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. 2020 Mar 6:8:157.
doi: 10.3389/fchem.2020.00157. eCollection 2020.

1D Copper(II)-Aroylhydrazone Coordination Polymers: Magnetic Properties and Microwave Assisted Oxidation of a Secondary Alcohol

Manas Sutradhar  1 , Elisabete C B A Alegria  1   2 , Tannistha Roy Barman  1 , M Fátima C Guedes da Silva  1 , Cai-Ming Liu  3 , Armando J L Pombeiro  1
Affiliations

1D Copper(II)-Aroylhydrazone Coordination Polymers: Magnetic Properties and Microwave Assisted Oxidation of a Secondary Alcohol

Manas Sutradhar et al. Front Chem. .

Abstract

The 1D Cu(II) coordination polymers [Cu3(L1)(NO3)4(H2O)2]n (1) and [Cu2(H2L2)(NO3)(H2O)2]n(NO3)n (2) have been synthesized using the aroylhyrazone Schiff bases N' 1,N' 2-bis(pyridin-2-ylmethylene)oxalohydrazide (H2L1) and N' 1,N' 3-bis(2-hydroxybenzylidene)malonohydrazide (H4L2), respectively. They have been characterized by elemental analysis, infrared (IR) spectroscopy, UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), single crystal X-ray diffraction and variable temperature magnetic susceptibility measurements (for 2). The ligand (L1)2- coordinates in the iminol form in 1, whereas the amide coordination is observed for (H2L2)2- in 2. Either the ligand bridge or the nitrate bridge in 2 mediates weak antiferromagnetic coupling. The catalytic performance of 1 and 2 has been investigated toward the solvent-free microwave-assisted oxidation of a secondary alcohol (1-phenylethanol used as model substrate). At 120°C and in the presence of the nitroxyl radical 2,2,6,6-tetramethylpiperydil-1-oxyl (TEMPO), the complete conversion of 1-phenylethanol into acetophenone occurs with TOFs up to 1,200 h-1.

Keywords: Cu(II) complexes; X-ray structure; coordination polymer; magnetism; microwave assisted oxidation of alcohols.

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Figures

Graphical Abstract
Graphical Abstract
Two new aroylhydrazone Cu(II) 1D coordination polymers [Cu3(L1)(NO3)4(H2O)2]n (1) and [Cu2(H2L2)(NO3)(H2O)2]n·(NO3)n (2) (H2L1 = N'1,N'2-bis(pyridin-2-ylmethylene)oxalohydrazide and H4L2 = N'1,N'3-bis(2-hydroxybenzylidene)malonohydrazide) have been structurally characterized. Their magnetic properties and the catalytic activity toward the solvent-free microwave-assisted oxidation of 1-phenylethanol have been explored.
Scheme 1
Scheme 1
Syntheses of 1 and 2.
Figure 1
Figure 1
(A) Ellipsoid plot of 1 with partial atom numbering scheme, (B) a fragment of its 1D polymeric chain with the metals in polyhedral representations (coordination number six represented in blue, and seven in violet), and (C) a view of the smooth wave-like nature of the chain. Symmetry operation to generate equivalent atoms: (i) 1.5–x, –y,1/2+z; (ii) 1.5–x, –y, −1/2+z.
Figure 2
Figure 2
(A) Ellipsoid plot of 2 with partial atom numbering scheme; (B) a fragment of 1D polymeric chain with the metals in polyhedral representation; (C) a view of the 1D chain. The nitrate counter-ions are omitted for clarity. Symmetry operations to generate equivalent atoms: (i) 1+x, y, z; (ii) −1+x, y, z.
Figure 3
Figure 3
Plot of χT vs. T for 2. The solid line represents the best theoretical fitting.
Figure 4
Figure 4
Magnetic topology of Cu14 cycle in 2 (the copper numbers do not show crystallographically independent copper ions, but rather are used to distinguish them).
Scheme 2
Scheme 2
MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone.
Figure 5
Figure 5
Influence of temperature and of presence of TEMPO additive on the yield of acetophenone from oxidation of 1-phenylethanol catalyzed by 1 and 2. Reaction conditions: 2.5 mmol of substrate, 5 μmol (0.2 mol% vs. substrate) catalyst 1 or 2, 5 mmol of t-BuOOH, 0.5 h, 80 or 120°C (5 or 20 W, respectively).
Figure 6
Figure 6
Influence of different additives on the yield of acetophenone from oxidation of 1-phenylethanol catalyzed by 1 and 2. Reaction conditions: 2.5 mmol of substrate, 5 μmol (0.2 mol% vs. substrate) catalyst 1 or 2, 5 mmol of t-BuOOH, 0.5 h, 80°C (5 W MW irradiation).
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