Palladium(II) and platinum(II) dihalogeno-complexes containing the polyfunctional phosphine 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine (dpnapy). Crystal structure of the metallocycle trans,trans-[{Pd(µ-dpnapy)Cl2}2]·2H2O

Abstract

The co-ordination chemistry of the polyfunctional phosphine 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine (dpnapy) with some palladium(II) and platinum(II) dihalogeno-complexes has been investigated. It reacts with [Pd(cod)Cl₂](cod = cycloocta-1.5-diene) in 1:1 molar ratio to give the metallocycle trans,trans-[{Pd(µ-dpnapy)Cl₂}₂]1, the molecular structure of which was unambiguously determined by an X-ray analysis. The structure consists of two trans-PdCl₂ moieties linked by two dpnapy molecules in a head-to-tail arrangement. In this way the two palladium atoms and two bridging dpnapy comprise a quasi-planar twelve-membered ring. Attempts to include a metal ion in the central cavity of the metallocycle failed. This result has been explained in terms of the geometric constraints of dpnapy, the structure of which was also determined. When [Pd(PhCN)₂Cl₂] was used as starting palladium source the reaction proceeded in a different way. Complexes of general formula [M(dpnapy)₂Cl₂](M = Pd 2 or Pt 3) containing two dangling dpnapy-P ligands were obtained from the reaction of dpnapy with [M(cod)Cl₂](M = Pd or Pt), [Pd(PhCN)₂Cl₂] and [Pt(dmso)₂Cl₂](dmso = dimethyl sulfoxide) when the ligand-to-metal molar ratio was 2:1. Complexes 2 and 3 have been used as starting materials for the synthesis of homo- and hetero-metallocycles. Unexpectedly, only homometallocycles have been isolated from these reactions, e.g. trans-[Pd(dpnapy)₂Cl₂] reacts with [{Rh(CO)₂Cl}₂] giving trans,trans-[{Pd(µ-dpnapy)Cl₂}₂] and cis,cis-[{Rh(CO)(µ-dpnapy)Cl}₂] the formation of which requires the transfer of a dpnapy ligand from the palladium to the rhodium complex. On the basis of IR and NMR findings, a mechanism is proposed for this and other related reactions.