Alimentación de los ventiladores

Water is only one of several small molecules that have been of recent interest to scientists. Small molecules such as O₂, N₂and CH₄, are also appealing targets for synthetic chemists. Whilst being essential for terrestrial biological activity, many of them are also implicated in global warming and ozone depletion due to the large industrial generation of small molecules, such as CO₂. Despite this, they are useful in many carbon-neutral energy supply schemes because of the large amount of energy that can be stored in their bonds. Consequently much research has been conducted on maximising our use of small molecules, with the process considered the current prime challenge for chemists, by some.65_{Some of the work in this thesis looks at the design and synthesis of new} homogeneous catalysts, with the aim to activate one or more of these gases.

Many small molecules are inert at standard temperature and pressure, and therefore require activation. Even O₂is relatively unreactive in its triplet ground state, being sluggish to react with common singlet state molecules due to the conservation of spin quantum numbers, requiring activa- tion energy to initiate reactions, often via singlet oxygen.66_{In order to provide the required energy} the substrate is normally adsorbed to the surface of a heterogeneous catalyst, whilst a homogeneous catalyst usually coordinates the substrate to the metal centre. Once coordinated it is able to undergo a variety of transformations, leading to the formation and release of the product. For the activation of small molecules the use of large bulky ligands is normally required, in order to minimise the available space around the metal centre and prevent the coordination of undesired compounds. The ligands are normally flexible, in order to accommodate the change in metal oxidation which will occur, for this reason chelation is common. The use of electron-donating or electron-withdrawing motifs on the ligands can also be used to fine tune the electronics of the catalyst, as can the use of

𝜋-bonding.

(3-tert-butylsalicylideniminato)cobalt, 1. This complex is stable at room temperature under air, however reacts with pure dioxygen in the presence of pyridine to form the octahedral cobalt(III) complex, 2, shown in scheme 1.4.67_{The binding causes a reduction of the bond order of dioxygen,} seen in the elongation of the O−O bond from 1.21 Å to 1.35 Å. The angle between cobalt and the bound dioxygen is 116.4° in the crystal structure, indicating that the oxygen is binding through one of its lone pairs.

Co O N O N N O O t_Bu t_Bu Co O N O N t_Bu t_Bu O2, Py Acetonitrile 1 2

Scheme 1.4. The activation of O₂by complex 1.67

Binding of carbon dioxide typically requires a nucleophillic metal centre and a coordination vacancy. Carbon dioxide is weakly electrophillic, and will bind to metals in a variety of manners. A common method is 𝜂1_{, where the carbon atom forms a sigma bond to the metal centre. This bond} is reinforced through back-bonding from the d_z2 orbital on the metal to the 𝜋∗orbitals on the CO₂,

which align. The 𝜂1_{bind mode is not robust however, and is only one of nine recognised binding} modes for CO₂.68_{An example of the activation of CO}

2is shown in scheme 1.5, wherein complex 3 is placed under a partial pressure of CO₂, after which X-ray suitable crystals of coordinated complex 4 are collected.69_{The O−C−O bond angle in the crystal structure is 126°, which indicates} that there is a substantial amount of 𝜋-back-bonding into the CO₂𝜋∗orbital. This is also shown in

the C−O bond elongation from 1.16 Å to 1.25 Å.

Rh As As CO2, 1.35 bar Acetonitrile As As Cl Rh As As As As Cl O O 3 4

Scheme 1.5. The activation of CO₂by complex 3.69

The activation of carbon monoxide has been exploited on an industrial scale for many decades, the famous example being the Monsanto process for the synthesis of acetic acid from methanol and

carbon monoxide.70_{Due to its polar nature and available 𝜋}∗_{orbitals, carbon monoxide is relatively} active for a small molecule, for instance it is infamous for having a higher binding coefficient with haemoglobin than oxygen. For this reason there are numerous academic texts committed to the field.71,72 _{One example complex for the activation of CO is the rhodium complex 5. Shown in} figure 1.10, complex 5 has an eightfold increase in activity for carbonylation over the original Monsanto catalyst.73 _{This gain in activity is due to an increase in electron donation to the metal} centre, combined with an optimisation of the steric environment of the catalyst.74

Ph2P S Rh Ph2 P _CO I 5

Figure 1.10. A rhodium complex capable of using carbon monoxide to form acetic acid from methanol. It is eight times more active than the original Monsanto catalyst.73

The activation of dinitrogen is an impressive feat. Nitrogen is incredibly inert, having a bond order of three and a bond strength of 940 kJ mol−1_.75_{Compared to carbon monoxide, with whom} it is isoelectric, it is more difficult to activate because it has no dipole, is a weaker 𝜎-donor and a poorer 𝜋-acceptor.76_{Despite this, there are numerous examples of activated dinitrogen with many} transition metals.77–79_{The degree of nitrogen activation depends on the amount of back-bonding} into the 𝜋∗_{orbitals of nitrogen; this can be measured, as with other small molecules, by observing} the N−N bond length in the coordinated complex. As one may expect, there are numerous binding modes for N₂. The most common however is 𝜇-𝜂1_:𝜂1_{, involving a bimetallic complex. An exam-} ple of this type of binding is shown in scheme 1.6, where the mononuclear niobium complex 6 reacts with N₂and sodium triethylborohydride to form the bimetallic complex 7.80_{The N−N bond} length in the activated complex 7 is 1.34 Å, compared to 1.10 Å for free nitrogen. This large bond elongation means that complex 6 is classed as a strong activator of N₂.76

N2, NaHBEt3 Toluene Nb NCy2 NCy2 Cy2N _Cl Nb Cy2N Cy2N Cy2N Nb NCy2 NCy2 NCy2 N N 2 6 7