Imido complexes defined

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. 

The development of well-defined ROMP initiators has expanded the scope of the metathesis reaction, permitting the controlled polymerisation of highly functionalised monomers. Schrock s well-defined olefin metathesis initiators became available in the mid 1980s and they were based on alkoxy imido complexes of tungsten and molybdenum of general... 

The first well-defined, stable d°-alkylidene species having catalytic activity in alkene metathesis was Ta(=CHtBu)(OtBu)2Cl(PMe3) (5) [16]. However, research efforts moved rapidly to Mo and/or W alkylidenes, first in the form of 0X0 complexes (6) [17, 26], and then with the more easily accessible, imido bisalkoxy alkylidene complexes of general formula M(=NR )(=CHR )(OR )2 (7) [18, 19, 27], as well as the isoelectronic Re-alkylidyne-alkylidene complexes Re(=CR )(=CHR )(OR )2 (8) [28]. In 2001, it was shown that the silica-supported, Re-alkylidyne-alkylidene complex (=SiO)Re(=CtBu)(=CHtBu)(CH2tBu) (12) was unexpectedly very active [21,29]. According to computational studies, which will be described later in this chapter, the remarkable reactivity of this silica-supported catalyst arises from the different nature of the X and Y ligands (referred to as dissymmetric complexes) [7]. In addition, new molecular precursors based on bis-alkyl and bis-pyrrolyl M(=NR )(=CHR )(X)2 complexes became available [30]. Consequently, from 2005, considerable research effort has been devoted toward generating silica-supported (14 to 16 in Scheme 6.4) [23, 31, 32] and molecular (17 and 18 in Scheme 6.4) [22, 33, 34] Mo- and W-imido complexes... 

The direct use of ammonia or dinitrogen and hydrogen to synthesize imido amido complexes is essentially pioneers territory [58, 59]. Recently, we have formed, by surface organometaUic chemistry, a well-defined Ta amido imido species by reaction with either ammonia [9] or with dinitrogen and dihydrogen at subatmospheric pressure [60]. 

The organometaUic starting reagents are the MCM-41 supported ](=SiO)2TaH] and ](=SiO)2TaH3] described in the previous section. The MCM-41 supported hydrides cleave N-H bonds of ammonia at room temperature to yield the weU-defined imido amido surface complex ](=SiO)2Ta(NH)(NH2)] [9]. Dihydrogen is released in the gas phase during the reaction (Scheme 2.20). 

Organohydrazine derivatives play an important role in the coordination chemistry of rhenium since they represent important intermediates in the preparation of nitrido, imido, or dinitrogen complexes as has been described in the previous sections. They are defined as complexes which possess metal-nitrogen-nitrogen bonds, which would (strictly applied) also include dinitrogen complexes, and indeed this class has attracted considerable attention in the exploration of biological nitrogen fixation. 

The coupling of alkynes, an old field of polymer chemistry, is on new tracks resulting from defined metal-alkyl (39) and metal-carbene (40) catalysts [73, 74] previously simple metal halides (e. g. NbCls) of unspecified active catalyst structures were often employed. The polymer stereochemistry can be switched from 100% trans (R = t-Bu) to 100% cis (R = C(Cp3)2CH3) in the case of the imido(carbene) complex 40 upon making poly(2,3-bistrifluoromethyl)nor-bomadiene [74]. 

Though hexavalcnt technetium is known to be highly sensitive to oxydation and disproportionation, for instance in the case of tetraoxotcchnetale(Vl) [30, numerous complex compounds of rc(VI) were synthesized and identified and proved to be rather stable under normal conditions. Imido and predominantly nitrido ligands frequently act in mono- and dinuclear complexes as strong stabilizing Tr-clcctron donors [31]. In fluoro complexes the coordination numbers 7 and 8 are verified. In addition catccholato and thiolato ligands form well defined compounds. EPR spectroscopy was shown to be an efficient tool to characterize the complexes. 

Well defined high valent Schrock type carbene complexes of Mo or W with imido ligands are nowadays favoured as very active and selective catalysts for Ring opening Metathesis Polymerization (ROMP) and Acyclic Diene Metathesis Condensation (ADMET). Low valent Fischer type carbene complexes are less active and often need cocatalysts to enhance activities [1]. 

Undoubtedly the most active homogeneous catalyst systems are the well-defined metal alkylidene complexes synthesised in the laboratories of Schrock, Basset and Grubbs. First examples are the complexes I [29] and II [30]. The bulkiness of imido and aryloxide ligands probably slows down dimerization of these electron-deficient organometallic complexes to inactive complexes and prevents to some extent the coordination of the functional group to the tungsten atom [37]. 

Schemes 6.3 and 6.4) [3-5, 7-9, 11, 35-44]. Other studies focused on the classical heterogeneous catalysts, such as M0O3 [6, 10, 45-54] and Re20y supported on silica or alumina [55-59]. We will first focus on the studies of well-defined complexes of general formula M(ER )(=CHR )(X)(Y) (M = Re, Mo, or W ER = alkylidyne, imido, or 0x0 X and/or Y = alkoxy, siloxy, alkyl, or pyrrolyl). Afterwards, we will summarize the contributions of classical heterogeneous catalysts with the aim of highlighting the differences and similarities with the well-defined catalysts. 

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