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. 2026 Sep 15;11(38):57026–57033. doi: 10.1021/acsomega.6c04181

Merging Solid State and Coordination Chemistry: Toward a Closed Loop in Transition-Metal Complex Recycling

Zahir K Al-Wadhahi 1, Sanobar Naaz 1, Arief C Wibowo 1,*
PMCID: PMC13629390  PMID: 42825162

Abstract

This study showcases a method for recovering transition-metal (TM) precursors from TM coordination complex solid waste generated in an undergraduate inorganic chemistry laboratory and reusing them as starting precursors to again form the same respective complexes. To achieve this goal, solid-state chemistry techniques like calcination, acid dissolution, and precipitation are applied by which TM oxides and halides can be regenerated from solid wastes of TM coordination complexes. This method marks our contribution toward accomplishing a circular economy by leveraging hazardous solid waste as valuable resources, providing environmental sustainability and economic benefits.

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Introduction

Solid waste is a hazardous waste that causes various risks if not stored and disposed of properly. Improper management of solid waste can lead to health risks and environmental pollution, including soil, water, and air pollution. − These may affect extensively to the world’s economy due to ecosystem damage that leads to an increase in health care expenditure. There are various sources of hazardous solid waste such as agricultural, industrial, medical, electronic or e-waste, and research institutes. − Among these sources, academic institutions act as a primary supplier as they generate several ranges of chemical wastes used in teaching and research laboratories. − Solid waste from this source includes used reagents, discarded chemicals, solid products, and byproducts from various experimental procedures. Improper disposal of this waste leads to various pollution and risks to human health and environment due to its toxicity, corrosivity, flammability, and their reactivity. At the Department of Chemistry, Sultan Qaboos University (SQU) alone, preliminary estimation suggests that solid chemical waste quantities are approximately over 30 kg annually from both undergraduate laboratory courses and different research projects.

It is universally accepted that coordination chemistry is a fundamental part of any inorganic laboratory course. The importance of coordination chemistry is widely recognized due to its versatility in designing functional materials like conductors, semiconductors, catalysts, sensors, and magnetic devices, − and spans its applications from molecular recognition, catalysis to bioinorganic chemistry. − The synthesis and characterization of various transition-metal (TM) coordination complexes are adopted in an inorganic laboratory course globally, − offering students to hone their skills and understanding in their synthesis and characterization in conjunction with their theory. Despite the advantages, the inorganic laboratory course generates solid waste, and a versatile mitigation method is needed to address this problem.

It is well documented in the solid-state chemistry realm − that a metal coordination complex, typically used as a metal precursor, undergoes a thermal decomposition reaction into (binary or multinary nano) metal oxide via a precise calcination. Such a binary oxide can be further reacted with hydrohalic acid (HX, X = Cl, Br, or I) to form an aqueous solution of metal halide, from which a state-of-the-art solar cell compound named perovskite halide, ABX3 (A = monovalent cation, B = divalent cation, X = halide anion), is typically synthesized in combination with halide of Cs­(I) or quaternary amine.

Inspired by the above, we implemented both techniques to recover TM oxides and/or halides from solid wastes of TM coordination complexes by thermally converting the TM coordination complex waste into TM oxide and dissolving it in a diluted HX to finally recover its TM halide solid. Ultimately, these revived metal precursors (TM halides or oxides) are further utilized to resynthesize the original TM coordination complexes, thus establishing a method toward a closed loop of TM coordination complex synthesis cycle as an appealing route to realize a sustainability, abating solid waste problem.

Experimental Section

Revival of TM Precursors

All procedures should be conducted safely inside a fume hood. It is imperative to wear personal protective equipment, such as acid-resistant gloves, safety goggles and a laboratory coat, and avoid ingestion and inhalation of K2Cr2O7. Attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectra were recorded on a PerkinElmer FTIR-Spectrum 400. UV/vis spectra were performed using an Infitek Microvolume UV/vis (Nano) spectrophotometer, and their detailed parameters can be found in the Supporting Information. Electron microscopy images were performed using Hitachi SU8220 scanning electron microscope-energy dispersive spectrometry (SEM-EDS). Elemental analyses were performed using PerkinElmer CHNS/O 2400 Series II. Solid waste of metal complexes which are collected from inorganic laboratory were (1) mixture of tris­(ethylenediamine)­nickel­(II) chloride dihydrate ([Ni­(en)3]­Cl2·2H2O; en = ethylenediamine), bis­(triphenylphosphine)­nickel­(II) chloride ([Ni­(PPh3)2Cl2]; PPh3 = triphenylphosphine) and bis­(triphenylphosphine)­nickel­(II) thiocyanate ([Ni­(PPh3)2(SCN)2]), (2) tris­(acetylacetonato)­manganese­(III) ([Mn­(acac)3]; acac = acetylacetonate), and (3) potassium tris­(oxalato)­chromium­(III) trihydrate (K3[Cr­(ox)3]·3H2O; ox = oxalate). All commercial reagents (metal precursors and ligands) were purchased from Sigma-Aldrich (ACS reagent grade) and were used as received. Concentrated hydrochloric acid (37 wt %, Merck), glacial acetic acid (≥99%, Sigma-Aldrich), and acetone (≥99.5%, Alfa Aesar) were used as received.

Recovery of NiCl2·6H2O, MnCl2·4H2O, and K2Cr2O7

Step-by-step procedures for the recovery of all TM precursors can be found in the Supporting Information.

About 5.0 g of complex of Ni­(en)3]­Cl2·2H2O or from a mixture of Ni­(en)3]­Cl2·2H2O, [Ni­(PPh3)2Cl2], and [Ni­(PPh3)2(SCN)2] in the approximate amounts of 4.0, 0.5, and 0.5 g, respectively, was measured, then carefully transferred into a crucible, and placed it in a tube furnace, and the temperature was raised to 550 °C within 1 h, dwelling there for 24 h under air, followed by cooling to RT naturally. After calcination, 0.85 g of the product was obtained with 78.(6) wt % of Ni­(II) recovered from the complex (79.(1)% yield; see the Supporting Information for detailed calculation of the average yield and its standard deviation). The phase purity is verified by its powder X-ray diffraction (PXRD) pattern compared to that of simulation (Figure S1), in which a mixed phase of NiO (major phase) and metallic Ni (minor phase) were identified (see the Supporting Information for details). Afterward, the product was transferred to a beaker with addition of 10 mL of deionized water, by stirring and heating the mixture at 60–65 °C. Then, 30 mL of concentrated hydrochloric acid (HCl, 37 wt %) was added dropwise and allowed the reaction to run for 1 h. After dissolving the product (full dissolution was confirmed despite having a mixed phase), the solution was cooled to room temperature, filtered into a beaker, and then filtrate was evaporated using a steam bath (or rotary evaporator with a cold trap) until solid green product formed. A cold acetone was used to wash the solid product then dried in air. Green colored nickel­(II) chloride hexahydrate (NiCl2·6H2 O) (1.4(0) g, 52.(23)% yield; 24.(7) wt % of the Ni­(II) content in the revived TM precursor; see the Supporting Information for detailed average yield and its standard deviation) was obtained and its phase purity was confirmed by matching the revived PXRD pattern with those of simulated and commercial patterns (Figure a).

1.

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PXRD patterns comparison of revived: (a) NiCl2·6H2O, (b) MnCl2·4H2O, and (c) K2Cr2O7 with those of simulated and commercial patterns.

Similar steps were exercised for revival of Mn salts from [Mn­(acac)3] complex with a slight modification. After calcination, a 1.1(2) g of product (see the Supporting Information for detailed calculation of the yield) was obtained and the phase purity was confirmed by PXRD measurement (Figure S2), in which a mixed phase of Mn2O3 (major phase) and Mn3O4 (minor phase) were identified (with 34.(8) wt % of Mn­(III) recovered from the complex; see the Supporting Information for detailed calculation on the average yield and its standard deviation). A 10 mL of HCl (37 wt %) was added dropwise to dissolve the mixture (product and a 10 mL of deionized water) while stirring and heating at 60–65 °C for 1 h. Similar to that of NiCl2·6H2O revival, a full dissolution of the product was observed despite having a mixed phase. Filtering, evaporating the filtrate using a steam bath (or rotary evaporator with a cold trap), and finally washing with cold acetone were done to isolate light pink solid of manganese­(II) chloride tetrahydrate (MnCl2·4H2O) (1.5(7) g, 55.(87)% yield; 27.(8) wt % Mn­(II) content in the revived TM precursor; see the Supporting Information for detailed average yield and its standard deviation) as confirmed by PXRD measurement by matching the revived PXRD pattern with those of simulated and commercial patterns (Figure b).

A different approach was employed for potassium dichromate (K2Cr2O7) revival. First, 5.0 g of the K3[Cr­(ox)3]·3H2O complex was carefully transferred into the crucible and put in the furnace and the temperature was raised to 550 °C within 1 h, dwelling there for 24 h under air, followed by cooling to RT naturally. After calcination, 1.1(2) g of yellow powder of potassium chromate (K2CrO4) (56.5(6)% yield; 26.(7) wt % Cr­(VI) content revived from the complex; see the Supporting Information for detailed average yield and its standard deviation) was obtained and its phase purity was validated by PXRD measurement (Figure S3). A 10 mL of deionized water was used to dissolve K2CrO4 while stirring at room temperature. Into the filtered solution, a 10 mL of glacial acetic acid was added, then boiled; heating was stopped when orange colored precipitate started to form. After cooling to room temperature, the orange solid was filtered using Buchner suction and washed with cold acetone, then dried in air to get orange colored K2Cr2O7 (0.52 g, 63.(4)% yield; 17.(7) wt % Cr­(VI) content in the revived TM precursor; see the Supporting Information for detailed average yield and its standard deviation) as confirmed by the PXRD measurement by matching the revived PXRD pattern with those of simulated and commercial patterns (Figure c).

Resynthesis of TM Complexes Using Recovered Metal Precursors

Step-by-step procedures for resynthesizing the regenerated TM precursors into their corresponding complex can be found in the Supporting Information.

After reviving all three metal precursors (NiCl2·6H2O, MnCl2·4H2O, and K2Cr2O7) successfully, they were used to resynthesize their respective complexes using the same way, based on the existing literature − (and our laboratory course manual). The resulting resynthesized complexes displayed distinct colors of crystalline solids, i.e., purple for [Ni­(en)3]­Cl2·2H2O (complex 1) (85.(8)% yield), dark green for [Mn­(acac)3] (complex 2) (48.(3)% yield), and dark green for K3[Cr­(ox)3]·3H2O (complex 3) (48.(6)% yield). Lower yields in complexes 2 and 3 are atypical results due to product loss during precipitation and isolation of the products. Note that for complex 3, an additional recrystallization step was done using the ethanol–water (1:1 v/v) mixture. PXRD data of all three complexes were matched with their simulated patterns (Figure a–c) with CCDC numbers of 610673, 1484899, and 2396211, respectively. Quantitative analysis of the compounds was performed by CHN analysis. Anal. Calcd (found) for complex 1 made from recycled source: C, 20.83 (19.45); H, 8.16 (8.07); N, 24.30 (21.02). Anal. Calcd (found) for complex 1 made from commercial source: C, 20.83 (18.90); H, 8.16 (7.95); N, 24.30 (21.60). Anal. Calcd (found) for complex 2 made from recycled source: C, 51.14 (52.13); H, 6.01 (5.79). Anal. Calcd (found) for complex 2 made from commercial source: C, 51.14 (50.97); H, 6.01 (5.57).

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PXRD patterns comparison of resynthesized: (a) complex 1, (b) complex 2, and (c) complex 3 made using recycled TM precursor with those of simulated and complexes made from commercial sources.

Powder X-ray Diffraction

Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku MiniFlex 600 with a Cu Kα radiation (λ = 1.5418 Å). The acquisitions were done in a 2θ range of 5 to 50° with a step size of 0.02° and a scan speed of 0.25°/min. All samples were finely ground and mounted on a zero-background sample holder prior to the measurement. Crystalline phases identification of metal oxides, metal salts, and metal complexes were based on comparison with simulated (and/or commercial source) PXRD patterns. Rietveld refinements were conducted using Match software on the observed patterns against respective cif file of the complexes.

Magnetic Susceptibility Measurements

Magnetic moments were determined using the Gouy balance method. The apparatus was calibrated using a standard compound. Each complex was filled into a uniform glass sample tube, and the apparent change in mass in a magnetic field was recorded. The magnetic susceptibility was calculated, and the effective magnetic moment (μeff) was derived via the spin-only formula. Diamagnetic corrections for the samples were applied using Pascal’s constants. The measured μeff values were used to conclude the resulting geometry, the electronic configuration of the metal center, as well as the number of unpaired electrons. Detailed measurement and calculation procedures can be found in the Supporting Information.

Results and Discussion

Our method of TM coordination complexes recycling, to the best of our knowledge, is a new technique that has not been reported before as we combine a method adopted in solid-state chemistry to recover metal precursor, then using coordination complex route to resynthesize the intended complex using the recovered metal precursor. The novel route is to air calcine coordination complexes solid waste to get metal oxide, then as such, the metal oxide is dissolved in certain molarity of HCl, which is atypical method to have metal precursor of perovskite halide as an aqueous solution of metal cation and chloride anion. Their method, however, ends there, without recovering the metal precursor (e.g., metal chloride) as a solid phase, and to the best of our knowledge, no one has reported such extended route to isolate solid metal precursors. Recognizing such an opportunity, we venture onto this route and successfully recover three different solid metal precursors, i.e., NiCl2·6H2O, MnCl2·4H2O, and K2Cr2O7. Further the condensed aqueous solution of HCl isolated from rotary evaporator step (instead of evaporating the solvent using a steam bath) can be reused to dissolve another as calcined metal oxide. Considering simple air calcination followed by dissolution in a reusable aqueous solution of HCl to recover solid metal precursor in which it is utilized as the reagent to resynthesize the complex, we believe that our proof-of-concept method is a novel and simple way toward recycling of metal complexes in an inorganic laboratory course.

The existing reports on recycling of coordination complexes do not use our method. A comprehensive review by Nag and co-workers summarized few method, e.g., biobased leaching, mechanochemistry, photocatalysis, ionic liquid and deep eutectic solvents, as well as AI-driven ligand design, all with common purpose to extract precious metals (PM) from secondary sources using ligand donor (or acceptor) to coordinate hence isolate PM. A strongly coordinating vinyl ligand was used to successfully recover homogeneous Pt hydrosilylation catalyst by preventing agglomeration of the Pt catalyst, hence deactivation, and the resulting Pt complex with vinyl ligand can be reversibly adsorbed and released by the electrosorbent. Further, Fassbach et al. comprehensively reviewed conventional methods, e.g., volatility- and liquid–liquid phase-based separation, precipitation/crystallization, and membrane nanofiltration, adopted in industries for recycling homogeneous (organometallic) catalysts, where they rely on utilizing physicochemical properties to separate expensive metal catalysts from the product. Overall, the existing mature technologies, widely applied in industries, utilize appropriate functional group(s), embedded within a certain “core”, or exploit physicochemical properties, to extract the targeted metal from a mixture of the metal with other compound(s). Our method, on the other hand, combines nano-oxide and perovskite halide synthetic strategies to regenerate metal precursors from metal complexes solid waste generated in an undergraduate inorganic laboratory course.

PXRD analysis was performed to confirm the phase purity and crystalline nature of the revived TM precursors and resynthesized TM complexes. Diffraction patterns were collected over a 2θ range of 5°–50°, and the resulting patterns were compared with simulated reference and/or commercial patterns. The PXRD patterns of the recovered metal precursors (1) NiCl2·6H2O, (2) MnCl2·4H2O, and (3) K2Cr2O7 are matched with their simulated and commercial patterns as shown in Figure . NiCl2·6H2O reflections were observed at 15.8°, 25.6°, and 30.6° 2θ, for MnCl2·4H2O showed prominent peaks at 16.0°, 17.9°, and 20.1° 2θ and for K2Cr2O7, diffraction peaks appeared at 13.4°, 18.1°, and 27.4° 2θ. All the observed peaks of revived precursors matched well with their simulated and commercial patterns, confirming the phase purity of the regenerated TM precursors and hence a successful recovery process for NiCl2·6H2O, MnCl2·4H2O, and K2Cr2O7. Further, SEM-EDS were conducted to complement PXRD bulk structural characterization on the revived TM precursors. SEM-EDS analysis was utilized to affirm metal-to-chloride-to-oxygen or metal-to-oxygen ratio of the revived TM precursors. The expected atomic ratios matching the compound formulas were obtained, Figure S4 (Supporting Information), supporting PXRD results.

Our successful TM complexes recycling route stems from a merge between metal oxide nanoparticle synthesis and the synthesis of metal halide perovskite, as mentioned earlier in Introduction section. The former was adopted to decompose a TM complex into a TM oxide with known general mechanism in the literature. − As made oxide is then used as one of the reagents to resynthesize the TM complex or transformed into a metal halide, based on metal halide perovskite mechanism reported in literature, prior to making the corresponding TM complex. As an example, complex 1 would undergo decomposition reaction when heated under air (see details in the Experimental Section) to form nickel­(II) oxide as the first part of our TM recycling method, with the following balanced equation (adopted based on the literature mentioned earlier − )­

2[Ni(C2H8N2)3]Cl2·2H2O(s)+O2(air)→Δ2NiO(s)+6C2H8N2+4H2O+2Cl2

As formed NiO would then be dissolved in aqueous HCl to form the regenerated NiCl2·6H2O polycrystalline powder, based on the following balanced equation (adopted based on the literature mentioned earlier)­

NiO(s)+2HCl+5H2O→NiCl2·6H2O(s)

Other recycled TM precursors’ possible mechanisms can be found in the Supporting Information.

We also attempted to recover CrCl3 instead of K2Cr2O7 from the corresponding complex with unsuccessful results. As we conducted our proof-of-concept method, we found that our recycling method cannot be applied to Cr complexes since once Cr–O bond is formed (after air calcination), such Cr­(III)–O bond cannot be broken simply by using aqueous HCl under mild temperature, in accordance with Taube’s rule, hence denotes limitations of our current recycling method. Therefore, we expect similar cases for other inert TM ions having a d3 (inner orbital), e.g., V­(II) and Mo­(III), and low spin d6 ions, such as Co­(III) and Rh­(III).

To move closer to sustainability, we also used rotary evaporation, instead of steam bath evaporation, for two purposes: to isolate crystalline, regenerated metal precursors and to recover the aqueous HCl solvent. We then successfully reused the recovered solvent to dissolve metal oxides and produce a second batch of metal chlorides. We also found that omitting the acetone wash and instead drying the recovered product in open air at room temperature overnight still gave the same phase-pure products, as determined by PXRD. Compared with routine disposal of solid waste and the purchase of fresh metal salts, these substituted recycling steps make our method more sustainable. These regenerated TM precursors were then used to resynthesize their respective TM complexes.

The PXRD patterns of complexes 1 and 2 displayed sharp, well-defined peaks at 12.7°, 18.7°, and 20.3° and 10.9°, 13.2°, and 16.5° 2θ, respectively, representing high crystallinity. The peak positions matched well with those of simulated patterns based on the single-crystal structures of [Ni­(en)3]­Cl2·2H2O and [Mn­(acac)3] as well as the PXRD patterns of the complexes made from commercial sources, Figure a,b. In addition, further Rietveld refinements (Figures S5 and S6, Supporting Information) confirm the successful synthesis and bulk phase purity of the complexes. Complex 3 appeared relatively lower in intensities, therefore it was not possible to obtain a good Rietveld refinement. Nevertheless, the observed PXRD pattern displayed prominent 2θ peaks at 8.8°, 17.8°, and 24.6° and aligned well with that of simulated pattern of K3[Cr­(ox)3]·3H2O as well as the pattern of complex synthesized from commercial source, confirming its bulk phase identity, Figure c. To complement structural characterizations, we also conducted FTIR and CHN analysis. FTIR spectra showcased similar bands for all complexes made from commercial and revived metal precursors, Figure , corroborating bulk structural characterization by PXRD. The signature bands for complex 1 made from recycled TM precursor, e.g., O–H (3467 cm–1), N–H (stretching, 3277 cm–1), C–H (stretching, 2941 cm–1), N–H (bending, 1595 cm–1), C–N (stretching, 1325 cm–1), and C–C (bending, 1018 cm–1), matched perfectly with the bands of complex 1 synthesized from commercial source. Complex 2 synthesized from recycled and commercial sources exhibited similar signature bands, e.g. CO (stretching, 1625 cm–1), C–H (stretching, 2910 cm–1), CC (stretching, 1604 cm–1). Similarly, the signature bands for complex 3 made from recycled TM precursor, e.g., O–H (3427 cm–1), CO (stretching, 1640 cm–1), C–O (stretching, 1254 cm–1), and C–C (bending, 894 cm–1), matched perfectly with the bands of complex 3 synthesized from commercial source. UV/vis spectra comparison of all complexes made from recycled TM precursors are matched perfectly with those made from commercial ones, displaying the expected peaks in accordance with complementary color principle for d–d transition and/or ligand-to-metal charge transition (LMCT), − Figure S7. Complex 1 compounds displayed yellow band region (∼560 nm) peak as the complementary color of the observed purple color compounds. Absorbances in red region (∼600 nm) were observed for complexes 2 and 3 as they were dark green in colors. CHN elemental analysis (Experimental Section) further confirms equivalent behavior of complexes made from commercial and recycled sources.

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FTIR spectra comparison of resynthesized: (a) complex 1, (b) complex 2, and (c) complex 3 made using recycled TM precursor with those complexes made from commercial sources.

To confirm reproducibility of our method, we repeated (up to 3 replicates for each complex; see the Supporting Information for average yields and their standard deviations) the steps of recovering metal precursors followed by resynthesizing the complexes back using the regenerated metal precursors. PXRD patterns of each batch of each metal precursor matched with that of the simulated pattern, validating the reliability of our recycling TM complexes method, Figure .

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PXRD patterns showing reproducibility of recycled: (a) NiCl2·6H2O, (b) MnCl2·4H2O, and (c) K2Cr2O7, comparing patterns of three batches with those of simulated patterns.

The magnetic moments of the resynthesized TM complexes were measured using the Gouy balance method to study their electronic configurations and coordination geometries. − The effective magnetic moment (μeff) for each complex was calculated from experimentally observed magnetic susceptibility. The data derived from measurements of all complexes are shown in Table with a comparison between complexes made from commercial and our regenerated metal precursors. Complex 1 showed an effective magnetic moment of 2.41 μB, compatible to the spin-only value of about two unpaired electrons with octahedra d8 system. Complexes 2 and 3 exhibited effective magnetic moments of 4.78 and 3.43 μB, affirming the possession of ∼4 (octahedra d4, high spin system) and ∼3 (octahedra d3 system) unpaired electrons, respectively. Furthermore, as can be seen in Table , a comparable effective magnetic moment of all complexes made from commercial and revived metal precursors is verified, emphasizing equivalent properties obtained for all complexes, regardless of the source of TM precursor. Effective magnetic moment of complex 2 is on par with that of literature result, i.e., 4.78 μB from the adopted Guoy method and 4.82 μB from the Evan method, asserting that the Guoy method is equivalent to magnetic susceptibility measurements using NMR, known as the Evan method.

1. Effective Magnetic Moment Comparison between Complexes Made from Recycled and Commercial TM Precursor Sources.

variables complex 1 (recycled) complex 1 (commercial) complex 2 (recycled) complex 2 (commercial) complex 3 (recycled) complex 3 (commercial)
R 0 –47 –46 –55 –47 –53 –42
mass of tube 1.6123 g 2.0292 g 1.7341 g 1.6745 g 1.6188 g 1.8574 g
height of sample 1.8 cm 1.7 cm 1.8 cm 1.6 cm 1.8 cm 1.8 cm
mass (tube + sample) 1.6629 g 2.0751 g 1.7862 g 1.7129 g 1.7055 g 1.906 g
R 173 187 763 669 457 240
temperature (K) 293 294 292 293 293 293
μeff 2.4068 μB 2.749 μB 4.7793 μB 4.9125 μB 3.4258 μB 3.539 μB
μs 2.8284 μB 2.8284 μB 4.8989 μB 4.8989 μB 3.8730 μB 3.8730 μB
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Conclusion

The recovery and characterizations of NiCl2·6H2O, MnCl2·4H2O, and K2Cr2O7 and their resynthesizing back to their respective coordination complexes offer an innovative method that combines the key concepts in solid-state and coordination chemistry. Despite not applicable to inert TM (such as d3) ions, our recycling method to regenerate K2Cr2O7 and TM chlorides (NiCl2·6H2O, MnCl2·4H2O) from their corresponding coordination complexes solid wastes displayed phase pure metal precursors with good reproducibility that can be readily employed to resynthesize the corresponding complexes in a good yield with bulk phase purity as confirmed by PXRD, FTIR, and CHN analysis. Further, a comparable quality of as produced complexes is observed between those complexes made from recycled and commercial sources, as proven by PXRD, FTIR, and UV/vis spectra. Their room-temperature magnetic moments stemmed from the resynthesized complexes, made from both recycled and commercial sources, also showed satisfactory results, confirming their paramagnetic characters, geometries, electronic configurations, as well as their number of unpaired electrons, equivalent to the literature results. We hope that our effort toward a closed loop in a TM coordination complex recycling hence producing recovered TM precursors will promote more works on a circular economy across the Sultanate of Oman and beyond.

Supplementary Material

ao6c04181_si_001.pdf (901.3KB, pdf)

Acknowledgments

The authors would like to acknowledge SQU Internal Grant no. IG/SCI/CHEM/24/06 and Block Funding Program, Ministry of Higher Education, Oman with grant no. RC/RG/SCI/C/24/219 for financial supports, CAARU-SQU for PXRD, SEM-EDS measurements, and Umayma Al-Rashdi/Najwa Al-Bulushi (Chemistry, SQU) for CHN analysis.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c04181.

  • Detailed procedures, balanced equations and % yield calculations, Figures S1–S7 (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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