Tautomerism of substituted salicylaldehyde and 2- diphenylphosphinebenzaldehyde 1′-phthalazinylhydrazones: X-ray crystallography and quantum chemical modeling

Article abstract of DOI:10.1134/S0022476613050168

1′-Phthalazinylhydrazones of salicylaldehyde, its substituted derivatives, and 2-diphenylphosphine-benzaldehyde are synthesized and studied. A description is given of the structures salicylaldehyde 1′- phthalazinylhydrazone (1a) and 2-diphenylphosphinebenzaldehyde 1′-phthalazinylhydrazone (2), which exist in the crystal in the hydrazonophthalazone tautomeric form. Molecules of hydrazone 1a form in the crystal infinite stacks of hydrogen bonded dimers with intermolecular π-stacking interactions. A quantum chemical calculation is made of the geometry and total energy of the possible tautomers in vacuum and in aqueous and chloroform solutions. The hydrazonophthalazone tautomers are shown to be the most stable in all cases. The X-ray crystallography results are compared with the calculated data.

Full text of DOI:10.1134/S0022476613050168

Journal of Structural Chemistry. Vol. 54, No. 5, pp. 952-959, 2013
Original Russian Text Copyright © 2013 by S. I. Levchenkov, L. D. Popov, I. N. Shcherbakov, G. G. Aleksandrov, A. A. Zubenko, V. A. Kogan
TAUTOMERISM OF SUBSTITUTED SALICYLALDEHYDE
AND 2-DIPHENYLPHOSPHINEBENZALDEHYDE
1′-PHTHALAZINYLHYDRAZONES:
X-RAY CRYSTALLOGRAPHY AND QUANTUM
CHEMICAL MODELING
1
© S. I. Levchenkov, L. D. Popov, I. N. Shcherbakov,
2
2
UDC 539.27:544.18:547.556.93
3 4
G. G. Aleksandrov, A. A. Zubenko, and V. A. Kogan
2
1′-Phthalazinylhydrazones of salicylaldehyde, its substituted derivatives, and 2-diphenylphosphine-
benzaldehyde are synthesized and studied. A description is given of the structures salicylaldehyde 1′-
phthalazinylhydrazone (1а) and 2-diphenylphosphinebenzaldehyde 1′-phthalazinylhydrazone (2), which
exist in the crystal in the hydrazonophthalazone tautomeric form. Molecules of hydrazone 1а form in the
crystal infinite stacks of hydrogen bonded dimers with intermolecular π-stacking interactions. A quantum
chemical calculation is made of the geometry and total energy of the possible tautomers in vacuum and in
aqueous and chloroform solutions. The hydrazonophthalazone tautomers are shown to be the most stable in
all cases. The X-ray crystallography results are compared with the calculated data.
DOI: 10.1134/S0022476613050168
Keywords: hydrazones, tautomerism, X-ray crystallography, hydrogen bond, density functional theory.
The reason for the interest in 1-hydrazinophthalazine (hydralazine) and its derivatives and their complexes with
transition metals is the inherent biological activity of these compounds, especially their hypotensive, antiinflammatory, and
anticancer activity [1-5]. An additional incentive is the fact that hydralazine metabolism involves the interaction with the
carbonyl compounds formed in the Krebs cycle [6-8]. The literature describes several hydralazine hydrazones with a number
of mono- and dicarbonyl compounds [9-16]. However, until recently there have been only two papers on salicylaldehyde 1′-
phthalazinylhydrazones [17, 18] although hydrazones of salicylaldehyde and its substituted derivatives are “classical” ligand
systems for modern coordination chemistry. It should be noted that these, as well as other studies, proceed from the
assumption that 1′-phthalazinylhydrazones exist in the hydrazone tautomeric form. However, a number of studies using X-ray
crystallography and quantum mechanical calculations have shown that the hydrazonophthalazone (diazine) tautomer is the
main tautomeric form of this type of compounds [11, 14-16, 19, 20].
This paper presents the results of physicochemical studies and quantum chemical modeling of 1′-phthalazinyl-
hydrazones of substituted salicylaldehydes (1) and 2-diphenylphosphinebenzaldehyde (2). The aldehyde component of the
1
Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia; s.levchenkov@gmail.com.
3
2
Chemistry Department, Southern Federal University, Rostov-on-Don, Russia. N. S. Kurnakov Institute of General and
4
Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia. North-Caucasian Zonal Research Veterinary Institute,
Novocherkassk, Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 54, No. 5, pp. 914-922, September-October, 2013.
Original article submitted February 2, 2013.
952
0022-4766/13/5405-0952

hydrazones were salicylaldehyde itself and its most synthetically accessible derivatives: 5-methoxy-, 5-bromo-, 5-nitro-, and
3,5-di-tert-butylsalicylic aldehydes.
EXPERIMENTAL
1′-Phthalazinylhydrazones of salicylaldehyde and its substituted derivatives of type 1 were synthesized
according to the following procedure. An equivalent amount of sodium acetate was added to a hot suspension of 0.002 mol 1-
hydrazinophthalazine hydrochloride in 40 ml ethanol, and then a hot solution of 0.002 mol of a given aldehyde in 10 ml
ethanol was poured into the suspension. The reaction mixture was boiled for 1-3 h, after which 50 ml distilled water were
added. The resulting precipitate was filtered and washed with water. Recrystallization from the ethanol–DMF mixture.
1a, R=H; m.p. 217°С. Elemental analysis: found, %: C 60.02, H 4.42, and N 20.94; for C15H12N4O calculated, %:
–1
C 68.17, H 4.58, and N 21.20. IR spectrum (ν, cm ): 3316 ν(OH), 3250 ν(NH), 1613, 1600 ν(C=N), 1270 ν(C–O). PMR
spectrum (δ, ppm): 11.973 s (1H, NH), 10.355 s (1H, OH), 8.571 s (1H, CHarom), 8.34 dd (1H, J1 = 6.3 Hz, J2 = 2.1 Hz,
CHarom), 7.965 s (1H, CHazometh), 7.66 m (3H, CHarom), 7.50 dd (1H, J1 = 7.5 Hz, J2 = 1.5 Hz, CHarom), 7.21 td (1H,
J1 = 7.3 Hz, J2 = 1.7 Hz, CHarom), 6.87 m (2H, CHarom).
1b, R=5-OCH3; m.p. 194°С. Elemental analysis: found, %: C 65.18, H 4.82, and N 18.90; for C16H14N4O2 calculated,
–1
%: C 65.30, H 4.79, and N 19.04. IR spectrum (ν, cm ): 3308 ν(OH), 3280 ν(NH), 1609, 1590 ν(C=N), 1266 ν(C–O). PMR
spectrum (δ, ppm): 12.178 s (1H, NH), 9.968 s (1H, OH), 8.655 s (1H, CHarom), 8.30 dd (1H, J1 = 7.2 Hz, J2 = 1.5 Hz, CHarom),
8.104 s (1H, CHazometh), 7.75 m (3H, CHarom), 7.57 d (1H, J = 2.4 Hz, CHarom), 6.83 m (2H, CHarom), 3.758 s (3H, CH3).
1c, R=3,5-(t-Bu)2; m.p. 221°С. Elemental analysis: found, %: C 73.57, H 7.60, and N 15.05; for C23H28N4O
–1
calculated, %: C 73.37, H 7.50, and N 14.88. IR spectrum (ν, cm ): 3300 ν(OH), 3255 ν(NH), 1611, 1598 ν(C=N), 1267
ν(C–O). PMR spectrum (δ, ppm): 12.193 s (1H, NH), 11.041 s (1H, OH), 8.660 s (1H, CHarom), 8.30 d (1H, J = 7.5 Hz,
CHarom), 8.140 s (1H, CHazometh), 7.75 м (3H, CHarom), 7.33 d (1H, J = 2.1 Hz, CHarom), 7.28 d (1H, J = 2.4 Hz, CHarom), 1.438
s (9H, t-Bu), 1.278 s (9H, t-Bu).
1d, R=5-Br; m.p. > 250°С. Elemental analysis, found, %: C 62.75, H 3.53, and N 16.70; for C15H11BrN4O
–1
calculated, %: C 52.50, H 3.23, and N 16.33. IR spectrum (ν, cm ): 3272 ν(OH), 3190 ν(NH), 1619, 1601 ν(C=N), 1268
ν(C–O). PMR spectrum (δ, ppm): 12.308 s (1H, NH), 10.347 s (1H, OH), 8.613 s (1H, CHarom), 8.34 d (1H, J = 2.9 Hz,
CHarom), 8.29 d (1H, J = 7.8 Hz, CHarom), 8.125 s (1H, CHazometh), 7.75 m (3H, CHarom), 7.35 dd (1H, J1 = 8.7 Hz, J2 = 2.7 Hz,
CHarom), 6.85 d (1H, J = 8.7 Hz, CHarom).
1e, R=5-NO2; m.p. > 250°С. Elemental analysis, found, %: C 58.16, H 3.52, and N 22.81; for C15H11N5O3
–1
calculated, %: C 58.25, H 3.58, and N 22.64. IR spectrum (ν, cm ): 3336 ν(OH), 3272 ν(NH), 1614, 1604 ν(C=N), 1286
ν(C–O). PMR spectrum (δ, ppm): 12.468 s (1H, NH), 11.65 s (1H, OH), 9.05 d (1H, J = 3.0 Hz, CHarom), 8.705 s (1H,
CHazometh), 8.32 d (1H, J = 8.1 Hz, CHarom), 8.172 s (1H, CHarom), 8.12 dd (1H, J1 = 9.0 Hz, J2 = 2.7 Гц, CHarom), 7.77 m (3H,
CHarom), 7.07 d (1H, J = 9.0 Hz, CHarom).
1′-Phthalazinylhydrazone of 2-diphenylphosphinebenzaldehyde (2) was synthesized as follows. An equivalent
amount of sodium acetate was added to a hot suspension of 0.03 mol 1-hydrazinophthalazine hydrochloride in 30 ml ethanol;
the mixture was boiled for 10 min; and 0.3 mol 2-diphenylphosphinebenzaldehyde was added. The reaction mixture was
refluxed for 4 h, after which 80 ml distilled water were added; the resulting mixture was left for a day. The yellow precipitate
was filtered and washed with water and ethanol. Recrystallization from ethanol.
M.p. 190°С. Elemental analysis: found, %: C 75.13, H 5.01, and N 12.75; for C27H21N4P calculated, %: C 74.99, H
–1
4.89, and N 12.96. IR spectrum (ν, cm ): 3250 ν(NH), 1621, 1600 ν(C=N). PMR spectrum (δ, ppm): 11.922 s (1H, NH),
8.95 d (1H, JP–H 4.5 Hz, CH=N), 8.5 m (1H, CHarom), 8.23 d (1H, J 7.8 Hz, CHarom), 8.121 s (1H, CHarom), 7.7 m (3H, CHarom),
7.4 m (8H, CHarom), 7.2 m (4H, CHarom), 6.7 m (1H, CHarom).
953

TABLE 1. Crystallographic Data and Experiment and Refinement Details for Compounds 1а and 2
Value
Parameter
1а
2
Gross formula
Molecular mass
Crystal size, mm
Temperature, K
λ, Å
C15H12N4O
C27H21N4P
264.29
432.45
0.32×0.12×0.12
0.26×0.18×0.12
200(2)
173(2)
0.71073
0.71073
Crystal system
Space group
a, b, c, Å
Triclinic
Triclinic
P-1
P-1
6.7859(16), 8.3854(18), 11.777(3)
8.6512(7), 10.7550(8), 11.6520(9)
89.136(4), 82.840(4), 70.487(4)
84.6000(10), 87.6740(10), 89.2160(10)
α, β, γ, deg
3
V, Å
626.5(2)
2
1078.39(14)
2
Z
3
1.401
0.093
276
1.332
ρ(x), g/cm
–1
0.151
μ, mm
F(000)
452
2.58-30.32
2.36-29.65
θ data collection domain, deg
Reflection index ranges
Measured/independent reflections
Reflections with I > 2σ(I )
Number of refined parameters
R1 (I > 2σ(I ))
–9 ≤ h ≤ 9, –11 ≤ k ≤ 11, –16 ≤ l ≤ 16 –12 ≤ h ≤ 12, –15 ≤ k ≤ 14, –16 ≤ l ≤ 16
7299/3682
2638
11938/5998
4547
181
289
0.0492
0.0378
wR2 (all reflections)
0.1430
0.1095
GOOF
1.000
1.000
–3
0.400/–0.326
904891
0.376/–0.416
853334
Δρmax/Δρmin, e/Å
CSDB No.
The elemental analysis was conducted using a Perkin-Elmer 240C instrument. The PMR spectra were recorded in
DMSO-d6 using a Varian Unity 300 spectrometer (300 MHz) in the pulse Fourier mode, with HMDS used as an internal
–1
standard. The IR spectra were recorded using a Varian Scimitar 1000 FT-IR instrument in the domain 400-4000 cm ; the
samples were prepared in the form of a suspension in paraffin oil.
X-Ray crystallographic study. Single crystals of compounds 1a and 2 for XRD were obtained by slow cooling of
the solutions in DMSO. The unit cell parameters and reflection intensities were measured with a Bruker Apex II diffracto-
meter [21] with a CCD detector (MoK radiation, graphite monochromator, and ω scanning). The absorption of X-ray
α
radiation was estimated semiempirically (Tmin/Tmax = 0.9489/0.9730) in the SADABS program [22]. The structures were
solved by the direct method and refined by the full-matrix LSM in an anisotropic approximation for nonhydrogen atoms
2
against F . Hydrogen atoms were put into geometrically calculated positions and refined in the rider model (Uiso(H) =
hkl
1.2Ueq(C) for all groups). All the calculations were made in the SHELX-97 programs [23]. The experiment details and
crystallographic data are given in Table 1; selected interatomic distances and valence angles are given in Table 2; and the
hydrogen bond characteristics are given in Table 3.
The quantum chemical calculations of the electronic and spatial structure of compound 1 were conducted in the
GAUSSIAN′03 program [24] within the density functional theory (DFT) in vacuum and in aqueous and chloroform solutions.
The solvent effect was considered within the polarizable continuum model (PCM) [25]. The calculations used the B3LYP hybrid
exchange–correlation functional [26] with the exchange part in the form proposed by Becke [27] and the Lee–Yang–Parr
954

TABLE 2. Selected Interatomic Distances (Å) and Valence Angles (deg) in the Structure of 1а and 2
Bond
d
Bond
d
d
Bond
O(1)–C(1)
N(1)–C(7)
N(1)–N(2)
N(2)–C(8)
P(1)–C(1)
P(1)–C(7)
Angle
1.358(2)
1.293(2)
1.3934(19)
1.307(2)
1.8358(15)
1.8421(14)
ω
P(1)–C(13)
C(13)–C(18)
C(18)–C(19)
C(6)–C(7)
N(3)–C(15)
N(3)–N(4)
Angle
1.8453(15)
1.418(2)
1.459(2)
1.447(2)
1.287(2)
1.3711(19)
ω
N(4)–C(8)
N(1)–C(19)
N(1)–N(2)
N(2)–C(20)
N(3)–C(20)
N(3)–N(4)
Angle
1.370(2)
1.274(2)
1.3981(17)
1.303(2)
1.369(2)
1.361(2)
ω
C(7)–N(1)–N(2)
C(8)–N(2)–N(1)
C(15)–N(3)–N(4)
C(8)–N(4)–N(3)
O(1)–C(1)–C(2)
C(1)–P(1)–C(13)
113.08(14)
113.73(13)
117.13(15)
126.29(14)
118.55(17)
101.86(6)
C(7)–P(1)–C(13)
C(1)–P(1)–C(7)
C(18)–C(13)–P(1)
N(1)–C(19)–C(18)
O(1)–C(1)–C(6)
N(2)–C(8)–N(4)
101.42(6)
100.92(6)
121.79(11)
122.94(14)
121.19(16)
125.30(15)
N(2)–C(8)–C(9)
N(4)–C(8)–C(9)
N(2)–N(1)–C(19)
N(1)–N(2)–C(20)
N(2)–C(20)–N(3)
N(4)–N(3)–C(20)
118.99(14)
115.71(14)
113.90(13)
111.08(12)
123.25(14)
127.12(14)
TABLE 3. Characteristics of Hydrogen Bonds in Crystals of Compounds 1а and 2
D–H…A
D–H, Å
H…A, Å
D…A, Å
∠DHA, deg
Compound 1а
1.89
O(1)–H(1)…N(1)
0.84
0.88
0.88
2.636(2)
2.666(2)
3.006(2)
147
102
134
N(4)–H(4A)…N(1)
2.34
i
N(4)–H(4A)…N(3)
2.32
Compound 2
2.80
N(3)–H(3B)…P(1)
N(3)–H(3B)…N(1)
0.88
0.88
3.663(2)
2.563(2)
169
104
2.20
i
*Crystallographic positions: –x, –y+1, –z.
correlation part [28]. The split-valence basis 6-311+G(d,p) was used as a basis set. The geometrical structure of the studied
molecules was optimized for all the natural variables without any symmetry restrictions. Potential energy surface minima
were identified for each structure by calculating the matrix of force constants and normal vibration frequencies. The
ChemCraft program [29] was used to prepare the data and presentation images and visualize the calculated results.
RESULTS AND DISCUSSION
The mobility of NH protons in type 1 and 2 hydrazones explains the possibility of existence of the two main
tautomeric forms: phthalazone (A) and hydrazone (B).
955

Fig. 1. Structure of hydrazone 1a (50% probability thermal vibration
ellipsoids).
Fig. 2. Structure of hydrazone 2 (50% probability thermal vibration
ellipsoids).
The fact that the NН proton signal in the PMR spectrum of hydrazones 1 and 2 is in a very weak field suggests that
the compounds exist in the phthalazone tautomeric form (A), which is also typical of other hydrazinophthalazine hydrazones
[11, 14-16, 19, 20]. This is also evidenced by the single crystal XRD data for salicylaldehyde 1′-phthalazinylhydrazone 1a
(Fig. 1) and 2-diphenylphosphinebenzaldehyde 1′-phthalazinylhydrazone 2 (Fig. 2).
Compound 1 crystallizes from DMSO in the hydrazone tautomeric form, which is evidenced by the lengths of the
C–C and C–N bonds in the hydrazone and heterocyclic fragments [30]. In the crystal this form is stabilized by two
intramolecular hydrogen bonds: the fairly strong O(1)–H(1)…N(1) bond, which closes the six-membered cycles, and the less
strong N(4)–H(4A)…N(1) bond (Table 4). The molecule as a whole is not planar; the mean square planes of the phthalazine
fragment (defined by the atoms N(3), N(4), and C(8)–C(15)) and benzene ring (the atoms С(1)–С(6)) form an angle of
15.27(8)° mostly due to the rotation around the =N–N= bond of the diazine chain (the dihedral angle С(7)=N(1)–N(2)=С(8)
is 172.43(15)°) and, to a less extent, around the С(8)=N(2) bond (the dihedral angle N(1)–N(2)–C(8)–С(4) is 4.9(2)°). The
mean square planes of the pyridazine and benzene cycles in the phthalazine fragment are slightly rotated relative to each
other by an angle of 3.11(8)°.
The crystal packing of compound 1а contains centrosymmetric hydrogen-bonded dimers, which are formed due to
i
the formation of the bond pair H(4A)…N(3) and H(4A) …N(3) (symmetry code (i): (i: –x, –y+1, –z) between the phthalazine
i
nitrogen atoms of neighboring hydrazone molecules (Table 4). These dimers are also observed to have one more pair of short
i
contacts of the CH…O type formed by the atoms H(15A)…O(1) and H(15) …O(1) (d = 2.57 Å, which is less than the sum of
i
the van der Waals radii by 0.15 Å). A number of π-stacking interactions are also observed between neighboring hydrazone
956

TABLE 4. Total Energy (au) and Relative Stability (ΔЕ, kcal/mol) of Tautomeric Forms of Substituted
Salicylaldehyde and 2-Diphenylphosphinebenzaldehyde 1′-Phthalazinylhydrazones in Vacuum
Tautomeric form
Compound
R
Phthalazone (A)
Hydrazone (B)
1a
1b
1c
1d
1e
2
H
5-OCH3
3,5-(t-Bu)2
5-Br
–873.2455254 (0.00 )
–987.7988255 (0.00)
–1187.8332897 (0.00)
–3446.7869700 (0.00)
–1077.8113094 (0.00)
–1602.1716878 (0.00)
–873.2278062 (11.12)
–987.7860817 (8.00)
–1187.8152992 (11.29)
–3446.7705614 (10.30)
–1077.7910442 (12.72)
–1602.1591011 (7.90)
5-NO2
—
molecules. First, they are observed between the phthalazine fragments with the molecule in the crystallographic position 1–x, –y,
–z, with the mutual orientation of the interacting molecules being such that the pyridazine cycle of one molecule is above the
benzene cycle of the other molecule to complementarily compensate for distortions in the phthalazine fragment. The distance
between the mean square planes of the cycles is 3.4766 Å for the benzene cycles and 3.4152(7) Å for the pyridazine cycles; the
distance between the centroids of the benzene and pyridazine cycles is 3.5647(13) Å. Second, a π-stacking interaction is
observed between the benzene rings of the salicyl aldiminate fragments with the molecule in the position 1–x, 1–y, 1–z. The
distances between the mean square planes of the benzene rings are 3.4632(8) Å; the intercentroid distances are 3.7575(15) Å.
Hydrazone 2 also exists in the crystal in the phthalazone tautomeric form. The hydrogen atom H(3B) forms weak
intramolecular hydrogen bonds with the atoms N(1) and P(1) (Table 4). In contrast to hydrazone 1a and other
phthalazinylhydrazones [5, 16, 19, 20], no intermolecular hydrogen bonds with the participation of the nitrogen atoms of the
phthalazine fragment are formed in the case of compound 2, which appears to be explained by steric hindrances created by
the benzene rings of the diphenylphosphine substitute.
The benzene ring of the benzaldehyde fragment and the phthalazine cycle in the molecule of 2 are almost coplanar;
the dihedral angle between their mean planes is 8.23(6)°. The mean planes of the carbon atoms C(1)–C(6) and C(7)–C(12) of
the phenyl groups form angles of 88.61(8)° and 72.27(8)°, respectively, with the benzene ring plane of the benzaldehyde
fragment; the dihedral angle between the mean planes of the phenyl groups is 87.96(8)°.
The intermolecular contacts established in the crystal lattice of hydrazone 2 include the fairly short hydrogen bonds
ii
ii
ii
of the type C–H…N that form the centrosymmetric dimers С(12)–H(12A)…N(2) and С(12) –H(12A) …N(2) (symmetry
code (ii): 1–x, 2–y, 1–z, dH…N = 2.52 Å, dC…N = 3.455(2) Å). The distance between the hydrogen and nitrogen atoms is less
than the sum of the van der Waals radii by 0.23 Å.
To evaluate the relative stability of the tautomeric forms of hydrazones 1 and 2 in more detail, a quantum chemical
calculation was made of the total energy and the electronic and spatial structure of the possible tautomers (the results are
shown in Table 4).
The molecular structure of the most stable rotamers of hydrazone 1a tautomeric forms is shown in Fig. 3.
According to the quantum chemical modeling results, the phthalazone tautomer A of hydrazone 1 is the most stable
in vacuum for any substitutes R. The hydrazone tautomer B is destabilized with respect to the phthalazone one by
approximately 8-13 kcal/mol. All the tautomers of type 1 compounds are observed to have a hydrogen bond between the
phenol proton and the azomethine nitrogen atoms. It should be noted that the existence of a quinone tautomeric form [14] for
salicylaldehyde hydrazones is virtually impossible; in the case of salicylaldehyde 1′-phthalazinylhydrazone, its energy in
vacuum would be greater than that of the phthalazone tautomer by 26 kcal/mol. The energies of the most stable possible
cyclic tautomers of compound 1 are greater by 13-14 kcal/mol than those of form 1a. The relative stability of the tautomers of
type 1 compounds is almost unchanged in aqueous and chloroform solutions: the phthalazone tautomers are the most stable,
and the hydrazone ones are destabilized by 6-8 kcal/mol in water and by 7-11 kcal/mol in chloroform.
957

Fig. 3. Molecular structure of the tautomeric forms of hydrazone 1a (R=H) in vacuum.
Fig. 4. Molecular structure of the tautomeric forms of hydrazone 2 in vacuum.
An almost similar pattern of relative tautomer stability is observed for hydrazone 2 (Table 4); the only difference is
that other conformations of the tautomeric forms are the most stable (Fig. 4), which is explained by the repulsion of the
undivided electron pairs of nitrogen and phosphorus atoms.
It should be noted that the total energy of conformer A2 whose geometry is very similar to the structure of the molecule
of 2 from the single-crystal XRD data, in vacuum is only 0.11 kcal/mol greater than that of the most stable form A1.
The relative stability of the studied forms of hydrazone 2 is the same in aqueous and chloroform solutions. The
relative energies of conformer A2 of the phthalazone form and the hydrazone tautomer are 1.84 kcal/mol and 5.98 kcal/mol
in water and 0.51 kcal/mol and 6.60 kcal/mol in chloroform.
Thus, both for 1′-phthalazinylhydrazones of substituted salicylaldehydes and for 1′-phthalazinylhydrazone of 2-
diphenylphosphinebenzaldehyde, the phthalazone tautomeric form is always more stable than the other tautomers.
It should be noted that the quantum chemical results for the spatial structure of the phthalazone tautomer of
hydrazone 1a are well consistent with those of the single-crystal XRD. The difference between the experimental and
calculated lengths of the C–C and C–N bonds is always (for the molecule in vacuum, water, and chloroform) no greater than
0.01 Å. However, the best agreement is observed between the molecule geometry from the XRD data and the geometry
calculated for the molecule in vacuum. The most substantial difference between the calculated geometry in vacuum and in
solution is observed for the dihedral angle between the mean planes of the benzene ring and phthalazine cycle: 16.4° in
vacuum, 26.9° in chloroform, and 33.4° in aqueous solution (15.3° from the XRD data).
In the case of hydrazone 2, the calculated geometry of conformation A2 of the phthalazone tautomer in vacuum is most
close to the geometry from the XRD data (Fig. 4). There is a good agreement both between the bond lengths and valence angles
and the dihedral angles between the cycles; in particular, the calculated dihedral angle between the mean planes of the benzene
ring of the benzaldehyde fragment and the phthalazine cycle in vacuum is 12.2° (8.2° from the XRD data).
Thus, at quantum chemical modeling of the tautomeric forms of substituted salicylaldehyde and 2-diphenyl-
phosphinebenzaldehyde 1′-phthalazinylhydrazones and the X-ray crystallographic study of salicylaldehyde and 2-diphenyl-
phosphinebenzaldehyde 1′-phthalazinylhydrazone were accomplished; the results are compared with the calculated data. The
hydrazonophthalazone tautomers are shown to be the most stable in all the cases.
958

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