4. V. I. Shapoval and G. V. Reznik, "The theory of nonsteady-state processes of electro- reduction of anions in molten salts," Ionnye Rasplavy, No. 2, 135-146 (1974).

5. A. V. Bandura and N. P. Novoselov, "The possibility of using semiempirical methods of quantum chemistry for the calculation of solvate complexes," Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol., 21, No. 4, 516-521 (1978).

6. A. V. Bandura and N. P. Novoselov, "Quantum-chemical investigation of the interaction of ions of alkali halide salts with molecules of protic and aprotic solvents," Teor. Ekspo Khim., 14, No. 2, 174-181 (1978).

7. R. J. Boyd and M. A. Whitehead, "An SCF-MO-CNDO study of equilibrium geometry forces, constants, and bonding energies: CNDO/BW. i. Parametrization," J. Chem. Soc. Dalton Trans., No. i, 73-77 (!972).

8. B. J. McAloon and P. G. Perkins, "Calculation on some phosphate ions by modified CNDO method," Theor. Chim. Aeta, 22, No. 3, 304-308 (1971).

9. L. E. Sutton (editor), Tables of Interatomic Distances and Configurations in Molecules and Ions, No. ii, Chem. Sot., London (1958).

i0. V. I. Lebedev, Ionic-Atomic Radii and Their Significance for Geochemistry and Chemistry [in Russian], Izd. Leningr. Univ.,Leningrad (1969).

ii. D. A. Tkalenko, "Chronopotentiometric investigation of the cathodic reduction of NO3- anions in NaNO3 melts," Elektrokhimiya, 15, No. 6, 891-894 (1978).

12. Yu. K. Delimarskii, D. A. Tkalenko, and N. A. Chmilenko, "Cathodic reduction of nitrate ions in K, Li, Ca/N03 melts," Elektrokhimiya, 18, No. i, 155-158 (1981).

CONFO~ATIONAL ANALYSIS OF TETRAPHENYL-I,2-CYCLOHEXYLENEDIPHOSPHINE DIOXIDE

AS A POTENTIAL LIGAND

A. P. Baranov, V. G. Dashevskii, T. Ya. Medved', G. M. Petov, E. I. Matrosov, and M. I. Kabachnik

UDC 541(63+49)

The present work is a continuation of our research into the conformational isomerism of

neutral bidentate ligands of the alkylenediphosphine dioxide series R--P--R'--P--R The two 2 II LI 2.

donor centers of these compounds are connected by a bridge, and in orde2to O clarify the role of this bridge in determining the complex-forming properties of these ligands, we investi- gated tetraphenyl-l,2-cyclohexylenediphosphine dioxide

Pt~ P~PPh The synthesis and properties of this dioxide have been described in [i]. As in our previous investigation [2], we attempted to explain the conformational rigidity of the ligand and the effect of the cyclohexylene bridge on the mutual arrangement of the phosphoryl groups, with the aid of a scheme for calculating atom-atom potential functions with the respective set of parameters [3].

The calculations [4] of tetraphenylethylenediphosphine dioxide (R' = -CH2--CH~--) con- formations showed that the single C--C bridging bond ensures the flexibility of the ligand: the stable conformation corresponds to a very wide minimum of potential surface and the phosphoryl group oxygen atoms are separated by a great distance. Thus, a dioxide with an ethylene bridge cannot chelate alkali-metal cations (as confirmed experimentally in [5]). If, however, there is a double bond in the bridge R' (R' =--CH-----CH--), the corresponding cis-vinylene

A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 17, No. 6, pp, 833-838, November-December, 1981. Original article submitted December 23, 1980.

0040-5760/81/1706-0657507.50 �9 1982 Plenum Publishing Corporation 657

\ ! J

e e ] ~ ~ ~ a e

Fig. i. A perspective representation of the equatorial- equatorial (ee), axial-axial (aa), and axial-equatorial (ae) arrangements of phosphorus atoms with respect to the chair conformation of the cyclohexane ring of tetraphenyl-l,2- cyc!ohexylenediphosphine dioxide.

dioxide becomes conformationally rigid (the oxygen atoms are adjacent in the stable confor- mation) and this specifies its high complex-forming ability. The question arises as to whether it is possible, having taken a ligand with a single C-C bond in the bridge, so to change its chemical structure as to achieve the conformational rigidity necessary for high selectivity with respect to cations of a particular radius. With this aim we synthesized tetraphenyl-l,2-cyclohexylenediphosphine dioxide. According to the general concepts for the conformational analysis of 1,2-disubstituted cyclohexane [6], two geometric cis and trans isomers are possible for the synthesized structure, the cis isomer corresponding to the axial-equatorial (ae) arrangement of ring substituents and the transisomer being able to exist in the form of two conformers with either axial-axial (aa) or equatorial-equatorial (ee) arrangement of substituents (Fig. i). From a complex formation viewpoint, this ligand can chelate metal ions without any substantial conformational rearrangement, being only in the latter conformation. We now attempt a theoretical verification of this hypothesis by deter- mining the distances between RO... 0 donor atoms at points of potential surface minima. Know- ing this distance we can estimate the cationic radius which is optimum for complex formation.

RESULTS OF CALCULATION

It is natural to assume that the cyclohexane ring is in the chair conformation. Taking into account the rigid structure of a six-membered ring, we will examine the rotation of the two phosphoryl groups around the P-Cring bonds. During the first stage of this investigation, instead of the phenyl substituents at the phosphorus atoms, we considered only carbon atoms (we showed in [7] that the potential surface of such hypothetical structures is similar to that of the corresponding tetraphenyl analogs). Thus, within the scope of the approximations introduced, the potential surface of 1,2-cyclohexylenetetraphenyldiphosphine dioxide becomes two-dimensional and can be represented by a conformational map.

Conformational energy maps as a function of phosphoryl group rotational angles ~ and ~2 are presented in Fig. 2a-c, respectively, for the ee, aa, and ae positions of phosphorus atoms with respect to the rigid cyclohexane ring. Conformational maps are produced by taking into account only the energy of nonvalence interactions and the torsional energy of rotation of angles ~ and ~=. Numbers of the isolines represent the relative values in kJ/mole (the energy of the global minimum of all three conformational maps is taken as the zero reference point),

The low-energy regions occupy quite a small part of the total potential surface owing to steric hindrances, and in particular, to the nonvalence interactions between the radicals at the phosphorus atom and the hydrogen atoms of the cyclohexane ring and also to the inter- actions between radicals. A comparison of conformational maps shows that the ee conformer of tetraphenyl-l,2-cyclohexylenediphosphine dioxide (ignoring the rotations of the phenyl rings) corresponds to the global minimum energy of the molecule. The optimum conformer of map 2b loses in comparison by ~20 kJ/mole, i.e., the aa conformer is less stable than the ee conformer. Finally, the ae arrangement of phosphoryl groups is also energetically less advantageous.

The reason that the axial arrangement of phosphorus atoms is so unstable is the steric hindrances caused by the so-called "bowsprit" interaction with carbon and hydrogen atoms of the cyclohexane ring (shown by the dashed lines in Fig. 3). In addition, O...H interactions play a destabilizing role. Bowsprit interactions also destabilize the ae form.

658

~7

I

I ",,~o ,Wo,, ",~,J'~e, I z,~? t ) \ ~ / /

'. q~.. -?

J ~ ~, deg a

/

~O/ro . .

J ' '2~,'X', 'L~),

~. deg

I

9'~~ I ,,

I . . . . . . . . [

O ~0 ~,-deg -30 0 .~ ,~, deg c -90 b

Fig. 2. Conformational energy maps for tetraphenyl-l,2- cyclohexylenediphosphine dioxide as a function of phos- phoryl group rotational angles ~i and ~2 for the ee, ~, and ae positions, respectively, of phosphorus atoms with respect to the chair conformation of the cyclohexane ring; the numbers on isolines are the relative energies in kJ/ mole; the zero reference point is taken to be the energy of the global minimum for all three conformational maps.

The conformational map for the ee form has three stable minima, corresponding to the conformers represented spatially in Fig. 3b-d. These differ in the arrangement of oxygen atoms (with different angles ~i and ~2 on the conformational map). In one case (Fig. 3b) the oxygen atoms are adjacent, at a distance of 0,25 nm from each other (a global minimum with coordi- nates ~i = 250 • 5 ~ and ~2 = 250 • 5=); the conformer shown in Fig. 3c corresponds to another minimum with coordinates q: = 150 ~ and ~2 = 250 ~ (or vice versa). In this case the distance between donor atoms in the ligand is somewhat greater, being ~0.28 nm. The small energy dif- ference (~1.3 kJ/mole) between these two conformations suggests an absence of steric con- strictions. However, the electrostatic repulsion of oxygen atoms, as will be shown below, gives priority to the latter conformation. The conformer represented in Fig. 3d corresponds to the local minimum, which lies 7.4 kJ/mole above the global (its coordinates are ~ = 140 ~ and q2 = 140Q), The energy of this conformer is determined by nonvalence interactions at the phosphorus atoms, which are shown by dashed lines. Our conclusion with respect to the sta- bility of the diequatorial conformation of phosphoryl groups in the chair form of the cyclo- hexane ring is in agreement with data obtained from a study of ~:P--{H} NMR spectra [I]. We did not calculate the conformations for the boat form, the twist form, or the various tran- sitional forms of the cyclohexane ring, since these forms are known to have a higher energy than the optimum ee conformation (this is because steric hindrances are absent from the latter).

In order to calculate the phenyi ring rotations, we sought the energy minimum of the

659

Fig. 3. Spatial representation of conformers corresponding to minima on the conformatlonal maps: a) for the minimum of the aa form; b, c, and d) for minima of the ee form; the dashed lines show nonvalence interactions which destabilize the respective conformations; the phenyl rings are omitted arbitrarily; the dashed lines with arrows indicate the distances between oxygen atoms.

ee and aa forms of the cyclohexane ring with respect to six independent variables: to the two angles of rotation ~, and ~2, the four angles of rotation around the P--Ph bonds, and also with respect to the independent valence angles at the phosphorus atoms, with parametrization taken from [8]. The potential function of the molecule was supplemented by an electrostatic con- tribution in the form of Coulomb's law, with charges obtained from the dipole moments of the bonds (+0.4 and-0.35 e, respectively, for P and 0 atoms [9]). Proceeding from the various starting points, we arrived at the same optimum values for angles ~ and ~, as we found for our hypothetical structure. In the case of phenyl-substituted compounds, the energy differ- ence of the aa and ee forms remained in the same order. However, as would be expected, this did not apply to the energy differences obtained for the three conformations of the ee form, when the electrostatic component and the phenyl ring rotations were taken into account. The conformation most energetically advantageous was the chair/half-chair (where the chair re- lates to the conformation of the cyclohexane ring and the half-chair to the figure formed by the substituents). The chair/chair conformation (Fig. 3d) has an energy disadvantage of 4.8 kJ/mole owing to the electrostatic repulsion of oxygen atoms. In spite of the fact that quite rough values were taken for the charges on atoms, the redistribution tendency for the depths of minima was reasonably obvious. The third local energy minimum for the ee conformer has an energy of ~15.6 kJ/mole and hence its population is very low. Thus the conformational study showed that two conformers existed: the chair/half-chair with oxygen atoms about 0.28 nm apart and the less stable chair/chair conformer with a distance between oxygen atoms of about 0.25 nm.

A confirmation of the theoretically determined preferential conformations for the tetra- phenyl-l,2-cyclohexylenediphosphine dioxide molecule is the good agreement between the dipole moment calculated for ee form (5.2 D for the chair/half-chair and 7.2 D for the chair/chair) and the experimentally measured dipole moment of 5.0 D.

The dipole moment of cyclohexane dioxide was determined by DehyeVs second method [ii] in benzene at 25 • 0.1~ The dielectric constant and the density of solutions were measured on a Dipole precision dielectrometer. Using data and GedestrandVs equation [ii], we obtained a value of 5.0 • 0.i D for the dipole moment. The dipole moments of the various conformers were calculated according to the additive vector scheme for the chair conformation of the cyclo- hexane ring, taking as the fragmentary moments of polar groupings the experimental dipole moments for the monoxide (~p~s~O=~Azk~O=4,5• measured under the same conditions [9]. W~th such an approach, the dipole moment of the bridge between P=O groups can be arbitrarily taken

660

as zero, thus reducing the solution to a simple two-vector problem, where the dioxide dipole moment is determined by the mutual orientation of P=O groups only. For the other confor- matlons considered, thecalculated dipole moment differed from the experimental value. For example, for the conformer in Fig. 3d, the calculated dipole moment is 0.0 D, a value which agrees with its low-energy population.

According to the hypotheses which we developed in [i0] with respect to the potential possibilities of conformers during complex formation, tetraphenyl-l,2-cyclohexylenediphosphine dioxide can behave as a bidentate ligand. An examination of the change in the distances be- tween donor centers of the energetically optimum forms of the ligand shows that the geometric possibilities of the "claw" allow the effective chelation of metal cations with a radius of 0.04-0.08 nm. For example, of the alkali-metal cations, the Li + cation would have an "ideal" radius (0.06 nm). The remaining alkali-metal cations would be too big for an effective capture by the bidentate ligand under consideration.

LITERATURE CITED

i. Yu. M. Polikarpov, G. V. Bodrin, E. I. Babkina, et al., "Synthesis and properties of certain derivatives of 1,2-cyclohexylenetetrachlorodiphosphine," Izv. Akad. Nauk SSSR, Set. Khim., No. 5, 1188-1191 (1977).

2. V. G. Dashevskil, A. P. Baranov, T. Ya. Medved', and M. I. Kabachnik, "A conformational analysis of tetramethylmethylenedlphosphlne, a bidentate ligand in metal chelate com- plexes," Teor. Eksp. Khim., 13, No. 3, 340-348 (1977).

3. V. G. Dashevskii, Conformations of Organic Molecules [in Russian], Khimiya, Moscow (1974). 4. A. P. Baranov, V. G. Dashevskii, T. Ya. Medved', and M. I. Kabachnik, "A conformational

analysis of tetramethyl vinylidene-, vinylene-, and ethylenediphosphine dioxides and some of their halide derivatives in relation to the problem of selectivity in complex for- mation," Teor. Eksp. Khim., 13, No. 4, 488-495 (1977).

5, K. B. Yatsimlrskii, M. I. Kabachnik, E. I. Sinyavskaya, et al., "The interaction of alkylenediphosphlne dioxides with alkali-metal cations," Teor. Eksp. Khim., 12, No. 6, 777-781 (1976)..

6. E. Illel, The Bases of Stereochemistry [Russian translation], Mir, Moscow (1971). 7. V. G. Dashevskli, A. P. Baranov, T. Ya. Medved', and M. I. Kabachnik, "The conformational

effect of phenyl substituents on the complex-forming ability of alkylenediphosphine diox- ides," Teor. Eksp. Khim., 15, No. 3, 255-264 (1979).

8. A. Kh. Plyamovatyi, V. G. Dashevskii, and M. I. Kabachnik, "Calculation of the confor- mations of 1,3,2-dioxaphospholane rings with tri- and tetracoordinated phosphorus within the framework of the AMII--EUEP,*" Dokl. Akad. Nauk SSSR, 235, No. i, 124-127 (1977).

9. E. I, Matrosov, G. M. Petov, and M. I. Kabachnik, "Calculation of dipole moments for bonds in organophosphorus systems, taking into account the moment of the free electron pair of trivalent phosphorus," Zh. Strukt. Khim., 15, No. 2, 255-259 (1974). M. I. Kabachnik, V. G. Dashevskii, T. Ya. Medved', and A. P. Baranov, "The principles of analyzing the spatial effects and selectivity of complex-formation," Teor. Eksp. Khim., 13, No. 3, 335-339 (1977). V. Io Minkin, O. A. Osipov, and Yu. A, Zhdanov, Dipole Moments in Organic Chemistry [in Russian], Khimiya, Leningrad (1968).

i0,

ii.

*Additive model of interatomic interactions with effective unshared electron pairs.

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