Structure, optical and magnetic properties of a novel homometallic coordination polymers: Experimental and Computational studies

Single crystal of 1D homometallic coordination polymer involving cobalt metal ion and P2Mo5 Strandberg type polyoxometallate cluster (C6H10N2)2[Co(H2O)4P2Mo5O23].6H2O, is prepared in aqueous solution and characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance, fluorescence and magnetism. Single crystal X-ray diffraction analysis reveals that this compound crystallizes in the triclinic system with space group P \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\,\bar{1}$$\end{document}1¯. The 3-(ammoniomethyl)pyridine C6H8N2 organic fragment is used merely as stabilizer for the promotion of topological structure. DRS data indicate that the synthesized material can be identified as a ferromagnetic semiconductor with optical bands gaps energy of 1.81 and 2.74 eV, respectively. The large value of refractive index observed in the visible region make the simple a promising candidate for visible optical communication devices and fluorescent emisson result provides that the complex belongs to a blue luminescent compounds. Moreover, magnetic measurements and electronic structure calculations show that P2Mo5 Strandberg polyoxoanion can be reported as a new class of ligand that is candidate to construct metal-inorganic framworks with long distance ferromagnetic superechange between Co(II) centers. The evidence from this study suggests that the syntesized polymer can become a great multifunctional material openning the door for the developpement of new coordination polymers based on Strandberg type polyxoxmetalate with potential applications.

Metal-inorganic hybrid frameworks are one of the brilliant candidates of multifunctional materials owing to their interesting topological structures and many potential behaviours and applications [1][2][3] . Among the versatile characteristics of multifunctional materials; structure, optical and magnetic properties accurately represent a huge challenge for the scientists of modern technology owing to their encouraging applications such as spintronics, high-density information storage and solar cell conversation 4,5 . Although, this ambition can be systematically generated by selecting the suitable starting reagents (metals ions, inorganic building units and organic groups).
As a fascinated class of inorganic metal-oxygen building units, Strandberg-type polyoxometalate P 2 Mo 5 appears to be a dominant class of metal-oxo cluster anions that exhibit numerous remarquable behaviors, such as high electron density, good stability and especially ligand to metal charge transfer (LMCT) optic phenomena 6,7 . Furthermore, the incorporation of a paramagnetic transition metal with P 2 Mo 5 polyoxoanion cluster open the door to explore new chemistry paths in fabricating heterostructures and ultracompact devices with magnetic and optical properties.
In the last few years, several P 2 Mo 5 complexes including discrete cluster, 1D chains, 2D layered structures and 3D frameworks have been reported [8][9][10] . For instance, J. Thomas groups reported a series of copper complexes based on the P 2 Mo 5 clusters and indicated the influence of pH and temperature in the crystallinity of the synthesized compounds 11 . In 2014, Z.You and al. synthetized a three new molybdophosphate complexes and revealed their Curie-Weiss paramagnetic behaviour 12 . Recently,we have successfully isolated a new inorganic-organic hybrid compound basrd on strandberg-type phosphomolybdate and copper cation and investigated the antiferromagnetic interactions between the copper centers 13 . However, some optical electronic properties like refractive index, dielectric constant and electronic structure calculation of these compounds families have not been investigated up to now. Thus, based on aformentioned consideration, we successfully isolated a multifunctional molecular material based on Strandberg-type P 2 Mo 5 cluster and cobalt cation Co(II). Even so, 3-(ammoniomethyl) pyridine organic fragment is used merely as stabilizer for the promotion of topological structural diversification. Additionally, X-ray diffraction, optical, magnetic and electronic structure calculations are carried as well.

Methods
Synthesis of the sample. All chemical elements were commercially purchased and used without further purification.
A 50 mL of an aqueous solution containing ammonium heptamolybdate (NH 4 ) 6 [Mo 7 O 24 ].4H 2 O (0.952 g -0.77 mmol) is slowly added to an aqueous solution of cobalt (II) chloride CoCl 2 .2H 2 O (0.166 g -1 mmol) and 3-picolylamine (0.216 g − 2 mmol). Experimental results indicate that reasonable yields (49%) of crystalline products can be obtained when the pH value of the mixture is adjusted to 3-4 with 85% H 3 PO 4 under continuous stirring. Two weeks after, purple crystals that are suitable for single crystal X-ray diffraction were obtained. Elemental analysis for C 12  X-ray diffraction and measurements of physical properties. Intensity data were collected on a Nonius Kappa CCD diffractometer with graphite-monochromated α MoK (λ = 0.71073 Å) radiation at room temperature. The crystal structure is solved by direct method and refined by full-matrix least-squares on F 2 using the SHELXTL-97 program 14 . Anisotropic thermal parameters were used to refine all non-hydrogen atoms. The H atoms were placed geometrically and refined using a riding model. In Addition, the Molecular Hirshfeld surfaces are generated by CrystalExplorer computer program 15 .
The crystallographic data and refinement details are summarized in Table 1. Selected bond lengths, bond angles and the magnitude of distortion are listed in Table S1. Bond distances and angles of H-bond network are given in Table S2.
For spectral measurements, the UV-Vis diffuse reflectance is performed on a Perkin-Elmer spectrophotometer type instrument Lambda-45 coupled to an integration sphere type RSA-PE-20 in the range of 200-700 nm with a speed of 960 nm. min −1 and an aperture of 4 nm. Infrared (IR) spectrum was recorded at a room temperature on a Nicolet IR200 FTIR spectrophotometer in the 4000-400 cm −1 region. Excitation and emission spectra were measured with Perkin-Elmer LS55 Fluorimeter using solid samples at room temperature.
Magnetic measurements are performed as well. The temperature dependence of the magnetization was carried out using a BS 2 magnetometer developed in Louis Neel Laboratory of Grenoble with an external magnetic field equal to 500 Oe in the temperature range 2-330 K. Magnetization measurements M (μ 0 H) were performed with variable field μ 0 H up to 10 5 Oe at 2 K. www.nature.com/scientificreports www.nature.com/scientificreports/ electronic structure calculations. We used the Full Augmented Plane Wave (FLAPW) method 16 which performs DFT calculations with the generalized gradient approximation (GGA). The Kohn-Sham equation and energy functional were evaluated consistently. For this doing, the space was divided into the interstitial and the non-overlapping muffin-tin spheres centered on the atomic sites. The basis function inside each atomic sphere consisted in linear expansion of the radial solution of a spherically potential multiplied by spherical harmonics. In the interstitial region, the wave function was taken as an expansion of plane waves and no shape approximation for the potential was introduced in this region consistently with the full potential method. The core electrons were described by atomic wave functions solved relativistically using the current spherical part. Spin polarized potential as well as the ferromagnetic states are considered. The atomic muffin-tin (MT) spheres, supposed not to overlap with each other, are taken as 1.80, 2.10, 1.64, 1.04, 1.12 and 0.55 a.u for Co, Mo, P, N, C and H atoms, respectively. The gap energy, which defines the separation of the valence and core state, was chosen equal to −6.0 Ry. The largest reciprocal vector G in the charge Fourier expansion, G max , was equal to 28 and the cut-off energy corresponding to the product of the muffin-tin radius and the maximum reciprocal space vector (RMT. K max ) was equal to 7. Inside the atomics spheres, the potential and charge density are expanded in crystal harmonics up to l max = 6. Calculations are performed with 16 inequivalent k-points in the irreducible Brillouin zone. Such a value is large enough to ensure both the gap and the magnetic moment. The convergence criterion was chosen to be the total energy and set at 10 −4 eV. For computation, our refined XRD lattice parameters are used.   [Table S1], indicate a strong distortion type of MoO 6 octahedra. The two crystallographically independent cobalt cations, located upon inversion centers, exhibit the same octahedral coordination environment made by four water molecules and two bridging oxo-groups from two adjacent {P 2 Mo 5 } clusters through the terminal oxygen atom of the PO 4 tetrahedron. The Co-O bond lengths and O-Co-O bond angles, ranging from 2.047(2) to 2.127(3)Å and 83.9(11) to 96.1(11)°, respectively are consistent with the normal ranges observed in other complexes 22,23 . As seen in Fig. 1b, adjacent {P 2 Mo 5 } anions are connected to the Co(1) and Co(2) octahedron through the terminal oxygen atoms of two opposite PO 4 tetrahedra to generate an infinite 1D zigzag chain structure along the [101] crystallographic direction. The distance between adjacent P 2 Mo 5 clusters is 9.566 Å and the shortest intrachain Co⋯Co distance is close to 9.556 Å. The protonated 3-(ammoniomethyl)pyridinium dication and the uncoordinated water molecules, located in the void spaces, connect adjacent chains into a 3D supramolecular framework  Table S2]. In addition, weak hydrogen contacts are present involving the carbon atom as H-donor and the oxygen atoms of the phosphomolybdate anions acting as an H-acceptor. The bond lenghs and angles of the (C 6 H 10 N 2 ) 2+ groups ranging, respectively, from 1.323(5)-1.514 (6) and 112.1(4)-123.1(4), are within normal ranges 24 . In order to study the propensity of two chemical species (X and Y) to be in contact, we have calculated the Hirshfeld contact surfaces, derived random contact and enrichment ratios (E XY ) 25 Table S3. It is interestingly to note that the value of E XY is expected to be usually larger than unity for pairs of elements with a high propensity to form contacts in crystals, while pairs that have a tendency to avoid contacts are associated with E XY values lower than unity. The proportion of surface contacts of chemical type on the molecular Hirshfeld surface highlighted that this compound has a large number of hydrogen and oxygen atoms on their surface (S H = 60% and S O = 32.55%). While (C, N and Co) atoms are rarely present at the molecular surface (S < 5%), decreasing the value of the random contacts (R < 5%). The list of the enrichment ratios reveals that the O…H; C…O, N…O and Co…O contacts are highly enriched which turn out to be favored in the crystal packing. The N…H contacts can be considered enriched (E NH = 1) while the H…H contacts are slightly enriched for structure (E HH = 0.88). On the other hand, the N…H and C…H contacts are impoverished with E NH and E CH ratio of 0.55 and 0.16, respectively. Similarly, the proportion of carbon atoms on the molecular surface (S C = 4.65%) as well as the lower value of the random contact (R CC = 0.21%) have further strengthened our conviction that π…π interactions tend to be disfavored in directing the packing of (C 6  was studied by means of diffuse reflectance spectroscopy (DRS), as shown in Fig. 3a. The spectrum shows a maximum reflectance (~20%) in the region between 200 nm and 350 nm corresponding to lower absorption. From www.nature.com/scientificreports www.nature.com/scientificreports/ 400 nm, a decrease in reflectance is seen due to fundamental absorption (valance band to conduction band) by the material [26][27][28] . The transmittance spectrum indicates that the transmission coefficient varies between 5% and 50% at the UV-visible region which means that the studied compound is translucent in the studied wavelength region 29 [ Fig. 3b].

Study of non-covalent interactions. Using
The acquired diffuse reflectance spectrum is converted to , where R is the reflectance, which is proportional to the absorption coefficient α. The energy band gap of the sample is calculated using the relational expression proposed by Tauc, Davis, and Mott 30 . In the region of high absorption, E g is connected to the absorption coefficient α of the following equation 31 : (F(R)hγ) 1/r = B(hγ -Eg), where B is constant, E g is the optical band gap, hυ is the energy of the incident photon and (r) is an index which may be equal to ½ and 2 assigned to the allowed direct and allowed indirect transition, respectively. In the actual experiment, the α in the Tauc equation is substituted with F(R) and the relational expression becomes: (F(R)hγ) 1/r = B(hγ -Eg), In order to determine the value of Eg for the samples, we take the natural logarithm and first order derivation of the Tauc's equation to get:   www.nature.com/scientificreports www.nature.com/scientificreports/ the bands localized states in the band gap, is an important parameter to characterize the disorder in the complex compound 31 . The absorption edge is found to be exponentially dependent on the incident photon energy and obeys the empirical Urbach rule Equation 32 : where α 0 is a constant and E u is the Urbach energy. Thus, by plotting Ln(α) as a function of energy hυ [ Fig. S3], the value of E u is found to be 0.22 eV.
The spectral behavior of the real part of the refractive index n(E) of the complex compound was determined from reflectance and transmittance data using the following equation 33

Ras
(1 ) , R is the reflectance, T is the transmittance, k is the extinction coefficient and t is the thickness of the slab (t = 1 mm). The variation of the real refractive index n as well as the extinction coefficient k with the energy in the range of 1.7 to 6 eV are illustrated in Fig. 5a. The refractive index shows a large dispersive behavior on the visible region. Contrariwise, at the UV part, the compound can be assumed as non dispersive, having a constant refractive value (n ∞ = 1.41). The observed variation of the refractive index with energy is exclusively due to polarizability changes and the anomalous dispersion may be correspond to the photon energy of the forbidden gaps energy that the compound holds 34 . The maximum of refractive index reached to 2.35 at 1.81 eV, this large value of "n" known in the visible region of electromagnetic spectrum reveals that the sample can become a promising candidate for visible optical communication devices. The imaginary part of the refractive index k, denotes absorption of optical energy by the semiconductor. In the spectral regions where the processes of absorption are weak or absent, as in the case of the sub band gap range, k is minimal, whereas in regions of strong absorption, the magnitude of k is large.
The real and imaginary parts of dielectric constant are calculated using the relation 35 : ε r = n 2 − k 2 and ε i = 2nk. Where ε 0 is the dielectric constant in the absence of any contribution from free carriers. The real part (ε r ) of the dielectric constant of the complex indicates the extent to which the velocity of light is reduced, while the imaginary part (ε i ) indicates the energy absorption by the complex 36 . The variation of ε r and ε i with the energy are shown in Fig. 5b, respectively. The comparison between k(λ) and ε i (λ) follow closely each other in the complex and the low values of ε i infer that the optical loss due to absorption is very weak. Therefore, the ratio between ε i and ε r gives information about the loss factor 37 . The ε r (E) curve presents a similar behavior of n (E) plot because of the smaller values of k 2 .
The solid state fluorescence properties of the complex compound are investigated at room temperature. As expected in Fig. 6a, upon photoexcitation at 350 nm, the title compound shows three main emission peaks in the visible region at 370, 402 and 461 nm. To understand the origin of these emission peaks, the emission spectra of the free P 2 Mo 5 cluster and 3-picolylamine has been compared. Thus, the 3-picolylamine shows a broad emission at 378 nm, while the free P 2 Mo 5 cluster display two main peaks at 408 and 470 nm upon excitation at 350 nm. Therefore, the luminescence emission peaks observed in the emission spectrun of the complex can be attributed to the π* → π transitions of the picolylammonium groups 38 and the LMCT (O → Mo) of the polyanions 39 , respectively. However, the obvious decrease of the emission intensity of the complex, indicate that the coordination of P 2 Mo 5 to Co 2+ changes slightly the energy levels of the Strandberg cluster and affects their emission intensities 40 . The chromaticity diagram CIE 1931, upon excitation at 350 nm, of the simple is shown in Fig. 6b, the CIE colour coordinate (x, y) of the fluorescent emission peaks are (0.13, 0.11), illustrates that (C 6 H 10 N 2 ) 2 [Co(H 2 O) 4 P 2 Mo 5 O 23 ].6H 2 O belongs to a blue luminescent compounds. From the above results, it is concluded that the obtained compound is suitable for blue display device.
As concerns the infrared spectroscopy, the IR spectrum of the complex compound shown in Fig. S4  Magnetic properties. Magnetic measurements have been performed on powder sample of (C 6 H 10 N 2 ) 2 [Co(H 2 O) 4 P 2 Mo 5 O 23 ].6H 2 O in temperature range from 2 to 300 K under a magnetic field of 500 Oe. As known, the valence states of Mo atoms are + 6 which indicate that they have no contribution to magnetism of compound and the magnetic character of the materials would reside in the Co(II) sites. Figure 7 shows that there is an abrupt PM -FM phase transition in the magnetization versus temperature (curve red) at T C = 232 K which is determined by the minimum of the temperature derivative of the magnetization curve [ Fig. 7 inset]. To analyze the magnetic phase transition in detail, we have also calculated the temperature dependence of the inverse susceptibility, χ −1 curve. As depicted in the blue curve Fig. 7, the shape of the curve between 300 and 225 K indicates the presence of a ferromagnetic exchange coupling between the neighboring Co 2+ ions which is expected also from the structure data. Between 225 and 10 K, the χ −1 curve is almost constant and it rapidly falls towards zero at 4 K. The Curie -Weiss temperature θ cw can be obtained by a linear regression of the paramagnetic region of the Curie -Weiss law [χ = C/(T − θ cw )]. Here, C and θ cw are the Curie constant and the Curie-Weiss temperature, respectively. From C value, the experimental effective paramagnetic moment can be calculated as: , where N A = 6.023.10 23 mol −1 is the number of Avogadro, µ B = 9.274.10 −21 emu is the Bohr magneton and K B = 1.38016.10 −23 J.K −1 is the Boltzmann constant 42 . The spectroscopic splitting factor g can be deduced as  The values of C, θ cw , µ eff exp , g, J and H eff for the applied magnetic field are tabulated in Table 2. The positive sign of the Curie-Weiss constant approves the ferromagnetic interactions between Co(II) centres and the large magnitude of J indicates a rather high ferromagnetic interaction between neighboring Co 2+ centers. This result improved the capability of Strandberg-type polyoxoanion to mediate ferromagnetic interactions between metal atoms. One notices that for our ferromagnet Tc ~ θ cw = N w C. According to this relation the magnitude of the Weiss constant, from the experimental value of T C and C is N w = 19 a.u. To investigate other magnetic properties, isothermal measurements were performed at 2 K. The results indicate that (C 6 H 10 N 2 ) 2 [Co(H 2 O) 4 P 2 Mo 5 O 23 ].6H 2 O compound possesses a reversible magnetic behavior without any evidence of hysteretic loops which represent a great advantage for efficient magnetic cooling [ Fig. 8a]. The intercept of the linear fit to the high field data with the ordinate-axis gives a saturation magnetization for the magnetically ordered structure. We calculate M sat = 11 emu/g, which corresponds to a saturation magnetic moment value μ S exp of 5.1 μ B /fu (2,55 μ B /Co II ) which is slightly larger than the spin value expected for two uncoupled high-spin Co(II) ions (µ S th = 3 μ B /Co II ). On the other hand, field dependent magnetization reveals a hysteresis curve typical of a soft magnetic state, with a remnant magnetization of 0.013 emu/g and a coercive field of 13 Oe [Fig. 8b]. From the dM dH vs H plots at 2 K of the simple it is clearly to see only one peak which is suggested the interaction between the chains in studied material is very easy to overcome [Fig. 8c]. theoretical investigation on electronic and magnetic structures. The Total Density of State (DOS) of (C 6 H 10 N 2 ) 2 [Co(H 2 O) 4 P 2 Mo 5 O 23 ].6H 2 O complex deduced from the band structure calculation is reported in Fig. 9. Here, the Fermi level is taken as reference. As seen, this DOS is not symmetrical with respect to energy axis, pointing out that the system is ferromagnetically ordered. Additionally, the total density of states has different band gaps in up spin channel (1.8 eV) and down spin channel (2.7 eV) which confirms the existence of a ferromagnetic semiconducting behavior 44   www.nature.com/scientificreports www.nature.com/scientificreports/ parts of Mo(3d) states which strongly hybridize with each other. Thus, the expected reason for the ferromagnetic semiconducting behavior in the simple may be due to the appreciable exchange splitting between the up and down spin channels of Co(3d) states as well as the hybridization of Co(3d) -Mo(3d) states which is more pronounced in down spin channel than in up spin channel.
In the present work the l-decomposed DOS of Co atoms have been studied in particular, only small contributions from Co(4 s) is revealed. It should be noticed that no contributions from Co(4p) is observed since is fully full and shifts to lower energies and behaves as core level. In addition, the Co oxidation state is calculated and found equal 2.5. Magnetic moment carried by Co atoms is computed as well and found equal 2.54 μ B , leading to a  www.nature.com/scientificreports www.nature.com/scientificreports/ magnetic moment per formula of 5.08 μ B which is in good agreement with our measured magnetization 5.1 μ B /f.u. In this context, we underline that, on the one hand, no magnetic moment is revealed on all atoms except Co atoms included in our compound.
On the other hand, we observe a transition from ferromagnetic state to paramagnetic state taking place at Curie temperature T C = 232 K from our experimental measurements. Consequently, it should be concluded that a magnetic long-range order is revealed between neighboring Co cations. Considering the large Co-Co distance excising 9.5 Å, this magnetic interaction type is a characteristic of a magnetic super-exchange between Co(II) centers. This observed magnetic behavior is in accordance with early reported investigations on similar magnetic compounds 45,46 .
The orbital decomposed d-band density of state of Co(II) ions are depected in Fig. 11. The analysis of the PDOS suggest that the octahedral symmetry around Co atoms splits the Co (II) 3d levels into a triply degenerate t 2g band and a doubly degenerate e g band. the five -fold 3d of both Co cations show that ↑ t 2g and ↑ e g levels are completely occupied while the ↓ t 2g and ↓ e g are partielly occupied. Such electronic configuration leading to a high spin state (HS) for Co cations. More importantly and according to the coupling model consider by B. Belhadji and al 47 , the obtained PDOS exhibits a competition between a ferromagnetic super-exchange, arising from the hybridization of the occupied ↓ e g states with the empty ↓ t 2g states and an antiferromagnetic super-exchange due to the hybridization of the occupied ↑ t 2g states with the empty ↓ t 2g states. To distinguish the macroscopic dominant magnetic interaction a comparison between the experimental results and the computational results is performed. Thus, as the magnetic measurement present a dominant ferromagnetic ordering in the simple consequencly the ferromagnetic super-exchange occuring from (occupied ↓ e g -empty ↓ t 2g ) 3d orbitals is stronger than the antiferromagnetic super-exchange occuring from occupied ( ↑ t 2g -empty ↓ t 2g ) 3d orbitals of Co atoms.

conclusion
In summary, we successfully synthesized a novel Strandberg-type hybrid complex based on the {P 2 Mo 5 } anions bridged by cobalt cations generating 1D zig zag chain structure. A network of hydrogen bonds, between the organic and inorganic components in the crystal, leads to a three-dimensional supramolecular architecture. The analysis of the DRS data as well as the total DOS calculation reveals that this polymer can be identified as a ferromagnetic semiconductor. The study of the luminescent properties at room temperature demonstrated that the obtained material is a blue emission compound. Refractive index and dielectric results reveals that the elaborated semiconductor can be used as a candidate for visible light device. Field dependent magnetization presents a hysteresis curve typical of a soft magnetic state as well electronic structure calculation and temperature dependent magnetization measurement confirms the presence of long-range ferromagnetic order between Co cations, this magnetic interaction type is a characteristic of a magnetic super-exchange. At the end, our experiments and theoretical analyses both verify the coexistence of ferromagnetism and semiconducting conductivity in the studied polymer, which makes (C 6