Synthesis and reactivity of mononuclear (pentachlorophenyl)rhodium(II) complexes. Structural relevance of rhodium-o-chlorine secondary bonding

The paramagnetic rhodium(II) complex [PBzPh(3)](2)[Rh(C6Cl5)(4)] (1a) is obtained from the arylation of [RhCl3(tht)(3)] with LiC6Cl5. Reaction of 1a with CO affords the rhodium(I) complex [PB(2)Ph(3)][Rh(C6Cl5)(2)(CO)(2)] (2). The oxidation of la with chlorine or iodine gives the rhodium(III) compound [PBzPh(3)][Rh(C6Cl5)(4)] (3a). The reaction of 3a with methanolic solutions of HCl leads to the breaking of only one of the Rh-C bonds, producing the monomeric complex [PB(2)Ph(3)][Rh(C6Cl5)(3)Cl] (4a). Treatment of 3a with potassium thiocyanate or thallium(I) acetylacetonate gives rise to the substitution of the chlorine ligand and to the formation of [PBzPh(3)][Rh(C6Cl5)(3)X] (X = SCN, (5), acac (6)). With CO, 3a yields the diacyl complex [PBzPh(3)][Rh{C(O)C6Cl5}(2)(C6Cl5)Cl] (7). All the complexes are stable to the air and moisture in the solid state and moderately stable in deoxygenated solutions. The crystal structures of compounds la, 3a, 4a, and 7 have been determined. Although these rhodium(II) or rhodium(III) complexes have four ligands, the coordination around the rhodium atoms is rather different, conforming in all cases to chiral molecular structures. Whereas complex la shows an almost perfect square-planar geometry around the rhodium center, complexes 3a, 4a, and 7 display distorted-octahedral environments. Two pentachlorophenyl groups in 3a and one in 4a behave as conventional sigma-bonded ligands, while two additional C6Cl5 groups in 3a and 4a and one in 7 coordinate as chelating ligands, being bonded through the C-ipso atom and through secondary bonding interactions of the o-Cl atoms. Complex 7 additionally incorporates two (pentachlorophenyl)acyl groups linked as terminal or chelate ligands. The secondary-bonded a-chlorine atoms exhibit Rh-Cl distances in the range 2.5859-2.8863(9) Angstrom.