A series of tetraazalene radical-bridged M2 (M = CrIII, MnII, FeII, CoII) complexes with strong magnetic exchange coupling

The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated.


Table of Contents
Experimental Section S2 Table S1: Crystallographic data for compounds S7 Table S2: Selected interatomic distances and angles for compounds S8 Figure S1: UV/visible/NIR for 5 S9 Figure S2: UV/visible/NIR for 2 and 6 S10 Figure S3: UV/visible/NIR for 3 and 7 S11 Figure S4: UV/visible/NIR for 4 and 8 S12 Figure S5: Variable-temperature dc magnetic susceptibility data for 4 S13 Figure S6: Low-temperature variable-field magnetization data for 5 S14 Figure S7: Low-temperature variable-field magnetization data for 6 S15 Figure S8: Low-temperature variable-field magnetization data for 7 S16 Figure S9: Low-temperature variable-field magnetization data for 8 S17 Figure S10: 100 K Variable-field data for 2 S18 Figure S11: 100 K Variable-field data for 3 S19 Figure S12: 100 K Variable-field data for 4 S20 Figure S13: 100 K Variable-field data for 5 S21 Figure S14: 100 K Variable-field data for 6 S22 Figure S15: 100 K Variable-field data for 7 S23 Figure S16: 100 K Variable-field data for 8 S24 Figure S17: In-phase ac magnetic susceptibility for 7 S25 References S26 Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2015 (Cp)Fe(C 6 Me 6 ). This compound was prepared according to a modified literature procedure.
Hexamethylbenzene (1.09 g, 6.73 mmol) was added to freshly ground Al metal (0.0830 g, 3.10 mmol), AlCl 3 (1.80 g, 13.5 mmol), and ferrocene (1.25 g, 6.73 mmol) in 20 mL methylcyclohexane, and the resulting mixture was heated at reflux for 12 h. The resulting dark mixture was then carefully quenched with H 2 O (50 mL), and the orange organic layer was removed. The remaining aqueous green solution was washed with petroleum ether (3 × 10 mL). To the aqueous solution was then added (NH 4 )PF 6 (1.30 g, 8.10 mmol) in H 2 O (10 mL), and the resulting suspension was filtered to give a dark yellow powder. This powder was washed with an excess of H 2 O (30 mL), dissolved in acetone (15 mL), and then was precipitated with Et 2 O (50 mL). The solid was collected on a nylon membrane filter (0.22 μm) and washed with successive aliquots of Et 2 O (3 × 5 mL) to afford the product as a bright yellow solid (0.577 g, 20% The compound [(Cp)Fe(C 6 Me 6 )]PF 6 (1.00 g, 2.34 mmol) was dissolved in THF (15 mL), and the resulting yellow solution was added to finely-ground NaHg amalgam (4% Na wt/wt, 6 g) to afford a dark green solution. After stirring for 6 h, the solution was dried under reduced pressure. The ensuing dark green residue was dissolved in hexanes (15 mL), and the solution was filtered through diatomaceous earth. The filtrate was then dried under reduced pressure to give the product as a green solid (0.588 g, 89%). 1 H NMR (C 6 D 6 ): −7.07 (s, 18H), −40.31 (s, 5H).
After stirring at ambient temperature for 12 h, the brown solution was filtered through diatomaceous earth, and careful layering of hexanes onto the filtrate afforded 3·2.5THF as dark brown needle-shaped crystals suitable for single-crystal X-ray diffraction. Subsequent washing of the crystals with cold ( −78 o C) ethanol followed by hexanes enabled removal of co-precipitated Li (BAr F 4  [(TPyA) 2 Co 2 ( NMePh L 2− )](BAr F 4 ) 2 (4). TPyA (0.052 g, 0.18 mmol) was dissolved in THF (5 mL), and this solution was added dropwise to a solution of [Co(MeCN) 6 ](BAr F 4 ) 2 (0.36 g, 0.18 mmol) in THF (3 mL). The resulting golden solution was stirred for 5 min at ambient temperature and then was treated with solid NMePh LH 2 (0.044 g, 0.089 mmol). To this mixture was added dropwise with stirring a solution of Li[N(SiMe 3 ) 2 ] (0.035 g, 0.21 mmol) in THF (5 mL), resulting in a dark brown solution. After stirring at ambient temperature for 12 h, the solution was filtered through diatomaceous earth, and careful layering of hexanes onto the filtrate gave 4·2.5THF as dark yellow, needle-shaped crystals suitable for singlecrystal X-ray diffraction. The obtained crystals were dried under reduced pressure at ambient temperature for 2 h in order to remove the solvent molecules, yielding 4 (0.18 g, 70%) as a dark yellow crystalline solid. Anal. Calcd. for C 134 H 90 F 48 Co 2 N 12 B 2 C 55.1, H 3.11, N 5.76 %. Found: C, 55.2; H, 3.23; N, 5.66 %.
[(TPyA) 2 Cr 2 ( NMePh L 3−• )](BPh) 3 ·1.4MeCN (5). TPyA (0.0720 g, 0.248 mmol) was dissolved in THF (5 mL), and this solution was added dropwise to a solution of [Cr(MeCN) 6 ](BAr F 4 ) 2 (0.503 g, 0.248 mmol) in THF (3 mL). The resulting brown solution was stirred for 5 min at ambient temperature and then was treated with solid NMePh LH 2 (0.0616 g, 0.124 mmol). To this mixture was added dropwise with stirring a solution of Li[N(SiMe 3 ) 2 ] (0.0430 g, 0.257 mmol) in THF (5 mL), resulting in a dark purple solution. After stirring at ambient temperature for 4 h, the solution was filtered through diatomaceous earth, and the resulting filtrate was dried under reduced pressure. The resulting residue was washed with hexanes (3 × 5 mL) and dried under reduced pressure for 12 h. Cold MeOH (10 mL) was then added at −78 o C and this solution was treated with a solution of Na(BPh 4 ) (0.152 g, 0.444 mmol) in MeOH (5 mL) at −78 o C, resulting in an immediate precipitate. The solid was collected on a nylon filter (0.22 μm) and then washed with successive aliquots of MeOH (3 × 5 mL) and diethyl ether (3 × 5 mL), giving a gray filtrate and bright purple powder. Slow vapor diffusion of diethyl ether into an acetonitrile solution of the powder at ambient temperature gave 5·2.9MeCN as long, purple plates suitable for single-crystal X-ray diffraction. The obtained crystals were dried under reduced pressure at ambient temperature for 2 h in order to remove the solvent molecules, yielding 5 (0.0734 g, 27%) as a dark yellow crystalline solid. Anal. Calcd. for C 144.8 H 130.2 Cr 2 N 13.4 B 3 : C, 79.2; H, 5.98; N, 8.55%. Found: C, 79.0; H, 6.14; N, 8.31%.
X-ray Structure Determination. Single crystals of 1, 2·0.4THF, 3·2.5THF, 4·2.5THF, and 5·2.9MeCN were coated with deoxygenated Paratone-N oil, mounted on a MicroMounts TM rod, and frozen under a stream of N 2 during data collection. Crystallographic data were collected at 100 K using a Bruker Kappa Apex II diffractometer equipped with an APEX-II detector, a CuKα l μS microfocus source, and MX Optics. Raw data were integrated and corrected for Lorentz and polarization effects with SAINT v8.27B. 9 Absorption corrections were applied using SADABS. Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structure. Structures were solved using direct methods in SHELXT and further refined with SHELXL-2014 10 operated with the OLEX2 interface. 11 Magnetic Measurements. All samples were prepared and manipulated with the rigorous exclusion of dioxygen under a dinitrogen atmosphere. Magnetic measurements of 2-8 were performed on All hydrogen atoms were placed at calculated positions using suitable riding models and refined using isotropic displacement parameters derived from their parent atoms. Thermal parameters were refined anisotropically for all non-hydrogen atoms. Crystallographic data for these compounds at 100 K are given in Table 1. Significant disorder of the otolyl groups on the bridging ligand was modeled with partial occupancies; however, some disorder that did not improve the final refinement of the solution was placed at one position with a full occupancy. At 100 K, all solvent molecules in 1, 2·0.4THF, and 4·2.5THF, one THF molecule in 3·2.5THF, and 4.6 MeCN molecules in 5 were severely disordered and could not be modeled properly. 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. These molecules were included in the final molecular formula. polycrystalline samples sealed in a 2 mL polyethylene bag. All data were collected using a Quantum Design MPMS-XL SQUID magnetometer from 1.8 to 300 K at applied dc fields ranging from 0 to +7 T. Dc susceptibility data were corrected for diamagnetic contributions from the sample holder and for the core diamagnetism of each sample (estimated using Pascal's constants 12 Mössbauer Measurements. Zero-field iron-57 Mössbauer spectra were obtained at 80 K with a constant acceleration spectrometer and a cobalt-57 rhodium source. Prior to measurements, the spectrometer was calibrated at 295 K with α-iron foil. Samples were prepared in a dinitrogen-filled glovebox with crystals of 3 and 7′ that were inserted into the cryostat prior to the measurement. The samples contained approximately 80 mg/cm 2 of compound. All spectra were analyzed using the WMOSS Mössbauer Spectral Analysis Software (www.wmoss.org). Fits to a minor doublet in the spectrum of 3 at 80 K gave an isomer shift of δ = 0.08(1) mm/s and a quadrupole splitting of ΔE Q = 0.73(3) mm/s, consistent with a small amount of high-spin Fe III -containing impurity in the sample.
). 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. Experimental errors for magnetic exchange coupling in compounds 5-8 were determined by averaging simulations of two independently prepared samples.

Other Physical Measurements.
Elemental analyses of all compounds were performed by Midwest Microlab (Indianapolis, IN). Infrared spectra were recorded on a Bruker Alpha FTIR spectrometer equipped with an attenuated total reflectance accessory. 1 H NMR spectra were collected at 500 MHz on a Varian Inova 500 system at 298 K. UV/Vis/NIR spectra were obtained using either a Varian Cary 60 spectrophotometer or a Varian Cary 5000 spectrophotometer. Cyclic voltammetry measurements were carried out in a standard one-compartment cell under nitrogen, equipped with a glassy carbon working electrode, a platinum wire as counter electrode, and a silver wire as reference electrode using a CHI 760c potentiostat. Analyte solutions were prepared with 0.05 M solutions of Na(BAr F 4 ) in THF (2-4) or 0.1 M solutions of (NBu 4 )PF 6 in MeCN (5). Ferrocene (Cp 2 Fe) was employed as an internal standard and all potentials were referenced to the [Cp 2 Fe] 0/1+ couple.