Solvent Control of Supramolecular Architectures Derived from 4,4

DOI: 10.1021/cg900552b

Solvent Control of Supramolecular Architectures Derived from 4,40 -Bipyridyl-Bridged Copper(II) Dipicolinate Complexes

2009, Vol. 9 4685–4699

Marina Felloni, Alexander J. Blake, Peter Hubberstey,* Claire Wilson, and Martin Schro¨der* School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. Received May 22, 2009; Revised Manuscript Received July 20, 2009

ABSTRACT: Treatment of [Cu(dipic)(OH2)3] [dipic2 = pyridine-2,6-dicarboxylate (dipicolinate)] 1 with diverse bipyridyl bridging ligands of varying length [pyrazine (pyz), 4,40 -bipyridine (bipy), trans-4,40 -azobis(pyridine) (azpy), 1,2-bis(pyridin4-yl)ethene (bpe), 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazene (pytz) and 1,4-bis{2-(pyridin-4-yl)ethenyl}benzene (bpeb)] under a variety of conditions yielded [Cu2(dipic)2(bipy)] 3 4H2O, 2, [Cu2(dipic)2(bpe)] 3 2H2O, 3, [{Cu(dipic)(OH2)}(μ-pyz)0.5] 3 H2O, 4, [{Cu(dipic)(OH2)}(μ-pyz)0.5], 5, [{Cu(dipic)(OH2)}(μ-bipy)0.5][{Cu(dipic)(OH2)0.75(OHMe)0.25}(μ-bipy){Cu(dipic)(OH2)}] 3 2.25H2O 3 0.5CH3OH, 6, {[{Cu(dipic)}2(μ-bipy)] 3 2H2O 3 CH2Cl2}¥, 7, [{Cu(dipic)(OH2)}(μ-bpe){Cu(dipic)}] 3 3H2O, 8, {[{Cu(dipic)}(μ-azpy)0.5] 3 CH2Cl2}¥, 9, {[{Cu(dipic)}(μ-azpy)0.5] 3 CH3OH}¥, 10, [{Cu(dipic)(OH2)}2(μ-pytz)] 3 2H2O, 11, [Cu2(dipic)2(bpeb)] 3 4H2O, 12 and [{Cu(dipic)(OHMe)}2(μ-bpeb)], 13. Complexes 4-11 and 13 were characterized by single crystal X-ray diffraction which confirmed the presence of binuclear building blocks in which two square-pyramidal Cu(II) centers are linked by bipyridyl bridges. The differences in the structures lie in the choice of ligand located at the apical site of the Cu(II) center, the basal sites being occupied by one N- and two O-donors of the mer-bound dipicolinate ligand and an N-donor from the bridging heterocyclic ligand. In the presence of excess coordinating solvent such as H2O or MeOH, recrystallization affords products with the apical sites occupied by solvent molecules to give extensive hydrogen-bonding networks within 3-D matrices in 4-6, 8, 11, 13. With reduced levels of coordinating solvent in the crystallizing medium, the apical sites are occupied by carboxylate oxygens of adjacent [Cu(dipic)] moieties giving 2-D coordination polymers of 63 topology as in 7, 9, 10.

*Corresponding authors. E-mail: [email protected], [email protected].

The d9 configuration of the Cu(II) cation favors either a square-planar (4-coordinate) or a square-pyramidal (5-coordinate) geometry. Tetragonally elongated octahedral (6-coordinate) geometry is also known. However, as the axially located ligands (L) exhibit long Cu 3 3 3 L separations and can be considered to be weakly bound, the coordination geometry is essentially square-planar. Coordination of the tridentate dipicolinate anion to Cu(II) cations would thus be expected to involve three of the coplanar binding sites of the square-planar and square-pyramidal geometries, leaving the fourth coplanar binding site and the apical position of the latter geometry free for development of extended crystal structures.11 For square-planar Cu(II) geometries, N,N0 -donor bipyridyl bridging ligands would be expected to occupy the fourth binding site via binding of heterocyclic N-donors, thereby generating binuclear molecules (Scheme 2). Linking these binuclear tectons into extended structures can thus be achieved by supramolecular π-π interactions between adjacent dipicolinate anions [Scheme 3(a)] or by hydrogen-bonding, for example via O-H 3 3 3 O contacts between solvent donor O-H groups and carboxylate oxygen acceptors [Scheme 3(b)]. For square-pyramidal Cu(II) geometries, however, coordinatively linked extended structures can also be envisaged involving Cu 3 3 3 O contacts between Cu(II) centers and apically located carboxylate oxygens of adjacent [Cu(dipic)] moieties [Scheme 4(a)]. An alternative mode of association would involve hydrogen-bonding interactions between apically located coordinated solvent molecules and the carboxylate oxygens of adjacent [Cu(dipic)] centers [Scheme 4(b)].

r 2009 American Chemical Society

Published on Web 10/19/2009

Introduction Porous transition metal-based network structures are of current interest owing to the need to develop new materials for emerging technologies and economies. Although hydrogenbonding1-3 and π-π stacking3-5 supramolecular contacts are significant in coordination polymer design and synthesis, transition metal-based network structures continue to rely primarily on coordinative interactions.3,6-9 Certain coordination geometries may dictate network architectures with linear, square-planar and tetrahedral building blocks leading to 1-D chain, 2-D sheet and 3-D network constructions, respectively. Most frameworks prepared thus far are based on cationic metal centers linked by neutral organic bridging ligands with noncoordinated anions and solvent molecules occupying structural cavities and channels.3,6-9 The use of coordinating anions provides the opportunity for controlling the available space more effectively. Considerable progress in metal-organic framework construction has been achieved using anionic bridging ligands such as aromatic multicarboxylates to link metal centers.8,9 However, less effort has been applied using nonbridging chelating anions in conjunction with neutral bridging ligands.10 Consequently, we have embarked on a project to investigate the potential of [Cu(dipic)] (dipic2- = pyridine-2,6-dicarboxylate or dipicolinate) in coordination polymer construction11 with a variety of neutral N,N0 -donor bridging molecules (Scheme 1) ranging in length from pyrazine (N 3 3 3 N 2.77 A˚) to 1,4-bis{2-(pyridin-4-yl)ethenyl}benzene (N 3 3 3 N 16.06 A˚).

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Scheme 1. View of the N,N0 -Donor Bipyridyl Bridging Ligands Used in This Work (Typical N 3 3 3 N Separations Are Given in Parentheses)

Felloni et al. Scheme 4. View of the Supramolecular Interactions for Elaboration of N,N0 -Donor Bipyridyl-Bridged Binuclear Complexes Based on Square-Pyramidal Cu(II) Centers: (a) Coordinative and (b) Hydrogen-Bonded through Coordinated Solvent Molecules

Scheme 5. View of the [Cu(dipic)(OH2)3] Precursor Molecule

Scheme 2. View of the Binuclear Molecule Expected from Coordination of Square-Planar Cu(II) by Dipicolinate Anion and N,N0 -Donor Bipyridyl Bridging Ligand

hydrogen-bonding potential (e.g., water, MeOH, CHCl3, CH2Cl2, toluene and nitrobenzene) upon the reaction of the precursor molecule [Cu(dipic)(OH2)3] (1, Scheme 5) and diverse heterocyclic bridging ligands. The range of molecular rods used (Scheme 1) also allows us to assess the role of bridge length on the construction of the resultant extended structures. Experimental Section

Scheme 3. View of the Potential Supramolecular Interactions for the Elaboration of N,N0 -Donor Bipyridyl Bridged Binuclear Molecules Based on Square-Planar Cu(II) Centers: (a) π-π Stacking and (b) Hydrogen-Bonding through Lattice Solvent Molecules

To assess the influence of protonated solvents on the crystal engineering of extended architectures formed by bipyridylbridged binuclear [Cu(dipic)] complexes, we have investigated the effect of solvents of varying coordinating ability and

With the exception of trans-4,40 -azobis(pyridine),12 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine13 and 1,4-bis{2-(pyridin-4-yl)ethenyl}benzene,14 which were prepared following literature procedures, all chemicals were bought from Aldrich and used as received. Elemental analysis (CHN) was performed by the Nottingham University School of Chemistry Microanalytical Service using a Perkin-Elmer 240B instrument. Infrared spectra were obtained (as KBr pressed pellets) using a Nicolet Avatar 360 FTIR spectrometer. The IR spectra of the bulk samples confirmed the presence of both dipicolinate anion and bridging ligand. Synthesis of [Cu(dipic)(OH2)3], 1. A solution of dipicolinic acid (0.167 g, 1 10-3 mol) in hot water (15 cm3) was neutralized with Na2CO3 (0.106 g, 1 10-3 mol) to afford sodium dipicolinate [Na2(C7H3NO4)] as a white powder in quantitative yield. A solution of CuCl2 3 2H2O (0.170 g, 1 10-3 mol) in hot water (15 cm3) was reacted with a solution of sodium dipicolinate (0.211 g, 1 10-3 mol) in hot water (15 cm3) and the resulting mixture heated to reflux for 1 h. Slow cooling of the solution to room temperature gave a microcrystalline sample of 1, which was recrystallized under hydrothermal conditions at 453 K. Anal. Found (calculated for C7H9CuNO7): C 29.50 (29.75); H 3.15 (3.20); N 4.70 (4.95). IR cm-1: 3441 s, 3060 w, 1685 s, 1662-1432 s, 1362 s, 808 w, 774 w. Synthesis of [Cu2(dipic)2(bipy)] 3 4H2O, 2. A solution of 1 (0.564 g, 2 10-3 mol) in boiling water (60 cm3) was mixed with a solution of 4,40 -bipy (0.156 g, 1 10-3 mol) in boiling water (40 cm3), and the resulting mixture was heated to reflux for 1 h. Slow cooling of the solution to room temperature gave 2 as a blue powder. Anal. Found (calculated for C12H11CuN2O6): C 42.75 (42.10); H 2.75 (3.20); N 8.35 (8.20). IR cm-1: 3386 s, 3078 w, 1670 s, 1636-1423 s, 1345 s, 1229 w, 1179 w, 1084 w, 833 s, 782 w.

Article Compound 2 was also formed when MeOH, rather than water, was used as solvent. Anal. Found (calculated for C12H11CuN2O6): C 42.30 (42.10); H 2.80 (3.20); N 8.20 (8.20). IR cm-1: 3424 s, 3080 w, 1660 s, 1636-1423 s, 1342 s, 1228 w, 1178 w, 1084 w, 833 s, 782 w. Although all attempts to recrystallize 2 proved to be unsuccessful, its empirical formula corresponds to that of the previously structurally characterized [{Cu(dipic)(OH2)}3(μ-bipy)1.5] 3 3H2O,15 which is an analogue of complex 6. Synthesis of [Cu2(dipic)2(bpe)] 3 2H2O, 3. A solution of 1 (0.564 g, 2 10-3 mol) in hot water (60 cm3) was mixed with a solution of bpe (0.182 g, 1 10-3 mol) in hot water (40 cm3) and the resulting mixture heated to reflux for 1 h. Slow cooling of the solution to room temperature gave 3 as a blue powder. Anal. Found (calculated for C13H10CuN2O5): C 46.20 (46.22); H 2.95 (2.98); N 8.30 (8.29). IR cm-1: 3417 s, 3071 w, 1670 s, 1641-1431 s, 1344 s, 1211 w, 1179 w, 1079 w, 1038 w, 851 s, 780 w. All attempts to recrystallize 3 proved to be unsuccessful. Synthesis of [{Cu(dipic)(OH2)}(μ-pyz)0.5] 3 H2O, 4. A solution of pyrazine (0.040 g, 5 10-4 mol) in MeOH (8 cm3) was carefully layered on top of a solution of 1 (0.282 g, 10-3 mol) in water (12 cm3). Blue needles of 4 were formed after 7 days. Anal. Found (calculated for C18H18Cu2N4O12): C 35.30 (35.45); H 2.95 (2.95); N 9.00 (9.20). IR cm-1 3391 s, 3082 w, 1625 s, 1595-1427 s, 1368 s, 809 w, 741 w. Using a metal:bipyridyl ligand molar ratio of 1:1 (pyrazine, 0.040 g, 5 10-4 mol in 6 cm3 MeOH; 1, 0.141 g, 5 10-4 mol in 14 cm3 of water) under the same reaction conditions gave the same product. Compound 4 was also obtained by layering a solution of pyrazine (0.040 g, 5 10-4 mol) in MeOH (8 cm3) on top of chloroform (12 cm3) covering solid 1 (0.141 g, 5 10-4 mol) (metal: bipyridyl ligand molar ratio 1:1). Blue needles with the same unit cell parameters as those of 4 were formed after 7 days. The room temperature (298 K) single crystal structure of this material has been reported.16 Nonetheless, X-ray diffraction data were collected at 150 K. Synthesis of [{Cu(dipic)(OH2)}(μ-pyz)0.5], 5. Solid 1 (0.282 g, 1 10-3 mol) was covered with toluene (12 cm3), and a solution of pyrazine (0.040 g, 5 10-4 mol) in MeOH (8 cm3) was carefully layered on top of the toluene solution. Blue needles of 4 were formed after 7 days. Anal. Found (calculated for C18H14Cu2N4O10): C 37.75 (37.70); H 2.40 (2.45); N 9.60 (9.75). IR cm-1 3358 s, 3085 w, 1635 s, 1596-1420 s, 1342 s, 771 w, 739 w. Synthesis of [{Cu(dipic)(OH2)}(μ-bipy)0.5][{Cu(dipic)(OH2)0.75(OHCH3)0.25}(μ-bipy){Cu(dipic)(OH2)}] 3 2.25H2O 3 0.5CH3OH, 6. A solution of bipy (0.078 g, 5 10-4 mol) in MeOH (8 cm3) was carefully layered on top of a solution of 1 (0.141 g, 5 10-4 mol) in water (12 cm3). Blue blocks of 6 were formed after 7 days. Anal. Found (calculated for C36.75H34Cu3N6O17.75): C 42.55 (42.65); H 2.80 (3.30); N 8.05 (8.10). IR cm-1: 3440 s, 3079 w, 1636 s, 1617-1423 s, 1342 s, 782 w, 737 w. Following a single crystal structural analysis, it was realized that the structure of 6, which has a very similar empirical formula to 2, is the same species as that reported previously on four separate occasions15 and obtained from aqueous solutions containing both 1 and 4,40 -bipyridine. Synthesis of {[{Cu(dipic)}2(μ-bipy)] 3 2H2O 3 CH2Cl2}¥, 7. Solid 1 (0.282 g, 1 10-3 mol) was covered with CH2Cl2 (12 cm3), and a solution of bipy (0.078 g, 5 10-4 mol) in MeOH (8 cm3) was carefully layered on top of the halogenated solvent. Blue tablets of 7 were formed after 20 days. Anal. Found (calculated for C25H20Cu2Cl2N4O10): C 41.30 (40.88); H 2.65 (2.75); N 7.85 (7.63). IR cm-1: 3442 s, 3089 w, 1645 s, 1615-1421 s, 1350 s, 775 w, 738 w. Synthesis of [{Cu(dipic)(OH2)}(μ-bpe){Cu(dipic)}] 3 3H2O, 8. A solution of bpe (0.0055 g, 3 10-5 mol) in MeOH (10 cm3) was carefully layered on top of a solution of 1 (0.017 g, 6 10-5 mol) in water (10 cm3). Blue tablets of 8 were formed after 7 days. Anal. Found (calculated for C26H22Cu2N4O12): C 43.90 (43.95); H 2.90 (3.10); N 8.15 (7.90). IR cm-1: 3400 s, 3082 w, 1636 s, 1596-1431 s, 1360 s, 781 w, 737 w. Synthesis of {[{Cu(dipic)}(μ-azpy)0.5] 3 CH2Cl2}¥, 9. Solid 1 (0.282 g, 1 10-3 mol) was covered with CH2Cl2 (7 cm3), and a solution of azpy (0.092 g, 5 10-4 mol) in MeOH (8 cm3) was carefully layered on top. Slow diffusion of the layered solvents afforded green plates of 9 after 10 days. Anal. Found (calculated for C13H9CuCl2N3O4): C 39.05 (38.49); H 2.50 (2.24); N 11.10 (10.36).


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IR cm-1: 3423 s, 3060 w, 1665 s, 1637-1427 s, 1382 s, 1274 w, 1181 w, 1083 w, 1055 w, 854 w, 775w. Synthesis of {[{Cu(dipic)}(μ-azpy)0.5] 3 CH3OH}¥, 10. Solid 1 (0.07 g, 2.5 10-4 mol) was covered with toluene (7 cm3), and a solution of azpy (0.023 g, 1.25 10-4 mol) in MeOH (13 cm3) was carefully layered on top of the toluene solution. Green plates of 10 were formed after 7 days. Anal. Found (calculated for C13H11CuN3O5): C 44.20 (44.20); H 3.30 (3.10); N 11.70 (11.90). IR cm-1 3423 s, 3082 w, 1645 s, 1589-1427 s, 1342 s, 774 w, 736 w. Compound 10 was also obtained using nitrobenzene instead of toluene, a result confirmed by comparison of unit cell parameters. Synthesis of [{Cu(dipic)(OH2)}2(μ-pytz)] 3 2H2O, 11. A solution of pytz (0.059 g, 2.5 10-4 mol) in MeOH (10 cm3) was carefully layered on top of a solution of 1 (0.141 g, 510-4 mol) in water (10 cm3). Purple plates of 11 were formed after 7 days. Anal. Found (calculated for C13H11CuN4O6): C 40.80 (40.75); H 2.75 (2.85); N 14.40 (14.65). IR cm-1 3434 s, 3078 w, 1641 s, 1594-1429 s, 1361 s, 782 w, 744 w. Synthesis of [Cu2(dipic)2(bpeb)] 3 4H2O, 12. A solution of bpeb (0.017 g, 6.25 10-5 mol) in MeOH (8 cm3) was layered on top of a solution of 1 (0.035 g, 1.25 10-4 mol) in water (12 cm3). Slow diffusion of the layered solvents afforded a green powder as the only product of the reaction. Anal. Found (calculated for C17H15CuN2O6): C 50.80 (50.18); H 3.35 (3.72); N 6.75 (6.89). IR cm-1: 3400 s, 3070 w, 1640 s, 1612-1433 s, 1345 s, 1203 w, 1179 w, 1087 w, 1038 w, 839 s, 778 w. Synthesis of [{Cu(dipic)(OHCH3)}2(μ-bpeb)], 13. Solid 1 (0.008 g, 2.80 10-5 mol) was covered with CH2Cl2 (9 cm3) and a solution of bpeb (0.004 g, 1.40 10-5 mol) in MeOH (11 cm3) was carefully layered on top of the halogenated solvent. Green laths of 13 were formed after 10 days. Anal. Found (calculated for C18H15CuN2O5): C 53.25 (53.60); H 3.50 (3.70); N 6.85 (6.95). IR cm-1 3426 s, 3075 w, 1639 s, 1605-1433 s, 1343 s, 779 w, 737 w. Crystallography. Single crystal X-ray diffraction data were collected at either 120 K (8, 11, 13) or 150 K (4-7, 9, 10) using either a Stoe Stadi-4 four-circle diffractometer (6), Enraf-Nonius KappaCCD (8, 11, 13), Bruker SMART 1000 CCD (4, 7, 9, 10) or Bruker SMART APEX CCD (5) area detector diffractometers. All instruments were equipped with Oxford Cryosystems open flow cryostats.17 Radiation used was graphite monochromated Mo KR radiation (λ = 0.71073 A˚). Pertinent details of crystal data, data collection and processing are given in Table 1. The structures were solved by direct methods using either SHELXS9718 (4-8, 10, 13) or SIR9219 (9, 11) and refined by full-matrix least-squares on F2 using SHELXL97.20 Wherever possible, hydrogen atoms on solvent molecules (H2O in 4, 5, 8 and 11 and MeOH in 13) were located in Fourier difference syntheses and refined with restraints [O-H = 0.82(1) A˚; Uiso(H) = 1.5Ueq(O)]. All other hydrogen atoms on solvent molecules (H2O in 6 and 7, CH2Cl2 in 7 and 9 and MeOH in 6 and 10) were omitted. Compound 7 crystallized in the noncentrosymmetric space group P21, and the Flack parameter refined to a value of 0.00(2). Compounds 6, 7, 9 and 10 exhibited disorder. In 6, one of the axially coordinated solvent molecules was best modeled as a H2O/ MeOH mixture (occupancy 75:25). The H2O molecule was hydrogenbonded to a lattice H2O molecule modeled over three positions (occupancy 35:20:20). A second lattice H2O molecule was modeled over three positions (occupancy 40:35:25) while a third solvent molecule was best modeled as a H2O/MeOH mixture (occupancy 70:30). In 7, two lattice H2O molecules were modeled over two (occupancy 50:50) or three (occupancy 50:25:25) positions. In 9, the two chlorine atoms of the lattice CH2Cl2 molecule were modeled over five positions (occupancy 85:45:27.5:27.5:15). In 10, a MeOH lattice molecule was modeled over three positions (occupancy 60:20:20). All fully occupied non-hydrogen atoms and the chlorine atoms of the disordered CH2Cl2 molecule in 9 with occupancy 27% or greater were refined with anisotropic displacement parameters. All other partially occupied non-hydrogen atoms were refined with isotropic displacement parameters. All structure diagrams were produced using CAMERON.21

Results and Discussion Treatment of hydrated copper(II) chloride with sodium dipicolinate, previously prepared by neutralization of

properties

Z T/K Fcalcd/g cm-3 cryst size/mm3 μ/mm-1 reflections measured unique, Rint obsd [F g 4σ(F)] no. of params R1 [F g 4σ(F)] wR2 [all data] goodness-of-fit ΔF (min, max)/e A˚-3

R/deg β/deg γ/deg V/A˚3

formula M cryst syst space group a/A˚ b/A˚ c/A˚ 92.087(4) 950.3(6) 2 150(2) 2.006 0.50 0.05 0.04 2.313 5868 2416, 0.076 1327 162 0.0412 0.0747 0.82 0.81, -0.50

92.540(1)

1053.3(2) 2 150(2) 1.922 0.45 0.36 0.14 2.098

6473 2525, 0.022 2189 175 0.0263 0.0763 1.03 0.46, -0.44

5 C18H14Cu2N4O10 573.41 monoclinic P21/n 7.810(2) 7.316(2) 16.644(4)

4

C18H18Cu2N4O12 609.44 monoclinic P21/n 5.7518(5) 15.3395(12) 11.9495(10)

6

8732 7469, 0.166 4566 581 0.0969 0.267 1.20 2.68, -1.24

C36.75H34Cu3N6O17.75 1034.32 triclinic P1 10.652(2) 10.765(2) 18.690(4) 84.13(3) 86.84(3) 62.61(3) 1892.8(7) 2 150(2) 1.815 0.23 0.23 0.18 1.762 8862 5840, 0.025 5050 390 0.0571 0.175 1.07 1.39, -0.84

1398.6(2) 2 150(2) 1.744 0.40 0.21 0.11 1.776

111.075(2)

C25H20Cl2Cu2N4O10 734.43 monoclinic P21 15.9467(12) 5.6327(4) 16.6869(18)

7

22522 5891, 0.065 4211 416 0.0470 0.126 1.05 0.60, -0.65

C26H22Cu2N4O12 709.56 triclinic P1 7.2105(14) 10.689(2) 18.039(4) 87.67(3) 86.66(3) 71.16(3) 1313.2(4) 2 120(2) 1.794 0.06 0.05 0.02 1.697

8

9

10656 3875, 0.159 1651 225 0.0949 0.266 0.99 1.59, -2.16

1562.6(13) 4 150(2) 1.724 0.20 0.09 0.03 1.761

113.542(7)

C13H9Cl2CuN3O4 405.67 monoclinic P21/c 16.294(7) 5.669(3) 18.452(8)

Table 1. Crystal Data for Compounds 4-8, 9-11 and 13 10

13571 3958, 0.025 2623 205 0.0418 0.121 1.07 1.40, -0.43

1531.3(2) 4 150(2) 1.530 0.37 0.24 0.06 1.452

110.126(1)

C13H11CuN3O5 352.79 monoclinic P21/c 15.425(1) 6.2317(4) 16.967(1)

11

10171 3265, 0.064 2312 229 0.0488 0.137 1.03 0.93, -0.83

1395.1(1) 4 120(2) 1.823 0.12 0.08 0.025 1.608

103.451(2)

C13H11CuN4O6 382.81 monoclinic P21/c 10.8029(5) 18.2700(7) 7.2679(3)

13

10046 3353, 0.141 1714 239 0.0688 0.167 1.01 0.55, -0.88

C18H15CuN2O5 402.86 triclinic P1 5.3171(4) 8.2913(5) 19.030(2) 97.062(2) 92.892(3) 106.014(5) 797.15(10) 2 120(2) 1.678 0.15 0.025 0.006 1.404

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Table 2. Results of Diffusion Experiments of 1 with Various Bridging Ligands bridging ligand

preparative route

pyz pyz pyz pyz bipy

MeOH/H2Oa MeOH/CH2Cl2b MeOH/CHCl3c MeOH/toluened MeOH/H2Oa

bipy bipy bpe bpe azpy azpy azpy azpy pytz pytz bpeb bpeb

MeOH/CH2Cl2b MeOH/toluened MeOH/H2Oa MeOH/CH2Cl2b MeOH/H2Oa MeOH/CH2Cl2b MeOH/toluened MeOH/nitrobenzenee MeOH/H2Oa MeOH/CH2Cl2b MeOH/H2Oa MeOH/CH2Cl2b

product [{Cu(dipic)(OH2)}(μ-pyz)0.5] 3 H2O no product [{Cu(dipic)(OH2)}(μ-pyz)0.5] 3 H2O [{Cu(dipic)(OH2)}(μ-pyz)0.5] [{Cu(dipic)(OH2)}(μ-bipy)0.5][{Cu(dipic)(OH2)0.75(OHMe)0.25}(μ-bipy){Cu(dipic)(OH2)}] 3 2.25H2O 3 0.5CH3OH {[{Cu(dipic)}2(μ-bipy)] 3 2H2O 3 CH2Cl2}¥ no product [{Cu(dipic)(OH2)}(μ-bpe){Cu(dipic)}] 3 3H2O no product no product {[{Cu(dipic)}(μ-azpy)0.5] 3 CH2Cl2}¥ {[{Cu(dipic)}(μ-azpy)0.5] 3 CH3OH}¥ {[{Cu(dipic)}(μ-azpy)0.5] 3 CH3OH}¥ [{Cu(dipic)(OH2)}2(μ-pytz)] 3 2H2O no product [Cu2(dipic)2(bpeb)] 3 4H2O [{Cu(dipic)(OHMe)}2(μ-bpeb)]

code 4 4 5 6 7 8 9 10 10 11 12 13

a MeOH/H2O: a MeOH solution of the bridging ligand was layered on top of an aqueous solution of 1. b MeOH/CH2Cl2: a MeOH solution of the bridging ligand was layered on top of CH2Cl2 covering solid 1. c MeOH/CHCl3: a MeOH solution of the bridging ligand was layered on top of CHCl3 covering solid 1. d MeOH/toluene: a MeOH solution of the bridging ligand was layered on top of toluene covering solid 1. e MeOH/nitrobenzene: a MeOH solution of the bridging ligand was layered on top of nitrobenzene covering solid 1.

Figure 1. View of the square-pyramidal coordination geometries of the Cu(II) centers in (a) 11, (b) 13 and (c) 10 [atom identification: copper, blue spheres; carbon, gray spheres; nitrogen, blue spheres; oxygen, red spheres; hydrogen, yellow spheres].

dipicolinic acid with sodium carbonate, in water under reflux gave a microcrystalline sample of [Cu(dipic)(OH2)3], 1. Recrystallization gave a quantitative crop of blue crystals, which were characterized using single crystal X-ray diffraction methods.22 Initial attempts to produce bipyridyl-bridged binuclear Cu(II) dipicolinates involved heating to reflux aqueous or methanolic solutions containing dissolved 1 and

dissolved bridging ligand (bipy or bpe) followed by slow cooling to room temperature. Blue powders, which analyzed for [Cu2(dipic)2(bipy)] 3 4H2O, 2, and [Cu2(dipic)2(bpe)] 3 4H2O, 3, were the only isolated products of the reaction, and all attempts to crystallize 2 and 3 were unsuccessful. Subsequently, two variants of diffusion experiments were used to generate crystalline samples of the desired products.

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Table 3. Interatomic Distances for the Square-Based Pyramidal Cu(II) Coordination Spheres in 4-11 and 13 (a) Interatomic Distances in 4-6, 11 and 13a Cu-N1 (dipic) Cu-O1 (dipic) Cu-O2 (dipic) Cu-N2 (bridge) Cu-O3 (axial)

4

5

6 Cu(1)

6 Cu(2)

6 Cu(3)

11

13

1.900(2) 2.010(2) 2.0272(12) 1.964(2) 2.197(2)

1.897(3) 2.010(3) 2.026(3) 1.968(3) 2.194(3)

1.902(9) 2.002(8) 2.032(7) 1.974(9) 2.332(8)

1.897(8) 2.030(8) 2.031(7) 1.944(8) 2.286(9)

1.922(8) 2.009(7) 2.048(7) 1.972(8) 2.310(7)

1.908(3) 1.997(2) 2.049(2) 1.948(3) 2.280(3)

1.905(4) 2.000(4) 2.033(4) 1.951(5) 2.363(4)

(b) Pertinent Interatomic Distances in 7-10b Cu-N1 (dipic) Cu-O1 (dipic) Cu-O2 (dipic) Cu-N2 (bridge) Cu-O3 (axial)

7 Cu(1)

7 Cu(2)

8 Cu(1)

8 Cu(2)

9

10

1.900(5) 2.016(5) 2.028(4) 1.975(5) 2.239(4)

1.911(5) 2.036(5) 2.057(5) 1.960(5) 2.209(5)

1.897(3) 1.982(2) 2.058(2) 1.952(3) 2.357(3)

1.908(3) 2.037(2) 2.038(3) 1.946(3) 2.328(3)

1.926(8) 2.041(7) 2.064(6) 1.995(8) 2.275(6)

1.907(2) 2.020(2) 2.039(2) 1.968(3) 2.203(2)

a In 4-6 and 11 the axial positions are occupied by H2O molecules; in 13 they are occupied by MeOH molecules. b Whereas in 7, 9 and 10 the axial positions are occupied by carboxylate oxygens, in 8 that of Cu(1) is occupied by a H2O molecule and Cu(2) by a carboxylate oxygen.

In the first, a methanolic solution of the bridging ligand (Scheme 1) was layered on top of an aqueous solution of 1. In the second, solid 1 was covered by dichloromethane, chloroform, toluene or nitrobenzene and a methanolic solution of the bridging ligand carefully layered on top. The results of these experiments are summarized in Table 2. Initial characterization using IR spectroscopy and elemental analytical data confirmed these to comprise ten different materials, the formulations of which are given in Table 2. Single crystals suitable for X-ray study were obtained for nine of the products, namely, 4-11 and 13. The structures of 416 and 615 have been described previously. Nonetheless, for comparative purposes they are included in the ensuing discussion. Coordination Geometries of the Cu(II) Centers. The cationic centers in all nine structurally characterized compounds adopt five-coordinate square-pyramidal geometries. They all comprise mer-oriented [Cu(dipic)] fragments with an N-donor of a bridging 4-pyridyl moiety located in the fourth basal position and, depending on the solvent mixture from which the material is crystallized, an oxygen atom from either a water molecule [4-6, 8, 11; Figure 1(a)], a methanol molecule [13; Figure 1(b)] or an adjacent [Cu(dipic)] moiety [7-10; Figure 1(c)] at the apical position. Selected interatomic distances are collected in Tables 3(a) and 3(b) for the solvated and nonsolvated Cu(II) coordination geometries, respectively. For both systems, the data are typical of squarepyramidal geometries with the apically located oxygen atom more remote [Cu-O = 2.194(3)-2.363(4) A˚] than the basally located nitrogen [Cu-N(dipic) = 1.897(3)-1.926(8) A˚, CuN(bridge) =1.944(8)-1.995(8) A˚] and oxygen atoms [Cu-O= 1.982(2)-2.064(6) A˚]. For the solvated compounds (4-6, 8, 11, 13), the tridentate dipicolinate anion binds asymmetrically, the slightly more remote oxygen acting as an acceptor to a hydrogen bond from either water or methanol [e.g., for 11, Cu-O (non-hydrogen-bonded) = 1.997(2) A˚, Cu-O (hydrogen-bonded) = 2.049(2) A˚; Table 3(a)]. A similar but less pronounced asymmetry is observed in the nonsolvated structures (7-10), the longer Cu-O contact being to the oxygen of the bridging carboxylate moiety [e.g., for 10, Cu-O (non-hydrogen-bonded) = 2.020(2) A˚, Cu-O (hydrogenbonded)=2.039(2) A˚; Table 3(b)]. The square-pyramidal Cu(II) centers are linked through N,N0 -donor bipyridyl bridges to give a binuclear building block analogous to that identified in Scheme 2. A typical

Figure 2. View of the molecular structure of 4 showing the connectivity within the binuclear building block. Table 4. N 3 3 3 N and Cu 3 3 3 Cu Separations in the Binuclear Building Blocks of 4-11 and 13 N,N0 -bipyridyl linker

4 5 6 6 7 8 9 10 11 13

N 3 3 3 N/A˚ Cu 3 3 3 Cu/A˚ pyrazine 2.761(2) 6.689(1) pyrazine 2.782(4) 6.709(2) 4,40 -bipyridine (non-centrosymmetric) 7.080(12) 10.987(5) 0 7.081(12) 11.025(5) 4,4 -bipyridine (centrosymmetric) 7.070(8) 10.987(3) 4,40 -bipyridine trans-1,2-bis(pyridin-4-yl)ethene 9.383(4) 13.263(2) 9.058(11) 12.980(3) trans-4,40 -azobis(pyridine) 0 9.008(11) 12.926(1) trans-4,4 -azobis(pyridine) 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine 11.027(4) 14.914(1) 3,6-bis[2-(pyridin-4-yl)ethen-1-yl]benzene 16.062(6) 19.929(2)

example, that in 4, is shown in Figure 2. The N,N0 -donor bipyridyl bridges vary considerably in length from pyrazine through 4,40 -bipyridine and 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine to 3,6-bis[2-(pyridin-4-yl)ethen-1-yl]benzene (Scheme 1) giving a wide range of Cu 3 3 3 Cu separations. These are collected in Table 4 together with the corresponding N 3 3 3 N separations. In the majority of structures, the linked Cu(II) centers are chemically similar and are often related by crystallographic symmetry. The exception is the binuclear building block in 8 in which one Cu(II) center has an apically located [Cu(dipic)] moiety while the other has an apically located water molecule. Formation of Extended Structures (a) Extended Structures of 4-6, 11 and 13. HydrogenBonding and Aromatic Stacking of Binuclear Building Blocks. The extended structures of 4-6, 11 and 13 are formed solely by hydrogen-bonding and aromatic π-π stacking interactions between the binuclear building blocks. In 4-6 and 11, the binuclear building blocks comprise two [Cu(dipic)(OH2)] monomers linked through a bipyridyl bridge; in 13 they differ in the coordinated solvent molecule, methanol

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Figure 3. Views of the structure of 5 showing the interactions between binuclear building blocks to form (a) a 1-D chain parallel to the b-axis, (b) a 2-D sheet parallel to the (101) plane and (c) a 3-D network [atom identification as for Figure 1]. Table 5. Hydrogen-Bonding Interactions in the 3-D Extended Structures of 4 and 5 O-H/A˚ H 3 3 3 O/A˚ O 3 3 3 O/A˚ O-H 3 3 3 O/deg 4 O1W-H1WA 3 3 3 O1R O1W-H1WB 3 3 3 O1 O1R-H1RB 3 3 3 O1 O1R-H1RA 3 3 3 O3 5 O1W-H2W 3 3 3 O2 O1W-H1W 3 3 3 O4

0.82 0.82 0.82 0.81

1.92 1.95 2.05 1.99

2.729(2) 2.756(2) 2.867(2) 2.795(2)

171 169 175 173

0.83 0.83

2.01 1.93

2.819(4) 2.751(4)

163 169

replacing water. In four of the structures, 4, 5, 11 and 13, the binuclear building blocks are centrosymmetric. In the fifth, 6, the asymmetric unit contains three crystallographically independent [Cu(dipic)(OH2)] moieties, two of which are linked through a 4,40 -bipyridine bridge to form a non-centrosymmetric binuclear building block while the third forms the basis of a centrosymmetric version. The two pyrazine-bridged structures, 4 and 5, differ solely in that 4 contains two lattice water molecules per binuclear building block which are absent in 5. Thus, 5 has a less extensive hydrogen-bonding arrangement, its extended structure depending on two crystallographically independent hydrogen-bonded contacts. Each binuclear building block has four O-H hydrogen-bond donors and thus has four H 3 3 3 O H-bond acceptor sites giving a total of

eight O-H 3 3 3 O contacts for each building block. Four (O1W-H2W 3 3 3 O2) interactions result in the formation of a chain of binuclear building blocks aligned along the b-axis [Figure 3(a); Table 5]. The remaining four (O1WH1W 3 3 3 O4) link the chains into sheets, which lie parallel to the (101) plane [Table 5; Figure 3(b)], and the sheets are linked by stacking interactions reminiscent of that identified in Scheme 3(a). The contacts between [Cu(dipic)] moieties on adjacent [Cu(dipic)]¥ chains, which generate the 3-D extended structure [Figure 3(c)], are shown in Figure 4(a), and pertinent data are summarized in Table 6. In 4, which incorporates lattice water molecules, elaboration of the extended structure depends on four hydrogenbonded contacts. Two of these (O1W-H1WA 3 3 3 O1R; O1R-H1RB 3 3 3 O1) link [Cu(dipic)] moieties through an intermediate lattice water molecule in a motif reminiscent of that shown in Scheme 3(b). When combined with the coordinative bond of the [Cu(dipic)(OH2)] tecton, these hydrogen-bonded contacts generate a helical arrangement [Scheme 6(a)], which creates a chain of binuclear building blocks, aligned along the a-axis [Figure 5(a)]. The other two hydrogen bonds (O1W-H1WB 3 3 3 O1; O1R-H1RA 3 3 3 O3) serve to link the chains into a 3-D network [Figure 5(b)].

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Figure 4. Examples of aromatic π-π stacking interactions between pairs of [Cu(dipic)] moieties in (a) 5 and (b) 4, 7, 9 and 10 and between [Cu(dipic)] and pyridine moieties in (c) 8 [atom identification as for Figure 1]. Table 6. Perpendicular Plane 3 3 3 Plane and Cu 3 3 3 Cu Separations Associated with the Stacking Interactions between [Cu(dipic)] Moieties on Adjacent [Cu(dipic)]¥ Chains in 4, 5 and 7-10 perpendicular plane 3 3 3 plane separation/A˚ stacking interaction 4 5 7 8 9 10

a

[Cu(dipic)]-[Cu(dipic)] [Cu(dipic)]-[Cu(dipic)] [Cu(dipic)]-[Cu(dipic)] pyridine-[Cu(dipic)] [Cu(dipic)]-[Cu(dipic)] [Cu(dipic)]-[Cu(dipic)]

Figure

average

rangea

4

3.36(6) 3.21(7) 3.41(10) 3.02(15) 3.36(11) 3.34(6)

3.24-3.50 3.10-3.39 3.226-3.628 2.72-3.22 3.247-3.618 3.264-3.476

b a 4 b 4 c 4 b 4 b 4

centroid-centroid contact/A˚ dihedral angleb/deg Cu 3 3 3 Cu separation/A˚ 4.079(2) 0.0c 7.389(1) 5.176(4) 0.0c 5.304(2) 3.946(5) 2.4(1) 7.780(3) 5.009(4) 4.32(5) 5.288(2) 3.898(5) 0c 7.792(3) 3.764(3) 0c 7.390(1)

a These data include all 13 atoms of the [Cu(dipic)] moiety. b The angle between the perpendiculars of the least-squares planes defined by the two [Cu(dipic)] moieties (for 4, 5, 7, 9, 10) and by the [Cu(dipic)] moiety and the coordinated pyridine moiety (for 8). c Constrained by symmetry to be parallel.

The short O 3 3 3 O and H 3 3 3 O interatomic distances and large O-H 3 3 3 O interatomic angles in both 4 and 5 (Table 5) are indicative of stable hydrogen-bonding networks, with sheets in 4 [Figure 4(b); Table 6] linked by stacking interactions similar to those observed in 5 [Figure 4(a); Table 6] and similar to that identified in Scheme 3(a). The main difference is the lateral disposition of the [Cu(dipic)] moieties on adjacent [Cu(dipic)]¥ chains, which gives a Cu 3 3 3 Cu separation of 5.304(2) A˚ in 5 and 7.389(1) A˚ in 4. The stacking interaction in 4 effectively forms a chain of binuclear building blocks aligned in the [101] direction. In the 4,40 -bipyridine-bridged complex, 6, two sets of the three crystallographically independent [Cu(dipic)(OH2)] moieties are arranged around an inversion center to give one centrosymmetric and two non-centrosymmetric binuclear building blocks. These three building blocks form the repeat unit of a chain, which lies along the [101] direction [Figure 6(a)]. The molecules are linked through (i) an O-H 3 3 3 O hydrogen

bond, similar to the arrangement in Scheme 4(b), between the coordinated water molecule and a coordinated carboxylate oxygen of the adjacent [Cu(dipic)(OH2)] moiety [Scheme 6(b)] and supported by (ii) a relatively long Cu 3 3 3 O coordinative contact between the Cu(II) axial site trans to the coordinated water molecule and an uncoordinated carboxylate oxygen of an adjacent [Cu(dipic(OH2)] fragment. Interatomic distances for this motif are collected in Table 7. There are no aromatic stacking interactions independent of the coordinative and hydrogen-bonded contacts in 6, and the limited solventaccessible space (7.9%)23 in the structure is occupied by a disordered arrangement of water and methanol molecules. These act as hydrogen-bond donors and acceptors in the aggregation of chains, first into 2-D sheets aligned parallel to the (212) plane and second a 3-D network [Figure 6(b)]. Hydrogen-bond distances are collected in Table 8. In the 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine-bridged complex, 11, the binuclear building blocks are linked into a 3-D

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Table 7. Interatomic Distances Associated with Chain Formation in 6 Cu1 3 Cu2 3 Cu3 3 a

Cu 3 3 3 O coordinative contact ˚ 2.951(9) 3 3 O4B/A ˚ O1C/A 2.766(9) 33 ˚ 3.042(9) 3 3 O1A/A

O 3 3 3 O hydrogen-bond contacta O1 3 3 3 O3C/A˚ 2.78(1) O3 3 3 3 O3B/A˚ 2.77(1) O2 3 3 3 O3A/A˚ 2.95(1)

Cu 3 3 3 Cu separation Cu1 3 3 3 Cu2/A˚ 5.291(5) Cu1 3 3 3 Cu3/A˚ 5.281(5) Cu2 3 3 3 Cu3/A˚ 4.997(5)

The water hydrogen atoms could not be located.

Scheme 6. Schematic of the Cu 3 3 3 O Coordinative and O-H 3 3 3 O Hydrogen-Bonding Interactions, Which Link the [Cu(dipic)(OH2)] Moieties To Generate (a) the Helical Chain in 4, (b) the Polymeric Chain of Binuclear Building Blocks in 6 and (c) Part of the Tetranuclear Clusters in 8

network by O-H 3 3 3 O hydrogen-bonds involving the two coordinated water molecules and two lattice water molecules [Figure 7(a); Table 9]. The structure incorporates a series of stacked sheets aligned parallel to the (102) plane and which comprise {Cu(dipic)}2(μ-N,N-bipyridyl) moieties, lattice water molecules and coordinated water molecules from adjacent sheets [Figure 7(b)]. Hydrogen-bonding contacts between the coordinated water molecule and carboxylate centers link these sheets (Table 9), and long-range Cu 3 3 3 O coordinative interactions to the apical site trans to the coordinated water molecule [Cu 3 3 3 O 3.315(2) A˚; O 3 3 3 Cu-N1 102.4(1), O 3 3 3 Cu-O1 100.1(1), O 3 3 3 CuO2 77.2(1), O 3 3 3 Cu-N2 68.4(1)] are also observed. These contacts give rise to an arrangement of the [Cu(dipic)] moieties similar to that in 6 [Scheme 6(b)]. As for 6, there are no π-π aromatic stacking interactions independent of the coordinative and hydrogen-bonded contacts. The 3,6-bis[2-(pyridin-4-yl)ethen-1-yl]benzene-bridged structure of 13 does not contain any lattice solvent molecules. Thus, the only hydrogen-bonding interaction linking the binuclear building blocks in 13 is an O-H 3 3 3 O contact between the coordinated methanol molecule and a coordi-

Figure 5. Views of the structure of 4 showing interactions between binuclear building blocks and lattice water molecules to form (a) a 1D chain parallel to the a-axis and (b) a 3-D network [atom identification as for Figure 1].

nated carboxylate oxygen on an adjacent binuclear building block (Table 9). The resultant chain, which lies along the a direction, has a stepladder-type architecture [Figure 8(a)]

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with the chains stacked to give sheets parallel to the (014) plane [Figure 8(b)]. The stacking of these sheets and Cu 3 3 3 Cu separation [5.317(2) A˚] are determined by a long Cu 3 3 3 O coordinative interaction involving a noncoordinated carboxylate oxygen atom located at the apical site trans to the coordinated water molecule [Cu 3 3 3 O 3.223(3) A˚; O 3 3 3 Cu-N1 73.1(1), O 3 3 3 Cu-O1 73.6(1), O 3 3 3 Cu-O2 98.6(1), O 3 3 3 Cu-N2 100.5(1)]. A manifestation of these contacts is a disposition of [Cu(dipic)] moieties similar to that observed in 6 [Scheme 6(b)]. As for 6 and 11, there are no aromatic stacking interactions independent of the coordinative and hydrogen-bonded contacts. (b) Extended Structure of 8; Tetranuclear Cluster Formation from Binuclear Building Blocks. Although the bipyridyl bridged Cu(II) centers in the majority of the structures described herein are chemically similar, indeed in many they are centrosymmetrically related, the two metal centers in the asymmetric unit of 8 differ in having an O-donor, from either a [Cu(dipic)] moiety or from a water molecule, bound at the apical position. This results in the formation of a tetranuclear cluster in which binuclear building blocks comprising trans-1,2-bis(pyridin-4-yl)ethane-linked [Cu(dipic)] and

Felloni et al.

[Cu(dipic)(OH2)] nodes are held together by both coordinative Cu 3 3 3 O and hydrogen-bonded O-H 3 3 3 O interactions (Figure 9). A schematic of the Cu 3 3 3 O coordinative and O-H 3 3 3 O hydrogen-bonding interactions, which link the [Cu(dipic)(OH2)] moieties within the tetranuclear cluster, is

Figure 7. Views of the structure of 11 showing (a) the hydrogenbonding and (b) weak Cu 3 3 3 O coordinative interactions between binuclear building blocks and lattice water molecules to form an extended 3-D network. The sheets, which stack parallel to the (102) plane comprise {Cu(dipic)}2(μ-N,N-bipyridyl) moieties, lattice water molecules (O1R) and coordinated water molecules (O1W) from adjacent sheets [atom identification as for Figure 1]. Table 8. Interatomic Distances Associated with the Aggregation of the Binuclear Building Blocks in 6 O 3 3 3 O hydrogen-bond contacta O1 3 3 3 O3WA/A˚ 2.73(1) O2 3 3 3 O2A/A˚ 2.95(1) O3 3 3 3 O1WA/A˚ 2.67(1) a

O 3 3 3 O hydrogen-bond contacta O1WA 3 3 3 O3A/A˚ 3.18(1) O1WA 3 3 3 3 O4B/A˚ 2.91(1) O2WA 3 3 3 3 O1WA/A˚ 2.94(1)

The water hydrogen atoms could not be located.

Table 9. Hydrogen-Bonding Interactions in the 3-D Extended Structures of 11 and 13 O-H/A˚ H 3 3 3 O/A˚ O 3 3 3 O/A˚ O-H 3 3 3 O/deg

Figure 6. Views of the structure of 6 showing the interactions between binuclear building blocks and lattice water molecules to form (a) a 1-D chain, which lies along the [101] direction and (b) 2-D sheets aligned parallel to the (212) plane which aggregate into a 3-D network [atom identification as for Figure 1].

11 O1W-H1WA 3 3 3 O3 O1W-H1WB 3 3 3 O2 O1R-H1RA 3 3 3 O1 O1R-H1RB 3 3 3 O1W C5L-H5L 3 3 3 O3 13 O1M-H1M 3 3 3 O4

0.82 0.82 0.82 0.81 0.93

2.12 2.12 2.22 2.19 2.23

2.922(4) 2.929(4) 3.032(4) 2.944(4) 3.033(5)

178 177 161 149 144

0.83

1.92

2.748(6)

174

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shown in Scheme 6(c). It is effectively a combination of the contacts identified in Schemes 4(a) and 4(b). Details of the hydrogen-bonding contacts are given in Table 10. The Cu 3 3 3 Cu separation generated by this motif is 5.131(2) A˚, comparable with that linking the building blocks in 6 [Scheme 6(b)]. The main difference is the length of the Cu 3 3 3 O coordinative interaction which is probably short enough for viable tetranuclear cluster formation in 8 [2.357(3) A˚], but not for polynuclear chain formation in 6


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[2.766(9), 2.951(9) and 3.042(9) A˚]. The shorter Cu 3 3 3 O coordinative interaction in 8 arises from the absence of the trans-located water molecule that is found in 6, and which is responsible for the aggregation of the [Cu(dipic)(OH2)] moieties into the polymeric chain in the latter. Thus, the 3D extended structure of 8 depends primarily on the hydrogen bonds formed by the remaining O-H donor of the coordinated water molecule and the six O-H donors of the three lattice water molecules (Figure 10; Table 10). The short O 3 3 3 O and H 3 3 3 O intermolecular distances and approximately linear associated O-H 3 3 3 O angles (Table 10) are indicative of a stable hydrogen-bonding network. There is also a stacking interaction between pyridine and [Cu(dipic)] moieties on adjacent dimers [Figure 4(c); Table 6] which gives rise to a Cu 3 3 3 Cu separation of 5.288(2) A˚. (c) Extended Structures of 7, 9 and 10. Coordinative Interactions between Binuclear Building Blocks. In the 4,40 -bipyridine and trans-4,40 -azobis(pyridine)-bridged structures, 7, 9 and 10, the binuclear building blocks comprise two [Cu(dipic)] nodes linked through a bipyridyl bridge. The primary interaction by which they are linked to form the extended structure is a Cu 3 3 3 O coordinative contact between a carboxylate oxygen of an adjacent [Cu(dipic)] moiety and the apical position of the Cu(II) coordination geometry, reminiscent of that identified in Scheme 4(a). This leads to a coordinatively bonded 2-D sheet of 63 topology [Figure 11(a)]. An alternative view of the structure is one in which chains of [Cu(dipic)] centers are bridged by bipyridyl linkers, 4,40 -bipyridine for 7 and trans-4,40 -azobis(pyridine) for 9 and 10. A simplified view illustrating the 63 sheet topology of 10 is shown in Figure 11(b); by including solely the Cu(II) centers and the links between them, it is clear that the sheet has a distorted herringbone architecture. In all three cases, the [Cu(dipic)]¥ chains are aligned parallel to the Table 10. Hydrogen-Bonding Interactions in the 3-D Extended Structure of 8 O-H/A˚ H 3 3 3 O/A˚ O 3 3 3 O/A˚ O-H 3 3 3 O/deg

Figure 8. Views of the structure of 13 showing (a) the stepladder architecture of the 1-D chain formed by hydrogen-bonding contacts between binuclear building blocks and (b) the stacking of the chains to give 2-D sheets aligned parallel to the (014) plane [atom identification as for Figure 1].

O1W-H1WB 3 3 3 O4B

Intracluster Contact 0.81 2.12 2.904(4)

O1W-H1WA 3 3 3 O1M O1M-H1MA 3 3 3 O1P O1M-H1MB 3 3 3 O3B O1P-H1PB 3 3 3 O3A O1P-H1PA 3 3 3 O1R O1R 3 3 3 O4Aa

Intracluster Contacts 0.81 1.95 2.723(4) 0.81 1.97 2.778(4) 0.82 2.14 2.907(4) 0.83 2.03 2.832(4) 0.82 2.03 2.764(4) 2.919(4)

a

162 159 171 156 162 149

The hydrogen atoms attached to O1R could not be located.

Figure 9. View of the molecular structure of 8 showing the tetranuclear cluster formation and intradimer hydrogen-bonding interactions [atom identification as for Figure 1].

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Figure 10. View of the extended structure of 8 showing the inter-tetranuclear cluster hydrogen-bonding interactions [atom identification as for Figure 1].

Figure 11. Views of (a) 7, perpendicular to the (101) plane, and (b) a simplified version (excluding all atoms other than copper) of 10, perpendicular to the (102) plane showing the formation of the coordinatively bonded 2-D sheets of 63 topology [atom identification as for Figure 1].

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Table 11. Cu 3 3 3 Cu Interatomic Distances/A˚ within the 63 Nets in 7, 9 and 10 interatomic distances between copper(II) centers separated by 7a 5.289(3), 5.324(3)a 10.987(3) 5.633(3), 5.633(3)a 17.601(3) 13.952(3) 10.500(3)

a single dipic linker a bipyridyl bridge two dipic linkers the longest diagonal the intermediate length diagonal the shortest diagonal a

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9

10

5.388(3) 12.980(3) 5.669(3) 19.020(3) 15.570(3) 12.601(3)

5.349(1) 12.926(1) 6.232(1) 18.735(1) 16.189(1) 12.238(1)

Two values are quoted owing to the absence of a center of inversion between the linked Cu(II) centers.

Figure 12. Views of the [Cu(dipic)]¥ chains in (a) 7 and (b) 10 [atom identification as for Figure 1]. Table 12. Interatomic Distances and Angles Associated with the Secondary Bonding Interaction within the [Cu(dipic)]¥ Chains in 7, 9 and 10 compound 7 (chain Cu1) 7 (chain Cu2) 9 10 a

Cu 3 3 3 O/A˚ 3.093(5) 3.166(5) 3.093(6) 3.716(2)

Cu 3 3 3 Cu/A˚ 5.633(3) 5.633(3) 5.669(3) 6.232(1)

Cu 3 3 3 Cu 3 3 3 Cu/deg 64.34(8) 63.87(8) 63.49(15) 71.25(5)

dihedral anglea 8.4(1) 7.8(1) 8.5(2) 63.4(1)

The angle between the perpendiculars of the least-squares planes defined by the Cu(dipic) moiety and the coordinated pyridine moiety.

Figure 13. View of the structure of 9 perpendicular to the (010) plane showing the channels at (0.5, y, 0.25) and (0.5, y, 0.75) which accommodate disordered solvent molecules (omitted to aid clarity) [atom identification as for Figure 1].

b axis. However, the orientation of the sheets differs; those in 7 lie parallel to the (101) plane while those in 9 and 10 lie parallel to the (102) plane. The 63 net in 7 is considerably smaller than those in 9 and 10, which differ little, as illustrated by the various Cu 3 3 3 Cu interatomic distances (Table 11). Typical examples of [Cu(dipic)]¥ chains in complexes 7 and 10, are shown in Figures 12(a) and 12(b), respectively. The principal contact linking the [Cu(dipic)] moieties in all

three chains is the apical Cu(II)-carboxylate oxygen coordinative interaction discussed earlier [Figure 1(c)]. For 7 and 9 a very weak Cu 3 3 3 O interaction involving the empty coordination site of the Cu(II) center trans to the apical Cu 3 3 3 O contact and an uncoordinated carboxylate oxygen atom of the next-but-one [Cu(dipic)] center acts as a secondary contact [Figure 12(a)]. The Cu 3 3 3 O distances vary from 3.093(5) to 3.166(5) A˚ (Table 12). The weakness of this interaction is illustrated by the fact that there is an extremely long Cu 3 3 3 O distance [3.716(2) A˚; Table 12] in 10 despite a similar chain architecture, the main difference being a different orientation of the bridging pyridine and the [Cu(dipic)] moieties [Figure 12(b)]. For 7 and 9 the pyridine and [Cu(dipic)] fragments are effectively coplanar with dihedral angles no greater than 8.5(2), while for 10 the dihedral angle is 63.4(1) (Table 12). The different orientations result not only in different Cu 3 3 3 O distances but in different Cu 3 3 3 Cu 3 3 3 Cu angles and Cu 3 3 3 Cu distances between alternate Cu(II) centers (Table 12). In all three compounds, the sheets are packed in identical fashion to give large channels, which lie parallel to the b axis (Figure 13), and these are filled with disordered solvent molecules. Each structure has two channels per unit cell. For 7, which has a lower symmetry than the other two structures, one channel, centered at (0, y, 0), accommodates H2O molecules only, while the other, centered at (0.5, y, 0.5), accommodates CH2Cl2 molecules only. For 9 and 10 the channels, centered at (0.5, y, 0.25) and (0.5, y, 0.75) are symmetry-related and accommodate CH2Cl2 and CH3OH molecules, respectively. The volume of the solvent-accessible channels revealed values of 13.9% and 14.4% for 7, 2 14.4% for 10 and 2 16.2% for 9.13 The increased value for 9 is consistent with the non-coplanar arrangement of the bridging pyridine and [Cu(dipic)] moieties in this compound. The sheets in 7, 9 and 10 are linked by stacking interactions

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between [Cu(dipic)] moieties on adjacent [Cu(dipic)]¥ chains, very similar to that which links [Cu(dipic)] centers in 4 ([Figure 4(b)]. Although the higher symmetry of the structures of 9 and 10 compared to 7 dictates that the interacting moieties are exactly parallel, that is not the case for 7, for which the dihedral angle is 2.4(1). Perpendicular plane 3 3 3 plane and Cu 3 3 3 Cu separations, together with dihedral angles, are compared with those for 4 and 5 in Table 6.

Felloni et al.

(3) (4)

(5)

Conclusions The precise reaction conditions and choice of solvent are of paramount importance in these systems. The N,N0 -bipyridyl bridging ligand length seems not to be as crucial since similar structures are formed by pyrazine (4, 5), bipyridine (6), 3,6bis(pyridin-4-yl)-1,2,4,5-tetrazene (11) and 1,4-bis{2-(pyridin4-yl)ethenyl}benzene (13). The critical feature is the choice of donor ligand located at the apical sites of the two squarepyramidal Cu(II) centers of the binuclear building block (Scheme 2) found in all structurally characterized compounds. Whereas crystallization from coordinating solvents (H2O or MeOH) leads to these sites being occupied by solvent molecules, with reduced levels of coordinating solvent in the crystallization medium they are occupied by carboxylate oxygens of adjacent [Cu(dipic)] moieties. This choice has significant ramifications in the elaboration of the extended structure. When the apical position is occupied by a carboxylate oxygen (7, 9, 10), a coordination polymer with 63 topology results with [Cu(dipic)] linked to form 1-D [Cu(dipic)]¥ chains which are bridged by bipyridyl bridges within a 2-D sheet. When the apical positions are occupied by solvent molecules, molecular clusters are formed rather than polymeric structures. If both apical positions are occupied by solvent molecules, then the binuclear building blocks are linked by extensive hydrogen-bonding networks into 3-D matrices (4-6, 11, 13). If one apical position is occupied by a solvent molecule and the other by a carboxylate oxygen, this results in the formation of tetranuclear clusters, which are linked by extensive hydrogen-bonding into 3-D networks (8).

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Acknowledgment. We thank EPSRC for provision of diffractometers and for access to the National Service for X-ray Crystallography at the University of Southampton, and the University of Nottingham for financial support (to M.F.). M.S. gratefully acknowledges receipt of a Royal Society Wolfson Merit Award and of an ERC Advanced Grant. Supporting Information Available: X-ray crystallographic files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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