Adaptive ligands,
which can adapt their coordination mode to the
electronic structure of various catalytic intermediates, offer the
potential to develop improved homogeneous catalysts in terms of activity
and selectivity. 2,2′-Diphosphinobenzophenones have previously
been shown to act as adaptive ligands, the central ketone moiety preferentially
coordinating reduced metal centers. Herein, the utility of this scaffold
in nickel-catalyzed alkyne cyclotrimerization is investigated. The
complex [(
p
-tol
L1
)Ni(BPI)]
(
p
-tol
L1
= 2,2′-bis(di(
para
-tolyl)phosphino)-benzophenone; BPI = benzophenone imine)
is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal
alkynes, selectively affording 1,2,4-substituted benzenes from terminal
alkynes. In particular, this catalyst outperforms closely related
bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction
pathway involving a hemilabile interaction of the C=O unit
with the nickel center. This is further borne out by a comparative
study of the observed resting states and DFT calculations.
Ligands
that can adapt their coordination mode to the electronic properties
of a metal center are of interest to support catalysis or small molecule
activation processes. In this context, the ability of imine moieties
to bind in either an η1(N)-fashion via σ-donation
of the lone pair or, less commonly, in an η2(C,N)-fashion
via π-coordination is potentially attractive for the design
of new metal–ligand cooperative systems. Herein, the coordination
chemistry of chelating ligands with a diphosphine imine framework
(PCNP) to nickel is investigated. The imine moiety binds in an η1(N)-fashion in a Ni(II)Cl2 complex. The uncommon
η2(C,N)-interaction is obtained in Ni(0) complexes
in the presence of a PPh3 coligand. Increasing the bulk
on the phosphine side-arms in the Ni(0) complexes, by substituting
phenyl for o-tolyl groups, leads to a distinct binding
mode in which only one of the phosphorus atoms is coordinated. In
the absence of a coligand, a mixture of two different dimeric Ni(0)
complexes is formed. In one of them, the imine adopts an uncommon
η1(N)η2(C,N) bridging mode of the
ligand to nickel, while the second one may involve reactivity on the
ligand by the formation of a new C–C bond by oxidative coupling.
The latter is supported by the isolation and structural characterization
of a crystalline bis-CO derivative featuring a C–C bond formed
by oxidative coupling of two imine moieties.
π-coordinating
units incorporated in the supporting ligand
of an organometallic complex may open up specific reactive pathways.
The diphosphine ketone supported nickel complex [(
p
‑tol
L1)Ni(BPI)] (
p
‑tol
1;
p
‑tol
L1 = 2,2′-bis(di-p-tolylphosphino)benzophenone; BPI = benzophenone imine)
has previously been shown to act as an active and selective alkyne
cyclotrimerization catalyst. Herein, DFT calculations support an adaptive
behavior of the ligand throughout the catalytic cycle, several elementary
steps being assisted by coordination or decoordination of the CO
moiety. A comparison with related bi- and tridentate phosphine ligands
reveals the key role of the hemilabile π-acceptor moiety for
the catalytic activity and selectivity of
p
‑tol
1 in alkyne cyclotrimerization.
A nickel pincer complex is found to catalyze alkylÀalkyl Kumada coupling reactions of 1,3-and 1,4-substituted cyclohexyl halides and tetrahydropyrans with an excellent diastereoselectivity. The mechanistic investigation of the coupling reactions provides evidence that the activation of alkyl halides is reversible.
Activation
of a Si–H bond is commonly a critical step in
catalytic hydrosilylation reactions. Herein, we investigate the cooperative
reactivity of Ni(0) centers bearing a side-bound imine ligand toward silanes. Such complexes
activate a Si–H bond of diphenylsilane, resulting in formal
hydrosilylation of the imine backbone, which acts as a hydride acceptor.
The resulting hydrosilazane motif engages either in coordination to
nickel via the Si–H bond, forming an 18-electron η2-Si–H complex, or oxidative addition to Ni to form
16-electron Ni(II) silyl/hydride complexes. DFT calculations suggest
a cooperative activation of the silane via ligand-to-ligand hydride
transfer. In addition, the silicon fragment readily exchanges with
external hydrosilanes, showing that the Si–N bond can be reversibly
cleaved under mild conditions.
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