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