Diruthenium Complexes with Bridging Diethynyl Polyaromatic Ligands

Article pubs.acs.org/Organometallics

Diruthenium Complexes with Bridging Diethynyl Polyaromatic Ligands: Synthesis, Spectroelectrochemistry, and Theoretical Calculations Jing Zhang,† Ming-Xing Zhang,† Chao-Fang Sun,† Meng Xu,† František Hartl,*,‡ Jun Yin,† Guang-Ao Yu,† Li Rao,*,† and Sheng Hua Liu*,† †

Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China ‡ Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K. S Supporting Information *

ABSTRACT: This work describes syntheses and electrochemical, spectroscopic, and bonding properties in a new series of dinuclear ruthenium(II) complexes bridged by polyaromatic (biphenyl, fluorene, phenanthrene, and pyrene) alkynyl ligands. Longitudinal expansion of the π-conjugated polyaromatic core of the bridging ligands caused a reduced potential difference between the anodic steps and reinforced their bridge-localized nature, as evidenced by UV/vis/near-IR and IR spectroelectrochemical data combined with DFT and TDDFT calculations. Importantly, the intricate multiple IR ν(CC) absorption bands for the singly oxidized states imply a thermal population of a range of conformers (rotamers) with distinct electronic character. This behavior was demonstrated with more accurate DFT calculations of selected nontruncated 1e− oxidized complexes in three different conformations. The combined experimental and theoretical data reveal that thermally populated rotamers featuring various mutual orientations of the ligated metal termini and the bridging diethynyl polyaromatic moieties have a significant impact on the electronic absorption and ν(CC) wavenumbers of the singly oxidized systems.

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INTRODUCTION Since the first report of the Creutz−Taube ion [(H3N)5Ru(μpyrazine)Ru(NH3)5]5+, there have been numerous experimental and theoretical attempts to elaborate the properties of related symmetrical diruthenium complexes due to the surprising stability of the RuIIRuIII mixed-valence states.1 It has been well documented that diruthenium complexes with metal centers linked by π-conjugated bridges allow for facile electron transfer along the whole molecular framework. Accordingly, pertinent studies have concentrated on elaborately selecting the conjugated bridging ligands and tuning the electronic effect of the ancillary ligands to subtly modulate the intramolecular electron-transfer properties in mixed-valence complexes.2,3 We have focused on dinuclear ruthenium complexes with a particular type of redox-active terminals, viz., (P,P′-dppe)(η5-C5Me5)Ru, which are interconnected by the conjugated bridge participating in the redox behavior. These complexes may become attractive candidates for molecular wires.4 Furthermore, a range of studies in our laboratory as well as other literature reports have convincingly elucidated that the horizontal scaling of π-conjugated bridges results in poor solubility and chemical stability of the oxidized species and an attenuation of the electronic transport in a mixed-valence system; introducing aromatic rings such as benzene or a heterocycle in the spacer can constitute an © XXXX American Chemical Society

attractive alternative to improve this instability and eventually to tune their physical or even chemical properties.5 Therefore, a reasonable control of the length and appropriate extension of the conjugation in the bridging ligands can afford improved stability and allow for efficient tuning of the electron-transfer properties.6 In addition, numerous studies have indicated that fluorene, phenanthrene, and pyrene exhibiting a highly rigid conjugated character have emerged as promising organic materials for use in nanoelectronics, electron transport materials, organic semiconductors, and organic electronic devices (organic light-emitting diodes, organic field-effect transistors, organic photovoltaic devices).7 However, investigations of these compounds acting as bridging ligands in organometallic and inorganic systems have been scarce in recent years.3c,8 Sparked by the aforesaid description, the appealing possibilities driven by the integration of the “(P,P′-dppe)(η5C5Me5)Ru” redox-active termini with the strong conjugation offered by specific polyaromatic ligands attracted our interest. Therefore, we have synthesized a series of dinuclear ruthenium complexes (Chart 1) bridged by polyaromatic (biphenyl, fluorene, phenanthrene, and pyrene) alkynyl ligands. These Received: March 31, 2015

A

DOI: 10.1021/acs.organomet.5b00276 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1. Studied Series of Diethynyl Polyaromatic Ligand-Bridged Diruthenium Complexes 1a−1d

Scheme 1a

a Reagents and conditions: (a) TMSA, [Pd(PPh3)4], CuI, THF/Et3N; (b) [RuCp*(dppe)Cl], KF, MeOH/THF. TMSA = trimethylsilylacetylene, dppe = 1,2-bis(diphenylphosphino)ethane, Cp* = pentamethylcyclopentadiene.

the spectral features, with a focus on the singly oxidized cationic species.

compounds were studied by electrochemistry, infrared and UV−vis−NIR spectroelectrochemistry, and density functional theory (DFT) and time-dependent (TD)-DFT calculations. Following the description of analogous aromatic dibenzoheterocycle systems in our previously reported work,4f our primary goal in the present work is the study of the diethynyl bridging ligand featuring four gradually expanding aromatic cores, keeping the (P,P′-dppe)(η5-C5Me5)Ru termini identical for the ease of direct comparison of electrochemical and spectral properties. Furthermore, the heteroaromatic systems exhibited a complicated and at the time poorly understood IR spectral behavior along the anodic exploration,4f and we anticipated the existence of the same effects in these diethynyl polyaromatic systems. Recently, a series of studies have revealed that various mutual orientations of the ligated metal termini and the bridging moieties have a significant impact on the charge delocalization and localization in the formally mixedvalence systems.6,9 To make a breakthrough, we further employed the elaborate rotational model introduced in the meantime by Kaupp, Low, Costuas, Lapinte, and co-workers6,9 in the DFT calculations to make a reasonable interpretation of

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RESULTS AND DISCUSSION Synthesis and Characterization. The general synthetic route to dinuclear ruthenium complexes 1a−1d is outlined in Scheme 1. Bridge precursors 4a−4d were obtained in 38−77% yields, using Pd/Cu-catalyzed Sonogashira coupling reactions between 4,4′-diiodo-1,1′-biphenyl, 2,7-dibromo-9H-fluorene, 2,7-dibromophenanthrene, 2,7-dibromopyrene, and (trimethylsilyl)acetylene, respectively. Subsequently, the TMS termini of compounds 4a−4d were deprotected in methanolic KF solution and reacted with [RuCl(dppe)Cp*] at 60 °C.3a The target complexes 1a−1d were obtained by filtration. Despite the different aromatic parts of the bridging ligands, the resonance signals of complexes 1a−1d such as those of dppe in the 1H and 31P NMR spectra and Ru−CC in the 13C NMR spectra have revealed no significant differences. Crystal Structures of Complexes 1c and 1d. Compounds 1c and 1d have been characterized by single-crystal Xray crystallography10 (Table S1, Supporting Information). Selected bond lengths (Å), and Ru···Ru atomic distances (Å), B


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Organometallics

Table 1. Selected Bond Lengths (Å), Angles (deg), and Interatomic Distances (Å) from the Crystal Structure of 1c, the DFTOptimized Truncated Structures of [1c-H]n+ (n = 0, 1, 2), and the Nontruncated Structure of trans-[1c]+ Ru(1)C(37) Ru(1)P(1, 2) C(37)C(38) C(38)C(39) C(39)C(40) C(40)C(41) C(41)C(42) C(39)C(44) C(42)C(42) C(42)C(43) C(43)C(44) C(43)C(46) C(45)C(46) Ru···Ru P(1)Ru(1)P(2) Ru(1)C(37)C(38) C(37)C(38)C(39)

1c

[1c-H]

[1c-H]+

trans-[1c]+

[1c-H]2+

1.983(7) 2.244(19), 2.263(17) 1.201(8) 1.491(6) 1.390 1.390 1.390 1.390 1.457(10) 1.390 1.389(4) 1.332(8) 1.343(9) 16.363 83.70 178.79 178.88

2.016 2.277, 2.228 1.228 1.423 1.396 1.410 1.429 1.396 1.454 1.429 1.410 1.439 1.358 16.504 93.04 178.44 178.47

1.972 2.299, 2.301 1.239 1.401 1.432 1.371 1.427 1.409 1.436 1.439 1.398 1.442 1.355 16.367 92.20 178.12 178.24

1.921 2.275, 2.281 1.202 1.362 1.361 1.354 1.386 1.384 1.388 1.378 1.326 1.394 1.312 15.864 82.99 173.3 178.0

1.934 2.320, 2.321 1.251 1.380 1.442 1.364 1.438 1.424 1.418 1.450 1.387 1.445 1.352 16.255 90.84 177.96 177.97

Ru units are coplanar for two solid-state structures. This orientation permits an optimum overlap between the π-orbitals of the ethynyl and aromatic parts of the bridge, which is further in favor of electron transport through the whole system. The Ru−C and Ru−P distances are in the normal ranges, and the CC bond lengths [1.201 (1c) and 1.198 (1d) Å] are typical for carbon−carbon triple bonding. All of the P−Ru−P angles are analogous and in full accord with values for a range of acetylide complexes reported previously.3g,11 In cationic derivatives [1c-H]+, [1c-H]2+ and [1d-H]+, [1d-H]2+, the Ru−P distances [2.30−2.32 Å] are somewhat longer compared to those [2.23−2.28 Å] found in neutral complexes [1c-H] and [1d-H], respectively, which is probably due to decreased backbonding from the partially oxidized metal center to the phosphine ligand. These changes induced by the stepwise 1e− oxidation include a slight expansion of the coordination spheres around the Ru centers as well as slight shortening of the Ru−C bonds due to an increasing contribution from the RuCC bonding. In addition, we have compared the bond lengths, angles, and interatomic distances (Ru···Ru) for the truncated structure [1c-H]+ and nontruncated trans-[1c]+ and found that all of the values for trans-[1c]+ are smaller than those observed for the truncated [1c-H]+ just as the comparison between [1bH]+ and trans-[1b]+, which embodies the importance of the steric interactions between the bridging ligand and the RuCp*(dppe) fragment. Electrochemistry. Cyclic and square-wave voltammograms (CVs and SWVs) of complexes 1a−1d are shown in Figure S2. Generally, complexes 1a−1d displayed two reversible anodic waves in dichloromethane solutions containing 10−1 M nBu4NPF6 as the supporting electrolyte. The redox potentials of the two consecutive one-electron oxidations are summarized in Table 3. Reference CV plots recorded for the corresponding TMS-terminated bridging ligands 4a−4d (Figure S1) show no redox waves in the potential interval −1.0 to +1.0 V (vs Ag/ Ag+). Notably, the first oxidation of analogous ruthenium alkynyl complexes was shown to occur within −0.3 to +0.2 V (vs Ag/Ag+).4a,e The studied ruthenium alkynyl complexes 1a− 1d (vide infra) fall within this anodic potential interval, too. The first oxidation potentials decrease in the following sequence: −0.056 V (1d), −0.061 V (1a), −0.068 V (1c),

and bond angles (deg) from the crystal structures and the DFToptimized truncated structures of [1c-H]n+ (n = 0, 1, 2) and nontruncated structure of trans-[1c]+ and [1d-H]n+ (n = 0, 1, 2) are listed in Tables 1 and 2, respectively. The analogous Table 2. Selected Bond Lengths (Å), Angles (deg), and Interatomic Distance (Å) from the Crystal Structure of 1d and the DFT-Optimized Structures of [1d-H]n+ (n = 0, 1, 2) 1d Ru(1)C(37) Ru(1)P(1, 2) C(37)C(38) C(38)C(39) C(39)C(40) C(40)C(41) C(41)C(42) C(39)C(44) C(42)C(42) C(42)C(43) C(43)C(44) C(41)C(46) C(43)C(45) C(45)C(46) Ru···Ru P(1)Ru(1)P(2) Ru(1)C(37)C(38) C(37)C(38)C(39)

2.005(5) 2.250(14), 2.269(12) 1.198(6) 1.441(6) 1.382(6) 1.394(6) 1.418(6) 1.395(7) 1.421(8) 1.420(6) 1.392(6) 1.435(6) 1.438(6) 1.349(6) 16.336 83.71 179.08 176.94

[1d-H]

[1d-H]+

[1dH]2+

2.016 2.278, 2.278 1.228 1.425 1.410 1.400 1.430 1.410 1.424 1.430 1.400 1.443 1.443 1.360 16.436 93.04 178.53 178.10

1.973 2.297, 2.302 1.239 1.403 1.419 1.391 1.438 1.420 1.412 1.438 1.391 1.445 1.445 1.357 16.331 92.16 178.31 177.97

1.941 2.315, 2.314 1.251 1.383 1.431 1.382 1.447 1.432 1.399 1.447 1.382 1.448 1.448 1.355 16.218 91.02 177.92 177.66

series [1a-H]n+ (n = 0, 1, 2) and [1b-H]n+ (n = 0, 1, 2) are also displayed in Tables S2 and S3 (Supporting Information), respectively. The extension “-H” indicates that the η5-C5Me5 and dppe ligands were replaced by η5-C5H5 and two PH3 ligands, respectively, in 1a−1d. The crystal structures of 1c and 1d (Figure 1) feature approximately linear Ru(1)−C(37)− C(38)−C(39) moieties with the Ru−CC and CC−C angles being 178.79°, 179.08° and 178.88°, 176.94°, respectively. As is shown in Figure 1, we can see that the Cp* and dppe ligands and C(37) complete a pseudo-octahedral environment at each metal center. The aryl spacer, ethynyl, and C


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Figure 1. Molecular structures of 1c and 1d showing the atom-labeling scheme. Hydrogen atoms have been removed for clarity.

Table 3. Electrochemical Data for Complexes 1a−1da complex

E1/2(1) (V)

E1/2(2) (V)

ΔE1/2 (mV)

1a 1b 1c 1d

−0.06 −0.13 −0.07 −0.06

0.01 0.00 0.05 0.06

70 130 120 120

the charges are localized over a relatively small number of atoms.12−14 Compared with the analogous diethynyl heteroaromatic system in our previously reported work,4f the ΔE1/2 values of 1a−1d are smaller than those observed for the diruthenium complexes with the dibenzoheterocyclic bridge cores containing nitrogen (150 mV), oxygen (139 mV), and sulfur (132 mV) atoms in the central ring. We assume that the observed difference is caused by the more localized oxidation of the larger conjugated polyaromatic system and less involved Ru moieties compared to the heteroaromatic system. In particular, the bridge oxidation is especially prominent for 1d, in line with the heavy localization of its frontier orbitals on the pyrene ring in the DFT calculations (vide infra). Apparently, the longitudinal expansion of the π-conjugated polyaromatic core in the bridging ligands is favorable for the localized oxidation of the bridge. UV−Vis−NIR and IR Spectroelectrochemistry. We further conducted spectroelectrochemical experiments by using an OTTLE cell to gain deeper insight into the stepwise oxidation of 1a−1d. UV−vis−NIR spectral changes for reversibly oxidized complexes 1b and 1c are shown in Figures 2 and 3, respectively, and the corresponding IR spectra of the neutral parents and oxidized mono- and dications in Figure 4. Figures S3 and S4 (Supporting Information) display spectral changes accompanying the stepwise oxidation of complexes 1a,

b

a Potential data were determined in CH2Cl2 containing 1 mM compound and 10−1 M n-Bu4NPF6; a Ag/Ag+ electrode (internal solution of 10−2 M AgNO3 + 10−1 M n-Bu4NPF6 in acetonitrile; salt bridge 10−1 M n-Bu4NPF6 in CH2Cl2) was used as a reference; the E1/2 value of the ferrocene/ferrocenium (Fc/Fc+) couple was found at +0.23 V under these conditions. bΔE1/2 = |E1/2(1) − E1/2(2)| denotes the potential difference between the individual one-electron oxidation processes at 100 mV s−1.

and −0.128 V (1b). This trend is well reproduced by the DFTcalculated HOMO energies (vide infra). Inversely, the separation of the anodic waves, ΔE1/2, for complexes 1a−1d decreases from 1b (127 mV) over 1c (122 mV) and 1d (120 mV) to 1a (68 mV). The ΔE1/2 value for 1b is larger than that for 1a, as the rigid fluorenyl moiety is more suited for electron delocalization than the free-rotating biphenyl unit.3c However, the ΔE1/2 values for the larger conjugated π-systems 1c and 1d are slightly smaller. In general, electronic coupling generally makes only a certain extent of contribution to ΔE1/2 and electrostatic contributions can be important in systems where D


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Figure 3. UV−vis−NIR spectral changes recorded during the oxidation 1c → [1c]+ (top) and [1c]+ → [1c]2+ (bottom) in CH2Cl2/10−1 M n-Bu4NPF6 at 298 K within an OTTLE cell.

Figure 2. UV−vis−NIR spectral changes recorded during the oxidation 1b → [1b]+ (top) and [1b]+ → [1b]2+ (bottom) in CH2Cl2/10−1 M n-Bu4NPF6 at 298 K within an optically transparent thin-layer electrochemical (OTTLE) cell.

DFT calculations on three different rotamers of 1b+ and 1c+ (vide infra). IR spectroelectrochemistry is a powerful tool for the investigation of structural changes on a relatively short time scale as a function of the redox state. Neutral complexes 1a−1d show single ν(CC) bands in the IR region near 2060 cm−1 (Figure 4, Figures S5, Supporting Information). The characteristic ν(CC) absorptions provide suitable spectroscopic probes to evaluate the contributions of metal centers and bridging ligands to redox processes. The vibrational frequencies of the triple bond (CC) for the stable oxidized species [1a]n+−[1d]n+ (n = 0, 1, 2) were collected using the OTTLE cell at room temperature in parallel with the reversible (isosbestic) UV−vis−NIR spectral changes. The relevant data are depicted in Figure 4, Figure S5 (Supporting Information), and Table 5. Upon the first oxidation to [1a]+−[1d]+, the original single ν(CC) absorptions at around 2060 cm−1 associated with the neutral parent species were replaced by two to five new absorption bands with additional shoulders shifted to smaller wavenumbers, similar to previously reported diethynyl heteroaromatic singly oxidized systems.4f This intricate appearance of the IR spectra may reasonably be ascribed to the population of local states on the potential-energy landscape with bridge core-localized and more delocalized electronic structures in fluid solution, reflecting the coexistence of several accessible conformers (rotamers) that differ from each other with respect to the relative orientations of the diethynyl aromatic-bridge plane with respect to Ru orbitals and ancillary coligands.6,17,22 This explanation is supported by calculations of

1c, and 1d. The electronic absorption spectral data are collected in Table 4. Intense absorption bands for neutral complexes 1a−1d in the UV region arise mainly from intraligand-centered transitions mixed with some MLCT contributions (Figures 2 and 3 and Figures S3 and S4, Supporting Information).3a,4,15−19 Upon gradual 1e− oxidation of neutral complexes 1a−1d to [1a]+− [1d]+, the UV bands gradually decreased in amplitude. At the same time, a moderately intense broad band appeared in the visible region between 20 000 and 15 000 cm−1 (500−700 nm), which is attributed to transitions of an LMCT (from ancillary ligand to metal) and MLCT (from metal to bridging ligand) nature on the basis of TDDFT calculations (see below). Furthermore, two distinct near-IR absorptions can be observed for [1a]+−[1d]+ at 14 000−12 000 and 6000 cm−1. TDDFT calculations (vide infra) have revealed their prevailing MLCT character. On further oxidation yielding dicationic species [1a]2+−[1d]2+, a new characteristic absorption band in the visible region arose at ca. 12 000 cm−1 (800 nm), belonging to MLCT and LMCT transitions (vide infra). Apparently, the broad NIR absorption characteristic for the monocations is absent for [1a]2+−[1d]2+. On the basis of the above data, the distinctive NIR absorption in the studied series of 1e−oxidized complexes can reasonably be referred to the participation of the π-conjugated diethynyl polyaromatic bridge;4c,4i,20,21 and this assignment has indeed been specifically corroborated by TDE


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Figure 4. IR spectra recorded in the ν(CC) region for complexes 1b and 1c in different oxidation states (0, +1, +2) generated in CH2Cl2/10−1 M n-Bu4NPF6 within an OTTLE cell at 298 K.

redox noninnocent character of the whole bridging ligands. The dominant bridge oxidation has also been reported for closely related systems.3a,4b,d,f,11d,23 DFT and TD-DFT Calculations. A theoretical study was conducted in order to shed light on the electronic characters of [1a]n+−[1d]n+ (n = 0, 1, 2). We performed DFT calculations using the B3LYP/6-31G* ([1a-H]n+−[1d-H]n+, n = 0, 1, 2) and BLYP35/6-31G* (for [1b]+ and [1c]+) model chemistry on representative model complexes, which has been used for similar systems in the literature.9,15,22−25 B3LYP calculations were carried out on truncated models of [1a-H]−[1d-H], [1aH]+−[1d-H]+, and [1a-H]2+−[1d-H]2+, in which η5-C5Me5 (Cp*) and dppe were replaced by η5-C5H5 (Cp) and two PH3 ligands. Figure 5 and Figures S6−S8 (Supporting Information) show the selected frontier orbitals of [1a-H]−[1d-H]. Lists of frontier molecular orbital energies and compositions for [1aH]n+−[1d-H]n+ (n = 0, 2) from Mulliken analysis are provided in Tables S4−S11 (Supporting Information), respectively. Selected frontier molecular orbitals in two-electron-oxidized species [1a-H]2+−[1d-H]2+ are displayed in Figure 6 and Figures S9−S11 (Supporting Information). For neutral [1a-H]−[1d-H], DFT calculations indicate that the HOMOs are delocalized across the entire metal−organic πsystem with contributions of 18−24% ([1a-H]−[1d-H]) from the ruthenium atoms and 70−76% ([1a-H]−[1d-H]) from the conjugated bridging moieties. In contrast to the delocalization in the HOMOs for [1a-H]−[1c-H], the LUMO and LUMO+1 orbitals for [1a-H]−[1c-H] are localized on the metal moieties, with no significant contribution from the bridging ligands. However, the LUMOs of the pyrene derivatives [1d-H] are exclusively resident on the pyrene ring, whereby the unoccupied metal fragment-centered orbital lies somewhat higher in energy.17 For the dicationic species [1a-H]2+−[1d-H]2+, the HOMOs are delocalized across the whole molecular framework including metal centers, ethynyls, and aromatic ring systems. The LUMOs exhibit basically the same constitution as the HOMOs in neutral species, being heavily weighted on the bridging ligands ([1a-H]2+−[1d-H]2+: 64−71%), with only a minor involvement of the metal centers. Taken as a whole, these DFT-calculated data make clear that the ground electronic states of [1a-H]2+−[1d-H]2+ feature appreciable

Table 4. UV−Vis−NIR Electronic Absorption of Diruthenium Complexes 1a−1d in Various Oxidation States (0, +1, +2) νmax (cm−1) (εmax (dm3 mol−1 cm−1))

complex 1a [1a]+ [1a]2+ 1b [1b]+ [1b]2+ 1c [1c]+ [1c]2+ 1d [1d]+ [1d]2+

25 906 26 041 27 173 24 630 24 630 11 737 24 509 24 875 12 165 29 239 29 940 12 165

(56 900) (35 500), (27 050), (87 000) (41 900), (12 950) (82 000) (43 650), (14 000) (11 280) (88 250), (18 750)

18 656 (7400), 13 297 (4400), 6506 (3200) 12 722 (9000) 18 382 (16 900), 12 315 (4400), 5586 (5835)

18 796 (13 800), 12 468 (4550), 6369 (3636)

19 305 (8650), 14 204 (3250), 6219 (2750)

Table 5. Spectroelectrochemically Determined ν(CC) Wavenumbers (cm−1) for [1a]n+−[1d]n+ complex [1a]n+ [1b]n+ n+

[1c]

[1d]n+

n=0 2067 (s) 2061 (s) 2058 (s) 2057 (s)

n=1

n=2

2059 (m), 2040 (s), 2018 (s), 1968 (m), 1929 (s) 2013 (vs), 1943 (vs)

1927 (s)

2035 (s), 2023 (s), 1930 (m)

1973 (vw), 1929 (m) 1973 (w), 1934 (m)

2031 (s), 1975 (m), 1935 (m)

1924 (m)

the ν(CC) wavenumbers on the DFT-optimized nontruncated cationic conformers in the following section. Subsequent oxidation to [1a]2+−[1d]2+ was accompanied by collapse of the multiple ν(CC) band pattern. The resulting IR spectra of the dications typically exhibit a medium to high intensity ν(CC) band lying slightly lower in energy than the lowest band of the cationic precursor (Table 5). On the whole, the changes in the ν(CC) bands are similar during the oxidation of all the studied systems; however, a closer examination of the spectral alterations shows clear differences in the ν(CC) band patterns for the specific aromatic cores. The substantial shifts of the ν(CC) bands along the two-step oxidation of 1a−1d clearly document that the ethynyl moieties are also involved in the redox processes, further manifesting the F


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Figure 5. Selected frontier molecular orbitals of complexes [1b-H] plotted with contour values of ±0.04 (e/bohr3)1/2.

Figure 6. Selected frontier molecular orbitals of complexes [1b-H]2+ plotted with contour values of ±0.04 (e/bohr3)1/2.

consistent with the experimental data (Δν = 137 and 129 cm−1 for 2e− oxidations of 1b and 1c, respectively). Notably, thermal population of a range of conformers with distinct (localized vs delocalized) electronic character has been indicated by the multiple IR ν(CC) absorption bands of [1a]+−[1d]+ observed with IR spectroelectrochemistry (vide supra), in line with the literature data for similar systems.6,9,22 The BLYP35 method reported by Kaupp et al.9,22 considering conformational dynamics has been successfully used to simulate the important characteristics of NIR and IR spectra for mixedvalence systems, especially the partially valence-localized electronic structures. Therefore, this method has also been an

contributions from the bridging linkers, which provides further evidence for the conclusions drawn from the aforementioned spectral data. We also calculated the ν(CC) wavenumbers (B3LYP: scaled by 0.9614; see the Computational Details) for neutral [1b-H] (2119 cm−1) and [1c-H] (2121 cm−1) and for the dications [1b-H]2+ (1983 cm−1) and [1c-H]2+ (1982 cm−1), and a single CC stretch for all the species was observed. In addition, we evaluated the relative shifts of the ν(CC) wavenumbers upon the 2e− oxidations of [1b-H] and [1c-H]: the relative shifts Δν of the main ν(CC) bands for both [1bH]n+ and [1c-H]n+ (n = 0, 2) are around 140 cm−1, which is G


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Organometallics

[1c]+: 2112, 2165 cm−1, scaled by 0.95; see the Computational Details), which synergistically contribute to the main intense ν(CC) features in the experimental IR spectra (Table 5). Additional shoulders perceptible in the IR ν(CC) profiles of the cationic complexes point to the existence of other rotamers observable on the IR spectroscopic time scale, which have not been calculated in this work. In the TD-DFT calculations (BLYP35), the fully optimized structures of [1b]+ all mainly exhibit an intense transition at around 5000 cm−1 and two weaker bands between 6000 and 9000 cm−1 (trans-[1b]+: 5023, 8621, and 8803 cm−1; cis-[1b]+: 4995, 8591, and 8711 cm−1; perp-[1b]+: 5531, 6920, and 8223 cm−1, Table 6), i.e., close to the broad absorption maxima in the NIR region of the experimental electronic absorption spectra (Table 4). In all the rotamer cases, the three absorptions correspond to the β-HOSO → β-LUSO, βHOSO-1 → β-LUSO, and β-HOSO-2 → β-LUSO excitations, respectively, all having largely MLCT characters (Figure S12). Similar to [1b]+, the TD-DFT results for the three conformers of [1c]+ show two excitations near 8000 (dominated by perprotamer absorption) and one near 4000 cm−1, well matching the main NIR features in the experimental spectrum (Table 4); they can again be assigned to the β-HOSO → β-LUSO, βHOSO-1 → β-LUSO, and β-HOSO-2 → β-LUSO transitions with prominent MLCT characters (Figure S13). The combined electronic absorption of the three investigated conformers no doubt accounts for the broad and flat absorptions in the NIR region characteristic for the experimental electronic absorption spectra of [1b]+ and [1c]+.

ideal choice for the monocationic systems studied in this project. The DFT calculations were carried out on the nontruncated complexes [1b]+ and [1c]+ to underline the importance of the steric interactions between the bridging ligand and the RuCp*(dppe) fragment in the monocations (seen in the section of crystal structures, vide supra), using the global hybrid functional BLYP35 with a suitable continuum solvent model (dichloromethane). The basis set employed was 6-31G* (Lanl2dz for Ru atom). After the data reported by Kaupp et al., reoptimization of the systems [1b]+ and [1c]+ afforded three typical conformations, viz., trans-, cis-, and perpendicular (perp-) orientations (Figure 7).9,22 In the transoid

Figure 7. Schematic representation of the relative orientation of the two ruthenium centers (black and gray) and the plane of the diethynyl polyaromatic bridge (blue dashed lines) in [1b]+ (trans-[1b]+, cis[1b]+, and perp-[1b]+) and [1c]+ (trans-[1c]+, cis-[1c]+, and perp[1c]+).

and cisoid forms, the fixed P−Ru−Ru−P dihedral angles Ω (defined as a dihedral angle between the half-sandwich metal complex end groups) are 180° and 0°, respectively, and the bridging ligand plane bisects the two P−Ru−P angles of the diphosphine ligands. In the perpendicular orientation, the dihedral angle Ω is 90° and the mirror plane of the bridging moiety bisects the P−Ru−P angle in the RuCp*(dppe) moiety (Figure 7).22a We calculated the relative energies for the different conformers of [1b]+ and [1c]+: the energies of the transoid and cisoid forms are almost equal for both [1b]+ and [1c]+, while the energies of perp-[1b]+ and perp-[1c]+ are higher than those of their corresponding transoid and cisoid forms by up to 4.20 and 9.45 kJ mol−1, respectively. Overall, the energetic differences between the rotamers of [1b]+ and [1c]+ are minor, which further verifies their coexistence put forward in the preceding IR spectroelectrochemical section. According to Figure 8, there are notable differences in the distribution of spin density for different conformers of [1b]+ and [1c]+, which is symmetrical for the transoid and cisoid rotamers but markedly asymmetrical for the perp-rotamers. The varied involvement of the ethynyl units in the oxidation of the rotamers is reflected in the multiple ν(CC) wavenumbers in the cumulative IR spectra of the cationic complexes. All the calculated rotamers of [1b]+ and [1c]+ exhibit delocalized electronic structures with spin density distributed over the molecular backbone including both the bridge and the metal centers. The prominent involvement of the bridge confirms the redox noninnocent character of the ligand. With regard to calculated ν(CC) wavenumbers within the harmonic approach for (a) trans-[1b]+, cis-[1b]+, perp-[1c]+, and (b) trans-[1c]+, cis-[1c]+, perp-[1c]+, symmetrical transoid and cisoid rotamers feature a single intense ν(CC) band, while both perp-[1b]+ and perp-[1c]+ show two stretching bands (trans[1b]+: 2101 cm−1, cis-[1b]+: 2102 cm−1; trans-[1c]+: 2093 cm−1, cis-[1c]+: 2096 cm−1; perp-[1b]+: 2127, 2175 cm−1; perp-

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CONCLUSIONS This contribution presents a series of new dinuclear ruthenium complexes bridged by diethynyl polyaromatic (biphenyl, fluorene, phenanthrene, and pyrene) ligands. A combination of cyclic voltammetry, square wave voltammetry, in situ IR and UV/vis/near-IR spectroelectrochemistry, and DFT/TDDFT calculations has been employed to investigate the electrochemical, spectroscopic, and electronic properties of these compounds. As demonstrated by the electrochemistry data, the π-conjugated nature of the bridging ligands has a significant effect on the redox processes, larger conjugation being favorable for more bridge-localized oxidation in these systems. The specific evolution of the UV/vis/NIR absorption upon stepwise 1e− oxidation demonstrates the significant contribution of the bridging ligands. In line with this anodic behavior IR spectroelectrochemistry has revealed marked changes in the ν(CC) wavenumbers of the ruthenium diethynyl complexes. The multiple ν(CC) absorption bands for the singly oxidized states imply thermal population of a range of conformers with distinct electronic characters. The results of accurate DFT calculations conducted for three different selected rotamers of [1b]+ and [1c]+ demonstrate that the thermally populated molecular conformations in solution with various mutual orientations of the metal termini and planar aryl diethynyl moieties inherently influence the observed spectroscopic characteristics, as proposed in the pioneering studies published by Kaupp, Low, Costuas, Lapinte, et al.9,17,22 The choice of a reasonable and accurate calculation method for validating the interpretation of the experimental data is very important. Therefore, this study adds valuable material to the growing body of homobimetallic complexes in which the bridging ligand heavily participates in redox processes. Further investigations in this field are beneficial for promoting diversity in the design of H


Article

Organometallics

Figure 8. Spin-density distributions in trans-[1b]+, cis-[1b]+, and perp-[1b]+ (Ru/CHCH/Ar/CHCH/Ru) and trans-[1c]+, cis-[1c]+, and perp[1c]+ (Ru/CHCH/Ar/CHCH/Ru) with the corresponding compositions. Contour values: ±0.04 (e/bohr3)1/2. 2H, Ar−H), 7.69 (d, JHH = 8.4 Hz, 2H, Ar−H), 8.01 (s, 2H, Ar−H), 8.52 (d, JHH = 12 Hz, 2H, Ar−H). 13C NMR (100 MHz, CDCl3): δ −0.01 (SiMe3), 95.35, 104.98 (CC), 121.49, 122.85, 127.09, 129.57, 129.71, 131.85, 132.32 (Ar). Anal. Calcd for C24H26Si2: C, 77.77; H, 7.07. Found: C, 77.31; H, 7.23. Synthesis of 2,7-Bis((trimethylsilyl)ethynyl)pyrene (4d). Compound 4d was prepared by an analogous method to that for 4c and purified on a silica gel column with petroleum ether elution to obtain a light yellow solid (150 mg) in 38% yield. 1H NMR (600 MHz, CDCl3): δ 0.34 (s, 18H, SiMe3), 8.25 (s, 4H, Ar−H), 7.98 (s, 4H, Ar− H). 13C NMR (100 MHz, CDCl3): δ 0.07 (SiMe3), 95.01, 105.49 (CC), 120.91, 123.87, 127.46, 128.38, 130.98 (Ar). Anal. Calcd for C26H26Si2: C, 79.13; H, 6.64. Found: C, 79.59; H, 6.87. General Synthetic Procedure for 1a−1d. Target compounds 1a−1d were prepared according to the synthetic route presented in Scheme 1. Preparation of 4,4′-{Cp*(dppe)RuCC}2-C12H8 (1a). A solution of [RuCp*(dppe)Cl] (386 mg, 0.58 mmol), 4a (100 mg, 0.29 mmol), and KF (200 mg, 3.46 mmol) in 20 mL of CH3OH and 5 mL of THF was heated to reflux under a nitrogen atmosphere for 24 h. The crude product was collected by filtration and washed with hexane. The solid was dissolved in dichloromethane and precipitated from slow diffusion with hexane. The solid was filtered and dried to give 1a as a yellow powder (296 mg, yield 70%). 1H NMR (400 MHz, CDCl3): δ 1.58 (s, 30H, CH3 of C5Me5), 1.98−2.17 (m, 4H, CH2 of dppe), 2.58−2.85 (m, 4H, CH2 of dppe), 6.65−6.97 (m, 4H, Ar−H), 7.07−7.69 (m,

new bimetallic complexes with potential applications in molecular electronics.

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EXPERIMENTAL SECTION

Materials. All manipulations were carried out under an argon atmosphere by using standard Schlenk techniques, unless otherwise stated. Solvents were predried, distilled, and degassed prior to use, except those for spectroscopic measurements, which were of spectroscopic grade. The reagents 4,4′-diiodo-1,1′-biphenyl (3a) and 2,7-dibromo-9H-fluorene (3b) were commercially available and used without further purification. The starting materials 2,7-dibromophenanthrene (3c),26 2,7-dibromopyrene (3d),27 4,4′-bis((trimethylsilyl)ethynyl)-1,1′-biphenyl (4a),28 2,7-bis((trimethylsilyl)ethynyl)-9H-fluorene (4b),29 and [RuCp*(dppe)Cl]10c were prepared by the procedures described in the literature. General Synthetic Procedure for 4c and 4d. Synthesis of 2,7Bis((trimethylsilyl)ethynyl)phenanthrene (4c). To a stirred solution of 3c (336 mg, 1 mmol), CuI (19 mg, 0.1 mmol), and [Pd(PPh3)4] (116 mg, 0.1 mmol) in triethylamine (20 mL) and THF (30 mL) under an argon atmosphere was added (trimethylsilyl)acetylene (294 mg, 3 mmol), and the mixture at 60 °C was refluxed for 24 h. The cold solution was filtered through a bed of Celite. The filtrate was evaporated under reduced pressure and purified by silica gel column chromatography (petroleum ether) to give a white solid (263 mg, 71%). 1H NMR (400 MHz, CDCl3): δ 0.32 (s, 18H, SiMe3), 7.66 (s, I


Article

Organometallics Table 6. Major NIR Electronic Excitations in [1b]+ and [1c]+ Determined by TD-DFT Methods complex +

[1b]

conformer

λ/nm [cm−1]

oscillator strength ( f)

trans

1991 [5023]

1.0860

β-HOSO → β-LUSO (99%)

1160 [8621] 1136 [8803] 804 [12 438]

0.0023 0.0051 0.0011

558 [17 921] 2002 [4995]

0.0125 1.0845

β-HOSO-1 → β-LUSO (89%) β-HOSO-2 → β-LUSO (89%) β-HOSO-3 → β-LUSO (59%) β-HOSO-1 → β-LUSO (30%) β-HOSO-6 → β-LUSO (96%) β-HOSO → β-LUSO (99%)

1164 [8591] 1148 [8711] 810 [12 346]

0.0002 0.0094 0.0014

560 [17 857] 1808 [5531]

0.0100 0.6770

1445 [6920] 1216 [8223] 829 [12 063]

0.2217 0.0003 0.0008

trans

565 [17 699] 2512 [3981]

0.0057 1.0424


cis

1293 [7734] 1281 [7806] 848 [11 792] 583 [17 153] 2538 [3940]

0.0017 0.0068 0.0034 0.0072 1.0438

β-HOSO-1 → β-LUSO (83%) β-HOSO-2 → β-LUSO (82%) β-HOSO-4 → β-LUSO (74%) β-HOSO-7 → β-LUSO (96%) β-HOSO → β-LUSO (99%)

1269 [7880] 1252 [7987] 861 [11 614]

0.0068 0.0018 0.0033

583 [17 153] 2293 [4361]

0.0072 0.8965


1361 [7347] 1310 [7633] 882 [11 338]

0.0434 0.0002 0.0022

584 [17 123]

0.0093

cis

perp

[1c]+

perp

major contributions


β-HOSO-1 β-HOSO-2 β-HOSO-7 β-HOSO-6 β-HOSO-6

36H, Ar−H), 7.80 (s, 8H, Ar−H). 13C NMR (100 MHz, CDCl3): δ 10.03 (CH3 of C5Me5), 29.37−30.27 (m, CH2 of dppe), 92.52 (CH3 of C5Me5), 109.80 (Ru−CC), 115.07 (Ru−C), 125.68−139.09 (m, Ar). 31P NMR (160 MHz, CDCl3): δ 80.86 (s, dppe). IR (KBr/cm−1): ν (CC) 2069 (w). Anal. Calcd for C88H86P4Ru2: C, 71.92; H, 5.90. Found: C, 71.45; H, 6.02. Preparation of 2,7-{Cp*(dppe)RuCC}2-C13H8 (1b). Compound 1b was prepared by an analogous method to that for 1a. [RuCp*(dppe)Cl] (348 mg, 0.52 mmol), 4b (90 mg, 0.25 mmol), KF (174 mg, 3 mmol), CH3OH (20 mL), and THF (5 mL) were used. 1b was obtained as a yellow solid (255 mg) in 69% yield. 1H NMR (400 MHz, CDCl3): δ 1.59 (s, 30H, CH3 of C5Me5), 1.98−2.13 (m, 4H, CH2 of dppe), 2.63−2.79 (m, 4H, CH2 of dppe), 3.57 (s, 2H, fluorene−H), 6.84 (s, 2H, Ar−H), 7.14−7.46 (m, 36H, Ar−H), 7.80 (m, 8H, Ar−H). 13C NMR (100 MHz, CDCl3): δ 10.05 (CH3 of C5Me5), 29.37−30.27 (m, CH2 of dppe), 31.56 (CH2 of fluorene), 92.49 (CH3 of C5Me5), 110.69 (Ru−CC), 118.19 (Ru−C), 126.41−142.52 (m, Ar). 31P NMR (160 MHz, CDCl3): δ 80.66 (s, dppe). IR (KBr/cm−1): ν(CC) 2062 (w). Anal. Calcd for C89H86P4Ru2: C, 72.15; H, 5.85. Found: C, 72.61; H, 6.01.

→ → → → →

β-LUSO β-LUSO β-LUSO β-LUSO β-LUSO

(95%) (87%) (73%) (39%) (73%)

assignment M−CC → LCT MLCT MLCT MLCT MLCT/L(Cp*)MCT MLCT L(Cp*, dppe)MCT/MLCT M−CC → LCT MLCT MLCT MLCT MLCT/L(Cp*)MCT MLCT L(Cp*, dppe)MCT/MLCT M−CC → LCT MLCT MLCT MLCT MLCT MLCT MLCT/L(Cp*, dppe)MCT M−CC → LCT MLCT MLCT MLCT MLCT/L(Cp*)MCT L(Cp*, dppe)MCT/MLCT M−CC → LCT MLCT MLCT MLCT MLCT/L(Cp*)MCT MLCT L(Cp*, dppe)MCT/MLCT M−CC → LCT MLCT MLCT MLCT MLCT MLCT MLCT

Preparation of 2,7-{Cp*(dppe)RuCC}2-C14H8 (1c). Compound 1c was prepared by an analogous method to that for 1a. [RuCp*(dppe)Cl] (304 mg, 0.46 mmol), 4c (80 mg, 0.22 mmol), KF (153 mg, 2.64 mmol), CH3OH (20 mL), and THF (5 mL) were used. 1c was obtained as a yellow solid (252 mg) in 69% yield. 1H NMR (400 MHz, CDCl3): δ 1.59 (s, 30H, CH3 of C5Me5), 1.93−2.17 (m, 4H, CH2 of dppe), 2.60−2.86 (m, 4H, CH2 of dppe), 6.98 (d, JHH = 8.8 Hz, 2H, Ar−H), 7.03−7.48 (m, 36H, Ar−H), 7.66−7.94 (m, 8H, Ar−H), 8.15 (d, JHH = 8.0 Hz, 2H, Ar−H). 13C NMR (100 MHz, CDCl3): δ 10.06 (CH3 of C5Me5), 29.12−29.36 (m, CH2 of dppe), 92.49 (CH3 of C5Me5), 110.25 (Ru−CC), 121.20 (Ru−C), 126.06−138.99 (m, Ar). 31P NMR (160 MHz, CDCl3): δ 79.50 (s, dppe). IR (KBr/cm−1): ν (CC) 2059 (w). Anal. Calcd for C90H86P4Ru2: C, 72.37; H, 5.80. Found: C, 72.85; H, 5.49. Preparation of 2,7-{Cp*(dppe)RuCC}2-C16H8 (1d). Compound 1d was prepared by an analogous method to that for 1a. [RuCp*(dppe)Cl] (352 mg, 0.53 mmol), 4d (100 mg, 0.25 mmol), KF (174 mg, 3 mmol), CH3OH (20 mL), and THF (5 mL) were used. 1d was obtained as a yellow solid (250 mg) in 65% yield. 1H NMR (400 MHz, CDCl3): δ 1.62 (s, 30H, CH3 of C5Me5), 1.96−2.20 (m, J


Organometallics

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4H, CH2 of dppe), 2.65−2.90 (m, 4H, CH2 of dppe), 7.16−7.48 (m, 36H, Ar−H), 7.64 (s, 4H, Ar−H), 7.84 (s, 4H, Ar−H), 7.86 (s, 4H, Ar−H). 13C NMR (100 MHz, CDCl3): δ 10.12 (CH3 of C5Me5), 29.36−29.42 (m, CH2 of dppe), 92.57 (C5Me5), 110.63 (Ru−CC), 121.83 (Ru−C), 126.44−138.78 (m, Ar). 31P NMR (160 MHz, CDCl3): δ 79.27 (s, dppe). IR (KBr/cm−1): ν (CC) 2070 (w). Anal. Calcd for C92H86P4Ru2: C, 72.81; H, 5.71. Found: C, 73.22; H, 5.48. Crystallographic Details. Single crystals of complexes 1c and 1d suitable for X-ray analysis were grown by slow diffusion of hexane into a solution of dichloromethane. Crystals with approximate dimensions of 0.20 × 0.10 × 0.10 mm3 for 1c and 0.25 × 0.10 × 0.10 mm3 for 1d were mounted on glass fibers for diffraction experiments. Intensity data were collected on a Nonius Kappa CCD diffractometer with Mo Kα radiation (0.710 73 Å) at room temperature. The structures were solved by a combination of direct methods (SHELXS-97)30 and Fourier difference techniques and refined by full matrix least-squares (SHELXL-97).31 All non-H atoms were refined anisotropically. The hydrogen atoms were placed in ideal positions and refined as riding atoms. The partial solvent molecules have been omitted. Further crystal data and details of the data collection are summarized in Table S1. Selected bond distances and angles are given in Tables 1 and 2, respectively. Physical Measurements. 1H, 13C, and 31P NMR spectra were collected on a Varian Mercury Plus 400 spectrometer (400 MHz). 1H and 13C NMR chemical shifts are relative to TMS, and 31P NMR chemical shifts are relative to 85% H3PO4. Elemental analyses (C, H, N) were performed with a Vario ElIII Chnso instrument. The electrochemical measurements were performed on a CHI 660C potentiostat (CHI USA). A three-electrode single-compartment cell was used for the solution of complexes and supporting electrolyte in dry CH2Cl2. The solution was deaerated by argon bubbling on a frit for about 10 min before the measurement. The analyte (complex, ligand) and electrolyte (n-Bu4NPF6) concentrations were typically 10−3 and 10−1 mol dm−3, respectively. A prepolished 500 μm diameter platinum disk working electrode, a platinum wire counter electrode, and a Ag/Ag+ reference electrode were used. Spectroelectrochemical experiments at room temperature were performed with an airtight optically transparent thin-layer electrochemical (OTTLE) cell (optical path length of ca. 200 μm) equipped with a Pt minigrid working electrode and CaF2 windows.32 The cell was positioned in the sample compartment of a Bruker Tensor FT-IR spectrometer (1 cm−1 spectral resolution, 8 scans) or a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. The controlled-potential electrolyses were carried out with a CHI 660C potentiostat. The concentration of samples was ca. 2 × 10−3 mol dm−3. Dry 10−1 M n-Bu4NPF6 was used as the supporting electrolyte. Computational Details. DFT calculations were performed with the Gaussian 09 program,33 at the B3LYP/6-31G* and BLYP3534/631G* levels of theory. Geometry optimizations were performed without any symmetry constraints, and frequency calculations on the resulting optimized geometries showed no imaginary frequencies. Electronic transitions were calculated by the TD-DFT method. The MO contributions were generated using the Multiwfn2.6.1_bin_Win package and plotted using GaussView 5.0. The solvation effects in dichloromethane are included for a part of the calculations with the conductor-like polarizable continuum model (CPCM).35 Calculated harmonic vibrational frequencies were scaled by an empirical factor of 0.95 (BLYP35) and 0.9614 (B3LYP).36,37

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F. Hartl). *E-mail: [email protected] (L. Rao). *E-mail: [email protected] (S. H. Liu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from National Natural Science Foundation of China (21272088, 21472059, 21402057) and the self-determined research funds of CCNU from the colleges’ basic research and operation of MOE (CCNU14A05009, CCNU14F01003).

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00276. Electrochemistry (complexes and bridging ligands), UV− vis−NIR and IR spectroelectrochemistry, calculated DFT data, and NMR information (PDF) CCDC 1012775 and 1012776 for 1c and 1d (CIF) K


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Organometallics

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