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t
Bu
i
Pr
Cl
Cl
−
+
i
Pr
2 LiP
i
Pr
2
B
P
•
+
B
B
•
P
B
i
Pr
−
t
Bu
t
Bu
i
Pr
t
Bu
3
2 LiP
i
Pr
2
i
Pr
i
Pr
i
Pr
i
Pr
•
•
P
P
P
•
P
i
Pr
i
Pr
i
Pr
B
B
BB
i
Pr
t
Bu
t
Bu
t
Bu
t
Bu
Scheme 10.2
anion, [R
2
BBR
2
]
•
−
(
2
) (Scheme 10.1), was first structurally characterized as the contact ion pair,
[Li(OEt
2
)
2
][MeO(Mes)BB(Mes)OMe], in which the Li
+
cation is bound to the anion by coordination
to the methoxy oxygens.
7
The broadness of the electron paramagnetic resonance (EPR) signal of this
paramagnetic species precluded the determination of hfcc values. However, the solvent-separated ion
pair, [K(18-crown-6)(THF)
2
][Mes
2
BB(Ph)Mes], which was obtained by one-electron reduction with
potassium in tetrahydrofuran (THF) followed by the addition of 18-crown-6, exhibits a seven-line EPR
pattern (
g
11
B
13 G due to the coupling to two boron centers (
11
B,
I
=
2
.
0063) with
a
(
)
=
=
3
/
2, 80.2 %)
radical.
8
consistent with the formation of a
-bond order of 0.5 was indicated by comparison of the
structural parameters of this radical with those of the neutral precursor, Mes
2
BB(Ph)Mes.
π
A
π
10.2.1.2 Biradicaloids
A biradicaloid is a closed shell species derived from a singlet biradical by a weak interaction between the
radical centers.
2,3
The term biradicaloid is applied to p-block element compounds in which the biradical
character is substantially less than that of organic biradicals.
3
The study of such biradicaloids raises
fundamental questions about the nature of chemical bonding, as well as the prospect of generating materials
with unique magnetic properties, as indicated by the following examples involving cyclic B
2
P
2
systems.
The boron-centered biradicaloid, (
i
Pr
2
P)
2
(B
t
Bu)
2
(
3
) is prepared by the reaction of the 1,2-dichloroborane,
Cl(
t
Bu)BB(
t
Bu)Cl, with two equivalents of LiP
i
Pr
2
.
9
The biradicaloid
3
is an isoelectronic analogue of the
carbon-centered biradicaloids (RP)
2
(CR
)
2
(Scheme 10.16); it is presumably formed by a rearrangement
of the initially formed acyclic P-B-B-P skeleton (Scheme 10.2).
The thermally stable (
200
◦
C), air-sensitive yellow crystals of
3
display a perfectly planar P
2
B
2
ring
with a transannular B-B distance of 2.57
˚
A(cf.1.76A for a B-B single bond), indicative of biradical
character for
3
.
9
>
80
◦
Cto
room temperature suggests a singlet ground state for
3
. This inference is supported by DFT calculations,
which predict that the singlet state is 17.2 kcal mol
−
1
lower in energy than the triplet state biradical, thus
suggesting coupling between the two radical sites. The structures of four-membered rings (R
2
P)
2
(BR
)
2
are markedly influenced by the nature of the substituents on boron and phosphorus. Replacement of the
tert
-butyl group on boron in
3
by a 2,3,5,6-tetramethylphenyl group generates a folded structure with
a B-B distance of 2.24 A, while changing the
iso
-propyl groups on phosphorus to phenyl substituents
produces an even more folded structure with a B-B distance of 1.99 A.
9c
Thus, it seems that the biradical
nature of the P
2
B
2
ring in
3
results from the combination of the steric strain imposed by the substituents
on the phosphorus and boron and the
The absence of an EPR signal both in solution and in the solid state from
−
σ
donor property of the
tert
-butyl group at boron.
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