科研之友
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摘要
The 2007 synthesis of the hexamethylpentalene complex cis-(eta(5), eta(5)-C8Me6) Co-2(CO)(4) by O'Hare and coworkers makes of interest a theoretical study of the unsubstituted binuclear pentalene cobalt carbonyls (C8H6) Co-2(CO)(n) (n = 6, 5, 4, 3). In this connection an unbridged unsubstituted cis-(eta(5), eta(5)-C8H6) Co-2(CO)(4) structure analogous to the experimental hexamethyl structure is found to be the lowest energy (C8H6) Co-2(CO)(4) structure. Furthermore, this (C8H6) Co-2(CO) n structure is predicted to be the product from reactions of pentalene derivatives with cobalt carbonyls based on the thermochemistry of CO dissociation energies in the sequence n = 6 -> 5 -> 4 -> 3 -> 2. The low-energy structures for the unsaturated (C8H6) Co-2(CO) n (n = 3, 2) derivatives also have the pentalene ligand bonded to each cobalt as a pentahapto ligand. They may be derived from either this unbridged cis-(eta(5), eta(5)-C8H6) Co-2(CO)(4) structure or a doubly CO-bridged variation thereof by removal of one or two CO groups. The carbonyl-rich (C8H6) Co-2(CO)(n) (n = 6, 5) structures, which may be considered as precursors to cis-(eta(5), eta(5)-C8H6) Co-2(CO)(4), illustrate considerable diversity of bonding modes of the pentalene ligand to the central Co-2 unit. Structures without direct Co-Co bonds are found exhibiting either cis or trans stereochemistry in which trihapto allylic and/or pentahapto cyclopentadienylic subunits are bonded to Co(CO)(3) and Co(CO)(2) moieties, respectively. In addition, (C8H6) Co-2(CO) n (n = 6, 5) structures exhibiting cis stereochemistry are found in which C=C bonds of a butadiene or fulvene subunit of the pentalene ligand replace CO groups in Co-2(CO)(8), retaining the formal Co-Co single bond.
