title: Ligand-promoted solvent-dependent ionization and conformational equilibria of Re(CO)3Br[CH2(S-tim)2] (tim = 1-methylthioimidazolyl). Crystal structures of Re(CO)3Br[CH2(S-tim)2] and {Re(CO)3(CH3CN)[CH2(S-tim)2]}(PF6).
abstract: The compounds Re(CO)3Br[CH2(S-tim)2] (1) and {Re(CO)3(CH3CN)[CH2(S-tim)2]}(PF6) (2), where tim is 1-methylthioimidazolyl, were prepared in high yields and characterized both in the solid state and in solution. The solid-state structures show that the ligand acts in a chelating binding mode where the eight-member chelate ring adopts twist-boat conformations in both compounds. A comparison of both solid-state IR data for CO stretching frequencies and the solution-phase voltammetric measurements for the Re(1+/2+) couples between 1, 2, and related N,N-chelates of the rhenium tricarbonyl moiety indicate that the CH2(S-tim)2 ligand is a stronger donor than even the ubiquitous dipyridyl ligands. A combination of NMR spectroscopic studies and voltammetric studies revealed that compound 1 undergoes spontaneous ionization to form {Re(CO)3(CH3CN)[CH2(S-tim)2]+}(Br-) in acetonitrile. Ionization does not occur in solvents such as CH2Cl2 or acetone that are less polar and Lewis basic (less coordinating). The equilibrium constant at 293 K for the ionization of 1 in CH3CN is 4.3 x 10(-3). The eight-member chelate rings in each 1 and 2 were found to be conformationally flexible in all solvents, and boat-chair conformers could be identified. Variable-temperature NMR spectroscopic studies were used to elucidate the various kinetic and thermodynamic parameters associated with the energetically accessible twist-boat to twist-boat and twist-boat to boat-chair interconversions.
authors.affiliation: Department of Chemistry, Marquette University, Milwaukee, WI 53213-1881, USA.
authors.forename: Rosalice M
authors.initials: RM
authors.lastname: Silva
authors.forename: Brendan J
authors.initials: BJ
authors.lastname: Liddle
authors.forename: Sergey J
authors.initials: SJ
authors.lastname: Lindeman
authors.forename: Mark D
authors.initials: MD
authors.lastname: Smith
authors.forename: James R
authors.initials: JR
authors.lastname: Gardinier
mesh_terms: D018360:Crystallography, X-Ray; D004563:Electrochemistry; D007477:Ions / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D012211:Rhenium / Q000737:chemistry; D012996:Solutions; D012997:Solvents; D013055:Spectrophotometry, Infrared
chemical_list: D007477:Ions; D008024:Ligands; D009942:Organometallic Compounds; D012996:Solutions; D012997:Solvents; C520738:bromotricarbonyl(bis(1-methylthioimidazolyl)methane)rhenium(III); C520739:tricarbonyl(acetonitrile)(bis(1-methylthioimidazolyl)methane)rhenium(III); D012211:Rhenium