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-ordi nate Lewis bases, specifically 4-methylpyridine (mpy) and the phosphin es 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)), t ogether with three bis adducts B-2(1,2-O2C6H4)(2)(mpy)(2), B-2(1,2-S2C 6H4)(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 a dopts a tetrahedral geometry, the remaining boron being trigonal plana r 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 bor ons tetrahedral. Bond-length changes associated with ligand co-ordinat ion are discussed. Multinuclear solution-state NMR studies have also b een 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 thes e studies indicate that intermolecular exchange of base between the pa rent compound, the mono- and the bis-adducts occurs in solution togeth er with intramolecular exchange between the two boron centres in the m onoadducts. The various rates of these processes, inasmuch as they hav e been determined accurately, are discussed and rationalised. The mono adducts B-2(1,2-O/S2C6H4)(2)(mpy) are nominally isoelectronic with the phenonium cation.