II Redox Potential and Spin

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes James N. McPherson,† Ross W. Hogue,‡ Folaranmi Sunday Akogun,‡ Luca Bondì,‡ Ena T. Luis,† Jason R. Price,§ Anna L. Garden,‡ Sally Brooker,*,‡ and Stephen B. Colbran*,† †

School of Chemistry, The University of New South Wales, Kensington, NSW 2052, Australia Department of Chemistry and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand § ANSTO, Australian Synchrotron, Clayton, VIC Australia Downloaded via MIDWESTERN UNIV on January 26, 2019 at 10:01:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R 1 )-4-(R 2 )-pyrrolide anions (dppR1,R2)−, varying in the nature of the R1 and R2 substituents [R1, R2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of neutral [CoII(dppR1,R2)2] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [CoII(dppMe,Me)2], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl3 solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn−Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible CoIII/II redox potentials are predictably tuned by choice of substituents R1 and R2, from −0.95 (Me,Me) to −0.45 (CN,Ph) V vs Fc+/Fc, with a linear correlation observed between E1/2(CoIII/II) and the Swain−Lupton parameters of the pyrrolide substituents.

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INTRODUCTION Cobalt complexes with physical properties that switch in response to external stimuli continue to be of great interest as centers in molecular devices.1−8 Octahedral complexes of the d7 Co(II) ion are paramagnetic, and they can exist in either high (HS) or low spin (LS) configurations. When the ligand field stabilization energy (LFSE) is similar to the pairing energy, spin crossover (SCO, i.e., spin state switching) often results. Thermal SCO is triggered by changes in temperature, so the switching temperature (T1/2, the temperature at which the populations of the LS and HS states are equal) is a key design parameter. Recent studies of solution phase SCO Fe(II) systems have uncovered relationships which allow T1/2 in solution to be predictably tuned through appropriate ligand selection.9−13 Correlations for solid state SCO are less common, due to the often confounding influence of crystal packing effects, but have also been identified.14−16 These methods have only been applied to Fe(II) to date. No predictor of the SCO behavior within Co(II) systems has yet been discovered. One reason is that the physical changes on spin state switching for Co(II) SCO (ΔS = 1) systems are generally more subtle and less symmetric,5,6,17−19 making prediction a more difficult challenge than in Fe(II) SCO (ΔS = © XXXX American Chemical Society

2) systems. Another reason is that Co(II) SCO is less often studied. Cobalt(II) systems are also redox activeoxidation to Co(III) is typically reversible and (usually) results in diamagnetic complexes. Redox processes at metal centers are known to be dependent on ligand electronics, typically measured by substituent Hammett parameters20−32 or by Lever’s electronic parameter (LEP).27,28,30,33,34 Thus, the redox potential of the CoIII/CoII couple, E1/2′(CoIII/CoII), is another obvious design parameter.35 The possible impact of this couple is nicely illustrated in a recent study by New and co-workers, who have developed a series of MRI contrast agents based on Co(tris(pyridylmethyl)amine) species [Co(II) is active, and Co(III) inactive] to probe hypoxia within living cells and cancer spheroids.36,37 Modulation of E1/2′(CoIII/ CoII) could allow other cobalt-based systems to be applied to cells under oxidative stress (as turn-off sensors). Cobalt(II) complexes capable of both SCO and reversible redox are rare but are also of considerable interest.23,38 Received: December 11, 2018

A

DOI: 10.1021/acs.inorgchem.8b03457 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (Left) Five bis(dipyridylpyrrolide)cobalt(II) complexes, [CoII(dppR1,R2)2], and (right) five specific dpp− ligands34 (Py = 2-pyridyl), employed in this study.

Figure 2. ORTEP Views from SC-XRD structures of the [Co(dpp)2] complexes studied in this work (with 50% thermal ellipsoids and H atoms omitted for clarity) and annotated with the average Co−N bond distances, quadratic elongation parameter ⟨λOh⟩, and XLS for each structure. For clarity, only one residue (residue 12) of the [Co(dppCO2Et,CO2Et)2] structure at 100 K is shown, and the metric data is the average of all data in the unit cell (weighted by % occupancy). Additional metric data are listed in Tables S2 and S3 in the Supporting Information. †For [Co(dppCO2Et,CO2Et)2] at 100 K, the ⟨λOh⟩ for the residue which was most stable during refinement of the SC-XRD data (residue 12, XLS = 95%) is given, rather than the cell average, due to difficulties associated with disordered residues.

The di(2-pyridyl)-3-(R 1 )-4-(R 2 )-pyrrolide anion, (dppR1,R2)−, shown in Figure 1, is an anionic analogue of terpyridine (tpy), with the central pyridyl ring replaced by a pyrrolide σ- and π-donor which can bear two substituents (R1, R2).39 These changes result in different geometric and electronic properties from those offered by tpy,40 and also offer an obvious advantage over the widely used tpy ligands: [CoII(dpp)2] complexes are neutral and so avoid the complications due to counterions that the analogous tpy complexes exhibit.18 In addition, homoleptic [CoII(dpp)2] complexes are easily prepared, with the first (R = H or CO2Et) derivatives obtained in the 1950s by Hein and co-workers.41,42 In 2006, Tour and co-workers used [Co(dppR,R)2] (R = H or SCN) complexes to construct single molecule transistors with Kondo temperatures above 50 K.43 In 2014, Colbran and co-workers invented a one-pot condensation protocol that provided easy entry to a range of Hdpp ligand precursors with diverse substitution at the C3/C4 positions of the pyrrole.34 This has greatly facilitated the present investigation of a series of five [CoII(dppR1,R2)2] complexes, Figure 1, in which the 3- and 4-substituents of the

pyrrolide donor (R1 and R2, respectively) are systematically varied. Remarkable and predictable tuning of the reversible CoIII/CoII redox couples is demonstrated, and the different magnetic responses across the series of Co(II)complexes to changing temperature, in both solid and solution state, are also reported.

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RESULTS AND DISCUSSION

Syntheses of Complexes. The five targeted [Co(dppR1,R2)2] complexes, Figure 1, were easily prepared by a 2:1 reaction of HdppR1,R2 ligand:cobalt dichloride hexahydrate, with excess triethylamine, in refluxing methanol under an inert atmosphere. The neutral complexes were poorly soluble in methanol, so the dark red-brown powders were collected in good yields (71−88%). Characterization was by elemental and thermogravimetric analysis, high-resolution mass spectrometry, paramagnetic 1H NMR spectroscopy, UV−vis−NIR electronic spectroscopy, variable temperature magnetic susceptibility measurements in both the solid and solution phase, and cyclic voltammetry. The complex [Co(dppMe,Me)2] was air sensitive B


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Inorganic Chemistry

Figure 3. Assignment of the 1H NMR spectrum of [CoII(dppMe,Me)2] through intermolecular electron transfer (chemical exchange) with the diamagnetic [CoIII(dppMe,Me)2][PF6] complex. The 1H NMR (500 MHz, acetone-d6) spectrum of the mixture was acquired (left), and then a series of 1H NMR spectra were collected following presaturation at the marked frequencies (B3−B6). During the presaturation at a particular marked frequency, electron transfer interconverts the Co(II) and Co(III) complexes, and so the intensity of the corresponding signal in the diamagnetic Co(III) complex is also reduced. The right-side shows the resulting difference spectra over the region for the pyridyl protons in the Co(III) complex, above the assigned 1H NMR (500 MHz, acetone-d6) spectrum of [CoIII(dppMe,Me)2][PF6]. The signals attributed to diamagnetic [CoIII(dppMe,Me)2][PF6], residual protio-solvent and water in the left-side spectrum are denoted as *.

in solution, which accords with its low CoIII/II redox couple (see below). Solid State Structures. Crystals suitable for single crystal X-ray diffraction (SC-XRD) structural analyses were obtained for [Co(dppR1,R2)2] (R1,R2 = Me,Me; CO2Me,Me; and CO2Et,CO2Et) by the slow diffusion of pentane into chloroform solutions, and for HdppCO2Me,Me (pentane into a benzene solution); for detailed descriptions of each SC-XRD structure, see the Supporting Information. Figure 2 presents views and salient metric data for the [Co(dpp)2] structures. For each complex, the average Co−N bond length, ⟨Co−N⟩, scales with the experimental solid state magnetic susceptibility (discussed later), with a slightly better quality of fit found when only the average axial Co−Npyrrolide bond length, ⟨Co− Npyr⟩, was examined (see Figure S5, Supporting Information). It was therefore possible to determine the relative fraction of LS cobalt(II) centers (XLS) within each structure from the ⟨Co−Npyr⟩ value (see eq S5, Supporting Information). A number of geometric parameters have been used to describe the distortion of coordination octahedra around SCO active iron(II) systems (e.g., φ, θ, and Σ in Table S2, Supporting Information);44,45 however, only Σ and Robinson’s quadratic elongation parameter, ⟨λOh⟩,46 show any trend with relation to XLS of the [Co(dpp)2] complexes studied in this work. The molecules of [Co(dppCO2Me,Me)2] in the structures determined at both 298 and 100 K were predominantly LS (XLS ≈ 65% at 298 K shifting to XLS ≈ 96% at 100 K). The shorter ⟨Co−Npyr⟩ values (and other structural parameters, see Table S2, Supporting Information) are consistent with those observed for other LS bis(terdentate)cobalt(II) complexes.5,7,18,38,47−49 The octahedral environments around these LS cobalt(II) centers are significantly distorted (with quadratic elongation parameters, ⟨λOh⟩,46 of 1.045 and Σ = 114.9° at 100 K) due to the combination of the ligand imposed tetragonal contraction along the Npyr−Co−Npyr axis and the complementary strong Jahn−Teller effect associated with the singly occupied eg orbital.18 We have recently shown that the relative ligand deformation ΔρL (the difference in the distance

between the outer donor atoms of a pincer ligand when metalbound, ρL(coord), and when free in its “coordination ready” planar syn;syn-conformation, ρL(relax)) is an easily measured parameter that can be used to approximate the strain energy which builds within a pincer ligand, such as dpp−, upon chelation (ΔEL(strain)).50 For [Co(dppCO2Me,Me)2], the DFT optimized geometry of (dppCO2Me,Me)− gives ρL(relax) of 4.899 Å (see Table S1, Supporting Information) and ρL(coord) ≈ 4.22 Å upon complexation with a cobalt(II) ion, which predicts ≈75 kJ mol−1 strain within each bound dpp− ligand in the [Co(dppCO2Me,Me)2] species. [Co(dppMe,Me)2] at 100 K exhibited ⟨Co−Npyr⟩ = 1.97 Å (XLS ≈ 0), consistent with increased population of the eg antibonding orbitals associated with the HS state of the cobalt(II) center. The coordination octahedra about the HS [Co(dpp)2] species were more distorted (with XLS ≈ 0: ⟨λOh⟩ = 1.071−1.073; and Σ = 154.0−157.5°) than for the LS analogues (with XLS ≥ 90%: ⟨λOh⟩ = 1.044−1.045; and Σ = 114.8−114.9°) as is the norm in SCO,6 despite the LS state of Co(II) experiencing increased Jahn−Teller distortion18 (see above). The reduced steric demands of the resulting more open, so less strained, dpp− ligands bound to the HS Co(II) centers are reflected in ρL(coord) = 4.38 Å and the estimated ρL(relax) ≈ 5.047 Å, which afford a reduced ΔEL(strain) ≈ 65 kJ mol−1.50 The structures of [Co(dppCO2Et,CO2Et)2] were less symmetric. In the structure determined at 298 K, the asymmetric unit is occupied by two unique molecules: Co1 with ⟨Co−Npyr⟩ = 1.9079(15) Å thus XLS ≈ 46%, and HS Co2 with ⟨Co−Npyr⟩ = 1.959(2) Å thus XLS ≈ 0. The dpp− ligands around Co1 are more strained (ρL(coord) = 4.216(3) Å and estimated ρL(relax) = 4.969 Å, ΔEL(strain) ≈ 70 kJ mol−1), and the coordination octahedron less distorted (with ⟨λOh⟩ = 1.053 and Σ = 130.9°) than in the HS Co2 complex (ρL(coord) = 4.302(3) Å, ΔEL(strain) ≈ 60 kJ mol−1, ⟨λOh⟩ = 1.073, and Σ = 157.5°). At 100 K, the symmetry is broken further with 16 independent [Co(dpp)2] molecules occupying the asymmetric unit, with a quadrupling of the b axis (see Figures S6, S7 and S8 and Tables S3 and S4, C


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Inorganic Chemistry Supporting Information for more details) and an overall ⟨Co− Npyr⟩ = 1.86 Å and XLS ≈ 88% across the 16 different (some disordered) residues. NMR Studies. Broad paramagnetically shifted peaks were observed in the 1H NMR spectra of all five [CoII(dpp)2] complexes in acetone-d6 at 298 K (Table S5). The spectrum of the exemplar complex [CoII(dppMe,Me)2] was unambiguously assigned using a methodology similar to that reported by Constable et al.51 First, the 1H NMR spectrum of the diamagnetic Co(III) analogue complex [CoIII(dppMe,Me)2][PF6] was assigned (Figures 3 (bottom right) and S16−S20 and Table S5, Supporting Information). Then a mixture of paramagnetic [CoII(dppMe,Me)2] and diamagnetic [CoIII(dppMe,Me)2][PF6] complexes was prepared in acetone-d6. These two complexes interconvert through electron transfer, which is sufficiently slow on the 1H NMR time scale at 298 K for both species to be observed simultaneously in the mixture (Figure 3, left). Rather than sensitizing the diamagnetic signals through chemical exchange spectroscopy as Constable et al. reported,51 a less subtle approach was employed. A series of 1H NMR spectra were collected following presaturation at the frequency of each signal assigned to the paramagnetic [CoII(dppMe,Me)2] species. The signal for the corresponding 1H environment in the diamagnetic [CoIII(dppMe,Me)2][PF6] was reduced during the presaturation (due to chemical exchange), and thus the identity of each presaturated signal was deduced from the difference spectra, as shown on the right in Figure 3. With the 1H NMR spectrum for [Co(dppMe,Me)2] assigned, the remaining paramagnetic signals were assigned by analogy through comparisons of the chemical shifts and by line width analyses (see Table S5, and the Supporting Information). The proton environments at the B5 and B4 positions (with similar fwhh: 12−21 Hz) were discriminated by inspection of the 1 H−1H COSY NMR spectra (with correlations for 3J(B3,B4) observed in all cases, see the Supporting Information). The signals in the 1H NMR spectra due to pyrrolide substituents were generally assigned by integration. These spectra confirm that the complexes are present as [Co(dpp)2] in solution. Electronic Spectra. The UV−vis spectra of the [Co(dpp)2] complexes in dichloromethane at 298 K were dominated by π* ← π and charge transfer (CT) bands between 250−500 nm (ε = 20 000−65 000 L mol−1 cm−1); see Figure S10. The vis-NIR regions of the electronic spectra show up to three broad features from 550−1100 nm; Figure S11. The d7 cobalt(II) center has 19 terms in a simple octahedral (Oh) environment, and the tetragonal distortion imposed on the cobalt(II) center by the dpp− ligand further splits these terms. Overlap of these d−d bands plus CT transitions results in the broad features observed throughout the visible−NIR region.43 The lowest energy feature at 10 400−10 850 cm−1 observed for all [Co(dpp)2] complexes was unobscured by nearby bands; Figure 4. Absorption maxima at ∼10 800 cm−1, with slightly lower ε values, were observed for the complexes that were HS in solution at all temperatures (see below), namely [Co(dppCN,Ph)2] (ν̃max: 10 820 cm−1; peak full width at half height (fwhh) = 1700 cm−1) and [Co(dppMe,Me)2] (10 780 cm−1; fwhh = 1390 cm−1). In contrast, the SCO-active complexes in solution had slightly more intense and broader features that tailed to lower energy. This was observed for [Co(dppCO2Et,CO2Et)2] (10 420 cm−1; fwhh = 3130 cm−1), [Co(dppCO2Me,4‑Py)2] (10 470 cm−1; fwhh = 2580 cm−1) and

Figure 4. NIR region of electronic spectra for the [Co(dpp)2] complexes studied in CH2Cl2 solution at 298 K, fitted to Gaussian peaks (see Figure S12, Supporting Information).

[Co(dppCO2Me,Me)2] (10 400 cm−1; fwhh = 2177 cm−1). In these cases there are equilibrium mixtures of HS/LS at 298 K in CDCl3 (see below). Thus, the electronic spectra provide an indicator of SCO activity in solution. Magnetic Susceptibility Studies. Solid State Measurements. Solid-state magnetic susceptibility measurements were performed on each [Co(dpp)2] complex from 50−400 K (Figure 5). As the samples converted to hydrates, C, H and N

Figure 5. Temperature dependence of the magnetism of the [Co(dpp)2] complexes in the solid state.

microanalysis and thermogravimetric assay were used to determine the water content immediately prior to each susceptibility measurement. [Co(dppMe,Me)2]·1/2H2O with χMT values of 2.4−2.9 cm3 K mol−1 over 50−300 K is consistent with a HS Co(II) species. The other complexes exhibit gradual SCO transitions without any hint of complicating discontinuities such as have been observed for some [Co(tpy) 2 ] 2 + systems. 5 2 [Co(dppCO2Et,CO2Et)2]·1/3H2O underwent the most complete SCO transition observed within the accessible temperature D


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Inorganic Chemistry range: the χMT value of 0.6−0.7 cm3 K mol−1 between 50−100 K is consistent with LS Co(II), and steadily increased above 100 K, reaching 3.0 cm3 K mol−1 at 400 K, consistent with transitioning to the HS state. [Co(dppCO2Me,Me)2]·1/4H2O and [Co(dppCO2Me,4‑Py)2]·H2O exhibit similar susceptibility plots to one another: like [Co(dppCO2Et,CO2Et)2]·1/3H2O, they too are LS at 50 K, with χMT values of 0.7 and 0.8 cm3 K mol−1, respectively. However, the transitions for [Co(dppCO2Me,Me)2]·1/4H2O and [Co(dppCO2Me,4‑Py)2]·H2O are more gradual than for [Co(dppCO2Et,CO2Et)2]·1/3H2O and are incomplete at 400 K, the upper limit of our instrument, with both reaching a χMT value of 2.5 cm3 K mol−1 at 400 K, consistent with a residual LS population. The magnetic responses on heating the samples were identical to those observed during cooling, with no thermal hysteresis observed (see Figure S13). SC-XRD for [Co(dppCO2Et,CO2Et)2], which underwent the more abrupt SCO transition, revealed a change in space-group and quadrupling of the crystallographic b-axis as the crystal was cooled from 298 K (χMT = 2.55 cm3 K mol−1, C2/c, b = 14.077(3) Å) to 100 K (χMT = 0.729 cm3 K mol−1, Pn, b = 56.061(11) Å). Two wave-like perturbations appear along the b-axis indicative of cooperative effects operating during the SCO in the solid state.53,54 At highest frequency, the adjacent Co···Co separations alternate ca. 7.19 and 6.83 Å, whereas at lower frequency the adjacent Co···Co separations parallel to the a-axis trend closer over each cell from 10.263(3) Å (max.) to 9.834(3) Å (min.); see Figure S8. The change in the octahedral distortion parameters (ΔΣ = 11.8° and Δφ = 3.3°) are comparable to those measured recently in similar cobalt(II) complexes which displayed cooperative SCO.55 In contrast, [Co(dpp CO2Me,Me ) 2] showed only slight contraction of the unit cell (in all dimensions) from 298 K (V = 3014.1(10) Å3) to 100 K (V = 2900.2(10) Å3). [Co(dppCN,Ph)2]·3/4H2O has a χMT value of 1.5 cm3 K mol−1 at 50 K, with an approximate 60% LS population at this temperature. It then undergoes a gradual and incomplete SCO to fully HS at 400 K (χMT = 3.3 cm3 K mol−1). Gradual and/or incomplete SCO, as observed for these [Co(dpp)2] complexes, is commonplace among SCO-active cobalt(II) complexes.5,7,18 Solution State Susceptibility Measurements. The magnetic susceptibilities of the four complexes that were SCO active in the solid state (see above), namely [Co(dppCN,Ph)2], [Co(dpp C O 2 E t , C O 2 E t ) 2 ], [Co(dpp C O 2 M e , 4 ‑ P y ) 2 ] and [Co(dppCO2Me,Me)2], were also investigated using the Evans 1H NMR method,56 in CDCl3 solution between 233 and 323 K (the temperature window accessible in CDCl3 solution). Figure 6 presents the variable temperature magnetic susceptibility data that was obtained, as well as the fits to these data sets. Three of the four complexes exhibited SCO in chloroform solution. Complexes [Co(dppCO2Et,CO2Et)2] and [Co(dppCO2Me,4‑Py)2] exhibited very similar behavior to one another, undergoing gradual and incomplete spin transitions from mostly HS at 323 K, with χMT values of 2.4 cm3 K mol−1 for [Co(dppCO2Et,CO2Et)2] and 2.5 cm3 K mol−1 for [Co(dppCO2Me,4‑Py)2], to about 50:50 LS:HS, with both having a χMT value of 1.9 cm3 K mol−1 at 233 K. Complex [Co(dppCO2Me,Me)2] is already at about 50:50 LS:HS at 318 K (χMT = 1.9 cm3 K mol−1), and this drops to 1.1 cm3 K mol−1 at 233 K, again consistent with a gradual and incomplete SCO within the measured temperature range. In contrast, [Co(dppCN,Ph)2] remained HS across the available temperature range, with χMT being 2.92−2.75 cm3 K mol−1 from 233−323

Figure 6. Variable temperature magnetic susceptibility in CDCl3 of [Co(dppCN,Ph)2] (black), [Co(dppCO2Et,CO2Et)2] (orange), [Co(dppCO2Me,4‑Py)2] (purple), and [Co(dppCO2Me,Me)2] (green). Solid lines represent the results of least-squares fitting of the data to eq 2; the resulting parameters are summarized in Table 1.

K, consistent with the literature values for HS Co(II) compounds.57 Thermodynamic parameters for the SCO transitions were obtained from fits of the magnetic susceptibility versus temperature data to eq 1,58 which reduces to eq 2 upon substitution of realistic values for the maximum (χMT)HS (= 3.0 cm3 K mol−1) and minimum (χMT)LS (= 0.65 cm3 K mol−1) values taken from the solid-state data. χM T =

χM T =

(χM T )HS − (χM T )LS

(

ΔSCOH RT

(

ΔSCOH RT

1 + exp

−

ΔSCOS R

−

ΔSCOS R

)

2.35 1 + exp

)

+ (χM T )LS (1)

+ 0.65 (2)

While the fits to eq 1 (with two additional variables) resulted in tighter correlations than for those to eq 2, we can provide no reasonable explanation for the different (χMT)HS values predicted by the fits to eq 1 for the structurally similar complexes {e.g., using eq 1, (χMT)HS is 2.1 cm3 K mol−1 for [Co(dpp CO2Me,Me ) 2 ] but 2.6 cm 3 K mol −1 for [Co(dppCO2Et,CO2Et)2]}. We therefore present the thermodynamic parameters from the fits of these data to eq 2 (see Figure 6) in Table 1. The thermodynamic parameters, ΔSCOH and ΔSCOS, obtained from fitting the data sets, Table 1, are consistent with known examples of cobalt(II) complexes in solution (ΔSCOH = 5−23 kJ mol−1 and ΔSCOS = 31−81 J mol−1 K−1),57,59,60 as well as for the much larger number of iron(II) complexes which have been studied in solution (ΔSCOH = 4− 41 kJ mol−1 and ΔSCOS = 22−146 J mol−1 K−1).11,57,59−61 The transition temperature obtained for the [Co(dppCO2Me,Me)2] complex, T1/2 = 314 K, is the highest of the three SCO-active compounds assayed, and is consistent with having a greater LS population than the other two between 233−323 K. That the magnetic behavior of complexes [Co(dppCO2Et,CO2Et)2] and [Co(dppCO2Me,4‑Py)2] are very similar in solution (Figure 6) but different in the solid state (Figures 5 and S13) illustrates that subtle influencessuch as crystal E


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Table 1. Thermodynamic Parameters (ΔSCOH/kJ mol−1; ΔSCOS/J K−1 mol−1) for the SCO-Active [Co(dpp)2] Complexes in CDCl3 Obtained from Fits of the Observed Data to Eq 2, along with the T1/2/K Values ΔSCOH ΔSCOS T1/2

[Co(dppCO2Et,CO2Et)2]

[Co(dppCO2Me,4‑Py)2]

[Co(dppCO2Me,Me)2]

6.2 ± 0.2 28.1 ± 0.6 221 ± 9

8.1 ± 0.1 35.7 ± 0.4 227 ± 5

9.1 ± 0.3 29 ± 1 314 ± 15

Figure 7. Left: Cyclic voltammograms (ν = 100 mV s−1) showing the CoIII/CoII couples of the [Co(dpp R1,R2)2] complexes in dichloromethane with 0.1 M [Bu4N][PF6] at a glassy carbon minidisk electrode and 298 K. Right: Plot of the potential of the CoIII/CoII couple for each [Co(dpp R1,R2 )2] complex against σ, where σ = αF − R; α = 2.69, and F and R are the sum of Hansch, Leo, and Taft’s modified Swain−Lupton field and resonance terms for the R1 and R2 pyrrolide substituents in each ligand.21,63 The bars in the data for each reversible process represent the anodic and cathodic peak potentials; the predicted CoIII/CoII couples for four hypothetical [Co(dpp R1,R2)2] species are shown as blue crosses (+).

Table 2. Redox Potentials (V) Observed for the [Co(dppR1,R2)2] Complexes in CH2Cl2 with 0.1 M [NBu4][PF6], vs. E1/2′(Fc+/ Fc) at 298 K, along with Theoretical CoIII/CoII Redox Potentials, E°[LS-CoIII/LS-CoII] and E°[LS-CoIII/HS-CoII], Estimated Using Free Energies (ΔG LS‑CoIII/CoII) and Solvation Energies (ΔGsolv) Determined by DFT (See the Supporting Information) complex

Ep,cirrev

E1/2′(CoIII/CoII)

E1/2′(dpp−/dpp)

E1/2′(dpp−/dpp)

E°[LS-CoIII/LS-CoII]

E°[LS-CoIII/HS-CoII]

CN,Ph

−2.24 −2.25 −2.30 −2.34 

−0.445 −0.605 −0.668 −0.829 −0.949

   +1.02 +0.70

   +1.12 +0.84

−0.493 −0.551 −0.613 −0.752 −0.843

−0.976 −1.440 −0.958 −1.100 −1.514

)2] [Co(dpp [Co(dppCO2Et,CO2Et)2] [Co(dppCO2Me,4‑Py)2] [Co(dppCO2Me,Me)2] [Co(dppMe,Me)2]

Supporting Information). No correlations were found between the δ 15N values of the donor atoms of the Hdpp ligands and the magnetic properties of their [Co(dpp)2] complexes, Figure S32. We also attempted to correlate the SCO activity of the Co complexes with a linear combination of electronic and our recently reported steric strain parameter for the ligands. A meaningfulpredictiverelationship could not be obtained. Electrochemistry. The electrochemical responses of the five [Co(dppR1,R2)2] complexes were studied by cyclic voltammetry in dichloromethane. Cyclic voltammograms (CVs) of each complex showed a reversible CoIII/CoII couple at a potential exquisitely sensitive to the pyrrolide substituents (R1, R2), Figure 7 and Table 2. The CoIII/CoII couples were completely free of the confounding electrode- and electrolytedependent irreversibility reported by Fontecave and coworkers in their study of the redox chemistry of [Co(tpyR)2]2+ complexes.35 CVs of the more electron rich complexes [Co(dppMe,Me)2] and [Co(dppCO2Me,Me)2] also revealed consecutive reversible oxidations of the two redox noninnocent dpp− ligands,34 separated by 100 mV, and centered at ca. 0.77 and 1.07 V, respectively (Table 2 and Figure S43). All complexes, except [Co(dppMe,Me)2], also showed an irreversible reduction process at very negative potential, ca. −2.3 V (Ep,cirrev, Table 2 and Figure S44).

packing and interion interactions in the solid state or solvent polarity,62 dielectric constant, or donicity in the solution statemay profoundly affect the magnetic behavior of a particular species. It is not practical to completely describe these environments, either experimentally or theoretically. The best that can be done is to minimize environmental influences (or at least attempt to make them constant throughout a study), and so susceptibility measurements in solution offer the best hope for giving results from which correlations can be drawn. We therefore searched for correlations between the solution state SCO activity (T1/2 values) found for the [Co(dpp)2] systems against those parameters previously reported to predict SCO activity of iron(II) systems.9−13 No correlation was found to Hamnett σM, σp or σp+ parameters, Figure S14, or to the more general Swain and Lupton’s field (F) and resonance (R) terms, Figure S15.21,63 Slattery and co-workers likewise found little correlation between the inductive influence of the 4′-substituents and the T1/2 values for a range of SCO-active homoleptic tpy cobalt(II) complexes.23 Brooker et al. recently reported a predictive correlation between the switching temperatures (T1/2) and the found or DFT calculated 15N chemical shift of the N donor atoms within the azine rings of unbound bi- and tridentate ligands (δNA).12 Hence the 15N chemical shifts (δ 15N) for each Hdpp ligand were measured in chloroform-d solution (Table S6, F


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Inorganic Chemistry

approach has been used previously to identify the MOs from which an electron is removed during oxidation.33,65,69 Unsurprisingly, a very strong correlation (R2 = 0.98) was also found between the observed E1/2′(CoIII/II) and the LUMO energies calculated for the five (LS) [CoIII(dpp)2]+ complexes (Figure 9), consistent with the reduction event involving this orbital, and confirming the reversibility of the CoIII/CoII redox processes as observed experimentally. Predictable Tuning of E1/2′(CoIII/CoII). The reversible CoIII/CoII redox potentials of the five complexes varied by 500 mV (Figure 7), ranging from −0.445 V for [Co(dppCN,Ph)2] to −0.949 V for [Co(dppMe,Me)2]. Lever electronic parameters (LEP) are not available for pyrrolide donors,27,28,30,33,34 and the calculation of the LEP for the (dppCO2Me,Me)− ligand from the RuIII/II couples of known [Ru(dppCO2Me,Me)Lx]z+ complexes (Lx: tpy, z = 1; bpy and Cl−, z = 0; or bpy and MeCN, z = 1)34 resulted in a large uncertainty (LEP = −0.01 ± 0.06 V). Therefore, the more general Swain−Lupton field (F) and resonance (R) parameters21,63 were used to characterize the electronic contributions of the two substituents on the fivemembered pyrrolide ring, and not the more common Hammett parameters that are derived from six-membered ring systems.20,21 Thus, the E1/2′(CoIII/CoII) values were fitted against the sum of F and R parameters for the two substituents R1 and R2 (then doubled, to account for the two ligands coordinated to the cobalt center) reported by Hansch et al.,21 to eq 3, giving eq 4 (for the line shown in the right-side plot, Figure 7) with an r2 of 0.887.

The ionization energy of a molecule is related to the HOMO eigenvalue, as is the electron affinity to its LUMO eigenvalue.64,65 Views of the Kohn−Sham HOMOs and LUMOs of all the [Co(dpp)2] complexes are presented in the Supporting Information to distinguish between ligand- and metal-centered MOs.66−68 Inspection of the redox-active frontier orbitals (the LS Co(II) HOMO and the Co(III) LUMO) reveals their predominant σ*(N(σsp)−Co(dx2−y2)) character (see Figure 8, and the Supporting Information), in contrast to the HOMOs for the HS Co(II) species with exclusive dpp− ligand character (see Figures S46−S50, Supporting Information).

Figure 8. An exemplar redox-active frontier orbital for the [Co(dpp)2] complexes (the LUMO of [CoIII(dppCO2Et,CO2Et)2]+ in this case) revealing appreciable σ*(N(σsp)−Co(dx2−y2)) character (drawn with isosurface value = 0.03 e/Å3).

E1/2′(CoIII /CoII) = E0 + fF + rR

(3)

E1/2′(CoIII /CoII) = − 1.12 + 0.487F − 0.181R

(4)

It is helpful to define a single parameter, σ, to quantify the electronic contribution of the four substituents on the cobalt center, eq 5:

The experimental E1/2′(CoIII/II) values are linearly correlated (Figure 9) with the HOMO energy for the LS states of the five [CoII(dpp)2] complexes (R2 = 0.98)far more strongly than with the HS states (R2 = 0.76)which again supports the preferential oxidation of the LS state in these systems. This

σ = αF − R , where α = f /r = 2.69

(5)

The inductive force term (F) in eq 5 dominates by greater than a factor of 2 (α = 2.69), while the resonance term (R) is opposite in sign. A similar approach, but for the RuIII/RuII couple of our three previously reported [Ru(dppR1,R2)(tpy)]+ complexes,34 for which Swain−Lupton parameters are available (R1,R2 = CO2Me,4-Py; CO2Me,Me; and Me,Me)21 returned a similar dependencethat is the RuIII/RuII redox potential increased (became less negative) with increasing F, but increasing R lowered the potential (but with F dominating by more than a factor of 3 in these casessee Figure S52). Equation 5 allows eq 4 to be simplified to eq 6: E1/2′(CoIII /CoII) = −1.12 + 0.181σ

(6)

Larger values of σ represent more electron withdrawing substituents, and eq 6 shows that, as anticipated, these correlate to higher E1/2′(CoIII/CoII) values. This correlation (Figure 7, right-side plot) allows us to predict E1/2′(CoIII/CoII) values for some other accessible homoleptic [Co(dpp)2] complexes, and in so doing also demonstrated that a range of E1/2′(CoIII/CoII) values from about −1.7 to 0 V should be accessible for this ligand class. The analysis predicts that [CoII(dppH,H)2] should be even more easily oxidized than [CoII(dppMe,Me)2], which we observed to slowly oxidize in air, while [CoII(dppSCN,SCN)2] should be more stable than [CoII(dppCN,Ph)2], which is air

Figure 9. Frontier orbital energies (DFT) versus the observed CoIII/II couples (E1/2′(CoIII/II)) for the five [Co(dpp)2] complexes. G


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Inorganic Chemistry stable. Tour et al. have already studied these complexes,43 albeit without a standard reference couple specified, but our predictions and their findings about sensitivity to molecular oxygen are entirely consistent. Future studies will include preparing fresh samples of those literature complexes, along with the new complexes shown predictively in Figure 7, as we look to prove the robustness of this correlation.

CoCl2.6H2O (approximately 0.25 mmol) was added, and the mixture was stirred at reflux for 2 h. After cooling to room temperature, the crude product, [Co(dppR1,R2)2], was collected by filtration, and washed with cold MeOH three times. Yields and the characterization data for each complex are presented in the Supporting Information. Magnetic Susceptibility Measurements. Variable temperature magnetic susceptibility data were collected at the University of Otago on a Quantum Design Versalab, a cryogen-free PPMS susceptometer, equipped with a vibrating sample mount (VSM) under an applied field of 1000 Oersteds. Data on [Co(dppCN,Ph)2]·3/4H2O, [Co(dppCO2Et,CO2Et)2]·1/3H2O and [Co(dppCO2Me,Me)2]·1/4H2O were collected in settle mode at 5 K intervals, while [Co(dppCO2Me,4‑Py)2]·H2O were collected in sweep mode at 5 K min−1, in the temperature range 400−50 K. Data on [Co(dppMe,Me)2]·1/2H2O were collected in the temperature range 300− 50 K in settle mode at 5 K intervals. The susceptibility−temperature data for cooling and heating overlaid. Samples were mounted in a polyethylene capsule for which a background diamagnetic correction was applied. Data were also corrected for the diamagnetism of the sample according to the approximation that χMdia = −0.5 × MW × 10−6 cm3 mol−1 (where MW = molecular weight of the complex).58 Solution magnetic susceptibility data on [Co(dpp CN,Ph )2 ] (0.006243 g cm−3), [Co(dppCO2Et,CO2Et)2] (0.006645 g cm−3), [Co(dppCO2Me,4‑Py)2] (0.006585 g cm−3), and [Co(dppCO2Me,Me)2] (0.006742 g cm−3) were measured in CDCl3 in the temperature range 233−323 K, by 1H NMR spectroscopy on a Varian 500 MHz VNMRS spectrometer using the Evans’ method.56 Pure CDCl3 was placed in a capillary NMR tube insert and the paramagnetic solution was placed in the outer tube. Temperatures are accurate to ±1 K. The mass susceptibility of the complexes was calculated from the shift of the residual CHCl3 peak in the paramagnetic solution compared to that in pure CDCl3, Δf in Hz, using eq 7:

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CONCLUSIONS Families of (dppR1,R2)− ligands, with systematic variation of the R1,R2 substituents present on the pyrrolide ring, are easily prepared. The resulting neutral [CoII(dpp)2] complexes of the tridentate, anionic dpp− ligands comprise severely tetragonally compressed octahedral cobalt(II) centers, due to a combination of Jahn−Teller and strained tridentate ligand bite effects. This occurs for both the HS and LS states of cobalt(II), but with the HS states exhibiting a longer average Co−N length and somewhat more distortion than the LS state. The strained bite is a consequence of the wider ligand bend angle, due to the five-membered pyrrolide core, over that provided by the pyridine core of the related tpy ligands. We anticipate the resulting longer Co−Ndistal (Co−Npy) bonds will result in different reactivities (e.g., hemilability) than the more common [Co(tpy)2]2+ systems, thus potentially leading to interesting applications in catalysis. Importantly these complexes also exhibit useful properties the oxidation and/or spin states of the cobalt centers can be reversibly switched in response to external stimuli. Variable temperature SC-XRD studies reveal a phase transition in the structure of the [Co(dppCO2Et,CO2Et)2] complex with the most abrupt spin crossover in the solid state, with two wave-like perturbations along the b crystallographic axis indicative of cooperativity. Three complexes show significant SCO activity around room temperature in solution. Predictable tuning of the redox potential, but not of solution SCO, is demonstrated. The former correlation, of E1/2′(CoIII/ CoII) values with the electronic nature of the pyrrolide substituents, σ (determined by use of the modified Swain− Lupton parameters for R1,R2), allows us to predict that the E1/2′(CoIII/CoII) can be tuned, using accessible substituents, by up to ca. 1.7 V. The DFT results suggest that the CoIII/CoII couples proceed through a LS Co(II)− LS Co(III) electron transfer pathway, thus minimizing the reorganization energy for the electron transfer. The energies of the redox active σ*(N(σsp)− Co(dx2−y2) orbital (the HOMO in the LS d7-Co(II) state and the LUMO in the LS d6-Co(III) state) also correlate with experimental E1/2′(CoIII/CoII) values, thus a quick DFT calculation allows prediction of the oxidation potential for a Co(dppR1,R2)2 complex in those cases where Swain−Lupton parameters for the substituents R1, R2 are unavailable. We expect these and similar [Co(dpp)2] complexes will find use as redox or magnetic centers in sophisticated molecular assemblies with targeted function.

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χg =

d − ds 3Δf + χ0 + χ0 0 4πmf m

(7)

where m is the concentration of the paramagnetic solution in g cm−3 and this was corrected for the temperature dependence of the density of chloroform, f is the spectrometer frequency in Hz, and d0 and ds are the densities of the solvent and solution, respectively. χ0 is the mass susceptibility of the solvent in cm3 g−1. This equation was simplified by taking an approximation that ds = d0 + m, which is reasonable for dilute solutions.70,71 This approximation leads to the second and third terms canceling out, giving eq 8:

χg =

3Δf 4πmf

(8)

Multiplying χg by the molecular weight (MW) gave the molar susceptibility χM. These χM values were then corrected for the diamagnetic contributions of each sample according to χMdia(sample) = −0.5 × MW × 10−6 cm3 mol−1 (where MW = molecular weight of the complex).58 Note: The χMT values, for two separate VT runs on the [Co(dppCN,Ph)2] complex, were averaged at each temperature; the χMT value was 2.8 cm3 K mol−1 at 298 K in both runs. Solution magnetic susceptibility data were fitted to eqs 1 and 2 by least-squares fitting performed using OriginPro version 9.1.0 from OriginLab Corporation. Electrochemistry. Solutions of compounds were of ca. 1 mM concentration in dichloromethane with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte and were sparged with high purity nitrogen. CVs were recorded at 100 mV s−1 under a nitrogen atmosphere using a GAMRY Reference 600 potentiostat using a standard three-electrode system comprising glassy carbon minidisk working electrode (1.0 mm diameter, freshly polished), a platinum wire counter electrode and an eDAQ leakless Ag/AgCl reference electrode. A background CV was always obtained between anodic and cathodic solvent discharge limits prior to adding sample. Ferrocene (Fc) was added as an internal standard at the end of each set of CV experiments and all potentials are quoted relative to the Fc+/Fc couple {E1/2 = 0 V, ΔEp ≈ 80 mV in dichloromethane}.

EXPERIMENTAL SECTION

General Synthetic Procedure for [Co(dpp)2] Complexes. Methanol was dried over activated 3 Å molecular sieves for 24 h prior to use. Reactions were performed under a dry, oxygen free atmosphere of dinitrogen using standard Schlenk techniques. Ligand precursors Hdpp were prepared according to the literature method.34 HdppR1,R2 (approximately 0.5 mmol) was combined with excess NEt3 (0.3 mL, 2 mmol) in MeOH (10 mL) and stirred at reflux for 15 min. H


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DFT Calculations. All dpp− ligand DFT calculations were performed with the Gaussian 2009 software package.72 Vacuum phase dpp− ligand geometry optimizations were performed using the B3LYP functional73−75 with the 6-31G** basis set.76 Dispersion corrections were included by adding the D3 version of Grimme’s empirical dispersion correction with Becke−Johnson damping.77 The inter-ring N−C−C−N torsion angles were constrained to 0.0° by the modredundant command. All [Co(dpp)2]z+ (z = 0, 1) complex DFT calculations were performed using the ORCA 4.0.1 code,78 using the B3LYP functional coupled with the D3 version of Grimme’s functional with BJ damping correction to account for dispersion forces.77,79 The experimental SCXRD structures of [CoII(dppMe,Me)2], [CoII(dppCO2,Me,Me)2] and [CoII(dppCO2Et,CO2Et)2] provided the initial Cartesian coordinates for the respective complexes, and the structure of [CoII(dppCO2Me,Me)2] was modified as required to generate input coordinates for the remaining [Co(dppCO2Me,4‑Py)2] and [Co(dppCN,Ph)2] complexes. The [Co(dpp)2] complexes were preoptimized with a def2-SVP basis set,80 to calculate the Hessian matrix faster and confirm the obtained structure as a minimum by the absence of imaginary frequencies, before a final optimization was performed using the def2-TZVPP basis set.80 Both optimizations were supported by a def2/J auxiliary basis set81 to take advantage of the RI-J approximation.78 Then a single point calculation on each of the final optimized structures was performed to calculate the solvation energies in CH2Cl2 with a conductor-like polarizable continuum solvation model CPCM.82 X-ray Crystallography. Pale yellow hexagonal plates of the HdppCO2Me,Me ligand suitable for SC-XRD were grown by slow diffusion of pentane vapor into a solution of the compound in benzene and the X-ray diffraction measurements were carried out on a Bruker kappa-II CCD diffractometer at 150 K using IμS Incoatec Microfocus Source with Mo−Kα radiation (λ = 0.710723 Å). Crystals of the [Co(dppR1,R2)2] complexes (R1,R2: CO2Et,CO2Et− brown blocks; CO2Me,Me−red-brown plates; and Me,Me−redbrown plates) suitable for SC-XRD studies were grown by the slow diffusion of pentane vapor into a solution of the compound in chloroform (benzene for R1, R2 = CO2Me, Me). Diffraction measurements were carried out using Si⟨111⟩ monochromated synchrotron X-ray radiation (λ = 0.71073 Å) on the MX1 Beamline at the Australian Synchrotron.83 For [Co(dppCO2Et,CO2Et)2] and [Co(dppCO2Me,Me)2], the single crystal was mounted and a full data set was first collected at 298 K, and then the sample was cooled and a second set of data for each was collected at 100 K. Crystallographic data have been deposited as entries CCDC 1873690 (HdppCO2Me,Me), 1873691 {[Co(dppMe,Me)2]·2(C5H12)}, 1873692 {[Co(dpp CO2Me,Me ) 2 ] at 298 K}, 1873693 {[Co(dppCO2Me,Me)2] at 100 K}, 1873694 {[Co(dppCO2Et,CO2Et)2] at 298 K}, and 1873695 {[Co(dppCO2Et,CO2Et)2] at 100 K}.

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Article

AUTHOR INFORMATION

Corresponding Authors

*(S.B.C.) E-mail: [email protected]. *(S.B.) E-mail: [email protected]. ORCID

James N. McPherson: 0000-0003-0628-7631 Folaranmi Sunday Akogun: 0000-0001-7613-5911 Ena T. Luis: 0000-0002-3191-8944 Anna L. Garden: 0000-0002-8069-3243 Sally Brooker: 0000-0002-5878-8238 Stephen B. Colbran: 0000-0002-1119-4950 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Government (PhD scholarship to J.N.M. and E.T.L.), the University of Otago (Ph.D. scholarship to R.W.H, F.S.A. and L.B.), the Marsden Fund (RSNZ), Australian Research Council (DP160104383) and UNSW Sydney for supporting this research. Access to the Australian Synchrotron was made possible by the Collaborative Access Program (MXCAP12368/13234). We gratefully acknowledge the Mark Wainwright Analytical Centre (MWAC) at UNSW Sydney (NMR and BMSF), the Campbell Microanalytical Laboratory at the University of Otago, the NeSI high performance computing (HPC) facilities (NZ), and Prof. Federico Totti (Florence) for providing access to the LaMM group HPC facilities.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03457. Experimental details and analytical data for complexes; computational studies; single-crystal X-ray structures; magnetic susceptibility studies; NMR studies; additional electrochemistry; correlations and data fits; coordinates of DFT optimized structures (PDF) Accession Codes

CCDC 1873690−1873695 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. I


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II Redox Potential and Spin

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