Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

Chapitre III
The slightly distorted square planar complexes 3 and 4 contain the chelate ligands PNth and PNox
and two terminal bromides. As expected, the trans influence of the phosphorus donor results in longer
Ni1-Br2 bonds compared to Ni1-Br1. Both ligands show analogous coordination geometries and
structural parameters. The Ni-N and Ni-P distances in 3 are much shorter than those observed in 1a.
Although satisfactory elemental analyses data were obtained, attempts to record good quality NMR
spectra failed. Consistently, red solutions of compounds 3 and 4 were found to be paramagnetic by the
Evans method, [4] displaying magnetic moments of ca. 1.3 and 1.0 µB, respectively. These data point
to the presence of a paramagnetic isomer in equilibrium with the square planar one(see below).
N
Cl
P Ni
Cl
Cl
Cl
Ni
P
P
N
N
P
Ni
Cl
N
Cl
Cl
Cl
Ni
N
P
1a, 2a 1b, 2b
GREEN RED
=
S N
PPh 2
PN th
for 1a, 1b
Scheme 1.
20-35 kbar
O N
PPh 2
PN ox
for 2a, 2b
A comparison between the FTIR spectra of 2a and 2b is shown in Figure S-3 (Supporting
Information). When compared to 2b, the low pressure form 2a displays several split absorptions, as a
result of the presence of two symmetry-independent ligands. In contrast to 1a but similarly to 3 and 4,
only one independent ligand is likely to be present in the unit cell of 1b.
Few examples of nickel complexes undergoing a fully characterized pressure induced
isomerization have been reported, e.g. the conversion from tetrahedral to dinuclear, square-pyramidal
structures, [6] or from mononuclear to tetrahedral square planar arrangements. [7] A structurally
characterized square-planar to octahedral isomerization was recently reported for a Pd thioether
species. [8] Our system represents, however, a more dramatic structural isomerization, involving at the
same time (i) the entropically favored fragmentation of a tetranuclear complex in four distinct square
planar molecules and (ii) an octahedral to square-planar rearrangement. The former is associated with
an overall denser unit cell of the mononuclear form (estimated density from X-ray powder diffraction
4
P
N
Ni
Cl
Cl
6