Binuclear methylborole nickel carbonyls

The existence of the bis(borole) nickel sandwich compound (η5-C4H4BCH3)2Ni suggests the possibility of mixed methylborole nickel carbonyls. The structures and thermochemistry of the binuclear methylborole nickel carbonyls (C4H4BCH3)2Ni2(CO)n (n = 3, 2, 1) have been compared with those of the isoelectronic cyclopentadienylcobalt derivatives (C5H5)2Co2(CO)n and cyclobutadiene nickel derivatives (C4H4)2Ni2(CO)3. For the tricarbonyls (n = 3) the lowest energy structures in all three systems have two terminal carbonyl groups, one bridging carbonyl group, and metal–metal single bonds. However, for the unsaturated derivatives (C4H4BCH3)2Ni2(CO)n (n = 2, 1) the pendant methyl groups are not “innocent bystanders”. Instead structures with one of the methyl hydrogens functioning as an agostic hydrogen to the nickel atom are competitive in energy with structures analogous to those of the (C5H5)2Co2(CO)n and (C4H4)2Ni2(CO)n analogues. Thus for (C4H4BCH3)2Ni2(CO)2 a structure with two bridging carbonyl groups and a formal NiNi double bond of length ∼2.37 Å and a structure with one bridging carbonyl group, one terminal carbonyl group, a C–H–Ni agostic hydrogen atom, and a Ni–Ni single bond of length ∼2.48 Å have essentially the same energies. For (C4H4BCH3)2Ni2(CO) the two lowest energy structures also have agostic C–H–Ni hydrogen atoms and Ni–Ni single bonds. The thermochemistry for the synthesis of (C4H4BCH3)2Ni2(CO)n derivatives from Ni(CO)4 + (η5-C4H4BCH3)2Ni is not favorable since such reactions are endothermic requiring, for example, 13 kcal/mol of energy for the reaction Ni(CO)4 + (η5-C4H4BCH3)2Ni → (C4H4BCH3)2Ni2(CO)3 + CO.

Graphical abstractThe lowest energy (C4H4BCH3)2Ni2(CO)3 structure has terminal methylborole ligands, one bridging carbonyl group, and an Ni–Ni single bond. However, in the lowest energy structures of the unsaturated derivatives (C4H4BCH3)2Ni2(CO)n (n = 2, 1), the methyl substituent bonds to a nickel atom through a C–H–Ni agostic bond.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The lowest energy (C4H4BCH3)2Ni2(CO)3 structures have a Ni–Ni bond and one bridging CO group. ► Structures with a methyl agostic hydrogen are found in (C4H4BCH3)2Ni2(CO)n (n = 2, 1). ► A low-energy (C4H4BCH3)2Ni2(CO)2 structure is also found with a NiNi double bond and two bridging CO groups.