Use of a redox-active ligand to reversibly alter metal carbonyl electrophilicity

Article abstract of DOI:10.1021/ja00093a023

The chelating ligand, 1 ,1′-bis(diphenylphosphino)cobaltocene (dppc) is used as an electrochemically tunable ligand for altering electrophilicity of the carbonyl carbon in metal carbonyl complexes. The Re carbonyl complexes [Re(CO)_4-n(CH₃CN)_ndppc]^2+/+, n = 0 (cis), 1 (fac), 2 (cis,cis), and [fac-Re(CO)₃(NCO)dppc]^+/0 were prepared and characterized in both states of charge indicated. Single crystal X-ray crystallography was performed for both states of charge of [Re(CO)₄dppc]^2+/+ with the oxidized and reduced forms crystallizing in the monoclinic space groups C2/c and P2₁/c, respectively. The most pronounced structural difference between the two forms is the Cp(centroid)-Co distance, which is 0.09 ? longer for the reduced form. Cyclic voltammetry shows that E_1/2 for the Co(III)/Co(II) couple is 200-400 mV more positive for coordinated dppc than for the free ligand. For each CO substituted, E_1/2 becomes ～100 or 200 mV more negative for substitution by CH₃CN or NCO^-, respectively. Solution IR spectroscopy shows that v_CO for the oxidized form of each species is typically ～15 cm^-1 higher than for the reduced form, but the number and relative intensities of the carbonyl absorptions are the same. In every redox pair the oxidized species has higher reactivity with respect to nucleophilic attack at the carbonyl carbon. In CH₃CN at 25.0 °C, [fac-Re(CO)₃-(CH₃CN)dppc]⁺² reacts with amine N-oxides (CH₃)₃NO, N-methylmorpholine N-oxide, and (CH₃)₂(C₆H₅)NO ～200 times faster than [fac-Re(CO)₃(CH₃CN)dppc]⁺ to form [cis,cis-Re(CO)₂(CH₃CN)₂dppc]^+2/+, while [cis-Re-(CO)₄dppc]⁺² reacts with N₃-5400 times faster than [cis-Re(CO)₄dppc]⁺ to form [fac-Re(CO)₃(NCO)dppc]^+/0, with ΔΔH^# = 3-4 kcal/mol in both cases. The ionic strength and dielectric strength dependence of the reactivity of [Re(CO)₄dppc]^+2/+ toward N₃- were assayed to investigate electrostatic contributions to the attenuation of reactivity of [Re(CO)₄dppc]^+2/+ toward N₃- caused by reduction of the dppc ligand.