Methylborabenzene ligands in binuclear iron carbonyl derivatives: High spin states and iron–iron multiple bonding

• Singlet cis and trans doubly bridged structures are predicted for (C5H5BMe)2Fe2(CO)4.
• A triplet triply bridged structure is predicted for (C5H5BMe)2Fe2(CO)3.
• High spin quintet and triplet structures are predicted for (C5H5BMe)2Fe2(CO)2.
• (C5H5BMe)2Fe2(CO)3 is thermodynamically unstable with respect to (C5H5BMe)2Fe2(CO)4 + (C5H5BMe)2Fe2(CO)2.
• Higher energy singlet (C5H5BMe)2Fe2(CO)2 structures are found with formal FeFe triple bonds.

The experimentally known methylborabenzene iron carbonyl (C5H5BCH3)2Fe2(CO)4 and its decarbonylation products (C5H5BCH3)2Fe2(CO)n (n = 3, 2) have been studied by density functional theory for comparison with their cyclopentadienyl analogs. The lowest energy (η6-C5H5BCH3)2Fe2(CO)4 structures are the experimentally known singlet doubly bridged cis-(η6-C5H5BCH3)2Fe2(CO)2(μ-CO)2 and the corresponding trans isomer similar to the corresponding (η5-C5H5)2Fe2(CO)2(μ-CO)2 system. Also the triplet triply bridged (η6-C5H5BCH3)2Fe2(μ-CO)3 is the lowest energy tricarbonyl structure similar to the cyclopentadienyl system. However, significant differences between the methylborabenzene and cyclopentadienyl derivatives are found in the dicarbonyl systems. A singlet (η5-C5H5)2Fe2(μ-CO)2 structure with a short FeFe distance of ∼2.1 Å was previously found to be the lowest energy structure in the cyclopentadienyl system. An analogous methylborabenzene structure (η6-C5H5BCH3)2Fe2(μ-CO)2 is found as the lowest energy singlet structure. However, an unsymmetrical quintet (η6-C5H5BCH3)FeFe(CO)2(η6-C5H5BCH3) structure is predicted to lie ∼22 kcal/mol below this singlet structure. The existence of this low-energy, high-spin (η6-C5H5BCH3)2Fe2(CO)2 structure makes (η6-C5H5BCH3)2Fe2(μ-CO)3 disfavored with respect to disproportionation into (η6-C5H5BCH3)2Fe2(μ-CO)4 + (η6-C5H5BCH3)2Fe2(μ-CO)2 unlike its cyclopentadienyl analog.

The lowest energy (η6-C5H5BCH3)2Fe2(CO)4 structures are the singlet doubly bridged cis- and trans-(η6-C5H5BCH3)2-Fe2(CO)2(μ-CO)2 structures. The triplet triply bridged (η6-C5H5BCH3)2Fe2(μ-CO)3 structure is the lowest energy tricarbonyl structure. An unsymmetrical quintet (η6-C5H5BCH3)FeFe(CO)2(η6-C5H5BCH3) structure is predicted to be the lowest energy dicarbonyl structure.Figure optionsDownload as PowerPoint slide