Two different hydrogen bond donor ligands together: A selectivity improvement in organometallic {Re(CO)3} anion hosts

Article abstract of DOI:10.1021/ic201120s

Rhenium(I) compounds [Re(CO)₃(Hdmpz)₂(ampy)] BAr′₄ and [Re(CO)₃(N-MeIm)₂(ampy)] BAr′₄ (Hdmpz = 3,5-dimethylpyrazole, N-MeIm = N-methylimidazole, ampy = 2-aminopyridine or 3-aminopyridine) have been prepared stepwise as the sole reaction products in good yields. The cationic complexes feature two different types of hydrogen bond donor ligands, and their anion binding behavior has been studied both in solution and in the solid state. Compounds with 2-ampy ligands are labile in the presence of nearly all of the anions tested. The X-ray structure of the complex [Re(CO)₃(Hdmpz) ₂(ampy)]⁺ (2) shows that the 2-ampy ligand is metal-coordinated through the amino group, a fact that can be responsible for its labile character. The 3-ampy derivatives (coordinated through the pyridinic nitrogen atom) are stable toward the addition of several anions and are more selective anion hosts than their tris(pyrazole) or tris(imidazole) counterparts. This selectivity is higher for compound [Re(CO)₃(N-MeIm) ₂(MeNA)]BAr′₄ (5·BAr′₄, MeNA = N-methylnicotinamide) that features an amido moiety, which is a better hydrogen bond donor than the amino group. Some of the receptor-anion adducts have been characterized in the solid state by X-ray diffraction, showing that both types of hydrogen bond donor ligands of the cationic receptor participate in the interaction with the anion hosts. DFT calculations suggest that coordination of the ampy ligands is more favorable through the amino group only for the cationic complex 2, as a consequence of the existence of a strong intramolecular hydrogen bond. In all other cases, the pyridinic coordination is clearly favored.

Full text of DOI:10.1021/ic201120s

ARTICLE
pubs.acs.org/IC
Two Different Hydrogen Bond Donor Ligands Together: A Selectivity
Improvement in Organometallic {Re(CO)3} Anion Hostsr
Laura Ion,† Sonia Nieto,† Julio Pꢀerez,*,† Lucía Riera,*,‡ Víctor Riera,† Jesꢀus Díaz,§ Ramꢀon Lꢀopez,||
Kirsty M. Anderson,^ and Jonathan W. Steed^
†Departamento de Química Orgꢀanica e Inorgꢀanica-IUQOEM, Universidad de Oviedo-CSIC, C/Juliꢀan Clavería, 8. 33006 Oviedo, Spain
‡Instituto de Síntesis Química y Catꢀalisis Homogꢀenea (ISQCH), Universidad de Zaragoza-CSIC, C/Pedro Cerbuna,
12. 50009 Zaragoza, Spain
§Departamento de Química Orgꢀanica e Inorgꢀanica, Universidad de Extremadura, Facultad de Veterinaria, 10071 Cꢀaceres, Spain
Departamento de Química Física y Analítica, Universidad de Oviedo, C/Juliꢀan Clavería, 8. 33006 Oviedo, Spain
^Departament of Chemistry, University of Durham, DH1 3LE Durham, United Kingdom
S Supporting Information
b
ABSTRACT: Rhenium(I) compounds [Re(CO)3(Hdmpz)2-
(ampy)]BAr04 and [Re(CO)3(N-MeIm)2(ampy)]BAr04 (Hdmpz=
3,5-dimethylpyrazole, N-MeIm = N-methylimidazole, ampy =
2-aminopyridine or 3-aminopyridine) have been prepared step-
wise as the sole reaction products in good yields. The cationic
complexes feature two different types of hydrogen bond donor
ligands, and their anion binding behavior has been studied both
in solution and in the solid state. Compounds with 2-ampy
ligands are labile in the presence of nearly all of the anions
tested. The X-ray structure of the complex [Re(CO)3(Hdmpz)2(ampy)]+ (2) shows that the 2-ampy ligand is metal-coordinated
through the amino group, a fact that can be responsible for its labile character. The 3-ampy derivatives (coordinated through the
pyridinic nitrogen atom) are stable toward the addition of several anions and are more selective anion hosts than their tris(pyrazole)
or tris(imidazole) counterparts. This selectivity is higher for compound [Re(CO)3(N-MeIm)2(MeNA)]BAr04 (5 BAr04, MeNA =
3
N-methylnicotinamide) that features an amido moiety, which is a better hydrogen bond donor than the amino group. Some of the
receptor-anion adducts have been characterized in the solid state by X-ray diffraction, showing that both types of hydrogen bond
donor ligands of the cationic receptor participate in the interaction with the anion hosts. DFT calculations suggest that coordination
of the ampy ligands is more favorable through the amino group only for the cationic complex 2, as a consequence of the existence of a
strong intramolecular hydrogen bond. In all other cases, the pyridinic coordination is clearly favored.
’ INTRODUCTION
biimidazole,3 for example), or (b) metal ligand coordination can be
used to place simple modentate ligands in positions such that their
hydrogen-bond donor groups can converge toward an external
anion.4 Using the latter strategy, we have prepared rhenium(I)
organometallic compounds bearing three azole ligands (Figure 1) in
a facial disposition and studied their behavior as anion receptors.
The use of N-alkylimidazole ligands (N-RIm) allowed us to
evaluate the effect of the counteranion present in the receptor.5
For compounds [Re(CO)3(N-RIm)3]X (Figure 1b), the best
hydrogen bond donor groups are the central CꢀH groups of the
imidazole ligands. The hostꢀguest interactions between the
cationic metal complex and the external anions are intrinsically
weak and therefore allowed the comparison of different counter-
anionꢀs Xꢀ (OTfꢀ, PF6ꢀ, BF4ꢀ, etc). The results showed that
BAr04 (Ar0 = 3,5-bis(trifluoromethyl)phenyl), an anion that has
been widely used in organometallic chemistry and catalysis as an
The incorporation of metal fragments in the structure of anion
receptors can be a useful tool.1 While the synthesis of purely
organic anion receptors can be a tedious task, often metal-based
receptors are straightforwardly prepared, in good yields and as
the only products of simple reactions. The role of the metal can
be crucial for the behavior of the host toward an external anion
guest, as (i) it can be a Lewis acidic center and therefore a key
component of the anion binding site; (ii) it can bear positive
charge so that the electrostatic attraction will contribute to the
overall hostꢀguest interaction; and (iii) it can act as part of a
redox, luminescent, or colorimetric reporter group, etc. One of its
multiple purposes is to preorganize hydrogen-bond donor func-
tional groups for anion binding by coordination of the ligands
bearing those groups to a metal center. This preorganization can
be achieved in two different ways: (a) metalꢀligand coordination
can be used to enforce a particular conformation of a ligand that
will result in an optimal organization of hydrogen-bond donor
groups (as it occurs with bis(carbamoyl)-2,20-bipyridines2 or
Received: May 26, 2011
Published: August 11, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
[Re(CO)3(Hdmpz)2(2-ampy)]BAr04 (2 BAr04),8 and [Re(CO)3-
3
(Hdmpz)2(3-ampy)]BAr04 (4 BAr04)8 were prepared as previously
3
reported. Tetrabutylammonium salts were purchased from Fluka or
Aldrich. Deuterated dichloromethane (Cambridge Isotope Labora-
tories, Inc.) was stored under nitrogen in a Young tube and used without
further purification. 1H NMR and 13C NMR spectra were recorded on a
Bruker Advance 300, DPX-300, or Advance 400 spectrometer. NMR
1
spectra are referred to the internal residual solvent peak for H and
13C{1H} NMR. IR solution spectra were obtained in a Perkin-Elmer FT
1720-X spectrometer using 0.2 mm CaF2 cells. NMR samples were
prepared under nitrogen using Kontes manifolds purchased from
Aldrich. Oven-dried 5 mm NMR tubes were subjected to several vacuum-
nitrogen cycles, filled with the solution of the receptor (prepared
separately in a Schlenk tube, typically in a 10ꢀ2 M concentration in
CD2Cl2) by means of a 1 mL syringe, and stoppered with rubber septa.
After the NMR spectrum of the receptor was recorded, the successive
aliquots of the tetrabutylammonium salt (typically 4 ꢁ 10ꢀ2 M in
CD2Cl2, separately prepared and kept in a septum-stoppered vial during
the titration) were injected through the septum using Hamilton micro-
syringes (10ꢀ100 μL). The volume of each addition was 10 μL before
reaching the saturation zone (nearly horizontal line of the titration
profile) and 20 or 40 μL afterward. When the change in δ is small, 20 μL
of salt solution was added from the beginning. Data were treated using
the WinEQNMR program.11
Computational Details. Quantum chemical computations were
carried out with the Gaussian 03 series of programs.12 Full geometry
optimizations of stable species were performed in the gas phase by
employing the hybrid density functional B3LYP13 with the 6-31G(d)
basis set for nonmetal atoms14 together with the LANL2DZ for Re15 and
by using the standard Schlegel’s algorithm.16 The B3LYP functional
combines the Becke’s three-parameter nonlocal hybrid exchange poten-
tial with the nonlocal correlation functional of Lee, Yang, and Parr. The
nature of the stationary points was verified by analytical computations of
harmonic vibrational frequencies. H, ΔS, and ΔG were also calculated
within the ideal gas, rigid rotor, and harmonic oscillator approxima-
tions.17 A pressure of 1 atm and a temperature of 298.15 K were assumed
in the calculations. For interpretation purposes, a natural bond orbital
(NBO) analysis was also performed.18
Figure 1. Tris(pyrazole) (a) and tris(imidazole) (b) rhenium com-
pounds previously used by our group as anion receptors.
alternative to less innocent counteranions such as hexafluoro-
phosphate, tetrafluoroborate, perchlorate, etc.,6 is advantageous
over more conventional counteranions for cationic receptors.
This fact is mainly due to the poorly coordinating character of
this tetraarylborate, which minimizes its competition with the
external anion for the cationic guest.
On the other hand, we have studied the interactions of
[Re(CO)3(Rpz)3]BAr04 (Rpz = 3(5)-tert-butylpyrazol, 3,5-di-
methylpyrazole; Figure 1a) with anions, both in solution and in
the solid state. These results have shown that there is a significant
hostꢀguest interaction in which at least two of the three pyrazole
ligands interact simultaneously with the same external anion.7
Herein, we report the study of the behavior of “mixed”
compounds of formula [Re(CO)3(Lz)2(Lz0)]BAr04 (Lz = 3,5-
dimethylpyrazole or N-methylimidazole; Lz0 = 2-aminopyridine,
3-aminopyridine, or N-methylnicotinamide) that bear more than
one type of hydrogen bond donor group.8 We have chosen the
fac-{Re(CO)3} scaffold, as it has previously been shown to be a
stable host, positively charged (so that Coulombic attraction
enhances the overall hostꢀguest interaction) and the presence of
CO ligands allows the employment of IR spectroscopy to quickly
and easily detect side reactions such as ligand substitution,
deprotonation, etc. The new compounds feature two imidazole
or pyrazole ligands (the same previously employed in fac-[Re-
(CO)3(Lz)3]+ receptors) and one amino or amidopyridine. These
pyridine derivatives display a binding site (usually the pyridinic
nitrogen) and a functionality that possesses good hydrogen
bonding donor groups, i.e., NꢀH groups from amide or amine
moieties.9 In fact, these ligands have been used previously for the
design of metal-based anion receptors.4
The modular synthesis of these complexes allows the prepara-
tion of hosts of the same general geometrical type, but in which
the ligands bearing the hydrogen bond donor groups and, in turn,
the exact size and shape of the anion-recognizing molecular cleft
can be varied. In this paper, it will be shown that complexes in
which two different types of such ligands coexist in the coordina-
tion sphere of the same metal can be prepared. Apart from
2-aminopyridine derivatives, these complexes are stable against
ligand redistribution reactions and against ligand substitution by
the external anionic guest, a condition that plagues hosts based
on metal complexes of monodentate ligands. In addition, it will
be shown that these hosts display enhanced selectivity toward the
smaller anions compared with their simpler counterparts with
only one type of hydrogen bond donor ligand.
Crystal Structure Determination. General Description: For
Compounds 1 ReO4, 3 Cl, 3 Br, 3 NO3 and 5 Br. For each
3
3
3
3
3
structure determination, a single crystal was mounted on a Bruker
diffractometer equipped with a SMART 1K or 6K CCD area detector
and Oxford Cryostream N2 cooling device, using graphite-monochro-
mated Mo Ka radiation (λ = 0.71073 Å). The structures were solved by
direct methods and refined with full-matrix least-squares technique on
F2 using the SHELXS and SHELXL programs. Information on the
individual data collections and refinements can be found in the re-
spective CIF files. CCDC-821877 (1 ReO4), CCDC-821878 (3 Br),
3
3
CCDC-821879 (3 Cl), CCDC-821880 (3 NO3), and CCDC-821881
3
3
(5 Br) contain supplementary crystallographic data for this paper.
3
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Centre,
12 Union Road, Cambridge CB2 1EZ, U. K.; fax: (+44) 1223ꢀ336033
or e-mail: deposit@ccdc.cam.ac.uk).
Synthesis of [Re(CO)3(N-MeIm)2(2-ampy)]BAr04 (1 BAr04).
3
N-MeIm (0.040 mL, 0.492 mmol) was added to a solution of [ReBr-
(CO)5] (0.100 g, 0.246 mmol) in toluene (20 mL), and the mixture was
refluxed for 30 min. The solvent was then evaporated to dryness under
vacuum. The residue was redissolved in CH2Cl2 (20 mL). AgOTf
(0.064 g, 0.246 mmol) was added, and the mixture was stirred in the
dark for 1 h. The resulting solution was filtered from the white solid
(AgBr) through Celite. NaBAr04 (0.209 g, 0.246 mmol) and 2-ampy
(0.024 g, 0.246 mmol) were added, and the reaction mixture was allowed
to stir at room temperature for 1 h. The solution was filtered off
’ EXPERIMENTAL SECTION
General. All manipulations were carried out under a nitrogen
atmosphere using Schlenk techniques. Compounds [ReBr(CO)5],10
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Inorganic Chemistry
ARTICLE
(via canula) from the white solid (NaOTf) and concentrated to a volume
Scheme 1. Synthesis of the “Mixed” Organometallic Com-
pounds [1ꢀ5] BAr04
of 5 mL; the addition of n-hexane (20 mL) caused the precipitation of
3
1 BAr04 as a pale brown solid, which was washed with with n-hexane
3
(2 ꢁ 20 mL). Yield: 0.300 g, 88%. IR (CH2Cl2): 2032s, 1919s (νCO). 1H
NMR (CD2Cl2): δ 7.76 [m, 8H, Ho, BAr04], 7.61 [s, 2H, CH, N-MeIm],
7.60 [m, 4H, Hp, BAr04], 7.55 [m, 1H, CH, 2-ampy], 7.28 [m, 1H, CH,
2-ampy], 7.00 [s, 2H, CH, N-MeIm], 6.93 [s, 2H, CH, N-MeIm], 6.79
[m, 1H, CH, 2-ampy], 6.51 [m, 1H, CH, 2-ampy], 5.41 [s, broad, 2H,
NH2, 2-ampy], 3.71 [s, 6H, CH3, N-MeIm]. 13C{1H} NMR (CD2Cl2):
δ 193.8 [CO], 193.5 [2 ꢁ CO], 161.3 [q (1JCB = 50.0 Hz), Ci, BAr04],
160.1, 149.2 [2-ampy], 141.2 [N-MeIm], 140.0 [2-ampy], 134.8 [Co,
BAr04], 130.4 [N-MeIm], 128.8 [q (2JCF = 31.6 Hz), Cm, BAr04], 124.6
[q (1JCF = 272.4 Hz), CF3, BAr04], 122.9 [N-MeIm], 117.5 [Cp, BAr04],
114.6 [2-ampy], 112.0 [2-ampy], 34.7 [CH3, N-MeIm]. Anal. Calcd for
C48H30BF24N6O3Re: C, 41.42; H, 2.17; N, 6.04. Found: C, 41.54; H,
2.11; N, 6.06%.
Synthesis of [Re(CO)3(N-MeIm)2(3-ampy)]BAr04 (3 BAr04).
3
The preparation was as described for 1 BAr04, starting from
3
[ReBr(CO)5] (0.100 g, 0.246 mmol), N-MeIm (0.040 mL, 0.492
mmol), AgOTf (0.064 g, 0.246 mmol), NaBAr04 (0.209 g, 0.246 mmol),
and 3-ampy (0.024 g, 0.246 mmol). Compound 3 BAr04 was obtained as
3
a white microcrystalline solid. Yield: 0.280 g, 82%. IR (CH2Cl2): 2031s,
1917s (νCO). 1H NMR (CD2Cl2): δ 7.89 [m, 1H, CH, 3-ampy], 7.75
[m, 8H, Ho, BAr04], 7.61 [m, 7H, CH + Hp and CH, 3-ampy + BAr04 and
N-MeIm], 7.16 [m, 2H, CH, 3-ampy], 6.99 [s, 2H, CH, N-MeIm], 6.76
[s, 2H, CH, N-MeIm], 4.03 [s, broad, 2H, NH2, 3-ampy], 3.71 [s, 6H,
CH3, N-MeIm]. 13C{1H} NMR (CD2Cl2): δ 197.1 [CO], 196.6 [2 ꢁ
CO], 164.1 [q (1JCB = 49.5 Hz), Ci, BAr04], 147.8, 144.5 [3-ampy], 143.5
[N-MeIm], 142.2 [3-ampy], 137.2 [Co, BAr04], 132.5 [N-MeIm], 131.3
[q (2JCF = 31.4 Hz), Cm, BAr04], 128.9 [3-ampy], 127.0 [q (1JCF = 272.7
Hz), CF3, BAr04], 126.6 [3-ampy], 125.2 [N-MeIm], 119.9 [Cp, BAr04],
37.1 [CH3, N-MeIm]. Anal. Calcd for C48H30BF24N6O3Re: C, 41.42; H,
2.17; N, 6.04. Found: C, 41.72; H, 2.16; N, 5.98%.
(used as a triflate abstractor, taking advantage of the low
solubility of sodium triflate in dichloromethane, and as the
way to introduce the noncoordinating BAr04ꢀ counteranion)
to afford the cationic compounds [Re(CO)3(Lz)2(ampy)]BAr04
(see Scheme 1).
Compounds [1ꢀ4] BAr04 were spectroscopically character-
3
ized in solution by IR and NMR spectroscopies (see the
Experimental Section). The IR spectra showed the typical
pattern for cationic complexes of the fac-{Re(CO)3} fragment,
whose νCO values are similar to those of the [Re(CO)3(Lz)3]+
complexes previously reported.5,7 1H and 13C NMR spectra
showed the equivalence of the two Hdmpz or N-MeIm ligands
present in each complex and only two signals for the three CO
ligands indicating the existence of a molecular mirror plane in
each compound. For compounds 1 BAr04 and 4 BAr04, the
Synthesis of [Re(CO)3(N-MeIm)2(MeNA)]BAr04 (5 BAr04).
3
Compound 5 BAr04 was prepared as described above for compound
3
1 BAr04 starting from [ReBr(CO)5] (0.100 g, 0.246 mmol), N-MeIm
3
(0.040 mL, 0.492 mmol), AgOTf (0.064 g, 0.246 mmol), NaBAr04
(0.209 g, 0.246 mmol), and MeNA (0.035 g, 0.246 mmol). Yield: 0.251
g, 71%. IR (CH2Cl2): 2033s, 1919s (νCO). 1H NMR (CD2Cl2): δ 8.90
[m, 1H, CH, MeNA], 8.53 [m, 1H, CH, MeNA], 8.20 [m, 1H, CH,
MeNA], 7.77 [m, 8H, Ho, BAr04], 7.65 [s, 2H, CH, N-MeIm], 7.60 [m,
4H, Hp, BAr04], 7.54 [m, 1H, CH, MeNA], 7.06 [s, 2H, CH, N-MeIm],
6.76 [s, 2H, CH, N-MeIm], 6.27 [s, broad, 1H, NH, MeNA], 3.77 [s, 6H,
CH3, N-MeIm], 3.00 [d (3JHH = 5.0 Hz), CH3, MeNA]. 13C{1H} NMR
(CD2Cl2): δ 194.3 [CO], 193.8 [2 ꢁ CO], 163.2 [CO, MeNA], 161.7 [q
(1JCB = 49.9 Hz), Ci, BAr04], 154.8, 153.1 [MeNA], 141.2 [N-MeIm],
136.8 [MeNA], 134.8 [Co, BAr04], 133.3 [MeNA], 130.0 [N-MeIm],
3
3
1
presence in the H NMR spectra of two different signals for
the two methyl groups indicated that a fast dissociation-recoor-
dination process of the pyrazole ligands does not take place.
All of the new compounds [1ꢀ4] BAr04 were found to be
3
stable in solution for at least two days at room temperature, and
no product of intermolecular ligand redistribution could be
128.9 [q (2JCF = 32.7 Hz), Cm, BAr04], 126.2 [MeNA], 124.6 [q (1JCF
=
spectroscopically detected. The behavior of [1ꢀ4] BAr04 to-
272.3 Hz), CF3, BAr04], 123.1 [N-MeIm], 117.5 [Cp, BAr04], 34.7 [CH3,
N-MeIm], 26.7 [CH3, MeNA]. Anal. Calcd for C50H32BF24N6O4Re: C,
41.88; H, 2.25; N, 5.86. Found: C, 41.97; H, 2.26; N, 5.99%.
3
ward several anions, as their tetrabutylammonium salts, were
investigated by means of IR and NMR spectroscopy. Com-
pounds 1 BAr04 and 2 BAr04, with the 2-ampy ligand, were
3
3
found to be labile in the presence of chloride, bromide, or nitrate
anions in solution, as was clearly evidenced by a 10ꢀ15 cmꢀ1
shift to lower wavenumber values in the IR spectra. In the case of
the bromide, the addition of an equimolar amount of [Bu4N]-
[Br] to the solution of compound 1 BAr04 or 2 BAr04 led to the
formation of the previously known neutral products [ReBr-
(CO)3(Hdmpz)2]19 or [ReBr(CO)3(N-MeIm)2],20 respectively,
indicating that the 2-ampy ligand is displaced by the bromide
anion. Compounds 1 BAr04 and 2 BAr04 were found to be stable
’ RESULTS AND DISCUSSION
The mixed compounds were prepared in a stepwise manner by
sequential substitution reactions from [ReBr(CO)5] (see Scheme 1).
The reaction of [ReBr(CO)5] with two equivalents of the azole
(N-methylimidazole, N-MeIm, or 3,5-dimethylpyrazole, Hdmpz)
in refluxing toluene afforded after 30 min the corresponding fac-
tricarbonyl complexes [ReBr(CO)3(Lz)2] (Lz = Hdmpz or N-
MeIm). The reactions of these neutral complexes with AgOTf
led to the substitution of the bromide by the labile triflate ligand,
which was subsequently replaced by the 2- or 3-aminopyridine
(2-ampy or 3-ampy) moiety in the presence of the NaBAr04 salt
3
3
3
3
ꢀ
only in the presence of the weakly basic anions ReO4 and
ClO4ꢀ. Slow diffusion of hexane into a concentrated solution of
an equimolar mixture of 1 BAr04 and [Bu4N][ReO4] at ꢀ20 ꢀC
3
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Inorganic Chemistry
ARTICLE
Figure 3. Molecular structure of 2 ClO4 showing the main hydrogen
3
bond interactions.
Figure 2. Molecular structure of 1 ReO4 showing the main hydrogen
3
bonds as dashed lines.
resembling the behavior found for 4 BAr04, which features the
3
same pyridinic ligand.8 For pyrazole compounds 2 BAr04 and
3
afforded crystals of the 1 ReO4 adduct,21 while the salt [Bu4N]-
4 BAr04, the dramatically different stability toward the substitu-
3
3
[BAr04] remained in solution.
tion reactions by external anions can be attributed to the different
As shown in Figure 2, the cation of 1 ReO4 is a fac-{Re-
coordination mode of the aminopyridine ligand: the amino
3
bonded 2-ampy in 2 BAr04 is labile, whereas the pyridine-ligated
(CO)3} pseudooctahedral complex, with a 2-ampy and two
N-MeIm ligands bonded to the metal center. 2-Aminopyridine
is coordinated in its most usual mode through the pyridinic
nitrogen. One NꢀH group of the amino moiety forms a
hydrogen bond with one oxygen atom of the perrhenate anion
(d(N2 O5) = 2.940 Å, —(N2ꢀH O5) = 174.0ꢀ), while
3
3-ampy in 4 BAr04 is stable. For N-methylimidazole derivatives
3
1 BAr04 and 3 BAr04, in which the ampy ligands are in the same
3
3
coordination mode, steric hindrance should be responsible for
the different stability found: the amino group ortho to the
pyridinic nitrogen makes the ligand bulkier and therefore
3 3 3
3 3 3
more labile than in 3 BAr04, in which the amino group is in
O4 and O6 atoms form weaker hydrogen bonds with imidazole
central CH groups of other cationic complexes (see Figure 2).
Therefore, the oxoanion interacts with at least three rhenium
complexes, through hydrogen bonds with NH and central CH
groups, but there is no evidence in the solid state of simultaneous
interaction of NH and NC(H)N groups of the same metal cation
with the oxygen atoms of a given external anionic guest.
3
a meta position.
Density functional theory (DFT) computations at the B3LYP/
6-31G(d) (LANL2DZ for Re) level of theory were employed to
fully optimize the structures of complexes 1ꢀ4 (see the Support-
ing Information for computational details). Given the different
species determined by X-ray diffraction, two isomeric forms were
considered in our calculations for each complex that is identified
by the corresponding Roman numeral. This is followed by either
the letter a for the amino-bonded isomer or letter b for the
pyridine-bonded one. In Figure 4, ball-and-stick representations
of the optimized structures for both idealized isomers are shown
for complexes 2 and 4. Figure S1 and Table S1 in the Supporting
Information collect the optimized structures for complexes 1 and
3 and all the energy data, respectively.
Consistent with the experimental observations, on the basis of
the Gibbs energy difference between the two optimized isomers,
IIa is more stable than IIb by 2.8 kcal/mol (Figure 4). This
unusual coordination of the 2-ampy ligand can be attributed to
the formation of a strong intramolecular hydrogen bond between
the pyridinic nitrogen and a NH-pyrazole group. In effect, a NBO
(natural bond orbital) analysis indicates that complex IIa shows a
strong interaction between the electron lone pair of the pyridine
nitrogen and the σ antibonding NꢀH of the pyrazole group with
a second order perturbation energy of 19.6 kcal/mol. Therefore,
the formation of the hydrogen bond between the pyridinic
nitrogen and pyrazole NꢀH goup is the main delocalization of
In contrast, the structure of 2 ClO4, determined by X-ray
3
diffraction (Figure 3), shows that, in this case, the 2-ampy ligand
is bonded to the Re atom through the amino group. The
participation of the amino group in the coordination of 2-ami-
nopyridine to a metal center is rare, and only a few examples of
complexes with neutral 2-ampy ligands acting as bridges are
known.22 N(k1)-bonding via the exocyclic amino group is rather
exceptional and may occur only if coordination at the pyridine
N-donor site is not feasible for steric reasons.23 In 2 ClO4, this
3
feature is attributed to the presence of an intramolecular hydro-
gen bond between the NꢀH group of one of the pyrazole ligands
and the pyridine nitrogen atom, characterized by N4 N6 =
3 3 3
2.893(7) Å and N4ꢀH N6 = 162.3(6)ꢀ. In addition, the
3 3 3
ClO4ꢀ anion forms weak hydrogen bonds with NꢀH groups of
pyrazole and 2-ampy ligands. Hydrogen bonds between the
perchlorate anion and one cationic host are shown in Figure 3,
as dotted lines (for a more complete view, see the Supporting
Information).
Compound 3 BAr04, with a 3-ampy ligand, was found to
3
be stable toward the addition of external anionic substrates
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Figure 5. Job plot of receptor 3 BAr04 toward chloride and nitrate
3
anions.
Table 1. Binding Constant Values for Compound 3 BAr04 in
3
CD2Cl2
anion
Ka/Mꢀ1
anion
Ka/Mꢀ1
Fꢀ
2129 ((323)
1220 ((56)
1405 ((204)
799 ((68)
NO3
490 ((39)
246 ((29)
547 ((4)
ꢀ
Figure 4. B3LYP/6-31G* optimized structures for cationic complexes 2
(a) and 4 (b).
Clꢀ
Brꢀ
Iꢀ
ReO4
CH3COOꢀ
ꢀ
the lone pair of the pyridine nitrogen. In contrast, for IIb, the
formation of a hydrogen bond between the NꢀH of pyrazole and
the amino group (NpyrazoleꢀH Namino) implies a second-
substitution of one N-MeIm by a 3-ampy ligand increases the
strength of the anion binding by the complex. More interestingly,
the Ka value for acetate, one of the more basic anions employed,
is quite low, a fact that can be attributed to a preference of the
receptor for small, spherical anions.
3 3 3
order perturbation energy of only 4.5 kcal/mol, the principal
delocation of the lone pair of the amino nitrogen being into the
pyridinic ring (32.5 kcal/mol).
For complex 4, analogous to 2 with a 3-ampy ligand, the
orientation of the amino moiety precludes the formation of the
mentioned hydrogen bond. Accordingly, the theoretical calcula-
tions have shown that the pyridine-bonded isomer, IVb, is
10.9 kcal/mol more stable than IVa (Figure 4b). This situation
is virtually identical to that of complex 3, [Re(CO)3(N-MeIm)2-
(3-ampy)]+, isomer IIIb (pyridine-bonded) being 10.2 kcal/mol
more stable than IIIa (aminoꢀbonded isomer).24 Finally, geo-
metry optimizations were undertaken for complex 1, [Re-
(CO)3(N-MeIm)2(2-ampy)]+, which contains a 2-ampy ligand
but does not features pyrazole NH groups able to form strong
hydrogen bonds. In this case, the pyridinic isomer (Ib) is
preferred over the amino-bonded one (Ia) by 3.2 kcal/mol.24
The smaller energy difference in the latter with respect to IIIa
and IIIb and IVa and IVb isomers is probably due to the more
steric hindrance of the amino group being in an ortho instead
of a meta position. This fact is in complete concordance with
the experimental finding of the 2-ampy ligand being much
more labile than 3-ampy in 1 BAr04 and 3 BAr04 receptors
Single crystals of the chloride, bromide, and nitrate adducts
were obtained by the slow diffusion of hexane into saturated
CH2Cl2 solutions at ꢀ20 ꢀC. The solid state structures were
determined by X-ray diffraction, and it was found that in every
case the Bu4N BAr04 salt crystallized separately. The metallic
3
cationic complex was found to be virtually identical in the three
adducts: a pseudooctahedral rhenium atom bearing three carbo-
nyl ligands in a facial disposition, two N-MeIm and one 3-ampy
ligand coordinated to the metal through the pyridine nitrogen.
The solid state structures of both halide adducts (Clꢀ and
Brꢀ)25,26 are quite complex, resulting in tridimensional net-
works of hydrogen bonds between the halides and amino
NꢀH and imidazole central CꢀH groups of the cationic
rhenium complexes (Figure 6). Each halide anion participates
in several hydrogen bonds, the stronger interactions being
those that involve the NꢀH groups of the amino moieties of
3-ampy ligands.
The solid state structure of the 3 NO3 adduct (shown in
3
3
3
(see above).
Figure 7)27 can be described as antiparallel ribbons of cationic
rhenium complexes, the nitrate anions being encapsulated be-
tween them. Within each ribbon, the rhenium complexes orient
their {Re(CO)3} fragments toward the outside and their hydro-
gen bond donor side toward the nitrate anions. Each nitrate
anion interacts at least with three metal cations, and it was
found that the stronger hydrogen bonds are those of the amino
group [d(N6 O7) = 2.919 Å, —(N6ꢀH O7) = 173ꢀ,
The addition of Bu4N X salts (X = Fꢀ, Clꢀ, Brꢀ, Iꢀ,
3
CH3COOꢀ, NO3ꢀ, and ReO4 ) to 3 BAr04 solutions in
ꢀ
3
1
CD2Cl2 caused noticeable shifts in the H NMR spectroscopic
signals of the amine NꢀH and imidazole NC(H)N groups.
Anion exchange was found to be fast, and 1:1 binding constants
(Job plots for chloride and nitrate anions, indicating formation of
1:1 adducts, are shown in Figure 5) were obtained from 1H NMR
titrations in CD2Cl2.11 The magnitudes of these binding con-
stants were found to be on the same order when they were
calculated either from the amino NꢀH or from the imidazole
3 3 3
3 3 3
d(N6 O8) = 3.116 Å, —(N6ꢀH O8) = 158ꢀ].
3 3 3
3 3 3
The NꢀH groups of amides are considerably stronger hydro-
gen bond donors than those of amines. Therefore, a com-
pound with methylnicotinamide as a ligand instead of 3-ampy
should bind anions more effectively than the 3-ampy analogue.
1
CꢀH downfield shifts in the H NMR spectra. The relative
magnitude of the binding constants (Table 1) showed that the
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Figure 7. Molecular structure of 3 NO3 hostꢀguest adduct, showing
3
the main hydrogen bond interactions as dashed lines.
Table 2. Binding Constant Values for Compound 5 BAr04 in
3
CD2Cl2
anion
Ka/Mꢀ1
anion
Iꢀ
Ka/Mꢀ1
Fꢀ
Clꢀ
Brꢀ
11802 ((1981)
13550 ((1520)
2358 ((203)
2294 ((247)
1738 ((149)
2060 ((340)
ꢀ
NO3ꢀ
NO3
ability of the amido NꢀH group to act as a hydrogen bond
donor group. In addition, there is a significant preference for
fluoride and chloride anions over other anions tested. Un-
fortunately, crystals of these halide adducts (5 F or 5 Cl)
3
3
could not be grown despite numerous attempts using different
conditions. The solid state structure of the bromide adduct,
which crystallized separately to the Bu4N BAr04 salt, was
3
determined by X-ray diffraction.28 As shown in Figure 8a,
the rhenium primary coordination sphere is in agreement with
the one deduced from spectroscopic data in solution, the
ReꢀN(MeNA) distance being noticeably longer [d(ReꢀN5) =
2.242(2) Å] than the ReꢀN(ampy) bond distances found
Figure 6. Molecular structure of (a) 3 Cl and (b) 3 Br adducts,
showing the main hydrogen bonds as dotted lines.
3
3
[2.189(4) Å for 3 Br, for example]. This fact can be attri-
3
buted to the higher electronic withdrawing ability of the
carbamoyl group present in the nicotinamide ligand. As
shown in Figure 8a, the bromide anion interacts simulta-
neously through hydrogen bonds with the amido NꢀH group
However, the strongly electron-withdrawing alkoxycarbonyl
substituent should make the pyridine ligand a weaker donor
than the 3-ampy, and therefore one would need to worry about
the stability of the rhenium complex. Compound [Re(CO)3(N-
[d(N6
Br1) = 3.263 Å, —(N6ꢀH
Br1) = 158.6ꢀ], and
3 3 3
3 3 3
MeIm)2(MeNA)]BAr04 (5 BAr04, MeNA = methylnicotinamide)
with the central CꢀH group of one NꢀMeIm ligand
[d(C4 Br1) = 3.601 Å, —(C4ꢀH Br1) = 155.9ꢀ]. The
3
was prepared following the same procedure used before for the
3 3 3
3 3 3
synthesis of compounds [1ꢀ4] BAr04, using methylnicotinamide
remaining imidazole NC(H)N group of the rhenium cation
forms a hydrogen bond with the CO group of the amido-
3
instead of an ampy ligand. Compound 5 BAr04 was obtained in
3
good yield (71%), as the only product of the reaction, as a yellow
analytically pure solid (see the Experimental Section). The complex
was found to be stable against ligand dissociation and redistribution.
pyridine ligand of a neighbor complex [d(C8 O4) = 3.254 Å,
3 3 3
—(C8ꢀH O4) = 170.71ꢀ], so the structure results in chains
3 3 3
(see Figure 8b).
The ability of 5 BAr04 to act as an anion receptor was determined in
In view of the solid state structure of the bromide adduct, it can
be suggested that the smaller size of fluoride and chloride should
allow them to approach closer to the hydrogen bond donor
groups. This would produce stronger hydrogen bonds and a
higher electrostatic attraction and therefore a stronger overall
receptorꢀanion interaction, in agreement with the behavior
found in solution for fluoride and chloride anions.
3
solution by measuring the association constants (Ka) in CD2Cl2.
The results for several anions are summarized in Table 2, and Job
plots showed that the adducts have a 1:1 receptor/anion ratio (the
Job Plots are given as Supporting Information).
The binding constants are higher than those of N-methylimi-
dazole rhenium receptors, discussed above, showing the higher
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Figure 8. (a) Molecular structure of the 5 Br adduct, showing the main intramolecular hydrogen bonds as dashed lines. (b) View of the intermolecular
3
interactions in 5 Br, which lead to infinite chains.
3
’ CONCLUSIONS
plots. This material is available free of charge via the Internet at
http://pubs.acs.org
Organometallic anion receptors derived from the {Re(CO)3}
fragment combining more than one class of hydrogen bond
donor ligands can be easily prepared. The substitution of an azole
by an amino or amido pyridine ligand in tris(imidazole) or
tris(pyrazole) [Re(CO)3(Lz)3]BAr04 compounds leads to more
selective anion receptors. The situation of the amino group in
ampy ligands is crucial for determining the stability of its
compounds. While 3 BAr04 and 4 BAr04, featuring a 3-ampy
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: riera@unizar.es.
’ ACKNOWLEDGMENT
3
3
ligand, were found to be stable toward the addition of external
Financial support from the Ministerio de Ciencia e Innovaciꢀon
(MICINN, project number CTQ2009-12366) and Principado
de Asturias (project number IB08-104) is gratefully acknowl-
edged. We thank the Engineering and Physical Sciences Research
Council for funding.
anions, compounds with the 2-ampy ligand, 1 BAr04 and
3
2 BAr04, are labile in the presence of external anions. The solid
3
state structure of 2 ClO4 revealed that the 2-ampy ligand is
3
bonded to the rhenium atom through the amino moiety, a very
rare coordination mode. DFT calculations along with an NBO
analysis have shown that, for complex 2, the amino-bonded isomer
is preferred over the pyridine-bonded isomer by 2.83 kcal/mol,
due to the formation of a strong intramolecular hydrogen bond
between the pyridine nitrogen and the pyrazole NꢀH group. In
the other complexes studied herein, the pyridinic coordination of
ampy ligands is favored, both experimentally and theoretically.
’ DEDICATION
rThis paper is dedicated to the memory of Professor F. Gordon
A. Stone.
Compound [Re(CO)3(N-MeIm)2(MeNA)]BAr04 (5 BAr04)
featuring a methylnicotinamide ligand showed a high preference
for fluoride and chloride anions.
’ REFERENCES
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Inorganic Chemistry
ARTICLE
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18.848(17) Å, R = 90.0ꢀ, β = 105.62(3)ꢀ, γ = 90.0ꢀ, 120(2) K, V =
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3
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