Chemistry Reference
In-Depth Information
Cl
Ir
F
Cl
O
(a)
(b)
(c)
Figure10.7
(a) [Cl
2
O
2
]
•
+
IrF
6
).
((a) Reprinted with permission from [66]. Copyright 1999 American Chemical Society; (b) Reproduced with
permissionfrom[67].CopyrightWiley-VCHVerlagGmbH&Co.KGaA.)
[Sb
2
F
11
]
−
), (b) EPR Spectrum of
49
, and (c) [(Cl
2
)
2
]
+
[IrF
6
]
−
(
49
; anion
=
(
50
·
short bonds within the Cl
2
and O
2
units (1.909(1) and 1.207(5) A, respectively), and a long Cl-O distance
(about 2.41 A) between these two units, cf. the sum of van der Waals radii for Cl and O
3.30 A. The
Cl-Cl distance is about 0.07 A shorter than that of Cl
2
, thus indicating that most of the positive charge, and
the unpaired electron, resides on the chlorine atoms. Consistently, the EPR spectrum of
49
(Figure 10.7b)
shows a seven-line pattern (
g
=
=
1
.
9988) due to the coupling of the unpaired electron with two equivalent
chlorine centers (
a
(
35
/
37
Cl)
2
, 24.2 %).
66
Subsequent attempts to produce the monomeric radical cation [Cl
2
]
•
+
from the reaction between Cl
2
and IrF
6
also resulted in a dimeric product in the form of the rectangular radical cation [(Cl
2
35
Cl,
I
37
Cl,
I
=
=
/
=
/
2.23 G;
3
2
, 75.8 %;
3
2
]
•
+
)
(
50
)
(Figure 10.7c).
67
Cl contacts
(1.941(3) and 2.936(7) A, respectively). The short chlorine - chlorine bond lengths are intermediate between
those observed for Cl
2
and [Cl
2
]
•
+
, suggesting delocalization of the unpaired spin density over both units.
The EPR spectrum of
50
as the [IrF
6
]
−
salt shows only an unresolved broad signal.
67
However, the original
EPR spectroscopic study of
50
supports the uniform delocalization of the unpaired electron over all four
chlorine atoms.
68
The structure of
50
displays two short Cl-Cl bonds and two long Cl
···
10.7 Summary and future prospects
In the past six to seven years a number of significant developments have occurred in the intriguing field of
stable radicals, primarily through the strategy of installing extremely bulky groups on the heavy p-block
element centers in order to provide kinetic stabilization of highly reactive species. For example, numerous
examples of heavy group 14 analogs of classic carbon-centered radicals, as well as their isoelectronic
anionic congeners from group 13, have been identified. A similar approach has also been successfully
employed to generate a variety of unsaturated cyclic and acyclic (neutral and charged) radicals involving
elements from groups 13 - 15. At the same time, several heterocyclic systems with biradicaloid character
have been comprehensively characterized.
To date the primary focus has been on the refinement of synthetic approaches and the elucidation of
both the molecular and electronic structures of these novel species. In the future, more attention will
likely be accorded to practical applications. For example, the incorporation of these paramagnetic building
blocks into oligomeric or polymeric systems with unique magnetic properties represents a major synthetic
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