Isolation of a potassium bis(1,2,3-triazol-5-ylidene)carbazolide: A stabilizing pincer ligand for reactive late transition metal complexes

Article abstract of DOI:10.1039/c3cc49385g

The synthesis and X-ray crystal structure of a potassium adduct of a monoanionic CNC-pincer ligand featuring two mesoionic carbenes is reported. Owing to the peculiar electronic and steric properties of this ligand, the first neutral stable Ni(ii)-hydride, and an unusual Cu(ii) complex displaying a seesaw geometry, have been isolated.

Full text of DOI:10.1039/c3cc49385g

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Isolation of a potassium bis(1,2,3-triazol-5-
ylidene)carbazolide: a stabilizing pincer ligand for
reactive late transition metal complexes†
Cite this: Chem. Commun., 2014,
50, 2431
Received 10th December 2013,
Accepted 15th January 2014
Daniela I. Bezuidenhout,*ab George Kleinhans,a Gregorio Guisado-Barrios,bc
David C. Liles,a Gael Ungb and Guy Bertrand*b
¨
DOI: 10.1039/c3cc49385g
www.rsc.org/chemcomm
The synthesis and X-ray crystal structure of a potassium adduct of a
monoanionic CNC-pincer ligand featuring two mesoionic carbenes
is reported. Owing to the peculiar electronic and steric properties
of this ligand, the first neutral stable Ni(II)-hydride, and an unusual
Cu(II) complex displaying a seesaw geometry, have been isolated.
It is well known that tridentate pincer ligands not only give rise
to robust catalysts, but also allow for isolating extremely
reactive metal centers.1 A large number of both neutral and
monoanionic pincer ligands featuring N-heterocyclic carbenes
(NHCs) have been prepared, and the corresponding complexes
Chart 1
used as catalysts for various chemical transformations.2 However, amido functionality, its mononuclear tridentate Ni(II)-hydride
only four complexes, featuring a strongly donating amido-moiety and Cu(II) complexes.
as the central coordinating atom, flanked by two NHC wing-tip
The planar carbazole backbone with its rigid geometry seemed
groups in a CNC-fashion, have been reported (A–C) (Chart 1).3 an attractive choice for the design of a bis(mesoionic carbene)amido
Moreover, pincer ligands based on the novel generation of pincer-type ligand. The synthesis of the dicationic salt precursor,
carbenes, namely mesoionic carbenes (MICs),4 which are even namely the bis(1,2,3-triazolium)carbazole 1, was achieved in 43%
stronger donors than NHCs, have been even less explored. For the yield by an adapted version of the formal 1,3-dipolar cycloaddition
CCC-tridentate binding mode, a handful of examples are known between a 1,3-diaza-2-azoniaallene salt and a 1,8-diethynylcarbazole
with imidazol-4-ylidenes,4,5 whereas with 1,2,3-triazol-5-ylidenes,6 (Scheme 1).6a Addition of 3 equivalents of potassium hexa-
binuclear bridged complexes7 or mononuclear complexes with methyldisilazide (KHMDS) to a THF solution of 1 at ꢀ78 1C resulted
bidentate ligands where the central C-atom does not ligate,8 only in the deprotonation of the carbazole, keeping unchanged the
are exclusively found. A neutral CNC-analogue of tridentate two pendant 1,2,3-triazolium moieties. The cationic salt 2 was
terpyridine, [2,6-bis(1,2,3-triazol-5-ylidene)-pyridine], is the only isolated in 93% yield as an air- and moisture-stable red solid. The
example of a bisMIC pincer acting as a tridentate ligand.9 Here monodeprotonation of 1 is indicated by the absence of the N–H
we report the synthesis of the first stable anionic CNC-tridentate resonance in the 1H NMR spectrum, and by the presence of a
ligand featuring terminal 1,2,3-triazol-5-ylidenes and a central triazolium C–H signal (2H) at 10.03 ppm (see ESI†). The structure of
1 was confirmed by an X-ray diffraction study (Fig. 1). When a large
excess of KHMDS (5 equivalents) was added to a diethylether
a Chemistry Department, University of Pretoria, Private Bag X20, Hatfield 0028,
suspension of 1 at ꢀ78 1C, the potassium salt 3 could be isolated
Pretoria, South Africa. E-mail: daniela.bezuidenhout@up.ac.za; Tel: +27 420 2626
b UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla, CA 92093-0343, USA.
1
in good yield (72%) after extraction with hexanes. In the H NMR
spectrum, the disappearance of the acidic N–H and triazolium C–H
signals confirmed the formation of the triply deprotonated
compound 3. In the 13C{1H} NMR spectrum, a singlet at 195 ppm,
characteristic of a K–Ccarbene adduct, was observed.10 Interestingly,
the bis(carbene)amido potassium adduct is indefinitely stable
in solution and in the solid state, in the absence of oxygen
and water.
E-mail: guybertrand@ucsd.edu; Tel: +1 858 534 5412
c
´
Departamento de Quımica Inorganica y Organica, Universitat Jaume I,
Avenida Vicente Sos Baynat s/n, 12071 Castellon, Spain
† Electronic supplementary information (ESI) available: Synthetic procedures and
analytical data; CIF files for single crystal X-ray structural analysis. CCDC 956921–
956923, 957055 and 972504. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc49385g
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Scheme 2 Synthesis of complexes 4–6.
A characteristic hydride signal appears at ꢀ6.30 ppm in the
1H NMR spectrum, and a single crystal X-ray diffraction study
confirmed the structural assignment (Fig. 1). Note that two
cationic Ni(carbene)–hydride complexes have been reported to
date,11 complex 4 being the first example of an isolated neutral
Ni–carbene hydride complex. Formation of the hydride complex
is presumed to occur via the triazolium C5–H activation, similar to
the C2–H activation of the imidazolium ring observed by Cavell
et al.11a The molecular structure (Fig. 1) shows a distorted square
planar geometry around the Ni center, analogous to other nickel
hydride complexes featuring pincer ligands with a central amido-
ligating atom,12 and with comparable Ni–N1 and Ni–H1 bond-
lengths (see ESI†) to these12 and other Ni–H pincer complexes.13
The two 2,6-diisopropylphenyl groups (Dipp) of the mesoionic
carbenes shield the hydride. The 1,2,3-triazol-5-ylidenes rings are
not coplanar with the carbazolide backbone: both rings are tilted
[C4–C3–C2–C1 = ꢀ7.7(3)1]. The two mesoionic carbenes and the
metal center (C1–N1–C1) form an angle of 177.56(10)1.
Scheme 1 Synthesis of ligand precursors 1 and 2, and ligand 3.
Fig. 1 Molecular structures of 1, 3–6 (thermal ellipsoids at 50% probability
level). Hydrogen atoms (except triazolium C–H’s, N–H and Ni–H), solvent
molecules and counteranions (PF6ꢀ) were omitted for clarity except in the
case of 1. The diisopropylphenyl-groups of 3 were drawn as wireframe
structures for the sake of clarity.
Encouraged by the stability of the Ni–H complex 4, the
synthesis of the first example14 of a Cu(II)–H complex was attempted.
Following a similar procedure as described above, CuCl2 was
Crystals of 3, suitable for an X-ray diffraction study, were employed as starting material (Scheme 2), and the paramagnetic
obtained from a mixture of deuterated benzene and toluene. [(CNC)Cu(II)Cl] complex 5 was obtained in 86% yield.
The structure of 3 (Fig. 1) features a [(CNC)–K–K–(CNC)] dimer
Although the Cu(II)–carbene fragment is implicated in copper
centered on a distorted rhombic K2C2 core, with the two three- catalysed carbene transfer reactions, it is usually too reactive to be
coordinated potassium atoms acting as a bridge between two isolated.15 Again, only two other examples of Cu(II)–carbene
CNC ligands. The two 1,2,3-triazol-5-ylidene rings are planar complexes are known.16 No usable NMR data could be obtained
with bond distances between that found for single and double due to the paramagnetic nature of the complex, but the single
bonds, and are not coplanar with the carbazolide backbone. crystal data confirms the structure of this unusual carbene
The bond angles [N2–C1–C2 = 100.49(14)1] and [N5–C37–C38 = complex. The C2–C1–Cu1–C1 torsion angle of 110.64(5)1 displays
100.66(15)1] in 3 are more acute than the corresponding angle significant deviation from planarity, and the Cl1–Cu1–N1 bond
[N2–C1–C2 = 104.06(19)1] in 1 as always observed when com- angle of 123.53(6)1 is indicative more of an unusual seesaw
paring a carbene and its precursor.4 Each potassium binds to geometry than the expected square planar arrangement around
one N atom and one carbene carbon from a CNC unit plus one the Cu-atom.16,17 Attempts at substituting the chloro ligand with a
carbene carbon from the adjacent (CNC) ligand. The bond hydrido employing superhydride (LiHBEt3) proved unsuccessful;
distances (K1–N1 = 2.624(15); K1–C1 = 2.869(17); K1–C370
=
it resulted in the reduction of Cu(II) to Cu(I) and the formation
2.9684(19) Å) are well in accordance with other potassium– of the brown-colored complex 6 [(CNC)Cu(I)] (Scheme 2). An
carbene bond lengths of 3.000(13)–3.048 Å;10 additionally, long independent route, utilizing CuI, KHMDS and 1 as precursor,
carbon–K distances are observed (K1–C37 = 3.158(17) Å).10b
also led to the isolation of 6. An X-ray diffraction study (Fig. 1)
When the cationic pincer derivative 1 was reacted with excess revealed the rarely found ‘‘naked’’ slightly distorted T-shape
base and 1.1 equivalent of nickel(II) dichlorodimethoxyethane adduct geometry of the Cu(I),16,18 in which the metal center is coordi-
([Ni(DME)Cl2]) in THF at ꢀ78 1C, the unexpected nickel-hydride nated to the central amido and two mesoionic carbenes. The
complex [(CNC)NiH] 4 was obtained in 39% yield (Scheme 2). 1,2,3-triazol-5-ylidenes rings are both tilted from the plane of
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This work was supported by the National Research Foundation
of South Africa, (DIB, Grant number 76226), the Fulbright Scholar
Program (DIB), and the DOE (GB, DE-FG02-13ER16370).
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