Download The coordination chemistry of Ga+ at Gallium-rich transition

4 Summary and Outlook
This thesis reports the electrochemical investigation, synthesis and characterization
of novel TM compounds containing low valent aluminum, gallium and substituent free
Ga
+
compounds.
Since the rst syntheses of AlCp*
62
63
and GaCp*
around 20 years
ago the chemistry of low valent group 13 elements with steric bulky groups R has seen
substantial progresses.
17, 20, 21, 127, 188
The chemistry of low valent group 13 elements with
steric less bulky groups however is much less developed. There are few reports about
GaMe as a ligand
43, 163
or substituent free Ga
+ 27, 28
+ 27
and In
containing compounds. In
addition, while experimental proof for the interesting redox properties was available,
73, 75
no systematic investigation of these properties has yet been carried out.
The focus of this thesis lies on the systematic investigation of the redox properties of TM
I
complexes containing E R (E = Al, Ga; R = Me, Cp*) ligands as well as the formation
+
of Ga containing compounds. This is investigated by the comparison of the reaction of
F
+
the gallium transfer reagent [Ga2 Cp*][BAr4 ] and the in-situ generation of Ga by one
F
electron oxidation with [FeCp2 ][BAr4 ].
4.1 Redox Chemistry of [Ni(ECp*)x(PPh3)y ]
Compounds (E = Al, Ga; x = 2, 4; y = 0, 2)
4.1.1 Summary
Paramagnetic Ni
I
complexes are used in a variety of catalytic processes.
115121, 189
The
ligands used for the stabilization of the paramagnetic nickel complexes are homoleptic
137
116, 134
or heteroleptic
phosphine complexes. Whereas all of the complexes, except
I
the three coordinated [Ni (PPh3 )3 ][BF4 ], used for the catalysis reactions are either unstable intermediates or neutral complexes. In general the eld of open shell 15 or 17 Ve
compounds is not a very intensively investigated eld of transition metal chemistry.
The compounds synthesized in this thesis are an interesting extension to the chemistry
of open shell compounds. The synthesis and characterization of the rst indenitely in
67
Chapter 4.
Summary and Outlook
solution or in solid state (under argon) stable cationic tetra-coordinated paramagnetic
I
homoleptic and heteroleptic Ni complexes with ECp* (E = Al, Ga) ligands could be
F
achieved. [Ni(AlCp*)4 ][BAr4 ] (1) the rst example of a homoleptic group 13 complex
of monovalent nickel could be synthesized in very low yields. The use of the heteroleptic
F
educt [Ni(AlCp*)2 (PPh3 )] for the oxidation with [FeCp2 ][BAr4 ] leads to the quantitaI
I
F
tive synthesis of the heteroleptic monovalent Ni compound [Ni (AlCp*)2 (PPh3 )][BAr4 ]
F
+
+
(2). The reaction of [Ni(GaCp*)2 (PPh3 )] with [C][BAr4 ] (C = FeCp2 or Ga2 Cp* ) interestingly did not yield the corresponding [NiGa(GaCp*)2 (PPh3 )] or a cluster. Instead
the
Ni
center
I
is
oxidized
forming
the
paramagnetic
compound
F
[Ni (GaCp*)2 (PPh3 )][BAr4 ] (3) in quantitative amounts. This reaction is further proof
+
that Ga can react as an oxidizing agent and secondly the variation of the ligands at
the metal center can switch the oxidation from the ligand to the metal center.
4.1.2 Outlook
The use of Paramagnetic Ni
indicate that the Ni
I
I
complexes as catalysts in dierent catalytic processes
complexes presented in this thesis should be investigated in relation
to their catalytical properties. The stability of the heteroleptic [Ni(AlCp*)2 (PPh3 )] in
comparison to the homoleptic [Ni(AlCp*)4 ] in the oxidized state indicates the stabilizing
eect of the phosphines. According to this reactions of the chemically inert [Ni(AlCp*)4 ]
with dierent phosphine ligands may be achieved by the reaction of a catalytic amount
F
of [FeCp2 ][BAr4 ] with [Ni(AlCp*)4 ] in the presence of an excess of the phosphine. AnI
F
other promising eld of activity may be the reactions of the [Ni (ECp*)2 (PPh3 )2 ][BAr4 ]
(E = Al, Ga) complexes with oxygen.
Both of these complexes react pyrophoric in
the solid state towards oxygen, but slow diusion of oxygen into a uorobenzene solution of the compounds 2 and 3 leads to clear colorless solutions without decomposition.
Also slow diusion of oxygen into peruoropolyalkylether containing crystals
of 2 and 3 leads to slow oxidation beginning from the edges of the crystals without
decomposition yielding colorless amorphous compounds.
I
F
This may indicate that the
[Ni (ECp*)2 (PPh3 )2 ][BAr4 ] (E = Al, Ga) complexes are usable for oxygen activation
and should be investigated.
68
Chapter 4.
Summary and Outlook
4.2 The Coordination Chemistry of Ga+ towards d10
Transition Metal Complexes
4.2.1 Summary
The coordination chemistry of substituent free Ga
been
widely
explored.
F
[PtGa(GaCp*)4 ][BAr4 ]
The
obtained
only
by
examples
the
+
towards d
are
reaction
F
of
m
10
-TM complexes has not
the
terminal
the
gallium
+
Ga
adduct
transfer
agent
F
[Ga2 Cp*][BAr4 ] with [Pt(GaCp*)4 ] and [(GaCp*)4 Pt-( 2 -Ga)-PtH(GaCp*)3 ][(BAr4 )2 ]
+
with a linear bridging Ga
unit synthesized by protolysis of the same educt with
F 27
[H(OEt2 )2 ][BAr4 ].
In the course of this thesis the one electron oxidation by
F
F
[FeCp2 ][BAr4 ] and the reaction of [Ga2 Cp*][BAr4 ] towards the homoleptic compounds
m
[M(GaCp*)4 ] (M = Ni, Pd, Pt) and [Pd3 ( 2 -GaCp*)4 (GaCp*)4 ] have been investi-
F
gated.
[NiGa(GaCp*)4 ][BAr4 ] (4) isformed in the reactions of [Ni(GaCp*)4 ] with
F
F
[C][BAr4 ] (C = Ga2 Cp*, FeCp2 ) as a congener to [PtGa(GaCp*)4 ][BAr4 ].
m
m
In con-
F
trast to this [Pd2 ( 2 -Ga)2 ( 2 -GaCp*)2 -(GaCp*)4 ][(BAr4 )2 ] (5) is obtained by the re-
F
actions of [C][BAr4 ] with [Pd(GaCp*)4 ] independently from stoichiometry. Compound
5 exhibits the longest Pd-GaR bond distance known so far (2.4704(11) Å). No con-
m
m
F
gener of [MGa(GaCp*)4 ][BAr4 ] (M = Ni, Pt) could be synthesized. [Pd3 ( 2 -Ga)2 ( 2 -
m
3 m2
F
GaCp*)( 3 -GaCp*)2 -(GaCp*)3 ][(BAr4 )2 ] (7) was synthesized by one electron oxidation
of [Pd (
-GaCp*)4 (GaCp*)4 ]. In comparison to the linear solid state structure of the
parent compound it possesses a trigonal bipyramidal geometry. The formation of both
Pd complexes 5 and 7 indicate that palladium compounds prefer the coordination of
two substituent free Ga
+
in the bent edge bridging bonding mode. In contrast to this
F
the reaction of [Pt(GaCp*)4 ] with [FeCp2 ][BAr4 ] does not lead to the gallium transfer
F
product [PtGa(GaCp*)4 ][BAr4 ]. Instead compound 6 forms the trigonal bipyramidal
m
m
m
F
Pt trimer[Pt3 2 -Ga( 2 -GaCp*)( 3 -GaCp*)2 (GaCp*)3 ][(BAr4 )2 ] independent from the
stoichiometry of the reaction. This is noteworthy as it is the only example so far of the
formation of dierent products between the reactions of in-situ generation of Ga
F
+
by
F
[FeCp2 ][BAr4 ] and the reaction of the gallium transfer reagent [Ga2 Cp*][BAr4 ].
4.2.2 Outlook
The formation of compounds 5 and 7 indicate the ability of TM-complexes to accom-
+
modate more than one Ga in one compound. The selection of a suitable two electron
oxidation agent or the use of a ferrocenium salts with higher charged anions should give
69
Chapter 4.
Summary and Outlook
access to compounds with more than two Ga
+
ligands. Another interesting aspect is the
reduction of the presented compounds which may lead to the formation of compounds
containing gallium in the oxidation state 0.
4.3 The Coordination Chemistry of Ga+ towards
Ruthenium Complexes
4.3.1 Summary
The
behaviour
of
+
Ga
towards
[Ru(H)2 (GaCp*)2 (PCy3 )2 ] is known.
28
the
ruthenium
With the addition of Ga
+
hydride
complex
a reductive elimination
F
of the hydrides occur and hydrogen is evolved aording [RuGa(GaCp*)2 (PCy3 )2 ][BAr4 ]
(10). Herein the synthesis of the congener [Ru(H)2 (GaCp*)2 (PPh3 )2 ] (8) and the be-
F
F
haviour towards [FeCp2 ][BAr4 ] and [Ga2 Cp*][BAr4 ] is reported.
Compound 8 is
synthesized by the substitution of two PPh3 ligands from [Ru(H)2 (PPh3 )4 ] by GaCp*.
F
Treatment of [Ru(H)2 (GaCp*)2 (PPh3 )2 ] with [C][BAr4 ] (C = Ga2 Cp*, FeCp2 ) leads to
the
addition
of
+
Ga
without
the
loss
of
the
hydrides
yielding
F
[RuGa(H)2 (GaCp*)2 (PCy3 )2 ][BAr4 ] (9). The second dierence between compound 9
and 10 besides the retaining of the hydrides is the great dierence in the bond length
+
of the terminal Ga (9: 2.684(1) Å; 10: 2.300(2) Å). This dierence is due to the higher
8
6
basicity of the d -Ru center in compound 10 compared to the d -Ru center in compound
9 and the corresponding better bonding to the strong
sv p
+
/ -acceptor properties of Ga .
The hydride positions in the crystal structure could not be rened from single crystal
X-Ray data and were determined by theoretical calculations. These calculations reveal
a low energy dierence between the possible isomers 9A (hydrides trans to GaCp), 9B
(hydrides trans to PMe3 ) and 9C (GaH + RuH) but model complex 9B can be disregarded due to it having the wrong structure. Model 9A and 9C can be distinguished by
the dierence in the bond length of the Ga
+
to the Ru center and the better tting of
the calculated IR-frequencies of 9A to the measured IR-spectrum. This indicates that
the model compound 9A may be seen as the most probable model for the positions of
+
the hydrides which are situated trans to the CaCp* ligands and surrounding the Ga .
The formation of [{Ru(GaCp*)3 (
h3
m
F
-(CH2 )2 C(CH2 ( -Ga)))}2 ][(BAr4 )2 ] (11) was ini-
tially achieved by Thomas Cadenbach by the reaction of [Ru(GaCp*)3 (TMM)] with
F 28
[Ga2 Cp*][BAr4 ].
In this work a second synthesis method involving the oxidation of
F
[Ru(GaCp*)3 (TMM)] by [FeCp2 ][BAr4 ] is presented.
70
Chapter 4.
Summary and Outlook
4.3.2 Outlook
The
striking
dierence
between
[Ru(H)2 (GaCp*)2 (PCy3 )2 ]
and
+
[Ru(H)2 (GaCp*)2 (PPh3 )2 ] after the addition of Ga , is the loss of the hydrides in the
rst case and the retention of the hydrides in the second case. Because convenient hydrogen storage and release is currently a hot topic in research, it may be an interesting
challenge to tune the properties of a ruthenium hydride complex by choosing the ap-
+
propriate phosphines. The addition of Ga to this complex may allow reversible release
of hydrogen at slightly elevated temperatures and the uptake of hydrogen at elevated
pressure.
71