Binuclear 1,2-diaza-3,5-diborolyl iron carbonyls: Effect of replacing ring CC units with isoelectronic BN units

•(Me4B2N2CH)2Fe2(CO)n (n = 5, 4, 3, 2) have been investigated by density functional theory.•The lowest energy (Me4B2N2CH)2Fe2(CO)n (n = 4, 3, 2) structures have two bridging CO groups.•No (Me4B2N2CH)2Fe2(CO)4 structure with only terminal CO groups was found at accessible energies.•Triplet triply bridged and doubly bridged (Me4B2N2CH)2Fe2(CO)3 structures have comparable energies.•The lowest energy (Me4B2N2CH)2Fe2(CO)2 structure is a triplet with two bridging CO groups.

The recently synthesized 1,2-diaza-3,5-diborolyl ligand is a BN analog of the cyclopentadienyl ligand that has been recently used for the synthesis of a BN analog of ferrocene. The binuclear 1,2-diaza-3,5-diborolyl iron carbonyl derivatives (Me4B2N2CH)2Fe2(CO)n (n = 5, 4, 3, 2) have now been investigated by density functional theory for comparison with the corresponding cyclopentadienyliron carbonyl systems, (C5H5)2Fe2(CO)n. The lowest energy (Me4B2N2CH)2Fe2(CO)n (n = 4, 3, 2) structures have two bridging CO groups. For (Me4B2N2CH)2Fe2(CO)3 a triplet triply bridged (Me4B2N2CH)2Fe2(μ-CO)3 structure is analogous to the experimentally known stable compounds (η5-C5R5)2Fe2(μ-CO)3 (R = H, CH3). From the thermochemistry results, the doubly bridged singlet (Me4B2N2CH)2Fe2(CO)2(μ-CO)2 structure appears to a promising synthetic objective. The energies of the triplet (Me4B2N2CH)2Fe2(CO)3 and (Me4B2N2CH)2Fe2(CO)2 structures are lower than those of the corresponding singlet structures. The Wiberg bond indices of the iron–iron bonds determined by the NBO method correlate well with the formal bond orders suggested by the FeFe distances and electron counting. The (Me4B2N2CH)2Fe2(CO)5 structures lack an iron–iron bond and require only ∼2 kcal/mol in energy for decarbonylation to (Me4B2N2CH)2Fe2(CO)4. In all of the (Me4B2N2CH)2Fe2(CO)n (n = 4, 3, 2) structures, the predicted FeN and FeB distances are close to those in the ferrocene analog [η5-(CH2)3N2(BPh)2CMe]2Fe.

Graphical abstractThe binuclear 1,2-diaza-3,5-diborolyl iron carbonyl derivatives (Me4B2N2CH)2Fe2(CO)n (n = 5, 4, 3, 2) have been investigated by density functional theory for comparison with the corresponding (C5H5)2Fe2(CO)n systems. The lowest energy (Me4B2N2CH)2Fe2(CO)n (n = 4, 3, 2) structures have two bridging CO groups. A pentacarbonyl (Me4B2N2CH)2Fe2(μ-CO)(CO)4 with a bridging CO group as the only link between the two halves of the molecule is found.Figure optionsDownload full-size imageDownload as PowerPoint slide