English

• 1. INTRODUCTION

  The coordination complexes of manganese(Ⅱ) and copper(Ⅰ) ions have been the subject of intensive research owing to their aesthetically pleasing structures, and intriguing physicochemical properties which are closely relevant to the design of new magnetic^[1-10] and catalytic^[11-20] materials. Coordination compounds chelated by derivatives of imidazo-[1, 5-a]pyridine were widely investigated in recent years^[21-23]. This series of ligands possess several nitrogen donors, and they are electron rich and highly conjugated. These features render them to be the suitable ligands for assembling versatile coordination complexes. Recent advances in this area revealed that the complexes supported by this series of ligands exhibited potential applications in DNA cleavage^[24-25], catalysis^[26-27], and photoluminescent materials^[28-30]. Therefore, the development of efficient synthetic methodologies for both imidazo[1, 5-a]pyridine scaffolds and their complexes is of great importance.

  With the motivations for synthesizing manganese(Ⅱ) and copper(Ⅰ) coordination complexes, and comprehensively studying the magnetic properties and catalytic activity of these compounds, we carried out the reactions of manganese(Ⅱ) and copper(Ⅱ) salts with 2, 6-bis(imidazo[1, 5-a]-pyridin-3-yl)pyridine (bipp) and 3-(pyridin-2-yl)imidazo[1, 5-a]pyridine (pip). Two complexes with formulae [Mn(bipp)-(SO₄)(H₂O)]_n (1) and [Cu(pip)₂]ClO₄ (2) were prepared (Scheme 1). The magnetic properties of 1 and catalytic activity of 2 toward ketalization reactions were investigated. Herein, the preparation and structures of compounds 1 and 2 were reported. The magnetic properties of 1 and catalytic behavior of 2 were also presented.

  Scheme 1

  

  Scheme 1.  Synthesis of compounds 1 (a) and 2 (b)

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  2. EXPERIMENTAL

  2.1 Physical measurements

  All manipulations were conducted under solvothermal conditions in air. All solvents and reagents were available from chemical suppliers and used directly. 2, 6-Bis(imidazo-[1, 5-a]pyridin-3-yl)pyridine (bipp) and 3-(pyridin-2-yl)imidazo[1, 5-a]pyridine (pip) were prepared by following the reported procedures^[21-23]. FT-IR spectroscopy studies were carried out on a Nicolet MagNa-IR 500 spectrometer with samples prepared as KBr disks. Elemental analyses of carbon, hydrogen and nitrogen atoms were performed with a Carlo-Erba EA1110 CHNO-S analyser. The magnetic susceptibilities of 1 were determined on an MPMS XL-7 SQUID magnetometer.

  2.2 Preparation of 1 and 2

  2.2.1   Preparation of [Mn(bipp)(SO₄)(H₂O)]_n (1)

  To a 10 mL thick glass tube were added MeOH (4 mL), bipp (0.1 mmol, 31 mg), and MnSO₄·H₂O (0.1 mmol, 17 mg). The tube was then sealed and heated to 90 ºС for 72 hours. Red block-shaped crystals of the product were obtained upon gradually cooling the reaction mixture to ambient temperature. These crystals were isolated, rinsed with MeOH and dried. The yield of the product was 95% calculated by MnSO₄·H₂O. Elemental analysis calcd. (%) for C₁₉H₁₅MnN₅O₅S: C, 47.51; H, 3.15; N, 14.58. Found (%): C, 47.59; H, 3.43; N, 14.63. Selected ATR IR data (cm⁻¹): 3158 (w), 2159 (w), 1631 (w), 1604 (w), 1591 (m), 1573 (m), 1553 (m), 1482 (s), 1359 (m), 1308 (w), 1263 (m), 1256 (m), 1173 (w), 1134 (w), 1114 (m), 1071 (s), 1037 (m), 921 (w), 908 (m), 831 (m), 844 (w), 792 (m), 753 (m), 740 (m), 683 (s), 670 (s), 663 (m), 619 (m).

  2.2.2   Preparation of [Cu(pip)₂]ClO₄ (2)

  To a 10 mL thick glass tube were added MeOH (4 mL), DMF (1 mL), Cu(ClO₄)₂·6H₂O (0.1 mmol, 17 mg) and pip (0.1 mmol, 20 mg). The tube was then sealed and heated to 90 ºС for 72 hours. Red block-shaped crystals of the product were obtained upon gradually cooling the reaction mixture to ambient temperature. These crystals were isolated, rinsed with EtOH and dried. The yield of the product was 70% calculated by Cu(ClO₄)₂·6H₂O. Elemental analysis calcd. (%) for C₂₄H₁₈ClCuN₆O₄: C, 52.09; H, 3.28; N, 15.19. Found: C, 51.95; H, 3.21; N, 15.03. The main ATR IR data (cm⁻¹): 1597 (m), 1474 (m), 1447 (w), 1407 (s), 1365 (m), 1260 (m), 1168 (m), 1084 (s), 1013 (w), 978 (w), 913 (w), 826 (w), 778 (m), 767 (m), 731 (m), 719 (m), 678 (m), 619 (m).

  2.3 Ketalization reactions catalysed by 2

  To a 10 mL Schlenk flask were added ketone (1.4 mmol), ethylene glycol (1.4 mmol), toluene (1 mL) and compound 2 (2 mg). The reaction mixture was heated to reflux with a Dean-Stark being connected to the bottom of a condenser for 24 h. Then compound 2 was removed by filtration. The yields of the products were calculated by ¹H NMR.

  2.4 General crystallographic details

  Diffraction data measurements for these two compounds were carried out on a Bruker Smart Apex Ⅱ diffractometer at 296.15 K. SHELXS package was employed to solve the structures of 1 and 2^[31]. All ordered non-hydrogen elements were refined anisotropically by full-matrix least-squares calculations against F^2[32]. X-ray powder diffraction data were determined on a Rigaku D/Max-2500 diffractometer utilizing a graphite monochromator and a Cu-target tube.

  3. RESULTS AND DISCUSSION

  3.1 Characterization and structure description of 1 and 2

  These two compounds were characterized by IR (Fig. S1), X-ray powder diffraction analysis (Fig. S3), and elemental analysis. The existence of the coordinated water in compound 1 was confirmed by measuring the TG curve (Fig. S2). The PXRD patterns of these two compounds (experimental and simulated PXRD patterns, Fig. S3) indicated that the phases of compounds 1 and 2 were pure. Compound 1 underwent several step weight loss. The first step from 25 to 225 ℃ was assigned to the removal of the coordinated H₂O molecule (calcd.: 3.74%; found: ca. 3.75%), suggesting high thermal stability of 1.

  The structure refinement details and crystal data are listed in Table S1. The main interatomic distances and angles are collected in Table 1. The structures of 1 and 2 are presented in Figs. 1 and 2. The main crystallographic parameters are summarized in the following descriptions.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

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  Compound 1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Mn(1)–O(1)	2.1638(18)		Mn(1)–N(3)	2.239(2)		Mn(1)–O(3)1	2.1682(18)
  Mn(1)–N(1)	2.249(2)		Mn(1)–O(2)	2.143(2)		Mn(1)–N(2)	2.288(2)
  1 x, 3/2–y, 1/2+z
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Mn(1)–O(3)1	168.53(7)		O(2)–Mn(1)–N(3)	108.38(8)		N(3)–Mn(1)–N(2)	71.38(7)
  O(1)–Mn(1)–N(3)	90.74(7)		O(2)–Mn(1)–N(1)	108.72(8)		N(1)–Mn(1)–N(2)	71.52(7)
  O(1)–Mn(1)–N(1)	93.14(7)		O(2)–Mn(1)–N(2)	179.64(8)		S(1)–O(1)–Mn(1)	134.40(11)
  O(1)–Mn(1)–N(2)	95.25(7)		N(3)–Mn(1)–N(1)	142.90(7)		S(1)–O(3)–Mn(1)2	141.53(11)
  O(3)1–Mn(1)–N(3)	90.24(8)		O(2)–Mn(1)–O(3)1	83.84(7)		O(2)–Mn(1)–O(1)	85.01(8)
  O(3)1–Mn(1)–N(1)	92.96(8)		O(3)1–Mn(1)–N(2)	95.89(7)
  1 x, 3/2–y, 1/2+z; 2 x, 3/2–y, 1/2+z
  Compound 2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–N(1)	2.0758(15)		Cu(1)–N(2)	2.0022(14)		Cu(1)–N(3)	2.0275(14)
  Cu(1)–N(4)	2.0702(15)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(2)–Cu(1)–N(1)	80.67(6)		N(3)–Cu(1)–N(1)	111.22(6)		N(2)–Cu(1)–N(4)	111.87(6)
  N(2)–Cu(1)–N(3)	146.19(6)		N(3)–Cu(1)–N(4)	81.66(6)		N(4)–Cu(1)–N(1)	136.87(6)

  Figure 1

  

  Figure 1.  (a) Asymmetric unit of compound 1. (b) Molecular structure of 1 showing the chain-like configuration and coordination polyhedrons of the Mn^Ⅱ ions. Symmetry codes: A: x, 3/2–y, 1/2+z

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  Figure 2

  

  Figure 2.  (a) Crystallographically determined structure of 2 with the ellipsoids of 30% probability level. (b) Coordination polyhedron of the Cu^I ion

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  Compound 1 crystalizes in monoclinic space group P2₁/c. M_r = 480.36, a = 10.6615(6), b = 15.4762(8), c = 12.6476(7) Å, β =113.3130(10)°, D_c = 1.665 g/cm³, V = 1916.47(18) ų, μ = 0.843 mm^‑1, F(000) = 980, R = 0.0380, wR = 0.0977, and goodness-of-fit is 1.094. Each asymmetric unit contains one Mn^Ⅱ ion, one bipp ligand, one SO₄^2- group and a coordination water molecule (Fig. 1). The Mn^Ⅱ center is coordinated by N1, N2 and N3 atoms from one bipp ligand, O1 and O3A atoms from two SO₄^2- groups and O2 atom from coordinated H₂O molecule, giving a [MnN₃O₃] core. Along the a axis, the adjacent Mn^Ⅱ ions are bridged by one O-S-O unit from one SO₄^2- anion. The Mn^Ⅱ…Mn^Ⅱ distance is 6.476 Å. Within compound 1, two similar mononuclear [Mn1A(bipp)(μ₃-O)] and [Mn1B(bipp)(μ₃-O)] units are bridged by the [MnN₃O₃] core in a mirror mode, leading to a chain structure of [Mn(bipp)(SO₄)(H₂O)]_n (Fig. 1). The average Mn–N length is 2.259 Å, and it is larger than that of Mn–O distance (2.158 Å). The bond angles of the Mn^Ⅱ ion from the axial position range from 71.38(7)° to 142.90(7)°, and bond angles from the bridged plane fall in the scope of 83.84(7)°~168.53(7)°. A slight distortion from the ideal octahedral structure with a CShM (continuous symmetry measurement) value of 3.237 was calculated by using SHAPE 2.1 software^[33].

  Compound 2 crystalizes in triclinic space group P$\overline{1}$. M_r = 553.43, a = 7.3860(4), b = 10.5325(6), c = 14.5829(8) Å, α = 88.959(2)°, β =78.193(2)°, γ = 80.407(2)°, D_c = 1.679 g/cm³, V = 1094.77(10) ų, μ = 1.168 mm^‑1, F(000) = 564, R = 0.0322, wR = 0.0852, and goodness-of-fit is 1.050. Compound 2 is composed of one [Cu(pip)₂]⁺ unit and one ClO₄^- counterion (Fig. 2). The Cu^I ion of [Cu(pip)₂]⁺ unit is chelated by two pip ligands. Thus the Cu^I centre displays tetrahedron geometry. The distances of Cu–N_pyridine bonds are 2.0758(14) and 2.0275(14) Å, respectively. The Cu–N_imidazole distances are 2.0702(15) and 2.0022(14) Å, respectively.

  3.2 Dc magnetic susceptibility studies of 1

  Dc magnetic susceptibility values of 1 were determined between 1.8 and 300 K. The χ_mT product for two asymmetric units at 300 K is 8.04 cm³·K·mol^-1 (Fig. 3). This number is smaller than that of expected. As the theoretical value for two non-interacted Mn^Ⅱ ions is 8.75 cm³·K·mol^-1 (g = 2.00, S = 5/2). Upon cooling from 300 to 70 K, the χ_mT value remains almost constant. With further cooling, the value of χ_mT sharply decreases to 1.56 cm³·K·mol^-1 at 1.8 K, proving that Mn^Ⅱ complex is antiferromagnetically coupled.

  Figure 3

  

  Figure 3.  χ_MT products of 1 between 1.8 and 300 K at 1000 Oe. The solid line represents the fitted values based on the model^[34]

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  To quantitatively assess the antiferromagnetical couplings in complex 1, we fitted the interactions of compound 1 by using the classical spin Heisenberg model (1-J) for a Mn₂^Ⅱ compound^[34].

  Fitting the magnetic susceptibilities of 1 using the reported model^[34] gave g = 1.95 and J = –0.34 cm^-1. The negative J value indicated weakly antiferromagnetic Mn^Ⅱ…Mn^Ⅱ couplings.

  3.3 Catalytic properties of 2

  Having the crystal structures of 1 and 2 in hand, we next studied the catalytic behavior of compound 2 for the ketalization reactions of various ketones. Ketalization reaction is an efficient methodology to protect carbonyl moiety of ketones^[35]. Typically, ketalization reaction is catalyzed by a Lewis acid. As the Cu^I ion of 2 is four-coordinated, it may serve as a Lewis acid to activate the oxygen of the carbonyl group.

  The protection of carbonyl groups of several ketones by ethylene glycol was conducted in toluene at 110 ℃. As can be seen in Table 2, when the substrates are aliphatic ketones, namely, acetone, 2-butanone, acetyl acetone and cyclohexanone (entries 1~3, 5), compound 2 can efficiently catalyze the reactions, generating the protected ketones with the yields of 88% (Fig. S6), 97% (Fig. S7), 97% (Fig. S8) and 99% (Fig. S10), respectively. The catalytic activity of complex 2 toward aromatic ketones is lower. It is found that a yield of only 5% (Fig. S9) was given for acetophenone.

  Table 2

  

  Table 2.  Ketalization of Ketones Catalysed by 2

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  Entry	Ketone	Product	Cat. (mol%)	Yield (%)
  1			0.2	88
  2			0.2	97
  3			0.2	97
  4			0.2	5
  5			0.2	99

  4. CONCLUSION

  In conclusion, two complexes [Mn(bipp)(SO₄)(H₂O)]_n (1) and [Cu(pip)₂]ClO₄ (2) were synthesized and characterized. They display diverse structures. The magnetic properties of compound 1 were studied, revealing antiferromagnetic interactions between metal centers. Compound 2 was able to catalyze the ketalization reactions of aliphatic and aromatic ketones. It exhibited high activity towards the ketalization of aliphatic ketones.

  
  
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• 

  Scheme 1  Synthesis of compounds 1 (a) and 2 (b)

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  Figure 1  (a) Asymmetric unit of compound 1. (b) Molecular structure of 1 showing the chain-like configuration and coordination polyhedrons of the Mn^Ⅱ ions. Symmetry codes: A: x, 3/2–y, 1/2+z

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  Figure 2  (a) Crystallographically determined structure of 2 with the ellipsoids of 30% probability level. (b) Coordination polyhedron of the Cu^I ion

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  Figure 3  χ_MT products of 1 between 1.8 and 300 K at 1000 Oe. The solid line represents the fitted values based on the model^[34]

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

  Compound 1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Mn(1)–O(1)	2.1638(18)		Mn(1)–N(3)	2.239(2)		Mn(1)–O(3)1	2.1682(18)
  Mn(1)–N(1)	2.249(2)		Mn(1)–O(2)	2.143(2)		Mn(1)–N(2)	2.288(2)
  1 x, 3/2–y, 1/2+z
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Mn(1)–O(3)1	168.53(7)		O(2)–Mn(1)–N(3)	108.38(8)		N(3)–Mn(1)–N(2)	71.38(7)
  O(1)–Mn(1)–N(3)	90.74(7)		O(2)–Mn(1)–N(1)	108.72(8)		N(1)–Mn(1)–N(2)	71.52(7)
  O(1)–Mn(1)–N(1)	93.14(7)		O(2)–Mn(1)–N(2)	179.64(8)		S(1)–O(1)–Mn(1)	134.40(11)
  O(1)–Mn(1)–N(2)	95.25(7)		N(3)–Mn(1)–N(1)	142.90(7)		S(1)–O(3)–Mn(1)2	141.53(11)
  O(3)1–Mn(1)–N(3)	90.24(8)		O(2)–Mn(1)–O(3)1	83.84(7)		O(2)–Mn(1)–O(1)	85.01(8)
  O(3)1–Mn(1)–N(1)	92.96(8)		O(3)1–Mn(1)–N(2)	95.89(7)
  1 x, 3/2–y, 1/2+z; 2 x, 3/2–y, 1/2+z
  Compound 2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–N(1)	2.0758(15)		Cu(1)–N(2)	2.0022(14)		Cu(1)–N(3)	2.0275(14)
  Cu(1)–N(4)	2.0702(15)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(2)–Cu(1)–N(1)	80.67(6)		N(3)–Cu(1)–N(1)	111.22(6)		N(2)–Cu(1)–N(4)	111.87(6)
  N(2)–Cu(1)–N(3)	146.19(6)		N(3)–Cu(1)–N(4)	81.66(6)		N(4)–Cu(1)–N(1)	136.87(6)

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  Table 2.  Ketalization of Ketones Catalysed by 2

  Entry	Ketone	Product	Cat. (mol%)	Yield (%)
  1			0.2	88
  2			0.2	97
  3			0.2	97
  4			0.2	5
  5			0.2	99

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