Nitrile-amidine coupling at Pt(IV) and Pt(II) centers. An easy entry to imidoylamidine complexes

Article abstract of DOI:10.1021/ic050037q

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 [Ph ₃PCH₂CO₂Me]Cl. Treatment of 5 with 3 equiv of Ph₃P=CHCO₂-Me 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 CDCl ₃ is used. When the platinum(II) complex trans-[PtCl ₂(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-[PtCl ₂(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-[PtCl ₂(RCN)₂] (R = Me, Et) with the amidine gives ca. 50-60% yield of [PtCl₂{NH=C(R)NHC(Ph)=MHPh}] (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.