6456
J. Phys. Chem. 1996, 100, 6456-6462
Nature of Bonding in Cyclic Conjugated Ylides La´ szlo´ Nyula´ szi* and Tama´ s Veszpre´ mi Department of Inorganic Chemistry, Technical UniVersity of Budapest, H-1521 Budapest, Gelle´ rt te´ r 4, Hungary ReceiVed: June 22, 1995; In Final Form: NoVember 21, 1995X
The conjugative ability of the λ5-PdC bond has been compared to its λ3-PdC counterpart at the MP2/631G*//MP2/6-31G* level of theory, using isodesmic reaction energies. Investigating heterobutadienes, it has been observed that compounds containing a λ3-PdC bond show similar delocalization energy as those with CdC units. As for λ5-phosphabutadienes, however, stabilization is achieved only in the case of C substitution. This behavior has been rationalized by perturbation theory arguments with the conclusion that while for the λ3-PdC and CdC bonds a two-way interaction, for the λ5-PdC system a one-way interaction is operational. Comparing cyclic systems containing λ3- and λ5-PdC bonds, similar structures (bond lengths) and stabilizations (as concluded from bond separation and homodesmic reaction energies) were obtained for six-membered rings including benzene and other rings containing one and three (symmetrically arranged) phosphorus atoms. The four-membered rings (1λ3,3λ3- and 1λ5,3λ5-diphosphacyclobutadienes), however, show completely different behavior, since the λ5-P derivative does not show destabilization upon ring formation and has equal bond lengths, while λ3-P rings are clearly antiaromatic. Rationalization has been given in terms of the differences in the one-way and two-way conjugative interactions.
Introduction A most important result in the 1980s in the field of maingroup chemistry was the synthesis of numerous EdC double bonded compounds (E being a second-row element). Among these compounds one of the greatest double bond strength and stability is exhibited by those containing σ2,λ3-phosphorus, as interpreted by the symmetry allowed overlap of the pz atomic orbitals.1,2 Furthermore, the conjugative ability of the λ3-PdC bond is rather high as was shown by the rotational barriers of BH2 and NH2 substituents attached to the carbon atom of the λ3-PdC unit.3 (Barriers for the rotation about the central single bond in case of phosphabutadienes, however, are lower.4) Energies of isodesmic reactions revealed that energy gain in conjugation is nearly the same for λ3-PdC and CdC bonded systems, and π-ionization energies of λ3-PdC systems are again nearly identical with those of the CdC systems.5 Aromatic stability of phosphabenzene was found to be just slightly less, than that of benzene.6-11 Azaphospholes10-11 and thiaphospholes12 were found to exhibit aromaticity comparable to the corresponding azoles and thiophenes. The recent synthesis and the structural characterization of ring systems containing σ4,λ5-phosphorus (such as the 1,1,3,3,5,5hexakis(dimethylamino)-1λ5,3λ5,5λ5-triphosphabenzene13) raised the question whether the λ5-PdC “double bond” is able to form aromatic compounds similarly to its σ2,λ3-P containing counterpart. In our recent work14 we pointed out by comparing the MP2/6-31G* structures of 1λ5,3λ5,5λ5-triphosphabenzene and 1λ5,3λ5-diphosphacyclobutadiene that although the geometry of 1λ5,3λ5,5λ5-triphosphabenzene is indicative of an aromatic structure (all PC bond lengths are equal; thus its Bird index,15 considered as the best measure of aromaticity,16 is 100, the same value as for benzene), this compound is considered nonaromatic, in accordance with its NMR13 and photoelectron spectra.17 Bird, in an investigation of the aromaticity of phosphorus compounds,18 concluded that aromaticity of planar ring systems containing pentavalent phosphorus is somewhat (by about 10X
Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6456$12.00/0
15%) less than that of their trivalent phosphorus containing counterparts. Ma¨rkl,19 on the other hand, stated that the λ5phosphabenzenes are not Hu¨ckel aromatic systems. In a recent review of Johnson20 it has been even more explicitly stated that “λ5-phosphorines (phosphinines) are not aromatic”. To the contrary, in the original work,21 cited by Johnson’s review20 the following statement has been found: “λ5-phosphorines (phosphinines) ... may be classified as Hu¨ckel aromatic systems”.21 Attaching BH2 and NH2 groups to the C site of H3PdCH2 resulted in a significant stabilization in the case of the boro substituent, as measured by isodesmic reaction energies.22 A simple model to account for the delocalization in case of compounds containing λ5-PdC unit was the “island model” introduced by Dewar23 for phosphazenes. This model considers explicitly the participation of d orbitals in the formation of pentavalent phosphorus and is considered to give the best description of hypervalent “double-bonded” systems even in the most recent literature.24,25 As recent investigations pointed out that the d-orbital participation (sp3d hybrid in the present case) is of little importance26 in the formation of hypervalent compounds, the validity of the “island model” should be questioned, as well. In our recent work27 dealing with the electronic structure of the λ5-PdC “double bond” it was pointed out that the electronic structure of conformer a (Scheme 1), which can be used to build possibly conjugated or aromatic systems, is similar to that in the allyl ion (the third “center” of the bonding is formed from the two hydrogens attached to phosphorus). Although this conformer corresponds to a first-order saddle point at the potential energy surface, its energy is just slightly (by less than 1 kcal/mol) higher than that of conformer b. The equivalence between this and the usually accepted dative-bond + backdonation model28,29 has been shown and the large charge separation as well as the low ionization energy characteristic for these substances was accounted for as well. Furthermore, it was shown by the analysis of the electron density that the bonding can be described similarly to the “normal” π-bond (in © 1996 American Chemical Society
Bonding in Cyclic Conjugated Ylides SCHEME 1
HPdCH2), without using the usual concept of “ylidic bonding” advocated even in the most recent review.20 By using the above model, an understanding of the bonding in longer chains and cycles can be attempted as well. The aim of the present work is to understand how far conjugative effects and aromatic stabilizationsknown to be usual in “normal” π-bonded systemssare operational in systems containing the σ4,λ5-PdC bond. Calculations Quantum chemical calculations have been carried out by the GAUSSIAN 9030 and GAUSSIAN 9231 packages on an IBM 3090 mainframe and an IRIS 4000 workstation. The basis set used was the standard 6-31G* type. All the geometries have been optimized at the MP2 level of theory. At the optimized geometries second-derivative calculations were carried out. Unless otherwise stated, the resulting structures were real minima on the potential energy surface as shown by the positive harmonic frequencies obtained. For calculating the structure of anions, the 6-31G* basis set was used in order to get comparable results with calculations carried out for the phosphorus ring systems. As it is known that geometries of anions are reasonably well described without using diffuse functions but such basis sets give poor results for energies,32 we will restrict the discussion to the geometries of these compounds. The calculations carried out at the MP2/631+G* level for the open-chain anions, indeed resulted in minor changes only in the structural parameters. Results and Discussion Before analyzing the bonding in ring systems, interactions in smaller linear units should be considered. The simplest conceivable compounds to study the conjugative interactions between double bonded systems are (hetero)butadienes. The rotational barrier of λ3-phosphabutadienes have recently been investigated by Bachrach et al. at the MP2/6-31G*//HF/6-31G* level of theory,4 thus here we did not consider the different rotamers but reoptimized the structures found to be minima in the previous study4 at the MP2/6-31G* level of theory. To investigate the conjugative ability, bond separation reaction energies were studied. For compounds containig λ5-phosphorus atom we considered geometries derived from conformer a (in Scheme 1) of H3PdCH2 (although it is not necessarily of minimum energy) in order to have the same sort of interactions, as in case of the possibly aromatic ring compounds (see later). (As a consequence, while calculating bond separation energies and bond lengths, the a conformer of H3PdCH2 has been considered throughout. Detailed investigation of the rotational barrier in λ5-phosphabutadienes will be carried out in a subsequent work.) Energies of bond-separation reactions for the butadiene congeneers according to the general reaction scheme below are compiled in Table 1:
HAdDsEdGH + DH3 + EH3 f HAdDH + HEdGH + DH2sEH2 (1) (A, D, E, and G being CH, P, and PH2 groups).
J. Phys. Chem., Vol. 100, No. 16, 1996 6457 TABLE 1: MP2/6-31G* Bond Lengths (Å), Double Bond (%),a and Bond-Separation Energy (∆E, kcal/mol) in Phosphabutadienes AdDsEdGb AdDsEdG
AdD
DsE
EdG
∆E
CCCC P(3)CCC P(5)CCC CP(3)CC CP(5)CC P(3)CP(3)C P(5)CP(5)C P(3)CCP(3) P(5)CCP(5) CP(3)P(3)C CP(5)P(5)C CXCC CCCX
1.343 1.689 1.673 1.675 1.659 1.690 1.668 1.699 1.659 1.680 1.659 1.401 1.373
1.456 (36%) 1.450 (39%) 1.447 (41%) 1.811 (25%) 1.799 (25%) 1.809 (26%) 1.758 (51%) 1.421 (54%) 1.472 (27%) 2.149 (21%) 2.176 (23%) 1.485 (21%) 1.413 (58%)
1.343 1.346 1.347 1.343 1.338 1.678 1.665 1.699 1.662 1.680 1.650 1.346 1.413
14.58 16.40 14.61 10.44 2.32 11.59 12.07 18.40 -0.42 8.70 4.93
a Double bond percentage was defined for a bond with a length of r as rX-Y - r/rX-Y - rXdY, where rX-Y and rXdY are the single and double bond lengths, respectively, The bond lengths for these prototype bonds are shown in Table 2. b X: CH-CH2-; P(3) and P(5) denotes λ3- and λ5-P atoms, respectively.
In the case of λ3-phosphorus compounds the bond-separation reaction energies are between 8 and 18 kcal/mol. The 14.58 kcal/mol bond separation energy of 1,3-butadiene is within this interval in agreement with our previous finding5 about the similar conjugative abilities of the λ3-PdC and CdC bonds. It should be noted as well that there is a slight but systematic decrease in the bond-separation energy if the union of the two double bonds happens to be at the P instead of the C site. (The concept of union was used by Dewar33 to describe the interaction of two π-systems connected by a single bond.) For the compounds containing λ3-phosphorus the bond separation energies are significantly higher (by about 6-12 kcal/ mol) than the rotational barriers.4 A possible explanation is that in the case of the isomer rotated by 90° there is some hyperconjugative interaction with the rest of the π-system. Similar effects are known to stabilize cations and radicals in β-position34 (for the β-PH2-substituted ethyl cation the energy difference between the planar and perpendicular geometriessstabilization has been achieved in the latter caseswas 7.2 kcal/mol34a). Another possible source of this difference might be that the bond-separation reaction is not homodesmic; thus changes in DH and EH energies upon changing the valence states of C and D are not considered. For compounds containig λ5-phosphorus, however, a much larger variance is shown in bond separation reaction energies than for their λ3-phosphorus-containing counterparts. Considering 1λ5,3λ5-diphosphabutadienes, the stabilization could simply be attributed at first sight to the attractive Coulomb forces between the oppositely charged central P and C atoms as could have been surmised by considering ylidic structures. Because H3PdCHsCHdCH2 (built from the polar PH3dCH2 and the nonpolar CH2dCH2 units) exhibits, however, similar bond separation energy as HPdCHsCHdCH2 the conjugative interaction should play an important role here, and although the attraction between opposite charges may have some stabilizing effect, it cannot be used as a sole argument for the other systems either. The results derived from bond-separation reactions can be rationalized by using perturbation theory.33 According to second-order perturbation theory, the energy of an interaction is determined mainly by the two HOMO-LUMO interactions between the reacting molecules. (It should be noted that for the systems with λ5-PdC units the LUMO is not of proper symmetry; thus, a higher lying unoccupied orbital should be
6458 J. Phys. Chem., Vol. 100, No. 16, 1996
Nyula´szi and Veszpre´mi
SCHEME 2
SCHEME 3
considered. For the sake of simplicity, however, in the following analysis the term LUMO will always be used for the lowest unoccupied orbital with the proper a′′ symmetry.) This stabilization is proportional to the (square of the product of the) MO coefficients at the interacting sites, while inversely proportional to the HOMO-LUMO energy gap. Since the highlying HOMO of H3PdCH2 (cf. with the low ionization energy35) is mainly localized at the carbon atom,27 a large stabilization should be expected if the union takes place at this center. Substitution at phosphorus (having large linear coefficient in the rather high-lying LUMO), however, should result in sizeable effects only if the substituent group has large MO coefficient in its high-lying HOMO. Indeed, significant stabilization was achieved on the λ5-PdC unit if substituted at the C site, while substitution at the P site results in small stabilization. (The only exception is PH3dCHsCHdPH3, where no sizeable stabilization has been obtained. Note, however, that in this case the MO coefficient of carbon atoms is small in both fragment LUMOs.) There is one compound, substituted at the P site, but still showing a significant stabilization; 1λ5,3λ5-diphosphabutadiene (PH3dCHsPH2dCH2). The stabilization in that case should mainly be attributed to the HOMO (at the C site) LUMO (at the P site) interaction (Scheme 2), both centers having large coefficients in the respective orbitals, while the other HOMOLUMO interaction is nearly ineffective, due to the small MO coefficients in the respective orbitals. Thus, the interaction present in 1λ5,3λ5-diphosphabutadienes is of “one-way” (donoracceptor like) type. A similar effect should be responsible for the observed stabilization of the λ5-PdC bond, substituted by the acceptor BH2 group22 at the C site. In the case of forming butadiene (1λ3,3λ3-diphosphabutadiene) from two ethylenes (λ3-phosphaethenes) the interactions should be equal (similar) at the different sites of the double bonds. Furthermore both HOMO-LUMO interactions have similar contributions, therefore in case of the usual π-bonding, the interaction is considered to be a two-way interaction (Scheme 2). For the allyl ion, however, the same effects as for H3PdCH2 should be observed, due to their similar π-type orbitals.27 To study this effect, substitution by a CHdCH2 group at different positions of an allyl ion (forming 2-methylenebutadienyl anion (c) and pentadienyl anion (d), respectively, in Scheme 3) have been considered. From the structures obtained at the MP2/631G* level of theory, it can be clearly seen, that for the CH2sPH2sCHdCH2 analogue (c) the bond lengths of the double bond part and allyl part are nearly unchanged (cf. with the values in Table 1), while for the H3PdCHsCHdCH2 analogue pentadienyl anion (d) the alternation of the bond lengths was significantly reduced.
Figure 1. Shape of the two uppermost occupied π-orbitals of butadiene and that of 1λ5,3λ5-diphosphabutadiene shown by 1 Å above the molecular plane, as plotted by the MOLDEN program.37
TABLE 2: MP2/6-31G* Bond Lengths (Obtained for the Simplest Hydrogenated Derivatives Containing the Respective Bond) (in angstroms) CH3-CH3 CH2dCH2 CdCsC- ion
1.524 1.334 1.391
PH2-CH3 PHdCH2 PH4-CH3 PH3dCH2
1.857 1.675 1.847 1.674
PH2-PH2 PHdPH PH4-PH4 PH3dPH3
2.183 2.039 2.231 2.145
The different behavior of allyl-like systems and “normal” double bonds can be seen on the π-orbitals as well. The interaction of the orbitals can again be discussed in terms of perturbation theory, as being proportional to the MO coefficients in the interacting orbitals. While for the “normal” double bonds orbital energies split due to their interaction, no (or only small) split is observed, if an allyl-like system is substituted at the central atom, having zero (nearly zero) linear coefficient in the HOMO. This finding is in agreement with the orbital energies and orbital shapes for anion (c), CH2dPH2sCHdCH2 and CH2dPH2sCHdPH3 if compared to those of the isolated building blocks. In case of the latter compound for example, the shape of the two uppermost occupied π-orbitals is shown in Figure 1. This picture is in complete contrast with that of the conventional delocalized π-systems (shown for 1,3-diphosphabutadiene in Figure 1). The interaction between the HOMOs of CH2dCH2 and H2CdPH3, however, is considerable in the case of CH2dCH-CHdPH3, as both HOMOs have large linear coefficients at the site of substitution. This is in agreement with the photoelectron spectrum of CH2dCHsCHdPMe3,35 where a sizeable split of the π-bands was observed (compared to the IEs of Me3PdCH2 and CH2dCH2). The length of the central “single” bond (Table 1) as compared to the bond length in the isolated molecules obtained at the same level of theory (Table 2) reveals some bond shortening in each case for both the λ3- and λ5-phosphorus-containing compounds. The shortening of these bonds as shown by the double bond percent, however, does not correlate satisfactorily with the stabilization of the bond-separation reactions. The most important geometrical features of λ3- and λ5phosphinines are shown in Figure 2. As is evident from the data, the structure of the two compounds is similar (in agreement with the X-ray structural data of some substituted derivatives18), the two hydrogens situated at phosphorus seemingly does not influence the bonding situation in the ring. The equalization of the CC bond lengths is slightly smaller for the pentavalent system, in agreement with the reported small decrease of the Bird index.18 The energies of the bond separation (2) and homodesmic (3) reactions (Table 3) of λ3- and λ5-phosphinines are similar
Bonding in Cyclic Conjugated Ylides
J. Phys. Chem., Vol. 100, No. 16, 1996 6459
Figure 2. Geometrical parameters (in angstroms with bold letters) and charge distribution (in italics) of the investigated four- and sixmembered rings.
TABLE 3: MP2/6-31G* Bond Separation Reactiona and Homodesmic Reactionb Energies (kcal/mol)
∆E bond sepn ∆E homodesm
71.91 28.16
68.66 27.24
53.26 20.55
Figure 3. Formation of the π-orbitals of λ5-phosphinine from those of λ3-phosphinine and two hydrogens. ∆E bond sepn ∆E homodesm
∆E bond sepn ∆E homodesm
60.77 26.01
-52.37 -81.53
62.03 21.80
-16.85 -40.02
13.96 -10.22
a Bond separation reactions for the six and three-membered rings (negative value means destabilization).
and only slightly less than for benzene:
C5H5X + 5CH4 + XH3 w 2CH2dCH2 + 2CH3sCH3 + CH2dXH + CH3sXH2 (2) C5H5X + 2CH2dCH2 + CH2dXH w CH2dCHsCHdXH + CH2dXsCHdCH2 + CH2dCHsCHdCH2 (3) where X ) CH, P, PH2, the stabilization of the ring with the pentavalent phosphorus being somewhat smaller than with the trivalent one. This finding is in agreement with the conclusion drawn from the bond-length distribution and is indicative that not only λ3- but λ5-phosphinines are aromatic systems. It is interesting to note here that the chemical reactivity of cyclic conjugated six-membered rings is different from that of the nonconjugated systems. Ma¨rkl36 detected the presence of a 3,4dihydro-1λ5-phosphanaphthalane derivative, by its Wittig reaction product, while its fully conjugated counterpart 1λ5phosphanaphthalane failed to undergo the Wittig reaction. Comparing the orbitals (Figure 3) and the uneven charge distribution (Figure 2) of the two systems, however, a different
picture emerges. Following the idea described for H3PdCH2,27 the respective orbitals can be constructed from those of P and the hydrogens of proper symmetry. Thus, in the formation of a 3c-4e bond (Figure 3, right side) on phosphorus, an orbital of b1 symmetry forms38 interacting with the ring π-orbitals (of the same symmetry). The result of this interaction is that the HOMO is significantly destablized (cf. the photoelectron spectra of λ3- and λ5- phoshinines39), and as the linear coefficient of phosphorus in the HOMO decreases (while it increases in the LUMO) phosphorus becomes more positive than in λ3- phosphinine. The charge distribution of λ5-phosphinine (Figure 2) shows some resemblance to that of the benzyl anion (recall the analogy between the H2(P) unit and the CH2- group) at the atoms being separated from the inductive effect of phosphorus by at least one σ-bond (ring atoms 3, 4, and 5). (It should be noted that in a previous work the charge distribution was compared to that in pentadienyl anion.21) The charge separation is somewhat more balanced for λ3-phosphinine (Figure 2), although the PC bond is still rather polar as has been shown previously by the comparing HPdCH2 with H3PdCH2.27 By analogy with benzene and cyclobutadiene (being built from CdC bonds), triphosphabenzenes and diphosphacyclobutadienes (made of λn-PdC, n ) 3 and 5 blocks) should be aromatic and antiaromatic species, respectively, according to Hu¨ckel’s rule. The above surmise is fully supported for the systems built from λ3-PdC units as has been shown before,40 in agreement with the similar conjugative abilities of the CdC and λ3-PdC bonds.5 Bond lengths in the case of the sixmembered ring triphosphabenzene are equal (Figure 2) and are intermediate between that of the single and double bonds (cf. the data in Table 2), while for 1λ3,3λ3-diphosphacyclobutadienes they are alternating, approaching the single- and doublebond length.40 Bond separation (4-5) and homodesmic (6-7) reaction energies (Table 3) predict stabilization and destabilization:
6460 J. Phys. Chem., Vol. 100, No. 16, 1996
Nyula´szi and Veszpre´mi
C3H3X3 + 3CH4 + 3XH3 w 3CH2dXH + 3CH3sXH2 (4)
SCHEME 4
C2H2X2 + 2CH4 + 2XH3 w 2CH2dXH + 2CH3sXH2 (5) C3H3X3 + 3CH2dXH w 3CH2dXsCHdXH
(6)
C2H2X2 + 2CH2dXH w 2CH2dXsCHdXH
(7)
where X ) CH, P, PH2, for the six- and four-membered rings, respectively, similarly to the carbon analogues. (It should be noted that the destabilization in the case of 1λ3,3λ3-diphosphacyclobutadiene is smaller than for cyclobutadiene. This fact can at least partly be attributed to the lower ring strain in the phosphorus compound, as the bonding angles of near 90° are preferred by phosphorus but not carbon.41 On the other hand, the stabilization energy of 1λ3,3λ3,5λ3-triphosphabenzene (the homodesmic reaction energy is smaller by 7.61 kcal/mol than that of benzene) can similarly be attributed to the ring strain caused by the near 90° bonding angle about phosphorus atoms). For λ5-PdC moieties, however, the picture seems to be controversial. The MP2/6-31G* treatment in the case of 1λ5,3λ5-diphosphacyclobutadiene was shown to be adequate, as in the CASSCF(2,2) calculation the weight of the reference was 0.99. As it can be seen in Figure 2, bond lengths are equal not only in case of six-membered rings, but in the four-membered ring 1λ5,3λ5-cyclodiphosphabutadiene as well,14 in agreement with the published crystal structural data of the corresponding dimethylamino-substituted derivatives.42 In case of 1λ5,3λ5cyclodiphosphazenes Trinquier43 reported similar structural characteristics based on HF/DZP calculations. To explain the equal P-N bond lengths, he used the island model of Dewar,23 although expressed that d orbitals are needed only to obtain qualitatively good results, but bonding can be described without explicit consideration of the d orbitals. The P-C bond length (Figure 2) in 1λ3,3λ3,5λ3-triphosphabenzene is similar, as for 1λ5,3λ5,5λ5-triphosphabenzene, while it is somewhat larger in the case of 1λ5,3λ5-diphosphacyclobutadiene, indicating a somewhat weaker bond in the latter case. Bond separation reaction energies (Table 3) show stabilization for both the sixand four-membered rings, homodesmic reaction energies, however, indicate stabilization for the six-membered rings only. The destabilization obtained in the homodesmic reaction for the four-membered ring 1λ5,3λ5-diphosphacyclobutadiene is smaller than for the other investigated four-membered systems (cyclobutadiene and 1λ3,3λ3-diphosphacyclobutadiene) and is likely to be attributed to ring strain. An estimation of ring strain can be given by the isodesmic reaction 8 written for the corresponding
C2P2H10 + 2CH4 + 2PH5 ) 4CH3sPH4
(8)
saturated ring (1λ5,3λ5-diphosphacyclobutane, C2P2H10). The destabilization in (8) was 20 kcal/mol at the MP2/6-31G* level of theory. Although the different hybridization will modify the ring-strain energy, by using this rough estimate the homodesmic reaction energy becomes thermoneutral, or indicates slight stabilization. The ring strain (estimated by a similar procedure) for the corresponding compound containing trivalent phosphorus (C2P2H6) was 10.0 kcal/mol. To understand the behavior of the four-membered rings second order perturbation theory can again be used. The stabilization energy of butadiene (built from two ethylenes) is the sum of C1,HOMO-C2,LUMO and C1,LUMO-C2,HOMO interactions (all other terms are smaller and thus being neglected here). As all the linear coefficients might be positive in that case, stabilization is achieved. If the butadiene system is closed at the other side as well to cyclobutadiene (Scheme 4), the linear
SCHEME 5
coefficients in both LUMOs are of opposite sign. Thus the interaction energy will cancel that obtained at the formation of butadiene and therefore the total interaction energy remains zero, resulting in two uncoupled double bonds (cf. with bond lengths) and an antiaromatic ring. Indeed, if the estimated 10 kcal/mol ring strain (see above) and the -16.86 kcal/mol bond separation energy is taken into consideration for 1λ3,3λ3-diphosphacyclobutadiene, the destabilization upon the formation of the fourmembered ring is about 7 kcal/mol only. For 1λ5,3λ5-diphosphacyclobutadiene the one-way C-P interaction (see Scheme 2), shown effective for the linear 1λ5,3λ5-diphosphabutadiene, should be operational in the fourmembered ring as well. As it is shown in Scheme 4 this interaction will not be affected, whether it is arranged in a cyclic way or not. This results in stabilization independent of the number of interacting λ5-PdC units. Indeed, the bond-separation energy shows stabilization, while the homodesmic reaction energy (in this case the two interactions between the λ5-PdC units are compared to that in two linear 1λ5,3λ5-diphosphabutadienes) is nearly thermoneutral, or somewhat positive, if the ca. 20 kcal/mol ring strain is considered. The stabilization in the homodesmic reaction is larger for the six-membered ring than what could be calculated from adding together interactions resulting from the three 1λ5,3λ5-diphosphabutadienes, as shown by the stabilization in the homodesmic reaction. This is presumably a result of the interactions between the lower lying occupied and the unoccupied orbitals. These interactions are similar to those resulting in an aromatic stabilization in case of benzene (i.e., the nodal properties of the interacting fragments are the same, as for interacting ethylenes). Due to the larger separation of the interacting orbitals, however, this stabilizing effect is smaller than in case of benzene. As it has been shown for 1λ5,3λ5-phosphabutadienes, firstorder perturbation in these molecules will not affect the energies and shapes of the HOMOs significantly. Similarly, the shape of these orbitals in the four- and six-membered rings are almost identical, each of the uppermost orbitals being localized on one of the carbon atoms. Thus, the orbital energies are also very similar in case of the four- and six-membered rings in agreement with their photoelectron spectra that were pointed out to be surprisingly similar.12 Recalling the analogy between the allyl ion and H3PdCH2,27 the structures of 1,3-dimethylenecyclobutadienyl (e) and 1,3,5trimethylenebenztriyl (f) (Scheme 5) has been calculated as well. The structure and the (anti)aromatic character of 2,4-dimethylenecyclobutadienediyl (e) have been the subject of previous investigations as well.44 It has been pointed out by using
Bonding in Cyclic Conjugated Ylides perturbation theory developed to describe delocalization by donor-acceptor interactions45 that this molecule is of electrondelocalizing type and belongs to those substituted cyclobutadiene derivatives46 that are not antiaromatic. Indeed both the fourand the six-membered rings have nonalternating bonds (see Figure 2), the bond length in the six-membered ring being somewhat shorter, similarly to the λ5-P derivatives. Conclusions While the λ3-PdC bond has similar conjugative ability to that of the CdC bond, in agreement with our previous findings,27 the conjugative ability of the λ5-PdC bond differs significantly from them, showing sizeable stabilization in some cases when coupled with another double bond, while in certain cases no stabilization is observed in bond-separation reaction energies of (phospha)butadienes. Using perturbation theory, it can readily be shown that the interaction energy is determined mainly by HOMO-LUMO interactions, and thus the linear coefficients27 of these orbitals at the place of substitution. Since the linear coefficients in the LUMO at the C site and in the HOMO at the P site are near zero for the λ5-PdC system (in analogy with the terminal and central carbon atoms of allyl ion), coupling the double bonds, results in small (or zero) stabilization energies. Sizeable interaction with the involvement of the λ5-PdC system can be achieved only if the HOMO is perturbed at the C site; thus this type of interaction is a “one-way conjugation” pointing from the C atom of the λ5-PdC system toward the other subunit. For ring systems the same type of interaction scheme can be used as for the open chain compounds. The “two-way interaction” characteristic for “normal” double bonds such as CdC is subject to orbital phase match in formation of conjugated aromatic systems resulting in Hu¨ckel aromatic or antiaromatic systems. For the “one-way interacting” double bonds as λ5-PdC, stabilization is expected in any ring system built from such units independent of the ring size. Thus, both four- and six-membered systems containing λ5-PdC units are stabilized by conjugative interaction. Ring strain that is larger in four- but smaller in six-membered rings than in case of CdC bonded compounds acts to modify the above effects. As for the question of the degree of conjugative ability and aromaticity of compounds containing λ5-PdC units, the present results show that some (probably the most important) aromaticity criteria are fulfilled in these systems (equalization of bond length, thus large Bird index, significant stabilization in homodesmic reaction energies, and difference in the chemical reactivity with respect to the open chain or not fully conjugated cyclic systems). Other usually accepted aromaticity criteria, such as the chemical shift in the 13C NMR spectrum or the different spectroscopic (photoelectron spectrum) and structural behavior of four- and six-membered rings, however, show differences from the benzene case and indicate that these systems are not aromatic. Since the definition of aromaticity is rather hazy,47 it is not wise to classify these stabilized cyclic conjugated systems with λ5-PdC units as aromatic. Since the consideration that the electronic structure of H3PdCH2 is similar to that of an allyl ion27 gives explanation for all the observed “aromatic” and “nonaromatic” characteristics, the usual description20 that these systems should be handled simply as ylides, discarding completely the concept of ylenes (thus they are completely different from double-bonded and conjugated systems), is misleading and needs reVision. Acknowledgment. Financial support from OTKA T 014955 and T 4097 is gratefully acknowledged. Financial support for
J. Phys. Chem., Vol. 100, No. 16, 1996 6461 the purchase of a workstation is also acknowledged from the Foundation of the Technical Education and for the OMFB. References and Notes (1) Feller, D.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1985, 107, 2596. (2) For a review of the chemistry of the σ2, λ5-PdC bond see: Regitz, M., Scherer, O. J., Eds. Multiple Bonds and Low Coordination in Phosphorus Chemistry; G. Thieme Verlag: Stuttgart, 1990. (3) Korkin, A. A. Int. J. Quantum. Chem. 1990, 38 245. (4) Bachrach, S. M.; Liu, M. J. J. Am. Chem. Soc. 1991, 113, 7929. (5) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J. J. Phys. Chem. 1993, 97, 4011. The similar chemical reactivity of the two bonds has been described in: Mathey, F. Acc. Chem. Res. 1992, 25, 90. Appel, R. Pure Appl. Chem. 1987, 59, 977. (6) Baldridge, K. K.; Gordon, M. S. J. Am. Chem. Soc. 1988, 110, 4204. (7) Janoschek, R. Chem. Ber. 1989, 122, 2121. (8) Bock, C. W.; Tratchman, M.; George, P. Struct. Chem. 1990, 1, 345. (9) Jonas, V.; Frenking, G. Chem. Phys. Lett. 1993, 210, 211. (10) Veszpre´mi, T.; Nyula´szi, L.; Re´ffy, J.; Heinicke, J. J. Phys. Chem. 1992, 96, 624. (11) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J.; Burkhardt, B.; Regitz, M. J. Am. Chem. Soc. 1992, 114, 9080. (12) Nyula´szi, L.; Va´rnai, P.; Krill, S.; Regitz, M. J. Chem. Soc., Perkin Trans. 2 1995, 315. (13) Fluck, E.; Heckmann, G.; Plassa, W.; Spahn, M.; Borrman, H. J. Chem. Soc., Perkin Trans. 1 1990, 1223. (14) Veszpre´mi, T.; Nyula´szi, L.; Va´rnai, P.; Re´ffy, J. Acta. Chim. Hung.Models Chem. 1993, 130, 691. (15) Bird, C. W. Tetrahedron 1985, 41, 1409. (16) Katritzky, A. R.; Barczynski, P.; Musumarra, G.; Pisano, D.; Szafran, M. J. Am. Chem. Soc. 1989, 111, 7. (17) Gleiter, R.; Veszpre´mi, T.; Fluck, E. Chem. Ber. 1991, 124, 2071. (18) Bird, C. W. Tetrahedron 1990, 46, 5697. (19) Ma¨rkl, G. λ3-Phosphinines, Aza-λ3-Phosphinines and λ3,λ3-Diphosphinines in Multiple Bonds and Low Coordination in Phosphorus Chemistry Regitz, M., Scherer, O. J., Eds.; G. Thieme Verlag: Stuttgart, 1990. (20) Johnson, A. W.; Kaska, W. C.; Ostoja Starzewski, K. A.; Dixon, D. A. Ylides and Imines of Phosphorus; John Wiley Inc. New York 1993. (21) Scha¨fer, W.; Schweig, A.; Dimroth, K.; Kanter, H. J. Am. Chem. Soc. 1976, 98, 4410. (22) Bestmann, H. J.; Kos, A. J.; Witzgall, K.; Schleyer, P. v. R. Chem. Ber. 1986, 119, 1331. (23) Dewar, M. J. S.; Lucken, E. A. C.; Whitehead, M. A. J. Chem. Soc. 1960, 2423. (24) Doyle, M. P.; Westrum. L. J.; Wolthuis, W. N. E.; See, M. M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. J. Am. Chem. Soc. 1993, 115, 958. (25) Bieger, K.; Tejeda, J.; Re´au, R.; Dahan, F.; Bertrand, G. J. Am. Chem. Soc. 1994, 116, 8087. (26) (a) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 1434. (b) Magnusson, E. J. Am. Chem. Soc. 1993, 115, 1051. (27) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J. J. Phys. Chem. 1995, 99, 10142. (28) Naito, T.; Nagase, S.; Yamataka, H. J. Am. Chem. Soc. 1994, 116, 10080. (29) Gilheany, D. G. Chem. ReV. 1994, 94, 1339. (30) GAUSSIAN 90, Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A.; Binkley, J. S.; Gonzalez, C.; Defrees, D. H.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, R. L.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1990. (31) GAUSSIAN 92, Revision C, Frisch, M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, N. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Repolge, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.: Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1992. (32) Euig, C. S.; Van Wayer, J. P. J. Am. Chem. Soc. 1990, 112, 109. (33) Dewar, M. J. S. The PMO Theory of Organic Molecules; Plenum Press: New York, 1975. (34) For leading references see: (a) White, J. C.; Cave, R. J.; Davidson, E. R. J. Am. Chem. Soc. 1988, 110, 6308. (b) Guerra, M. J. Am. Chem. Soc. 1992, 114, 2077 (35) Ostoja Starzewski, K. A.; Bock, H. J. Am. Chem. Soc. 1976, 98, 8486. (36) Ma¨rkl, G. Angew. Chem., Int. Ed. Engl. 1963, 2, 153. (37) MOLDEN 2.5 Schaftenaar G. Caos/Camm Center, The Netherlands. (38) A similar interaction was noted before as hyperconjugation with the lone pairs of oxygen situated at the λ5-phosphorus atom and was used to explain the lowering of the first IE of λ5-phosphinine with respect to λ3-phosphinine.39a
6462 J. Phys. Chem., Vol. 100, No. 16, 1996 (39) (a) Schweig, A.; Scha¨fer, W.; Dimroth, K. Angew. Chem. 1972, 84, 636. (b) Schweig, A.; Scha¨fer, W.; Ma¨rkl, G. Tetrahedron Lett. 1973, 39, 3743. (40) The structure obtained in the present study for λ3,λ3-diphosphacyclobutadiene is similar to that one published recently by Schoeller and Busch (MCSCF/DZP level of theory): Schoeller, W. W.; Busch, T. Angew. Chem. 1993, 105, 635. (41) Schoeller, W. W. Bonding Properties of Low Coordinated Phosphorus Compounds in Multiple Bonds and Low Coordination in Phosphorus Chemistry Regitz, M.; Scherer, O. J., Eds.; G. Thieme Verlag: Stuttgart, 1990. (42) The structure of the 1,1,3,3-tetrakis(dimethylamino)-1λ5,3λ5-diphosphabutadiene (Svara, J.; Fluck, E.; Riffel, U. Z. Naturforsch. B 1985, 40, 1258) and that of 1,1,3,3,5,5-hexakis(dimethylamino)-1λ5,3λ55λ5-triphoshabenzene (Gleiter, R.; Veszpre´mi, T.; Fluck, E. Chem. Ber. 1989, 129, 2071) has been determined by X-ray diffraction. It should be noted that the calculated structures being similar to that obtained for the substituted derivatives were not real minima but saddle points (having one and two imaginary frequencies for the four and six-membered rings, respectively) on the potential energy surface, according to second-derivative calculations at both the HF/6-31G* and MP2/6-31G* levels of theory. Minima have been found at slightly lower energies (in case of 1λ5,3λ5-diphosphacyclobutadiene 0.43 and 1.19 kcal/mol at the HF and MP2 levels of theory, respectively, and 0.23 kcal at the HF/6-31G* level of theory for 1λ5,3λ5,5λ5triphosphabenzene). These minima have equal bond lengths in case of the four-membered ring, by somewhat (0.005-0.008 Å) longer PC bonds, than
Nyula´szi and Veszpre´mi for the other structure, furthermore the skeleton is still planar. For the sixmembered systems, the ring atoms are slightly out of the plane. Hydrogen atoms at carbon, however, are out of the plane of the ring for both fourand six-membered rings, by 30-40°, depending on the level of theory used. These structures are derivable from the perpendicular (b) conformer of PH3dCH2. The potential curve of this out-of-plane motion is rather shallow, explaining (together with the possible substituent effect of the NEt2 group on phosphorus) why this structure has not been reported in the crystal structure investigations. Detailed analysis of these structures will follow in a subsequent paper. In all of the present investigations the planar structures have been considered to have results consistent with λ5-phosphinine, which was shown to be a real minimum in the planar form according to the secondderivative calculations. (43) Trinquier, G. J. Am. Chem. Soc. 1986, 108, 568. (44) Fukui, K.; Inagaki, S. J. Am. Chem. Soc. 1975, 97, 445. Inagaki, S.; Fujimoto, H.; Fukui, K. J. Am. Chem. Soc. 1976, 98, 4693. Inagaki, S.; Hirabayashi, Y. J. Am. Chem. Soc. 1977, 99, 7418. Inagaki, S.; Hirabayashi, Y. Chem. Lett. 1982, 709. (45) Inagaki, S.; Iwase, K.; Kawata, H. Bull. Chem. Soc. Jpn. 1985, 58, 601. (46) Roberts, J. D. Chem. Soc., Spec. Publ. 1958, No. 12, 111. (47) Aromaticity has been defined in a quite hazy way as “having an electron organization like that of benzene”: Lewis, D.; Peters, D. Facts and Theories of Aromaticity; Macmillan: London, 1975.
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