Chemistry Reference
In-Depth Information
]
2
−
•
is generated, as the dilithium
derivative, by one-electron oxidation of the trilithium salt of the corresponding tetrakisimidophosphate
trianion [P(N
t
Bu)
3
(NSiMe
3
)
The phosphorus(V)-containing dianion radical [P(N
t
Bu)
3
(NSiMe
3
)
]
3
−
(a tetraimido analogue of orthophosphate PO
4
3
−
)
.
52
The formally neutral
}
•
exists as deep blue crystals that adopt highly distorted PN
3
Li
3
X cubic
radical
{
Li
2
[P(N
t
Bu)
3
(NSiMe
3
)
]
structures (
38a
,X
O
t
Bu), in which an entrapped LiX molecule provides one edge of
the cube (Scheme 10.15). In an extremely dilute THF solution, solvation of the Li
+
counterions causes
cleavage of the cubic structure to give the solvent-separated ion pair
39
and a molecule of (THF)
3
LiI.
The EPR spectrum of
39
displays a complicated, approximately fifty-line signal (
g
=
I;
38b
,X
=
=
2
.
0063) that is best
31
P)
simulated with hyperfine couplings to one phosphorus atom (
a
(
=
23.1 G), two equivalent nitrogen
14
N)
14
N)
centers (
a
(
=
5.38 G), two unique nitrogen atoms (
a
(
=
7.38 and 1.93 G), and a single lithium
0.30 G). The relatively small
31
P hfc constant in
39
, compared to those in
34
and
36
,
indicates a small amount of spin density located at the phosphorus center. This conclusion is supported by
DFT calculations that predict a SOMO consisting predominantly of nitrogen-based 2p orbitals for
39
.
52
The 1,3-diphosphacyclobuten-4-yl radical, [(Mes*C)
2
(P
t
Bu)P]
•
(
40
,Mes*
7
Li)
nucleus (
a
(
=
=
2,4,6-
t
Bu
3
C
6
H
2
)
,isiso-
lated as deep red crystals by treatment of the phosphaalkyne Mes*C
P with
t
BuLi followed by one-electron
oxidation with iodine (Scheme 10.16).
53
The structure of
40
shows a nearly planar four-membered ring
with trigonal planar carbon atoms, and both pyramidal and two-coordinate phosphorus atoms. The bond
parameters suggest a pure single bond for the P-C bonds involving the three-coordinate phosphorus atom
and partial double bond character for the other two P-C bonds. The room temperature EPR spectrum
of
40
in toluene exhibits a four-line central signal with small satellites (
g
≡
=
2
.
0025), from which hfc
31
P
α
)
=
31
P
β
)
=
13
C)
constants of
a
30.2 G were obtained. The relatively low
31
P hfcc values compared to those observed in phosphinyl and diphosphanyl radicals (
34, 36
), as well as
the metrical parameters, support an allylic radical in which the unpaired electron is delocalized mainly in
the CPC unit as depicted in Scheme 10.16 (cf. cyclotetrasilenyl
15
, Scheme 10.7).
Ab initio
calculations,
however, predict that 80 - 90 % of the spin density resides on the two carbon atoms.
(
20.4 G,
a
(
10.2 G and
a
(
=
10.4.1.2 Biradicaloids
The treatment of the cyclic anion formed from Mes*C
Pand
t
BuLi with methyl iodide, instead of iodine,
provides a novel route to the carbon-centered biradicaloid [(Mes*C)
2
(P
t
Bu)(PMe)]
••
(
41
) with a four-
membered P
2
C
2
ring (Scheme 10.16).
54
≡
A number of different derivatives of
41
, [(RC)(R
C)(PMes*)
2
]
••
t
Bu
P
1
/
2
l
2
R
R
•
t
Bu
P
40
P
t
BuLi
R
C
P
t
Bu
R
R
-
P
P
Li
+
R
R
•
•
Mel
t
Bu
t
Bu
P
R =
Me
41
t
Bu
Scheme 10.16
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