Dinitrogen complexes tantalum

Fryzuk MD, Johnson SA, Patrick BO, Albinati A, Mason SA, Koetzle TF. New mode of coordination for the dinitrogen ligand formation, bonding, and reactivity of a tantalum complex with a bridging N2 unit that is both side-on and end-on.JAm Chem Soc. 

Parallel to the work of Bergman et al., Cummins et al. published a closely related system obtained from a vanadium(in) precursor and mesityl azide leading to a vanadium(V) diazenylimido complex, which loses dinitrogen already at room temperature. Interestingly, the loss of dinitrogen from the vanadium(V)diazenylimido complex follows second-order kinetics and crossover was found in a double labelling experiment, which is in contrast to the observations of Bergman et al. with tantalum complexes. 

The starting tantalum hydride(s) also cleave the dinitrogen triple bond at low pressure of hydrogen and at 250 °C to yield the surface Ta amido imido complex [(=SiO)2Ta(=NH)(NH2)] (Scheme 2.22) [60]. 

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. 

Compounds (17) and (19) are formally 14-electron species. By contrast, the /z-dinitrogen complexes [ M (N2)(Cp )2 2(t/-N2)] (M = Ti, Zr) are respectively 16-electron and 18-electron species, and the low oxidation state of the metal is preserved. The spectroscopic data also illustrate the differences between the tantalum and these Tin or Zr11 /4-dinitrogen compounds. The l5N spectrum of (16) shows a singlet at 414p.p.m.,306 a chemical shift comparable to that... 

Complexes of tantalum with dinitrogen have suddenly become relatively common. The initial synthesis was an indirect one, but direct reduction under N2 is also possible (334). 

An alternative route to tantalum dinitrogen complexes is shown in Eq. (2). 

Niobium and tantalum belong to the metals that are able to bind dinitrogen. The development of this area was first due to Schrock and coworkers, who in 1982 could obtain stable tantalum /X-N2 compounds in high yields by reducing Alkylidene complexes under dinitrogen at normal pressure.This route can be circumvented in a single pot synthesis by reacting the pentahalides with sUylated hydrazines (equation 12). 

Scheme 4 Mechanism of the production of bridging dinitrogen in niobium and tantalum dithiocarbamate complexes (R2dtc, etc. Omitted for clarity)...

A few derivatives of oxyanions have also been prepared protactinium (V) forms hexanitrato complexes, Pa(N03)e , by reaction of the hexachloro complex with dinitrogen pentoxide, in contrast to niobium and tantalum which, under similar conditions, yield only tetranitrato complexes, M0(N03)4 (35). Neptunium (V) nitrates, NPO2NO3 and Np0(N03)3 have also been reported 71). Protactinium (V) sulfato-and selenato complex acids, H3PaO( 804)3 and H3Pa0(Se04)3, have been obtained from aqueous solution (13), but no fully sulfated or sele-nated species have been recorded. 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

Reaction of Parahydrogen with a Tantalum-Dinitrogen Complex 448... 

Fryzuk MD, MacKay BA, Patrick BO. Hydrosilylation of a dinuclear tantalum dinitrogen complex cleavage of N2 and functionalization of both nitrogen atoms. Jz4m Chem Soc. 2003 125 3234-3235. 

Figure 71. A proposed mechanistic pathway for the generation of hydrazine upon protonation of tantalum dinitrogen complexes.

Kinetics and mechanisms of hydrazine-forming reactions, involving coupled redox and substitution processes, from dinitrogen-bridged dinuclear diethyldithiocarbamate complexes of niobium and tantalum have been reported and discussed. ... 

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