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A Family of Octahedral Magnetic Molecules Based on [NbIV(CN)8]4− Mirosław Arczyński,† Michał Rams,‡ Jan Stanek,‡ Magdalena Fitta,§ Barbara Sieklucka,† Kim R. Dunbar,*,∥ and Dawid Pinkowicz*,†,∥ †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland Marian Smoluchowski Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland § Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland ∥ Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States ‡

S Supporting Information *

ABSTRACT: A building block approach has been used to prepare a new family of hexanuclear magnetic molecules Mn4Nb2, Fe4Nb2, and Co4Nb2 of general formula {[MII(tmphen)2]4[NbIV(CN)8]2}·solv (M = Mn, Fe, Co; tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline; solv = MeOH and/or H2O). Mn4Nb2 exhibits a magnetocaloric effect at temperatures close to 1.8 K, and Fe4Nb2 undergoes an incomplete gradual spin crossover and a photomagnetic response related to light-induced excited spin state trapping.

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intriguing spin transitions,48−50 photomagnetic effects,5,50,51 and single-molecule magnet (SMM) behavior.5,45,52 We noted with interest that the hexanuclear octahedral molecules (6-OC) are actually quite rare and relatively underexplored, with only three members having been reported, viz., {[MnII(bpy)2]4[MoIV(CN)8]2}, {[MnII(bpy)2]4[WIV(CN)8]2},38 and the aforementioned {[MnII(bpy)2]4[NbIV(CN)8]2},27 of which only the Nb congener exhibits a high-spin ground state due to intramolecular magnetic interactions; the Mo and W analogues are simple paramagnets. Herein we report the missing members of the cyanidebridged {[MII(tmphen)]4[NbIV(CN)8]2}·solv M4Nb2 octahedral magnetic molecule family (Figure 1), exhibiting a variety of functionalities depending on the nature of the M center (M = MnII, FeII, CoII, tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and solv = H2O and/or MeOH). {MnII(tmphen)]4[NbIV(CN)8]2}·solv (Mn4Nb2) is a high-spin molecule, and {[FeII(tmphen)]4[NbIV(CN)8]2}·solv (Fe4Nb2) exhibits a steplike spin crossover behavior and photomagnetism involving the light-induced excited spin state trapping (LIESST) phenomenon. Finally, {[CoII(tmphen)]4[NbIV(CN)8]2}·solv (Co4Nb2) exhibits magnetic anisotropy typical for octahedral CoII centers.

INTRODUCTION Cyanide chemistry has played a prominent role in the field of molecular magnetism from its inception.1 The unique coordination capabilities of the cyanide anion allow for the use of a modular design approach2 with respect to the choice of transition metal and auxiliary ligands, a strategy that has led to the isolation of a wide variety of magnetic molecules,3−7 coordination polymers,8−10 and, in general, molecular magnetic materials exhibiting multifunctionality.11−13 In addition to the plethora of 3d magnetic systems based on cyanide, there are exciting opportunities for the chemistry of 4d and 5d transition metals owing to their rich redox chemistry, expanded coordination spheres, and stronger spin−orbit coupling.14−17 These fascinating possibilities notwithstanding, coordination compounds based on cyanide complexes of early 4d and 5d transition metals are underrepresented due to their elusive character and relatively high oxygen and moisture sensitivity, one such example being octacyanoniobate(IV).18,19 Several 2-D20,21 and 3-D22−24 coordination polymers based on this anion have been reported, some of which exhibit unique combinations of physical and chemical properties,25,26 but there are only three examples of discrete assemblies involving this “fragile” cyanometalate; there are two molecules with manganese(II)27 and one involving nickel(II) as a second metal center.28 Attempts have been made to prepare other discrete congeners such as CoII-NbIV without success.29 Among the viable molecular motifs based on cyanide ligands, there are molecular squares (SQ),5,30−34 pentanuclear trigonal bipyramids (5-TBP),7,35−37 hexanuclear octahedra (6-OC),27,38 octanuclear cubes (8-CU),39 and pentadecanuclear six-capped body-centered cubes (15-CU),40−47 a number of which exhibit © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Chemicals and Reagents. All chemicals are of analytical grade. The reagents 3,4,7,8-tetramethylphenanthroline, MnCl2·6H2O, FeCl2· 4H2O, CoCl2·6H2O, 18-crown-6 ether (Sigma-Aldrich), and MnCl2 anhydrous (Acros Organics) were used as received. K4[Nb(CN)8]· 2H2O was prepared according to literature procedures.18 Analytical Received: December 28, 2016 Published: March 21, 2017 4021

DOI: 10.1021/acs.inorgchem.6b03134 Inorg. Chem. 2017, 56, 4021−4027

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Inorganic Chemistry

solution of 0.210 mmol (49.4 mg) of 3,4,7,8-tetramethylphenanthroline with 0.104 mmol (24.7 mg) of CoCl2·6H2O in 9 mL of methanol. The resulting purple solution was left undisturbed for crystallization for 24 h. After this time, purple block crystals were harvested. Yield: 55%. Elemental analysis (%) was performed ∼3 min after removing the sample from the mother liquor and after 24 h of vacuum drying. Anal. Calcd for freshly prepared [Co(tmphen)2]4[Nb(CN)8]2·15H2O· 8MeOH: C, 56.13; H, 5.888; N, 13.78. Found: C, 56.17; H, 5.491; N, 13.78. Calcd for vacuum-dried [Co(tmphen)2]4[Nb(CN)8]2·8.5H2O: C, 59.69; H, 5.044; N, 15.47. Found: C, 59.62; H, 4.882; N, 15.38. The purity of the sample immersed in mother solution was also confirmed by powder X-ray diffraction (Figure S1 in the Supporting Information). Magnetic Measurements. Magnetic susceptibility measurements for all compounds were performed using Quantum Design SQUID magnetometers (MPMS3 and MPMS-XL) on crushed samples that were moistened with mother liquor solutions and sealed in glass tubes under vacuum to protect the crystals from the loss of crystallinity. The mass of the samples was checked after drying them under vacuum. Photomagnetic studies for Fe4Nb2 were performed using a 300 W xenon lamp and a suitable set of band-pass filters. The sample of Fe4Nb2 for photomagnetic measurements was also protected from solvent loss by sealing it in a foil bag with a droplet of the mother solution. Diamagnetic corrections were applied by including the sample holders, and the solvent and by using Pascal’s constants.53 Single-Crystal Structure Determination. Single-crystal diffraction data for Mn4Nb2 and Co4Nb2 were collected at 110 K on a Bruker ApexII diffractometer (Mo Kα, graphite monochromator), and the data for Fe4Nb2 were collected on an Agilent SuperNova diffractometer (Cu Kα, graphite monochromator) also at 110 K (see Table S1 in the Supporting Information). Single-crystal diffraction data for Mn4Nb2 impurity were collected at 120 K on a Bruker D8QuestEco Photon50 diffractometer (Mo Kα, graphite monochromator). Data reduction, cell refinements, and absorption corrections were performed using SAINT and SADABS or Crysalis, respectively. Intensities of reflections were corrected for sample absorption using the multiscan method. All structures were solved using direct methods and refined anisotropically using weighted full-matrix least squares on F2 using SHELX.54 All three compounds are isostructural. The crystallization solvent molecules (MeOH and/or H2O) exhibit considerable disorder and could not be refined. The disorder is related to the rapid loss of crystallization solvent when the crystals are exposed to ambient conditions; therefore, the SQUEEZE procedure was applied to all three structural models. The crystallization solvent related disorder also affects some of the tmphen ligands; therefore, in some cases ISOR restraints were used on some of the C atoms of the tmphen ligands. The H atoms were located at the idealized calculated positions and refined as riding atoms. The data for Mn4Nb2 were collected up to θmax = 20.780° due to the relatively small crystal size. In case of Fe4Nb2 PLATON and CHECKCIF suggested the higher symmetry space group I41/acd. The structure was solved and refined anisotropically using this space group, which led to much higher R indices (Table S2 in the Supporting Information). Moreover, in this space group the asymmetric unit contains only one Fe site with short Fe−N bond lengths typical for a low-spin configuration, which is in contradiction to the magnetic and Mössbauer data suggesting at least one high-spin FeII center still present at 110 K. Therefore, the structural model obtained in the lower symmetry space group I41 (similar to Mn4Nb2 and Co4Nb2) is considered to be more appropriate. Other Physical Methods. Powder X-ray measurements were performed at room temperature for all M4Nb2 samples immersed in the mother liquor in borosilicate glass capillaries (0.7 mm in diameter) using a PANalytical X’Pert Pro MPD diffractometer with Cu Kα radiation (graphite monochromator; Figure S1 in the Supporting Information). Elemental CHN analyses were performed using an ELEMENTAR Vario Micro Cube CHNS analyzer for freshly prepared and for vacuum-dried samples (several hours under dynamic vacuum).

Figure 1. Crystal structure of Fe4Nb2 as a representative example of the M4Nb2 family. Color scheme: Fe, green; Nb, yellow; C, gray; N, blue. H atoms are omitted for clarity. grade methanol (POCH) was used for the syntheses of Fe4Nb2 and Co4Nb2. The preparation of Mn4Nb2 was performed using anhydrous methanol distilled in the presence of magnesium methanolate. Preparation of [Mn(tmphen)2]4[Nb(CN)8]2·14H2O·7MeOH (Mn4Nb2) (Formula Based on Elemental Analysis). The synthesis was performed under an inert atmosphere using anhydrous MeOH. An octacyanoniobate(IV) solution was prepared by dissolving 0.025 mmol (12.0 mg) of K4[Nb(CN)8]·2H2O in 15 mL of anhydrous methanol and 0.2 mmol (52 mg) of 18-crown-6 ether. This mixture was then rapidly added to a solution of 0.400 mmol (94.5 mg) of 3,4,7,8-tetramethylphenanthroline with 0.050 mmol (6.3 mg) of anhydrous MnCl2 in 15 mL of methanol. The solution was left undisturbed for 5 days, which led to the formation of orange-yellow block crystals. When the synthesis was performed using reagent grade MeOH under ambient conditions and with MnCl2·4H2O, needleshaped crystalline impurities of a second phase appeared. This phase has been identified as the Mn4Nb2 cage molecule that crystallizes in a tetragonal P43 space group and showing different molecular packing in comparison to the main I41 phase. Yield: 11%. Elemental analysis (%) was performed ∼5 min after removing the sample from the mother liquor and after 24 h of vacuum drying. Anal. Calcd for freshly prepared [Mn(tmphen)2]4[Nb(CN)8]2·14H2O·7MeOH: C, 56.91; H, 5.820; N, 14.07. Found: C, 56.92; H, 5.486; N, 13.89. Calcd for vacuum-dried [Mn(tmphen)2]4[Nb(CN)8]2·10H2O: C, 59.84; H, 5.162; N, 15.51. Found: C, 59.86; H, 4.959; N, 15.32. The purity of the sample immersed in mother solution was also confirmed by powder X-ray diffraction (Figure S1 in the Supporting Information). Preparation of [Fe(tmphen)2]4[Nb(CN)8]2·6H2O·15MeOH (Fe4Nb2) (Formula Based on Elemental Analysis). A 0.049 mmol (24.0 mg) quantity of K4[Nb(CN)8]·2H2O was dissolved in 3 mL of reagent grade methanol by the addition of ∼0.4 mmol (105 mg) of 18-crown-6 ether (fast stirring required). The transparent yellow solution was rapidly added to 3 mL of a methanolic solution of 0.212 mmol (50.0 mg) of 3,4,7,8-tetramethylphenanthroline with 0.101 mmol (20.0 mg) of FeCl2·4H2O and stirred for a few seconds. The resulting red solution was left for 3−5 h for crystallization. After this time, dark-red block crystals were collected. Yield: 18%. Elemental analysis (%) was performed ∼3 min after removing the sample from the mother solution and after 24 h of vacuum drying. Calcd for freshly prepared [Fe(tmphen)2]4[Nb(CN)8]2·6H2O·15MeOH: C 57.89, H 5.900, N 13.62; found: C 57.83, H 6.105, N 13.58. Calcd for vacuumdried [Fe(tmphen)2]4[Nb(CN)8]2·11H2O: C 59.36, H 4.890, N 15.32; found: C 59.39, H 5.19, N 15.40. The purity of the sample immersed in mother solution was also confirmed by powder X-ray diffraction (Figure S1 in the Supporting Information). Preparation of [Co(tmphen)2]4[Nb(CN)8]2·15H2O·8MeOH (Co 4 Nb 2 ) (Formula Based on Elemental Analysis). An octacyanoniobate(IV) solution was prepared by dissolving 0.049 mmol (24.0 mg) of K4[Nb(CN)8]·2H2O in 10 mL of methanol with ∼0.4 mmol (105 mg) of 18-crown-6 ether. It was then added to a 4022


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MnII and HS CoII centers, respectively.57,58 Conversely, the corresponding bonds in Fe4Nb2 at 110 K strongly suggest the presence of low-spin (LS) FeII centers with average Fe−N bond lengths being 2.01, 1.98, 1.98, and 1.96 Å for Fe1, Fe2, Fe3, and Fe4, respectively (Table S5). These distances are much less than those expected for HS FeII (ca. 2.2 Å)26,59−61 and are in good agreement with values reported for LS FeII compounds (ca. 1.9−2.0 Å).48,62,63 Magnetic data (see below), however, indicate that at 110 K a fraction of the FeII centers is still in the HS state (approximately 11%). Due to the extreme sensitivity of the crystals, attempts to determine the crystal structure at temperatures close to the room temperature were unsuccessful. Magnetic and Photomagnetic Properties. The three compounds are isostructural, but the magnetic nature of the 3d metal center (MnII, FeII, or CoII) has a profound impact on their magnetic properties and additional functionalities. Magnetic properties of all three compounds were investigated from 300 to 1.8 K and from 0 to 5 T (or 0 to 7 T). The χT(T) dependence for Mn4Nb2 at 0.1 T is presented in Figure 2. The

RESULTS AND DISCUSSION Syntheses. All three compounds were obtained as crystalline samples within several hours by the addition of a methanol solution of octacyanoniobate(IV) to a methanol solution of the [MII(tmphen)2]2+ in situ prepared precursors. Note that the MeOH-insoluble potassium octacyanoniobate(IV) is rendered soluble by using an excess of 18-crown-6 ether. Mn4Nb2 was prepared using anhydrous methanol, anhydrous MnCl2, and an excess of tmphen ligand in order to suppress the crystallization of unidentified byproducts. The Fe4Nb2 and Co4Nb2 derivatives do not require anhydrous conditions, and the Fe:tmphen and Co:tmphen ratios need to be slightly less than or exactly 1:2. Crystal Structures. All three compounds crystallize as relatively large block crystals (approximately 0.2 × 0.2 × 0.2 mm; their colors are orange for Mn4Nb2, dark red for Fe4Nb2, and purple for Co4Nb2). All compounds are extremely sensitive to solvent lossFigure S2 in the Supporting Information presents the experimental PXRD patterns for pristine and vacuum-dried Co4Nb2 as an example. Even when the crystals are kept under mother liquor that is in contact with air, a slow deterioration of the crystals occurs due to moisture uptake (0.5−3 h). Single-crystal X-ray diffraction revealed that the compounds crystallize in the tetragonal I41 space group (Table S1 in the Supporting Information) with the asymmetric units consisting of two halves of neighboring molecules (Figure S3− S5 in the Supporting Information). However, the structure of Fe4Nb2 can also be solved and refined in I41/acd (Table S2 in the Supporting Information). The asymmetric unit for Fe4Nb2 in I41/acd is much smaller and is presented in Figure S6 in the Supporting Information. Moreover, if the synthesis of Mn4Nb2 is performed under ambient conditions using reagent grade MeOH, it can contain needle-shaped impurities identified as Mn4Nb2 molecules crystallizing in the tetragonal P43 space group with slightly different crystal-packing arrangements (Table 2 and Figure S7 in the Supporting Information). The CN-bridged metallic cores of the M4Nb2 family are best described as pseudo-octahedral cages (Figure 1) with the 3d metal centers bound to two tmphen ligands occupying equatorial positions of the pseudo-octahedron and the two octacyanidoniobate(IV) moieties serving as caps for the equatorial plane by providing four bridging cyanide ligands each. The 3d metal ions are six-coordinate and adopt highly distorted octahedral geometries, whereas the coordination spheres of the Nb centers are nearly square antiprismatic (SAPR) according to the continuous shape measure analysis using SHAPE software55,56 (Tables S3 and S4 in the Supporting Information). Each molecule of Mn4Nb2, Fe4Nb2, and Co4Nb2 is intrinsically chiral due to the characteristic propeller-like arrangement of the tmphen coligands. Both enantiomeric forms are present, however, due to racemic twinning (Flack parameters ca. 0.5 for all three compounds). The molecules are packed in such a way that the terminal cyanides point toward the tmphen ligands of neighboring molecules (Figures S8 and S9 in the Supporting Information). The closest intermolecular metal−metal distances are ca. 10.4 Å, and there are no direct contacts between the molecules. The atoms of the H2O and MeOH crystallization solvent could not be located from the Fourier maps due to severe disorder. Bond distances and angles for Mn4Nb2 and Co4Nb2 (Table S5 in the Supporting Information) are typical for Mn− Nb(CN)8 and Co−Nb(CN)8 compounds with high-spin (HS)

Figure 2. χT(T) at 0.1 T and M(H) at 1.8 K for Mn4Nb2 (black circles, experimental data; red solid lines, best fit).

room-temperature χT value is 17.3 cm3 K mol−1, which is close to the spin-only value of 18.3 cm3 K mol−1 expected for four MnII (S = 5/2) and two NbIV (S = 1/2) noninteracting magnetic centers, assuming average g = 2.0. The χT(T) plot exhibits a flat minimum at ∼230 K due to antiferromagnetic interactions between the CN-bridged NbIV and MnII ions and then increases abruptly and reaches a maximum of 43.8 cm3 K mol−1 at 3.5 K that corresponds well with the spin-only χT value of 45.0 cm3 K mol−1 expected for a high-spin S = 9 molecule where MnII and NbIV spins are coupled antiferromagnetically. The M(H) dependence recorded at 2.0 K confirms the high-spin ground state of Mn4Nb2. The 17.9 μB saturation value, which is close to 18.0 μB expected for an S = 9 system, is almost reached at a relatively low field of ∼2.0 T. Both χT(T) and M(H) were fit using a simple Hamiltonian assuming only one type of Heisenberg exchange interaction between NbIV and MnII (PHI software by Chilton et al.):64 Ĥ = −2JNbMn (S Nb1 + S Nb2)(SMn1 + SMn2 + SMn3 + SMn4 ) + μB H ∑ giSiz i

where the exchange coupling constant JNbMn and the g factor of niobium(IV) gNb are the fitting parameters and SNb = 1/2, SMn = 5/2, and gMn = 2.0 are fixed. The best fit with JNbMn = −9.4(1) 4023


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Inorganic Chemistry cm−1 and gNb = 2.0(1) is depicted as the red solid line in Figure 2. The values of both parameters JNbMn and gNb are similar to those reported in the literature. 19,27,57,65 The Mn 4 Nb 2 compound exhibits a magnetocaloric effect (MCE) as determined by isothermal magnetization measurements up to 7 T in the 2−30 K range (Figure S10 in the Supporting Information). The entropy change was calculated using the integrated Maxwell relation (Figure 3).66,67 The isothermal

a weakly resolved steplike feature around 220 Kmost likely due to the two-step-like SCO transition at different Fe sites in each molecule (Figure S11 in the Supporting Information). Such a magnetic behavior is characteristic of a FeII spincrossover transition with very weak cooperativity. Moreover, analysis of the maximum and minimum χT values suggests that the transition is incomplete (note that this cannot be caused by sample desolvation or any related effect because all magnetic measurements were performed for samples completely immersed in the mother solution MeOH). At 325 K there is a fraction of FeII centers in the LS state (approximately 15% FeII per mole of Fe4Nb2), and below 50 K a fraction is in the HS state (∼10% FeII per mole of the Fe4Nb2). Further decrease of the χT(T) value below 50 K is due to zfs effects and antiferromagnetic interactions. The M(H) curve measured at 1.8 K (Figure 4, inset) increases rapidly in the 0−3 T range but does not reach saturation even at 7 T, where the value is 1.76 μB most likely due to the NbIV centers (S = 1/2) being coupled antiferromagnetically with the remaining HS FeII centers. Temperature-dependent 57Fe Mössbauer spectra (Figure 5) collected at three different temperatures show two doublets,

Figure 3. Entropy change of Mn4Nb2 calculated from isothermal magnetization measurements. Solid lines were calculated on the basis of the fitting of the magnetization data (Figure S8 in the Supporting Information). For details see the text and the Supporting Information.

M(H) curves in Figure S10 yield entropy changes ΔSm as shown in Figure 3. The maximum value of ΔSm is 26.5 J mol−1 K−1 (T = 2 K, ΔH = 7 T), and it is slightly larger than the limiting value 24.5 J mol−1 K−1 expected for an S = 9 molecule calculated using the formula ΔSlim = R ln(2S + 1).68 The entropy change calculated per unit of mass of Mn4Nb2 is 8.3 J K−1 kg−1 (Mmol = 3186 g/mol). Although these results do not outperform the best MCE materials,69 it is one of the very few examples of a magnetocaloric effect in a CN-bridged molecule.70 The χT(T) dependence for Fe4Nb2 recorded at 0.1 T is presented in Figure 4. The compound exhibits the highest χT value of 13.0 cm3 K mol−1 at 325 K, which is slightly lower than the expected 15.3 cm3 K mol−1 for four HS FeII (S = 2, and assuming gFe = 2.257,71) and two NbIV (S = 1/2, assuming gNb = 1.98). With cooling, the χT(T) value decreases gradually from 13.0 cm3 K mol−1 at 325 K to ca. 1.6 cm3 K mol−1 at 50 K with

Figure 5. 57Fe Mössbauer spectra of Fe4Nb2 with fitted contributions of the high-spin (HS) and low-spin (LS) FeII.

confirming the presence of two different spin states of FeII in Fe4Nb2. The doublet with a smaller quadrupole splitting (Δ) and lower isomer shift (δ) is assigned to LS FeII and that with a larger quadrupole splitting (Δ) and higher isomer shift (δ) is HS FeII (Table S6 in the Supporting Information). The fractions of the HS state are 65%, 44%, and 17% at 293, 200, and 80 K, respectively. These data confirm that Fe4Nb2 reveals gradual spin crossover behavior. The character of this SCO transition might be related to weak intramolecular interactions between the FeII centers within each molecule. The photomagnetic effect for Fe4Nb2 was studied in the magnetometer sample chamber at 10 K and 1000 Oe magnetic field using several different wavelengths (436, 470, 640, 850, and 740 nm). The strongest increase in the magnetization was observed using 740 nm light (Figure S12 in the Supporting Information). The observed effect is a consequence of a lightinduced excited spin state trapping (LIESST) effect at the LS FeII centers (Figure 4, inset). The light-induced metastable HS state of FeII undergoes quite fast relaxation even at the temperature of irradiation, but complete relaxation to the initial state was only achieved by heating above 60 K, which is the TLIESST for this compound (with 0.8 K/min heating rate).

Figure 4. χT(T) at 0.1 T and M(H) at 1.8 K for Fe4Nb2 before (black points) and after irradiation with 740 nm at 10 K (blue circles). 4024


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Inorganic Chemistry The magnetic moment increases upon irradiation from ∼1.2 to 2.0 cm3 K mol−1 at 10 K, which was observed consistently in several attempts and was independent of the amount of the sample or the measurement setup. This suggests that only some of the LS FeII centers per molecule can be excited to the HS state with light. The SCO and photomagnetic behavior of Fe4Nb2 are similar to those of the pentanuclear trigonalbipyramidal (TBP) molecules Fe II 3 Co III2 and Fe II 3 Fe III2 reported by Dunbar and co-workers.72 The χT(T) dependence for Co4Nb2 measured at 0.1 T is presented in Figure 6. The χT(T) curve gradually decreases

only moderate magnetic interactions between NbIV and CoII spin centers. Finally, compounds Fe4Nb2 and Co4Nb2 are the first examples of discrete bimetallic assemblies based on octacyanoniobate(IV) with metal centers other than NiII and MnII.

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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.6b03134. CShM analysis parameters, additional structural diagrams, and spectral and magnetic data (PDF) Crystallographic data for Co4Nb2 (CIF) Crystallographic data for Fe4Nb2 (CIF) Crystallographic data for Mn4Nb2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for K.R.D.: [email protected]. *E-mail for D.P.: [email protected]. ORCID

Kim R. Dunbar: 0000-0001-5728-7805 Dawid Pinkowicz: 0000-0002-9958-3116 Notes

The authors declare no competing financial interest.

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Figure 6. χT(T) at 0.1 T and M(H) at 1.8 K for Co4Nb2.

ACKNOWLEDGMENTS This work was supported by the Polish Ministry of Science and Higher Education within the Iuventus Plus Programme (grant agreement no. 0370/IP3/2015/73) and by the National Science Centre within the Opus 8 research project (decision no. DEC-2014/15/B/ST5/04465). The research in the Dunbar laboratories was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences Division (Grant Nos. DE-FG02-02ER45999 and DE-SC0012582). K.R.D. also thanks the Robert A. Welch Foundation (A-1449). M.A. gratefully acknowledges the financial support of the Polish Ministry of Science and Higher Education within the “Diamentowy Grant” Programme, grant agreement no. 0041/ DIA/2015/44. The authors thank Mr. Sławomir Bojarowski from the University of Warsaw for the single-crystal XRD measurement of Fe4Nb2. Part of the research was carried out with equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.00-12-023/08).

from 12.8 cm3 K mol−1 at 300 K to 4.0 cm3 K mol−1 at 1.8 K with a more abrupt decrease at low temperatures. At 300 K gCo is assumed to be 2.5 with S = 3/2 and the expected χT value is 12.5 cm3 K mol−1 (gNb = 2.0). This value of the gCo is based on the literature search for distorted octahedral CoII ions.73,74 Since the χT(T) dependence profile can be influenced by magnetic exchange interactions, crystal field effects, and spin− orbit coupling, it is not possible to draw any conclusions regarding the sign of the exchange interaction JCo−Nb between the CN-bridged CoII and NbIV. Both ferromagnetic and antiferromagnetic interactions are possible.57,58 From the inset in Figure 6 it can be seen that the M(H) curve reaches 7.8 μB at 1.8 K and 7 T without an obvious saturation due to the magnetic anisotropy of the CoII centers. The compound does not show any signs of slow magnetic relaxation. All attempts to fit/simulate the experimental curves were unsuccessful due to severe overparametrization.

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CONCLUSIONS In summary, a family of hexanuclear octahedral magnetic molecules based on the paramagnetic eight-coordinate 4d cyanometalate [NbIV(CN)8]4− has been achieved via a buildingblock approach. Isolation of three isostructural molecules with a variation in the 3d cations resulted in different magnetic and physical properties for each molecule. Mn4Nb2 exhibits strong antiferromagnetic coupling through the NbIV−CN−MnII linkage, leading to a high-spin ground state of S = 9 for the molecule. The Fe4Nb2 analogue exhibits gradual thermal spin crossover and a LIESST-type photomagnetic effect at low temperatures which has never been reported previously for this class of hexanuclear magnetic molecule. The magnetic nature of Co4Nb2 requires further studies due to the significant spin− orbit coupling, magnetic anisotropy of the CoII centers, and

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Inorganic Chemistry

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