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J. Phys. Chem. B 2000, 104, 8382-8390
Structure Characterization of the Copper(II) Complex of Poly(4-vinylpyridine) by a Combination of EPR, ENDOR, and Molecular Modeling Techniques Gunnar Jeschke* Max-Planck-Institut fu¨ r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany ReceiVed: January 20, 2000; In Final Form: May 23, 2000
A combination of EPR and orientation-selective pulse ENDOR methods provides enough constraints to characterize the structure of the metal center in the copper(II) complex of poly(4-vinylpyridine). The structure derives from the propeller structure of the tetrapyridino copper(II) complex in pyridine glass determined by the same methods. It features more disorder with respect to the angle between the pyridine ring and complex plane but only moderate tetrahedral distortion. The enhanced resolution of high-field EPR allows one to detect another metal center at higher copper loading of the polymer that probably involves only three pyridyl side group ligands. No evidence is found for an inhomogeneous metal distribution in the polymer if care is taken to equilibrate the structure during preparation. The EPR and ENDOR results are supported by molecular modeling based on density functional and force-field calculations. This combination of experimental and theoretical techniques provides a more detailed account of the principles governing the structure of this macromolecule-metal complex than EPR and ENDOR alone.
Introduction Synthetic macromolecule-metal complexes (MMC) have found application as ion conductors, catalysts, gas separation membranes, and templates for preparing nanosized metal clusters.1,2 A better understanding of the principles of structure formation in such systems is now needed, in particular, if the materials are to be tailored for certain applications. As has been noted in the outlook chapter of ref 1, a lack of suitable characterization techniques has become a bottleneck in this pursuit. Traditional methods such as X-ray crystallography fail because there is no long-range order. Scattering techniques are of only limited use, as there is usually also not much order at medium length scales. On the other hand, modern electron paramagnetic resonance (EPR) and especially electron-nuclear double resonance methods (ENDOR) have recently contributed considerably to our understanding of structure and function of metal complexes of biomacromolecules.3,4 Such methods may also be attractive for synthetic MMCs, but some obstacles must be overcome. EPR and ENDOR have so far been used on metalloproteins mainly to obtain first and rough information on coordinated amino acids, to refine structures obtained by X-ray crystallography, and to obtain detailed insight into electronic structure. For synthetic MMCs, the geometric structures often have to be determined from scratch. Furthermore, metalloproteins usually feature well-defined molecular geometries even if they cannot be crystallized. In contrast, some variation in the structure is expected even in the first coordination sphere of the metal in the case of synthetic MMCs. In the present contribution, we present the first detailed study on the structure of a synthetic MMC based on a combination of EPR and ENDOR methods. As a model system, we use the copper(II) complex of poly(4-vinylpyridine) (CuP4VP) that is of interest because of its ability to catalyze the technically important oxidative polymerization of 2,6-dimethylphenol. This complex has been studied before by several authors,5-8 but no * Author to whom correspondence should be addressed. Fax: +49-6131379 100. E-mail:
[email protected].
detailed model for its structure has emerged so far. We provide such a model for the geometry of the first coordination sphere and discuss aspects of strain and higher metal loading. The experimental results are supported and complemented by BP,9-11 BLYP,9,12 and B3LYP9,12 density functional computations of the geometry of the tetrapyridino copper(II) complex and of small oligomer model complexes, as well as by force-field calculations. Experimental Section Sample Preparation and Characterization. All chemicals were used without further purification. Poly(4-vinylpyridine) (8 mmol, MW ) 60 000 g mol-1; Polysciences) were dissolved in 100 mL of the solvent (absolute ethanol or ethanol/water mixture) under reflux conditions. A solution of the appropriate amount of the metal salts (CuCl2, NiCl2, ZnCl2; Aldrich) in 10 mL of the same solvent was slowly added, and the combined solution was kept under reflux for a further 30 min. Products were obtained by evaporation of the solvent under reduced pressure and characterized by differential scanning calorimetry (Mettler Toledo STARe). The solution of 1 mol % CuCl2 in dry pyridine was transferred to an EPR tube and shock-frozen in liquid nitrogen to obtain a transparent, glassy sample. EPR and ENDOR Spectroscopy. EPR spectra were measured on Bruker ESP 380e and ESP 680 spectrometers at Xand W-band frequencies, respectively. Pulse EPR and ENDOR measurements at X-band frequencies were performed at 15 K with an ENDOR probehead EN 4118X-MD-4 (Bruker), whereas continuous-wave (CW) EPR experiments were performed at 80 K and ambient temperature. A TeraFlex probehead (Bruker) was employed for the W-band experiments at 15 K. Microwave pulse lengths of 64 ns for the inversion pulse and 16 and 32 ns for the detection subsequence were used in the Davies ENDOR experiment. Mims ENDOR was performed with 24-ns microwave pulses and a minimum interpulse delay of 124 ns between the first two pulses. Radio-frequency pulse lengths were 5 µs in all ENDOR experiments. EPR spectra were simulated with the Simfonia software (Bruker) considering the natural abun-
10.1021/jp000256r CCC: $19.00 © 2000 American Chemical Society Published on Web 08/10/2000
Structure of the Cu(II) Complex of Poly(4-vinylpyridine) dance of the two copper isotopes (63,65Cu) and hyperfine couplings to the directly coordinated nitrogen nuclei. Hyperfine couplings to protons, the nuclear quadrupole interactions of copper and nitrogen, and nuclear Zeeman terms were neglected. EPR-correlated ENDOR spectra were computed with a homemade MATLAB (The Math Works) program through numerical diagonalization of the spin Hamiltonians. First, resonance fields and eigenvectors for the electron spin were 3 1 computed for the four multiplet lines with mCu I ) (- /2, - /2, 1/ , 3/ ) taking into account only the g tensor and the copper 2 2 hyperfine tensor. For each of these lines, ENDOR frequencies and EPR superhyperfine splittings were then calculated by including the hyperfine interaction and nuclear Zeeman interaction for 1H and additionally the nuclear quadrupole interaction for 14N. This two-step approach is justified because the coupling to protons and nitrogen nuclei does not significantly influence the quantization axis of the electron spin. All computations were performed for 63Cu and 65Cu, and the spectra were added with the appropriate ratio. Simulation of an entire 2D pattern based on 5400 orientations on the unit sphere takes about 5 min for 14N and about 3 min for 1H on a 600-MHz Pentium PC. For fits of angular distributions, the principal values of the interaction tensors were first determined from the ENDOR projections of the 2D spectrum and from fits of EPR spectra. A set of 2D spectra was then computed for the entire angular range of interest (0-90°) in 1° increments and used for the fitting of the angles 1 (14N) and 2 (1H). Computational Chemistry. Density functional calculations were performed with the Spartan,13 Titan,14 and ADF15,16 software packages on SGI O2 and Pentium PC computers. Starting structures were obtained from a modified Merck molecular force field17-19 (see below) and were optimized using the Becke-Perdew density functional9-11 in Spartan. The numerical DN* basis set of the Spartan package was applied. For two complexes, computations with the B3LYP functional9,12 were performed for comparison (Titan). A 6-31G* basis set was used for the light elements, and an effective core potential of double-ζ quality including the outermost core orbitals was used for copper.20 B3LYP geometry optimizations in Titan for the tetrapyridino copper(II) complex of pyridine were also performed for two deliberately distorted starting structures instead of the force-field structure (1 ) 0°, 2 ) 30° and 1 ) 0°, 2 ) 85°; see main text for angle definition), and the same final structure was obtained in these cases. For this complex, the results were also verified by a computation with the BLYP functional9,12 in ADF (Slater-type basis function set IV of triple-ζ quality). The Spartan implementation of the Merck molecular force field17-19 was extended by an atom type for a pyridine nitrogen involved in a coordinative bond with copper. Standard rules were applied for the angular terms of the copper coordination sphere,21 whereas the equilibrium length (2.020 Å) and force constant (2.000 mdyn Å-1) of the Cu-N bond were adjusted to fit the DFT structures of the oligomer complexes (see text).
J. Phys. Chem. B, Vol. 104, No. 35, 2000 8383
Figure 1. Experimental (exp.) and simulated (sim.) X-band CW EPR spectra of copper(II) complexes of poly(4-vinylpyridine). Simulation parameters are given in Table 1 and isotropic line broadening with 20 G line width has been used. a) Spectrum for a ligand-to-copper ratio of 100:1. b) Pseudomodulation derivative of the spectrum for a ligandto-copper ratio of 50:1.
Results and Discussion Coordination Polyhedron. Continuous-wave (CW) EPR spectra have been used before to determine the type of coordination polyhedron in CuP4VP.6,8 As shown in Figures 1 and 2 and Table 1, the spectrum can be simulated by assuming axial g and copper hyperfine tensors with g| > g⊥ and coinciding unique axes of the two tensors. This indicates square-planar coordination. The coinciding unique axes suggests C4 pseudosymmetry at copper, with the symmetry axis perpendicular to
Figure 2. Experimental (exp.) and simulated (sim.) W-band electron spin-echo-detected EPR spectra of copper(II) complexes of poly(4vinylpyridine) for two ligand-to-copper ratios L:Cu. Simulation parameters are given in Table 1 and isotropic line broadening with 200 G line width has been used.
the complex plane. The g and A values are close to those found in the copper complexes of pyridine22 and 4-ethylpyridine6 and
8384 J. Phys. Chem. B, Vol. 104, No. 35, 2000
Jeschke
TABLE 1: Spin Hamiltonian Parameters of the Copper(II) Center in Poly(4-vinylpyridine) at High Ligand-to-Copper Ratio complex
g|
g⊥
ACu | mTa
CuPVP CuPVP8,d Cu(py)422,e Cu(etpy)46
2.262 2.21 2.263 2.260
2.055 2.06 2.053 2.066
16.2 15 17.8 17.6
ACu ⊥ mTa
AN| MHzb,c
AN⊥ MHzb,c
1.3 n.a. 1.7 2.6
36.4 n.a. 33.8 n.a.
40.3 n.a. 37.4 n.a.
a Principal axis directions coincide with the g tensor. b Directions refer to the principal axis system of the nitrogen hyperfine tensor. c Obtained from ENDOR measurements. d Resin with a cross-linking degree of 4%. e Pyridine (0.4 M) in a mixture of 60% v/v of glycerol with water.
in a CuP4VP resin with a 4% degree of cross-linking.8 The nitrogen hyperfine parameters and tensor orientations have been determined independently by orientation-selective ENDOR (see below). The number of pyridyl sidegroups functioning as copper ligands can be determined by simulating the nitrogen superhyperfine structure visible in the derivative of the CW EPR spectrum calculated by pseudomodulation23 with a modulation amplitude of 5 G. The satisfying agreement between the experimental and simulated spectra in Figure 1b is obtained by assuming four ligands, in agreement with the C4 pseudosymmetry found above. Determining ligand numbers via the superhyperfine structure is a more direct and more widely applicable approach than the earlier determination by Kirsh et al.6 via comparison of g and A values with those of lowmolecular-weight complexes. This is because the g and A values may depend on the particulars of the coordination geometry which may differ between the MMC and complexes with low molecular weight. The remaining deviations in the spectrum are mainly due to strain, i.e., to a variation in the coordination geometries as discussed below. The 10-fold increase in the line width parameter in going from the X band to the W band indicates that g strain provides the dominant contribution to line broadening, so that for this sample g values cannot be determined to a significantly higher precision at high field. We can estimate that gy - gx < 0.002, as no rhombicity of the g tensor can be detected even at high field. Square-planar copper complexes often feature one or two more weakly bound axial ligands. One might speculate that these ligands are the counterions of the copper, in our case chloride ions, or, alternatively, molecules of the solvent used during preparation. There is some indication that solvent molecules play a role, namely, a slightly more blueish color of the MMC prepared in the presence of water as compared to those obtained from absolute ethanol. However, we do not find any significant features due to axial ligands in the ENDOR spectra. This excludes the presence of water ligands at a well defined distance. In fact, it is reasonable to assume that the number of such weakly bound ligands (between zero and two), their kind, and their position are determined by steric requirements that vary strongly among the centers in the MMC. Geometry of the First Coordination Sphere. In the following, we use the term first coordination sphere for the four pyridyl side groups of the polymer that are coordinated to the copper. Assume for the moment that all four pyridine rings have the same orientation with respect to copper and the complex plane, which is a reasonable first approximation given the C4 pseudosymmetry. The orientation of the pyridine ring can then be characterized by two parameters 1 and 2, where 1 is the angle between the Cu-N bond and the complex plane and 2 is the angle between the pyridine ring plane and the complex plane (Figure 3). Note that 2 corresponds to a structure variation
Figure 3. Simplified model of the complex geometry of the copper(II) complexes of pyridine and poly(4-vinylpyridine) and definition of the angles 1 and 2. The model consists of the copper ion in the center of the square coordination plane with the four nitrogen nuclei at the corners, a coplanar light hexagon symbolizing a pyridine ligand with 1 ) 2 ) 0, and a dark hexagon symbolizing a rotated pyridine ligand. The z axis coincides with the unique axes of the g and copper hyperfine tensors. The positions of protons H2 and H6 are indicated. (a) The angle 1 quantifies the tetrahedral distortion. (b) The angle 2 quantifies an out-of-plane rotation of the pyridine ring about the Cu-N axis.
that conserves square-planar coordination whereas 1 quantifies tetrahedral distortion. To determine the position of the pyridine ring uniquely, we still need one distance between the copper atom and an atom of the ligand. A complete picture of the coordination geometry thus requires only two angles and one distance in this simple case. The approximation of four exactly equivalent ligands can be relaxed by allowing for a distribution of the three parameters. The angles 1 and 2 can be related to angles between principal axis frames of interaction tensors in the EPR spin Hamiltonian. The unique axes of the g and copper hyperfine tensor axis coincide with an axis perpendicular to the complex plane for symmetry reasons. The nitrogen hyperfine tensor also features approximately axial symmetry, with the unique axis coinciding with the Cu-N bond if this bond has σ character.24 One can thus infer 1 from the angle between the two unique axes. Once 1 is known, 2 can be determined by measuring one more angle between the complex plane and a vector between the copper atom and a ligand atom. The most easily accessible ligand atoms for this are the protons in the 2- and 6-positions of the pyridine ring. These protons H2 and H6 are symmetryrelated for 1 ) 0. Their hyperfine coupling tensor is also approximately axial, with the unique axis coinciding with the Cu-H vector. The spin density on the nitrogen nucleus of the same ligand causes only a slight rhombicity (
