EPR STUDY OF THE TOLUENE SOLUTION PROPERTIES OF THE CHROMIUM(III) ALKYL COMPLEXES [CPCRMECL]2, [CP-ASTERISK-CRRCL]2 (R = ME, ET, CH2SIME3), AND [CP-ASTERISK-CRMEBR]2 - DIMER MONOMER EQUILIBRIA IN SOLUTION

EPR signals have been observed for a series of Cr(III)-alkyl complexes dissolved in toluene and the solution and frozen-solution EPR spectra have been used to characterize the structures and ground states of these complexes in solution. Both ligand (alkyl, halide) and metal (Cr-53) hyperfine splitting of these signals indicates that in the noncoordinating solvent toluene: CpCrMeCl exists entirely as a dinuclear complex, consistent with its dichloride-bridged solid-state X-ray structure; Cp*CrMeCl exists as an analogous dinuclear complex at higher concentrations but dissociates to a mononuclear structure upon dilution; Cp*CrEtCl and Cp*Cr(CH2SiMe3)Cl both exist entirely as monomeric coordinatively unsaturated structures; Cp*CrMeBr, though somewhat more complicated than the chloride complexes, exists in a dinuclear structure which appears to dissociate into a mononuclear species at low concentrations. Reactions of [CpCrMeCl]2 Or [Cp*CrMeCl]2 with trimethylphosphine or 4-tert-butylpyridine result in loss of the solution EPR spectrum and appearance of typical S = 3/2 Cr(III) frozen-solution EPR signals, indicating formation of the three-legged piano stool adducts Cp(*)CrMeLCl. While the origin of the EPR signal from the antiferromagnetically coupled ground state of the dinuclear complexes has not been clearly established, the coordinatively unsaturated mononuclear complexes Cp*CrRCl (R = Me, Et, CH2SiMe3) have an S = 1/2 ground state. The unprecedented large and well-resolved isotropic and anisotropic halide hyperfine coupling in these complexes suggests significant Cr(III)-Cl pi-bonding in a planar two-legged piano stool structure, which may be important in stabilizing these formally 13-electron complexes. Both increased steric properties of the Cp and alkyl ligands and decreased concentrations shift the dimer-monomer solution equilibrium toward the monomeric species; reactivity of the dichloride-bridged dinuclear molecules may be associated with these coordinatively unsaturated mononuclear complexes.