Unsymmetrical Zirconacyclopentadienes from Isolated Zirconacyclopropenes with 1-Alkynylphosphine Ligands

The reaction of one equiv of 1-alkynylphosphines, R₂PCCR′ (R = Et, ⁱPr, or Ph and R′ = Ph or Mes), with Cp₂Zr(pyr)(η²-Me₃SiCCSiMe₃) resulted in formation of monoalkyne complexes. In the case where R = Et, ⁱPr, or Ph and R′ = Ph, a “ligand free” zirconacyclopropene complex is produced. These complexes are stabilized by intermolecular donation of the phosphorus lone-pair in the dimeric complexes [Cp₂Zr(η²-R₂PCCPh)]₂ (R = Et, ⁱPr, or Ph). However, with R = Ph and R′ = Mes, the zirconocyclopropene-pyridine complex Cp₂Zr(pyr)(η²-Ph₂PCCMes) is formed. Homocoupling of the 1-alkynylphosphines was demonstrated by reaction of a second equiv of Ph₂PCCPh with [Cp₂Zr(η²-Ph₂PCCPh)]₂ to give the diphosphinozirconacyclopentadiene Cp₂Zr[2,5-(Ph₂P)₂-3,4-Ph₂C₄] with high regioselectivity (77%). The zirconacyclopropene complexes also react with one equiv of PhCCPh or EtCCEt to give zirconacyclopentadienes in which the phosphino substituent preferentially adopts the 2-position (α) of the zirconacyclopentadiene ring. These unsymmetrical zirconacyclopentadienes undergo substitution of the R₂PCCR′ moiety with the less bulky alkynes PhCCPh or EtCCEt. The substituents on the 1-alkynylphosphines significantly influence the rates of alkyne substitution such that sterically more demanding substituents in either the α- (-PⁱPr₂) or β- (-Mes) position of the zirconacycle lead to faster exchange. The α-phosphinozirconacyclopentadienes were readily converted to 1-phosphinobutadienes via reaction with benzoic acid. The zirconacyclopentadiene Cp₂Zr[2-Ph₂P-3,4,5-Ph₃C₄] was converted to the corresponding thiophene oxide by the oxo-transfer reaction with sulfur dioxide. In the case of ((1Z,3E)-3-ethyl-2-phenylhexa-1,3-dienyl)diphenylphosphine and (3,4,5-triphenylthiophen-2-yl oxide)diphenylphosphine, the molecules were isolated as their phosphine oxides. Reactions of the zirconacyclopropene complexes [Cp₂Zr(η²-Ph₂PCCPh)]₂ and Cp₂Zr(pyr)(η²-Ph₂PCCMes) with the diyne (F₅C₆)CC−1,4-C₆H₄−CC(C₆F₅) gave bis(zirconacycle)s terminated with phosphino groups. These bis(zirconacycle)s were converted to the corresponding phosphino-terminated oligomers by protonolysis with hydrochloric acid. In addition, the Ph₂PCCMes moiety of Cp₂Zr[2-Ph₂P-3-Mes-4-(C₆F₅)C₄]−1,4-C₆H₄−Cp₂Zr[2-Ph₂P-3-Mes-4-(C₆F₅)C₄] was exchanged with PhCCPh to give the phenylene(zirconacyclopentadiene) Cp₂Zr[2,3-Ph₂-4-(C₆F₅)C₄]−1,4-C₆H₄−Cp₂Zr[2,3-Ph₂-4-(C₆F₅)C₄].