Classical and non-classical redox reactions of Pd(ii) complexes containing redox-active ligands

Article abstract of DOI:10.1039/c4cc04863f

Reactivity studies of a Pd(ii)-verdazyl complex reveal novel ligand-centred reduction processes which trigger pseudo-reductive elimination at Pd. Reaction of the complex with water induces a ligand-centred redox disproportionation. The reduced verdazyl ligands can also be reversibly protonated. This journal is

Full text of DOI:10.1039/c4cc04863f

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Classical and non-classical redox reactions of
Pd(II) complexes containing redox-active ligands†
Cite this: Chem. Commun., 2014,
0, 11676
5
a
b
b
c
Corey A. Sanz, Michael J. Ferguson, Robert McDonald, Brian O. Patrick and
a
Received 26th June 2014,
Accepted 6th August 2014
Robin G. Hicks*
DOI: 10.1039/c4cc04863f
www.rsc.org/chemcomm
Reactivity studies of a Pd(II)–verdazyl complex reveal novel ligand- herein we demonstrate that ligand-centered redox processes
centred reduction processes which trigger pseudo-reductive elimina- trigger unexpected and unique metal-centered reactivity.
ꢀ
tion at Pd. Reaction of the complex with water induces a ligand-centred
The phosphine verdazyl ligand vdP was prepared using
redox disproportionation. The reduced verdazyl ligands can also be standard procedures for the synthesis of 1,5-diisopropyl-6-
1
0,11
ꢀ
reversibly protonated.
oxoverdazyl radicals (see ESI†).
PdCl (MeCN) afforded the complex ( vdP)PdCl
The recent resurgence of interest in redox-active ligand (RAL) to our previously reported syntheses of related verdazyl–PdCl
Reaction of vdP with
ꢀ
2
2
2
, analogously
2
7
,12
complexes stems in part from the possibility of discovering and complexes (Scheme 1).
The electronic structure of the radical
developing new stoichiometric and catalytic reactions. A major ligand, as probed by EPR and UV-vis spectroscopy, is entirely
motivation for exploring this paradigm is to enable multi- representative of N,N -diisopropyl-6-oxoverdazyl derivatives; a
electron chemistry at base metals. Accordingly, much of the the EPR hyperfine coupling to the phosphorus nucleus (9.3 G)
1
0
2
activity in this field has focused on RAL complexes of first row indicates only a very small amount of spin density at P. Analo-
transition metals; studies involving heavier metals are far less gously, the electronic spectrum (Fig. S6, ESI†) and EPR spectrum
3
ꢀ
common.
Palladium complexes are employed as catalysts in a huge corresponding data for other verdazyl–palladium complexes,
(Fig. S9, ESI†) of ( vdP)PdCl are qualitatively consistent with the
2
1
2
range of reactions. Some of these reactions incorporate auxiliary the spin is largely confined to the tetrazine ring of the verdazyl.
ꢀ
redox reagents, e.g., copper for the Wacker oxidation and Sono-
The molecular structures of radical vdP and Pd complex
4
5
ꢀ
gashira reaction. Benzoquinone and TEMPO have also been ( vdP)PdCl are shown in Fig. 1. The structural metrics asso-
explored in this context, and Pd–benzoquinone or Pd–TEMPO ciated with the free ligand are normal for 6-oxoverdazyls.
2
1
1,13
complexes may well be (labile) intermediates in the catalytic The changes in tetrazine ring bond lengths upon coordina-
6
reactions they enable. These and other studies are suggestive of tion to Pd are minor, consistent with related verdazyl–Pd
1
2
ꢀ
a potentially rich chemistry associated with Pd–RAL complexes, complexes. The larger chelate ring size of the vdP ligand
but this concept is an under-explored one. We have recently forces a non-coplanar arrangement of the palladium ion’s
reported on the electrochemical properties of Pd complexes square plane with that of the NCCCP chelate ring. The two
7
8,9
containing verdazyls, a new class of RAL. Herein we present Pd–Cl bond lengths are markedly different: the Pd–Cl bond
preliminary investigations of the chemical redox properties of trans to the phosphine is 2.38 Å whereas the Pd–Cl trans to N is
related complexes based on a new phosphine/verdazyl ligand. only 2.27 Å. This observation can be ascribed to the trans
These studies constitute the first explorations of chemical reac- influence of the phosphine ligand.
tivity of any type of a metal complex of a verdazyl radical, and
a
Department of Chemistry, University of Victoria, PO Box 3065 STN CSC, Victoria,
BC V8W 3V6, Canada. E-mail: rhicks@uvic.ca
b
X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta,
Edmonton, Alberta, T6G 2G2, Canada
c
Crystallography Laboratory, Department of Chemistry, University of British Columbia,
Vancouver, BC V6T 1Z1, Canada
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization details. CCDC 997842–997845. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc04863f
Scheme 1
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addition has occurred to the (p*, N–N antibonding) ligand SOMO
and the Pd ions remain in their +2 oxidation state. The reduction
is fully reversible: treatment of (vdP) Pd Cl with PhICl regene-
2
2
2
2
rates the monopalladium radical complex (eqn (2)). The chloride
loss/addition accompanying reduction/oxidation renders these
processes non-classical versions of reductive elimination/oxidative
addition, in which the ligand has been reduced/oxidized conco-
mitant with ligand loss/addition at the metal. Such processes
16
have been noted in RAL complexes of other metals but not for
palladium.
ꢀ
ꢀ
2
Fig. 1 Structures of vdP (left) and ( vdP)PdCl (right). Thermal ellipsoids
ꢀ
ꢁ
represented at 50%. Hydrogen atoms removed for clarity. Selected bond
lengths for vdP (Å): N1–N2 1.360(2), N3–N4 1.368(2), N2–C2 1.331(3),
N3–C21.326(3). Selected bond lengths for ( vdP)PdCl
2( vdP)PdCl
2
+ 2Fc* - (vdP)
2
Pd
2
Cl
2
+ 2Fc* + Cl
(1)
(2)
ꢀ
ꢀ
ꢀ
2
(Å): N1–N2 1.350(6),
(vdP) Pd Cl + PhICl - 2( vdP)PdCl + PhI
2
2
2
2
2
N3–N4 1.372(6), N2–C2 1.318(7), N3–C2 1.341(7), N3–Pd1 2.065(4),
P1–Pd1 2.2243(16), Cl1–Pd1 2.3822(17), Cl2–Pd1 2.2678(16).
2 2 2
Reaction of (vdP) Pd Cl with HCl leads to ligand protonation
and generation of a mononuclear Pd complex (H-vdP)PdCl2
ꢀ
The electrochemical properties of ( vdP)PdCl
2
differ from those containing the protonated (reduced) verdazyl (also known as
7
17
of related palladium complexes containing verdazyl ligands.
Whereas the latter generally possess reversible one-electron base regenerates the binuclear complex (vdP)
the so-called ‘‘leuco’’ verdazyl ). Deprotonation via addition of
2
Pd Cl . The struc-
2
2
reduction processes, reduction of the former is at best quasi- ture of the leuco complex is shown in Fig. 3. The tetrazine bond
reversible (Fig. S7, ESI†). This result was somewhat surprising in metrics are consistent with a fully reduced and protonated (at
light of the qualitative similarities of the electronic structures of the
radical palladium complexes (see above). In order to understand rapidly oxidized to the corresponding radical, this leuco Pd
possible reasons for these differences in electrochemistry, we complex is air stable; more powerful oxidants (PhICl ) react with
. Reaction with deca- the leuco complex via ligand oxidation to give ( vdP)PdCl . The
2
N ) ring. In stark contrast to free leuco verdazyls which are
2
ꢀ
ꢀ
explored chemical reductions of ( vdP)PdCl
2
2
methylferrocene leads not to the anionic species [(vdP)PdCl ] – but (leuco) verdazyl ligand stability to (de)protonation also contrasts
2
instead to an air-stable, binuclear species (vdP) Pd Cl whose the behaviour of Pd p-complexes of benzoquinone (BQ), in which
2
2
2
structure is shown in Fig. 2 (see also eqn (1)). The structure consists protonation of formally Pd(0)–BQ induces intramolecular electron
1
8
of two square planar Pd ions linked by bridging chlorides. The transfer, leading to Pd(II) and free 1,4-hydroquinone.
ꢀ
(
m-Cl)
2
Pd
2
core is a fairly common structural motif in Pd(II)
The radical complex ( vdP)PdCl
2
is unexpectedly – and in
chemistry but is normally accessed by either chloride abstraction contrast to previously reported verdazyl–Pd complexes – very
1
4
from (LL)PdCl
synthesis from a monoanionic ligand LX and a PdCl
2
(where LL is a neutral bidentate ligand) or direct moisture-sensitive; reaction with water produces the reduced
15
source; in binuclear complex described above and one equivalent of
the present system it is the conversion of an LL type ligand ( vdP) to uncoordinated radical species in which the phosphine has
an LX ligand via ligand reduction that leads to binuclear complex been converted to its phosphine oxide vdPQO (eqn (3)). The
2
ꢀ
ꢀ
formation. The N–N bonds in the tetrazine ring are significantly source of the oxygen in the latter was confirmed to be water
ꢀ
18
longer than in the radical complex ( vdP)PdCl
2
, indicating electron from H
scavenged by added base (NEt
of PdCl is also generated. This reaction can also be conducted
2
O studies (see ESI†). The HCl by-product can be
3
), and a (presumed) equivalent
2
Fig. 2 Structure of (vdP)
2
Pd
2
Cl
2
. Thermal ellipsoids represented at 50%. Hydro-
2
Fig. 3 Structure of (H–vdP)PdCl . Thermal ellipsoids represented at 50%.
gen atoms removed for clarity. Selected bond lengths (Å): N1–N2 1.373(4),
N3–N4 1.421(4), N2–C2 1.300(5), N3–C2 1.354(5), N3–Pd1 2.054(3),
P1–Pd1 2.2273(10), Cl1–Pd1 2.4048(10), Cl1 –Pd1 2.4354(10).
Hydrogen atoms except H2 removed for clarity. Selected bond lengths (Å):
N1–N2 1.417(3), N3–N4 1.417(3), N2–C2 1.368(3), N3–C2 1.292(3), N3–Pd
2.0562(19), P1–Pd 2.2192(6), Cl1–Pd1 2.2791(6), Cl2–Pd1 2.3586(6).
0
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ꢀ
starting from the vdP ligand, Pd(MeCN)
generates the same products without the ‘‘extra’’ PdCl
eqn (4)). In these reactions, water induces a redox dispropor-
2
Cl
2
and water, which
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(
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equivalents are reduced by one electron to their corresponding
anions, and the oxidation process involves conversion of the
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some resemblance to the well-known reaction of Pd(OAc) with
2
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9
oxidation by product.
7
8
ꢀ
ꢀ
3
( vdP)PdCl
2
+ H
2
O - (vdP)
2
Pd
2
Cl
2
+ vdPQO + 2HCl + ‘‘PdCl
2
’’
(3)
ꢀ
ꢀ
3
vdP + 2(MeCN)
2
PdCl
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+ H
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O - (vdP)
2
Pd
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Cl
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+ vdPQO
(4)
9
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1
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In summary, we have presented fundamental reactivity
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studies constitute the very first investigations of the chemical 12 S. D. J. McKinnon, J. B. Gilroy, R. McDonald, B. O. Patrick and
R. G. Hicks, J. Mater. Chem., 2011, 21, 1523–1530.
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1
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to the observation of non-classical versions of oxidative addition
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1
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2
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11678 | Chem. Commun., 2014, 50, 11676--11678
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