**3. Concluding remarks and future scope**

Here we present a recent development of molecular catalysts toward clean and renewable fuels using earth-abundant metals. We have highlighted a series of Co-,

Path A:

Path B:

(mol cat�<sup>1</sup>

corresponds to a TOF of 2.0 molH2 (mol cat�<sup>1</sup>

(ppy = 2-phenylpyridine, dtbpy = 4,4<sup>0</sup>

6108 for complex (**12)** and 10,014 for complex (**13)**.

catalysts for WRCs in the near future (**Figure 8**).

**2.4 Ni electrocatalysts for HER**

**108**

½ � ð Þ bztpen Cu II ð Þ <sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> <sup>H</sup><sup>þ</sup> ! �0*:*03 V ½ � ð Þ bztpen Cu III ð Þ<sup>H</sup> <sup>2</sup><sup>þ</sup> (4)

½ � ð Þ bztpen Cu II ð Þ <sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> <sup>H</sup><sup>þ</sup> ! �0*:*03 V ½ � ð Þ bztpenH Cu Ið Þ <sup>2</sup><sup>þ</sup> (6)

½ � ð Þ bztpenH Cu Ið Þ <sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> <sup>H</sup><sup>þ</sup> ! �0*:*85 V ½ � ð Þ bztpenH Cu II ð Þ<sup>H</sup> <sup>2</sup><sup>þ</sup> (7)

) cm�<sup>2</sup> was calculated on a faradic efficiency of approximately 96%, which

)s�<sup>1</sup> cm�<sup>2</sup>


(TEA) photosystems as sacrificial reductant (SR) under optimal condition upon 6 h of irradiation of UV–visible light, the turnover number (TON) of which is calculated as

Based on the control potential electrolysis experimental data, the authors proposed photocatalytic hydrogen evolution mechanism. In the first step, excited PS system takes out one electron from TEA and donates to CuII center of complex **(13)**. The protonated Cl-substituted pyridyl unit accepts that electron and kicks out the Cu-Cl center for the dissociation of Cl ligand which is substituted at the apical position which is more labile (longest bond length Cu-Cl). After that, the CuI species accept one e� and one H+ from the reduced Cl-substituted pyridinium moiety; CuII-H center is formed as a key intermediate which lead to H2 evolution. This executive mechanism provides us guidelines to design more efficient Cu-based

Professor Richard Eisenberg and coworker [77] synthesized a sequence of nickel bis(chelate) complexes; all complexes attained square planar geometry and examined photocatalytic as well as electrocatalytic behavior for hydrogen evolution.

of [(bztpen)Cu](BF4)2.


According to the author's studies on the mechanism of this process, the controlled potential electrolysis of complex **11** was measured at pH 2.5 in phosphate buffer at �0.90 V, over 2 h in a glassy carbon electrode. TON 1.4 � <sup>10</sup><sup>4</sup> mol H2

Moreover, Wang et al. [76] fabricated and examined two Cu complexes with TMPA = tris(2-pyridyl)methylamine and Cl-TMPA 1-(6-chloropyridin-2-yl)methyl-*N*,*N*-bis(pyridin-2-ylmethyl)methaneamine for photocatalytic H2 evolution behavior. They observed both in Cu(II) complexes [Cu(TMPA)Cl]Cl (**12)** and [Cu(Cl-TMPA) Cl2](**13)** that **(13)** is far efficient for photocatalytic H2 production than (**12)**, due to the presence of more labile Cl ligand with longer Cu-Cl bond length and a dangling Clsubstituted pyridyl unit in the second coordination sphere, which both contribute to a higher photocatalytic activity of complex (**13**). TMPA acts as a tetradentate ligand and coordinate with Cu(II) in a distorted trigonal manner; Cl-TMPA acts as a tridentate ligand coordinate to Cu(II) with two chloride ions in a distorted square pyramidal manner, leaving one Cl-substituted pyridyl group in the second coordination sphere structure which is given in **Table 1**. ESI-Ms data favor the formation of Cu-hydride intermediate for hydrogen evolution. The authors investigated the photocatalytic H2 production activities in the presence of a multicomponent [Ir(ppy)2(dtbpy)]Cl

½ � ð Þ bztpenH Cu II ð Þ<sup>H</sup> <sup>2</sup><sup>þ</sup>!½ � ð Þ bztpen Cu II ð Þ <sup>2</sup><sup>þ</sup> <sup>þ</sup> H2 (8)

½ � ð Þ bztpen Cu III ð Þ<sup>H</sup> <sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> <sup>H</sup><sup>þ</sup> ! �0*:*85 V ½ � ð Þ bztpen CuðII <sup>2</sup><sup>þ</sup> <sup>þ</sup> H2 (5)

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

Ni-, Cu-, Zn-based complexes for HER. We have recapitulated the fundamental principles of hydrogen and oxygen evolution reactions with molecular complexes. The designing and fabrication of the molecular complexes with redox-active ligands have been discussed in details; HER activity of the complexes strongly dependent on redox-active ligands as well the central metal ions are discussed in detail. A mechanistic approach and transfer of electron and proton during the homogeneous electrocatalyst and photocatalysts cycle are given in point. Although reasonable progress has been made in the development of metal complexes based electrocatalysts and chromospheres for photocatalytic hydrogen production, still several issues exist which need further improvement: (i) some photocatalytic systems suffer from low activities and short life times which is manifested in the instability of catalytic systems and so concern on the systems with modest water splitting activity and poor stability of the complexes. (ii) most of the complexes are not soluble in water leading to the use of organic solvent or mixture of organicwater solvent. From the future prospective, it is required to develop redox-active ligands with substituted functional group to increase the solubility of complexes in water. More experimental, spectroscopic, magnetic, and theoretical investigations is still needed to be carried out in order to understand the ligand- and metal-centered electron transfer processes. (iii) In addition, the overpotential requirements for most of the organic ligands are still very high, a chelating ligands giving much lower thermodynamic potentials and much smaller oxidation potential that should be utilized in future. (iv) In the regard of future growth in this field, with the need to design molecular complexes that can be immobilized on the surface of the electrode, for this purpose addition of suitable functional group in the ligand is necessity. These complexes can also be supported by the development of surface of the solid photocatalyst, like TiO2, BiVO4, etc. to demonstrate efficient photoelectrochemical cell.
