Boron-boron bond oxidative addition to rhodium(I) and iridium(I) centres

The reaction between the diborane(4) compound B-2(1,2-O2C6H4)(2) and either of the rhodium(I) complexes [RhCl(PPh3)(3)] or [{Rh(mu-Cl)(PPh3)(2)}(2)] afforded the colourless rhodium(III) bis(boryl) species [RhCl(PPh3)(2)-{B(1,2-O2C6H4)}(2)]. Similar reactions have been carried out with the diborane(4) compounds B-2(1,2-O-2-4-(BUC6H3)-C-t)(2), B-2(1,2-O-2-3,5-(Bu2C6H2)-C-t)(2), B-2(1,2-O-2-3-MeC6H3)(2), B-2(1,2-O-2-4-MeC6H3)(2), B-2(1,2-O-2-3-MeOC6H3)(2), B-2(1,2-S2C6H4)(2), B-2(1,2-S-2-4-MeC6H3)(2) and B-2[R,R-1,2-O2CH(CO2Me)CH(CO2Me)](2) affording analogous rhodium complexes all of which have been characterised spectroscopically. The complexes derived from the reactions with B-2(1,2-O2C6H4)(2) and B-2(1,2-O-2-3-MeC6H3)(2) have also been characterised by X-ray crystallography, the structures comprising a five-co-ordinate rhodium centre with a square-based-pyramidal geometry in which the apical site is occupied by a boryl group and the phosphines are mutually trans in basal positions. Reactivity studies have also been carried out for [RhCl(PPh3)(2){B(1,2-O2C6H4)}(2)]. Hydrolysis or alcoholysis with catechol afforded [RhH2Cl(PPh3)(3)] and either B-2(1,2-O2C6H4)(2)(mu-O) or B-2(1,2-O2C6H4)(3) and addition of the phosphines PMe3, PEt3 and PMe2Ph afforded the new bis(boryl) compounds cis,mer -[RhCl(PMe3)(3){B(1,2-O2C6H4)}(2)], [RhCl(PEt3)(2)-{B(1,2-O2C6H4)}(2)] and cis,mer-[RhCl(PMe2Ph)(3)(B(1,2-O2C6H4)}(2)] the PEt3 complex having been characterised by X-ray crystallography and shown to be similar to the PPh3 complex. The iridium analogue [IrCl(PEt3)(2){B(1,2-O2C6H4)}(2)] was also prepared from the reaction between [IrCl(PEt3)(3)] and B-2(1,2-O2C6H4)(2) and shown by X-ray crystallography to be isomorphous with the rhodium complex. Reactions between [RhCl(PPh3)(2){B(1,2-O2C6H4)}(2)] and the phosphines PPr31, P(C6H11)(3), 1,2-bis(diphenylphosphino)ethane (dppe) and 1,2-bis(dicyclohexylphosphino)ethane (dcpe) are also described although these do not result in new rhodium boryl complexes. The reaction between [(RhCl(dppe)}(2)] and B-2(1,2-O2C6H4)(2) afforded a compound tentatively assigned as [Rh(dppe)(2){B(1,2-O2C6H4)}] with analogous compounds being formed with the diborane(4) compounds B-2(1,2-O-2-3-MeC6H3)(2) and B-2(1,2-O-2-4-MeC6H3)(2). Finally, the reaction between [Rh(PMe3)(4)]Cl and the diborane(4) compound B-2(1,2-O2C6H4)(2) is described which affords cis,mer-[RhCl(PMe3)(3) {B(1,2-O2C6H4)}(2)]. Analogous reactions with B-2(1,2-O-2-3,5-(Bu2C6H2)-C-t)(2), B-2(1,2-O-2-3-MeC6H3)(2) and B-2[R,R-1,2-O2CH(CO2Me)CH(CO2Me)](2) afforded similar products.