**1.4 Experimental approach: kinetics and mechanisms of some selected transition metal complexes of Fe(II) and Fe(III)**

The redox reactions of a few selected coordination compounds of the transition metal, iron, in its two oxidation states, i.e., +2 and +3, are briefly discussed. The mixed ligand complexes such as dicyanobis(phenanthroline)iron(III) and dicyanobis(bipyridine)iron(III) oxidize hexacyanoferrate(II), acetylferrocene, and 1-ferrocenylethanol by outer-sphere mechanism [8–11, 13] in the aqueous-organic media. The effect of optimized parameters on the kinetics of the redox reactions helped to propose the operated mechanism and rate laws (**Figures 2**–**5**) [8, 10].

#### **Figure 2.**

*Kinetics of the redox reaction between Fe(III) and Fe(II) complexes. The abbreviations; ɛ.Kobs/AF/FEt/HCF, correspond to the multiplication product of the molar absorptivity of [FeII(phen)2(CN)2] and observed zero-order rate constant/acetylferrocene/1-ferrocenylethanol/hexacyanoferrate(II).*

#### **Figure 3.**

*Kinetics of the redox reaction between Fe(III) and Fe(II) complexes. The abbreviations; k*<sup>0</sup> *obs/AF/FEt/HCF, correspond to the observed pseudo-first-order rate constant/acetylferrocene/1-ferrocenylethanol/ hexacyanoferrate(II).*

An outer-sphere electron transfer mechanism was proposed for each reaction because the oxidants and reductants are substitution inert and outer-sphere reactants.

Meanwhile, the oxidation of tris(bipyridine)iron(II) by ceric and bromate ions in aqueous-acidic media was reported to follow an outer-sphere mechanism with a second-order rate law (**Figures 6** and **7**) [12, 14]. The iron complexes of the chelating agents such as 1,10-phenanthroline and 2,2<sup>0</sup> -bipyridine and the ligand such as cyanide ion are either good outer-sphere oxidants and/or reductants and show high stability towards ligand substitution [27, 28].

and/or to optimize the utilization of oxygen in an effective way to transport and storage. Hemeproteins are those structures which maintain and control these oxygen reduction reactions. Such management and control of protein was surfaced by studying the mechanism of the redox reaction of aquopentacyanoferrate(II) ([FeII(CN)5H2O]3�) with coordinated dioxygen of human oxyhemoglobins (HbO2) [29]. This reaction yielded hydrogen peroxide (H2O2) and aquomethemoglobin (metHb∙H2O) and the oxidation product of aquopentacyanoferrate(II), i.e., [FeIII(CN)5H2O]2�. The reaction was found to undergo overall second-order

*-bipy)3]*

*<sup>4</sup>*� *by [FeIII(phen/bpy)2(CN)2]*

*<sup>+</sup> by acetylferrocene, 1-ferrocenylethanol, and*

*<sup>+</sup> [10].*

*2+ by ceric sulfate in the aqueous-acidic media [13–14].*

*2+ by bromate ion in the aqueous-acidic media [12].*

**Figure 4.**

**Figure 5.**

**Figure 6.**

**Figure 7.**

**7**

*hexacyanoferrate(II) [8].*

*Proposed rate laws: reduction of [FeIII(phen)2(CN)2]*

*Introductory Chapter: Redox - An Overview DOI: http://dx.doi.org/10.5772/intechopen.92842*

*Proposed rate law: oxidation of [FeII(CN)6]*

*Proposed rate law: oxidation of [FeII(bpy)3]*

*Proposed rate law: oxidation of [FeII(2,2*<sup>0</sup>

Protein as a nutrient is an important structure and is critical to aerobic life because of its control of oxygen reduction reactions. This management is crucial either to avoid or to minimize the production of destructive products such as hydroxyl radicals, peroxide, and superoxide as a result of the destructive reduction *Introductory Chapter: Redox - An Overview DOI: http://dx.doi.org/10.5772/intechopen.92842*

**Figure 4.**

*Proposed rate laws: reduction of [FeIII(phen)2(CN)2] <sup>+</sup> by acetylferrocene, 1-ferrocenylethanol, and hexacyanoferrate(II) [8].*

$$-\frac{\dot{w}\_{\perp}\left[\vec{h}^{\prime}\vec{w}(\ell\ell')\right]}{\dot{\ell}\ell} = \dot{k}\_{\perp} + \dot{k}\_{\perp}\left[\vec{h}^{\prime}\vec{w}(\ell\ell')\right]\left[\vec{h}^{\prime}\vec{w}(\ell\ell')\right]\_{\vec{h}} + \frac{\dot{k}\_{\perp}\left[\vec{h}^{\prime}\vec{\nu}(\ell\ell')\right]}{\dot{\ell}\ell\_{\perp}\left[\vec{h}^{\prime}\vec{w}(\ell\ell')\right]\_{\vec{h}}}$$

#### **Figure 5.**

*Proposed rate law: oxidation of [FeII(CN)6] <sup>4</sup>*� *by [FeIII(phen/bpy)2(CN)2] <sup>+</sup> [10].*

$$-\frac{\delta\left[\overline{\boldsymbol{\nu}}\_{\boldsymbol{\mathsf{F}}}\overline{\boldsymbol{\nu}}\_{\boldsymbol{\mathsf{F}}}\left(\left\{\boldsymbol{\delta}\boldsymbol{\mathsf{p}}\boldsymbol{\mathsf{v}}\right\}\_{\boldsymbol{\mathsf{F}}}\right)^{\boldsymbol{\mathsf{i}}-\mathsf{i}}\right]}{\delta\boldsymbol{\mathsf{v}}}=\frac{\delta\left[\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{F}}}\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{e}}}\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{e}}}\left[\left[\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{e}}}\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{e}}}\left[\left[\boldsymbol{\delta}\boldsymbol{\mathsf{e}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}\right]^{\boldsymbol{\mathsf{i}}}\right]\left[\left[\boldsymbol{\delta}\boldsymbol{\mathsf{e}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}^{\boldsymbol{\mathsf{e}}}\right]\left[\left[\boldsymbol{\delta}\boldsymbol{\mathsf{e}}\right]\_{\boldsymbol{\mathsf{e}}}^{\boldsymbol{\mathsf{e}}}\ldots\right]\_{\boldsymbol{\mathsf{e}}}}}{\delta\boldsymbol{\mathsf{w}}\_{\boldsymbol{\mathsf{e}}}\overline{\boldsymbol{\kappa}}\_{\boldsymbol{\mathsf{e}}}\left[\left[\boldsymbol{\delta}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}\left[\left[\boldsymbol{\kappa}\_{\boldsymbol{\mathsf{e}}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}\left[\left[\boldsymbol{\kappa}\_{\boldsymbol{\mathsf{e}}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}\left[\left[\boldsymbol{\kappa}\_{\boldsymbol{\mathsf{e}}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}\right]\right]\left[\left[\boldsymbol{\kappa}\_{\boldsymbol{\mathsf{e}}}\boldsymbol{\mathsf{w}}\right]\_{\boldsymbol{\mathsf{e}}}^{\boldsymbol{\mathsf{e}}}\ldots\left[\left[\boldsymbol{\$$

#### **Figure 6.**

An outer-sphere electron transfer mechanism was proposed for each reaction because the oxidants and reductants are substitution inert and outer-sphere

*Kinetics of the redox reaction between Fe(III) and Fe(II) complexes. The abbreviations; k*<sup>0</sup>

*correspond to the observed pseudo-first-order rate constant/acetylferrocene/1-ferrocenylethanol/*

*Kinetics of the redox reaction between Fe(III) and Fe(II) complexes. The abbreviations; ɛ.Kobs/AF/FEt/HCF, correspond to the multiplication product of the molar absorptivity of [FeII(phen)2(CN)2] and observed*

*zero-order rate constant/acetylferrocene/1-ferrocenylethanol/hexacyanoferrate(II).*

chelating agents such as 1,10-phenanthroline and 2,2<sup>0</sup>

show high stability towards ligand substitution [27, 28].

Meanwhile, the oxidation of tris(bipyridine)iron(II) by ceric and bromate ions in aqueous-acidic media was reported to follow an outer-sphere mechanism with a second-order rate law (**Figures 6** and **7**) [12, 14]. The iron complexes of the

such as cyanide ion are either good outer-sphere oxidants and/or reductants and

Protein as a nutrient is an important structure and is critical to aerobic life because of its control of oxygen reduction reactions. This management is crucial either to avoid or to minimize the production of destructive products such as hydroxyl radicals, peroxide, and superoxide as a result of the destructive reduction


*obs/AF/FEt/HCF,*

reactants.

**6**

*hexacyanoferrate(II).*

**Figure 3.**

**Figure 2.**

*Redox*

*Proposed rate law: oxidation of [FeII(bpy)3] 2+ by ceric sulfate in the aqueous-acidic media [13–14].*

$$\begin{split} Ric\_{\text{H}} &= \left\{ k\_{\text{s}} \left[ \left\{ \text{Pr} \left\{ 2, 2'-\text{H} \left( \text{p.v} \right) \right\}\_{\text{s}}^{2-} \right\} \right] \left\| H \, \text{H} \, \text{r} \, O\_{\text{s}} \right\| \right. \\ &\left. \left. + \frac{k\_{\text{s}} \left( \text{K}\_{\text{p.v}} \left[ \left\{ \text{H}, 2'-\text{H} \left( \text{p.v} \right) \right\}\_{\text{s}}^{+} \right] \left\| H \right\| \, \text{H} \, \text{r} \, O\_{\text{s}}^{-} \right) \right] \right\} \\ &\left. \left( 1 + \text{K}\_{\text{s}} \left\{ \text{H}, \text{Br} \left( \text{O}\_{\text{s}} \right)^{+} \right\} \right) \right) \end{split}$$

**Figure 7.** *Proposed rate law: oxidation of [FeII(2,2*<sup>0</sup> *-bipy)3] 2+ by bromate ion in the aqueous-acidic media [12].*

and/or to optimize the utilization of oxygen in an effective way to transport and storage. Hemeproteins are those structures which maintain and control these oxygen reduction reactions. Such management and control of protein was surfaced by studying the mechanism of the redox reaction of aquopentacyanoferrate(II) ([FeII(CN)5H2O]3�) with coordinated dioxygen of human oxyhemoglobins (HbO2) [29]. This reaction yielded hydrogen peroxide (H2O2) and aquomethemoglobin (metHb∙H2O) and the oxidation product of aquopentacyanoferrate(II), i.e., [FeIII(CN)5H2O]2�. The reaction was found to undergo overall second-order

kinetics with a first order in each oxidant (HbO2) and reductant ([FeII(CN)5H2O]3), respectively. The results declared that the structures of the reactants such as protein and external donor control the kinetics of the electron transfer with an inner-sphere mechanism that involves direct electron transfer from the aquopentacyanoferrate(II) to bound dioxygen that yields peroxide, subsequently. Another study surfaced the effect of binding sites and protonation on the kinetics of the electron transfer reaction (s) of blue copper proteins [30]. The oxidants with different binding sites such as [CoIII(4,7-DPSphen)3] <sup>3</sup>, [FeIII(CN)6] <sup>3</sup>, and [CoIII(phen)3] 3+ were used to oxidize parsley plastocyanin. In each reaction, regardless of the binding sites, and prior to electron transfer, a strong association between protein and complex occurs. The variation in the binding sites varied the reduction potential and affected the rate of electron transfer, consequently. The reductant (plastocyanin) is a copper protein that consisted of type 1 copper, which is involved in electron transport from photosystem II to photosystem I at the surface of the thylakoid membrane. A single copper here utilizes oxidation states I and II. The structure of poplar plastocyanin PCuII contains Cu(II) coordinated with two histidines, one cysteine, and one methionine in a distorted tetrahedral arrangement.

oxidants such as ozone (O3), hydrogen peroxide (H2O2), and UV light with the

This concise review of the redox reactions and their applications surfaced the crucial role of redox processes. The importance of redox processes is undoubtedly tremendous. The applications encompass energy production, technological development to treat and maintain water resources, and advances in materials chemistry. These advances may lead the life to its standard in an economic and cost-effective way. Redox reactions are also an important facet of biological and biochemical world to carry out life and its routine practices. For example photosynthesis, respiration and digestion are among the common ones. Precisely, we can sum up with one sentence that "redox" is basically the key to sustaining life on this planet.

1 Department of Chemistry, Shaheed Benazir Bhutto Women University, Peshawar,

3 Department of Chemistry, University of Malakand, Chakdara Dir Lower, Pakistan

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 National Center of Excellence in Physical Chemistry, University of Peshawar,

4 Institute of Chemical Sciences, University of Peshawar, Peshawar, Pakistan

5 Department of Physical Chemistry, Institute of Chemistry, University of

OH) which degrades

, Muhammad Sufaid Khan<sup>3</sup> and Hamsa Noreen4,5

addition of catalysts that may lead to yield hydroxyl radical (•

*Introductory Chapter: Redox - An Overview DOI: http://dx.doi.org/10.5772/intechopen.92842*

**2. Conclusion**

**Author details**

Rozina Khattak<sup>1</sup>

Peshawar, Pakistan

Pakistan

**9**

\*, Murtaza Sayed<sup>2</sup>

Campinas, Campinas, São Paulo, Brazil

provided the original work is properly cited.

rznkhattak@sbbwu.edu.pk

\*Address all correspondence to: rznkhattak@yahoo.com;

such pollutants, dyes and organic compounds, for example [35–40].

It has always been of interest to probe the details of the transfer of electron(s) and proton(s) because of successfully unveiling strategies of energy conversion in both of the fields, biology and chemistry. The energies as well as mechanism are strongly influenced by the coupling of electron and proton transfer. This defines the need to build up multiple redox equivalents to carry out those reactions that involve multielectrons. This also explains those mechanistic pathways through which electron and proton transfer occur simultaneously to avoid intermediates of high energy [31]. The theoretical background of the proton-coupled electron transfer reactions in solutions and proteins and electrochemistry was reviewed and discussed [32]. The theoretical treatment was based on the calculations of multistate continuum theory wherein the solvent provides dielectric continuum, the solute is treated as a multistate valence bond model, and quantum mechanical approach is used for transferred proton or hydrogen nucleus. The rate expression of electronically nonadiabatic electron transfer and proton-coupled electron transfer depends upon the reorganization energies of solute (inner-sphere) and solvent (outer-sphere) and also upon electronic coupling. For proton-coupled electron transfer, this is the average of the proton vibrational wave functions of the reactants and the products. The compensation of the smaller outer-sphere solvent reorganization energy for proton-coupled electron transfer by the larger energy needed to coupling for electron transfer appears with a similar rate for both electron transfer and protoncoupled electron transfer in calculations. A comparative theoretical study supported the reviewed outcomes through the proton-coupled electron transfer, single proton transfer, and single electron transfer reactions in iron bi-imidazoline complexes [33].

#### **1.5 Advanced oxidation processes for water treatment**

The oxidation of organic compounds by a number of oxidants either of inorganic nature or organic nature has been of interest. These redox reactions are usually catalyzed by transition metals. The kinetics of the oxidation of pyridinecarbaldehyde isonicotinoyl hydrazone to isonicotinoyl picolinoyl hydrazine was studied, and the mechanism was proposed in the view of results obtained in aqueous solution [34]. The reaction was catalyzed by iron(III). Advanced oxidation processes (AOPs) are used to remove pollutants/contaminants such as organic and inorganic compounds from water and wastewater by oxidation of these unwanted compounds. The process involves a number of chemical reactions consisting of

oxidants such as ozone (O3), hydrogen peroxide (H2O2), and UV light with the addition of catalysts that may lead to yield hydroxyl radical (• OH) which degrades such pollutants, dyes and organic compounds, for example [35–40].
