Nitrile−Amidine Coupling at Pt(IV) and Pt(II) Centers. An Easy Entry to Imidoylamidine Complexes

Treatment of trans-[PtCl4(RCN)2] (R = Me, Et, Ph, NEt2) with 2 equiv of the amidine PhC(NH)NHPh in a suspension of MeCN (R = Me), CHCl3 (R = Et, Ph), or in CHCl3 solution (R = NEt2) results in the formation of the imidoylamidine complexes trans-[PtCl4{NHC(R)NC(Ph)NHPh}2] (1−4) isolated in good yields (66−84%). The reaction of soluble complexes 3 and 4 with 2 equiv of Ph3PCHCO2Me in CH2Cl2 (40 °C, 5 h) leads to dehydrochlorination resulting in a chelate ring closure to furnish the platinum(IV) chelates [PtCl2{NHC(R)NC(Ph)NPh}2] (R = Ph, 5; R = NEt2, 6), accordingly, and the phosphonium salt [Ph3PCH2CO2Me]Cl. Treatment of 5 with 3 equiv of Ph3PCHCO2Me at 50 °C for 5 d resulted in only a 30% conversion to the corresponding Pt(II) complex [Pt{NHC(NEt2)NC(Ph)NPh}2] (15). The reduction can be achieved within several minutes, when Ph2PCH2CH2PPh2 in CDCl3 is used. When the platinum(II) complex trans-[PtCl2(RCN)2] is reacted with 2 equiv of the amidine, the imidoylamidinato complexes [PtCl(RCN){NHC(R)NC(Ph)NHPh}] (8−11) and [PhC(NH)NHPh]·HCl (7) are formed. The reaction of trans-[PtCl2(RCN)2] with 4 equiv of the amidine under a prolonged reaction time or treatment of [PtCl(RCN){NHC(R)NC(Ph)NHPh}] (8−11) with 2 more equiv of the amidine yields the complex bearing two chelate rings [Pt{NHC(R)NC(Ph)NHPh}2] (12−15). The treatment of cis-[PtCl2(RCN)2] (R = Me, Et) with the amidine gives ca. 50−60% yield of [PtCl2{NHC(R)NHC(Ph)NHPh}] (16 and 17). All of the platinum compounds were characterized by elemental analyses; FAB mass spectrometry; IR spectroscopy; 1H, 13C{1H}, and 195Pt NMR spectroscopies, and four of them (4, 6, 8, and 15) were also characterized by X-ray crystallography. The coupling of the Pt-bound nitriles and the amidine is metal-mediated insofar as RCN and PhC(NH)NHPh do not react in the absence of the metal centers in conditions more drastic than those of the observed reactions. The nitrile−amidine coupling reported in this work constitutes a route to the synthesis of imidoylamidine complexes, some of them exhibiting luminescent properties.