Studies on 1,2-phenylenedioxoborylcyclopentadienes and some of their metal (Ti, Zr, Fe) complexes

Various properties of 1,2-phenylenedioxoborylcyclopentadienes C5H4BO2C6H4-1,2 (≡C5H5BCAT), its 4-tBu (≡C5H5BCATt) and 3,5-tBu2 (≡C5H5BCATtt) analogues are reported. Under ambient conditions [Zr(η5-C5H4BCAT)2Cl2] (3a) in PhMe was unreactive towards ethene, but in the presence of a large excess of methylaluminoxane (MAO) it catalysed the conversion of propene into a low average molecular weight atactic polypropylene. Whereas various Ti(IV) complexes [Ti(η5-C5H4BX)Cl3] [BX = BCAT (2a), BCATt (2b), BCATtt (2c)] did not react under these conditions with C2H4, a number of corresponding Zr and Hf analogues, as well as the related metallocene dichlorides, afforded polyethylene of moderate polydispersity and average molecular weight, but were less active than [ZrCp2Cl2]. From 11B NMR spectral data, it is concluded that the product of treating 3a with MAO is possibly [Zr(η5-C5H4BMe2)2Cl2]. 11B NMR chemical shifts on the three series of compounds Cl-BX (10a–10c), Me3SiC5H4–BX (1a–1c) and 2a–2c fall within the narrow range 27.2 (2a) to 29.5 (1a) with 10a at 29.0. Cyclic voltammetry on a number of the metal complexes, including 2a–2c, [Zr(η5-C5H4BCATt)2Cl2] (3c) and [Fe(η5-C5H4BCATt)2] (9), showed three reduction waves, but only the first of these was reversible for 3c. For 2a–2c there was evidence for formation of successively [2]− and solvated TiCl3 (together with oxidation of the anion [C5H4BCAT]−); support for these Ti(III) species came from the EPR spectra of the in situ generated species obtained by the controlled potential electrolysis of 2a and comparison with similar data derived from [Ti{η5-C5H3(SiMe3)2-1,3}2Cl2]; while from 3c, the formation of [3c]− is inferred.

17 Compounds, including 2a, 3a and 9 have been studied with respect to (i) (modest) catalytic activity for C2H4 or C3H6 polymerisation, and (ii) the electronic effect of the boryl substituent (using 11B NMR spectroscopy, cyclic voltammetry and EPR spectra of reduced species). (BCAT = BO2C6H4-1,2, BCATt = the 4-tBu analogue).Figure optionsDownload as PowerPoint slide