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P=O Moiety as an Ambidextrous Hydrogen Bond Acceptor Elena Yu. Tupikina, Michael Bodensteiner, Peter M. Tolstoy, Gleb S. Denisov, and Ilya G. Shenderovich J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11299 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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P=O Moiety as an Ambidextrous Hydrogen Bond Acceptor Elena Yu. Tupikina,† Michael Bodensteiner,‡ Peter M. Tolstoy,§ Gleb S. Denisov,† and Ilya G. Shenderovich*,‡ †

Institute of Physics, Saint-Petersburg State University, Ulianovskaya st. 1, 198504, St. Petersburg, Russian Federation



Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstr. 31, 93040, Regensburg, Germany

§

Center for Magnetic Resonance, Saint-Petersburg State University, Universitetsky pr. 26, 198504, St. Petersburg, Russian Federation

ABSTRACT Hydrogen bond patterns of crystals of phosphinic, phosphonic, and phosphoric acids and their co-crystals with phosphine oxides were studied using 31P NMR and singlecrystal XRD. Two main factors govern these patterns and favor or prevent the formation of co-crystals. The first one is a high protonaccepting ability of the P=O moiety in these acids. As a result, this moiety effectively competes with other proton acceptors for hydrogen bonding. For example, this moiety is a stronger proton acceptor than the C=O moiety of carboxylic acids. The second factor is the

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inclination of the P=O moiety of both the acids and the oxides to form two hydrogen bonds at once. The peculiarity of these bonds is that they weaken each other to a little degree only. In order to highlight this point we are using the term “ambidextrous”. These two features should govern the interactions of P=O moiety with water and other proton donors and acceptors in molecular clusters, the active cites of enzymes, soft matter, and at surfaces.

INTRODUCTION Higher energy and directionality are two factors that differentiate conventional B···HA hydrogen bonds (Hbonds) from van der Waals interactions, where A and B stand for oxygen, nitrogen or a halogen. For these reasons, it is rarely a problem to predict the configuration of a simple Hbonded complex in the gas or liquid state and the effects of solvent polarity and H/D isotope substitution on geometry of such complexes. The situation is different in the solid state. Hbonding remains critical for the structural features of crystalline and amorphous materials.1,2 At the same time, the effect of weaker but abundant van der Waals interactions cannot be ignored anymore. Although these weak interactions cannot compete with conventional Hbonds, they impact on the stability of different Hbonded patterns and the crystal structure as a whole. Among the most amazing examples are nuclear quantum effects on the H/D isotopic polymorphism in the complex of pentachlorophenol with 4methylpyridine35 and on the ligand-receptor recognition.68 fThe latter is responsible for specific activity of deuterated drugs that just entered clinical practice.9 Much less studied phenomenon is the potential ability of some protonaccepting groups to form several Hbonds at once. For example, the electron density function at the P=O moiety of

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diphenylphosphonic acid in the crystal state suggests that this moiety has a potential to form up to three hydrogen bonds at once.10,11 There is a number of publications that specifically highlight the ability of the P=O moiety to form two hydrogen bonds at once.12,13 At the same time, it remains unknown how these conjugated hydrogen bonds affect each other. Here we purposefully avoid the term “bifurcated hydrogen bond” because its meaning has to be specified in each specific case.14 The situation is especially complex for amorphous materials for which the morphology cannot be evaluated by Xray diffraction (XRD). However, in some of these materials the morphology can be successfully studied using solidstate Nuclear Magnetic Resonance (NMR) spectroscopy. The only requirement of this method is the presence in the given material of NMR active nuclei whose NMR parameters depend on intermolecular interactions. Depending on the chemical composition of the system under study, these nuclei usually are 1H, 2H, 13C, 15N, 19F, and 31P. The 1

H and 2H nuclei are rarely the best choice seeing a small range of chemical shifts. The use of 13C

and 15N NMR often requires isotope labelling. In contrast, 19F and 31P are blameless. The objective of this work was to evaluate the general structural patterns in the co-crystals of phosphinic (R2POOH), phosphonic (RPO(OH)2), and phosphoric (PO(OH)3) acids with phosphine oxides (R3PO). We aimed also to learn about the factors that favor or prevent the formation of a certain pattern. To facilitate our task, the composition and morphology were initially studied using 31

P NMR and then, when it was possible, were confirmed by XRD analysis. Phosphinic,

phosphonic, and phosphoric acids represent three different types of Hbond pattern. These Hbonds are perturbed by interaction with phosphine oxides that results in the change of Hbond pattern. This choice provides two independent 31P spinlabels that are sensitive to morphology. In Figure 1 are shown substances that have been used as model systems in this work. As the model acids, we used dimethylphosphinic (A1) and diphenylphosphinic acid (A2),

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methylphosphonic (A3) and phenylphosphonic acid (A4), and phosphoric acid (A5). As the model phosphine oxides, we used triphenylphosphine oxide (B1), trioctylλ5phosphanone (trioctylphosphine oxide, B2), and hexamethylphosphoric triamide (B3). The gasphase proton affinities of these bases are about 910, 950, and 960 kJ/mol respectively.15 They cover the range from weak to strong bases.16 B1 (the melting point (MP) ≈ 430 K) is an effective crystallization aid; it forms hemihydrates.1719 B2 (MP ≈ 325 K) is neither hygroscopic nor watersoluble. In contrast, B3 (MP ≈ 280 K) is very hygroscopic. These differences insure that the general structural patterns governed by the interactions of P=O moiety can be identified.

Figure 1. Substances used as model systems in this work. Triphenylphosphine oxide (B1), trioctylλ5phosphanone (B2), hexamethylphosphoric triamide (B3), dimethylphosphinic acid (A1), diphenylphosphinic acid (A2), methylphosphonic acid (A3), phenylphosphonic acid (A4), and phosphoric acid (A5). In Fig. 2 are shown Hbond crystal patterns of phosphinic and phosphonic acids whose substituents are not bulky and/or cannot participate in strong specific interactions. These patterns

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exhibit one general feature, namely, Hbonded chains.2022 However, the structures of these chains are different for these two types of acids. In the case of phosphinic acids there are two Hbonds per molecule and each POH···O=P pair is bound by a linear Hbond, Fig. 2a. Using the graph set approach this can be identified as a chain C11 (4).2324 Incorporation of a proton acceptor in such structure can be energetically unfavorable because of a competition with the P=O moiety for the same mobile proton. In contrast, there are four Hbonds per molecule of phosphonic acid, Fig. 2b. Here, each P=O moiety is involved in two Hbonds, representing a chain C22 (8) of cyclic dimers R22 (8). It is an open question whether the anti-cooperative effect between these two bonds is large or small. However, it is reasonable to expect that incorporation of a proton acceptor can be energetically favorable.25 We are not aware about the crystal structure of A5. What is known is that A5 forms with B1 a crystal whose structure contained an Hbonded acid dimer terminated by four molecules of the base.26

Figure 2. Hbond crystal patterns of phosphinic (a) and phosphonic acids (b).

EXPERIMENTAL SECTION

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All chemicals were commercial products and used without additional purification. The solidstate

31

P NMR measurements were performed on an Infinityplus spectrometer system

(Agilent) operated at 7 T, equipped with a variabletemperature ChemagneticsVarian 6 mm pencil CPMAS probe. The 31P{1H} static and MAS CP NMR spectra were recorded using a cross polarization contact time of 1 ms, the typical 90opulse lengths were about 4.0 sec. The spectra were indirectly referenced to H3PO4 (85% in H2O). The numerical NMR parameters have been extracted from the experimentally obtained spectra using the WSolids1 program package.27 The Gaussian 09.C.01 program package was used for the model calculations at the B3LYP/ccpVDZ level of approximation.28 The geometry optimization was done at the standard convergence criteria. The B3LYP functional produces reasonable geometries, harmonic frequencies, and lattice energies for molecular crystals with Hbonds of different strength2932 and does not require the use of different empirical corrections.33 Xray diffraction data for the single crystals of A3/B1, A5/B1, and A5/B3 were collected at Rigaku Oxford Diffraction SuperNova diffractometers. Using Olex2,34 the structures were solved with the ShelXT structure solution program,35 using intrinsic phasing methods. The model was refined with version 2016/6 of ShelXL using least squares minimization.36 The atomic coordinates, the bond lengths, the angles, and the thermal parameters have been deposited at the Cambridge Crystallographic Data Centre (CCDC) with numbers 1577504, 1577505, and 1577506.

RESULTS Structural changes are monitored in this study using parameters of the 31P NMR chemical shift tensors (CST) of the partners.37,38 These parameters are the anisotropy (Δσ), the principal components (11, 22, 33), and the isotropic value (iso) of CST. Δσ = zz – (xx + yy)/2, where ii

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are the principal components of CST ordered according to their separation from

iso = (11 + 22 + 33)/3 in the following way: zz  iso  xx  iso  yy  iso.

31

P CST

parameters of polycrystalline B1, B2, A1, A2, A3, A4 and frozen B3 are collected in Table 1. These parameters are hard to measure for A5 because of its hygroscopy.

Table 1. Experimental 31P NMR CST of the model bases and acids. Substance

T, K

a

B1d

300

B1e

δisob

δ11c

δ22c

δ33c

–195 28.8

96

91

–101

300

–190 27.4

95

86

–99

B2h

300

–179 47.9

108

108

–71

B3h

210

–135 25.4

90f

50f

–65f

A1h

300

–84

58.0

105

69

3

A2g

300

–133 29.0

107

40

–59

A3g

300

–56

38.1

66

48

1

A4h

300

–78

21.1

71

23

–31

a

The anisotropy of CST, ppm. bThe average value of the principal components, ppm. cThe principal components of CST, ppm. dData are taken for the orthorhombic polymorph from ref 19. eData are taken for the monoclinic polymorph from ref 19. fThe margin of error is about 5 ppm. gData are taken from ref 19. hThis work.

Phosphinic acids. Samples A1/B1 and A2/B1 were obtained by slow evaporation of solutions of A1/B1 and A2/B1 equimolar mixtures in ethanol/dichloromethane. The 31P{1H} CPMAS NMR spectrum of sample A1/B1 is shown in Fig. 3a. This spectrum can be perfectly simulated using a superposition of the 31P{1H} CPMAS spectra of bulk A1 (Fig. 3b) and B1 (Fig. 3c). Similarly, the

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31

P{1H} CPMAS NMR spectrum of sample A2/B1 can be simulated using a superposition of the

spectra of bulk A2 and B1.

Figure 3.

31

P{1H} CPMAS spectra at 300 K of A1/B1 (a) and A2/B2 (d) samples. Spectra

simulated using NMR parameters of bulk A1 (b), B1 (c), A2 (e), and B2 (f).

Sample A2/B2 was obtained by heating an equimolar mixture of A2 and B2 to 370 K for 2 h. Thus, powdered A2 was dissolved in molten B2. The mixture hardened at 300 K. The

31

P{1H}

CPMAS NMR spectrum of this sample is shown in Fig. 3d. The spectrum can be simulated using a superposition of the 31P{1H} CPMAS spectra of bulk A2 (Fig. 3e) and B2 (Fig. 3f). Both the direct addition of B3 to solid A1 and A2 and evaporation of solutions of A1/B3 and A2/B3 mixtures in ethanol/dichloromethane gave rise to oily products. In the case when A2 and

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B3 were in the 3:1 molar ratio, A2 and B3 gave in 31P{1H} CP spectra narrow peaks at 24.8 and 21.1 ppm respectively. The mixture froze at about 200 K. The resulted spectrum could be simulated using the NMR parameters of bulk A2 and B3. Phosphonic acids. Slow evaporation of a solution of A3/B1 equimolar mixture in solution in ethanol gave rise to a crystalline precipitate. The 31P{1H} CP and CPMAS NMR spectra of this precipitate are shown in Fig. 4a. The deconvolution of these spectra shows that A3 and B1 form a complex of the 1:1 stoichiometry. The NMR parameters of A3 and B1 in this complex are collected in Table 2. The crystal structure of this complex is discussed in a separate section below.

Figure 4.

31

P{1H} CP NMR static and MAS spectra at 300 K of A3/B1 co-crystal (a) and its

characteristic structural pattern with four Hbonds (b). The numerical data stand for O…O distances.

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Table 2. Experimental 31P NMR CST of solid complexes formed between the model bases and acids. Complex

T, K

a

δisob

δ11c

δ22c

δ33c

A3/B1

300

–158

33.4

86

86

–72

A3/B1

300

–78

32.2

66

51

–20

A4/B1

300

–156

31.2

83

83

–73

A4/B1

300

–63

19.3

57

23

–23

A3/B2

270

–146

57.8

108

103

–38

A3/B2

270

–84

26.5

62

48

–30

A4/B2

295

–142

55.5

108

98

–39

A4/B2

295

–90

14.4

69

21

–45

A5/2B1

300

–188

18.9

82

82

–107

–162

33.7

88

88

–74

A5/2B1

300

–40

1.9

15

15

–25

A5/B2

215

–114

62.0

106

94

–14

A5/B2

215

–34

1.8

22

3

–21

A5/B3

300

–79

28.7

55

55

–23

A5/B3

300

–65

1.1

23

23

–42

Parameters correspond to the substance labelled in bold. aThe anisotropy of CST, ppm. bThe average value of the principal components, ppm. cThe principal components of CST, ppm.

Similarly, B1 forms co-crystals of the 1:1 stoichiometry with A4. The corresponding

31

P{1H}

NMR parameters are collected in Table 2. We did not attempt to make an XRD measurement for this sample.

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Slow evaporation of a solution of A3/B2 equimolar mixture in ethanol gave rise to an oily product that hardens at 270 K. The 31P{1H} CP and CPMAS NMR spectra of this precipitate are shown in Fig. 5a,b. The deconvolution of these spectra shows that A3 and B2 form a complex of the 1:1 stoichiometry, Fig. 5c,d. The NMR parameters are collected in Table 2. The quality of the crystals was insufficient for the XRD measurement. Sample 2A4/B2 was obtained by heating a mixture of A4 and B2 in the 2:1 molar ratio to 370 K for 2 h. After cooling to room temperature, the mixture becomes heterogeneous; there are an oily phase and a solid phase. The

31

P{1H} CPMAS NMR spectrum of this sample at 295 K is

shown in Fig. 5i. The mixture hardened upon cooling. Its 31P{1H} CPMAS NMR spectrum at 240 K is shown in Fig. 5j. These spectra cannot be simulated using the parameters of A4/B2 sample and of bulk A4 and B2. The positions of signals in these spectra depend on temperature. Mixtures of A3 with B3 and A4 with B3 are homogeneous oily products at room temperature for both the 1:1 and 2:1 molar ratios. At 300 K the static 31P{1H} NMR spectrum of sample A3/B3 exhibits two peaks of equal integral intensity centered at 25.9 and 25.2 ppm. At 200 K the static 31

P{1H} CP NMR spectrum of this sample exhibits a signal that can be roughly simulated using

the following parameters: 𝛿11 = 𝛿22 = 55 ppm, 𝛿33 = −34 ppm. There is only one peak at 25.3 ppm in the 31P{1H} CPMAS NMR spectrum. The linewidth of the peak is about 1 kHz. The NMR parameters of sample 2A3/B3 are very similar to the former ones. At 300 K there are two peaks centered at 26.6 and 26.3 ppm whose integral intensity ratio is about 1:2. At 200 K the NMR parameters are: 𝛿11 = 𝛿22 = 57 ppm, 𝛿33 = −34 ppm, 𝛿𝑖𝑠𝑜 = 26.7 ppm. It is not possible to justify a deconvolution of these static spectra into two or three 31P CSTs. We did not study the 31P NMR parameters of samples A4/B3 and 2A4/B3.

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Figure 5.

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P{1H} CP static (a) and MAS (b) NMR spectra at 270 K of A3/B2 sample and the

deconvolution of the MAS spectrum on the A3 (c) and B2 (d) components. 31P{1H} CP static (e) and MAS (f) NMR spectra at 295 K of A4/B2 sample and the deconvolution of the MAS spectrum on the A4 (g) and B2 (h) components. 31P{1H} CP MAS NMR spectra of 2A4/B2 sample at 295 K (i) and 240 K (j).

Phosphoric acid. Slow evaporation of a solution of 1:2 A5/B1 mixture in dichloromethane gave rise to a crystalline precipitate. The 31P{1H} CP NMR spectra of this precipitate are shown in Fig.

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6a. The deconvolution of these spectra shows that A5 and B1 form a complex of the 1:2 stoichiometry. The NMR parameters of A5 and two non-equivalent B1 in this complex are collected in Table 2. The crystal structure of this complex is discussed in a separate section below.

Figure 6. 31P{1H} CP static and MAS NMR spectra of A5/2B1 co-crystal (a) and its characteristic structural pattern with six Hbonds (b). The numerical data stand for O…O distances. The unit cell contains two crystallographically independent acid dimers.

Slow evaporation of solutions of 1:1 and 1:2 A5/B2 mixtures in dichloromethane gave rise to oily products. The 31P{1H} CP NMR spectra of these samples at low temperatures are shown in Fig. 7. The static spectrum of A5/B2 sample (Fig. 7a) can be simulated using two 31P CSTs, Fig.

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7b,c. The numerical parameters of these CSTs are collected in Table 2. The MAS spectrum of this sample exhibits two signals of the linewidth of 1 kHz, Fig. 7d. The MAS spectrum of A5/2B2 sample exhibits the same two signals and a signal at ca. 48 ppm that can be attributed to bulk B2, Fig. 7e.

Figure 7.

31

P{1H} CP NMR static (a) and MAS (d) spectra of A5/B2 sample at 215 K. The

deconvolution of the static spectrum into the B2 (b) and A5 (c) components.

31

P{1H} CPMAS

NMR spectrum of A5/2B2 sample at 230 K (e). Asterisks indicate spinning sidebands.

Slow evaporation of a solution of A5/B3 equimolar mixture in dichloromethane gave rise to a crystalline precipitate. The 31P{1H} CP and CPMAS NMR spectra of this precipitate are shown in Fig. 8a. The deconvolution of these spectra shows that A5 and B3 form a complex of the 1:1 stoichiometry. The NMR parameters of A5 and B3 in this complex are collected in Table 2. The crystal structure of this complex is discussed in a separate section below.

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Figure 8. 31P{1H} static and MAS CP NMR spectra of A5/B3 co-crystal (a) and its characteristic structural pattern with six Hbonds (b). The numerical data stand for O…O distances.

Model calculations. In order to understand structural patterns of phosphine oxides Hbonding we made a number of model calculations. The level of approximation that was used in these calculations is not sufficient to obtain realistic numeric values. Our aim was to observe a trend how these parameters change with respect to the Hbond structure. In Figure 9 are shown optimized structures of the acid-base complexes under consideration. We have used hydrogen fluoride (HF) as the model acid because of its small size and high protondonating ability, which exclude any other possible noncovalent interactions that can compete with the P=O…H interaction under question.39 Phosphine oxides of a low and high protonaccepting ability were modelled by H3PO (Fig. 9a,b) and B3 (Fig. 9c,d) respectively. 2,4,6trimethylpyridine (collidine; Coll) was

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taken for comparison because its complexes with HF had been studied both experimentally and theoretically in details, Fig. 9e,f.4044

Figure 9. Optimized structures of model acid-base complexes: H3PO/HF (a), H3PO/2HF (b), B3/HF (c), B3/2HF (d), Coll/HF (e), and Coll/2HF (f). The PH and CH protons are omitted for clarity. Qualitative energetic and structural parameters of the model acidbase complexes are collected in Table 3. The parameter 𝐸HB that we associate with the total energy of Hbonding was calculated in the following way: 𝐸HB = 𝐸compl − 𝐸Base − 𝑛 ∙ 𝐸HF , where 𝐸compl is the HartreeFock energy of the model complex, 𝐸Base is the HartreeFock energy of a solitary molecule of the corresponding base, 𝐸HF is the HartreeFock energy of a solitary molecule of HF, 𝑛 is the number

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of HF molecules in the complex. H3PO and B3 behave similarly; namely, their P=O groups are ready to form two Hbonds at once, Fig. 9b,d. Although the formation of the second Hbond results in a decrease of the average perHbond interaction energy, the effect is only about 10 % for both bases. It goes without saying that this decrease is associated with an increase of the length of these Hbonds and that the interaction energies are larger for B3. In contrast, Coll can form only one Hbond; the addition of the second molecule of HF changes the structure to [CollH]+…[FHF]–.4044

Table 3. Energetic and structural parameters of HBonds in the model acidbase complexes. Complex

EHB,a kJ/mol

O…H,b Å

H3PO/HF

49

1.63

H3PO/2HF

87

1.67

B3/HF

61

1.54

B3/2HF

113

1.60

Coll/HF

64



a

Hbond energy, see text for details. bThe O…H distance.

XRD structures. The crystal samples turned out to be hygroscopic and especially in case of A5/2B1 of poor quality. A5/2B1 revealed additional non-merohedral twinning. Nevertheless, we were able to determine the structures for the three combinations: A3/B1, A5/2B1, and A5/B3. Geometrical and displacement parameter restraints were applied to disordered parts of the structures. Wherever possible bridging hydrogen positions were located from the difference Fourier map and refined without geometrical restraints.

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The structural pattern of A3/B1 crystal exhibits a cyclic dimer of A3 terminated at each end by a B1 molecule, representing a motif D11 (2) R22 (8) D33 (11) Fig. 4b. In A5/2B1 crystal cyclic dimers of A5 are terminated at the both ends by four B1 molecules, a motif D11 (2) R22(8) D33 (11) D22 (7) Fig. 6b. The simplified structural pattern of A5/B3 cocrystal exhibits an alternating chain of cyclic dimers of A5 and two molecules of B3, a motif D11 (2) R22(8) D33 (11) D22 (7) D22 (6) D12 (3) R24 (12) Fig. 8b. The P=O group of each B3 is involved in two Hbonds with two cyclic dimers of A3.

DISCUSSION Solid samples A1/B1, A2/B1, and A2/B2 represent the physical mixtures of the components. Oily mixtures A1/B3 and A2/B3 are converting to the physical mixtures of the components upon freezing. The latter happens about 80 K below the melting point of B3. Thus, phosphinic acids A1 and A2 interact with phosphine oxides in the liquid state but do not form co-crystals with them. In this sense, phosphinic acids deviate from benzoic acids. The latter form co-crystals of 1:1 and 2:1 acid:base composition with a large variety of bases including phosphine oxides.4547 The structural units of these co-crystals resemble Hbonded complexes in solution.4850 Let us compare Hbond features of carboxylic and phosphinic acids. The O…O distance in a crystal of 2nitrobenzoic acid (pKa ≈ 2.2) is about 2.66 Å.51 For phosphonic acids the O…O distances are 2.50 Å and 2.452.48 Å for A3 (pKa ≈ 2.9) and A4 (pKa ≈2.3) respectively.16,20,21,52 The pKa’s of these acids are similar. However, in the case of phosphinic acids the Hbonds are shorter and presumably stronger. Bulk benzoic acids, including 2nitrobenzoic acid, tend to aggregate in cyclic dimers while phosphinic acids prefer chains. However, this cannot be the reason for the difference. There are phosphinic acids that aggregate in cyclic dimers due to steric reasons. The O…O distances in these dimers are about 2.50 Å or shorter as well.5355 Some ideas can be extracted from the XRD structure of

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3(hydroxy(phenyl)phosphoryl)propanoic acid.56 In Figure 10 is shown a scheme of the Hbond network in this structure. The POH and P=O moieties of neighboring molecules form an Hbond chain. Besides, the P=O moieties are Hbonded to the COH moieties of the molecules of the other chain. Thus, there are two Hbonding chains of C(7)C(5) type. Each P=O group is involved in two H-bonds, whereas C=O groups remain excluded from the Hbond network. The protonaccepting ability of the C=O group is low while the orientation of the lone pairs of the P=O group favors to the double Hbonding.10 This pattern is present for different species of such type.5759 In some cases the inclination of P=O groups to multiple Hbonding is compensated by water.60,61 The differences in the protonaccepting properties of the C=O and P=O moieties should be important for specific biochemical activity of the latter.6264 We conclude that whereas even a weak base can compete with the C=O moiety of a carboxylic group for a proton, a much stronger base is needed in order to outperform the proton accepting ability of the P=O moiety. The proton accepting abilities of B1 and B2 are not sufficient to perturb the Hbond network of crystalline A1 and A2. B3 is strong enough for that but it is not a good partner for cocrystallization. We assume that substituted pyridines can be a reasonable alternative, especially taking into account the possibility to use the isotropic 15N NMR chemical shift as a measure of Hbond geometry.6569

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Figure 10. The structure of a polymorph of 3(hydroxy(phenyl)phosphoryl)propanoic acid, Ref. 56. The numerical data stand for the O…O distances of COH···O=P and POH···O=P H-bonds. Our qualitative model calculations support these tentative conclusions. The formation of the second Hbond by the P=O group under discussion results only in a small decrease of the perHbond interaction energy. This decrease is smaller even than that for fluorine anion, one of the strongest Hbond acceptor.70 Thus, the anti-cooperativity effect for Hbonding to the P=O group is very small. In order to highlight this effect we prefer to introduce a term “ambidextrous”. P=O moiety is an ambidextrous Hbond acceptor that is P=O moiety is able to form two Hbond at once without a marked reduction in the strength of each of them. Whether the onebond or the twobond structural patterns are energetically favorable in the crystal structure under discussion depends on two factors: the per-Hbond interaction energy of the specific acidbase pair and the effect of these two structural patterns on the total energy of van der Waals interactions. The larger is the protonaccepting ability of a P=O group, the larger is the perHbond interaction energy,

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and the larger is the probability that this group will be involved in multiple Hbonding in the crystalline state. The model calculations do not exhibit any specificity for the perHbond interaction energy in phosphine oxides as compared to pyridines. The gasphase proton affinities of H3PO, B3, and Coll are about 900, 960, and 990 kJ/mol and correlate with the perHbond interaction energies in their complexes with HF that are about 49, 61, and 64 kJ/mol respectively. In addition to the aforementioned, the ambidextrous nature of P=O moieties should affect the structure of Hbonded complexes in the gas and liquid states. For example, when a silica surface is functionalized with propionic acid moieties, the effective acidity of the surface can be manipulated by adding or removing of a small amount of water.71 In contrast, residual water cannot be removed from a silica surface functionalized with phosphonic acid moieties even at 420 K in high vacuum.72 It would be of great interest to compare the structures of small acidbase clusters of pyridines and phosphine oxides. While the former are studied in many details, the latter are rather terra incognita.48,7376 Of special interest in this context are compounds that contain P=O and POH moieties; such compounds can be effective NMR sensors of noncovalent interactions in complex systems.7780 Phosphonic acids A3 and A4 are packed in the crystal in a chain arrangement with a double Hbonding at the P=O moieties, Fig. 2b. This is a weak point that can be attacked by a base. Indeed, the structural pattern in co-crystal A3/B1 is the cyclic dimer of A3 terminated at the both ends by a B1 molecule, Fig. 4b. The NMR parameters of co-crystal A4/B1 leave no doubt that its crystal structure is very similar. Most probably, the same is true for co-crystals A3/B2 and A4/B2. However, the NMR parameters of sample 2A4/B2 suggest, that all these co-crystals can be unstable and change to a glassy products if the acid is in excess. This trend is confirmed by the

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NMR parameters of samples A3/B3 and 2A3/B3. These oily products become glassy at low temperature. Here, a crystal is not formed even at the equimolar ratio, A3/B3. Phosphoric acid forms co-crystals with all studied phosphine oxides. However, the structural patterns are different. In the co-crystal with B1 the cyclic dimer of A5 is terminated at the both ends by four B1 molecules, Fig. 6b. This cluster does not form Hbonds with the neighboring ones. In contrast, in the co-crystal with B3 the cyclic dimer of A5 is terminated at the both ends by four B3 molecules that form Hbonds with the next cyclic dimer, Fig. 8b. Thus, there are 1D Hbond bands of alternating cyclic dimer and B3 pairs. As a result, the composition of the cocrystal changes to 1:1. The same composition was observed for co-crystal A5/B2. Thus, this crystal structure should be similar to the one of A5/B3. We come explicitly to the question about a competition between the total energies of Hbonds and van der Waals interactions. The best approach to answering this question is the topological analysis of charge density distribution function (ρ(r)) based on Bader’s “Atoms in Molecules” (AIM) theory.81 According to AIM, the bond critical point (bcp) and the bond path connecting the corresponding atoms are indicative of a bonding interaction, and the topological parameters of ρ(r) reflect the mechanism of this interaction and its strength. The experimental distribution of ρ(r) can be derived from highresolution Xray diffraction data.82 The energy of the interactions can be estimated using the socalled Espinosa’s correlation scheme.83,84 Although the numerical accuracy of such estimates presumably depends on the bond energy,85,86 the qualitative validity of this approach has been demonstrated for hydrogen bonds and weak noncovalent interactions. 8790 However, such analysis requires crystals of an extremely high quality. We did not have such crystals. Alternatively, periodic quantum chemical calculations can be of aid here.91,92 The progress here has been reviewed recently.93 Nevertheless, the reported structures support the

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conclusion that the increase in energy of Hbonding interaction results in higher probability of double Hbonding of P=O moieties in the crystalline state.

CONCLUSIONS In this work, we have studied interaction of phosphinic, phosphonic, and phosphoric acids with phosphine oxides in order to evaluate the general structural patterns of their cocrystals and to learn about the factors that favor or prevent the formation of a certain pattern. Our main conclusion is that these patterns are affected by the inclination of P=O moiety to form two Hbond at once without a marked reduction in the strength of each of them. We say that P=O moiety is an ambidextrous hydrogen bond acceptor. The larger is the protonaccepting ability of this moiety in the molecule under discussion, the higher is the probability of double Hbonding. We also reach the following specific conclusions: (i) The structural pattern of Hbond networks formed by phosphinic acids are governed by a high protonaccepting ability of their own P=O moiety and its inclination to double Hbonding. Thereby, these Hbond networks differ from ones formed by carboxylic acids. None of phosphine oxides studied in this work forms co-crystals with phosphinic acids at room temperature. However, the formation of co-crystals is not excluded for phosphine oxides exhibiting the protonaccepting ability larger than that of B2, for example, similar to the protonaccepting ability of B3. Alternatively, phosphinic acids could form co-crystals with highly basic substituted pyridines. (ii) The structural pattern of Hbond networks in the co-crystals of phosphonic acids with phosphine oxides exhibits a cyclic dimer of the acid terminated at the both ends by the molecules of the oxide, a motif D(3) R22 (8)D(3). However, if the acid is in excess, it does not form the second

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crystalline phase of bulk acid. Instead, the components form an oily product that becomes glassy upon cooling and represents a solid solution of the base in the acid. (iii) The structural pattern of Hbond networks in the co-crystals of phosphoric acid with phosphine oxides depends on the basicity of the oxide. For B1 the cyclic dimer of the acid is terminated from the both ends by four B1 molecules. This cluster represents an isolated structural unit, a motif D(7) R22(8)D(7). There are only van der Waals interactions between these units. In contrast, P=O groups of B2 or B3 form multiple Hbonds that connect the cyclic dimer of the acid into a chain. This Hbond network goes through the whole co-crystal, a chain of R24 (12) and R22 (8).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. X-ray diffraction structure for single crystal A3/B1, A5/2B1, A5/B3 (Table S1).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +49-941-9434027.

ACKNOWLEDGMENT

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We thank Prof. Dr.Sc. Konstantin A. Lysenko for helpful discussions. This work was supported by the Russian Foundation of Basic Research (Project 170300590) and the German-Russian Interdisciplinary Science Center (GRISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD). The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (www.gausscentre.eu) for funding this project by providing computing time on the GCS Supercomputer SuperMUC at Leibniz Supercomputing Centre (LRZ, www.lrz.de).

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