Ramon Costa

Performance of the M06 family of exchange-correlation functionals for predicting magnetic coupling in organic and inorganic molecules

The performance of the M06 family of exchange-correlation potentials for describing the electronic structure and the Heisenberg magnetic coupling constant (J) is investigated using a set of representative open-shell systems involving two unpaired electrons. The set of molecular systems studied has well defined structures, and their magnetic coupling values are known experimentally. As a general trend, the M06 functional is about equally as accurate as B3LYP or PBE0. The performance of local functionals is important because of their economy and convenience for large-scale calculations; we find that M06-L local functional of the M06 family largely improves over the local spin density approximation and the generalized gradient approximation.

Restricted Ensemble-Referenced Kohn-Sham versus Broken Symmetry Approaches in Density Functional Theory: Magnetic Coupling in Cu Binuclear Complexes †

The performance of density functional theory in estimating the magnetic coupling constant in a series of Cu(II) binuclear complexes is investigated by making use of two open shell formalisms:  the broken symmetry and the spin-restricted ensemble-referenced Kohn-Sham methods. The strong dependence of the calculated magnetic coupling constants with respect to the exchange-correlation functional is confirmed and found to be independent of whether spin symmetry is imposed or not. The use of a method which guarantees the spin state does not improve the correlation with the experiment and indeed shows some worsening due to an overestimation of the ferromagnetic interactions. However, with the present exchange-correlation functionals, a rather systematic deviation is found. Therefore, it would be possible to develop improved density functionals which will allow for a rigorous treatment of open shell systems in density functional theory.

Synthesis, structure and magnetic properties of trinuclear complexes of copper(II) with different amines as terminal ligands

Abstract Three new trinuclear copper(II) complexes were synthesized: {[(ettmd)Cu]2[μ-Cu(pba)]}(ClO4)2 (1), {[(pmd)Cu]2[μ-Cu(Me2pba)]}(ClO4)2·3H2O (2) and {[(bapa)Cu]2[μ-Cu(Me2pba)]} (3) where Cu(pba) = 1,3-propanediyl-bis)oxamato)cuprate(II), Cu(Me2pba) = 2,2-dimethyl-1,3propanediyl-bis(oxamato)cuprate(II), ettmd = 4-ethyl-1,1,7,7-tetramethyldiethylenetriamine; pmd = N,N,N′,N″, N″-pentamethyldiethylenetridetermined by X-ray analysis. The crystals of 1 are monoclinic, space group P21/c, with a = 18.463(4), b = 14.402(3), c = 16.537(4) A, β = 109.00(3)°, Z = 4 and R = 0.072 for 2447 reflections observed. The crystals of 2 are triclic, space group P 1 − , with a = 19.610(4), b = 16.626(4), c = 16.007(3) A, α = 115.75(3)°, β = 89.97(3)°, γ = 100.86(4)°, Z = 4 and R = 0.068 for 7342 reflections observed. The crystals of 3 are orthorhombic, space group Pcba, a = 40.492(4), b = 16.363(3), c = 10.944(3) A, Z = 8 and R = 0.034 for 1634 reflections observed. The central copper(II) ions of compounds 1 and 2 are square-pyramidal, the apical positions being occupied by a perchlorate anion. In 3 the central copper ion is square-planar. The terminal copper ions of compounds 1, 2 and 3 are all pentacoordinated with geometries that are intermediate between square-pyramidal and trigonal-bipyramidal. If the coordination is described in terms of square-pyramidal geometry, the basal plane of each terminal copper atom is approximately perpendicular to the basal plane of the central copper ion. The distortion towards trigonal-bipyramidal geometry is largest for compound 1 and smallest for 3. Magnetic susceptibility measurements were carried out for this series of complexes. The magnetic data were interpreted using the Heisenberg Hamiltonian and led to J values of −137.1, −128.1 and −92.1 cm−1 for 1, 2 and 3, respectively. The intensity of the magnetic coupling is discussed in terms of the relative orientation of magnetic orbitals.

New series of triply bridged dinuclear Cu(II) compounds: synthesis, crystal structure, magnetic properties, and theoretical study.

Five new triply bridged dinuclear Cu(II) compounds have been synthesized, and their magnetic properties have been measured and characterized. The magnetic coupling constants (J) of these compounds plus a previously structurally characterized compound of the same type have been derived by appropriate fitting of the experimentally measured molar susceptibility variation with the temperature. Two of the compounds are ferromagnetically coupled, and three are antiferromagnetically coupled with J values in the [+150, -40] cm(-1) range. The validity of the structural aggregate Addisons parameter as a qualitative magneto-structural correlation is confirmed. The origin of the magnetic interactions and the magnitude of the magnetic coupling have been analyzed by means of density functional theory-based calculations using a variety of state of the art exchange-correlation potentials. It is shown that the long-range separated LC-ωPBE provides the overall best agreement with experiment for this family as well as for a set of previously reported hetero triply bridged dinuclear Cu(II) compounds, especially for ferromagnetic systems.

Iron carbonyl complexes of heterocyclic α-diimines: systematic synthesis, crystal structures of [Fe(CO)3(L)] and [Fe2(CO)7(L)] (L=2,2′-bipyridine and 1,10-phenantroline), and their AIM analysis ☆

Heterocyclic α-diimines {2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me 2 bpy), 1,10-phenantroline (phen), 4,7-dimethyl-1,10-phenantroline (4,7-Me 2 phen) and 2,9-dimethyl-1,10-phenantroline (2,9-Me 2 phen)} react, in a wide range of conditions, with iron clusters of nuclearity one, two or three. Two kinds of compounds, [Fe(CO) 3 (α-diimine)] (type 1 ) and [Fe 2 (CO) 7 (α-diimine)] (type 2 ), are afforded in all cases. We propose a reaction mechanism to explain this behaviour. The crystal structures of compounds [Fe(CO) 3 (bpy)] ( 1a ), [Fe(CO) 3 (phen)] ( 1b ), [Fe 2 (CO) 7 (bpy)] ( 2a ) and [Fe 2 (CO) 7 (phen)] ( 2b ) at 173 K were determined by single-crystal X-ray diffraction methods. In contrast, reaction of the pentanuclear iron cluster [Fe 5 C(CO) 15 ] with bpy led to the tetranuclear salt [Fe(bpy) 3 ][(μ-H)Fe 4 C(CO) 12 ] 2 ( 3 ). The electronic structures of the mono- and di-iron derivatives with bpy and phen were analysed using the atoms in molecules (AIM) theory. Their comparative study seems to rule out the existence of FeFe bond in the dinuclear complexes.

Spin adapted versus broken symmetry approaches in the description of magnetic coupling in heterodinuclear complexes

The performance of a series of wave function and density functional theory based methods in predicting the magnetic coupling constant of a family of heterodinuclear magnetic complexes has been studied. For the former, the accuracy is similar to other simple cases involving homodinuclear complexes, the main limitation being a sufficient inclusion of dynamical correlation effects. Nevertheless, these series of calculations provide an appropriate benchmark for density functional theory based methods. Here, the usual broken symmetry approach provides a convenient framework to predict the magnetic coupling constants but requires deriving the appropriate mapping. At variance with simple dinuclear complexes, spin projection based techniques cannot recover the corresponding (approximate) spin adapted solution. Present results also show that current implementation of spin flip techniques leads to unphysical results.

Synthesis, characterization and magnetic properties in trinuclear complexes of Cu(II) derived from 2-hydroxy-1,3-propylenebis(oxamato)

Abstract The aim of this paper is to vary the magnetic coupling between three copper(II) ions through the oxamato bridge in a predictable manner. Starting from the planar monomeric fragment [Cu(pbaOH)] 2− (where pbaOH is 2-hydroxy-1,3-propylenebis(oxamato)) we have synthesized four trinuclear complexes of Cu(II): {[Cu(dien)] 2 [Cu(pbaOH)]}(ClO 4 ) 2 ( 1 ); {[Cu(pmedien)] 2 [Cu(pbaOH)]}(ClO 4 ) 2 ( 2 ); {[Cu(petdien)] 2 [Cu(pbaOH)]}(ClO 4 ) 2 ( 3 ) and {[Cu(bipy)] 2 [Cu(pbaOH)]}(ClO 4 ) 2 ( 4 ) with dien = diethylenetriamine; pmedien = N,N,N ′ ,N ′ ,N ″-pentamethyldiethylenetriamine; petdien = N,N,N ′ ,N ′ ,N ″-pentaethyldiethylenetriamine and bipy = 2,2′-bipyridyl. According to the literature data and electronic spectra we propose that terminal Cu(II) ions in 1 and 2 are close to a square pyramid environment with a basal plane perpendicular to the basal plane of the Cu(II) central. In 3 the suggested environment is an intermediate between a square pyramid and a trigonal bipyramid. Finally, in 4 the two copper(II) terminal ions are in planar or square-pyramid surroundings. The values of J in 1 – 4 were deduced from the temperature dependence of the magnetic susceptibility. J was found to be equal to −93.2 cm −1 in 1 , −111.6 cm −1 in 2 , −196 cm −1 in 3 and −358 cm −1 in 4 . This agrees with our hypothesis about the geometry of terminal Cu(II) ions.

Handling Magnetic Coupling in Trinuclear Cu(II) Complexes.

The problem of deriving three different two-body magnetic couplings in three electrons/three centers in a general geometric arrangement is investigated using the trinuclear Cu(II) HAKKEJ complex as a real case example. In these systems, one quartet and two doublet low lying electronic states exist, which define the magnetic spectra. However, the two possible linearly independent energy differences do not provide enough information to extract the three magnetic coupling constants. Here, we show how to obtain these parameters without making any assumption on the symmetry of the system from a combination of density functional- and wave function-based calculations. The density functional calculations explore various broken symmetry solutions and relate the corresponding energy to the expectation value of the Heisenberg Hamiltonian. This allows one to obtain all magnetic couplings, although their magnitude strongly depends on the exchange-correlation functional. Interestingly, a constant ratio between the magnetic coupling constants along a series of investigated functionals is found. This provides an additional equation to be used when relying on energy differences between spin states, which in turn allow solving the Heisenberg spectrum. The magnetic couplings thus obtained are compared to the experiment. Implications for the appropriate interpretation of the experiment and for the study of more complex systems are discussed.

Synthesis and characterization of two copper(II) complexes with protonated 6-benzylaminopurine as ligand. Crystal and molecular structure of [CuCl3(6-benzylaminopurineH)2]Cl·H2O

Abstract Two new mononuclear complexes of Cu(II), [CuX3(6-benzylaminopurineH)2]X·H2O (X = C1, Br) have been obtained by direct reaction of CuX2·aq and 6-benzylaminopurine in hydrochloric or hydrobromic acid. The chloro complex crystallizes in the monoclinic system, a = 13.508(3), b = 21.181(4), c = 10.661(2) A, β = 110.644(2)°, Z = 4. The crystal structure consists of mononuclear units where Cu(II) displays a distorted bpt geometry. Cu and three chlorine atoms are placed in the equatorial plane while the N atoms belonging to the imidazole ring of 6-benzylaminopurine protonated ligands are in the apical sites. The EPR spectrum at room temperature presents three g values g1 = 2.285, g2 = 2.133 and g3 = 2.035. They clearly indicate a distortion of bpt to sp geometry according to the molecular structure.

Heterodimetallic copper(II) compounds containing ferrocenecarboxylato(−1) and triamines as ligands

The syntheses and characterization of two new copper(II) complexes [Cu(petdien){η2-O2C[(η5-C5H4)Fe(η5-C5H5)]}][BPh4] 1 and [Cu(pmdien)(H2O){η1-O2C[(η5-C5H4)Fe(η5-C5H5)]}](ClO4) 2 (petdien = N,N,N′,N″,N″-pentaethyldiethylenetriamine, pmdien = N,N,N′,N″,N″-pentamethyldiethylenetriamine) are reported. The structures of both compounds have been determined by X-ray diffraction and reveal that these complexes contain a heterodinuclear Cu(II)–Fe(II) cation in which the ferrocenecarboxylato(−1) ligand acts as a chelating agent in 1 or as a monodentate group in 2. The influence of the steric effects induced by the tridentate amines in the mode of binding of the ferrocenecarboxylato group is discussed. Magnetochemical studies of these compounds are also reported.