Synthesis and Reactions of Palladium and Platinum Complexes Bearing Diphosphinidenecyclobutene Ligands: A Thermally Stable Catalyst for Ethylene Polymerization

Abstract

Diphosphaalkenes are phosphorus analogues of diimines and form stable chelate rings with transition metal species. [1±3] The coordination structures resemble those of diimine complexes. However, owing to the unique electronic nature of PC bonds, diphosphaalkene complexes are expected to possess rather intriguing chemical properties, significantly different from the diimine analogues. Thus, we have been interested in the reaction chemistry of diphosphaalkene complexes, particularly in catalytic systems; data for such systems is extremely limited. [2] Herein we examine the synthesis, structures, and catalytic properties of some methylpalladium and platinum complexes (1 ± 4) coordinated with 3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene derivatives. [4] A cationic monomethylpalladium complex derived from 1 exhibits good catalytic activity for ethylene polymerization. P P R R M Me Me tBu tBu tBu tBu tBu tBu 3: M = Pd, R = H 4: M = Pt, R = Ph 1: M = Pd, R = Ph 2: M = Pd, R = SiMe 3The diphosphaalkene ligands in 1 ± 4 were chosen with reference to findings reported by Brookhart et al. on nickeland palladium-based catalysts for ethylene polymerization. [5,6] There the key to high catalytic activity was the use of a-diimine ligands having bulky aryl groups at the nitrogen atoms. The diphosphinidenecyclobutene ligands possess similar structural features.Complexes 1 and 2 were prepared by ligand displacement of [PdMe 2 (tmeda)] (tmeda tetramethylethylenediamine) with phenyl-and trimethylsilyl-substituted ligands in Et 2 O and isolated as orange crystals in 94 % and 63 % yields, respectively. Complex 3 was not obtained by this method but prepared in 60 % yield by using [PdMe 2 (cod)] (cod cyclooctadiene) in place of [PdMe 2 (tmeda)]. The platinum complex 4 was synthesized from [Pt 2 Me 4 (SMe 2 ) 2 ] in 89% yield. All complexes were characterized by NMR spectroscopy and elemental analysis (see Supporting information).The X-ray structure of 4, [7] which clearly shows chelate coordination of the diphosphinidenecyclobutene ligand (Figure 1). The two aryl rings on the phosphorus atoms are almost Figure 1. Molecular structure of 4. Hydrogen atoms are omitted for clarity. Selected bond lengths [] and angles [8]: Pt-C(1) 2.093(3), Pt-P 2.

Phosphaorganische Chemie: Panorama und Perspektiven

“… 338 In beiden Fällen ähneln die Aktivität der Katalysatoren und die Eigenschaften der entstehenden Polymere den entsprechenden Parametern der all‐Kohlenstoff‐Katalysatoren. Außerdem ist der kationische Komplex [Pd( 45 )(Me)] + zu nennen, der angesichts seiner Strukturverwandtschaft zu den Pd‐Diimin‐Katalysatoren ein zuverlässiger Katalysator für die Ethylenpolymerisation ist 341…”

Bulky N-Substituted 1,3-Benzazaphospholes: Access via Pd-Catalyzed C−N and C−P Cross Coupling, Lithiation, and Conversion to Novel P═C−PtBu2 Hybrid Ligands

“…Trivalent phosphorus compounds are extremely versatile ligands for coordination chemistry and find use in homogeneous transition-metal catalysis by stabilizing metals in low-valence states; they present versatile tuning abilities via steric and electronic properties . Bulky and basic phosphanes are currently attracting particular attention, but the endeavor to explore the potential of PC compounds, electron-rich aromatically stabilized phospholides, and sandwich complexes thereof, or electron-deficient neutral PC systems, is also evident and has been highlighted in recent overviews. – Catalysis with electron-deficient σ 2 - P -ligands had been rather rare, , but current studies with diphenylphosphanyl-phosphaalkene σ 3 /σ 2 - P , P ′-hybrid ligands are promising and have led to isolable Pd(II) and Au(I) P , P ′-chelate complexes that are catalytically active in various reactions . In studies on anellated heterophospholes, particularly 1,3-benzazaphospholes (L), we obtained stable LM(CO) 5 complexes (M = Cr, Mo, W) and detected π-acidic properties of the double-bonded phosphorus. , Coordination of transition metals in higher oxidation numbers to neutral benzazaphospholes has not yet been observed, only to benzazaphospholide anions. , Even [Rh(COD)Cl] 2 did not react with 1 H -1,3-benzazaphospholes except by heating in the presence of base.…”

Phosphorus Heterocycles: From Laboratory Curiosities to Ligands in Highly Efficient Catalysts

“…Combination of [PdMe 2 (tmeda)] and the diphosphinidene cyclobutanes 21 b, c gave the dimethylpalladium complexes 49 b and 49 c in 94 and 63 % yield, respectively. The preparation of 49 a required [PdMe 2 (cod)] as a starting material (60 % yield, Scheme ) 43…”

“…Best results were obtained at 70 °C and at a pressure of 9.81×10 5 Pa. Catalyst activity remained unchanged at higher pressures, although the average molecular weight decreased significantly. A remarkable feature of Yoshifuji's catalyst is its thermal stability, up to 100 °C no tendency for decomposition was noticed 43…”