Formation of carbon-carbon-bonded dimers in the reduction of [Co(II)salophen] [salophen equals N,N'-o-phenylenebis(salicylideneaminato)]: Their reactivity with electrophiles to form Co-C bonds

The reduction of cobalt-Schiff base complexes has been explored as a function of the nature of the ligand. In the case of substituted salen complexes, reduction occurred at the metal with the formation of bifunctional Co(I)-Na complexes, while for salophen derivatives the reductive coupling of imino groups was observed. Such C-C bonds function as electron shuttles in chemical reductions. Reduction of [Co(MeOsalen)] (1) with Na metal in THF led to a monomeric [Co(MeOsalen)Na(THF)(2)] (2A) and a dimeric [{Co(MeOsalen)Na(THF)}(2)] (2B) compound. Compounds 2A and 2B exemplify a bifunctional acid-base system with both centers in close proximity. The analogous reduction of [Co(salophen)] (3) and [Co(MeOsalophen)] (5) led to dimeric cobalt(II) derivatives containing a bridging C-C bond across two units in [Co-2(salophen)(2)Na-2(THF)(6)] (4) and [Co-2(MeOsalophen)(2)Na-2(THF)(4)] (6), respectively. However, such cobalt(II) centers behave as cobalt(I), since the C-C bond provides a pair of electrons to the metal couple during the chemical reaction. Alkylation at the cobalt has been observed in complexes 4 and 6 with the concomitant formation of a Co(III)-C bond in [Co(salophen)(Me)] (9), [Co(salophen)(COOMe)(THF)] (10), [Co(MeOsalophen)(Me)(H2O)] (11), and [Co(MeOsalophen)(COOMe)(H2O)] (12). The cleavage and formation of the C-C bond linking the two salophen units have been observed in the reversible reactions of 4 and 6 with carbon dioxide leading to the CO2 adducts [Co(salophen)NaCO2(THF)] (13) and [Co(MeOsalophen)NaCO2(THF)] (14). Crystallographic data: 2 is triclinic, space group P (1) over bar, a = 14.651(1) Angstrom, b = 15.355(1) Angstrom, c = 11.671(11), alpha = 105.70(1)degrees, beta = 102.66(1)degrees, gamma = 97.65(1)degrees, Z = 2, and R = 0.044; 4 is monoclinic, space group P2(1)/c, a = 11.270(2) Angstrom, b = 17.594(3) Angstrom, c = 16.606(3) Angstrom, alpha = gamma = 90 degrees, beta = 90.45(2)degrees, Z = 2, and R = 0.053; 6 is triclinic, space group P (1) over bar, a = 11.857(4)Angstrom, b = 12.639(4) Angstrom, c = 10.553(5), alpha = 106.28(3)degrees, beta = 101.17(3)degrees, gamma = 103.04(3)degrees, Z = 1, and R = 0.057.