Synthesis, electrochemistry, kinetics and density functional theory calculations on imino and thenoyl-bidentate complexes of rhodium

English: In this study L,L’-BID ligands were synthesized where the L and L’ coordinating atoms were varied between O, NH and NPh. The ligands were (CH3COCHCOHCH3) (Hacac), (CH3COCHCNH2CH3) (1a), (CH3COCHCNHPhCH3) (1b), (CH3CNHPhCHCNPhCH3) (1c). The dicarbonyl Rh complexes [Rh(CH3COCHCNHCH3)(CO)2] (2a), [Rh(CH3COCHCNPhCH3)(CO)2] (2b), [Rh(CH3CNPhCHCNPhCH3)(CO)2] (2c)) as well as the phosphine Rh complexes ([Rh(CH3COCHCNHCH3)(CO)(PPh3)] (3a), [Rh(CH3COCHCNPhCH3)(CO)(PPh3)] (3b)) were also synthesized and studied. All ligands (Hacac, 1a - 1d) displayed a single reduction peak ( <-2.5 VvsFcH/FcH+) during electrochemical analysis. From the cyclic voltammetry (CV) data it is found that the electronegativity of the L,L’ atoms, and their groups, have a significant electronic effect on the reduction potential. The observed trend was with the more positive reduction potential: Hacac>1b>1c>1a. This trend was the same as the trend in the electronegativies of atoms or groups. Complexes 3a and 3b, including selected complexes containing O.O’-BID ligands ([Rh(CH3COCHCOHCH3)(CO)(PPh3)], [Rh(C4H3SCOCHCOCPh)(CO)(PPh3)] M1, [Rh (C4H3SCOCHCOCC4H3S)(CO)(PPh3)] M2 and [Rh(C4H3SCOCHCOCF3)(CO)(PPh3)] M3) all displayed a single RhI/III oxidation peak and a single reduction peak, that is coupled to the oxidation peak. The [Rh(L,L’-BID)(CO)(PPh3)] complexes displayed a similar trend to that observed for the free, unbound ligands, namely: [Rh(CH3COCHCOHCH3)(CO)(PPh3)]>3b>3a. The kinetic results, followed in situ by UV/VIS, IR and NMR, for the oxidative reaction between CH3I and complexes 3a and 3b indicated that complex 3b displayed only one oxidative addition step in the reaction leading to an Rh(III)-alkyl, while complex 3a displayed 2 steps: the first step is the formation of a Rh(III)-alkyl product and the second step the formation of a Rh(III)-acyl product. Density functional theory calculations of the optimized geometries and energies of the reactants, possible products and transition states for the above mentioned reactions corroborated the experimentally observed results. For complex 3b it was found that a single transition step is present in the reaction leading to the formation of a Rh(III)-alkyl product. For complex 3a it was found that there are 2 transition states in the oxidative addition reaction. The first is the formation of the Rh(III)-alkyl product. The second is the formation of the Rh(III)-acyl product from the alkyl product through CO insertion. The Rh(III)-acyl product has the lowest energy.