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Structural characterization and catalytic activity of rhodium pyrazolylborate complexes in alkyne hydrothiolation Fraser, Lauren Rae

Abstract

A series of hydrobis- and hydrotris(pyrazolyl)borate bis(triphenylphosphine) rhodium (I) complexes were synthesized and structurally characterized. These complexes are of the general form [BpRRh(PPh₃)₂] {BpR = H₂BR'₂ , R' = 3,5-dimethylpyrazolyl (2), pyrazolyl (3)}, and [TpRRh(PPh₃)₂] {TpR = HBR'₃ , R' = 3,5-dimethylpyrazolyl (1), pyrazolyl (4), 3-methylpyrazolyl (5), 3-phenylpyrazolyl (6), or 3-phenyl-5-methylpyrazolyl (7)}. Wilkinson's catalyst, [ClRh(PPh₃)₃], and the corresponding potassium salt of the ligands were mixed together in THF or toluene to produce known complexes 1-4 and new complexes 5-7. Both solid state and solution phase characterization were carried out for these complexes. The X-ray crystal structures were obtained for complexes 2, 4 and 5-7. All showed approximate square planar geometry with coordination of two pyrazolyl rings. IR spectroscopy (KBr pellet) was performed on complexes 1, 2 and 4-7 and the BH stretching frequencies were in the range of κ²-coordination. ¹H and ³¹P{¹H} NMR spectroscopy was performed on all seven complexes and variable temperature NMR spectroscopy for complexes 1 and 4-7 to examine the solution phase structures of these complexes. Complexes 1-7 were then used in alkyne hydrothiolation reactions with alkyl thiols as catalysts and their activities were examined. It was found that tris(pyrazolyl)borate complexes were superior to bis(pyrazolyl)borate complexes. As well, tris(pyrazolyl)borate rhodium complexes with substitution at the 3- and 5-positions on the pyrazolyl rings gave the best selectivity and yields, favoring the branched alkyl vinyl sulfides. Thus, complexes 1 and 7 have shown to be effective catalysts in alkyne hydrothiolation when using alkyl thiols to give regioselectively the branched isomer. A general method to produce branched alkyl vinyl sulfides has been discovered and will be presented in the body of this thesis.

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