Experimental and Theoretical Investigation of a Series of Novel

Mar 22, 2016 - Angewandte Chemie International Edition 2017 56 (30), 8849-8854 ... Debangsu Sil , Firoz Shah Tuglak Khan , Sankar Prasad Rath. Chemist...
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Experimental and Theoretical Investigation of a Series of Novel Dimanganese(III) μ‑Hydroxo Bisporphyrins: Magneto−Structural Correlation and Effect of Metal Spin on Porphyrin Core Deformation Debangsu Sil, Susovan Bhowmik, Firoz Shah Tuglak Khan, and Sankar Prasad Rath* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: The synthesis, structure, and properties of a new family of five ethane-bridged dimanganese(III) μ-hydroxo bisporphyrins with the same core structure but different counteranions are reported here. Additions of 10% Brønsted acids such as HI, HBF4, HSbF6, HPF6, and HClO4 to a dichloromethane solution of the dichloro dimanganese(III) bisporphyrin produces complexes having a remarkably bent μhydroxo group with I3−, BF4−, SbF6−, PF6−, and ClO4− as counteranions, respectively. The X-ray structures of all complexes have been determined, which have revealed the presence of two equivalent high-spin manganese(III) centers with equally distorted porphyrin rings in the complexes, in sharp contrast with the case for the diiron(III) μ-hydroxo bisporphyrin analogues. 1H NMR spectra have shown highly deshielded meso resonances, unlike the case for the diiron(III) analogues, where the meso resonances are highly shielded. The variable-temperature magnetic data have been subjected to a leastsquares fit which provides a moderate antiferromagnetic coupling through the hydroxo bridge between two zero-field split Mn(III) centers with coupling constant (J) values ranging from −29.5 to −38.6 cm−1. Fairly good correlations are observed for J with Mn−O(H) distances and Mn−O(H)−Mn angles for all the complexes except for that having an I3− counteranion. DFT calculations support the stabilization of two equivalent high-spin Mn(III) porphyrin cores in the complexes and have also explored the role of metal spin in controlling porphyrin ring deformation. Unlike diiron(III) μ-hydroxo bisporphyrin complexes, the dimanganese(III) analogues do not have easily accessible spin states of the metal attainable by subtle environmental perturbations and, therefore, can only stabilize the high-spin state with a variety of counteranions.

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INTRODUCTION Oxo-/hydroxo-bridged dimetal cores are common structural motifs found in various proteins and enzymes. Diiron centers bridged by oxo and hydroxo groups are prevalent among the active sites of proteins involved in O2 metabolism,1−5 although not those of hemes, and the manganese centers in PS II have been suggested to be linked by oxo and/or hydroxo groups.6 The high versatility of the oxo-/hydroxo-bridged dimetal core, formed just by simple protonation or deprotonation, allows the enzymes a simple way to control the property of the intermediates in the catalytic cycles. Recently, our group has reported a series of μ-oxo and μhydroxo complexes of ethane- and ethene-bridged diiron(III) bisporphyrins.7,8 The two iron(III) centers in the μ-hydroxo complexes are found to be inequivalent.8 Moreover, the spin states of the iron(III) center were found to be dependent on the counteranions used.8 The most extreme case was observed with the I3− counteranion, where one iron(III) center is in the high-spin (S = 5/2) state while the other is in the admixedintermediate spin state (S = 3/2 with a minor contribution of S = 5/2).8d When BF4−, PF6−, and SbF6− counteranions are used, two iron(III) centers are also found to be inequivalent with admixed-intermediate spin states.8a,c This is quite unusual © XXXX American Chemical Society

considering the similar coordination environments of the two iron(III) centers. Such unusual stabilization of two different spin states in the same molecule was proposed to be due to the unequal ring deformation of the two porphyrin centers. The porphyrin unit with greater core deformation was found to stabilize the intermediate spin state of iron(III). An extensive computational study on ethane-bridged diiron(III) μ-hydroxo bisporphyrin has revealed that subtle environmental perturbation can change the spin state ordering in the complex.8a So far, porphyrin ring deformation has been considered as one of the reasons behind the alteration of the spin state;8−12 however, an alternate possibility where a change in the spin state could also induce different ring deformation has never been explored. These attractive features have prompted us to investigate the subject further by moving the metal ion from iron to manganese. We report here a hitherto unknown family of dimanganese(III) bisporphyrins bridged by a single hydroxo group between two Mn centers with a variety of counteranions. Unlike diiron(III) μ-hydroxo complexes, the dimanganese(III) Received: September 28, 2015

A

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1

and porphyrin eg (π*) orbitals.14 The mixing of these orbitals give rise to “hyper spectra” that are characterized by an intense Soret band at low energy (470−480 nm) with additional transitions at higher energy (360−380 nm).14a UV−visible spectroscopic data of 1 show two intense bands at 365 and 480 nm (Soret band) along with Q bands at 573 and 615 nm in dichloromethane. Addition of strong Brønsted acids having counteranions weakly coordinating to 1 results in a blue shift of the Soret band due to the formation of the dimanganese(III) μhydroxo bisporphyrin complex 1·X. For a comparison, the UV−visible spectra of 1 and 2·PF6 are shown in Figure 1 as a representative case. Similar spectral changes have also been observed during the formation of other μ-hydroxo complexes reported here.

analogues can only stabilize the high-spin state under similar conditions. The synthesis, structure, and properties of a series of dimanganese(III) μ-hydroxo bisporphyrins have been demonstrated in the present study. Counteranions with different sizes and shapes have been used, and their effects are compared with those of the reported ethane-bridged diiron(III) μ-hydroxo bisporphyrins. This study also analyses the variation of exchange coupling with the structural parameters and focuses on the relation between metal spin state and porphyrin deformation. Density functional theory (DFT) calculations have been employed which further support the experimental observations.

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RESULTS AND DISCUSSION Synthesis. 1,2-Bis[chloromanganese(III)octaethylporphyrinyl]ethane (1) was synthesized by refluxing 1,2-bis(meso-octaethylporphyrinyl)ethane13 and MnCl2 in a degassed solvent mixture of CHCl3 and MeOH (1/1) in moderate yield. When a dichloromethane solution of 1 was stirred with a 10% aqueous solution of HI containing I2, a color change from bright red to brown was observed due to formation of the ethane-bridged dimanganese(III) μ-hydroxo bisporphyrin 2·I3. The complex was isolated as a solid in excellent yield and was structurally characterized. Similarly, other ethane-bridged dimanganese(III) μ-hydroxo bisporphyrins with BF4−, SbF6−, PF6−, and ClO4− counteranions were prepared by treating a solution of 1 in dichloromethane with dilute acids of the corresponding anions. All of the complexes were isolated as crystalline solids in excellent yields and were structurally characterized. Conductivity measurements at 295 K of the μ-hydroxo complex (2·X) in dichloromethane further support the formulations of a 1/1 electrolyte in solution. Details of the synthetic procedures and characterization of the complexes are given in the Experimental Section. Scheme 1 outlines the synthetic protocol and includes a list of all of the complexes reported here, along with their abbreviations. Enhanced interest in electronic spectra of Mn(III) porphyrins arises from the unique mixing of metal eg orbitals

Figure 1. UV−visible spectra (at 295 K) in dichloromethane for 1 (blue line) and 2·PF6 (red line).

Crystallographic Characterization. Dark red crystals suitable for an X-ray diffraction study were grown in air at room temperature by slow diffusion of n-hexane into a benzene solution of 2·I3, 2·BF4, and 2·PF6, while for 2·SbF6 and 2·ClO4 dichloromethane and toluene solutions were used, respectively. Figure 2 gives the X-ray structure of 2·I3 along with its packing diagram, while Figure 3 demonstrates the X-ray structures of 2· BF4, 2·ClO4, 2·SbF6, and 2·PF6 and Figures S1−S4 in the Supporting Information represent their packing diagrams. In all of the dimanganese(III) μ-hydroxo bisporphyrins, the two B

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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indicating no significant interactions between the cation and anion in the solid state. Table 2 compares selected structural parameters among dimanganese(III) μ-hydroxo bisporphyrins 2·X, their diiron(III) analogues8 4·X (Chart 1), and also the previously reported [{MnIII(OEP)}2(OH)]ClO415a and [{MnIII(TPP)}2(OH)]ClO4.15b Mn−Np (average) bond lengths are 2.000(8) and 2.006(8) Å for core-I and core-II, respectively, in 2·I3, 1.990(4) and 2.005(4) Å in 2·BF4, 1.985(4) and 1.980(4) Å in 2·PF6, and 2.005(4) and 1.991(4) Å in 2·ClO4, while a value of 1.991(4) Å is found in 2·SbF6. As can be seen, the Mn−Np distances are nearly similar in both cores, which are also comparable to the values reported previously for [{MnIII(OEP)}2(OH)]ClO4, 2.006(7) Å, and [{MnIII(TPP)}2(OH)]ClO4, 2.008(7) Å.15 The manganese atoms are displaced by 0.40 and 0.42 Å for core-I and core-II, respectively, in 2·I3, 0.31 and 0.28 Å in 2·BF4, 0.21 Å in 2·SbF6, 0.31 and 0.30 Å in 2·PF6, and 0.35 and 0.38 Å in 2·ClO4 from the least-squares plane of the C20N4 porphyrinato core. These values are in the range of five-coordinate high-spin Mn(III) porphyrins reported earlier.15,16 However, displacements of ∼0.20 Å in [{MnIII(OEP)}2(OH)]ClO4 and [{MnIII(TPP)}2(OH)]ClO4 have been reported.15 Thus, the metal displacements are relatively greater in the case of the dimanganese(III) μ-hydroxo bisporphyrin complexes relative to those in their unbridged analogues. Moreover, the metal displacement in the diiron(III) μ-hydroxo bisporphyrin 4·X is found to be greater than that in the dimanganese(III) analogue 2·X. Observed Mn−O distances are 1.972(7) and 1.991(6) Å in 2·I3, 2.008(3) and 2.005(3) Å in 2·BF4, 1.9596(12) Å in 2·SbF6, 1.995(2) and 1.997(2) Å in 2·PF6, and 2.057(3) and 2.035(3) Å in 2·ClO4. The Mn−O(H) bond distances are reported as 1.998(2) and 2.024(2) Å for [{MnIII(OEP)}2(OH)]ClO4 and 2.028(18) and 2.025(23) Å for [{MnIII(TPP)}2(OH)]ClO4.15 As can be seen, the Mn−O distance of 2·X is consistent with a

Figure 2. (A) Perspective view of 2·I3 showing 50% thermal contours for all non-hydrogen atoms at 100 K. H atoms and solvent molecules have been omitted for clarity. (B) Diagram illustrating the packing of 2·I3·C6H6 in the unit cell. H atoms have been omitted for clarity.

manganese centers are in a five-coordinate square-pyramidal geometry. Except for 2·SbF6 and 2·PF6, which crystallize in monoclinic crystal systems with C2/c and P21/c space groups, respectively, all other molecules crystallize in the triclinic crystal system with the P1̅ space group. Selected bond lengths and bond angles are reported in Table 1. The counteranions exist as isolated ions in the solid for all of the μ-hydroxo complexes reported here, and the nonbonding distances between the metal and the nearest atom of the counteranions are more than 5 Å,

Figure 3. Perspective views of (A) 2·SbF6, (B) 2·BF4, (C) 2·PF6, and (D) 2·ClO4 showing 50% thermal contours for all non-hydrogen atoms at 100 K. H atoms and solvent molecules have been omitted for clarity. C

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Inorganic Chemistry Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for 2·X Mn1−O1 Mn1−N1 Mn1−N2 Mn1−N3 Mn1−N4 Mn2−O1 Mn2−N5 Mn2−N6 Mn2−N7 Mn2−N8 Mn1−O1−Mn2 O1−Mn1−N1 O1−Mn1−N2 O1−Mn1−N3 O1−Mn1−N4 O1−Mn2−N5 O1−Mn2−N6 O1−Mn2−N7 O1−Mn2−N8

2·I3

2·BF4

2·SbF6

2·PF6

2·ClO4

1.972(7) 2.001(7) 1.995(8) 2.013(8) 1.992(8) 1.991(6) 2.000(8) 2.009(8) 2.004(8) 2.012(8) 142.4(3) 99.3(3) 96.0(3) 101.3(3) 102.5(3) 96.7(3) 95.1(3) 102.4(3) 104.3(3)

2.008(3) 1.988(4) 1.992(4) 1.988(4) 1.992(4) 2.005(3) 2.004(4) 2.006(4) 2.013(4) 1.997(4) 149.90(15) 103.52(13) 101.35(14) 92.48(14) 90.83(14) 97.94(13) 99.55(14) 93.37(14) 92.18(13)

1.9596(12) 1.987(4) 1.995(4) 1.986(5) 1.996(4)

1.995(2) 1.979(4) 1.991(4) 1.988(4) 1.984(3) 1.997(2) 1.979(4) 1.970(4) 1.986(4) 1.986(4) 152.42(12) 97.19(12) 99.38(13) 97.40(12) 94.25(12) 96.09(13) 94.92(12) 97.44(12) 100.21(12)

2.057(3) 2.009(4) 2.004(4) 2.002(4) 2.004(5) 2.035(3) 1.990(4) 1.993(4) 1.983(4) 1.997(4) 144.28(19) 91.59(15) 98.38(15) 101.51(15) 97.35(16) 94.64(15) 99.28(16) 99.77(16) 99.86(15)

159.2(2) 96.81(16) 98.27(15) 93.12(16) 96.09(15)

Table 2. Selected Structural Parameters of Dimanganese(III) and Diiron(III) μ-Hydroxo Porphyrins M−O(H) (Å)

M−Np (Å)a

core-I core-II core-I core-II core-I

1.972(7) 1.991(6) 2.008(3) 2.005(3) 1.9596(12) 1.995(2) 1.997(2) 2.057(3) 2.035(3) 1.998(2)

2.000(8) 2.006(8) 1.990(4) 2.005(4) 1.991(4) 1.985(4) 1.980(4) 2.005(4) 1.991(4) 2.006(7)

core-II core-I

2.024(2) 2.028(18)

2.007(5) 2.008(7)

core-II core-I core-II core-I core-II core-I core-II core-I core-II

2.025(23) 1.897(3) 1.934(3) 1.925(3) 1.967(3) 1.911(2) 1.922(2) 1.933(4) 1.940(4)

2.008(8) 2.051(4) 2.007(4) 2.019(4) 1.967(3) 2.063(3) 2.054(3) 1.976(5) 1.970(5)

compound 2·I3 2·BF4 2·SbF6 2·PF6 2·ClO4 [{MnIII(OEP)}2OH] ClO4 [{MnIII(TPP)}2OH] ClO4 4·I3 4·BF4 4·ClO4 4·SbF6

core-I core-II core-I core-II

M−O(H)−M (deg) 142.4(3)

ΔM24 (Å)b

152.73(11)

0.40 0.42 0.31 0.28 0.21 0.31 0.30 0.35 0.38 0.23

160.4(8)

0.19 0.19

149.90(15) 159.2(2) 152.42(12) 144.28(19)

0.20 0.55 0.48 0.41 0.39 0.57 0.48 0.35 0.35

142.5(2) 148.5(2) 141.2(1) 152.11(19)

M···M (Å)c

Δ24 (Å)d

ΔCm (Å)e

3.909(1)

0.25 0.27 0.27 0.21 0.24 0.26 0.28 0.23 0.30 0.13

0.49 0.53 0.51 0.40 0.45 0.52 0.56 0.44 0.56 0.25

3.993(2)

0.43 0.24

0.15 0.06

0.22 0.21 0.31 0.22 0.31 0.17 0.13 0.26 0.29

0.09 0.40 0.56 0.45 0.59 0.29 0.12 0.52 0.58

3.752(1) 3.876(1) 3.855(1) 3.877(1) 3.895(1)

3.627(1) 3.747(1) 3.615(1) 3.759(1)

twist angle (deg)f

ref

12.99

this work

16.65

this work

18.95 15.65

this work this work

12.07

this work

4.3

15a

29.9(6)

15b

12.88

8c

14.85

8c

10.85

8c

16.3

8a

a Average value. M = Fe/Mn. bDisplacement of M from the least-squares plane of the C20N4 porphyrin core. cNonbonding distance. dAverage displacement of atoms from the least-squares plane of the C20N4 porphyrin core. eAverage deviation of the meso carbons from the least-squares plane of the C20N4 porphyrin core. fAverage value of four N−M−M′−N′ dihedral angles.

152.73(11) and 160.4(8)° have been reported for the related unbridged analogues [{Mn III (OEP)} 2 (OH)]ClO 4 and [{MnIII(TPP)}2(OH)]ClO4, respectively.15 As can be seen, the Mn−O(H)−Mn angle decreases in the ethane bridged analogue of the complex by more than 8°. Moreover, 2·I3 revealed the smallest known Mn−O(H)−Mn angle of 142.4(3)° among all the complexes reported here and elsewhere15 and this angle is also similar to that in the diiron(III) analogue of the complex. A considerably greater Mn−O(H)−Mn angle is observed in 2·SbF6 in comparison to

bridging hydroxide ligand rather than a bridging oxo ligand, whose expected value falls within the range 1.78−1.84 Å.7,15 It should be noted here that the inverse relationship between Mn−Np and Mn−O(H) distances has not been observed in Mn(III) porphyrin complexes reported here, which was observed earlier in the case of diiron(III) μ-hydroxo bisporphyrin analogues.8 The bridging Mn−O(H)−Mn angles for 2·I3, 2·BF4, 2·SbF6, 2·PF6, and 2·ClO4 are 142.4(3), 149.90(15), 159.2(2), 152.42(12) and 144.28(19)°, respectively. However, values of D

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Inorganic Chemistry Chart 1

other μ-hydroxo complexes reported here, which is probably to minimize the inter-ring interactions in the molecule. The distance between two least-squares planes observed in 2·SbF6 is 4.241 Å, which is relatively less than the 4.432, 4.313, 4.363, and 4.45 Å observed for 2·I3, 2·BF4, 2·PF6, and 2·ClO4, respectively. A shorter mean plane separation probably causes an enhanced inter-ring interaction in 2·SbF6, leading to an increase in the Mn−O(H)−Mn angle. In addition, the relative orientations of the two porphyrin rings in 2·X were more staggered in comparison to that in the unbridged complex [{MnIII(OEP)}2(OH)]ClO4, where it was nearly eclipsed (torsional angle of 4.3°). The greater torsional angles in 2·X suggests more interporphyrin interaction in the molecule in comparison to the unbridged analogue [{MnIII(OEP)}2(OH)]ClO4, which is also reflected in the greater Mn···Mn separation in the latter. The porphyrin rings are highly distorted in all of the complexes reported here and are best appreciated by turning to Figure 4, where the out-of-plane displacements of porphyrin core atoms are compared. As can be seen, both porphyrin rings of 2·X are deformed almost equally, leading to the stabilization of two equivalent manganese(III) centers. This is in sharp contrast to the case for the diiron(III) μ-hydroxo bisporphyrin complex 4·X, reported earlier by us,8 where two porphyrin rings are deformed to an unequal extent, which leads to the stabilization of two different spins of iron in a single molecular framework. It would be easier to understand the situation by turning to Figure 5, where the out-of-plane displacements of porphyrin core atoms are compared between 2·I3 and 4·I3. It is clearly evident that, with identical coordination environments and counteranions, just a change in the metal ion from iron(III) to manganese(III) leads to such a large difference in the deformation between the two porphyrin rings. A possible explanation of this phenomenon could lie in the easily accessible spin states (S = 5/2, S = 3/2) of iron(III), which can be modulated easily by slight modification of external stimuli, viz., axial coordination, porphyrin ring deformation, etc.8a In identical coordination environments, the spin state of the metal ion is known to be guided by the extent of porphyrin ring deformation; a highly deformed core stabilizes intermediate spin while a planar core prefers the high-spin state of iron. However, in the case of dimanganese(III) μ-hydroxo bisporphyrin complexes, the five-coordinate Mn(III) ion can only stabilize the high-spin (S = 2) state, which leads to nearly equal ring distortion. In other words, when a spin state change

Figure 4. Out-of-plane displacements (in units of 0.01 Å) of porphyrin core atoms of (A) 2·I3, (B) 2·BF4, (C) 2·SbF6, (D) 2·PF6, and (E) 2· ClO4 from the mean plane of the C20N4 porphyrinato core. The horizontal axis represents the atom number in the macrocycle (inset shows the numbering scheme), showing the bond connectivity between atoms.

of the metal center is allowed, it leads to unequal core deformation stabilizing different spin states (vide infra). 1 H NMR. Figure 6 compares the 1H NMR spectra (in CDCl3) of 1 and 2·I3 at 295 K. As expected, the signals are found to be very broad in comparison to those for the related iron complexes. Assignments of the peaks were done on the basis of relative intensities and line widths and also by comparison with the 1H NMR spectra of MnIII(OEP)Cl17 and MnIII(5-Me-OEP)Cl. The presence of a C2 axis of symmetry in 1 and 2·I3 suggests that its 1H NMR pattern should be similar to that of meso-substituted five-coordinate Mn(III) porphyrins of type XMnIII(meso-R-OEP). As expected, the 1H NMR spectrum of MnIII(5-Me-OEP)Cl shows the presence of two meso proton resonances at 46.46 and 43.62 ppm in a 1:2 intensity ratio and the four methylene resonances at 23.23, 22.43, −0.37, and −1.47 ppm, along with appearance of a new signal at 35.41 ppm which has been assigned to the CH3 group E

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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giving rise to an overall broad signal which, however, produces a clear separation at 318 K (Figure 6). The signal at 21.01 ppm has been assigned to the bridging −CH2 protons by comparing with the 1H NMR spectra of 1 and MnIII(5-Me-OEP)Cl. The four peaks at −2.20, 12.43, 13.12, and 14.33 ppm have been assigned to the methylene protons by analogy with the 1H NMR spectra of MnIII(OEP)Cl, 1, and MnIII(5-Me-OEP)Cl. The methyl resonance was observed at 1.97 ppm. The smaller isotropic shifts observed in the 1H NMR spectra of 2·I3, in comparison to those of MnIII(OEP)Cl, 1, and MnIII(5-MeOEP)Cl, reflect weak paramagnetic character as a consequence of antiferromagnetic coupling between the two manganese centers through the hydroxo bridge (vide infra). The 1H NMR spectra of other dimanganese(III) μ-hydroxo complexes are very similar to that of 2·I3. The 1H NMR spectrum of 1 is strikingly different from that of diiron(III) counterpart, 3 (Chart 1). 3 shows eight relatively sharp methylene peaks in the highly deshielded region (∼40 ppm), two meso resonances in the highly shielded region with a 2:1 intensity ratio, and a highly deshielded bridging −CH2− resonance.7a In the case of 1, however, the peaks are very broad and are relatively less shifted; the methylene resonances are shifted in both highly deshielded and shielded regions over a wide range between −12.57 and 23.40 ppm, and the bridging −CH2− resonance (at 35.49 ppm) is found to be relatively less deshielded than that in 3. The most discernible difference lies in the position of the meso resonances, which are found in the highly deshielded region (44.73 and 49.46 ppm) in 1 as opposed to the highly shielded (−55.0 and −74.7 ppm) signals observed in 3. Similar differences have also been observed in the 1H NMR spectra between 2·X and 4·X, and thus, the spectral patterns are very much dependent on the metal ion (Fe3+/Mn3+) used. Figure 7 shows the Curie plots (δ vs 1000/T) of the methylene, bridging, and meso protons for 2·I3 over the

Figure 5. Out-of-plane displacements (in units of 0.01 Å) of porphyrin core atoms of (A) 2·I3 and (B) 4·I3 from the mean plane of the C20N4 porphyrinato core. The horizontal axis represents the atom number in the macrocycle, showing the bond connectivity between atoms. The inset shows the numbering scheme of the porphyrin ring.

Figure 7. Curie plot (δ vs 1000/T) of the proton signals of 2·I3. The solid lines are linear least-squares fits of the experimental data which are then extrapolated to 1000/T = 0.

1

Figure 6. H NMR in CDCl3 of (A) 1 at 295 K and 2·I3 at (B) 295 K and (C) 318 K.

at the meso position (C5). The 1H NMR spectrum of the dimeric complex 1 was found to be similar to that of MnIII(5Me-OEP)Cl, which showed two meso signals at 44.73 and 49.46 ppm in a 1:2 intensity ratio, the bridging −CH2 resonance at 35.49 ppm, three broad methylene resonances at 23.40, 22.30, and −12.57 ppm, and the methyl resonance at 2.52 ppm. The 1 H NMR spectra of 2·I3 at 295 K produces two meso resonances at 27.97 and 29.34 ppm in a 1:2 intensity ratio, as also obtained in the case of 1 and MnIII(5-Me-OEP)Cl. The meso signals are found overlapping with one another, thereby

temperature range of 318−243 K, which display a linear dependence. However, the variation of the chemical shift of the methylene protons is relatively less over the temperature range, suggesting the presence of substantial antiferromagnetic coupling between the two manganese(III) centers. Moreover, the observed linear dependence for each resonance demonstrates that the high-spin (S = 2) ground state is not in thermal equilibrium with other spin states.12a Extrapolation of δ vs 1000/T plots to 1000/T = 0 do not yield the diamagnetic value for any of the protons, which strongly suggests that there is F

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Comparison of Structural Parameters and Magnetic Properties for 2·X, [{MnIII(OEP)}2OH]ClO4, [{MnIII(TPP)}2OH]ClO4, and 4·X compound

M−O(H)a

M−O(H)−Mb

Jc

Dc

ref

2·I3 2·BF4 2·SbF6 2·PF6 2·ClO4 [{MnIII(OEP)}2OH]ClO4 [{MnIII(TPP)}2OH]ClO4 4·I3

1.9815(7) 2.0065(3) 1.9596(12) 1.996(2) 2.046(3) 2.011(18) 2.026(17) 1.897(3) 1.934(3) 1.925(3) 1.967(3) 1.911(2) 1.922(2) 1.933(4) 1.940(4)

142.4(3) 149.90(15) 159.2(2) 152.42(12) 144.28(19) 152.73(11) 160.4(8) 142.5(2)

−29.5 −34.7 −38.6 −35.1 −32 −35.5 −37 −4.5

−7.11 −7.4 −8.7 −8.6 −8.2 −8 −10.8 10

this work this work this work this work this work 15a 15b 8c

148.5(2)

−36

141.2(1)

−42

152.11(19)

−36.5

10

8a

−37.6

10

8a

4·BF4 4·ClO4 4·SbF6

core-I core-II core-I core-II core-I core-II core-I core-II

4·PF6 a

10

8c 8c

Values in Å. M = Mn/Fe. bValues in deg. cValues in cm−1.

centers in the molecule, and no visible effects upon changing the counteranions are observed in solution. The reversal in shifting of the meso resonances between Fe(III) and Mn(III) bisporphyrins is quite intriguing and needs some discussion. The bonding interaction between the Fe(III) dz2 orbital with porphyrin a2u results in positive spin density on the meso carbons, and hence the meso protons are shifted to a highly shielded region. This bonding interaction, however, reduces with decreasing metal displacement (ΔM24) from the porphyrin mean plane.12a,b,18 A lower displacement of Mn (see Table 2) suggests that as we move from Fe(III) to Mn(III) the interaction between the metal dz2 orbital with the porphyrin a2u orbital decreases. However, this alone cannot account for the shifting of the meso protons in the Mn(III) bisporphyrins to the highly deshielded region, which can be explained in terms of an anomalous spin polarization mechanism proposed by Cheng et al.18b Electron correlation is maximized for high-spin Mn(III) complexes with four unpaired electrons and a vacant dx2−y2 orbital, which favors the transfer of α spin from porphyrin to the metal while leaving a net β spin density on the nitrogen donor orbital. Spin polarization along the porphyrin bonding framework rationalizes the negative spin density on the meso carbon atom, which eventually has shifted the meso proton resonance to a highly deshielded region. It is, however, interesting to note here that both the meso and the bridging −CH2 resonances of 1 and 2·X are shifted toward highly deshielded regions, which also suggests significant σ-spin distribution pattern at the porphyrin periphery. Moreover, large differences in the chemical shifts of the methylene resonances are also observed (Figure 6) in the manganese(III) porphyrins (MnIII(5-Me-OEP)Cl, 1, and 2·X). The exact reason for all such behaviors is still unknown and requires further investigation. Magnetic Studies. Variable-temperature magnetic susceptibility measurements have been carried out for all the dimanganese(III) μ-hydroxo bisporphyrins reported here in the solid state at an applied magnetic field of 0.1 T over the temperature range of 5−300 K. Using the software PHI,19 the susceptibility data were fitted to a model of an exchangecoupled dinuclear manganese(III) complex containing small residual mononuclear manganese(III) as an impurity. The

significant second-order Zeeman contributions to the dipolar shifts.12a,b It is interesting to compare the 1H NMR spectra here between the dimanganese(III) μ-hydroxo bisporphyrin 2·X and diiron(III) μ-hydroxo bisporphyrin 4·X. The meso signals are found in the highly shielded region in 4·X, but in 2·X, they are highly deshielded (∼30 ppm). Sixteen highly deshielded methylene (spanning between 68.8 and 16.1 ppm) and four highly shielded meso signals (at −3.5, −10.3, −15.9, and −35.5 ppm) have been observed in the case of μ-hydroxo complex 4· I3, which confirms the presence of two inequivalent Fe(III) centers within a single molecular framework that can also be seen by the presence of two distinct sets of signals.8d In one set, eight relatively broad methylene signals (spanning from 68.8 to 21.2 ppm) and two broad meso signals at −10.3 and −35.5 ppm are observed due to the nearly high-spin (S = 5/2) nature of iron for core-I. In the other set, however, eight sharp methylene peaks appear in a relatively narrow region (from 19.9 to 16.1 ppm) along with two sharp meso resonances at −3.5 and −15.9 ppm that are due to the admixed-intermediate-spin state of iron present in core-II.8d In addition, the 1H NMR spectra differ significantly with the counteranions used in 4·X. While the overall spectral patterns and shifts to lower and higher shielding of the resonances are the same, the actual numbers are widely different. For example, in 4·BF4, 16 sharp ethylene proton signals are observed in the narrow range of 28.4−14.3 ppm along with four sharp meso signals (at −1.0, −3.8, −11.7, and −15.6 ppm), which demonstrates the presence of two inequivalent Fe centers with admixed-intermediate spin in the molecule.8c Similar spectral patterns are also observed for 4·PF6 and 4·SbF6.8a However, only eight sharp methylene (between 21.6 and 16.1 ppm) and two meso proton (at −3.8 and −15.6 ppm) signals are observed in 4·ClO4, which suggest two equivalent iron centers in intermediate-spin states.8c An extensive computational study has revealed that the energetic differences between S = (5/2, 3/2) and S = (3/2, 3/2) are very small, and therefore, subtle environmental perturbations can change the spin-state ordering in the diiron(III) μ-hydroxo complex. In sharp contrast, only one set of signals is observed in 2·X, which is due to two equivalent Mn(III) porphyrin G

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry exchange Hamiltonian is given by H = −2JS1S2, where S1 = S2 = 2. Each manganese(III) center of the dimer is treated as a zerofield split high-spin species (S = 2) with a g value set at 2.0. Keeping in mind the axially elongated structure around each Mn(III) center in the dimer, a negative sign is expected for D (zero-field splitting parameter);13 thus, D was constrained to negative values in the present calculations. The parameters obtained for all the complexes are given in Table 3 along with the structural parameters that are likely to influence the coupling constant (J) value. Figure 8 compares the χMT vs T

Figure 8. χMT vs T plots for 2·I3 (purple △), 2·BF4 (red ⬡), 2·SbF6 (blue ○), 2·PF6 (black □), 2·ClO4 (dark blue ◇), and 4·I3 (green ⬠). The solid lines are the best fits calculated using the values given in the text.

Figure 9. Plots of (A) average Mn−O(H) bond length vs −J and (B) Mn−O(H)−Mn angle vs −J for μ-hydroxo dimanganese(III) bisporphyrins (black ■) and {[MnIII(OEP)]2(OH)}(ClO4)8b (red ●).

plots of polycrystalline samples of 2·X along with 4·I3 over the temperature range of 5−300 K. The χMT values for all of the complexes decrease upon a decrease in the temperature, which is indicative of antiferromagnetic coupling between the two manganese(III) centers. From the best fit, J values of −29.5, −34.7, −38.6, −35.1, and −32 cm−1 are obtained for 2·I3, 2·BF4, 2·SbF6, 2·PF6, and 2· ClO4, respectively. Previously reported complex of μ-hydroxo dimanganese(III) porphyrins available for magnetic comparison are [{MnIII(OEP)}2(OH)](ClO4)15a and [{MnIII(TPP)}2(OH)](ClO4).15b The magneto−structural data for these complexes are Mn−O(H) = 2.011(18) Å, Mn−O(H)−Mn = 152.73° and J = −35.5 cm−1 for the OEP complex and Mn−O(H) = 2.026(17) Å, Mn−O(H)−Mn = 160.4° and J = −37 cm−1 for the TPP complex. Magnetic susceptibility measurements of a reported nonheme dimanganese complex bridged by a single hydroxo group, [{MnIII(5CH3)salen}2OH]ClO4·3H2O, has J = −25.36 cm−1, Mn−O(H) = 2.021(3) and 2.082(3) Å, and Mn−O(H)−Mn = 141.4°.20 The greater −J value for [{MnIII(TPP)}2(OH)](ClO4) in comparison to that for [{MnIII(OEP)}2(OH)](ClO4) has been explained on the basis of a larger Mn−O(H)−Mn angle in the former.15 Figure 9 shows plots of varying −J with the Mn− O(H) bond length and Mn−O(H)−Mn angle for all of the complexes with OEP type ligands. The −J value of complexes 2·X is found to increase with an increase in Mn−O(H)−Mn angle in the complexes, and a plot of Mn−O(H)−Mn vs −J shows a good linear correlation except for 2·I3, which deviates to a large extent. The exchange interaction also increases with decreasing Mn−O(H) bond distance; a plot of Mn−O(H) vs −J also shows a linear correlation except for the complex 2·I3, which has been found to deviate from linearity. Such anomalous behavior of 2·I3 may be attributed to the large size and shape of the I3− counteranion; similar behavior has also

been observed earlier with the diiron(III) μ-hydroxo bisporphyrin of the ion. Moreover, the J values of other complexes are quite similar to those of the related diiron(III) μhydroxo complexes. It is interesting to note the difference in magnetic behavior between 2·X and 4·X.8a,c J values of −29.5, −34.7, −38.6, −35.1, and −32 cm−1 are observed for 2·I3, 2·BF4, 2·SbF6, 2· PF6, and 2·ClO4, respectively. However, the coupling constant J (−4.5 cm−1) was close to 0 in 4·I3 while significantly higher values of −36, −42, −37.6, and −36.5 cm−1, respectively, were observed for 4·BF4, 4·ClO4, 4·PF6, and 4·SbF6.8a,c Apart from the complexes with I3− counteranions, the J values observed for other complexes are, however, more or less similar and do not seem to depend much on the counteranion size. It seems likely that a large counteranion would result in a decrease in the intradimer and interdimer interactions, leading to a relatively weaker coupling between the two metal centers in 2·I3 and 4·I3 (which has a large I3− counteranion), in an ethene-bridged μhydroxo complex with a large I5− counteranion, and also in {[FeIII(TPP)]2(OH)}+ with very large counteranions such as [CB11H6Cl6]− and [F20-BPh4]−.21b Computational Studies. In an attempt to better understand the interdependence between spin state and porphyrin nonplanarity, we performed a series of computational studies on 2·BF4 and 2+ (after removal of the BF4− counteranion). Density functional calculations have been carried out with the unrestricted B3LYP hybrid functional22−24 using the Gaussian 09, revision B.01, package.25 Geometry optimizations were performed for 2+ (without BF4− counteranion) and 2·BF4 using the LANL2DZ basis set for manganese atoms and the 631G(d,p) basis set for all other atoms. Chloroform was used for solvent correction in all of the calculations reported herein. Atom coordinates have been obtained directly from the crystal H

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry structure of 2·BF4, and the counteranion has been removed in the case of 2+. The complex 2·BF4 has two Mn(III) centers each with two possible spin states, high-spin (S = 2) and low-spin (S = 1), which, in principle, could result in 16 possibilities of coupling the spins of the two Mn(III) centers in the molecule. For example, a spin of S = +2 on one Mn(III) center can couple with spins of +2, + 1, −1, and −2 of the other Mn(III) center. We chose only to focus on ferromagnetic interactions between spin states with combinations (2, 2) and (2,1), which would give some idea regarding the interrelationship between the spin state and porphyrin nonplanarity, as well as the preferential stabilization of spin states in the μ-hydroxo dimanganese(III) bisporphyrin. Figure 10 presents the spin-state energies of the S = (2, 2) and S = (2, 1) states of 2·BF4 and 2+ (without BF4−

Figures S5 and s6 in the Supporting Information display the optimized structures of 2+ and 2·BF4 with S = (2, 2) and S = (2, 1) states. Geometry optimization slightly elongates the Mn− O(H) bond distances from 2.008/2.005 Å in the crystal structure to 2.081/2.077 Å. The Mn−O(H) distances in the optimized structures act as clear markers of the manganese spin state. It has been observed that the Mn−O(H) distance for the low-spin Mn(III) center is shorter than that of the high-spin state. The Δ24 values, which are measures of the extent of porphyrin ring deformation, are virtually identical between the optimized structure and the crystal structure of the complex. This is quite significant for our calculations, since the goal is to observe the effect of spin state change on the extent of porphyrin ring deformation. The Δ24 values of the two porphyrin cores are similar in the case of the S = (2, 2) state, while in the S = (2, 1) state, the value is found to be smaller for the porphyrin having the low-spin Mn(III) center. This can be better visualized by turning to Figure S7 in the Supporting Information, which compares the out-of-plane displacement plots for the optimized geometries of 2·BF4 obtained using S = (2, 2) and S = (2, 1) states. The core deformations of both rings are found to be similar in the S = (2, 2) state. A similar observation was also found for 2+. In the geometry optimization using the S = (2, 1) state, it has been found that porphyrin containing the low-spin Mn(III) center is less distorted in comparison to that with the high-spin state. Therefore, changing the spin state of one Mn(III) center from high-spin (S = 2) to low-spin (S = 1) results in unequal core deformation between two porphyrin rings. This further supports our observation from the out-of-plane displacement plots (Figure 4) of the crystal structures of complexes 2·X (vide supra), where both of the porphyrin cores in a molecule are deformed almost equally. Therefore, the theoretical results along with our experimental findings further confirm the key role played by the spin state of the metal in controlling porphyrin ring deformation. As the dimanganese(III) μ-hydroxo complexes stabilize only the high-spin (S = 2) state of Mn(III), we find the geometry with the S = (2, 2) state is energetically more stable than the S = (2, 1) state. It is to be noted here that the presence of the counteranion leads to an increase in energy difference between the S = (2, 2) and S = (2, 1) states. This suggests that counteranions, along with metal spin state, may have some role in influencing porphyrin ring deformation, which was observed in the diiron(III) μ-hydroxo bisporphyrin 4·X. The spin states of the iron(III) center in both ethane- and ethene-bridged diiron(III) μ-hydroxo bisporphyrins were found to be dependent on the counteranions used.8 An extensive computational study on the ethane-bridged diiron(III) μhydroxo bisporphyrin has revealed that the energetic differences between the S = (5/2, 3/2) and S= (3/2, 3/2) states are very small, and therefore, a subtle change through external perturbations can change the spin state ordering in these complexes.8a Both porphyrin rings in a complex show considerable ruffling but not by the same degree, hence leading to the stabilization of two different spin states of iron. For example, in the X-ray structure of 4·I3, the I3− counteranion has been found to be in close proximity with one of the porphyrin rings, leading to a larger ring deformation in compared to the other ring (Figure 5B).8b−d Moreover, it has been observed that both ethane- and ethene-bridged μ-hydroxo complexes having the same counteranion also have identical structure and spin state behavior. As observed in the solid state, a tightly associated counteranion can produce a significant steric effect

Figure 10. Relative spin-state energies of S = (2, 2) and S = (2, 1) states of (A) 2+ (without BF4− counteranion) and (B) 2·BF4− as calculated using unrestricted B3LYP and B97D functionals in DFT. ΔE and ΔE+ZPE values are relative to the S = (2, 2) state.

counteranion) as calculated using the B3LYP and B97D functional. It has been found that the S = (2, 2) state is energetically more favorable than S = (2, 1) for both 2+ and 2· BF4. This is in accord with our experimental findings, where both manganese(III) centers are in the high-spin (S = 2) state. It has, however, been observed that the presence of the counteranion in 2·BF4 stabilizes the S = (2, 2) over the S = (2, 1) state to a greater extent than is found in the case of 2+, where there is no counteranion. Inclusion of the zero point energy (ZPE) correction increases the energy gap between the S = (2, 2) and S= (2, 1) states by ∼1 kcal/mol for both 2+ and 2·BF4. Table S1 in the Supporting Information gives the structural parameters for all of the optimized geometries of 2·BF4 and 2+ along with the relative energies (in kcal/mol). As can be seen, the alteration of spin state in the μ-hydroxo dimanganese(III) bisporphyrins seems to be an energy-demanding phenomenon. In contrast, μ-hydroxo diiron(III) bisporphyrins, 4·X, were earlier found to have the quartet and sextet states quite close in energy and their relative ordering depended on subtle environmental perturbations.8a I

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

it would have resulted in unequal ring deformation. However, such a geometry would be energetically less favorable than that where the two Mn(III) centers are in the high-spin (S = 2) state, by amounts of ∼15 and ∼20 kcal/mol for 2+ and 2·BF4, respectively. The spin state is, therefore, found to be the controlling factor behind the extent of ring deformation when all other factors remain identical.

on the porphyrin ring in the solution phase also as a consequence of the counteranion’s close proximity.8b,26 The different spin-state behavior of 4·X has been attributed to the properties of the counteranion X, which are also known to operate under the influence of variety of phenomena related to steric effects, charge polarization, stability of the ion pair formations, etc., although the exact reason is still unknown and requires further investigation.8a−c DFT calculations on the dimanganese(III) μ-hydroxo bisporphyrin 2·X demonstrate that the S = (2, 2) state is stabilized to a large extent (ΔE ≈ 20 kcal/mol) in comparison to the mixed spin S = (2, 1) state. Thus, other than high-spin, no other easily accessible spin states are available for Mn(III) and subtle environmental perturbations resulting from possible interactions with the counteranions can hardly influence the porphyrin ring deformation (and produce two equivalent Mn(III) porphyrin centers) which has been observed experimentally and is further supported by DFT. This has resulted in almost no visible structural and spectroscopic changes upon changing the counteranion in 2·X, which is, however, in sharp contrast with the diiron(III) analogues. Spin state is found to be the controlling factor behind the extent of ring deformation when all other factors remain identical. Summary. A series of dimanganese(III) μ-hydroxo bisporphyrins with a variety of counteranions having different sizes and shapes have been synthesized and characterized by various spectroscopic techniques. Crystallographic studies reveal a remarkably bent complex with the smallest known Mn−O(H)−Mn angle, while the manganese centers are equivalent with similar porphyrin ring deformations and are in the high-spin (S = 2) state. This is in sharp contrast with the analogous diiron(III) μ-hydroxo bisporphyrins, where two iron centers are inequivalent with the stabilization of two different spin states of iron. In addition, the two porphyrin rings were found to be distorted to different extents in the diheme analogues, which has been proposed to be the key factor in stabilizing the different spin states of iron. 1 H NMR spectra of all dimanganese(III) μ-hydroxo bisporphyrins reported here are similar, and no counteranion dependence has been observed. The striking features of the spectra are the highly deshielded meso proton signals, which are, however, shifted to a highly shielded region in their diiron analogues. This can be explained in terms of an anomalous spin polarization mechanism which rationalizes the negative spin density on the meso carbons in Mn(III) porphyrins. Variable-temperature magnetic susceptibility measurements have revealed that the two manganese(III) centers in a complex are antiferromagnetically coupled via a hydroxo bridge. The coupling constants (J) vary between −29 and −38.7 cm−1 and are found to be linearly correlated (with the exception of 2·I3) with structural parameters such as Mn−O(H) distance and Mn−O(H)−Mn angle. Other than the complex with an I3− counteranion, the J values observed for other complexes are, however, similar and do not seem to depend much on the counteranions used. The large size and cylindrical shape of the I3− ion might be responsible for the decrease in the intradimer as well as interdimer interactions, leading to a relatively weak antiferromagnetic coupling between the two Mn(III) centers in 2·I3. DFT calculations reproduces the experimental ground state, S = (2, 2), and predict that if there were a possibility of the two Mn(III) centers in the bisporphyrin units to be present in two different spin states, viz. high-spin (S = 2) and low-spin (S = 1),

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EXPERIMENTAL SECTION

Materials. 1,2-Bis(meso-octaethylporphyrinyl)ethane was synthesized by modifying the literature method.13 Reagents and solvents were purchased from commercial sources and were purified by standard procedures before use. Syntheses. 1. A 100 mg portion of 1,2-bis(mesooctaethylporphyrinyl)ethane (0.091 mmol) was dissolved in 50 mL of degassed chloroform. To the above solution was added 300 mg of MnCl2·4H2O (1.56 mmol) dissolved in 50 mL of dry methanol. The solution thus obtained was refluxed for 3 h and then washed with 10% HCl(aq) solution. The organic layer was separated, dried over anhydrous Na2SO4, and then evaporated to complete dryness. The solid product was purified by column chromatography on silica gel using chloroform:methanol (98:2) as eluent. Yield: 87 mg (75%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 365 (6.1 × 104), 480 (5.5 × 104), 573 (1.3 × 104), 615 (5.8 × 103). Complexes 2·X (X = I3, BF4, SbF6, PF6, ClO4) were prepared using a general procedure; details for one representative case are described below. 2·I3. A 100 mg portion of 1 (0.078 mmol) was dissolved in 50 mL of dichloromethane. A 50 mL portion of 10% aqueous HI solution containing 64 mg of I2 was added, and the mixture was stirred for 15 min at room temperature. The organic layer was then separated, dried over anhydrous Na2SO4, and evaporated to complete dryness. The solid compound thus obtained was dissolved in a minimum volume of dichloromethane and carefully layered with n-hexane. After the solution stood for 7−8 days in air at room temperature, a dark crystalline solid was obtained which was then collected by filtration, washed well with the mother liquor, and dried under vacuum. Yield: 110 mg (88%). Conductivity (CH2Cl2, 10−4 M solution at 295 K): ΛM = 25 Ω−1 cm2 mol−1. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 364 (4.6 × 104), 465 (3.8 × 104), 568 (1.2 × 104), 611 (7.5 × 103). 2·BF4. Yield: 85 mg (83%). Conductivity (CH2Cl2, 10−4 M solution at 295 K): ΛM = 23 Ω−1 cm2 mol−1. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 360 (4.9 × 104), 464 (4.1 × 104), 566 (1.1 × 104), 613 (6.5 × 103). 2·SbF6. Yield: 98 mg (86%). Conductivity (CH2Cl2, 10−4 M solution at 295 K): ΛM = 22 Ω−1 cm2 mol−1. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 361 (4.2 × 104), 469 (3.9 × 104), 567 (1.4 × 104), 612 (6.9 × 103). 2·PF6. Yield: 95 mg (90%). Conductivity (CH2Cl2, 10−4 M solution at 295 K): ΛM = 23 Ω−1 cm2 mol−1. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 364 (4.5 × 104), 463 (1.8 × 104), 571 (1.2 × 104), 612 (5.7 × 103). 2·ClO4. Yield: 88 mg (86%). Conductivity (CH2Cl2, 10−4 M solution at 295 K): ΛM = 25 Ω−1 cm2 mol−1. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 363 (4.6 × 104), 470 (4.2 × 104), 567 (1.3 × 104), 611 (8.5 × 103). Computational Methods. DFT calculations have been carried out by employing the unrestricted B3LYP22−24 hybrid functional using the Gaussian 09, revision B.01, package.25 The method used was Becke’s three-parameter hybrid exchange functional, the nonlocal correlation provided by the Lee, Yang, and Parr expression, and the Vosko, Wilk, and Nusair 1980 correlation functional (III) for local correction. The basis set was LanL2DZ for the manganese atom and 631G(d,p) for all other atoms. Calculations were instigated from the crystal structure coordinates of 2·BF4 reported in this paper. Frequency calculations on all of the optimized geometries ensured that there were no imaginary frequencies. Chloroform was used for solvent correction in all of the calculations reported here. To see the J

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 4. Crystal Data and Data Collection Parameters of 2·X formula T (K) formula wt cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z dcalcd, g cm−3 μ, mm−1 F(000) cryst size, mm3 no. of unique data completeness to θ = 25.00°, % no. of params refined GOF on F2 R1a (I > 2σ(I)) R1a (all data) wR2b (all data) largest diff peak and hole, e Å−3 a

2·I3·C6H6

2·BF4·4.25C6H6

2·SbF6·CH2Cl2

2·PF6·2C6H6

2·ClO4·1.5(toluene)

C163H189I6Mn4N16O2 100(2) 3385.46 triclinic P1̅ 14.840(5) 14.997(5) 19.268(5) 87.856(5) 74.193(5) 71.326(5) 3903(2) 1 1.440 1.559 1713 0.22 × 0.18 × 0.12 13208 91.0 914 1.038 0.0910 0.1695 0.2107 1.404 and −1.351

C199H231B2F8Mn4N16O2 100 (2) 3272.38 triclinic P1̅ 14.656(5) 15.833(5) 23.555(5) 96.246(5) 106.556(5) 113.735(5) 4638(2) 1 1.172 0.330 1735 0.18 × 0.15 × 0.10 16032 98.2 1064 1.038 0.0882 0.1239 0.2792 1.683 and −0.472

C75H89Cl2F6Mn2N8Sb 100 (2) 1535.07 monoclinic C2/c 17.832(5) 24.159(5) 18.505(5) 90 116.234(5) 90 7151(3) 4 1.426 0.863 3168 0.20 × 0.16 × 0.12 7018 99.7 457 1.031 0.0728 0.1201 0.1970 0.861 and −0.876

C86H100F6Mn2N8OP 100 (2) 1516.59 monoclinic P21/c 14.9243 (11) 27.767(2) 19.2760(15) 90 105.733(2) 90 7688.7(10) 4 1.310 0.416 3196 0.20 × 0.15 × 0.10 14275 99.8 972 1.001 0.0678 0.1387 0.1863 0.678 and −0.441

C84H101ClMn2N8O5 100(2) 1448.06 triclinic P1̅ 15.0609(11) 15.1516(11) 18.4813(13) 91.405(2) 109.5290(10) 109.816(2) 3693.9(5) 2 1.302 0.437 1536 0.24 × 0.18 × 0.10 13414 98.0 928 1.055 0.0749 0.1288 0.2317 1.557 and −0.759

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. magnetic data for 2·PF6 are J = −35.1 cm−1, D = −8.6 cm−1, p = 0.07%, and TIP = 8.6 × 10−7 cm3 mol−1. The parameters obtained from fitting the magnetic data for 2·ClO4 are J = −32 cm−1, D = −8.2 cm−1, p = 0.36%, and TIP = 5.1 × 10−6 cm3 mol−1. It should also be noted here that an accurate determination of D would require a highresolution EPR at liquid He temperature. However, this would not change the J value to a large extent. X-ray Structure Solution and Refinement. Single-crystal X-ray data were collected at 100 K on a Bruker SMART APEX CCD diffractometer equipped with a CRYO Industries low-temperature apparatus, and intensity data were collected using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The data integration and reduction were processed with SAINT30 software. An absorption correction was applied.31 The structure was solved by direct methods using SHELXS-97 and was refined on F2 by full-matrix least-squares techniques using the SHELXL-2014 program package.32 Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were included in calculated positions. In the refinement, hydrogens were treated as riding atoms using SHELXL default parameters. Crystallographic data and data collection parameters are given in Table 4. CCDC files 1406054, 1406053, 1406052, 1406056, and 1406055 for 2·I3, 2·BF4, 2·SbF6, 2·PF6, and 2·ClO4 respectively, contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

effects of dispersion, we ran a set of single-point energy calculations on the UB3LYP optimized structures using the unrestricted density functional method B97D,27 which incorporates a force-field-like pairwise dispersion correction developed by Grimme, and similar basis set combinations. Visualizations of the optimized geometries, molecular orbitals, and the corresponding diagrams were made using Chem Craf t software.28 Instrumentation. UV−vis spectra were recorded on a PerkinElmer UV/vis spectrometer. Molar conductances were measured at room temperature in dichloromethane on an Elico type CM-82 T conductivity bridge. 1H NMR spectra were recorded on a JEOL 500 MHz instrument. The spectra for paramagnetic molecules were recorded over a 100 kHz bandwidth with 64K data points and a 5 ms 90° pulse. For a typical spectrum between 2000 and 3000 transients were accumulated with a 50 μs delay time. The residual 1H resonances of the solvents were used as secondary references. Magnetic Measurements. Magnetic susceptibility data were collected using a Quantum Design MPMS SQUID magnetometer over the temperature range 5−300 K. The magnetic data were fitted, using the software PHI,19 to a model considering exchange coupling between two zero field split (D being the zero field splitting parameter), high-spin (S = 2) manganese(III) centers. The presence of a small amount of mononuclear manganese(III) impurity and temperature-independent paramagnetism have also been taken into account. Data were collected at an applied magnetic field of 0.1T and corrected for diamagnetism using Pascal’s constant.29 The plots are shown in Figure 8. Considering the axially elongated geometry around the manganese centers in our complex, we have restricted to negative D values for fitting. The value of g has been kept fixed at 2.00. The parameters obtained after fitting the magnetic data for 2·I3 are J = −29.5 cm−1, D = −7.11 cm−1, p = 0.29%, and TIP = 5 × 10−6 cm3 mol−1. The parameters obtained from fitting the magnetic data for 2· BF4 are J = −34.7 cm−1, D = −7.4 cm−1, p = 0.25%, and TIP = 8.4 × 10−7 cm3 mol−1. The parameters obtained from fitting the magnetic data for 2·SbF6 are J = −38.6 cm−1, D = −8.7 cm−1, p = 0.24%, and TIP = 4.7 × 10−6 cm3 mol−1. The parameters obtained from fitting the

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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.5b02226. Packing diagrams of 2·BF4, 2·SbF6, 2·PF6, and 2·ClO4, optimized geometries of 2+ and 2·BF4, comparison of structural parameters for the optimized geometries of 2+ and 2·BF4, and out of plane displacement plots of the K

DOI: 10.1021/acs.inorgchem.5b02226 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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optimized geometries of 2·BF4 in the S = (2, 2) and S = (2, 1) states (PDF) Crystallographic data for 2·I3 (CIF) Crystallographic data for 2·BF4(CIF) Crystallographic data for 2·SbF6 (CIF) Crystallographic data for 2·PF6 (CIF) Crystallographic data for 2·ClO4 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.P.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Science and Engineering Research Board (SERB), New Delhi, India, and the Council of Scientific and Industrial Research, New Delhi, India, for financial support. D.S. and F.S.T.K. thank the UGC of India for their fellowships. We thank Prof. Devesh Kumar for some helpful discussion on DFT calculations.

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DEDICATION Dedicated to Professor Narayanasami Sathyamurthy on the occasion of his 65th birthday. REFERENCES

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