USE OF A REDOX-ACTIVE LIGAND TO REVERSIBLY ALTER METAL-CARBONYL ELECTROPHILICITY

The chelating ligand, 1,1'-bis(diphenylphosphino)cobaltocene (dppc) is used as an electrochemically tunable ligand for altering electrophili city of the carbonyl carbon in metal carbonyl complexes. The Re carbon yl complexes [Re(CO)(4-n)(CH3CN)(n)dppc](2+/+), n = 0 (cis), 1 (fac), 2 (cis,cis), and [fac-Re(CO)(3)(NCO)dppc](+/0) were prepared and chara cterized in both stales of charge indicated. Single crystal X-ray crys tallography was performed for both states of charge of [Re(CO)(4)dppc] (2+/+ with the oxidized and reduced forms crystallizing in the monocli nic space groups C2/c and P2?1/c, respectively; The most pronounced st ructural difference between the two forms is the Cp(centroid)-Co dista nce, which is 0.09 Angstrom longer for the reduced form. Cyclic voltam metry shows that E(1/2) for the Co(III)/Co(II) couple is 200-400 mV mo re positive for coordinated dppc than for the free ligand. For each CO substituted, E(1/2) becomes similar to 100 or 200 mV more negative fo r substitution by CH3CN or NCO-, respectively. Solution IR spectroscop y shows that v(CO) for the oxidized form of each species is typically similar to 15 cm(-1) higher than for the reduced form, but the dumber and relative intensities of the carbonyl absorptions are the same. In every redox pair the oxidized species has higher reactivity with respe ct to nucleophilic attack at the carbonyl carbon. In CH3CN at 25.0 deg rees C, [fac-Re(CO)(3)- (CH3CN)dppc](+2) reacts with amine N-oxides (C H3)(3)NO, N-methylmorpholine N-oxide, and (CH3)(2)(C6H5)NO similar to 200 times faster than [fac-Re(CO)(3)(CH3CN)dppc](+) to form [cis,cis-R e(CO)(2)(CH3CN)(2)dppc](+2/+), while [cis-Re(CO)(4)dppc](+2) reacts wi th N-3(-) 5400 times faster than [cis-Re(CO)(4)dppc](+) to form [fac-R e(CO)(3)(NCO)dppc](+/0), with Delta Delta H double dagger = 3-4 kcal/m ol in both cases. The ionic strength and dielectric strength dependenc e of the reactivity of [Re(CO)(4)dppc](+2/+) toward N-3(-) were assaye d to investigate electrostatic contributions to the attenuation of rea ctivity of [Re(CO)(4)dppc](+2/+) toward N-3(-) caused by reduction of the dppc ligand.