Preparation and interconversion of dimeric di-µ-hydroxo and tri-µ-hydroxo complexes of platinum(II) and palladium(II) with 2,2′-bipyridine and 1,10-phenanthroline

Abstract

Treatment of [Pt(L)I₂][L = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)] with AgNO₃ in acetone gives the nitrato complexes [PtL(ONO₂)₂]. The palladium analogues were prepared from [Pd(L)Cl₂] in dilute nitric acid. Dissolution of [ML(ONO₂)₂](M = Pd or Pt) in water results in the formation of the hydroxo-bridged dimers [LM(µ-OH)₂ML][NO₃]₂ plus nitric acid. Reaction of [M(L)Cl₂] with AgNO₃ in water gives [LM(µ-OH)₂ML][NO₃]₂ directly as the sole product. The dimers are resistant to substitution, although prolonged heating in aqueous nitric acid reforms [ML(ONO₂)₂]. The dimers add 1 mol of OH^– to form the very stable trihydroxo-bridged compounds [LM(µ-OH)₃ML]⁺(M = Pt, deep red; M = Pd, deep yellow) where each metal is five-co-ordinate. These complexes are slowly cleaved by hydroxide to give [ML(OH)₂], which was also prepared either by base hydrolysis or by reaction of [M(L)Cl₂] with Ag₂O. Addition of HX (X = NO₃, or ClO₄) to [PtL(OH)₂] affords [LPt(µ-OH)₃PtL]⁺, [LPt(µ-OH)₂PtL]²⁺ or [PtL(ONO₂)₂] at pH 8, 4, and 1 respectively. The complexes have been characterised by i.r., u.v., and n.m.r. (¹⁹⁵Pt, ¹³C, and ¹H) spectroscopy.