Metal-N2 complexes

Since this time, more detailed studies indicate that metal-N2 complexes indeed are formed in these reactions. Examples have been isolated by careful adjustment of conditions. That these reactions proceed to near-stoichiometric production of NH3 and that the bis-r/5-cyclopentadienyl titanium core (and its pentamethyl-substituted analogue) limits the coordination possibilities at the metal held promise of relatively easy determination of both the structure of the com-... 

Figure 2. Structures of some Group IV metal-N2 complexes (a) [rf-C5Me5)2Zr(N2)]2(N2) as determined in Ref 30 (b) [(rf-C5Me5)2Ti 2(N2) as determined in Ref 31, compare with the suggested structure in Figure la (c) probable structure of [(yf-C5H5)2Ti-ii-(irf,rf-C5H4)Ti(if-C5H5)]2-(N2) based on the known structure of (rf-C5H5)3(r)t rf-C 5H4)Ti2, THF as...

The side-on -coordinated N2 attains a nitride-like structure and is comparatively reactive. Thus, the formation of a side-on metal N2-complex in the nitrogenase is a reasonable way for N2-activation (for rev. see Ref.170)), but it should be mentioned that end-on -complexes have also been discussed as models171). Recent experiments showed170) that side-on -metal N2-complexes built up on reduced oxo-molybdenum in the valence state +4 and coordinated by cystein lead to diimid-for-mation followed by a rapid disproportion or decomposition of diimid and subsequent reduction of the generated hydrazine to NH3. The reaction sequence is given by ... 

A surprising variety of reagents is capable of generating metal-N2 complexes. Within six months of the first report of such a compound, most of the common reagents now used were known to be eflFective. In chronological order, the early reagents used were hydrazine hydrate (J), the azide ion (2), acyl azides (10), and nitrogen (N2) (ii). More recently, substituted hydrazines 12,13) and sulfonyl azide (14) have been used, as well as some more complex systems that will be described later. 

An example of a serendipitous discovery in a field related to diazo chemistry is the first in vitro product of a reaction of molecular nitrogen with a transition metal complex (Allen and Senoff, 1965). As discussed in the context of diazo-metal complexes (Zollinger, 1995, Sec. 3.3), the metal —N2 bonds are similar to C —N2 bonds in organic diazo compounds. The paradigm that N2 is (almost) inert in chemical reactions probably explains why it took so long for N2 complexes to be discovered. ... 

Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

Forming a hydrogen bond between (i )-homocitrate and His 442 could effectively release electron density into the cluster. Studies on structurally defined N2 complexes have shown that the binding of N2 to a metal site and its ability to be protonated are favored by electron-rich sites. Thus, it is postulated that the electron-richness of the... 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Figure 6.3 Energy changes for N2 reduction in the absence (top) or presence (bottom) of metal-sulfur complexes. (Adapted with permission from Figure 7b of Sellman, D., Sutter, J. Acc. Chem. Res., 1997, 30, 460-469. Copyright 1997, American Chemical Society.)...

Examples of the former criteria are as follows metal centers with a low electron-richness (high Eg) bind preferably ligands that are strong electron donors (low Pi values) coordination of N2 is favored by a high electron-richness (low Eg) and a high polarizability (P) of the metal center [10, 11]. Tr ns-[ReCl(N2)(dppe)2] is an example of a rather stable N2 complex with such features (the Eg and p values of the binding metal center are 0.68 V and 3.4, respectively [21]). 

A number of new metal-dinitrogen complexes of type M(N2) have been detected in the reaction of metal atoms with N2 under matrix isolation conditions. Most strikingly, nickel atoms and nitrogen give Ni(N2)4 (45). However, this is strictly a matrix species, and it decomposes on removal of the matrix gases even at very low temperatures. It appears to be stable up to -150°C isolated in an SF6 matrix. Mixed CO/N2 complexes have also been prepared in matrices but their stabilities outside the matrix are not reported (72). 

Another uncertainty lies in the mode of binding of N2 and other substrates. Does N2 bind end-on to Mo, does it slide between Fe atoms within the coenzyme, or does it bind in some other way While N2 is unre-active, it forms nitrides with metals and complexes with some metal chelates. These complexes are generally of an end-on nature, e.g., N=N-Fe. Stiefel suggested that N2 first forms a complex of this type with an iron atom of the MoFe-protein.44 Then an atom of Mo(IV) could donate two electrons to the N2 (Eq. 24-9, step a) to form a complex of N2 and Mo(VI). Addition of two protons (Eq. 24-9, step b) would yield a molecule of diimide, which would stay bound at the... 

As discussed earlier, the enzymic reaction catalyzed by glutamine synthetase requires the presence of divalent metal ions. Extensive work has been conducted on the binding of Mn2+ to the enzyme isolated from E. coli (82, 109-112). Three types of sites, each with different affinities for Mn2+, exist per dodecamer n, (12 sites, 1 per subunit) of high affinity, responsible for inducing a change from a relaxed metal ion free protein to a conformationally tightened catalytically active protein n2 (12 sites) of moderate affinity, involved in active site activation via a metal-ATP complex and n3 (48 sites) of low affinity unnecessary for catalysis, but perhaps involved in overall enzyme stability. The state of adenylylation and pH value alter the metal ion specificity and affinities. 

Extensive studies of the mechanism of formation of dialkylhydrazido(2—) complexes from [Mo(N2)2(dppe)2] (M = Mo, W) have revealed a free radial mechanism for addition of the first alkyl group with loss of one N2 molecule as the rate-determining step.484 Addition of the second alkyl group proceeds via an SN 2 mechanism (see Scheme 6) and the effect of the metal and coligands on reaction rates has been studied in detail.485 Acoyl and aroyl hydrazido(2 —) complexes can be obtained by reaction of the N2 complexes with acoyl or aroyl halides in the presence of acid (equation 177).486... 

---