O-Semiquinonic PCP-pincer nickel complexes with alkyl substituents: Versatile coordination sphere dynamics

Article abstract of DOI:10.1039/b801350k

o-Semiquinonic nickel pincer complexes (R₂PCP)Ni(SQ) show a versatile coordination sphere dynamics via swing or fan oscillations depending on the steric properties of the phosphorus substituents. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801350k

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o-Semiquinonic PCP-pincer nickel complexes with alkyl substituents: versatile
coordination sphere dynamics†
Konstantin A. Kozhanov,*a Michael P. Bubnov,a Vladimir K. Cherkasov,a Nina N. Vavilina,a
Larisa Yu. Efremova,a Oleg. I. Artyushin,b Irina L. Odinetsb and Gleb A. Abakumova
Received 24th January 2008, Accepted 10th March 2008
First published as an Advance Article on the web 7th April 2008
DOI: 10.1039/b801350k
o-Semiquinonic nickel pincer complexes (R2PCP)Ni(SQ) show a versatile coordination sphere dynamics
via “swing” or “fan” oscillations depending on the steric properties of the phosphorus substituents.
Introduction
Experimental
Pincer complexes have been of a great interest in recent decades.
Being fine catalysts of certain reactions they have an excellent
chemical stability.1
Previously we have reported a number of spin-labeled o-
semiquinonic nickel complexes with diphenylphosphine pincer
ligands.2 An unusual structural isomerisation of these complexes
via unique “swing” oscillation has been reported (Scheme 1).2
Materials and methods
Starting materials are commercially available (Aldrich, Dalchem,
Strem) and were used without purification unless otherwise noted.
The quinones and 1,5-bis(bromomethyl)-2,4-dimethylbenzene
were obtained according to known procedures.3,4 (DME)NiBr2
was prepared by reaction of Ni powder with Br2 in
dimethoxyethane.5 All synthetic procedures were carried out in
evacuated ampoules using freshly purified solvents6 (except 95%
aqueous ethanol which was used as received).
1H and 31P NMR spectra were obtained on a “Bruker” DPX-
200 spectrometer (200 MHz, CDCl3, d/ppm). EPR spectra were
recorded on Bruker EMX spectrometer, operating in X-band
(working frequency ∼9.6 GHz) and equipped with an NMR-
gaussmeter and variable-temperature unit. HFC constants and g-
factor values were obtained by simulation in WINEPR SimFonia
(v. 1.25).
Scheme 1 Structural isomerism of o-semiquinonic PCP-pincer nickel
complexes.
Since the coordination sphere rearrangements play the certain
role in the catalytic processes, the studies of the factors having an
effect on the coordination sphere dynamics deserve consideration.
In present work we have obtained and thoroughly investigated a
number of new o-semiquinonic pincer complexes (R2PCP)Ni(SQ),
where R = t-Bu, i-Pr or cyclo-C6H11 (Cy), and SQ = 3,6- or 3,5-
di-tert-butyl-o-benzosemiquinone.
Molecular modelling was provided using “Hyperchem 8.0”
software. The molecular mechanics “MM+” method with “bonds
dipoles” options and all of force field components using the
“Fletcher–Reeves” algorithm was applied for geometry optimiza-
tion.
Synthetic procedures
2,6-Bis(dialkylphosphinomethyl)arylbromonickel
complexes
7
(R2PCP)NiBr were obtained by metallation using (DME)NiBr2
of the corresponding PCP-ligands. The latter were in turn
obtained by dialkylphosphine addition to the corresponding
2,6-bis(bromomethyl)arene followed by dehydrobromination
of the intermediate phosphonium salt formed.7 Thallium o-
benzosemiquinonates Tl(SQ) were obtained by reaction of the
corresponding o-benzoquinones with the thallium amalgam8 and
were used in situ.
aG. A. Razuvaev Institute of Organometallic Chemistry of Russian
Academy of Sciences, Tropinina str., 49, Nizhny Novgorod, 603950, Russia.
E-mail: kostik@iomc.ras.ru; Fax: +7(831)4627497; Tel: +7(831)4627682
o-Semiquinonic nickel pincer complexes 1a–b, 2a–b, 3b
To THF solution of 1 eq. of (R2PCP)NiBr a solution of 1 eq. of
Tl(SQ) was added. The mixture was kept for 30 min, and THF was
replaced by toluene. After completion of the reaction (1–2 week,
monitored by EPR) the solution was filtered. Complex 1a was
isolated from hexane as greenish-brown needles (32% yield), while
other complexes were investigated without isolation.
bA. N. Nesmeyanov Institute of Organoelement Compounds of Russian
Academy of Sciences, Vavilova str. 28, 119991, Moscow, Russia. E-mail:
odinets@ineos.ac.ru; Fax: 7(495)1355085
† Electronic supplementary information (ESI) available: Full synthetic
procedures, NMR, EPR (including simulations) and molecular modeling
data and details of kinetic calculations. See DOI: 10.1039/b801350k
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Table 1 Parameters of EPR spectra for investigated complexes
Hyperfine coupling constants/G
31P
Complex
T/K
gi
SQ back proton
a
1a
1b
2a
2b
320
210
320
220
300
190
290
2.0071
2.0071
2.0054
2.0054
2.0055
2.0054
2.0054
2.0043
2.0054
2.0062
2.0060
2.0056
2.0052
2.0064
2.0041
aP1 = aP2 = 21.7
aP1 = 27.8, aP2 = 14.7
aP1 = aP2 = 17.9
aP1 = 22.7, aP2 = 13.4
aP1 = aP2 = 19.8
aP1 = 25.3, aP2 = 15.7
aP1 = aP2 = 16.6
aP1 = aP2 = 14.4
aP1 = 22.2, aP2 = 14.2
aP1 = aP2 = 17.0
aP1 = aP2 = 24.2
aP1 = aP2 = 25.6
aP1 = aP2 = 20.3
aP1 = aP2 = 18.4
aP1 = aP2 = 24.4
—
aH1 = 3.6, aH2 = 2.2
a
—
aH = 2.5
aH1 = aH2 = 2.8
aH1 = 3.8, aH2 = 2.2
aH = 2.7
200 (isomer 1)
200 (isomer 2)
300
290
180
aH = 2.2
aH = 2.5
3b
4a
aH = 2.5
aH1 = aH2 = 2.8
aH1 = 3.4, aH2 = 2.2
aH = 2.6
4b
330
200 (isomer 1)
200 (isomer 2)
aH = 2.4
aH = 3.4
a Unobserved due to broadened lines.
2.2 G) become averaged (aH1 = aH2 = 2.8 G) due to fast tautomeric
interconversion. In the case of 1a these protonic HFC constants
remain unchanged with temperature (210–280 K), but the phos-
phorus constants become averaged in the temperature range 210–
320 K. In general, the size of HFC constants of the magnetic
nucleus of the ligand (31P) bonded to the metal-o-semiquinone
fragment arises from two mechanisms:10 spin polarization of r-
bonds O–Ni–P and direct interaction of the Ni–P orbital with
p* orbital of the semiquinone containing unpaired electron.
The second path is more effective and in the case of pincer-o-
semiquinonato–nickel complexes it determines the phosphorus
splitting constants. As the consequence this value is very sensitive
to geometrical features and particularly to the angle between the
Ni–P axis and the o-semiquinonic plane11 (particularly to the nor-
mal to this plane). For complex 1a EPR structural perturbations
can be explained by “fan” oscillations (Scheme 2) without “swing”
oscillation (Scheme 1). In fact, different phosphorus splitting
constants are the consequence of different positions of phosphorus
nuclei relative to o-semiquinonic ring. Rapid “fan” oscillations
result in averaged phosphorus splitting constants. Decreasing of
temperature slows down interconversion and both phosphorus
nuclei can be observed separately.
Results and discussion
The reactivity of initial (R2PCP)NiBr towards Tl(SQ) is much
lower in comparison with the earlier studied (Ph2PCP)NiBr.2
Moreover, complex 3b is unstable§ and complex 3a is not formed.
This fact is in a good agreement with steric protection of the metal
centre.9
Surprisingly, the temperature dependence of the EPR spectrum
of 1a (Fig. 1, Table 1) drastically differs from that for 4a.2 So,
while at low temperatures 4a demonstrates hyperfine splitting on
two equivalent phosphorus nuclei (aP1 = aP2 = 25.6 G), for 1a
the phosphorus nuclei are not equivalent (aP1 = 27.8 G, aP2
=
14.7 G). Moreover, with increasing temperature, nonequivalent
semiquinonic proton HFC constants for 4a (aH1 = 3.4 G, aH2
=
Scheme 2 “Fan” oscillation in 1a.
The same is true for complex 1b. In its EPR spectrum at low
temperatures (Fig. 2, Table 1) only one signal with nonequivalent
phosphorus atoms is observed (aH = 2.5 G, aP1 = 22.7 G, aP2
=
13.4 G) whereas for its analog2 4b a superposition of two spectra
Fig. 1 EPR spectra of 1a.
is observed (aH = 2.4 G, aP1 = aP2 = 18.4 G and aH = 3.4 G, aP1
=
aP2 = 24.4 G) indicating the presence of two structural isomers. The
temperature dependence of the spectrum of 1b (Fig. 2) is similar
to that for 1a (Fig. 1).
§ Full decomposition of 3b occurred during two months after synthesis in
solution at r.t.
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Fig. 3 EPR spectrum of 2a.
protons (aH1 = 3.8 G, aH2 = 2.2 G) is observed. With an increase
in temperature the averaging of phosphorus constants in observed
(as for 1a), but the same process is observed also for the protonic
constants (as for 4a) indicating that both “swing” and “fan”
oscillations take place (Scheme 3).
Fig. 2 EPR spectrum of 1b.
It should be noted that the central component of the phosphorus
triplet in the high-temperature spectra of 1a and 1b is broadened.
Such broadening is caused by a fast interconversion process (on
the EPR time scale) and can be described by eqn (1) (with regard
to equal concentration of interconverting tautomers):12
DC = c es(dH)2/4
(1)
where c e is the electron magnetogyric ratio, s is the reduced lifetime
of exchanging particles, dH is root-mean-square field difference
between exchanging lines. In our case the outer components of
the phosphorus triplet are not broadened due to interconvertion,
so DC is the difference between line widths of the central and
the outer components. So it is possible to calculate the tautomer
lifetime (t = 2s with regard to equal life times of the interconverting
tautomers) at different temperatures and, correspondingly, activa-
tion parameters (DH‡ and DS‡) of the interconversion reaction
(eqn (2)):2
Scheme 3 “Swing” and “fan” motions for 2a (schematic view along the
basal O–Ni–C direction).
1/t = (kBT/h)exp(DS‡/R)exp(DH‡/RT)
(2)
where kB, h and R are the Boltzmann, Planck and absolute gas
constants, respectively. After linearization and linear regression
analysis (ESI†) the following values were obtained: for 1a: DH‡ =
A similar transformation of the EPR spectrum is observed
for 2b (Fig. 4, Table 1). At low temperature the superposition
of two spectra is observed indicating the coexistence of two
structural isomers: one with equivalent and another one with
nonequivalent phosphorus nuclei (isomer 1: aP1 = aP2 = 14.4 G,
aH= 2.2 G; isomer 2: aP1 = 22.2 G, aP2 = 14.2 G, aH = 2.5 G).
The temperature increase leads to averaging of the central part
of the spectrum (Fig. 4). Conclusions made for 4b2 allow to
attribute each low-temperature spectrum to each structural isomer
(Scheme 4). “Swing” oscillation is responsible for interconversion
of isomers. The presence of two spectra (both isomers) at low
temperature means that “swing” oscillation occurs. At the same
time nonequivalence of phosphorus in isomer 2 reflects a “sloped”
conformation and averaging of the spectrum indicates the presence
of “fan” oscillations for isomer 2. Equivalence of the phosphorus
53
40
7 kJ mol−1, DS‡ = 79
6 kJ mol−1, DS‡ = 46
14 J mol−1 K−1, for 1b DH‡
=
7 J mol−1 K−1. These values are
quite different from earlier obtained ones for “swing” motion of
4b molecules: DH‡ = 16.1 0.8 kJ mol−1, DS‡ = −33.2 2.0 J
mol−1 K−1 for direct reaction and DH‡ = 15.9 0.8 kJ mol−1, DS‡ =
−28.3 2.0 J mol−1 K−1 for reverse reaction.2 This can serve as an
additional evidence for different interconversion mechanisms for
1a and 1b in comparison with 4a and 4b.
In the case of 2a and 2b the coordination sphere dynamics is
more complicated. In the EPR spectrum of 2a at low temperature
(Fig. 3, Table 1) hyperfine splitting on two nonequivalent phos-
phorus atoms (aP1 = 25.3 G, aP2 = 15.7 G) and two nonequivalent
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oscillation; but only one of isomers exists as a mixture of two
tautomers which interconvert through a “fan” oscillation.
Absence of “fan” oscillation for isomer 1 can be explained by
steric reasons, particularly relatively small i-Pr substituents at the
phosphorus atom together with the absence of tert-Bu groups in
ortho-position to oxygen trans- to the Ni–C bond.
Unfortunately, the complicated character of molecular dynam-
ics of 2a and 2b leads to transformation of all the central part of
the EPR spectra upon temperature change making it impossible
to separate the individual processes and hence evaluation of the
activation parameters of the two different tautomeric interconver-
sions.
Finally, in the case of 3b no considerable perturbations in
the EPR spectrum upon temperature change were observed.
Moreover, the phosphorus HFC constants remain unchanged with
temperature (aP1 = aP2 = 17.0 G, aH = 2.5 G). Neither “fan”
nor “swing” movements in the metal coordination sphere appear
to operate, and the o-semiquinonic ring occupies an orthogonal
position relative to the pincer fragment, as in complexes 4a–b.2
Thus, two conformations of the complexes (R2PCP)Ni(SQ) are
possible: with “sloped” (1a–b, 2a–b) and orthogonal (3b, 4a–
b) positions of o-semiquinonic and pincer fragments relative to
each other. Such difference cannot be described only in terms
of the substituent bulkiness. Most probably, the difference in
configuration of chelate fragments is a result of axial asymmetry
of i-Pr and Cy-substituents along the C–P-bond (in contrast to
Ph and t-Bu). Asymmetry leads to concerted rotation of both
substituents at the phosphorus atom and consequently appearance
of local energy minima.
In order to investigate the driving force of forming of the
“sloped” configuration which results in the “fan” type of motion
we made some structural calculations for 1a, 2a, 2b and 4a in the
molecular model system “HyperChem 8.0” (see ESI†). Geometry
optimization of molecules of 1a, 2a and isomer 2 of 2b leads to
“sloped” configuration in contrast to 4a and isomer 1 of 2b which
have geometries close to orthogonal. Table 2 presents the resulting
O–Ni–P angles of the inner coordination core of the geometry
optimized molecules. These angles determine the HFC constants
in the EPR spectra.
Thus, results of the modelling are in a good agreement with EPR
data and our suggestions about the structure of these complexes.
On the contrary, the “swing” oscillations ability is in good
agreement with the steric considerations.9 So, complexes 4a–b and
2a–b having small substituents at phosphorus atoms reveal this
dynamics, whereas 1a–b and 3b do not.
Fig. 4 EPR spectrum of 2b (A) and simulated spectrum for the 170 K
spectra of separate isomers and their sum (B).
Conclusions
A new type of coordination sphere dynamics have been ob-
served. Some spin-labeled o-semiquinonato-pincer nickel com-
plexes possess “fan” additionally to “swing” oscillations of the
o-semiquinone fragment relative to the pincer ligand. “Fan”
oscillations are proposed to be connected with axial asymmetry of
substituents at the phosphorus atoms in the pincer ligand. “Swing”
oscillations occur when the substituents at the phosphorus atom
are relatively small. Additionally, an unusual case was observed:
one of structural isomers of complex bearing i-Pr substituents
at the phosphorus atom shows “fan” oscillations of the ligands
whereas the other one do not.
Scheme 4 “Swing” and “fan” motions for 2b
nuclei in isomer 1 corresponds to an orthogonal configuration of
ligand planes without “fan” oscillation. Complex 2b thus exists
as a mixture of two isomers, interconverting through a “swing”
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Table 2 Results of molecular modelling (energies and angles O–Ni–P)
Angles/◦
Complex
Energy/kcal mol−1
P1–Ni–O1b
P1–Ni–O2b
P2–Ni–O1b
P2–Ni–O2b
1a
118.52
72.06
51.67
53.51
95.16
91.22
96.02
96.46
100.52
96.06
109.81
96.91
103.57
99.81
97.48
95.96
94.59
92.3
95
96.02
109.69
105.21
104.54
111.47
2a
2b-1a
2b-2a
4a
107.47
a Isomers 1 and 2 of 2b. b O1 is basal, O2 is apical.
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