Synthesis of cis- and trans-ReOMe2(bipy)Cl and related cationic oxorhenium(V) complexes

Abstract

The purpose of this study was to prepare high oxidation-state rhenium oxomethyl complexes, including cationic compounds, and test selected examples for olefin polymerization activity. Reaction of ReOMe₂(PMe₃)₂Cl with pyridine N-oxide and 2,2′-bipyridine (bipy) gave cis-ReOMe₂(bipy)Cl. Conversely, treatment of ReO(bipy)Cl₃ with AlMe₃ gave trans-ReOMe₂(bipy)Cl. The compound cis-ReOMe₂(bipy)Cl isomerized quantitatively to trans-ReOMe₂(bipy)Cl upon heating in benzene (ΔH ^‡ = 29(3) kcal mol^–1, ΔS ^‡ = 6(9) e.u.; at 79 °C, k_obs = 1.34(9) × 10^–2 min^–1 and t_1/2 = 52 min). Reaction of cis-ReOMe₂(bipy)Cl with AgX (X^– = PF₆^–, BF₄^–, or BPh₄^–) in acetonitrile gave the cationic acetonitrile adducts [ReOMe₂(bipy)(CH₃CN)]X. Treatment of the PF₆^– salt with PR₃ (PR₃ = PMe₃, P(OMe)₃, PMe₂Ph, P(n-Bu)₃, and PMePh₂) yielded the cationic PR₃ adducts cis-[ReOMe₂(bipy)(PR₃)][PF₆]. The BF₄^– salt of the PMe₃ adduct, cis-[ReOMe₂(bipy)(PMe₃)][BF₄], was prepared by the reaction of ReOMe₂(PMe₃)₂Cl with [Ag(CH₃CN)₂][BF₄] in acetonitrile followed by treatment with bipy in CH₂Cl₂. Single crystal X-ray crystallographic analyses of cis-ReOMe₂(bipy)Cl, trans-ReOMe₂(bipy)Cl, and cis-[ReOMe₂(bipy)(PMe₃)][BF₄] revealed that they have distorted octahedral structures. Analogs of cis-ReOMe₂(bipy)Cl, trans-ReOMe₂(bipy)Cl, and cis-[ReOMe₂(bipy)(PMe₃)][BF₄] were also prepared in which the bipy ligand was replaced with 1,10-phenanthroline (phen). An additional analog of cis-[ReOMe₂(bipy)(PMe₃)][BF₄] was prepared in which the bipy ligand was replaced by 4,4′-dimethyl-2,2′-bipyridine. In preliminary experiments, ethylene was polymerized using both cis-ReOMe₂(bipy)Cl and trans-ReOMe₂(bipy)Cl as catalyst precursors.