Complexes of Heteroscorpionate Trispyrazolylborate Ligands. Part 10. Structures and Fluxional Behavior of Rhodium(I) Complexes with Heteroscorpionate Trispyrazolylborate Ligands, Tp‘ ‘Rh(LL) (LL = (CO)₂ or COD)

Heteroscorpionate Tp‘ ‘ ligands constructed of two 3-phenyl-5-methylpyrazolyl and a variable third pyrazolyl residue (3-methyl-5-phenyl-, 3,5-dimethyl-, and 3,5-diethylpyrazolyl) coordinate to rhodium(I) in a κ2 fashion to form Tp‘ ‘Rh(LL) complexes (where LL = COD or (CO)2). One of the two 3-phenyl-5-methylpyrazolyl and the less sterically hindered pyrazolyl coordinate through N(2) atoms, while the axially appended 3-phenyl-5-methylpyrazolyl is in side-on conformation. The Tp‘ ‘Rh(COD) complexes possess C1 symmetry in the solid state. The energy of activation of the exchange between coordinated and uncoordinated pyrazolyls was estimated from variable-temperature 1H NMR measurements. Two ΔG⧧ values were found for heteroscorpionate complexes. The ΔG⧧1 − ΔG⧧2 depends on a difference in steric demands of competing pyrazoles. For the homoscorpionate TpPh,MeRh(COD) complex two rotational isomers were identified by low-temperature 1H NMR spectroscopy, which originate from slow rotation of the dangling 3-phenyl-5-methylpyrazolyl residue. All studied Tp‘Rh(COD) complexes were found to be catalytically active in regioselective polymerization of phenylacetylene.