J. Braz. Chem. Soc., Vol. 18, No. 7, 1379-1387, 2007. Printed in Brazil - ©2007 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Article Article Article Article Article Molecular Properties of Coordination Compounds of the Croconate Ion with First-Row Divalent Transition Metals: a Quantum Mechanical Study D. E. C. Ferreira,a H. F. Dos Santos,b W. B. De Almeidaa and G. M. A. Junqueira*,b,c aDepartamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte-MG, Brazil bDepartamento de Química, Instituto de Ciências Exatas, Universidade Federal de Juiz de Fora, 36036-900 Juiz de Fora-MG, Brazil cDepartamento de Química, Universidade de Coimbra, 3004-535 Coimbra, Portugal Neste artigo são relatadas geometrias, propriedades magnéticas e espectroscópicas de 2+ 2+ 2+ 2+ 2+ 2+ complexos de croconato [M(C5O5)(H2O)4] (M= Mn , Fe , Co , Ni , Cu e Zn ), calculados com Teoria do Funcional de Densidade. O estado fundamental de todos os complexos obtido foi de alto spin, em acordo com a proposta experimental. As estruturas e freqüências vibracionais calculadas também estão em acordo com o experimento, embora as comparações feitas sejam com estruturas do estado sólido. As propriedades ópticas não lineares (ONL) foram calculadas para todos os compostos analisados, usando o método Time Dependent Hartree-Fock (TDHF) dentro da abordagem estática, sugerindo que estes complexos de metais de transição possam ser utilizados no desenvolvimento de novos materiais baseados em moléculas. In this paper we report geometries, magnetic and vibrational spectroscopic properties of 2+ 2+ 2+ 2+ 2+ 2+ croconate complexes [M(C5O5)(H2O)4] (M= Mn , Fe , Co , Ni , Cu and Zn ) calculated at the Density Functional Theory level. The ground state of all complexes was found to be of high spin, in accordance with the experimental proposal. The calculated structures and vibrational frequencies were also in agreement with experiment, even though comparisons were made with the solid state structure. The calculated nonlinear optical (NLO) properties were for all the compounds analyzed, using the Time Dependent Hartree-Fock (TDHF) method within the static approach, suggesting that these transition metal complexes might be considered as lead molecules to the development of novel based-molecular materials. Keywords: oxocarbons, coordination compounds, vibrational spectroscopy, nonlinear optical properties, TD-DFT Introduction coordination compounds have been widely studied concerning spectroscopic,3-8 electromagnetic9-12 and 2– 13-16 Croconate (C5O5) is one of the monocyclic ions of nonlinear optical properties. Crystallographic 2– the oxocarbon series [(CnOn) ], where the derivatives with structures of the squarate and croconate complexes with n equal to 3-6 constitute the main representatives (Figure first-row transition metals have been described in the 1). These compounds were firstly recognized as a series literature9,17-22 showing molecules with an infinite of molecules by West et al.1 in the 60‘s. Their structures polymeric chain where the metal ions are surrounded by 17,18,22 present Dnh symmetry characterized by high degree of six oxygen atoms on a distorted octraedron. The electron delocalization around the ring that confers an oxocarbon ion is usually chelated through oxygen atoms extra stability to the ionic compounds. For rings larger to one metal ion and singly-coordinated to another metal, than five members it has been found2 a non-planar form leaving two of its five oxygen atoms (in the case of as being more favorable than the Dnh isomer. Over the croconate) uncoordinated. The divalent croconate past years, the free oxocarbon ions as well as their complexes with Mn2+, Fe2+, Cu2+, Ni2+, Co2+ or Zn2+ were found to be trihydrated with general formula *e-mail: [email protected] • M(C5O5) 3H2O and present similar powder X-ray 1380 Molecular Properties of Coordination Compounds of the Croconate Ion J. Braz. Chem. Soc. O O O OO O O O O -2 -2 -2 -2 O O O O O O O O O (a)(b) (c) (d) Figure 1. The main oxocarbon representatives: (a) deltate, (b) squarate, (c) croconate, (d) rhodizonate. diffraction patterns, differing only in the interplanar d- results by Cundari et al.33 showed that ECP approaches spacing which is often associated to the π-stacking are capable of predicting the geometry of transition metal interactions.22,23 The magnetic properties of isostructural complexes. The magnetic properties as well as harmonic croconate complexes have been investigated in the last vibrational frequencies, infrared intensities and Raman few years.17,22 The observed magnetic moments of Mn2+ activities were also obtained for the complexes at each (5.84), Fe2+ (5.40), Co2+ (5.11), Ni2+ (3.26) and Cu2+ (1.98 level of calculation mentioned before. Bohr magnetons) indicate these compounds as high-spin In this work the hyperpolarizabilities (β) were obtained complexes.22 In addition to the interesting chemical using the Time Dependent Hartree-Fock (TDHF) method physics aspects of these species that have motivated many within the static field approach.34-36 The TDHF method studies,1-23 some nonlinear optical applications have also solves the wave function of the ground electronic state in been reported.24 Nonlinear optics (NLO) is associated to the presence of the electric field within variational method the interactions of applied electromagnetic fields in for the calculation of both static as well as dynamic suitable materials to generate new fields altered in polarizabilities. The frequency-dependent first hyper- frequency or other physical properties. At the molecular polarizability (dynamic property) is the response of the level the effect of the light is to polarize the charge system at a determinate frequency to oscillatory applied distribution and modifies the propagated field. Generally, fields, and adds an additional level of complexity in the conjugated molecules asymmetrized by interacting calculations.35 However, the use of the TDHF within static electron donor and acceptor groups are highly polarizable treatment has produced good results in NLO molecular electronic systems with NLO properties. Consequently, studies, regarding to the experimental data.37-43 The the oxocarbon derivatives have already been used as hyperpolarizability tensor β is defined by expanding the xerographic photoreceptors and their nonlinear optical dipole moment of a molecule in the presence of an electric properties investigated.13-16,24 In spite of the extensive field F in a Taylor series (equation 1), where i, j, k, … 1-24 0 literature about oxocarbon species, the understanding represent x, y, z, … and p i is the dipole moment in the of their coordination compounds at a molecular level is absence of the field, α is the linear polarizability, β is the still needed. Our group has used quantum mechanical first hyperpolarizability and γ is the second methods and Monte Carlo simulation to study the hyperpolarizability. oxocarbon ions and their complexes in gas phase and aqueous solution.2,25-29 In the present work, we analyze 1 1 p = p 0 + α 0 ⋅ F + β ⋅ F F + γ ⋅ F F F +... structural, magnetic, spectroscopic and nonlinear optical i ij j 2! ijk j k 3! ijkl j k l (1) properties of the coordination compounds of the croconate ion with first-row divalent transition metals with different For static fields, the hyperpolarizability may be multiplicities. determined as derivative of the induced dipole with respect to the field which may be expressed as a derivative of the Theoretical details energy ε with respect to the electric field F (equation 2). The geometries of the [M(C O )(H O) ] (M= Mn2+, ∂ 2 p ∂ 2 ⎛ ∂ε ⎞ ∂ 3ε 5 5 2 4 β = = ⎜ − ⎟ = − Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) compounds were optimized ∂F 2 ∂F 2 ⎝ ∂F ⎠ ∂F 3 (2) in gas phase at the Density Functional Theory (DFT) level with the BP86 and B3P8630,31 functional, employing the The β hyperpolarizability is a tensor with 27 β standard split-valence basis-set 6-31G(d) and 6- components ijk with i, j, k = x, y e z (Figure 2a). Generally, 31++G(d,p) for C, O, H atoms and the effective core the nonzero components are reducible due to the symmetry 32 44 β β potential (ECP) LANL2DZ for the metal ions. Previous aspects. For example, if the program code use ijk= ikj, Vol. 18, No. 7, 2007 Ferreira et al. 1381 it provides 18 components (Figure 2b). The GAUSSIAN- initial guess was a perfect octahedron and the geometry 45 β β 03 program uses ij= ji hence provides a tensor with only optimized without any constraint. Distinct electronic states 10 elements (Figure 2c). Concerning the calculation of (i.e. different multiplicities) were considered and the final β the total molecular hyperpolarizability ( mol) we reported optimized geometries are depicted in Figure 4. As can be the three independent values for b as a quasi-Pythagorean seen the geometries undergo a significant distortion upon problem and solve the complete expression for calculating optimization. The relative energy and spin density on the the magnitude of the total molecular β (equations 3 and metal center are given in Table 1. It can be observed that 46 β 4). In the present approach, mol is obtained from the the higher spin compounds are the most energetically trace elements of the hyperpolarizability tensor, being favorable at both levels of theory studied, except for Fe2+ invariant to an arbitrary rotation of x,y,z axis.34 The and Ni2+ complexes at the BP86 level, where the triplet calculations were performed with the GAUSSIAN-03 and singlet states were found as the electronic ground program.45 states, respectively. Our results with the B3P86 functional are in better agreement with the magnetic susceptibility 2221/2 ββββmol=++() X Y Z (3) and EPR measurements that show the croconate complexes with the divalent metal ions as high-spin species.23 The ββββ=++∴==1 (),,;,,k xyzi xyz determination of the spin density has been used as a i3 ∑ ikk kik kki (4) ik, criterion to predict molecule-based paramagnetic materials.47 The DFT methods have provided good results ⎡⎤⎡⎤xxxyxxzxx xxx yxx zxx ⎡⎤ xxx for both the signs and magnitudes of spin densities at a 48 ⎢⎥⎢⎥xxyyxyzxy xxy yxy zxy ⎢⎥ xxy lower computation cost.