The related PNP-platform bearing non-arene C 2 bridgeheads between the central
amino-N and the flanking phosphine-P donors [192, 193] display redox activity,
whereby the reversibility as well as the locus of the oxidation depends both on the
metal present (Co or Ni) and on the level of dehydrogenation of the C 2 bridges, as
detailed by the group of Schneider. The redox-chemistry of the
Ni-di(vinylphosphino)amido derivative was characterized in detail using XAS,
EPR, CV, XRD, spectroelectrochemistry and DFT [194]. On top of this ligandcentred redox-chemistry, this platform also allows for proton and H atom transfer
reactivity. Strikingly, the Co analog showed exclusive oxidation (and reduction)
chemistry at the metal centre, not the ligand backbone, resulting in the isolation of a
rare square planar Co
III complex that was crystallographically characterized [195].
Diarylamido-monophosphine analogs as well all-nitrogen diarylamido-bisamino
analogs have also been established as redox-active ligand frameworks. Ozerov and
co-workers prepared a series of PNZ ligands, with Z being either a differently
substituted phosphorus-based donor or an imine donor [196]. A series of group
10 metal complexes (M ¼ Pd, Pt, some Ni) was prepared, complemented by the
corresponding Rh(CO) complexes and the electronic properties scrutinized by CV as
well as IR spectroscopy (for the Rh-carbonyl species). Quasi-reversible one-electron
redox events were observed across the whole series, with several observations
supporting ligand-centred redox, including (1) redox potentials were mainly affected
by substituents conjugated with the diarylamido π system but remote from the metal
centre; (2) only small changes were observed for the E ½ values across the various
metals; and (3) structural changes in the diarylamido backbone largely effected the
redox potential. The IR spectroscopic study of the various Rh-carbonyl complexes
allowed for insight in the donor strength of the pincer ligands, and although there did
not appear to be a clear relationship between ν CO as measured by IR and the E ½
obtained by CV across the whole ligand scope (i.e. the most electron-donating ligand
is not the most easily oxidized), some trends could be identified within smaller
subsets of ligands wherein only substituents at specific sites were varied. The Fiedler
group reported on dinuclear cobalt complexes bearing a diarylamido as bridging
moiety to span two binding pockets consisting of Schiff-base type salicylaldimine
chelates that also feature redox activity in themselves, overall creating a pentadentate
donor sphere accommodating two metals [197]. As a result, several electrochemical
oxidative events are apparent by CV. Ozerov developed monophosphinemonoimine-diarylamido ligand scaffolds that could form dinuclear palladium complexes, each bound to a redox-active pincer unit [198]. Depending on the length of
the –(CH 2 ) n linker between both imine units, electronic communication between
both redox-active pockets upon oxidation by CV was either present (short) or absent
(long).
Vicic et al. examined the (redox-) chemistry of the Ni(CF 3 )(NNN) complex, with
NNN being Nickamine originally successfully developed by Hu for Ni-catalysed
cross-coupling of alkyl halides [199]. The trifluoromethyl complex Ni(CF 3 )
(NNN
Me
) showed a quasi-reversible oxidation wave in CV at +0.80 V vs. Ag/Ag
+
in THF, likely coupled to follow-up chemistry, as scan rate variations led to
unsymmetrical current flow for the oxidation vs. the re-reduction process
170
J. I. van der Vlugt
amino-N and the flanking phosphine-P donors [192, 193] display redox activity,
whereby the reversibility as well as the locus of the oxidation depends both on the
metal present (Co or Ni) and on the level of dehydrogenation of the C 2 bridges, as
detailed by the group of Schneider. The redox-chemistry of the
Ni-di(vinylphosphino)amido derivative was characterized in detail using XAS,
EPR, CV, XRD, spectroelectrochemistry and DFT [194]. On top of this ligandcentred redox-chemistry, this platform also allows for proton and H atom transfer
reactivity. Strikingly, the Co analog showed exclusive oxidation (and reduction)
chemistry at the metal centre, not the ligand backbone, resulting in the isolation of a
rare square planar Co
III complex that was crystallographically characterized [195].
Diarylamido-monophosphine analogs as well all-nitrogen diarylamido-bisamino
analogs have also been established as redox-active ligand frameworks. Ozerov and
co-workers prepared a series of PNZ ligands, with Z being either a differently
substituted phosphorus-based donor or an imine donor [196]. A series of group
10 metal complexes (M ¼ Pd, Pt, some Ni) was prepared, complemented by the
corresponding Rh(CO) complexes and the electronic properties scrutinized by CV as
well as IR spectroscopy (for the Rh-carbonyl species). Quasi-reversible one-electron
redox events were observed across the whole series, with several observations
supporting ligand-centred redox, including (1) redox potentials were mainly affected
by substituents conjugated with the diarylamido π system but remote from the metal
centre; (2) only small changes were observed for the E ½ values across the various
metals; and (3) structural changes in the diarylamido backbone largely effected the
redox potential. The IR spectroscopic study of the various Rh-carbonyl complexes
allowed for insight in the donor strength of the pincer ligands, and although there did
not appear to be a clear relationship between ν CO as measured by IR and the E ½
obtained by CV across the whole ligand scope (i.e. the most electron-donating ligand
is not the most easily oxidized), some trends could be identified within smaller
subsets of ligands wherein only substituents at specific sites were varied. The Fiedler
group reported on dinuclear cobalt complexes bearing a diarylamido as bridging
moiety to span two binding pockets consisting of Schiff-base type salicylaldimine
chelates that also feature redox activity in themselves, overall creating a pentadentate
donor sphere accommodating two metals [197]. As a result, several electrochemical
oxidative events are apparent by CV. Ozerov developed monophosphinemonoimine-diarylamido ligand scaffolds that could form dinuclear palladium complexes, each bound to a redox-active pincer unit [198]. Depending on the length of
the –(CH 2 ) n linker between both imine units, electronic communication between
both redox-active pockets upon oxidation by CV was either present (short) or absent
(long).
Vicic et al. examined the (redox-) chemistry of the Ni(CF 3 )(NNN) complex, with
NNN being Nickamine originally successfully developed by Hu for Ni-catalysed
cross-coupling of alkyl halides [199]. The trifluoromethyl complex Ni(CF 3 )
(NNN
Me
) showed a quasi-reversible oxidation wave in CV at +0.80 V vs. Ag/Ag
+
in THF, likely coupled to follow-up chemistry, as scan rate variations led to
unsymmetrical current flow for the oxidation vs. the re-reduction process
170
J. I. van der Vlugt