Controlling the thermoelectric properties of organo-metallic coordination polymers through backbone geometry

Poly(nickel-benzene-1,2,4,5-tetrakis(thiolate)) (Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(nickel-benzene-1,2,3,4-tetrakis(thiolate)) (Ni-ibtt). Comparative analysis of Ni-ibtt and Ni-btt reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel–sulfur (Ni–S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Ni-btt possesses a linear backbone, whereas Ni-ibtt exhibits a more undulating, zig-zag-like structure. Consequently, Ni-ibtt demonstrates slightly higher solubility and an increased bandgap in comparison to Ni-btt. The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While Ni-btt exhibits p-type behavior, Ni-ibtt demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.


General Experimental
All reagents and solvents were obtained from commercial suppliers and used without further purification.Reactions were carried out under nitrogen atmosphere using standard Schlenck techniques and reported yields refer to purified and spectroscopically pure compounds, unless otherwise stated.Concentration under reduced pressure was performed by rotary evaporation (25 -40 °C) at an appropriate pressure.Thin layer chromatography (TLC) was performed using Merck Si60 F254 pre-coated TLC aluminium plates and flash column chromatography was performed using Merck Geduran Si 60 silica gel (40-63 μm particle size). 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker NMR spectrometer Avance III 400 at 298 K.Chemical shifts (δ) are reported in parts per million (ppm), using the residual solvent peaks as internal standard.For 1 H-NMR (δ): CDCl3 7.26.For 13 C-NMR (δ): CDCl3 77.16.Coupling constants (J) are given in Hertz (Hz) and spin multiplicities denoted as follows: s = singlet, d = doublet, hept = heptet.Mass spectrometry data was collected on a Thermo Accela LC/LTQ (EI) and Waters LCT Premier QTOF (ESI).
Bruker Platinum ATR with a single reflection diamond attachment was used to record Fouriertransform infrared spectra (FTIR).Renishaw inVia Raman spectrometer equipped with a Leica DM LM microscope was used to perform the Raman spectroscopy at room temperature under ambient conditions.A 785 nm diode laser was chosen as the excitation source for all the samples with a maximum power of 500 mW.Thermal stabilities were examined by a TA instruments Q500 TGA under nitrogen atmosphere over a range of 293 K to 1073 K at a constant heating rate of 20 K min -1 .Energy dispersive X-ray fluorescence (ED-XRF) data was collected by a Malvern Panalytical Epsilon 4 spectrometer equipped with a silver anode X-ray tube.X-ray diffraction (XRD) was performed by a STOE SEIFERT diffractometer using Mo Kα (0.7093 Å) radiation with a detected angular range of 2° < 2θ < 40° and a step size of 0.05° at 1 s per step.MDI Jada 6 software was used for the XRD data analysis.X-ray photoelectron spectroscopy (XPS) was performed by a Thermo Scientific K-Alpha spectrometer with a monochromated Aluminium (Al) Kα X-ray source (1486.7 eV).All XPS data was processed with the CasaXPS software and calibrated against the C 1s peak to 284.6 eV.EPR spectra were measured at 77 K using an EMX X-band (~10 GHz) spectrometer (Bruker, Germany) with a cylindrical microwave resonator and a liquid nitrogen Dewar.The concentration of unpaired electron was estimated by comparing the area of observed absorption peak with that of CuSO4 × 5 H2O standard sample (Cu 2+ , d9, S = 1/2).DC magnetization measurements were performed using a MPMS3 magnetometer (Quantum Design, USA) for a temperature of 2-300 K and magnetic field of 0 -7 T. Electrical conductivity and Seebeck measurements were performed on OMCP pellets.The pellets were prepared by grounding the OMCP samples into fine powder with mortar and pestle before compressing the powder into round-shaped pellets (12.5 mm diameter; thickness between 0.8 -1.2 mm) using KBr pellet dies applying a pressure of around 2.6 GPa.The thermoelectric characterisation was conducted on a Netzsch SBA 548 Nemesis thermoelectric set up under Helium environment using a four-probe technique and dual Peltier configuration, respectively.Synthetic procedures 1,2,3,4-Tetrakis(isopropylthio)benzene (iTIB) 1,2,3,4-tetrafluorobenzene (0.9 g, 6 mmol) and sodium hydroxide (1.83 g, 45.7 mmol) were dissolved in 20 mL of anhydrous N,N-dimethylacetamide (DMA) under an inert atmosphere (N2), followed by the dropwise addition of 2-propanethiol (2.90 mL, 31.3 mmol) at room temperature.The solution was then refluxed at 100 °C for 2 h.Afterwards the solution was cooled to room temperature and 30 mL of brine was slowly added under stirring.The crude product was extracted with ethyl acetate (4 × 25 mL) and the combined organic layers filtered through a silica plug to remove residual DMA.After solvent evaporation, the product (iTIB) was recovered as a light-yellow oil (2.04 g, 91%). 1

Poly[nickel-benzene-1,2,3,4-tetrakis(thiolate)] (Ni-ibtt)
iTIB (1.51 g, 4 mmol) was dissolved in anhydrous N, N-dimethylacetamide (75 mL) under inert atmosphere (N2).Sodium pieces (5.2 g, 226 mmol) were then added to the solution and heated to 100 °C for 24 hours.Afterwards 15 mL of degassed and deionised water were added dropwise into the reaction slurry to yield an orange-brown solution.Nickel acetate tetrahydrate (1.02 g, 4 mmol) dissolved in 20 mL of degassed water was added dropwise over 15 minutes, during which the solution gradually turned deep black in colour.The mixture was further heated at 100 °C for another 24 hours.The crude black solid was collected by centrifugation and further purified by Soxhlet extraction with deionized water for 24 h, followed by methanol for 24 hours.The purified Ni-ibtt was dried for 24 hours in vacuum at 100 °C and recovered as a black powder (0.75g, 72%).

Density functional theory
Cluster Density Functional Theory (DFT) calculations we performed on oligomeric cluster models of the Ni-btt and 100%-trans Ni-ibtt coordination polymers.These models consisted of four nickel ions bridged by three benzene-1,2,4,5-tetrathiolate or benzene-1,2,3,4tetrathiolate ligands, respectively, and capped with terminal benzene-1,2-dithiolate capping ligands, see Fig. S19, while we also performed calcations on their octomer equivalents with eight nickel atoms and nine ligands, see Fig. 4 in the main text.The octamer calculations were performed to get a general idea of the shape of the polymers and the tetramer oligomers were used for a full exploration of possible electronic states of the materials.The cluster calculations used a combination of the B3LYP hybrid functional, [1][2][3][4] incorporating 20% Hartree-Fock exchange, the Grimme's D3 dispersion correction 5 and the def2-SVP basis-set 6 and were performed using Turbomole 7.5. 7,8 ultiple spin-states were explored for each oligomer while also calculations on cationic and anionic versions of the oligomers were performed to calculate the oligomers ionisation potential and electron affinity.
0][11][12] These calculations use the projector augmented wave method, with scalar-relativistic pseudopotentials and 3p electrons included in the valence shell for Ni. 13 In line with our previous study on Ni coordination polymers, 14 the hybrid functional HSE06, incorporating 25% Hartree-Fock exchange and a screening parameter of 0.11 bohr -1 was used to describe the exchange-correlation interactions. 15,16 he periodic models of the coordination polymers were constructed based the central part of the oligomeric models and consist of a single chain in one dimension (along c for Ni-btt, and a for Ni-ibtt), with a cell size of 30 Å in the other two dimensions to prevent interaction between periodic images.A plane wave energy cutoff of 600 eV and k-point meshes of 1 × 1 × 4 and 2 × 1 × 1 respectively were sufficient to converge the total energy of each polymer to within 1 meV per atom.Multiple initial spin states were trialled for each polymer, and the structures allowed to relax until the forces on each atom were below 0.01 eV Å -3 .The sumo package was used for plotting the electronic band structures, 17 with the high-symmetry path through the Brillouin zone taken from Bradley and Cracknell. 18Curvature effective masses were obtained using a five-point parabolic fit to the band edges using the effmass package, and Kane dispersion parameters were calculated to assess non-parabolicity. 19able S3.(S)HOMO and (S)LUMO energies and -IP and -EA values predicted for the oligomeric models of Ni-ibtt and Ni-btt (B3LYP/def2-SVP).

Figure S8 .
Figure S8.FTIR spectra of TIB and iTIB.The spectra have been stacked for clarity.
Fig. S19 Structures of the tetramer cluster models of Ni-ibtt, top, and Ni-btt, bottom, used in the cluster calculations.

Figure S20 .
Figure S20.Frontier orbitals for the electronic ground state of Ni-ibtt, left, and Ni-btt, right.In the case of Ni-btt two occupied and two unoccupied orbitals are shown because the openshell singlet ground state of Ni-btt is calculated with unrestricted DFT and thus spin-polarised molecular orbitals with one electron per orbital and not two as in the restricted calculations for closed-shell Ni-ibtt.