3-[4-Phenoxyphenyl]pyrazole (Hpzpp) and 3-[4-butoxyphenyl]pyrazole (Hpzbp) in rhodium chemistry crystal structures of 3-[4-phenoxyphenyl]pyrazole, [Rh(μ-pzpp)(COD)]2·1/2CH2Cl2 and [Rh(μ-pzbp)(COD)]2

Article abstract of DOI:10.1016/S0022-328X(96)06890-8

The novel pyrazoles containing 3-[4-phenoxyphenyl] (pp) and 3-[4-butoxyphenyl] (bp) substituents, Hpz^pp and Hpz^bp, have been synthetized and characterized, and the crystalline structure of 3-[4-phenoxiphenyl]pyrazole (Hpz^pp) is also reported. Rh(I) compounds [Rh(Cl)(Hpz^R)(LL)] and [Rh(μ-pz^R)(LL)]₂ (LL = NBD, COD, 2CO; R = pp, bp) have been prepared in order to explore the influence of the alkoxy- or aryloxyphenyl substituents on the pyrazol ring of some features such as the presence of dynamic processes or the preference of determined isomers in the complexes. The molecular structures of complexes [Rh(Cl)(Hpz^R)(LL)] and [Rh(μ-pz^R)(LL)]₂ (LL = NBD, COD, 2CO; R = pp, bp) have been studied by IR and ¹H and ¹³C NMR spectroscopies. ¹H NMR spectra of compounds [Rh(Cl)(Hpz^R)(LL)] (LL = NBD, COD, 2CO; R = pp, bp) indicate that the presence of a metallotropic equilibrium only depends on the steric characteristics of the ancillary ligands. On the other hand, complexes [Rh(μ-pz^R)(CO)₂]₂ (R = pp, bp) are formed as a mixture of the head-to-head (H-H) and head-to-tail (H-T) configurational isomers. By contrast, [Rh(μ-pz^R)(LL)]₂ (LL = NBD, COD; R = pp, bp) have been obtained as only one isomer in both the solid state and the solution. The crystalline structures of complexes [Rh(μ-pz^pp)(COD)]₂·1/2-CH₂Cl₂ and [Rh(μ-pz^bp)(COD)]₂ have been solved, showing the presence of the H-T configurational isomer in both cases. The ¹H NMR spectra of [Rh(μ-pz^R)(LL)]₂ (LL = NBD, COD, 2CO; R = pp, bp) show that the ortho protons of the C₆H₄ group of the substituents on the pyrazol ring are considerably deshielded. Furthermore, the X-ray structures of [Rh(μ-pz^pp)(COD)]₂·1/2CH₂C1₂ and [Rh(μ-pz^bp)(COD)]₂ complexes show an Rh-H(ortho) distance of ca. 2.7 A, characteristic of a weak preagostic interaction.