Pentafluorophenyl CopperâPyridine Complexes: Synthesis

Article pubs.acs.org/Organometallics

Pentafluorophenyl Copper−Pyridine Complexes: Synthesis, Supramolecular Structures via Cuprophilic and π-Stacking Interactions, and Solid-State Luminescence Ami Doshi,† Anand Sundararaman,† Krishnan Venkatasubbaiah,†,§ Lev N. Zakharov,‡ Arnold L. Rheingold,‡ Mykhaylo Myahkostupov,† Piotr Piotrowiak,† and Frieder Jak̈ le*,† †

Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States

‡

S Supporting Information *

ABSTRACT: The effect of the binding of pyridine ligands to pentafluorophenyl copper, [C6F5Cu]4 (1), on structural features and photophysical properties has been investigated through a combined multinuclear NMR, X-ray crystallography, and photoluminescence study. Reaction of 1 with 2 equiv of pyridine yields a novel pyridine complex, 3, in which the tetranuclear framework of 1 is retained. Complex 3 features a rhombus-shaped tetracopper core with a short diagonal Cu···Cu distance of 2.5941(6) Å between the dicoordinate copper centers and a longer distance between the pyridine-coordinated copper centers of 4.178(1) Å. In contrast, treatment of 1 with 4 equiv of pyridine results in complete breakdown of the tetranuclear aggregate to give the formally dicoordinate species C6F5Cu(py) (4-H). Reaction of 1 with 2,2′-bipyridine results in formation of the tricoordinate complex C6F5Cu(2,2′-bipy) (5). Aggregate breakdown in species 4 and 5 is reflected in a significantly reduced chemical shift difference Δδ(19Fmeta/para) and a strong downfield shift of the copper-bound carbon atoms in the 13C NMR spectra in comparison to 3. A dynamic equilibrium is established at ratios of py/C6F5Cu ranging from 0 to 2. The solid-state structures of all compounds have been determined by single-crystal X-ray crystallography. The supramolecular assembly of complex 3 via arene−arene π-interactions leads to a network structure with solvent-filled channels propagating through the lattice along the crystallographic c axis. The 2,2′-bipyridine complex 5 also shows π-stacking as the dominant feature in the extended solid-state structure. In contrast, a different mode of supramolecular assembly is found for 4-H in that cuprophilic interactions lead to assembly into one-dimensional copper chains with equidistant Cu···Cu contacts of 2.8924(3) Å. However, the closely related complexes 4-R with methyl or chloro substituents in either the ortho or the para position form supramolecular stacks with structural features that, again, are dominated by offset perfluoroarene−arene interactions with intermolecular plane-to-plane separations of ca. 3.3−3.6 Å. The dicoordinate copper atoms are aligned in onedimensional chains with alternating short and long Cu(I)···Cu(I) distances of 3.531(1)/3.698(1) Å in 4-pMe, 3.2454(5)/ 4.2970(5) Å in 4-oMe, 3.521(1)/3.784(1) Å in 4-pCl, and 3.4797(6)/3.8363(6) Å in 4-oCl. Compound 4-H is strongly blue fluorescent at 460 nm in the solid state, but yellow-green fluorescent at 77 K, resulting in an interesting example of luminescence thermochromism. In contrast, the substituted compounds 4-R display strong luminescence only at liquid nitrogen temperature. In all cases, the fluorescence emission band is in the range of ca. 410−425 nm and thus at significantly different energy from that in 4-H, which strongly suggests that the short Cu···Cu contacts in 4-H give rise to unique luminescence properties.

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INTRODUCTION Organocopper compounds have emerged as an important class of organometallic compounds. They have attracted considerable interest from both inorganic and organic chemists because of the rich structural diversity and unusual bonding patterns, as well as their important roles as intermediates in coupling reactions and as mild and selective reagents and catalysts in organic synthesis.1−3 An interesting aspect from a structural perspective is that monomeric species RCu remain elusive, whereas well-defined homoleptic organocopper species [RCu]n with varying degrees of aggregation (n = 2−8) have been identified in solution and also structurally characterized in the solid state.2,4,5 Aggregation typically occurs through © 2011 American Chemical Society

bridging of two or more copper centers with an organic moiety.4,6 The degree of aggregation depends on a number of factors, including steric and electronic effects, and the presence of external donor ligands.4,7,8 For example, unsubstituted phenylcopper adopts a polymeric structure and hence is insoluble in common organic solvents, whereas the presence of substituents in the ortho position of the aromatic ring leads to well-defined soluble oligomers as a result of the added steric Special Issue: Fluorine in Organometallic Chemistry Received: October 15, 2011 Published: December 1, 2011 1546

dx.doi.org/10.1021/om200989b | Organometallics 2012, 31, 1546−1558

Organometallics

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bulk.4 Thus, 2,4,6-triisopropylphenylcopper features a tetranuclear square-planar aggregate due to the presence of sterically demanding isopropyl groups in the ortho positions of the phenyl ring.9 van Koten and co-workers have demonstrated that smaller arylcopper aggregates can be stabilized by the intramolecular coordination with chelating amino or alkoxy groups in the ortho position of the aromatic ring.10 It is also well known that treatment of organocopper species with strongly coordinating solvents or external ligands can lead to the breakdown of the aggregated structure.11−13 Smaller organocopper aggregates are obtained, for example, by using N, P, or S compounds as donors. For instance, phenylcopper, which adopts a polymeric structure in the absence of an external Lewis base, has been isolated as the dimethylsulfide solvate [PhCu]4(SMe2)2 with a square-planar tetranuclear copper(I) core,14 while a monomeric species PhCuL was obtained in the presence of the sterically demanding tridentate ligand l,l,l-tris[(diphenylphosphino)methyl]ethane (triphos).15 However, in general, it is difficult to predict whether monomeric species or partially aggregated complexes will be formed, and surprisingly, few nonaggregated species RCuLn have been isolated and structurally characterized. In most complexes, either bulky groups 15−18 and/or chelating ligands19−22 enforce the monomeric structure.23 Lewis base additives have been related to a beneficial effect of strong donor solvents, such as DMF, NMP, or HMPA, on the reactivity of organocopper compounds in organic synthesis, including their use in carbocupration, stereoselective conjugate additions, and allylic substitution reactions.2,4,8 We became interested in the coordination behavior and photophysical properties of pentafluorophenyl copper (1), because we reasoned that the unique combination of steric demand and the electron-withdrawing effect of the pentafluorophenyl group should enable us to isolate and structurally characterize otherwise inaccessible Lewis base adducts.24 Compound 1 was first introduced by Cairncross and Sheppard in 1968, and they determined the aggregation number in benzene to be 3.75−3.85 by cryoscopy and to be 3.95 by vapor pressure osmometry, suggesting that a tetranuclear aggregate is present in solution.25 Using single-crystal X-ray diffraction, we confirmed that, in the absence of Lewis bases, 1 adopts a tetrameric structure also in the solid state.26 More importantly, we discovered the formation of π-complexes, such as [C6F5Cu]4(toluene)2) (2), in which the tetranuclear aggregate structure is maintained and investigated the assembly of 1 into offset supramolecular stacks via multiple Cu···π interactions in the presence of extended π-systems.26−28 In stark contrast, the presence of an excess of pyridine results in the breakdown of the tetranuclear structure with formation of a strongly blueluminescent monomeric complex C6F5Cu(py) (py = pyridine).29 The aim of the current work was to further investigate the factors that determine the aggregation behavior of pentafluorophenyl copper−pyridine complexes in solution and the solid state. Special emphasis is given to the role of π-stacking and cuprophilic interactions on the supramolecular assembly and the photophysical properties.

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(Innovative Technologies; alumina/copper columns for hydrocarbon solvents), and the chlorinated solvents were subsequently distilled from CaH2 and degassed via several freeze−pump−thaw cycles. Pentafluorophenyl copper toluene solvate (2) was prepared according to a literature procedure.24 Compound 2 was heated to 80 °C under vacuum to yield the solvent-free complex 1 as a colorless white powdery solid. The synthesis of C6F5Cu(py) (4-H) has been reported previously.29 2-Picoline, 4-picoline, 2-chloropyridine, 4-chloropyridine hydrochloride, and 2,2′-bipyridine were purchased from ACROS. Pyridine was degassed and distilled from calcium hydride prior to use. 2-Picoline, 4-picoline, and 2-chloropyridine were used as received, and 4-chloropyridine hydrochloride was treated with 6 M sodium hydroxide solution and extracted with ether. After removal of ether at 0 °C under vacuum, 4-chloropyridine was dissolved in toluene, and the solution was degassed via several freeze−pump−thaw cycles prior to use. The 499.91 MHz 1H, 125.68 MHz 13C, and 470.2 MHz 19F NMR spectra were acquired on a Varian INOVA NMR spectrometer equipped with a 5 mm dual broad-band gradient probe (Nalorac, Varian Inc., Martinez, CA). The 100.5 MHz 13C NMR spectrum was recorded on a Varian VXR-S spectrometer. All NMR data were recorded at ambient temperature. 1H and 13C NMR spectra were referenced internally to the solvent peaks, and 19F NMR spectra were referenced externally to α,α′,α″-trifluorotoluene (0.05% in C6D6; δ = −63.73 ppm). The abbreviations Pf and py are used for pentafluorophenyl and pyridyl, respectively. Variable-temperature solid-state luminescence data were measured on a Varian Cary Eclipse Fluorescence spectrophotometer equipped with an Oxford Instruments Cryostat, model Optistat DN. For phosphorescence measurements, a delay time td of 0.1 ms and a gate time tg of 1.0 ms were used with excitation and emission slit widths of 5/10 nm. For the lifetime measurements, thoroughly degassed samples were excited with 5 ns ∼80 mJ pulses at 355 nm (third harmonic of a Q-switched Nd:YAG laser, Quantel, Brilliant). The emission was dispersed through a monochromator (Oriel M257) and detected with a Hamamatsu R928 photomultiplier. The transients were recorded using a Tektronix SCD 1000 digital oscilloscope controlled by a Labview subroutine. Fluorescence decays were fitted as single exponentials using the Igor software package by Wavemetrics, Inc. DFT calculations were performed with the Gaussian 03 program.30 Geometries and electronic properties were calculated by means of the B3LYP hybrid density functional with the basis set of 6-311++G (d,p). The input files and orbital representations were generated with Gaussview (scaling radii of 75%). Excitation data were calculated using TD-DFT (B3LYP).30 Elemental analyses were obtained from Quantitative Technologies, Inc., Whitehouse, NJ, and Schwarzkopf, Woodside, NY. Melting points and decomposition temperatures were determined in sealed capillary tubes and are not corrected. Spectral Data for 1 in CDCl3 and d5-Pyridine. 19F NMR (470.2 MHz, CDCl3, 20 °C): δ = −104.1 (m, 2F, ortho-F), −141.5 (t, J(19F, 19F) = 20 Hz, 1F, para-F), −158.1 (m, 2F, meta-F). 13C NMR (100.5 MHz, CDCl3, 20 °C): δ = 154.7 (dd, J(19F, C) = 244/22 Hz, Pf-C2,6), 145.7 (dm, J(19F, C) = 262 Hz, Pf-C4), 137.2 (dm, J(19F, C) = 260 Hz, Pf-C3,5), 98.7 (t, J(19F, C) = 52 Hz, Pf-C1). 19F NMR (470.2 MHz, C5D5N, 20 °C): δ = −110.6 (m, 2F, ortho-F), −163.4 (t, J(F, F) = 20 Hz, 1F, para-F), −164.1 (m, 2F, meta-F); 13C NMR (125.7 MHz, C5D5N, 25 °C): δ = 150.2 (dd, J(19F, C) = 219 Hz/33 Hz, Pf-C2,6), 138.0 (dm, J(19F, C) = 241 Hz, Pf-C4), 136.6 (dm, J(19F, C) = 253 Hz, Pf-C3,5), 130.3 (t, J(19F, C) = 80 Hz, Pf-C1). Synthesis of [C6F5Cu]4(py)2 (3). Neat pyridine (80 μL, 1.0 mmol) was added dropwise to a solution of 1 (0.46 g, 0.50 mmol) in CH2Cl2 (10 mL) at room temperature. Upon addition of pyridine, the solution turned yellow. The solution was layered with 10 mL of pentane and stored at −38 °C. Colorless needle-like crystals formed after 24 h. The crystals contain one molecule of pentane per main molecule, but were dried for elemental analysis and for the determination of the yield. Yield: 0.44 g (82%). For 3: Tm: 145 °C (dec.). 1H NMR (500 MHz, CDCl3, 20 °C): δ = 8.35 (d, J = 5.0 Hz, 4H, Py-H2,6), 7.94 (t, J = 7.5 Hz, 2H, Py-H4), 7.50 (m, 4H, Py-H3,5). 19 F NMR (470.2 MHz, CDCl3, 20 °C): δ = −108.2 (br m, 8F, ortho-

EXPERIMENTAL SECTION

All reactions and manipulations were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inertatmosphere glovebox (Innovative Technologies). Hydrocarbon and chlorinated solvents were purified using a solvent purification system 1547


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Table 1. Experimental Data for X-ray Diffraction Studies 3 empirical formula formula weight T, K wavelength, Å cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalcd, g cm−3 μ, mm−1 F(000) cryst size, mm limiting indices

θ range, deg reflns collected independent reflns absorption correction refinement method data/restraints/ parameters goodness-of-fit on F2 final R indices [I > 2σ(I)]a R indices (all data)a peak/hole (e Å−3) a

4-pMe

4-oMe

4-pCl

4-oCl

5

C39H22Cu4F20N2 1152.75 218(2) 0.71073 monoclinic C2/c 20.1931(13) 12.4763(9) 15.8809(11) 90 90.6490(10) 90 4000.7(5) 4 1.914 μ(Mo Kα) 2.225 2264 0.30 × 0.15 × 0.10 −25 ≤ h ≤ 25 −15 ≤ k ≤ 15 −20 ≤ l ≤ 19 1.92−27.00 13688 4364 semiempirical from equivalents full-matrix leastsquares on F2 4364/4/302

C12H7CuF5N 323.73 100(2) 0.71073 triclinic P1̅ 5.9779(4) 7.2228(6) 13.4698(10) 91.634(2) 90.298(2) 95.320(2) 578.83(8) 2 1.857 μ(Mo Kα) 1.935 320 0.30 × 0.30 × 0.20 −7 ≤ h ≤ 7 −9 ≤ k ≤ 9 −17 ≤ l ≤ 17 1.51−27.49 4730 2463 semiempirical from equivalents full-matrix leastsquares on F2 2463/0/172

C12H7CuF5N 323.73 100(2) 1.54178 monoclinic P21/c 6.10850(10) 7.5034(2) 25.0325(5) 90 95.5830(10) 90 1141.91(4) 4 1.883 μ(Cu Kα) 3.228 640 0.25 × 0.21 × 0.17 −7 ≤ h ≤ 7 −7 ≤ k ≤ 8 −29 ≤ l ≤ 29 3.55−65.01 8950 1918 semiempirical from equivalents full-matrix leastsquares on F2 1918/0/174

C11H4ClCuF5N 344.15 100(2) 0.71073 triclinic P1̅ 5.9903(9) 7.2800(10) 13.0816(19) 88.434(2) 86.879(2) 83.596(2) 565.95(14) 2 2.019 μ(Mo Kα) 2.214 336 0.20 × 0.20 × 0.10 −7 ≤ h ≤ 7 −9 ≤ k ≤ 9 −16 ≤ l ≤ 16 2.82−27.53 4782 2449 semiempirical from equivalents full-matrix leastsquares on F2 2449/0/172

C11H4ClCuF5N 344.15 100(2) 1.54178 monoclinic P21/c 12.5523(2) 7.1538(2) 24.8910(5) 90 98.5210(10) 90 2210.46(8) 8 2.068 μ(Cu Kα) 5.561 1344 0.18 × 0.16 × 0.10 −14 ≤ h ≤ 14 −8 ≤ k ≤ 8 −29 ≤ l ≤ 28 5.42−64.44 13171 3539 semiempirical from equivalents full-matrix leastsquares on F2 3539/0/344

C16H8CuF5N2 386.79 100(2) 0.71073 monoclinic P21/c 8.7282(9) 24.282(3) 7.0340(7) 90 112.065(2) 90 1381.6(2) 4 1.859 μ(Mo Kα) 1.640 768 0.25 × 0.19 × 0.10 −11 ≤ h ≤ 11 −31 ≤ k ≤ 25 −9 ≤ l ≤ 8 1.68−27.50 8470 3127 semiempirical from equivalents full-matrix leastsquares on F2 3127/0/249

1.048

1.064

1.215

1.074

1.136

0.967

R1 = 0.0324 wR2 = 0.0938 R1 = 0.0410 wR2 = 0.0991 0.612/−0.217

R1 = 0.0422 wR2 = 0.1168 R1 = 0.0464 wR2 = 0.1207 1.379/−0.721

R1 = 0.0250 wR2 = 0.0686 R1 = 0.0253 wR2 = 0.0688 0.322/−0.255

R1 = 0.0275 wR2 = 0.0750 wR2 = 0.0296 wR2 = 0.0767 0.769/−0.420

R1 = 0.0338 wR2 = 0.0893 R1 = 0.0370 wR2 = 0.0927 0.575/−0.346

R1 = 0.0272 wR2 = 0.0628 R1 = 0.0368 wR2 = 0.0649 0.384/−0.236

R1 = ∑∥Fo| − |Fc∥/∑|Fo|; wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.

F), −151.2 (br, 4F, para-F), −161.2 (br m, 8F, meta-F). 13C NMR (125.7 MHz, CDCl3, 20 °C): δ = 152.1 (d, J(19F, C) = 232 Hz, PfC2,6), 148.7 (Py-C2,6), 142.4 (dm, J(19F, C) = 251 Hz, Pf-C4), 139.0 (Py-C4), 136.8 (dm, J(19F, C) = 256 Hz, Pf-C3,5), 125.7 (Py-C3,5), 106.4 (t, J(19F, C) = 59 Hz, Pf-C1). Elemental analysis for C34H10Cu4F20N2 (1080.61), Calcd: C, 37.79; H, 0.93; N, 2.59%. Found: C, 36.86; H, 0.71; N, 2.68%. The slightly low C value for the elemental analysis is likely due to incomplete combustion as a result of the large content of fluorine or the high air sensitivity of the material. Synthesis of C6F5Cu(4-picoline) (4-pMe). Neat 4-picoline (0.067 g, 0.72 mmol) was added dropwise to a solution of 2 (0.16 g, 0.14 mmol) in toluene (5 mL) at room temperature. Upon addition of 4-picoline, an intense yellow color developed. The reaction solution was kept at −35 °C for 1 day, and the pure product was isolated from the reaction solution as pale yellow crystals. The product obtained was washed with hexanes and dried under high vacuum. Yield: 0.13 g (72%). For 4-pMe: Tm = 130−134 °C. Tdec = 148−155 °C. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.52 (d, J = 6.0 Hz, 2H, Py-H2,6), 7.38 (d, J = 6.0 Hz, 2H, Py-H3,5), 2.50 (s, 3H, Me). 19F NMR (470.2 MHz, CDCl3, 25 °C): δ = −112.9 (m, 2F, ortho-F), −160.8 (t, J(19F, 19F) = 20 Hz, 1F, para-F), −164.0 (pst, J = 19 Hz, 2F, meta-F). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ = 152.5 (Py-C4), 149.3 (dd, J(19F, C) = 224/32 Hz, Pf-C2,6), 149.4 (Py-C2,6), 138.8 (dm, J(19F, C) = 245 Hz, Pf-C4), 136.5 (dm, J(19F, C) = 253 Hz, Pf-C3,5), 126.7 (Py-C3,5), 122.2 (t, J(19F, C) = 72 Hz, Pf-C1), 21.8 (Me). Elemental analysis for

C12H7CuF5N (323.73), Calcd: C, 44.52; H, 2.18; N, 4.33%. Found: C, 43.21; H, 2.12; N, 3.96%. The slightly low C value for the elemental analysis is likely due to incomplete combustion as a result of the large content of fluorine or the high air sensitivity of the material. Synthesis of C6F5Cu(2-picoline) (4-oMe). Neat 2-picoline (0.050 g, 0.58 mmol) was added dropwise to a solution of 2 (0.13 g, 0.12 mmol) in toluene (5 mL) at room temperature. Upon addition of 2-picoline, an intense yellow color developed. The crude product was obtained as a colorless microcrystalline solid, which was washed with hexanes. Yield: 0.13 g (84%). Slow evaporation of toluene at RT gave pale yellow “single crystals” of 4-oMe suitable for X-ray diffraction analysis. For 4-oMe: Tm = 141−144 °C. Tdec = 147−150 °C. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.59 (d, J = 5.0 Hz, 1H, Py-H6), 7.89 (pst, J = 8.0 Hz, 1H, Py-H4), 7.46 (d, J = 8.0 Hz, 1H, PyH3), 7.38 (pst, J = 7.0 Hz, 1H, Py-H5), 2.93 (s, 3H, Me). 19F NMR (470.2 MHz, CDCl3, 25 °C): δ = −113.1 (m, 2F, ortho-F), −160.9 (t, J(19F, 19F) = 20 Hz, 1F, para-F), −164.0 (m, 2F, meta-F). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ = 159.5 (Py-C2), 149.7 (dd, J(19F, C) = 220 Hz/33 Hz, Pf-C2,6), 149.6 (Py-C6), 139.5 (Py-C4), 138.7 (dm, J(19F, C) = 244 Hz, Pf-C4), 136.4 (dm, J(19F, C) = 253 Hz, Pf-C3,5), 125.9 (Py-C3), 122.7 (Py-C5), 122.8 (t, J(19F, C) = 71 Hz, Pf-C1), 26.0 (Me). Elemental analysis for C12H7CuF5N (323.73), Calcd: C, 44.52; H, 2.18; N, 4.33%. Found: C, 44.38; H, 2.17; N, 4.17%. Synthesis of C6F5Cu(4-chloropyridine) (4-pCl). A solution of 4-chloropyridine (ca. 0.3 mmol) in toluene (5 mL) was added to a 1548


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solution of 2 (0.13 g, 0.12 mmol) in toluene, and the resulting dark yellow mixture was kept at −35 °C for 1 day. Crystallization from toluene gave pale yellow crystals of 4-pCl, which were also used for Xray diffraction analysis. Yield: 0.11 g (68%). For 4-pCl: Tm = 143−145 °C. Tdec = 147−150 °C. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.59 (d, J = 6.5 Hz, 2H, Py-H2,6), 7.60 (d, J = 6.5 Hz, 2H, Py-H3,5). 19F NMR (470.2 MHz, CDCl3, 25 °C): δ = −112.6 (m, 2F, ortho-F), −159.9 (t, J(19F, 19F) = 18 Hz, 1F, para-F), −163.7 (m, 2F, meta-F). 13 C NMR (125.7 MHz, CDCl3, 25 °C): δ = 150.7 (Py-C2,6), 149.9 (dd, J(19F, C) = 222 Hz/27 Hz, Pf-C2,6), 148.5 (Py-C4), 139.0 (dm, J(19F, C) = 245 Hz, Pf-C4), 136.5 (dm, J(19F, C) = 253 Hz, Pf-C3,5), 126.6 (Py-C3,5), 121.3 (t, J(19F, C) = 75 Hz, Pf-C1). Elemental analysis for C11H4ClCuF5N (344.15), Calcd: C, 38.39; H, 1.17; N, 4.07%. Found: C, 37.53; H, 1.17; N, 4.01%. The slightly low C value for the elemental analysis is likely due to incomplete combustion as a result of the large content of fluorine or the high air sensitivity of the material. Synthesis of C6F5Cu(2-chloropyridine) (4-oCl). Neat 2-chloropyridine (0.056 g, 0.49 mmol) was added dropwise to a solution of 2 (0.11 g, 0.099 mmol) in toluene (5 mL) at room temperature. Upon addition of 2-chloropyridine, an intense yellow color developed. The solution was kept at −35 °C for a day, which yielded light yellow feathery crystals. Yield: 0.12 g (88%). Slow evaporation of toluene at RT gave pale yellow “single crystals” of 4-oCl suitable for X-ray diffraction analysis. For 4-oCl: Tm = 123−127 °C. Tdec = 130−133 °C. 1 H NMR (500 MHz, CDCl3, 25 °C): δ = 8.5 (d, J = 5.0 Hz, 1H, PyH6), 7.90 (pst, J = 8.0 Hz, 1H, Py-H4), 7.55 (d, J = 8.5 Hz, 1H, PyH3), 7.46 (pst, J = 6.0 Hz, 1H, Py-H5). 19F NMR (470.2 MHz, CDCl3, 25 °C): δ = −110.5 (br, 2F, ortho-F), −156.3 (br, 1F, para-F), −162.9 (br, 2F, meta-F). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ = 151.7 (Py-C2), 150.9 (d, J(19F, C) = 228 Hz/28 Hz, Pf-C2,6), 150.4 (Py-C6), 140.8 (Py-C4), 140.4 (dm, J(19F, C) = 249 Hz, Pf-C4), 136.5 (dm, J(19F, C) = 254 Hz, Pf-C3,5), 125.9 (Py-C3), 123.6 (Py-C5), 115.0 (t, J(19F, C) = 67 Hz, Pf-C1). Elemental analysis for C11H4ClCuF5N (344.15), Calcd: C, 38.39; H, 1.17; N, 4.07%. Found: C, 38.25; H, 1.07; N, 4.02%. Synthesis of C6F5Cu(2,2′-bipy) (5). A solution of 2,2′-bipyridine (712 mg, 4.56 mmol) in THF (10 mL) was added dropwise to a solution of 1 (1.00 g, 1.08 mmol) in THF (20 mL) at room temperature. Upon addition, the color of the solution became intense reddish brown. This solution was stored at −38 °C for 24 h. Orangered, needle-like crystals formed and were isolated by decantation of the mother liquor and dried under high vacuum for 30 min. Yield: 1.18 g (70%). For 5: Tm: = 90 °C (dec.). 1H NMR (500 MHz, CD2Cl2, 20 °C): δ = 8.91 (br, 2H, bipy-H6,6′), 8.16 (d, J = 7.0 Hz, 2H, bipyH3,3′), 8.04 (pst, 2H, bipy-H4,4′), 7.59 (br, 2H, bipy-H5,5′). 19F NMR (470.2 MHz, CD2Cl2, 20 °C): δ = −111.1 (m, 2F, ortho-F), −164.3 (t, J(19F, 19F) = 20 Hz, 1F, para-F), −165.1 (m, 2F, meta-F). 13C NMR (100.5 MHz, THF-d8, 20 °C): δ = 153.7 (bipy-C2,2′), 150.7 (bipyC6,6′), 150.2 (dd, J(19F, C) = 226/32 Hz, Pf-C2,6), 139.6 (bipy), 138.1 (dm, J(19F, C) = 244 Hz, Pf-C4), 136.7 (dm, J(19F, C) = 255 Hz, Pf-C3,5), 128.9 (t, J(19F, C) = 83 Hz, Pf-C1), 127.1 (bipy), 122.4 (bipy). Elemental analysis for C16H8CuF5N2 (386.79), Calcd: C, 49.68; H, 2.08; N, 7.24%. Found: C, 49.64; H, 1.59; N, 7.21%. Crystal Structure Determinations. X-ray diffraction intensities were collected at 100 and 218 K (3) on a Bruker Apex (Kα Mo radiation, λ = 0.71073 Å) or Apex 2 (Kα Cu radiation, λ = 1.54178 Å) diffractometers. For all structures, SADABS (Sheldrick, G. M. SADABS (2.01): Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998) absorption correction was used. Non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as an idealized contribution. A solvent pentane molecule in 3 is highly disordered about a 2-fold axis. This solvent pentane molecule was refined as disordered over two positions and with restrictions. The standard C− C bond lengths were used as targets in the refinement for corresponding C−C distances, and similar carbon atoms were refined with the same thermal parameters. H atoms in the disordered pentane molecule were not taken into consideration. Crystallographic data and details of data collections and refinements of the structures are given in

Table 1. The X-ray structures of 3, 4-pMe, 4-oMe, 4-pCl, 4-oCl, and 5 have been deposited at the Cambridge Crystallographic Data Center (CCDC) as supplementary publications nos. CCDC-852837 to 852842. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: (+44) 1223-336-033. E-mail: [email protected]).

■

RESULTS AND DISCUSSION Synthesis. [C6F5Cu]4 (1) was prepared according to a literature procedure by Sheppard and Cairncross25 and recrystallized from toluene to give the toluene solvate [C6F5Cu]4(toluene)2 (2). Compound 2 was either used directly or after drying in high vacuum at 80 °C to remove the toluene solvent molecules, yielding the base-free, purified species 1. Addition of 2 equiv of pyridine to 1 in CH2Cl2 at RT resulted in a light yellow solution, from which colorless needlelike crystals of 3 were collected in 82% yield after layering with hexanes at −38 °C (Scheme 1). When 2 was treated in toluene Scheme 1. Synthesis of Pentafluorophenyl Copper−Pyridine Complexes

or CH2Cl2 with an excess of different pyridine derivatives, airsensitive colorless-to-pale yellow needle-like crystals of compounds 4-R (4-H, 4-pMe, 4-oMe, 4-pCl, 4-oCl) were isolated after recrystallization from toluene at −38 °C in high yield. The red-colored bipyridine species 5 was obtained upon crystallization of an equimolar mixture of 1 and 2,2′-bipyridine in THF solution at −38 °C. All complexes were fully characterized by 1H, 13C, and 19F NMR spectroscopies and elemental analysis, and the solid-state structures were determined by single-crystal X-ray diffraction. The pyridine complexes 3 and 4-R are thermally stable to about 125−150 °C, whereas compound 5 decomposes at ca. 90 °C. A comparison with the reported decomposition temperature for the dioxane complex31 of 1 (200−220 °C) suggests that coordination of pyridine ligands leads to destabilization of the pentafluorophenyl copper complex. This is consistent with early reports that the isolation and crystallographic characterization of organocopper pyridine complexes is complicated by their relatively low thermal stability.32 We found that the pyridine complexes can be stored at low temperature (−20 °C) under a nitrogen atmosphere for extended periods of time. However, upon exposure to air, they quickly decompose, as evidenced by a rapid color change to greenish-brown. 1549


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Solution Structures. The pyridine complexes 3 and 4-R are readily soluble in both polar and nonpolar aprotic solvents, whereas the 2,2′-bipyridine complex 5 shows only moderate solubility. We have previously demonstrated that dissolution of 1 in strongly coordinating solvents, such as acetonitrile or DMSO, leads to aggregate breakdown, which is reflected in a smaller separation of the 19F NMR signals for the para- and meta-fluorine atoms, Δδm,p, a downfield shift of the copperbound carbon atom, δ(13Ci),33 and an increase in the coupling constant 2J(13Ci, 19F) to the ortho-fluorine atoms.26 Only a moderate downfield shift for the ipso-carbons in 3 (δ(13Ci) 106.4) relative to 1 (δ(13Ci) 98.7), a slight increase in the coupling constant 2J(C, F) from 52 to 59 Hz, and a moderate decrease in Δδm,p from 16.6 to 10.0 ppm are observed. These data are similar to those recorded for the tetranuclear toluene complex 2 and thus indicative of coordination of pyridine without breakup of the tetranuclear structure.26 In contrast to 3, complexes 4-R show a strong decrease of Δδ(19Fm,p) to ∼3.1−6.6 ppm, which suggests the breakdown of the tetranuclear aggregate and formation of monomeric pyridine complexes (Figure 1, Table 2).34 The signal for the

chloropyridine (partially) dissociates from the complex, possibly leading to higher aggregates. The latter is also evident from the chemical shift of the ipso-C atom of the C6F5 group, which is between those of the other pyridine complexes (δ in the range of 121.3−122.8) and that of the base-free tetramer 1 (δ 98.7). Further evidence comes from the relatively small coupling constant 2J(13Ci, 19F) of 67 Hz. Strong broadening of the 19F NMR signals for compound 4-oCl at room temperature is also consistent with the presence of a dynamic equilibrium. Although the observed spectral features could be the result of hindered rotation due to the sterically demanding chlorine substituent, a dissociation equilibrium is more likely, given that Δδ(19Fm,p) of 4-oCl increases with increasing concentration. Variable-temperature 19F NMR data for compound 4-oCl were acquired at temperatures ranging from −50 °C to +30 °C. Further broadening of the signals was observed at low temperature. In contrast, low-temperature NMR spectroscopy of complexes 4-H, 4-pMe, 4-oMe, and 4-pCl showed no evidence of a dynamic process down to −50 °C. For the 2,2′-bipyridine complex 5, we observed a more dramatic decrease of Δδm,p to 0.8 ppm, which is accompanied by a slight further downfield shift of the ipso-C in the 13C NMR spectrum to δ(13Ci) 128.9 and an increase in the coupling constant to 2J(13Ci, 19F) = 83 Hz. This trend is likely a result of the trigonal environment with two nitrogen donors at copper. Interestingly, similar data were observed when 1 was dissolved in neat d5-pyridine (Δδm,p = 0.7 ppm, (δ(13Ci) 130.3, 2J(13Ci, 19 F) = 80 Hz), which suggests that, under those conditions, multiple pyridine molecules coordinate to Cu.35 To further probe the dynamic nature of complexation in solution, we performed a titration study, in which we treated 1 with varying amounts of pyridine in the noncoordinating solvent CDCl3 (Figure 2). At ratios of Py/CuC6F5 ranging

Figure 1. 19F NMR spectra of compounds 4-R in CDCl3 at 25 °C.

Table 2. Comparison of Selected 19F and 13C NMR Data complex

solventa

Δδ(19Fm,p)/ ppm

δ(13Ci )/ ppm

1 3 4-H 4-pMe 4-oMe 4-pCl 4-oCl 5 1

CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CD2Cl2/THFb pyridine-d5

16.6 10.0 3.4 3.2 3.1 3.8 6.6 0.8 0.7

98.7 106.4 121.8 122.2 122.8 121.3 115.0 128.9 130.3

2

J(13Ci, 19F)/ Hz 52 59 70 72 71 75 67 83 80

Figure 2. NMR data for the titration of a solution of 1 in CDCl3 with pyridine at RT.

from 0 to 2, neither the 1H nor the 19F NMR spectra showed more than one set of signals. However, the 19F NMR signals experience a gradual shift and are broadened, suggesting that a fast equilibrium between complexes with varying numbers of pyridine ligands, as outlined in Scheme 2, is established in solution. At −80 °C, both [C6F5Cu]4 (1) and C6F5Cu(py) (4-H) give rise to only one set of signals, but a number of different species were observed at ratios intermediate between Py/CuC6F5 = 0:1 and 2:1. Most notably, at a ratio of Py/CuC6F5 = 0.5:1, not only resonances for [C6F5Cu]4(py)2 (3) but also smaller sets of signals for 4-H and one additional species were detected. As expected, the relative amount of the actual 4:2 complex 3 in

Data were acquired at RT, at ca. 2 × 10−1 M concentration. b19F NMR data were acquired in CD2Cl2 and 13C NMR data in THF-d8.

a

copper-bound carbon atom in the 13C NMR spectra of species 4-R (115.0−122.8 ppm) is also strongly downfield shifted and shows an enhanced coupling of 2J(C, F) = 67−75 Hz in comparison with the respective resonance for the intact tetranuclear species 3 (59 Hz). Notably, the 2-chloropyridine complex 4-oCl shows a slightly larger value for Δδ(19Fm,p) than the other complexes 4-R, which indicates that, in solution, 21550


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Scheme 2. Proposed Equilibria between Oligonuclear and Mononuclear Arylcopper Complexes

than for Cu1···Cu1A with 4.178(1) Å. This is a feature that is characteristic of tetranuclear copper species in which two copper atoms are coordinated by donor ligands.13,14,26,27,36,37 The Cu−C distances at Cu1 and Cu1A (average = 2.061(3) Å) in 3 are significantly longer than those at Cu2 and Cu3 (average = 1.974(2) Å), indicating a highly unsymmetrical bridging mode of the aryl groups similar to that in 2. This type of bond alternation is less pronounced14,37 or not observed at all13,36 in organocopper sulfide complexes. As previously suggested by van Koten and co-workers for other donor-supported tetranuclear arylcopper species,4 the long copper−carbon bonds at Cu1 support a description of 3 as a contact ion pair consisting of two [(C6F5)2Cu]− cuprate anions bridged by two pyridine-stabilized copper cations via the ipso-carbon atoms of the pentafluorophenyl rings. All complexes 4-R were also subjected to X-ray diffraction analysis. Single crystals of 4-oMe and 4-oCl were grown via slow solvent evaporation from a mixture of toluene and hexanes, whereas needle-like crystals of 4-H, 4-pMe, and 4-pCl were obtained from the reaction mixture at low temperature. Structure plots are presented in Figure 4, and selected geometric parameters are listed in Table 3. The crystal structure of 4-H has been communicated previously, but the data are provided in Table 3 for comparison purposes.29 The molecular structures of complexes 4-R confirm the formation of mononuclear dicoordinated species C6F5Cu(L) in the solid state with a linear or nearly linear coordination geometry at copper (N−Cu−C from 174.66(8) to 178.54(6)°). The Cu−C bonds of all complexes are within a narrow range from 1.887(2) to 1.901(3) Å, and thus considerably shorter than those found for the tetranuclear species 3 (1.955(2)− 2.076(2) Å), which shows bridging rather than terminal aryl groups. The distances are similar to those of other dicoordinate organocopper−ligand complexes, such as [(C6H2-2,4,6-t-Bu3)Cu(Me2S)],17 with Cu−C = 1.886(3) Å, a related dimeric complex containing a chelating oxazolinyl group (Cu−C = 1.899(5) Å),22 and N-heterocyclic carbene−copper(I) complexes.38 The copper−nitrogen distances (1.897(2)−1.917(2) Å) are significantly shorter than the ones in 3 (2.023(2) Å), but comparable to those in the oxazolinyl complex mentioned above (Cu−N = 1.902(4) Å)22 and a related dimeric pyridylalkyl species [2-(SiMe3)2C(Cu)C5H4N]219 (Cu−N = 1.910(3) Å). Copper−nitrogen distances in dicoordinate copper cations [py2Cu]+ are also found in the same range (1.86−1.96 Å).39,40 Noteworthy is that the Cu−N bond lengths for the two independent molecules of 4-oCl (1.909(2), 1.917(2) Å) are slightly longer than those in the other complexes, suggesting weaker coordination of the pyridine ligand to Cu. The latter is consistent with the conclusions drawn from the solution NMR studies of 4-oCl.

solution diminishes as the ratio of Py/CuC6F5 is either increased or decreased. The amount of the third species, which shows broad signals even at −80 °C (δ(19F) −105.1, −147.2, −158.8; Δδm,p = 11.6 ppm), significantly increases at a smaller ratio of Py/CuC6F5 = 0.25:1. On the basis of these observations, we tentatively assign this species to a 4:1 complex [C6F5Cu]4(py). Solid-State Structures. A single-crystal X-ray diffraction study of 3 confirms that the tetranuclear copper aggregate remained intact, while two pyridine molecules are coordinated to opposite copper centers (Figure 3). Importantly, the

Figure 3. Plot of the molecular structure of 3; a cocrystallized molecule of pentane is omitted for clarity. Selected interatomic distances (Å) and angles (°): Cu(1)−N(1) 2.023(2), Cu(1)−Cu(2) 2.4496(4), Cu(1)−Cu(3) 2.4687(4), Cu(2)···Cu(3) 2.5940(6), Cu(1)···Cu(1A) 4.179(1), Cu(1)−C(1) 2.045(3), Cu(1)−C(7) 2.077(2), Cu(2)−C(1) 1.991(2), Cu(3)−C(7) 1.956(2), Cu(1)− Cu(2)−Cu(3) 58.528(11), Cu(1)−Cu(3)−Cu(2) 57.812(10), Cu(2)−Cu(1)−Cu(3) 63.660(15), Cu(1)−Cu(2)−Cu(1A) 117.06(2), Cu(1)−Cu(3)−Cu(1A) 115.62(2), C(1)−Cu(2)−C(1A) 139.40(15), C(7)−Cu(3)−C(7A) 156.64(14).

structural parameters are different from those of 1 and other homoleptic tetranuclear organocopper complexes that lack interactions with donor molecules.26 Indeed, the structure is more similar to that of the toluene complex 2, except for that the four copper atoms in 3 lie in a plane rather than adopting a butterfly arrangement as in the structure of 2 (2; interplanar angle Cu1Cu3Cu2)//Cu1ACu3Cu2 = 37.7°). The pentafluorophenyl groups are positioned alternately above and below the Cu4 plane, which leads to strong puckering of the eightmembered Cu4C4 ring in 3. The separation between opposite Cu centers is much smaller for Cu2···Cu3 with 2.5940(6) Å 1551


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Figure 4. Plots of the molecular structures of (a) 4-pMe, (b) 4-oMe, (c) 4-pCl, and (d) 4-oCl. Only one of two independent molecules of 4-oCl is shown. Geometric parameters are summarized in Table 3.

Table 3. Comparison of Selected Bond Lengths [Å], Interatomic Distances [Å], and Angles [°] for Complexes 4-R

a

4-oCl Cu1−C1 Cu1−N1 N1−C7 N1−C11 C−X (X = Cl, CH3) C1−Cu1−N1 Py//Pf Cu1···Cu1A Cu1A···Cu1B C1−Cu1···Cu1A C1A−Cu1A···Cu1B C1−Cu1···Cu1B N1−Cu1···Cu1A N1−Cu1···Cu1B N1A−Cu1A···Cu1B C7−N1−Cu1···Cu1B C11−N1−Cu1···Cu1B C2−C1−Cu1···Cu1B C6−C1−Cu1···Cu1B Cu1···Cu1A···Cu1B

4-Hb

4-pMe

4-oMe

4-pCl

1.8913(17) 1.9022(15) 1.356(2) 1.346(2)

1.891(3) 1.900(2) 1.352(3) 1.345(3) 1.501(4) 177.88(8) 2.89 3.531(1) 3.698(1) 95.89(7) 83.27(7) 96.32(7) 85.24(7) 95.46(7) 84.88(7) 65.79(20) 112.49(19) −117.29(21) 65.26(20) 175.16(1)

1.8963(19) 1.9072(16) 1.353(3) 1.351(3) 1.498(3) 174.66(8) 5.32 3.2454(5) 4.2970(5) 92.56(7) 89.42(7) 91.44(7) 92.69(5) 85.23(5) 93.89(5) 60.73(14) −119.27(15) 122.00(18) −58.19(16) 168.22(1)

1.8873(19) 1.8972(16) 1.354(2) 1.345(2) 1.7212(18) 176.34(7) 2.34 3.521(1) 3.784(1) 98.83(5) 77.94(5) 100.54(5) 82.50(4) 100.21(4) 81.07(7) 115.12(14) −62.23(14) −119.67(16) 63.10(14) 170.54(1)

178.54(6) 0.0 2.8924(3) 2.8924(3) 89.900(5) 89.900(5) 90 90.097(5) 90.097(5) 90 90 90 90 90 179.716(15)

mol A

mol B

1.901(3) 1.898(3) 1.917(2) 1.909(2) 1.338(4) 1.340(4) 1.349(4) 1.349(4) 1.727(3) 1.723(3) 177.75(11) 176.19(11) 44.21 43.12 3.4797(6) 3.8363(6) 100.14(9)/100.01(9) 103.85(8)/103.00(9) 90.40(9)/92.45(8) 80.31(7)/75.55(7) 75.59(7)/80.53(7) 93.08(7)/87.04(7) −109.82(20) −109.98(20) 72.85(20) 70.33(02) −116.40(23) −113.80(22) 63.35(23) 68.76(24) 155.80(2)

Symmetry operations used to generate equivalent Cu atoms for 4-H: −x, −y, z + 1/2; −x, y + 1/2, −z + 1/2; x, −y + 1/2, −z; −x, −y, −z; x, y, −z − 1/2; x, −y − 1/2, z − 1/2; −x, y − 1/2, z. For 4-pMe: (i) −x + 1, −y + 1, −z; (ii) −x + 1, −y + 2, −z; (iii) x, 1 + y, z. For 4-pCl: (i) −x + 2, −y, − z + 1; (ii) −x + 2, −y + 1, −z + 1; (iii) x, 1 + y, z. For 4-oMe: (i) −x + 2, −y + 1, −z + 1; (ii) −x + 2, −y, −z + 1; (iii) x, y − 1, z. For 4-oCl: (i) x − 1, y, z; (ii) x − 1, y − 1, z; (iii) 1 + x, 1 + y, z; (iv) 1 + x, y, z; (v) x, y − 1, z; (vi) x, 1 + y, z. bReference 29. a

The pentafluorophenyl groups and the pyridine rings are coplanar for 4-H and nearly so for 4-pMe (2.89°) and 4-pCl (2.34°); however, large dihedral angles of 43.12° and 44.21°, respectively, were observed for the two independent molecules in the complex 4-oCl. The latter is attributed to the high steric demand of the chloro substituent in the 2-position of the pyridine ring. Interestingly, the methyl group in the 2-position of 4-oMe leads to only a slight twisting of the two aromatic rings of 5.32°. The crystal structure of 5 also reveals a monomeric species, but the copper atoms are in an unusual trigonal-planar coordination geometry3d,18 as a result of chelation of the 2,2′bipyridine ligand (Figure 5). The Cu−N bond lengths of 2.018(2) and 2.082(2) Å are not equivalent, and both distances

are slightly longer than those in the monomeric compounds 4R, indicating weaker binding in the trigonal geometry in comparison with the typical linear dicoordinate geometry. The unsymmetric coordination of the bipyridine in 5 further manifests itself in an angle C(1)−Cu(1)−N(1) of 148.30(7) °, which is larger than the C(1)−Cu(1)−N(2) angle of 131.35(7)°. A more symmetric structure has been reported for the complex 4-MeOC6F4Cu(phen) (phen = phenanthroline) with Cu−N = 2.0720(18) and 2.0949(19) Å,3d whereas the gold complex ArAu(2,2′-dmphen) (Ar = 2,4,6-(NO2)3C6H2; dmphen = 2,9-dimethyl-1,10-phenanthroline)41 shows much stronger distortions from trigonal-planar geometry with two very different Au−N bonds of 2.1363(3) and 2.5753(3) Å. The Cu−C bond of 1.907(2) Å is slightly longer than that in 1552


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Figure 5. Plot of the molecular structure of 5. Selected bond lengths (Å) and angles (°): Cu(1)−N(1) 2.017(2), Cu(1)−N(2) 2.082(2), Cu(1)−C(1) 1.907(2), C(11)−C(12) 1.491(3), C(1)−Cu(1)−N(1) 148.30(7), C(1)−Cu(1)−N(2) 131.35(7), N(1)−Cu(1)−N(2) 79.69(6), C(6)−C(1)−Cu(1) 124.2(1), C(2)−C(1)−Cu(1) 123.0(1), (bipy)//(Pf) 46.5, angle sum at Cu 359.34.

Figure 6. View of the extended structure of 3 along the crystallographic c axis, illustrating the formation of channels that are occupied by pentane solvent molecules. Only one position of the disordered pentane molecule is given for clarity.

compounds 4-R, but significantly shorter than that in the tetranuclear species 3. Supramolecular Structures via Cuprophilic and πInteractions. All the complexes described in here form extended supramolecular structures in the solid state via either π-stacking interactions or short Cu···Cu contacts as a result of so-called cuprophilic interactions. Supramolecular assembly of molecular materials via π-stacking of arenes and perfluoroarenes continues to be a topic of much current interest.42,43 Aurophilic interactions, the attractive forces between closedshell d10 metal ions of gold, have also long been recognized and are now commonly used in supramolecular chemistry.44 They are generally considered to be similar in strength to hydrogen bonding interactions and rely, to a large extent, on relativistic effects. In contrast, attractive interactions between closed-shell Cu(I)···Cu(I) pairs have only recently been proposed based on experimental and theoretical studies, and the relevance and strength of such interactions remain controversial.40,45,46 On the basis of MP2 calculations, Schwerdtfeger et al. concluded that cuprophilic interactions between neutral pairs [RCuL]2 should be attractive by up to −4 kcal mol−1 and that the interaction potential is very shallow in the range from ca. 2.5 to 3.5 Å.47 The complex [C6F5Cu]4(py)2 (3) displays an interesting extended structure, in which intermolecular π-interactions (Py//Py, Pf//Pf, and Py-Pf) lead to channels propagating along the crystallographic c axis (Figure 6), which are occupied by pentane solvent molecules. Whereas short intramolecular Cu···Cu contacts (2.5940(6) Å) are observed within the Cu4 aggregates, as discussed above, the intermolecular Cu···Cu contacts are very long (>7 Å), precluding the presence of any Cu···Cu interactions. Extended structures that form as a result of π-interactions are also observed in the crystal structure of the 2,2′-bipyridine complex 5 (Figure 7). Both the bipyridyl units and the pentafluorophenyl groups form offset stacks along the crystallographic c axis. The orientation of the bipyridyl units alternates from layer to layer with a 90° offset between pairs of bipyridyl units. Consequently, only one of the pyridyl rings and the central five-membered CuN2C2 chelate ring match up with the respective parts of the adjacent molecule (distance between

Figure 7. Two different views of a fragment of the crystal packing of 5 showing offset π-stacking along the crystallographic c axis between Cu(2,2′-bipy) fragments and between the pentafluorophenyl groups. Cu···Cu = 5.631 Å.

centroids = 3.382 Å). The shortest intermolecular C···C and C···N distances fall in the range from 3.3 to 3.6 Å, and interestingly, they are augmented by a short Cu···C distance of 3.323 Å to the meta-pyridyl position.48 The Cu···Cu separations of 5.631 Å, on the other hand, are very long, which is insofar surprising as dimers with short Cu···Cu contacts have been reported for the closely related phenanthroline (phen) complex 4-MeOC6F4Cu(phen) (Cu···Cu = 2.5770(6) Å).3d Inspection of the extended structures of the dicoordinate organocopper pyridine complexes 4-R reveals, in all cases, chains of Cu atoms that progress throughout the entire crystal lattice. However, the stacking direction, and thus the orientation of the chains, as well as the Cu···Cu separations vary considerably. The parent compound 4-H, which 1553


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Figure 8. Plots of the extended structures of 4-R (stacking direction for 4-H is the crystallographic c axis; for all substituted derivatives, it is the b axis). (a) View sideways, from left to right: 4-H, 4-pMe, 4-pCl, 4-oMe, 4-oCl. (b) View along the crystallographic a axis: 4-H, 4-pMe, 4-pCl, 4-oMe. View along the crystallographic c axis: 4-oCl. For 4-H, the Cu chain is slightly tilted toward the front to show three-dimensionality. (c) Space filling illustration of the tilting of the stacks.

crystallizes in the Pbcm space group, adopts a very unusual structure in that the copper atoms in 4-H are arranged in onedimensional chains with Cu···Cu distances of 2.8924(3) Å.29 These Cu···Cu distances are among the very shortest reported for unsupported Cu(I)···Cu(I) contacts (Figure 8).49 All atoms, including the hydrogen atoms, reside on a crystallographic mirror plane. Consequently, all copper atoms are equidistant and the direction of the chain is perpendicular to the plane of the molecules. Importantly, the ligands in adjacent RCuL fragments adopt a staggered conformation, thereby avoiding any π-stacking and allowing the molecules to approach very closely. Hence, aggregation leads to supramolecular structures that are unprecedented in organocopper chemistry.50,51 Intrigued by these unusual observations, we set out to further study the effects of placing electron-donating Me groups and electron-withdrawing Cl substituents at the ortho and para positions of the pyridyl ring on the packing of complexes C6F5Cu(py-R) (4-R). Surprisingly, in all cases, a packing pattern that is very different from that of 4-H was observed. Compounds 4-R form offset stacks that progress along the crystallographic b axis with the RCuL monomers aligned in such a way that the C6F5 groups undergo π-interactions with the pyridyl moieties of the adjacent molecules and vice versa (Figure 8). In each stack, the successive molecules adopt an eclipsed, rather than a staggered, arrangement and are offset so that the resulting stacks are tilted by an angle of α = 23.9° for (4-pMe), 26.8° for (4-oMe), 24.7° for (4-pCl), and 21.2° for (4-oCl). This tilting is the result of lateral slippage of the aromatic groups, a phenomenon that is commonly encountered

for stacks involving perfluoroarene−arene interactions (see Figure S1, Supporting Information).43,46,52 The tilt angle was measured using the vector perpendicular to the plane containing the pentafluorophenyl ring. The Cu···Cu distances between adjacent molecules alternate, thereby leading to formation of dimers with relatively short Cu···Cu contacts of 3.245−3.531 Å that are, in turn, connected through longer Cu···Cu contacts ranging from 3.698 to 4.297 Å to form infinite Cu chains. The alternation is most pronounced for 4-oMe with Cu···Cu distances of 3.245 and 4.297 Å, respectively. Noteworthy is also that, for 4-oMe, relatively short contacts of 3.245 Å coincide with the largest deviation from linear coordination geometry at copper (C1−Cu1−N1 = 174.66(8) °). For 4-oCl, the formation of a zigzag Cu chain is apparent and reflected in a relatively small angle Cu1···Cu1A···Cu1B of 155.79(2)°. The latter is likely related to the presence of the sterically demanding 2-chloro substituent, which also leads to tilting of the pyridyl and C6F5 groups with respect to each other. From the crystallographic studies, we conclude that, unlike the parent complex 4-H, the substituted pyridine complexes 4R show multiple face-to-face pentafluoroarene−arene πinteractions. On the basis of the Cu···Cu contacts of 2.8924(3) Å in 4-H, which are significantly shorter than the sum of the van der Waals radii of two Cu(I) centers of 3.78 Å and close to the sum of the covalent radii of 2.64 Å,53 the presence of cuprophilic interactions is likely to play a significant role. Indeed, recent theoretical studies on 4-H are in agreement 1554


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with this assessment.54 The Cu···Cu separations for complexes 4-R are comparative much larger (>3.2 Å), and cuprophilic interactions are expected to be weaker. The supramolecular assembly in those complexes is, to a large extent, governed by perfluoroarene−arene π-interactions, which appears to prevent the Cu centers in neighboring molecules from approaching more closely.46 Photophysical Properties. Intrigued by the observation of infinite copper chains from the X-ray data, we decided to investigate the photoluminescence properties of complexes 4-R in the solid state. The parent compound 4-H displays strong blue fluorescence (λmax = 460 nm, τ = 30 ns) in the solid state at RT upon excitation at λ = 330 nm. This band is also observed in phosphorescence mode, possibly as a result of a delayed fluorescence effect. The 460 nm band is accompanied by a weaker long-lived emission band (τ = 70 μs) that extends to ca. 700 nm (Figure 9). The emission wavelength was found

that the observed delayed fluorescence is caused by triplet− triplet annihilation (T1 + T1 → S1 + S0) rather than by conventional repopulation of the singlet excited state. This mechanism, which is common for polymers, solids, and concentrated solutions, would also account, at least in part, for the increased singlet emission at high temperatures. Triplet migration or “hopping” across the solid is faster at room temperature than at 77 K and facilitates the annihilation process.57 In stark contrast to the observed strong blue fluorescence of 4-H, among the substituted derivatives, only 4-pMe proved to be (weakly) emissive in the solid state at room temperature. Upon excitation at 325 nm, 4-pMe displayed a weak yellowgreen luminescence with a maximum at 558 nm. At 77 K, all substituted compounds exhibited a band in the range of 411− 425 nm (Figure 10a). Again, these bands correspond to decay

Figure 10. (a) Solid-state fluorescence and (b) phosphorescence spectra of compounds 4-H (λexc = 330 nm) and 4-R (λexc = 325 nm) at 77 K.

Figure 9. (a) Variable-temperature solid-state fluorescence and (b) phosphorescence spectra of 4-H (λexc = 330 nm).

from singlet excited states as they are absent when the experiment was performed in phosphorescence mode. However, they all are significantly higher energy than that of 4-H (475 nm at 77 K). No clear correlation with the electronic effects of the pyridine substituent was observed, suggesting that the transition is most likely not simply due to a pyridine-based MLCT state, but rather involves orbitals centered at Cu and/or the C6F5 moiety. In the case of 4-pMe, the band at 411 nm is accompanied by a broad emission at ca. 600 nm, which is red shifted from the room-temperature band at 558 nm. Theoretical calculations by Yang and Su and co-workers predict an emission at 449 nm for C6F5Cu(py) due to charge transfer from the pyridine-centered LUMO to the coppercentered HOMO-2.54 Interestingly, they also calculated the emission spectra for a dimeric species and found that the emission maximum shifted from ca. 460 nm at a Cu···Cu separation of 3.8 Å to 550 nm at a Cu···Cu separation of 2.8 Å.

to be virtually independent of the excitation wavelength (275− 375 nm), indicating that, although the excitation pathway is the same for the two bands, they decay independently. The luminescence properties of 4-H proved to be strongly temperature-dependent, and upon cooling, the emission color changes gradually from blue to a bright yellow-green color. This so-called luminescence thermochromism55 was traced back to an increase in the intensity of the band at 580 nm relative to that at 460 nm (Figure 9). The latter all but disappears at 77 K, revealing a weak shoulder band at ca. 500 nm. On the basis of the luminescence lifetimes, the emission at 460 nm is assigned to fluorescence from a metal-to-ligand charge transfer (MLCT) state, whereas the band at 580 nm is likely due to emission from a lower-lying triplet excited state.56 Because the two emissive states are separated by more than 4000 cm−1, that is, 20 times the value of kBT at room temperature, it is more likely 1555


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strong luminescence only at liquid nitrogen temperature. In all cases, the fluorescence emission band is in the range of ca. 410−425 nm and thus at significantly different energy from that in 4-H. Triplet−triplet annihilation phenomena are observed for compounds 4-H and 4-oMe, suggesting that the shorter Cu···Cu contacts in these compounds give rise to unique luminescence properties.

Taking into consideration the theoretical results by Yang and Su and co-workers,54 our observations might suggest that the larger Cu···Cu separations in the substituted derivatives 4-R in comparison with those in 4-H result in a higher energy emission due to a smaller contribution of intermolecular orbital mixing between Cu centers in neighboring molecules. The fluorescence features discussed above are accompanied by decay from a triplet excited state that results in emission maxima in the range from 545 to 565 nm for the substituted species 4-R and, for 4-oMe, a feature at 425 nm that is attributed to delayed fluorescence (Figure 10b). Again, we notice that the phosphorescence for 4-H is significantly more bathochromic than that for any of the other derivatives, independent of whether electron-withdrawing or electrondonating substituents are attached to the pyridine ligand. On the basis of our observations, it is clear that the luminescence features for compound 4-H are distinct from those of all other derivatives, indicating that the unique solid-state packing with short Cu···Cu separations affects the photophysical properties, presumably due to mixing of Cu-centered orbitals of neighboring molecules. There also appears to be a correlation between short Cu···Cu contacts and the observation of delayed fluorescence via triplet−triplet annihilation, which is most prominent for 4-H (Cu polymer with Cu···Cu = 2.892 Å) and for 4-oMe (Cu dimers with Cu···Cu = 3.245 Å).

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ASSOCIATED CONTENT

S Supporting Information *

Illustration of the offset (slippage) of the π-systems in the stacks of 4-R and results from DFT and TD-DFT calculations on complexes 4-R. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Address § School of Chemical Sciences, National Institute of Science Education and Research, Institute of Physics Campus, Bhubaneswar-751005, Orissa, India.

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ACKNOWLEDGMENTS Acknowledgment is made to the National Science Foundation and the Alfred P. Sloan Foundation for support of this research.

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CONCLUSIONS We have demonstrated that, depending on the amount of pyridine as an external ligand added to solutions of [C6F5Cu]4 (1), either a tetranuclear species [C6F5Cu]4(py)2 (3) or a mononuclear species C6F5Cu(py) (4-H) can be isolated. NMR data for a complex with 2,2′-bipyridine (5) are strikingly similar to those recorded for solutions of 1 in d5-pyridine, indicating that, in neat pyridine, more than one ligand is bound to Cu and most likely a tricoordinate species of the type C6F5Cu(py)2 is present. The solid-state structures of these compounds, determined by single-crystal X-ray crystallography, reveal different elements of supramolecular assembly. Arene−arene π-interactions in 3 lead to a network structure with channels propagating through the lattice along the crystallographic c axis. Similarly, the 2,2′bipyridine complex 5 shows formation of offset π-stacks as the dominant feature in the extended solid-state structure. In contrast, the supramolecular assembly of 4-H is based on cuprophilic interactions and leads to one-dimensional copper chains with equidistant Cu···Cu contacts of 2.8924(3) Å. To further investigate this highly unusual structural motif, we prepared a series of complexes, in which the pyridine ring is substituted in the ortho or para position with an electrondonating methyl group or an electron-withdrawing chloro substituent. Again, offset supramolecular stacks are formed with infinite Cu···Cu chains. However, the Cu···Cu distances are significantly longer in all cases. We attribute this difference to the presence of offset perfluoroarene−arene interactions with intermolecular plane-to-plane separations of ca. 3.3−3.6 Å, which ultimately limit how close the Cu centers can approach each other (Cu···Cu in the range of 3.2454(5)−4.1970(5) Å). For 4-H, no π-stacking is observed, as neighboring molecules are oriented at an angle of 90° relative to each other. Compound 4-H is strongly blue fluorescent at 460 nm in the solid state at room temperature, but shows yellow-green luminescence at 77 K, giving rise to luminescence thermochromism. In contrast, the substituted compounds display

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