Thiosemiquinoid Radical-Bridged CrIII2 ... - ACS Publications

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

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Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling Carol Hua, Jordan A. DeGayner, and T. David Harris* Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

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ABSTRACT: Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)2Cr2(RL4−)]2+ (OLH4 = 1,2,4,5-tetrahydroxybenzene, OSLH4 = 1,2-dithio-4,5-dihydroxybenzene, SLH4 = 1,2,4,5-tetrathiobenzene, TPyA = tris(2pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between CrIII centers in these complexes, with exchange constants of J = −2.83(3) (OL4−), −2.28(5) (OSL4−), and −1.80(2) (SL4−) cm−1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)2Cr2(RL3−•)]3+. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal− semiquinoid radical coupling, with exchange constants of J = −352(10) (OL3−•), − 401(8) (OSL3−•), and −487(8) (SL3−•) cm−1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

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INTRODUCTION

The vast majority of compounds referenced above comprise clusters or solids where paramagnetic metal ions are connected by diamagnetic one- or two-atom bridging ligands, such as oxide or cyanide.3,4 Here, magnetic coupling between the metal ions is mediated through the diamagnetic linker via a superexchange coupling mechanism. Since magnetic coupling strength drops precipitously with increasing number of atoms in the linker, monatomic bridging ligands are typically necessary to facilitate strong coupling between metal centers. However, such ligands greatly limit the ability to predict and chemically tune the structure and function of the resulting compounds. As an alternative, organic linkers can impart a wide range of chemical programmability and tunability in molecules and networks, yet their multiatomic compositions lead to superexchange interactions of miniscule strengths. One

The realization of molecule-based magnets, ranging from discrete multinuclear complexes to extended framework materials, relies on the implementation of strong magnetic coupling between spin centers. For instance, in zero-dimensional molecular species, the energetic separation between spin ground state and excited states is directly correlated to the exchange coupling constant, J.1 The isolation of the ground state is necessary for applications such as multinuclear singlemolecule magnets and potentially high-relaxivity MRI contrast agents.2 In addition, the relaxation barrier of one-dimensional single-chain magnets increases with J, and as such, the synthesis of magnetically anisotropic chain compounds with strong intrachain coupling represents a route to hightemperature single-chain magnets.3 Likewise, in two- and three-dimensional permanent magnets, which may find use as lightweight bulk magnets or as gas separation media, the magnetic ordering temperature is directly proportional to J.4 © XXXX American Chemical Society

Received: March 7, 2019

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DOI: 10.1021/acs.inorgchem.9b00674 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

literature procedures. Dry and deoxygenated THF, CH3CN, Et2O, MeOH, and hexane were obtained from a Pure Process Technology solvent purification system and subsequently stored over 3 or 4 Å molecular sieves prior to use in a dinitrogen-filled glovebox. Dry and deoxygenated triethylamine was obtained by drying over CaH2 followed by distillation under N2. Microanalyses were carried out at Northwestern University. Electrospray ionization (ESI) mass spectrometry data were acquired in CH3CN or MeOH with a 100 μL/min flow rate on a Bruker AmaZon-SL instrument and were collected in the range m/z = 50−2000. Synthesis of [(TPyA)2Cr2(OL4−)](BArF4)2 (1). CrCl2 (8.8 mg, 0.071 mmol) and TPyA (20.8 mg, 0.0713 mmol) were dissolved in THF (2 mL) and stirred for 5 min to give a pale yellow solution. After the solution was stirred for 5 min, a suspension of OLH4 (5.0 mg, 0.035 mmol) in triethylamine (7.3 mg, 0.071 mmol) and THF (1 mL) was added to yield a red solution. Solid NaBArF4 (63.4 mg, 0.0713 mmol) was then added to this solution, yielding an immediate color change to brown and then to a dark purple solution. The solution was stirred at ambient temperature for 6 h, and then the solvent was removed under reduced pressure. The ensuing purple residue was dissolved in CH2Cl2 (2 mL) and then filtered through diatomaceous earth. Hexane was layered on top of the filtrate to yield 1 as a dark purple solid (55 mg, 92%) after 3 days. ESI-MS (ESI+, CH3CN): 411.16 (calculated [M]2+ = 411.09, 60%), 274.38 (calculated [M]3+ = 274.06, 100%); (ESI−, CH3CN): 863.11 (calculated [BArF4]− = 863.06, 100%) amu. Elemental analysis: found C, 50.75; H, 2.65; N, 4.30%. Calcd. for C106H62B2Cr2F48N8O4·2THF: C, 50.84; H, 2.92; N, 4.16%. [(TPyA)2Cr2(OSL4−)](BPh4)2 (2). CrCl2 (12.2 mg, 0.100 mmol) and TPyA (28.8 mg, 0.100 mmol) were dissolved in THF (2 mL) and stirred for 5 min to give a pale yellow solution. After the solution was stirred for 5 min, a solution of OSLH4 (5.0 mg, 0.035 mmol) in triethylamine (20.1 mg, 0.200 mmol) and MeOH (1 mL) was added to yield a dark brown solution. Solid NaBPh4 (33.9 mg, 0.100 mmol) was then added to this solution, yielding no observable color change. The solution was stirred at ambient temperature for 6 h, and then the solvent was removed under reduced pressure. The ensuing brown residue was dissolved in acetonitrile (1 mL) and then filtered through diatomaceous earth. Gaseous diethyl ether was diffused into the filtrate to yield 2 as brown needle-shaped crystals (55 mg, 74%) after 3 days. ESI-MS (ESI+, CH3CN): 427.09 (calculated [M]2+ = 427.07, 100%); (ESI−, CH3CN): 319.18 (calculated [BPh4]− = 319.17, 100%) amu. Elemental analysis: found C, 71.59; H, 5.43; N, 8.32%. Calcd. for C90H78B2Cr4N8S2O2·2.4CH3CN·C4H10O: C, 71.23; H, 5.76; N, 8.74%. [(TPyA)2Cr2(SL4−)](BArF4)2 (3). CrCl2 (12.0 mg, 0.0966 mmol) and TPyA (28.2 mg, 0.0966 mmol) were dissolved in THF (2 mL) and stirred for 5 min to give a pale yellow solution. After the solution was stirred for 5 min, a solution of SLH4 (10 mg, 0.0486 mmol) in triethylamine (19.7 mg, 0.1932 mmol) and THF (1 mL) was added to yield a brown solution. Solid NaBArF4 (84.2 mg, 0.0966 mmol) was then added to this solution, yielding no observable color change. The solution was stirred at ambient temperature for 6 h, and then the solvent was removed under reduced pressure. The ensuing brown residue was dissolved in CH2Cl2 (2 mL) and then filtered through diatomaceous earth. Hexane was layered on top of the filtrate to yield 3 as brown plate-shaped crystals (110 mg, 87%) after 3 days. ESI-MS (ESI+, CH3CN): 443.09 (calculated [M]2+ = 443.05, 100%); (ESI−, CH3CN): 863.12 (calculated [BArF4]− = 863.06, 100%) amu. Elemental analysis: found C, 50.03; H, 5.39; N, 3.72%. Calcd. for C106H62B2Cr2F48N8O4·2.8THF: C, 50.00; H, 5.02; N, 3.98%. [(TPyA)2Cr2(OL4−)](BArF4)3 (4). To a solution of 1 (20.0 mg, 0.0119 mmol) in THF (1 mL) was added dropwise with stirring a solution of [(C5Me5)2Fe]BArF4 (14.0 mg, 0.0119 mmol) in THF (0.5 mL) to yield a color change from deep purple to maroon. The solution was stirred for 10 min, and then the solvent was removed under reduced pressure. The ensuing maroon residue was dissolved in a minimal amount of CH2Cl2 (∼0.5 mL), and then hexane (10 mL) was then added to cause the precipitation of a dark pink solid. The solid was filtered using a Büchner funnel and washed with hexane (3 × 5 mL) before being dried under reduced pressure to give 4 (45 mg, 70%).

strategy to surmount this challenge is to install an unpaired electron onto the organic linker to engender strong direct coupling interactions between radical and metal-based unpaired electrons. Indeed, this approach has seen considerable success, as exemplified by organonitrile5 and nitronyl nitroxide radical6 linkers. Nevertheless, the bis(monodentate) binding modes and low charges of these ligands limit their use in the directed assembly of compounds and, in the case of extended networks, often hamper the isolation of crystalline products. Bis(bidentate) benzoquinoid molecules constitute an ideal class of bridging ligand for synthesizing metal−organic magnets with strong coupling (see Figure 1). These ligands

Figure 1. Redox series of the deprotonated benzoquinoid ligands of general form L2− (left), L3−• (center), and L4− (right); E = O, S.

readily undergo redox chemistry to generate the semiquinoid radical, form strong metal−ligand bonds with predictable binding modes and structures, and are amenable to a wide range of substitution chemistry. Indeed, numerous molecules,7 a chain compound,8 and several extended networks9 based on tetraoxolene radical linkers, which feature oxygen atoms in all four donor positions, have been shown to exhibit exceptionally strong magnetic coupling between metal ions and radical linkers. More recently, we have shown that moving from oxygen to nitrogen donors in dinuclear complexes leads to much stronger coupling, owing to the more radially diffuse orbitals of nitrogen relative to oxygen.10 Along these lines, moving from oxygen to sulfur donors in semiquinoid linkers should likewise provide a route to stronger magnetic coupling between metal ions and linkers. While numerous studies of transition-metal complexes containing 1,2,4,5-tetrathiobenzene-based bridging ligands with Fe,11 NiII,12 PtII,12b,c RhIII,13 IrIII,13 TiIV,14 ZrIV,14 HfIV,14 and RuII12c have been reported, to our knowledge, a radical form of this ligand has been isolated only once, in a NiII2 complex.12b Moreover, a benzene tetrathiolate radical has not yet been investigated in the presence of paramagnetic metal centers nor has the magnetic exchange coupling been determined. Herein, we present the synthesis and detailed characterization of the series of benzoquinoid-bridged CrIII2 complexes [(TPyA)2Cr2(RL4−)]2+ (OLH4 = 1,2,4,5-tetrahydroxybenzene, OSLH4 = 1,2-dithio-4,5-dihydroxybenzene, SLH4 = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine), and their subsequent one-electron oxidation to the semiquinoid-bridged complexes [(TPyA)2 Cr 2 ( RL 3−•)] 3+ . These complexes enable the first measurements of magnetic coupling between metal ions and thiosemiquinoid radicals, and they demonstrate the value of moving from oxygen to sulfur donors in the design of radical-based magnetic molecules and materials.

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EXPERIMENTAL SECTION

General Considerations. Purchased chemicals were used as obtained and without further purification. The compounds 1,2,4,5tetrahydroxybenzene (OLH4),15 bis-tetraphenylphosphonium bromide 4,5-dihydroxybenzene-1,2-dithiol (OSLH4),16 1,2,4,5-tetrathiolbenzene (SLH4),17 [Cp2Fe]BPh4,18 [Cp2Fe]BArF4,19 tris(2-pyridylmethyl)amine (TPyA),20 and NaBArF421 were synthesized according to B


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Inorganic Chemistry

Figure 2. Synthesis and subsequent chemical oxidation of dichromium compounds 1−6.

Figure 3. Crystal structures of [(TPyA)2Cr2(OL4−)]2+ (left), [(TPyA)2Cr2(OSL4−)]2+ (middle), [(TPyA)2Cr2(SL4−)]2+ (right), as observed in 1, 2′, and 3′, respectively. Purple, yellow, red, blue, and gray spheres represent Cr, S, O, N, and C atoms, respectively; H atoms are omitted for clarity. UV/Vis/NIR Spectroscopy. UV/vis/NIR spectra were obtained at ambient temperature using a CARY5000 spectrophotometer in transmission mode over the wavenumber range of 5000−35000 cm−1. Samples were dissolved in either CH3CN or THF to give solutions with approximate concentrations of 1 × 10−4 M. The background spectrum was acquired with the corresponding solvent used for sample analysis. Electrochemistry. Electrochemical measurements were performed using a CHI 760c potentiostat inside a dinitrogen-filled glovebox. Cyclic voltammograms were recorded using a glassy carbon working electrode (3.0 mm diameter), a platinum wire auxiliary electrode, and an Ag/Ag+ wire quasi-reference electrode in 0.1 M (NBu4)PF6 in CH3CN. Ferrocene was added as an internal standard upon completion of each experiment. All potentials are given in V vs [Cp2Fe]0/1+. UV/Vis/NIR Spectroelectrochemistry. The transmission spectra of the electrogenerated species were collected in situ in 0.1 M [(nC4H9)4N]PF6/CH3CN electrolyte over the range 5000−35000 cm−1 using a OTTLE cell obtained from the University of Reading. The cell consisted of a Pt mesh working and counter electrode and a Ag/Ag+ quasi-reference electrode. The applied potential was controlled using an CHI 760c potentiostat. Continuous scans of the sample were taken, and the potential increased gradually until a change in the spectrum was observed. Magnetic Measurements. All samples were prepared under a dinitrogen atmosphere with rigorous exclusion of dioxygen. Compounds were sealed in a 2 mL polyethylene bag, and data were collected using a Quantum Design MPMS-XL SQUID magnetometer from 1.8 to 300 K at applied dc fields from 0 to 7 T. Direct current (dc) susceptibility data were corrected for diamagnetic contributions from the sample holder and for the core diamagnetism of each sample (estimated using Pascal’s constants27). M vs H curves, constructed from data collected from 0 to 3 T at 100 K, confirmed the absence of significant ferromagnetic impurities in all samples (see Figures S10− S15).

Elemental analysis: found C, 49.02; H, 2.59; N, 3.37%. Calcd. for C138H74B3Cr2F72N8O4·0.9CH2Cl2·1.5C6H14: C, 49.10; H, 2.70; N, 3.10%. [(TPyA)2Cr2(OSL4−)](BPh4)3 (5). To a solution of 2 (20.0 mg, 0.013 mmol) in acetonitrile (1 mL) was added dropwise with stirring a solution of [(C5H5)2Fe]BArF4 (6.7 mg, 0.013 mmol) in acetonitrile (0.5 mL) at −35 °C to yield a color change from brown to aqua. The solution was stirred for 10 min before diethyl ether (10 mL) was added to cause the precipitation of a blue-green solid. The solid was filtered using a Büchner funnel and washed with diethyl ether (3 × 5 mL) before being dried under reduced pressure to give 5 (18 mg, 74%). Elemental analysis: found C, 75.15; H, 5.32; N, 6.22%. Calcd. for C114H98B3Cr2N8O2S2·1.2CH3CN: C, 75.09; H, 5.50; N, 6.02%. [(TPyA)2Cr2(SL4−)](BArF4)3 (6). To a solution of 3 (20 mg, 0.0077 mmol) in THF (1 mL) was added dropwise with stirring a solution of [(C5H5)2Fe]BArF4 (8.03 mg, 0.00765 mmol) in THF (0.5 mL) at −78 °C to yield a color change from brown to deep green. The solution was stirred for 10 min, and then the solvent was removed under reduced pressure. The ensuing dark green residue was dissolved in a minimal amount of CH2Cl2 (∼0.5 mL), and then hexane (15 mL) was added to cause the precipitation of a green oil. The oil was washed with hexane (3 × 5 mL) before being dried under reduced pressure to give 6 (21 mg, 78%). Elemental analysis: found C, 48.36; H, 2.40; N, 3.17%. Calcd. for C138H74B3Cr2F72N8S4·1.35THF: C, 48.29; H, 2.39; N, 3.13%. X-Ray Structure Determination. Single crystals of 1, 2′, and 3′ were coated with deoxygenated Paratone-N oil and mounted on a MicroMountsTM rod before being frozen under a stream of cold N2 during the data collection. Data were collected on a Bruker Kappa APEX-II CCD diffractometer employing a monochromated CuKα 1 μS microfocus source and MX Optics. The data integration and reduction were undertaken with SAINT and XPREP,22 and subsequent computations were carried out with the WinGX22 and Olex223 graphical user interface. All structures were solved by direct methods with SHELXT24 and extended and refined with SHELXL.25 The non-hydrogen atoms in the asymmetric unit were modeled with anisotropic displacement parameters. A riding atom model with group displacement parameters was used for the hydrogen atoms. An empirical absorption correction determined with SADABS26 was applied to the data. Crystallographic data for these compounds at 100 K are given in Table S1. At 100 K, the solvent molecules in 1 and 3′ were severely disordered and could not be modeled. These species were therefore treated as a diffuse contribution to the overall scattering without specific atom positions using the solvent masking procedure implemented in OLEX2.23

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RESULTS AND DISCUSSION Synthesis and Structures of Reduced Complexes. The nonradical-bridged Cr2 complexes were synthesized by the addition of a solution of the protonated ligand RLH4 and Et3N to a solution of CrCl2 and tris(2-pyridylmethyl)amine (TPyA) in THF, followed by addition of solid NaBR4 (R = 3,5bis(trifluoromethyl)phenyl (ArF4) for OLH4; SLH4 and Ph for OS LH4) and drying of the ensuing solution under reduced C


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Inorganic Chemistry pressure (see Figure 2). Liquid diffusion of hexane into a CH2Cl2 (OLH4 and SLH4) solution or vapor diffusion of Et2O into MeCN (OSLH4) solutions of the compounds gave purple [(TPyA)2Cr2(OL4−)](BArF4)2 (1, 92%), brown [(TPyA) 2 Cr 2 ( O S L 4 − )](BPh 4 ) 2 (2, 74%), or brown [(TPyA)2Cr2(SL4−)](BArF4)2 (3, 87%). The formation of CrIII in these compounds may stem from a spontaneous, solvent-assisted oxidation. Liquid or vapor diffusion methods gave crystals of 1, [(TPyA) 2 Cr 2 ( OS L 4− )]Br 2 (2′), or [Cr2(TPyA)2(SL4−)](BPh4)2·CH3CN·C4H10O (3′) that were suitable for single-crystal X-ray diffraction analysis. In 2′, the presence of presence of Br− likely stems from a small amount of residual Ph4PBr remaining from the synthesis of 2 (see Experimental Section). Within all compounds, the cationic complexes crystallized exclusively as one stereoisomer, with the TPyA ligands orientated trans to one another (see Figure 3). Within each cationic complex, two [(TPyA)M]2+ units are connected by the bridging ligand RL4− and are related through a crystallographic inversion site. The resulting positional disorder of the E (E = O, S) donors in 2′ gives Cr−E bond distances that represent an average of Cr−O and Cr−S. Each CrIII center exhibits a slightly distorted octahedral coordination geometry, with four N donors from the TPyA capping ligand and two cis-disposed E atoms from RL4−. All complexes feature Cr−N bond lengths in the range of 2.050(5) to 2.100(8) Å, which are consistent with reported bond distances for S = 3/2 CrIII in similar coordination environments.7p,28 In moving from 1 to 2′ to 3′, a distinct systematic increase in the Cr−E bond length is observed, with mean Cr−E distances of 1.887(3), 2.255(5), and 2.319(5) Å, respectively. In accord with the Cr− N distances, these values are consistent with S = 3/2 CrIII ligated by O7p or S donors29 in similar coordination environments. Similarly, the mean C−E distance increases from 1.359(2) to 1.554(6) to 1.767(4) Å in 1, 2′, and 3′, respectively, as is expected in moving from O to S donors. The presence of exclusively CrIII in 1, 2′, and 3′ indicates that oxidation from CrII to CrIII occurs during the synthesis of these complexes. Similar spontaneous oxidation reactions have been observed in other chromium benzoquinoid complexes.7l,10a,30 The C−C bond distance in benzoquinoid ligands is highly diagnostic of the ligand oxidation state, with a shortening of C1−C2 expected for RL4− relative to more oxidized forms due to the higher degree of aromaticity within the ring. The C1− C2 bond distances of 1.415(4), 1.380(2), and 1.396(8) Å for 1, 2′, and 3′, respectively, are in close agreement with the values of 1.413(4),7a 1.383(4),31 and 1.398(9)32 Å previously reported for metal complexes of EL4− and therefore confirm the assignment of EL4− in 1, 2′, and 3′. Electrochemical Properties. To probe the redox chemistry of 1−3, cyclic voltammograms were obtained for solutions of these compounds in CH3CN using 0.1 M (NBu4)PF6 as the electrolyte (see Figures 4 and S1). For 1, three distinct one-electron redox processes are assigned to O n− L , based on the previously reported complex [(CTH)2Cr2(OL4−)]2+ (CTH = rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane).7n Specifically, the reversible processes at −1.23 and +0.166 V vs [Cp2Fe]0/1+ are assigned to the OL4−/3−• and OL3−•/2− couples, respectively. The quasi-reversible process at +0.527 V is attributed to O 2−/1−• L , and its lack of reversibility likely stems from the relative instability of OL1−•. Finally, the irreversible reductions processes at −1.78 and −2.10 V are assigned to two distinct reduction events of CrIII to CrII. Exposure of a solution of 1 to

Figure 4. Cyclic voltammograms, collected at a scan rate of 100 mV/ s, for 1 (black, bottom), 2 (red, middle), and 3 (blue, top) in CH3CN with 0.1 M (NBu4)PF6 as an electrolyte.

air under ambient conditions resulted in an immediate color change from purple to pink, stemming from oxidation of OL4− to OL3−•. The cyclic voltammogram for 2 displays a reversible oneelectron process at −0.109 V vs [Cp2Fe]0/1+, which we assign to the OSL4−/3−• couple, and a quasi-reversible process at +0.65 V, which we assign to the OSL3−•/2− couple. As in the assignments above, the two irreversible processes in the cathodic region at −1.60 and −2.32 V are attributed to reduction of the CrIII to CrII. Compound 2 is stable in air at ambient conditions for several hours but then undergoes a color change from brown to a dark green. A similar color change, corresponding to the one-electron oxidation of OSL4−, has been reported for the related compound (Ph4P)2[Ni(OSL3−•)2].16 In the cyclic voltammogram for 3, the reversible process assigned to SL4−/3−• appears at −0.095 V vs [Cp2Fe]0/1+. This process occurs at a significantly lower potential than the analogous event at +0.5 V for 1,2,4,5-tetraisopropylbenzene (see Figure S1) but at a higher potential than −0.34 to −0.53 V previously reported for [(P−P)2Ni2(SL)]0/1+ (P−P = 1,2bis(diphenylphosphino)ethane, 1,2-bis(diphenylphosphino)ethylene, and 1,2-bis(dicyclohexylphosphino)ethane).12b A further broad quasi-reversible process at +0.662 V in 3 is assigned to the SL3−•/2− couple, and the lack of reversibility likely stems from the relative instability of the C−S double bonds of SL2−. Finally, an irreversible reduction occurs in the cathodic region, at −1.87 V, and is assigned to reduction of CrIII to CrII. Compound 3 is stable in air under ambient conditions and can be stored at ambient temperature in air for months without noticeable degradation. Across the series of complexes, the RL4−/3−• redox process shifts from −1.23 to −0.109 V to −0.095 V vs [Cp2Fe]0/1+ upon moving from 1 to 2 to 3. This shift toward more anodic potentials reflects the less electronegative nature of S compared to O and the greater propensity for the formation of CO double bonds. In addition, spectroelectrochemical measurements were carried out on a solution of 3 to further probe its redox chemistry. The UV/vis/NIR spectrum for 3 features three primary bands at 21066, 25800, and 32166 cm−1, which are assigned to the charge transfer transitions 4A2g(F) → 4T2g, 4 A2g(F) → 4T1g(F), and 4A2g(F) → 4T1g(P) for CrIII in an D


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Inorganic Chemistry octahedral coordination geometry (see Figure 5a).7o The additional band at 39583 cm−1 can be assigned as an

either THF or CH3CN were treated with one equivalent of either [Cp2Fe]+ or [(C5Me5)2Fe]+. In general, the oxidized compounds were isolated through addition of Et2O or hexane to the ensuing solution to yield a solid precipitate of [(TPyA)2Cr2(OL3−•)](BArF4)3 (4), [(TPyA)2Cr2(OSL3−•)](BPh4)3 (5), or [(TPyA)2Cr2(SL3−•)](BArF4)3 (6) (see Figure 2). Note that the UV/vis/NIR spectra collected for chemically generated and isolated 6 is identical to the species generated in the spectroelectrochemical experiment (see Figures 5b and S3). The UV/vis/NIR spectrum for 1 in MeCN exhibits bands at 12340, 19820, and 30500 cm−1. The band at 19820 cm−1 is assigned to a charge transfer band of OL4−, in accord with a broad band in the range 19700−20500 cm−1 reported for a series of compounds containing chloranilic acid (CA) and various phenanthroline coligands, [Cr2(CA)4(N−N)4](ClO4) (N−N = 5-methyl-1,10-phenanthroline, 2,9-dimethyl-1,10phenanthroline, and 5-chloro-1,10-phenanthroline).7o Likewise, the band at 30500 cm−1 is assigned to a π → π* transition of OL4−, nearly identical in energy to that of 30490 cm−1 previously observed for related chloranilate-bridged CrIII complexes.7p Upon treatment of 1 with one equivalent of [Cp2Fe](BArF4) to give 4, a color change from purple to bright pink was observed. The emerging low-energy transitions at 14930 and 15680 cm−1 are indicative of formation of a CrIII center ligated by OL3−• (see Figures 5b and S4).7g−n These bands occur at relatively low energies for a 4A2g → 4 T2g transition, suggesting that OL3− acts as a considerable πdonor to the CrIII ion.33 The highly detailed absorption band at 15395 cm−1 for 4 is associated with vibronic processes, and previous studies have determined that the energy difference between the bands correspond well to stretching bands observed in resonance Raman spectra.7f−p Finally, the band at 26270 cm−1 corresponds to an LMCT process from OL3−• to CrIII.7p The UV/vis/NIR spectrum for 2 in MeCN exhibits bands at 15680, 26270, and 30425 cm−1, close to those of 14947, 24096, and 31949 cm −1 reported for (PPh 4 ) 2 [Fe(OSLH22−)2].16 The similarity of these absorption energies strongly suggests that the transitions correspond to the bridging ligand. The weak charge transfer bands associated with the octahedral CrIII centers are obscured by these bands. Upon treatment of 2 with one equivalent of [Cp2Fe][BPh4] to give 5, a color change from brown to blue was observed. The corresponding sharp absorption band at 12515 cm−1 is assigned to the radical species, as it occurs at a nearly identical energy as that of 12500 cm−1 observed upon aerobic oxidation of (PPh4)2[Fe(OSLH22−)2] (see Figures 5b and S5).16 The NIR absorption bands for 4−6, which are assigned to π → π* and LMCT transitions, exhibit a progressive red shift upon moving from 4 to 6 (Figure 5b). Specifically, the π → π* band undergoes a shift from 19940 cm−1 for 4 to 15725 cm−1 for 6, while the LMCT band moves from 15425 cm−1 for 4 to 10110 cm−1 for 6. The red shift in both the π → π* and LMCT transitions upon increasing the number of S atoms reflects the more diffuse orbitals of S, which results in greater electron delocalization onto the heteroatom and thus less energy required for the charge transfer process.34 The stability of the radical-bridged species was evaluated by monitoring the UV/vis spectra of solutions of 4−6 in MeCN under a dinitrogen atmosphere. Here, the wavelength corresponding to the point of largest change in the spectrum was identified, and the signal intensity at that wavelength was

Figure 5. (a) UV/vis/NIR spectroelectrochemistry of complex 3 upon the application of a potential of +0.55 V in an electrolyte solution of 0.1 M (NBu4)PF6 in CH3CN, where the arrows indicate the spectral evolution. (b) UV/vis/NIR spectra of chemically isolated 4−6. The asterisk denotes a detector change at 12500 cm−1.

intraligand charge transfer (ILCT) corresponding to a π → π* transition from the aromatic rings of the TPyA capping ligand.33 Upon the application of a positive potential of +0.55 V vs Ag/AgCl, two distinctive bands appeared in the NIR region at 10105 and 15683 cm−1. The band at 10105 cm−1 is due to a LMCT process from the S donor atoms to the CrIII center. The π → π* transition appearing at 15683 cm−1 involves a SOMO acceptor orbital which contains significant contributions from the S p orbitals (see Figure 5a).12b With the appearance of these bands in the NIR region, a band at 22450 cm−1 concomitantly increases in intensity. The assignment of one of these NIR bands SL4−/3−• is further supported by an analogous spectroelectrochemical experiment carried out on 1,2,4,5-tetraisopropylbenzene, where the application of a potential of +1.225 V results in a distinctive band at 13140 cm−1, along with the disappearance of a band at 25200 cm−1 (see Figure S2). Both processes correspond well to those observed for 3. Finally, the spectral changes associated with application of the positive potential were fully reversible upon subsequently applying a potential of 0 V, demonstrating the reversibility of redox chemistry in 3. Chemical Oxidation. Given the accessibility of RL3−• determined by electrochemical measurements of 1−3, these oxidized complexes should be accessible through chemical oxidation. Toward that end, solutions of complexes 1−3 in E


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Inorganic Chemistry plotted as a function of time. The decay curves were then fitted to a second-order rate equation to provide values of half-life (t1/2) for each compound (see Figure S6). Compound 4 exhibited the longest half-life, with t1/2 > 200 h, followed by 5 (t1/2 = 31.4 h) and then 6 (t1/2 = 4.1 h). Note that the half-life of 6 could be increased to 21 h when stored at −40 °C. Magnetic Properties. To probe the presence of magnetic exchange coupling in 1−3, variable-temperature dc susceptibility data were collected on microcrystalline samples in the range 2−300 K under an applied field of 0.5 or 1 T (see Figure 6). At 300 K, the plot of χMT vs T for all compounds exhibits a

The value of J progressively decreases on moving from 1 to 2 to 3. The decreasing trend of J is a result of the increase in distance between CrIII centers, as lengthening of the C−E and Cr−E bonds occurs upon the inclusion of more S atoms. A similar trend of decreasing J was observed in O- and S-bridged cyclopentadienyl manganese(II) cages, where MnII centers were linked by OSiPh3 or SSiPh3 groups. Upon replacing the O atom for S, the exchange coupling value decreased from J = −4.4 to −3.0 cm−1.36 Similarly, substitution of Se for S in two bis(μ-methoxo)dichromium(III) complexes resulted in a decrease from J = −1.96 to −0.49 cm−1.37 The decrease in J upon the substitution of atoms lower on the periodic table within a group has also been observed in Group 15 elements, where replacing a bridging P for As resulted in a decrease of J from −13.5 to −1.5 cm−1.38 Plots of χMT vs T were similarly constructed for 4−6 to examine the presence of chromium-semiquinoid radical coupling (see Figure 7). Owing to the thermal instability of

Figure 6. Variable-temperature dc magnetic susceptibility data for 1 (green), 2 (red), and 3 (blue), collected under an applied field of 1 T. The black lines represent fits to data.

value of χMT = 3.70−3.75 cm3K/mol, in accord with the value of χMT = 3.75 expected for two magnetically isolated CrIII ions with g = 2.00. Upon decreasing temperature, χMT undergoes a decline in all compounds, with that of 1 occurring at the highest temperature, followed by 2 and then 3, reaching minimum values of χMT = 0.18−0.22 cm3K/mol at 2 K. The decrease in χMT with decreasing temperature is indicative of weak intramolecular antiferromagnetic coupling between CrIII centers to give an S = 0 ground state. To quantify this coupling, the data were fit using the program PHI35 and the ̂ spin Hamiltonian Ĥ = −2J(Ŝ Cr1 · Ŝ Cr2) to give exchange constants of J = −2.8(3) (g = 1.98), − 2.2(5) (g = 1.97), and −1.8(2) cm−1 (g = 1.98) for 1−3, respectively (see Table 1). The value of J obtained for 1 is comparable to that of −2.0 cm−1 reported by Guo and McCusker for a chloranilatebridged CrIII2 complex featuring tris(2-aminoethyl)amine (tren) capping ligand,7n−p yet is considerably smaller than those of J = −6.5 cm−1 and −7.8 cm−1 reported for analogous complexes capped by phenanthroline derivatives.7o To our knowledge, the values of J determined for 2 and 3 represent the first such values reported for derivatives of OSL4− and SL4−.

Figure 7. Variable-temperature dc magnetic susceptibility data for 4 (green), 5 (red), and 6 (blue), collected under an applied field of 1 T. (Inset) Spin ladder showing the spin energy levels for compounds 1− 6, as calculated from fits to the magnetic susceptibility data.

these radical-bearing complexes, variable-temperature dc susceptibility data were collected from 2 K only up to 280 K. At 280 K, compounds 4−6 exhibit values of χMT = 4.06, 4.17, and 4.24 cm3K/mol. While these values are close to that of χMT = 4.125 cm3K/mol expected for two magnetically isolated CrIII centers (S = 3/2) and an organic radical (S = 1/2), the distinct values for the three compounds suggests that the presence of magnetic coupling strong enough to manifest at 280 K. In support of that hypothesis, χMT increases with decreasing temperature, reaching maximum values of χMT = 4.38 (120 K), 4.37 (120 K), and 4.36 (110 K) cm3K/mol for 4−6, respectively. These values are in excellent agreement with that of χMT = 4.375 cm3K/mol expected for an S = 5/2 ground state (g = 2), which arises from non-compensated

Table 1. Summary of Parameters Obtained from Fits to Magnetic Dataa −1

J (cm ) D (cm−1) g

1

2

3

4

5

6

[(TPyA)2Cr2(NL3−•)]3+

−2.83(3)  1.98

−2.28(5)  1.97

−1.80(2)  1.98

−352(10) +0.92 1.97

−401(8) +0.85 1.97

−487(8) +0.87 1.99

−626(7) +0.6 1.83

Provided values of g for 1−3 were obtained from fits to variable-temperature magnetic susceptibility data, while those for 4−6 were obtained from fits to low-temperature magnetization data. Values for [(TPyA)2Cr2(NL3−•)]3+ were obtained from ref 10a. a

F


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Inorganic Chemistry antiferromagnetic coupling between the radical of RL3−• and two CrIII centers in each complex. To quantify the strength of metal-radical coupling, the data were fit using the program PHI35 and the spin Hamiltonian Ĥ = −2J[Ŝ L · (Ŝ Cr1 + Ŝ Cr2)] to give exchange constants of J = −352(10) (g = 1.97), − 401(8) (g = 1.97), and −487(8) cm−1 (g = 1.99) for 4−6, respectively (see Table 1). The value of J obtained for 4 is considerably smaller than that of J = −487(8) cm−1 previously reported for [(CTH)2Cr2(OL3−•)]3+.7n This difference may stem from the use of the aromatic TPyA vs the aliphatic CTH as a capping ligand. Indeed, the influence of the capping ligand on the energy of the molecular orbitals is reflected in the LMCT transition, which occurs at 14850 cm−1 for [(CTH)2Cr2(OL3−•)]3+ vs 15425 cm−1 for 4. Remarkably, moving from RL4− to RL3−• in 1−6 leads to an increase in J of over 2 orders of magnitude. Indeed, the strong coupling in 4−6 results in even the lowest-energy excited state lying well above the entire spin manifold of 1−3 (see Figure 7, inset). Moreover, in substituting S for O donors in moving from 4 to 5 to 6, the value of J significantly increases. This progressive increase can most likely be attributed to the stronger interaction of the more diffuse p orbitals of S and associated π* orbital of RL3−• with the spin-bearing T2g orbitals of CrIII, which overwhelms the opposing effects of increasing Cr−E and C−E distances. The lower energy required for charge transfer is directly observed in the LMCT and π → π* electronic transitions involving the SOMO of RL3−• and π -type T2g orbitals of Cr in 4−6. Specifically, a distinct red shift is associated with substitution of S for O, with 15425 cm−1 (4), 12500 cm−1 (5), and 10110 cm−1 (6) for the LMCT process and 19940 cm−1 (4), 16385 cm−1 (5), and 15725 cm−1 (6) for the π → π* process. This trend suggests that the strengthening of magnetic coupling across the series can be predominantly attributed to alterations in the energy levels of the orbitals involved in bonding between CrIII and donor atom of RL3−• upon moving from O to S. The values of J obtained for 4−6 are considerably smaller than that of J = −626(7) cm−1 previously reported for the related tetraazalene radical-bridged CrIII2 complex [(TPyA)2Cr2(NL3−•)]3+ (NL3−• = N,N′,N″,N‴tetra(2-methylphenyl)-2,5-diamino-1,4-diiminobenzoquinone).10a This difference may stem from the considerably stronger Lewis basicity of tetraazalene trianion relative to the chalcogen-based ligands. Finally, to assess the presence of magnetic anisotropy in compounds 4−6 and to confirm the presence of an S = 5/2 ground state, low-temperature magnetization data were collected at integer dc fields from 1 to 7 T (see Figures S7− S9). The resulting plots of reduced magnetization show minimal splitting of the isofield curves and reach saturation near the value of M = 5 μB expected for an S = 5/2 ground state, indicating the presence of only minimal magnetic anisotropy. To quantify this effect, the data were fit using the program PHI35 to give axial zero-field splitting parameters of D = +0.92 (g = 1.97), + 0.85 (g = 1.97), and +0.87 (g = 1.99) cm−1, for 4−6, respectively, consistent with octahedral CrIII.

strength of metal−radical coupling increases from J = −352(10) to −401(8) to −487(8) cm−1, and this progression can be correlated to the energy of absorption bands in the UV/ vis/NIR spectra. Work is underway to incorporate thiosemiquinoid linkers into chain compounds and extended networks in order to realize high-barrier single-chain magnets and permanent magnets with high ordering temperatures, respectively.

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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.9b00674. Additional experimental details, UV/vis/NIR spectra, low-temperature magnetization data, and crystallographic data (PDF) Accession Codes

CCDC 1881953−1881955 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carol Hua: 0000-0002-4207-9963 T. David Harris: 0000-0003-4144-900X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by the National Science Foundation (DMR-1351959) and the Army Research Office (W911NF-14-1-0168/P00005 and W911NF-15-1-0331). C.H. gratefully acknowledges the American−Australian Association for a Dow Chemical Company Fellowship and the Australian Government for an Endeavour Postdoctoral Fellowship.

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REFERENCES

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SUMMARY AND OUTLOOK The foregoing results demonstrate the impact on magnetic coupling of moving from O to S in metal−semiquinoid complexes and provide the first quantitation of coupling between a metal and thiosemiquinoid linker. In moving from O to S donors in the complexes [(TPyA)2Cr2(RL3−•)]3+, the G


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Inorganic Chemistry

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