The reversible one-electron oxidation of trans-[Mn′L(CO)(Ph2PCH2PPh2)2]z+: synthesis and characterisation of stable manganese(II) derivatives

Abstract

The complexes trans-[MnL(CO)(dppm)₂]^z(1; z= 0, L = Br, CN, or NCS; z=+1, L = NCMe, CNMe, CNBu^t, or CO; dppm = Ph₂PCH₂PPh₂) undergo diffusion-controlled, one-electron oxidation at a platinum electrode in CH₂Cl₂. The observed linear correlation between oxidation potential, E°, and ligand constant, P_L, has allowed (i) a comparison of the electron-richness and polarisability of the binding site [Mn(CO)(dppm)₂]⁺ with those of related square-pyramidal species, and (ii) comments to be made concerning the linkage isomerism of the thiocyanate ligand. All of the manganese(II) complexes trans-[MnL(CO)(dppm)₂]^z+1(2) except for the bromide (2; z= 0, L = Br) have been generated by chemical oxidation (with [NO][PF₆] or [N₂C₆H₄F-p][PF₆]) or by controlled potential electrolysis. The paramagnetic compounds have been characterised by i.r. and e.s.r. spectroscopy, and (2; z=+1, L = CNMe or CNBu^t) have been isolated as crystalline salts of the [PF₆]^– anion. The irreversible oxidation of (1; z= 0, L = Br), either electrolytically or by [Fe(η-C₅H₅)₂][PF₆] in CH₂Cl₂, leads to the formation of (1; z=+1, L = CO), but in the presence of acetonitrile or CNBu^t the reaction gives (1; z=+1, L = NCMe or CNBu^t).