Chemistry Reference
In-Depth Information
O
O
N
O
P
P
O
N
N
NMe
P
Me
N
N
N
•
Ph
75
(c)
(a)
(b)
Ph, R
Figure 7.5
Optimized structure (a) and positive spin density plot (b) for 6-phosphaverdazyl
x
(R
=
=
NMe
2
).
135
(c)Structureof6-phosphaverdazyl/cyclotriphosphazenespirocycle
75
.
136
AcO
OAc
AcO
OAc
B
R'
R"
B
R'
R"
B(OAc)
3
R'
R"
NH
NN
N
CoCp
2
N
NN
N
N
NN
N
CoCp
2
R
R
R
9
76
77
Scheme 7.27
Phosphaverdazyl radicals are persistent in solution but slowly decompose, most likely due
not
to the
presence of phosphorus but due to disproportionation via hydrogen atom transfer from the N-methyl
groups (Scheme 7.20). The electronic structures of these radicals have been probed through EPR
133 - 136
and computational
135,137
studies and fundamentally resemble verdazyls in their electronic structure. The
31
P hyperfine couplings (
small
5 G) are indicative of small spin density at the heteroatom. Some 6-
phosphaverdazyl derivatives have a nitrogen atom attached directly to the 6-phosphaverdazyl phosphorus
(e.g.,
71
,R
<
NMe
2
and phosphazene-verdazyl spirocycle
75
(Figure 7.5c)), and the non-negligible
spin density on these nitrogen atoms has been suggested to arise from a through-space, spiroconjugation
mechanism (Figure 7.5a, 7.5b for
71
,R
=
NMe
2
).
135,136
The only other inorganic verdazyls are boron-containing verdazyl radical anions. Reactions of formazans
with “diboron tetraacetate”
138
(more likely boron triacetate; generated
in situ
from boric acid (B(OH)
3
)
and acetic anhydride) gives formazan/B(OAc)
2
complexes
76
(“boratatetrazine”) (Scheme 7.27), which
can be reduced with cobaltocene to give highly reactive, but persistent borataverdazyl radical anions.
139
=
7.3 Tetraazapentenyl radicals
Reactions of triethyl orthoformate with
N,N
-diarylhydrazines (Ar
substituted phenyl or carbazolyl)
produces 1,2,4,5-tetrazapentenes
78
, which can be oxidized (PbO
2
) to the corresponding tetraazapentenyl
radicals
79
(Scheme 7.28).
140
These radicals can be considered as hydrazyls in which a hydrazyl-type
radical center is conjugated to a hydrazone (R
2
N-N
=
C) moiety, permitting the unpaired electron to delo-
calize between two structurally equivalent fragments. Radicals
79
are persistent enough to be isolated
as crystalline, monomeric radicals, although in solution they gradually decompose. Heating solutions of
=
Search WWH ::
Custom Search