Unsymmetrical Zirconacyclopentadienes from Isolated Zirconacyclopropenes with 1-Alkynylphosphine Ligands

Abstract

The reaction of one equiv of 1-alkynylphosphines, R2PCCR′ (R = Et, i Pr, 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, i Pr, 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, i Pr, 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- (-P i Pr2) 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].

Coordination Behavior and Reactivity of α‐Heterosubstituted Trimethylsilylalkynes at Titano‐ and Zirconocene

“…This structural motif was also observed by Tilley et al in the reaction of R 2 P–C≡C–Ph (R = Et, i Pr, Ph) and [Cp 2 Zr] 20. The fact that titanium forms an isostructural complex―despite being much smaller―is not surprising since PMe 3 ‐stabilized titanacyclopropenes have long been known 21.…”

“…In alkyne 4 phosphorus is included as a heteroatom, since it has shown a good ability to coordinate to group 4 metallocenes 8,20. Methyl groups were chosen as substituents for the phosphorus atom to minimize steric repulsion at this center, possibly allowing it to coordinate intramolecularly to form the four‐membered heterometallacycle mentioned above (Scheme , C).…”

“…Tilley et al observed that the 13 C NMR spectroscopic signals of the alkyne moiety appear in two groups:20 the phenyl‐bound carbon at δ = 197.9–200.9 ppm and the phosphorus‐bound one at much higher field ( δ = 144.1–154.9 ppm). The same is found for complex 9b , even if the difference between the groups is much less pronounced [ δ = 174.2 (P– C ≡C) and 191.0 ppm (C≡ C –Si)].…”

Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes

“…In the reaction of Cp 2 Zr(py)(η 2 ‐btmsa) with Ph 2 P‐C≡C‐PPh 2 a dinuclear complex was formed. Such a type of complexation was described before by Tilley in various zirconocene complexes of mono‐phosphanyl‐substituted alkynes, which were also stabilized by intermolecular interactions of the lone pairs of the phosphorus atoms . The dimerization led to an increasing stability and it was impossible to obtain the monomeric complex by adding THF or pyridine.…”

“…Reactions of P‐substituted silyalkynes with the metallocene complexes Cp 2 Ti(η 2 ‐btmsa) and Cp 2 Zr(py)(η 2 ‐btmsa) give isostructural dimers which consist of two metallacyclopropene units with a dative bond of phosphorus of one metallacyclopropene to the metal of the other moiety (Scheme ) . Such compounds were observed before when using mono‐ and di‐phosphanyl‐substituted alkynes , …”