Tantalum complexes organometallic

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

Mashima, K. Kaidzu, M. Tanaka, Y. Nakayama, Y Nakamura, A. Hamilton, J. G Rooney, J. J. Control of stereoselectivity in the ring-opening metathesis polymerization of norbornene by the auxiliary ligands butadiene and o-xylylene in well-defined pentamethylcyclopentadiene tantalum carbene complexes. Organometallics 1998,17,4183 195. 

Michiue K, Oshiki T, Takai K, Mitani M, Fujita T. Tantalum complexes incorporating tris(pyrazolyl)borate ligands syntheses, stmctures, and ethylene polymerization behavior. Organometallics. 2009 28 6450-6457. 

Galajov M, Garcia C, Gomez M, Gomez-Sal P, Temprado M. Synthesis and DFT, multinuclear magnetic resonance, and X-ray stmctural studies of iminoacyl imido hydridotris(3,5-dimethylpyrazolyl)borate niobium and tantalum(V) complexes. Organometallics. 2014 33 2277-2286. 

Hadi GAA, Fromm K, Blaurock S, Jelonek S, Hey-Hawkins E (1997) Organometallic tantalum complexes with phosphine, phosphanido and phosphinidene ligands. Syntheses... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

Mechanistically, the unusual reactivity of the starting tantalum hydrides [(sSiO)2Ta H] and [(=SiO)2Ta (H)3] towards ammonia to yield the amido imido complex [(=SiO)2Ta(=NH)(NH2)] can be fully rationalized in terms of classical molecular organometallic elementary steps. Scheme 2.21 offers an example of a such sequence of elementary steps with the noteworthy close analogy with methane activation by the same hydrides described above. 

In summary, the tantalum hydride system adds to the few previously reported well-defined organometallic complexes capable of cleaving N-H bonds of ammonia to yield either an amido or an imido complex, and achieves unprecedented dinitrogen N=N triple bond cleavage with dihydrogen on isolated tantalum atoms to yield reduction of both N atoms. 

We then studied group 5 metals, especially tantalum-for which the laboratory already had great experience. Because of the studied reaction, alkyl or hydride-type compounds such as those developed in the laboratory could not be employed. Consequently, we became interested in alkoxo-type derivatives, either synthesized by reaction of the grafted complex with an alcohol or obtained by direct synthesis starting from an alkoxy-tantalum compound grafted on silica. In all cases, resulting complexes have been characterized by surface organometallic chemistry techniques, especially EXAFS and solid-state NMR (ID and 2D with C-labeled compounds). Indeed various compounds bonded by one, two or three surface bonds have been prepared and characterized. 

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)... 

Metal-mediated Protein Modification Niobium Tantalum Organometallic Chemistry Polynuclear OrganometaUic Cluster Complexes. 

Carbonyl Complexes of the Transition Metals Mechanisms of Reaction of Organometallic Complexes Niobium Tantalum Organometallic Chemistry. 

---