Isostructural Crystals of Bis(Guanidinium) Trioxofluoro-Phosphate/Phosphite in the Ratio 1/0, 0.716/0.284, 0.501/0.499, 0.268/0.732, 0/1—Crystal Structures, Vibrational Spectra and Second Harmonic Generation

Abstract

The title structures of bis(guanidinium) trioxofluorophosphate, bis(guanidinium) trioxofluorophosphate-phosphite (0.716/0.284), bis(guanidinium) trioxofluorophosphate-phosphite (0.501/0.499), bis(guanidinium) trioxofluorophosphate-phosphite (0.268/0.732), and bis(guanidinium) phosphite are crystal-chemically isotypic. Their structures correspond to the structure of bis(guanidinium) trioxofluorophosphate which was determined by Prescott, Troyanov, Feist & Kemnitz (Z. Anorg. Allg. Chem. 2002, 628, 1749–1755). The P and O atoms of the substituted trioxofluorophosphate and phosphite anions share the same positions while the P-F and P-H_hydrido are almost parallel and oriented in the same direction. Two symmetry-independent anions and two of three symmetry-independent cations are situated on the crystallographic mirror planes. The ions are interconnected by N-H⋯O hydrogen bonds of moderate strength. The most frequent graph set motif is $R_2^2\left(8\right),$ which involves interactions between the primary amine groups and the trioxofluorophosphate or phosphite O atoms. Fluorine, as well as the hydrido hydrogen, avoids inclusion into the hydrogen-bond network. The Hirshfeld surface analysis was also performed for the comparison of intermolecular interactions in the title structures of bis(guanidinium trioxofluorophosphate and bis(guanidinium) phosphite. The title crystals were also characterized by vibrational spectroscopy methods (FTIR and Raman) and the second harmonic generation (SHG). The relative SHG efficiency considerably decreases from bis(guanidinium) trioxofluorophosphate to bis(guanidinium) phosphite for the fundamental 1064 nm laser line.

1. Introduction

The present work’s aim was to prepare a series of mixed crystals in which the trioxofluorophosphate anion was substituted by the phosphite anion in different ratios. The mixed crystals with trioxofluorophosphate and phosphite anions were observed in a few cases [1]. In these structures, the oxygens of the trioxofluorophosphate and phosphite anions occupy the same positions while the P-F and P-H_hydrido are nearly parallel with the same orientation. The searches in the Cambridge Structural Database [2] as well as in the Inorganic Crystal Structure Database (2022) [3] have revealed that no other mixed phosphites and trioxofluorophosphates were included in either of these databases since 2012.

The structure of bis(guanidinium) trioxofluorophosphate was determined at 180 K [4]; the structure was assigned the refcode XOMQAR in the Cambridge Structural Database [2] (version 5.43 with updates until September 2022). Since this structure was non-centrosymmetric achiral [5], we expected that the series of title structures with phosphite anions might also be non-centrosymmetric. We also hoped that they might possess comparatively interesting non-linear optical (NLO) properties analogous to the structure of carbamoylguanidinium hydrogen phosphite [6,7], which also forms mixed crystals with the trioxofluorophosphate anion in all concentration ratios [1]. Trioxofluorophosphate anion [PO₃F]^2- has a large hyperpolarizability which makes it a candidate for synthesis of new materials suitable for the second harmonic generation [8]. Some of the trioxofluorophosphate structures turned out to be promising NLO materials such as (NH₄)₂PO₃F (diammonium trioxofluorophosphate), NaNH₄PO₃F.H₂O (sodium ammonium trioxofluorophosphate monohydrate) and already mentioned bis(guanidinium) trioxofluorophosphate [8].

In this paper, which also extends our previous studies of promising inorganic salts of guanidine [9,10], we report the preparation, crystal structure determination and spectroscopic and NLO properties of a series including bis(guanidinium) trioxofluorophosphate—compound (I), mixed crystals bis(guanidinium) trioxofluorophosphate-phosphite—compounds (II)–(IV) and bis(guanidinium) phosphite—compound (V). In addition, our attention was focused on examining the expected effect of a variable content of [PO₃F]²⁻ and [PO₃H]²⁻ anions in the title structures on SHG efficiency.

2. Materials and Methods

2.1. Synthesis and Crystallization

Colourless transparent crystals of the title compounds were prepared by neutralization of aqueous solutions of bis(guanidinium) carbonate (Aldrich, St. Louis, MO, USA, 99%), H₂PO₃F and H₃PO₃ (Aldrich, 99%). In all the cases, the cation/anion molar ratio equalled to 1. For the title mixed crystals, the amounts of the anions were combined in molar ratios 1:0, 3:1, 1:1, 1:3 and 0:1.

Fluorophosphoric acid H₂PO₃F was prepared by the exchange of water-diluted (NH₄)₂PO₃F through a column with cation-exchanging resin (Amberlite) in the H-cycle and immediately added into the aqueous solutions of bis(guanidinium) carbonate and bis(guanidinium) phosphite dropwise. The volume of the eluted solution of H₂PO₃F was about 50 mL. The colourless crystals were prepared from the aqueous solutions by crystallization in a desiccator over P₄O₁₀ over three weeks. Bis(ammonium) trioxofluorophosphate monohydrate (NH₄)₂PO₃F·H₂O was prepared by the method described in [11] while the product which was obtained by heating of urea, H₃PO₄ and NH₄F was recrystallized from water. The aqueous solution of bis(guanidinium) phosphite was prepared by neutralization of the corresponding amounts of bis(guanidinium) carbonate (CH₆N₃)₂CO₃ and phosphoric acid H₃PO₃.

In particular, for bis(guanidinium) trioxofluorophosphate there were 0.48 g (CH₆N₃)₂CO₃ and 0.40 g (NH₄)₂PO₃F·H₂O used; for bis(guanidinium) trioxofluorophosphate-phosphite (3:1) there were 0.64 g of (CH₆N₃)₂CO₃ used, 0.40 g of (NH₄)₂PO₃F·H₂O and 0.07 g of H₃PO₃ used; for bis(guanidinium) trioxofluorophosphate-phosphite (1:1) there were 0.96 g of (CH₆N₃)₂CO₃, 0.40 g of (NH₄)₂PO₃F·H₂O and 0.22 g of H₃PO₃ used; for bis(guanidinium) trioxofluorophosphate-phosphite (1:3) there were 0.96 g of (CH₆N₃)₂CO₃, 0.20 g of (NH₄)₂PO₃F·H₂O and 0.32 g of H₃PO₃ used; for bis(guanidinium) phosphite there were 1.10 g of (CH₆N₃)₂CO₃ and 0.5 g of H₃PO₃ used. It seems that the title crystals are highly soluble in water because they started to crystallize in quite a reduced volume of the solution which was estimated to be about 5 mL. Hereafter the title compounds will be referred as (I), (II), (III), (IV), and (V) in correspondence with the decreasing content of trioxofluorophosphate anion [PO₃F]²⁻.

2.2. Structure Determination and Refinement

The intensity data were collected on a SuperNova dual (Cu at zero point) diffractometer (Agilent Technologies, Oxfordshire, UK) equipped with an Atlas S2 CCD detector. The absorption correction was carried out by the analytical method based on the knowledge of the sample shape except for the phosphite where the multi-scan method was applied. The applied software for data collection and reduction was [12]. The structures were solved by [13] and refined by [14]. It should be noted that the software used [14] does not refine the weighting scheme. It uses experimentally obtained reflection weights. The calculated goodness-of-fit S bears its original meaning, indicating the amount of information available in data with respect to the information provided by the structural model. The extinction correction was carried out by the method [15]. It turned out that the structural data of the pure trioxofluorophosphate, (I), were somewhat worse. Therefore, the indicator wR(F²) is somewhat worse than that in other title structures. The weighting scheme of (I) also differs from the other title structures (

Table 1. Crystal data and details of data collection and refinement for compounds (I), (II) and (III).

Compound	I	II	III
Chemical formula	FO3P·2(CH6N3)	(CH6N3)4(PO3F)0.744(PO3H)0.256 (PO3F)0.687(PO3H)0.313	(CH6N3)4(PO3F)0.507(PO3H)0.493 (PO3F)0.494(PO3H)0.506
Diffractometer	SUPERNOVA Dual	SUPERNOVA Dual	SUPERNOVA Dual
Detector	AtlasS2 (CCD)	AtlasS2 (CCD)	AtlasS2 (CCD)
Shape and size of the sample (mm)	prism 0.183 × 0.232 × 0.399	prism 0.047 × 0.227 × 0.414	prism 0.068 × 0.096 × 0.447
colour	colourless	colourless	colourless
Mr	218.1	426.02	418.29
Crystal system, space group	Monoclinic, Cm	Monoclinic, Cm	Monoclinic, Cm
T (K)	95	95	95
a, b, c (Å)	13.191 (3)	13.1423 (4)	13.1069 (7)
	7.2940 (6)	7.2997 (1)	7.3028 (3)
	11.644 (3)	11.6364 (10)	11.6406 (16)
β (°)	119.80 (4)	119.614 (6)	119.560 (11)
V (Å3)	972.2 (5)	970.51(11)	969.18 (18)
Z	4	2	2
X-ray density (g/cm3)	1.4903	2.63	2.59
F000	456	447	440
μ (mm−1)	2.681	2.627	2.587
Radiation, λ(Å)	Cu Kα, 1.54184	Cu Kα, 1.54184	Cu Kα, 1.54184
monochromator	mirror	mirror	mirror
θ – limit (°)	4.38 – 75.08	4.37 – 75.02	4.37 – 74.82
hmin, hmax	−16, 11	−15, 16	−16, 16
kmin, kmax	−8, 8	−9, 9	−8, 8
lmin, lmax	−11, 14	−14, 14	−14, 14
Absorption correction	Analytical [12]	Analytical [12]	Analytical [12]
Tmin, Tmax	0.559, 0.713	0.554, 0.908	0.536, 0.842
No. of measured, independent and observed [I > 3σ(I)]	3702, 1519, 1512	6952, 2050, 2047	7256, 2129, 2124
Rint	0.018	0.02	0.016
(sin θ/λ)max (Å−1)	0.627	0.627	0.626
Refinement/ Weighting scheme	|F|2, w = 1/[σ2(I) + 0.0016(I)2]	|F|2, w = 1/[σ2(I) + 0.0004(I)2]	|F|2, w = 1/[σ2(I) + 0.0004(I)2]
R[F2 > 3σ(F2)]	0.036	0.022	0.021
wR(F2)	0.102	0.06	0.056
S	2.17	2.16	2.07
No. of reflections	1520	2050	2129
No. of parameters	171	177	177
Extinction correction [14]	2000(200)	730(100)	660(90)
No. of constraints	14	16	16
No. of restraints	12	16	16
Δρmax, Δρmin (e Å−3)	0.55, −0.25	0.16, −0.18	0.24, −0.27
No. of the Friedel pairs	460	968	1052
Flack param.	0.06 (3)	0.003 (14)	−0.006 (12)

Note: The used software does not refine the weighting scheme. It uses experimentally obtained weights of reflections and therefore the calculated S has its original meaning, indicating the amount of information available in data with respect to the information provided by the structure model. Such S rarely reaches 1.

). All the hydrogens in the title structures were clearly discernible in the difference electron density map except for the hydrido hydrogen, which belonged to molecule H₃PO₃ in the mixed crystals. The displacement parameters of the primary amine hydrogens were constrained to U_iso(H_{primary amine}) = 1.2U_eq (N_{primary amine}). Since the hydrido hydrogens could not be clearly resolved in the difference electron density maps of the mixed crystals (Figure 1) the following model was adopted: The P1-F1 and P2-F2 distances were restrained to 1.574 and 1.576 Å, respectively, with the weight [16] equal to 0.001 Å for both distances. These values were taken from the refinement of the title structure of bis(guanidinium) trioxofluorophosphate. The positional parameters of the hydrido hydrogens were restrained by the distance restraints P1-H1 and P2-H2 equal to the respective values 1.28 and 1.31 Å with the weight equal to 0.01 Å. The latter distance values were taken from the refinement of the title structure of bis(guanidinium) phosphite. The distance restraints were necessary to obtain a reasonable model because in the mixed crystals of bis(guanidinium) trioxofluorophosphate-phosphite (0.716/0.284), bis(guanidinium) trioxofluorophosphate-phosphite (0.501/0.499), and bis(guanidinium) trioxofluorophosphate-phosphite (0.268/0.732) the unrestrained refinement of F1/H1 and F2/H2 atoms yielded the respective P1-F1 distances 1.560(3), 1.538(3), 1.478(8) Å and the respective P1-F2 distances 1.5606(16), 1.553(2) and 1.529(7) Å.

The occupational parameters of F1 and H1 as well as F2 and H2 were refined under the condition that the sum of these occupancies equalled to 1. The refined values of the occupational parameters F1|F2 converged to the respective values 0.744(7)|0.687(7); 0.507(7)|0.494(6); 0.270(9)|0.265(9), which corresponded to the mixed crystals with the proportion of the trioxofluorophosphate in descending order. (The given values in the formula are the average of the occupations pertinent to each site; the content of the phosphite is the complement to 1.) The positional parameters of the primary amine hydrogens were restrained by the distance 0.88 Å with the weight equal to 0.02 Å. The values of the Flack parameters for each of the title structures indicated that all the samples were inversion-twin monodomain crystals. The structures of bis(guanidinium) trioxofluorophosphate and bis(guanidinium) trioxofluorophosphate-phosphite (0.732/0.268) have inverted coordinates (1 − x, 1 − y, 1 − z) regarding bis(guanidinium) trioxofluorophosphate-phosphite (0.501/0.499) and bis(guanidinium) trioxofluorophosphate-phosphite (0.284/0.716) and bis(guanidinium) phosphite in order to maintain the Flack parameter approximating to 0.0 in all the title structures.

The aryl and the methyl H atoms were constrained with the C_aryl-H = 0.93, C_methyl-H = 0.96 and C_methine-H = 0.98 Å and U_iso(H_aryl/methine) = 1.2U_eq(C_aryl/methine), and U_iso(H_methyl) = 1.5U_eq(C_methyl). The positional parameters of the secondary amine H atoms H1na and H1nb were refined freely while U_iso(H_{secondary amine}) = 1.2U_eq(N_{secondary amine}). The details regarding the X-ray diffraction experiment and the refinement are given in Table 1 and

Table 2. Crystal data and details of data collection and refinement for compounds (IV) and (V).

Compound	IV	V
Chemical formula	(CH6N3)4(PO3F)0.271(PO3H)0.729 (PO3F)0.265(PO3H)0.735	HO3P·2(CH6N3)
Diffractometer	SUPERNOVA Dual	SUPERNOVA Dual
Detector	AtlasS2 (CCD)	AtlasS2 (CCD)
Shape and size of the sample (mm)	prism 0.183 × 0.232 × 0.399	prism 0.047 × 0.227 × 0.414
colour	colourless	colourless
Mr	409.92	200.1
Crystal system, space group	Monoclinic, Cm	Monoclinic, Cm
T (K)	95	95
a, b, c (Å)	13.0598 (4)	13.0231 (4)
	7.3078 (2)	7.3185 (3)
	11.6352 (9)	11.6405 (4)
β (°)	119.509 (6)	119.484 (3)
V (Å3)	966.39(10)	965.77 (7)
Z	2	4
X-ray density (g/cm3)	1.4087	1.3765
F000	433	424
μ (mm−1)	2.546	2.493
Radiation, λ(Å)	Cu Kα, 1.54184	Cu Kα, 1.54184
monochromator	mirror	mirror
θ – limit (°)	4.37 - 74.69	4.37 - 75.02
hmin, hmax	−15, 16	−15, 16
kmin, kmax	−8, 8	−9, 9
lmin, lmax	−14, 13	−14, 14
Absorption correction	Analytical [12]	Multi-scan [12]
Tmin, Tmax	0.586, 0.850	0.672, 0.779
No. of measured, independent and observed [I > 3σ(I)]	4134, 1766, 1760	6465, 2066, 2047
Rint	0.013	0.018
(sin θ/λ)max (Å−1)	0.626	0.624
Refinement/ Weighting scheme	|F|2, w = 1/[σ2(I) + 0.0004(I)2]	|F|2, w = 1/[σ2(I) + 0.0004(I)2]
R[F2 > 3σ(F2)]	0.022	0.016
wR(F2)	0.069	0.044
S	2.35	1.45
No. of reflections	1766	2066
No. of parameters	177	163
Extinction correction [14]	1200(120)	1970(80)
No. of constraints	16	16
No. of restraints	16	12
Δρmax, Δρmin (e Å−3)	0.22, −0.28	0.16, −0.10
No. of the Friedel pairs	694	1005
Flack param.	−0.023 (19)	0.016 (11)

. The employed graphical programs were PLATON [17] and DIAMOND [18]. The Hirshfeld surfaces (see reference [19]) were visualized by [20].

Supplementary publications CCDC 2211221–2211225, for bis(guanidinium) trioxofluorophosphate/phosphite in the ratio 1/0, 0.716/0.284, 0.501/0.499, 0.268/0.732, 0/1, respectively, contain crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

2.3. Vibrational Spectroscopy

Infrared spectra were recorded using the DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectroscopic technique (samples mixed with KBr at 1:50 ratio and ground in agate mortar) on a Thermo Scientific (Madison, WI, USA) Nicolet 6700 FTIR spectrometer with 2 cm⁻¹ resolution and Happ-Genzel apodization in the 400–4000 cm⁻¹ region.

The Raman spectra of the title salts were taken on many unoriented small crystals (size about 0.1 mm) which were separated from the crucible. The spectra were recorded on a Thermo Scientific (Madison, WI, USA) DXR Raman Microscope interfaced to an Olympus (Olympus Optical Co. Ltd., Tokyo, Japan) microscope (objectives 10× and 50×) using a depolarized 780 nm frequency-stabilized single mode laser (10 mW laser power, 45–3410 cm⁻¹ spectral range, 400 lines/mm grating). The spectrometer was calibrated using a software-controlled calibration procedure employing multiple neon emission lines (wavelength calibration), multiple polystyrene Raman bands (laser frequency calibration) and standardized white light sources (intensity calibration). The Raman spectra samples were also collected on a dispersive confocal Raman microscope MonoVista CRS+ (Spectroscopy &Imaging GmbH, Warstein, Germany) interfaced to an Olympus microscope (objectives 20× and 50×) using a 785 nm diode excitation laser (10 mW laser power, 40–3800 cm⁻¹ spectral range, 300 lines/mm grating). The spectrometer was wavelength- and intensity-calibrated using a software-controlled auto-alignment and calibration procedure with mercury and Ne-Ar lamps.

2.4. Second Harmonic Generation

The second harmonic signal was studied on powder samples of each compound using the Kurtz-Perry powder technique [21] with potassium dihydrogen phosphate (KDP) as a reference material. As a light source BMI (Evry, France) Q-switched Nd-YAG laser (1064 nm, ~6 ns pulses of 2 mJ, 20 Hz repetition rate) was used and the filtered second harmonic signal at 532 nm was detected by a photomultiplier and a boxcar averager.

3. Results

3.1. Crystal Structure

Information regarding the crystal data and the refinement of the title crystal-chemically isotypic structures [22] is given in Table 1 and Table 2. The refinement of the data which was extracted from CIF of XOMQAR regarding the structure determination of bis(guanidinium) trioxofluorophosphate [3] confirmed that the previous structure determination referred to the same phase as it is described in this article.

Each of the title structures consists of three symmetry-independent guanidinium cations and two symmetry-independent anions (trioxofluorophosphate or phosphite). The anions are situated in special positions, i.e., on the symmetry planes (the Wyckoff position 2a). Two of the symmetry-independent cations, i.e., those with C1 and C2 atoms, as well as both anions, are situated in a special position (the Wyckoff position 2a), too. The guanidinium cation centered on C3 is in a general position 4b. The molecules are shown in Figure 2 and Figure 3 and in the Figures S1–S3, Supplementary Materials. Figure 4 shows the packing of the title molecules in bis(guanidinium) trioxofluorophosphate.

As in the previous structure determinations of the mixed trioxofluorophosphates and phosphites, the respective P and O atoms of the latter molecules occupy the same positions. On the other hand, the fluorine and the hydrido hydrogen atoms are oriented in the same direction, while P-F and P-H_hydrido bonds are nearly parallel. In the title structures of the mixed crystals, the electron density along the P-F direction is smeared, and the hydrido hydrogen is not observable (Figure 1). Interestingly, in the title mixed crystals, the maxima of the electron density of the P-F/H_hydrido region are shifted towards each P atom compared to the pure trioxofluorophosphate. Thus, chemically reasonable models could be obtained by the refinement with the restrained P-F and P-H_hydrido distances (see the section of this article which describes the refinement).

The lattice parameter b is almost unaffected by the substitution of the trioxofluorophosphate by the phosphite. This observation agrees with the fact that the P-F or P-H_hydrido are almost perpendicular to the unit-cell axis b.

The molecules are interconnected by N-H⋯O hydrogen bonds of moderate strength [23]. The hydrogen bonds are listed in

Table 3. Hydrogen-bond interactions in the title structure determinations for compounds (I)–(V).

Compound	H-Bond	D-H (Å)	H-A (Å)	D···A (Å)	D-H···A (°)	Compound	H-Bond	D-H (Å)	H-A (Å)	D···A (Å)	D-H···A (°)
I	N1-H1n1···O2 ii	0.92(3)	1.97(3)	2.874(3)	167(3)	I	N5-H1n5···O2 iv	0.82(3)	2.39(4)	3.118(5)	148(4)
II		0.874(16)	1.998(16)	2.8677(12)	173.0(17)	II		0.860(19)	2.33(2)	3.096(2)	149(2)
III		0.879(15)	1.993(15)	2.8656(11)	171.8(14)	III		0.85(2)	2.14(2)	2.980(2)	170(2)
IV		0.90(2)	1.97(2)	2.8610(17)	174(3)	IV		0.86(2)	2.30(3)	3.066(3)	148(3)
V		0.889(13)	1.973(13)	2.8564(10)	171.8(12)	V		0.826(16)	2.31(2)	3.0541(16)	149.7(16)
I	N2-H1n2···O1 iii	0.86(3)	2.06(3)	2.911(3)	172(5)	I	N5-H2n5···O1	0.85(5)	2.17(5)	2.988(5)	162(3)
II		0.847(16)	2.070(16)	2.9106(12)	171(3)	II		0.85(3)	2.14(3)	2.988(2)	171(2)
III		0.840(15)	2.072(16)	2.9079(12)	173(2)	III		0.85(2)	2.14(2)	2.980(2)	170(2)
IV		0.867(19)	2.05(2)	2.9049(17)	171(3)	IV		0.87(3)	2.13(3)	2.980(3)	167(3)
V		0.849(13)	2.066(14)	2.9051(11)	170(2)	V		0.87(2)	2.12(2)	2.9733(18)	169.4(16)
I	N2-H2n2···O4 x	0.85(3)	2.22(3)	2.976(4)	148(3)	I	N6-H1n6···O2	0.85(5)	2.10(5)	2.940(5)	171(3)
II		0.853(14)	2.214(16)	2.9676(15)	147.1(18)	II		0.85(3)	2.10(3)	2.939(2)	169.0(16)
III		0.847(13)	2.221(15)	2.9686(16)	147.1(17)	III		0.82(2)	2.13(2)	2.946(2)	170.2(14)
IV		0.862(16)	2.21(2)	2.967(2)	147(2)	IV		0.85(3)	2.10(3)	2.945(3)	169.5(19)
V		0.838(11)	2.213(13)	2.9650(12)	149.4(15)	V		0.85(2)	2.11(2)	2.9475(18)	171.0(12)
I	N3-H1n3···O4 i	0.85(3)	2.08(3)	2.915(4)	169(3)	I	N6-H2n6···O4 iv	0.84(3)	2.18(3)	2.972(4)	158(4)
II		0.852(19)	2.06(2)	2.9106(17)	173.5(16)	II		0.854(15)	2.123(17)	2.9698(17)	171(2)
III		0.843(17)	2.067(18)	2.9040(15)	171.6(19)	III		0.863(14)	2.121(16)	2.9668(18)	166(2)
IV		0.85(2)	2.06(2)	2.901(2)	170(2)	IV		0.861(17)	2.123(19)	2.965(2)	166 (3)
V		0.848(15)	2.055(16)	2.8953(13)	170.6(17)	V		0.828(13)	2.154(15)	2.9650(13)	166.3(18)
I	N4-H1n4···O3	0.88(4)	2.07(4)	2.943(4)	171(4)	I	N7-H1n7···O3 v	0.86(3)	2.17(3)	2.987(4)	161(3)
II		0.86(2)	2.08(2)	2.9343(16)	171.9(17)	II		0.861(16)	2.129(18)	2.9791(17)	169.1(18)
III		0.852(18)	2.086(19)	2.9300(15)	170.7(18)	III		0.865(15)	2.128(17)	2.9752(17)	166.1(17)
IV		0.85(2)	2.09(2)	2.931(2)	170(3)	IV		0.859(19)	2.13(2)	2.970(2)	167(2)
V		0.841(15)	2.089(16)	2.9268(14)	174.3(15)	V		0.837(13)	2.144(14)	2.9671(13)	167.6(14)
I	N4-H2n4···O4 iv	0.90(4)	2.03(4)	2.909(5)	166(3)	I	N7-H2n7···O2 iv	0.85(3)	1.97(3)	2.810(4)	167(4)
II		0.87(2)	2.05(2)	2.897(2)	167.5(19)	II		0.855(15)	1.990(14)	2.8225(15)	164(2)
III		0.87(2)	2.03(2)	2.8914(17)	168.1(17)	III		0.852(15)	2.005(13)	2.8277(15)	162(2)
IV		0.88(3)	2.01(3)	2.884(3)	171(2)	IV		0.856(17)	1.982(16)	2.826(2)	169(3)
V		0.846(19)	2.035(18)	2.8761(15)	172.8(15)	V		0.848(13)	2.013(12)	2.8372(11)	163.8(18)

Note: Symmetry codes: (i) x, −y + 1, z; (ii) x + 1, −y + 1, z; (iii) x + 1, y, z; (iv) x − ½, −y + ½, z; (v) x − ½, y + ½, z; (x) x, y, z − 1.

, which shows the contraction of the donor-acceptor bond lengths towards the phosphite end of the series with two exceptions for N6-H1n6⋯O2 and N7-H1n7⋯O3^v. Cf. P-O bond lengths, which are about 0.2 Å shorter in the trioxoflourophosphate than in the phosphite anion. The comparison of the other bond lengths is listed in Table S1, Supplementary Materials.

The most common graph set motifs [24] are $R_2^2\left(8\right)$ in which all the cations are involved: C1-N1-H1n1⋯O2ⁱⁱ-P1^vi-O1^vi⋯H1n2ⁱ-N2ⁱ; C1-N2-H1n2⋯O1ⁱⁱⁱ-P1ⁱⁱⁱ-O2ⁱⁱⁱ-H1n1ⁱ-N1; C2-N4ⁱ-H1n4ⁱ-O3-P2^vii-O4ⁱ-H1n3-N3; C2-N4-H2n4-O4^iv-P2^v-O4^v-H2n4ⁱ-N4ⁱ; C2-N3-H1n3ⁱ -O4-P2-O3-H1n4-N4; C3-N6-H2n6-O4^iv-P2^iv-O3⋯H1n7-N7; C3-N6-H1n6-O2-P1-O1- H2n5-N5 with the symmetry codes (i) x, 1 − y, z; (ii) 1 + x, 1 − y, z; (iii) 1 + x, y, z; (iv) −1/2 + x, 1/2 –y, z; (v) −1/2 + x, 1/2 + y, z; (vi) 1 + x, 1 + y, z; (vii) x, 1 + y, z.

Moreover, there are also present graphs set motifs $R_2^1\left(6\right)$ for C3-N5-H1n5…O2^v…H2n7-N7 as well as $R_4^2\left(10\right)$ C1^viii-N2^viii-H2n2^viii⋯O4⋯H1n3ⁱ-N3 -H1n3⋯O4ⁱ⋯H2n2^ix-N2^ix with the symmetry codes (viii) x, y, 1 + z, (ix) x, 1 − y, 1 + z.

Fluorine in the title trioxofluorophosphates (Figure 4) avoids hydrogen bonding, as was observed previously. This phenomenon is similar to that observed in C-F bonded fluorine [25,26] and can be generalized to other fluorine-containing molecules with oxygen ligands [27]. The closest interatomic distances to the fluorines or the hydrido hydrogens are given in

Table 4. The closest intermolecular interactions to anionic F and H_hydrido hydrogen in the title structures for compounds (I)–(V).

Compound	H-interaction	D-H (Å)	H-A (Å)	D···A (Å)	D-H···A (°)
I	N5-H1n5···F1 xi	0.83(4)	2.86(3)	3.421(4)	126(3)
II	N5-H1n5···F1 xi	0.860(19)	2.824(18)	3.419(2)	127.8(18)
II	N5-H1n5···H1 xi	0.860(19)	2.82(13)	3.45(16)	131(5)
III	N5-H1n5···F1 v	0.824(18)	2.838(18)	3.425(3)	129.9(18)
III	N5-H1n5···H1 v	0.824(18)	2.87(6)	3.50(7)	134(3)
IV	N5-H1n5···F1 v	0.86(2)	2.81(2)	3.407(7)	128(2)
IV	N5-H1n5···H1 v	0.86(2)	2.86(6)	3.50(7)	132(3)
V	N5-H1n5···H1 v	0.826(16)	2.883(19)	3.524(16)	136.0(15)
I	N2-H1n2···F2 viii	0.80 (3)	3.05(4)	3.293 (4)	101(3)
II	N2-H1n2···F2 viii	0.847(16)	3.05(2)	3.289 (2)	98.7(15)
II	N2-H1n2···H2 viii	0.847(16)	3.14(14)	3.34 (12)	96.3(19)
III	N2-H1n2···F2 x	0.840 (15)	3.035(18)	3.425(3)	100.1(13)
III	N2-H1n2···H2 x	0.840 (15)	3.08(8)	3.50(7)	98.0(14)
IV	N2-H1n2···F2 x	0.867(19)	3.04(3)	3.407(7)	99.6(18)
IV	N2-H1n2···H2 x	0.867(19)	2.98(6)	3.50(7)	98.7(18)
V	N2-H1n2···H2 x	0.849(13)	3.09(2)	3.524(16)	97.4(11)
I	N2-H2n2···F2 viii	0.83 (3)	2.93(4)	3.293(4)	109(3)
II	N2-H2n2···F2 viii	0.853 (14)	2.97(2)	3.289(2)	104.4(15)
II	N2-H2n2···H2 viii	0.853 (14)	2.97(11)	3.34(12)	109(2)
III	N2-H2n2···F2 x	0.847 (13)	2.95(2)	3.288(3)	105.9(15)
III	N2-H2n2···H2 x	0.847 (13)	2.92(6)	3.31(6)	110.0(17)
IV	N2-H2n2···F2 x	0.826(16)	2.99(3)	3.294(6)	103.3(19)
IV	N2-H2n2···H2 x	0.826(16)	2.88(5)	3.23(5)	106(2)
V	N2-H2n2···H2 x	0.838 (11)	2.92(2)	3.305(14)	110.3(13)

Note: Symmetry codes: (v) x − ½, y + ½, z; (viii) x, y, z + 1; (x) x, y, z − 1; (xi) x + ½, y − ½, z.

. The fingerprints of the Hirshfeld surface with w(r) = 0.5 [19,20,28] neatly illustrate the prevailing N-H⋯O intermolecular interactions over F⋯H and H_hydrido in bis(guanidinium) trioxofluorophosphate as well as in bis(guanidinium) phosphite (Figure 5 and Figure S4, Supplementary Materials).

3.2. Vibrational Spectroscopy

The FTIR and the Raman spectra of compounds (I)–(V), which were recorded at room temperature, are presented in

Table 5. FTIR and Raman spectra of compounds (I)–(V).

	gu2PO3F				gu2(PO3F)x(HPO3)y										gu2HPO3
					Compound (II)				Compound (III)		Compound (IV)
	Compound (I)				x = 0.716 y = 0.284				x = 0.501 y = 0.499		x = 0.268 y = 0.732				Compound (V)
	FTIR		Raman		FTIR		Raman		Raman		FTIR		Raman		FTIR		Raman
Assignment	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	cm−1	Int. *	Assignment
v N-H(…O)	3325	71			3330	91					3330	73			3310	70			v N-H(…O)
	3120	84			3115	99					3110	86			3100	83
	2830	50			2830	72					2835	48			2835	48
?											2377	10							?
					2307	62	2305	33	2307	9	2305	50	2308	10	2305	52	2306	14	v PH
					2299	62	2298	17	2300	9	2297	51	2298	29	2296	54	2297	13
?	2266	21
	2182	sh
	2120	sh
											2040	8			2040	12			?
											2015	8			2015	12
δ NH2, v CN	1683	87	1682	5							1694	67	1697	6	1697	84	1697	2	δ NH2, v CN
	1669	sh	1660	sh	1667	99	1677	5	1665	3	1658	27	1659	6	1663	77	1660	3
	1588	62	1586	6	1588	80	1592	6	1591	2	1589	61	1587	sh	1588	59	1595	2
			1568	sh					1567	4			1564	5	1571	sh
?	1389	43
	1381	35
					1256	62
															1221	12			?
ρ NH2	1166	sh							1160	2					1170	44	1165	2	ρ NH2
vas PO3, ρ NH2	1104	80			1104	98									1110	sh			vas PO3, ρ NH2
ρ NH2, δ PO3	1053	sh	1059	5							1065	88			1064	93	1059	3	δ PO3, ρ NH2
															1046	sh
					1022	72					1022	59			1022	62			δ PH, ρ NH2
vs CN3	1012	49	1013	100	1010	74	1011	134	1013	100	1010	56	1013	95	1010	57	1012	100	vs CN3
vs PO3	997	72	1000	sh	997	89			1000	25	995	66	1001	sh
	979	sh	975	5	973	75	973	13	977	16	974	75	977	30	975	80	976	56	vs PO3
?	916	7
	876	37													859	sh			?
v PF	811	79	813	13	811	86	811	11	816	3	810	67	811	3
δ NCN	750	62			752	73					756	64			757	64			δ NCN
γ N-H(…O), τ NH2			675	6					678	2	661	85			660	84	678	3	γ N-H(…O), τ NH2
											649	86
	625	80			621	85
δ PO3	580	80	576	8	581	85	572	3	564	sh	560	90	570	11	570	90	569	17	δs PO3
							556	2	552	9					560	88
δ NCN, τ NH2	549	89	547	11			546	5			541	100			544	93	550	9	δ NCN, τ NH2
									525	9	529	98	534	7			532	sh
δ FPO3, δ NCN, τ NH2	521	100	517	18	521	100	519	19
					481	77			473	6					478	87			δ PO3
					464	80	463	8	466	6	464	98	465	10	463	100	469	14
ρ PO3			377	10			377	9	385	6			387	3
External modes			183	23			185	27	189	10			189	13			188	sh	External modes
			143	84			143	100	146	51			147	59			146	38
			123	47			130	sh	130	38			131	47			130	37
									112	36							115	34
			84	45			84	78	88	95			88	100			90	94

* The intensities of IR and Raman bands are presented using a relative scale of 0–100.

and depicted in Figure 6a–d. Unfortunately, the compound (III)—Gu₂(PO₃F)₀.₅₀₁ (HPO₃)₀.₄₉₉—was not prepared in a sufficient amount for the infrared spectra collection by DRIFT technique; therefore, it is missing in Table 5 as well as in Figure 6c,d.

The number of the normal modes in all the title salts was determined by the extended nuclear site group analysis [29]. All the crystals belong to the monoclinic system with Cm ($C_s^3$) space group and contain 30 atoms per asymmetric units (Z = 4). Twenty atoms occupy the Wyckoff position 4b (C₁), while 10 atoms occupy the Wyckoff position 2a (C_s). The asymmetric unit contains three guanidinium(1+) cations and two anions—phosphites [HPO₃]²⁻, trioxofluorophosphates [PO₃F]²⁻ or their mixture according to their variable proportion. The symmetry analysis of the optical vibrational modes yielded 60A′ + 60A″ representations for the internal modes and 28A′ + 29A″ representations for the external modes (see Table S2, Supplementary Materials). The correlation diagrams concerning guanidinium cations, occupying 1 (C₁) and m (C_s) sites, trioxofluorophosphate and phosphite anions, which occupy m (C_s) sites, are presented in Table S3a–c, Supplementary Materials. However, the well-resolved Davydov splitting (see Tables S2 and S3, Supplementary Materials) was not observed in the recorded IR and Raman spectra of the prepared salts, except for a doublet at ~2306 cm⁻¹ and ~2300 cm⁻¹, which will be discussed below.

A detailed assignment of the recorded bands is given in Table 5. It is based on previous studies of guanidine salts [9,10], trioxofluorophosphates [30,31,32,33] and phosphites [34,35,36,37,38,39,40] as well as on the present structure determinations. The assignment of the bands of the stretching and out-of-plane bending vibrations of the N-H bonds (see Table 5) involved in the hydrogen interactions is based on the correlation curves [41], which concern a vibrational band position and the corresponding hydrogen bond lengths.

The overall character of the recorded vibrational spectra is in accordance with the expected vibrational manifestations of the hydrogen-bonded salts containing guanidinium cations and phosphite and/or trioxofluorophosphate anions. The following paragraphs summarise and highlight selected aspects observed in the spectra.

The Raman spectra of all the compounds are dominated by very strong bands of symmetric stretching vibrations of guanidinium (CN₃) skeleton at ~1010 cm⁻¹ and by strong bands of external modes located below 200 cm⁻¹. The increasing proportion of the phosphite anion in compounds (II)–(V) is reflected by the increasing intensity of the bands of symmetric stretching PO₃ vibrations at ~975 cm⁻¹ as well as by changes in the intensities and the band area ratio concerning the doublet (~2306 and ~2300 cm⁻¹) of the P-H stretching vibrations. The changes in the intensities of the bands forming this doublet, i.e., comparison of the relative band areas, are listed in

Table 6. Comparison of the relative peak area of two bands of stretching PH vibrations.

Compound	Stoicheiometric Coefficients x/y	Area of the Band at 2300 cm−1	Area of the Band at 2306 cm−1	Ratio of the Band Areas
II	0.716/0.284	941	1679	0.56
III	0.501/0.499	1046	763	1.37
IV	0.268/0.732	3195	1137	2.81
V	0/1	763	680	1.12

and depicted in Figure 7. It is obvious that the increasing proportion of the phosphite anion in compounds (II)–(IV) leads to the increase of the intensity of the band at ~2306 cm⁻¹ and simultaneously to the decrease of the intensity of the band at ~2300 cm⁻¹. Surprisingly, the ratio of band areas of this doublet of the compound (III) is similar to that of in the compound (V), i.e., in the pure phosphite compound. The formation of such a doublet (present in both Raman and IR spectra) could be explained by the Davydov splitting of the vibrational bands due to the presence of more formula units in the unit cell or by the Fermi resonance with some overtone or a combination mode. Similar splitting of ν P-H band was also observed in previously studied organic phosphites [42,43].

Strong, wide and structured bands (3500–2600 cm⁻¹ region) of stretching vibrations of NH₂ groups involved in N-H...O hydrogen bonds dominate the IR spectra of all the compounds. The increasing proportion of the phosphite anions in compounds (II), (IV) and (V) is indicated by the increasing intensity of the bands of stretching P-H vibrations (doublet at ~2305 and ~2300 cm⁻¹), symmetric stretching (~975 cm⁻¹) and bending (~465 cm⁻¹) vibrations of PO₃ groups while a decrease in intensity or disappearance of the bands characteristic for trioxofluorophosphate anions—e.g., bands at 1104, 997 and 811 cm⁻¹—is observed.

3.3. Second Harmonic Generation

Basic measurements of the relative SHG efficiency of the title series of compounds show that the efficiency of the powdered samples is below the KDP standard (1064 nm laser irradiation). The resulting values of relative second harmonic signal intensity I^(2ω) with frequency 2ω equalled 0.6, 0.45, 0.4, 0.35 and 0.3 I^(2ω)(KDP) for the series of compounds (I)–(V), respectively. This means that the efficiency of SHG in the title compounds grows with the proportion of the trioxofluorophosphate anions.

4. Conclusions

The title structure is a new example of a series of isostructural compounds with mixed trioxofluorophosphate and the phosphite anions which were thus far rarely observed. The N-H...O hydrogen bonds with prevailing $R_2^2\left(8\right)$ graph set motifs are the most important intermolecular interactions in the title structures. Fluorine, as it is common in trioxofluorophosphates, is a poor hydrogen-bond acceptor.

The overall character of the recorded FTIR and Raman spectra agrees with crystal structure determinations. Both types of the vibrational spectra are sensitive to the composition of the title structures.

Relative efficiency of SHG (1064 nm laser irradiation) in the series of the title compounds grows with the proportion of [PO₃F]²⁻ anions. This result confirms the potential of the trioxofluorophosphate anion in the crystal engineering of novel molecular NLO materials.