Self-Assembly of Discrete Metallocycle versus Coordination Polymer

Main IR (cm−1): 3250m, 3021w, 1600s, 1571m, 1530m, 1459m, 1330m, ..... Figure 4d, these 2-D layers extend parallel to the crystallographic bc plane ...
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Self-Assembly of Discrete Metallocycle versus Coordination Polymer Based on Silver(I) and Di-2- and Di-3-pyridines with Flexible Spacer Zhao-Peng Deng,† Li-Na Zhu,† Shan Gao,*,† Li-Hua Huo,† and Seik Weng Ng‡ Laboratory of Functional Materials, School of Chemistry and Materials Science, Heilongjiang UniVersity, Harbin 150080, People’s Republic of China, and Department of Chemistry, UniVersity of Malaya, Kuala Lumpur 50603, Malaysia

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3277–3284

ReceiVed January 25, 2008; ReVised Manuscript ReceiVed May 16, 2008

ABSTRACT: Four flexible bis(pyridyl) ligands, namely, 1,3-bis(2-pyridylaminomethyl)benzene (L1), 1,3-bis(3-pyridylaminomethyl)benzene (L2), 1,4-bis(3-pyridylaminomethyl)benzene (L3), and 1,4-bis(2-pyridylaminomethyl)benzene (L4), have been synthesized, and the reactions of these ligands with AgX salts (X ) NO3-, ClO4-) lead to the formation of six new complexes, [Ag(L1)]2(ClO4)2 (1), [Ag(L2)ClO4]n (2), [Ag(L1)NO3]n (3), [Ag(L3)(PPh3)2]n(NO3)n · nCH3OH (4), [Ag2(L4)(PPh3)2(NO3)2]n · nH2O (5), and [Ag(L4)NO3]n (6), exhibiting from zero-dimensional (0-D) to three-dimensional (3-D) frameworks. In complex 1, the L1 ligand bridges two silver atoms to form a dinuclear with a 24-membered macrometallacyclic ring, while the L1 ligand coordinates to silver atoms to generate a 2-D (4,4) network with two pyridine rings oriented in a divergent fashion (or trans conformation) in complex 3. With a similar trans conformation, L2 in complex 2, L3 in complex 4, and L4 in complex 5 connect the adjacent silver atoms or Ag2O2 units which formed by the bridging of nitrate anions in a η2:µ2 fashion to give rise to infinite helical chains, respectively. Complex 6 exhibits a 3-D framework with diamond topology, which is combined by two kinds of helical chains: one is a -Ag-NO3-Ag-NO3- chain with the nitrate anions in a η1:η1:µ2 fashion, and another is a -Ag-L4-Ag-L4- chain with the L4 ligand in trans conformation. All of these complexes are photoluminescent in the solid state with spectra that closely resemble those of the ligand precursor. Introduction The crystal engineering of silver coordination polymers based on multifunctional N-donor spacers, such as the bipyridylcontaining tectons, has received much attention.1-4 The interest arises not only from the Ag(I) ion holding a diverse coordination geometry due to its variable coordination number, but also from the fascinating structural diversities and photoluminescent properties. In designing polymers, the properties of ligands such as various coordination modes, variable lengths, and relative orientation of donor atoms play a fundamental role in determining the structural outcome of target polymers. In the past few years, extensive studies have been carried out using rigid bridging ligands such as 4,4′-bipyridine-type,5 Schiff-base,6 2,4,6-tri(4-pyridyl)-1,3,5-triazine,7 etc., and a variety of one-, two-, and three-dimensional (1-D, 2-D, and 3-D) silver polymeric networks with beautiful topologies and inclusion behaviors have been obtained.8 By contrast, the flexible ligands have many degrees of freedom and hence few conformational restraints, as well as the unpredictable nature of such systems.9 When the flexible bipyridyl ligands adopting different conformations react with silver salts, interesting and unusual structures may facilitate the formation of helixes and other novel supramolecular architectures. So far, the different spacers in these flexible ligands involve (i) aliphatic hydrocarbon;10 (ii) aromatic and heterocyclic ring;11 (iii) amide;12 (iv) di-Schiff base;13 (v) ether;14 (vi) reduced di-Schiff base,15-18 and other more complicated ones,19 in which the reduced di-Schiff base ligands are less reported. Sun and co-workers reported some silver(I) coordination polymers constructed from the reduced di-Schiff base ligands, namely, 1,2-bis(4-pyridylmethylamino)ethane,15 1,2-bis(3-pyridylmethylamino)ethane,16 and N,N′bis(3-pyridylmethyl)-1,4-benzenedimethylamine17 with long

Chart 1. Scheme of the Flexible Bis(Pyridyl) Ligands in This Work

spacers -CH2NHCH2CH2NHCH2- and -CH2NHCH2C6H4CH2NHCH2-, respectively. Recently, Effendy and Piero have represented mono- or dinuclear silver(I) complexes sustained by a short spacer -CH2NHCH2- of bis(2-picolyl)amine.18 Based on the above considerations, we focus our attention on the silver complexes constructed from the reduced di-Schiff base ligands with a larger N · · · N separation between the termina pyridyl groups, in the hopes of obtaining new architectures and probing the influence of flexibility and length of such ligands on the helical structures. Here in this paper we have synthesized four flexible bis(pyridyl) ligands with a longer spacer -NHCH2C6H4CH2NH-, namely, 1,3-bis(2-pyridylaminomethyl)benzene (L1), 1,3-bis(3-pyridylaminomethyl)benzene (L2), 1,4-bis(3pyridylaminomethyl)benzene (L3) and 1,4-bis(2-pyridylaminomethyl)benzene (L4) (Chart 1), and report the preparation and crystal structures of six novel Ag(I) complexes [Ag(L1)]2(ClO4)2 (1), [Ag(L2)ClO4]n (2), [Ag(L1)NO3]n (3), [Ag(L3)(PPh3)2]n(NO3)n.nCH3OH (4), [Ag2(L4)(PPh3)2(NO3)2]n · nH2O (5) and [Ag(L4)NO3]n (6), which were obtained by the reactions of the above four flexible bis(pyridyl) ligands and AgX (X ) ClO4-, NO3-). Experimental Section

* To whom correspondence should be addressed. † Heilongjiang University. ‡ University of Malaya.

All chemicals were of A. R. grade and used without further purification. The ligands (L1, L3, L4) were prepared according to

10.1021/cg800091y CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

3278 Crystal Growth & Design, Vol. 8, No. 9, 2008

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Table 1. Crystallographic Data of Complexes 1-6 complex

1

2

3

molecular formula formula weight space group a/Å b/Å c/Å β/° V/Å3 Z Dc/g m-3 µ (Mo KR)/ mm-1 F(000) reflections collected unique reflections no. of params GOF on F2 final R indices [I g 2σ(I)]

C36H36N8O8Cl2Ag2

C18H18N4O4ClAg

C18H18N5O3Ag

R (int) largest difference peak, hole/ e Å-3

995.37

497.68

460.24

P21/c

P21/n

P21/n

8.9572(18) 21.677(4) 10.138(2) 106.25(3) 1889.8(7) 2 1.749 1.241

7.7827(7) 16.1812(13) 14.9377(15) 98.664(3) 1859.7(3) 4 1.778 1.261

9.877(2) 10.211(2) 18.348(4) 97.28(3) 1835.6(7) 4 1.665 1.127

1000 18252

1000 13240

928 14122

4324

3277

3260

253

271

244

1.042

1.021

1.100

R1 ) 0.0356

R1 ) 0.0693

R1 ) 0.0395

wR2 ) 0.0974 0.0344 0.661, -0.530

wR2 ) 0.1727 0.0818 0.862, -0.633

wR2 ) 0.0675 0.0418 0.566, -0.750

Complex 1 Ag(1)-N(1) Ag(1)-N(4)i N(1)-Ag(1)-N(4)i N(2)-H(5) · · · O(4)ii

2.139(2) 2.142(2) 163.54(9) 150.9

N(2) · · · O(4)ii N(3) · · · O(2) N(3)-H(14) · · · O(2)

3.039(4) 3.318(5) 159.5

Complex 2 Ag(1)-N(4)i 2.128(8) Ag(1)-N(1) 2.134(8) i 167.5(3) N(4) -Ag(1)-N(1) N(4)i-Ag(1)-Ag(1)ii 94.0(2)

Ag(1)-Ag(1)ii N(3) · · · O(2)iV N(1)-Ag(1)-Ag(1)ii N(3)-H(3N) · · · O(2)iV

3.3760(16) 3.238(19) 92.5(2) 150.9

Complex 3 Ag(1)-N(4)i Ag(1)-N(1) Ag(1)-O(1) N(4)i-Ag(1)-N(1) N(4)i-Ag(1)-O(1) N(1)-Ag(1)-O(1)

2.190(4) 2.215(4) 2.537(4) 151.22(15) 116.71(14) 89.11(14)

Ag(1)-N(3) Ag(1)-N(1) Ag(1)-P(1) Ag(1)-P(2) N(3)-Ag(1)-N(1) N(3)-Ag(1)-P(1) N(1)-Ag(1)-P(1) N(3)-Ag(1)-P(2) N(1)-Ag(1)-P(2)

2.400(3) 2.437(3) 2.4611(7) 2.4707(8) 90.62(10) 117.83(7) 100.11(7) 102.55(7) 113.76(8)

Ag(1)-N(1) Ag(1)-P(1) Ag(1)-O(1) Ag(1)-O(4) Ag(2)-N(4)i N(1)-Ag(1)-P(1) N(1)-Ag(1)-O(1) P(1)-Ag(1)-O(1) N(1)-Ag(1)-O(4) P(1)-Ag(1)-O(4) O(1)-Ag(1)-O(4)

2.254(4) 2.3826(13) 2.521(4) 2.563(4) 2.226(4) 139.33(10) 102.66(16) 114.39(13) 98.65(15) 110.63(12) 66.78(13)

Ag(1)-N(3) Ag(1)-N(1) N(3)-Ag(1)-N(1) N(3)-Ag(1)-O(1) N(1)-Ag(1)-O(1)

2.172(3) 2.177(3) 169.64(13) 93.6(2) 90.1(2)

Ag(1)-O(1)ii N(3) · · · O(2)iV N(3) · · · O(1)iV N(3)-H(14) · · · O(2)iV N(3)-H(14) · · · O(1)iV

2.690(4) 3.076(6) 3.261(6) 139.6 136.9

Complex 4 N(2)-O(1) N(4)-O(1)iii O(4)-O(2) P(1)-Ag(1)-P(2) N(2)-H(2N) · · · O(1) N(4)-H(4N) · · · O(1)iii O(4)-H(4) · · · O(2)

2.907(5) 3.076(5) 2.915(9) 126.45(3) 148.6 149.0 151.0

Complex 5

complex

4

5

6

molecular formula formula weight space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z Dc/g m-3 µ (Mo KR)/ mm-1 F(000) reflections collected unique reflections no. of params GOF on F2 final R indices [I g 2σ(I)]

C55H52N5O4P2Ag

C54H50N6O7P2Ag2

C18H18N5O3Ag

1016.83

1172.71

460.24

P1j

P21/n

P21/n

10.6580(3) 14.9873(5) 16.3809(4) 78.7423(8) 78.9459(7) 82.0829(8) 2504.93(13) 2 1.348 0.517

9.805(2) 25.549(5) 21.396(4) 90.00 96.87(3) 90.00 5321.4(18) 4 1.461 0.852

9.2795(5) 11.7789(6) 17.0213(10) 90.00 103.5359(15) 90.00 1808.79(17) 4 1.690 1.144

1052 40209

2376 51208

928 17430

11390

12130

4127

606

648

281

1.099

1.066

1.032

R1 ) 0.0424

R1 ) 0.0494

R1 ) 0.0391

wR2 ) 0.1278 0.0306 1.372, -0.577

wR2 ) 0.1318 0.0671 0.792, -0.937

wR2 ) 0.0904 0.0447 0.533, -0.630

R (int) largest difference peak, hole/ e Å-3

Table 2. Selected Bond Lengths [Å] and Angles [°] for Complexes 1-6a

the literature methods.20 The L2 ligand was prepared according to the literature method of L1. Elemental analyses were performed on a CARLO ERBA 1106 analyzer. The IR spectra were recorded on a Bruker Equinox 55 FT-IR spectrometer using KBr pellet. Luminescence spectra were measured on a Perkin-Elmer LS 55 luminance meter. CAUTION! Although not encountered in our experiments, metal perchlorates are potentially explosive. They should be handled carefully. Synthesis of [Ag(L1)]2(ClO4)2 (1). A mixture of AgClO4 (103.7 mg, 0.5 mmol) in acetonitrile and L1 (145.2 mg, 0.5 mmol) in methanol was stirred for 5 min, and then filtered. The resulting clear solution was allowed to evaporate slowly at room temperature for three weeks, affording colorless crystals, which were collected by filtration, washed

Ag(2)-P(2) Ag(2)-O(4) N(2) · · · O(5) N(3) · · · O(3)ii N(4)i-Ag(2)-P(2) N(4)i-Ag(2)-O(4) P(2)-Ag(2)-O(4) N(2)-H(1) · · · O(5) N(3)-H(10) · · · O(3)ii

2.3670(13) 2.441(4) 3.049(7) 3.077(7) 136.31(10) 99.47(16) 120.59(12) 146(6) 127.4

Complex 6 Ag(1)-O(1) Ag(1)-O(2)i N(3)-Ag(1)-O(2)i N(1)-Ag(1)-O(2)i O(1)-Ag(1)-O(2)i

2.788(9) 2.811(7) 97.9(2) 87.8(2) 125.3(2)

a Symmetry codes (in 1): i-x + 1, -y + 1, -z + 1. iix - 1, y, z. (in 2): i-x + 1/2, y - 1/2, -z + 1/2. ii-x, -y + 1, -z + 1. iV-x + 1, -y + 1, -z. (in 3): i-x + 1/2, y - 1/2, -z + 1/2. ii-x, -y, -z + 1. iVx + 1/2, -y + 1/2, z - 1/2. (in 4): iii-x + 1, -y + 2, -z + 1. (in 5): ix + 1/2, -y + 3/2, z + 1/2. iix - 1/2, -y + 3/2, z - 1/2. (in 6): i-x + 3/2, y - 1/2, -z + 3/2.

with ether successively, and dried in air. Yield: 60% based on Ag. Anal. Calcd for C36H36N8O8Cl2Ag2: C 43.44, H 3.65, N 11.26%; Found: C 43.48, H 3.68, N 11.22%. Main IR (cm-1): 3362s, 3035w, 1610s, 1571s, 1507s, 1459m, 1324s, 1282m, 1173m, 921w, 772s, 693m. Synthesis of [Ag(L2)(ClO4)]n (2). This complex was obtained with a yield of 54% (based on Ag) by the same method used for the preparation of (1) using L2 instead of L1. Anal. Calcd for C18H18N4O4Cl2Ag: C 43.44, H 3.64, N 11.26%; Found: C 43.47, H 3.68, N 11.23%. Main IR (cm-1): 3382m, 3072w, 1601s, 1583s, 1519m, 1480w, 1425s, 1383s, 1306m, 1248w, 1087vs, 798m, 700m, 619m. Synthesis of [Ag(L1)NO3]n (3). This complex was obtained with a yield of 65% (based on Ag) by the same method used for the preparation of (1) using AgNO3 instead of AgClO4. Anal. Calcd for C18H18N5O3Ag: C 46.98, H 3.94, N 15.22%; Found: C 46.95, H 3.97, N 15.26%. Main IR (cm-1): 3324m, 3021w, 1608s, 1566s, 1524s, 1445m, 1330m, 1300s, 1160m, 894w, 764m, 696w. Synthesis of [Ag(L3)(PPh3)2]n(NO3)n · nCH3OH (4). A suspension of AgNO3 (85.0 mg, 0.5 mmol) in methanol was treated with L3 (145.2 mg, 0.5mmol). Ammonia-water was droped until a clear colorless solution emerged, then an aqueous solution containing PPh3 (262.3 mg, 1.0 mmol) was added and the reaction mixture was stirred for 10 min. The clear solution produced was filtered and allowed to evaporate slowly at room temperature for six weeks, and colorless crystals suitable

Self-Assembly of Discrete Metallocycle

Crystal Growth & Design, Vol. 8, No. 9, 2008 3279 673641 for 1, 673642 for 2, 673640 for 3, 673639 for 4, 673638 for 5, and 673637 for 6. Selected bond lengths and angles are listed in Table 2.

Results and Discussion

Figure 1. (a) View of the dinuclear structure of complex 1. (b) The chain structure formed by H-bonding interactions. for X-ray diffraction were obtained. Yield: 55% based on Ag. Anal. Calcd for C55H52N5O4P2Ag: C 64.97, H 5.15, N 6.89%; Found: C 64.95, H 5.18, N 6.85%. Main IR (cm-1): 3412m, 3321m, 3058m, 1590s, 1521m, 1480s, 1433s, 1385s, 1310s, 1187w, 1092m, 795m, 744s, 696s, 512s. Synthesis of [Ag2(L4)(PPh3)2(NO3)2]n · nH2O (5). This complex was obtained with a yield of 68% (based on Ag) by the same method used for the preparation of (4) using L4 instead of L3. Anal. Calcd for C54H50N6O7P2Ag2: C 55.31, H 4.30, N 7.17%; Found: C 55.34, H 4.32, N 7.13%. Main IR (cm-1): 3444m, 3328m, 3049w, 1600m, 1569m, 1521m, 1480m, 1460w, 1440m, 1385s, 1293m, 1167w, 1099m, 819w, 744m, 690m, 513m. Synthesis of [Ag(L4)NO3]n (6). This complex was obtained with a yield of 65% (based on Ag) by the same method used for the preparation of (3) using L4 instead of L1. Anal. Calcd for C18H18N5O3Ag: C 46.98, H 3.94, N 15.22%; Found: C 46.95, H 3.98, N 15.20%. Main IR (cm-1): 3250m, 3021w, 1600s, 1571m, 1530m, 1459m, 1330m, 1159m, 825w. X-ray Crystallographic Measurements. Table 1 provides a summary of the crystal data, data collection and refinement parameters for the complexes 1-6. All diffraction data were collected at 295 K on a RIGAKU RAXIS-RAPID diffractometer with graphite monochromatized Mo-KR (λ ) 0.71073 Å) radiation in ω scan mode. All structures were solved by direct method and difference Fourier syntheses. All nonhydrogen atoms were refined by full-matrix least-squares techniques on F2 with anisotropic thermal parameters. The H atoms on carbon were placed in calculated positions with C-H ) 0.93 Å (aromatic) or 0.97 Å (aliphatic) and amine N-H ) 0.86 Å, and U(H) ) 1.2Ueq (C, N) in the riding model approximation, and the hydroxyl H atom of methanol molecule were located in difference Fourier synthesis maps and refined in the riding model approximation, with O-H distance restraint (0.85(1) Å) and U(H) ) 1.5Ueq (O). The perchlorate anion in complex 2, water molecule in complex 5, and nitrate anions in complexes 4 and 6 are disordered. All calculations were carried out with the SHELXL97 program.21 The CCDC reference numbers are

[Ag(L1)]2(ClO4)2 (1) and [Ag(L2)ClO4]n (2). The formulation of [Ag(L1)]2(ClO4)2 (1) and [Ag(L2)ClO4]n (2) was confirmed by the single-crystal X-ray diffraction method. The X-ray structural analysis revealed the [2 + 2] discrete supramolecular array of 1 and the layer structure of 2. Complex 1 possesses the structure of 2:2 self-assembled metallomacrocycle as shown in Figure 1a. The Ag(I) cation is charge balanced by the perchlorate anion and shows approximately linear geometry with the N-Ag-N angle of 163.54(9)°. Such bending of the N-Ag-N bond should be due to the influence of the weak interaction between the perchlorate anion and Ag(I) cation with the Ag-O separation of 2.908(3) Å being considerably less than the sum of the van der Waals radii (3.24 Å).22 Each L1 ligand with two pyridine units oriented in a convergent fashion (cis conformation) cooperatively coordinates two silver atoms, producing a 24-membered macrometallacyclic ring in which the Ag · · · Ag separation is 4.732(8) Å. The binuclear units are further linked together through intermolecular H-bonding interactions between uncoordinated perchlorate ions and N atoms of benzenebis(methylamine), generating a 1-D chain along the a-axis (Figure 1b) with the shortest adjacent Ag · · · Ag distance being 5.119(8) Å. In complex 2, each L2 ligand with two pyridine units oriented in a divergent fashion (or trans conformation) connects two Ag(I) cations to form an infinite helix-like chain along the b-axis (Figure 2b) with a pitch of 16.2 Å, in which the Ag-O separation is 2.763(8) and 2.867(8) Å, respectively, and the N-Ag-N angle is 167.5(3)° (Figure 2a). Such bending of the N-Ag-N bond is similar to that of complex 1. Adjacent helical chains are connected via the 4-membered Ag2O2 ring to generate an infinite quasi- 2-D layer structure (Figure 2c). The Ag(I) cations in the Ag2O2 units were 3.374(6) Å apart, which is shorter than the summed van der Waals radii of two silver atoms (3.44 Å),23 indicating the existence of some weak Ag · · · Ag interactions.24 Additionally, π-π interactions are observed between opposed pyridine rings and opposed benzene rings, with centroid-centroid distances of 3.807(3) and 3.886(2) Å, respectively. The stability of the 2-D layer structure is further consolidated by the collaboration of weak Ag-O, Ag · · · Ag, and π-π stacking interactions. [Ag(L1)NO3]n (3). Replacing the weakly coordinating perchlorate anion by more strongly coordinating nitrate anion in a similar reaction leads to the formation of the 2-D grids. In complex 3, L1 ligands coordinate Ag(I) cations to generate an infinite helix-like chain along the b-axis (Figure 3b) with two pyridine rings oriented in a divergent fashion which is different from that of complex 1. The pitch of 10.2 Å is relatively smaller than that of complex 2. Each Ag(I) cation is four-coordinated in a distorted tetrahedral geometry by two oxygen atoms of different nitrate anions and two nitrogen atoms of different L1 ligands, and each silver center makes N-Ag-N bond angle of 151.22(15)° (Figure 3a). The coordinating nitrate anions bridge the distinct chains and form a 2-D grid structure. In terms of topology, reducing the dinuclear [Ag2(NO3)2] as four-connected nodes (Figure 3c), while the L1 ligands are viewed as linkers, this layer can be represented topologically as a (4,4) sheet (Figure 3d). [Ag(L3)(PPh3)2]n(NO3)n · nCH3OH (4) and [Ag2(L4)(PPh3)2(NO3)2]n · nH2O (5). X-ray diffraction studies reveal that the structure of complex 4 consists of methanol solvent mol-

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Figure 2. (a) Perspective view of the asymmetric unit complex 2 with 50% ellipsoid probability. (b) Space-filling view of the Ag(I)-ligand helical chain along the b-axis. (c) The 2-D layer structure extended through weak Ag-O, Ag · · · Ag and π-π stacking interactions.

ecules, infinite 1-D helical cationic coordination chains in which the Ag(PPh3)2 fragments are bridged by L3 ligands as well as counterbalanced nitrate anions (Figure 4a). In a chain structure, the geometry of the Ag(I) cation is described as distorted tetrahedral P2N2 coordination, and the bond angles around the Ag(I) cation decrease in the order: P-Ag-P > P-Ag-N > N-Ag-N. Like L1 in complex 3 and L2 in complex 2, each L3 ligand links two adjacent Ag atoms, utilizing its two pyridyl nitrogen atoms in divergent fashion, which results in a helical cationic chain running with the pairs of PPh3 groups alternately directed “left” and “right” along the chain (Figure 4b). It is noteworthy that each uncoordinated nitrate ion accepts two hydrogen bridges from adjacent chains, generating a quasi- 2-D layer structure (Figure 4c). As indicated in Figure 4d, these 2-D layers extend parallel to the crystallographic bc plane and stack along the a-axis to generate 16.38 × 11.84 Å2 1-D paral-

lelogram-like channels, in which the methanol solvent molecules and coordinated PPh3 molecules are located as the guest. Different from complex 4, two types of crystallographically independent silver centers exist in complex 5 (Figure 5a), and there are no significant differences between coordination environments around Ag(I) cations. Two silver atoms Ag1, Ag2 having distorted tetrahedral NO2P environment are linked by two nitrate anions in a η2:µ2 mode and generate a 4-membered Ag2O2 ring with a Ag · · · Ag distance of 4.264(3) Å, which are further coordinated by two PPh3 molecules, forming a [Ph3PAg(µ2NO3)]2 unit. The L4 ligand is also oriented in a trans conformation, and holds the [Ph3PAg(µ2-NO3)]2 units together to generate a 1-D helical chain structure extended along the c-axis (Figure 5b) with the pitch being calculated to be 22.44 Å. [Ag(L4)NO3]n (6). Removing the bulky ligand PPh3 from the system of complex 5 results in the formation of high-

Self-Assembly of Discrete Metallocycle

Crystal Growth & Design, Vol. 8, No. 9, 2008 3281

Figure 3. (a) Perspective view of the asymmetric unit complex 3 with 50% ellipsoid probability. (b) Space-filling view of the Ag(I)-ligand helical chain along the b-axis. (c) Ag polyhedron including the bridging modes of nitrate anions. (d) Schematic representation of the (4,4) topology (black stick, L3).

dimensional complex 6. As shown in Figure 6a, two crystallographically independent “half” L4 ligands occupy different inversions in the middle of the benzene rings, respectively. Each Ag(I) cation displays a distorted tetrahedral geometry, with two oxygen donors from two nitrate anions and two nitrogen donors from two L4 ligands. The N-Ag-N bond angle of 169.64(13)° is significantly lager than that of complex 3, owing to the longer Ag-O distances. Interestingly, the nitrate anions act as a new coordination fashion of η1:η1:µ2 which is different from η2:µ2 of complexes 3, 5 and counteranion of complex 4, and bridge the adjacent Ag(I) cations to generate a -Ag-NO3-Ag-NO3- helix chain along the b-axis with a pitch of 11.78(6) Å (Figure 6b). Simultaneously, L4 ligands serve as trans bridges to link the Ag(I) cations, forming a -Ag-L4-Ag-L4- helix chain along the a-axis with its pitch being 9.28(5) Å (Figure 6c). Combinations of the two kinds of helices give rise to the formation of a 3-D framework structure (Figure 6d). A better insight into the nature of this intricate architecture can be achieved by the application of a topological approach, reducing multidimensional structures to simple node-and-linker nets. As depicted above, each four-coordinated Ag atom can be looked as a tetrahedral node, and each L4 ligand and nitrate ion can be looked as the same linkers. According to the simplification principle, the resulting structure of complex 6 is a 4-connected 3-D framework, and its vertex symbol is (62 · 62 · 62 · 62 · 62 · 62), which is a typical diamond topology, as displayed in Figure 6e. Luminescent Properties. Recently, the luminescent properties of silver complexes containing bis(pyridyl) ligands have been reported.25 The luminescent properties of complexes 1-6 and free ligands L1-L4 in the solid state at room temperature were investigated. Complexes 1-6 and free ligands L1-L4 are all excited at the same excitation wavelength of 243 nm, where

free ligands L1-L4 exhibit emission maximum at 364, 401, 380 and 367 nm, respectively. In contrast to the free ligand L1, red shifts of 8 and 9 nm have appeared in complexes 1 and 3, respectively, while complex 2 exhibits the same emission peak with free ligand L2. Complexes 4 and 5 show emission with maxima at 390 and 368 nm, respectively. Compared with the emission of the free ligand L3, a red shift of 10 nm has been observed upon the formation of complex 4. The peak of 390 nm for complex 6 exhibits a red shift (about 23 nm) with respect to the free ligand L4. The resemblance between the emission spectra of complexes 1-6 and their relative free ligands L1-L4 implies that the emission bands may be tentatively assigned as intraligand (IL) π-π* transitions.26 In contrast to the free ligands, it is found that the intensity of luminescence is lowered in complexes 1-3, while complexes 4-5 exhibit enhanced luminescence properties. These phenomena may be attributed to the influence of the introduced PPh3 molecule on the freedom of flexible ligands. In complexes 4-5, the bulky PPh3 molecules restrain the twisted degrees of flexible ligands (L3, L4) and reduce the loss of energy via radiationless decay of the intraligand emission excited state, which cause the enhancement of fluorescence intensity. Nevertheless, the ligands (L1, L2) in complexes 1-3 are not hindered by the bulky PPh3 molecule, and possess the relatively larger torsion of conformations, which arise from twisted intramolecular charge transfer (TICT)27 and lead to the decrease of fluorescence intensity. Conclusions In summary, a family of six new luminescent silver complexes 1-6 based on four flexible bis(pyridyl) ligands L1-L4 has been prepared. These ligands can exhibit the formation of either discrete complexes of the metallomacrocycle type for the

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Zhu et al.

Figure 4. (a) Molecular structure of complex 4 with 50% ellipsoid probability. (b) Space-filling view of the helical chain structure with nitrate anions, PPh3 molecules, methanol molecules were omitted for clarity. (c) 2-D layer structure extended through H-bonding interactions. (d) Illustration of the channels along the a-axis, with nitrate anions, PPh3 molecules, methanol molecules omitted for clarity.

Figure 5. (a) Molecular structure of complex 5 with 50% ellipsoid probability. (b) Space-filling view of the helical chain structure extended along the c-axis with the disorder water molecules and PPh3 molecules were omitted for clarity.

convergent disposition of the coordination sites (complex 1) or infinite coordination networks for the divergent orientation of

the monodentate sites (complexes 2-6), owing to the flexible nature of their spacer. Moreover, the structures of these silver

Self-Assembly of Discrete Metallocycle

Crystal Growth & Design, Vol. 8, No. 9, 2008 3283

Figure 6. (a) Molecular structure of complex 6 with 50% ellipsoid probability. (b) View of the Ag(I)-nitrate anion helical chain along the b-axis. (c) View of the Ag(I)-ligand helical chain along the a-axis. (d) Schematic representation of the 3-D network: blue, Ag(I)-nitrate anion helical chain; green and red, Ag(I)-ligand helical chains along different directions. (e) Topological representation of complex 6 showing the diamond topology (turquoise ball, silver atoms; purple stick, nitrate anions; bright green stick, L4).

complexes are also profoundly influenced by the anions: from simple 0-D macrocyclic dinuclear, various 1-D helical chains, 2-D (4,4) topology networks to classic 3-D noninterpenetrating diamond frameworks. The bulkier ClO4- anion in complexes 1 and 2, for its weakly coordinating function, can only act as counteranion and generate lower-dimensional structure. On the contrary, the smaller nitrate anion, owing to its stronger coordination and various coordination modes, can afford comparatively higher-dimensional structures. The dissociative nitrate anion facilities the formation of the 1-D chain structure of complex 4, while the η2:µ2 and η1:η1:µ2 fashion lead to the 1-D chain structure for complex 5, 2-D grid for complex 3, and 3-D framework for complex 6. Meanwhile, the presence of PPh3 prevents the 1-D chain from being further extended into a higher-dimensional network by covalent bonding (e.g., 2-D or 3-D), which may be attributed to the larger steric hindrance of terminal ligand PPh3 in complexes 4 and 5. This is in accord with the fact that transformation of polymeric silver(I) complexes composed of N-heterocyclic ligands to their tertiary phosphine derivatives has resulted in the formation of monomers, oligomers, or helical polymers.28 This study clearly indicates that the flexibility of their spacer, as well as the coordination ability, mode and bulk of the anions play important roles in crystal engineering, and the ligands can adopt various conformations to maximize the intra- and intermolecular forces. Acknowledgment. This work is supported by Natural Science Foundation of Heilongjiang Province (No. B200501) and the Scientific Fund for Remarkable Teachers of Heilongjiang Province

(No. 1054 G036). We thank the University of Heilongjiang and the University of Malaya for supporting this study. Supporting Information Available: X-ray crystallographic files in CIF format of complexes 1-6, and the other materials are available free of charge via the Internet at http://pubs.acs.org.

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