An EPR study of the intramolecular dynamics in o-semiquinonic nickel complexes with a diphosphorous pincer ligand

Article abstract of DOI:10.1039/b308701h

Square pyramidal o-semiquinonic nickel complexes with 2,6- bis(diphenylphosphinomethyl)phenyl have a flexible coordination sphere and exist in solution as the mixture of tautomers rapidly, according to EPR time scale, interconverting at room temperature.

Full text of DOI:10.1039/b308701h

An EPR study of the intramolecular dynamics in o-semiquinonic nickel
complexes with a diphosphorous pincer ligand†
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 (in Cambridge, UK) 24th July 2003, Accepted 4th September 2003
First published as an Advance Article on the web 22nd September 2003
Square pyramidal o-semiquinonic nickel complexes with
2,6-bis(diphenylphosphinomethyl)phenyl have a flexible co-
ordination sphere and exist in solution as the mixture of
tautomers rapidly, according to EPR time scale, inter-
converting at room temperature.
to those in known analog [2,6-bis(di-cyclo-hexylphosphinome-
thyl)phenyl]bromonickel,9 but angles P–Ni–P are different. In
complex 1 it is 141.677° and in a literature analog it is 167.57°.
This difference can be explained by steric hindrances of tert-
butyl substituents of o-semiquinone.
Complex 1 is paramagnetic in the solid state as well as in
solution. The room temperature EPR spectrum in solution
(triplet of triplets) reflects solid state structural features.
Spectrum results from hyperfine coupling with two equivalent
protons (H4 and H5) of semiquinone and two equivalent
phosphorous nuclei being in apical positions towards the
semiquinonic plane: a(H4) = a(H5) = 2.8 G, a(P1) = a(P2) = 24.2
G, g = 2.0060 (CH2Cl2, T = 300 K). Investigation of the
temperature dependence of the EPR spectrum reveals flexibility
of its coordination sphere. Decreasing the temperature causes
broadening of the central components of proton triplets.
Simulation allows evaluation of the approximate HFC constants
on protons: a(H4)  2.2 G, a(H5)  3.4 G, a(P1) = a(P2) = 25.5
G, g = 2.0056 (CH2Cl2, T = 205 K). Nonequivalence of back
bonded protons in the EPR spectrum reflects the asymmetry of
spin density distribution which is a consequence of complex
geometry. The trans-position for O1 is occupied by an aryl
ligand, while that for O2 remains vacant. This type of trans
effect appearing as nonequivalence of back bonded protons in
the EPR spectrum has been already described for semiquinonic
complexes of platinum and palladium.10 Motion, which causes
exchange of a vacant site and a valent bond, corresponds to
interconversion of tautomers (Scheme 1). At ambient tem-
perature, tautomers interconvert rapidly (in EPR time scale), but
cooling slow down this process.
Introduction of a o-semiquinone ligand as a spin-label mon-
itored by EPR spectroscopy in many cases allows us to study
structural features1 and dynamic processes2 occurring in the
inner coordination sphere of complexes in solution. In some
cases it is a useful tool to investigate redox reactions including
paramagnetic centres3 and evaluate their thermodynamic and
kinetic parameters.4 Tautomeric interconversions of semi-
quinonic complexes were studied as well.5
Here we report a new spin-labeled nickel compound with a
pincer ligand. o-Semiquinone, coordinated to a diamagnetic
nickel atom, contains a single unpaired electron. Its EPR
spectrum provides information about the complex structure and
its behaviour in solution.
Complexes were synthesized by exchange reaction of
[2,6-bis(diphenylphosphinomethyl)phenyl]bromonickel† with
a thallium salt of the corresponding o-semiquinone.
A structural study‡ of [2,6-bis(diphenylphosphinomethyl)-
phenyl](3,6-di-tert-butyl-o-benzosemiquinono)nickel 1 indi-
cate that the complex has a distorted square-pyramidal geome-
try (Fig. 1) similar to many five-coordinated complexes of
platinum group metals with diphosphorous pincer ligands.6,7
Bond lengths in the quinone fragment are typical for an anion-
radical coordination mode.8 Distances Ni–C and Ni–P are close
Scheme 1 Equilibrium of tautomers of complex 1.
Bonds lengths distribution in semiquinone chelate cycle
allow us to attribute HFC constants to H4 and H5. An anion-
radical of o-semiquinone can be presented as the superposition
of resonance structures. Scheme 2 displays two boundary ones.
If structure a predominates, it would result in value a(H4) being
larger than a(H5). It would also lead to a shorter O1–M bond
distance compared to O2–M. Predominance of structure b
produces the opposite constant and bond lengths situation.
The fact that bond length Ni–O2 (2.060 Å) is longer than Ni–
O1 (1.924 Å), while C2–O2 (1.289 Å) is shorter than C1–O1
(1.298 Å) in structure 1, can be attributed to the predominant
Fig. 1 ORTEP view of complex 1 with 50% thermal ellipsoids. Hydrogen
atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): 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).
† Electronic supplementary information (ESI) available: synthetic proce-
dures. See http://www.rsc.org/suppdata/cc/b3/b308701h/
Scheme 2 Resonance structures of o-semiquinone coordination mode.
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presence of structure b compared to a. It allows us to attribute
a = 3.4 G to H5 being in para-position to O2, and consequently
a = 2.2 G to H4. It should be mentioned that the sum of the
protonic HFC constants at low temperature is equal to the
doubled constant at room temperature: 2.2 + 3.4 = 2 3 2.8 =
5.6 G. It means that spin density distribution between metal and
quinonic fragment remains unchanged under temperature
variation.
In contrast to complex 1 having symmetrical 3,6-di-
substituted semiquinone, complex 2 with unsymmetrical 3,5-di-
tert-butyl-o-benzosemiquinone can exist in two structural
isomers. We failed to obtain crystals of 2 suitable for
crystallographic analysis, but the temperature dependence of the
EPR spectrum indicates dynamic equilibrium of these two
isomers (Fig. 2). The EPR spectrum of [2,6-bis(diphenylphos-
phinomethyl)phenyl](3,5-di-tert-butyl-o-benzosemiquinono)-
nickel 2 (triplet of doublets) at 330 K reflects splitting on two
equivalent phosphorous nuclei and one back bonded proton:
aav(P1) = aav(P2) = 20.3 G, aav(H) = 2.6 G, g = 2.0052. The
hydrogen, occupying the ortho- position to oxygen has low spin
density and often is invisible in EPR spectrum. Low field
component of phosphorous triplet is broadened even at 330 K.
Decreasing of temperature results in the appearance of super-
position of two spectra with different HFC constants on both
phosphorous and hydrogen nuclei and differently broadened
components.
Isomer 1. The HFC constant on phosphorous and g-factor of
Isomer 2 are slightly larger than the same values of Isomer 1. It
corresponds to larger contribution of phosphorous-metal orbi-
tals in orbitals occupied by unpaired electron and can be caused
by steric reasons.
Temperature broadening of the low-field component of the
EPR spectrum caused by exchange between isomers allows
evaluation of kinetic parameters of direct and reverse reac-
tions:
Isomer 1 ? Isomer 2: DH#  16.1 ± 0.8 kJ mol21; DS# 
233.2 ± 2 J mol21*K
Isomer 2 ? Isomer 1: DH#  15.9 ± 0.8 kJ mol21; DS# 
228.3 ± 2 J mol21*K
It should be noted that enthalpies of both reactions are the
same within experimental error, and equilibrium primarily is
driven by entropy.
Interconversion of isomers corresponds to pendulum oscilla-
tions of three- and double-coordinated ligands relative to one
another around the P–Ni–P axis (Scheme 4). It could be
assumed that isomers interconvert through a trigonal-bipyr-
amidal transition state and entropic factor is connected with
steric considerations. The rate of motion is of the order 108–109
sec21, which is close to the rates of solvate sphere re-
organisation.
Scheme 4 Ligand motions corresponding to tautomers interconversion.
In summary, two novel five-coordinate spin-labeled nickel
complexes with pincer ligands were shown to exist in solution
as a mixture of interconverting tautomers. Pendulum oscilla-
tions of ligands around the P–Ni–P axis are responsible for
migration of vacant site. Tautomerism is driven by entropy.
This work was supported by Russian Foundation of Basic
Research (grant: 01–03–33065), INTAS: 00–00677 and Rus-
sian President grant supporting scientific schools: 1649.2003.3.
Spectral investigations were carried out in the Analytic Center
of The Institute of Organometallic Chemistry of RAS supported
by Russian Foundation of Basic Research.
Notes and references
‡ Crystal data of complex 1. C46 H47 Ni O2 P2; M = 752.49; T = 100 K;
l = 0.71073 Å; monoclinic, space group P2(1)/n; a = 21.1434(16) Å, b =
9.4577(7) Å, c = 22.2619(16) Å, a = 90°, b = 117.4680(10)°, g = 90°;
V = 3949.8(5) Å; Z = 4, DCalc = 1.265 g sm23; F(000) = 1588; q(deg)
1.81–26.00; reflections collected: 32601 unique: 7748 (Rint = 0.0512); final
Fig. 2 View of the EPR spectra of complex 2 at different temperatures
(toluene) and its simulation for T = 210 K.
R indices [I > 2s(I)] R1 = 0.0355, wR2 = 0.0840; R indices (all data) R1
0.0481, wR2 = 0.0882. CCDC 212481.
=
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.
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These two spectra belong to different structural isomers
differing by tert-butyl substituent orientation (Scheme 3). Spin
density on the oxygen atom in the position trans- to the vacant
site is larger than that on the other position. It results in an
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semiquinone ring. In Isomer 2 this site is occupied by a
hydrogen, while in Isomer 1 this position is occupied by tert-
butyl. Therefore, spectrum 2 with a larger HFC constant on the
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Scheme 3 Structural isomers of complex 2.
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