Tris(bipyridine)Metal(II)-Templated Assemblies of 3D Alkali-Ruthenium Oxalate Coordination Frameworks: Crystal Structures, Characterization and Photocatalytic Activity in Water Reduction

A series of 3D oxalate-bridged ruthenium-based coordination polymers with the formula of {[ZII(bpy)3][MIRu(C2O4)3]}n (ZII = Zn2+ (1), Cu2+ (3, 4), Ru2+ (5, 6), Os2+ (7, 8); MI = Li+, Na+; bpy = 2,2’-bipyridine) and {[ZnII(bpy)3](H2O)[LiRu(C2O4)3]}n (2) has been synthesized at room temperature through a self-assembly reaction in aqueous media and characterized by single-crystal and powder X-ray diffraction, elemental analysis, infrared and diffuse reflectance UV–Vis spectroscopy and thermogravimetric analysis. The crystal structures of all compounds comprise chiral 3D honeycomb-like polymeric nets of the srs-type, which possess triangular anionic cages where [ZII(bpy)3]2+ cationic templates are selectively embedded. Structural analysis reveals that the electronic configuration of the cationic guests is affected by electrostatic interaction with the anionic framework. Moreover, the MLCT bands gaps values for 1–8 can be tuned in a rational way by judicious choice of [ZII(bpy)3]2+ guests. The 3D host-guest polymeric architectures can be used as self-supported heterogeneous photocatalysts for the reductive splitting of water, exhibiting photocatalytic activity for the evolution of H2 under UV light irradiation.


Introduction
In recent years, the depletion of fossil fuels and the environmental problems caused by their combustion have stimulated research on the development of new renewable energy production technologies. So far, several approaches have been proposed in order to address this challenge. Among those explored, the system combining photocatalysts and solar energy as a clean and abundant energy resource is recognized to be of great promise. Currently, enormous attention has been paid to photocatalytic hydrogen production from water, which is a promising way to produce hydrogen as a potential clean energy source [1,2]. In this line, the hybridization of organic and inorganic materials opens up a new field in the design and preparation of applicable photocatalysts for water splitting reaction by the integration of useful organic and inorganic characteristics within a single composite [3,4]. n network. Thereby, tris-bipyridine complexes are quantitatively and homogeneously distributed within the polymeric framework. Moreover, the chemical variation and combination of the metal ions in the oxalate backbone, as well as in the tris-bipyridine cation offer unique opportunities for the rational design of a photoactive coordination polymer with the desired photochemical and photophysical properties, such as light-induced electron transfer and excitation energy transfer in the solid state.

X-Ray Structure Determinations
Tetrahedral-shaped single crystals of Compounds 1´8 ( Figure S1) were selected for single-crystal X-ray diffraction analyses. The intensity data were collected at room temperature on an Oxford-Gemini X-ray diffractometer using for Compounds 2 and 4 graphite-monochromatic Mo-Kα (λ = 0.71073 Å) and for 1, 3, 5´8, Cu-Kα (λ = 1.54184 Å) radiation. The CrysAlisPro software was used for cell refinement and data reduction. Images were collected at a 55-mm fixed crystal-detector distance, using the oscillation method, with 1 oscillation and variable exposure time per image. The structures were solved by direct methods using the SIR92 program [81]. The refinement was performed by SHELX-97 using full-matrix least squares on F 2 [82]. All non-H atoms were anisotropically refined. The hydrogen atoms of the 2,2'-bipyridine ligand were placed geometrically, and the hydrogen atoms of the water molecule in Compound 2 could not be located, but were included in the formula. Flack's absolute parameter (x) was used to determine the space group of compounds [83]. Crystallographic data for 1´8 (CCDC#1404961-1404964, #1404970-1404973) have been deposited with Cambridge Crystallographic Data Centre. The detailed crystallographic data are summarized in Table S1 (Supplementary Materials). Topological and geometrical analysis of 1´8 was obtained using TOPOS 4.0 software [84]. X-ray powder diffraction patterns were collected with a X'Pert Philips X-ray diffractometer (CuKα radiation, λ = 1.5418 Å) at room temperature. The powder diffraction patterns indicate that all compounds are isostructural and show analogous patterns to the simulated patterns from the atomic coordinates of the crystal structures of 1´8 ( Figures S2-S5, Supplementary Materials).

Characterization Methods
The IR spectra were recorded on a Bruker Tensor-27 spectrophotometer as KBr pellets in the 4000-500 cm´1 region. Microanalyses (C, H, N) were carried out by the use of a Perkin-Elmer model 2400B elemental analyzer. X-ray microanalysis (SEM/EDX) confirmed the ratio Ru:Z II to be 1:1 (Z II = Zn 2+ , Cu 2+ , Os 2+ ), by using JEOL JSM-6100 scanning microscopy (SEM) coupled with an INCA Energy-200 dispersive X-ray microanalysis system (EDX) with a PentaFET ultrathin window detector. As shown in Figure S6 (Supplementary Materials), the microcrystalline texture of the samples consists of microcrystals that repeat the same habit as those obtained single crystals, indicating that powder products have been obtained as pure phases. A Mettler-Toledo TGA/SDTA851 was used for the thermal analyses in a nitrogen and air dynamic atmosphere (50 mL/min) at a heating rate of 10˝C/min. Approximately 10 mg of powder sample were thermally treated, and blank runs were performed. A Pfeiffer Vacuum TermoStar™ GSD301T mass spectrometer was used to determine the evacuated vapors. The masses 15 (NH 3 ), 18 (H 2 O), 44 (CO 2 ) and 46 (NO 2 ) were tested by using a detector C-SEM, operating at 1200 V, with a time constant of 1 s. A Cary 6000i (Varian) spectrophotometer was used to measure diffuse reflectance spectra in the range 200-1800 nm using a polytetrafluoroethylene (PTFE)-coated integrating sphere.

Photocatalytic Hydrogen Evolution
Reactions were carried out at room temperature in a 100-mL gastight cell that was custom-designed in order to allow purging and irradiation of the suspension. The gastight cell was a 100-mL two-necked, flat-bottomed flask with a water refrigerator. The cell volume was 100 mL, of which gases occupied 83 mL. In each experiment, 10 µmol of heterogeneous catalyst were dispersed in a mixture containing 10 mL H 2 O and 7 mL TEA (triethylamine). Reaction mixtures were deoxygenated with three cycles of evacuation and purging with argon. The samples' solutions were illuminated with UV light at room temperature by a 500-W mercury lamp (HELIOS ITALQUARTZ Apparatus, Model UV50F-85P503I5, ď366 nm) for 12 h. During reaction, magnetic stirring was used to prevent sedimentation of the catalyst. For experiments performed with visible light irradiation, the xenon lamp (150 W, ě417 nm) was used as the light source. Reaction products were analyzed by mass spectrometry taking regular aliquots (0.5 mL) of the reactor headspace gas through a septum using a gastight syringe. Mass spectrometry analyses were performed using an OmniStar™ (Pfeiffer Vacuum) gas analysis module connected to AutoChem II 2920 (Micromeritics) catalyst characterization system. A cold trap was used with Ar as the carrier gas. Each gas aliquot was quantified using the calibration graph, which had been previously obtained using standard 10% (v/v) H 2 in Ar and 5% (v/v) O 2 in He gas mixtures (Air Liquid, Paris, France).
The detailed crystal data and structure determination parameters of ruthenium-based coordination polymers 1-8 are summarized in Table S1. The CPs 1-8 crystalize in the cubic chiral space group P2 1 3 with the asymmetric unit consisting of a complete oxalate ligand, the Ru 3+ and Na + /Li + ions of the anionic network, the Z II metal center (Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ) and the complete bpy ligand of the cationic template ( Figure S7a). Each Ru 3+ and Na + /Li + ion is surrounded by six oxygen atoms of the oxalate ligand forming a distorted octahedral coordination environment (Figure 1a,b,d,e) with the mean Ru´O and Na/Li´O bond lengths, which are within the range observed for analogous compounds [65].
The detailed crystal data and structure determination parameters of ruthenium-based coordination polymers 1-8 are summarized in Table S1. The CPs 1-8 crystalize in the cubic chiral space group P213 with the asymmetric unit consisting of a complete oxalate ligand, the Ru 3+ and Na + /Li + ions of the anionic network, the Z II metal center (Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ) and the complete bpy ligand of the cationic template ( Figure S7a). Each Ru 3+ and Na + /Li + ion is surrounded by six oxygen atoms of the oxalate ligand forming a distorted octahedral coordination environment (Figure 1a,b,d,e) with the mean Ru−O and Na/Li−O bond lengths, which are within the range observed for analogous compounds [65]. Selected bond distances and distortion parameters of Ru III , Z II and M I coordination environments for Compounds 1−8 are given in Table 1. Interestingly, the {Ru(C2O4)3} and {M I (C2O4)3} structural units (SBU) manifest the same Δ or Λ-configuration in the chiral 3D anionic networks (Figure 1a,b,d,e). Thus, Compounds 1, 4-6 and 8 build SBU with the Λ-form configuration, while 2-3 and 7 are constructed with the Δ-form.
In this type of structure, the oxalate ligand exhibiting μ-coordination mode ( Figure S7b) links in an alternate manner Ru 3+ and Na + /Li + metal centers to form helical substructures, where Ru···Na/Li distances ranged from 5.46-5.63 Å. As shown in Figure 1c  Selected bond distances and distortion parameters of Ru III , Z II and M I coordination environments for Compounds 1´8 are given in Table 1  In this type of structure, the oxalate ligand exhibiting µ-coordination mode ( Figure S7b) links in an alternate manner Ru 3+ and Na + /Li + metal centers to form helical substructures, where Ru¨¨¨Na/Li distances ranged from 5.46-5.63 Å. As shown in Figure 1c,f, helical substructures with three-fold axis interpretation spread along the b-axis and, depending on the {Ru(C 2 O 4 ) 3 } and {M I (C 2 O 4 ) 3 } SBUs' conformations (∆ or Λ), exhibit left-or right-handed rotation. Repeatedly connected adjacent helices form a porous anionic 3D framework with honeycomb-like channels running along the [111] crystallographic direction (Figure 2a). According to topological analysis performed using TOPOS 4.0 software [84], resulting 3D anionic networks are three-connected uninodal nets with a 10 3 -a array topology (Figure 2b; also denoted as the srs-type net) [85]. (6), Os 2+ (8)) coordination polymers; the configuration and structural distortion parameters of [Z II (bpy) 3 ] 2+ (Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ) guests compared to the corresponding [Z II (bpy) 3 ] 2+ cation in salts 1 . , where θn is one of the twelve N-Z II -N angles in the coordination sphere [90]. 3 The mean quadratic elongation: = ∑

Bonds
, where ‹d› and dn are the mean Z II -N bond length and the six Z II -N bond lengths in coordination polyhedra, respectively [91]. In fact, the {[M I Ru(C2O4)3] 2− }n (M I = Na + , Li + ) anionic frameworks are cage-like structures with three-fold cavities formed as a result of helical substructure interconnection. The tris-chelating cationic [Z II (bpy)3] 2+ (where Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ; bpy = 2,2'-bipyridine) complex acting as the charge balanced template fits the large anionic cavities in a specific and highly symmetrical manner (Figure 3a). Interestingly, the cationic entity acts as a structural (appropriate size/shape),   3 ] 2+ (where Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ; bpy = 2,2'-bipyridine) complex acting as the charge balanced template fits the large anionic cavities in a specific and highly symmetrical manner (Figure 3a). Interestingly, the cationic entity acts as a structural (appropriate size/shape), stoichiometric and chiral template, which repeats the homochiral conformational characteristics (∆ or Λ), such as SBUs in the polymeric network, resumed in Table 1. The role of bulky [Z II (bpy) 3 ] 2+ cations in oxalate-based anionic coordination arrays has been previously investigated and has a significant effect on the network structure formation [72,92].
For the sake of topological simplification of the 3D framework structures, the anionic cavities are generalized as the self-dual natural tile characteristic for 10,3-net topologies and can be described as a triangle vertex figure with 14 vertices and three faces ( Figure 3b). As illustrated in Figure 3c, the [10 3 ] tiles sharing one face reconstruct porous spaces of the anionic network to form the 3D honeycombed architecture.  Applying the models of Voronoi-Dirichlet polyhedra [93], an accessible volume of three-fold anionic cages in {[M I Ru(C2O4)3] 2− }n (M I = Na + , Li + ) nets, the volume of cation [Z II (bpy)3] 2+ (Z II = Zn 2+ , Cu 2+ , Ru 2+ and Os 2+ ) incorporated in the networks and their volume in free salts were calculated and summarized in Figure 4. The volume of anionic cages in the {[NaRu(C2O4)3] 2− }n framework are slightly bigger than those in the {[LiRu(C2O4)3] 2− }n, which is caused by the difference between the ionic radii of Na and Li metal centers incorporated in the framework. Notably, the cationic template [Z II (bpy)3] 2+ (where Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ) selectively residing in the anionic cages of 1, 3-8 undergoes a 6.9%-14.4% expansion compared to the corresponding cationic complex in the free salt forms.  Applying the models of Voronoi-Dirichlet polyhedra [93], an accessible volume of three-fold anionic cages in {[M I Ru(C 2 O 4 ) 3 ] 2´} n (M I = Na + , Li + ) nets, the volume of cation [Z II (bpy) 3 ] 2+ (Z II = Zn 2+ , Cu 2+ , Ru 2+ and Os 2+ ) incorporated in the networks and their volume in free salts were calculated and summarized in Figure 4. and was observed in analogous compounds {[Z II (bpy)3](H2O)[LiCr(C2O4)3]}n (Z II = Ni 2+ , Ru 2+ ) [71] and {[Z III (bpy)3](X)[NaM III (C2O4)3]}n (M III = Cr 3+ , Al 3+ , Rh 3+ ; Z III = Cr 3+ , Rh 3+ , Co 3+ ; X = ClO4 − , PF6 − ) [72][73][74][75][76], The special packing arrangement of [Z III (bpy)3] 3+ or [Z II (bpy)3] 2+ cations creates cubic-shaped cavities able to encapsulate small anions (ClO4 − or PF6 − ) or neutral molecules (H2O). In the case of 2, three pairs of parallel aligned, adjacent bpy ligands, perpendicularly oriented to each other, form the cubic-shaped vacancies in which the water molecules reside with full occupancy of this site.  However, in the actual case of the structure of 2, the capture of water molecules into these cavities is expected, taking into account the aqueous preparation of the compound. Figure 5 shows the packing arrangement of three adjacent tris-chelated [Zn II (bpy) 3 ] 2+ cations exhibiting the cubic-shaped cavity, which is drawn with the frontal bpy ligand, partially omitted in order to have a free view into the cage where the H 2 O molecule is entrapped. The volume of this cubic cage in Compound 2 is about 45 Å 3 ( Figure S8). Consequently, the size decreasing of the [Zn II (bpy) 3 ] 2+ cationic template observed in 2 can be explained as a result of a steric pressure effect introduced by incorporation of additional water molecules into the cubic-shaped cavities. However, in the actual case of the structure of 2, the capture of water molecules into these cavities is expected, taking into account the aqueous preparation of the compound. Figure 5 shows the packing arrangement of three adjacent tris-chelated [Zn II (bpy)3] 2+ cations exhibiting the cubic-shaped cavity, which is drawn with the frontal bpy ligand, partially omitted in order to have a free view into the cage where the H2O molecule is entrapped. The volume of this cubic cage in Compound 2 is about 45 Å 3 ( Figure S8). Consequently, the size decreasing of the [Zn II (bpy)3] 2+ cationic template observed in 2 can be explained as a result of a steric pressure effect introduced by incorporation of additional water molecules into the cubic-shaped cavities.

Infrared Spectroscopy
The IR spectra of 1-8 are very similar ( Figure S9

Thermogravimetric Analysis
The thermal stability of 1-8 in air and nitrogen atmospheres was investigated. The

Infrared Spectroscopy
The IR spectra of 1-8 are very similar ( Figure S9

Thermogravimetric Analysis
The thermal stability of 1-8 in air and nitrogen atmospheres was investigated. The thermogravimetric curves (TG and derivative TG), SDTA and mass spectrometry analysis of evacuated vapors for 1-8 in both air and nitrogen atmospheres are depicted in Figures S10-S11 and S13-S14 (Supplementary Materials), respectively. As represented, the thermogravimetric analysis results demonstrate similar decomposition behaviors, confirming the isomorphic nature of Compounds 1-8. The degradation processes occurred in one single step simultaneously in both air and nitrogen atmospheres and very closely resemble each other. As summarized in Table S2 (Supplementary Materials), in air atmosphere, degradation of 1-8 proceeds through one continuous stage in which a mass loss of 63.6%-75.2% (depending on the compositional characteristics) is observed in the range 180-600˝C. This mass loss is associated with a broad exothermic peak on the SDTA and DSC curves (Figures S10-S12, Supplementary Materials) and corresponds to simultaneous decomposition of the organic template and ligand. The associated mass spectrometry m/z 18, 44 and 46 curves are in good agreement with the TG/dTG curves and occur as one broad maximum coinciding with the maximum of mass loss in dTG curves, suggesting continuous structure collapsing and oxidational degradation of the ligands.
Oppositely, in nitrogen atmosphere, the pyrolysis of Compounds 1-8 proceeds in three steps (Table S3,

UV-Vis Spectroscopy
The room-temperature UV-Vis-NIR diffuse reflectance spectra of the powder samples corresponding to 1-8 are represented in Figure 6. All spectra consist of three groups of bands: the high energy bands observed between 200 and 330 nm are assigned to the πÑπ* transition of bpy ligands; the intense broad band at ca. 400 nm corresponds to the MLCT transition in [Na/LiRu(C 2 O 4 ) 3 ] 2´u nits; whereas the weaker bands in the VIS-NIR region have been assigned to ligand-field transitions within the [Z II (bpy) 3 ] 2+ cationic templates of Compounds 1-8. Figure 6a shows a comparison of the diffuse-reflectance spectra of 1, 2 and [Zn(bpy) 3 ](ClO 4 ) 2 compounds. As expected, the spectrum of the [Zn(bpy) 3 ] 2+ complex does not appear to have d-d transitions due to the close shell electronic configuration (t 2g 6 e g 4 ) for the d 10 Zn 2+ ion [68]. However, the spectra of 1 and 2 exhibit a broad adsorption band ca. 700 nm, which was assigned to the d-d (Ru 3+ ) spin-forbidden 2 T 2 Ñ 4 T 2 transition within the [Na/LiRu(C 2 O 4 ) 3 ] 2´f ramework units [94]. The Vis-NIR spectral region of 3 and 4 coordination polymers templated by the [Cu(bpy) 3 ] 2+ cationic complex (Figure 6b) reveal the adsorption band of ca. 690 nm that was assigned to the 2 E g Ñ 2 T 2g single electron transition, which is expected in the octahedral crystal field for the Cu 2+ ion ( 2 T 2g ) with the t 2g 5 e g 4 excited electronic state [95]. Figure 6a shows a comparison of the diffuse-reflectance spectra of 1, 2 and [Zn(bpy)3](ClO4)2 compounds. As expected, the spectrum of the [Zn(bpy)3] 2+ complex does not appear to have d-d transitions due to the close shell electronic configuration (t2g 6 eg 4 ) for the d 10 Zn 2+ ion [68]. However, the spectra of 1 and 2 exhibit a broad adsorption band ca. 700 nm, which was assigned to the d-d (Ru 3+ ) spin-forbidden 2 T2→ 4 T2 transition within the [Na/LiRu(C2O4)3] 2− framework units [94]. The Vis-NIR spectral region of 3 and 4 coordination polymers templated by the [Cu(bpy)3] 2+ cationic complex (Figure 6b) reveal the adsorption band of ca. 690 nm that was assigned to the 2 Eg→ 2 T2g single electron transition, which is expected in the octahedral crystal field for the Cu 2+ ion ( 2 T2g) with the t2g 5 eg 4 excited electronic state [95]. Normally, the octahedral coordination of Cu 2+ ions undergoes Jahn-Teller distortion, leading to the trigonally-distorted pseudo-D3 symmetry and can be observed in the corresponding spectrum of the [Cu(bpy)3](ClO4)2 compound (Figure 6b), where d-d transitions appeared as a medium-strong band of ca. 680 nm, and a sharp band of ca. 1100 nm should be treated as the trigonal field and assigned to 2 E→ 2 E and 2 E→ 2 A1 transitions, respectively [96]. Based on these observations, the fact that the {[M I Ru(C2O4)3] 2− }n (M I = Na + , Li + ) anionic framework rigidly restricts Jahn-Teller distortion in the guest [Cu(bpy)3] 2+ cationic complex is concluded. Furthermore, the corresponding structural distortion parameters (bond angle variance (σ 2 ) and mean quadratic elongation (λ)) calculated for the [Cu(bpy)3] 2+ complex in 3 and 4 frameworks, which are summarized in Table 1, suggest that the coordination environment of the Cu 2+ ion in the template cationic complex exhibits more regularized octahedral geometry than that found for the corresponding free salt form.
The diffuse-reflectance spectra of Compounds 5 and 6 are similar with respect to the corresponding [Ru(bpy)3](ClO4)2 complex, and the Vis-NIR region consists of several high intensity bands (Figure 6c), which are attributed to electron transitions within the low-spin [Ru(bpy)3] 2+ complex, where the Ru 2+ ion possesses the t2g 5 eg 1 electronic configuration [95]. Thus, the absorption band of ca. 450 nm is assigned to the 1 A1→ 1 T1 transition. Moreover, the shoulder centered at 480 nm corresponds to the t2g→π* metal-ligand charge transfer ( 1 MLCT) transition, while the broad shoulder observed at 560 nm belongs to a spin-forbidden third t2g→π* metal-ligand charge transfer ( 3 MLCT) transition [97]. Similarly, Compounds 7 and 8 exhibit diffuse-reflectance spectra close to the [Os(bpy)3](PF6)2 complex. As shown in Figure 6d, the Vis-NIR region of spectra consists of several overlapped bands located from 410 -520 nm and was assigned to the 1 A1→ 1 T2 and 1 A1→ 1 T1 d-d transitions, which are expected for the low-spin [Os(bpy)3] 2+ complex with the Os 2+ ion in the t2g 5 eg 1 Normally, the octahedral coordination of Cu 2+ ions undergoes Jahn-Teller distortion, leading to the trigonally-distorted pseudo-D 3 symmetry and can be observed in the corresponding spectrum of the [Cu(bpy) 3 ](ClO 4 ) 2 compound (Figure 6b), where d-d transitions appeared as a medium-strong band of ca. 680 nm, and a sharp band of ca. 1100 nm should be treated as the trigonal field and assigned to 2 EÑ 2 E and 2 EÑ 2 A 1 transitions, respectively [96]. Based on these observations, the fact that the {[M I Ru(C 2 O 4 ) 3 ] 2´} n (M I = Na + , Li + ) anionic framework rigidly restricts Jahn-Teller distortion in the guest [Cu(bpy) 3 ] 2+ cationic complex is concluded. Furthermore, the corresponding structural distortion parameters (bond angle variance (σ 2 ) and mean quadratic elongation (λ)) calculated for the [Cu(bpy) 3 ] 2+ complex in 3 and 4 frameworks, which are summarized in Table 1, suggest that the coordination environment of the Cu 2+ ion in the template cationic complex exhibits more regularized octahedral geometry than that found for the corresponding free salt form.
The diffuse-reflectance spectra of Compounds 5 and 6 are similar with respect to the corresponding [Ru(bpy) 3 ](ClO 4 ) 2 complex, and the Vis-NIR region consists of several high intensity bands (Figure 6c), which are attributed to electron transitions within the low-spin [Ru(bpy) 3 ] 2+ complex, where the Ru 2+ ion possesses the t 2g 5 e g 1 electronic configuration [95]. Thus, the absorption band of ca. 450 nm is assigned to the 1 A 1 Ñ 1 T 1 transition. Moreover, the shoulder centered at 480 nm corresponds to the t 2g Ñπ* metal-ligand charge transfer ( 1 MLCT) transition, while the broad shoulder observed at 560 nm belongs to a spin-forbidden third t 2g Ñπ* metal-ligand charge transfer ( 3 MLCT) transition [97]. Similarly, Compounds 7 and 8 exhibit diffuse-reflectance spectra close to the [Os(bpy) 3 ](PF 6 ) 2 complex. As shown in Figure 6d, the Vis-NIR region of spectra consists of several overlapped bands located from 410-520 nm and was assigned to the 1 A 1 Ñ 1 T 2 and 1 A 1 Ñ 1 T 1 d-d transitions, which are expected for the low-spin [Os(bpy) 3 ] 2+ complex with the Os 2+ ion in the t 2g 5 e g 1 ground state [95].
Similarly to [Ru(bpy) 3 ] 2+ -contained compounds, the diffuse-reflectance spectra of 7 and 8, as well as [Os(bpy) 3 ](PF 6 ) 2 exhibit characteristic shoulders localized from 560 800 nm, which are attributed to the t 2g Ñπ* metal-ligand charge transfer (MLCT) along with the spin-forbidden third t 2g Ñπ* metal-ligand charge transfer ( 3 MLCT) transition [98]. The band gaps of 1-8 were estimated from Tauc plots [99] obtained from UV-Vis diffuse-reflectance data transformed by the Kubelka-Munk function ( Figure S16). The band gaps (E g ) were determined extrapolating the intersection point between the baseline and the linear portion of the adsorption edge in a plot represented as function (αhυ) 3/2 against energy (hυ, eV). The optical adsorption related to E g in the region of MLCT transition, which is assumed to be directly forbidden, can be assessed at 2.54 eV for 1, 2.31 eV for 2, 2.68 eV for 3, 2.67 eV for 4, 2.10 eV for 5, 2.11 eV for 6, 1.68 eV for 7 and 1.67 eV for 8, respectively. The determined values of band gaps for coordination polymers 1-8 follow the order 3 « 4 > 1 > 2 > 5 « 6 > 7 « 8.

Photocatalytic Activity
The photocatalytic splitting of water for hydrogen production using Compounds 1-8 under UV (ď366 nm) and VIS (ě417 nm) light irradiation was examined. In a typical experiment, the reactions were performed in a reactor equipped with a refrigerated 500-W Hg-lamp (ď366 nm) and using 10 µmol of heterogeneous catalyst 1-8 dispersed in a water (H 2 O)/triethylamine (TEA) mixture (v/v = 1.4:1), where TEA acts as the electron donor. The amounts of H 2 produced over 1-8 photocatalysts under 8 h of UV light irradiation are depicted in Figure 7.
As seen in Figure 7 (left), the heterogeneous catalysts 1-8 are active in the photoreductive water splitting reaction, forming 1.26 µmol (TON of 0.12) of H 2 after 8 h under UV light irradiation. Catalysts 7 and 8 exhibit the highest photocatalytic performance, compared to the activity of the other compounds, and their activities decrease through the sequence 8 > 7 > 6 > 5 > 2 > 1 « 4 > 3. Interestingly, this sequence of photocatalytic activity is directly opposed to the calculated band gaps for these compounds (Figure 7, right). Therefore, the synergistic effects of the smallest band gap and chemical nature of the [Z II (bpy) 3 ] 2+ cationic template are the main factors determining the photocatalytic activities of 1-8 under UV light irradiation. Blank reactions were performed to ensure that H 2 production was light-promoted and conducted over a heterogeneous catalyst. One blank was UV-illuminated without the catalyst, and another was in the dark with the catalyst under the same experimental conditions. No H 2 was detected in the above two blank tests. A "hot filtration" test was conducted with 6, in which the heterogeneous catalyst, previously exposed to 8 h of reaction under UV light, was removed by centrifugation, and the transparent uncolored reactant solution was returned into the photolysis cell (previously degassed and filled with Ar) for an additional consecutive photocatalytic run. As a result, no H 2 was detected, which indicates that the detected photoactivities are promoted by heterogeneous catalysts rather than by leached soluble species. experimental conditions. No H2 was detected in the above two blank tests. A "hot filtration" test was conducted with 6, in which the heterogeneous catalyst, previously exposed to 8 h of reaction under UV light, was removed by centrifugation, and the transparent uncolored reactant solution was returned into the photolysis cell (previously degassed and filled with Ar) for an additional consecutive photocatalytic run. As a result, no H2 was detected, which indicates that the detected photoactivities are promoted by heterogeneous catalysts rather than by leached soluble species.  Additionally, the photocatalytic activities of 1-8 were examined under VIS light irradiation under the same reaction conditions. As shown in Figure 8, all coordination compounds also catalyze the photoreduction of water to H 2 , albeit less efficiently. These differences in photocatalytic activities     3 ] 3´c omponents of the 1-8 frameworks, this leads to the decreasing of the photocatalytic performance in the water-splitting reaction.
Taking into account the above-mentioned statements, we can propose that the reaction includes the following steps: promotion of the [Ru III (C 2 O 4 ) 3 ] 3´s tructural unit of the framework to its excited state under UV light irradiation; resonant energy migration from the exited ([Ru III (C 2 O 4 ) 3 ] 3´) * unit to [Z II (bpy) 3 ] 2+ cationic guest through Forster and Dexter energy transfer mechanisms (see the additional references in the Supplementary Materials), causing the latter to go into the exited state; the exited species ([Z II (bpy) 3 ] 2+ )* transfers an electron, located on one bpy ligand, to the water proton and returns to its initial state through the oxidation of a sacrificial reductant TEA.
In order to confirm the recyclability of photocatalysts, the photocatalytic reaction of reductive water splitting was repeated four times with Compound 6, where after each catalytic cycle, the heterogeneous solid was separated by centrifugation, washed several times with distillated water and reused in the next consecutive photocatalytic run. As shown in Figure S17, the amounts of H 2 evolved after 8 h of UV light irradiation in each consecutive photocatalytic cycle decrease slightly, probably due to the loss of catalyst upon recycling manipulation procedures. Moreover, the closely similar photocatalytic activities of recycled catalyst suggest that Compound 6 does not undergo photodecomposition or deactivation, at least after four repeated catalytic runs. Additionally, to confirm the stability of heterogeneous catalyst, after each recycling run, the reused material 6 was checked by XRD, and as evidenced from the comparison of those diffractograms ( Figure S18), photocatalyst 6 maintains its crystallinity and structural integrity during the water splitting reaction. These results indicate that coordination polymers 1-8 behave as stable, active and reusable heterogeneous catalysts for the photoreductive water-splitting reaction. Moreover, we compare the photocatalytic activities of 1-8 with other known MOFs and CPs able to photo-split water to H 2 ( Table 2).
The presented results reveal that coordination polymers 1-8 show moderate photocatalytic activity towards H 2 generation under VIS light compared to known MOFs/CPs; meanwhile, under UV light, they exhibit higher photocatalytic efficiencies. It is reasonable to conclude that {M I Ru(C 2 O 4 ) 3 ] 2´} n (M I = Na, Li) anionic frameworks selectively templated by [Z II (bpy) 3 ] 2+ (Z II = Zn 2+ , Cu 2+ , Ru 2+ , Os 2+ ) cationic complexes can be viewed as designable and efficient heterogeneous catalysts for UV light-promoted photoreactions.
These results highlight that rational synthesis of 3D anionic architectures using a target cationic guest, such as [Z II (bpy) 3 ] 2+ , provides a powerful route for the construction of multifunctional guest-encapsulated CPs with a predictable structural topology and desirable properties.

Acknowledgments:
The authors thank FEDER and Spanish MINECO for financial support under Projects MAT2013-40950-R, UCAN08-4E-008, MAT2012-38664-C02-1 and Consolider ORFEO. Alla Dikhtiarenko also thanks the Spanish Ministerio de Educación, Cultura y Deporte for their pre-doctoral FPU grant (AP2008-03942). We would like to thank Jorge Gascon for fruitful discussions and valuable suggestions during the preparation of the manuscript.
Author Contributions: Alla Dikhtiarenko designed the experiments and performed the syntheses and basic analyses. Rafael Valiente and Pedro Villanueva-Delgado performed UV-Vis measurements and interpreted the data. José R. García and José Gimeno supervised the research work. Alla Dikhtiarenko, José R. García and José Gimeno wrote the paper. All authors were involved in reading and approving the final manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: