title: Magnetism and variable temperature and pressure crystal structures of a linear oligonuclear cobalt bis-semiquinonate.
abstract: The crystal structure of the first oligomeric cobalt dioxolene complex, Co3(3,5-DBSQ)2((t)BuCOO)4(NEt3)2, 1, where DBSQ is 3,5-di-tert-butyl-semiquinonate, has been studied at various temperatures between 20 and 200 K. Despite cobalt-dioxolene complexes being generally known for their extensive ability to exhibit valence tautomerism (VT), we show here that the molecular geometry of compound 1 is essentially unchanged over the full temperature range, indicating the complete absence of electron transfer between ligand and metal. Magnetic susceptibility measurements clearly support the lack of VT between 8 and 300 K. The crystal structure is also determined at elevated pressures in the range from 0 to 2.5 GPa. The response of the crystal structure is surprisingly dependent on the dynamics of pressurisation: following rapid pressurization to 2 GPa, a structural phase transition occurs; yet, this is absent when the pressure is increased incrementally to 2.6 GPa. In the new high pressure phase, Z' is 2 and one of the two molecules displays changes in the coordination of one bridging carboxylate from μ2:κO:κO' to μ2:κ(2)O,O':κO', while the other molecule remains unchanged. Despite the significant changes to the molecular connectivity, analysis of the crystal structures shows that the phase transition leaves the spin and oxidation states of the molecules unaltered. Intermolecular interactions in the high pressure crystal structures have been analysed using Hirshfeld surfaces but they cannot explain the origin of the phase transition. The lack of VT in this first oligomeric Co-dioxolene complex is speculated to be due to the coordination geometry of the terminal Co-atoms, which are trigonal bipyramidally coordinated, different from the more common octahedral coordination. The energy that is gained by a hs-to-ls change in Oh is equal to Δ, while in the case of the trigonal bipyramidal (C3v), the energy gain is equal to the splitting between d(z(2)) and degenerate d(x(2) - y(2))/d(xy), which is significantly less.
authors.affiliation: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark. jacobo@chem.au.dk.
authors.forename: Jacob
authors.initials: J
authors.lastname: Overgaard
authors.affiliation: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark. jacobo@chem.au.dk.
authors.forename: Louise H
authors.initials: LH
authors.lastname: Møller
authors.affiliation: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark. jacobo@chem.au.dk.
authors.forename: Mette A
authors.initials: MA
authors.lastname: Borup
authors.affiliation: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark. jacobo@chem.au.dk.
authors.forename: Maxime
authors.initials: M
authors.lastname: Tricoire
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.
authors.forename: James P S
authors.initials: JP
authors.lastname: Walsh
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University Mainz, D-55128 Mainz, Germany and Graduate School Materials Science in Mainz, Staudingerweg 9, D-55128 Mainz, Germany.
authors.forename: Marcel
authors.initials: M
authors.lastname: Diehl
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University Mainz, D-55128 Mainz, Germany.
authors.forename: Eva
authors.initials: E
authors.lastname: Rentschler