Niobium complexes protonation

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

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]. 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Some researchers investigated multiphase additives. Kojima et al. [101] ball-milled MgHj with a nano-Ni/Al Oj/C composite catalyst. The mixture decomposed at a really low temperature of 200°C but in vacuum. A complex composite catalyst BCN/Ni/Pd/SWNT (where BCN is barium-calcium niobium high-temperature proton conducting compound and SWNT is single-wall-nanotubes) was also used by Yoo et al. [102] high-temperature proton conducting for ball-milled MgH. It desorbed 3 wt.%H2 in about 3,600 s at 230-250°C in vacuum. 

Winter and co-workers used the proton NMR spectra of niobium(v) complexes with 5-phenyltetrazolate ligand recorded at temperatures between —80 and +40°C in toluene-structural analysis <2001IC6451>. One- and two-dimensional 111 NMR methods in solution and solid state were used for analyzing the structure of irbesartan 83 <1998J(P2)475> as well as for the analysis of the structure of coordination compounds of ruthenium(n) and iron(n) with 5-aryltetrazolates <2002OM3774, 2003JOM(669)135, 2006IC695>. 

Complex formation between Nb(OH)s and methylamine has been established582 by an n.m.r. study. Two signals are observed in the n.m.r. spectrum of an aqueous solution of methylamine containing Nb(OH)5. One of these is due to the methyl protons of the amine and the other to the rapidly exchanging protons of the OH, NH2, and H20 groups. The i.r. spectra of solutions and precipitates formed by Nb(OH)5 and methylamine or monoethanolamine have been interpreted583 in terms of Nb—N co-ordination. Niobium oxohalide complex formation with tri-n-octylamine has been studied by reverse ebullioscopy.584... 

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). 

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