Experimental and Theoretical Studies of the Photoreduction of Copper

J. Phys. Chem. C 2008, 112, 1335-1344

1335

Experimental and Theoretical Studies of the Photoreduction of Copper(II)-Dendrimer Complexes Haiying Wan, Shenggang Li, Tatyana A. Konovalova, Shelby F. Shuler, David A. Dixon,* and Shane C. Street* Chemistry Department, The UniVersity of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336 ReceiVed: July 23, 2007; In Final Form: October 25, 2007

Zero valent Cu polyamidoamine (PAMAM) dendrimer nanocomposites were synthesized using UV irradiation starting from the aqueous Cu(II)/dendrimer system. The size of the nanoparticles is strongly dependent on the generation of the dendrimers and the pH of the solution. Larger nanoparticles are obtained with higher generation dendrimers, as well as in more basic solution. EPR spectra show that the sites bonded to the Cu(II) ion are significantly different at different pH values. Density functional theory (DFT) calculations have been used to predict the structures and EPR spectra of the Cu(II)-dendrimer complexes. At pH ) 3, the hydrated ion complexes Cu(H2O)62+ or Cu(H2O)52+ are present, as expected and reported previously. At pH ) 7.8, a chelating complex with two tertiary amine sites with or without two amide oxygen sites is present. At pH ) 11, the Cu(II) ion binds to either the primary amine and amide oxygen sites on a single branch or to two tertiary amines and four amide oxygen sites on all four branches. Our results show the importance of the amide sites in Cu(II)-dendrimer complexes in neutral and basic solutions.

Introduction Dendrimers are a class of molecules with unique architectures and dimensions in contrast to traditional linear polymers.1 They consist of three distinct regions: a core, layers of branched units, and terminal groups usually highly substituted with a large number of functional groups, as shown in Figure 1. The large number of functional groups on highly branched dendrimers as shown in Table 1 can have a significant impact on their physical properties in the solid state and solution.2,3 Highly branched dendrimers were initially proposed to have a dense-shell structure (with a less dense core) by de Gennes and Hervet,4 whereas Lescanec and Muthukumar5 later argued that dendrimers with flexible bonds should have a dense-core structure (with less density on the outer shells). Theoretical/computational studies so far appear to support the dense-core model.6 Dendrimers have been used as templates for particle formation and/or protective agents for the agglomeration of monometallic7-14 and bimetallic metal particles.13,15,16 Most of the procedures for synthesizing such systems are based on the complexation of dendrimers with metal ions followed by chemical reduction.13 A few reports have shown the formation of metallic nanoparticles by photochemical reduction.9,17-20 Au, Ag, and Pt nanoparticles have been prepared by UV irradiation following the complexation of precursor metal ions with dendrimer. However, the detailed mechanism and kinetics by which metal ions are converted to metal clusters via photoreduction remain largely unclear. The structure of copper(II)-dendrimer complexes have been studied extensively both experimentally and computationally. Turro and co-worker used electron paramagnetic resonance (EPR) spectroscopy to probe the Cu(II)-dendimer complexes.21,22 The structures of Cu(II) complexes with NH2* Corresponding author. E-mail: [email protected]; sstreet@ bama.ua.edu.

Figure 1. Structure of the PAMAM G0-NH2 dendrimer.

TABLE 1: Selected Properties of PAMAM Dendrimers

generation

mol. wgt. (g/mol)

diameter (Å)

no. primary amines

no. tertiary amines

0 4

517 14 214

15 45

4 64

2 62

terminated dendrimers were shown to be dependent on the dendrimer generation, pH, temperature, Cu(II) concentration, and the aging of the sample.22 At acidic pH (pH e 3.5), they proposed that the Cu(H2O)62+ complex predominates. At intermediate pH (4 e pH e 5), they proposed that three components were present: (1) Cu(H2O)62+, (2) Cu2+ complexes with two terminal primary amines, and (3) Cu2+ complexes with two terminal primary amines and two internal tertiary amines. At higher pH (pH g 6), they proposed that the third set of complexes were the only ones present. Crooks and co-workers used UV-vis spectroscopy and spectrophotometric titration to study the copper(II)-dendrimer complexes.7,13 The Cu(II) d-d transition in the Cu(II) complex with the OH-terminated G4 dendrimer is much more intense than that in the Cu(H2O)62+ complex, and is blue-shifted from ∼810 nm to ∼605 nm. A strong ligand-to-metal charge-transfer (LMCT) transition was also observed at ∼300 nm. Furthermore,

10.1021/jp075780d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008

1336 J. Phys. Chem. C, Vol. 112, No. 5, 2008

Figure 2. UV-visible spectra of CuCl2 aqueous solution and Cu(II)/ G4-NH2 solutions before and after irradiation.

their titration experiments yielded a linear relationship between the number of pairs of the outermost tertiary amine groups and the number of Cu(II) ions absorbed by different generations of OH-terminated dendrimers, consistent with the dense-core model of the dendrimer structure. By using matrix-assisted laser desorption ionization (MALDI) mass spectrometry, Zhou et al. reached the same conclusion.23 Recently, Gentle and co-workers carried out extended X-ray absorption fine structure (EXAFS) experiments on the Cu(II) complexes with NH2-terminated G4 dendrimer.24 They suggested the formation of five- and six-membered rings generated by the copper(II) ion chelating with the primary/tertiary amines as well as with amides on the basis of their EXAFS data. The participation of the terminal primary amine nitrogen and amide nitrogen sites in the coordination was also indicated by their NMR measurements. The EXAFS technique was also used by Diallo et al., together with bench scale measurements of the proton and Cu(II) binding to dendrimers as a function of pH.25 Diallo et al. suggested that complex formation involves both the internal tertiary and terminal primary amine groups. At pH ) 5.0, no significant binding was observed. At pH ) 9, the uptake of the Cu(II) ion increased linearly with metal ion-dendrimer loadings. However, at pH ) 7.0, the Cu(II) uptake followed an initial increase with a gradual leveling off, and then a second increase. On the basis of their EXAFS experiments at pH ) 7.0, Diallo et al.25 attributed the first binding step to the formation of octahedral complexes with two to four tertiary amine groups and two axial water molecules, and the second step to the formation of Cu(II)-water complexes. High-level theoretical calculations on the molecular structure of the copper(II)-dendrimer complexes are still lacking. This is predominantly due to the large size of the dendrimer molecules, the large number of potential structures of the complexes, and the role of environmental effects such as solvent, pH, and counterions. Tarazona-Vasquez and Balbuena have carried out Hartree-Fock (HF) and density functional theory (DFT)26 calculations on the G0 dendrimer terminated with the NH2 and OH groups as well as their complexes with metal ions (Cu2+, Ag+, Au3+, Pt2+) and metal atoms (Cu, Ag, Au, Pt).27-29 In their studies on the copper(II)-dendrimer complexes,28 they found that the Cu(II) ion prefers to form a tetracoordinated complex with two internal tertiary amine groups and the oxygen sites on the two amide groups, although it can also form a bidentate complex with the two amide sites alone. Hydrated Cu(II) ions can also form complexes with the dendrimer, although they tend to have a lower coordination number with

Wan et al. the dendrimer itself. They also considered the effect of pH on the structures of the complexes in their molecular dynamics studies.29 In the current work, we describe a photochemical method for synthesizing copper nanoparticles with narrow particle-size distributions by means of ultraviolet (UV) irradiation following complexation of the precursor Cu(II) ion to amine-terminated poly(amidoamine) (PAMAM) dendrimers with ethylenediamine (EDA) core in aqueous solution. The Cu(II) ion was chosen to demonstrate this concept because its complexation to the PAMAM dendrimer yields UV-vis and EPR spectra that can be readily interpreted.7,22 The Cu nanoparticles were studied with high-resolution transmission electron microscopy (HRTEM). Furthermore, DFT calculations were performed to model the Cu(II)-dendrimer complexes. Calculated copper(II)-ligand binding energies and magnetic parameters (g tensors and hyperfine coupling constants A) were utilized to predict the possible structures of the Cu(II)-dendrimer complexes. Experimental and Computational Methods In a typical experiment, 10 mM aqueous (Milli-Q, Millipore) solution of CuCl2 (Fisher) was mixed with 0.1 mM G0 or G4 NH2-terminated PAMAM dendrimer (Aldrich). The molar ratio of Cu to the number of primary amine groups of the dendrimers was kept as 1:4, which corresponds to about 16 Cu(II) ions per G4 dendrimer or 1 Cu(II) ion per G0 dendrimer. UV irradiation was provided by eight lamps (8 W per lamp, 253.7 nm) in a Rayonet photochemical reactor (model RPR-600, Southern New England Ultraviolet Inc.) at ambient temperature for 12 h. The solution was purged with nitrogen gas for ∼30 min prior to the irradiation. The pH of the solution was controlled in the range 2-12 by adding 0.1 M solutions of HCl (Fisher) or NaOH (Fisher). The starting pH was set as 7.8, which corresponds to near complete neutralization of all generations of the dendrimers. The reduction and irradiation effects were monitored by a Cary 50 UV-vis spectrophotometer with a 1 cm path length. Images of the copper-dendrimer nanocomposites were obtained with a FEI Technai Supertwin Transmission Electron Microscope. EPR measurements were performed with a Varian E-12 spectrometer equipped with a rectangular cavity. The magnetic field was measured with a Bruker ESR 035M gaussmeter, and the microwave frequency was measured with a HP 5245L frequency counter. EPR spectra were recorded by using the Scientific Software Services’s setup consisting of a PIDM, a data acquisition computer, connected to a Varian E-12, and the EWWIN data acquisition software. Diphenylpicrylhydrazyl (DPPH) (g ) 2.00351) was used to calibrate the magnetic field. Dendrimer samples were placed in quartz EPR tubes (o.d. 4 mm, WILMAD) and measured at 77 K. The experimental spectra were simulated by using the XSophe computer simulation software using Powder-Matrix Diagonalization Approach and a Gaussian line shape.30 DFT calculations were performed to predict the molecular structures, energetics, and magnetic properties of a large number of model complexes. We employed the B3LYP hybrid exchangecorrelation functional31,32 and the DFT-optimized polarized double-ζ DZVP2 basis set.33 Geometries were optimized with the Berny algorithm using redundant internal coordinates.34 Because of the potential presence of multiple local minima for some of the model complexes, we optimized the structures starting from different initial geometries. Both steric effects and hydrogen-bonding interactions were considered in choosing the starting geometries. In general, the structures do not have symmetry and have substantial flexibility with a number of

Photoreduction of Copper(II)-Dendrimer Complexes

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1337

Figure 3. HRTEM images of Cu/G4-NH2 nanoparticles formed upon UV irradiation.

Figure 4. HRTEM images of Cu/G0-NH2 nanoparticles formed upon UV irradiation at pH ) 3.0 (a), pH ) 7.8 (b), and pH ) 11.0 (c).

low-energy conformers. For the largest model complex, we used the SAM1/d method as implemented in the AMPAC 8 program to perform the initial geometry optimization.35 To ensure the optimized geometries are minima on the potential energy hypersurface and to obtain the zero-point vibrational energies and other thermodynamic properties, we calculated the harmonic vibrational frequencies using analytic second derivative techniques. In addition, the solvation free energies were calculated with the COSMO model.36 The g tensors and hyperfine coupling constants A were calculated at the B3LYP level. All of the calculations were carried with the Gaussian 03 program package37 on the Opeteron-based Cray XD-1 computer at the Alabma Supercomputing Center and the Xeon-based Dell Linux cluster at the University of Alabama. Results and Discussions Figure 2 shows the absorption spectra of the CuCl2 aqueous solution and the CuCl2/G4-NH2 composite solution before and after irradiation. The CuCl2 solution without dendrimer gives a broad, weak absorption band ∼800 nm (12 000 cm-1, ) 11, f ) 2.3 × 10-4), which corresponds to the d-d transition of the d 9 Cu(II) ion having undergone a strong Jahn-Teller distortion of the octahedral 2Eg state38,39 Upon complexation of the Cu2+ with the dendrimer, the d-d transition increases in intensity and blue-shifts to ∼580 nm. Similar observations have been made previously by Crooks and co-workers for the G4OH dendrimer.7 After UV irradiation, this peak diminishes and gives rise to the lower energy shoulder.

Figure 3 shows electron micrographs of the copper nanoparticles obtained in the presence of G4-NH2 after irradiation. The images show that Cu nanoparticles of spherical shape with an average diameter of 5.5 nm were formed. The value of the spatial frequency (SF) from HRTEM is ∼0.21 nm, consistent with the Cu cubic (111) plane. The diffraction patterns in the selected area were analyzed, and the particles were found to be metallic Cu nanoparticles. The cell parameter derived from the diffraction pattern is 0.362 nm, consistent with the reported values of the copper cubic phase40 (Joint Committee on Powder Diffraction standards (JCPDS) number 01-1242). Possible structures of the Cu(II)-PAMAM dendrimers suggested from previous experimental and computational studies involve the internal tertiary amine groups, the amide groups, the terminal primary amine groups, and the solvent water molecules.22,24,25,28,29 To better understand the structures of these complexes, we replaced the G4 dendrimer with the much simpler G0 dendrimer, which has only four primary and two tertiary amines as shown in Table 1. Figure 4 displays the Cu/G0-NH2 nanoparticles formed at various pH values. At pH ) 3, the average diameter of the nanoparticles formed is ∼1 nm and the nanoparticles are clearly isolated from each other. As the solution becomes more basic, the average size of the nanoparticles increases. At pH ) 11, the nanoparticles have an average diameter of ∼3 nm, larger than that of G0-NH2 itself at pH ) 3. Thus, the structure and number of metal complexes strongly affect the size and distribution of the copper nanoparticles formed upon UV irradiation.


Figure 5. Structures of singly and doubly coordinated model Cu(II) complexes. Copper ligand bond lengths and bond angles are shown in angstroms and degrees, respectively.

To estimate the binding capabilities of the nitrogen- and oxygen-based ligands to the Cu(II) ion, we calculated the structures of a number of singly and doubly coordinated model complexes (Figure 5) and their total binding energies (Table 2) according to the types of ligands present in the dendrimers. The total binding energies were calculated as the energy difference between the complexes and their components at 0 K. The calculated copper-nitrogen bond lengths range from 1.93 Å in 1 to 2.05 Å in 6, whereas the copper-oxygen bond distances are slightly shorter, from 1.90 to 1.94 Å. The binding energies of the singly coordinated complexes are in the order 1 < 3a < 2 < 3b. The larger binding energy of N(CH3)3 to the Cu2+ ion as compared to NH2CH3 is consistent with the trend found for the M+-L (M ) Cu, Al, Ga, In; L ) NHn(CH3)3) complexes: NH3 < NH2CH3 < NH(CH3)2 < N(CH3)341 and with the measured gas-phase proton affinities at 298 K: 204.0 kcal/mol for NH3, 214.9 kcal/mol for NH2CH3, 222.2 kcal/mol for NH(CH3)2, and 226.8 kcal/mol for N(CH3)3.42 The Cu(II) binding energy to the oxygen site on the amide is much larger than that to the nitrogen site on the amide by ∼35 kcal/mol. Our calculated proton affinity at 0 K for the oxygen site on the amide is

Wan et al. ∼12 kcal/mol larger than that of the nitrogen site: 210.1 kcal/mol for the oxygen site and 197.7 kcal/mol for the nitrogen site. Similarly, the Cu(II) ion prefers to bind to the oxygen site on the amide rather than to the nitrogen site. The binding energy of the Cu(II) ion to the amide oxygen is larger than those to NH2CH3 and N(CH3)3, and the binding energy to the amide nitrogen is less than that of N(CH3)3. This is inconsistent with a simple proton affinity model because the PA of CH3NHCOCH3 (212.4 kcal/mol)42 is less than the PAs of NH2CH3 and N(CH3)3. Simultaneous binding of the Cu(II) ion to both the N and O sites on the amide leads to a decrease in the binding energy. The total binding energy for binding two NH2CH3 to form 4 is ∼100 kcal/mol more than that of 1. The binding energy of the chelating complex 5 is ∼17 kcal/mol larger than that of 4. The proton affinity of (CH3)2NCH2CH2N(CH3)2 (242.1 kcal/ mol)42 is also much larger than that of N(CH3)3, although this cannot completely account for the large difference between the binding energies of 2 and 5. The above binding energies clearly show the importance of the oxygen sites on the amide groups (largely ignored in most previous studies on copper(II)-dendrimer complexes except in the work of Gentle and co-workers, and Tarazona-Vasquez and Balbuena),24,28 as well as the nitrogen sites on the tertiary amine groups. In aqueous solutions, the pKb values of the ligands are 4.75 for NH3, 3.34 for NH2CH3, 3.27 for NH(CH3)2, 4.20 for N(CH3)3, and 3.60 and 5.74 for (CH3)2NCH2CH2N(CH3)2.42,43 The amide is a much weaker base than the above alkylamines with pKb ≈ 14.44 In aqueous solution, the Cu(II) ion is penta- or hexacoordinated,45 so we calculated the structures of a number of tetra-, penta-, and hexacoordinated model complexes with water included in the first solvent shell as shown in Figure 6 and Table 2. Four of the six copper-oxygen bonds in 7 are ∼2.03 Å long, and the other two are ∼2.28 Å located at the axial positions. This results from the Jahn-Teller distortion of the d 9 Cu(II) ion and the ideal Oh symmetry is distorted to an approximate D4h symmetry. The orientations of the water ligands are such that they can form weak hydrogen bonds with the neighbor water ligands. When the water ligands are replaced by the ammonia ligands, the ammonia ligand prefers the equatorial position. Replacement of one of the equatorial water ligands with an ammonia ligand significantly lengthens the copper-oxygen bond distances for the axial water ligands from 2.28 to 2.42 Å, whereas the copper-oxygen bond distances for the equatorial water ligands elongate only slightly by about 0.05 Å. The copper-nitrogen bond length is 2.00 Å, slightly shorter than the copper-oxygen bond distances for the equatorial water ligands. Replacement of the second equatorial water ligand results in both the trans 9a and cis 10a configurations. The copper-oxygen and copper-nitrogen bond lengths in the cis configuration are similar to those in the Cu(H2O)5(NH3)2+ complex. The trans configuration has slightly shorter coppernitrogen and axial copper-oxygen bond distances and longer equatorial copper-oxygen bond lengths than the cis configuration. The cis and trans conformers, however, have very similar total binding energies, indicating they are similar in terms of stability. Further replacement of two equatorial water ligands results in a nearly D4h configuration, with the remaining two water ligands weakly bonded to copper(II) in a trans configuration. Geometry optimization starting from the cis conformer results in the exclusion of one of the water ligands from the first solvation shell and a structure similar to that of 15 with one addition water ligand hydrogen bonded to the water and



TABLE 2: Total Binding Energies in kcal/mol for the Copper(II) Complexesa molecule

binding energies

Cu(NH2CH3)2+ (1) Cu(N(CH3)3)2+ (2) Cu(N-CH3NHCOCH3)2+ (3a) Cu(O-CH3NHCOCH3)2+ (3b) Cu(NH2CH3)22+ (4) Cu(N,N-(CH3)2NCH2CH2N(CH3)2)2+ (5) Cu(N,O-CH3NHCOCH3)2+ (6) Cu(H2O)62+ (7) Cu(H2O)5(NH3)2+ (8) trans-Cu(H2O)4(NH3)22+ (9a) cis-Cu(H2O)4(NH3)22+ (9b) trans-Cu(H2O)2(NH3)42+ (10) Cu(H2O)(NH3)52+ (11) Cu(H2O)52+ (12) Cu(H2O)3(NH3)22+ (13) Cu(H2O)2(NH3)32+ (14) Cu(H2O)(NH3)42+ (15) Cu(NH3)42+ (16) Cu(H2O)4(NH2CH3)22+ (17) Cu(H2O)2(NH2CH3)42+ (18) Cu(H2O)3(NH2CH3)22+ (19) Cu(H2O)5(N(CH3)3)2+ (20) Cu(H2O)3(NH2CH3)(N(CH3)3)2+ (21) Cu(NH2CH3)3(N(CH3)3)2+ (22) Cu(H2O)2(N(CH3)3)22+ (23) Cu(H2O)3((CH3)2NCH2CH2N(CH3)2)2+ (24) Cu(H2O)3(H2NCH2CH2NHCOCH3)2+ (25a) Cu(H2O)3(H2NCH2CH2NHCOCH3)2+ (25b) Cu(H2O)2((CH3)2NCH2CH2N(CH3)2)22+ (26) Cu(H2O)((CH3)2NCH2CH2N(CH3)2)22+ (27) Cu(H2O)2(H2NCH2CH2NHCOCH3)22+ (28) Cu(H2O)(H2NCH2CH2NHCOCH3)22+ (29) Cu(H2O)2(CH3NHCOCH2CH2N(CH3)CH2CH2N(CH3)CH2CH2 CONHCH3)2+ (30) Cu((CH3NHCOCH2CH2)2NCH2CH2N(CH2CH2CONHCH3)2)2+ (31)

192.4 226.5 200.2 235.9 290.0 307.4 216.7 355.4 368.5 383.5 382.2 402.2 405.2 335.0 365.6 377.7 387.0 369.3 392.8 418.1 377.3 379.2 379.8 392.2 367.2 388.7 322.2 380.0 413.0 382.0 424.0 392.5 438.1 443.5

a

The experimental proton affinities of the ligands are 204.0 for NH3, 214.9 kcal/mol for NH2CH3, 226.8 kcal/mol for N(CH3)3, 212.4 kcal/mol for CH3NHCOCH3, and 242.1 kcal/mol for (CH3)2NCH2CH2N(CH3)2. See ref 42.

ammonia ligands. This structure is ∼5 kcal/mol more stable than 10, indicating the importance of the hydrogen-bonding interaction in determining the stability of the complexes. When the fifth water ligand is replaced, the copper-oxygen bond length lengthens to ∼3.00 Å, much longer than the longest value of 2.28 Å in 7, and the axial copper-nitrogen bond length is also much longer than the equatorial ones by about 0.2 Å. The penta- and tetracoordinated complexes are similar to the hexacoordinated ones without one or two of the axial water ligands. The total binding energies of these complexes follow the order of 7 < 8 < 9b ≈ 9a < 10 ≈ 11 for the hexacoordinated species and 12 < 13 < 14 < 15 for the pentacoordinated species. Loss of one water ligand in the hexacoordinated complexes 7, 9a, and 10 to the pentacoordinated complexes 12, 13, and 15 reduces the total binding energy by 15-20 kcal/mol, and the binding energy of tetracoordinated complex 16 is ∼30 kcal/mol smaller than that of hexacoordinated complex 10. These calculations, of course, do not include any other solvation effects, which may change these values. It is known that the sixth coordination of ammonia to the copper(II) ion occurs only in liquid ammonia.39 We also calculated the structures of a number of copper(II) complexes with larger ligands: NH2CH3, N(CH3)3, (CH3)2NCH2CH2N(CH3)2, H2NCH2CH2NHCOCH3, CH3NHCOCH2CH2N(CH3)CH2CH2N(CH3)CH2CH2CONHCH3 (EDA core with only two branches and without terminal primary amines), and (CH3NHCOCH2CH2)2NCH2CH2N(CH2CH2CONHCH3)2 (G0 without terminal primary amines). The calculated Cu-NH2CH3 bond lengths in 17 to 19 are very close to the calculated CuNH3 bond lengths. The coordination number of the copper(II)

ion appears to decrease in the presence of bulky tertiary amine ligands. The total binding energies of the complexes with NH2CH3 (17 and 18) are ∼10 and ∼15 kcal/mol larger than those with NH3 (9a and 10). The total binding energies of the complexes with the chelating ligand (CH3)2NCH2CH2N(CH3)2 (24 and 27) are ∼5 and 35 kcal/mol smaller than those with NH2CH3 (17 and 18) because of the missing water ligand, whereas the total binding energy of complex 26 is only ∼5 kcal/ mol less than that of 18. Similarly, the total binding energies of the complexes with the chelating ligand H2NCH2CH2NHCOCH3 (25b and 29) are also ∼10 and 25 kcal/mol smaller than those with NH2CH3 (17 and 18), but the total binding energy of the complex 28 is ∼5 kcal/mol more than that of 18. We note that the structure of complex 25a is different from that of 25b in that the N site on the amide group also binds the Cu(II) ion in 25a, which destabilizes the complex despite the increasing coordination number. This is consistent with the calculated total binding energies for complexes 3b and 6. Complexes 30 and 31, where the Cu(II) ion binds two tertiary amines and two to four amide oxygen sites, have the highest total binding energies of all of the complexes. We also calculated the reaction free energies at 298 K for the exchange of water with ammonia, and these are listed in Table 3. Both the free energies in the gas phase and in aqueous solution are given. Within 2 kcal/mol, the solution and gasphase values for the exchange reactions are the same. Replacement of the first two water molecules in 7 with ammonia is exothermic by ∼30 kcal/mol. Because the cis and trans conformers 9a and 9b have similar energies, the existence of different conformers does not significantly affect the reaction


Wan et al.



Figure 6. Structures of tetra-, penta-, and hexacoordinated model Cu(II) complexes. Selected copper-ligand bond lengths are shown in angstroms.

TABLE 3: Calculated Reaction Free Energies at 298.15 K in kcal/mol for the Substitution of Ammonia with Water for the Copper(II) Complexes in the Gas Phase and Solution Phase reaction

gas phase

solution phase

Cu(H2O)62+ (7) + NH3 f Cu(H2O)5(NH3)2+ (8) + H2O Cu(H2O)5(NH3)2+ (8) + NH3 f Cu(H2O)4(NH3)22+ (9a) + H2O Cu(H2O)4(NH3)22+ (9a) + 2NH3 f Cu(H2O)2(NH3)42+ (10) + 2H2O Cu(H2O)2(NH3)42+ (10) + NH3 f Cu(H2O)(NH3)52+ (11) + H2O Cu(H2O)52+ (12) + 2NH3 f Cu(H2O)3(NH3)22+ (13) + 2H2O Cu(H2O)3(NH3)22+ (13) + NH3 f Cu(H2O)2(NH3)32+ (14) + H2O Cu(H2O)2(NH3)32+ (14) + NH3 f Cu(H2O)(NH3)42+ (15) + H2O

-13.5 -14.2 -18.1 -1.5 -31.4 -12.4 -7.4

-14.2 -13.8 -19.4 -1.6 -29.7 -13.3 -8.9

free energies. Further replacement of two water molecules is less exothermic, ∼20 kcal/mol. As mentioned above, the four ammonia ligands always occupy the equatorial positions, and thus there is no other distinct conformer in this case. The replacement of the fifth water molecule by ammonia is barely exothermic. The pentacoordinated complexes follow the same trend. The structures of the copper(II)-dendrimer complexes are also likely to be influenced by the availability of the different ligands at different pH values and by the steric constraints imposed by the dendrimer. The reaction free energy for the reaction Cu(H2O)62+ + 2CH3NH3+(H2O)3 f Cu(H2O)4(CH3NH2)22+ + 2H3O+(H2O)3 was calculated to be positive, 5.4 kcal/ mol in the gas phase and 6.9 kcal/mol in aqueous solution. This shows that the Cu(II) ion is unlikely to coordinate with the primary amines by replacing the protons when these primary amines are protonated. EPR spectroscopy provides information on Cu(II) binding to the different sites of the dendrimer as a function of pH. Figure 7 shows the typical powder spectra of Cu(II) (I ) 3/2) with axial symmetry: gzz (gII) > gxx ≈ gyy (g⊥) and Azz > Axx ≈ Ayy. The experimental spectra were simulated with reasonable accuracy and the magnetic properties, g tensors, and hyperfine coupling constants (A), were determined from the spectral simulations. The best-fit values at the three different pH values are given in Table 4. The g tensors and hyperfine coupling constants of Cu(II) at the three different pH values at 77 K are very close to the three signals at 130 K reported previously.22 The g and A values at pH ) 3 are in reasonable agreement

with those measured by Lewis et al. for the Cu(II) solution with 60% glycerine and 40% water (gzz ) 2.400 ( 0.001, gxx ≈ gyy ) 2.099 ( 0.002, Azz ) 160.9 ( 0.6 G, Axx ≈ Ayy ) 15.9 ( 1.3 G)46 with the largest difference found for Azz. Table 4 lists the calculated g tensors and hyperfine coupling constants. The experimental value of gzz (2.40) is best matched by that of 7 (2.40) or that of 12 (2.38) because the calculated gzz values for the rest of complexes are significantly smaller. The calculated values for the other two components are higher than the experimental values by 0.06 for 7 and by 0.03 and 0.06 for 12. Our experimental value of Azz (129 G) is smaller than the absolute values of the calculated results for 7 (-197 G) and 12 (-200 G). However, our calculated values for Azz are in quite good agreement with that from Lewis et al.46 The calculated Axx and Ayy components of the hyperfine coupling constants were substantially overestimated. Recently, de Almeida et al. calculated these parameters for Cu(II) aqua ions47 at the DFT level with a modified B3LYP exchange-correlation functional. They suggested that the restricted DFT method is more suited for the calculations of these parameters because of the spin contamination problems with the unrestricted DFT method and their choice of functional. However, there is very little spin contamination in these molecules with the regular B3LYP functional (the expectation value of S 2 is 0.751 for Cu(H2O)62+), and our calculated values for the g tensors are in somewhat better agreement with the experimental values than theirs (gzz ) 2.35, gxx ) 2.14, gyy ) 2.12 for Cu(H2O)62+ with Ci symmetry). Although their value for Azz of -167.6 G is quite close to that from the work of Lewis et al.,47 the other two


Figure 7. EPR spectra of Cu2+-G0 dendrimer complexes prepared at (a) pH ) 3; (b) pH ) 7.8; (c) pH ) 11 and measured at 77 K. EPR parameters: microwave frequency, 9.36 GHz; microwave power, 5 mW; modulation amplitude, 10 G. Solid line, experimental; dotted line, simulated.

components of the A values (Axx ) 68.6 G, Ayy ) 52.4 G), were overestimated just as found by us. Our calculated values are thus consistent with a penta or hexa-aquo complex at pH ) 3. This result is also consistent with the fact that the dendrimer N sites are all protonated at this pH and the only unprotonated binding sites might be the amide. This is consistent with the conclusions of Turro and co-workers, and Diallo et al.22,25At pH ) 7.8, the experimental value of gzz (2.21) is close to that of a number of structures: 18 (2.19), 19 (2.21), 21 (2.20), 22 (2.20), 23 (2.21), 24 (2.20), 27 (2.19), 28 (2.24), 29 (2.23), and 30 (2.20). The agreement is good considering the accuracy of the experimental simulations and that of the calculations. Replacement of two NH3 ligands in 9a with two NH2CH3 ligands in 17 reduces the gzz value from 2.27 to 2.23, and losing one water ligand in 9a to form 13 slightly reduces the gzz value to 2.25. The other two components of the g tensor at this pH are close to those predicted for 18, 22, 24, 26, 27, 28, 29, and 30 from the above complexes to within 0.04. For the hyperfine coupling constants, the calculated Azz values of 18 (-189), 22 (-165), 24 (-204), 26 (-195), 27 (-181), 29 (-157), and 30

Wan et al. (-203) are consistent with the experimental value of (187. The experimental values of the other two components are best fit by the calculated values for 18, 24, 26, 27, and 30. The primary amines are mostly protonated at this pH,25,48,49 so we can probably exclude 18. In 26 and 27, the Cu(II) ion binds to four tertiary amine nitrogen sites from two EDA cores. However, this requires two different G0 molecules to form the complex, which is not likely due to steric effects. However, such a structure may play a role for higher generation dendrimers. In 24 and 30, the Cu(II) ion binds to two tertiary amine nitrogen sites from one EDA core with or without two amide oxygen sites by forming five- and/or six-member ring structures. These are consistent with the results by Gentle and co-workers, and Diallo et al.24,25 However, these are not consistent with the proposed structures of Turro and co-workers.22 At pH ) 11, all of the binding sites are expected to be deprotonated and can bind to the copper(II) ion. At this pH, the calculated gzz values for 17 (2.29), 20 (2.25), 25a (2.27), 25b (2.28), and 31 (2.25) are closest to the measured value of gzz (2.27); the other two components of the experimental g tensor are best matched by the calculated values of 25a, 25b, and 31 to within 0.05. The calculated Azz values of -188, -171, and -192 for 25a, 25b, and 31 agree well with the experimental value of (197, whereas one of the predicted Axx and Ayy values is too large for 25a and 25b. Because 25a is ∼60 kcal/mol less stable than 25b, we exclude 25a. Both 25b and 31 suggest that the Cu(II) ion binds to the O sites on the amides. In 25b, the Cu(II) ion binds to the terminal primary amine and the amide oxygen sites on only one branch to form a six-member ring, whereas in 31 the Cu(II) ion binds to two tertiary amines and four amide oxygen sites on all four branches to form five- and six-member rings. This result is again not consistent with the proposed structures from Turro and co-workers at high pH.22 At pH ) 11, the concentration of OH- is expected to be fairly high, and thus its binding with Cu(II) needs to be considered. Structures 32-36 are obtained by replacing one H2O ligand with OH- in structures 7, 24, 25b, 27, and 29, respectively. In the optimized structures, the OH- ligand always occupies the equatorial position. Thus, an H2O ligand in structures 32-34, a tertiary amine ligand in 35, and an oxygen site on the amide are displaced from the first solvation shell. This is due to the much stronger interaction between Cu(II) and OH- as compared to Cu(II) binding to the other neutral ligand. However, the gzz values of 33-36 are too small as compared to experiment at this pH, and thus these structures cannot be important structures at this pH. Although the gzz value of Cu(OH)(H2O)5+ (32) is very close to the experimental value at this pH, it is unlikely to be the dominant species at this pH because no precipitate formed and the color of the solution is dark-blue, indicative of complex formation with the dendrimer. The reason that OH- is not involved in the complex formation is probably due to differential solvation effects because the solvation of the OH- and the Cu(II) complex is likely to be substantially larger than the solvation of the singly positively charged Cu(II)-OH- complex. Finally, transient nitrogen-based organic radicals have been detected in previous EPR studies of Cu(II)-dendrimer complexes, where the radical spectra appeared as a multiline signal superimposed on the high-field portion of the Cu(II) signal.21 Previous workers hypothesized a redox reaction probably from Cu(II) to Cu(I). However, we think this organic radical signal is also responsible for the second step of the redox reaction from Cu(I) to Cu(0). We are currently using EPR and DFT to study these photoinduced organic radicals in metal-dendrimer systems. Our studies indicate that the organic radical is formed



TABLE 4: Experimental and Calculated g tensors (gzz > gxx ≈ gyy) and Hyperfine Coupling Constants (Azz > Axx ≈ Ayy) for the Tetra-, Penta-, and Hexacoordinated Copper(II) Model Complexes property

g tensor gzz

experiment - this work pH ) 3 pH ) 7.8 pH ) 11 experiment - ref 22 signal A signal B signal C calculated - this work Cu(H2O)62+ (7) Cu(H2O)5(NH3)2+ (8) trans-Cu(H2O)4(NH3)22+ (9a) cis-Cu(H2O)4(NH3)22+ (9b) trans-Cu(H2O)2(NH3)42+ (10) Cu(H2O)(NH3)52+ (11) Cu(H2O)52+ (12) Cu(H2O)3(NH3)22+ (13) Cu(H2O)2(NH3)32+ (14) Cu(H2O)(NH3)42+ (15) Cu(NH3)42+ (16) Cu(H2O)4(NH2CH3)22+ (17) Cu(H2O)2(NH2CH3)42+ (18) Cu(H2O)3(NH2CH3)22+ (19) Cu(H2O)5(N(CH3)3)2+ (20) Cu(H2O)3(NH2CH3)(N(CH3)3)2+ (21) Cu(NH2CH3)3(N(CH3)3)2+ (22) Cu(H2O)2(N(CH3)3)22+ (23) Cu(H2O)3((CH3)2NCH2CH2N(CH3)2)2+ (24) Cu(H2O)3(H2NCH2CH2NHCOCH3)2+ (25a) Cu(H2O)3(H2NCH2CH2NHCOCH3)2+ (25b) Cu(H2O)2((CH3)2NCH2CH2N(CH3)2)22+ (26) Cu(H2O)((CH3)2NCH2CH2N(CH3)2)22+ (27) Cu(H2O)2(H2NCH2CH2NHCOCH3)22+ (28) Cu(H2O)(H2NCH2CH2NHCOCH3)22+ (29) Cu(H2O)2(CH3NHCOCH2CH2N(CH3)CH2CH2N(CH3)CH2CH2 CONHCH3)2+ (30) Cu((CH3NHCOCH2CH2)2NCH2CH2N(CH2CH2CONHCH3)2)2+ (31) Cu(OH)(H2O)5+ (32) Cu(OH)(H2O)2((CH3)2NCH2CH2N(CH3)2)+ (33) Cu(OH)(H2O)2(H2NCH2CH2NHCOCH3)+ (34) Cu(OH)((CH3)2NCH2CH2N(CH3)2)2+ (35) Cu(OH)(H2NCH2CH2NHCOCH3)2+ (36)

by photoinduced ligand to metal charge transfer (LMCT), which is also responsible for the reduction of Cu(II) to Cu(0). This mechanism is consistent with the observed nanoparticle size variations at different pH values and our present EPR and DFT studies. In neutral and basic pH solutions, the Cu(II) ions are predicted to be bonded to the dendrimer directly, which facilitates LMCT processes resulting in larger nanoparticles. In acidic pH solutions, however, the Cu(II) ions are surrounded by H2O, and thus LMCT processes are less likely. This work will be reported in a future publication. Conclusions Zero valent Cu dendrimer nanocomposites were synthesized by using UV irradiation starting from the aqueous Cu(II)/ dendrimer system. The size of the nanoparticles strongly depends on the generation of the dendrimers and the pH of the solution. Larger nanoparticles can be obtained with a higher generation of dendrimers, as well as in more basic solution. Density functional theory (DFT) calculations have been used to predict the structures and EPR spectra of the Cu(II)-dendrimer complexes. The calculated total binding energies of these model complexes show that it is favorable for Cu(II) to form hexacoordinated or pentacoordinated complexes. The g tensors and hyperfine coupling constants were calculated for these com-

coupling constant (Gauss)

gxx

gyy

Azz

Axx

Ayy

2.40 2.21 2.27

2.09 2.08 2.03

2.08 2.09 2.11

(129 (187 (197

(9 (3 (10

(20 (37 (40

2.43 2.21 2.30

2.11 2.03 2.04

2.09 2.08 2.08

-135 -197 -174

-10 -11 -6

-10 -13 -9

2.40 2.32 2.27 2.28 2.21 2.24 2.38 2.25 2.24 2.21 2.21 2.29 2.19 2.21 2.25 2.20 2.20 2.21 2.20 2.27 2.28 2.18 2.19 2.24 2.23 2.20

2.15 2.14 2.02 2.09 2.06 2.07 2.12 2.05 2.04 2.05 2.06 2.00 2.05 2.03 2.00 2.00 2.05 2.01 2.06 2.06 2.06 2.06 2.05 2.06 2.06 2.05

2.16 2.06 2.19 2.10 2.07 2.08 2.15 2.10 2.11 2.08 2.06 2.23 2.10 2.10 2.00 2.00 2.10 2.17 2.07 2.14 2.10 2.05 2.07 2.08 2.08 2.06

-197 -178 297 -202 -200 -164 -200 184 -158 -198 -197 366 -189 347 243 398 -165 475 -204 -188 -171 -195 -181 -149 -157 -203

107 56 -6 66 42 74 95 123 41 54 28 104 41 173 -10 165 37 256 14 10 36 -5 1 110 99 11

91 121 99 58 42 90 89 -99 116 25 28 122 13 70 6 186 93 323 -2 82 106 -12 20 71 58 22

2.25 2.26 2.18 2.21 2.19 2.20

2.08 2.13 2.06 2.07 2.07 2.06

2.08 2.04 2.04 2.05 2.04 2.05

-192 160 -170 160 -168 -207

40 -125 28 108 11 16

41 41 54 -94 59 22

plexes and compared to the experimental data in order to assign the EPR spectra. The spectrum at pH ) 3 was assigned to the hydrated ion complexes Cu(H2O)62+ or Cu(H2O)52+. At pH ) 7.8, we attribute the spectrum to a chelating complex with two tertiary amine sites with or without two amide oxygen sites. At pH ) 11, the Cu(II) ion appears to bind to either the primary amine and amide oxygen sites on a single branch or two tertiary amines and four amide oxygen sites on all four branches. These assignments show the importance of the amide sites in the Cu(II)-dendrimer complex in neutral and basic solutions. The formation of the five-member and/or six-member ring structures is consistent with the results from recent EXFAS experiments.24,25 Although our EPR data are nearly identical to those from Turro and co-workers,22 our predicted structures derived on the basis of extensive DFT calculations differ from theirs. Acknowledgment. Financial support for this work by the National Science Foundation (CTS-0608896) through the NIRT program, the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) (catalysis center program) and the Robert Ramsay Chair Foundation is gratefully acknowledged. S.S. and H.W. acknowledge the support of NSF MRSEC grant DMR0213985 and MINT Center at The University of Alabama.

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