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COPPER(II)-ARENE COMPLEXES ON MONTMORILLONITE and increases G H 2 . Efficient removal of cap- reduces GHs and increases GH2O2. These effects are additive and empirical equations were proposed: they make it possible to predict a primary molecular yield formed in an irradiated solution containing simultaneously larger concentrations of efficient scavengers for both free-radical precursors (OH and ea,-). 3. Unifying curves were obtained for GM/GMOdependence on reactivities toward OH or ea,-. These ex-
perimental plots agree reasonably well with the diffusion-kinetic theoretical predictions which were calculated for experimental conditions where the secondary spur reactions are not expected to occur.
Acknowledgments. The authors are indebted to Dr.
A. 0. Allen (Brookhaven National Laboratory, Upton, PIT. I-.) for his interest in this work and to Mr. M. Borovi6anin for technical assistance in the experiments.
Interlamellar Metal Complexes on Layer Silicates. I.
Copper(11)-Arene
Complexes on Montmorillonite by Thomas J. Pinnavaia* and M. M. Mortland" Department of Chemistry and the Departments of Crop and Soil Sciences and Geology, Michigan State University, East Lansing, Michigan 48823 (Received August 4* 1971) Publication costs assisted by Michigan State University
The adsorption of toluene and other methyl-substituted benzenes on the interlamellar surfaces of copper (11) montmorillonite has been studied by spectroscopic methods. In addition to physically adsorbed molecules interacting with the silicate surface there are present chemisorbed species which are coordinated through R electrons to the exchangeable copper(I1) ions. In each case the coordinated species is similar to that in the previously studied type I benliene complex in which the ring is planar and aromaticity is retained on coordination. None of the methyl-substituted benzenes formed complexes analogous to the type I1 benzene complex in which the ring is distorted and aromaticity is lost. Possible structures for the type I complexes are discussed.
Introduction Montmorillonite is a naturally occurring layer aluminosilicate whose structure is similar to that of pyrophyllite except that there is isomorphous replacement of Ala+ by Mg2+in the octahedral sheet. The resulting negative charge is balanced by exchangeable cations positioned between the aluminosilicate layers, and the large internal surface area (ca. 800 m2/g) thus becomes available for the adsorption of a variety of molecules. The adsorption mechanisms, which have been recently reviewed, differ considerably, depending 011 the nature of the interlamellar cation and the adsorbate molecule. I n the case of transition metal ion exchange forms of the mineral and adsorbate molecules containing donor atoms, mechanisms involving the formation of discrete donor-acceptor complexes are common. The coordination chemistry of the metal ion is often related to that of the ion in homogeneous solution, but it can sometimes be greatly influenced by the unique environment a t the silicate surface. A case in point is the formation of two types of copper(I1)-arene complexes2~3
in the adsorption of benzene on copper(I1) montmorillonite. One complex possesses an apparent lLgreenJ' color (type I), and the other is "red" (type 11). Under conditions where the type I complex is formed, the degree of hydration of the mineral is higher than that needed to form the type I1 complex. Thus the two complexes can be interconverted simply by adding or removing controlled amounts of water. The nature of the coordinated benzene in the two complexes differs markedly, as judged by infrared spectroscopy. I n the type I complex the benzene ring is planar and retains its aromaticity, whereas in the type I1 complex the ring is greatly distorted and the T electrons are probably localized. These two chemisorbed species are especially interesting in view of the fact that no copper(I1)-arene complexes have yet been observed in homogeneous solution. hloreover, the donation of T (1) M. M. Mortland, Aduan. Agron., 2 2 , 7 5 (1970). (2) H. E. Doner and M. M. Mortland, Science, 166, 1406 (1969), (3) M. M .Mortland and T. J. Pinnavaia, Nature (London), 229, 75
(1971). T h e Journal of Physical Chemistry, Vol. 76, N o . 26, 1971
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THO MA^ J. PINNAVAIA AND M.
electrons from an aromatic hydrocarbon to a metal ion acceptor has not been previously observed for an adsorption process on layer aluminosilicates and related three-dimensional zeolites. The present study investigates by spectroscopic means the nature of adsorbed toluene and other methylsubstituted benzenes on copper(I1) montmorillonite in order to establish their relationships, if any, to the two types of benzene complexes,
Experimental Methods Materials. Naturally occurring montmorillonite (Wyoming bentonite, API No. 25) was purchased from Ward's Katural Science Establishment. The copper(11)exchange form was prepared by treating the < 2 - ~ g fraction of the mineral with 1.0 N CuCl2, centrifuging, and discarding the supernatant liquid. The procedure was repeated three times to maximize the exchange of the original cations (mainly Ka+). Excess CuClz was removed by washing with distilled water until no test for C1- was obtained with AgY03. The approximate composition of the anhydrous mineral is Cuo 19 [All 53Feo.i&~go.sal[Ah 0&%.95]0io(OH)2. Arcne-copper(I1) montmorillonite complexes were prepared by placing a sample of the air-dried mineral, usually in the form of a film, in a PsOj desiccator containing a beaker of the appropriate reagent grade aromatic hydrocarbon. The deeply colored complexes were formed within 24 hr under these conditions. Infrared Spectra. Spectra in the region 4000-600 cm-' were obtained on a Beckmarr IR-7 spectrophotometer. Samples were prepared by evaporating in the open atmosphere aqueous suspensions of the mineral on a polyethylene surface. The rwuiting thin, selfsupporting films (ca. 1 mg/cm2) were then peeled away from the polyethylene surface and used to form the desired complex. The films were mounted in a brass cell equipped with XaC1 windows in order to protect them from atmospheric moisture. Since the film samples are highly oriented with the planes of the silicate sheets lying parallel to the film surface, it was possible to investigate pleochroic effects by observing diff ercnces in absorption intensities with the film positioned 90 and 45" to the path of the spectrophotometer beam. All differences in absorbance and relative optical densities were estimated from band heights. Electronic Xpectra. Electronic spectra of mull samples in the region 26,000-250 nm were obtained on a Cary 14 spectrophotometer. The mulls were prepared by evaporating a suspension of copper(l1) montmorillonite directly onto a quartz glass window, forming the desired complex, and then coating the sample with mineral oil. All spectra were run against a reference sample of copper(I1) montmorillonite in order to compensate for radiation losses due to scattering and adsorptions of the mineral itself. The spectrum of each T h e Journal of Physical Chemistry, Vol, 75?
16,2.971
XI,MORTLAND
complex was obtained on at least two independent samples to ensure reproducibility of the absorption bands. It mas not possible to observe the internal transitions of the aromatic ligands below 250 nm because of the absorptions of physically bound species also present on the surface and severe radiation scattering in this region. Esr Xpectra. The esr spectrum of the copper(I1)toluene complex was recorded at room temperature on a Varian E-4 spectrometer. The sample was prepared by freeze-drying a suspension of the mineral in a quartz-glass esr tube fitted with a Teflon stopcock and then forming the complex. X - R a y Diffraction. Powder diffraction patterns were obtained by conventional means on a Phillips X-ray diffractometer using Cu radiation and an Ni filter.
Results When the exchangeable alkali metal and alkaline earth cations of naturally occurring montmorillonite are replaced by copper(II), the layer silicate mineral adopts the pale blue color characteristic of aqueous Cu2+. At room temperature a5d ca. 50% relative humidity the 001 spacing (12.4 A) indicates that the interlamellar surface occupied by the cation is also covered nith a monolayer of water. The water content corresponds to ca. 14 water molecules per eoppcr(11) ion. The nidal ion may achieve a coordination number of 6 by binding to three of these water molecules and to three oxygen atoms of the silicate structure. The remaining water molecules occupy outer spheres of coordination. Recent studies by Farmer and Russell4 on copper(I1) and other cation-exchange forms of montmorillonite indicate that the adsorbcd water is highly structured with the water in outer spheres of coordination forming dielectric links via hydrogen bonding between the cation and the negaCivcly charged silicate structure. Dehydration of the mineral by evacuation at temperatures below 150" or by drying over Pz05leads to preferential removal of outer-sphere water and frees some of the silicate surface for the adsorption of a variety of neutral molecules. The removal of all interlamellar water causes adjacent silicate layers to come together, and internal adsorption of many molecules is prevented or greatly retarded. Copper(I1) montmorillonite dried over PzO5 at room temperature binds toluene by physical adsorption to the silicate structure and by complex form$on with the copper(I1) ions, A 001 spacing of 15.8 A upon adsorption verifies that the binding occurs on the interlamellar surfaces. The two forms of bound toluene are readily distinguished by infrared spectroscopy. The infrared bands characteristic of both forms are identified in spectrum C: of Figure 1. Included in the (4) V. C. Farmer and J . D. Russell, Trans. Faraday (1971).
SOC., 67, 2737
COPPER(II)-ARENE COMPLEXES ON MONTMORILLONITE WAVE NUMBER
Cm-’
Figure 1. Infrared spectra for (A) liquid toluene, (B) copper(I1) montmorillonite, and (C) the toluene-copper(I1) montmorillonite complex, Bands labeled “Pi’ and “L” are assigned to physically adsorbed and ligand toluene, respectively.
figure are spectra for liquid toluene and copper(I1) montmorillonite. Although we are lacking quantitative equilibrium data, the ratio of ligand to physically adsorbed toluene increases with decreasing degree of hydration, I n addition, there is a differential loss of the two species upon heating at 105”, the ligand being relatively stable while the physically adsorbed toluene is greatly diminished. Thus the assignment of bands in spectrum C was facilitated by interconverting the two forms as well as observing their relative stabilities. Four normal modes are observed in the region 1700600 cm-’ for physically adsorbed toluene: the C-C stretching vibrations vlga6 and Y19bj respectively, at 1495 and -1458 cm-l, the C-H out-of-plane deformation vll at 738 cm-l, and the out-of-plane skeletal vibration v4 at -695 cm-l. The other normal modes expected in this region either are too weak to observe or are obscured by vibrations of the adsorbed water or of the silicate structure. At least one internal vibration of the CHa group is also observed, uix., the symmetric deformation at 1380 cm-l. The asymmetric CH, deformation probably lies near the Y l g b vibration a t -1458 cm-l. Little or no pleochroism (cf. Experimental Section) was observed for the symmetric CH3 deformation and the vlgR and Y l g b vibrations for which the transition moments lie in the plane of the aromatic ring. However, an appreciable pleochroic effect (ea. 20% increase in absorbance) was observed for the vll vibration in which the transition moment is perpendicular to the ring. Much larger pleochroic effects would be expected if the plane of the molecule was perfectly parallel to the silicate surface. Thus the plane of the physically adsorbed toluene is tilted but probably more nearly parallel than perpendicular to the silicate surface. All of the observed vibrational frequencies are essentially unshifted (
