The structural and theoretical study of 1H-3,5-di-phenyl-1,2,4-diazaphosphole in the solid state

Article abstract of DOI:10.1016/j.tet.2007.06.069

The N-, P-containing five-membered heterocyclic compound 1H-3,5-di-phenyl-1,2,4-diazaphosphole (1) was prepared in good yield and has been structurally characterized. 1H-3,5-Di-phenyl-1,2,4-diazaphosphole (1), crystallizing in two unexpected cyclic dimers

Full text of DOI:10.1016/j.tet.2007.06.069

Tetrahedron 63 (2007) 9129–9133
The structural and theoretical study of 1H-3,5-di-phenyl-1,2,4-
diazaphosphole in the solid state
b
c
a,
Li Wan,a Ibon Alkorta,b, Jose Elguero, Jie Sun and Wenjun Zheng
*
*
ꢀ
aDepartment of Chemistry, Fudan University, Handan Road 220, Shanghai 200433, China
b
´
Instituto de Quꢀımica Medica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain
cShanghai Institute of Organic Chemistry, Chinese Academy of Science, Fenglin Road 354, Shanghai 200032, China
Received 17 May 2007; revised 19 June 2007; accepted 21 June 2007
Available online 27 June 2007
Abstract—The N-, P-containing five-membered heterocyclic compound 1H-3,5-di-phenyl-1,2,4-diazaphosphole (1) was prepared in good
yield and has been structurally characterized. 1H-3,5-Di-phenyl-1,2,4-diazaphosphole (1), crystallizing in two unexpected cyclic dimers
with N–H/N hydrogen bonds, presents in the solid state a dynamic proton disorder implying a dynamic equilibrium within both dimers.
The conformations of the phenyl rings, the disorder of the NH protons, and the intermolecular hydrogen bond of several 1,2,4-diazaphospholes
(1–5) in the solid state have been rationalized by DFT [B3LYP/6-311++G(d,p)] calculations.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
about the motif of intermolecular NH-hydrogen bond was
not reported in the published paper.3 From a molecular
point of view, 1H-1,2,4-diazaphospholes can be considered
4-phosphapyrazoles; thus, their conformations in the solid
state may be compared to those of the corresponding
pyrazoles. Indeed, for 2 and 3, this is the case, and the
conformations are similar to those of 1H-pyrazole4 and
1H-3,5-di-tert-butyl-pyrazole, respectively.5 While 1H-3,5-
di-phenyl-pyrazole features a cyclic tetramer with N–H/N
hydrogen bonds,6 the solid state structure of 1H-3,5-di-
phenyl-1,2,4-diazaphosphole (1) has not yet been solved
owing to the difficulty in obtaining crystals for X-ray diffrac-
tion analysis. Such problem presumably stems from an
extraordinary arrangement of the molecules in the crystal
lattice. Recently, some of us investigated the dynamic
properties of 1 by solid state 13C and 15N CPMAS NMR
spectra as well as by an approach to GIAO/ab initio cal-
culations.7 The results indicated a probable oligomeric
aggregate of 1 with a dynamic behavior of intermolecular
solid state proton transfer (ISSPT). 1H-3,5-Di-phenyl-
1,2,4-diazaphosphole (1) has attracted much interest as a
potential candidate in coordination chemistry,8 in accurate
electron density studies,8 as well as for analysis of the solid
state properties of these heterocycles.7 As a first step, we
have now optimized the preparation of this compound,
determined its crystal structure to confirm its molecular
constitution, and carried out a theoretical analysis using
B3LYP/6-311++G(d,p) calculations9,10 to understand the
origin of the proton disorder present in the solid state
structure.
The nitrogen- and phosphorus-containing five-membered
heterocycles prepared by the reaction of 1,3-bis(dimethyl-
amino)-2-phosphaallyl chlorides and hydrazine in chloro-
form were formulated as 1H-1,2,4-diazaphospholes (1–3)
on the basis of spectroscopic studies (Scheme 1).1
Cl–
R
R
P
R
R
H2N NH2
+
- Me2NH2+ Cl-
- Me2NH
Me2N
P
NMe2
N
N
H
1 R = Ph
2 R = H
3 R = tBu
Scheme 1. 1H-3,5-Di-substituted-1,2,4-diazaphospholes 1–3.
Subsequent work showed that, in the solid state, the
structures of 1H-1,2,4-diazaphophole (2) and 1H-3,5-di-
tert-butyl-1,2,4-diazaphosphole (3) crystallize in a helix of
order 3 and a dimer, respectively, in which significant inter-
molecular disorder NH-hydrogen bonds were revealed.2 The
structure of 5-diisopropylamino-3-trifluormethyl-1H-1,2,4-
diazaphosphole (4) is also known, but further information
Keywords: Azaphospholes; Ab initio calculations; 1H-1,2,4-Diazaphosp-
holes; Hydrogen bonding; X-ray.
* Corresponding authors. Tel.: +34 91 5622900; fax: +34 91 5644853
(I.A.); tel./fax: +86 21 65642408 (W.Z.); e-mail addresses: ibon@iqm.
csic.es; wjzheng@fudan.edu.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.06.069

9130
L. Wan et al. / Tetrahedron 63 (2007) 9129–9133
2. Results and discussion
2.1. Synthesis of compound 1 (Scheme 1)
Previously, Schmidpeter and Willhalm reported that the re-
action of 1,3-bis(dimethylamino)-2-phosphaallyl chlorides
and hydrazine led to formation of 1H-3,5-di-phenyl-1,2,4-
diazaphosphole (1). This compound could be isolated in
mild yield following extraction with ether.1 In the course
of an investigation on 3,5-di-phenyl-1,2,4-diazaphospholide
metal complexes,8 we found that the yield of 1 was signifi-
cantly improved by addition of excess of hydrazine in the re-
action system, which could be washed off with water from
the combined extracting solvent phase (diethyl ether). The
resulting residue, after removal of ether, was sublimed at
about 200 ꢀC in high vacuum (0.01 mmHg) affording 1 as
white pure solid in 85% yield. Notably, 1 should be kept in
dark due to the light sensitivity otherwise it will decompose
into a yellowish solid within few days.
2.2. Structural characterization of compound 1
The colorless crystals of 1 were obtained by a method of
solid phase recrystallization in high vacuum at 200 ꢀC by
€
using a Buchi Glass Oven (B-585) apparatus. The structure
was solved with SHELXS-9711 and refined by full-matrix
least-squares analysis with SHELXL-9712 using 5220 inde-
pendent reflections (qmax¼27ꢀ) with I>2s(I). The results of
the analysis confirm formula 1.
Compound 1 crystallizes in the monoclinic space group P21/c.
Molecular dimensions are shown in Figures 1 and 2. As illus-
trated in these figures, and concerning the conformation of
the phenyl rings attached to the heterocycle, the average
planes formed by the phenyl groups and the heterocycle
ring show that they are not coplanar (see Section 4, Table
1). Due to the difference in average phenyl dihedral angles,
there are two molecular conformations [A (N1, N2) and
B (N3, N4)] present in the crystal in a ratio of 1:1. Being
the average phenyl dihedrals 26.88(18)ꢀ for the conformation
A and 23.95(20)ꢀ for B, the total average phenyl dihedral of
the molecules A and B is 25.42(19)ꢀ [(26.88(18)+23.95(20))/
2¼25.42(19)ꢀ]. However, given only single set of the signals
in the spectrum, the solid state 13C CPMAS NMR spectro-
scopic data for 1 seem to suggest a dynamic process between
A and B at room temperature, in agreement with those ob-
served by some of us previously.7 Notably, intermolecular
hydrogen bonds in the solid state with disordered NH protons
involving nitrogen atoms are found, evidenced by the broad
signal in the solid state 15N CPMAS NMR spectrum as
well as the almost identical internal angles at nitrogen atoms
(vide infra). More interesting, however, is the presence of the
intermolecular hydrogen bonds only between the nitrogen
atoms of two molecules with an identical conformation
(A–A or B–B) (Fig. 2). Since the nitrogen atoms in the
molecules are excellent hydrogen bonding formers, the dis-
crete molecules are linked into two different cyclic dimers
by the intermolecular hydrogen bonding interactions
Figure 1. A stereoview of part of the crystal structure of 1H-3,5-di-phenyl-
1,2,4-diazaphosphole.
di-phenyl-pyrazole.6 Additionally, the heterocyclic rings
for A and B in 1 are almost planar with mean devia-
˚
˚
tions 0.0011 A and 0.0032 A from the ring plane. There
are no significant interactions among adjacent dimers.
˚
The bond lengths of N(1)–N(2) (1.341(2) A), P(1)–
˚
C(1) (1.7404(18) A) in the dimer A–A and N(3)–N(4)
˚
(1.344(2) A), P(2)–C(15) (1.7393(18) A) in the dimer B–B
˚
are slightly longer than those found in 1H-1,2,4-diazaphosp-
2
˚
˚
hole (N–N 1.323(3) A, P–C 1.710(3) A). The angles of C–
P–C [C(1)–P(1)–C(2) 87.01(9)ꢀ in A, C(15)–P(2)–C(16)
87.30(9)ꢀ in B] are comparable to those found in 1H-3,5-
di-tert-butyl-1,2,4-diazaphosphole (C–P–C 87.3(2)ꢀ).2 The
internal angles at nitrogen atoms of the heterocyclic ring
are slightly different for the molecule A (N1 114.09(19)ꢀ
vs N2 113.11(19)ꢀ) and for the molecule B (N3
114.11(18)ꢀ vs N4 112.99(17))ꢀ, clearly implying the dy-
namic proton disorder on both nitrogen atoms even though
the proton is somewhat closer to one of the two nitrogen
atoms (N1 and N3).
Table 1. Selected torsion angles [ꢀ] of 1
˚
[N(1)–H(1)/N(2A) 2.884(2) A; N(2)–H(1A)/N(1A)
˚
2.884(2) A] (symmetry codes: [ꢁx+1, ꢁy+1, z]), and [N(3)–
C(4)–C(1)–C(3)–P(1)
27.5 (2)
26.25 (15)
21.7 (2)
˚
˚
P(1)–C(2)–C(9)–C(10)
P(2)–C(15)–C(17)–C(18)
C(28)–C(16)–C(23)–P(2)
H(3)/N(4A) 2.890(2) A; N(4)–H(3A)/N(3A) 2.890(2) A]
(symmetry codes: [ꢁx, ꢁy+1, ꢁz]). The conformation of 1
is therefore in contrast to that of the tetrameric 1H-3,5-
26.2 (2)

L. Wan et al. / Tetrahedron 63 (2007) 9129–9133
9131
Figure 2. An ORTEP plot of two dimeric 1H-3,5-di-phenyl-1,2,4-diazaphospholes showing displacement ellipsoids at 50% probability level.
2.3. Computational studies
the attractive interaction between an aromatic CH and the
pyridinic nitrogen lone pair while they are twisted in the
case of 5-phenyl group due to a repulsive interaction between
the CH and the NH.17 In the case of 1, the twisted conforma-
tions of the phenyl groups are obviously caused by an intra-
molecular interaction of adjacent groups in the similar
manner. On thebasis of thesolidstateNMRresults,7 itis clear
that a dynamic equilibrium is present between the two dimers
of 1. The calculated total average dihedral angle is consistent
with that found from the crystallographic data (Scheme 3).
To gain a better understanding of the conformation of the
phenyl rings in 1, the disorder of the NH protons and the
intermolecular hydrogen bond of 1H-1,2,4-diazaphospholes
in the solid state, we have performed a series of B3LYP/6-
311++G(d,p) calculations of compounds 1–5 (Scheme 2).
Previously, some of us published the results of B3LYP/6-
31G* calculations on compounds 3 and 5.7 Table 2 shows
the results obtained at the B3LYP/6-311++G(d,p) level on
compounds 1–5, the last one being unknown.
3. Conclusion
It is clear that in the case of 4 the NH proton is localized
while in the cases of 1, 2 and 3, the proton is delocalized be-
tween both nitrogen atoms in a 50:50 disorder. In the case of
4, the other tautomer (5-diisopropylamino-3-trifluormethyl-
1H-1,2,4-diazaphosphole) is expected to be much less stable
since in pyrazoles and in 1H-1,2,4-triazoles the CF3 group
‘prefers’ the 3-position,15,16 and for this reason there is no
disorder. The difference, on the basis of the solid state
NMR results,7 is that the disorder is static in the case of 2
(helical chain) and 3 (cyclic dimer) while it is dynamic in
the case of 1 (two cyclic dimers).
In summary, the chemists’ interest in compound 1 arises
from the very unusual existence of two different cyclic di-
mers linked by intermolecular disordered hydrogen bonds,
including a dynamic equilibrium in the solid state. Such con-
formations in solid state are unprecedented in the structures
of five-membered heterocycles. A theoretical study of sev-
eral 1H-1,2,4-diazaphospholes using the DFT [B3LYP/6-
311++G(d,p)] calculations promoted the understanding of
the observed structural features as to the twisted conforma-
tion of the phenyl rings of 1, the disorder of the NH protons
and the intermolecular hydrogen bonds. It is now clear that
three 1H-1,2,4-diazaphospholes with identical substituents
at positions 3 and 5, Ph 1, H 2 and t-Bu 3, crystallize in three
The phenyl groups in the analogous case of 1H-3,5-disubsti-
tuted pyrazoles, are almost coplanar for the 3-phenyl because
P
P
P
P
P
(i-Pr)2N
C6H5
C6H5
CF3 F3C
CF3
H
H
t-Bu
t-Bu
N
N
N N
N
N
N
N
N N
H
H
H
H
H
1 (this work)
4 (KORHII)
5
2 (HELMOA)
3 (HELQAQ)
Scheme 2. The 1H-1,2,4-diazaphospholes studied.
Table 2. Calculated and experimental geometries of compounds 1–5 (angles in deg)
Compd
CN(H)Ncalcd
109.4
CNN(H)calcd
118.8
Average
114.10
Refcode13
CN(H)Nexp
CNN(H)exp
1
1
2
3
4
5
This work
This work
HELMOA14
HELQAQ
KORHII
—
114.2
112.3
112.2
113.3
107.7
114.3
112.5
112.3
113.4
117.7
108.2
109.5
108.3
108.5
117.9
119.0
118.8
117.5
113.05
114.25
[113.35]
113.00

9132
L. Wan et al. / Tetrahedron 63 (2007) 9129–9133
14.3º
14.3º
24.2º
24.2º
34.1º
34.1º
P
P
P
N
N
N
N
N
N
N
N
118.8º
118.8º
114.1º
109.4°
109.4º
114.1°
fast
Average:
H
H
H
H
H
H
114.1º
H
H
109.4º
109.4º
N
N
N
N
114.1º
118.8°
118.8º
P
P
P
Experimental:
CNN angle = 113.3º
Ph dihedral angle = 25.4º
Scheme 3. The dynamic situation present in compound 1.
€
different structures: two dimers with ISSPT, helix (no
ISSPT) and dimer (no ISSPT), respectively. The relationship
between nature of 1H-3,5-substituted pyrazoles and the hy-
drogen bond pattern present in the crystal was recently re-
ported;18,19 however, the corresponding structural studies
of 1H-1,2,4-diazaphospholes are still at the initial stages.
Thus, these compounds deserve further structural investiga-
tions using X-ray crystallography, solid state 13C and 15N
CPMAS NMR (15N labeled) and theoretical calculations.20
the solid phase recrystallization on a Buchi Glass Oven
(B-585). 1H NMR spectra were recorded with a Bruker AV-
500 spectrometer (500 MHz for 1H NMR). Chemical shifts
are reported as parts per million (ppm) in d units on the scale
downfield from tetramethylsilane (TMS). X-ray data were
collected on a Bruker Smart APEX-2 diffractometer.
4.2. Synthesis of 1H-3,5-di-phenyl-1,2,4-diazaphosphole
(1)
To a solution of 1,3-bis(dimethylamino)-2-phosphaallyl
chlorides (50 mmol) in chloroform (150 mL),1 hydrazine
(55 mmol) was added slowly. After stirred for 18 h at
room temperature, the solution was refluxed until the
brown-red color disappeared. The volatile components
were then removed under reduced pressure, and the resulting
residue was extracted with ether (5ꢂ20 mL). The combined
organic phase was washed with water (2ꢂ10 mL) to remove
the remaining hydrazine. After the removal of ether on a
rotate distillation apparatus, the resulting residue was
sublimed at 200 ꢀC in high vacuum (0.01 mmHg) to afford
white solid (10.12 g, 85.0%), which was kept in dark due
to the light sensitivity (Mp 218–220 ꢀC). The spectra data
are identical to those reported in the literature.1
4. Experimental section
4.1. General experimental procedures
The starting materials N,N-dimethylbenzamide (98%) and
oxalyl chloride (98%) were commercially available from
Acros, tris(trimethylsilyl)phosphine (95%) from Aldrich,
hydrazine from SCR, and used without further purification.
All preparations were performed under an atmosphere of
dry N2-free O2, employing either Schlenk-line techniques or
an inert-atmosphere vacuum glovebox. Solvents were dried
according to standard methods. Access to the required
1H-3,5-di-phenyl-1,2,4-diazaphosphole (1) was achieved
by an optimized method referring to the literature.1 Melting
points were taken on a hot-plate microscope apparatus and
were uncorrected. The single crystals were prepared by
4.3. Crystal growth of 1H-3,5-di-phenyl-1,2,4-diaza-
phosphole (1)
The purified 1H-3,5-di-phenyl-1,2,4-diazaphosphole pow-
der was charged into a glass tube (diameter 2 cm, 10 cm
ꢀ
˚
Table 3. Bond lengths [A] and angles [ ] of 1
€
long), and then compacted. The tube was set on a Buchi
P(1)–C(1)
N(1)–C(1)
N(1)–N(2)
P(2)–C(15)
N(3)–C(15)
N(3)–N(4)
1.7404 (18)
1.329 (2)
1.341 (2)
1.7393 (18)
1.336 (2)
1.344 (2)
C(1)–N(1)–N(2)
C(2)–N(2)–N(1)
C(15)–N(3)–N(4)
N(1)–C(1)–C(3)
N(1)–C(1)–P(1)
N(2)–C(2)–C(9)
114.09 (19)
113.11 (19)
114.11 (18)
118.95 (17)
112.73 (15)
119.03 (17)
Glass Oven (B-585) and heated in high vacuum
(0.01 mmHg) at 200 ꢀC for about 24 h to give monocrystals
of sufficient size suitable for X-ray diffraction analysis in the
solid phase of sample. During which time, a partial amount
of 1H-3,5-di-phenyl-1,2,4-diazaphosphole was sublimed on
˚
Table 4. Hydrogen bonds [A and deg] of 1
D–H/A
d(D–H)
d(H/A)
d(D/A)
<(DHA)
N(4)–H(30)/N(3)#1
N(2)–H(10)/N(1)#2
N(3)–H(3)/N(4)#1
N(3)–H(3)/N(3)#1
N(1)–H(1)/N(2)#2
N(1)-H(1)/N(1)#2
0.803 (19)
0.81 (4)
0.84 (3)
0.84 (3)
0.81 (3)
0.81 (3)
2.21 (3)
2.16 (4)
2.19 (3)
2.73 (3)
2.17 (3)
2.72 (3)
2.890 (2)
2.884 (2)
2.890 (2)
3.143 (3)
2.884 (2)
3.167 (3)
142 (4)
148 (3)
141 (3)
112 (2)
146 (3)
116 (2)

L. Wan et al. / Tetrahedron 63 (2007) 9129–9133
9133
Table 5. Crystal data and structure refinement details of 1
€
2. Polborn, K.; Schmidpeter, A.; Markl, G.; Willhalm, A. Z.
Naturforsch. B 1999, B54, 187–192.
Crystal data
Mol. Formula
Mr
1
€
3. Grobe, J.; Le Van, D.; Hegemann, M.; Krebs, B.; Lage, M.
Chem. Ber. 1992, 125, 411–414.
C14H11N2P
238.22
Crystal system
Space group
Monoclinic
P21/c
ꢀ
4. Berthou, J.; Elguero, J.; Rerat, C. Acta Crystallogr., Sect. B
1970, B26, 1880–1881.
5. Llamas-Saiz, A. L.; Foces-Foces, C.; Cano, F. H.; Jimenez, P.;
˚
a/A
9.5341 (13)
14.0033 (19)
18.473 (2)
103.342 (2)
2399.7 (6)
8
ꢀ
˚
b/A
˚
Laynez, J.; Meutermans, W.; Elguero, J. Acta Crystallogr.,
Sect. B 1994, B50, 746–762.
6. Aguilar-Parrilla, F.; Scherer, G.; Limbach, H.-H.; Foces-Foces,
M. C.; Cano, F. H.; Smith, J. A. S.; Toiron, C.; Elguero, J. J. Am.
Chem. Soc. 1992, 114, 9657–9659.
c/A
b/ꢀ
˚
V/A
3
Z
Dx/g cmꢁ3
1.319
Crystal size/mm
Crystal color
m(Mo Ka)/mmꢁ1
Total data
0.476ꢂ0.332ꢂ0.180
Colorless
0.206
13,891
5220
2880
0.0607
27.0
0.0436
ꢀ
7. Claramunt, R. M.; Lopez, C.; Schmidpeter, A.; Willhalm, A.;
Elguero, J.; Alkorta, I. Spectroscopy 2001, 15, 27–32.
8. Zheng, W. J.; Zhang, G. Z.; Fan, K. N. Organometallics 2006,
25, 1548–1550.
9. Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971,
54, 724–728.
Unique data
Observed data
Rint
ꢀ
qmax
/
R[F2>2s(F2)]
10. Frisch, M. J.; Pople, J. A.; Krishnan, R.; Binkley, J. S. J. Chem.
Phys. 1984, 80, 3265–3269.
11. Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467–473.
12. Sheldrick, G. M. SHELXS-97, Program for X-ray Crystal
the B€uchi Glass Oven. However, we were unable to obtain
the monocrystals of sufficient size from sublimation.
€
€
Structure Refinement; Gottingen University: Gottingen,
Germany, 1997.
13. Cambridge Structural Database, version 5.28 (November
2006), Allen, F. H. Acta Crystallogr., Sect. B 2002, B58,
380–388.
14. There is also a HELMOA01 structure but the reported angles
are almost identical (both 112.26ꢀ), Marsh, R. E. Acta
Crystallogr., Sect. B 2002, B58, 893–899.
4.4. X-ray structural analysis
Data collection: SMART;21 cell refinement: SAINT;21 data
reduction: SAINT and SHELXTL;21 program used to solve
structure: SHELXS-97;11 program used to refine structure:
SHELXL-97;12 molecular graphics: SHELXL;12 software
used to prepare material for publication: SHELXL.12 Crys-
tallographic data for 1 have been deposited in the Cambridge
Crystallographic Data Centre (deposition number: CCDC
630888) (Tables 3–5).
ꢀ
15. Abboud, J. L. M.; Cabildo, P.; Can˜ada, T.; Catalan, J.;
Claramunt, R. M.; de Paz, J. L. G.; Elguero, J.; Homan, H.;
Notario, R.; Toiron, C.; Yranzo, G. I. J. Org. Chem. 1992,
57, 3938–3946.
ꢀ
16. Tipping, A. E.; Jimenez, P.; Ballesteros, E.; Abboud, J.-L. M.;
Yanez, M.; Essefar, M.; Elguero, J. J. Org. Chem. 1994, 59,
ꢀ˜
1039–1046.
17. Klein, O.; Aguilar-Parrilla, F.; Lopez, J. M.; Jagerovic, N.;
Acknowledgements
ꢀ
Elguero, J.; Limbach, H.-H. J. Am. Chem. Soc. 2004, 126,
11718–11732.
The synthesis was performed at Fudan University, Shanghai,
China, and support from NSFC (Grant no. 20571017); the
ꢀ
theoretical analysis was carried out at Instituto de Quımica
Medica, CSIC, Madrid, Spain, and support from the Minis-
18. Foces-Foces, C.; Alkorta, I.; Elguero, J. Acta Crystallogr.,
Sect. B 2000, B56, 1018–1028.
ꢀ
ꢀ
19. Alkorta, I.; Elguero, J.; Foces-Foces, C.; Infantes, L. Arkivoc
2006, 2, 15–30 and references therein.
terio de Educacion y Ciencia of Spain (Project number
CTQ2006-14487-C02-01/BQU). X-ray data were collected
at Shanghai Institute of Organic Chemistry, Chinese Acad-
emy of Science.
20. We have calculated [B3LYP/6-311++G(d,p)] the barrier to
the double proton transfer in the case of the parent 1H-1,2,4-
diazaphosphole dimer. For a concerted process and after ZPE
correction the barrier amounts to 52.8 kJ molꢁ1. In the case
of 3,5-di-phenyl-4-bromopyrazole, also a dimer, the experi-
References and notes
mental barrier is 53.1 kJ molꢁ1
.
21. SMART, SAINT and SHELXTL; Bruker AXS: Madison, WI,
2000.
1. Schmidpeter, A.; Willhalm, A. Angew. Chem., Int. Ed. Engl.
1984, 23, 903–904.