Chemistry of metal hydrides : XVIII. The formation of secondary carbene complexes from imidoyl derivatives

Abstract

Protonation of trans-PtCl(CHNR)(PEt 3) 2 occurs reversibly to give the cation trans-[PtCl(CHNHR)(PEt 3) 2] + while methylation gives trans-[PtCl(CHNMeR)(PEt 3) 2] +. The further secondary carbene complexes trans-[PtL(CHNMeR)(PEt 3) 2] 2+ (L = PPh 3, CNR) and trans-[Pt(CNR)(CHNHR)(PEt 3) 2] 2+ were prepared from these and the complex trans-[Pt(CNR)(CHNHR)(PMe 2Ph) 2] 2+ by a similar route. The latter compound reacts with p-toluidine to give trans-[Pt{C(NHR) 2}(CHNHR)(PMe 2Ph) 2] 2+ which may be deprotonated by the stronger base triethylamine to the formimidoyl-carbene complex trans-[Pt{C(NHR) 2}(CHNR)(PMe 2Ph) 2] +. trans[Pt(CNR)(CHNHR)(PEt 3) 2] 2+ does not react with p-toluidine but gives the bisimidoyl complex trans-Pt{C(OMe)NR}(CHNR)(PEt 3) 2 on reaction with methoxide ion. The platinum secondary carbene complexes are characterized by a broad low-field C(carbene)H signal in their 1H NMR spectra and a strong absorption due to v(C(carbene)N) in the 1500–1600 cm −1 region of their infrared spectra. The infrared spectra of the p-tolylaminocarbene complexes indicate some hydrogen bonding interaction with the counteranion. The 1H NMR spectra of the carbene complexes provide evidence for restricted rotation about the CN bond, the ratio of the isomers varying with solvent. NH signals at very low field coupled to the C(carbene)H signals were visible for trans-[PtCl(CHNHR)(PMe 2Ph) 2]Cl and trans-[Pt(CNR)(CHNHR)L 2]X 2 (L = PEt 3, X = ClO 4; L = PMe 2Ph, X = PF 6) when recorded in chloroform- d. For trans-[Pt(CNR)(CHNHR)(PEt 3) 2](ClO 4) 2 in acetonitrile- d 3, however, the NH signal and the coupling were lost due to rapid NH exchange. Addition of acid, or cooling, always resulted in the appearance of the NH signals and the CHNH coupling. The magnitude of this coupling (≈10 or ≈18 Hz) identified the signals for each isomer.