SCHEDULED MAINTENANCE
Lewis-base adducts of the diborane(4) compounds B2(1,2-E2C6H4)2 (E = O or S)
Abstract
The reactivity of the diborane(4) compounds
B
2
(1,2-E
2
C
6
H
4
)
2
(E = O or S) have been studied with respect to their
ability to co-ordinate Lewis bases, specifically 4-methylpyridine (mpy)
and the phosphines PMe
2
Ph and PEt
3
. Four
monoadducts have been characterised by X-ray crystallography, namely
B
2
(1,2-O
2
C
6
H
4
)
2
(mpy),
B
2
(1,2-S
2
C
6
H
4
)
2
(mpy),
B
2
(1,2-S
2
C
6
H
4
)
2
(PMe
2
Ph) and
B
2
(1,2-S
2
C
6
H
4
)
2
(PEt
3
), together with three bis adducts
B
2
(1,2-O
2
C
6
H
4
)
2
(mpy)
2
,
B
2
(1,2-S
2
C
6
H
4
)
2
(mpy)
2
and
B
2
(1,2-S
2
C
6
H
4
)
2
(PMe
2
Ph)
2
. In the former set of compounds the one
base is co-ordinated to one boron centre which adopts a tetrahedral
geometry, the remaining boron being trigonal planar as in the structure
of the parent compound. For the bis adducts each of the two borons is
co-ordinated to a mpy or phosphine with both borons tetrahedral.
Bond-length changes associated with ligand co-ordination are discussed.
Multinuclear solution-state NMR studies have also been performed for
solutions containing
B
2
(1,2-O
2
C
6
H
4
)
2
and mpy,
B
2
(1,2-S
2
C
6
H
4
)
2
and mpy and
B
2
(1,2-S
2
C
6
H
4
)
2
and PEt
3
. All of these studies indicate that intermolecular
exchange of base between the parent compound, the mono- and the
bis-adducts occurs in solution together with intramolecular exchange
between the two boron centres in the monoadducts. The various rates of
these processes, inasmuch as they have been determined accurately, are
discussed and rationalised. The monoadducts
B
2
(1,2-O/S
2
C
6
H
4
)
2
(mpy) are nominally isoelectronic with the phenonium cation.
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