Figure 1 and Fig. 2 demonstrate that at room temperature, the vanadyl porphyrin complex on the surface of γ-Al2O3 is mainly in the form VOTPP/γ-Al2O3-2, with a minor fraction of forms VOTPP/γ-Al2O3-1 and VOTPP/γ-Al2O3-3, which are not well resolved. With a gradual increase in the temperature of heating up to 400°C, a decrease in the contribution of form VOTPP/γ-Al2O3-2 in the EPR spectrum is observed, accompanied by an increase in the contribution of forms VOTPP/γ-Al2O3-3 and then VOTPP/γ-Al2O3-4.
The heating of the sample at temperatures up to 450°C under forevacuum conditions, followed by subsequent cooling of the sample to room temperature, and exposure to air in an ampoule, results in the broadening of the spectra. The relative contributions of different forms remained essentially unchanged. However, holding the sample in air at normal pressure, room temperature, and 40% humidity for several days led to complete restoration of the spectrum to its original state observed at 20°C prior to heating the sample (Fig. 3). Reversible transition of forms VOTPP/γ-Al2O3-3,4 to forms VOTPP/γ-Al2O3-1,2 upon water adsorption on the γ-Al2O3 surface indicates that the affinity of the water to the adsorption sites, on which VOTPP is adsorbed in forms 3 and 4, is higher, and the water added to the system displaces VOTPP to the sorption sites where VOTPP is adsorbed in forms VOTPP/γ-Al2O3-1,2. Upon subsequent evacuation and heating, the reappearance of forms 3 and 4 is observed again.
Figure 4 shows the correlation between gxy and Axy and between gz and Az spin-Hamiltonian parameters for the detected forms of VOTPP/γ-Al2O3-1,2,3,4. Figure 4 also presents the data available in the literature on the parameters of the VOTPP spin Hamiltonian in various solvents [3, 30–36]. There is a considerable variability in the corresponding spin Hamiltonian parameter values for even same solvent (for example, see values for VOTPP/CHCl3). Of course, analyzing this data it is necessary to take into account the inaccuracy of calibration of the spectrometer field, the broadening of lines in concentrated solutions, the difference in the temperatures at which spectra are recorded, and the presence of impurities in the solution that are easily coordinated along the axial axis of the complex. Despite all the above circumstances, a reproducible and unambiguous trend appears, consisting of a simultaneous decrease in Axy (by ~ 5%) and Az (by ~ 2%) and an increase in gz (by ~ 0.005) when moving from weakly polar solvents (having a dielectric constant ϵ < 5) to moderately polar ones (5 < ϵ < 15).
There is well known a strong correlation between the z-components of spin-Hamiltonian parameters and the equatorial environment of the vanadyl complex [37, 38]. As an illustration, Fig. 4 depicts the typical values for vanadyl in the presence of four sulfur atoms (4S) and four nitrogen atoms (4N) in a non-porphyrin equatorial environment. The influence of the local environment polarity along the axial axis of vanadyl complexes on the spin-Hamiltonian parameters is discussed less frequently in the literature [3, 12, 13]. At the same time, for the VO(acac)2 [12, 13], VOTPP and VOOEP [3] complexes, it was reliably shown that an increase in the polarity of the porphyrin environment results in a decrease in the EPR hyperfine interaction constant Aiso and value of giso, indicating a stronger delocalization of the unpaired electron due to the vanadyl interaction with polar groups of molecules located on the (V = O)2+ axis. For VOTPP and VOOEP, a decrease in each of the gxy, Axy, and Az components and an increase in gz with increasing solvent polarity were shown (since the decrease in gxy and the increase in gz occur by approximately the same amount, the value of giso = (2gxy + gz)/3 decreases in the same way as for VO(acac)2).
The spin-Hamiltonian parameters identified in earlier studies for VOTPP adsorbed on Al2O3 and Al2O3-based catalysts exhibit a greater degree of dispersion (see Fig. 4) [11, 20–25]. Nevertheless, it can be assumed that in these systems, VOTPP typically occurs in a polar environment. In some cases, VOTPP is observed in a local environment that is more polar than pyridine, a solvent with a maximum dielectric constant in which spin-Hamiltonian parameters are available for VOTPP.
Figure 4 Correlation diagrams between spin-Hamiltonian parameters gxy and Axy (a) and between spin-Hamiltonian parameters gz and Az (b) for the vanadyl porphyrins adsorbed on γ-Al2O3 or supported on γ-Al2O3 catalysts: VOTPP/γ-Al2O3-1,2,3,4 [this work], Boscan porphyrins/γ-Al2O3 [22, 23], VOTPP/(γ-Al2O3,MoxNiyAl2O3), VOOEP/Al2O3, VOOEP/Ni-Mo-Al2O3, VOOEP/CoMo/Al2O3, VOOEP/CoMoSx/Al2O3 [24], VOOEP/Al2O3 [26], HDS catalyst [27]; VOTPP frozen in low polar solvents: CS2 [28, 30], CCl4 [28], o-xylene [3], piperidine [34], toluene [3, 30, 35], benzene [28], CHCl3 [28, 30–33]; mid-polar solvents: CH2Cl2 [3, 28, 30], C2H2Cl4 [36], methylcyclohexane-1,3-diamine (MCDA) [3], Tetrahydrofuran [33], pyridine, 2-methyltetrahydrofuran [30] CH2Cl2:pyridine = 4:1 (vol:vol) [28]; and captured in solid: triphenylphosphine, H2TPP [30], ZnTPP [39]. Parameter values for the 4N type complex of the trans-[VIVOX(capca)] with various ligands X in the 6th position [40] are also shown. The arrows show the change in parameters for the trans-[VIVOX(capca)] complex when the neutral ligand is replaced by a charged cation in the 6th position of the complex. For convenience, typical parameters for vanadyl complexes with four nitrogen (4N) and sulfur (4S) atoms in an equatorial environment [37, 38] are also shown
Figure 4 illustrates that the polarity of the environment of VOTPP/γ-Al2O3-2 form is close to that of solvents with medium polarity, and VOTPP/γ-Al2O3-1 form corresponds to VOTPP, which is in a low-polar environment, suggesting that these are likely complexes consisting of VOTPP adsorbed in such a way that the phenyl groups, due to steric hindrance, prevent contact between the vanadyl fragment and the polar groups of the surface. A further decrease in the values of Axy, Az, gxy, and an increase in gz for forms 3 and 4 is likely due to the coordination of VOTPP to the more polar centers of the surface.
Figure 4 also shows that the obtained spin-Hamiltonian parameters for 2 and 3 forms closely coincide with those identified for VOTPP/γ-Al2O3 in the work [28]: forms CVK-1 и CVK-2 (CVK – the first letters of the surnames of the authors of the article [28]), respectively. In that study, an increase in the relative fraction of the form CVK-2 was also recorded with an increase in the annealing temperature of the sample from 100 to 300 ° C, in addition, this form was also unstable when the samples were exposed to air. The authors [28] proposed that the form CVK-1 may be attributed to a VOTPP molecule, which is bounded via V4+ of the vanadyl group to a Lewis basic surface site of the γ-Al2O3 surface, and form CVK-1 is VOTPP whose vanadyl group is bound through the oxygen to an Lewis acid site. In our opinion forms 2, 3, and 4 are probably VOTPP adsorbed on the γ-Al2O3 surface sites with increasing Lewis acidity. These facts are consistent with known data, which indicate that during heat treatment of γ-Al2O3, the number and strength of Lewis acid sites increases as water is desorption from the surface [41].
The parameters Az, Axy, and Aiso of the VOTPP/γ-Al₂O₃-3 form were found to be 9%, 24%, and 15% lower than those of the VOTPP/γ-Al₂O₃-1 form, respectively. The respective values for the VOTPP/γ-Al₂O₃-4 form are 12%, 35%, and 22% lower than those of the VOTPP/γ-Al₂O₃-1 form. In homogeneous solutions, the alterations in spin-Hamiltonian parameters resulting from variations in solvent polarity are considerably less pronounced. For example, for VO(acac)2, the maximum reduction in Aiso is about 6% at the transition from low-polar and high-polar solvents, and for VOTPP, the maximum reduction in Az, Axy, and Aiso is 3%, 5%, and 3.5%, respectively, at the transition from low-polar to mid-polar solvents. However, in these systems, the polar solvent molecules coordinating along the axial axis of the complex are neutral. In the study [42], vanadyl complexes were obtained in which the vanadyl is coordinated along the axial axis with anions SCN−, COO−, Cl−, N3−, PhCOO−. For these complexes, with a constant equatorial environment of 4N and substitution of neutral axial ligands with the mentioned anions, the decrease in Az, Axy, Aiso ~10%. Thus, charged ligands on the axial axis of vanadyl complexes, with a constant equatorial environment, may lead to several times greater reductions in Az, Axy, Aiso constants than those observed in typical neutral solvents.
The theory of EPR spectra of transition metals [33] gives the following expressions for gxy и gz:
$$\varDelta {g}_{xy}={g}_{xy}-{g}_{e}=-2\lambda \frac{{{\beta }_{2}^{\text{*}}}^{2}{{\epsilon }_{\pi }^{\text{*}}}^{2}}{\varDelta {E}_{xz,yz}}$$
1
,
$$\varDelta {g}_{z}={g}_{z}-{g}_{e}=-8\lambda \frac{{{\beta }_{2}^{\text{*}}}^{2}{{\beta }_{1}^{\text{*}}}^{2}}{\varDelta {E}_{{x}^{2}-{y}^{2}}}$$
2
,
where λ ≅ 150 см-1 [43] is the spin-orbit coupling constant, β2*, β1* and επ* – the contributions of dxy, dx2-y2, dxz,yz wave functions of vanadium into antibonding orbitals b2g*, b1g* и eπ*, and ∆Exz,yz and ∆Ex2-y2 are the energies for b2g* → eπ* and b2g* → b1g* transitions, respectively.
Figure 5 shows a correlation diagram of the ∆Exz,yz and ∆Ex2−y2 (assuming that β2*2 = β1*2 = επ*2 = 1) for various forms of VOTPP/γ-Al2O3.Since energy level splitting ∆Exz,yz and ∆Ex2−y2 characterize the energy of the V = O and V-N bonds, respectively [3], it is possible to conclude that the increase in the acidity/basicity of the γ-Al2O3 surface sites on which VOTPP is adsorbed is associated with a weakening of the V = O bond energy and strengthening of the V-N bond.
The observed phenomenon can be explained by the coordination VOTPP along the axial axis to surface centers of different polarity/acidity.