Transition metal chemistry of low valent group 13 organyls

Article abstract of DOI:10.1002/ejic.200400569

The coordination of low-valent group 13 organyls E^IR [E = Al, Ga, In; R = Cp*, C(SiMe₃)₃] to transition metals has attracted increasing interest over the past decade. Complexes and cluster compounds of these new ligands with a number of transition metals have been isolated and characterised. The E^IR moiety is formally isolobal with CO and PR₃ (R = alkyl, Cp*) or carbenes (R = chelating group) with varying σ-donor and π-acceptor properties depending on the organic group R as well as the group 13 metal E. In this review, different ways of forming M-E bonds such as substitution reactions of labile ligands or insertion of E^IR into transition metal halide bonds are described. Furthermore, the reactivity of homoleptic complexes M_a(E^IR) _b, is discussed, outlining the use of these new complex types in bond activation reactions. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400569

MICROREVIEW
Transition Metal Chemistry of Low Valent Group 13 Organyls
[a]
[a]
[a]
[a]
Christian Gemel, Tobias Steinke, Mirza Cokoja, Andreas Kempter, and
Roland A. Fischer*[
a]
Keywords: Group 13 elements / Transition metals / Main group elements / Carbene homologues
I
The coordination of low-valent group 13 organyls E R [E = Al,
as well as the group 13 metal E. In this review, different ways
Ga, In; R = Cp*, C(SiMe
increasing interest over the past decade. Complexes and
cluster compounds of these new ligands with a number of
3
)
3
] to transition metals has attracted
of forming M−E bonds such as substitution reactions of labile
ligands or insertion of E R into transition metal halide bonds
I
are described. Furthermore, the reactivity of homoleptic com-
I
transition metals have been isolated and characterised. The
plexes M
a
(E R)
b
is discussed, outlining the use of these new
I
E R moiety is formally isolobal with CO and PR
3
(R = alkyl,
complex types in bond activation reactions.
Cp*) or carbenes (R = chelating group) with varying σ-donor ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
and π-acceptor properties depending on the organic group R
Germany, 2004)
1
. Introduction
stabilised by coordination of Cp*[1Ϫ3] and the subsequent
development of convenient synthetic procedures for AlCp*
Since the pioneering discovery by H. Schnöckel in 1989
[
4,5]
[6,7]
and GaCp*
by H. Roesky and P. Jutzi, much work
I
I
that the low valent group 13 metals Al and Ga can be
has been devoted to the coordination of these compounds
[
8,9]
to transition metal centres.
[
a]
I
Anorganische Chemie II, Organometallics and Materials
Chemistry, Ruhr Universität Bochum,
The E R fragment is isolobal with CO and PR , i.e. it
3
exhibits σ-donation as well as π-accepting properties. The
extent of σ-donation and π-acceptance is, of course, depen-
dent on the nature of the metals, but also to a large extent
Universitätsstrasse 150, 44780 Bochum, Germany
Fax: (internat.) ϩ 49-(0)234-321-4174
E-mail: roland.fischer@ruhr-uni-bochum.de
Roland A. Fischer (far right), born in 1961 in Rosenheim, Germany, studied chemistry at the
Technische Universität München (1981Ϫ1986). In 1989 he received his doctorate for his work on
alkyne complexes of rhenium under the guidance of W. A. Herrmann. After a postdoctoral fellow-
ship at the University of California at Los Angeles with H. D. Kaesz (1989Ϫ1990), he returned
to the Technische Universität München and completed his habilitation in 1995 on the coordination
chemistry of aluminium, gallium and indium at transition metals and on the use of these complexes
as MOCVD sources for intermetallic phases. From 1996 to 1997 he was Professor of Inorganic
Chemistry at the Ruprecht-Karls-University Heidelberg and in 1998 he moved to the Ruhr-Univer-
sität Bochum. His research interests deal with various aspects of organometallic and materials
chemistry with some emphasis on precursor chemistry for MOCVD. His scientific work has
been honoured among others with the Heinz-Meier-Leibnitz award from the Bundesministerium für Bildung und Forschung (Federal Ministry
for Education and Science) (1993), a Karl-Winnacker scholarship (1995) and the award of the Alfried-Krupp-von-Bohlen-und-Halbach
Foundation (1996).
Christian Gemel (centre) was born in 1971 in Vienna, Austria. From 1990Ϫ1996 he studied Chemistry at the Technical University of Vienna,
receiving his Doctorate in 1997 for his work on organoruthenium chemistry in the group of Karl Kirchner. After postdoctoral visits to Kenneth
G. Caulton, Indiana University (1998) and Klaus Richard Poerschke, MPI Mülheim (2000Ϫ2001), he joined the group of Prof. Roland
Fischer in 2002. He became a permanent research staff member in 2004.
Tobias Steinke (far left), born in 1974 in Bochum, Germany trained as a teacher of chemistry from 1995Ϫ2000. In 2001 he started to work
on his PhD under the guidance of Prof. Fischer on the coordination chemistry of ECp* to transition metals. His doctoral work has been
awarded with the Wolfgang-Seel-Preis.
Mirza Cokoja (centre left) was born in 1979, Oberhausen, Germany and studied chemistry at the Ruhr University Bochum from 1998Ϫ2003.
He received his Diploma in 2003 for his work on the insertion reactions of ECp* into RuϪCl Bonds.
Andreas Kempter (centre right) was born in 1978, Sigmaringen, Germany and studied chemistry at the Ruhr University Bochum from
1
999Ϫ2004. He received his Diploma in 2004 for his work on the coordination chemistry of sterically encumbered low valent group 13 metals.
MICROREVIEWS: This feature introduces the readers to the author’s research through a concise overview of the
selected topic. Reference to important work from others in the field is included.
Eur. J. Inorg. Chem. 2004, 4161Ϫ4176
DOI: 10.1002/ejic.200400569
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
[
24]
on the organic group R. For R ϭ Cp*, for example, the [Fe(CO) (AlCp*)] (3b), are also available by salt metath-
4
III
[25Ϫ27]
vacant p-orbitals of the group 13 metal are partially popu- esis from carbonylmetallates and E RX .
2
lated by the π-donor orbitals of the Cp* groups thus dimin-
ishing the π-acceptor properties of the ligand. In fact, the
contribution of the σ-donation has been calculated to be
very large, resulting in the highest contribution of the bond
energy being the electrostatic attraction between the par-
tially positive group 13 metal and the partially negative
[
10]
transition metal, following the order Al Ͼ Ga Ͼ In.
In contrast to the chemistry of transition metal borylene
complexes which has been recently reviewed by H.
[
11Ϫ13]
I
Braunschweig,
the free ligands E R are stable and iso-
lable. This facilitates access to their transition metal com-
plexes since classic organometallic synthetic approaches can
A recent example of ligand substitution on metal car-
I
be applied. Substitution of labile ligands by E R is the most bonyl fragments is the reaction of [fac-(RCN)
M(CO)
]
3
3
general route of forming transition metal-group 13 element (R ϭ Me, M ϭ Mo; R ϭ Et, M ϭ W) with GaCp* giving
bonds. Homoleptic as well as heteroleptic complexes the monomeric compounds [fac-(GaCp*) M(CO) ] (4a:
It was shown that compound 4a can
3
3
I
[28]
[
M (E R) L ] are accessible from a wide range of transition M ϭ Mo, 4b: W).
a b c
metal complexes such as carbonyl, olefin or phosphane act as a building block for the synthesis of dinuclear com-
I
2
complexes. The ease of oxidation of E R provides less com- plexes affording [Mo
(CO) {µ -(GaCp*)} ] (5) upon reac-
3
2
6 3
I
mon synthetic pathways. Thus, E R can act as in situ reduc- tion with [fac-(MeCN)
Mo(CO) ] (Scheme 1).
3
ing agents towards higher oxidation state transition metal
complexes [L MX ], giving either insertion products if
n
n
the oxidised group 13 metal remains coordinated to the
transition metal centre or low oxidation state complexes
I
[
M (E R) ] by cleavage of X ER.
a
b
2
In this review we wish to highlight recent developments
in this particular field from a mainly synthetic point of
view. Fundamental bonding and general structural issues
have been intensively discussed in the literature
[
10,14Ϫ17]
I
previously.
However, interest in E R has somewhat
diminished during the last few years most likely because the
chemistry of these new species has been considered to be
quite unexciting. In contrast, our recent results which focus
particularly on the transition metal chemistry of AlCp*,
GaCp* and InCp* have revealed the unexpected potential
of these compounds in classic organometallic and cluster
chemistry. Herein we hope to convince the reader that these
‘‘exotic’’ ligands and related compounds hold much prom-
ise for the future.
2
Scheme 1. Synthesis of [Mo
2
(CO)
6
{µ -(GaCp*)}
3
] (5)
2
. Carbonyl Complexes M (CO) (ER)
In contrast to the well explored substitution reactions of
ECp*, the addition to coordinatively unsaturated centres is
a
b
c
I
Since the first isolation of E R, investigations of their only known for the in situ formed [(dcpe) Pt] complex
2
properties as ligand towards transition metals have mostly [dcpe ϭ bis(dicyclohexylphosphanyl)ethane] leading to
I
[29,30]
involved metal carbonyl fragments. In particular, the reac- [(dcpe) Pt(E R) ].
The unsaturated 14 VE fragment
2
2
I
tion of E R with homoleptic transition metal carbonyls has [(dcpe) Pt] is thermally formed from [(dcpe) PtH(neopen-
been studied in detail. Hence, the complexes [Mn (CO) {µ - tyl)] and trapped in the presence of E R ligands (Scheme 2).
An unsaturated carbonyl system which has been studied
b: Ga ) or [Ni (µ -GaCp*) (CO) ] were obtained by in more detail is the reaction of GaCp* with the multiple
2
2
2
I
2
8
[
18]
2
[19]
[
1
InC(SiMe ) ]} ],
[Co (CO) (µ -ECp*) ] (1a: E ϭ Al,
3
3
2
2
6
2
[
6]
2
[6]
4
4
6
treatment of the homoleptic metal carbonyl clusters bonded dimeric complexes [CpM(CO) ] (M ϭ Mo, W).
2
2
I
[20]
2
with E R. The compounds [Cr(CO) (ECp*)] (E ϭ Al,
Reaction with GaCp* yields the addition products
[Fe (CO) {µ - [Cp(CO) M(µ-GaCp*)] (6a: M ϭ Mo, 6b: M ϭ W)
5
Ga,[
6]
In ), [(GaCp*) Mo(CO) ],
[21]
[22]
2
4
2
6
2
2
[23]
[6]
[28]
In[C(SiMe ) ]} ] (2)
obtained by substitution of chelating olefin ligands such as
and [Fe(CO) (GaCp*)] (3a) were (Scheme 3).
3
3
2
4
1
An unusual feature of 6a and 6b is the η -coordination
cyclooctene, norbornadiene, cyclooctatetraene and cyclo- mode of the Cp* moieties bound to the bridging Ga atoms,
heptatriene, respectively. Some carbonyl complexes, such as presumably a consequence of the steric overcrowding in the
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Transition Metal Chemistry of Low Valent Group 13 Organyls
MICROREVIEW
Scheme 2. Trapping of the unsaturated fragment [(dcpe)
2
Pt] with
I
E R
4
Figure 1. Monomeric Clusters [M(ECp*) ]
series [M(ECp*) ] (M ϭ Ni, Pd, Pt and E ϭ Al, Ga, In),
4
only [Pt(InCp*) ] seems to be thermally unstable, decom-
4
posing at temperatures above Ϫ30 °C. This indicates weaker
σ-donor properties of InCp* compared with InC(SiMe ) ,
3
3
AlCp* or GaCp*.
3
.2 Cluster Complexes [M (ECp*) ] and [M (ECp*) ]
2
5
3
8
2 2
Scheme 3. Synthesis of [Cp(CO) M(µ-GaCp*)] (6a: M ϭ Mo, 6b:
M ϭ W)
Compared with the above examples, neutral homoleptic
cluster compounds of the type [M (ECp*) ] (M ϭ Pd, Pt;
a
b
b Ͼ a Ͼ 1) represent quite a novel and more promising
I
molecules. The GaCp* coordination in the tungsten com- field in the coordination chemistry of E R (E ϭ Al, Ga,
plex is surprisingly weak and shows a temperature depen- In). Recent studies concerning the synthesis of cluster com-
I
dent association/dissociation equilibrium.
pounds have been focused on the coordination of E R to
transition metal carbonyl fragments [(CO) M ]. Consti-
n
a
tution and structural characteristics of new GaCp*-substi-
[
6]
[34]
3
. Homoleptic Complexes of the Type
tuted Ni4
and Rh carbonyl clusters
or [{(CpNi)(µ2-
6
[
35]
[
M (ECp*) ] (M ؍ Ni, Pd, Pt)
AlCp*)}2] are directly related to classical transition metal
carbonyl cluster structures. In contrast, the novel homolep-
tic series [M (ECp*) ] has no direct structural analogue in
a
b
3.1 Monomeric Complexes [M(ECp*)4]
a
b
classical carbonyl or phosphane cluster chemistry, although
The COD ligands in [Pt(COD) ] and [Ni(COD) ]
COD ϭ 1,5-cyclooctadiene) can be readily replaced by
2
2
recently the coordination chemistry of PR has been ex-
3
(
[
36]
tended to include a bridging mode. These complexes ex-
hibiting bridging and terminal E R ligands are still rare in
ECp* or E[C(SiMe ) ] yielding monomeric complexes of
3
3
I
I
[22,31,32]
the type [M(E R) ] (M ϭ Pt, Ni; R ϭ Cp*; Figure 1).
4
comparison with other MϪE complexes. In principle, ac-
Analogous Pd compounds can be obtained by treatment of
Pd(tmeda)(CH ) ] with ECp*. In contrast to the reaction
cess to [M (ECp*) ] proceeds via two different reaction
a
b
[
3
2
pathways: (i) by building block synthesis and (ii) via direct
of [Pd(tmeda)(CH ) ] (tmeda ϭ N,N,NЈ,NЈ-tetramethyl-
3
2
I
methods. In both cases, E R reacts with a transition metal
ethylenediamine) with PR giving Pd(PR ) which usually
3
3 n
complex bearing labile ligands (e.g. olefins) with full substi-
tution of the ligands, the nature of the products being deter-
mined mainly by kinetic factors.
proceeds via a ligand-assisted reductive elimination of
ethane, the reaction with ECp* includes a methyl group
migration and reduction of the palladium centre by
[
33]
ECp*.
I
3.2.1 Building Block Synthesis of Homoleptic Clusters
All complexes [M(E R) ] are kinetically inert, indicating
4
[MPt(GaCp*)5]
the high bond energy of the ECp* to transition metal link-
age. Substitution reactions with PR or CO do not take
The monomeric compounds [M(GaCp*) ] (7a: M ϭ Pt,
3
4
I
I
place and a coalescence of [M(E R) ] and the free E R is 7b: M ϭ Pd; Figure 1) can serve as building blocks for the
4
not observed in the NMR spectra. Also, the synthesis of synthesis of the dinuclear cluster complexes [MPt(GaCp*)5]
heteroleptic complexes [M(ECp*) (EЈCp*) ] (n ϩ m ϭ 4) (8a: M ϭ Pt, 8b: M ϭ Pd) with terminal and bridging
n
m
is not possible by ligand substitution. Interestingly, in the GaCp* ligands (Scheme 4). The reaction of [Pt(COD)2]
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
a b
Scheme 4. Building block synthesis of clusters M (ECp*)
with an equimolar amount of [M(GaCp*) ] and subsequent
The first direct synthesis of a homoleptic cluster complex
4
2
2
addition of GaCp* yields [Pt (GaCp*) (µ -GaCp*) ] (8a) was reported in 2000 (Scheme 5). [Pt (GaCp*) (µ -GaCp*) ]
2
2
3
2
2
3
and the analogous ‘‘trimetallic’’ heterodinuclear compound (8a) is quantitatively formed from the reaction of
2
[
PtPd(GaCp*) (µ -GaCp*) ] (8b).
[Pt(C H ) ] with an excess of GaCp* and could be isolated
2
3
2 4 3
[
37]
It has been shown that the [M(GaCp*) ] species form in analytically pure form in high yields.
4
reactive yet isolable dinuclear associates with [Pt(COD)2].
Based on variable temperature NMR spectroscopic data
2
these intermediates were identified as [Pt (GaCp*)(µ -
2
2
2
GaCp*) (η -COD)] (9a) and [PtPd(GaCp*)(µ -GaCp*) -
3
3
2
[33]
(
η -COD)] (9b).
3
.2.2 Direct Synthesis of Homoleptic Clusters [M (ECp*) ]
a b
The reaction of ECp* with transition metal complexes
having olefin ligands generally leads to a full substitution
of the olefins by ECp*. However, in most cases the ratio of
the reactants does not directly determine the composition
of the products. Instead, the formation of mono-, di- or
trinuclear products is influenced by kinetic factors, i.e. by
the reaction temperature, the nature of the transition metal-
olefin complex or simply the solubilities of the starting ma-
terials and products.
2
2 2 3
Scheme 5. Synthesis of [Pt (GaCp*) (µ -GaCp*) ] (8a) and
2
[
3 4 4
Pd (InCp*) (µ -InCp*) ] (10a)
The monomeric complex [Pt(GaCp*) ] (7a) which is
4
quantitatively formed in the reaction of [Pt(COD) ] with
2
GaCp* was not observed in this reaction. It was shown
Figure 2. Dimeric clusters [M
2
(ECp*)
5
]
3 8
Figure 3. Trimeric clusters [Pd (ECp*) ]
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Transition Metal Chemistry of Low Valent Group 13 Organyls
2
MICROREVIEW
later that [Pt (GaCp*) (µ -GaCp*) ] (8a) is also kinetically leptic clusters are formed. At Ϫ30 °C in hexane the reaction
2
2
3
2
stable in the presence of free GaCp* and does not react to yields the dinuclear compound [Pd (GaCp*) (µ -GaCp*) ]
2
2
3
[
33]
give [Pt(GaCp*) ] (7a).
(8c) in quantitative yields. Only trace amounts (Ͻ 3%) of
When [(tmeda)Pd(CH ) ] is treated with an excess of the monomeric compound [Pd(GaCp*) ] (7b) are observed.
4
3
2
4
2
InCp*, the trinuclear complex [Pd (InCp*) (µ -InCp*) ] The reaction in toluene at room temperature yields the trin-
3
4
4
2
(
10a) is quantitatively formed. Complex 10a is also kine- uclear complex [Pd (GaCp*) (µ -GaCp*) ] (10b) in high
3 4 4
tically inert and does not react to give mononuclear yield, while the dimeric species 8c is formed in only very
[38]
[
Pd(InCp*) ] in the presence of InCp*.
small amounts (Ͻ 2%) under these conditions (Scheme 6).
4
A good illustrative example of the subtle kinetic control
of cluster formation is the reaction of [Pd (dvds) ] with
GaCp* (dvds ϭ 1,3 divinyl-1,1,3,3-tetramethyldisiloxane).
2
3
3
.2.3 Reactivity of Homoleptic Cluster Complexes
[M (ECp*) ] (M ؍ Pd, Pt)
a
b
Depending on the reaction conditions, two different homo-
While the syntheses and structural chemistry of MϪE
complexes and their use as precursors to new materials have
attracted widespread interest, remarkably little is still
known about their general chemical reactivity. The coordi-
nate MϪE bond in such complexes is relatively polar and
[
16]
strong when E ϭ Al, Ga
and the steric demand of the
I
[22]
E R ligands is rather large. Thus, neither dissociative nor
associative reactions are very likely to occur at the central
metal. Hence, the organometallic chemistry of such com-
pounds would appear to be rather unfruitful. Indeed, the
electronically and sterically saturated monomeric complexes
[39]
[
M(ECp*) ] turn out to be completely kinetically inert.
4
In sharp contrast, however, the related unsaturated cluster
complexes [M (ER) ] (b Ͼ a Ͼ 1) react with a selection of
a
b
ligands (Cp*Al, CO, phosphanes, isocyanides) to give new
2
Scheme 6. Synthesis of [Pd
2
(GaCp*)
2
(µ -GaCp*)
3
] (8c) and
di- and trinuclear substitution products (Scheme 7).[
40]
2
[
3 4 4
Pd (GaCp*) (µ -GaCp*) ] (10b)
2
Scheme 7. Reactivity of [M
2
(GaCp*)
2
(µ -GaCp*)
3
]
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
On the basis of quantum chemical calculations by Frenk-
ing, the substitution of GaCp* or InCp* by AlCp* in
homoleptic complexes is a thermodynamically favourable
[16]
2
reaction.
Indeed, treatment of [M (GaCp*) (µ -GaCp*) ]
2 2 3
with AlCp* quantitatively yields the ‘‘trimetallic’’ com-
2
pound [Pt (GaCp*) (µ -AlCp*) ] (8e) or the fully substi-
2
2
3
2
tuted dinuclear complex [Pd (AlCp*) (µ -AlCp*) ] (8d).
2
2
3
2
The reaction of [M (GaCp*) (µ -GaCp*) ] with PPh
2
2
3
3
quantitatively yields the mono- or disubstituted complexes
2
[
MPt(GaCp*)(PPh )(µ -GaCp*) ] (8h: M ϭ Pd, 8i: M ϭ
3 3
2
Pt) and [Pd (PPh ) (µ -GaCp*) ] (8g). Whereas the substi-
2
3 2
3
tution reactions at Pd centres giving 8f and 8g readily occur
at room temperature, the substitution of GaCp* by PPh in
3
the related Pt Ga complex 8a requires both a longer reac-
2
5
tion time and a higher temperature (90 °C). In the case of
the ‘‘trimetallic’’ compound 8b, the substitution can only
be observed at the Pd centre, leaving the Pt-GaCp* bond
unchanged. This is in good agreement with the fluxionality
Figure 4. Triangular cluster 10c
I
of [M (E R) ] in solution (vide infra). Whereas ligands at-
2
5
tached to a palladium centre usually show fast exchange
between the terminal and bridging positions, such processes
are slower on platinum centres. A dissociative mechanism
of the bridging ligands forming unsaturated metal centres
as intermediates is assumed for the substitution reactions
I
as well as for the fluxional processes of [M (E R) ].
2
5
Generally, heteroleptic monomeric complexes of the type
[
M(GaCp*)4Ϫx(L) ] (x ϭ 1Ϫ4) are not accessible via substi-
x
tution reactions. However, when the chelating diphosphane
ligand dppe [dppe ϭ bis(diphenylphosphanylethane)] is ad-
2
ded to [M (GaCp*) (µ -GaCp*) ], all Cp*Ga units are sub-
2
2
3
stituted and the monomeric complexes [M(dppe) ] (M ϭ
2
Pd, Pt) are formed quantitatively. The reaction of
2
[
Pt (GaCp*) (µ -GaCp*) ] (8a) with the strong π-acceptors
2
2
3
CNtBu or CO gives the disubstitution products
2
2
[
Pt (CNtBu) (µ -GaCp*) ] (8j) and [Pt (CO) (µ -GaCp*) ]
2 2 3 2 2 3
(
8k), respectively, with the GaCp* ligands preferring the
Scheme 8. Synthesis of [Pd
InCp*) (dppe) ] (10d)
3
(InCp*)
3
(PPh
2
3 3 3
) ] (10c) and [Pd -
bridging positions.
(
3
2
In contrast to the substitution reactions of [M (GaCp*) -
2
2
2
(
µ -GaCp*) ] complexes, where the cluster core is always
3
The third InCp* ligand is found in a µ -coordination mode
retained, the reaction of the trinuclear compound
Pd (InCp*) ] (10a) with PPh or dppe leads to a rearrange-
parallel to the Pd plane. Additionally, each Pd centre is
3
[
3
8
3
coordinated by one or two terminal phosphane ligands.
ment of the linear Pd core to give the triangular Pd clus-
3
3
ters [Pd (InCp*) (PPh ) ] (10c) and [Pd (InCp*) (dppe) ]
3
3
3 3
3
3
2
3
.2.4 Solution Structures and NMR Spectroscopy
(10d) (Scheme 8).
2
In both complexes two InCp* units cap both faces of a
Whereas [Pt (GaCp*) (µ -GaCp*) ] (8a) exhibits a static
2
2
3
Pd triangle resulting in a trigonal bipyramidal geometry. NMR spectrum at room temperature, i.e. two well dis-
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Transition Metal Chemistry of Low Valent Group 13 Organyls
MICROREVIEW
tinguishable signal sets (singlets) for the bridging and the NHCs (nitrogen heterocyclic carbenes) are widely used as
terminal GaCp* ligands, the corresponding palladium clus- supporting and directing ligands for reactivity and selec-
2
ter [Pd (GaCp*) (µ -GaCp*) ] (8c) is dynamic in solution tivity in organometallic chemistry, almost nothing is known
2
2
3
1
I
and gives rise to only one Cp* signal set. The H NMR about the corresponding potential of E R ligands.
2
spectrum of [PtPd(GaCp*) (µ -GaCp*) ] (8b) at room tem-
The reaction of [Ni(COD)
perature shows two resonances with an integral ratio of 1:4. AlCp* in benzene does not yield the expected complex [Ni-
Clearly the fluxional process involves only the Pd centre of (AlCp*) ] (7c) but instead gives [NiH(AlCp*) (AlCp*Ph)]
2
] with four equivalents of
2
3
4
3
39]
[
the dimeric structure, leaving the terminal Pt bound GaCp* (11) in almost quantitative yields (Scheme 9).
unchanged. With regards to the dependence on the group
1
3 metal, it was found that the fluxional process in
2
[
Pd (AlCp*) (µ -AlCp*) ] (8d) is slower than in
2 2 3
2
[
Pd (GaCp*) (µ -GaCp*) ] (8c). These results are in very
2
2
3
good agreement with the calculated bond energies in
[
M{E(CH )} ] (M ϭ Ni, Pd, Pt; E ϭ B Ϫ Tl) by Frenk-
3
4
[
16]
ing. For the transition metal the order Ni ഠ Pt Ͼ Pd can
be observed, while for the group 13 elements the trend is B
Ͼ Al Ͼ Ga ഠ In Ͼ Tl.
1
2
The H NMR spectrum of [Pd (GaCp*) (µ -GaCp*) ]
3
4
4
(
10b) at room temperature exhibits two resonances with an
2
integral ratio of 1:3, whereas [Pd (InCp*) (µ -InCp*) ]
3
4
4
(10a) shows this signal splitting only at Ϫ80 °C. The reason
for the unusual ratio of 1:3 is still uncertain and is not in
agreement with the solid-state structure. A dissociation of
the trinuclear component into two [Pd(ECp*) ] fragments
3
and one [Pd(ECp*) ] (E ϭ Ga, In) unit cannot be excluded.
2
However, the addition of an excess of InCp* does not yield
the saturated mononuclear species [Pd(InCp*) ]. The reac-
4
tivity of these cluster complexes with phosphanes (PPh or
3
dppe) suggests that the nuclearity is sustained in solution
Scheme 9. Synthesis of 7c, 11 and 12
(vide supra). Thus a triangular structure of 10a and 10b
similar to the triangular phosphane clusters 10c and 10d in
solution has been suggested. The structure bears two axial
and six equatorial ligands. The equatorial ligands (terminal
as well as bridging) are presumed to be fluxional in solu-
tion, giving a coalesced signal corresponding to six ligands.
As shown by NMR spectroscopy, the C H in 11 is not
6
6
exchanged by C D , showing the kinetic inertness of com-
6
6
plex 11. It is assumed that the activation of benzene, which
does not occur in the case of [Ni(CO) ], [Ni(PR ) ] or other
n
3 n
1
0
d
Ni complexes, takes place on a reactive, low-coordinate
fragment [Ni(AlCp*) ] (n Ͻ 4) giving an intermediate of
n
the type [(AlCp*) Ni(H)(C H )]. The reactive fragment
n
6
5
3
.3 Bond Activation Reactions by M(ECp*)n
[Ni(AlCp*) ] is not formed from [Ni(AlCp*) ] (7c) by a dis-
n
4
sociative process but, rather, is an intermediate in the course
The donor acceptor bond in [L MǟECp*] has been cal- of the formation of kinetically inert 7c and is apparently
n
culated to be rather polar in the case of E ϭ Al and Ga. trapped in the presence of suitable reaction partners prior
The electron density of the transition metal centre on to the formation of 11. The subsequent migration of the
coordination should therefore increase consider- phenyl group to an AlCp* ligand, accompanied by oxi-
[
10,16,17,24,31,41]
ably.
This is an intrinsically interesting feature dation of the aluminium and formation of a strong AlϪC
especially from the point of view of classical bond acti- bond, as well as the coordination of a fourth equivalent of
vation reactions being relevant in organometallic chemistry AlCp* contribute to the driving force of the whole acti-
(
H , CϪH, SiϪH, BϪH, etc.). Whereas CO, phosphanes or vation reaction.
2
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
Similarly, in the presence of HSiEt (Scheme 9), the for- position between the Fe and Al centres. It is noteworthy
3
0
mation of the hydrosilyl complex [Ni(AlCp*) (H)(SiEt )] that the Fe source is inert in the presence of Cp*Ga or
3
3
(12) can be observed. In contrast to 11, the hydride may be Cp*In, accentuating again the role of Cp*Al as a suitable
found in a terminal and not in a bridging position, showing ligand for bond activation reactions.
III
the electron deficient nature of the Al centre in 11. Com-
However, bond activations are not only restricted to
pound 12 selectively reacts with C H giving 11 via cleavage Cp*Al, as shown by the reaction of [Cp*Rh(CH ) L] (L ϭ
6
6
3 2
of HSiEt , indicating that [Ni(AlCp*) ] is the probable rel- dimethyl sulfoxide, pyridine) with GaCp* (Scheme 10). In
3
3
evant intermediate for the CϪH activation of benzene.
the first step of this reaction sequence, the ligand L is sub-
stituted by GaCp* giving [Cp*Rh(GaCp*)(CH ) ] (14).
3
2
This is isostructural with the starting complex. Complex 14
is thermally unstable and slowly reacts further in solution
at room temperature to give the zwitterionic rhodium spec-
5
[43]
ies [Cp*Rh{(η -C Me )Ga(CH ) }] (15).
5
4
3 3
3
Figure 5. [NiH(AlCp*) (AlCp*Ph)] (11)
Another example of CϪH bond activation was found in
6
4
the reaction of [(η -C H CH )Fe(η -C H )] with AlCp*
6
5
3
4
8
giving a product with the empirical formula [Fe(AlCp*) ]
5
Scheme 10. Reaction of [Cp*Rh(CH
with GaCp*
3
)
2
L] (L ϭ dmso, pyridine)
[
42]
(13). The solid-state structure of 13 indeed exhibits a cen-
tral iron atom in a distorted trigonal bipyramidal coordi-
nation environment. However, two CϪH bonds of two Cp*
ring methyl groups are attached to adjacent AlCp* ligands,
giving an unusual Cp*AlϪCH (C Me )AlϪCH (C Me )Al
The mechanism of this reaction sequence is still uncer-
tain. However, the driving force presumably includes the
migration of the methyl groups to ECp* (E ϭ Al, Ga) ac-
companied by oxidation of the group 13 metal centre and
the formation of a strong EϪC bond. This is a rather gen-
eral point in the reactions of MϪC bonds with ECp* and
is very similar to the classical alkyl migration to transition
metal bonded electrophilic π-ligands such as CO and olef-
ins.
2
5
4
2
5
4
chelating system (Figure 6). The hydrides occupy a bridging
4
. Insertion Reactions of ECp* into
Metal؊Halide and Metal؊Metal Bonds
I
Insertion reactions of E X (X ϭ halide) have been known
for a long time and date back to the studies of Graham,
Chatt and Hsieh more than 30 years ago. In particular, the
insertion of InX (X ϭ Cl, Br) has been the focus of much
research. In the 1960s, Graham et al. reported insertions of
InBr into the metalϪmetal bonds of [Co (CO) ] and
2
8
2
[
Mn (CO) ] which they assumed to give [(µ -Br) -
2 10 2
[44Ϫ47]
In{Co(CO) } ] and [BrInMn(CO) ] , respectively.
4
2 2
5 2
Later, Chatt et al. studied the insertion of InCl into the
Figure 6. [Fe(AlCp*)
5
] (13)
PtϪCl bond of [trans-Pt(Cl)(SiPh ){PMe (Ph)} ] and
3
2
2
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Transition Metal Chemistry of Low Valent Group 13 Organyls
MICROREVIEW
claimed that the product was [trans-Pt(InCl )(SiPh )- and their ability to stabilise intermediates stimulated a re-
2
3
[
48]
(
PMe Ph) ].
A. T. T. Hsieh reported a number products examination of these classic insertion reactions. Jutzi re-
2
2
from the insertion of InX into metal-metal and metal-halide ported on the insertion of GaCp* into the FeϪCl bonds of
bonds of carbonyl and carbonyl free complexes of tungsten, FeCl2 giving [Cp*Fe(GaCp*) (GaCl ·thf)] (16) and re-
2
2
I
molybdenum, manganese, iron, mercury and the metals of vealing the potential of E R as reducing and Cp*-transfer
[
49Ϫ52]
the cobalt triad.
Hsieh also investigated the behaviour reagents together with the capacity to behave as stabilising
[
58]
of InCl towards Ph PAuCl, suggesting the formation of ‘‘spectators’’ (Scheme 11).
3
[
53]
Ph PAuInCl .
Due to the insolubility of the products,
3
2
however, the structural proposals were solely based on in-
frared spectroscopy, elemental analyses and comparisons of
the melting points.
The actual complexity of these reactions was first shown
[
54]
by H. Schmidbaur
who isolated gold/indium clusters
from a reaction analogous to that of Hsieh.[ Treatment
of (PPh )AuCl with InCl in THF yielded the In dimer
53]
II
3
[
In Cl (THF) ], whereas addition of a chelating ligand such
2 4 4
as dppe led to the cluster [(dppe) Au In Cl (THF) ]. A
2
3
3
6
6
Scheme 11. Reaction of FeCl with GaCp*
2
slight change in the stoichiometry gave the ionic cluster
ϩ
Ϫ [55]
[
(dppe) Au] [(dppe) Au In Br (THF)] . The reaction of
2 2 3 3 7
InBr with [CpNi(Br)(PR )] (R ϭ Me, Ph) typically yields
3
ϩ
Ϫ
[56,57]
4.2 Rhodium- and Ruthenium؊ECp* Coordination
Compounds and Clusters
ionic [CpNi(PR ) ] [InBr ] ‘‘ate’’ complexes,
while
3
2
4
only special conditions such as the presence of strong
Lewis-bases (e.g. Ph PϭO, quinuclidine) suppress this reac-
tion so that NiϪIn bonded compounds can be isolated.
In general, depending on stoichiometry as well as re-
action conditions, a variety of insertion products may
3
These examples clearly show that insertion reactions of be formed from the reactions of ECp* (E ϭ Ga, In)
I
6
[59,60]
E X are extremely sensitive to the reaction conditions and and the isolobal d complexes [{Cp*RhCl } ]
or
2
2
[
61]
that many possible reaction pathways have to be con- [{(p-cymene)RuCl } ]
(Scheme 12 and 13). The reaction
2
2
I
I
sidered. The access to E R species with enhanced solubility of [{RhCp*Cl } ] with six equivalents of E R in toluene
2
2
Scheme 12. Reactions of ECp* with [{Cp*RhCl
2 2
} ]
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
2 2
Scheme 13. Reactions of GaCp* with [{(p-cymene)RuCl } ]
at room temperature leads to simple, monomeric insertion some of the reaction intermediates, facilitating the Cp*ϪCl
products [RhCp*(ECp*) Cl ] (17a: E ϭ Ga, 17b: E ϭ In). exchange between two Ga centres.
3
2
The analogous ruthenium complex [(p-cymene)Ru-
Changing the stoichiometric ratio of the reactants
(
GaCp*) Cl ] (18) can be obtained from the reaction of or the reaction conditions leads to new structural types
3
2
GaCp* with [{(p-cymene)RuCl } ] only by prior treatment for the insertion products. The reaction of [{RhCp*Cl } ]
2
2
2 2
II
of the dimeric starting complex with pyridine. The forma- with only one equivalent of ECp* gives the Rh
tion of the intermediate monomer [(p-cymene)RuCl (pyrid- dimer [{RhCp*Cl}2] (22) which is also accessible
2
[
62,63]
ine)] is followed by insertion of GaCp* into the RuϪCl from the reaction of the precursor with Na/Hg
bonds and substitution of pyridine by GaCp*. If dmso is elemental gallium.
or
[
64]
I
I
The analogous Ru ϪRu complex
used instead of pyridine, the substitution can be suppressed [{(p-cymene)RuCl} ] (23), which has not been reported
2
yielding [(p-cymene)Ru(GaCp*) (Cl) (DMSO)] (19). How- in literature before, can be obtained by treatment of
2
2
ever, in contrast to the isolobal Rh complex, only GaCp* [{(p-cymene)RuCl } ] with one equivalent of GaCp* in
2
2
gave defined reaction products, whereas InCp* yielded in- toluene. Several attempts to use classic reducing agents (Li,
tractable product mixtures under all conditions tested. K, C K, sodium naphthalide) for this reduction failed, re-
The compound [Cp*Rh(GaCp*) Cl ] (17a) reacts readily sulting only in the uncontrolled formation of Ru metal.
8
3
2
in the presence of Lewis acids. When 17a is treated with
Further reaction of 23 with four equivalents of GaCp*
one equivalent of GaCl in toluene, the Lewis acid-base ad- gives the dimeric cage compound 24 in very good yields.
3
duct [Cp*Rh(GaCp*) (GaCl )] (20) is immediately formed. The cube-like cage core consists of two Ru(p-cymene), four
2
3
3
Complex 20 is also formed as a side product in the thermal GaCp* ligands and two µ -bridging chlorine atoms. Com-
decomposition of complex 17a, showing the lability of the plex 24 can also be prepared by direct treatment of GaCp*
Cp*ϪGa bond. In the case of the Ru(p-cymene) fragment, with [{(p-cymene)RuCl } ], albeit in very low yields.
2
2
the corresponding complex [(p-cymene)Ru(GaCp*)2-
Several products can be obtained from the reaction of
(
GaCl )] (21) can be isolated in the stoichiometrically quan- [{Cp*RhCl} ] with ECp*. However, the molecular struc-
3
2
titative yield of 67% (‘‘quantitative’’ is based on the amount tures of these products are not clear yet. The homoleptic
of chlorine in the starting material) if [{(p-cymene)RuCl } ] cluster complex [(RhCp*) (GaCp*) ] (25) could be isolated
2
2
2
3
is treated directly with GaCp* in the absence of pyridine. as a thermal decomposition product of one of these inser-
Clearly the dimeric structure starting material is retained in tion products.
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Transition Metal Chemistry of Low Valent Group 13 Organyls
The reaction of [{Cp*RhCl } ] with only three equiva-
MICROREVIEW
2
2
lents of InCp* (instead of six as in the synthesis of 17b)
2
leads to the salt [Cp* Rh][Cp*Rh(InCp*){In Cl (µ -Cp*)}]
2
2
4
(26). The molecular structure of the anionic part exhibits a
distorted tetrahedral RhIn core. The striking feature of the
3
anion is the Cp* ring bridging two indium centres. The C5
ring takes an almost parallel arrangement to the ClϪInϪCl
planes with deviations smaller than 10°. Complex 26 is the
first example of an intramolecular bridging Cp* ring. The
only structural analogies are the solid-state structures of
Et GaCp and EtGaCp , consisting of linear polymers with
2
2
[65]
intermolecular bridging C H ligands.
5
5
3 2
Figure 8. [(p-cymene)Ru(GaCp*) Cl ] (18)
Cp* groups in a ratio of 2:2:1. Also, complex 26 does not
show any decoalescence even at Ϫ80 °C.
4.2.2 Solid State Structures
The molecular structures of [RhCp*(ECp*) Cl ] (17a,
3
2
1
7b) and [(p-cymene)Ru(GaCp*) Cl ] (18) exhibit cage-like
3
2
motifs with EϪClϪE bridges. Generally, only one of the
chlorine atoms is found in a bridging position, while the
second chlorine is terminally coordinated. The strong dif-
ference in hapticity of the Cp* rings is a common feature
of these structures. The Cp* ring on the ECl unit is σ-
2
3 2
Figure 7. [Cp*Rh(ECp*) Cl ] (17a: E ϭ Ga, 17b: E ϭ In)
bonded, while the Cp* group in the ECp* ligand showing
5
no chloride interactions is clearly η -bonded. The bridging
chloride atom exhibits some ground state cis effect on the
coordination of the Cp* on the ECl unit which shows a
3
2
4
.2.1 Solution Structures and NMR Spectroscopy
‘‘nonclassical’’ hapticity best described as η or η . The free
p-electron pairs at the Cl atoms effectively compete with the
π-density at the Cp* for the empty p-orbitals on the group
The monomeric insertion products 17a and 17b as well
as 18 can be regarded as acid-base adducts between Cp*
ECl2 and the basic d transition metal centres in [Cp*
1
3 metal centre. Therefore, the fluxional process in solution
is reflected in the solid-state structures which can be re-
8
Rh(ECp*)2] and [(p-cymene)Ru(ECp*)2], respectively.
However, the NMR spectroscopic data are not consistent
with simple acid-base adducts. Instead, in all these com-
plexes the three ECp* ligands are equivalent on the NMR
time scale down to Ϫ80 °C. Clearly a fast exchange of the
chlorides between all group 13 atoms takes place, resulting
in three coalesced ECp* ligands. A similar fluxional process
was observed in the acid-base adducts of GaCp* and
GaCp*X (X ϭ Cl, Br). In this case, coalescence of the
2
[
66]
Cp* rings at Ϫ80 °C in [D ]toluene was observed.
calculations on the systems R EϪER and their isomers
DFT
8
3
R EϪER revealed that the difference in energy between
2
2
these two forms is strongly dependent on both the groups
R as well as the group 13 metal E. The adducts Cl BϪGaH
3
and Cl BϪInH for example, exhibit bridging chlorines in
3
the energetic minimum.[
67]
Additionally, in solution, a fast Cp* exchange of the
InCp* units in the anionic part of complex 26 can be as-
sumed. The NMR spectra show three signal sets for the Figure 9. [Cp*
2
2
Rh][Cp*Rh(InCp*){In
2
Cl
4
(µ -Cp*}] (26)
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
garded as trapped ‘‘intermediates’’ of a chlorine exchange removed by treatment with NaBPh , giving the cationic
4
ϩ
Ϫ
between two ECp* centres.
complex [Cp*Ru(GaCp*) ] [BPh ] (27c). In contrast, 17a,
3 4
In contrast, the GaCl3 complexes [Cp*Rh(GaCp*)2- 17b or 18 do not show any reaction with NaBPh . Notably,
4
(
GaCl )] (20) and [(p-cymene)Ru(GaCp*) (GaCl )] (21) are the cation 27c can also be obtained by treatment of
3
2
3
Ϫ
simple acid-base adducts without GaϪClϪGaCp* inter- [{RuCp*Cl } ] with GaCp*, thus having [Cp*GaCl ] as
actions. Consequently, all Cp* units in both complexes are the counterion instead of [BPh ] .
η -bonded to the Gallium centres.
2
2
3
Ϫ
4
5
The reaction of GaCp* with [(Ph P) RuCl ] leads to a
3 3 2
complete substitution of the PPh ligands and insertion of
3
GaCp* into the ruthenium-halide bonds (Scheme 15).
The product is the octahedral homoleptic complex
[
Ru(GaCp*) Cl ] (28) which exhibits fluxional chloride li-
6 2
gands and coalescence of all GaCp* moieties.
2 2
Figure 10. [Ru(p-cymene)(GaCp*) Cl] (24)
4.3 Reaction of ECp* with other Ruthenium Complexes
In contrast to [{(p-cymene)RuCl } ], [{Cp*RuCl} ] reacts
2
2
4
both with GaCp* and InCp* to give the monomeric ‘‘pi-
ano-stool’’ complexes [Cp*Ru(ECp*) Cl] [E ϭ Ga (27a), In
3
(27b)] (Scheme 14).
In solution, all three ECp* ligands of 27a and 27b are
3 3 2
Scheme 15. Reaction of GaCp* with [(Ph P) RuCl ]
equivalent on the NMR time scale down to Ϫ80 °C, due to
a fast exchange of the chlorides between the three group 13
metals. In the solid state the chloride is either terminally
coordinated to gallium (27a) or bridging two indium atoms 5. Coordination Chemistry of Sterically Bulky
(
27b). As in complexes 17a, 17b and 18, the influence of Ligands
the chlorine coordination on the hapticity of adjacent Cp*
ligands can be observed.
While the coordination chemistry of ECp* is a well ex-
The reaction of ECp* with [Cp*RuCl] can be described plored field, sterically encumbered low-valent group 13 li-
4
I
as redox-neutral. The neutral p-cymene ligand is a stronger gands such as E Trip {E ϭ Al, Ga, In; Trip ϭ Bis[2,5-
π-acceptor in comparison with the anionic Cp* ligand and (2,4,6-iPr C H )phenyl]},
thus has a more polarised bond to ruthenium. This situ- Tp ϭ Tris(pyrazoyl)hydroborato]
[
14,15,68Ϫ73]
I
E Tp [E ϭ Al, Ga, In;
3
6
3
[
74Ϫ82]
I
or E (DDP) (E ϭ
ation leads to an increased electron density on all metal Al, Ga; DDP ϭ 2-Diisopropylphenylamino-4-diisopro-
[
83Ϫ88]
centres. Indeed, the chloride of compound 27a can be easily pylphenylimino-2-pentene
have been little used as li-
I
Scheme 14. Synthesis of [Cp*Ru(E Cp*)
3
Cl]
4172
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Transition Metal Chemistry of Low Valent Group 13 Organyls
MICROREVIEW
gands in transition metal chemistry. The iron carbonyl com-
I
plexes [(CO) FeϪE R] (E ϭ Ga; R ϭ DDP, Trip, Tp; E ϭ
4
[
14,89Ϫ91]
In,
R
ϭ
Tp)
and the tungsten complex
I
[91]
[
(CO) WϪIn Tp]
are the only such complexes reported
5
so far. The difference in the steric demand as well as in the
electronic properties of these ligands compared with ECp*
points to a different coordination chemistry. As an example
for this class of bulky ligands, the coordination chemistry
[
92]
of Ga(DDP) has recently been studied in more detail.
The reactions of ML (M ϭ Ni, Pd, Pt; L ϭ olefin) with
n
GaCp* lead to a full substitution of the olefinic ligands and
give homoleptic monomeric, di- or trimeric clusters in all
cases (vide supra). However, according to Scheme 16, the
reaction of [Pd (dvds) ] with Ga(DDP) does not lead to
2
3
complete substitution but, rather, gives the monosubsti-
tuted complex [{(DDP)Ga}Pd(dvds)] (29a). Similarly, the
reaction of [Pt(COD) ] with Ga(DDP) leads to the disubsti-
2
tuted complex [(1,3-COD)Pt{Ga(DDP)} ] (29b). Clearly,
complete substitution of the olefins is prevented by the high
2
Figure 11. [{(DDP)Ga}Pd(dvds)] (29a)
steric demand of Ga(DDP).
limits the kinetic access to halide transfer reactions. A reac-
tion mixture of [{Cp*RhCl } ] and Ga(DDP) shows oxi-
2
2
I
dised gallium species but the highly reactive Rh -products
are not stable under these conditions and nonspecific de-
composition is observed.
A representative example of the difference between
Ga(DDP) and GaCp* is their reactivity towards
[
(Ph P) RuCl ]. Whereas GaCp* causes complete substi-
3 3 2
tution of the Ph P ligands and the insertion of GaCp* into
3
the ruthenium-halide bonds with the conservation of the
II
oxidation state of Ru (vide supra), Ga(DDP) leads to a
II
0
III
selective reduction of Ru to Ru . A free Ga -species was
1
detected by H NMR spectroscopy which, in contrast to
Cp*GaCl , is not acidic and does not stabilise the Ru cen-
tre. Instead, the highly reactive Ru is stabilised by ortho-
0
2
0
metallation of one of the phenyl-rings by reoxidation of the
ruthenium centre (Scheme 17). Similar orthometallation re-
actions are known to take place in the reactions of
[
(PPh ) RuCl ] with other reducing agents such as potass-
3 3 2
[
93,94]
ium naphthalide.
Scheme 16. Reactions of Ga(DDP) with [Pd
Pt(COD)
2 3
(dvds) ] and
[
2
]
In contrast to [M(ECp*) ] complexes, which are kine-
4
tically inert, the electronically unsaturated complexes 29a
and 29b are substitutionally labile and form dimeric,
Ga(DDP) bridged complexes on reaction with CO.
Ga(DDP) shows similar reducing abilities to GaCp*.
However, the steric demand of the organic group as well as
the reduced Lewis acidity of the gallium centre somehow Scheme 17. Reaction of [(PPh
3 3 2
) RuCl ] with Ga(DDP)
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C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer
MICROREVIEW
In contrast, reactions of Ga(DDP) with transition metal compounds, an insertion of a Ga(DDP) into a goldϪhalide
halides in the oxidation state ϩI do not give Ga(DDP)Cl2 bond with full migration of the chloride is observed.
but, rather, lead to insertion of Ga(DDP) into the transition
metal halide bond.
I
The Rh -compound [(Ph P) RhCl] reacts with Ga(DDP)
3
3
6. Summary and Outlook
to give the monomeric complex [(PPh ) RhCl{Ga(DDP)}]
3
2
(
30) (Figure 12). However, the chloride can be found in a
Since their discovery, the low valent group 13 organyls
GaCp* and AlCp* have attracted much attention as ligands
towards transition metals. They are formally isolobal with
CO, phosphanes and NHCs but in contrast, their bonds to
transition metals have been calculated to be very polar, the
strongest contribution being the coulombic attraction be-
tween the metals. Early research concentrated on transition
metal carbonyl fragments in order to elucidate the general
properties of this new class of ligand. However, recent re-
search is focussing on more reactive transition metal frag-
ments, without competing strong π-acceptor ligands such as
CO. The strong polarity of the metalϪmetal bond is quite
interesting in the context of organometallic chemistry. In
particular, for bond activation reactions the high increase
bridging position between the gallium and the rhodium
centre. This is the first example of a halide-bridged tran-
sition-metal group 13 metal complex reported so far. Inter-
estingly, the Lewis acidic properties of coordinated
Ga(DDP) are not sufficiently strong to cause a complete
migration of the chlorine atom.
I
in electron density of transition metal centres by E R is po-
tentially useful or, qualitatively speaking, it should facilitate
oxidative addition reactions. In particular, the reactivity of
I
the unsaturated fragments [M(E R) ] is not understood and
n
will necessitate further research. In addition, the chemistry
I
R
of sterically demanding ligands E R with R ϭ DDP, Tp
or similar substituents holds much promise for the dis-
covery of novel and unusual reactions. Based on the above
examples we speculate that these exotic ‘‘metallic’’ ligands
I
E R may well be very useful in a classical sense as ancillary
ligands in organometallic chemistry. However, it should be
mentioned here that the title compounds [M (ECp*) ] may
3 2
Figure 12. [(PPh ) RhCl{Ga(DDP)}] (30)
a
b
also be regarded as the basic building blocks of Schnöckels
[
95]
elaborate cluster [Al (AlCp*) ]
showing that ECp*, in
Whereas the reaction of [(PPh )AuCl] with GaCp* leads
to unselective decomposition, the reaction with Ga(DDP)
38
12
3
general, seems to be a suitable stabilising ligand for large
I
metal clusters. The use of [M (E R) ] as building blocks for
yields the complexes [(Ph P)Au{Ga(DDP)}Cl] (31a) and
a
b
3
the synthesis of larger transition metal ECp* clusters as
well as molecular precursors for intermetallic M/E nano-
particles seems worthy of exploration.
[
{(DDP)Ga}Au{Ga(DDP)}Cl] (31b) (Scheme 18). In these
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Scheme 18. Reaction of [(PPh )AuCl] with Ga(DDP)
4174
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 4161Ϫ4176

Transition Metal Chemistry of Low Valent Group 13 Organyls
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