Tantalum complexes cyclopentadienyl

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

Note added in proof Very recent work by Schrock has extended this tantalum ylide chemistry very considerably and some cyclopentadienyl tantalum methylenes and benzylidenes could be isolated and fully characterized (82b). Among the new compounds the complex (C5H4CH3)2Ta(CH3)(CH2) is particularly noteworthy. Its crystal structure has been determined and the temperature dependence of its nmr spectra has been carefully investigated (17b). The ylidic carbon is in a trigonal planar configuration ... 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

Tetrachloro(cyclopentadienyl) complexes of niobium and tantalum, and their ring-substituted derivatives can be prepared by two general, methods. One method uses tin reagents of cyclopentadienes and the other uses trimethylsilyl reagents, which are reacted with niobium or tantulum pentachloride. Here the former method (Method 1) is described for the synthesis of Ta(ii -C5Me5)Cl4, and the latter (Method 2) for the synthesis of Nb(T -C5Me5)Cl4. 

Group 5 metal catalysts based on various cyclopentadienyl and dianionic mixed ligand and related complexes of tantalum (169,170), niobium (171,172), and vanadium (173,174) have also been described. 

In contrast to the tantalum compounds considered above, the niobium complexes can exist as conformers of different stability at low temperatures. For example, in the H NMR spectrum of complex 4, the intensity ratio between the signals of the conformers is about 3 1 [ 32 ]. The crystal structures of the bis-amido compound 5 and the methylamido compound 6 all show that the lone pair of the amido ligand lies preferentially in the equatorial binding plane of the cyclopentadienyl and imido ligands, a conformation due to the ir bonding interaction of the lone pair with the niobium centre. If rotation about the Nb-Namido bond becomes slow on the NMR time scale then two conformers will be observed, one with the amido proton oriented toward the cyclopentadienyl ring and one with it oriented away. It is supposed that the major conformer is that with the amido proton oriented toward the cyclopentadienyl ring, which minimizes the steric interactions present [32]. 

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