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 CN 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 CHNH coupling. The magnitude of this coupling (≈10 or ≈18 Hz) identified the signals for each isomer.
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