Accessibility of 17-Electron Structures for Cyclopentadienylchromium(III) Compounds. 1. Experimental Studies on the Dichloride and Dimethyl Compounds

Abstract

Adducts of the CpCrX2 (X = Cl, CH3) fragments with the bidentate ligands Me2PCH2PMe2 (dmpm), Me2PCH2CH2PMe2 (dmpe), and Ph2PCH2CH2PPh2 (dppe) are described. Depending on the amount of ligand used, dinuclear 2:1 adducts or mononuclear 1:1 adducts are obtained. The dinuclear adducts have a CpX2Cr(μ-η1:η1-L-L)CrX2Cp structure featuring 15-electron Cr centers, as shown by X-ray structural determinations on [CpCrCl2]2(dppe), [CpCrCl2]2(dmpe), and [CpCr(CH3)2]2(dmpe). The reaction between [CpCrCl2]2(L-L) and L-L affords the mononuclear CpCrCl2(L-L) derivatives. A 31P-NMR monitoring of this reaction for L-L = dmpm reveals a quantitative reaction, followed by release of free dmpm. This phenomenon is interpreted as a dmpm-induced isomerization of [CpCrCl2]2(dmpm) to [CpCrCl(dmpm)]+[CpCrCl3]-. The mononuclear adducts have a 15-electron configuration CpCrX2(η1-L-L) with a dangling phosphine ligand, both in the solid state, as shown by an X-ray crystal structure of CpCrCl2(dmpm), and in solution, as shown by 31P-NMR, UV−visible, and EPR spectroscopies. The hypothetical 17-electron configuration where the bidentate L-L ligand adopts a chelating configuration, CpCrX2(η2-L-L), is not experimentally observed for any of the systems investigated.