Angewandte
Communications
Chemie
Homogeneous Catalysis
A Bis(silylene)-Substituted ortho-Carborane as a Superior Ligand in
the Nickel-Catalyzed Amination of Arenes
Dedicated to Professor Herbert W. Roesky
Abstract: The synthesis and structure of the first 1,2-bis-
(NHSi)-substituted ortho-carborane [(LSi:)C]2B10H10 (termed
SiCCSi) is reported (NHSi = N-heterocyclic silylene; L =
PhC(NtBu)2). Its suitability to serve as a reliable bis(silylene)
chelating ligand for transition metals is demostrated by the
formation of [SiCCSi]NiBr2 and [SiCCSi]Ni(CO)2 complexes.
The CO stretching vibration modes of the latter indicate that
the SiII atoms in the SiCCSi ligand are even stronger s donors
than the PIII atoms in phosphines and CII atoms in N-
heterocyclic carbene (NHC) ligands. Moreover, the strong
donor character of the [SiCCSi] ligand enables [SiCCSi]NiBr2
to act as an outstanding precatalyst (0.5 mol% loading) in the
catalytic aminations of arenes, surpassing the activity of
previously known molecular Ni-based precatalysts (1–
10 mol%).
N
-Heterocyclic silylenes (NHSis),[1] the silicon analogues of
N-heterocyclic carbenes (NHCs), are no longer laboratory
curiosities but valuable building blocks for the synthesis of
new functional silicon compounds[2] that are accessible in
multigram quantities; they can even serve as very effective
s donor ligands for transition metals (TMs).[3] In fact, the
electronic nature of NHSi ligands, with a singlet electronic
ground state akin to NHCs, leads to a strong s-donor and p-
acceptor character of the divalent Si center toward TM sites.
Experimental data indicate that the s-donor strengths of
NHSi ligands can be greatly varied by modifying the nature of
the heterocyclic backbone around the divalent Si atom,[3a] and
DFT calculations indicate that they can compete with or even
exceed the electronic features of commonly used NHCs or
phosphine ligands with respect to s-donor and p-acceptor
strengths, ligand-to-metal charge transfer, and steric param-
eters.[4] Therefore, NHSis are attractive steering ligands in
homogeneous catalysis.[5] Currently, the chelating bis-
(silylene) ligands A and D, and the SiXSi pincer-type NHSi
Scheme 1. Multidentate NHSi ligands.
ligands B (X = CH) and C (X = N) have been synthesized in
our laboratory (Scheme 1).[6] TM complexes of the latter type
of ligands can act as very active precatalysts in the borylation
of benzenes,[7a] Sonogashira coupling,[7b] Kumada–Corriu
coupling,[7c] the cyclotrimerization of alkynes, the co-cyclo-
trimerization of alkynes and organocyanides to form pyridi-
nes,[7d] the hydrosilylation of ketones,[7e–g] and the reduction of
amides to amines.[7h] However, compared to the rich struc-
tural diversity of phosphine and NHC ligands, multidentate
NHSi ligands are still scarce. Herein we report the synthesis of
the first bis(NHSi) ligand (termed SiCCSi; Scheme 1) with an
ortho-carborane backbone,[8] and its superior function in the
Ni-catalyzed amination of arenes (Buchwald–Hartwig cou-
pling reactions).
SiCCSi was synthesized and isolated in 84% yield through
a salt metathesis reaction of (LiC)2B10H10 with N,N’-di-tert-
butyl(phenylamidinato)chlorosilylene (Scheme 2). Single-
crystal X-ray diffraction analysis of SiCCSi revealed that
the SiII atoms of the NHSi motifs pointed towards each other
[*] Y.-P. Zhou,[+] Dr. S. Raoufmoghaddam,[+] Prof. Dr. M. Driess
Technische Universitꢀt Berlin, Department of Chemistry:
Metalorganics and Inorganic Materials, Sekr. C2
Strasse des 17. Juni 135, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
ꢀ
with an Si Si distance of 3.267 ꢀ to form a pre-organized
chelating “pocket” for metal coordination (Figure 1, top).
This allows SiCCSi to serve as a reliably strong chelating SiII
ligand for NiII, affording [SiCCSi]NiBr2 after reacting with
L2NiBr2 (L2 = 1,2-dimetheoxyethane). The 29Si NMR signal at
d = 58.7 ppm is drastically downfield shifted compared to the
“free” ligands (d = 18.9 ppm) and other known NHSi-NiII
complexes.[6,7a,b] Single-crystal X-ray analysis of [SiCCSi]-
NiBr2 revealed that the NiII center is, as expected, in a square-
Dr. T. Szilvꢁsi
Department of Inorganic and Analytical Chemistry
Budapest University of Technology and Economics
Szent Gellꢂrt tꢂr 4, 1111 Budapest (Hungary)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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