Addition of H₂ on (Sulfur, Phosphorus, Sulfur)-Pincer-Based Rhodium(I), Iridium(I), Palladium(II), and Platinum(II) Complexes: Reactivity and Regioselectivity

The addition of H2 on (sulfur, phosphorus, sulfur)-pincer-based 16-VE complexes was studied from both experimental and theoretical points of view. The reaction of H2 with [M(SPS)(PPh3)] (M = Rh, Ir) led to the formation of dihydride M(III) complexes, which was fully characterized with iridium as metal. No reaction occurred with the isoelectronic 16-VE Pt(II) [Pt(SPS)Cl] complex. This reaction was also studied by means of DFT/B3PW91 calculations on the model complexes [M(SPS)(PH3)] (M = Rh, Ir) and by B3PW91//ONIOM(B3PW91:UFF) calculations on real systems. The regioselectivity observed and the greater reactivity of the Ir compound were properly reproduced and related through a thermodynamic cycle to the electronic properties of the metal complex and to the energetics of the M−H bonds under formation. For both Rh and Ir complexes, the transition state for the formation of the dihydride product is a molecular dihydrogen complex located about 9 kcal mol-1 above the reactants. The reactivity of Pd(II) and Pt(II) analogues ([M(SPS)(X)], X = Cl, PH3+) was also studied, and the formation of M(IV) dihydrides was found to be much less favorable, especially for M = Pd. Non-oxidative-addition processes leading to molecular dihydrogen complexes or to an heterolytic cleavage of H2 were also explored, but none of them were found to be favored on ΔG grounds.