Orfho-hydroxyphenylhydrazo-β-diketones: Tautomery. Coordination ability, and catalytic activity of their copper(II) complexes toward oxidation of cyclohexane and benzylic alcohols

Article abstract of DOI:10.1021/ic101516k

New hydrazone o-HO-phenylhydrazo-β-diketones (OHADB),R1NHN=CR2R3 [R1 = HO-2-C₆H₄,R2 = R3 = COMe (H₂L1,1),R2R3 = COCH₂C(Me)₂CH₂CO (H₂L2,2),R2 = COMe, R3 = COOEt (H₂L4,4

Full text of DOI:10.1021/ic101516k

918 Inorg. Chem. 2011, 50, 918–931
DOI: 10.1021/ic101516k
Ortho-Hydroxyphenylhydrazo-β-Diketones: Tautomery, Coordination Ability, and
Catalytic Activity of Their Copper(II) Complexes toward Oxidation of
Cyclohexane and Benzylic Alcohols
†
†
Maximilian N. Kopylovich, Kamran T. Mahmudov, M. Fatima C. Guedes da Silva,†,‡ Pawel J. Figiel,†
ꢀ
Yauhen Yu. Karabach,† Maxim L. Kuznetsov,† Konstantin V. Luzyanin,† and Armando J. L. Pombeiro*,†
†
´
ꢀ
Centro de Quımica Estrutural, Complexo I, Instituto Superior Tecnico, TU Lisbon, Av. Rovisco Pais, 1049-001,
‡
ꢀ
Lisbon, Portugal, and Universidade Lusofona de Humanidades e Tecnologias,
ULHT Lisbon Av. Campo Grande no. 376, 1749-024, Lisbon, Portugal
Received July 28, 2010
New hydrazone o-HO-phenylhydrazo-β-diketones (OHADB), R1NHNdCR2R3 [R1 = HO-2-C6H4, R2 = R3 = COMe
(H2L1, 1), R2R3 = COCH2C(Me)2CH2CO (H2L2, 2), R2 = COMe, R3 = COOEt (H2L4, 4); R1 = HO-2-O2N-4-C6H3, R2R3 =
COCH2C(Me)2CH2CO (H2L3, 3), R2 =COMe, R3 =COOEt (H2L5, 5), R2R3 = COMe (H2L6, 6A)], and their Cu(II)
complexes [Cu2(CH3OH)2(μ-L1)2] 7, [Cu2(H2O)2(μ-L2)2] 8, [Cu(H2O)(L3)] 9, [Cu2(μ-L4)2]n 10, [Cu(H2O)(L5)] 11,
[Cu2(H2O)2(μ-L6)2] 12A and [Cu(H2O)2(L6)] 12B were synthesized and fully characterized, namely, by X-ray analysis
(4, 5, 7-12B). Reaction of 6A, Cu(NO3)2 and ethylenediamine (en) leads, via Schiff-base condensation, to [Cu-
{H2NCH2CH2NdC(Me)C(COMe)dNNC6H3-2-O-4-NO2}] (13), and reactions of 12A and 12B with en give the Schiff-base
polymer [Cu{H2NCH2CH2NdC(Me)C(COMe)dNNC6H3-2-O-4-NO2}]n 14. The dependence of the OHADB tautomeric
equilibria on temperature, electronic properties of functional groups, and solvent polarity was studied. The OHADB
from unsymmetrical β-diketones exist in solution as a mixture of enol-azo and hydrazo tautomeric forms, while in the
solid state all the free and coordinated OHADB crystallize in the hydrazo form. The relative stabilities of various
tautomers were studied by density functional theory (DFT). 7-14 show catalytic activities for peroxidative oxidation
(in MeCN/H2O) of cyclohexane to cyclohexanol and cyclohexanone, for selective aerobic oxidation of benzyl alcohols
to benzaldehydes in aq. solution, mediated by TEMPO radical, under mild conditions and for the MW-assisted solvent-
free synthesis of ketones from secondary alcohols with tert-butylhydroperoxide as oxidant.
Introduction
surprisingly low impact in coordination chemistry. It was
postulated1,2 that ADB may exist in three tautomeric forms
(enol-azo, keto-azo, and hydrazo; see below) allowing their
application for the design of functional materials attributed
to smart hydrogen bonding, bistate molecular switches, self-
assembled layers, liquid crystals, and so forth.2 However, ex-
perimentally it was found that the studied ADB are stabilized
in the hydrazo form with formation of a hydrogen bond be-
tween the hydrazone =N-NH- moiety and a carbonyl group
giving a six-membered cycle (Scheme 1). In contrast to the well-
studied chemistry of β-diketones, the number of publications
devoted to ADB coordination compounds is very limited,3
and the structurally characterized complexes are usually
simple diligand monomeric chelates.3a,c Thus, to promote
the versatility of coordination of ADB, we decided to focus
on a functionalized form, in particular the ortho-hydroxyphenyl
Azoderivatives of β-diketones (ADB, Scheme 1) represent
a class of compounds known for a long time but with a
*To whom correspondence should be addressed. Fax: (þ351) 218464455.
E-mail: pombeiro@ist.utl.pt.
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r
pubs.acs.org/IC
Published on Web 01/06/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 919
Scheme 1. Intramolecular Hydrogen Bonds in ADB and OHADB
substituted ADB (OHADB, Scheme 1), in view of the promis-
ing coordination features of these compounds.
prepare different types of copper(II) complexes by varying
the synthetic conditions and by using amines as reagents for
Schiff-base template condensations.7
In fact, a comparison of our previous observations4,5 with
literature data6 shows that OHADB drastically differ in phys-
icochemical, analytical, and coordination properties from
β-diketones and simple ADB. Relevant differences between
OHADB and ADB can be accounted for by the formation, in
the former, of one further intramolecular H-bond, whereby
the =N-NH hydrogen can be shared between the CdO(2)
and -O(10)H groups (Scheme 1). As a result, a transition of
the hydrazo proton to the carbonyl moiety is facilitated and
the mole fraction of the enol-azo form, acid properties, and
reactivity should increase. This phenomenon is worth to be
investigated in detail, in particular to try and find a depen-
dence of the relative amounts of the tautomers (hydrazo and
enol-azo, see below) on the OHADB structure, temperature,
solvent polarity, and so forth.
Ligand lability can be favorable to catalytic activity, and
our preliminary studies have shown that related copper(II)
complexes containing labile sites (coordinated water molec-
ules) are significantly active in some oxidation reactions.4a
Other copper complexes with N,O-ligands display a high cata-
lytic activity in cyclohexane oxidation, for example, some
multinuclear Cu compounds that mimic the particulate meth-
ane monooxygenase function.8 On the other hand, the applica-
tion of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) radical
(or its derivatives) with copper complexes for the aerobic oxida-
tion of benzyl alcohols to aldehydes has been studied intensively
during the past years, with regard to the selective oxidation
of alcohols by dioxygen or air.9 Hence, the current work also
aimed to apply the OHADB-Cu(II) complexes as catalyst pre-
cursors for the above types of oxidations.
Thus, having in mind the above-mentioned considerations,
the main objectives of the present study are as follows: (i) to
synthesize new ADB with an ortho-OH group at the aro-
matic ring (i.e., OHADB), as well as with other substituents;
(ii) to study the influence of temperature, solvents, and sub-
stituents on the tautomeric equilibria of the obtained OHADB;
(iii) to synthesize new complexes of these OHADB with Cu(II)
and study the structural influences of substituents and synthetic
conditions; (iv) to check the catalytic activity of the synthesized
copper(II) complexes in the above-mentioned oxidation reac-
tions.
In addition, the ortho-OH group close to the hydrazone
moiety can create one more metal chelating site with a stabi-
lizing effect on the respective complex. This would facilitate
the formation of complexes, for example, with Cu(II), hope-
fully with different primary structures and overall topologies.
Thus, another aim is to test the versatility of OHADB to
(4) (a) Mahmudov, K. T.; Kopylovich, M. N.; Guedes da Silva, M. F. C.;
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Results and Discussion
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Nicola, C.; Garau, F.; Karabach, Y. Yu.; Martins, L. M. D. R. S.; Monari, M.;
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Jezierska, J.; Ozarowski, A.; Kirillov, A. M.; Kopylovich, M. N.; Pombeiro,
A. J. L. Chem. Commun. 2006, 4605. (e) Kirillov, A. M.; Karabach, Y. Yu.;
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Synthesis of OHADB and Their Structural Properties.
3-(2-Hydroxyphenylhydrazo)pentane-2,4-dione (H2L1, 1),
5,5-dimethyl-2-(2-hydroxyphenylhydrazo)cyclohexane-1,
3-dione (H2L2, 2), 5,5-dimethyl-2-(2-hydroxy-4-nitrophe-
nylhydrazo)cyclohexane-1,3-dione (H2L3, 3), 1-ethoxy-2-
(2-hydroxyphenylhydrazo)butane-1,3-dione (H2L4, 4),
1-ethoxy-2-(2-hydroxy-4-nitrophenylhydrazo)butane-1,3-
dione (H2L5, 5), and 3-(2-hydroxy-4-nitrophenylhydrazo)-
pentane-2,4-dione (H2L6, 6A) (Scheme 2) were synthesized
via the Japp-Klingemann reaction10 between the respective
aromatic diazonium salts and β-diketones in an ethanolic
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J. Org. Chem. 1964, 29, 2959.

920 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
Scheme 2. Ortho-Hydroxy Substituted Azoderivatives of β-Diketones (OHADB) Studied in This Work
Scheme 3. Possible Tautomeric Transitions of OHADBa
a Hypothetical forms which have no experimental support are shown in {} brackets.
solution containing sodium acetate. The compound 6A
was described by us earlier,4a and thus will not be discussed
in detail now, but will be considered for comparative
purposes.
reported for the simple ADB.1-5,10-12 Moreover, the two
CdO resonances in the 13C{1H} NMR spectra in the δ
163.8-206.3 range and the single CdN resonance at
about 129-137 indicate that 1-3 and 6A exist in solution
in the hydrazo form (Scheme 3, Supporting Information,
Table S2).
The IR spectra of the OHADB compounds 1-6A
(relevant features are provided in the Supporting Informa-
tion, Table S1) reveal the presence of OH and NH vibra-
tions at about 3180-3468 and 2952-3100 cm-1, corres-
pondingly. The carbonyl stretchings [ν(CdO) 1638-1670,
The 1H NMR spectra of the OHADB from unsymme-
trical β-diketones (4, 5) in DMSO-d6 at 20-25 ꢀC consist
of two sets of signals, indicating they exist in this sol-
vent as a mixture of two tautomeric forms (enol-azo and
hydrazo, see below). To identify these forms, and to determine
their relative amounts, two-dimensional (2D) correlation
and variable temperature NMR experiments in both protic
(CD3OD) and aprotic (CD3CN) solvents were performed
(Supporting Information, Figures S1, S2, Table S2). These
data indicate that in both solvents the tautomeric equi-
librium exists with comparable amounts of both forms,
although dependent on the conditions and the substitu-
ents. Thus, with a decrease of temperature and of the
electron-withdrawing properties of the substituents, the
tautomeric balance shifts to the hydrazo form, while a de-
crease in solvent polarity increases the mole fraction of
ν(CdO H) 1616-1632 cm-1] and the ν(CdN) (1569-
3 3 3
1601 cm-1) vibrations indicate that, in the solid state,1-6A
are stabilized in the H-bonded hydrazone forms. This is
also supported by X-ray and NMR data.
The 1H NMR spectra of the OHADB from symmetric
β-diketones (1-3, and 6A) show only one set of signals for
each compound in CD3OD, CD3CN or DMSO-d6. The
broad signal at δ about 14-15 is assigned to =N-NH
adjacent to the aryl unit. Two methyl groups of the
β-diketone moiety yield two resolved singlets, presumably
because of the formation of an intramolecular six-mem-
bered H-bonded ring (Scheme 3, hydrazo) as it has been
(11) (a) Kitaev, Y. P.; Budnikov, G. K.; Maslova, L. Izv. Akad. Nauk
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Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 921
Scheme 4. Schematic Representations of Complexes 7-11
the enol-azo form of 5(the opposite is observed for 4). More-
over, in the more polar CD3CN (as compared to CD3OD),
the concentration of the enol-azo tautomer (less polar form)
decreases faster with a temperature drop. It should be
mentioned that these NMR experiments do not allow to
distinguish between the E- and Z- enol-azo tautomers
(Scheme 3). The third tautomeric form, namely, keto-azo,
was not detected in solution under any experimental
conditions, presumably because of its lower stability in
comparison with the others. Gradient enhanced 2D cor-
relation NMR spectra allow to discriminate the carbon
signals of the carbonyl groups from the imine moieties for
each tautomer (Supporting Information, Figure S2),
showing that tautomerization occurs on the side of the
molecule that bears the stronger electron-donor ethoxy
substituent.13 Hence, using those data, one can predict
and tune the tautomeric equilibrium, and that can be of a
particular importance for many applications.2,5,11
All the previous studies on the tautomery of ADB de-
scribe their existence (both in solution and in solid phase)
only in the hydrazo form with formation of the hydrogen
bond involving the hydrazone=N-NH- moiety and a car-
bonyl group forming a six-membered cycle (Scheme 1).1,2,11,12
It was also shown that the presence of electron-withdraw-
ing and -donating functional groups at the β-diketone
fragments and at the aromatic ADB ring influences the
sequence: -COOH (2.587(2)) g -H (2.580(4)) g -NO2
(2.578(1)) > -CH3 (2.562(1)) > -CN (2.541(5)) for sub-
stituents in ortho position of the aromatic part of ADB.
In the case of OHABD, the presence of the ortho-OH
group must also significantly change the O N distance
3 3 3
because of one more intramolecular H-bond formation
between this -OH group and the hydrogen atom of the
hydrazone =N-NH- moiety (Scheme 1). This should
strengthen the tension on the 6-membered cycle and
weaken the N-H bond. The tension rises with the tem-
perature and hence the N-H bond should weaken further
and promote the proton transfer to the O2 of the carbonyl
moiety. On the other hand, the more asymmetric and
longer the groups attached to the carbonyl moieties and
to the aromatic part of OHADB are, the more uncom-
pensated would be the movement of the molecule and its
amplitude. Hence, the easier should be the tautomeric
conversion to the enol-azo form. It should also be men-
tioned that the ortho-OH group of OHADB increases
the number of possible isomers (Scheme 3), which can
promote the coordination versatility.
To further study the structural features of OHADB and
to find out which tautomeric form is stabilized in the solid
state, we performed the X-ray analyses of 4 and 5 at 299
(or 295) and 150 K (Supporting Information, Table S2,
Scheme S1). The study at different temperatures was per-
formed to find out if the temperature can affect the tauto-
meric competition with intramolecular proton transfer,
as was found by us in solution by NMR (see above) and
reported2c for the related ketohydrazone-azoenol system
in the solid phase. However, all the X-ray structures of
O2 N2 distance (Scheme 1) within that hydrogen
3 3 3
bonded six-membered cycle,1d-f,2b,2c ranging from 2.54 to
˚
2.70 A, and corresponding, for example, to the following
(13) Hansch, C.; Leo, A.; Taft, W. R. Chem. Rev. 1991, 91, 165.

922 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
Figure 1. X-ray molecular structures of complexes7-11 with atom numberingschemes. Ellipsoids are drawn at 50% probability. Symmetry operations to
generate equivalent atoms:(i) -x, 1 - y, 2- z (7); (i) 1- x, -y, 2 - z (8); (i) 1/2 - x, -1/2 - y, -z and (ii) 1/2 - x, -1/2 þ y, 1/2þ z (10). In 11 only one ofthe
molecules of the asymmetric unit is shown.
the studied compounds show that they crystallize in
the hydrazo form I (Supporting Information, Figure S3,
Scheme 3).
depicted in Figure 1, with the most relevant bond dis-
tances and angles displayed in Table 1.
[Cu2(CH3OH)2(μ-L1)2] (7) and [Cu2(H2O)2(μ-L2)2] (8)
are similar; they crystallize as binuclear species, the for-
mer with two methanol and the latter with two water
ligands, each on the vertex of the square-pyramidal metal
coordination sphere (Scheme 4, Figure 1). The copper
atoms, however, are not in the plane defined by the co-
ordinated atoms, being slightly shifted in the direction of
Therefore, on the basis of IR, NMR, theoretical calcula-
tions (See Supporting Information, Tables S3, S4), and
X-ray structural analyses, we can conclude that the studied
OHADB derived from symmetric β-diketones exist in solu-
tion and in the solid phase mainly in the hydrazo form, while
those derived from unsymmetric β-diketones are present as
a mixture of hydrazo and enol-azo forms in solution,
although in the solid phase the hydrazo form predominates.
Complexation of OHADB to Copper(II). Acetone,
methanol, or ethanol solutions of 1-6B (for details see
Experimental Section) and an aqueous solution of Cu-
˚
the coordinated solvent molecules (0.220 and 0.129 A in
7 and 8, respectively). In both structures, only half of the
molecule is in the asymmetric unit, the metal is in a gen-
eral position and the inversion center is in the center of a
Cu(μ-O)2Cu core. The β-diketone fragments act as tri-
dentate units by means of the deprotonated aromatic OH
group in ortho position (O1), which bridge the copper(II)
atoms, of one of the nitrogen atoms (N1) and of one oxygen
atom (O2) from a carbonyl group. The pentacoordinated
Cu(II) atoms belong to three different metallacycles: one
(NO3)2 2.5H2O were mixed, refluxed for a short time,
3
and then left for slow evaporation. X-ray quality crystals of
7-12B were usually formed directly in the reaction mixtures.
Molecular representations of the isolated complexes 7-11
are shown in Scheme 4 while their molecular structures are

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 923
˚
Table 1. Selected Bond Distances (A) and Angles (deg) for Compounds 7-14
7
8
9
10
11
12
13
14
C-O (coordinated carbonyl)
C-O (free carbonyl)
N-N
1.275(2)
1.224(3)
1.283(2)
1.9163(17)
1.269(3)
1.233(3)
1.279(3)
1.923(2)
1.271(5)
1.240(5)
1.295(5)
1.930(3)
1.244(6)a
1.275(5)a
1.21(5)a
1.30(4)
1.277(11)a
1.225(12)a
1.289(11)a
1.904(8)a
1.210(11)
1.294(9)
1.942(7)a
1.289(5)a
1.896(4)a
1.308(4)
1.906(3)a
Cu-N
1.91(3)
Cu(μ-O)2Cu Core
102.09(14)
— CuOCu (deg)
— OCuO (deg)
101.59(6)
78.41(6)
3.0352(5)
1.9694(14)
1.9476(14)
102.90(9)
77.10(9)
3.0561(7)
1.9629(18)
1.9449(19)
78.13(12)
3.0178(9)
1.958(3)
1.932(3)
˚
Cu Cu (A)
3 3 3
˚
longest Cu-O (A)
shortest Cu-O (A)
˚
6-Membered Metallacycle
— O-Cu-N (deg)
91.44(6)
1.9031(13)
93.85(9)
1.8960(19)
91.05(14)
1.915(3)
92.18(14)a
1.896(3)a
90.6(12)a
1.91(3)a
89.6(3)a
1.946(6)a
˚
Cu-O (A)
5-Membered Metallacycle
— O-Cu-N (deg)
84.55(6)
1.9477(16)
83.97(8)
1.9449(19)
85.07(13)
1.935(3)
83.95(14)a
1.954(3)a
85.5(12)a
1.92(3)a
85.6(3)a
86.2(3)
1.924(6)
85.87(13)
1.942(3)
1.946(6)a
˚
Cu-Oortho (A)
a Average value.
endo ring of the Cu2O2 core, which is the central planar
ring of the molecule, and two fused six- and five-mem-
bered exo metallacycle rings. The Cu-O bond distance in
the six-membered ring is generally shorter than that in the
five-membered one (Table 1), a fact that, together with
the larger O-Cu-N angle in the former, can be justified
by the decrease in ring constraint.
group in the aromatic part of the diketone in 9 has no
clear effect in bond length and angles, as compared with
those of 8, since the dimensions in the six- and five-mem-
bered rings are not so dissimilar in both cases (Table 1).
The water ligand in 9 is able to establish intermolecular
H-bonds with the coordinated O1 atom of a vicinal mo-
lecule and the non-coordinated O3 from another one
˚
Specifically in the structure of 7, the H10B hydrogen
of the coordinated methanol interacts with the π cloud of
[O10-H10A O1, d(D A) 2.698(4) A, 133.00ꢀ; O10-
3 3 3 3 3 3
˚
H10B O3, d(D A) 2.663(5) A, 167.00ꢀ] (Supporting
3 3 3
3 3 3
the Cu1-O2-C8-C7-N2-N1 metallacycle (H-centroid
Information, Figure S8). The minimum Cu Cu distance
between two adjacent molecules is as short as 5.081 A.
3 3 3
˚
˚
3.191 A). Intermolecular H-bonds [O10-H10 O3,
3 3 3
d(D A) 2.675(2) A, 129.00ꢀ] between the molecules lead
to one-dimensional (1D) chains along the crystallogra-
phic b axis; the cohesion between these layers is obtained
The structure of 10, [Cu2(μ-L4)2]n, is polymeric with
bimetallic repeating units of the type of those found for
7 and 8. The interconnection of these units is achieved by
means of coordination of both the COOEt carbonyl oxygens
which take up the apical positions of the square-pyramid
coordination sphere around the copper(II) atoms of two
differently oriented bimetallic units. Such a coordination
leads to quite dissimilar Cester-Oester-Cu angles [115.6(3)
and 155.0(3)ꢀ] and to a grid-type network where the
Cu(μ-O)2Cu planes in adjacent units make angles of 81.19ꢀ
(Figure 2). The C-O bond distances of the carbonyl
oxygens of the ligand skeleton are considerably different
˚
3 3 3
by van der Waals H π interactions involving the methyl
3 3 3
hydrogen H91C and the C1 < C6 phenyl ring (C-H
3 3 3
˚
centroid, 3.228 A, 155.2ꢀ). The minimum distance between
copper atoms in adjacent layers is of 6.6928(6) A (Support-
˚
ing Information, Figure S5). In the structure of 8, on the
other hand, intermolecular H-bonds between the coordi-
nated water molecules and the carbonyl oxygens of two
vicinal molecules [Supporting Information, Figure S6:
˚
O10-H10A O3, d(D A) 2.792(3) A, 159(4)ꢀ; O10-
3 3 3
3 3 3
˚
˚
H10B O3, d(D A) 2.858(3) A, 161(3) ꢀ] lead to 1D
[1.271(5) and 1.217(6) A] leading to a lower average value
3 3 3
3 3 3
˚
[1.244(6) A] as compared to the previously discussed
chains. Additionally, intermolecular π π interactions are
3 3 3
observed between the C1 < C6 phenyl ring and the Cu1-
N1-N2-C11-C16-O2 metallacycle; the centroid
centroid distance of 3.337 A indicate a reasonably strong
structures (Table 1).
The asymmetric unit of [Cu(H2O)(L5)] (11; Scheme 4,
Figure 1) comprises two mononuclear square-planar
molecules with a relative orientation that makes an angle
of 25.35ꢀ measured by the planes defined by the corre-
sponding metal coordinated atoms. Bond angles and dis-
tances in both molecules are quite similar, but the torsion
angles for the C-C-Oester-Ccarbonyl groups are consid-
erably different (156.49ꢀ and 176.93ꢀ in the Cu1 and Cu2
molecules, respectively). The most significant H-bonds in
this structure (Supporting Information, Figure S10) in-
volve the water ligands of both molecules; in each of them,
they interact with the carbonyl oxygen of the ester group
in an adjacent molecule and with the aromatic oxygen of
another one. The crystal structure is further stabilized by
3 3 3
˚
π
π interaction (Supporting Information, Figure S6).
3 3 3
In the three-dimensional (3D) packing diagram of 8 the
molecules are aligned in two different layers, with a rela-
tive angle of 73.43ꢀ (measured by the planes defined by the
Cu2O2 cores). Such alignment leads to a supramolecular
network which is distinct from that of 7 (Supporting
Information, Figure S7). Because of the bulkiness of the
chelating ligands, the minimum intermolecular Cu
˚
Cu distance in 8 (8.0307 A) is longer than that in 7 (see
3 3 3
above).
In contrast with the previous cases, [Cu(H2O)(L3)] (9)
is mononuclear with a tetracoordinated square-planar
metal center (Scheme 4, Figure 1). The inclusion of a nitro
medium intense π π interactions between the aromatic
3 3 3

924 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
ring and the six-membered ring of the Cu2 and Cu1
molecules, respectively (centroid centroid distances of
free (in 10) or coordinated to CuII of another unit, meaning
that coordination to copper(II) does not proceed by a
3 3 3
simple direct attack of CuII to the H-bonded CdO
˚
3.475 A). This feature agrees with theoretical calculations
0
3 3 3
5
of the model [Cu(H2O)(L )] 110 with the methoxy group
HN OH moiety. A more complicated mechanism should
3 3 3
instead of the ethoxy (Supporting Information, Scheme
S2) which indicate a strong delocalization of electron
density in the six-membered metallacycle.
be considered, conceivably via an initial attack of CuII to
a carbonyl oxygen not involved in H-bond, followed
by rupture of H-bonds and ligand deprotonation.
It was demonstrated10c that the six-membered H-bonded
cycle (Scheme 1) in unsymmetrical ADB usually involves
the carbonyl group of the moiety with the stronger
electron-donor group. That is also confirmed in this work
for H2L4 and H2L5; in both cases the hydrazo hydrogen is
bonded to the ethoxycarbonyl group of the diketone moiety
(Supporting Information, Scheme S1, S2, Figure S3). How-
ever, in the corresponding complexes 10 and 11 (Scheme 4,
Figure 1) the carbonyl adjunct to the ethoxy group is either
Condensation Reaction of an OHADB Ligand. Com-
plexes of the above type can be used for further and ready
synthesis of Schiff-base species, and the viability of this
approach is illustrated for the case of H2L6 (6A). In fact,
although free 6A does not undergo condensation with an
amine, for example, ethylenediamine (en), such a reaction
readily occurs upon coordination to CuII, as shown below.
Hence, free 6A is simply deprotonated by en to give the
(H2en)L6 salt (6B) (Scheme 5, Route I). The reaction of 6B
withCu(NO3)2 hydrate in methanol/water affords the mono-
nuclear complex [Cu(H2O)2(μ-L6)] 12B (Route II) which
converts easily into the dinuclear [Cu2(H2O)2(μ-L6)2] 12A
by simple refluxing a methanol solution of the former
(Scheme 5, Route III). 12A can be also formed in a direct
way from reaction of H2L6 (6A) with Cu(NO3)2 hydrate
(Route IV).4a
The X-ray diffraction analysis of [Cu(H2O)2(μ-L6)] (12B)
confirms that copper(II) is pentacoordinated with one
water ligand in an equatorial position and the other one
in the apical position of a square-pyramid geometry (Sup-
porting Information, Figure S10a). The bond distances and
angles are in agreement with the previously discussed struc-
tures (see above and Table 1) but, similarly to what was
found in 11, the torsion angles for the C-C-Ccarbonyl-C
ketone groups are different (173.13ꢀ and 167.34ꢀ in the
Cu1 and Cu2 molecules, respectively). The asymmetric
unit of 12B, like that of 11, also comprises two molec-
ules; nevertheless, the angle of the planes defined by the
square environment of each Cu(II) ion is only of 8.43ꢀ.
Figure 2. 3D packing diagram of compound 10 (arbitrary view). The
grid-type network is achieved by means of dissimilar Cester-Oester-Cu
angles. The Cu(μ-O)2Cu planes in adjacent units make angles of 81.19ꢀ.
Scheme 5. Other Reactions of Copper(II) with 6A and Its Modified Form 6B

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 925
Figure 3. X-ray molecular structures of complexes 13 (a) and 14 (b) with atom numbering scheme and packing diagram of 14 showing a fragment of two
1D chains viewed along the crystallographic b axis (c).
˚
Moreover, it is interesting to compare the relative orien-
tation of the molecules in the asymmetric units of both
structures: in 11, one molecule is rotated about 180ꢀ in the
horizontal plane relative to the other one, but in 12B the
180ꢀ horizontal rotation is associated with a reflection
also in the horizontal plane (Supporting Information,
Figure S10b). Consequently, the equatorial water mole-
cules in both molecules of 12B are oriented in the same
direction, while in 11 they are pointing to opposite sides.
1.210(11) A, respectively, indicate double bond characters
and a π-electron distribution within the β-diketone frag-
ment. Several intermolecular hydrogen bonds were found
in the structure, mainly involving the N21 as H donor, and
O4 and O11 as acceptors (Supporting Information, Figure
S11). Moreover, reasonably strong H π interactions
3 3 3
were located involving the methyl groups of the six-mem-
bered metallacycles and the phenyl rings of neighboring
˚
molecules (C11-H11 centroid of 3.171 A). Such con-
3 3 3
Another relevant feature of 12B is the π π interaction
involving the five-membered metallacycle rings with a
tacts result from the relative positions of the molecules of
13 in space since their planes (defined by the atoms in the
coordination sphere of the metal) make angles of 87.84ꢀ
leading to a 3D grid-type network.
3 3 3
˚
short centroid centroid distance of 3.168 A. Both intra-
3 3 3
and intermolecular H-bond interactions were found for
12B, the former involving methylic hydrogens and N2
Interestingly, reactions of 12A and 12B with en (Routes
VI and VII, respectively) provide a coordination polymer,
14 (Figure 3b), with the repeating unit that is a Schiff base
complex very similar to 13 (it also bears one six- and two
five-membered metallacycles). The Cu1 atom has a square-
pyramidal coordination sphere with the tetradentate N3O
ligand occupying the equatorial positions and the apical
site being engaged with the O4 atom of the carbonyl group
from an adjacent molecule. Although O4 is coordinated to
(in the Cu1 molecule) or the carbonyl O24 (in the Cu2
˚
molecule) [C15-H15B N2, d(D A) 2.684(16) A,
3 3 3
3 3 3
˚
103.00ꢀ; O31-H31B O24, d(D A) 2.770(14) A,
3 3 3 3 3 3
102.00ꢀ].
In the presence of Cu(NO3)2 hydrate, condensation of
en with one CdO group of 6A readily occurs at pH 2 to give
the Schiff base complex [Cu{H2NCH2CH2NdC(CH3)C-
(C(dO)CH3)dNNC6H3-2-O-4-NO2}] 13 (Route V). The
pH value of 2 is crucial, sincefor otherpH values this metal-
promoted (namely, by template effect)7 reaction does not
proceed.
˚
copper, the C14-O4 double bond distance (1.220(5) A) is
not significantly longer than that in13 (1.210(11) A). How-
˚
ever, the carbonyl O4 atom of each unit is 1.038 A away
˚
The molecular structure of 13 (Figure 3a) reveals a dis-
torted square planar geometry around the copper(II) ion
and confirms its N3O environment. The C12-N22, C13-
N2 and C14-O4 bond lengths of 1.292(11), 1.319(11), and
from the plane of the molecule (defined by the atoms in
the coordination sphere of the metal) and the group is
considerably twisted (N2-C13-C14-O4 torsion angle
˚
of 155.64ꢀ), as compared with 13 (0.880 A and 174.15ꢀ,

926 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
Scheme 6. Oxidations of Cyclohexane and Benzyl Alcohol Using
OHADB-Cu Complexes As Catalysts
and 18% under conventional heating or microwave irra-
diation, correspondingly (entries 15-17).
For the establishment of the mechanism type, experi-
ments in the presence of carbon and oxygen radical traps
(TEMPO and diphenylamine, respectively) were performed.
Consequently, the addition of TEMPO or Ph2NH in a
stoichiometric amount relative to cyclohexane leads to a
marked inhibition of the products formation (compare
entries 19 and 20 with 1), signifying that the peroxidative
oxidation of cyclohexane proceeds mainly via a radical
pathway. Apparently, an hydroxyl radical (HO•) which
can be derived from the reduction of hydrogen peroxide
by CuI (the latter being formed upon oxidation of H2O2
to HOO• by CuII) abstracts a hydrogen atom from cyclo-
hexane providing the formation of cyclohexyl radical
(Cy•), which reacts with O2 dissolved in the reaction mix-
ture to form cyclohexylperoxyl radical (CyOO•).8g,h The
latter can undergo either a dismutation (giving ultimately
cyclohexanol, cyclohexanone, and dioxygen) or a reduc-
tion by CuI to the corresponding anion that can be pro-
tonated to cyclohexyl hydroperoxide (CyOOH). This can
also be formed by H-abstraction of CyOO• from H2O2,
and can further undergo metal-assisted decomposition
to yield CyOO• or the alkoxyl radical (RO•) which ab-
stracts hydrogen from cyclohexane forming Cy• and
cyclohexanol.8,14-18
respectively). The 1D coordination polymer thus formed
spreads along the crystallographic c axis (Figure 3c) in
ladder type chains, extending to the second dimension by
means of intermolecular hydrogen bonds. The structure
is further stabilized by means of π π interactions in-
3 3 3
volving the phenyl and the six-membered metallacycle rings
of successive steps of the ladder, with centroid centroid
3 3 3
˚
distances of 3.404 A.
Catalytic Activity of OHADB-Copper(II) Complexes.
The oxidations of cyclohexane and benzyl alcohol (Scheme 6)
were chosen as model reactions to test the catalytic potential
of the synthesized copper compounds 7-14 for oxidations of
alkanes and alcohols, respectively.
The involvment of CyOOH in the formation of the
alcohol and ketone is proved by the following observa-
tions. At the end of reaction, CyOOH is also present,
apart from the alcohol and ketone. However, it is not
stable and decomposes at the high temperature of the GC
injector/column to the corresponding alcohol and ketone
which are the detected products. Following the reported
method established by Shul’pin,17,18 we estimated the
amount of CyOOH at the end of the reaction by perform-
ing the GC analyses without and with addition of triphe-
nylphosphine, which quantitavely converts CyOOH into
CyOH (with formation of OPPh3) (compare entries 1 and 2).
CyOOH is thus shown to be the major product at the
end of the reaction (ca. 15% yield), while the alcohol and
ketone are present in lower amounts (ca. 1 and 9% yields,
respectively).
The complexes exhibit good catalytic activities in the
peroxidative oxidation of cyclohexane by aqueous H2O2,
under mild conditions (at 25 ꢀC), to afford cyclohexanol
(CyOH) and cyclohexanone (Scheme 6a, Table 2). The
total yield, in a single batch, reaches 16-25% (entries 1-6
and 12-14) for the substrate to catalyst molar ratio of 100,
whereas turnover number (TON) values up to 60 (entry 8)
are reached for the substrate to catalyst molar ratio of
5000. The activities of the mononuclear complexes 9 and
11, bearing the copper atom with a square planar coordi-
nation mode and one ligated water, are similar under stan-
dard conditions (product yields lie in 17-19% range, entries
4 and 6). The mononuclear complexes 12B (which contains
copper with a square pyramidal coordination and two ligated
water molecules) and 13 (with a distorted square planar
geometry) exhibit a slightly higher activity (overall yield of
22-23%, entries 12 and 13). The catalytic activities of the
dinuclear complexes 7 and 8 are different (25 and 16% cor-
respondingly, entries 1-3). Such differences possibly reflect
the different chelating ligand properties and labilities of the
coordinated methanol and water. The reactions performed
under higher substrate to catalyst molar ratios (5000) and
increased reaction time (24 h) showed similar activities of
the dinuclear Cu(II) complexes 7 and 8 (TONs of 55 and
60), and, although smaller, of the mononuclear com-
plexes 9 and 11 (TONs of 35 and 40) (entries 7, 8 and 9,
11, respectively). The polymeric compound 10 exhibits
the lowest catalytic activity in the series, what possibly is
related to the absence of labile ligands. Generally, the com-
plexes bearing ligands derived from pentane-2,4-dione
(7, 12B, 13) are more active than the others. The reactions
performed in the presence of 7 with a low oxidant loading
(H2O2/cyclohexane molar ratio of 2) and at a higher tem-
perature (50 ꢀC) lead in 1 h to the overall yields of about 17
Since [Cu2(H2O)2(μ-L6)2] (12A)4a and other copper(II)
complexes with N,O-ligands19 proved to be good cata-
lysts for the aerobic oxidation of benzyl alcohols medi-
ated by TEMPO in aqueous media, we have now applied
(14) Kirillov, A. M.; Kopylovich, M. N.; Kirillova, M. V.; Karabach, Y.
Yu.; Haukka, M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Adv. Synth.
Catal. 2006, 348, 159.
(15) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. Rev.
2004, 104, 939. (b) Costas, M.; Chen, K.; Que, L., Jr. Coord. Chem. Rev. 2000,
200-202, 517. (c) Roelfes, G.; Lubben, M.; Hage, R.; Que, L., Jr.; Feringa, B. L.
Chem.;Eur. J. 2000, 6, 2152.
(16) Hogan, T.; Sen, A. J. Am. Chem. Soc. 1997, 119, 2642.
(17) (a) Shul’pin, G. B. J. Mol. Catal. A: Chem. 2002, 189, 39. (b) Shul'pin,
G. B. In Transition Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm, C.,
Eds.; Wiley-VCH: New York, 2004; Vol. 2. (c) Shul'pin, G. B. Organic Reactions
Catalyzed by Metal Complexes; Nauka: Moscow, 1988.
(18) (a) Nizova, G. V.; Krebs, B.; Suss-Fink, G.; Schindler, S.; Westerheide,
L.; Cuervo, L. G.; Shul’pin, G. B. Tetrahedron 2002, 58, 9231. (b) Suss- Fink, G.;
Gonzalez, L.; Shul'pin, G. B. Appl. Catal. A: Gen. 2001, 217, 111. (c) Shul'pin, G. B.;
Suss-Fink, G. J. Chem. Soc., Perkin Trans. 2 1995, 2, 1459. (d) Kodera, M.;
Shimakoshi, H.; Kano, K. Chem. Commun. 1996, 1737. (e) Shul'pin, G. B. C. R.
Chim. 2003, 6, 163.
(19) Figiel, P. J.; Kirillov, A. M.; Karabach, Y. Yu.; Kopylovich, M. N.;
Pombeiro, A. J. L. J. Mol. Catal. A: Chem. 2009, 305, 178.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 927
Table 2. Peroxidative Oxidation of Cyclohexane to Cyclohexanol and Cyclohexanonea
yieldb of products, %
entry
catalyst
n(C6H12)/n(Cat.)
n(H2O2)/n(C6H12
)
reaction time, h
Alc.
Ket.
totalc
A/Kd
TONe
1
7
7
8
9
10
11
7
8
9
10
11
12B
100
100
100
100
100
100
5000
5000
5000
5000
5000
100
100
100
100
100
100
100
100
100
10
10
10
10
10
10
2
2
2
2
2
10
10
10
2
2
2
2
10
10
6
6
6
6
6
6
24
24
24
24
24
6
6
6
1
1
7.1
15.1
5.3
6.9
5.9
5.4
0.5
0.6
0.3
0.2
0.3
9.6
10.0
6.0
7.6
11.1
8.2
2.7
2.6
2.5
16.6
9.7
23.7
24.8
15.6
18.9
15.9
17.1
1.1
1.2
0.7
0.5
0.8
23.4
22.3
16.2
16.7
18.2
17.0
9.1
0.4
1.6
0.5
0.6
0.6
0.5
0.8
1.0
0.8
0.7
0.6
0.7
0.8
0.6
0.8
1.6
0.9
0.4
0.8
0.4
23.7
24.8
15.6
18.9
15.9
17.1
55
60
35
25
40
23.4
22.3
16.2
16.6
18.2
17.0
9.1
2f
3
4
5
6
7
8
9
10
11
12
13
14
15g,h
16g,i
17j,i
18g,h
19k
20l
10.3
12.0
10.0
11.7
0.6
0.6
0.4
0.3
0.5
13.9
12.3
10.2
9.1
7.1
8.8
6.4
3.4
7.0
13
12A
7
7
7
Cu(NO3)2
7
7
1
1
6
6
6.0
9.5
6.0
9.5
a Selected data; reaction conditions (unless stated otherwise): C6H12 (1 mmol for entries 1-6, 12-20 and 5 mmol for entries 7-11), MeCN 4 mL,
catalyst precursor (0.01 mmol for entries 1-6, 12-20 and 0.001 mmol for entries 7-11), n(HNO3)/n(Cat) = 10, H2O2 (10 mmol added as an aqueous
30% solution), 25 ꢀC. b Moles of product/100 mols of C6H12
.
c Cyclohexanol þ cyclohexanone. d Alcohol (cyclohexanol)/ketone (cyclohexanone) molar
ratio. e Overall TON values (moles of products/mol of catalyst). f GC analysis performed upon addition of PPh3. g In sealed tube under microwave
irradiation (10W, 50 ꢀC), H2O2 (2 mmol, aqueous 30% solution). h 1 mL MeCN. i 0.5 mL MeCN. j In sealed tube with conventional heatingat 50 ꢀC, H2O2
(2 mmol, aqueous 30% solution). k In the presence of TEMPO (1 mmol). l In the presence of diphenylamine (1 mmol).
Table 3. Aerobic Oxidation of Benzyl Alcohols to the Corresponding Benzaldehydesa
entry
catalyst
substrate
product
time, h
yield,b %
1
2
3
4
5
6
7
8
9
9
11
8
10
7
12B
12A
13
9
9
9
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
4-Me-benzyl alcohol
4-Cl-benzyl alcohol
2-Cl-benzyl alcohol
cinnamyl alcohol
1-phenylethanol
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
4-Me-benzaldehyde
4-Cl-benzaldehyde
2-Cl-benzaldehyde
cinnamaldehyde
acetophenone
6
22
99
72
52
35
63
79
99
65
48
67
81
62
50
22
22
22
22
22
22
22
22
22
22
22
22
22
9
10
11
12
13
14
9
9
a Conditions: substrate (3 mmol), catalyst (0.03 mmol (1 mol %)), TEMPO (0.15 mmol (5 mol %)) in aqueous solution of K2CO3 (10 mL, 0.1M), 50 ꢀC,
1 atm, air. b Moles of product/100 mols of substrate.
the other synthesized copper(II) complexes 7-13 to this
reaction (Scheme 6b, Table 3). Oxidation of benzyl alco-
hol (3 mmol) in alkaline (0.1 M K2CO3) aqueous solution,
in the presence of a catalytic amount of the monomeric
complex 9 (0.03 mmol, 1 mol % vs substrate) and TEMPO
(0.15 mmol, 5 mol % vs substrate) results in 22% of ben-
zaldehyde yield after 6 h of reaction (entry 1). Prolonging
the reaction time to22h led toa nearly quantitativeyield of
benzaldehyde (99%, entry 2). Unexpectedly, among all the
tested complexes, the most efficient ones are those with the
strong electron-withdrawing nitro group at the phenyl ring
(complexes 9, 11, 12B, and 12A, entries 2, 3, 7 and 8). The
other tested complexes (7, 8, and 10) shown moderate
activities (entries 4-6).
Substituted benzyl alcohols can also be converted to the
corresponding aldehydes, as known for other systems,20
with moderate to good yields (entries 10-12). Allylic
alcohol (cinnamyl alcohol, entry 13) is also oxidized to
cinnamaldehyde in good yield. The oxidation of a sec-
ondary benzyl alcohol (1-phenylethanol, entry 14) leads
to the corresponding ketone in moderate yield. The limi-
tation of the current system, as usually observed for other
catalysts,21 concerns the failure of application to aliphatic
alcohols (results not included in Table 3). As proposed
earlier,21c Cu/TEMPO catalytic systems follow a mecha-
nism similar to that of the galactose-oxidase enzymes,
(21) (a) Gamez, P.; Arends, I. W. C. E.; Reedijk, J.; Sheldon, R. A. Chem.
Commun. 2003, 2414. (b) Punniyamurthy, T.; Rout, L. Coord. Chem. Rev. 2008,
252, 134. (c) Lin, L.; Liuyan, J.; Yunyang, W. Catal. Commun. 2008, 9, 1379.
(d) Palombi, L.; Bonadies, F.; Scetti, A. Tetrahedron 1997, 53, 15867. (e) Figiel,
P. J.; Kopylovich, M. N.; Lasri, J.; Guedes da Silva, M. F. C.; Silva, J. J. R. F.;
Pombeiro, A. J. L. Chem. Commun. 2010, 46, 2766.
(20) (a) Sheldon, R. A. J. Mol. Catal. A: Chem. 2006, 251, 200. (b) Sheldon,
R. A.; Arends, I. W. C. E. Adv. Synth. Catal. 2004, 346, 1051. (c) Minisci, J. Mol.
Catal. A: Chem. 2003, 204-205, 63.

928 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
Table 4. MW-Assisted Oxidation of Benzyl Alcohol and 1-Phenylethanol with TBHP to the Corresponding Carbonyl Compounds with Complex 9 as Catalysta
entry
substrate
product
temp., ꢀC
time, min
yield,b %
TON,c (TOF)d
1
2
3
4
5
benzyl alcohol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
benzaldehyde
acetophenone
acetophenone
acetophenone
acetophenone
50
80
80
80
80
60
15
30
60
20.1
26.1
39.0
82.7
89.0
101
131 (523)
195 (390)
414 (414)
445 (223)
120
a Conditions: substrate (5 mmol), catalyst (0.01 mmol, 0.2 mol %), TBHP (10 mmol), 50 ꢀC (benzyl alcohol) or 80 ꢀC (1-phenylethanol), 10W. b Moles
of product/100 mols of substrate. c Overall TON values (moles of products/mol of catalyst). d TOF = TON/h (values in brackets).
involving coordination of the alcohol and TEMPO to
the copper center, and hydrogen abstraction from the
alcohol R-carbon by TEMPO. As a result, the formation
of TEMPOH and of a subsequent alkoxy-Cu(II) complex
is postulated. The latter reacts with another molecule of
TEMPO, leading to the aldehyde and TEMPOH, with
regeneration of the copper catalyst.
(mediated by TEMPO), in aqueous medium, of benzyl alco-
hols to the corresponding benzaldehydes (up to 99% yield),
or for the solvent-free microwave-assisted oxidation of ben-
zyl alcohol and 1-phenylethanol to the corresponding car-
bonyl products.
Experimental Section
The synthesized complexes were also tested as catalyst
precursors for the solvent-free low power (10 W) micro-
wave-assited (MW) peroxidative oxidation21d,e of benzyl
alcohol and 1-phenylethanol with tert-butylhydroperoxide
(TBHP) to benzaldehyde and acetophenone, respectively
(Scheme 6c, Table 4). Complex 9 showed the highest acti-
vity, for example, leading to a benzaldehyde yield of about
20% in 1 h (entry 1), while the use of the other catalyst pre-
cursors resulted in about 15% average yield. Thus, 9 was
chosen for the studies of the MW-assisted oxidation of
1-phenylethanol, which proceeds more efficiently (aceto-
phenone yields of ca. 83 or 89% were achieved in 1 or 2 h,
entries 4 or 5, respectively). Despite the good catalytic
activity of the studied complexes, they are less active than
some copper(II) triazapentadiene compounds we have
previously reported.21e
Materials and Instrumentation. All the synthetic work was
performed in air and at room temperature. All the chemicals
were obtained from commercial sources (Aldrich) and used as
received. Infrared spectra (4000-400 cm-1) were recorded on a
BIO-RAD FTS 3000MX instrument in KBr pellets. 1D (1H,
13C{1H}) and 2D (1H,1H-COSY, 1H,13C-HMQC, 1H,13C-
HSQC and 1H,13C-HMBC) NMR spectra were recorded on Bruker
Advance IIþ 300.13 (75.468 carbon-13) and 400.13 (100.61 carbon-
13) MHz (UltraShield Magnet) spectrometers at ambient tem-
perature. C, H, and N elemental analyses were carried out by the
ꢀ
Microanalytical Service of the Instituto Superior Tecnico. Elec-
trospray mass spectra were run with an ion-trap instrument
(Varian 500-MS LC Ion Trap Mass Spectrometer) equipped
with an electrospray (ESI) ion source. For electrospray ioniza-
tion, the drying gas and flow rate were optimized according to
the particular sample with 35 psi nebulizer pressure. Scanning
was performed from m/z 100 to 1200 in methanol solution. The
compounds were observed in the positive mode (capillary vol-
tage = 80-105 V). Chromatographic analyses were undertaken
by using a Fisons Instruments GC 8000 series gas chromato-
graph with a DB-624 (J&W) capillary column (FID detector)
and the Jasco-Borwin v.1.50 software.
Conclusions
A series of new OHADB were synthesized and fully
characterized. The symmetric ones are stabilized in solution
in the hydrazo form, as known for other ADB, but, for the
first time, it was found that the unsymmetric OHADB exist in
solution in enol-azo and hydrazo forms, though the tauto-
meric balance is dependent on the solvent polarity and on the
electron-withdrawing properties of the substituents at the
aromatic ring. These results can help to tune the tautomeric
balance to the desired form for different types of applications
or further syntheses. X-ray structural analyses show that
OHADB exist in the solid state in the hydrazo form with the
H-N proton forming hydrogen bonds between one CdO of
the β-diketone fragment and the ortho-OH group of the aro-
matic part of the molecule, and that intramolecular hydrogen
bonded rings are formed at the side bearing the stronger
electron-withdrawing substituents.
Synthesis of the ortho-Hydroxy Substituted Phenylhydrazo-β-
diketones (OHADB). These compounds were prepared via the
Japp-Klingemann reaction involving diazotization of aromatic
amines with following azocoupling of the thus formed diazo-
nium salt and a dione.10
Diazotization. A 0.025 mol portion of an ortho-aminophenol
was dissolved in 50 mL of water upon addition of 1.0 g of
crystalline NaOH. The solution was cooled in an ice bath to 0 ꢀC
and 1.725 g, 0.025 mol of NaNO2 was added with subsequent
addition of 5 mL 33% HCl in portions of 0.2 mL for 1 h under
rigorous stirring. During the reaction the temperature of the
mixture must not exceed þ5 ꢀC. A suspension of the unstable
2-hydroxyphenyldiazonium chloride was obtained which was
then used as such for the next stage (see below).
Azocoupling. A 5.0 g portion of CH3COONa was added to a
mixture of 0.025 mol of β-diketones with 50 mL of ethanol. The
solution was cooled in an ice bath, and a suspension of the ortho-
aminophenol diazonium salt (prepared according to the proce-
dure described above) was added in two equal portions under
vigorous stirring for 1 h. Along this stage, the pH must be main-
tained in the range of 8-10; crystalline CH3COONa may be
added if needed. On the next day, the formed precipitate of the
ligand was filtered off, washed with water, recrystallized from
ethanol, and dried in air. The characterizations of the ligands
Upon complexation with copper(II), the studied OHADB
lead to structurally different mono-, bi-, and polynuclear
complexes; also a Schiff base template synthesis was per-
formed showing the possibility to form new versatile imino
derivatives. OHADB behave as promising ligands toward
complexes of different nuclearities forming extended 1D or
2D assemblies via H-bonding or π π interactions involv-
3 3 3
1
were carried out by IR, element analysis, H and 13C NMR
ing, for example, metallacycle rings.
The complexes act as efficient and selective catalysts for
the mild peroxidative oxidation of cyclohexane to the corre-
sponding alcohol and ketone, for the aerobic oxidation
spectroscopies and are given below.
Compound 1. Yield: 72% (based on pentane-2,4-dione), black
powder soluble in methanol, ethanol, acetone and insoluble in

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 929
water and chloroform. Anal. Calcd for C11H12N2O3 (M = 220):
C, 60.00; H, 5.45; N, 12.73. Found: C, 58.88; H, 5.47; N, 12.20.
IR (KBr, selected bands, cm-1): 3468 ν(OH), 3079 ν(NH), 1668
193.5 (C-O). Hydrazone, δ: 14.1 (CH3), 30.6 (CH3), 60.8 (CH2),
113.5, 116.2, and 128.8 (CAr-H), 130.7, (CdN), 135.9 (CAr-NH-N),
142.9(CAr-NO2), 143.6 (CAr-OH), 162.1 and 196.6 (CdO).
Preparation of Compound 6B. 0.01 mol of 6A was dissolved in
ethanol and 0.01 mol ethylenediamine was added and heated at
70 ꢀC for 1 h. The precipitate was separated and recrystallized
from methanol. Yield: 73% (based on 6A), red powder soluble
in methanol, ethanol, acetone and insoluble in chloroform.
Anal. Calcd for C13H19N5O5 (M = 325.32): C, 48.00; H, 5.89;
N, 21.53. Found: C, 48.04; H, 5.96; N, 20.97. IR (KBr, selected
bands, cm-1): 3346 ν(NH), 1672 ν(CdO), 1653 ν(CdO), 1636
ν(CdO), 1610 ν(CdN), 1508 ν(C-N) and 742 ν(Ar). 1H NMR
(300.13 MHz, DMSO-d6). Enol-azo, δ: 2.23 (s, 3H, CH3), 2.75
(s, 3H, CH3), 3.44 (s, 2H, CH2), 3.46 (s, 2H, CH2), 7.09-7.68
(3H, Ar-H). Hydrazone, δ: 2.44 (s, 6H, CH3), 3.44 (s, 2H, CH2),
3.46 (s, 2H, CH2), 7.09-7.68 (3H, Ar-H). 13C{1H} NMR
(75.468 MHz, DMSO-d6). Enol-azo, δ: 18.5 (CH3), 31.3 (CH3),
47.6 (2CH2), 110.6 (CAr-NdN), 110.8 (C-N), 111.0, 113.7 and
134.5(CAr-H), 144.6 (CAr-NO2), 146.1 (CAr-O), 167.2 (C-O),
197.0 (CdO). Hydrazone, δ: 26.5 (2CH3), 47.6 (2CH2), 111.0,
113.7 and 134.5(CAr-H), 136.9 (CdN), 140.5 (CAr-NH-N), 144.6
(CAr-NO2), 146.1 (CAr-O), 196.4 (2CdO).
ν(CdO), 1632 ν(CdO H), 1599 ν(CdN) and 750 ν(Ar).
3 3 3
1H NMR (300.13 MHz, DMSO-d6), δ: 2.41 (s, 3H, CH3), 2.48 (s,
3H, CH3), 6.91-7.67 (4H, Ar-H), 10.51 (s, 1H, OH), 14.58 (s, 1H,
NH). 13C{1H} NMR (75.468 MHz, DMSO-d6), δ: 26.5 (CH3), 31.2
(CH3), 114.9, 115.8, 120.2, and 126.2 (CAr-H), 129.3 (CAr-NH-N),
133.2 (CdN), 146.3 (CAr-OH), 196.2 and 196.4 (CdO).
Compound 2. Yield: 75% (based on 5,5-dimethylcyclohexane-
1,3-dione), dark brown powder soluble in methanol, ethanol,
acetone and insoluble in water and chloroform. Anal. Calcd for
C14H16N2O3 (M = 260): C, 64.61; H, 6.15; N, 10.78. Found: C,
64.09; H, 6.38; N, 10.70. IR (KBr, selected bands, cm-1): 3180
ν(OH), 2952 ν(NH), 1650 ν(CdO), 1616 ν(CdO H), 1594
3 3 3
ν(CdN) and 759 ν(Ar). 1H NMR (300.13 MHz, DMSO-d6), δ:
1.02 (s, 6H, CH3), 2.50 (s, 2H, CH2), 2.57 (s, 2H, CH2), 6.92-
7.64 (3H, Ar-H), 10.75 (s, 1H, OH), 15.25 (s, 1H, NH). 13C{1H}
NMR (75.468 MHz, DMSO-d6), δ: 28.0 (CH3), 30.4 (CH3), 51.8
(CH2), 51.8 (CH2), 38.9 (Cipso), 116.0, 120.3, 127.3, and 128.0
(CAr-H), 130.3 (CAr-NH-N), 137.0 (CdN), 147.2 (CAr-OH), 204.2
and 206.0 (CdO).
General Procedure for the Syntheses of the Copper(II) Com-
plexes. To 90 mL of a 1.11 ꢀ 10-2 M acetone (4), methanol (1, 2,
Compound 3. Yield: 83% (based on 5,5-dimethylcyclohexane-
1,3-dione), dark brown powder soluble in methanol, ethanol,
acetone and insoluble in water and chloroform. Anal. Calcd for
C14H15N3O5 (M = 305): C, 55.08; H, 4.92; N, 13.77. Found: C,
55.00; H, 4.98; N, 13.68. IR (KBr, selected bands, cm-1): 3448
5, 6B) or ethanol (3) solution of 1-6B, 10 mL of a 1 ꢀ 10-1
M
aqueous solution of Cu(NO3)2 2.5H2O were added under stirr-
3
ing, and the mixture was refluxed for 10 min and left for slow
evaporation. The products (7-12B, correspondingly) usually
started to precipitate in the reaction mixture after 2 days at room
temperature; after 4 days they were filtered off, washed with a
small amount of ethanol, and dried in air. Specific details for
each compound are given below.
ν(OH), 3100 ν(NH), 1665 ν(CdO), 1628 ν(CdO H), 1596
3 3 3
ν(CdN) and 748 ν(Ar). 1H NMR (400.13 MHz, DMSO-d6), δ:
0.90-1.03 (s, 6H, CH3), 2.50 (s, 2H, CH2), 2.62 (s, 2H, CH2),
7.22-7.74 (3H, Ar-H), 10.27 (s, 1H, OH), 14.91 (s, 1H, NH).
13C{1H} NMR (75.468 MHz, DMSO-d6), δ: 28.0 (CH3), 30.3
(CH3), 52.0 (CH2), 52.0 (CH2), 38.7 (Cipso), 110.7, 114.5, and
115.3 (CAr-H), 131.7 (CAr-NH-N), 136.1 (CdN), 144.8 (C Ar-NO2),
149.0 (CAr-OH), 203.4 and 206.3 (CdO).
Compound 7. Yield 50% (based on Cu). Black powder soluble
in acetone, methanol, ethanol, acetonitrile, and DMSO. Anal.
Calcd for C24H28N4O8Cu2 (M = 627.6): C, 45.93; H, 4.50; N,
8.93. Found: C, 46.08; H, 4.50; N, 9.16. MS (ESI): m/z: 656
[MþH]þ. IR (KBr, selected bands, cm-1): 1639 ν(CdO), 1631
ν(CdO), 1584 ν(CdN), 755 ν(Ar). UV-vis (pH 4.5): λmax = 428
Compound 4. Yield: 61% (based on 1-ethoxybutane-1,3-dione),
black powder soluble in methanol, ethanol, acetone and insoluble
in water and chloroform. Anal. Calcd for C12H14N2O4 (M = 250):
C, 57.60; H, 5.60; N, 11.20. Found: C, 57.75; H, 5.79; N, 11.07. IR
(KBr, selected bands, cm-1): 3238 ν(OH), 2963 ν(NH), 1661
nm, Δλ = 45 nm, ε = 10900 L mol-1 cm-1 8b
.
Compound 8. Yield 73% (based on Cu). Dark brown powder
soluble in acetone, methanol, ethanol, acetonitrile, and DMSO.
Anal. Calcd for C28H32N4O8Cu2 (M = 679.7): C, 49.48; H, 4.75;
N, 8.24. Found: C, 50.06; H, 4.66; N, 8.46. MS (ESI): m/z: 666
[MþH]þ. IR (KBr, selected bands, cm-1): 1655 ν(CdO), 1619
ν(CdO), 1588 ν(CdN), 758 ν(Ar). UV-vis (pH 5): λmax = 449
ν(CdO), 1632 ν(CdO H), 1601 ν(CdN) and 744 ν(Ar).
3 3 3
1H NMR of a mixture of tautomeric enol-azo and hydrazone
forms (300.13 MHz, DMSO-d6). Enol-azo, δ: 1.03-1.07 (s, 3H,
CH3), 2.39 (s, 3H, CH3), 4.21-4.25 (s, 2H, CH2), 6.93-6.97 (4H,
Ar-H), 10.41 (s, 1H, HO-Ar), 12.59 (s, 1H, HO-enol). Hydrazone,
δ: 1.25-1.33 (s, 3H, CH3), 2.39 (s, 3H, CH3), 4.27-4.32 (s, 2H,
CH2), 7.00-7.56 (4H, Ar-H), 10.49 (s, 1H, Ar-OH), 14.68
(s, 1H, NH). 13C{1H} NMR (75.468 MHz, DMSO-d6). Enol-
azo, δ: 14.0 (CH3), 26.6 (CH3), 60.3 (CH2), 114.3 (C-N),
114.8(CAr-NdN), 120.1, 125.0, 126.0, 127.2, (CAr-H) 146.2,
(CAr-OH), 165.2 (CdO), 193.2 (C-O). Hydrazone, δ: 14.2 (CH3),
30.4(CH3),60.8(CH2), 125.9, 115.6, 115.8 and 120.2, (CAr-H), 129.3
(CdN), 129.5 (CAr-NH-N), 145.7 (CAr-OH), 162.8 and 195.8 (CdO).
Compound 5. Yield: 69% (based on 1-ethoxybutane-1,3-dione),
dark brown powder soluble in methanol, ethanol, acetone and
insoluble in water and chloroform. Anal. Calcd for C12H13N3O6
(M = 295): C, 48.81; H, 4.41; N, 14.23. Found: C, 48.76; H, 4.41;
N, 14.05. IR (KBr, selected bands, cm-1): 3399 ν(OH), 3093
nm, Δλ = 76 nm, ε = 9750 L mol-1 cm-1
.
Compound 9. Yield 71% (based on Cu). Dark brown powder
soluble in acetone, methanol, ethanol, acetonitrile, and DMSO.
Anal. Calcd for C14H15N3O6Cu (M = 384.8): C, 43.69; H, 3.93;
N, 10.92. Found: C, 43.87; H, 3.84; N, 10.62. MS (ESI): m/z: 757
[MþH]þ. IR (KBr, selected bands, cm-1): 1647 ν(CdO), 1604
ν(CdN), 746 ν(Ar). UV-vis (pH 4): λmax = 457 nm, Δλ = 43
nm, ε = 17300 L mol-1 cm-1
.
Compound 10. Yield 47% (based on Cu). Greenish black
powder soluble in acetone, methanol, ethanol, acetonitrile, and
DMSO. Anal. Calcd for C24H24N4O8Cu2 (M = 623.6): C, 46.23;
H, 3.88; N, 8.98. Found: C 46.71; H 3.94; N 9.06. MS (ESI): m/z:
369 [MþH]þ. IR (KBr, selected bands, cm-1): 1668 ν(CdO),
1611 ν(CdO), 1599 ν(CdN), 755 ν(Ar). UV-vis (pH 5): λmax
=
ν(NH), 1656 ν(CdO), 1632 ν(CdO H), 1569 ν(CdN), 738
3 3 3
404 nm, Δλ = 149 nm, ε = 9000 L mol-1 cm-1
.
ν(Ar). 1H NMR of a mixture of tautomeric enol-azo and hydra-
zone forms (300.13 MHz, DMSO-d6). Enol-azo, δ: 1.27-1.34
(s, 3H, CH3), 2.44-2.50 (s, 3H, CH3), 4.27-4.29 (s, 2H, CH2),
7.64-7.87 (3H, Ar-H), 11.44 (s, 1H, Ar-OH), 12.31 (s, 1H, HO-
enol). Hydrazone, δ: 1.27-1.34 (s, 3H, CH3), 2.50-2.51 (s, 3H,
CH3), 4.30-4.34 (s, 2H, CH2), 7.64-7.87 (3H, Ar-H), 11.44 (s,
1H, Ar-OH), 14.25 (s, 1H, NH). 13C{1H} NMR (75.468 MHz,
DMSO-d6). Enol-azo, δ: 13.9 (CH3), 26.6 (CH3), 61.4 (CH2),
110.0 (C-N), 110.2, (CAr-NdN), 114.0, 116.1 and 135.8,
(CAr-H), 145.4 (CAr-NO2), 145.8 (CAr-OH), 163.8 (CdO) and
Compound 11. Yield 58% (based on Cu). Dark brown powder
soluble in acetone, methanol, ethanol, acetonitrile, and DMSO.
Anal. Calcd for C12H13N3O7Cu (M = 374.8); C, 38.46; H, 3.50;
N, 11.21. Found: C, 38.97; H, 3.56; N, 11.49. MS (ESI): m/z: 409
[MþH]þ. IR (KBr, selected bands, cm-1): 1686 ν(CdO), 1638
ν(CdO), 1604 ν(CdN), 747 ν(Ar). UV-vis (pH 3.5): λmax = 424
nm, Δλ = 41 nm, ε = 15500 L mol-1 cm-1
.
Synthesis of complexes 12A and 12B and their interconversion.
Compound 12A. The preparation of the complex 12A from 6A is

930 Inorganic Chemistry, Vol. 50, No. 3, 2011
Kopylovich et al.
similar to the synthesis of the compounds 7-11 described above
M
elemental analysis, also by comparison of IR spectra and the
X-ray unit cell with those of the genuine compound.
and reported by us before.4a Thus, to 90 mL of 1.11 ꢀ 10-2
ethanol solution of H2L6, 10 mL of 1 ꢀ 10-1 M aqueous solution
Computation Details. The full geometry optimization of all
structures was carried out at the DFT/HF hybrid level of theory
using the Becke’s three-parameter hybrid exchange functional
in combination with the gradient-corrected correlation func-
tional of Lee, Yang, and Parr (B3LYP)22 with the help of the
Gaussian-98 program package.23 The restricted approximations
for the structures with closed electron shells and the unrestricted
methods for the structures with open electron shells were
employed. Symmetry operations were not applied for all struc-
tures. The geometry optimization was carried out using a
relativistic Stuttgart pseudopotential that described 10 core
electrons and the appropriate contracted basis sets (8s7p6d)/
[6s5p3d]24 for the copper atoms and the 6-31G(d) basis sets for
other atoms. The Hessian matrix was calculated analytically for
structures to prove the location of correct minima (no imaginary
frequencies) and to estimate the thermodynamic parameters, the
latter being calculated at 25 ꢀC. Solvent effects (δEs) were taken
into account at the single-point calculations on the basis of the
gas-phase geometries at the CPCM-B3LYP/6-31þG(d)//gas-
B3LYP/6-31G(d) level of theory using the polarizable con-
tinuum model25 in the CPCM version26 with DMSO and water
taken as solvents. The enthalpies and Gibbs free energies in solu-
tion (Hs and Gs) were estimated by addition of the solvent effect
δEs to the gas-phase values Hg and Gg. Several possible con-
formations for each structure were calculated and only the most
stable ones are discussed.
of Cu(NO3)2 2.5H2O and 5-6 drops of 0.01 M HCl (to reach
3
pH 2) were added. The mixture was stirred under solvent reflux
for 10 min and left for slow evaporation. Greenish-black crystals
started to form in the reaction mixture after 2 d at room tem-
perature; after 4 d they were filtered off and dried in air. Yield
70% (based on Cu). Anal. Calcd for C22H22Cu2N6O12 (M =
689.54): C 38.32, H 3.22, N 12.19. Found: C 38.32, H 3.14, N
11.98. IR (KBr, selected bands, cm-1): 3468 ν(OH), 1652
ν(CdO), 1611 ν(CdO H), 1602 ν(CdN), 745 ν(Ar).
3 3 3
Compound 12B. A methanol/water (5/1, v/v) solution (50 mL)
of 6B (0.01 mol) was added to 0.01 mol copper(II) nitrate hy-
drate, and the mixture was stirred under reflux for 5 min and
then left for slow evaporation. Yield 50% (based on Cu). Greenish
black powder soluble in acetone, methanol, ethanol, aceto-
nitrile, and DMSO. Anal. Calcd for C11H13N3O7Cu (M =
362.8): C, 36.42; H, 3.61; N, 11.58. Found: C, 36.75; H, 3.62;
N, 11.76. IR (KBr, selected bands, cm-1): 1673 and 1636
ν(CdO), 1599 ν(CdN), 746 ν(Ar). UV-vis (pH 5): λmax
=
439 nm, Δλ = 47 nm, ε = 19200 L mol-1 cm-1
.
Conversion of 12B to 12A. A methanol solution (10 mL) of
12B (0.01 mol) was refluxed for 8 h, whereafter it was left at
room temperature for slow evaporation. After about 2 d, crys-
tals of 12A started to form, and their identity was confirmed by
elemental analysis and also by comparison of IR data and the
X-ray unit cell with those of the genuine compound.
X-ray Structure Determinations. X-ray quality single crystals
of ligands (4 and 5) and complexes were grown by slow evapo-
ration at room temperature of methanol (7), water-methanol
(8, 12), water-ethanol (9, 11, 13), or water-acetone (10) solu-
tions. They were immersed in cryo-oil, mounted in a Nylon loop,
and measured at a temperature of 150 K (also 299 K for 4 and 5).
Intensity data were collected using a Bruker AXS-KAPPA
APEX II diffractometer with graphite monochromatic Mo-KR
(λ 0.71073) radiation. Data were collected using ω scans of 0.5ꢀ
per frame and full sphere of data were obtained. Cell parameters
were retrieved using Bruker SMART software and refined using
Bruker SAINT27 on all the observed reflections. Absorption
corrections were applied using SADABS.28 Structures were
solved by direct methods by using the SHELXS-97 package28
and refined with SHELXL-97.29a Calculations were performed
using the WinGX System-Version 1.80.03.29b All hydrogens
were inserted in calculated positions. Least square refinements
with anisotropic thermal motion parameters for all the non-
hydrogen atoms and isotropic for most of the remaining atoms
(some exceptions in the structures of 10, 11, and 12 because of a
poorer quality of crystals) were employed. Crystallographic and
selected structural details are listed in Supporting Information,
Tables S1 and S2 respectively. CCDC 784258-784269 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Syntheses of the Schiff-Base Cu Complexes. Compound 13.
To an aqueous-ethanol (20/80, v/v, 50 mL) solution of 6 (10
mmol) were added Cu(NO3)2 2.5H2O (10 mmol), ethylenedia-
3
mine (10 mmol), and 5 mL of 0.01 M HCl (pH 2); then the mix-
ture was refluxed for 5 min and left for slow evaporation. The
crystals of 13 started to form in the reaction mixture after about
2 d at room temperature. The product was filtered off, washed
with a small amount of water, and dried in air. Yield 34% (based
on Cu). Greenish black powder soluble in acetone, methanol,
ethanol, acetonitrile, and DMSO. Anal. Calcd for C13H15N5O4-
Cu (M = 368.8): C, 42.33; H, 4.10; N, 18.99. Found: C, 42.45; H,
4.03; N, 18.63. IR (KBr, selected bands, cm-1): 1646 ν(CdO),
1601 ν(CdN), 1560 ν(CdN), 749 ν(Ar).
Synthesis of 14 from 12B. An ethanol-water (1:1, v/v, 50 mL)
solution with 0.01 mol of 12B, en (0.01 mol), and 1 mL of 0.01 M
HCl (pH 2) was refluxed for 5 min and then evaporated at room
temperature; after about 3 d, crystals of 14 started to form. Yield
37% (based on Cu). Dark brown crystals soluble in acetone,
methanol, ethanol, acetonitrile, and DMSO. Anal. Calcd for
C13H15N5O4Cu (M = 368.8): C, 42.33; H, 4.10; N, 18.99. Found:
C, 41.16; H, 4.22; N, 18.55. IR (KBr, selected bands, cm-1): 1634
ν(CdO), 1588 ν(CdN), 1546 ν(CdN), 748 ν(Ar).
Conversion of 12A to 14. An ethanol solution (50 mL) with
0.01 mol of 12A and en (0.01 mol) was refluxed for 8 h, and then
was left for evaporation at room temperature; after about 2 d,
crystals of 14 started to form; their identity was confirmed by
Catalytic Studies. Oxidation of Cyclohexane. The reaction
mixtures were prepared as follows: to 1-10.0 μmol of the com-
plex 7-13 contained in the reaction flask were added 4 mL of
MeCN, 0.01-0.1 mmol HNO3, 1-5.0 mmol C6H12, and 10.0
mmol H2O2 (30% in H2O), in this order. The reaction mixture
was stirred for 6 h at room temperature (ca. 25 ꢀC) and air
atmospheric pressure, then 90 μL of cycloheptanone (as internal
standard) and 9.0 mL of diethyl ether (to extract the substrate
(22) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.;
Parr, R. G. Phys. Rev. 1988, B37, 785.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
Jr., R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Peterson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9;
Gaussian, Inc.: Pittsburgh, PA, 1998.
(24) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866.
(25) Tomasi, J.; Persico, M. Chem. Rev. 1997, 94, 2027.
(26) Barone, V.; Cossi, M. J. Phys. Chem. 1998, 102, 1995.
(27) APEX2 & SAINT; Bruker AXS Inc.: Madison, WI, 2004.
(28) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
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Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 931
and the products from the reaction mixture) were added. The
resulting mixture was stirred for 15 min, and then a sample taken
from the organic phase was analyzed by GC. The GC analyses of
the aqueous phase showed the presence of only traces (less than
0.05%) of oxidation products. Blank experiments were per-
formed and confirmed that no cyclohexane oxidation products
(or only traces, below 0.3%) were obtained in the absence of the
metal catalyst. The formation of CyOOH was proved by using
Shul’pin’s method17,18 based on the effect of addition of PPh3
(prior to GC analysis) on the amounts of cyclohexanol and
cyclohexanone (PPh3 converts CyOOH into CyOH). For the
experiments performed in a sealed tube, the following reaction
procedure was used: to 10.0 μmol of complex 7 contained in the
Pyrex tube were added 0.5-1 mL of MeCN, 0.1 mmol HNO3,
1 mmol C6H12, and 2.0 mmol H2O2 (30% in H2O), in this order.
The tube was closed and placed in the focused microwave
reactor (or oil bath). The system was left under stirring and
irradiation for 1 h at 50 ꢀC. After the reaction mixture was
cooled down, 3.5-3 mL of MeCN, 90 μL of cycloheptanone (as
internal standard), and 9.0 mL of diethyl ether (to extract the
substrate and the products from the reaction mixture) were
added. The resulting mixture was stirred for 15 min, and then a
sample taken from the organic phase was analyzed by GC.
Aerobic Oxidation of Alcohols in Aqueous Solutions. The
reactions were carried out according to the procedure described
earlier.4a,19 They were performed in flasks fitted with water
circulating condensers under atmospheric pressure of air. All
reagents were placed into the flask, and the resulting mixture
was then heated with continuous stirring in an oil bath. The
reaction mixtures after the oxidation reaction were neutralized
by 1 M HCl, and then 10 mL of EtOAc was added for the extrac-
tion. The organic phase was analyzed by gas chromatography
using acetophenone as the internal standard.
Microwave-Assisted Oxidation of Alcohols with TBHP.
To 0.01 mmol of the catalyst contained in a Pyrex tube were
added 5 mmol of the alcohol substrate [i.e., benzyl alcohol or
1-phenylethanol] and 10 mmol of TBHP. The tube was closed and
placed in the focused microwave reactor. The system was left under
stirring and irradiation for 15-240 min at 50 ꢀC (in case of benzyl
alcohol) or 80 ꢀC (in case of 1-phenylethanol). The power of 10 W
was selected for all experiments. After the reaction, the mixture was
allowed to cool down, 5 mL of MeCN and internal standard (i.e.,
75 μL of acetophenone in the case of benzyl alcohol oxidation and
300 μL of benzaldehyde in the case of 1-phenylethanol oxidation)
were added. Small aliquots (about 50 μL) were then taken, diluted
10 times by MeCN, and subjected to the GC analysis.
Acknowledgment. This work has been partially supported
by the Foundation for Science and Technology (FCT), Portugal,
and its PPCDT (FEDER funded). K.T.M., P.J.F., and Y.Y.K.
express gratitude to the FCT for their postdoc and doctoral
fellowships. M.N.K., MLK, and K.V.L. are grateful to the FCT
^
and IST for research contracts within the Ciencia 2007 and
Ciencia 2008 scientific programs. The authors thank the Portu-
^
guese NMR Network (IST-UTL Centre) for providing access to
the NMR facility.
Supporting Information Available: Tables listing crystallo-
graphic data, atomic coordinates, bond lengths and bond
angles, anisotropic displacement parameters, hydrogen coordi-
nates and isotropic displacement parameters, torsion angles and
hydrogen bonds, calculated total energies, enthalpies and Gibbs
free energies. X-ray crystallographic data for 4, 7-14 in CIF
format. Packing diagram of 4, 7-14. This material is available
free of charge via the Internet at http://pubs.acs.org.