Synthesis, electrochemistry and density functional theory calculations on chrome and cobalt carbene and betadiketonato complexes

Two series of metal(β-diketonato)3 complexes were synthesized: tris(β-diketonato)chromium(III) complexes (where β-diketonato = [R1COCHCOR2]- = acac, ba, dbm, tfba, tfth, tffu, tfaa, hfaa) and tris(β-diketonato)cobalt(III) complexes (where β-diketonato = acac, ba, dbm, tfba, tfaa). These paramagnetic complexes were characterized by mass spectroscopy, X-ray crystallography and melting point measurements. The electrochemical behaviour of tris(β- diketonato)chromium(III) complexes was investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and square wave voltammetry (SWV). The reduction potentials of the CrIII(β-diketonato)3 → CrII(β-diketonato)3 process correlate linearly with the following electronic parameters: 1) the acid dissociation constant (pKa) of the uncoordinated β-diketonato ligands, 2) the total electronegativities (Σx) of the side groups (R1 and R2) on the β-diketonato ligands, and 3) the total Hammett sigma constant (Σσ) of the side groups (R1 and R2) on the β-diketonato ligands. DFT-calculations were done on both series of these metal(β-diketonato)3 complexes. Molecular geometry, spin state and respective population of possible isomers were calculated in this study. The understanding of crystal structures was complemented by using DFT-results. For CrIII(β-diketonato)3 complexes, linear dependence was found between their reduction potentials and electron affinity (EA), as well as between reduction potentials and the energies of the lowest unoccupied molecular orbitals (ELUMO). Electrochemical behaviour of chromium(0) carbene complexes was investigated by cyclic voltammetry. The difference in oxidation potential of fac/mer isomers was observed for the first time for chromium(0) carbene complexes and discussed in this study. DFT-calculations revealed the following linear relationships with the redox process of Cr(0) carbene complexes: 1) the linear dependence between the cathodic potential (Epc) and energies of the lowest unoccupied orbitals (ELUMO), and 2) the linear dependence between the anodic potential (Epa) and energies of the highest occupied orbitals (EHOMO).