Communication
cies may prevent further chemistry at the metal, although alky-
lation of the nitrogens was possible. Similarly, although Flow-
ers and Cossairt et al. very recently showed the power of phos-
phine-facilitated direct metalation to make two PNHCs in one
For the first application of 1, metals of square planar geome-
try were targeted so that the fourth coordination site would
certainly be in close proximity to the NH wingtips. Although to
the best of our knowledge, none of the Group 10 metals have
successfully engaged in PNHC formation by direct metalation,
[
6g]
step, their product was coordinatively saturated. Kunz et al.
reported intriguing species B, made using a three-step protect-
ing-group strategy, but without an indication of its chemistry,
II
II
Pd and Pt were tested first, because of their extensive CÀH
metalation literature. The Pt analogue 2c was formed by
[
12]
[5d]
other than aqueous acid stability.
using Cl Pt(COD) at 1008C for six days (Scheme 1). After experi-
2
For our purpose of wanting to create a stable pincer PNHC
framework with one site available for chemistry and catalysis,
the Kunz ligand framework of C1–7 was very attractive, be-
cause of indications from X-ray crystal structure of favorable
mentation, it was found that the best way to make 2b was to
use [Pd(OAc) ] to enable metalation, followed by treatment
2
with aqueous NaCl. More remarkably, the Ni analogue 2a
could be made in similar way, at higher temperature. Carboxyl-
ate-assisted CÀH bond activation is widespread, in which vari-
[10]
accommodation of a metal in the binding pocket. However,
the formidable challenge of synthesizing PNHC complexes de-
mands different synthetic routes than normal NHC analogs. We
envisioned 1 (Scheme 1) as starting point, with tert-butyl sub-
[
13]
ous proton-shuttling roles are possible. To ease the purifica-
tion of metalation products, the crude samples of 3b and
c were converted cleanly to the chloride species 2a and b,
which were more readily purified by silica-gel column chroma-
tography.
The chloride complexes 2a–c were converted to the acetate
or triflate analogues by using the respective silver salts
(
Scheme 1). Bubbling CO through a solution of the triflate
complexes 5a–c in benzene gave 6a–c. Removal of solvent by
oil-pump vacuum from solutions of 6a and b led to CO loss
and recovery of 5a and b, suggesting that the MÀCO bond is
weaker in the Ni and Pd cases, than for Pt. Bubbling ethene
through solutions of 5a–c in benzene gave only Pt analogue
7
c with no reaction in the Ni and Pd cases. The Pt hydride spe-
cies 4c could be crystallized, whereas attempts at isolating the
Pd and Ni hydrides were unsuccessful.
To give insight into structure and hydrogen bonding, Table 1
compares the coordination sphere of seven PNHC complexes
[
14]
prepared in this study to those for C1–C3, aprotic analogs of
Kunz. With the exception of 6c, discussed below, the struc-
tures of the new PNHC pincers are all close to ideal square
planar geometry, with C-M-C and N-M-X angles near 1808. The
metal bond lengths in the Pd and Pt species (e.g., 2b and c)
are quite similar, as expected for second- and third-row conge-
ners, whereas the first-row Ni complex, 3a, showed shorter
bonds than Pd analogue 3b.
Scheme 1. (a) Preparation of bis PNHC complexes. (b) AgOAc, acetone, rt,
1
2–15 h; (c) NaBH
5c); AgOTf, acetone, rt, 1 h (5c); (e) CO or C
c were characterized in situ.
4
, THF, 708C, 20 h; (d) AgOTf, THF, rt, 40 min (5a) or 7.5 h
(
2
H
4
(1 atm), [D ]benzene; 6a–
6
The role of hydrogen bonding involving the PNHC ligands
was determined by using a combination of X-ray crystallogra-
phy, NMR, and IR spectroscopies (with the latter confirming
solid-state studies, see below). Complexes with acetato and tri-
flato ligands (3, 5) exhibit intramolecular hydrogen bonding,
whereas intermolecular interaction was seen in 6. The structure
of 3b (Figure 2a) clearly shows the acetato ligand and one NH
interacting, whereas the structure of 5b (Figure 2b) shows the
triflato ligand reaching both NH moieties. In both cases, the
planarity of the pincer ligand is essentially maintained, with
some flexibility to accommodate the asymmetry introduced by
the tetrahedral sulfur of the triflate in 5b. The planarity of the
pincer framework can be gauged by noting the dihedral angle
between the bond between PNHC carbene C and protic N
atoms, and the bond between metal and coordinated atom of
X or L, which in the case of 3b is 3.2 and 4.78 for the two inde-
pendent molecules in the unit cell, and in the case of 5b is
slightly larger (9.5, 11.18). Greater distortion and stronger hy-
stituents on imidazoles to disfavor metal N coordination.
Herein, we report the successful synthesis of Pd and Pt
bis(PNHC) complexes, and even Ni analogs, all using direct
metalation. Moreover, we show effects of PNHC hydrogen
bonding, as well as preliminary studies of reactivity, including
anti-Markovnikov alkyne functionalization and catalysis of hy-
dration to aldehyde. The air stability of the pincer complexes
can be an attractive feature for further applications in catalysis.
The synthesis of 1 in multigram quantities was accomplished
in 37% overall yield in three steps from carbazole. The tert-bu-
[11a]
tylation method of Hou and co-workers
gave 3,6-di-tert-
butyl-carbazole, which was subjected to electrophilic iodina-
[
11b]
tion, described by Inoue and Nakada
on a similar com-
giving 1,8-di-iodo-3,6-di-
tert-butyl-carbazole. Copper-catalyzed coupling described by
+
À [11c]
pound, utilizing PhCH NEt ICl2
,
2
3
[
10]
[11d]
Kunz, but using 4-tert-butylimidazole gave 1.
Chem. Eur. J. 2015, 21, 10988 – 10992
10989
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Marelius, David C.
