[200]. The electrochemical oxidation potential for this CF 3 complex is equal in
magnitude to that of its precursor, NiCl(NNN
Me ), which indicates that the locus of
the redox is the ligand. Strikingly, the latter complex was reported to show irreversible oxidation behaviour in THF with NBu 4 PF 6 as electrolyte [201]. DFT calculations support the proposition that the oxidation is ligand-centred, with the HOMO of
the neutral complex being largely ligand-based and centred on the amido nitrogen
and upon oxidation 41% spin density residing on the amido nitrogen in the cation [Ni
(CF 3 )(NNN
Me
)]
+
. The Vicic group also reported on a series of Cu and Ni complexes
with bis(oxazolinyl)amino (boxam) ligands [202]. For the Ni species, a dependency
of oxidation potential on the co-ligand X in the series Ni(X)(boxam) was observed,
with E ox ranging from À0.17 V for X ¼ Cl to +0.42 V for X ¼ CF 3 (referenced
against Fc/Fc
+
) and with varying levels of reversibility. Despite the likely intermediacy of substrate radical species in, e.g. the alkyl halide cross-coupling, with
potential intermediacy of a Ni
III species, no evidence for the redox involvement of
these ligands in any catalytic process has been reported to date.
The group of Gardinier has developed the chemistry of diarylamido systems with
flanking pyrazole units featuring neutral β nitrogen donors for high-valent Rh
III
dichloride [203] as well as Re
I
-tricarbonyl [204] complexes. Furthermore, one
homoleptic Ga
III complex was reported, which exhibited weak electronic communication between both ligand fragments, proposedly via superexchange [205]. A
series of homoleptic Ni
II -bis-ligand species were reported that had different parasubstituents in the 4 and 4
0 positions of the diarylamido backbone to further elucidate
the effects that tailor the redox-chemistry, such as the (weak) electronic communication between ligands in ligand-mixed valent mono-oxidized derivatives, as
deduced from the intervalence charge transfer (IVCT) transition [206]. In all cases,
the combination of spectroscopic, electrochemical and computational data suggest
quasi-reversible but localized redox-chemistry to occur strictly in the diarylamido
backbone. Also a dinucleating analog was prepared and coordinated to Re(CO) 3 as a
bis-anionic fragment [207].
3.4 Neutral Pincer Systems
Although not strictly reporting on ligand-centred radical species, Agapie reported on
bisphosphino-arene pincer-like PareneP species within a variety of mononuclear and
dinuclear complexes. The central arene fragment was shown to show redox activity,
for instance, in the installment of a dinuclear M 2 -polycarbonyl (M ¼ Fe, Co) core
[208]. X-ray crystallographic determination of the solid-state structures revealed
features consistent with reduction of the arene ring to a bis-allyl motif, concomitant
with formal oxidation of the M 2 core by two electrons to yield M
I
-M
I fragments.
More recently, based on this original PareneP motif, the group converted the central
arene motif into a capped 1,2-catechol derivative that still proved amenable to
partial, i.e. η
2 or η
4 arene coordination, to, e.g. Mo [209]. Reaction of the siloxane
or catecholborane capped derivatives with O 2 led to deprotection and oxidation of
Redox-Active Pincer Ligands
171
magnitude to that of its precursor, NiCl(NNN
Me ), which indicates that the locus of
the redox is the ligand. Strikingly, the latter complex was reported to show irreversible oxidation behaviour in THF with NBu 4 PF 6 as electrolyte [201]. DFT calculations support the proposition that the oxidation is ligand-centred, with the HOMO of
the neutral complex being largely ligand-based and centred on the amido nitrogen
and upon oxidation 41% spin density residing on the amido nitrogen in the cation [Ni
(CF 3 )(NNN
Me
)]
+
. The Vicic group also reported on a series of Cu and Ni complexes
with bis(oxazolinyl)amino (boxam) ligands [202]. For the Ni species, a dependency
of oxidation potential on the co-ligand X in the series Ni(X)(boxam) was observed,
with E ox ranging from À0.17 V for X ¼ Cl to +0.42 V for X ¼ CF 3 (referenced
against Fc/Fc
+
) and with varying levels of reversibility. Despite the likely intermediacy of substrate radical species in, e.g. the alkyl halide cross-coupling, with
potential intermediacy of a Ni
III species, no evidence for the redox involvement of
these ligands in any catalytic process has been reported to date.
The group of Gardinier has developed the chemistry of diarylamido systems with
flanking pyrazole units featuring neutral β nitrogen donors for high-valent Rh
III
dichloride [203] as well as Re
I
-tricarbonyl [204] complexes. Furthermore, one
homoleptic Ga
III complex was reported, which exhibited weak electronic communication between both ligand fragments, proposedly via superexchange [205]. A
series of homoleptic Ni
II -bis-ligand species were reported that had different parasubstituents in the 4 and 4
0 positions of the diarylamido backbone to further elucidate
the effects that tailor the redox-chemistry, such as the (weak) electronic communication between ligands in ligand-mixed valent mono-oxidized derivatives, as
deduced from the intervalence charge transfer (IVCT) transition [206]. In all cases,
the combination of spectroscopic, electrochemical and computational data suggest
quasi-reversible but localized redox-chemistry to occur strictly in the diarylamido
backbone. Also a dinucleating analog was prepared and coordinated to Re(CO) 3 as a
bis-anionic fragment [207].
3.4 Neutral Pincer Systems
Although not strictly reporting on ligand-centred radical species, Agapie reported on
bisphosphino-arene pincer-like PareneP species within a variety of mononuclear and
dinuclear complexes. The central arene fragment was shown to show redox activity,
for instance, in the installment of a dinuclear M 2 -polycarbonyl (M ¼ Fe, Co) core
[208]. X-ray crystallographic determination of the solid-state structures revealed
features consistent with reduction of the arene ring to a bis-allyl motif, concomitant
with formal oxidation of the M 2 core by two electrons to yield M
I
-M
I fragments.
More recently, based on this original PareneP motif, the group converted the central
arene motif into a capped 1,2-catechol derivative that still proved amenable to
partial, i.e. η
2 or η
4 arene coordination, to, e.g. Mo [209]. Reaction of the siloxane
or catecholborane capped derivatives with O 2 led to deprotection and oxidation of
Redox-Active Pincer Ligands
171