Phosphinochalcogenoic esters, thioesters, and amides have attracted considerable attention due to their applications as drug intermediates , , , , ligands for the design of metal complexes , , , flame retardants for polymeric materials , as well as building blocks in organic and elementoorganic synthesis , , , , . Thioselenophosphinic S-esters and amides are of particular interest as promising selenium sources for the preparation of metal selenide thin films  and selenium-containing nanoparticles .
The oxidative transition-metal free cross-coupling reaction between secondary phosphine selenides and various phenols, thiols or amines using the carbon tetrachloride/triethylamine system under mild conditions resulted in previously unknown phosphinochalcogenoic O-esters, S-esters, and amides in high yields , , . The starting secondary phosphine selenides were prepared via the Trofimov-Gusarova reaction (a halogen-free method from red phosphorus, styrenes and elemental selenium) , , , , .
Previously, we have carried out the conformational analysis of some secondary bis(2-phenylethyl)- and bis(2-phenylpropyl)phosphine arylalkylphosphine selenides ,  and tris(2-pyridyl)phosphine selenide . The data on synthesis, polarity, and conformational analysis using a complex of physical methods (IR spectroscopy, dipole moments, and quantum chemical calculation) of these compounds are presented in .
Results and discussion
In this study, we have determined previously unknown polarities of O-phenyl diphenethylphosphinoselenoate 1, O-(naphthalen-1-yl) diphenethylphosphinoselenoate 2, S-ethyl diphenethylphosphinoselenothioate 3, S-phenyl diphenethylphosphinoselenothioate 4, and P,P-diphenethyl-N-phenylphosphinoselenoic amide 5, carried out quantum chemical calculations of the dipole moments of their possible conformers, and calculated polarities of these conformers according to the vector-additive scheme.
For determining the experimental values of the dipole moments, we used the second Debye method based on the measurement of the dielectric constant of the dilute solutions of the polar substance in a nonpolar solvent. The experimental dipole moments of 1–5 are listed in Table 1, their values are sufficiently high and are typical for the polarities of the compounds of tetra-coordinated phosphorus (2.5–5.0 Debye) .
In the calculations of dipole moments according to the vector-additive scheme (Table 2) we used the theoretical geometric parameters and the following bonds moments: m(P=>Se)=4.00 D, calculated from μexp Et3P=Se ; m(P→N)=0.31 D, calculated from μexp (Me2N)3P ; m(O→P)=0.51 D, calculated from μexp MeC(CH2O)3P=O ; m(S→P)=0.80 D, calculated from μexp 2-methyl-1,3,2-dithiaphosphinane 2-sulfide ; m(Csp3→Р)=0.55 D, calculated from μexp Me3P ; m(Csp3→Csp2)=0.75 D, calculated from μexp C6H5CН3 ; m(СPh→N)=1.73 D, calculated from μexp C6H5NН2 ; m(H→N)=1.31 D, calculated from μexp C6H5NН2 ; m(CPh→O)=0.37 D, calculated from μexp C6H5OH ; m(CPh→S)=0.30 D, calculated from μexp C6H5SH ; m(Csp3→S)=1.14 D, calculated from μexp (CH3)2S ; m(Cnaphthyl→O)=0.33 D, calculated from μexp α-naphthol .
We identified the possible conformations of isolated molecules of 1–5 using the method of density functional theory – B3PW91, and broadened basis 6-311++G(df,p), calculated their theoretical polarities and relative energies, and compared theoretical results with experimental data. According to the data of the theoretical calculations for each of the compounds 1–5, three energetically possible conformers were found (relative energy of preferred conformers is not more than 5.1 kJ·mol−1), which arise as a result of internal rotation about to the P–Csp3, P–O, P–S or P–N bonds.
The possible conformers of 1–5 are shown in Fig. 1–3, 5 and 6, while their relative energy values, theoretical and calculated according to vector-additive scheme dipole moments are listed in Table 2. The percentage of possible conformers (n, %) was found using values of calculated according vector-additive scheme dipole moments of these forms:
In each of three conformers 1а–с (Fig. 1, Table 2) with the symmetrical structure, the phosphorus atom has a pyramid coordination, and substituents at the phosphorus atom are arranged like a propeller relative to the P=Se group. Most energetically advantageous conformer is 1а with gauche,gauche,gauche-orientation of the substituents (Csp3–Csp3 and P–O bonds) relative to the P=Se bond (dihedral angles Se=P–C7–C10 54°, Se=P–C8–C9 56°, Se=P–O–C6 48°). Phenyl rings are located trans relative to the P–Csp3 bonds (dihedral angles P–С7–С10–С17 176°, P–C8–C9–C11 176°). In conformers 1b and 1c the substituents at the phosphorus atom are gauche,gauche,gauche-oriented as well, and the value of the angles are slightly differed from 1a (1b: dihedral angles Se=P–C7–C10 46°, Se=P–C8–C9 55°, Se=P–O–C6 48°; 1с: dihedral angles Se=P–C7–C10 58°, Se=P–C8–C9 47°, Se=P–O–C6 54°), phenyl rings are located gauche and trans in 1b, and trans in 1c relative to the P–Csp3 bonds (1b: dihedral angles P–С7–С10–С17 70°, P–C8–C9–C11 179°; 1с: dihedral angles P–С7–С10–С17 177°, P–C8–C9–C11 175°).
Theoretical and calculated using additive scheme dipole moments of conformers 1а–с are in agreement with the experimental data (Table 2). Relative energies as well as theoretical and calculated dipole moments of all three conformers 1a–c are very close to each other, which indicates the conformational equilibrium in 1. Comparison of the polarity data (Table 2) shows that in the conformational equilibrium of compound 1 the prevailed form is conformer 1b (76%); conformers 1a and 1c are almost equally present in the equilibrium.
Exchange of phenoxyl substituent for a bulkier naphthyl fragment in 2 does not lead to the significant changes in the arrangement of substituents at the phosphorus atom. For phosphinoselenoate 2, there are three energetically preferable conformers (Fig. 2, Table 2). Conformers 2a and 2b are almost equal in relative energy. In both conformers, substituents at the phosphorus atom which has pyramidal structure are symmetrically arranged gauche,gauche,gauche (mutual orientation of the Csp3–Csp3 and P–O bonds relative to the P=Se bond; 2а: dihedral angles Se=P–C1–C4 45°, Se=P–C2–C3 57°, Se=P–O–C17 66°; 2b: dihedral angles Se=P–C1–C4 53°, Se=P–C2–C3 58°, Se=P–O–C17 65°).
The phenyl rings of the phenylethyl fragments are located gauche or trans relative to the Csp3–Csp3 bonds (2а: dihedral angles P–С1–С4–С11 71°, P–C2–C3–C5 −178°; 2b: dihedral angles P–С1–С4–С11 176°, P–C2–C3–C5 176°). In conformer 2c, one of the phenylethyl fragments is oriented trans, and second phenylethyl as well as the phenoxyl group are oriented gauche relative to the P=Se bond (dihedral angles Se=P–C1–C4 46°, Se=P–C2–C3 178°, Se=P–O–C17 59°), the phenyl rings maintain gauche and trans positions relative to the Csp3–Csp3 bonds (dihedral angles P–С1–С4–С11 73°, P–C2–C3–C5 176°).
Comparison of the values of theoretical, calculated and experimental dipole moments of 2a, 2b, and 2c conformers also supports conformational equilibrium in solution of 2, however, while in 1 one conformer predominates, in 2 the percentages of 2a, 2b, and 2c forms are practically equal.
In phosphinoselenothioate 3, oxygen-containing fragment at the phosphorus atom replaced by the thioethyl group. The minimum of relative energy corresponds to the conformer 3a, and two other preferred conformers 3b and 3c have higher energies by 3.1 and 3.6 kJ mol−1 respectively (Fig. 3, Table 2). In conformer 3a the symmetrically arranged phenylethyl substituents have gauche orientation relative to the P=Se bond (dihedral angles Se=P–C1–C4 −55°, Se=P–C2–C3 54°), while the thioethyl group is in eclipsed cis position (dihedral angle Se=P–S–C17 is only 4°). The phenyl rings of the phenylethyl fragments are trans arranged relative to the Csp3–Csp3 bonds (dihedral angles P–С1–С4–С11 178°, P–C2–C3–C5 −179°). In less symmetrical conformers 3b and 3c, the arrangement of substituents at the phosphorus atom remains (3b: dihedral angles Se=P–C1–C4 −59°, Se=P–C2–C3 −55°, Se=P–S–C17 4°; 3c: dihedral angles Se=P–C1–C4 45°, Se=P–C2–C3 49°, Se=P–S–C17 22°, which corresponds to the gauche,gauche,cis-orientation of the Csp3–Csp3 and S–Csp3 bonds relative to the P=Se group). In phenylethyl fragments of 3b aromatic rings are trans arranged relative to the Csp3–Csp3 bonds (dihedral angles P–С1–С4–С11 174°, P–C2–C3–C5 177°), and in 3с – gauche and trans (dihedral angles P–С1–С4–С11 71°, P–C2–C3–C5 179°).
In all three conformers 3a–c eclipsed orientation of the P=Se and S-Csp3 bonds of the thioethyl fragment (Fig. 4) is favorable for the formation of intramolecular hydrogen interaction P=Se⋯H–Csp3 (Se⋯H distance in 3a – 2.76 A, 3b – 2.76 A, 3c – 2.81 A) . These H-contacts define the lower value of polarity of phosphine selenide 3 in comparison with the compounds 1, 2, 4, and 5 (Table 1), since it is well known that compounds with an intramolecular hydrogen bond are usually characterized by low values of dielectric permittivity and, accordingly, dipole moments .
Furthermore, analysis of the experimental frequencies of the stretching vibrations of the P=Se bond in 1–5 shows lowest value of ν P=Se 527 cm−1 for the phosphinoselenothioate 3  (ν P=Se 583 cm−1 for 1 ; ν P=Se 587 cm−1 for 2 ; ν P=Se 574 cm−1 for 4 ; and ν P=Se 577 cm−1 for 5 ), which also supports to the existence of intramolecular P=Se⋯H–Csp3 contact.
The values of theoretical, calculated according to additive scheme, and experimental dipole moments are in good agreement (Table 2). Based on a comparison of the polarities of 3a–c conformers, we believe that in solution phosphinoselenothioate 3 exists as conformational equilibrium, in which 3a and 3b forms are equally probable, and the conformer 3c is minor.
Replacement of ethyl radical for a phenyl in 4 leads to a change of this substituent orientation (Fig. 5, Table 2). In conformer 4a, substituents at the phosphorus atom are arranged symmetrically in the form of a propeller, and have gauche,gauche,gauche-orientation of the P–Csp3 bonds relative to the P=Se bond (dihedral angles Se=P–C7–C10 53°, Se=P–C8–C9 51°, Se=P–S–C6 51°), the phenyl rings under the rotation about the Csp3–Csp3 bonds occupy trans position relative to the P–Csp3 bonds (dihedral angles P–С7–С10–С17 −176°, P–C8–C9–C11 −177°). Energies of conformers 4b and 4c are close to that of 4a, the substituents at the phosphorus atom are arranged gauche,gauche,gauche (4b: dihedral angles Se=P–C7–C10 45°, Se=P–C8–C9 49°, Se=P–S–C6 50°; 4с: dihedral angles Se=P–C7–C10 −50°, Se=P–C8–C9 56°, Se=P–S–C6 −31°), the phenyl rings are trans-oriented relative to the P–Csp3 bonds in 4с (dihedral angles P–С7–С10–С17 178°, P–C8–C9–C11 −176°) and gauche,trans – in 4b (dihedral angles P–С7–С10–С17 71°, P–C8–C9–C11 −179°). Among the energetically preferred conformers 4a-c two are equally probable: 4а corresponding to the energy minimum and 4c (Table 2), their calculated using additive scheme dipole moments are almost equal to the experimental values for 4 both in benzene and in dioxane. Based on the correlation of the theoretical and experimental polarity data, there is a conformational equilibrium of 4a, 4b, and 4c in solution.
For compound 5, three preferred conformers were found according to quantum chemical calculations (Fig. 6, Table 2). The energy minimum corresponds to conformer 5a, in which the Csp3–Csp3 bonds of phenylethyl substituents at the P atom are arranged gauche,gauche relative to the P=Se bond (dihedral angles Se=P–C7–C10 45°, Se=P–C8–C9 55°), and the phenyl ring at the nitrogen atom is trans oriented relative to the P=Se bond (dihedral angle Se=P–N–C6 −162°).
Relative energies of 5a and 5b conformers are almost equal, the arrangement of phenylethyl fragments in 5a and 5b is similar (dihedral angles Se=P–C7–C10 40°, Se=P–C8–C9 48°), but in 5b the N–Csp2 bond is gauche oriented relative to the P=Se bond (dihedral angle Se=P–N–C6 is 54°), and the same orientation of the Csp3–Csp3 and N–Csp2 bonds is observed in 5с (dihedral angles Se=P–C7–C10 42°, Se=P–C8–C9 53°, Se=P–N–C6 54°). The phenyl rings are gauche,trans-oriented relative to the P–Csp3 bond in 5а (dihedral angles P–С7–С10–С17 72°, P–C8–C9–C11 −178°) and 5с (dihedral angles P–С7–С10–С17 72°, P–C8–C9–C11 −176°), and gauche,gauche – in 5b (dihedral angles P–С7–С10–С17 73°, P–C8–C9–C11 73°).
In solution of amide 5, 5a, 5b, and 5c forms are in conformational equilibrium (ratio 1.2:1.7:1 respectively), which is supported by the similarity between values of the calculated using vector additive scheme and experimental dipole moments. The theoretical dipole moments are in a good agreement with calculated and experimental data.
In order to explain slightly lower theoretical values of dipole moments relative to the experimental values we carried out quantum chemical calculation of the dimer of 1 (Fig. 7). According to the quantum chemical calculation, the energy of the dimer is lower by 36.1 kJ·mol−1 (i.e. energy gain is twice the energy of the monomer). The dipole moment of dimer 1 increases and is equal to 8.5 D (see polarity value of 1 in Table 2).
Both molecules of dimer 1 have symmetrical propeller structure, and the substituents at the phosphorus atoms in these molecules maintain gauche,gauche,gauche-orientation (the Csp3–Csp3 and Р–О bonds) relative to the P=Se bond (dihedral angles Se1=P1–C7–C10 36°, Se1=P1–C8–C9 33°, Se1=P1–O1–C6 63°; Se2=P2–C29–C32 37°, Se2=P2–C30–C31 31°, Se2=P2–O2–C28 63°), all Csp2–Csp3 bonds of the phenylethyl fragments are also gauche-oriented relative to the Csp3–P bond (dihedral angles P1–С7–С10–С17 74°, P1–C8–C9–C11 77°; P2–С29–С32–С39 72°, P2–C30–C31–C33 80°).
In dimer of 1 in contrast to the isolated molecules the formation of contacts between the selenium atom of the P2=Se2 group of one of the molecules and different hydrogen atoms is possible, both intramolecular P2=Se2⋯H24–Csp2(OPh) (2.710 Å), P=Se2⋯H38–Csp2(Ph1) (2.886 Å), P=Se2⋯H42–Csp2(Ph2) (2.896 Å), and intermolecular P=Se2⋯H5–Csp2(Ph) (2.801 Å), and P=Se2⋯H24–Csp3 (2.857 Å), which stabilize the dimer structure. It can be concluded that the lower value of the theoretical dipole moment of phosphinoselenoate 1 as compared to the experimental value can be attributed to the partial dimerization of the molecules in solution.
O-Phenyl diphenethylphosphinoselenoate 1 and O-(naphthalen-1-yl) diphenethylphosphinoselenoate 2 were synthesized according to procedure , S-ethyl diphenethylphosphinoselenothioate 3 and S-phenyl diphenethylphosphinoselenothioate 4 were prepared according to procedure , and P,P-diphenethyl-N-phenylphosphinoselenoic amide 5 was synthesized using procedure . The solvents were purified using standard procedures.
The experimental values of the dipole moments were determined according to the second Debye method. Physical parameters of 1 and 3–5 were measured from series consisting of 4–6 solutions in benzene or 1,4-dioxane at 25°C, while the measurements for 2 were only conducted in benzene because of its poor solubility in 1,4-dioxane. The dielectric permittivity of solutions of 1–5 was determined on a BI-870 instrument (Brookhaven Instruments Corporation), the accuracy is ±0.01. The refractive indices of solutions were determined on a RA-500 refractometer (Kyoto Electronics), the accuracy is ±0.0001.
The experimental dipole moments were calculated by the formula :
where M is the molecular weight of a substance, d is the density of the solvent, α and γ are the slope tangents of the straight lines in the coordinates εi−wi and ni2−wi; and εi, ni, and wi are the dielectric permittivity, refractive index, and weight fraction of the solute in ith solution, respectively. The coefficients α and γ were calculated by the formulas: α=(εi−ε0)/ωi and γ=(ni2–n02)/ωi, where ε0 and n0 are the dielectric permittivity and refractive index of the solvent, respectively.
Quantum chemical calculations were carried out using density functional theory (DFT) method B3PW91 with 6-311++G(df,p) basis set by the GAUSSIAN 09 program  with the total optimization of geometry. Correspondence of the obtained stationary points to the energy minima was proved by calculation of the second derivatives in all cases. The calculations were performed in Kazan Federal University and Kazan Branch of Joint Supercomputer Center of the Russian Academy of Sciences.
We carried out experimental and theoretical conformational analysis of O-phenyl diphenethylphosphinoselenoate, O-(naphthalen-1-yl) diphenethylphosphinoselenoate, S-ethyl diphenethylphosphinoselenothioate, S-phenyl diphenethylphosphinoselenothioate, and P,P-diphenethyl-N-phenylphosphinoselenoic amide by the method of dipole moments and DFT calculations. In all cases, the theoretical results are in a good agreement with the experimental data obtained. We concluded that the conformations of the examined compounds fit into the overall conformational picture for compounds of tetra-coordinated phosphorus with alkyl and aryl substituents , namely, these derivatives of bis(2-phenylethyl)selenophosphinic acid exist as a conformational equilibrium of non-eclipsed gauche and trans forms with propeller arrangement of the substituents relative to the P=Se bond, and eclipsed cis orientation of thioethyl substituent may be caused by the formation of intramolecular hydrogen interaction Se⋯H.
The studied polyfunctional compounds are prospective ligands for the preparation of metal complexes, precursors for the design of biologically active compounds, single-source precursors of nanomaterials, and building blocks for synthesis of novel organophosphorus compounds. Besides theoretical value, the results of the present study can be useful for prediction and rationalization of reactivity of phosphinoselenoic esters, thioesters, and amides.
YAV, RRK, DVC, and EAI are grateful to the Russian Foundation for Basic Research (Grant 16-03-00100) for the financial support.
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About the article
Published Online: 2017-02-07
Published in Print: 2017-03-01
Citation Information: Pure and Applied Chemistry, Volume 89, Issue 3, Pages 393–401, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2016-0802.http://creativecommons.org/licenses/by-nc-nd/4.0/.