9377
J. Phys. Chem. 1992,96,9377-9382
Theoretlcal Study on the Interactions between a Metal Chelate and a Clay: Monte Carlo Simulations Hisako Sato, Device Development Center, Hitachi Ltd., Imai. Ome-shi. Tokyo 198, Japan
Akihiko Yamagishi,* Department of Polymer Science, Faculty of Science, Hokkaido University, Sapporo 060, Japan
and Shigeki &to Department of Chemistry, Faculty of Science, Kyoto University, Sakyo- ku, Kyoto 606, Japan (Received: December 6, 1991)
Monte Carlo simulations are used to investigate the intercalation structures of a metal complex within a smectite clay. The investigated model system is [M(phen)#+ (phen = 1,lO-phenanthroline)intercalated between two linked [SiO4lCand [AlO4Is tetrahedra sheets. The free energy of binding, the roles of the upper and lower sheets in determining the orientation of a bound chelate, and the effects of water medium on the binding free energy are investigated. For interlayer distances from 10 to 18 A, the chelate is bound by the sheets with the 3-fold (e3) symmetry axis perpendicular to the sheet surface. The chelate rotates around the C3axis by 15-60O from the equilibrium position. A comparison with experimental data shows that agreement with Monte Carlo simulationsis satisfactory, although the theoretical treatments predict more details about the bound structures of the chelate than the experimental results.
Introduction We have been studying the adsorption of metal complexes by a clay, focusing on the stereochemical effects in the interactions among the adsorbed molecules.' When optically active metal chelates such as [M(phen)J2+ (phen = 1,lO-phenanthroline) or [ M ( b p ~ ) ~ ](bpy ~ + = 2,2'-bipyridyl) are adsorbed, the chirality of these chelates has a remarkable effect on the adsorption behaviors. For example, the racemic mixture of [ M ( ~ h e n ) ~ (M ]~+ = Fe and Ru) is adsorbed to the amount of 2 times the excess of the cation exchange capacity (CEC) of a clay, while the enantiomer is adsorbed within the CEC? In the case of [ M ( b p ~ ) ~ ] ~ + (M = Ru), however, the racemic mixture is adsorbed within the CEC, while the enantiomer is adsorbed to 2 times the excess of the CEC.3 The results suggest that the bound chelates interact stereoselectively in the interlayer space of a clay and that the interaction is affected drastically by the slight changes in the ligand structures. Recently the binding structure of a molecule on a solid surface has been a subject of theoretical investigations. For example, the orientation and binding energy of a bound species have been predicted successfully for several systems such as organic molecules in the cavity of zeolite, water molecules adsorbed on a talc surface, and metal ions or complexes intercalated in the interlayer space of a clay.cb In these studies, the binding energies are calculated as the sum of the electrostatic and short-range interaction energies between an adsorbed molecule and a solid support under various configurations. The structures of pillared clays (anilinium vermiculite) have been investigated theoretically by use of the lattice energy minimization method.' According to the method, the stable structure is predicted by minimizing the total energy of the whole system, including a host lattice and guest molecules. In the calculation, the deformation of a host lattice due to the bindings of guest molecules is taken into account. In the present paper, we intend to investigate the binding structures of a metal chelate intercalated between the clay layers. For that purpose, Monte Carlo simulations are used to obtain the thermal distributions of configurationsof a bound chelate. The investigated system is a [ M ( ~ h e n ) ~ ion ] ~ +that is intercalated between two sheets composed of [SiO4lCand [do4]+ tetrahedra (denoted by a "cluster"). The sheet is regarded as a model of the tetrahedral sheet of a clay layer. The results are compared with the binding states of [ R u ( ~ h e n ) ~as ] ~recently + determined by the X-ray diffraction and electric dichroism measurements.8
Method We assume that the binding energy of a metal complex with a cluster (E,,) is expressed by the sum of the long-range electrostatic (E,) and the short-range interaction energies (E,): Eb = E, + E, (1) These energies are expressed in terms of the atom-atom pair potentials between the metal complex and the cluster. The electrostatic energy is given in atomic units as
E, = XXei*ej*/rij i
l
(2)
where ei* is the effective charge on the ith atom of the cluster and e,* the effective charge on the jth atom of the metal complex. rij denotes the interatomic distance. The electrostatic effective charges (e(ES)i*) are used for the effective charges of the cluster, el*. To determine e(ES)i*, we first carry out ab initio molecular orbital (MO) calculations (HOND07, QCPE 544)9 for a small cluster such as ([SiO4lC), (n = 1-6).Io The restricted Hartree-Fock (RHF) wave function with the STO-3G minimal basis set is employed. The electrostatic potential at the point r
V,(r) = Jdr'dr)/Ir
- r l + CWlr - rrl i
(3)
is calculated using the electron density p(r) obtained from the MO calculation and the nuclear charge of the ith atom, 2,.The electrostatic effective charges are determined by fitting the potential V,(r) to the function V,(r) = Ce(ES)i*/lr - ril i
(4)
The electrostatic potential is calculated at about 600 points for each cluster.l1 As for the effective charges of the chelate, e,*, the Mulliken atomic charges (e(MK)i*) are used.l2 e(MK)i* is obtained by ab initio calculations of [ M ( ~ h e n ) ~ in ] ~ which + no electronic interactions between the ligands and a central metal ion (11) is assumed. The short-range interaction energy E, between the ith atom of a cluster and the jth atom of a metal complex is represented by the Lennard-Jones potential:
Ew
5
CC4u0ij[(aij/rij)'~ - (gij/rijI6I f J
0022-3654/92/2096-9377503.0~/0 0 1992 American Chemical Society
(5)
9378 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Sato et al. to it, the electrostatic energy is modified with the following tapering function:
1
1
forxl
SO," Model Silicate Sheet
Figure 1. Model sheet of linked [SiO4I4+and [A1O4I5-tetrahedra used in the present calculations.
in which the first and second terms represent the attractive and repulsive interactions, respectively. The energy and length parameters, Uojjand uij, are approximated by uoj ujj
= (Qu,)1'*
=
'/2(Uj
(6)
+ Uj)
with the values of Vi and ui intrinsic to the ith atom. The short-range interactions are calculated for the pairs of hydrogen or carbon atoms in the chelate and oxygen atoms in a cluster.13 A cluster employed in the simulation calculation is composed of 40 [SO4]&and 8 [AlO4I5-tetrahedra linked by oxygen atoms as shown in Figure 1. The S i 4 distance is taken from the experimental value for a-silica, 1.628 A.14 The A 1 4 distance is assumed to be the same as the S i 4 distance, although the former is reported to be about 0.05 A longer than the latter.13 The surface has a size of 36.832 A X 3 1.896 A and a total charge of -8e. Thus the density of negative charge is about -e/150 A2. We further assume that the structure of a [SO4]&moiety has the normal tetrahedral form. Since we use the cluster model to simulate the clay surface, the H atoms are attached to the terminal 0 atoms to represent the SP3hybridization of terminal 0 atoms. The OH distance is taken to be 0.958 A from the reported value of a water m01ecule.l~When the effect of the OH group in an octahedral sheet is examined, an OH group is placed at the center of each hexagonal hole. The direction of the OH bond is perpendicular to the cluster surface. The horizontal position of the 0 atom is the same as that of the apical 0 atom in [SiO4]&.The OH distance is taken to be 0.958 A. The simulations of a bound state are based on Monte Carlo technique using a Metropolis algorithm." The Monte Carlo box is a rectangular prism in which the upper and lower planes of the prism are the same as shown in Figure 1. The x and y axes coincide with the a and b axes of the unit cell of a 2:l type clay layer, respectively. The z axis is taken to be normal to the surface of a sheet. The origin of the coordinates is located at the lower right edge of the lower sheet. The length of the z axis is varied from 10 to 20 A. A single chelate molecule, [ M ( ~ h e n ) ~ ] ~is' , placed inside the prism. The net charge inside the prism is therefore -14e. To satisfy the charge balance, cations should coexist inside the prism. In the present simulations, however, no other cation is added because the purpose of the present calculation is to study the structure of an isolated bound chelate that may be formed at the initial stage of adsorption. The periodic boundary conditions are applied in the x, y directions. In calculating the long-range electrostatic energy, we apply the method of trucating the electrostatic interactions at the proper distance.llb According
(7 )
- RT)
= ( R c ,-~RT)/(R,
Y
forO
