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Article

Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear MnIII Complexes: Synthesis, Crystal Structures, and Spectroscopic Properties

by
Hao-Ran Jia
,
Jian Chang
,
Hong-Jia Zhang
,
Jing Li
and
Yin-Xia Sun
*
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(7), 272; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst8070272
Submission received: 22 May 2018 / Revised: 18 June 2018 / Accepted: 21 June 2018 / Published: 28 June 2018
(This article belongs to the Section Crystal Engineering)
Browse Figures
Versions Notes

Abstract

:
Three polyhydroxyl-bridged tetranuclear MnIII complexes [Mn4(L1a)2(μ3-OMe)2(μ2-OMe)2(MeOH)2] (1), [Mn4(L2a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (2), and [Mn4(L3a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (3) derived from Mnn+-promoted reactivity of Schiff base ligands (HL1 = 1-(4-{[(E)-3,5-dichlorine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime, HL2 = 1-(4-{[(E)-3-bromine-5-chloro-2-hydroxybenzylidene]amino}phenyl) ethanone O-benzyloxime, and HL3 = 1-(4-{[(E)-3,5-dibromine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime) have been synthesized and characterized. In the MnIII complexes 1, 2, and 3, the newly formed ligands (L1a)4−, (L2a)4−, and (L3a)4− are derived from the chemoselective cleavage of the C=N bond in the original Schiff base ligands HL1, HL2, and HL3 to form corresponding halogenated salicylaldehyde, 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde, respectively. Then, the further addition of acetone to two halogenated salicylaldehyde molecules in situ α,α double aldol reaction promoted by Mnn+ ions in the presence of base to give the new ligands ((Lna)4−. X-ray crystallographic analyses of the MnIII complexes 1, 2, and 3 show that the three complexes are all tetranuclear structure and crystallizes in the triclinic system, space group P-1. The four MnIII ions and bridging alkoxido groups are arranged in a face-shared dicubane-like core with two missing vertices. In the three MnIII complexes, the asymmetric unit contains two kinds of different MnIII ions (Mn1 and Mn2), where the MnIII ions are all hexacoordinated with slightly distorted octahedral geometries. Simultaneously in the synthesis of multinuclear Mnn+ complexes above, we explored the crystal structure, spatial configuration, and spectroscopic properties of the multinuclear MnIII complexes with different halogen substituents.

1. Introduction

There is a wide range of research space in aldol condensation reactions, in particular, in its asymmetric and catalytic field, via an asymmetric organic catalyst or chelating agent of the metal complex unit. Normally, the B, Ti, or Sn ions because of their specific Lewis acid properties are used for promoting enolization followed by aldol addition [1,2,3]. To a lesser extent, the other first row of transition metal Zr, Co, Ni, Cu, or Zn ions also have been used [4,5,6]. In addition, we have recently reported a tetranuclear ZnII complex using a double aldol ligand formed in situ by α,α-double aldol addition of acetone to 3,5-dichlorosalicylaldehyde promoted by ZnII ion in the presence of base [7]. In efforts to design new multidentate ligands which can form novel polynuclear complexes with attractive structural features, we are probing into this reaction using Lewis acid metal assistance. This favorable reaction will allow to obtaining ligands that are extremely difficult to separate by classical anionic organic chemistry under conventional conditions. This, in turn, will allow the synthesis of novel-innovative metal complexes that are pre-restricted by ligand design [8,9,10,11,12,13,14,15,16,17]. Based on this, the transition metal Mn ion has become our target of choice due to its various oxidation states and Lewis acidity [18], which acts as a potential promoter to study the extent of the in situ aldol reaction of acetone and salicylaldehyde derivatives. Up to now, the use of Mn ions for the synthesis of aldol products has been relatively little explored, as reported, a unique one-pot α,α-double aldol addition of acetone to two o-vanillin molecules promoted by Mnn+ ions in situ, leading to a novel multidentate ligand, further obtained a rare defect-dicubane {Mn4} complex [19]. Therefore, we hope that we can synthesize a variety of multinuclear Mn complexes, explore the spatial structure and crystal parameters, and perhaps determine the laws.
Moreover Schiff base ligands and their complexes have been application in many fields [20,21,22,23], such as biological activity reagents [24,25,26,27,28,29,30,31,32,33,34], magnetic materials [35,36,37,38,39,40,41,42,43], luminescent materials [44,45,46,47,48,49,50,51,52]. Based on this, we designed and synthesized three Schiff base ligands with different halogen substituents and their MnIII complexes 1, 2, and 3, respectively. In the synthesis of this MnIII complexes, due to the hydrolysis chemoselective cleavage of the C=N bond of the original Schiff base ligands (HL1, HL2, and HL3), forms the corresponding halogenated salicylaldehyde molecules and further the α,α double aldol addition of acetone to two halogenated salicylaldehyde molecules promoted by Mnn+ ions in situ, leading to a unique corresponding multidentate polyhydroxyl ligand (Lna)4−, acetone-disalicylaldehyde aldol (Scheme 1). On the basis of the synthesis of multinuclear MnIII complexes, we have investigated the crystal structure and spatial configuration of the multinuclear MnIII complexes with different halogen substituents. To the best of our knowledge, a few aldol additions were previously reported as a one-pot reaction or a specific Mnn+-promoted reaction.

2. Experiments

2.1. Reagents and Physical Measurements

4-aminoacetophenone, O-benzylhydroxylamine and 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde were purchased from Aldrich and used without further purification. The other reagents and solvents were analytical grade reagents from Tianjin Chemical Reagent Factory. C, H, and N analyses were carried out with a GmbH VariuoEL V3.00 automatic elemental analyzer (Hanau, Germany). FT-IR spectra were recorded on a VERTEX70 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany), with samples prepared as KBr (500–4000 cm−1). UV–vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer (Kyoto, Japan). X-ray single crystal structure was determined on a Bruker Smart Apex-II CCD diffractometer (Karlsruhe, Germany). 1H NMR spectra were recorded using a Mercury-400BB spectrometer (Varian, Palo Alto, CA, USA) at 500 MHz. Melting points were measured by the use of a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company (Beijing, China) and the thermometer was uncorrected. Fluorescent spectra were performed on aLS-55fluorescence photometer (Perkin-Elmer, Norwalk, CA, USA).

2.2. Synthesis of HL1, HL2, and HL3

HL1, HL2, and HL3 were synthesized according to an analogous method reported previously in the literature [53,54,55,56,57,58,59,60]. The synthetic route involved in the synthesis of HL1, HL2, and HL3 are given in Scheme 2.
1-(4-Aminophenyl)ethanone O-benzyl oxime was synthesized according to an analogous method reported early [7]. To an ethanol solution (5 mL) of 1-(4-aminophenyl)ethanone (272.0 mg, 2.2 mmol) was added an ethanol solution (5 mL) of O-benzylhydroxylamine (270.0 mg, 2.2 mmol). The mixture solution was stirred at 328 K for 18 h. Cooled to room temperature, and the precipitate was filtered and washed successively with ethanol and n-hexane, respectively. The product was dried under vacuum and purified with recrystallization from ethanol to obtain 411.80 mg of 1-(4-aminophenyl)ethanone O-benzyl oxime. Yield, 85.2%. m.p. 352–353 K. Anal. Calcd. for C15H16N2O (%): C, 74.97; H, 6.71; N, 11.66; Found (%): C, 74.92; H, 6.76; N, 11.63. 1H NMR (500 MHz, DMSO-d6) δ = 2.16 (s, 2H, CH2), 2.52 (s, 3H, CH3), 5.15 (s, 2H, Ar-NH2), 6.84 (d, J = 8.1 Hz, 2H, Ar–H), 7.67–7.20 (m, 7H, Ar–H).
Add ({4-amino}phenyl)ethanone O-benzyloxime (240.0 mg, 1.0 mmol) into ethanol solution (7 mL) of 3,5-dichlorosalicylaldehyde (191.5 mg, 1.0 mmol). The mixture solution was stirred at 333 K for 18 h. After cooling to room temperature, the precipitate was filtered and washed successively with ethanol and ethanol/n-hexane (1/4), respectively. The product was dried under reduced pressure to obtain 301.84 mg HL1, Yield, 79.2%. m.p. 402–403 K. Anal. Calcd. for Anal. Calcd. for C22H18Cl2N2O2 (HL1) (%): C, 63.93; H, 4.39; N, 6.78. Found (%): C, 63.99; H, 4.42; N, 6.75. 1H NMR (500 MHz, DMSO-d6) δ = 2.25 (s, 3H, –CH3), 5.23 (s, 2H, Ar–CH2–O), 7.32–7.79 (m, 11H, Ar–H), 9.06 (s, 1H, CH=N), 14.35 (s, 1H, OH).
The ligands HL2 and HL3 were prepared by a method similar to that of HL1 except substituting 3,5-dichlorosalicylaldehyde with 3-bromine-5-chlorosalicylaldehyde or 3,5-dibrominesalicylaldehyde, respectively. HL2: 351.33 mg, Yield, 76.8%. m.p. 429–431 K. C22H18BrClN2O2 (HL2) (%): C, 57.73; H, 3.96; N, 6.12. Found: C, 58.04; H, 4.02; N, 6.08. 1H NMR (500 MHz, DMSO-d6) δ = 2.25 (s, 3H, –CH3), 5.23 (s, 2H, Ar–NH2), 7.32–7.87 (m, 11H, Ar–H), 9.04 (s, 1H, CH=N), 14.49 (s, 1H, OH). HL3: 419.84 mg. Yield, 83.8%. m.p. 441–442 K. C22H18Br2N2O2 (HL3) (%): C, 52.62; H, 3.61; N, 5.58. Found: C, 52.71; H, 3.57; N, 5.75. 1H NMR (500 MHz, DMSO-d6) δ = 2.25 (s, 3H, –CH3), 5.23 (s, 2H, Ar–CH2–O), 7.32–7.93 (m, 11H, Ar–H), 9.08 (s, 1H, CH=N), 14.55 (s, 1H, OH).

2.3. Syntheses of MnIII Complexes 1, 2, and 3

Complex 1: A solution of Mn(OAc)2·4H2O (2.5 mg, 0.01 mmol) in methanol (5 mL) was added dropwise to a solution of HL1 (10.2 mg, 0.02 mmol) in acetone (3 mL) containing three drops of trimethylamine at room temperature. The color of the mixed solution turned red-brown immediately, then stirred for 0.5 h at room temperature. The mixture solution was filtered and the filtrate was allowed to stand at room temperature for about two weeks. The solvent was partially evaporated and obtained several red-brown prismatic single crystals suitable for X-ray crystallographic analysis. The yield was 52% (based on the total available Mn). Anal. Calcd. for C40H40Cl8Mn4O16 (%): C, 37.53; H, 3.15. Found: C, 37.59; H, 3.21.
Complex 2: Complex 2 and 3 were synthesized by adding a solution of Mn(OAc)2·4H2O (2.5 mg, 0.01 mmol) in methanol (5 mL) dropwise to HL2 (10.3 mg, 0.02 mmol) in acetone (3 mL), HL3 (10.4 mg, 0.02 mmol) in acetone (3 mL), respectively, and others steps are same as complex 1. The yields were 56% and 59% (based on the total available Mn), respectively. Complex 2: C38H36Cl4Br4Mn4O16 (%): C, 31.92; H, 2.54. Found: C, 32.17; H, 2.38. Complex 3: C38H36Br8Mn4O16 (%): C, 28.39; H, 2.26. Found: C, 28.37; H, 2.28.

2.4. X-Ray Crystallography

The selected single crystals of complexes 1, 2, and 3 were put in a sealed tube, and the measurement was performed on a Bruker Smart Apex-II CCD diffractometer (Karlsruhe, Germany). The reflections were collected by a graphite monochromated Cu Ka radiation (λ = 1.54184 Å) at 293(2) K and 296.15(2) K for MnIII complexes 1 and 3, respectively, and that of 2 was collected by a graphite monochromated Mo Ka radiation (λ = 0.71073 Å) at 296.15(2) K. The SMART and SAINT software packages [61] were used for data collection and reduction respectively. Absorption corrections based on multiscans using the SADABS software [62] were applied. The structures were solved by direct methods and refined by full-matrix least-squares against F2 using the SHELXL program [63]. All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and isotropically fixed in the final refinement. Details of the crystal parameters, data collection, and refinements for complexes 1, 2, and 3 are summarized in Table 1. Supplementary crystallographic data for this paper have been deposited at the Cambridge Crystallographic Data Centre (1538187, 1538179, and 1538146 for complexes 1, 2, and 3) and can be gained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

3. Results and Discussion

3.1. Synthesis

Synthesis of MnIII complexes 1, 2, and 3 consisting of the addition of Schiff base ligands (HL1, HL2, and HL3) to the acetone solution containing a small amount of triethylamine, respectively, followed by addition of a solution of Mn(OAc)2·4H2O in methanol (5 mL). It is worth noticing that the C=N bond of the original Schiff base ligands HL1, HL2, and HL3 have chemoselectively cleaved due to the hydrolytic action and gave the corresponding halogenated salicylaldehyde derivatives, 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde, respectively. Then a unique one-pot α,α double aldol addition in situ of acetone to two corresponding halogenated salicylaldehyde molecules promoted by Mnn+ ions, resulting to the corresponding new multidentate ligands H4L1a, H4L2a, and H4L3a, acetone-disalicylaldehy aldol (Scheme 1), respectively. In the process of synthesis of MnIII complexes 1, 2, and 3, triethylamine acts as a base in this reaction forming bridging oxides, methoxides, and ligands after deprotonation.
It is worth noting that, in order to explore the oxidation of manganese ions, the same reactions to synthesis the MnIII complexes 1, 2, and 3 were carried out under air-free conditions to prevent the oxidation of MnII to MnIII. As well as the reaction were carried out under acetone-free and Mn salt-free conditions (see Supporting Information). However, these reactions did not give any crystals of the complex product nor any aldol product in the solution (analyzed by NMR), which leads us to believe that MnIII is more likely to promote the α,α double aldol addition, and the acetone is also a necessary factor.
Based on the above findings, our proposed mechanism to rationalize the formation of the new generated polyhydroxyl multidentate ligands, H4L1a, H4L2a, and H4L3a, by Mnn+-promoted α,α-double aldol additions [7,19] is described in Scheme 3. Firstly, the initial Schiff base ligands (HL1, HL2, and HL3) underwent hydrolysis to get 1-(4-aminophenyl)ethanone O-benzyl oxime and corresponding salicylaldehyde derivatives (3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, 3,5-dibrominesalicylaldehyde), respectively (Scheme 3A). Then, a molecule of acetone is deprotonated by the hydroxide moiety of the base, triethylamine, yielding enolate i, which is stabilized by the keto-enolate mesomeric effect as well as coordination to Mnn+ (Scheme 3B). Moreover, the phenol position of salicylaldehyde derivatives is also deprotonated by the hydroxide base leading to a Mnn+-phenolate complex ii. The concomitant coordination of the Mnn+ ion to the neighboring carbonyl oxygen atom of salicylaldehyde derivatives allows the activation of the aldehyde function which undergoes the first aldol addition of the acetone enolate i , yielding the first aldol product iii (Scheme 3C). The latter can then be further deprotonated either in the α or α′ position from the carbonyl of the acetone residue. Deprotonation of iii occurs predominantly in the α position due to the thermodynamic conditions used which favor deprotonation on the most substituted α position (Scheme 3D). The formation of enolate iv, stabilized by the keto-enolate mesomeric effect, is also promoted by Mnn+ coordination assistance. Enolate iv can undergo another aldol addition on a second molecule of salicylaldehyde derivatives, activated by its coordination to Mnn+, leading to the ligand acetone-di-salicylaldehyde aldol ν (H4L1a, H4L2a and H4L3a) which is quadruply deprotonated and coordinated to two Mnn+ ions

3.2. IR Spectra

The FT-IR spectra of MnIII complexes 1, 2, and 3 exhibit various bands in the 500–4000 cm−1 region. The most important FT-IR bands are listed in Table 2. In the MnIII complexes 1, 2, and 3, the bands appear at about 3437, 3416, and 3445 cm−1, respectively, are attribute to O–H stretching frequency coordination methanol or water molecules, which confirmed by the crystal structure [64,65,66,67,68,69]. No characteristic C=N stretching band is found in complexes 1, 2, and 3 indicating that the ligands HL1, HL2, and HL3 are converted into the polyhydroxy deprotonation ligands and coordinated to the MnIII ions. The frequency of Ar–O and C–O stretching vibration shows a strong band at 1244 and 1156 cm−1, 1296 and 1167 cm−1, and 1275 and 1173 cm−1 in the MnIII complexes 1, 2, and 3, respectively [70,71,72,73]. The characteristic stretching of the carbonyl (C=O) group appears at 1587, 1602, and 1620 cm−1 as the strong bands in the MnIII complexes 1, 2, and 3, respectively. The moderate and weak vibrations appearing around 698 and 503 cm−1 in 1 (698 and 503 cm−1 in 2, 698 and 503 cm−1 in 3) correspond to the asymmetric and symmetric stretching vibrations of the Mn–O–Mn, respectively, indicating that the Mn-O bond forms at the MnIII ions and the oxygen atoms.

3.3. UV–vis Absorption Spectra

The UV–vis spectra of the MnIII complexes 1, 2, and 3 were recorded in 1.0 × 10−5 mol·L−1 DMF solution at room temperature shown in Figure 1.
A broad absorption band at 443, 417, and 410 nm in complexes 1, 2, and 3 are observed, respectively, which can be attributed to the L→M charge-transfer transitions. In the complex 2, the peak at 297 can be assigned to the n–π* charge transition of the C=O bond of the new ligand (L2a)4− unit, probably arising from the charge transfer of an oxygen atom in the methoxy group to the MnIII center. The shoulder band at 472 nm can attribute to the d-d transitions for elongated octahedral MnIII ions [74,75,76,77], however, which are not observed in complexes 1 and 3 maybe because of the weaker d–d transitions absorption peak of MnIII ions are obscured by the stronger L→M charge-transfer absorption peak of the complexes.

3.4. Crystal Structure of MnIII Complexes 1, 2, and 3

Crystal structures and atom numberings of complexes 1, 2, and 3 are depicted in Figure 2, Figure 3 and Figure 4, respectively. Selected bond lengths and angles for the MnIII complexes 1, 2, and 3 are listed in Table S1. X-ray crystal structure analyses revealed that the MnIII complexes 1, 2, and 3 take on similar crystal structures, which all are centrosymmetric tetranuclear cluster. The crystals of MnIII complexes are all solved as triclinic space group P-1. All the MnIII complexes 1, 2, and 3 consist of four MnIII ions, two (Lna)4− (n = 1 (1), n = 2 (2), n = 3 (3)) moieties, two μ3-OMe and two μ2-OMe units, two coordinated methanol molecules (in 1) or water molecules (in 2 and 3) (Figure 2a). The MnIII ions and bridging alkoxido groups are arranged in a face-shared dicubane-like core with two missing vertices, which is typical Mn4 “defect-dicubane” structure known [78]. The planar Mn4 rhombus can be described as being composed of two Mn3 triangular faces, each held together by a μ3-oxygen atom [O6 or O6a (a = 1 − x, 1 − y, 1 − z)] of a CH3O− ligand with distances to the MnIII atoms in the range 2.004(5)–2.265(5) Å in 1 (1.996(6)–2.272(6) Å in 2, 2.009(4)–2.256(4) Å in 3), which are nearly equal to that in the similar complexes [MnIII4(AcVn2)2(μ3-OMe)2(μ-OMe)2(MeOH)2] [19]. The Mn2–O6–Mn1, Mn2–O6–Mn1a, and Mn1–O6–Mn1a (a = 1 − x, 1 − y, 1 − z) angles are 101.4(2)°, 93.9(2)°, and 99.1(2)° in 1, respectively (101.4(3), 93.3(3)°, and 100.4(3)° in 2; 101.1(2)°, 93.4(2)°, and 100.3(2)° in 3). The two MnIII (Mn1 and Mn1a (a = 1 − x, 1 − y, 1 − z)) ions lie on the common edge of the two triangular faces. The external edges of each triangle are hold by μ2-oxygen atoms (O2, O2a (a =1 − x, 1 − y, 1 − z)) and (O7, O7a (a = 1 − x, 1 − y, 1 − z)) with distances of 1.866(6) and 2.226(5) Å of O2 to Mn1 and Mn2 in 1 (1.863(6) and 2.210(6) Å in 2, 1.863(4) and 2.227(4) Å in 3) as well as 1.922(6) and 1.968(6) Å of O7 to Mn1 and Mn2 in 1 (1.904(6) and 1.967(6) Å in 2, 1.918(4) and 1.966(5) Å in 3), respectively (Figure 2b), which are similar to the complexes of Schiff base ligands previously reported [79]. The Mn1–O2–Mn2 and Mn1–O7–Mn2a (a = 1 − x, 1 − y, 1 − z) angle is 99.5(2)° and 108.1(2)° in 1 (99.8(3)° and 108.3(3)° in 2, 99.6(2)° and 107.6(2)° in 3), respectively.
In the crystal structures of MnIII complexes 1, 2, and 3, the asymmetric unit contains two kinds of different MnIII ions, the metal center Mn1 is hexa-coordinated by three O (O7, O6, O6a (a = 1 − x, 1 − y, 1 − z)) atoms from one μ2-OMe and two μ3-OMe units, respectively, and the remaining three O atoms (O1, O2, and O5) are from a multidentate polyhydroxyl (Lna)4− ligands. The coordination geometry of Mn1 can be described as a slightly distorted octahedron geometry based on the corresponding bond lengths and angles (Figure 2c). The equatorial donor atoms O1, O2, O6, and O7 are nearly co-planar, with slight deviation from the mean plane: 0.050(2), −0.053(1), 0.051(2), and −0.048(3) Å in 1 (0.051(3), −0.054(2), 0.052(2), and −0.050(1) Å in 2, 0.050(2), −0.053(4), 0.050(2), and −0.048(3) Å in 3), respectively, and the Mn1 atom deviates by only 0.091(2) Å in 1 (0.095(3) Å in 2, 0.093(1) Å in 3) from this plane. As expected for the MnIII ion, the axial Mn1–O5 and Mn1–O6a (a = 1 − x, 1 − y, 1 − z) distances of 2.194(6) Å and 2.265(5) Å in 1 (2.210(7) Å and 2.272(6) Å in 2, 2.208(5) Å and 2.256(4) Å in 3), respectively, are longer than the four Mn-O distances in equatorial sites with Mn1–O1 = 1.899(6) Å, Mn1–O2 = 1.866(6) Å, Mn1–O6 = 2.005(5) Å, Mn1–O7 = 1.922(6) Å in 1 (Mn1–O1 = 1.882(7) Å, Mn1–O2 = 1.863(6) Å, Mn1–O6 = 1.996(6) Å, Mn1–O7 = 1.904(6) Å in 2 and Mn1–O1 = 1.885(5) Å, Mn1–O2 = 1.863(4) Å, Mn1–O6 = 2.008(4) Å, Mn1–O7 = 1.918(4) Å in 3). Obvious bond elongation, due to Jahn–Teller effect, is observed for the apical bonds O5–Mn1–O6a, which is expected for high-spin MnIII as well as on the basis of charge considerations. This phenomenon is often found in the MnIII complexes [80]. The Mn2 atoms are also hexa-coordinated by one O atom (O8) of solvent methanol (in 1) or water molecule (in 2 and 3), two O atoms (O7, O6) from μ2-OMe and μ3-OMe units, respectively, and the remaining three O atoms (O2, O3, O4) come from the (Lna)4− ligands, which generate an elongated octahedron around Mn2 on the basis of the corresponding bond lengths and angles (Figure 2c). The square bases of the octahedron for Mn2 consist of O3, O4, O6, O7a (a = 1 − x, 1 − y, 1 − z), with the apical position occupied by O2 and O8. Similarly, the apical bonds along O2-Mn2-O8 direction are elongated. The Mn∙∙∙Mn separations are 3.253(2) Å for Mn1∙∙∙Mn1a (a = 1 − x, 1 − y, 1 − z), 3.133(2) Å for Mn1∙∙∙Mn2a, 3.149(2) Å for Mn1∙∙∙Mn2a (a = 1 − x, 1 − y, 1 − z), and 5.374(2) Å for Mn2∙∙∙Mn2a (a = 1 − x, 1 − y, 1 − z) in 1 (that of 3.282(2) Å, 3.123(3) Å, 3.138(2) Å and 5.332(2) Å 2, 3.277(1) Å, 3.133(1) Å, 3.135(1) Å and 5.343(1) Å in 3). This phenomenon is consistent with the literature report [19,81,82].
The molecular structures of MnIII complexes 1, 2, and 3 are similar to each other, but because of the intra- and inter-molecular hydrogen bond interactions (Table 3), their supramolecular structures are different. As illustrated in Figure 5, in the MnIII complex 1, there are three pairs of intramolecular (O8a–H8A···O1, C18–H18B···O7 and C19a-H19C···O4 (a = 1 − x, 1 − y, 1 − z)) hydrogen bond interactions which are helpful to stabilize the whole architecture. Furthermore, the complex 1 molecules interlinks into one 1D infinite chains along the a axis by a pair of weak intermolecular C19b–H19Ab···O5 (b = 2 − x, 1 − y, 1 − z) hydrogen bond interactions and the other 1D infinite chains [83,84] along the b axis by a pair of C11c-H11c···O3 (c = 1 − x, 2 − y, 1 − z) hydrogen bond interactions, respectively (Figure 6 and Figure 7). Thus, these two 1D chains interlink to each other resulting to the crystal packing of the MnIII complex 1 shown a 2D-layer supramolecular structure [85,86,87,88,89] parallel to the ab-planes (Figure 8).
In complex 2, three pairs of intra-molecular O8–H8B···O1, C18–H18A···O7, and C19a–H19Ba···O4 (a = 1 − x, 1 − y, 1 − z) hydrogen bonds are formed (Figure 9), which are helpful to stabilize the whole architecture. The –O8H8B of coordinated water is hydrogen-bonded to the O1 atom of the (L2a)4− ligand. The proton (–C18H18A) of the μ3-MeO− units is hydrogen-bonded to the O7 atom of the μ2-MeO− units. The proton (–C19aH19Ba (a = 1 − x, 1 − y, 1 − z)) of the μ2-MeO− units is hydrogen-bonded to the O4 atom of the (L2a)4− ligand. Synchronously, the independent complexes are held together from zero-dimension into 1D supramolecular chains along the crystallographic b axis through a pair of intermolecular C11c–H11c···O3 (c = 1 − x, 2 − y, 1 − z) hydrogen bonding (Figure 10).
In the crystal structure of complex 3, there are a pair of intramolecular hydrogen bonds C19a–H19Ba···O4 (a = 1 − x, 1 − y, 1 − z) to stabilize the whole architecture (Figure 11, Table 3). Meanwhile, the complex 3 interlinks into one 1D infinite chain along the b axis by a weak intermolecular C11c–H11c···O3 (c = 1 − x, 2 − y, 1 − z) hydrogen bond interaction [90,91,92,93] (Figure 12).

3.5. Fluorescence Properties

The emission spectra of the Mn(III) complexes 1, 2, and 3 were investigated in dilute DMF solution (5.0 × 10−5 mol/L) at room temperature (Figure 13). The complexes exhibit the bluish violet photoluminescence with maximum emissions at 486, 494, and 501 nm (π-π*) upon excitation at 320 nm. The changes of the maximum emission wavelength and fluorescence intensity may be related to the different substituents on complexes 1, 2, and 3 [94,95,96,97,98,99,100]. As discussed above, because of the coordination of MnIII ions to the ligand, which resulting in increasing of the delocalization of electrons and reducing the energy gaps between the π-π* molecular orbitals of the ligand in the complexes (from 3, 1 to 2).

4. Conclusions

Based on the above data, description and discussion, three tetranuclear MnIII complexes with defective double-cubane cores, namely [Mn4(L1a)2(μ3-OMe)2(μ2-OMe)2(MeOH)2] (1), [Mn4(L2a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (2), and [Mn4(L3a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (3) have been synthesized and characterized. X-ray crystal structure determinations revealed that the structural features of complexes 1, 2, and 3 are similar except for the differences in the coordinated solvent molecules and the substituent of the ligands. There are worth noting that when the Schiff base ligand reacted with MnIII acetate tetrahydrate, they undergo an one-pot chemoselective cleavage of the C=N bond and further the α,α double aldol addition of acetone to two salicylaldehyde derivatives molecules promoted by Mnn+ ions in situ, leading to the novel multidentate polyhydroxyl ligand (L1a)4−, (L2a)4−, and (L3a)4−. This has proved an effective route to obtain the multidentate ligands and their tetranuclear MnIII compounds. In these MnIII complexes, all hexa-coordinated MnIII atoms adopt elongated slightly distorted octahedral geometries. In addition, the MnIII complex 1 possess a self-assembling infinite 2D supramolecular structure, whereas complexes 2 and 3 show the 1D chain. Interestingly, the existence of substituent effect in complexes 1, 2, and 3 may be responsible for the slight differences in their coordination geometries, supramolecular structure, and fluorescence properties.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/2073-4352/8/7/272/s1, Table S1: Selected bond lengths (Å) and angles (°) for MnIII complexes 1, 2 and 3.

Author Contributions

Y.-X.S. conceived and designed the experiments; H.-R.J. and J.C. performed the experiments; H.-J.Z. and J.L. analyzed the data; H.-R.J., and Y.-X.S. wrote the paper.

Funding

This research received no external funding.

Acknowledgments

This work was supported by Program for Excellent Team of Scientific Research in Lanzhou Jiaotong University (201706), which is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 08 00272 sch001 550
Scheme 1. Synthesis of the multidentate polyhydroxyl ligand (Lna)4− and their corresponding MnIII complexes 1, 2, and 3.
Scheme 1. Synthesis of the multidentate polyhydroxyl ligand (Lna)4− and their corresponding MnIII complexes 1, 2, and 3.
Crystals 08 00272 sch001
Crystals 08 00272 sch002 550
Scheme 2. Synthetic route of the ligands HL1, HL2, and HL3.
Scheme 2. Synthetic route of the ligands HL1, HL2, and HL3.
Crystals 08 00272 sch002
Crystals 08 00272 sch003 550
Scheme 3. Proposed mechanism for the Mnn+-promoted α,α double aldol additions. (A) Hydrolysis of Ligands HL1, HL2, and HL3. (B) Deprotonation of the acetone. (C) Activation of the salicylaldehyde derivatives and aldol addition. (D) Deprotonation of the aldol product and second aldol addition.
Scheme 3. Proposed mechanism for the Mnn+-promoted α,α double aldol additions. (A) Hydrolysis of Ligands HL1, HL2, and HL3. (B) Deprotonation of the acetone. (C) Activation of the salicylaldehyde derivatives and aldol addition. (D) Deprotonation of the aldol product and second aldol addition.
Crystals 08 00272 sch003
Crystals 08 00272 g001 550
Figure 1. UV–vis absorption spectra of the MnIII complexes 1, 2, and 3 in DMF solution (c = 1 × 10−5 mol/L).
Figure 1. UV–vis absorption spectra of the MnIII complexes 1, 2, and 3 in DMF solution (c = 1 × 10−5 mol/L).
Crystals 08 00272 g001
Crystals 08 00272 g002 550
Figure 2. (a) Molecule structure and atom numbering of complex 1 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).
Figure 2. (a) Molecule structure and atom numbering of complex 1 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).
Crystals 08 00272 g002
Crystals 08 00272 g003 550
Figure 3. (a) Molecule structure and atom numbering of complex 2 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 2 (symmetry code: a = 1 − x, 1 − y, 1 − z).
Figure 3. (a) Molecule structure and atom numbering of complex 2 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 2 (symmetry code: a = 1 − x, 1 − y, 1 − z).
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Crystals 08 00272 g004 550
Figure 4. (a) Molecule structure and atom numbering of complex 3 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 3 (symmetry code: a = 1 − x, 1 − y, 1 − z).
Figure 4. (a) Molecule structure and atom numbering of complex 3 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for MnIII atoms of complex 3 (symmetry code: a = 1 − x, 1 − y, 1 − z).
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Crystals 08 00272 g005 550
Figure 5. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).
Figure 5. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).
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Crystals 08 00272 g006 550
Figure 6. Part of 1D supramolecular structure containing C19b–H19Ab···O5 hydrogen bond interactions along a axis of the MnIII complex 1 (symmetry code: b = 2 − x, 1 − y, 1 − z).
Figure 6. Part of 1D supramolecular structure containing C19b–H19Ab···O5 hydrogen bond interactions along a axis of the MnIII complex 1 (symmetry code: b = 2 − x, 1 − y, 1 − z).
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Crystals 08 00272 g007 550
Figure 7. Part of 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions along b axis of the MnIII complex 1 (symmetry codes: c = 1 − x, 2 − y, 1 − z).
Figure 7. Part of 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions along b axis of the MnIII complex 1 (symmetry codes: c = 1 − x, 2 − y, 1 − z).
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Crystals 08 00272 g008 550
Figure 8. View of the 2D layered motif within MnIII complex 1 (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
Figure 8. View of the 2D layered motif within MnIII complex 1 (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
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Crystals 08 00272 g009 550
Figure 9. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 2 (symmetry codes: a = 1 − x, 1 − y, 1 − z).
Figure 9. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 2 (symmetry codes: a = 1 − x, 1 − y, 1 − z).
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Crystals 08 00272 g010 550
Figure 10. 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions in complex 2 along b axis (symmetry codes: c = 1 − x, 2 − y, 1 − z).
Figure 10. 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions in complex 2 along b axis (symmetry codes: c = 1 − x, 2 − y, 1 − z).
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Crystals 08 00272 g011 550
Figure 11. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 3 (symmetry codes: a = 1 − x, 1 − y, 1 − z).
Figure 11. Part of intramolecular C–H···O hydrogen-bonding interactions in MnIII complex 3 (symmetry codes: a = 1 − x, 1 − y, 1 − z).
Crystals 08 00272 g011
Crystals 08 00272 g012 550
Figure 12. Part of 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions along the b axis in complex 3 (symmetry codes: c = 1 − x, 2 − y, 1 − z).
Figure 12. Part of 1D supramolecular structure containing C11c–H11c···O3 hydrogen bond interactions along the b axis in complex 3 (symmetry codes: c = 1 − x, 2 − y, 1 − z).
Crystals 08 00272 g012
Crystals 08 00272 g013 550
Figure 13. Emission spectra of the MnIII complexes 1, 2 and 3 (λex = 320 m) in DMF solution (5.0 × 10−5 mol/L).
Figure 13. Emission spectra of the MnIII complexes 1, 2 and 3 (λex = 320 m) in DMF solution (5.0 × 10−5 mol/L).
Crystals 08 00272 g013
Table
Table 1. Crystal data and structure refinement for MnIII complexes 1, 2, and 3.
Table 1. Crystal data and structure refinement for MnIII complexes 1, 2, and 3.
Compound123
Empirical formulaC40H40Cl8Mn4O16C38H36Br4Cl4Mn4O16C38H36Br8Mn4O16
Formula weight1280.081429.871607.71
Temperature/K293(2)296.15296.15
Crystal systemtriclinictriclinictriclinic
Space groupP-1P-1P-1
a/Å9.3045(13)9.4483(15)9.3658(5)
b/Å11.792(3)11.392(2)11.2509(7)
c/Å12.408(3)12.650(4)12.7732(7)
α/°70.12(2)72.261(6)72.341(4)
β/°75.412(16)75.408(6)76.598(4)
γ/°80.928(15)81.766(6)81.729(4)
Volume/Å31235.1(5)1251.7(5)1243.68(13)
Z111
ρcalc g/cm31.7211.8972.147
μ/mm−112.6964.45616.170
F(000)644.0700.0774.0
Crystal size/mm30.28 × 0.26 × 0.240.26 × 0.25 × 0.240.28 × 0.25 × 0.22
RadiationCu Kα (λ = 1.54184)Mo Ka (λ = 0.71073)Cu Kα (λ = 1.5418)
2θ range/°7.756 to 131.9963.764 to 50.0167.412 to 131.958
Index ranges−10 ≤ h ≤ 5, −13 ≤ k ≤ 13, −14 ≤ l ≤ 14−8 ≤ h ≤ 11, −13 ≤ k ≤ 13, −15 ≤ l ≤ 15−9 ≤ h ≤ 10, −13 ≤ k ≤ 13, −14 ≤ l ≤ 15
Reflections collected736174948978
Independent reflections4258 [Rint = 0.0927, Rsigma = 0.1358]4201 [Rint = 0.0532, Rsigma = 0.0759]4135 [Rint = 0.0386, Rsigma = 0.0485]
Data/restraints/parameters4258/3/3144201/0/3014135/0/301
Goodness-of-fit on F21.0771.0231.101
R (I ≥ 2σ(I))R1 = 0.0765, wR2 = 0.1792R1 = 0.0904, wR2 = 0.2548R1 = 0.0627, wR2 = 0.1871
R (all data)R1 = 0.1275, wR2 = 0.2256R1 = 0.1329, wR2 = 0.2905R1 = 0.0743, wR2 = 0.1979
Largest diff. peak/hole/eÅ−30.571/−1.0791.011/−0.8401.23/−0.81
Table
Table 2. Selected FT-IR data for MnIII complexes 1, 2, and 3 (cm−1).
Table 2. Selected FT-IR data for MnIII complexes 1, 2, and 3 (cm−1).
Complexν(O–H)ν(Ar–O, C–O)ν(C=O)ν(Mn–O)ν(C=C) Benzene Ring Skeleton
134371244, 11551587698, 5031449, 1412
234161296, 11671602695, 4941449,1352
334451275, 11731620694, 4711476, 1368
Table
Table 3. Main hydrogen bonds [Å,°] for MnIII complexes 1, 2, and 3.
Table 3. Main hydrogen bonds [Å,°] for MnIII complexes 1, 2, and 3.
CompoundD–H···Ad(D–H)d(H···A)d(D···A)∠D–H···A
1O8A–H8A···O10.852.012.767(1)149
C18–H18B···O70.962.593.159(1)118
C19–H19C···O40.962.652.992(1)115
C11–H11···O30.982.573.470(1)153
C19–H19A···O50.962.603.406(1)142
2O8–H8B···O10.852.062.830(1)150
C18–H18A···O70.962.583.128(1)116
C19–H19B···O40.962.402.954(2)116
C11–H11···O30.982.383.286(1)154
3C19–H19B···O40.962.452.991(1)115
C11–H11···O30.982.283.182(8)152

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Jia, H.-R.; Chang, J.; Zhang, H.-J.; Li, J.; Sun, Y.-X. Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear MnIII Complexes: Synthesis, Crystal Structures, and Spectroscopic Properties. Crystals 2018, 8, 272. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst8070272

AMA Style

Jia H-R, Chang J, Zhang H-J, Li J, Sun Y-X. Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear MnIII Complexes: Synthesis, Crystal Structures, and Spectroscopic Properties. Crystals. 2018; 8(7):272. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst8070272

Chicago/Turabian Style

Jia, Hao-Ran, Jian Chang, Hong-Jia Zhang, Jing Li, and Yin-Xia Sun. 2018. "Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear MnIII Complexes: Synthesis, Crystal Structures, and Spectroscopic Properties" Crystals 8, no. 7: 272. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst8070272

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