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

Abstract

The reaction of Co 2+( N-MeIm) 2(Cl) 2 ( 1), with Li 2H 2Hbab (H 4Hbab=1,2-bis(2-hydroxybenzamido)benzene) affords the binuclear complex [Co 2 2+ (H 2Hbab) 2( N-MeIm) 2] ( 2). Single crystal X-ray crystallography shows that the two Co 2+ 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 Fe 2+ complex, [Fe 2(H 2Hbab) 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 [Co 2+Co 3+(H 2Hbab) 2( N-MeIm) 2] + ( 3), and the oxidized complex [Co 2 3+ (H 2Hbab) 2( N-MeIm) 2] 2+ ( 4). The UV–Vis spectrum of 2 shows ligand field transitions at 600 nm ( ε M=160) and 540 nm ( ε M=120) and a phenolate-to-Co 2+ ligand-to-metal charge-transfer (LMCT) band at 327 nm ( ε M=23 600). Oxidation of the dicobalt core results in a bathchromic shift of the LMCT band ( 3: 305 nm ( ε M=17 400), 334 nm (sh, ε M=14 500); 4: 295 nm ( ε M=21 000), 357 nm ( ε M=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 ( S 1= S 2=3/2; μ B=6.90) are consistent with a weakly interacting high-spin Co 2+ dimer. Analogous measurements for 3 ( S 1=3/2, S 2=0; μ B=4.78) and 4 ( S 1= S 2=0; μ B=0.25) indicate the presence of a single high-spin Co 2+ 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 Co 2+ (d 7) dimer, 2, versus the isostructural Fe 2+ (d 6) 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 Co 2+ and Fe 2+ catalyzed reactions.