Binuclear Carbonylheptalenechromium Complexes: Partition of Heptalene into a Complexed Heptafulvene Subunit and an Uncomplexed 1,3‐Diene Subunit for Coordination to a Multiply Bonded Pair of Chromium Atoms

Abstract

The possibility of extending carbonylchromium chemistry of the bicyclic non‐benzenoid aromatic hydrocarbon heptalene from the known mononuclear (η6‐C12H10)Cr(CO)3 to binuclear derivatives of the type C12H10Cr2(CO)n (n = 6, 5, 4, 3) is examined by density functional theory. The lowest energy C12H10Cr2(CO)6 structures have each heptalene ring bonded to an independent Cr(CO)3 unit as a hexahapto or heptahapto ligand with Cr···Cr distances too long (4.4 to 6.0 Å) for direct bonding. The lowest energy C12H10Cr2(CO)5 structure has an eight‐carbon heptafulvene subunit of the heptalene unit coordinated to a carbonyl‐bridged Cr2(CO)4(μ‐CO) unit as an octahapto ligand leaving an uncomplexed cis‐1,3‐diene subunit in the heptalene ligand. The short Cr≡Cr distance in this structure of ca. 2.36 Å suggests the formal triple bond required to give each chromium atom the favored 18‐electron configuration. A slightly higher energy C12H10Cr2(CO)5 structure by ca. 3 kcal/mol has all 12 heptalene carbon atoms bonded to an unbridged Cr2(CO)5 with a Cr–Cr distance of ca. 3.3 Å corresponding to the formal single bond required to give each chromium atom the favored 18‐electron configuration. The global minima for the more highly unsaturated C12H10Cr2(CO)n (n = 4, 3) complexes are predicted to have a four‐electron donor bridging η2‐μ‐CO group as indicated by short Cr–O distances consistent with a direct bond and a low ν(CO) frequency consistent with a low formal C–O formal bond order.