Ligand-Promoted Solvent-Dependent lonization and Conformational Equilibria of Re(CO)_3Br[CH_2(S-tim)2](tim = 1-methylthioimidazolyl).Crystal Structures of Re(CO)_3Br[CH_2(S-tim)_2]and{Re(CO)_3(CH_3CN)[CH_2(S-tim)_2]}(PF_6)

摘要

The compounds Re(CO)_3Br[CH_2(S-tim)_2](1)and{Re(CO)_3(CH_3CN)[CH_2(S-tim)2]}(PF_6)(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 CH_2(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(CH_3CN)[CH_2(S-tim)_2]~+}(Br~-)in acetonitrile.lonization does not occur in solvents such as CH_2Cl_2 or acetone that are less polar and Lewis basic(less coordinating).The equilibrium constant at 293 K for the ionization of 1 in CH_3CN 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.