Synthesis and conformation of 9,10-dihydroplatina-anthracenes: nuclear magnetic resonance and X-ray crystal structure studies

Abstract

The platinacycles [Pt(C₆H₄CH₂C₆H₄)L₂](1a)–(1h)[L = PEt₃, PMe₃, PMe₂Ph, PMePh₂, or PPh₃; L₂= cyclo-octa-1,5-diene(cod), Ph₂PCH₂CH₂PPh₂, or Ph₂PCH₂PPh₂] have been made and characterised by ¹H, ¹³C, ³¹P, and ¹⁹⁵Pt n.m.r. spectroscopy. The crystal structure of (1a; L = PEt₃) has been determined, showing that the platinacycle has a boat conformation with a substantial dihedral angle (105.3°) between the aromatic rings. The boat conformation metallacyclic ring in complex (1a) is rigid on the n.m.r. time-scale up to at least 100 °C in dimethyl sulphoxide. The platinum(IV) complex [Pt(C₆H₄CH₂C₆H₄)I₂(PEt₃)₂](2) has been made by oxidative addition of I₂ to (1a). Its crystal structure also shows a boat conformation, but the ring is very much flatter (dihedral angle 151.2°), and the octahedral geometry at Pt causes one PEt₃ group to lie beneath the platinacycle. Complex (2) is fluxional at ambient temperatures due to rapid ring inversion but this can be frozen out at low temperatures and ΔG^‡ of 41 kJ mol^–1 estimated for the inversion process at T_c. Mean bond distances are: Pt–P 2.323(10)(1a), 2.386(3)(2), Pt–l 2.785(20)(2), Pt–C 2.05(1)(1a), 2.08(1)Å(2). Both crystals are monoclinic, space groups P2₁/n and P2₁/c: (1a), a= 10.748(2), b= 15.259(4), c= 14.885(4)Å, and β= 90.77(2)°; (2), a= 17.375(5), b= 9.681 (3), c= 19.192(5)Å, and β= 120.72(2)°. For (1a), R= 0.048 for 2.725 observed diffractometer collected reflections [I/σ(I) > 3.0], and for (2), R= 0.053 for 3 311 observed reflections.