Investigation of Geometrical and Conformational Isomers of Cobalt(II) Bis(diphenylacetato)-bis(γ-picolinate) by Dynamic NMR Spectroscopy and Molecular Mechanics

Article abstract of DOI:10.1023/A:1009545026244

Cobalt(II) bis(diphenylacetato)-bis(γ-picolinate) is synthesized and its geometrical and conformational isomers are investigated using dynamic NMR and molecular mechanics methods. The complex in the solution is shown to exist as cis and trans isomers, which are detected at 263 K in the course of a slow intermolecular ligand exchange. At T < 223 K, the intramolecular rotation of the phenyl radicals about the ^-O₂C-CH(Ph)₂ and ^-O₂CCH(Ph)-Ph bonds slows, which results in the formation of different conformers. The energies of the retarded intermolecular exchange and of the retarded rotation of the phenyl radicals were calculated from ¹H NMR and molecular mechanics data.

Full text of DOI:10.1023/A:1009545026244

Russian Journal of Coordination Chemistry, Vol. 27, No. 1, 2001, pp. 57–60. Translated from Koordinatsionnaya Khimiya, Vol. 27, No. 1, 2001, pp. 64–67.
Original Russian Text Copyright © 2001 by Novitskii, Chekal, Gulya.
Investigation of Geometrical and Conformational Isomers
of Cobalt(II) Bis(diphenylacetato)-bis(g-picolinate) by Dynamic
NMR Spectroscopy and Molecular Mechanics
G. V. Novitskii1, Al. Chekal2, and A. P. Gulya1
1Moldova State University, ul. Matteevicha 60, Chisinau, 277009 Moldova
2Al. I. Kuza University, Jassy, Romania
Received December 30, 1999
Abstract—Cobalt(II) bis(diphenylacetato)-bis(γ-picolinate) is synthesized and its geometrical and conforma-
tional isomers are investigated using dynamic NMR and molecular mechanics methods. The complex in the
solution is shown to exist as cis and trans isomers, which are detected at 263 K in the course of a slow intermo-
–
lecular ligand exchange. At T < 223 K, the intramolecular rotation of the phenyl radicals about the O2C–
CH(Ph)2 and –O2CCH(Ph)–Ph bonds slows, which results in the formation of different conformers. The energies
of the retarded intermolecular exchange and of the retarded rotation of the phenyl radicals were calculated from
1H NMR and molecular mechanics data.
Dynamic NMR spectroscopy is successfully ligand produces significant steric hindrances.
employed in stereochemical studies of labile cobalt(II)
3
4
complexes in solutions. It was established [1–4] that in
isotropic paramagnetic shifts of cobalt(II) coordination
compounds, the predominating pseudocontact (dipole)
component is specified by the coordinates of the reso-
nant nucleus and the g factor of the complex.
2
CH3
γ
O
1
C C
H
β
HO
α
N
It is worth noting that the NMR spectroscopic
method can be used for studying paramagnetic com-
plexes, especially geometrical and conformational iso-
mers [5], provided that the spin-electron relaxation in
these systems varies within 10–9–10–11 s [6]. Earlier [7],
we investigated the geometrical and conformational
isomerism of adducts of cobalt(II) mono- and dime-
thylacetate with γ-picoline by dynamic NMR spectro-
scopy. It was established that the signals registered
under slow (in terms of the NMR time scale) intramo-
lecular ligand exchange are due to the cis, trans isomers
and to different conformers. The latter, however, appear
(γ-Pic)
(HDfa)
EXPERIMENTAL
Magnetic measurements were carried out using the
Gouy method; Hg[Co(CNS)4] was used as the standard
for calibration.
The IR absorption spectra were taken on a Specord
M-80 spectrophotometer with the samples suspended
in mineral oil.
only upon the retarded rotation about the Csp2 –Csp3
1
The H NMR spectra of the complex dissolved in
bond:
CDCl3 were recorded on a Bruker WP-80 instrument
equipped with a device for low-temperature measure-
ments. Tetramethylsilane was used as the internal stan-
dard.
R
O
C(sp2)
C(sp3)
(R = H, CH3).
CH3
–O
H
Synthesis of [Co(γ-Pic)2(Dfa)2] (I). Cobalt(II) ace-
tate (2.5 g, 0.01 mol) was dissolved in 70 ml of meth-
anol. Sodium diphenylacetate (4.68 g, 0.02 mol) was
then added with heating and stirring. The mixture was
allowed to stand for 24 h; the precipitate was filtered off
Analogous conformers were also detected for the cis
isomers of adducts of Co(II) adamantanate [8] and
Co(II) trifluoroacetate [9] with γ-picoline.
This work was undertaken to study the conforma- and dried at 140°ë. The obtained lilac-colored sub-
tional transformations occurring in the Co(II) diphenyl- stance was dissolved in chloroform containing γ-
acetate complex with γ-picoline, in which the Dfa– picoline. After the solvent was partially evaporated, the
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58
NOVITSKII et al.
γ-CH
3
β-H
(a)
α-H
cis
trans
(b)
20
10
α-H
CH
trans
trans cis
cis
200
160
120
80
40
0
–40
δ, ppm
(c)
50
30
10 δ, ppm
1
Fig. 1. H NMR spectra of [Co(γ-Pic) (Dfa) ] in CDCl at (a) T = 300, (b) T = 233, and (c) T = 213 K.
2
2
3
title compound precipitated as pink crystals readily sol- of bands of the stretching vibrations of the carboxyl
uble in organic solvents.
According to elemental analysis data, the composi-
tion of the compound is [Co(γ-Pic)2(Dfa)2].
group (νas(COO) = 1601 and νs(COO) = 1403 cm–1) are
evidence of its bidentate coordination [11].
1
The H NMR spectrum taken at room temperature
(Fig. 1a) shows six signals with chemical shifts of 90.0,
25.5, 20.4, 10.8, 9.4, and –5.4 ppm and an intensity of
ratio 2 : 2 : 1 : 8 : 2 : 3, indicating that the complex com-
position remains unchanged in a solution. The signals
at 90.0, 25.5, and –5.4 ppm are assigned to the protons
of the respective α-CH, β-CH, and γ-CH3 groups of the
coordinated γ-picoline [6–8]. The other signals corre-
spond to the protons in positions 1, 2(2') + 3(3'), and 4
in the diphenylacetate moiety.
For CoC40H36N2O4
anal. calcd. (%): Co, 8.85;, C, 71.96; H, 5.40; N, 4.20.
Found (%):
Co, 8.80;, C, 72.01; H, 5.44; N, 4.23.
RESULTS AND DISCUSSION
The magnetic measurements of the obtained com-
pound gave µeff = 4.97 µB, which suggests an octahedral
environment and a high-spin state of cobalt(II) in struc-
ture I [10].
Three regions of intermolecular and intramolecular
ligand exchange in solution I can be identified on the
The IR spectrum of compound I contains bands of temperature dependence of the dynamic NMR parame-
stretching vibrations ν(C=N) = 1632 and ν(C=C) = ters: 298–273 K (rapid exchange), 273–253 K (average
1520 cm–1 of the coordinated γ-picoline. The positions exchange), and the region below 253 K (slow
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 1
2001

INVESTIGATION OF GEOMETRICAL AND CONFORMATIONAL ISOMERS
59
exchange) (Figs. 1a, 1b). In the slow exchange region,
E, kkal/mol
30
signals from α-CH, β-CH, and γ-CH3 groups split into
two equal-intensity signals, which we assigned to cis
and trans isomers [6–8].
(a)
20
10
γ-Pic
γ-Pic
γ-Pic
γ-Pic
0
The 1H NMR spectrum of the diphenylacetate moi-
ety at 233 K (Fig. 1b) exhibits 12 signals of different
intensity. With a decrease in temperature to 213 K
(Fig. 1c), the number of signals increased even more,
while the number of lines corresponding to γ-picoline
moiety remained the same. This fact may be explained,
as in [7], by the presence of conformers due to magnet-
ically inequivalent positions occupied by two phenyl
radicals relative to the anisotropic dipole surfaces of a
central atom during retarded rotation about the bond.
0
100
200
300
400
α, deg
E, kkal/mol
30
(b)
20
10
Ph
Ph
–O
O
C
C
0
H
0
100
200
300
400
β, deg
To establish the details of the conformational pro-
cesses occurring as a result of the change in the geom-
etry of the acido ligand, we estimated the energy of the
retarded rotation around the bonds:
Fig. 2. Calculated energies of (a) retarded rotation 1 and (b)
retarded rotation 2 vs. the angles of internal rotation.
–O2C
CH(Ph)2
–O2C
CH(Ph)
Ph
and
α1
β5
β1
O
H
α2
(Rotation 1)
(Rotation 2)
H
α5
β4
Ph
β2
Calculations were carried out by the molecular
mechanics technique using the MMC program with
standard parametrization [12–14].
Ph
CO
Ph
α4
O
α3
β3
The data obtained show that the most stable confor-
mations
can be realized at the following torsion angles:
Angle
α1
α2
α3
α4
α5
β1
β2
β3
β4
β5
12.82
αi(βi), deg
44.93
109.87
24.87
112.73
67.60
93.93
86.63
47.02
120.16
The plots of the potential energy vs. the angle of the cis–trans transition and the conformational isomer-
internal rotation about the above-mentioned bonds are ization of compound I using dynamic NMR data.
shown in Fig. 2. For rotation 2, the potential energy
curve exhibits two maxima with nearly equal energy
(26.37 and 29.17 kkal/mol) and two minima, while the
curve for rotation 1 contains three maxima and three
minima (Fig. 2).
The calculations were performed by the Eyring
equation at the coalescence point [15, 16]:
∆G#T = 19.14Tc[9.97 + log(Tc/∆δ)](J/mol),
where Tc is the coalescence temperature and ∆δ is the
To correlate the experimental and calculated data difference between the chemical shifts of the signals
(assuming ∆S# ~ 0), we estimated the Gibbs energies for from the two isomers. The coalescence point for sepa-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 1
2001

60
NOVITSKII et al.
Intensity, relative units
(and below) that the conditions for retarded rotation are
reached for conformers separated by a barrier of
10 kcal/mol (Fig. 2b). In other words, while the phenyl
radical in conformer C retards its rotation, in conform-
ers A and B, the same radical continues to rotate.
0.8
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Ph
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O
O
O
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H
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O
Ph
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(B)
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conformers has not yet been reached; it is only at 223 K
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 1
2001