Second-sphere interaction of anions with a weakly binding metal complex host: Probing the effect of counteranions

Article abstract of DOI:10.1039/b715599a

The reaction of [Re(OTf)(CO)₅] with N-methylimidazole (MeIm) afforded [Re(CO)₃(MeIm)₃]OTf (1). The reactions of 1 with KPF₆, NaBPh₄ and NaBAr′₄ (Ar′ = 3,5-bis(trifluoromethyl)phenyl) afforded [Re(CO)₃(MeIm) ₃]PF₆ (2) [Re(CO)₃(MeIm)₃]BPh ₄ (3) and [Re(CO)₃(MeIm)₃] BAr′₄ (4) respectively. An analogous reaction using N-phenylimidazole (PhIm) yielded [Re(CO)₃(PhIm)₃] BAr′₄ (7). These new compounds were characterized by IR and NMR, and the structures of 1 and 2 were determined by X-ray diffraction. Compounds [Re(CO)₃(MeIm)₃]₂[PtCl₆] (5), [Re(CO)₃(MeIm)₃][HSO₄] (6), [Re(CO) ₃(PhIm)₃][Br] (8) and [Re(CO)₃(PhIm) ₃][NO₃] (9) were crystallized from equimolar mixtures of either 4 or 7 and the tetrabutylammonium salt of the corresponding anion, and their structures were determined by X-ray diffraction. The solution behavior of 1-4, 7 toward several anions was studied spectroscopically, including the quantitative determination of binding constants by ¹H NMR. The cationic tris(imidazole)complexes are stable against imidazole-by-anion substitution, and the main hydrogen bonding interactions involve the imidazole NC(H)N groups. The binding constants for compounds 1-4 with several external anions follow the order 1 < 2 < 3 < 4, indicating that the strength of the cationic complex-counteranion interaction follows the order OTf^- > PF₆^- > BPh₄^- > BAr′₄^-. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715599a

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An international journal of inorganic chemistry
www.rsc.org/dalton
Number 7 | 21 February 2008 | Pages 841–964
ISSN 1477-9226
PAPER
Julio Pérez et al.
COMMUNICATION
Second-sphere interaction of
anions with a weakly binding metal
complex host
Enbo Wang et al.
A new polyoxometalate-based
3d–4f heterometallic aggregate

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PAPER
www.rsc.org/dalton | Dalton Transactions
Second-sphere interaction of anions with a weakly binding metal complex
host: probing the effect of counteranions†‡
Julio Pe´rez,*a Luc´ıa Riera,*a Laura Ion,a V´ıctor Riera,a Kirsty M. Anderson,b Jonathan W. Steedb and
Daniel Miguelc
Received 10th October 2007, Accepted 21st November 2007
First published as an Advance Article on the web 13th December 2007
DOI: 10.1039/b715599a
The reaction of [Re(OTf)(CO)5] with N-methylimidazole (MeIm) afforded [Re(CO)3(MeIm)3]OTf (1).
The reactions of 1 with KPF6, NaBPh4 and NaBArꢀ4 (Arꢀ = 3,5-bis(trifluoromethyl)phenyl) afforded
[Re(CO)3(MeIm)3]PF6 (2) [Re(CO)3(MeIm)3]BPh4 (3) and [Re(CO)3(MeIm)3]BArꢀ4 (4) respectively. An
analogous reaction using N-phenylimidazole (PhIm) yielded [Re(CO)3(PhIm)3]BArꢀ4 (7). These new
compounds were characterized by IR and NMR, and the structures of 1 and 2 were determined by
X-ray diffraction. Compounds [Re(CO)3(MeIm)3]2[PtCl6] (5), [Re(CO)3(MeIm)3][HSO4] (6),
[Re(CO)3(PhIm)3][Br] (8) and [Re(CO)3(PhIm)3][NO3] (9) were crystallized from equimolar mixtures of
either 4 or 7 and the tetrabutylammonium salt of the corresponding anion, and their structures were
determined by X-ray diffraction. The solution behavior of 1–4, 7 toward several anions was studied
spectroscopically, including the quantitative determination of binding constants by 1H NMR. The
cationic tris(imidazole)complexes are stable against imidazole-by-anion substitution, and the main
hydrogen bonding interactions involve the imidazole NC(H)N groups. The binding constants for
compounds 1–4 with several external anions follow the order 1 < 2 < 3 < 4, indicating that the strength
−
−
−
of the cationic complex–counteranion interaction follows the order OTf− > PF6 > BPh4 > BArꢀ4
.
In addition to acting as a Lewis acid, a transition metal
fragment can be used (as an alternative to organic scaffolds) as an
element of geometric organization. Thus, the metal coordination
of simple ditopic molecules featuring both heteroatoms bearing
lone electron pairs (i.e., hydrogen bond acceptors) and hydrogen-
bond donor groups (such as N–H groups) have been used for the
synthesis of new anion receptors.5 In this context, we have recently
studied the interactions of [Re(CO)3(Hpz)3]BArꢀ4 compounds
(Hpz = generic pyrazole, Arꢀ = 3,5-bis(trifluoromethyl)phenyl)
with anions.6 In these species, N-coordination of the pyrazoles
to the relatively inert Re center in a metal-enforced mutually fac
geometry allows the simultaneous interaction of at least two of
the N–H groups with external anionic guests (Chart 1a, ) and,
therefore, a significant overall interaction.
Introduction
Coordination of an N-alkylimidazole to a Lewis-acidic metal
center could be envisaged as a way to increase the hydrogen-bond
donor ability of the imidazole NC(H)N group. The combination
of their r-donor and p-acceptor properties makes imidazoles
good ligands for a variety of transition metal centers, including
organometallic fragments with the metal in a low oxidation state.1
Coordination of the parent imidazole (or other N–H containing
imidazole derivatives) to metal centers have been found to increase
the acidity of the N–H group,2 but the effect on the C–H group, a
weaker hydrogen-bond donor, remains unexplored.
Imidazolium salts are most often prepared by means of the
reaction of an N-alkylimidazole with a RX (e.g., alkyl bromide or
iodide) electrophile. The complex resulting from coordination of
a N-alkylimidazole to a cationic, Lewis acidic metal center, could
be regarded as an analog of an imidazolium cation. The NC(H)N
group of imidazolium cations is a relatively strong hydrogen-bond
donor. The importance of its interactions with anions within ionic
liquids has been noted,3 and organic molecules containing several
imidazolium groups in a mutually convergent disposition have
recently emerged as a new type of artificial host for anions.4
aDepartamento de Qu´ımica Orga´nica e Inorga´nica -IUQOEM, Facultad
de Qu´ımica, Universidad de Oviedo-CSIC, 33006 Oviedo, Spain. E-mail:
japm@uniovi.es, lrm@fq.uniovi.es; Fax: +34 985103446
bDepartment of Chemistry, University of Durham, Durham, UK DH1 3LE
cDepartamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad
de Valladolid, 47005 Valladolid, Spain; Fax: +34 983 423013
† CCDC reference numbers 642013–642018. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b715599a
‡ Electronic supplementary information (ESI) available: 1H NMR titration
profiles and Job Plots. See DOI: 10.1039/b715599a
Chart 1 (a) Geometry of our recently reported tris(pyrazole) complexes,6
and (b) the tris(N-alkylimidazole) complexes reported in this work.
A drawback of cationic receptors of anions is that the accompa-
nying counteranion competes with the external anionic guest for
878 | Dalton Trans., 2008, 878–886
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the receptor.5c We have proposed that the employment of the highly
The IR mCO bands of 1, indicative of a fac-Re(CO)3 geometry, occur
at wavenumber values (2025 and 1907 cm−1 in CH2Cl2) signifi-
cantly lower than those of the structurally related tris(pyrazole)
compounds (2039 and 1929 cm−1 for [Re(CO)3(Hdmpz)3]BArꢀ4
(Hdmpz = 3,5-dimethylpyrazole) and 2038 and 1933 cm−1 for
[Re(CO)3(HtBupz)3]BArꢀ4 (HtBupz = 3(5)-tert-butylpyrazole) in
CH2Cl2).6 The difference reflects that N-methylimidazole is a
ligand more electron-releasing than Hdmpz or HtBupz.
The structure of the cationic tris(imidazole) complex in 1 con-
sists of a rhenium atom coordinated to three carbonyl ligands in
a mutually facial disposition, in agreement with the solution data,
and to the non-substituted nitrogen atoms of three molecules of
N-methylimidazole. Despite the difference in the donor character
of the heterocyclic ligands inferred from the differences in the IR
spectra (see above), the Re–N and Re–C distances and the angles
about Re in 1 (see Table 1) are closely similar to those found in the
tris(pyrazole) complexes mentioned above.6
−
charge-delocalized BArꢀ4 tetraarylborate anion should minimize
−
such interference.6 Whereas BArꢀ4 has become quite popular in
organometallic chemistry and catalysis as an alternative to less
innocent counteranions such as perchlorate, triflate, tetrafluorob-
orate, hexafluorophosphate, tetraphenylborate, etc.,7 our recent
work is the first example of its use as counteranion of cationic
anion hosts. Intrinsically weak host–guest interactions, such as
those expected to occur between the C–H groups of imidazole
ligands and external anions, appear as a good benchmark test
for the magnitude of the effect of having a less coordinating
counteranion. N-coordination of three N-alkylimidazoles to the
fac-{Re(CO)3} fragment could afford a geometry similar to
that of the above mentioned tris(pyrazole) complexes,6 the C–
H groups being now the ones able to simultaneously interact
with external anions (Chart 1b). Therefore, we sought to prepare
cationic rhenium tricarbonyl tris(N-alkylimidazole) complexes
with several different low-interacting counteranions, including
BArꢀ4−, and compare the magnitudes of their interaction with
external anions. Our results are discussed below.
The crystalline structure of 1 features a network of weak
hydrogen bonds between the C–H groups of the cationic tris(N-
methylimidazole) complex and the oxygen atoms of the triflate
anion. The stronger (but still very weak; note the angle, far
from linear) among these hydrogen bonds are those involving
Results and discussion
˚
the imidazole NC(H)N group (C(21) · · · O(4) = 3.24 A and
C(21) · · · H · · · O(4) = 134◦). Only one NC(H)N group of each
cationic complex interacts with each triflate anion. At least
in the solid state, thus, there is no convergence of several
NC(H)N hydrogen-bond donor groups from a single rhenium
complex toward a given anion. This is in contrast to the related
tris(pyrazole) complexes, in which the N–H groups of two pyrazole
ligands of each complex converge toward one oxygen atom of even
weak hydrogen-bond acceptors such as acetone or the perchlorate
anion.6 The difference can be attributed to the weaker hydrogen
bond donor ability of the imidazole C–H bonds compared with
the pyrazole N–H bonds.
The reaction of [Re(OTf)(CO)5] with a three-fold molar amount
of N-methylimidazole (MeIm) in refluxing toluene afforded
[Re(CO)3(MeIm)3]OTf (1) in high yield (Scheme 1). We have
previously found that the similar reaction using pyrazoles (Hpz)
affords a mixture of products, presumably the compounds
[Re(CO)3(Hpz)3]OTf and [Re(OTf)(CO)3(Hpz)2].6a In contrast,
1 was obtained as the single product of the above mentioned
reaction, the difference reflecting the fact that imidazoles are better
ligands than pyrazoles.1 Compound 1 was characterized by IR,
NMR (1H and 13C) and single-crystal X-ray diffraction (see Fig. 1).
Thus, at least in the solid state, where the distance between ions
is small, the multitude of other non-covalent interactions at play
overcomes the formation of two or three hydrogen bonds between
one given anion and the NC(H)N groups of one given cationic
complex. For instance, in the structure of 1, weak hydrogen bonds
occur between the triflate oxygens and the other C–H groups,
including those of the methyl groups, as displayed in Fig. 1.
Rhenium(I) tris(N-alkylimidazole) compounds were previously
unknown; however, analogs containing the parent imidazole were
reported by Alberto et al.8
Scheme 1 Synthesis of compound 1.
Compounds [Re(CO)3(MeIm)3]PF6 (2), [Re(CO)3(MeIm)3]-
BPh4 (3) and [Re(CO)3(MeIm)3]BArꢀ4 (4) were cleanly obtained
by the anion exchange reactions of 1 with the salts KPF6, NaBPh4
and NaBArꢀ4 respectively in CH2Cl2 (see Scheme 2).
Scheme 2 Synthesis of compounds 2–4.
Compounds 2–4 were spectroscopically characterized (see Ex-
perimental); in addition, the structure of 2 was determined by
Fig. 1 Structure of 1 showing the main hydrogen bonds as dotted lines.
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◦
ꢀ
˚
Table 1 Selected bond distances (A) and angles ( ) for compound [Re(CO)3(MeIm)3]OTf (1) and the tris(pyrazole) complexes [Re(CO)3(Hpz)3]BAr 4
(Hpz = Hdmpz, HtBupz)
˚
Bond Distances/A
1
[Re(CO)3(Hdmpz)3]+
2.204(6)
2.186(8)
2.195(7)
1.938(11)
[Re(CO)3(HtBupz)3]+
2.191(4)
2.187(4)
2.193(4)
1.936(6)
Re(1)–N(1)
Re(1)–N(3)
Re(1)–N(5)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
2.202(2)
2.204(2)
2.182(2)
1.914(3)
1.906(3)
1.912(3)
1.946(11)
1.926(11)
1.918(6)
1.910(7)
Bond Angles/◦
C(1)–Re(1)–C(2)
C(2)–Re(1)–N(1)
N(1)–Re(1)–N(3)
C(1)–Re(1)–N(3)
C(3)–Re(1)–N(5)
C(1)–Re(1)–C(3)
C(1)–Re(1)–N(5)
N(1)–Re(1)–C(3)
N(1)–Re(1)–N(5)
87.01(13)
93.14(11)
86.63(9)
93.24(11)
174.61(10)
89.26(12)
93.80(11)
90.30(10)
86.63(9)
85.8(5)
91.2(3)
87.2(2)
95.9(4)
176.4(4)
86.4(4)
92.6(3)
97.0(3)
84.1(2)
89.0(2)
92.0(2)
85.75(16)
93.19(19)
176.3(2)
87.8(2)
94.8(2)
93.8(2)
83.47(16)
X-ray diffraction (Fig. 2). Because the only difference within
the 1–4 family of compounds is the counteranion, and the four
complexes feature low interacting counteranions, the IR and
NMR spectroscopic data of the cationic complexes are very similar
in 1–4.
In principle, one could imagine that the C3 symmetry of the
tris(N-methylimidazole) complexes could allow them to interact in
a three-point fashion with a C3-symmetric anion such as a triflate
or with an Oh-symmetric anion such as hexafluorophosphate
(Chart 2).
Chart 2 Possible three-point contact structures in 1 and 2.
The fact that this interaction is not found in the solid state
could be attributed to the low hydrogen-bond acceptor capability
of these anions. However, neither the octahedral, dianionic
2−
complex PtCl6 in compound [Re(CO)3(MeIm)3]2[PtCl6] (5),
presumably a much better hydrogen-bond acceptor (besides being
dianionic, metal–chloride bonds have been found to be good
hydrogen-bond acceptors),9 nor the hydrogensulfate anion in
[Re(CO)3(MeIm)3][HSO4] (6) (see Experimental for the synthesis
and characterization, and Fig. 3 and 4 for plots of the structures)
were found to establish three-point hydrogen bond connections
with the [Re(CO)3(MeIm)3]+ cation.
In 5, the hydrogen bond between the NC(H)N group of one of
the imidazole ligands and one of the chlorides is charact◦erized by
˚
C(11) · · · Cl(1) = 3.76 A and C(11) · · · H · · · Cl(1) = 148 . Again,
Fig. 2 Structure of compound 2 showing the main hydrogen bond
interactions.
only one of such interactions exists within a given ion pair. Hydro-
gen bonds involving the other types of C–H groups have shorter
distances and similar angles in this structure; thus C(24) · · · Cl(3) =
◦
˚
The structure of 2 consists of a cation virtually identical to
3.55 A and C(24) · · · H · · · Cl(3) = 141 for a hydrogen bond
−
˚
the one described for 1, with weak hydrogen bonds to PF6
.
involving a methyl C–H group, and C(33) · · · Cl(1) = 3.69 A and
The shorter of these hydrogen bonds involves the NC(H)N
C(33) · · · ·H · · · Cl(1) = 151◦ (not shown in Fig. 3) for one of the
other C–H bonds of the imidazole ring. However, this feature
of the solid state structure does not extend to the solution phase.
Rather, as it will be discussed below, the NC(H)N group is by large
the C–H group that more significantly takes part in the formation
in hydrogen bonds in solution.
˚
group of a ligated imidazole, with C(11) · · · F(1) = 3.32 A and
C(11) · · · H · · · F(1) = 155◦ (Fig. 2). As in the structure of 1, only
one of the N-methylimidazole ligands of each complex interacts
with a given hexafluorophosphate anion, and the other C–H bonds
of the imidazole ligands are involved in weaker hydrogen bonds.
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Fig. 3 (a) Molecular structure of [Re(CO)3(MeIm)3]2[PtCl6] (5) adduct. (b) View of the ribbons (see text) in the structure of 5.
C(24) · · · H · · · O(14) = 175◦). Like in the structure of the hex-
achloroplatinate adduct 5, the (dianionic) hydrogensulfate dimers
in 6 occupy the space in between rows of cationic complexes (with
which they interact through charge-assisted C · · · H · · · O hydrogen
bonds), and the latter present their Re(CO)3 faces to each other. In
6, in between these Re-dianion–Re ribbons, “hydrophobic” voids
are occupied by molecules of dichloromethane (see ESI‡).
Attempts to crystallize adducts of the [Re(CO)3(MeIm)3]+
cation with other anions were unsuccessful. Better crystallinity
was found for the adducts obtained via anion exchange from
the tris(N-phenylimidazole) compound [Re(CO)3(PhIm)3]BArꢀ4
(7), synthesized in a manner analogous to that described above
for 4. Thus, single crystals of [Re(CO)3(PhIm)3][Br] (8) and
[Re(CO)3(PhIm)3][NO3] (9), grown by slow diffusion of hexane
into concentrate dichloromethane solutions of the equimolar
mixtures of 7 and either [Bu4N][Br] or [Bu4N][NO3] at room tem-
perature, were employed to determine the crystalline structures of
these two compounds by X-ray diffraction. The results, displayed
in Fig. 5a showed the presence of [Re(CO)3(PhIm)3]+ cations with a
rhenium first coordination sphere like the one described above for
the N-methylimidazole derivatives, involved in a complex network
of hydrogen bonds to the anions. In addition to the imidazole
NC(H)N group, the phenyl substituents were found to act as ad-
ditional sources of hydrogen-bond donor groups. Thus, as depicted
in Fig. 5a, the stronger hydrogen bonds to a given bromide in 8
are those to the NC(H)N groups of two of the imidazole ligands
Fig. 4 View of the molecular structure of a {[Re(CO)3(MeIm)3][HSO4]}2
pair in 6.
The view in Fig. 3b shows that the solid state structure of 5
can be described as neutral ribbons each consisting of a row of
hexachloroplatinate dianions sandwiched between two rows of
cationic rhenium complexes. Within each ribbon, the rhenium
complexes orient their hydrogen-bond donor Re(MeIm)3 side
toward the inner row of anions, whereas their outward Re(CO)3
sides face the Re(CO)3 periphery of another ribbon. Within each
ribbon, the main interactions are charge-assisted C–H · · · Cl–Pt
hydrogen bonds, whereas the ribbons are linked mainly by van der
Waals forces. Obviously, this is only an approximation, as some
C · · · H · · · OC–Re interactions exist between ribbons.
˚
of a cationic complex (C(11) · · · Br = 3.66 A, C(11) · · · H · · · Br =
167◦), but there are also two hydrogen bonds to the C–H groups
of the ortho carbons of the phenyl substituents on these same
◦
˚
imidazoles (C(19) · · · Br = 3.66 A, C(19) · · · H · · · Br = 139 ).
This bromide forms two weaker hydrogen bonds to the NC(H)N
◦
˚
In the structure of 6 (Fig. 4a), the hydrogensulfate anions are
paired via self-complementary hydrogen bonds forming pseudo-
chair-like rings. Both “free” oxygens of each sulfate unit form
a hydrogen bond, one with the NC(H)N group of an imidazo◦le
(C(21) · · · Br = 3.90 A, C(21) · · · H · · · Br = 179 ) and ortho CH
◦
˚
phenyl (C(25) · · · Br = 3.94 A, C(25) · · · H · · · Br = 174 ) groups
of the “third” imidazole ligand of a neighbor cationic complex.
When viewed along the plane of that “third” phenylimidazole,
the structure consists of chains in which anion and cation occupy
alternating positions, and adjacent chains run in an anti-parallel
fashion (see ESI‡). Fig. 5b shows that each of the two imidazoles
˚
ligand (C(11) · · · O(11) = 3.12 A and C(11) · · · H · · · O(11) = 134 )
and the other with a CH3 group of a different imidazole ligand
˚
of the same rhenium complex (C(24) · · · O(14) = 3.29 A and
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Fig. 5 (a) Molecular structure of [Re(CO)3(PhIm)3][Br] (8). (b) View of the inter-chain ring stacking present in the molecular structure of 8.
of a given complex that converge toward a bromide anion inserts
parallel to two consecutive imidazoles of the same type of an
adjacent, anti-parallel chain, suggesting some degree of inter-
chain p-stacking.
The structure of the nitrate adduct 9 is considerably more com-
plex than the ones previously discussed, with an asymmetric unit
containing two non-equivalent cationic complexes and two non-
equivalent nitrate anions. In contrast with the structure of 8, now
the phenyl and imidazole rings within each phenylimidazole ligand
are not coplanar. The main hydrogen bonds occur between nitrate
oxygens and NC(H)N and ortho CH phenyl groups. As depicted
in Fig. 6, each nitrate anion (only one of the two non-equivalent
nitrate anions is displayed, but the environment of the other is
qualitatively similar) interacts mainly with four CH groups of
three cationic complexes.
The cationic tricarbonyltris(imidazole) complexes in 1–4, 7
were found to be stable (i.e., not to undergo substitution or
deprotonation) toward the anions fluoride, chloride, bromide,
iodide, nitrate, hydrogensulfate, dihydrogenphosphate and acetate
both in dichloromethane and in acetonitrile. 1H NMR studies of
the interaction of compounds 1–4, 7 with the tetrabutylammonium
salts of the anions mentioned above showed that the signals
of the complex that undergo larger shifts upon anion addition
are those due to the NC(H)N groups of the ligated imidazoles.
When CD3CN was used as solvent, this anion-induced change
in chemical shift was much smaller than the one found when
tris(pyrazole) compounds [Re(CO)3(Hpz)3]BArꢀ4 were used as
hosts in the same solvent.6 For some anions, such as chloride, ni-
trate or hydrogensulfate the magnitude of this change in chemical
shift in CD3CN solution was sufficient to allow the construction
of Job plots, that showed the formation of 1 : 1 adducts between
the anions and the cationic rhenium complex. Binding constants
were calculated for 3 (see ESI‡) by 1H NMR titrations (fast
Fig.
6 Molecular structure of one of the two non-equivalent
[Re(CO)3(PhIm)3][NO3] pair present in the structure of 9.
anion exchange was found).10 Again, the values of these constants
were much smaller than those found for the tris(pyrazole) hosts
mentioned above,6 indicating that the interaction between the
tris(imidazole) complexes and the anions is much weaker, as
expected because, in general, CH groups are weaker hydrogen
bond donors than N–H groups. The weakness of these interactions
in solution agrees with the presence of only weak hydrogen bonds
in the crystalline structures discussed above. In this regard, we note
that on the tris(imidazole) complexes reported here, C–H groups
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are the only hydrogen bond donors (i.e., they are not assisted by
simultaneous formation of stronger (e.g., those involving N–H or
O–H groups) hydrogen bonds. Moreover, unlike in purely organic
tris(imidazolium) receptors, the complexes reported here carry a
single positive charge.
of insoluble silver halides. Tetraphenylborate interacts with the
cationic complexes less than triflate or hexafluorophosphate.
However, the interactions between hydrogen-bond donor groups
−
and the p electron density of the BPh4 rings are well known.
For instance, Kiviniemi et al. have studied the behavior of
BPh4 toward azolium cations.11 Finally, the strongly electron-
−
To amplify the magnitude of the binding constants and compare
1
their values for the hosts having different counteranions, the H
withdrawing effect of the trifluoromethyl groups on the aryl rings
−
NMR titration experiments of 1–4, 7 with tetrabutylammonium
salts were carried out in CD2Cl2, a less competitive solvent. In
this solvent, the anion-induced shift is also much higher for
the imidazole NC(H)N groups (several tenths of ppm); albeit
detectable, shifts in the CH3 (in 1–4) or o-C6H5 (in 7) groups
are generally an order of magnitude lower. In contrast with the
deprotonation of the [Re(CO)3(Hpz)3]BArꢀ4 compounds effected
by fluoride,6 and with the vanishing of the N–H signals observed
upon fluoride addition to many anion receptors containing N–H
groups, the signals of the C–H groups of hosts 1–4, 7 remained
sharp when these complexes were titrated with [Bu4N][F]. Again
fast exchange was found in every instance, Job plots indicated the
presence of 1 : 1 adducts, and binding constants in CD2Cl2 for
of the BArꢀ4 counteranion make it the more innocent; i.e., the
one that interacts less with the cationic complexes. The magnitude
of the effect revealed by Table 2 is small, so that when strong
interactions between a cationic receptor and an anionic guest are
−
at play, having BArꢀ4 as counteranion instead of, for instance,
PF6−, does not offer a significant advantage in the sense of
affording higher binding constants. This being true, it should be
−
kept in mind that the BArꢀ4 salts are endowed with other often
−
desirable properties: (a) BArꢀ4 salts are much more soluble in
organic solvents, the difference becoming particularly important in
−
those of moderate polarity (e.g. CH2Cl2), and (b) BArꢀ4 does not
undergo hydrolysis (it even resists sulfuric acid!),12 in contrast with
−
the documented hydrolysis of PF6 to difluorophosphate in the
1
the anions listed above were obtained for 1–4, 7 using H NMR
presence of, for instance, electrophilic metal centers,5c,13 or with the
relatively easy cleavage of the B–C bonds of BPh4− in the presence
of strong acids or electrophiles.7a More generally, our results
suggest that, in those instances in which host–guest interactions
are intrinsically weak (such as hydrogen bonds or other non-
covalent interactions between a catalyst and its substrate), the
BArꢀ4− anion, already widely used in other fields of chemistry, can
be a better choice than other more conventional counteranions.
titrations (Job plots and titration curves are given as ESI‡). The
results are shown in Table 2. For comparison, binding constants
were also obtained for the tris(N-phenylimidazole) compound 7.
First, the comparison between [Re(CO)3(MeIm)3]BArꢀ4 (4) and
[Re(CO)3(PhIm)3]BArꢀ4 (7) shows that the interactions of the latter
with anions in solution are weaker, in spite of the involvement
of some of the phenyl C–H groups in hydrogen bonding. In-
terestingly, the IR mCO bands of 7 (2032 and 1917 cm−1) occur
at wavenumber values slightly higher than those of 4 (2029 and
1912 cm−1), so the phenyl substituents act as electron withdrawing
groups compared with the methyl groups. That should make the
C–H groups of 7 better hydrogen-bond donors. Therefore, given
that this is not what is actually observed, the predominant effect
of the phenyl substituents on 7 must be to sterically hinder the
approach of anions.
Conclusions
A comparative study of the behavior of [Re(CO)3(MeIm)3]OTf
(1), [Re(CO)3(MeIm)3]PF6 (2) [Re(CO)3(MeIm)3]BPh4 (3) and
[Re(CO)3(MeIm)3]BArꢀ4 (4) toward several anions showed that
the cationic tris(imidazole) rhenium complex forms 1 : 1 adducts
with the anions, that the main hydrogen bond interactions within
these adducts are those between the anion and the imidazole
Second, a comparison of compounds 1–4 indicates that the
magnitude of the interaction of the cationic complex with external
anions varies in the order of 1 < 2 < 3 < 4. In as much as the
only difference between these compounds is the counteranion,
the strength of the cationic complex–counteranion interaction
−
NC(H)N groups, and that the tetraarylborate BArꢀ4 (Arꢀ =
3,5-bis(trifluoromethyl)phenyl) is the anion that interacts less
with the cationic complex, thus leading to larger complex–anion
binding constants. These results suggest that BArꢀ4−, so far used
in organometallic chemistry and catalysis, but not in supramolec-
ular chemistry, would be advantageous over more conventional
counteranions for cationic supramolecular receptors, and that, in
general, its use would enhance hydrogen bonding between polar
substrates and cationic complexes.
must follow the opposite order, i.e., OTf− > PF6 > BPh4
>
−
−
BArꢀ4−. Triflate, and specially hexafluorophosphate, are often used
as counteranions of cationic receptors of anions. In part, this is
due to the fact that their silver salts are commercially available,
providing an easy route to anion exchange driven by precipitation
Table 2 Binding constant values for compounds 1–4, 7 in CD2Cl2
Anion
1, Ka/M−1
2, Ka/M−1
3, Ka/M−1
4, Ka/M−1
7, Ka/M−1
F−
Cl−
Br−
I−
NO3
HSO4
81
77
79
60
38
61
65
8
9
1
1
1
3
6
107
100
62
57
38
78
105 16
2
2
5
5
3
2
400
176 16
174
151
132 11
373 43
7
1002 112
291 19
261 18
292 20
830 152
144 14
1
1
101
78
5
6
−
234 16
168 21
78
200 18
−
a
7
−
COOCH3
243
4
290 19
a
−
1H NMR signals of interest were obscured by those of the BArꢀ4 anion, precluding calculation of binding constants.
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thermal parameter. Calculations were made with SHELXTL and
PARST.21 Crystal and refinement details are collected in Table 3
Experimental
General
Synthesis of [Re(CO)3(MeIm)3]OTf (1)
All manipulations were carried out under a nitrogen atmosphere
using Schlenk techniques. Compound [Re(OTf)(CO)5]14 was pre-
pared as previously reported. Tetrabutylammonium salts were
purchased from Fluka or Aldrich. Deuterated acetonitrile and
dichloromethane (Cambridge Isotope Laboratories, Inc.) were
stored under nitrogen in Young tubes and used without further
A mixture of [Re(OTf)(CO)5] (0.200 g, 0.420 mmol) and MeIm
(0.100 mL, 1.260 mmol) in toluene (25 mL) was refluxed for 3 h.
The resulting solution was slowly cooled to room temperature
affording colorless crystals of 1, one of which was employed
for an X-ray structure determination. Yield: 0.261 g, 93%; IR
1
purification. H NMR and 13C NMR spectra were recorded on
1
(CH2Cl2, cm−1): 2025, 1907 (mCO); H NMR (CD2Cl2): d = 7.54
a Bruker Avance 300, DPX-300 or Avance 400 spectrometer.
NMR spectra are referred to the internal residual solvent peak
(s, 3H, CH MeIm), 7.05 (s, 3H, CH MeIm), 7.02 (s, 3H, CH
MeIm), 3.79 (s, 9H, CH3 MeIm). 13C NMR (CD2Cl2): d = 195.5
(CO), 141.0, 131.4, 122.5 (CH MeIm), 34.7 (CH3 MeIm); 19F
NMR (CD2Cl2): d = −79.0; elemental analysis calcd (%) for
C16H18F3N6O6ReS: C 28.87, H 2.73, N 12.63; found: C 28.85,
H 2.63, N 12.51.
1
1
for H and 13C{ H} 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 10−2 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 × 10−2 M in CD2Cl2,
separately prepared and kept in a septum-stoppered vial during
the titration) were injected through the septum using Hamilton
microsyringes (10–100 lL). The volume of each addition was 10 lL
before reaching the saturation zone (nearly horizontal line of the
titration profile), and 20 or 40 lL afterwards. When the change in
d is small, 20 lL of salt solution were added from the beginning.
Data were treated using the WinEQNMR program.10
Synthesis of [Re(CO)3(MeIm)3]PF6 (2)
To a solution of [Re(CO)3(MeIm)3]OTf (1) (0.100 g, 0.150 mmol)
in CH2Cl2 (20 mL) and MeCN (2 mL), KPF6 (0.028 g, 0.150 mmol)
was added and the mixture was stirred at room temperature for
3 h. The resulting solution was filtered via canula and concentrated
under reduced pressure to a volume of 10 mL. Addition of
hexane (30 mL) caused the precipitation of a white solid which
was washed with hexane (2 × 20 mL). Yield: 0.080 g, 87%; IR
1
(CH2Cl2, cm−1): 2025, 1908 (mCO); H NMR (CD2Cl2): d = 7.48
(s, 3H, CH MeIm), 7.06 (s, 3H, CH MeIm), 7.10 (s, 3H, CH
MeIm), 3.78 (s, 9H, CH3 MeIm). 13C NMR (CD2Cl2): d = 195.5
[CO], 140.9, 131.4, 122.6 [CH MeIm], 34.6 [CH3 MeIm]; 31P NMR
(CD2Cl2): −144.6 [sep (1JPF = 711.7 Hz) PF6]; 19F NMR (CD2Cl2):
−73.13 [d (1JPF = 711.7 Hz), PF6]; elemental analysis calcd (%)
for C15H18F6N6O3PRe: C 27.24, H 2.74, N 12.70; found: C 27.55,
H 2.52, N 12.65. Slow diffusion of hexane into a concentrated
solution in CH2Cl2 at room temperature afforded colorless crystals
of compound 2, one of which was employed for an X-ray analysis.
Crystal structure determination. General description
For compounds 5 and 6. Data collection was performed at
150(2) K on a Nonius KappaCCD single crystal diffractometer,
˚
using Cu-Ka radiation (k = 1.5418 A). Images were collected up
to 2h = 140◦ at a 29 mm fixed crystal-detector distance, using
the oscillation method. Data collection strategy was calculated
with the program Collect.15 Data reduction and cell refinement
were performed with the program HKL Denzo and Scalepack.16
A semi-empirical absorption correction was applied using the
program SORTAV.17
Synthesis of [Re(CO)3(MeIm)3]BPh4 (3)
To a solution of [Re(CO)3(MeIm)3]OTf (1) (0.200 g, 0.301 mmol)
in CH2Cl2 (20 mL), NaBPh4 (0.103 g, 0.301 mmol) was added
and the mixture was stirred at room temperature for 1 h. The
resulting solution was filtered off the white solid (NaOTf) via
canula and concentrated under reduced pressure to a volume of
5 mL. Addition of hexane caused the precipitation of a white solid
which was washed with hexane (2 × 20 mL). Yield: 0.240 g, 96%;
IR (CH2Cl2, cm−1): 2027, 1911 (mCO); 1H NMR (CD2Cl2): d = 7.36
(m, 11H, CH MeIm and BPh4), 7.02 (m, 8H, BPh4), 6.90 (m, 4H,
BPh4), 6.80 (s, 3H, CH MeIm),6.72 (s, 3H, CH MeIm), 3.41 (s, 9H,
For compound 2. A crystal was attached to a glass fiber and
transferred to a Bruker AXS SMART 1000 diffractometer with
graphite monochromatized Mo-Ka X-radiation and a CCD area
detector. A hemisphere of the reciprocal space was collected up
to 2h = 48.6◦. Raw frame data were integrated with the SAINT18
program. An empirical absorption correction was applied with the
program SADABS.19
CH3 MeIm). 13C NMR (CD2Cl2): d = 195.0 (CO), 164.1 [q (1JCB
=
49.4 Hz), Ci BPh4], 140.7, [CH MeIm], 135.9 [Co BPh4], 130.2 [CH
MeIm], 125.7 [Cm BPh4], 122.8 [CH MeIm], 121.8 [Cp BPh4], 34.5
[CH3 MeIm]; elemental analysis calcd (%) for C39H38BN6O3Re: C
56.05, H 4.58, N 10.06; found: C 55.80, H 4.49, N 9.72.
For compounds 1, 8 and 9. A single crystal was mounted on
a Bruker diffractometer equipped with a SMART 1 K CCD
area detector and Oxford Cryostream N2 cooling device, using
˚
graphite-monochromated Mo-Ka radiation (k = 0.71073 A). .
Structure solution and refinement (all). The structures were
solved by direct methods with SHELXTL.20 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were set in
calculated positions and refined as riding atoms, with a common
Synthesis of [Re(CO)3(MeIm)3]BArꢀ4 (4)
Compound 4 was prepared as described above for compound
3 from [Re(CO)3(MeIm)3]OTf (1) (0.070 g, 0.105 mmol) and
884 | Dalton Trans., 2008, 878–886
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©

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This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 878–886 | 885
©

View Article Online
NaBArꢀ4 (0.093 g, 0.105 mmol). Yield: 0.090 g, 62%; IR
(VA012C05) and European Union (grant UE-05-ERG-516505
to L. R.) for support of this work. We thank Ministerio de
Educacio´n y Ciencia for a Ramo´n y Cajal contract (L. R.) and
for a FPU predoctoral fellowship (L. I.). K. M. A. thanks the
EPSRC for a post-doctoral grant.
1
(CH2Cl2, cm−1): 2029, 1912 (mCO); H NMR (CD2Cl2): d = 7.74
(s, 8H, Ho BArꢀ4), 7.58 (s, 4H, Hp BArꢀ4), 7.54 (s, 3H, CH MeIm),
7.00 (s, 3H, CH MeIm),6.72 (s, 3H, CH MeIm), 3.74 (s, 9H, CH3
MeIm). 13C NMR (CD2Cl2): d = 194.8 (CO), 161.7 [q (1JCB = 49.8
Hz), Ci BArꢀ4], 140.9, [CH MeIm], 134.8 [Co BArꢀ4], 130.2 [CH
MeIm], 128.9 [q (2JCF = 31.4 Hz), Cm BArꢀ4], 124.6 [q (1JCF
=
272.4 Hz), CF3 BArꢀ4], 122.6 [CH MeIm], 117.5 [Cp BArꢀ4], 34.6
[CH3 MeIm]; elemental analysis calcd (%) for C47H30BF24N6O3Re:
C 40.91, H 2.19, N 6.09; found: C 40.92, H 2.19, N 6.06.
Notes and references
1 R. B. King and K.-N. Chen, Inorg. Chem., 1977, 16, 3372.
2 (a) J. A. Winter, D. Caruso and R. Shepherd, Inorg. Chem., 1988, 27,
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1851.
Synthesis of [Re(CO)3(MeIm)3]2[PtCl6] (5)
3 (a) P. A. Hunt, B. Kirchner and T. Welton, Chem.–Eur. J., 2006, 12,
6762; (b) F. C. Gozzo Fabio, L. S. Santos, R. Augusti, C. S. Consorti, J.
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To a solution of [Re(CO)3(MeIm)3]BPh4 (3) (0.027 g, 0.030 mmol)
in CH2Cl2 (15 mL), [nBu4N]2[PtCl6] (0.050 g, 0.060 mmol) was
added and the mixture was stirred at room temperature for 15 min.
The resulting yellow solution was filtered via canula and concen-
trated under reduced pressure to a volume of 5 mL. Addition
of hexane caused the precipitation of a yellow solid which was
washed with hexane (2 × 20 mL). Slow diffusion of hexane into a
concentrated solution of 5 in CH2Cl2 at room temperature afforded
yellow crystals, one of which was employed for an X-ray structure
determination. Yield: 0.040 g, 93%; IR (CH2Cl2, cm−1): 2024, 1906
1
(mCO); H NMR (CD2Cl2): d = 7.59 (s, 3H, CH MeIm), 7.05 (m,
6H, CH MeIm), 3.88 (s, 9H, CH3 MeIm). 13C NMR (CD2Cl2): d =
198.1 (CO), 143.4, 133.9, 125.2 [CH MeIm], 38.3 [CH3 MeIm];
elemental analysis calcd (%) for C30H36Cl6N12O6PtRe2: C 25.01, H
2.52, N 11.66; found: C 25.36, H 2.56, N 11.32.
Synthesis of [Re(CO)3(PhIm)3]BArꢀ4 (7)
8 R. Alberto, R. Schibli, R. Waibel, U. Abram and A. P. Schubiger,
Coord. Chem. Rev., 1999, 190–192, 901.
A mixture of [Re(OTf)(CO)5] (0.150 g, 0.316 mmol) and PhIm
(0.120 mL, 0.948 mmol) in toluene (25 mL) was refluxed for
3 h. The solvent was evaporated to dryness. The residue was
redissolved in CH2Cl2 (20 mL), NaBArꢀ4 (0.280 g, 0.316 mmol)
was added and the reaction mixture was stirred for 1 h at room
temperature. The resulting colorless solution was filtered off the
white solid (NaOTf) and concentrated under reduced pressure
to a volume of 5 mL. Addition of hexane (20 mL) caused the
precipitation of a microcrystalline solid that was washed with
hexane (2 × 10 mL). Yield: 0.310 g, 63%; IR (CH2Cl2, cm−1): 2032,
1919 (mCO); 1H NMR (CD2Cl2): d = 8.11 (s, 3H, CH PhIm), 7.77
(s, 8H, Ho BArꢀ4), 7.56 (m, 13H, Hp BArꢀ4 and C6H5 PhIm), 7.46
(m, 9H, PhIm), 7.06 (s, 3H, CH PhIm). 13C NMR CD2Cl2): d =
194.2 (CO), 161.7 [q (1JCB = 49.8 Hz), Ci BArꢀ4], 139.4, [CH PhIm],
135.3 [PhIm], 134.8 [Co BArꢀ4], 130.8 [ CH PhIm], 130.4 [PhIm],
129.7 [CH PhIm], 128.9 [q (2JCF = 31.0 Hz), Cm BArꢀ4], 124.5 [q
(1JCF = 272.0 Hz), CF3 BArꢀ4], 122.0 [PhIm], 121.3 [PhIm], 117.5
[Cp BArꢀ4]; elemental analysis calcd (%) for C62H36BF24N6O3Re: C
47.55, H 2.32, N 5.37; found: C 47.69, H 2.15, N 5.37.
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Acknowledgements
We thank Ministerio de Educacio´n y Ciencia (Grants CTQ2006–
08924 and CTQ2006–07036/BQU), Junta de Castilla y Leo´n
21 (a) M. Nardelli, Comput. Chem., 1983, 7, 95; (b) M. Nardelli, J. Appl.
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886 | Dalton Trans., 2008, 878–886
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The Royal Society of Chemistry 2008
©