Bent metallocene

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

Prior to this 1996 study, there had been no reports of boratabenzene complexes of early transition metals.42 An X-ray crystal structure of the catalyst revealed a C2-symmetric geometry that resembles Cp2Zr-based bent metallocenes. The bond lengths suggest a strong B-N it interaction (rotational barrier measured by NMR 18 kcal/mol) and a very weak Zr B interaction ( t 5 coordination of the boratabenzene ring). 

FIGURE 14.1 (See color insert following page 302.) Frontier molecular orbitals of (a) bent metallocenes (Cp2M M = Ti, Zr) and (b) carbene. 

Triple-decker sandwiches that are capped at one or both ends by C2B3 rings are important synthons for the construction of larger stacks via coordination to metal ions (see the following subsection), and many of these have been synthesized and their reactivity investigated.75,78 Bent triple-deckers such as 6879,80 that resemble bent metallocene complexes have been prepared, structurally characterized, and shown to be amenable to the introduction of halo- and organosubsitutuents at boron.80... 

The binary borohydride species Zr( III 11)4 and U(BH4)4 have been investigated by quantum mechanical techniques and, for the zirconium case, also by gas-phase electron diffraction. All confirm that these simple molecules have a staggered conformation of borohydride ligands.15 In a related study, the hafnium analog Hf(BH4)4 has also been analyzed and is essentially isostructural.16 These studies show the molecules to possess tetrahedral symmetry with all of the BH4 ligands triply (i.e., if) bridging. Photoelectron spectra [He(i)] of the half and bent metallocene complexes Zr(7]S-CsHs)(BH4)4, M(7]S-CsHs)2(BH4)2 (M = Zr, Hf), and Ta(7]S-CsHs)2(BH4) have been determined.17... 

Figure 2. Contour diagram in the yz plane of the three most important molecular orbitals of the generic d° bent metallocene Cp Mt of Gi symmetry. Solid and dashed lines correspond to positive and negative contour of the wave function.

The benzyl derivative Cp Ta( j -02)Bn was structurally characterized by X-ray diffraction the 02-ligand is side-on coordinated and lies in the equatorial plane of the bent metallocene fragment. The 0-0 distance of 1.477(8) A and the 0-0 stretching frequency (vo-o = 863cm ) are consistent with a peroxo hgand (02 ) coordinated to tantalum in its highest possible formal oxidation state (+V). Notably, base appears to stabihze these complexes i.e., in the presence of triethylamine Cp Ta( 7 -02)Me did not decompose even when heated to 80 °C. 

There are very few examples of silicon-bridged bent-metallocenes of type 42 in Scheme 14 described in the literature. The class of silicon-bridged [l]-ferrocenophanes is most important due to the exceptional structure and reactivity, and representative examples 43-52 are listed in Scheme 15. 

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

The organosamarium(II) complex Sm( / 5-CsMc5)2 has the bent-metallocene structure, which can be attributed to polarization effects its THF solvate Sm( / 5-C5Mc5)2(TI If )2 exhibits the expected pseudo-tetrahedral coordination geometry. Sm( j5-C5Me5 )i] is a very powerful reductant, and its notable reactions include the reduction of aromatic hydrocarbons such as anthracene and of dinitrogen, as illustrated in the following scheme ... 

We have recently used a stereochemical trick to solve this pending structural problem. The anellation of the chiral (.v-/ruH.v-r 4-C4H6)IV1 subunit with a chiral bent metallocene leads to the formation of a pair of diastereomeric (s-/nmv- b u tad i e n e) m e t a 11 oce ne complexes. Crystals of a single diastereomer were obtained that were free from the otherwise so often observed disorder phenomena. The actual example was prepared by... 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

In most cases that were reported in the literature so far, the (s-cis-ri4-diene)metallocene complexes (and their s-trans- counterparts), were synthesized by treatment of the respective bent metallocene dichloride precursor with a suitable conjugated-diene-magnesium reagent. However, in a few cases alternative entries and procedures have been developed and described. None of these has found such a widespread application as the original procedure, but some interesting chemistry has evolved among those methods. 

Even non-conjugated di-olefins may be used in these systems as precursors for the (conjugated diene)metallocene synthesis. They become isomerized under the reaction conditions at the bent metallocene system.81 Even vinylcyclopropanes yield the corresponding (l,3-diene)metallocene complexes when treated with the reagent 71. The three-membered ring is readily opened under the typical reaction conditions.82... 

The (s-cA-butadiene) Group 4 metallocenes adopt a a2,7i-type structure. The actual strength of the n-bonding component and, hence, the metallacyclopentene character of the complexes depends very much on the substitution pattern of the diene ligand6 and it is also strongly influenced by the nature of the bent metallocene unit. These various influences were recently analyzed for some ansa-metallocene/ 1,3-diene combinations by means of computational chemistry,83,84 and the results were compared with the dynamic features (AGfw of the ring-flip inversion process, solid... 

Once the butadiene/metal-carbonyl/ketone (or aldehyde or nitrile) template coupling at the Group 4 bent metallocene unit has been carried out and subsequent modifications have been made, the corresponding... 

It is likely that the stereochemically decisive influence between the respective substituents at the front of the bent metallocene wedge takes place in the actual transition state (155//) of the insertion. By a detailed kinetic study using the unique chemical and spectroscopic features of the 128 —>-148 transformation we could ascertain that the actual... 

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