Cobalt PorphyrinâThiazyl Radical Coordination Polymers: Toward

Article Cite This: J. Am. Chem. Soc. 2017, 139, 14620-14637

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Cobalt Porphyrin−Thiazyl Radical Coordination Polymers: Toward Metal−Organic Electronics Delia A. Haynes,*,† Laura J. van Laeren,† and Orde Q. Munro*,‡ †

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa School of Chemistry, University of the Witwatersrand, Private Bag 3, PO WITS 2050, Johannesburg 2000, South Africa

‡

S Supporting Information *

ABSTRACT: Herein we delineate an unusual one-dimensional coordination polymer (CP), 3, prepared from S = 1/2 Co(TPP), 1 (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and S = 1/2 4(4′-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, 2. The atypically long S−S distance for CP 3 (2.12 Å) reflects fractional electron transfer from the formally Co(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP 3 involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co(TPP)]0.5+ hemication (Co2.5+) bound to a dithiadiazolide hemianion (pyDTDA0.5−). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP 3 and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals 1 and 2 involves coordination of the Co(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co(TPP)(S-py-DTDA), 3a. Polymerization and crystallization of 3a affords diamagnetic CP 3. Dissolution of CP 3 in DMSO favors Co−S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound CoIII(TPP)(N-py-DTDA−)(OSMe2) complex (λmax, 436 nm). However, monomerization of CP 3 in dry 1,2dichloroethane affords bright green diamagnetic CoIII(TPP)(N-py-DTDA−), 3b, with multiple MLCT bands in the 800−1100 nm NIR region and a red-shifted Soret band (λmax, 443 nm). Implications for the use of CP 3 in electronic devices are discussed based on its density of states.

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electron-withdrawing ligands (hfac−, or 1,1,1,5,5,5-hexafluoroacetylacetonate) have been successful in forming coordination complexes through the heterocyclic N atom. Thus, Preuss and co-workers have investigated the coordination of 4-(2′-pyridyl)and 4-(2′-pyrimidyl)-1,2,3,5-dithiadiazolyl radicals to several transition metal centers12 and, more recently, Ce(III)13 and Ln(III).14 The first N-coordinated DTDA metal complex was described in 2004;12a subsequent work suggested that magnetic coupling between metal ions may be mediated by DTDA ligands,12b though not in the case of coordination polymers of Mn(II), Co(II), and Ni(II) with 4-(2′-cyanofuryl-5′)-1,2,3,5dithiadiazolyl as the bridging ligand.15 Organometallic complexes incorporating DTDA ligands and [Cr(Cp)(CO)2] (where Cp = cyclopentadienyl anion) have also been described.16 These represent the only known DTDA−metal complexes where coordination to a metal center occurs through the heterocyclic S atoms with retention of the S−S bond. In all of these materials, the DTDA moiety acts as a bidentate ligand with coordination to the metal occurring through both sulfur atoms (an η2-S−S binding mode). Examples of 4-(4′-pyridyl)-

INTRODUCTION Thiazyl radicals exhibit significant potential as building blocks for molecular materials with unusual or intrinsically exploitable physical properties.1 Examples of magnetic2 and conducting3,4 materials derived from such radicals are known and underpin the active development of new thiazyl radicals. One family of synthetically tractable thermally and kinetically stable thiazyl radicals, the 1,2,3,5-dithiadiazolyls (DTDA), has been widely investigated both in solution and in the solid state. The magnetic behavior of both DTDA radicals and their metal complexes strongly depends on the arrangement of molecules in the solid state and on intermolecular interactions. 2 Consequently, there have been various attempts at directing the crystal structures of these materials.5 The first reported crystal structure of a DTDA radical appeared in 1980.6 Since then, a large number of derivatives have been structurally characterized, and two exhibit magnetic ordering in the solid state.7,8 The behavior of DTDA radicals as ligands has also been investigated9 and reviewed.10 Early work focused on zerovalent metals, where coordination tends to occur via the sulfur atoms and cleavage of the S−S bond is commonplace.11 Reactions of the DTDA radical with divalent metals chelated by strongly © 2017 American Chemical Society

Received: July 25, 2017 Published: September 19, 2017 14620

DOI: 10.1021/jacs.7b07803 J. Am. Chem. Soc. 2017, 139, 14620−14637

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Journal of the American Chemical Society

122 and radical 2 (synthesized via the method usually employed in our laboratory23) in THF. Specifically, 1 was added to a suspension of 2 in THF in a 1:1 mol ratio. The resulting bloodred solution was heated gently prior to removing an aliquot and layering it with an equal volume of n-hexane. This yielded crystals of the THF solvate of CP 3, namely 3·n(THF), in low but reproducible yield.24 Most preparations of CP 3 afford a diamagnetic (EPR-silent) solid. Occasionally, a broad lowintensity EPR signal with g ≈ 2.00 is detected for unground solid CP 3. The origin of this signal is unclear but may reflect the presence of excess 2 possibly as a capping group on a small fraction of the CP chain ends (see Figure S12), which would account for an available free electron in the chain. Crystal structure of 3·n(THF). Crystals of the THF solvate of CP 3 were filtered and air-dried; these remained stable under ambient conditions for several months. The single crystal structure of 3·n(THF), where n = 1.71, was determined, revealing a unique metal-bridging coordination mode for the DTDA radical. Crystals of 3·n(THF) (monoclinic, Pn) contain two metalloporphyrin−radical pairs and four part-occupancy THF molecules in the asymmetric unit (Supporting Information, SI, Figure S1 and Tables S1 and S2). Each of the crystallographically independent Co(TPP)(py-DTDA) pairs represents the repeat unit of a coordination polymer in which radical 2 bridges adjacent Co centers through Co−S and Co− Npy bonds (Figure 1a). For the first and second independent molecules (A and B), the Co−S distances measure 2.2509(4) and 2.2597(4) Å, respectively. Similarly, the independent axial Co−Npy distances are 2.0996(3) and 2.0892(3) Å. Each cobalt center is thus hexacoordinate, with the sulfur and pyridyl nitrogen atoms of different axial py-DTDA ligands bound trans to one another. The chains thus formed run along the c-axis and are stacked on top of one another along the crystallographic a-axis, such that the crystallographically unique parallel stacks of chains alternate along the b-axis (Figure 1b), resulting in a polar packing arrangement. The THF molecules reside in channels between the chains along the a-axis (Figure S2, SI). As shown in Figure 1a, the porphyrin core conformations are S4-ruffled25 and characterized by mean absolute perpendicular meso-carbon displacements, |Cm|, of 0.44(4) and 0.40(3) Å relative to the 25-atom mean plane of the metalloporphyin (molecules A and B, respectively).26 The S4-ruffled conformation is evidently brought about by the plane of the axial pyridine ring in each of the two independent molecules bisecting a Np−Co−Np angle within the porphyrin core and being positioned above opposite meso-carbons in the macrocycle;27 the relevant dihedral angles measure 43.3° and 43.1° for molecules A and B, respectively. Coordination of the DTDA (CN2S2 ring) to the Co center results in an out-of-plane deviation for the CN2S2 ring with the metal-bound S atom displaced toward the Co atom. The S−S bond distance in neutral 2 has not been previously determined, but the mean S−S bond length in all 1,2,3,5dithiadiazolyl radicals catalogued in the CSD 28 (117 structures)29 is 2.089(10) Å (Figure S3). The S−S bond distances in 3 are 2.120(2) and 2.122(2) Å for molecules A and B, respectively, consistent with a fully intact S−S bond in both coordination polymers (the sum of the van der Waals radii of two sulfur atoms is 3.60 Å30). The 43 structurally characterized metal complexes of 1,2,3,5-dithiadiazolyl radicals in the CSD fall in to two classes: those where coordination is via the DTDA nitrogen atom(s) (38 structures) and those involving an η2-S−S binding mode (5 structures). The mean S−S distance in the N-

1,2,3,5-dithiadiazolyl (py-DTDA) acting as a Lewis base via the pyridyl nitrogen are uncommon but exist in the form of an adduct with triethylborane and in the complex fac-Mn(CO)3Br(py-DTDA) 2.17 Reaction of these adducts with Pd(0) complexes resulted in cleavage of the S−S bond. Based on the alluring behavior of DTDA radicals in the solid state and the range of supramolecular architectures that can be designed using porphyrin scaffolds,18 we elected to investigate the reaction between a metalloporphyin with a doublet ground state, specifically the low-spin 3d7 Co(II) complex of 5,10,15,20-tetraphenylporphyrin, Co(TPP) (1), and a DTDA radical capable, in principle, of bridging metal centers, namely, 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl, py-DTDA (2) (Scheme 1). Scheme 1. Compound Structures and Formal Charge Assignments for CP 3

This reaction produces an unusual coordination polymer, CP 3, in which 2 forms a bridging ligand to adjacent cobalt ions to generate formally noninteger redox states for both the metal ion and bridging ligand. We have also investigated the dissociation of solid 3 in coordinating and noncoordinating solvents; evidence is presented for five- and six-coordinate Co(III) porphyrins axially ligated by a N-bound (Npyridyl−Co) py-DTDA anion, a hitherto unknown coordination mode for a species derived from a 1,2,3,5-dithiadiazolyl radical. Collectively, our analysis of (i) CP 3 in the solid state, (ii) the reaction between 1 and 2 in solution to form 3a, a monomer precursor of CP 3, and (iii) the unique species produced upon dissolution of solid CP 3 lays the foundation that will underpin potential applications of this and related systems in topical areas such as solution-fabricated metal−organic electronics.19

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RESULTS AND DISCUSSION Synthesis. In our initial synthetic approach, radical 2 was synthesized via the known one-pot procedure20 and reduced with a Zn/Cu redox couple in THF. Before removal of THF from the reaction mixture, a solution of [CoIII(TPP)]Cl and Ag[SbF6] in THF was added to the flask. An aliquot of the reaction mixture was placed in a tube and layered with hexane. This yielded thin, iridescent purple-blue plates of CP 3 (as its THF solvate), a novel CP, evidently formed by reduction of [CoIII(TPP)]Cl and subsequent (or concomitant) bridging of the metal centers by 2.21 We have since found that CP 3 can be reproducibly synthesized directly from the Co(II) porphyrin 14621


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Figure 1. (a) One of the crystallographically independent coordination polymer (CP) chains formed in 3·n(THF), catena-μ-4-(4′-pyridyl)-1,2,3,5dithiadiazolyl-5,10,15,20-tetraphenylporphinatocobalt-THF (1/1.71). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids (50% probability level) and atomic numbering for the py-DTDA ligand highlight the central repeat unit. Key mean bond lengths (Å): Co−Np, 1.968(7); Co−Npy, 2.094(5); Co−S, 2.256(5); S−S, 2.121(1); S−N, 1.640(22); N−C(DTDA), 1.336(18); Npy−C, 1.343(5). (b) Crystal packing in 3·n(THF) viewed along the a-axis. The two crystallographically independent polymeric chains of 3 are shown in orange and blue; THF molecules are shown in violet-red. (c) Space-filling view along one CP chain in 3 highlighting the stepped array of py-DTDA ligands (carbon atoms in green) on the front side of the chain.

Figure 2. (a−c), DFT-calculated (in vacuo) structures and highest-energy occupied molecular orbitals of 2; (d) plot of the variation of the S−S bond distance of 2, d(S−S), with redox state, q. The solid lines are nonlinear fits of the data to third order polynomials. The in vacuo curve (red) may be scaled to fit the mean S−S distance (blue) observed for all neutral DTDA radicals in the solid state (117 structures, CSD). Interpolation of the S−S distance determined crystallographically for 3·n(THF) predicts a fractional charge for the py-DTDA bridging ligand in the solid state of close to −0.5e. The cubic function describing the blue curve is d(S−S) = 2.0893(4) − 7.68(3) × 10−2q + 4.96(5) × 10−3q2 + 9.85(9) × 10−3q3; R2 = 0.9998.

In the case of a resonance hybrid, the expectation is that the mean Co−Np and Co−Npy distances (Np and Npy refer to pyrrole and pyridine N atoms, respectively) in CP 3 should fall between the limits typical of Co(II) and Co(III) porphyrins. However, because oxidation of a S = 1/2 Co(II) porphyrin removes the unpaired electron from the 3dz2 orbital (the 3dx2−y2 orbital is vacant), there is a negligible difference in the Co−Np bond distances for the two oxidation states, despite Co(III) having a smaller ionic radius. Analysis of the Co(II) and Co(III) porphyrins axially ligated by ≥1 pyridine ligand in the CSD (25 structures; 16 Co2+ complexes) reflects this since the

coordinated structures is 2.090(8) Å, whereas ligation via the DTDA S−S bond is associated with a longer S−S distance (2.134(4) Å). One explanation for this trend is that the metal ion delocalizes significant electron density into the π-SOMO (singly occupied molecular orbital) of the DTDA ring (antibonding with respect to the S−S bond),31 thereby reducing the S−S bond order. For CP 3, complete transfer of an electron from Co(II) to coordinated 2 would give a formally Co(III)-bound 1,2,3,5-dithiadiazolide anion, while fractional electron transfer would reflect a resonance hybrid. 14622


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that the electronic structure of 2 is unusual in CP 3 and involves significant anionic character. Since the estimated charge from the DFT-simulations is close to −0.5e, the X-ray and DFT data indicate that CP 3 may be formulated as a resonance hybrid structure in which the redox state of the cobalt ion is +2.5 and that of the bridging py-DTDA ligand −0.5. The material overall is charge-neutral and diamagnetic in this model. (Most preparations of polycrystalline 3 are EPRsilent.) Noninteger oxidation states are known for group 9 and 10 metal ions. Thus, several CPs of the −M−M−X− type, where M = Pt or Ni and X = I, exhibit average valence (AV) states for the metal ions best formulated as M2.5+,41 while the +2.67 oxidation state exists for cobalt in the oxide Co3O4.42 Furthermore, ligands with noninteger metrical oxidation states (ranging from −0.5 to −2.3) occur in complexes of aromatic catecholates.43 Clearly, the metrical oxidation states of −0.5 and +2.5 determined here for 2 and Co, respectively, in CP 3 appear feasible. Since dithiadiazolide anions are notoriously unstable both in the solid state and in solution,44 though detectable by cyclic voltammetry (E1/2 = −0.95 V),45 CP 3 represents a rare example of a material with a partially anionic DTDA and good solid-state stability. In some respects, this stability parallels that observed for products isolated from radical coupling reactions between R-DTDA• and the 17electron radical [CrI(Cp)(CO)3]•.16 In these diamagnetic 18electron complexes, where R-DTDA replaces one CO ligand, the S−S bond is coordinated in η2 fashion to the metal. The S− S bonds (2.11−2.15 Å) reflect full or partial anionic character for the DTDA ring (Figure 2), in agreement with the formal oxidation states assigned to the system (Cr2+/R-DTDA−).16c Cationic DTDA species, in contrast, have been crystallographically characterized in the +1 (S−S distance ∼2.01 Å, e.g., [HCN2S2][Br]46) and +0.5 (S−S distance ∼2.07 Å) oxidation states.46 The latter noninteger oxidation state reflects the AV state typical of monocationic bis(DTDA) salts such as 1,3- or 1,4-[(S2N2C)C6H4(CN2S2)][X] (where X = I, Br).46,47 Importantly, the scaled DFT-calculated curve of Figure 2 is commensurate with the X-ray data for these cationic DTDA species, upholding its generality. IR Spectroscopy. FTIR spectroscopy, being both widely accessible and of diagnostic value for crystalline R-CNSSN• radicals,48 is central to confirming the 1D chain structure in polycrystalline CP 3. The IR spectrum of CP 3 is shown along with spectra for 2 and Co(TPP)Cl for comparison in Figure 3. Diagnostic vibrational modes crucial to detecting 2 sandwiched between cobalt ions in the CP have been assigned with the assistance of frequency analysis on the fully optimized in vacuo DFT-calculated structure (B3LYP49/SDD50 level of theory)51 of two polymer repeat units, μ-{Co(TPP)(S-py-DTDA)}2 or more simply {3}2. Co(TPP) band assignments are available elsewhere.52 The DFT-calculated and experimental IR spectra of 2 in various states were also required to guide vibrational mode assignments made for CP 3 (Figure S5; Table S4). Eight lowfrequency vibrational modes (565, 581, 751, 846, 1052, and 1139 cm−1) give rise to spectral features for CP 3 that signal the presence of 2 as an ambidentate bridging ligand. The bands are labeled a through f in Figure 3, which also illustrates the displacement vectors for each vibrational mode. Full band assignments comparing DFT-calculated and experimental vibrational modes are given in the SI (Figure S6 and associated text). For brevity, only the more important vibrational modes of the CP are discussed here.

mean Co−Np distance measures 1.97(1) and 1.96(1) Å for Co2+ and Co3+, respectively (Figure S4, Table S3). The Co−Np bond lengths in CP 3 average 1.963(5) and 1.973(5) Å for the two independent molecules, matching those in both oxidation states.32 Significantly, the axial Co−Npy distance of CP 3 (2.09(1) Å) falls between those typical of Co3+ (2.21(11) Å) and Co2+ (2.03(7) Å) porphyrins with pyridyl ligands. This, considered with the unusually long S−S bond for the bridging ligand, provides structural evidence for a bonding formalism in which a full (or partial) 1,2,3,5-dithiadiazolide anion is bound to a full (or partial) Co(III) porphyrin. Redox States and the Structure of py-DTDA. To understand what happens to the structure of 2 when electron transfer alters its redox state, as appears to be significant when 2 bridges 1 in CP 3, we used DFT simulations at the HSEH1PBE33/6-31G(d,p)34 level of theory to calculate the structures of py-DTDA in the oxidation states −2, −1, 0, +1, and +2 (Figure 2). The SOMO of 2 is of π-symmetry and localized on the DTDA ring, consistent with the literature.35,36 Reduction to form the diamagnetic anion pairs up the unpaired spin of the radical; this raises the energy of the SOMO from −5.479 eV to +0.618 eV for the HOMO of the anion. Oxidation of the radical, in contrast, removes the unpaired spin in the SOMO such that the HOMO − 1 level of the neutral radical, a σ-symmetry bonding MO, becomes the HOMO of the diamagnetic cation. Marked geometrical changes accompany electron density changes in the DTDA ring. The N−S and S−S distances increase by 7.6% and 6.4%, respectively, as the oxidation state changes from +1 to −1, while the C−N distance exhibits the reverse trend, shortening somewhat (1.9%) in going from the cation to the anion. Contraction of the C−N bond length during DTDA ring reduction requires commensurate widening of the N−C−N bond angle from 117° in the cation to 129° in the anion. The structural perturbations calculated here for the DTDA ring of py-DTDA in different redox states, particularly the S−S distance, therefore place the earlier qualitative bonding assessments of Banister et al.37 and Rawson et al.20,38 for related neutral DTDA-type radicals (and their charge transfer salts39) on a more formal quantitative footing. As seen in Figure 2d, the S−S bond distance varies linearly with molecular charge for the redox states −1, 0, and +1 because the electron density changes involve mainly the DTDA ring for these states. In contrast, the redox states −2 and +2 involve addition and removal of an electron from MOs that are centered mainly on the pyridine ring. Collectively, this gives rise to the nonlinear dependence of S−S distance on molecular charge. Solvation dampens the degree of elongation or contraction of the S−S bonds with changing redox state; the effect is more pronounced at the extremities of the charge range. Importantly, Figure 2d may be used to estimate the redox state of the bridging py-DTDA ligand in CP 3 from the crystal structure. Specifically, we scaled the data for the in vacuo simulation (red curve) so that the calculated S−S distance for the neutral radical matched that for the mean S−S bond length in 117 selected 1,2,3,5-dithiadiazolyl radicals catalogued in the CSD (2.089(10) Å). This gives a graph (blue curve) that gauges the dependence of the S−S distance on redox state for DTDA species in the solid state and is of fundamental significance in assigning the oxidation state. Interpolation from the mean X-ray S−S distance of 2.121(1) Å for CP 3 suggests the charge on the bridging DTDA ligand is −0.41e.40 This confirms our earlier postulate from analysis of the X-ray data 14623


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stretching modes for the porphyrin and aryl rings give the expected, though broadened, 3-peak pattern (2995, 3023, 3052 cm−1) seen for Co(TPP)Cl. Two prominent broad bands unique to the CP appear at a much lower frequency, specifically 2851 cm−1 (band g, anti- and symmetric C−H stretching modes for the pyridine β-CH groups closest to the DTDA ring) and 2922 cm−1 (band h, anti- and symmetric C−H stretching modes for the pyridine α-CH groups pointing toward the porphyrin ring). Despite the limited DFT model used, we can confidently assign bands g and h to the νa(C−H) and νs(C−H) stretching modes of py-DTDA in the polymer chain. Both bands are broad because they encompass the symmetric and antisymmetric modes for the CH groups and the material is polymeric. Interestingly, vibrational modes g and h in the CP have the opposite frequency order (assignments) relative to the uncoordinated free radical (Figure S5, Table S4). Although puzzling at first glance, the structure of 3 clearly suggests that vibrations of the α-C−H groups on the pyridine ring (those adjacent to the pyridyl N atom) will require a higher excitation energy because of their close interaction with the porphyrin ring and, consequently, the higher steric repulsion barrier to normal vibration of these groups; no such restrictions to vibrations of the α-C−H groups exist for the unbound free radical 2. The key signature of 1D chain formation in CP 3 is undoubtedly the marked shift of bands g and h for 2 to a lower frequency range than the TPP ν(C−H) bands. This reflects the extended 1D structure of solid CP 3 and points to an unusual electronic state for the bridging ligand, which weakens the C− H bond force constants. The experimental frequency difference for bands g and h, |Δνα̅ −β|, measures 71(1) cm−1. We reasoned that changes in the electron density in the DTDA ring with molecular redox state might have an unequal impact on the αand β-C−H stretching vibrations in both unbound 2 and the bridging ligand in CP 3. In principle, this would lead to different frequency responses to changes in the charge state of the molecule for the α- and β-CH groups. Figure 4 confirms this notion for 2 using the DFT-calculated antisymmetric vibrational modes for analysis. Specifically, the α- and β-C−H stretching vibrations decrease monotonically with increasing negative charge over the range +1 through −2; the variation is more pronounced for the α-CH groups than the β-CH groups. The expected linear relationship between vibrational frequency and C−H bond distance (and thus the underlying bond stretching force constant) holds irrespective of the position of the C−H group on the pyridine ring (Figure 4b). Usefully, the absolute frequency separation between the αand β-C−H stretching vibrations, |Δν̅α−β|, may be used to gauge the redox state of py-DTDA in CP 3, or, for that matter, any other coordination compound of 2. As shown in Figure 4c, |Δν̅α−β| follows a simple quadratic charge dependence, allowing use of an experimentally determined frequency difference for the material to estimate the molecular charge state of 2. Since the experimental value of |Δνα̅ −β| for 3 is 71(1) cm−1, solution of the quadratic equation indicates that the charge on pyDTDA in the coordination polymer is −0.60e. The 3D plot of band frequency against molecular charge and C−H distance for py-DTDA gives an excellent fit to an equation describing a 3D plane, confirming the concerted dependence of the C−H stretching frequencies on both molecular charge and the stretching force constant (C−H bond distance). Collectively, analysis of the S−S bond distance in py-DTDA as a function of molecular charge (Figure 2) and |Δν̅α−β|

Figure 3. Infrared spectra (normalized absorbance) for 2, 3, and Co(TPP)Cl (reference). (a) Low frequency spectra. (b) High frequency spectra. Calculated frequency data at the B3LYP/SDD level of theory for two polymer repeat units (i.e., a dimer) are included for selected diagnostic vibrational modes and are corrected by the pertinent NIST frequency scale factor (0.961). Note: the absolute scales for the two sets of spectra are unequal. Peaks unique to 3 that confirm the presence of the py-DTDA ligand in the coordination polymer chain structure are marked with a star symbol (★). Other peaks of potential diagnostic value are marked with a solid circle (●). Band assignments for eight diagnostic modes involving the bridging ligand in CP 3 are illustrated schematically.

The signature low frequency mode of CP 3 is the antisymmetric N−S stretching vibration νa(N−S) (band c) for the DTDA ring of 2 (978 cm−1, Table S4), which exhibits the largest shift (227 cm−1) to lower frequency (751 cm−1) upon formation of the 1D CP and is well-matched by the DFTcalculated frequency (732 cm−1). The significant shift to lower frequency for the N−S stretching mode reflects increased electron population of the π-antibonding SOMO (Figure 2) and concomitant reduction of the N−S bond order, confirming the partial anionic character of the DTDA ring in the CP (as deduced from the X-ray data). The high frequency IR spectrum of CP 3 is pivotal to correctly gauging the redox state of py-DTDA. The C−H 14624


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(initially Co2+) to the bridging py-DTDA ligand upon assembly of CP 3 forms a py-DTDA0.5− hemianion as the bridging ligand in the material. This allows us to write the diamagnetic polymer repeat unit (Scheme 1) as Co2.5+(TPP)(S-py-DTDA0.5−). Thermally driven valence tautomerism in cobalt complexes reportedly accounts for the valence- and spin-state interconversion CoIII(L)(catecholate)(semiquinonate) ↔ CoII(L)(semiquinonate)2, where L is a neutral bidentate ligand such as a bipyridine or bipyrazine derivative.53 Here, valence tautomerism (Scheme 2) accounts for the electronic structure Scheme 2. DTDA Ring−Metal Valence Tautomerism in CP 3

of CP 3 adequately because the electron responsible is simply transferred intramolecularly from the metal ion to the coordinated DTDA ring of radical 2 (or vice versa) by direct orbital overlap in the Co−S bond. Solid-State Electronic Spectrum. The diffuse reflectance UV−vis−NIR spectrum of polycrystalline CP 3 is shown in Figure 5a. The spectral data were smoothed and the background subtracted before deconvolution and analysis (Figure S8). The UV bands are weaker in intensity than the visible bands (in contrast to solution transmission spectra where the UV bands are normally 2−3 orders of magnitude more intense than the visible bands). This is not uncommon for porphyrin diffuse reflectance spectra54,55 but may, in fact, reflect the intrinsic electronic structure of CP 3. The main bands in the electronic spectrum of CP 3 are well-resolved and may be assigned using the normal notation for metalloporphyrins. The sharp Soret band (B(0,0); 411 nm) coupled with the wavelength maxima and intensity ratio observed for the two Q bands (550 and 594 nm) reflect the presence of a six-coordinate metalloporphyrin. There are also three distinct bands at λ > 600 nm that are atypical for ordinary Co(II) or Co(III) porphyrins; these excited states reflect the presence of py-DTDA as an axial ligand and may be tentatively assigned as d → π* MLCT bands involving excitation of d electrons to a vacant π-symmetry MO located mainly on the DTDA ring of the bridging ligand in CP 3. By way of comparison, the electronic spectrum of CoII(TPP)56 does not match that of CP 3. Disregarding any solvent-dependent shifts, the Soret (405 nm) and Q(1,0) bands (530 nm)57 of CoII(TPP) recorded in DCM are blue-shifted relative to the band energies observed for solid CP 3. Furthermore, the Q(0,0) band is unresolved and the weak, broad NIR band for CoII(TPP) (750−950 nm, λmax ∼856 nm), which Antipas and Gouterman assigned to the allowed LMCT excitation a2u(π, TPP) → dz2,56 is fundamentally different to the low-lying CT states observed for CP 3. The electronic spectra for six-coordinate CoIII(TPP) derivatives (Soret band ∼430 nm; Q bands ∼545 and ∼580 nm)58 also do not adequately match the electronic spectrum of CP 3, although the visible region bands of solid CP 3 are quite similar in energy to those reported (λmax vis = 555, 598, and 645

Figure 4. (a) Relationship between the in vacuo DFT-calculated [HSEH1PBE/6-31(d,p)] C−H stretching frequencies (antisymmetric modes), C−H bond distances, and molecular redox states of pyDTDA. The equation of the best-fit mean plane through the data is ν̅ = 1.74(7) × 104 + 6(2)q − 1.32(6) × 104d. The surface shows that the calculated C−H stretching frequencies increase linearly with decreasing C−H bond distance and increasing positive molecular charge in a strongly concerted manner (R2 = 0.986). (b) Independent fit of the frequency dependence on the C−H bond distance: ν̅ = 1.85(9) × 104 − 1.42(8)d. (c) Graph of the frequency difference, |Δν̅α−β|, between the α- and β-C−H stretching modes of py-DTDA as a function of molecular charge. The quadratic function describing the relationship is |Δν̅α−β| = 52(2) − 26.2(9)q + 8.7(8)q2, where q is the charge in electron units.

(Figures 4c and S7) permits facile estimation of the redox state of 2 in CP 3. Limits to the approach are that (i) the S−S bond distance can be experimentally measured from a single crystal X-ray structure and (ii) well-resolved bands can be assigned to the α- and β-C−H stretching modes of the pyridyl group, allowing measurement of |Δν̅α−β|. Since all materials are easily analyzed by IR spectroscopy, the second approach (Figure 4) will be universal for systems in which 2 is present either in isolated or in coordinated form. To prove the validity of the DFT simulations, sublimed 2 has an experimental frequency difference |Δν̅α−β| of 53 cm−1 (Table S4); the intercept for the quadratic function (q = 0) is 52(2) cm−1, consistent with the experimental frequency difference for the bands of a chargeneutral radical. Valence Tautomerism in CP 3. Taking the average charge determined on the py-DTDA ligand in CP 3 from the two estimates at hand (−0.41e, X-ray data; −0.60e, IR data), we get q = −0.5(1)e. This conceptually tidy result suggests how best to formulate a bonding description for 3 and thus a foundation for the fundamental electronic structure of the material. Fractional electron transfer of 0.5 units of charge from the cobalt ion 14625


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was a poor match for the experimental spectrum; agreement was considerably better for n ≥ 4. From Figure 5b, the calculated band maxima (543, 567, 615(sh), and 693 nm) are located ca. 10−40 nm to the blue of the experimental band maxima, while the overall shape and band intensity ratios of the calculated spectrum closely match those of the experimental envelope (bands I through V).60 The calculated electronic spectrum of {3}4 over the full UV−vis−NIR range (Figure S9, 140 singlet excited states) therefore predicts a low B/Q-band intensity ratio ( 3), terminating at ΔE∞ (LUMO−HOMO) = 0.669(6) eV for a chain of infinite length. The magnitude of ΔE(LUMO−HOMO) diminishes by 50% for each 1.7 unit length increase in the CP chain relative to the terminal FMO energy gap (Figure S10). The first spin-allowed excited state energy (f > 0 for n = 1), follows a similar pattern. The DFT simulations therefore predict that controlling the chain length of CP 3 would modulate the FMO energy gap. Density of States for CP 3. Although full band structure calculations are beyond the scope of the present DFT study, the density of states (DOS) was analyzed as a function of increasing chain length (Figure 6c). For n ≥ 10, the DOS spectrum develops into a series of well-resolved maxima between −15 and +4 eV for the 14-mer, {3}14, and exhibits two noteworthy features. First, the midpoint energy, Em, between the HOMO and LUMO (−3.56 eV), which would be the chemical potential, μ, for an intrinsic semiconductor at zero temperature, lies close to the edge (ca. −2.93 eV) on the shoulder of peak b (ostensibly the conduction band). This suggests that at infinite chain length, CP 3 could be an intrinsic n-type semiconductor depending on the actual chemical potential for the material. Second, the HOMO and LUMO are close enough in energy to overlap when a bandwidth of 0.3 eV is used for Gaussian convolution of the one-electron states for {3}14. The DOS envelope is thus never exactly zero for the extended chain structure (n ≥ 14) and lack of a discrete band gap is therefore possible (in theory) for the material (n → ∞). One caveat is that location of the HOMO and LUMO on opposite termini of the CP chain creates “chain-end states”. However, such states are experimentally significant in graphene

Figure 5. (a) Solid state electronic spectrum of polycrystalline CP 3 deconvoluted into 15 constituent bands (Voigt functions); selected band assignments are shown. Low-lying charge transfer bands I−III reflect excited states with substantial transfer of metal electron density (dz2) to the bridging py-DTDA ligand (Scheme 1). (b) TD-DFT electronic spectrum (lowest 80 excited states; fwhm = 1400 cm−1) for a tetramer segment of the X-ray structure of 1D coordination polymer 3 (i.e., {3}4). The deconvoluted experimental spectral envelope of bands I−V is superimposed (black line) for comparison. Excited states making up the DFT-calculated envelope (scaled to fit the intensity of the experimental envelope) are indicated; selected states are highlighted (red).

nm) by Hodgson et al. for the 6-coordinate Co(III) complex CoBr(TpivPP)(Py) in solution, where TpivPP is 5,10,15,20tetrakis(o-pivalamidophenyl)porphyrin.59 The band energies for CP 3 thus exhibit characteristics typical of both Co(II) (mostly in the UV region) and Co(III) (mostly in the visible region) TPP derivatives. Although solvent-, temperature-, and ligand-dependent shifts in the electronic spectra of metalloporphyrins are not insignificant and could account for our observations, the spectral data for CP 3 appear to support our earlier notion that the cobalt ion in CP 3 exists in a noninteger redox state poised between +2 and +3 (Scheme 1). The foregoing NIR band assignments (d → π* MLCT) for the lowest-energy states of CP 3 were confirmed using TDDFT simulations on an extended chain model {3}n of the material (Figure 5b). As might be expected, the calculated spectrum of a single repeat unit in the chain structure (n = 1) 14626


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nanoribbons62 and would presumably also become important for the electronic structure of nanowires fabricated with short chains of CP 3. The solid-state electronic structure of {3}14 is reminiscent of MMX-type CPs [M2(CH3CS2)4I]n, where M = Pt or Ni, which have a metallic band63 between the valence and conduction band edges below the Fermi level and thus metallic charge conduction (when M = Pt) above room temperature.64 If the states marked by peaks a and b in Figure 6c give rise to typical valence and conduction bands, respectively, then their separation, ΔEb−a, equates to an optical band gap of 593 nm (2.09 eV). (Note: the actual energy gap is ∼1.27 eV if the band edges are used as opposed to the peaks in the DOS spectrum.) Interestingly, ΔEb−a is identical to the experimental energy for the Q(0,0) state in the solid state spectrum of 3 (Figure 5a). A key transition in the calculated spectrum falling within the Q(0,0) absorption band envelope is labeled IV (581 nm) in Figure 5b. From Table S7, transition IV involves electron transfer down the chain of {3}4 from CP repeat unit [3] to repeat unit [2] (Figure 6b), suggesting that carrier mobility along a vector defined by the principal axis of the CP chain is feasible. The narrow band gap for CP 3 (∼1.27 eV) and band spread suggests that the conduction electron population, Ncb, could be quite high (theoretically ∼4.6 × 108 electrons/cm3) at 298 K.65,66 (Pure semiconductor-grade silicon, for comparison, has a band gap of 1.12 eV67 and Ncb = 8.5 × 109 electrons/cm3 at 298 K.) The gap energy of ∼1.27 eV taken from the DOS band edges of CP 3 is ∼0.57 eV lower than that of pentacene (1.84 eV) and up to ∼1.5−2.0 eV smaller than many typical molecular semiconductors used in OLEDs.68 Finally, the dependence of ΔE(LUMO−HOMO) on CP chain length mirrors previous DFT simulations on covalent porphyrin wires69 and graphene nanoribbons, which predict a similar monotonic decrease in the calculated band gap for the material as a function of increasing chain length or ribbon width.70,71 For graphene, the ribbon width may be used to modulate the band gap of the material, as shown experimentally72 and theoretically.73 For CP 3, control of the chain length could similarly allow modulation of the electronic behavior of the material (alongside the many chemical modifications that may be made to the porphyrin and pyDTDA bridging ligand). CP 3 clearly has the potential to be used in electronic devices as a transistor component or molecular wire,19,41 thereby contributing to a topical trajectory4 in a field hitherto dominated by magnetic studies on thiazyl radical complexes. Recent progress in controlling selfassembly74,75 and performing conductance measurements76,77 on 1D metalloporphyrin wires suggests that studies with CP 3 might be fruitful. In the remainder of the paper, we focus on delineating the solution behavior of the system because understanding the molecular species and how they assemble or disassemble will be crucial in any efforts at device fabrication with CP 3. EPR and DFT Study on the Formation of 3. The reaction between the monomeric precursors 1 and 2 of CP 3 was studied using EPR spectroscopy (Figure 7). A solution of 2 in DMSO gives the expected 1:2:3:2:1 pentet centered at 3485 G (g = 2.007) due to coupling between the radical electron and the two I = 1 nitrogen nuclei (aN = 5.5 G). Addition of aliquots of 1 dissolved in DMSO to this solution of 2 results in monotonic reduction of the DTDA radical’s signal intensity until its complete disappearance with sufficient addition of 1. The EPR data are consistent with added 1 trapping 2 in

Figure 6. (a) Graph of the DFT-calculated FMO energy gap and first excited state energies for 1D CP 3 (X-ray coordinates) vs chain length. Energy levels of the 12 highest and 12 lowest energy filled and unfilled MOs, respectively, are indicated (right) for the tetramer (n = 4). The MLCT transition (MO 852 → LUMO 861) leading to band II in the spectrum (695 nm) is illustrated. (b) Selected MOs for {3}4 relevant to assigning key transitions in the visible−NIR electronic spectrum of solid CP 3. Porphyrins are numbered [1]−[4] so that the HOMO resides on unit [1] in the chain. Isosurfaces ±0.02 au. (c) Density of states (DOS) spectrum of 3 for selected coordination polymer chain lengths. The Gaussian convolution algorithm employed peaks of unit height and a fwhm of 0.30 eV for each state. Occupied and virtual MO energies for the 14-mer chain are indicated by green and red bars, respectively.

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only way in which the diamagnetic reaction product could form would be for intramolecular electron transfer to occur between the formally Co(II) ion and the DTDA ring over a distance >6 Å. Such an electron transfer would produce the Co(III) complex of the N-bound py-DTDA anion and would seemingly be less likely. Importantly, the phenyl radical analogue of 2 (PhDTDA, phenyl-1,2,3,5-dithiadiazolyl) showed equivalent reactivity to 2 when reacted with 1, affording an EPR-silent adduct in solution (Figure S11). Since Ph-DTDA may only coordinate to 1 via the DTDA ring and gives the same EPR reaction profile as 2, the model proposed for the interaction of 2 with 1 in Figure 8a fits the available data. To elucidate the identity of the initial species formed in solution between 1 and 2, we used DFT simulations in DCM and DMSO solvent continua to gauge the relative stability of the diamagnetic five-coordinate N- and S-bound py-DTDA adducts of 1. The simulations were also used to assess the nature of the interacting molecular orbitals for the most stable product complex. As shown in Figure 8b, the S-bound pyDTDA adduct of 1, compound 3a, is the thermodynamically favored reaction product by 62.46 kJ mol−1 in DCM and 48.22 kJ mol−1 in DMSO. The predicted product species (3a) was confirmed by the 1H NMR spectrum of a solution of 1 mixed with 2 in DMSO-d6, which showed signals from CoII(TPP)(Spy-DTDA)(O-DMSO) (i.e., the DMSO solvate of 3a) with no evidence of 3b or its DMSO adduct, as judged by the absence of the upfield pyridine α-CH and β-CH resonances (5−6 ppm) typical of 3b (vide inf ra). Three noteworthy points emerge from analysis of the ligand field energy level diagram in Figure 8b: (i) the N-bound pyDTDA− anion of 3b is a strong field ligand, while S-bound pyDTDA in 3a is of intermediate ligand field strength, (ii) the reaction of 1 with 2 to give 3a involves spin-pairing and donation of electron density by 2 into the 3dz2 orbital of the Co(II) ion of 1 to form the Co−S bond, and (iii) depopulation of the 3dz2 and 3dx2−y2 orbitals of 3b (when compared with those of 3a) reflects formation of the Co(III) porphyrin axially coordinated to py-DTDA−.85 In short, 3a is a five-coordinate Sbound Co(II) complex of neutral 2, while 3b is a fivecoordinate N-bound Co(III) complex of 2−. Species 3b is more than a curiosity, becoming especially relevant when CP 3 monomerizes in noncoordinating solvents (vide inf ra). Finally, correct assignment of the species formed when 1 reacts with 2 in solution underpins an understanding of how CP 3 forms. Specifically, the LUMO of 3a (which is simply the antibonding counterpart to the HOMO in Figure 8a) has significant 3dz2 character (32%, Figure S12). Polymerization of 3a presumably involves donation from a filled σ-symmetry MO incorporating the pyridyl nitrogen atom lone pair of one molecule of 3a (MO206) to the LUMO acceptor (MO221) of a second molecule of 3a. Formation of the dative covalent Co− Npy bond first permits dimerization and thence oligomerization of 3a to {3}n in solution. Macromolecular CP 3 would precipitate or crystallize when the solubility product of {3}n is exceeded (eq 1).

Figure 7. EPR study of the reaction of py-DTDA, 2, with added aliquots of a DMSO solution of CoII(TPP), 1, at 295 K. The spectral progression (top to bottom) reflects accumulative quenching of the signal from free 2 (top panel) with increasing [1] until only EPR-silent 3a is present (bottom panel). The last two spectral traces are plotted on the same intensity scale to highlight complete quenching of the signal. Similar spectra were obtained in CH2Cl2 and THF solutions of 1 and 2. Use of the phenyl analogue of 2, Ph-DTDA (phenyl-1,2,3,5dithiadiazolyl), which lacks the pyridyl N-donor atom, afforded similar spectra.

solution to afford an EPR-silent diamagnetic adduct at 295 K. Since py-DTDA is an ambidentate ligand capable of coordinating to cobalt through either the pyridyl nitrogen or DTDA sulfur atoms, the key question is which coordination mode is preferred for the initial reaction product? In coordinating solvents like DMSO and DMF, Co(II) porphyrins are typically four-coordinate78 (only Co3+ porphyrins form definitive 6-coordinate S- and O-bound complexes with DMSO79). A five-coordinate complex is thus likely formed when 2 reacts with four-coordinate 1, consistent with the reaction of Co(II) porphyrins with other ligands80,81 and NO.82 Because the SOMO of 2 is a π-symmetry MO with large amplitude on the two sulfur atoms (Figure 2) and the unpaired electron in 1 resides in the 3dz2 orbital, σ overlap of the 3dz2 orbital with a lobe of the π-symmetry SOMO on one of the DTDA ring sulfur atoms would allow formation a Co−S bond (Figure 8). In this bonding scenario, the two unpaired electrons in 1 and 2 spin-pair to form the bonding electron pair that makes up the Co−S bond. The reaction amounts to straightforward radical coupling and is clearly akin to the reaction between Co(II) porphyrins and nitric oxide (NO).83,84 The alternative scenario in which the pyridyl nitrogen atom of 2 coordinates to the Co ion of 1 places the unpaired spins on the metal ion and axial ligand at least 6.3 Å apart. Unless there is a direct orbital pathway allowing spin-pairing to occur, the

n1 + n2 ⇌ n3a ⇌ {3}n ⇌ {3}n(s)

(1)

Co(II) porphyrin CPs such as those bearing trans meso-pyridyl donor ligands (5,15-dipyridyl-10,20-diphenylporphyrin, H2DPyP) are known86 and highlight the feasibility of CP 3 formation from pyridyl-based building blocks (3a) in which the metal ion is formally Co(II). Only once oligomerization gets underway in the present system will the shift in electronic 14628


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Figure 8. (a) Scheme delineating the EPR-observed reaction between py-DTDA, 2, and Co(TPP), 1. Molecular orbitals for DFT-calculated singlet state (S = 0) species (CH2Cl2 solvent continuum, 298.15 K) are indicated (isosurfaces ±0.02 au). The reaction between radicals 1 and 2 is a thermodynamically favored radical coupling event yielding diamagnetic 3a. (b) Structural data for the S-bound (left) and N-bound (center) adducts that may be formed in the reaction between 1 and 2. The rightmost diagram depicts DFT-calculated ligand field energy level diagrams (NBO analysis) for the three cobalt porphyrin species. The true Cartesian axes for 3a and 3b (C2 symmetry) are rotated by 45° in the xy plane relative to 1. Labels for the 3d orbitals dx2−y2 and dxy have been switched for 3a and 3b to match the Cartesian axes of 1, which follow the convention employed in ligand field theory (x- and y-axes collinear with Co−Np bonds). Natural electron populations for the dz2 and dx2−y2 orbitals are given (the dπ orbitals are effectively doubly occupied with populations ranging from 1.93−1.97e). The plotted d-orbital energies for 1 are the average of the α- and β-spin orbitals.

structure to that of the valence hybrid (Co2.5+ oxidation state) delineated for solid CP 3 occur. Although it is normal to write a reaction such as eq 1 as a fully reversible process, it is important to stress that because a unique AV electronic state is attained in solid CP 3, dissociation and monomerization of the CP proceeds via a dif ferent mechanism (eq 2) from oligomerization (eq 1), as discussed below. Interestingly, the stoichiometric reaction of 1 with 2 to ultimately yield the zigzag chains of solid CP 3 shares some similarities with the coordination chemistry of the bicyclic C2N3S3 radical 1,3,5-trithia-2,4,6-triazapentalenyl (abbreviated TTTA). Specifically, Fujita and Awaga87 reported the first CP involving TTTA, which, like py-DTDA, is ambidentate and may coordinate to metal ions via its N or S atoms or both. Stoichiometric reaction of Cu(hfac)2 with TTTA involves coordination via the N atoms on adjacent rings of the bridging radical, yielding a crystalline ferromagnetic 1D zigzag CP with weak intermolecular antiferromagnetic interactions. CP 3, in contrast, is diamagnetic principally due to Co−S bond formation and direct spin coupling (Figure 8a). Solution Phase Electronic Spectroscopy. CP 3 dissolves in coordinating solvents such as DMSO or noncoordinating chlorinated solvents (particularly DCM and 1,2-dichloroethane, DCE). While DCM partly monomerizes CP 3 (Figure S13),

DCE fully monomerizes the polymer at higher dilutions (400 μM); this changes dramatically with increasing dilution to bright green. As shown in the photographs (Figure 9a), the bright green color is most pronounced at [3] = 52 μM, becoming yellow-green with further dilution. The color change reflects the marked changes in the relative intensities of the visible-region bands at 549, 589, and 662 nm in the spectrum (Figures 9a−c). More specifically, the red-wavelength band at 662 nm is the most intense visible band at concentrations intensity Q(0,0)/589 nm), the optical spectra support the idea that oligomerization involves chain elongation by concentration-driven repetitive coupling of monomeric 3a or 3b (Figure 8a) to give CP 3. We will return to an assessment of dimer formation in Figure 9c (blue curve, 662 nm data) once the structure of the monomeric species involved in the equilibrium has been assigned.

Figure 10. Possible main routes for the monomerization of CP 3 in DCE (illustrated using a tetramer, {3}4, for simplicity). Selected DFTcalculated parameters are indicated for all stable ground states. Triplet state species (S = 1) require a Co2+ oxidation state, while singlet state species (S = 0) may contain Co2+ (metal ion and charge-neutral pyDTDA spins antiferromagnetically coupled) or Co3+ (anionic pyDTDA−). Assignment of the formal metal ion and ligand oxidation state may be made from the S−S bond distance and the curves of Figure 2.

Monomerization Routes for 3. Regarding the mechanism of hetero- or homolytic bond cleavage for {3}n, monomers in different spin states are possible and are of diagnostic value if they can be identified. Using the route labels of Figure 10, there are four possible cleavage scenarios: (I) Homolysis of the Co− Npy bonds in the chain to yield paramagnetic (S = 1) or antiferromagnetically coupled (S = 0) CoII(TPP)(S-pyDTDA•), denoted species 3a. (I′) Heterolysis of the Co−Npy bonds to give (S = 0) CoIII(TPP)(S-pyDTDA−), or species 3a′. (II) Heterolysis of the Co−S bonds in the chain to produce (S = 0) CoIII(TPP)(N-pyDTDA−), or species 3b. (II′) Homolysis of the Co−S bonds to give (S = 1) CoII(TPP)(N-pyDTDA•), denoted species 3b′. One expects the relative lability/ thermodynamic strength of the Co−Npy and Co−S bonds and the nature of the solvent (coordinating vs noncoordinating) to affect monomerization of the CP by one or more of the routes depicted in Figure 10. DFT calculations indicated that species 3a′ is unlikely in any spin state, while 3b′ is only possible in the triplet state (Figure 10 and SI). With this in mind, TD-DFT simulations were used to calculate the electronic spectrum of each stable monomer species depicted in Figure 10. The best match of the

Figure 9. Changes in the electronic spectra of 3 (normalized absorbance, A*) with concentration in the visible−NIR (a) and UV−visible (b) regions at 298(1) K. The starting solution was produced by dissolving microcrystalline 3·n(THF) in dry 1,2dichloroethane to a stock concentration of 451 μM based on the molar mass of the polymer repeat unit. The photographs in part a show the color change of the solution from brown-red to yellow-green with increasing dilution. (c) Absolute scale UV−vis−NIR spectra for 3 as a function of concentration. The inset highlights the nonlinear Beer−Lambert law behavior at 549 and 662 nm.

cm−1) at high dilution. This behavior is consistent with oligomerization of the chromophore with increasing concentration or, conversely, depolymerization of {3}n with increasing dilution. The variation in absorbance with total concentration, C0, was well-fitted by the Box−Lucas exponential model, A549 = 14630


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transition HOMO → LUMO + 3, or more informatively written: dz2,π(DTDA) → dπ*,π*(py-DTDA). Band a may thus be assigned to an intraligand (IL) π−π* transition of the pyDTDA axial ligand. Although the calculated band (1058 nm) is 528 cm−1 higher in energy (when scaled by +39 nm) than that measured experimentally (1002 nm), the transition is unique in this spectral region and constitutes a key diagnostic band characteristic of the metal-bound anion (py-DTDA−) in 3b. The remaining bands diagnostic of 3b are discussed in relation to Table S10, while the monomerization equilibrium may be written quite simply (eq 2) if we tentatively assume that stepwise dissociation of the oligomer at the chain ends is favored:90

experimental electronic spectrum in dilute DCE, especially in the NIR region, was that for monomer 3b (Figure 11; Figures S14−S16).89 The bands marked a−g in Figure 11 have been assigned on the basis of the DFT-calculated transitions given in Tables S9 and S10 (SI). For brevity, only the most important transition for structure assignment will be discussed here. Band a at 1002 nm arises from a 1B excited state derived from the

{3}n ⇌ {3}n − m + m3b

(2)

Notably, the low-intensity bands e (764 nm), f (743 nm), and k (558 nm) in the experimental electronic spectrum of 3b have no counterpart in the DFT-calculated spectrum of the complex. These states are not vibronic in origin and are assigned to 1, which is present in the solution at a low concentration and derived, we surmise, from disproportionation of 3b into its charge-neutral constituents through intramolecular (or possibly intermolecular) electron-transfer: CoIII(TPP)(N ‐py‐DTDA−) ⇌ CoII(TPP) + py‐DTDA• 3b

1

2

(3)

As noted in Figure S15b, disproportionation of 3b is possibly linked to thermal population of the lowest-energy singlet excited state (1A, 1271 cm−1) in the system, which involves transfer of an electron from the anionic DTDA ring into the σ* MO of the Co−N bond (LMCT), thereby favoring dissociation by concomitantly reducing the Co−N bond order and the Co(III) porphyrin in the excited state. 1H NMR spectroscopy at high dilution (vide inf ra) confirms the presence of all three species and the expected 1:1 mol ratio of the products. The NMR and electronic spectra collectively reveal one possible pathway by which 3b might isomerize to 3a and permit repolymerization to {3}n (eq 1). Additionally, independent redox-mediated dissociation events involving the end groups of oligomer chains {3}n (Figure 10) might afford low, equimolar concentrations of 1 and 2 in the system. The identification of 3b as the monomeric species produced when oligomeric {3}n depolymerizes in DCE points to Co−S bond heterolysis (route II, Figure 10) being mechanistically favored, despite the relative energy of 3b (S = 0) being higher than that of 3b′ (S = 1) and 3a (S = 0). To account for why the least stable species is formed upon dissolution of CP 3, kinetic effects might be as significant as thermodynamic effects. Although we have an idea of the relative stabilities of the monomeric species from the DFT simulations in DCE, we have no knowledge of the actual thermodynamics of the depolymerization reaction. Considering the kinetics of depolymerization, the intermediate Co2.5+ oxidation state for the metal ion in 3 coupled with the anionic character of the bridging ligand might underpin a classic kinetic trans effect91 wherein the powerful σ-donor ligand (based on the parameter Δ3b in Figure 8 and related data in Table S5) labilizes the trans Co−S bond and, consequently, induces 1D chain scission to form kinetic product 3b. The X-ray data (Figure 1) point to a potential structural trans influence91 operating in unison with (or underpinning) the kinetic trans effect since the longer Co− S bond (2.25(1) Å) is ostensibly weaker than the shorter Co−

Figure 11. (a) Visible−NIR spectrum of 3 dissolved in DCE (298 K) and diluted to a concentration of 2.2 μM (from Figure 9a). The experimental spectrum (UV-normalized absorbance scale) has been deconvoluted using Voigt functions to locate the bands. Bands e, f, and k are likely due to 1. The DFT-calculated envelope (fwhm, 1200 cm−1; broken blue line) was scaled to fit the peak height of band g in the experimental spectrum. The inset shows the TD-DFT calculated spectrum of singlet state species 3b in DCE with transition energies and oscillator strengths. The DFT-calculated wavelengths are scaled (+39 nm) based on the energy difference between the experimental (442 nm) and calculated (403 nm) Soret band maxima in the UV region (Figure S15a). (b) Selected MO energy levels for 3b (C2 symmetry). Six frontier MOs that contribute significantly to the visNIR electronic spectrum of 3b are rendered with isosurfaces at the ±0.02 au level. Abbreviations: H, HOMO; L, LUMO. 14631


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poor intermolecular HOMO−LUMO overlap. (Formation of π−π dimers in neutral DTDA analogues, while bonding in terms of SOMO−SOMO overlap,20 is nevertheless weak such that spectroscopic observations below ∼250 K are mandatory to measure equilibrium constants >1.12c,96) Nonplanar 5-coordinate Co(III) porphyrins are known to form tight centrosymmetric π−π dimers97,98 with a pyrrole ring (β-C) of one porphyrin in contact (3.41−3.52 Å) with the Co(III) ion of the partner (as suggested for π-{3b}2). Because π-{3b}2 involves 5-coordinate monomers, the axial ligands likely block further aggregation to higher molecular weight (MW) π-stacked J-aggregates,99 accounting for the lack of spectroscopic evidence for such species. Since eq 6 fits the experimental data well at higher concentrations, π-{3b}2 is present over a wide concentration range (25−400 μM). Because the spectral data also reflect an equilibrium mixture containing oligomeric {3}n (discussed above), the species resulting from depolymerization of CP 3 may be summarized by two main equilibria and one irreversible step (Scheme 3). The stoichiometric coefficients n and m in Scheme 3 are concentration-dependent (divergent nonlinear curves; Figure 9c). NMR Evidence for 3a and 3b. As noted briefly earlier (and in Figure S17), addition of 1 to a solution of radical 2 in DMSO-d6 generates complex 3a (or 3a·DMSO with O-bound DMSO) in which the pyridyl α-CH (7.91 ppm) and β-CH (7.27 ppm) protons resonate as discrete doublets in the expected region for a marginally shielded pyridyl ligand. This suggests that coordination via one of the two S atoms of the DTDA ring is favored. The mean (rotationally averaged) DFTcalculated chemical shifts for the pyridyl α-CH (8.63 ppm) and β-CH (6.77 ppm) protons for 6-coordinate 3a·DMSO or 5coordinate 3a (α-CH, 8.61 ppm; β-CH, 6.71 ppm) support the above experimental assignments and confirm the EPR-silent reaction product formed in our earlier radical quenching experiment (Figure 7). In the case of 3a·DMSO, the O-bound axial ligand (solvent) is energetically favored by −28.1 kJ mol−1 relative to the S-bound linkage isomer. The 1H NMR signals are broadened for 3a·DMSO relative to a typical diamagnetic Co(III) porphyrin, possibly due to spin−orbit mixing of a lowlying triplet excited state into the 1A ground state (Figure S17). Regarding species 3b, Figure 12 shows the 1H NMR spectra CP 3 dissolved in DCE-d4 at two key concentrations: 16 and 83 μM. The concentrations (and solvent) were chosen to match points in the dilute region of Figure 9. At [3] = 16 μM, the solution should comprise exclusively monomeric 3b, thereby permitting its definitive assignment. Based on the deviations from the Beer−Lambert law seen at 83 μM in Figure 9c, we expect a composition comprising monomeric 3b, oligomeric {3}n, and possibly π-{3b}2 (least abundant component). The 1 H NMR spectrum at 16 μM shows several diagnostic signals that confirm the structure of monomeric 3b. Specifically, the pyridyl α- and β-CH protons of the axial ligand resonate as doublets at 0.075 and 5.49 ppm, respectively, consistent with significant shielding by the porphyrin ring current and, importantly, coordination of the pyridyl N atom to the metal. The DFT-calculated chemical shifts for the pyridyl α-CH (0.20 ppm) and β-CH (5.12 ppm) protons closely match those observed experimentally. Together, the DFT and NMR data support the structure of 3b with py-DTDA− coordinated to Co(III) via its pyridyl nitrogen atom and identify the product of monomerization of CP 3 in DCE.

Nax bond (2.10(1) Å) in crystalline 3, favoring heterolysis of the 1D CP chain at the Co−S sites. This idea is broadly consistent with the generally lower bond dissociation enthalpies for X−S bonds (e.g., O−S, 523 kJ mol−1) compared with X−N bonds (e.g., O−N, 632 kJ mol−1).92 Dimerization of 3b. The behavior and origin of the band at ca. 660 nm in the visible spectrum in DCE is central to understanding the species in equilibrium with oligomer {3}n. As shown in Figure 9a, this red-wavelength band increases in intensity relative to the Soret (UV) band as the solution is diluted. The 600 nm band, at highest dilution, may be assigned largely to a monomeric species derived from cleavage of {3}n. This band is essentially absent from the spectrum of solid CP 3 (Figure 5), confirming that it is not due to oligomeric {3}n. The absorbance measured at 662 nm exhibits a negative nonlinear deviation from the limiting Beer−Lambert law (ε = 8190(91) M−1 cm−1), consistent with the dimerization model of eqs 4−6,93a where M and D are monomer and dimer species, respectively, KD is the dimerization constant, and εM and εD are the molar absorptivities of the monomer and dimer species, respectively. (The monomer, M, is species 3b, as delineated earlier.) The terms C0 and AT in eq 6 are the experimental concentration (total) and absorbance at 662 nm, respectively: 2M ⇌ D KD =

[D] [M]2

A T = εMC0 − (2εM − εD) ⎡ 4K C + 1 − (1 + 8K C ) ⎤ D 0 ⎥ ⎢ D 0 ⎢⎣ ⎥⎦ 8KD

(4)

(5)

(6)

The nonlinear least-squares fit of eq 6 to the data in Figure 9c gives the following parameters: KD = 2.29(7) × 103 M−1, εM = 1.02(6) × 104 M−1, εD = 7.31(16) × 103 M−1 (R2 = 0.999). The magnitude of KD determined from the least-squares fit (log KD = 3.36) is typical for weaker types of π−π dimers formed by charge-neutral and cationic metalloporphyrins in various media,93−95 possibly due to the inherent ability of organic solvents to disperse π-aggregates,95 or the nonplanarity of 3b, which might negate sterically efficient π-stacking. (For comparison, KD values are often >105 M−1 in aqueous media.93) The broad, red-shifted Soret band (λmax = 447 nm, [3] = 41 μM, Figure 9b) supports the formation of a metalloporphyrin π−π dimer, which may be denoted π-{3b}2 (Scheme 3). Dimerization by self-association of the DTDA rings of 3b may be ruled out on theoretical grounds because the DTDA rings are negatively charged, which results in negligible LUMO amplitude on the DTDA ring (Figure 11) and commensurately Scheme 3. Species Distribution and Schematic Structure of the π−π Dimer Detected by Optical Spectroscopy during the Monomerization of CP 3 in DCE

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Co−Npy distance in the X-ray structure of 6-coordinate CP 3 (2.094(5) Å, Figure 1) is 11% longer than the DFT-calculated distance for 3b (1.878 Å in DCE). From Figure 12, the o-CH phenyl protons of TPP change negligibly in chemical shift upon oligomerization of 3b into chain structure {3}n; further discussion of these signals is given in the SI (Figure S18). A number of lower intensity signals (marked *) with a similar pattern to those of {3}n, but at distinct chemical shifts, are present in the 1H NMR spectrum of the 83-μM solution of 3 (e.g., 8.04, 7.98, and 7.59 ppm). These are assigned to π-{3b}2 based on its low concentration (Beer− Lambert law plot, Figure 9c) and the normal observation that self-associating porphyrin rings in π−π dimers100 induce localized magnetic field anisotropies and hence perturbations in proton through-space shieldings that reflect the specific structure of the dimer.101,102 Significantly, signals due to 1 and 2 occur in the 1H NMR spectra in DCE-d4 at a 1:1 mol ratio. The presence of 1 and 2 was dependent on the solvent and method used to dissolve CP 3; the free radical concentration was always markedly lower (sometimes zero) in DCM-d2 and DMSO-d6. Since the free radical content of the solution is solvent-dependent (and radicals 1 and 2 are present in a 1:1 mol ratio), the evidence suggests that disproportionation of 3b occurs to some extent. Partial Co−S bond homolysis of oligomeric {3}n during monomerization (alongside more favorable Co−S bond heterolysis) would also account for the presence of radicals 1 and 2. Notably, a low concentration of 1 was also identified in the electronic spectrum of CP 3 dissolved in DCE (Figure 11a). Finally, the 1H NMR spectrum of CP 3 dissolved in DMSOd6 at 298 K reflects the presence of the six-coordinate mixedligand complex CoIII(TPP)(N-py-DTDA−)(O-DMSO), where 2 is N-bound and anionic (Figure 13a). The DFT-calculated structure of the S-bound DMSO linkage isomer was 57.7 kJ mol−1 higher in energy than the O-bound mixed ligand complex, accounting for the single species observed in the experimental 1H NMR spectrum (Figure S18) and allowing clean structural assignment. The DFT-calculated S−S bond distance (2.25 Å) in the DTDA ring of CoIII(TPP)(N-pyDTDA−)(O-DMSO) and the strongly shielded pyridyl α-CH (0.42 ppm) and β-CH (4.84 ppm) proton resonances that match the experimental chemical shifts for these diagnostic signals (0.59 and 5.43 ppm) confirm that monomerization of CP 3 in DMSO occurs by Co−S bond heterolysis to give a single species, diamagnetic 3b·DMSO. The 59Co NMR spectrum of 3b·DMSO was used to gauge the nuclear shielding engendered by N-bound 2−, particularly since shielding data exist for Co(III) porphyrins axially ligated by conventional Ndonors (amines,58 imidazoles,103 and pyridines104). The 59Co NMR spectrum (Figure 13a) of the solution containing CP 3 dissolved in DMSO-d6 at 298 K (δCo = 8093.48(4) ppm, ω1/2 = 6142(16) Hz), affirms the diamagnetic low-spin d6 Co(III) oxidation state for the monomer assigned as 3b·DMSO from its 1 H NMR spectrum. The 59Co chemical shift is significantly shielded (on the order of ∼200 ppm) relative to the values reported for bis(amine), ca. 8200−8600 ppm,58 and bis(imidazole), ca. 8200−8450 ppm,103 complexes of Co(III) porphyrins at ∼298 K. This suggests that the ligand field splitting parameter (ΔE, or simply Δ) is larger for 6-coordinate 3b·DMSO than for the related bis(N-donor) ligand complexes of Co(III) porphyrins, all other factors being equal. A large value of Δ induced by a more powerful axial σ-donor ligand (as

Figure 12. 1H NMR spectra (500 MHz, 302 K) of 3 recorded in dry DCE-d4 at two concentrations: 16 μM (monomeric; turquoise spectrum) and 83 μM (black spectrum). The major species are CoII(TPP) (1, red labels), py-DTDA (2, purple labels), and diamagnetic 5-coordinate CoIII(TPP)(N-py-DTDA−) (3b, blue labels). Selected peak integrals for the latter species are given within the peak envelope. Signal expansions are shown along with the DFTcalculated structure of the five-coordinate monomer and mean calculated chemical shifts for the magnetically unique protons in the molecule. Peaks marked with an asterisk (*) are due to higher oligomers or most likely π-{3}2. The mole ratio of CoII(TPP) to free py-DTDA is 1:1 from the integrals of the signals at 9.43 and 8.22 ppm, respectively.

The 1H NMR spectrum at [3] = 83 μM further delineates the solution chemistry of 3b. Specifically, the key diagnostic signals for py-DTDA− bound to CoIII(TPP) exhibit a downfield shift relative to their frequencies in the 16-μM solution. Since Figure 9c indicates that both oligomeric {3}n and monomeric 3b are present, the more intense downfield pyridyl α-CH (0.686 ppm) and β-CH (5.53 ppm) proton signals may be assigned to oligomeric {3}n, in which the py-DTDA bridging ligands are sandwiched between Co(TPP) units in a growing oligomer chain. The marked downfield shift (0.61 ppm) of the pyridyl α-CH resonance upon oligomerization of monomeric 3b into the 1D chain structure is noteworthy. This reflects translation of the axial pyridyl group away from the porphyrin ring, diminished ring current shielding of the pyridyl α-CH protons, formation of a 6-coordinate complex, and commensurately longer bond distances to the axial ligand. The trend evident in the NMR data is supported by the fact that the mean 14633


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Figure 13. (a) Summary of the chemical pathways, spectroscopic, structural, and DFT-calculated energy parameters, and bond lengths of key species involved in the formation and dissolution of CP 3. The 59Co NMR spectrum of CoIII(TPP)(N-py-DTDA−)(O-DMSO), or 3b·DMSO, is depicted in the lower right corner. Experimental chemical shifts for the protons of the py-DTDA ligand are indicated. (b) ESI+ mass spectrum of CP 3 dissolved in acetonitrile. The simulated spectral patterns are given in the SI (Figure S19). Full structure assignments for fragments W−Z are indicated (with calculated monoisotopic masses). The coordination polymer dissociates into desolvated fragment W, which then fragments further into X−Z.

Y, 58%, C50H36CoN7; [M + H]+). Regarding the low mass range of the MS spectrum (Figure S21), the disulfide-free axial ligand cleaved from fragment Y (pyridine-4-carboximidamide, C6H7N3; [M + H]+ = 122.0715 Da) is a major species present in the light fragments (besides 2). Clearly, loss of disulfide from metal-bound py-DTDA to leave the resultant ring-opened ligand fragment (C6H7N3) coordinated to the cobalt ion in Y proves that py-DTDA is N-bound in fragment W. In short, cleavage of an acetonitrile solution of CP 3 yields exclusively desolvated structure 3b, consistent with all foregoing experiments in DCE. The charge assignments shown in Figure 13b for the metal (Co3+) and axial ligand in fragment W (pyDTDA−) are based on the NMR and electronic spectra of monomerized CP 3 (species 3b, vide supra). Finally, fragment Z in the MS is unusual in structure but ostensibly consistent with loss of dinitrogen or hydrazyl radical from the complex. This fragment also confirms that py-DTDA is N-bound in fragment W.

noted earlier; Figure 8) reduces the magnitude of the paramagnetic shielding term and increases the nuclear shielding at the metal.105 The relatively shielded 59Co chemical shift for the monomeric dissolution product of CP 3 in DMSO is thus a hallmark of axial ligation by the powerful σ-donor ligand pyDTDA−. Mass Spectrometry. The MS of CP 3 dissolved in acetonitrile (Figures 13b and S19) unambiguously confirms the structure of 3b at m/z = 853.1500 (fragment W). MS-MS analysis (Figure S20) of the peak at m/z = 853.1500 shows the presence of 1 (m/z = 671.17) and 2 (m/z = 182.9952) as well as some diagnostically useful fragments derived from 2. The MS data suggest that dissolution of CP 3 in acetonitrile monomerizes the chain structure to give a molecular ion peak consistent in mass (but not necessarily ligand coordination mode) with the 5-coordinate repeat unit depicted in Scheme 1. From the mass of fragment W alone, it is not possible to ascertain whether py-DTDA is N- or S-bound to cobalt in the complex. Fortunately, analysis of the full fragmentation pattern resolves the ambiguity. Dissolution of 3 in a nucleophilic solvent such as acetonitrile likely cleaves the Co−S bonds in the polymer exclusively to form the solvated species Co(TPP)(N-py-DTDA)(NCCH3), which then loses the weakly bound solvent to form fragment W, or 3b (853.1500 Da). This species loses either one or both DTDA ring sulfur atoms to yield clean fragments with the N-bound axial ligand at m/z = 824.2006 (fragment X, 94%, C50H35CoN7S; M+) and 793.2198 (fragment

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SUMMARY AND CONCLUSIONS We have studied the first dithiadiazolyl−metalloporphyrin complex using 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl (compound 2, py-DTDA) as the ligand and CoII(TPP), 1, as the metalloporphyrin. Because of the ambidentate nature of pyDTDA, a diamagnetic 1D coordination polymer (CP 3) is formed in solution and in the solid state by a radical coupling reaction between 1 and 2, which, through Co−S bond 14634


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Journal of the American Chemical Society formation, initially gives species 3a, monomeric five-coordinate CoII(TPP)(S-py-DTDA). Subsequent oligomerization of 3a by repetitive Co−Npy bond formation yields CP 3 with py-DTDA ligands bridging cobalt porphyrins in the 1D chain. X-ray, FTIR, and DFT data collectively show that CP 3 is a valence tautomer (Co2+ ↔ Co3+) with intramolecular electron transfer between the metal ion and DTDA ring. The resonance hybrid formally comprises the redox states Co2.5+ and pyDTDA 0.5− . Anionic and coordinated forms of 2 are unprecedented. DFT calculations on the X-ray structure of CP 3 (14-mer) show that the FMO energy gap of the material decreases exponentially to a limiting value of 0.669(6) eV at infinite chain length. CP 3 could, in theory, be useful as a semiconductor, molecular wire, or flexible transistor component. The X-ray data provide a 3D model of the CP critical to thinking about how oligomers might be connected to source/ drain electrodes and surfaces when the search for applications for this unusual material commence. Finally, dissolution of CP 3 in noncoordinating solvents proceeds by Co−S bond heterolysis (main pathway) to give 5coordinate low-spin CoIII(TPP)(N-py-DTDA−), species 3b, as evidenced by electronic and NMR spectroscopy. Some Co−S bond homolysis (or disproportionation of 3b) occurs based on low-intensity signals from 1 and 2 in the spectra of solutions of 3b. In coordinating solvents such as DMSO, six-coordinate mixed-ligand species are formed, CoIII(TPP)(N-py-DTDA−)(Solv), where Solv is the relevant neutral solvent ligand, or simply 3b·(Solv). Understanding the solution species required to form CP 3 or those produced when it dissolves will be pivotal to solution processing methods (electronic device fabrication) going forward. Our summary of the complex chemistry defining this system (Figure 13a) should assist those wishing to craft functional materials with CP 3.

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Initiative of the Department of Science and Technology and NRF (Grant No 64799, O.Q.M.). We also thank Mr. Craig Grimmer (University of KwaZulu-Natal) for recording additional NMR spectra and providing access to an X-band EPR spectrometer.

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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/jacs.7b07803. X-ray crystal structure data for 3 (CIF) DFT calculated structures (ZIP) Full experimental details and additional EPR, FTIR, and NMR spectral data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Delia A. Haynes: 0000-0002-8390-5432 Orde Q. Munro: 0000-0001-8979-6321 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.H. acknowledges the National Research Foundation (NRF) of South Africa and Stellenbosch University for funding. We thank Dr. Jaco Brand of the Central Analytical Facility (CAF, Stellenbosch University) for recording 59Co NMR spectra and NMR laboratory services. L.J.vL. thanks the NRF for a Ph.D. bursary. This work is based on research supported by WITS University and the South African Research Chairs 14635


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