J. Phys. Chem. 1983,87, 1903-1905
1903
Aminophosphenium Ion and Its CIS-Trans Isomerization Georges Trlnquler
and Marie-Rose Marre+
Laboratoire de Physique Quantique (E.R.A. au C.N.R.S.No. 821) and Laboratoire des HMrocycles du Phosphore et de /'Azote (E.R.A.au C.N.R.S. No. 926), Universit6 Paul Sabatier, 31062 Toulouse Cedex, France (Received: September 20, 1982; In Finel Form: December 20, 1982)
The geometrical and electronic structures of isolated model aminophosphenium ion H2N-PH+ are investigated theoretically. The expected conjugation of the nitrogen ?r lone pair is clearly shown. Accurate calculations result in an internal NP rotational barrier of 55 kcal/mol. The rotational barriers in H2N-XH+ and HN-XH systems (X = P, CH) are discussed. Introduction A recent report by Harrison' provided theoretical results on the simple phosphenium ions PH2+,PHF+, and PF2+. Since some structural data on diaminophosphenium ions are also a ~ a i l a b l e ,we ~ - ~undertook theoretical investigations on the aminophosphenium ion as an extension of our theoretical study on model phospha(II1)azene and aminophosphinidene molecule^.^ Although the evidence for diaminophosphenium derivatives is clearly established, that for monoaminophosphenium ions such as [Me2NPC1+,AlC1J2is less obvious. This work deals with (1)the geometrical and electronic structure of the monoaminophosphenium ion H2N-PH+ in its closed-shell singlet state: (2) its cis-trans isomerization by rotation around the N-P bond or inversion on phosphorus, and (3) its formation energy as the proton affinity of singlet HN=PH or H2N-P. The ab initio calculations were carried out with the PSHONDO program' for the SCF calculations. The PSHONDO program results from the introduction of the effective potentials of Durand and Barthelat8 in the HONDO program.g The CIPSI programlOJ' was used for the CI. The geometries were optimized at the SCF level with a double { + d ( t d ( P ) = 0.57, id(") = 0.95) basis set (DZd). On each optimized geometry, SCF and CI calculations were performed with a double { + polarization basis set (DZP) obtained from the former by adding a set of p functions on each hydrogen (q,(H) = 0.8). The CI by the CIPSI algorithm consists of the diagonalization of a subspace of the most important determinants (about 40 in our cases) combined with a second-order Moller-Plesset perturbation of the other determinants.
Results and Discussion The optimized geometry of the aminophosphenium ion is reported in Figure la. This geometry is found to be planar as a consequence of the conjugation of the nitrogen a lone pair in the vacant pr atomic orbital (AO) of the phosphenium center. This further results in a short NP bond which shows its multiple character. A typical NP single bond length is computed at about 1.67 A,12-14 whereas we have computed the typical N P double bond in HN=PH at 1.56 A.5 Therefore the NP bond in the ground-state aminophosphenium ion is close to a double bond (1). The structure of P(i-Pr2N)2+is known from
1
* Laboratoire de Physique Quantique. Laboratoire des HBtiirocycles du Phosphore e t de 1'Azote. 0022-3654/83/2087-1903$01.50/0
X-ray diffraction experiments on [P(i-Pr2N)2+,A1C14-].2-4 Its basic skeleton is planar, the P N bond lengths are 1.61 and 1.62 A, and the NPN angle is 115'. Since two NP conjugations occur in this molecular ion, it seems reasonable for the NIJ bonds to be longer than the one calculated without the correlation effect in our model. The difference in the valence angles on phosphorus should be due to steric hindrance occurring in P(i-Pr2N)2+.Moreover, the calculated valence angles reported in Figure l a are in good agreement with ab initio results for PH2+' and P(NHJ2+.4 The 0.03-Alengthening of the NP bond from HN=PH to H2N-PH+ is comparable to the NC bond lengthening c 1.27 A)15 occurring from methylenimine HN=CH2 ( d ~ = to methylenimonium ion H2N-+CH2 ( ~ N C= 1.29 according to STO-3G calculations on both species. The evidence for conjugation is shown by the a net charges (Figure 2a). Delocalization of the nitrogen a lone pair to the phosphorus vacant orbital splits the unitary a positive charge. The total net atomic charges somewhat modify this repartition through c effects. When the NP bond is rotated keeping the planar nitrogen, the perpendicular form has the following structure and is 49.5 (SCF, DZd) or 49.0 kcal/mol (SCF, DZP) above the planar form: PN, 1.638 A; PH, 1.431 8;NH, 1.019 A; LNPH, 103.4'; LHNH, 113.8'. The CI increases this energy difference to 56.9 kcal/mol (DZP basis set). When (1)Harrison, J. F. J.Am. Chem. SOC. 1981,103, 7406. (2)Thomas, M. G.; Schultz, C. W.; Parry, R. W. Znorg. Chem. 1977, 16,994. (3)Cowley, A. H.; Ushner, M. C.; Szobota, J. C. J. Am. Chem. SOC. 1978,100,7784. (4)Cowley, A. H.; Cushner, M. C.; Lattman, M.; McKee, M. L.; Szobota, J. S.; Wilburn, J. C. Pure Appl. Chem. 1980,52,789. 1982,104,6969. (5)Trinquier, G. J. Am. Chem. SOC. (6)It is strongly presumed that H2N-PH' has a singlet ground state, with a high singlet-triplet separation. PHZCwas calculated to have a singlet ground state with a singlet-triplet separation of -16 kcal/mol (ref 1). Substituting a hydrogen atom with an amino group is known to stabilize a closed-shell singlet state, as discussed for H2G&NH2 (Trinquier,G.; Barthelat, J. C.; Satg6, J. J. Am. Chem. SOC.1982,104,5931) and :N-NH2 (Davis, J. H.; Goddard, 111, W. A. Ibid. 1977,99,7111)and as calculated for HC-NH2 (Baird, M. C.; Taylor, K. F. J.Am. Chem. SOC. 1978,100,1333).See also Trinquier, G.; ThBse, Universit4 Paul-Sabatier, Toulouse, France, 1981. (7)Daudey, J. P. Private communication. (8)Durand, Ph.; Barthelat, J. C. Theor. Chim. Acta 1975, 38,283. (9)Dupuis, M.; Rys, J.; King, H. F. J. Chem. Phys. 1976,65, 111. (10)Huron, B.; Malrieu, J. P.; Rancurel, P. J. Chem. Phys. 1973,58, 5745. (11)An improved version of CIPSI was used Pelissier, M.; ThBse, Universiti Paul-Sabatier, Toulouse, 1980. (12)Cowley, A. H.; Mitchell, D. J.; Whangbo, M. H.; Wolfe, S. J. Am. Chem. SOC.1979,101,5224. (13)Barthelat, M.; Mathis, R.; Mathis, F. J.Mol. Struct. 1981,85,351. (14)For a review on P-N bond distances, see Clardy, J. C.; Kolpa, R. L.; Verkade, J. G. Phosphorus 1976,4,133. (15)Howell, J. M. J. Am. Chem. SOC.1976,98,886. Pearson, R.; Lovas, F. J. J. Chem. Phys. 1977,66,4149. (16)Kollman, P. A.; McKelvey, J.; Gund, P. J.Am. Chem. SOC.1975, 97,1640.
@ 1983 American Chemical Society
1904
The Journal of Physical Chemistry, Vol. 87,
No. 11, 1983
Trinquier and Marre
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Figure 2. Net atomic charges and net ?r charges (in parentheses) obtained with the DZP basis set: (a) planar aminophosphenium ion; (b) nonplanar aminophosphenium ion; (c) neutral trans-phosphazene from ref 5.
1.015
H
C
Figure 1. SCF (Dzd) optbnized geometries for aminophosphenkwn bn: (a) preferred planar geometry; (b) transition state for rotation around the NP bond; (c) transition state for inversion on phosphorus atom, in A and degrees.
TABLE I: Summary of t h e Relative Energies, in kcal/mol
the constraint of planarity on nitrogen is removed, the nitrogen becomes pyramidal. The pyramidalization curve is very flat. Two minima correspond to pyramidalizations which are cis and trans with repsect to the PH bond. The cis side is preferred over the trans side by 0.1 kcal/mol; the maximum of the curve occurs for a slight trans pyramidalization at 0.4 kcal/mol. For each minimum, a simultaneous optimization of pyramidalization and valence angles on nitrogen was carried out leading to nearly degenerate energies with a slight preference for cis pyramidalization. Finally, the cis-pyramidalized form was fully optimized but one must remember that pyramidalization gives a small energy gain and that a trans-pyramidalized form is nearly degenerate with the cis-pyramidalized form. From a technical viewpoint, the inclusion of d A O s in the nitrogen basis set is certainly an important requirement here. The geometry of this nonplanar pyramidalized form, which is the transition state for internal rotation around the NP bond, is reported in Figure lb. In this transition state, conjugation of the nitrogen lone pair no longer occurs. The NP bond length (1.65 A) is close to a single NP bond length (.;;1.67 A). The geometry of the nitrogen center ressembles that of a typical sp3 amino group. The positive charge remains localized mainly on the phosphorus atom (cf. Figure 2b). Since energy benefit due to pyramidalization is very small, the rotational barrier is computed after CI a t 55.1 kcal/mol (see Table I). The in-plane inversion through linear phosphorus was also explored. The NPH angle was kept equal to B O o , and the geometry was optimized and is reported in Figure IC. The molecule is planar and conjugated with a NP bond
H,N-PH'
--
SCF DZd planar nonplanar (with planar nitrogen ) nonplanar (with pyramidalized nitrogen) planar (with linear phosphorus) H N = P H (trans) t H' H,N-P ( ' A , ) + H'
GI,
DZP
0
DZP
0 49.5
49.0
0 56.9
48.8
48.0
55.1
1 0 4 . 1 102.9
95.8
221.3 219.9 222.7 227.0
shorter than in the relaxed planar form. As expected, linearization on phosphorus requires a lot of energy (more than 100 kcal/mol at the SCF level). This time, correlation effects diminish this energy difference, due to strong conjugation in the linear phosphorus form (see Table I). The cis-trans isomerization in H2N-PH+ proceeds therefore via internal rotation or torsion around the N-P bond. The rotational barrier is predicted to be 55 kcal/ mol. For a typical NP double bond, the rotational barrier in HN=PH was calculated under the same conditions at 40 kcal/mol only. These results could seem surprising. A similar trend was noticed by K~llman'~J'from HN=CH2 (aErOt = 57 kcal/mol) to H2N=CH2+ (AE,,, = 71 kcal/ mol). So, resonance stabilization in the methylenimonium (aminophosphenium) ion would be greater than the strength of the C-N (P-N) ?r bond broken in methylenimine (phospha(II1)azene). We suggest that the lower rotational barrier for the imine with respect to the amino ion might come from specific differential effects in the (17)Kollman, P. A.; Trager, W. F.; Rothenberg, S.; Williams, J. E.J. Am. Chem. SOC.1973,95, 458.
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983
Amlnophosphenlum Ion
TABLE 111: Valence Molecular Orbital Energy Levels of Planar H,NPH+ (DZP Basis Set)
TABLE 11: Comparison of Internal Rotational Barriers systems HN=PH H,N'-'PH+ HN=CH, H,N%H,+ H,N'-'CH(NH,)' H,N-C( NH, ),+ Me,N'-'PNMe,+ Me,N=PN(SiMe ) (i-Pr) N2PN (j-P;) ,'+ +
method
A E r O t , kcal/mol
CIa CIa SCF" SCF SCF SCFe SCFe exptlt exptlg exptlg
40 55 48 55,b 57c 71:" 72f 28 14 15 16 11
MO's
DZ basis set, ref 18. Extended basis set, ref 1 9 . DZ basis set, ref 16. e DZ basis set, ref 1 7 . f DZ basis set, ref 20. t 'H dynamic NMR, ref 4 .
nonconjugated forms, the resonance stabilizations being comparable in the planar-conjugated forms of neutral imine and amino cation. The nonconjugated neutral imine is stablized by a "residual" two-center three-electron interaction between the imino lone pair and the unpaired u electron of carbon in methylenimine 2 or phophorus in phospha(II1)azene 3 --..
N
'
N
P
2
E,
6a' u * (NP) 2a" Pn (P) or n * ~ p(LUMO) 5a' no (P) (HOMO) l a " nn ( N ) or n ~ p 4a' 3 a' 2a' 1a'
" DZP basis set, this work.
C
1905
'
3
eV
-2.12 -6.04 -18.61 -20.10 -23.45 -26.61 -29.92 -40.39
TABLE IV: Comparison o f Some Computed Proton Affinities, in kcal/mol SCF NH, H,N--CH,a HN=CH, HN=PHe
" Reference 1 6 .
220" 21 8b 230 226d 22aa 223b 221
Reference 22. Reference 1 7 . e This work.
CI 214'
220 Reference 2 3 .
For the planar aminophosphenium ion the SCF (DZP) calculations give the valence MO pattern displayed in Table 111. Energy levels have been reported for diaminophosphenium derivative~.~*~8' In a diamino system, the more important MO level ordering is P* (on N, P, N) (LUMO) nonbonding P (on N, N) (HOMO) nonbonding u lone pair (on P) bonding u (on N, P, N)
4
5
(in that case, the interaction is twofold due to the lone pair of phosphorus). This is no longer possible in the corresponding nonconjugated amino cations 4 and 5. 'H dynamic NMR experiments have been performed on (R2N)2P+derivative^.^.^ For (Me2NI2P+,they resulted in a rotational barrier of 15 kcal/mol (see Table 11). This is much smaller than our calculated value on H2N-PH+ (55 kcal/mol). As for the trend in the NP bond lengths, a lower rotational barrier can be expected when two amino groups are linked to the phosphenium center, for two reasons: (1) the conjugation of each amino group is smaller; (2) when one amino group rotates, the other amino lone pair can then fully delocalize into the phosphenium 3pzvacant AO. This is nicely illustrated in the calculations by Kollman et al.16 on the series H2N-CH2+, H2NCH+-NH2, and (H2N)&+. The corresponding calculated N-C rotational barriers are reported in Table 11. From H2N-CH2+ to H2N-CH+-NH2, the rotational barrier decreases by 43 kcal/mol which is a 61% decrease. Our results therefore seem consistent. At this point, one must say that there is no evidence for any P N rotational barrier in [Me2NPC1+,A1C14-]from 'H NMR experiments. As noticed by Thomas et al.2this may be related to the rapid exchange of C1- groups in the solution through bridging with AlC14- or other C1- donors. (18) Macaulay, R.; Burnelle, L. A.; Sandorfy, C. Theor. Chim. Acta (Bed.) 1973, 29, 1. (19) h h n , J. M.; Munsch, B. Theor. Chim. Acta (Bed.) 1968,12,91. (20) Apeloig, Y.; von R. Schleyer, P.; Pople, J. A. J . Am. Chem. SOC. 1977,99, 1291.
The main difference between the two systems is that in monoaminophosphenium, the two crucial MO's which are involved for electrophile or nucleophile reactivity or for bonding to a transition metal are now the frontier orbitals, namely, the n, (HOMO) and pI (or u*Np)(LUMO), both located mainly on phosphorus. The formation energies of the aminophosphenium ion from HN=PH or H2N-P are given in Table I. If the zero-point energies are neglected, these energy differences correspond to the proton affmities of lA' HN=PH and 'Al H2N-P. From a computational viewpoint, one can see in Table I that the correlation effects only slightly change the SCF calculated proton affinities; moreover they correspond to what is expected from the NP bond multiplicity of each species. Kollman et al." noticed a somewhat similarity in the proton affinities of sp3- or sp2-bonded nitrogens. Comparing the proton affinity of NH=PH with some other selected proton affinities (Table IV) supports this trend. On the other hand, the fair similarity between the proton affinities for HN=PH and H2N-P is a simple consequence of their close thermodynamic stabilities. Acknowledgment. We acknowledge the benefit of helpful discussions with Dr. R. Wolf and we thank Dr. J. P. Malrieu for reading the manuscript. Registry No. H2N-PH+, 85282-60-6; HN=PH, 61559-67-9. (21) Hutchins, L. D.; Paine, R. T.; Campana, C. F. J . Am. Chem. SOC. 1980, 102,4522. (22) Eades, R. A,; Weil, D. A,; Ellenberger, M. R.; Farneth, W. E.; Dixon, D. A.; Douglas, C. H. J . Am. Chem. SOC.1981, 103, 5372. (23) Eades, R. A.; Scanlon, K.; Ellenberger, M. R.; Dixon, D. A.; Marynick, D. S. J . Phys. Chem. 1980,84, 2840.
