Photochemistry and Redox Chemistry of an Unsymmetrical Bimetallic

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Photochemistry and Redox Chemistry of an Unsymmetrical Bimetallic Copper(I) Complex Oliver Back, Jana Leppin, Christoph Förster, and Katja Heinze* Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: The bimetallic copper(I) complex Cu2L2 (cis-1) is formed with high diasteroselectivity from [Cu(NCCH3)4][BF4] and HL (4-tert-butyl phenyl(pyrrolato-2yl-methylene)amine) in a kinetically controlled reaction. cis-1 features a rather short Cu···Cu distance of 2.4756(6) Å and is weakly emissive at room temperature in solution. Oxidatively triggered disproportionation of cis-1 yields elemental copper and the mononuclear copper(II) complex CuL2 (trans-2). One-electron reduction of trans-2 gives cuprate [2]− with a bent bis(pyrrolato) coordinated copper(I) entity. The imine donor atoms of [2]− can insert an additional copper(I) ion giving exclusively the bimetallic complex cis-1 closing the oxidation−elimination−reduction−insertion cycle.

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INTRODUCTION Mono- and bimetallic copper active sites are very common in biological contexts.1−6 Frequently, these are associated with rapid, reversible electron transfer processes.1−6 For example, cytochrome c oxidase, the terminal enzyme for aerobic aspiration, features a binuclear copper site, the so-called CuA center, for rapid electron transfer.3−6 This CuA center comprises a bis(thiolato)-bridged mixed-valence Cu(1.5)− Cu(1.5) unit with a Cu···Cu distance of 2.43 Å. Upon reduction to the CuI−CuI state by cytochrome c, this distance increases by 0.08−0.22 Å.3−5 In this reduced state, no bond exists between the CuI centers, while in the mixed-valent state a formal bond order of 0.5 is achieved between the copper centers. Furthermore, mono- and bimetallic copper(I) complexes have found great interest due to their interesting photophysical properties7−11 and potential applications in dye-sensitized solar cells12 and in light-emitting devices.13 For example, the lowest energy emitting excited state of [Cu(phen)2]+ has triplet metalto-ligand charge transfer character (phen = 1,10-phenanthroline).7,8,10 Moreover, dual emission from both singlet and triplet excited states has been observed for other CuILn complexes, which has been accounted for by thermally activated delayed fluorescence (TADF).14−16 However, nonradiative deactivation of the MLCT state is believed to occur for fourcoordinate copper(I) complexes in solution via a flattening distortion of the D2d symmetric CuL4 structure and addition of a fifth ligand to the CuII center in the excited state.7,8 The chemical reactivity of electronically excited states,14,15,17 as well as the redox-induced thermal chemical reactivity18,19 of binuclear copper(I) complexes, is less well-explored. Here we describe the formation of the bimetallic copper(I) complex Cu2L2 (cis-1) featuring two chemically distinct copper(I) centers (HL = 4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine20−22). The underlying reason for the © XXXX American Chemical Society

observed exclusive diastereoselective formation of the cis isomer cis-1 is discussed, as well as its photochemistry and redox chemistry. In fact, all these aspects are related as we discuss below.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under inert atmosphere (Schlenk techniques, glovebox). Diethyl ether, toluene, and petroleum ether (40−60 °C) were distilled from sodium. Acetonitrile was distilled from calcium hydride and THF from potassium. Methanol, 99.9%, Extra Dry, AcroSeal was used as received from the supplier. The ligand HL was prepared according to literature procedures.20 All other reagents were used as received from commercial suppliers (Acros, Sigma-Aldrich). NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer at 400.31 MHz (1H) and 100.657 MHz (13C). All resonances are reported in ppm versus the solvent signal as internal standard CD3CN (1H, δ = 1.94; 13 C, δ = 1.32, 118.26 ppm) and C6D6 (1H, δ = 7.16; 13C, δ = 128.26 ppm).23 UV−vis spectra were recorded on a Varian Cary 5000 spectrometer in 1 cm cuvettes. Emission spectra were recorded on a Varian Cary Eclipse spectrometer. IR spectra were recorded with a BioRad Excalibur FTS 3100 spectrometer as KBr discs. FD mass spectra were recorded on a Thermo Fisher DFS mass spectrometer with a LIFDI upgrade. Electrospray ionization (ESI+, ESI−) mass spectra were recorded on a Micromass QT of Ultima API mass spectrometer. Elemental analyses were performed by the microanalytical laboratory of the chemical institutes of the University of Mainz. Electrochemical experiments were carried out with a BioLogic SP-50 voltammetric analyzer using platinum wire as counter and working electrodes and a 0.001 M Ag/AgNO3 electrode as reference electrode. The cyclic voltammetry measurements were carried out at a scan rate of 20−800 mV s−1 using 0.1 M [nBu4N][B(C6F5)4] as supporting electrolyte in acetonitrile. Potentials are referenced against the ferrocene/ferrocenium couple (E1/2 = 220 ± 5 mV under the experimental conditions). X-Band CW EPR spectra were recorded on Received: June 9, 2016

A

DOI: 10.1021/acs.inorgchem.6b01400 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry a Magnettech MS 300 spectrometer with a frequency counter HewlettPackard 5340A at a microwave frequency of 9.39 GHz in solution (298 K). Mn2+ in ZnS was used as external standard. Simulations were performed with the program package Easyspin for MatLab (R2015a).24 Crystal Structure Determinations. Intensity data were collected with a Bruker AXS Smart1000 CCD diffractometer with an APEX II detector and an Oxford cooling system and corrected for absorption and other effects using Mo Kα radiation (λ = 0.71073 Å). The diffraction frames were integrated using the SAINT package, and most were corrected for absorption with MULABS.25,26 The structures were solved by direct methods and refined by the full-matrix method based on F2 using the SHELXTL software package.27,28 All non-hydrogen atoms were refined anisotropically, while the positions of all hydrogen atoms were generated with appropriate geometric constraints and allowed to ride on their respective parent atoms with fixed isotropic thermal parameters. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC-1044691 (cis-1) and 1458703 (trans-2). Copies of the data can be obtained free of charge upon application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [fax (0.44)1223-336-033; email [email protected]]. Crystallographic Data of cis-1. Data follow: C30H34Cu2N4 (577.71); orthorhombic; Pbcn; a = 18.6865(6) Å, b = 8.8626(3) Å, c = 16.5367(6) Å, V = 2738.66(16) Å3; Z = 4; density, calcd = 1.401 g cm−3, T = 173 K, μ = 1.578 mm−1; F(000) = 1200; crystal size 0.41 × 0.17 × 0.08 mm3; θ = 2.18−27.91°; −24 ≤ h ≤ 24, −11 ≤ k ≤ 11, −21 ≤ l ≤ 21; rflns collected = 59849; rflns unique = 3264 [R(int) = 0.0588]; completeness to θ = 27.91° = 99.7%; semiempirical absorption correction from equivalents; max and min transmission 1.1853 and 0.7836; data 3265; restraints 0, parameters 169; goodnessof-fit on F2 = 1.088; final indices [I > 2σ(I)] R1 = 0.0383, wR2 = 0.1175; R indices (all data) R1 = 0.0522, wR2 = 0.1235; largest diff peak and hole 1.013 and −1.118 e Å−3. Crystallographic Data of trans-2. Data follow: C30H34CuN4 (577.71); triclinic; P1;̅ a = 10.021(4) Å, b = 10.316(4) Å, c = 13.898(5) Å, α = 78.506(10)°, β = 88.588(10)°, γ = 73.641(10)°; V = 1350.1(9) Å3; Z = 2; density, calcd = 1.265 g cm−3, T = 223 K, μ = 0.833 mm−1; F(000) = 542.0; crystal size 0.35 × 0.20 × 0.04 mm3; θ = 1.496−28.119°; − 13 ≤ h ≤ 13, − 13 ≤ k ≤ 13, − 18 ≤ l ≤ 18; rflns collected = 26292; rflns unique = 6534 [R(int) = 0.0595]; completeness to θ = 25.242° = 99.9%; semiempirical absorption correction from equivalents; max and min transmission 0.967 and 0.819; data 6534; restraints 0, parameters 322; goodness-of-fit on F2 = 0.946; final indices [I > 2σ(I)] R1 = 0.0395, wR2 = 0.0937; R indices (all data) R1 = 0.0614, wR2 = 0.1002; largest diff peak and hole 0.479 and −0.251 e Å−3. Density Functional Theory Calculations. DFT calculations were carried out using the ORCA program package (version 3.0.2).29 All calculations were performed using the PBE030 functional in combination with the D3 correction31 and employ the RIJCOSX approximation.32 Relativistic effects were calculated at the zeroth order regular approximation (ZORA) level. The ZORA keyword automatically invokes relativistically adjusted basis sets.33 To account for solvent effects, a conductor-like screening model (COSMO) modeling toluene was used in all calculations.34 Geometry optimizations and TD-DFT calculations (50 vertical transitions) were performed using Ahlrichs’ split-valence double-ξ basis set def2-SV(P) which comprises polarization functions for all non-hydrogen atoms.35,36 The presence of energy minima/saddle points was checked by numerical frequency calculations. Computed free Gibbs enthalpies were used to compare the relative energies of the optimized structures. Explicit counterions and/or solvent molecules were not taken into account. Synthesis of cis-1. [Cu(MeCN)4][BF4] (278 mg; 0.88 mmol) was dissolved in acetonitrile (25 mL), and elemental Cu (250 mg; 3.94 mmol) was added. The mixture was stirred for 24 h at 25 °C. The solution was filtered and added to a solution of HL (200 mg; 0.88 mmol) in acetonitrile (25 mL). Triethyl amine (1 mL; 7.14 mmol) was added, resulting in a yellow precipitate, which was filtered, dried

under reduced pressure, and recrystallized from acetonitrile to obtain cis-1 as yellow needles in 79% yield (401 mg; 0.69 mmol). Anal. Calcd for C30H34Cu2N4 (577.7)·0.5CH3CN: C, 62.24; H, 5.98; N, 10.54. Found: C, 62.33; H, 6.02; N, 10.40. 1H NMR (C6D6): δ = 7.89 (s, 1H, H7), 7.45 (s, 1H, H11), 7.08 (d, 3JHH = 8.4 Hz, 2H, H3,5), 6.98 (d + s, 3 JHH = 8.6 Hz, 2H + 1H, H2,6 + H9), 6.58 (s, 1H, H10), 1.19 (s, 9H, H13) ppm. 13C NMR (C6D6): δ = 158.8 (C7), 149.2 (C1), 148.7 (C4), 141.2 (C11), 136.3 (C8), 127.5 (C9), 126.5 (C3,5), 122.5 (C2,6), 114.2 (C10), 34.4 (C12), 31.5 (C13) ppm. MS (FD, toluene): m/z (int) = 513.1 (100, [CuL2]+), 577.1 (75, [Cu2L2]+). IR (KBr): ν̃ = 2962 (m, CH), 2902 (m, CH), 2864 (m, CH), 1610 (m, CN), 1338 (s), 1047 (s), 898 (m), 829 (m), 746 (m), 608 (w), 588 (m) cm−1. UV−vis (toluene): λmax (ε) = 309 (10900), 369 (25650), 403 (12400 L mol−1 cm−1) nm. Synthesis of trans-2. HL (500 mg; 2.21 mmol) was dissolved in acetonitrile (25 mL), and copper(II) chloride (148 mg; 1.11 mmol) was added. Triethyl amine (1 mL; 7.14 mmol) was added to give a dark brown solution. After the reaction mixture stirred for 5 min at 25 °C, the solvent was removed under reduced pressure, and the residue was extracted with diethyl ether (5 × 5 mL) to obtain a dark brown solution. After removal of the solvent under reduced pressure, the black solid was washed with methanol (4 × 5 mL) and dried under reduced pressure. The product trans-2 was obtained as a dark brown solid in 54% yield (307 mg; 0.60 mmol). Anal. Calcd for C30H34CuN4 (513.2)·0.5CH3OH: C, 69.09; H, 6.84; N, 10.57. Found: C, 69.10; H, 7.01; N, 10.61. MS (FD, toluene): m/z (int) = 513.1 (100, [CuL2]+). IR (KBr): ν̃ = 2962 (m, CH), 2902 (m, CH), 2864 (m, CH), 1610 (m, CN), 1581 (m, CN), 1296 (s), 1041 (s), 898 (m), 835 (m), 744 (m), 603 (w), 584 (m) cm−1. UV−vis (toluene): λmax (ε) = 312 (14050), 361 (13700), 397 (15100), 476 (sh, 220 L mol−1 cm−1) nm. EPR (THF, 298 K): g1 = 2.0853, g2 = 2.0854, g3 = 2.1521; A(63/65Cu) = 5, 5, 155 G (from the simulation; correlation time τc = 0.1, 0.1, 0.1 ns); Aiso(14N) = 11.6 G (directly from the experimental spectrum). EPR (THF, 77 K): g1 = 2.0655, g2 = 2.0745, g3 = 2.2319; A1,2,3(63/65Cu) = 10, 25, 155 G (from the simulation). Synthesis of [CoCp2][2]. trans-2 (32.4 mg; 63.1 μmol) was dissolved in diethyl ether (2 mL), and a solution of cobaltocene (12.0 mg; 63.4 μmol) in diethyl ether (3 mL) was added to give a reddish brown solution. After the reaction mixture stirred for 5 min at room temperature, a light brown solid precipitated. The suspension was filtered with a syringe filter to obtain a light brown solid, which was washed with diethyl ether (4 × 3 mL) to yield a reddish brown solid in 65% yield (28.7 mg; 40.8 μmol). Anal. Calcd for C40H44CoCuN4 (703.28): C, 68.31; H, 6.31; N, 7.97. Found: C, 67.93; H, 6.20; N, 8.28. 1H NMR (CD3CN): δ = 8.42 (s, 1H, H7), 7.27 (d, 3JHH = 8.4 Hz, 2H, H3,5), 7.01 (d + s, 3JHH = 8.6 Hz, 2H + 1H, H2,6 + H11), 6.65 (s, 1H, H9), 6.15 (s, 1H, H10), 5.62 (s, 5H, CpH), 1.19 (s, 9H, H13) ppm. MS (ESI+): m/z (int) = 189.0 (100, [CoCp2]+), 235.22 (55), 242.30 (35, [HL + O]+). MS (ESI−): m/z (int) = 225.7 (19, [L]−), 513.4 (100, [CuL2]−), 528.4 (25, [CuL2 + O−H]−), 738.3 (45, [CuL3]−). IR (KBr): ν̃ = 3082 (m, CHCp), 2960 (m, CH), 2902 (m, CH), 2864 (m, CH), 1610 (m, CN), 1538 (s, CN), 1392 (s), 1174 (s), 1037 (s), 871 (m), 842 (m), 744 (m), 617 (w), 568 (m) cm−1. UV−vis (CH3CN): λmax (ε) = 265 (27500), 308 (18900), 369 (26000), 483 (850), 533 (380 L mol−1 cm−1) nm. One-Electron Oxidation of cis-1 by [FcH][PF 6 ] for EPR Investigation. To a solution of cis-1 (5.0 mg; 8.65 μmol) in THF (8 mL) was added a solution of ferrocenium hexafluorophosphate (2.86 mg; 8.65 μmol) in THF (2 mL). After a few minutes, EPR spectra were recorded at room temperature. EPR (THF, 298 K): g1 = 2.0853, g2 = 2.0854, g3 = 2.1521; A(63/65Cu) = 5, 5, 155 G (from the simulation; correlation time τc = 0.1, 0.1, 0.1 ns); Aiso(14N) = 11.6 G (directly from the experimental spectrum). EPR (THF, 77 K): g1 = 2.0655, g2 = 2.0745, g3 = 2.2319; A1,2,3(63/65Cu) = 10, 25, 155 G (from the simulation). The EPR data matched those of trans-2. One-Electron Oxidation of cis-1 for Oxidatively Triggered Disproportionation. A solution of cis-1 in toluene (2.5 mL, c = 2.5 × 10−5 mol L−1; 6.25 × 10−8 mol) was placed in a UV−vis cuvette under inert atmosphere. A solution of ferrocenium hexafluorophosphate (0.8 mL, c = 2.7 × 10−7 mol L−1; 2.16 × 10−10 mol) in toluene B


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Inorganic Chemistry Scheme 1. Synthesis and Reactivity of the Dicopper(I) Complex cis-1 (Atom Numbering for NMR Assignments)

was added. UV−vis spectra of the reaction mixture were recorded over a period of 1 h (see Supporting Information). Reaction of [CoCp2][2] with CuI. To a solution of [CoCp2][2] (5.0 mg, 7.1 μmol) in acetonitrile (5 mL) was added [Cu(NCCH3)4][BF4] (2.3 mg, 7.3 μmol). After 5 min, the solvent was removed under reduced pressure, and the solid was extracted with petroleum ether (40−60 °C) (5 mL). After removal of the solvent under reduced pressure, a yellow solid was obtained. A 1H NMR spectrum was recorded in C6D6, and the data matched those of cis-1. 1H NMR (C6D6): δ = 7.89 (s, 1H, H7), 7.45 (s, 1H, H11), 7.08 (d, 3JHH = 8.4 Hz, 2H, H3,5), 6.98 (d + s, 3JHH = 8.6 Hz, 2H + 1H, H2,6 + H9), 6.58 (s, 1H, H10), 1.19 (s, 9H, H13) ppm (see Supporting Information).

resonances confirming the presence of a single isomer (Figure 1; Supporting Information, Figure S2) or a fast interconversion

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RESULTS AND DISCUSSION The binuclear copper(I) complex 1 is conveniently obtained as a yellow powder from [Cu(NCCH3)][BF4] and 1 equiv of HL20 in CH3CN in the presence of NEt3 (Scheme 1). Mass spectrometry of the product in toluene under inert conditions (liquid injection field desorption ionization; LIFDI; Supporting Information, Figure S1) confirms the binuclear nature of 1 showing a prominent peak at m/z = 576/578 with the expected isotopic pattern of a Cu 2 species fitting to a Cu 2 L 2 stoichiometry. Under less inert conditions, only peaks at m/z = 513/515, assigned to the mononuclear complex [CuL2]+, are observed suggesting some (possibly redox-based) reactivity of 1 (see below). In principle, cis and trans isomers are conceivable for 1. However, NMR spectroscopy merely reveals a single set of

Figure 1. 1H NMR spectrum of 1 in C6D6. The asterisk denotes the solvent resonance.

between the two isomers. Heating of 1 in C6D6 does neither shift nor broaden the 1H NMR resonances appreciably, and no further resonances appear. Hence, under these conditions, no hints for isomerization or dissociation of 1 are found. Density functional theory (DFT) calculations find the trans isomer trans-1 slightly preferred over the cis isomer cis-1 (by 4 kJ mol−1 with a SVP basis set; by 1 kJ mol−1 with a def2-TZVP C


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Inorganic Chemistry

Table 1. Selected Bond Distances [Å] and Angles [deg] of cis-1 Determined by XRD and of cis-1, 3[cis-1], and [cis-1]+ Determined by DFT Methods Cu1···Cu2 Cu1−N2 Cu2−N1 Cu1−Cu2−N1 Cu2−Cu1−N2 N2−Cu1−N2A N1−Cu2−N1A N2−Cu1−Cu2−N1A N2−Cu1−Cu2-N1 C7−N1−C1−C6 phenyl−phenyl centroid

cis-1 (XRD)

cis-1 (DFT)

trans-1 (DFT)

2.4756(6) 1.839(2) 1.8890(19) 94.25(6) 91.80(7) 176.39(13) 171.49(12) −135.75(9) 44.25(9) 56.4(3) 6.310

2.477 1.846 1.879 95.6 92.8 174.4 168.8 −135.5 41.5 53.0 6.286

2.448 1.855 1.872 95.0 90.3 174.6 173.0 −133.6 47.3 41.2 7.945

3

[cis-1] (DFT)

[cis-1]+ (DFT)

2.314 1.827 1.854 95.3 90.2 179.6 169.4 −127.5 52.5 22.5 6.878

2.339 1.836 1.864 94.1 90.2 179.7 171.9 −127.6 52.4 44.2 6.862

N1−Cu2−N1A and N2−Cu1−N2A angles of 171.5° and 176.4°, respectively. Both copper ions point slightly inward to the metallacycle (in,in conformation) suggesting an attractive interaction between the copper(I) centers,37 leading to a comparatively short Cu···Cu distance in cis-1 of 2.4756(6) Å in spite of the above stated twist of the ligands. The distance is significantly shorter than that found in Cu2(OAc)2 with a threeatom bridging acetato ligand (2.544(4) Å),38 in the trinuclear pyridylpyrrolido complex Cu3L13 (2.6875(4), 2.7924(4), 2.8572(4) Å)39 and in the dinuclear complex Cu2L22 (HL2 = 2-hydroxy-1,10-phenanthroline) featuring tridentate ligands and tris-ligated copper centers (2.679(3), 2.661(2), 2.673(2) Å in several polymorphs).40 The Cu···Cu distance in cis-1 is similar to that found by Cotton et al. in Cu2L32 with a NCN bridging ligand (HL3 = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine; 2.4527(10) Å). However, in this case, the copper(I) ions prefer an out,out conformation.41 In a Ci symmetrical NHC complex [Cu2L42]2+ (L4 = bis(N-heterocyclic carbene)) with a rigid backbone, a short Cu···Cu distance (2.4923 Å) is realized which has been ascribed to the rigidity of the bridging di-NHC ligands.42 Ci symmetrical NHC-picolylbridged chlorido dicopper(I) complexes feature Cu···Cu distances around 2.5226(8)−2.5744(9) Å, and cuprophilic interactions have been invoked to explain their high emission quantum yields in the solid state.14 In all cases, classical metal− metal bonding is absent due to the closed-shell d10/d10 electron configurations of the metal centers; yet, cuprophilic interactions have been invoked in some instances.14,37 The electronically distinct CuI centers in cis-1 might furthermore support some electrostatic attractive interactions, explaining the in,in conformation of the copper(I) centers and the short distance. Similarly, an unsupported Cu···Cu interaction (2.810(2) Å) between formally differently charged copper(I) fragments [Cu(pyR)2][CuCl2] has been reported by Siemeling and ascribed to a cuprophilic interaction. However, also in this case some Coulomb attractions might be present as well.43 DFT calculations on cis-1 give a copper−copper distance of 2.477 Å and correctly reproduce the in,in conformation (Table 1). Complex cis-1 displays an absorption band at around 369 nm extending to the visible spectral region (>400 nm) which accounts for the yellow color (Figure 3a). A shoulder appears on the low energy side at 403 nm by deconvolution into Gaussian bands. On the basis of time-dependent DFT calculations (Supporting Information, Table S1), the band at 369 nm is ascribed to MLCT excitations calculated at 313 and 300 nm. The difference electron densities of these transitions

basis set) (Table 1). The transition state of the isomerization is calculated to be high in energy (81 kJ mol−1 with a SVP basis set; 84 kJ mol−1 with a def2-TZVP basis set relative to the cis isomer). This quite substantial barrier excludes a fast interconversion between cis-1 and trans-1. Hence, the DFT results predict a mixture of cis-1 and trans-1 which cannot interconvert. Yet, this mixture is not observed experimentally, and an explanation for this discrepancy will be suggested below. Crystallization from a CH3CN solution provides single crystals of 1 suitable for X-ray diffraction studies (Figure 2,

Figure 2. Molecular structure of cis-1 in the solid state (hydrogen atoms omitted for clarity, thermal ellipsoids at 50% probability level, distances in Å).

Table 1). The structural analysis reveals that exclusively the cis isomer cis-1 is present resulting in a C2 symmetric twisted dimetallacycle with electronically distinct copper(I) centers (a 1,6-dicupra-2,5,7,10-tetraazacyclodeca-2,9-diene scaffold, Figure 2). In spite of pointing in the same direction, no significant π−π interactions of the aryl rings of the ligands are present (Figure 2, distance between centroids >6 Å). The conceivable metallacycle trans-1 with local Ci symmetry and electronically identical CuI centers is not observed. Again, this is in contrast to the thermodynamic predictions by the DFT calculations. This apparent discrepancy will be discussed below. In cis-1, one copper center is ligated by two pyrrolato nitrogen donor atoms and the other one by two imine nitrogen donor atoms, respectively, suggesting a formally zwitterionic description [Cu(imine)2]+ and [Cu(pyrrolato)2]−. Hence, the unfavorable chelating ligation of L− with a small bite angle at a single copper(I) ion is avoided by the bridging coordination. The two bridging ligands are twisted relative to each other, resulting in a highly nonplanar complex geometry (Table 1). Both copper(I) centers are almost linearly coordinated with D


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Inorganic Chemistry

Figure 3. (a) UV−vis absorption spectrum (black, solid line), excitation spectrum (black, dotted line; λobs = 485 nm), and emission spectrum (red line) of cis-1 in toluene and (b) TD-DFT calculated absorption spectrum of cis-1 with prominent transitions marked.

reveal that both copper centers and both imine units of the bridging ligands (π*) are involved suggesting a delocalized description of these excited 1MLCT states (Figure 4a). The weak absorptions calculated at 429 and 411 nm correspond to weakly allowed MLCT transitions from 3dx2−y2 orbitals of the copper centers to the π* orbitals of the imine units (Figure 4a). Irradiation with λexc = 370 nm resulted in a weak, broad, faintly structured emission band peaking at 485 nm at room temperature in toluene solution. The excitation spectrum (λobs = 485 nm) follows the absorption spectrum suggesting that the emissive state can be populated from several excited 1MLCT states (Figure 3a). The significant energy difference between excitation at 370 nm and the emission maximum suggests a different orbital parentage and multiplicity of the emitting state. DFT calculations of the lowest energy triplet state 3[cis-1] find that both copper centers and both bridging ligands are involved, consistent with the calculated lowest 1MLCT state (Figure 4a,b). Hence, the nature of the lowest energy 3MLCT state is best described as d(Cu2; 3dx2−y2) → π*(L, L′). In accordance with the charge transfer nature, the calculated Cu··· Cu distance in 3[cis-1] decreases by 0.16 Å to 2.314 Å with respect to cis-1 (Table 1) reflecting the increased copper− copper bond order. Similar to the increase in bond order in the MLCT states, oxidation to the mixed-valent cation [cis-1]+ should strengthen the Cu···Cu interaction. Indeed, DFT calculations on mixedvalent [cis-1]+ determine the Cu···Cu distance as 2.339 Å (Table 1, Figure 4c). The experimentally recorded cyclic voltammograms of cis-1 in CH3CN/[nBu4N][B(C6F5)4] are shown in Figure 5. The electrochemical study reveals that only irreversible waves are observed upon scanning to positive voltages causing the appearance of several waves which correspond to follow-up

Figure 4. (a) DFT calculated difference electron densities of relevant excited 1MLCT Franck−Condon states of cis-1 (green, electron depletion; yellow, electron increase; isosurface value 0.005 au), (b) spin density of the lowest energy 3MLCT state 3[cis-1] (isosurface value 0.03 au), and (c) spin density of [cis-1]+ (isosurface value 0.03 au). Distances in Å.

products. After several oxidative/reductive cycles, an increasing anodic copper stripping peak44,45 is observed at 0.005 V, and the employed Pt electrode has gained a red lustrous appearance (Figure 5). Obviously, elemental copper or copper(I) oxides (from traces of water) are electrodeposited on the electrode after oxidation/reduction. Similar electroplating has been recently observed using copper(II) complexes with pyridyl and amino ligands under reductive scans.46−48 Furthermore, a new, low intensity, quasireverible wave around −1.2 V versus ferrocene grows during these scans which is assigned to the reduction of a follow-up product (Figure 5g). For elucidation of the oxidation events and oxidation products in more detail, cis-1 is chemically oxidized by 1 equiv of ferrocenium hexafluorophosphate in THF (Scheme 1). The room temperature EPR spectrum of the resulting solution reveals a quartet resonance typical for hyperfine coupling of the unpaired electron with a single copper nucleus (Figure 6; I(63/65Cu) = 3/2; Experimental Section). In fact, the coupling pattern perfectly matches the EPR resonance of the CuL2 complex trans-2, which has been independently prepared from CuCl2, HL, and NEt3 for comparison. The facile loss of one copper atom from the initially formed [cis-1]+ is also evident from the FD mass spectrum of cis-1, especially under noninert conditions (Experimental Section, Supporting Information, Figures S1 and S3). The solid state structure of 2 has been E


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Inorganic Chemistry

Figure 7. Molecular structure of trans-2 in the solid state including the centrosymmetric dimeric arrangement (hydrogen atoms omitted for clarity, thermal ellipsoids at 50% probability level, distance in Å).

Table 2. Selected Bond Distances [Å] and Angles [deg] of trans-2 Determined by XRD and of trans-2 and [2]− Determined by DFT Methods Cu1−N1 Cu1−N2 Cu1−N3 Cu1−N4 N2−Cu1−N3 N1−Cu1−N2 N1−Cu1−N4

2 (XRD)

2 (DFT)

[2]− (DFT)

1.9649(7) 2.06466(18) 1.9598(17) 2.0465(18) 97.94(7) 82.35(7) 99.01(7)

1.947 2.020 1.947 2.019 101.2 83.0 100.4

1.912 2.412 1.914 2.386 157.5

thermodynamically favored. After elimination of [CuI(solv)n]+ species from [cis-1]+, the cis isomer cis-2 should form initially (Scheme 1). Yet, isomerization to trans-2 is certainly feasible, either in the four-coordinate CuIIL2 complex or in a fivecoordinate CuIIL2(solv) adduct (solv = THF, CH3CN). Cu−N distances of trans-2 are similar to those of comparable complexes.49 In the solid state, two complexes trans-2 form a centrosymmetric dimeric aggregate with the pyrrolate π-system of one complex coordinating the copper(II) center of the other in a η5-like fashion with a copper···centroid(pyrrolate) distance of 3.43 Å (Figure 7). Likely due to the aggregation and stacking of the nearly planar four-coordinate complex trans-2 in the solid state (see above), slow tumbling of trans-2 in THF solution is observed in the solution EPR spectra in THF. This had to be included in the EPR spectrum simulation in the slowmotional regime (Figure 6). The EPR data from the solution spectra match those in frozen solution at 77 K (Supporting Information, Figure S4).49c The UV−vis spectrum of trans-2 in toluene features a broad absorption band around 361 nm tailing to approximately 500 nm leading to the dark brown color (Supporting Information, Figure S5). In the low energy region, a small shoulder around 476 nm is observed originating from d−d transitions of the d9−CuII electron configuration. According to DFT calculations, the initially formed mixedvalent complex [cis-1]+ features a rather delocalized spin density distribution (Figure 4c). Attempts to trap the mixedvalent [cis-1]+ species by treating a THF solution of cis-1 with a THF solution of [FcH][PF6] and rapidly freezing the sample by immersion in liquid nitrogen lead only to EPR silent species. Warming the sample to room temperature immediately generates the EPR signature of trans-2; hence, the mixedvalent cation [cis-1]+ appears to react quite fast to give trans-2. Although the Cu···Cu distance should decrease in [cis-1]+, a (solvated) copper(I) species is obviously eliminated yielding

Figure 5. (a−f) Cyclic voltammograms of cis-1 in CH3CN/ [nBu4N][B(C6F5)4] (consecutive oxidative scans), (g) reductive scan after scans a−f, and (h) a photograph of the Pt electrode after several oxidative/reductive cycles.

Figure 6. X-band EPR spectra of cis-1 + [FcH][PF6] (black) and 2 (blue) in THF at 298 K, and the corresponding simulated spectrum (red; slow-motional regime; g1 = 2.0853, g2 = 2.0854, g3 = 2.1521; A(63/65Cu) = 5, 5, 155 G; correlation time τc = 0.1, 0.1, 0.1 ns; hyperfine coupling to 14N omitted in the simulation).

determined by XRD (Figure 7, Table 2). Comparable complexes have been reported before, and in all cases the trans isomer is observed.49 The same holds for the nearly square planar copper(II) complex trans-2. As all reported routes lead to the trans isomer, the trans configuration is F


Article

Inorganic Chemistry the mononuclear copper(II) complex trans-2 (Tables 1 and 2). Yet, nucleophilic attack of a solvent molecule at the copper centers of [cis-1]+ should be feasible, facilitating copper(I) elimination. A further viable alternative mechanistic pathway for the formation of trans-2 would be disproportionation of [cis1]+ into cis-1 and [cis-1]2+ with the latter eliminating a (solvated) copper(II) species. In the CV experiments, the eliminated solvated copper(I) or copper(II) species are reduced to elemental copper at the platinum electrode (Figure 5). Formation of trans-2 is also observed by oxidation of cis-1 with iodine, and no stabilized mixed-valent species of the type (μI)[Cu2L2] has been observed, although such mixed-valent iodido-bridged [Cu−I−Cu]2+ complexes have been recently stabilized with bridging amidinato ligands.18 UV−vis spectroscopic monitoring of the oxidation process revealed that the formation of trans-2 from cis-1 is catalytic in the oxidant [FcH][PF6] (Supporting Information, Figure S6). Addition of 0.03 equiv of [FcH][PF6] to cis-1 causes quantitative formation of trans-2 within a few minutes according to UV−vis spectroscopy. Hence, a radical chain reaction is proposed with [FcH]+ acting as initiator, giving trans-2 and solvated copper(I) ions. In the propagation step, the [Cu(solv)n]+ species oxidizes cis-1 yielding trans-2, a further [Cu(solv)n]+ species, and elemental copper. In the termination step, [Cu(solv)n]+ oxidizes ferrocene to the ferrocenium ion (Scheme 2). Scheme 2. Proposed Oxidatively Induced Disproportionation of cis-1 into trans-2 and Elemental Copper Catalyzed by [FcH]+

Figure 8. (a−f) Cyclic voltammograms of trans-2 in CH3CN/ [nBu4N][B(C6F5)4] (consecutive oxidative scans) and (g) reductive scan after scans a−f.

According to the cyclic voltammogram of trans-2 in CH3CN/[nBu4N][B(C6F5)4] (Figure 8a−f), the copper(II) complex trans-2 is irreversibly oxidized (oxidative decomposition, probably ligand dissociation), yielding the copper stripping peak after reduction, similar to the cyclic voltammograms of cis-1 (Figure 5). Expectedly, the stripping peak is less intense for trans-2. In the reductive scan, a quasireversible reduction wave of trans-2 to the mononuclear cuprate [2]− appears at −1.17 V versus FcH/[FcH]+ (Figure 8g).49b,c This nicely matches the formation of the 2/[2]− couple from cis-1 after copper elimination (Figure 5g). Consequently, trans-2 was chemically reduced to [2]− by cobaltocene (E1/2 = −1.33 V vs FcH/[FcH]+)50 giving the diamagnetic cobaltocenium cuprate [CoCp2][2] (Scheme 1) which was characterized by NMR, IR, and UV−vis spectroscopy as well as by mass spectrometry and elemental analysis (Experimental Section and Supporting Information, Figures S7−S10). The related cuprate K[CuL12] with pyridylpyrrolido ligands [L1]− has been prepared from Cu3L1339 and potassium and characterized by NMR spectroscopy and mass spectrometry.51 A less than four-coordinate structure has been suggested for K[CuL12].51 According to DFT calculations, the anionic

copper(I) complex [2]− is essentially two-coordinate with two pyrrolato donor atoms coordinating the copper(I) center (Cu− N1/N3 1.912/1.914 Å, Table 2), while the two imine nitrogen atoms are rather nonbonding (Cu−N2/N4 2.412/2.386 Å). The two pyrrolates coordinate in a bent fashion with a N1− Cu−N3 angle of 157° (Figure 9). The planes of the pyrrolates are not coplanar in the DFT optimized geometry. The phenyl rings stack with a centroid−centroid distance of 3.8 Å, although this π stacking might be flexible in solution. A coordination pocket with the imine donor atoms (N2···N4 at 3.0 Å) thus forms. This preorganized arrangement with free imine donor atoms should allow for copper(I) insertion giving the bimetallic copper(I) complex cis-1 (Scheme 1). Indeed, addition of 1 equiv of [Cu(NCCH3)4][BF4] to [CoCp2][2] resulted in the formation of cis-1 (Supporting Information, Figure S11). This observed reactivity gives a mechanistic rationale for the exclusive formation of the cis isomer cis-1. Treatment of [Cu(NCCH3)4][BF4] with L− first gives the mononuclear cuprate [CuL2]− ([2]−) with two coordinated pyrrolates and two free imine coordination sites. Straightforward insertion of a G


Article

Inorganic Chemistry

HL or NHC-picolyl.14 In order to obtain stable and luminescent copper(I) complexes for light-emitting devices, strategies to impede such dissociative reactivity might be the usage of more rigid macrocyclic or cage ligands accommodating two copper(I) centers, instead of two hemilabile chelating ligands and furthermore steric protection of the metal centers to avoid nucleophilic attack in the excited MLCT states. The detailed investigation of the redox properties of cis-1 was the basis to the understanding of the unexpected diastereoselectivity during formation of cis-1 and of potential excited state deactivation pathways of electronically excited cis-1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01400. Spectra of cis-1, trans-2, and [CoCp2][2]; Cartesian coordinates of DFT optimized geometries (PDF) Crystal data for cis-1 (CIF) Crystal data for trans-2 (CIF)

Figure 9. DFT calculated optimized geometries of (a) trans-2 (including spin density with an isosurface value 0.03 au) and (b) [2]− (distances in Å).

further CuI ion gives cis-1. Formation of trans-1 would require splitting of a Cu−N(pyrrolato) bond which is obviously not observed. Hence, the diastereoselective formation of cis-1 is kinetically determined, and the DFT calculated slight preference of trans-1 (vide supra) cannot be realized by this pathway. As the MLCT states of cis-1 appear to feature significant [Cu21.5] character (apart from the reduced ligand in the MLCT state), which is similar to that of the reactive mixed-valent species [cis-1]+, an analogous dissociative reactivity could be expected for the MLCT excited states. This reactivity might add to parasitic nonradiative pathways with respect to designing luminescent bimetallic copper(I) complexes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49613127277. Author Contributions

The manuscript was written through contributions of all authors. C.F. solved the X-ray diffraction crystal structures. All authors have given approval to the final version of the manuscript. Notes

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The authors declare no competing financial interest.

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CONCLUSION The bimetallic copper(I) complex Cu2L2 (cis-1) is formed with high diasteroselectivity as the asymmetric cis isomer from [Cu(NCCH3)4][BF4] and HL (4-tert-butyl phenyl(pyrrolato-2yl-methylene)amine) in a kinetically controlled reaction. Due to the asymmetric charge distribution within the Cu2 core and consequently an increased electrostatic attraction of the copper(I) centers, a rather short Cu···Cu distance of 2.4756(6) Å is realized. cis-1 is weakly emissive at room temperature in solution, likely the result of radiative relaxation from the lowest 3MLCT state. Oxidatively triggered disproportionation of cis-1 yields elemental copper and the mononuclear copper(II) complex CuL2 (trans-2). Interestingly, the removal of an electron from a Cu−Cu antibonding orbital initially shortens the Cu···Cu distance according to DFT, yet the positive charge allows for nucleophilic attack at the Cu2 unit leading to [CuI(solv)n]+ elimination and formation of trans-2. One-electron reduction of trans-2 gives the cuprate [2]− with a bent bis(pyrrolato) coordinated copper(I) center. The imine donor atoms of [2] − are well-preorganized and can accommodate and insert a further copper(I) ion giving exclusively the bimetallic complex cis-1 closing the cycle (Scheme 1). This sequence of coordination of the ligand donor atoms explains the diastereoselective formation of cis-1. The dissociative redox activity of cis-1 might also account for its rather poor luminescent properties. Such a redox-analogous decomposition pathway might be a general parasitic, irreversible deactivation channel of bimetallic copper(I) complexes in solution; especially with dissymmetric bridging ligands such as

ACKNOWLEDGMENTS We thank Regine Jung-Pothmann for X-ray data collection and Petra Auerbach and Dr. Mihail Mondeskhi for collecting the LIFDI mass spectra. Parts of this research were conducted using the supercomputer Mogon and the advisory services offered by Johannes Gutenberg-University Mainz (www.hpc. uni-mainz.de), which is a member of the AHRP and the Gauss Alliance e.V. This article is dedicated in memoriam to Prof. Malcolm H. Chisholm.

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Photochemistry and Redox Chemistry of an Unsymmetrical Bimetallic

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