Ruthenium biimidazole complexes as anion receptors

Article abstract of DOI:10.1039/b510016j

The behavior of the compound [RuCl(cym)(H₂biim)] [BAr′₄] (cym = η⁶-para-isopropylmethylbenzene, Ar′ = 3,5-bis(trifluoromethyl)phenyl), synthesized from [{RuCl(cym)} ₂(μ-Cl)₂], H₂biim and NaBAr′ ₄, has been studied as a receptor of anions both in solution and in the solid state. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510016j

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Ruthenium biimidazole complexes as anion receptors{
Laura Ion,a Dolores Morales,*a Julio Pe´rez,*a Luc´ıa Riera,a V´ıctor Riera,a Richard A. Kowenickib and
Mary McPartlinc
Received (in Cambridge, UK) 14th July 2005, Accepted 13th October 2005
First published as an Advance Article on the web 14th November 2005
DOI: 10.1039/b510016j
with H2biim followed by anion metathesis with NaBF4.6
The behavior of the compound [RuCl(cym)(H2biim)][BAr94]
The [RuCl(cym)(H2biim)]+ cation seemed a good candidate for
our studies because of the ease of preparation, stability towards air
and moisture, and the relative substitutional inertness usually
attributed to [RuCl(g6-arene)(L–L)]+ complexes. In order to
minimize the competition with external anions we preferred the
BAr94 (Ar9 = 3,5-bis(trifluoromethyl)phenyl) counteranion.7 Thus,
the [RuCl(cym)(H2biim)][BAr94] salt (1) was prepared in virtually
quantitative yield by the reaction of [{RuCl(cym)}2(m-Cl)2],
(cym
= = 3,5-bis-
g6-para-isopropylmethylbenzene, Ar9
(trifluoromethyl)phenyl), synthesized from [{RuCl(cym)}2-
(m-Cl)2], H2biim and NaBAr94, has been studied as a receptor
of anions both in solution and in the solid state.
The occurrence of two N–H bonds able to simultaneously bind a
hydrogen bond acceptor in molecules as simple as ureas1 has
prompted the study of their behavior as receptors of anions.2 The
same feature occurs in the syn conformation of 2,29-biimidazole
(H2biim). However, the strong self-association of H2biim mole-
cules results in a low solubility and competes against anion
binding. Thus, we have found that the low solubility of H2biim in
CD3CN makes it NMR-invisible in this solvent, whereas in
DMSO-d6, in which H2biim is sparingly soluble, d (N–H) does not
change appreciably upon addition of, for instance, an equimolar
amount of tetrabutylammonium chloride. This behavior can be
modified in two ways. The first one is the functionalization of
biimidazole with appropriate organic substituents. This strategy
has been recently used by Sessler et al.3a to change the solid state
structural pattern, and by Causey and Allen to study the behavior
towards anions in solution.3b The second way would be to
coordinate H2biim to a metal fragment, which would: (a) enforce
the syn conformation and therefore preorganize H2biim optimally
for anion binding, (b) suppress self-association, (c) enhance the
polarization of the biimidazole N–H bonds, making them better
hydrogen bond donors, and, for cationic metal fragments, (d) add
coulombic attraction to the hydrogen bond interaction.
Biimidazole has been extensively used as a ligand and, in several
instances, X-ray diffraction studies have showed the presence of
hydrogen bonding between its N–H groups and an external
anion.4 Extended structures based on this motif have been
exploited for crystal engineering.3a,4d However, the solution
behavior of H2biim complexes as receptors of anions has never
been quantitatively studied.5 Our first results in this area are the
subject of the present paper.
H2biim8 and NaBAr94 (see Scheme 1), and spectroscopically
9
characterized.10{ A band at 3207 cm21 in the IR spectrum of 1
(KBr pellet) was assigned to n(N–H). In the 1 : 1 adducts obtained,
on addition of tetrabutylammonium chloride or nitrate (see
below), these bands shifted to 3099 and 3113 cm21, respectively,
…
suggesting N–H X interactions. Compound 1 was found to be
very soluble, even in moderately polar dichloromethane, a feature
common to most BAr94 salts.
The two N–H groups of coordinated H2biim appeared as a
singlet at d 11.43 in the 1H NMR of 1 in CD3CN. The addition of
tetrabutylammonium bromide, nitrate, hydrogensulfate, iodide or
perrhenate shifted this signal to higher frequencies. Fast anion
exchange was found in each case (titration curves are shown in
Fig. 1 and in the Electronic Supplementary Information{), and
binding constants were determined using the WinEQNMR
program (see Table 1).11
The N–H 1H NMR signal broadened and finally vanished when
less than an equimolar amount of Bu4NCl was added to the
2
…
CD3CN solution of 1, indicating a strong 1 Cl interaction in
this solvent. The same was found for the more basic fluoride,
dihydrogenphosphate and cyanide anions. In the more competitive
DMSO-d6 solvent, the N–H 1H NMR signal remained visible
when 1 was titrated with Cl2, but not with F2, H2PO42 or CN2.
Carmona et al. reported the high yield preparation of the
compound [RuCl(cym)(H2biim)][BF4] (cym = g6-para-isopropyl-
methylbenzene) by means of the reaction of [{RuCl(cym)}2(m-Cl)2]
aDepartamento de Qu´ımica Orga´nica e Inorga´nica-IUQOEM, Facultad
de Qu´ımica-CSIC, Universidad de Oviedo, Oviedo, 33006, Spain.
E-mail: mdms@fq.uniovi.es; japm@fq.uniovi.es; Fax: (+34) 98510 3446;
Tel: (+34) 98510 3465
bChemistry Department, University of Cambridge, Lensfield Road,
Cambridge, UK CB2 1EW
cDepartment of Health and Human Sciences, London Metropolitan
University, Holloway Road, London, UK N7 8DB
{ Electronic Supplementary Information (ESI) available: General X-ray
information and 1H NMR titration plots of 1 in DMSO-d6 and CD3CN.
See DOI: 10.1039/b510016j
Scheme 1 Synthesis of [RuCl(cym)(H2biim)][BAr94] (1).
Chem. Commun., 2006, 91–93 | 91
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Fig. 1 1H NMR titration plot of receptor [RuCl(cym)(H2biim)][BAr94]
(1) in CD3CN with Br2.
a
Table 1 Binding constants values for 1 in CD3CN and DMSO-d6
Anion
Br2
K in CD3CN/M21
K in DMSO-d6/M21
4527 (¡841)
4828 (¡587)
5920 (¡370)
1114 (¡207)
145 (¡28)
—
579 (¡92)
451 (¡20)
651 (¡88)
—
—
970.3 (¡21)
2
NO3
2
HSO4
I2
2
ReO4
Cl2
a
Errors are given in parentheses
To obtain values of the binding constants, for a comparison to be
made between the different anions, the titrations of 1 with Br2,
Fig. 2 (a) View of the crystalline structure of the adduct
2 2
NO3 and HSO4 were repeated in DMSO-d6, the results of
[RuCl(cym)(H2biim)][Cl]. (b) View of the crystalline structure of the
[RuCl(cym)(H2biim)][NO3] adduct.
which are showed in Table 1. In this solvent, the interaction
between 1 and the anions I2 and ReO4 was found to be too
2
inorganic anions. The lack of selectivity can, at least in part, be
attributed to ion pairing. In this context, the behavior of neutral
metal biimidazole complexes is currently being investigated in our
lab, and will be reported in a future publication.
weak, resulting in no observable change in the chemical shift of the
N–H signals of 1.
The supramolecular adducts [RuCl(cym)(H2biim)][Cl] and
[RuCl(cym)(H2biim)][NO3] were structurally characterized by
means of single crystal X-ray diffraction, and graphical representa-
tions of the results are displayed in Fig. 2.§ Interestingly, these
adducts crystallized separately from [Bu4N][BAr94], which
remained in solution." In the structure of the chloride adduct
there are two molecules per asymmetric unit. In one of them
(shown in Fig. 2a), the chloride anion interacts with the two N–H
groups of only one of the Ru complexes. In the second, the
chloride anion is disordered and is located over two sites in a 1 : 1
ratio. Each of these 50% occupancy chlorides interacts with the
two N–H groups from one metal complex and with one N–H
group from a neighbouring metal complex.
We thank Ministerio de Ciencia y Tecnolog´ıa (grant BQU2003-
08649) and European Union (ERG to L. R.) for their support
of this work. L. I. thanks Ministerio de Educacio´n y Ciencia for
a FPU pre-doctoral fellowship. D. M. and L. R. thank
Ministerio de Educacio´n
y Ciencia for Ramo´n y Cajal
fellowships. R. A. K. would like to acknowledge the States of
Guernsey and the Cambridge University Domestic and
Millennium funds.
Notes and references
{ Synthesis of [RuCl(cym)(H2biim)][BAr94] (1): To a solution of
[{RuCl(cym)}2(m-Cl)2] (0.100 g, 0.163 mmol) in CH2Cl2 (20 mL) and
MeCN (0.25 mL), NaBAr94 (0.290 g, 0.326 mmol) and H2-biim (0.048 g,
0.359 mmol) were added. After stirring for 24 h, filtration and evaporation
of the filtrate gave an orange solid that was washed with hexane (2 6
20 mL) and dried in vacuo. Yield: 0.380 g, 87%. 1H NMR (CD3CN): d
11.43 (s br, 2 H, NH of H2biim), 7.69 (m, 14 H, BAr94 and H2biim), 7.38
In the structure of the nitrate adduct, shown in Fig. 2b, each
N–H group interacts with one nitrate oxygen, there being no
additional contacts. Strong hydrogen bond interactions are
…
…
indicated by the N Cl (3.097(9) and 3.107(8) s) and N
O
(2.719(6) and 2.867(6) s) distances.
(d, 3JHH = 1.56 Hz, 2 H, H2biim), 5.86, 5.65 (AA9BB9 system, JAB = JA9B9
=
The results shown in Table 1 indicate that 1 is a non-selective
receptor. It is attractive to speculate that the complementarity in
shape between host and guest found for [RuCl(cym)-
(H2biim)][NO3] could partly compensate for the higher basicity
of chloride and bromide with respect to nitrate, resulting in
binding constants of the same order.
6.2 Hz, 4 H, cym), 2.71 (m, 1 H, CH of iPr), 2.16 (s, 3 H, CH3 ) and 1.11 (d,
4JHH = 7.2 Hz, 6H, CH3 of Pr). 13C {1H}NMR (CD3CN): d 161.6 (q,
i
1JCB = 49.9 Hz, Ci of BAr94), 138.2 (s, H2biim), 134.7 (s, Co of BAr94),
131.0 (s, H2biim), 129.0 (q, 2JCF = 31.7 Hz, Cm of BAr94), 124.5 (q, 1JCF
=
271.2 Hz, CF3 of BAr94), 121.0 (s, C-iPr, cym), 120.3 (s, H2biim), 117.7 (s,
Cp of BAr94), 103.2 (s, C-Me, cym), 83.1 (s, CA of cym), 81.5 (s, CB of cym),
30.9 (s, CH of iPr), 21.2 (s, CH3 of iPr) and 18.0 (s, CH3 of cym). Anal. calc.
for C48H32BClF24RuN4: C, 45.46; H, 2.54; N, 4.42. Found: C, 45.81; H,
2.39; N, 4.12%.
To conclude, we have investigated for the first time the solution
behavior of metal biimidazole complexes as anion receptors.
The compound [RuCl(cym)(H2biim)][BAr94] has been found to
establish strong, non-selective interactions with several simple
§ Crystal data for adduct [RuCl(cym)(H2biim)][Cl] (dichloromethane and
water co-crystallized, see ESI{): C16.50H23Cl3N4ORu, crystal dimensions
92 | Chem. Commun., 2006, 91–93
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0.23 6 0.18 6 0.10 mm, triclinic, space group P-1, a = 11.094(2), b =
12.523(3), c = 17.560(4) s, a = 104.49(3), b = 92.84(3), c = 110.07(3)u,
V = 2193.9(8) s3, Z = 4, T = 293(2) K, Dc = 1.516 g cm23, Mo-Ka
radiation (l = 0.71073 s), 13548 reflections collected, 4506 independent
reflections (3.61 ¡ h ¡ 21.00u), R1 = 0.0610, wR2 = 0.1430, GOF on F2 =
1.107, CCDC 279201. Crystal data for adduct [RuCl(cym)(H2biim)][NO3]:
C16H20ClN5O3Ru, crystal dimensions 0.21 6 0.12 6 0.05 mm,
orthorhombic, space group Pbca, a = 16.048(3), b = 13.798(3), c =
3 (a) W. E. Allen, C. J. Fowler, V. M. Lynch and J. L. Sessler, Chem.–
Eur. J., 2001, 7, 721–729; (b) C. P. Causey and W. E. Allen, J. Org.
Chem., 2002, 67, 5963–5968.
4 Some recent examples: (a) M. G. B. Drew, V. Fe´lix, I. S. Gonc¸alves,
F. E. Ku¨hn, A. D. Lopes and C. C. Roma˜o, Polyhedron, 1998, 17,
1091–1102; (b) S. Fortin and A. L. Beauchamp, Inorg. Chem., 2001, 40,
105–112; (c) S. Fortin, P. Fabre, M. Dartiguenave and A. L. Beauchamp,
J. Chem. Soc., Dalton Trans., 2001, 3520–3527; (d) R. Atencio,
M. Chaco´n, T. Gonza´lez, A. Bricen˜o, G. Agrifoglio and A. Sierraalta,
Dalton Trans., 2004, 505–513; (e) R. Atencio, K. Ram´ırez, J. A. Reyes,
T. Gonza´lez and P. Silva, Inorg. Chim. Acta, 2005, 358, 520–526.
5 Beauchamp and co-workers have recently reported that hydrogen bond
association, structurally characterized in the solid state, persists in
CH2Cl2 solution for Re(III) H2biim complexes and chloride and
carboxylate anions, see ref. 4b,c.
16.470(4) s V = 3646.9(13) s3, Z = 8, T = 180(2) K, Dc = 1.701 g cm23
,
Mo-Ka radiation (l = 0.71073 s), 23164 reflections collected, 4170
independent reflections (3.55 ¡ h ¡ 27.48u), R1 = 0.0503, wR2 = 0.1040,
GOF on F2 = 1.014, CCDC 279202. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b510016j
" Typically, the four ions are present in the crystals obtained from mixtures
of a cationic receptor (added as its salt with a given counteranion) and
tetrabutylammonium salt of the target anionic guest.
6 D. Carmona, J. Ferrer, A. Mendoza, F. J. Lahoz and L. A. Oro,
Organometallics, 1995, 14, 2066–2080.
7 S. Nieto, J. Pe´rez, V. Riera, D. Miguel and C. Alvarez, Chem. Commun.,
2005, 546–548.
8 E. E. Bernarducci, P. K. Bharadwaj, R. A. Lalancette, K. Krogh-
Jespersen, J. A. Potenza and H. J. Schugar, Inorg. Chem., 1983, 22,
3911–3920.
9 M. Brookhart, B. Grant and A. F. Volpe, Jr., Organometallics, 1992, 11,
3920–3922.
10 We have recently prepared the analogous phenanthroline compound
[RuCl(cym)(phen)][BAr94] using the same procedure: C. Mene´ndez,
D. Morales, J. Pe´rez, V. Riera and D. Miguel, Organometallics, 2001, 20,
2775–2781.
1 (a) P. J. Smith, M. V. Reddington and C. S. Wilcox, Tetrahedron Lett.,
1992, 41, 6085–6088; (b) T. R. Kelley and M. H. Kim, J. Am.
Chem. Soc., 1994, 116, 7072–7080; (c) J. Scheele, P. Timmerman and
D. N. Rheinhoudt, Chem. Commun., 1998, 2613–2614.
2 For general overviews of the supramolecular chemistry of anions, see:
(a) Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-
James and E. Garc´ıa-Espan˜a, Wiley, New York, 1997; (b)
F. P. Schmidtchen and M. Berger, Chem. Rev., 1997, 97, 1609–1646;
(c) Coord. Chem. Rev., 2003, 240, ed. P. A. Gale (this volume is specially
dedicated to this area); (d) For metal-containing receptors of anions,
see: P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40,
486–516.
11 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.
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