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

Abstract

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 ¹H and ¹³C 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 ¹H and ³¹P chemical exchange spectra indicate a further slower dynamic equilibrium 1⇌2 with loss of dpnapy. The binuclear complex [{Rh(cod)Cl}₂(µ-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 ¹H and ¹³C 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 ¹H chemical exchange spectrum shows a slow general scrambling of the cod olefinic protons and the presence of the equilibrium 2⇌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)(µ-dpnapy)Cl}₂]·2CH₂Cl₂3. 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.