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Ferromagnetic Exchange Coupling in a Family of MnIII Salen-Type Schiff-Base Out-of-Plane Dimers Chihiro Kachi-Terajima,*,† Rikako Ishii,† Yoshiaki Tojo,† Masato Fukuda,† Yasutaka Kitagawa,‡ Mizuki Asaoka,‡ and Hitoshi Miyasaka*,§ †

Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba 274-8510, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan § Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: A series of Mn I II saltmen dimers, [Mn 2 (5Rsaltmen)2(X)2](A)2n (saltmen2− = N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate); R = H, Cl, Br, MeO, Me; X = H2O, ReO4−, NO3−, N3−, NCS−, A− = ClO4−, PF6−, CF3SO3− for X = H2O) were synthesized and structurally and magnetically investigated to understand the correlation between their intradimer ferromagnetic (FM) interaction and single-molecule magnet (SMM) behavior. All complexes had a similar di-μ-phenolate-bridged out-of-plane dimer structure but displayed different bridging Mn−Oph* distances depending on the R substituents of the saltmen ligand and axial X ligand. Magnetic susceptibility studies revealed intradimer FM coupling (JMn−Mn*), resulting in an ST = 4 ground state for all dimers. However, the magnitude of FM coupling strongly depended on R and X. JMn−Mn* increased with decreasing Mn−Oph* distance but decreased with decreasing Mn−X distance with a relation of H2O ≈ ReO4− > NO3− > N3− ≈ NCS− with a linear trend for R = H, Cl, Me but not for R = Br, MeO. Theoretical investigations revealed that a larger orbital overlap stabilized a FM spin configuration through competition between the orbital degeneracy and on-site Coulomb repulsion of out-of-phase and in-phase orbitals. Most dimers showed typical SMM behavior. The dimers with larger JMn−Mn* tended to have higher blocking temperatures.

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and axial X ligands on the MnIII center. The exchange coupling in the MnIII−(Oph)2−MnIII bridge of linear tetranuclear complexes [Mn(5-Rsaltmen)Ni(pao) (bpy)2]2(ClO4)4 (5Rsaltmen = N,N′-(1,1,2,2-tetramethylethylene)bis(5-Rsalicylideneiminate); bpy = 2,2′-bipyridine) depended on the R substituent (R = H, Cl, Br, and MeO).29 However, the correlation between the structure and intradimer magnetic interaction for the series of MnIII salen-type dimers, consequently providing SMM behavior, is still not apparent. In fact, there are several papers that report an antiferromagnetic (AFM) ground state in MnIII salen-type dimers24,30,31 and a magneto-structural correlation study for bis-μ-alkoxo MnIII dimers.32 There has been no comprehensive study on the exchange interactions in MnIII salen-type out-of-plane dimers. A systematic investigation of magneto-structural correlations in the series of MnIII saltmen dimers will help us to understand their SCM systems.13−18 Here, we present a large series of [Mn2(saltmen)2]2+ dimers, [Mn2(5-Rsaltmen)2(X)2](A)2n (n = 0 or 1) (R = H, Cl, Br, MeO, Me; X = axial ligands with H2O,

INTRODUCTION With the development of superparamagnetic nanosized clusters, called single-molecule magnets (SMMs),1−6 and superparamagnetic isolated chains, known as single-chain magnets (SCMs),7−11 a class of MnIII salen-type out-of-plane dimer complexes (salen2− is N,N′-(ethylene)bis(salicylideneiminate)) have been attracting much attention. This is because superparamagnetic clusters not only have potential to be SMMs with a ferromagnetic (FM) ST = 4 ground state12 but also act as extendible uniaxial anisotropic building blocks for the design of SCMs, as well as their monomer forms.13−23 In fact, the unverified knowledge of “the invariable ST = 4 magnetic nature of MnIII salen-type dimers” provided a strategy to produce the first FM-type SCMs, [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 (saltmen2− = N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate); pao− = pyridine-2-aldoximate; L1 = 4picoline, 4-t-butylpyridine, N-methylimidazole; A− = ClO4−, BF4−, PF6−, ReO4−),16 and the related SCMs.13−18 However, SMM behavior in [Mn2(saltmen)2]2+ dimers with strong uniaxial anisotropy was eventually proved in [Mn2(saltmen)2(ReO4)2]12 and a few derivatives.24−28 Therefore, we decided to explore the variation of FM coupling in the dimers induced by changing the R substitution of the saltmen © XXXX American Chemical Society

Received: April 8, 2017 Revised: May 8, 2017 Published: May 15, 2017 A

DOI: 10.1021/acs.jpcc.7b03336 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C ReO4−, NO3−, N3−, NCS−; A− = noncoordinating counteranion of ClO4−, PF6−, CF3SO3− when X = H2O) (see Chart 1), where

nediamine in methanol/water with salicylaldehyde or 5-chloro-, 5-bromo-, 5-methoxy, or 5-methylsalicylaldehyde, respectively. Syntheses of [Mn 2 (saltmen) 2 (H 2 O) 2 ](ClO 4 ) 2 (1), [Mn2(saltmen)2(NCS)2] (5), [Mn2(5-Clsaltmen)2(H2O)2](ClO4)2 (6), and [Mn2(5-Brsaltmen)2(H2O)2](ClO4)2 (11) were performed according to literature methods.15,33 CAUTION: Perchlorate salts are potentially explosive and should only be handled in small quantities. [Mn2(saltmen)2(NO3)2] (3). To a boiling methanol solution (160 mL) of H2saltmen (3.244 g, 10.0 mmol) was added Mn(O2CCH3)3·2H2O (2.861 g, 10.0 mmol) and subsequently NaNO3 (0.892 g, 10.5 mmol). The resulting solution was heated under reflux for 30 min, and then water (40 mL) was slowly added during heating. The filtrate was left to stand for 1 week at room temperature to form block crystals of 3. The crystals were collected by suction filtration. Yield: 3.413 g, 3.88 mmol, 78% (based on Mn). Anal. Calcd for C40H44N6O10Mn2: C, 54.67; H, 5.05; N, 9.56. Found: C, 54.67; H, 5.00; N, 9.56. IR (KBr): ν(CN), 1607 cm−1; ν(NO3), 1385 cm−1. [Mn2(5-Clsaltmen)2(ReO4)2] (7). Block crystals were obtained by slow diffusion of a solution of 6 (0.059 g, 0.053 mmol) in methanol (10 mL) into a solution of NaReO4 (0.051 g, 0.187 mmol) in methanol (5 mL). Yield: 0.041 g, 0.029 mmol, 56% (based on Mn). Anal. Calcd for C40H40N4O12Cl4Mn2Re2: C, 34.49; H, 2.89; N, 4.02. Found: C, 34.46; H, 2.90; N, 4.08. IR (KBr): ν(CN), 1605 cm−1; ν(Re−O), 924 cm−1. [Mn2(5-Clsaltmen)2(NO3)2] (8). The synthetic procedure for 8 is similar to that for 3, starting from Mn(O2CCH3)3·2H2O (1.341 g, 5.00 mmol), NaNO3 (0.433 g, 5.10 mmol), and H25Clsaltmen (1.967 g, 5.00 mmol). Yield: 1.515 g, 2.98 mmol, 60% (based on Mn). Anal. Calcd for C40H40N6O10Cl4Mn2: C, 47.27; H, 3.97; N, 8.27. Found: C, 47.04; H, 3.97; N, 8.21. IR (KBr): ν(CN), 1611 cm−1; ν(NO3), 1385 cm−1. [Mn2(5-Clsaltmen)2(N3)2] (9). Slow diffusion of a solution of 6 (0.056 g, 0.099 mmol) in methanol (6 mL) into a solution of NaN3 (0.014 g, 0.215 mmol) in methanol (6 mL) yielded brown crystals of 9. Yield: 0.040 g, 0.082 mmol, 83% (based on Mn). Anal. Calcd for C40H40N10O4Cl4Mn2: C, 49.20; H, 4.13; N, 14.34. Found: C, 48.91; H, 4.12; N, 14.47. IR (KBr): ν(N3), 2039 cm−1; ν(CN), 1609 cm−1. [Mn2(5-Clsaltmen)2(NCS)2] (10). Slow diffusion of a solution of 6 (0.056 g, 0.099 mmol) in methanol (6 mL) into a solution of NaNCS (0.016 g, 0.197 mmol) in methanol (6 mL) yielded brown crystals of 10. Yield: 0.048 g, 0.095 mmol, 96% (based on Mn). Anal. Calcd for C42H40N6O4Cl4Mn2S2: C, 50.01; H, 4.00; N, 8.33. Found: C, 49.71; H, 3.90; N, 8.48. IR (KBr): ν(NCS), 2056 cm−1; ν(CN), 1611 cm−1. [Mn2(5-Brsaltmen)2(ReO4)2] (12). The experimental procedure for 7 was used to produce 12 using 11 (0.072 g, 0.055 mmol) instead of 6. Yield: 0.071 g, 0.045 mmol, 82% (based on Mn). Anal. Calcd for C40H40N4O12Br4Mn2Re2: C, 30.59; H, 2.57; N, 3.57. Found: C, 30.50; H, 2.60; N, 3.55. IR (KBr): ν(CN), 1601 cm−1; ν(Re−O), 922 cm−1. [Mn2(5-Brsaltmen)2(NO3)2] (13). The synthetic procedure for 13 was similar to that presented for 3, starting from Mn(O2CCH3)3·2H2O (1.341 g, 5.00 mmol), NaNO3 (0.433 g, 5.10 mmol), and H25-Brsaltmen (2.411 g, 5.00 mmol). Yield: 2.167 g, 1.81 mmol, 73% (based on Mn). Anal. Calcd for C40H40N6O10Br4Mn2: C, 40.23; H, 3.38; N, 7.04. Found: C, 40.13; H, 3.56; N, 7.05. IR (KBr): ν(CN), 1607 cm−1; ν(NO3), 1385 cm−1.

Chart 1

R = H with X = H2O, 1 (A− = ClO4−);33 ReO4−, 2;12 NO3−, 3; N3−, 4;24 NCS−, 5;33 R = Cl with X = H2O, 6 (A− = ClO4−); ReO4−, 7; NO3−, 8; N3−, 9; NCS−, 10; R = Br with X = H2O, 11 (A− = ClO4−);15 ReO4−, 12; NO3−, 13; N3−, 14; NCS−, 15; R = MeO with X = H2O, 16 (A− = ClO4−); ReO4−, 17; NO3−, 18; N3−, 19; NCS−, 20; H2O, 21 (A− = PF6−); H2O, 22 (A− = CF3SO3−); and R = Me with X = H2O, 23 (A− = ClO4−); ReO4−, 24; NO3−, 25; N3−, 26; NCS−, 27, and discuss the effects arising from ligand R-substitution and MnIII−(μ2-Oph)2− MnIII core modification on their FM properties that influence SMM behavior.

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EXPERIMENTAL SECTION General Procedures and Materials. All chemicals and solvents were of commercial grade and used without further purification. H25-Rsaltmen (R = H, Cl, Br, MeO, Me) ligands were synthesized by the reaction of 1,1,2,2-tetramethylethyleB


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The Journal of Physical Chemistry C [Mn2(5-Brsaltmen)2(N3)2] (14). Brown crystals were obtained by slow diffusion of a solution of 11 (0.065 g, 0.050 mmol) in methanol (10 mL) into a solution of NaN3 (0.012 g, 0.185 mmol) in methanol (5 mL). Yield: 0.041 g, 0.036 mmol, 72% (based on Mn). Anal. Calcd for C40H40N10O4Br4Mn2: C, 41.62; H, 3.49; N, 12.13. Found: C, 41.08; H, 3.40; N, 12.31. IR (KBr): ν(N3), 2039 cm−1; ν(CN), 1603 cm−1. [Mn2(5-Brsaltmen)2(NCS)2] (15). NaSCN (0.111 g, 1.369 mmol) was added to a solution of 11 (0.652 g, 0.500 mmol) in methanol (50 mL). The mixture was stirred for 3 h at room temperature. The brown microcrystals that appeared were collected by suction filtration. Recrystallization was performed to obtain large crystals of 15 in methanol (200 mL). Yield: 0.322 g, 0.271 mmol, 54% (based on Mn). Anal. Calcd for C42H40N6O4Br2Mn2S2: C, 42.52; H, 3.40; N, 7.08. Found: C, 42.35; H, 3.46; N, 7.00. IR (KBr): ν(CN), 1605 cm−1; ν(NCS), 2066 cm−1. [Mn2(5-MeOsaltmen)2(H2O)2](ClO4)2 (16). The synthetic procedure for 16 was similar to that for 3, starting from Mn(O2CCH3)3·2H2O (0.361 g, 1.35 mmol), NaClO4 (0.184 g, 1.47 mmol), and H25-MeOsaltmen (0.509 g, 1.32 mmol). Yield: 0.587 g, 0.529 mmol, 78% (based on Mn). Anal. Calcd for C44H58N4O18Cl2Mn2: C, 47.62; H, 5.09; N, 5.05. Found: C, 47.45; H, 5.08; N, 5.03. IR(KBr): ν(CN), 1605 cm−1; ν(Cl− O), 1097, 1147 cm−1. [Mn2(5-MeOsaltmen)2(ReO4)2] (17). Block crystals of 17 were obtained by a counterion exchange reaction from ClO4− to ReO4−. NaReO4 (0.030 g, 0.110 mmol) in ethanol (7 mL) was layered on a solution of 16 (0.055 g, 0.050 mmol) in ethanol (7 mL). Black block crystals were formed after 5 days. The crystals were collected by suction filtration and washed with a minimum amount of ethanol. Yield: 0.017 g, 0.012 mmol, 24.7% (based on Mn). Anal. Calcd for C44H52Mn2N4O16Re2: C, 38.43; H, 3.81; N, 4.07. Found: C, 37.96; H, 3.86; N, 4.12. IR (KBr): ν (CN), 1593 cm−1; ν (Re−O), 923 cm−1. [Mn2(5-MeOsaltmen)2(NO3)2] (18). The synthetic procedure for 19 was similar to that for 3, starting from Mn(O2CCH3)3· 2H2O (1.341 g, 5.00 mmol), NaNO3 (0.510 g, 6.00 mmol), and H25-MeOsaltmen (1.922 g, 5.00 mmol). Yield: 1.628 g, 1.63 mmol, 65% (based on Mn). Anal. Calcd for C44H52N6O14Mn2: C, 52.91; H, 5.25; N, 8.41. Found: C, 52.84; H, 5.26; N, 8.48. IR (KBr): ν(CN), 1599 cm−1; ν(NO3), 1385 cm−1; ν(MeO), 2833, 1042 cm−1. [Mn2(5-MeOsaltmen)2(N3)2] (19). To a solution of 16 (0.550 g, 0.496 mmol) in methanol (20 mL) was added NaN3 (0.071 g, 1.092 mmol). The mixture was stirred for 3 h at room temperature. The green-brown microcrystals that appeared were collected by suction filtration. Large block crystals were obtained by recrystallization from methanol (50 mL). Yield: 0.161 g, 0.168 mmol, 34% (based on Mn). Anal. Calcd for C44H52N10O8Mn2: C, 55.12; H, 5.47; N, 14.61. Found: C, 55.08; H, 5.30; N, 14.74. IR (KBr): ν(CN), 1599 cm−1; ν(N3), 2041 cm−1. [Mn2(5-MeOsaltmen)2(NCS)2] (20). The same procedure as that presented for 20 was used to prepare 21 from 16 (0.219 g, 0.198 mmol) and NaSCN (0.036 g, 0.444 mmol). Yield: 0.074 g, 0.075 mmol, 38% (based on Mn). Anal. Calcd for C46H52N6O8Mn2S2: C, 55.75; H, 5.29; N, 8.48. Found: C, 55.48; H, 5.25; N, 8.45. IR (KBr): ν(CN), 1597 cm−1; ν(NCS), 2077 cm−1. [Mn2(5-MeOsaltmen)2(H2O)2](PF6)2 (21). The synthesis of 17 was similar to that of 3, starting from Mn(O2CCH3)3·2H2O

(2.287 g, 8.53 mmol), NH4PF6 (1.521 g, 9.33 mmol), and H25MeOsaltmen (3.250 g, 8.45 mmol). Yield: 4.035 g, 3.36 mmol, 79% (based on Mn). Anal. Calcd for C44H56N6O10Mn2F12P2: C, 44.01; H, 4.70; N, 4.67. Found: C, 43.93; H, 4.67; N, 4.64. IR (KBr): ν(CN), 1603 cm−1; ν(P−F), 557, 839 cm−1. [Mn2(5-MeOsaltmen)2(H2O)2](CF3SO3)2 (22). The synthetic procedure for 18 was similar to that for 3, starting from Mn(O2CCH3)3·2H2O (0.270 g, 1.01 mmol), NaCF3SO3 (0.172 g, 1.00 mmol), and H25-MeOsaltmen (0.384 g, 1.00 mmol). Yield: 0.400 g, 0.331 mmol, 66% (based on Mn). Anal. Calcd for C46H56N4O16F6Mn2S2: C, 45.70; H, 4.67; N, 4.63. Found: C, 45.59; H, 4.62; N, 4.64. IR (KBr): ν(CN), 1603 cm−1; ν(CF3SO3), 1286, 1032, 637 cm−1. [Mn2(5-Mesaltmen)2(H2O)2](ClO4)2 (23). The synthesis of 23 was similar to that of 3, starting from Mn(O2CCH3)3·2H2O (0.181 g, 0.514 mmol), NaClO4 (0.114 g, 0.812 mmol), and H25-Mesaltmen (0.134 g, 0.500 mmol) and using ethanol instead of methanol. Yield: 0.135 g, 0.129 mmol, 50% (based on Mn). Anal. Calcd for C44H56Mn2N4O14Cl2: C, 49.90; H, 5.18; N, 5.19. Found: C, 50.53; H, 5.40; N, 5.36. IR (KBr): ν(CN), 1600 cm−1; ν(Cl−O), 1099 cm−1. [Mn2(5-Mesaltmen)2(ReO4)2] (24). [Mn2(5Mesaltmen)2(H2O)2](PF6)2 (0.123 g, 0.10 mmol) was added to methanol (20 mL) and stirred. The resulting solution was added to a solution of NaReO4 (0.082 g, 0.30 mmol) in methanol (10 mL), stirred for 30 min, and then filtered. The filtrate was left to stand for 2 days at room temperature, resulting in the formation of block crystals. Yield: 0.107 g, 0.082 mmol, 82% (based on Mn). Anal. Calcd for C44H52Mn2N4O12Re2: C, 40.30; H, 4.00; N, 4.27. Found: C, 40.21; H, 4.26; N, 4.22. IR (KBr): ν(CN), 1597 cm−1; ν(Re−O), 924 cm−1. [Mn2(5-Mesaltmen)2(NO3)2] (25). The synthetic procedure for 25 was similar to that presented for 3, starting from Mn(O2CCH3)3·2H2O (0.403 g, 1.5 mmol), NaNO3 (0.124 g, 1.5 mmol), and H25-Mesaltmen (0.528 g, 1.5 mmol). Yield: 0.383 g, 0.41 mmol, 55% (based on Mn). Anal. Calcd for C44H52Mn2N6O10: C, 56.53; H, 5.61; N, 8.99. Found: C, 56.19; H, 5.55; N, 8.85. IR (KBr): ν(CN), 1600 cm−1; ν(NO3), 1385 cm−1. [Mn2(5-Mesaltmen)2(N3)2] (26). The synthesis of 26 was similar to that of 3, starting from Mn(O2CCH3)3·2H2O (2.693 g, 10.04 mmol), NaN3 (0.691 g, 10.63 mmol), and H25Mesaltmen (3.438 g, 9.754 mmol). The resulting black microcrystals were collected by suction filtration and washed with a small amount of methanol. Yield: 2.501 g, 2.795 mmol, 55.68% (based on Mn). Anal. Calcd for C44H52Mn2N10O4: C, 58.52; H, 5.86; N, 15.82. Found: C, 59.06; H, 5.86; N, 15.65. IR (KBr): ν(CN), 1617 cm−1; ν(N3), 2047 cm−1. Single crystals suitable for X-ray analysis were obtained by slow diffusion of a methanol solution of NaN3 into a methanol solution of Mn(CH3COO)3·2H2O and H25-Mesaltmen. The XRD powder pattern simulated from the single-crystal structure matched that of the experimental XRD pattern of the black microcrystals obtained during synthesis (Figure S1). [Mn2(5-Mesaltmen)2(NCS)2] (27). The synthetic procedure for 27 was similar to that for 24, starting from [Mn2(5Mesaltmen) 2(H2O) 2](PF 6)2 (0.307 g, 0.25 mmol) and NH4NCS (0.089 g, 1.2 mmol. Yield: 0.085 g, 0.091 mmol, 36% (based on Mn). Anal. Calcd for [Mn2(5-Mesaltmen)2(NCS)2], C46H52Mn2N6O4S2: C, 59.60; H, 5.65; N, 9.07. Found: C, 59.40; H, 5.66; N, 8.98. IR (KBr): ν(CN), 1597 cm−1; ν(NCS), 2065 cm−1. C


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The Journal of Physical Chemistry C

2,12 3, 4,24 6, 8, 11,15 13, 16, 18, 20−23, 25, and 26 were synthesized by method (i), while most of the compounds with X = ReO4−, N3−, or NCS−, that is, 5,33 7, 9, 10, 12, 14, 15, 17, 19, 24, and 27, were prepared by method (ii) using the sodium or ammonium salt of the corresponding counteranion. The high coordination affinity of X = ReO4−, NO3−, N3−, and NCS− enabled easy exchange of the axial ligand from H2O to X, and the low solubility of the X-coordinated materials allowed isolation of their pure crystals in high yield. It should be noted that recrystallization should be performed carefully for the compounds with X = ReO4− and N3−. This is because ReO4− and N3−, which can act as bridging ligands, might encourage molecular rearrangement to a chain form with the same formula. Indeed, to prevent this, [Mn2(saltmen)2(ReO4)2] (2) was synthesized by a reported method12 (its recrystallization resulted in molecular rearrangement to form the ReO4-bridged dimer [Mn(saltmen)(ReO4)(μ-ReO4)Mn(saltmen)(H2O)]). Although a few exceptions are found, the out-of-plane dimer form is relatively stable for many complexes of [Mn(5Rsaltmen)]+ with R = H, Cl, Br, MeO, and Me possessing monodentate X axial ligands. General Structural Description of the Dimers. All compounds, except for 8, were structurally characterized by single-crystal X-ray analysis (the twin form of the crystal of 8 made it difficult to complete its analysis, but its similar dimer structure was confirmed). The crystallographic data for all compounds are summarized in Tables S1−S4, and selected bond distances and angles are listed in Tables S5−S10. Representative ORTEP drawings of the dimer series with R = Me, 23−27, are depicted in Figure 1 (those of the other complexes are given in Figures S2−S4). Note that 19 and 25 have two independent molecules in their unit cell, both of which are almost structurally identical (see Tables S9 and S10, respectively). All compounds form an out-of-plane dimeric structure selfbridged by two phenolate oxygen atoms of the 5-Rsaltmen2− ligands, producing a di-μ2-phenolate bridging motif with an inversion center between MnIII ions. This basic structural feature is identical to that observed in the previously reported [Mn2(saltmen)2]2+ family.12,15,24−29,33 Each MnIII site has an axially elongated octahedral geometry (quasi-tetragonal geometry) as typically seen in MnIII high-spin complexes with Jahn− Teller distortion, in which the equatorial coordination sites are occupied by a N2O2 atom set of the 5-Rsaltmen2− tetradentate ligand with average bond distances of ⟨Mn−N⟩av = 1.987 Å and ⟨Mn−O⟩av = 1.884 Å, and the two elongated axial (Jahn−Teller axis) positions are occupied by the dimer-forming phenolate oxygen O1* with Mn−O1* = 2.3471(13)−3.7218(5) Å and the apical ligand X with ⟨Mn−X⟩av = 2.179 Å. This bridging motif, especially the Mn−O1* bond distance and relevant angles related to a parallelogram consisting of Mn−O1−Mn*− O1*, is very important to interpret the intradimer magnetic properties of these compounds; this relationship is the main subject of this work (vide infra). The bond distances and angles of the bridging motifs of the compounds are summarized in Table 1 together with their magnetic parameters described hereafter. Crystal Packing of the Dimers. To interpret the magnetic properties of this type of Mn dimer, a precise evaluation of interdimer interactions, which are closely associated with the packing behavior of dimers, is crucial. Even in this series, we found four types of crystal packing (Scheme 1). These classifications are not related to the space group of the crystal

Physical Measurements. Infrared spectra were measured using the KBr disk method on Shimadzu FT-IR-8600 and JASCO FT/IR−4100 spectrophotometers. Elemental analyses were performed using a PerkinElmer 2400 CHN elemental analyzer. Magnetic susceptibility measurements were conducted on a Quantum Design superconducting quantum interference device magnetometer (MPMS-XL) in the temperature range from 1.8 to 300 K. The dc magnetic field ranged from −5 to 5 T or from −7 to 7 T. We performed ac measurements at various frequencies from 0.5 to 1488 Hz with an ac field amplitude of 2 or 3 Oe. The measurements were performed on polycrystalline samples that were restrained by n-eicosane to avoid reorientation induced by the applied magnetic field. Experimental data were corrected by the trace background values of the sample holder and n-eicosane matrix and by the diamagnetic contribution of each sample calculated from Pascal constants.34 Crystallography. Crystal data were collected on a Rigaku CCD diffractometer (Saturn 70) with graphite-monochromated Mo−Kα radiation (λ = 0.71070 Å) for 3, 6, 7, 9, 12−16, 18, 20, and 22. Data for 12 were collected on a Rigaku AFC7R diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). The structures were solved by direct methods (SIR92,35 SIR9736) for 6, 7, 9, 13, 14, 16, 18, and 22 or heavyatom Patterson methods (PATTY)37 for 3, 12, 15, and 20. All structures were expanded using Fourier techniques (DIRDIF99). 38 All calculations were performed using the CrystalStructure crystallographic software package.39 Data for 10, 17, 19, 21, and 23−27 were collected on a Bruker SMART APEX and APEX II CCD area detector diffractometer with a graphite monochromator and Mo Kα radiation (λ = 0.71073 Å). An empirical absorption correlation was applied using the SADABS program.40 The structures were solved by direct methods (SHELXS 97) and refined by full-matrix least-squares calculations on F2 (SHELXL-97, SHELXL-2014/7 for 24 and 27) with the SHELX-TL program package.41,42 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were fixed in calculated positions and refined with a riding model. The final cycle of full-matrix least-squares refinement on F2 based on observed reflections (I > 2.00σ(I)) was used, with unweighted and weighted agreement factors of R1 = Σ∥Fo| − | Fc∥/Σ|Fo| (I > 2.00σ(I) and all data) and wR2 = [Σ(w(Fo2 − Fc2)2)/Σw(Fo2)2]1/2 (all data), respectively. A Sheldrick weighting scheme was used. Crystal data and details of the structure determination for all compounds are summarized in Tables S1−S4. CCDC-1535047−1535049 for 3−7, CCDC1535053−1535066 for 9−23, CCDC-1535057 and 1535068 for 25 and 26, and CCDC-1542904 and 1542903 for 24 and 27 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/ data_request/cif. Computer Calculations. All calculations were carried out using the Gaussian09 program package.43

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RESULTS AND DISCUSSION Synthesis. The present compounds were synthesized by one of the following two synthetic methods: (i) one-pot reaction in an alcohol/water mixture containing all starting materials of H25-Rsaltmen, MnIII(O2CCH3)3·2H2O, and the sodium salt of the counteranion that became X− or A− of the products, and (ii) the counterion exchange reaction of [Mn2(5Rsaltmen)2(H2O)2](A)2 (A− = ClO4−, PF6−). Compounds 1,33 D


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The molecular packing of 26 is depicted in Figure S9 as a representative example. Magnetic Properties of Dimers: Direct Current (dc) Magnetic Measurements. Magnetic properties of some MnIII dimers have already been reported.12,24,33 Magnetic interactions between MnIII ions through the biphenolate bridge are typically FM, and their magnitude is influenced by the distance of the Mn−O1* bridge,33 although some exceptions with AFM coupling have been reported.24,30,31 The dc magnetic susceptibility of polycrystalline samples restrained by n-eicosane was measured in the temperature range from 1.82 to 300 K at 1 kOe. The magnetic behavior of the dimers can be classified into three types: Type I, including 2,12 3, 4,24 5,33 7, 8, 10, 12, 13, 17, and 24−27; Type II, 1,33 6, 11, 16, and 21−23; and Type III, compounds 9, 14, 15, and 18−20. The temperature dependence of χT of 3, 16, and 9 as representative examples of Type I, II, and III, respectively, is depicted in Figure 2. The χT values of all dimers were about 6.0 cm3 K mol−1 at 300 K. This value corresponds to the spin-only value of 6.0 cm3 K mol−1 (for g = 2.0) for a set of two MnIII ions (S = 2). In Type-I compounds, the χT product increases gradually as temperature falls to reach a maximum value of around 9 cm3 K mol−1 and then decreases to about 8 cm3 K mol−1 at 1.82 K (Figure S10). This behavior is typical for [Mn2(saltmen)2(X)2]2+ complexes that exhibit a FM interaction between MnIII ions with strong uniaxial anisotropy, i.e., zero-field splitting (ZFS). The decrease of the χT product at low temperature could possibly be associated with an interdimer AFM interaction, as well as the effect of ZFS. The χT products for the Type-II series decreased more rapidly than those of the Type-I compounds at low temperature (below 10 K) (Figure S11). The Type-III complexes showed different magnetic properties from those of the other types of dimers (Figure S12). The χT product of Type-III compounds was almost constant above 50 K and then decreased to less than 5 cm3 K mol−1 at 1.82 K without forming a peak; this series behaved as though the MnIII dimers were AFM coupled. The magnetic data were simulated using a dimer model with the following spin Hamiltonian: H = −2JMn−Mn*(SMn·SMn*) + 2DMnSMn,z2 + zJ′⟨ST⟩, where JMn−Mn* is the exchange coupling constant in a MnIII dimer, DMn is the ZFS parameter for a MnIII ion, and zJ′ is the total interdimer interaction taken into account in the framework of the mean-field approximation (i.e., z represents the number of neighboring dimers interacting with a dimer, and J′ is the mean value of one couple of intermolecules).44,45 The best-fit parameters for all dimers are listed in Table 1 together with their intradimer Mn−O1* and Mn···Mn* distances. Independent of their type of magnetic behavior, JMn−Mn* of all dimers were evaluated to be FM (JMn−Mn* > 0), and it was estimated that DMn < 0 in the range from −2.39 to −9.25 K. The magnitudes of JMn−Mn*, g, and DMn are consistent with those of previously reported MnIII dimers.12,24,33 The JMn−Mn* value of Type-I compounds is relatively larger than those of Type-II and -III ones, revealing the existence of a strong FM interaction between Mn ions via the biphenolate bridge. Although the Type-I compounds 3, 5,33 7, 10, and 12 possessed interdimer short contacts (see Scheme 1), the interdimer interactions (zJ′) in these compounds were estimated to be negligibly small. The magnetic properties of 2,12 17, 24, 26, and 27 with the orthorhombic Pbca space group were classified as Type I, although 17 and 24 have close

Figure 1. ORTEP drawings of a series of dimers with 5-Mesaltmen ligands: 23, 24, 25 (one unit in an asymmetric set), 26, and 27. Hydrogen atoms are omitted for clarity (50% probability thermal ellipsoids; symmetry operations (*) − x, −y + 1, −z for 23; −x + 2, −y, −z + 1 for 24; −x + 2, −y + 2, −z + 1 for 25; −x, −y + 1, −z + 1 for 26; −x, −y, −z + 2 for 27).

lattice but to the molecular arrangements induced by supramolecular interactions. Compounds 1,33 6, 11,15 16, 21, 22, and 23, which have the axial ligand X = H2O and a noncoordinating counteranion (A− = ClO4−, PF6−, or CF3SO3−), make up the first group, even though their R substituents and A− counteranions are different. The representative packing diagram of 16 is depicted in Figure S5. These dimers form supramolecular one-dimensional columns mediated by π−π stacking and hydrogen bonding between neighboring dimers. Each column runs along the a axis, which is separated from the nearest columns by A− counteranions. Compounds 3, 5,33 7, 10, 12, 15, 18, and 20 (monoclinic space group); 13 and 25 (triclinic space group); and 17 and 24 (orthorhombic Pbca space group) make up the second group. It should be noted that no significant interdimer π−π contacts or hydrogen bonding was found between neighboring dimers in this group, but interdimer short contacts of less than 3.6 Å were found between adjacent 5-Rsaltmen ligands. The third group consists of compounds 4,24 9, 14, and 19, which have an N3− axial ligand. These compounds have a close contact between the 5-Rsaltmen ligand and terminal N atom of the N3− ligand in addition to the short contact between adjacent 5-Rsaltmen ligands found in the second group. Figures S6, S7, and S8 show near interdimer contacts ( NO3− > N3− ≈ NCS−, which differs from the spectrochemical series ranking of ReO4− < N3− < NO3− < H2O < NCS−.52,53 This is not surprising because different metal centers were used. The magnetochemical series found for the spin-state mixing in iron(III) porphyrins displayed the order H2O < ReO4− < NCS− < N3−.54,55 Thus, the JMn−Mn* vs Mn−X trend with the JMn−Mn* ∝ Mn−X relationship in our study (Figure 3b) is more similar to the magnetochemical series than the spectrochemical one, which is the reverse trend to the JMn−Mn* ∝ (Mn−O1*)−1 relationship. There is no clear correlation between JMn−Mn* and other structural parameters such as the Mn−O−Mn* angle (Figure S16). Computational Evaluation of JMn−Mn*. To clarify the mechanism of the FM behavior of the series of the MnIII saltmen dimer complexes, we carried out density functional

Figure 3. (a) Correlation between the Mn−O1* distance and FM exchange parameter JMn−Mn* of the dimer series. The plot focuses on the R substituent on the saltmen2− ligand. (b) Correlation between the Mn−X distance and JMn−Mn* classified by the axial X ligand. H


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The Journal of Physical Chemistry C theory (DFT) calculations. Within the family of dimer complexes, we focused here on the neutral species (15 complexes; Table S11). Dimers 1,33 6, 11, 16, 21, 22, and 23 with a positive charge accompanying the counteranion (ClO4−, PF6−, and CF3SO3−) were excluded to avoid the effect of external electrostatic field confounding the evaluation of magnetic interactions. To calculate the electronic structures of the neutral dimers, we first constructed model structures (Table S11) based on atomic coordinates obtained from X-ray crystal structure analyses. All DFT calculations were performed using the Becke three-parameter Lee−Yang−Parr hybrid functional set (B3LYP),56 with Lanl2dz and 6-31+G* basis sets employed for Re and other atoms, respectively. The spinpolarized electronic structures were approximated by a spinunrestricted broken symmetry method.57,58 To examine the magnetic interactions between MnIII ions, we evaluated the effective exchange integral (Jab) value (i.e., orbitalaveraged exchange integral value) in the following Heisenberg Hamiltonian H̑ = −2Jab Sȃ ·S̑ b

Figure 5. Relationship between experimentally obtained JMn−Mn* and calculated Jab. The dashed line shows where JMn−Mn* equals Jab.

values are exceptions. This is attributed to an issue with the functional set of the DFT method used (vide infra). Therefore, we only considered the ten dimers 2, 3, 4, 5, 7, 10, 13, 17, 24, and 27 showing a linear scale in the Jab vs JMn−Mn* plot (Figure 5 and Table S1) in our discussion of the origin of their FM exchange interactions. As a representative example, the magnetic orbitals of the AFM state of 2 are depicted in Figure 6a. This figure contains eight magnetic orbitals, i.e., four occupied (HONO−HONO− 3) and four unoccupied (LUNO−LUNO+3) orbitals, because

(1)

where Sȃ and Sb̑ are total spin operators for Mn a and b (here Sa = Sb = 4/2), respectively. Note that Jab corresponds to JMn−Mn* in the simulation of the experimental data (vide supra). To calculate Jab, we used Yamaguchi’s approximate spin-projection (AP) procedure Jab =

E AFM − EFM 2 ⟨S ⟩ − ⟨S̑ ⟩AFM ̑ 2 FM

(2)

2̑ XM

where E and ⟨S ⟩ denote the total energy and expectation values of S2, respectively, for the state XM, which is either an AFM or FM coupling state.59−61 To explain the magnetic behavior of the dimers, natural orbital (NO) analyses were carried out. The NOs were determined by diagonalizing their first-order density matrices as XM

ρ(r, r′) =

∑ niϕi*(r)ϕi(r′)

(3)

i th

62,63

where ni denotes the occupation number of the i NO (ϕi) and is related to the overlap between α and β orbitals. If ni is close to 1.0, α and β orbitals are completely polarized, and two electrons are localized at each Mn site. In the series considered here, ni should be equal to 2.0 because α and β orbitals are fully delocalized over the two Mn sites. In addition, the overlap between the spin-polarized α and β orbitals (Ti) in the AFM state can be estimated from ni as follows Ti = ni − 1

(4)

The NOs are usually arranged in the order of ni, and the highest occupied NO (n > 1) and lowest unoccupied NO (n < 1) are formally defined as HONO and LUNO, respectively. The calculated Jab values are summarized in Table 1 together with experimental JMn−Mn* ones, and calculated total energies and ⟨S2̑ ⟩ values used to evaluate Jab are given in Table S12. Although the experimentally obtained JMn−Mn* values are within about a few Kelvin, the absolute values of Jab quantitatively reproduced them. However, the five complexes 14, 15, 18, 19, and 20 displayed negative Jab values, albeit very small, while all of their experimental results were positive. The relationship between Jab and JMn−Mn* plotted in Figure 5 reveals the reliability of the calculated Jab values for most of the complexes; only the five complexes 14, 15, 18, 19, and 20 with small |Jab|

Figure 6. (a) NOs of magnetic orbitals of 2 (left: snapshots, right: simplified diagrams), where α and β NOs are superposed in the same picture. (b) Correlation between Jab and averaged overlap (Tave) of the magnetic orbitals. The dashed line represents the least-squares fit. I


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The Journal of Physical Chemistry C of the four spins of each MnIII ion. These orbitals mainly consisted of localized Mn atomic orbitals, as shown in the schematic orbital representations (Figure 6a). The other dimer complexes displayed quite similar NO configurations to those of 2. Table S13 reveals that ni of the magnetic orbitals is close to 1.0, indicating that the α and β orbitals are fully polarized on each Mn atom because of the strong static electron correlation. The overlap (Ti) can be calculated using ni in eq 4. The relation between Jab and the orbital-averaged overlap (Tave) Tave

1 = 4

1 ∑ Ti = 4 i

∑ (ni − 1) i

two orbitals become energetically degenerate. This situation is favorable for FM interactions. From this viewpoint, we evaluated the orbital energy difference (Δε) between out-ofphase and in-phase orbitals and compared the Δε values for AFM and FM states, Δε(AFM) and Δε(FM), respectively (Figure 7b). If Δε(AFM) is much larger than Δε(FM), the complex preferably adopts the FM state. Here, we define ΔΔε in eqs 6a−6c, and examine the relation between ΔΔE and Jab values.

(5)

where i = HONO−HONO−3, was examined by plotting these values, as shown in Figure 6b. Usually, a smaller overlap induces FM coupling because of the orbital energy degeneracy. However, in these complexes, the intensity of the FM interaction is proportional to the orbital overlap between magnetic orbitals; namely, a larger overlap provides stronger FM coupling. A similar phenomenon was observed for the FM behavior of pyrazolate-bridged binuclear CuII complexes reported by Okawa et al.,64 in which large orbital overlap resulted in FM coupling between CuII spins, which the authors explained by orbital complementarity. Therefore, we focused on the phase relations of occupied and unoccupied magnetic orbitals. As shown in Figure 6a, HONOs and LUNOs are composed of out-of-phase and in-phase orbitals, respectively, in terms of d−d interactions, except for the HONO−2 and LUNO+2 pair. For example, HONO−3 and LUNO+3 consisted of dz2-type orbitals of MnIII ions. To determine the phase relations between two MnIII ions, whole molecular and electronic structures must be taken into account. However, as illustrated in Figure 7a, with increasing overlap between two dz2-type orbitals, which leads to direct interaction between MnIII ions, the HONO−3 (out-of-phase) and LUNO+3 (inphase) become more unstable and stable, respectively, as the

Δε(AFM) = εin ‐ phase(AFM) − εout ‐ of ‐ phase(AFM)

(6a)

Δε(FM) = εin ‐ phase(FM) − εout ‐ of ‐ phase(FM)

(6b)

ΔΔε = Δε(AFM) − Δε(FM)

(6c)

where ε is the orbital energy. Under the mean-field approximation, ΔΔε is associated with the Coulomb repulsion at the Mn ion (see the Appendix in the Supporting Information). The calculated ΔΔε values have a positive correlation with Jab, as shown in Figure 7c, which means that the orbital degeneracy is closely related to the Jab value. In addition, this result supports the assumption that a larger overlap induces larger degeneracy to provide a FM spin arrangement. To consider the relation between the orbital overlap and Jab values in detail, we next examined the relation between the structural parameters and Jab values using the three structural parameters of Mn−Mn distance (rMn−Mn), Mn−O1* distance (rMn−O), and Mn−X distance (rMn−X), as illustrated in Figure 8.

Figure 7. Illustrations of the relationship between (a) orbital overlap and orbital energy gap (ΔΔE) and (b) orbital energy and spin coupling state. (c) Correlation between Jab and ΔΔE between out-ofphase and in-phase orbitals. The dashed line represents the leastsquares fit.

Figure 8. Correlations between (a) Jab and structural parameters (bond lengths of Mn−O*, Mn···Mn*, and Mn−X) and (b) averaged overlap and structural parameters. Dashed lines represent the leastsquares fits for respective structural parameters. J


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26; and Type D, 6, 11,15 16, and 21−23. These classifications are associated with the magnitudes of JMn−Mn* and zJ′. The Type-I dimers in the dc data classification are classified here into Type A, B, and C based on the magnitude of JMn−Mn*. Type-I dimers 2,12 4,24 7, 8, and 12 with a relatively large JMn−Mn* displayed higher blocking temperatures than other dimers (Type A). As a representative example of a Type-A complex, the ac susceptibility data for 8 are depicted in Figure 9a (the data for 7 and 12 are presented in Figure S17), showing the typical slow magnetization dynamics of a SMM. Linear fitting of the plot of τ(T) vs T−1 with an Arrehenius relation τ(T) = τ0 exp(Δeff/(kBT)) gave τ0 = 6.3 × 10−7 s and Δeff/kB = 13.4 K for 8 (the parameters for other compounds are listed in Table 1). Smaller effective energy barriers than the expected values of about 26 K (Δ/kB = |DST|ST2 with DST = −1.65 and ST = 4) should result from the shortcut of the thermal barrier by quantum tunneling of the magnetization (QTM). Type-I dimers 3, 5,33 13, 24, 25, and 27 with JMn−Mn* of intermediate magnitude also showed ac frequency dependence, indicating the possibility of SMM behavior (Type B). However, their blocking temperature is much lower than 1.8 K because of the absence of clear χ″ peaks under zero dc magnetic field. This is likely because excited states are still located close to the ST = 4 ground state even at low temperature under zero dc field. However, applying dc fields confirmed their SMM behavior in the ST = 4 ground state (Figures 9b and S18−S20). Type-I dimers 10, 17, and 26 with low JMn−Mn* have no ac frequency dependence at temperatures above 1.8 K because like the Type-B complexes their weak FM interaction between Mn centers may produce obscure spin states at temperatures above 1.8 K (Type C in Figures 9c and S21). In addition, Type-III dimers 9, 14, 15, 18, 19, and 20, which have relatively large zJ′ (comparable to JMn−Mn*), are also categorized as Type C. In fact, the χ′T values of the compounds at low temperature are much smaller than Ms = 10 cm3 K mol−1 for ST = 4 with g = 2 and decrease with falling temperature. Type-II dimers 1, 6, 11, 16, 21, 22, and 23 with an H2O axial ligand possess moderate interdimer interactions and are assigned as displaying Type-D ac magnetic behavior. The Type-D dimers display the ac frequency behavior of SMMs (Figures 9d and S22−S24), but the magnitude of the χ″ peak continuously decreases with lowering frequency, which follows the decrease of χ′T with falling temperature. This behavior could be caused by the presence of interdimer AFM interactions. The obtained Arrhenius parameters for the Type-A, -B, and -D compounds are summarized in Table 1. These parameters are similar across the series and comparable to that of previously reported dimers.12,24−28 Therefore, the dimers in this report exhibit SMM behavior regardless of their ligand substituents.

The Jab value depends on all of these parameters but in a very narrow range for rMn−X compared with those for rMn−Mn and rMn−O. In addition, the trend of Jab vs rMn−X (i.e., positive correlation) is opposite to those of Jab against rMn−Mn and rMn−O (i.e., negative correlation), which results from the fact that rMn−X is strongly related to rMn−O because they are mainly located on the same dz2 orbital, so a change of rMn−O strongly affects rMn−Mn. Tave between Mn ions (see eq 5) also displayed similar trends: a positive correlation with rMn−X and negative correlations with rMn−Mn and rMn−O, as shown in Figure 8b. These results for both Jab and Tave provide the same conclusion that the FM interaction in the series of Mn saltmen dimers is tuned by the orbital overlap determined by the molecular structure: when the orbital overlap between Mn ions is larger, the FM interaction becomes stronger. Finally, we briefly comment on the error found for the five complexes 14, 15, 18, 19, and 20, for which negative Jab values were calculated even though their JMn−Mn* values were estimated to be positive (see Figure 5). It has been reported that calculated Jab values strongly depend on the mixing ratio of the Hartree−Fock (HF) parameter in the functional set.65,66 The B3LYP functional set has been a de facto standard in recent years; however, it sometimes overestimates the stability of the AFM state. It has been reported that functional sets with a large ratio of HF exchange are applicable to magnetic molecules with small overlaps between atomic orbitals.65,66 In this sense, the functional set used to calculate Jab should be examined. We recalculated Jab of 14, 15, 18, 19, and 20 using a BHandHLYP functional set, which included a larger HF ratio (50%) than that in B3LYP (20%).67 As summarized in Table 2, Table 2. Jab Values Calculated at the BHandHLYP/6-31+G* Level of Theory complex

Jab [K]

14 15 18 19 20

0.7 1.0 1.1 0.7 1.3

the Jab values calculated using BHandHLYP are positive, suggesting that the negative Jab values in the previous calculations originate from the overstabilization of the AFM state by the B3LYP functional set, for which the calculated total energies and ⟨S̑2⟩ values are listed in Table S14. The choice of functional set will be discussed further in the future. Superparamagnetic Properties: Alternating Current (ac) Magnetic Measurements. As found in the dc magnetic studies, all dimers exhibited intramolecular FM coupling as well as large uniaxial anisotropy, which induces a ST = 4 ground state. These characteristics create a finite-sized energy barrier Δ/kB = |D|ST2 between ms = ±4 levels; therefore, all compounds have the potential to be SMMs, like 2 and 4.12,24 The ac susceptibility of polycrystalline samples was measured in the temperature range from 1.8 to 5 K under a 3-Oe ac field at frequencies of 0.5−1488 Hz. Most dimers exhibited strong frequency dependence of their in-phase (χ′) and out-of-phase (χ″) components of the ac magnetic susceptibility. The ac magnetic susceptibility behavior of the dimers can be roughly classified into four types: Type A, 2,12 4,24 7, 8, and 12; Type B, 3, 5,33 13, 24, 25, and 27; Type C, 9, 10, 14, 15, 17−20, and

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CONCLUSION Uniaxially anisotropic MnIII salen-type dimers have a potential to be SMMs if they have an ST = 4 ground state. In fact, several dimer compounds with ST = 4 have already been reported, most of which displayed SMM characteristics12,24−28 and were used as FM units for SCMs.13−17 We answered several fundamental questions about FM MnIII salen-type dimer units regarding FM coupling between MnIII ions in MnIII salen-type out-of-plane dimers, the mechanism behind FM coupling, and the relationship between JMn−Mn* and SMM behavior. To do this, we investigated the magnetic properties of a series of K


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Figure 9. Temperature dependence of the χ′T product and the imaginary part of ac susceptibility χ″ measured at several frequencies for (a) 8 under zero dc field (Type A), (b) 3 under a 200-Oe dc field (Type B), (c) 9 and 10 under a zero dc field (Type C), and (d) 16 under a zero dc field (Type D).

[Mn2(5-Rsaltmen)2(X)2]0/2+ dimers. All dimers exhibited FM interactions between their Mn ions, regardless of the R substituents of the saltmen Schiff-base ligands and axial X ligands, revealing the complexes all possessed an ST = 4 spin ground state. Some of the compounds exhibited very weak FM interactions, which may be associated with the coordination geometry around the MnIII ions (i.e., Z-type and W-type). The most interesting observation was that the FM interaction

became stronger with decreasing Mn−O1* and Mn···Mn* distances. The increase of orbital overlap promoted FM coupling; this is not intuitive because larger orbital overlaps are commonly advantageous for AFM coupling. To explain this unusual behavior, the mechanism behind the FM behavior of the series was examined by theoretical calculations. The calculated Jab values were consistent the experimental values, indicating that the FM interaction is closely related to the L


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The Journal of Physical Chemistry C orbital overlap and ΔΔε value, which is the energy difference between out-of-phase and in-phase orbitals. In addition, the ΔΔε value was also closely related to the on-site Coulomb repulsion at MnIII ions. Consequently, the FM interaction in the series of MnIII saltmen dimers is concluded to result from competition between the orbital degeneracy and the on-site Coulomb repulsion of out-of-phase and in-phase orbitals; i.e., orbital overlap is advantageous for FM coupling in this case. The magnetization dynamics of MnIII saltmen dimer SMMs is also related to the magnitude of JMn−Mn*, which stabilizes the ST = 4 ground state at finite temperatures. These magnetostructural correlations in the series of MnIII saltmen dimers verify their effectiveness as FM units with an ST = 4 ground state and uniaxial anisotropic units similar to the SMM units sometimes used for expansion to SCMs.

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and millimeter band EPR evidence for a ground S = 10 state in [Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O. J. Am. Chem. Soc. 1991, 113 (15), 5873−5874. (3) Boyd, P. D. W.; Li, Q.; Vincent, J. B.; Folting, K.; Chang, H. R.; Streib, W. E.; Huffman, J. C.; Christou, G.; Hendrickson, D. N. Potential building blocks for molecular ferromagnets: [Mn12O12(O2CPh)16(H2O)4] with a S = 14 ground state. J. Am. Chem. Soc. 1988, 110 (25), 8537−8539. (4) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, U.K., 2006. (5) Gatteschi, D.; Sessoli, R. Quantum Tunneling of Magnetization and Related Phenomena in Molecular Materials. Angew. Chem., Int. Ed. 2003, 42 (3), 268−297. (6) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. Single-Molecule Magnets. MRS Bull. 2000, 25 (11), 66−71. (7) Bogani, L.; Vindigni, A.; Sessoli, R.; Gatteschi, D. Single chain magnets: where to from here? J. Mater. Chem. 2008, 18 (40), 4750. (8) Miyasaka, H.; Julve, M.; Yamashita, M.; Clérac, R. Slow Dynamics of the Magnetization in One-Dimensional Coordination Polymers: Single-Chain Magnets. Inorg. Chem. 2009, 48 (8), 3420−3437. (9) Coulon, C.; Miyasaka, H.; Clérac, R. Single-Chain Magnets: Theoretical Approach and Experimental Systems. In Single-Molecule Magnets and Related Phenomena; Winpenny, R., Ed.; Structure and Bonding; Springer: Berlin, 2006; Vol. 122, pp 163−206. (10) Billoni, O. V.; Pianet, V.; Pescia, D.; Vindigni, A. Static and dynamic properties of single-chain magnets with sharp and broad domain walls. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84 (6), 64415. (11) Miyasaka, H.; Clérac, R. Synthetic Strategy for Rational Design of Single-Chain Magnets. Bull. Chem. Soc. Jpn. 2005, 78 (10), 1725− 1748. (12) Miyasaka, H.; Clérac, R.; Wernsdorfer, W.; Lecren, L.; Bonhomme, C.; Sugiura, K.; Yamashita, M. A Dimeric Manganese(III) Tetradentate Schiff Base Complex as a Single-Molecule Magnet. Angew. Chem., Int. Ed. 2004, 43 (21), 2801−2805. (13) Miyasaka, H.; Saitoh, A.; Yamashita, M.; Clerac, R. A MnIII2NiII single-chain magnet separated by a thick isolating network of BPh4− anions. Dalton. Trans. 2008, No. 18, 2422−2427. (14) Saitoh, A.; Miyasaka, H.; Yamashita, M.; Clerac, R. Direct evidence of exchange interaction dependence of magnetization relaxation in a family of ferromagnetic-type single-chain magnets. J. Mater. Chem. 2007, 17 (19), 2002−2012. (15) Miyasaka, H.; Nezu, T.; Sugimoto, K.; Sugiura, K.; Yamashita, M.; Clérac, R. [Mn2III(5-Rsaltmen)2NiII(pao)2(L)]2+: An ST=3 Building Block for a Single-Chain Magnet That Behaves as a SingleMolecule Magnet. Chem. - Eur. J. 2005, 11 (5), 1592−1602. (16) Miyasaka, H.; Clérac, R.; Mizushima, K.; Sugiura, K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. [Mn2(saltmen)2Ni(pao)2(L)2](A)2 with L = Pyridine, 4-Picoline, 4-tert-Butylpyridine, N-Methylimidazole and A = ClO4−, BF4−, PF6−, ReO4−: A Family of Single-Chain Magnets. Inorg. Chem. 2003, 42 (25), 8203−8213. (17) Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. Evidence for Single-Chain Magnet Behavior in a MnIII−NiII Chain Designed with High Spin Magnetic Units: A Route to High Temperature Metastable Magnets. J. Am. Chem. Soc. 2002, 124 (43), 12837−12844. (18) Ferbinteanu, M.; Miyasaka, H.; Wernsdorfer, W.; Nakata, K.; Sugiura, K.; Yamashita, M.; Coulon, C.; Clérac, R. Single-Chain Magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of MnIII−FeIII− MnIII Trinuclear Single-Molecule Magnet with an ST = 9/2 Spin Ground State. J. Am. Chem. Soc. 2005, 127 (9), 3090−3099. (19) Miyasaka, H.; Madanbashi, T.; Sugimoto, K.; Nakazawa, Y.; Wernsdorfer, W.; Sugiura, K.; Yamashita, M.; Coulon, C.; Clérac, R. Single-Chain Magnet Behavior in an Alternated One-Dimensional Assembly of a MnIII Schiff-Base Complex and a TCNQ Radical. Chem. - Eur. J. 2006, 12 (27), 7028−7040. (20) Miyasaka, H.; Takayama, K.; Saitoh, A.; Furukawa, S.; Yamashita, M.; Clérac, R. Three-Dimensional Antiferromagnetic Order of Single-Chain Magnets: A New Approach to Design Molecule-Based Magnets. Chem. - Eur. J. 2010, 16 (12), 3656−3662.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03336. Figures S1−S24 and Tables S1−S14 and the appendix (PDF) File containing CIF files for the dimers (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +81-47-4721319. *E-mail: [email protected]. Tel.: +81-22-215-2030. ORCID

Chihiro Kachi-Terajima: 0000-0001-5058-8357 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Hiroki Hiraga (Tohoku University) for synthesizing and characterizing the dimers and Prof. Toshiaki Saito (Toho University) for support with the magnetic measurements. This work was supported by Grants-in-Aid for Scientific Research (No. 16H02269, 25620041, and 26410093) and for Innovative Areas (“Coordination Programming” Area 2107, No. 24108714 and 24108721 and “π-System Figuration” Area 2601, No. 15H00983) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, E-IMR, the Sumitomo Foundation, and the Asahi Glass Foundation. This work was also performed under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposal No. 14K0008 and 15K0118). Y.K. acknowledges Toyota Physical and Chemical Research Institute Scholars. C.K.-T. was partly supported by the HighTech Research Center Project (2005−2009) and the Supported Program for Strategic Research Foundation at Private Universities (2012−2016) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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