**2.3 ZnII and CuII complexes for HER**

Grapperhaus et al. [74] recently reported two homogeneous electrocatalysts for H2 production. They derived bis(thiosemicarbazones) ligand from 1,2-diones, considered as a kind of multitalented redox non-innocent system. Tetra-coordinated N2S2 is able to bind with low-valent transition metals centered and formed to stable neutral complexes (**9, 10**) (**Table 1**). ZnII complex **9** containing a redox-active

**Figure 6.** *Possible pathway for catalytic hydrogen evolution, involving PCET processes.*

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

moiety diacetyl-bis (N-4 methyl-3-thiosemicarbazide) exhibits the homogeneous catalysis of electro-driven H2 evolution through proton reduction with a maximum of TOF 1170 s<sup>1</sup> in CH3OH and 11,700 s<sup>1</sup> in CH3CN at an overpotential of 0.756 V and 1.074 V, CH3COOH used as proton source. Bulk electrolysis showed that the TON of H2 evolution of ZnII is 37 in over 2.5 h experiments. To make this more comparative, Grapperhaus and coworker synthesized another CuII complex with similar ligand diacetyl-bis(N-4-methyl-3-thiosemicarbazide) and examined H2 evolution reaction. CuII complex **10** exhibits a maximum TOF of 10,000 s<sup>1</sup> in CH3CN and 5100 s<sup>1</sup> in DMF at an overpotential of 0.80 and 0.76 V, respectively. Controlled potential electrolysis confirmed CuII complex act as an excellent electrocatalyst to produce H2 with a minimum faradic efficiency of 81% and TON as high as 73 during experiment over 23 h. They examined HER mechanism of complex **10** through DFT computational studies. In the proposed mechanism, initially the protonation occurs at the hydrazino nitrogen ligand. This was followed by an electrochemical step as a formation of reduced Cu<sup>I</sup> species [CuI (HL)]. The second protonolysis occurs at the opposite hydrazine nitrogen of the ligand to yield [CuI (H2L)]<sup>+</sup> . Further one-electron reduction of [Cu<sup>I</sup> (H2L)]<sup>+</sup> leads to the formation of the H2 evolution illustrated in **Figure 7**. Hence, here it is worth to mention that the identity of metal ions at the active site affects the HER mechanism.

Professor Wang and group proposed [75] a significant homogeneous mononuclear copper electrocatalyst for H2 production attributed to diamine-tripyridine ligand; complex **11** attains trigonal bipyramidal geometry. According to the author, this ionic copper complex [Cu(bztpen)]2+ with a five coordinating nitrogen atom shows a Jahn-Teller effect. Electrochemical and spectroscopic studies supported that the H2 generation reaction takes place by two successive proton-coupled reduction processes. On the experimental observations of DPV, CV, UV–vis, and <sup>1</sup> H-NMR spectroscopic study, the authors proposed two possible pathways: path (A), protonation takes place at the CuI centered in the first step (Eqs. (4) and (5)), and path(B), protonation occurs at one of the nitrogen atoms of the ligand (Eqs. (6), (7), and (8)). H2 generation reaction in path B takes place by two successive proton-coupled reduction processes. Protonation occurs at the ligand centered in the first reduction step, followed by the Cu<sup>I</sup> centered in the second step. This provides [(bztpenH)CuII(H)]2+copper hydride species, which release H2 and regenerate CuII catalyst.

**Figure 7.** *Plausible mechanism for proton reduction in complex [CuL].*

involves the transfer of electron and proton in which the oxidation number changes

Similarly, cobalt bis(iminopyridine) complex **8** was prepared for electrocatalytic water-splitting reaction [73]. The ligand-centered redox activity was observed during the cyclic voltammetry, suggesting the considerable role of redox-active ligand which is completely involved to stabilize the cobalt metal in higher oxidation state. The two reduction potentials were observed for the CoIII/CoII (quasi reversible) and CoII/CoI (reversible) couples at 0.34 V and 0.86 V (vs. Ag/AgC), respectively. The improved water reduction was attributed to the assimilation of a redox-active-

Grapperhaus et al. [74] recently reported two homogeneous electrocatalysts for H2 production. They derived bis(thiosemicarbazones) ligand from 1,2-diones, considered as a kind of multitalented redox non-innocent system. Tetra-coordinated N2S2 is able to bind with low-valent transition metals centered and formed to stable neutral complexes (**9, 10**) (**Table 1**). ZnII complex **9** containing a redox-active

by PCET process, and the oxidation number changes from CoI to CoII. In the last step, H2 is produced in dihydrogen bond through an intramolecular mechanism. The authors also confirmed cobalt diimine-dioxime catalysts **6** and **7** active for H2 evolution under light-driven conditions in the presence of photosensitizers, associated with Ru, Ir, or Re derivatives. The photocatalytic activity of **6**-Br2 was observed in mixed H2O/CH3CN solvent in the presence of TEA and cyclometalated iridium-based photosensitizer [72]. Turnover numbers (TON) determined after continuous 4 h UV–visible light irradiation **6**-Br2 showed 300 (H2 mol cat<sup>1</sup>

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

whereas **7**-Br2 complex shows only 50 (H2 mol cat<sup>1</sup>

ligand to cobalt which facile reduction of [Co-H]n+ species.

*Possible pathway for catalytic hydrogen evolution, involving PCET processes.*

**2.3 ZnII and CuII complexes for HER**

. In the second steps, further electron and proton transfer takes place

),

) as shown in **Table 2**.

by CoII to Co<sup>I</sup>

**Figure 6.**

**106**

Path A:

$$\left[\left(\text{bztpen}\right)\text{Cu(II)}\right]^{2+} + \text{e}^- + \text{H}^+ \xrightarrow{-0.03\,\text{V}} \left[\left(\text{bztpen}\right)\text{Cu(III)H}\right]^{2+} \tag{4}$$

$$\left[ (\text{bztpen}) \text{Cu(III)H} \right]^{2+} + \text{e}^- + \text{H}^+ \xrightarrow{-0.85\text{ V}} \left[ (\text{bztpen}) \text{Cu(II})^{2+} + \text{H}\_2 \tag{5}$$

Path B:

$$\left[\left(\text{bztpen}\right)\text{Cu(II)}\right]^{2+} + \text{e}^- + \text{H}^+ \xrightarrow{-0.03\,\text{V}} \left[\left(\text{bztpen}\right)\text{Cu(I)}\right]^{2+} \tag{6}$$

$$[(\text{bztpenH})\text{Cu}(\text{I})]^{2+} + \text{e}^- + \text{H}^+ \xrightarrow{-0.85\text{ V}} [(\text{bztpenH})\text{Cu}(\text{II})\text{H}]^{2+} \tag{7}$$

$$\left[\left(\text{bztpenH}\right)\text{Cu}(\text{II})\text{H}\right]^{2+} \longrightarrow \left[\left(\text{bztpen}\right)\text{Cu}(\text{II})\right]^{2+} + \text{H}\_2\tag{8}$$

Fluorescein (Fl) as the photosensitizer along with triethanolamine (TEOA) as the sacrificial electron donor was used in water under basic medium (pH = 9.8). Bis (chelate) complexes (**14–17**) contain bdt(bisdithiolate) and their derivatives having S, O, or N as donors for coordination to the Ni center. The photochemical study reveals that only complexes **15** and **17** exhibited similar activity of hydrogen production in terms of TON (6000 mol of H2 per mole of catalyst in 100 h). In

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

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

contrast, the TON for the sample complex **16** was much lower (900). Complex **14** was observed to be inactive for hydrogen generation. To understand the better significant result, the authors pursued electrochemical experiments under a fixed potential to examine the catalytic activity of complexes. In the absence of acid, both monoanionic complexes **14** and **17** exhibit only one reversible redox couple at 0.56

reductions. The neutral complexes **15** and **16** exhibit two reversible reduction waves

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-,

at 0.11 and 0.94 V and at 0.89 and 1.53 V, respectively. However, on increasing the concentration of acetic acid to these solutions, more negative catalytic waves were observed in cyclic voltammetry study. Complex **14** shows much more negative reduction potential (2.25) which suggested this system is inactive for H2 generation (as we discussed earlier). As is the case for complex **14**, **complex 16** exhibits a catalytic wave potential at 2.03 V which showed poor activity for H2 generation as a consequence of not favorable electron transfer step from Fl to the catalyst. Moreover, complexes **15** and **17** show activity for light-driven H2 generation; these two complexes display electrocatalytic wave potential at significantly

/[NiL2]

<sup>2</sup>ligand-based

and 0.45 V, respectively, which is attributed to the [NiL2]

*Photocatalytic H2 evolution mechanism for complex [Cu(Cl-TMPA)Cl2] 12.*

**Figure 8.**

**109**

less negative at 1.64 and 1.62 V, respectively.

**3. Concluding remarks and future scope**

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 (mol cat�<sup>1</sup> ) cm�<sup>2</sup> was calculated on a faradic efficiency of approximately 96%, which corresponds to a TOF of 2.0 molH2 (mol cat�<sup>1</sup> )s�<sup>1</sup> cm�<sup>2</sup> of [(bztpen)Cu](BF4)2.

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 (ppy = 2-phenylpyridine, dtbpy = 4,4<sup>0</sup> -di-*tert*-butyl-2,2<sup>0</sup> -bipyridine) and triethylamine (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 6108 for complex (**12)** and 10,014 for complex (**13)**.

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

#### **2.4 Ni electrocatalysts for HER**

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.

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

**Figure 8.** *Photocatalytic H2 evolution mechanism for complex [Cu(Cl-TMPA)Cl2] 12.*

Fluorescein (Fl) as the photosensitizer along with triethanolamine (TEOA) as the sacrificial electron donor was used in water under basic medium (pH = 9.8). Bis (chelate) complexes (**14–17**) contain bdt(bisdithiolate) and their derivatives having S, O, or N as donors for coordination to the Ni center. The photochemical study reveals that only complexes **15** and **17** exhibited similar activity of hydrogen production in terms of TON (6000 mol of H2 per mole of catalyst in 100 h). In contrast, the TON for the sample complex **16** was much lower (900). Complex **14** was observed to be inactive for hydrogen generation. To understand the better significant result, the authors pursued electrochemical experiments under a fixed potential to examine the catalytic activity of complexes. In the absence of acid, both monoanionic complexes **14** and **17** exhibit only one reversible redox couple at 0.56 and 0.45 V, respectively, which is attributed to the [NiL2] /[NiL2] <sup>2</sup>ligand-based reductions. The neutral complexes **15** and **16** exhibit two reversible reduction waves at 0.11 and 0.94 V and at 0.89 and 1.53 V, respectively. However, on increasing the concentration of acetic acid to these solutions, more negative catalytic waves were observed in cyclic voltammetry study. Complex **14** shows much more negative reduction potential (2.25) which suggested this system is inactive for H2 generation (as we discussed earlier). As is the case for complex **14**, **complex 16** exhibits a catalytic wave potential at 2.03 V which showed poor activity for H2 generation as a consequence of not favorable electron transfer step from Fl to the catalyst. Moreover, complexes **15** and **17** show activity for light-driven H2 generation; these two complexes display electrocatalytic wave potential at significantly less negative at 1.64 and 1.62 V, respectively.
