Alkoxo and aryloxo derivatives of metals

Alkoxo and Aryloxo Derivatives of
Metals
Elsevier, 2001
Author: D.C. Bradley, R.C. Mehrotra, I.P. Rothwell and A. Singh
ISBN: 978-0-12-124140-7

Foreword, Page xi
1 - Introduction, Pages 1-2
2 - Homometallic Alkoxides, Pages 3-181
3 - Heterometallic Alkoxides, Pages 183-228
4 - X-Ray Crystal Structures of Alkoxo Metal Compounds, Pages 229-382
5 - Metal Oxo-alkoxides, Pages 383-443
6 - Metal Aryloxides, Pages 445-669
7 - Industrial Applications, Pages 671-686
Index, Pages 687-704


Foreword
The value of a book may well be judged by the number of times a person has to
buy it, for, while many books once read gather dust upon a shelf, those more often
sought can sometimes be seldom found. Over 20 years ago, I was fortunate to receive
a complimentary copy of “Metal Alkoxides” by Bradley, Mehrotra and Gaur. As one
interested in alkoxide metal chemistry, this proved a valuable reference for me and
my research group. In fact, I had to purchase two subsequent copies and probably
would have purchased more were it not for the fact that the book became out of print
and unavailable except through the library. Now I have received the galley proofs of
the second edition entitled “Alkoxo and Aryloxo Derivatives of Metals” by Bradley,
Mehrotra, Rothwell and Singh. After 20 years, virtually every field of chemistry must
have changed to the extent that a new edition would be appropriate. However, it is
unlikely that any field of chemistry, save computational chemistry, will have changed
as much as that of the chemistry of metal alkoxides and aryloxides during the period

1978–2000. The explosion of interest in metal alkoxides has arisen primarily for two
reasons. First and foremost, we have witnessed the tremendous growth of materials
chemistry spurred on by the discovery of high temperature superconducting oxides
and by the increasingly important role of other metal oxides to technology. Metal
alkoxides, mixed metal alkoxides and their related complexes have played an essential
role in the development of new routes to these materials either by sol-gel or chemical
vapor deposition techniques. In a second area of almost equal magnitude, we have
seen the growth of a new area of organometallic chemistry and catalysis supported
by alkoxide or aryloxide ancillary ligands. As a consequence of these major changes
in chemistry, virtually any issue of a current chemistry journal will feature articles
dealing with metal alkoxides and aryloxides. Thus, although the present book owes its
origins, and to some extent its format, to the first edition, its content is largely new. For
example, while the first edition reported on but a handful of structurally characterized
metal alkoxides, this second edition carries a whole chapter dealing with this topic, a
chapter with over 500 references to publications. The second edition is therefore most
timely, if not somewhat overdue, and will be a most valuable reference work for this
rapidly expanding field of chemistry. I only hope that I can hold on to my copy more
successfully than I did in the first instance.
Malcolm H. Chisholm FRS
Distinguished Professor of Mathematical and Physical Sciences
The Ohio State University
Department of Chemistry
Columbus, OH 43210-1185 USA
January 2001

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1
Introduction

In 1978 the book entitled “Metal Alkoxides” was published.1 It contained over one
thousand references and attempted to summarize most of what was known about metal
alkoxides up to that time. A striking feature was the dearth of X-ray crystal structures and so structural aspects necessarily involved speculation based on the results of
molecular weight determinations, combined where possible with spectroscopic data.
The intervening years have witnessed a spectacular advance in our knowledge of
the chemistry of the metal alkoxides, a development which has been driven primarily
by research activity resulting from the realization that these compounds have great
potential as precursors for the deposition of metal oxide films for microelectronic
device applications and in bulk for producing new ceramic materials. Simultaneously
a tremendous advance occurred in X-ray crystallography with the advent of computercontrolled automated diffractometers and with improvements in the techniques for
growing and mounting single crystals of the air sensitive metal alkoxides. Consequently
the number of structures solved has become so large that in this book a separate chapter
with over 500 references has been devoted to crystal structures with much of the data
summarized in tabular form. In addition, considerable advances have been made in the
synthesis and characterization of a range of new alkoxides of the alkali metals, alkaline
earths, yttrium and the lanthanides which together with other new developments has
led to a chapter on Homometallic Alkoxides containing well over 1000 references.
Similarly the chapter on Heterometallic Alkoxides (previously described as Double
Metal Alkoxides) has been expanded to include many novel compounds, with particular
emphasis on the recently authenticated species containing two, three and even four
different metals in one molecule.
Another area that has expanded in recent years concerns the Industrial Applications
of metal alkoxides. Besides the previously mentioned deposition of metal oxides in the
microelectronic and ceramics industries there have also been major developments in
the catalytic activity of early transition metal alkoxo compounds in several important
homogeneous reactions. This has stimulated a growing interest in the mechanisms of
reactions catalysed by metal alkoxides.
Metal Oxo Alkoxides are implicated as intermediates in the hydrolysis of metal
alkoxides to metal oxides and their importance in the sol–gel process has led to much

research activity in this area. Accordingly we have allocated a whole chapter to the
Metal Oxo Alkoxides.
In the 1978 book very little space was devoted to metal aryloxides because this
area had received scant attention, but the intervening years have seen a resurgence of
activity involving the synthesis and characterization of many novel compounds and

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2

Alkoxo and Aryloxo Derivatives of Metals

studies on their catalytic activity. Therefore we have added a separate chapter dealing
with this important topic.
In this book we are giving the relevant references at the end of the seven chapters rather than placing them all at the end of the text in the hope that this will be
more convenient for the reader. Finally, the authors acknowledge their indebtedness to
all of their former research students, postdoctoral assistants, and colleagues for their
invaluable contributions to the research which has provided much of the information
collected in this publication.

REFERENCE
1.

D.C. Bradley, R.C. Mehrotra, and D.P. Gaur, Metal Alkoxides, Academic Press, London
(1978).

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2
Homometallic Alkoxides

1 INTRODUCTION

Metal alkoxides [M(OR)x ]n (where M D metal or metalloid of valency x ; R D 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 described1,2 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-1980s3 – 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 applications11 – 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 (OBut ,
OCHPri2 , OCHBut2 , OCMeEtPri , OCBut3 ) as well as multidentate (OCR0 CH2 OPri 2 )
(R0 D But or CF3 ), OCR002 CH2 X (R00 D Me or Et, X D OMe, OEt, NMe2 ) ligands,21 – 24
(ii) fluorinated tertiary alkoxo (OCMe CF3 2 , OCMe2 CF3 , OC CF3 3 , etc.) moieties,21 – 23 and (iii) ligands containing intramolecularly coordinating substituents
(OCBut2 CH2 PMe2 , OCH2 CH2 X (X D OMe, OEt, OBun , NR2 , PR2 )).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 elimination9,10,21

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Alkoxo and Aryloxo Derivatives of Metals

type reaction(s) (Eq. 2.1):
M—OCHR0 R00

! M—H C R0 R00 CDO
#
M C 12 H2

2.1

These, therefore, are not generally isolable under ambient conditions unless special
types of chelating alkoxo ligands21 are used.
Although single crystal X-ray studies presented considerable difficulties in the earlier
stages,25 the development of more sophisticated X-ray diffraction techniques has led
to the structural elucidation of a number of homo- and heteroleptic alkoxides17 – 23 and

actual identification of many interesting metal oxo-alkoxide systems (Chapter 4).
In this chapter we shall discuss the synthesis,3,4,26 chemistry and properties of
homometallic alkoxides with more emphasis on homoleptic alkoxides [M(OR)x ]n and
M(OR)x .Ln with occasional references to metal oxo-alkoxides MOy OR x 2y and
metal halide alkoxides M(OR)x y Xy .Lz (where x D valency of metal, L D neutral
donor ligand, X D halide, and n, y and z are integers). The discussion will generally
exclude organometallic alkoxides and a considerable range of metal-organic compounds
containing alkoxo groups, as in these systems the alkoxo groups play only a subsidiary
role in determining the nature of the molecule.

2 METHODS OF SYNTHESIS

Metal alkoxides in general are highly moisture-sensitive. Stringent precautions are,
therefore, essential during their synthesis and handling; these involve drying of all
reagents, solvents, apparatus, and the environment above the reactants and products.
Provided that these precautions are taken, the preparation of metal alkoxides, although
sometimes tedious and time consuming, is relatively straightforward.
The method employed for the synthesis3,4,8,17,21 of any metal/metalloid alkoxide
depends generally on the electronegativity of the element concerned. Highly electropositive metals with valencies up to three (alkali metals, alkaline earth metals, and
lanthanides) react directly with alcohols liberating hydrogen and forming the corresponding metal alkoxides. The reactions of alcohols with less electropositive metals
such as magnesium and aluminium, require a catalyst (I2 or HgCl2 ) for successful
synthesis of their alkoxides. The electrochemical synthesis of metal alkoxides by anodic
dissolution of metals (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb) and even metalloids
(Si, Ge) in dry alcohols in the presence of a conducting electrolyte (e.g. tetrabutylammonium bromide) appears to offer a promising procedure (Section 2.2) of considerable
utility. It may be worthwhile to mention at this stage that the metal atom vapour technique, which has shown exciting results in organometallics, may emerge as one of the
potential synthetic routes for metal alkoxides also in future.
For the synthesis of metalloid (B, Si) alkoxides, the method generally employed
consists of the reaction of their covalent halides (usually chlorides) with an appropriate
alcohol. However, the replacement of chloride by the alkoxo group(s) does not appear to
proceed to completion, when the central element is comparatively more electropositive.

In such cases (e.g. titanium, niobium, iron, lanthanides, thorium) excluding the strongly
electropositive s-block metals, the replacement of halide could in general be pushed

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Homometallic Alkoxides

5

to completion by the presence of bases such as ammonia, pyridine, or alkali metal
alkoxides.
Another generally applicable method, particularly in the case of electronegative
elements, is the esterification of their oxyacids or oxides (acid anhydrides) with alcohols
(Section 2.6), and removing the water produced in the reaction continuously.
In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic
alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions
of metal dialkylamides with alcohols has provided a highly convenient and versatile
route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals,
particularly in their lower valency states.
The metal–hydrogen and metal–carbon bond cleavage reactions have also been
exploited in some instances (Section 2.10.2).
The following pages present a brief summary of the general methods used for the
synthesis of metal and metalloid alkoxides applicable to specific systems. Tables 2.1
and 2.2 in Section 2.1 (pp. 6–14) list some illustrative compounds along with their
preparative routes and characterization techniques.
2.1

Reactions of Metals with Alcohols (Method A)

The facility of the direct reaction of a metal with an alcohol depends on both the
electropositive nature of the metal and the ramification of the alcohol concerned.
In view of the very feeble acidic character of nonfluorinated alcohols [even
weaker than that of water: pKa values (in parentheses) of some alcohols
are CH3 OH(15.8), CH3 CH2 OH(15.9), CH3 2 CHOH(17.1), CH3 3 COH(19.2),
CF3 CH2 OH(12.8), CH3 CF3 2 COH(9.6), CF3 2 CHOH(9.3), CF3 3 COH(5.4)], this
route is more facile with lower aliphatic and fluorinated alcohols.
2.1.1 s-Block Metals
2.1.1.1 Group 1 metals (Li, Na, K, Rb, Cs)
The more electropositive alkali metals react vigorously with alcohols by replacement
of the hydroxylic hydrogen (Eq. 2.2):
M C 1 C y ROH

!

1
1
[MOR.yROH]n C H2 "
n
2

M D Li, Na, K, Rb, Cs; R D Me, Et, Pri , But ;3,6,26,27
M D Li; R D But , CMe2 Ph;28
t 29

M D K, Rb, Cs; R D Bu ;
M D K, Rb; R D But ;29

2.2
y D 0.

y D 0.
y D 1.

y D 0.

The alkali metals react spontaneously with sterically compact aliphatic alcohols
(MeOH, EtOH, etc.) and the speed of the reaction increases with atomic number of the
metal, Li < Na < K < Rb < Cs, corresponding to a decrease in ionization potential of
the alkali metals. The ramification of the alkyl group is also important, as shown by the

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Alkoxo and Aryloxo Derivatives of Metals

Table 2.1 Examples of some homoleptic alkoxides
Compound1

Method of
preparation2

Characterization
techniques3

Group 1
[LiOMe]1
[LiOBut ]6

A
A

[LiOCMe2 Ph]6

A

[LiOCBut3 ]2

J-2

[LiOCBut3 thf ]2
[LiOCBut2 CH2 PMe2 ]2

J-2
J-3

[LiOCBut2 CH2 PPh2 ]2

J-3

[LiOCBut2 CH2 PPh2 ]2 But2 CO

J-3

X-ray
IR; 1 H, 13 C, 7 Li
NMR; MW
IR; 1 H, 13 C, 7 Li

NMR; MW;
X-ray
1
H, 13 C, 7 Li NMR;
X-ray
X-ray
1
H, 13 C, 7 Li, 31 P
NMR; X-ray
1
H, 13 C, 7 Li, 31 P
NMR; X-ray
1
H, 13 C, 7 Li, 31 P
NMR; X-ray

[MOMe]1
(M D Na, K, Rb, Cs)
[NaOBut ]6
[NaOBut ]9
[MOBut .HOBut ]1
M D K, Rb)
[MOBut ]4
M D K, Rb, Cs
[NafOCH CF3 2 g]4
Group 2
[Be OMe 2 ]n
[Be OBut 2 ]3
[Be OCEt3 2 ]2
[Be OCMe2 CH2 OMe 2 ]2

[Be OCEt2 CH2 OMe 2 ]2
[BefOC CF3 g2 ]3 .OEt2
Mg OMe 2 .3.5MeOH
[Ca -OR OR thf ]2 . toluene
[Ca OR

2

A
A
A
A
A
J-2
E-3, J-2
J-2
J-2
I
I
E-2
A
E-2

2

thf 3 ].THF

E-2

CafOC CF3 3 g2

Ca3 OCHBut2 6
Ca2 [OCBut CH2 OPri 2 ]4
Ca[OCBut CH2 OPri CH2 CH2 NEt2 ]2
Ca9 OC2 H4 OMe 18 HOC2 H4 OMe 2

A
I
I
I
A

Sr[OC CF3 3 ]2
Sr2 [OCBut CH2 OPri 2 ]4
Ba OBut 2
Ba OCEt3 2
Ba OCMeEtPri 2

A
I
A
A
A

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Reference
28a
28
28
396

230
22
422
422

X-ray
X-ray
X-ray
IR; 1 H, 13 C NMR;
MW; X-ray
1
H, 13 C NMR;
X-ray
IR; 1 H, 19 F NMR;
X-ray

a, b, c, d
e, f
e
29

IR
IR; 1 H NMR; MW
IR; 1 H NMR; MW
IR; 1 H NMR; MS
IR; 1 H NMR; MS
1
H, 19 F NMR; MW
X-ray
IR; 1 H, 13 C NMR;

X-ray
IR; 1 H, 13 C NMR;
X-ray
19
F NMR

214, 385
385
385
340
340
396
38
147

IR; MS; X-ray
IR; MS
IR; 1 H, 13 C NMR;
X-ray
19
F NMR
IR; MS
1
H NMR
1
H NMR
1
H NMR

29, g

397

147
47,
53,
53,
53,
50

53, 340
340
340
340

47
53, 340
47
47
47


Homometallic Alkoxides

7

Table 2.1 (Continued )
Method of
preparation2

Compound1

Ba OCHBut2 2
Ba[OCH CF3 2 ]2
Ba[OC CF3 3 ]2
[Ba OBut 2 HOBut 2 ]4

A
A
A
I

Ba2 [OCBut CH2 OEt 2 ]4
Ba2 [OCBut CH2 OPri 2 ]4
Ba2 OCPh3 4 thf 3

E-2
A, I
A
A

Ba[O CH2 CH2 x CH3 ]2
(x D 2, 3)
Scandium, Yttrium, and Lanthanides
[ScfOCH CF3 2 g3 NH3 2 ]2

I

Ln OPri 3
Ln D Y, Pr, Nd, Sm, Eu,
Gd, Tb, Dy, Ho, Et, Tm, Yb, Lu

A

Ln OPri 3
Ln D Y, Dy, Yb
Ln OPri 3
Ln D Pr, Nd
Ln OR 3
Ln D Pr, Nd;
R D Bun , Bui , Bus , But , Amn ,
Amt , Prn CH(Me), Prn CMe2
Gd OPri 3
Er OPri 3
Ln OMe 3
Ln D Gd, Er
Ho OPri 3
[YfOCH CF3 2 g3 thf 3 ]
[YfOCMe2 CF3 g3 ]n
[YfOCMe2 CF3 g3 thf 2.5 ]
[YfOCMe CF3 2 g3 ]n
[YfOCMe CF3 2 g3 NH3 0.5 ]
[YfOCMe CF3 2 g3 NH3 3 ]
[YfOCMe CF3 2 g thf 3 ]
[YfOCMe CF3 2 g3 OEt2 0.33 ]
fYfOCMe CF3 2 g3 diglyme g
[YfOCMe CF3 2 g3 HOBut 3 ]
fY OCH CF3 2 g3 NH3 0.5 ]
[YfOCH CF3 2 g3 thf 3 ]
[Y3 OBut 9 HOBut 2 ]

A

[Y3 OAmt

9

HOAmt 2 ]

E-2
G

Characterization
techniques3
1

H NMR
F NMR
19
F NMR
1
H, 13 C NMR;
X-ray
19

IR; MS
1
H, 13 C NMR;
X-ray
IR; 1 H, 13 C NMR;
MS
IR; 1 H, 19 F NMR;

MS; X-ray
IR; 1 H NMR (Y,
La, Lu);
UV-Vis (Pr, Nd,
Sm, Ho, Er)
IR; 1 H NMR
(Ln D Y)
MW

Reference
47
47
47
549
53, 340
53, 340
48
52

349
55

54
153

MW

153

E-2

E-2
E-3

IR; MW
IR; MW
IR

157
157
157

E-2
I
I
I
I
I
I
I
I
I
I
I
I
I

MW
IR; MS; X-ray
1
H, 19 F NMR

1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
1
H, 19 F, 89 Y NMR
IR; 1 H, 13 C, 89 Y
NMR; MS
IR; 1 H, 13 C, 89 Y
NMR; MS

158
349
349a
349a
349a

349a
349a
349a
349a
349a
349a
349a
349a
345

I

345

(continued overleaf )

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Alkoxo and Aryloxo Derivatives of Metals

Table 2.1 (Continued )
Compound1

Method of
preparation2

Characterization

techniques3

I

IR; 1 H, 13 C, 89 Y
NMR; MS
IR; 1 H, 13 C NMR;
X-ray
1
H, 13 C NMR; MS;
X-ray
1
H, 13 C NMR; MS

[Y OR 3 ]2
R D CMe2 Pri , CMeEtPri , CEt3
[Y OC2 H4 OMe 3 ]10

A

[La3 OBut

I

HOBut 2 ]

9

[La OR 3 ]2
R D CMe2 Pri , CMeEtPri

[La3 OBut 9 thf 2 ]

I

164
349
349b
349
959
348a
348
56
349b
349b
349c
349c
355

E-2
G
G

MW
MW
MW

141
141
143
143

141a

E-2

IR; 1 H, 13 C NMR;
X-ray
IR; 1 H, 13 C NMR;
X-ray
IR; 1 H NMR; X-ray

165

I

La4 OCH2 But

I

Actinides
[Th OPri 4 ]n
[Th OEt 4 ]n
[Th OR 4 ]n
R D Bun , Pentn , CH2 But
R D CMe3 , CMe2 Et, CMeEt2 ,
CMe2 Prn , CMe2 Pri , CEt3 ,
CMeEtPrn , CMeEt,Pri
Th4 OPri 16 Py 2
Th2 OCHEt2
[Th OBut

4

E-1
E-2
I
150Ž C, vacuum
G

Py

J-3

Py 2 ]

E-2

8

345

IR; 1 H, 13 C NMR
IR; MS; X-ray
IR; 1 H NMR; X-ray
IR; MS; X-ray
IR; 1 H NMR; X-ray
IR; 1 H NMR; X-ray
IR; 1 H NMR; X-ray
IR
IR
IR

1
H, 19 F NMR; MS
1
H, 19 F NMR; MS
IR; 1 H, 13 C NMR;
MS; X-ray

[LafOCMe CF3 2 g3 thf 3 ]

Ce OCBut3 3
[Ce OCHBut2 3 ]2
Ce OR 4
R D Me, Et, Prn , Bun , Bui ,
CH2 But
Ce OBut 4 thf 2
[PrfOCMe CF3 2 g3 NH3 2 ]2
[PrfOCMe CF3 2 g3 NH3 4 ]
[PrfOCMe2 CF3 g3 ]3
[Nd OCBut3 3 thf ]
Nd4 OCH2 But 12
Nd2 OCHPri2 6 thf 2
[Nd OPri 3 .Pri OH]4
[EufOCMe CF3 2 g3 ]n
[Eu2 fOCMe CF3 2 g6 NH3 2 ]
[EufOCMe CF3 2 g3 thf 3 ]
[EufOCMe CF3 2 g3 diglyme ]
[Lu OCMe2 CH2 OMe 3 ]2

345

E-2
I
I
I
I
I
I
A
I
I
I
I
I

I

[Ce OPri 4 .Pri OH]2

57

160

[La OCPh3 3 ]2

12

345

H, 13 C NMR;
X-ray

IR; 1 H, 13 C NMR;
X-ray
IR; 1 H, 13 C NMR;
MS; X-ray
IR; 1 H, 13 C NMR;
X-ray
MW
1
H, 13 C NMR;
X-ray
MW
X-ray
MW

E-2

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1

Reference

346
349c
348a
143
460
344
344
143

165
398


Homometallic Alkoxides

9

Table 2.1 (Continued )
Method of
preparation2

Compound1
Th2 OBut

8

HOBut

Th2 OCHPri2

8

U OMe 4
U OR 4
R D Et, But
U2 OBut 8 HOBut
U OCHBut2 4
UfOCH CF3 2 g4 thf 2

UfOC CF3 3 g4 thf 2
U OEt 5
U OR 5
R D Me, Prn , Pri , Bus , Bun , Bui
U OBut 5
U OCH2 CF3 5
Pu OPri 4 .Pri OH
Group 4
[Ti OEt 4 ]4
M OR 4
M D Ti, Zr
R D MeCH2 CH2 2 CH2 ,
Me2 CHCH2 CH2 , MeCH Et CH2 ,
Me3 CCH2 , CHEt2 , CHMePrn ,
CHMePri , CMe2 Et
Ti OR 4
R D CMe2 Et, CMeEt2
[Zr OR 4 ]n
R D Et, Pri , Bun , Bus
R D Pri , Prn , Bun , Amn
[Zr OPri 4 .Pri OH]2
[Hf OR 4 ]n
R D Et, Pri
R D Me, Et, Pri , But , Amt
[Hf OPri 4 .Pri OH]2
Group 5
V OR 4
R D Me, Et, Pri , But
[Nb OR 5 ]2
R D Me, Et, Prn , Bun , n-pentyl

[Ta OR 5 ]n
R D Me, Et, Prn , Bun ,
MeCH2 CH2 CH2 (and its isomers),
MeCH2 CH2 CH2 CH2 (and its
isomers)

J-3
J-3

Characterization
techniques3
IR; 1 H, 13 C NMR;
X-ray
1
H NMR;
thermochemical
data; X-ray

E-3, I
I
J-3
E-3
E-2
E-2
J-1
J-1

398
399
213

213

IR; 1 H NMR; UV-Vis
1
H NMR; eff ; MS
19
F NMR; eff
19
F NMR; eff

G
E-1
E-1
E-1
E-1

Reference

330
229
h
h
379
289
289
289
144

G

H NMR
X-ray
MW

134
548
434
273

G

MW; LvŁ ; S

274

MW
MW
IR; 1 H, 13 C NMR;
X-ray
MW

145
145a
460

MW
IR; 1 H, 13 C NMR;
X-ray

277

460

E-1
E-1
E-3, I
E-1
G
E-1

1

E-3, I
E-1, G

UV-Vis;
MW

E-1, G

MW

eff ;

ESR

277

331, 333
588, 589, 590
279, 468

280, 312, 469

(continued overleaf )

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10

Alkoxo and Aryloxo Derivatives of Metals

Table 2.1 (Continued )
Method of
preparation2

Compound1
[Nb OPri 5 ]2
[M OEt 5 ]2
M D Nb, Ta
[M OR 5 ]2
M D Nb, Ta
R D Me, Et, Bui , Pri
[Ta OR 5 ]2
R D Me, Et, Bun , Pri
[Ta OC2 H4 OMe 5 ]

B, E-1
B
E-1
G

Group 6
[Cr OCHBut2 2 ]2

Cr OCHBut2

Mo2 [OCMe2 CF3 ]6
[Mo2 OCMe2 Et 6 ]
Mo2 OPri 8 MoDMo

4
4

HOPri

Mo2 OCH2 But

4

Mo2 OPri

4

W2 OPri
W2 OPri

4

MoD
DMo

Py

NHMe2

6
6

Py

IR; MS

81

G

MS

81

X-ray
UV-Vis; ESR
IR; MS
IR; MS; X-ray
IR; UV-Vis; MS
UV-Vis; eff ; MW

i
226
340

340
226
168
331
467, 471

2

W2 OBut 6 WÁW
W4 OR 12
R D Pri , CH2 But

I
E-2
E-2
E-3
J-1
I
I
J-3
J-3

Mo2 OR 4 HOR 4
R D c-pentyl, c-hexyl

4

A

E-3

Mo2 OR 6 MoÁMo
R D But , CMe2 Ph, Pri , CH2 But
Mo2 [OCMe CF3 2 ]6

Mo OBut
Mo2 OPri

608
79
566
470

E-2
I
E-3
E-2
I
I

4

4

Reference

IR; MS; X-ray
MS
X-ray (M D Nb)
1

H NMR

E-3

Cr[OCBut CH2 OPri 2 ]2
[Cr OCMe2 CH2 OMe 3 ]
Cr OCHBut2 3 thf
Cr OBut 4

Characterization
techniques3

J-3
J-3
I
I (Cpyridine)
I
I

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Thermochemical
data; MS;
UV-Vis; eff
IR; UV-Vis; eff ;
MS; X-ray
1
H NMR; X-ray
(R D CH2 But )
1

H, 13 C NMR;
X-ray
1
H, 13 C NMR
1
H, 13 C NMR
1

H NMR; MW;
X-ray
1
H NMR; MW
IR; 1 H NMR;
UV-Vis; X-ray
IR; 1 H NMR;
UV-Vis; X-ray
(c-pentyl)
IR; 1 H NMR;
UV-Vis; X-ray
IR; 1 H NMR;
UV-Vis; X-ray
X-ray
IR; 1 H NMR; MS;
X-ray
IR; 1 H NMR; MS
MW

226
360, 361, 362
170

170
170
371, 372
371
375
375
375
375
j
365
365
365


Homometallic Alkoxides

11

Table 2.1 (Continued )
Method of
preparation2

Compound1
W4 OPri

12 /W2

OPri

6

M4 OCH2 R 12
M D Mo, W
R D c-C4 H7 , c-C5 H9 ,
c-C6 H11 , Pri
W4 OEt 16
Group 7
[Mn OR 2 ]n
R D primary, secondary, and
tertiary alcohols
[Mn OCHBut2 2 ]2

Crystallization of
W2 OPri 6 from
dimethoxyethane
alcoholysis of
M2 OBut 6

Characterization
techniques3
1

Reference

H NMR; MS;
X-ray
k

I

IR; 1 H, 13 C, 95 Mo
NMR;
X-ray (M D Mo;
R D c-C4 H7 )
1
H NMR; X-ray

I

Reflectance spectra;

l

366

eff

I

IR; UV-Vis; ESR;
MW
X-ray
IR; MS
1
H NMR; X-ray

226

Mn[OCBut CH2 OPri 2 ]2
Re3 OPri 9

I
I
E-2

Group 8
[FefOCBut CH2 OEt 2 g2 ]2

I

IR; MS

340

Group 9
[Co OCHBut2 2 ]2
[Co OCPh3 2 ]2 .n-C6 H14

I
I

IR; UV-Vis; MW
IR; 1 H NMR;
UV-Vis; X-ray
IR; 1 H NMR;
UV-Vis; X-ray
IR; MS
IR; MS

226

351a

Co OCPh3

2

thf

2

Co[OCBut CH2 OPri 2 ]2
[Co[OC CF3 CH2 OPri 2 ]2 ]2
Group 10
[Ni OR 2 ]n
R D Me, Et, Prn , Pri , But , Amt ,
t-C6 H13
Ni[OCBut CH2 OPri 2 ]2
dppe Pt OMe 2
Pt OCH2 CH2 PPh2 2
Pt OCMe2 CH2 PPh2

E-3
E-2
E-2
E-2
E-2

2

Pt OCMe2 CH2 PPh2 .3.5H2 O

Group 11
[Cu OCHBut2 ]4
[CuOBut ]4
CufOCH CF3 2 g PPh3
Cu OCHPh2 PPh3 3
[CuOCEt3 ]4

I (THF used as
solvent)
I
I

3

I
E-3
E-3
E-3
J-2
J-2
E-2

351
340
173, 174

351a
340
340
6, 219

IR; MS
H, 31 P NMR; X-ray
IR; 1 H, 31 P NMR;
MS; X-ray
IR; 1 H, 31 P NMR;
X-ray
1
H, 31 P NMR; X-ray

340
177, 177a
178

MW

226
m
n, o
p
p
47

1

IR; 1 H NMR; ESR
IR; 1 H NMR; X-ray
IR; 1 H NMR
1
H, 13 C NMR; MW

178a
178b

(continued overleaf )

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12

Alkoxo and Aryloxo Derivatives of Metals

Table 2.1 (Continued )
Method of
preparation2

Compound1
Cu OCEt3

Cu[OCH CF3 2 ]2 L
L D tmeda, teed, bipy, (py)2
Cu[OCMe CF3 2 ]2 L
L D tmeda, bipy, (py)2
Group 12
M[OCBut CH2 OPri 2 ]2
M D Zn, Cd
[Zn OR 2 ]n
R D CEt3 , CEtMe,
C2 H4 OMe, C2 H4 OC2 H4 OMe,

C2 H4 NMe2 , CHMeCH2 NMe2 ,
C2 H4 NMeC2 H4 NMe2
Cd9 OC2 H4 OMe 18 HOC2 H4 OMe

47

I

IR; MS

340

I

IR; 1 H NMR

339b

I

IR; 1 H, 13 C, 113 Cd
NMR; X-ray
IR

339c

IR; 1 H, 13 C, 27 Al
NMR; MW; MS
X-ray

6

E-2, G
E-2, G

2

[Cd OBut 2 ]n

I

Group 13
[Al OPri 3 ]4

A

[Al OBut 3 ]2
[Ga[OR 3 ]n
R D Me, Et, Prn , Pri
[InfOCMe2 CF3 g3 ]2
[In OPri 3 ]n
Group 14
Ge OCBut3

A
I
E-2
I
E-2

2

[Sn OBut 2 ]2
[Sn OPri 4 .Pri OH]2

I
I
E-2

Sn OBut 4
[Pb OBut 2 ]n

I
I

[Pb OR 2 ]n
R D Pri , But , CMe2 Et, CEt3

I

CH2 CH2 OMe,
CHMeCH2 NMe2
[Pb OPri 2 ]x

E-2

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1

Reference

H, 13 C NMR;
UV-Vis
UV-Vis; ESR; X-ray
(L D tmeda)
UV-Vis; ESR; X-ray
(L D tmeda)

E-2

2

Characterization
techniques3

X-ray
IR; 1 H NMR
IR; 1 H, 13 C NMR;
X-ray
IR; 1 H NMR
1

H, 13 C NMR;
X-ray
1
H, 119 Sn NMR
1
H, 13 C, 119 Sn NMR
X-ray

X-ray
IR; 1 H, 207 Pb NMR;
MS
IR; 1 H, 13 C NMR;
MW; X-ray
(R D Pri ,
C2 H4 OMe;
n D 1. R D But ,
n D 3)

175
175

339c

579, 580
71
338
187
358
187
352
352
190
190, 191
190
194
612

192

Homometallic Alkoxides

13

Table 2.1 (Continued )
Method of
preparation2

Compound1
Group 15
Bi OR 3
R D Me, Et, Prn , Pri
Bi OBut 3
[Bi OC2 H4 OMe 3 ]

E-2
E-2; I

[Bi OCH CF3 2 ]3 thf ]2

E-2

[BifOC CF3 3 g3 ]
Group 16
Se OR 4
R D Me, Et
R D CH2 CF3
Te OR 4

R D Me, Et, Pri
R D CH2 CF3
R D C CF3

3

Characterization
techniques3

E-2

Bi C
3 CF3 3 CCOCl
E-1, E-2
E-1
E-2
E-1
Te C
4 CF3 3 CCOCl

Ł

Reference
202

IR; 1 H, 13 C NMR
IR; 1 H, 13 C NMR;
MS; X-ray, MW
IR; 1 H, 19 F NMR;
X-ray

IR; 19 F NMR

203
205, 339

1

r, s
s

1

H, 13 C,
H, 13 C,
NMR

77
77

Se NMR
Se, 19 F

206, 206a
q

r
1

H, 13 C, 125 Te,
NMR

IR; 19 F NMR

19

F
q

Lv D Latent heat of vaporization.
bpy D 2,20 -bipyridine; diglyme D bis (2-methoxyethyl) ether (ligand); dppe D 1,2-bis
(diphenylphosphino)ethane, Py D pyridine (ligand); teed D N,N,N0 ,N0 -tetraethylethylenediamine
(ligand); tmeda D N,N,N0 ,N0 -tetramethylethylenediamine (ligand); 2 Methods A–J (J-1–J-7) as
described in text; 3 ESR D electron spin resonance; eff D magnetic moment; MS D mass
spectrum; MW D molecular weight; UV-Vis D ultraviolet and visible.
a
E.Weiss, Helv. Chim. Acta, 46, 2051 (1963); b E. Weiss and W. Biăucher, Angew. Chem., 75,
1116 (1963); c E. Weiss, Z. Anorg. Allg. Chem., 332, 197 (1964); d E. Weiss and H. Alsdorf, Z.
Anorg. Allg. Chem., 372, 2061 (1970); e J.E. Davies, J. Kopf, and E. Weiss, Acta Crystallogr., 38,
2251 (1982); f E.Weiss, Angew. Chem., Int. Ed. Engl., 32, 1501 (1993); g E. Weiss, H. Alsdorf,
and H. Kăuhr, Angew. Chem. Int. Ed. Engl., 6, 801 (1967); h R.A. Andersen, Inorg. Nucl. Chem.,
Lett., 15, 57 (1979); i B.D. Murray, H. Hope, and P.P. Power, J. Am. Chem. Soc., 107, 169
(1985); j M.H. Chisholm, D.L. Clark, J.C. Huffman, and M. Hampden-Smith, J. Am. Chem.
Soc., 109, 7750 (1987); k M.H. Chisholm, K.Folting, C.E. Hammond, M.J. Hampden-Smith, and
K.G. Moodley, J. Am. Chem. Soc., 111, 5300 (1989); l B. Horvath, R. Moseler, and E.G. Horvath,
Z. Anorg. Allg. Chem., 449, 41 (1979); m T. Greiser and E. Weiss, Chem. Ber., 104, 3142 (1976);
n
T. Tsuda, T. Hashimoto, and T. Saegusa, J. Am. Chem. Soc., 94, 658 (1972); o T.H. Lemmen,
G.V. Goeden, J.C. Huffman, R.L. Geerts, and K.G. Caulton, Inorg. Chem., 29, 3680 (1990); p K.
Osakada, T. Takizawa, M. Tanaka, and T. Yamamoto, J. Organomet Chem., 473, 359 (1994);
q
J.M. Canich, G.L. Gard, and J.M. Shreeve, Inorg. Chem., 23, 441 (1984); r N.Temple and W.

Schwarz, Z. Anorg. Allg. Chem., 474, 157 (1981); s D.B. Denny, D.Z. Denny, P.T. Hammond,
and Y.F. Hsu, J. Am. Chem. Soc., 103, 2340 (1981).
1

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14

Alkoxo and Aryloxo Derivatives of Metals

Table 2.2 Examples of a few selected heteroleptic alkoxides
Method of
preparation2

Compound1
[Y5 O

OPri

13 ]

1 H, 13 C, 89 Y

References

NMR; MW;
MS; X-ray
IR; 1 H NMR; MS;
X-ray (Ln D Yb)

IR; X-ray
1 H, 13 C NMR; X-ray
IR; 1 H, 13 C NMR; X-ray
1 H, 13 C NMR

60
161
160
383

IR; 1 H NMR; X-ray
X-ray
1 H NMR
IR; 1 H NMR; X-ray
1 H, 13 C NMR

959
159
227
a
228

G
G
D
D
D
E-3
E-3
E-3

IR
IR,
IR;
IR;
IR;
IR;
IR;
IR;

272
272
108
108
108a
592
235
233

1H

CoBr OMe .2MeOH
Ni(OMe)Cl
[Cu OMe fOCH CF3 2 g]n
Cu3 OBut 4 [OC CF3 3 ]2

G
WCl4 PMe2 Ph 2
C TlOCH2 CF3
E-2

ˇ-Hydrogen
elimination
from Re3 OPri 9
E-3
E-3
G
G

Cu4 OBut 6 [OC CF3 3 ]2

G

[Ln5 O OPri 13 ]
Ln D Sc, Y, Yb
[Nd5 O OPri 3 HOPri 2 ]
[Y3 OBut 7 Cl2 thf 2 ]
[Y3 OBut 8 Cl thf 2 ]
[CeOCBut3 2 OBut 2 ]
[Nd OCBut3 2 Cl thf ]2
[Nd6 OPri 17 Cl]
But3 CO 2 UCl2 thf 2
UO2 OBut 2 Ph3 PO 2
But3 CO 2 MCl2
M D Ti, Zr
Ti OPri 2 [OCH CF3 2 ]2
Ti[OCH(CF3 )2 ]2 (OEt)2 (HOEt)
[TiCl3 OPri HOPri ]2
[TiCl2 OPri 2 HOPri ]2
[TiCl2 OCH2 CH2 Cl 2 HOCH2 CH2 Cl ]2
VCl OMe 2

VO OPri 3
CrO2 OR 2
R D CH2 CCl3 , CH2 CF3 , CH2 CH2 Cl
Mo4 OPri 10 OMe 2
[W OCH2 CF3 2 Cl2 PMe2 Ph 2 ]
Re3 OCHEt2 8 H
Re3 OPri 8 H

[Zn OCEt3 fN SiMe3 2 g]2
[Zn2 OOCMe 3 OMe ]
[Al OPri OBut 2 ]2
[Al OPri 2 acac ]2
In5 O OPri 13
[Sn OBut 3 OAc py ]
1 acac

A

Characterization
techniques3

A; E-2
A
E-2
E-2
Ce OCBut3 2
C But OOBut
E-3
E-2
E-3

E-2
E-3

1 H, 19 F

NMR; X-ray
1 H, 13 C NMR; X-ray
1 H, 13 C NMR; X-ray
1 H, 13 C NMR; X-ray
reflectance spectra; eff
1 H, NMR; MW
1 H, 19 F NMR; MW

59
58

NMR; X-ray
b
IR; 1 H, 13 C, 31 P NMR; X-ray 171
IR; 1 H,
IR; 1 H,

13 C
13 C

NMR
NMR; X-ray

IR; reflectance spectra; eff
IR; reflectance spectra; eff

IR
IR; 1 H, 19 F NMR; UV-Vis;
eff ; X-ray
IR; 1 H, 19 F NMR; UV-Vis;
eff ; X-ray
IR; 1 H NMR; MW; X-ray
IR; 1 H NMR; X-ray

I
Zn OAc 2
C Bun2 Sn OMe 2
1 H NMR; MW; MS
G
Al OPri 3 C acacH 1 H, 13 C, 27 Al NMR; X-ray
E-2
IR; 1 H, 13 C NMR; MS; X-ray
1 H, 13 C, 17 O, 119 Sn NMR;
Sn OBut 4
X-ray
C Me3 SiOAc C Py

174
174
595
541
175
651
651
688
431a

492
726
58
431

D acetylacetonate; py D pyridine; thf D tetrahydrofuran ligand ; 2 For methods see text (Section 2);

3 For abbreviations see footnote of Table 2.1.
a C.J. Burns, D.C. Smith, A.P. Sattelberger, and H.B.

Gray, Inorg. Chem., 31, 3724 (1992); b M.H. Chisholm,
C.E. Hammond, M. Hampden-Smith, J.C. Huffman, and W.G. Van der Sluys, Angew. Chem., Int. Ed. Engl.,
26, 904 (1987).

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15

Homometallic Alkoxides

reactivity sequence of alcohols, MeOH > EtOH > Pri OH > But OH, towards an alkali
metal. This order of reactivity is understandable from an electronic viewpoint which
predicts a decrease in the acidity of the hydroxyl hydrogen in the same order.
2.1.1.2 Group 2 metals (Be, Mg, Ca, Sr, Ba)
Group 2 metals, being less electropositive than group 1 metals, react sluggishly even
with sterically compact alcohols and require a catalyst (iodine or mercury(II) chloride)
particularly in cases of lighter group 2 metals (Be and Mg)30 – 34 to yield insoluble,
polymeric, and nonvolatile metal dialkoxides.
The reaction of magnesium with methanol had been reported26 to form solvates of different compositions: Mg OCH3 2 .3CH3 OH and Mg OCH3 2 .4CH3 OH,35,36 which have

been shown by X-ray diffraction studies to have the compositions Mg OCH3 2 .2CH3 OH37
and Mg OCH3 2 .3.5CH3 OH,38 respectively.
With sterically less demanding alcohols, alkoxides of the heavier alkaline earth
metals (Ca, Sr, Ba) [M(OR)2 ]n R D Me, Et, Pri had been prepared by a number
of workers39 – 44 by reactions of metals with alcohols. These are also oligomeric or
polymeric, and nonvolatile.
Interest in the synthesis and chemistry of soluble and volatile alkaline earth metal
alkoxides experienced a sudden upsurge in the 1990s,21 – 23 owing to the discovery of
superconducting ceramics45,46 containing Ba and Ca.
Reactions of sterically demanding monodentate alcohols47 with heavier alkaline earth
metals (M0 ) have been reported to yield soluble derivatives:
M0 C 2R0 OH
0

! M0 OR0

0

2

C H2 "
0

(2.3)
0

M D Ba; R D CMe3 , CEt3 , CHMe2 , CH CF3 2 . M D Ca, Sr; R D C CF3 3 .
By contrast, reaction of barium granules with Ph3 COH does not appear to take place,
even in the presence of I2 or HgCl2 as a catalyst, in refluxing tetrahydrofuran (THF)
over three days. However, the same reaction in the presence of ammonia as a catalyst

yields X-ray crystallographically characterized dimeric derivative [H3 Ba6 O OBut 11
OCEt2 CH2 O (thf)3 ].48 It may be inferred that ammonia reacts initially with barium
to form Ba NH2 2 , which undergoes proton transfer and anion metathesis to yield the
desired alkoxide derivative.
Although the reactions of heavier alkaline earth metals with alcohols are generally
straightforward, yielding the expected homoleptic derivatives, in some instances it
has been reported that the reaction follows a different course to yield an intriguing
product as in the case of the formation of X-ray crystallographically characterized49
oxo-alkoxide cluster of the composition H3 Ba6 O OBut 11 OCEt2 CH2 O (thf), in the
reaction of Ba with But OH in THF. The reasons for the formation of such an unusual
product in a simple reaction of the above type (Eq. 2.3) are not yet well understood,
but it tends to indicate that either adventitious hydrolysis or alkene/ether elimination
may be the main factor. Furthermore, the formation of OCEt2 CH2 O ligated product
in this reaction indicates that the diolate ligand is probably formed in a side-reaction
involving the solvent tetrahydrofuran molecules.
2-Methoxyethanol (a chelating alcohol) has been shown50 to react with calcium
filings in refluxing n-hexane to yield an X-ray crystallographically authenticated product

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16

Alkoxo and Aryloxo Derivatives of Metals

according to the following reaction:
Ca C 4HOC2 H4 OMe

n-hexane

! 19 [Ca9 OC2 H4 OMe

18 ].2

HOC2 H4 OMe C H2 " 2.4

By contrast, a similar reaction with barium granules followed a different course51
to yield [H4 Ba6 O OCH2 CH2 OMe 14 ] which has been characterized by single-crystal
X-ray diffraction studies.
Recently, it has been reported that monomeric Ba[O CH2 CH2 O n CH3 ]2 (n D 2 or 3)
products are obtained in the reactions of barium granules with an oligoether alcohol52
in tetrahydrofuran (Eq. 2.5):
Ba C 2HO CH2 CH2 O n CH3

THF

! Ba[O CH2 CH2 O n CH3 ]2 C H2 "

2.5

where n D 2 or 3.
The factor(s) determining the variation in the nature of products in the reaction of
Ca/Ba with chelating alcohols obviously require further investigations.
Interestingly, the reaction of barium with a sterically demanding alcohol having
donor functionality yields a volatile derivative53 with excellent solubility (even in
n-pentane) (Eq. 2.6):
Ba C 2HOCBut CH2 OPri

THF/NH3

! Ba[OCBut CH2 OPri 2 ]2

2

2.6

H2

2.1.2 Group 3 and the f-block Metals
The method involving direct reaction of a metal with alcohol was extended by Mazdiyasni et al .54 for the formation of scandium, yttrium, and lanthanide alkoxides using
mercuric chloride (10 3 –10 4 mol per mol of metal) as a catalyst:
Ln C 3Pri OH
(excess)

HgCl2 cat.

!

heat

3
1
[Ln OPri 3 ]n C H2 "
n
2

2.7

Ln D Sc, Y, Dy, and Yb.
Mercuric chloride appears to form an amalgam with the metal which reacts with

isopropyl alcohol to yield the triisopropoxide. Mazdiyasni et al .54 also noticed that the
use of HgCl2 in stoichiometric ratio resulted in the formation of alkenoxide contaminated with chloride. For example, the reaction of yttrium metal, isopropyl alcohol, and
mercuric chloride in 1:3:4 molar ratio yielded yttrium isopropeneoxide55 and hydrogen
chloride:
Y C 3HOCH CH3

2

! Y[OC CH3 DCH2 ]3 C 4Hg C 8HCl C 12 H2
2.8

C 4HgCl2

The above route has also been utilized for the synthesis of neodymium56 and
yttrium57 alkoxides as shown by Eqs (2.9) and (2.10):
4Nd C 16Pri OH
10Y C 30HOC2 H4 OMe

! [Nd OPri 3 .Pri OH]4 C 6H2 "
! [Y OC2 H4 OMe 3 ]10 C 15H2 "

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2.9
2.10


Homometallic Alkoxides

17

By contrast, interesting oxo-isopropoxides of the type Ln5 O OPri 13 , where Ln D
Sc,58 Y,58,59 Nd,60 and Yb58 have been isolated from the reaction mixtures resulting
from the interaction of metal chips and isopropyl alcohol, out of which, the last three
have been characterized by X-ray crystallography.
In restrospect, the isolation of oxo-alkoxide products, in the straightforward reactions
of metals with alcohols has generated a new interest in metal oxo-alkoxide products
(Chapter 5), a large number of which have been reported in the extensive investigations of Turova et al .61 – 63 employing the solubility and vapour pressure studies of
M(OR)x –ROH systems.
Bradley et al .58 have suggested that the formation of Ln5 O OPri 13 occurs by
the mechanism of metal alkoxide decomposition involving elimination of an ether,64
according to Eq. (2.11):
5Ln OPri

3

! Ln5 O OPri

13

C Pri2 O

2.11

Obviously, more quantitative work is essential to explore the extent and course
of side-reactions in the interactions of metals with different alcohols under varying
experimental conditions.
2.1.3 p-Block Elements
Aluminium alkoxides may be prepared65 – 74 by reaction of an alcohol with aluminium
activated by I2 , HgCl2 , or SnCl4 under refluxing conditions, for example:

2Al C 6ROH
(excess)

1%HgCl2


! 2Al(OR)3 C 3H2 "

2.12

where R D primary, secondary, or tertiary alkyl groups.
Aluminium triethoxide was first prepared in 1881 by Gladstone and Tribe65 by
the reaction of aluminium metal with ethanol in the presence of iodine as a catalyst.
Wislicenus and Kaufman66 in 1893 reported an alternative method of preparing normal
as well as isomeric higher alkoxides of aluminium by reacting amalgamated aluminium
with excess of refluxing alcohol. Hillyer67 prepared aluminium trialkoxides by the
reaction of metal with alcohols in the presence of SnCl4 as a catalyst. Tischtschenko68
in 1899, however, pointed out that the above reactions involving catalysts were useful
for the preparation of primary and secondary alkoxides of aluminium, but the reaction
of metal with tert-butyl alcohol was very slow even in the presence of catalysts. A
successful synthesis of aluminium tri-tert-butoxide described by Adkins and Cox71
in 1938, involved the reaction of amalgamated aluminium with refluxing tert-butyl
alcohol.
By contrast, in a reaction similar to Eq. (2.12), indium forms In5 O OPri 13 , the
structure of which has been established by X-ray crystallography.58
Although metallic thallium did not appear to react with alcohols75 even under
refluxing conditions, the reaction of ethyl alcohol with the metal partly exposed to
air does occur, resulting in the formation of liquid thallous ethoxide, for which the
following course of reactions has been suggested: (Eqs 2.13–2.15):
2Tl C 12 O2

! Tl2 O

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2.13


18

Alkoxo and Aryloxo Derivatives of Metals

Tl2 O C EtOH
TlOH C EtOH

2.2

! TlOEt C TlOH
TlOEt C H2 O

2.14
2.15

Electrochemical Technique (Method B)

The possibility of synthesizing metal alkoxides by the anodic dissolution of metals
into alcohols containing conducting electrolytes was demonstrated for the first time
by Szilard76 in 1906 for the methoxides of copper and lead. Since then this technique
has proved to be most promising. For example, the electrochemical method for the
preparation of ethoxides of Ti, Zr, Ta, Si, and Ge77 was patented by the Monsanto

Corporation in 1972, and was later applied by Lehmkuhl et al .78 for the synthesis of
Fe(II), Co, and Ni alkoxides M(OR)2 (R D Me, Et, Bun , and But ).
Turova et al .79 have substantially widened the scope of this technique by the synthesis
of a wide variety of homoleptic metal alkoxides and oxo-metal alkoxides: (i) soluble
M(OR)n , M D Sc, Y, La, lanthanide,80 Ti, Zr, Hf, Nb,79 Ta81 when R D Me, Et, Pri ,
Bun ; MO(OR)4 , M D Mo, W when R D Me, Et, Pri ;82 – 86 2-methoxyethoxides of Y,
lanthanide, Zr, Hf, Nb, Ta, Fe(III), Co, Ni, Sn(II),87 and (ii) insoluble metal alkoxides
such as Bi(OMe)3 ;88 Cr(OR)3 , R D Me, Et, MeOC2 H4 ;89 V(OR)3 ;86 Ni(OR)2 , R D Me,
Prn , Pri ;90 Cu(OR)2 , R D Bun , C2 H4 OMe;91 Re4 O2 (OMe)16 .92
Besides the above, Banait et al . have also employed the electrochemical reactions
of some (including polyhydroxy) alcohols for the synthesis of alkoxides of copper93
and mercury.94
In 1998, the anodic oxidation of molybdenum and tungsten95 in alcohols in the presence of LiCl (as electroconductive additive) was found to yield a variety of interesting
oxo-metal alkoxide complexes, some of which have been authenticated by singlecrystal X-ray crystallograpy.
The electrode ionization reactions of alcohols and anode polarized metals in the
presence of an electroconductive additive, followed by the interaction of the generated
intermediate species and the formation of the final products can by illustrated96 by the
following reactions (Eqs 2.16 and 2.17):
M
nROH C ne
nHž
MnC C nRO

! MnC C ne

anode

! nRO C nHž
n
! H2 cathode

2

2.16

2.17

! M(OR)n

where M D anode metal and ROH D an appropriate alcohol.
This process has great promise for the direct conversion of the less electropositive
metals to their alkoxides owing to its simplicity and high productivity as well as its
continuous and non-polluting character (with hydrogen as the major by-product).
The electrochemical technique appears to have been successfully employed in Russia
for the commercial production96 of alkoxides of Y, Ti, Zr, Nb, Ta, Mo, W, Cu, Ge,
Sn, and other metals.

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Homometallic Alkoxides

2.3

19

Reactions of Metal Atom Vapours with Alcohols (Method C)

Although the development of metal atom vapour technology over the past three decades
has shown tremendous utility for the synthesis of a wide range of organometallic
compounds (many of which were inaccessible by conventional techniques),97 the use

of this technique for the synthesis of metal alkoxides and related derivatives does not
appear to have been fully exploited.98 In 1990, Lappert et al .99 demonstrated the utility
of this technique for the synthesis of M—O—C bonded compounds by the isolation
of alkaline earth metal aryloxides.
2.4

Direct Reactions of Metal Halides with Alcohols (Method D)

By far the most common synthetic technique for metal alkoxides (Eq. 2.18) is the
replacement of halides from an appropriate metal halide by alkoxo groups.
MCln C x C y ROH

MCln

x (OR)x (ROH)y

C x HCl "

2.18

Halides of alkaline earth, lanthanide, actinide, and later 3d (Mn, Fe, Co, Ni) metals on
interactions with alcohols form crystalline molecular adducts like MgBr2 .6MeOH,100
CaBr2 .6MeOH,100 LnCl3 .3Pri OH101 – 103 where Ln is a lanthanide metal,
ThCl4 .4EtOH,104,105 MCl2 .2ROH (M D Mn, Fe, Co, Ni; R D Me, Et, Prn , Pri ).106 Apart
from the alkaline earth metal (Ca, Sr, Ba) halides, all of these undergo alcoholysis in
the presence of a suitable base to yield the corresponding homoleptic alkoxide or
chloride-alkoxide derivatives (Sections 2.5.1, 2.5.2, and 2.5.3).
Interesting variations in the extent of alcoholysis reactions of tetravalent metal (Ti,
Zr, Th, Si) chlorides may be represented107 by Eqs (2.20–2.23), to which CCl4 has
been added for comparison.

CCl4 C ROH (excess)
SiCl4 C 4ROH
TiCl4 C 3ROH (excess)
2ZrCl4 C 6ROH (excess)

!no reaction

2.19

!Si(OR)4 C 4HCl "

2.20

!TiCl2 (OR)2 .ROH C HCl "

2.21

!ZrCl2 (OR)2 .ROH
C ZrCl3 (OR).2ROH C 3HCl "

ThCl4 C 4ROH (excess)

!ThCl4 .4ROH

2.22
2.23

Depending on the nature of the metal (M), the initial metal chloride (MCln ) or a
product MClx y (OR)y forms an addition complex with alcohol molecules (ROH)
without enough perturbation of electronic charges for the reaction to proceed further.

The reactions of metal tetrachlorides MCl4 (M D Ti, Zr, Th) towards ethyl alcohol
show a gradation TiCl4 > ZrCl4 > ThCl4 .107
Although no clear explanation is available for the varying reactivity of different metal
chlorides with alcohols, it is interesting to note that final products of similar compositions have been isolated in the reactions of tetraalkoxides of these metals with HCl.
For example, the reaction of Ti OPri 4 with HCl leads finally to Ti OPri 2 Cl2 .Pri OH
(Section 4.11.2).

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20

Alkoxo and Aryloxo Derivatives of Metals

Specific intermediate products according to Eq. (2.18) may be isolated by controlling
the conditions (solvent, stoichiometry, or temperature). For example, the equimolar
reaction of TiCl4 with Pri OH in dichloromethane at room temperature has been
shown108 to yield the dimeric complex [TiCl3 HOPri -Cl ]2 . The above reaction
in 2–3 molar ratios gives the dimeric complexes [TiCl2 OPri HOPri -Cl ]2 and
[TiCl2 OPri HOPri -OPri ]2 as outlined in Scheme 2.1 on the basis of X-ray
structures of the products.
Pri

H
Cl
− 2HCl

OPri

Cl

Cl
2TiCl4 + 4HOPri

O

Ti
PriO

Ti
Cl

O
Pri

Cl
Cl

H
+ 2HOPri

Pri

H
Cl
Cl

Pr
O

i

Ti
PriO
O
Pri

O
OPri

Ti
O
Pri
H

Pri

H

O
− 2HCl

Cl

OPri

Cl
Pri

Ti

O

Cl
Cl

Cl
H

Scheme 2.1

Interestingly, the reaction of metallic uranium in isopropyl alcohol in the presence of
stoichiometric amounts of iodine109 has been shown to afford a mixed iodide–isopropoxide of uranium(IV), UI2 OPri 2 Pri OH 2 (Eq. 2.24):
U C 2I2 C 4Pri OH

Pri OH

! UI2 OPri

2

Pri OH 2 C 2HI

2.24

This type of reaction appears to have considerable promise for the preparation of other
polyvalent metal–iodide–isopropoxide complexes.
Out of the p-block elements, anhydrous chlorides of electronegative elements
boron,110 – 112 silicon,2,113 – 117 and phosphorus118,119 react vigorously with alcohols to
yield homoleptic alkoxo derivatives [M(OR)x ] (Eq. 2.25). Although no detailed studies
have been made, AlCl3 120,121 and NbCl5 122,123 undergo only partial substitution, while

GeCl4 124,125 does not appear to react at all with alcohols.
MClx C x ROH

! M(OR)x C nHCl "

2.25

M D B, x D 3; Si, x D 4; P, As x D 3
The reactions indicated above occur with primary and secondary alcohols only, and
have been studied mainly with ethyl and isopropyl alcohols. With a tertiary alcohol
(But OH), silicon tetrachloride yields almost quantitatively Si(OH)4 and But Cl.126 This

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Homometallic Alkoxides

21

has been shown by Ridge and Todd127 to be due to facile reactivity of HCl initially
evolved to yield But Cl and H2 O, which hydrolyses SiCl4 .
SiCl4 C 4But OH

! Si(OH)4 C 4But Cl "

2.26

The reaction of AsCl3 with an excess of CF3 CH2 OH128 gives As OCH2 CF3 3 , which
could be oxidized with chlorine in the presence of CF3 CH2 OH to As OCH2 CF3 5 as
shown by the following reaction (Eq. 2.27):

AsCl3 C 3CF3 CH2 OH

! As OCH2 CF3

3HCl

3

CCl2 , C2HOCH2 CF3

! As OCH2 CF3

5

C 2HCl "

2.27

Following the earlier observations of Fischer,129 Klejnot130 observed that the reaction
of WCl6 with ethyl alcohol can be represented by Eqs (2.28) and (2.29):
WCl6 C 2C2 H5 OH
Cl2 C C2 H5 OH

! WCl3 (OEt)2 C 12 Cl2 C 2HCl "

2.28

! CH3 CHO C 2HCl "

2.29

Chloro-alkoxo derivatives of W(V) can be prepared by the direct reactions of WCl5
with alcohols at 70Ž C.131,132 The reaction between WCl4 and the alcohols ROH
(R D Me, Et) leads to the (WDW 8C -containing derivatives W2 Cl4 (OR)4 (HOR)2 ,133
which have been characterized by X-ray crystallography.
2.5 Reactions of Simple and Complex Metal Chlorides or Double Nitrates
with Alcohols in the Presence of a Base (Method E)

On the basis of the earlier observations,3,4,26 it appears that except for a few metal(loid)
halides, most of these undergo only partial solvolysis or no solvolysis even under
refluxing conditions. Thus in order to achieve the successful preparation of pure
homoleptic metal alkoxides, the use of a base such as ammonia, pyridine, trialkylamines, and alkali metal alkoxides appears to be essential. While alkali alkoxides
provide anions by direct ionization, the role of other bases (žžB) could be to increase
the concentration of alkoxide anions according to Eqs (2.30)–(2.32):
B C ROH

(HB)C C OR

2.30

OR C M—Cl

! M—OR C Cl

2.31

ž
ž

C

(HB) C Cl

C

! (BH) Cl

2.32

Of the commonly employed bases (NH3 , NaOR, KOR) for completion of the reactions and preparation of soluble metal alkoxides, NH3 appears to have some distinct
advantages including: (i) passage of anhydrous ammonia in a reaction mixture of an
anhydrous metal chloride and alcohol produces heat by neutralization of the liberated
HCl with NH3 ; the cooling of the reaction mixture is an index of the completion of the
reaction, (ii) precipitated NH4 Cl can be filtered easily, (iii) excess NH3 can be easily
removed by evaporation, whereas (iv) heterobimetallic alkoxides like NaAl(OR)4 and
KZr2 (OR)9 tend to be formed with excess of alkali alkoxides.

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22

Alkoxo and Aryloxo Derivatives of Metals

2.5.1 The Ammonia Method (E-1)
The addition of a base, typically ammonia, to mixtures of metal(loid) halides and
alcohols allows the synthesis of homoleptic alkoxides for a wide range of metals and
metalloids. Anhydrous ammonia appears to have been employed for the first time by
Nelles134 in 1939 for the preparation of titanium tetra-alkoxides (Eq. 2.33):
TiCl4 C 4ROH C 4NH3

(excess)

benzene

! Ti(OR)4 C 4NH4 Cl #

2.33

Zirconium tetra-alkoxides were prepared for the first time in 1950 by the ammonia
method,135 as earlier attempts136 to use the alkali alkoxide method did not give a
pure product, owing to the tendency of zirconium to form stable heterobimetallic
alkoxides137 (Chapter 3) with alkali metals.
The ammonia method has, therefore, been successfully employed3,4,21,26 for the
synthesis of a large number of alkoxides of main-group and transition metals according
to the following general reaction (Eq. 2.34):
MClx C x ROH C x NH3

benzene

! M(OR)x C x NH4 Cl #

2.34

Owing to the highly hydrolysable nature of most of the alkoxide derivatives, stringently anhydrous conditions are essential for successful preparation of the alkoxides.
Apart from careful drying of all the reagents as well as solvents, gaseous ammonia
should be carefully dried by passage through a series of towers packed with anhydrous calcium oxide, followed preferably by bubbling through a solution of aluminium
isopropoxide in benzene.
Benzene has been reported to be a good solvent for the preparation of metal alkoxides
by the ammonia method, as its presence tends to reduce the solubility of ammonium
chloride, which has a fair solubility in ammoniacal alcohols. In addition, the ammonium chloride precipitated tends to be more crystalline under these conditions, making

filtration easier and quicker. Although most of the earlier laboratory preparations have
been carried out in benzene, the recently emphasized carcinogenic properties of this
solvent suggests that the use of an alternative solvent should be explored.
2.5.1.1 Group 3 and f-block metals
To date no unfluorinated alkoxides of scandium, yttrium, and lanthanides18,21 in the
common C3 oxidation state appear to have been prepared by the ammonia method.
By contrast, yttrium and lanthanides (Ln) fluoroalkoxide derivatives of the types
LnfOCH CF3 2 g3 138 and LnfOCH CF3 2 g3 .2NH3 139 have been isolated by this route.
It might appear that the ammonia method is applicable to the synthesis of a large
number of metal alkoxides, but there are certain limitations. For example, metal chlorides (such as LaCl3 )140 tend to form a stable and insoluble ammoniate M NH3 y Cln
instead of the corresponding homoleptic alkoxide derivative. Difficulties may also arise
if the metal forms an alkoxide which has a base strength comparable with or greater than
that of ammonia. Thorium provides a good example of this type where the ammonia
method has not been found to be entirely satisfactory.141 For example, during the
preparation of thorium tetra-alkoxides from ThCl4 and alcohols, Bradley et al .142 could

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Homometallic Alkoxides

23

obtain only thorium trialkoxide monochlorides owing to the partial replacement of chlorides. These workers observed that the alcoholic solutions of Th(OEt)4 or Th OPri 4
were alkaline to thymolphthalein. On the other hand anhydrous ammoniacal alcohols
were acidic to this indicator. Thus thorium tetra-alkoxides tend to be more basic than
ammonia and the following feasible equilibria (Eqs 2.35 and 2.36) may be responsible
for the formation of Th(OR)3 Cl instead of the expected tetra-alkoxides.
Th(OR)4 C NH4 C

Th(OR)3 C C NH3 C ROH

2.35

Th(OR)3 C C Cl

Th(OR)3 Cl

2.36

However, it was observed141 that treatment of alcoholic solutions of thorium tetrachloride with sodium alkoxides gave thorium tetra-alkoxides.
In search of a convenient method for the synthesis of tetra-alkoxides of cerium(IV) and
plutonium(IV), which do not form stable chlorides, the complex chlorides C5 H6 N 2 MCl6
(M D Ce(IV), Pu(IV); C5 H6 N D pyridinium) method proved to be convenient starting
materials:
C5 H6 N 2 MCl6 C 6NH3 C 4ROH

! M(OR)4 C 6NH4 Cl # 2C5 H5 N

2.37

where M D Ce143 or Pu144 and R D Pri .
Bradley et al .145 had earlier reported that dipyridinium hexachlorozirconate
C5 H6 N 2 ZrCl6 , which can be prepared from the commonly available ZrOCl2 .8H2 O,
also reacted smoothly with alcohol in the presence of ammonia to form the tetraalkoxides Zr(OR)4 .
During an attempt to prepare tetra-tert-alkoxides of zirconium and cerium by the
reactions of C5 H6 N 2 MCl6 (M D Zr, Ce) with tert-butyl alcohol, Bradley and coworkers143,144 had noticed the formation of MCl OBut 3 .2C5 H5 N as represented by
Eq. (2.38):
C5 H6 N 2 MCl6 C 3But OH C 5NH3

! MCl OBut 3 .2C5 H5 N C 5NH4 Cl # 2.38

As the product reacts with primary alcohols (Eq. 2.39) in the presence of ammonia to
give heteroleptic alkoxides, M(OR) OBut 3 , steric reasons have been suggested as a
possible explanation for the partial replacement reactions with tert-butyl alcohol:
MCl OBut 3 .2C5 H5 N C EtOH C NH3

! M(OEt) OBut

C 2C5 H5 N C NH4 Cl #
2.39
It is, however, somewhat intriguing that dipyridinium hexachloro derivatives of
zirconium and cerium146 undergo complete replacement with Cl3 C.CMe2 OH, which
should apparently be an even more sterically hindered alcohol than But OH:
3

C5 H6 N 2 MCl6 C 4Cl3 C.CMe2 OH C 6NH3
! M OCMe2 CCl3

4

C 2C5 H5 N C 6NH4 Cl #

2.40

Reactions of MCl4 (M D Se, Te) with a variety of alcohols (MeOH, EtOH, CF3 CH2 OH,
But CH2 OH, Me2 CHOH) in 1:4 molar ratio in THF using Et3 N as a proton acceptor afford
corresponding tetra-alkoxides.146a

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