Ligand Stereoelectronic Effects in Complexes of Phospholanes, Phosphinanes, and Phosphepanes and Their Implications for Hydroformylation Catalysis

Convenient syntheses are described for the five-, six-, and seven-membered phosphacycles PhP(CH₂)_x-1, where x = 5 (L^a₅), 6 (L^a₆), 7 (L^a₇), and Bu^tP(CH₂)_x-1 where x = 5 (L^b₅), 6 (L^b₆), 7 (L^b₇). Treatment of [PtCl₂(cod)] with L^a_5-7 gives cis-[PtCl₂(L^a_5-7)₂] (1a_5-7), whereas with L^b_5-7 a mixture of cis-[PtCl₂(L^b_5-7)₂] (1b_5-7) and trans-[PtCl₂(L^b_5-7)₂] (2b_5-7) is obtained. Metathesis of 1a₇ with NaI gives a mixture of cis-[PtI₂(L^a₇)₂] (3a₇) and trans-[PtI₂(L^a₇)₂] (4a₇). The crystal structures of 1a₅, 1a₆, 1a₇, and 4a₇ have been determined. Comparison of the structures of 1a₇ and 4a₇ reveals that L^a₇ has variable steric bulk, with the crystallographically determined cone angle ranging from 137° (smaller than L^a₅) to 172° (larger than L^a₆), depending on the particular twist-chair seven-membered-ring conformations adopted. The complex cis-[PtCl₂(L^b₆)] (1b₆) is fluxional on the NMR time scale at ambient temperatures, as a result of restricted PtP rotation. Treatment of [Rh₂Cl₂(CO)₄] with L^a_5-7 or L^b_5-7 gives the expected trans-[RhCl(CO)(L^a_5-7)₂] (5a_5-7) or trans-[RhCl(CO)(L^b_5-7)₂] (5b_5-7), and from the ν_CO values, it is deduced that the donor strengths to rhodium(I) are in the order L^b_5-7 > L^a_5-7 and, within the L^a and L^b series, L₇, L₆ > L₅. An investigation into the kinetics of the oxidative addition of MeI to 5a_5-7 showed that the rate of reaction is in the order 5a₅ > 5a₇ > 5a₆; i.e., the smallest ligand gives the highest rate. It is postulated that the flexible L^a₇ ligand adopts a lower bulk conformation and the order of decreasing rate is then in the order of increasing bulk. The rate of reaction of MeI with 5b₅ is 4 times faster than with 5a₅, but oxidative addition was not observed with 5b₆ or 5b₇, perhaps because steric congestion destabilizes the rhodium(III) product. A study of the rhodium-catalyzed hydroformylation of 1-octene is reported using ligands L^a_5-7 and L^b_5-7, but no systematic trends were observed, and the results for L^a_5-7 were similar to those for the acyclic analogue PhPEt₂. An anomalous but reproducible result is that the catalyst derived from L^b₇ shows negligible hydroformylation activity but rapid octene isomerization activity. The overall conclusion is that, with the rhodium complexes of the simple phosphacycles described here, no special effect of the rings was observed in the hydroformylation catalysis. An α-substituent effect is identified as a common feature in several high-activity hydroformylation catalysts derived from phosphinanes.