Catalysis is usually considered in terms of the energy difference between initial and transition states to increase the rate of reaction required to either enable an alternative reaction pathway with a lower energy transition state to occur or increase the energy of the initial state of the reagents, e.g., by changing the solvent or adsorbing molecules on a specific surface. However, in certain classes of reactions, this might not be sufficient because the reagents and products possess different spin states, and the spin conservation rule formally prohibits such processes (e.g., formation of water from H2 and O2, oxidation of NO, and oxidative complexation of Co3+). Even overcoming the energetic barrier, the system cannot resolve into stable products without change of the electronic spin multiplicity state. Such reactions can still occur, but they require specific non-thermodynamic conditions. Usually, they are referred to as processes where spin is not conserved.
The most straightforward way to change the electronic spin of a system is a spin-flip of an electron by interaction with the spin of the atom nucleus. This event is realized through spin-orbital coupling, and thus its probability strictly depends on the atomic number of the element. Starting from 3d metals, it might occur even more frequently because of higher coupling efficiency with d orbitals, so spin-crossing is a conventional way to explain why spin-forbidden reactions take place. However, the probability of spin flipping of light atoms is negligible (for hydrogen it is of the order of 10-15 s-1). Thus, when considering metal-free chemistry, one should consider the second way of spin prohibition rule cancellation: spin catalysis, which is not related to interaction of the unpaired electron with particles within an atom.
Spin catalysis is defined as “phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst” [1]. More simply, to obtain the desired (and more thermodynamically favorable) spin state, one of the reacting particles can exchange its magnetic moment with the third body (spin catalyst), which presumably has pseudo-spin- degenerate ground or low-lying excited states. Thus, the overall spin state of the system is still conserved but the reaction can proceed (Fig. 1).
It should be emphasized that the spin catalyst term might indicate both molecular and solid states, with the latter often being much more convenient for industrial application.
Historically, spin catalysis has been investigated in the paradigm of spin chemistry concurrently with chemically induced dynamic nuclear polarization and reactions in external magnetic fields, mainly for processes in solution, and investigated by electron paramagnetic resonance spectroscopy. The theory of such processes can be found in the works of Buchachenko, Minaev, and Schwarz [2, 3, 4, 5]. Only brief excerpts are given here.
First, we will discuss a radical pair in the gas phase. It is obvious that the pair can recombine to form a covalent bond only in the singlet state (i.e., in the case when the spins of the radicals are opposite). The probability of such a collision is 25% because two radicals can equiprobably form one singlet and three triplet states, with the latter being anti-bonding. However, in the case of the presence of a third radical (spin catalyst), one of the triplet states (1T) can turn into a reactive singlet (1S) through spin-spin interaction with R3, with the probability of this process exponentially decreasing with increasing distance between the interacting radicals. It should be noted that to make spin catalysis effective, the values of the spin-spin interactions in the R1-R3 and R2-R3 pairs should be different, which can be realized either by the different nature of the interacting radicals R1 and R2 (e.g., different spin-centering atoms) or different distances d(R1-R3) and d(R2-R3) (Fig. 1(b)).
The equilibrium of the RA-RB radical pair in dilute media is shown in Scheme 1. For covalent bond homolytic cleavage, constant k1 is much larger than k-1, so the overall rate is mostly controlled by diffusion step 2. Singlet-triplet transformation is formally forbidden because of the spin conservation rule and depends on the probability of the spin flip, which is extremely improbable. Hence, this stage is the subject of spin catalysis, which opens the possibility of reaction 3, so that overall rate of the reaction comprises diffusion of both singlet (k2) and triplet (k4) radical pairs (Scheme 1(b)). The probability of interaction between the radical pair and the paramagnetic admixture can be estimated by the Smoluchowski equation. The reverse reaction can be described with a similar set of equations, so it is clear that the elementary reactions of the recombination of radicals and covalent bond homolytic cleavage are both spin catalysis phenomena, because they enable additional reaction pathways to form the singlet or triplet radical pair, respectively [7, 8, 9]. However, the opposite direction of the processes might require different catalysts.
It should be emphasized that there is no opposition between conventional catalysis and spin catalysis, and the same material can serve as both a conventional catalyst and a spin catalyst, decreasing the barrier of the reaction and providing a radical center for spin-exchange (examples will be discussed below). Moreover, a lot of modern materials combine classical and spin-catalyst properties, although this has never been emphasized. Some processes in nature occurring at polynuclear magnetic metalo-centers also might combine the effects of conventional catalysis (by activation of substrates), spin-crossing (owing to interaction with d metals), and spin catalysis (owing to low-energy ferromagnetic-antiferromagnetic transitions in metal-oxide clusters) [10, 11, 12, 13, 14].
The main problem for practical implementation of spin catalysis in the gas phase is rather trivial: the probability of three-particle collision is low in dilute systems. In this regard, catalysis by solids has a number of advantages. First, the requirements for the solid to be a spin catalyst have to be defined:
(1) Active centers should have a small energy difference between at least two spin states (e.g., singlet and triplet). Presumably, this difference should be less than the energy of thermal motion at the temperature of the process.
(2) The solid should have high specific surface area (this coincides with the requirements for conventional heterogeneous catalysts).
(3) Optionally, the materials should be spin-conductive (i.e., spin centers should be conjugated and thus have a common electronic system). Although this requirement is not obligatory, it can give the catalyst valuable additional properties, as will be shown below.
We will now discuss the role that each of the above- mentioned criteria plays in the efficiency of the spin catalyst. Isolated free radicals in the absence of an external magnetic field are spin-degenerate by default. However, this does not apply for complex spin centers, like polynuclear complexes of paramagnetic d-metal ions, which often tend to have antiferromagnetic ordering. However, if the energy difference between the antiferromagnetic (ground) and ferromagnetic (excited) states is small, there should be no requirement for additional energetic activation of the spin catalyst to be able to exchange spin with one of the reacting species. Certainly, the lower this difference, the larger the partition of states for spin catalysis in each moment of time because of the Boltzmann distribution.
A large specific surface area allows more spin centers to be available for the interaction, similar to conventional heterogeneous catalysts. Additionally, adsorption of reagents specifically orients them relative to the spin catalyst, so the distance between different atoms in the substrate and the spin catalyst is more or less defined (Fig. 2), and thus the effect of spin catalysis is less stochastic.
In the previous cases, we considered only localized effects of the spin catalyst. However, what happens if a material is spin-conductive? The spin centers are then conjugated and have a common electronic system, so they are correlated (only the limit of the speed of light and possible defects in the structure hinder conductivity). This means that all of the species in the gas of liquid phase that are close to the surface of the catalyst particle are also correlated via the electronic system of the catalyst. If something happens to the catalyst (e.g., it is charged either positively or negatively, excited or quenched, becomes radical or diamagnetic) all of the species within the effective distance from the surface of the particle (which is still defined by the exponential law) are affected. In this case, the probability of the spin-spin interaction, which is a crucial factor for spin catalysis, greatly increases because there is no need for three-particle collision. The radicals can then exchange their spins via the electronic system of the catalyst when they are separated by large distances (up to hundreds of micrometers as long as the spin catalyst retains conductivity at such a range) (Fig. 2(d)).
Spin conductivity of the catalyst (or support) also provides the possibility to remotely control the charge and spin state of the catalytic centers through application of an external electric or magnetic field. The main problem of reactions in a magnetic field is that the reacting system should be placed into the cavity of a strong magnet to induce different population of the spin levels [15]. Using a conductive support, it might be possible to separate the place of magnetic field application and the reaction space, and also to tune the material of the support to be more susceptible to the effect of the magnetic field. Hence, in this way, non-thermodynamic control over the reaction is possible by choosing appropriate external field parameters.
Conventional conductors, i.e., metals, possess the properties of a spin catalyst by default if an electric current (being a combination of two coherent spin currents) is running through them. This shows the peculiarities of electrocatalysis, which also can be explained by taking the spin-related properties of the catalyst and substrates into consideration.
For application of spin catalysis in real processes, a few points should be stressed. First, a lot of processes in nature and industry already apply this phenomenon (the most important example is combustion, where triplet O2 molecules convert into singlet products). Second, spin catalysis might have as many problems as benefits, for instance, initiating undesired chain radical reactions. The aim is to determine the right way to enable the target processes and suppress unwanted processes. Third, spin inhibition (cancellation of spin-changing reactions by introducing spin scavengers) might be as useful as spin catalysis. Fourth, spin catalysis/inhibition can be stimulated not only by the concentration of the magnetic particles (molecular radicals or paramagnetic solids), but also by external physical effects, such as magnetic and electric fields, and current (particularly, spin-polarized current), because there are both theoretical and experimental reports of their effect on the energy of the transition states and them “triggering” different reaction pathways [16, 17, 18, 19, 20, 21, 22, 23, 24, 25].
Spin catalysis in homogeneous media (by soluble or volatile species) has been intensively studied [26]. We can refer to such applications as initiation and inhibition of radical polymerization (in rubbers, foams, glues, etc). Hereafter, we focus on application of spin catalysis in heterogeneous systems. Several areas of application can be proposed, each requiring more detailed investigation. Below each of these areas will be briefly discussed.
A lot of effort has been made to develop effective catalysts to both prevent (in selective oxidation) and promote combustion (e.g., in removal of volatile pollutants). One of the main factors is the reactivity of the triplet (ground state) 3O2 molecule, because the final products (and semi-products) of its reaction are usually singlet state, so changing the spin state of the system is often a limiting step. It should be noted that the complicated character of the elementary processes occurring during combustion is usually because of the necessity to deal with the spin conservation rule, leading to detours in the pathways of the reactions. One way to enhance the reactivity of dioxygen, apart from activating it by increasing its energy, is transformation into the singlet state (Fig. 3, top left), which is known to be one of the most forbidden transitions in nature [27]. By finding an appropriate spin catalyst, this transition can be either inhibited or prohibited (for instance, by introducing more effective spin scavenger), depending on the demands of the process.
Many industrial processes require mild oxidation of the reagent, e.g., oxidation of hydrocarbons to alcohols (with methane-to-methanol conversion being the most difficult) [28]. Practical investigation is not only to increase conversion and selectivity, but also to decrease the temperature of the process, making the reaction more localized and thus decreasing diffusion of half-products, which might be unselectively oxidized. Here, a spin catalyst, which is able to prevent recombination of broken C-H bonds and activate O2 to the singlet state (or allow its easy dissociation), might be useful.
Transformation to a biradical state is both necessary and sufficient in a lot of cyclization reactions (e.g., anti-Hückel cycloaddition) [29]. A spin catalyst might increase the rate of the reaction by enhancing transformations between different spin states of such biradicals. Another example is cyclization of n-hexane to c-hexane, which occurs through cleavage of primary C-H bonds. It is known that geometrical prearrangement on the surface of the catalyst increases the selectivity of this cyclization. However, there is still not an effective catalyst for selective cleavage of these C-H bonds, which can be achieved by geometrically placing the ends of the n-hexane close to the spin center.
In contrast to cyclization, change of the spin state might be crucial for cycle opening (Fig. 3, bottom left). For instance, in the synthesis of maleic anhydride from benzene or phtalic anhydride, the naphthalene C=C bond is cleaved via cycloaddition of O2, which requires transition to the singlet state so can be promoted by spin catalysis.
Molecules like H2 or Cl2 are difficult to activate using standard acid-base catalysts because of their zero dipole moment and low polarizability. However, spin catalysts, which are able to induce spin changes, might enable their transition to the active triplet state or lead to their homolytic dissociation. The radicals evolved can either initiate chain reactions or donate/accept an electron, transforming into reactive ions. These properties are widely exploited in the water-gas shift reaction and synthesis of phosgene.
Nitromethane (CH3NO2), a monopropellant, can be formed by coupling CH3• and NO2• radicals (Fig. 3, top and bottom right) [30]. However, the temperature of this process should be low to prevent chain decomposition of the compound. A spin catalyst might increase the rate of the coupling reaction at low temperatures by increasing the partition of singlet pairs formed upon collision of radicals.
Finally, we will give possible candidates for spin catalysts. First, magnetic oxides and polynuclear magnetic clusters containing different paramagnetic metal ions, e.g., Mn(II/III/V), Fe(III), Co(II), and Cu(II) [10-14, 31-35]. Second, polynuclear complexes of transition metals (in low-temperature catalysis) grafted on supports [14]. Third, materials with non-integer oxidative states of metals (e.g., bronzes) that are also conductive [36]. Finally, different types of carbon materials that possess half-metallicity, i.e., equal stability of several multiplicity states [37], and are good conductors (e.g., B- or N-doped carbon particles, where partially localized spins should be observed [38, 39]).
There is one significant problem with investigation of spin-catalyzed reactions: most of these processes are of radical nature, and thus tend to fall into the non-linear regime, which makes them difficult to predict. The physics (and mathematical background) of non-linear processes is still unclear, so there is not a convenient tool to describe and, more importantly, predict a cascade of spin-catalyzed reactions. However, the merits of such studies might be as good as unexpected. As well as the ordinary tuning the reactions (e.g., low-temperature selective oxidation or metal-free catalysis), a unique tool to non-thermodynamically control reactions could be developed, with elementary processes being triggered by an external electric or magnetic field. This would have clear advantages in control over reactor media, which is now mainly achieved by changing the temperature, pressure, and flow rates (parameters with a long delay of response). Change of reaction rates by an external magnetic field would allow immediate response of the system, which is much preferable. Additionally, fine control over elementary reactions opens a way to suppress undesirable by-processes through decrease of the temperature, and thus makes them less stochastic.
2015, Vol. 36 Issue (10): 1656-1661
PDF (576 KB)
2015, Vol. 36 Issue (10): 1656-1661
PDF (576 KB)