Synthesis, molecular structure and palladium(II) and platinum(II) complex chemistry of 3-(ferrocen-1-yl)-1-(pyridin-2-yl)pyrazole

Abstract

Treatment of the potassium salt of 3(5)-(ferrocen-1-yl)pyrazole L¹ with 2-bromopyridine in diglyme (2,5,8-trioxononane) at 130 °C for 3 d, followed by an aqueous quench, yielded the compound 3-(ferrocen-1-yl)-1-(pyridin-2-yl)pyrazole L² in 37% recrystallised yield. A single-crystal structure determination of L² confirmed the proposed 1,3-disubstitution pattern at the pyrazole ring, the substituted cyclopentadienyl and two heterocyclic rings being approximately coplanar. Treatment of L² with a stoichiometric amount of [{MCl(C₃H₅)}_x](M = Pd, x= 2; M = Pt, x= 4) and NH₄PF₆ gave [M(η³-C₃H₅)(L²)]PF₆(M = Pd 1 or Pt 2) while a similar reaction with [PdCl₂(NCPh)₂] afforded [PdCl₂(L²)]3. Treatment of 3 with 1 molar equivalent of K₂C₂O₄, Na₂(cat) or Na(acac)–NH₄PF₆ yielded [Pd(L)L²][L = C₂O₄ ^2–4 or catecholate (cat)5] and [Pd(acac)(L²)]PF₆6(acac = acetylacetonate) as analytically pure solids. The ¹H NMR behaviour of 4–6 is complex and suggestive of extensive ligand dissociation in solution; molecular models imply that this may reflect steric interactions between the pendant ferrocenyl moiety and metal-bound anionic chelate O-donors. The UV/VIS and electrochemical data show that the ferrocenyl group becomes significantly electron-poorer upon co-ordination of L², reflecting an inductive interaction with the co-ordinated M²⁺(M = Pd or Pt) electrophiles.