Synthesis and structural study of 2′- and 2′,6′- positioned methyl- and nitro-substituted 3-(arylhydrazono)pentane-2,4-diones

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

J. Phys. Org. Chem. 2007; 20: 716–731

Published online 2 August 2007 in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/poc.1225

Synthesis and structural study of 2(- and

2(,6(-positioned methyl- and nitro-substituted

3-(arylhydrazono)pentane-2,4-diones

1

2

*

Jan Marten, Wilhelm Seichter, Edwin Weber and Uwe Bo¨ hme

¹Institut fu¨ r Organische Chemie, TU Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg/Sachsen, Germany

²Institut fu¨ r Anorganische Chemie, TU Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg/Sachsen, Germany

Received 5 February 2007; revised 2 April 2007; accepted 3 May 2007

ABSTRACT: A systematic series of ortho-methyl- and nitro-substituted arylhydrazones 2–6 formed by

Japp–Klingemann reaction between pentane-2,4-dione and the respective aryldiazonium salts have been synthesized

and studied by X-ray crystal structure analysis, with added quantum chemical calculations. The optimized molecular

geometries based on DFT calculations, enabling determination of relevant rotational barriers, and the calculated bond

and ring critical points, using the method of ‘atoms in molecules’, were found to correspond with the experimental

data, involving speciﬁc molecular conformations and hydrogen-bonded ring structure dependent on the ortho-

substitution, thus making possible reliable structural prediction of this compound class. Copyright # 2007 John Wiley

& Sons, Ltd.

Supplementary electronic material for this paper is available in Wiley InterScience at http://www.mrw.interscience.

wiley.com/suppmat/0894-3230/suppmat/

KEYWORDS: 3-(arylhydrazono)pentane-2,4-diones; synthesis; crystal structures; DFT calculations

INTRODUCTION

yet been proved to exist in respective arylhydrazones,

although implying a promising target for the design of

functional materials attributed to smart hydrogen bond-

ing¹¹or photo-triggered structural switching.¹²

Substitution in the ortho positions of the aryl ring

relative to the hydrazone function of 1 might be another

parameter of inﬂuence to control structural properties of

this compound class. Thus the series of arylhydrazones

2–6 (Scheme 1) featuring dimensionally similar but

electronically different methyl and nitro groups in a

systematic pattern of substitution have been synthesized

and structurally studied, including DFT calculations of

the geometric facts.

Coupling of a 1,3-dicarbonyl compound with an aromatic

diazonium salt to yield an arylhydrazone is a synthetic

process commonly known as the Japp–Klingemann

reaction.¹Using this method has given rise to a variety

of arylhydrazones that derive from different arylamines

and 1,3-dicarbonyl compounds,^1–3being described as

effective complexants for transition metal ions,^3–6or

antineoplastic components.⁷All of them show a

characteristic intramolecular hydrogen bridge linking

one of the carbonyl groups to the NH-moiety of the

hydrazone unit. This has recently been corroborated by

X-ray crystal studies.^3,8It has also been observed that in

hydrazones of this type based on an unsymmetrically

substituted 1,3-dione, the six-membered H-bonded ring

will always be generated at the steric most favorable side

of the molecule.⁹Moreover, as shown from the crystal

structures,^3,8the carbonyl groups are in a conformational

anti position. On the other hand, tautomerism, which is a

typical feature of 1,3-dicarbonyl compounds,¹⁰has not

RESULTS AND DISCUSSION

Synthesis

The hydrazones 1–6 (Scheme 1) were synthesized via

Japp–Klingemann reaction¹between the respective

aromatic diazonium salts and pentane-2,4-dione in a

methanolic solution containing sodium acetate.³The

diazonium salts were obtained by usual diazotation of the

corresponding aniline.¹³

*Correspondence to: E. Weber, Institut fu¨r Organische Chemie, TU

Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg/Sach-

sen, Germany.

E-mail: edwin.weber@chemie.tu-freiberg.de

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

717

O

electronic properties of the aryl substituents inﬂuence

bonding parameters and thus p-electron delocalization

within the conjugated system and to what extent the

molecular conformation is affected by non-covalent

intermolecular interactions, hence supplying potential

reasoning for clearing up this point in dispute.

O

N

NH

Molecular structures. All molecules adopt the EZE-

conformation which represents, in this order, the

2

1

R

—

alignment of the non-H-bridged O(2) C(4), the hydro-

—

gen-bonded O(1) C(2) fragment, and the aryl group

—

17

—

with respect to the C(3) N(1) double bond. Previous

—

studies revealed that this conformation is also preferred in

the solid phase structures of other compounds of this

kind.^8,2,18The H-bridged hydrazone ring and the aromatic

ring in the compounds deviate more or less from

coplanarity. The dihedral angles between the

least-squares planes of these molecular units

(Scheme 2) range between 2.5(1) and 17.2(2)8, being

largest for the 2,6-dinitroarylhydrazone, 5 for reasons

which will be discussed below.

1

2

3

4

5

6

R¹= H ,

R²= H

R¹= CH₃, R²= H

R¹= CH₃, R²= CH₃

R¹= NO₂, R²= H

R¹= NO₂, R²= NO₂

R¹= CH₃, R²= NO₂

Scheme 1. Compounds discussed

Table 2 reﬂects the general relationship between bond

lengths within the C(3)-N(1)-N(2)-C(6) sequence of the

compounds showing that a relative increase of the

N(1)—N(2) bond length corresponds with a decrease of

the respective N(2)—C(6) and N(1)—C(3) bond lengths.

Furthermore, the trend of the N(2)ꢀꢀꢀO(1) distances

indicates that electron-donating ortho-substituents of

the aromatic ring tend to strengthen the intramolecular

hydrogen bond whereas it is weakened by electron-

withdrawing groups. Another interesting feature concerns

the bond angle at C(3) which includes a signiﬁcant

decrease of the angle N(1)–C(3)–C(4) [112.2(3)–

114.0(1)8] accompanied by an increase of the angle

N(1)–C(3)–C(2) [122.5(2)–124.9(1)8]. It is evident from

Tables 2 and 3 that these variations depend on the

electronic character of the substituents and correlate with

hydrogen bond parameters.

Compound 1 (Fig. 1a) exhibits a nearly planar overall

conformation with bond distances and angles which are

similar to those found in other non-functionalized

b-diketoarylhydrazones.¹⁸Surprisingly, introduction of

methyl groups in the ortho position of the aromatic ring,

as in 2 and 3, have only little inﬂuences on the molecular

conformation and bond parameters. Also, the presence of

the electron-withdrawing nitro group in 4 hardly affects

the distance N(2)ꢀꢀꢀO(1), being very similar to that of 1,

although in 4 the hydrogen at N(2) is involved in a

bifurcated intramolecular hydrogen bond with the nitro

X-ray structural study

Relating to the problem, crystal structures of six

3-(arylhydrazono)pentane-2,4-diones containing ortho-

substituents of different electronic nature have been

studied. They involve the unsubstituted parent compound

1, its 2-methyl and 2,6-dimethyl-substituted analogs 2

and 3, the 2-nitro and 2,6-dinitro derivatives 4 and 5 as

well as the 2-nitro-6-methylarylhydrazone 6. Relevant

crystallographic data are summarized in Table 1. A

selection of bond distances and angles including

hydrogen bond parameters is given in Tables 2 and 3,

respectively. ORTEP plots of the molecules 1, 5, and 6 are

displayed in Fig. 1. The packing diagrams of compounds

1 and 6 are illustrated in Figs 2 and 3, respectively.

Packing diagrams of 2–5 are given in Figs SI–SIV,

respectively.

A common feature of the compounds is the presence of

an intramolecular six-membered p-conjugated hydro-

gen-bonded ring containing an extraordinary strong

N—HꢀꢀꢀO hydrogen bond with NꢀꢀꢀO bond distances

˚

ranging over 2.542(3) and 2.597(2) A. Investigations

based on the Resonance Assisted Hydrogen Bond

(RAHB) model, which were applied to related com-

pounds by Gilli et al.,¹⁴suggests a synergistic mutual

reinforcement of intramolecular hydrogen bonding and

p-delocalization within structure elements like the

˚

group [(N(2)ꢀꢀꢀO(3), 2.617(7) A], forcing the substituent

—

H—N—N C—C O system. This model, however,

into coplanarity with the aromatic ring. Interestingly, in

the monosubstituted compounds 2 and 4 with potential

syn or anti position of the substituent (CH₃, NO₂) relative

to the nitrogen atom N(1), only the anti conformation is

shown. This is reasonable for compound 4 due to the nitro

group participating in a bifurcated hydrogen bond, unlike

the methyl substituent in compound 2 being unable in this

—

is criticized recently.¹⁵On the other hand, calculations

have shown that the p-electron delocalization in a related

system correlates well with parameters describing the

H-bond strength such as the electron density at the HꢀꢀꢀO

bond critical point.¹⁶Characterization of the present

crystal structures therefore should be focused on how

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

718

J. MARTEN ET AL.

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

Table 2. Selected conformational parameters of the compounds 1–6

719

Compound

1

2

3

4

5

6

˚

Bond lengths (A)

N(1)—N(2)

N(1)—C(3)

N(2)—C(6)

C(2)—C(3)

C(3)—C(4)

C(2)—O(1)

C(4)—O(2)

1.301(4)

1.321(4)

1.414(4)

1.469(5)

1.476(5)

1.234(4)

1.219(4)

1.297(1)

1.321(1)

1.411(1)

1.470(1)

1.473(1)

1.230(1)

1.207(1)

1.284(3)

1.333(3)

1.418(3)

1.488(4)

1.465(4)

1.234(3)

1.215(3)

1.314(1)

1.397(1)

1.484(1)

1.487(1)

1.229(1)

1.211(1)

1.333(2)

1.307(2)

1.386(2)

1.495(2)

1.498(2)

1.226(2)

1.216(2)

1.312(3)

1.317(4)

1.389(3)

1.478(4)

1.485(4)

1.226(4)

1.212(4)

Bond angles (8)

C(3)—N(1)—N(2)

N(1)—N(2)—C(6)

N(1)—C(3)—C(2)

N(1)—C(3)—C(4)

C(2)—C(3)—C(4)

C(3)—C(2)—O(1)

C(3)—C(4)—O(2)

Torsion angles (8)

O(1)–C(2)–C(3)–N(1)

C(2)–C(3)–N(1)–N(2)

C(3)–N(1)–N(2)–C(6)

C(2)–C(3)–C(4)–O(2)

Dihedral angle (8)

mpla(1)^aꢀꢀꢀmpla(2)^b

mpla(1)^aꢀꢀꢀmpla(3)^c

mpla(1)^aꢀꢀꢀmpla(4)^d

122.4(3)

119.4(3)

123.7(3)

112.2(3)

124.1(3)

119.1(3)

121.2(3)

121.31(9)

120.77(9)

123.41(10)

112.92(9)

123.65(9)

119.80(10)

122.65(10)

121.5(2)

124.7(2)

122.5(2)

113.9(2)

123.6(2)

119.3(2)

121.7(3)

121.52(6)

118.02(6)

124.21(7

112.30(7)

123.50(7)

118.91(7)

121.66(10)

120.01(14)

118.82(13)

124.86(14)

113.96(14)

121.16(14)

119.17(15)

119.68(15)

120.1(2)

122.5(2)

123.5(3)

113.9(3)

122.6(3)

119.7(3)

121.7(3)

ꢁ5.5(5)

3.6(5)

ꢁ5.21(17)

1.87(16)

ꢁ0.9(4)

ꢁ0.1(4)

ꢁ179.7(2)

1.8(5)

0.22(13)

ꢁ0.99(12)

179.63(6)

5.06(15)

14.1(3)

ꢁ6.0(3)

ꢁ7.5(5)

1.8(4)

ꢁ178.8(3)

ꢁ177.37(9)

177.30(15)

26.6(3)

ꢁ178.4(2)

ꢁ9.5(5)

ꢁ6.30(18)

ꢁ12.8(5)

3.53(0.88)

5.1(0.27)

3.35(0.57)

2.47(0.10)

3.02(0.14)

—

17.72(0.18)

67.04(0.08)

8.70(0.09)

7.13(0.18)

74.42(0.19)

—

^aThe mpla(1) means the mean plane given by C(6)-C(7)-C(8)-C(9)-C(10)-C(11).

^bmpla(2) means the mean plane given by the sequence H(2)-N(2)-N(1)-C(3)-C(2)-O(1).

^cmpla(3) means the mean plane given by N(3)-O(3)-O(4) in 4, 5, 6.

^dmpla(4) means the mean plane given by N(4)-O(5)-O(6) in 5.

respect. Possibly, also a steric interference between the

ortho-substituent and the N(1) atom in the syn position is

a barrier to form this conformer. Additional indication to

this fact is that in compounds 3, 5, and 6 having an

ortho-disubstituted aromatic ring, substituent-induced

intramolecular strain causes a marked decrease of the

bond angle N(2)–C(6)–C(11) whereas the N(2)–C(6)–

C(7) angle is increased. This effect is most distinctive in

the dimethyl derivative 3, in which these angles are 113.3

and 124.48, respectively. In the 2,6-disubstituted com-

pounds 5 and 6, the nitro group at C(7) is inclined with

regard to the aromatic plane resulting in reduction of

intramolecular strain to give more regular bonding angles

at C(6).

In the crystal structure of 5 (Fig. 1b), the molecule

exhibits a highly bent geometry along the C₅O₂fragment

which can be seen from enlarged torsion angles of the

atomic sequences N(1)-C(3)-C(2)-O(1) and C(2)-C(3)-

C(4)-O(2) being 14.1(3) and 26.6(3)8, respectively. The

H-bridged hydrazone ring, however, is less affected by

this distortion. The largest atomic distance from its mean

plane is found for atoms N(2) and C(3) being only 0.07(1)

involved in hydrogen bonding, is rotated 67.0(1)8 out of

the aromatic plane.

Unlike 4 and 5, the nitro substituent of compound 6

(Fig. 1c) is arranged syn with respect to N(1) and

therefore excluded from intramolecular hydrogen bond

interaction. Hence 6 corresponds in the parameters of

hydrogen bonding with compound 3. Moreover, a distinct

twist [74.4(2)8] of the nitro group of 6 out of the aromatic

ring plane prevents from mutual interaction of the

aromatic and nitro p-electrons.

Packing structures. Due to the planar geometry, the

crystal structures of all studied compounds, excepting

compound 6, are characterized by a sheet-like alignment

of molecules of which Fig. 2 is illustrating a representa-

tive example. As the molecules lack strong outwardly H

donor sites, intermolecular cross-linking is restricted to

C—HꢀꢀꢀO contacts and pꢀꢀꢀp stacking interactions which,

however, give rise to speciﬁc types of supramolecular

pattern.

In the crystal structure of 1 (Fig. 2), the molecules are

linked together to double strands running along the crys-

tallographic b-axis. Both carbonyl oxygens are involved

in hydrogen bonding to the aromatic parts of neighboring

˚

and 0.06(1) A apart. Similar to the monosubstituted

compound 4, one of the nitro groups in 5 is involved in an

intramolecular bifurcated hydrogen bond [(N(2)ꢀꢀꢀO(1),

˚

2.597(2) A] and thus is nearly coplanar with the aromatic

˚

molecules [O(1)ꢀꢀꢀH(11), 2.55 A; O(2)ꢀꢀꢀH(9), 2.63 A].

˚

Offset arene–arene p-contacts with a separation of 4.22 A

ring [Q ¼ 8.7(1)8], while the other nitro group, not being

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

720

J. MARTEN ET AL.

˚

Table 3. Distances (A) and angles (deg.) of hydrogen bond interactions of 1–6

Atoms involved

Distances

Angle

D–HꢀꢀꢀA

(1)

Symmetry

DꢀꢀꢀH

DꢀꢀꢀA

HꢀꢀꢀA

D–HꢀꢀꢀA

N(2)–H(2)ꢀꢀꢀO(1)

x, y, z

ꢁ1 þ x, ꢁ1 þ y, z

0.91

0.96

0.93

2.580(4)

3.582(5)

3.461(5)

1.85

2.63

2.55

136.4

171.5

167.3

C(9)–H(9)ꢀꢀꢀO(2)

C(11)–H(11)ꢀꢀꢀO(1)

ꢁx, 1 ꢁ y, ꢁz

(2)

N(2)–H(2)ꢀꢀꢀO(1)

x, y, z

1 þ x, 1 þ y, ꢁ1 þ z

0.94

0.93

2.562(1)

3.499(2)

1.82

2.58

134.3

168.1

C(9)–H(9)ꢀꢀꢀO(2)

(3)

N(2)–H(2)ꢀꢀꢀO(1)

x, y, z

0.5 þ x, 0.5 ꢁ y, ꢁ0.5 þ z

0.86

0.93

2.538(9)

3.384(5)

1.85

2.48

136.8

163.9

C(10)–H(10)ꢀꢀꢀO(2)

(4)

N(2)–H(2)ꢀꢀꢀO(1)

N(2)–H(2)ꢀꢀꢀO(3)

C(10)–H(10)ꢀꢀꢀO(2)

C(8)–H(8)ꢀꢀꢀO(3)

x, y, z

0.79

0.93

2.578(1)

2.617(1)

3.269(1)

3.344(1)

2.02

2.01

2.64

2.56

127.6

132.7

125.3

141.8

x, y, ꢁ1 þ z

x, ꢁ1 þ y, z

(5)

N(2)–H(2)ꢀꢀꢀO(1)

N(2)–H(2)ꢀꢀꢀO(5)

C(1)–H(1B)ꢀꢀꢀO(2)

C(1)–H(1A)ꢀꢀꢀO(4)

C(5)–H(5A)ꢀꢀꢀO(3)

C(1)–H(1A)ꢀꢀꢀO(6)

C(5)–H(5C)ꢀꢀꢀO(6)

x, y, z

0.79

0.98

2.597(2)

2.605(2)

3.411(2)

3.123(2)

3.578(2)

3.323(2)

3.423(2)

2.01

2.57

2.64

2.63

2.53

2.46

130.9

130.4

143.2

139.3

163.1

138.2

167.6

x, 1 ꢁ y, 0.5 þ z

1 þ x, y, z

x, 1 ꢁ y, ꢁ0.5 þ z

0.5 þ x, 0.5 ꢁ y, 0.5 þ z

ꢁ0.5 þ x, 0.5 ꢁ y, ꢁ0.5 þ z

(6)

N(2)–H(2)ꢀꢀꢀO(1)

x, y, z

0.5 þ x, 0.5 ꢁ y, 0.5 þ z

0.86

0.96

2.542(3)

3.442(5)

1.86

2.64

135.3

140.9

C(1)–H(1A)ꢀꢀꢀO(3)

(centroid-to-centroid) stabilize the crystal packing in

direction of the a-axis.

replace aromatic rings in this speciﬁc pꢀꢀꢀp stacking

arrangement. By way of contrast, in the crystal structure

of 3, only the H-bonded six-membered rings are subjected

to stacking interactions, possibly because of the sterical

barrier of the ortho-substituents, which prevents effective

face-to-face aromatic interactions.

In the compounds 4–6, the nitro substituents provide

additional coordination sites which allow a higher degree

of intermolecular cross-linking. In the crystal structure of

the monosubstituted derivative 4 (Fig. SIII), only one of

the nitro oxygens takes part in an intramolecular

N—HꢀꢀꢀO and a weaker C—HꢀꢀꢀO contact to an adjacent

A similar layered arrangement of molecules is also

found in the crystal structures of the methyl derivatives 2

(Fig. SI) and 3 (Fig. SII). The presence of methyl groups,

however, restricts intermolecular hydrogen bonding to the

oxygen O(2) forming either linear (2) or zigzag (3)

C—HꢀꢀꢀO bonded single strands. In the structure of 2, the

aromatic rings and the H-bonded six-membered rings are

arranged in a stacking mode, which suggests attractive

forces between them. Obviously, non-aromatic molecular

entities featuring a conjugated p-electron system can

Figure 1. ORTEP plots of the molecular structures of compounds 1 (a), 5 (b), and 6 (c). The thermal ellipsoids are drawn at the

50% probability level and the hydrogen bonds are shown as broken lines

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

721

Figure 2. Packing diagram of 1 viewed down the crystallographic a-axis. The hydrogen bonds are shown as broken lines

molecule. Attachment of an additional nitro substituent,

as in the 2,6-dinitro-substituted compound 5 (Fig. SIV)

drastically changes the molecular assembly in the crystal.

With the exception of the nitro oxygen O(5), which is

involved in intramolecular hydrogen bonding, all other

strong acceptors are involved in intermolecular cross-

linking, thus stabilizing the crystal structure by a close

network of non-covalent interactions. Different from the

crystal structures described above, the 2-nitro-6-methyl-

substituted derivative 6 shows a typically columnar

packing mode with the molecules of the neighboring

stack inclined to one another (Fig. 3).

compounds under investigation. Optimized geometries

are denoted with formula number and following letters (a

or b) in order to distinguish calculated geometries from

experimental data.

Parent compound 1 and methyl-substituted

derivatives 2 and 3. A comparison of the optimized

geometry of 2a with the X-ray structure of 2 shows very

good agreement of bonding parameters (Fig. 4a). There

are only small deviations in the overall geometry of the

molecule with respect to the angle between the planes 1

and 2 (Scheme 2) formed by the hydrazone unit (plane 1)

and the phenyl ring including ortho-substituents (plane

2). The optimized molecule 2a is completely planar while

the molecule 2 in the solid state shows an angle of

3.538(0.88) between both planes. This difference is more

pronounced in the structure overlay of 3a and 3. The angle

between the planes of 3 in the solid state is 3.358(0.57).

DFT calculations

Quantum chemical calculations have been performed to

obtain a better insight into the structural features of the

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

722

J. MARTEN ET AL.

Figure 3. Packing diagram of 6. Hydrogen bonds are shown as broken single lines; arene–arene interactions are marked as

broken double lines; interactions between aromatic rings and H-bridged hydrazone rings are displayed as dotted lines

On the other hand, the optimized geometry of the

molecule in the gas phase (3a) has an angle of 21.148

between planes 1 and 2. This non-planar geometry of the

unhindered molecule in the gas phase suggests a repulsive

steric interaction between the methyl groups in ortho

positions of the phenyl ring and the hydrazone unit.

The quantum chemical investigation of rotational

barriers is a suitable tool to investigate such kind of

steric interactions more closely.¹⁹For that purpose,

relaxed potential energy surface (PES) scans have been

performed with rotation of the phenyl group around the

bond N(2)—C(6), being schematically shown for 1a, 2a,

and 3a in Fig. 5. We can assume that all steric and

electronic interactions between the phenyl groups and the

hydrazone unit are switched off at a torsion angle of 908.

Therefore the torsional potentials of 2a and 3a were

scaled in that way that all three functions intersect at 908.

It is possible to draw information about the different steric

interactions of 1a, 2a, and 3a by comparing these graphs.

O

N

H

N

Plane 1

R₂

R₁

Figure 4. Structure overlay of the optimized geometries of

2a (a) and 3a (b) with the geometries of 2 and 3 from X-ray

structures, respectively. The aromatic carbon atoms of the

arene unit were positioned on top of each other

Plane 2

Scheme 2. Representation of planes discussed in the text

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

723

(a)

30

25

20

15

10

E (1a)

E(2a)+1.8

5

0

∆

E₂E₃

E(3a)+19.21

∆E₁

0

50

100

150

angle (°)

(b)

O

H

O

H

N

H

N

CH

3

H C

3

CH

3

Figure 5. Schematic representation (a) of PES scans of 1a, 2a, and 3a for rotation of the aryl ring around N(2)–C(6). Illustration

(b) of the rotational conformers of 2a

The torsional potential for 1a gives information about

the conjugative interaction between the p-system of the

phenyl ring and the hydrazone unit. This torsional

potential has a maximum at 908, with 25.4 kJ/mol higher

energy than the molecule in its minimum geometry with a

torsion angle of 08. The substitution of one ortho position

at the phenyl ring of 1 by a methyl group, as in 2, leads to

torsion potential with two different minima [E(2a) in

Fig. 5]. The global minimum of this molecule is at a

torsion angle C(7)–C(6)–N(2)–N(1) of 08. This global

minimum is 1.8 kJ/mol higher in energy than the global

minimum for 1a (~E₁in Fig. 5) thus indicating that there

is only a small steric repulsion. The maximum is also at

908 due to the complete inhibition of conjugative

interaction. Going further to 1808 leads to a local

minimum which is 14.3 kJ/mol higher in energy than the

global minimum. This can be explained with the repulsive

interaction between the methyl group in ortho position

and the free electron pair at N(1). The steric repulsion

between the methyl group and the nitrogen atom N(1) can

be estimated with 16.1 kJ/mol (~E₂in Fig. 5). The

rotational barrier is changed, if there are two methyl

groups in both ortho positions of the phenyl ring. The

global minimum is no longer at a torsion angle of 08, but

at a torsion angle of about 358. This conformation is

obviously the best compromise between minimal steric

repulsion and optimal conjugative interaction. The

overall rotational barrier of 3a is much smaller than

for the molecules discussed before. This can be attributed

to the steric repulsion between the methyl groups in both

ortho positions with the hydrazone unit which disfavors

torsion angles close to 0 or 1808. The steric repulsion

between two methyl groups in ortho position and the

hydrazone unit can be estimated with 21 kJ/mol (~E₃in

Fig. 5). The respective torsion angle for 3 is much smaller

in the solid state [3.4(0.6)8 between the mean plane given

by C(6)-C(7)-C(8)-C(9)-C(10)-C(11) and the mean plane

given by the sequence H(2)-N(2)-N(1)-C(3)-C(2)-O(1)],

suggesting a consequence of packing effects. However,

one should bear in mind that these values were calculated

for an isolated molecule in the gas phase and depend also

on the method and basis set combination. Therefore, these

values should be regarded as a qualitative description of

the rotational barrier and the electronic and steric effects

connected with this rotation.

Nitro-substituted compounds 4 and 5. The struc-

ture overlay of the optimized geometry of 4a with the

geometry of 4 from X-ray structure analysis shows a

nearly perfect ﬁt (Fig. 6a), which is a remarkable case

where gas phase and solid state molecular geometry

closely correspond. The molecule is completely planar

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

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724

J. MARTEN ET AL.

level in a previous work.²¹Further rotation results in a

continuing increase of energy since the methyl group at

C(1) comes into spatial conﬂict with H(2).

The PES scan of 5a was scaled to have the same

maximum energy as the PES scan of 4a (Fig. 7). The

rotation of the phenyl ring around the bond N(2)—C(6) of

5a gives a symmetric PES with two minima at 30 and

1508. These minima represent the best compromise

between (a) steric repulsion between the nitro group and

the free electron pair at N(1), (b) the attractive interaction

between one oxygen atom of the nitro group and H(2),

and (c) the maximized conjugative interaction between

phenyl ring and the hydrazono group. The maximum of

this PES scan is at 908 with 26.2 kJ/mol higher energy

than the minima. All three interactions mentioned above

are switched off at 908. Only the nitro groups attain

maximal conjugative interaction with the phenyl ring at

this torsion angle.

A comparison of the PES scans of 4a and 5a allows to

draw some interesting conclusions. The energy difference

~E₄represents the destabilizing effect between the nitro

group and the free electron pair at N(1) and can be

estimated to 6.5 kJ/mol in these compounds. The energy

difference ~E₅can be interpreted as the stabilizing effect

between one oxygen atom of the nitro group and H(2)

with a value of 10.1 kJ/mol.

Figure 6. Structure overlay of the optimized geometries of

4a (a) and 5a (b) with the geometries of 4 and 5 from X-ray

structures, respectively. The aromatic carbon atoms were

positioned on top of each other

In order to obtain more information about the bonding

situation in 3-(arylhydrazono)pentane-2,4-diones, includ-

ing possible hydrogen bonds, another method of quantum

chemical analysis has been used. Compound 4a which

shows a remarkably close ﬁt to the solid state structure

was chosen as a representative example. For that purpose,

the Atoms in Molecules (AIM) Theory can be utilized.²²

This method partitions the electron density of a molecule

into individual non-overlapping atomic fragments by

rigorously deﬁned interatomic surfaces.²³Various proper-

ties can be derived from the electron density distribution.

The electron density in a molecule is a three-dimensional

function which has four types of extrema: maxima,

minima, and two types of saddle points.²⁴The maxima

coincide with the nuclear positions. Minima are found in

the center of cage molecules and are called cage critical

points. Saddle points being speciﬁc to a maximum in one

dimension and a minimum in two dimensions are called

ring critical points. These are found in the center of

atomic rings. Saddle points resulting from a maximum in

two dimensions and a minimum in one dimension are

called bond critical points. When a bond critical point is

found between two atoms, we can draw an atomic

interaction line. When the molecule is in an equilibrium

geometry, the atomic interaction line is called a bond

path. A bond path, however, is not identical with a bond in

the sense used by Lewis. Bond paths of this particular

type are observed for predominately ionic bonds, covalent

bonds, weak hydrogen bonds, and even interactions

between anions. Unfortunately, the topological analysis

of electron density cannot discriminate between attractive

allowing maximal conjugation of the p-system. Further-

more, a bifurcated hydrogen bond from N(2)—H(2) to

O(1) and O(3) is indicated on the basis of the structural

data (cf. Fig. SIII). To obtain some information about

these stabilizing effects, three different torsion potentials

have been investigated: (a) the rotation of the phenyl ring

around the bond N(2)—C(6); (b) the rotation of the nitro

group around the bond C(11)—N(3); (c) the rotation of

the carbonyl group at C(2)—O(1) around C(2)—C(3).

The rotational barrier of the phenyl ring around the

bond N(2)—C(6) of 4a gives a PES (Fig. 7) which is

similar to the rotation of the phenyl ring in 2a. There is a

global minimum at 08 and a local minimum at 1558. The

maximum is at 958, which is 32.9 kJ/mol higher in energy

compared to the global minimum. Rotation of the nitro

group out of the planar conformation not only reduces

p-conjugation with the phenyl ring but also breaks the

hydrogen bond. This barrier amounts to 23.6 kJ/mol for

4a, a value which is very similar to the barrier of

nitrobenzene (24.2 kJ/mol).²⁰Hence, from this data there

is no remarkable effect which could be ascribed to the

presence of a hydrogen bond between N(3)—O(3) and

H(2)—N(2). Rotation of the carbonyl group by varying

the torsion angle O(1)–C(2)–C(3)–N(1) impairs both the

p-conjugation of the 3-(hydrazono)pentane-2,4-dione

unit and the potential hydrogen bond between O(1)

and N(2)—H. The total energy of the molecule increases

about 22.6 kJ/mol when this torsion angle is at 908. The

intramolecular H-bond energy for ketohydrazones was

estimated with 29.4 kJ/mol at the B3LYP/6-31 þ G(d,p)

Copyright # 2007 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2007; 20: 716–731

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SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

725

35

(a)

30

25

20

15

10

5

∆E₅

∆E₄

4a

5a+6.5

0

50

100

150

angle (°)

(b)

O

H

O

H

O

H

N

O

N

O

N

O

N

O

(c)

O

H

O

H

N

O

N

O

N

O

N

O

Figure 7. Schematic representation (a) of PES scans of 4a and 5a for the rotation of the aryl ring around N(2)–C(6). Illustration

of the conformational rotation of 4a (b) and 5a (c), with attractive interactions symbolized by a broken line, repulsive interactions

by a zigzag line

and repulsive interactions but tells us that there are

interactions between the atoms, discussed in more detail

in recent literature.^25–28In order to avoid improper

conclusions from the topological analysis of electron

density, the calculated torsional potentials from above are

supportive, allowing us to distinguish unambiguously

between attractive and repulsive interactions.

Figure 8a shows the electron density distribution of 4a

in the plane of the atoms. The AIM analysis allows to

derive a molecular graph which is shown in Fig. 8b. The

atoms are connected by atomic interaction lines going

through the bond critical points. These lines correspond

widely to our association of covalent bonds in this organic

molecule. There are distinct properties of the electron

density at the bond critical points that provide us with

useful information about the nature of the bond.²⁴The

ﬁrst property is the value of the electron density, which is

given in atomic units (e/Bohr³) in Table 4. It is possible to

draw conclusions about the bonding degree (single/

double/triple bond) by comparing this value with suitable

reference molecules. The second value is the ellipticity,

which represents the shape of the electron density

distribution in a plane perpendicular to the bond path at

the bond critical point. While double bonds show

signiﬁcant bond ellipticity, single and triple bonds do not.

The bond critical point between C(2) and O(1) of 4a

has electron density of 0.40 e/Bohr³and a very small bond

ellipticity between. These values are very similar to the

Copyright # 2007 John Wiley & Sons, Ltd.

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726

J. MARTEN ET AL.

Figure 8. Electron density distribution (a) and representation of bond and ring critical points (b) of 4a

—

molecules hydrazine and HN NH, that is, a partial

—

values of formaldehyde. The low ellipticity of the C

O

double bond can be explained with the presence of free

electron pairs at the oxygen atom in the plane

perpendicular to the p-orbitals of the double bond. The

values of the electron density of the C—N-bonds

[C(3)—N(1) 0.348 and N(2)—C(6) 0.302 e/Bohr³] are

double bond character is shown. Three bond critical

points between H(2) and its neighboring atoms were

found (Table 4). The bond between N(2) and H(2) can be

regarded as a typical single covalent bond. There are also

bond critical points between H2 and O1 and O3,

respectively. The value of electron density at the bond

critical point for these interactions is one order of

magnitude smaller than for N(2)—H(2). Hence, on the

basis of the topological analysis we can assume the

presence of hydrogen bonds between these atoms. Similar

—

between the values of methylamine (0.267) and H C

2

NH (0.389), meaning that both bonds of 4a have partial

double bond character; a statement which is supported by

the ellipticity. The same is true for the bond N(1)—N(2),

when making use of a comparison with the reference

Table 4. Electron density and bond ellipticity at selected bond critical points in 4a and reference molecules

Molecule

Bond critical point

Electron density (e/Bohr³)

Ellipticity

4a

C(2)–O(1)

C(3)–N(1)

N(2)–C(6)

N(1)–N(2)

0.40

0.074

0.274

0.136

0.105

0.348

0.302

0.388

N(2)–H(2)

H(2)ꢀꢀꢀO(1)

H(2)ꢀꢀꢀO(3)

C–C

0.322

0.036

0.029

0.243

0.348

0.402

0.410

0.267

0.389

0.273

0.476

0.036

0.055

0.071

0

0.388

0

0.072

0.041

0.237

0.037

0.17

Ethane

Ethene

Acetylene

Formaldehyde

Methylamine

—

C

O

—

C–N

—

N

—

H C NH

C

—

Hydrazine

2

N–N

—

HN NH

—

N

—

N

Copyright # 2007 John Wiley & Sons, Ltd.

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SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

727

values for hydrogen bonds have been found in other

molecules.^29,30A weak interaction is also suggested

between O(2) and C(1) based on the presence of a ring

critical point (Fig. 8b).

been synthesized and studied by X-ray crystal structure

analysis, with added quantum chemical calculations.

Due to the good agreement between the optimized

geometries based on DFT calculations and the exper-

imental data from crystal structure analysis, relevant

rotational barriers could be determined. These give an

obvious indication that repulsive forces between an

ortho-positioned methyl group and the nitrogen-free

electron pair are stronger than for a nitro group and the

electron pair in this type of molecules, corresponding to

the structural ﬁndings. Moreover, good correspondence

was found between the bond and ring critical points,

calculated by the method of ‘atoms in molecules’ and the

experimental data, conﬁrming the conjugate character of

the six-membered hydrogen-bonded ring of the hydra-

zone unit. This is a result not conﬂicting with the IRAHB

model based on p-electron delocalization.¹⁶Thus, it is

shown that DFT calculations carried out on a series of

arylhydrazones, formed of pentane-2,4-dione and ortho-

methyl- and nitro-substituted diazonium salts, are a useful

way to make reliable prediction of their molecular

conformations. This might be useful in the design of

functional materials attributed to smart hydrogen bond-

ing,¹¹photo-triggered structural switching,¹²and targeted

metal ion complexes.³¹

Mixed methyl- and nitro-substituted derivative

6. Two rotamers of 6 are imaginable, depicted as 6a and

6b in Fig. 9. The experimentally determined structure in

the solid state is similar to the optimized structure 6a. At

the B3LYP/6-311 þ G(d,p) level of theory, the optimized

geometry 6a is more stable than 6b. The energy difference

between both rotamers is 4.5 kJ/mol in electronic energy

(including zero point correction) and 3.1 kJ/mol in Gibbs

free energy. Different effects of steric repulsion are

cumulated in this molecule between the substituents in

ortho position (NO₂and CH₃) and the hydrazono group.

Furthermore, the nitro group is in conjugative interaction

with the phenyl ring. The minima at 50 and 1308 in the

energy proﬁle for the rotation of the nitro group in 6a

(Fig. 10) represent therefore a compromise between

minimized steric repulsion and retained conjugative

interaction between the nitro group and the phenyl ring.

The nitro group and the nitrogen atom N(1) are not able to

form any stabilizing orbital interaction. This is shown

exemplarily in Fig. 11 with HOMO-2 [free electron pair at

N(2)] and HOMO-3 [electron pair at O(3) and O(4) plus

phenyl p-orbital].

EXPERIMENTAL

Synthesis

CONCLUSIONS

Melting points were determined with a hot-stage

microscope (VEB Dresden Analytik) and are uncor-

rected. Elemental analyses were determined on a Heraeus

CHN rapid analyzer. The IR spectra were obtained with a

Nicolet 510 FT-IR spectrometer. ¹H and ¹³C-NMR

spectra were recorded using a Bruker Avance DPX 400

(400 MHz) instrument. The chemical shifts (d) are

reported as ppm relative to SiMe₄. Mass spectra were

recorded with a DEI-MS (Finnigan SSQ 710), EI-MS

(Bruker Daltonik, ESQUIRE-LC ion trap, solvent:

acetonitrile/water/5% formic acid; ﬂow rate 3 ml/min;

ion polarity: positive), or GC-MS (Finnigan MAT 8230,

source mode: chemical ionization with isobutane, ion

polarity: positive).

As the starting point of this investigation, an ortho-

mononitro (4) and ortho-methyl-and nitro-disubstituted

arylhydrazone (6) were found in unexpectedly different

conformations regarding the arylhydrazone bond. Trying

to solve this strange behavior, a systematic series of

respective ortho-mono- and disubstituted arylhydrazones

including the unsubstituted parent compound (1–6) have

Compounds 1–6: General procedure

A solution of the diazonium salt was prepared under

cooling (0–5 8C) from the respective aniline (20 mmol) in

hydrochloric acid (3 N, 40 ml; in the case of compounds

1–4 and 6) or conc. sulfuric acid (20 ml; for compound 5)

and a conc. aqueous solution of sodium nitrite (1.37 g,

20 mmol) according to the standard procedure.¹³The cold

solution of the diazonium salt was added under cooling

(0 8C) and stirring to a mixture being composed of

Figure 9. Optimized geometries (B3LYP/6-311 þ G(d,p)) of

6a and 6b

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728

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20

18

16

14

12

10

8

6

4

2

0

40

60

80

100

120

140

160

180

200

angle (°)

Figure 10. Energy proﬁle for conformational rotation of the nitro group in 6a

pentane-2,4-dione (2.1 ml, 2.04 g, 20 mmol), NaOH

(1.0 g, 25 mmol), sodium acetate (8.2 g, 100 mmol),

methanol (160 ml), and water (160 ml). The mixture

was allowed to warm to room temperature and stirred for

1 h. The precipitate which had formed was collected,

washed with water, and recrystallized from ethanol.

Speciﬁc details for each compound are given below.

3-(2-Methylphenylhydrazono)pentane-2,4-dione

(2). 2-Methylaniline (2.14 g, 20 mmol) was used to

afford 96% yellow powder; m.p. 97–99 8C (found, C

65.86, H 6.49, N 12.60; C₁₂H₁₄N₂O₂requires C 66.04, H

6.47, N 12.84%); IR (KBr), n ¼ 3308 (m, b, N—H), 1669

—

(s, C O), 1623 (s, C OꢀꢀꢀH), 1584 (s, C N), 1519 (s,

—

ꢁ1

C), 1373–1326 (s, CO—CH ), 760 (s, Ar—H) cm ;

3

—

C

—

¹H-NMR (400 MHz, CDCl₃):d ¼ 2.40 (s, 3H, CH₃CO),

2.51 (s, 3H, CH₃CO), 2.63 (s, 3H, CH₃), 7.10 (m,

³J_H-H¼ 7.6 Hz, 1H, Ar—H), 7.20 (m, ³J_H-H¼ 7.2 Hz, 1H,

3-(Phenylhydrazono)pentane-2,4-dione (1). Freshly

distilled aniline (1.8 ml, 1.86 g, 20 mmol) was used to

afford 95% yellow powder; m.p. 85–87 (lit.³²m.p. 88 8C).

3

Ar—H), 7.28 (m, J_H-H¼ 7.6 Hz, 1H, Ar—H), 7.75 (m,

³J_H-H¼ 8.0 Hz, 1H, Ar—H), 14.98 (s, 1H, NH).

3

˚

Figure 11. HOMO-2 (a) and HOMO-3 (b) of 6a (0.02 e/A isosurface)

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SYNTHESIS AND STRUCTURES OF 3-(ARYLHYDRAZONO)PENTANE-2,4-DIONES

729

¹³C-NMR (100.6 MHz, CDCl₃):d ¼ 17.00 (Ar—CH₃),

99 8C (found, C 55.05, H 5.02, N 15.88; C₁₂H₁₃N₃O₄

requires C 54.75, H 4.98, N 15.96%); IR (KBr), n ¼ 3330

26.68, 31.56 (CH₃—CO), 115.02 (Ar), 125.68 (C—CH₃),

—

125.68 (Ar), 127.50 (Ar), 131.09 (Ar), 133.84 (C N),

—

(m, b, N—H), 1675 (s, C O), 1635 (s, C OꢀꢀꢀH), 1606

—

;

139.78 (C—N), 197.19, 197.86 (C O); MS (GC-MS),

—

(s, C N), 1535 (s, C C), 1359–1308 (s, CO—CH ),

—

3

m/z ¼ 218 [M þ H]^þ(97%).

791 (s, Ar—H) cm^ꢁ1

¹H-NMR (400 MHz, CDCl₃):

d ¼ 2.33 (s, 3H, CH₃CO), 2.49 (s, 3H, CH₃CO), 2.63 (m,

3H, C—CH₃), 7.31 (m, 1H, Ar), 7.63 (d, ³J_H-H¼ 7.6 Hz,

1H, Ar—H), 7.8 (d, ³J_H-H¼ 8.4 Hz, 1H, Ar—H), 14.26 (s,

1H, NH); ¹³C-NMR (100.6 MHz, CDCl₃): d ¼ 19.00

3-(2,6-Dimethylphenylhydrazono)pentane-2,4-

dione (3). 2,6-Dimethylaniline (2.42 g, 20 mmol) was

used to afford 98% yellow powder; m.p. 74–76 8C (found,

C 66.96, H 6.94, N 11.89; C₁₃H₁₆N₂O₂requires C 67.22,

H 6.94, N 12.06%); IR (KBr), n ¼ 3327 (m, b, N—H),

(CH₃), 26.49, 31.70 (CH₃—CO), 123.51 (Ar), 125.14

—

(Ar), 133.43 (C—CH ), 135.30 (C—NO ), 135.31 (C

—

3

2

—

1671 (s, C O), 1616 (s, C OꢀꢀꢀH), 1588 (s, C N),

N), 135.44 (Ar), 141.64 (C—N), 197.11, 197.33 (C O);

—

1522 (s, C C), 1355–1322 (s, CO—CH ), 758 (s,

MS (ESI, 150 8C), m/z ¼ 264 [M þ H]^þ40%).

—

3

Ar—H) cm^ꢁ1; ¹H-NMR (400 MHz, CDCl₃): d ¼ 2.40 (s,

3H, CH₃CO), 2.45 (s, 3H, CH₃CO), 7.07 (m, 2H, Ar—H),

7.12 (m, 1H, Ar—H), 14.97 (s, 1H, NH); ¹³C-NMR

(100.6 MHz, CDCl₃): d ¼ 19.38 (Ar—CH₃), 26.92, 31.54

X-ray crystallography

—

(CH —CO), 126.38 (Ar), 129.66 (Ar), 129.86 (C N),

The X-ray diffraction data of 3 and 6 were collected on a

CAD-4 diffractometer in the v–2u scan mode

—

3

—

133.70 (C—CH ), 138.07 (C—N), 197.15, 197.65 (C

3

O); MS (ESI, 150 8C), m/z ¼ 233 [M þ H]^þ(100%).

(l_CuKa¼ 1.5418 A, graphite monochromator). X-ray

˚

diffraction studies of 1, 2, 4, and 5 were carried out on

a Bruker-AXS APEX II diffractometer with a CCD area

detector (l_MoKa¼ 0.71073 graphite monochromator):

Frames were collected with v and f rotation at 10 s

per frame. The net intensities were corrected for Lorentz

and polarization effects. Absorption correction was

carried out with SADABS (SAINT-NT).³³Preliminary

structure models were derived by application of direct

methods ³⁴and were reﬁned by full-matrix least-squares

calculation based on F²values for all unique reﬂections.

The non-hydrogen atom positions were reﬁned aniso-

tropically. The nitrogen-bonded hydrogen atom H(2) in 3

and 6 was included in the models in calculated positions,

whereas this hydrogen in structures 1, 2, 4, and 5 was

extracted from difference electron density maps and was

hold riding on its parent nitrogen atom during subsequent

calculations.

3-(2-Nitrophenylhydrazono)pentane-2,4-dione

(4). 2-Nitroaniline (2.76 g, 20 mmol) was used to afford

99% yellow powder; m.p. 137 8C (found, C 53.05, H 4.64,

N 16.86; C₁₁H₁₁N₃O₄requires C 53.01, H 4.45, N

—

16.50%); IR (KBr), n ¼ 3270 (m, b, N—H), 1686 (s, C

—

O), 1643 (s, C OꢀꢀꢀH), 1606 (s, C N), 1494 (s, C C),

1519 (s, C—NO₂), 1318–1308 (s, CO—CH₃), 790 (s,

Ar—H) cm^ꢁ1; ¹H-NMR (400 MHz, CDCl₃): d ¼ 2.49 (s,

3H, CH₃CO), 2.57 (s, 3H, CH₃CO), 7.48 (d,

3

³J_H-H¼ 9.2 Hz, 2H, Ar—H), 8.29 (d, J_H-H¼ 9.2 Hz,

2H, Ar—H), 14.48 (s, 1H, NH). ¹³C-NMR (100.6 MHz,

CDCl₃): d ¼ 26.6, 31.78 (CH₃—CO), 115.80 (Ar), 125.79

—

(Ar), 15.09 (C N), 144.61 (C—NO ), 146.63 (C—N),

—

2

þ

—

196.76, 198.73 (C O); MS (DEI), m/z ¼ 249 [M þ H]

—

(10%).

3-(2,6-Dinitrophenylhydrazono)pentane-2,4-dio-

ne (5). 2,6-Dinitroaniline (3.66 g, 20 mmol) was used to

afford 82% yellow powder. In this case we used 20 ml

concentrated sulphuric acid instead of the hydrochloric

acid; m.p. 153–155 8C (found, C 44.87, H 3.44, N

Supplementary material

Crystallographic data for the structures in this paper have

been deposited with the Cambridge Crystallographic

Data Centre as supplementary publication numbers

CCDC-633705 (1), CCDC-633706 (2), CCDC-633707

(3), CCDC-633708 (4) CCDC-633709 (5), and

CCDC-255318 (6). Copies of the data can be obtained,

free of charge, on application to CCDC, 12 Union Road,

Cambridge CB2 1EZ, UK (fax: þ44-1223-336033,

e-mail: deposit@ccdc.cam.ac.uk).

18.64; C₁₁H₁₀N₄O₆requires C 44.90, H 3.43, N 19.04%);

—

IR (KBr), n ¼ 3266 (m, b, N—H), 1675 (s, C O), 1617

—

(s, C OꢀꢀꢀH), 1538 (s, C C), 1504 (s, C—NO ),

—

2

1360–1320 (s, CO—CH₃), 740 (s, Ar—H) cm^ꢁ1

;

¹H-NMR (400 MHz, CDCl₃): d ¼ 2.39 (s, 3H, CH₃CO),

3

2.63 (s, 3H, CH₃CO), 8,18 (m, J_H-H¼ 8.0 Hz, 2H,

3

Ar—H), 8.36 (m, J_H-H¼ 8.4 Hz, 1H, Ar—H), 15.17 (s,

1H, NH); ¹³C-NMR (100.6 MHz, CDCl₃): d ¼ 26.36, 31.77

(CH₃—CO), 120.36 (Ar), 130.25 (Ar), 132.82 (C—NO₂),

—

137.64 (C N), 142.27 (C—N), 197.01, 198.04 (C O);

—

MS (GC-MS), m/z ¼ 294 [M þ H]^þ(100%).

—

Quantum chemical calculations

3-(2-Methyl-6-nitrophenylhydrazono)pentan-

e-2,4-dione (6). 2-Methyl-6-nitroaniline (3.04 g,

20 mmol) was used to afford 98% yellow powder; m.p.

The Quantum chemical calculations were carried out

using the GAUSSIAN 03 series of programs.³⁵Geome-

tries were fully optimized at the density functional theory

J. Phys. Org. Chem. 2007; 20: 716–731

DOI: 10.1002/poc

730

J. MARTEN ET AL.

level (DFT), using Becke’s three-parameter hybrid

exchange functional and the correlation functional of

Lee, Yang, and Parr (B3LYP).^36,37Geometry optimiz-

ations and harmonic frequencies were calculated for all

elements with the polarized 6-31G(d,p) basis set for

2a–5a and with the 6-311 þ G(d,p) basis set for 6a and

6b.^19,38,39The stationary points were characterized by

zero imaginary frequencies.

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Relaxed PES scans have been performed with the

Opt ¼ ModRedundant utility in Gaussian 03 with B3LYP/

6-311 þ G(d,p). This option includes the speciﬁcation of

redundant internal coordinates. In these cases, a speciﬁc

torsion angle has been changed in ﬁve degree steps. On

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optimized, restricting only the torsion angle to the

speciﬁed value. This method allows access to a deﬁned

section of the PES. Rotational barriers which include the

breaking and formation of hydrogen bonds strongly

depend from the basis set. Therefore, different basis sets

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tested for that purpose: 6-31G(d,p); 6-31 þ G(d,p);

6-311 þ G(d,p). The lowest and probably most realistic

barrier was obtained with B3LYP/6-311 þ G(d,p). The

calculation of the PES scans included up to 36

optimization steps. Therefore, the use of computational

expensive electron correlation methods like CCSD or

MP2 is impractical. The same holds for more extensive

basis sets. DFT methods fail to describe any stabilization

originated from the dispersion energy.⁴⁰Therefore, one

should keep in mind that the presented PES proﬁles

represent estimations of the rotational barriers.

Three-dimensional MO plots were generated with

´

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3

GaussView ⁴¹with a contour value of 0.02 e/A . The AIM

˚

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previously optimized geometries and could be denoted as:

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`

2895.

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`

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B3LYP/6-311 þ G(2d,p)//(B3YLP/6-31G(d,p).

The

wavefunction ﬁles for the AIM analysis were generated

in Cartesian coordinates with a basis set containing 6d

functions. The option SCF ¼ Tight was used to prevent

‘charge leakage’ as described by Popelier.⁴²The electron

density topology was analyzed using the AIMPAC

programs which can be obtained from the ofﬁcial Atoms

in Molecules Download Site at http://www.chemistry.mc-

¨

and Reﬁnement. University of Gottingen: Germany, 1997.

¨

master.ca/aimpac/ and AIM2000 by F. Biegler-Konig and

¨

J. Schonbohm.

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T, Kudin KN, Burant JC, Millam JM, Iyengar SS, Tomasi J, Barone

V, Mennucci B, Cossi M, Scalmani G, Rega N, Petersson GA,

Nakatsuji H, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J,

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MC, Farkas O, Malick DK, Rabuck AD, Raghavachari K, Fores-

man JB, Ortiz JV, Cui Q, Baboul AG, Clifford S, Cioslowski J,

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Acknowledgements

We gratefully acknowledge ﬁnancial support by the

German Ministry of Science and Technology (BMBF-Project

‘BioMon’ 02110120).

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DOI: 10.1002/poc

Article Doi

Synthesis and structural study of 2′- and 2′,6′- positioned methyl- and nitro-substituted 3-(arylhydrazono)pentane-2,4-diones

DOI: 10.1002/poc.1225

Source and publish data:

Authors:

Article abstract of DOI:10.1002/poc.1225

Full text of DOI:10.1002/poc.1225

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