Reaction of a redox-active ligand complex of nickel with dioxygen probes ligand-radical character

Article abstract of DOI:10.1021/ja9065943

(Chemical Equation Presented) Bis(imino)pyridine complex [Ni{2,6-(ArN=CMe)₂C₅H₃N}Cl] (where Ar = 2,6-ⁱPr₂C₆H₃) was synthesized by reduction of the corresponding dichloride comple

Full text of DOI:10.1021/ja9065943

Published on Web 10/14/2009
Reaction of a Redox-Active Ligand Complex of Nickel with Dioxygen Probes
Ligand-Radical Character
Tony D. Manuel and Jan-Uwe Rohde*
Department of Chemistry, The UniVersity of Iowa, Iowa City, Iowa 52242
Received August 4, 2009; E-mail: jan-uwe-rohde@uiowa.edu
Redox-active ligands profoundly influence the electronic proper-
ties of transition metal complexes and may participate in their
multielectron redox chemistry. For example, such ligands are
capable of enabling oxidative addition chemistry at formal d0 metal
centers and the activation of molecular hydrogen and oxygen.1,2
Bis(imino)pyridines constitute a particularly versatile group of
redox-active ligands as they have afforded 3d metal complexes that
promote a variety of bond-forming and bond-breaking transforma-
tions.3 The ligands were shown to accept up to three electrons,4,5
and as suggested by spectroscopic and structural data, the reduction
of complexes of these ligands often places the electron density into
molecular orbitals of predominantly ligand character.5-8 An
Figure 1. Molecular structure of 2. Displacement ellipsoids are drawn at
the 50% probability level; hydrogen atoms have been omitted for clarity.
Color key: turquoise ) Ni, blue ) N, gray ) C, green ) Cl.
intriguing aspect of these compounds is that, even though the
reducing equivalents are not stored directly at the metal center,
corresponding distances in 1 and the free ligand.13,14 Distances
small-molecule reactivity such as N2 activation still occurs at the
metal center.3b,9-11
similar to those in 2 were found in related complexes of other first-
row transition metals (i.e., Cr,15 Fe,16 and Co9) and were attributed
We have investigated the reactivity of bis(arylimino)pyridine
to localization of the added electron in a ligand-based orbital.5,8,15
complexes with dioxygen to gain further insights into the small-
molecule reactivity involving this class of ligands. In the present
Consistent with the crystallographic data, the EPR spectrum of 2
report, we describe a one-electron reduced Ni complex that activates
in frozen toluene solution displays an isotropic resonance signal
O2 to cause the cleavage of a ligand C-C bond. The ligand
centered at g ) 2.016 (peak-to-peak separation ≈ 25 G). Thus,
oxygenation points to a mechanism that entails direct electron
complex 2 may be best described as a ligand radical anion complex
of NiII (Scheme 1, depicting only one resonance structure). It is of
note that other one-electron reduced Ni complexes of bis(arylimi-
no)pyridine ligands were described as NiI complexes of the
unreduced ligand on the basis of their EPR parameters.6,17
Complex 2 is stable in solution but is air-sensitive. Upon addition
of O2 to a toluene solution of 2 at 60 °C, the characteristic
absorption bands of 2 in the visible and near-IR region disappeared
under concomitant formation of new bands at ca. 400, 480, and
515 nm and appearance of isosbestic points at 375 and 430 nm
(Figure 2). These observations suggest that a new compound, 3,
was produced from 2. On a preparative scale, 3 was isolated as an
orange-red solid and recrystallized from nitromethane at -25 °C
transfer from the ligand to O2. In contrast, the O2 reaction of a
redox-active ligand complex of an early transition metal resulted
in the formation of metal-bound peroxo groups while leaving the
redox-active ligands intact.2b Furthermore, metal-centered reduction
of O2 by genuine NiI complexes of redox-inactive ligands is evident
from the observation of Ni-Ox species.12
Scheme 1. Synthesis of Compounds 2 and 3a
a Ar ) 2,6-iPr2C6H3; (i) 1 equiv of Na(Hg), N2; (ii) O2, 60 °C.
Reduction of [Ni{2,6-(ArNdCMe)2C5H3N}Cl2], 1 (where Ar )
2,6-iPr2C6H3),13 with a sodium amalgam under an inert gas
atmosphere afforded the new complex [Ni{2,6-(ArNdCMe)2-
C5H3N}Cl], 2 (Scheme 1). Insights into the molecular and electronic
structure of 2 were obtained by X-ray crystallography and EPR
spectroscopy. The crystallographic analysis on a black single crystal
of 2·2C7H8 confirmed that the Ni center is coordinated by the three
nitrogen atoms of the tridentate bis(imino)pyridine and a chloride
ligand in a distorted square-planar geometry (Figure 1 and Sup-
porting Information). Notable parameters in the ligand framework
are the C-C distances adjacent to the pyridine ring, i.e., 1.451(4)
Å for C1-C6 and 1.446(4) Å for C5-C8, which are shorter, and
the imine C-N distances, which are slightly longer than the
Figure 2. Reaction of 1 mM 2 (black) in toluene with O2 at 60 °C (product
solution containing 3, orange), as monitored by electronic absorption
spectroscopy (path length, 0.5 cm). Inset: Time course of the reaction (λ )
820 nm).
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10.1021/ja9065943 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
to afford single crystals of 3·2CH3NO2. The structure determination
unveiled an unexpected modification of the tridentate supporting
ligand. Akin to 2, the Ni center in 3 is situated in a distorted square-
planar environment consisting of three nitrogen donor atoms and a
chloride ligand (Figure 3). However, one of the two acetimino
groups of the tridentate ligand in 2 was transformed into an amidato
group with a C-O distance of 1.236(2) Å and a C-N distance of
1.348(2) Å. Furthermore, the C1-C6 and C5-C8 distances of
1.472(3) and 1.506(3) Å are longer than those in 2 and consistent
with C(sp2)-C(sp2) single bonds, suggesting that the newly formed
iminoethylpyridylcarboxamidato ligand is in its closed-shell form.
Consequently, the Ni center remains in the +II oxidation state
(Scheme 1). While the Ni-N and Ni-Cl distances in 3 are slightly
shorter than those in 2, the most significant contraction is observed
for the Ni-N3 distance (1.892(2) Å in 3 vs 1.942(2) Å for the
average of the Ni-N2 and Ni-N3 distances in 2) due to the
conversion of the imino donor into the anionic amidato donor.
one-electron oxidation. Thus, the O2 reactivity of these ligand-
radical complexes varies significantly with the nature of the metal
center. Further investigations of the mechanisms and the factors
influencing this reactivity are warranted.
In conclusion, a reduced bis(imino)pyridine nickel complex (2)
was synthesized and characterized as a NiII-ligand-radical species.
The ligand-centered oxygenation observed in the reaction of this
complex with O2 contrasts with the prevailing metal-centered
chemistry of redox-active bis(imino)pyridine ligands and demon-
strates that they can be directly involved in the activation and
conversion of a small molecule.
Acknowledgment. Acknowledgment is made to the University
of Iowa and the Donors of the American Chemical Society
Petroleum Research Fund (46587-G3) for support of this research.
We thank Dr. Dale C. Swenson for the collection of X-ray
diffraction data and Dr. Sarah C. Larsen and Mr. William M. Ames
for access to and assistance with their EPR spectrometer.
Supporting Information Available: Experimental procedures,
characterization data of all compounds, details of the crystal structure
determinations (Tables S1-S4, PDF), and crystallographic information
files of 2 and 3 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 3. Molecular structure of 3. Displacement ellipsoids are drawn at
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Compound 3 was further characterized by ESI-MS of the reaction
solution and the isolated product. In both cases, the mass spectra
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1
in nearly quantitative yield. The H NMR spectrum of 4 exhibits
the expected resonance signals of an asymmetrically disubstituted
pyridine ring and of two sets of isopropyl substituents. The presence
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the peaks arising from the NdCCH3 and ArCH(CH3)2 protons of
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[Co{2,6-(ArN)CMe)2C5H3N}Cl].9 Under the same conditions as
those used for 2, this complex was readily oxidized to a CoII
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(19) The methine C-H bonds in 2 are nearby an open coordination site of the
Ni center but remain unaltered. Furthermore, the presence of more easily
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thracene) does not affect the oxidation of 2 to 3.
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