Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine

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Multi-Electron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand Kate M. Waldie, Srinivasan Ramakrishnan, Sung-Kwan Kim, Jana K. Maclaren, Christopher E. D. Chidsey, and Robert M. Waymouth J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01047 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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Journal of the American Chemical Society

Multi-Electron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand

Kate M. Waldie,† Srinivasan Ramakrishnan,† Sung-Kwan Kim, Jana K. Maclaren, Christopher E. D. Chidsey, Robert M. Waymouth*

Department of Chemistry, Stanford University, Stanford, CA, 94305

*[email protected]

ABSTRACT The dicationic complex [CpCo(azpy)(CH3CN)](ClO4)2 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (-0.16 V versus Fc0/+) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of CoIII systems: two sequential one-electron reductions to CoII at -0.4 V and CoI at -1.0 to -1.3 V versus Fc0/+. The doubly-reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory (DFT) calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low energy azo π* orbitals in the intermediary redox state and additional

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stabilization of 2 from structural reorganization, leading to a two-electron reduction. This unprecedented result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.

INTRODUCTION The development of catalysts based on earth-abundant materials that can store electricity in chemical bonds, either via the electrocatalytic reduction of protons to H21 or CO2 to liquid fuels such as formic acid or methanol2 is highly desirable for the storage of energy from renewable resources.3 These multi-step fuel-forming reactions require electrocatalysts capable of mediating multi-electron redox processes near the thermodynamic potential for the reaction of interest in order to obtain maximum energy efficiency; the addition of the second electron is typically more difficult than the first due to electrostatic repulsion. As a result, two separate oneelectron redox events at potentials E1 and E2 are observed, where E1 > E2 is referred to as normal ordering of potentials.4 If E1 is at the thermodynamic potential for the reaction of interest, the overpotential for the multi-electron catalytic reaction will decrease as the potential difference between E1 and E2 is minimized. To achieve two-electron redox behavior, the second reduction must be thermodynamically more favorable than the first such that the difference between the redox potentials ΔE (E2 – E1) is negative, a situation known as potential inversion.4 The electrostatic penalty for second electron addition may be overcome through structural reorganizations that minimize electrostatic repulsion4 or preferential stabilization of the doublyreduced species from, for example, aromaticity5 or solvation6. Two-electron redox behavior at mononuclear first-row metal complexes is unusual.7-16 An early example is [Ni(bipy)3]2+ (bipy = 2,2’-bipyridine), which undergoes two-electron reduction to Ni0 at -1.65 V and subsequently


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loses one bipy ligand.10 Kubiak and co-workers12 showed that replacement of bipy with the bulky 6,6’-dimesityl-2,2’-bipyridine (mesbpy) ligand in [Mn(mesbpy)(CO)3Br] resulted in a potential compression of 300 mV between E1 and E2, leading to reversible two-electron behavior. In nature, several enzymatic reactions involving multi-electron transfers are catalyzed by metalloproteins containing first row transition metals, facilitated by the presence of redox active groups, either coordinated to the metal directly or within the metal’s second coordination sphere. These groups serve to store and release electrons as required, enabling a net multi-electron transfer.17-19 Taking inspiration from biological systems, the introduction of redox active ligands in homogeneous transition metal complexes is well established,20-21 and recent reports have demonstrated the critical roles of redox active ligands in several stoichiometric and catalytic transformations with first-row metals.22-25 Despite advances in the utilization of redox active ligands, the ability to transfer more than one electron at the same potential still remains a challenge with first row transition metal complexes. Azopyridines have been extensively explored for materials chemistry due to their unique electronic and structural properties.26 These compounds represent an intriguing family of redox active ligands.19,27 Their redox non-innocence arises from the low-lying azo N=N π* orbital, which enables facile ligand reduction by one or two electrons to yield the radical anion or hydrazido dianion, respectively. The electron storing capacity of the azo ligands can stabilize high-oxidation state metals,7,13,28-31 while back-donation into the azo π* orbital can stabilize lowvalent metal centers without the formal ligand reduction.7 Additionally, two-electron behavior has been observed with first-row metal complexes containing azo ligands in some cases: [Co(L)2Cl] (L = 2-(arylazo)pyridine ligand) displays a two-electron reduction feature13, while [Ni(L)3]2+ and [Ni(L)2Cl2] are both reduced by two electrons to yield [Ni(L)2] assigned as a high-



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spin NiII center antiferromagnetically coupled to two azo radical anions.14 The origin of this reactivity for these systems remains poorly understood. We posited that the strong π-acidity of the azopyridine ligand would facilitate sequential delivery of two electrons to a CoIII complex at less negative potentials compared to a redoxinnocent ligand system. In this report, we show that the CoIII complex 1 bearing the phenylazopyridine (azpy) ligand exhibits a reversible two-electron reduction at a very mild potential, whereas the analogous complexes 3 and 4 containing the weaker π-acids 2,2’bipyridine and pyrazolylpyridine (Figure 1) are reduced in two separate one-electron events with ΔE1,2 = 600 – 900 mV. This represents an unusual example13,20 of a reversible two-electron transfer for a CoIII complex, and an unexpectedly dramatic compression of redox potentials for the two electron transfers. Experimental and theoretical analysis are employed to characterize the structural and electronic changes that occur upon reduction of 1 to the isolable doubly-reduced neutral complex 2 (Figure 2). Further computational studies are used to model the electron transfer chemistry of 1 and to provide insights on the factors that enable facile multi-electron redox reactions at transition metal centers bearing redox-active ligands.

2(ClO4) Co

N

N

N

Co

NCCH 3

N

N N

1

2 2(ClO4)

N

Co N

2(ClO4)

NCCH 3

N

3

Co N

NCCH 3 NH

4

Figure 1. Structures of CpCo complexes.


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2(ClO4) N

Co N N

NCCH3

0 2e

Co

E1/2 = -0.16 V vs. Fc0/+

N

1

N

+

CH3CN

N 2

Figure 2. Two-electron reduction of 1 to 2 (Fc = Ferrocene).

RESULTS Synthesis. Azopyridine complex 1 was synthesized according to the literature procedure for related cyclopentadienyl (Cp) cobalt compounds.32 Treatment of [CpCo(CO)2] with iodine in diethyl ether yields the diiodide complex, to which the phenylazopyridine ligand in acetonitrile is added in the presence of excess silver perchlorate (Scheme 1). Complex 1 is stable for a limited time in acetonitrile, but air stable as a solid. The 2,2’-bipyridine (bipy, 3) and pyrazolylpyridine (Hpypz, 4) analogues were prepared in the same fashion. The synthesis of neutral cyclopentadienyl CoI complexes of the form [(Cp-R5)Co(L2)] (R = H, CH3) are known33-35, but are typically obtained in low to moderate yields. Here, we find that azopyridine complex 2 can be prepared in 90% yield by reduction of 1 in acetonitrile with two equivalents of cobaltocene (E1/2 = -1.33 V versus ferrocene/ferrocenium, Fc0/+)36 (Scheme 1). The reaction proceeds quantitatively, yielding 2 as dark red crystals following separation from the cobaltocenium perchlorate salt via extraction of the product into diethyl ether. Notably, 2 is stable as a solid or in solution for several days, even in the presence of oxygen and/or water.



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2(ClO4) (i)

Co OC

CO

(ii)

Co OC

I

I

Co

N N

N

(iii) NCCH 3

1

Co N

N N 2

Scheme 1. Synthesis of azopyridine complexes 1 and 2. Conditions: (i) I2, diethyl ether, 25°C, 3 days. (ii) azpy, 3 eq. AgClO4, CH3CN, 25°C, 2 h. (iii) 2 eq. cobaltocene, CH3CN, 25°C, 45 min.

The 1H NMR spectrum of 1 shows two sets of aromatic resonances corresponding to the pyridyl and phenyl ring protons. The Cp signal is at δ 6.31 ppm, very similar to that of the bipyridine and pyrazole complexes 3 and 4. The neutral azopyridine complex 2 is also diamagnetic and displays a highly resolved 1H NMR spectrum. The Cp signal is shifted upfield to δ 4.84 ppm, comparable to that reported for the Cp chemical shift for [CpCo(bipy)].35 No changes in the chemical shifts or peak widths of the 1H NMR signals are observed up to 75°C in acetonitrile-d3, suggesting that higher spin states are not thermally accessible over this range of temperatures.

Electrochemistry. The electrochemical behavior of complexes 1, 3, and 4 was investigated in acetonitrile and referenced to the ferrocene/ferrocenium redox couple (Fc0/+) and is summarized in Table 1. The bipyridine and pyrazolylpyridine complexes 3 and 4 exhibit two sequential reversible one-electron reductions E1 and E2 with DE1,2 of 600 - 900 mV; this behavior is typical of CoIII complexes [CpCo(L2)(CH3CN)]2+.34,37 The CoII/CoI reduction occurs nearly 300 mV more negative for the pyrazole complex 4 compared to the bipyridine complex 3, which would suggest that pyrazolylpyridine is a weaker π-acid ligand than bipy.


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In contrast, the voltammogram of the azopyridine complex 1 displays markedly different behavior. As shown in Figure 3, 1 exhibits two reversible reductions at -0.16 and -1.82 V versus Fc0/+. The peak-to-peak separation for the redox couple at -0.16 V is only 45 mV, compared to 75 mV for ferrocene at the same scan rate (100 mV/s). Reversibility of this feature is maintained up to 2000 V/s (Figure S18). The peak height of the first reduction is twice as large as that of the -1.82 V couple at the same scan rate. Also, controlled-potential electrolysis of 1 at -0.40 V is consistent with a two-electron transfer (Figure S22). These data indicate that the redox couple at -0.16 V is a two-electron process, which is most reasonably attributed to two one-electron steps, where the addition of the second electron occurs more readily than the first.4,38-39 As expected, the voltammogram of 2 in the anodic direction exhibits a reversible two-electron wave -0.16 V versus Fc0/+ (Figure S19). The two-electron oxidation of [CpCo(azpy)] 2 occurs at a more positive potential than the one-electron oxidation of [CpCo(CO)(PPh3)] (-0.38 V versus Fc0/+, CH2Cl2),40 but the two-electron reduction of 1 is more facile than the one-electron reduction of 3 and 4. We undertook detailed experimental and theoretical studies to determine the origin of this remarkable two-electron behavior.



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Figure 3. Cyclic voltammograms of 1 (black), 3 (blue), and 4 (green) in acetonitrile (1 mM cobalt in 0.1 M Bu4NClO4). Scan rate 100 mV/s.

Table 1. Cyclic voltammetry data of 1, 3, and 4a Complex Ligand E1/2 (V)b ∆Ep (mV)c ia/icd E1/2 (V)b ∆Ep (mV)c ia/icd 1 azpy -0.16 45 1.13 -1.82 75 1.0 3 bipy -0.40 70 1.0 -1.01 65 0.95 4 Hpypz -0.36 70 0.83 -1.28 65 0.73 a Conditions: 1 mM [Co] in 0.1 M Bu4NClO4 in CH3CN, glassy carbon working electrode, Pt auxiliary electrode, Ag/AgNO3 reference electrode, 100mV/s. b E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials, respectively. Potentials versus Fc0/+. c∆Ep = Epa - Epc. dia = anodic peak current, ic = cathodic peak current. Determined from the full CV scan shown in Figure 3.

Characterization. Solid-State Structure. Complexes 1 and 2 were characterized by X-ray crystallography, and selected bond lengths and angles are summarized in Table 2. The iodide complex [CpCo(Hpypz)I]I 4-I was also characterized by X-ray crystallography: this structure is presented in the Supporting Information.


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Crystals of 1 suitable for X-ray diffraction were obtained from diffusion of diethyl ether into acetonitrile solution. As shown in Figure 4, 1 exhibits a “piano-stool” geometry at cobalt where the metal center is ligated by the η5-cyclopentadienyl anion, bidentate azopyridine ligand, and acetonitrile. A similar coordination geometry is seen in the structure of the pyrazole complex 4-I (Figure S3). The N3-C10 length is shorter than expected for a pure single bond, indicating some delocalization over the co-planar pyridine and azo fragments. However, this delocalization does not extend onto the phenyl ring, as evidenced by the longer N2-C11 bond length and the dihedral angle between this ring and the azo plane (21.9°). The N-N bond length is 1.27 Å, which falls into the range typically observed for neutral azo ligands,26 consistent with the formulation of 1 as a CoIII complex.

Figure 4. Structure of 1 and 2, 50 % probability ellipsoids. Hydrogen atoms are omitted for clarity, as well as the perchlorate counterions of the dicationic complex 1.



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Table 2. Comparison of the experimental and calculated values for selected bond lengths and angles for 1 and 2.

Co-N1 Co-N2 Co-N4 Co-Cp N2-N3 N2-C11 N3-C10 N1-Co-N2 N1-Co-N4 N2-Co-N4

1

1

2

2

(expt)

(calc)

(expt)

(calc)

Bond lengths (Å) 1.930(2) 1.952 1.869(2) 1.947(2) 1.966 1.823(2) 1.909(2) 1.906 1.680 1.728 1.680 1.267(3) 1.278 1.348(3) 1.431(3) 1.417 1.428(3) 1.399(4) 1.379 1.351(3) Bond angles (°) 79.88(9) 80.46 81.41(9) 91.60(9) 92.36 94.54(9) 93.93 -

1.875 1.838 1.691 1.338 1.429 1.347 81.56 -

Single crystals of 2 for X-ray analysis were isolated by slow cooling of a hexanes/diethyl ether solution. The coordination geometry of 2 is that of a two-legged piano stool where the cyclopentadienyl and Co-azopyridine planes are nearly perpendicular. The phenyl ring is twisted out of the azo plane by 53.4°. There is a substantial shortening of the Co-azopyridine bonds, with the Co-N1 and Co-N2 lengths decreasing by 0.06 and 0.12 Å compared to 1, respectively.41 The N-N bond has increased from 1.27 to 1.35 Å for 2, halfway between typical bond lengths of neutral azo and dianionic hydrazido ligands.26 The C-C bond lengths in the pyridine ring are not equivalent but rather alternate between short (1.36 Å) and long (1.41 Å), indicative of some loss of aromaticity. Calculated Structures. Density functional theory (DFT) was employed to provide further insights on the coordination geometry and electronic structure of these complexes. Several functionals and basis sets were evaluated; calculations using pure functionals afforded results that are in better agreement with the experimental crystallographic, spectroscopic, and electrochemical data than those calculated with hybrid functionals. The best overall agreement 10 ACS Paragon Plus Environment

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with all experimental data was obtained using the BP86 functional42-43 with a TZVP44 basis set (see Computational Details) and the computational data described herein are reported with this level of theory, unless otherwise noted. The DFT-calculated structure of 1 is in very good agreement with the X-ray structure: key bond lengths and angles are consistent within 0.05 Å and 0.8°, respectively (Table 2). Upon two-electron reduction of 1 to the neutral complex 2, contraction of the coordination sphere at cobalt and dearomatization of the azopyridine ligand is well reproduced in the DFT-optimized structure, which accurately predicts the key bond lengths of 2 within 0.015 Å. The lowest energy structure of 2 is a closed-shell singlet with no net spin (S = 0), as observed experimentally by 1H NMR. Optimizations using the broken-symmetry approach45-48 starting from the triplet-optimized geometry with BP86 yielded the antiferromagnetically couplet singlet biradical that is higher in energy than the corresponding closed shell singlet. Furthermore, the calculated structure of the antiferromagnetically coupled state (Figure S23) is inconsistent with the X-ray structure of 2 irrespective of the density functional employed. In particular, the Co-N2 bond distance is longer (by ca. 0.1 Å) compared to the experimental value, and the dihedral angle between the phenyl and azo planes is substantially less due to partial delocalization of the azopyridine ligand radical onto the phenyl ring. Hybrid functionals with Hartree-Fock exchange such as B3LYP predict the inverse scenario, with the antiferromagnetically coupled biradical as the lowest energy state (Figure S24). However, the optimized structure and calculated spectroscopic signatures of this biradical are very different from those measured experimentally. Taken together, the DFT calculations are most consistent with the spin-paired diamagnetic description of 2. Infrared & Resonance Raman Spectroscopy. The N-N stretching frequency is often used as an indicator of the extent of reduction of azo ligands.13,49 For 1, the N-N stretching frequency



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is 1388 cm-1, which is lower than ν(N-N) for free phenylazopyridine (1420 cm-1) and is consistent with coordination of a neutral ligand to cobalt. The assignment of the azo stretching frequency was corroborated by analysis of the mono-15N labeled complex 1-15N, in which only the azo N-phenyl nitrogen is labeled. Here, the N–N stretch is 1370 cm-1, which is in reasonable agreement with the predicted isotopic shift calculated for a 14N-15N harmonic oscillator. The N-N stretching frequency of 2 could not be reliably assigned based on its infrared spectrum due to the presence of several absorbances in this region. Instead, the resonance Raman (rR) spectrum was recorded in carbon tetrachloride at 77 K (Figure 5). Excitation into the visible transition at 20 200 cm-1 (vide infra) leads to strong rR features at 1300, 1235, and 1183 cm-1. Analysis of the 15N-monolabeled complex 2-15N reveals that only the two lower energy modes show an isotopic shift, decreasing by 12 and 17 cm-1 for the 1235 and 1183 cm-1 bands, respectively. These energies and isotopic shifts are well reproduced by frequency calculations on 2-14N and 2-15N (Figure 5, inset). By comparison to the DFT-calculated spectra, the bands at 1235 and 1183 cm-1 are assigned to the N-Cphenyl and the N-N azo stretching modes, respectively.

Figure 5. Resonance Raman (rR) spectra of 2-14N (black, solid) and 2-15N (black, dashed) in CCl4 at 77 K (λex = 20 200 cm-1). Inset: DFT-calculated vibrational spectra for 2-14N (red, solid) and 2-15N (red, dashed).


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UV/Vis Spectroscopy. The electronic spectrum of 1 and 2 in acetonitrile is shown in Figure 6a, and Gaussian fits of these spectra are given in the Supporting Information. The absorbance profile of 1 displays a weak d-d transition at 20 400 cm-1 (ε = 1300 M-1 cm-1), which is also observed for bipyridine and pyrazole complexes 3 and 4 (Figure S7). Also present are two strong charge-transfer (CT) transitions at 26 000 cm-1 (ε = 10 300 M-1 cm-1) and 29 900 cm-1 (ε = 9100 M-1 cm-1). Upon reduction to 2, an intense CT band is still present at 27 000 cm-1 (ε = 10 600 M-1 cm-1), and there are additional lower energy features: two strong transitions at 23 900 cm1

(ε = 7200 M-1 cm-1) and 20 200 cm-1 (ε = 12 900 M-1 cm-1), and a weak band at 14 300 cm-1 (ε =

700 M-1 cm-1).



ε (M-1 cm-1)

(a) 15000 10000 5000 0 25000 20000 15000 10000 Energy (cm-1)

ε (M-1 cm-1)

(b) 15000 10000

0.08 0.06 0.04

5000 0

0.02

Oscillator Strength

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 25000 20000 15000 10000 Energy (cm-1)

Figure 6. (a) Electronic spectrum of 1 (black) and 2 (red) in acetonitrile. (b) Electronic spectrum in acetonitrile (red) and calculated stick spectrum (blue) for 2 based on gas-phase TD-DFT wavelengths and oscillator strengths, and calculated spectrum (navy) with Gaussian functions of fwhm = 850 cm-1. (c) Representative natural transition orbitals of 2 for the 19 200, 18 700, and 14 700 cm-1 electronic transitions.

Time-Dependent DFT. The origins of the key transitions in the UV/Vis absorption spectrum of 2 were investigated by time-dependent density functional theory (TD-DFT) calculations in the gas phase.50-52 The energies of the calculated transitions (Figure 6b) reproduce the main features of the experimental spectrum in acetonitrile. The majority of the calculated excited states for 2 have significant contributions from more than one transition; therefore, a natural transition orbital (NTO) analysis53 was performed.


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As seen in Figure 6b, several calculated transitions contribute to the band at 27 000 cm-1 in the experimental spectrum, with the major contributions being from the two transitions at 27 300 and 26 200 cm-1. The NTO pairs for these states (see Table S4) both correspond to phenyl π ® LUMO, which has significant azo π* and cobalt dyz character. The band at 23 900 cm-1 in the experimental spectrum is mainly composed of the transitions at 24 900 & 24 000 cm-1. The dominant NTO pairs for these transitions are both assigned as metal-to-ligand, specifically, cobalt dxz ® pyridine π* and cobalt dyz with significant azo π* contributions ® pyridine π*, respectively. The strong band at 20 200 cm-1 in the experimental spectrum corresponds to excited states derived from transitions at 19 200 and 18 700 cm-1, which are attributed to the approximate HOMO ® π* with significant cobalt and azopyridine contributions based on the respective NTO pairs. The weak band at 14 300 cm-1 is assigned to the transition at 14 700 cm-1, which is cobalt dxz ® LUMO. The dominant NTO pairs for the transitions at 19 200, 18 700, and 14 700 cm-1 are shown in Figure 6c, and other select NTO pairs are presented in the Supporting Information. X-Ray Photoelectron Spectroscopy. The electronic structure for complexes 1 and 2 in the solid state was probed using X-ray photoelectron spectroscopy (XPS). The Co 2p1/2 and 2p3/2 binding energies for 1 are 795.4 and 780.5 eV, respectively (Figure 7). These binding energies are comparable to the XPS data for 3 and 4 (Table 3). The splitting between the Co 2p/2 and 2p3/2 peaks for these complexes is 14.9 eV, which is typical of low-spin CoIII systems.54-55 For the neutral azopyridine complex 2, there is a 1 – 1.2 eV positive shift in both Co 2p binding energies compared to the CoIII complexes. The direction and magnitude of this shift is consistent with reduction at cobalt. The 2p1/2 – 2p3/2 peak separation is 0.2 eV larger than for the CoIII complexes, and matches well with the peak separations observed for a representative CoI



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standard, CpCo(COD) 7, where COD = 1,5-cyclooctadiene. Notably, satellite features typically characteristic of the CoII oxidation state56-57 are not present for both Co 2p signals.

Figure 7. X-ray photoelectron spectroscopy (XPS) data for 1 (black) and 2 (red). Table 3. Co 2p X-ray photoelectron spectroscopy binding energies (eV) Complex

Co 2p1/2

Co 2p3/2

ΔBEa

1 795.4 780.5 14.9 2 794.4 779.3 15.1 3 795.4 780.5 14.9 4 795.3 780.4 14.9 7 794.8 779.8 15.0 a ΔBE = difference in Co 2p1/2 and Co 2p3/2 binding energies in eV.

Computational Studies. The calculated potential for one-electron reduction of 1 to the monocationic species [CpCo(azpy)(CH3CN)]+ is -0.17 V versus Fc0/+. The calculated spin density of this mono-reduced species is predominantly localized on the metal (Figure S25), consistent with a CoII assignment. The low-spin state (S = ½) for the mono-reduced cation is energetically favored by 11 kcal/mol compared to the high-spin quartet. Similarly, the most stable state of the 16 ACS Paragon Plus Environment

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singly reduced forms of complexes 3 and 4 is the low-spin doublet where the unpaired electron is also localized on cobalt (Figures S26 and S27). These results are in line with the low-spin CoII formulation assigned by electron paramagnetic resonance (EPR) spectroscopy for the related species [Cp*Co(bipy)]+, generated in situ by electrochemical reduction of the CoIII-Cl complex.37 For the second electron transfer, the calculated redox potentials for the reduction of the five-coordinate complexes [CpCo(N-N)]+ are in much better agreement with the experimental values compared to reduction of the acetonitrile complexes [CpCo(N-N)(CH3CN)]+, suggesting that the two-electron reduction occurs via a stepwise ECE’ mechanism in which the acetonitrile ligand is lost prior to addition of the second electron. Our calculations correctly predict the experimentally observed trend in compression of the reduction potentials for 4 > 3 > 1. Furthermore, the reduction potential for [CpCo(azpy)]+ to generate 2 is calculated to be -0.14 V versus Fc0/+, 30 mV more positive than the calculated first reduction potential. In order to quantify the importance of structural reorganization to the reduction processes for this series of complexes, theoretical square schemes were constructed for the two electron transfer steps for 1, 3, and 4 (Figure 8). This method of analysis, introduced by Baik, Schauer, and Ziegler58, separates the reduction energies into two components: the energy difference associated with electron addition in the absence of structural changes (vertical electron attachment), and the energy difference for structural relaxation. In the theoretical square scheme, vertical electron attachment and structural relaxation energies are represented by the horizontal and vertical lines, respectively, and the overall reduction process is indicated by the diagonal.



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Figure 8. Theoretical square schemes for the first (ESCF,1, left) and second (ESCF,2, right) reduction processes, separated into vertical electron attachment steps (horizontal) and associated structural changes (vertical). S = acetonitrile.

Table 4. Electronic energy contributions (ΔESCF, eV) of vertical electron attachment and structural relaxation associated with the one-electron redox processes E1 and E2 Complex

N-N

ESCF,1

ESCF,2

1a

1b

2a

2b

1

azpy

-4.26

-0.09

-4.22

-0.16

3

bipy

-3.72

-0.29

-3.57

-0.07

4

Hpypz

-3.75

-0.27

-3.46

-0.02

The electronic energy change ΔESCF calculated in a polarizable continuum solvent model for acetonitrile for each (a) electron addition and (b) accompanying structural reorganization step in Figure 8 for 1, 3, and 4 are summarized in Table 4. The ΔESCF values for the first and second electron additions (steps 1a and 2a) scale approximately with the π-acidity of the bidentate ligand: Hpypz < bipy E1) resulting in a two-electron feature,4 but also represents a substantial decrease of nearly 1 V in the potential required to reach the neutral complex compared to 3 and 4.



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The relative importance of structural reorganizations to the reduction energies and potential ordering can be estimated by theoretical square schemes (Figure 8).58,68-69 This analysis reveals that structural relaxations do not play a large role (< 0.1 eV) in the first one-electron reduction for 1, but contribute to the energetics for the reduction of 3 and 4 (0.29 and 0.27 eV, respectively). The structural changes associated with this reduction are primarily lengthening of the Co-NCCH3 bond, which is consistent with anticipated Jahn-Teller distortions for the reduction of CoIII (d6) to low-spin CoII (d7), as well as with the Co-NCH3CN σ-antibonding interaction in the SOMO of [CpCo(N-N)(CH3CN)]+. For the second reduction of the bipyridine and pyrazole systems, vertical electron attachment is the main factor governing the energetics, with structural reorganizations accounting for < 2 % of the change in electronic energy. Considering the electron attachment energies only, addition of the first electron to 3 and 4 is substantially easier than the second electron addition. This normal ordering of potentials is further enforced by the greater stabilization from structural relaxation in [CpCo(N-N)(CH3CN)]+; that is to say, ΔΔESCF,b, defined as ΔESCF,2b – ΔESCF,1b or the difference between the reorganizational energy contributions for steps 2b and 1b, is positive for 3 and 4. While structural reorganization is negligible for the first electron addition to 1, the stabilization provided by structural relaxations for the second one-electron transfer is larger (0.16 eV). The vertical electron attachment energies for steps 1a and 2a are equivalent within the accuracy of our DFT methods (ΔΔESCF,a = ΔESCF,2a – ΔESCF,1a = 0.04 eV), and thus for 1, this analysis indicates that structural relaxation upon reduction to 2 is an important factor that makes the second reduction more energetically favored than the first. The geometry changes associated with the second reduction of 1 are readily evident in the DFT-calculated and X-ray structures of 2 (vide supra): there is substantial lengthening of the azo N-N bond and contraction of the Co-


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Nazo bond. The close approach of the azo ligand is enabled due to rotation of the phenyl ring out of the ligand plane, thereby avoiding unfavorable steric interactions with the cyclopentadienyl ligand while maintaining the crucial planar p-system of the azopyridine core. The same degree of Co-N contraction is not possible with bipyridine and pyrazole due to the rigidity of these ligands. The frontier molecular orbitals of the five-coordinate [CpCo(N-N)]+ complexes offer further insight into the two-electron redox behavior of 1. As mentioned previously, the SOMO for the monocation is predominantly metal-centered in this ligand series, consistent with the unpaired electron residing on cobalt (vide supra). In the case of [CpCo(bipy)]+ and [CpCo(Hpypz)]+, the SUMO is also primarily localized to the Co dz2 orbital with only small contributions from the ligand π* orbitals (Figures S29 – S30). A very different situation exists for the azopyridine system: the azo π* orbital is much lower energy compared to bipy and Hpypz,27 which brings it into better energetic alignment with the cobalt d orbitals and results in more effective orbital mixing. Consequently, the SUMO of [CpCo(azpy)]+ has substantial metal dz2 and azo π* character with a metal-ligand π-bond between Co and Nazo. The greater overlap between cobalt and azopyridine orbitals lowers the energy of the SUMO, making it more energetically accessible for electron transfer. Furthermore, localization of both the SOMO and SUMO on cobalt in [CpCo(bipy)]+ and [CpCo(Hpypz)]+ results in a large electrostatic penalty for population of the SUMO. For [CpCo(azpy)]+, delocalization of the SUMO across the metal and azo ligand decreases Coulombic repulsion for the second electron addition, resulting in the approximately equal vertical electron attachment energies for the first and second electron transfers (steps 1a and 2a). To further underscore the importance of metal-ligand frontier orbital alignment in affecting the potential difference DE1,2 between sequential one-electron reduction events, we



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computed the two one-electron reduction potentials for a series of analogous CoIII complexes where the azo moiety is replaced with an imine. The electron-withdrawing nature of the imine ligand is systematically varied by changing R1 and R2 (Figure 10). As the π-acidity of the imine ligand is increased from A to L, the calculated DE1,2 decreases, becoming negative beyond complex I and redox potential inversion is predicted.4 Therefore, these calculations suggest that two-electron redox behavior of 1 is not an isolated case and may be generalized to other first row metal complexes by careful tuning of the ligand π* orbital energies for optimal overlap with the metal d orbitals. While further experimental and theoretical studies are warranted, these observations provide a framework on which to base the design of metal and ligand coordination environments to facilitate multi-electron transfer reactions.

Figure 10. DFT-calculated reduction potentials for imine analogues of 1.

CONCLUSION


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We have shown that the dicationic CoIII complex 1 is able to accept two electrons at -0.16 V versus Fc0/+ in acetonitrile. This redox process is chemically and electrochemically reversible, in both cases generating the neutral species 2. DFT calculations predict that the first electron transfer is metal-based to generate the CoII monocationic species [CpCo(azpy)(CH3CN)]+. Following loss of the acetonitrile ligand, addition of the second electron populates the SUMO of [CpCo(azpy)]+, a metal d-azo π* hybrid orbital, which leads to elongation of the azo N-N bond and shortening of the Co-Nazo bond in 2. The new electron pair is delocalized across the metal and ligand with a π-bonding interaction between cobalt and azo nitrogen. We conclude that the two-electron redox behavior of 1 arises from the following: (a) the structural relaxations associated with the second electron transfer provide additional significant stabilization for 2; and (b) the low-energy π* orbital of the strongly π-acidic azopyridine ligand is optimally poised for overlap with the cobalt d orbitals, which reduces the electrostatic penalty for electron transfer to [CpCo(azpy)]+ relative to the bipyridine or pyrazole systems. These results provide guiding principles for the design of monometallic first row coordination compounds that undergo twoelectron redox reactions.

EXPERIMENTAL SECTION Materials and Methods. All experiments were carried out under a nitrogen or argon atmosphere using standard vacuum line, Schlenk, and glove box techniques. Solvents were dried following standard methods and degassed via three freeze-pump-thaw cycles. All reagents were used as received unless otherwise described. Phenylazopyridine (azpy)70, CpCoI2(CO)71, [CpCo(bipy)(CH3CN)][ClO4]2 332, were prepared as previously described. 15N-labeled phenylazopyridine was synthesized according to the same procedure70 for the unlabeled ligand



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using 15N-nitrosobenzene, prepared from 15N-aniline (Sigma-Aldrich) via oxidation with H2O2/sodium tungstate.72 Tetrabutylammonium perchlorate (Sigma-Aldrich) was recrystallized from ethanol, dried under reduced pressure, and stored in an inert atmosphere glove box. Ferrocene (Sigma-Aldrich) was sublimed under vacuum, and stored in an inert atmosphere glove box. Acetylferrocene (Sigma-Aldrich) was recrystallized from hexanes, dried under reduced pressure, and stored in an inert atmosphere glove box. CAUTION: While we experienced no difficulties with the use of perchlorate salts, they should be regarded as potentially explosive and handled with care. Instrumentation. 1H and 13C NMR spectra were recorded on Varian 300, 400, or 500 MHz spectrometers. All NMR spectra were taken at room temperature unless stated otherwise. Residual solvent proton and carbon peaks were used as reference. Chemical shifts are reported in parts per million (δ). UV/Vis absorption spectra were obtained on an Agilent Cary 6000i UV/Vis/NIR spectrometer. IR spectra were recorded in a solid KBr disk on a Bruker Vertex 70 FTIR spectrometer. Resonance Raman spectra were obtained using a series of lines from Kr+ and Ar+ ion lasers. Incident power ranged from 5 to 20 mW in a 135 backscattering configuration. Samples were immersed in a liquid nitrogen finger dewar. High-resolution mass spectra were obtained by LC/ESI-MS on a Waters Acquity UPLC and Thermo Fisher Exactive Orbitrap mass spectrometer. X-ray photoelectron spectroscopy was performed on a PHI VersaProbe system using an Al Kα X-ray source (1486.6 eV). Solid samples were ground into a fine powder and dispersed onto adhesive tape or indium foil. Spectra were calibrated to the carbon 1s binding energy (284.5 eV). Electrochemical experiments were performed using a WaveNow USB Potentiostat (Pine Research Instrumentation) at ambient temperature in an inert atmosphere glove box. For cyclic


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voltammetry, the electrochemical cell consisted of a three-electrode setup using a glassy carbon working electrode (3 mm diameter, Bioanalytical Systems, Inc.), platinum auxiliary electrode, and Ag/AgNO3 non-aqueous reference electrode (Bioanalytical Systems, Inc.). Controlled potential electrolysis was performed using a custom built gas-tight cell with a Teflon cap having openings for each electrode or component: glassy carbon working electrode (3 mm diameter, Bioanalytical Systems, Inc.) for cyclic voltammetry, carbon cloth (Fuel Cell Store) working electrode for bulk electrolysis, Ag/AgNO3 non-aqueous reference electrode (Bioanalytical Systems, Inc.), and platinum coil auxiliary electrode separated from the cell solution by a 20 mm diameter fine glass frit. The glassy carbon electrode was polished between each scan. All potentials are referenced to the Fc0/+ couple (0.0 V) using acetylferrocene (0.25 V versus Fc0/+) as an internal reference. X-Ray Crystallography. Single crystals for X-ray analysis were mounted on a Kapton loop using Paratone N hydrocarbon oil or perfluorinated ether oil. All measurements were made on a Bruker APEX II CCD detector X-Ray diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Frames corresponding to an arbitrary sphere of data were collected using ω-scans of 0.3° counted for a total of 10 seconds per frame. Data were integrated using the Bruker SAINT software program73 to a maximum θ-value of 28.26°, analyzed for agreement and possible absorption using XPR74, and corrected for Lorentz and polarization effects. Absorption corrections were applied using the SADABS program.73 No decay correction was applied. Structures were solved by direct methods,73 expanded using Fourier techniques, and refined by full-matrix least-squares procedures based on F2.75 Hydrogen atoms were included in ideal positions and refined isotropically in riding model with Uiso = 1.5Ueq(X) for methyl groups and Uiso = 1.2Ueq(X) for other atoms, where Ueq(X) are thermal parameters of parent atoms. Non-



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hydrogen atoms were refined anisotropically. Crystallographic data for 1, 2, and 4-I are presented in the Supporting Information. Synthesis. [CpCo(azpy)(CH3CN)][ClO4]2 1. A solution of CpCoI2(CO) (0.20 g, 0.50 mmol) in acetonitrile (5 mL) was added drop-wise to a mixture of AgClO4 (0.31 g, 1.5 mmol) and phenylazopyridine (0.10 g, 0.55 mmol) in acetonitrile (10 mL) with stirring under N2. The color of the solution changed from dark violet to red with precipitation of AgI. This suspension was stirred at room temperature for 2 hours. After filtration, the filtrate was evaporated in vacuo. The crude residue was recrystallized from acetonitrile/diethyl ether (1:1) affording 1 as a red powder. Yield 50% (0.14 g). 1H NMR (500 MHz, CD3CN) δ 9.71 (d, J = 5.3 Hz, 1H), 8.80 (d, J = 7.8 Hz, 1H), 8.59 (td, J = 7.8, 0.9 Hz, 1H), 8.38 (d, J = 7.8 Hz, 2H), 8.11 (ddd, J = 7.3, 5.8, 1.3 Hz, 1H), 7.94 (t, J = 7.4 Hz, 1H), 7.84 (t, J = 7.9 Hz, 2H), 6.31 (s, 5H), 1.96 (s, 3H). 13C NMR (101 MHz, CD3CN) δ 166.6, 157.4, 156.4, 144.6, 136.4, 132.3, 131.5, 130.7, 125.4, 94.0. HRMS. Calcd for C16H14N3ClO4Co [(M - CH3CN + ClO4)+]: m/z 405.9999. Found: m/z 405.9997. [CpCo(azpy)] 2. Cobaltocene (0.038 g, 0.20 mmol) was added to a solution of [CpCo(azpy)(CH3CN)][ClO4]2 1 (0.055 g, 0.10 mmol) in acetonitrile (6 mL). The mixture was stirred at room temperature under N2 for 45 minutes. After removal of the solvent in vacuo, the residue was diluted with diethyl ether and filtered to remove the cobaltocenium perchlorate salt. The filtrate was evaporated in vacuo to yield 2 as a dark red solid. Yield 28 mg (90 %). 1H NMR (500 MHz, CD3CN) δ 10.22 (d, J = 6.4 Hz, 1H), 8.13 (d, J = 7.4 Hz, 2H), 7.61 (d, J = 8.6 Hz, 1H), 7.48-7.42 (m, 3H), 7.36 (ddd, J = 8.5, 6.9, 1.5 Hz, 1H), 6.98 (td, J = 6.5, 1.3 Hz, 1H), 4.84 (s, 5H). 13C NMR (101 MHz, CD3CN) δ 156.2, 131.6, 129.6, 127.0, 124.5, 120.3, 112.6, 78.7. HRMS. Calcd for C16H15N3Co [(M + H)+]: m/z 308.0592. Found: m/z 308.0598.


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[CpCo(15N-azpy)] 2-15N. The same procedure to prepare the unlabeled complex 2 was followed here, except 15N-labeled phenylazopyridine was used in place of azpy. Yield 18 mg (86 %). NMR spectra are consistent with the unlabeled sample. HRMS. Calcd for C16H1514N215NCo [(M + H)+]: m/z 309.0563. Found: m/z 309.0561. [CpCo(Hpzpy)(CH3CN)][ClO4]2 4. A solution of CpCoI2(CO) (1.00 g, 2.46 mmol) in acetonitrile (50 mL) was added drop-wise to a mixture of AgClO4 (1.53 g, 7.38 mmol) and Hpzpy (0.39 g, 2.71 mmol) in acetonitrile (50 mL) with stirring under N2. The color of the solution changed from dark violet to red with precipitation of AgI. The suspension was stirred at room temperature for 72 hours. After filtration, the filtrate was evaporated in vacuo and the solid residue washed with acetone (3 x 10 mL). The crude solid was recrystallized from acetonitrile/diethyl ether (1:1) at room temperature affording 4 as a red crystalline solid. Yield 59% (0.74 g). 1H NMR (300 MHz, CD3CN) δ 13.16 (br s, 1Hz, N-H of pz), 9.66 (ddd, J = 5.7, 1.4, 0.8 Hz, 1H), 8.31 (d, J = 2.9 Hz, 1H), 8.27 (td, J = 7.8, 1.4 Hz, 1H), 8.09 (ddd, J = 7.9, 1.5, 0.8 Hz, 1H), 7.78 (ddd, J = 7.6, 5.7, 1.5 Hz, 1H), 7.17 (d, J = 2.9 Hz, 1H), 6.36 (s, 5H), 1.96 (s, 3H). 13C NMR (125 MHz, CD3CN) δ 158.3, 154.6, 152.9, 142.4, 140.1, 127.6, 124.2, 106.6, 90.5. HRMS. Calcd for C13H11N3Co [(M - CH3CN - H)+]: m/z 268.0279. Found: m/z 268.0281. Computational Details. Density functional theory (DFT) calculations76-78 were performed using the Gaussian0979 (G09) Rev D.01 software package. Intermediates were optimized in the gas phase on an ultrafine grid using the “tight” convergence criterion in G09™ to obtain the lowest energy structures along with their electronic energies. Six different density functionals (BP86,42-43 B3P86,43,80 BLYP,42,80-82 B3LYP,42,80-82 M06-L,83 and M0684) were employed, with TZVP44 basis set on all the atoms. Solvation energies for the optimized structures were obtained at the same level of theory using the SMD85 (Solvation Model Density) model for



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acetonitrile. Harmonic analyses in the gas-phase were performed on all gas-phase optimized structures in order to obtain bond frequencies and thermal corrections to the free energy, which were then added to the electronic energies to obtain the absolute free energies in the gas phase and solvated phase.86-87 Broken-symmetry45-48 singlet states were optimized starting from the triplet optimized geometry with the keyword guess=mix using the corresponding unrestricted density functional. Spin density diagrams and MO pictures were made using Gaussview5 from the spin densities and MO energies calculated on a grid of 803 points at an isovalue of 0.02 and 0.08, respectively. The in silico vibrational spectra were plotted based on the frequencies and intensities given by the harmonic analyses described above in the gas phase. TD-DFT50-52 calculations were performed at the same level of theory. Redox potentials were obtained using ferrocene as the reference according to equation (1), wherein the standard free energy and the corresponding reduction potential for each reaction was calculated using equations (2) and (3). Standard state corrections were made to the free energies in order to account for the change in going from 1 mol per 24.46 L (gas phase) to 1 M (solution phase). For stoichiometric loss of an acetonitrile ligand during reduction, a correction of RT ln(19.15) = 1.75 kcal/mol was applied to the free energy. )45 *

)* Ox($%&') + Cp. Fe($%&') ⇄ Red($%&') + Cp. Fe* ($%&') ° $%&'

° $%&' @ D E;

ΔG°$%&' = G:;< =>? @ + GBC ° E($%&')

=

°($%&')

°($%&')

− GGH=@ + GBCD E;

° −ΔG($%&')

nF

(1) (2) (3)

ASSOCIATED CONTENT Supporting Information. Crystallographic data files (CIF format), tabulated X-ray data, and molecular structures obtained from single crystal X-ray diffraction. 1H NMR spectra, 32 ACS Paragon Plus Environment

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UV/Vis spectra and Gaussian fits. Additional cyclic voltammograms and controlled-potential electrolysis studies. Computational details including DFT-optimized coordinates and energies of 1, 2 (singlet), 2 (broken-symmetry singlet), 2 (triplet) complexes, TD-DFT excitation energies and oscillator strengths. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: [email protected]

Author contributions: †

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This material is based on work supported by the Global Climate and Energy Program at Stanford, and the National Science Foundation (CHE-1565947). K.M.W. and S.R. are grateful for Center for Molecular Analysis and Design (CMAD) Fellowships. K.M.W. is also grateful for a Gabilan Stanford Graduate Fellowship and a National Science and Engineering Research Council of Canada (NSERC) Postgraduate Scholarship. We thank Dr. Andrew J. Ingram and Dr.



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Diego Solis-Ibarra for assistance with X-ray crystallography, Kyle Sutherlin for assistance with rR spectroscopy, and Olivia Hendricks for assistance with X-ray photoelectron spectroscopy. Prof. T.D.P. Stack and Prof. E.I. Solomon are acknowledged for helpful discussions.

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Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine

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