a computational investigation of phosphaalkynes: a new building block

Journal of Molecular Structure (Theo&em), 256 (1992) 17-27 Elsevier Science Publishers B.V., Amsterdam

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A computational investigation of phosphaalkynes: a new building block

Keerthi Jayasuriya Geo-Centers Corporation, U.S. Army Research Development and Engineering Center, Bldg. 3028, Picatinny Arsenal, NJ 07806-5000 (USA)

(Received 15 May 1991)

Abstract

The electronic properties of C = P, C =P and C-P bonds and their aza analogues, C = N, C = N and C-N bonds, were investigated using an ab initio self-consistent field molecular orbital ap- proach at the 4-31G* level. The cyclodimerization and cyclotetramerization of the monomer, methylphosphaacetylene ( CHB-C = P) were analyzed by the molecular electrostatic potential (MEP) method. Based on the MEP maps, two reactive sites attractive towards an incoming electrophile were identified for phosphaacetylene, but its aza analogue, acetonitrile, showed only a single reactive site. The -C = P group, in contrast to the -C 3 N group, tends to form ‘side-on’ bonds with transition metals, and this has been reinforced by MEP maps of phosphaacetylene, indicating a larger negative region near the n-bonds.

INTRODUCTION

Cubane (C&H,) (I) is an interesting polyhedral hydrocarbon because of its high symmetry (0,) and its highly strained structure. Despite its high heat of formation (140 kcal mol- ’ ) [ 1 ] and high strain energy (156 kcal mol-l) [ 21, it is surprisingly thermally stable. There is considerable interest in replacing some of the CH units in cubane with other elements such as nitrogen and its heavier homolog phosphorus, in expectation of discovering unusual physical and chemical properties. For example, 1,3,5,7-tetraazacubane (II), which is an aza analogue of cubane and isoelectronic with cubane, has not yet been synthesized. The presence of four nitrogen atoms in alternative corners of the cube tends to distort the geometry of the cube in II, thus considerably increasing the strain energy [ 31. Although II, or any substituted derivative of II, has not yet been synthesized, substituted tetraphosphacubane (III) (1,3,5,7-tetra-t- butyltetraphosphacubane), was synthesized recently by thermal cyclooligomerization of t-butylphosphaacetylene [ 41. Alkynes and nitriles, which con- tain carbon-carbon and carbon-nitrogen triple bonds respectively, are well

Correspondence to: K. Jayasuriya, Geo-Centers Corporation, U.S. Army Research Develop- ment and Engineering Center, Bldg. 3028, Picatinny, Arsenal NJ 07806-5000, USA.

0166-1280/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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established starting materials in synthetic organic chemistry. In contrast, the reactive nature of phosphaalkynes has not been explored until recently.

The objective of this work was to investigate, by a computational procedure, the electronic properties of C-P, C = P and C E P bonds and compare them with the C-N, C = N and C = N bonds respectively. In this study, methylphosphaacetylene (1-phosphapropyne) (IV), 1,3dimethyldiphosphacyclobuta- diene (V), tetraphosphacubane (VI) and their aza analogs (VII, VIII and II) are used as model compounds to investigate the nature of the carbon-phosphorus and carbon-nitrogen bonds in these compounds. In order to facilitate the computations, the bulky t-butyl group in IV, V, VII and VIII was replaced by a methyl group. This should not affect the main focus of the investigation, which is the electronic properties of the carbon-phosphorus and carbon-nitrogen bonds in these molecules.

I II -

W

CH-C-_-p 3

IV -

CH-C-_-N 3

III - t--Butyl

r P 0% 7 -P

P

VI -

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METHOD

Ab-initio self-consistent field (SCF) molecular orbital calculations were carried out using the GAUSSIAN 88 program [ 51. All the geometries were optimized at the 4-31G* level to retain the consistency of the calculated structures. The electrostatic potential calculations were carried out on the optimized structures of II-VIII at the 4-31G* level to analyze the reactivity of C-P, C = P and C=P bonds and their aza analogs. The electrostatic potential V( r ) that is created at any point r in the surrounding space by the nuclei and electrons of the molecule is given by

(1)

where 2, is the charge on nucleus A located at RA and p(r) is the electronic density function of the molecule. The first term on the right-hand side of eqn. (1) represents the contribution of the nuclei and the second term gives the effect of the electrons. Therefore, an approaching electrophile will initially tend to go to those regions where V(r) is negative, reflecting the predominant effect of electrons in these regions. The electrostatic potential is exactly equal in magnitude to the electrostatic interaction energy between the static charge distribution of the system and a positive unit point charge located at r. The electrostatic potential method is a well-established technique for interpreting and predicting the chemical reactivity of organic molecules [ 6-81.

RESULTS AND DISCUSSION

Structures

The carbon-phosphorus bond lengths reflect the nature of the carbon-phosphorus bonds present in the optimized structures of IV-VI (see Table 1). The carbon-phosphorus bond length varies from the C-P single bond in VI to the extremely short C = P triple bond in IV. Complex IV posesses C,, symmetry and the C = P bond consists of a single 2p,-3p, bond and two 2p,-3p, bonds with a length of 1.51 A, whereas the methyl group is bonded with an sp-sp3 bond length of 1.46 A. The cyclodimerizaton of IV ( [ 2 + 2lcycloaddition) yields V which contains two types of C-P bonds: the C=P double bond length and the C-P single bond length average out to 1.65 and 1.91 A, respectively. Com- parison of the internal ring angles of V with cyclobutadiene reveals that the C-P-C angles are on average 9” smaller, whereas the P-C-P angles are 8.7” larger. The structure of VI indicates a distorted cube-like frame with C-P-C angles on average 5.1’ smaller, whereas the P-C-P angles are 4.8” larger than those of unsubstituted cubane. The optimized C-P bond length in VI is 1.943 A,

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TABLE 1

The optimized geometries of IV, V and VI

Molecule Bond Bond length (8ngstram)

Bond angle Value

(degrees 1

IV c-c CZP C-H

V C-P 1.914 C=P 1.656 c-c 1.487 C-H 1.087

VI C-P c-c C-H

1.466 (1.46)” 1.513 (1.54) 1.085 (1.108)

1.943 (1.881)b 1.518 1.085

C-CEP 179.9 C-C-H 110.6 H-C-H 108.2

C-P-C P-C-P P-C-C C-C-H

C-P-C 84.9

P-C-P 94.8 C-C-H 111.6

81.2 98.7

128.9 112.2

“Experimental values are given in parentheses [ 151. bFrom X-ray crystal structure data [ 41.

TABLE 2

The calculated total energy (4-3lG*) and dipole moment of III and IV-VIII

Molecule Total energy (hartree)

Dipole moment

(debye )

IV -417.7194 1.33 V - 835.43494 0.0 VI - 1515.0142 0.0 VII - 131.79793 4.0 VIII - 263.50222 0.0 II -370.96155 0.0

which is longer than the experimental P-C bond lengths found for III. The calculated total energies and dipole moments of IV-VI and their aza analogs at the 4-31G* level are given in Table 2.

Moleculur Electrostatic Potentials

The C E P triple bond can be viewed as the result of the juxtaposition of a t.r- bond and two n-bonds. Figure 1 shows the molecular electrostatic potential of methylphosphaalkyne (IV) in a plane bisecting the symmetry axis of the C E P bond. The two negative regions that surround the C = P bond region reach a minimum of - 17.1 kcal mol-’ on a line perpendicular to the symmetry axis of the C E P bond at its midpoint. The corresponding minimum near the mid-

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*.-----W\ MINIMA (kcal/mol) /’ \

Fig. 1. Calculated electrostatic potential of methylphosphaacetylene (IV) in a plane bisecting the symmetry axis of the C=P bond. Dashed contours correspond to negative potential; the zero contour is shown. The magnitudes of the other contours are (in kcal mol-I): - 15.68, -9.4, -6.3, -3.1, -0.06,0.06, 3.1, 7.5, 31.4, 62.7,313.7 and 627.5. The locations and values of the minima are indicated.

point of the C = C bond in acetylene is - 25.2 kcal mol-l at the 4-3lG* basis level. Unlike in acetylene, there is another minimum near the terminal phosphorus atom, which reflects the strength of the lone pair of phosphorus, with the minimum reaching only -5.4 kcal mol-‘. The negative electrostatic potential region can be regarded as a reactive site attractive to incoming electro- philes. From the MEP maps it is clear that there are two distinct possibilities of reactivity sites in IV, the C s P triple bond and the lone pair of the terminal phosphorus atom, for an approaching electrophile. The magnitude of the minima near the midpoint of the C = P bond, contrasted to the weak minimum due to the phosphorus lone pair, clearly indicates that this is the favored site for initial reaction with an electrophile. This is indeed the case. It was found that phosphaalkynes with alkyl substituents are always bound ‘side-on’ in transition metal complexes and it appears that the lone pair of the phosphorus atom does not interact significantly with the metal atom [g-lo]. The nature of the molecular electrostatic potential maps of phosphaalkyne and alkynes in prin- cipal confirms the similarity of the reactivity of these two compounds.

In contrast, the aza analog of IV shows a large negative region due to its localized lone pair, with the minima reaching -58.4 kcal mol-‘. The shape

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MINIMA (kcal/mol)

Fig. 2. Calculated electrostatic potential of acetonitrile (VII) in a plane bisecting the symmetry axis of the C=N bond. Dashed contours correspond to negative potential; the zero contour is shown. The magnitudes of the other contours are (in kcal mol-‘): -31.4, -15.68, -9.4, -6.3, -3.1, -0.06,0.06,3.1, 7.5, 31.4, 62.7, 313.7 and 627.5. The locations and values of the minima are indicated.

and the magnitude of the molecular electrostatic potential map of acetonitrile (VII) is very different from its phospha analog (IV) (see Fig. 2). Close ex- amination of the MEP maps of phosphaalkynes and acetonitriles are of interest regarding their coordination chemistry. The large negative potential surrounding the C 3 P bond region in IV clearly indicates that ‘side-on’ bonding to metal atoms is the most preferred for phosphaalkynes. However, the coordination of phosphaalkyne to metal atoms can affect the donor properties of the phosphorus lone pair, which can subsequently bind to a third metal atom [ 111. The nitriles are known to bond to transition metal atoms via the nitrogen lone pair where the negative region is localized. In acetonitrile, the negative potential is localized in the nitrogen lone-pair region, whereas in phosphaalkyne a large negative potential is associated with the triple bond.

The MEP of V, which is the dimer of the phosphaalkyne, through a plane bisecting a C=P double bond, shows a minimum of - 10.7 kcal mol-’ due to the 2p,-3p, bond between carbon and phosphorus atoms in the ring, whereas

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MINIMA (kcal/mol)

1 -0.66

2 -11.23

3 -11.67

4 -9.09

Fig. 3. Calculated electrostatic potential of 1,3-diiethyldiphosphacyclobutadiene (V) in a plane 1.75 a.u. above the molecular plane. Dashed contours correspond to negative potential, the zero contour is shown. The magnitudes of the other contours are (in kcal mol-I): -9.4, -6.3, -3.1, -0.06,0.06,3.1,7.5 and 31.4. The locations and values of the minima are indicated.

the MEP map through the C-P bond does not show any minima, as expected. The MEP map in the plane of the molecule shows the strength of the phosphorus lone pair, with the minimum reaching - 20.7 kcal mol-‘. A rather interesting feature is found in the MEP map generated in a plane 1.75 a.u. above the molecular plane of V (see Fig. 3). The negative potential present between the carbon-phosphorus bond, with the minimum reaching - 11.67 kcal mol-‘, reflects the 2p,-3p, bonding region between the carbon and phosphorus atoms. Figure 3 reflects the displacement of the electron density from phosphorus to carbon, which reinforces the differences in electronegativities of carbon and phosphorus atoms (carbon = 2.5; phosphorus = 2.1) [ 121. Although the minimum is found between the C=P bond in a plane above the molecular plane, most of the negative region is directly above the carbon atom, which is more electronegative than phosphorus. The MEP map of VIII in the molecular plane gives a minimum of -70.3 kcal mol-‘, which indicates the strength of the

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MINIMA (kcal/mol)

Fig. 4. Calculated electrostatic potential of tetraphosphacubane (VI) in a diagonal plane bisecting the cube. Dashed contours correspond to negative potential; the zero contour is shown. The magnitudes of the other contours are (in kcal mol-‘): -9.4, -6.3, -3.1, -0.06,0.06,3.1,7.5,31.4, 62.7,313.7 and 627.5. The locations and values of the minima are indicated.

localized lone pairs on the nitrogen atom, which is more electronegative than the carbon atom.

The MEP map through a diagonal plane bisecting the tetraphosphacubane (VI) molecule shows a negative region due to the lone pairs of phosphorus with the minimum reaching -25.9 kcal mol-l (see Fig. 4). Unlike cubane, in which the corresponding plane indicates negative potentials between the C-C bonds, (an unusual feature found only in strained ring systems [ 131) there is no negative region present near the C-P bond. The corresponding MEP map of tetraazacubane gives a minimum of -61.1 kcal mol-l localized near the nitrogen atom’s lone pair of electrons.

The cyclotetramerization of alkyl substituted phosphaalkyne (t-butyl-C = P) yields tetra-t-butyl-tetraphosphacubane [4] and a plausible explanation of this cyclotetramerization is not possible without the assumption of an initial cyclodimerization of phosphaalkyne to the corresponding 1,3diphos

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P P

P+21 dimerization

v

[2+21

I

dimerization

Scheme 1.

phete (see Scheme 1). The MEP maps of the model compound methyl-substituted phosphaalkyne (IV) reinforce the mechanistic view of the cyclodimerization of phosphaalkyne, which assumes an initial head-to-tail dimerization of the phosphaalkyne, yielding the 1,3-diphosphete [ 41. The negative potential regions around the C = P bond and the lone pair of the phosphorus atom of IV are the most likely sites (electron-rich sites) for an initial electrophilic attack, the electrostatic potential generated around IV helps an initial head-to-tail dimerization.

The relative energy of a dimer indicates its stability compared to the un-

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bound monomer. The relative energies for V and VIII at the 4-31G* level are 2.4 and 58.7 kcal mol-l respectively, and are derived as

Relative Energy = Edimer - 2 ( Em,,nOmer) (2)

Both V and VIII are energetically unstable with respect to their monomers by 2.4 and 58.7 kcal mol-‘, respectively. However, the energy difference between 1,3dimethyldiphosphacyclobutadiene (V) and its monomer (IV) is relatively small. It is possible to postulate that the cyclodimerization of phosphaalkyne is energetically feasible, possibly facilitated by the formation of a suitable transition metal complex. The synthesis of a 1,3diphosphacyclobutadienecob- alt (I) complex has been accomplished by dimerization of phosphaalkyne [ 141. In fact, thermal cyclooligomerization of phosphaalkyne has been accomplished to yield alkyl substituted tetraphosphacubane [ 41, According to the relative energy of VIII, it is most unlikely that VIII can be realized through the cyclodimerization of acetonitrile in this manner. This study indicates that cyclooligomerization of phosphaalkyne to yield respective dimers and tetramers is thermodynamically favorable.

SUMMARY

This analysis provides a quantitative interpretation of the reactivities of the C = P, C = P and C-P bonds, using a molecular electrostatic potential method. The main focus was on the cyclodimerization and cyclotetramerization of alkyl-substituted phosphaalkyne to yield tetraphosphacubane. Based on the negative electrostatic potential generated around these molecules, their reactivities and properties were examined. These properties were compared with their aza analogs.

ACKNOWLEDGMENT

The author wishes to express his appreciation to the U.S. Army Research Development and Engineering Center (ARDEC ) Computer Center for its sup- port of this work.

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a computational investigation of phosphaalkynes: a new building block

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