An electrically conductive metallocycle: densely packed molecular hexagons with π-stacked radicals

Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni6(NDI-Hpz)6(dma)12(NO3)6]·5DMA·nH2O (PMC-hexagon) (NDI-Hpz = N,N′-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H2O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2–2.1) × 10−4 S cm−1 (pressed pellet), which is superior to that of most NDI-based conductors including metal–organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.


General Information
IR spectra were collected with KBr pellets on a JASCO FT/IR-4200 spectrometer at room temperature (RT). UV-Vis-NIR absorption spectra were collected with KBr pellets on a JASCO V-670 spectrophotometer at RT. KBr pellets were prepared in a glovebox (MBRAUN UNILAB1200/780) filled with Ar gas, and then sealed in a custom cell for IR and UV-Vis-NIR measurement under an inert atmosphere. Thermogravimetry (TGA) analysis was carried out on a SHIMADZU DTG-60H with a heating rate of 5 °C/min under a constant nitrogen gas flow (0.1 L/min). The ESR spectrum was acquired by using a JEOL JES-FA100. The static magnetic susceptibility was measured on polycrystalline sample in the temperature range of 1.8-300 K using a superconducting quantum interference device (SQUID) magnetometer. The calculation of the intrinsic diamagnetic correction was conducted using common Pascal's constants. [1] Single-crystal X-ray diffraction (SXRD). The diffraction data for PMC-hexagon were collected on a Rigaku XtaLAB AFC10 diffractometer with a HyPix-6000HE hybrid pixel array detector, graphite monochromated Mo Kα radiation (λ = 0.7107 Å) and a cryogenic equipment GN-2D/S. The crystal structure was solved using direct methods (Sir2019 [2] ), followed by Fourier synthesis. Structure refinement was carried out using full-matrix leastsquares procedures with SHELXL [3] on F 2 in the Yadokari-XG 2009 software. [4] The SQUEEZE method was applied to remove the electron density in 1D pore channels, because it was too dispersed to fabricate a meaningful molecular structure. [5] The CCDC number of crystallographic data for PMC-hexagon was assigned to CCDC

2091485.
Electrical conductivity measurement. Polycrystalline powder of PMC-hexagon was loaded into an alumina ring that was set on the one of the pellet dices. Then the other pellet dice was inserted to the alumina ring and the pressure was applied by using Specac Mini-Pellet Press. The 3 mmΦ pressed pellet prepared in the insulating alumina ring was connected to Agilent E5260A via the pellet dices, which act as electrode, and the current-voltage (I-V) characteristic was collected. Synthesis of PMC-hexagon, pellet fabrication and the I-V measurement of the pellets were carried out in a glovebox (MBRAUN UNILAB1200/780) filled with Ar gas.
The temperature dependence of the electrical conductivity was measured in a liquid He cryostat of a Quantum Design PPMS (Physical Property Measuring System) model 6000 by using the two-probe method in direct current (DC) mode with Keithley sourcemeter model 2611. The cooling rate was 1 K/min. The 3 mmΦ pressed pellet was prepared in the glovebox and connected to the sample puck by gold wires (20 μmΦ) and carbon paste (Dotite XC-12 in diethyl succinate) under air. Then, the carbon paste was dried in the glovebox.
Computational method. Amsterdam Modeling Suite (AMS) packages [6,7] were applied for the calculations of transfer integral of PMC-hexagon. The transfer integral between adjacent NDI-Hpz molecules was investigated by the B3LYP-D3/TZP method [8] without structural optimization. To estimate the g-factor of Ni 2+ ion, quantum calculations were conducted by using the ORCA4.2.1 package program suite. [9,10] CASSCF (8,5) calculations using the minimal active spaces comprising only d-electrons were performed. Cartesian coordinates of the initial geometry was used from crystal structure with replacing NDI-core to hydrogen atom. The def2-SVP basis set and RIJCOSX was used for all atoms. To consider the effect of the dynamic correlation, NEVPT2 was employed on top of the converged CASSCF wave function. [11] Adsorption measurement. N2 sorption isotherm was performed using microtracbel Belsorp Max with G1 grade (>99.9999 %) gas. Solid PMC-hexagon was activated by heating at 120 °C for an hour under high-vacuum condition with turbo molecular pump. Sorption measurement was performed by volumetric method, and the volume of adsorbed gas was calculated from the ideal gas equation with second virial coefficient (H2: 3.436 × 10 -9 , N2: -4.264 × 10 -7 ). N2 sorption isotherms were measured for PMC-hexagon at 77 K. Temperature control during sorption measurement was done by using Dewar bottle. Adsorption isotherm was measured in the pressure range of P/P0 = 10 -5 -0.997 (P0 = 101.325 kPa) and desorption isotherm was measured down to P/P0 = 0.1. The volume of sample space was measured after sorption measurement by using G2 grade (>99.999 %) He gas, and dead volume correction was applied to the raw data.  The static magnetic property of PMC-hexagon was measured with a temperature range of 1.8-300 K as shown in Fig. S3. In the simple case, the MT values of S =1 spin (Ni 2+ ion) and S =1/2 spin (NDI radical) are expected to 1.000 and 0.375 cm 3 K mol −1 , respectively at RT. Assuming that the amount of NDI •species is 84% of the total NDI cores from the occupancy of hydrogen-bonded DMA molecules, the total MT value is expected to 1.315 cm 3 K mol −1 . In practice, however, the experimental MT value (1.516 cm 3 K mol −1 at 300 K) is higher than the expected value due to the contribution of several factors such as spin-orbit coupling, which is typically represented as the deviation of g-factor from 2. We first estimated the g-factors of Ni 2+ ion from the coordination geometry obtained by SXRD analysis. The quantum calculations (CASSCF and NEVPT2) using the ORCA4.2.1 package program was conducted and the results are shown in Table S2. [a] Zero-field splitting parameter.

Crystallographic information of PMC-hexagon
Since the anisotropy of g-factors is very small, the isotropic g-factor (giso) was used for the next step. The temperature dependence of MT value was fitted by [Eq.(1)] using least-square method with the fixed giso and three variables (zero-field splitting parameter (D), temperature-independent paramagnetism (TIP) and molar ratio of effective NDI •species (n). The results of fitting parameters are listed in Table S3.  The fitting using giso obtained by CASSCF calculation gave negative n value. It is obviousely inconsistent with the existence of NDI •species confirmed by the ESR spectrum (Fig. S2). On the other hand, the fitting using giso obtained by NEVPT2 calculation gave reasonable parameters. As shown in Fig. S3, the calculated curves using these parameters are in good agreement with the MT-T plot. The D values were so small that it is not suitable to determine their signs from the fitting. In both cases, the amount of effective NDI •species is in the range of 10 to 15%. These small n values suggest the antiferromagnetic interaction between radical spins and/or spin quenching by partial dimerization of NDI cores. Fig. S4. Current-voltage (I-V) characteristics of PMC-1 measured in pressed pellets at RT. Whole process from the synthesis to the measurement was carried out under argon atmosphere as well as PMC-hexagon (Fig. 5a).

H NMR spectra of NDI-Hpz and PMC-hexagon in DMSO-d6
To estimate the amount of DMA molecules in the crystal, 1 H NMR spectra of NDI-Hpz and PMC-hexagon (pristine and heated samples) dissolved in DMSO-d6 solution were acquired at RT. A drop of conc. H2SO4 was added to dissolve PMC-hexagon perfectly. In this acidic condition, NDI-Hpz exists as protonated state, as supported by the single peak attributed to the hydrogen atoms in a protonated pyrazole group (a and b in Fig. S6). The amount of DMA molecules were calculated by comparing the integration values of a or b with z in Fig. S7 and S8, taking into consideration that a and b reflect four hydrogen atoms per a NDI-Hpz moleluce and z reflect three hydrogen atoms per a DMA molecule. The integration value of z was used rather than those of x and y because z is farther from the large residual DMSO solvent peak. Fig. S9 indicates that DMA molecules were almost removed by the heating, whereas the complicated spectrum implies the deterioration of the sample.      after N 2 sorption measurement pristine sample simulation 1 H NMR spectrum of PMC-hexagon after the measurement of N2 sorption isotherm Fig. S12. 1 H NMR spectrum of PMC-Hexagon after the measurement of N2 sorption isotherm. The solid was dissolved in DMSO-d6 with a drop of conc. H2SO4 at RT. Note that strong H2O signal, which was overlapped with NDI-pyz signals, was carefully removed at the baseline correction process in order to obtain reliable integration values of hydrogens from NDI-pyz ligand. Compared with Fig. S7 and S8, the integration values of x, y and z are nearly half, indicating the elimination of DMA molecules by the activation process.