Monomeric complexes of Re(CO)3+ ion with tridentate N∩N∩O-ligands - Schiff base derivatives of salicylic aldehyde

Article abstract of DOI:10.1016/j.inoche.2014.05.007

Simple synthetic procedures, reactions of Re(CO)₅Cl with potentially tridentate N∩N∩OH ligands (Schiff bases prepared from aliphatic or aromatic amines and salicylic aldehyde) lead to formation of monomeric complexes of fac-Re(CO)₃

Full text of DOI:10.1016/j.inoche.2014.05.007

Inorganic Chemistry Communications 46 (2014) 103–106
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Inorganic Chemistry Communications
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Monomeric complexes of Re(CO)+3 ion with tridentate N∩N∩O−ligands
— Schiff base derivatives of salicylic aldehyde
c
c
Marek Grzegorczyk a, Andrzej Kapturkiewicz a,b, , Fabiola W. Sanjuan-Szklarz , Jacek Nowacki
⁎
a
Institute of Chemistry, Faculty of Sciences, University of Siedlce, 3 Maja 54, 08-110 Siedlce, Poland
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Simple synthetic procedures, reactions of Re(CO)5Cl with potentially tridentate N∩N∩OH ligands (Schiff bases
prepared from aliphatic or aromatic amines and salicylic aldehyde) lead to formation of monomeric complexes
of fac-Re(CO)+3 ion. Three obtained complexes have been characterized by means of elemental analyses and IR,
UV–vis, and EI-MS techniques. Molecular structures of the synthesized species were investigated using X-ray
diffraction measurements. Depending on the nature of N∩N∩OH ligand the investigated Schiff bases form with
fac-Re(CO)+3 ion bidentate or tridentate chelates with N∩N, N∩O− or N∩N∩O− coordination types.
© 2014 Elsevier B.V. All rights reserved.
Received 21 February 2014
Received in revised form 28 April 2014
Accepted 9 May 2014
Available online 18 May 2014
Keywords:
Rhenium carbonyls
Schiff base complexes
N∩N∩OH ligands
X-ray crystallography
EI-MS, IR and UV-vis spectroscopy
The Schiff bases, derived from salicylic aldehyde with various types
of amines (mono-, di- and polyamines) are widely studied ligands due
to their versatility and flexibility in binding to various transition metal
ions with formation of stable complexes [1]. The researchers' interest
in these species has been attracted by their potential applications as
pharmaceuticals [2], catalysts [3], optical and magnetic materials [4,5],
liquid crystals [6] as well as the model systems for biological macromol-
ecules [7]. Among many other investigated transition metal ions also
fac-Re(CO)+3 complexes with Schiff bases have also been studied
[8–18]. In most cases special attention was paid to negatively charged
ligands — derivatives of acetylacetone or salicylic aldehyde, although
fac-Re(CO)+3 complexes with neutral Schiff bases are also known [19].
The reported investigations were inspired by possible practical applica-
tions of fac-Re(CO)+3 chelates as photoactive materials [20], electrolumi-
nescent materials for OLED devices [21], active species in photocatalytic
reduction of CO2[22,23] or therapeutic radiopharmaceuticals based on
186Re/188Re radionuclides [24,25].
investigations were mainly devoted to imine derivatives of
acetylacetone [15–18], whereas ligands derived from salicylic alde-
hyde(s) are distinctly less elaborated [11–13]. The aim of this work
was to pursue the studies on the fac-Re(CO)+3 ion complexes with an-
ionic N∩N∩O− tridentate ligands to investigate this still largely unex-
plored class of the rhenium carbonyls. Moreover, because one can
expect that the studied ligands may coordinate fulfilling all three avail-
able coordination places, the performed investigations could allow syn-
thesis of rather rare complexes of fac-Re(CO)+3 ion with tridentate
chelators. Here we present the preliminary results concerning prepara-
tion and structural characterization of fac-Re(CO)+3 complexes with 2-
[[2-(propylamino)ethyl]-imino]methyl]phenol — HL1, 2-[[(pyridin-2-
ylmethyl)-imino]methyl]phenol — HL2, and 2-[[2-(pyridin-2-yl)ethyl]-
imino]methyl]phenol — HL3, potentially tridentate chelators (with struc-
tures depicted in Fig. 1).
The investigated imine ligands were synthesized by condensation of
an appropriate amine with salicylic aldehyde performed in refluxing
methanol. The investigated complexes were prepared (with ca.
50–60% yields) by refluxing of equimolar amounts of Re(CO)5Cl
and given ligand (HL1, HL2, and HL3, correspondingly) in toluene solu-
tions under a dry argon atmosphere for about 6 h. The precipitated
crude products were further purified by crystallization from hexane/
dichloromethane (1:1 v/v) mixtures. The obtained products were
preliminary characterized by means of EI-MS spectra and elemental
analysis. The recorded EI-MS spectra of the synthesized complexes (cf.
Fig. 2) point unambiguously to their monomeric nature. In all three in-
vestigated cases there are observed characteristic signals corresponding
Investigations dedicated to complexes of fac-Re(CO)+3 ion with
bidentate [8–12] as well as with polydentate [13–18] anionic Schiff
bases have been reported. Whereas bidentate N∩O− ligands form com-
plexes with stable fac-Re(CO)3(N∩O−) fragment, tridentate N∩N∩O−
chelators may form Re(CO)3(N∩N∩O−
) species. The reported
⁎
Corresponding author at: Institute of Physical Chemistry, Polish Academy of Sciences,
Kasprzaka 44/52, 01-224 Warsaw, Poland. Tel.: +48 22 6323221; fax: +48 22 6323333.
E-mail address: akapturkiewicz@ichf.edu.pl (A. Kapturkiewicz).
http://dx.doi.org/10.1016/j.inoche.2014.05.007
1387-7003/© 2014 Elsevier B.V. All rights reserved.

104
M. Grzegorczyk et al. / Inorganic Chemistry Communications 46 (2014) 103–106
Fig. 1. Structural formulae of the investigated HL1, HL2, and HL3 tridentate N∩N∩OH Schiff bases.
to fragments which can be ascribed to Re(CO)3(N∩N∩ O−),
Re(CO)2(N∩N∩O−), Re(CO)(N∩N∩O−), and Re(N∩N∩O−) species,
respectively. The recorded EI-MS spectra exhibit also characteristic
signal patterns due to the natural abundance of the 186Re and 188Re iso-
topes. The observed signal patterns correspond to the presence of one
rhenium atom in the EI-MS fragments.
(evaporation at temperatures above 240 °C) abstraction of HCl molecule
from the preliminary Re(CO)3(H+N∩N∩O−)Cl or Re(CO)3(N∩N∩OH)Cl
products with conversion to the Re(CO)3(N∩N∩O−) species take place.
Obtained X-ray results show that the investigated rhenium(I) com-
plexes contain fac-Re(CO)+3 core in a distorted octahedral geometry,
but with the coordinated ligands attached in a distinctly different man-
ner. Ligand HL3 coordinates to fac-Re(CO)+3 ion in the expected
tridentate way with formation of one five-membered and one six-
membered chelate rings. Thus complex with HL3 ligand can be directly
synthesized according to the following reaction
EI-MS results may suggest that neutral complexes with general for-
mula of Re(CO)3(N∩N∩O−) are formed in reaction of Re(CO)5Cl with
each of the investigated ligands. To some extent, however, the results
of the elemental analysis [26] collide with structures deduced on the
basis of the above described EI-MS results. Only for a complex formed
in reaction with HL3 ligand the agreement between the experimentally
found and the required contents of C, H, and N could be regarded as sat-
isfactory, whereas for complexes synthesized from HL1 and HL2 ligands
the elemental analysis data point to additional presence of chloride in
their structures [26]. Correspondingly for these two complexes the
experimentally found contents of C, H, and N are different from that
expected for Re(CO)3(N∩N∩O−) formulae. Therefore, because we
were able to grow suitable crystals by slow evaporation of synthesized
complexes dissolved in appropriate solvent [27–29], we decided to
carry out X-ray diffraction measurements as the decisive proof of their
structures. The ORTEP schemes for the investigated complexes are
presented in Fig. 3. Taking into account the results from the performed
X-ray investigations one can obtain satisfactory agreement between the
experimentally found and the required contents of C, H, and N for all the
prepared complexes [26]. Moreover the experimentally found contents
of Cl for complexes with HL1 and HL2 ligands remain also in agreement
with that required. Thus one can conclude that the recorded EI-MS spec-
tra for Re(CO)3(H+N∩N∩O−)Cl and Re(CO)3(N∩N∩OH)Cl complexes,
with molecular peaks different from that expected for the synthesized
species, indicate thermal instability of these chelates. The synthesized
complexes seem to be stable up to temperature corresponding to toluene
boiling point, but under condition required to record EI-MS spectra
ReðCOÞ5Cl þ HL3→ReðCOÞ3ðN∩N∩O−Þ þ 2CO þ HCl:
ð1Þ
Contrary to that, ligands HL1 and HL2 in complexes Re(CO)3(H+
N∩N∩O−)Cl and Re(CO)3(N∩N∩OH)Cl are coordinated to fac-
Re(CO)+3 core in the bidentate manner acting as N∩O− and N∩N
ligands, respectively. In both Re(CO)3(H+N∩N∩O−)Cl and
Re(CO)3(N∩N∩OH)Cl complexes additional coordination of Cl− ion
to fac-Re(CO)+3 core takes place. Reactions leading to their formation
can be formulated as follows
ReðCOÞ5Cl þ HL1→ReðCOÞ3ðHþN∩N∩O−ÞCl þ 2CO
ReðCOÞ5Cl þ HL2→ReðCOÞ3ðN∩N∩OHÞCl þ 2CO:
ð2Þ
ð3Þ
As mentioned above directly synthesized Re(CO)3(H+N∩N∩O−)Cl
or Re(CO)3(N∩N∩OH)Cl complexes are unstable at conditions required
for recording EI-MS spectra. EI-MS signals can be observed only for
Re(CO)3(N∩N∩O−) fragments with lack of signals expected for
Re(CO)3(H+N∩N∩O−)Cl or Re(CO)3(N∩N∩OH)Cl species indicating
efficient abstraction of HCl molecule from primary products of the
performed syntheses.
ReðCOÞ3ðHþN∩N∩O−ÞCl→ReðCOÞ3ðN∩N∩O−Þ þ HCl
ReðCOÞ3ðN∩N∩OHÞCl→ReðCOÞ3ðN∩N∩O−Þ þ HCl
ð5Þ
ð6Þ
Abstraction of HCl from Re(CO)3(H+N∩N∩O−)Cl or Re(CO)3(N∩-
N∩OH)Cl molecules may be a combined effect of the temperature and
the ionization process. However, our results from TG/DTG studies per-
formed for solid samples indicate that temperature effect seems to be
more crucial, because abstraction of HCl is observed at temperatures
below that required for recording of EI-MS spectra. The recorded TG
and DTG thermograms for solid sample of Re(CO)3(H+N∩N∩O−)Cl
(cf. Fig. 4) show the mass loss (corresponding to the release of one
HCl molecule) starting at ca. 235 °C followed by the thermal decompo-
sition occurring above ca. 265 °C. The observed temperature range re-
quired for the conversion of Re(CO)3(H+N∩N∩O−)Cl into plausible
Re(CO)3(N∩N∩O−) remains in nice agreement with the results
from the performed EI-MS measurements as well as with the thermal
stability of Re(CO)3(H+N∩N∩O−)Cl in boiling toluene. Principally,
performing syntheses in solvent with appropriately high boiling point,
one could also prepare Re(CO)3(N∩N∩O−) chelates with HL1 or HL2
ligand but this option seems to be unfeasible in view of the results
from the performed thermogravimetric analysis.
Structures of the products obtained by the thermal decomposition of
Fig. 2. EI-MS spectra of complexes synthesized from HL1 (top), HL2 (middle), and HL3
(bottom) ligands. The most intense peaks correspond to Re(N∩N∩O− + ions.
)
Re(CO)3(H+N∩N∩O−)Cl and Re(CO)3(N∩N∩OH)Cl complexes remain

M. Grzegorczyk et al. / Inorganic Chemistry Communications 46 (2014) 103–106
105
Fig. 3. ORTEP drawings of Re(CO)3(H+N∩N∩O−)Cl (left), Re(CO)3(N∩N∩OH)Cl (middle), and Re(CO)3(N∩N∩O−) (right) complexes showing 50% probability ellipsoids. N and O
symbols specify the coordinating centers of N∩N∩O− ligands.
an open question. Most probably, however, the thermally induced HCl
abstraction leads to structures similar to that for Re(CO)3(N∩N∩O−
of the three carbonyl ligands. For complex Re(CO)3(N∩N∩OH)Cl, how-
ever, only two strong IR absorption bands (at 1907 and 2021 cm−1
)
)
chelate synthesized directly from HL3 ligand, but this plausible option
requires further studies. The result of the performed thermogravimetric
analysis suggests also that reactions of Re(CO)5Cl with HL1 or HL2 li-
gands performed at the temperatures high enough for HCl abstraction
will most probably lead to contaminated products due to fairly small
difference between temperatures required for HCl releasing from and
further decomposition of the formed Re(CO)3(N∩N∩O−) species. Con-
trary to HL2 and HL3, HL1 ligand behaves as N∩O− type chelator. Despite
that, contrary to previously reported N∩O− type ligands, lack of further
have been observed, similarly as it was previously found for many fac-
Re(CO)+3 diimine complexes [34,35]. In the same way the recorded
UV–vis spectra (cf. Fig. 6) are similar to other monomeric fac-Re(CO)3+
with diimine ligands [19,20]. Relatively intense absorption bands
observed at 230–310 nm can be straightforwardly assigned to intra-
ligand π–π* transitions, whereas the low intensity long wavelength
absorption (located above 340 nm) attributed to the spin allowed
1*MLCT d(Re)–π*(ligand) transitions. Such UV–vis spectra pattern is
typical for fac-Re(CO)+3 with aromatic ligands and remains in agree-
ment with the presence of an octahedral environment around the
central rhenium(I) ion.
conversion of Re(CO)3(H+N∩N∩O−)Cl into Re2(CO)6(N∩N∩O−
)
2
dimer, analogous to that previously reported Re2(CO)6(N∩O−)2[11,
30–34], is not observed. Most probably it can be ascribed to the presence
of relatively strong N\H+ bond in Re(CO)3(H+N∩N∩O−)Cl and corre-
spondingly to a larger amount of energy required, as compared to
Re(CO)3(N∩OH)Cl species, for the abstraction of HCl molecule as neces-
sary for further dimerization.
Our results indicate clearly that in the case of fac-Re(CO)+3 com-
plexes the coordination ability of potentially tridentate N∩N∩OH li-
gands depends strongly on their nature. Depending on the electronic
and structural factors three different classes of fac-Re(CO)+3 chelates
are affordable. To obtain the desired Re(CO)3(N∩N∩O−) chelates,
both, electronic and structural (steric) factors of N∩N∩OH ligand must
be appropriate to adopt fac-Re(CO)+3 core. The proper molecular struc-
ture of N∩N∩OH ligand is necessary to form stable five-membered or
six-membered chelate rings. In a similar way the electronic, electron-
donating and electron-accepting properties of all three coordination
centers must be able to form stable Re\N and Re\O− bonds. The re-
sults presented in this communication suggest that the Npyr and Nimine
coordination centers interact with fac-Re(CO)+3 core much stronger as
The X-ray structures of the synthesized complexes remain in nice
agreement with the results of IR and UV–vis spectroscopic studies. In
the recorded IR spectra (cf. Fig. 5) the observed patterns of IR absorption
in the region of CO stretching vibration have been found. The IR spec-
trum of complex Re(CO)3(H+N∩N∩O−)Cl shows three vCO stretching
vibrations at 1855, 1894 and 2014 cm−1. Similarly for complex
Re(CO)3(N∩N∩O−) three vCO stretching vibrations are seen at 1869,
1898 and 2011 cm–1 remaining consistent with a facial coordination
Fig. 4. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves recorded
for Re(CO)3(H+N∩N∩O−)Cl complex. Insert presents differential scanning calorimetry
(DSC) curve.
Fig. 5. IR spectra (the carbonyl stretching region) recorded in KBr pellets for Re(CO)3(H+
N∩N∩O−)Cl (bottom), Re(CO)3(N∩N∩OH)Cl (middle), and Re(CO)3(N∩N∩O−) (top)
complexes synthesized from HL1, HL2, and HL3 ligands, respectively.

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M. Grzegorczyk et al. / Inorganic Chemistry Communications 46 (2014) 103–106
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Fig. 6. UV–vis absorption spectra of the investigated HL1, HL2, HL3 ligands (dashed lines)
and Re(CO)3(H+N∩N∩O−)Cl, Re(CO)3(N∩N∩OH)Cl, and Re(CO)3(N∩N∩O−) complexes
(solid lines) in acetonitrile solutions. The spectra of HL2 & Re(CO)3(N∩N∩OH)Cl and HL3
& Re(CO)3(N∩N∩O−) pairs are shifted along the y-axis by factors 2.5 × 104 M−1 cm−1
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C, 35.35; H, 3.72; N, 5.22; Cl 6.89% and C, 37.68; H, 2.16; N, 5.34; Cl, 6.79%,
whereas for C15H18ClN2O4Re and C16H12ClN2O4Re the following C, 35.19; H, 3.54;
N, 5.47; Cl, 6.92% and C, 37.10; H, 2.34; N, 5.41; Cl, 6.85% are required. Corresponding
values for Re(CO)3(N∩N∩O−) and C17H13N2O4Re: found C, 40.86; H, 2.85; N, 6.01%
and required C, 41.21; H, 2.64; N, 5.65%.
compared with the phenolic oxygen or the nitrogen atom of aliphatic
amines. The last type of coordination centers seems to possess the
weakest ability to form efficiently stable Re\N bonds. The obtained re-
sults point undoubtedly to distinctly different coordination abilities of
coordination centers within the investigated N∩N∩OH ligands.
[27] Selected crystallographic data for Re(CO)3(H+N∩N∩O−)Cl (MM = 511.96) crystal-
lized from acetonitrile, C15H18ClN2O4Re, orthorhombic, Pbca, a = 11.1455(2)
Å, b = 15.9622(3) Å, c = 18.8619(4) Å, 100°K, V = 3355.66(11) Å3, Z = 8.
19221 reflections measured, 3780 independent (Rint = 0.0681). R1 = 0.0407,
wR2 = 0.0466 (all data).
[28] Selected crystallographic data for Re(CO)3(N∩N∩OH)Cl (MM = 517.93) crystal-
lized from toluene, C16H12ClN2O4Re, monoclinic, P21/c, a = 10.0330(3) Å, b = 12.
8760(3) Å, c = 13.4643(3) Å, β = 108.651(3)°, 100°K, V = 1648.03(7) Å3, Z = 4.
18233 reflections measured, 3926 independent (Rint = 0.0279). R1 = 0.0269,
wR2 = 0.0443 (all data).
Appendix A. Supplementary data
Crystallographic data for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre and
allocated the deposition numbers CCDC 987986, 987987, and 987988
for C1, C2, and C3 complexes, respectively. Copies of the data can be ob-
tained free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EW, UK (E-mail: deposit@ccdc.cam.ac.uk). Supplementary data to
this article can be found online at 10.1016/j.inoche.2014.05.007. These
data include MOL files and InChiKeys of the most important compounds
described in this article.
[29] Selected crystallographic data for Re(CO)3(N∩N∩O−) (MM = 495.50) crystallized
from dichloromethane, C17H13N2O4Re, triclinic, P − 1, a = 7.5446(5) Å, b = 8.
3788(4) Å, c = 12.9370(7) Å, α = 77.496(4)°, β = 76.974(5)°, γ = 89.570(4)°,
100 K, V = 777.13(7) Å3, Z = 2. 18233 reflections measured, 2725 independent
(Rint = 0.0369). R1 = 0.0185, wR2 = 0.0391 (all data).
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