Unusual Noncovalent Interaction Between the Chelated Cu(II) Ion and

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Unusual Noncovalent Interaction Between the Chelated Cu(II) Ion and the π Bond in the Vitamin B13 Complex, cis-Diammine(orotato)copper(II): Theoretical and Vibrational Spectroscopy Studies K. Helios,† R. Wysokin´ski,† W. Zierkiewicz,† L. M. Proniewicz,‡ and D. Michalska*,† Faculty of Chemistry, Wrocław UniVersity of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland, Faculty of Chemistry, Jagiellonian UniVersity, R. Ingardena 3, 30-060 Krako´w, Poland ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: April 22, 2009

The crystal structure of the Cu(II) complex with Vitamin B13 (orotic acid), cis-[Cu(oro)(NH3)2] has revealed the presence of unusual, noncovalent π-type interaction between the chelated Cu(II) ion and the CdC bond of the uracilate ring [Michalska et al. Polyhedron 2007, 26, 4303]. In this work, the origin and strength of this interaction is thoroughly investigated. Comprehensive studies of the molecular structures and vibrational spectra of the title complex have been performed by using the unrestricted density functional theory methods, B3LYP, and the newly developed M05-2X functional. Calculations at the UMP2 level were also carried out for comparison. A variety of basis sets have been employed in the DFT calculations, including aug-cc-pVTZ, D95V(d,p), SDD, and LanL2DZ. The 63Cu/65Cu isotope substitution technique was applied to identify the copper-ligand vibrations in the infrared spectra. The clear-cut assignment of all the bands in the FT-IR and Raman spectra of the title complex has been made on the basis of the calculated potential energy distribution, PED. It is shown that an extremely intense band at 1210 cm-1 in the Raman spectrum of cis-[Cu(oro)(NH3)2] is diagnostic for the N-1 deprotonation of the uracilate ring and coordination to the copper(II) ion. The B3LYP functional performs better than M05-2X in predicting vibrational frequencies of this complex in the solid state. Intermolecular interactions in crystal were modeled by the supramolecular system consisting of cis[Cu(oro)(NH3)2], ethylene (above), and formaldehyde (below the copper coordination plane). The stable structure of this system has been predicted only by the M05-2X and MP2 methods, which include dispersion energy, whereas the B3LYP calculations failed in geometry optimization. The distance between the Cu atom and the CdC bond, predicted by the M05-2X method (3.00 Å) is similar to the van der Waals contacts between the stacking bases in DNA. The calculated interaction energy between the chelated Cu(II) complex and ethylene amounts to -7.33 kcal mol-1, which is similar to that determined for stacked uracil dimer. It is concluded that the London dispersion energy plays a significant role in the noncovalent interaction between the chelated Cu(II) ion and the uracilate ring in the crystal of cis-[Cu(oro)(NH3)2]. Many copper enzymes in their active sites contain the chelated Cu(II) ion and the aromatic groups (Phe, Tyr and Trp) as the potential binding sites; therefore, the noncovalent copper(II)-π interaction can be very important for the structure and functioning of these enzymes. 1. Introduction Orotic acid (6-carboxyuracil, vitamin B13) is a key intermediate in biosynthesis of nucleic acids, being the only precursor in the pathway to formation of all pyrimidine nucleotides in living organisms.1,2 Vitamin B13 also plays the main role in the metabolism of vitamins B6 (folic acid) and B12 (cobalamine). Orotic acid has also attracted growing attention in medicine, since it is used as the carrier for some metal ions in curing syndromes associated with a deficiency of Mg2+, Ca2+, Cu2+, and Zn2+ ions.3-5 The treatment with Mg-orotate yields excellent results in the prevention and therapy of heart and vessels diseases,6 and it markedly improves the liver enzymes activity.7 Moreover, platinum(II) complexes with orotic acid and diaminocyclohexane ligands have revealed some antitumor properties.8 In view of the biological importance of orotic acid, the coordination chemistry of this ligand has been the subject of very intensive studies.9-18 * Corresponding author. E-mail: [email protected]. † Wrocław University of Technology. ‡ Jagiellonian University.

Recently,9 we have reported the crystal and molecular structure of cis-diammine(orotato)copper(II), cis-[Cu(oro)(NH3)2], and demonstrated the presence of unusual, noncovalent copper(II)-π interaction between the chelated Cu(II) ion and the CdC double bond of the uracil ring. In this complex, the copper(II) cation is chelated by the carboxylate oxygen atom and the deprotonated ring nitrogen atom of the orotate ligand. Two ammonia nitrogen atoms complete the square-planar environment around copper in the basal plane. In the crystal, one carbonyl oxygen atom (O4a) from the neighboring uracil ring forms a long copper-oxygen axial bond, whereas the sixth apical copper-binding site is located, surprisingly, at the π(CdC) bond of the other uracil ring, as illustrated in Figure 1. The distance between the copper atom and the midpoint of the CdC bond (3.293 Å) is similar to the van der Waals contacts between the stacking bases in DNA. It seems that the noncovalent Cu(II)-π binding force is very important in stabilizing the columnar, polymeric structure of this complex. In the past decade, much interest has been focused on the significance of the cation-π noncovalent interactions in protein folding, the functioning of ionic channels in membranes, and

10.1021/jp901912v CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

Cu(II)-cis-[Cu(oro)(NH3)2] Noncovalent Interaction

J. Phys. Chem. B, Vol. 113, No. 23, 2009 8159 M+ (or M2+) interacting with olefins or aromatic molecules.21-26 Interaction between the chelated copper(II) complex and the CdC double bond has not been studied as yet. The main goal of this work is to provide detailed insights into the molecular and electronic structure and the nature of bonding in cisdiammine (orotato)copper(II). Ab initio MP2 and density functional theory (DFT) methods, including the newly developed functional M05-2X,32,33 have been used to study the structure and vibrational spectra of the isolated cis-[Cu(oro)(NH3)2] complex. The M05-2X method belongs to the new generation of DFT methods and shows very good performance for noncovalent interactions, especially weak interactions, π · · · π stacking, and hydrogen bonding. Thus, it is interesting to examine the performance of the M05-2X vs B3LYP functionals in calculations of the structure and vibrational spectra of the title complex. The clear-cut assignment of the experimental Raman and FT-IR spectra of cis-[Cu(oro)(NH3)2] has been made on the basis of the calculated potential energy distribution (PED). To aid the assignment of the copper-ligand vibrations, the 63Cu/65Cu isotope substitution method has been applied. The intermolecular interactions were studied in the supramolecular system consisting of cis-[Cu(oro)(NH3)2], ethylene (above), and formaldehyde (below the copper coordination plane). The optimized structure of this system has been obtained only in calculations by the M05-2X and MP2 methods, which include dispersion energy.

Figure 1. The view of the dimer unit of cis-[Cu(oro)(NH3)2] displaying the pseudooctahedral environment around the copper ions.

in various molecular recognition processes involving aromatic side chains of proteins.19,20 A large number of experimental and theoretical studies have been carried out on the factors that control the binding geometry, strength, and specificity of a cation-π interaction for alkali metal cations 21,22 and transition metal monocations, including Cu+.23-26 Copper cations play an essential role in many intracellular metabolic processes.27 Wilson’s and Menkes diseases are two genetically inherited disorders of Cu metabolism. Copper deficiency is related to cardiac myopathy and ischemic heart disease.28 Moreover, it is suggested that the interaction between copper cations and the β-amyloid peptide (containing aromatic tyrosine side chain) is associated with the pathogenesis of Alzheimer’s disease.29 Copper-containing oxidases (amine oxidases, galactose oxidase, tyrosinase, ceruloplasmin, laccase) catalyze the oxidation of a wide variety of substrates ranging from small molecules, such as methane, to large peptides. Elucidation of crystal structures for many of these enzymes has revealed that the tyrosine residue is coordinated to the Cu(II) ion, in the active site of enzyme.30,31 For example, in galactose oxidase, the Tyr residue occupies an axial position in the distorted square-pyramidal geometry around copper. Additionally, the aromatic indole ring of tryptophan (Trp) is also present in the active site.31 Hence, it is evident that the aromatic groups of amino acids (Phe, Tyr, and Trp) in proteins can be viewed as important binding sites for the copper ion. It should be emphasized that earlier theoretical studies on cation-π interaction were carried out for a bare metal cation

2. Methods 2.1. Experimental Section. The crystals of cis-[Cu(oro)(NH3)2] were obtained as described in ref 9. The complex analogs with pure stable isotopes, 63Cu and 65Cu, were obtained in a microscale, in a similar way. In each synthesis, 0.05 mmol of copper(II) isotope (in the form of copper(II) nitrate or copper(II) sulfate) was added to a solution containing 0.05 mmol of orotic acid, 5 cm3 of concentrated ammonia, and 5 cm3 of ethanol. The reaction mixture was heated at 313 K under reflux for 1 h. Upon cooling, the complex precipitated in the form of dark-purple crystals. The FT-infrared (MIR) spectra of cis-[Cu(oro)(NH3)2] were measured in KBr pellets on a Bruker IFS 113 V spectrometer, in the range 4000-400 cm-1 (with a resolution of 2 cm-1). The far-infrared (FIR) spectra (600-50 cm-1) of each isotopic species, 63Cu and 65Cu, were recorded on IFS 66/S Bruker spectrometer using Nujoll mull technique and polyethylene discs. The accuracy of the readings was (1.0 cm-1. The Raman spectrum of the copper(II) complex was recorded on a Jobin Yvon spectrometer, model T6400, using a CCD camera as a detector (Princeton Instruments). Excitation was provided by an argon laser (514.5 nm line). For the measurements, a few milligrams of the compound was placed in a capillary tube and measured with a resolution of 2 cm-1. 2.2. Theoretical. The complex investigated is an open-shell system (d9 electron configuration of Cu(II) cation), which requires the use of the unrestricted methods for calculations of an electronic structure. Since a spin contamination of the UHF wave function may occur, therefore, the expectation value of the total spin, Sˆ2, should be examined. The final Sˆ2 was equal to 0.7503 and 0.7500, in UMP2 and unrestricted DFT calculations, respectively. This is in perfect agreement with the value of 0.7500 corresponding to the doublet ground state wave function with no spin contamination and confirms the validity of the theoretical results. The calculated ground electronic state for the title complex is doublet, 2A′.

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In the first part, calculations were performed for the isolated cis-[Cu(oro)(NH3)2] complex. The optimized geometry, harmonic frequencies, IR intensities and Raman scattering activities were computed using the density functional gradient corrected three-parameter hybrid B3LYP functional34,35 and the Mo¨llerPlesset second-order perturbation (MP2) method. The newly developed DFT method, M05-2X, which has recently become available to us, is also employed in this study. This new method includes spin kinetic energy density in both the exchange and correlation functionals; moreover, it is completely free of selfcorrelation error.32,33 All calculations were based on an unrestricted mechanism; however, for clarity, U will be omitted from the UMP2, UB3LYP, and UM05-2X abbreviations, in the remaining text. A variety of basis sets have been used in computations. The largest basis set is aug-cc-pVTZ, which is the correlationconsistent, polarized valence, triple-ζ basis set augmented with diffuse functions on all atoms.36,37 For the title complex, it employs 875 contracted basis functions (1710 primitives), including the all electron (21s,17p,9d,3f,2g)/[8s,7p,5d,3f,2g] contracted basis set for Cu. It should be mentioned that these calculations were computationally very expensive. The SDD relativistic effective core potential supplemented by valence basis sets on all atoms38 and the effective core potential of Hay and Wadt39 with the concomitant basis set were employed (the latter basis set is denoted as I). We have also used the combined basis sets: LanL2DZ for Cu in conjunction with the polarized valence double-ξ basis set (D95V(d,p))40 for all ligands (denoted as II). The basis set III utilized the aug-cc-pVTZ basis set for all nonmetal atoms in conjunction with LanL2DZ for copper. A natural bond orbital (NBO) analysis was applied separately to R and β spin density matrices, as described by Carpenter and Weinhold for open-shell species.41,42 This method has provided the character of valence hybrid orbitals on atoms. Each natural bonding orbital, σAB, can be written in terms of two directed valence hybrids, hA, hB on atoms A and B, with corresponding polarization coefficients, cA and cB:

σAB ) cAhA + cBhB

(1)

Polarization coefficients vary smoothly from covalent (cA ) cB) to ionic (cA , cB) limit. To provide the detailed vibrational assignment of the experimental spectra, a normal-mode analysis was carried out, and the potential energy distribution was calculated at each level of theory. The nonredundant set of 60 internal coordinates for the complex was defined, as recommended by Pulay et al.43 The symmetrized internal coordinates for the ligands were analogous to those reported in our earlier studies on 1-methyluracil.44 The procedure for normal coordinate analysis was described previously,44,45 and calculations were performed using the Balga program.46 In the region below 1500 cm-1, the B3LYP-calculated frequencies show very good agreement with experiment; therefore, they are not scaled in this work. However, the calculated harmonic frequencies higher than 1500 cm-1 are all overestimated in comparison to the experimental ones. This is mainly caused by the neglect of anharmonicity, the incomplete treatment of electron correlation, and basis set truncation effects. To aid comparison between the predicted and observed frequencies, various scaling strategies have been devised.47,48 The procedure developed by Pulay and co-workers47 uses about a dozen parameters to scale force constants in internal coordinates. Schlegel et al.48 have shown that the direct scaling of the

Figure 2. The optimized molecular structure of cis-[Cu(oro)(NH3)2], and the numbering of atoms.

computed harmonic frequencies by two scaling factors, one below and one above 1800 cm-1 (dual scaling), greatly improves the agreement between the theoretical and experimental results. In this work, we have also employed two scaling factors: 0.920 (above 1800 cm-1) and 0.957 (in the region of 1800-1500 cm-1) for the B3LYP-calculated harmonic frequencies. These factors were determined by minimizing the root-mean-square errors between the theoretical and observed frequencies. It should be mentioned that the B3LYP predicted ν(NH) frequency of the N-Hi bond involved in the intramolecular hydrogen bond in the title complex is slightly lower than experimental; therefore, the frequency of the corresponding mode 8 was not scaled. The scaling factors for frequencies determined in this work will be used in our further theoretical study of the vibrational spectra of the Vitamin B13 complexes with transition metal ions. In the second part of our theoretical studies, we performed full geometry optimization of the supramolecular system consisting of cis-[Cu(oro)(NH3)2], ethylene (above the copper complex), and formaldehyde (below the complex). An attempt to optimize this structure using the unrestricted B3LYP method was unsuccessful. The stable structure of this supramolecule has been obtained at the MP2/I, M05-2X/I, and M05-2X/II levels of theory. The binding energy between the ethylene molecule, A, and the rest of the supramolecule, B (cis-[Cu(oro)(NH3)2 bonded with formaldehyde), was calculated as the difference between the total electronic energy of the supramolecular system and the sum of the energies of A and B. This energy was then corrected for basis set superposition error (BSSE) using the counterpoise (CP) method.49 All computations were performed with the Gaussian 03 (Rev. E.01) set of programs.50 3. Results 3.1. Structure. The optimized molecular structure and the numbering of atoms of cis-[Cu(oro)(NH3)2] are shown in Figure 2. In this complex, the copper atom is chelated by the carboxylate oxygen atom O1 and the N1-deprotonated nitrogen atom of the uracilate ring (the numbering of atoms in the ring refers to that commonly used for uracil and its derivatives). The two ammonia nitrogen atoms, N4 and N5, complete the squareplanar basal plane. Table 1 lists the theoretical bond lengths and angles calculated by the MP2, M05-2X, and B3LYP methods using various basis sets and the experimental geometrical parameters obtained from the X-ray data for crystal.9 As follows from this comparison, the theoretical Cu-N1 and Cu-O1 bond lengths are slightly shorter, whereas the Cu-ammonia (Cu-N4 and Cu-N5) bond lengths are slightly longer than experimental, regardless of the method used in calculations. This discrepancy can be caused by the fact that the theoretical values correspond to an isolated

Cu(II)-cis-[Cu(oro)(NH3)2] Noncovalent Interaction

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TABLE 1: Comparison of Experimental and Theoretical Bond Lengths (Å) and Angles (°) of cis-[Cu(oro)(NH3)2], Calculated by the MP2, M05-2X and B3LYP Methods with Various Basis Sets Cu-N1 Cu-O1 Cu-N4 Cu-N5 N1-C2 C2-N3 N3-C4 C4-C5 C5-C6 C6-N1 C6-C7 C7-O1 C7-O3 C2-O2 C4-O4 O1-Cu-N1 N1-Cu-N5 N4-Cu-N5 N4-Cu-O1 C7-O1-Cu C6-N1-Cu N1-C2-N3 N1-C2-O2 C2-N3-C4 N3-C4-O4 N3-C4-C5 C4-C5-C6 C5-C6-N1 C6-N1-C2 C5-C6-C7 C6-C7-O3 C6-C7-O1 O3-C7-O1

exptla

MP2/Ib

M05-2X/Ib

B3LYP/Ib

B3LYP/SDD

M05-2X/IIc

B3LYP/IIc

B3LYP/IIId

B3LYP/aug-cc-pVTZ

1.999(1) 1.959(1) 1.996(1) 1.955(1) 1.363(1) 1.377(1) 1.372(1) 1.439(1) 1.350(1) 1.365(1) 1.517(1) 1.278(1) 1.230(1) 1.233(1) 1.234(1) 82.5(1) 96.6(1) 90.4(1) 90.5(1) 116.4(1) 112.1(1) 117.6(1) 124.3(1) 126.5(1) 120.3(1) 114.0(2) 118.4(1) 125.2(1) 117.9(1) 121.0(2) 119.6(1) 114.9(1) 125.6(1)

1.985 1.928 2.095 2.055 1.390 1.407 1.436 1.473 1.386 1.420 1.537 1.367 1.265 1.296 1.273 84.5 97.7 96.9 80.9 117.1 112.2 117.2 123.4 127.0 120.0 113.1 119.7 123.8 119.2 122.3 122.1 112.3 125.6

1.970 1.917 2.061 2.026 1.363 1.382 1.413 1.456 1.358 1.391 1.517 1.338 1.238 1.276 1.248 83.6 97.5 97.7 81.3 117.5 112.6 117.4 123.0 125.6 119.8 113.6 119.0 124.2 119.4 121.9 121.8 112.4 125.9

1.979 1.942 2.078 2.041 1.376 1.390 1.425 1.457 1.369 1.394 1.518 1.349 1.246 1.281 1.255 83.3 97.8 98.2 80.7 116.9 113.1 117.0 123.3 126.7 119.9 113.2 119.6 123.9 119.7 122.2 122.7 112.7 124.6

1.964 1.926 2.070 2.025 1.376 1.391 1.425 1.458 1.369 1.395 1.520 1.349 1.246 1.281 1.255 84.0 97.6 97.4 81.0 116.7 112.8 117.2 123.1 126.6 119.9 113.2 119.6 124.0 119.5 122.2 122.5 112.6 124.8

1.967 1.900 2.077 2.032 1.357 1.373 1.404 1.454 1.353 1.378 1.522 1.315 1.214 1.250 1.221 84.5 95.8 96.9 82.7 116.8 111.5 117.0 123.6 127.3 119.9 113.0 118.7 124.8 119.3 121.1 121.2 113.2 125.6

1.977 1.923 2.099 2.049 1.368 1.382 1.414 1.456 1.363 1.380 1.523 1.322 1.221 1.253 1.228 84.0 97.5 97.6 81.9 116.5 112.1 116.6 123.9 127.5 120.0 112.7 119.3 124.4 119.7 121.6 122.2 113.4 124.3

1.958 1.911 2.097 2.036 1.360 1.375 1.407 1.445 1.351 1.375 1.518 1.313 1.211 1.245 1.218 84.3 97.1 96.4 82.2 116.4 112.0 116.7 123.8 127.3 120.0 112.6 119.5 124.4 119.5 121.7 122.2 113.4 124.4

1.968 1.912 2.099 2.039 1.361 1.375 1.407 1.445 1.351 1.375 1.518 1.314 1.211 1.245 1.219 84.1 96.9 96.7 82.3 116.6 112.0 116.6 123.8 127.3 120.0 112.7 119.5 124.3 119.6 121.7 122.1 113.4 124.4

a X-ray data from ref 9 (the estimated standard deviation in parentheses). b LanL2DZ basis set on all atoms, denoted as I. c D95V(d,p) basis set on all nonmetal atoms and LanL2DZ basis set on Cu, denoted as II. d aug-cc-pVTZ basis set on all nonmetal atoms and LanL2DZ basis set on Cu, denoted as III.

cis-[Cu(oro)(NH3)2] in the gas phase; therefore, the intermolecular interactions are neglected. Both the Cu-N1 and Cu-O1 bond lengths calculated by the B3LYP functional with two basis sets (I and II) are closer to experiment than those predicted by the M05-2X functional with the same basis sets. A further enlargement of the basis set in the B3LYP calculations (e.g. the use of aug-cc-pVTZ) does not improve the results. The copper-orotate (Cu-N1 and Cu-O1) atom distances are underestimated by 0.031 and 0.047 Å, respectively, whereas the copper-ammonia bond lengths are overestimated by about 0.1 Å, in comparison with experiment. It should be noted, however, that the bond lengths in the uracilate ring predicted at the B3LYP/aug-cc-pVTZ level of theory show the best agreement with experiment. For example, the calculated N1-C2 (1.361 Å) and C2-N3 (1.375 Å) bond lengths nearly reproduce the experimental values, 1.363(1) Å and 1.377(1) Å, respectively. As is seen in Table 1, calculations with the MP2 method using the LanL2DZ basis set seriously overestimate all atom distances in the orotate ligand. It is evident that the MP2 method requires a larger basis set for accurate prediction of the molecular geometry. All the bond lengths calculated by M05-2X are consistently shorter than those computed by the B3LYP functional (with the same basis sets). In some cases, this difference amounts to nearly 0.01 Å (e.g. for C2-N3, N3-C4, and C5dC6 bonds). It seems that the M05-2X functional performs better than B3LYP in prediction of the bond lengths in the uracil ring.

Examination of the results listed in Table 1 clearly indicates that the accurate atom distances can be obtained only with these basis sets, which contain the polarization functions (and diffuse functions) on all nonmetal atoms; for example, the D95V(d,p) or aug-cc-pVTZ basis sets. This is particularly important for the bonds involving oxygen or nitrogen atoms. For example, the C7-O1 distances calculated at the MP2/I and B3LYP/I levels of theory (without polarization functions), are equal to 1.367 and 1.349 Å, respectively, whereas the experimental value is much lower, 1.278(1) Å. The O1-Cu-N1 bond angle in the coordination ring is quite well predicted in calculations at all levels of theory. Similarly, the calculated N1-Cu-N5 bond angle shows very good agreement with experiment. However, the other two bond angles, N4-Cu-N5 and N4-Cu-O1, show some discrepancies between the theoretical and experimental values. This is caused by the fact that in crystal, the N4 nitrogen atom is involved in the intermolecular hydrogen bonding with other complex unit, which may lead to some distortions of the geometry around copper. The calculated bond angles of the uracil ring are accurate to within 1° in all theoretical methods. In summary, the new M05-2X density functional method predicts the copper-orotate (Cu-O1 and Cu-N1) bond lengths in a slightly worse agreement with experiment in comparison to the results obtained by the B3LYP method with the same basis set. On the other hand, M05-2X performs better than B3LYP in predicting geometrical parameters for the uracil ring.

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TABLE 2: The NBOa Description of the Bonds between Copper and Ligands in the cis-[Cu(oro)(NH3)2] Complex cAhA + cBhBb

σ(A-B) bond σ(Cu-O1) σ(Cu-N1) σ(Cu-N4) σ(Cu-N5)

0.376 0.386 0.326 0.347

(d1.3sp2)Cu (d1.0sp2)Cu (d0.9sp2)Cu (d0.9sp2)Cu

+ + + +

0.926 0.922 0.946 0.938

total occupancyc (sp3.5)O1 (sp2.5)N1 (sp4)N4 (sp4.5)N5

1.879 1.866 1.930 1.915

a Calculations performed by the unrestricted M05-2X method using the D95V(d,p) basis set for nonmetal atoms and LanL2DZ for Cu. b hB is an average hybrid orbital for R and β spin electrons. c The sum for R and β spin electrons.

3.2. NBO Analysis. The natural bond orbital (NBO) analysis of cis-[Cu(oro)(NH3)2] has provided detailed insight into the bonding in this complex. The electronic ground state is 2A′. In calculations performed by the unrestricted M05-2X method using the II basis set, the natural electron configuration of Cu is [core] 3d9.284s0.324p0.31. Thus, 18 core electrons and 9.91 valence electrons give the total of 27.91 electrons on the Cu cation in this complex. This is consistent with the calculated natural charge on Cu, equal to +1.09e (it corresponds to the difference between the nuclear charge (Z ) 29) and 27.91e). It is interesting to note that in cis-[Cu(oro)(NH3)2], the effective natural charge on the chelated copper(II) ion is close to +1. In the case of an open-shell system, the NBO analysis is performed separately for R and β-spin electrons.42,51 The results obtained for R-spin electrons show strong donations from various spn natural hybrid orbitals on N1, O1, N4, and N5 atoms to the Cu acceptor molecular orbitals. Each donation corresponds to a chemical picture of the L f Cu coordination bond from a lone pair orbital on the ligand atom to copper. Calculations performed for β-spin electrons have revealed the hybridization of Cu atomic orbitals and formation of strongly polarized σ bonds between Cu and the ligand atoms. Thus, the NBO results for R and β-spin electrons complement each other in the description of the copper-ligand bonds in the planar complex. These results are collected in Table 2. The hybrid orbitals on Cu correspond to an idealized dsp2 hybridization of transition metal M2+ cations in square-planar complexes. According to NBO, the hybrids are a mixture of 4s, 4px, 4py, and 3dxy atomic orbitals (with a minor contribution from the 3dx2-dy2 orbital). As is seen in this table, σ(Cu-O1) is a bond resulting from the overlap of a d1.3sp2 hybrid on Cu with an sp3.5 hybrid on the O1 atom. Similarly, the σ(Cu-N1) bond is formed by an overlap of a dsp2 hybrid on copper with an sp2.5 hybrid on N1. Both the σ bonds are strongly polarized toward the O1 and N1 atoms. The calculated total electron population (occupancy) on each bond is much lower than the idealized occupancy of 2e. This indicates a strong electron delocalization within the chelate ring. Two ammonia nitrogen atoms, N4 and N5, donate electron density from the sp4- and sp4.5-type hybrids, respectively, to two d0.9sp2 hybrid orbitals on Cu. The NBO method provides the donor-acceptor interaction energy between the orbitals of the conjugated bonds. The strength of this interaction can be estimated by the second-order perturbation theory.41 The selected interacting orbitals and the interaction energies (E2) are available in the Supporting Information. As follows from these results, the π orbitals of the C7dO3, C5dC6, and C4dO4 bonds are strongly conjugated. This leads to a relatively high occupancy of the π*(C5dC6) antibonding orbital, which should be formally empty. A gain of electron population in the π*(CdC) orbital can be directly correlated with a weakening of the corresponding

CdC bond.41 Thus, it is expected that the C5dC6 bond in the orotate ligand is weaker than that in uracil, which will be confirmed by the vibrational spectra (see the next section). Furthermore, NBO analysis shows that π electron delocalization builds a partial double bond between the N1 and C2 atoms. All these results indicate that the N1-deprotonated uracilate ring displays strong π-donating capability, and the strength of the copper-N1 bond should be similar to that between Cu(II) and pyridine. 3.3. Vibrational Spectra. Theoretical vibrational spectra were computed by the unrestricted B3LYP method using several basis sets: SDD, LanL2DZ, D95V(d,p), and aug-cc-pVTZ. Moreover, calculations of the spectra were also performed by the novel M05-2X functional using the D95V(d,p) basis set on all nonmetal atoms, and the LanL2DZ basis set on Cu (II basis set). To allow the comparison of the corresponding vibrational frequencies, normal coordinate analysis was carried out at each level of theory. The detailed examination of all the theoretical results has revealed that the best overall agreement between the experimental and theoretical spectra gives the B3LYP method with the II or III basis sets (the latter denotes the augcc-pVTZ basis set for all nonmetal atoms and LanL2DZ for Cu). However, calculations of the spectra at the B3LYP/III level are computationally very expensive, whereas the results are very similar to those obtained with the II basis set. The performance of the unrestricted M05-2X functional in calculations of the vibrational frequencies is very disappointing. All the calculated frequencies are seriously overestimated in comparison with experiment. For example, the predicted ν(C7dO3) carboxyl and ν(C4dO4) carbonyl stretching vibrations are larger than experiment by about 200 cm-1 (see the Supporting Information). Recently, Riley and co-workers52 performed a critical assessment of the performance of 37 DFT methods for their ability to accurately calculate molecular properties. These authors have concluded that B3LYP is the most accurate functional for calculating vibrational frequencies. Our conclusions from this work support their findings and show that the B3LYP/II level of theory is more reliable than the M05-2X/II method for predicting vibrational spectra of the Cu(II) complex. Therefore, in this paper, the theoretical frequencies, infrared intensities, Raman scattering activities, and PED calculated at the B3LYP/II level of theory are shown in Tables 3 and 4, whereas all other theoretical results are compared in the Supporting Information (Tables 1S and 2S). Orotate Ligand Vibrations. Figure 3 illustrates the experimental FT-IR and Raman spectra of cis-[Cu(oro)(NH3)2] in the range from 3500 to 600 cm-1. Table 3 lists the observed bands and the theoretical results obtained for this region. As is seen in Figure 3, the Raman spectrum of cis[Cu(oro)(NH3)2] is dominated by an extremely intense band at 1210 cm-1. According to PED, this band corresponds to mode 23, which is generated by the uracilate ring stretching vibration (69%) coupled with the in-plane δ(C5-H) bending vibration (17%), as shown in Table 3. A similar strong band has been observed in the FT-Raman spectra of cis-[Pt(oro)(NH3)2]10 and [Ni(oro)(H2O)4],11 at 1217 and 1215 cm-1, respectively. In all these complexes, the metal ion binds to the deprotonated N1 nitrogen atom of the uracilate ring. In the FT-Raman spectrum of anhydrous orotic acid, the corresponding very strong band occurs at 1251 cm-1.9 Thus, it can be concluded that the shift of this intense Raman band to the range 1220-1210 cm-1 is diagnostic for the N1 deprotonation of the uracil ring and formation of the N1-metal bond in these complexes.

Cu(II)-cis-[Cu(oro)(NH3)2] Noncovalent Interaction

J. Phys. Chem. B, Vol. 113, No. 23, 2009 8163

TABLE 3: Experimental IR and Raman Bands and Theoretical Frequencies (cm-1), Infrared Intensities (IIR, km mol-1), and Raman Scattering Activities (SR Å4 amu-1) for cis-[Cu(oro)(NH3)2] Calculated at the B3LYP Level of Theory theora

exptl no.

IR

Raman

1 2 3 4 5 6 7 8

3339 3317 3264 ov 3181 3135 3105 2980 2872 2812

9 10 11 12 13 14 15 16 17 18 19 20 21 22

1661 vs 1639 vs ov 1614 s ov 1580 sh ov 1548 w 1479 m 1412 m 1374 s 1315 m 1292 s 1268 s 1240 sh ov 1141 w 1050 w 1021 m 944 m 894 br ov 843f w 802 m ov 773 m 715 w 668 w 619 w ov 607 w ov

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

m m br m sh sh br w w

3339 3319 ov 3257 3184 ov 3104

w w w m m

1674 w 1643e sh 1631 1615 1605 1576 ov ov 1470 1420 1364 1315 ov 1266

s sh sh m m m w m w

1210 vs 1050 w 950 w ov 844 w 800 m 775 br 668 w ov ov ov 602 s

ωb

IIR

SR

band assignment, PED (%)c

3328 3319 3298 3247 3182 3334 3002 2942d

32 19 54 36 13 68 9 777

72 71 55 160 117 106 66 55

1701 1631 1699 1615 1605 1578 1574 1550 1470 1395 1372 1288 1315 1198

644 32 385 545 21 25 2 141 158 204 40 76 489 260

73 1 173 17 5 56 19 15 33 30 13 3 4 16

1216 1157 1043 1028 935 866 807 676 796 772 773 732 673 640 636 596 582

12 3 18 18 17 18 135 64 26 16 36 0 15 47 43 4 12

24 3