Experimental Observation of Supramolecular ... - ACS Publications

Experimental Observation of Supramolecular Carbonyl-π/π-π/ π-carbonyl and Carbonyl-π/π-π/π-anion Assemblies Supported by Theoretical Studies Somnath Ray Choudhury,† Patrick Gamez,*,‡ Arturo Robertazzi,§ Chih-Yuan Chen,| Hon Man Lee,| and Subrata Mukhopadhyay*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3773–3784

Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India, Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, CNR-INFM SLACS and Dipartimento di Fisica, UniVersita` di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Italy, and Department of Chemistry, National Changhua UniVersity of Education, Changhua, Taiwan 50058 ReceiVed April 19, 2008; ReVised Manuscript ReceiVed June 15, 2008

ABSTRACT: Mononuclear nickel(II) and copper(II) complexes, namely, (C6H9N2)2[Ni(ntaH)2] (1), (C5H7N2)2[Cu(mal)2(H2O)2] (2), and (C5H7N2)4[Ni(mal)2(H2O)2](NO3)2 (3) [ntaH3 ) nitrilotriacetic acid; C6H8N2H ) protonated 2-amino-4-picoline; malH2 ) malonic acid; C5H7N2 ) protonated 2-aminopyridine] have been synthesized in water and their crystal structures have been determined by single crystal X-ray diffraction. In all the complexes, robust bimolecular cyclic hydrogen bonding R22(8) motifs are observed between the protonated heteroaromatic N-rings and the metal-carboxylate complexes. Moreover, the aromatic molecules are engaged in carbonyl · · · π and anion · · · π interactions with the noncoordinated carbonyl moieties of the metal complexes and with a nitrate ion respectively, giving rise to remarkable carbonyl · · · π/π · · · π/carbonyl · · · π and carbonyl · · · π/π · · · π/π · · · anion interactions having sandwich type topologies. In 1 and 3, the carbonyl · · · π interactions are one of the weak forces responsible for the stabilization of the final assemblies, whereas in 2, it strongly contributes to the stabilization of the one-dimensional tape generated from monomeric [Cu(mal)2(H2O)2]2- units. Density functional theory studies reveal a high stability of the unique lone pair · · · π/π · · · π/lone pair · · · π supramolecular self-assembly observed in compound 2 and confirm the favorable formation of the lone pair · · · π/π · · · π/π · · · anion array present in compound 3. Compounds 1-3 are compared in terms of synthetic aspects and supramolecular interactions with analogous complexes, that is, {[Cu(mal)2](picH)2 · 5H2O}n (4), {[Cu(mal)2](picH)2 · 2H2O}n (5), (picH)2[M(mal)2(H2O)2] · 4H2O (M ) Ni/Co/Mn) (6-8), recently obtained with picoline (picH ) protonated picoline). The self-assembly pathways involved in the recognition of the heterocyclic amines by the metal-carboxylate complexes and the occurrence of lone pair (l.p.) · · · π and anion · · · π interactions are examined and related. Introduction Self-assembly is the fundamental molecular recognition process adopted by nature to generate the elegant and intricate molecular machinery from which life is built. Appropriate complementarities between the substrate and the ligand are the necessary prerequisite for molecular recognition. Various weak dispersive interactions, such as hydrogen bonds, π-π stacking, hydrophobic, charge-transfer, electrostatic as well as metal ion coordination, represent the backbone of self-assembly processes and supramolecular architectures.1-3 Hydrogen bonding still remains the most reliable and widely used means of enforcing molecular recognition.4 The self-assembly of molecular building blocks through molecular recognition has led the way in the development of a number of functional all-organic and hybrid inorganic-organic materials.5 The supramolecular interactions with π aromatic clouds, such as C-H · · · π, cation-π, π-π stacking, have been demonstrated extensively both experimentally and theoretically.6 In recent years, crystallographic, as well as theoretical evidence for “anion-π interactions”,7,8 which was primarily thought to be improbable due to the electron-donating character of anions and the expected repulsive interactions with aromatic π-systems, are increasingly reported. Anion-π interactions start being recog* To whom correspondence should be addressed. E-mail: smukhopadhyay@ chemistry.jdvu.ac.in (S.M.); [email protected] (P.G.). † Jadavpur University. ‡ Leiden University. § Universita` di Cagliari. | National Changhua University of Education.

nized by the scientific community as an important type of supramolecular interaction as are cation-π and π-π contacts.8a Likewise, experimental proofs for carbonyl-π interactions, and more generally for lone pair (l.p.)-π interactions, are scarce in the literature.6c,9 Searches of the Cambridge Structural Database (CSD) revealed the occurrence of carbonyl-π interactions in some crystal structures of proteins,6c,10 as well as in some other organic crystals and between aromatic analytes and polyacrylate derivatives supported on silica.11 In a comprehensive review, Egli et al. have pointed out the different possible orientations of a carbonyl group over the interacting π-face of aromatic rings.6c The carbonyl (a) may be stacked onto the plane of the ring, (b) may form an angle 0° < R < 90° with the ring plane, or (c) may be perpendicular to the ring plane.6c In relation to anion · · · π interactions, carbonyl · · · π interactions are expected to be energetically favorable in the case of electron-deficient aromatic rings and destabilizing for electron-rich rings.9a Ruiz-Pe´rez et al. has remarkably exploited the different coordination modes of the malonate dianion with transition metal ions. It has been shown that extended magnetic networks of diverse dimensionalities (1D, 2D, and 3D) can be chemically constructed from malonato-bridged polymetallic complexes. These coordination polymers behave as ferro-, ferri-, or canted antiferro-magnets.12 In previous studies with first-row transition metal malonate complexes in combination with 2-amino-4picoline, a common feature has been noticed in all compounds so far synthesized. Actually, a preferred recognition pattern between the malonate and the picoline group is observed, that is, the formation of a R22(8) hydrogen bonding synthon.13 It is

10.1021/cg800403p CCC: $40.75  2008 American Chemical Society Published on Web 08/14/2008

3774 Crystal Growth & Design, Vol. 8, No. 10, 2008

also noteworthy that this recognition is enforced in situ through the protonation of the aminopicoline group. This molecular recognition phenomenon between aminopicoline derivatives and metal-malonate complexes has been further studied by subtle alterations of both the metallic unit and the auxiliary ligand. These slight modifications are aimed at observing whether the same type of hydrogen bonding association is still achieved or not. Therefore, nitrilotriacetic acid was used in place of malonic acid to prepare the nickel(II) complex 1, and 2-aminopyridine was used instead of picoline for the preparation of the copper(II) complex 2 and the nickel(II) complex 3. Interestingly, in all the complexes, no changes are observed in the formation of the supramolecular pairing of the two synthons. The same R22(8) cyclic motif is found in both solid-state structures, which involves several N-H · · · O bonds. Moreover, the occurrence of supramolecular associations like “carbonyl · · · π/π · · · π/ carbonyl · · · π” (in 1 and 2) and “carbonyl · · · π/π · · · π/π · · · anion” (in 3) are the outstanding features observed in the three compounds. These remarkable molecular interactions between four supramolecular synthons have been investigated by theoretical calculations (complexes 1-3), which have shown that the associations are energetically favorable. Experimental Section Physical Measurements. IR spectra were recorded on a PerkinElmer RXI FT-IR spectrophotometer with the sample prepared as a KBr pellet in the range 4000-600 cm-1. Elemental analyses (C, H, N) were performed on a Perkin-Elmer 240C elemental analyzer. The TGA of complexes 2 and 3 were obtained using a Mettler Toledo TGA/ SDTA 851e system. Materials. All reactions were carried out in aerobic condition and in water as the solvent. Nitrilotriacetic acid (Lancaster), malonic acid (Aldrich), copper(II) acetate monohydrate (Lancaster), nickel(II) carbonate (Lancaster), nickel(II) nitrate hexahydrate (Lancaster), 2-amino4-picoline (Aldrich) and 2-aminopyridine (Aldrich) were used as received. Freshly boiled, doubly distilled water was used throughout the present investigation. Synthesis of (C6H9N2)2[Ni(ntaH)2] (1). Basic nickel(II) carbonate NiCO3 · 2Ni(OH)2 · 4H2O (0.376 g, 1 mmol) was suspended in 35 mL of water. This suspension was allowed to react with nitrilotriacetic acid (1.146 g, 6 mmol) at 80 °C to give a clear light blue solution. A hot (60 °C) aqueous solution (20 mL) of 2-amino-4-picoline (0.648 g, 6 mmol) was added dropwise to the above solution with continuous stirring. The reaction mixture thus obtained was further heated at 60 °C for an hour with continuous stirring. The solution was then cooled to room temperature and left unperturbed for the slow evaporation of the solvent. Block-shaped, pale blue, single crystals suitable for X-ray analysis were obtained after several weeks from the mother liquor. The crystals were filtered off, washed with cold water, and dried in air (yield: 32%). Anal. Calcd. for C24H32N6O12Ni: C, 43.99; H, 4.92; N, 12.82%. Found: C, 43.69; H, 4.55; N, 12.44%. Main IR absorption bands observed for 1 (KBr pellet/cm-1) are 3111 (b), 2929 (w), 1724 (s), 1686 (s), 1641 (m), 1577 (b), 1425 (m), 1389 (m), 1329 (s), 762 (vs), 727 (vs). Synthesis of (C5H7N2)2[Cu(C3H2O4)2(H2O)2] (2). Copper(II) acetate monohydrate (0.199 g, 1 mmol) was dissolved in 20 mL of water. This solution was allowed to react with malonic acid (0.208 g, 2 mmol) dissolved in 10 mL of water, giving a clear blue solution. A warm aqueous solution (10 mL) of 2-amino-pyridine (0.188 g, 2 mmol) was added dropwise to the above blue solution with continuous stirring. The resulting reaction mixture was subsequently warmed to 60 °C and stirred for an hour at this temperature. The solution was then cooled to room temperature (the pH of this solution was 4.2) and left unperturbed for the slow evaporation of the solvent. Blue single crystal plates suitable for X-ray analysis were obtained after several weeks from the mother liquor which were isolated by filtration. The crystalline material was washed with cold water and dried in air. (Yield: 50%). Anal. Calcd. for C16H22N4O10Cu: C 38.91, H, 4.49, N, 11.34%; found: C, 38.22; H, 3.89; N, 10.91%. Main IR absorption bands observed for 2 (KBr pellet/ cm-1) are: 3367 (b), 3172 (w), 2921 (w), 1672 (s), 1639 (m), 1596

Choudhury et al. (vs), 1485 (s), 1429 (vs), 1386 (w), 1361 (s), 765 (vs), 744 (vs). Compound 2 was analyzed by thermogravimetry (see Supporting Information). Synthesis of (C5H7N2)4[Ni(C3H2O4)2(H2O)2](NO3)2 (3). Nickel(II) nitrate hexahydrate (0.291 g, 1 mmol) dissolved in 25 mL of water was allowed to react with malonic acid (0.208 g, 2 mmol) in water (25 mL) at 60 °C resulting in a clear light green solution. A warm aqueous solution (20 mL) of 2-amino-pyridine (0.376 g, 4 mmol) was added dropwise to the above green solution with continuous stirring. The pH of this solution was adjusted to 5.5 by the addition of diluted NaOH solution. The reaction mixture thus obtained was further heated at 60 °C for an hour with continuous stirring. The resulting solution was then cooled down to room temperature and kept unperturbed for the slow evaporation of the solvent. After a few weeks, flat, pale green, single crystals suitable for X-ray analysis were obtained. The crystals were collected by filtration, washed with cold water, and dried in air (Yield: 42%). Anal. Calcd. for C26H36N10O16Ni: C, 38.87; H, 4.52; N, 17.43%. Found: C, 37.95; H, 4.11; N, 16.72%. Main IR absorption bands observed for 3 (KBr pellet/cm-1) are: 3341 (b), 3158 (w), 2905 (w), 1681 (s), 1636 (m), 1598 (vs), 1482 (s), 1429 (vs), 1356 (m), 767 (s), 739 (s). Compound 3 was analyzed by thermogravimetry (see Supporting Information). X-ray Crystal Structure Determination of 1-3. A crystal with dimension 0.10 × 0.19 × 0.51 mm3 for 1 was used for data collection on an Bruker SMART APEX II diffractometer equipped with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 150(2) K. A total of 7604 reflections were measured to give 3309 unique reflections (Rint ) 0.021) for 1. 2828 data [I > 2σ(I)] were used for solution and refinement by full-matrix least-squares on F2 with the SHELX-9714 package. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on their parent atoms. The final R and Rw values are 0.0316 and 0.0658, respectively. A single crystal of 2 with dimension 0.26 × 0.49 × 0.50 mm3 was mounted on a Bruker SMART APEX II diffractometer equipped with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 150(2) K. The data collection and refinement were processed as described for 1. It has to be mentioned here that crystal data for 2 were initially collected on a Bruker SMART 1K CCD area-detector diffractometer equipped with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 298(2) K (instead of 150(2) K). A preliminary report of its structure was recently published in Acta Crystallogr. 2007, E63, m1331, where 2 was prepared by mixing stoichiometric amounts of the reagents, followed by the adjustment of the pH of the resulting aqueous solution to 5.2 through addition of NaOH. In the present study, however, the crystals of 2 were collected from an aqueous solution (whose pH value was 4.2), obtained from the simple mixing of the reagents without the addition of external NaOH. It thus appears that the slight modification of the synthetic procedure (vide infra) did not result in the formation of a different coordination compound. A single crystal of 3 with dimension 0.11 × 0.19 × 0.43 mm3 was mounted on a Bruker SMART APEX II diffractometer equipped with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 150(2) K. The data collection and refinement were processed as described above. Information concerning crystallographic data collection and refinement for all the complexes are listed in Table 1. Computational Details. The use of the most popular density functional theory (DFT) functionals is not appropriate for systems dominated by dispersive forces similar to those investigated here.15-20 Unfortunately, high-level ab initio methods (such as MP2 and CCSD),21,22 which correctly account for dispersion interactions, become rapidly prohibitive with the size of the system. A valuable alternative is constituted by the hybrid BHandH DFT functional,23,24 that has recently been shown to perform remarkably well in describing dispersion interactions with a similar accuracy to that of CCSD calculations, but at an affordable CPU cost.15 In the last two years, the BHandH functional has been successfully applied to a variety of systems, showing significant results.21,25-28 In this work, all calculations were carried out using the Gaussian03 suite of programs.29 The Becke’s “half-and-half” functional, BHandH,23 is an ad hoc mixture of exact (HF) and local density approximation exchange, coupled with Lee, Yang, and Parr’s expression24 for the correlation energy. BHandH was used in combination with Pople’s30 basis sets 6-31+G(d) to estimate interaction energies (∆E, calculated

Carbonyl-π/π-π/π-carbonyl and -anion Assemblies

Crystal Growth & Design, Vol. 8, No. 10, 2008 3775

Table 1. Crystallographic Data for Compounds 1-3 compound

1

2

3

formula M crystal system space group a/ Å b/ Å c/ Å R (°) β (°) γ (°) F(000) V/ Å3 Z T/K theta min-max [°] λ(Mo KR)/ Å µ(Mo KR)/ mm-1 R1, I > 2σ(I) (all) wR2, I > 2σ(I) (all) S(GOF) total reflections independent reflections observed data [I > 2σ(I)] min. and max. resd. density [e/ Å3]

C24H32N6O12Ni 655.27 triclinic P1j (No. 2) 8.6389(7) 8.7583(8) 9.8337(9) 85.482(2) 68.146(2) 88.461(2) 342 688.42(11) 1 150(2) 2.2 to 28.1 0.71073 0.781 0.0316 0.0658 1.42 7604 3309

C16H22N4O10Cu 493.93 triclinic P1j (No. 2) 6.9946(7) 7.8872(9) 9.5558(10) 96.301(2) 108.469(2) 104.092(2) 255 474.86(9) 1 150(2) 2.7 to 28.6 0.71073 1.217 0.0215 0.0620 1.03 5100 2391

C26H36N10O16Ni 803.34 triclinic P1j (No. 2) 7.1107(2) 10.8105(2) 11.5483(3) 89.331(2) 86.593(2) 76.129(2) 418 860.31(4) 1 150(2) 1.8 to 28.6 0.71073 0.652 0.0333 0.0865 1.07 10779 4377

2828

2333

3617

-0.55, 0.56

-0.35, 0.34

-0.33, 0.39

via supermolecule approach with no correction for the Basis Set Superposition Error) of supramolecular assemblies which were built from crystal structures determined by X-ray diffraction (see Results and Discussion). As in previous studies,31 the Atoms-in-Molecules (AIM)32 theory was performed directly on experimental structures via single point calculations to estimate the intermolecular interactions. AIM is based upon those critical points where the gradient of the density, ∇F, vanishes. In particular, two bonded atoms are connected with a bond path through the bond critical point (see Results and Discussion). Importantly, the electron density at the bond critical points roughly correlates with the strength of chemical bonds and interactions.15,32-35 Calculations on complex 1, complex 3, and assembly B were performed in order to test the effect of the optimization on the interaction energies (Supporting Information). In particular, the interaction energies calculated via single point on the X-ray structures and those obtained from optimization show a similar qualitative trend (Table S1, Supporting Information).

Figure 1. ORTEP diagram of 1 with atom numbering scheme. The thermal ellipsoids are drawn at the 50% probability level. Unlabeled half-structural domain is generated by the symmetry operation of an inversion center (1 - x, -y, -z). Table 2. Theoretical Data for the Three Different Self-Assemblies Investigated to Analyze the Formation of the Supramolecular Carbonyl · · · π/π · · · π/carbonyl · · · π Array Observed in 2 (see Results and Discussion) supramolecular assemblies ∆E (kcal mol-1) Flone pair · · · π (au) Fπ · · · π (au) 0.0133 0.0210

0.106

Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1 Ni(1)-O(3) Ni(1)-O(5) O(3)-Ni(1)-O5 O(3)-Ni(1)-N1 O(3)-Ni(1)-O5a

Results and Discussion Crystal Structure Description of 1. The structure of 1 is solved in the triclinic space group P1j with the asymmetric unit consisting of one-half of the molecular anion [Ni(ntaH)2]2- and one C6H8N2H+ cation. The full anion is generated by the symmetry operation of an inversion center. An ORTEP diagram of 1 is shown in Figure 1. Selected bond lengths, angles, and supramolecular interactions are listed in Tables 3 and 4. The nickel(II) atom, located on an inversion center, is in an octahedral coordination environment. The basal plane around the metal ion is constituted of the oxygen atoms O3 and O5 from one nta unit and their symmetry related counterparts O3**, O5** (** ) 1 - x, -y, -z), belonging to another nta unit. The nitrogen atoms N1 and its symmetry related counterpart N1** (** ) 1 - x, -y, -z) occupy the trans axial positions forming a NiO4N2 chromophore. Nitrilotriacetic acid (nta) acts as a dianionic tridentate ligand with one remaining carboxylic acid group and binds to nickel(II) in a facial fashion. The atoms constituting the equatorial plane are coplanar, therefore indicating almost perfect octahedral coordination geometry around the metal. The Ni1-O3 and Ni1-O5 bond lengths are 2.0309(13) and 2.0472(13) Å, respectively. The Ni-N1 bond lengths is

+60.7 -115.0 -214.0

assembly A assembly B assembly C

a

2.0309(13) 2.0472(13) 89.68(5) 82.69(6) 90.32(5)

Ni(1)-N(1)

2.1223(15)

O3-Ni(1)-N1a O5-Ni(1)-N1 O5-Ni(1)-N1a

97.31(6) 81.04(5) 98.96(5)

Symmetry code: 1 - x, -y, -z. Table 4. Relevant H-Bonds in Compound 1

D-H · · · A O1-H1 · · · O6 N3-H3 · · · O4b N4-H4A · · · O6c N4-H4B · · · O3b C1-H1A · · · O5d C10-H10 · · · O1e C11-H11 · · · O2f a

D-H [Å]

H · · · A [Å]

D · · · A [Å]

D-H · · · A [°]

0.84 0.86 0.89 0.87 0.99 0.96 0.95

1.80 1.92 1.93 1.95 2.48 2.30 2.52

2.5788(18) 2.788(2) 2.811(2) 2.813(2) 3.067(2) 3.165(2) 3.172(2)

154 176 171 170 117 150 126

a Symmetry code ) x, 1 + y, z. b Symmetry code ) -x, -y, 1 - z. Symmetry code ) 1 - x, -y, -z. d Symmetry code ) 1 - x, -y, -z. e Symmetry code ) 1 - x, 1 - y, -z. f Symmetry code ) -1 + x, y, z. c

2.1223(15) Å, which is somewhat longer than the equatorial bonds. The O3-Ni1-O5 bite angle is 89.68(5)°, which is very close to the ideal value of 90°. The N1-Ni1-O3 and N1-Ni1-O5 bite angles of 82.69(5)° and 81.04(5)°, respectively suggest that N1 is slightly tilted toward O3 and O5. The Ni-O as well as the Ni-N bond distances lie in the normal range for such octahedral nickel(II) complexes.36


Choudhury et al.

Figure 2. Formation of a 1D chain along the b axis. Monomeric [Ni(ntaH)2]2- units link one another by strong self-complementary O1-H1 · · · O6 hydrogen bonds leading to the formation of R22(18) hydrogen bonding arrangement. Each monomeric [Ni(ntaH)2]2- unit also recognizes two picoline ligands through N4-H4B · · · O3 and N3-H3 · · · O4 hydrogen bonds generating a R22(8) cyclic motif.

Adjacent monomeric anionic [Ni(ntaH)2]2- units are connected to one another by strong self-complementary O1-H1 · · · O6 [2.5788(18) Å] hydrogen bonds, giving rise to the formation of a R22(18) hydrogen bonding motif (Figure 2). This molecular association is generated through carboxyl-carboxylate interactions, which have been previously described in the literature.37 This self-assembly propagates along the b axis, generating an infinite 1D chain. Each [Ni(ntaH)2]2- unit is also involved in hydrogen bonding interactions with two aminopicolinium cations (C6H8N2H+) through the formation of R22(8) motifs (Figure 2). This recognition pattern between picoline and a carboxylate end is found to be comparable to those observed with other Cu(II) systems involving picoline and malonic acid.13a,b The slight variations of the N-H · · · O bond distances may be attributed to the distinct crystal packing environments of the different complexes. The same type of hydrogen bonding synthon is observed in complexes bearing 2-amino-4,6-dimethylpyrimidine ligands as well.37 In 1, the R22(18) synthons are generated via strong N4-H4A · · · O6 (2.811 Å) hydrogen bonds producing a 2D sheet, where the protonated picoline plays an important role (Figure 3). Such a crucial role of the picoline ligand has never been observed in previous related complexes.13 The formation of a 3D supramolecular network is ensured by three additional weak interactions. Primarily, the 2D sheets are interlinked by C10-H10 · · · O1 [3.165(2) Å] and C(11)-H(11) · · · O(2) [3.172(2) Å] hydrogen bonding interactions. These contacts are further supplemented by strong face-to-face π-π stacks between the pyridyl rings [R(1) ) N(3)/C(7)/C(8)/C(9)/C(10)/C(11), centroid · · · centroid distance of 3.4935(11) Å, symmetry code: -x, 1 - y, 1 - z, dihedral angle ) 0°, the angle between centroid-centroid joining line and the normal from the first ring center to the second ring plane is 11.55°, slippage ) 0.699 Å] of 2-amino-4-picoline ligands which protrude out of the adjacent 2D sheets (Figure 4). In addition, an unusual contact between a carboxylate CdO group of the ntaH ligand and the π system of an aminopicoline molecule is observed, which is responsible as well for the strengthening of the 3D assembly along the c axis (Figure 4). Indeed, the uncoordinated carboxylate oxygen atom O4 (which is also involved in a strong H bond; see Table 4) is in contact

Figure 3. Aminopicoline-assisted formation of a 2D sheet through N4-H4A · · · O6 hydrogen bonds.

with the pyridyl ring of 2-amino-4-picoline [C(4)-O(4) · · · Cg(5) ) 3.4746(15) Å, Cg(5) is the centroid of the ring defined by the atoms N(3)/C(7)/C(8)/C(9)/C(10)/C(11), the angle C(4)O(4)-Cg(5) is 84.91(10)°]. The shortest separation distances reflecting this interaction are O4 · · · N3 ) 3.177(2) Å and O4 · · · C7 ) 3.256(2) Å, and are therefore below the sum of the corresponding van der Waals radii. The occurrence of these CdO (l.p.)/pyridine contacts produces a unique sandwich-type carbonyl(l.p.) · · · π/π · · · π/carbonyl(l.p.) · · · π topology. A recent analysis of the CSD for lone pair · · · π interactions by Egli et al.6c showed that a significant carbonyl (π)-π (aromatic) stacking interaction is suggested by an angle ω ranging from 0° to 24°, ω being the dihedral angle between the CdO bond and the plane of the aromatic ring. In 1, the orientation of the carbonyl group is almost parallel to the ring (ω ≈ 5°). As stated by Egli et al.,6c this orientation potentially allows potential hydrogen bonding interactions with the O lone pairs. A hydrogen bond is indeed observed for O4 which interacts with the nitrogen atom N3 of a neighboring aminopicoline molecule (N3-



Figure 5. Representation of the X-ray crystal structure of 2. The dotted lines indicate the hydrogen bonds. Unlabeled atoms are generated by the inversion operation (1 - x, 1 - y, 1 - z). Table 5. Selected Bond Lengths (Å) and Angles (deg) for 2

Figure 4. Perspective view of the unique, sandwich-type carbonyl(l.p.) · · · π/π · · · π/carbonyl(l.p.) · · · π interaction (shown by brass). Each 2-amino-4-picoline ring is interacting with a CdO group on one side and with a second aminopicoline molecule on the other side, leading to the formation of 3D assembly.

H3 · · · O4 ) 2.788(2); Table 4). Moreover, the distance D (corresponding to the distance between the carbonyl oxygen atom and the ring centroid) and the angular distribution [deviation of the angle R (R is the angle CdO-Cg) from 120°] are 3.47 Å and 35.08°, respectively. These values are within the mean values, that is, 3.58 and 30.6°, calculated by Egli et al.,6c on the basis of favorable stacking CdO (l.p.) · · · π interactions found in the CSD. Such a l.p. · · · π interaction between a CdO group and a π ring was thought to weaken hydrogen bonds, to be responsible of thermal dehydration behavior, and for preventing the formation of new Cu-O bonds in the crystal lattice of [Cu2(sgly)2(H2O)] · H2O, whose D and angular deviation values were respectively 3.564(3) Å and 41.38°.38 Even though similar interactions between a (carboxylate) carbonyl group and an aromatic ring have been noticed before,39-41 this supramolecular feature was documented by Addadi and Lahav for the first time in 1979.39 Later, Moorthy et al. interpreted the solid-state photochemical reactivity of some coumarin derivatives in terms of CdO · · · π interactions.42 Very recently, Santos-Contreras et al.43 reported two organic compounds, namely, ethyl-6-chloro-2-oxo-2H-chromene-3-carboxylate and ethyl 6-bromo-2-oxo-2H-chromene-3-carboxylate, where it has been found that the CdO · · · π along with other weak dipolar interactions are responsible for the formation of 3D networks. However, this interaction has not been thoroughly exploited so far as a routine tool in the design and construction of supramolecular structures. DFT single point calculations give an interaction energy of -125 kcal mol-1 for 1 which is inferior to the values obtained for larger systems such as compound 3 or assembly C (see below). In contrast to the other complexes reported in the present study (complexes 2 and 3; see below), the atoms-in-molecules (AIM) analysis indicated that the lone pair · · · π interactions observed in 1 are not strongly contributing to the overall stabilization of the carbonyl(l.p.) · · · π/π · · · π/carbonyl(l.p.) · · · π

Cu(1)-O(1) Cu(1)-O(3) Cu(1)-O(5) C(1)-O(1) C(1)-O(2) O(1)-Cu(1)-O(3) O(1)-Cu(1)-O(5) O(1)-Cu(1)-O(3)a O(1)-Cu(1)-O(5)a O(3)-Cu(1)-O(5) O(3)-Cu(1)-O(5)a a

1.9367(10) 1.9332(11) 2.6205(12) 1.2749(16) 1.2427(15) 92.18(4) 95.89(4) 87.82(4) 84.11(4) 85.06(4) 94.94(4)

C(3)-O(3) C(3)-O(4) C(1)-C(2) C(2)-C(3) Cu(1)-O(1)-C(1) Cu(1)-O(3)-C(3) O(1)-C(1)-O(2) C(1)-C(2)-C(3) O(3)-C(3)-O(4) O(3)-C(3)-C(2)

1.2802(17) 1.2427(17) 1.5263(18) 1.5099(17) 125.78(8) 126.36(8) 122.93(12) 118.96(10) 121.44(11) 119.69(12)

Symmetry code ) 1 - x, 1 - y, 1 - z. Table 6. Relevant H-Bonds in Compound 2

D-H · · · A

D-H [Å]

H · · · A [Å]

D · · · A [Å]

D-H · · · A [°]

N1-H1A · · · O4a N1-H1B · · · O2b N2-H2 · · · O3a O5-H5A · · · O4a O5-H5B · · · O2c C5-H5 · · · O1b C6-H6 · · · O2d C7-H7 · · · O4e

0.88 0.88 0.77 0.82 0.79 0.95 0.95 0.95

2.11 2.32 1.97 1.98 2.24 2.40 2.52 2.45

2.9194(15) 3.1619(16) 2.7416(15) 2.7909(17) 3.0016(16) 3.1738(16) 3.4669(17) 3.2424(17)

152 160 173 170 163 139 172 141

a Symmetry code ) 2 - x, 1 - y, 1 - z. b Symmetry code ) 2 - x, 1- y, 2 - z. c Symmetry code ) x, 1 + y, z. d Symmetry code ) 2 x, -y, 2 - z. e Symmetry code ) 2 - x, -y, 1 - z.

assembly. Instead, fairly significant C-H · · · π and C-H · · · O bonding interactions were found between the aminopyridine ring and the coordinated ligand, with an overall electron density of 0.0632 au. Crystal Structure Description of 2. Complex 2 is also solved in the triclinic space group P1j with the asymmetric unit consisting of half of the molecular anion [Cu(C3H2O4)2(H2O)2]2and one C5H7N2+ cation. The full anion is generated by the symmetry operation of an inversion center. The structure of 2 is shown in Figure 5. Selected bond lengths, angles, and supramolecular interactions are listed in Tables 5 and 6. The copper(II) ion, located on an inversion center, is in an octahedral coordination environment whose equatorial plane is formed by the oxygen atoms O1 and O3 from one malonate unit and their symmetry related counterparts O1**, O3** (** ) 1 - x, 1 y, 1 - z) from a second malonate unit. Two water molecules (O5 and O5**, ** ) 1 - x, 1 - y, 1 - z) occupy the trans axial positions, thus generating a CuO4O2 chromophore. The Cu-O bond distances in the equatorial plane vary between 1.933 and 1.937 Å, and the angle subtended at the copper atom by the malonate ligand is 92.18°. The value of the apical Cu(1)-O(5) bond length is 2.621 Å, which is significantly longer than the equatorial bond distances and suggests that the coordination


Choudhury et al.

Figure 6. Formation of 1D tape in (C5H7N2)2[Cu(C3H2O4)2(H2O)2] (2) through the association of discrete [Cu(mal)2(H2O)2]2- monomeric units. The occurrence of hydrogen bonding interactions (O5-H5A · · · O4) along the a axis generates a R22(12) cyclic motif. Each monomeric unit is connected to two 2-aminopyridine molecules as well, giving rise to the formation of R22(8) hydrogen bonding synthons.

Figure 7. Dual recognition of 2-aminopyridine units with the coppermalonate framework.

polyhedron of the copper atom in the anionic unit is essentially an elongated octahedron. All coordination bonds and angles in the anionic unit are within the range of values previously observed for related malonate-containing copper(II) complexes.44 Malonate ligands usually adopt an envelope conformation in which only the methylene group is significantly displaced from the chelate ring plane. The monomeric anionic units are interlinked to each other via strong self-complementary O5-H5A · · · O4 [2.7909(17) Å] hydrogen bonds which give rise to a R22(12) cyclic motif, ultimately generating an infinite 1D tape along the crystallographic a axis. Each monomeric anionic unit also recognizes two aminopyridinium cations (C5H7N2+) through doubly coordinated carboxylate ends, leading to R22(8) hydrogen bonding assemblies involving the hydrogen bonds N2-H2 · · · O3 and N1-H1A · · · O4 (Figure 6). Apart from these two supramolecular contacts, the aminopyridinium cation (C5H7N2+) is further recognized by another monocopper unit through the formation of a R22(8) motif involving the hydrogen bonds N1-H1B · · · O2 and C5-H5 · · · O1. This dual recognition induces the self-assembly of the monomeric units along the c axis (Figure 7). Each resulting 1D tape is connected to an adjacent chain by complementary O5-H5B · · · O2 [3.0016(16) Å] hydrogen bonds, forming a 2D network along the b axis (Figure 8). The 3D network of 2 is generated by weak to moderate hydrogen bonding interactions. The H bonds N1-H1B · · · O2 [3.1619(16) Å], C5-H5 · · · O1 [3.1738(16) Å], C6-H6 · · · O2

Figure 8. Two-dimensional assembly of monomeric units of 2 via complementary O5-H5B · · · O2 hydrogen bonds, generating a R22(12) cyclic motif. 2-Aminopyridine molecules are omitted for clarity.

[3.4669(17) Å] and C7-H7 · · · O4 [3.2424(17) Å] produce a R22(12) cyclic motif. The noncoordinating O2 atom is orientated toward the π-face of a 2-aminopyridine moiety. The distance between O2 and the centroid of the aminopyridine ring is 3.2341(13) Å [symmetry code: x, y, z, angle C1-O2 · · · Cg ) 119.32(9)°, C1 · · · Cg ) 3.9924(15) Å]. In this case, the carbonyl group approaches the π-face of the aminopyridine with an angle, suggesting a significant l.p. · · · π interaction. The 2-aminopyridine ring is further stacked over a second aminopyridine molecule [R(1) ) N(2)/C(4)/C(5)/C(6)/C(7)/C(8); symmetry code: 3 - x, 1 - y, 2 - z]. The centroid-centroid distance is 4.3591(9) Å and the dihedral angle amounts to 0° (the angle between the centroid-centroid joining line and the normal from the first ring center to the second ring plane is 42.11° and the slippage is 2.923 Å). This parallel-displaced stack (evidenced by a long Cg · · · Cg distance) protrudes out of the adjacent 1D chain along the c axis. The amino nitrogen atom N1 lies only 3.30 Å above the π face of the 2-aminopyridine ring revealing


Figure 9. Perspective view of the carbonyl(l.p.) · · · π/π · · · π/carbonyl(l.p.) · · · π interaction (shown by coral) involving the oxygen atoms O2. Each 2-aminopyridine ring is sandwiched between a CdO group and another aminopyridine, leading to the formation of 3D network.

an unusual stacking of -NH2 group over the aromatic-π cloud. The N1 lone pair appears to interact with the π cloud of the aromatic moiety, producing a unique, multilayered carbonyl (l.p.) · · · π/π · · · π/carbonyl(l.p.) · · · π supramolecular association (Figure 9). In the case of complex 2, the outstanding self-assembly has been thoroughly investigated by theoretical calculations to examine its stability. In this work, DFT-BHandH calculations coupled with the 6-31+G(d) basis sets were performed on the different assemblies depicted in Figure 10 to estimate the binding energies for the crystal structures determined via X-ray diffraction techniques. In addition, the atoms-in-molecules (AIM)32,33 theory was applied to visualize the single lone pair · · · π and π · · · π intermolecular interactions as reported previously.15,25,45 As shown in Table 2, the self-assemblies B and C (see Figure 10) have negative binding energies (∆E), while the aminopyridiunium stack (assembly A; Figure 10) is not stable as evidenced by the positive ∆E value. This instability may be expected because each ring bears a positive charge which gives rise to repulsive interactions. Indeed, the DFT optimization at the BHandH/6-31+G(d) level shows that the rings move away from each other, that is, the aminopyridinium dimer is not a minimum. Interestingly, the integration of the copper coordination unit in the calculations drastically affected this scenario. First, the interaction between the copper complex and one pyridiunium ring (assembly B; Figure 10) led to a stable supramolecular pairing, as ∆E is -115 kcal mol-1. The AIM analysis displayed in Figure 11 (assembly B) confirmed that one O · · · N (lone pair · · · π) interaction holds together the two molecules, with the electron density at the bond critical point being equal to 0.0133 au (as a reference, BH and H calculated values for bond critical points for the water, argon and benzene dimers are 0.0343, 0.0059, 0.0141 au, respectively).15 The large interaction energy suggested that electrostatics may primarily contribute to the overall binding. Similarly, the interaction of two copper complexes with the aminopyridiunium ring dimer (assembly C, Figure 10) brings the overall binding energy to -214 kcal mol-1 (Table 2), that is, the entire system is very stable. In particular, π · · · π


interactions stabilize the aminopyridiunium ring dimer with an overall electron density of 0.106 au, which is rather large compared to neutral purely π-stacked systems.15 Importantly, the interactions between the copper complex and the aminopyridiunium ring are stronger in the assembly C with respect to assembly B. In particular, the C · · · O interactions in assembly C have electron density of 0.0210 au to compare to 0.0133 of N · · · O interactions in assembly B (Figure 11 and Table 2). As above, such large energies are most likely due to strong electrostatic interactions between the molecules of the supramolecular array. Crystal Structure Description of 3. Complex 3 is solved in the triclinic space group P1j with the asymmetric unit consisting of half of the molecular anion [Ni(C3H2O4)2(H2O)2]2-, two crystallographically independent C5H7N2+ cations and a nitrate anion. The full anion is generated by the symmetry operation of an inversion center. An ORTEP diagram of 3 is shown in Figure 12. Selected bond lengths, angles and supramolecular interactions are listed in Tables 7 and 8. The nickel(II) ion, located on an inversion center, is in an octahedral coordination environment whose equatorial plane is formed by the oxygen atoms O1 and O3 from one malonate unit and their symmetry related counterparts O1**, O3** (** ) x, y - 2, z) from a second malonate unit. Two water molecules (O5 and O5**, ** ) x, y - 2, z) occupy the trans axial positions, thus generating a NiO4O2 chromophore. The Ni-O bond distances in the equatorial plane vary between 1.996 and 2.019 Å, and the angle subtended at the nickel atom by the malonate ligand is 90.52°. The value of the apical Ni(1)-O(5) bond length is 2.112 Å, which is longer than the equatorial bond distances and suggests that the coordination polyhedron of the nickel atom in the anionic unit is a slightly distorted octahedron. All coordination bonds and angles in the anionic unit are within the range of values previously observed for related malonate-containing Ni(II) complexes.46 Malonate ligands usually adopt an envelope conformation in which only the methylene group is significantly displaced from the chelate ring plane. The monomeric anionic units are interlinked to each other via strong self-complementary O5-H5A · · · O4 [2.7369(17) Å] hydrogen bonds which give rise to a R22(12) cyclic motif, ultimately generating an infinite 1D tape along the crystallographic a axis. Each monomeric anionic unit also recognizes four aminopyridinium cations (C5H7N2+) through doubly coordinated carboxylato ends, leading to R22(8) hydrogen bonding assemblies involving the hydrogen bonds N2-H2C · · · O2, N1-H1A · · · O1 and N3-H3A · · · O3, N4H4A · · · O4 (Figure 13). Two dangling nitrate ions are also attached with one monomeric unit by O5-H5B · · · O6 and O5H5B · · · O8 hydrogen bonds. The noncoordinating O2 atom is orientated toward the π-face of a 2-aminopyridine moiety. The distance between O2 and the centroid of the aminopyridine ring is 3.1607(15) Å [symmetry code: x, 1 + y, z, angle C1-O2 · · · Cg ) 130.85(10)°, C1 · · · Cg ) 4.0741(17) Å]. As for 2, the carbonyl group in 3 also approaches the π-face of the aminopyridine with an angle, suggesting a significant l.p. · · · π interaction. The 2-aminopyridine ring is further stacked over a second aminopyridine molecule [R(1) ) N(1)/C(4)/C(5)/C(6)/ C(7)/C(8); R(2) ) N(3)/C(9)/C(10)/C(11)/C(12)/C(13) symmetry code: x, y, z]. The centroid-centroid distance is 4.1743(12) Å and the dihedral angle amounts to 7.39°. Unlike 2, the paralleldisplaced stack (long Cg · · · Cg distance) protrudes out of the same monomeric unit. The amino nitrogen atoms N2 and N4 lie only 3.32 and 3.38 Å above the π face of the parallel stacked 2-aminopyridine ring, respectively (in 2, these two distances are identical), revealing an unusual stacking of the -NH2 group


Choudhury et al.

Figure 10. Representations of the supramolecular assemblies investigated by DFT calculations; A: aminopicoline stacks; B: carbonyl · · · interaction; C: carbonyl · · · π/π-π/π · · · carbonyl self-assembly. Table 7. Selected Bond Lengths (Å) and Angles (deg) for 3 Ni(1)-O(1) Ni(1)-O(3) Ni(1)-O(5) C(1)-O(1) C(1)-O(2) O(1)-Ni(1)-O(3) O(1)-Ni(1)-O(5) O(1)-Ni(1)-O(3)a O(1)-Ni(1)-O(5)a O(3)-Ni(1)-O(5) O(3)-Ni(1)-O(5)a a

1.9963(11) 2.0194(11) 2.1120(13) 1.2792(18) 1.2316(18) 90.52(4) 87.49(5) 89.48(4) 92.51(5) 86.99(5) 93.01(5)

C(3)-O(3) C(3)-O(4) C(1)-C(2) C(2)-C(3) Ni(1)-O(1)-C(1) Ni(1)-O(3)-C(3) O(1)-C(1)-O(2) C(1)-C(2)-C(3) O(3)-C(3)-O(4) O(3)-C(3)-C(2)

1.2692(18) 1.2452(18) 1.521(2) 1.515(2) 127.87(10) 127.52(10) 122.74(14) 120.13(12) 122.23(14) 119.35(13)

Symmetry code: ) x, y - 2, z. Table 8. Relevant H-Bonds in Compound 3 D-H · · · A

Figure 11. Schematic views of atoms in molecules topology of the supramolecular assemblies B and C. The blue lines represent the bond paths. The red and silver balls are the bond critical points for the lone pair · · · π and π · · · π interactions, respectively. Only the electron density relative to intermolecular interactions is reported. Further details on the Atoms-in-Molecules theory and topology are reported in the Supporting Information.

N1-H1A · · · O1a N2-H2C · · · O2a N2-H2D · · · O7b N3-H3A · · · O3c N4-H4A · · · O4c N4-H4B · · · O7 N4-H4B · · · O8 O5-H5A · · · O4d O5-H5B · · · O6e O5-H5B · · · O8e C12-H12A · · · O2b C13-H13A · · · O6a

D-H [Å] H · · · A [Å] D · · · A [Å] D-H · · · A [°] 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.83 0.88 0.88 0.95 0.95

1.89 1.98 2.30 1.89 2.17 2.59 2.39 1.91 2.17 2.41 2.35 2.55

2.7530(17) 2.8419(19) 3.139(2) 2.7668(16) 2.980(2) 3.392(2) 3.199(3) 2.7369(17) 2.999(2) 3.180(2) 3.183(2) 3.434(3)

165 168 160 172 153 152 152 178 159 147 146 156

a Symmetry code ) 1 + x, -1 + y, z. b Symmetry code ) 1 - x, 1 - y, 1 - z. c Symmetry code ) 1 - x, 1 - y, -z. d Symmetry code ) 1 - x, 2 - y, -z. e Symmetry code ) -x, 2 - y, -z.

Figure 12. ORTEP diagram for 3 with the atom numbering scheme. The thermal ellipsoids are drawn at the 50% probability level. Unlabeled half-structural domain is generated by the symmetry operation of an inversion center (x, y - 2, z).

over the aromatic-π cloud. This coupled carbonyl (l.p.) · · · π/ π · · · π interaction within the monomeric units adds stability to the formation of the 1D tape. Interestingly, one dangling nitrate anion belonging to an adjacent monomeric unit (along the b axis) is in contact with this multilayered carbonyl (l.p.) · · · π/ π · · · π assembly from the open opposite face of the 2-aminopyridine ring [O8 · · · Cg ) 3.510(2) Å, N5 · · · Cg ) 3.6991(17) Å]. The shortest separation distances reflecting this anion-π interaction is N5 · · · C5 ) 3.237 Å, which is below the sum of the corresponding van der Waals radii. Three relatively weak hydrogen bonds N4-H4B · · · O8, N2-H2D · · · O7, C13-H13A · · · O6 along with the multilayered interaction carbonyl (l.p.) · · · π/ π · · · π/π · · · anion contribute to the 2D assembly in 3, which propagates along the b axis (Figure 14). The overall 3D association is rather simple in 3 and guided by two moderately

weak hydrogen bonds, that is, N2-H2D · · · O7 and C12-H12A · · · O2. Single point calculations on 3 were carried out as reported in the Computational Details to investigate the formation of this supramolecular aggregate assembled through lone pair · · · π, anion · · · π and π-π interactions. The overall interaction energy equals -243 kcal mol-1, confirming the strong interactions occurring in the system. As expected, π-π interactions were found between the two aminopyridine rings with an overall electron density of 0.0390 au, which is a lower value compared to the one obtained for assembly C (vide infra). The AIM analysis suggests that this weaker interaction may be caused by the presence of the NO3- anion, which plays a crucial role. This anion is involved in several interactions (including O-H · · · O and N-H · · · O) with both the malonato ligand and the aminopyridine rings, with an overall electron density of 0.0825 au. Indeed, when the calculations are carried out after removal of the NO3- ion, the interaction energy drops to -187 kcal mol-1. Synthetic Observations. Recently, a series of related complexes, namely, {[Cu(mal)2](picH)2 · 5H2O}n13a (4), {[Cu(mal)2](picH)2 · 2H2O}n13b (5), (picH)2[M(mal)2(H2O)2] · 4H2O13c (M )



Figure 13. Formation of 1D tape in 3 through the association of discrete [Ni(mal)2(H2O)2]2- monomeric units. The occurrence of hydrogen bonding interactions (O5-H5A · · · O4) along the a axis generates a R22(12) cyclic motif. Each monomeric unit is connected to four 2-aminopyridine molecules as well, giving rise to the formation of R22(8) hydrogen bonding synthons.

Figure 14. Two-dimensional assembly of monomeric units of 3 via carbonyl(l.p.) · · · π/π · · · π/π · · · anion interactions. The other 2-aminopyridine and nitrate molecules are omitted for clarity.

Ni/Co/Mn), (6-8), have been reported where the malonate dianion acts as the primary ligand directly coordinated to the metal center. Conversely, the protonated heterocyclic bases functions as secondary (auxiliary) ligands, which do not

coordinate directly to the metal center, but are hydrogen bonded to the malonate oxygen atoms. It has also been demonstrated that the pH of the reaction media has an effect on the formation of compounds 4 and 5 from stoichiometric quantities of the reactants.13b Herein, the same synthetic procedure has been applied using 2-aminopyridine instead of 2-amino-4-picoline. Surprisingly, no effect of the pH of the reaction media (in the range 4.2-5.3) on the nature of the products formed is observed in the present study, as verified by crystallography. The presence or absence of a methyl group at the para position of the pyridine ring appears to be the main feature affecting the resulting solidstate structures of 2-5. Therefore, not only the pH, but also some other factors determine the final self-assembly process. The nature of these other factors and their exact role in the selfassembly of such molecules are poorly understood at this stage of the investigation. When this synthetic procedure is applied to other first-row transition metals, that is, Ni, Co, Mn, the exclusive production of the compounds with formula (picH)2[M(mal)2(H2O)2] · 4H2O (M ) Ni/Co/Mn) (6-8) could be observed, when the pH was adjusted to nearly 5.5. No product could be isolated from these systems when the reactions were performed at pH 4.2.13c As a result, the pH-dependent formation of the twin ternary systems 4 and 5 was found to be exclusive. Full crystallographic data in CIF format for 1-3 have been deposited with the Cambridge Crystallographic Data Centre (CCDC Nos. 680133, 680134, 685529). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (fax +44(0)-1223-336033 or email: [email protected]. Discussion In all the complexes mentioned earlier, one basic feature is found to be common and important, that is, the molecular


Choudhury et al.

Figure 16. Tripodal recognition of protonated 2-amino-4-picoline molecules with malonate moieties in 6-8, forming R22(8) and R22(7) hydrogen bonding motifs on both sides of the water/metal complex assembly. Figure 15. Recognition of protonated 2-amino-4-picoline molecules by malonate moieties in 4 and 5, leading to R22(8) hydrogen bonding patterns.

recognition of the heterocyclic bases (picoline and aminopyridine) with the different metal carboxylate coordination units. This supramolecular association generates primarily a R22(8) hydrogen bonding motif. The protonated heterocyclic molecule interacts with the carboxylate oxygen atoms forming two N-H · · · O hydrogen bonds. Such molecular pairing is also observed in protein-nucleic acid interactions (for instance, the interaction between a protonated cytosine and the carboxylate group of a protein).47 It is also interesting to mention that this recognition is enforced in situ through the protonation of the heterocyclic bases. In complexes 4 and 5, the recognition mode of the picoline molecule by the copper-malonate entity is the same (Figure 15). Interestingly, in 4 and 5, the copper malonate polymeric sheets are identical. Indeed, similar topological arrangements of the copper(II) centers and the malonates constituting the bc plane are present in both complexes. Primarily, it was expected that the same sheet would be generated while replacing 2-amino-4-picoline by 2-aminopyridine during the synthesis. However, the X-ray structures of 2 revealed that a different 2D sheet was generated and that, unlike 4 and 5, discrete monomeric [Cu(mal)2(H2O)]2- units are formed. Nevertheless, the recognition of the 2-aminopyridine molecules by the discrete [Cu(mal)2(H2O)]2- coordination units remains intact and the robust R22(8) motif is also operative. In 3, the 1D association is similar to that of 2, most likely because the R22(12) synthon is operative in both the complexes. In contrast to 2, each monomeric [Ni(mal)2(H2O)]2- unit in 3 recognizes four aminopyridinium cations. In complexes 6-8, the protonated picoline molecules are connected to each side of the [M(mal)2(H2O)2]2- chain and are H bonded to the malonate terminals creating a R22(8) motif (Figure 16). The binding of picoline to the layer is further strengthened by an additional C-H · · · O hydrogen bond with the nearest oxygen atom of the adjacent malonate terminal across the metal, generating a R22(7) motif. These two H bonding motifs act in unison and a tripodal recognition of the protonated picoline molecules enhance the stability of the supramolecular assembly. The robust R22(8) hydrogen bonding pattern developed between the picoline derivative and the malonate persists even when the latter is coordinated to a range of transition metals. It has to be further noted that in all these complexes, the picolinic nitrogen

atom gets protonated in situ, which is a prerequisite for this robust R22(8) motif to prevail. Remarkably, when the malonate is substituted by a nitrilotriacetate (compound 1), an equivalent H bonding association between the picoline ring and the metal organic unit is observed. The carbonyl(l.p.) · · · π interactions observed in 2 significantly contribute to the stability of 1D tape generated by the supramolecular association of the [Cu(mal)2(H2O)]2- units. The resulting multilayer sandwich topology plays an important role, together with relatively strong hydrogen bonds, in the formation of the 3D network. In 3, the generation of the 1D tape and the contribution from the carbonyl(l.p.) · · · π interaction are comparable to those found in 2. However, significant changes in the 2D and 3D assemblies are observed which may originate from the presence of the nitrate anions (which actually weakens the stability of the supramolecular association, as compared with 2). This multilayer carbonyl (l.p.) · · · π/π · · · π/π · · · anion interaction appears to play an important role in the assembly of the 2D network, whereas hydrogen bonds dictate the 3D assembly. Conclusions Supramolecular self-assemblies are generally stabilized by a variety of favorable intermolecular interactions. Hydrogen bonds and aromatic π-stacks are well accepted supramolecular interactions which are involved in the formation of intricate architectures, such as DNA. Understanding these noncovalent bonds is very important for the rational design of remarkable frameworks. In the present study, three metal-organic compounds have been prepared whose solid-state structures evidence the generation of outstanding 3D networks through noncovalent interactions. Indeed, the X-ray structures of 1-3 reveal the presence of H bonding and π-π contacts. In addition, the three compounds exhibit carbonyl (lone pair) · · · π interactions which contribute to the creation of supramolecular assemblies showing remarkable lone carbonyl · · · π/π · · · π/carbonyl · · · π and carbonyl · · · π/π · · · π/ π · · · anion patterns. Theoretical investigations based on the BHandH DFT functional and on the AIM analysis support the experimental findings of an intricate intermolecular interactions network which characterizes the studied complexes. In 1995, Egli and co-workers indicated that lone pair · · · π interactions were involved in the stabilization of the structure of Z-DNA, justifying its conformational stability at elevated ionic strength in spite of poor base stacking interactions.48



However, since this first observation of lone pair · · · π bonding contacts, crystallographic proofs of such noncovalent bonds are rare. The results herein reported strongly indicate that the interactions between electron-rich molecules with π-acidic rings are certainly of importance and should be considered by the supramolecular chemist to build multidimensional molecular structures. Acknowledgment. S.M. is grateful to Jadavpur University for partial financial support of this work. H.M.L. is grateful to the National Science Council of Taiwan for financial support of this work. P.G. is indebted to the Chemical Research Council of The Netherlands.

(8)

Supporting Information Available: DFT test calculations for compound 1, compound 3 and assembly B; thermogravimetry analysis (TGA) of compounds 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Lehn, J.-M. Supramolecular Chemistry-Concepts and PerspectiVes; VCH: Weinheim, Germany, 1995. (b) Fujita, M. Molecular SelfAssembly Organic versus Inorganic Approaches, Struct. Bonding (Berlin), Vol. 96; Springer-Verlag: Berlin, Germany, 2000. (c) MacGillivray, S. L. R.; Atwood, J. L. Nature 1997, 389, 469. (d) Biradha, K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 6431. (e) Childs, L. J.; Alcock, N. W.; Hannon, M. Angew. Chem., Int. Ed. 2001, 40, 1079. (f) Bond, A. D. Chem. Commun. 2002, 1664. (g) Kidowaki, M.; Tamaoki, N. Chem. Commun. 2003, 290. (2) (a) Braga, D.; Grepioni, F.; Orpen, A. G. Crystal Engineering: From Molecules and Crystals to Materials; NATO Science Series, Mathematical and Physical Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999; Vol. 538. (b) Seddon, R. K.; Zaworotko, M. Crystal Engineering: The Design and Application of Functional Solids; NATO Science Series, Mathematical and Physical Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999; Vol. 539. (c) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids; Elsevier Science Publishers B. V.: Amsterdam, The Netherlands, 1989. (3) (a) Honman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107. (b) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998, 98, 1375. (c) Wu, C. D.; Hu, A. G.; Zhang, L.; Lin, W. B. J. Am. Chem. Soc. 2005, 127, 8940. (d) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudsonk, S. D.; Duank, H. Nature 2002, 419, 384. (e) Corna, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencla, S. Nature 2004, 431, 287. (4) (a) Zhang, Y.; Yang, Z.; Yuan, F.; Gu, H.; Gao, P.; Xu, B. J. Am. Chem. Soc. 2004, 126, 15028. (b) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1997. (c) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. (5) (a) Nørgaard, K.; Bjørnholm, T. Chem. Commun. 2005, 1812. (b) Kepert, C. J. Chem. Commun. 2006, 695. (6) (a) Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97, 1303. (b) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210. (c) Egli, M.; Sarkhel, S. Acc. Chem. Res. 2007, 40, 197. (7) (a) Gamez, P.; Mooibroek, T. J.; Teat, S. J.; Thilagar, P. Acc. Chem. Res. 2007, 40, 435. (b) Schneider, H.-J. Angew. Chem., Int. Ed. 1991, 30, 1417. (c) Quinõnero, D.; Garau, C.; Frontera, A.; Ballester, P.; Costa, A.; Deya`, P. M. Chem. Phys. Lett. 2002, 359, 486. (d) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. ChemPhysChem 2003, 4, 1344. (e) Garau, C.; Quinõnero, D.; Frontera, A.; Ballester, P.; Costa, A.; Dey, a`, P. M. New J. Chem. 2003, 27, 211. (f) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. Chem. Phys. Lett. 2003, 382, 534. (g) Garau, C.; Quinõnero, D.; Frontera, A.; Costa, A.; Ballester, P.; Deya`, P. M. Chem. Phys. Lett. 2003, 370, 7. (h) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. Chem. Phys. Lett. 2004, 399, 220. (i) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. J. Phys. Chem. A 2004, 108, 9423. (j) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. Chem. Phys. Lett. 2004, 392, 85. (k) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. Rec. Res. DeV.

(9) (10)

(11) (12) (13)

(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)

Chem. Phys. 2004, 5, 227–255. (l) Quinõnero, D.; Garau, C.; Frontera, A.; Ballester, P.; Costa, A.; Deya`, P. M. J. Phys. Chem. A 2005, 109, 4632. (m) Garau, C.; Frontera, A.; Quinõnero, D.; Ballester, P.; Costa, A.; Deya`, P. M. Eur. J. Org. Chem. 2005, 179. (n) Garau, C.; Quinõnero, D.; Frontera, A.; Ballester, P.; Costa, A.; Deya`, P. M. J. Phys. Chem. A 2005, 109, 9341. (o) Frontera, A.; Saczewski, F.; Gdaniec, M.; Dziemidowicz-Borys, E.; Kurland, A.; Deya`, P. M.; Quinõnero, D.; Garau, C. Chem. Eur. J. 2005, 11, 6560. (p) Maheswari, P. U.; Modec, B.; Pevec, A.; Kozlevcar, B.; Massera, C.; Gamez, P.; Reedijk, J. Inorg. Chem. 2006, 45, 6637. (a) Han, B.; Lu, J.; Kochi, J. K. Cryst. Growth Des. 2008, doi 10.1021/ cg701138n. (b) Mascal, M.; Armstrong, A.; Bartberger, M. D. J. Am. Chem. Soc. 2002, 124, 6274. (c) Yaroslav, R. S.; Lindeman, S. V.; Rosokha, S. V.; Kochi, J. K. Angew. Chem., Int. Ed. 2004, 43, 4650. (d) Kim, D.; Tarakeshwar, P.; Kwang, S. K. J. Phys. Chem. A 2004, 108, 1250. (e) Alkorta, I.; Rozas, I.; Elguero, J. J. Am. Chem. Soc. 2002, 124, 8593. (f) Berryman, O. B.; Hof, F.; Hynes, M. J.; Johnson, D. W. Chem. Commun. 2006, 506. (g) Demeshko, S.; Dechert, S.; Meyer, F. J. Am. Chem. Soc. 2004, 126, 4508. (h) Casellas, H.; Massera, C.; Gamez, P.; Manotti Lanfredi, A. M.; Reedijk, J. Eur. J. Inorg. Chem. 2005, 2902. (i) de Hoog, P.; Gamez, P.; Mutikainen, I.; Turpeinen, U.; Reedijk, J. Angew. Chem., Int. Ed. 2004, 43, 5815. (a) Gautrot, J. E.; Hodge, P.; Cupertinob, D.; Helliwella, M. New J. Chem. 2006, 30, 1801. (b) Mooibroek, T. J.; Teat, S. J.; Massera, C.; Gamez, P.; Reedijk, J. Cryst. Growth Des. 2006, 6, 1569. (a) Thomas, K. A.; Smith, G. M.; Thomas, T. B.; Feldmann, R. J. Proc. Natl. Acad. Sci. U. S. A. 1982, 79, 4843. (b) Kim, C.-Y.; Chandra, P. P.; Jain, A.; Christianson, D. W. J. Am. Chem. Soc. 2001, 123, 9620. (a) Gambaro, A.; Ganis, P.; Manoli, F.; Polimeno, A.; Santi, S.; Venzo, A. J. Organomet. Chem. 1999, 583, 126. (b) Ihara, H.; Uemura, S.; Okazaki, S.; Hirayama, C. Polym. J. (Tokyo) 1998, 30, 394. Rodrı´guez-Martin, Y.; Hernańdez-Molina, M.; Delgado, F. S.; Pasań, J.; Ruiz-Pe´rez, C.; Sanchiz, J.; Lloret, F.; Julve, M. CrystEngComm 2002, 4, 522. (a) Choudhury, S. R.; Jana, A. D.; Colacio, E.; Lee, H. M.; Mostafa, G.; Mukhopadhyay, S. Cryst. Growth Des. 2007, 7, 212. (b) Choudhury, S. R.; Jana, A. D.; Chen, C.-Y.; Dutta, A.; Colacio, E.; Lee, H. M.; Mostafa, G.; Mukhopadhyay, S. CrystEngComm, accepted for publication. (c) Choudhury, S. R.; Jana, A. D.; Dey, B.; Das, S.; Chen, C.-Y.; Lee, H. M.; Mukhopadhyay, S. Manuscript submitted to Eur. J. Inorg. Chem. Sheldrick, G. M. SHELXL97: Program for the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. Waller, M. P.; Robertazzi, A.; Platts, J. A.; Hibbs, D. E.; Williams, P. A. J. Comput. Chem. 2006, 27, 491. van de Vondele, J.; Lynden-Bell, R.; Meijer, E. J.; Sprik, M. J. Phys. Chem. B 2006, 110, 3614. Cerny´, J.; Hobza, P. Phys. Chem. Chem. Phys. 2007, 9, 5291. Swart, M.; van der Wijst, T.; Fonseca Guerra, C.; Bickelhaupt, F. J. Mol. Model. 2007, 13, 1245. Hobza, P.; Sponer, J. Chem. ReV. 1999, 99, 3247. Meijer, E. J.; Sprik, M. J. Chem. Phys. 1996, 105, 8684. Wang, W. Z.; Pitonaac, M.; Hobza, P. ChemPhysChem 2007, 8, 2107. Jurecka, P.; Hobza, P. J. Am. Chem. Soc. 2003, 125, 15608. Becke, A. D. J. Chem. Phys. 1993, 98, 5648. Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785. Magistrato, A.; Robertazzi, A.; Carloni, P. J. Chem. Theory Comput. 2007, 3, 1708. Overgaard, J.; Waller, M. P.; Piltz, R.; Platts, J. A.; Emseis, P.; Leverett, P.; Williams, P. A.; Hibbs, D. E. J. Phys. Chem. A 2007, 111, 10123. Zhao, Y.; Truhlar, D. G. J. Chem. Theory Comput. 2007, 3, 289. Meyer, M.; Steinke, T.; Su¨hnel, J. J. Mol. Model. 2007, 13, 335. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;. Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;


(30) (31)

(32)

(33) (34) (35) (36)

(37) (38) (39) (40)

Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian, Inc.: Pittsburgh, PA 2003. Hehre, W. J.; Ditchfie, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. See for example, (a) Matta, C. F.; Castillo, N.; Boyd, R. J. J. Phys. Chem. B 2006, 110, 563–578. (b) Robertazzi, A.; Platts, J. A. Chem. Eur. J. 2006, 12, 5747. (c) Hibbs, D. E.; Overgaard, J.; Platts, J. A.; Waller, M. P.; Hursthouse, M. B. J. Phys. Chem. B 2004, 108, 3663– 3672. (a) Bader, R. F. W. Chem. ReV. 1991, 91, 893. (b) Bader, R. F. W. Atoms in Molecules-A Quantum Theory; University Press: Oxford, 1990. Boyd, R. J.; Choi, S. C. Chem. Phys. Lett. 1986, 129, 62. Rybarczyk-Pirek, A.; Dubis, A. T.; Grabowski, S. J.; NawrotModranka, J. Chem. Phys. 2006, 320, 247. Howard, S. T.; Lamarche, O. J. Phys. Org. Chem. 2003, 16, 133. (a) Ghosh, S. K.; Bharadwaj, P. K. Inorg. Chim. Acta 2006, 359, 1685. (b) Zhang, L.-J.; Liu, B.-X.; Ge, H.-Q.; Xu, D.-J. Acta Crystallogr. 2006, E62, m2180. (c) Ghosh, S. K.; Ribas, J.; Bharadwaj, P. K. Cryst. Growth Des. 2005, 5, 623. (d) Wang, Y.; Okabe, N. Inorg. Chim. Acta 2005, 358, 3407. (e) Aromı´, G.; Gamez, P.; Roubeau, O.; CarreroBerzal, P.; Kooijman, H.; Spek, A. L.; Driessen, W. L.; Reedijk, J. Eur. J. Inorg. Chem. 2002, 1046. (f) Amoroso, A. J.; Burrows, M. W.; Dickinson, A. A.; Jones, C.; Willock, D. J.; Wong, W. T. J. Chem. Soc., Dalton Trans. 2001, 225. (g) Soldatov, D. V.; Ripmeester, J. A. Chem. Eur. J. 2001, 7, 2979. (h) Francese, G.; M. Romero, F.; Neels, A.; Stoeckli-Evans, H.; Decurtins, S. Inorg. Chem. 2000, 39, 2087. Hemamalini, M.; Muthiah, P. T.; Butcher, R. J.; Lynch, D. E. Inorg. Chem. Commun. 2006, 9, 1155, and references therein. Yang, X.; Wu, D.; Ranford, J. D.; Vittal, J. J. Cryst. Growth Des. 2005, 5, 41. Addadi, L.; Lahav, M. Pure Appl. Chem. 1979, 51, 1269. Bernstein, J.; Cohen, M. D.; Leiserowitz, L. In The Chemistry of Quinonoid Compoundsmed; Patai, S., Ed.; J. Wiley & Sons: London, 1974; p 37.

Choudhury et al. (41) Ueno, K.; Nakanishi, H.; Hasegawa, M.; Sasada, Y. Acta Crystallogr. 1978, B34, 2034. (42) Moorthy, J. N.; Samant, S. D.; Venkatesan, K. J. Chem. Soc., Perkin Trans. 2 1994, 1223. (43) Santos-Contreras, R. J.; Martıńez-Martıńez, F. J.; Garcıá-Baéz, E. V.; Padilla-Martıńez, I. I.; Peraza, A. L.; Ho¨pfl, H. Acta Crystallogr. 2007, C63, o239. (44) (a) Chattopadhyay, D.; Chattopadhyay, S. K.; Lowe, P. R.; Schwalbe, C. H.; Mazumdar, S. K.; Rana, A.; Ghosh, S. J. Chem. Soc., Dalton Trans. 1993, 913. (b) Gil de Muro, I.; Mautner, F. A.; Insausti, M.; Lezama, L.; Arriortua, M. I.; Rojo, T. Inorg. Chem. 1998, 37, 3243. (c) Ruiz-Pe´rez, C.; Sanchiz, J.; Hernańdez-Molina, M.; Lloret, F.; Julve, M. Inorg. Chim. Acta 2000, 298, 245. (d) Ruiz-Pe´rez, C.; Sanchiz, J.; Hernańdez-Molina, M.; Lloret, F.; Julve, M. Inorg. Chem. 2000, 39, 1363. (e) Ruiz-Pe´rez, C.; Sanchiz, J.; Hernańdez-Molina, M.; Lloret, F.; Julve, M. Inorg. Chem. 2000, 39, 3845. (f) Rodrı´guezMartin, Y.; Sanchiz, J.; Ruiz-Pe´rez, C.; Lloret, F.; Julve, M. Inorg. Chim. Acta 2001, 326, 20. (g) Rodrı´guez-Martin, Y.; Sanchiz, J.; RuizPe´rez, C.; Lloret, F.; Julve, M. Inorg. Chim. Acta 2001, 318, 159. (h) Sanchiz, J.; Rodrı´guez-Martin, Y.; Ruiz-Pe´rez, C.; Mederos, A.; Lloret, F.; Julve, M. New J. Chem. 2002, 36, 1624. (i) Rodrı´guez-Martin, Y.; Hernańdez-Molina, M.; Delgado, F. S.; Pasań, J.; Ruiz-Pe´rez, C.; Sanchiz, J.; Lloret, F.; Julve, M. CrystEngComm 2002, 4, 440. (j) Sain, S.; Maji, T. K.; Mostafa, G.; Lu, T. H.; Chaudhuri, N. R. New J. Chem. 2003, 27, 185. (k) Delgado, F. S.; Sanchiz, J.; Ruiz-Pe´rez, C.; Lloret, F.; Julve, M. Inorg. Chem. 2003, 42, 5938. (45) Robertazzi, A.; Platts, J. A. J. Phys. Chem. A 2006, 110, 3992. (46) (a) Delgado, F. S.; Hernańdez-Molina, M.; Sanchiz, J.; Ruiz-Pe´rez, C.; Rodrı´guez-Martin, Y.; Lo´pez, T.; Lloret, F.; Julve, M. CrystEngComm 2004, 6, 106. (b) Ruiz-Pe´rez, C.; Hernańdez-Molina, M.; Sanchiz, J.; Lo´pez, T.; Lloret, F.; Julve, M. Inorg. Chim. Acta 2001, 298, 245. (47) Vallee, B. L.; Auld, D. S. Acc. Chem. Res. 1993, 26, 543. (48) Egli, M.; Gessner, R. V. Proc. Natl. Acad. Sci. U. S. A. 1995, 92, 180–184.

CG800403P

Experimental Observation of Supramolecular ... - ACS Publications

Recommend Documents