Reductions of challenging organic substrates by a nickel complex of a noninnocent crown carbene ligand

Article abstract of DOI:10.1021/ja107703n

The first crown-tetracarbene complex of Ni(II) has been prepared, and its crystal structure determined. The complex can be reduced by Na/Hg, with an uptake of two electrons. The reduced complex reductively cleaves arenesulfonamides, including those derived from secondary aliphatic amines, and effects Birch reduction of anthracenes as well as reductive cleavage of stilbene oxides. Computational studies show that the orbital that receives electrons upon reduction of the complex 2 is predominantly based on the crown carbene ligand and also that the HOMO of the parent complex 2 is based on the ligand.

Full text of DOI:10.1021/ja107703n

Published on Web 10/20/2010
Reductions of Challenging Organic Substrates by a Nickel Complex of a
Noninnocent Crown Carbene Ligand
Neil J. Findlay, Stuart R. Park, Franziska Schoenebeck, Elise Cahard, Sheng-ze Zhou,
Leonard E. A. Berlouis, Mark D. Spicer,* Tell Tuttle,* and John A. Murphy*
WestCHEM, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Thomas Graham Building,
295 Cathedral Street, Glasgow, G1 1XL, United Kingdom
Received March 26, 2010; E-mail: john.murphy@strath.ac.uk
Scheme 1. Synthesis of Nickel-Containing Complexes and the
X-ray Crystal Structure of 2 (Only the Cation Is Shown)
Abstract: The first crown-tetracarbene complex of Ni(II) has been
prepared, and its crystal structure determined. The complex can
be reduced by Na/Hg, with an uptake of two electrons. The
reduced complex reductively cleaves arenesulfonamides, includ-
ing those derived from secondary aliphatic amines, and effects
Birch reduction of anthracenes as well as reductive cleavage of
stilbene oxides. Computational studies show that the orbital that
receives electrons upon reduction of the complex 2 is predomi-
nantly based on the crown carbene ligand and also that the
HOMO of the parent complex 2 is based on the ligand.
We recently reported1 the synthesis of a new family of metal
N-heterocyclic carbene (NHC) complexes, consisting of a macro-
cyclic (crown) tetra-NHC enclosing either a single Pd(II) ion or
two ions of Cu(I) or Ag(I). In view of the exceptional σ-donor
properties of NHC ligands,2 this macrocyclic tetra-NHC should be
able to affect significantly the redox properties of transition metals.
We are interested in strong and selective reducing agents3,4 and
reasoned that reduced metal crown NHC complexes3h should be
particularly powerful in this regard. Fort and co-workers showed5
that Ni(0)/imidazolium chloride/NaOiPr and related systems were
effective for reducing aryl halides (including aryl fluorides) to
arenes. In that case, a standard Ni(0)/Ni(II) cycle, featuring oxidative
addition and reductive elimination, was entirely consistent with their
results. We now report the synthesis of the Ni(II) crown tetra-NHC
complex 26 and its two-electron reduction to the complex 4 which
shows remarkable electron transfer chemistry.
extraordinary power to inhibit reduction of the Ni(II) ion, compared
to all previous neutral ligands.9,10 No oxidation of the Ni(II)
complex was observed in CV in the range 0 to +2.5 V. This shows
that the crown carbene ligand spectacularly inhibits both reduction
and oxidation at nickel.
Nickel(II) complex 2 was synthesized from macrocycle 1
(Scheme 1) and recrystallized from methanol. The crystal structure
of 2 reveals a square planar nickel atom coordinated by all four
NHC ligands. The ligand conformation topologically resembles the
cover of a tennis ball. To study the redox activity of 2, it was treated
with excess Na/Hg, and an aliquot of the supernatant solution was
removed for titration against iodine. A 1 equiv amount of the
reduced nickel complex converted 1 equiv of iodine to 2 mol of
iodide, indicating that the reduced nickel complex was a two-
electron donor.7 Hence, the reduced complex 4 could be viewed
formally as a Ni(0) complex (but see below for a more accurate
description).
To test the reactivity of the reduced complex 4 formed from
amalgam reductions, aliquots of the resulting red solution were
separated from the amalgam and added to substrates. The carbonyl
compounds, PhCHO, 5, and PhCOMe, 7, were reduced exclusively
to the (d,l)-isomer of the corresponding pinacol products 6 (64%)
and 8 (69%) respectively,7,11 while benzophenone 9 afforded
benzhydrol 10 (65%) (Scheme 2). These selective reductions
contrast with other Ni-based reductions of ketones and aldehydes,
where mixtures of pinacol and alcohol products are seen.12,13
Complex 4 did not lead to reduction of aliphatic ketones.
Our attention now turned to one of the most challenging reductive
transformations in organic chemistry, the Birch reduction of
aromatic rings,14 routinely involving alkali metals in liquid NH3.14c,d
Reaction of anthracene 11a with 4 afforded 9,10-dihydroanthracene
12a (55%) following an inverse quench14c,d of the reaction mixture.
More electron-deficient 9-substituted anthracenes 11b-e worked
well also (54-85%). Complex 4 was also reacted with p-
The corresponding PF6 salt (3) was prepared for analysis by
cyclic voltammetry (CV).8 Reduction of the complex revealed an
irreversible electron transfer at -2.4 V vs Ag/AgCl corresponding
to a single electron transfer (calibrated against Fc/Fc+). This peak
was partly hidden when a Pt cathode was used but fully revealed
when a glassy carbon electrode was used. A second peak was not
observed before the onset of solvent (DMF) reduction. Such a highly
negative potential implies that the neutral tetracarbene ligand has
9
15462 J. AM. CHEM. SOC. 2010, 132, 15462–15464
10.1021/ja107703n  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Reductions of Carbonyl Compounds and of
Anthracenes
Figure 1. HOMO (left) and LUMO (right) of the Ni(II) crown carbene
complex 2. System charge is 2+.
toluenesulfonamides including 13 and 15, which are among the
most difficult functional groups for reduction, despite common
application as both protecting and activating groups.15 Even the
least reactive toluenesulfonamides, those derived from secondary
aliphatic amines, e.g. 13, underwent efficient reductive cleavage,
although this required an excess (4 equiv) of the reducing agent to
achieve a reasonable rate3c (Scheme 3; see Supporting Information
(SI) for further examples). The reductive nature of the cleavages
was established by adding iodomethane to the products from
reaction of 15. This afforded sulfone 17 (72%) via methylation of
the p-toluenesulfinate anion. Finally, complex 4 also reduced cis-
and trans-stilbene oxides 18 and 19 respectively to (E)-stilbene 20
and 1,2-diphenylethanol 21.4 The formation of common products
from both epoxide isomers is consistent with a common intermedi-
ate, as expected for an electron transfer mechanism.
Figure 2. (Left) SOMO of the Ni crown carbene complex 22 [system
charge is 1+ ] and (right) SOMO of the Ni crown carbene complex 23
[system charge is 3+].
Computational studies reveal the true nature of the redox
processes. Complex 2 is formally a Ni(II) complex with the crown
carbene ligand. However, the addition of an electron to this system
populates the LUMO of 2, which resides primarily on the crown
carbene ligand (Figure 1). As such, the monocationic complex (22)
can be considered as a Ni(II) species with a radical anion ligand,
rather than involving reduction of the metal center. This is confirmed
upon inspection of the single occupied MO (SOMO) of 22 (Figure
2), which is closely related to the LUMO of 2. There is a minor
difference in the interaction of the Ni d-orbital with the carbene
ligands, which is due primarily to the distortion of the geometry of
the first coordination sphere. That is, there is a slight deviation from
planarity about the metal center upon reduction of the system (Table
1). Nonetheless, the pz-orbitals on the carbene C atoms clearly
dominate in both the LUMO of 2 (Figure 1) and the SOMO of 22
(Figure 2).
Further reduction of the crown carbene complex results in the
addition of a second electron to the crown carbene ligand. The
second electron is clearly donated into the half-filled orbital that is
predominantly ligand-based, encompassing the carbenic carbon of
each of the four imidazolylidene rings (Figure 3) with only a small
component on the Ni ion; 4 may best be viewed as a ligand dianion
complex of Ni(II). The HOMO of the neutral compound 4 (Figure
3) is, to all intents-and-purposes, identical to the SOMO of 22
(Figure 2). This results in a closed shell system, with the Ni(II)
Scheme 3. Reductive Cleavage of Organic Substrates Using
Complex 4
Table 1. Selected X-ray Crystallographic and Calculated Structural
Parameters for 2, 4, 22, and 23
2
4
22
23
X-ray
Calcd
Calcd
Calcd
Calcd
d(Ni-C), Å
1.930
1.930
1.930
1.930
90.01
90.00
90.02
89.96
1.962
1.962
1.962
1.963
91.96
92.00
91.95
91.97
1.963
1.962
1.963
1.963
90.29
90.40
90.34
90.33
1.933
1.957
1.957
1.932
87.50
92.79
87.50
87.10
1.900(7)
90
90
90
90
∠C-Ni-C, deg
(cis)
∠C-Ni-C, deg
(trans)
180
180
179.77 158.57 171.16 178.01
179.82 158.67 171.16 176.86
d(Ccarbene-N), Å
1.331(6) 1.366
1.366
1.391
1.404
101.5
1.377
1.381
103.0
1.370
1.370
104.7
∠N-Ccarbene-N, deg 102.6(6) 104.2
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C O M M U N I C A T I O N S
(2) While NHCs are substantially stronger σ-donors than phosphines, their
π-acceptor ability has become more widely acknowledged recently,
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with the anthracene (11d) or with acetophenone (7); and (iii) treatment of
macrocycle (1) with Na/Hg did not lead to reduction of the anthracene
(11d) [see SI for further information].
(8) Complex 3; 0.01 M soln in 0.1 M Bu4NPF6/DMF; glassy carbon working
electrode, Pt counter electrode, Ag/AgCl/KCl(sat.) reference electrode. CV
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Figure 3. HOMO of the nickel crown carbene complex 4. System charge
is 0.
metal center coupled to the doubly anionic crown carbene ligand.
A more pronounced tetrahedral distortion is observed at the metal
center in this case. Furthermore, in both 4 and 22 the Ccarbene-N
distances increase by about 0.03 Å and the N-C-N angle becomes
more acute by about 2° (with respect to 2), emphasizing the ligand-
based nature of the electron transfer (Table 1). While ligand
noninnocence has been long-established in square planar nickel
chemistry (especially with dithiolene ligands),16 it has not previously
been observed in NHC ligands.17,18
In order to test the conformational stability of the reduced
complex 4, the geometry of the first coordination sphere was
distorted to break one or two of the Ni-NHC bonds. However,
upon optimization, the distorted species immediately collapses back
to the stable square planar arrangement.
Studies of the putative oxidized species 23 (Figure 2) similarly
show that the electron transfer is from a ligand-based orbital, rather
than the Ni, with the square planar geometry around the metal
remaining largely undisturbed. An attempt to stabilize electrons
around the metal center in the parent species 2 by coordination
expansion, allowing one or two solvent molecules to access the
axial sites at the metal center, was also unsuccessful. Attempts to
optimize the solvent complexes resulted in their expulsion from
the first coordination sphere and formation of outer sphere
complexes via electrostatic interactions (Figures S2 and S3 in SI).
Thus, we conclude that the crown carbene ligand spectacularly
inhibits the redox processes at the Ni center as a result of (i)
inflexibility of the metal coordination sphere, (ii) inability to
undergo coordination expansion due to the ligand conformation,
and (iii) the low energy of the filled metal-based orbitals relative
to the MOs of the crown carbene.19
Acknowledgment. We thank EPSRC Mass Spectrometry Cen-
tre, Swansea, and EPSRC, WestCHEM, and the Glasgow Centre
for Physical Organic Chemistry (GCPOC) for funding.
Supporting Information Available: Experimental procedures,
spectroscopic data, .cif file for 2 computational results and cyclic
voltammetry data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) For another interpretation of noninnocence in carbenes, see: Romain, C.;
Miqueu, K.; Sotiropoulos, J. M.; Bellemin-Laponnaz, S.; Dagorne, S.
Angew. Chem., Int. Ed. 2010, 49, 2198–2201.
(19) In nickel dithiolene systems [Ni(dth)2]n- (n ) 2, 1, 0)15 the ligand π-orbitals
are also at higher energies than the metal-based orbitals and reduction results
in population of predominantly ligand-based orbitals.
References
(1) McKie, R.; Murphy, J. A.; Park, S. R.; Spicer, M. D.; Zhou, S.-Z. Angew.
Chem., Int. Ed. 2007, 46, 6525–6528.
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