Doubly [SiMe 2 ]-bridged metallocenes (1,2-SiMe 2 ) 2 {η 5 -C 5 H 2 -4-R}{η 5 -C 5 H-3,5-(CHMe 2 ) 2 }ZrCl 2 (R ) H (1a), CHMe 2 (1b), SiMe 3 (1c), CHMe(CMe 3 ) (1d), (+)-menthyl (1e)), when activated by methylaluminoxane (MAO), catalyze propylene polymerization with high activities. The preparations and X-ray structures of the dilithio salt of an enantiopure, doubly silylene-bridged bis(cyclopentadienyl 1e), are reported. The C ssymmetric systems 1a-c are highly regiospecific and syndiospecific (>99.5%) in neat propylene. At lower propylene concentrations, polymers with lower molecular weights and tacticity (mostly m-type stereoerrors) are obtained. The microstructures of polymers produced under differing reaction conditions are consistent with stereocontrol dominated by a site epimerization process, an inversion of configuration at zirconium resulting from the polymer chain swinging from one side of the metallocene wedge to the other without monomer insertion. The relative importance of chain epimerization (at the β carbon) has been established by parallel polymerization of 2-d 1 -propylene and d 0 -propylene with 1b/MAO at low propylene concentrations. The C 1symmetric systems 1d,e/MAO display an unusual dependence of stereospecificity on propylene concentration, switching from isospecific to syndiospecific with increasing propylene pressure, consistent with a competitive unimolecular site epimerization process and a bimolecular chain propagation. The microstructures of the polypropylenes produced by 1d/MAO and 1e/MAO with [r] ≈ 50% resemble the hemiisotactic microstructure produced by Me 2 C(η 5 -C 5 H 3 -3-Me)(η 5 -C 13 H 8 )ZrCl 2 (2b)/MAO. Contrastingly, the hemiisotactic polypropylene microstructure obtained with 2b/MAO is found to be maintained at all propylene concentrations examined.

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The reduction of [Fe(CO),-L,X,] (L=P(OMe),, X = Br (la), I (lb); L = P(OiP&, X = Br (2a), I (2b); L = PEt,, X = Br (3a), I (3b)) with Zn in dioxane (la,b-2a,b) or PhLi in ether (3a,b) led to formation of the corresponding dicarbonyl(halo)bis(phosphorus donor)iron(I) complexes (L = P(OMe),, X = Br (4a), I (4b); L = P(OiPr),. X = Br (Sa), I (5b); L = PEt,, X = Br (6a), I (6b)). Slightly contaminated 5a,b and pure 6a,b were isolated as stable crystalline blue or blue-green complexes. Complexes 4a,b were obtained, with only minor impurities, by cornproportionation of 1 a,b and the dicarbonyl(ha1o)bis-(trimethy1phosphite)ferrate anions 7a,b and characterized in THF solution. The comproportionation products 5 a,b-6 a,b were obtained in high yields by reaction of 2a,b-3a,b with the dinitrogen complexes 13-15. Further reduction of 4a,b-6a,b or exhaustive reduction of 1 a,b-3a,b with sodium amalgam or iBuLi in THF afforded the nonisolable dicarbonyl-(halo)bis(phosphorus donor)ferrate(o) anions (7a,b-9a,b). The latter were characterized by acidification with trifluoroacetic acid or acetic acid yielding stable Results and DiscussionThe reduction of complexes 1 a,b-2a,b with excess Zn in dioxane and of complexes 3a,b with three equivalents of phenyllithium in ether proceeded with loss of one halogen substituent to afford the paramagnetic blue or green 17-electron iron@) compounds 4a,b-6a,b (Scheme 1). In the reduction of la,b with Zn, some [Fe(CO),{ P(OMe),} ,] and [ Fe(CO), { P(OMe),} 3]were isolated as by-products, in addition to 4 a,b.["] While 5 a,b could be isolated in analytically pure form, 6a,b were characterized as spectroscopically pure compounds.When sodium amalgam in THF was used as the reducing agent and the reaction temperature was kept below -30 "C, the Chem. Eur. J. 1995, I . No. 8 Q VCH Verlagsgesellschafi mhH, 0 6 9 4 5 1 Wernheim. 1995 094?-6539/95/0108-0541 S 3O.W+.25/0

The complexes of the type [ReH(CO)5 -"(PMe,),,] (n = 2 , l ; n = 3, 2; n = 4,3) were reacted with aldehydes, C02, and RC=CCOOMe (R = H, Me) to establish a phosphine-substitutional effect on the reactivity of the Re-H bond. In the series 1-3, benzaldehyde showed conversion with only 3 to afford a (benzy1oxy)carbonyltetrakis(trimethy1phosphine)rhenium complex 4. Pyridine-2-carbaldehyde allowed reaction with all hydrides 1-3. With 1 and 2, the same dicarbonyl[(pyridin-2-yl)methoxy-O,N]bis(tnmethylphosphine)rhenium 5b was formed with the intermediacy of a [(pyridin-2-yl)methoxy-O]-ligated species and extrusion of CO or PMe,, respectively. The analogous conversion of 3 afforded the carbonyl[(pyridin-2-yl)methoxy-O,N]tris(trimethylphosphine)rhenium(I) 7b. While 1 did not react with CO,, 2 and 3 yielded under relatively mild conditions the formato-ligated [R~(HCOZ)(CO)(L)(PM~,)~] species (8 (L = CO) and 9 (L = PMe3)). Methyl propiolate and methyl butynoate were transformed, in the presence of 1, to [Re{C(C02Me)=CHR}(CO)3(PMe3)J systems (10a (R = H), and 10b (R = Me)), with prevailing a -metallation and trans-insertion stereochemistry. Similarly, HC=CC02Me afforded with 2 and 3, the a metallation products [Re{C(C0,Me)=CH2}(CO)(L)(PMe3)3] 11 (L = CO) and 12 (L = PMe,). The methyl butyonate insertion into 2 resulted in formation of a mixture of the (2)-and (E)-isomers of 4 PMe, 4 Me3P Me3P PMe, 1 2 3 stereoisomers which display hydride/PMe, cis-and hydride/CO trans-configurations. The syntheses of 2 and 3 were recently reported [5], and 1 was obtained in high yield by reduction of [ReCl(CO),(PMe,),] with Na in THF and subsequent acidification with H,O.

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