WO2014027913A1

WO2014027913A1 - Catalyst for fixation of molecular nitrogen

Info

Publication number: WO2014027913A1
Application number: PCT/RU2012/000673
Authority: WO
Inventors: Alexei Alexeevich GRIDNEV
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-08-15
Filing date: 2012-08-15
Publication date: 2014-02-20
Anticipated expiration: 2015-02-15

Description

CATALYST FOR FIXATION OF MOLECULAR NITROGEN.

DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION

Nitrogen fixation is a process by which nitrogen (N₂) in the atmosphere is converted into ammonia (NH₃). _Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK.] Atmospheric nitrogen or elemental nitrogen (N₂) is relatively inert: it does not easily react with other chemicals to form new compounds. Fixation processes free up the nitrogen atoms from their diatomic form (N₂) to be used in other ways.

Artificial fertilizer production is now the largest source of human-produced fixed nitrogen in the Earth's ecosystem. Ammonia is a required precursor to fertilizers, explosives, and other products. The most common method is the Haber process. The Haber process requires high pressures (around 200 atm) and high

temperatures (at least 400 °C). This process uses natural gas as a hydrogen source and air as a nitrogen source. [

http://en.wikipedia.org/wiki Nitrogen_fixation]

Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of reducing the energy required for this conversion. However, such research has thus far failed to even approach the efficiency and ease of the Haber process. Catalytic chemical nitrogen fixation at temperatures considerably lower than the Haber process is an ongoing scientific endeavor. Several molybdenum catalyst were reported [John J. Curley, Emma L. Sceats, and Christopher C. Cummins J. Am. Chem. Soc, 2006, 128 (43), pp 14036- 14037; Dmitry V. Yandulov and Richard R. Schrock Science. Vol. 301. (2003), no. 5629, pp. 76-78]. However, this catalytic reduction fixates only a few nitrogen molecules. Nitrogen was converted to ammonia and hydrazine by A. E. Shilov [Catalytic reduction of molecular nitrogen in solutions A.E. Shilov

Russian Chemical Bulletin Volume 52, Number 12,(1970), pages 2555-2562]. A. E. Shilov concluded that for chemical nitrogen fixation the optimum catalyst should be a polynuclear complex. He stated that "The reaction proceeds as a multielectron process, and the limiting step involves the electron transfer from a reducing agent".

The major problem of nitrogen fixation in solutions is formation of catalyst capable of transferring several electrons to dinitrogen simultaneously. This requirements comes from unusual properties of dinitrogen molecule. Dinitrogen molecule is very stable thermodynamically, especially the third bond. However, second and first bond are much wicker. Hence, for fixation of nitrogen both the third and, at least, second bond should be opened simultaneously. Formation of hydrazine from dinitrogen is thermodynamically viable process only if two bonds between nitrogen atoms are opened simultaneously

[http://en.wikipedia.org/wiki/Nitrogen]. It requires 4 electron donation from the catalyst to dinitrogen molecule.

The scope of the present invention is to provide catalyst capable for such multielectron transfer from the catalyst to dinitrogen molecule.

STATEMENT OF THE INVENTION

In accordance with the present invention, the catalyst comprising metallochelate with two and more metal atom surrounded by a macrocyclic aromatic ligand. This ligand with conjugated system of π-electron is capable to form at least two aromatic structures. This is quite unusual feature of invented catalysts as no examples of ligands with "tunable' aromaticity is known so far. For the simplicity, we call multunuclear chelates with tunable aromaticity "G-chelates".

Aromaticity as a phenomenon has many explanations and definitions that we would not like to discuss. For the purposes of this invention we would rather use the simplest and classic features of the aromatic compounds. Specifically, it is planar molecular structure with 4n+2 conjugated π-electrons (Huckel's rule, [Doering, W. v. E. (September 1951), Abstracts of the American Chemical Society Meeting, New York, p. 24M].

Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules. If molecule has two aromatic structures then it should be stable in each of them.

Description of G-chelate structure.

Chelate I is an example to start (M is a transition metal).

I

Simple playing with double bonds of G-chelate I shows that it can have 2 stable π-electron structures, I and II, due to the change of "n" parameter in the

Huckel's formula of the aromaticity, 4n+2. In the G-chelate I we have 26 electrons, n=6, while in G-chelate II we have 22 conjugated electrons, or n=5.

Note that metal atoms, M, changed valence from 3 to 1. It is important and we will return to that issue later. Apparently, carbon atoms can be replaced with any other heteroatoms, like G-chelate III. Heteroatoms can change distribution of electron densities causing change in reactivity patterns in G-chelate III ( bottom part of G-chelate III has electron-donating groups while upper part has electron- withdrawing groups).

Replacing of methylene groups with methyne ones is possible. In this case (IV) we have two systems of conjugated electrons with aromatic features. 26- electrone (n=6) aromatic system is formed by outer double bonds, while inner double bonds and M-atoms form another conjugated system, M-N-C-C-N-M. The inner system can exist in two forms. For four valent metals we have structure IV and for two valent metal we have structure V. In G-chelate V two d- electrons are required for the formation of aromatic 6-conjugated electron ring. In G-chelate IV empty d-orbitals participate in the ring formation. Quantum calculations may answer the question which structure is more favorable thermodynamically. Most likely, it depends on the origin of metal atoms and their axial ligands.

In G-chelates I-IV distance between metal atoms is calculated to be about 340 pm, which is too long for the most metal-metal bonds which are in the 200-250 pm range. Thus, some expanded porphyrins which have molecular structure close to ligand structure of IV (isoamethrin, rubyrin, amethrin) are capable to form bimetallic chelates but only with Cl(-) or OH(-) bridges between metal atoms instead of direct metal -metal bond [Sessler J. L.; Tomat E. Acc. Chem. Res. 40 (2007)371-8]. On the other hand, there is no indication in the article whether or not any attempt was made to synthesize bimetallic complexes of expanded porphyrins with metal-metal bonds.

Many metals, like Mo, Sc, Ti, Ta, Cr, W, Mn and even Fe are capable to form metal-metal bond in chemical compounds in the range of 270-330 pm of lengths [5-9]. Unlike carbon or nitrogen, heavy metals are capable to adjust their bond length in a broad range, up to 60 pm, depending on ligands. Hence, simple 25-50 pm shift of each M to the center of the molecule will reduce M-M distance from 340 pm to 250-300 pm that is acceptable for many metal-metal bonds.

Additional opportunities come from certain flexibilitiy of macrocyclic ligands. Flexibility of the macrocyle increases if we remove methyne or methylene bridge between bispyrrolic fragments in I-IV. Insignificant change of bond angles between pyrrole rings in the bispyrrole fragment together with bond angles of methyne bridges let macrocycle VI be noticeably stretched vertically with simultaneous horizontal shrinking. This move of the macrocycle will reduce distance between metal atoms. The same effect could be expected by replacing methylene bridge in I with ethylene bridges (VII). Distance between nitrogen atoms in structures VIII and IX is about 200 pm that fits perfectly, as it was mentioned before, M-M bond length for the most of transition metals. However, unlike G-chelate I, 22- electron aromatic system of structure VIII (n=5) can not become neither 18-electron (n=4), nor 26-electron aromatic macrocycle.

Therefore, structure VIII can not be considered a G-chelate since aromatic conjugation of its π-electrons of its equatorial ligand can not be "tuned'. 26- electron aromatic structure IX, also, has problems with drawing a 22-electron version. Only an assumption of single unpaired electron delocalized on the outer fragments of the macrocycle and out of the aromayic system of conjigation let us accomplish drawing of 22-electron G-chelate (X). This unpaired electron is delocalyzed on 10 carbon atoms that should substantially reduce its energy.

Much better drawing results of aromatic structure of VIII type can be achieved by replacing of one carbon atom in VIII with a hetero atom (N, S, O). 26- electron G-chelate XI produces 22-electron G-chelate XII without complications.

Expanding number of conjugation electrons by replacing of 5+5 fused rings of VIII with 6+5 fused rings we can obtain both 22-electron ring structure (XIII, n=5) and 26-electron structure(XIV, n=6). Similar structures can be produced by replacing of a carbon atom in XIII with a heteroatom (26-electron structure XV and its 22-electron version XVI). Both XIV and XVI require unpaired electron outside conjugation path to achieve aromatic conjugation in the most of π- electrons. Derealization of this unpaired electron is limited in XIV to only 2 carbon atoms (bottom and upper pyrrole rings). In XVI unpaired electron delocalized on 6 carbon atoms. Presence of a heteroatom in XVI reduce energy of unpaired electron even further as compared with XIV.

Fused two 5 -member rings in VIII, XI and XII macrocycles reduce size of central dentate cavity of the macrocycles as compared to IX because of pointing methyne bridge at larger angle. To accommodate larger metal atoms ethylene bridges instead of methyne bridges can be applied, like in 30-electron G-chelate XVII, which can be easily redrawn into 26-electron version XVIII.

Three metal G-chelates could have three types of symmetry, linear, triangle and L-shaped. Linear three metal atom G-chelate XIX can form 30-electron (XIX), 34-electron (XX, conjugation path is shown in bold lines for single bonds) and 26-electron (XXI) structures. In XXI we see one unpaired electron like in X, XIV and XVI but it is spread on 14 carbon atoms (upper and bottom part of the molecule), which should provide better stability of this electron than in G- chelates with 2 metal atoms. Two unpaired electrons in XX are included in the main conjugation system so that no formal objection against aromaticity and stability of XX can be anticipated.

We can draw linear three metal G-chelates by replacing pyrrolic moieties in XIX with pyridine ones (XXII and XXIII). In 34-electron G-chelate XXII we, also, see two unpaired electrons that are included in the main conjugation path.

Apparently, one may conclude that presence of unpaired electrons in aromatic path of conjugation of π-electrons is intrinsic feature of G-chelates based on naphthyridine and its homologues.

XXII XXIII

With increasing number of metal atoms number of G-chelates with different geometrical arrangement of metal atoms in the chelate increases drastically. Because of that let's consider only two the most simple of them with four atoms of metals. The "linear" G-chelelate XXIV has 34-electron conjugation system that can form a 38-electron version XXV with two unpaired electron in the conjugation pathway. The last feature could be anticipated as XXV is

constructed with naphthyridine homologue.

XXIV XXV

It is easy to show delocalization of these 2 unpaired electrons along the whole aromatic conjugation path. Aromatic 30-electron version of this linear four-metal G-chelate, XXVI, is, also, like three-metal one, has one unpaired electron delocalized outside the aromatic conjugation path.

Having compared linear G-chelates with different number of metals, one may deductively conclude that, first, linear G-chelates with larger number of metal atoms are possible. Second, with increasing number of metals in linear G- chelates several ways of aromatic conjugation of π-electrons are possible.

The other simplest structure with four metal atoms, a "square" ones, is

exem lified with G-chelates XXVII - XXXII.

[5+5] fused rings may form 34 electron XXVII and 30-electronXXVII in the most classic way, i.e., without necessity of unpaired electron neither in the "main" conjugation system, nor in the conjugation that stays aside of this "main" conjugation system. [6+6] and [6+5] fused rings in the macrocycle (30 electron XXIX and 26 electron XXX, 34 electron XXXI and 30 electron XXXII) does re uire presence of such unpaired electrons.

XXVII XXVIII

XXXI XXXII

Above, we mentioned that change in conjugation path of π-electrons in G- chelates requires changes in valency of metal atoms. In G-chelates I-II and XI- XII and others, four electrons are pushed from a macrocycle to metal atoms. This transformation is very unlikely as either I or XI are more or less

thermodynamically preferable than II or XII, correspondingly. However, reactive molecules present in the solution should change the situation by formation of axial bonds with metal atoms in the chelate. Thus, substrate molecule would trigger off transfer of G-chelate from one stable state to another due to the change in the pattern of aromatic conjugation.

Nitrogen fixation by G-chelates.

Simultaneous opening of two bonds in dinitrogen molecule is crucial for ambient nitrogen fixation as it was indicated above. It means, that nitrogen fixation catalyst should be able to form four bonds with dinitrogen molecule in a single step. Ability of G-chelates simultaneously transfer 4 electrons to metal atoms ideall matches this requirement. For example

XXXIII

In case 4-nuclei XXXIV is another example (dinitrogen in XXXIV is located above the planar molucule of G-chelate). Hydrolysis (alcoho lysis,

hydrogenolysis, etc.) of XXXIII and XXXIV will lead to formation of hydrazine. Besides nitrogen fixation, G-chelates can be used as efficient catalyst for many other rocesses due to their unique ability to form four bonds simultaneously.

XXXIV

In G-chelates transfer from one aromatic pattern of π- conjugation to another is a pure electronic process that requires no change in geometry of the ligand or charge transfer. This transfer requires only a substrate molecule to form a complex with M-atoms. Because of this unique feature, G-chelates might have catalytic properties in various reactions and nitrogen fixation would be only one of them.

Claims

1. Catalyst for nitrogen fixation in solution comprises two and more elements chelated by macrocyclic ligand with conjugated system of π-electrons capable to form at least two aromatic macrocyclic conjugations.

2. Catalyst of claim 1, wherein elements are transition metals.

3. Catalyst of claim 1, wherein elements are non-metallic elements of third, fourth and fifth periods of the Mendeleev table of elements.