3,5-Dimethylpyrazole: Reactivity with Zinc(II) Carboxylates and its Determination Method

General Description

The reactivity of 3,5-dimethylpyrazole with zinc(II) carboxylates varies significantly with solvent choice, leading to mononuclear complexes in methanol and bridged binuclear complexes in dimethylformamide (DMF). The presence of solvent molecules can influence structural diversity in complexes, particularly with p-nitrobenzoic acid. In addition, a validated HPLC-DAD method was developed for accurately determining 3,5-dimethylpyrazole in nitrogen fertilizer samples. Optimization of analysis parameters ensured accurate separation, with recovery rates between 95.6% and 103.3%, supporting effective monitoring of the nitrification inhibitor for sustainable agricultural practices. This method enhances crop productivity while minimizing environmental impacts.

Figure 1. 3,5-Dimethylpyrazole

Reactivity with Zinc(II) Carboxylates

Formation of Mononuclear Zinc Complexes in Methanol 

When 3,5-dimethylpyrazole reacts with zinc(II) acetate dihydrate and aromatic carboxylic acids like benzoic acid derivatives (R = C6H5, p-CH3-C6H4, p-NO2-C6H4), mononuclear zinc complexes are formed in methanol. These complexes have the general formula [Zn(HDMP)2(RCO2)2]. The presence of methanol as a solvent stabilizes these mononuclear structures, where 3,5-dimethylpyrazole acts as a bidentate ligand coordinating to the zinc ion alongside the carboxylates. 

Binuclear Complex Formation

In contrast, the reaction of 3,5-dimethylpyrazole with zinc(II) acetate dihydrate and aromatic carboxylic acids in dimethylformamide (DMF) results in the formation of binuclear complexes. These complexes are bridged by 3,5-dimethylpyrazolato ligands, denoted as [Zn2(mu-DMP)2(HDMP)2(RCO2)2]. The solvation properties of DMF facilitate the bridging of zinc ions by the 3,5-dimethylpyrazolato ligands, leading to the formation of larger, binuclear structures. ¹

Solvated Mononuclear Complexes with p-Nitrobenzoic Acid 

Interestingly, when zinc(II) acetate dihydrate reacts with p-nitrobenzoic acid and 3,5-dimethylpyrazole in various solvents, including DMF, solvated mononuclear complexes are obtained. These complexes exhibit structural diversity due to the presence of solvent molecules around the central zinc ion. For instance, the complex [Zn(HDMP)2(p-NO2-C6H4CO2)2] observed in DMF contains two structurally independent molecules within its asymmetric unit (Z' = 2), indicating different coordination environments influenced by the solvent. ¹

In summary, the reactivity of 3,5-dimethylpyrazole towards zinc(II) carboxylates is highly dependent on the solvent environment. Methanol favors the formation of stable mononuclear complexes, while DMF induces the formation of larger, binuclear complexes bridged by 3,5-dimethylpyrazolato ligands. Solvents like DMF also facilitate the solvation of complexes, leading to structurally diverse coordination environments around the zinc center. These findings underscore the solvent-induced reactivity and the versatility of 3,5-dimethylpyrazole as a ligand in coordination chemistry with zinc(II) carboxylates. ¹

Determination Method

Method Development and Optimization

A study focused on developing an HPLC-DAD method for the determination of 3,5-dimethylpyrazole in nitrogen fertilizer samples containing 3,5-dimethylpyrazolium glyceroborate, a nitrification inhibitor. Key parameters such as eluent composition, column material, eluent flow rate, column oven temperature, and detection wavelength were systematically optimized during method development. These optimizations ensure accurate separation and quantification of 3,5-dimethylpyrazole within complex fertilizer matrices, crucial for reliable inhibitor determination. ²

Validation of the HPLC-DAD Method

The developed method underwent rigorous validation to ensure its reliability and accuracy. Parameters evaluated included linearity, limit of detection (LOD), limit of quantification (LOQ), specificity, stability, and precision (both intra-day and inter-day) and accuracy. Validation experiments confirmed the method's ability to accurately quantify 3,5-dimethylpyrazole across a range of concentrations typically found in fertilizer samples. Specificity tests ensured that the method selectively identifies and quantifies 3,5-dimethylpyrazole in the presence of other matrix components, crucial for robust analysis of complex fertilizer formulations. ²

Application in Fertilizer Analysis

Upon successful method development and validation, the HPLC-DAD method was applied to analyze nitrogen fertilizer samples for the presence of 3,5-dimethylpyrazole. The method demonstrated excellent performance with recovery rates ranging from 95.6% to 103.3%, indicating high accuracy in quantifying DMP. Additionally, relative errors were minimal, ranging from 0% to 4.61%, further confirming the method's reliability and reproducibility in real-world sample analysis. This validated method provides a practical tool for monitoring and ensuring the proper dosage of 3,5-dimethylpyrazolium glyceroborate nitrification inhibitor in agricultural practices, contributing to effective nitrogen management and sustainable crop production. ²

In summary, the development and validation of the HPLC-DAD method for 3,5-dimethylpyrazole determination represent a critical step in ensuring accurate quantification of this important nitrification inhibitor in nitrogen fertilizers. The method's robustness and reliability make it a valuable analytical tool for agricultural research and industry applications focused on optimizing fertilizer formulations and enhancing crop productivity while minimizing environmental impact. ²

Reference

1. Sarma R, Kalita D, Baruah JB. Solvent induced reactivity of 3,5-dimethylpyrazole towards zinc (II) carboxylates. Dalton Trans. 2009; (36): 7428-7436.

2. Şahan S, Şahin U, Başaran M, Uzun O, Güneş A. Determination of 3,5 - dimethylpyrazolium glyceroborate nitrification inhibitor in nitrogen fertilizer samples: HPLC-DAD method development and validation for 3,5 - dimethylpyrazole. J Chromatogr B Analyt Technol Biomed Life Sci. 2017; 1068-1069: 277-281.

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