Synthesis and structural characterisation of the first homoleptic organometallic nickel(III) compound

Article abstract of DOI:10.1039/a607668h

The homoleptic, square-planar, d⁷, 15-electron species [NBu₄][Ni^III(C₆Cl₅)₄] 2 is obtained by low-temperature oxidation of [NBu₄]₂[Ni^II(C₆Cl

Full text of DOI:10.1039/a607668h

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Synthesis and structural characterisation of the first homoleptic organometallic
nickel(iii) compound
Pablo J. Alonso, Larry R. Falvello, Juan Fornie´s,* Antonio Mart´ın, Babil Menjo´n and Gerardo Rodr´ıguez
Instituto de Ciencia de Materiales de Arago´n, Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C., Pza. S. Francisco s/n,
E-50009 Zaragoza, Spain
The homoleptic, square-planar, d7, 15-electron species
[NBu4][NiIII(C6Cl5)4] 2 is obtained by low-temperature
oxidation of [NBu4]2[NiII(C6Cl5)4] 1 with Cl2.
[g4(SxBx + SyBy) + g∑SzBz], where mB is the Bohr magneton and
g∑ = 1.92 and g4 = 2.84. This would imply a low-spin (strong
ligand field) situation for the 3d7 ion. Since the NiIII centre in 2
is in a square-planar environment, the unpaired electron should
be expected to occupy an a1g (z2) orbital.12 In terms of the
crystal-field approximation, a second order of perturbation for
such an ion would give g∑ = ge and g4 = ge 2 6 l/D, where l
is the spin–orbit constant and D the difference in energy
between the a1g (z2) and eg (xz,yz) orbitals. Since l < 0 for NiIII,
the former expression predicts that g4 > ge while g∑ lies close
to the free-electron value in agreement with our observation.**
This behaviour is in keeping with that reported for other
nickel(iii) species,13 the only remarkable feature being in our
case the high g anisotropy observed.
In the last few years the chemistry of NiIII has evolved very
rapidly1,2 partly because of its relevance to some metalloprotein
systems.3,4 In spite of this burgeoning development, organome-
tallic compounds of NiIII are still extremely rare, the only
isolated examples of this class of compounds being, to our
knowledge, those reported by van Koten and coworkers.5 They
contain a single Ni–C bond and are stabilised by using
polydentate ligands. We now report on the synthesis
and characterisation of the first homoleptic organometallic
nickel(iii) compound, which contains four non-assisted Ni–C s
bonds.
Low-temperature chlorination of the substrate [NBu4]2[NiII-
(C6Cl5)4] 16 renders the complex [NBu4][NiIII(C6Cl5)4] 2,
which has been isolated as a green solid in good yield.†
Complex 2 is air- and moisture-stable but because of its rather
limited thermal stability it is best formed and handled at low
temperature. At room temperature it readily decomposes both in
solution and in the solid state, undergoing reductive elimination
of C6Cl5–C6Cl5.‡ This observation is of particular interest
because of its relevance to the nickel-mediated synthesis of
biaryls, which has been shown to proceed in some instances via
unstable aryl intermediates of NiIII.7
The 2–1 couple is not only chemically but also electrochem-
ically related by a reversible redox process at a remarkably low
potential (E1/2 = 20.11 V).§ The IR spectrum of 2 shows the
absorptions corresponding to the X-sensitive mode of the C6Cl5
group and to the n(Pt–C) mode8 at higher wavenumbers than
those observed for the nickel(ii) parent compound (cf. 1, 813
and 581 cm21; 2, 827 and 603 cm21, respectively). This shift
can be associated with the increase in the oxidation state of the
metal centre in keeping with previous observations in the
perhalogenophenyl chemistry of palladium and platinum.8a,9
The structure of the anion [NiIII(C6Cl5)4]2 as found in the
dietherate 2·2Et2O by X-ray diffraction methods is shown in
Fig. 1.¶ The Ni atom is in an almost square-planar environment.
The C6Cl5 groups define a helix around the metal centre with
two possible orientations, D and L, both of which are present in
the crystal measured (centrosymmetric space group). The mean
dihedral angle formed by the C6Cl5 groups and the coordination
plane is 63.2(3)°. The mean Ni–Cipso distance [200.7(8) pm] is
slightly longer than found in other pentachlorophenyl deriva-
tives of NiII (mean value 192.2 pm).10 This seeming elongation
is at variance with the expected dependence of the M–C
distance on the oxidation state of M. This difference, however,
must be treated with caution because it could merely be due to
the recently demonstrated flexibility of bond distances.11 The
EPR spectrum of a powder sample of 2 is given in Fig. 2∑ in
which two features can be observed: one at ca. 340 mT having
a parallel character and the other one at ca. 230 mT having a
perpendicular character. This spectrum can be interpreted as
due to a paramagnetic S = 1/2 entity in an axial symmetry
including only an anisotropic electronic Zeeman interaction and
can be described with the following spin-Hamiltonian H = mB
Fig. 1 Thermal ellipsoid diagram of the anion of 2. Selected distances (pm)
and angles (°): Ni–C(1) 200.1(8), Ni–C(7) 201.4(7); C(1)–Ni–C(7) 90.1(3),
C(1)–Ni–C(1A) 178.6(5), C(7)–Ni–C(1A) 90.0(3), C(7)–Ni–C(7A) 178.9(4).
270
300
180 210 240
330 360 390
B / mT
Fig. 2 EPR spectrum of a powder sample of 2 (X-band, 77.3 K)
Chem. Commun., 1997
503

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It can be concluded that the pentachlorophenyl group is a
ligand especially well suited to stabilising unusual oxidation
states, at least in the platinum-group metals.9,14 Studies aimed at
testing the reactivity of this unprecedented nickel(iii) compound
are in progress.
We thank the Direccio´n General de Ensen˜anza Superior
(Projects PB95-0003-CO2-01 and PB95-0792) for financial
support.
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Footnotes
† Experimental procedure: to a yellowish suspension of [NBu4]2[NiII-
(C6Cl5)4] 1 (0.5 g, 0.32 mmol) in CH2Cl2 (7 cm3) at 240 °C was added Cl2
dissolved in CCl4 (0.18 mmol). The system changed immediately to green
and, after addition of MeOH (20 cm3) at 280 °C, the resulting green solid
was filtered off in a low-temperature device, subsequently washed with cold
Et2O (3 3 10 cm3) and dried (2, 70% yield). Anal. Found: C 36.5, H 2.8, N
1.1; C40H36Cl20NNi requires: C 37.0, H 2.8, N 1.1%. IR(Nujol; cm21):
selected absorptions associated with the C6Cl5 groups:8 827vs (X-sensitive
vibr.), 675vs, 603s [n(Pt–C)] and 275m.
‡ The fate of the metal-containing fragment could not be determined so far
and hence we still do not know whether this reductive elimination process
takes place inter- or intra-molecularly.
§ The cyclic voltammogram of 1 at 100 mV s21 scan rate in 0.1 m NBu4PF6
shows Epa = 20.055 V and Epc = 20.168 V vs. SCE.
¶ Crystal data for 2·2Et2O: C48H56Cl20NNiO2, M = 1446.65, monoclinic,
space group C2/c (no. 15), a = 1987.3(2), b = 2167.8(1), c = 1487.2(1)
pm, b = 110.026(5)°, U = 6.031(2) nm3, Z = 4, F(000) = 2940,
Dc = 1.593 g cm23, l(Mo-Ka) = 71.073 pm, 344 parameters refined with
5300 reflections with I > 2s(I) to R = 0.0736, Rw = 0.1577. Atomic
coordinates, bond lengths and angles, and thermal parameters have been
deposited at the Cambridge Crystallographic Data Centre (CCDC). See
Information for Authors, Issue No. 1. Any request to the CCDC for this
material should quote the full literature citation and the reference number
182/365.
∑ EPR data were taken on a Varian E-112 spectrometer working in the
X-band. Measurements at liquid-nitrogen temperature (77.3 K) were taken
using a quartz immersion Dewar. The magnetic field was measured with a
Bruker ER035M gaussmeter. The diphenylpicrylhydrazyl resonance signal
[g = 2.0037(2)] was used to determine the microwave frequencies.
** Other contributions such as charge-transfer, covalency, etc. can modify
the principal g-values so as to make the crystal-field description inadequate.
In fact, either positive or negative g-shifts are observed for square-planar
low-spin d7 complexes.12
References
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Pergamon, Oxford, 1987, vol. 5, ch. 50, p. 1; R. I. Haines and
Received, 11th November 1996; Com. 6/07668H
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Chem. Commun., 1997