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.

Graphical abstractThe 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 full-size imageDownload as PowerPoint slide