Synthesis and structure of Rh(I) tris(pyrazolyl)borate complexes of the type TpPh,MeRhL1L2

The complex TpPh,MeRh(CO)2 (1) was prepared from the reaction of the appropriate potassium pyrazolyl borate, TpPh,MeK, with Rh(acac)(CO)2 in warm benzene. Reaction of Rh(acac)(CO)2 with TpPh,MeK and PPh3 gave the compound TpPh,MeRh(CO)(PPh3) (2). The complexes 1 and 2 were characterized by elemental analysis and IR, 1H-, 13C-, and 31P-NMR spectroscopies. The structures of both compounds were determined by X-ray crystallography. The complex 1 crystallizes in the triclinic space group P1̄ with a=11.227(2), b=12.219(2), c=12.262(2) Å, α=63.61(3), β=87.67(3), γ=88.17(3)°, V=1505.4(4) Å3, and Z=2. Crystals of 2 are triclinic, space group P1̄, with a=11.114(2), b=11.913(2), c=18.071(4) Å, α=82.69(3), β=82.02(3), γ=64.01°, V=2123.9(7) Å3, Z=2. The fluxional behavior of 1 and 2 was studied by variable-temperature 1H-NMR spectroscopy. In the case of 1 the κ2↔κ3 conversion is fast on the NMR time-scale even at −90°C, whereas complex 2 is a unique example of two well-separated κ2↔κ3 equilibria (by 21 kJ mol−1 ΔG≠ gap).