Dinuclear Ruthenium Complex Based on a π-Extended Bridging

Article pubs.acs.org/IC

Dinuclear Ruthenium Complex Based on a π‑Extended Bridging Ligand with Redox-Active Tetrathiafulvalene and 1,10Phenanthroline Units Bin Chen,† Zhong-Peng Lv,† Carol Hua,‡ Chanel F. Leong,‡ Floriana Tuna,§ Deanna M. D’Alessandro,*,‡ David Collison,§ and Jing-Lin Zuo*,† †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ School of Chemistry, The University of Sydney, New South Wales 2006, Australia § School of Chemistry and Photon Science Institute, The University of Manchester, Manchester M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: The synthesis of a π-extended bridging ligand with both redox-active tetrathiafulvalene (TTF) and 1,10phenanthroline (phen) units, namely, bis(1,10-phenanthro[5,6-b])tetrathiafulvalene (BPTTF), was realized via a selfcoupling reaction. Using this ligand and Ru(tbbpy)2Cl2 (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), the dinuclear ruthenium(II) compound [{Ru(tbbpy)2}2(BPTTF)](PF6)4 (1) has been obtained by microwave-assisted synthesis. Structural characterization of 1 revealed a crossed arrangement of the TTF moieties on adjacent dimers within the crystal structure. The optical and redox properties of 1 were investigated using electrochemical, spectroelectrochemical, electron paramagnetic resonance (EPR), and absorption spectroscopic studies combined with theoretical calculations. One exhibits a rich electrochemical behavior owing to the multiple redox-active centers. Interestingly, both the ligand BPTTF and the ruthenium compound 1 are EPR-active in the solid state owing to intramolecular charge-transfer processes. The results demonstrate that the TTF-annulated bis(phen) ligand is a promising bridging ligand to construct oligomeric or polymeric metal complexes with multiple redox-active centers.

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INTRODUCTION Tetrathiafulvalene (TTF) and its derivatives are promising components for molecule-based multifunctional materials owing to their π-electron-donor ability and multiple readily accessible redox states.1−3 Significant attempts have thus been devoted to modifying the TTF core with various functional groups capable of binding to metal ions so as to enhance synergic effects between the electrochemical activity of the TTF backbone and the optical or magnetic properties of the metal ions.4−8 Among them, several bipolar TTF derivatives with the acceptor−donor−acceptor (A−D−A) structure, such as bis(quinoxaline)-TTF,9 bis(tetracyanoquinodimethane)-TTF,10 bis(naphthoquinone)-TTF,11 bis(pyrazine)-TTF,12 and bis(dipyrido[3,2-a:2′,3′-c]phenazine)-TTF,13 have been investigated for their intramolecular charge-transfer (ICT) properties and their potential applications in the field of molecular wires.3 It is well-known that 1,10-phenanthroline (phen) has a powerful chelating ability with metal ions and also acts as an electron-deficient acceptor for the construction of donor− acceptor (D−A) systems.7,14 The association of the TTF unit with phen is of interest for new multifunctional materials based on coordination compounds. Shatruk et al. prepared the © 2016 American Chemical Society

versatile phen ligand with TTF (linking one phen ligand to TTF) for the first time.15,16 In their work, the photophysical and electrochemical behaviors of monometallic ruthenium(II) complexes containing this type of TTF-annulated phen ligand were studied. Jia et al. have also investigated a dinuclear ruthenium complex based on the bis(dipyrido[3,2-a:2′,3′c]phenazine)-TTF ligand, which displayed interesting photophysical properties that could be modulated via a solvent.13 In the present work, a novel π-extended bridging ligand with both TTF and phen units has been successfully prepared for the first time. The new ligand exhibits a bipolar A−D−A structure and, importantly, is an ideal bridging ligand for connecting metal ions and redox-active TTF centers. The ligand thus provides an ideal platform for the construction of model systems (oligomeric or polymeric metal complexes) with easily tunable properties, allowing for the investigation of complexes containing multiple redox-active centers and ICT interactions. On the basis of this ligand and Ru(tbbpy)2Cl2 (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), the interesting Received: February 22, 2016 Published: April 12, 2016 4606

DOI: 10.1021/acs.inorgchem.6b00437 Inorg. Chem. 2016, 55, 4606−4615

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Inorganic Chemistry

Ru(tbbpy)2Cl2 and tetrapyridophenazine, the latter possessing a similar conjugated structure and solubility to BPTTF.18,19 It is well-known that ruthenium(II) polypyridine complexes have been of great interest in solar-energy conversion and photochemistry for their photoredox activity and their high chemical stability.20 In this work, we successfully prepared the dinuclear complex [{Ru(tbbpy)2}2(BPTTF)](PF6)4 (1) under microwave irradiation in ethylene glycol/acetone. The purple product was isolated upon the addition of an aqueous solution of ammonium hexafluorophosphate, followed by purification by column chromatography in a yield of above 40%. The results suggest that microwave-assisted molecular assembly is an effective strategy to incorporate insoluble TTF-based ligands into molecule-based materials. Because of the existence of many tert-butyl groups in the 2,2′-bipyridine coligand, the ruthenium compound 1 is soluble in most organic solvents and the proposed structure was unambiguously characterized by 1H NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), elemental analysis, and single-crystal X-ray diffraction analysis. Three major peaks in the ESI-MS spectrum could be assigned to [M − 4PF6]4+, [M − 3PF6]3+, and [M − 2PF6]2+ for compound 1 (Figure S2, SI). The chemical shift range of 8.6−7.1 ppm in the 1H NMR spectrum reveals characteristic peaks that can be explicitly assigned to hydrogen atoms of bipyridine and phen moieties (Figure S3, SI). Crystal Structure Description. A suitable crystal of compound 1 was obtained by slow vapor diffusion of diethyl ether into a nitromethane solution; however, the sample quickly lost crystallinity when separated from the mother liquor. The compound crystallizes in the monoclinic space group C2/c with two independent molecules in the asymmetric unit, namely, TTF-A and TTF-B (Figure 1). All of the

dinuclear ruthenium(II) complex, [{Ru(tbbpy)2}2(BPTTF)](PF 6 ) 4 (1; BPTTF = bis(1,10-phenanthro[5,6-b])tetrathiafulvalene), has been obtained by microwave-assisted synthesis, as indicated in Scheme 1. The results demonstrate Scheme 1. Synthesis of the BPTTF Ligand and Its Dinuclear Ruthenium Compound 1

the coordinating ability of the TTF-annulated bis(phen) ligand. A combined experimental and computational study of compound 1 is presented herein, where the rich electrochemical behavior and charge-transfer-induced electron-transfer processes are interrogated using electrochemical, spectroelectrochemical, electron paramagnetic resonance (EPR), and absorption spectroscopic studies.

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RESULTS AND DISCUSSION Synthesis and Characterization. In previous work, the preparation of 1,3-dithiole-2-ono[4,5-f ][1,10]phenanthroline, a potential precursor for novel TTF ligands, was achieved via multiple steps from phen under rigorous reaction conditions.16,17 In these prior works, Hudhomme and Shatruk et al. reported that they failed to realize the self-coupling reaction from the precursor for synthesis of the symmetrical πconjugated TTF-fused phen ligand.16,17 In the present case, after the triethylphosphite-mediated coupling reaction of 1,3dithiole-2-ono[4,5-f ][1,10]phenanthroline at a higher temperature (150 °C) in the presence of o-dichlorobenzene, the novel π-extended bridging ligand (BPTTF) with both TTF and phen units was successfully prepared. The resulting yellow product was sparingly soluble in common solvents, such that it could only be characterized by IR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry [MALDI-TOF-MS; Figure S1, Supporting Information (SI)] as well as elemental analysis. The new BPTTF ligand shows a bipolar A−D−A structure. Theoretically, it is a useful bridging ligand for connecting metal ions and redox-active TTF centers in order to construct oligomeric or polymeric metal complexes. However, owing to the rigid planar and extended π-conjugated system, the solubility of the BPTTF ligand is poor, thus preventing the efficient synthesis of metal complexes under conventional reaction conditions. In recent years, Rau and co-workers reported a series of microwave-assisted reactions between

Figure 1. Schematic diagram of the asymmetric unit for 1. Hexafluorophosphate anions are omitted for clarity.

ruthenium ions are octahedrally coordinated by two nitrogen atoms from terminal phen units of BPTTF and four nitrogen atoms from two chelating bipyridine ligands. The average Ru− N bond lengths of Ru1−N, Ru2−N, and Ru3−N range from 2.058(2) to 2.083(2) Å, which are consistent with those in other ruthenium(II) complexes (average Ru−N distance of 2.064 Å), whereas the average Ru4−N bond length [2.022(2) Å] is close to that of the 3+ state for Ru−N bonds (∼2.014 Å).21 The corresponding bond lengths of less than 2.0 Å for Ru4−N24 and Ru4−N21 are 1.895(2) and 1.969(2) Å, respectively, as is also verified in other ruthenium(III) complexes.21,22 The decrease in the Ru−N bond lengths 4607

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Å (C111−C112) and 1.757 Å, respectively. On the basis of structural data, the charge-sensitive central CC bond lengths of the TTF cores [1.318(3) Å for TTF-A and 1.347(3) Å for TTF-B] are within the range of distances of neutral TTF molecules11,25 and are far from the bond lengths of its radical cation (1.38−1.40 Å).26 This result suggests that, in the solid state, steric effects in the dimer bring about a longer CC bond length and a shorter C−S bond length, instead of a radical state in TTF-B. The TTF-B core shows a boatlike conformation with a dihedral angle of 42.97(9)° for the S5− C103−C104−S6 and S7−C118−C117−S8 planes. This structural distortion is consistent with other literature reports for TTF-based ligands.27,28 It is a relatively rare example among TTF-based complexes of a system in which two distinct conformations of the same molecule can be observed in one crystal structure. Typically, alternating parallel stacks of TTF molecules are usually observed in columns with a head−head or head−tail fashion owing to the relatively strong intermolecular interactions derived from shorter π···π and S···S contacts. Interestingly, the interaction between neighboring TTF molecules in 1 leads to the formation of cross-stacked dimers with a short intermolecular S2···S7 contact of 3.73 Å and a C9···C115 contact of 3.28 Å. In addition, this dimer exhibits weak π···π contacts with an interplanar separation of 3.90 Å (Figure S4, SI). The compound is a rare example containing the crossed arrangement of TTF units owing to the significant steric strain generated by the terminal [Ru(tbbpy)2] metal cores. Electrochemical Studies. Because of its poor solubility in most common organic solvents, the redox properties of the noninnocent BPTTF ligand were investigated using solid-state cyclic voltammetry (CV; Figure S5, SI). The two quasireversible oxidation processes at onset potentials of 0.80 and 1.02 V (vs Fc/Fc+; Fc = ferrocene) were assigned to TTF-based oxidation to its radical-cation state followed by a further oneelectron oxidation to its dication species. These assignments are consistent with the study by Jia et al. for a related dinuclear ruthenium complex based on the bis(dipyrido[3,2-a:2′,3′c]phenazine)-TTF ligand, which also displayed two oxidation processes for the radical-cation and dication species.13 For the ligand BPTTF, the cathodic region was also characterized by one quasi-reversible reduction at −1.14 V and two irreversible

accompanies the increase in electrostatic attraction, as illustrated by the oxidation state assignments.23 The true situation for Ru4 in this complex may be intermediate between integer oxidation numbers (2+/3+), and this will be further discussed in EPR studies. The bite angles of N−Ru−N [average 79.92(8)°] are significantly smaller than the ideal value of 90°. Furthermore, the annulated TTF backbones exhibit quite distinct conformations in the dimer structure, as illustrated in Figure 2. For TTF-A, the charge-sensitive central CC bond

Figure 2. Distinct conformations of BPTTF ligands in 1.

length of 1.318(3) Å (C13−C14) is in agreement with the characteristic lengths of neutral TTF compounds,24 while the mean bond length of the C−S bonds around the central CC bonds is 1.769 Å. The entire molecular structure of BPTTF is nearly planar, with a dihedral angle of 12.26(9)° between the S1−C5−C6−S2 and S3−C20−C19−S4 planes. In contrast to TTF-A, however, the bond lengths of the corresponding CC and C−S bonds for TTF-B are 1.347(3)

Figure 3. (a) Cyclic voltammogram and (b) differential pulse voltammogram of 1 at 100 mV s−1 in a 0.1 M TBAPF6/CH3CN electrolyte. The arrow shows the direction of the forward scan. 4608

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Inorganic Chemistry Table 1. Electrochemical Half-Wave Potentials (V vs Fc/Fc+) for 1 and Reference Compounds oxidation EOx1(TTF/TTF•+)

EOx2(TTF•+/TTF2+)

EOx3(Ru2+/Ru3+)

ERed1(phen/phen•−)

ERed2(bpy/bpy•−)

ref

1 TTFb EdtTTFphenc TTF-dppzb [Ru(bpy)2(phen)](PF6)2a [Ru(tbbpy)2(phen)](PF6)2b [Ru(bpy)2(EdtTTFphen)](PF6)2c [Ru(bpy)2(TTF-dppz)](PF6)2d

0.45 −0.10 0.17 0.29

0.75 0.37 0.52 0.64

0.85

−1.60

−1.98

0.89 0.84 1.04 0.99

−1.61 −1.84 −1.80 −1.54 −1.35

−2.04 −2.08 −1.92 −1.79

this work 29 16 18 30 31 16 18

a

a

reduction

complex

0.26 0.29

0.58 0.61

Measured in CH3CN. bMeasured in CH2Cl2. cMeasured in CH3CN/CH2Cl2 = 3/2. dMeasured in CH3CN/CH2Cl2 = 1/5.

Table 2. Experimental and DFT-Calculated Selected Bond Lengths (Å) and Angles (deg) for Compound 1 X-ray

DFT

TTF-A

bond length (Å)

TTF-B

bond length (Å)

bond length (Å)

C13−C14 C13−S1 C13−S2 C14−S3 C14−S4 C5−S1 C6−S2 C20−S3 C19−S4 C5−C6 C19−C20 N1−Ru1 N2−Ru1 N5−Ru1 N6−Ru1 N7−Ru1 N8−Ru1 N3−Ru2 N4−Ru2 N9−Ru2 N10−Ru2 N11−Ru2 N12−Ru2

1.318(3) 1.825(2) 1.718(3) 1.781(3) 1.751(3) 1.760(3) 1.779(3) 1.753(3) 1.735(3) 1.375(3) 1.392(4) 2.094(2) 2.070(2) 2.042(2) 2.082(2) 2.037(2) 2.174(2) 2.094(2) 2.096(2) 2.035(2) 2.063(2) 2.026(2) 2.035(2)

C111−C112 C111−S5 C111−S6 C112−S7 C112−S8 C103−S5 C104−S6 C118−S7 C117−S8 C103−C104 C117−C118 N13−Ru3 N14−Ru3 N17−Ru3 N18−Ru3 N19−Ru3 N20−Ru3 N15−Ru4 N16−Ru4 N21−Ru4 N22−Ru4 N23−Ru4 N24−Ru4

1.347(3) 1.806(3) 1.773(3) 1.685(3) 1.765(3) 1.760(2) 1.744(2) 1.756(2) 1.780(3) 1.385(3) 1.371(4) 2.080(2) 2.086(2) 2.059(2) 2.071(2) 2.070(2) 2.054(2) 2.064(2) 2.114(2) 1.969(2) 2.032(2) 2.058(2) 1.895(2)

1.350 1.781 1.781 1.781 1.781 1.768 1.768 1.768 1.768 1.374 1.374 2.119 2.120 2.112 2.107 2.112 2.106 2.119 2.119 2.112 2.107 2.107 2.112 DFT

X-ray TTF-A

bond angle [deg]

TTF-B

bond angle [deg]

bond angle [deg]

C14−C13−S2 C14−C13−S1 S2−C13−S1 C13−C14−S4 C13−C14−S3 S4−C14−S3 N5−Ru1−N6 N2−Ru1−N1 N7−Ru1−N8 N11−Ru2−N12 N9−Ru2−N10 N3−Ru2−N4

124.5(2) 118.9(2) 116.31(14) 123.1(2) 122.2(2) 114.31(14) 79.81(8) 80.43(8) 81.48(8) 80.30(8) 78.88(9) 77.56(8)

C112−C111−S6 C112−C111−S5 S6−C111−S5 C111−C112−S7 C111−C112−S8 S7−C112−S8 N20−Ru3−N19 N17−Ru3−N18 N13−Ru3−N14 N21−Ru4−N22 N24−Ru4−N23 N15−Ru4−N16

122.6(2) 125.1(2) 112.15(13) 121.9(2) 120.16(19) 117.88(14) 79.68(8) 79.65(8) 79.99(8) 81.79(9) 80.47(9) 78.98(8)

122.81 122.76 114.40 122.75 122.79 114.43 77.65 78.37 77.60 77.60 77.66 78.37

reductions at onset potentials of −2.34 and −2.50 V for the phen moieties. The electrochemical properties of 1 were measured in an acetonitrile solution, and all potentials are reported versus the Fc/Fc+ couple. The BPTTF complex exhibits three oxidation and two reduction events (Figure 3a). The major oxidation

processes at 0.45 and 0.75 V are also ascribed to the successive oxidations of the neutral TTF core to its radical cation and dication, respectively. The oxidation potentials of the TTF units are shifted anodically in comparison to the corresponding TTF systems in Table 1, as expected because of the electronwithdrawing nature of the phen moieties. The third major 4609

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Figure S7 (SI). The energies of the first five HOMOs for 1•+ show that, except for the one on HOMO−2 with α spin, the other four single electrons can be paired based on their coincident orbitals and degenerate energies. Figure S8 (SI) also illustrates that the net spin distribution of 1•+ originates from the unpaired electron on HOMO−2(α). Because the calculated energy of HOMO−2(α) is only 0.02 eV lower than that of the singly occupied MO with α spin [SOMO(α)], it is reasonable to assume that the unpaired electron on HOMO−2(α) is lost first in the second oxidation step. This small energy gap between MOs on TTF and Ru2+ also explains why the second TTF oxidation process was very close to the oxidation of Ru2+ in the CV and DPV data shown in Figure 3. All of these calculations confirm that the two oxidation processes of the TTF core precede oxidation of Ru2+ in the electrochemical measurement. As shown in Figure 5, the HOMO is delocalized across the whole TTF fragment without any contribution from the metal-centered orbitals. As a result, the electronic communication between the TTF unit and Ru2+ is negligible in solution. The four LUMOs are all located on the phen fragment with a small range in energy of about 0.13 eV, whereas the contributions of LUMO+4 and LUMO+5 are mainly from the bipyridine units. UV/Vis Absorption Spectrum and Spectroelectrochemistry (SEC). Owing to the good solubility upon complexation, the UV/vis spectrum of 1 was recorded in an acetonitrile solution. TD-DFT calculations were performed using the B3LYP and CAM-B3LYP functionals, with the results indicating that the latter overestimated the transition energies for compound 1.33 Thus, we used B3LYP calculations to interpret the UV/vis spectrum. The results of TD-DFT calculations are listed in Table S2 (SI) to better understand the absorption spectrum. Four major absorption bands around 209, 285, 320, and 448 nm were observed (Figure 6). Among

oxidation couple at 0.85 V is likely to correspond to two overlapping one-electron oxidations of Ru2+ to Ru3+ because the potentials are consistent with the oxidation processes observed in similar divalent ruthenium compounds (Table 1). The separation between the potentials of the second and third oxidation processes is small and is only observed by differential pulse voltammetry (DPV; Figure 3b). Two reduction waves observed in the cathodic region (−1.60 and −1.98 V) correspond to the reduction of the phen and bipyridine moieties. The assignments in CV and DPV reflect the redox processes as illustrated in Figure S6 (SI). Theoretical Calculations. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to elucidate the electronic structures of the BPTTF ligand and compound 1. These computational methods have proven indispensible for analysis of ruthenium polypyridyl complexes.32 The comparison between the calculated optimized structure and crystal structures for the TTF-annulated molecule is listed in Table 2. The corresponding frontier molecular orbitals (MOs) for the ligand are shown in Figure 4.

Figure 4. Frontier MOs of the ligand BPTTF.

DFT calculations reveal that the highest occupied (HOMO) and lowest unoccupied (LUMO) MOs are mainly located on the TTF and phen fragments, respectively, similar to other A− D−A-type compounds.11 It is evident that there is an orbital overlapping the HOMO and LUMO, which may facilitate ICT. For compound 1, the energy diagram of selected MOs is shown in Figure 5 and the relevant orbital energies appear in

Figure 5. Selected frontier MOs of compound 1. Hydrogen atoms have been omitted for clarity.

Figure 6. UV/vis spectrum of 1 at room temperature in CH3CN (black line) and calculated TD-DFT transitions in the diruthenium complex (vertical red bars with heights illustrating oscillator strengths).

Table S1 (SI). Theoretical calculations indicate that the contributions of the π-type HOMO and LUMO are similar to the nature of frontier MOs in the ligand BPTTF. The gap between the π-type HOMO, which is mainly centered on the TTF unit, and ruthenium-based HOMO−1 or HOMO−2 is approximately 0.7 eV. The energies of the MOs for the TTF radical cation (1•+) have also been calculated, as shown in

these peaks, the intense absorption bands (around 209 and 285 nm) arise from spin-allowed π−π* transitions. The band around 320 nm is ascribed to ligand-to-ligand charge-transfer transitions. As observed in other ruthenium polypyridyl complexes,16,19 metal-to-ligand charge-transfer (MLCT) transitions between dπ(Ru) → π*(bpy) and dπ(Ru) → π*(phen) are centered around 448 nm, while a low-energy tail that 4610

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Figure 7. Spectroelectrochemical data of 1 in a 0.1 M TBAPF6/CH3CN electrolyte over the potential ranges of (a) 0−1.15 and (b) 1.15−1.50 V. The arrows show the direction of spectral change.

Figure 8. X-band (9.8 GHz) EPR spectra of (a) the ligand BPTTF and (b) 1 in the solid state at room temperature.

oligomers during the oxidation processes of 1 owing to the bulky octahedral coordination sphere of the terminal Ru2+ centers, as observed in another dinuclear ruthenium complex.13 Following SEC oxidation, an attempt to regain the starting complex revealed approximately 90−95% conversion (Figure S10, SI), suggesting that the SEC processes proceeded without appreciable chemical degradation of the sample. The potentials applied to the SEC cell are greater than those applied during CV because of the higher cell resistance. Solid-state UV/vis/near-IR (NIR) spectrometry of the ligand BPTTF and complex 1 was performed to confirm the presence of charge-transfer processes in the solid state (Figure S11, SI). For the ligand BPTTF, a band observed around 15000 cm−1 is characteristic of a TTF radial cation. For complex 1, a series of broad MLCT bands were also observed in the visible region of the spectrum. EPR Spectra and SEC. X-band EPR measurements for both the ligand and metal complex were performed at room temperature to confirm whether intramolecular electron transfer exists in the solid state. The EPR signal due to the ligand-based processes of BPTTF shows a sharp peak at g = 2.009, which is in accordance with the characteristic EPR peak of the radical cation TTF•+ (Figure 8a),34 the presence of which was also evident from the UV/vis spectrum of the ligand in the solid state (Figure S11, SI). The signal indicates that there is an ILCT process in the ligand BPTTF, which can be explained on the basis of the shape of the frontier MOs (Figure

extends above 520 nm corresponds to the intraligand chargetransfer (ILCT) excitation from the HOMO [π(TTF)] to π* on the phen units. SEC was employed in order to better understand the change in the electronic behavior as the redox states of compound 1 were manipulated (Figure 7). A potential bias over the range 0−1.15 V was applied to 1 dissolved in a 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 )/ CH3CN electrolyte. A gradual change was observed, whereby new bands developed at 15000 and 30000 cm−1, in agreement with generation of the TTF radical-cation species (Figure S9, SI), for dinuclear ruthenium complexes incorporating TTFbased ligands.13 As the potential was increased anodically from 1.15 to 1.50 V, two new bands around 32500 cm−1 increase in intensity because of formation of the TTF dication species; however, an increase in intensity at 15000 cm−1 also suggests the presence of a higher concentration of the TTF radical cation. Close inspection of the cyclic voltammogram reveals that these two oxidation processes occur reasonably close to one another (0.45 and 0.75 V vs Fc/Fc+ for TTF/TTF•+ and TTF•+/TTF2+, respectively), and in addition to the slower time scale of the SEC measurement compared to CV, it is not unexpected that both the radical-cation and dication species are simultaneously observed as the system approaches equilibrium. During SEC oxidation of 1, the MLCT transition at 22000 cm−1 also diminishes in intensity because of oxidation of Ru2+ to Ru3+. There is no evidence for π−π-bonded dimers or 4611

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that the radical generated at room temperature disappears upon application of a reductive potential and reversibly reforms upon application of an oxidizing potential. Variable-temperature measurements in the solution state were also undertaken to further interrogate the diruthenium complex. The EPR spectrum of the complex retains a sharp signal in solution upon reduction of the temperature to 240 K. The signal at 240 K is broader and of a higher intensity than that observed at room temperature owing to the reduced rotational movement of the molecule, where the anisotropy of the system starts to become apparent (Figure S12a, SI). When the experiment was performed at 210 K, the electrolyzed species were generated at room temperature before being flashfrozen and inserted into the spectrometer. The frozen solution EPR spectrum of the radical cation of TTF displays rhombic anisotropy (g values: 2.0027, 2.0065, and 2.0150), indicating that the environment where the electron is located has low symmetry. The assignments for the EPR signals have been verified by spectral simulation (Figure S12b and Table S3, SI) No EPR response was observed in the solution state at room temperature. The compound 1 exists as single dinuclear complex molecular form in solution, and all of the oxidation states of the ruthenium atoms are 2+, as verified by the abovementioned measurements (ESI-MS, 1H NMR, CV, and DPV). This is due to the strong electrostatic repulsion of the two adjacent positive charges. Nevertheless, dimeric species are stabilized by the lattice because of the relatively strong intermolecular interactions derived from shorter π···π and S··· S contacts. When the small anion PF6 is used, a reversible dimerization from the solution to solid state occurs, as demonstrated in the crystal structure.37 Scheme 2 provides a detailed description of electronic structural forms in their different states for compound 1.

4). Compared to the ligand, the ruthenium complex, however, exhibits not only a signal for the phen-based radical anion (g = 2.003) but also a typical broad peak for ruthenium(III) [g⊥(g1,g2) = 2.133 and g∥(g3) = 1.992; Figure 8b], which is typical behavior of [Ru(bpy)2(L)] complexes involving partial metal-based oxidation and predominantly ligand-centered reduction.35 The g-factor anisotropy, as calculated by g1 − g3 = 0.141, is located between that of a genuine ruthenium(III) species (>0.5) and the typical values for radical complexes of ruthenium(II) ( 2σ(I)]a R1, wR2 (all data)a a

C98H108F24N12P4Ru2S4 2364.22 monoclinic C2/c 44.902(9) 40.884(8) 38.996(8) 90 95.497(3) 90 71257(25) 16 0.882 291(2) 0.308 0.67−28.00 19328 −56 ≤ h ≤ 56 −54 ≤ k ≤ 54 −51 ≤ l ≤ 34 85473/0/2639 1.037 0.0507, 0.1209 0.0770, 0.1269

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

Synthesis of Bis(1,10-phenanthro[5,6-b])tetrathiafulvalene (BPTTF). To a solution of 1,3-dithiole-2-ono[4,5-f ][1,10]phenanthroline (0.73 g, 2.7 mmol) in 1,2-dichlorobenzene (20 mL) was added triethylphosphite (9 mL). The reaction mixture was stirred at 150 °C for 12 h under a nitrogen atmosphere. After cooling to room temperature, the solution was filtered and the precipitate was washed with methanol, giving the product as a yellow solid. Yield: 56.5%. IR (KBr, cm−1): 1651(w), 1594(s), 1493(m), 1415(s), 915(m), 791(s), 731(s). MS (MALDI-TOF, CH3OH): 508.85 [M+]. Anal. Calcd for C26H12N4S4: C, 61.39; H, 2.38; N, 11.01; S, 25.22. Found: C, 61.24; H, 2.43; N, 11.13; S, 24.95. 4613

DOI: 10.1021/acs.inorgchem.6b00437 Inorg. Chem. 2016, 55, 4606−4615

Article

Inorganic Chemistry Synthesis of [{Ru(tbbpy) 2 } 2 (BPTTF)](PF 6 ) 4 (1). cis-[Ru(tbbpy)2Cl2] (0.284 g, 0.4 mmol) and the BPTTF ligand (0.102 g, 0.2 mmol) were suspended in ethylene glycol/acetone (80 mL/5 mL). The mixture was reacted under microwave irradiation to form a deeppurple solution (microwave setup: 5 min, 125 °C; 6 h, 125 °C). After the solution was cooled to room temperature, it was filtered off from the unconverted BPTTF. Following evaporation to remove acetone, the saturated aqueous solution of NH4PF6 (0.800 g) was added to the filtrate, resulting in a reddish-brown precipitate. The solid was purified using neutral alumina chromatography changing from CH2Cl2 to CH2Cl2/CH3OH (100:1). Yield: 42.3%. 1H NMR (500 MHz, acetonitrile-d3, ppm): δ 1.36 (CH3-tert-butyl, 36H, s), 1.44 (CH3tert-butyl, 36H, s), 7.22 (H5, 4H, dd, J = 6.0, 2.0 Hz), 7.42 (H6, 4H, d, J = 6.0 Hz), 7.46 (H5′, 4H, dd, J = 6.1 and 2.1 Hz), 7.66 (H6′, 4H, d, J = 6.0 Hz), 7.79 (Hm, 4H, dd, J = 8.4 and 5.3 Hz), 8.07 (Ho, 4H, dd, J = 5.3 and 1.1 Hz), 8.33 (Hp, 4H, dd, J = 8.3 and 1.1 Hz), 8.47 (H3, 4H, d, J = 2.0 Hz), 8.51 (H3′, 4H, d, J = 2.1 Hz). M (C98H108F24N12P4Ru2S4) = 2364.24 g mol−1. MS (ESI, CH3OH): m/z 446.67 ([M − 4PF6]4+, 100%), 643.42 ([M − 3PF6]3+, 93%), 1037.58 ([M − 2PF6]2+, 45%). IR (KBr, cm−1): 2962(m), 1615(m), 1540(w), 1482(m), 1415(m), 1203(w), 837(s), 557(m). Anal. Calcd for C98H108F24N12P4Ru2S4: C, 49.79; H, 4.60; N, 7.11; S, 5.43. Found: C, 49.69; H, 4.43; N, 7.04; S, 5.32.

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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.6b00437. MALDI-TOF-MS spectrum of BPTTF, ESI-MS spectrum of 1, 1H NMR spectrum of 1, schematic diagram of the dimer in 1, solid-state CV and SEC of BPTTF, DFT calculation results, and EPR spectra (PDF) X-ray crystallographic files in CIF format for 1 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (Grant 2013CB922101), the National Natural Science Foundation of China (Grant 91433113), the Natural Science Foundation of Jiangsu Province (Grant BK20130054), and the Australian Research Council (Grant DP110101671). We also acknowledge support from the EPSRC National UK EPR Research Facility and Service at the University of Manchester.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00437 Inorg. Chem. 2016, 55, 4606−4615