8200
J. Phys. Chem. 1993,97, 8200-8206
Pentaaza- and PentaphosphacyclopentadienideAnions and Their Lithium and Sodium Derivatives: Structures and Stabilities Mikhail N. Glukhovtsev,t Paul von R. Schleyer,. and Christoph Maerker Institut fur Organische Chemie der Friedrich- Alexander- Uniuersitiit, Erlangen- NCirnberg, Henkestrasse 42, 0-8520 Erlangen, Germany Received: February 22, 1993 Both the pentazole, Ns- (4), and pentaphosphole, Ps- (5), anions favor planar Dsh geometries, in contrast to hexazine (N6) and to hexaphosphabenzene (Pa), which have D2 twist-boat structures. While the pentazole anion 4 is thermodynamically unstable relative to N3- N2, a barrier of 19.4 kcal/mol at MP4SDTQ/63l+G*//MP2(full)/6-3lG* AZPE(HF/6-31G*) inhibits the dissociation. In contrast to 4,5 is stable both toward P3- P2 dissociation (AE = 71.6 kcal/mol) and toward 2(5) 2P3- P4(Td) disproportionation (AE = 116.4 kcal/mol). The lithium salt, NsLi, favors the planar CZ, structure 15, in contrast to the cyclopentadienyllithium, (CH)sLi (27), and to the sodium PsNa derivative (12), both of which prefer CsO symmetry. While 15 is 33.5 kcal/mol unstable relative to dissociation into N3Li (C-,,) and N2 (MP4SDTQ/ 6-31+GS//MP2/6-31G* AZPE(MP2/6-31G*), pyramidal 1 2 is highly stable toward disproportionation into P3Na (either C3,, triplet 34 or Ca singlet 35) and P4 (T,j). Comparison of the calculated IR spectra for 12 and for 35 with the experimental spectrum for 12 shows clearly that 35 may be present.
+
+
+
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+
+
I. Introduction Inorganicanalogs of aromatic monocyclichydrocarbons attract attention for several reasons.‘ To what extent are these isoelectronic (or valence isoelectronic) molecules similar in structure and stability. Benzene 1 and borazine 2 provide a classical example.2 Polynitrogen and polyphosphorus monocycles deserve particular interest.Ia4 The use of N, isomers as high-energy density materials is under consideration.3
H
Are the polynitrogen (4) and polyphosphorus (5) analogs of 3 stable, even though the N6 and P6 analogs (6 and 7) of benzene (1) are not?””J In contrast to their hydrocarbon analogs 3and 1,4-7 are known only as either c o m p l e ~ e s ~ or J ~highly - ~ ~ unstable trapped molecules.l.3J4 Some mixed metallocene complexes of PSare known experimentally,e.g., 8 and 9.hJl The X-ray P-P distance is 2.13 A in 8.ll The minimum energy geometries of benzene analogs hexazine (6) and hexaphosphabenzene (7) are quite different from the geometry of benzene; namely, nonplanar D2 twist-boat structures, 10 and 11, are favored819(Chart I). Both 10 and 11are thermodynamicallyunstable (MP4SDTQ/ 6-31G1*//MP2/6-31G** + AZPE(MP2/6-31G*)) by 212.3 kcal/mol toward N6 (10) 3N2 decomposition and by 65.0 kcal/mol toward 2P6 (11) 3P4(Td) disproportionation, re~pectively.~.~ In contrast, benzene is stable by 146.7 kcal/mol (MP4SDTQ/6-3 lG**//MP2/631G** + B E ( M P 2 / 6 - 3 lG*);9 143.7 kcal/mol (exptl’) toward dissociation into 3HC=CH. A C5, structure with a planar P5 moiety was assigned to 12 on the basis of IR spectra.12b However, this conclusion has been challenged because of the discrepancies between the experimental*2band calculated (HF/DZP”) frequencies for 12. Stabilization of the Ng- ring form by formation of complex 13 was predicted by EH calculations.16 The interactions between the N or P lone pairs and the metal orbitals (e.g., the e’] orbitals of the CpM-MCp fragment in complexes like 9 and e’l u-orbitals of the X5 ring) stabilize the inorganic ring systems substantial-
--
2
1
3
The cyclopentadienide anion 3has greater aromatic character (based on various aromaticity criteria, see, e.g., refs lc-d, 4) than benzene. This is indicated, for example, by the topological resonance energies, TRES (TRE(1) = 0.276, TRE(3) = 0.318),5 and by the homodesmotic6 (eq 1) and isodesmic (eqs 2 and 2a) reaction energies:
ly.16,17
The ring current index (RCI) as an aromaticity criterion (the lowest bond order in a ring also serves as an index of aromaticityl8) indicates that Pg- (5) is highly aromatic with the index of 1.73 while benzene and cyclopentadienideanion have RCI(1) = 1.75 and RCI(3) = 1.72, re~pectively.~~ To what extent do the N3A € = 60 kcaVmd (HF/3-210)” 9-anions, 4 and 5, exhibit aromatic character? Do they = 64.9 kcaVmd ( M P ~ S D T Q ( ~ C ) / ~ ~ ~ G * / / M P Z ( this ~ ~ Iwork ~ ) / ~ - ~ ~ G *and , prefer planarity or the distorted geometrieslike those of hexazine = 64 kcavmd ( 0 ~ 1 ) ~ (10) and of hexaphosphabenzene (ll)? How high is the Ns- + Nz? Is it possible to dissociation barrier of Ns- (4) + CH3- + 4CH4 H3CCH3 + 2H3CCH2- + 2H2C=CH2 (2b) stabilize N5- as a NsLi salt? What structure is best, pyramidal A € = 87 kcaVmd (HF/3-210)” 14 or planar, 15 and 16? Is NsLi stable toward dissociation into N3Li and dinitrogen? What are the relative stabilities of the = 90.2 kcaVmd (MP4SDTO(fc)16-31+Gf//MP2(lu11)/6-31G*, this work analogous forms of PsNa? Is the pyramidal structure 12 assigned in the experimental work12bthe most stable PsNa isomer? Are t AlexandervonHumboldtResearch Fellow. Permanentaddress: Institute structures 17and 18competitive with 12 in stability? We provide of Physical and Organic Chemistry, Rostov University, 194/3 Stachky Ave., answers to these questions in this paper. Rostov on Don, 344104 Russia. A € = 12.4 kcaV mol MW(fc)/6-31+G*//MPZ(fc)/6 -31+G*
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0022-3654/93/2097-8200$04.00/0
Q 1993 American Chemical Society
Pentaaza- and Pentaphosphacyclopentadienide Anions
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8201
CHART I
P -P
N-N
W“N
LOA N
N 5
4
FeCp* 8
6
7
ckp* 9 Cp’=C,Me,
II. Computational Methods
III. Results and Discussion
Using the GAUSSIAN-92 program,?(’geometry optimizations were carried out at HF/6-3 lG* and then at MP2(fu11)/6-3 1G*.21a Single point energies were computed at M P ~ S D T Q ( ~ C(only )~’~ valence electrons are involved in the MPn treatment, in contrast to all electron MPn(ful1) calculations) with the 6-3 1+G*2Zand the 6-31 lG*23basis sets and the MP2(fu11)/6-31G* geometries. For the P3Na pyramidal triplet and the C, planar singlet, as well as the linear singlet and triangular triplet of P3-, single point calculations were performed with the quadratic configuration interaction method including single and double substitutions with triples correction ((U)QCISD(T)),24 Analytical second derivatives established the nature of the stationary points at the HF/ 6-31G* and, for 4,5,14-16, the MP2(fu11)/6-31G* levels.25For reference P3- and P3Na molecules MP2(fc)/6-3 lG* frequency calculationswere performed. Zero point energies (ZPE) derived from the MP2/6-3 lG* and HF/6-3 1G* frequency calculations are scaled (by 0.9626and by 0.91:’ respectively, for first-row molecules and by 0.9426 and 0.91:’ respectively, for second-row molecules) and used to correct the relative energies. Projections to the pure spectroscopic states of the D3h cyclic N3- and P3triplets (PMP4)28also were carried out.
CeometrieS. Both D5h structures of 4 and 5 were calculated to be minima at MND029and at the Hartree-Fock le~e1.~5.*32 However, these results cannot be considered to be definitive since equilibrium geometries may changedrastically at correlatedlevels (note hexazine (IO) and hexaphosphabenzene (ll)).8-9 At MP2/ 6-31G*, both planar structures 4 and 5 are minima (Table I), in contrast to 6 10 and 7 11. The MP2(fu11)/6-31GS outof-planee”2vibration frequencies for 4 and 5 are fairly large, 760 and 212 cm-*,respectively (Table I). In contrast to 6 and 7, the lowest (e”2) frequencies only differ slightly between the MP2 and H F levels, e.g., at HF/DZP + diffuse functions v(e”2) = 224 cm-lfor515(cf.212cm-lat MP2/6-31G*). Onlythee’lstretching vibrations for 4 and 5 have nonzero (but weak) IR intensities.
Na
13
Li
17
-
The N-N and P-P bond lengths in 4 and 5 (Figure 1) are shorter than those in HzXXH2 (X = N: 1.435 A, MP2/6-31G*; 1.447 A, exptl.33 X = P 2.230 A, MP2/6-3 1G*) but longer than those in HX=XH (trans) (X = N: 1.265 A, MP2/6-31G*; 1.252 A, e~pt1.3~ X = P: 2.043 A, MP2/6-31G*). The bond distancesin 4 and 5 manifest the structural criteria of aromaticity (for a discussion of whether this is a meaningful criterion of aromaticity, see ref 35). Thus, the six-*-electron five-membered X5- (X = N, P) rings, 4 and 5, are stable toward nonplanar distortions, in contrast to 6 and 7. Like pyrazole (19), the pentazole,NsH (20), also favors a C, planar structure which is a minimum at the MP2 leve1.8~~6
12
14
-
15
16
18
0
N-N
It
\\
N H
N.N, N
19
20
H
Stabilitiesof the N5-and P5-a h Ions. While the six-*-electron ions 4 and 5 obey the Hiickel rule, aromatic character alone cannot assure stability of a molecule, e.g., if dissociation products or the isomers are more favorable. Indeed, 4 is unstable toward dissociation (eq 3). In contrast, the analogous reaction (eq 4) for 5 is endothermic. The total energies of the species involved in reactions 3-5 are given in Table 11;the geometries of the N3- and P3- D..h structures were optimized at MP2/6-3 1+G*.
8202 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Glukhovtsev et al.
TABLE I: TOWbrgies, Bond Lensths,and IR Frequencies (Unscaled) Calculated for the N5-and Ps-4 ‘Structures, 4 and 5, at Various Computational Levels 5 4 MP2(f~ll)/6-31G*
Eta. au R(XX),A ( X = N, P)
-213.036 45 1.342
HF/DZP + diff. functions15 -1703.700 00 2.095
MPZ(fu11)/6-3lGb -1704.300 10 2.109
e”2 (out-of-planedeformation) 760 224’ e’) (in-plane deformation) 1081 336 1124 509 8’1 (breathing) 1146 532 e‘2 (stretching) llUb 564b e’l (stretching) a Compare with 233 cm-1 at HF/3-21G, unscaled value.32 b The only IR active vibration.
P
212 312
467 513
546b
TABLEII: ToWEaergies,Bond Vibrational Energies of the speciea hvo ved in andEquations zero 3-6’
-P
1. M A
structure
R(W,
A ( X = N or P)
Eta, au
ZPE, kcal/mol -163.848 91’ 1.218 6.6 1.415 -163.717 19 4.6b -109.279 8 1 3.1 1.130 -1022.538 03 2.w 1.987 -1022.536 25d 2.168 2.lb -1363.397 34 2.195 4.0 -68 1.676 00 1.o 1.932 -273.097 25 1.342 13.5 -1704.331 28 5.2 2.109 At UQCISD(T)/6-31+Gb//MP2(f~ll)/6-31+Gb, the N3- D-h D3h energy differences is 7 1.1 kcal/mol (Eta(N3-JLh.21) -163.82703 au). At MP2(fc)/6-31Gb. e At MP2(fu11)/6-31+Gb. At UQCISD(T)/6-31G(2df)//MP2/6-31+Gb, the D3h triplet 24 is 4.1 kcal/mol lower in energy than the linear P3- singlet 22 (Ew(24) = -1022.65907 au). e Total energies were calculated at MP4SDTQ(fc)/6-31+Gb with the MP2(fu11)/6-31+Gb geometries for the anions and MP2(ful1)/631Gbgcometricsfortheneutralmolacules. Zeropointvibrationalencrgies (unscaled) were calculated at MP2(fu11)/6-31Gb, unless indicated otherwise.
-
W C2“
p=p--.p
W Dlb
21
Figure 1. MPZ(ful1)/6-3lGbgeometriesof the (CH)s-, Ns-, and Ps- Dsk structures as well as of the Cb transition state for the dissociation into N3-
4
+
-
+
N-N-N- (21, D-h) N S N hE = -23.4 kcal/mol (MP4SDTQ/6-31+GL/ /MP2(fu11)/6-3 1G* AZPE(MP2/6-31G*) (3)
22
/&
+ N2.
N-
+
1-
/&
N
P-
P
24
23
this energy difference is small in comparison with the energies of reactions 4 and 5 evaluated with 22 instead of 24. The formation of the tetrahedral P4 (white phosphorus) is more favorable than formation of P2 (see, e.g., ref 38). Nevertheless, in contrast to 7: 5 also is stable even toward disproportionation (eq 5 ) (MP4SDTQ/6-3l+G* AZPE(MP2(full)/6-3lG*); the The most stable N3- isomer is linear singlet 21.” Both 21 and MP2/6-31G* geometries for 5 and P4(Td) as well as the N3-D3hcyclic triplet 23 areminima at MP2(fu)/6-31G*, but the MP2(fu11)/6-3 1+G* geometry for the linear P3- anion 22 were cyclic triplet is 82.7 kcal/mol higher in energy than 21 at used): MP~SDTQ/~-~~+G*//MP~(~ (Table U ) /11). ~ - ~At ~+G* QCISD(T)/6-3 1+G1//MP2(fu11)/6-3 1+G*, this energy dif2P; (5) 2P-P-P- (22) P4(Td) ference, 7 1.1 kcal/mol, is much more than the difference of 42.7 hE = 116.4 kcal/mol ( 5 ) kcal/mol,calculated at UHF/6-3 11+G*.37b While the possibility of the formation of 23 in a solid N2 matrix bombarded with The reaction energy (eq 5 ) may be overestimated somewhat, 5-keV Ne atoms has been ~uggested,’~b23 should be a very since the proper description of tetrahedral P4 needs a basis set unstable N3- isomer. augmented with f-type polarization functions.38 Nevertheless, 5 The cyclic €5-D3h triplet 24 was calculated to be 8.4 kcal/mol obviously is a stable species. (MP3/DZP//HF/DZP)”a lower in energy than linear singlet We also calculated the barrier for dissociation (eq 3). While 22. While both 22 and 24 are minima at MP2/6-31GS, singlet only C, symmetry constraints were applied, the transition-state 22 is 1.1 kcal/mol more stable than triplet 24 at PMP4(fc)/6structure retains CZ, symmetry (Figure 1, Table I). At 31+G*//MP2/6-31+G* (Table 11). However,at UQCISD(T)/ MP4SDTQ/6-31+G*//MP2/6-31G*AZPE(HF/6-31G*), 6-3lG(2df)//MP2/6-3 1+G* this order reverses, and triplet 24 the barrier is rather high (19.4 kcal/mol). The MP4SDQ/6is 4.1 kcal/mol lower in energy than singlet 22. Nevertheless, 31+G*//HF/3-21G + AZPE(HF/3-21G) calculations’6gavea
5
P-P-P- (229 D-h) + m P PE = 71.6 kcal/mol (MP4SDTQ/6-31+G*/ /MP2(fu11)/6-3 lG* + AZPE(MP2/6-3 1G*) (4)
+
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Pentaaza- and Pentaphosphacyclopentadienide Anions
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8203
TABLE III: Total (au) and Relative (kcal/mol) Energies of the N&i Isomers Calculated at Various Computational Levels MP2/6-3 1G* structure 14 c5, 1s c2, 16 Cb
Em1 7.4 (6.6)b 0 7.0 (6.6)
Em -280.492 00 (0)' -280.503 83 (0) -280.492 70 (1)
MP4SDTQ/6-3 1+G*//MP2/6-3 1G* Etot -280.530 56 -280.545 57 -280.537 30
MP4SDTQ/6-3 11G*//MP2/6-31G1 ElOf -280.618 93 -280.630 87 -280.620 19
Ere1
9.4 (8.6)b 0 5.2 (4.8)
Em1 7.5 (6.7)b 0 6.7 (6.3)
Number of imaginary frquencies calculated at MP2(full)/6-3lG* is given in parentheses. Relative energies calculated with AZPE(MP2(full)/ 6-31G*) correction are given in parentheses. For 14, the unscaled ZPE(MP2(fu11)/6-31G* value is 14.2 kcal/mol).
& 2.135A . 0l 74Ae
v
Figure 2. HF/6-31GC geometries of the (CH)SLi C5, pyramidal (27) and CZ, planar (28) structures. While the former is a minimum, the latter corresponds to a transition state.
22 kcal/mol barrier. Thus, 4 possesses significant kinetic stability toward dissociation (reaction 3). While C2 open-chainstructure 25 is the most stable N6 isomersJoa* (at MP4SDQ/6-31+G*/ /HF/3-21G AZPE(HF/3-21G)), the open-chain N5- isomer 26 was found to be 53.6 kcal/mol higher in energy than 4.16v31
+
16 C2"
Figure 3. MP2(fu11)/6-31G* geometries of the NsLi C5, pyramidal (14) and Cb planar structures, 15 and 16.
Li
, N )
1 /": 7 Li
Li
I
N&i and P&a Complexes. Baudler and her group have reported the experimental generation of P5Na;lZa pyramidal C5, structure 12 has been assigned to this species.12b However, Hamilton and Schaefer15 find that the experimental IR e'l frequency is too large (815 cm-1). This value is even larger than the frequency for P2 (781 cm-1) and does not agree with the 564-cm-1 value calculated for 12at HF/DZP + diffuse functions.15 Do P5Na and NsLi prefer the pyramidal structures 12 and 14 to the planar isomers 17, 18 and 15,16, respectively? Such a preferenceis related to the well-documented pyramidal structure of lithium cyclopentadienide-(CH)5Li, 276b-39(see X-ray data on derivative 27aS9)-which exemplifies "three-dimensional aromaticity".6b*m (However, the hydrogens in 27 disfavor Li+ positions in the ring plane.) Our calculations do show that the planar CZ,structure 28 is not a minimum but the transition state for Li migration from one face to the other. The HF/6-31G* energy of 28 is 52.0 kcal/mol higher than that of 27 (Figure 2).
H 27a The C-C bond lengths change very little from 1.412 A in D5h (CH)5-(3) to 1.410Ain 27 (MP2(fu11)/6-31G*) (Figures 1 and 2). We have found planar 15 to be 7.5 kcal/mol (MP4SDTQ/ 6-31+G*//MP2/6-31G*) lower in energy than pyramidal 14 (Table 111, Figure 3). Only 15 was calculated at MPZ/DZP by Ferris and Bartlett,36 but they did not discuss the alternative pyramidal geometry. Structure 16 containing monocoordinated lithium is 6.7 kcal/ mol higher in energy at MP4SDTQ/6-31+G*//MP2/6-31G* and corresponds to the transition state for the Li migrationaround the Ns ring (Scheme I). Planar structure 15 is unstable toward decomposition (eq 6) (at MP4SDTQ/6-3l+G*//MP2/6-3lG* + AZPE(MP2/6-
8204 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Glukhovtsev et al.
SCHEME I
TABLE IV: Total tau) and Relative (kcal/mol) Energies of the P&Ja Isomers Calculated at Various Levels ~
MP2(f~11)/6-31G*/ /MP2(f~ll)/6-3lG*
HF/6-31G*
structure 12 Csu 17 CZ, 18 CZ,
ElOt -1865.518 33 (0)" -1865.502 02 (1) -1865.499 37 (0)
Ere1 0 10.2 (lO.O)b 11.9 (11.7)
Em -1866.150 10 -1866.125 08 -1866.120 25
MP4SDTQ/6-3 1G*/ /MP2/6-31GS
Ere1
Etn -1866.160 75 -1866.136 51 -1866.131 76
0 15.7 (15.5)*
18.7 (18.5)
MP4SDTQ/6-31+G*/ /MP2/6-31G*
Ere1 0 15.2 (15.0) 18.2 (18.0)
Etn -1866.17000 -1866.146 54 -1866.142 42
Ere1 0 14.7 (14.5) 17.3 (17.1)
a Number of imaginary frequencies calculated at HF/6-31GS is given in parentheses. *Relative energies calculated with AZPE(HF/6-31G*) correction are given in parentheses. For 12, the unscaled ZPE(HF/6-31G*) = 6.2 kcal/mol.
Total (au) and Relative (kcal/mol) Energk of Pfla Structures 31-36 Calculated at Various Computational Levels
TABLE V
HF/3-2 1G*
structure
&I
34, C30 (3A2) 35, CZ, 36,c-h 37, CZ,
-1178.162 95 (0)" -1 178.106 66 (0) -1 178.090 21 (2) -1 178.084 55 (2) -1177.931 97 (1) -1178.145 85 (1)
38,cz,
39, CZ,('Bz)
MP2(f~11)/6-3lG*//MP2(f~ll)/6-3lG* Ere1 0 35.3 45.6 49.2 144.9 10.7
Etot -1 184.372 76 (0) -1 184.365 01 (0) -1 184.332 70 (2) -
QCISD(T)/6-3 1GS//MP2(fu11)/6-3 1G*
Ere1
Em
0 4.9 (5.0)b 25.1 (24.8)
-1 184.380 29 (-1 184.380 28)c -1184.366 20 (-1 184.374 50)e -
-
&I
-
-
-
-
-
0 8.8 (8.9)'"
-
0 Number of imaginaryfrequenciesis given in parentheses. Relative energiescalculated with AZPE(MP2/6-3 1G*) correction are givm in parentheses. The PMP4 values with and without AZPE(MP2/6-31G*) correction are 3.7 and 3.6 kcal/mol, respectively. The PMP4/6-31G* values are given in E ~ ( 3 4 )= -1 184.38536 au (PMP4) and Ea(35) = -1 184.38019 au. parentheses. At MP4SDTQ/6-31+GS//MP2/6-31G*,
-
31G*)):
15
N3Li (29,C-J
+ N,
AE = -33.5 kcal/mol
(6)
However, Ferris and Bartlett', have computed a 16.2 kcal/mol barrier for this dissociation, indicating that some degree of kinetic stability can be expected. Among the N3Li structures 29-33,29 is the most stable (MP2(fu11)/6-3 1G*//MP2(fu11)/6-3 1G*relative energies (kcal/mol) are given in parentheses); where the MP2(fu11)/6-3lG* data for 29-32 have been taken from ref 41 (calculations of Prof. 0. Charkin). At HF/6-31G*, both 30 and 33 are transition states, in contrast to minimum 29. We calculated 29 to be a minimum at MP2/6-31G* as well. Thus, the lowest energy N3Li isomer, 29,is employed in reaction 6.
12
C5"
u n
2.108A 2.101A
N
N
~('JN-L~
17
C2v
W
N A
31, .C, (78.9)
30, C, (6.6) Li
N-L-N \
4
N 32, C3J3AJ (85.6)
AN\
NQI ,Li N
33,G ( 2 7 . 2 )
In contrast to the NSLi situation, pyramidal PSNa (12) is 14.5 and 17.1 kcal/mol lower in energy than the alternative planar isomers 17 and 18, respectively (MP4SDTQ/6-31+G*//MP2(fu11)/6-31G* AZPE(HF/6-31G*)) (Table IV, Figure 4). Planar 17 with dicoordinate sodium is a transition state for the sodium migration from the one ring face to the other. Planar structure 18 having monocoordinatesodium, 2.6 kcal/mol higher in energy than 17 (Table IV), is a minimum, in contrast to the analogous N5Li structure 16. The changes of the PP bond lengths in 17 relative to those in 5 are minor in comparison to the changes in the N-N bond lengths in 15 relative to those in 4 (Figures 1,
+
Figure4. MP2(fu11)/6-31G* geometriesof thePsNa &pyramidal (12) and CB planar structures, 17 and 18.
3, and 4). The counterion, Na+, affects the geometry of 17 very little, in contrast to the behavior of 15. Is 12 stable toward disproportionation into P3Na and P4( Td) and toward the P3Na Pz dissociation? We need to establish the most stable P3Na isomer. The relative stabilities of the P,Na structures differ qualitatively from those of the analogous N3Li species. In contrast to the LiN3 isomers 29-33, the most stable
+
&
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8205
Pentaaza- and PentaphosphacyclopentadienideAnions
TABLE VI: Fundamental Harmonic Vibrational Frequencies and IR and Raman Intensities Calculated for 12 at HF/6-31G* mode symmetry el a1 e2 e2 a1 e2 el
2.1801
34 C3"
Y, cm-'
I(IR), km/mol
Z(Raman), A'/amu
125 206 244 345 505 535 560
4.41 54.39 0.00 0.00 0.05 0.00 0.57
7.21 0.84 0.37 19.68 86.21 5.37 0.06
TABLE VII: Fundamental Harmonic Vibrational Frequencies and IR Intensities Calculated for 35 at MP2(fc)/6-31G* ~
2.6274.
v, cm-1
Z(IR), km/mol
b2 bi a1
105 (99)' 123 (116) 176 (165) 309 (290) 464 (436) 837 (787)
0.72 (17.07)b 0.13 (0.02) 9.86 (2.78) 36.38 (1.78) 1.04 (123.81) 11.18 (18.16)
a1
a1 b2 35 CP"
~
mode symmetry
~~
a Frequencies scaled by 0.94 are given in parentheses. b Raman intensities(A4/amu) calculated at HF/6-3 lG* are given in parentheses.
W @I
963A-@I
9 6 i ' A * 2 5 2 3 A a
36 C o w
Figure 5. MPZ(fu11)/6-31G* geometries of the P3Na isomers 34-36.
isomer of Nap3 is the pyramidal C3utriplet 34 (Table V, Figure 5). The planar CZ,singlet 35 is the second in stability.
35
34
36
D
39 ('B3
38
37
Pyramidal structure 12 is highly stable toward disproportionations (eqs 7 and 7a) and is also stable with respect to P3Na P2 dissociations (eqs 8 and 8a) (MP4SDTQ/6-31+G*//MP2/ 6-3 1G *).
+
2PsNa (12)
2PsNa (12)
P,Na (12)
P,Na (12)
-
-
2P,Na (35, C,)
+ P4(Td) AE = 114.5 kcal/mol (7)
2P,Na (34, C,,, ,A,)
P,Na (35, C,)
+ P4(Td)
AE = 108.0 kcal/mol (7a)
+ P, hE = 71.5 kcal/mol (8)
P3Na (34, C3", ,A,)
+ P, AE = 68.2 kcal/mol (8a)
The highest frequency for 12 at HF/6-31G* is 560 cm-1 (el, the scaled value is 510 cm-1) (Table VI). This is considerably lower than the experimentally observed intense IR band at 8 15
cm-l.lZb Moreover, our calculated 560-cm-l frequencyhas a weak IR intensity (0.57 km/mol). The Raman a1 band (460 cm-*, scaled value; Raman intensity is 86 A4/amu, Table VI) agrees with the experimental value of 463 cm-1. On the basis of the HF/DZP calculations, Hamilton and Schaefer suggested that the discrepanciesbetween the calculated and the experimental IR spectra might be due to the presence of NaP3.l5 They considered the calculated IR frequencies for the linear P3- D-,, singlet. In fact, the lowest energy Nap3 singlet has a bent rather than a linear structure for the P3 moiety (35) (Table VII). For 35, our MP2/6-31G* highest frequency, 837 cm-l (787 cm-l, scaled value, asymmetrical PP stretch, Table VII), fits the experimental value of 815 cm-l fairly well. The second highest frequency at 464 cm-' (unscaled, the scaled value is 436 cm-l) is close to the 463-cm-l experimental frequency. In contrast to 35, the highest IR frequency of the pyramidal triplet 34 computed at UMP2(fu11)/6-3lG* is only 577 cm-l (IR intensity, 0.12 km/mol). The second highest frequency (e) is at 435 cm-l (weak IR intensity of 0.61 km/mol). The most intense a1 1R band for 35 is 269 cm-1 ( I = 45.4 km/mol). This differs considerably from the experimental IR data for 12. Hence, the presence of 34 can be ruled out. Therefore, it appears likely that the reported experimentalIR spectrum for 1212bwas contaminated by the presence of Nap3 (35); the 815-cm-1 IR band can be assigned to this species.
IV. Conclusions We conclude the following: (i) The pentazole anion, Ns- (4), and pentaphosphole anion, Ps- (5), favor planar D5h structures, in contrast to hexazine (Nb) and hexaphosphabenzene (P6), which have 0 2 twist-boat structures. (ii) Species obeying the Hiickel rule are not necessarily stable thermodynamically or kinetically toward dissociation and isomerization. The pentazole anion 4 is thermodynamically unstable toward dissociation into N3- + Nz. The barrier is 19.4 kcal/mol at MP4SDTQ/6-3 1+G*//MP2(fu11)/6-3 lG* + AZPE(HF/631G*). In contrast to 4, 5 is stable with respect to both P3- + Pz dissociation (AE = 7 1.6 kcal/mol) and 2(5) 2P3- P4( Td) disproportionation (AE = 116.4 kcal/mol). (iii) The lithium salt, NSLi, favors a planar CZ,structure 15, in contrast to cyclopentadienyllithium, (CH)sLi (27), and to the sodium PSNa complex (12). While 15 is 33.5 kcal/mol unstable relative to dissociation into N3Li (GU) + Nz (MP4SDTQ/6-
-
+
8206 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
+
31+G*//MP2/6-31G* AZPE(MP2/6-31G1), pyramidal 12 is highly stable toward the P3Na P2dissociation and also toward disproportionationinto P3Na (either C3, triplet 34 or C , singlet 35) + P,(Td). (iu) Comparison of the experimental IR spectrum of PSNa with the calculated frequenciesfor 12 and for P3Na (35) clearly shows that the latter may be present. This confirms a suggestion of Hamilton and Schaefer.*s
+
Acknowledgment. The Alexander von Humboldt Foundation is greatly acknowledged for a Research Fellowship for M.N.G. C.M. thanks gratefully the Fond der Chemischen Industrie for a Kekule-Scholarship. This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the Stiftung Volkswagenwerk, and the Convex Computer Corporation. Wealso thank Prof. 0.J. Scherer for helpfulcomments. References and Notes (1) (a) Benaon,F. R. TheHigh Nitrogen Compounds;Wiley: New York, 1984. (b) Quin, L. D. The Heterocyclic Chemistry of Phosphorus; Wiley: New York, 1981. (c) Balaban, A. T.; Bancin, M.; Ciorba, V. Annulenes, Benzlcr-,Hetero-, Homo-Derivatives, and Their Valence Isomers;CRC Press: Boca Raton, FL, 1987; Vol. 3. (d) Glukhovtsev, M. N., Simkin, B. Ya.; Minkin, V. I. Adv. Heterocyclic Chem., in press. (e) Janoschek, R. Chem. Ber. 1992,125,2687. ( f ) Hluser, M.; Schneider, U.; Ahlrichs, R. J . Am. Chem. Soc. 1992,114,9551.(g) Matsunaga, N.; Cundari, T. R.; Schmidt, M. W.; Gordon, M.S . Theor. Chim. Acta 1992,83,57. (2) (a) Power, P. P. The Chemistry of Inorganic RingSystem. Studies in Inorganic Chemistry; Steudel, R., Ed.; Elsevier: Amsterdam, 1992;Vol. 14,pp 15-24. (b) Fink, W. H.; Richards, J. C. J. Am. Chem.Soc. 1991,113, 3392. (3) (a) Engelke, R. J . Phys. Chem. 1989,93,5722.(b) Lee,T. J.; Rice, J. E. J. Phys. Chem. 1991,94, 1215. (c) Lauderdale, W. J.; Stanton, J. F.; Bartlett, R. J. J. Phys. Chem. 1992,96,1173.(d) Bliznyuk,A. A.;Shen, M.; Schaefer, H. F. Chem. Phys. Lett. 1992,198,249. (4) (a) Balabnn,A. T.; Bancin, M.;Ciorba, V.Annulenes,Benzlcr-,Hetero-, Homlcr-Derivatives,and Their ValenceIsomers; CRC Press: Boca Raton, FL, 1987;Vol. 1. (b) Garrat, P. J. Aromaticity; Wiley: New York, 1986. ( 5 ) Gutman, I., Milun, M.; Trinajstic, N. J. Am. Chem. Soc. 1977,99, 1692. (6) (a) George, P.; Trachtman, M.; Bock, C. W.; Brett, A. M. Theor. Chim. Acta 1975,38,121.(b) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (7) Cox, J. D.;Pilcher, G. Thermochemistry of Organic and Organometallic compounds; Academic Press: New York, 1970. (8) Glukhovtsev,M. N.; Schleyer, P. v. R. Chem. Phys. Lett. 1992,198, 547. (9) Glukhovtsev,M.N.;Schleyer, P. v. R. J. Am. Chem.Soc., submitted. (10) (a) Huber, H.; Ha, T. K.; Nguyen, M. T. J. Mol. Struer. (Theochem) 1983,105,351.(b) Ramek, M. J. Mol. Struct. (Theochem) 1984,109,391. (c) Ha,T.-K.;Nguyen, M. T. Chem. Phys. Lett. 1992,195,179.(d) Nguyen, M. T.; Hegarty, A. F. J. Chem.Soc., Chem. Commun. 1986,383.(e)Nagase, S.;Ito, K. Chem. Phys. Lett. 1986,126,43. (1 1) (a) Scherer, 0.J.; Sitzmann, H.; WolmershHuser, G. Angew. Chem. 1985, 97, 358. (b) Scherer, 0.J.; Blath, C.; Braun, J.; Winter, R. The Chemistryof InorganicRingSystem. Studies in Inorganic Chemistry;Steudel, R., Ed.; Elsevier: Amsterdam, 1992;Vol. 14,pp 193-208. (c) Snodgrass,J. T.; Coe, J. V.; Friedhoff, C. B.; McHugh, K. M.; Bowen, K. H. Chem. Phys. Letr. 1985,122,352.
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