Molecular Assemblies Containing Unsupported [FeIII−(μ2:η2-RCO2)−CuII] Bridges

Formate is an inhibitor of cytochrome oxidases and also effects conversion of the bovine heart enzyme from the “fast” to the “slow” cyanide-binding form. The molecular basis of these effects is unknown; one possibility is that formate inserts as a bridge into the binuclear heme a 3−CuB site, impeding the binding of dioxygen or cyanide. Consequently, Fe−Cu−carboxylate interactions are a matter of current interest. We have initiated an examination of such interactions by the synthesis of the first examples of [FeIII−(μ2:η2-RCO2)−CuII] bridges, minimally represented by FeIII−L + CuII-O2CR → [FeIII−(RCO2)−CuII] + L. A series of Cu(II) precursor complexes and solvate forms have been prepared and their structures determined, including [Cu(Me5dien)(O2CH)]+ (3), [Cu(Me5dien)(O2CH)(MeOH)]+ (4), [Cu(Me6tren)(O2CH)]+ (5), and [Cu(Me5dien)(OAc)]+ (6). [4](ClO4) was obtained in monoclinic space group P21 /n with a = 8.166(3) Å, b = 15.119(5) Å, c = 15.070(4) Å, β = 104.65(2)°, and Z = 4. [5](ClO4)/[6](ClO4) crystallize in orthorhombic space groups Pnma/Pna21 with a = 16.788(2)/14.928(5) Å, b = 9.542(1)/9.341(4) Å, c = 12.911(1)/12.554(4) Å, and Z = 4/4. In all cases, the carboxylate ligand is terminal and is bound in a syn orientation. Also prepared for the purpose of structural comparison was [Fe(OEP)(O2CH)], which occurred in monoclinic space group P21 /c with a = 13.342(2) Å, b = 13.621(2) Å, c = 19.333(2) Å, β = 106.12(2)°, and Z = 4. The desired bridges were stabilized in the assemblies [(OEP)Fe(O2CH)Cu(Me5dien)(OClO3)]+ (9), [(OEP)Fe(OAc)Cu(Me5dien)]2+ (10), and {(OEP)Fe[(O2CH)Cu(Me6tren)]2}3+ (11), which were prepared by the reaction of 3, 6, and 5, respectively, with [Fe(OEP)(OClO3)] in acetone or dichloromethane. [9](ClO4)/[10](ClO4)2·CH2Cl2 crystallize in triclinic space group P1̄ with a = 9.016(3)/13.777(3) Å, b = 15.377(5)/13.847(3) Å, c = 19.253(5)/17.608(4) Å, α = 78.12(3)/96.82(3)°, β = 86.30(4)/108.06(3)°, γ = 76.23(3)/114.32(3)°, and Z = 2/2. Each assembly contains a [FeIII−(RCO2)−CuII] bridge but with the differing orientations anti-anti (9) and syn-anti (10, 11). The compound [11](ClO4)2(SbF6) occurs in orthorhombic space group Pbcn with a = 12.517(6) Å, b = 29.45(1) Å, c = 21.569(8) Å, and Z = 4. Complex 11 is trinuclear; the Fe(III) site has two axial formate ligands with bond distances indicative of a high-spin configuration. Structural features of 9−11 are discussed and are considered in relation to the possible insertion of formate into the binu