Synthesis, Structures, and Reactions of Methylplatinum(II) and -palladium(II) Complexes Bearing Diphosphinidenecyclobutene Ligands (DPCB-Y)

Abstract

Dimethylplatinum(II) complexes coordinated with 1,2-diaryl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutenes (DPCB-Y; aryl = 4-methoxyphenyl, phenyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl] are prepared by ligand displacement of Pt2Me4(μ-SMe2)2 with corresponding DPCB-Y, which is a novel class of ligands having sp2-hybridized phosphorus as coordination atoms. Comparison of the NMR data with those of diimine- and diphosphine-coordinated analogues indicates intermediate magnitude of trans-influence of DPCB-Y ligands. This observation is supported by X-ray structural analysis. On the other hand, structural deviations of the coordinated DPCB-Y from free DPCB-Y suggest the occurrence of strong π-back-donation from the platinum to the ligands. Reflecting the strong π-back-donation, the ethylene protons in [PtMe(η2-C2H4)(DPCB-Y)]OTf appear at δ 5.22−5.02 in the 1H NMR spectra, the chemical shifts of which are significantly lower than those so far reported for platinum(II) complexes (ca. δ 4.0) and comparable to that of free ethylene (δ 5.30). Ethylene polymerization using methylpalladium catalysts bearing DPCB-Y ligands is also reported.

Synthesis and catalytic properties of cationic palladium(II) and rhodium(I) complexes bearing diphosphinidinecyclobutene ligands

“…Probably, dp orbital levels of square planar complexes are not so sensitive to r-donors on the coordination plane, and therefore the dp-pp interaction is highly dependent on the Pd-P bond length. The occurrence of strong p-back donation in 1a is also evidenced by the purple color of this complex, which is markedly red-shifted from the yellow color of [Pd(g 3 …”

“…1) tend to adopt a parallel orientation with respect to the diphosphinidenecyclobutene skeleton (B) upon coordination [3]. This is due to the occurrence of strong p-back donation from metal to DPCB ligand.…”

“…GLC analysis was performed on a Shimadzu GC-8A instrument equipped with a TCD and a Silicone OV-1 column. DPCB-Y ligands were prepared as previously reported [3].…”

“…We recently found that the 1,2-diaryl-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutenes (DPCB-Y) shown in Chart 1 serve as particularly useful ligands [2]. Thus, while the DPCB-Y ligands structurally resemble diimine ligands, they possess extremely low-lying p * orbitals located around the phosphorus, and exhibit a strong pacceptor property towards transition metals [3]. We have documented that this property is useful for catalysis, leading to hitherto unknown reactivity and selectivity in hydroamination of dienes [4], direct conversion of allylic alcohols into N-and C-allylation products [5], (Z)-selective hydrosilylation of alkynes [6], cross-coupling reactions [7], and so on.…”

“…The 31 P NMR signals appeared at d 135.7 (1a), 127.3 (1b), and 143.1 (1c), respectively. The chemical shifts are 34-38, 28-31, and 8-12 ppm higher than that of free DPCB-Y [3], PdMe 2 (DPCB-Y) [3], and [Pd(g 3 -allyl)-(DPCB-Y)]OTf [5b], respectively. 2a-c) and related complexes (2d,e)…”