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

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Abstract
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 a3−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 binuclear sites of two oxidases whose structures were recently determined. The present results contribute to the series of molecular assemblies with the bridge groups [FeIII−X−CuII], X = O2-, OH-, and RCO2-, all with a common high-spin heme, thereby allowing an examination of electronic structure as dependent on the bridging atom or group and bridge structure. (Me5dien = 1,1,4,7,7-pentamethyldiethylenetriamine; Me6tren = tris(2-(dimethylamino)ethyl)amine; OEP = octaethylporphyrinate(2−).)