ORTHO-METALATION REACTIONS IN BINUCLEAR DIRHODIUM COMPOUNDS - MOLECULAR-STRUCTURE AND REACTIVITY OF A MONOMETALLATED COMPOUND WITH THE PHOSPHINE-P(ORTHO-CLC6H4)(C6H5)2

The complex Rh2(O2CCH3)3[(C6H4)P(o-ClC6H4)(C6H5)](HO2CCH3)2 [(1)(HO2CCH3)2] has been characterized by X-ray crystallography. This is a new example of a fully characterized Rh4(2+) unit with only one ortho-metalated phosphine bridging the two rhodium atoms. It crystallizes in space group P1BAR with unit cell dimensions of a = 14.324 (3) angstrom, b = 11.627 (3) angstrom, c = 11.105 (3) angstrom, alpha = 65.25 (2)-degrees, beta = 112.57 (3)-degrees, gamma = 100.43 (2)-degrees, V = 1551 (1) angstrom 3, and Z = 2. Some important distances (angstrom) are as follows: Rh-Rh, 2.401 (1); Rh(1)-P, 2.209 (2); Rh(2)-C(2), 2.026 (7); Rh-O (trans to O), in the range of distances 2.009 (5)-2.051 (4); Rh(1)-O(27) (trans to P) 2.185 (6); Rh(2)-O(29) (trans to C) 2.241 (4). Compounds of formulation Rh2(O2CCH3)3[(C6H4)P(o-ClC6H4)(C6H5)](P(p-XC6H4)3) (X = CH3 (2), H (3), Cl (4)) can be obtained in solution from (1)(HO2CCH3)2 by axial ligand exchange. Compounds 2-4 undergo thermal reactions in CHCl3/HO2CCH3 (3:1 mixture) yielding doubly metalated compounds of formula Rh2(O2CCH3)2[(C6H4)P(o-ClC6H4)(C6H5)][(XC6H3)P(XC6H4)2](HO2CCH3) (X = CH3 (5), H (6), Cl (7)). If an excess of the corresponding P(p-XC6H4)3 ligand is used, the metalation reaction is considerably faster, and adducts of molecular formula Rh2(O2CCH3)2[(C6H4)P(o-ClC6H4)(C6H5)][XC6H3)P(XC6H4)2](P(p-XC6H4)3) are formed (X = CH3 (8), H (9), Cl (10)). In both conditions, with or without excess phosphine, the reaction rates increase in the order Cl < H < CH3. The two metalated phosphines bridge the rhodium atoms in a head-to-tail (compounds 5 and 6) or head-to-head fashion (compounds 7-10). Two different reaction pathways are proposed in order to account for these structural differences.