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

The series of N-carbazolyl phosphines PPh3-n(NC12H8)(n) (n = 1, L-1; n = 2, L-2; n = 3, L-3) has been synthesised using BuLi to generate the N-carbazolyl lithium salt, followed by reaction with the appropriate chlorophosphine. The reactions between [Rh(mu-Cl)(CO)(2)](2) and four equivalents of L-1 or L-2 gave [RhCl(CO)(L-1)(2)] 1 and [RhCl(CO)(L-2)(2)] 2, though attempts to synthesise the analogous complex using L-3 resulted in the formation of [Rh(mu-Cl)(CO)(L-3)](2) 3 instead. The inability of L-3 to cleave the chloride bridges can be related to its considerable steric requirements. The electronic properties of L1-3 were assessed by comparison of the v(CO) values of the [Rh(acac)(CO)(L1-3)] complexes 4 - 6. The increase in number of N-carbazolyl substituents at the phosphorus atom results in a decrease of the sigma-donor and increase in the pi-acceptor character in the order L-1 < L-2 < L-3. In the reactions of L1-3 with [PdCl2(cod)] only L-1 was able to displace cod from the metal centre and form [PdCl2(L-1)(2)] 7. The use of [PdCl2(NCMe)(2)] instead of [PdCl2(cod)] resulted in the formation of the complexes [PdCl2(L-1)(2)] 7 from L-1, the cyclometallated complex [Pd(mu-Cl){P(NC12H8)(2)(NC12H7)- kappa(2)P, C}](2) 8 from L-3, and a mixture of [PdCl2(L-2)(2)] 9 and [Pd(mu-Cl){ PPh(NC12H8)(NC12H7)-kappa(2)P, C}](2) 10 from L-2. The reaction of L-3 with [Pd(OAc)(2)] produced the cyclometallated complex [Pd(mu-O2CCH3){P(NC12H8)(2)(NC12H7)-kappa(2)P, C}](2) 11. The reaction of L-3 with [Pd-2(dba)(3)] . CHCl3 produced the 14-electron complex [Pd(L-3)(2)] 12. The X-ray crystal structures of six complexes are reported, all of which show the presence of C - H...Pd hydrogen bonding.