Sterically hindered electron-withdrawing ligands: the reactions of N-carbazolyl phosphines with rhodium and palladium centres

Abstract

The series of N-carbazolyl phosphines PPh_3−n(NC₁₂H₈)_n (n = 1, L¹111111; n = 2, L²222222; n = 3, L³333333) has been synthesised using BuLi to generate the N-carbazolyl lithium salt, followed by reaction with the appropriate chlorophosphine. The reactions between [Rh(μ-Cl)(CO)₂]₂ and four equivalents of L¹111111 or L²222222 gave [RhCl(CO)(L¹111111)₂] 1 and [RhCl(CO)(L²222222)₂] 2, though attempts to synthesise the analogous complex using L³333333 resulted in the formation of [Rh(μ-Cl)(CO)(L³333333)]₂3 instead. The inability of L³333333 to cleave the chloride bridges can be related to its considerable steric requirements. The electronic properties of L^1–31–31–31–31–31–31–3 were assessed by comparison of the ν(CO) values of the [Rh(acac)(CO)(L^1–31–31–31–31–31–31–3)] complexes 4–6. The increase in number of N-carbazolyl substituents at the phosphorus atom results in a decrease of the σ-donor and increase in the π-acceptor character in the order L¹111111 < L²222222 < L³333333. In the reactions of L^1–31–31–31–31–31–31–3 with [PdCl₂(cod)] only L¹111111 was able to displace cod from the metal centre and form [PdCl₂(L¹111111)₂] 7. The use of [PdCl₂(NCMe)₂] instead of [PdCl₂(cod)] resulted in the formation of the complexes [PdCl₂(L¹111111)₂] 7 from L¹111111, the cyclometallated complex [Pd(μ-Cl){P(NC₁₂H₈)₂(NC₁₂H₇)-κ²P,C}]₂8 from L³333333, and a mixture of [PdCl₂(L²222222)₂] 9 and [Pd(μ-Cl){PPh(NC₁₂H₈)(NC₁₂H₇)-κ²P,C}]₂10 from L²222222. The reaction of L³333333 with [Pd(OAc)₂] produced the cyclometallated complex [Pd(μ-O₂CCH₃){P(NC₁₂H₈)₂(NC₁₂H₇)-κ²P,C}]₂11. The reaction of L³333333 with [Pd₂(dba)₃]·CHCl₃ produced the 14-electron complex [Pd(L³333333)₂] 12. The X-ray crystal structures of six complexes are reported, all of which show the presence of C–H⋯Pd hydrogen bonding.