[Co4O4]4+ cubane core as a Bronsted base:: Preparation and properties of [Co4O3(OH)(O2CR)2(bpy)2]3+ and [Co4O2(OH)2(O2CR)2(bpy)2]4+ salts

The preparation and properties are described of complexes resulting from the mono- and diprotonation of the [Co4O4](4+) unit to give products containing [Co4O3(OH)](5+) and [Co4O2(OH)(2)](6+), respectively. Treatment of [Co4O4(O2CC6H4-p-OMe)(2)(bpy)(4)](2+) (bpy = 2,2'-bipyridine) in MeCN with 70% perchloric acid leads to isolation of [Co4O2(OH)(2)(O2CC6H4-p-OMe)(2)(bpy)(4)](ClO4)(4) (5) in 61% yield. Treatment of [Co4O4(O2CC6H4-p-Me)(2)(bpy)(4)](2+) in MeCN with an acidic solution of(NH4)(3)[Ce(NO3)(6)] leads to isolation of [Co4O3(OH)(O2CC6H4-p-Me)(2)(bpy)(4)]-[Ce(NO3)(6)] (6) in 96% yield. Complex 5 . 3MeCN . H2O crystallizes in monoclinic space group C2/c with (at -155 degrees C) a = 33.838(8) Angstrom, b = 13.826(3) Angstrom, c = 29.944(7) Angstrom, beta = 98.84(1)degrees, and Z = 8. Complex 6 . PhCN . H2O crystallizes in orthorhombic space group Pbca with (at -154 degrees C) a = 22.603(4) Angstrom, b = 34.759(6) Angstrom, c = 18.167(3) Angstrom, and Z = 8. The complexes contain [Co4O2(OH)(2)](6+) (5) and [Co4O3(OH)](5+) (6) distorted-cubane cores, the sites of protonation being apparent by (i) the approach of ClO4- or [Ce(NO3)(6)](3-) anions to within hydrogen-bonding distances (O ... O approximate to 2.7 Angstrom) and (ii) the longer Co-OH- bond lengths compared with Co-O2- bond lengths. Peripheral ligation is provided by chelating bpy and syn,syn-bridging RCO2- groups. The Co-III atoms are six-coordinate and approximately octahedral. Solution studies show that the diprotonated species is a strong diprotic acid analogous to H2SO4; the first deprotonation goes to completion, but the monoprotonated species is a weak acid and is in equilibirium with the nonprotonated species, pH studies in aqueous solution yield a pK(a2) value of 3.15 for the second deprotonation. Electronic and H-1 NMR spectra of the diprotonated complexes in various solvents are consistent with the monoprotonated complex being the major species in solution. The combined results demonstrate that the [Co4O4](4+) core is capable of acting as a Bronsted base, undergoing either one or two protonation reactions.