EPR study of intramolecular dynamics in o-semiquinonic nickel complexes with PCP-pincer ligand in solution

Article abstract of DOI:10.1039/b407972h

A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphraomethyl)phenyl were investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.

Full text of DOI:10.1039/b407972h

View Article Online / Journal Homepage / Table of Contents for this issue
F U L L P A P E R
EPR study of intramolecular dynamics in o-semiquinonic nickel
complexes with PCP-pincer ligand in solution
Konstantin A. Kozhanov, Michael P. Bubnov,* Vladimir K. Cherkasov, Georgy K. Fukin
and Gleb A. Abakumov
G.A. Razuvaev Institute of organometallic chemistry of Russian Academy of Sciences,
49, Tropinina str., 603950, Nizhny Novgorod, Russian Federation. E-mail: bmp@imoc.sinn.ru;
Fax: +7 8312 127497; Tel: +7 8312 127682
Received 26th May 2004, Accepted 12th July 2004
First published as an Advance Article on the web 11th August 2004
A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl were
investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution
as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is
entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution
depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.
Synthetic procedures
Introduction
Pincer complexes consist of a metal center and a pincer skeleton.
The pincer skeleton is a tridentate ligand which is connected to the
metal via a metal–carbon -bond and two coordinative bonds. The
most common type of pincer skeleton is an aryl fragment having
two arms as substituents in aryl 2,6-positions. Due to the rigid
chelating skeleton providing excess electron density on the metal,
this class of complexes determines its specific properties. First
discovered in the early 1970s, the chemistry of pincer complexes
has been extensively developed over the last ten years. This class of
compounds has found application in wide range of organic synthe-
ses as catalysts1,2 and is fundamental to the creation of new materials
such as gas sensors, switches, self assembling dendrimers etc.3
o-Semiquinone ligand is a radical anion of o-quinone containing
an unpaired electron in -aromatic system. It coordinates to metal
through two oxygen atoms forming five-membered chelate ring. It
has already been used as a spin label for EPR spectroscopy.4 In many
cases it has allowed us to study structural features and dynamic pro-
cesses occurring in the inner coordination sphere of complexes in
solution.4,5 It is also a useful tool to investigate thermodynamic and
kinetic parameters of inner-sphere dynamics.6 Tautomeric intercon-
versions of o-semiquinonic complexes were studied as well.7
Combination of the pincer skeleton and o-semiquinone on nickel
atom allowed us to observe inner-sphere dynamics consisting of
pendulum (or swing) oscillations of one ligand relative to the other
one.8 This motion is responsible for migration of vacant site and this
was not mentioned in literature before. The present report develops
this research by increasing the number of semiquinonic ligands in
order to investigate the effect of ring substituents in o-semiquinone
on inner-sphere dynamics and spin-density distribution.
1,3-Bis(diphenylphosphinomethyl)bromobenzene. The com-
pound was obtained according to known procedures.9 1H NMR
(200 MHz, CDCl3, /ppm): 3.60 s (4H, –CH2–P); 6.53–6.72 m (3H,
C6H3); 7.3–7.5 m (20H, –PPh2). 31P NMR (CDCl3, /ppm): −12.15.
2,6-Bis(diphenylphosphinomethyl)phenylbromonickel.
Nickel tetracarbonyl (0.335 g, 1.957 mmol) was condensed to
frozen solution of 1,3-bis(diphenylphosphinomethyl)bromobe
nzene in 50 ml of toluene. Mixture was allowed to warm up to
ambient temperature. Warming was accompained by gas elimina-
tion. After the end of gas elimination solution was heated in water
bath (~80 °C). Solution colour has changed from pale-yellow to
red–brown and yellow precipitate formed. After the end of gas
elimination mixture was cooled and filtered. Yellow solid was
recrystallized from THF. Yield: 0.49 g (45%). Anal.(%) Found:
C 62.52; H 4.43; Ni 9.75; P 10.26; Br 13.87. C32H27P2BrNi Calc.:
C 62.75; H 4.41; Ni 9.64; P 10.13; Br 13.07. IR (Nujol, cm−1):
1470s, 1440s, 1425w, 1410w, 1385s, 1315w, 1185w, 1105m,
1070w, 1030w, 1000w, 965w, 845m, 830w, 780w, 755cp, 745m,
710w, 695s, 590w, 530w, 510m, 485w, 445w. 1H NMR (200 MHz,
CDCl3, /ppm): 3.83 s (4H, –CH2–P); 7.4 m (12H, o,p-Harom) and
7.8 m (8H, m-Harom) - Ph; 7.0 m (3H, ,-C6H3).
Experimental
Materials and methods
Thallium o-benzosemiquinolates. Thallium salts of o-benzo-
semiquinones were obtained from thallium amalgam and
corresponding o-quinone according to known procedure12 and used
further in situ.
Starting materials are commercially available (Aldrich, Fluka,
Strem) and were used without purification unless otherwise noted.
1,3-Bis(diphenylphosphinomethyl)bromobenzene and quinones
were obtained according known procedures.9,10 All synthetic
procedures were carried out under vacuum using dried, distilled
solvents.11
1H-NMR spectra were obtained on “Bruker” DPX-200 spectro-
meter (200 MHz, CDCl3, /ppm). X-band EPR spectra were
recorded on Bruker ER 200 D-SRC spectrometer with ER041 MR
microwave bridge, ER 4105 DR double resonator and ER 4111 VT
variable temperature unit; spectral parameters were obtained by
simulation using WinEPR SimFonia v1.25 (Bruker). IR spectra
were recorded on Specord M-80.
2,6-bis(Diphenylphosphinomethyl)phenylnickel o-benzo-
semiquinolates. Complexes 1–4, 7 and 8 (for complex numbers
see Table 1 later) were obtained by exchange reaction of 2,6-
bis(diphenylphosphinomethyl)phenylbromonickel with thallium
salt of corresponding o-semiquinone (Method A) (see Scheme 1).
Complexes were investigated by EPR in solution. The molecular
structure of 1 was determined by X-ray crystallography. Complexes
5 and 6 were obtained by a substitution reaction of a less electron-
accepting o-quinone by a more electron-accepting one (Method B).
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2
2 9 5 7

View Article Online
of the Ni, P1, P2, O1 and C15 atoms from the averaged equatorial
plane is 0.314 Å. The O2 atom occupies the axial position. Three
atoms of the pincer ligand (P1, P2 and C15) take part in the forma-
tion of two five-membered metallocycles. These metallocycles
are not planar. The Ni1–P1, Ni1–P2 and Ni1–C15 distances are
2.167(1), 2.187(1) and 1.926 (2) Å. These values are close to the
Ni–P(2.173(1), 2.177(1)) and Ni–C (1.920(3)) distances in [1,3-bis-
(diphenylphosphinomethyl)C6H3]bromonickel.16 The P1P2
distance in 1 is 4.112 Å, that is significantly shorter than the similar
distance in [1,3-bis(diphenylphosphinomethyl)phenyl]bromo-
nickel16 (4.314 Å). The respective distance in the free 1,3-bis-
(diphenylphosphinomethyl)benzene molecule is 5.356 Å, that is,
significantly larger than in the metal complexes. The bite angle in
1 [P1–Ni1–P2 141.68(2)°] is significantly smaller than those in
the known metal complexes (165.14(3)° for Ni, 162.33(9)° for Pd,
164.8(1)° for Pt16). By the way, the phosphorus atoms in PPh2 groups
in 1 occupy cis-positions relative to the aryl fragment bonded to
metal, whereas the phosphorus atoms in PPh2 groups in the bromo-
metal complexes16 occupy trans-positions relatively to the aryl
fragments. These differences can be explained by steric hindrance
of tert-butyl substituents of o-semiquinone. The bond lengths in the
quinone fragment are typical for an radical-anionic coordination
mode.17 The dihedral angle between the planes of semiquinonic and
benzene fragments in 1 is 75.8°. This allows to decrease the steric
hindrance in the coordination sphere of the Ni(1) atom.
The unpaired electron localized in the -system of o-semi-
quinone provides information about the complex geometry and
features of spin density distribution.4,5 The results of EPR-spectral
investigations are listed in the Table 1. The room temperature EPR
spectrum of 1 (Fig. 2) is consistent with its composition: the triplet
of triplets is the result of the coupling of the unpaired electron
with two equivalent phosphorus atoms (31P, 100%)18 lying in the
base of square pyramid. The relatively large5 phosphorus HFC
constant is caused by complex geometry. Phosphorus atoms are
situated above and below semiquinone plane. The triplet with a
small coupling constant results from splitting on two equivalent
protons (1H, 100%)18 in back positions (H4 and H5) of semiquinone
ring (Figs. 1 and 2). Cooling of the solution leads to broadening of
central components of proton triplets. It is the result of exchange
of two particles differing by proton coupling constants. The asym-
metrical spin density distribution within o-semiquinone is the
consequence of complex structure. In the solid state atom O2 (Fig. 1)
has a vacant site in trans-position whereas O1 has Ni–aryl covalent
bond. Apparently, in solution at room temperature the complex
molecule undergoes fast pendulum (or swing) oscillations of one
ligand relative to the other, which averages both proton coupling
constants. This motion is responsible for migration of vacant site.
Cooling slows down these motions and non-equivalence of protons
H4 and H5 appears in EPR spectrum. Down from ~200 K in CH2Cl2
the exchange becomes slow and doublet of doublets appears instead
of proton triplet. Simulation allows to evaluate aH4 and aH5 coupling
constants. Analysis of the bond length distribution in chelate ring
allows us to attribute each coupling constant to particular proton.
The bond length Ni–O2 (2.0595(10) Å) is longer than Ni–O1
(1.9238(10) Å). At the same time C2–O2 (1.2890(17) Å) is shorter
than C1–O1 (1.2975(17) Å). The o-semiquinone coordination unit
can be presented as superposition of two resonance structures A and
B responsible for proton coupling constants (Scheme 2). Difference
of bonds lengths in chelate unit signify predominance of structure
A compared to B. Consequently the larger HFC constant a = 3.4 G
can be attributed to H5 occupying the para-position towards O2
which has an excess of spin density. Thus, HFC constant a = 2.2 G
should be attributed to H4. It should be mentioned that the sum of
proton HFC constants at low temperature is equal to double constant
at room temperature: 2.2 + 3.4 = 2 × 2.8 = 5.6 G, which means that
the spin density distribution between metal and organic fragment is
unchanged upon temperature variation.
Scheme 1 Synthesis of complexes 1–4, 7 and 8.
Resulting solutions were investigated by EPR in situ. Solvents
and concentration were varied to obtain optimal resolution of the
spectra.
Method A. To solution of 1,3-bis(diphenylphosphinomethyl)-
phenylbromonickel (0.1184 g, 0.1927 mmol, 1 eq.) in CH2Cl2
(20 ml) the solution of coressponding thallium o-semiquinolate,
obtained from 0.1927 mmol (1 eq.) of quinone, was added. Solvent
was changed to toluene, and resulting solution was filtered to
remove thallium bromide.
Method B. An excess (about five-fold) of an o-benzoquinone
was added to solution of complex of a less acceptor o-semiquinone.
Mixture was stored at room temperature until the signal of initial
semiquinonic complex in EPR spectrum disappeared.
Single crystal X-ray diffraction study of 1. Structural study
of 1 has already been published.8 X-ray quality single crystal of
1 were obtained from hexane/CH2Cl2 solution by slow evapora-
tion. Intensity data were collected at 100 K temperature on a
Smart Apex diffractometer with graphite monochromated Mo–K
radiation ( = 0.71073 Å) in the – scan mode ( = 0.3°, 10 s on
each frame). C46H47NiO2P2, M = 752.49, monoclinic, space group
P2(1)/n, a = 21.1434(16) Å, b = 9.4577(7) Å, c = 22.2619(16) Å,
 = 117.4680(10)°, V = 3949.8(5) Å3, Z = 4, calc = 1.265 g cm−3,
 = 0.609 mm−1, 1.81° ≤  ≤ 26°, 32601 reflections collected of
which 7748 were independent [R(int) = 0.0512]. The intensity data
were integrated by SAINT program.13 The structure was solved by
direct methods and was refined on F2 using all reflections with
SHELXTL.14 All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms were found from Fourier synthesis and refined
isotropically. SADABS15 was used to perform area-detector scaling
and absorption corrections. The final R indices are R1 = 0.0355
[I > 2(I)], wR2 = 0.0882. The selected distances and bond angles
are shown in Fig. 1.
CCDC reference number 212481.
See http://www.rsc.org/suppdata/dt/b4/b407972h/ for crystallo-
graphic data in CIF or other electronic format.
Fig. 1 ORTEP view of complex 1 with 50% thermal ellipsoids. Hydrogen
atoms omitted for clarity. Selected bonds lengths (in Å) and angles (°): C1–
O1 1.2975(17), C2–O2 1.2890(17), Ni–O1 1.9238(10), Ni–O2 2.0595(10),
Ni–C15 1.9261(15), C15–Ni–O1 177.96(6), O1–Ni–O2 81.60(4), O1–
Ni–P1 98.31(4), O1–Ni–P2 95.47(3), O2–Ni–P2 101.58(3), O2–Ni–P1
115.70(3), P1–Ni–P2 141.677(16).
Compounds including symmetrical semiquinonic fragments are
also presented by complexes 4 and 5. Observation of tautomerism
in complex 4 by EPR is impossible because of the absence of
magnetic nuclei in the back position of the semiquinone ring. In the
Results and discussion
X-ray diffraction study of 1 has shown that the Ni atom has a
distorted square-pyramidal coordination (Fig. 1). The deviation
2 9 5 8
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2

View Article Online
we were unable to determine both proton and fluorine coupling
constants for low temperature EPR spectra even by simulation,
therefore we cannot exactly attribute each EPR spectrum to each
structural isomer.
Spectrum 1: a1(P1) = a1(P2) = 23.8 G.
Spectrum 2: a2(P1) = a2(P2) = 25.5 G.
Fig. 2 EPR spectrum of complex 1 at different temperatures.
A more complicated EPR-spectral picture is observed in the
cases of complexes 7 and 8. Both complexes (as 2 and 3) also
can exist as two structural isomers. EPR spectrum of 7 reflects
HFC with phosphorus nuclei and one back-bonded proton on the
semiquinone ring (Table 1, Fig. 5). No splitting is observed for the
proton in the ring 6-position. Low field component of the spectrum
is broadened even at 330 K. Decreasing of temperature leads to
appearance of a superposition of two spectra belonging to different
structural isomers differing in the tert-butyl substituent positions.
These spectra differ in both the proton and phosphorus coupling
constants as well as in linewidth.
Scheme 2 Resonance structures of o-semiquinone coordination mode.
EPR spectrum of 5, the hyperfine structure is well resolved but no
temperature dependence was observed.
More interesting structural features were found in the case of 4-
substituted 3,6-di-tert-butyl-o-benzosemiquinonic complexes 2 and
3. These molecules can exist as two structural isomers differing by
position of a substituent in ring 4-position relatively inner coordina-
tion core (Scheme 3).As a consequence they differ by semiquinonic
proton HFC constant.
Spectrum 1: a1(P1) = a1(P2) = 18.4 G, a1(H) = 2.4 G, g = 2.0041.
Spectrum 2: a2(P1) = a2(P2) = 24.4 G, a2(H) = 3.4 G, g = 2.0063.
According to the argument above, spectrum 2, with the larger pro-
ton coupling constant, is attributed to the isomer having hydrogen in
para-position to oxygen being trans- to the vacant site.
Scheme 3 Structural isomers of 2 and 3.
EPR spectrum of 2 at 300 K (triplet of doublets) results from
hyperfine coupling with two phosphorus nuclei and one back-
bonded proton, a(H5) = 3.22 G, a(P1) = a(P2) = 21.7 G, g = 2.0052.
Decreasing of temperature leads to an appearance of the superposi-
tion of two spectra (Fig. 3):
Fig. 4 EPR spectra of complex 3 at different temperatures (Et2O).
Spectrum 1: a1(P1) = a1(P2) = 19.1 G, a1(H) = 2.4 G.
Spectrum 2: a2(P1) = a2(P2) = 21.2 G, a2(H) = 3.6 G.
These two spectra belong to two different structural isomers differ-
ing by methoxy substituents positions according to Scheme 3. As it
noted above spin density on the oxygen atom in the trans-position
to vacant site is larger than that on the other oxygen atom. It results
in an excess of the spin density in corresponding para-position of
the semiquinone ring. In isomer 2 this site is occupied by a proton,
while in isomer 1 this position is occupied by the methoxy group.
Therefore, spectrum 2 with the larger proton HFC constant can be
attributed to isomer 2, and spectrum 1 to isomer 1.
The same situation is observed for 3 (Fig. 4). Its EPR spec-
trum is a triplet of doublets of doublets at 270 K, a(H5) = 3.12 G,
a(P1) = a(P2) = 23.75 G, a(F4) = 8.2 G, g = 2.0061, and the superposi-
tion of two spectra at 170 K. Due the complicated superposition
Fig. 5 View of EPR spectra of complex 7 at different temperatures (in
toluene) and its simulation for T = 210 K.
The EPR spectrum of 8 is more complicated (Fig. 6).
At 300 K it is a triplet of triplets, aav(P1) = aav(P2) = 21.6 G,
aav(H4) = aav(H5) = 2.81 G. Spectrum components are broadened
even at 300 K. A decrease in temperature causes the appearance of
superposition of two spectra.
Spectrum 1: a1(P1) = a1(P2) = 18.4 G, a1(H4) = 3.6 G, a1(H5) = 2.15 G.
Spectrum 2: a2(P1) = a2(P2) = 25.8 G, a2(H4) = 2.2 G, a2(H5) = 3.4 G.
Fig. 3 EPR spectrum of complex 2 at different temperatures (Et2O).
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2
2 9 5 9

View Article Online
2 9 6 0
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2

View Article Online
Table 2 Kinetic parameters of direct and reverse reactions
Isomer 1 → Isomer 2
H‡/kJ mol−1
Isomer 2 → Isomer 1
Compound
S‡/kJ mol−1 K−1
−33.2 ± 2
H‡/kJ mol−1
15.9 ± 0.8
S‡/kJ mol−1 K−1
−28.3 ± 2
7
8
16.1 ± 0.8
18.2 ± 0.8
−27.6 ± 2
18.2 ± 0.8
−22.9 ± 2
both cases is driven by enthropy. The rate of interconversion is of
the order 108–109 s−1, which is close to the rates of solvate sphere
reorganisation.
It is necessary to make some intermediate conclusion. It was
shown by means of EPR spectroscopy that complexes 2, 3, 7 and
8 in solution exist as mixture of two structural isomers. The rate
of interconversion depends on temperature. For complex 6 the
equilibrium of isomerization cannot be observed due to broadened
spectrum components. Surprisingly, no transformation of the EPR
spectrum with changing temperature has been observed for complex
5. No significant influence of the redox properties of semiquinone
ligand on equilibrium position was found. Addition of Lewis bases
(e.g. pyridine, PPh3, THF), which are able to occupy vacant site
in square pyramidal complex, does not affect the EPR spectral
picture.
Phosphorus HFC constants reflect features of the geometry and
spin density distribution in the complex. What defines these values?
Complexes 1–6 include semiquinonic ligands which are functiona-
lized derivatives of 3,6-di-tert-butyl-o-benzosemiquinone. In these
cases the steric influences of o-semiquinones on the inner coordina-
tion core are the same. Therefore, spin density distribution is deter-
mined by electronic factors of back substituents of semiquinone.
Room temperature phosphorus HFC constants for complexes 2–5
in general are lower that the same constant for 1. This phenomenon
is observed despite of a quite different effect of fluoro, methoxy
and nitro substituents on redox potential of o-semiquinone.20 EPR
spectrum of 6 is triplet with broad lines, a(P1) = a(P2) = 31.9 G,
g = 2.0137. Splitting on the back proton is not oserved due to the
large linewidth. The largest values of phosphorus HFC constant
and g-factor together with broadened lines for 6 reflect a higher
contribution of a group metal-pincer orbital to the orbital containing
unpaired electron. We propose that, in general, mesomeric donor
substituents (–OCH3, –F) lead to a decrease in phosphorus HFC
constants, whereas the mesomeric acceptor substituent (–NO2)
leads to an increase.
Complexes 1, 7 and 8 include semiquinones having only alkyl
substituents without functional groups. Spin density distributions
in these cases are determined predominantly by steric factors. For
complex 1 tautomers are identical to each other (Scheme 4) and
consequently have the same phosphorus coupling constant. For
complexes 7 and 8 there are two observations to be emphasized:
(1) the isomer with the larger phosphorus coupling constant always
has broader lines and a larger g-factor; (2) different isomers of
complexes 7 and 8 have very similar phosphorus HFC constants
(Table 1, complexes 7 and 8, low temperature, spectra 1 (of
isomer 1): a(P) = 18.4 G (for both 7 and 8), spectra 2 (of isomer 2):
a(P) = 24.4 (for 7) and 25.8 (for 8) G). The difference between phos-
phorus coupling constants for each isomer of each complex (7 or 8)
is larger than the respective difference in the case of 2, 3 contain-
ing functionalized semiquinones. This difference is a(P) = 24.4 −
18.4 = 6.0 G for 7 and 25.8 − 18.4 = 7.4 G for 8. For complexes 2
and 3 they are: a(P) = 21.2 − 19.1 = 2.1 G and 25.5 − 23.8 = 1.7 G
respectively. Using the argumentation mentioned above, which
allows to attribute each spectrum to each isomer we can define both
spectra 2 of 7 and 8 as belonging to isomer having a tert-butyl group
in ortho-position to oxygen in the base of the tetragonal pyramid
(scheme 5, underlined). According to the kinetic parameters this
isomer is entropically disfavoured. This isomer of complexes 7 and
8 has a phosphorus HFC constant close to that in 1, where both
tautomers have a tert-butyl group in this position.
Fig. 6 EPR spectra of complex 8 at different temperatures (CH2Cl2)
and its simulation for T = 190 K.
It is difficult to attribute each splitting unambiguously to a parti-
cular proton, especially because some coupling constants were
determined with the help of simulation.
It could be noted that the high-field components of the phos-
phorus triplet of both isomers of 7 and 8 are recorded at about the
same field. So, this component could not be significantly broadened
due to fast interconversion, and the broadening of low field compo-
nent can be described by known equation:19
 = ePIPII(H)2,
where  = III/(I + II) is the reduced lifetime of exchanging
particles, PI and PII are the probabilities of corresponding isomer
formation, H is field difference between low field components, e
is the electron gyromagnetic constant, and  is difference of line
width between low field and high field components.
This equation allows us to evaluate the lifetimes of each isomer
in the temperature range where fast interconversion is observed.
At high temperature, when fast exchange takes place, the current
phosphorus HFC constant remains essentially unchanged. Conse-
quently, the isomer ratio, necessary to calculate PI, PII and I, II is
independent of temperature and can be obtained by double integra-
tion of low field components in the low temperature spectrum.
The temperature dependence of isomer lifetime allows us to
determine the activation parameters (H‡ and S‡) of direct and
reverse interconversion reactions using the following equation:19
1/ = (kBT/h)exp(S‡/R)exp(H‡/RT)
where  is the lifetime of the corresponding isomer, and kB, h and R
are the Boltzmann, Planck and absolute gas constants respectively.
This equation becomes linear in the following form:
ln(h/kBT = S‡/R + H‡/R(1/T)
The slope and intercept of this regression are H‡/R and S‡/R
correspondingly, that allows us to evaluate the kinetic parameters
of the direct and reverse reactions in Table 2.
It should be noted that the enthalpies of direct and reverse reac-
tions are the same within experimental error, and equilibrium in
It would seem that functionalized substituents essentially affect
the spin density distribution between the semiquinone and the
nickel-pincer fragment, but they cause a small perturbation in the
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2
2 9 6 1

View Article Online
3 M. Albrecht and G. van Koten, Angew. Chem., Int. Ed. Engl., 2001,
40, 3750–3781.
4 G. A. Razuvaev, V. K. Cherkasov and G. A. Abakumov, J. Organomet.
Chem., 1978, 160, 361–371.
5 V. I. Nevodchikov, G. A. Abakumov, V. K. Cherkasov and G. A. Razuvaev,
J. Organomet. Chem., 1981, 214, 119–124.
6 G. A. Abakumov, G. A. Razuvaev, V. I. Nevodchikov and V. K. Cherkasov,
J. Organomet. Chem., 1988, 341, 485–494.
7 M. I. Kabachnik, N. N. Bubnov,A. I. Prokof’ev and S. P. Solodovnikov,
Sov. Sci. Rev., Sect. B, 1981, 3(XXIV), 197–297, N22.
Scheme 4 Equilibrium of tautomers of complex 1.
8 K. A. Kozhanov, M. P. Bubnov, V. K. Cherkasov, G. K. Fukin and
G. A. Abakumov, Chem. Commun., 2003, 2610.
9 J. R. Dilworth, Y. Zheng and D. V. Griffiths, J. Chem. Soc., Dalton
Trans., 1999, 1877–1881.
10 S. A. Chesnokov, V. K. Cherkasov, Yu. V. Chechet, V. I. Nevodchikov,
G. A. Abakumov and O. N. Mamysheva, Russ. Chem. Bull., 2000,
49(9), 1506–1511; V. A. Garnov, V. I. Nevodchikov, G. A. Abakumov,
L. G. Abakumova, Ju. A. Kursky and V. K. Cherkasov, Bull. Acad.
Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1985, 34(12), 2589–2591;
G. A. Abakumov, V. K. Cherkasov, L. G. Abakumova and V. I.
Nevodchikov, Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.),
1990, 39(5), 984–986; G. A. Abakumov, V. K. Cherkasov, L. G.
Abakumova, V. I. Nevodchikov, N. O. Druzhkov, N. P. Makarenko
and Ju. A. Kursky, J. Organomet. Chem., 1995, 491, 127; N. L.
Komissarova, I. S. Belostotskaya, E. V. Dzhuaryan and V. V. Ershov,
Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1973, 1380;
K. Ley and E. Muller, Chem. Ber., 1956, 89, 1402; A. N. Prokof’ev,
V. B. Vol’eva, N. A. Novikova, I. S. Belostotskaya, V. V. Ershov
and M. N. Kabachnik, Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl.
Transl.), 1980, 2707.
11 D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purification of
Laboratory Chemicals, Pergamon, Oxford, 1980.
12 V. A. Muraev, G. A. Abakumov and G. A. Razuvaev, Dokl. Acad. Sci.
USSR, 1974, 217(5), 1083–1086.
13 SAINTPlus Data Reduction and Correction Program v. 6.02a, Bruker
AXS, Madison, WI, 2000.
14 G.M. Sheldrick , SHELXTL v. 6.12, Structure Determination Software
Suite, Bruker AXS, Madison, WI, 2000.
Scheme 5 Isomer of complexes 7 and 8 showing a large phosphorus
HFC constant in low temperature EPR spectrum.
symmetry of spin density distribution in different isomers. On the
other hand, alkyl substituents do not affect the spin density distri-
bution between o-semiquinone and nickel-pincer fragments, but
essentially influence its symmetry in different isomers.
In summary, a number of novel square-pyramidal spin-labeled
nickel complexes with pincer ligands were shown to exist in
solution as mixtures of interconverting isomers. Pendulum (or
swing) oscilations of the semiquinonic ligand around the P–Ni–P
axis are responsible for the migration of the vacant site. Isomerism
is driven by enthropy. Functional substituents in the back position
of o-semiquinone do not affect the equilibrium but influence the
spin density distribution between organometallic and o-semiqui-
nonic fragments. This influence is determined by the mesomeric
properties of substituents. The phosphorus HFC constant is affected
sterically by bulkiness of substituent being in ortho-position towards
the oxygen occupying basal site in coordination polyhedron.
15 G.M. Sheldrick, SADABS v.2.01, Bruker/Siemens Area Detector
Absorption Correction Program, Bruker AXS, Madison, WI, 1998.
16 F. Bachechi, Struct. Chem., 2003, 14(3), 263–269.
17 C. G. Pierpont and R. M. Buchanan, Coord. Chem. Rev., 1981, 38,
45–87.
18 B. A. Goodman and J. B. Reynar, Advances in Inorganic Chemistry
and Radiochemistry, Academic Press, New York, 1970, vol. 13,
pp. 135–362.
19 J. E. Wertz and J. R. Bolton, Electron Spin Resonance: Elementary
Theory and Practical Applications, McGraw-Hill, New York, 1972,
pp. 207–237.
Acknowledgements
This work was supported by Russian Foundation for Basic
Research (grants: 04-03-32409, 04-03-32413), INTAS: 00-00677
and Russian President grants supporting scientific schools:
1649.2003.3 and 1652.2003.3. Spectral investigations were carried
out in Analytic Center of Institute of organometallic chemistry of
RAS supported by the Russian Fundation for Basic Research.
References
20 S. A. Chesnokov,V. K. Cherkasov, G. A. Abakumov, O. N. Mamysheva,
Yu. V. Chechet and V. I. Nevodchikov, Bull. Acad. Sci., Div. Chem. Sci.
(Engl. Transl.), 2001, 12, 2258–2263.
1 J. T. Singleton, Tetrahedron, 2003, 59, 1837–1857.
2 M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759–1792.
2 9 6 2
D a l t o n T r a n s . , 2 0 0 4 , 2 9 5 7 – 2 9 6 2