Inorganic Chemistry
ARTICLE
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(6) Designation of the pincer ligand in 1 as LXX-type is warranted in
the sense that it is, in its closed shell form, a dianionic ligand (or a diradical if
the covalent bonding model is applied). On the other hand, it is instructive
to note that the Ni-N distances between different halves of the dimer are
shorter than those in each half (see X-ray structure of 1 in ref 5j), and that
this trend is confirmed by the calculated Ni-N bond orders (vide supra).
These observations imply greater covalent character in the Ni-N bonds that
are trans to the aryl moiety and suggest, in turn, that the dimeric complex 1
can be viewed as two (LXL0)Ni halves bridged through primarily Ni-amide
type interactions involving the L0 moieties.
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(19) For an interpretation of computed bond orders, please see:
Gorelsky, S. I.; Basumallick, L.; Vura-Weis, J.; Sarangi, R.; Hedman, B.;
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(22) It should be noted that the calculated sum of spin present on the
four atoms in Ni2N2 core in [1]þ is 1.06, but the sum of all atomic spin
3
densities in [1]þ is still 1.0. This is because some ligand atoms possess
3
negative spin densities due to spin-polarization effects.
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(24) We have also considered an alternative reaction pathway
involving initial oxidation at the ligand to generate an intermediate
featuring a Ni-coordinated aminyl radical. Such a species could undergo
H-abstraction by the in situ generated succinimide radical to give 3, or
abstract H from the reaction medium to generate 4. However, this
scenario would require more than 2 equiv of oxidant for the formation of
the ultimate product, 2.
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Inorg. Chem. 2009, 48, 6159. This complex features a dianionic XLX-type
SNS ligand; no structural studies have been conducted on this species
because of its limited solubility.
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(27) For a recent discussion on the characterization and potential
applications of metal complexes bearing aminyl radicals see: Hicks, R. G.
Angew. Chem., Int. Ed. 2008, 47, 7393.
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Fan, H.; Huffman, J. C.; Meyer, K.; Mindiola, D. J. Inorg. Chem. 2008, 47,
10479. It should be added that, strictly speaking, this compound is not a
pincer-type species because the two neutral donor moieties of the PNP
ligand are bound to different nickel centers, while it is the central amido
moiety that serves as bridge. Thus, the PNP ligand spans across the two
nickel centers and does not impose a meridional coordination environ-
ment on them.
(11) It should be noted that a wider-than-anticipated difference in
the potentials for anodic and cathodic waves (on the order of ca. 0.080-
0.120 V) is a common feature of the cyclic voltammograms obtained for
DCM solutions of all nickel pincer complexes examined by us. That one-
electron processes are at work in these systems is supported by the
results obtained from square-wave-voltammetry experiments and by the
fact that, under the experimental conditions employed during our
studies, ferrocene also produces similar peak-to-peak separations.
(12) All redox potentials noted in this report are referenced to the
Cp2Fe/[Cp2Fe]þ couple.
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(13) Analogous amide-to-imine transformations in pincer-type com-
plexes have been reported by Milstein’s group; see ref.2n and Zhang, J.;
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