Self-assembly of cyclometallated Pd(II) compounds directed by C-H?π interactions: A structural evidence for metalloaromaticity in a cyclopalladated ring

Article abstract of DOI:10.1016/j.inoche.2010.09.037

Structural studies on the two binuclear cyclometallated compounds [Pd(C²,N-bzan)(μ-N₃)]₂ (1) and [Pd(C ²,N-bzan)(μ-Pz)]₂ (2) (bzan = N-benzylideneaniline) have been carried out in this work. In both c

Full text of DOI:10.1016/j.inoche.2010.09.037

Inorganic Chemistry Communications 14 (2011) 83–86
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Inorganic Chemistry Communications
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Self-assembly of cyclometallated Pd(II) compounds directed by C–H⋅⋅⋅π interactions:
A structural evidence for metalloaromaticity in a cyclopalladated ring
Anderson M. Santana a,b, Janaína G. Ferreira a,c, Antonio C. Moro a, Sahra C. Lemos a, Antonio E. Mauro a,
,
⁎
a
c
Adelino V.G. Netto a, , Regina C.G. Frem , Regina H.A. Santos
⁎
a
Instituto de Química de Araraquara, UNESP-Univ Estadual Paulista, C.P.355, 14801-970, Araraquara-SP, Brazil
Instituto de Ciências Exatas e da Terra, ICET-UFMT,78060-900, Cuiabá-MT, Brazil
Instituto de Química de São Carlos, USP, C.P. 780, 13560-970, São Carlos-SP, Brazil
b
c
a r t i c l e i n f o
a b s t r a c t
Structural studies on the two binuclear cyclometallated compounds [Pd(C2,N-bzan)(μ-N3)]2 (1) and [Pd(C2,
N-bzan)(μ-Pz)]2 (2) (bzan=N-benzylideneaniline) have been carried out in this work. In both compounds,
C–H⋅⋅⋅π interactions play an important role in the molecular self-assembly. In complex 1, ortho-CH bonds
from bzan moiety interact with the cyclopalladated ring at a distance of Cbzan⋅⋅⋅Cg 3.448(6) Å, providing a
new structural evidence for the metalloaromaticity of the cyclometallated ring.
Article history:
Received 15 September 2010
Accepted 30 September 2010
Available online 7 October 2010
Keywords:
© 2010 Elsevier B.V. All rights reserved.
Supramolecular chemistry
C–H⋅⋅⋅π interactions
Cyclopalladated
Metalloaromaticity
Crystal engineering represents an important branch of supramo-
lecular chemistry and has currently attracted many research interests
in solid and material science. The rapid development in this field
shows that a variety of organic components with specific functional
groups have been used to build supramolecular structures, sustained
by either metal coordination or noncovalent intermolecular interac-
tions, aiming at creating novel materials with useful properties [1].
Among these intermolecular forces, the hydrogen bond is arguably
the most powerful organizing element in the design of supramolec-
ular systems, because of its strength and high degree of directionality
[2]. In the past few years, there has been a growing interest on the role
played by C–H⋅⋅⋅A weak forces (A=O, N, Cl, and π) in self-assembly
reactions, conformation of molecules and chiral recognition [3].
Particularly, the C–H⋅⋅⋅π interactions are very attractive due to the
wide range of C–H group acidity and π-basicity as well as the frequent
occurrence of C–H groups in almost all organic molecules [3,4].
Despite their extreme weak nature, the cooperative effects of C–H⋅⋅⋅π
interactions are surprisingly noticeable and should not be under-
estimated. Of more recent interest, and not yet fully explored, is the
soft interaction formed by C–H acids and the delocalized π bonds of a
chelate ring [5]. It is well-established that metallocycles possessing
delocalized π system can manifest aromatic properties (metalloar-
omaticity) similar to their organic counterparts [6]. Therefore, new
supramolecular strategies of molecular assembly can arise from the
identification and detailed understandingof the roleof C–H⋅⋅⋅πchelate ring
interactions in the organization of chelated-based molecules within
the crystal.
In our previous investigations, the cooperative action of weak
C–H⋅⋅⋅π hydrogen bonds in directing self-assembly of the cyclopal-
ladated compounds [Pd(C2,N-dmba)(μ-N3)]2 and [Pd2(C2,N-dmba)2
(μ-N3)(μ-Cl)] (dmba=N,N-dimethylbenzylamine) during the crys-
tallization was described [7]. In order to increase our knowledge
about the role of C–H⋅⋅⋅π forces in molecular assembly of
cyclometallated Pd(II) species, we decided to investigate derivatives
of N-benzylideneaniline (bzan) since it possesses abundance of CH
donor groups and two phenyl rings in its molecular structure. As
a part of our ongoing studies on the structures and properties of
transition metal-based compounds [8], we describe in this paper
the resolution of the crystal structures of a known compound, [Pd
(C2,N-bzan)(μ-N3)]2 (1) [9], and a new pyrazolato analogue, [Pd(C2,
N-bzan)(μ-Pz)]2 (2). To the best of our knowledge, [Pd(C2,N-bzan)
(μ-Pz)]2 is the first cyclopalladated compound containing double
pyrazolate-bridges whose structure was determined by single-
crystal X-ray diffraction. It is worth mentioning that the structures
[Pd(C2,N-deba)(μ-dmPz)]2 (deba=N,N-diethylbenzylamine; dmPz=
3,5-dimethylpyrazolate) [10] and [Pd(C⌢E)(μ-Pz)]2 (C⌢E=trimesity-
larsine and trimesitylphosphine; Pz=pyrazolate) [11] have been
established only by spectroscopic evidences.
⁎
Corresponding authors. Mauro is to be contacted at Tel.: +55 16 33016625;
The cyclopalladated [Pd(C2,N-bzan)(μ-Pz)]2 was prepared by
mixing [Pd(C2,N-bzan)(μ-N3)]2 [9] with the pyrazolate ligand in
acetone [12]. Careful recrystallization from acetone solutions gave
fax: +55 16 33227932. Netto, Tel.: +55 16 33016626.
E-mail addresses: mauro@iq.unesp.br (A.E. Mauro), adelino@iq.unesp.br
(A.V.G. Netto).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.037

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A.M. Santana et al. / Inorganic Chemistry Communications 14 (2011) 83–86
yellow crystals of good quality for X-ray diffraction studies [13]. The
coordination of the pyrazolato ligand in [Pd(C2,N-bzan)(μ-Pz)]2 was
detected by the disappearance of the νasN3 band at 2056 cm−1, which
is observed in the spectrum of the starting azido-compound [9],
and by the presence of its typical IR absorptions at 3114 cm−1 (νCH)
and 1190 cm−1 (δCH). Some characteristic IR bands of C,N-chelating
bzan were observed at 3050 (νCH) and 748 cm−1 (δCH). Besides the
expected resonances of the C,N-chelating bzan, the 1 H NMR spectrum
of [Pd(C2,N-bzan)(μ-Pz)]2, in CDCl3, showed the characteristic AMX
system corresponding to the pyrazolate groups [14].
The ORTEP representations of compounds [Pd(C2,N-bzan)(μ-N3)]2
(1) and [Pd(C2,N-bzan)(μ-Pz)]2 (2) with the atom labeling scheme are
presented in Figs. 1 and 2, respectively.
Compound [Pd(C2,N-bzan)(μ-N3)]2 (1) consists of a dimer in
which each bzan moiety is bonded to the Pd(μ-1,1-N3)2Pd unit
through an iminic nitrogen atom and an aromatic carbon atom (C9),
providing two equivalent five-membered cyclometallated chelate
rings in a trans arrangement. The coordination geometry at the Pd
atoms is essentially square planar and defined by two N atoms from
the end-on azido groups, one C atom from aromatic ring and the N
atom from the bzan ligand. The Pd–C bond length of 1.986(5) Å found
in [Pd(C2,N-bzan)(μ-N3)]2 is shorter than the value predicted from the
sum of the covalent radii of carbon and palladium (2.081 Å) [15], but
comparable to the values found for [Pd(C2,N-bzan)(μ-N,S-CN3S2)]2
(1.997(5) Å) [9] and [Pd(C2,N-bzan)(μ-SCN)]2 (1.981(7) Å)[16]. The
Pd···Pd distance of 3.2432(6) Å found in [Pd(C2,N-bzan)(μ-N3)]2
is slightly longer than the value found for [Pd(C2,N-dmba)(μ-N3)]2
(3.1915(8) Å) (dmba=N,N-dimethylbenzylamine) [7].
Fig. 2. ORTEP drawing with labeling scheme for [Pd(bzan)(μ-pz)]2 (2). Displacement
ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and bond
angles (º) are: Pd–N 2.033(6); 2.039(6), Pd–C9 1.970(7); 2.009(6), N–C7 1.283(9); 1.28
(1), C1–C2 1.36(1); 1.38(1), C1–C6 1.36(1); 1.38(1), C2–C3 1.38(1); 1.39(1), C3–C4
1.38(1); 1.36(1), C4–C5 1.37(1); 1.37(2), C5–C6 1.38(1); 1.38(1), C7–C8 1.436(9); 1.48
(1), C8–C9 1.396(9); 1.40(1), C8–C13 1.40(1); 1.41(1), C9–C10 1.403(9); 1.37(1), C10–
C11 1.40(1); 1.40(1), C11–C12 1.35(1); 1.37(1), C12–C13 1.37(1); 1.36(1), Pd1–N1a
2.096(6), Pd1–N2b 2.001(6), Pd2–N1b 2.125(6), Pd2–N2 1.984(6), N1–N2 1.365(9);
1.353(9), N1–C14 1.35(1); 1.33(1), N2–C16 1.33(1); 1.34(1), C14–C15 1.35(1); 1.36
(1), C15–C16 1.36(1); 1.38(1), N–Pd–N1 95.8(2); 99.3(2), N–Pd–C9 81.3(2); 80.9(3),
N1–Pd–C9 175.5(2); 176.6(2), Pd–N–C1 125.1(4); 126.5(4), Pd–N–C7 113.8(5); 114.9
(5), Pd–N1–N2 119.0(4); 115.7(4), Pd–N2–N1 120.5(4); 117.1(4), N1–Pd–N2 88.0(2);
85.8(2).
The molecular structure of compound [Pd(C2,N-bzan)(μ-Pz)]2 (2)
features two square planar palladium centres which are bridged by
two exobidentate pyrazolato ligands. Each bzan molecule is con-
nected to the “Pd(μ-Pz)2Pd” core through the iminic nitrogen atom
(Na and Nb) and the aromatic carbon atom (C9a and C9b), providing
two equivalent five-membered cyclometallated chelate rings in a
trans arrangement. The main structural feature of 2 is the boat-like
conformation of the “Pd(μ-Pz)2Pd” six-membered metallacycle, being
consistent for all cases where two pyrazolato-type ligands bridge two
palladium atoms. The Pd⋅⋅⋅Pd separation of 3.2432(6) Å in 2 falls
within the typical range of the corresponding metal–metal distances
found in similar binuclear Pd(II) pyrazolato compounds (3.0–3.4 Å)
[17]. The dihedral angle formed by the best least squares coordination
planes of the metals is 65.4(2)°. Interestingly, there is a disparity in
the Pd–Npyrazolato bond length which may be ascribed to the different
trans influences of the C and N atoms. The Pd1–N1a distance of 2.096
(6) Å, with the N atom occupying a nearly trans position to the C atom
(C9a), is significantly longer than the Pd1–N2b of 2.001(6) Å where
the N atom is trans to the iminic N atom (Na). The same trend is also
observed for the other Pd atom (Pd2). The bond distances and angles
involving the chelating bzan moiety in 2 compare well with those
found in 1 and related compounds [9,16].
A cooperation of intermolecular C–H⋅⋅⋅π interactions is responsi-
ble for the stabilization of the crystal structure of 1 (Fig. 3). The iminic
C7 atom forms a close contact with an aromatic ring Cg(2) (the ring
centroid defined by C1⋅⋅⋅C6 atoms) of an adjacent molecule (C7⋅⋅⋅Cg
(2) 3.753(5) Å, H7⋅⋅⋅Cg(2) 3.09 Å, C7–H7⋅⋅⋅Cg(2) 129.5°, symmetry
code 1−x,−y,1−z). The H5 atom of the aromatic C5 atom takes part
in an intermolecular C–H⋅⋅⋅π interaction involving the aromatic ring
Cg(3) (defined by C8⋅⋅⋅C13 atoms: C5⋅⋅⋅Cg(3) 3.716(7) Å, H5⋅⋅⋅Cg(3)
3.08 Å, C5–H5⋅⋅⋅Cg(3) 126.9°, symmetry code ½−x,−½+y, ½−z).
The most remarkable structural feature in the crystal packing of
1 is that the H4 and H11 atoms of an molecule point towards the
centre of the palladium chelate ring Cg(1) of two neighboring species
Fig. 1. ORTEP drawing with labeling scheme for [Pd(bzan)(μ-N3)]2 (1). Displacement
ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and bond
angles (º) are: Pd–N 2.043(9), Pd–C9 1.986(5), N–C7 1.272(6), C1–C2 1.387(8), C1–C6
1.381(8), C2–C3 1.383(8), C3–C4 1.374(8), C4–C5 1.360(9), C5–C6 1.376(8), C7–C8
1.441(7), C8–C9 1.405(7), C8–C13 1.384(7), C9–C10 1.380(7), C10–C11 1.385(8), C11–
C12 1.374(8), C12–C13 1.377(7), Pd–N1 2.067(4), Pd–N1* 2.174(4), N1–N2 1.215(7),
N2–N3 1.143(8), N–Pd–N1 179.6(2), N–Pd–C9 81.1(2), N1–Pd–C9 173.1(2), Pd–N–C1
124.6(3), Pd–N–C7 113.8(3), Pd–N1–N2 121.1(3), N1–N2–N3 176.8(5). * Symmetry
code: −x, −y, −z.

A.M. Santana et al. / Inorganic Chemistry Communications 14 (2011) 83–86
85
Fig. 3. A view of C–H⋅⋅π interactions in 1 built up by symmetry operations ½−x, −½+y, ½−z, ½−x, −½+y, ½−z, 1−x, −y, 1−z, 3/2−x, ½+y, ½−z. Key geometrical parameters:
H10⋅⋅⋅N2 2.57 Å, C10⋅⋅⋅N2 3.240(7) Å, C10–H10⋅⋅⋅N2 129.0°, H4⋅⋅⋅Cg(1) 3.04 Å, C4⋅⋅⋅Cg(1) 3.448(6) Å, C4–H4⋅⋅⋅Cg(1) 108.4°, H(5)⋅⋅⋅Cg(3) 3.08 Å, C(5)⋅⋅⋅Cg(3) 3.716(7) Å, C5–
H5⋅⋅⋅Cg(3) 126.9°, H7⋅⋅⋅Cg(2) 3.09 Å, C7⋅⋅⋅Cg(2) 3.753(5) Å, C(7)–H(7)⋅⋅⋅Cg(2) 129.5 °, H(11)⋅⋅⋅Cg(1) 3.11 Å , C(11)⋅⋅⋅Cg(1) 3.652(6) Å, C(11)–H(11)⋅⋅⋅Cg(1) 108.4°.
Fig. 4. A view of C–H⋅⋅π interactions in 2 built up by symmetry built up by inversion symmetry operation. Key geometrical parameters: H(4b)⋅⋅⋅Cg(3a) 2.84 Å, C(4b)⋅⋅⋅Cg(3a) 3.629
(1) Å, C(4b)–H(4b)⋅⋅⋅Cg(3a) 142 °, H(11a)⋅⋅⋅Cg(4b) 2.79 Å, C(11a)⋅⋅⋅Cg(4b) 3.567(9) Å, C(11a)–H(11a)⋅⋅⋅Cg(4b) 141°.

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A.M. Santana et al. / Inorganic Chemistry Communications 14 (2011) 83–86
(c) A.V. Khripun, V.Y. Kukushkin, G.I. Koldobskii, M. Haukka, Inorg. Chem.
Commun. 10 (2007) 250–254;
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J.G. Ferreira, R.H.A. Santos, Inorg. Chem. Commun. 9 (2006) 493–496.
[3] G.R. Desiraju, T.R. Steiner, The Weak Hydrogen Bond in Structural Chemistry and
Biology, Oxford University Press, Oxford, 1999.
(C4⋅⋅⋅Cg(1) 3.448(6) Å, H4⋅⋅⋅Cg(1) 3.04 Å, C4–H4⋅⋅⋅Cg(1) 108.4°,
symmetry code ½−x,−½+y, ½−z; C11⋅⋅⋅Cg(1) 3.652(6) Å, H11⋅⋅⋅Cg
(1) 3.11 Å, C11–H11⋅⋅⋅Cg(1) 108.4°, symmetry code 3/2−x, ½+y,
½−z). Many authors have considered the existence of C–H⋅⋅⋅π(metal
chelate ring) in structures whose H⋅⋅⋅Cg distances are b3.0 Å [5].
However, according to DFT calculations reported by Zaric et al. [18],
the interaction energy is greater than 1.5 kcal/mol for the range of
2.4–2.9 Å, while it is greater than 1 kcal/mol to H⋅⋅⋅Cg distances up to
3.4 Å, indicating that attractive interaction is expected even at the
H⋅⋅⋅Cg distances longer than 3.0 Å. Therefore, the intermolecular
C–H⋅⋅⋅π (Pd chelate ring) in 1 may be thought to be a structural
evidence for the metalloaromaticity of the cyclopalladated ring.
As can be observed in the crystal packing of 2 (Fig. 4), each
molecule of [Pd(C2,N-bzan)(μ-Pz)]2 (2) is assembled by C–H⋅⋅⋅π
interactions involving the C4b-H4b donor group and the phenyl ring
Cg(3a) (the ring centroid defined by C8a⋅⋅⋅C13a atoms; C4b⋅⋅⋅Cg(3a)
3.629(1) Å, H4b⋅⋅⋅Cg(3a) 2.84 Å, C4b–H4b⋅⋅⋅Cg(3a) 142°, symmetry
code 3/2−x, ½+y, ½−z). On the other hand, the C11a–H11a moiety
interacts with the pyrazolate aromatic ring Cg(4b) (defined by
N1b⋅⋅⋅C16b atoms: C11a⋅⋅⋅Cg(4b) 3.567(9) Å, H11a⋅⋅⋅Cg(4b) 2.79 Å,
C11a–H11a⋅⋅⋅Cg(4b) 141°, symmetry code 3/2−x, ½+y, ½−z).
In conclusion, this study showed that the cooperative effect of
C–H⋅⋅⋅π interactions plays a pivotal role for directing self-assembly
of organopalladated molecules 1 and 2 during the crystallization. At
this point, it seems important to point out that the manifestation of
the C–H⋅⋅⋅π(metal chelate ring) interactions in these complexes
appears to be sensitive to the nature of the central core. Systematic
solid state studies on C–H⋅⋅⋅π hydrogen bonds in other cyclopalla-
dated compounds are currently underway in order to investigate
the ability of cyclopalladated chelate ring to act as π acceptor
towards CH acids and their potential utility in crystal engineering.
[4] M. Nishio, M. Hirota, Y. Umezawa, The CH⋅⋅⋅π Interaction, Evidence, Nature and
Consequences, Wiley-VCH, New York, 1998.
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J. Niclós-Gutiérrez, Inorg. Chem. 41 (2002) 6956–6958;
(c) Z.D. Tomić, V.M. Leovac, S.V. Pokorni, D. Zobel, S.D. Zaric, Eur. J. Inorg. Chem.
(2003) 1222–1226;
(d) Z.D. Tomić, S.B. Novaković, S.D. Zarić, Eur. J. Inorg. Chem. (2004) 2215–2218.
[6] H. Masui, Coord. Chem. Rev. 219 (2001) 957–992.
[7] E.T. de Almeida, A.E. Mauro, A.M. Santana, S.R. Ananias, A.V.G. Netto, J.G. Ferreira,
R.H.A. Santos, Inorg. Chem. Commun. 10 (2007) 1394–1398.
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(1987) 1273–1277;
(b) A.V.G. Netto, R.C.G. Frem, A.E. Mauro, Mol. Cryst. Liq. Cryst. 374 (2002) 255–260;
(c) A.V.G. Netto, R.C.G. Frem, A.E. Mauro, R.H.A. Santos, J.R. Zoia, Trans. Metal
Chem. 27 (2002) 279–283;
(d) K. Tomita, A.C.F. Caíres, V.A.D. Neto, A.E. Mauro, Acta Cryst. C 50 (1994) 1872–1873;
(e) A.E. Mauro, S.H. Pulcinelli, R.H.A. Santos, M.T.P. Gambardella, Polyhedron 11
(1992) 799–803;
(f) J. Zukerman Schpector, E.E. Castellano, A.E. Mauro, Acta Cryst. C 42 (1986)
302–303.
[9] A.M. Santana, A.E. Mauro, E.T. de Almeida, A.V.G. Netto, S.I. Klein, R.H.A. Santos, J.R.
Zóia, J. Coord. Chem. 53 (2001) 163–172.
[10] S. Trofimenko, Inorg. Chem. 10 (1971) 1372–1376.
[11] E.C. Alyea, S.A. Dias, F. Bonati, Trans. Metal Chem. 6 (1981) 24–27.
[12] Complex 2 was synthesized by the dropwise addition of a solution containing
0.025 g (0.37 mmol) of pyrazole and 0.5 mL (3.6 mmol) of N, N, N-triethylamine
to an acetone suspension containing 0.1000 g (0.15 mmol) of [Pd(bzan)(μ-N3)]2.
The resulting solution was stirred for 1 h, the solvent was removed under reduced
pressure and the yellow solid was washed with pentane and dried in vacuum.
Yield: 95%. C32H26N6Pd2 (707.44): calcd. C 54.32, H 3.71, N 11.88; found C 54.69, H
3.80, N 11.62.
[13] Crystal Date were collected on a Enraf–Nonius CAD4 difractometer at room
temperature using KαMo radiation (λ=0.71073 Å). The structures were solved
by direct methods and refined by full-matrix least squares on F2 using SHELXS-97
software. All non-hydrogen atoms were refined anisotropically. The structural
analysis was performed by PLATON program. Crystallographic data for 1:
monoclinic, space group P21/n, a=10.5695(6) Å b=10.1215(6) Å, c=11.9116
(7) Å, Z=2, α=γ=90° β=103.462(5)° V=1239.3(1) Å3, Dc =1.761 g/cm3,
μ=1.483 mm−1, crystal dimension: 0.15 · 0.10 · 0.10 mm, of 3781 reflections
collected, 3606 were independent [Rint =0.046]. The R values are R1 =0.041, and
wR2 =0.119 [IN2σ(I)] and GOF=0.97, max/min residual electron density: 0.138/
−0.993 e/Å3. For 2: monoclinic, space group P21/n, a=12.234(1) Å, b=11.7350
(7) Å, c=20.588(2) Å, Z=4, α=γ=90° β=100.636(8)° V=2905.0(4) Å3,
Dc =1.617 g/cm3, μ=1.27 mm−1, crystal dimension 0.10 · 0.10 · 0.05 mm, of
8880 reflections collected, 8444 were independent [Rint =0.050]. The R values are
R1 =0.050 and wR2 =0.156 [IN2σ(I)] and GOF=0.93; max/min residual electron
density: 0.572/−0.669 e/Å3. Crystallographic data have been deposited with the
Cambridge Crystallographic Data Center, Summary of Data CCDC Nos.
737228/737229.
Acknowledgements
The authors wish to acknowledge CNPq, FAPESP and CAPES for
financial support.
Appendix A
Additional materials, consisting of atomic coordinates and equivalent
isotropic displacement parameters for non-hydrogen atoms, H-atom
coordinates and isotropic displacement parameters, interatomic bond
lengths and bond angles have been deposited with the Cambridge
Crystallographic Data Centre, CCDC, no. 737228 (complex 1), and 737229
(complex 2). Copies free of charge of this information may be obtained
free of charge from de Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44 1223 336 033; email:deposit@ccdc.cam.ac.uk).
[14] NMR data (ppm) given as δ (multiplicity, J, [integration], assignment): 1H NMR
(500 MHz, CDCl3): 8.14 (s, [1H], HC=Nbzan), 8.11 (s, [1H], HC=Nbzan), 7.40 (d, 2 Hz,
[2H], H5Pz), 7.24–7.05 (m, [18H], CHbzan), 6.55 (d, 2 Hz, [2H], H3Pz), 5.83 ppm (t, 2 Hz,
[2H], H4Pz). 13C{1H} NMR (125 MHz, CDCl3): 175.6 (HC=Nbzan), 148.9 (Cbzan–Pd),
138.4 (C5Pz), 137.5 (C3Pz), 135.4–122.5 (Carom), 104.4 ppm (C4Pz).
[15] L. Pauling, The Nature of Chemical Bond, Cornell University Press, NY, 1960.
[16] A.M. Santana, A.V.G. Netto, E.T. de Almeida, A.E. Mauro, J. Souza Jr., R.H.A. Santos, Z.
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