Reactions with Monomeric Lewis Bases

Alt and Jung synthesized a wide variety of bridged metallocene dichloride complexes where the metal is Zr or Hf. These metallocene-containing units had a vinyl group appended to the bridging carbon that has subsequently polymerized giving a polymer with metallocene arms radiating from the central carbon-backbone core. One such structure is 8, where M=Zr and n= 1,3,5. 

The polymers containing this type of metallocene were activated with MAO and the PE polymerized. The general melting points for the PEs obtained ranged from a low of about 132°C to a high of about 140°C. Activities ranged from about 9 X 10 to 3 X 10 gPE/mmole Zrh for the catalyst systems. 

CONDENSATION REACTIONS WITH MONOMERIC LEWIS BASES 

Electronegative halides attached to the Cp2M metal centers can undergo condensation reactions. The metallocene dihalide acts as the Lewis acid. When the 

Lewis base is monofimctional tiie products will be monomeric, as shown later for a metallocene dichloride. 

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Condensation Reactions with Monomeric Lewis Bases 121... 

V. CONDENSATION REACTIONS WITH MONOMERIC LEWIS BASES... 

Complexes (1063) have been obtained by the reaction of Ni(CO)4 with monomeric Lewis base-stabilized aluminum pnictines [dmap-AlMe2-E(SiMe3)2] (E = P, As, Sb).2555... 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

The heterocycles can be cleaved by reaction with 4-(dimethylamino)pyri-dine, yielding Lewis base-stabilized monomeric compounds of the type dmap—M(R2)E(Tms)2 (M = Al, Ga E = P, As, Sb, Bi). This general reaction now offers the possibility to synthesize electronically rather than kinetically stabilized monomeric group 13/15 compounds. These can be used for further complexation reactions with transition metal complexes, leading to bimetallic complexes of the type dmap—M(Me2)E(Tms)2—M (CO) (M = Al, Ga E = P, As, Sb M = Ni, Gr, Ee). 

Neutral benzol,3,2-diazaphospholes or their tetrameric cycloaddition products react with hard Lewis acids to give N-coordinated Lewis acid-base complexes [13, 80, 81] this reaction can be used to disassemble the otherwise stable oligomers into monomeric units at ambient temperature. 

The coordination of the phosphine P(ft-Pr)2Ph to the Lewis acidic Ga center is essential for the synthesis of both compounds. In the absence of any Lewis base, the most likely reaction product would be the heterocubane [ClGaSbSi(/-Pr)3]4. However, in analogy to the results observed for reactions of heterocycles [R MER with Lewis bases, leading to base-stabilized monomeric compounds, both the formation of 84 and 85 can be explained by reaction of such a heterocubane intermediate with the phosphine base. According to the description of heterocycles as head-to-tail adducts, heterocubanes may be described as Lewis acid-base adducts between two four-membered rings as shown in Fig. 45. 

Mixed-valence compounds. The ground state of monomeric species RM (M = B, Al, Ga, In), which have an electron lone pair at the metal center, is singlet and the singlet-triplet energy gap increases with increasing atomic number. Consequently, these compounds are able to act as Lewis bases. Reactions with Lewis-acidic... 

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