Palladium complexes of P,P-, P,N- and P,S- donor ligands based on the P-N-P motif

Abstract

The diphosphazane ligands of the type Ph?P(E)N(R)P(E?)Ph? (1–5) exhibit diverse palladium–allyl chemistry, including various fluxional processes in solution. The symmetrical ligands Ph?PN(CHMe?)PPh? (1) and Ph?P(S)N(CHMe?)P(S)Ph? (2) effectively form only one isomer in solution because the achiral nature of these ligands cannot discriminate the different allyl-face coordination in NMR spectra. The presence of syn/anti isomers in the case of complex Pd(?³-1,3-Me?-C?H?){?²-Ph?PN(CHMe?)PPh?} (15) indicates that complexes 12–21 are fluxional in solution, but the lack of asymmetry in the auxiliary ligand does not allow one to study the dynamic behavior by NMR spectroscopy. Incorporation of a chiral center in the ligand backbone introduces the possibility that palladium–allyl complexes can exist as a pair of different face-coordinated diastereomers. Diastereomerism is observed for allyl complexes bearing the homodonor chiral ligand Ph?PN((S)-CHMePh)PPh? (3) only when the allyl moiety is unsymmetrical. The study of the fluxional process for the complex Pd(?³-1,3-Me?-C?H?){?²-Ph?PN((S)-CHMePh)PPh?} (25) shows that opening of the ?³-allyl group to the ?-bonded intermediate is not selective, as the ligand Ph?PN((S)-CHMePh)PPh? consists of identical donating atoms. The introduction of an achiral unsymmetrical P,S-ligand Ph?P(S)PN(CHMe?)PPh? (4) brings out the possibility of geometrical isomerism, as observed for unsymmetrical allyl complexes with ligand 4. The stereodynamic behavior of allyl complexes bearing the P,S-ligand is dictated by the electronic difference between the two donating centers. As a result, the ?-bonded intermediate is formed selectively at the trans position with respect to the sulfur center. Incorporation of a chiral center in the P,S-ligand complicates the situation by increasing the number of possible isomers, as allyl face-coordination can lead to different diastereomers. Unlike the case observed for the chiral homodonor ligand Ph?PN((S)-CHMePh)PPh? (3), the complexes of the heterodonor chiral ligand Ph?P(S)PN((S)-CHMePh)PPh? (5) show diastereomerism even when the allyl group is symmetrical and unsubstituted. Thus, two conditions are essential for allyl complexes to exist as NMR-distinguishable diastereomers: (i) the auxiliary ligand should be chiral, and (ii) either the allyl part or the auxiliary ligand should be unsymmetrical. Despite the correct assignment of several allylic arrangements, including various geometrical isomers with the aid of 2D NMR experiments, the exact assignment of the allyl diastereomers-i.e., the relative arrangement of the coordinated allyl moiety with respect to the auxiliary ligand-remains inconclusive owing to the absence of any intruding reporter group in the auxiliary ligand. This problem will be addressed in the next section. The P-chiral diphosphazane monosulfide ligand Ph?P(S)N(CHMe?)PPh(N?C?HMe?-3,5) favors P,S-coordination over N,S-coordination owing to the formation of a more stable five-membered ring around the palladium center. Complex 44 exists in solution as two isomers, of which the endo-isomer is found to be the major one. There is exchange between these two isomers by two different mechanisms operating simultaneously. The cis–trans isomerization is observed to be the major pathway over the syn–anti isomerization process. The 1,3-dimethyl analogue of complex 45 exists as a mixture of three isomers in solution, but the solid-state structure shows only one isomer, namely the exo, syn/syn-isomer (45a). On dissolution in CDCl?, 45a transforms into two other isomers, 45b and 45c. The formation of 45b and 45c can be rationalized by considering cis–trans isomerization and syn–anti isomerization, respectively, which is consistent with the observation of two simultaneous exchange processes for complex 44. The primary step of the exchange process involves selective opening of the ?³-allyl group to a ?-bonded intermediate at the trans position with respect to the more ?-acceptor phosphorus center, followed by rotation around the Pd–C bond. Increase of steric bulk of the allyl moiety (e.g., Pd(?³-1,3-Ph?-C?H?){?²-Ph?P(S)N(CHMe?)PPh(N?C?HMe?-3,5)} (46)) slows the exchange, as the presence of two phenyl groups hinders rotation around the Pd–C bond. However, at a higher temperature (50 °C), exchange occurs via a cis–trans isomerization process. The ligand system chosen in the present study is a potential candidate for studying the isomerization dynamics of palladium–allyl complexes in detail, as all the isomers are clearly observable even at ambient temperature. The diphosphazane ligands of the type Ph?P(E)N(R)PPh(N?C?HMe?-3,5) bearing a stereogenic phosphorus center exhibit various fluxional processes in solution, and the study of their exchange behavior can help in the assignment of diastereomeric structures. The preferred mode of coordination with the ligands Ph?PN(R)PPh(N?C?HMe?-3,5) {R = CHMe? (6) or (S)-CHMePh (7)} is through phosphorus and nitrogen rather than the usually observed P,P-coordination. This mode of coordination electronically stabilizes the palladium–allyl bonding and leads to the formation of a six-membered chelate ring. On the other hand, the monosulfide ligand Ph?P(S)N(R)PPh(N?C?HMe?-3,5) (8) displays P,S-coordination. The ¹³C NMR spectrum is a valuable tool to elucidate the coordination mode of these ligands as well as the geometrical disposition of the allyl substituents with respect to the coordinated phosphorus center. The steric and electronic effects of the auxiliary ligands play a key role in the stabilization of palladium–carbon ?- and ?-bonds. The formation of a palladium dimer with X?PN(Me)PX? (X = OPh) ligand indicates that this ligand, having two phosphonite phosphorus atoms, behaves as a strong electron-accepting ligand, which does not permit metal–allyl ?-interaction. As a result, a moderately stable allyl complex is observed only when the electron-donating allyl moiety bearing two phenyl groups is introduced. On the other hand, when the electron-donating ability of the auxiliary ligand is increased by incorporating eight methyl groups at the 2- and 6-positions of the phenyl rings on the ligand X?PN(Me)PX? (X = OC?H?Me?-2,6), the palladium–allyl complexes are the only products. At the same time, the methyl groups at the 2- and 6-positions of the phenyl rings exert a significant steric effect in determining the relative abundance of two isomers of complexes 56 and 57. The isomers with unusual allylic arrangements—anti- (56b) and syn/anti- (57b)-are formed in exceptionally high amounts compared to other allyl complexes of diphosphazane ligands employed in the present investigation. The diphosphazane ligands bearing the axially chiral 1,1?-binaphthylene-2,2?-dioxy moiety, (rac)-(C??H??O?)PN(CHMe?)PY?, give a stable 1,3-dimethyl allyl complex (58) only when Y = Ph. However, both ligands (rac)-(C??H??O?)PN(CHMe?)PY? (Y = Ph or OPh) give stable 1,3-diphenyl allyl complexes. The reactions of palladium–carbon ?-bonded complexes with diphosphazane ligands have been studied. The operation of a pronounced electronic effect is strikingly illustrated by the formation of a single isomer in the case of diphosphazane monosulfide ligands Ph?P(S)N(R)PPh? (R = CHMe? or (S)-CHMePh), wherein the palladium–carbon ?-bond takes up the position trans to sulfur with its less ?-electron-acceptor nature than the position trans to the relatively greater ?-acceptor phosphorus atom.