Lucia
Casali
a,
Torvid
Feiler
b,
Maria
Heilmann
b,
Dario
Braga
a,
Franziska
Emmerling
*b and
Fabrizia
Grepioni
*a
aDepartment of Chemistry “G. Ciamician”, University of Bologna, 40126 Bologna, Italy. E-mail: fabrizia.grepioni@unibo.it
bBAM Federal Institute for Materials Research and Testing, 12489 Berlin, Germany. E-mail: franziska.emmerling@bam.de
First published on 18th January 2022
In situ monitoring of mechanochemical reactions between dicyandiamide (DCD) and CuX2 salts (X = Cl−, NO3−), for the preparation of compounds of agrochemical interest, showed the appearance of a number of phases. It is demonstrated that milling conditions, such as the amount of water added in wet grinding and/or the milling frequency, may affect the course of the mechanochemical reactions, and drive the reaction towards the formation of different products. It has been possible to discover by in situ monitored experiments two novel crystalline forms, namely the neutral complexes [Cu(DCD)2(OH2)2(NO3)2] (2) and [Cu(DCD)2(OH2)Cl2]·H2O (4), in addition to the previously known molecular salt [Cu(DCD)2(OH2)2][NO3]2·2H2O (1, DIVWAG) and neutral complex [Cu(DCD)2(OH2)Cl2] (3, AQCYCU), for which no synthesis conditions were available. Compounds 2 and 4 were fully characterized via a combination of solid-state techniques, including X-ray diffraction, Raman spectroscopy and TGA.
Among the eco-sustainable methods mechanochemistry stands out,2,3 as it is a synthetic approach promoted by the input of mechanical energy in the absence (grinding) or in the presence (kneading) of a minimum amount of solvent, thus overcoming solvent-related problems such as solubility and solvolysis typically found in solution chemistry. The mechanochemical method can provide a wide spectrum of materials,4 ranging from metal–organic complexes to novel organic molecules, regardless of the relative solubilities of the starting components. In addition to this, the success of mechanochemistry is due to fast and quantitative reactions, along with a simple implementation.
In light of this, mechanochemistry has attracted significant interest as an alternative method for obtaining pure compounds.5 Despite the wide application, the mechanisms of milling reactions are still not fully understood. Typically, mechanistic information is deduced from ex situ experiments in a stepwise manner: milling is interrupted at fixed time intervals, the milling vessel opened, and the sample removed for ex situ analysis. However, this procedure unavoidably affects the reaction conditions, and air-sensitive or fast converting intermediates/phase changes cannot be detected under these conditions. Therefore, the ability to monitor these reactions directly in situ, without the need to interrupt the milling process, helps in the detection, isolation, and characterization of new phases/intermediates, as also in the optimization of the synthetic procedure.6
In this work we investigate the mechanochemical synthesis of metal–organic complexes through the combination of in situ techniques, i.e. time-resolved X-ray diffraction (XRD) coupled with Raman spectroscopy,7,8 which provide real-time information about the solid-state transformations occurring during the milling process. The investigated complexes, based on copper(II) and dicyandiamide (DCD), are intended for agrochemical applications in the context of a more sustainable agriculture. Copper(II) serves the dual purpose of micronutrient and inhibitor towards urease,9 the soil enzyme responsible of the fast hydrolysis of urea into ammonium. DCD, on the other side, is supplied to the soil with the aim of inhibiting ammonia monooxygenase (AMO),10 which catalyzes the conversion of ammonium into hydroxylamine (NH2OH), precursor of greenhouse gases such as NO2, NO.11 We have already shown that mechanochemically synthesized co-crystals of urea could represent a novel class of fertilizers, proving that co-crystallization can be a new promising route for the delivery of agrochemicals.12–14 Moreover, it was recently shown that in situ monitoring of mechanochemical reactions is a powerful approach to explore the solid-state reactivity of agrochemicals.15
To this goal we selected from the CSD database two coordination complexes based on DCD and copper salts, i.e. [Cu(DCD)2(OH2)2][NO3]2·2H2O (1) (refcode DIVWAG16) and [Cu(DCD)2(OH2)2Cl2] (3) (refcode AQCYCU17). For both compounds, however, no details could be found on synthetic procedures and conditions; as single crystal data are deposited in the CSD structural database, it is likely that the two compounds were obtained from solution. We decided to try a mechanochemical approach, as in situ Raman spectroscopy in monitoring the mechanochemical synthesis of DCD-based coordination compounds had proven to be efficacious in the case of zinc complexes.18 Our results show that the milling conditions affect the course of the mechanochemical reactions, with different phases appearing depending on the amount of the milling water and on the grinding conditions. In addition to the known compounds 1 and 3, novel crystalline forms, i.e. the neutral complexes [Cu(DCD)2(OH2)2(NO3)2] (2) and [Cu(DCD)2(OH2)Cl2]·H2O (4) were detected, isolated and structurally characterized via solid-state methods. Analogous syntheses were conducted for sake of comparison in aqueous solution or via slurry in water (see below).
The in situ strategy for the simultaneous, real-time analysis of milling reactions is thus a powerful tool for the (i) isolation and characterization of new materials, (ii) optimization of the synthetic procedure, and (iii) understanding of the reactivity of the investigated systems, thus highlighting the advantages of mechanochemistry over conventional synthetic methods.
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Fig. 1 Time-evolution of the milling reaction between DCD and Cu(NO3)2·3H2O: powder X-ray diffraction (left) coupled with Raman spectroscopy (right). |
In 1 the Nimino atoms belonging to DCD molecules are directly coordinated to the copper ion, which is in a square planar geometry (see Fig. 2). Therefore, the variation in the absorption of this vibrational mode is reasonably due to a variation in the coordination network involving DCD, with a concomitant change of the coordination geometry around copper, possibly related to a variation in the content of the crystallization water. The mechanochemical reaction was then performed with different amounts of milling water (Fig. 3), and we found out that the formation of 1, as also the kinetics of formation, is correlated to the amount of milling water. In the absence of water pure solid 2 was invariably obtained as soon as the reaction started. By addition of water, up to 10 μL, compound 1 was first obtained, but after two minutes it converted into 2. Finally, with the addition of 80 μL of water, compound 1 formed immediately, and remained stable for the whole duration of the experiment. This trend is in agreement with a transformation of 1 into 2 following the release of crystallization water.
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Fig. 3 In situ monitoring of formation and stability of compounds 1 and 2 as a function of time and amount of milling water. Milling frequency 50 Hz (unless differently specified). |
The milling frequency also affected the kinetics of the reaction: by reducing the milling frequency to 20 Hz, while keeping constant the water amount at 50 μL, the lifetime of compound 1 increased to ca. 60 minutes, against the 5 minutes observed for the mechanochemical reaction at 50 Hz (see ESI†).
TGA measurements on 1 and 2 evidence the difference in the water content and in the interaction of the water molecules with the copper cation (see ESI†). The TGA trace for 1 shows two weight losses, one in the range 60–80 °C and the second in the range 100–120 °C, corresponding to a stepwise dehydration process, involving first the two water molecules on the second coordination sphere, then the two water molecules directly linked to the copper cation. The TGA trace of 2, on the contrary, only presents an event at 100–120 °C, corresponding to the loss of two water molecules, thus indicating that compound 2 is a dihydrate and the water molecules are likely bound to the metal center; this information was crucial for the structural determination of 2 from powder diffraction data (see Scheme 1 and Fig. 4).
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Scheme 1 Schematic representation of the compounds discussed in this work (OW atoms bound/not bound to the metal cations are in blue/cyan, respectively). |
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Fig. 4 Hydrogen bonding patterns in crystalline [Cu(DCD)2(OH2)2(NO3)2]: no direct interactions are observed between DCD molecules (compare with Fig. 1), as the hydrogen bonding DCD dimers present in crystalline 1 are replaced here by dimers between DCD molecules and nitrate anions. Additional HB-rings involving the NO3− ions are evidenced in light green. |
The DCD molecules are organized in hydrogen bonded dimers with the nitrate anions, which act as hydrogen bond acceptors of the double N–HDCD donor [N(H)DCD⋯O− 2.924(3) Å and 3.068(3) Å]. Such crystal arrangement is consistent with the data in the CSD database: among the 7 structures containing DCD and nitrate ions, only DIVWAG (1) does not display this kind of interaction. Such crystal arrangement results in the formation of parallel layers of the dimers (one layer is shown in Fig. 4), connected to each other via the water molecules [C–NDCD⋯(H)Owater 2.839(2) Å and N–O−nitrate⋯(H)Owater 2.815(2) Å].
A solution synthesis and a slurry in water were also performed with hydrated copper nitrate and DCD. Crystallization from an aqueous solution of the two reagents yielded a physical mixture of compounds 1 and 2; no single crystals of 2 were obtained. The slurry experiment, on the other hand, yielded pure compound 2, as this is the thermodynamically stable phase in the presence of water (see ESI†).
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Fig. 5 Time evolution of the milling reaction between DCD and CuCl2·2H2O with 50 μL of water at 50 Hz. The appearance of the phase X is highlighted by the orange rectangle. |
The formation of the expected compound 3 was almost instantaneous, to the point that the peaks of the reagents were not detectable; the formation of 3, however, was accompanied by the concomitant formation of an unknown crystalline form, named X, which disappeared within the first 60 seconds.
In order to identify form X, we performed the same mechanochemical synthesis with a lesser amount of milling water: in this way the lifetime of X increased, to the point that, in the absence of water, it became the only crystalline form present. Interestingly, pure solid X transforms into a physical mixture of X and of the new compound [Cu(DCD)2(OH2)Cl2]·H2O (4) upon exposure to air.
Manual grinding at ambient conditions – with no intentional addition of water – yielded pure solid 4 (Fig. 6): manual grinding in open air evidently caused stoichiometric water uptake from the atmosphere.
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Fig. 6 In situ monitoring of formation and stability of compounds X, 3 and 4 as a function of time and amount of milling water. Milling frequency 50 Hz. |
Finally, for sake of comparison, we performed the synthesis from a water solution of the reagents and via slurry. The first method yielded a physical mixture of 3 and X, in agreement with the mechanochemical synthesis in the presence of water. Interestingly, the slurry method resulted solely in the formation of compound 4 (see ESI†).
TGA analysis performed on powder of pure form 4 proved that the compound contains two water molecules (see ESI†), as in the case of 3. The difference between the two compounds could be evidenced upon structural resolution from powder X-ray data. (see ESI†): Compounds 3 and 4, although sharing the same stoichiometry, differ for the coordination geometry around the copper cation, which changes from octahedral in 3 to square-pyramidal in 4 (Scheme 1), as one of the two coordinated water molecules in 3 becomes a crystallization water molecule in 4 (see Fig. 7 for a comparison of the two packing arrangements).
The copper cation in crystalline 4 is bound to two DCD ligands, one water molecule and two chloride anions. A second water molecule interacts exclusively with the first water molecule and one DCD molecule. The crystallographically independent DCD molecules are involved in different hydrogen bond networks: one of the molecules is hydrogen bond acceptor of the non-coordinated water [C–NDCD⋯(H)Owater 2.783(4) Å], while the second molecule is hydrogen bond donor toward the non-coordinated water molecule [N(H)DCD⋯Owater 2.928(4) Å] and two chloride anions in equatorial position [N(H)DCD⋯Cl− 3.211(1) Å and 3.256(5) Å]. The chloride anion in the axial position is instead the hydrogen bond acceptor of the coordinated water molecule [O(H)water⋯Cl− 3.211(4) Å]. All these interactions result in the formation of hydrogen bonded wavy chains, which are held together by hydrogen bonds between the axial chloride anions and the coordinated water molecule, giving rise to a wavy layered packing (Fig. 7a). This packing arrangement can be compared with the one of crystalline 3 (Fig. 7b), where DCD molecules interact directly with each other along infinite HB-chains, which interact on one side with the copper cations, and on the other side with the chloride anions, thus forming layers, as shown in the bottom part of Fig. 7b.
With regard to DCD and Cu(NO3)2·3H2O, the first attempted mechanochemical reactions always provided different outcomes, i.e. the pure compound 1 or a physical mixture of 1 and 2 in different proportions. The combination of X-ray diffraction with Raman spectroscopy allowed us to (i) isolate and characterize the new compound 2, (ii) understand the relationship between the two phases and (iii) optimize the synthetic procedure to obtain pure 1 or 2, along with the understanding that the amount of milling water deeply influenced the kinetics as also the outcome of the synthesis.
Concerning DCD and CuCl2·2H2O, all the mechanochemical syntheses provided the expected compound 3. However, in situ monitoring via X-ray diffraction revealed the appearance of a transient phase X, whose lifetime depended on the amount of milling water. In attempting to isolate this phase, a new phase 4 was obtained by manual grinding of the reagents and characterized through solid-state methods.
The presented case-studies underlined the importance of a precise synthetic protocol describing the mechanochemical synthesis. The explored systems turned out to be highly sensitive to the amount of water or the milling conditions. When performing mechanochemical reactions, a generic reference to a catalytic amount of milling water may thus be imprecise; the use of unspecified water addition, together with the use of only one milling frequency, may prevent the discovery of products only accessible through the utilization of different reaction conditions.
Along with the benefits arising from in situ strategies, in this work we also highlighted the advantages of the mechanochemical method over solution ones. For both complexes the synthesis from solution always (i) resulted in a physical mixture of different phases and (ii) never provided single crystals of the new compounds, while our mechanochemical approach allowed the preparation and characterization of pure crystalline compounds. Work is in progress to expand this strategy to additional hydrate metal systems.
Footnote |
† Electronic supplementary information (ESI) available: X-ray powder patterns, Rietveld refinements, Raman spectra, TGA, DSC. CCDC 2128518 and 2128519. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ce01670a |
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