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

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.