(II) dibromide species having a P-H bond. Upon treating with NEt 3 , (PP
H
P)Co(Br) 2
was converted to a P-P bond-coupled dimer 42, which probably undergoes
dehalogenation coupled with the reduction of cobalt(II) (Fig. 19). Consequently, a
phosphinyl radical appears as a form of a predicted intermediate species, (PPP)
CoBr. The DFT analysis on the intermediate species displays reasonable effective
spin densities found at the central phosphorus atoms in both a ferromagnetic- and an
antiferromagnetic-coupled state, thus supporting a phosphinyl radical coordination
at a cobalt(I) center. This result suggests that both P- and Co-based frontier orbitals
are similar in energy, and thus inner-sphere electron transfer can effectively occur
between P and Co depending on the coordination environment of a (PPP)Co
scaffold. When a phenoxide group is introduced to 42, a phenoxide dicobalt
(I) species (43) forms leading to interestingly transformation [43]. By treating with
tert-butyl isocyanides to the solution of 43, two monomeric cobalt(I) species are
generated in a 1:1 ratio; a phosphinite cobalt(I) phenoxide (PP
OPh P)Co(OPh) (44)
and a diamagnetic bis-isocyanide species, (PPP)Co(CN
t Bu) 2 (45). Upon coordination of a π-acidic ligand, one phenolate migrates from a cobalt(I) center to a P-P
moiety to form a P-O bond, while a phosphide moiety is regenerated to give 45.
Compared to the cooperativity of a (PPP)Ni scaffold typically undergoing a
two-electron process by using a Ni(II/0) couple, a (PPP)Co scaffold prefers a
single-electron process occurring at a central phosphide moiety of a PPP ligand,
while a cobalt ion remains in its +1 oxidation state.
5 N-group Transfer Reactions of a M-P Moiety
To a (PPP)Ni scaffold, introducing a redox-noninnocent ligand, such as anilide,
allows to broaden the scope of metal-ligand cooperativity between P and Ni
[44]. Similar to other alkoxide group transfer, the cooperative transformation of a
ditolylamido nickel(II) species (46) to (PP
NTol2 P)Ni(CO) (47) displays when 46 is
exposed to CO(g). Thus, not only P-O and P-S bonds, a new P-N bond is successfully generated from the reaction of 46 with CO via the metal-ligand cooperative
transformation operating at a P-Ni moiety, as shown in Fig. 20. Interestingly, when
CO(g) is charged to the solution of a phosphide nickel(II) anilido complex (PPP)Ni
(NHMes) (46, Mes ¼ 2,4,6-trimethylphenyl), a P-P-coupled dinuclear nickel(0) species, (P 2 P-PP 2 ){Ni(CO)} 2 (48), is generated along with the formation of isocyanate
and amine (Fig. 20) [44]. This reaction involves multiple single-electron transfer
processes; two electrons are provided from both anilide and phosphide ligands to
reduce a nickel(II) ion to give 48. When light-induced one-electron transfer occurs
from anilide, an open-shell phosphide-Ni(I) specie should be generated.
Corresponding inner-sphere electron transfer leads to form a N radical, which is
responsible for H-atom abstraction. According to the DFT evaluations, a significant
spin density is found at a nickel ion in a (PPP)Ni moiety. Interestingly, when a CO
ligand coordinates to a corresponding nickel(I) center, umpolung-type inversion of
spin density between Ni and P occurs to give a nickel(0) mono-carbonyl species
Metal-Ligand Cooperativity of Phosphorus-Containing Pincer Systems
89
H
P)Co(Br) 2
was converted to a P-P bond-coupled dimer 42, which probably undergoes
dehalogenation coupled with the reduction of cobalt(II) (Fig. 19). Consequently, a
phosphinyl radical appears as a form of a predicted intermediate species, (PPP)
CoBr. The DFT analysis on the intermediate species displays reasonable effective
spin densities found at the central phosphorus atoms in both a ferromagnetic- and an
antiferromagnetic-coupled state, thus supporting a phosphinyl radical coordination
at a cobalt(I) center. This result suggests that both P- and Co-based frontier orbitals
are similar in energy, and thus inner-sphere electron transfer can effectively occur
between P and Co depending on the coordination environment of a (PPP)Co
scaffold. When a phenoxide group is introduced to 42, a phenoxide dicobalt
(I) species (43) forms leading to interestingly transformation [43]. By treating with
tert-butyl isocyanides to the solution of 43, two monomeric cobalt(I) species are
generated in a 1:1 ratio; a phosphinite cobalt(I) phenoxide (PP
OPh P)Co(OPh) (44)
and a diamagnetic bis-isocyanide species, (PPP)Co(CN
t Bu) 2 (45). Upon coordination of a π-acidic ligand, one phenolate migrates from a cobalt(I) center to a P-P
moiety to form a P-O bond, while a phosphide moiety is regenerated to give 45.
Compared to the cooperativity of a (PPP)Ni scaffold typically undergoing a
two-electron process by using a Ni(II/0) couple, a (PPP)Co scaffold prefers a
single-electron process occurring at a central phosphide moiety of a PPP ligand,
while a cobalt ion remains in its +1 oxidation state.
5 N-group Transfer Reactions of a M-P Moiety
To a (PPP)Ni scaffold, introducing a redox-noninnocent ligand, such as anilide,
allows to broaden the scope of metal-ligand cooperativity between P and Ni
[44]. Similar to other alkoxide group transfer, the cooperative transformation of a
ditolylamido nickel(II) species (46) to (PP
NTol2 P)Ni(CO) (47) displays when 46 is
exposed to CO(g). Thus, not only P-O and P-S bonds, a new P-N bond is successfully generated from the reaction of 46 with CO via the metal-ligand cooperative
transformation operating at a P-Ni moiety, as shown in Fig. 20. Interestingly, when
CO(g) is charged to the solution of a phosphide nickel(II) anilido complex (PPP)Ni
(NHMes) (46, Mes ¼ 2,4,6-trimethylphenyl), a P-P-coupled dinuclear nickel(0) species, (P 2 P-PP 2 ){Ni(CO)} 2 (48), is generated along with the formation of isocyanate
and amine (Fig. 20) [44]. This reaction involves multiple single-electron transfer
processes; two electrons are provided from both anilide and phosphide ligands to
reduce a nickel(II) ion to give 48. When light-induced one-electron transfer occurs
from anilide, an open-shell phosphide-Ni(I) specie should be generated.
Corresponding inner-sphere electron transfer leads to form a N radical, which is
responsible for H-atom abstraction. According to the DFT evaluations, a significant
spin density is found at a nickel ion in a (PPP)Ni moiety. Interestingly, when a CO
ligand coordinates to a corresponding nickel(I) center, umpolung-type inversion of
spin density between Ni and P occurs to give a nickel(0) mono-carbonyl species
Metal-Ligand Cooperativity of Phosphorus-Containing Pincer Systems
89