Crystal structure and electrochemical properties of [Ni(bztmpen)(CH3CN)](BF4)2 {bztmpen is N-benzyl-N,N′,N′-tris[(6-methylpyridin-2-yl)methyl]ethane-1,2-diamine}

The structure and electrochemical properties of a nickel tripyridine–diamine complex are reported. The complex has two redox couples at −1.50 and −1.80 V (versus F c +/0) based on nickel.


Chemical context
Nickel complexes with polypyridine-amine ligands are of great interest in catalytic reactions. For example, nickel complexes containing N5-pentadentate ligands with different amine-to-pyridine ratios have been studied for electrochemical H 2 production in water at pH = 7 and the complex with a diamine-tripyridine ligand displays a TON (turn-over number) of up to 308000 over 60 h electrolysis at À1.25 V vs the standard hydrogen electrode (SHE), with a Faradaic efficiency of 91% (Zhang et al., 2014). The nickel-based complex Ni-PY 5 {PY 5 = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine} has been found to act as an electro-catalyst for oxidizing water to dioxygen in aqueous phosphate buffer solutions (Wang et al., 2016). The rate of water oxidation catalyzed by the Ni-PY 5 complex is enhanced remarkably by the proton-acceptor base HPO 4 2À , with a rate constant of 1820 M À1 s À1 . A stable configuration is important for the stability of a catalyst. In the title complex, the Ni 2+ cation is chelated by five N-atom sites, so the configuration is stable. With the reductive dissociation of acetonitrile, the title complex would give an open site for a catalytic reaction. Herein, we describe the crystal structure and electrochemical properties of the title complex.

Structural commentary
In the title complex ( Fig. 1), the coordination sphere of the nickel(II) atom adopts a normal octahedral geometry. The Ni 2+ cation lies almost in the equatorial plane. One pyridine nitrogen atom (N1) and two amino nitrogen atoms (N2, N3) as well as the nitrogen atom of an acetonitrile ligand (N4) form the equatorial plane. The latter can easily be dissociated from nickel. The axial positions are occupied by two pyridine nitrogen atoms (N5, N6). The Ni-N bond lengths for the two axial pyridine-nitrogen atoms [Ni-N5 = 2.209 (3) and Ni-N6 = 2.187 (3) Å ] are significantly longer than that for the other four nitrogen atoms [Ni-N1 = 2.151 (3), Ni-N2 = 2.082 (3), Ni-N3 = 2.188 (2), Ni-N4 = 2.061 (3) Å ]. The presence of the 6-methyl substituent hinders the approach of the pyridine group to the Ni 2+ core. A a result of the steric hindrance from the methyl substituent, the three atoms N5, Ni1 and N6 are not completely linear in the axial direction, with a contact angle of 170.89 (9) . Two intramolecular C-HÁ Á ÁN contacts occur (Table 1).
Generally, the reduction of a metal complex is accompanied by the dissociation of the ligand, or the weakest ligand if more than one ligand is present, which could induce the appearance of an open site for a catalytic reaction (Knoll et al., 2014;Johnson et al., 2016). The introduction of o-methyl in the title complex is in favor of the dissociation of acetonitrile. On the cathodic scan under Ar, the title complex features one reversible couple at À1.50 V and one half-reversible couple at À1.80 V (vs F c +/0 ) based on nickel, assigned to Ni II/I and Ni I/0 respectively (Fig. 2) Cyclic voltammograms of the title complex (1 mM) with a varied scan range under Ar in CH 3 CN with 0.1 M n Bu 4 NBF 4 as the supporting electrolyte.

Figure 1
The structures of the molecular components in the title compound, showing 50% probability displacement ellipsoids. H atoms have been omitted for clarity. assigned to the reduction of pyridine. The free ligand bztmpen itself is electrochemically silent in the potential range (Fig. 3). The coordination with nickel leads to a positive shift of the reduction on pyridine. The good reversibility of the couple indicates a negligible change in the configuration of the title complex after one electron reduction. The second reduction might result in a change of the configuration. Analogues in the absence of o-methyl show only one redox couple more negative than À1.50 V (vs F c +/0 ; Zhang et al., 2014). The positive shift of the first redox couple for the title complex results from the weaker electron-donating ability of the pyridine ligands, which are farther from the nickel core. The electrochemical properties of these analogues are consistent with the differences shown in the structure.

Supramolecular features
In the title crystal, no classical hydrogen bonds have been found. Weak C-HÁ Á ÁF contacts (Table 1) link the components into a three-dimensional network. The crytal paacking is illustrated in Fig. 4.

Figure 4
Packing plot of the molecular components in the title compound viewed down the a axis. C-HÁ Á ÁF weak bonds are shown as dotted lines.  is most similar to the title complex (Zhang et al., 2014). Under reductive conditions, Ni 2+ (bztpen) displays a high activity on electro-catalytic water reduction. The title complex possesses a higher steric hindrance than Ni 2+ (bztpen), which affects evidently the bond lengths, especially in the axial direction.  (Wang et al., 2016). Ni 2+ (PY 5 ) has been found to act as an electro-catalyst for oxidizing water to dioxygen in an aqueous phosphate buffer solution.

Synthesis and crystallization
The