Self-Assembly with 2,6-Bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine: Silver(I) and Iron(II) Complexes

A new “click” ligand, 2,6-bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine (L) featuring a tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (tripy) pocket and two pyridyl (py) units was synthesized in modest yield (42%) using the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The coordination chemistry of the ligand with silver(I) and iron(II) ions was examined using a battery of solution (1H and DOSY (diffusion ordered spectroscopy) nuclear magnetic resonance (NMR), infrared and absorption spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)), and solid state (X-ray crystallography, elemental analysis) techniques. When treated with silver(I) ions, the ligand forms discrete [Ag(L)]+ (X−, where X− = BF4−, NO3− or SbF6−) complexes in dimethyl sulfoxide (DMSO) solution but these complexes crystallize as coordination polymers. The addition of [Fe(H2O)6](BF4)2 to an acetonitrile solution of the ligand forms the expected monomeric octahedral [Fe(L)2]2+ complex and treatment of the iron(II) complex with AgBF4 generates a heterometallic linear coordination polymer.


Introduction
The generation of self-assembled metallosupramolecular architectures has been studied extensively in the past 30 years [1]. The main factors controlling the outcome of the self-assembly process are now reasonably well understood, appropriate matching of the coordination geometry preference of the metal ion(s), the denticity of the ligands, and the length and flexibility of the linking units between binding sites on the ligands usually provides good-to-excellent control on the resulting assembly. The combination of a suitable metal ion with a rigid bridging ligand will reliably generate either discrete macrocyclic or cage architectures or continuous metal-organic framework materials. However, bridging ligands with more flexibility provide less control on the outcome of the self-assembly process and can lead to mixtures or coordination polymers [2]. Coordination polymers can also be generated by exploiting rigid ligands with divergent coordinating units. As these metallosupramolecular systems can be readily self-assembled, as opposed to generated through (potentially) more laborious step-by-step synthetic procedures, they have been examined for a wide range of applications, including gas sorption [3,4], drug delivery [5,6], and catalysis [7,8]. Metallosupramolecular systems with interesting biological [9], photophysical [10,11] and redox [12][13][14][15] properties have also been generated. Recently, as a part of efforts to further widen the potential applications of metallosupramolecular architectures, there has been a focus on the design and synthesis of heterometallic [16,17] complexes/assemblies.
The silver(I) complexes of L were prepared by adding an acetonitrile or acetone solution of AgX (1 equiv., where X = BF4 − , NO3 − or SbF6 − ) to an acetonitrile or acetone solution of the ligand (L 1 equiv.). The resulting solutions were stirred at room temperature for 5 min in the absence of light (Scheme 1) and colorless solids immediately precipitated in each case. Repeating the synthesis in dimethylformamide (DMF), resulted in colorless solutions. Vapor diffusion of diethyl ether into the DMF solutions generated either X-ray quality colorless crystals or microcrystalline powders in good yields (50-92%). IR spectra of the isolated colorless solids show absorption bands resulting from C-H stretching (3100-2900 cm −1 ) and from the skeletal vibrations of the aromatic rings (1600-1400 cm −1 ) confirming the presence of the ligands in the isolated materials, while elemental analysis indicated that complexes with 1:1 metal:ligand ratios were obtained.
The complexes were only soluble in polar DMF or DMSO, 1 H NMR spectroscopy (500 MHz, d6-DMSO, 298 K) of the three complexes showed the downfield shifting of ligand protons Ha (Δδ = 0.12-0.17 ppm) and Hc (Δδ = 0.25-0.32 ppm), relative to the "free" ligand consistent with complex
The silver(I) complexes of L were prepared by adding an acetonitrile or acetone solution of AgX (1 equiv., where X = BF 4 − , NO 3 − or SbF 6 − ) to an acetonitrile or acetone solution of the ligand (L 1 equiv.). The resulting solutions were stirred at room temperature for 5 min in the absence of light (Scheme 1) and colorless solids immediately precipitated in each case. Repeating the synthesis in dimethylformamide (DMF), resulted in colorless solutions. Vapor diffusion of diethyl ether into the DMF solutions generated either X-ray quality colorless crystals or microcrystalline powders in good yields (50-92%). IR spectra of the isolated colorless solids show absorption bands resulting from C-H stretching (3100-2900 cm −1 ) and from the skeletal vibrations of the aromatic rings (1600-1400 cm −1 ) confirming the presence of the ligands in the isolated materials, while elemental analysis indicated that complexes with 1:1 metal:ligand ratios were obtained.

Heterometallic Silver(I)-Iron(II) Complex
Having shown that silver(I) and iron(II) ions would form homometallic complexes with L, we attempted to generate a self-assembled heterometallic silver(I)-iron(II) coordination polymer. The [Fe(L) 2 ](BF 4 ) 2 complex (1 equiv.) was dissolved in acetonitrile and AgBF 4 (2 equiv.) was added at RT resulting in the formation of an orange precipitate. The orange solid was quite insoluble but could be dissolved in DMSO. However, the color immediately changed from orange to colorless consistent with decomposition of the iron(II) complex, as seen above for the [

Heterometallic Silver(I)-Iron(II) Complex
Having shown that silver(I) and iron(II) ions would form homometallic complexes with L, we attempted to generate a self-assembled heterometallic silver(I)-iron(II) coordination polymer. The [Fe(L)2](BF4)2 complex (1 equiv.) was dissolved in acetonitrile and AgBF4 (2 equiv.) was added at RT resulting in the formation of an orange precipitate. The orange solid was quite insoluble but could be dissolved in DMSO. However, the color immediately changed from orange to colorless consistent with decomposition of the iron(II) complex, as seen above for the [  In one of the disordered silver-pyridyl components of the structure, the silver(I) ions are each coordinated to two pyridyl units (Ag-N range from 2.22 to 2.38 Å) with a water (H2O) ligand sandwiched between adjacent [Ag(py)2] + units (Figure 7c). One of the silver(I) ions was essentially two coordinate with a distorted linear coordination geometry (N9A-Ag1A-N9A 164°). The other silver(I) ion was three coordinate bound to two pyridyl units and a water (H2O) ligand. The bond angle of the [Ag(py)2] + units was not linear (N1A-Ag1A-N1A 121°) suggesting that the silver(I) is coordinated to a third ligand giving a trigonal coordination geometry. The silver(I) ions were both close to a H2O In one of the disordered silver-pyridyl components of the structure, the silver(I) ions are each coordinated to two pyridyl units (Ag-N range from 2.22 to 2.38 Å) with a water (H 2 O) ligand sandwiched between adjacent [Ag(py) 2 ] + units (Figure 7c). One of the silver(I) ions was essentially two coordinate with a distorted linear coordination geometry (N9A-Ag1A-N9A 164 • ). The other silver(I) ion was three coordinate bound to two pyridyl units and a water (H 2 O) ligand. The bond angle of the [Ag(py) 2 ] + units was not linear (N1A-Ag1A-N1A 121 • ) suggesting that the silver(I) is coordinated to a third ligand giving a trigonal coordination geometry. The silver(I) ions were both close to a H 2 O molecule (Ag1A-O1 2.66 Å, N1A-Ag1A-O1 93 • , Ag2A-O1 2.67 Å) suggestive of a bridging interaction (Figure 7c). In the second disordered silver-pyridyl component of the structure the silver(I) ions are each coordinated to two pyridyl units with a water (H 2 O) ligand coordinated to only one of the [Ag(py) 2 ] + units (Figure 7d). There is one short silver-water interaction/bond (Ag1B-O32 2.27 Å) while the other silver ion is 3.65 Å away from the water ligand, too long for any type of bonding.
CAUTION: Azides are explosive and care should be taken when handling them. Reactions were carried out on small scale. No problems were encountered during the course of this work.
The addition of AgBF 4 to the iron(II) complex generates a heterometallic silver(I)-iron(II) linear coordination polymer. The collected results indicate that L, and related, hybrid "click" ligands featuring chelating pockets appended with pyridyl donors, could be exploited to generate a wide range of new metallosupramolecular architectures. A range of other octahedral metal ions could potentially be coordinated within the tripy pockets of the ligand, while other cis-protected or naked metal ions could be used to interact with the peripheral pyridyl units to generate a vast family of discrete or polymeric systems. Efforts in these directions are underway.
Supplementary Materials: The following are available online, 1 H-, 13 C-NMR, UV-Vis and HR-ESI-MS spectral data and crystallographic data in CIF format.