**2. Photochemical hydrogen production from a series of 3D transition metal complexes bearing o-phenylenediamine ligand**

Masaki Yoshida et al. [65] developed a series of 3D-transition metal complexes with *o*- phenylenediamine (opda) ligands for hydrogen production due to the following properties: (**a**) aromatic amine undergoes homolytic N-H bond cleavage by photoexcitation [66] which is applicable for hydrogen production under mild condition, and (**b**) opda complexes have extensively been obsessed as reversible multielectron or multiproton pooling ability because of its multistep redox

processes between *o*-phenylenediamine(opda), semibenzoquinodiamine (s-bqdi), and *o*-benzoquinodiimine (bqdi) which is useful for reversible hydrogen production. [M(opda)3], M = Mn2+, Fe2+, Co2+, Ni2+, and Zn+2 complexes **(1–5)** shown in **Table 1** have photochemical hydrogen production ability.

Schiff base condensation of butanedione-monoximeon diamine compounds but not widely used for HER. Cobalt diimine-dioxime catalysts are active for H2 evolution in aqueous solution, both after immobilization on electrode materials and in lightdriven homogeneous conditions. The electrocatalytic activity of complexes (**6**) and (**7**) for hydrogen evolution was supported gas chromatography and by cyclic voltammetry study illustrated in **Figure 5** with the appearance of an irreversible wave. In the case of the BF2-associated complex (7), the electrcatalytic wave which

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal…*

to the H-bridged complex (6), electrocatalytic H2 evolution occurs at potentials

couple in contrast

developed toward negative potentials corresponds to the CoII/CoI

positively shifted with regard to the CoII/CoI couple.

*DOI: http://dx.doi.org/10.5772/intechopen.92854*

**103**

a. Photochemistry of aromatic amine (opda)


In the past M. Yoshida and coworkers proposed [Fe-opda] for photochemical HER mechanism at photoirradiation of λ = 298 nm; ππ\* excitation occurred in complex with N-H bond homolysis process, followed by H2 elimination [67]. After this process, the opda ligands in the complex were partially oxidized to bqdi or sbqdi ligands. This mechanism is based on the deep-rooted photochemical N-H bond activation of aromatic amines. Theoretical study and ultrafast spectroscopic studies of amino benzene support that the photochemical N-H fission occurs by the photoexcitation to higher-lying ππ\* level which leads to the formation of the πσ\* state [68]. Photochemical mechanism for HER of all complexes is shown in **Figure 4.**

All opda-based metal complexes display photochemical HER activities with the formation of almost one equivalent of H2 gas. However, the HER was not observed at all in the dark in all complexes, which suggests that the HER was obsessed by photochemical reaction. Moreover, they observed remarkable decrease in hydrogen evolution reaction, while the ligand is replaced with aromatic amines. This experiment suggested that the photo-induced HER activities of the complexes in this case are weakly dependent on the central metal ion and strongly dependent on the redox-active ligand. Further to check the metal ion dependency, examine the catalytic hydrogen production in the presence of hydroquinone (HQ; 10 equiv) as a sacrificial electron-proton donor. The photochemical H2 production from [M-opda] (7.98 <sup>10</sup><sup>2</sup> mmol) with HQ (7.98 <sup>10</sup><sup>1</sup> mmol) in THF (4 mL) under an N2 atmosphere at 20°C for 190 h turns over the number for all the complexes given in **Table 2**. Difference in TON may be caused by the stability of each complexes.

### **2.1 Cobalt diimine-dioxime complexes for HER**

V. Artero and coworker synthesized cobalt diimine-dioxime complexes as molecular catalysts for H2 evolution [69, 70]. This synthesized ligand (N2 , N2 propanediylbis-butan-2-imine-3-oxime) has emerged many years ago through *Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

Schiff base condensation of butanedione-monoximeon diamine compounds but not widely used for HER. Cobalt diimine-dioxime catalysts are active for H2 evolution in aqueous solution, both after immobilization on electrode materials and in lightdriven homogeneous conditions. The electrocatalytic activity of complexes (**6**) and (**7**) for hydrogen evolution was supported gas chromatography and by cyclic voltammetry study illustrated in **Figure 5** with the appearance of an irreversible wave. In the case of the BF2-associated complex (7), the electrcatalytic wave which developed toward negative potentials corresponds to the CoII/CoI couple in contrast to the H-bridged complex (6), electrocatalytic H2 evolution occurs at potentials positively shifted with regard to the CoII/CoI couple.

processes between *o*-phenylenediamine(opda), semibenzoquinodiamine (s-bqdi), and *o*-benzoquinodiimine (bqdi) which is useful for reversible hydrogen production. [M(opda)3], M = Mn2+, Fe2+, Co2+, Ni2+, and Zn+2 complexes **(1–5)** shown in

In the past M. Yoshida and coworkers proposed [Fe-opda] for photochemical HER mechanism at photoirradiation of λ = 298 nm; ππ\* excitation occurred in complex with N-H bond homolysis process, followed by H2 elimination [67]. After this process, the opda ligands in the complex were partially oxidized to bqdi or sbqdi ligands. This mechanism is based on the deep-rooted photochemical N-H bond activation of aromatic amines. Theoretical study and ultrafast spectroscopic studies of amino benzene support that the photochemical N-H fission occurs by the photoexcitation to higher-lying ππ\* level which leads to the formation of the πσ\* state [68]. Photochemical mechanism for HER of all complexes is shown in **Figure 4.** All opda-based metal complexes display photochemical HER activities with the formation of almost one equivalent of H2 gas. However, the HER was not observed at all in the dark in all complexes, which suggests that the HER was obsessed by photochemical reaction. Moreover, they observed remarkable decrease in hydrogen evolution reaction, while the ligand is replaced with aromatic amines. This experiment suggested that the photo-induced HER activities of the complexes in this case are weakly dependent on the central metal ion and strongly dependent on the redox-active ligand. Further to check the metal ion dependency, examine the catalytic hydrogen production in the presence of hydroquinone (HQ; 10 equiv) as a sacrificial electron-proton donor. The photochemical H2 production from [M-opda] (7.98 <sup>10</sup><sup>2</sup> mmol) with HQ (7.98 <sup>10</sup><sup>1</sup> mmol) in THF (4 mL) under an N2 atmosphere at 20°C for 190 h turns over the number for all the complexes given in **Table 2**. Difference in TON may be caused by the stability of each complexes.

V. Artero and coworker synthesized cobalt diimine-dioxime complexes as molecular catalysts for H2 evolution [69, 70]. This synthesized ligand (N2

propanediylbis-butan-2-imine-3-oxime) has emerged many years ago through

,

**Table 1** have photochemical hydrogen production ability.

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

a. Photochemistry of aromatic amine (opda)

b. Redox properties of opda complex

**2.1 Cobalt diimine-dioxime complexes for HER**

N2

**102**

**Table 1.** *Structure of redox-active ligands, electrocatalysts, and photocatalysts.*

#### **Figure 4.**

*Plausible mechanism for photochemical HER with [M-opda] complexes.*
