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Structural, Electronic, and Computational Studies of Heteroleptic Cu(I) Complexes of 6,6′-Dimesityl-2,2′-bipyridine with FerroceneAppended Ethynyl-2,2′-bipyridine Ligands Jonathan E. Barnsley,† Synøve Ø. Scottwell,† Anastasia B. S. Elliott,† Keith C. Gordon,*,† and James D. Crowley*,† †

Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9016, New Zealand S Supporting Information *

ABSTRACT: Optical characterization and computational modeling of three ferrocene-appended ethynyl-2,2′-bipyridine ligands and the associated heteroleptic copper(I) complexes of 6,6′-dimesityl-2,2′-bipyridine are reported. These dyes have been studied using electrochemical analysis, electronic absorption, and Raman and resonance Raman spectroscopies, coupled with density functional theoretical approaches. For the complexes, optical spectra are dominated by a low energy copper(I) centered metal to ligand charge transfer (MLCT) transition; this is modulated by the presence of pendant ferrocene units and the extent of conjugation of the ferrocenyl bipyridine backbone. Electronic tuning due to ferrocene is shown to result in a redshift of the MCLT transition of up to ∼0.2 eV, while an elongation of conjugation appears to result in an increased MLCT intensity of around 50%.

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INTRODUCTION Dye sensitized solar cells have provided a promising, green alternative to generate electricity from solar radiation.1,2 In recent years the efficiencies of these cells have reached 11.9%.3 Typically ruthenium(II) sensitizers are used due to their high absorbance through metal to ligand charge transfer (MLCT) transitions that occur in the 450−650 nm visible region.4,5 Due to the material cost of ruthenium and toxicity issues, alternative sensitizers are sought after. Copper(I) based dyes have been noted as potential replacements for the ruthenium systems, owing to their lower material cost, and to the fact that they have MLCT transitions in the visible region.6−8 Bipyridine (bipy) based ligands used in the generation of copper(I) dyes often exploit bulky units such as mesityl (Mes) groups to enable the formation of heteroleptic complexes.9,10 The use of bulky groups α to the chelating N atoms on the ligands can increase the excited state lifetime of the copper(I) complexes as it restricts the ability of the excited state (with a copper(II) center) to twist and form exciplexes, which is a primary deactivation mechanism.6,7,11 Maximizing the absorption profile of copper(I) sensitizers is important if these dyes are to compete with the more intensely absorbing ruthenium(II) dyes. To do this, it is desirable to increase the extinction coefficient and to find methods to shift the MLCT transitions of these complexes to the red.6,7 A number of differing approaches have been explored, including the implementation of σ donating ligands12−14 and π-stacking interactions,15,16 while, for ruthenium(II) dyes, π conjugation © XXXX American Chemical Society

including electronic effects from donating and accepting units has also been noted to increase the molar extinction coefficients.17−19 In contrast to ruthenium(II) dyes, the preferred geometry about the metal for copper(I) systems changes upon photoexcitation. This is because copper(I) adopts a tetrahedral coordination sphere and copper(II) a 5-coordinate geometry (most often trigonal bipyramidal). A consequence of this phenomenon is tuning of λmax. If the trigonal bipyramidal geometry is readily accessible, the λmax shifts to the red, while a sterically hindered or π-stacked geometry requires greater energy to undergo the MLCT process generating the formally oxidized copper(II) state. Tuning via this method can shift the MLCT band by up to 0.45 eV.20−25 The geometry of the ground state also provides an approach to alter the optical profile in copper(I) dyes. Distortion results in the breakdown of D2d microsymmetry about the copper and allows for a normally symmetry forbidden transition to occur, broadening the possible spectral profile of copper(I) based dyes.16,26 Furthermore, a number of studies have shown that extending the conjugation length of the acceptor ligand-based MO shifts the MLCT band toward the red.15,16,27−30 One way to potentially encompass a number of these desirable outcomes is to utilize electronic rich and conjugated Received: June 1, 2016

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DOI: 10.1021/acs.inorgchem.6b01300 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry pendant units with the aim of distorting the ground state geometry from D2d around the copper(I) center. We have explored the properties of the CC-ferrocenyl units in generating these effects. Ferrocenyl units are readily incorporated into materials and have the ability to behave as a “molecular hinge” due to free rotation about the iron(II)− cyclopentadiene (Cp) bond; they also act as a conjugated electron donor unit.31,32 Ferrocene derivatives have been noted for emission quenching properties,33,34 yet they also have been exploited in both single molecule dyes and as part of molecular assemblages, to enhance the desirable photophysical properties of these systems.33,35−45 Additionally, ferrocene provides welldocumented and behaved redox properties, that can act as a probe of the electronic structure of the molecules to which they are attached.34,39 Herein we report the optical properties of three ethynyl ferrocene linked bipy ligand systems and the associated copper(I) complexes with varying degrees of conjugation and electronic influence from ferrocene (Figure 1 and Scheme 1).

Scheme 1. Synthesis of Ligand 3 and the Cu(I) Complex 3Cua

Conditions: (i) CuI, [PdCl2(PPh3)2], triethylamine, THF, 60 °C (24 h); (ii) [Cu(CH3CN)4]PF6, 6,6′-dimesityl-2,2′-bipyridine, acetone, 25 °C (30 min).

a

ligand 3 and the copper(I) complex 3Cu were synthesized using procedures analogous to those used to generate 1, 2, 1Cu, and 2Cu. A Pd(0) catalyzed Sonogashira cross-coupling between 5,5′-dibromo-2,2′-bipyridine and ethynylferrocene (Scheme 1) provided 3 in good yield (89%). The copper(I) complex of 3 was obtained in 93% yield by complexation with the in situ generated [Cu(diMesbipy)(CH3CN)2]+ (where diMesbipy = 6,6′-dimesityl-2,2′-bipyridine).46 The new ligand (3) and complex (3Cu) were characterized using 1H NMR, 13C NMR, and IR spectroscopies, high resolution electrospray ionization mass spectrometry (HRESMS), and elemental analysis (Supporting Information). Ground State Geometries. The molecular structures of 1Cu and 3Cu were obtained using X-ray crystallography (Figure 2). Unsurprisingly, the structures of these systems (1Cu and 3Cu) proved to be very similar to that previously reported for 2Cu.31 The cyclopentadiene (Cp) rings of the ferrocene units were eclipsed as expected, and the coordination geometries about the copper(I) ions of the complexes are all nearly identical. Heteroleptic tetrahedral coordination environments are observed in each system with the copper(I) ions coordinated to a diMesbipy and one of the bipyridylethynylferrocene ligands. Each copper(I) ion is found in a distorted tetrahedral geometry, with the complexes containing one long Cu−N bond (2.020(2) and 2.037(6) Å) and one short Cu−N bond (2.007(3) and 2.006(9) Å) to the diMesbipy ligand. This distortion allows one of the mesityl groups to maximize the π−π interactions with the plane of the bipy unit on the ferrocenyl ligands. This is more pronounced in 3Cu (centroid− centroid distances of 3.713 and 5.049 Å for the “top” mesityl group, and 4.713 and 4.142 Å for the “bottom” mesityl group)

Figure 1. Two ferrocene-appended ethynyl-2,2′-bipyridine (1 and 2) ligands and the corresponding copper(I) complex (1Cu and 2Cu) investigated in this study.

The electronic perturbation of the MLCT transition is monitored using a suite of techniques including electrochemical, UV−vis, resonance Raman, Fourier transform− Raman (FT-Raman), and time-dependent density functional theory (TD-DFT) methods. With electronic perturbation due to ferrocene and an increase of conjugation along the ferrocenyl bipy backbone, the intensity of the MLCT transition was increased by 50% and shifted to the red by 0.2 eV.

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RESULTS AND DISCUSSION Ligand and Complex Synthesis. We have previously reported the synthesis of 1, 2, 1Cu, and 2Cu as part of our work on stimuli responsive molecular actuators.31 The new B


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Inorganic Chemistry

Table 1. Largest N−Cu−N Bond Angles and τ4a Geometry Parameter for the Cu(I) Complexes Compound 1Cu

Largest angles/deg

[Cu(diMesbipy)(bipy)]BF424

Exp. Cal. Exp.

125.40, 129.20, 128.17, 131.47, 129.82, 129.95, 129.20, 130.14, 129.88,

CuFc-bipy32

Exp.

125.99, 130.98

2Cu

Exp. Cal. Exp. Cal.

3Cu

a

130.87 130.32 132.83 132.77 130.02 129.85 132.12 131.27 131.6

This parameter is calculated using Houser’s formula τ4 =

where α and β are the two largest N−Cu−N angles.47

τ4 0.735 0.698 0.702 0.679 0.696 0.696 0.699 0.685 0.698 0.715 360 ° − (α + β) , 144 °

0.679), indicating that they share the same distorted tetrahedral geometry, in agreement with the known [Cu(diMesbipy)(bipy)]BF4 complex, which has a τ4 of 0.698 (Table 1). With respect to the ethynyl linker, minimal geometric influence is observed when compared to a previously reported 1Cu analogue CuFc-bipy ([Cu(diMesbipy)(5-ferrocenyl-2,2′-bipyridine)]+) which exhibited a crystal structure derived τ4 of 0.715.32 While close to the τ4 of the broken D2d symmetry of [Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,10-phenanthroline), it is apparent that the presence of the pendant ferrocene groups does not impact the geometry of the copper(I) center to any significant degree in the solid state.26 This is evidence that molecular fluxionality and increased electron density, as a result of the ferrocene substituents, are insufficient to distort the copper(I) tetrahedral geometry. Solid state geometries are consistent with the ground state geometries predicted by density functional theory calculations (DFT). These show negligible differences in experimental and calculated τ4 values for all complex species (Table 1). The small deviations between calculated and experimental angles are attributed to solid-state packing interactions observed in the crystal. Electrochemistry. The electrochemical properties of the new ligand and complex (3 and 3Cu) were investigated using a combination of differential pulse voltammetry (DPV) and cyclic voltammetry (CV), and compared to that of the previously investigated species of interest (Table 2).24,31,32 The diferrocenyl ligand 3 displayed the expected reversible oxidation of the ferrocenyl groups at a similar potential to ligands 1 and 2, with an E° of 0.70 V. Only a single oxidation process is observed for the ferrocenyl groups, indicating that there is no sign of electronic coupling between the organometallic units.48 The copper complexes all showed two reversible redox processes, for which 1Cu and 2Cu are assigned as CuII/I and ferrocenyl oxidation, respectively.31,32 The CV of 3Cu followed the same pattern, with the copper-centered oxidation followed by ferrocene-centered oxidation. This was confirmed using DPV, with the area under the curve for the second oxidation (E° = 0.73 V) being approximately double that of the first (E° = 0.63 V), consistent with the two electron oxidation of the two ferrocenyl groups, versus the single electron oxidation of the copper. Comparison to [Cu(diMesbipy)(bipy)]+ suggests that 1Cu has minimal perturbation (E° = 0.72 vs 0.73 V for CuII/I

Figure 2. Front and side views of (a) 1Cu (selected bond lengths (Å) and angles (deg): Cu1−N1: 2.007(3), Cu1−N2: 2.020(2), Cu1−N3: 2.008(3), Cu1−N4: 2.005(2), N1−Cu1−N2: 81.0(1), N3−Cu1−N4: 82.6(1), N1−Cu1−N3: 125.4(1), N2−Cu1−N4: 130.9(1)), and (b) 3Cu (selected bond lengths (Å) and angles (deg): Cu1−N1: 2.006(9), Cu1−N2: 2.037(6), Cu1−N3: 1.994(8), Cu1−N4: 2.046(7), N1− Cu1−N2: 81.0(3), N3−Cu1−N4: 81.6(3), N1−Cu1−N3: 125.9(3), N2−Cu1−N4: 114.4(3)), where the thermal ellipsoids are shown at the 25% probability level. Solvent, counterions, and hydrogen atoms are omitted for clarity.

than in 1Cu (centroid−centroid distances of 4.344 and 3.920 Å for the “top” mesityl group, and 3.796 and 4.587 Å for the “bottom” mesityl group) (Figure S7). The resulting “clamped down” orientation has been observed previously in similar bipyridine systems, due to the π−π interactions between the mesityl and bipyridine units (Figure S10).24 The distortion away from a normal tetrahedral coordination geometry can be quantified using the four-coordinate geometry index, τ4.47 The τ4 value (see Table 1) ranges from 1.00 for a perfect tetrahedron, to 0.00 for a square planar geometry. All three examined structures had similar τ4 values (0.735 to C


Article

Inorganic Chemistry Table 2. Electrochemical Data for Ligands 1−3 and the Corresponding Copper(I) diMesbipy Ccomplexes 1Cu−3Cu in CH2Cl2 E°/Va Compound 1 1Cu 2 2Cu 3 3Cu [Cu(diMesbipy)(bipy)]BF424 CuFc-bipy32

CuII/I 0.72 0.81 0.63 0.73 0.71

Fc+/0 0.68 0.79 0.78 0.85 0.70 0.73 0.78

1 × 10−3 M in analyte, 0.1 M Bu4NPF6, referenced to [Fc*]+/0 = 0.00 V. a

oxidation) felt due to functionalization with the ethynyl linked ferrocene unit, while 2Cu and 3Cu exhibit shifts of E° = +0.09 and −0.09 V, respectively, when compared to 1Cu.24,31 The copper oxidation for 3Cu (E° = 0.63 V) is shifted in the cathodic direction relative to both 1Cu and 2Cu, which indicates a more readily oxidized copper center. The E° for CuII/I is known to be highly sensitive to the geometry of the copper(II) and copper(I) state; thus, any change in steric encumbrance will shift the observed potential over a 400 mV potential window.24,49,50 However, no ground state geometry factors appear to be in play, as both X-ray crystallography data and DFT predictions show similar τ4 values for all complexes. This finding suggests that the tuning of the copper oxidation may not be through structural constraints but rather is a consequence of an electronic interaction. For each ligand, binding to [Cu(diMesbipy)]+ resulted in an anodic shift for the Fc+/0 oxidation (+0.03 for 3Cu to +0.11 V for 1Cu), an electrochemical indication of electronic coupling between the copper and ferrocenyl moieties. The trend (2Cu > 1Cu > 3Cu) is reflected in both the oxidation of ferrocene (two centers for 2Cu for a total shift of +0.14 V) and the oxidation of copper, evidence that the ferrocene moiety is indeed tuning the copper(I) to copper(II) oxidation. This tuning is negligibly affected by the presence of the ethynyl linker, where CuFc-bipy exhibits oxidation values of 0.71 and 0.78 V for the CuI/CuII and Fc0/I oxidations, respectively.32 Such a result reiterates the potency of ethynyl linker to effectively mediate electronic connectivity. Ground-State Vibrational Spectroscopy. Figure 3 shows the experimental FT-Raman data for the compound series, and assignments are obtained through DFT calculations (Table S1). The mean absolute deviations (MADs) of experimental versus calculated ṽ using Raman vibrational frequencies can be used to quantify how effectively the calculated optimized geometry has modeled the compound in question. The technique used to generate MADs has been described in the literature and is adopted here.51−56 Typically a MAD of less than 15 cm−1 is regarded as satisfactory, especially in the case of large, dynamic molecules such as the studied compound series. For every compound, a MAD of less than 15 cm−1 was attained and has been included with an associated scaling factor in Table S2. Previous work has shown DFT methodologies implementing a CAM-B3LYP/6-31G(d) functional/basis set combination to be an effective approach on ferrocenyl ethynyl systems.31 The MADs shown here further emphasize the applicability of this approach, balanced with computational demand. FT-Raman

Figure 3. FT-Raman data for the compound series, collected at 1064 nm on neat samples. Red lines indicate modes of interest.

spectra of the ligand systems 1, 2, and 3 are dominated by a ferrocenyl bipy band at ∼1590 cm−1 and an alkyne band at ∼2210 cm−1. The medium intensity bands at 1173 and 1499 cm−1 result from a vibrational coupling of ferrocenyl Cp-CCbipy units. This is consistent with a considerably polarizable πconjugated system. Spectra for 1 and 2 are similar to variations seen mostly in the 1167 cm−1 band (1173 cm−1 for 1) and a spitting of the 1305 cm−1 band observed for 1 to 1316 and 1302 cm−1 bands for 2. These vibrations are of similar ferrocenyl Cp-CC-bipy nature; however, they are coupled between both arm units in 2, resulting in an overall higher cross section due to a greater polarizability. The splitting of the 1305 cm−1 band is not predicted by DFT and may be due to a structural deformation around the “molecular hinge” ferrocene unit. 3 exhibits a stronger Raman spectrum, i.e. a greater signalto-noise ratio, than both 1 and 2. This results from the increase of the ferrocenyl bipy backbone (Cp-CC-bipy-CC-Cp) length and conjugation. Associated backbone vibrations become more intense relative to other present modes, as predicted by DFT calculations. On binding with [Cu(diMesbipy)]+, a number of spectroscopic changes occur. For 1Cu and 2Cu, the ferrocenyl bipy breathing mode at ∼990 cm−1 disappears and is replaced by the higher energy [Cu(diMesbipy)]+ breathing vibration at ∼1010 cm−1. The shift in energy is attributed to a perturbed bonding network due to the presence of the copper, and the spreading of electron density due to the more electronegative nitrogen D


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Inorganic Chemistry

structure with the phen ligand tilting toward one of the two mesityl moieties. TD-DFT calculations predict two transitions, one that donates to ferrocenyl bipy and one to diMesbipy (Table S3). The diMesbipy involved transition is shifted to higher energy by around ∼0.4 eV and is predicted to have a lower extinction coefficient as seen by Fraser et al.24 Electrochemical (CuI/II) and absorption (λmax) data, relative to the [Cu(diMesbipy)(bipy)]BF4 standard, are tabulated in Table 3. For all compounds there is a redshift of the MLCT

and carbon atoms. The ferrocenyl Cp-CC breathing mode at ∼1170 cm−1 undergoes a splitting to form two peaks with complexation, and in 2Cu, separate peaks are overlapping to form a single broad peak. In the 1305 cm−1 region there is a deformation of the Cp-CC-bipy bands resulting in a number of peaks at 1283, 1313, and 1324 cm−1 for 1Cu and 2Cu, while in 3Cu the 1305 cm−1 peak shifts to 1283 cm−1. The ferrocenyl Cp-CC-bipy band at 1499 cm−1 also undergoes a redshift upon complexation to the [Cu(diMesbipy)]+ unit, while a new ferrocenyl Cp-CC-bipy band at 1374 cm−1 becomes apparent for 1Cu and 2Cu. The considerable deformation of these ferrocenyl Cp-CC-bipy modes indicates a highly connected π-conjugated system. The strong ferrocenyl bipy mode region at ∼1580 cm−1 also becomes sharpened and only one band is observed. Electronic Absorption Spectroscopy. Electronic absorption spectra for the ligands and complexes are shown in Figure 4. These are displayed as the molar concentration; that is, for

Table 3. Experimental Data Relative to [Cu(diMesbipy)(bipy)]BF4 and Ordered with Respect to Fc:Cu Compound

Fc:Cu

Δλmax/eV

ΔE0 CuI/II/V (per Cu)

2Cu 1Cu CuFc-bipy32 3Cu

0.5 1 1 2

−0.08 −0.12 −0.11 −0.20

0.08 −0.01 −0.02 −0.10

band from 0.08 to 0.20 eV for 2Cu through 3Cu, respectively, while a decrease in CuI/II oxidation potential is observed from +0.08 to −0.10 for 2Cu through 3Cu, respectively. As crystallography and DFT show negligible structural distortion as the ligand is systematically altered, the origin of the tuning is unlikely to be due to the geometry of the complexes. When compared with the ratio of ferrocenyl to copper (Fc:Cu), a linear red-shifting trend is observed as Fc:Cu increases. This data are consistent with a ferrocene based tuning for these systems and that of CuFc-bipy.32 As minimal differences are seen between CuFc-bipy and 1Cu, the ethynyl unit appears to successfully facilitate the equal electronic communication seen for the more proximal CuFc-bipy. Emission experiments were carried out in degassed solutions (CH2Cl2), and negligible emission was detected, consistent with the ferrocenyl copper(II) quenching observed in the literature.34 Resonance Raman. The absorption and electrochemical data suggest that ferrocenyl-bipy type ligands are having an electronic effect on the copper(I) complexes. It is difficult to experimentally extract the contribution from the two active chromophores (copper(I) → ferrocenyl-bipy and copper(I) → diMesbipy) from the electronic absorption data alone. Resonance Raman spectroscopy offers a useful method for probing such transitions because the vibrational modes of the resonant acceptor ligand will be preferentially enhanced.25,59−63 In the case of these complexes, where the energy of the LUMOs on each ligand is suitably close, the transition may populate either one or both of the diimine ligands. The fundamental vibrational modes of these individual ligands provide support for involvement with this Franck−Condon state. It is important to consider that the resonance Raman intensity is related to the structural distortion (Δq) and the polarizability of the ligands, where one ligand may be more polarizable and, hence, contribute more strongly to the spectrum. Resonant Raman spectra for 3, as a function of excitation wavelength, are shown in Figure 5a. What is striking about these data is that they appear almost identical to the nonresonance Raman spectrum, despite the fact that the excitation wavelength covers the ligand π−π* transition at 385 nm and the ferrocenyl based transition at 480 nm. The lack of distinctive resonance effects attributable to ferrocence vibra-

Figure 4. UV−vis absorption spectra for all compounds (CH2Cl2).

2Cu there are twice as many coppers per molecule relative to 1Cu and 3Cu. The experimental and calculated (TD-DFT) data are presented in Table S3. Each ligand (1, 2, and 3) shows two characteristic bands at 385 and 480 nm. These bands are attributed to ligand π−π* and ferrocenyl transitions, respectively.57,58 Consistent with these assignments, the ferrocene band has twice the intensity for 3 relative to 1 and 2. The ε for these transitions (∼2000 M−1 cm−1 per ferrocenyl unit) suggests that the ethynyl linker plays a small role in this transition, and this is consistent with TD-DFT calculations (Table S3). Complexation with [Cu(diMesbipy)]+ leads to the appearance of a strong MLCT absorption, ranging in λmax from 486 to 511 nm. These transitions are red-shifted from the parent [Cu(diMesbipy)(bipy)]+ complex (λmax 472 nm, ε 5400 M−1 cm−1). For 1Cu and 2Cu the transitions have approximately the same intensity as the parent complex (ε ∼ 5000 M−1 cm−1 per Cu); however, 3Cu has ε ∼ 7500 M−1 cm−1; that is, per copper(I) center, 3Cu has an absorption intensity boosted by 50% compared to that of 1Cu, 2Cu, and [Cu(diMesbipy)]+. The 3Cu complex has appreciable absorption to 600 nm, but there is no clear evidence of the symmetry broken shoulder seen for [Cu(dpp)2]+.16 This is consistent with the crystallographic data which suggests the D2d microsymmetry is maintained in these compoundsthis is in contrast to the related [Cu(diMesbipy)(phen)]+, which shows a “pac-man” E


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Inorganic Chemistry

in 3Cu is consistent with the lack of the band I shoulder in UV−vis spectra. The presence of the alkyne bands in the resonance Raman spectra suggests its involvement in the MLCT process terminating on the ferrocenyl-bipy, consistent with UV−vis, electrochemistry, and DFT data.

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CONCLUSION The optical properties of three ferrocene-appended ethynyl2,2′-bipyridine ligand systems and the corresponding [Cu(diMesbipy)]+ complexes have been reported. Each of the copper(I) complexes were shown to display a broad absorption band between 350 and 450 nm. Resonance Raman spectroscopy and DFT calculations indicated that these absorption bands were metal to ligand charge-transfer in nature from the copper(I) center to the diMesbipy and the ferrocenyl bipy residues.25,59−63 The increased electron density as a result of the ferrocene substituents does not lead to a vast increase of π−π stacking, and in turn enough distortion of D2d microsymmetry for the forbidden band I to occur. Experimental and theoretical results show negligible geometric tuning typically seen for copper(I) MLCT states, while electronic tuning effects and an increased conjugation length are deemed responsible for a red-shifted transition (0.2 eV) and increased intensity of 50%both highly desirable characteristics of copper(I) based sensitizing dyes. While the redshift observed here was modest, we are targeting the synthesis of other more highly functionalized bipy and 1,10-phenanthroline ligands with extended conjugation in order to further enhance the visible absorption properties of these Cu(I) systems.64 Efforts toward such systems with ferrocene and other electron donor groups are underway.65

Figure 5. FT and resonance Raman data for (a) 3 and (b) 3Cu (neat solid and 10−4 M CH2Cl2, respectively).

tions may be a consequence of the relatively low ε for the ferrocenyl-based electronic transition at 480 nm coupled with the localized nature of that transition. However, it is clear that a number of MLCT transitions, both to the diMesbipy ligand and to the ferrocenyl ligand, are present at this excitation wavelength. The appreciable enhancement at 351 nm, resonant with the π−π* transitions, suggests this is a delocalized transition across the entire ligand framework. The bands at 1561 and 1379 cm−1 are diMesbipy ligand in nature and are depicted in Figure 6. The 1452 cm−1 mode is

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EXPERIMENTAL SECTION

General Methods. Unless otherwise stated, all reagents were purchased from commercial sources and used without further purification. The solvents used were of laboratory grade, with petrol referring to the fraction of petroleum ether boiling in the range 40−60 °C, and ether referring to diethyl ether. Dry tetrahydrofuran (THF) was obtained by passing the solvent through an activated alumina column using a PureSolv solvent purification system (Innovative Technologies Inc., MA). 0.1 M Ammonium hydroxide/ethylenediaminetetraacetic acid (NH4OH/EDTA) solution was made up by mixing 30 g of EDTA with 900 mL of water and 100 mL of NH4OH. All reagents were purchased from commercial sources and used without further purification, unless otherwise specified. The ligands (1 and 2) and corresponding copper(I) complexes (1Cu and 2Cu) were synthesized using our reported methods.315,5′-Dibromo-2,2′-bipyridine was synthesized via a previously published procedure,66 as was 6,6′-dimesityl-2,2′-bipyridine.46 All spectroscopic measurements were made with Sigma-Aldrich spectroscopic or HPLC grade solvents. Spectral data was analyzed using GRAMS A/I (ThermoScientific) and OriginPro v9.0 (Origin Lab Corporation). 1 H and 13C NMR spectra were recorded on either a 400 MHz Varian/Agilent 400-MR or Varian 500 MHz AR spectrometer at 298 K. Chemical shifts (δ) are reported in parts per million (ppm) and referenced to residual solvent peaks (CDCl3: 1H δ 7.26 ppm, 13C δ 77.16 ppm). Coupling constants (J) are reported in Hertz (Hz). Standard abbreviations indicating multiplicity were used as follows: m = multiplet, q = quartet, t = triplet, dt = double triplet, d = doublet, dd = double doublet, s = singlet. IR spectra were recorded on a Bruker ALPHA FT-IR spectrometer with an attached ALPHA-P measurement module. Microanalyses were conducted at the Campbell Microanalytical Laboratory at the University of Otago. Electrospray mass spectra (ESMS) were collected on a Bruker micrOTOF-Q spectrometer.

Figure 6. Eigenvector diagrams for vibrations ν257, ν282, and ν297 for 3Cu.

ferrocenyl ligand based, with a vibrational contribution from the electronically connected ferrocenyl moiety. These three bands are not present in 3 and represent bands coincident with the excitation process. Resonance Raman and TD-DFT data are consistent with the assignment of the lowest energy band to be MLCT in nature, identical to that of 1Cu and 2Cu. The spectra observed for 3Cu are significantly different from those observed for [Cu(diMesbipy)(bipy)]+ and [Cu(diMesbipy)(phen)]+ complexes.24 In the resonance Raman spectra of [Cu(diMesbipy)(phen)]+, the enhancement of a band at 1009 cm−1 was attributed to torsional motion associated with the symmetry broken (band I) MLCT transition.16 The lack of enhancement of this torsional mode F


Article

Inorganic Chemistry

minimum residual electron densities 1.813 and −1.711 e Å−3; τ4 = 0.699. CCDC: 1479983. Electrochemical Methods. Cyclic voltammetric (CV) experiments in CH2Cl2 were performed at 20 °C on solutions degassed with argon. A three-electrode cell was used with Cypress Systems 1.4 mm diameter glassy carbon working, Ag/AgCl reference and platinum wire auxiliary electrodes. The solution was ∼10−3 M in electroactive material and contained 0.1 M Bu4NPF6 as the supporting electrolyte. Voltammograms were recorded with the aid of a Powerlab/4sp computer-controlled potentiostat. Potentials are referenced to the reversible formal potential (taken as E° = 0.00 V) for the decamethylferrocene [Fc*]+/0 process.71 Under the same conditions, E° calculated for [FcH]+/0 was 0.55 V (CH2Cl2).72 Computational Methods. Geometry and vibrational calculations were generated using the Gaussian 09W program package which implemented the coulomb attenuating model Becke, three-parameter, Lee−Yang−Parr (CAM-B3LYP) functional employing the basis set 631G(d) and a DMF polarizable continuum model.73 Calculated vibrational spectra were generated using Gaussum v2.2.5 software, and scaled using scaling factors listed in Table S2 to account for anharmonicity.74 TD-DFT calculations were also implemented, using the same functional and basis set. Molecular orbitals were visualized using Gauss View 5.0 W (Gaussian Inc.) while vibrations were visualized using Molden.75 FT-Raman Methods. FT-Raman spectra were recorded using a Bruker MultiRAM spectrometer implementing a 1064 nm excitation at 50 mW power, a resolution of 1 cm−1 and 100 coadded scans. Signal was detected using a D418T Ge detector. Resonance Raman Methods. Resonance Raman spectra were collected using a setup which has been previously described.62 In short, it is composed of an excitation beam and collection lens in a 135° backscattering arrangement. Scattered photons were focused on the entrance slit of an Acton SpectraPro500i spectrograph with a 1200 grooves/mm grating, which disperses the radiation in a horizontal plane on a Princeton Instruments Spec10 liquid-nitrogen-cooled CCD detector. A Coherent Innova I-302 krypton ion laser was used to provide excitation wavelengths (λex) of 350.7, 406.7, and 413.1 nm, a solid-state CrystaLaser was used for 448.0, 532.0, and 593.7 nm while a Coherent Innova Sabre argon-ion laser for 457.9, 488.0, and 514.5 nm. Notch filters (Kaiser Optical, Inc.) or long-pass filters (Semrock, Inc.) matched to these wavelengths were used to remove the laser excitation line. Sample concentrations were typically 1 × 10−3 mol L−1. UV−Vis Methods. Electronic absorption spectra were measured for 1 × 10−5 M solutions implementing an OceanOptics USB2000 spectrometer.

Preparation of 3. Ethynylferrocene (0.30 g, 1.433 mmol, 3.00 equiv), 5,5′-dibromo-2,2′-bipyridine (0.15 g, 0.478 mmol, 1.00 equiv), CuI (0.018 g, 0.096 mmol, 0.20 equiv), and [PdCl2(PPh3)2] (0.020 g, 0.029 mmol, 0.06 equiv) were combined under a nitrogen atmosphere. Dry triethylamine (0.8 mL, 5.73 mmol, 12.00 equiv) and dry THF (25 mL) were added and the solution stirred at 60 °C for 24 h. CHCl3 (50 mL) and NH4OH/EDTA (50 mL) were added, and the aqueous layer extracted with CHCl3 (2 × 50 mL). The organic phase was washed with saturated aqueous NaCl (2 × 100 mL), dried over Na2SO4, and the solvent removed in vacuo. Column chromatography (silica gel, gradient 100% petrol, then 5:1 petrol/CHCl3, then 100% CHCl3, then 95:5 CHCl3/acetone) was used to obtain the product as an orange solid (0.243 g, 89%). 1H NMR (400 MHz, CDCl3) δ 8.76 (d, J = 2.0 Hz, 2H, Hd), 8.39 (d, J = 8.2 Hz, 2H, Hf), 7.89 (dd, J = 8.2, 2.1 Hz, 2H, He), 4.55 (t, J = 1.9 Hz, 4H, Hc), 4.30 (t, J = 1.9 Hz, 4H, Hb), 4.28 (s, 10H, Ha); 13C NMR (125 MHz, CDCl3) δ 153.8, 151.6, 139.1, 121.3, 120.6, 93.5, 83.1, 71.8, 70.2, 69.4, 64.5; IR: ν (cm−1) 2205, 1482, 1447, 812, 481; HRESI-MS (MeOH) m/z = 573.0748 [3+H]+ (calc. for C34H25Fe2N2 573.0712); UV−vis (CH2Cl2) λmax (ε/ L mol−1 cm−1): 354 (40300), 408 (13800), 481 (4800); Anal. Calc. for C34H24Fe2N2•0.75H2O: C, 69.71; H, 4.39; N, 4.78. Found: C, 69.83; H, 4.20; N, 4.79. Preparation of 3Cu. [Cu(CH3CN)4]PF6 (19 mg, 0.051 mmol, 1.00 equiv) was dissolved in acetone (3 mL) and 3 (29 mg, 0.051 mmol, 1.00 equiv), also dissolved in acetone (3 mL), was added. The mixture was stirred for 15 min to give a yellow solution. 6,6′-dimesityl2,2′-bipyridine (20 mg, 0.051 mmol, 1.00 equiv) was dissolved in acetone (3 mL) and added to this solution. The mixture was stirred for a further 15 min, then filtered through a cotton wool plug and vapor diffused (acetone/petrol) in order to obtain dark red crystals (56 mg, 93%). 1H NMR (400 MHz, CDCl3) 8.48 (d, J = 8.2 Hz, 2H, Hg), 8.24 (t, J = 7.8 Hz, 2H, Hh), 8.12 (s, 2H, Hd), 7.93 (dd, J = 8.4, 2.0 Hz, 2H, He), 7.83 (d, J = 8.4 Hz, 2H, Hf), 7.54 (d, J = 8.4 Hz, 2H, Hi), 6.21 (s, 4H, Hk), 4.55 (t, J = 1.8 Hz, 4H, Hc). 4.34 (t, J = 1.9 Hz, 4H, Hb), 4.26 (s, 10H, Ha), 1.91 (s, 6H, Hl), 1.83 (s, 12H, Hj); 13C NMR (125 MHz, CDCl3) δ 158.7, 152.2, 149.8, 148.3, 139.0, 138.6, 138.2, 137.1, 134.8, 127.6, 127.2, 123.0, 120.6, 120.4, 96.6, 81.6, 72.0, 70.3, 69.9, 63.4, 20.9, 20.4; IR: ν (cm−1) 2208, 1486, 823, 556, 455; HRESI-MS (MeOH): m/z = 1027.22 [3Cu-PF6]+ (calc. for C62H52CuFe2N4 1027.22); UV− vis (CH2Cl2) λmax (ε/ L mol−1 cm−1): 368 (35900), 395 (20600), 503 (11300); Anal. Calc. for C62H52CuFe2N4PF6•0.1H2O: C, 63.37; H, 4.48; N, 4.77. Found: C, 63.09; H, 4.73; N, 4.91. Crystallographic Methods. Crystals of 1Cu were grown via vapor diffusion of petroleum ether into acetone, and of 3Cu via vapor diffusion of diethyl ether into acetone. Both structures were collected on a Bruker Kappa Apex II are detector diffractometer using monochromated Mo Kα radiation (0.71073 Å) at low temperature (100 K). SADABS67 was used for absorption correction. The structures was solved by direct methods using either SIR-9768 or XSeed69 and refined against F2 using anisotropic thermal displacement parameters for all non-hydrogen atoms using SHELXL-9770 software. Hydrogen atoms were placed in calculated positions and refined using a riding model. Due to the extent of disordered solvent in the crystal lattices, the SQUEEZE routine within PLATON was implemented, the details of which are contained within the CIFs. Crystallographic Data for 1Cu. C56H56CuF6FeN4O2P, Mr 540.70, triclinic, P-1, a = 10.9313(14) Å, b = 14.207(2) Å, c = 19.792(3) Å, α = 71.505(6)°, β = 76.440(6)°, γ = 78.322(7)°, V = 2806.3(7) Å3, Z = 2, ρc = 1.280 Mg/m3; 37710 reflections collected, 11469 independent reflections (Rint = 6.22%), which were used in all calculations; R1 = 0.0623, wR2 = 0.1685 for I > 2σ(I) and R1 = 0.0935, wR2 = 0.1849 for all unique reflections; maximum and minimum residual electron densities 1.285 and −0.564 e Å−3; τ4 = 0.735. CCDC: 1046338. Crystallographic Data for 3Cu. C62H52CuF6Fe2N4P, Mr 1173.29, triclinic, P-1, a = 14.1641(11) Å, b = 15.7908(11) Å, c = 29.130(2) Å, α = 95.364(5)°, β = 92.364(4)°, γ = 114.839(4)°, V = 5863.3(8) Å3, Z = 4, ρc = 1.329 Mg/m3; 122186 reflections collected, 24061 independent reflections (Rint = 11.36%), which were used in all calculations; R1 = 0.0952, wR2 = 0.2556 for I > 2σ(I) and R1 = 0.1697, wR2 = 0.2896 for all unique reflections; maximum and

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01300. 1 H NMR, 13C NMR, and mass spectrometry characterization data; X-ray crystallography unit cells and cavity surfaces; mean absolute deviations between FT and calculated Raman spectra and scaling factors; FT-Raman vibrations and DFT based assignments; ferrocene free rotation (β) values from crystallography and DFT; experimental and calculated electronic absorption data; resonance Raman data collected with 351, 406, 413, 448, 457, 488, 514, 532, and 594 nm excitations; and XYZ optimized structures (PDF) Crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(Keith Gordon) E-mail: [email protected]. Fax: +64 3 479 7906. Phone: +64 3 479 7599. G


Article

Inorganic Chemistry *(James Crowley) E-mail: [email protected]). Fax: +64 3 479 7906. Phone: +64 3 479 7731.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dan Preston and Dr. Karl Shaffer are thanked for their preliminary synthetic efforts on this project. JEB, SØS, and ABSE thank the University of Otago for Ph.D. scholarships. JDC thanks the Marsden Fund (Grant no. UOO1124) and the University of Otago (Division of Sciences Research Grant) for supporting this work. The support of the MacDiarmid Institute is gratefully acknowledged.

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