Convenient Access to Gallium(I) Cations through Hydrogen Elimination from Cationic Gallium(III) Hydrides

Article abstract of DOI:10.1021/acs.inorgchem.9b02136

Although gallium hydrides X_nGaH_3-n (X = monoanionic substituent) are usually stable compounds, cationic arene-solvated species [H₂Ga(arene)₂]⁺ spontaneously eliminate dihydrogen at room temperature to afford the arene-solvated gallium(I) compounds [Ga(PhF)₂][CHB₁₁Cl₁₁] (1) and [Ga(Ph₃CH)][B(C₆F₅)₄] (3). A key requirement appears to be the presence of a weakly coordinating anion. Use of the more basic triflimide anion, [NTf₂]^-, reverses the stability, i.e., the gallium(III) hydride H₂GaNTf₂ (4) is more stable than the gallium(I) compound GaNTf₂ (5). The experimental results are supported by DFT calculations. Compounds 1 and 3 can be used as catalysts for the oligomerization of 2,4,4-trimethyl-1-pentene and the hydrosilylation of benzophenone and 1-hexene.

Full text of DOI:10.1021/acs.inorgchem.9b02136

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Convenient Access to Gallium(I) Cations through Hydrogen
Elimination from Cationic Gallium(III) Hydrides
,
†
†
‡
Rudolf J. Wehmschulte,* Roberto Peverati, and Douglas R. Powell
†Chemistry Program, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901-6975, United
States
‡
Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251,
United States
*
S Supporting Information
ABSTRACT: Although gallium hydrides X GaH
(X =
3−n
n
monoanionic substituent) are usually stable compounds, cationic
+
arene-solvated species [H Ga(arene) ] spontaneously eliminate
2
2
dihydrogen at room temperature to afford the arene-solvated
gallium(I) compounds [Ga(PhF) ][CHB Cl ] (1) and [Ga-
2
11 11
(
Ph CH)][B(C F ) ] (3). A key requirement appears to be the
3 6 5 4
presence of a weakly coordinating anion. Use of the more basic
triflimide anion, [NTf ] , reverses the stability, i.e., the
−
2
gallium(III) hydride H GaNTf (4) is more stable than the gallium(I) compound GaNTf (5). The experimental results are
2
2
2
supported by DFT calculations. Compounds 1 and 3 can be used as catalysts for the oligomerization of 2,4,4-trimethyl-1-
pentene and the hydrosilylation of benzophenone and 1-hexene.
INTRODUCTION
involving group 13 compounds include the elimination of H2
■
upon thermolysis of (HGaCl ) and (H GaCl) to afford
2
2
2
2
Although the chemistry of gallium is dominated by the +3
1
6
17
1
Ga[GaCl
elimination of Cp*H (Cp* = [Me C ] ) from Cp* AlH as
5 5 2
18
4
]
and Ga[GaCl H], respectively, and the
3
oxidation state, the recently developed access to simple
−
2
gallium(I) cations is beginning to lead to their application as
an alternative approach to (Cp*Al)4.
catalysts in synthetic chemistry. For example, [Ga(arene)2 or 3]-
3
,4
The experimental data are supported by quantum-chemical
calculations, and results of an exploratory survey of the
catalytic activities of the new compounds in the hydrosilylation
[
Al{OC(CF ) } ] catalyzes the polymerization of isobutene,
3 3 4
the hydroarylation of alkynes, and the transfer hydrogenation
of a trisubstituted olefin, and the in situ generated, crown-
5
of 1-hexene, benzophenone, and CO and the oligomerization
2
ether stabilized gallium(I) triflate catalyzes C−C bond
of 2,4,4-trimethyl-1-pentene are also included.
formation involving allyl and allenyl boronic esters and acetals,
6
ketals, and aminals. Furthermore, a gallium(I) site in a Ga-
RESULTS AND DISCUSSION
■
[
exchanged zeolite is believed to be an active site for the
catalytic dehydrogenation of light alkanes, but this finding has
been challenged by another paper.
The syntheses of the molecular gallium(I) compounds
mentioned above involve the oxidation of gallium metal with a
mild oxidant, usually a silver salt such as Ag[Al{OC(CF ) } ]
or AgOTf (OTf = triflate, Tf = CF SO ), but an alternative
approach, the use of a benzene radical cation, [Me C ][Al-
7
The first crystals of the gallium(I) species [Ga(C H F) ]-
6
5
2
8
CHB Cl ], 1, were obtained in an attempt to synthesize
11 11
[
Cl Ga][CHB Cl ] according to eq 1.
2
11 11
GaCl + [Et Si][CHB Cl ] → [Cl Ga][CHB Cl ] + Et SiCl
2
3
3
11 11
2
11 11
3
3
3 4
(
1)
6
3
2
The crystal structure (Figure 1) showed some residual electron
density close to the gallium center in the approximate region
where hydride substituents would be expected. However, the
6
6
9
{
“
OC(CF ) } ] as an oxidizer, was recently reported. Green’s
3 3 4
GaI”, which seems to be a mixture of several subvalent gallium
1
1
0−12
failure to detect a gallium hydride signal in the H NMR
species,
is prepared by the oxidation of gallium metal with
−
1
spectrum or an absorption around 2000 cm (νGa−H) in the
iodine, and the comproportionation reaction of gallium metal
and gallium(III) chloride to afford Ga[GaCl ] may also be
19
71
IR spectrum, followed by the detection of a signal in the Ga
4
NMR spectrum at −703 ppm, which is in the typical region for
viewed as an oxidation of gallium metal with gallium(III)
2
1
3
gallium(I) compounds, convinced us that we had indeed
chloride. Here, we present an alternative route to gallium(I)
−
isolated a gallium(I) species. As this finding was supported by
preliminary DFT calculations, which indicated that the
compounds [Ga(arene) ][A] (A = [CHB Cl ] , [B-
n
11 11
−
(
C F ) ] ), namely, the reductive elimination of dihydrogen
6 5 4
+
from [H Ga] species at room temperature. Contrary to
2
1
4
transition-metal chemistry, reductive elimination is still
rather uncommon in main group chemistry. Examples
Received: July 17, 2019
1
5
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02136
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Δ,C H Br
6
5
Ga + Ag[CHB Cl ] ⎯⎯⎯⎯⎯⎯⎯⎯ ⎯→ [Ga(C H Br)][CHB Cl ]
1
1
11
6
5
11 11
−
Ag
2
(
3)
Given the relative ease with which compound 1 can be
prepared through the reductive elimination route, we were
interested if this method could be extended to more readily
−
available counterions such as [B(C F ) ] and [N-
6
5 4
−
(
[
SO CF ) ] . For both anions, suitable precursors such as
Ph C][B(C F ) ] and HN(SO CF ) are commercially
2 3 2
3
6
5
4
2
3 2
available. The Slattery−Krossing method may not be ideal to
2
2
use with Ag[B(C F ) ] due to the latter’s thermal lability and
6
5 4
23
its multistep synthesis. On the other hand, the sensitivity of
the [B(C F ) ] anion toward strong Lewis acids could have
−
Figure 1. Structure of 1 (50% ellipsoids). Selected distances (Å) and
angles (deg): Ga(1)···centroid = 2.721(2), 2.948(1); Ga(1)···Cl(10)
6
5 4
+
been detrimental in the presence of the Lewis acid [H Ga] .
2
=
3.400(2); Ga(1)···Cl(11) = 3.397(2); Ga(1)···Cl(12) = 3.353(3),
Fortunately, our new method, the addition of [Ph C][B-
3
and centroid−Ga(1)−centroid = 119.8°.
(
C F ) ] to freshly prepared (H GaCl) in the presence of
6 5 4 2 2
excess Et SiH (eq 4), afforded the target compound,
3
[
Ga(Ph CH)][B(C F ) ], 3, after crystallization, albeit in
3 6 5 4
+
low yields (19%).
gallium(I) cation [Ga(C H F) ] was more stable than the
6
5
2
+
gallium(III) cation [H Ga(C H F) ] , an optimization of the
synthetic procedure was developed, as well as an extension to
additional species with the weakly coordinating anions
2
6
5
2
1
/2(H GaCl) + [Ph C][B(C F ) ]
2
2
3
6 5 4
+
Et SiH
3
⎯
⎯⎯⎯⎯⎯⎯ →⎯ [Ga(Ph CH)][B(C F ) ] + H
−
−
3
6 5 4
2
(
WCAs) [B(C F ) ] and [NTf ] .
−Et SiCl
6
5 4
2
3
(4)
3
Hydrosilanes have long been known to convert chlorogal-
lanes into hydrogallanes at or below room temperature, as
The mother liquor contained numerous compounds derived
20
−
according to eq 2.
from the decomposition of the [B(C F ) ] anion as shown by
6
5 4
1
9
F NMR spectroscopy (see Figure S15). As the silylium salt
GaCl + Me SiH → 1/2(HGaCl ) + Me SiCl
(2)
3
3
2 2
3
[Et Si][B(C F ) ] was not isolated in this synthesis, the
3
6 5 4
It is likely that residual Et SiH from the synthesis of the
triphenylmethane side product was found to be coordinated to
the gallium(I) cation due to its higher basicity compared to
fluorobenzene.
3
silylium ion reagent [Et Si][CHB Cl ] caused the formation
3
11 11
+
of an intermediate [H Ga] ion, which then eliminated H to
2
2
afford compound 1. In order to test this hypothesis, the
reaction was performed as above (see eq 1) followed by the
Several attempts were made to prepare [Ga(arene) ][N-
n
(SO CF ) ], and the best procedure appears to be a two-step
2
3 2
addition of excess Et SiH, which gave the target compound in
approach (eqs 5 and 6). First, the reaction of (H GaCl) with
HN(SO CF ) in hexanes in the presence of excess Et SiH
2 3 2 3
3
2 2
4
8% yield in the form of colorless crystals. A variation of this
procedure, the addition of the silylium salt to in-situ generated
resulted in the formation of a colorless, hexane insoluble solid,
1
9
(
H GaCl)
2
in the presence of excess Et SiH, avoids the
which is tentatively identified as H GaN(SO CF ) , 4, based
2
3
2
2
3 2
1
presence of an intermediate and likely highly reactive
on NMR and IR data. The H NMR spectrum shows a strong
broad signal at 4.68 ppm and the IR spectrum a peak at 2080
[
4
Cl Ga][CHB Cl ], leading to an overall cleaner product in
2% yield. It is not clear at this point if the silylium ion
2 11 11
−
1
cm , both indicative of the Ga−H unit. For example, the
abstracts a chloride or hydride ion from (H GaCl) (Scheme
compound H Ga(C H -2-CH NMe ), which also features a
2
2
2
6
4
2
2
1
1
). In the latter case, the excess Et SiH would convert the
bidentate monoanionic substituent, displays the Ga−H H
NMR signal at 5.59 ppm and the Ga−H stretch at 1856
cm . The significantly higher frequency of the Ga−H
stretch for 4 may be explained by the higher positive charge on
the gallium center, and this finding is also supported by DFT
calculations (see below and in the Supporting Information).
The corresponding data for (H GaCl) are 5.46 ppm and 2021
3
+
+
intermediate [HGaCl] into [H Ga] , which then undergoes
2
−1
24
reductive elimination to form the arene-coordinated Ga(I)
cation 1.
For comparison, we have also obtained [Ga(C H Br)]-
6
5
2
[
CHB Cl ], 2, by the Slattery−Krossing method (eq 3) in
1
1
11
analogy to the synthesis of the indium compound [In-
2
2
21
−1 19
(
C H Br) ][CHB Cl ].
cm . Solutions of compound 4 in C D slowly eliminate H
6 6 2
6
5
1.5
11 11
Scheme 1. Possible Pathways for the Formation of 1
B
DOI: 10.1021/acs.inorgchem.9b02136
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
at room temperature, and after several hours the 71Ga NMR
reflecting the higher basicity of the carborate anion. Closer
Ga···X contacts lead to longer Ga···C distances and narrower
centroid−Ga−centroid angles. On the other hand, compound
1 is rather robust, whereas compound 3 can experience anion
decomposition under harsh conditions (vide infra); [Ga-
(benzene) ][GaCl ] is known to suffer from various
28
spectrum shows a weak peak at −645 ppm (w1 = 1100 Hz),
/2
indicating the presence of Ga(I) in an asymmetric environ-
ment. A microcrystalline solid of a compound tentatively
identified as GaN(SO CF ) , 5, was obtained after leaving a
2
3 2
hexane/C D solution of 4 standing at room temperature for 4
6
6
2
4
1
3
weeks. This reaction can be accelerated by heating at 60−90
decomposition modes dependent on conditions. In com-
pound 3, each gallium center is coordinated to phenyl groups
from two different triphenylmethane molecules, leading to a
coordination polymer (Figure S47) similar to the silver
compound [Ag(Ph CH)][MeCB F ]. In addition, there
are two additional phenyl groups in contact with the gallium
°
C, but it is accompanied by the formation of a fine gray
precipitate and an eventual weakening of the Ga(I) Ga NMR
7
1
signal, likely due to decomposition of 5.
2
9
+
Et SiH/+HN(SO CF )
3 2
3
11 11
3
2
1
/2(H GaCl) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ →⎯ H Ga[N(SO CF ) ]
2
2
2
2
3 2
−
H /−Et SiCl
2
2
3
4
center in an η fashion with rather long average distances of
.461 and 4.128 Å, resulting in an overall bowl-shaped
(5)
3
Δ
environment for the gallium center (Figure S48).
H Ga[N(SO CF ) ] ⎯⎯ ⎯→ Ga[N(SO CF ) ]
2
2
3 2
2
3 2
Compounds 1−3 are readily soluble in moderately polar
aromatic solvents such as fluoro- and bromobenzene. As long
as the coordination environment of gallium is symmetrical,
−
H
2
4
5
(6)
Interestingly, the putative intermediates [H Ga(arene) ]-
2
n
7
1
[
WCA] spontaneously decompose to the gallium(I) com-
Ga NMR spectroscopy is an excellent analytical tool with a
−
−
pounds 1 and 3 for [WCA] = [CHB Cl ] and
[
1
1
11
rather high sensitivity; solutions of 1−3 with concentrations of
−
B(C F ) ] ; whereas, the more basic triflimide counterion
6 5 4
ca. 10−20 mM give rise to strong signals in the Ga(I)
2
,25
stabilizes the gallium(III) hydride 4, and conversion to the
gallium(I) species 5 requires energy. This is also supported by
DFT calculations (vide infra).
The structures of 1 (Figure 1) and 3 (Figure 2) were
determined by single crystal X-ray diffraction and show that
region
Hz.
of −650 to −750 ppm and peak widths of 150−300
A preliminary computational investigation of the stabilities
of the cations [Ga(arene) ] (arene = C H , C H F) and syn
and anti GaN(SO CF ) , 5, was conducted with respect to the
precursors [H Ga(arene) ] and syn and anti H GaN-
+
2
6
6
6
5
2
3 2
+
2
2
2
(
SO CF ) , 4, using density functional theory calculations.
2 3 2
No interactions with the anion were assumed for the cation
calculations, and the choice of the arene-coordinated
precursors is based on the existence of an arene-coordinated
+
30
cation in [(2,6-Mes C H O) Al]
and the fact that the
2
6
3
2
former represent a minimum on the potential energy surface.
For the identification of the structural minima, geometry
optimizations were performed using the B97-D3(BJ) ex-
3
1,32
change−correlation functional
and the def2-SV(P) basis
3
3
set, starting from random initial orientations of the
fragments. The energetic results have been further refined
using single-point calculations on the optimized structures
using the ωB97M-V and the MN15 functionals with the
def2-TZVP basis set. The energetic results show that
34
35
3
3
+
+
[
Ga(C H ) ] and [Ga(C H F) ] are more stable than the
6 6 2 6 5 2
corresponding gallium(III) hydride cations by 11.4 and 10.6
kcal/mol, respectively (see Table S4). The results also reflect
the experimentally observed higher stability of the gallium(III)
triflimide 4 over the gallium(I) species 5 by 9.4 kcal/mol.
Figure 2. Structure of 3 (50% ellipsoids). H-atoms are omitted for
clarity. Selected distances (Å) and angles (deg): Ga(1)···centroid =
.601(2), 2.675(2); Ga(1)···F(2A) = 3.123(2); and centroid−
Ga(1)−centroid = 140.7°.
+
Furthermore, the calculated data for the [Ga(C H F) ] cation
6
5
2
agree very well with the XRD data for 1 (see Table S3), and
2
the calculated harmonic frequency for the IR active asymmetric
−
1
Ga−H stretch (2092 cm ) for the syn isomer of 4 is close to
−
1
the observed one (2080 cm ).
the gallium(I) centers are coordinated by two arenes in a
slightly distorted η fashion and have also moderate (1, Ga···Cl
An exploratory reactivity study focused on the activity of
compounds 1 or 2 and 3 as catalysts for the polymerization of
2,4,4-trimethyl-1-pentene and the hydrosilylation of benzo-
6
=
3.353−3.400 Å) and weak (3, Ga···F = 3.123 Å) contacts to
the counterion. This bent sandwich geometry is typical for
many gallium(I) arene complexes,
known compounds [Ga(benzene) ][GaCl ] and [Ga-
phenone, 1-hexene and CO . The Krossing group has shown
2
1
3,25
including the long-
that [Ga(arene) ][Al{OC(CF ) } ] compounds are active
2
3 3 4
2
6
catalysts for the polymerization of isobutene to highly reactive
2
4
27
3,4
(
[
{
C H Me ) ][GaCl ], as well as the more recently reported
Ga(C H F) ][Al{OC(CF ) } ] and [Ga(C H Me ) ][Al-
OC(CF ) } ]. The gallium environment in compound 3 is
polyisobutylene. For operational simplicity the olefin 2,4,4-
6
3
3 2
4
2
,3
trimethyl-1-pentene (bp = 101 °C) was chosen here as an
6
5
2
3 3
4
6
3
3 2
3
alternative to isobutene (bp = −7 °C), although it is known to
3
3 4
2
,3
36
similar to that in [Ga(C H F) ][Al{OC(CF ) } ], whereas
resist polymerization. Both compounds 1 and 3 readily
6
5
2
3 3 4
the Ga···C and Ga···Cl distances in compound 1 are closer to
catalyzed an exothermic reaction that led to a mixture of
dimers, trimers, and tetramers still possessing a double bond
26
those of the classic [Ga(benzene) ][GaCl ] (Table S3),
2
4
C
DOI: 10.1021/acs.inorgchem.9b02136
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
see Figures S21−S26). Furthermore, a strong Ga(I) signal is
supplies from a previous project (PRF 52856-ND3). The
authors also thank the National Science Foundation (CHE-
1726630) and the University of Oklahoma for the purchase of
the X-ray diffractometer.
7
1
detected in the Ga NMR spectra after three cycles, indicating
the robustness of the system. Benzophenone is rapidly (30 min
for 3) reduced to diphenylmethane using Et SiH and a catalyst
3
loading of 1%, and the intermediate Ph C(H)OSiEt is not
2
3
detected even in a 1:1 Ph CO:Et SiH reaction. As before, a
2
3
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2
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̈
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■
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Rudolf J. Wehmschulte: 0000-0003-0719-0620
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