Bis(2,6-pyrazolyl)pyridines as a New Scaffold for Coordination Polymers

Two coordination polymers, Fe(LOBF3)(CH3COO)(CH3CN)2]n•nCH3CN and [Fe(LO−)2AgNO3BF4•CH3OH]n•1.75nCH3OH•nH2O (LO− = 3,3′-(4-(4-cyanophenyl)pyridine-2,6-diyl)bis(1-(2,6-dichlorophenyl)-1H-pyrazol-5-olate)), were obtained via a PCET-assisted process that uses the hydroxy-pyrazolyl moiety of the ligand and the iron(II) ion as sources of proton and electron, respectively. Our attempts to produce heterometallic compounds under mild conditions of reactant diffusion resulted in the first coordination polymer of 2,6-bis(pyrazol-3-yl)pyridines to retain the core N3(L)MN3(L). Under harsh solvothermal conditions, a hydrogen atom transfer to the tetrafluoroborate anion caused the transformation of the hydroxyl groups into OBF3 in the third coordination polymer of 2,6-bis(pyrazol-3-yl)pyridines. This PCET-assisted approach may be applicable to produce coordination polymers and metal–organic frameworks with the SCO-active core N3(L)MN3(L) formed by pyrazolone- and other hydroxy-pyridine-based ligands.


Introduction
Coordination polymers (CPs) and metal-organic frameworks (MOFs) are crystalline materials with a periodic n-dimensional structure made of metal ions and organic ligands [1]. Featuring unique (such as permanent porosity [1]) and highly tunable properties, MOFs have found use in sensing [2], catalysis [3], gas storage and separation [4], the applications relying on the delicate control over the adsorption/desorption ability of MOFs towards guest molecules. To achieve this control [5,6], one of the key strategies is to incorporate a switchable component that allows MOFs to undergo a reversible transformation triggered by an external stimulus (light, temperature, pressure or presence of guest compounds) [7]. Such a transformation affects the pore size, so there is no need for high-temperature and low-pressure conditions for desorption of guests [7].
Recently, we proposed a counterintuitive ligand design with ortho-dichloro-functionalized N-phenyl groups to produce the first SCO-active iron(II) complexes of N,N′disubstituted 3-bpp [43]. Among other things, it allowed us to induce an SCO centered around room temperature [44] by decorating the pyridine moiety of 5,5-dihydroxy-substitited 3-bpp with the p-cyanophenyl group (Scheme 3).
Recently, we proposed a counterintuitive ligand design with ortho-dichloro-functionalized N-phenyl groups to produce the first SCO-active iron(II) complexes of N,N -disubstituted 3-bpp [43]. Among other things, it allowed us to induce an SCO centered around room temperature [44] by decorating the pyridine moiety of 5,5-dihydroxy-substitited 3-bpp with the p-cyanophenyl group (Scheme 3).
By bonding to another metal ion via this additional coordination site, the homoleptic iron(II) complex [Fe(L OH ) 2 ](BF 4 ) 2 (L OH = 4-(2,6-bis(1-(2,6-dichlorophenyl)-5-hydroxy-1Hpyrazol-3-yl)pyridin-4-yl)benzonitrile) may act as a linker to produce CPs and MOFs. Our attempts to synthesize those with different sources of metal ions as nodes resulted in rare homoleptic CPs to contain the 3-bpp ligand and the first one to retain the core N 3 (L)MN 3 (L) via the proton-coupled electron transfer (PCET) reaction. This reaction, which is mostly found in hydroxo complexes [45] or organic compounds, such as aminosubstituted phenols [46], involves the transfer of electrons and protons from one atom to another. It is useful in homolytic activation of X-H (X = S, N, O, C) and C=Y (e.g., Y = O) bonds [47][48][49] and in asymmetric coupling reactions [48,50]. The homoleptic complex [Fe(L OH ) 2 ](BF 4 ) 2 with the hydroxyl groups as the source of the protons may potentially undergo a PCET similar to iron(II) complexes with bidentate imidazole-based ligands [51], although no such examples were yet reported for 1-bpp, 3-bpp or tpy ligands. [Fe(L OH )2](BF4)2 with the hydroxyl groups as the source of the protons may potentially undergo a PCET similar to iron(II) complexes with bidentate imidazole-based ligands [51], although no such examples were yet reported for 1-bpp, 3-bpp or tpy ligands. Scheme 3. The complex [Fe(L OH )2](BF4)2 as a potential linker to produce CPs in this study [44]. PhCl2 stands for ortho-dichloro-functionalized N-phenyl groups.

Results and Discussion
To obtain CPs and MOFs by exploiting an additional coordination site of the iron(II) complex [Fe(L OH )2](BF4)2, we used two different approaches: reactant diffusion [52] at room temperature and solvothermal synthesis, which is often used to produce high-quality crystals for X-ray diffraction [25,53]. As the sources of the metal ion to serve as a node, different inorganic salts were chosen, such as ZnCl2, Zn(OAc)2, FeCl2, (CH3CN)2PdCl2, AgNO3, CuSO4, NiCl2 and Co(OAc)2. They feature a good solubility in methanol, which does not cause the decomposition of the polymeric product.
Use of the excess of FeCl2, CuSO4 and AgNO3 (10 eq.) under mild conditions of reactant diffusion [52] in methanol to retain the core N3(L)MN3(L) resulted in non-soluble crystalline products [54]; with other inorganic salts, only a minor color change of the solution was observed. X-ray diffraction of the obtained crystalline products showed them to be  Figure 2) with methanol as both the co-ligand and lattice solvent. In the latter two cases, one of the OH groups of each 3-bpp ligand is deprotonated. The deprotonation of the N-heterocyclic ligand and the simultaneous oxidation of the metal ion, as in a PCET-based process, was earlier observed in iron(II) complexes of 2,6-diimidazolyl pyridines [51]. In contrast, the deprotonation of only one OH group of 6,6′-dihydroxy terpyridine in its copper(II) complexes retained the oxidation state of the metal ion [55], as in [Fe(L O− )2]•5CH3OH.

Results and Discussion
To obtain CPs and MOFs by exploiting an additional coordination site of the iron(II) complex [Fe(L OH ) 2 ](BF 4 ) 2 , we used two different approaches: reactant diffusion [52] at room temperature and solvothermal synthesis, which is often used to produce high-quality crystals for X-ray diffraction [25,53]. As the sources of the metal ion to serve as a node, different inorganic salts were chosen, such as ZnCl 2 , Zn(OAc) 2 , FeCl 2 , (CH 3 CN) 2 PdCl 2 , AgNO 3 , CuSO 4 , NiCl 2 and Co(OAc) 2 . They feature a good solubility in methanol, which does not cause the decomposition of the polymeric product.
Use of the excess of FeCl 2 , CuSO 4 and AgNO 3 (10 eq.) under mild conditions of reactant diffusion [52] in methanol to retain the core N 3 (L)MN 3 (L) resulted in non-soluble crystalline products [54]; with other inorganic salts, only a minor color change of the solution was observed. X-ray diffraction of the obtained crystalline products showed them to be  (Figure 2) with methanol as both the co-ligand and lattice solvent. In the latter two cases, one of the OH groups of each 3-bpp ligand is deprotonated. The deprotonation of the N-heterocyclic ligand and the simultaneous oxidation of the metal ion, as in a PCET-based process, was earlier observed in iron(II) complexes of 2,6-diimidazolyl pyridines [51]. In contrast, the deprotonation of only one OH group of 6,6 -dihydroxy terpyridine in its copper(II) complexes retained the oxidation state of the metal ion [55], as in [Fe(L O− ) 2 ]•5CH 3 OH.
In [Fe(L OH ) 2 ][FeCl 4 ]•5CH 3 CN, which features the anion FeCl 4 − resulting from the coordinative nature of the chloride anion, the Fe-N bond lengths and continuous shape measures [56] (Table 1)      [a] θ is the 'twist' angle between the two least-squares planes of the 3-bpp ligands; φ is the 'ro angle NPy-Fe-NPy; β is the rotation angle of the p-cyanophenyl group relative to the pyridine γ is the rotation angle of the dichlorophenyl group relative to the pyrazol-3-yl plane; S(OC), S and S(ebcT) are octahedral, trigonal-prismatic and edge-bicapped tetrahedral symmetry mea respectively; S(SS) and S(T) are seesaw and tetrahedral symmetry measures, respectively. value in brackets is for the coordination bond with the acetonitrile molecule. [c] The value f angle NPy-Fe-NCN is given. [d] The value in brackets is for the minor component of the disorde cyanophenyl group.  [a] θ is the 'twist' angle between the two least-squares planes of the 3-bpp ligands; φ is the 'rotation' angle N Py -Fe-N Py ; β is the rotation angle of the p-cyanophenyl group relative to the pyridine plane; γ is the rotation angle of the dichlorophenyl group relative to the pyrazol-3-yl plane; S(OC), S(TPR) and S(ebcT) are octahedral, trigonal-prismatic and edge-bicapped tetrahedral symmetry measures, respectively; S(SS) and S(T) are seesaw and tetrahedral symmetry measures, respectively. [b] The value in brackets is for the coordination bond with the acetonitrile molecule. [c] The value for the angle N Py -Fe-N CN is given. [d] The value in brackets is for the minor component of the disordered p-cyanophenyl group.
The coordination of the iron(II) ion by the p-cyanophenyl group of the ligand does not occur, probably owing to the kinetics of the reaction, which causes the complex [Fe(L OH ) 2 ][FeCl 4 ]•5CH 3 CN to precipitate from the reaction mixture faster than the iron(II) ion coordinates. Under these conditions, the deprotonation of one of the OH groups was not expected in contrast to the oxidation of iron(II) to iron(III) [57]. In the absence of a strong base, the former may arise from a PCET [46] with both the electron and proton transferred towards the anion NO3 − similar to iron(II) complexes with imidazole-based ligands [51]. The resulting iron(III)-containing CP is not stable towards DMF, which causes its decomposition to produce a dark brown solution featuring signals of unidentified diamagnetic compounds in its 1 (Figure 2, top), with one of the two OH groups in each 3-bpp ligand being deprotonated. Under these conditions, the deprotonation may occur through a multi-site PCET process with an electron transfer from the solvent (methanol to produce formaldehyde) rather than from the iron(II) ion, which keeps its oxidation state 2+, similar to copper complexes of 6,6 -dihydroxy terpyridine [55]. The deprotonation of the OH groups causes the shortening of the bonds Fe-N (Table 1) [32] and the first one to retain a core N 3 (L)MN 3 (L) (Figure 2, top). As follows from X-ray diffraction, the Fe-N bond lengths (Table 1) fall into the range typical of (pseudo)octahedral complexes of iron(III) in the low-spin state [21]. Three cores [Fe(L O− ) 2 ] coordinate the silver(I) ion via the p-cyanophenyl substituent of the 3-bpp ligand and one of the two deprotonated OH groups (Ag-N 2.147(7) and 2.178(7) Å, Ag-O 2.498(7) Å) to produce a coordination double chain along the diagonal of the crystallographic plane a0c (Figure 2, bottom). The seesawshaped coordination environment, as gauged by continuous symmetry measures [56], of the silver(I) ion is completed by the oxygen atom of the coordinated methanol molecule (Ag-O 2.448 (7)  Under these conditions, the deprotonation of one of the OH groups was not expected in contrast to the oxidation of iron(II) to iron(III) [57]. In the absence of a strong base, the former may arise from a PCET [46] with both the electron and proton transferred towards the anion NO 3 − similar to iron(II) complexes with imidazole-based ligands [51]. The resulting iron(III)-containing CP is not stable towards DMF, which causes its decomposition to produce a dark brown solution featuring signals of unidentified diamagnetic compounds in its 1 H NMR spectra.
Mild conditions of reagent diffusion allowed us to keep the core N 3 (L)MN 3  Under solvothermal conditions often used to obtain new CPs and MOFs [25,53], the complex [Fe(L OH ) 2 ](BF 4 ) 2 was kept at 140 • C for 24 h in a sealed ampule with a solution of the transition metal salt in DMF or DMF/AcN (1:1). In most cases, no crystalline products were obtained; 1 H NMR spectroscopy of the reaction mixtures revealed a variety of paramagnetic compounds as a sign of the decomposition of the parent complex [Fe(L OH ) 2 ](BF 4 ) 2 . The only exception was cobalt acetate tetrahydrate, which produced the third homoleptic CP with a 3-bpp ligand [32], a 1D-CP of iron(III) [Fe(L OBF 3 )(CH 3 COO)(CH 3 CN) 2 ]n•nCH 3 CN with the OH groups transformed into OBF 3 upon the reaction with BF 4 (Figure 3, top). The pseudo-octahedral coordination environment of the metal ion, as gauged by continuous symmetry measures [56], is formed by the three nitrogen atoms of the tridentate 3-bpp ligand, the nitrogen atom of the p-cyanophenyl substituent of the other 3-bpp ligand and two nitrogen atoms of the solvent acetonitrile molecules; the Fe-N bond lengths (Table 1) fall into the range typical of low-spin complexes of iron(III) [21]. The 3-bpp ligand acts as a bridge to produce 1D-coordination polymer chains along the crystallographic axis b (Figure 3, bottom). Parallel-displaced stacking interactions between the parallel dichlorophenyl groups of the neighboring chains that pack them along the crystallographic axis c into the layers; the appropriate intercentroid and shift distances are 3.933(7) and 1.956(15) Å. Acetate anions and solvent acetonitrile molecules occur between these layers. upon the reaction with BF4 (Figure 3, top). The pseudo-octahedral coordination envir ment of the metal ion, as gauged by continuous symmetry measures [56], is formed by three nitrogen atoms of the tridentate 3-bpp ligand, the nitrogen atom of the p-cyanop nyl substituent of the other 3-bpp ligand and two nitrogen atoms of the solvent aceto trile molecules; the Fe-N bond lengths (Table 1) fall into the range typical of low-s complexes of iron(III) [21]. The 3-bpp ligand acts as a bridge to produce 1D-coordinat polymer chains along the crystallographic axis b (Figure 3, bottom). Parallel-displa stacking interactions between the parallel dichlorophenyl groups of the neighbor chains that pack them along the crystallographic axis c into the layers; the appropr intercentroid and shift distances are 3.933(7) and 1.956(15) Å. Acetate anions and solv acetonitrile molecules occur between these layers.  The 1D-CP [Fe(L OBF3 )(CH 3 COO)(CH 3 CN) 2 ] n •nCH 3 CN apparently resulted from the heat-induced dissociation [58] of the complex [Fe(L OH ) 2 ](BF 4 ) 2 and further coordination of the "open-shell" iron(II) ion by the p-cyanophenyl group of the 3-bpp ligand in acetonitrile. Long-term heating also causes other reactions to occur, such as OH bond activation [59]. At this temperature, the PCET process involves the tetrafluoroborate anion, as it is the only potential source of BF 3 moiety. The hydrogen atom transfer via a PCET towards the anion BF 4 apparently produces HF and BF 3 (Lewis acid) and the latter reacts with the deprotonated ligand (Lewis base), thereby resulting in the formation of the CP [Fe(L OBF3 )(CH 3 COO)(CH 3 CN) 2 ] n •nCH 3 CN (see Figure S1 of Supplementary Materials). Here, cobalt acetate acts only as a donor of the acetate anion rather than a source of the metal ion to coordinate the p-cyanophenyl group, owing to higher stability of the iron(II)cyano species compared to those of cobalt(II) [60]. In such a multicomponent system of cobalt acetate, [Fe(L OH ) 2 ](BF 4 ) 2 and DMF/AcN, however, it is quite difficult to pinpoint the exact mechanism of the PCET-assisted transformation. The reason behind it may be high temperature or the presence of oxygen, but it is still arguable.

Materials and Methods
Synthesis. All synthetic manipulations were carried on air unless stated otherwise. Solvents were purchased from commercial sources and purified by distilling from conventional drying agents in an argon atmosphere prior to use. The iron(II) complex [Fe(L OH ) 2 ](BF 4 ) 2 was synthesized as reported previously [44].
[  Figure S2 of Supplementary Materials), which was filled with methanol above the level of these vials and kept for two days until all the reagents diffused into the 20 mL vial and grey crystals of the product appeared. These were centrifuged, washed with methanol and dried in vacuum. Yield: 43 mg (56%). Anal. Calc. for C 62 solvate molecule of diethyl ether or methanol, respectively, which was severely disordered and thereby treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON [64]. Crystal data and structure refinement parameters are given in Table 2 2 ] n •nCH 3 CN; they are the second and the third homoleptic CPs to contain the 3-bpp ligand and the heterometallic one is the first to retain the core N 3 (L)MN 3 (L). These coordination polymers were obtained by applying a novel approach that includes a PCET from the hydroxyl group of the pyrazolone moiety to the iron(II) ion. The deprotonated 3-bpp scaffold, which appeared in the neutral complex [Fe(L O− ) 2 ]•5CH 3 OH via a potential multi-site PCET process similar to copper complexes with 6,6'-dihydroxy terpyridine, stabilized the low-spin state of the metal ion-as gauged by X-ray diffraction-owing to its anionic character. The proposed approach can be applied to other ligands with acidic protons, such as substituted pyrazolones [65,66], and other metal ions that are prone to oxidation to an SCO-active form, such as manganese(II) and cobalt(II) [67], to produce switchable CPs and MOFs.
Under harsh solvothermal conditions, retaining the core N 3 (L)MN 3 (L) with a neutral ligand, such as 1-or 3-bpp and tpy, is hardly possible. The heating initiated a PCET between the complex [Fe(L OH ) 2 ](BF 4 ) 2 and the tetrafluoroborate anion, which is believed to be inert towards reactive species, such as cation radicals [68]. Our investigation of this PCET reaction as a catalytic variant of the hydrogen atom transfer for a reductive ketone coupling [69] is currently underway.