Donor-Type Nickel–Dithiolene Complexes Fused with Bulky Cycloalkane Substituents and Their Application in Molecular Conductors

Donor-Type Nickel–Dithiolene Complexes Fused with Bulky Cycloalkane Substituents and Their Application in Molecular Conductors. Abstract: The effects of substituents on the arrangement of metal–dithiolene complexes based on π -conjugated systems, which are extensively used to synthesize various functional materials, have not been studied adequately. New donor-type nickel–dithiolene complexes fused with bulky cycloalkane substituents [Ni(C n -dddt) 2 ] (C 5 -dddt = 4a,5,6,6a-pentahydro-1,4-benzodithiin-2,3-dithiolate; C 6 dddt = 4a,5,6,7,8,8a-hexahydro-1,4-benzodithiin-2,3-dithiolate; C 7 -dddt = 4a,5,6,7,8,9,9a-heptahydro-1,4-benzodithiin-2,3-dithiolate; and C 8 -dddt = 4a,5,6,7,8,9,10,10a-octahydro-1,4-benzodithiin-2,3-dithiolate) were synthesized in this study. All the complexes were crystallized in cis-[Ni(cis-C n dddt) 2 ] conformations with cis-oriented ( R , S ) conformations around the cycloalkylene groups in the neutral state. Unique molecular arrangements with a three-dimensional network, a one-dimensional column, and a helical molecular arrangement were formed in the crystals owing to the ﬂexible cycloalkane moieties. New 2:1 cation radical crystals of [Ni(C 5 -dddt) 2 ] 2 (X) (X = ClO 4 − or PF 6 − ), obtained by electrochemical crystallization, exhibited semiconducting behaviors ( ρ rt = 0.8 Ω cm, E a = 0.09 eV for the ClO 4 − crystal; 4.0 Ω cm, 0.13 eV for the PF 6 − crystal) under ambient pressure due to spin-singlet states between the dimers of the donor, which were in accordance with the conducting behaviors under hydrostatic pressure ( ρ rt = 0.2 Ω cm, E a = 0.07 eV for the ClO 4 − crystal; 1.0 Ω cm, 0.12 eV for the PF 6 − crystal at 2.0 GPa). the donor abilities of novel donor complexes [Ni(C n -dddt) 2 with two cycloalkane rings (C 5 -dddt = 4a,5,6,6a-pentahydrro-1,4-benzodithiin-2,3-dithiolate; C 6 -dddt = 4a,5,6,7,8,8a-hexahydro-1,4-benzodithiin-2,3-dithiolate; C 7 -dddt 4a,5,6,7,8,9,9a-heptahydro-1,4-benzodithiin-2,3-dithiolate, and C 8 -dddt = 4a,5,6,7,8,9,10,10a-octahydro-1,4-benzodithiin-2,3-dithiolate) based on electrochemical measurements and electronic absorption spectra. The packing motifs of the neutral [Ni(C n -dddt) 2 ( n = 5, 6, 7, 8) discussed based on X-ray crystallographic analysis results. Furthermore, electronic features of new cation radical crystals -dddt) ClO and PF resistivity, donor sheets parallel to the bc planes. Although the donor molecule is fused with the bulky cyclopentane moieties, the two-dimensional donor sheet arrangements through the S ··· S contacts preferentially are constructed in the radical cation crystals, which are also observed in the organic radical cation crystals based on the BEDT-TTF and BETS molecules exhibiting metallic and superconducting behaviors [14–18].


Introduction
Organic π-conjugated molecules, including tetrathiafulvalene (TTF) skeletons, are among the most common building blocks for the realization of various functional materials [1]. The TTF molecule has an electron donor ability, which can help realize various charge transfers and produce radical cation crystals. The TTF-TCNQ (TCNQ: 7,7,8,8tetracyanoquinodimethane) is a prototype of charge-transfer compounds, where the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands of the open-shell donors and acceptors, respectively, contribute to the conduction [2,3]. It is the first organic conductor to exhibit a metallic conductivity within a wide temperature range with a minimum of 59 K, where a sharp metal-to-insulator transition is observed [4]. Moreover, TTF has attracted significant research attention with the discovery of superconducting salts [5][6][7][8] based on TTF derivatives such as atetramethyltetraselenafulvalene (TMTSF) [9], bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) [10,11], and dimethyl(ethylenedithio)diselenadithiafulvalene (DMET) [12,13]. The conducting properties of molecular-based materials are significantly influenced by their molecular arrangements in Scheme 1. Organic components of molecular conductors.
Metal bis-dithiolene complexes can exhibit various oxidation states [47] and have smaller HOMO-LUMO gaps than those of the organic donors [48,49], which is advantageous for the components of molecular conductors. Electron-acceptor type [M(dmit)2] (M = Pd, Pt; dmit = 1,3-dithiole-2-thion-4,5-dithiolete) complexes with small gaps between the HOMO and LUMO form dimer structures. These structures induce crossing HOMO-LUMO band inversions through metal-metal interactions in the solid state, which provide various conducting, magnetic, and other physical properties by combining with closedshell cations [47,[50][51][52]. The electron-donor-type metal bis-dithiolene complexes [M(dddt)2] (M = Ni, Pd, Pt, and Au; dddt = 5,6-dihydro-1,4-dithiine-2,3-ditholate) are candidate materials for the development of molecular conductors due to their orbital symmetries, which are similar to those of BEDT-TTF [53][54][55][56]. A cation radical crystal, [Ni(dddt)2]3(AuBr2)2, is the first metal based on the donor-type metal-dithiolene complexes exhibiting metallic behavior down to at least 1.3 K [57]. Recently, Kato et al. reported a single-component molecular Dirac electron system based on Pd(dddt)2 that exhibits temperature-independent resistivity under high hydrostatic pressures [58][59][60][61][62][63]. The conducting and magnetic properties exhibited by the molecular crystals based on the dithiolene complexes are significantly dependent on the molecular arrangements of the metal-dithiolene complexes and organic molecular conductors. In the case of molecular conductors based on metal-dithiolene complexes, the molecular arrangements of the complexes are dependent on the combination of counter cations and anions in the crystals [47,48]. However, there are few reports on the substituent effects on the molecular arrangement, which influences the physical properties, despite the advantages of the Metal bis-dithiolene complexes can exhibit various oxidation states [47] and have smaller HOMO-LUMO gaps than those of the organic donors [48,49], which is advantageous for the components of molecular conductors. Electron-acceptor type [M(dmit) 2 ] (M = Pd, Pt; dmit = 1,3-dithiole-2-thion-4,5-dithiolete) complexes with small gaps between the HOMO and LUMO form dimer structures. These structures induce crossing HOMO-LUMO band inversions through metal-metal interactions in the solid state, which provide various conducting, magnetic, and other physical properties by combining with closed-shell cations [47,[50][51][52]. The electron-donor-type metal bis-dithiolene complexes [M(dddt) 2 ] (M = Ni, Pd, Pt, and Au; dddt = 5,6-dihydro-1,4-dithiine-2,3-ditholate) are candidate materials for the development of molecular conductors due to their orbital symmetries, which are similar to those of BEDT-TTF [53][54][55][56]. A cation radical crystal, [Ni(dddt) 2 ] 3 (AuBr 2 ) 2 , is the first metal based on the donor-type metal-dithiolene complexes exhibiting metallic behavior down to at least 1.3 K [57]. Recently, Kato et al. reported a single-component molecular Dirac electron system based on Pd(dddt) 2 that exhibits temperature-independent resistivity under high hydrostatic pressures [58][59][60][61][62][63]. The conducting and magnetic properties exhibited by the molecular crystals based on the dithiolene complexes are significantly dependent on the molecular arrangements of the metal-dithiolene complexes and organic molecular conductors. In the case of molecular conductors based on metal-dithiolene complexes, the molecular arrangements of the complexes are dependent on the combination of counter cations and anions in the crystals [47,48]. However, there are few reports on the substituent effects on the molecular arrangement, which influences the physical properties, despite the advantages of the components of molecular conductors [64,65]. Thus, we designed novel donor-type metal bis-dithiolene complexes [Ni(C n -dddt) 2 ] formed by a [Ni(dddt) 2 ] skeleton fused with bulky cycloalkane rings and investigated the substituent effects on the arrangement of the complexes and their physical properties (Scheme 2). components of molecular conductors [64,65]. Thus, we designed novel donor-typ bis-dithiolene complexes [Ni(Cn-dddt)2] formed by a [Ni(dddt)2] skeleton fus bulky cycloalkane rings and investigated the substituent effects on the arrangeme complexes and their physical properties (Scheme 2). Donor organic molecules fus bulky substituents such as cycloalkane and dioxane moieties can form unique m arrangements in the crystals [66], which exhibit superconducting and metallic be [45,46,[67][68][69]. Scheme 2. Metal-dithiolene complexes as components of molecular conductors.

Scheme 2.
Metal-dithiolene complexes as components of molecular conductors.

Elemental Analyses
The analyses of the carbon, hydrogen, and nitrogen elements were performed using a PERKIN-ELMER 240 C elemental analyzer for 1, 3, 7; and a Yanako MT-6 CHN CORDER for 2, 4, 5, 6, and 8. The elemental analyses of the nickel and sulfur atoms of the neutral complexes 5-8 were performed using a SHIMADZU ICPS-1000IV by the Chemical Analysis Team of AD&SD in RIKEN.

Electrochemical Measurements
Cyclic voltammograms of monoanionic complexes 1-4 were obtained using acetonitrile solutions (1 × 10 −3 mol L −1 ) with (Bu 4 N)(ClO 4 ) (0.1 M) as a supporting electrolyte and recorded using a BAS ALS/[H] CH Instruments Model 610 electrochemical analyzer combined with an Ag/Ag + reference electrode, platinum counter electrode, and glassy carbon working electrode at room temperature. The scan rate of the measurements was 100 mV/s.

Electronic Absorption Spectra
The electronic absorption spectra (300-1600 nm) of 1-8 in CH 2 Cl 2 , CH 3 CN, and benzene solutions (5 × 10 −5 mol L −1 ) at room temperature were measured by the Chemical Analysis Team of AD&SD in RIKEN using a Shimadzu UV-3100 system.

Electrical Resistivity
The temperature dependence of the electrical resistivity was measured using the standard four-probe method at ambient pressure for 9 and 10. Gold wires (15 µm in diameter) were attached to the crystal with a carbon paste. Resistivity measurements under the hydrostatic pressure of the salt were performed within the range from 0.3-1.8 GPa using a clamp-type piston-cylinder high-pressure cell [71,72].

Magnetic Susceptibility
The magnetic susceptibilities of 9 (15 mg) and 10 (20 mg) were measured in a magnetic field of 1 T using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS-XL7) within the temperature range of 2-300 K.

Band Calculation
The intermolecular overlap integrals (S) between the frontier orbitals were calculated based on the extended Hückel molecular orbital (MO) method. The semi-empirical parameters for the Slater-type atomic orbitals were obtained from the literature [73][74][75] Crystallographic data for the single crystals of L2-L4, 5-8, and 9-10 were obtained using a Rigaku MicroMax-007 diffractometer with a multilayer mirror monochromated Mo-Kα (λ = 0.71073 Å) and charge-coupled device detector at 93 K. Structural refinements were carried out using the full-matrix least-squares method on F 2 [76]. The structures were determined and refined using SHELXL-2018 in the Olex2 software package [77,78]. The parameters were refined using anisotropic temperature factors, with the exception of the hydrogen atoms, which were parametrically refined using the riding model with a fixed C-H bond distance of 0.95 Å. The crystallographic data for L2-L4, 5-8, and 9-10 are summarized in Tables 2-4, respectively.

Synthesis of Complexes 1-8 3.1.1. Syntheses and Molecular Structures of Precursors L1-L4
One of the most effective methods for synthesizing various TTF derivatives and their precursors involves [2 + 4] cycloadditions of (C 3 S 5 ) x (as 4π) and a molecule that involves one of its external double bonds (as 2π) [79]. This reaction is stereospecific, similar to the [2 + 4] cycloaddition of tetrathiooxalate with alkanes, thus providing a cis-oriented compound [80]. Precursors L1-L4 were synthesized using a modified version of a method outlined in a previous study, by the [2 + 4] cycloadditions of (C 3 S 5 ) x and cycloheptene, cyclohexene, cycloheptene, and cyclooctene [81]. The cycloalkylene residues were cisoriented with respect to the newly formed 1,4-dithiene rings. Single crystals of L1-L4 were obtained by recrystallization from their dichloromethane solutions, which were suitable for X-ray crystallographic analyses, with the exception of L1. Figure 1 presents the molecular structures of two crystallographically independent molecules A and B in the crystal of L2. The fused cyclohexane moieties in molecules A and B formed cis-oriented (R,S) conformations at the two carbons C(4)-C(5) and C(13)-C(14), respectively. Stable chair-type configurations of the cyclohexylene groups were observed for molecules A and B. A similar cis-oriented conformation was observed in the crystals of 4,5-(l,4-dioxanediyl-2,3-dithio)-l,3-dithiole-2-thione and 4,5-(cis-cyclohexylenedithio)-1,3-dithiole-2-one [82,83].  7), respectively. The comparison of the torsion angles between molecules A and B revealed that the fused cyclohexylene group in molecule A deviated more from the C 3 S 5 plane at the S(1) and S(2) positions than that of molecule B.

Syntheses and Molecular Structures of Precursors L1-L4
One of the most effective methods for synthesizing various TTF derivatives and their precursors involves [2 + 4] cycloadditions of (C3S5)x (as 4π) and a molecule that involves one of its external double bonds (as 2π) [79]. This reaction is stereospecific, similar to the [2 + 4] cycloaddition of tetrathiooxalate with alkanes, thus providing a cis-oriented compound [80]. Precursors L1-L4 were synthesized using a modified version of a method outlined in a previous study, by the [2 + 4] cycloadditions of (C3S5)x and cycloheptene, cyclohexene, cycloheptene, and cyclooctene [81]. The cycloalkylene residues were cis-oriented with respect to the newly formed 1,4-dithiene rings. Single crystals of L1-L4 were obtained by recrystallization from their dichloromethane solutions, which were suitable for X-ray crystallographic analyses, with the exception of L1. Figure 1 presents the molecular structures of two crystallographically independent molecules A and B in the crystal of L2. The fused cyclohexane moieties in molecules A and B formed cis-oriented (R,S) conformations at the two carbons C(4)-C(5) and C(13)-C(14), respectively. Stable chairtype configurations of the cyclohexylene groups were observed for molecules A and B. A similar cis-oriented conformation was observed in the crystals of 4,5-(l,4-dioxanediyl-2,3dithio)-l,3-dithiole-2-thione and 4,5-(cis-cyclohexylenedithio)-1,3-dithiole-2-one [82,83].   and cyclooctane moieties formed cis-oriented (R,S) conformations at the two carbons C(4) and C(5), respectively, in addition to molecule L2. Twist-chair-and boat-chair-type configurations of the cycloheptylene and cyclooctylene groups were formed in L3 and L4, which were observed in the crystals of CnDT-EDO-TTF (n = 7, 8) [45]. The C 3 S 5 moieties of L3 and L4 were planar structures, as observed in the crystal of L2; where the bond lengths C(1)=S (5) Figure 2 presents the molecular structures of L3 and L4. The fused cycloheptane and cyclooctane moieties formed cis-oriented (R,S) conformations at the two carbons C(4) and C(5), respectively, in addition to molecule L2. Twist-chair-and boat-chair-type configurations of the cycloheptylene and cyclooctylene groups were formed in L3 and L4, which were observed in the crystals of CnDT-EDO-TTF (n = 7, 8) [45]. The C3S5 moieties of L3 and L4 were planar structures, as observed in the crystal of L2; where the bond lengths C(1)=S (5)

Molecular and Crystal Structures of Neutral Complexes 5-8
We successfully obtained single crystals of the neutral complexes that were suitable for X-ray crystallographic analysis by recrystallization from CS2 solutions of complexes 5-8. The synthetic procedure for the neutral complexes is shown in Scheme 3. The precursors of L1-L4 formed cis-oriented (R,S) conformations around the cycloalkylene groups due to the [2 + 4]
Crystals 2021, 11, x FOR PEER REVIEW sum of the van der Waals radius (3.70 Å) were observed between the S (3.574 Å) and S(1)-S(4′) atoms (3.604 Å), which formed a three-dimensional ture in crystal 5.  A similar molecular geometry of trans-[Ni(cis-C6-dddt)2] was re [94]. However, the packing motif of crystal 6 was significantly differ reported crystal constructed by the centrosymmetric molecular arr [Ni(cis-C6-dddt)2] with an inversion center on the nickel atom. In th non-centrosymmetric molecular arrangement with a helix axis along the trans-[Ni(cis-C6-dddt)2] molecules was observed ( Figure 6). The prevented the formation of stacks of the complexes and intermo through the S···S contacts. The shortest intermolecular S···S distance w S(1) and S (8). The shortest Ni-Ni distance was 8.030 Å, which was the A similar molecular geometry of trans-[Ni(cis-C6-dddt) 2 ] was reported by Bai et al. [94]. However, the packing motif of crystal 6 was significantly different from that of the reported crystal constructed by the centrosymmetric molecular arrangement of trans-[Ni(cis-C6dddt) 2 ] with an inversion center on the nickel atom. In the case of crystal 6, a noncentrosymmetric molecular arrangement with a helix axis along the b axis formed by the trans-[Ni(cis-C6-dddt) 2 ] molecules was observed ( Figure 6). The cyclohexylene rings prevented the formation of stacks of the complexes and intermolecular interactions through the S···S contacts. The shortest intermolecular S···S distance was 3.749 Å between S(1) and S (8). The shortest Ni-Ni distance was 8.030 Å, which was the length of the a axis.
One  Figure 8 presents the crystal structure of 7. The cycloheptylene moieties were in contact along the c axis. There was no intermolecular S···S contact, given that the [Ni(dddt)2] cores were completely separated by the steric hindrance of the cycloheptane moieties.     through the S-S contacts. The shortest distance was 3.775 Å between column. The columns interacted through the side-by-side intermolecu tween S(3) and S(3′) with a distance of 3.444 Å.   Crystals 2021, 11, x FOR PEER REVIEW through the S-S contacts. The shortest distance was 3.775 Å between S(2) and S(3 column. The columns interacted through the side-by-side intermolecular S···S con tween S(3) and S(3′) with a distance of 3.444 Å.

Electronic Absorption Spectra of 1-8
The electronic absorption spectra of monoanionic complexes 1-4 and neutral complexes 5-8 in CH2Cl2 were measured. Monoanionic complexes 1-4 exhibited a strong and broad absorption band at approximately 1036-1201 nm in the near-infrared region, which can be attributed to a π-π * transition between their HOMOs and LUMOs (Table 7) [95,96]. With respect to neutral complexes 5-8, the HOMO-LUMO absorption bands were shifted to the higher energy region (982-1040 nm), which corresponds to the energy gap between and Complexes 1-4.

Electronic Absorption Spectra of 1-8
The electronic absorption spectra of monoanionic complexes 1-4 and neutral complexes 5-8 in CH 2 Cl 2 were measured. Monoanionic complexes 1-4 exhibited a strong and broad absorption band at approximately 1036-1201 nm in the near-infrared region, which can be attributed to a π-π * transition between their HOMOs and LUMOs (Table 7) [95,96]. With respect to neutral complexes 5-8, the HOMO-LUMO absorption bands were shifted to the higher energy region (982-1040 nm), which corresponds to the energy gap between HOMO-1 and the HOMOs of the monoanionic complexes due to the weak shoulder absorptions observed at approximately 1000 nm. For example, monoanionic complex 4 exhibited two absorptions at 1201 nm and 1040 nm, which can be attributed to the HOMO-LUMO and HOMO-1 to HOMO gaps, respectively ( Figure 12). The HOMO-LUMO gaps of the neutral complexes estimated by the spectra were within the range 1.19-1.26 eV. The spectroscopic behaviors of the complexes were similar to those of other bis(ethylene-1,2-dithiolato) complexes and M(S 2 C 2 R 2 ) 2 [85][86][87][88][89]93,94,97,98]. Table 7. Wavelength at absorption maxima (λ max ) and absorption coefficients (ε) for the π-π* transitions of 1-8 in CH 2 Cl 2 .      Figure 14 and Figure S2 present the crystal structures of 9 and 10. In the crystals, similar molecular arrangements of the donors and anions were observed with a two-fold axis on one of the Cl-O and P-F bonds of the anions along the b axis, respectively. In 9, the perchlorate anions were disordered at the two sites by rotation around the two-fold axis. The cis-[Ni(cis-C5-dddt) 2 ] conformation would induce dimerized structures of the donors in 9 and 10, which differ from the molecular arrangement in the cases of the 3:2 crystals of [Ni(dddt) 2 ] 3 (A) 2 significantly. The dimerized structures of the donors were formed by the S···S contacts (3.665 Å and 3.664 Å for 9 and 10, respectively), and the dimers were arranged into columnar structures through the S···S contacts (3.682 Å and 3.556 Å for 9 and 10, respectively) along the a-c axis directions in 9 and 10. Intermolecular side-by-side interactions through the S···S contacts (3.507-3.637 Å and 3.500-3.629 Å for 9 and 10, respectively) between the columns were observed along the b axis, thus providing two-dimensional donor sheets parallel to the bc planes. Although the donor molecule is fused with the bulky cyclopentane moieties, the two-dimensional donor sheet arrangements through the S···S contacts preferentially are constructed in the radical cation crystals, which are also observed in the organic radical cation crystals based on the BEDT-TTF and BETS molecules exhibiting metallic and superconducting behaviors [14][15][16][17][18].

Structures and Physical Properties of Radical Cation
tals 2021, 11, x FOR PEER REVIEW around the two-fold axis. The cis-[Ni(cis-C5-dddt)2] conformation wo ized structures of the donors in 9 and 10, which differ from the molecu the cases of the 3:2 crystals of [Ni(dddt)2]3(A)2 significantly. The dime the donors were formed by the S···S contacts (3.665 Å and 3.664 Å fo tively), and the dimers were arranged into columnar structures throug (3.682 Å and 3.556 Å for 9 and 10, respectively) along the a-c axis dire Intermolecular side-by-side interactions through the S···S contacts ( 3.500-3.629 Å for 9 and 10, respectively) between the columns were ob axis, thus providing two-dimensional donor sheets parallel to the bc pl donor molecule is fused with the bulky cyclopentane moieties, the tw nor sheet arrangements through the S···S contacts preferentially are con ical cation crystals, which are also observed in the organic radical catio the BEDT-TTF and BETS molecules exhibiting metallic and supercon [14][15][16][17][18].

Molecular Orbitals and Energy Band Calculations of 9 and 10
To reveal the electronic features of the salts, molecular orbitals (MO) and energy ban structures were determined using the extended Hückel and tight-binding methods Figure 15 presents the determined HOMOs and LUMOs of the donor molecules of 9 an 10 in their neutral states. The highly symmetric HOMOs and LUMOs with the nickel orbitals of the donors were in good agreement with the molecular orbitals of th [M(dddt)2] complex calculated based on the first-principles density-functional theory [56 62].

Molecular Orbitals and Energy Band Calculations of 9 and 10
To reveal the electronic features of the salts, molecular orbitals (MO) and energy band structures were determined using the extended Hückel and tight-binding methods. Figure 15 presents the determined HOMOs and LUMOs of the donor molecules of 9 and 10 in their neutral states. The highly symmetric HOMOs and LUMOs with the nickel d orbitals of the donors were in good agreement with the molecular orbitals of the [M(dddt) 2 ] complex calculated based on the first-principles density-functional theory [56][57][58][59][60][61][62].

Molecular Orbitals and Energy Band Calculations of 9 and 10
To reveal the electronic features of the salts, molecular orbitals (MO) and energy band structures were determined using the extended Hückel and tight-binding methods. Figure 15 presents the determined HOMOs and LUMOs of the donor molecules of 9 and 10 in their neutral states. The highly symmetric HOMOs and LUMOs with the nickel d orbitals of the donors were in good agreement with the molecular orbitals of the [M(dddt)2] complex calculated based on the first-principles density-functional theory [56][57][58][59][60][61][62].  The calculated overlap integrals (S) of the crystals between the HOMOs are summarized in Table 8 with a schematic diagram of their donor arrangements. The symbols a1-b4 indicate directions of intermolecular interactions between the donors. Figure S3 presents the corresponding molecular arrangement of the donor molecules viewed along the end-on direction in the crystal. The significant dimerization of the crystals was observed, given that the face-to-face interactions (a2) were significantly stronger than the other interactions (a1) in the columns. Dominant side-by-side interactions (b1 and b4) connected the dimers, thus forming two-dimensional sheets in the crystals. Figure 16 presents the calculated band structure of 9, which is similar to that of crystal 10. Both the upper bands crossed the Fermi level, thus forming the small two-dimensional Fermi surfaces of the crystals.

Electrical Conductivities and Magnetic Properties of 9 and 10
The temperature dependence of the resistivities was measured for single crystals of 9 and 10 K using a standard four-probe direct current (DC) method ( Figure 17). The salts exhibited semiconducting behaviors (9: ρrt = 0.8 Ω cm and Ea = 0.09 eV; 10: ρrt = 4.0 Ω cm and Ea = 0.13 eV). Although the energy band calculations for 9 and 10 suggested a metallic nature, crystals 9 and 10 exhibited semiconducting behaviors under ambient pressure. The results of the resistivity measurements and the energy band calculations indicate that the crystals forms a Mott insulating state or a spin-singlet state under ambient pressure [104]. In order to reveal the electronic structures of the crystals, temperature dependence of the resistivities under hydrostatic pressures and magnetic susceptibilities of 9 and 10 were measured. The resistivity measurements under various hydrostatic pressures (0. The calculated overlap integrals (S) of the crystals between the HOMO rized in Table 8 with a schematic diagram of their donor arrangements. Th b4 indicate directions of intermolecular interactions between the donors. sents the corresponding molecular arrangement of the donor molecules vie end-on direction in the crystal. The significant dimerization of the crystals given that the face-to-face interactions (a2) were significantly stronger tha teractions (a1) in the columns. Dominant side-by-side interactions (b1 and the dimers, thus forming two-dimensional sheets in the crystals. Figure 1 calculated band structure of 9, which is similar to that of crystal 10. Both th crossed the Fermi level, thus forming the small two-dimensional Fermi s crystals.

Electrical Conductivities and Magnetic Properties of 9 and 10
The temperature dependence of the resistivities was measured for single crystals of 9 and 10 K using a standard four-probe direct current (DC) method ( Figure 17). The salts exhibited semiconducting behaviors (9: ρ rt = 0.8 Ω cm and E a = 0.09 eV; 10: ρ rt = 4.0 Ω cm and E a = 0.13 eV). Although the energy band calculations for 9 and 10 suggested a metallic nature, crystals 9 and 10 exhibited semiconducting behaviors under ambient pressure. The results of the resistivity measurements and the energy band calculations indicate that the crystals forms a Mott insulating state or a spin-singlet state under ambient pres-sure [104]. In order to reveal the electronic structures of the crystals, temperature dependence of the resistivities under hydrostatic pressures and magnetic susceptibilities of 9 and 10 were measured. The resistivity measurements under various hydrostatic pressures (0.3-1.8 GPa) exhibited slightly smaller activation energies than those under ambient pressure (9: ρ rt = 0.2 Ω cm and E a = 0.07 eV; 10: ρ rt = 1.0 Ω cm and E a = 0.12 eV at 1.8 GPa). The temperature dependencies of the magnetic susceptibilities of 9 and 10 were in accordance with the Curie-Wiess law. However, significantly small χ values at 300 K indicate diamagnetic features (χ = 2.9 × 10 −5 cm 3 mol −1 for 9 and χ = 2.3 × 10 −5 cm 3 mol −1 for 10). These results suggest that the crystals formed spin-singlet states between the dimers through the S···S contacts to the b1 direction (Table 8), which is in accordance with their conducting behaviors.
Crystals 2021, 11, x FOR PEER REVIEW GPa) exhibited slightly smaller activation energies than those under ambient pres ρrt = 0.2 Ω cm and Ea = 0.07 eV; 10: ρrt = 1.0 Ω cm and Ea = 0.12 eV at 1.8 GPa). The t ature dependencies of the magnetic susceptibilities of 9 and 10 were in accordan the Curie-Wiess law. However, significantly small χ values at 300 K indicate diam features (χ = 2.9 × 10 −5 cm 3 mol −1 for 9 and χ = 2.3 × 10 −5 cm 3 mol −1 for 10). These suggest that the crystals formed spin-singlet states between the dimers through contacts to the b1 direction (Table 8), which is in accordance with their conducting iors.

Conclusions
New donor-type nickel-dithiolene complexes based on the [Ni(dddt)2] s fused with bulky cycloalkane substituents were synthesized as new components lecular conductors. All the neutral crystals were crystallized in cis-[Ni(cis-Cn-ddd formations with cis-oriented (R,S) conformations around the cycloalkylene groups the [2 + 4] cycloadditions. The flexible cycloalkane moieties in the molecules can unique molecular arrangements in their neutral crystals. The three-dimensional n and one-dimensional columnar structures were constructed through intermolecu contacts in crystals 5 and 8. No intermolecular S···S contacts were observed in 6 However, [Ni(C6-dddt)2] formed a helical molecular arrangement in the crystal. N ion radical crystals [Ni(C3-dddt)2]2(ClO4 − or PF6 − ) with dimerized structures were s fully obtained by electrochemical crystallization, which formed different electroni tures of the [Ni(dddt)2]3(X)2 crystals [55,57,[99][100][101][102][103]. The radical cation crystals we lators due to the spin-singlet states between the dimers through the strong intermo S···S contacts. However, the donors with cycloalkane substituents can adjust the tions of the inter-and intra-dimer interactions through the S···S contacts in their cation crystals by modification of the cycloalkane moieties, and allow for the rea of various cation radical crystals with unique donor arrangements by introducing tetrahedral, and octahedral anions. Moreover, neutral crystals are potential candid new single-component molecular conductors [58][59][60][61][62][63].
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Fi Molecular structure of 10 along vertical and parallel directions on the molecular plane w labels. Figure S2: Crystal structure of 10 along the (a) a, (b) b, and (c) c axes. Hydrogen at omitted for clarity. The dashed lines indicate sulfur-sulfur distances shorter than the sum o der Waals radius (<3.70 Å), CIFs: CCDC numbers 2103657-2103665 for 5-8, 9, and 10.

Conclusions
New donor-type nickel-dithiolene complexes based on the [Ni(dddt) 2 ] skeleton fused with bulky cycloalkane substituents were synthesized as new components of molecular conductors. All the neutral crystals were crystallized in cis-[Ni(cis-Cn-dddt) 2 ] conformations with cis-oriented (R,S) conformations around the cycloalkylene groups due to the [2 + 4] cycloadditions. The flexible cycloalkane moieties in the molecules can induce unique molecular arrangements in their neutral crystals. The three-dimensional network and onedimensional columnar structures were constructed through intermolecular S···S contacts in crystals 5 and 8. No intermolecular S···S contacts were observed in 6 and 7. However, [Ni(C 6 -dddt) 2 ] formed a helical molecular arrangement in the crystal. New cation radical crystals [Ni(C 3 -dddt) 2 ] 2 (ClO 4 − or PF 6 − ) with dimerized structures were successfully obtained by electrochemical crystallization, which formed different electronic structures of the [Ni(dddt) 2 ] 3 (X) 2 crystals [55,57,[99][100][101][102][103]. The radical cation crystals were insulators due to the spin-singlet states between the dimers through the strong intermolecular S···S contacts. However, the donors with cycloalkane substituents can adjust the proportions of the inter-and intra-dimer interactions through the S···S contacts in their radical cation crystals by modification of the cycloalkane moieties, and allow for the realization of various cation radical crystals with unique donor arrangements by introducing linear, tetrahedral, and octahedral anions. Moreover, neutral crystals are potential candidates for new single-component molecular conductors [58][59][60][61][62][63].
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/cryst11101154/s1, Figure S1: Molecular structure of 10 along vertical and parallel directions on the molecular plane with atom labels. Figure