The effect of coordination of alkanes, Xe and CO2 (η1-OCO) on changes in spin state and reactivity in organometallic chemistry: a combined experimental and theoretical study of the photochemistry of CpMn(CO)3

Abstract

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragment ³CpMn(CO)₂ formed upon photolysis of CpMn(CO)₃. Experimental data are provided for reactions in n-heptane and perfluoromethylcyclohexane (PFMCH), as well as in PFMCH doped with C₂H₆, Xe and CO₂. In PFMCH we find that the conversion of ³CpMn(CO)₂ to ¹CpMn(CO)₂(PFMCH) is much slower (τ = 18 (±3) ns) than the corresponding reactions in conventional alkanes (τ = 111 (±10) ps). We measure the effect of the coordination ability by doping PFMCH with alkane, Xe and CO₂; these doped ligands form the corresponding singlet adducts with significantly variable formation rates. The reactivity as measured by the addition timescale follows the order ¹CpMn(CO)₂(C₅H₁₀) (τ = 270 (±10) ps) > ¹CpMn(CO)₂Xe (τ = 3.9 (±0.4) ns) ∼ ¹CpMn(CO)₂(CO₂) (τ = 4.7 (±0.5) ns) > ¹CpMn(CO)₂(C₇F₁₄) (τ = 18 (±3) ns). Electronic structure theory calculations of the singlet and triplet potential energy surfaces and of their intersections, together with non-adiabatic statistical rate theory, reproduce the observed rates semi-quantitatively. It is shown that triplet adducts of the ligand and ³CpMn(CO)₂ play a role in the kinetics, and account for the variable timescales observed experimentally.