Diphospholyl and triphospholyl zirconium π-complexes: Ziegler–Natta oligomerization catalysts and reactive intermediates in P–C cage formation by hydrolysis

Abstract

2,4,5-Tri( t-butyl)-1,3-diphospholyl sodium salt ( 2a) and 3,5-di( t-butyl)-1,2,4-triphospholyl sodium salt ( 3a) react with ZrCl 4 and CpZrCl 3 to form (η 5-2,4,5-tri( t-butyl)-1,3-diphospholyl)ZrCl 3 ( 4), Cp(η 5-2,4,5-tri( t-butyl)-1,3-diphospholyl)ZrCl 2 ( 5), and Cp(η 5-3,5-di( t-butyl)-1,2,4-triphospholyl)ZrCl 2 ( 6) in the complete absence of water, respectively. Surprisingly, bent sandwich complex 6 exhibits the NMR spectroscopic characteristics of a hindered ring ligand rotation but the cyclopentadienyl ligand of 5 rotates freely in the NMR time scale in spite of the superior space demand of its 2,4,5-tri( t-butyl)-1,3-diphospholyl ligand. This contradiction is discussed on the basis of an attractive interligand P–C interaction between homo- and heterocyclic π-ligands, which is stronger for 6. Water even in trace amounts present in the reaction mixture changes the course of the reactions completely. No zirconium π-complexes are accessible this way, but only oligo- or polycyclic organophosphorus compounds. Compound 3a and ZrCl 4 form the asymmetric tricyclic P 6( t-BuC) 4H 2 isomer 8 with two P C double bonds as a dimer of 1,2,4-triphosphol. It is accompanied by small amounts of HCl addition product 9, where one of the P C double bonds is eliminated. Compound 8 contains six, and 9 eight stereogenic centers, but both form one pair of enantiomers each only. A single stereoisomer of P 6( t-BuC) 4H 2 cage 10 is formed if (1-trimethylstannyl)-3,5-di( t-butyl)-1,2,4-triphosphol 3b is used as the P 3( t-BuC) 2 source in combination with Cp ∗ZrCl 3 in the presence of trace amounts of water. Compound 10 is a meso-compound, which is composed by dimerization of either two homochiral units of ( R)-3,5-di( t-butyl)-1,2,4-triphosphol 3c R or by two units of its S-enantiomer 3c S. No trace of cyclic addition products has been identified, which would represent the heterochiral combination of enantiomers 3c R + 3c S. P 2- and P 3-zirconocene dichloride derivatives 5 and 6 have been tested as Ziegler–Natta alkene oligomerization catalysts. Both are catalytically active with 1-hexene as the substrate, but cannot compete with the activity of the phosphorus-free original catalyst Cp 2ZrCl 2.