Low-spin versus high-spin ground state in pseudo-octahedral iron complexes

Pseudo-octahedral complexes of iron find applications as switches in molecular electronic devices, materials for data storage, and, more recently, as candidates for dye-sensitizers in dye-sensitized solar cells. Iron, as a first row transition metal, provides a weak ligand-field splitting in an octahedral environment. This results in the presence of low-lying (5)T excited states that, depending on the identity of iron ligands, can become the ground state of the complex. The small energy difference between the low-spin, (1)A, and high-spin, (5)T, states presents a challenge for accurate prediction of their ground state using density functional theory. In this work, we investigate the applicability of the B3LYP functional to the ground state determination of first row transition metal complexes, focusing mainly on Fe(II) polypyridine complexes with ligands of varying ligand field strength. It has been shown previously that B3LYP artificially favors the (5)T state as the ground state of Fe(II) complexes, and the error in the energy differences between the (1)A and (5)T states is systematic for a set of structurally related complexes. We demonstrate that structurally related complexes can be defined as pseudo-octahedral complexes that undergo similar distortion in the metal-ligand coordination environment between the high-spin and low-spin states. The systematic behavior of complexes with similar distortion can be exploited, and the ground state of an arbitrary Fe(II) complex can be determined by comparing the calculated energy differences between the singlet and quintet electronic states of a complex to the energy differences of structurally related complexes with a known, experimentally determined ground state.     

The metal complexes using organic ligands with photo-excited high-spin states

In this work, 4-pyridyl-phenylanthracene-iminonitroxide radical 2 was synthesized, which can make the coordination to metal ions. It was confirmed by the time-resolved ESR experiments that 2 has a photo-excited quartet (S = 3/2) high-spin state. Cu(II)(hfac)₂(2)₂ and Mn(II)(hfac)₂(2)₂ were synthesized by using 2 as a ligand. Their magnetic properties on the ground states were analyzed by three-spincluster model S₁ − S_M − S₂ (S₁ = S₂ = S_M = 1/2 for Cu(II)(hfac)₂(2)₂ and S₁ = S₂ = 1/2, S_M = 5/2 for Mn(II)(hfac)₂(2)₂). The exchange interactions (J/k_B) between 2 and the metal ions were very weak (J/k_Bs were ferromagnetic for Cu(II)(hfac)₂(2)₂ and antiferromagnetic for Mn(II)(hfac)₂(2)₂). The molecular orbital calculations of 2 have suggested the strong interaction between the paramagnetic center of the metal ions and the photo-excited quartet high-spin state.     

Hexacyanometalate molecular chemistry: heptanuclear heterobimetallic complexes; control of the ground spin state

Following a bottom-up approach to nanomaterials, we present a rational synthetic route from hexacyanometalates [M(CN)(6)](3-) (M=Cr(III), Co(III)) cores to well-defined heptanuclear complexes. By changing the nature of the metallic cations and using a localised orbital model it is possible to control and to tune the ground state spin value. Thus, with M=Cr(III), d(3), S=3/2, three heptanuclear species were built and characterised by mass spectrometry in solution, by single-crystal X-ray diffraction and by powder magnetic susceptibility measurements, [Cr(III)(CNbondM'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II), L(n)=polydentate ligand), showing spin ground states S(G)=9/2 [Cu(II)], with ferromagnetic interactions J(Cr,Cu)=+45 cm(-1), S(G)=15/2 [Ni(II)] and J(Cr,Ni)=+17.3 cm(-1), S(G)=27/2 [Mn(II)], with an antiferromagnetic interaction J(Cr,Mn)=-9 cm(-1), (interaction Hamiltonian H=-J(Cr,M) [S(Cr)Sigma(i)S(M)(i)], i=1-6). With M=Co(III), d(6), S=0, the heptanuclear analogues [Co(III)(CN-M'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II)) were similarly synthesised and studied. They present a singlet ground state and allow us to evaluate the weak antiferromagnetic coupling constant between two next-nearest neighbours M'-Co-M'.     

Molecular states of atoms. I. Orbital energy parameters

Atomic valence state energies are analyzed to obtain values of orbital energy parameters that may be used in semiempirical molecular orbital calculations. Difficulty in defining the interaction between orbitals with non-integer electron populations is systematically avoided by distinguishing between a valence state and a molecular state of an atom, only the latter state having non-integer spin paired orbital occupancy. Application of the virial theorem to the molecular state enables a value for the orbital kinetic energy to be obtained from the valence state orbital energy parameters once an arbitrary configuration is defined as reference. The orbitals then are eigenfunctions of the atomic Fock operator for that reference molecular state and, with their energy parameters, may be employed as a fixed basis set for molecular orbital calculations.     

Bonding in the ground state and excited states of copper-alkene complexes

The ground state and ¹B₂ excited state of Cu(C₂H₄)⁺ and of CuX(C₂H₄) (X  F, Cl) have been investigated by the Hartree-Fock-Slater (HFS) method. The main metal-ligand interactions in the ground state are ethene π → Cu 4s donation and Cu 3d_π → ethene π^* backdonation, which have comparable contributions to the metal-ligand bond strength. The excitation of CuX(C₂H₄) does not involve an alkene π → metal charge transfer (LMCT), but instead is metal 3d → alkene π^* charge transfer (MLCT) in character. The implications for the photochemistry of olefin-copper(I) complexes are discussed.     

Metastable high-spin states in chemical ionization in hydrocarbon combustion

The chemical ionization mechanism for HCO ₂ ⁺ is considered from MINDO/3 CI calculations and ones on the spin-orbit interaction elements, which has been employed in examining the sections of the potential-energy surfaces along the reaction coordinates in the formation of HCO ₂ ⁺ and HCO ₂ ⁺ .Karaganda University. Translated from Teoreticheskaia i ksperimental'naya Khimiya, Vol. 27, No. 6, pp. 672–677, November–December, 1991. Original article submitted April 24, 1989.     

Self-assembly of heterobimetallic d-f hybrid complexes: sensitization of lanthanide luminescence by d-block metal-to-ligand charge-transfer excited states

Carboxylate-bearing d-transition metal bipyridyls form ternary complexes with seven-coordinate lanthanide centers. The neodymium,- ytterbium-, and erbium-containing complexes exhibit lanthanide-centered emissions sensitized by the MLCT states of the d-block components.     

Potential energy surface and rovibrational states of the ground Ar-HI complex

A potential energy surface for the ground electronic state of the Ar-HI van der Waals complex is calculated at the coupled-cluster with single and double excitations and a noniterative perturbation treatment of triple excitations [CCSD(T)] level of theory. Calculations are performed using for the iodine atom a correlation consistent triple-zeta valence basis set in conjunction with large-core Stuttgart-Dresden-Bonn relativistic pseudopotential, whereas specific augmented correlation consistent basis sets are employed for the H and Ar atoms supplemented with an additional set of bond functions. In agreement with previous studies, the equilibrium structure is found to be linear Ar-I-H, with a well depth of 205.38 cm(-1). Another two secondary minima are also predicted at a linear and bent Ar-H-I configurations with well depths of 153.57 and 151.57 cm(-1), respectively. The parametrized CCSD(T) potential is used to calculate rovibrational bound states of Ar-HI/Ar-DI complexes, and the vibrationally averaged structures of the different isomers are determined. Spectroscopic constants are also computed from the CCSD(T) surface and their comparison with available experimental data demonstrates the quality of the present surface in the corresponding configuration regions.     

Electronic structure of molecular complexes in their ground and excited states. I.

Using a recently modified INDO method the equilibrium intermolecular distance and heats of formation of various types of molecular complexes (CT, HB, CTTS, etc.) have been calculated. The results obtained are in reasonable agreement with available ab initio studies.     

Electronic structure of molecular complexes in their ground and excited states. II.

Using a modified INDO method the potential energy curves for ground and some excited states (with an appreciable amount of charge transfer character) of wide range molecular complexes have been calculated. The calculation performed for CT, HB, CTTS, etc. systems shows that potential energy curves could be regarded as criterion for classification of molecular complexes into several groups with different properties. Analysis of proton motion in ground and excited states of H₃N-HCl and hydrogen maleate anion have also been performed.     

Ligand oxidations in high-spin nickel thiolate complexes and zinc analogues

Oxidations of a trigonal-bipyramidal, high-spin Ni(II) dithiolate complex of a pentadentate, N3S2-donor ligand, N1,N9-bis(imino-2-mercaptopropane)-1,5,9-triazanonane) nickel(II), and the structurally analogous Zn(II) complex, lead to oxidations of the ligand. Oxidation of the Ni(II) complex with I2 produces a novel Ni(II) macrocyclic cationic complex containing a monodentate disulfide ligand (2). Crystals of the I3- salt of the complex form in the triclinic space group P(1) with cell dimensions a=8.508(3) A, b=9.681(2) A, c=14.066(4) A, angles alpha=90.97(2) degrees , beta=91.61(3) degrees , gamma=90.83(2) degrees , and Z=2. The structure was refined to R=6.31% and Rw=16.63% (I > 2sigma(I)). Oxidation of the Ni(II) complex with O2 leads to the formation of a novel pentadentate bis-iminothiocarboxylate complex with trigonal-bipyramidal geometry (3). This neutral product crystallizes in the monoclinic space group P21/c with cell dimensions a=13.625(3) A, b=7.605(5) A, c=14.902(4) A, angles alpha=gamma=90 degrees, beta=102.81(2) degrees , and Z=4. The structure was refined to R=7.18% and Rw=17.86% (I > 2sigma(I)). Oxidation of the Zn(II) dithiolate analogue with O2 leads to the formation of the Zn(II) complex of the pentadentate bis-iminothiocarboxylate ligand. The neutral complex is isomorphous with the Ni(II) complex and crystallizes in the monoclinic space group P2(1)/c with cell dimensions a=13.8465(4) A, b=7.6453(2) A, c=15.0165(6) A, angles alpha=gamma=90 degrees , beta=103.2140(11) degrees , and Z=4. The structure was refined to R=3.96% and Rw=9.45% (I > 2sigma(I)). Details of the crystal structures are reported. Kinetics of the O2 reactions show that the reactions of the Ni(II) and Zn(II) dithiolates follow the rate law, Rate=k2[1][O2], with k2=1.81 M(-1) s(-1) for the Ni(II) complex and k2=1.93 x 10(-2) M(-1) s(-1) for the Zn(II) complex. The O2 oxidation of the high-spin Ni(II) thiolate complex was found to follow a similar oxidation mechanism to those of low-spin Ni(II) complexes, which form transient persulfoxide intermediates that yield S-oxidation products. In the case of the high-spin system reported here, the transient persulfoxide intermediate gives rise to an alternative ligand oxidation product, a bis-iminothiocarboxylate complex, because of the reactivity of the ligand, which contains a methylene with acidic H atoms alpha to the thiolate sulfur. The proposed mechanism is supported by studies of the analogous Zn dithiolate complex, which gives rise to the analogous bis-iminothiocarboxylate product (5).     

The energy mismatch in multiphonon relaxation

Modifications are presented for zero temperature to the energy gap law which take into account the energy mismatch (the difference between the energy gap and the nearest excited level of the effective vibrator). The results for the decay rate of rare earth ions in f → f excitation show a sub-linear dependence on the square width of the spectrum of the electron—phonon interaction for small width.     

Intramolecular NH/pi complexes of 2-allylaniline derivatives in the ground and excited states

The photochemical and photophysical properties of three 8-allyl-1,2,3,4-tetrahydroquinolines (1a-c) have been studied. These compounds exhibit a 2-allylaniline-like photochemical behavior, undergoing photocyclization to lilolidines (3a-c). The absorption, emission, and excitation spectra of 1a-c, employing convenient model compounds for comparison, demonstrate the formation of a NH/pi intramolecular ground-state complex (AB). This species can absorb light at long wavelengths (330-340 nm), giving rise to the corresponding excited complex AB*. Emission from AB* is red-shifted (420 nm) with respect to that observed when the monomer A is excited (lambda(exc) = 300 nm). These experimental results have been rationalized by means of density-functional theory calculations.     

X-ray fluorescence determination of the molar ratio of metals in heterobimetallic complexes

A method was developed for the X-ray fluorescence determination of the molar ratio of metals in heterobimetallic complexes. The procedure includes simple rapid sample preparation and a novel algorithm of measurements and data processing. It was demonstrated that the procedure can be also used for the determination of the metal ratio in films of complexes applied to oxide supports.     

Equilibrium of low- and high-spin states of Ni(II) complexes controlled by the donor ability of the bidentate ligands

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy(Bz)2 (Py(Bz)2 = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine), and a beta-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH2Cl2 solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH2Cl2, which are nearly the same as those of 3 and 4 in CH2Cl2. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)2 ligand in 1.     

The potential energy surfaces of the ground and excited states of carbon dioxide molecule

Potential energy surfaces (PESs) of the ¹A_l(¹Σ _g ⁺ ), ¹B₂ and ³B₂ electronic states of CO₂ have been computed as a function of the two bond distances and the bond angle. The calculations were based on the complete active space self consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic structure models. From our calculations no crossing point between ¹B₂ and ³B₂ states was found, but there is a crossing point located between ¹B₂ and ³A₂ state on the PESs. The energy of the crossing point is lie 0.23 eV above the CO + O (³P), which is in agreement with the value of 0.27 eV on the experiment. This implies that the mechanism of the recombination of an oxygen atom with a carbon monoxide molecule: CO(X ¹Σ⁺, ν) + O(³P)→³CO₂*→¹CO₂*→CO(X ¹Σ⁺, ν = 0) + O(¹ D) may occur through the ³A₂ state crossing the ¹B₂ state. The equilibrium geometries and adiabatic excitation energies of ^1,3B₂, ^1,3A₂ states of CO₂ were reported and discussed in this paper, too.     

Ferromagnetic interactions in heterobimetallic chains formed through the secondary coordination of dithiolene complexes

A novel paramagnetic ( S = 1/2) copper dithiolene complex based on the tfadt ligand (tfadt: 3-trifluoromethyl-acrylonitrile-2,3-dithiolate) is prepared as its n-Bu 4N (+) salt and crystallized with [Ni(cyclam)] (2+) into infinite, one-dimensional chains through CN...Ni interactions, avoiding any direct (antiferromagnetic) overlap between the dithiolene complexes. An unprecedented ferromagnetic interaction within the heterobimetallic chains between the S = 1/2 [Cu(tfadt) 2] (2-) and the S = 1 [Ni(cyclam)] (2+) tectons is observed, despite the fact that the SOMOs of both [Cu(tfadt) 2] (2-) (with d x (2) - y (2) symmetry) and [Ni(cyclam)] (2+) (with d x (2) - y (2) and d z (2) symmetry) have the same e g symmetry. The experimental exchange interaction deduced from the fit of the magnetic susceptibility ( J exp/ k B = +5.0 K) was confirmed by theoretical calculations ( J calc/ k B = +7.3 K), and a rationale is given for the presence of an intrachain ferromagnetic interaction.