Probing Electronic Communications in Heterotrinuclear Fe–Ru–Fe

Oct 28, 2015 - *E-mail: [email protected]., *E-mail: [email protected]., ... methods were used to elucidate the spectroscopic signatures of the...
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Probing Electronic Communications in Heterotrinuclear Fe−Ru−Fe Molecular Wires Formed by Ruthenium(II) Tetraphenylporphyrin and Isocyanoferrocene or 1,1′-Diisocyanoferrocene Ligands Victor N. Nemykin,*,† Semyon V. Dudkin,†,∥ Mahtab Fathi-Rasekh,† Andrew D. Spaeth,‡ Hannah M. Rhoda,† Rodion V. Belosludov,*,§ and Mikhail V. Barybin*,‡ †

Department of Chemistry and Biochemistry, University of Minnesota, Duluth, 1039 University Drive, Duluth, Minnesota 55812, United States ‡ Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States § Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: Two new heterotrinuclear Fe−Ru−Fe complexes of ruthenium(II) tetraphenylporphyrin axially coordinated with a pair of isocyanoferrocene ((FcNC)2RuTPP, 1) or 1,1′-diisocyanoferrocene (([C5H4NC]2Fe)2RuTPP, 2) ligands [Fc = ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2−) anion] were synthesized and characterized by UV−vis, magnetic circular dichroism, NMR, and FTIR spectroscopies as well as by electrospray ionization mass spectrometry and single-crystal Xray diffraction. Isolation of insoluble polymeric {([C5H4NC]2Fe)RuTPP}n molecular wires (3) was also achieved for the first time. The redox properties of the new trinuclear complexes 1 and 2 were probed using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation methods and correlated to those of the bis(tertbutylisocyano)ruthenium(II) tetraphenylporphyrin reference compound, (t-BuNC)2RuTPP (4). In all cases, the first oxidation process was attributed to the reversible oxidation of the RuII center. The second and third reversible oxidation processes in 1 are separated by ∼100 mV and were assigned to two single-electron FeII/FeIII couples, suggesting a weak long-range iron−iron coupling in this complex. Electrochemical data acquired for 2 are complicated by the interaction between the axial η1-1,1′diisocyanoferrocene ligand and the electrode surface as well as by axial ligand dissociation in solution. Spectroelectrochemical and chemical oxidation methods were used to elucidate the spectroscopic signatures of the [1]n+, [2]n+, and [4]n+ species in solution. DFT and time-dependent DFT calculations aided in correlating the spectroscopic and redox properties of complexes 1, 2, and 4 with their electronic structures.

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INTRODUCTION Polynuclear organometallic (especially ferrocene-containing) supramolecular systems with adjustable redox, electron-transfer, and photophysical characteristics have been envisioned as prospective building blocks for application in molecular electronics.1 Because of their well-defined properties, many porphyrins,2 tetraazaporphyrins,3 phthalocyanines,4 corroles,5 subphthalocyanines,6 and BODIPYs and azaBODIPYs7 with ferrocenyl substituents, linked to a core π-system via conjugated fragments, have been considered in this regard within the past two decades. Such ferrocenyl−aromatic π-system assemblies often exhibit long-range metal−metal electronic coupling and are attractive as potential molecular random-access memory components, catalysts for electro- and photocatalytic transformations, active components for light-harvesting, redoxswitchable fluorescence markers, and molecular wires.8 For most of these applications, ferrocenyl substituents should be connected to the parent π-system either directly or through a πconjugated linking group. On the other hand, reports on porphyrinoids and BODIPYs in which the central atom is © XXXX American Chemical Society

attached to a redox-active ferrocenyl moiety directly or via a conjugated linking group are quite rare.9 In coordination chemistry, organic isocyanides can act as both strong σ-donors and good π-acceptors,10 which can easily support formation of axially coordinated complexes with iron(II) and ruthenium(II) porphyrins and phthalocyanines of general formula (P)ML2 or (P)MLX (P = porphyrin or phthalocyanine, L = organic isonitrile, X = additional axial ligand).11 In a series of papers, Hanack and co-workers explored formation of oligomeric complexes between iron(II) phthalocyanine (PcFe) or ruthenium tetraazaporphyrin (RuTAP) and diisocyanoarenes, which were found to have semiconducting properties.12 Such scaffolds, if the simple diisocyanoarene ligands are modified with redox-active units, can be viewed as prototypes for molecular wires. In our first paper, we explored an axial coordination of PcFe with redoxactive isocyanoferrocene and found a weak long-range metal− Received: July 18, 2015

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DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

mixture was stirred at room temperature while the reaction progress was monitored by TLC on alumina plates. After 30 min, a precipitate formed, which was filtered, washed with small amounts of cold hexanes and hexanes/DCM, and dried under reduced pressure. Anal. Calcd for C68H44Fe2N8Ru·0.45CH2Cl2: C, 67.17; N, 9.16; H, 3.70. Found: C, 67.17; N, 9.21; H, 4.35. 1H NMR (20 °C, 500 MHz, CDCl3): δ 8.56 (s, 8H, β-pyrrole), 8.21 (m, 8H, m-Ph), 7.69 (m, 12H, o-Ph, p-Ph), 3.64 (t, 4H, J = 2.5 Hz, β-H, noncoordinated C5H4NC), 3.42 (t, 4H, J = 2.5 Hz, β-H, coordinated C5H4NC), 2.81 (t, 4H, J = 2.5 Hz, α-H, noncoordinated C5H4NC), 2.77 (t, 4H, J = 2.5 Hz, α-H, coordinated C5H4NC). 13C NMR (20 °C, 125 MHz, CDCl3): δ 142.5 (α-pyrrole), 142.0 (Cipso, Ph), 133.3 (Cortho, Ph), 130.9 (β-pyrrole), 126.1 (Cpara, Ph), 125.5 (Cmeta, Ph), 120.4 (Cmeso), 68.7 (α-C, noncoordinated C5H4NC), 66.9 (β-C, noncoordinated C5H4NC), 66.9 (β-C, coordinated C5H4NC), 66.0 (α-C, coordinated C5H4NC). UV−vis [DCM; λ, nm (log ε, M−1 cm−1)]: 417 (5.25), 536 (3.56). IR (KBr): ν(NC) 2112, 2090 cm−1. HR ESI MS: calcd for C68H44Fe2N8Ru, 1186.1448; found, 1186.1617 [M]+. Synthesis of {([C5H4NC]Fe)2RuTPP}n (3). A freshly prepared sample of complex 2 was washed with dry hexanes, toluene, and DCM at room temperature until the yellow color of the free isonitrile ligand was no longer observed in the washings. The precipitate was then dried under reduced pressure. The resulting polymer was not soluble in any nonpolar solvents, aromatic hydrocarbons, DCM, chloroform, acetone, or THF. Anal. Calcd for C56H36FeN6Ru·3H2O·1.65CH2Cl2: C, 60.52; H, 3.99; N, 7.35. Found: C, 60.54; N, 7.96; H, 3.99. IR (KBr): ν(NC) 2081 cm−1. Instrumentation. A Varian Unity INOVA NMR instrument was used to evaluate spectra taken at 500 MHz for 1H and 125 MHz for 13 C nuclei, respectively. The 1H and 13C chemical shifts are reported in parts per million relative to TMS as an internal standard. All UV−vis data were obtained on a JASCO-720 spectrophotometer at room temperature. An OLIS DCM 17 CD spectropolarimeter with a 1.4 T DeSa magnet was used to obtain all magnetic circular dichroism (MCD) data. IR data were obtained on a PerkinElmer Spectrum 100 FT-IR spectrometer at room temperature for samples pressed in KBr pellets. Electrochemical measurements were conducted using a CHI620C electrochemical analyzer utilizing the three-electrode scheme. Unless stated otherwise, platinum working, platinum auxiliary, and Ag/ AgCl pseudoreference electrodes were employed in a 0.05 M solution of TFAB in DCM for electrochemical experiments. In all cases, the redox potentials are referenced to the FcH/FcH+ couple using decamethylferrocene as an internal standard. Spectroelectrochemical data were collected using a custom-made 1 mm cell, a working electrode made of platinum mesh, and a 0.15 M solution of TFAB in DCM. High-resolution (HR) electrospray ionization (ESI) mass spectra were recorded using a Bruker MicrOTOF-III system for freshly prepared samples dissolved in THF under an ambient atmosphere. Chemical titration experiments were typically conducted using 1.0 × 10−6 to 3 × 10−6 M solutions of porphyrin complexes and ∼1.0 × 10−3 M stock solutions of the oxidant (Fe(ClO4)3 or “magic blue”) added in 0.1−0.3 equiv increments. Elemental analyses were performed by Atlantic Microlab, Inc. in Atlanta, GA. Computational Details. All computations were performed using the Gaussian 09 software package running under Windows or UNIX OS.20 Molecular orbital contributions were compiled from single-point calculations using the QMForge program.21 In all single-point calculations, the TPSSh exchange-correlation functional22 was used. The triple-ζ quality effective core potential SDD basis set23 was used for all atoms in all calculations. Frequencies were calculated for all optimized geometries to ensure that the final geometries represent minima on the potential energy surface. TDDFT calculations were conducted for the first 80 (complexes 1 and 2) or 50 (complex 4) excited states to ensure that all charge transfer (CT) and π−π* transitions of interest were accounted for. X-ray Crystallography. Single crystals of complexes 1 and 2 suitable for X-ray analysis were obtained by slow evaporation of toluene or toluene/DCM solutions of the corresponding compounds at room temperature, respectively. Experimental data for all samples were collected using a Rigaku Rapid II X-ray diffractometer with a

metal coupling in the (FcNC)2FePc system (FcNC = isocyanoferrocene).13 One of the serious drawbacks of the (FcNC)2FePc complex is that its first one-electron oxidation involves the phthalocyanine macrocycle, whereas for an ideal axial coordination-based molecular wire oxidation of the central metal ion or a redox-active axial ligand is more preferable. It is well-known that the oxidation potential of a porphyrin macrocycle in general is higher compared to that of the phthalocyanine core.14 Thus, in this Article, we introduce the first example of a porphyrin featuring axially coordinated isocyanoferrocene ligands. Because of the low chemical stability of iron(II) porphyrins, a RuII ion in RuTPP (TPP = 5,10,15,20tetraphenylporphyrin(2−) ligand) was employed as the central atom for accommodating axial coordination. In addition, we also tested the 1,1′-diisocyanoferrocene ligand in a similar coordination reaction as the first redox-active, potentially bidentate fragment needed for the formation of polymeric molecular wires. The optical and redox properties of the new (FcNC) 2 RuTPP (1) and ([C 5 H 4 NC] 2 Fe) 2 RuTPP (2) ([C5H4NC]2Fe = 1,1′-diisocyanoferrocene) complexes as well as their electronic structures were elucidated using a variety of spectroscopic methods and X-ray crystallography as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. We were able to establish a convenient way to form insoluble polymeric {([C5H4NC]2Fe)RuTPP}n molecular wires (3). The properties of (FcNC)2RuTPP and ([C5H4NC]2Fe)2RuTPP will be compared to and contrasted with the earlier reported “reference” (t-BuNC)2RuTPP complex (4).

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EXPERIMENTAL SECTION

Materials. All commercial reagents were ACS grade and were used without further purification. All reactions were performed under a dry argon atmosphere with flame-dried glassware. Toluene was distilled over sodium metal. Dichloromethane (DCM) and hexanes were distilled over CaH2. Tetrabutylammonium tetrakis(pentafluorophenyl)borate (TFAB, (NBu4)[B(C6F5)4]),15 isocyanoferrocene (FcNC),16 1,1′-diisocyanoferrocene ([CNC5H4]2Fe),17 and reference porphyrin 418 were prepared according to literature procedures. Synthetic Work. Synthesis of (FcNC)2RuTPP (1). Under an argon atmosphere, commercially available (OC)RuTPP (0.10 g, 0.13 mmol) was added to a solution of the FcNC ligand (0.30 g, 1.42 mmol) in 12 mL of a 1:1 (v/v) mixture of toluene and DCM. The reaction mixture was stirred at room temperature for 4 h, and then all solvents were removed under reduced pressure. The residue was washed several times with hexanes, dried under vacuum, and recrystallized from a mixture of dry DCM/toluene to form analytically pure complex 1. Yield: 0.043 g (28.1%). Anal. Calcd for C66H46Fe2N6Ru·0.16CH2Cl2: C, 69.15; H, 4.06; N, 7.31. Found: C, 69.15; H, 4.17; N, 7.05. 1H NMR (20 °C, 500 MHz, CDCl3): δ 8.54 (s, 8H, β-pyrrole), 8.24 (dd, J = 6.6, 2.9 Hz, 8H, m-Ph), 7.69 (m, 12H, o-Ph, p-Ph), 3.28 (t, 4H, β-H, C5H4NC, J = 2.0 Hz), 3.19 (s, 10H, C5H5), 2.68 (t, α-H, C5H4NC, J = 2.0 Hz). 13C NMR (20 °C, 125 MHz, CDCl3): δ 142.6 (α-pyrrole), 142.3 (Cipso, Ph), 133.3 (Cortho, Ph), 130.6 (β-pyrrole), 125.9 (Cmeta, Ph), 125.3 (Cpara, Ph), 120.3 (Cmeso), 68.9 (C5H5), 64.3 (β-C, C5H4NC), 64.2 (α-C, C5H4NC), 52.4 (ipso-C, C5H4NC). Because of the often19 broad nature of the 13C NMR signal for the isocyano carbon atom, it was not clearly observed in the spectrum of (FcNC)2RuTPP. UV−vis [DCM; λ, nm (log ε, M−1 cm−1)]: 419 (5.73), 534 (4.15). IR (KBr): ν(NC) 2094 cm−1. HR ESI MS: calcd for C66H46Fe2N6Ru, 1136.1543; found, 1136.1547 [M]+. Synthesis of ([C5H4NC]2Fe)2RuTPP (2). Under an argon atmosphere, the (C5H4NC)2Fe ligand (0.090 g, 0.381 mmol) was added to a solution of commercially available (OC)RuTPP (0.050 g, 0.067 mmol) in 6 mL of a DCM and toluene (1:1, v/v) mixture. The B

DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry curved IPDS (improved point detection system) detector employing graphite-monochromatized Mo Kα radiation (λ = 0.71075 Å). The structures of complexes 1 and 2 were solved by direct methods using the SIR-92 program.24 All non-hydrogen atoms were located from analysis of a difference Fourier map and refined through isotropic and, subsequently, anisotropic approximations. The toluene solvent molecule in the X-ray structure of complex 2 was found to be severely disordered and thus was removed using the PLATON SQUEEZE procedure. All hydrogen atoms were placed in their geometrically expected positions. The isotropic thermal parameters of all hydrogen atoms were fixed to the values of the equivalent isotropic thermal parameters of the corresponding carbon atoms using riding model constraints so that Uiso(H) = 1.2Ueq(C) for the hydrogen atoms. Both structures reported herein were completely refined via the full-matrix least-squares method using the Crystals for Windows program.25 Complete crystallographic information is available in the Supporting Information (CIF). CCDC 1411393 and CCDC 1411394 contain the supplementary crystallographic data for all compounds. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving. html (or from the Cambridge Crystallographic Data Centre, 12 Union Rd., Cambridge CB2 1EZ, U.K.; fax (+44) 1223-336-033 or e-mail [email protected]).

isocyanides are shifted upfield compared to those documented for the corresponding free ligands. To understand the origin of the relatively low stability of complex 2 in solution, we probed its formation in CDCl3 using 1H NMR titrations. Two titration experiments were conducted. In the first experiment, up to a 5fold excess (12 equiv) of the 1,1′-diisocyanoferrocene ligand was gradually added to a solution of (OC)RuTPP (Figure 1

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RESULTS AND DISCUSSION Synthesis, Spectroscopy, and X-ray structures. The trinuclear complexes 1 and 2 were synthesized following the synthetic strategy developed earlier for the preparation of other (RNC)2RuTPP complexes (Scheme 1).11,13,18 We used a large Scheme 1. Synthetic Strategy for Preparation of the (RNC)2RuTPP Complexes

Figure 1. Transformation of the 1H NMR spectra of the (OC)RuTPP complex upon stepwise addition of the 1,1′-diisocyanoferrocene ligand to its solution in CDCl3 at 25 °C: 1, (OC)RuTPP, no ligand; 2, 1.2 equiv of ligand; 3, 2.4 equiv of ligand; 4, 3.6 equiv of ligand; 5, 4.8 equiv of ligand; 6, 6.0 equiv of ligand; 7, 7.2 equiv of ligand; 8, 8.4 equiv of ligand; 9, 9.6 equiv of ligand; 10, 10.8 equiv of ligand; 11, 12.0 equiv of ligand. Labeling legend: a, b, and c indicate β-pyrrolic, 1,1′diisocyanoferrocene, and meso-phenyl fragment signals for complex 2, respectively; d indicates the resonances for the free axial ligand.

and Figure S1). These NMR data clearly reflect a dynamic equilibrium that involves reversible binding of the sterically crowded bidentate axial ligand to the RuTPP core. Similar complexity in the NMR patterns was previously discussed for the interactions of RuTPP and sterically crowded phosphorusor nitrogen-based bidentate ligands.27 Indeed, upon addition of the axial ligand to the initial (OC)RuTPP complex (RuTPP:ligand ratio of 1:1.2), the β-pyrrolic 1H NMR resonances of the latter disappear and at least eight different porphyrin-based species can be identified in the 1H NMR spectrum, along with the signals for the free axial diisocyanide ligand (Figure 1, Scheme 2). Upon further increasing the axial ligand’s concentration (up to a 5-fold excess), the 1H NMR pattern simplifies to suggest the presence of two major species in solution. Thus, at a RuTPP:ligand ratio of 1:12, which corresponds to a 5-fold excess of the axial ligand for complex 2, one of the two major species constitutes a biscoordinated complex (2), whereas the other is the free axial ligand (Figure 1). In addition, three minor species were identified in this 1H NMR spectrum. One of these compounds exhibits the βpyrrolic 1H resonance at 8.40 ppm, which has ∼10% of the intensity compared to that of the β-pyrrolic 1H signal observed for 2. Other features in the above 1H NMR spectrum include signals for diastereotopic phenyl ortho- and meta-hydrogen atoms as well as four triplet resonances corresponding to the axially coordinated ligand. The integrated resonance intensities

excess of 1,1′-diisocyanoferrocene in the synthesis of complex 2 to minimize formation of oligomers of the general formula {[(CNC5H4)Fe(C5H4NC)]RuTPP[(CNC5H4)Fe(C5H4NC)]RuTPP}n featuring bridging 1,1′-diisocyanoferrocene linkers. The known “reference” complex 418 was synthesized and isolated using a similar protocol. While complexes 1 and 4 exhibit good thermal stability in solutions, heating of complex 2 even in hydrocarbon solvents or treating this complex with toluene or DCM leads to the formation of increasing amounts of a dark-colored insoluble substance, which most likely represents a polymeric {[(CNC5H4)Fe(C5H4NC)]RuTPP[(CNC5H4)Fe(C5H4NC)RuTPP]}n molecular wire (3). This polymer was characterized by elemental analysis and IR spectroscopy. Similar to other axially coordinated diamagnetic ruthenium and iron porphyrins and phthalocyanines,26 the NMR signals for the axial ligands in complexes 1 and 4 with monodentate C

DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 2. 1H NMR Titration-Based Possible Transformations of the (OC)RuTPP Complex into ([C5H4NC]2Fe)2RuTPP and {([C5H4NC]2Fe)RuTPP}n Compounds

crowded axial ligand. Indeed, the torsion angle between the two isocyanide groups, determined from the X-ray structure of complex 2, is ∼58°, and thus, the noncoordinated isocyanide fragment is still located in close proximity to the ruthenium center. Thus, axially coordinated 1,1′-diisocyanoferrocene may act as an η2 bidentate ligand to form a hexacoordinated 1:1 (RuTPP:ligand) complex and free 1,1′-diisocyanoferrocene (Scheme 2). The CN stretching bands in the IR spectra of complexes 1−3 are shifted to lower energies compared to the corresponding spectra of the free ligands.16−18 For 1, the ν(CN) band occurs at 2094 cm−1, whereas the free isocyanoferrocene ligand16 absorbs at 2120 cm−1 (26 cm−1 difference). The IR spectrum of complex 2 features two ν(C N) bands. One is significantly (28 cm−1) red-shifted from the ν(CN) band observed for the free ligand,17 whereas the other is close in energy (8 cm−1 red-shifted) to the ν(CN) of 1,1′-diisocyanoferrocene. These observations clearly indicate that only one isocyanide group (2090 cm−1) is coordinated to the ruthenium center in complex 2, while the second CN substituent (2112 cm−1) remains noncoordinated. Because of the η1:η1-μ bridging coordination of the 1,1′-diisocyanoferrocene linker in polymer 3, there is only one isocyanide stretching band present in its IR spectrum. Moreover, since both isocyanide groups in this complex interact with the ruthenium ions, the CN stretching vibration in polymer 3 undergoes the largest (37 cm−1) red shift compared to the free ligand. The ESI mass spectra of complexes 1, 2 (recorded for samples containing an excess of the axial ligand), and the reference complex 4 are shown in Figure S3 and confirm formation of the bis(isocyanide) coordination to the ruthenium porphyrin core in these compounds.

indicate a 1:1 RuTPP:isocyanide ratio in the complex, whereas the presence of four 1H resonances from the isocyanide ligands and the diastereotopic nature of the phenyl hydrogen atoms clearly suggest that this minor species is a pentacoordinated ([C5H4NC]2Fe)RuTPP complex. The remaining two minor species contribute less than 1% to the resonance intensities of the entire 1H NMR pattern and, similar to the previous report, 27 can be tentatively formulated as oligomeric {([C 5 H 4 NC] 2 Fe) 2 RuTPP} n and hexacoordinated η 2 ([C5H4NC]2Fe)RuTPP complexes (Scheme 2). The proportion of the former oligomeric species increases after storage of the freshly prepared NMR sample for several days, while the contribution of the latter complex rises with decreasing concentration of the axial ligand (Figure 1). In the second titration experiment, (OC)RuTPP was titrated into a solution of the free ligand (Figure S2). In this case, formation of the biscoordinated complex 2 as the only reaction product was observed in solution only in the presence of a large excess of the free axial ligand. Both 1H NMR titration data sets suggest that, unlike complexes 1 and 4, complex 2 can only be stabilized in the presence of an excess of the axial ligand in solution. Indeed, when several freshly prepared samples of the biscoordinated complex 2 were washed with cold hexanes (to remove the excess free axial ligand), dried, and redissolved in CDCl3, the 1H NMR spectrum of the resulting solution was consistent with only ∼55% complex 2, the remaining ∼45% of the ruthenium porphyrin contribution being from the porphyrins with a single axial ligand coordinated to the ruthenium center. Free 1,1′-diisocyanoferrocene was present in this mixture as well. Such mediocre stability of complex 2 in solution and formation of the polymeric molecular wire 3 can be attributed to the bidentate capability of the sterically D

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alternatively, they can be attributed to the Q0−0 transition and the metal to ligand charge-transfer (MLCT) (Ru → π*, TPP) band. These two alternative assignments are discussed below in the context of our DFT and TDDFT calculations. Overall, the influence of the ferrocene-containing axial ligands on the UV− vis and MCD spectra of (RNC)2RuTPP complexes is negligible at best as these spectra are very similar to the UV−vis spectra of all earlier reported alkyl and aryl isocyanide complexes of ruthenium tetraarylporphyrins.11,18 The molecular structure of the reference complex 4 is known,18 while the solid -state structures of the new complexes 1 and 2 were determined in this work by X-ray crystallography (Figure 3, Figures S4 and S5).23 After exploring several solvent systems for crystallization, we found that room temperature evaporation of saturated solutions of complexes 1 and 2 in toluene afforded good-quality single crystals suitable for X-ray experiments. Taking into consideration the relatively low stability of the latter complex in solution, crystals of 2 were grown in the presence of excess free axial ligand. While the solid-state structure of complex 1 is solvent-free, a toluene solvent molecule is present in the X-ray crystal structure of complex 2. The CAMERON drawings of the molecular structures of complexes 1 and 2 are illustrated in Figure 3. Key crystallographic information and selected metric data for these complexes are provided in Table 1 and Table S1. Both complexes 1 and 2 crystallize in monoclinic unit cells with the ruthenium(II) atom located at the molecular center of symmetry. For complexes 1 and 2, the central ruthenium atom features a distorted octahedral C2N4 coordination with the Ru− N bonds being 0.06−0.08 Å longer than the Ru−C bonds. The Ru−CN bond lengths in complexes 1 and 2 are very close to those documented for similar ruthenium porphyrin complexes axially coordinated with 4-cyano-1-isocyano-2,6-diisopropylbenzene,11 reflecting σ-donor:π-acceptor ratios10 of the FcNC and (CNC5H4)2Fe ligands similar to that of the former aryl isocyanide. However, the Ru−CN bond distance in reference compound 4 is slightly longer than that observed for aryl isocyanides coordinated to the same macrocyclic platform. The isocyanide CN bond distances in both complexes 1 and 2 are in a typical range for the transition-metal isocyanide-containing complexes exhibiting a modest extent of back-bonding.10 It is instructive to point out that the CN bond length of the noncoordinated isocyanide group in complex 2 is 0.02 Å shorter than the CN bond distance of the coordinated isocyanide substituent, which echoes the difference in their ν(CN) vibrations. The Ru−C−N and isocyanide C−N−C angles are close to linear, and the porphyrin core is planar in both complexes. The ferrocenyl groups in both complexes assume nearly eclipsed conformations, and no significant disorder of the Cp rings was encountered in the Fourier density map. The Fe−C bond distances are within the typical range expected for the isocyanoferrocene ligands.10 The torsion angle between coordinated and noncoordinated isocyanide groups in the crystal structure of complex 2 is ∼58°. As a result, the terminal carbon atom of the noncoordinated isonitrile group forms a close contact (∼2.64 Å) with the hydrogen atom of the phenyl group of the porphyrin. Moreover, this isocyanide group is located at only ∼4 Å above the pyrrole ring of the porphyrin core, which is only slightly larger than the sum of the van der Waals radii of two carbon atoms. Again, such an arrangement may facilitate the dissociation of one axial ligand and the transformation of the remaining ligand into a bidentate platform. The Fe···Fe distances in 1 and 2 were found to be

The UV−vis and MCD spectra of complexes 1, 2, and 4 are shown in Figure 2. Similar to our previously reported

Figure 2. Experimental UV−vis (top) and MCD (bottom) spectra of complexes 4 (A), 1 (B), and 2 (C).

(RNC)2FePc complexes,13 the UV−vis and MCD spectra of the compounds reported herein are very similar to each other. Indeed, the Q-band region of the UV−vis spectra of all three complexes consists of a single band observed between 529 and 536 nm as well as a prominent shoulder with λmax at ∼580 nm. These two features in the UV−vis spectra of 1, 2, and 4 are associated with two Faraday A-terms centered around 535 and 580 nm in the corresponding MCD spectra. The Soret band region in the UV−vis spectra of the (RNC)2RuTPP complexes is dominated by a single Soret band with λmax around 418 nm, which is associated with a Faraday A-term in the corresponding MCD spectra. As expected for the effective 4-fold symmetry of the porphyrin core in the (RNC)2RuTPP complexes, ΔHOMO > ΔLUMO (ΔHOMO is the energy difference between two highest energy occupied porphyrin-centered π-orbitals, and ΔLUMO is the energy difference between two lowest energy unoccupied porphyrin-centered π-orbitals), which is reflected in the negative-to-positive (in ascending energy) sequence of the MCD signals.28 The intense UV−vis band and the MCD Aterm signal of the Soret band can be easily assigned to the porphyrin-centered π−π* transition.29 Assignment of the Qband region in the UV−vis and MCD spectra of 1, 2, and 4 is less straightforward. Indeed, the MCD A-terms around 535 (stronger) and 580 (weaker) nm can be assigned as a vibronic Q0−1 component and as a Q0−0 transition, respectively,29 or E

DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Molecular structures of complexes 1 (left) and 2 (right) as 50% thermal ellipsoids. The toluene solvent molecule of crystallization observed in the structure of complex 2 is omitted. All hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1 and 2 Complex 1 Ru(1)−N(1) Ru(1)−N(2) C(23)−Ru(1) C(23)−N(3) C(24)−N(3) C(24−28)−Fe(1) (average)

2.0674(4) 2.052(4) 1.990(6) 1.165(7) 1.385(7) 2.0418(6)

Ru(1)−N(1) Ru(1)−N(2) C(23)−Ru(1) C(23)−N(3) C(24)−N(3) C(24−28)−Fe(1) (average) C(29)−N(4) C(34)−C(7) C(34)−H(221)

2.054(3) 2.048(3) 1.970(5) 1.173(5) 1.385(6) 2.0342(5) 1.390(7) 3.541(5) 3.846(7)

N(1)−Ru(1)−N(2) N(2)−Ru(1)−C(23) N(1)−Ru(1)−C(23) Ru(1)−C(23)−N(3) C(24)−N(3)−C(23) C(29−33)−Fe(1) (average)

90.74(17) 93.11(18) 89.19(19) 173.0(5) 169.4(6) 2.0432(6)

N(1)−Ru(1)−N(2) N(2)−Ru(1)−C(23) N(1)−Ru(1)−C(23) Ru(1)−C(23)−N(3) C(24)−N(3)−C(23) C(29−33)−Fe(1) (average) C(34)−N(4) C(34)−H(51) C(34)−C(3)

90.10(13) 88.12(15) 86.64(15) 173.6(4) 177.6(4) 2.0314(6) 1.153(8) 3.768(8) 3.967(6)

Complex 2

between 11.62 and 11.70 Å, while the Fe−Ru distances were found to be between 5.81 and 5.85 Å. The intermolecular contacts in the solid-state structures of both complexes 1 and 2 are rather weak (Supporting Information, Figures S4 and S5). DFT and TDDFT Calculations. DFT and TDDFT calculations were employed to correlate the spectroscopic and redox properties of the (RNC)2RuTPP complexes with their electronic structures. A DFT-predicted energy diagram for all three complexes reported herein is shown in Figure 4, while the corresponding frontier molecular orbital drawings are given in Figure 5 and Figures S5−S8. In addition, the molecular orbital compositions for these complexes are listed in Table 2 and Table S2. For all complexes, the classic Gouterman30 “a2u”- and “a1u”-type (in D4h point group notation) porphyrin-centered πorbitals were predicted to be the HOMO and HOMO − 4 (Table 2, Figures 4 and 5). In all cases, DFT predicted that the a2u MO has higher energy compared to the porphyrin-centered

Figure 4. DFT-predicted orbital energy diagrams for (RNC)2RuTPP complexes.

F

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Figure 5. DFT-predicted frontier molecular orbitals for (RNC)2RuTPP complexes.

configuration is typically expected for low-spin distorted octahedral ruthenium(II) porphyrins.11,18,32 Because of the axial compression and electronic characteristics of the isocyanide ligands, one might expect that the rutheniumcentered dxz and dyz MOs should be less stable compared to the dxy orbital. In agreement with this prediction, our DFT calculations suggest that the energies of the predominantly ruthenium-centered dxz (HOMO − 1) and dyz (HOMO − 2) orbitals in all (RNC)2RuTPP complexes are slightly higher (0.05−0.06 eV) compared to that of the predominantly ruthenium-centered dxy (HOMO − 3) orbital. It is interesting to note that the DFT-predicted energies of the dxz, dyz, and dxy MOs are close to each other, with the largest difference being only 0.14 eV. Such near-degeneracy probably arises from two competing phenomena: an axial compression and the πaccepting capability of the axial isocyanides. A very small energy gap between the DFT-predicted porphyrin-centered HOMO and the predominantly ruthenium-centered HOMO − 1 makes it ambiguous to pinpoint the centricity of the first oxidation process as the ground-state calculations cannot predict spin polarization in a single-electron oxidized species. Indeed, previously published electron paramagnetic resonance (EPR), electrochemical, and chemical oxidation data for the reference complex 418 and a few other ruthenium porphyrins with axial isocyanide ligands,11 as well as our spectroelectrochemical and chemical oxidation data discussed below, suggest that the first oxidation process in (RNC)2RuTPP complexes is rutheniumcentered. A subtle (0.09−0.2 eV) stabilization of the ruthenium-centered HOMO − 1 compared to the porphyrincentered HOMO can be explained by the presence of a small (10%) amount of Hartree−Fock exchange in the Tao− Perdew−Staroverov−Scuseria hybrid (TPSSh) exchange-correlation functional. Indeed, it has been pointed out that the energies of the metal-centered and n-type MOs tend to decrease with an increase of the extent of Hartree−Fock exchange in the exchange-correlation functional, while the relative energies of the π-orbitals in the same systems are much less sensitive to this phenomenon.34a We also tested several “pure” local density approximation (LDA), generalized gradient approximation (GGA), and meta-GGA exchange-correlation functionals to improve agreement between theory and experiment for the (RNC)2RuTPP systems. Although several exchange-correlation functionals predicted a ruthenium-centered HOMO closely followed by the porphyrin-centered HOMO − 1 (Supporting Information, Table S3), when these

Table 2. DFT-Predicted Molecular Orbital Compositions for (RNC)2RuTPP Complexesa Complex 4 composition (%) Ru

porphyrin

Ph

t-BuNC

96.33 30.06 35.90 37.98 76.44 81.33 83.34

3.17 0.46 0.43 0.45 19.62 11.02 10.82

0.02 1.45 9.83 8.40 3.48 1.16 0.94

MO

energy (eV)

symmetry

210 211 212 213 214 215 216

−4.779 −4.53 −4.466 −4.456 −4.366 −1.976 −1.974

au ag ag ag au ag ag

MO

energy (eV)

symmetry

262 263 264 265 266 267 268

−4.892 −4.722 −4.665 −4.579 −4.477 −2.096 −2.09

au ag ag ag au ag ag

MO

energy (eV)

symmetry

Ru

porphyrin

Ph

CNFcCN

274 275 276 277 278 279 280

−4.954 −4.845 −4.791 −4.736 −4.539 −2.174 −2.134

au ag ag ag au ag ag

0.57 70.42 47.43 53.55 0.54 5.16 5.12

96.11 28.81 41.61 30.19 74.12 82.60 83.98

3.05 0.47 0.60 0.30 18.59 9.67 9.49

0.26 0.30 10.36 15.96 6.74 2.57 1.41

0.48 68.03 53.85 53.18 0.46 6.49 4.91 Complex 1

composition (%) Ru

0.62 69.58 48.20 53.59 0.57 7.14 4.41 Complex 2

porphyrin

Ph

FcNC

96.19 29.53 39.93 26.10 74.10 80.54 84.33

3.09 0.38 0.47 0.26 18.29 10.24 10.24

0.10 0.51 11.39 20.04 7.05 2.08 1.02

composition (%)

a

Data for the HOMO and LUMO are shown in bold.

a1u MO, which is a typical case for tetraarylporphyrins.31 These two porphyrin-centered π-orbitals are closely spaced with three predominantly ruthenium-centered HOMO − 1 to HOMO − 3 orbitals. For instance, the HOMO to HOMO − 1 gap is only 0.09−0.2 eV for the porphyrins considered herein. The HOMO energy decreases by 0.17 eV upon going from complex 4 to complex 2, which reflects the electron-withdrawing capabilities of the axial ligands. The (dxy)2, (dxz)2, (dyz)2 electronic G

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Inorganic Chemistry functionals were used in TDDFT calculations, no reasonable agreement between the predicted and experimental spectra of complexes 1−3 was documented (Figure S9). In contrast, TDDFT calculations with the TPSSh exchange-correlation functional resulted in similar UV−vis spectra for complexes 1− 3 and predicted energies of the Ru → TPP charge-transfer transitions to be at lower energies than the porphyrin-centered Q-band, which is in excellent agreement with the experimental data. Thus, the TPSSh exchange-correlation functional offers a reasonable compromise between the ground-state electronic structures and excited-state properties of complexes 1−3. DFT predicts several ferrocenyl-centered MOs below the porphyrincentered a1u-type orbital. These ferrocenyl-centered orbitals are ∼0.5 eV more stable in the case of complex 2 compared to complex 1, which reflects the electron-withdrawing influence of an additional isocyanide substituent (Figure 4, Table S2). Due to their delocalized nature, ferrocenyl-centered MOs can potentially support an electron-transfer pathway for the metal−metal electronic coupling documented for complex 1 on the basis of electrochemical experiments. The LUMO and LUMO + 1 in all complexes were predicted to constitute nearly degenerate porphyrin-centered π*-MOs, similar to the “eg” set of Gouterman’s four-orbital model,30 and are well-separated in energy from the other porphyrin-centered π*-orbitals. In the case of the ferrocenyl-containing compounds, LUMO and LUMO + 1 are followed by a set of higher energy unoccupied ferrocenyl-centered MOs. These are well-separated (∼0.8 eV) in energy in compound 1 but are significantly closer (∼0.4 eV) in complex 2 (Figure 4). In a simplified approach,29,33 DFT-predicted electronic structures suggest that, in addition to classic porphyrin-centered π−π* transitions, the electronic absorption spectra of (RNC)2RuTPP complexes should be further enriched with a number of low-energy Ru → π* (TPP) charge-transfer bands and (in the case of ferrocenyl-containing axial ligands) with Fc → π* (TPP) and Ru → Fc charge-transfer transitions. Since the TDDFT method was shown to provide a reasonable accuracy for the energies of π−π* and charge-transfer transitions in porphyrins and their analogues, we used this approach to assign the observed bands for the (RNC)2RuTPP systems.34 The TDDFT-predicted UV−vis spectra of all (RNC)2RuTPP complexes along with the calculated intensities for major transitions are shown in Figure 6. Symmetry requirements for the Ci point group restrict that only 1Au excited states would contribute to TDDFT-predicted intensities for the (RNC)2RuTPP complexes. Thus, although from the electronic structure standpoint numerous low-energy MLCT bands originating from the predominantly ruthenium-centered dxy, dxz, and dyz orbitals to TPP π*-MOs could be expected, they are symmetry forbidden as the LUMO and LUMO + 1 have ag symmetry. In the case of reference compound 4, the Q-band region is dominated by the contributions from excited states 6 and 7, which resemble those predicted by a classic Gouterman model, a2u (HOMO), a1u (HOMO − 4) → eg (LUMO, LUMO + 1, au → ag in the Ci point group) single-electron transitions.30 The Soret band region for this complex is dominated by two nearly degenerate excited states (excited states 9 and 10, Supporting Information, Table S4). These, again, originate from admixture of two Gouterman classic a2u, a1u → eg (au → ag in the Ci point group) single-electron transfers. According to TDDFT calculations, the first two intense MLCT bands originating from HOMO − 1 to HOMO − 3 (Ru) → LUMO + 2 (TPP, π*, excited states 13 and 14) should appear at 357

Figure 6. Experimental UV−vis (top) and TDDFT-predicted (bottom) spectra of complexes 4 (A), 1 (B), and 2 (C). The vertical blue bars represents zero intensity MLCT transitions.

and 356 nm, respectively, which reflects the LUMO to LUMO + 2 energy gap. Thus, both Q-band and Soret band regions in the reference complex 4 can be described by the porphyrincentered π−π* transitions, while the low-energy MLCT (Ru → TPP) bands should have negligible intensity because of the symmetry considerations. In the case of complexes 1 and 2, the low-energy Q-band region can be described by a set of symmetry-forbidden MLCT (Ru → TPP) transitions followed by two porphyrin-centered π−π* transitions dominated by the HOMO, HOMO − 4 → LUMO, LUMO + 1 excitations, which is similar to that of the reference complex 4. Again, in both complexes, the Soret band region is dominated by the classic porphyrin-centered π−π* transitions. Unlike the predominantly ruthenium-centered d orbitals, combinations of two sets of individual ferrocenyl-centered occupied and unoccupied orbitals result in delocalization over both ferrocenyl-based orbitals of au and ag symmetries. Because of the availability of low-energy unoccupied ferrocene-centered orbitals of au symmetry, predominantly Ru → Fc transitions would have nonzero intensities. Indeed, TDDFT predicts that the lowest energies for such bands would be observed at ∼407 nm for complex 1 (excited states 26 and 27, Table S4) and at ∼447 nm for complex 2 (excited states 17 and 18, Table S4). The lowenergy shift of these predominantly Ru → Fc transitions for complex 2 is a result of the electron-withdrawing effect of the unbound isocyanide group, which is associated with ∼0.4 eV stabilization of the ferrocenyl-centered unoccupied MOs in this complex. It should be noted, however, that the TDDFTpredicted intensities for these bands are about an order of magnitude lower compared to the Soret band intensity. In addition, symmetry-allowed low-intensity predominantly Fc → Fc (or substituted ferrocene → substituted ferrocene) H

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Inorganic Chemistry transitions between the Q-band and Soret band were also predicted by the TDDFT method in the ∼495 and ∼425 nm regions in both ferrocenyl-containing porphyrins. Overall, the Q-band region in all three compounds described herein is dominated by the classic Gouterman π−π* transitions,30 while the low-energy MLCT (Ru → TPP) bands are symmetryforbidden. Similarly, the Soret band region for all three complexes is dominated by the porphyrin-centered π−π* transitions, while the TDDFT-predicted intensities of the predominantly Fc → Fc (or substituted ferrocene → substituted ferrocene) and Ru → Fc (or Ru → substituted ferrocene) bands in ferrocene-containing porphyrins are significantly smaller. Not surprisingly, such dominance of the porphyrin-centered π−π* transitions in the Q-band and Soret band regions for complexes 1, 2, and 4 explains the similarities in the UV−vis and MCD spectra of these compounds. Redox Properties. The redox characteristics of the (RNC)2RuTPP complexes were assessed through electrochemical (cyclic voltammetry (CV) and differential pulse voltammetry (DPV), Figure 7 and Figure S10), spectroelec-

Figure 9. Spectroelectrochemical oxidation of complex 1 in DCM/ 0.15 M TFAB solution at room temperature: (A) first oxidation; (B) combined second and third oxidations.

Figure 7. DPV (red) and CV (blue) electrochemical data for complexes 4 (top) and 1 (bottom) recorded in DCM/0.05 M TFAB solutions. In all cases, the CV data were recorded at a 100 mV/s scan rate.

Figure 10. Spectroelectrochemical oxidation of complex 2 in DCM/ 0.15 M TFAB solution at room temperature.

Table 3. Half-Wave Potentials (V) for (RNC)2RuTPP in DCM/0.05M TFAB Solution at Room Temperaturea complex

RuII/RuIII

FeII/FeIII

TPP2−/TPP1−

4 1 2

−0.023 0.033 0.187

0.437, 0.533

0.713 0.997

a All potentials are referenced to the FcH/FcH+ couple and are ±5 mV.

below also correlate well with the earlier published chemical oxidation data on the same complex and allow assignment of the first oxidation couple to the RuII/RuIII process. The second oxidation process is assigned to the porphyrin ring oxidation.14 The CV and DPV data for complex 1 were recorded using the noncoordinating TFAB electrolyte because of its well-known ability to increase the resolution between redox waves in mixedvalence compounds.35 The electrochemical profile of complex 1 in a DCM/TFAB system consists of four reversible oxidation waves, with the second and third processes being in close proximity (ΔE1/2 ≈ 100 mV) to each other. The first oxidation potential is ∼60 mV more positive than the similar process documented for reference compound 4 and is assigned to the RuII/RuIII couple on the basis of the spectroelectrochemical and chemical oxidation data. The closely spaced second and third

Figure 8. Spectroelectochemical oxidation of the reference complex 4 in DCM/0.15 M TFAB solution at room temperature.

trochemical (Figures 8−10), and chemical oxidation (Figures S11−S13) experiments. Table 3 summarizes the half-wave potential data relevant to the redox processes of (RNC)2RuTPP compounds. In the case of the reference complex 4, two reversible oxidation processes in CV and DPV experiments were observed for solutions in DCM/TFAB, which correlate well with the earlier reports on this compound (Figure 7).18 Spectroelectrochemical and chemical oxidation data presented I

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Figure 11. NIR portion of the electronic spectra for transformation of [1]+ (black line) into the mixed-valence [1]2+ (red line) and [1]3+ (blue line) in DCM/0.15 M TFAB solution at room temperature (left). One of several possible band deconvolutions of the NIR transition envelope (right).

and a decrease of the reversibility of the first oxidation process. This oxidation event (Figure S10) occurred almost instantaneously with the use of a platinum working electrode, while it was slower when a glassy carbon electrode was employed (we did not explore use of a gold working electrode because of its known high affinity toward adsorbing 1,1′-diisocyanoferrocene37). The reversibility of the first redox event can be easily restored after the working electrode is polished, and such behavior is suggestive of the interaction of the noncoordinated isocyanide group with the electrode surface. Due to the adsorption of complex 2 on the electrode surface as well as the high mobility of the axial ligand and the continuous formation of the insoluble polymer 3 during the electrochemical experiments, we were unable to observe well-separated oxidation waves for the second and third oxidation events for complex 2. Despite these complications, the high coordination affinity of the uncoordinated isocyanide group in complex 2 may be considered a desirable characteristic as using polymer 3 would require anchoring these molecular wires to an electroactive surface. Spectroelectrochemical and chemical oxidation data pertaining to the first oxidation process for various (RNC)2RuTPP complexes are very close to each other (Figures 8−10 and Figures S11−S13). During the first oxidation event, the Soret band decreases in intensity and undergoes a blue shift from ∼420 to ∼400 nm, while the Q-band at 530 nm transforms into three new bands at ∼510, ∼560, and ∼640 nm. All of these changes are very similar to those observed earlier for oneelectron oxidation of reference complex 4,18 for which formation of the [4]+ complex was confirmed by a variety of spectroscopic methods. Thus, the first oxidation event for all complexes was attributed to the RuII/RuIII couple. Changes in the UV−vis spectra of [1]+ upon its transformation into the mixed-valence [1]2+and [1]3+ species during the second and the third oxidation processes are shown in Figures 9 and 11 and Figures S12 and S14. In the case of the chemical and spectroelectrochemical oxidations, the appearance of several new bands in the 600−800 nm region was observed in the corresponding UV−vis spectra. The overall intensities of these bands are close to what might be expected for porphyrincentered π−π* transitions, although the presence of the lower intensity, lower energy FeII → RuIII, RuIII → FeIII, and TPP → FeIII charge-transfer transitions cannot be excluded either. The appearance of an intervalence charge-transfer (IVCT) band in the NIR region of the electronic spectrum of [1]2+ (FeII−RuIII− FeIII core) can be considered a marker for the formation of the mixed-valence complex in solution.38 The NIR spectral transformations during the spectroelectrochemical and chemical oxidation experiments are shown in Figure 11 and Figure

redox waves are assigned to stepwise oxidations of two axial ferrocenyl moieties as these potentials are close to the FeII/FeIII redox couple of the free ligand.13,16 The fourth redox process corresponds to the oxidation of the porphyrin macrocycles on the basis of the similarity of its potential to that of the earlier reported (RNC)2RuTPP complexes.11,18 The ∼100 mV separation between the stepwise oxidation of two ferrocenyl fragments (Kc = 49; see, however, a cautionary warning on the use of Kc values obtained from the electrochemical data for analysis of the mixed-valence compounds)36 is similar to the 80 mV separation observed earlier for the (FcNC)2FePc complex13 and suggests electronic coupling between the axial isocyanoferrocene ligands separated by ca. 11.62 Å in complex 1. Collecting CV and DPV data on complex 2 turned out to be a very complicated task due to several reasons. First, as clearly evident from the 1H NMR data discussed above, complex 2 is only stable in the presence of the excess axial ligand. The mobility of the axial ligand makes it difficult to collect a full range of electrochemical data as an excess of the axial ligand, which is necessary to use to stabilize complex 2, would mask redox processes associated with the axial ferrocenyl fragment oxidations in 2. Second, a problem arises due to the high affinity of the isocyanide group toward metal coordination and even glassy carbon electrode surfaces. Indeed, interaction of 1,1′-diisocyanoferrocene with a gold surface and formation of polymeric structures with gold(I) salts are well documented.37 In all cases, it was proposed that the 1,1′-diisocyanoferrocene coordinates with the metal centers in the μ-η1:η1-motif.37 Thus, it could be expected that both the free ligand and the noncoordinated isocyanide group in complex 2 can be immobilized on the electrode surface, thereby complicating analysis of the electrochemical experiments. Indeed, the first oxidation event for complex 2 is fully reversible during the initial electrochemical scan involving a freshly polished electrode. The E1/2 value for the first oxidation of 2 is close to that documented for the RuII/RuIII couple in the oxidation of other (RNC)2RuTPP complexes,11,18 and this assignment was further confirmed by the spectroelectrochemical and chemical oxidation experiments. The RuII/RuIII half-wave potential for the oxidation of complex 2 is ∼150 mV more positive compared to the corresponding potential for complex 1, which reflects the electron-withdrawing influence of the additional isocyanide substituents in the former complex. Knowing the central metal-centered first oxidation process is important for prospective usage of complex 2 and polymer 3 as molecular wires because such an electronic structure would better facilitate electron transfer along the axial direction. Expanding the scanning range to involve the axial isocyanide ligands’ potentials resulted in a gradual shift of the oxidation potentials J

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oxidation of the dicationic complex to form [1]3+ with an FeIII− RuIII−FeIII core. The DFT and TDDFT calculations were emloyed to correlate the spectroscopic and redox properties of complexes 1, 2, and 4 with their electronic structures.

S14. These changes are indicative of a rather complex redox picture. Indeed, during oxidation of the initial [1]+ complex with an FeII−RuIII−FeII core to form the mixed-valence [1]2+ dication with an FeII−RuIII−FeIII core, we observed growth of a broad NIR band between ∼1000 and 2600 nm with a poorly defined maximum at ∼1620 nm. Upon further transformation of the mixed-valence [1]2+ to [1]3+ (FeIII−RuIII−FeIII core), this broad diffuse NIR band evolved into a more defined and intense NIR peak with λmax ≈ 1465 nm. A very similar growth in intensity and high-energy shift of the most intensive NIR transition were observed earlier for several organometallic complexes of general formula FcCCMCCFc,39 which is close to an axial FcNCRuCNFc group in complex 1. Similar to the previous reports,39 one of the possible assignments of this band could be LMCT, but the other CT transitions cannot be excluded. It is interesting to note, however, that no such behavior was documented for the reported earlier mixed-valence [(FcII/IIICN)2FeIIPc(1−)]2+ complex and fully oxidized [(FcIIICN)2FeIIPc(1−)]3+ compound.13 The overlap of the IVCT with other NIR transitions is quite common for mixed-valence compounds and often complicates data analysis.38−40 To locate the IVCT band in [1]2+, we attempted band deconvolution analysis for its NIR envelope (Figure 11). Although the NIR region of the spectrum of [1]2+ cannot be described by a single LMCT Gaussian function, the position and width of the possible IVCT band cannot be precisely determined through a fitting algorithm as many possible fitting solutions could be found. Because of the above complication, accurate analysis of the IVCT band is impossible to perform, and therefore, it is difficult to estimate the extent of metal−metal coupling in the mixedvalence [1]2+ complex.

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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.5b01614. Additional spectroscopic, crystallographic, and DFT data for (RNC)2RuTPP complexes and complete ref 20 (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address ∥

S.V.D.: Tomsk Polytechnic University, 634050 Tomsk, Russian Federation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the National Science Foundation (NSF) (Grants CHE-1401375, CHE-1464711, MRI-0922366, and MRI-1420373) and Minnesota Supercomputing Institute grants to V.N.N. as well as NSF Grant CHE-1214102 to M.V.B. is greatly appreciated. R.V.B. is grateful for the support of the HITACHI SR16000-M1 supercomputing facility by the Computer Science Group and E-IMR center at the Institute for Materials Research, Tohoku University, Sendai. This work was partially supported by ICC-IMR (International Collaboration Center, Institute for Materials Research) of Tohoku University. We are also thankful to the Ministry of Education and Science of the Russian Federation (Grant VIU-110-IPR and “Science” No. 4.2569.2014/K project).

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CONCLUSIONS New heterotrinuclear Fe−Ru−Fe complexes 1 and 2 involving ruthenium(II) tetraphenylporphyrin axially coordinated with two isocyanoferrocene or 1,1′-diisocyanoferrocene ligands, respectively, were characterized by UV−vis, MCD, NMR, and IR spectroscopies, as well as by ESI MS spectrometry and X-ray crystallography. The preparation and isolation of insoluble polymeric molecular wire 3 was also achieved for the first time. The 1H NMR titration experiments suggested significantly more pronounced lability of the 1,1′-diisocyanoferrocene ligand compared to that of isocyanoferrocene in these complexes. The redox properties of 1 and 2 were probed by employing electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation methods and were correlated with those of the reference compound 4. In all cases, the first oxidation process was attributed to a reversible oxidation of the RuII center. The second and third reversible oxidation events for 1 are separated by ∼100 mV and were assigned to two singleelectron FeII/FeIII couples, suggesting a weak long-range metal−metal coupling in this complex despite a fairly large distance (11.5 Å) between the two iron centers. Electrochemical data obtained for 2 are complicated by the interaction between the uncoordinated isocyanide substituent of the axial 1,1′-diisocyanoferrocene ligand and electrode surface as well as by the axial ligand dissociation processes. Spectroelectrochemical and chemical oxidation methods were used to elucidate the spectroscopic signatures of the [1]n+, [2]n+, and [4]n+ species in solution. A very broad absorption band was observed in the near-infrared region for the mixed-valence [1]2+ with an FeII− RuIII−FeIII core that transforms into a more defined band upon

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01614 Inorg. Chem. XXXX, XXX, XXX−XXX