Synthesis, structural properties, and reaction chemistry of hydrido alkyl diaminocarbene complexes of platinum(II). Cleavage of the metal-carbene and metal-alkyl bonds and synthesis of formamidines and cis hydrido carbene derivatives

Article abstract of DOI:10.1021/om00119a015

Hydrido alkyl carbene complexes of the type PtH(R_X)[C(N(CH₂)_nCH₂)NHR](PPh ₃) (n = 2, R_X = CF₃ (1), CH₂CN (2), CH₂CF₃ (3); n = 4, R_X = CF₃ (4); R = p-MeOC₆H₄) have been prepared by reaction of the parent isocyanide precursors PtH(R_X)(CNR)(PPh₃) with azetidine or piperidine. Complexes 1-4 have cis Pt-H and Pt-R_X bonds with the carbene ligand trans to hydride. They have been characterized by analytical data and their IR and ¹H, ¹⁹F, ³¹P, and ¹³C NMR spectra. Compound 2 was also structurally characterized by X-ray analysis. The triclinic crystal, space group P1, has lattice parameters a = 12.998 (3) ?, b = 11.833 (2) ?, c = 10.257 (2) ?, α = 100.83 (4)°, β = 80.72 (3)°, γ = 114.20 (4)°, and Z = 2. The structure was refined to R = 0.022 and R_w = 0.022. The coordination geometry around the Pt atom in 2 is distorted square planar. The geometry of the carbene ligand is essentially planar. The plane of the carbene intersects the platinum square plane at an angle of 73.7 (2)°. The molecules of 2 are associated in the solid state to give dimers through intermolecular interactions between the nitrogen atom of CH₂CN and the aminocarbene proton of an adjacent molecule. Important bond lengths are Pt-C(carbene) = 2.069 (4) ?, Pt-H = 1.61 (4) ?, Pt-CH₂CN = 2.116 (7) ?, and Pt-P = 2.252 (2) ?. The isocyanide complex PtH(CF₃)(CNR)(PPh₃) reacts with excess HNEt₂ in refluxing THF for 3 days to give a mixture of isomers with the carbene ligand trans to hydride (5a), PPh₃ (5b), or trifluoromethyl (5c). Complexes 1-5 are stable in the solid state and in solution, and no reductive elimination of HR_X is observed in refluxing THF for several hours even in the presence of diphenylacetylene. However, complexes 2 and 3, but not 1 and 4, react with equivalent amounts of PPh₃ to give trans-(PPh₃)₂PtH[C(NCH₂CH₂CH ₂)NR] (6) and formation of CH₃CN and CH₃CF₃, respectively. Isotopic experiments with N-D and Pt-D derivatives indicate that this reaction proceeds through the protonolysis of the metal-alkyl bond by the aminocarbene proton, promoted by PPh₃. Complex 6 reacts with HBF₄ to give the hydrido carbene derivative trans-{(PPh₃)₂PtH[C(NCH₂CH₂CH ₂)NHR]}BF₄ (7). Similarly the reactions of 2 with equimolar amounts of diphosphines yield (P-P)PtH[C(NCH₂CH₂CH₂)NR] (P-P = Ph₂PCH₂CH₂PPh₂ (8), Ph₂PCH=CHPPh₂ (9)) and CH₃CN. Complexes 8 and 9 react with HBF₄ to produce the cis hydrido carbene derivatives {(P-P)PtH[C(NCH₂CH₂CH₂)NHR]}BF₄ (10 and 11, respectively). On the other hand, complexes 1 and 4 react with diphosphines to give metal-carbene bond cleavage and formation of (P-P)PtH(CF₃) and the formamidines HC(NCH₂CH₂CH₂)=N(R) and HC[N(CH₂)₄CH₂]=N(R), respectively.