S. J. McLain

Tantalacyclopentane complexes and their role in the catalytic dimerization of olefins

-Disubstituted and cs,t3-disubstituted tantalacyclopentane complexes are intermediates in the selective catalytic dimerization of RCH=CH2 (R = Me, Pr, CH2CHMe2, CH2CMe3) to a mixture of the tail-to-tail and the head-to-tail dimers, respectively. Deuterium labelling studies show that neither dimer forms by reductive elimination from an alkenyl hydride intermediate directly. The most satisfactory explanation is that the tantalum hydride adds back to the alkenyl double bond to give a tantalacyclobutane complex which then rearranges to the olefin. The fact that formation of the metallacyclobutane ring is probably a relatively slow step of the reaction can explain why the type of dimer changes from nearly exclusively the tt-dimer when R = Me to exclusively the ht-dimer when R = CH2CMe3. INTRODUCTION Tantalacyclopentane complexes were first discovered as products of the reaction of TaCp(CHCMe3)C12 (Cp = 5-C5H5) with olefins (Ref. 1). An example is shown in equation 1. TaCp(CHCMe3)C12 + 3MeCH=CH2 Cl2CpTa + (1) The intermediate in this reaction is believed to be a tantalacyclobutane complex (2; equation 2) which rearranges selectively to a 2,4,4-trimethyl-l-pentene complex. 2,4,4-Trimethyl-l+MeCHCH2 +MeCH=CH2 Cl2CpTa )—_ C12CpTa-II Cl2CpTa4j, 1 (2) pentene must be displaced by propylene to give a propylene complex which then reacts with another equivalent of propylene to give 1. Excess propylene is dimerized primarily to 2,3dimethyl-l-butene by 1 but the catalytic activity is relatively short-lived (ca. 20 turnovers). We have since found that the analogous ii5-C5Me5 system (Cp = n5-C5Me5Tis wellbehaved and amenable to more detailed study. RESULTS AND DISCUSSION An ethylene or propylene complex analogous to that shown in equation 2 can be prepared as shown in equation 3 (Ref. 2). The propylene complex is especially useful for preparing other LiCp 1⁄2Zn(CH2CH2R)2 Ta(CH2CMe3)Clk —p TaCp(CH2CMe3)C13 • TaCp(CH2CMe3)(CH2CH2R)Cl2 (3) R=HorMe + -CMek TaCp (RCH=CH2 ) Cl2 Author to whom correspondence should be sent.

EXAFS study of nickel tetracarbonyl and nickel clusters in zeolite Y

Adsorption and thermal decomposition of Ni(CO)4 in the cage system of zeolite Y nhave been studied with EXAFS, electron microscopy and IR spectroscopy , Ni(CO)4 nis adsorbed as an intact molecule in both cation - free zeolite Y and NaY. Symmetry nchanges of the molecule in NaY are assigned to the formation of Na—OC-IMi bridges. nThermal treatment of the Ni(CO)4/NaY adduct leads to loss of CO concomitant with nthe formation of a binodal Ni phase. A major part of the forms clusters with ndiameter between 0.5 and about 1.5 nm, in addition to larger crystallites n(5-30 nm), sticking at the outer surface of the zeolite matrix., nThe Ni-Ni scattering amplitude indicates increasing average particle size with nincreasing temperature.