Ligand conformations and spin states in open metallocenes of the first row transition metals having U-shaped 2,4-dimethylpentadienyl ligands

Abstract

Open metallocenes containing a metal sandwiched between two U-shaped pentadienyl or substituted pentadienyl ligands have been synthesized by Ernst and co-workers using reactions of pentadienyl or substituted pentadienyl anion reagents with metal halides. The complete series of such open metallocenes (2,4-Me₂C₅H₅)₂M of the first row transition metals (M = Ti to Ni) has now been examined using density functional theory. The experimentally known lowest energy structures of the early transition metals are open metallocene structures with two pentahapto U-shaped η⁵-2,4-Me₂C₅H₅ ligands. Such structures can exist as syn-eclipsed, gauche-eclipsed, and/or anti-eclipsed conformations of similar energies. The theoretical predictions of the lowest energy conformations of the open metallocenes are in excellent agreement with available experimental data. Singlet and triplet spin state structures are preferred energetically for the Ti and Cr derivatives, respectively. For (η⁵-2,4-Me₂C₅H₅)₂V, doublet and quartet spin state structures are spaced in energy by 7.6 kcal mol⁻¹. The (2,4-Me₂C₅H₅)₂Mn energy surface is complicated with doublet, quartet, and sextet spin state structures closely spaced in energy. In the sextet spin state of (2,4-Me₂C₅H₅)₂Mn only the end carbon atoms of the 2,4-Me₂C₅H₅ ligand are within bonding distance of the manganese atom. This corresponds to tetrahedral C₄Mn coordination similar to that found experimentally in the sextet ground state t-butyl derivative (2,4-tBu₂C₅H₅)₂Mn as well as the related Mn(CN)₄²⁻. The low energy structures for the “open ferrocene” (η⁵-2,4-Me₂C₅H₅)₂Fe are singlet spin state structures with two U-shaped pentahapto η⁵-2,4-Me₂C₅H₅ ligands but otherwise analogous to ferrocene. The low energy (2,4-Me₂C₅H₅)₂M structures of the late transition metals Co and Ni have at least one trihapto η³-2,4-Me₂C₅H₅ ligand. The lowest energy (2,4-Me₂C₅H₅)₂Ni structure is a singlet spin state structure with two trihapto η³-2,4-Me₂C₅H₅ ligands and a 16-electron nickel environment analogous to the well-known bisallylnickel, (η³-C₃H₅)₂Ni.