Organometallics
Communication
crystallization attempts were unsuccessful, compound 5 was
successfully characterized by NMR spectroscopy and elemental
analysis.13 Yet, when 5 was dissolved in Et2O/MeCN 10/1,
small crystals of 5-MeCN were obtained. This is in line with
previous observations in our group about the facile displace-
ment of one of the stilbene units in nucleophilic coordinating
solvents.12,13 Interestingly, complexes 3−5 have stability
comparable to that of complex 2: they are stable for months
in air if stored in the freezer (−18 °C); however, signs of
decomposition could be observed after several days if they
were left opened to air in the benchtop.13 Despite their
similarities in structure, complexes 2−5 have severe differences
in electronics; yet, a clear trend in their stability toward
oxidation could not be clearly deduced. Then, we speculated
that the steric contribution of these substituents could play a
major role. To validate this hypothesis, we increased the bulk
at the para positions of the stilbene by introducing a tBu group.
Following the general procedure as for 1−5, Ni(4‑tBustb)3 (6)
was obtained as a yellow-orange solid in gram quantities and in
high yield (95%, Figures 1B and 2). To our delight, 6 exhibited
a remarkable stability to temperature and oxidation, and it
could simply be stored in air on the benchtop (1 month). In
contrast to 1−5, complex 6 also displayed high stability in
solution, showing no signs of decomposition in a variety of
solvents. Strikingly, in the solid state, this 16-electron complex
could be heated up to 60 °C for 1 h, showing no visual signs of
decomposition. Finally, X-ray photoelectron spectroscopy
(XPS) unequivocally confirmed that the oxidation state of Ni
in complex 6 is Ni(0).13 Such outstanding physical properties
highlight the robustness of 6 and certify its superior stability in
comparison to the other binary 16-electron Ni(0) olefin
complexes known.14 Figure 2 (bottom) shows the ORTEP
drawings of these family of complexes. Complexes 2−4 and 6
reveal certain common features in the solid state: three stilbene
units are wrapped around the Ni center in a propeller
arrangement, rendering a distorted-trigonal-planar geometry
resembling that of 1, 2 and t,t,t-Ni(CDT).14c
Figure 1. (A) Ni(4‑CF3stb)3: advantages and drawbacks. (B)
Ni(4‑tBustb)3: a superior Ni(0) source for Ni catalysis.
stilbene derivatives as ligands. A survey of the substitution
pattern of the aryl groups revealed that steric hindrance play a
fundamental role toward protecting the Ni(0) center from
oxidation. The stability and catalytic activity of these new
complexes were benchmarked with complexes 1 and 2. From
this analysis, Ni(4‑tBustb)3 (6) was identified as an extremely
superior Ni(0) complex, with features that circumvent the
limitations of 2 (Figure 1B). Hence, 6 (1) is stable at room
temperature and can be stored opened to air on the bench for long
periods of time (ca. 1 month), (2) displays higher stability in
solution with various solvents, (3) presents faster kinetic
profiles than 2, (4) is catalytically competent in reactions
where 2 was proven to be either inefficient or inactive, and (5)
can be prepared in multigram quantities with high yields
(95%).
The unexpectedly high stability toward oxidation observed
in our previous work for complex 2 over that of 112 posed the
question as to whether this is the result of steric factors
rendered by the CF3 groups or is a consequence of the electron-
withdrawing effect posed by this group on the stilbene unit. To
shed light on this question, we synthesized various binary 16-
electron complexes, featuring different steric and electronic
substitutions at the meta and para positions of the stilbene.
The procedure previously optimized for the multigram
synthesis for 2 (Ni(acac)2, stilbene, and AlEt3)12 proved
successful for the preparation of complexes 3−6 (Figure 2,
top). At the onset, we prepared complex 3, bearing fluorine
atoms at both para positions of the phenyl rings, which render
the stilbene unit slightly electron deficient (67% yield). When
the fluorine atoms were replaced by a more electron donating
group such as Me, complex 4 was obtained in good yield
(74%). At this point, we questioned whether the highly
compact arrangement of the stilbenes around the Ni would
permit substitution at the meta position of the aryl ring.
Gratifyingly, the corresponding Ni(0) complex bearing three
(E)-1,2-bis(3,5-dimethylphenyl)ethene ligands (5) could
successfully be synthesized in high yield. Although several
A detailed analysis of the solid-state structures revealed a
priori unexpected features (Figure 3). Although the stilbene
ligands are electronically different among themselves, similar
C1C2 distances of the ethene moiety coordinated to Ni
were observed (1.39 Å)comparing well with the distance in
t,t,t-Ni(CDT) (1.37 Å)14c and coinciding with that of
Ni(COD)2 (1.39 Å) (Figure 3).15 This striking observation
suggests a comparable π back-donation from the d orbitals of
the Ni to the empty π* orbital of the stilbenes in 1−4 and 6.
Moreover, similar geometries for the propeller structure were
observed, with torsion angles (θ = C2−C1−Ni−C3 = 28.3−
30.3°) slightly lower than that of t,t,t-Ni(CDT) (32.0°).14c Yet,
differences could be observed in the torsion between the
ethene moiety and the ipso carbons of the aryl groups (ψ =
C1−C2 = C3−C4). Indeed, these angles range from 158.4 to
161.7° in complexes 1−4; however, complex 6 presents a
much higher torsion (ψ = 156.2°), which is ascribed to the
t
repulsion between the three Bu units pointing outward at the
edges of the complex. This structural feature suggests that the
tBu groups are experiencing an extreme steric situation and, as
a result, they are offering minimal space available for oxygen to
go through, thus profoundly protecting the Ni center.
Although speculative at this point, the origin of the superior
stability of 6 could be the result of attractive London
t
dispersion forces (LDF) by the Bu units, which hold the
stilbenes together in the solid state.16
B
Organometallics XXXX, XXX, XXX−XXX