Synthesis of an anionic Au(I) hydroamination precatalyst supported by charged hydrido-carboranyl phosphine ligands

Article abstract of DOI:10.1016/j.poly.2018.09.027

Herein is reported the synthesis of an anionic gold complex bearing two charged hydrido carboranyl phosphine ligands. The compound is fully characterized by multinuclear NMR spectroscopy, high resolution mass spectrometry, as well as a single crystal X-ra

Full text of DOI:10.1016/j.poly.2018.09.027

Accepted Manuscript
Synthesis of an Anionic Au(I) Hydroamination Precatalyst Supported by Charg-
ed Hydrido-Carboranyl Phosphine Ligands
Jack F. Kleinsasser, Sarah E. Lee, Christopher A. Lugo, Varun Tej, Scott G.
McArthur, Vincent Lavallo
PII:
S0277-5387(18)30579-5
https://doi.org/10.1016/j.poly.2018.09.027
POLY 13426
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
27 July 2018
12 September 2018
14 September 2018
Please cite this article as: J.F. Kleinsasser, S.E. Lee, C.A. Lugo, V. Tej, S.G. McArthur, V. Lavallo, Synthesis of
an Anionic Au(I) Hydroamination Precatalyst Supported by Charged Hydrido-Carboranyl Phosphine Ligands,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.09.027
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Synthesis of an Anionic Au(I) Hydroamination Precatalyst Supported by Charged
Hydrido-Carboranyl Phosphine Ligands
Jack F. Kleinsasser, Sarah E. Lee, Christopher A. Lugo, Varun Tej, Scott G. McArthur, Vincent
Lavallo
Department of Chemistry, University of California Riverside, Riverside CA 92521 USA
Abstract
Herein is reported the synthesis of an anionic gold complex bearing two charged hydrido
carboranyl phosphine ligands. The compound is fully characterized by multinuclear NMR
spectroscopy, high resolution mass spectrometry, as well as a single crystal X-ray diffraction
study. In addition, we probe this compound’s ability to catalyze the hydroamination of alkynes
and compare its behavior to a related highly active catalyst supported by a perchlorinated
carboranyl phosphine.
Keywords: Gold Catalysis; hydroamination; carborane anion; carborane ligand; electrostatic
effects
1
. Introduction
Neutral phosphine ligands are ubiquitous ancillary ligands in homogenous catalysis. Such species
are typically appended with alkyl or aryl groups, to manipulate the steric environment and
electronic properties of the metal center. Neutral closo-carborane clusters,[1-3] specifically
H C B H congeners, are alternative R-groups,[4, 5] which have been heavily investigated over
2
2
10 10
the last 60 years. However, despite these efforts little progress has been made in developing
competitive or superior catalysts, compared to classical systems supported by ligands with
hydrocarbon R-groups. This fact is perhaps partially explained by the tendency of such clusters to
undergo catalyst deactivation by cage degradation[4], B-H cyclometallation[6] reactions, or other

degradation pathways.[7, 8] In contrast to neutral H C B H clusters, their anionic analogue
2
2
10 10
[
HCB H111-] and its derivatives,[9] are not susceptible to chemical cage degradation[10] and are
11
more resistant to cyclometallation.[11]
Several years ago we began implementing a variety of ligands functionalized with such
closo-carborane anions, including phosphine[11-16] and N-heterocyclic Carbenes.[17-20] So far
we have found two examples of phosphine supported systems that display remarkable reactivity
1
[13] and catalytic activity 2[15], respectively (Fig. 1). Compound 1 is a dianionic Pd(0) species
that undergoes the oxidative addition of certain chloroarenes in less than 10 minutes at ambient
temperature. No other isolable Pd(0) species has been reported to undergo such rapid oxidative
addition reactions. The extreme reactivity of 1 was found both experimentally and computationally
to be the result of electrostatic effects of coordinating two charged phosphine ligands to Pd(0).
Electrostatic repulsion between the ligands, results in facile ligand dissociation of one phosphine,
allowing immediate access to highly reactive mono phosphine ligated Pd(0). Compound 2 is a
zwitterionic Au(I) species, which functions as a single component Au(I) catalyst that does not need
+
an activator, such as Ag or acid. This catalyst displays the highest activity yet reported for the
hydroamination of amines with alkynes.[21, 22] Turn over numbers (TONs) of around 100,000 in
a 24 hr period were observed for some substrates. The high activity of 2 is thought to result from
the size of the ligand, which prevents double phosphine substitution, but more importantly the
electrostatic stabilization of the Au(I) cation by field effects of the proximal carborane anion.

H
H
2 2Li+
Cl
-
H
Cl
H
Cl
Cl
Cl
H
Cl
H
H
Cl
H
C
Cl
H
C
Cl
Cl
H
P
Pd
P
P
H
Cl
Au
S
H
1
H
2
H
C
H
H
H
H
H
H
H
H
Figure 1. Dianionic Pd(0) complex 1 is the most active isolable compound for the oxidative
addition of arylchlorides. Zwitterionic Au(I) complex 2 is the most active catalyst for the
hydroamination of alkynes.
Here, we report our efforts to synthesize a linear monoanionic dicoordinate Au(I) complex
analogous to 1. The rational for this approach was that if facile ligand dissociation, similar to 1,
was observed when two charged phosphine ligands were attached to Au(I), we might be able to
achieve an even more active system than 2.
2
. Materials and Methods
General Considerations: Unless otherwise stated, all manipulations were carried out using
standard Schlenk or glovebox techniques (O , H O < 1ppm) under a dinitrogen or argon
2
2
atmosphere. Solvents were dried on K or CaH and distilled under argon before use. The ligand
2
Li(THF) [P(C H ) CB H ] 3 was prepared according to the literature procedure.[16] The amines
3
3
7 2
11 11
were dried over CaH and distilled before use. The alkynes were dried over molecular sieves and
2
used without further purification. All other reagents were purchased from commercial vendors and
used without further purification. Known catalytically produced imines from Table 1 were

1
13
identified by comparison to their reported spectroscopic data for H NMR and C NMR (See
Supplementary Information for characterization of products). NMR spectra were recorded at room
temperature on Bruker Avance 300MHz, Bruker Avance 400MHz, or Bruker Avance 600MHz
1
13
spectrometers. NMR chemical shifts are reported in parts per million (ppm). H NMR and C
1
1
NMR chemical shifts were referenced to residual protio solvent. B NMR chemical shifts were
3
1
externally referenced to BF OEt . P NMR chemical shifts were externally referenced to 80%
3
2
H PO in H O. The mass spectrometry data was collected on an Agilent LCTOF Multimode-
3
4
2
ESI/APCI with direct injection.
.1 Synthesis of 4 Au((P(C H ) CB H ) )Li
2
3
7 2
11 11 2
In a vial equipped with a stir bar, 3 (757 mg, 1.57 mmol) and ClAuSMe (462 mg, 1.57 mmol)
2
were added and subsequently dissolved in 10 mL of methylene chloride. The mixture was stirred
and a white precipitate rapidly formed. After one hour the mixture was filtered through a glass
pipette plugged with glass microfiber to remove LiCl. The solvent was removed under vacuum
and remaining solid was washed with pentane. The compound was dried under vacuum overnight
to afford the purified product (yield = 701.0 mg, 1.35 mmol, 86.1%). Crystals suitable for X-ray
diffraction were obtained by layering a CH Cl solution of 4 with pentane. m.p. = 110°C. NMR
2
2
1
3
2
H (300 MHz, CD Cl , 25°C): δ = 3.79 (m, 12H, THF), 2.53 (d-sep, J (H,H) = 7.2 Hz, J (H,P) =
2
2
1
11
4
.0 Hz, 1H, CH), 1.95 (m, 12H, THF), 1.44 (m, 6H, CH ), 1.35 (m, 6H, CH ); H[ B] (300 MHz,
3 3
3
1
1
CD Cl , 25°C): 2.13 (br-s, 5H, BH), 1.96 (br-s, 1H, BH), 1.72 (br-s, 5H, BH); P[ H] (162 MHz,
2
2
1
3
1
11
THF-d , 25°C): 83.6; C[ H] NMR (101 MHz, CD Cl , 25°C): 69.0, 28.8, 25.8, 22.6, 19.2; B
8
2
2
1
1
(
96 MHz, CD Cl , 25°C): -2.9 (d, 1B, J (B,H) = 120 Hz), −11.7(d, 5B, J (B,H) = 120 Hz), −12.9(d,
2 2

1
5
7
B, J (B,H) = 120 Hz); HRMS: (Multimode-ESI/APCI) [M]- m/z calc’d for H B C PAu:
34 11 11
15.5239; found: 715.5432.
.2 General procedure for catalytic experiments
A standard solution of 4 was prepared in dry CH Cl and the desired amount of catalyst was
2
2
2
immediately transferred via micropipette to screw capped (PTFE) vials. The CH Cl solution was
2
2
removed by placing under vacuum for 10 minutes and a stir bar was subsequently added. The
amines and alkynes were then transferred via micropipette (or added directly if solid) to the screw
capped vials which were then sealed and heated to the indicated temperature and stirred for the
1
allotted times (Table 1, Results and Discussion). Yields were calculated by integration of the H
NMR, via direct comparison of the imine formed to the amine consumed (no side reactions were
observed). The validity of this approach was confirmed by obtaining isolated yields of several
compounds. The products were isolated via column chromatography or by crystallization at -25˚C
in hexanes.
3
. Results and Discussion
Given our success at preparing the diphosphine substituted Pd(0) species 1[13] we imagined
it should certainly be possible to form a similar compound with Au(I) as gold is a larger atom.
Thus, we reacted two equivalents of ligand 3 with ClAu(SMe ) in methylene chloride solvent,
2
and a white precipitate of LiCl immediately formed. After filtration and concentration of the
supernate the residue was analyzed by mass spectrometry (negative-ion mode), which showed a
molecular ion peak corresponding to the formation of a gold complex bearing two carboranyl
1
phosphines. Analysis of the H NMR spectrum of the residue in CD Cl reveals the existence of
2
2
THF, which

Figure
2.
Synthesis of anionic Au(I) complex 4.
suggests the presence of a lithium cation coordinated by THF, consistent with the formation the
anionic complex 4. Integration of the THF peaks shows that there are three THF molecules
coordinated to the Li cation. Additionally, we see the signature isopropyl multiplets at 2.53 ppm
and 1.44 through 1.35 ppm, indicating the presence of the carboranyl phosphine. One resonance
3
1
11
is present in the P NMR at 81.2 ppm, indicating a single phosphorus product. Analyzing the B
NMR presents three peaks at -2.8, -11.7, and -12.9 ppm, respectively, in a 1:5:5 ratio showing
that the local C v symmetry of the cluster is preserved.
5
3
.1 Crystal Structure
A single crystal X-ray diffraction study unambiguously confirmed the structure of 4. In the solid-
state, 4 displays the expected perfectly linear geometry (P-Au-P angle = 180˚) of a symmetrical
dicordinate Au(I) complex, as well as a single lithium counter cation coordinated by THF (Figure
3
). The P-Au-P bond lengths are 2.3079(5) and 2.3099(2) Å, close to both the reported P-Au bond
length for the neutral ortho-dicarba-closo-decaborane-substituted phosphine–AuCl complex
2.232(3) Å)[23] and the anionic carborane substituted phosphine-AuCl in 2 (2.2477(12) Å).[15]
(

The closest B-H distance to the Au(I) center is 3.001(5) Å, which is outside of the range for
covalent Au-H interactions, demonstrating that the carborane anion substituent does not
interact strongly with the cationic Au center.
Figure 3. Solid-state structure of 4. Thermal ellipsoids drawn at the 50% probability level and
+
hydrogen atoms omitted for clarity. Note: THF molecules coordinated to Li disordered over two
positions. Color code: grey = carbon, brown = boron, violet = phosphorus, yellow = gold, red =
oxygen, pink = lithium.
3
.2 Catalytic Activity
We next investigated the catalytic activity of complex 4 towards the hydroamination of alkynes
with primary amines (Table 1) and compared its behavior to the previously reported catalyst 2[15]
(Figure 1). Unlike 2, complex 4 did not produce any hydroamination products with any of the
chosen substrates at ambient temperature, run under neat conditions. In fact, reasonable rates of
conversion were not observed until 80 °C. Our initial test substrates, aniline and
phenylacetylene, were reacted with a 1.0% catalyst loading, producing the corresponding imine in
9
0% yield (TON = 90) after 24 hours (Table 1, entry 1). Dropping the catalyst loading to 0.5 and

Table 1. Hydroamination of alkynes with primary amines in the presence of catalyst 4.
Ar
Catalyst
Neat
N
R1
R2
Ar NH2
R1
R2
o
[a][b]
Entry Ar
R1 R2
Catalyst
Loading [%]
1.0
T[ C] Yield [%]
TON
24 hr
90
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Ph
Ph
Ph
H
H
H
Ph
Ph
Ph
80
80
80
80
80
80
80
80
80
80
80
80
80
80
90
0.5
86
172
765
710
80
0.1
77 (74)
71
Mes H Ph
Dipp H Ph
0.1
1.0
80 (74)
90 (86)
90
Ph
H
4-FC H4
1.0
90
6
Mes H 4-FC H4
1.0
90
6
Dipp H 4-FC H4
1.0
87 (83)
92
85
6
Ph
H
4-MeOC H 0.1
920
950
780
65
6
6
0
1
2
3
4
Mes H 4-MeOC H 0.1
95 (92)
78
6
6
Dipp H 4-MeOC H 0.1
6
6
Ph
H
n-C H9
1.0
1.0
1.0
65
4
Mes H n-C H9
55
55
4
Dipp H n-C H9
50
50
4
[
a] The yield was determined by NMR spectroscopy by the direct integration of the peak for the
amine starting material with respect to the peak for the imine product. [b] The yield of the isolated
product is given in parentheses. Dipp = 2,6-diisopropylphenyl, Mes = mesityl (2,4,6,-
trimethylphenyl).
0
.1 mol% respectively, resulted in reasonable but lower yields (86%: TON = 177; 77%: TON =
7
65; respectively) (Table 1, entries 2 and 3). Comparatively, 2 catalyzes this reaction with a 0.1
mol% loading at ambient temperature in one hour to produce the product in > 95% yield (TON
950). Furthermore, catalyst 2 achieves an identical yield even with 0.01 mol% loading but at only
>
slightly elevated temperatures (50 °C). Complex 4 shows similar performance with the larger
mesityl amine (71%; TON = 710), but the very sterically demanding 2,6-diisoppropylamine
required the utilization of higher catalyst loading (1 mol%) to a achieve any appreciable conversion
(Table 1, entries 4 and 5, respectively). On the other hand, complex 2 catalyzes these reactions at
5
0 °C with 0.001% loading to achieve maximum turnovers of 67,000 and 85,000, respectively. In

contrast to 4, catalyst 2 displays a clear trend in improved yields with bulkier amines for all
substrates tested, which was rationalized by sterically related amine substrate and imine product
inhibition. Catalyst 4 mediated the hydroamination of the electron poor alkyne para-
fluorophenylacetylene with all three sterically differentiated amines providing the imine products
in 85-90% yields (table 1, entries 6-8). However, appreciable conversions were not observed below
1
.0 mol% catalyst loading. In contrast, the zwitterionic catalyst 2 mediates these reactions at 50
°
C with 0.001 mol% loading and achieves TONs between 54,000-92,000. Complex 4 is highly
active for the hydroamination of the electron rich alkyne, para-methoxyphenylacetylene,
producing the imine products in 78-92% yield at 0.1 mol% loading (Table 1, entries 9-11).
However, complex 2 is again far more active for these reactions, achieving 90,000-95,000
turnovers for the same amines at 0.001 mol% loading. Complex 4 was less active for the
hydroamination of the alkyl substituted alkyne, 1-hexyne, affording the ensuing imines in 50-65%
yields at 1.0 mol% loading (Table1, entries 12-14). Comparatively, catalyst 2 is an order of
magnitude more active for the same substrates at the same temperature. Additionally, we attempted
to react an internal alkyne, biphenylacetylene, but the resulting TONs were less than 5% after 24
hours which were not included in the table. Catalyst 2 on the other hand, could achieve yields
between 67 and 89%, depending on the amine, at 80 °C with 0.1 mol% loading.
The difference in reactivity between 2 and 4 can likely be explained, in part, by the fact that
in contrast to our initial hypothesis, repulsive electrostatic effects in this system are not strong
enough to induce facile phosphine dissociation, as they are in the analogous Pd(0) species. Since
Au(I) is formally positively charged, in contrast to Pd(0), some of the electrostatic repulsion
between the anionic carborane substituents is negated, rendering phosphine ligand dissociation
3
1
less favorable. Indeed, we monitored by P NMR the hydroamination reaction of aniline and

3
1
phenylacetylene at both 1.0% and 0.1% catalyst loading and saw no change in the P at ambient
temperature, confirming that the phosphines of 4 are not labile without external heating. In
addition, at completion of the heated reactions with 4 we visually notice the formation of colored
Au nanoparticles and metal plating on the reaction vessels, whereas with complex 2 we see no
such visual cues of decomposition. This observation is in line with the far greater chemical stability
of the perchlorinated carborane present in 2. Lastly, although not quantified but empirically noted
based on the difference of C-H acidities of the [HCB H ] and [HCB Cl111-] anions,[24] the
1
-
1
1
11
11
perchlorinated carboranyl substituent should be a far stronger electron withdrawing group
compared to its hydric congener, thus rendering this phosphine a weaker donor and the Au a
stronger -acid towards alkynes.
4
. Conclusion
An anionic gold complex 4 bearing two charged hydrido carboranyl phosphine ligands is
reported and fully characterized. In contrast to the previously reported mono-phosphine ligated
zwitterionic Au(I) species 2, supported by a phosphine with a perchlorinated carborane anion
substituent, 4 is much less catalytically active for the hydroamination of alkynes. The difference
in reactivity can likely be explained by the overall superior stability and -acidity of 2, and also
the reluctance of one of the phosphines in 4 to dissociate. Given these results, subsequent
investigations into such systems will focus on monophosphine ligated zwitterionic species with
enhanced -acidity.
Acknowledgments
We are grateful to the National Science Foundation of the USA for funding this work (Grant CHE-
1
455348). We thank Fook S. Tham (U.C.R.) for X-ray crystallographic analysis.
Appendix A

Supplementary data CCDC <1850911> contains the supplementary crystallographic data for <4>.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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[
[
[
1
[

Figure

Synopsis: An anionic Au(I) complex supported by two charged carboranyl phosphines is
reported. Its catalytic activity for the hydroamination of alkynes is investigated and compared
to the most active known system.
Figure
Synopsis: An anionic Au(I) complex supported by two charged carboranyl phosphines is
reported. Its catalytic activity for the hydroamination of alkynes is investigated and compared
to the most active known system.