Chemistry Reference
In-Depth Information
2
Homometallic Alkoxides
1
INTRODUCTION
Metal alkoxides [M(OR)
x
]
n
(where M D metal or metalloid of valency
x
;RD simple
alkyl, substituted alkyl, or alkenyl group; and
n
D degree of molecular association),
may be deemed to be formed by the replacement of the hydroxylic hydrogen of an
alcohol (ROH) by a metal(loid) atom.
Historically, the first homoleptic alkoxo derivatives of elements such as boron and
silicon had been described
1
,
2
as early as 1846, but later progress in the alkoxide
chemistry of only half a dozen metals was rather slow and sporadic till the 1950s;
since then the chemistry of alkoxides of almost all the metals in the periodic table has
been systematically investigated. With a few exceptions, systematic investigations on
the structural aspects of metal alkoxides till the mid-1980s
3-10
were limited to studies
on molecular association, volatility, chemical reactivity, and spectroscopic (IR, NMR
and electronic) as well as magnetic properties. It is only since the early 1980s that
definitive X-ray structural elucidation has become feasible and increasingly revealing.
The rapidly advancing applications
11-16
of metal alkoxides for synthesis of ceramic
materials by sol - gel/MOCVD (metallo-organic chemical vapour deposition) processes
(Chapter 7) have more recently given a new impetus to intensive investigations on
synthetic, reactivity (including hydrolytic), structural, and mass-spectroscopic aspects
of oxo-alkoxide species.
17-21
Some of the exciting developments since 1990 in metal alkoxide chemistry have been
focussing on the synthesis and structural characterization of novel derivatives involving
special types of alkoxo groups such as (i) sterically demanding monodentate (OBu
t
,
OCHPr
i
2
, OCHBu
t
2
,OCMeEtPr
i
, OCBu
t
3
) as well as multidentate (OCR
0
CH
2
OPr
i
2
)
(R
0
D Bu
t
or CF
3
), OCR
0
2
CH
2
X(R
00
D Me or Et, X D OMe, OEt, NMe
2
) ligands,
21-24
(ii) fluorinated tertiary alkoxo (OCMe
CF
3
2
,OCMe
2
CF
3
,OC
CF
3
3
, etc.) moie-
ties,
21-23
and (iii) ligands containing intramolecularly coordinating substituents
(OCBu
t
2
CH
2
PMe
2
,OCH
2
CH
2
X(XD OMe, OEt, OBu
n
,NR
2
,PR
2
)).
21
,
22
Compared
to simple alkoxo groups, most of these chelating/sterically demanding ligands possess
the inherent advantages of enhancing the solubility and volatility of the products by
lowering their nuclearities owing to steric factors and intramolecular coordination.
Solubility and volatility are the two key properties of metal alkoxides which provide
convenient methods for their purification as well as making them suitable precursors
for high-purity metal oxide-based ceramic materials.
It is noteworthy that the homoleptic platinum group metal (Ru, Rh, Pd, Os, Ir, Pt)
alkoxides are kinetically more labile possibly owing to
ˇ
-hydrogen elimination
9
,
10
,
21
