Michael D. Fryzuk

The continuing story of dinitrogen activation

Abstract This review attempts to survey the recent literature concerning the coordination chemistry and the reactivity patterns of metal–dinitrogen complexes. In order that this work may stand alone, a certain amount of background information is included; however, the emphasis is on synthesis and reactivity patterns of recently discovered dinitrogen complexes. In addition, some effort is made to discuss new trends in dinitrogen chemistry as well as to point out underdeveloped topics in this area.

Side-on end-on bound dinitrogen: an activated bonding mode that facilitates functionalizing molecular nitrogen.

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.

The hydride route to the preparation of dinitrogen complexes

That the inert dinitrogen molecule can act as a ligand to a transition metal complex is one of the key discoveries in inorganic chemistry of the past century. This feature article summarises a body of work up to 2010 that describes a particularly attractive route to dinitrogen complexes that involves the direct reaction of N(2) with metal hydride derivatives. This process is shown to be general across the transition series and, depending on the metal, different levels of activation of the coordinated dinitrogen unit are observed.

Synthesis and Reactivity of Side-On-Bound Dinitrogen Metal Complexes

An up-to-date account of the synthesis of side-on-bound dinitrogen complexes of the lanthanides, the actinides, and the transition elements over the past 40 years is given. In addition, the reactivity of these derivatives is summarized. There have been many complicated multinuclear cluster complexes with the N2 imbedded in a fashion that corresponds to side-on N2. There have been some suggestions, as early as 1960, that side-on dinitrogen complexes should exist. However, a key date in this area is 1988, which is when the disamarium complex (Cp*2Sm)2(μ-η2:η2-N2) was reported. It is this date that is used in this account as the real starting point for the area of side-on dinitrogen coordination chemistry. After 1988, side-on dinitrogen complexes are reviewed from the point of view of synthesis, structure (N−N bond lengths, where applicable), and reactivity. What becomes apparent is that while there have been many new side-on dinitrogen complexes discovered recently, investigations into their reactivity patt...

Complexes of groups 3, 4, the lanthanides and the actinides containing neutral phosphorus donor ligands

Revue synthetique de travaux concernant la reactivite de complexes avec des derives du phosphore. Synthese bibliographique

Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF(6) (1) {[PCP] = o-(i)Pr(2)PC(6)H(4)(NC(3)H(4)N)o-C(6)H(4)P(i)Pr(2)} to Ni(COD)(2) results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF(6) (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC-diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C-C coupling of the NHC carbon and a presumed Ni-ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni-H bond rather than into the Ni-carbene linkage; this is based on the observed deuterium scrambling, which requires reversible beta-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni-C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.

Inner-sphere two-electron reduction leads to cleavage and functionalization of coordinated dinitrogen

Activation of molecular nitrogen by transition metal complexes is an area of current interest as investigations using the inert N2 molecule to produce higher-value organonitrogen compounds intensify. In an attempt to extend the addition of hydride reagents E-H (where E = BR2, AlR2, and SiR3) to the dinitrogen complex ([NPN]Ta)2(μ-H)2(μ-η1:η2-N2) [1; where NPN = (PhNSiMe2CH2)2PPh], the reaction with zirconocene chlorohydride, [Cp2Zr(Cl)H]x, was examined. The crystalline product formed in 35% yield was determined to be ([NP(N)N]Ta)(μ-H)2(μ-N)(Ta[NPN])(ZrCp2) (2) in which the coordinated N2 has been cleaved to form a phosphinimide bridging between Ta and Zr and a triply bridging nitride. The mechanism of this reaction was examined to determine the fate of the chloride and hydride ligands attached to Zr in the starting zirconocene reagent. Using the zirconocene dihydride dimer ([Cp2ZrH2]2), a higher yield of 2 was obtained (76%), and H2 was also observed by 1H NMR spectroscopy. To probe the origin of the eliminated H2, the dideuterated dinitrogen complex ([NPN]Ta)2(μ-D)2(μ-η1:η2-N2) (d2-1) was allowed to react with ([Cp2ZrH2]2), which resulted in the formation of ([NP(N)N]Ta)(μ-D)2(μ-N)(Ta[NPN])(ZrCp2), (d2-2), with no evidence of hydrogen for deuterium scrambling between the starting zirconocene dihydride and the ditantalum dinitrogen complex. Studies into the use of preformed Zr(II) and Ti(II) reagents were also performed. The proposed mechanism involves initial adduct formation that facilitates inner-sphere electron transfer to cleave the N-N bond to form a species with bridging nitrides, one of which is transformed by nucleophilic attack of a phosphine donor to generate the observed phosphinimide.

Inorganic chemistry: Ammonia transformed

Ammonia is produced industrially by combining nitrogen and hydrogen gas, catalysed over a solid iron surface. How about a catalytic reaction that could take place in solution? The first steps have now been taken.

Synthesis, structure and hydrogenation of η3-benzyl diphosphine complexes of rhodium and iridium

Abstract The preparation of (COD)Rh(η 3 -CH 2 Ph) is described starting from [(COD)Rh] 2 (μ-Cl) 2 by the addition of either Zn(CH 2 Ph) 2 or Mg(CH 2 Ph) 2 (THF) 2 . The addition of the bulky chelating diphosphines t Bu 2 P(CH 2 ) 3 P t Bu 2 , i Pr 2 P(CH 2 ) 3 P i Pr 2 , i Pr 2 P(CH 2 ) 2 -P i Pr 2 , i Pr 2 PCH 2 P i Pr 2 and Cy 2 PCH 2 PCy 2 to (COD)Rh(η 3 -CH 2 Ph) yields the coordinatively unsaturated, four-coordinate rhodium complexes of the form P 2 Rh(η 3 -CH 2 Ph). Iridium complexes of the form P 2 Ir(η 3 -CH 2 Ph) (where P 2  t Bu 2 P(CH 2 ) 3 P t Bu 2 and i Pr 2 P(CH 2 ) 3 P i Pr 2 ) can be prepared from [P 2 Ir] 2 (μ-Cl) 2 and Zn(CH 2 Ph) 2 or Mg(CH 2 Ph) 2 (THF) 2 . Reaction of the benzyl complexes with H 2 (1 atm) yields binuclear hydride derivatives of varying composition depending on the chelate ring size of the coordinated diphosphine. For the diphosphines with only a single methylene in the backbone, binuclear hexahydride complexes are formed in which the diphosphine is binucleating. The X-ray structure of { i Pr 2 P(CH 2 ) 3 P i Pr 2 }Rh(η 3 -CH 2 Ph) shows a square planar geometry about rhodium with alternating single and double bonds in the η 3 -coordinated benzyl fragment. Crystals of {1,3-bis(diisopropylphosphino)propane}rhodium(η 3 -benzyl)are monoclinic, a = 10.540(3), b = 15.030(9), c = 14.858(5) A, β = 92.91(3)°, Z = 4, D c = 1.329 g cm −3 , space group P 2 1 / n . The structure was solved by the Patterson method and was refined by full-matrix least-squares procedures to R = 0.036 and R w = 0.043 for 4152 reflections with I >- 3σ( I ).

Carbon–Nitrogen Bond Formation by the Reaction of 1,2‐Cumulenes with a Ditantalum Complex Containing Side‐On‐ and End‐On‐Bound Dinitrogen

Synthetic ammonia serves as the key precursor for the synthesis of many nitrogen-containing chemicals and is produced by the Haber–Bosch process on the 100 million ton scale annually. Given the energy-intensive nature of this reaction, there is considerable interest in the search for alternative methods to generate nitrogen-containing molecules directly from N2. [2] One approach towards this goal is the functionalization of coordinated dinitrogen to form new nitrogen–carbon bonds. Typically, reagents such as alkyl halides or triflates can be employed for this purpose. A major drawback of those conversions is the formation of inorganic salts or transition metal halides as byproducts, rendering these reactions atom-inefficient. However, concerted addition reactions that involve the activated dinitrogen unit and substrates with C X double or triple bonds (X = C, N, O, S) could potentially lead to catalytic cycles for the production of nitrogen-containing heterocycles without any waste products. Unfortunately, only a very few stoichiometric cycloadditions at coordinated N2 units have been discovered to date. In all cases, highly activated side-on bridging h:hN2 complexes of Group 4 metals have been employed to achieve the desired nitrogen–carbon bond formations by reaction with arylalkynes, isocyanates, carbon dioxide, or carbon monoxide (via migratory insertion). The question we posed was: Could a Group 5 ditantalum dinitrogen complex with the side-on/end-on coordination mode engage in cycloaddition-type processes? Our initial attempts to try atom-efficient cycloadditiontype chemistry with the previously reported side-on end-on bound dinitrogen in [{(NPN)Ta}2(m-H)2(m-h :h-N2] (1; NPN = PhP(CH2SiMe2NPh)2) [3d] led to a different behavior. For example, with phenyl acetylene and simple alkenes such as propene, we observed displacement of the N2 unit or migratory insertion reactions into the tantalum hydrides, respectively (Scheme 1). In contrast, a variety of E H hydride reagents (EH = R2BH, R2AlH, RSiH3) have been shown to add across the side-on/end-on N2 unit of 1 to generate new functionalized nitrogen fragments and the formation of terminal Ta H bonds. As these E H additions can be considered to be additions across a quasi-Ta N multiple bond, we continued our search for other reagents that could engage in [2+2] chemistry with the side-on/end-on h:h-N2 moiety. Herein, we report our studies on the reaction of 1,2-cumulenes of the type X=C=Y (X, Y= NR, O, S) with the dinitrogen complex 1. Owing to its low cost and its recent successful application in dinitrogen functionalization with Group 4 metals, CO2 was chosen as a substrate for preliminary experiments. Parent dinitrogen complex 1 reacts instantaneously with carbon dioxide, but yields intractable solids that have so far resisted characterization. However, the bis(N-phenylimino) analogue of CO2, N,N’-diphenyl carbodiimide, reacts cleanly with one equivalent of 1 to give 2 as a single product in 70 % isolated yield (Scheme 2). Complex 2 gives rise to two doublets (JP,P = 16.0 Hz) at d = 3.0 and 8.6 ppm in the P{H} NMR spectrum, verifying that it is an unsymmetrical dinuclear species. Along with a fairly crowded aromatic region and a complicated pattern for the methylene protons, eight resonances for the silyl methyl groups and two signals for the bridging hydrides (at d = 11.9 and 12.1 ppm) are observed in the proton NMR spectrum. The latter signals show complex splitting patterns, as both Scheme 1. Reactivity of 1 towards C C double and triple bonds.