Transition metal dinitrogen complex

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Structure of [Ru(NH3)5(N2)] . RuA5N2.png
Structure of [Ru(NH3)5(N2)] .
Ball-and-stick model of ReCl(dppe)2N2 ReCl(dppe)2N2-3D-balls.png
Ball-and-stick model of Re Cl (dppe)2 N2
Fe(0)-N2 complex. PetersFeN2-attrane.svg
Fe(0)-N2 complex.

Transition metal dinitrogen complexes are coordination compounds that contain transition metals as ion centers the dinitrogen molecules (N2) as ligands. [2]

Contents

Historical background

Transition metal complexes of N2 have been studied since 1965 when the first complex was reported by Allen and Senoff. [3] This diamagnetic complex, [Ru(NH3)5(N2)]2+, was synthesized from hydrazine hydrate and ruthenium trichloride and consists of a [Ru(NH3)5]2+ centre attached to one end of N2. [4] [5] The existence of N2 as a ligand in this compound was identified by IR spectrum with a strong band around 2170–2100 cm−1. [4] In 1966, the molecular structure of [Ru(NH3)5(N2)]Cl2 was determined by Bottomly and Nyburg by X-ray crystallography. [6]

The dinitrogen complex trans-[IrCl(N2)(PPh3)2] is made by treating Vaska's complex with aromatic acyl azides. It has a planar geometry. [7]

The first preparation of a metal-dinitrogen complex using dinitrogen was reported in 1967 by Yamamoto and coworkers. They obtained [Co(H)(N2)(PPh3)3] by reduction of Co(acac)3 with AlEt2OEt under an atmosphere of N2. Containing both hydrido and N2 ligands, the complex was of potential relevance to nitrogen fixation. [8]

From the late 1960s, a variety of transition metal-dinitrogen complexes were made including those with iron, [9] molybdenum [10] and vanadium [11] as metal centers. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen. Biological nitrogen fixation probably occurs via the binding of N2 to those metal centers in the enzyme nitrogenase, followed by a series of steps that involve electron transfer and protonation. [12]

Bonding modes

In terms of its bonding to transition metals, N2 is related to CO and acetylene as all three species have triple bonds. A variety of bonding modes have been characterized. Based on whether the N2 molecules are shared by two more metal centers, the complexes can be classified into mononuclear and bridging. Based on the geometric relationship between the N2 molecule and the metal center, the complexes can be classified into end-on or side-on modes. In the end-on bonding modes of transition metal-dinitrogen complexes, the N-N vector can be considered in line with the metal ion center, whereas in the side-on modes, the metal-ligand bond is known to be perpendicular to the N-N vector. [13]

Mononuclear, end-on

As a ligand, N2 usually binds to metals as an "end-on" ligand, as illustrated by [Ru(NH3)5N2]2+. Such complexes are usually analogous to related CO derivatives. This relationship is illustrated by the pair of complexes IrCl(CO)(PPh3)2 and IrCl(N2)(PPh3)2. [14] In these mononuclear cases, N2 is both as a σ-donor and a π-acceptor. The M-N-N bond angles are close to 180°. [2] N2 is a weaker pi-acceptor than CO, reflecting the nature of the π* orbitals on CO vs N2. For this reason, few examples exist of complexes containing both CO and N2 ligand.

Transition metal-dinitrogen complexes can contain more than one N2 as "end-on" ligands, such as mer-[Mo(N2)3(PPrn2Ph)3], which has octahedral geometry. [15] In another example, the dinitrogen ligand in Mo(N2)2(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia. [16] Because many nitrogenases contain Mo, there has been particular interest in Mo-N2 complexes.

Bridging, end-on

N2 also serves as a bridging ligand with "end-on" bonding to two metal centers, as illustrated by {[Ru(NH3)5]2(μ-N2)}4+. These complexes are also called multinuclear dinitrogen complexes. In contrast to their mononuclear counterpart, they can be prepared for both early and late transition metals. [2]

In 2006, a study of iron-dinitrogen complexes by Holland and coworkers showed that the N–N bond is significantly weakened upon complexation with iron atoms with a low coordination number. The complex involved bidentate chelating ligands attached to the iron atoms in the Fe–N–N–Fe core, in which N2 acts as a bridging ligand between two iron atoms. Increasing the coordination number of iron by modifying the chelating ligands and adding another ligand per iron atom showed an increase in the strength of the N–N bond in the resulting complex. It is thus suspected that Fe in a low-coordination environment is a key factor to the fixation of nitrogen by the nitrogenase enzyme, since its Fe–Mo cofactor also features Fe with low coordination numbers. [17]

The average bond length of those bridging-end-on dinitrogen complexes is about 1.2 Å. In some cases, the bond length can be as long as 1.4 Å, which is similar to those of N-N single bonds. [18] Hasanayn and co-workers have shown that the Lewis structures of end-on bridging complexes can be assigned based on π-molecular-orbital occupancy, in analogy with simple tetratomic organic molecules. For example the cores of N2-bridged complexes with 8, 10, or 12 π-electrons can generally be formulated, respectively, as M≡N-N≡M, M=N=N=M, and M-N≡N-M, in analogy with the 8-, 10-, and 12-π-electron organic molecules HC≡C-C≡CH, O=C=C=O, and F-C≡C-F. [19]

Mononuclear, side-on

In comparison with their end-on counterpart, the mononuclear side-on dinitrogen complexes are usually higher in energy and the examples of them are rare. Dinitrogen act as a π-donor in these type of complexes. Fomitchev and Coppens has reported the first crystallographic evidence for side-on coordination of N2 to a single metal center in a photoinduced metastable state. When treated with UV light, the transition metal-dinitrogen complex, [Os(NH3)5(N2)]2+ in solid states can be converted into a metastable state of [Os(NH3)5(η2-N2)]2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm−1.

Some other examples are considered to exist in the transition states of intramolecular linkage isomerizations. Armor and Taube has reported these isomerizations using 15N-labelled dinitrogen as ligands. [20]

Bridging, side-on

In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector, which can be considered as side-on fashion. One example is [(η5-C5Me4H)2Zr]2(μ 2,η 2,η2-N2). [21] The dimetallic complex can react with H2 to achieve the artificial nitrogen fixation by reducing N2. [22] A related ditantalum tetrahydride complex could also reduce N2. [23]

Reactivity

Hypothesized cycle for M-catalysed nitrogen fixation according to Chatt et al. ChattCycle.svg
Hypothesized cycle for M-catalysed nitrogen fixation according to Chatt et al.

Cleavage to nitrides

When metal nitrido complexes are produced from N2, the intermediacy of a dinitrogen complex is assumed. Some Mo(III) complexes also cleave N2: [24]

2 Mo(NR2)3 + N2 → (R2N)3Mo-N2-Mo(NR2)3
(R2N)3Mo-N2-Mo(NR2)3 → 2 N≡Mo(NR2)3

Attack by electrophiles

Some electron-rich metal dinitrogen complexes are susceptible to attack by electrophiles on nitrogen. When the electrophile is a proton, the reaction is of interest in the context of abiological nitrogen fixation. Some metal-dintrogen complexes even catalyze the hydrogenation of N2 to ammonia in a cycle that involves N-protonation of a reduced M-N2 complex. [25] [26]

See also

Related Research Articles

Nitrogen fixation is a chemical process by which molecular nitrogen (N
2), which has a strong triple covalent bond, is converted into ammonia (NH
3) or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. The nitrogen in air is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbe-mediated process that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif).

<span class="mw-page-title-main">Nitrogenase</span> Class of enzymes

Nitrogenases are enzymes (EC 1.18.6.1EC 1.19.6.1) that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only family of enzymes known to catalyze this reaction, which is a key step in the process of nitrogen fixation. Nitrogen fixation is required for all forms of life, with nitrogen being essential for the biosynthesis of molecules (nucleotides, amino acids) that create plants, animals and other organisms. They are encoded by the Nif genes or homologs. They are related to protochlorophyllide reductase.

The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five (n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or non-bonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period, lending stability to the complex. Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive. The rule is not helpful for complexes of metals that are not transition metals. The rule was first proposed by American chemist Irving Langmuir in 1921.

Metal nitrido complexes are coordination compounds and metal clusters that contain an atom of nitrogen bound only to transition metals. These compounds are molecular, i.e. discrete in contrast to the polymeric, dense nitride materials that are useful in materials science. The distinction between the molecular and solid-state polymers is not always very clear as illustrated by the materials Li6MoN4 and more condensed derivatives such as Na3MoN3. Transition metal nitrido complexes have attracted interest in part because it is assumed that nitrogen fixation proceeds via nitrido intermediates. Nitrido complexes have long been known, the first example being salts of [OsO3N]−, described in the 19th century.

A transition metal oxo complex is a coordination complex containing an oxo ligand. Formally O2-, an oxo ligand can be bound to one or more metal centers, i.e. it can exist as a terminal or (most commonly) as bridging ligands (Fig. 1). Oxo ligands stabilize high oxidation states of a metal. They are also found in several metalloproteins, for example in molybdenum cofactors and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is potassium ferrate (K2FeO4), which was likely prepared by Georg E. Stahl in 1702.

A metal carbido complex is a coordination complex that contains a carbon atom as a ligand. They are analogous to metal nitrido complexes. Carbido complexes are a molecular subclass of carbides, which are prevalent in organometallic and inorganic chemistry. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis, olefin metathesis, and related catalytic industrial processes. Ruthenium-based carbido complexes are by far the most synthesized and characterized to date. Although, complexes containing chromium, gold, iron, nickel, molybdenum, osmium, rhenium, and tungsten cores are also known. Mixed-metal carbides are also known.

<span class="mw-page-title-main">Bis(dinitrogen)bis(1,2-bis(diphenylphosphino)ethane)molybdenum(0)</span> Chemical compound

trans-Bis(dinitrogen)bis[1,2-bis(diphenylphosphino)ethane]molybdenum(0) is a coordination complex with the formula Mo(N2)2(dppe)2. It is a relatively air stable yellow-orange solid. It is notable as being the first discovered dinitrogen containing complex of molybdenum.

<span class="mw-page-title-main">Tris(trimethylsilyl)amine</span> Chemical compound

Tris(trimethylsilyl)amine is the simplest tris(trialkylsilyl)amine which are having the general formula (R3Si)3N, in which all three hydrogen atoms of the ammonia are replaced by trimethylsilyl groups (-Si(CH3)3). Tris(trimethylsilyl)amine has been for years in the center of scientific interest as a stable intermediate in chemical nitrogen fixation (i. e. the conversion of atmospheric nitrogen N2 into organic substrates under normal conditions).

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

<span class="mw-page-title-main">Phosphenium</span> Divalent cations of phosphorus

Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates.

<span class="mw-page-title-main">Transition metal imido complex</span>

In coordination chemistry and organometallic chemistry, transition metal imido complexes is a coordination compound containing an imido ligand. Imido ligands can be terminal or bridging ligands. The parent imido ligand has the formula NH, but most imido ligands have alkyl or aryl groups in place of H. The imido ligand is generally viewed as a dianion, akin to oxide.

A transition metal phosphido complex is a coordination complex containing a phosphido ligand (R2P, where R = H, organic substituent). With two lone pairs on phosphorus, the phosphido anion (R2P−) is comparable to an amido anion (R2N−), except that the M-P distances are longer and the phosphorus atom is more sterically accessible. For these reasons, phosphido is often a bridging ligand. The -PH2 ion or ligand is also called phosphanide or phosphido ligand.

<span class="mw-page-title-main">Nontrigonal pnictogen compounds</span>

Nontrigonal pnictogen compounds refer to tricoordinate trivalent pnictogen compounds that are not of typical trigonal pyramidal molecular geometry. By virtue of their geometric constraint, these compounds exhibit distinct electronic structures and reactivities, which bestow on them potential to provide unique nonmetal platforms for bond cleavage reactions.

<span class="mw-page-title-main">Abiological nitrogen fixation using homogeneous catalysts</span> Chemical process that converts nitrogen to ammonia

Abiological nitrogen fixation describes chemical processes that fix (react with) N2, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses an iron-based heterogeneous catalysts and H2 to convert N2 to NH3. This article focuses on homogeneous (soluble) catalysts for the same or similar conversions.

Jonas C. Peters is the Bren Professor of Chemistry at the California Institute of Technology and Director of the Resnick Sustainability Institute. He has contributed to the development of catalysts and photocatalysts relevant to small molecule activation.

Karsten Meyer is a German inorganic chemist and Chair of Inorganic and General Chemistry at the Friedrich-Alexander University of Erlangen-Nürnberg (FAU). His research involves the coordination chemistry of transition metals as well as uranium coordination chemistry, small molecule activation with these coordination complexes, and the synthesis of new chelating ligands. He is the 2017 recipient of the Elhuyar-Goldschmidt Award of the Spanish Royal Society of Chemistry, the Ludwig-Mond Award of the Royal Society of Chemistry, and the L.A. Chugaev Commemorative Medal of the Russian Academy of Sciences, among other awards. He also serves as an Associate Editor of the journal Organometallics since 2014.

<span class="mw-page-title-main">Transition metal nitrate complex</span> Compound of nitrate ligands

A transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds.

Main-group element-mediated activation of dinitrogen is the N2 activation facilitated by reactive main group element centered molecules (e.g., low valent main group metal Ca, dicoordinate borylene, boron radical, carbene, etc.).

In organometallic chemistry, metal tetranorbornyls are compounds with the formula M(nor)4 (M = a metal in a +4 oxidation state) (1-nor = 4bicyclo[2.2.1]hept-1-yl) and are one of the largest series of tetraalkyl complexes derived from identical ligands. Metal tetranorbornyls display uniform stoichiometry, low-spin configurations, and high stability, which can be attributed to their +4 oxidation state metal center. The stability of metal tetranorbornyls is predominately considered to be derived from the unfavorable ß-hydride elimination. Computational calculations have determined that London dispersion effects significantly contribute to the stability of metal tetranorbornyls. Specifically, Fe(nor)4 has a stabilization of 45.9 kcal/mol−1. Notable metal tetranorbornyls are those synthesized with metal centers of cobalt, manganese, or iron.

In 1956, Longuet-Higgins and Orgel predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate eg orbital of cyclobutadiene has the correct symmetry for π interaction with the dxz and dyz orbitals of the proper metal. The compound was synthesized three years after the prediction and it serves as a beautiful case of theory before experiment. This successful attempt opens the door for the formation of novel compounds containing other organic ligands which in their free state are highly reactive molecules. Of all those reactive molecules, trimethylenemethane (TMM) has the most natural derivation from the cyclobutadiene complexes and in 1966, Emerson and co-workers reported the first trimethylenemethane (TMM) transition metal complex, (CO)3FeC(CH2)3, which became the starting point of the legends of trimethylenemethane complexes. Some good reviews on this aspect could be served as further resources for this topic.

References

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