Quantum Chemical Study of the Interligand Electron Transfer in Ru Polypyridyl Complexes.

Quantum chemical calculations have been performed to study the photocycle of [Ru(bpy)3]2+, a complex that is extensively used as an electron donor in photocatalytic reactions. After the initial spin-allowed excitation from the nonmagnetic ground state to a singlet state of metal-to-ligand charge transfer character, the system undergoes a rapid intersystem crossing to a triplet state of equal character. The calculations indicate a lifetime of 10 fs, in good agreement with experimental estimates. Important factors for this extremely fast intersystem crossing are the large spin-orbit coupling and the large vibrational overlap of the states involved. Both MLCT states are delocalized over the three bipyridine ligands, but the delocalized electron can easily increase its degree of localization. The hopping parameters have been calculated and found to be large for the localization on two ligands and subsequently on one. The combination of localization and geometry relaxation creates a rather long-lived trapped triplet MLCT state with a calculated lifetime of 9 µs. The addition of methyl groups on the bipyridine ligands decreases the ligand field and consequently lowers the metal-centered triplet states. This could eventually lead to opening of a fast deactivation channel of the 3MLCT states to the initial nonmagnetic states via the triplet ligand field states as occurs in the corresponding Fe(II) complex.

Role of the Imide Axial Ligand in the Spin and Oxidation State of Manganese Corrole and Corrolazine Complexes.

Electronic structure calculations have been performed on four different Mn corrole and corrolazine complexes to clarify the role of the imide axial ligand on the relative stability of the different spin states and the stabilization of the high-valent Mn ion in these complexes. Multiconfigurational perturbation theory energy calculations on the DFT-optimized geometries show that all complexes have a singlet ground state except the complex with the strongest electron-withdrawing substituent on the imide axial ligand, which is found to have a triplet ground state. The analysis of the σ and π interaction between the metal and imide ligand shows that this spin crossover is caused by a subtle interplay of geometrical factors (Mn-N distance and coordination angle) and the electron-withdrawing character of the substituent on the imide, which reduces the electron donation to the metal center. The analysis of the multiconfigurational wave functions reveals that the formally Mn(V) ion is stabilized by an important electron transfer from both the equatorial corrole/corrolazine ligand and the axial imide. The macrocycle donates roughly half an electron, being somewhere between the closed-shell trianionic and the dianionic radical form. The imide ligand transfers 2.5 electrons to the metal center, resulting in an effective d-electron count close to five in all complexes.

Spin-crossover in phenylazopyridine-functionalized Ni-porphyrin: trans-cis isomerization triggered by π-π interactions.

Reversible, room-temperature light-induced spin-crossover has been reported in a Ni-porphyrin functionalized with a phenylazopyridine (PAPy) ligand (Venkataramani et al., Science, 2011, 331, 445). Upon light irradiation (500 nm), the azopyridine moiety induces a change in the Ni(II) coordination sphere from square planar (n = 4) to square pyramid (n = 5), leading to a change in the total spin of the molecule from S = 0 to S = 1. The trans-cis isomerization in the azopyridine ligand has been proposed to trigger the spin-crossover effect. However, the radiation used to induce the HS state is about 135 nm red-shifted with respect to the radiation used for trans-cis isomerization of the N=N double bond in other compounds. To elucidate the light-induced spin-crossover mechanism of this Ni(II) compound, a combined DFT/CASSCF/CASPT2 study has been performed to determine the most stable cis and trans conformers with n = 4 or n = 5, and to characterize the excitation that triggers the SCO process. π-π interactions between porphyrin and PAPy are shown to play an essential role in the spin crossover.

Electronic structure aspects of the complete O2 transfer reaction between Ni(II) and Mn(II) complexes with cyclam ligands.

This work explores the electronic structure aspects involving the complete intermolecular O2 transfer between Ni(ii) and Mn(ii) complexes, both containing N-tetramethylated cyclams (TMC). The energy of the low-lying states of reactants, intermediates and products is established at the CASSCF level and also the DDCI level when possible. The orthogonal valence bond analysis of the wave functions obtained from CASSCF and DDCI calculations indicates the dominant superoxide nature of all the adducts participating in the reaction, and consequently that the whole reaction can be described as the transfer of the superoxide O2(-) between Ni(ii) and Mn(ii) complexes, without any additional change in the electronic structure of the fragments.

On the reaction mechanism of the complete intermolecular O2 transfer between mononuclear nickel and manganese complexes with macrocyclic ligands.

The recently described intermolecular O2 transfer between the side-on Ni-O2 complex [(12-TMC)Ni-O2](+) and the manganese complex [(14-TMC)Mn](2+), where 12-TMC and 14-TMC are 12- and 14-membered macrocyclic ligands, 12-TMC=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long-range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two-step reaction, with a first rate-determining bimolecular step and predict the exothermic character of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a µ-η(1):η(1)-O2 coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction.

The role of macrocyclic ligands in the peroxo/superoxo nature of Ni-O2 biomimetic complexes.

The impact of the macrocyclic ligand on the electronic structure of two LNi-O2 biomimetic adducts, [Ni(12-TMC)O2](+) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and [Ni(14-TMC)O2](+) (14-TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), has been inspected by means of difference-dedicated configuration interaction calculations and a valence bond reading of the wavefunction. The system containing the 12-membered macrocyclic ligand has been experimentally described as a side-on nickel(III)-peroxo complex, whereas the 14-membered one has been characterized as an end-on nickel(II)-superoxide. Our results put in evidence the relationship between the steric effect of the macrocyclic ligand, the O2 coordination mode and the charge transfer extent between the Ni center and the O2 molecule. The 12-membered macrocyclic ligand favors a side-on coordination, a most efficient overlap between Ni 3d and O2 π* orbitals and, consequently, a larger charge transfer from LNi fragment to O2 molecule. The analysis of the ground-state electronic structure shows an enhancement of the peroxide nature of the Ni-O2 interaction for [Ni(12-TMC)O2](+), although a dominant superoxide character is found for both systems.

Comparing the peroxo/superoxo nature of the interaction between molecular O2 and ß-diketiminato-copper and nickel complexes.

Adducts resulting from the interaction between molecular oxygen and ß-diketiminato-copper and nickel complexes have been recently described in the literature as peroxo and superoxo complexes, respectively. The nature of the interaction is analyzed by means of DDCI calculations and an orthogonal valence bond reading of the ground state wavefunction for each system. Our results reveal that there is not any substantial difference between these systems, both presenting a marked leading superoxo nature, which is in line with the fact that LCu-O(2) and LNi-O(2) adducts present similar O-O distances and quite close O-O stretching vibration modes.

The electronic structure of Ullman's biradicals: an orthogonal valence bond interpretation.

This paper addresses the electronic structure of Ullman's organic biradical, bis(nitronyl) nitroxide, in the frame of ab initio wave function based methods, both using standard (delocalized) molecular orbital methods and a valence bond strategy based on orthogonal localized orbitals. The aim of this study is to clarify the origin of the magnetic coupling of this molecule and to understand the recently observed unexpected behavior of the computed magnetic coupling constant with respect to the use of different theoretical methods. A detailed analysis of the physical effects that govern the correct description of the main characteristics of the electronic structure, in particular the spin density, reveals that the wave functions of the radical fragments are better seen as arising from the interaction of three unpaired electrons and that the intuitive picture of one unpaired electron per fragment is oversimplified and can lead to incorrect results. Furthermore, we establish that both the triplet and the singlet wave functions are well represented by the antisymmetrized product of the two fragment wavefunctions, thus corroborating previous observations concerning the low weight of the ionic structures (hopping of one electron from one fragment to the other) in the biradical singlet wavefunction. The use of the orthogonal valence bond formalism has also allowed us to extract a set of parameters for the fragment (direct exchange, hopping integral, on-site repulsion) which can be used for larger magnetic systems in studies based on model Hamiltonians.

Insights on the photomagnetism in copper octacyanomolybdates.

High level ab initio calculations on the photoinduced high-spin molecule [Mo(CN)(2)(CN-Cu(tris(2-aminoethyl)amine)(6)](8+) are reported. The calculations indicate that the mechanism of the photoinduced transformation from a paramagnetic to a ferromagnetic state involves a local Mo d-d transition followed by the deformation of the coordination sphere from dodecahedron to square antiprism. Subsequently, Mo loses a ligand and becomes seven coordinated in a pentagonal bipyramid coordination. The resulting Mo(IV)(S = 1) ion interacts ferromagnetically with the five remaining Cu(II) ions through the cyanide bridges. The estimated coupling is about +50 K and the resulting magnetic susceptibility curve resembles the experimental one taking into account that part of the sample is magnetically deactivated during the measurement. The calculated potential energy profile along the linear interpolated reaction coordinate shows a small barrier for the reverse reaction in agreement with the thermal reversibility of the photoinduced state. Moreover, we find that the reverse reaction can be induced by light.

Electronic structure and relative stability of 1:1 Cu-O2 adducts from difference-dedicated configuration interaction calculations.

A computational strategy to analyze Cu-O(2) adducts based on the use of difference-dedicated configuration interaction (DDCI) calculations is presented. The electronic structure, vertical gaps and nature of the metal-O(2) interaction, and the extension of the charge transfer between both fragments have been investigated. Relative stabilities between isomers are determined from triplet states CCSD(T) calculations. The key point of the here proposed strategy rests on the use of a rationally designed active space, containing only those orbitals, which optimize the interaction pathways between LCu and O(2) fragments. The procedure has been tested on a broad set of model and synthetic biomimetic systems, the results compared with previous theoretical evaluations and/or available experimental data. Our study indicates that this strategy can be considered as an alternative approach to multireference second-order perturbation theory methods to deal with this type of systems with remarkable biradical nature.

Extraction of magnetic coupling parameters in 2-dimensional magnetic honeycomb layers.

Quantum chemical calculations have been performed to determine the magnetic coupling between transition metals in the anionic layers of bifunctional materials. These layers are characterized by a hexagonal grid of oxalato-bridged transition metal ions, [M(2)(ox)(3)](2-), with M = Cu(2+), Ni(2+), Co(2+), Fe(2+). No experimental information is available for the coupling constants in these lattices. The magnetic coupling parameters are calculated through an embedded cluster containing two metal ions and the oxalato ligands coordinated to these. The calculated parameters are used to construct a larger model for which the temperature dependence of the magnetic susceptibility is calculated. The curves obtained compare reasonably well with experimental curves, validating the theoretical estimates of the magnetic coupling between the transition metals in the hexagonal lattices.

Analysis of the magnetic coupling in binuclear systems. III. The role of the ligand to metal charge transfer excitations revisited.

In magnetic coordination compounds and solids the magnetic orbitals are essentially located on metallic centers but present some delocalization tails on adjacent ligands. Mean field variational calculations optimize this mixing and validate a single band modelization of the intersite magnetic exchange. In this approach, due to the Brillouin's theorem, the ligand to metal charge transfer (LMCT) excitations play a minor role. On the other hand the extensive configuration interaction calculations show that the determinants obtained by a single excitation on the top of the LMCT configurations bring an important antiferromagnetic contribution to the magnetic coupling. Perturbative and truncated variational calculations show that contrary to the interpretation given in a previous article [C. J. Calzado et al., J. Chem. Phys. 116, 2728 (2002)] the contribution of these determinants to the magnetic coupling constant is not a second-order one. An analytic development enables one to establish that they contribute at higher order as a correlation induced increase in the LMCT components of the wave function, i.e., of the mixing between the ligand and the magnetic orbitals. This larger delocalization of the magnetic orbitals results in an increase in both the ferro- and antiferromagnetic contributions to the coupling constant.

On the applicability of multireference second-order perturbation theory to study weak magnetic coupling in molecular complexes.

The performance of multiconfigurational second-order perturbation techniques is established for the calculation of small magnetic couplings in heterobinuclear complexes. Whereas CASPT2 gives satisfactory results for relatively strong magnetic couplings, the method shows important deviations from the expected Heisenberg spectrum for couplings smaller than 15-20 cm(-1). The standard choice of the zeroth-order CASPT2 Hamiltonian is compared to alternative definitions published in the literature and the stability of the results is tested against increasing level shifts. Furthermore, we compare CASPT2 with an alternative implementation of multiconfigurational perturbation theory, namely NEVPT2 and with variational calculations based on the difference dedicated CI technique.

Role of the coordination of the azido bridge in the magnetic coupling of copper(II) binuclear complexes.

It is well-known that the azido bridge gives rise antiferromagnetic (AF) or ferromagnetic (F) coupling depending on its coordination mode, namely end-to-end or end-on, respectively. The aim of the present work is to analyse the factors contributing to this different magnetic behaviour. The difference dedicated configuration interaction (DDCI) method is applied to several binuclear Cu(II) azido-bridged models with both types of coordination. In end-on complexes, the direct exchange and the spin polarisation contributions are found to be responsible for the ferromagnetic coupling. In end-to-end complexes, both the direct exchange and the spin polarisation are small and the leading term is the antiferromagnetic dynamical polarisation contribution. The most relevant physical effects are included in the DDCI calculations so that good quantitative agreement is reached for the coupling constant as well as the spin densities.