Crystal structures and electrochemical properties of nickel(II) complexes with N,N′,N′′,S-tetradentate Schiff base ligands

Nickel(II) Schiff base complexes containing thiolate S and polyamine N donor atoms exhibit electrocatalytic activity for proton reduction. The piperazine moiety in the Schiff base ligand gives a smaller bite angle, which is effective in reducing the overpotential.


Chemical context
Sulfur donor atoms bound to iron or nickel ions are commonly found in the active site of hydrogenase enzymes in nature. In [NiFe] hydrogenases, cysteine sulfurs are bound to the metal centers, and in [FeFe] hydrogenases, the amine moiety in the azadithiolate ligand bound to the iron centers is essential to the catalytic function (Lubitz et al., 2014). Nickel(II) complexes with sulfur and nitrogen donor atoms are efficient precatalysts or real catalysts for the electro-and photoreduction of protons (Han et al., 2012;Martin et al., 2015;Luo et al., 2017;Inoue et al., 2020). It has been pointed out that the hemilabile pyridine ligand in [Ni(C 5 H 4 NS) 3 ] is protonated in the photocatalytic hydrogen production (Han et al., 2012). The pendant amines as a proton acceptor site are also important for developing efficient electrocatalysts for hydrogen production (Helm et al., 2011;Stewart et al., 2013). In this context, thiolate complexes with pendant amino groups are good candidates for the development of proton-reduction catalysts.
The nickel(II) complex [Ni(C 11 H 16 N 3 S)]Cl (Bouwman et al., 1999) contains an N,N 0 ,N 00 ,S-tetradentate Schiff base ligand with terminal thiolate and amine moieties. The terminal amino group that is bound to the Ni center is a potential protonacceptor site. For instance, the Schiff base ligands derived from salicylaldehydes and 1-(2-aminoethyl)piperazine give square-planar and/or octahedral nickel(II) complexes, in which the terminal piperazinyl group binds to Ni in the bidentate chelate mode and the monodentate mode with protonation (Mukhopadhyay et al., 2003). Furthermore the cationic complex [Ni(C 11 H 16 N 3 S)]Cl is water-soluble, which makes it possible to investigate its catalytic performance in aqueous media. In the electrocatalytic proton reduction, the electrochemical properties of the precatalysts are directly related to the formation of real catalysts. Therefore the tuning of the redox properties is also required in the ligand design.
In this work we synthesized two water-soluble N 3 S Schiff base nickel(II) complexes, [Ni(C 12 H 18 N 3 S)]Cl (1) and [Ni(C 13 H 18 N 3 S)]PF 6 (2), in which the N,N 0 ,N 00 ,S-tetradentate Schiff base ligands contain an additional N-methyl group and a terminal piperazine moiety, respectively. The electrochemical properties of these complexes were investigated by cyclic voltammetry in water, and compared with those of [Ni(C 11 H 16 N 3 S)]Cl (3) without N-substituents.

Structural commentary
The complex cations in 1 and 2 consist of an Ni 2+ ion and a monoanionic N,N 0 ,N 00 ,S-tetradentate ligand, giving a square-planar geometry. The asymmetric unit in 1 comprises the complex cation and a chloride anion, whereas in 2 a hexafluorophosphate anion and a dichloromethane molecule are incorporated into the crystal lattice.
In complex 1, the methylene chains in the two N,N-chelate rings and the methyl group on the tertiary amine N atom are Perspective view of the complex cation of 2 with displacement ellipsoids at the 50% probability level. Table 1 Selected geometric parameters (Å , ) for 1.  N1-Ni1-N3 162.95 (10) N1-Ni1-S1 98.90 (7) N1-Ni1-N2 87.80 (10) N3-Ni1-S1 97.67 (8) N3-Ni1-N2 76.05 (10) N2-Ni1-S1 171.77 (7) N1-C8-C9-N2 45.1 (3) Figure 1 Perspective view of the complex cation of 1 with displacement ellipsoids at the 50% probability level. Hydrogen atoms of the minor occupancy component of the disordered region are omitted for clarity. disordered over two sets of sites. These two models are enantiomers to each other. The conformation of the methylene chains is dependent on the configuration of the methyl group. The N,S-chelate ring in 1 does not show disorder, and the benzene ring and the NiN 3 S coordination plane are almost coplanar. The dihedral angle between the least-squares planes is 9.74 (8) . The corresponding interplanar angle in 2 is 20.92 (12) . Because there is no significant difference in the conformation of the central chelate ring between 1 [N1-C8-C9A-N2 = 42.9 (2) ] and 2 [N1-C8-C9-N2 = 45.1 (3) ], this bending is due to the rigid structure of the piperazine chelate, which fixes the direction of the methylene groups on the tertiary amine N atom.

Supramolecular features
The crystal structure of 1 shows hydrogen bonds between the terminal amine nitrogen atom in the complex cation and two chloride ions with the N3Á Á ÁCl1 and N3Á Á ÁCl1(Àx + 1 2 , y À 1 2 , Àz + 1 2 ) distances of 3.2245 (17) and 3.1948 (17) Å , respectively (Table 3), which are similar to that of 3 (Bouwman et al., 1999). Each chloride ion bridges two complex cations through the hydrogen bonds. Thus in the crystal, the cations and anions pack together to form a zigzag hydrogen-bonded chain along the b-axis direction (Fig. 3). The disorder found in the complex cation does not affect the chain structure. There areinteractions between the hydrogen-bonded chains through the planar N,S-chelate moieties including the benzene rings [centroid-centroid distances = 3.7378 (12) and 3.8965 (13) Å ].

Database survey
The two N,N 0 ,N 00 ,S-tetradentate Schiff base ligands studied here have not been reported so far for other transition-metal ions. A similar Schiff base structure that contains benzenethiolate and polyamines is found in a trinuclear nickel(II) complex with a C 3 -symmetric ligand based on a 1,3,5trimercaptobenzene backbone (Feldscher et al., 2014). An analogous mononuclear nickel(II) complex that has a phenol O atom instead of the thiol S atom in 2 shows a piperazine bite angle of 76.65 (8) (Mukhopadhyay et al., 2003), which is comparable to that of 2.
8.16 ppm and four aromatic protons in the range 7.07-7.65 ppm (Fig. 4). In the aliphatic region, eight multiplet signals and a singlet signal are due to methylene and methyl groups, respectively. The COSY spectrum of 1 shows cross peaks between the azomethine proton and the two methylene protons at 4.01 and 4.35 ppm; thus, they were attributed to the CH 2 group adjacent to the C N group. Similar spectroscopic features appear for 2 in the aromatic region, whereas six sets of signals due to methylene protons are observed in the aliphatic region. The two sharp signals at 2.83 and 4.31 ppm for 2 were attributed to the central N,N-chelate moiety, and the latter is assigned to the CH 2 group adjacent to C N on the basis of the COSY correlation. This observation is consistent with the fast conformational change of the central chelate ring in 2. Furthermore, the similar signal pattern for the terminal methylene protons at 3.96 and 4.21 ppm suggests a boat conformation of the piperazine moiety that binds to Ni in the bidentate chelate mode.

Electrochemical Properties
The redox behavior of the N,N 0 ,N 00 ,S-tetradentate Schiff base nickel(II) complexes 1, 2, and 3 was investigated by cyclic voltammetry. Measurements were performed in 5 Â 10 À4 M (1 M = 1 mol dm À3 ) aqueous solution containing KNO 3 (0.1 M) at a scan rate of 0.1 V s À1 . The working electrode was a glassy carbon disk electrode with a diameter of 3 mm, the auxiliary electrode was a platinum wire, and the reference electrode was Ag/AgCl/saturated KCl. All complexes exhibit irreversible reduction and oxidation processes (Fig. 5). In the reduction process, the cathodic wave appeared at À1.31 V for 1, À1.19 V for 2, and À1.34 V for 3. The anodic peaks in the reverse scan (-0.52 V for 1; À0.70, À0.40 V for 2, and À0.48 V for 3) suggest the adsorption of the reduced species. In the oxidation process, the anodic wave appeared at 0.73 V for 1, 0.79 V for 2, and 0.68 V for 3. In both processes, the redox potentials of 1 are slightly shifted to more positive values than those of 3, which suggests that the electronic and steric effects of the methyl group on the central N atom are not so significant. The voltammogram of 2 shows further positive shifts, and the shift in the reduction process is more pronounced. This is probably related to the smaller bite angle of the terminal piperazine chelate, which reduces the electron-donating ability of the Schiff base ligand toward the nickel center. The proton-reduction abilities of complexes 1 and 2 were compared in a buffer solution of pH 4.6 (0.1 M acetic acid/ sodium acetate). A catalytic current was observed during the reduction process, giving a peak at À1.28 V for 1 and À1.23 V for 2 (Fig. 6). This suggests that the reduced species of the nickel(II) complex is catalytically active for proton reduction. The reduction potential for 2 is more positive than that for 1, and thus the piperazinyl arm in 2 is effective in reducing the overpotential for proton reduction.

Synthesis and crystallization
General Procedures. NMR spectra were recorded on a Bruker AVANCE 300 or a JEOL EX-400 spectrometer at room temperature. Cyclic voltammetric measurements were performed at room temperature with an ALS/DY2325 voltammetric analyzer (Bioanalytical System Ins.) under N 2 . Elemental analyses were performed by the Analytical Research Service Center at Osaka City University or A Rabbit Science Co., Ltd.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All non-hydrogen atoms were refined anisotropically. A methyl group and two methylene groups bound to the central N atom of two N,N-chelating moieties in 1 were modeled as disordered over two positions each, and the occupancy factors refined to 0.864 (3) and 0.136 (3). Hydrogen atoms on the disordered C atoms and the adjacent C atoms that belong to the minor site were placed in calculated positions with C-H(methyl) = 0.98 Å and C-H(methylene) = 0.99 Å and refined using a riding model with U iso (H) = 1.5U eq (C) and 1.2U eq (C), respectively.