A cationic methylpalladium complex bearing a diphosphinidenecyclobutene ligand generated by the treatment of the corresponding dimethyl complex 1 with H(OEt2)2[BAr4] (Ar=3,5‐(CF3)2C6H3), exhibits good catalytic activity for ethylene polymerization in solution (up to 128 kg h−1 (mol cat.)−1). The catalyst is thermally stable and promotes the polymerization even at 100 °C without notable decomposition.

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.

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