Skirting the oxo-wall: characterization and catalytic reactivity of binuclear Co2+/3+ 1,2-bis(2-hydroxybenzamido)benzene complexes with comparison to their isostructural Fe2+/3+ analogs. Implications of d-electron count on oxygen atom transfer catalysis

The reaction of Co2+(N-MeIm)2(Cl)2 (1), with Li2H2Hbab (H4Hbab=1,2-bis(2-hydroxybenzamido)benzene) affords the binuclear complex [Co2 2+(H2Hbab)2(N-MeIm)2] (2). Single crystal X-ray crystallography shows that the two Co2+ centers in 2 are related by a crystallographically imposed center of symmetry with the coordination sphere around each metal center composed of amide oxygens, terminal and bridging phenolate oxygen atoms and a single nitrogen from N-methylimidazole. 2 is isostructural to the Fe2+ complex, [Fe2(H2Hbab)2(N-MeIm)2] (5), previously reported from our laboratory (A. Stassinopoulos, G. Schulte, G.C. Papaefthymiou, J.P. Caradonna, J. Am. Chem. Soc. 113 (1991) 8686–8697). Stoichiometric iodometric oxidation of 2 yields the mixed valence complex [Co2+Co3+(H2Hbab)2(N-MeIm)2]+ (3), and the oxidized complex [Co2 3+(H2Hbab)2(N-MeIm)2]2+ (4). The UV–Vis spectrum of 2 shows ligand field transitions at 600 nm (εM=160) and 540 nm (εM=120) and a phenolate-to-Co2+ ligand-to-metal charge-transfer (LMCT) band at 327 nm (εM=23 600). Oxidation of the dicobalt core results in a bathchromic shift of the LMCT band (3: 305 nm (εM=17 400), 334 nm (sh, εM=14 500); 4: 295 nm (εM=21 000), 357 nm (εM=15 000)). EPR spectra of 2 and 3 at 4 K show broad resonances from 500 to 4500 Gauss indicative of the presence of strong zero-field splitting effects, while 4 is EPR silent. Solution magnetic susceptibility measurements for 2 (S1=S2=3/2; μB=6.90) are consistent with a weakly interacting high-spin Co2+ dimer. Analogous measurements for 3 (S1=3/2, S2=0; μB=4.78) and 4 (S1=S2=0; μB=0.25) indicate the presence of a single high-spin Co2+ for 3 and a diamagnetic core for 4, consistent with the latter’s 1H NMR spectrum. Complexes 2–4 exhibit the ability to catalytically oxidize a variety of organic substrates (olefins, sulfides) using iodosylbenzene (OIPh) as oxygen atom donor. Reactions with olefin substrates (norbornene, cyclooctene, styrene) yield solely epoxide products while cyclohexene afforded 85% epoxide with small amounts (15%) of allylic oxidation products. Oxygen atom transfer to olefins occurs with high but not exclusive retention of stereochemistry. The mechanistic implications of the significantly different product distributions obtained with the Co2+ (d7) dimer, 2, versus the isostructural Fe2+ (d6) dimer, 5 (A. Stassinopoulos, J.P. Caradonna, J. Am. Chem. Soc. 112 (1990) 7071–7073), are presented. These data strongly infer the operation of two independent mechanisms for the Co2+ and Fe2+ catalyzed reactions.