Side-on End-on Bound Dinitrogen: An Activated Bonding Mode That Facilitates Functionalizing Molecular Nitrogen

Abstract

Molecular nitrogen is the source of all of the nitrogen necessary to sustain life on this planet. How it is incorporated into the biosphere is complicated by its intrinsic inertness. For example, biological nitrogen fixation takes N(2) and converts it into ammonia using various nitrogenase enzymes, whereas industrial nitrogen fixation converts N(2) and H(2) to NH(3) using heterogeneous iron or ruthenium surfaces. In both cases, the processes are energy-intensive. Is it possible to discover a homogeneous catalyst that can convert molecular nitrogen into higher-value organonitrogen compounds using a less energy-intensive pathway? If this could be achieved, it would be considered a major breakthrough in this area. In contrast to carbon monoxide, which is reactive and an important feedstock in many homogeneous catalytic reactions, the isoelectronic but inert N(2) molecule is a very poor ligand and not a common industrial feedstock, except for the above-mentioned industrial production of NH(3). Because N(2) is readily available from the atmosphere and because nitrogen is an essential element for the biosphere, attempts to discover new processes involving this simple small molecule have occupied chemists for over a century. Since the first discovery of a dinitrogen complex in 1965, inorganic chemists have been key players in this area and have contributed much fundamental knowledge on structures, binding modes, and reactivity patterns. For the most part, the synthesis of dinitrogen complexes relies on the use of reducing agents to generate an electron-rich intermediate that can interact with this rather inert molecule. In this Account, a facile reaction of dinitrogen with a ditantalum tetrahydride species to generate the unusual side-on end-on bound N(2) moiety is described. This particular process is one of a growing number of new, milder ways to generate dinitrogen complexes. Furthermore, the resulting dinitrogen complex undergoes a number of reactions that expand the known patterns of reactivity for coordinated N(2). This Account reviews the reactions of ([NPN]Ta)(2)(mu-H)(2)(mu-eta(1):eta(2)-N(2)), 2 (where NPN = PhP(CH(2)SiMe(2)NPh)(2)), with a variety of simple hydride reagents, E-H (where E-H = R(2)BH, R(2)AlH, RSiH(3), and Cp(2)ZrCl(H)), each of which results in the cleavage of the N-N bond to form various functionalized imide and nitride moieties. This work is described in the context of a possible catalytic cycle that in principle could generate higher-value nitrogen-containing materials and regenerate the starting ditantalum tetrahydride. How this fails for each particular reagent is discussed and evaluated.

Theoretical Study on N-N Activation by Thiolate-bridged Dinuclear Dinitrogen Transition-metal Complexes

“…双核金属 配合物结合 N 2 的键型模式具有多样性, 以 end-on 键型 (端基配位键型), side-on 键型(侧基配位键型), 以及 side-on-end-on 键 型 ( 端 基 -侧 基 配 位 键 型 ) 为 主 要 形 式 [13,14] . 事实上, 大多数含有较强活化双氮的配合物都 不是单核的, 而且这些配合物很少是 18 电子配合物.…”

Molecular Pnictogen Activation by Rare Earth and Actinide Complexes

“…There are reviews on transition metal N2 activation which cover N2 binding modes [27][28][29], multimetallic N2 activation [30,31], the relevance of metal hydride complexes to N2 activation [32,33], N2 cleavage and functionalisation [34,35] (including electrochemical [36] and photolytic N2 cleavage [37]), and N2 activation at bare metal atoms [38] and using surface organometallic chemistry [39]. Specific reviews have also focused on activation by group 4 metals [40][41][42], iron [31,43,44], molybdenum [24,[45][46][47], and the mid-to-late transition metal centres [48].…”

Activation of Dinitrogen‐Derived Hafnium Nitrides for Nucleophilic NC Bond Formation with a Terminal Isocyanate

“…[1] Because of its high atmospheric abundance and non-toxicity, molecular nitrogen is an attractive synthon for the construction of N À C bonds if the kinetic and thermodynamic barriers associated with N 2 reduction can be surmounted. Inspired by reports by Sobota [2] and Fryzuk, [3] our laboratory has been exploring "ligand-induced N 2 cleavage", whereby a reducing transition metal is combined with an incoming ligand to supply the requisite six electrons to cleave the NN bond. [4] With strongly activated zirconocene [5] and hafnocene [6] dinitrogen complexes, this approach has proven to be a general method for nitrogen-carbon bond formation using carbon monoxide as the incoming ligand.…”

“…Coordination of an anionic (Xtype) ligand reduces hafnium-nitrogen multiple bonding and likely increases the nucleophilicity of the nitride. This in turn promotes N À C bond formation with the terminal isocyanate to assemble the ureate core, which is trapped by [Me 3 Si] + to yield 2. Because 1 can only be generated for short periods in solution, subsequent studies were conducted with the pyridine-stabilized dihafnocene nitride, [{(h 5 -C 5 H 2 -1,2,4-Me 3 ) 2 Hf} 2 (NCO)(pyridine)(m 2 -N)] (3), as this compound can be isolated and stored in the solid state.…”

“…Monitoring the addition of one equivalent of CH 3 OTf to a C 6 (Figure 2) assigned as Figure 3 and established the identity of 4 as the dihafnocene ureate complex, [{(h 5 -C 5 H 2 -1,2,4-Me 3 ) 2 Hf} 2 (m 2 -OTf)(m 2 -NCONH)]. [22] The hydrogen atom on the ureate core was located in the difference map and there was no evidence for a methyl group from the CH 3 OTf addition. The core of the molecule contains a bridging parent ureate ligand, [NCONH] 3À , along with a triflate anion that spans both hafnium atoms.…”