Unexpected transformation of a schiff base pyridine N -oxide in the presence of Pr(NO 3) 3 · 6H 2O

Article abstract of DOI:10.1080/10426507.2012.743134

The synthesis of (E)-((6-(((3-hydroxy-4-methoxybenzylidene)amino)methyl)-N- oxide-pyridin-2-yl)methyl)diphenylphosphine oxide 1 and its unexpected transformation during efforts to study the coordination chemistry with Pr(NO₃)₃·6H₂O are described.

Full text of DOI:10.1080/10426507.2012.743134

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Unexpected Transformation of a Schiff
Base Pyridine N-Oxide in the Presence of
Pr(NO3)3 · 6H2O
Sabrina Ouizem a , Diane A. Dickie a , Michael J. Katz b & Robert T.
Paine a
a Department of Chemistry and Chemical Biology , University of New
Mexico , Albuquerque , NM , USA
b Department of Chemistry and Materials Research Center ,
Northwestern University , Evanston , IL , USA
Accepted author version posted online: 13 Nov 2012.Published
online: 08 May 2013.
To cite this article: Sabrina Ouizem , Diane A. Dickie , Michael J. Katz & Robert T. Paine (2013)
Unexpected Transformation of a Schiff Base Pyridine N-Oxide in the Presence of Pr(NO3)3
· 6H2O, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:1-3, 111-115, DOI:
10.1080/10426507.2012.743134
To link to this article: http://dx.doi.org/10.1080/10426507.2012.743134
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Phosphorus, Sulfur, and Silicon, 188:111–115, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.743134
UNEXPECTED TRANSFORMATION OF A SCHIFF BASE
PYRIDINE N-OXIDE IN THE PRESENCE OF Pr(NO3)3·6H2O
Sabrina Ouizem,1 Diane A. Dickie,1 Michael J. Katz,2
and Robert T. Paine1
1Department of Chemistry and Chemical Biology, University of New Mexico,
Albuquerque, NM, USA
2Department of Chemistry and Materials Research Center, Northwestern
University, Evanston, IL, USA
GRAPHICAL ABSTRACT
Abstract The synthesis of (E)-((6-(((3-hydroxy-4-methoxybenzylidene)amino)methyl)-N-
oxide-pyridin-2-yl)methyl)diphenylphosphine oxide 1 and its unexpected transformation
during efforts to study the coordination chemistry with Pr(NO3)3·6H2O are described.
Received 4 September 2012; accepted 19 October 2012.
Financial support for this study at the University of New Mexico was provided by the Division of Chem-
ical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, and U.S. Department of Energy
(Grant DE-FG02-03ER15419 (R.T.P). In addition, funds from the National Science Foundation assisted with the
purchases of the X-ray diffractometer (CHE-0443580) and NMR spectrometers (CHE-0840523 and -0946690).
Address correspondence to Robert T. Paine, Department of Chemistry and Chemical Biology, University of
New Mexico, Albuquerque, NM 87131, USA. E-mail: rtpaine@unm.edu
111

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S. OUIZEM ET AL.
Keywords Pyridine N-oxide; schiff base; lanthanide, complex
INTRODUCTION
In recent decades, considerable effort has been devoted to the development of new
chelating ligands for Ln(III) and An(III) ions that would provide more efficient extractants
than the well-known carbamoylmethylphosphine oxide (CMPO) ligands, widely employed
in liquid–liquid extraction (LLE) processes (Scheme 1).1 In continuation of our research
focused on ligands derived from pyridine N-oxide platforms, such as illustrated by the
NOPOPO ligands,2 we developed a particular interest in the impact of the introduction of
an amine group, especially a Schiff base, on coordination of such multifunctional ligands
with Ln(III) ions.
Scheme 1
RESULTS AND DISCUSSION
Amine 13 reacts with vanillin in the presence of catalytic amounts of sulfuric acid
and pyridinium p-toluenesulfonate in dichloromethane to give the corresponding imine 2
in moderate yield (Scheme 2). The composition and structure of the molecule are fully
supported by HRMS, IR, and 31P, 1H, 13C NMR spectroscopies.
Scheme 2
The coordination chemistry of 2 was initially examined with Pr(NO3)3·6H2O by
using a ligand/metal ratio of 1:1 in a mixture of chloroform and methanol. Since Schiff base
pyridine ligands are known to be relatively soft N-donor ligands4 and lanthanide ions prefer
hard donor centers, it was anticipated that 2 would preferentially coordinate to the Pr(III)
via the oxygen atoms of the pyridine N-oxide and the phosphine oxide functional groups.
In addition, it was expected that the nitrogen of the imine or possibly the hydroxyl group
on the aryl ring might provide additional hemilabile interactions. However, a preliminary

SCHIFF BASE PYRIDINE N-OXIDE
113
Figure 1 Preliminary crystal structure determination for [Pr(3)(3−)](NO3)2. H-atoms and outer-sphere nitrate
ions removed for clarity (Color figure available online).
X-ray crystallographic molecular structure determination for the complex isolated from the
1:1 combination of 2 with Pr(NO3)3 reveals a surprising result. First, the structure shows
that ligand 2 has undergone significant chemistry, forming 3, which is not observed to
occur in the absence of the Pr(III) ion. Secondly, two molecules of the rearranged ligand
3 coordinate to the Pr(III) ion as illustrated in Scheme 3. One of the molecules can be
considered to contain a neutral isoindoline ring at its central core, while the second has an
anionic isoindoline ring.

114
S. OUIZEM ET AL.
Scheme 3
The ligand containing the anionic isoindoline fragment is bonded in a NONOPO-
PON pentadentate mode with bond lengths Pr1-N1:2.648(11), Pr1-O1:2.708(7), and
Pr1-O2:2.282(7) Å. The neutral isoindoline-based ligand coordinates in a NONOPOPO
tetradentate mode with Pr1-O3:2.351(7), Pr1-O4:2.732(7), and Pr1···N2:3.462(11) Å (Fig-
ure 1). It is noted that the crystal used for the preliminary structure determination suffers
from some disorder. The Pr(III) is disordered over two sites between the isoindoline frag-
ments, and the OMe and OH groups in bo−th ligands are disordered over two sites on the aryl
rings. In addition, the outer sphere NO3 ions and lattice solvent molecules are severely
disordered. Therefore, additional discussion of the metrical parameters from this data set
is not appropriate.
The interesting ligand rearrangement and coordination chemistry encourage further
study of 2 and its derivatives. Comprehensive spectroscopic and structural studies are in
progress that should elucidate the formation pathway for the rearranged ligand molecule,
and additional coordination chemistry of 2 and 3 with other Ln(III) ions is in progress.
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