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In-Depth Information
R
R
R
R
h
ν
•
•
2
P
Cl
2
P
PP
R'
R'
R'
R'
35
34a
; R = 2,6-(CF
3
)
2
C
6
H
3
R' = N
i
Pr
2
,N
c
Hex
2
34b
; R = N
i
Pr
2
R' = N(SiMe
3
)
2
34c
; R,R' = CH(SiMe
3
)
2
Scheme 10.12
an electron-rich alkene (Scheme 10.12).
44
The monomeric radicals
34
dimerize to the corresponding
diamagnetic diphosphines, [R
2
P]
2
(
35
), both in the solid state and upon cooling the solutions of these
odd-electron species.
45,46
=
CH(SiMe
3
)
2
substituents (
34c
), resulting in derivatives that are indefinitely stable both in solution and in
the gas phase.
47
The gas phase electron diffraction structures of
34b
(R/R
=
Remarkable stability has been achieved, however, for example with R
R
=
46
N(SiMe
3
)
2
/N
i
Pr
2
)
and
34c
(R
=
0
◦
, respectively) around
the central phosphorus atom. Most significantly, the structure of the extremely stable radical
34c
shows a
syn,syn
conformation for the CH(SiMe
3
47
0
◦
and
CH(SiMe
3
)
2
)
reveal a V-shaped arrangement (
∠
NPN
=
99
.
∠
CPC
=
104
.
)
2
groups, in contrast to the
syn,anti
arrangement observed for the
dimeric congener
35c
. Based on DFT calculations, the disparity in the conformations of
34c
and
35c
is a
significant contributor to the remarkable stability of the monomeric paramagnetic species
34c
in solution,
that is the transformation between
34c
and
35c
not only involves P-P bond cleavage/formation, but also an
isomerization of the substituents. Calculations predict an endothermic process of 95 kJ mol
−
1
for the P-P
bond cleavage upon going from
35c
to
34c
, while the subsequent isomerization of the paramagnetic species
34c
,togivethe
syn,syn
conformation, releases an energy of 67.5 kJ mol
−
1
for each of the monomeric
units, thus giving an overall exothermic process by about 40 kJ mol
−
1
for the conversion of
35c
to
34c
.
47
Therefore, the absence of dimerization and the stability of
34c
in solution is attributed not to the kinetic
stability imparted by the bulky ligands, but to the energy required for the conformational change prior to
the dimerization.
A phosphinyl radical [R
2
P]
•
(R
(
34d
) that is
stable both in solution and in the solid state is produced by one-electron reduction of the chlorine-containing
precursor.
48
=
NV[N(Np)Ar]
3
;Np
=
neopentyl, Ar
=
3,5-Me
2
C
6
H
3
)
Similarly to
34b
and
34c
, the solid state structure of
34d
shows a V-shaped phosphorus center
9
◦
)
(
. The stability of the radical
34d
is achieved by delocalization of the unpaired electron
onto the vanadium centers, which form a redox couple (IV/V) (Scheme 10.13). The delocalization is also
evident from the EPR spectrum of
34d
. Whereas the “true” phosphinyl radicals
34a
-
c
show hfc constants
of 75.9 - 96.3 G to the central phosphorus atom (
31
P,
I
∠
NPN
=
110
.
2
, 100 %) with significantly smaller coupling
to the substituents, the hfcc value of 42.5 G to the phosphorus atom in
34d
is substantially smaller and
the coupling of
a
=
1
/
51
V)
(
=
23.8 G to the two transition metal centers indicates extensive delocalization of
Ar
Ar
Ar
Ar
Ar
Ar
RN
NR
RN
NR
RN
Ar
NR
Ar
P
P
P
Ar
V
N
N
V
Ar
Ar
V
N
N
V
Ar
V
N
N
V
NR
RN
RN
RN
NR
NR
RN
NR
RN
NR
RN
NR
Ar
Ar
Ar
Ar
Ar
Ar
34d;
R = CH
2
t
Bu
Ar = 3,5-Me
2
C
6
H
3
Scheme 10.13
Resonance-stabilized radical
34d
by the vanadium (IV/V) redox couple
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