Article pubs.acs.org/IC
Effect of Axial Ligands on the Spectroscopic and Electrochemical Properties of Diruthenium Compounds Machima Manowong,† Baocheng Han,*,‡ Thomas R. McAloon,‡ Jianguo Shao,§ Ilia A. Guzei,∥ Siyabonga Ngubane,⊥ Eric Van Caemelbecke,†,# John L. Bear,† and Karl M. Kadish*,† †
Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States Department of Chemistry, University of WisconsinWhitewater, 800 West Main Street, Whitewater, Wisconsin 53190-1790, United States § Department of Chemistry, Midwestern State University, 3410 Taft Boulevard, Wichita Falls, Texas 76308, United States ∥ Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States ⊥ Molecular Science Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa # Department of Chemistry, Houston Baptist University, 7502 Fondren Road, Houston, Texas 77074-3298 United States ‡
S Supporting Information *
ABSTRACT: Three related diruthenium complexes containing four symmetrical anionic bridging ligands were synthesized and characterized as to their electrochemical and spectroscopic properties. The examined compounds are represented as Ru2(dpb)4Cl, Ru2(dpb)4(CO), and Ru2(dpb)4(NO) in the solid state, where dpb = diphenylbenzamidinate anion. Different forms of Ru2(dpb)4Cl are observed in solution depending on the utilized solvent and the counteranion added to solution. Each Ru25+ form of the compound undergoes multiple redox processes involving the dimetal unit. The reversibility as well as potentials of these diruthenium-centered electrode reactions depends upon the solvent and the bound axial ligand. The Ru25+/4+ and Ru25+/6+ processes of Ru2(dpb)4Cl were monitored by UV−vis spectroscopy in both CH2Cl2 and PhCN. A conversion of Ru2(dpb)4Cl to [Ru2(dpb)4(CO)]+ was also carried out by simply bubbling CO gas through a CH2Cl2 solution of Ru2(dpb)4Cl at room temperature. The chemically generated [Ru2(dpb)4(CO)]+ complex undergoes several electron transfer processes in CH2Cl2 containing 0.1 M TBAClO4 under a CO atmosphere, and the same reactions were seen for a chemically synthesized sample of Ru2(dpf)4(CO) in CH2Cl2, 0.1 M TBAClO4 under a N2 atmosphere, where dpf = N,N′-diphenylformamidinate anion. Ru2(dpb)4(NO) undergoes two successive one-electron reductions and a single one-electron oxidation, all of which involve the diruthenium unit. The CO and NO adducts of Ru2(dpb)4 were further characterized by FTIR spectroelectrochemistry, and the IR spectral data of these compounds are discussed in light of results for previously characterized Ru2(dpf)4(CO) and Ru2(dpf)4(NO) derivatives under similar solution conditions.
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INTRODUCTION
to one-electron additions or one-electron abstractions at the dimetal unit as described in eqs 1−4.
The structural, electrochemical, and spectroscopic properties of numerous diruthenium complexes with a Ru25+ core and a paddle-wheel structure have been reported in the literature1−27 since the first synthesis and physical characterization of Ru2(OAc)4Cl (OAc = acetate anion) by Stephenson and Wilkinson in 1966.28 The majority of investigated Ru25+ complexes have contained an axially bound chloride anion and are formulated in their neutral form as Ru2L4Cl or Ru2(OAc)x(L)4‑xCl (x = 1−4) where L is a symmetrical or unsymmetrical anionic bridging ligand.1,6,8,16,18,19,21,24−26,29,30 Four redox reactions are generally observed for these types of diruthenium compounds in nonaqueous solvents such as CH2Cl2, benzonitrile (PhCN), or acetonitrile (CH3CN). The electrode reactions are almost always reversible and correspond © 2014 American Chemical Society
Ru 2 5 + + e− ⇄ Ru 4 +
(1)
Ru 2 4 + + e− ⇄ Ru 3 +
(2)
Ru 2 5 + ⇄ Ru 6 + + e−
(3)
Ru 2 6 + ⇄ Ru 7 + + e−
(4)
The E1/2 values for reduction and oxidation of the diruthenium unit will depend upon the type and number of anionic bridging ligands as well as upon the type and number of Received: March 31, 2014 Published: July 8, 2014 7416
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Article
Packard model 8453 diode array spectrophotometer using cells with path lengths of 10, 1, 0.1, and 0.01 cm, the exact value depending on concentration. Infrared spectroelectrochemistry experiments were carried out using a Nicolet 6700 FT-IR spectrometer by Thermo Scientific. A homemade three-electrode cell, whose design has been reported in the literature,35 was used for spectroelectrochemistry experiments. The IR spectra of each electrooxidized or electroreduced complex under N2 were obtained as difference spectra measured against the corresponding unoxidized or unreduced compound. Mass spectra were recorded using an electrospray ionization (ESI-MS) or MALDI-TOF mass spectrometer at the University of Houston Mass Spectrometry Laboratory. Elemental analysis was carried out by Atlantic Microlab, Inc., Norcross, GA. Ferrocene (Fc) was used as internal standard in the electrochemical studies; the E1/2 of the Fc/Fc+ couple in CH2Cl2 with 0.1 M TBAClO4 was measured as 0.48 V versus SCE. Calculation of Formation Constants by UV−Vis Spectroscopy. Changes of the UV−vis spectra of Ru2(dpb)4Cl in CH2Cl2 containing 0.1 M TBAClO4 were monitored during a titration with PhCN, and the resulting spectral data then used to calculate the formation constant using the Hill equation (see eq 5)36
axial ligands which could be anionic or neutral. For example, the E1/2 for reduction of Ru2(F5ap)4Cl in CH2Cl2 was reported as −0.35 V,23 where F5ap = 2-(2,3,4,5,6-pentafluoroanilino)pyridinate anion while Ru2(ap)4Cl is reduced at −0.87 V versus SCE17 under the same solution conditions, a difference of more than 500 mV with changes in substituents on the ap bridging ligands (where ap =2-anilinopyridinate anion). Likewise, the E1/2 for reduction of Ru2(ap)4CN is located at −0.73 V while [Ru2(ap)4(CN)2]− is reduced at −1.24 V in the same solvent,31 again a difference of more than 500 mV in E1/2 for the Ru5+/4+ process with change of axial ligands. Changes in the type of axial ligand, while maintaining the same number and type of bridging ligands, not only will affect the redox potentials as described above but also may lead in some cases to a change in electronic configuration and/or chemical reactivity of the diruthenium complexes in their various dimetal oxidation states.13,32,33 Indeed, Ru2(dpf)3(OAc)(BF4) where dpf = N,N′-diphenylformamidinate anion was shown to exist as a quantum admixture of spins whereas Ru2(dpf)3(OAc)Cl exists only in an S = 1 spin state.13 The reaction of CO with Ru2(dpf)3(OAc)(BF4) (or most likely [Ru2(dpf)3(OAc)]+ in solution) was shown to afford the monoCO adduct, [Ru2(dpf)3(OAc)(CO)]+, whereas no reaction at all is seen between Ru2(dpf)3(OAc)Cl and carbon monoxide which does not bind to the Ru25+ form of the examined compound.33 In earlier studies from our laboratory we have reported how systematic changes in the bridging ligands would affect redox potentials of the dimetal unit,8,17,34 and we have now turned our attention to systematically investigating the effect of neutral and anionic axial ligands on the redox potentials of a series of Ru25+ compounds with the same four anionic bridging ligands. The investigated diruthenium compounds are represented as Ru2(dpb)4X, [Ru2(dpb)4X2]−, and [Ru2(dpb)4]+ (dpb = diphenylbenzamidinate anion) where X is a neutral or negatively charged axial ligand or a solvent molecule. The redox reactions given by eqs 1−4 were examined in CH2Cl2 and PhCN containing a variety of added anions, and the products of each electron addition or electron abstraction were characterized by cyclic voltammetry combined with thin-layer UV−vis and IR spectroelectrochemistry.
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log[(Ai − A 0)/(A f − Ai )] = log K + n log[PhCN]
(5)
where Ai is the absorbance in solutions with a specific concentration of added PhCN, A0 is the initial absorbance where [PhCN] = 0.0 M, and Af is the final absorbance of the coordinated species. The slope of the log[(Ai − A0)/(Af − Ai)] versus log [PhCN] plot gives n, the number of PhCN species bound to the diruthenium center, and the value of log K is evaluated from the intercept of the line at log [PhCN] = 0.0. Calculation of Formation Constants by Electrochemical Titration Method. Change in half-wave potentials for reduction or oxidation of Ru2(dpb)4Cl in CH2Cl2 containing 0.1 M TBAClO4 were monitored during a titration with PhCN in order to calculate the formation constants for ligand binding using eq 6
(E1/2)c = (E1/2)s − 0.059 log
KMLp K(MLq)−
− (p − q)0.059 log[L] (6)
where (E1/2)c and (E1/2)s are half-wave potentials for the ligated and unligated diruthenium compound, and p and q are equal to the number of ligands bound to the oxidized and reduced forms of the compound. KMLp and K(MLq)- are the formation constants of the MLp and (MLq)−, respectively. When the ligand concentration is equal to 1.0 M or when p = q, the difference in half-wave potentials between (E1/2)c and (E1/2)s will directly give the ratio of stability constants for binding of an axial ligand to the two different oxidation states of the redox couple. Synthesis of Starting Materials. Diruthenium acetate, Ru2(OAc)4Cl,28 and N,N′-diphenylbenzamidine (Hdpb)37 were prepared following procedures reported in the literature. Ru2(dpb)4Cl (1). In a round-bottom flask, Ru2(OAc)4Cl (1.9 g, 4.03 mmol) was added to N,N′-diphenylbenzamidinate (50.0 g, 184 mmol) and heated (>170 °C, 10 h) under vacuum (10−2 mm Hg) to melt the ligand. The unreacted N,N′-diphenylbenzamidinate ligand was sublimed (12 h) from the reaction mixture under vacuum (160 °C at 10−2 mm Hg). Upon cooling, the dark green residue obtained was dissolved in CH2Cl2 (30 mL) and filtered, and the filtrate collected was vacuum distilled to remove solvent. The residue obtained was recrystallized from a CH2Cl2/hexane solvent mixture to give a green solid with a yield of about 70%. Mass spectral data [m/z, fragment]: 1289, Ru2(dpb)4+; 1323, Ru2(dpb)4Cl+. UV−vis (CH2Cl2, nm (ε × 10−3 L mol−1 cm−1)): 443 (4.7), 556 (1.5), 715 (2.9), 778 (3.0), 854 (1.0). Ru2(dpb)4(CO) (2). In a Schlenk flask, Ru2(dpb)4Cl (0.050 g, 0.0378 mmol) was dissolved in CH2Cl2 (20 mL) and purged with nitrogen (2 min). Excess sodium borohydride was added to the CH2Cl2 solution and stirred until a color change from yellow green to red was observed. CO gas was then passed through the solution until it turned green (30 min), after which the solution was filtered. The
EXPERIMENTAL SECTION
General Procedures and Reagents. Ultra-high-purity grade nitrogen gas (N2), nitric oxide (NO), and research purity grade carbon monoxide (CO) were purchased from Matheson Trigas or Praxair. Both N2 and NO were passed through anhydrous calcium sulfate and sodium hydroxide pellets to eliminate traces of water prior to use while CO was used without further purification. Tetra-n-butylammonium perchlorate (TBAClO4, Fluka Chemicals Co.), tetra-n-butylammonium hexafluorophosphate (TBAPF6, Sigma-Aldrich), tetra-n-butylammonium fluoride (TBAF, Fluka Chemicals Co.), tetra-n-butylammonium chloride (TBACl, Fluka Chemicals Co.), tetra-n-butylammonium iodide (TBAI, Sigma-Aldrich), and tetra-n-butylammonium bromide (TBABr, TCI America) were all used as received without further purification. Dichloromethane (CH2Cl2, 99.8%, EMD Chemicals) was used as received whereas PhCN (Sigma-Aldrich) was distilled over P2O5 under vacuum before use. Physical Measurements. Cyclic voltammetry was carried out with an EG&G model 263A potentiostat by using a three-electrode system made up of a glassy carbon or platinum disc working electrode, platinum wire counter electrode, and a homemade saturated calomel electrode (SCE) as the reference electrode. A fritted glass bridge was used to separate the SCE electrode from the supporting electrolyte/ solvent mixture. UV−vis measurements were recorded on a Hewlett7417
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Figure 1. Molecular structures of (a) Ru2(dpb)4(CO) 2-A and (b) Ru2(dpb)4(NO) 3. The H atoms have been omitted for clarity. filtrate was subsequently vacuum distilled to remove the solvent. The solid residue was recrystallized from a hexane/dichloromethane mixture to give a green solid (yield 90%). MS [m/z, fragment]: 1289, Ru2(dpb)4+. UV−vis (CH2Cl2, nm (ε × 10−3 L mol−1 cm−1)): 411 (11.2), 472 (3.90), 586 (3.60), 703 (2.85). IR (KBr, cm−1): 1924 (s) (νco), 2927 (m), 2854 (m), 1725 (w), 1594 (m), 1485 (s), 1285(s), 1272 (s), 1251(s), 1211 (w), 1114 (s), 1024 (w), 937 (w) and 900 (s). Ru2(dpb)4(NO) (3). In a Schlenk flask, Ru2(dpb)4Cl (0.050 g, 0.0378 mmol) was dissolved in CH2Cl2 (20 mL) and purged with nitrogen (2 min). Nitric oxide (NO) gas was then passed through the solution until color of solution turned red (20 min). Nitrogen gas was used to purge out unreacted NO gas, and the color of the solution changed to green. The solution was then filtered, and the filtrate was subsequently vacuum distilled to remove the solvent. The solid residue was recrystallized from a hexane/dichloromethane mixture, and the solvents were decanted and the solids redissolved in dichloromethane (yield 90%). UV−vis (CH2Cl2, nm (ε × 10−3 L mol−1 cm−1)): 419 (5.5), 446 (5.0), 529 (4.0), 700 (8.9). MS [m/z, fragment]: 1289, Ru2(dpb)4+. IR (KBr, cm−1): 1778 (s) (νNO), 3666 (w), 3411 (br), 3066 (m), 1588 (m), 1486 (s), 1246 (s), 1207 (w), 1116 (s), 1071 (w), 1018 (m), 925 (s), 769 (m), 702 (s). Single Crystal X-ray Diffraction Studies. Measurements were made with a Bruker SMART APEXII diffractometer equipped with a CCD area detector. The programs used in the X-ray diffraction studies were as follows: data collection, Apex2 (Bruker, 2009); cell refinement and data reduction, Bruker SAINT v7.12A (Bruker-Nonius, 2004); structure refinement and validation, SHELXTL (Sheldrick, 2008); and PLATON (Spek, 1990). Green single crystals of 2 were obtained by slow evaporation of a CH2Cl2 solution. The unit cell contains one diruthenium complex (2A, with atoms Ru1 and Ru2) in a general position, one diruthenium complex (2-B with atom Ru3) on the intersection of two 2-fold axes, one duruthenium complex (2-C with atoms Ru4) on a crystallographic 2-fold axis, and some diffusely diffracting solvent molecules that were not identified, vide inf ra. The molecular structure of complex 2-A is reliably established, and the complex was refined anisotropically. It is a problem-free part of the structure. Complex 2-B resides on the intersection of two 2-fold axes, and thus, only one-quarter of 2-B is symmetry-independent. Strangely enough, this complex is present in the lattice only 81.4(2)% of the time. The C89 phenyl ring is disordered over two positions with the minor component being present 47.9(6)% of the time. The O2 carbonyl is equally disordered over two positions. This complex was refined with constraints and restraints. Complex 2-C resides on a crystallographic 2-fold axis. The Ru atom is disordered over two positions with the major component being
present 64.4(3)% of the time. The other atoms exhibit large thermal displacement ellipsoids, but it was not possible to model the whole molecule disorder of the complex. Due to the disorder of atom Ru4 and unique position of the rest of the complex, some Ru−N and Ru− C distances are not chemically reasonable. This complex was refined with constraints and restraints. The phenyl ring C114 and the O3 carbonyl are equally disordered over two positions. Additionally, there were several partially occupied solvent molecules present in the asymmetric unit. Bond length restraints were applied to model the molecules, but the resulting isotropic displacement coefficients suggested the molecules were mobile. In addition, the refinement was computationally unstable. Option SQUEEZE of the program PLATON was used to correct the diffraction data for diffuse scattering effects and to identify the solvent molecules. PLATON calculated the upper limit of volume that can be occupied by the solvent to be 4098 Å3, or 16.8% of the unit cell volume. The program calculated 938 electrons in the unit cell for the diffuse species. It was not possible to confidently identify the solvent because compound 2 had been exposed to MeCN, CH2Cl2, EtOH, hexanes, and MeOH, meaning that any combination of these solvents could be present. The solvent molecules are disordered over several positions. It should be noted that all derived results in the following tables are based on the known contents. No data are given for the diffusely scattering species. Dark sea green single crystals of 3·xsolvent were obtained by slow evaporation of a CH2Cl2 solution. There are also two molecules of dichloromethane solvent per Ru complex 3 in the lattice. There is positional disorder in the structure. In the dinuclear complex, the phenyl ring at N3 is disordered over two positions in a 0.505(4):0.495(4) ratio. The disordered rings were refined isotropically with an idealized geometry. In one of the crystallization solvent dichloromethane molecules atoms Cl4 is disordered over two positions in a 0.852(5):0.148(5) ratio, and atoms Cl4 and Cl4a were refined with constraints. There was an additional solvate molecule present in the asymmetric unit. Bond length restraints were applied to model the molecules, but the resulting isotropic displacement coefficients suggested that the molecules were mobile. In addition, the refinement was computationally unstable. Option SQUEEZE of the program PLATON was used to correct the diffraction data for diffuse scattering effects and to identify the solvate molecule. PLATON calculated the upper limit of volume that could be occupied by the solvent to be 559 Å3, or 7.8% of the unit cell volume. The program calculated 58 electrons in the unit cell for the diffuse species. It is very likely that this solvate molecule is disordered over several positions. All derived results in the following tables are based on the known contents. No data are given for the diffusely scattering species. 7418
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RESULTS AND DISCUSSION Synthesis. Ru2(dpb)4Cl (1) was synthesized by reaction of Ru2(OAc)4Cl with molten dpb bridging ligand. Subsequent recrystallization in CH2Cl2 yielded the pure complex 1. The electrosynthesis of Ru2(dpf)4(CO) under a CO atmosphere was first reported by Bear and Kadish,38 and a few years later, the chemical synthesis of [Ru2(dpf)3(OAc)(CO)]BF4 was reported by Barral et al.33 In the present study, Ru2(dpb)4(CO) (2) was prepared by reduction of Ru2(dpb)4Cl using NaBH4 and CH2Cl2 as solvent. CO gas was then passed through the solution to yield 2 which was purified by recrystallization in a hexane/acetone solvent mixture. Ru2(dpb)4(NO) (3) was synthesized by a chemical reaction of deaerated Ru2(dpb)4Cl in CH2Cl2 saturated with nitric oxide; the excess NO was then removed by purging with nitrogen. Subsequent purification by recrystallization yielded pure samples of Ru2(dpb)4(NO) (3). The molecular structures of 2 and 3 are shown in Figure 1, whereas the crystallographic data are tabulated in Table 1 and
selected metric parameters are listed in Table 2. All other structural data for 2 and 3 are given in the Supporting Information. In both complexes as well as in Ru2(dpf)4(NO), atom Ru1 is in a distorted octahedral environment ligating to four N atoms in the equatorial plane, to the second Ru atom in one apical position and to a C or N atom in the other. The coordination environment of atom Ru2 is distorted octahedral with one vacant apical position, rendering this metal center fivecoordinate. The Ru−Ru bond distances in 2 and 3 measure 2.4789(8) and 2.4119(5) Å. These bonds are shorter than the Ru−Ru bond distances in the corresponding dpf complexes. This trend is consistent with the fact that metal−metal bond distances in diruthenium complexes usually greatly depend on the type of equatorial ligands. The Ru−Ru bond lengths of all complexes in Table 2 are within the expected ranges of Ru−Ru bond distances in diruthenium complexes. In addition, the Ru− CO bond distances are appreciably longer than the Ru−NO bond lengths, and the change in Ru−Lax (Lax = CO or NO) bond distance is larger for the dpf complex. The Ru−Ru−Lax bond angles of diruthenium complexes in Table 2 are all close to 180°. The bond angles are slightly different from each other for the dpb complexes with values of 178.4° or 178.5°, but are identical to each other for the dpf complexes with a value of 180°. All Ru−N distances are typical. In all three compounds atom Ru2 is displaced from the plane defined by the fourcoordinated N atoms toward the CO or NO ligand by ∼0.15 Å whereas atom Ru2 is displaced in the opposite direction from its equatorial plane to a smaller extent (
