Complejos monociclopentadienilo de titanio en bajo estado de oxidación para la activación de dinitrógeno
Authors
Horno Martín, Estefanía delDate
2022Embargo end date
2024-01-22Affiliation
Universidad de Alcalá. Departamento de Química Orgánica y Química Inorgánica; Universiad de Alcalá. Programa de Doctorado en QuímicaKeywords
Síntesis de fertilizantes
Combustibles alternativos
Tecnologías químicas-Investigación
Document type
info:eu-repo/semantics/doctoralThesis
Version
info:eu-repo/semantics/acceptedVersion
Rights
Attribution-NonCommercial-NoDerivatives 4.0 Internacional
Access rights
info:eu-repo/semantics/openAccess
Abstract
Chapter 1. Introduction
By the end of the XIXth century, the reserves of NaNO3, a natural fertilizer, were running out, so finding an alternative way of producing fertilizers which were able to ensure the sustainment of the increasing population was mandatory. In this context, the Haber-Bosch method for the synthesis of ammonia was developed. Since then, this process has been used to produce ammonia in industry, although it has been slightly modified.3 The NH3 is employed as feedstock in the synthesis of fertilizers which are known to be responsible of the sustenance of the 60% of global population. Furthermore, ammonia is currently considered a promising candidate as an alternative fuel, due to the advantages that it offers over conventional hydrocarbons or even hydrogen.5
Despite all its strengths, the main drawback of the use of ammonia for these purposes consists of the enormous environmental costs associated to the large-scale synthesis of this molecule.6 These are mainly due to the severe conditions of pressure and temperature required in the Haber-Bosch process, along with the use of H2 obtained from fossil fuels. Nowadays, the search of a more sustainable alternative to this method is one of the most active research areas in the chemical technologies.
Although dinitrogen is one of the most inert molecules known, in 1965 Allen and Senoff reported the first metal complex in which N2 appeared coordinated to a metal centre.9 Only two years later, the first metal system capable of incorporating molecular nitrogen was published. Since then, diverse coordination modes of dinitrogen have been described in many derivatives, being the most abundant the mononuclear complexes containing dinitrogen in an 1-N2 (end-on) disposition bonded to elements of Groups 6 to 9.9 Examples of dinuclear derivatives with dinitrogen in a -1:1-N2 (bridging end-on) bonding are also numerous, while the -2:2-N2 (bridging side-on) mode still remains scarce. Generally, -2:2-N2 coordination exhibits a higher energy content when comparing to the -1:1-N2 mode.
The coordination of dinitrogen to a metal centre involves certain weakening of the NN bond. The degree of activation can be tentative measured attending to the elongation of the nitrogen-nitrogen distance, although considering the variation of the wavenumber in IR or Raman shift for the NN vibration is more accurate to stablish comparisons. If the activation is strong enough, scission of the original N2 unit can be achieved, leading to
Summary
192
the formation of two nitrido groups. In 1996, Cummins reported the first example of this transformation.28
Considering the behaviour shown by certain metal systems towards dinitrogen, these kinds of complexes are regarded as candidates to take part in a mild synthesis of ammonia. Indeed, Schrock reported in 2003 the first example of a catalytic synthesis of ammonia under mild conditions using a molybdenum(III) complex.31 Later, he proposed a mechanistic cycle with many isolated intermediates in different oxidation states involved in the process.32
In order to incorporate and activate dinitrogen, metal centres must have electrons available to be transferred to N2, so metals in low oxidation states are required. To obtain these low-valent metal systems, different synthetic strategies can be designed. Among the most used are the utilisation of a strong reductant agent towards a higher-valent species under a N2 atmosphere or the reaction of hydride complexes with dinitrogen. In the latter case, the release of H2 by a reductive elimination process provides the metal centre with the electrons requires to bind N2. In 2013, Hou and co-workers published a study in which polymetallic titanium hydride complexes fixed dinitrogen molecules that were subsequently functionalized by the remaining hydrides on the metals to form NH moieties.40 Related to this result, our group reported in 2017 a study about the treatment of [TiCp*Me3] with a H2/N2 mixture at atmospheric pressure and room temperature. The resultant compound showed a cube-type structure in which two of the vertices were nitrido ligands, demonstrating that dinitrogen had been activated under mild conditions presumably via a generated low-valent titanium hydride intermediate.46
In view of these precedents and as a continuation of these investigations, in this Thesis we describe our efforts on the preparation of monocyclopentadienyl titanium compounds in low oxidation states. The results obtained in the reaction of some of these low-valent complexes with dinitrogen are also discussed.
Chapter 2. Reaction of trialkyl titanium derivatives with amine-borane adducts.
Treatment of [TiCp*Me3] (1) with ammonia-borane afforded the paramagnetic dinuclear titanium(III) complex [{TiCp*(NH2BH3)}2(-NH2BH2NHBH3)] (7), which decomposes in solution above 80 ºC to give the trinuclear hydride derivative [{TiCp*(-H)}3{-N(BH3)3}] (8). This hydride compound 8 could be also obtained by the reaction of [TiCp*(CH2SiMe3)3] (2) with NH3BH3 at 80 ºC. When complex 1 was treated with
Summary
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NH2tBuBH3, the reaction afforded a solution from which a small fraction of crystals composed by molecules of [TiCp*(NHtBuBH3)2] (9) and [TiCp*(NHtBuBH3)(NHtBuBH2Me)] (10) was isolated.
Reaction of [TiCp*Me3] (1) with dimethylamine borane, NHMe2BH3, produced a mixture of the paramagnetic trinuclear compound [{TiCp*(-H)}3(3-H)(3-NMe2BH2)] (11) and the diamagnetic octahydride complex [(TiCp*)4(-H)8] (12). Both species could be obtained as pure samples in separate experiments when [TiCp*(CH2SiMe3)3] (2) was used as the titanium initial reagent. Thus, the treatment of 2 with NHMe2BH3 at 65 ºC under a hydrogen atmosphere gave 11, which can be described as a trititanium tetrahydride system stabilized with a NMe2BH2 moiety. If the heating of compound 2 under hydrogen atmosphere took place in the absence of the dimethylamineborane adduct, the reaction resulted in the precipitation of the octahydride complex 12.
In order to complete this study with borane reagents, we investigated the reactions of 1 with BH3(thf). The treatment of 1 with four equivalents of BH3(thf) led to the diamagnetic derivative [{TiCp*(BH3Me)}2(-B2H6)] (13). However, the addition of five equivalents of BH3(thf) to complex 1 gave the paramagnetic complex [{TiCp*(-B2H6)}2] (14). Both compounds are titanium(III) species, and their magnetic behaviour could be explained by theoretical calculations, since a singlet state was the most stable electronic structure for 13, whereas a triplet state was preferred for 14.
Chapter 3. Reduction processes of halide titanium complexes.
Thermolysis of [TiCp*Cl2Me] (3) at 180 ºC in hexane led to the precipitation of the titanium(III) complex [{TiCp*Cl(-Cl)}2] (15) in a clean process with methane and ethene as volatile by-products. The treatment of 3 with excess of pinacolborane in hexane at 65 ºC afforded a mixture of 15 and the paramagnetic trimer [{TiCp*(-Cl)2}3] (16). Although the solid structure of 15 revealed a dinuclear motif, this compound showed and intriguing behaviour in solution. NMR spectroscopy experiments on aromatic hydrocarbon solutions of 15 and DFT calculations for several [(TiCp*Cl2)n] aggregates are consistent with the existence of an equilibrium between 15 and the paramagnetic tetramer [{TiCp*(-Cl2)}4] (15’) in solution. In contrast, compound 15 readily dissolves in tetrahydrofuran to give a solution of the mononuclear adduct [TiCp*Cl2(thf)] (17).
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194
The reactivity of 15 towards different agents was studied. Attempts of substitution of the chloride ligands for several alkyl or amido groups were unsuccessful, probably because of disproportionation processes, but the treatment of 15 with [Li{N(SiMe3)2}] at room temperature in toluene led to the precipitation of the amido titanium(III) complex [{TiCp*(-Cl){N(SiMe3)2}}2] (18). Compound 15 in solution at temperatures higher than 100 ºC undergoes also disproportionation as demonstrated by reaction with cobaltocene and N-(4-methylbenzylidene)aniline yielding the ionic paramagnetic compound [CoCp2][TiCp*Cl3] (19) and the diamagnetic diazatitanacyclopentane derivative [TiCp*Cl{N(Ph)CH(p-tolyl)}2] (20), respectively.
When 15 was treated with excess tetrahydrofuran or ammonia, the dinuclear core was not preserved, and the resultant products were the mononuclear titanium(III) adducts 17 and [TiCp*Cl2(NH3)2] (21). In contrast, the reactions with softer Lewis bases, as 2,6-dimethylphenylisocianide or tert-butylisocyanide, yielded the dinuclear titanium(III) complexes [{TiCp*Cl(-Cl)(CNR)}2] (R = 2,6-Me2C6H3 (22), tBu (23)). Complex 22 in solution is stable up to 120 ºC, whereas 23 decomposes at 55 ºC in toluene leading to the titanium(IV) dinuclear derivative [(TiCp*Cl2)2(-2:2-tBuN=C-C=NtBu)] (24) through an oxidative coupling reaction.
On the other hand, the treatment of [TiCp*X3] (X = Cl (4), Br (5), I(6)) with one or two equivalents of LiAlH4 in toluene at room temperature resulted in the formation of the halide-bridged dimers [{TiCp*X(-X)}2] (X = Cl (15), Br (25), I (26)). In contrast, the reactions of [TiCp*X3] with 2 equivalents of LiBH4 in tetrahydrofuran at room temperature led to the titanium(III) tetrahydridoborato complexes [{TiCp*(BH4)(-X)}2] (X = Cl (27), Br (28)). Compound 27 had been previously reported by Girolami and co-workers,98 but through a more complicated procedure. In addition, the treatment of [TiCp*Cl3] with an excess of LiBH4 at 85 ºC led to complete substitution of the halide ligands to afford the mononuclear titanium(III) derivative [TiCp*(BH4)2(thf)] (29). The attempted sublimation of 29 provoked the release of the thf ligand and formation of [{TiCp*(BH4)(-BH4)}2] (30). A more appropriate method of preparation of complex 30 is the treatment of [TiCp*Cl3] (4) with an excess of powdered LiBH4 in toluene. Complex 30 in tetrahydrofuran regenerates 29, and the reaction of 30 with pyridine produces the analogous mononuclear titanium(III) compound [TiCp*(BH4)2(py)] (31).
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Finally, the treatment of the trihalide derivatives [TiCp*X3] (X = Cl (4), Br (5)) with LiAlH4 in ethereal solvents as tetrahydrofuran or diethylether gave the hydride-bridged heterometallic complexes [{TiCp*(-H)}2{(-H)2AlXL}2] (L = thf, X = Cl (32), Br (33); L = OEt2, X = Cl (34)). These compounds can be described as titanium(II) hydride systems stabilized with two {(-H)2AlXL} fragments. Theoretical calculations revealed the presence of a Ti-Ti metal bond in these complexes as well as the existence of strong interactions between the electrons of this bond and the empty s orbitals of the aluminium atoms.
Chapter 4. Activation of dinitrogen with titanium complexes.
Whereas the reduction of [TiCp*Br3] (5) with magnesium produced the paramagnetic magnesium-titanium(III) derivative [{TiCp*Br(-Br)2}2Mg(thf)2] (35), the treatment of [TiCp*Cl3] (4) with the same reducing agent led to the trinuclear compound [{TiCp*(-Cl)}3(3-Cl)] (36). This complex is also paramagnetic, and according to theoretical calculations, the most stable electronic structure is a doublet state in which one of the metal centres is regarded as titanium(II) while the oxidation states for the others is +2.5. However, the structural parameters and the NMR data suggest that 36 is better described by three resonance forms, assigning a formal oxidation state of +2.33 for each titanium atom.
The reaction of 36 with azobenzene gave 4 and the imido-bridged titanium(IV) complex [{TiCp*Cl(-NPh)}2] (37) via cleavage of the N=N bond. Similarly, exposition of solutions of 36 to dinitrogen at room temperature led to the formation of [{TiCp*(-Cl)}3(3-1:2:2-N2)] (38). Compound 38 represents the first well-defined example of the 3-1:2:2 coordination mode of the dinitrogen molecule in a metal complex. Spectroscopic data and theoretical calculations indicate that the dinitrogen unit in this complex should be considered a hydrazido [N2]4- ligand. Treatment of 38 with an excess of hydrogen chloride in solution produced NH4Cl and [TiCp*Cl3] (4). This inspired us to design a cyclic production of NH4Cl on a relatively large scale, performing the reaction of 4 with excess of magnesium in thf alternating N2 and HCl atmospheres.
On the other hand, the reduction of [{TiCp*(BH4)(-BH4)}2] (30) with magnesium in thf under N2 afforded the diamagnetic derivative [{TiCp*(BH4)(thf)}2(-1:1-N2)] (39). Complex 39 reacted with (LutH)(BPh4) to give the ionic compound
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[{TiCp*(thf)2}2(-1:1-N2)](BPh4)2 (40). Both complexes 39 and 40 exhibit a -1:1 coordination mode for the dinitrogen unit. According to spectroscopic and crystallographic data we proposed the existence of certain electronic delocalisation in the {Ti(-N2)Ti} moiety.
The treatment of 39 with triflic acid (HOTf) afforded the dinuclear titanium(III) derivative [{TiCp*(BH4)(-O2SOCF3)}2] (41) with N2 and H2 release. Reaction of [{TiCp*(BH4)(-BH4)}2] (30) with lutidinium triflate (LutH)(OTf) in 1:2 proportion also led to 41, but, if an excess of the lutidinium salt was used, the trinuclear titanium(III) complex [{TiCp*(-O2SOCF3)2}3] (42) was obtained. In contrast, treatment of 30 with (LutH)(BPh4) in presence of thf or pyridine resulted in the ionic compounds [TiCp*(BH4)L2](BPh4) (L = thf (43), py (44)).
Finally, the reaction of derivative [{TiCp*(BH4)(thf)}2(-1:1-N2)] (39) with two equivalents of (LutH)(OTf) gave a mixture of species, from which the ionic compounds [(TiCp*)4(3-N)2(3-NH)2][TiCp*(BH4)(OTf)2] (45) and [{TiCp*(3-NH)}4][TiCp*(NH3)(OTf)3] (46) were isolated although in very low yields. When complex 39 was treated with an excess of (LutH)(OTf), again a mixture of products was obtained. From this mixture, a poor fraction of crystals of the mononuclear titanium(III) complex [TiCp*(NH3)2(OTf)2] (47) could be isolated.
Chapter 5. Experimental section.
The experimental procedures for the synthesis of all the complexes are described in this chapter. All manipulations were carried out under argon or dinitrogen atmosphere using Schlenk line or glovebox techniques. Solvents were refluxed over an appropriate drying agent and distilled just prior to use. The starting reagents were purchased or prepared according to published methods.
The structural characterization of the new compounds was performed by elemental analysis as well as IR and NMR spectroscopic techniques. The molecular structures for most of the complexes were unambiguously determined by X-ray diffraction experiments.
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