Tuning Electron Delocalization and Transfer Rates in Mixed-Valent

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Tuning Electron Delocalization and Transfer Rates in Mixed-Valent RuO Complexes Through ‘Push-Pull’ Effects 3

Tyler M Porter, Gabriele C. Canzi, Steven A Chabolla, and Clifford P. Kubiak J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05485 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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The Journal of Physical Chemistry

Tuning Electron Delocalization and Transfer Rates in MixedValent Ru3O Complexes Through ‘Push-Pull’ Effects Tyler M. Porter, Gabriele C. Canzi, Steven A. Chabolla, Clifford P. Kubiak*. Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, M/C 0358, La Jolla, California 92093-0358.

ABSTRACT: Electron transfer rates in a series of oxo-centered triruthenium clusters featuring an extended aromatic ancillary ligand of the type [Ru3(OAc)6(µ3-O)(CO)(L)(pep)] where L = 4cyanopyridine (cpy), pyridine (py), or 4-dimethylaminopyridine (dmap) and pep = 4(phenylethynyl)pyridine were investigated. The electron self-exchange rate constants for the 0/– couple were determined by 1H NMR line broadening experiments and found to range from 4.3 to 9.2 (x 107 M–1 s–1) in deuterated acetonitrile (ACN-d3). Relative rates of self-exchange can be rationalized based on increased contact area between self-exchanging pairs, and a push-pull modulation of electron density between the pep vs. ancillary pyridine ligands. Faster selfexchange was observed with increasing electron-donating character of the ancillary pyridine ligand substituent (dmap > py > cpy) and this was attributed to increased orbital overlap between self-exchanging pairs. These results are supported by trends observed in 1H NMR contact shifts of the pep ligand which were found to depend on the electron-donating or -withdrawing nature of the ancillary pyridine ligand.

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INTRODUCTION The study of electron transfer (ET) processes across weak, non-covalent interactions is essential to the broader understanding of long range ET in both biological and artificial systems.1-7 There has been considerable interest in studying ET across hydrogen bonds in the past decade, while other non-covalent interactions remain relatively unexplored.3,8-13 Electron transport through π interactions remains an area of considerable interest, as such structures are often involved in ET in natural and artificial systems.14-17 The ease with which π interactions form in supramolecular systems makes them ideal candidates for self-assembling motifs in the development of molecular devices.17-19

Several theoretical and experimental

studies on electron transfer through π interactions have shown the electronic coupling (Hab) has a significant dependence on the overlap between exchanging units.17,20,21

Minor structural

variations can perturb this interaction and inhibit ET. We have previously demonstrated that large electronic couplings can be observed in oxocentered triruthenium acetate clusters in the form of covalently linked dimers,22-26 freely diffusing monomers,27,28 and recently in hydrogen-bonded dimers.11,12 It is clear that the large electronic couplings observed in these clusters make them excellent starting points to investigate ET processes occurring across non-covalent interactions. Herein we report studies of electron self-exchange rates in a series of oxo-centered triruthenium clusters featuring an extended aromatic ancillary ligand 4-(phenylethynyl)pyridine (pep) (Figure 1). The facile synthesis, large contact area, and weak electron-withdrawing nature of the pep ligand offer an excellent opportunity for gaining understanding of the nature of electronic coupling across π interactions.

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Figure 1. Oxo-centered triruthenium clusters of the type [Ru3(µ3-O)(OAc)6(CO)(L1)(L2)] where L1 = 4-cyanopyridine (X = CN, cpy), pyridine (X = H, py), or 4-dimethylaminopyridine (X = N(CH3)2, dmap) and L2 = 4-(phenylethynyl) pyridine (pep), cpy, py, or dmap.

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RESULTS AND DISCUSSION:

Figure 2. 1H NMR of 3 with increasing mole fraction of reduced species from top to bottom. The kinetics of electron self-exchange for [Ru3(µ3-O)(OAc)6(CO)(L)(pep)]0/- pairs (1–3) were determined by NMR line broadening experiments (Figure 2) with rate constants determined from equation 1. ∆

(1)

Here, χred and χox are the mole fractions of reduced and oxidized species, ∆ν is the difference in chemical shift between reduced and oxidized species in Hertz, Wro is the peak width at half maximum of the mixture in question, Wred and Wox are the peak widths at

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half maximum of the pure reduced and oxidized species, and Ctot is the total concentration in moles per liter.29 Mole fractions of the exchanging species were determined by IR spectroscopy. Mole fraction ratios were found by integration of the ν(CO) bands of the neutral (1944 cm–1 (1), 1942 cm–1 (2), 1939 cm–1 (3)) and singly reduced (1905 cm-1 (1–), 1902 cm-1 (2–), 1899 cm-1 (3–) species. The ν(CO) region of the spectra was fit to two wellresolved Voigt line shapes, giving an upper bound to ket of 1011 M–1 s–1. Representative spectra for ν(CO) shifts upon reduction in acetonitrile are shown as Figure 3a for complex 3.

Figure 3. (a) IR spectra of the ν(CO) region for 3 in ACN with varying mole fractions of [red]/[ox], experimental mole fractions were determined from integration of ν(CO) bands. (b) Plot of determined from integration of ν(CO) bands versus chemical shift of pep protons. The exchange was found to be in the fast regime,29 where k(Ctot)≫2π(∆ν), as the chemical shifts for the exchanging species were the averages of the diamagnetic and paramagnetic chemical shifts weighted by their respective mole fractions (Figure 3b). The electron self-

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exchange rate constants for 1–3 were found to range from 4.3 to 9.2 x 107 M-1 s-1 in ACN-d3 (Table 1) and are comparable in magnitude for redox couples of the homoleptic clusters 4– 6.27,28,30

Table 1. Electron transfer rate constants for 1–3 and 4–6 in ACN-d3. Complex

–

kET ACN (x 10 7 M

–1 -1

s )

Pyridine pKa

1 CO(cpy)(pep)

4.3(2)

1.9

2 CO(py)(pep)

6.8(2)

5.1

3 CO(dmap)(pep)

9.9(6)

9.2

4 CO(cpy)2

13(3)

1.9

5 CO(py)2

1.8(5)

5.1

6 CO(dmap)2

0.7(5)

9.2

In clusters 1-3, the rate constants scale linearly with pyridine ligand pKa and the fastest self-exchange was observed when the most electron-donating ligand was used. The near linearity in the logarithmic plot of rate constant versus pyridyl ligand pKa (Figure S15) suggests that the electron-donating nature of the pyridyl ligands contribute to the activation energy, expressed below in the standard Marcus-Hush formalism (Equation 2).31,32

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! "#

(2)

It is important to note that equation 2 is only applicable in the adiabatic regime in the context of the Marcus-Hush formalism, and that Hab only needs to be a few percent of λ to achieve adiabaticity.23,27,28 In these systems, the reorganization energies for the 0/– couple are estimated to be on the order of ~10 000 cm-1 while Hab is estimated to be in the adiabatic regime at 10-15% of λ.23,27,28 These conditions yield rate constants that are consistent with those experimentally observed on the order of 107 M-1 s-1. The observed (~40 cm-1) shift in the ν(CO) stretching frequency for 1–3 suggests that the inner-sphere reorganization energies are comparable amongst the three clusters and therefore it is unlikely that differences in nuclear reorganization and inner-shell barriers contribute much to the observed differences in rate.28 It should be noted that the ν(CO) bands are not as important as the low frequency modes that make the dominant contributions to the reorganization energy; they are used however, as probes of relative electronic redistribution within the clusters. Our findings here are supported by earlier electron self-exchange studies conducted on similar clusters of the type [Ru3(µ3-O)(OAc)6(CO)(L)2] (4–6, Figure 1, L1 = L2 = cpy, py, or dmap). In this previous work, the rate determining factor for self-exchange of the 0/– couple was found to be donor-acceptor overlap of the ancillary pyridyl ligands.27,28 Self-exchange proceeds through electron delocalization onto the π* system (A2'' SOMO) of the ancillary ligands, leading to large reorganization energies (λ) that are compensated by strong donor-acceptor couplings (Hab).28 This mechanism is consistent with an effective orbital overlap picture and is mediated by

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the contact area of the ancillary ligand in self-exchanging pairs. As charge density is increasingly delocalized out to the ancillary pyridyl ligand, larger rate constants for self-exchange are observed. In 4–6 as the ancillary ligand becomes more electron-withdrawing in nature, decreasing pyridine pKa, electron self-exchange rate constants were found to increase over an order of magnitude. While complexes 1–3 appear to follow the same mechanism for self-exchange as 4–6, an inverted trend is observed in the variation of rate constants with pyridyl ligand pKa (Table 1). These differences are best explained through consideration of the relative electron-withdrawing nature of both pep and pyridine ligands as summarized through the Hammett parameter (σ) in lieu of ligand pKa. Here the pyridine substituents (including the phenylethynyl group) can be assigned an empirical value that describes the electron-withdrawing nature of the substituents (Table 2). Table 2. Hammett σ parameter for para substituents. 33 Substituent

σp

–CN

0.66

–CC(C6H5)

0.16

–H

0.00

–N(CH3)2

-0.83

The substituent effects are most readily seen in the 1H NMR where paramagnetic contact shifts relate to the degree of electron density delocalized out to each ligand. The paramagnetic contact shift in pep and pyridine protons can be represented as the average change in ligand proton chemical shift (∆δ) upon reduction where greater shifts relate to increased or decreased electron density delocalized out to each respective ligand. Through this definition, the substituent

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effects are most clearly seen in the plot of ∆δ vs σp where the linear relationships observed conform to an electron density ‘push-pull’ effect (Figure 4).

Figure 4. Plot of average chemical shift (∆δ) of pep and pyridine protons upon reduction in ACN-d3 vs σp. The positive linear trend observed for the pyridine protons with increasing σp indicates increased electron density shifts to the pyridine ligand as more electron-withdrawing substituents are used. This effect is further illustrated as the negative linear trend in pep proton chemical shift with increasing σp; electron density shifts away from the pep ligand as more electronwithdrawing substituents are used on the pyridine ligand. These results highlight how the degree of electron distribution can be tuned with ancillary ligand substitution, which in turn modulates the degree of electronic coupling in a self-exchange ET. By pushing more charge density onto the pep ligand, better donor-acceptor overlap produces larger cluster-to-cluster couplings and faster rates of ET (Figure 5a).

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Figure 5. (a) Plot of kET vs σp showing increasing rates of ET with decreasing electron-donating nature of the pyridine substituent. (b) Cyclic votammogram of 1–3 (~3 mM) in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. A three electrode set-up was used with a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The electrochemistry of 1–3 further reflects the ability for the ancillary ligands to tune the electronic structure of the clusters. As the pyridine substituent becomes more electronwithdrawing, decreased electron density is found on the cluster, and the first (cluster-based) reduction shifts to more positive potentials. Furthermore, at more negative potentials, a second one-electron, reversible reduction (cluster-based) is observed, and shifts to more positive potentials with increasing strength of electron-withdrawing substituents (Figure 5b). This same behavior is seen in the homoleptic clusters 4–6, (Table 3) however, the additional one-electron reduction is only reversible in 4, becoming irreversible in 5 (Ea = –2.36 V) and unobservable in 6. Table 3. Reduction potentials of complexes 1–6 in volts referenced to Fc+/0. Complex

E1/2(1) (V)

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E1/2(2) (V)

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1 CO(cpy)(pep)

-1.11

-1.74

2 CO(py)(pep)

-1.22

-2.00

3 CO(dmap)(pep)

-1.31

-2.11a

4 CO(cpy)2

-1.05

-1.60

5 CO(py)2

-1.27

-2.36a

6 CO(dmap)2

-1.47

---

a

Second reduction in 3 and 5 is irreversible and the reported voltage corresponds to the peak anodic current (Epa). This difference in redox behavior of 1–3, compared to 4–6, highlights the pep ligand’s ability to accept increased electron density and stabilize additional cluster-based redox states. Further comparison of the reduction potentials for heteroleptic (1–3) and homoleptic (4–6)

clusters demonstrates how the ancillary ligands modulate the electronic structure of the clusters. In 1 the first reduction potential is shifted 60 mV more negative in comparison to 4, while the reduction potential for 3 is shifted 160 mV more positive relative to 6. This is shown in the overlaid CV of the homoleptic and heteroleptic clusters (Figure S34a). The positive shift in the reduction potential for 3 arises from the electron-withdrawing nature of the pep ligand in comparison to the electron-donating nature of the dmap ligand. This difference pushes electron density onto the pep ligand and shifts the reduction potential of the cluster more positive with respect to the homoleptic cluster 6. The negative shift for 1, has similar origins, however, the pep ligand is now weakly electron-donating in comparison to a cyanopyridine ligand. This small difference then reflects increased electron density on the cluster with respect to 4 (which features two cpy ligands), producing a negative shift in the first reduction potential. The plot of the difference in reduction potentials for 1–3 and 4–6 vs σp (Figure S34b) most clearly shows this effect. Here the shifts correlate well with the electron-withdrawing nature of the pyridine substituents and a slight positive shift can be observed for 2 reflecting the electron-withdrawing

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nature of the pep ligand. The shifts in reduction potentials and their relationship to σp are consistent with the proposed mechanism suggested by NMR. The electronic and geometric effects of the ancillary ligands in self-exchange ET were investigated in greater detail through density functional theory (DFT) calculations. The initial geometries were adapted from crystal structures of similar tri-substituted clusters ([Ru3(µ3O)(OAc)6(L)3][PF6], L = cpy or py).11 While several orientations of the ancillary ligands are possible, due to the computational expense associated with the calculations, we focused only on four: When the pep and pyridine ligands are considered parallel or perpendicular to the plane of the ruthenium atoms (Figure 6). Geometry optimizations for 1–3 showed that the lowest optimized single point energy was when both ancillary ligands were parallel to the plane of ruthenium atoms while the highest was with both ligands perpendicular to the plane of ruthenium atoms (∆E ≈ 2.3 kcal/mol). The two asymmetric conformers were found to be nearly isoenergetic with total energy separations of ∆E ≤ 0.5 kcal/mol and with ∆E ≈ 1 kcal/mol separations between the highest and lowest energy conformations. Representative structures for (3)– along with the singly occupied molecular orbital (SOMO) are shown in Figure 6.

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Figure 6. SOMOs for (3)– for each calculated conformation of the ancillary ligands. While all four conformations are nearly isoenergetic, the ancillary ligand torsional angles have a significant impact on the orbital structure of these clusters. The four conformations show nearly symmetric HOMOs (Figure S42) and SOMOs (Figure 6) with the majority of orbital density localized in the core; while rotating the ancillary ligands to the parallel conformation shows a high degree of orbital density localized on the pep ligand. In addition to the dependence on torsional angle, the SOMOs of the totally parallel conformation of (1)––(3)– were compared where orbital localization was dependent on the donor ability of the ancillary ligand substituent (Figure 7).

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Figure 7. SOMOs of the anionic clusters (1)––(3)– from DFT calculations highlighting the orbital localizations dependence on ancillary ligand. All three clusters display significant orbital density in their cores, which is modulated in response to the electron-donating (or withdrawing) effects of the pyridine ligands. In 1, a significant amount of the SOMO is localized on the cyanopyridine ligand with none on the pep ligand; while in 3 no orbital density is observed on the dimethylaminopyridine ligand, yet a significant amount is localized on the pep ligand. This is a clear illustration of the push-pull effects noted in the dependence of self-exchange rates with ligand substitution pattern. These results are also consistent with the observed experimental data where the paramagnetic contact shifts observed in 1H NMR depend on the ancillary ligand. This highlights the ability of the pyridine substituent to tune the cluster electronic states and ultimately the mechanism of selfexchange in these systems. While electron self-exchange in 1–3 is consistent with the effective orbital overlap picture, the ancillary ligand primarily involved in this interaction can be modulated through ancillary ligand substitution.

CONCLUSIONS These results build upon the previous studies of electron self-exchange interactions in triruthenium mixed valent clusters and extend the concept of effective orbital overlap as a primary

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factor in determining electron transfer rates. By incorporating an extended aromatic ancillary ligand, a greater effective contact area was allowed leading to better donor-acceptor overlap, and ultimately, large electronic couplings. Through consideration of the Hammett parameter, the degree of coupling in these systems was shown to be influenced by both ancillary ligands and a ‘push-pull’ effect of the electron density. By incorporation of more electron-donating ancillary ligand substituents, electron self-exchange rate constants were shown to increase by an order of magnitude in comparison to similar systems. Studies are currently underway to better understand how non-covalent interactions relate to electronic coupling and rates of electron transfer between exchange pairs. EXPERIMENTAL: Preparation and Purification. All chemicals were used as received unless otherwise noted. The complexes 1, 2, and 3 were synthesized following previously reported procedures.11,24,34 ACN-d3 (D, 99.8%) was obtained from Cambridge Isotope Laboratories and was dried and stored over 3 Å molecular sieves for several days. Decamethylcobaltocene was received from Sigma-Aldrich, stored in a glove box freezer at -20 ºC and used as received. All other solvents used in this study were obtained from Fisher Scientific and deoxygenated and dried on a custom drying system over alumina columns in an argon atmosphere. [Ru3(µ µ3-O)(OAc)6(CO)(L)(pep)] (L = cpy (1), py (2), dmap (3)). A 20 mL scintillation vial was charged with 200 mg (0.23 mmol) of [Ru3(µ3-O)(OAc)6(CO)(L)(MeOH)] (Where L = cpy, py, or dmap), approximately 10 mL of dichloromethane, and set to stir. In a separate vial, 1 eq. of 4-(phenylethynyl)pyridine (pep) was dissolved in approximately 5 mL of dichloromethane and subsequently added to the original scintillation vial. After stirring 24 hours, the reaction mixture was reduced to dryness and purified using flash chromatography on silica gel with

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DCM:MeOH (95:5%) as an elutant. The second, blue-green band, identified as [Ru3(µ3O)(OAc)6(CO)(L)(pep)] was collected and isolated as dark blue, microcrystalline solid. The airstable solid was stored under an inert atmosphere in a glove box until future use. [Ru3(µ µ3-O)(OAc)6(CO)(cpy)(pep)] (1): Yield: 179.6 mg; 76%. 1H NMR (ACN-d3, 500 MHz): δ (ppm) = 9.13 (2H, m, m-pep(NCH)), 9.07 (2H, m, m-cpy(NCH)), 8.38 (2H, m, o-cpy(NCHCH)), 8.19 (2H, m, o-pep(NCHCH)), 7.71 (2H, m, o-pep(–CC–CCH)), 7.51 (2H, m, m-pep(–CC– CCHCH)), 7.50 (1H, s, p-pep(–CC–CCHCHCH)) 2.02 (6H, s, CH3COO), 2.02 (6H, s, CH3COO), 1.82 (6H, s, CH3COO). 13C {1H} NMR (ACN-d3, 500 MHz): δ (ppm) = 224.1 (CO), 193.6 (CH3COO), 193.1 (CH3COO), 193.0 (CH3COO), 153.4 (o-pep(NCH)), 153.3 (ocpy(NCH)), 136.9 (p-pep(NCHCHC)), 133.2 (o-pep(–CC–CCH)), 131.1 (m-pep(–CC– CCHCH)), 129.9 (p-pep(–CC–CCHCHCH)), 127.5 (m-cpy(NCHCH)), 127.3 (m-pep(NCHCH)), 125.1 (p-cpy(NCHCHC–CN)), 122.4 (pep(–CC–C), 116.9 (NCHCHC–CN), 97.4 (–CC–(C6H5)), 86.8 ((NC5H4)–CC–), 23.8 (CH3COO), 23.7 (CH3COO), 22.0 (CH3COO). Anal. Calcd for C32H31N3O14Ru3: C, 39.03%; H, 3.17%; N, 4.27%; found: C, 39.29%; H, 3.70%; N, 4.34%; IR (cm-1): 1944, 1611.0, 1571, 1504, 1427, 1354. UV-Vis (nm) 278 (28300 ± 200 M-1 cm-1), 403 (10800 ± 100 M-1 cm-1), 593 (7300 ± 200 M-1 cm-1). ESI-MS (m/z) [M + H+]+calcd: 988.90 (100.0%), 989.90 (92.5%). Found: 988.22, 989.91. Reduction potentials (V, vs. Fc+/0, ACN): +2/+, 0.90; +/0, 0.25; 0/–, –1.11; –/–2, –1.74. [Ru3(µ µ3-O)(OAc)6(CO)(py)(pep)] (2): Yield: 199.2 mg; 83%. 1H NMR (ACN-d3, 500 MHz): δ (ppm) = 9.18 (m, 2H, o-py(NCH)), 9.15 (m, 2H, o-pep(NCH)), 8.36 (m, 1H, p-py(NCHCHCH)), 8.23 (m, 2H, m-pep(NCHCH)), 8.16 (m, 2H, m-py(NCHCH)), 7.75 (m, 2H, o-pep(–CC–CCH)), 7.54 (m, 2H, m-pep(–CC–CCHCH)), 7.53 (m, 1H, p-pep(–CC–CCHCHCH)), 2.00 (s, 6H, CH3COO), 1.99 (s, 6H, CH3COO), 1.78 (s, 6H, CH3COO). 13C {1H} NMR (ACN-d3, 500 MHz):

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δ (ppm) = 224.7 (CO), 193.1 (CH3COO), 192.8 (CH3COO), 192.7 (CH3COO), 153.6 (opep(NCH)), 153.4 (o-cpy(NCH)), 141.9 (p-py(NCHCHCH)), 136.8 (p-pep(NCHCHC)), 133.2 (o-pep(–CC–CCH)), 131.1 (m-pep(–CC–CCHCH)), 130.0 (p-pep(–CC–CCHCHCH)), 127.3 (m-pep(NCHCH)), 126.0 (m-py(NCHCH)), 122.3, (pep(–CC–C)), 97.3 (–CC–(C6H5)), 86.9 ((NC5H4)–CC–), 23.8 (CH3COO), 23.7 (CH3COO), 22.0 (CH3COO). Anal. Calcd for C31H32N2O14Ru3: C, 38.79%; H, 3.36%; N, 2.92%; found: C, 38.47%; H, 3.70%; N, 2.98%. IR (cm-1): 1942, 1610, 1573, 1503, 1427, 1354. UV-Vis (nm): 284 (33357 ± 100 M-1 cm-1), 400 (9100 ± 100 M-1 cm-1), 584 (7760 ± 40 M-1 cm-1). ESI-MS (m/z) [M + H+]+ calcd: 963.91 (100.0%), 964.91 (92.5%). Found: 963.32, 965.42. Reduction potentials (V, vs. Fc+/0, ACN): +2/+, 0.86; +/0, 0.22; 0/–, –1.22; –/–2, –2.00. µ3-O)(OAc)6(CO)(dmap)(pep)] (3): Yield: 179.9 mg; 78%. 1H NMR (ACN-d3, 500 [Ru3(µ MHz): δ (ppm) = 9.20 (m, 2H, o-pep(NCH)), 8.95 (m, 2H, o-dmap(NCH)), 8.24 (m, 2H, mpep(NCHCH)), 7.74 (m, 2H, o-pep(–CC–CCH)), 7.54 (m, 2H, m-pep(–CC–CCHCH)), 7.53 (m, 1H, p-pep(–CC–CCHCHCH)), 7.32 (m, 2H, dmap(NCHCH)), 3.30 (s, 6H, –N(CH3)2), 1.95 (s, 6H, CH3COO), 1.94 (s, 6H, CH3COO), 1.72 (s, 6H, CH3COO).

13

C {1H} NMR (ACN-d3, 500

MHz): δ (ppm) = 224.5 (CO), 192.3 (CH3COO), 192.0 (CH3COO), 191.9 (CH3COO), 158.5 (pdmap(NCHCHC), 154.4 (o-dmap(NCH)), 153.1 (o-pep(NCH)), 136.4 (p-pep(NCHCHC)), 133.2 (o-pep(–CC–CCH)), 131.1 (m-pep(–CC–CCHCH)), 129.9 (p-pep(–CC–CCHCHCH)), 127.3 (m-pep(NCHCH)), 122.4 (pep(–CC–C)), 108.9 (m-dmap(NCHCH)), 97.1 (–CC–(C6H5)), 87.0 ((NC5H4)–CC–), 39.9 (–N(CH3)2), 23.9 (CH3COO), 23.9 (CH3COO), 22.2 (CH3COO). Anal. Calcd for C33H37N3O14Ru3: C, 39.52%; H, 3.72%; N, 4.19%; found: C, 39.25%; H, 4.02%; N, 4.04%. IR (cm-1): 1939, 1611, 1576, 1540, 1504, 1426, 1392, 1353. UV-Vis (nm): 286 (38000 ± 2000 M-1 cm-1), 399 (11000 ± 1000 M-1 cm-1), 589 (8000 ± 1000 M-1 cm-1). ESI-MS (m/z) [M +

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H+]+ calcd: 1006.95 (100.0%), 1007.95 (92.5%). Found: 1007.19, 1008.28. Reduction potentials (V, vs. Fc+/0, ACN): +2/+, 0.77; +/0, 0.18; 0/–, –1.31; –/–2, –2.11. Sample Preparation. A 5 to 6 mM stock solution of each complex was prepared in deuterated acetonitrile in a glove box under a nitrogen atmosphere for use in NMR, IR, and UV-Visible experiments. Five aliquots of the un-reduced stock solution was added to five separate J-young NMR tubes to give volumes of 100, 200, 300, 400, 500 $L respectively. The remaining stock solution was cooled to –20 ºC in a freezer and decamethylcobaltocene was added over a period of 30 minutes to fully reduce the sample by one electron (confirmed by 40 cm-1 shift in CO IR bands). The reduced solution was then filtered over glass wool and added to the J-Young tubes containing 100, 200, 300, and 400 $L of the neutral species in aliquots to give a total solution volume of 500 $L while a sixth separate J-Young tube was charged with 500 $L of the remaining reduced solution. This afforded four 500 $L samples containing varying ratios of neutral to reduced clusters, one 500 $L sample in the neutral state, and one 500 $L sample in the reduced state. After collecting 1H NMR, the samples were injected into a Specac sealed liquid IR cell with CaF2 windows and a path length of 0.1 mm where the IR and UV-Visible spectra were subsequently collected. All samples were analyzed immediately and were stable for days unless subjected to atmosphere. NMR Data Collection and Analysis. 1H NMR spectra were collected on a JEOL or Varian 500 MHz NMR spectrometer and analyzed using iNMR software. A total of 64 scans of 16384 data points (0.46 Hz resolution) were collected from +20.0 to –10.0 ppm. Peak widths at half height were measured by fitting each absorbance to Lorentzian line shapes using iNMR software and each rate constant is an average of at least four values calculated from equation 1. All spectra were recorded at ambient temperatures of the instrument (20-23 ºC).

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Infrared Data Collection and Analysis. Infrared spectra were collected on a Bruker Equinox 55 FTIR spectrometer, using a SPECAC sealed liquid IR cell with CaF2 windows and a path length of 0.1 mm. After solvent subtraction, CO peaks were fit to Voigt functionals using Igor Pro software to obtain the ratios of neutral to reduced clusters in solution, which was subsequently used to determine the mol fraction of the samples. UV-Visible Data Collection and Analysis. UV-visible spectra were collected on a Shimadzu UV-3600 UV/vis/NIR spectrometer. Samples were enclosed in a SPECAC sealed liquid IR cell with CaF2 windows and a path length of 0.1 mm. Electrochemical Measurements. Electrochemistry was performed on a BASi Epsilon potentiostat

in

dried,

degassed

acetonitrile

with

0.1

M

tetrabutylammonium

hexaflurorophosphate (TBAPF6, recrystallized from MeOH and vacuum dried at 80 ºC) and about 3 mM analyte at a scan rate of 100 mV/s under a nitrogen atmosphere. The electrode setup consisted of a glassy carbon disk (3 mm diameter) working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode separated from solution by a CoarlPor® tip. All experiments were referenced to an internal standard of ferrocene/ferrocenium. DFT Calculations. Calculations were performed in the ORCA software suite (version 3.0.3) using the B3LYP/G functional with the RIJCOSX approximation.35-39 Ruthenium atoms were treated with the ECP[DEF2-TZVP/J] basis set while the DEF2-SVP/J basis set was used for all other atoms.40-43 Dispersion corrections were applied using the Becke-Johnson damping scheme (D3BJ) and solvation was accounted for using the COSMO solvation model in methylene chloride.44,45 Analytical frequency calculations were run at the same level of theory, without the COSMO solvation model to confirm optimized geometries as minima on the potential energy surface. Molecular graphics and analyses were performed with the UCSF Chimera package.

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Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIGMS P41-GM103311).46

ASSOCIATED CONTENT Supporting Information. A selection of cyclic voltammograms, X-ray crystallographic information tables, and IR-SEC plots are available online at http://pubs.acs.org. Corresponding Author *Phone: 858-822-2665. 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. Funding Sources National Science Foundation (NSF) Grant CHE-1461632. ACKNOWLEDGMENT We acknowledge support for this work from the NSF through the grant CHE-1461632. S. A. C. thanks the NSF for support through a GFRP award. We also acknowledge Dr. Anthony Mrse of the UCSD NMR facilities for assistance and advice in NMR studies and the W. M. Keck Foundation through computing resources at the W. M. Keck Laboratory for Integrated Biology II. The authors would further like to thank Dr. Charles Machan, Dr. Jane Henderson, and Mark Reineke for insightful conversations about these systems.

Notes The authors declare no competing financial interest.

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Tuning Electron Delocalization and Transfer Rates in Mixed-Valent

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