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)

Document Type

Article

Language

eng

Publication Date

2006

Publisher

American Chemical Society

Source Publication

Inorganic Chemistry

Source ISSN

0020-1669

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 Re1+/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 × 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.

Comments

Inorganic Chemistry, Vol. 45, No. 17 (2006): 6794-6802. DOI.

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