Coordinated dinitrogen

Molybdenum(0) also forms a variety of dinitrogen complexes (41), especially when there are phosphine ligands in the molybdenum coordination sphere (see Fig. 7c). This type of complex has been extensively studied because the coordinated dinitrogen is reduced to ammonia upon acidification. 

Dinitrog en Complexes. The relative iaertness of molecular nitrogen is well known, however, some Cp —Zr compounds coordinate dinitrogen and substantially iacrease its reactivity. The nitrogen molecule can be coordinated either ia a terminal position or as a bridge ia dimeric stmctures. 

The chemical reactivity of coordinated N2 has been extensively studied because of its potential relevance to the catalytic and biological fixation of N2 to NH3 (p. 1035). For other recent work on the reactions of coordinated dinitrogen see refs. 41-44... 

The bulky ruthenium TMP complex Ru(TMP) is very electron deficient in the absence of any coordinating ligand, and a tt-complex with benzene has been proposed. In fact, it readily coordinates dinitrogen, forming the mono- and bis-N adducts Ru(TMP)(N2)(THF) and Ru(TMP)(N2)2, - As a result, the use of the TMP ligand for careful stereochemical control of the chemistry at the metal center, which has been very successful for the isolation of elusive rhodium porphyrin complexes, is less useful for ruthenium (and osmium) because of the requirement to exclude all potential ligands, including even N2,... 

The Ge—N—N—W chain found in the complexes lrans- WI(NNGcR3 )(PR3 L]52 (4) results from the novel coupling reaction of a coordinated dinitrogen molecule with R3GeCl... 

Using protons from water (and an energy input) for reaction with metal-coordinated dinitrogen (the metalloenzyme nitrogenase)... 

Also, it is worth mentioning (a) the process of N=N triple bond cleavage by Mo(III) complex [6], and (b) the reactivity of coordinated dinitrogen with electrophiles, including the protons and organic free radicals [7], and coordinated [8] and free [9] dihydrogen. 

Below, we plan to discuss only the reactivity of the coordinated dinitrogen with dihydrogen molecules. 

Amongst the model compounds able to coordinate dinitrogen, some can coordinate one dinitrogen and some two dinitrogen molecules. The following coordination modes have been identified for such complexes. 

The N-N distance of the linearly coordinated dinitrogen molecule (1.12 A) is slightly longer than that of the nitrogen molecule in the gaseous state (1.10 A). 

Under the same experimental conditions, [Ru(NH3)5(N2)]2+ gives rise to a completely irreversible oxidation (Ep = +0.80 V), indicating that the trication undergoes a still faster loss of the coordinated dinitrogen.50... 

There are two types of interesting and important reactions involving the coordinated dinitrogen, its reduction to hydrazine and ammonia, and the reactions leading to carbon-nitrogen bond formation. 

The principal interest of diazenido complexes of molybdenum(II) is that they are formed by the alkylation or protonation of coordinated dinitrogen. They thus represent a unique group of... 

The base members of the series [Mo(N2H)X(dppe)2] (X = F or Br) were obtained initially by treatment of [Mo(N2H2)X(dppe)2]+ (themselves obtained from reaction of [Mo(N2)2(dppe)2] with HX) with exactly one mole of NEt3.138 If an excess of base is used, reaction under N2 regenerates the parent [Mo(N2)2(dppe)2]. They react with acid, primarily to regenerate the hydrazide(2 - ) complexes, and kinetic studies suggest that they are also formed as intermediates during the conversions of coordinated dinitrogen to hydrazide(2 - ).139... 

The other diazenido complexes of Mou are generally carbonyl species and almost all are phenyldiazenido complexes. None is obtained via coordinated dinitrogen. 

The first nickel(0) complex containing coordinated dinitrogen, [ Ni(PCy3)2 2N2], was prepared by Jolly and Jonas in 1968, as an unstable dark-red compound (equation 56). 5... 

Unusual and complicated dinitrogen complexes have been reported to be formed by reactions (61)237,23S and (62).239 In both complexes (57) and (58) the N2 molecules are bound in a side-on mode to nickel atoms. The N—N bond distances (135 pm and 136 pm, respectively) are the longest ones found in any complex with coordinated dinitrogen. 

Interest in these species stems largely from study of the mechanism of conversion of coordinated dinitrogen to hydrazine and ammonia. It seems abundantly clear that hydrazido(2—) species are involved in the conversion, in simple molecular systems at least, of N2 to NH3, and there is evidence to suggest their involvement enzymatically. 

Species not mentioned in this chapter include the coordination chemistry of dinitrogen, and of hydrazines, hydroxylamine and hydroxylaminato anions. The chemistry of coordinated dinitrogen has been extensively reviewed, and its protonation has been discussed in detail.1... 

The interest in these ligands has stemmed largely from their proven involvement in the chemistry of coordinated dinitrogen. However, they are of considerable intrinsic interest as their electronic flexibility permits them to bind to transition metals with a wide variety of bonding modes. 

Both protonated and alkyl-substituted hydrazido(2—) complexes are readily available from the protonation or alkylation of coordination dinitrogen (equations 163 and 164). 

These are not accessible from coordinated dinitrogen and the preparative routes involve alkylation (equation 172) or protonation (equation 173) of hydrazido(2—) precursors or direct use of the hydrazine with elimination of hydrogen halide (equation 174). 

The simplest member of this class is the NNH2 ligand, which, as mentioned above, is a crucial and persistent intermediate in the protonation of coordinated dinitrogen. Indeed it remains the only coordinated protonated nitrogen ligand identifiable between dinitrogen and ammonia.455,456 Spectroscopic studies have provided a detailed mechanistic picture of the protonation of [Mo(N2)2(depe)2] (M = Mo, W depe = Et2PCH2CH2PEt2).472... 

An interesting dichlorodiazomethane complex [WBr(NNCCl2)(dppe)2]Br has been obtained by reaction of [WBr(NNH2)(dppe)2]Br with chloroform in the presence of base and diphenyliodonium bromide.487 A mechanism involving free radicals has been postulated. If the reaction is carried out in organic solvents organonitrogen species are formed by attack of solvent radicals on the coordinated dinitrogen.487... 

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