pubs.acs.org/OrganometallicsPublished on Web 01/05/2010r 2010 American Chemical Society

Organometallics 2010, 29, 509–511 509

DOI: 10.1021/om9008263

Two New Receptors for Carboxylate Anions with an IR-Active

Reporting Group

Xiaohui Tian,*,†,‡ Li Song,† Yizhong Yuan,†,‡ and Jiaping Lin*,†,‡

†Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science andEngineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237,

China and ‡KeyLaboratory forMaterials Physics of Shanghai, School ofMaterials Science andEngineering,East China University of Science and Technology, Shanghai 200237, China

Received September 23, 2009

Summary: Two new urea derivatives bearing a tricarbonyl-chromium moiety showed high selectivity and sensitivitytoward carboxylate anions in competitive hydrogen bondingsolvents, which can be monitored by the changes of the IRspectrum of the carbonyl groups complexed with the organicmetal.

There is an increasing interest in the design of singlemolecular sensors that selectively recognize specific anionsin complex media.1 These molecular sensors would havepotential applications in various biological and industrialdetections.2 Receptors for carboxylate anions are importantfor the recognition of a variety of biomolecules, especiallyamino acids.3,4 Many neutral molecules with one or moreurea subunits have shown to be excellent carboxylate recep-tors.5 Their recognition is achieved through the formation ofN-H 3 3 3O hydrogen bonds, and the recognition selectivitycan be effectively improved by the introduction of electron-withdrawing substitutions like -NO2.

6 However, owing tothe poor solubility of the urea compounds and the compe-titiveH-bond interactionwith the solvent,7 little progress hasbeen made in the carboxylate anion recognition by the ureasystem in hydrogen bond forming medium. Because of thelack of appropriate reporting groups in some urea receptors,the identification of carboxylate anion recognition is mainlyachieved by NMR spectroscopic methods.8

The Cr(CO)3 moiety is known to be capable of stabilizingboth benzylic cations and anions, and behaves as a netelectron-withdrawing group. In this work, we report twonew complexes [(N,N0-diphenyl-urea)Cr(CO)3] 3 and [(N-methyl-N0-phenyl-urea)Cr(CO)3] 4, inwhich eachurea-boundCr(CO)3-benzene fragment can act as a good hydrogenbond donor and acceptor (Figure 1). As a consequence, the

urea subunit of 3 gives a definite hydrogen-bonding com-plex with two ethanol molecules, whereas the urea subunitof 4 donates and accepts a hydrogen bond to/from anotherone, giving rise to a homodimer. We hoped that thesecompounds would be good candidates for constructingnovel aggregates. Each complex studied is able to interactwith carboxylate anions in ethanol and MeCN solution,giving rise to a FTIR as well as UV-vis signal of the anionpresence.The syntheses of 3 and 4 were carried out by refluxing the

solution of N,N0-diphenyl-urea 1 and N-methyl-N0-phenyl-urea 2 with (NH3)3Cr(CO)3 in dioxane for 5 h, respectively(25% and 31% yield). 1H NMR, elemental analysis, andmass spectra are consistent with the proposed structuresof 3 and 4.These two complexes were observed to exhibit high solu-

bility and stability in solution, for example, each complexforms formed ethanol solutions, showed no signs of de-composition after several weeks in air, at least 1 order ofmagnitude more concentrated in comparison to the corres-ponding free organics, whichwere the least soluble in ethanolsolvent.The X-ray structures of 3 and 4 were determined and

shown in Figure 2 with selected bond lengths. The geometryof N(1) in each crystalline forms is close to a trigonal planarmolecular geometry,9,10 indicating sp2 hybridization. Thelone pair electrons on the nitrogen can interact with the areneπ-system, allowing the withdrawing of electron densityfrom the tricarbonylchromium moiety.11 This view isfurther supported by comparison of their hydrogen bondingdonor-acceptor distances, thus, distances of H(1) 3 3 3O(5)[2.008(18) A for 3 and 2.050(17) A for 4] were significantlyshorter than the distances of H(2) 3 3 3O(5) [2.270(2) A for 3and 2.450(2) A for 4].12 More interestingly, the urea subunitof each complex can act simultaneously as a good hydrogenbond donor and acceptor, This is solely due to the presenceof the Cr(CO)3moiety, with an ability to stabilize negative aswell as positive charges in benzylic positions. As shown inFigure 2, Compound 3 crystallized with two ethanol mole-cules in the unit cell. In this case, the urea subunit, already

*To whom correspondence should be addressed. For X.T.: phone, 86021 64252167; fax, 86 021 54288175; E-mail: tianxh@263.net. For J.L.:jlin@ecust.edu.cn.(1) (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486.

(b) Sessler, J. L.; Camioso, S.; Gale, P. A.Coord. Chem. Rev. 2003, 240, 17.(2) Snowden,T. S.;Anslyn, E.V.Curr.Opin.Chem.Biol. 1999, 3, 740.(3) Brooks, S. J.; Gale, P. A.; Light, M. E. Chem. Commun. 2005, 37,

4696.(4) Yakovenko, A. V.; Boyko, V. I.; Kalchenko, V. I.; Baldini, L.;

Casnati, A.; Sansone, F.; Ungaro, R. J. Org. Chem. 2007, 72, 3223.(5) Fitzmaurice, R. J.; Kyne, G. M.; Douheret, D.; Kilburn, J. D.

J. Chem. Soc., Perkin Trans. 2002, 2, 841.(6) Amendola, V.; Esteban-G�omez, D.; Fabbrizzi, L.; Licchelli, M.

Acc. Chem. Res. 2006, 39, 343.(7) Boiocchi, M.; Boca, L. D.; G�omez, D. E.; Fabbrizzi, L.; Licchelli,

M.; Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507.(8) Gunnlaugsson, T.;Glynn,M.; Tocci (n�eeHussey),G.M.;Kruger,

P. E.; Pfeffer, F. M. Coord. Chem. Rev. 2006, 250, 3094.

(9) Salmain, M.; Vessieres, A.; Jaouen, G.; Butler, I. S. Anal. Chem.1991, 63, 2323.

(10) Shen, K. Y.; Tian, X. H.; Zhong, J. H.; Lin, J. P.; Shen, Y.; Wu,P. Y. Organometallics 2005, 24, 127.

(11) Tian, X.H.; Shi,W.; Shen,K.Y.; Li, C. Y.; Lin, J. P.; Che, Y. C.;Zhang, P. J. Organomet. Chem. 2006, 691, 994.

(12) Che, Y. C.; Tian, X. H.; Chen, H.; Tang, Z. Y.; Lin, J. P.New J.Chem. 2006, 30, 883.

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510 Organometallics, Vol. 29, No. 3, 2010 Tian et al.

involved in the interaction of two NH 3 3 3O bonds, is also aproton acceptor for the OH 3 3 3O interaction that occurswith OH donor group of another ethanol; thus, the two andmore molecules 3 can be linked via ethanol molecules.[Crystal data for 3: C18H18CrN2O5, M = 394.34, mono-clinic, space group P21/c, a = 19.4093(18), b = 6.9993(6),c = 13.6169(12) A, β = 94.866(2)�, V = 1843.2(3) A3,T =293(2) K, Z = 4, 10421 reflections measured, 3998unique (Rint = 0.0756), which were used in all calculations.The finalR1 andwR2were 0.0556 and 0.1423 (all data).] Thiskind of ethanol binding, exerting the effect of solvents on theassembly of host molecules, is in sharp contrast to the case ofpreviously reported urea-hydrogencarbonate complexes, inwhich the two HN-CO-NH groups serve as the onlyproton donors to be linked via two hydrogencarbonate ions.In comparison, 4 assembled to form a homodimer throughcomplementary hydrogen bonding, thus ethanol cannot actas a molecule bridge in this crystal packing. [Crystal datafor 4: C11H10CrN2O4, M = 286.21, orthorhombic, spacegroup P212121, a = 9.2068(9), b = 11.1401(11), c =11.8643(11) A, V=1843.2(3) A3, T = 293(2) K, Z = 4,7203 reflections measured, 2643 unique (Rint = 0.0525),which were used in all calculations. The final R1 and wR2were 0.0354 and 0.0811 (all data).] This property of 4, whichis a potential self-organizing recognition process, is perhaps

the most promising for possible development toward amolecular duplex system through adapted substitution onthe aromatic moiety and/or on urea nitrogen’s, leading to aspecific molecular recognition unit for constructing supra-molecular structures.In this study, we showed that a FTIR method instead of

NMR can easily follow the binding of 3 and 4 with AcO-

anion. As shown in Figures 3 and 4, the two strong bandscorresponding to the symmetric and antisymmetric ν(CO)modes at 1959.8 and 1877.0 cm-1 for 3 and 1956.3 and1873.9 cm-1 for 4 in the solid state shifted to higher wave-numbers when dissolved in ethanol solvent. With the addi-tion of AcO- to the ethanol solution, ν(CO)modes shifted tolower wavenumber and reached at 1956.1 and 1873.9 cm-1

for 3 and 1953.6 and 1872.8 cm-1 for 4. These observationsrevealed that each complex has a stronger capability oftrapping AcO- anion; despite that, the hydrogen bondinginteraction of the two receptors with ethanol still existed inthe present case or not. To verify the hydrogen bonding eventin solution that each receptor selectively bound with AcO-

instead of ethanol under competitive conditions, a controlexperiment was carried out using MeCN to replace ethanolas the solvent. No noticeable wavenumber shifts were ob-served when the solids of the two receptors were dissolved in

Figure 1. The structures of complexes 3 and 4.

Figure 2. ORTEP plot (50% thermal ellipsoids) of the X-raycrystal structures of compound 3-ethanol (a) and dimer 4 (b).Selected bond lengths (A) and angles (deg) are: (a) H(2) 3 3 3O(5),2.270(2); H(1) 3 3 3O(5), 2.008(18); N(1)-C(9), 1.381(3), N(2)-C(11), 1.408(3); N(2)-H(2) 3 3 3O(5), 150(3); N(1)-H(1) 3 3 3O(5),160(2); C(10)-N(1)-C(9), 127.11(19); C(10)-N(2)-C(11),128.10(2). (b) H(1) 3 3 3O(4), 2.050(17); H(2) 3 3 3O(4), 2.450(2);N(1)-C(9), 1.392(3); N(2)-C(11), 1.455(3); N(1)-H(1) 3 3 3O(4), 170(2); N(2)-H(2) 3 3 3O(4), 141(2); C(10)-N(1)-C(9),125.10(19); C(10)-N(2)-C(11), 122.2(2).

Figure 3. FTIR spectra of 3 in ethanol (2.8 mmol 3 dm-3)

containing 0-28 mmol 3 dm-3 of [Bu4N]AcO.

Figure 4. FTIR spectra of 4 in ethanol (2.8 mmol 3 dm-3)

containing 0-28 mmol 3 dm-3 of [Bu4N]AcO.

Communication Organometallics, Vol. 29, No. 3, 2010 511

MeCN. The addition of AcO- anion in this case shifted theseν(CO)modes to 1955.0 and 1971.0 cm-1 for 3 and 1952.1 and1968.9 cm-1 for 4, which remained unchanged upon furtheraddition of ethanol.A ν(CO) titration plot for each receptor with AcO- anion

in ethanol solution is shown in Figure 5. Complex 3 wasclearly saturated at much lower AcO- concentrations than 4

(ca. 6 rather than 14 mmol dm-3). Perhaps a dimerization isoccurring like that observed upon crystallization of 4 and inthe meantime disturbs the interaction of 4with AcO- anion,giving the high order species, e.g., 2:1. This result offers anapproach to optimize the binding properties of a givenreceptor by using different conjugated substitutions to ob-tain a AcO- affinity appropriate for its concentration rangeunder investigation.The binding properties of 3 and 4 with AcO- were further

accessed by UV-vis spectroscopy. There were two strongabsorption bands appeared at 251 and 319 nm for 3 inMeCN. On addition of ethanol from 0 to 2.0 equiv, theintensity of both bands increases. With further addition ofAcO- to theMeCN solution of the mixture of 3 and ethanol,the absorption band at 251 nm disappears and appears atabout 264 nm, red-shifted by 13 nm, meanwhile, there is a

slight red-shift of the band at 319 nm along with decreasein the intensity. In contrast, no noticeable changes wereobserved upon addition of ethanol to the MeCN solutionof the mixture of 3 and AcO-, although the ethanol is rathera hydrogen donor, whereas AcO- anion is an acceptor.From the UV-vis spectroscopic titrations of 3 with AcO-

and ethanol in MeCN solution, the association constants13

of the 3-acetate and 3-ethanol were calculated to be 6.85 �104 and 4.62 � 103 M-1 (errors < 10%), respectively. Theselectivity for AcO- is almost 15-fold higher compared tothat of ethanol. Similar, but less remarkable spectral changeswere also observed for 4. These spectral changes due to theweak bonding interaction were not large enough to calculatethe association constants precisely. The substitution at-tached to the NH subunit in our system affected the bindingselectivity, and thus phenyl substitution at theNHgroup of 3enhanced the AcO- selectivity and sensitivity.In conclusion, we showed that the Cr(CO)3 moiety in two

new urea based receptors for carboxylate acted not only as anIR-active reporting source but also as an electron acceptor toenhance the binding selectivity. Chromium complexationlikely results in spatial arrangements of atoms that increasethe solubility14,15 and in particular enables the urea subunit tobe a good H-bond donor capable of differentiating betweenAcO- and ethanol. This is in sharp contrast to the reportedurea receptors7 that are selective for AcO- anions only in theabsence of competitive H-bond solvents like alcohols. Ourcompounds have the potential to be used in the detection ofcarboxylate anions in various biological processes, especiallyin water, which is currently being investigated.

Acknowledgment. We gratefully acknowledge the sup-port by Shanghai Key Project for Basic Research (no.09JC1404300), Shanghai Leading Academic DisciplineProject (B502), and Shanghai Key Laboratory Project(08DZ2230500).

Supporting Information Available: Characterization of 3 and4 and UV-vis titration experiments profile (PDF, CIF). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Changes in ν(CO)sym and ν(CO)asym for 3 and 4 uponaddition of [Bu4N]AcO in ethanol.

(13) Connors, K. A. Binding Constants: The Measurement of Mole-cular Complex Stability; John Wiley & Sons: New York, 1987.

(14) Whittall, I. R.; McDonagh, A. M.; Humphrey, M. G.; Samoc,M. Adv. Organomet. Chem. 1999, 43, 349.

(15) Gilbert, T. M.; Hadley, F. J.; Simmons, M. D.; Bauer, C. B.;Rogers, R. D. J. Organomet. Chem. 1996, 510, 83.