Peroxosolvates: Formation Criteria, H2O2 Hydrogen Bonding, and

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Peroxosolvates: formation criteria, HO hydrogen bonding, and isomorphism with the corresponding hydrates Ivan Yu. Chernyshov, Mikhail V. Vener, Petr V. Prikhodchenko, Alexander G. Medvedev, Ovadia Lev, and Andrei V. Churakov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01449 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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Crystal Growth & Design

Peroxosolvates: formation criteria, H2O2 hydrogen bonding, and isomorphism with the corresponding hydrates Ivan Yu. Chernyshov,†,‡ Mikhail V. Vener,†,‡ Petr V. Prikhodchenko,† Alexander G. Medvedev,†,§ Ovadia Lev,§,* and Andrei V. Churakov†,* †

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,

Leninskii prosp. 31, Moscow 119991, Russia ‡

Department of Quantum Chemistry, Mendeleev University of Chemical Technology, Miusskaya

Square 9, Moscow 125047, Russia §

The Casali Center of Applied Chemistry, The Institute of Chemistry, The Hebrew University of

Jerusalem, Jerusalem 91904, Israel.

ABSTRACT: The Cambridge Structural Database has been used to investigate the detailed environment of H2O2 molecules and hydrogen-bond patterns within “true” peroxosolvates in which the H2O2 molecules do not interact directly with the metal atoms. A study of 65 crystal structures and over 260 hydrogen bonds reveals that H2O2 always forms two H-bonds as proton donors and up to four H-bonds as a proton acceptor, but the latter can be absent altogether. The necessary features of peroxosolvate coformers are clarified. 1) Coformers should not participate in redox reactions with H2O2, and should not catalyze its decomposition. 2) Coformers should be

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Brønsted bases or exhibit amphoteric properties. The efficiency of the proposed criteria for peroxosolvate formation is illustrated by the synthesis and characterization of several new crystals. Conditions preventing the H2O2/H2O isomorphous substitution are essential for peroxosolvate stability: 1) Every H2O2 in the peroxosolvate has to participate in five or six hydrogen bonds. 2) The distance between the two proton acceptors forming H-bonds with the H2O2 molecule should be longer than the distance defined by the nature of the acceptor atoms.

1. INTRODUCTION Рeroxosolvates – solid adducts of H2O2 and various compounds – are widely known as commercial solid sources of hydrogen peroxide.1 In fact, the first synthesis of a perhydrate, (sodium percarbonate) was reported by Tanatar,2 who soon afterwards, also published his work on the synthesis of urea perhydrate.3 Today, these compounds are the two most widely used peroxosolvates. Peroxosolvates are used as tractable models for theoretical studies of hydrogenbonded networks at the molecular level. This is important due to the ubiquity of hydrogen peroxide in environmentally relevant and biological processes.4–6 The hydrogen peroxide generated in cells is a key metabolite in redox signaling and oxidative stress.5,7 A major cellular source of H2O2 resides in mitochondria, and therefore, the transmembrane transport of intracellular hydrogen peroxide is an essential biological process.8,9 The redox activity of hydrogen peroxide is well known, but hydrogen peroxide adduct formation is often just as important. Some aquaporins (peroxiporins) act as transmembrane proteins to facilitate the transport of H2O2 across cell membranes.10–12 There are 97 characterized peroxosolvates according to the Cambridge Structure Database (CSD),13 and version 2016-1 of the Inorganic Crystal Structure Database (ICSD).14 This is


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several orders of magnitude less than the number of characterized crystallohydrates.15 A major group of peroxosolvates includes 65 crystals in which the H2O2 molecules do not interact directly with the metal atoms. These are denoted as “true” peroxosolvates. The second group consists of 32 crystals in which the oxygen atoms in H2O2 coordinate with metal cations (primarily alkaline).16–18 The detailed analysis of crystals from the second group is beyond the scope of this article. During the last decade, 33 peroxosolvates were synthesized and characterized by single crystal X-ray analysis. This makes up more than half of the 65 “true” peroxosolvates reported to date (Figure 1). Most of the recently reported peroxosolvates were synthesized from reactions with a high concentration (>80%) of H2O2. Thus, some compounds, for instance some amino acids, only form peroxosolvates with H2O2 concentrations >80%.19,20 This can be attributed to the competition between the H2O2 and H2O molecules. When a compound forms isomorphous (isostructural) hydrates and peroxosolvates, the formation of mixed hydrates/peroxosolvates or pure hydrates is also possible.21 Therefore, isomorphism with hydrates precludes the practical application of peroxosolvates. The peroxosolvates reported by Tanatar (sodium percarbonate and urea perhydrate) do not form isomorphous hydrates, and therefore these compounds can be produced from dilute aqueous hydrogen peroxide, without being affected by humidity.


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Figure 1. Cumulative number of “true” peroxosolvates by year according to CSD and ICSD. In recent years we have demonstrated that H2O2/H2O isomorphism is quite abandoned, but the only other example of isomorphous H2O2/H2O substitution was mentioned by Pedersen in his work on the structure of ammonium oxalate peroxosolvate.22 Based on X-ray experiments, he reported that this phenomenon results in the shortening of the O–O bond distance (down to 1.35 – 1.40 Å), and in the appearance of strong residual peaks of electron density (up to 0.4 e•Å-3) at the center of the O–O bonds. Pedersen noted that the scale of both effects is proportional to the fraction of water molecules at the peroxide sites. These features are indistinguishable when low H2O fractions and X-ray data with poor accuracy are used. Therefore, the formalization of qualitative and quantitative structural criteria for isomorphous H2O2/H2O substitution is important. In addition, the search for new peroxosolvates with potential commercial value involves the selection of coformers that can form strong H-bonds with hydrogen peroxide.


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The aim of this paper is twofold: (1) to formulate the mandatory criteria for peroxosolvate formation, and (2) to clarify the conditions necessary for H2O2/H2O isomorphous substitution (IS). Achieving these goals involves the identification of the specific features of the crystal structure, and the H-bond networks of isomorphous peroxosolvates and crystallohydrates. Database analysis followed by DFT computations of the model systems is used in this study to accomplish this task. The efficiency of the proposed criteria is illustrated by the synthesis and characterization of several new crystals; melamine peroxosolvate (1), and two peroxosolvates of thymine with different degrees of IS, (2) and (3), are used as test cases.

2. RESULTS AND DISCUSSION

Analysis of the CSD revealed the necessary features of the peroxosolvate coformers: 1. Coformers should not participate in redox reactions with H2O2, and should not catalyze its decomposition. 2. Coformers should be Brønsted bases or exhibit amphoteric properties. If the coformer is a strong base, then deprotonation of H2O2 becomes possible, with subsequent formation of ionic or complex hydroperoxide/peroxide (NH4+OOH-,23 [Sn(OOH)6]2-

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) moieties. Thus, while

crystallohydrates can also be formed from acids, no “true” peroxosolvates can be formed from acidic coformers.13 Therefore, carboxylic acids readily form crystallohydrates, but none form peroxosolvates. Analysis of the H-bond networks is pertinent to gather insight and improve the second requirement of peroxosolvate coformers.


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2.1. Specific features of H-bond networks in peroxosolvates. Our statistics are based on 260 H-Bonds in 65 peroxosolvates (Table S1). All of the currently known peroxosolvates contain between one and four crystallographically independent H2O2 molecules (Table 1). Each hydrogen peroxide molecule may form a different number of conventional H-bonds as proton donors (DH-bonds) and proton acceptors (AH-bonds). We concluded the following general rules: 1) H2O2 forms two DH-bonds in all known peroxosolvates. 2) H2O2 can form up to four AHbonds, and the AH-bond may be absent altogether. 3) The total number of H-bonds ranges between two and six. Notably, the maximal theoretical number is also six. Table 1. Structural features of “true” peroxosolvatesa) Number of crystallographically independent H2O2

Number of crystals

Number of H2O2 with the specified number of DHbonds 0

1

Number of H2O2 with the specified number of AH-bonds

2

0

1

2

3

4

1

41

–

–

39b)

17

7

11

2

4

2

18

–

–

32

23

3

3

2

1

3

5

–

–

12

3

4

5

–

–

4

1

–

–

4

1

2

–

–

–

Sum

65

–

–

87

44

16

19

4

5

a)

The crystals are described further in the Supporting Information. The number of fragments here and below may not correspond to the number of crystals. This is due to the presence of atoms with undefined positions in some crystals. b)

There are many crystals with one or two AH-bonds, while three or four AH-bonds are rarely realized (Table 1). This is attributed (in accordance with ref. 25) to an insufficient number of acidic protons in many coformers. One of the few exceptions where H2O2 participates in six Hbonds is urea•H2O2.26 Figure 2 illustrates a new 3D-structure of melamine peroxosolvate (C3H6N6•H2O2), wherein the H2O2 molecule also forms the maximal number of H-bonds. Special


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attention should be paid to crystals without AH-bonds. In this case other types of noncovalent interactions should be taken into account, e.g. CH···O contacts.27–29

Figure 2. H-bond network in a melamine peroxosolvate crystal C3H6N6•H2O2; d(OH···N) = 2.706–2.721 Å, d(NH···O) = 2.947–3.187 Å. The structure of the crystallohydrates has been well reviewed.30,31 Water molecules can form between one and four H-bonds, with three H-bonds being the most frequent (both 2DH + AH and DH + 2AH variants occur with the same frequency).32,33 The energy of the average DH- and AH-bonds formed by an H2O molecule is approximately equal.32 The difference between AH- and DH-bonds is particularly evident in hydrogen peroxide dihydrate H2O2•2H2O.34 The O···O distance in the HO–H···OH2 fragment (2.741 Å) is much longer than the corresponding distance in the HOO–H···OH2 fragment (2.685 Å). On the other hand, the O···O distances in the HO–H···O2H2 and HO–H···OH2 fragments are similar. This is in accord with the fact that stronger Brønsted acids form stronger DH-bonds.35 Taking into


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account the negative correlation between the H-bond length and H-bond energy in the crystals,36 it can be concluded that the AH-bonds in H2O and H2O2 are relatively similar, while the DHbonds formed by hydrogen peroxide molecules are stronger. This conclusion is in agreement with the corresponding sublimation enthalpies. The sublimation enthalpy (59 kJ/mol) of ice Ih 37 is lower than the corresponding value (65 kJ/mol) of the hydrogen peroxide crystal.38 The same general conclusion has been previously established by the solid-state DFT computations of isomorphous serine hydrate and peroxosolvate.39 The energy of the DH-bonds formed by H2O2 with the CO2– group of the serine is ~10 kJ/mol larger than the energy of a similar DH-bond formed by H2O.39 However, the energy of AH-bonds formed by H2O and H2O2 are rather similar (28.5 and 31.2 kJ/mol respectively).39 As a result, the total energy of the serineH2O2 bonds is ~20 kJ/mol larger than those in serine-H2O. The H2O/H2O2 IS requires similar overall binding strength of H2O and H2O2. However, the DH-bonds of H2O2 molecules have higher energy compared to those of H2O molecules which leads to relatively higher common hydrogen bonding energy of the hydrogen peroxide containing crystal in isomorphous hydrates and peroxosolvates. Therefore, the H2O2 DH-bonds can be treated as structure-forming interactions in peroxosolvates. 2.2. Criteria for H2O2/H2O IS in peroxosolvates. One of the conditions of isomorphism is invariance of the structural motif (isomorphism/pseudoisomorphism).40 Indeed, the existence of a (pseudo)isomorphous crystallohydrate for the considered peroxosolvate is strong evidence for possible H2O2/H2O IS. In accordance, 10 out of 12 peroxosolvates with H2O2/H2O IS contain an isomorphous or pseudoisomorphous crystallohydrate. (Tables S1, S2). When H2O2 is isomorphously substituted with H2O, the oxygen in the water molecules becomes located near the center of the substituted hydrogen peroxide O–O bond. In these


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crystals, the H2O molecule forms DH-bonds with the same atoms as the H2O2 molecules, and the lone electron pairs of H2O/H2O2 interact in the same environment (Figure 3). Therefore, the existence of H2O2/H2O IS implies similar interaction energies for H2O2 and H2O within their environment. For H2O2/H2O IS to take place, the substituting H2O molecule has to form the same number and type of H-bonds as the H2O2 molecule in the peroxosolvate. The following set of conditions is necessary for IS to occur: 1) There should be water DH-bonds in the isomorphous crystallohydrate. 2) The number of AH-bonds in the isomorphous crystallohydrate and peroxosolvate should be identical, and H2O2/H2O IS is unfavorable if H2O2 forms more than two AH-bonds. In addition, the orientation of the AH-bonds should also be considered. Thus, 3) H2O2/H2O IS is unfavorable if the coformer does not adjust to the orientation of the electron lone pairs of the water molecules.

Figure 3. 1D H-bond motif formed by the H2O2/H2O isomorphously substituted H2O2 molecule in CAZHAN [Ref. 19]. The formation of two DH-bonds in the X···HOOH···Y and X···HOH···Y fragments becomes impossible with increasing of the X to Y distance above the critical value. These values were denoted as break distances, Dbr(H2O2) and Dbr(H2O) for corresponding fragments. The distance


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between the protons in the H2O2 molecules depends on the conformation of the molecule (cisconformer ~1.8 Å and for trans-conformer ~2.6 Å). Both are larger than those found in H2O (~1.5 Å). Therefore, Dbr(H2O) should be smaller than Dbr(H2O2). This statement is illustrated by the distribution of the O to O distance in both the peroxosolvates and crystallohydrates (Figure 4). According to CSD analysis, Dbr(H2O) ~ 5.6 Å for crystallohydrates with an O···HOH···O fragment. Thus, the majority of the considered crystallohydrates are characterized by an O to O distance 6.4 Å. Similar changes occur in the 2HCN•H2O2 system at D > 7.5 Å. The theoretical Dbrc(H2O) value was determined without taking into account the environmental effects (long-range electrostatics, crystal packing, etc.) and should therefore be treated with caution. The Dbr(H2O) value (~5.6 Å) determined from CSD statistics, can be applied as a criterion for H2O2/H2O IS. The DFT computations confirm the result from the CSD analysis. They also demonstrate that at smaller D values, the changes in the H-bond topology are more marked in the X···HOH···Y fragment when compared to those in the X···HOOH···Y fragment. We therefore concluded that IS will not occur when D >Dbr(H2O).


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Figure 5. Dependence of the computed relative energy E–E0 of the HCN···HOH···NCH (a), and HCN···HOOH···NCH (b) model systems on the N···N distance D. E0 is the global energy of the corresponding system with two equivalent H-bonds.


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Accordingly, H2O2/H2O IS does not occur in crystals with H2O2 sitting in the inversion center or at the reflection plane (CAZHUH, GADOXP10, KUMRER, VAYGUY01, BAFJUQ, VAYMAJ, or CAXCAF, respectively, Table S1). The H–O–O–H angle is ~180° or ~0° in these crystals. Notably, the cis-conformation of H2O2, with an H-O-O-H angle

Peroxosolvates: Formation Criteria, H2O2 Hydrogen Bonding, and

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