J. Phys. G e m . 1993,97, 4011-4015
4011
A New Look at the Similarities of the Conjugative Ability and Reactivity of -P=C and C=C Double Bonding LBsz16 NyulBszi, TamBs Veszprhmi,’ and J6zsef R6ffy Department of Inorganic Chemistry, Technical University of Budapest, H - 1521 Budapest, Gellbrt tbr 4, Hungary Received: June IO, 1992
Comparison of *-ionization energies of 28 compounds containing -X=C ( (X = CH, N, P) double bonds reveals a larger similarity between the carbon and phosphorus than between nitrogen and phosphorus containing species. Ring fragmentation reaction heats of the corresponding azaphospholes and azoles show a similar behavior. All of these facts suggest that, although the strength of the -P=C bond is known to be significantly smaller, than that of the C=C or C=N bond, in conjugative interactions -P=C is more comparable to C=C than to N=C s-systems. By using perturbation arguments, earlier observations about the similarities in chemical reactions of the -P-C and C=C bonds are understandable.
Introduction A most important result in the field of main group chemistry from the 1980s is probably the synthesis and characterization of numerous compounds containing doubly bonded main group elements. Review articles appeared’-15 which summarized research work about Si=C, Si-Si, -P=C, -+N, and S=C bonded compounds. Contemporary interest in these compounds is continuous. The nature of these double bonds has been investigated by theoretical means as well,lG34 and their strength has been estimated by different ways.I6-l9 The strength of double bonds between different elementsof the periodic table has been discussed in detail in the benchmark paper of Kutzelnigg19and calculated by Schmidt et al.17andvon Schleyer et a1.I6 It has been established that “the C-C, C=N and N = O *-bonds are about equally strong...when a single third period atom is substituted the ?r-bond strength decreases.”I7 This is in agreement with the classical double bond rule.35 This rule, based on the presumed poor overlap between 2p and 3p orbitals (however, the poor overlap itself has been disproved later36-37), concludes that double bonds involving third row elements are much weaker than bonds between second row atoms. The frequent citation of this classical rule in reviews8J0J7 indicatesthat, although it has been suggestedto “stand the double bond rule on its hand by predicting all possible double bonds...will beisolated,”I7thedouble bond rule inits original sensestill survives in the minds of chemists. As a consequence, all the findings concerning the strength of an E=C double bond or the necessarily existing conjugative ability of azaphospholes and related compounds are usually considered only as exceptions. In the investigationof the strength of the double bonds formed between different second (and third) row elements and carbon (or silicon), it was concluded that an important factor in determining the double bond strength is electronegativity.I6As the electronegativity of carbon (2.5) is somewhat higher than that of phosphorus (2,1), the measured strength of the -P=C bond (P being trivalent) is smaller than the correspondingvalue for the C=C bond (38.03*(58%) and 65.039kcal/mol (loo%), respectively). Similar results have been obtained by ab initio quantum-chemical calculations as well, calculating rotational barriers [it should be noted that rotational barrier of HP==CH2 at the 3-21G(*) level of theory was obtained as 107 kcal/mol,4° but this value seems to be an artifact due to the neglection of electron correlation] about the double bond17 (43.1 (67%) and 65.4 kcal/moI (IO(?%), respectively) and using the heats of properly selected isodesmicreactionsI6(49.4 (71 %) and 69.6 kcall
mol (100%) respectively); thus the -P=C double bond strength can be considered as 60-70% that of the C-C bond. The N = C bond strength, however, is comparable (100-1 lO%I69I7)to that of the C = C bond. One would expect that the strength of conjugative interactions between wsystems parallels the *-bond strength of the respective bond. The behavior of the -P==C bond in conjugative interactions has been discussed in the case of aromatic molecules and in the case of chainlikecompounds as well, comparingusually the -PIC and C=N bonds. The aromatic stabilization of phosphabenzene has been found to be about 88% of and no significant difference between the aromaticity of imidazole and 1H- 1,3azaphosphole has been observed.43Comparing the aromaticity of benzene to Si& and P6, Janoschekpointed out that phosphorus is more closely related to carbon than silicon is.44 Investigating the conjugationby calculating rotation barriers about the “single” bonds in H,X=YH,-ZH2 systems (X, Y = C, N, Si, and P; Z = B and N; n = 1,2; m = 0, 1); however, Korkin45stated that “multiple bonds in the third period are weaker than are their counterparts in the second one” and “the main differencebetween silicon and phosphorus ...is the availability of the lone pair” (for phosphorus). His calculated rotational barriers for the HE=CH-XHz and CH2=E-XH~ molecules (E = CH, P X = N, B), however, are very close for carbon and phosphorus while being much smaller for E=SiH derivatives. All these data indicate that, although never addressed explicitly, the conjugative ability of the -Mbond is larger than it could be surmised by considering the double bond strength only. There are even more striking indications concerning the chemical reactivity of the C = C and -P=C bonds, as several reactions known to be common for C = C double bonds result in analogous products if one of the carbon atoms is replaced by phosphorus; furthermore, reaction rates are similar in the comparable reactions,46regardless of the expectations based on the weaker bond. Chemical reactivity, however, is not merely determined by the strength of a given bond. It is suggested that kinetic factors which are characterizable by the activation energy might play a significant role as well. If the transition state of a reaction happens to be of reactant type, its energy can be estimated by using perturbation methods on the reactant’s wave f ~ n c t i o n , 4 ~ ~ ~ * thus allowing to use ground state wave functions to estimate activation energies. In this context it is worth to investigate the response of the -P=C system for perturbations. The reactivity of the -P=C bond has been investigated by using FMO considerations, concluding in an ambient reactivity, as a result of the closeness of the ?r and n p levels of the parent
0022-365419312097-4011%04.00/0 0 1993 American Chemical Society
NyulPszi et al.
4012 The Journal of Physical Chemistry, Vol. 97, No. 16, 1993
phosphaethene.49-5' (The above-mentioned similarity between the chemistry of the C = C and -F-C bonds referred to the double bond reactions.) Using the concept of electron demand49 in a comparative theoretical investigation of the reactivity of -P==C and C=C bonded it has been found by comparing experimental ionization energies that enamines are more electronrich than amino-substitutedmethylenephosphenes. The difference was attributed to the less (nlr) overlap in case of the phosphorus compound. It should be mentioned, however, that the compared derivatives had different substituents which might account for the observed differencesin the ionization energy values. As far as the polarity of the -P=C bond is concerned (it might influence the chemical reactivity, too), it is generally agreed that phosphorus is positively charged while carbon is negatively charged.8J0~53In the a-orbital itself, however, phosphorus is only slightly more positive than carbon.49 The intent of the present study is to compare the response of the -P=C, C-C, and N-C bonds to perturbations caused by conjugativeinteractions in a quantitative way for a great number of different species to obtain information on their relative conjugative ability. For such a comparison,an isodesmic reaction (designated previously as a ring fragmentation reactions4) will be considered. In these reactions an aromatic *-system will be separated to an -X=C( (X = P, N, or CH) subunit and to the rest of the molecule, thus providing information on the relative energies of the -X=C( bond as being separated or built into the aromatic system. Photoelectron spectra of several conjugated compounds containing -P=C bond will be discussed and compared to their C = C bond containing analogues, in order to get some information about the perturbationcaused by conjugative interactions on the *-x=c energy levels.
Calculations Calculations have been carried out by the MICROMOL program package.55 In order to get comparable results to our previous ~ o r k s the ~ ~split 3 ~ valence ~ 4-31G basis set was used augmented with polarization functions at phosphorus (E = 0.57). Full geometry optimization for all the compounds studied has been carried out. Second derivatives have been calculated numerically at the optimum geometries obtained. Since no imaginary values appeared among the calculated harmonic frequencies, all of the optimum geometries are real minima on the potential energy surface.
Results and Discussion
I) than for the corresponding phosphorus (and carbon) analogues, resulting in a somewhat higher slope (1.094) with a regression coefficient of only 0.950 if *-ionization energies of N - C containing compoundsare plotted against the corresponding C - C containing compounds (Figure 2). Thus we conclude that r-systems incorporating phosphorus atom are more related to the carbon derivatives than to those of the nitrogen containing compounds. The above conclusions can be tested by quantum-chemical calculations as well, investigatingnot only changes in the r-system, but the effect of the incorporation of the X=€ bond on the total energy of aromatic systems by using isodesmic reaction^.^^ All of these reactions have been considered with X = P, CH, and N thus allowing the comparison of the conjugative abilities of the -M,C I C , and N = C bonds. For this purpose the following ring fragmentation reactions have been considered:
w CH2=CH-CH=CH2
I
+
XH=CH2
+ CH34H3 + XH&H3
CHZ=CH-Nt+
+ XH=C& + X H d H 3 + NH&H3
CHp=CH-NHz
+ XH=CHz + X H d H 2 + CH34H3
CH2=N-NH2
+ XH=CH2 + X H d H p + CH3-CH3
HN=CH-NHp
+ XH=CH2 + XHt-NH2 + NH2-CH3
HN=CH-NHZ
+ XH=CH2 + XH2-NH2 + NH24H3
In order to get information about the perturbation of the
w M O s of the-X=C( bond when incorporated into an aromatic system, photoelectron spectroscopic data (using Koopmans' theorem) have been investigated. The available photoelectron spectroscopic data on the *-ionization energies of the -P=C bonded compounds together with the known *-ionization energies of the corresponding carbon compounds are compiled in Table I. While making these comparisons, great care has been taken to use ionization energies of similarly substitutedderivativeswhich means that the differing substituent can only be an alkyl group. These data suggest that the assigned *-ionization energies for the compounds investigated to date span a reasonably large IE range (-7.5-13 eV). This appears to be a result of the splitting of the energy levels due to the conjugative interaction in the r-system. The correspondingdata of the-and C = C bonded compounds are plotted in Figure 1, showing an impressively good (nearly y = x type) correlation: slope 0.974; intercept 0.2 eV; correlationcoefficient 0.987. This unexpectedly good correlation suggeststhat the interaction of a -P=Cand a C=C moiety with the rest of the *-system is of similarmagnitude,and both r-systems have a similar response for this type of perturbation. The *-ionization energiesof the correspondingcompounds containing N - C moiety, however, are at considerably higher values (Table
HN=N-NH,
+ HX=CH2 + X H d H 2 + NH&H3
The heats of these ring fragmentation reactions for the corresponding azaphospholes and azoles are compiled in Table 11. These show a reasonably good agreement between the energy stabilization caused by conjugation of the -M and C=C bonded compounds. The corresponding values when carbon is replaced by nitrogen are in each case higher, than when replaced by phosphorus. This fact suggests a larger stabilization caused by conjugation (this concurs with our previous findingss4). Furthermore, the average of the stabilization energy differences between the corresponding C=€ and E 4 (E = P, N) compounds investigated is 0.74 and -5.44 kcal/mol for the phosphorus and nitrogencontainingcompounds, respectively. This would indicate again that the stabilization of the phosphorus compounds shows more resemblance to that of the carbon than to the nitrogen derivatives. It should be mentioned, however,
-P-C and C=C Double Bonding TABLE I:
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4013
Ionization Energies of Corresponding -M, C=C, and N=C Compounds (in eV)
T
lr
CH>=CHP
I
- 1
HP=CH+
H N=CH 2' CH3-CH=CH*" C H 3 - W C H 2d HWCH-CH3' CH3-N=CH2' HN=CH-CH3"
lr
lr
U
10.51 10.30 12.44
+
10.03 9.60 9.75 11.37 11.43
0
0
10.18 10.05 11.65
CIHC=CH2" CIP=CHf CIN=CH2' 0
x = C" x = Pf
9.25 9.2 9.7
9.25 9.8 10.5
12.38 12.1 12.6
x =cc x = Ph
8.22 (8.35) 8.44 8.81
9.22 9.19 10.38
12.6 12.8 14.03
X = Nf V
Q
X
A
D
I
1 (
1 1
X = Nf x = C , R = Hij X = P, R = t-Buk X=N,R=Hi X = C, R = H' X = P, R = ?-Bum
x = C" x = P" X X X X
= NP = C, R = HP = P, R = Meo = N, R = Me4
9.15 (8.6) 8.6 10.00
9.88 (9.3) 9.5 10.56
8.56 (8.0) 7.93 7.71 7.68 8.24
8.36 8.41 8.53
9.76 9.56 10.58
11.03 10.9 11.18
8.37 (8.2) 8.15 8.74
8.99 (8.7) 8.8 1 9.16
10.40 (10.2) 9.88 11.29
11.73 (11.5) 11.7 12.3
a Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He1 Photoelectron Spectra; Halsted Press: New York, 1980. Lacombe, S.;Gonbeau, D.; Cabioch, J.-L.; Pellerin, B.; Denis, J.-M.; Pfister-Guillouzo, G. J . Am. Chem. SOC.1988, 110,6964. Bock, H.; Dammel, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 504. Bock, H.; Bankmann, M. Angew. Chem., Int. Ed. Engl. 1986, 25, 265. Lacombe, S.; Pellerin, B.; Guillemin, J. C.; Denis, J. M.; Pfister Guillouzo, G. J . Org. Chem. 1989, 54, 5958. JBatich, C.; Heilbronner, E.; Hornung, V.; Ashe, A. J.; Clark, D. T.; Cobley, U. T.; Kilcast, D.; Scanlan, I. J . Am. Chem. SOC.1973,95,928. As benzene has D6h symmetry and consequently degenerated levels, the average of the r-levels is better for comparison purposes. This value is 10.29 eV for benzene, while 10.37 eV for phosphabenzene. g Nyul&szi,L.; T6th, T.; Zsombok, Gy.; Csonka, G . ;Rtffy, J.; Veszprtmi, T.; Nagy, J. J . Mol. Struct. 1990,218,201 and references cited therein. Note, that the first band of (1 H)-1,3-azaphosphole consist of closely spaced unresolved vibronic lines, thus can only be characterized by the band maximum at the center of the band. In case of pyrrole, however, the center of the band is a t 8.35 eV (by 0.13 eV away from the vertical value). Veszprtmi, T.; Nyuliszi, L.; Rtffy, J.; Heinicke, J. J . Phys. Chem. 1992, 96, 623. 'Palmer, M. H.; Beveridge, A. J. Chem. Phys. 1987, 111, 249. Cradock, S.; Findlay, R. H.; Palmer, M. H. Tetrahedron 1973, 29, 2173. NyulBszi, L.; Veszprtmi, T.; Rtffy, J.; Burkhardt, B.; Regitz, M. J . Am. Chem. SOC.1992, 114,9080. The effect of the tBu group on the IE of the carbon derivatives here and later on was estimated to be 0.6 eV (see k , The modified IE values are listed in parentheses. Bloch, M.; Brogli, F.; Heilbronner, E.; Jones, T. B.; Prinzbach, H.; Schweikert, 0. Helu. Chim. Acta 1978, 61, 1388. Heinicke, J.; Kovics, I.; Nyuliszi, L. Chem. Ber. 1991, 124, 493. Dolby, L. J.; Hanson, G.; Koenig, T. J . Org. Chem. 1976, 41, 3537. oNyuliszi, L.; Csonka, G.; Riffy, J.; Veszprtmi, T.; Heinicke, J. J . Organomet. Chem. 1989, 373, 49. p Giinsten, H.; Klasinc, L.; Ruscic, B. 2. Naturforsch. 1976, 31a, 1051. Note, that the IE values of the corresponding methyl derivative should be lower by about 0.2 eV as concluded from the effect of 2-methyl substitution " I ) . The corrected values are listed in parentheses. 4 Guimon, C.; Pfister-Guillouzo, G.; Salmona, G.;Vincent, E. J. J . Chim. Phys. 1978, on indole (see . 75, 859.
'
that the scatter of the ring fragmentation heats is larger than in the case of A ionization energies if the phosphorus and carbon containing derivatives are compared. This fact can partly be attributed to the repulsion of the in plane lone pairs of phosphorus and nitrogen. Repulsion of neighboring nitrogen lone pairs is known to have a destabilizing effect on azoles,s7 and the same effect, although to a lesser extent,19might be operational between phosphorus and nitrogen lone pairs too. As a consequence, the stabilization effects calculated for phosphorus and nitrogen are somewhat lower than the stabilization of the a-system only. Further large difference can be seen in case of benzene. This compound should have a high degree of aromaticity4' and as a consequence large ring fragmentation energy too. The observed similarity in n-interactions of the -P-C and C = C bonds, which is seemingly in contrast to their bond strength, might be understandable, considering the role of the electronegativity in the formation of the *-system. It has been pointed out before that the strength of the E=C bonds16 is related to the electronegativities of the constituting atoms. Electronegativity of phosphorus is somewhat smaller than that of carbon in the atomic ground state, while based on the analysis of orbital splittings of group V heterobenzenes, Waluk et al.S8 concluded that the
'E k C l eV. 12.
10-
Figure 1. Correlation between *-ionization energies of C=C and -P=C bondedcompounds. ThesymbolsusedarelistedinTableIfor thedifferent compounds.
Nyuliszi et al.
4014 The Journal of Physical Chemistry, Vol. 97, No. 16, 1993
8
10
IEC,
c
12 eV
Figure 2. Correlation between r-ionization energies of C = C and N = C bonded compounds. The symbols are identical to those used for the corresponding.phosphorus derivatives.
TABLE XI: Heats of Ring Fragmentation Reactions for the Corresponding Compounds Containing -P=€ and C = C Double Bonds (kcal/mol)
x=c X=P X=N
61.40 52.12 61.13
33.40 36.11 39.41
33.40 38.30 42.63
rx9 N-N
I
x=c X=P
X=N
43.83 45.26 49.45
21.12
31.00 36.11
YX\)
N‘X
c);
x=c X=P
X=N
43.86 42.88 49.10
N-N
I
21.11 26.83 3 1 .I1
45.40 44.26 50.34
moieties it is concluded that all comparable IE values identified so far are nearly matching (near unit slope and zero intercept, regression coefficient 0.987) if -P=C and C=C bonded compounds are considered,while they are considerablyhigher values in case of the C=N bond. As the splitting of the energy levels is a characteristic measure of the conjugative interaction, the nearly equivalent r-ionization energies of the corresponding -P=C and C - C compounds indicates the similar conjugative behavior of the two bonds. Ring fragmentationreactions modeling the effect of conjugation on the total energy of the molecule give support to the above conclusion too. Investigatingnine aromatic molecules containing -P==C, C - C , and N = C moieties the average stabilization was found 98%, 100% (40.24 kcal/mol), and 114%, respectively, indicatingthat theconjugativeabilityof the-bondisnearly the same than for the C=C bond but is somewhat smaller than that of the N = C bond. The noted similarities between the responses on perturbative interactions investigated in both ring fragmentation and orbital energies (using Koopmans’ theorem) additionally disclosed that the observed similar changes in the total energies are primarily due to changes in the *-orbital energies. The similarity in the conjugative ability of the -P=C and C=C bonds might be somewhat puzzling as the strength of the -P=Cdouble bond has been found’6J7J8J9 about 60-70% that of the C=C bond. It should be considered, however, that the strength of the -P=C bond itself is determined by the interaction between the constituting atoms (HP and CH2groups), while in case of conjugated systems phosphorus is already influenced by the molecular environment in the -P=C bond. By analogy with phosphabenzene,58the effective electronegativity of phosphorus and carbon in -+C bonded systems can be considered to be nearly the same value. The above observationsgive some simple explanations for the observed similar reactivity of the -P=C and C=C bonds by using perturbation arguments. Because the response of C = C bond to perturbations caused by the conjugative interactions is more analogous to that of the -p-Cthan to the N = C bond, it seems understandable that perturbations caused by a substrate in chemical reactions will be similar in case of -P=C and C - C bonds, resultingin similaractivationenergies consequently similar chemical behavior.
I
45.40 38.66 49-89
effective a-orbital electronegativitiesof phosphorus, arsenic and antimony are even higher than that of carbon. The effective electronegativities are influenced by the actual bonding angles predetermined by the geometry of the molecules containing the -P=C moiety and the polarizing effect of the molecular environment. (In case of azaphospholesthe bonding angle about phosphorus is nearly 95O according to our calculations, that is between the bonding angle of phosphabenzene and that of the atomic p orbitals.) The observed similarities in the conjugative ability of the -P=C and C=C bonds indicate that the effective electronegativitiesof carbon and phosphorus in conjugated systems are very close to each other.
Conclusions Although it is generallyconsidered that C=C and C=N bonds are of similar strength and -P=Cbond is somewhat weaker,l6-I7 in the present investigation we conclude that in conjugative interactions -P=C bond is similar to the C - C bond, while the C=N bond has somewhat stronger conjugative ability. From the investigation of the r-ionization energies of several conjugated molecules containing -M, C-C, and N==C
Acknowledgment. The authors are grateful to R-COMP for allowing to use their computer facilities. Financial support from OTKA 642 is acknowledged too. The authors are highly indebted to Dr. D. Robare for his help while preparing the manuscript. References and Notes (1) Cowley, A. H.Acc. Chem. Res. 1984, 17, 386. (2) Cowley, A. H.Polyhedron 1984, 3, 389. (3) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (4) Miiller, G.Nachr. Chem. Techn. Lab. 1986, 34, 778. (5) (a) Brook, A. G.; Baines, K. M . Adu. Organomet. Chem. 1986,25, 2 . (b) Grev, R. S. Adu. Organomet. Chem. 1991,33, 125. (6) Issleib, K.; Schmidt, H.; Wirkner, Ch. Z . Anorg. Allg. Chem. 1981, 473, 85. (7) Sommer, L. H.; Parker, D. R. J. Organomet. Chem. 1976,110, C1. (8) van der Knaap, Th. A.; Klebach, Th. C.; Visser, F.; Bicklehaupt, F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M . Tetrahedron 1984,40,765. (9) Appel, R.; Knoll, F.; Ruppert, 1. Angew. Chem. Inr. Ed. Engl. 1981, 40, 765. (10) Appel, R.; Knoll, F. Adu. Inorg. Chem. 1989, 33, 259. (1 1) Trinquier, G.; Malrieu, J.-P. Complementaryviews on the homopolar double-bond structure. In The Chemistry of Double-bonded Functional Groups; Patai, S.,Ed.; John Wiley & Sons: Chichester, U.K., 1989; p 2. (12) Corey, J.H. Historicaloverviewandcomparisonofsiliconwithcarbon. In The Chemistry of Organic Silicon Compounds; Patai, S.,Rappaport, Z., Eds.; John Wiley & Sons: Chichester, U.K., 1989; p 33. (13) Niecke, E.; Gudat, D. Angew. Chem. Int. Ed. Engl. 1991, 30, 217. (14) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew. Chem. 1991, 103, 916. (15) Seppelt, K. Angew. Chem. 1991, 103, 399. (16) Schleyer, P. v. R.; Kost, D. J . Am. Chem. Soc. 1988, 110, 2105.
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