Prescott, Hillary A.: The Crystal Structures and Thermal Behavior of Hydrogen Monofluorophosphates and Basic Monofluorophosphates with Alkali Metal and N-containing Cations

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Kapitel 6. Discussion

6.1 A Structural Comparison to the Hydrogen Chalcogenates and Other Oxoacid Salts

The similarity of the basic sulfate and monofluorophosphates due to their isosterism was commented on quite early [18, 94, 95]. Certain basic sulfates and monofluorophosphates are isostructural and do have comparable solubilities [17, 19]. On the other hand, the crystal structures of the hydrogen monofluorophosphates proved to be quite different when compared with those of the hydrogen sulfates. Only a few examples were observed, in which the hydrogen monofluorophosphate / basic monofluorophosphates and the corresponding sulfate were isostructural. Unfortunately, not all of the structures presented in this thesis have an analogous sulfate, whose structure has been previously determined. Thus, the comparison is not complete, yet leads to interesting conclusions about the structural influence of fluorine in the hydrogen and basic monofluorophosphates.

The hydrogen monofluorophosphates with K, Rb, Cs, NH4

Despite the parallel compositions of MHPO3F and MHSO4 with M = K, Rb, Cs, NH4, these compounds are not isostructural. In the case of potassium, the orthorhombic structure of KHSO4 [9, 39] contains cyclic dimers and infinite chains of HSO4 tetrahedra. Branched


88

chains like the ones found in KHPO3F were only observed in alpha-NaHSO4 [41].

The Rb structures of both classes of compounds are monoclinic, but have different lattice parameters. They also vary in their patterns of hydrogen-bonded tetrahedra. The structure of RbHSO4 [8, 96] includes two crystallographically different chains of tetrahedra parallel to the b-axis. This type of structure has not been observed in the hydrogen monofluorophosphate structures. The alpha-RbHPO3F, instead, has tetramers similar to those found in the nonisostructural AgHSO4 [42, 43]. Yet, the isotypism of the Rb and NH4 hydrogen sulfates [97] was imitated by the isostructural hydrogen monofluorophosphates: alpha-RbHPO3F and alpha-NH4HPO3F. The beta-RbHPO3F structure with O/F disordering could not be compared structurally to the hydrogen sulfates, but does have lattice constants comparable to a high-pressure RbHSO4 modification [98] (a = 7.354, b = 7.354, c = 7.758 Å, gamma = 110.84(4)°), in which the hydrogen positions could also not be determined.

In the room temperature phase of CsHSO4 [6], the HSO4 tetrahedra are connected by hydrogen bonds to form chains instead of dimers like those formed in CsHPO3F. Interestingly enough, some similarity is found between the monoclinic lattice constants of CsHPO3F and the high-temperature, tetragonal phase, CsHSO4 (a = 5.714 Å and c = 14.212 Å) [99].

Analogous to the alkali metal compounds, the alpha and beta modifications of NH4HPO3F and those of the hydrogen sulfate, NH4HSO4 [7, 100], are not isostructural [77]. The nonferroelectric, RT modification of NH4HSO4 [100] does contain two unique, distorted SO4 tetrahedra linked by two short hydrogen bonds (2.514(6) and 2.598(5) Å), but the units form chains, not tetramers, in the b-direction. A difference between the NH4HPO3F and NH4HSO4 structures is also seen in the N···O hydrogen bonding. In the case of the sulfate, each O atom is involved in one N-H···O bond including the O atoms, which are hydrogen donors in the two O-H···O bonds. In the NH4HPO3F structures, the hydrogen donor atoms, O3 and O6, are not acceptors in N-H···O bonds. This and the absence of N-H···F bonds force the other O atoms to participate in more than one hydrogen bond. The only exception is O5 in á-NH4HPO3F, which is a single acceptor due to the missing hydrogen bond with H10.

Similarities are observed between the alpha modification of NH4HPO3F and the acid salt, alpha-(NH4)2SeO4(H3PO4) [101]. The lattice constants of these two salts (both P21/n) are almost identical with a = 7.540(3), b = 15.516(5), c = 7.741(3) Å, and beta = 106.75(3) for alpha-(NH4)2SeO4(H3PO4). The structures also have common features. The phosphoric acid adduct consists of tetramers of selenium and phosphorus tetrahedra with a similar


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orientation and packing to those in alpha-NH4HPO3F. The tetramers in alpha-(NH4)2SeO4(H3PO4) are then linked to each other along the b-axis by an additional hydrogen bond. This hydrogen bond is imitated in the NH4HPO3F structure by the short F····F distance (2.731 Å) probably induced by the packing of the tetramers in the structure. The F····F distance is longer than the hydrogen bond (2.503(3) Å) in alpha-(NH4)2SeO4(H3PO4). In the case of the beta modifications, the compounds have similar lattice constants, but different space groups.

In comparison to the hydrogen monofluorophosphates with the MHPO3F composition, some structural similarity was found between the unique hydrogen monofluorophosphate, K3[H(PO3F)2] and the corresponding sulfate. The formula with a M/H ratio of 3:1 was only obtained for the potassium hydrogen monofluorophosphate, whereas it is quite common among the acid sulfates and selenates with the general formula, M3[H(XO4)2]: (NH4)3[H(SO4)2], Na3[H(SO4)2], K3[H(XO4)2], Rb3[H(XO4)2], and Cs3[H(SeO4)2] (X = S or Se), which are all isostructural except for the sodium salt. The space group of K3[H(PO3F)2], C2/c (A2/a), is identical to that of K3[H(SO4)2] at RT [44], but the compounds have different lattice constants and ratios. In both structures, the K1 atom has a special position; there is only one unique tetrahedron; and the hydrogen bond is situated around the center of symmetry. The hydrogen atom was assigned the special position at (0, 0, ½) in K3[H(SO4)2] instead of a disordered general position as in K3[H(PO3F)2], but hydrogen disordering at RT seemed to be very clear in K3[H(SO4)2] [44]. A refinement with the hydrogen atom position directly on the center of symmetry resulted in an increased R factor for the structure of Rb3H(SeO4)2 in [48], which was also observed for the refinement of K3[H(PO3F)2]. The hydrogen bond length, O···O, in K3[H(PO3F)2] of 2.451(8) Å was one of the shortest found for the hydrogen monofluorophosphates presented here and was only slightly shorter than the 2.493(1) Å found in [K3H(SO4)2] [44]. Thus, some common structural features exist between the K3[H(PO3F)2] and [M3H(XO4)2] salts. However, the arrangement of the tetrahedra is much different probably due to fluorine and its limited involvement in the metal coordination in comparison with oxygen. This could account for a variation in the lattice parameters between the hydrogen monofluorophosphate and hydrogen sulfates.

The guanidinium compounds

Differences between the (hydrogen) monofluorophosphates were observed in the structures with not only ammonium and the alkali metal cations, but also guanidinium. Both the guanidinium monofluorophosphate and hydrogen monofluorophosphate are not isostructural with the corresponding sulfate and hydrogen sulfate [35]. The structures of


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the basic salts are quite different in symmetry; the structure of the sulfate is cubic (P4132), that of the monofluorophosphate monoclinic (Cm), yet some structural similarities do exist between the two. In both structures, three guanidinium cations and two tetrahedra are found in the asymmetric unit. The tetrahedra have special positions on rotation axes in the sulfate and on the mirror plane in the monofluorophosphate. Two of the guanidinium cations are also situated on a special position with the third in a general position. This also applies to both structures. In addition, the long N···O hydrogen bonds have comparable ranges for the N···O distances of 2.87(1)-3.15(2) Å in the sulfate and 2.911(4)-3.128(4) Å in the monofluorophosphate. One of which is bifurcated in the sulfate. Bifurcated hydrogen bonds were not observed in either the hydrogen monofluorophosphates or basic monofluorophosphates. Instead one of the oxygen atoms participates in two hydrogen bonds to two different nitrogen atoms. In both cases, all of the oxygen atoms participate in hydrogen bonding. As expected, an automatic reduction in symmetry results, when oxygen is replaced by fluorine in the tetrahedra. The non-involvement of fluorine in hydrogen bonding also varied the structural features of the hydrogen bond system with each guanidinium cation connected to three SO4 tetrahedra in the sulfate and 3-4 PO3F tetrahedra in the monofluorophosphate.

The acid salts with guanidinium are symmetrically more closely related: both are monoclinic, P21/n (sulfate) and P21/c (monofluorophosphate), but structurally quite diverse. The SO4 tetrahedra are hydrogen-bonded to form chains instead of cyclic dimers like those in [C(NH2)3]HPO3F. The O-H···O bond is also slightly longer than that observed in the monofluorophosphate (2.623(3) compared to 2.562(4) Å). In addition, each of the O atoms on sulfur participates in two N···O hydrogen bonds independent of its function in the O-H···O hydrogen bonding. In the hydrogen monofluorophosphate, only the O atoms, which do not function as a hydrogen donor in O-H···O bond, participate in the hydrogen bonding to the guanidinium cations. A similar behavior was observed in the ammonium structures. Thus, a noninvolvement of two hydrogen atoms in hydrogen bonding is observed in the structure of the hydrogen monofluorophosphate not seen in the structure of the hydrogen sulfate. This noninvolvement could be caused by the absence of a sufficient amount of hydrogen acceptors in the structure due to the nonparticipation of both the fluorine atom and the hydrogen donor O atoms in the hydrogen bonding. It could also be an effect of the packing (fluorine directed away from the N atoms and hydrogen bonds) making hydrogen bonding with these hydrogen atoms unfavorable. In the hydrogen sulfate, all of the guanidinium hydrogen atoms and all of the O atoms participate in hydrogen


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bonding.

Thus, isotypism of the sulfate and monofluorophosphates mentioned in [17, 19] was not reflected in the anhydrous hydrogen sulfates and hydrogen monofluorophosphates. At the most, common structural features were observed between the structures. The presence of fluorine in the structures seemed to play a significant role in the variations found. The comparison of the hydrates, NaHPO3F·2.5H2O, [N(CH3)4]HPO3F·H2O, and Na2PO3F·10H2O, to the corresponding sulfates and acid salts also showed the influence of fluorine on the crystal structure.

The hydrates

NaHPO3F·2.5H2O

A hydrate identical to NaHPO3F·2.5H2O [81] is not known for the sodium hydrogen sulfates: alpha-NaHSO4 [41], beta-NaHSO4 [36], or the monohydrate [102]. However, the hydrate of sodium hydrogen monofluorophosphate is isostructural to the phosphite, NaHPO3H·2.5H2O, with the lattice parameters: space group C2/c, a = 19.177(3), b = 5.2869(7), c = 12.672(2) Å, beta = 109.82(3)° with V = 1208.67(3) Å3 and Z = 8 [103]. In the hydrogen phosphite, the hydrogen atom on phosphorus does not participate in hydrogen bonding or metal coordination. On the basis of this isostructural behavior, one can assume that the hydrogen (phosphite) and fluorine (monofluorophosphate) atoms have equivalent positions and functions in the structures. This strongly supports the general observation that fluorine does not participate in the hydrogen bonding or sodium coordination in the structure. The comparison of the hydrogen monofluorophosphates to the hydrogen phosphites has been made before in [28] and leads to interesting conclusion about the similarity of hydrogen and fluorine when bonded to phosphorus.

[N(CH3)4]HPO3F·H2O

The cubic tetramethyl ammonium acid salts, [N(CH3)4]HPO3F·H2O and [N(CH3)4]HSO4·H2O [103], are isostructural with the lattice constants, a = 9.691(2) and 9.750(1) Å, respectively. Slight differences in the structure were observed in the disorder of the tetrahedral anion. In the sulfate structure, all of the tetrahedral oxygen atoms have general positions and are disordered, whereas in the monofluorophosphate, the fluorine position on phosphorus has a special position on the C3 axis and is not disordered. The special position of fluorine is probably caused by the HPO3F tetrahedral symmetry and the orientation of fluorine away from hydrogen bonding and towards the inert part of the structure: the [N(CH3)4]+ cation; both of which do not apply to the sulfate structure. The variation in tetrahedral disorder and the nonparticipation of fluorine in the hydrogen


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bonding could lead to the observed change in the occupancies of the two orientations of the Ow atom from 0.5/0.5 in the sulfate to 0.888(4)/0.112(4) in the monofluorophosphate. Consequently, the hydrogen atom positions also vary in their disorder between the two structures. These small variations, however, are not significant enough to affect the overall isotypism of the compounds. A similar effect was observed in the decahydrate of sodium monofluorophosphate.

Na2PO3F·10H2O

The decahydrate, Na2PO3F·10H2O [81], is isostructural with the corresponding sulfate, Na2SO4·10H2O (Glauber's salt). The sulfate structure has been studied at room temperature with X-ray [104] and neutron [105] single crystal structure analysis. The P-F bond corresponds to the S-O6 bond in [105]. The bridging of the NaO6 octahedra and the interconnection of the PO3F tetrahedra to each other via two water molecules are structural features of both compounds. The lengths of the hydrogen bonds in the sulfate (2.75 to 3.01 Å) [105] are similar to those found in the monofluorophosphate (2.718(2) to 3.023(2) Å). One difference between the two structures is the bond lengths in the tetrahedron. All of the S-O bonds lengths are within the range of 1.4-1.5 Å [105], whereas the PO3F tetrahedron is distorted with three P-O bonds (1.5 Å) and a P-F bond (1.6 Å).

Differences between the structures were also found in bonding and disorder. In the crystal structure analysis of [104], disordered hydrogen bonds were assumed to be present in both the tetramers with water molecules. The authors correlated the possible disorder in the structure to a residual entropy, which was found earlier experimentally [106]. In the neutron diffraction study [105], the exact positions of the hydrogen atoms were determined. Both the SO4 tetrahedron and the hydrogen atoms in the ring systems are in fact disordered. The two orientations for the SO4 tetrahedron are rotated about 30° around one of the S-O bond. The disorder of the rings and tetrahedron was confirmed as the source for the zero point entropy. The occupancies were refined to 0.5 for both positions of the disordered hydrogen atoms in the rings and to 0.753/0.247 for the major and minor configurations of the O atoms on sulfur.

The degree of disorder found in Na2PO3F·10H2O and Na2SO4·10H2O [105] differs slightly affecting the hydrogen bond system. In Na2PO3F·10H2O, disorder is only found in the Ow5/Ow11 tetramer and disordering of the PO3F tetrahedron is questionable. Although very weak peaks with distances of 1.481, 1.423, and 1.869 Å from the P atom were found shifted 23.2 to 30.2 º from O2, O3, and F, respectively, this weak disorder is rotated around the P-O1 axis not equivalent to the S-O axis of rotation in the sulfate. An additional


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refinement of the occupancies yielded values of about 0.97/0.03 compared to occupancies of 0.753/0.247 in Na2SO4·10H2O [105]. Therefore, the second orientation of the PO3F tetrahedron was neglected in the final refinement. The disorder of the Ow5/Ow11 ring also varies from that in Na2SO4·10H2O [105] in its assigned occupancies. The occupancies refined and fixed for the disordered hydrogen atoms are 0.67 and 0.33 instead of an equal distribution between the two positions. One explanation for the discrepancies in disorder between the structures could be the measurement temperature: 160 for Na2PO3F·10H2O and 296.5 K for Na2SO4·10H2O [105].

The structure of Na2SO4·10H2O measured at 180 K [107] has the same type of disorder, but the occupancies vary from those found at RT [105]. The new occupancies of the different configurations were refined to 0.938/0.062 for the tetrahedron and 0.569/0.431 for one of the tetramers. The disorder in the second tetramer (0.5/0.5) is not noticeably influenced by temperature, but the temperature does appear to have a significant effect on the disorder of the SO4 tetrahedron. Fluorine could be the controlling factor for the variation in disorder of the hydrogen atoms in the tetramer of both structures. The replacement of SO4 with PO3F also leads to interesting and subtle variations in the hydrogen bond system. The F atom bonds to two water molecules like that found for the corresponding O atom in sulfate. However, it does not participate in a third bond found in sulfate. Thus, by substituting an O atom with a F atom, the disorder is reduced in the structure and the hydrogen bonding of the water molecules to the PO3F tetrahedron is slightly varied.

The thermal stabilities of the salts also differed. Glauber's salt, Na2SO4·10H2O melts at 305 K [55], whereas the monofluorophosphates has a lower (incongruent) melting point of 283±2 K suggesting a more unstable structure, which may be expected on the basis of fluorine's unusual involvement in the hydrogen bonding.

6.2 Structural Features

The structural features of the hydrogen-bonded HPO3F tetrahedra were all types that had been observed before in acid salts of oxoacids, except for that of beta-RbHPO3F due to O/F disordering. Yet, some variations did exist.

Hydrogen-bonded chains were, interestingly enough, observed in the hydrogen sulfates and selenates of larger cations: Rb+ [8, 33] and Cs+ [6, 34]. [C(NH2)3]HSO4 also formed chains of HSO4 tetrahedra. In the case of the hydrogen monofluorophosphates, the smaller


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cations, Na+ and [NH2Et2]+, form such a pattern, whereas cyclic dimers were formed in the structures with larger cations, Cs+ and [NHEt3]+, and [C(NH2)3].

The following correlation between cation size and pattern of the hydrogen-bonded HPO3F tetrahedra was observed:

Na (infinite chains) rarr K (branched chains) rarr Rb/NH4 (tetramers) rarr Cs (cyclic dimers)

with a similar trend for the diethyl and triethylammonium structures.

[NH2Et2] (infinite chains) rarr [NHEt3] (cyclic dimers)

The structural pattern of cyclic dimers found in a variety of the hydrogen monofluorophosphates, CsHPO3F, [NHEt3]HPO3F, [C(NH2)3]HPO3F, and [N,N´-dmuH]HPO3F, was not very common for the hydrogen sulfates [32]. The only hydrogen sulfate, which contains cyclic HSO4 dimers, was beta-NaHSO4 [36], which was indirectly confirmed with the determination of the isostructural NaHSeO4 [37]. The frequent occurrence of this pattern in the hydrogen monofluorophosphate suggests the structural stability of this pattern. Its rarity in the hydrogen sulfates implies the structural influence of fluorine in the hydrogen monofluorophosphates.

6.3 Fluorine

Some general bonding characteristics were observed for the fluorine atom in the structures of the hydrogen and basic monofluorophosphates and seem to have a direct influence on the crystal structure formed. The characteristics included

In the structures with sodium and N-containing cations, the fluorine atom is only bound to phosphorus. Metal coordination and N···F hydrogen bonds were not observed. Thus, the sodium cations were coordinated solely by oxygen atoms in the hydrates, NaHPO3F·2.5H2O, Na2PO3F·10H2O, and Na5[N(CH3)4]PO3F·18H2O. The presence of crystal water and its abundance in two of the sodium structures: the decahydrate and mixed salt, in which more than one sodium cation is present, suggest that crystal water is necessary for cation coordination. The only other hydrate structure found was that of tetramethylammonium. The noninvolvement of fluorine in the metal coordination and presence of crystal water have also been observed in other hydrates, in which the cations have an equivalent or smaller radius than that of Na (Tab. 55), such as CaPO3F·2H2O [73],


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CuPO3F·2H2O [108], ZnPO3F·2.5H2O [109], and Ni(H2O)6(NH4)2(PO3F)2 [110]. Therefore, it can be concluded that the crystal water is essential for the complete coordination of the metal and a resulting stabilization of the structure.

Tab. 55 Cation radii [111, 112], number of metal-fluorine bonds per fluorine atoms, avg. M-F distance, avg. P-F distances, and VF in the alkali metal hydrogen monofluorophosphates (Å)

Structure

NaHPO3F·2.5H2O

KHPO3F

K3[H(PO3F)2]

alpha-RbHPO3F

CsHPO3F

Cation radius

1.16

1.52

1.52

1.66

1.81

M-F bonds

-

1-2

4

2

1

Avg. d(M-F)

-

2.895

3.079

3.106

3.194(3)

Avg. d(P-F)

1.564(2)

1.573

1.594(3)

1.579

1.577(2)

VF

0.95

1.01-1.15
1.08 (avg.)

1.14

1.08 and 1.12
1.10 (avg.)

1.04

Anhydrous structures were obtained for the hydrogen monofluorophosphates with larger metal cations, K+, Rb+, and Cs+ (Tab. 55); the coordination of these cations was fulfilled by both fluorine and oxygen atoms. The number of M-F bonds found per fluorine atom in the structure varied from 1-4. In the MHPO3F structures, the fluorine atoms coordinate with 1-2 metal cations, whereas the fluorine atom in the structure of K3[H(PO3F)2] is extensively involved in the coordination of four different potassium cations based on the high M/F ratio. Consequently, the P-F bond is lengthened and the total fluorine bond valency of 1.14 is one of the highest observed. This high valency for fluorine suggests structural instability and explains why this type of acid salt was not obtained for other hydrogen monofluorophosphates, although it is a common compositon among the sulfates. The high valencies of the F4 atom in KHPO3F and F2 in alpha-RbHPO3F also suggest a chemical instability of these compounds, which was reflected by the pseudo-orthorhombic twinning of KHPO3F and difficult recrystallization of alpha-RbHPO3F.

The compounds with N-containing cations and sodium have average total bond valencies of fluorine that are less than 1.0 with the exception of 1.06 for Cs3(NH4)2(HPO3F)3(PO3F)2 due to fluorine coordination of cesium (Tab. 56). The low values (< 1.0) reflect an almost complete valency of fluorine based only on the bond to phosphorus. A similar observation was noted in [73]. Thus, the fluorine atom is content by its bond to phosphorus and is basically isolated in the structure aside from the P-F bond. This "isolation" is demonstrated by the packing of the (H)PO3F tetrahedra in the structures. The P-F bond is often directed towards a location in the structure (indicated with * in Tab. 56), where no hydrogen donors or hydrogen atoms are situated. Consequently, no N-H···F hydrogen bonds were observed. This was illustrated most clearly in the structure of Na/[NMe4] (Fig. 16b), in which the [NMe4]+ cation is located in a cavity with the P-F bonds pointed


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towards the central N atom of the cation. In the structure of [N,N´-dmuH]HPO3F, the methyl groups provide an inert space in the structure for the fluorine atoms (P-F bond), which gives the structure added stability and could explain the successful synthesis and analysis of this salt despite its very low pH. The absence of these stabilizing methyl groups and additional hydrogen bonds was probably responsible for the failed synthesis of the uronium hydrogen monofluorophosphate.

Tab. 56 Structures with N-containing cations and the total fluorine bond valency

Structure

VF (avg.)

[PipzH2][HPO3F]2*

0.95

[NH2(CH2CH3)2]HPO3F*

0.95

[NH(CH2CH3)3]HPO3F*

0.95

[N,N´-dmuH]HPO3F*

0.97

[C(NH2)3]HPO3F*

0.98

alpha-NH4HPO3F

0.96

beta-NH4HPO3F (RT)

0.96

[N(CH3)4]HPO3F·H2O*

0.96

Cs3(NH4)2(HPO3F)3(PO3F)2

1.06

Na5[NMe4](PO3F)3·18H2O*

0.93

[C(NH2)3]2PO3F*

0.95

Similar packing behavior is observed in the structure of NaHPO3F·2.5H2O, in which the P-F points away from the O-H···O bond between the HPO3F tetrahedra and the chains of the NaO6 octahedron and hence, the Ow-H···O(w) hydrogen bonds. The fluorine atom does not participate in the O(w)-H···X hydrogen bonding. This applied to all of the structures presented here regarding the O-H···X bonding between the tetrahedra. This also holds true for almost all of the structures in the case of Ow-H···X bonding except for the decahydrate of Na2PO3F. In the Na2PO3F·10H2O structure, two hydrogen bonds are found which do involve fluorine as a hydrogen acceptor. Extended bonding of the fluorine atom has a direct effect on the P-F distance, which has been noticed before in mixed alkali metal monofluorphosphates [113]. Consequently, this structure has the longest P-F distance of 1.6082(9) Å and the lowest bond valency for fluorine (0.91) (hydrogen bonds are not considered in the calculation of the VF) (Tab. 57). These O-H···F bonds connect the tetrahedron to two different water molecules. The question of why unusual O···F bonds are observed can be answered by considering the hydrate structure of Na5(NMe4)(PO3F)3·18H2O. This structure with Na+ and [NMe4]+ cations also has a high number of water molecules; however, O···F hydrogen bonds were not observed. Instead two of the hydrogen atoms do not participate in the hydrogen bonding at all, which can be directly derived from the presence of fluorine instead of oxygen on phosphorus. A similar


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nonparticipation of individual hydrogen atoms was also observed in the acid salts: alpha-NH4HPOF and [C(NH2)3]HPO3F, but is more involved on the basis of O-H···O bonding and will be discussed later. A look at the F/H2O ratio in the hydrates shows that the F/H2O ratio has increased from 1:6 in Na5[NMe4](PO3F)3·18H2O to 1:10 in the decahydrate. Thus, the high F/H2O ratio found in the decahydrate is probably the deciding factor for the forced O···F bonding. Interestingly enough, O···F hydrogen bonding has only been observed in hydrated fluorides and fluorometallate hydrates such as FeSiF6·6H2O, where no oxygen atoms are bonded to the central atom [30]. A noted feature of the O···F bonds in Na2PO3F·10H2O is their long distances of 2.837(2) and 3.003(2) Å. These O···F hydrogen bonds are comparable in length to O···F bonds found in the hydrates of the hexafluorosilicates and fluorometallates [114, 30]. The long O···F distances and the lower melting point of the salt compared to that of Na2SO4·10H2O reflect fluorine‘s hesistance to participate in additional bonds.

6.4 The Tetrahedral Bonding

Certain trends are observed in the bonding of the (H)PO3F tetrahedron within the hydrogen monofluorophosphates (Group I and II) and basic monofluorophosphates (Group III) shown in Tab. 57. Group I includes the hydrogen monofluorophosphates with N-containing cations; the akali metal hydrogen monofluorophosphates belongs to Group II. The beta-RbHPO3F and Cs3(NH4)2(HPO3F)3(PO3F)2 are handled separately based on their unique structural features: O/F disordering and the presence of HPO3F and PO3F tetrahedra, respectively. Based on the data in Tab. 57, the average P-ODH and P-F distance increase from 1.544 and 1.560 for Group I to 1.559 and 1.577 Å for Group II, respectively, because of metal coordination to the oxygen and fluorine atoms. The longest P-F distance is observed for the basic monofluorophosphates (Group III, 1.588 Å). A parallel trend is found for the average P-O distance from Group I to Group III. Although the metal coordination explains the slightly longer distance found for Group II (1.486 Å) compared to that of Group I (1.482 Å), this does not apply to Group III for compounds with sodium and N-containing cations. In this case, the longer P-O distance of 1.508 Å can be attributed to extensive hydrogen bonding.


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Tab. 57 Avg. bond distances and VF for the given structures /Å
(Values were averaged for structures with several bonds.)

Structure

VF

d(P-F)

d(P-ODH)

d(P-OHdisd)

d(P-O)

 

 

 

 

 

 

Group I

 

 

 

 

 

[PipzH2][HPO3F]2

0.95

1.564(1)

1.549(1)

 

1.493

[NH2(CH2CH3)2]HPO3F

0.95

1.566(1)

1.545(1)

 

1.481

[NH(CH2CH3)3]HPO3F

0.95

1.566(2)

 

1.523

1.452(3)

[N,N´-dmuH]HPO3F

0.97

1.554(2)

1.542(2)

 

1.495

[C(NH2)3]HPO3F

0.98

1.544(3)

1.531(3)

 

1.480

alpha-NH4HPO3F

0.96

1.562

1.548

 

1.490

beta-NH4HPO3F (RT)

0.96

1.566

1.547

 

1.485

[N(CH3)4]HPO3F·H2O

0.96

1.563(1)

 

1.500(1)

 

 

 

 

 

 

 

Avg.

0.96

1.560

1.544

 

1.482

 

 

 

 

 

 

Group II

 

 

 

 

 

NaHPO3F·2.5H2O

0.95

1.564(2)

1.563(2)

 

1.492

KHPO3F

1.08

1.573

1.556

 

1.486

alpha-RbHPO3F

1.10

1.579

1.557

 

1.487

CsHPO3F

1.04

1.577(2)

 

1.528(2)

1.477(3)

K3[H(PO3F)2]

1.14

1.594(3)

 

1.543(4)

1.490

 

 

 

 

 

 

Avg.

1.06

1.577

1.559

 

1.486

 

 

 

 

 

 

Group III

 

 

 

 

 

Na2PO3F·10H2O

0.91

1.6082(9)

 

 

1.508

Na5[NMe4](PO3F)3·18H2O

0.93

1.586

 

 

1.509

[C(NH2)3]2PO3F

0.95

1.571

 

 

1.506

 

 

 

 

 

 

Avg.

0.93

1.588

 

 

1.508

 

 

 

 

 

 

beta-RbHPO3F

1.07

1.538(4)

1.541(4)

1.513(4)

1.485(4)

 

 

 

 

 

 

 

 

 

 

 

 

Cs3(NH4)2(HPO3F)3(PO3F)2

 

 

 

 

 

HPO3F

1.09

1.572

1.547

 

1.479

PO3F

1.06

1.574(4)

 

 

1.493

 

 

 

 

 

 

The bond distances in the Cs3(NH4)2(HPO3F)3(PO3F) are inconsistent with these trends. Similar P-F distances of 1.572 and 1.574(4) Å are found for the HPO3F tetrahedra and nondisordered PO3F tetrahedron, respectively, and are only slightly shorter than that found in Group II. Both distances are practically identical to the 1.573 distance for KHPO3F, although that of the PO3F tetrahedron should theoretically be longer. The P-ODH distance (1.547 Å), on the other hand, approaches that of the Group I compounds (1.544 Å), which is feasible due to the presence of NH4+ cations in the structure. Differences are seen in the P-O bond lengths: 1.479 averaged for the HPO3F tetrahedra and 1.493 for the PO3F tetrahedron. The P-O bond of the HPO3F tetrahedra has the shortest distance overall and that of PO3F tetrahedron lies between the average distance found for Group II and Group


99

III as does the composition of the compound with HPO3F and PO3F tetrahedra.

The beta-RbHPO3F compound demonstrates the shortest P-F bond on the basis of O/F disordering in the structure [115]. The P-O bond distance (P-O1) has an expected distance of 1.485(4) Å, which lies directly between the lengths for Group I and II, the P-O3/FA distance is much longer with a length of 1.538(4) Å. This length is difficult to interpret based on the missing hydrogen atom, but may, at least partially, correspond to a P-ODH bond. It is only slightly shorter than the average for Group I, but deviates significantly from lengths observed in the alkali metal hydrogen monofluorophosphates (Group II).

The P-OHdisd distance for the oxygen atom involved in the disordered hydrogen bond has a distance of 1.513(4) Å in beta-RbHPO3F. The hydrogen atom position is disordered not only in beta-RbHPO3F, but also in the structures of [NHEt3]HPO3F, [N(CH3)4]HPO3F·H2O, K3[H(PO3F)2], and CsHPO3F predominantly with an occupancy of 0.5. Consequently, the corresponding oxygen atom is both a hydrogen donor and hydrogen acceptor (½D+½A) and should have a P-O distance between P-O and P-ODH lengths. The P-OHdisd distances varied from 1.500(1) for the hydrate to 1.543(4) for K3[H(PO3)2] and are, as expected, all between the P-O and P-ODH distances of Groups I and II. Similar P-O½D+½A distances of 1.523 (averaged) and 1.528(2) Å were found for the [NHEt3]HPO3F and CsHPO3F structures, respectively, which both feature cyclic dimers of HPO3F tetrahedra. The short P-OHdisd distance in [N(CH3)4]HPO3F·H2O can be accounted for by the changed occupancy from 0.5 as in the other structures to 0.33. In the case of K3[H(PO3)2], the same trend is observed for the P-O½D+½A distance as was found for the P-F length. The extensive metal coordination results in a P-O½D+½A bond longer than that found in the other structures. The P-O½D+½A in beta-RbHPO3F lies between that found for CsHPO3F and [N(CH3)4]HPO3F·H2O.

6.5 The Hydrogen Bonding

The hydrogen bonding in the hydrogen monofluorophosphates and basic monofluorphosphates was as diverse as the structural patterns and compositions (Tab. 58). The hydrogen bonds classified as O···O (O-H···O bond between the (H)PO3F tetrahedra), Ow···O (Ow-H···O bond between the crystal water and the (H)PO3F tetrahedron), Ow···Ow (Ow-H···Ow bond between molecules of crystal water), and N···O (N-H···O bond between the N-containing cation and the (H)PO3F tetrahedra) are summarized in (Tab. 58). They


100

have strengths ranging from very strong (<2.5 Å) to strong (2.5-2.65 Å) to medium (2.65-2.8 Å) to weak (>2.8 Å) [31].

The HPO3F tetrahedra were connected to each other with both strong and very stong O-H···O bonds. Very strong bonds were found between the tetrahedra in the potassium salts and alpha-RbHPO3F. The only other very strong hydrogen bond found was between the carbonyl group of the dimethyl uronium cation and the HPO3F tetrahedron. The bond is one of the shortest seen in the hydrogen monofluorophosphates but not as short as the 2.421(3) Å bond in [OC(NH2)2]·H3PO4 [116]. A medium strength and very strong O···O hydrogen bonds observed in Cs/NH4 structure between the HPO3F and disordered PO3F tetrahedra could be due to the PO3F disorder and based on inaccuracy and high standard deviations, but it is not clear if that is the main reason.

The strong Ow···O bond found in [NMe4]HPO3F·H2O is the only one of its type and is the result of the disordered hydrogen position between the HPO3F tetrahedron and the crystal water. This is probably due to the bulky cation preventing the interlinking of the HPO3F tetrahedra. Another rather exotic hydrogen bond was the strong N···O bond in [NHEt3]HPO3F which is caused by the low functionality of the oxygen atoms (one function per oxygen atom) in the structure due to the absence of metal coordination and a total of two hydrogen atoms in the structure.

It seems that the strength of the N···O bond is inversely proportional to the HN/H(H)PO3F ratio. In [NHEt3]HPO3F, the ratio is 1 and a strong bond is observed. In the [NH2Et2] and [PipzH2] structures, one medium and one weak N···O bond are found with an increased HN/H(H)PO3F ratio of 2. Medium and weak N···O bonds also occur in the Cs/NH4 structure, in which a ratio of 8:5 (1.6) exists. In the guanidinium structures with very high HN/H(H)PO3F ratios, only weak N···O bonds are observed. One exception is the structure of the uronium salt, [N,N´-dmuH]HPO3F. Here, although there are only two HN hydrogen atoms for the one HPO3F tetrahedron, only weak N···O bonds are found based on the additional hydrogen atom of the carbonyl group. Thus, the functionalities of the oxygen atoms must also be considered (Tab. 59). Four hydrogen acceptor functions are present in the uronium structure, whereas only three are found in the [NH2Et2] and [PipzH2] structures.


101

Tab. 58 Hydrogen bond distance (XY) for the structure sorted by bond type and strength; the structures are listed by type of structural pattern (Å)
he bonds with disordered hydrogen positons are indicated with (di).

Structure

O···O

 

 

Ow···O

 

 

Ow···Ow

 

N··· O

 

 

 

very strong

strong

med

strong

medium

weak

medium

weak

strong

medium

weak

Chains

 

 

 

 

 

 

 

 

 

 

 

NaHPO3F·2.5H2O

 

2.566(2)

 

 

 

2.838(3)-2.972(2)

2.791(2)

 

 

 

 

[NH2Et2]HPO3F

 

2.529(2)

 

 

 

 

 

 

 

2.761(2)

2.837(2)

[PipzH2][HPO3F]2

 

2.541(2)

 

 

 

 

 

 

 

2.677(2)

2.822(2)

 

 

 

 

 

 

 

 

 

 

 

 

Branched Chains

 

 

 

 

 

 

 

 

 

 

 

KHPO3F

2.497(5)

2.520(5)-2.590(5)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isolated Dimers

 

 

 

 

 

 

 

 

 

 

 

K3[H(PO3F)2]

2.451(8) (di)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cyclic Dimers

 

 

 

 

 

 

 

 

 

 

 

CsHPO3F

 

2.527(2) (di)

 

 

 

 

 

 

 

 

 

[NHEt3]HPO3F

 

2.515(2) (di)

 

 

 

 

 

 

2.622(2)

 

 

[C(NH2)3]HPO3F

 

2.562(4)

 

 

 

 

 

 

 

 

2.898(4)-3.042(4)

[N,N´-dmuH]HPO3F

2.488(2) CO···O

2.562(2)

 

 

 

 

 

 

 

 

2.884(3), 2.942(3)

 

 

 

 

 

 

 

 

 

 

 

 

Tetramers

 

 

 

 

 

 

 

 

 

 

 

alpha-NH4HPO3F

 

2.508(3), 2.535(3)

 

 

 

 

 

 

 

 

2.800(3)-2.951(4)

beta-NH4HPO3F (RT)

 

2.539(2), 2.568(2)

 

 

 

 

 

 

 

 

2.881(2)-3.043(2)

alpha-RbHPO3F

2.486(7)

2.561(5)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Complex/Hydrate

 

 

 

 

 

 

 

 

 

 

 

Cs/NH4

2.37(2)-2.47(2)

2.50(1)-2.57(2)

2.67(2)

 

 

 

 

 

 

2.734(8)-2.798(8)

2.800(8)-2.86(2)

[NMe4]HPO3F·H2O

 

 

 

2.637(2) (di)

 

 

 

 

 

 

 

Na2PO3F·10H2O

 

 

 

 

2.718(2)-2.793(2)

2.802(2)-3.023(2)

2.771(2)-2.790(2)

2.827(2)-2.857(1)

 

 

 

Na/[NMe4]

 

 

 

 

2.677(3)-2.798(3)

2.806(3)-2.988(3)

2.790(3)-2.796(3)

2.802(3)-2.973(3)

 

 

 

[C(NH2)3]2PO3F

 

 

 

 

 

 

 

 

 

 

2.820(4)-3.128(4)

 

 

 

 

 

 

 

 

 

 

 

 

beta-RbHPO3F

 

2.560(8) (di)

 

 

 

 

 

 

 

 

 

 

O...F

 

 

 

 

 

 

 

 

 

 

Na2PO3F·10H2O

2.837(2), 3.003(2)

 

 

 

 

 

 

 

 

 

 


102

Tab. 59 Functions of the (H)PO3F oxygen and fluorine atoms in the structures

Structure

O1/O4/O7/O10

O2/O5/O8/O11

O3/O6/O9/O12

F1/F2/F3/F4

 

M-O

OA

M-O

OA

O½D+½A

M-O

OA

O½D+½A

OD

M-F

FA

Chains

 

 

 

 

 

 

 

 

 

 

 

NaHPO3F·2.5H2O

2

1

 

3

 

 

1

 

1

 

 

[NH2Et2]HPO3F

 

2

 

1

 

 

 

 

1

 

 

[PipzH2][HPO3F]2

 

1

 

2

 

 

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Branched Chains

 

 

 

 

 

 

 

 

 

 

 

KHPO3F

1/3/3/3

1/0/0/0

2/3/2/2

1/0/1/1

 

2/1/2/1

 

 

1/1/1/1

2/1/1/2

 

 

 

 

 

 

 

 

 

 

 

 

 

Isolated Dimers

 

 

 

 

 

 

 

 

 

 

 

K3[H(PO3F)2]

5

 

5

 

 

3

 

1

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

Cyclic Dimers

 

 

 

 

 

 

 

 

 

 

 

CsHPO3F

3

 

6

 

1

 

 

 

 

1

 

[NHEt3]HPO3F

 

1

 

 

1

 

 

1

 

 

 

[C(NH2)3]HPO3F

 

3

 

2

 

 

 

 

1

 

 

[N,N´-dmuH]HPO3F

 

2

 

2

 

 

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tetramers

 

 

 

 

 

 

 

 

 

 

 

alpha-NH4HPO3F

 

3/3

 

2/1

 

 

 

 

1/1

 

 

beta-NH4HPO3F (RT)

 

3/3

 

2/2

 

 

 

 

1/1

 

 

RbHPO3F

2/3

 

3/2

1/1

 

2/2

 

 

1/1

2/2

 

 

 

 

 

 

 

 

 

 

 

 

 

Complex/Hydrate

 

 

 

 

 

 

 

 

 

 

 

Cs3NH4 (avg.)

2

1

2

1

 

3

 

 

1

2

 

[NMe4]HPO3F·H2O

 

 

 

 

 

 

 

1

 

 

 

Na2PO3F·10H2O

 

4

 

3

 

 

3

 

 

 

2

Na5[NMe4]

0/1/0

2/2/3

 

3/3/4

 

 

3/3/3

 

 

 

 

[C(NH2)3]2PO3F

 

2/2

 

4/4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

beta-RbHPO3F

3

 

2

 

1

2

 

 

 

2

 


103

The functionality of the oxygen atom was also helpful in explaining the noninvolvement of individual hydrogen atom in hydrogen bonding. As was mentioned before, in the structures, alpha-NH4HPO3F and [C(NH2)3]HPO3F, single hydrogen atoms were not involved in the hydrogen bond system. The comparison of the alpha-NH4HPO3F to its sulfate analogy made clear that an oxygen atom does not act as both an hydrogen acceptor and donor in different hydrogen bonds in the hydrogen monofluorophosphates. The function of the oxygen atom as a hydrogen donor rules out it's ability to also act as a hydrogen acceptor in the structures of the hydrogen monofluorophosphates. This limitation on the oxygen atom's functions and the nonparticipation of fluorine in the hydrogen bonding leads to the noninvolvement of hydrogen atoms in the hydrogen bond system of the hydrogen monofluorophosphates due to the insufficient amount of hydrogen acceptors. This was confirmed by a complete hydrogen bond system in [C(NH2)3]2PO3F, in which this limitation is no longer valid.

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