[Co4O4]4+ Cubane Core as a Brønsted Base: Preparation and Properties of [Co4O3(OH)(O2CR)2(bpy)2]3+ and [Co4O2(OH)2(O2CR)2(bpy)2]4+ Salts

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Abstract
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 °C) a = 33.838(8) Å, b = 13.826(3) Å, c = 29.944(7) Å, β = 98.84(1)°, and Z = 8. Complex 6·PhCN·H2O crystallizes in orthorhombic space group Pbca with (at −154 °C) a = 22.603(4) Å, b = 34.759(6) Å, c = 18.167(3) Å, 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 ≈ 2.7 Å) 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 CoIII 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 pKa2 value of 3.15 for the second deprotonation. Electronic and 1H 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 Brønsted base, undergoing either one or two protonation reactions.

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