Polyfunctional phosphine ligands. Synthesis of 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine and its co-ordination properties

The new trinucleating ligand 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine (dpnapy) has been prepared and its co-ordination chemistry with rhodium(I) complexes investigated. Reactions with [{Rh(cod)Cl}2] (cod = cycloocta-1,5-diene) lead to different products depending on the ligand to metal ratio. The P-monodentate ligand complex [Rh(cod)(dpnapy)Cl] 1 is formed when a 2:1 ligand:dimer ratio is used. The cod region of the H-1 and C-13 NMR spectra of 1 is temperature dependent. The fluxional behaviour can be explained by the formation of a five-co-ordinate intermediate displaying rapid intramolecular exchange of the cod olefinic and CH2 protons. A lower activation energy process, not frozen at 215 K, is also observed. At room temperature (r.t.) the two-dimensional H-1 and P-31 chemical exchange spectra indicate a further slower dynamic equilibrium 1 half arrow right over half arrow left 2 with loss of dpnapy. The binuclear complex [{Rh(cod)Cl}2(mu-dpnapy)] 2, in which dpnapy acts as bridging bidentate ligand using the phosphorus and the terminal nitrogen atoms, is the reaction product when a 1 : 1 ligand: dimer ratio is used. The H-1 and C-13 NMR data for complex 2 are consistent with a dimeric structure of low symmetry. Complex 2 is also involved in several dynamic processes. At r.t. the two-dimensional H-1 chemical exchange spectrum shows a slow general scrambling of the cod olefinic protons and the presence of the equilibrium 2 half arrow right over half arrow left 1 with participation of [{Rh(cod)Cl}2]. The ligand reacts with [{Rh(CO)2Cl}2] giving as the only isolable product the metallocycle [{Rh(CO)(mu-dpnapy)Cl}2].2CH2Cl2 3. The reaction proceeds through several steps involving the formation of cis-dicarbonyl intermediates as indicated by IR and NMR spectra. The structure of 3 has been determined by X-ray crystallography. It consists of two Rh(CO)Cl units, joined together by two dpnapy molecules in a head-to-tail arrangement.