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CH/ p interactions of p -system of acetylacetonato chelatering:
Comparison of CH/ p interactions of Ni(II)-acetylacetonato
chelate and benzene rings
Milos K. Milc ic, Vesna B. Medakovic , Snezana D. Zaric *
Department of Chemistry, University of Belgrade, Studentski trg
16, P.O. Box 158, 11001 Belgrade, Serbia and Montenegro
Received 17 February 2006; received in revised form 10 May 2006;
accepted 13 June 2006Available online 22 June 2006
Abstract
The calculations have been done for CH/ p interaction with p
-system of Ni(II)-acetylacetonato chelate ring. The results show
thatthere is an attractive electrostatic interaction, while
dispersion component is a major source of attractive interacting
energies. The inter-action was compared with CH/ p interaction
between two benzene rings. The comparison shows that two
interactions are quite similar,enabling to estimate the energy of
CH/ p interaction with p -system of Ni(II)-acetylacetonato chelate
ring to be about 10.5 kJ/mol. Theresults indicate that CH/ p
interactions of chelate ring in various molecular systems can be as
important as CH/ p interactions of phenylring. 2006 Elsevier B.V.
All rights reserved.
Keywords: CH/ p interaction; DFT; MP2; Chelate
1. Introduction
There is intensive research of noncovalent interactionsinvolving
p -systems [142]. It was shown that these interac-tions are very
important for many biological molecules[14]. Analyzing CH/ p
interactions in proteins shows thatthese interactions play an
important role in the stabilityof proteins [1].
Transition metal complexes can be involved in XH/ pinteractions
in two ways: as hydrogen atom donor or
hydrogen atom acceptor [2339]. In the rst case, par-tially
positive hydrogen atoms of coordinated ligandscan interact with
aromatic ring. This type of interactionis also called metal ligand
aromatic cation- p (MLAC p )interactions [2933]. In the second
case, there is an inter-action of CH bond with p -systems of planar
chelaterings with delocalized bonds [3439]. The capability of
p -systems of chelate rings to be involved in
noncovalentinteractions in a similar way as aromatic rings could
beconnected with metalloaromaticity of chelate rings [34 42].
There are many unsolved questions about noncovalentinteractions
of chelate rings and about metalloaromatic-ity of chelate rings.
Recent theoretical study of metallo-benzene complexes, analogues of
benzene where oneCH unit has been replaced by an organometallic
frag-ment, showed that they may be aromatic [43]. However,
there is still very little known about the aromaticity of
chelate rings with heteroatoms. One of these chelate ringsthat show
some of the aromatic properties is acetylaceto-nato chelate ring
[44]. In order to elucidate noncovalentinteractions of potentially
aromatic chelate rings, andsimilarity with interactions of organic
aromatic rings,here we present theoretical results about CH/ p
interac-tion of p -system of Ni(II)-acetylacetonato chelate ringand
comparison with CH/ p interaction of p -systems of benzene.
Electrostatic and dispersion component of CH/ p interactions of
Ni(II)-acetylacetonato chelate ring
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights
reserved.
doi:10.1016/j.ica.2006.06.022
* Corresponding author. Tel.: +381 11 3282 111; fax: +381 11 184
330.E-mail addresses: [email protected] ,
[email protected]
(S.D. Zaric ).
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44274430
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have been estimated for the rst time. Moreover, a com-parison
with very accurate calculations on CH/ p interac-tions in benzene
dimer enabled to estimate interactingenergies.
2. Computational methods
All ab initio and DFT energy calculations were carriedout using
the GAUSSIAN 98 program [45]. The model systemfor calculations of
CH/ p interactions of benzene withp -system of acetylacetonato
chelate ring ( Fig. 1a) was madefrom XIBRUV crystal structure [46],
which was taken fromCSD [47].
Since crystal structures sometimes display
unrelaxedintramolecular geometries yielding distorted wave
func-tions and wrong energies, we rst optimized the
crystalcoordinates of the individual CH/ p partners. In the caseof
benzenebenzene interactions, we optimized benzenemolecule in D 6 h
geometry. All optimizations have beendone using DFT, specically
Becke three-parameterexchange functional (B3) [48] and the
LeeYangParr cor-relation functional (LYP) [49], level of theory.
TheLANL2DZ basis set was chosen for nickel atom and the6-31G **
basis set was chosen for carbon, oxygen, nitrogenand hydrogen
atoms. These optimized structures for bothmodel systems were then
reassembled into geometries opti-mal for CH/ p interactions ( Fig.
1).
MP2 single point calculations, with the same basis set,were done
using B3LYP optimized structures. The interac-tion energy D E is
dened as the difference between theenergy of the complexes AB and
the energy of the isolated
partners, i.e. D E = E (A B ) E (A) E (B ). The
standardcounterpoise method was applied to correct MP2 calcu-lated
interaction energies for the basis set superpositionerror (BSSE)
[50].
3. Results and discussion
The calculations have been done for CH/ p interaction of benzene
molecule with acetylacetonato chelate ring of Ni(II) complex ( Fig.
1a). Ni(II)-acetylacetonato chelatering was a part of the model
system made from XIBRUV
crystal structure. Single point calculations were done
fordistances from 2.1 to 3.5 A between center of the ring ( X )and
hydrogen atom from benzene molecule ( Fig. 2). Forcomparison,
calculations with the same methods have alsobeen done for CH/ p
interaction in T-shape benzene dimer(Figs. 1b and 2).
For the calculations, B3LYP and MP2 methods wereused. Because of
large basis set superposition error(BSSE) in case of MP2 method,
only MP2 data correctedfor BSSE are presented. B3LYP method was
used as themost popular DFT method, MP2 was used since
previouscalculations on benzene dimer showed that MP2 methodis a
reasonably good method for calculating the geometryof T-shape
benzene with CH/ p interactions [1922]. Also,comparison of B3LYP
and MP2 methods enables tomake a conclusions about the nature of
the CH/ p interac-tion of chelate ring. Namely, B3LYP method can
well rec-ognize electrostatic component but not
dispersioncomponent, while MP2 method can well recognize
disper-sion component.
The interacting energies with B3LYP method for ben-zenechelate
and benzenebenzene are quite similar, atthe minima the energies are
2.86 and 2.69 kJ/mol,respectively (Table 1 , Fig. 2). Negative
energies obtainedfor B3LYP method show that there is an attractive
electro-
static interaction for both Ni(II)-acetylacetonato chelate(Fig.
1a) and benzene ring ( Fig. 1b). Results for benzeneare in
agreement with previous results that show thatT-shaped benzene
dimer is stabilized by the attractive elec-
Fig. 1. Model systems for calculations of CH/ p interaction (a)
of benzenering with Ni(II)-acetylacetonato chelate ring of complex
made from thecrystal structure XIBRUV [46] and (b) between two
benzene molecules in
T-shape position.
-8
-6
-4
-2
0
2
4
6
8
10
12
2.1 2.3 2. 5 2.7 2.9 3.1 3.3 3.5 3.7
Distance ()
E i (
k J / m o l )
B3LYP (Bz-Bz)
B3LYP (Bz-Ch)
MP2 (Bz-Bz)
MP2 (Bz-Ch)
Fig. 2. Diagram showing the results of single point energy
calculations of CH/ p interactions for different H X distances for
benzenebenzene (Bz
Bz) and benzenechelate (BzCh) ( Fig. 1).
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trostatic interaction [22]. Somewhat larger B3LYP energyfor
Ni(II)-acetylacetonato chelate ring shows that the elec-trostatic
components are stronger for the interactions of
Ni(II)-acetylacetonato chelate ring.
The interacting energies with MP2 method for benzene chelate and
benzenebenzene are again very similar; at theminima the energies
are 6.78 and 6.60 kJ/mol, respec-tively (Table 1 , Fig. 2). The
origin of the larger energyfor chelate ring is stronger
electrostatic component, aswas shown in B3LYP calculations.
A comparison of the energies obtained with MP2 andB3LYP methods
shows that B3LYP method underesti-mates interactions, as was
anticipated since DFT methodsare not able to describe dispersion
component of the inter-actions. In both cases, calculated B3LYP
interactingenergy is by 3.9 kJ/mol smaller than MP2 energy (
Table1). It indicates that dispersion component is a major
com-ponent in interacting energy and that CH/ p interaction of
Ni(II)-acetylacetonato chelate ring has almost the samecontribution
of dispersion component as CH/ p interactionof benzene. Again, the
results for benzene are in agreementwith previous results that show
that for T-shaped benzene
the major source of attraction is dispersion interaction
[22].For both methods the minima are deeper for benzene chelate
interaction, and whole curves are at somewhatlower energy (Fig. 2).
At the same time, the minima of the curves are at shorter distances
for benzenechelate thanfor benzene dimer interaction. For MP2
method the min-ima are at 2.6 and 2.7 A , while for B3LYP method
theyare at 2.8 and 2.9 A , respectively (Table 1 ). Both
energiesand distances are indicating that CH/ p interaction of
Ni(II)-acetylacetonato chelate ring in the model system of XIBRUV
crystal structure ( Fig. 1a) is somewhat strongerthan CH/ p
interaction of benzene ring ( Fig. 1b).
The results, for the rst time, show that CH/ p interac-tions of
benzene ring and Ni(II)-acetylacetonato chelatering are quite
similar ( Fig. 2). Previous, very accurate cal-culations on
T-shaped benzene dimer show that the inter-acting energy is about
10.0 kJ/mol [1922]. Based onthat, we can estimate that CH/ p
interaction with Ni(II)-acetylacetonato chelate ring in our model
system(Fig. 1a) is about 10.5 kJ/mol.
It was shown that CH/ p interactions of phenyl rings arevery
important in different molecular systems, especially inbiomolecules
[14]. Because of similar interacting energiesof p -system of
acetylacetonato chelate and phenyl rings,CH/ p interactions of
chelate rings can have similar impor-
tance in molecular systems with transition metal com-
plexes. These interactions can inuence the interactionsof metal
complexes in solution and other environments.
Acknowledgement
The authors would like to thank Prof. M. B. Hall for the
support. This work was supported by the Serbian Ministryof
Science (Grant 142037).
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Table 1Interaction energies at the minima for benzenechelate and
benzene benzene using MP2 and B3LYP methods
Benzenechelate Benzenebenzene
E i (kJ/mol) Distance (A ) E i (kJ/mol) Distance (A )
B3LYP 2.86 2.8 2.69 2.9
MP2
6.78 2.6
6.60 2.7
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