Chiral Ruthenium(II) Complexes as Supramolecular Building Blocks

Nov 30, 2016 - Hasti Iranmanesh†, Kasun S. A. Arachchige†¶, Mohan Bhadbhade‡, William A. Donald†, Jane Y. Liew§, Kenny T.-C. Liu†, Ena T. ...
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Chiral Ruthenium(II) Complexes as Supramolecular Building Blocks for Heterometallic Self-Assembly Hasti Iranmanesh,† Kasun S. A. Arachchige,†,¶ Mohan Bhadbhade,‡ William A. Donald,† Jane Y. Liew,§ Kenny T.-C. Liu,† Ena T. Luis,† Evan G. Moore,*,§ Jason R. Price,∥ Hong Yan,⊥ Jiajia Yang,†,⊥ and Jonathon E. Beves*,†,⊥ †

School of Chemistry and ‡Mark Wainwright Analytical Centre, UNSW Australia, Sydney, Australia § School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland, Australia ∥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia ⊥ Key State Laboratory for Coordination Chemistry, Nanjing University, Nanjing, China S Supporting Information *

ABSTRACT: A series of enantiopure ruthenium(II) polypyridyl complexes are reported that feature pendant pyridyl groups suitable for building larger self-assembled structures. The complexes are characterized in detail in solution using NMR spectroscopy, cyclic voltammetry, and photophysical methods and in the solid state using single-crystal X-ray crystallography. The complexes are luminescent, displaying long excited-state lifetimes that are quenched when the pendant pyridyl groups are protonated. Reaction with cadmium(II) ions results in the formation of a mixed-metal one-dimensional coordination polymer, which was characterized by single-crystal X-ray crystallography.

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encapsulated,50−55 used as structural elements,56−61 or doped into MOF structures,62−65 including by postsynthetic modification.66−68 Some of these examples have been shown to be active as visible light catalysts for CO2 reduction,59,60 and others have shown promise for hydrogen production.69−72 Related cyclometalated iridium(III) complexes have also been successfully built into cadmium(II)- or copper(II)-containing networks that show solid-state luminescence.73,74 To the best of our knowledge, examples of ruthenium(II)-based photoredox catalysts that demonstrate stimulus−responsive behavior have not yet been reported, although these features might allow their catalytic properties to be controlled externally, such as in response to guest molecules. In this study, the preparation and characterization of a series of enantiopure ruthenium(II) complexes that feature pendant groups capable of recognizing protons or metal ions is reported, together with their structural, redox, and photophysical properties.

INTRODUCTION The introduction of photo- and redox-active metal complexes into extended structures allows their useful properties to be combined with the structural integrity of coordination polymer frameworks or discrete molecular assemblies. One successful strategy for incorporation of metal complexes into extended structures is their use as metalloligands,1−11 where the periphery of a complex is decorated with metal-ion binding groups to allow controlled self-assembly when combined with labile metal ions. Palladium(0)-cross coupling reactions, either on free ligands12 or directly upon metal complexes,13−15 are an efficient means to synthesize metalloligands, including pyridylfunctionalized ligands, which have been used for molecular wires16 and sensing applications.17 Ruthenium(II) complexes have attracted considerable research attention, with applications ranging from light harvesting18−21 and water splitting21 to luminescent materials22 and, more recently, photoredox catalysis.23−26 The prototypical photoactive complex [Ru(bpy)3]2+ (where bpy = 2,2′-bipyridine),27 and related complexes,28−30 have been shown to facilitate visible light photoredox catalysis,31 with impressive performance demonstrated for various reaction types ranging from cycloadditions to reductive couplings.28,32−40 Examples of discrete molecular cages containing ruthenium(II) centers remain rare,11,41−48 despite their potential to act as solution-state reaction vessels.49 However, [Ru(bpy)3]2+-type units have been incorporated into extended materials suitable for photoredox catalysis, such as metal−organic frameworks (MOFs), where they have been © XXXX American Chemical Society

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EXPERIMENTAL SECTION

General. The numbering scheme adopted for ligands 3 and 4 and complexes [Ru(phen)2(X)](PF6)2 (X = 1−6) are shown in Schemes 1 and 2. 1H and 13C{1H} NMR assignments were made using twodimensional (2D) NMR methods (COSY, ROESY, NOESY, HSQC, HMBC) and are unambiguous unless stated otherwise. cis-Ru(phen)2Cl2 (phen = 1,10-phenanthroline) was prepared according to Received: August 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b02007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. (a) Pd(0)-Mediated Suzuki Coupling Reactions (right) and Sonogashira Coupling Reactions (left) upon [Ru(phen)2(1)](PF6)2, (b) Pd(0)-Mediated Suzuki Coupling Reaction upon Ligand 1 and the Numbering Scheme Adopteda

(i) DMF/DME/NEt3, CuI, Pd(PPh3)4, 80 °C, 24 h; (ii) water/DMF, Cs2CO3, Pd(PPh3)4, 80 °C, 48 h; (iii) DMF, Cs2CO3, Pd(PPh3)4, 80 °C, 48 h.

a

Scheme 2. (a) Pd(0)-Mediated Suzuki Coupling Reactions (right) and Sonogashira Coupling Reactions (left) upon [Ru(phen)2(2)](PF6)2, (b) Pd(0)-Mediated Suzuki Coupling Reaction upon Ligand 2 and the Numbering Scheme Adopteda

(i) DMF/DME/NEt3, CuI, Pd(PPh3)4, 80 °C, 24 h; (ii) water/DMF, Cs2CO3, Pd(PPh3)4, 80 °C, 48 h; (iii) DMF, Cs2CO3, Pd(PPh3)4, 80 °C, 48 h.

a

a modified literature procedure.75 [Ru(phen)2(py)2]Cl2,76 4-pyridineboronic acid pinacol ester,77 and 4-ethynylpyridine78 were prepared according to the literature. Ligands 1, 2,79 3,80 and 674 have been previously reported. Details of synthetic procedures can be found in the Supporting Information (Sections S2 and S3). Resolution of Complexes. Both Λ and Δ isomers of [Ru(phen)2(py)2](PF6)2 were prepared following the procedure reported by Keene.81,82 Na(−)-AsOTart was prepared from D-(−)-tartaric acid following the reported procedure.81 Na(+)-AsOTart was used to isolate [Λ-Ru(phen)2(py)2](PF6)2, and Na(−)-AsOTart was used to isolate [Δ-Ru(phen)2(py)2](PF6)2. Single-Crystal X-ray Diffraction. A summary of crystallography data and refinement parameters is shown in Table 1, and data were

deposited with the Cambridge Structural Database (CCDC entries: 1493091, 1493092, 1493095, 1493096, 1493103, 1493166, 1493162, 1499345). Single crystals of [Ru(phen)2(2)](PF6)2, [Ru(phen)2(3)](PF6)2, and [Ru(phen)2(4)](PF6)2 were measured using a Bruker kappa-II CCD diffractometer at 150 K with an IμS Incoatec Microfocus Source and Mo Kα radiation (λ = 0.710 723 Å). Single crystals of [Ru(phen)2(1)](PF6)2, [Ru(phen)2(5)](PF6)2, Δ-[Ru(phen)2(4)](PF6)2, and ligand 2 were measured using Si monochromated synchrotron X-ray radiation (λ = 0.710 23 Å) at the MX1 Beamline of the Australian Synchrotron,83 while single crystals of Δ-[Ru(phen)2(2)](PF6)2 and {[Ru(phen)2(4).CdCl3](PF6)}0.5 were measB

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Inorganic Chemistry Table 1. Crystallographic Data and Refinement Parameters [Ru(phen)2(1)](PF6)2 ·MeCN empirical formula formula weight temperature/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z ρcalcg/cm3 μ/mm−1 independent reflections

{[Ru(phen)2(2)](PF6)2}2

[Ru(phen)2(3)](PF6)2

C36H26BrF12N7P2Ru 1027.56 100(2) monoclinic C2/c 22.203(4) 13.674(3) 27.303(6) 90 94.76(3) 90 8261(3) 8 1.652 1.515 9805 [Rint = 0.0468, Rsigma = 0.0239] 9805/612/546 1.109 R1 = 0.0871, wR2 = 0.2232

[Ru(phen)2(4)](PF6)2 3H2O

C34H22Br2F12N6P2Ru C39H27F12N7P2Ru 1065.40 984.68 150(2) 150(2) monoclinic monoclinic Cc P21/c 18.8064(10) 10.9641(8) 18.7873(10) 30.769(3) 22.3898(12) 12.2946(11) 90 90 102.726(3) 100.830(4) 90 90 7716.5(7) 4073.8(6) 8 4 1.834 1.605 2.654 0.558 12425 7167 [Rint = 0.1124, Rsigma = 0.0875] [Rint = 0.0790, Rsigma = 0.0858] data/restraints/parameters 12 425/263 149/102 837 7167/0/588 goodness-of-fit on F2 1.03748 1.067 final R indexes [I ≥ 2σ(I)] R1 = 0.050 3488, wR2 = R1 = 0.0485, wR2 = 0.1139 0.120 4182 final R indexes [all data] R1 = 0.0969, wR2 = 0.2284 R1 = 0.057 056, wR2 = 0.123 211 R1 = 0.0742, wR2 = 0.1228 largest diff. peak/hole/e Å−3 4.18/−1.56 1.80/−0.89 0.70/−0.65 Δ{[Ru(phen)2(4)](PF6)2}2·H2O·1 [Ru(phen)2(5)](PF6)2 /2Et2O Δ{[Ru(phen)2(2)](PF6)2}2·H2O·EtOH

C44H36F12N8O3P2Ru 1115.82 150.0 triclinic P1̅ 12.7248(11) 14.3433(11) 14.7081(12) 72.936(3) 81.380(3) 71.364(3) 2427.1(3) 2 1.527 0.483 8363 [Rint = 0.0689, Rsigma = 0.0812] 8363/0/640 1.075 R1 = 0.0659, wR2 = 0.1668

empirical formula formula weight temperature/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z ρcalcg/cm3 μ/mm−1 independent reflections

C44H30CdCl3F6N8PRu 1135.55 100(2) monoclinic C2/c 16.373(3) 24.228(5) 20.184(4) 90 105.82(3) 90 7703(3) 4 0.979 0.636 8877 [Rint = 0.1430, Rsigma = 0.0695]

data/restraints/ parameter goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff. peak/hole/ e Å−3

C41H27F12N7P2Ru 1008.70 100(2) triclinic P1̅ 11.440(2) 14.710(3) 14.990(3) 76.03(3) 78.88(3) 86.64(3) 2401.8(9) 2 1.395 0.475 11 395

C70H52Br4F24N12O2P4Ru2 2194.89 100 orthorhombic P212121 18.237(4) 32.482(7) 12.878(3) 90 90 90 7629(3) 4 1.911 2.690 21 456

C180H134F48N32O3P8Ru4 4357.22 100(2) monoclinic P21 12.638(3) 23.853(5) 16.772(3) 90 95.94(3) 90 5028.8(18) 1 1.439 0.461 23 185

[Rint = 0.0770, Rsigma = 0.0496] 11 395/63/569

[Rint = 0.0522, Rsigma = 0.0271]

[Rint = 0.0630, Rsigma = 0.0497]

21 456/63/1069

23 185/289/1296

8877/69/280

1.019 R1 = 0.0765, wR2 = 0.2021 R1 = 0.0953, wR2 = 0.2139 5.26/−0.81

1.043 R1 = 0.0480, wR2 = 0.1251

1.022 R1 = 0.0494, wR2 = 0.1232

1.137 R1 = 0.1074, wR2 = 0.3015

R1 = 0.0497, wR2 = 0.1263

R1 = 0.0618, wR2 = 0.1302

R1 = 0.1313, wR2 = 0.3196

2.77/−1.74

0.51/−1.13

3.43/−1.71

ured using Si monochromated synchrotron X-ray radiation (λ = 0.710 23 Å) at the MX2 Beamline of the Australian Synchrotron.

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R1 = 0.0797, wR2 = 0.1732 0.89/−0.65 {[Ru(phen)2(4)·CdCl3] (PF6)}0.5

2,2′-bipyridine (1) and 5,5′-dibromo-2,2′-bipyridine (2) were prepared as described in the literature,79 except purification was performed by recrystallization from 1:1 CHCl3/EtOH rather than by column chromatography. These ligands could be reacted directly to form ligands 3−6, with full details provided in the Supporting Information.84 Ligands 1 and 2 were reacted with cis-Ru(phen)2Cl2 under microwave conditions85 in ethane1,2-diol leading to isolated yields of 87% and 94% for the

RESULTS AND DISCUSSION

Synthesis of Complexes [Ru(phen)2(X)](PF6)2, (X = 1− 6). A variety of [Ru(phen)2(X)](PF6)2 complexes as shown in Figure 1 were prepared in this study, where phen = 1,10phenanthroline and X = ligands 1−6. The ligands 5-bromoC

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characterized in solution using 2D multinuclear NMR techniques to allow unambiguous assignment of both 1H and 13 C NMR signals. To assign the signals corresponding to the protons of the phen ligands in the monosubstituted complexes [Ru(phen)2(1)]2+, [Ru(phen)2(3)]2+, and [Ru(phen)2(5)]2+, one-dimensional (1D) selective nuclear Overhauser effect (NOE) experiments were performed as previously described (Supporting Information Section S5 Figures S39 and S40). The 1 H NMR spectra and assignments are shown in Figure 2. Some general trends can be noted by comparison of these 1H NMR spectra. First, the introduction of substituents onto the 5,5′-positions of the bipyridine ligand has a similar influence on the adjacent signals in both the mono- and disubstituted complexes. As examples, the HA4 and HA6 signals (see Scheme 1 and Scheme 2 for numbering) are shifted downfield consistently between the two series, relative to the parent [Ru(phen)2(bpy)]2+ complex. Introduction of the bromo substituent results in small downfield shifts for the HA6 proton (Δδ +0.08 ppm for [Ru(phen)2(1)]2+and +0.04 ppm for [Ru(phen)2(2)]2+), and more significant downfield shifts for the HA4 signals (Δδ +0.18 ppm for [Ru(phen)2(1)]2+ and +0.17 ppm for [Ru(phen)2(2)]2+). Similar trends were observed upon introduction of pyridyl groups, with downfield shifts for the HA6 proton (Δδ +0.19 ppm and +0.20 ppm); and for the HA4 protons (Δδ by +0.30 and +0.33 ppm), for the monosubstituted ([Ru(phen)2(3)]2+) and disubstituted ([Ru(phen)2(4)]2+) complexes respectively, relative to the parent [Ru(phen)2(bpy)]2+ complex. The HA4 and HA6 signals also displayed similar trends on the introduction of acetylenepyridyl groups in [Ru(phen)2(5)]2+ and [Ru(phen)2(6)]2+. In the monosubstituted cases, the signals for the unsubstituted pyridyl group are largely unaffected upon introduction of substituents on the other ring, and these electronic effects are entirely as expected. More interesting to note are the changes in the phenanthroline 1H NMR signals, which are effectively influenced primarily by through-space effects and are sensitive to changes in coordination geometry. Let us start by considering the protons

Figure 1. Ruthenium(II) complexes prepared in this study. py = 4pyridyl.

[Ru(phen)2(1)](PF6)2 and [Ru(phen)2(2)](PF6)2 complexes, respectively. We note that extended heating of complexes containing 1 or 2 results in dehalogenation of the ligand,14 so particular care is required when performing coupling reactions upon these units. These building blocks were further elaborated with additional metal ion binding sites using Pd(0) cross-coupling reactions such as those we have described previously.14 Specifically, reaction of [Ru(phen)2(1)](PF6)2 or [Ru(phen)2(2)](PF6)2 with excess 4-pyridineboronic acid pinacol ester, Pd(PPh3)4, and Cs2CO3 in degassed water/dimethylformamide (DMF) gave [Ru(phen)2(3)](PF6)2 and [Ru(phen)2(4)](PF6)2 with isolated yields of 71% and 73%, respectively (Schemes 1 and 2). Similarly, the reaction of [Ru(phen)2(1)](PF6)2 or [Ru(phen)2(2)](PF6)2 with excess 4-ethynylpyridine, Pd(PPh3)4, and CuI in degassed 1,2-dimethoxyethane (DME)/DMF/NEt3 gave [Ru(phen)2(5)](PF6)2 and [Ru(phen)2(6)](PF6)2 with isolated yields of 60% and 54%, respectively (Schemes 1 and 2). In all cases the complexes were purified by column chromatography (SiO2, acetonitrile/water/saturated KNO3(aq) mixtures), with the center of the main orange band being collected and concentrated with excess ammonium hexafluorophosphate to precipitate the target complexes as hexafluorophosphate salts. Nuclear Magnetic Resonance Characterization. Each of the [Ru(phen)2(X)](PF6)2 complexes (X = 1−6) was

Figure 2. 1H NMR spectra (CD3CN, 500 MHz, K2CO3 to ensure no protonation) of (a) [Ru(phen)2(bpy)](PF6)2, (b) [Ru(phen)2(1)](PF6)2, (c) [Ru(phen)2(2)](PF6)2, (d) [Ru(phen)2(3)](PF6)2, (e) [Ru(phen)2(4)](PF6)2, (f) [Ru(phen)2(5)](PF6)2, (g) [Ru(phen)2(6)](PF6)2. D

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Inorganic Chemistry adjacent to the phenanthroline nitrogen atoms, which we labeled HC2 and HC2′ in the disubstituted complexes. The HC2′ signal, which is on the ring trans to the equivalent phenanthroline ligand, is shifted downfield (+0.12 and +0.09 ppm for [Ru(phen)2(4)]2+ and [Ru(phen)(6)]2+, respectively, relative to [Ru(phen)2(bpy)]2+), whereas the HC2′ signal, which is on the ring trans to the bipyridine ligand, is essentially unaffected in [Ru(phen)2(4)]2+ (Δδ +0.03) but shifted upfield in [Ru(phen)(6)]2+ (Δδ −0.08 ppm) relative to [Ru(phen)2(bpy)]2+. If we consider the more complicated monosubstituted complexes it can be seen that the trends discussed above are broadly mirrored. First, it must be noted that the lower symmetry of these complexes results in four nonequivalent pyridyl rings corresponding to the phenanthroline ligands, labeled C, C′, C″, and C‴ (see Scheme 1 for labeling). In these cases, pairs of signals overlap (ring C′ with ring C″; ring C with ring C‴), with the exception of the signals corresponding to the protons adjacent to the nitrogen atoms, HC2, HC2′, HC2′′, and

Figure 3. UV−visible absorption spectra (left) and normalized emission spectra (right) of the complexes synthesized in this study. All samples were measured at ∼2 × 10−6 mol/L (MeCN) and excited at 450 nm.

HC2′′′. To assign these nonequivalent signals, 1D selective NOE experiments were performed (Supporting Information section S5 Figures S39 and S40). Specifically, the 1H NMR signals of the phenanthroline rings nearest to the substituted bpy ring (HC2 and HC2′′) show NOE coupling to the bipyridine HA6′ signal, in line with recent assignments of the analogous signals of related complexes.85 The signals that correspond to the protons close in space to the substituted bipyridyl ring (i.e., HC2 and HC2″) have similar chemical shifts to the analogous signals of the disubstituted complexes. For example, the HC2 signal is shifted downfield (Δδ = +0.10, +0.14, +0.11 ppm) in the monosubstituted complexes [Ru(phen) 2 (1)] 2+ , [Ru(phen)2(3)]2+, and [Ru(phen)2(5)]2+, and similar shifts were observed in the analogous disubstituted complexes (Δδ = +0.04, +0.12, and +0.09 ppm, all relative to [Ru(phen)2(bpy)]2+). The signals of phenanthroline rings close in space to the unsubstituted pyridyl ring (e.g., HC2′ and HC2′′′) are largely unaffected by the substitution (Δδ < 0.07 ppm), as might be expected. This pattern of NMR signals is likely common for unsymmetrical ruthenium(II) polypyridyl complexes and offers some interesting insight into the structures of the complexes. The solid-state structures of these complexes (vide infra) show the complexes adopt near-ideal octahedral geometry, suggesting the observed solution-state NMR signal shifts are the result of electronic rather than steric effects. We expect these 1H NMR assignments to be general for a wide variety of related ruthenium(II) polypyridyl complexes and will prove useful benchmarks for future NMR assignments. In complexes [Ru(phen)2(3)]2+, [Ru(phen)2(4)]2+, [Ru(phen)2(5)]2+, and [Ru(phen)2(6)]2+, the HB2 signals corresponding to protons on pendant pyridyl rings exhibit strong sensitivity to acid and water content and become extremely broad upon protonation. Similar behavior has been perceived in 4′-pyridyl decorated 2,2′:6′,2″-terpyridine complexes of ruthenium(II)86 and in bis(2,2′-bipyridine)(5,5′-(3-pyridyl)2,2′-bipyridine) ruthenium(II) complexes.14 Photophysical Studies. The solution-state absorption and emission spectra of the [Ru(phen)2(X)](PF6)2 complexes (X = 1−6) were measured in acetonitrile and are shown in Figure 3. In each case an intense ligand-centered (LC) π−π* absorption is observed at 263 nm, as expected for ruthenium(II)

polypyridyl complexes,87 in addition to typical spin-allowed MLCT d−π* absorption bands at lower energies (MLCT = metal-to-ligand charge transfer). In all complexes, these 1MLCT transitions range from 443 to 450 nm, which are similar to [Ru(phen)2(bpy)]2+, which has a typical 1MLCT transition at 448 nm.87,88 Notably, these 1 MLCT transitions are also slightly broader in comparison to [Ru(phen)2(bpy)]2+, and additional bands are also apparent at longer wavelengths. For example, for [Ru(phen)2(6)]2+, an additional peak is evident centered at 498 nm. This suggests the complexes exhibit multiple 1MLCT excited states, with the lowest-energy transition corresponding to a bpy-localized 1 MLCT excited state, and the higher-energy bands corresponding to phen-localized 1MLCT exited-state states. For complexes with pendant pyridyl groups, the 1MLCT absorption was found to be sensitive to acid, with protonation resulting in a broadening of the band towards longer wavelengths, and a decreasing in the intensity of the absorption. Data for complex [Ru(phen)2(4)]2+ is shown in Figure 4, and related data for the other complexes can be found in the Supporting Information (Figures S44−S46). The introduction of an extended acetylene spacer also resulted in new ligandcentered transition, which decreases in energy and increases in intensity on going from mono- to disubstitution, yielding an intense absorption at 348 nm for the [Ru(phen)2(6)](PF6)2 complex. Upon protonation of the pyridyl groups, this band undergoes a red shift to 359 nm with no appreciable change in intensity. Each of the [Ru(phen)2(X)]2+ complexes (X = 1−6) displays expected 3MLCT photoluminescence in acetonitrile solution at room temperature, and their absorption and emission data are summarized in Table 2. Most notably, as substituents are introduced onto the bipyridine unit, the peak emission wavelength is shifted to lower energy, with [Ru(phen)2(6)]2+ having the lowest-energy emission at 696 nm, which is redshifted by almost 90 nm with respect to the parent [Ru(phen)2(bpy)]2+ complex. In addition to the changes noted in the 1MLCT absorption, the corresponding 3MLCT emission of complexes containing appended pyridyl groups (i.e., [Ru(phen)2(3)]2+ to [Ru(phen)2(6)]2+) is quenched upon the addition of excess 1

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which also demonstrated significant emission quenching upon protonation.92 Solution photoluminescence quantum yield (PLQY) and 3 MLCT excited state lifetimes (τ) were determined for the fully deprotonated complexes, together with several reference complexes measured under identical conditions as their bishexafluorophosphate salts in aerated acetonitrile, with results as summarized in Table 2. Each of the [Ru(phen) 2(X)]2+ complexes (X = 1−6) display room-temperature emission, with PLQY and τ values that are comparable to those of the [Ru(bpy)3]2+ and [Ru(phen)3]2+ complexes. Notably, the PLQY value we obtain for the former is also in excellent agreement with recent reports from the literature.90 By comparison to [Ru(phen)2(bpy)]2+ as a model, the bromofunctionalized [Ru(phen)2(1)]2+ and [Ru(phen)2(2)]2+ complexes display slightly longer 3MLCT excited-state lifetimes of ca. 200 and 290 ns, respectively, primarily due to a decrease in the nonradiative deactivation rate constant (knr). Similarly, complexes with pendant pyridyl groups also have slightly longer excited-state lifetimes ranging from ca. 170 to 245 ns. Considering the entire [Ru(phen)2(X)]2+ series of complexes (X = 1−6), we also note that the nonsymmetric monosubstituted complexes (X = 1, 3, 5) have consistently higher radiative deactivation rate constants (kr) compared to the symmetrically disubstituted complexes (X = 2, 4, 6), although the reasons for these differences remain unclear. Lastly, as noted, the steady-state 3MLCT emission of the [Ru(phen)2(3−4)]2+ complexes bearing pendant pyridyl groups is effectively quenched upon titration with TFA, and corresponding time-resolved emission profiles collected in the presence of excess acid displayed multiexponential decay behavior. In particular, a decrease in the observed lifetimes was noted, yielding better fits to a biexponential decay including shorter-lived components on the 10−20 ns time scale. This behavior can be readily attributed to the presence of competing excited-state protonation equilibria in solution, such as that observed for other ruthenium(II) polypyridyl complexes including [Ru(bpy)2(bpm)]2+ or [Ru(bpy)2(bpz)]2+ (bpm = 2,2′-bipyrimidine, bpz = 2,2′-bipyrazine) featuring coordinated ligands with additional basic nitrogen atoms sensitive to aqueous solution pH.93

Figure 4. UV−visible absorption spectra (left) and emission spectra (right, λex = 415 nm) of [Ru(phen)2(4)]2+ upon titration with aliquots of trifluoroacetic acid in acetonitrile.

trifluoroacetic acid (TFA), due to protonation of the pendant nitrogen atoms. An example titration for [Ru(phen)2(4)](PF6)2 is shown in Figure 4, and data for the other complexes can be found in the Supporting Information (Figures S44−S46). Addition of acid had no significant effect on the absorption or emission of the bromo-functionalized complexes [Ru(phen)2(1)]2+ and [Ru(phen)2(2)]2+, as shown in Figure S48, which confirms the interaction between the acid and noncoordinated pyridyl nitrogen atoms on the complexes is responsible for the observed changes in optical properties. The observed luminescence quenching can be attributed to the proximity of additional positive charge on the pendant pyridyl ring adjacent to the metal center,91 providing additional nonradiative deactivation pathways, and similar results have been observed for a [Ru(bpy)2(2,2′-bipyrazine)]2+ complex, Table 2. Summary of Photophysical Dataa complex [Ru(bpy)3](PF6)2

[Ru(phen)3](PF6)2b [Ru(phen)2(bpy)](PF6)2 [Ru(phen)2(1)](PF6)2 [Ru(phen)2(2)](PF6)2 [Ru(phen)2(3)](PF6)2 [Ru(phen)2(4)](PF6)2 [Ru(phen)2(5)](PF6)2 [Ru(phen)2(6)](PF6)2

abs λmax/nm MLCT

ε 1 × 10−3/M−1 cm−1 at 450 nm

b

89

452 45289 45187 446b 447c,87 448b 44287,88 447 444 449 443 445 438, 498

13

ems λmax/nm b

ΦPL/% b

τ/ns b

kr 1 × 104/s−1 b

knr 1 × 106/s−1 6.2b

1.9 1.890 4.0c,90 1.0b

158

12.0

97b

10.3b

10.2b

1887,88

616 61589 62190 598b 604c,87 608b

1.5

116

12.9

8.5

13.8 11.8 13.1 10.3 11.9 8.6

627 640 643 666 654 696

2.0 2.5 2.3 1.7 1.9 0.9

203 291 214 245 224 169

9.9 8.6 10.7 6.9 8.5 5.3

4.8 3.4 4.6 4.0 4.4 5.9

18c

,87

a All measurements performed in aerated acetonitrile except where specified. Photoluminescence quantum yields (Φ), lifetimes (τ), and rate of radiative (kr) and nonradiative (knr) decay. bMeasured in this study unless otherwise reference. cIn aerated water. Data for reference complexes [Ru(phen)3]2+, [Ru(phen)2(bpy)]2+, and [Ru(bpy)3]2+ was in agreement with literature values;87 see Supporting Information for details.

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DOI: 10.1021/acs.inorgchem.6b02007 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 3. Electrochemical Data of Selected Ru(II) Complexesa Ru(II) complexes [Ru(bpy)3](PF6)2e [Ru(phen)3](PF6)2e [Ru(phen)2(py)2](PF6)2e [Ru(phen)2(bpy)](PF6)2e [Ru(phen)2(1)](PF6)2 [Ru(phen)2(2)](PF6)2 [Ru(phen)2(3)](PF6)2 [Ru(phen)2(4)](PF6)2 [Ru(phen)2(5)](PF6)2 [Ru(phen)2(6)](PF6)2

ligand reductionsb/V vs Fc/Fc+

M2+/3+/V vs Fc/Fc+ +0.90 +0.90 +0.90 +0.90 +0.94 +0.97 +0.94 +0.97 +0.93 +0.96

−1.63irr −1.67irr −1.59 −1.45 −1.51 −1.31

−1.72 −1.76 −1.78 −1.75 −1.75 −1.76 −1.86 −1.80 −1.81 −1.68

−1.89 −1.89 −1.96 −1.92 −1.91 −1.91

−2.15 −2.21

−1.96

−2.17 −2.19 −2.17 −2.12 −2.23

−1.92

−2.23

Eox − Ered/Vc

emission/eVd

2.62 2.66 2.68 2.69 2.57 2.64 2.53 2.42 2.44 2.27

2.01 2.07 n/d 2.04 1.98 1.94 1.93 1.86 1.90 1.78

a

All cyclic voltammetry measurements were performed in MeCN solution with 0.1 M [nBuN]PF6. Glassy carbon, platinum, and leakless Ag/AgCl electrodes were used as working, counter, and reference electrodes, respectively. Reference and potentials quoted are vs Fc+/Fc. bAll processes are reversible, except where noted, irr = irreversible. cEox − Ered = first oxidation potential − first reversible reduction potential. dCalculated using emission λmax values in Table 2. n/d = none detected. eData measured in this study, for literature values see ref 87.

and Δ-[Ru(phen)2(py)2](PF6)2 complexes were prepared using a literature procedure,76,81 and reaction of ligands 1 or 2 with these complexes under microwave conditions (160 °C, 30 min) in ethane-1,2-diol led to isolation of Λ- or Δ-[Ru(phen)2(1)](PF6)2 and Λ- or Δ-[Ru(phen)2(2)](PF6)2, respectively. Subsequent reaction of Λ- or Δ-[Ru(phen)2(1)](PF6)2 or Λand Δ-[Ru(phen)2(2)](PF6)2 under microwave conditions (120 °C, 1 h) with excess 4-pyridineboronic acid pinacol ester, Pd(PPh3)4, and Cs2CO3 in degassed DME/DMF furnished enantiopure Λ- and Δ-[Ru(phen)2(3)](PF6)2 or Λand Δ-[Ru(phen)2(4)](PF6)2 with isolated yields of 74% and 76%, respectively. Similarly, reaction of Λ- and Δ-[Ru(phen)2(1)](PF6)2 or Λ- and Δ-[Ru(phen)2(2)](PF6)2 under microwave condition (120 °C, 1 h) with excess 4(trimethylsilylethynyl)pyridine, Pd(PPh3)4, CuI, and K2CO3 in degassed MeOH/DMF/NEt3 gave Λ- or Δ-[Ru(phen)2(5)](PF6)2 and Λ- or Δ-[Ru(phen)2(6)](PF6)2 with isolated yields of 66% and 54%, respectively. Importantly, using microwave conditions resulted in significantly shorter reaction times compared to the more traditional coupling reaction conditions described for the racemic complexes reported in Section 3 of the Supporting Information. This method also showed no evidence of racemization. Circular dichroism (CD) measurements as shown in Figure 5 show that the spectra for the Λ and Δ isomers of the

Electrochemistry. The electrochemical behavior of the complexes was investigated using cyclic voltammetry, with individual results summarized in Table 3, together with selected ruthenium(II) reference complexes measured under identical conditions. All of the complexes display a reversible oneelectron oxidation peak, corresponding to the metal-centered Ru(II)/Ru(III) redox couple, with E1/2 values ranging from +0.93 to +0.97 V (vs Fc+/Fc) as expected based on the redox couples of typical Ru(II)/Ru(III) complexes.87 In all cases, the redox potential was shifted to more positive values upon going from mono to difunctionalized ligands. For the [Ru(phen)2(X)]2+ (X = 1−6) complexes, the first reduction process occurs within a potential range from −1.31 to −1.68 V (vs Fc+/Fc), and in all cases corresponds to a bipyridine-ligand-centered reduction. These values indicate the functionalized bipyridine ligands used in this study are all more readily reduced than the unfunctionalized bipyridine or phenanthroline ligands, which display their first ligand-based reductions in the range from −1.72 to −1.76 V under identical conditions. The bromo-functionalized derivatives [Ru(phen)2(1)]2+ and [Ru(phen)2(2)]2+ show an irreversible ligand-based reduction at −1.63 and −1.67 V, respectively, in addition to the expected series of three reversible ligand reductions at more negative potentials, while for the other complexes, a total of up to four reversible ligand-based reductions were observed. The band gaps calculated from electrochemical data using the first reversible oxidation and first reversible reduction suggest much lower values than those obtained from the 1MLCT absorption maxima, in agreement with the observed broad absorption data. For example, [Ru(phen)2(6)]2+ shows an additional broad absorption band centered at 500 nm, which extends beyond 600 nm (