Critical Role of Anions in Platinum(II) Precursors upon the Structural Motifs of Six-Membered Cycloplatinated N,N',N″-Triarylguanidines.

Abstract

The reactions of cis-[Pt(OAc)2(DMSO)2] with 2 equiv of sym N,N′,N″-triarylguanidines, [ArN=C(NHAr)2], in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(OAc){κ1N(ArN=C(NHAr)2)}] [sym = symmetrical; Ar = 2-MeC6H4 (1) and 2,4-Me2C6H3 (2)], in 82 and 84% yields, respectively. The salt metathesis reaction of 1 with 1 equiv of AgTFA in CH2Cl2 at room temperature (RT) afforded [Pt{κ2(C,N)}(TFA){κ1N(ArN=C(NHAr)2)}] (3) in 94% yield. The reaction of cis-[Pt(TFA)2(DMSO)2] with 1 equiv of [ArN=C(NHAr)2] in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(TFA)(DMSO)] [Ar = 2-MeC6H4 (4), 4-MeC6H4 (5), 2,4-Me2C6H3 (6), and 2-(MeO)C6H4 (7)], in ≥73% yields. The reaction of trans-[PtCl2(PhCN)2] with 2 equiv of [ArN=C(NHAr)2] in toluene under reflux condition for 48 h afforded trans-[PtCl2{ArN=C(NHAr)2}2] [Ar = 2-MeC6H4 (8) and 2,4-Me2C6H3 (9)] in 90 and 45% yields, respectively. Complexes 8 and 9 were separately refluxed in MeOH for 8 h to afford six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(μ-Cl)]2 (10 and 11), in 93 and 96% yields, respectively, with concomitant formation of the respective guanidinium salts, [(ArNH)3C]Cl, as the byproduct. Platinacycle 10 was treated with 2 equiv of AgTFA in CH2Cl2 at RT to afford six-membered cycloplatinated guanidine, [Pt{κ2(C,N)}(μ-TFA)]2 (12), in 94% yield. The new compounds were characterized by analytical techniques and multinuclear NMR (1H, 13C, and 195Pt) spectroscopy, and further, molecular structures of 10 compounds were determined by single-crystal X-ray diffraction. The structural motif in 1·1/2CH2Cl2 and 3 is novel in that it contains a planar six-membered [Pt{κ2(C,N)}] unit and a nonplanar eight-membered [Pt{κ2(N,O)}] ring, wherein OAc and the guanidine ligands are linked through a N–H···O hydrogen bond. The six-membered cycloplatinated structural motifs present in 10/11·C7H8 and 12·CH2Cl2 are also unprecedented in the literature. The number and nature of solution species of new complexes were unambiguously investigated by detailed NMR studies. The critical role of anions in Pt(II) precursors upon the course of cycloplatination and thus the motifs in the products were addressed. Plausible mechanisms of cycloplatination reactions are discussed.