A synthetic, spectroscopic and computational study of copper(II) complexes supported by pyridylamide ligands

Accepted Manuscript A Synthetic, Spectroscopic and Computational Study of Copper(II) Complexes Supported by Pyridylamide Ligands Magnus A. Pauly, Ethan M. Erwin, Douglas R. Powell, Gerard T. Rowe, Lei Yang PII: DOI: Reference:

S0277-5387(15)00673-7 http://dx.doi.org/10.1016/j.poly.2015.11.015 POLY 11660

To appear in:

Polyhedron

Received Date: Accepted Date:

9 August 2015 9 November 2015

Please cite this article as: M.A. Pauly, E.M. Erwin, D.R. Powell, G.T. Rowe, L. Yang, A Synthetic, Spectroscopic and Computational Study of Copper(II) Complexes Supported by Pyridylamide Ligands, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.11.015

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A Synthetic, Spectroscopic and Computational Study of Copper(II) Complexes Supported by Pyridylamide Ligands Magnus A. Pauly,a Ethan M. Erwin,a Douglas R. Powell,b Gerard T. Rowe,c＊ Lei Yang a＊ a

Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, USA Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA c Department of Chemistry & Physics, University of South Carolina-Aiken, Aiken, South Carolina 29801, USA ＊ Corresponding author. E-mail: [email protected], [email protected]. Fax: +1(501)4503623 b

Abstract: Three Cu(II) complexes supported by pyridylamide ligands N-2-acetamidopyridine (Haap) and 2-(N-(2-Pyridyl)carbamoyl)pyridine (H2pcp) have been synthesized. The structures of the three complexes have been characterized by X-ray crystallography, showing the metal sites with square pyramidal geometry in [Cu2(Haap)2(OAc)4] (1), trigonal bipyramidal geometry in [Cu(H2pcp)Cl2(DMF)] (2) and square planar geometry in [Cu2(2pcp)2(OAc)2] (3). Complexes 1 and 2 possess the neutral form of the ligands, and exhibit interesting intra- and intermolecular interactions in the outer coordination sphere. The deprotonated form of H2pcp in complex 3 displays a chelating-bridging coordination mode, giving rise to a dinuclear copper cluster. The detailed Hirshfeld surface analysis about the three complexes led to a better understanding of the weak interactions observed in X-ray crystal structures. Evidence offered by a close examination on FT-IR, UV-vis, EPR and NMR supported the structural characterization and provided deep insights into the electronic structures of the complexes. Cyclic voltammetry studies revealed the different reduction/oxidation behavior of the metal centers in the different coordination environments. Density functional theory calculations were performed to validate the experimental evidence about the coordination structures, spectroscopic properties and electronic structures. Keywords: pyridylamide ligands, Copper(II), X-ray structures, Hirshfeld surface, spectroscopic characterization, Cyclic voltammetry, Density functional theory

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Introduction Copper complexes supported by the ligand frameworks functionalized with amide groups have garnered great interest due to their diverse structural features [1], spectroscopic properties [2] and catalytic activities [3]. These studies have found that the nuclearity and supramolecular conformation of the complexes can be tuned by utilizing either the neutral or anionic form of the amide group. For a neutral amide group, the C=O oxygen is the potential coordination site, and the N–H group acts as a hydrogen bond donor to interact with surrounding units. In the anionic form, the amidate group stabilizes the copper atom through the negatively charged nitrogen donor due to its strong σ-donating ability, and the C=O group either connects other copper centers to form metal clusters, or acts as a hydrogen bond acceptor. In our laboratory, we are particularly interested in the copper complexes constructed with pyridylamide ligands. Our previous work on the ligand N-2-acetamidopyridine (Haap, Scheme 1 left) and its derivatives has shown the combination of pyridyl donors with amide groups played a key role in the subtle control of metal center geometries, electronic properties and intermolecular interactions [4]. As a continuing effort to enhance the structure, reactivity and biological relevance of the copper complexes, we employed pyridylamide ligands with the potential to incorporate intra- and intermolecular interactions (e.g. hydrogen bond and π based packing) into the outer sphere coordination environment. In this manuscript, application of the ligand Haap in the reaction with Cu(OAc)2 afforded a dimeric paddlewheel complex with interesting intermolecular interactions. In addition, ligand 2-(N-(2-Pyridyl)carbamoyl)pyridine (H2pcp, Scheme 1 right) derived from the ligand Haap was used as a supporting platform in the construction of new copper(II) compounds. The synthesis and characterization of two copper(II) complexes with ligand H2pcp showed intriguing structural features and spectroscopic properties. Our results demonstrated that ligand H2pcp featuring a dipyridyl amide backbone not only afforded new copper(II) complexes with different nuclearity, but also resulted in the desired second-sphere hydrogen bond or π


π interactions. Hirshfeld surface analysis revealed more detail of these weak interactions. Spectroscopic and electrochemical measurements have also been performed to examine the electronic properties and reduction/oxidation behavior of these complexes. Computational analysis based on the geometry optimization, vibrational frequency calculation and natural transition orbitals was found to be in good agreement with the experimental data.

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Scheme 1. Pyridylamide ligands Haap and H2pcp applied in this work.

Experimental Section Physical Measurements 1

H NMR spectra were recorded on Bruker AVANCE 300 MHz spectrometer at room

temperature. Chemical shifts (δ) were referenced to residual solvent signal. UV-vis spectra were recorded on a Cary 50 spectrometer. X-band EPR spectra were recorded on Resonance Instrument 8400 EPR spectrometer. The g factors and A coupling constants of EPR spectra were obtained from simulation using Bruker SimFonia software (version 1.25). Elemental analyses were carried out by Atlantic Microlabs, Norcross, GA. FT-IR spectra were collected on a Thermo Nicolet Magna 560 FTIR spectrometer with an ATR attachment. Solution magnetic susceptibility was measured by using Evans’ method in DMSO-d6 at room temperature. Cyclic voltammograms were measured using EG&G Princeton Applied Research Scanning Potentiostat with a three-electrode cell, carbon Pt disk working electrode, Pt wire auxiliary electrode and Ag/AgCl glass reference electrode. All measurements were performed in either DMF or CH3CN solution containing 1 mM analyte and 0.1 M tetrabutylammonium tetrafluoroborate at room temperature with N2 protection. Recrystallized ferrocene was used as the internal standard. Chemicals All reagents were obtained from commercial sources and used as received unless otherwise noted. Ligand N-2-acetamidopyridine (Haap) and 2-(N-(2-Pyridyl)carbamoyl)pyridine (H2pcp) were synthesized according to literature [5]. Synthesis of the Metal Complexes [Cu2(Haap)2(OAc)4] (1) A solution of Cu(OAc)2 (0.067 g, 0.37 mmol) in 1 mL DMF was added to a stirred solution of Haap (0.050 g, 0.37 mmol) in 2 mL DMF. The green suspension was stirred for 1 h and 20 mL of Et2O was added to the mixture. The resulted green powder was collected and washed with Et2O (5 mL × 3). Dissolving the product in 3 mL hot DMF afforded a clear blue solution. Slow

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evaporation of the filtrate at room temperature for about three weeks led to the formation of blue crystals suitable for X-ray crystallographic characterization (0.076 g, 65% yield). Anal. Calc. for C22H28N4O10Cu2: C, 41.57; H, 4.44; N, 8.82. Found: C, 41.23; H, 4.81; N, 8.52. FT-IR (cm-1): 1702 (νC=O of Haap), 1606, 1579 (νacetate), 1543, 1437 (νacetate), 1416 (νacetate), 1371, 1305, 1287, 1254, 1164, 1005, 964, 785, 682, 631, 602, 555, 469, 440, 418, 405. UV-vis [DMF, λmax, nm (ε, M-1cm-1)]: 382 (420), 699 (360). [Cu(H2pcp)Cl2(DMF)] (2) A solution of CuCl2 (0.034 g, 0.25 mmol) in 1 mL CH3CN was added to a stirred solution of H2pcp (0.050 g, 0.25 mmol) in 2 mL CH3CN. The bright green suspension was stirred for 2 h and the solvent was removed under vacuum. The green powder was collected and washed with Et2O (5 mL × 3). Dissolving the product in 2 mL DMF afforded a clear deep green solution. Vapor diffusion of Et2O into the filtrate of the solution for about two weeks led to the formation of green crystals suitable for X-ray crystallographic characterization (0.051 g, 50% yield). Anal. Calc. for C14H16Cl2N4O2Cu: C, 41.34; H, 3.96; N, 13.77. Found: C, 41.22; H, 3.68; N, 13.89. FTIR (cm-1): 1633 (νC=O of H2pcp), 1599, 1595, 1572, 1534, 1493, 1458, 1429, 1376, 1351, 1272, 1253, 1146, 1116, 1097, 1059, 1044, 1025, 1006, 935, 919, 909, 778, 756, 737, 710, 690, 644, 623, 608, 531, 500, 451, 418. UV-vis [DMF, λmax, nm (ε, M-1cm-1)]: 421 (4700), 891 (110). [Cu2(2pcp)2(OAc)2] (3) A solution of Cu(OAc)2 (0.046 g, 0.25 mmol) in 1 mL DMF was added to a stirred solution of H2pcp (0.050 g, 0.25 mmol) in 2 mL DMF. The dark blue solution was stirred for 2 h and 25 mL Et2O was added to the mixture. The resulted deep blue powder was collected and washed with Et2O (5 mL × 3). Dissolving the product in 2 mL DMF afforded a clear deep blue solution. Vapor diffusion of Et2O into the filtrate for one week led to the formation of deep green crystals suitable for X-ray crystallographic characterization (0.066 g, 82% yield). Anal. Calc. for C26H22N6O6Cu2: C, 48.67; H, 3.46; N, 13.10. Found: C, 48.86; H, 3.59; N, 12.95. FT-IR (cm-1): 1677 (νC=O of 2pcp), 1633 (νacetate), 1597, 1585, 1563, 1473, 1428, 1350 (νacetate), 1332, 1294, 1274, 1154, 1086, 1044, 1029, 940, 783, 760, 691, 674, 652, 618, 502, 457, 438, 409. UV-vis [DMF, λmax, nm (ε, M-1cm-1)]: 296 (shoulder, 14200), 649 (280). X-ray Crystallography X-ray crystallographic data were collected on crystals with dimensions of 0.20 × 0.20 × 0.10 mm for 1, 0.55 × 0.45 × 0.30 mm for 2, 0.38 × 0.25 × 0.21 mm for 3. Data were collected at 100 K

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using a diffractometer with a Bruker APEX CCD area detector [6] and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All three structures were solved by direct methods and refined by full-matrix least-square methods on F2 [7]. The crystal parameters, data collection and refinement are summarized in Table 1. Non-hydrogen atoms were refined with anisotropic displacement parameters, and the positions of hydrogen bonded to carbons and nitrogens were initially determined by geometry and were refined using a riding model. For complex 3, the selected crystal was twinned by a 2-fold rotation around the (0 1 0) direction. Restraints on the positional and displacement parameters of the solvent were required. Table 1. Summary of crystal data and refinement parameters for complexes 1–3. 1 2 3 Formula C22H28N4O10Cu2 C14H16Cl2N4O2Cu C29H29N7O7Cu2 fw 635.56 406.75 714.67 Temp (K) 100(2) 100(2) 100(2) Space group P-1 P21/n P21/n a (Å) 7.4585(16) 7.5467(3) 9.6090(13) b (Å) 8.3408(18) 12.7659(6) 26.602(4) c (Å) 10.350(2) 17.0298(8) 11.5168(16) α (deg) 98.063(3) 90 84.809(2) β (deg) 94.669(3) 102.574(2) 96.607(2) γ (deg) 101.396(3) 90 82.865(2) Z 1 4 4 3 V (Å ) 621.0(2) 1601.31(12) 2924.4(7) ρcalcd (g/cm3) 1.700 1.687 1.623 µ (mm-1) 1.777 1.711 1.515 R1 [I > 2σ(I)] 0.0311 0.0240 0.0331 wR2 [I > 2σ(I)] 0.0871 0.0672 0.0976 2 GOF on F 1.000 1.000 0.984

Hirshfeld Surface Analysis In order to investigate the important intra- and intermolecular interactions occurring within and among the coordination units, the analysis of Hirshfeld surface was performed by using CrystalExplorer (version 3.0) software [8]. The calculation was based on the CIF files from the X-ray crystal structures of the metal complexes. The two-dimensional fingerprint diagrams were plotted from the Hirshfeld surface calculation to highlight the types of interaction and the corresponding areas on the Hirshfeld surface. Computational Details

All calculations were carried out using Gaussian 09, Revision C.01 [9]. Geometry optimizations were carried out using the pure meta-GGA functional of Truhlar and Zheng (M06-L) [10] and 5

the 6-311++G(d,p) basis set on all atoms. Dunlap’s density fitting approximation was employed for all calculations [11] and N,N’-dimethylformamide solvation environment was modeled using the polarizable continuum model [12]. The identification of energy minima was confirmed through the use of analytical frequency calculations. For comparison to experimental infrared spectra, a scaling factor of 0.976 was applied to calculated frequencies [13]. Electronic spectra were predicted with TD-DFT calculations using the M06-L geometries, the τdependent gradient-corrected correlation functional of Voorhis and Scuseria (VSXC) [14] and the 6-311++G(d,p) basis set on all atoms. The N,N’-dimethylformamide solvation environment was modeled using the polarizable continuum model [12]. State-selective solvation were not included in the prediction of spectra. Visualization of selected electronic excitations was achieved through the use of natural transition orbitals [15]. Natural transition orbitals allow an excited state with contributions from several electronic transitions to be represented as a pair of orbitals that can be seen as the hole and particle associated with that state. Unrestricted natural orbitals were employed for visualization of the unpaired spin orbitals for the purposes of qualitatively estimating unpaired electron density delocalization onto both metal centers of the paddlewheel complex 1.

Results and discussion Synthesis and Characterization The ligands Haap and H2pcp were synthesized based on the reported procedures [5]. Metalation of Haap with Cu(OAc)2 in the absence of base generated a dinuclear copper(II) complex [Cu2(Haap)2(OAc)4] (1), as depicted in Scheme 2. Slow evaporation of the DMF solution yielded the blue crystals of 1. Despite many attempts of using the deprotonated form of Haap obtained from the application of bases (e.g. NaH and Et3N) to react with metal salts, the effort to isolate characterizable products was fruitless. Interestingly, from the reaction of neutral ligand H2pcp with CuCl2 in a 1:1 ratio, a mononuclear complex [Cu(H2pcp)Cl2(DMF)] (2) with intramolecular hydrogen bonding interactions in the secondary coordination sphere was isolated (Scheme 3). The proton transferred from the amide N–H group to the pyridine nitrogen, provoking the coordination of amidate nitrogen to the metal center. Meanwhile, the formed pyridinium group interacts with the chloride ligand through an intramolecular hydrogen bonding interaction. The reaction of one equiv. of ligand and two equiv. of CuCl2 afforded the same mononuclear

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complex with a slightly different packing structure. The reaction of the deprotonated form of H2pcp with CuCl2 only afforded uncharacterizable powder. The treatment of neutral ligand H2pcp with Cu(OAc)2 in DMF led to the deprotonation of the amide group and produced a dinuclear complex [Cu2(2pcp)2(OAc)2] (3), in which a tridentate coordination mode of the two ligand molecules were observed. Complexes 1, 2 and 3 were stable in solid and solution states when exposed to air, and no obvious changes were observed in months.

Scheme 2. Synthesis of complex [Cu2(Haap)2(OAc)4] (1).

Scheme 3. Synthesis of complexes [Cu(H2pcp)Cl2(DMF)] (2) and [Cu2(2pcp)2(OAc)2] (3). Crystal Structures Complexes 1–3

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The X-ray crystal structure of 1 showed the classic paddlewheel centrosymmetric dinuclear configuration, which has been reported in many other works [16]. Four bridging acetate ligands between the two copper centers (Figure 1 left) support each copper(II) atom through four Cu–O bonds with an average distance of 1.953 Å, forming the basal plane of the square pyramidal geometry. The copper atom lies out of the basal plane by about 0.137 Å. The axial position is occupied by a pyridyl nitrogen with a long Cu–N bond of 2.256 Å due to the Jahn-Teller effect. The τ5 value of 0.002 [17] confirms the square pyramidal geometry of the copper observed in the X-ray crystal structure. The Cu


Cu distance is about 2.633 Å, which is comparable to those reported paddlewheel complexes with the similar structure [16]. For the two pairs of acetates in the trans-positions, one pair is perpendicular to the pyridyl rings, while the other pair is co-planar with the two pyridylamide ligands, characterized by two intramolecular hydrogen bonds. The hydrogens from the N–H groups interact with the two acetate oxygen atoms in the trans positions (O


H = ~ 1.930 Å, Table 2). Intermolecular hydrogen bonding interactions involving C–H hydrogens, amide oxygens and acetate oxygens lead to the formation of a supramolecular network (Figure S1). In addition to the hydrogen bonding interactions, interesting C–H


π interactions are observed and the detailed information from the Hirshfeld surface analysis can be found in the next section. These non-covalent π-based interactions are especially attractive because the aromatic ligands in the paddlewheel complexes with bridging acetate can act as secondary building units (SBU) to construct supramolecular architectures with novel photophysical properties [18]. The monomeric structure of 2 consists of a five-coordinate copper(II) center, one H2pcp ligand, two chlorides and one DMF molecule (Figure 1 right). The N2Cl2O coordination environment of the copper(II) center is fulfilled by amidate N2 and DMF O2 on the axial positions, and pyridyl N3 with two chlorides on the equatorial positions. The N2–Cu1 bond length (1.998 Å) is significantly shorter than the Cu–Nimino bonds observed in the copper complexes with Schiff base ligands, suggesting the stronger σ donating character of the amidate group. The bond angle of N2–Cu1–O2 is about 169.6°, and the three bond angles N3–Cu1–Cl1, N3–Cu1–Cl2 and Cl1– Cu1–Cl2 on the equatorial plane are 111.4°, 140.1° and 108.4°, respectively (Table 2). The τ5 value of 0.49 suggests the five-coordinate copper(II) site actually has a mid-way geometry between the standard trigonal bipyramidal and square pyramidal geometries [17]. This unique structural distortion is attributed to the presence of four different intramolecular hydrogen

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bonding interactions in the second-sphere coordination environment. First, the H11 from the coordinated pyridine ring interacts with DMF oxygen O2 with an O


H distance of 2.50 Å, pulling the DMF molecule towards the pyridine and causing the bend of N2–Cu1–O2 axis (N2– Cu1–O2 = 169.6°). Second, H1N from the pendent pyridinium interacts with Cl1 through a distance of 2.22 Å, giving rise to a longer Cu1–Cl1 bond length (2.478 Å) and small Cl1–Cu1– Cl2 angle (108.4°). Meanwhile, the strong H1N


Cl1 hydrogen bond also results in the tilted pyridinium group around the C5–N2 bond, forming a large torsion angle of 27.35° between the pronated heterocycle ring and the rest of the ligand backbone. This is quite surprising when considering the double bond character of C5–N2 bond arising from the resonance effect (see Scheme 4 and the discussion below). H1A from the DMF molecule and the Cl2 atom participate in the third H-bond interaction (H1A


Cl2 = 2.612 Å), which pulls the Cl2 to form a large N3– Cu1–Cl2 bond angle (140.1°) on the equatorial plane. Although the fourth hydrogen bond between H4 and O1 (O


H = 2.35 Å) does not directly impact the core structure of the metal center, it may facilitate the formation of the small N2–Cu1–N3 bond angle (80.9°) by pulling the C=O group and the coordinated pyridine ring on the opposite side. The monomeric units are associated with each other through intermolecular hydrogen bonds and π


π stacking (see Hirshfeld analysis).

Figure 1. X-ray structures of complexes 1 (left) and 2 (right). Thermal ellipsoids are shown at the 30% level. Most of the hydrogen atoms are omitted for clarity. The dotted lines represent intramolecular hydrogen bonding interactions. One of the most interesting features of 2 is the migration of amide N–H proton to the pyridine ring to form the anionic amidate and the pendant cationic pyridinium group. Previous study

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based on the resonance structures of pyridylamide motif (Scheme 4) disclosed the two-fold effect of this proton transfer: (a) the strong ligand field created by the amidate nitrogen and (b) stabilization of structure by the intramolecular hydrogen bonding interaction involving pyridinium N–H group [1g,1h]. Although the coordinated pyridine ring with N3 donor is not involved in this proton transfer directly, it might stabilize the anionic amidate nitrogen by chelating the metal center though a five-member ring. Our previous work involving Haap with one methyl group (Scheme 1) on the tail of the amide arm didn’t show such a proton transfer [4], which suggests the coordination-stabilization function of the pyridyl group with N3 donor. The presence of the resonance structures in 2 can be confirmed by comparing the bond lengths of the ligands in 1 and 2 (Figure 1). For instance, due to the resonance structure c in Scheme 4, the C5– N1 bond length in the pyridinium (1.356 Å) of 2 is longer than the C5–N1 bond length in the pyridine of 1 (1.338 Å), while the C5–N2 bond in 2 (1.379 Å) is shorter than the C5–N2 bond in 1 (1.395 Å). Meanwhile, as a result of the imidic acid form in resonance structure d, the carbonyl group in complex 2 is 0.019 Å longer than that of 1.

Scheme 4. Proton migration and resonance structures of ligand H2pcp.

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Figure 2. X-ray structures of 3. Thermal ellipsoids are shown at the 30% level. Most hydrogen atoms are omitted for clarity. The X-ray structure of 2 demonstrated that intramolecular proton transfer is critical for the formation of the mononuclear complex with second-sphere hydrogen bonding interactions. Further tests of H2pcp with Cu(OAc)2 gave rise to a new dimeric copper(II) complex. The X-ray crystal structure of the 3 revealed a very interesting µ1,3-κ2 (Namidate,Npy) chelating-bridging coordination mode of anionic ligand 2pcp- (Figure 2). Among the previously reported transition metal coordination compounds involving ligand H2pcp, only a few examples displayed such a coordination mode [19]. The dimeric unit consists of two copper(II) centers, two deprotonated ligands and two terminal acetates. The two copper centers are connected by two anionic 2pcp- ligands in a perpendicular fashion. Each copper center is chelated by the pyridyl and amidate nitrogen donors from one ligand with an average bite angle (Npy–Cu–Namidate) of 82.2°. The four-coordinate environment of the copper atom is fulfilled by the pyridyl nitrogen from the second ligand and one acetate terminal ligand. The τ4 values are 0.21 for Cu1 and 0.15 for Cu2, indicating the distorted square planar geometries [20]. As a result of the chelating-bridging coordination mode, both ligand backbones possess large torsion angles (39.0° and 47.4°) between the pyridine-2-carboxamidate chelating moiety and the bridging pyridine ring. The average Cu–N and Cu–O bond is about 1.986 and 1.947 Å. Although the acetate anions act as monodentate ligands through the coordination of O1C and O1D donors, the distances of O2C


Cu1 (2.654 Å) and O2D


Cu2 (2.666 Å) suggest the presence of weak axial interaction (dash lines in Figure 2). The large Cu


Cu distance around 3.18 Å indicates the lack of direct interaction between the two copper(II) centers. Another interesting structural feature of 3 is the distorted saddle-shaped conformation (Figure 3). The upper part has a small groove with a width of 5.069 Å and depth of 3.741 Å, and the lower part has a large groove with a width of 8.621 Å and a depth of 3.687 Å. The square planar entities of both copper(II) centers in the lower part have a sector conformation with an included angle of 76°. This larger and deeper groove provides enough space for the insertion of the pyridine-2-carboxamidate moiety from another dimeric unit, forming a face-to-face

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conformation between the two pyridyl rings with a distance of 3.024 Å, which falls in the range of intermolecular π


π interactions (Figure 7 in Hirshfeld analysis).

Figure 3. Saddle-shaped conformation of 3. Table 2. Selected bond lengths (Å), bond angles (°) and intramolecular hydrogen bonds (Å) of complexes 1–3. Complex 1 Cu1–N1 2.255(2) Cu1–O1 1.971(2) Cu1–O2 1.942(2) Cu1–O3 1.955(2) Cu1–O4 1.945(2) Cu1


Cu1A 2.63 O1–Cu1–O2 88.95(9) O1–Cu1–O3 167.88(8) O1–Cu1–O4 89.63(9) O2–Cu1–O3 89.82(9) O2–Cu1–O4 167.99(8) O3–Cu1–O4 89.07(9) N1–Cu1–O1 98.05(8) N1–Cu1–O2 94.53(9) N1–Cu1–O3 94.07(9) N1–Cu1–O4 89.63(9) H-bonding parameter H2N


O1 1.93 Complex 2 Cu1–N2 Cu1–N3 1.9984(11) 2.0321(12) Cu1–O2 Cu1–Cl1 1.9882(10) 2.4784(4) Cu1–Cl2 2.2938(4) N2–Cu1–O2 N3–Cu1–Cl1 169.55(4) 111.41(3) N3–Cu1–Cl2 Cl1–Cu1–Cl2 140.07(3) 108.406(14) N2–Cu1–N3 N2–Cu1–Cl1 80.93(5) 96.07(3) N2–Cu1–Cl2 O2–Cu1–N3 92.24(4) 89.14(4) O2–Cu1–Cl1 O2–Cu1–Cl2 90.53(3) 93.34(3) H-bonding parameters H5


O1 2.35 H11


O2 2.50 H1N


Cl1 2.22 H1A


Cl2 2.61 Complex 3 Cu(1)–O(1C) 1.9406(13) Cu(1)–N(2B) 1.9625(14) Cu(1)–N(1A) 1.9952(15) Cu(1)–N(3B) 1.9990(15) Cu(2)–O(1D) 1.9522(13) Cu(2)–N(2A) 1.9685(15) Cu(2)–N(3A) 1.9919(16) Cu(2)–N(1B) 1.9981(15) O2C


Cu1 2.65 O2D


Cu2 2.66

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Cu1


Cu2 O(1C)–Cu(1)–N(2B) N(2B)–Cu(1)–N(1A) N(2B)–Cu(1)–N(3B) O(1D)–Cu(2)–N(2A) N(2A)–Cu(2)–N(3A) N(2A)–Cu(2)–N(1B)

3.18 172.56(6) 94.83(6) 81.99(6) 171.82(6) 82.48(6) 94.69(6)

O(1C)–Cu(1)–N(1A) O(1C)–Cu(1)–N(3B) N(1A)–Cu(1)–N(3B) O(1D)–Cu(2)–N(3A) O(1D)–Cu(2)–N(1B) N(3A)–Cu(2)–N(1B)

89.70(6) 96.04(6) 157.50(6) 93.09(6) 91.11(6) 167.10(6)

Hirshfeld Surface Analysis Hirshfeld surface analysis and the corresponding two-dimensional fingerprint plot are very useful approaches to evaluate the intermolecular contacts, including hydrogen bond, C–H


π interaction, π


π stacking, C–H


X (X = halogen) interactions, etc. [21]. In a map of shape index, the concave and convex shape are represented by red (negative) and blue (positive) colors, which are distinct for the identification of C–H


π interaction and π


π stacking. In a map of dnorm values, the interaction distances are normalized based on the distances from “a point on the surface to the nearest nucleus” outside (de) and inside (di) the surface, as well as the vdW radii of the atoms [22]. The red and blue colors represent the distance shorter and longer than the sum of vdW radii, so the map can provide rich and unbiased information about the close contacts such as hydrogen bonding interactions. The information obtained from Hirshfeld surface analysis can be condensed into a fingerprint plot, which is a two-dimensional matrix-like summary of the interaction frequency and strength, and relative surface area. In an effort to assess the structural features based on the supramolecular architectures guided by intermolecular interactions, we performed Hirshfeld surface analysis based on the X-ray structures of complexes 1–3. For 1, the left picture in Figure 4 shows the C–H


π interactions among three dimeric units in the X-ray crystal structure. The right picture is the Hirshfeld surface of unit 1 (top view) mapped with shape index. The two pyridine aromatic rings in unit 1 are the C–H


π acceptors (labeled as 1A and 2A, A stands for acceptor) that interact with C–H groups from unit 2 and unit 3 with distance of 2.601 Å and 2.639 Å, respectively. These two pyridine acceptors correspond to the two large red spots (1A and 2A) in the right of picture of Figure 4. Meanwhile, two C–H hydrogens from the acetates of unit1 act as C–H


π donors (labeled as 1D and 2D, D stands for donor) to interact with the pyridine rings of unit 2 and unit 3 separately, corresponding to the circled blue areas (1D and 2D) in the right of picture of Figure 4. The assignment of the C–H


π interactions can also be confirmed by the Hirshfeld surface mapped with de (Figure S2 left), in

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which the two orange depression areas are pyridine acceptors (1A and 2A) and two blue elevated areas are C–H donors (1D and 2D). In addition to the C–H


π interactions, the intermolecular hydrogen bonding interactions also substantially facilitate the supramolecular packing of complex 1 (Figure S3). The left picture in Figure S3 shows a pair of hydrogen bonds between two neighboring dimeric units. C–H hydrogens interact with amide oxygens through a distance of 2.351 Å, and such interactions align the molecules into a one-dimensional chain arrangement. This is illustrated in the dnorm Hirshfeld surface on the right of Figure S3. The two intense red spots labeled as A and D represent the acceptor and donor of the strong intermolecular hydrogen bonding contacts. Several small red areas are associated with weaker interactions among the units. The breakdown of the two-dimensional fingerprint plot (Figure S2 right) highlights the percentage contribution of the C–H


π (10.5%) and hydrogen bonding (28.9%) interactions to the whole Hirshfeld surface of 1.

Figure 4. C-H


π interactions (left) among the three units of 1 and Hirshfeld surfaces mapped with shape index (right) for the unit 1. 1A and 1D represent the acceptor and donor for the C– H


π interaction with a distance of 2.601 Å. 2A and 2D represent the acceptor and donor for the C–H


π interaction with a distance of 2.639 Å. Two major types of intermolecular interactions are observed in the packing structure of 2. Figure 5 shows the ladder-like π


π stacking among three monomeric units (left) and the corresponding Hirshfeld surfaces mapped with shape index (middle and right) of unit 1. The coordinated pyridyl group in unit 1 interacts with the pyridyl groups from unit 2 (above) and unit 3 (below)

14

with a closest C


C distance of 3.292 Å and 3.413 Å respectively. These two contacts give rise to the characteristic paired red and blue triangle regions on the Hirshfeld surface (Figure 5 middle and left). In the fingerprint plot shown in Figure 6, these features appear as the blue/green region (de = di = ~1.7 Å) in the center. In addition to the π


π interactions, the mononuclear unit features four significant intermolecular hydrogen bonding interactions across the backbone of the ligand, chloride and DMF molecule (Figure S4). For the unit 1 in Figure S4, the carbonyl oxygen of the ligand and one of the hydrogens on the pyridinium group act as the hydrogen bond acceptor (labeled as 1A) and donor (labeled as 1D) to interact with two other units through a H


O distance of 2.418 Å. In addition, the unit 1 pairs with the third unit through two hydrogen bonds (H


Cl = 2.755 Å) involving Cl- ligand (labeled as 2A) and DMF molecule (labeled as 2D), forming a fourteen-member supramolecular macrocycle structure. By examining the Hirshfeld surfaces mapped with dnorm (Figure S4 right), the four hydrogen bonding interactions can be easily identified as the red spots in Figure S4, in which 1A and 1D represent the acceptor and donor of C–H


O hydrogen bond, while 2A and 2D represent the acceptor and donor of C– H


Cl hydrogen bond. From the positions of the four small spikes on the fingerprint plot, it is clear that the C–H


O contacts (de + di = ~2.4 Å, Figure S5 left) are shorter than the C–H


Cl contacts (de + di = ~2.7 Å, Figure S5 right), which matches with the X-ray crystal data.

Figure 5. The π


π interactions among the three units of 2 (left, closest C


C distance = 3.292 Å and 3.413 Å) and Hirshfeld surface mapped with shape index (middle: front view; right: back view) for unit 1.

15

Figure 6. Two-dimensional fingerprint plot for a single unit of 2. For hydrogen bonding interactions, please see Figure S4 in supporting information. As discussed in the X-ray crystal structure of 3, intermolecular π


π interaction is observed between the two pyridyl rings in the lower part of the dimeric unit. The two pyridyl groups overlap with a closest C


C distance of 3.024 Å, corresponding to the large deep red and blue triangle pair on the Hirshfeld map (Figure 7 middle). This intermolecular π


π interaction is represented by the bright blue center of fingerprint plot (Figure 7 right). The dimeric unit of 3 is surrounded by complicated intermolecular hydrogen bonding interactions. Figure S6 is the Hirshfeld surface mapped with dnorm for the dimeric unit of 3 only, and a slightly disordered DMF molecule next to the dimeric unit was not included in the map. It shows the three major red spots that represent the dominant C–H


O interactions with H


O distances of 2.38~2.66 Å. A careful inspection based on the positions of the small wings labeled as 1, 2 and 3 in the fingerprint plot (Figure 7 right) confirms the presence of these contacts with different distances. The fingerprint plot is also featured by a small sharp wing at de = 0.9 and di = 1.1 Å, which may originate from the H


H contact between the pyridyl hydrogen and the interstitial DMF solvent molecule (not shown).

16

Figure 7. Left and middle: Intermolecular π


π interaction between two units of 3 (closest C


C distance = 3.024 Å) and the corresponding Hirshfeld surface mapped with shape index; Right: fingerprint plot of the dimeric unit of 3. For hydrogen bonding interaction 1, 2 and 3, please see Figure S6 in supporting information.

Spectroscopic and Magnetic Characterization The solid state IR spectra of complexes 1–3 were collected using ATR unit at room temperature (Figure S7). DFT calculations were performed to assign the major vibrational frequencies (see DFT calculation section). For 1, the sharp peak at 1702 cm-1 with medium intensity is assigned as the amide C=O stretching. The two pairs of acetates give an asymmetric O–C–O stretching frequency at 1579 cm-1. The symmetrical stretching of two O–C–O pairs is observed at 1437 cm1

and 1416 cm-1 with similar intensity. The mononuclear complex 2 has an amide C=O stretching

frequency at 1633 cm-1. Complex 3 shows a C=O stretching peak of amide groups at 1677 cm-1. The asymmetrical and symmetrical stretching modes of acetate ligands gave peaks at 1633 cm-1 and 1350 cm-1. The UV-vis spectra of 1, 2 and 3 in DMF solutions are presented in Figure 8. The typical d-d transition bands are observed for all three complexes. The green solution of 1 in DMF has an absorption manifold at 699 nm with a molar absorptivity value (ε) of 360 M-1cm-1. This feature is quite typical for copper(II) complexes with square-pyramidal geometries [23]. Compared with those previously reported dimeric copper acetate adducts, the blue shifted d-d transition of the 1 indicated the stronger interaction of the Cu–O bonds, which cause the stronger ligand field around the copper(II) center. This observation matches with the shorter average Cu–O bond length (1.953 Å) and longer Cu–Npy bond (2.255 Å) in the X-ray crystal structure. The green color of 2 arises from a broad band at 891 nm with ε = 110 M-1cm-1. A stronger band (ε = 4700

17

M-1cm-1) in the near UV region (421 nm) is due to the Cl-  copper(II) charge transfer excitation [24]. The blue complex 3 exhibits a band at 649 nm with ε = 282 M-1cm-1 arising from a dxy ݀୶మ ି୷మ transition (see computational analysis). This observation is in agreement with those reported copper complexes with square planar geometries [25]. To further compare the electronic properties of the three complexes, the reflectance UV-vis spectra in 300-900 nm region were collected at room temperature (Figure 8 right). In solid state, complex 1 showed a very similar d-d transition around 700 nm as in solution state, suggesting the presence of the same rigid dimeric structure in both solution and solid states. However, the dd transition bands for complexes 2 and 3 blue shifted from 891 nm and 649 nm in DMF solution to ~850 nm and 580 nm in solid state, respectively. This observation might be attributed to the coordination of DMF solvent molecules to the copper center in solution state, thus exerted a weaker ligand field.

Figure 8. Solution (left) and reflectance (right) UV-vis spectra of 1–3. The effective magnetic moments (µeff) of 1, 2 and 3 in solution state were measured by using Evan’s method in DMSO-d6 at room temperature. The µeff of 1 is 1.51 µB/copper, which is lower than the spin-only (s = 1/2) value (1.73 µB) of a single copper(II) ion with a d9 configuration. This observation indicates an antiferromagnetic coupling between the two copper(II) centers in 1, as those previously reported copper dimers with acetate bridging ligands [26]. For complex 2, the µeff of 1.99 µB confirms the monomeric structure of this compound. The µeff value of 3 (1.81 µB/copper) is close to the spin only behavior of copper(II), reflecting the absence of any significant interactions between the two copper centers in the dimeric arrangement. The results of magnetic moment measurement are supported by the paramagnetic NMR data of the three

18

compounds. 1H NMR of the complex 1 shows broad ligand signals at room temperature (Figure S8). While for 2 and 3, no ligand peaks are observed in 1H NMR spectra, confirming the strong paramagnetic behavior of the two complexes. The X-band EPR spectra of polycrystalline and solution samples of 1, 2 and 3 were measured at room temperature. For the power sample of 1, no EPR signals were observed in the 2000-5000 gauss. Interestingly, the solution sample of 1 in DMF is featured with a seven-line hyperfine pattern, indicating the coupling of electrons with two copper nuclei (I = 3/2, Figure 9). Similar hyperfine signal was also observed from the DMSO solution of 1 (Figure S9). The coupling constant A of ~70 G is about the half of the coupling constants from the mononuclear copper(II) species with four line patterns, suggesting the presence of possible Cu


Cu interactions [27]. To our best knowledge, only a very few paddlewheel dicopper(II) complexes with acetate ligands possess such a seven line hyperfine EPR pattern [28]. Table 3 summarizes the g values and A coupling constants obtained from the simulation of the experimental data. Considering the possible coordination of DMF molecules to the Cu(II) centers in 1, we are concerned that the seven-line hyperfine signal may come from the solvent derived species in which Haap ligands are replaced by DMF molecule(s). Therefore, a control experiment was performed in an effort to gain a better understanding about the EPR properties of 1. [Cu2(OAc)4(DMF)2] with the similar quadruple acetate bridging dimeric structure was synthesized by dissolving Cu(OAc)2 in DMF and recrystallized through vapor diffusion of Et2O into the blue solution. The crystal structure of the resulted blue crystals was obtained by X-ray crystallography (Figure S10, Table S1 and Table S2), which showed a very similar structure as the previous reports [29]. No EPR signals were observed for solid sample of [Cu2(OAc)4(DMF)2] under the identical conditions of 1. In contrast to the seven-line hyperfine signal observed from the DMF and DMSO solutions of 1, EPR measurement of [Cu2(OAc)4(DMF)2] in both DMF and DMSO solutions only show a broad signal around g = 2.010 (Figure S11). In addition, the lower µeff value of 1 (1.51 µB/copper) than the µeff of [Cu2(OAc)4(DMF)2] (1.64 µB/copper) suggests the less spin density localized on each copper center when Haap is the axial ligand, causing the better interaction of electrons with both copper centers.

19

Figure 9. Experimental and simulated X-band EPR spectra of 1 in DMF at room temperature. Table 3. Summary of g values and A coupling constants of 1–3 in solid state and solution state at room temperature. Solid

Complex 1

Solution

g

A (Gauss)

-

-

gx = 2.106 Complex 2

gy = 1.994

-

gz = 1.866 Complex 3

1.947

-

A (Gauss)

g 2.012 (DMF)

73 (DMF)

2.008 (DMSO)

70 (DMSO)

2.028 (DMF)

-

1.990 (CH3CN)

-

1.992 (DMSO)

-

1.980 (MeOH)

105 (MeOH)

1.975 (DMF)

130 (DMF)

1.974 (DMSO)

131 (DMF)

20

Figure 10. Experimental and simulated X-band EPR spectra of 2 in solid state. The EPR measurement of 2 in solid state showed interesting features (Figure 10). A rhombic pattern was observed in experimental spectrum (black) and its simulation (red) provided wellresolved gx = 2.106, gy = 1.994 and gz = 1.866 values. No hyperfine coupling was observed in the measurement. This EPR behavior is quite close to those reported five-coordinate copper(II) complexes with a mid-way structure between the perfect trigonal bipyramidal and square pyramidal geometries [30]. The calculated ratio of R = (gy-gz)/(gx-gy) is 1.44, indicating a dz2 ground state when the R value is larger than 1 [30a,30b]. In DMF, CH3CN and DMSO solutions, complex 2 only showed similar axial signals at g = 2.028, 1.990 and 1.992 in the absence of any hyperfine structures (Figure S12). Interestingly, the MeOH solution of 2 gave a partially resolved hyperfine-four line pattern at room temperature, which is in accord with the mononuclear copper d9 configuration (Figure S13). Simulation of the experimental spectrum suggested the g = 1.980 and A = 105 Gauss.

21

Figure 11. X-band EPR spectra of 3 in solid state (left) and in DMF (right) at room temperature. Simulation of solution EPR spectrum is red. The EPR of 3 at room temperature only displayed a broad band with a center at giso = 1.947 (Figure 11 left). No evidence of hyperfine splitting was obtained, and this could be ascribed to the short spin-lattice relaxation time in the polycrystalline state. The symmetrical shape of the signal is consistent with the square planar coordination environment of the copper(II) center in the X-ray crystal structure. However, the giso value of the signal is less than the free electron (g = 2.0023), suggesting the weak covalency of the copper-ligand interaction [25c]. The DMF (Figure 11 right) and DMSO solutions (Figure S14) of 3 showed a partially resolved four line patterns with a g value of ~1.975 and A coupling constant of ~130 Gauss, indicating no direct interactions between the copper centers due to the large Cu


Cu distance (3.177 Å). Electrochemistry In order to measure the susceptibility of the three copper(II) complexes toward reduction and oxidation, the electrochemical behavior of 1 (DMF), 2 (CH3CN) and 3 (DMF) were examined by cyclic voltammetry (CV) at room temperature with the protection of N2 gas. Both ligands Haap and H2pcp didn’t show any redox waves within the scanning range under the same experimental conditions. Recrystallized ferrocene was used as an internal standard. The results are shown in Figure 12.

22

Figure 12. Cyclic voltammograms of 1 (DMF), 2 (CH3CN) and 3 (DMF) containing 0.1 M NBu4BF4 as the supporting electrolyte. Scanning rate is 100 mV/s. Ferrocene was added as an internal standard. In DMF solution, complex 1 exhibited a reversible couple at E1/2 = -0.145 V (Epa = 0.013 V, Epc = -0.296 V), which could be assigned as the reduction of Cu(II) to Cu(I) (Figure 12 left). The peak current ratio (ipa/ipc = 1.02, scanning rate = 100 mV/s) is close to unity, suggesting a chemically reversible redox process. In an effort to confirm the origin of this couple, the electrochemical properties of [Cu2(DMF)2(OAc)4] was investigated at the same conditions. A similar reversible reduction wave was observed at E1/2 = -0.248 V (Epa = 0.010 V, Epc = -0.506 V, Figure S15). The reduction potential of 1 is more positive than that of [Cu2(DMF)2(OAc)4] by almost 0.1 V, indicating a stronger electron donating property of the ligand Haap than DMF. For complex 2, a partially reversible wave was observed at E1/2 = 0.553 V (Figure 12 middle), which could be attributed to the Cu(II)/Cu(I) redox couple. In addition, a strong irreversible redox wave observed at -0.162 V could be resulted from copper(0) generated during the redox process (Figure S16). Indeed, a copper metal film deposited on the surface of the working electrode was clearly visible after the measurement. The complex 3 showed only one redox feature in -1.0~+1.4 V window (Figure 12 right). The reversible Cu(II)/Cu(I) redox couple occurred at E1/2 = -0.160 V (Epa = -0.039 V, Epc = -0.320 V) with a peak current ratio ipa/ipc of 0.98 at a scan rate of 100 mV/s. In order to obtain a better understanding of the two reversible couples of complexes 1 and 3, multiple scans at different scanning rates were performed and the ipc data were plotted against the square root of the scanning rates (Figure S17 for 1 and Figure S18 for 3). The ∆Ep values of both complexes increased slightly with the increased scanning rates. The internal standard

23

ferrocene/ferrocenium (Fc/Fc+) couple shows the similar ∆Ep increase under the same experimental conditions, indicating the reductions of both complexes are electrochemically reversible. The plots of ipc against the square root of the scanning rate for both complexes show straight lines, suggesting the redox of the copper centers is a diffusion-controlled pathway. Theoretical Calculations In order to better understand the structural and spectroscopic features of complexes 1–3, we studied these compounds using density functional theory calculations. First, the minimized geometries of all compounds were calculated at the M06-L/6-311++G(d,p) level of theory (Figure 13) with DMF solvation effects included via the polarizable continuum model (Table S3–S5). Unlike what was observed in its crystallographically determined structure, the calculated minimized structure of 1 does not maintain the pyridylamide ligand’s coplanarity with the acetate paddlewheel, with the pyridine ring rotating 43° with respect to the acetate ligands. This discrepancy still existed when geometry minimizations were carried out with implicit solvation, larger basis sets, and different functionals, so we attribute the coplanar feature in X-ray crystal structure to crystal packing effects, while the lack of coplanarity in the calculated structure is like due to the Haap ligand occupying a less sterically hindered conformation with respect to the nearby acetate groups.

Figure 13. DFT minimized geometries of complexes 1–3. The crystal structures of 1 and 2 contain intramolecular hydrogen bond that was maintained in the minimized structures of these compounds. The effect of these hydrogen bonding interactions could also be seen in the predicted vibrational spectra (Table S7–S9). The vibrational frequency of carbonyl group is particularly sensitive to the presence of hydrogen bonds, causing it to shift to lower frequencies by ca. 50 cm-1 [31]. Complex 2 contains the same H2pcp ligand tautomer,

24

but with the protonated pyridine group rotating into a position that allows the acidic hydrogen atom to interact with a metal-bound chloride ligand. The structural information of the copper monomer allows for a nearly direct observation of the effect of intramolecular hydrogen bonds on the vibrational frequencies. The calculated amide carbonyl stretch frequencies in 2 and 3 are 1682 cm-1 and 1680 cm-1, which are higher than the experimental frequency 1633 cm-1 and 1677 cm-1 (Table S7–S9).

Figure 14. Predicted electronic spectra of 1 (black line), 2 (red line), and 3 (blue line), modeling each electronic transition as a Gaussian curve with a standard deviation of 0.4 eV. Time dependent density functional theory calculations carried out at the VSCX/6-311++G(d,p) level of theory provided insight into the origin of the UV-vis absorptions of 1–3 (Table S10– S12). With DMF solvation effects included, the predicted spectra closely matched experimental line shapes and λmax values (Figure 14). The predicted extinction coefficients are not correct in an absolute sense, and are overestimates by a factor of about 4. TDDFT does correctly predict the relative absorbances of the different compounds. In general, the predicted oscillator strength (f) of a given electronic transition is less accurate than its predicted energy, especially when the Franck-Condon contribution to the line shape is not taken into account [32]. Each of these near-IR/visible spectra are comprised of several electronic excitations that involve frontier electrons being promoted into the SOMOs of each of these compounds. The frontier orbitals involved in visible electronic excitations are depicted in the orbital energy diagram in Scheme 5. This scheme describes the most intense single electronic excitation for each compound, with the red arrows representing the electronic transitions that contribute to that

25

excited state. Interpretation of these multi-component excitations can be difficult with molecular orbital images. Natural transition orbitals provide a way to visually interpret these excitation processes as a pair of orbitals that represent the hole and particle associated with that particular excited state [15]. The NTOs for the electronic absorptions depicted in Scheme 5 can be seen in Figure 15. It can be seen that these transitions all involve electron density being excited into the highest energy d-orbital predicted by crystal field theory for each geometry (i.e., ݀୶మ ି୷మ for the square pyramidal and square planar geometries of complexes 1 and 3; ݀୸మ for the trigonal bipyramidal geometry of complex 2). The electrons involved in the transitions depicted for 1 and 2 originate from d-orbitals. In contrast, the hole for the transition of 3 has a large degree of ligand C=O π* participation, giving the transition more LMCT character than those found in the other two compounds (Figure 14).

Scheme 5. Frontier beta-spin orbitals with transition lines depicting contributions to the most intense electronic absorption from TD-DFT calculations performed on 1 (A), 2 (B), and 3 (C).

26

Figure 15. Natural transition orbital depictions of the hole-particle pairs of the most intense electronic absorptions predicted by TD-DFT calculations on 1 (A), 2 (B), and 3 (C). To better understand the seven-line pattern observed in the EPR spectrum of 1, we performed an unrestricted natural orbital (UNO) analysis to illustrate the interaction of unpaired electrons with copper center. The two HOMO images of 1 are shown in Figure 16. The p orbitals from the bridging acetates are interacting with ݀୶మ ି୷మ orbitals of copper centers, causing delocalization of the unpaired electrons (e.g., in natural orbitals with eigenvalues of 1.000) on both copper sites. It is noteworthy that the axial nitrogen donor is not involved in the delocalization of the unpaired electron density. This is probably due to the weak Cu–Npy bond interaction caused by JahnTeller effect in 1. Actually, the Cu–Npy distance (2.255 Å) is longer than most of the Cu–N bonds in paddlewheel Cu(II) complexes with axial nitrogen donors [16,26].

27

Figure 16. Natural orbitals containing the unpaired electrons of 1 showing the extent of unpaired spin delocalization in the compound.

Conclusions In this article, three new copper(II) complexes supported by pyridylamide ligands Haap and H2pcp were prepared and fully characterized to investigate their structural and spectroscopic properties. The structures of these complexes featured diverse coordination modes and vast array

of intra-/intermolecular interactions in the outer sphere of the metal sites. In complex 3, the amidate form of ligand H2pcp provided a strong σ donor to support the dimeric structure with a distorted saddle-shaped conformation. Detailed Hirshfeld surface analysis revealed more information about the nature of the weak contacts among the coordination units in the crystal packing structure. The electronic properties of the copper(II) complexes were evaluated by UV-

vis, EPR and cyclic voltammetry. The blue shifted d-d transition bands in both solution and solid states, in conjunction with the low potential of complex 3 relative to the complexes 1 and 2 suggested a strong ligand field exerted by the anionic form of ligand H2pcp. Computational studies confirmed the significant structural impact from the intra- and intermolecular interactions. The calculated IR and UV-vis spectra added great insight into the coordination environments and electronic structures of the complexes. Future work to expand the ligands with additional

functionality, develop more binding interaction of coordination units and construct novel supramolecular networks is currently underway. Supporting Information Packing structure of complex 1; , Hirshfeld surface pictures; 1H NMR spectra of complexes 1; IR and EPR spectra of complexes 1–3; Crystal structure and data of [Cu2(DMF)2(OAc)4]; Cyclic 28

voltammogram of complexes 1 and 3; TD-DFT calculated electronic transitions of complexes 1– 3; DFT calculated vibrational frequencies of complexes 1–3; DFT optimized geometry of complexes 1–3. CCDC-1061581 (complex 1), 1039739 (complex 2), 1039741 (complex 3) and 1061582 ([Cu2(DMF)2(OAc)4], supporting information) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

Acknowledgments This work was supported by the start-up fund provided by the College of Natural Sciences and Mathematics (CNSM) and University Research Council Fund (URCF) from University of Central Arkansas. We also thank Dr. Nan Xu for help with Echem work. _________________ [1] a) X. Wang, M. Le, H. Lin, J. Luan, G. Liu, J. Zhang, A. Tian, Inorg. Chem. Comm. 2014, 49, 19-23; b) L. M. Luis Miguel, S. O. Hisila, R. E. Navarro, M. L. Lorena, R. SugichMiranda, O. L. Karen, Polyhedron 2014, 79, 338-346; c) A. S. Jullien, C. Gateau, C. Lebrun, I. Kieffer, D. Testemale, P. Delangle, Inorg. Chem. 2014, 53, 5229-5239; d) W. Xu, W. J. Pan, Y. Q. Zheng, J. Coord. Chem. 2013, 66, 4415-4429; e) G. Mukherjee, K. Biradha, Cryst. Growth Des. 2013, 13, 4100-4109; f) S. P. Gavrish, Y. D. Lampeka, P. Lightfoot, V. B. Arion, B. K. Keppler, K. Wozniak, Crys. Growth Des. 2012, 12, 4388-1396; g) D. Wang, S. V. Lindeman, A. T. Fiedler, Eur. J. Inorg. Chem. 2013, 2013, 4473-4484; h) K. Gudasi, R. Vadavi, R. Shenoy, M. Patil, S. A. Patil, M. Nethaji, Inorg. Chim. Acta 2005, 358, 37993806; i) A. Rajput, R. Mukherjee, Coord. Chem. Rev. 2013, 257, 350-368. [2] a) H. Gilchoubian, G. Moayyedi, N. Reisi, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 2015, 138, 913-924; b) P. Comba, M. Kubeil, J. Pietzsch, H. Rudolf, H. Stephan, K. Zarschler, Inorg. Chem. 2014, 53, 6698-6707; c) I. Marti, A. Ferrer, J. Escorihuela, M. I. Burguete, S. V. Luis, Dalton Trans. 2012, 41, 6764-6776; d) U. P.

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Three new copper(II) complexes supported by pyridylamide ligands were synthesized. X-ray crystal structures showed desired intra- and intermolecular interactions, which were confirmed by Hirshfeld surface analysis. The electronic structures of these complexes were illustrated by experimental evidence and computational studies.

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