A unique formyl iodoargentate exhibiting luminescent and photocurrent response properties

Lili Yanga, Jian Zhou*a, Litao Anb, Shumei Cao*a and Jun Hua
aChongqing Key Laboratory of inorganic functional materials, College of chemistry, Chongqing normal university, Chongqing, 401331, P.R. China. E-mail: Jianzhou888888@163.com
bJiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, College of Chemistry and Chemical Engineering, Huaiyin Noraml University, Huaian 223300, China

Received 8th July 2019 , Accepted 17th September 2019

First published on 17th September 2019


A decomposition and self-assembly reaction affords a novel formyl iodoargentate [H2L]n[Ag2I3(μ-CHO)]n (1, L = 2,6-bis(1-imdazoly)pyridine) with an unprecedented CHO link mode, which provides the only example of iodoargentate incorporating the unstable formyl species under hydrothermal conditions. 1 exhibits luminescent and photocurrent response properties.


Metal formyl complexes (MFCs) are of particular interest as possible intermediates in the homogeneous metal catalyzed reduction of CO to generate various organic oxygenates.1 So far, numerous MFCs are known for d-/f-block metals, as exemplified by Mn,2 Zr,3 Ru,4 Ta,5 Mo,6 Rh,7 Pt,1b,8 Re9 and Ce.10 Conversely, no silver formyl complexes have been obtained until now to the best of our knowledge. The reported MFCs are usually prepared by the reaction of metal hydrides and CO or metal carbonyl complexes under inert atmosphere protection, but it is difficult to prepare MFCs under mild hydrothermal conditions, mainly because the formyl species is comparatively unstable with respect to sensitivity against water and oxygen.1a,11

The soft Lewis acidic Ag+ with 3d10 electronic configuration, which displays usually the interesting features such as biological, luminescent and catalytic properties,12 coordinates easily with the soft Lewis basic I ion in the presence of diverse structural directing agents, resulting in a variety of iodoargentates with novel properties, such as thermochromism,13 dielectric,14 photocatalysis,15 nonlinear optics,16 and semiconductor.17 However, the Ag+ ion interacts in a difficult manner with the hard Lewis basic C or O donors of formyl species in the presence of I anion. If both I anion and formyl species could be combined into the same framework structure in a synthesis, a new class of formyl iodoargentates with much more diversity of structures and unexpected properties would be obtained. Guided by this idea, we have successfully obtained the novel formyl iodoargentate [H2L]n[Ag2I3(μ-CHO)]n (1), which represent the only example of iodoargentate incorporating the unstable formyl species under hydrothermal conditions.

Light yellow block crystals of 1 were synthesized by hydrothermal reaction of AgI, KI, and 2,6-bis(1-imdazoly)pyridine in a mixed solvent of H2O, HI and DMF at 120 °C. The DMF is converted into formyl species by decomposition processes under hydrothermal conditions,18 but the mechanism of the reaction is still unclear. From the EDS mapping images in Fig. S1, the distribution of C, N, O, Ag and I is relatively uniform. The EDS analysis (Fig. S2) shows that a molar ratio of Ag[thin space (1/6-em)]:[thin space (1/6-em)]I is 2.14[thin space (1/6-em)]:[thin space (1/6-em)]2.85, consistent with the stoichiometry. The phase purity of 1 is verified by powder X-ray diffraction (PXRD) determination (Fig. S3).

1 crystallises in the monoclinic centrosymmetric space group P21/n with four formula units in the unit cell,§ and its asymmetric unit contains one protonated 2,6-bis(1-imdazoly)pyridine H2L2+ cation, one CHO anion, two Ag+ ions and I anions (Fig. 1a). The crystal structure of 1 consists of protonated H2L2+ cation and 1-D formyl iodoargentate chain [Ag2I3(μ-CHO)2+]n. Each Ag+ ion is coordinated by four I anions and two C/O atoms from two CHO ligands in a distorted octahedral fashion (Fig. S4). Two coordination environments are also observed for iodide ions, namely μ2-I2/I3 and μ4-I1 bridging modes. The Ag–I bond lengths lie between 2.7911(10) and 3.0056(12) Å [dav(Ag–I) = 2.8810 Å], compared with the corresponding bond distances (2.768(5)–3.1442(16) Å) of other iodoargentates with [AgI6] octahedron.19 The Ag–C bond distances vary from 2.111(14) to 2.223(12) Å and Ag–O bond distances are in the range of 2.108(9)–2.375(8) Å, which are in agreement with those (2.059(6)–2.312(4) Å for Ag–C and 2.129(6)–2.495(3) Å for Ag–O) observed in other compounds.20 The C–O distance of 1.485(15) Å is slightly longer than that of C–O single-bond length (about 1.43 Å) in alcohols or ethers, and is significantly activated (the C–O single-bond length). The similar activation is also observed in [(η5-CSMe4Et)TaCl2]2(μ-H)(μ-CHO) molecule (1.496 (14) Å for C–O).6 One imdazol and pyridyl of L is protonated, leading to the formation of H2L2+ cation that compensates the negative charge of [Ag2I3(μ-CHO)2+]n. The three heterocyclic rings of HL2+ cation are rigid, but these rings are connected by two rotatable C–N bonds, so it is a hinge-like HL2+ cation. The NCN atoms of two imdazol rings of HL2+ cation point in the opposite direction. One of the imdazol rings is almost coplanar with the pyridyl ring, but the other is highly twisted, the dihedral angles between the planes are 2.258° and 25.255°, respectively.


image file: c9dt02828e-f1.tif
Fig. 1 (a)The asymmetric unit of 1 with the labeling scheme, (b) flower-basket-shaped cluster [Ag4I9(CHO)], (c) 1-D formyl iodoargentate chain [Ag2I3(μ-CHO)2+]n.

Four Ag+ ions, nine I anions, and one CHO ligand are assembled into a new flower-basket-shaped cluster [Ag4I9(CHO)] (Fig. 1b). Four Ag+ ions are arranged in a quadrilateral manner, which is situated at the waist position of the flower basket cluster. Four I anions bridge four Ag⋯Ag edges, respectively, leading to a roughly quadrilateral petaline [Ag4I4] structure. One I anion caps the quadrilateral at the bottom to create a [Ag4I] tetragonal pyramid, while one CHO ligand lies at the handle position of the flower basket bridging four Ag+ ions. The adjacent flower-basket-shaped [Ag4I9(CHO)] units are aligned in an antiparallel fashion, and connected to each other via I and CHO bridges to give a new 1-D formyl iodoargentate chain [Ag2I3(μ-CHO)2+]n (Fig. 1c). The Ag⋯Ag distances are in the range of 3.0816(17)–3.3224(13) Å, and significantly shorter than the sum of the van der Waals radii for two Ag atoms (3.44 Å), indicating the existence of argentophilic interactions.13a The 1-D chain [Ag2I3(μ-CHO)2+]n is closely related to 1-D chain [Ag2X3]n.21 The backbone of [Ag2X3]n might be viewed as an ancestor of [Ag2I3(μ-CHO)2+]n, which is built up from the chain [Ag2X3]n decorated by CHO ligands. The H2L2+ cations are bridged via N–H⋯N H-bonds to form a 1-D [H2L2+]n chain (Fig. 2a). These [H2L2+]n chains interact further via π–π stacking interactions (Fig. 2b and Fig. S5) between pyridyl and imdazol rings with centroid-to-centroid distances of 3.578–3.775 Å, resulting in a 3-D supramolecular architecture with 1-D circular channels that are filled by 1-D [Ag2I3(μ-CHO)2+]n chains (Fig. 2c).


image file: c9dt02828e-f2.tif
Fig. 2 (a) 1-D [H2L2+]n chain constructed by N–H⋯N H-bonds, (b) π–π stacking interactions between pyridyl and imdazol rings, (c) the crystal packing diagram of 1, showing 1-D circular channels.

There are several unique characteristics of 1. Firstly, although the link modes of the CHO or CH2O ligand have emerged in some MFCs (Scheme 1),1–11 the CHO ligand in 1 is the first example of C and O contacts with two Ag+ ions, respectively, and it shows an unprecedented μ4-1,2κC:3,4κO link mode. Secondly, iodoargentates with octahedrally coordinated Ag+ center are very scarce, and appear to be limited to those of Tl2AgI3,19b [Ag(CH2I2)3][pftb]19a and [(Hpy)(Ag5I6)]n,22 mainly because the [AgI4] tetrahedral is common building unit within the iodoargentates, but other [AgI4X2] (X = C or O) octahedrons are only observed in 1. Thirdly, numerous 1-D iodoargentate chains based on the [AgI4] tetrahedra have been reported, as exemplified by [AgI2]n,16 [Ag2I3]n,21 [Ag5I94−]n,23 [Ag6I93−]n,24 and [Ag11I154−]n,25 but 1-D hybrid iodoargentate chains decorated by organic ligands are only encountered in 1-D chain [Ag2I3(μ-CHO)2+]n of 1. Finally, various aromatic imdazol or pyridyl derivatives with excellent redox behavior have been used as the structure-directing agents for the syntheses of novel iodoargentates with versatile structural chemistry and optoelectronic properties, while 2,6-bis(1-imdazoly)pyridine or its derivatives containing imdazol and pyridyl groups, which possess larger conjugated systems, are only found in 1. Therefore, 1 shows a new structural type.


image file: c9dt02828e-s1.tif
Scheme 1 Link modes of the CHO or CH2O ligand, E is the novel link mode observed in 1.

The IR spectrum of 1 exhibits that a single absorption peak at 3119 cm−1 is directly related to the stretching vibrations of vC–H bond. Compared with the IR spectrum of L, the vC–N stretching bands at 1096–1057 cm−1 can belong to a useful marker of the protonation of the imdazol and pyridyl rings (Fig. S6).26 The band at 1233 cm−1 is characteristic of vC–O stretching, which is confirmed the presence of C–O group. The solid-state UV-vis spectra of 1, L and AgI are shown in Fig. 3a. The spectrum of L ligand exhibits a strong peak at 275 nm, while a similar type of absorption peak (280 nm) was also found in 1, which can be assigned to the π–π* transitions of L or H2L2+. The spectrum of AgI shows two peaks (324 nm and 223 nm), similar speaks at (328 nm and 237 nm) were also observed 1, which can be belong to the electronic excitation located at the AgI or 1-D formyl iodoargentate chain [Ag2I3(μ-CHO)2−]n. The absorption edge of 1 (2.16 eV) shows a remarkable red shift in contrast with the bulk AgI (2.81 eV) and L ligand (3.48 eV), which could be related to intermolecular interactions between H2L2+ and iodoargentate chain [Ag2I3(μ-CHO)2−]n. Similar red shift phenomenon is also observed other iodoargentates.13 The absorption edge is compared with the value of other iodoargentate [HCP][Ag2I3] (1.89 eV),13a [Pr(DMF)8]2Pb3Ag10I22 (2.57 eV),27 and [TM(2,2-bipy)3]Ag5I7 (1.94–2.58 eV, TM = Zn, Ni, Co),28 which exhibit the properties of a wide-band-gap semiconductor. L shows an emission band at 326 nm with an excitation at 280 nm wavelength (Fig. S8). This value is in agreement with that (323–331 nm) of the reported free L molecule.29 On excitation at 333 nm, 1 exhibits emission bands at 416 nm and 438 nm (Fig. 3b). Compared with emission peak of L, the emission at 416 nm could be related to the interaction of H2L2+ cation and [Ag2I3(μ-CHO)2−]n anion, while the emission at 438 nm is probably caused by multimetal-centred processes or charge transitions among the [Ag2I3(μ-CHO)2−]n skeleton.


image file: c9dt02828e-f3.tif
Fig. 3 (a) The solid-state UV-vis spectra of 1, L and AgI at room temperature. (b) Emission spectrum of 1 at room temperature.

Considering that the absorption edge of 1 is in the energy range suitable for visible-light photoelectric conversion applications, we investigated its photocurrent response, using a three electrode photoelectrochemical cell with the microcrystal sample modified ITO working electrode. The photocurrent–time (IT) curve with on–off models is shown in Fig. 4 (the on–off interval of 50 s). Upon the repetitive irradiation, a clear photocurrent response is observed. The photocurrent intensity of 1 is about 1.6 μA cm−2, indicating good photoelectric conversion properties in contrast with the other iodoargentate systems.30 The photocurrent response of L was also examined under the same condition. L exhibits a slightly lower photocurrent response, which demonstrates that L has electron-donating/accepting capacities. The photocurrent generation mechanism of 1 may be explained as follows. Under irradiation, electron-rich [Ag2I3(μ-CHO)2−]n polyanions can transfer their electrons to H2L2+ cations with strong electron acceptor property by charge transfer interactions to produce [Ag2I3(μ-CHO)2−]n˙ radicals. The electrons on H2L2+˙ radicals move to ITO substrates, and then the anodic current is generated.


image file: c9dt02828e-f4.tif
Fig. 4 Photocurrent response in the light on–off process for 1 and L (applied potential 0.9 V).

Conclusions

One new formyl iodoargentate containing an unprecedented μ4-1,2κC:3,4κO link mode of the CHO ligand has been hydrothermally prepared and structurally characterized, which offers the only example of an iodoargentate containing an unstable formyl species under hydrothermal conditions. 1 has luminescent and photocurrent response properties, making 1 a good candidate for potential multifunctional materials. This result demonstrates that varied iodoargentate units incorporating different CHO link modes may realize the design of other formyl iodoargentates with unexpected properties. Further work is in progress.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NNSF of China (No. 21671029), the NSF of Chongqing (No. cstc2018jcyjAX0157), Program for leading talents of scientific and technological innovation in Chongqing (No. CSTCCXLJRC201707), the Innovation Program for Chongqing's Overseas Returnees (No. cx2018008) and Program for Excellent Talents in Chongqing Higher Education Institutions. The authors are also grateful to Chongqing Normal University for financial support (No.14CSLJ02).

Notes and references

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Footnotes

Electronic supplementary information (ESI) available: Crystal data in CIF format, CIF file, SEM image, EDS elemental mappings, EDS spectrum, XRD, and some figures. CCDC 1937433. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt02828e
Synthesis of 1: The reagents of AgI (0.0384 g, 0.163 mmol), KI (0.0777 g, 0.468 mmol), 2,6-bis(1-imdazoly)pyridine (0.0056 g, 0.027 mmol) and a mixed solvent (1 mL, VH2O[thin space (1/6-em)]:[thin space (1/6-em)]VHI[thin space (1/6-em)]:[thin space (1/6-em)]VDMF = 17[thin space (1/6-em)]:[thin space (1/6-em)]50[thin space (1/6-em)]:[thin space (1/6-em)]7) were placed in a thick Pyrex tube (ca. 20 cm long). The sealed tube was heated at 120 °C for 7 days to yield yellow block-shaped crystals of 1. The crystals were washed with distilled water, dried and stored under vacuum (32% yield based on AgI). Anal. Calc. (found %) for C12H12Ag2I3N5O (1): C 17.18 (17.26), H 1.44 (1.52), N 8.35 (8.41). IR (cm−1): 3119 (s), 1606 (s), 1527 (ms), 1468 (s), 1450 (vs), 1284 (ms), 1233 (ms), 1158 (ms), 1096 (ms), 1057 (s), 1004 (ms), 924 (ms), 853 (w), 793 (ms), 737 (s), 641 (w), 611 (ms), 525 (w).
§ Crystal data of 1: Mr = 838.71, monoclinic, space group P21/n, a = 7.0423(3), b = 19.1127(10), c = 13.4120(7) Å, β = 103.4771(16)°, V = 1755.51(15) Å3, Z = 4, ρcalcd = 3.172 g cm−3, F(000) = 1520, GOF = 1.029. A total of 11[thin space (1/6-em)]566 reflections were collected, 3607 of which were unique (Rint = 0.0337). R1/wR2 = 0.0418/0.0995 for 213 parameters and 2735 reflections (I > 2σ(I)). CCDC 1937433 contains the supplementary crystallographic data for this paper.

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