J . Phys. Chem. 1988, 92, 4886-4892
4886
Bis(ylides) of Phosphorus(V) and Their Structural Isomers: An ab Initio Study Kerwin D. Dobbs, James E. Boggs, Andrew R. Barron, and Alan H. Cowley* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: December 14, 1987)
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The structural isomers XPCH2CH2 (l), XH2CP=CH2 (2), and XP(=CH2), (3) (X = H or C1) have been investigated by nonempirical molecular orbital methods. The consequences of changing the substituent,X, from H to C1have been investigated by the use of Mulliken population analyses. Only minor structural changes are evidenced for 1 and 2; however, for 3, when X = H, a planar structure is calculated, while for the corresponding chloro-substituted compound a significant twisting of the methylene groups is predicted. The steric and electronic factors responsible for this twisting are discussed. The relative energies of and the frontier orbital compositions for 1-3 are considered in relation to the available experimental information.
The three most commonly recognized structural isomers of empirical formula CzR4PXin which phosphorus is bonded to two carbons are phosphiranes, 1, phosphaalkenes, 2, and bis(methylene)phosphoranes, 3. X
X
v
I
Although the parent phosphirane, 1 ( R = X = H), was synthesized over 2 decades ago,' surprisingly few compounds of this class have been published in the interim. By contrast, phosphaalkenes, 2, have received considerable attention,2and, in fact, these compounds continue to assume an important role both as ligands and as reagents. The bis(methylene)phosphoranes, 3, are the most recently reported members of this series. Such compounds are of particular interest because of their relationship to the well-known Wittig reagents (R3P=CHz). In the present work ab initio molecular orbital calculations have been carried out on all three isomers. To facilitate analysis of the computational results, we limited the substituent R to H, and X has been taken to be either H or C1. The choice of C1 as X is based on its simplicity as a nonhydrogen substituent and on the fact that chloro-substituted derivatives of 3 have been isolated. It is recognized that there are other possible structural isomers with the same empirical formula such as the E and Z conformers 5. However, these of 1-phosphapropene, 4, and ~inylphosphine,~
4
5
molecules were found to be of higher energy than phosphirane; hence, the present discussion is restricted to those molecules that contain phosphorus bonded to two carbons. After completion of the present work, a detailed ab initio molecular orbital study on the addition and insertion reactions of phosphinidene (PH) with ethylene appeared in the l i t e r a t ~ r e . ~This study included the structures of phosphirane, vinylphosphine, and their phosphaalkene isomers. The major differences between the present investigation and this latter one are as follows: (1) our calculations have used a larger basis set (6-31G*, with d functions on P, C, and C1 versus 4-31G with a set of d functions only on P (the basis set used in ref 4 will henceforth be referred to as 4-31G(*)), (2) we have (1) Wagner, R. I.; Freeman, L. D.; Goldwhite, H.; Rowsell, D. G. J . Am.
Chem. SOC.1967, 89, 1102.
(2) Appel, R.; Knoll, F.; Ruppert, I . Angew. Chem., Inr. Ed. Engl. 1981, 20, 73 1. (3) First experimental evidence of vinylphosphine: Lasne, M.-C.; Ripoll, J.-L.; Thuillier, A. J . Chem. SOC.,Chem. Commun. 1986, 1428. (4) Gonbeau, D.; Pfister-Guillouzo, B. Znorg. Chem. 1987, 26, 1799.
0022-3654/88/2092-4886$01.50/0
TABLE I: Total Enereies (hartreed
-
RHF/6-31G*// RHF/6-31G*
MP2/6-3 lG*// RHF/6-31G*
CI H
-419.328 26 -878.25741 -419.327 72 -878.222 85 -419.24468
CI
-878.16597
-419.715 58 -878.782 28 -419.71537 -878.748 68 -419.63581 (-419.636 79)' -878.696 52 (-878.698 26)"
molecule
X
XPCHICH, XH2CP=CH,
H CI H
XP(=CH2)2
'Value obtained from a geometry optimization of this molecule at the MP2 level of theory (MP2/6-31G*//MP2/6-3lG*).
examined in detail the structure of bis(methylene)phosphorane, and (3) we have discussed the structural effects of the substituent Cl on 1-3. A discussion of the relative energies of 1-3 has also been included in this report to better understand the available experimental data on these types of compounds.
Computational Methods Molecular geometries for 1-3 (R = H; X = H and C1) were fully optimized with the 6-31G* basis set,s which includes a set of six Gaussian d functions on the heavy atoms. Calculations with polarization p functions on hydrogen atoms (RHF/6-3 lG**) result in marginal differences for the geometrical parameters. Additional geometry optimizations have been carried out on the Z and E conformers of 1-phosphapropene, 4, and on vinylphosphine, 5. Equilibrium geometries and total energies for these molecules are shown in Figure 1. None of these species lies lower in energy than phosphirane. To explore the effects of d functions on HP(==CH2)2, we obtained optimized structures for this molecule with two other basis sets. One basis set, 6-31G, lacks d functions on the heavy atoms, and the other basis set, 6-31G(C*), incorporates a set of d functions only on carbon. The 6-31G and 6-31G* basis sets were also used to optimize the geometrical parameters for ethylene. All structures for 1-3 have been established to be local minima by normal-mode analyses of the 6-3 lG* energy surfaces. To determine the relative energy of each species with electron correlation included, we performed second-order M011er-Plesset6 (MP2) calculations with the 6-31G* basis set at the 6-31G* geometries, and these calculations are denoted MP2/6-3 1G*// RHF/6-3 1G*. Geometry optimizations have also been carried out at the MP2 level with the 6-31G* basis set for bis(methy1ene)phosphorane and its chloro-substituted analogues. These latter calculations are denoted MP2/6-3 lG*//MP2/6-3 1G*. Both core and valence orbitals were included in all correlation ( 5 ) (a) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lerr. 1972,66,217. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra 1973,28, 213. (c) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J . Chem. Phys. 1982, 77, 3654. (6) (a) Mmller, C.; Plesset, M. S . Phys. Reu. 1934, 46, 618. For more recent developments see: (b) Binkley, J. S.; Pople, J. A. Znt. J . Quantum Chem. 1975, 9, 229.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4887
Bis(y1ides) of Phosphorus(V)
H,
H
-
E(RHF) -419.32654 E(MP2) = -419.71222
(CS)
1.084
H H
.
E(RHF) E(MP2)
--
.419.32681 -419.71241
, /
(b)
'(-.
1 076
107.5
H, H '.. ' 1CI 1081 7 8
