DFT study of the fixation of CO by SPS-based pincer Rh(I) and Ir(I) complexes: Regioselectivity and reactivity

The regioselectivity of the addition of CO on SPS-type pincer-based metal complexes is studied by means of DFT calculations on the unsubstituted model complexes [M(SPS)(PH3)] (M=Rh and Ir). The reaction leads to the decoordination of a sulphur centre and to the formation of the four-coordinate square-planar complex [M(SP)(PH3)(CO)], but the five-coordinate syn-SS complex [M(SPS)(PH3)(CO)] is found to be very close in energy (ΔE<4 kcal mol−1). MQ/MM calculations on the real complex [M(SPS)(PPh3)(CO)] with phenyl substituents however favor this five-coordinated structure, in good agreement with the X-ray structure reported for the Rh compound. The influence of the nature of the metal centre on the thermicity of the CO addition is related through a thermodynamic cycle to the electronic properties of the metal complex and to the energetics of the M-CO bond. It is shown that the larger exothermicity predicted for the Ir complex is due to the energetics of the metal-carbon bond under formation (ΔEbond(Ir-CO)>ΔEbond(Rh-CO)).