Ruslan
Shekurov
a,
Vera
Khrizanforova
a,
Leysan
Gilmanova
a,
Mikhail
Khrizanforov
a,
Vasily
Miluykov
a,
Olga
Kataeva
ab,
Zilya
Yamaleeva
a,
Timur
Burganov
a,
Tatiana
Gerasimova
a,
Airat
Khamatgalimov
ac,
Sergey
Katsyuba
a,
Valeri
Kovalenko
ac,
Yulia
Krupskaya
d,
Vladislav
Kataev
d,
Bernd
Büchner
de,
Volodymyr
Bon
f,
Irena
Senkovska
f,
Stefan
Kaskel
f,
Aidar
Gubaidullin
a,
Oleg
Sinyashin
a and
Yulia
Budnikova
*a
aArbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS, Arbuzov Str. 8, 420088 Kazan, Russia. E-mail: yulia@iopc.ru
bA.M. Butlerov Chemistry Institute of the Kazan Federal University, Kremlevskaya str. 18, 420000, Russia
cKazan National Research Technological University, Karl Marx Str. 68, Kazan 420015, Russia
dIFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany
eInstitute of Solid State and Materials Physics, Technical University Dresden, D-01062 Dresden, Germany
fChair of Inorganic Chemistry, Technische Universität Dresden, Bergstr. 66, 01062 Dresden, Germany
First published on 21st January 2019
New redox active 1D helical coordination polymers M(fcdHp) (M(II) = Zn(1), Co(2)) have been obtained by utilizing the 1,1′-ferrocenylenbis(H-phosphinic) acid (H2fcdHp) ligand and Zn or Co nitrate salts. Complexes 1 and 2 are isomorphic, crystallizing in the chiral space groups P4122 and P4322, respectively. Their redox, electrocatalytic and other properties are described. These compounds incorporated into carbon paste electrodes and exhibited reversible redox reactions, arising from the ferrocenyl moiety. These coordination polymers are efficient as electrocatalysts for the reduction of protons to hydrogen. Using N,N-dimethylformamidium ([DMF(H)+]) as the acid in the acetonitrile solution, Co CP (2) displays a turnover frequency of 300 s−1, which is among the fastest rates reported for any CP electrocatalyst in CH3CN. This high rate of catalytic reaction comes at the cost of the 820–840 mV overpotential at the potential of catalysis. As the hydrogen evolution reaction (HER) catalysts, the CPs exhibited in 0.5 M H2SO4 the overpotential η10 of 340 or 450 mV, onset overpotential of 220 or 300 mV (vs. RHE), Tafel slope of 110 or 120 mV dec−1, correspondingly for 1 and 2, and considerable long-term stability for the HER.
The chemical and hydrothermal stability of CPs is crucial for most potential applications. Although the judicious choice of the metal ion and organic linker provides the redox activity within CPs,15 the redox reaction often changes the coordination environment of metal ions, thus leading to the destruction of the framework. One way to overcome this issue is to construct the framework with stable ligands for the redox reaction. In this context, ferrocene-based coordination polymers are excellent candidates because they contain a stable ferrocene moiety and two carboxylate, phosphinates or other coordination sites.16 Some phosphonyl-substituted ferrocene derivatives have been reported and their complexation behavior towards transition metal cations has been explored.17 Some research groups have reported on the syntheses of ferrocene-based coordination polymers,18 but there are only a few reports of the solid-state electrochemical properties of coordination polymers containing the 1,1′-ferrocenedicarboxylate ligand,19 and the data for CPs with linkers based on phosphinate derivatives are absent. And, while the majority of carboxylate-based CPs display low stability in aqueous media,19 phosphinate derivatives give more hydrothermally stable CPs.20 Perhaps then surprisingly, phosphinates have obtained much less attention for the preparation of CPs,21 especially in comparison with carboxylic acid based linkers.19d Only monophosphinic acids,22–24 bisphosphinic acids with a short spacer,25,26 or bisphosphinic acids with a flexible spacer27 and recently phenylene-1,4-bis(methylphosphinic acid)20 have been used for the preparation of CPs. But the conformational lability of 1,1′-ferrocenediphosphinate can provide many interesting structures and allows the ligand to adopt a conformation most suitable for metal coordination.28
Thus, the synthesis of new ferrocene-based coordination polymers with phosphinate linkers is very important and can lead to new redox-active structures that are steady during redox processes in various environments, including acids and water, fundamentally interesting as electrocatalysts for hydrogen evolution, oxygen reduction, as electrochemical sensors and for other applications. The key to successful implementation is not only to construct the redox active framework itself, but also to hybridize the framework with an electrode, which allows the investigation of redox transformations of CPs and catalytic currents in the presence of a proton donor or other subjects of inquiry.
Herein we report on the synthesis, crystal structure, and the physico-chemical (redox, electrocatalytic, spectroscopic and magnetic) properties of redox-active Co(II)- and Zn(II)-based one-dimensional CPs with a ferrocene-containing diphosphinate ligand. The choice of cobalt and zinc derivatives is due to the importance of searching for catalysts or metal-based materials based on wide-spread non-platinum metals. An interesting additional observation has been found, that the enantiopure pairs of chiral CPs were obtained as a conglomerate by spontaneous resolution from the methanol/DMF solution of the ligand and metal nitrates.
1a: C10H10FeO4P2Zn, M = 377.34 g mol−1, crystal size (mm) 0.180 × 0.120 × 0.120, temperature 150(2) K, tetragonal, space group P4122 (No. 91), a = 8.254(2) Å, c = 18.660(4) Å, V = 1271.1(7) Å3, Z = 4, ρcalc = 1.972 g cm−3, μ = 3.279 mm−1, θ range: from 2.70° to 25.62°, 3369 reflection collected (−10 ≤ h ≤ 8, −10 ≤ k ≤ 7, −22 ≤ l ≤ 19), 1194 independent reflections (Rint = 0.0527), 1051 observed reflections with I ≥ 2σ(I), 86 refined parameters, R = 0.0302, wR2 = 0.0500, Flack parameter 0.03(2), and max. residual electron density 0.271 (−0.266) e Å−3.
1b: C10H10FeO4P2Zn, M = 377.34 g mol−1, crystal size (mm) 0.365 × 0.108 × 0.078, temperature 296(2) K, tetragonal, space group P4322 (No. 95), a = 8.250(2) Å, c = 18.655(4) Å, V = 1296.7(7) Å3, Z = 4, ρcalc = 1.974 g cm−3, μ = 3.283 mm−1, θ range: from 2.47° to 28.62°, 23622 reflection collected (−11 ≤ h ≤ 11, −11 ≤ k ≤ 11, −24 ≤ l ≤ 24), 1602 independent reflections (Rint = 0.0471), 1313 observed reflections with I ≥ 2σ(I), 86 refined parameters, R = 0.0320, wR2 = 0.0724, Flack parameter −0.01(1), and max. residual electron density 0.480 (−0.362) e Å−3.
2: C10H10FeO4P2Co, M = 370.90 g mol−1, tetragonal, P4122 (No. 91), a = 8.2200(12) Å, c = 18.840(4) Å, V = 1273.0(5) Å3, Z = 4, ρcalc = 1.935 g cm−3, λ = 0.88561 Å, T = 293 K, θmax = 34.1°, reflections collected/unique 1374/1341, Rint = 0.023, R1 = 0.0749, wR2 = 0.2287, S = 1.15, Flack parameter 0.23(3), and the largest diff. peak 1.23 e Å−3 and hole −0.70 e Å−3.
Powder samples were characterized by the UV-vis/DR technique using a Jasco V-650 spectrophotometer (Jasco International Co. Ltd., Hachioji, Tokyo, Japan) equipped with an integrating sphere accessory for diffuse reflectance spectra acquisition. BaSO4 powder was used as the reference for baseline correction.
The Zn and Co CPs are isotypic; hence the structural description is restricted to the Zn CPs. Both contain two crystallographically independent [ML]n entities with an identical coordination environment. The single crystal X-ray diffraction study showed that compounds 1 and 2 are one-dimensional coordination polymers with 8-membered macrocycles M–O–P–O–M–O–P–O, similar to that observed earlier for other metal phosphinates.36 Crystallizing in the chiral space groups P4122 and P4322, Zn-based coordination polymers 1a (left rotation) and 1b (right rotation) are axial enantiomers. The cobalt based CP 2a can be considered as an isomorphic structure to 1a. In all structures the ferrocene molecule is found in a slightly twisted conformation with a very small shift of the cyclopentadienyl rings, where phosphinic groups are located opposite one another. The zinc and cobalt ions in compounds 1a and 2a are coordinated by four oxygen atoms from symmetrically dependent phosphinic groups showing a tetrahedral coordination geometry (Fig. 1).
Fig. 1 A fragment of a polymeric chain of compound 2 showing spiro-fused 8-membered rings and the coordination geometry of Co(II). |
The M–O bond lengths range from 1.926(3) to 1.936(4) Å for the phosphinate oxygen atoms in 1a. The result is a 4-fold helix, in which the Co⋯Co distance between the adjacent metal centers in the chain is 4.867 Å, while the pitch of the helix is given by the unit cell c parameter, 18.659(4) for 1a and 18.840(4) Å for 2a. In such a manner each metal ion is interconnected by two ligand molecules forming 8-membered macrocycles M–O–P–O–M–O–P–O. This leads to the formation of infinite 1D helical chains along the [001] direction that generates the axial chirality in the structures. The length of the helix loop corresponds to the length of the c axis, namely 18.6 Å (Fig. 2). In the case of 1a and 2a, the dextrorotatory R-enantiomers were isolated and in the case of 1b, a levorotatory S-enantiomer was confirmed by the single crystal X-ray diffraction study.
Each crystal was found to be enantiomerically pure. All the helices in a given crystal have the same handedness, either all having a right-handed thread or all being left-handed.
The space groups P4122 and P4322 form an enantiomeric pair, and within a given batch of crystals, there is a statistical 1:1 mixture of crystals corresponding to each of the two space groups. In the crystal structure, helixes are interacting via weak C–H⋯O bonds and van der Waals interactions, as well as P–H⋯O hydrogen bonds (see the discussion of IR and Raman spectra below). The crystal structure is stabilized by a number of C–H ⋯O interchain interactions. These, involving phosphinate and Cp from adjacent chains, have C⋯O distances in the range of 3.111–3.435 Å. This results in a dense packing of the chains, with no space for any lattice solvent molecules between the chains (Fig. 3). By analyzing the large number of reported chiral CPs that exhibit helix architecture, it can be observed that the structures of most of them (around 64%) exhibit only 2- and 3-fold screw axes; the CPs exhibiting high symmetry 4-fold screw axes are rather rare (about 22%).
It is well-known that spontaneous resolution generally yields a conglomerate (racemic mixture of chiral crystals). Although the ligand chirality is not a critical prerequisite for the formation of homochiral CPs and several publications report the synthesis of chiral CPs starting from achiral ligands,37 it should be noted that in many reports the yielded crystals are separated from the reaction mixtures in the form of racemates or conglomerates.38
The combination of a labile ferrocene fragment in the ligand together with five-valent phosphorus used in this work allows obtaining coordination polymers demonstrating optical activity. The synergism of both fragments leads to the formation of axial coordination polymers demonstrating spontaneous resolution during crystallization. Earlier in our studies, it was demonstrated that ferrocenylphosphinic acids themselves also demonstrate the formation of supramolecular polymeric structures. We have shown that the phosphorus atom in this series of monosubstituted ferrocenylphosphinic acids is chiral and the formation of enantiomorphous hydrogen-bonded chains with chiral recognition occurs in these compounds.29
The IR and Raman spectra of the two CPs are very similar, both in terms of frequencies and intensities (Fig. S1–S3†). The positions of νP–H bands in the spectra are red-shifted by ca. 44 cm−1 with respect to the spectrum of H2fcdHp (Fig. S1, Table S1†), suggesting that P–H⋯O short contacts revealed by the X-ray study (vide supra) represent hydrogen bonding. Red shifts are also found for the Raman bands of stretching vibration ν(Fe–C) of ferrocenyl fragments: from 310 and 316 cm−1 for H2fcdHp to 280 and 290 cm−1 for 2 and to 283 and 290 cm−1 for 1. These red shifts, as well as the blue shift of the ν(C–H) bands of ferrocenyl fragments both in the IR and Raman spectra of 1D-CPs, indicate a redistribution of electron density in ferrocenyl moieties caused by the formation of 1D-chains of the CPs.
The comparison of experimental X-ray diffraction and vibrational spectroscopy data on 1 and 2 shows the similarity of the 1D coordination polymers, nonetheless there are differences in the UV-Vis spectra of their crystals. Due to the structural similarity of both CPs, it is possible to evaluate the role of metal nature on the electronic properties of 1 and 2.
Fig. 4 Experimental UV-Vis spectra of ferrocene (blue), H2fcdHp (green), complex 1 (black) and complex 2 (red). |
Bands below 500 nm in the UV-Vis spectra of the CPs and the acid are attributed to the Fc moiety.42 The formation of CP leads to the appearance of strong bands in the spectrum of complex 2 at 547, 585, and 633 nm associated with the 4A2(F) → 4T1(P) transitions specific for tetrahedral Co2+ chromophores.43
Complex 1 does not exhibit the d–d electronic transition because the d-orbitals of Zn(II) are completely occupied. Interesting, that in the spectra of complexes 1 and 2 the Fc bands are shifted to 477 nm relative to 454 nm in the spectrum of ferrocene and initial acid. The reason for this effect may be a transformation of the Fc moiety from a staggered conformation adopted in solid ferrocene at room temperature44 to an eclipsed conformation found in the crystals of complexes 1 and 2.
Fig. 5 Thermal analysis data of 1 (black) and 2 (red) CPs: (a) thermogravimetry (TG, solid lines) curves and curves of the first derivative of TG (DTG, dashed lines); (b) DSC curves. |
One of the possible explanations of the weight increase in the ranges of 350–400 °C and 500–750 °C is the initial oxidation of P–H bonds in Zn(II) and Co(II) phosphinates to P–OH bonds and the following oxide formation during further oxidation. The corresponding thermal effects on the DSC curves (Fig. 5b) accompany these two mass loss steps: there are two endothermic peaks at 485–491 °C and 883–956 °C.
The ferrocene fragment of 1 and 2 in the solid state was found to reversibly oxidize at 0.43–0.53 V vs. Fc+/Fc with ΔE = 105–110 mV (Table 1), that is, at potentials comparable to the ferrocene-1,10-diyl-bis(H-phosphinic acid precursor (E1/2 = 0.35 V in MeOH)29 and was much more difficult compared to unsubstituted ferrocene (0 V and ΔEp = (Eap − Ecp) = 60 mV.45 That is, the oxidation potential is determined primarily by the influence of the acceptor nature of the phosphinate group. Polymer 1 has one oxidation peak and one pronounced reduction peak at high negative potentials (Fig. 7, Table 1). Polymer 2 has two pronounced oxidation peaks (Fig. 7) and one reduction peak. The second oxidation peak of 2 can be attributed to the oxidation of a CoII/III couple; it is not observed for the zinc analog.
Fig. 7 Cyclic voltammograms obtained with a modified CPE of polymers 1 and 2 (paste composition: graphite + ionic liquid + MOF) in 2 ml CH3CN (0.1 M Bu4NBF4). Potentials vs. Fc+/Fc. |
Compound | Oxidation | Reduction | ||
---|---|---|---|---|
E p, V | E 1/2, V | ΔEf–bp, mV | E p, V | |
1 | 0.53 | 0.43 | 105 | −2.57 |
2 | 0.43 | 0.36 | 110 | −1.33 |
1 and 2 were found to be active electrocatalysts for hydrogen production using [(DMF)H]OTf46 as the proton source in acetonitrile solution (Fig. 8). At a potential of approximately −1.3 V (ref. Fc+/Fc), for both CPs, significant increases in the reduction current are observed in the presence of proton donors. The direct acid reduction on the surface of modified electrodes was also observed at −1.95 V (Fig. 8). Thus the new catalytic peak at less negative potential provides 650 mV of voltage economy. The overpotential (difference between the standard potential of the acid reduction46 and the potential of the catalytic process) for 1 and 2 is 820–840 mV. When using N,N-dimethylformamidium ([DMF(H)+]) as the acid in the acetonitrile solution, 2 CP displays a turnover frequency of 300 s−1, calculated from the catalytic current47 measured in the presence of the acid, which is among the fastest rates reported for any CP electrocatalysts in non-aqueous solutions. For 1, such a TOF calculation cannot be applied, since there is an increase in the current of the new peak.
Fig. 8 Cyclic voltammograms obtained with a modified CPE for 2 in the presence of various amounts of [(DMF)H]OTf in acetonitrile. Conditions: 0.1 V s−1 scan rate, 0.1 M Bu4NBF4. |
These catalysts were found to be stable in water and under strongly acidic conditions, with only trace decomposition observed over 2 weeks in the presence of 0.5 M H2SO4 or [DMF(H)+] (in the absence of current).
The electrocatalytic HER activities of CPs were investigated in the 0.5 M H2SO4 solution by linear sweep voltammetry (LSV) (Fig. 9, Table 2). As the hydrogen evolution reaction (HER) catalysts, the CPs exhibited in 0.5 M H2SO4 the overpotential η10 (at 10 mA cm−2 current density) of 340 or 450 mV, the onset overpotential of 220 or 300 mV (vs. RHE), the Tafel slope of 110 or 120 mV dec−1, correspondingly, high cycle durability and considerable long-term stability for the HER. Thus, these CPs can continuously work for 1000 cycles with negligible activity loss. TOFs in 0.5 M H2SO4 were evaluated by the literature method.7 For 1 and 2 TOFs (at 300 mV) were 4.5 10−3 and 1.5 10−3 s−1 respectively. Although this is not a record activity in the aqueous solutions of H2SO4, it is quite satisfactory among the known organometallic structures. And for the Zn and Co redox active coordination polymers based on the ferrocene-containing diphosphinate ligand, such activity is observed for the first time.
−Eonset, mV | η (at 10 mA cm−2), mV | Tafel slope, mV dec−1 | |
---|---|---|---|
1 | 220 | 340 | 110 |
2 | 300 | 450 | 120 |
We conducted the dynamic study of the coordination polymer state in water and acids over time. To control the state, we used both voltammetry and powder XRD vs. time. This allowed us to assert that in aqueous neutral media, polymers did not change their structure over a week, at least. And in acidic media (0.5 M H2SO4) the zinc polymer showed traces of decomposition after a week, and the cobalt polymer after one day (color change, the disappearance of peaks or the presence of new peaks in voltammograms and in the powder diffraction pattern, the drop of catalytic current gains) (see ESI†).
These discovered electrocatalytic properties of redox active coordination polymers based on the ferrocene-containing diphosphinate ligand make them a potential matrix for cost-effective catalysts in electrochemical hydrogen production. These electrocatalysts consisting of widely distributed metal (Fe, Zn, Co) are promising alternatives to high-cost Pt catalysts for the HER. Purposeful modification of the structure and ligand environment of the metals in these bimetallic coordination polymers will, one may hope, increase the efficiency of catalysis.
Footnote |
† Electronic supplementary information (ESI) available: Powder XRD, IR, Raman, and other information for the coordination polymers. CCDC 1861598, 1861599 and 1866808. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt04618b |
This journal is © The Royal Society of Chemistry 2019 |