Diastereomerization Dynamics of a Bistridentate RuII Complex

Article pubs.acs.org/IC

Diastereomerization Dynamics of a Bistridentate RuII Complex Martin Jarenmark,† Göran Carlström,† Lisa A. Fredin,‡ Joachim Hedberg Wallenstein,§ Isa Doverbratt,† Maria Abrahamsson,§ and Petter Persson*,‡ †

Centre for Analysis and Synthesis, and ‡Theoretical Chemistry Division, Department of Chemistry, Chemical Center, Lund University, Box 124, SE-22100 Lund, Sweden § Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Sweden S Supporting Information *

ABSTRACT: The unsymmetrical nature of a new tridentate ligand bis(quinolinyl)-1,3-pyrazole (DQPz) is exploited in a bistridentate Ru(II) complex [Ru(DQPz)2]2+ to elucidate an unexpected dynamic diastereomerism. Structural characterization based on a combination of nuclear magnetic resonance spectroscopy and density functional theory calculations reveals the first quantifiable diastereomerization dynamics for Ru complexes with fully conjugated tridentate heteroaromatic ligands. A mechanism that involves a large-scale twisting motion of the ligands is proposed to explain the dynamic interconversion between the observed diastereomers, and the analysis of both experiments and calculations reveals a potential energy landscape with a transition barrier for the diastereomerization of ∼70 kJ mol−1. The structural flexibility demonstrated around the central transition metal ion has implications for integration of complexes into catalytic and photochemical applications.

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INTRODUCTION Ruthenium complexes are widely used as light-harvesters in molecular solar energy conversion technologies;1 as efficient molecular catalysts for water oxidation and organic reactions;2 and as molecular sensors, anticancer drug candidates, and luminescent probes in medicinal applications.3 In particular, due to the excellent excited state properties of many ruthenium polypyridyl complexes such as [Ru(bpy)3]2+ (bpy =2,2′bipyridine), they serve as prototype systems for the development of photophysical and photochemical applications based on metal complexes.4 There is a significant increase in the number of possible stereoisomers in six-coordinate complexes compared to the typical tetrahedral carbon stereocenter, opening up new opportunities for stereoisomer selectivity and control, relevant for selective catalysis and medicinal chemistry.5 Also, [Ru(bpy)3]2+ is chiral due to the spatial orientation of the ligands around the central transition metal ion, which has been exploited in enantioselective photocatalysis.6 Asymmetry introduced in the ligand, either by pre-existing stereogenic centers or conformational locking of the formed chelate rings, adds further stereochemical possibilities. There has been significant interest in developing transition metal complexes for photochemical applications based on aromatic tridentate ligands, such as the prototype 2,2′:6′,2″terpyridine (tpy) (Figure 1a).7 The nonchiral homoleptic [Ru(tpy)2]2+complex, however, suffers from a short excited state lifetime that makes it less useful for photochemical © XXXX American Chemical Society

Figure 1. (a) Tridentate ligands DQPz, tpy and DQP. (b) Structure of mer-[Ru(DQPz)2]2+ with the pyrazole dipole moment indicated by red arrows. The dihedral angles ϕflip and ϕQ−Q are between the two arrows and between the two C8−C9 bonds of the quinolines on each ligand, respectively. (c) Representations of possible mer-stereoisomers and their interconversion, viewed along the pyrazole−Ru−pyrazole axis as indicated in (b). The polarity arrow at the back is dashed to highlight that it is located behind the metal center.

applications.7 This has been largely attributed to the distorted octahedral geometry arising from acute bite angles associated with the two five-membered chelate rings formed by each Received: December 16, 2015

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DOI: 10.1021/acs.inorgchem.5b02893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

lockmass correction. The ion source positions were level B, setting 1.0. The ion source settings were the following: ion spray voltage 3.2 kV, source heater 50 °C, sheath gas 10, auxiliary gas and sweep gas 0, capillary temperature 275 °C, and S-lens 60%. Combustion analysis was performed by Eurofins BioPharma Product Testing (Sweden) on a FlashEA 1112 (Thermo Fisher Scientific) CHN-analyzer using helium as carrier gas and a copper catalyst for reduction of nitrous gases. Sulfur and halogens were removed by silver cobolt oxide. Synthesis. 1-(8-Quinolinyl)-3-bromopyrazole (Q1PzBr). To a dry 100 mL two-necked round-bottom flask was added 522 mg (2.51 mmol) of 8-bromoquinoline, 517 mg (3.52 mmol) of 3-bromopyrazole, 480 mg (2.52 mmol) of copper(I) iodide, and 2.145 g of finely ground dry potassium carbonate. Then 0.67 mL (5.58 mmol) of transN,N′-dimethyl-1,2-cyclohexanediamine and 26 mL of degassed anhydrous toluene were added. The mixture was heated to reflux and intense stirring maintained. After 22 h of reflux, conversion of 8bromoquinoline did no longer proceed, as indicated by TLC (SiO2, ethyl acetate/heptanes 1:1), so heating was removed and solution filtered to remove solid that was extracted with several portions of toluene. The combined filtrates were evaporated under reduced pressure, which yielded a brown oil that was chromatographed on silica eluating with ethyl acetate/heptanes 1:6 to yield 125 mg (18%) of a white solid. Elemental analysis C12H8BrN3: calcd % C, 52.58; H, 2.94; N, 15.33; observd C, 52.59; H, 2.82; N, 15.01%; 1H NMR (500 MHz, CDCl3) ppm 8.98 (dd, 1H, J = 1.7 Hz, J = 4.1 Hz), 8.71 (d, 1H, J = 2.4 Hz), 8.26 (dd, 1H, J = 1.5 Hz, J = 8.3 Hz), 8.22 (dd, 1H, J = 1.2 Hz, J = 7.6 Hz), 7.82 (dd, 1H, J = 1.1 Hz, J = 8.1 Hz), 7.65 (dd, 1H, J = 7.9 Hz, J = 7.9 Hz), 7.50 (dd, 1H, J = 4.2 Hz, J = 8.3 Hz), 6.55 (d, 1H, J = 2.4 Hz). 1,3-Bis(8-quinolinyl)pyrazole (DQPz). To a 50 mL two-necked round-bottom flask was added 111 mg (0.404 mmol) of Q1PzBr, 105 mg (0.607 mmol) of 8-quinolinyl boronic acid, 94.8 mg (0.082 mmol) of tetrakis(triphenylphosphine) palladium(0), and 206 mg (0.972 mmol) of potassium phosphate tribasic. After purging the flask with N2 gas, 7.0 mL of degassed dimethylformamide and 1.8 mL of degassed deionized water were added and the solution was heated at 100 °C and stirred intensely for 3 h. The reaction mixture was allowed to cool and was then treated with 25 mL of 1.0 M NaOH(aq), 10 mL of deionized water, and 40 mL of ethyl acetate. The mixture was shaken thoroughly before separating the phases and extracting the aqueous layer with 3 × 15 mL of ethyl acetate. The combined organic phases were washed with 15 mL of 0.5 M NaOH(aq), 15 mL of brine, and 15 mL of deionized water before drying them over anhydrous Na2SO4. After filtering off the Na2SO4 , the solvent was evaporated to yield 380 mg of a brown oil that was chromatographed on silica by eluting with ethyl acetate/heptane 1:2, yielding 85 mg (71%) of a yellowish sticky solid. Elemental analysis C21H14N4·0.1H2O: calcd % C, 77.81; H, 4.42; N, 17.28; observd C, 77.42; H, 4.20; N, 16.95; 1H NMR (500 MHz, CD3CN) ppm 9.02 (dd 1H, J = 1.6 Hz, J = 4.1 Hz), 9.01 (dd 1H, J = 1.6 Hz, J = 4.1 Hz), 8.88 (d, 1H, J = 2.5 Hz), 8.46 (dd, 1H, J = 1.5 Hz, J = 7.3 Hz), 8.42 (dd, 1H, J = 1.8 Hz, J = 8.4 Hz), 8.35 (dd, 1H, J = 1.8 Hz, J = 8.3 Hz), 8.32 (dd, 1H, J = 1.4 Hz, J = 7.6 Hz), 7.96 (d, 2H, J = 8.2 Hz), 7.75 (dd, 1H, J = 7.8 Hz, J = 7.8 Hz), 7.69 (dd, 1H, J = 7.3 Hz, J = 8.1 Hz), 7.61 (dd, 1H, J = 4.2 Hz, J = 8.4 Hz), 7.59 (d, 1H, J = 2.5 Hz), 7.54 (dd, 1H, J = 4.1 Hz, J = 8.3 Hz). [Ru(DQPz)2](PF6)2·CH3OH. To a 5 mL vial of borosilicate glass was added 26.2 mg (0.0541 mmol) of Ru(dimethyl sulfoxide)4Cl2 [Ru(DMSO)4Cl2], 35.0 mg of (0.109 mmol) of DQPz, and 2.0 mL of ethylene glycol. The mixture was heated at 180 °C and stirred intensely for 30 min and was then cooled on a water bath. Then 2.0 mL of methanol was added and the solution was filtered. To the filtrate 0.80 mL of a 0.5 M sodium hexafluorophosphate solution in methanol/water 1:1 was added dropwise, and a red solid started forming. To complete the precipitation the mixture was kept at room temperature overnight, and the following day the solid was filtered off and washed with 3 × 0.5 mL of a mixture of methanol/water 1:2; after drying under vacuum 24.7 mg (44%) of a red solid was isolated. Elemental analysis C42H28F12N8P2Ru·CH3OH: calcd % C, 48.37; H, 3.02; N, 10.49; observd C, 48.09; H, 2.60; N, 10.46; 1H NMR (500 MHz, CD3CN) ppm 8.74 (m, 2H) 8.47 (d, 1H, J = 4.0 Hz), 8.38 (d,

ligand upon coordination. Considerable progress toward useful light-harvesting metal complexes has recently been achieved with tridentate ligands using design strategies involving either expanded coordination cages or cyclometalated or carbene heteroaromatic ligands,8 including significant recent advancements in prolonging the comparatively short excited state lifetimes of Fe(II) complexes through the use of tridentate Nheterocyclic (NHC) carbene ligands.9 Understanding the structural and electronic properties of such complexes is needed to facilitate further development of new ligands. Introduction of an expanded cage structure involving sixmembered chelate rings using the 2,6-bis(8-quinolinyl)pyridine (DQP) ligand (Figure 1a) gave significantly improved excited state lifetimes.10 In particular, the expanded coordination cage motif has in several cases been found to have a profound influence on the structure around the metal ion in terms of improved octahedral coordination environment at the expense of significant conformational distortion of the ligand backbone. For example, in [Ru(DQP)2]2+ the long-lived meridional (mer) form of the complex, mer-[Ru(DQP)2]2+, consists of a pair of enantiomers with C2 symmetric helical twists that introduce chirality into the complex. As no diastereotopic atoms exist, chiral auxiliary counterions had to be used to differentiate between the enantiomers, but no interconversion between conformers could be detected even at elevated temperatures.11 This behavior is in stark contrast to other complexes with more flexible nonplanar ligands, such as bidentate and tridentate nonconjugated chelating ligands with alkyl (−CH2−) linkers, where interconversion between conformers instead is often too fast to be observed or quantified,12 even when the enantiomers can be crystallized separately.13 In a few complexes, the interconversion is slow enough to be observed; however, the stereoisomers could only be differentiated by nuclear magnetic resonance (NMR) due to the presence of diastereotopic hydrogen atoms.12c−e Here, we use the new biheteroaromatic ligand 1,3-bis(8quinolinyl)pyrazole (DQPz, Figure 1a) to make a homoleptic Ru complex (Figure 1b), which, due to the ligand’s unsymmetrical central ring and conformational twist upon coordination, simultaneously adopts two types of chiralities. By combining NMR spectroscopy with first-principles calculations, the asymmetry of the complex is exploited to quantify the dynamical flip between two diastereomers. The flip involves a large-scale ligand rearrangement around the metal core, without any bond breaking, and the combined evidence from the NMR and density functional theory (DFT) provides key information about the energy landscape for the ligand rearrangement around the Ru core.

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EXPERIMENTAL SECTION

General. All chemicals were purchased through Sigma-Aldrich, unless noted otherwise, and used as received. Anhydrous solvents were used fresh from new bottles or dried further over 4 Å molecular sieves activated at 250 °C. Solvents were, if necessary, degassed by bubbling N2 gas through them for >30 min. The silica used for flash chromatography was 230−400 mesh and purchased from SigmaAldrich. The NMR spectra reported in the section Synthesis below were recorded on a Varian UNITY Inova 500 MHz spectrometer, and the deuterated solvents were of at least 99.9% deuterated grade; all 1H NMR resonances are referenced to the solvent residual signals (CDCl3: 7.26 ppm, CD3CN: 1.94 ppm). Electrospray ionization in positive mode was measured on a Thermo Scientific LTQ Velos Pro Orbitrap instrument. The mass spectrometer was operated in FTmode at a resolution of 30 000, and Leucine Enkephalin was used for B


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Inorganic Chemistry

Figure 2. Calculated geometries of representative (a) cis−fac-, (b) trans−fac-, and mer-[Ru(DQPz)2] (c) C-Ra and (d) C-Sa stereoisomers. Distances between hydrogens are given in Ångström (Å). Structures optimized using PBE0/LANL2DZ. 1H, J = 4.1 Hz), 8.23 (d, 1H, J = 6.7 Hz), 8.18 (d, 1H, J = 6.9 Hz), 8.05 (m, 7H), 7.89 (d, 1H, J = 4.1 Hz), 7.79−7.69 (m, 4H) 7.68−7.54 (m, 6H), 7.07 (m, 2H), 7.01 (m, 2H). High-resolution mass spectrometry (HRMS) (ESI+, CH3 CN) m/z {rel. intensity} 373.07387 [Ru(DQPz)2]2+ {100} (calc. C42H28N8Ru2+ 373.07347). X-ray Crystallography. The dark red crystal was fixed to a glass fiber using Epoxy glue, and the single crystal data was collected at room temperature on an Oxford Diffraction Xcalibur EOS CCD diffractometer with graphite monochromatized Mo−Kα radiation (λ = 0.710 73 Å) operated at 50 kV and 40 mA, with a detector distance of 50 mm and θmax = 29.4°. The Oxford CrysAlisPro RED software was used for data processing, including Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.14 Structural solution was accomplished using charge flipping as implemented in Superflip.15 All refinements were performed using the JANA2006 software.16 The refined structural model is shown in Figure S3 in the Supporting Information (SI). The compound [Ru(DQPz)2](PF6)2 crystallized poorly as the cis− fac isomer, and of several chosen crystals only one could be used for structural studies. Due to the symmetrical ligand and low-quality data, the noncoordinated pyrazole N could not be localized. Therefore, the two possible positions have been refined with the occupancy 0.5 C and 0.5 N, in both ligands, and both these positions are colored blue in the plotted image. The low amount of observed reflections makes it hard to model Ru and it has a high residual electron density, which looks normal in the difference Fourier map. Modeling Ru in a more complex manner does not make sense, and it does not improve the model. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were theoretically generated. A summary of the crystallographic data and structure refinement details is given in Table S1, and a list of selected bond distances and angles is given in Table S2. NMR. A few milligrams of [Ru(DQPz)2]2+ was dissolved in approximately 600 μL of CD3CN (99.8%D, Aldrich). NMR experiments were acquired at 297.8 K on a Bruker Avance III HD 500 MHz NMR spectrometer running Topspin 3.2 patch level 6 and equipped with a 5 mm SmartProbe BBFO probehead. The probe temperature was determined using a Varian 100% methanol reference sample.17 NMR chemical shifts (δ) are referenced internally to the residual solvent signal at 1.94 ppm for 1H, and indirectly to a 1H chemical shift of 0 ppm for tetramethylsilane, Si(CH3)4, for 13C and 15 18 N. All 2D 1H experiments were acquired using a spectral width of 1500 Hz (3 ppm) in the detected dimension and 1000 Hz (2 ppm) in the indirect dimension. Standard 2D pulse sequences supplied by Bruker as part of Topspin release 3.2 were used for all assignment experiments except for the TOCSY experiment. The NOESY experiment was acquired with a mixing time of 500 ms and suppression of zero-quantum coherences using the method of Thrippleton and Keeler.19 To get superior resolution in the indirect dimension, the TOCSY experiment was run with broadband homo decoupling in F1 using the PSYCHE method.20 The pulse sequence for the F1-PSYCHE-TOCSY experiment was downloaded from the Manchester NMR Methodology Group (http://nmr.chemistry. manchester.ac.uk). The experiment was run with a mixing time of 100 ms, using DIPSI-2 mixing at a RF field corresponding to a 38 μs 90° pulse. Shaped pulses used in the PSYCHE-part and for

suppression of zero quantum coherences were created using Shape Tool of Topspin version 3.2, following the instructions in the Supporting Information of Foroozandeh et al. 2014,20 with the exception that each chirp pulse in the double chirp element only has a single frequency sweep from high to low and from low to high, respectively. They were applied at an RF amplitude of 46 Hz. The PSYCHE gradient strength was optimized to 2.4% (1.3 G cm−1). Dephasing of zero quantum coherences was achieved by simultaneous application of a 180° chirp pulse of 30 ms duration and 893 Hz RF amplitude, and a gradient pulse having an amplitude of 2.8% (1.5 G cm−1)19 before and after the TOCSY mixing element. The 1D nuclear Overhauser effect (NOE) experiments were acquired using a pulse program written in-house, as in Stott et al. (1997).21 One hundred-sixty transients were accumulated, using an acquisition time of 2 s and a relaxation delay (D1) of 6 s. A 80 ms Gaussian cascade Q3 pulse22 was used for selective inversion, and two 1.75 ms hyperbolic secant broadband inversion pulses were used within the mixing time. Ten NOE experiments were performed with mixing times between 50 ms and 1 s. The integral ratio of exchange peak to irradiated peak was used for quantification using the PANIC approach.23 In this approach, the linear range of the NOE buildup curve is substantially extended to longer mixing times, by scaling the NOE cross-peak intensity by the corresponding intensity for the inverted peak. Thereby can more data points be used in the linear fit of the NOE cross-peak intensity versus mixing time, resulting in a better defined slope. Only data from mixing times of 200 ms or less, which were within the initial rate approximation, were used to fit the scaled peak integrals versus mixing times, leading directly to the rate constant as the slope of the corresponding plot. The data were processed with Topspin 3.2, using appropriate window functions, in some cases linear predicted in the indirect dimension, and zero-filled in both dimensions before Fourier transformation. The NMR chemical shift assignments were performed with the aid of the software package Sparky.24 Computation. All calculations were performed with Gaussian09.25 Complete optimizations, constrained transition state scans, and the transition state search were performed at the PBE0/LANL2DZ level of theory. Energies were calculated with a complete acetonitrile polarizable continuum model (PCM) and a triple-ζ basis set, 6311G(d,p), for all atoms but Ru, where the SDD Stuttgart/Dresden effective core potential (ECP)26 was employed.

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RESULTS AND DISCUSSION

The biheteroaromatic bidentate ligand 3-quinolinylpyrazole has previously been shown to be able to adopt various conformations upon coordination to Ru(II).27 By adding a second 8-quinolinyl group to this framework, more stable complexes based on the tridentate ligand DQPz were sought (Figure 1a). The ligand was synthesized in two steps and then reacted with Ru(DMSO)4Cl2 at 180 °C for 30 min. The red colored complex [Ru(DQPz)2][PF6]2 was then precipitated by addition of aqueous NaPF6 and isolated in moderate yield. Dark-red needlelike crystals of [Ru(DQPz)2][PF6]2 could be grown but diffracted poorly, and only one in several gave sufficient data to determine a structure. This turned out to be a C


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Inorganic Chemistry

α and β respectively, present in a ratio of 0.53:0.47, with similar chemical shifts but clearly resolved in many cases. Complete 1 H, 13C, and 15N NMR chemical shift assignments were unambiguously obtained for each species at 297.8 K in CD3CN, using standard methods and two-dimensional (2D) COSY, NOESY, 13C-HSQC, 13C-HMBC, and 15N-HMBC experiments, together with a 2D F1-PSYCHE-TOCSY20 experiment with much increased resolution in the indirect dimension. Briefly, each of the five different spin systems of the α and β forms were easily identified from the COSY and TOCSY spectra; see Figure 4. The numbering system used for the ligand atoms is displayed in Figure 3b. The 5H protons of the two forms were assigned based on that the 4H and 5H protons belong to the only two-spin spin system and the low-field

cis−fac isomer (see Figure S3 in SI), similar to structures observed for the pyridine tridentate analogue DQP.28 Powder diffraction was performed on the microcrystalline bulk sample but did not correlate with the pattern from the single crystal data; instead two separate phases were evident in the powder diffraction analysis (Figure S4 in SI). For bis-tridentate complexes, three main configurational isomers are possible: mer, cis−fac, and trans−fac, each with several possible stereoisomers (a mer-isomer is shown in Figure 1b). Representative DFT optimized geometries of [Ru(DQPz)2]2+ stereoisomers are shown in Figure 2 (energies of all isomers available in SI), where all optimizations were performed with Gaussian09, at the PBE0/LANL2DZ level, and energies were calculated with an acetonitrile polarizable continuum model (PCM) and the 6-311G(d,p) basis set for all atoms but Ru, where a SDD effective core potential was employed. The calculated energies (SI) of the cis−fac-, trans− fac-, and mer-isomers of [Ru(DQPz)2]2+ show the fac-isomers to be >50 kJmol−1 higher in energy than the mer, which clearly is the thermodynamically most stable structure. To get decisive information on the configuration of the complex, we turned to NMR. A 1D 1H NMR spectrum (Figure 3a) of [Ru(DQPz)2]2+ shows two different species, designated

Figure 3. 500 MHz NMR spectra of [Ru(DQPz)2]2+ in CD3CN at 297.8 K: (a) 1H NMR with assignments of the various resonances and (b) 2D NOESY with 500 ms mixing time showing NOE cross-peaks in black and exchange cross-peaks in red. Labels α and β denote the two different sets of resonances corresponding to the two diastereomeric species present.

Figure 4. 500 MHz 2D 1H F1-PSYCHE-TOCSY (100 ms) at 297.8 K of [Ru(DQPz)2]2+ dissolved in CD3CN. The different spin systems are identified, with the α-form shown in red and the β-form in black. D


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Inorganic Chemistry

should be compared to the strong cross peak observed between the 5 and 7″ protons (circled in blue), whose internuclear distance is 2.20−2.48 Å in all isomers. Since there is no reason why the NOE for 2′/2″ should be considerably weaker, or absent, in relation to 5/7″ of the same ligand and molecule, this strongly points to the meridional coordination geometry of [Ru(DQPz)2]2+, for both species present. For similar but symmetric ligands, such as 2,6-bis(quinolinyl)pyridine (DQP), a similar analysis could not differentiate between trans-fac- and mer-isomers, since, due to symmetry reasons, no NOE would be observable.28 This highlights the advantages of introducing asymmetry even if spectra become more crowded. The mer-[Ru(DQPz)2]2+ complex has double chirality, introduced by (i) the unsymmetrical ligands making two absolute configurations possible, which can be described by the C/A-convention adopted by IUPAC, and (ii) the ligand conformation, i.e., twisting, of the ligand backbone upon coordination to the metal, described by axial chirality labels Ra or Sa.29 (Details on the assignments are found in SI.) Hence, four combinations are possible: C-Ra, C-Sa, A-Ra, and A-Sa and the differences between them can be schematically illustrated by indicating the pyrazole plane and polarity with an arrow pointing toward the N-connected quinoline, as shown in Figure 1b. In the C-Ra structure (Figure 2c) the distance between the 2′ and 2″ protons of the two different ligands is 3.78 Å and consequently could yield a weak NOE, as is indeed observed for species α in Figure 3b. This would also hold true for the ASa enantiomer. However, for the corresponding diastereomers C-Sa (Figure 2d) or A-Ra, this distance would be 5.72 Å, a distance too long to give a significant NOE for 2′/2″. A NOE cross peak between the 2′/2′ protons of separate ligands should not be observable, since they, due to symmetry, have the same resonance frequency. This explains why there is no analogous weak NOE for species β, and hence it can be assigned to the enantiomeric pair C-Sa and A-Ra of mer-[Ru(DQPz)2]2+. This analysis shows how the inherent unsymmetrical nature of DQPz can be used as an efficient structural probe, allowing for an unambiguous determination of the stereochemistry of its isomers. The intense exchange cross peaks observed for almost all resonances in the 2D NOESY spectra are both notable and surprising. They occur between the two species α and β, proving that these two diastereomeric forms are in slow exchange. To determine the exchange rate between the two forms of [Ru(DQPz)2]2+ in solution, a series of selective 1D double pulse field gradient spin echo (DPFGSE) NOE experiments21,30 with various mixing times were acquired at 297.8 K. The resonance from the 2′β proton was selectively inverted, and the integrated intensity of the 2′β and 2′α protons in the spectra were analyzed using the PANIC approach.23 Analysis of all data points with a mixing time of 200 ms or less gave a rate constant for the exchange from the βform to the α-form, kβα = 2.1 s−1. Using the known ratio of the two species at equilibrium, together with the condition for detailed balance

chemical shift expected for a nitrogen-bound CH group. The 2D 13C-HSQC experiments provided the carbon chemical shifts of the 13CH groups, and the 2D 13C-HMBC experiment provided important through-bond connectivities between the different spin systems, directly connecting the 2′, 3′, 4′ and 5′, 6′, 7′ spin systems of one quinoline with each other, and the 2″, 3″, 4″ and 5″, 6″, 7″ spin systems of the other quinoline. Especially useful were the 13C-HMBC connectivities observed between 7′H/9′C and 2′H/9′C for one quinoline ring, and between 7″H/9″C and 2″H/9″C for the other quinoline (Figure 5), together with 3′H/10′C, 6′H/10′C, 3″H/10″C,

Figure 5. 500 MHz 2D 13C-HMBC (optimized for J = 8 Hz) at 297.8 K of [Ru(DQPz)2]2+ dissolved in CD3CN. The 9′C and 9″C chemical shifts are indicated by the horizontal lines, with the α-form shown in red and the β-form in black.

and 6″H/10″C. Observed NOEs between 4H/7′H and between 5H/7″H identified 7′H and 7″H, and allowed for positioning of the quinoline rings on the pyrazole (Figure 3b). With assigned proton resonances, the nitrogen chemical shift assignments were obtained from a 15N-HMBC experiment. The obtained NMR chemical shifts are shown in Table S3 in the Supporting Information. A detailed analysis of the calculated structures revealed that the mer- and fac-isomers (Figure 2, with relevant H−H distances included) should be distinguishable by differences in the nuclear Overhauser effect (NOE) between the 2′ and 2″ protons of the quinolines. For both the cis- and trans-facisomers, the 2′ and 2″ protons of the same ligand are about 2 Å apart, which should give strong NOE cross peaks in a 2D NOESY experiment. For both mer-isomers, however, they are at a distance of more than 4.8 Å from each other, which typically is too large a distance to observe a NOE. It is clear from the 2D NOESY (Figure 3b) that no cross peak between 2′β/2″β and only a weak one for 2′α/2″α is observed (circled in green in Figure 3b). Note that the black and red antiphase peak observed at the edge of the circle for the 2′β/2″β cross peak is an artifact due to a residual zero-quantum cross peak between 2′β and 4′β. The lack of large NOE enhancements

kβα[β ]eq = kαβ[α]eq

(1)

the exchange rate for the interconversion of the two forms, kex = kαβ + kβα, was determined, kex = 3.9 s−1. Using the Eyring equation E


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Inorganic Chemistry ΔG‡ = −RT ln

kexh k bT

dissociates simultaneously and the ligand rotates around the NPz-metal bond. Neither of these mechanisms is likely to occur at room temperature due to unfavorable energetics of simultaneously dissociating several coordinating groups. A less unlikely dissociative mechanism results from consecutive dissociation and rearrangement of each quinoline group on one ligand with a trans-fac-intermediate (cf. Figure 2b). The facisomer would then be in dynamic equilibrium with the mer and hence always be present in solution. The barrier for the interconversion between trans- and cis-fac-isomers is likely low so the latter would also be present, which through a dynamic resolution effect could explain why it could be crystallized despite not being observed in NMR. However, since the energy difference between the fac- and mer-isomers was calculated to be more than 50 kJmol−1 (vide supra), the molar ratio mer/fac would be about 109 (Keq = eΔG°/RT) at any given time, and it seems highly unrealistic that it would be possible to grow X-ray quality crystals of sufficient size from such low concentrations of the cis-fac-isomer, unless the solubility of that particular isomer is dramatically lower. It is more likely that the facisomers were present at a higher concentration from the beginning, although still below the NMR detection limit. They probably form due to kinetic effects during synthesis, which previously have been shown to be important for the ratio between mer and fac in the preparation of [Ru(DQP)2]2+.28 Hence, based on the energetics of the isomers and processes described, it seems unlikely that the diastereomerization occurs through any form of ligand dissociation. The transition path between the two distinct [Ru(DQPz)2]2+ diasteromeric forms is accessible at room temperature but has a high enough energy to result in a slow interconversion that can be observed by NMR. This case falls between the prototype complex [Ru(tpy)2]2+ that has a single ground state minimum with planar ligands and [Ru(DQP)2]2+, which is structurally locked in either of its two twisted structures (δ/λ corresponding to Ra/Sa in Ru(DQPz)2)11 (Figure 7). This

(2)

where R is the universal gas constant, T is the temperature, kb is Boltzmann’s constant, and h is Planck’s constant, this exchange rate corresponds to a barrier of ΔG‡ = 70 kJ mol−1 at 297.8 K. The facile interconversion between the diastereomers can be explained through a mechanism that involves a flip of the ligand conformation between Ra and Sa. This interconversion involves both twisting of the Q−Q dihedral angles (φQ−Q) and a change of the dihedral angle between the pyrazole nitrogens (φflip), i.e., the angle between the two polarity arrows in Figure 1b. The two diastereomer structures were optimized separately, with only a very small calculated energy difference between the C-Ra and C-Sa forms of 0.19 kJ mol−1, consistent with the nearly equal population of the two forms observed by NMR. Scanning either the φQ−Q or φflip angle from one minimum, either Ra or Sa, toward the other minimum leads to smoothly increasing energy before a distinct flip of the ligands (details in SI). The ligands remain coordinated to the metal throughout the entire scans despite the large distortions undergone by each of the ligands and the significant distortion in overall ligand conformation, suggesting that ligand dissociation is not required for diastereomerization. A transition state (TS) search resulted in a structure where the quinoline groups are close to planar with the pyrazole group (φQ−Q ∼ 8°, and the φflip angle between the polarity arrows of the pyrazoles is ∼90°, with energy ∼65 kJ mol−1 (Figure 6) relative to the ground state minima of the two

Figure 6. Calculated energies of the C-Ra (A-Sa), C−Sa (A-Ra), and the transition state between the two isomers along the ϕflip and ϕQ−Q reaction coordinates where the red arrows show the pyrazole polarity in each complex. PBE0/6-311G(d,p)[N,C,H]SDD(ECP)[Ru]/PCM(MeCN)//PBE0/LANL2DZ.

Figure 7. Schematic illustration of the different conformations and associated ground state potential energy landscapes in [Ru(DQPz)2]2+ compared to the prototype complexes [Ru(tpy)2]2+ and [Ru(DQP)2]2+.

diastereomers. This matches well with the TS barrier of 70 kJ mol−1 determined by NMR. This TS has one imaginary frequency and efficiently converts to both minima upon relaxation. Notably, while each ligand passes through a conformation where the φQ−Q is 0° during the diastereomerization, from the initial scans it is clear that the motion of one ligand induces the change in the other leading to a TS where one ligand has φQ−Q slightly greater than 0° and the other is slightly less. A mechanism for the diastereomerisation including ligand dissociation could also be possible if for instance one ligand completely dissociates, is rotated 180°, and then is coordinated again, alternatively if both quinoline groups of one ligand

intermediate nature of the [Ru(DQPz)2]2+ ground state potential energy surface is a result of the more open ligand structure of the DQPz ligand, with a five-membered central pyrazole unit compared to the DQP ligand with a central pyridine. The observed intramolecular rearrangement dynamics are related to a type of ground state flexibility known for some complexes of tridentate ligands.12c−e,g However, this has previously not been quantified for Ru complexes of fully conjugated tridentate polyaromatic ligands, despite their significance for a range of applications. From a broader perspective, the importance of large-scale ground state structural dynamics impacts the fundamental F


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Inorganic Chemistry

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assessment of modern ligand design strategies for lightharvesting, catalytic, and medicinal applications, based on the concepts of ligand coordination strain as well as configurational and conformational stability. Ligand twisting with concomitant interligand stacking and potential structural dynamics needs to be considered in a range of structurally related complexes.8a,b,d The ground state dynamics discussed here also complement a growing interest in time-resolved X-ray spectroscopic techniques for characterization of the excited state structural dynamics in transition metal complexes.31 In conclusion, this is the first quantitative demonstration and characterization of diastereomerization dynamics in Ru(II) complexes with fully conjugated tridentate heteroaromatic ligands. Both NMR and DFT indicate a TS barrier around 70 kJ mol−1 with slow interconversion between two diastereomers, C-Ra and C-Sa (or A-Ra and A-Sa). This clearly demonstrates the need to take ground state dynamics into account in the design of transition metal complexes for photochemical and catalytic applications, and the advantage of unsymmetrical ligands as a dynamical probe to monitor such processes.

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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.5b02893. Assignment of chirality descriptors, X-ray crystallographic and powder diffraction data, table of NMR chemical shift assignments, and calculated structures (PDF) X-ray crystallographic data of cis-fac-[Ru(DQPz)2](PF6)2 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ångpanneföreningens forskningsstiftelse (Åforsk), the Swedish Research Council (VR), the Crafoord Foundation, and the Knut and Alice Wallenberg (KAW) Foundation for financial support. M.A. and J.H.W. thank the Area of Advance Nano at Chalmers for financial support. P.P. acknowledges support from the NSC and LUNARC supercomputing facilities.

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REFERENCES

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Diastereomerization Dynamics of a Bistridentate RuII Complex

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