H4)(2)(mpy), B-2(1,2-S2C6H4)(2)(PMe(2)Ph) and B-2(1,2-S2C6H4)(2)(PEt(3)), together with three bis adducts B-2(1,2-O2C6H4)(2)(mpy)(2), B-2(1,2-S2C6H4)(2)(mpy)(2) and B-2(1,2-S2C6H4)(2)(PMe(2)Ph)(2). In the former set of compounds the one base is co-ordinated to one boron centre which adopts a tetrahedral geometry, the remaining boron being trigonal planar as in the structure of the parent compound. For the bis adducts each of the two borons is co-ordinated to a mpy or phosphine with both borons tetrahedral. Bond-length changes associated with ligand co-ordination are discussed. Multinuclear solution-state NMR studies have also been performed for solutions containing B-2(1,2-O2C6H4)(2) and mpy, B-2(1,2-S2C6H4)(2) and mpy and B-2(1,2-S2C6H4)(2) and PEt(3). All of these studies indicate that intermolecular exchange of base between the parent compound, the mono- and the bis-adducts occurs in solution together with intramolecular exchange between the two boron centres in the monoadducts. The various rates of these processes, inasmuch as they have been determined accurately, are discussed and rationalised. The monoadducts B-2(1,2-O/S2C6H4)(2)(mpy) are nominally isoelectronic with the phenonium cation.