Transmetalation of Acyclic Tungsten Carbenes to Coinage Metals: Distinct Behavior of Silver toward Carbene Transfer and Hydrolysis

Article abstract of DOI:10.1021/acs.organomet.0c00675

The transmetalation of acyclic monoamino and alkoxo carbenes from tungsten to group 11 metals has been studied for all three metals by analyzing the new metal carbenes formed and also their decomposition products in solution. The ease of transmetalation follows the trend Au > Cu > Ag, and the gold carbenes can be isolated: a copper carbene could be detected, but we found no experimental evidence of the formation of any silver carbene. The electrophilic character of copper and gold carbenes is supported by the identity of the products formed upon hydrolysis of the carbenes involved. Silver shows a distinct behavior, and the presence of Ag+ ions in an acetonitrile solution of a tungsten aminocarbene leads to the formal protonation of the carbene fragment to give an iminium salt. This outcome, a seemingly nucleophilic behavior, is unexpected for group 11 carbenes, but DFT calculations show that the protonation of a putative cationic silver carbene is energetically accessible.

Full text of DOI:10.1021/acs.organomet.0c00675

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Transmetalation of Acyclic Tungsten Carbenes to Coinage Metals:
Distinct Behavior of Silver toward Carbene Transfer and Hydrolysis
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Alberto Toledo, Vanesa Salamanca, Teresa Perez-Moro, and Ana C. Albeniz*
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ABSTRACT: The transmetalation of acyclic monoamino and alkoxo carbenes from
tungsten to group 11 metals has been studied for all three metals by analyzing the
new metal carbenes formed and also their decomposition products in solution. The
ease of transmetalation follows the trend Au > Cu > Ag, and the gold carbenes can
be isolated: a copper carbene could be detected, but we found no experimental
evidence of the formation of any silver carbene. The electrophilic character of copper
and gold carbenes is supported by the identity of the products formed upon
hydrolysis of the carbenes involved. Silver shows a distinct behavior, and the
presence of Ag+ ions in an acetonitrile solution of a tungsten aminocarbene leads to
the formal protonation of the carbene fragment to give an iminium salt. This
outcome, a seemingly nucleophilic behavior, is unexpected for group 11 carbenes,
but DFT calculations show that the protonation of a putative cationic silver carbene
is energetically accessible.
monomeric silver carbene has been disclosed.14g In general,
those studies show that the M−C(carbene) bond length
conforms to a single bond and that the metal to carbene back-
bonding is quite small or often negligible.
INTRODUCTION
■
The carbene chemistry of group 11 metals spans from the
useful and robust spectator N-heterocyclic carbene (NHC),1
diamino-type carbene (NAC),2 and cyclic alkylamino carbene
(CAAC) ligands,3 widely used in catalysis, to the reactive
carbene fragments from diazo derivatives or other precursors,
used as reagents and incorporated into the products. All group
11 metals are involved in catalytic reactions that may form
intermediate reactive metal carbenes in one of the steps of the
process.4 For example, the mechanisms of alkene cyclo-
propanation,5 carbene insertion reactions into C−H bonds of
alkanes,6 coupling reactions of diazo compounds,7 and the
Wolff rearrangement8 involve the formation of elusive metal−
carbene species from the diazo derivatives used as reagents and
this has been supported by DFT calculations.9 Cyclo-
heptatrienes as carbene precursors by a retro-Buchner reaction
have been used in a gold-catalyzed cyclopropanation reaction
with the likely participation of a gold carbene species.10 Even if
no reactant carbene source is present, many gold-catalyzed
transformations of unsaturated compounds are explained by
the formation of reactive species that can be considered gold−
carbenes with more or less metal stabilized carbocation
character.11 This subject has been discussed in the literature,
and efforts have been made to synthesize and study
nonstabilized gold carbenes that could give information
about the nature of those complexes.12 As a result, a number
of weakly stabilized AuCRR′ carbenes (R = R′ = hydrocarbyl,
H) have been reported and their structures analyzed.13
Nonstabilized carbenes of copper are even more elusive than
gold carbenes, but nonetheless examples of CuCRR′
derivatives have been reported.14 Only one example of a
There is still a lot to learn about reactive group 11 metal
carbenes, and there are few studies that compare the behavior
of the three metals toward the same carbene fragment and
reaction conditions.9a,14g,15 When there are enough data,
structural or otherwise, to compare the three metals, significant
differences between them and deviations from a periodic
behavior are found. The work we describe here explores the
formation of group 11 metal complexes with C(X)R carbenes
(X = NR2, OR) by transmetalation reactions from tungsten
carbenes. Since the X group is electron donating and is capable
of partially compensating the electron deficiency on the
carbene carbon, an intermediate reactivity is expected for the
M−C(X)R fragments, in comparison to NHCs or CRR′
groups (Scheme 1).16 It might be useful to get information
about the character and reactivity of the carbenes formed by all
three metals. This strategy has been used before by us in the
study of the reactivity of palladium carbenes and their
migratory insertion reactions.17 Transmetalation of mono-
amino or monoalkoxo carbenes between group 6 and another
Received: October 14, 2020
Published: December 23, 2020
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Scheme 1. Reactivity Trend for Metal Carbenes
transition metal is often the synthetic method of choice for
these unstable carbene fragments that are difficult to generate
by other means.18 There are some examples of this reaction
involving group 11 metals that have led to the successful
isolation of copper19 and gold monoheteroatom-containing
carbenes.20 With these precedents we have analyzed and
compared the behavior of analogous precursor complexes of
Cu, Ag, and Au toward tungsten C(X)R′ carbenes (X = NR2,
OR), for both transmetalation and their hydrolysis reactions.
Figure 1. Molecular structures of complexes 2a (a) and 2b (b)
(ORTEP plots; 40% probability ellipsoids). Hydrogens have been
omitted for clarity. Selected distances (Å) and angles (deg): 2a, Au1−
C1, 1.985(5); C1−N1, 1.295(6); Cl1−Au1−C1, 177.62(14); 2b,
Au1−C1, 2.000(13); C1−O3, 1.277(13); Cl1−Au1−C1, 179.3(3).
involvement in the stabilization of the electrophilic carbene,
but this is lost in solution, where free rotation of the ring
1
occurs, as shown by its H NMR pattern.
RESULTS AND DISCUSSION
■
The reaction of the neutral gold carbenes 2a and 2b with
AgSbF6 in acetonitrile led to their complete conversion to the
corresponding cationic Au(I) carbenes 3a and 3b. Unfortu-
nately, their isolation was not possible, but they were stable
enough to be characterized by NMR. Even at low temperature,
both complexes slowly evolve in acetonitrile solution to new
species which were identified as the cationic Au(I) bis-
carbenes 4a and 4b (Scheme 2). A faster reaction rate for the
The direct transmetalation of the tungsten carbenes was tested
using neutral and cationic group 11 precursor complexes with
weakly coordinated ligands that could favor the carbene for
ligand substitution. [MCl(tht)] (M = Cu, Ag, Au; tht =
tetrahydrothiophene) and [Cu(MeCN)4]PF6, [Au(MeCN)2]-
BF4, and AgBF4 in acetonitrile were reacted with [W(CO)5{C-
(X)Ph]} (X = NEt2, 1a; X = OMe, 1b). The results obtained
for each metal are described below.
Synthesis and Reactions of Gold Carbene Complexes.
The gold(I) carbenes 2a and 2b were synthesized in good
yields by direct and smooth transmetalation from the
tungsten(0) carbenes 1 to [AuCl(tht)] in either CH2Cl2 or
acetonitrile at room temperature (eq 1).16 On comparison of
Scheme 2. Formation of Cationic Gold Carbene Derivatives
2a and 2b, the 13Ccarbene NMR resonance is 44 ppm higher for
the alkoxocarbene, in agreement with the higher electrophilic
character expected for this carbene (2b) vs the aminocarbene
2a.
rearrangement to the bis-alkoxocarbene was observed in
comparison to the amino analogue. The identity of the
Au(I) bis-carbenes was supported by the reaction of different
amounts of the tungsten monoaminocarbene 1a with 1 equiv
of the cationic Au(I) complex [Au(MeCN)2]BF4. When the
reaction was carried out using equimolar amounts of Au and W
complexes, both mono- and bis-carbenes were formed (100%
conversion of 1a; 3a:4a = 5:1 molar ratio after 10 min in
solution at 25 °C). On the other hand, only the bis-carbene
complex 4a was formed by the addition of 2 equiv of 1a
(Scheme 2). 13C NMR reveals significant shifts in the carbenic
carbon signals depending on the other ligand coordinated to
the gold center, and thus, in comparison to the chloro (2) or
acetonitrile (3) derivatives, the higher trans influence of the
carbene fragment (4) is patent in the higher downfield shift
observed (Schemes 1 and 2).
The X-ray crystal structures of 2a and 2b were determined,
and they show two independent molecules in the unit cell (see
the Supporting Information).21 Both complexes show a linear
coordination for the gold center, where the Au−Ccarbene bond
distances conform to the typical range reported before for gold
carbenes20b−f,21 and are consistent with a single bond between
an sp2 carbon atom and the metal (Figure 1).22 The C−X
bond lengths are short and show a double-bond character as a
result of the lone pair donation from X to the electrophilic
carbene carbon. Therefore, the representation used for the
group 11 carbenes in eq 1 and throughout the paper reflects
these structural parameters and bonding situation.16 Aurophilic
interactions with an adjacent molecule are weak for 2b (Au−
Au distance 3.4481(2) Å) and even less important for 2a
(shortest Au−Au distance 3.6762 Å).20f The alkoxocarbene
exhibits the so-called anti conformation, with the methyl group
oriented toward the gold center, as observed before in other
transition-metal Fischer alkoxocarbene complexes.20f,23 The
planar arrangement for the phenyl ring may indicate some
The formation of the bis-carbene complexes was also
observed by the transmetalation of the alkoxocarbene from
1b to the cationic gold(I) aminocarbene 3a (generated in situ
from 2a as shown in Scheme 2). The reaction involves the
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exchange of both carbene fragments and a mixture of the
homoleptic biscarbenes 4a, 4b was observed along with the
new mixed complex 5, bearing two different carbene moieties
(eq 2).
conversion and eventually to some methyl benzoate as
byproduct.
These reactions show the electrophilic character of the
carbene carbon in these gold complexes, more pronounced for
the alkoxocarbene than the aminocarbene. The hydrolysis of
Fischer carbenes is well-documented in other transition-metal
carbenes, and the mechanism for the hydrolysis of group 6
metal carbenes was deeply studied by Bernasconi et al. and
others.24 The results described in Table 1 suggest that a similar
mechanism may operate for the electrophilic Au(I) Fischer
carbenes. The reaction is believed to proceed in two stages
involving a tetrahedral intermediate (A, Scheme 3) as a result
The reactivity of the neutral Au(I) monoamino- and alkoxo-
carbenes in the presence of acids or bases was studied (eq 3).
Scheme 3. Hydrolysis Processes for Gold Carbenes 226
a
Table 1. Hydrolysis Reactions of Gold Carbenes
conversion (%)
entry [Au]
additive
conditions
6
7
8
4
of a nucleophilic attack on the carbenic carbon. At this point,
two different pathways are possible according to the
concentration of OH−. At low concentrations, the elimination
of HX and the formation of the Au(I) acyl is faster than a
second nucleophilic attack.25 The subsequent hydrolysis with
water provokes a demetalation, leading to benzaldehyde (8).
At higher OH− concentrations, the second nucleophilic attack
on the former carbenic carbon is favored, resulting in the
formation of the gem-diol B (Scheme 3). According to the
nature of the substituent, when X = OMe, the elimination of
water occurs, leading to methyl benzoate (7), while the
formation of the carboxylate 6 indicates the elimination of HX
as observed if X = NEt2.
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2b
2b
2b
2b
days, 50 °C
days, 50 °C
1.5 h, 25 °C
24 h, 25 °C
48 h, 25 °C
5 min, 25 °C
1.5 h, 25 °C
24 h, 25 °C
b
H2O
c
NBu4OH
78
HBF4.OEt2
19
59
5
b
H2O
15
80
24
64
c
NBu4OH
HBF4·OEt2
a
Reaction conditions: 0.014 mmol of 2 in 0.6 mL of CD3CN, 0.014
mmol of additive. Conversions were determined by 1H NMR.
b
c
Addition of two drops of water. Solution in MeOH.
Reactions of Tungsten Carbenes with Ag and Cu
Derivatives. A new carbene was detected during the reaction
of the tungsten aminocarbene 1a with [CuCl(tht)] in
acetonitrile at room temperature (Scheme 4). The slow
transmetalation (10% in 24 h at room temperature) prevented
the isolation of the copper(I) carbene. Only a maximum
conversion of 50% to 9 was achieved by heating at 50 °C due
to its competing decomposition to benzaldehyde (8, 25%) and
NHEt2 under these conditions (entry 2, Table 2). NMR
characterization of 9 revealed a 13C shift at 239.8 ppm,
Table 1 collects the phenyl-containing products formed, which
give a complete account of the fate of the carbene fragment;
the concomitant formation of other byproducts such as
methanol of diethylamino derivatives also occurs (see
below). 2a is stable in solution and remains unchanged for
days at 50 °C (entry 1, Table 1). No trace of hydrolysis
products was detected even in the presence of water on heating
at 50 °C (entry 2, Table 1). The addition of NBu4OH to a
solution of 2a in CD3CN led to the formation of benzoate (6)
(entry 3, Table 1). On the other hand, the gold(I)
alkoxocarbene 2b is less stable and undergoes faster hydrolysis,
leading to methyl benzoate (7), benzaldehyde (8), and MeOH
as products (entries 5−7, Table 1). It is worth noting that a
change in color of the solution from yellow to purple is usually
observed with time, indicating the common decomposition to
Au nanoparticles. The reactivity of the Au(I) carbenes toward
a strong Brønsted acid such as HBF4 was also explored,
revealing that both alkoxo and amino carbenes lead to the
corresponding cationic bis-carbenes 4 (entries 4 and 8, Table
1) with little or no decomposition of the carbene fragment; the
higher reactivity of the alkoxocarbene leads to a higher
Scheme 4. Reactions of Cu and Ag Complexes with 1a
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a
Table 2. Reactions of Carbenes 1 with Cu and Ag Complexes
conversion (%)
entry
[W]
[M]
time
M-carb
7
8
10
11
1
2
3
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
days
4 h
15
25
51
13
[CuCl(tht)]
50 (9)
[Cu(MeCN)4]PF6
1/4 [AgCl(tht)]4
AgBF4
24 h
24 h
24 h
24 h
24 h
3 h
b
4
5
6
7
8
72
5
13
4
66
22
18
[CuCl(tht)]
19
24
2
77
[Cu(MeCN)4]PF6
1/4 [AgCl(tht)]4
AgBF4
b
9
24 h
30 h
10
9
a
b
Reaction conditions: 0.022 mmol of 1a and 0.022 mmol of [M] in 0.6 mL of CD3CN at 50 °C. Conversions were determined by 1H NMR. The
same results were obtained using [AgBr(tht)]4.
deshielded by 16 ppm in comparison to the analogous neutral
gold(I) carbene 2a (224 ppm). Attempts to isolate 9 or to
obtain suitable crystals for X-ray diffraction from the reaction
mixture failed, and the two-coordinated Cu(I) complex
represented in Scheme 4 is tentative. DFT calculations show
that the formation of 9 and [W(CO5(tht)] is thermodynami-
cally favored and the associated free energies for the process
are very similar for the two-, three- or four-coordinated Cu(I)
complexes [CuCl(carbene)(MeCN)n] (n = 0, 1, 2; ΔG = −6,
−6.4, and −7.5 kcal mol−1, respectively, see Table S4 in the
Supporting Information). Therefore, the formation of solvento
three- or four-coordinated complexes, most probably in
equilibrium with the dicoordinated complex in acetonitrile
solution, cannot be ruled out. The lability of the ligands in
three-coordinated cationic copper carbene complexes has been
studied before.27
formation of 9 or undetected for other precursor complexes)
and the lack of transmetalation for silver.
However, the reaction of 1a with AgBF4 revealed an
interesting reactivity because of the unexpected formation of
the iminium salt 10 when the mixture was heated at 50 °C in
acetonitrile (Scheme 4 and entry 5, Table 2). Other silver salts
AgY with weakly coodinating anions led to the same product
but, in general, proved to be less efficient in the decomposition
of the carbene fragment. Under the same conditions of entry 5,
Table 2: AgOTf, 69% 10; AgSbF6, 45% 10; AgOTs, 34% 10;
AgTFA, 27% 10, 28% 8. The formation of the iminium salt 10
from a metal carbene is a formal protonation of the carbene
carbon, a very unusual reaction for the electrophilic Fischer
carbenes involved in these reactions, as will be discussed
below.
Comparison of the Behavior of Group 11 Metals
toward Transmetalation. The experimental results de-
scribed above indicate a clear trend in the ease of trans-
metalation from tungsten to group 11 metals: Au > Cu > Ag.
The same trend is observed in the thermodynamic parameters
of the whole transmetalation processes, as shown in eqs 5 and
The reaction of 1a with [Cu(MeCN)4]PF6 led to hydrolysis
products (Scheme 4 and entry 3, Table 2) with no detection of
a cationic Cu(I) carbene intermediate. Similarly, no trace of
new carbenes was observed when the tungsten alkoxocarbene
1b was reacted with Cu(I) complexes (eq 4). The addition of
[Cu(MeCN)4]PF6 leads to decomposition to the hydrolysis
products, by the same routes depicted in Scheme 3. The
carbene dimerization derivative 11 (mixture of cis and trans
isomers) was obtained as the major product if [ClCu(tht)] was
used (eq 4 and entries 7 and 8, Table 2). It is known that some
Cu complexes,18b,19,28 as well as other transition-metal
derivatives,18a,29,30 promote this carbene homocoupling in
good to excellent yields.
No trace of a new metal carbene complex was observed by
direct transmetalation from the tungsten(0) carbenes 1 to
[AgCl(tht)]4 or AgBF4 in acetonitrile. Moreover, the
decomposition data in Table 2 shows that the presence of
[AgCl(tht)]4 does not affect the decomposition rate of 1a (cf.
entries 1 and 4, Table 2), and a similar behavior was observed
for the reaction of either [AgCl(tht)]4 or AgBF4 and 1b (cf.
entries 6, 9, and 10, Table 2). In contrast, the presence of
copper strongly affects the decomposition of the tungsten
derivatives 1 (cf. entries 1−3 and 6−8, Table 2), which points
to the transfer of the carbene fragment to Cu (observed in the
6, calculated by DFT and collected in Table 3 (see the
Experimental Section for details). The formation of the
dicoordinated group 11 carbene complex was considered in
all cases for comparison. Other coordination numbers were
also explored in the reagents ([M(MeCN)n]+) and products,
but the resulting energy values do not alter the observed trend
(see above and Tables S4 and S5 in the Supporting
Information). The transmetalation of the amino carbene
group from complexes 1a to both neutral and cationic group
11 metal complexes is thermodynamically more favored than
the transmetalation of the alkoxo carbene group from 1b. For
every combination of analogous reactants, the values in Table 3
show that, as far as thermodynamics is concerned, the ease of
transmetalation follows the trend Au > Cu > Ag. These results
are in agreement with the complete carbene transfer
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Table 3. Calculated Free Energies for the Formation of the
a
Group 11 Metal Carbenes
b
entry
X
[M]
ΔG (kcal mol−1
)
M-carb
1
2
3
4
5
6
7
8
NEt2
NEt2
NEt2
OMe
OMe
OMe
NEt2
NEt2
NEt2
OMe
OMe
OMe
[CuCl(tht)]
[AgCl(tht)]
[AuCl(tht)]
[CuCl(tht)]
[AgCl(tht)]
[AuCl(tht)]
[Cu(NCMe)2]+
[Ag(NCMe)2]+
[Au(NCMe)2]+
[Cu(NCMe)2]+
[Ag(NCMe)2]+
[Au(NCMe)2]+
−6
−0.23
−9.3
+1
+7.4
−2.9
−3.9
−2.1
−16.3
+4.3
+5.3
9
2a
2b
3a
3b
9
10
11
12
−9.2
a
b
DFT calculations according to eq 5 and 6. Isolated (2) or
Figure 2. Calculated bimetallic intermediates in the transmetalation
pathway (ΔG, kcal mol−1).
characterized in solution (3, 9).
Electrophilic Character of the Group 11 Metal
Carbenes and the Role of Silver in the Formation of
the Iminium Salt 10. The hydrolysis products observed for
the gold carbenes or the reactions of the copper derivatives
with 1 are consistent with the expected reactivity of an
electrophilic carbene: i.e., a carbene fragment bound to a late-
transition-metal fragment with no or very weak back-donation
ability. We calculated the three analogous cationic metal
carbenes [M{CPh(NEt2)}(NCMe)]+ (M = Cu, Ag, Au (3a)),
and indeed, the frontier orbitals for all three metal carbenes
show similar electron distributions. However, the decom-
position of 1a in the presence of AgBF4 leads cleanly to the
iminium salt 10, which is formally the result of the carbene
protonation, a reaction pathway unexpected for a putative
silver carbene. The generation of a free carbene by dissociation
from silver could produce a nucleophilic center amenable to
protonation. However, a less selective reaction should be
expected from such reactive species and DFT calculations
show a high dissociation energy, not very different from the
copper or tungsten carbenes which do not show that reactivity
pattern under the same conditions (see Table S6 in the
Supporting Information).
experimentally observed for gold in every case, whereas the
formation of a copper carbene complex is observed in only one
of the examples corresponding to the reaction in entry 1, Table
3. Nonetheless, the more reluctant formation of carbene 9 (M
= Cu, 10% in 24 h) in comparison to 2a (M = Au, complete
conversion in 30 min) must have a kinetic origin, since the free
energy values for both reactions are not too far apart (entries 1
and 3, Table 3). No silver carbene was detected and, as shown
in Table 2 (entries 4, 9, and 10), the extent and rate of
decomposition of the starting tungsten carbenes do not
indicate the intermediacy of a more reactive metal carbene.
One exception is the reaction of AgBF4 with 1a (entry 5, Table
2), which shows a distinct and faster decomposition pattern
and corresponds to the energetically most favorable trans-
metalation process calculated for silver (entry 8, Table 3).
Heterobimetallic intermediates with bridging carbenes have
been experimentally detected and/or isolated by Furstner et al.
̈
during the transmetalation of chromium or tungsten carbenes
to gold13b,20d and calculated by Sierra et al. for the
transmetalation of carbene fragments from chromium to
palladium, copper, or rhodium.18b Thus, we decided to
calculate the analogous heterobimetallic intermediates for the
transmetalation of 1a to [M(CH3CN)2]+, where M = Ag, Au.
This is the process that shows the lowest free energy among
those transmetalations that involve silver and also a distinct
reactivity pattern. The reactivity observed for the carbene
fragment could derive from a complete transfer of the carbene
to silver or from an arrested transmetalation to give a bimetallic
intermediate with a bridging carbene. The intermediates 1a-M
(M = Ag, Au) depicted in Figure 2 were found, where M
interacts with the carbenic carbon and one of the carbonyl
groups of the tungsten center. The Au−Ccarbene distance in the
calculated 1a-Au (2.784 Å) is longer than the Au−CO distance
(2.245 Å), indicating an important interaction of the gold
center with one of the carbonyl groups. In comparison with
this derivative, the intermediate 1a-Ag shows similar distances
between the metal and both fragments (Ag−Ccarbene, 2.594 Å;
Ag−C(O), 2.544 Å). The calculated energy values for 1a-M
(M = Au, Ag) are not very different but suggest again that the
transmetalation of the tungsten monoaminocarbene to silver is
less favored in comparison to gold since the silver
heterobimetallic intermediate is about 3 kcal mol−1 higher in
energy than the gold analogue.31
We carried out decomposition reactions of 1a in the
presence and absence of AgBF4 under different conditions to
corroborate that silver is playing a genuine role (eq 7 and
Table 4). Despite the plausible presence of water due to the
highly hygroscopic character of the silver salt, this is not
responsible for the reactivity observed by itself, since no trace
of the iminium salt was detected when water was deliberately
added to a solution of 1a (entry 3, Table 4). Water does not
accelerate the reaction in the presence of silver (entry 4, Table
4), and just an increase in the hydrolysis of the iminium salt to
8 was observed, as was tested independently. When D2O was
used, the deuterated aldehyde 8-D was observed, indicating
that the source of the proton in 10 is water.32 Only a couple of
reports can be found in the literature describing the
protonation of a tungsten carbene with HCl or HBr.33 Indeed,
the addition of HBF4·OEt2 to complex 1a led to the iminium
salt, a reaction that is still faster if AgBF4 is additionally present
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a
evolution of the silver species to the insoluble silver oxide can
provide the thermodynamic driving force of the process.
This means that a nucleophilic behavior for silver carbenes
Table 4. Decomposition Reactions of 1a According to Eq 7
conversion (%)
entry
MX
additive
conditions
10
8
might be possible. Furstner at al. have suggested this type of
̈
behavior for silver polymetallic complexes with bridging
nonstabilized diarylcarbene fragments, recently disclosed by
this group.35
1
2
3
4
5
6
7
8
9
6 days, 50 °C
26 h, 50 °C
26 h, 50 °C
20 h 50 °C
24 h, 25 °C
5 h, 50 °C
24 h, 50 °C
20 h, 50 °C
24 h, 50 °C
15
AgBF4
AgBF4
72
b
H2O
4
36
b
H2O
According to this, we also explored if the interaction of water
with bimetallic silver complexes could lead to the direct
protonation of the bridging carbene. No intermediate was
found from the interaction of H2O with the calculated
bimetallic Ag−W complex 1a-Ag (Figure 2). The calculated
dimeric complex [(MeCN)Ag{μ-CPh(NEt2)}Ag(NCMe)]2+
evolves in the presence of a water molecule to the monomeric
silver carbene [Ag{CPh(NEt2)}(NCMe)]+ and [Ag(NCMe)-
(H2O)]+ (see the Supporting Information for details).
HBF4
HBF4
67
90
AgBF4
ZnCl2
Zn(OTf)2
AgOTf
17
9
69
a
Reaction conditions: 0.022 mmol of 1a, 0.022 mmol of AgBF4 or
zinc salt, and 0.022 mmol of additive in 0.6 mL of CD3CN.
b
1
Conversions were determined by H NMR. Two drops of water.
CONCLUSION
■
(cf. entries 5 and 6, Table 4). According to this, the role of a
metal cation could be to increase the acidity of the water
present, by coordination, and therefore increase the decom-
position rate. However, the Lewis acidic character of silver
does not explain the formation of the iminium salt, since no
trace of 10 was observed when a Lewis acid of higher acidity
such as Zn(II) was added, even in the presence of water (cf.
entries 7−9, Table 4).34
These experiments show that silver is important to trigger
the decomposition observed. Since the carbene transmetalation
from 1a to a cationic acetonitrile silver complex is the
thermodynamically most favored process for this metal,
according to the data in Table 3, we explored the possibility
of the protonation of the putative silver carbene [Ag{CPh-
(NEt2)}(NCMe)]+ (c1) by water. An energetically accessible
pathway was found through a transition state that involves the
simultaneous interaction of water with silver and the carbene
carbon (Figure 3). The process has an activation energy of
25.3 kcal mol−1, which could be overcome under the reaction
conditions used: the formation of the iminum salt is a slow
reaction that requires heating for hours. The products formed
from TS1, 10 and Ag(OH)NCMe, are not very stable, but the
The reaction of tungsten(0) Fischer carbenes with neutral and
cationic complexes of the group 11 metals revealed different
behavior for each metal. Direct transmetalation to gold occurs
leading to the instantaneous formation of the carbene
complexes, and a series of neutral and cationic Au(I) carbenes
have been prepared and characterized. Transmetalation to
Cu(I) may occur, but only one copper monoaminocarbene
species has been detected during our studies due to their lower
stability. No experimental evidence of the formation of silver
carbenes was found. DFT calculations mirror the experimental
results and show that the formation of group 11 carbenes by
direct transmetalation of tungsten(0) Fischer carbenes is
thermodynamically favored for gold(I), possible for copper(I),
and endergonic for most silver(I) precursors.
The hydrolysis of the isolated gold carbenes reveals the
typical behavior of an electrophilic metal carbene. The same
type of reactivity is observed by an analysis of the carbene
decomposition products in the reactions of the tungsten(0)
carbenes with copper complexes.
On the other hand, the presence of silver does not induce
the decomposition of the tungsten methoxycarbene 1b in all
cases or the monoamino carbene for the neutral silver
precursors. This is an indirect indication that transmetalation
of the carbene fragment to silver is not taking place. However,
the reaction of the tungsten(0) monoaminocarbene 1a with
silver salts with weakly coordinating anions, and specially
AgBF4 in acetonitrile, leads to an unprecedented carbene
demetalation reaction by the formation of an iminium salt.
This is a formal protonation of the carbene acting as a
nucleophilic center. The transmetalation of the carbene from
1a to a silver acetonitrile complex is thermodynamically
favored, and we found by DFT calculations that the
protonation of the resulting putative cationic silver carbene
with water is possible, although energetically demanding. This
shows a distinct behavior for silver among group 11 metals and
the possibility of a mirror reactivity (carbene as a nucleophile)
for the elusive silver complexes with less stabilized carbenes.
EXPERIMENTAL SECTION
■
General Methods. 1H, 13C{1H}, and 31P{1H} NMR spectra were
recorded on Bruker AV-400, Agilent MR400, and Agilent MR500
spectrometers (LTI-UVa). Chemical shifts (in δ units, ppm) were
referenced to SiMe4 (1H and 13C) and H3PO4 (85%, 31P). The
spectral data were recorded at 293 K unless otherwise noted.
Heteronuclear 1H−13C HSQC and HMBC experiments were used to
Figure 3. Free energy profile for the protonation of a silver carbene by
water. Energies are given in kcal mol−1.
43
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Article
help with the signal assignments. All reactions were carried out under
an atmosphere of N2. Tungsten carbenes 1a36 and 1b,37 [AuCl-
(tht)],38 [Au(CH3CN)2]BF4,39 [Cu(CH3CN)4]PF6,40 [CuCl(tht)],41
Characterization of 5. A vial was charged with AgSbF6 (0.027 g,
0.077 mmol), and a solution of 2a (0.02 g, 0.051 mmol) in 0.6 mL of
CD3CN was added under an atmosphere of nitrogen at −30 °C. After
2 min, the tungsten carbene 1b (0.023 g, 0.051 mmol) was added to
the mixture. The yellowish suspension was filtered, transferred to a 5
mm NMR tube, and characterized by NMR at 293 K. It contained the
42
[AgBr(tht)]4, and [AgCl(tht)]4 were synthetized according to the
literature methods. Solvents were dried using an SPS PS-MD-5
solvent purification system prior to use. Silver salts, tetrabutylammo-
nium hydroxide (1 M solution in MeOH), tetrafluoroboric acid
etherate, zinc chloride, and zinc triflate are commercially available and
were purchased from commercial sources. The reactions using the
light-sensitive silver compounds were carried out in the absence of
light.
1
mixture 4a:4b:5 = 1.6:1:3.6 mol ratio. Data for 5 are as follows. H
NMR (400 MHz, δ, CD3CN): 8.32 (m, 2H, Hbortho), 7.90 (m, 1H,
Hbpara), 7.67 (m, 2H, Hbmeta), 7.55 (m, 2H, Hameta), 7.43 (m, 1H,
Hapara), 7.23 (m, 2H, Haortho), 4.99 (s, 3H, OCH3), 4.30 (q, J = 7.32
Hz, 2H, CH2CH3), 3.68 (q, J = 7.32 Hz, 2H, CH′2CH3), 1.62 (t, J =
7.32 Hz, 3H, CH2CH3), 1.25 (t, J = 7.32 Hz, 3H, CH2CH′3).
13C{1H} NMR (100.61 MHz, δ, CD3CN): 288.8 (Cbcarbene), 239.1
Synthesis of 2a. [AuCl(tht)] (0.26 g, 0.83 mmol) was added to a
solution of the tungsten carbene 1a (0.4 g, 0.83 mmol) in 30 mL of
CH2Cl2. The yellow solution was stirred for 20 min and filtered
through Celite and activated carbon. The solvent was evaporated to
ca. 2 mL, and pentane (5 mL) was added. The yellow solid was
filtered, recrystallized in CH2Cl2/pentane, and dried under vacuum
(Cacarbene), 143.8 (Caipso), 143.1 (Cbipso), 140.3 (Cbpara), 135.6
(Cbortho), 130.7 (Cbmeta), 130.2 (Cameta), 130 (Capara), 124.4 (Caortho),
72.5 (OCH3), 57.8 (CH2CH3), 50.1 (C′H2CH3), 15.6 (CH2CH3),
14.3 (CH2C′H3).
1
Detection and Characterization of 9. [CuCl(tht)] (0.023 g,
0.123 mmol) was added under an atmosphere of nitrogen to a
solution of 1a (0.015 g, 0.031 mmol) in 0.6 mL of CD3CN. The
yellowish solution was heated for 1 h at 50 °C and characterized by
NMR at 293 K. 1H NMR (400 MHz, δ, CD3CN): 7.43−7.32 (m, 3H,
Hmeta + Hpara), 7.02 (m, 2H, Hortho), 4.10 (q, J = 7.23 Hz, 2H,
CH2CH3), 3.50 (q, J = 7.23 Hz, 2H, CH′2CH3), 1.52 (t, J = 7.23 Hz,
3H, CH2CH3), 1.17 (t, J = 7.23 Hz, 3H, CH2CH′3). 13C{1H} NMR
(100.61 MHz, δ, CD3CN): 239.8 (Ccarbene), 144.3 (Cipso), 128.2
(Cpara), 129 (Cmeta), 122.8 (Cortho), 58.7 (CH2CH3), 47.7 (C′H2CH3),
15.4 (CH2CH3), 14.4 (CH2C′H3).
(0.28 g; 86% yield). H NMR (400 MHz, δ, CD3CN): 7.47 (m, 2H,
Hmeta), 7.36 (m, 1H, Hpara), 7.09 (m, 2H, Hortho), 4.23 (q, J = 7.25 Hz,
2H, CH2CH3), 3.53 (q, J = 7.25 Hz, 2H, CH′2CH3), 1.55 (t, J = 7.25
Hz, 3H, CH2CH3), 1.17 (t, J = 7.25 Hz, 3H, CH2CH′3). 13C{1H}
NMR (100.61 MHz, δ, CD3CN): 223.8 (Ccarbene), 144.7 (Cipso), 129.9
(Cmeta), 129.3 (Cpara), 123.8 (Cortho), 57.9 (CH2CH3), 49.2
(C′H2CH3), 14.6 (CH2CH3), 13.9 (CH2C′H3). Anal. Calcd for
C11H15AuClN: C, 33.56; H, 3.84; N, 3.56; Found: C, 33.66; H, 3.62;
N, 3.50.
Synthesis of 2b. This complex was synthesized using a procedure
similar to that described above for 2a but using the tungsten carbene
1
General Procedure for the Decomposition of the Carbene
Complexes. For gold complexes, a 5 mm NMR tube was charged
with the neutral Au(I) carbene (2a or 2b, 0.014 mmol) and 0.6 mL of
CD3CN, and finally stoichiometric amounts of the additive were
added if necessary. For copper and silver complexes, under a N2
atmosphere a 5 mm NMR tube was charged with the silver or copper
salt or complex (0.02−0.03 mmol), 0.6 mL of CD3CN, equimolar
amounts of the tungsten carbene (1a or 1b), and the additive when
needed. The decomposition products of the carbene fragment were
characterized by NMR by comparison with authentic samples (6−8).
The identity of 10 was independently corroborated by treatment of
PhCHNEt with (OEt3)BF41(see the Supporting Information).
Data for 10 are as follows. H NMR (500 MHz, δ, CD3CN): 8.83
(s, 1H, Himinium), 7.85 (m, 1H, Hpara), 7.83 (m, 2H, Hortho), 7.76 (m,
2H, Hmeta), 4.09 (q, J = 7.36 Hz, 2H, CH2CH3), 4.04 (q, J = 7.36 Hz,
2H, CH′2CH3), 1.51 (t, J = 7.36 Hz, 6H, CH2CH3 + CH2CH′3).
13C{1H} NMR (125.76 MHz, δ, CD2Cl2): 171.2 (Ciminium), 137.3
1b. Red solid (0.26 g; 88% yield). H NMR (400 MHz, δ, CD3CN):
8.35 (m, 2H, Hortho), 7.88 (m, 1H, Hpara), 7.62 (m, 2H, Hmeta), 4.94
(s, 3H, OCH3). 13C{1H} NMR (100.61 MHz, δ, CD3CN): 267.9
(Ccarbene), 143.2 (Cipso), 139.4 (Cpara), 136.0 (Cortho), 130.5 (Cmeta),
72.6 (OCH3). Anal. Calcd for C8H8AuClO: C, 27.26; H, 2.29;
Found: C, 27.33; H, 2.32.
Characterization of 3a and 4a. A vial was charged with AgSbF6
(0.04 g, 0.116 mmol), and a solution of 2a (0.03 g, 0.076 mmol) in
0.6 mL of CD3CN was added under an atmosphere of nitrogen at 243
K. The gray suspension was warmed to room temperature for 30 min
and filtered through a pad of Celite. The colorless solution, containing
complexes 3a and 4a in a 20:1 mole ratio, was introduced in a 5 mm
NMR tube and characterized by NMR. Data for 3a are as follows. 1H
NMR (400 MHz, δ, CD3CN, 243 K): 7.51 (m, 2H, Hmeta), 7.41 (m,
1H, Hpara), 7.12 (m, 2H, Hortho), 4.22 (q, J = 7.33 Hz, 2H, CH2CH3),
3.62 (q, J = 7.33 Hz, 2H, CH′2CH3), 1.55 (t, J = 7.33 Hz, 3H,
CH2CH3), 1.19 (t, J = 7.33 Hz, 3H, CH2CH′3). 13C{1H} NMR
(100.61 MHz, δ, CD3CN, 243 K): 216.9 (Ccarbene), 143.1 (Cipso),
130.1 (Cpara), 130 (Cmeta), 124.2 (Cortho), 58.9 (CH2CH3), 49.7
(C′H2CH3), 14.9 (CH2CH3), 14 (CH2C′H3). Data for 4a are as
follows. 1H NMR (400 MHz, δ, CD3CN, 243 K): 7.46 (m, 2H,
Hmeta), 7.38 (m, 1H, Hpara), 7.05 (m, 2H, Hmeta), 4.18 (q, J = 7.31 Hz,
2H, CH2CH3), 3.57 (q, J = 7.31 Hz, 2H, CH′2CH3), 1.52 (t, J = 7.31
Hz, 3H, CH2CH3), 1.17 (t, J = 7.31 Hz, 3H, CH2C′H3). 13C{1H}
NMR (100.61 MHz, δ, CD3CN, 243 K): 239 (Ccarbene), 143.7 (Cipso),
129.9 (Cpara), 129.5 (Cmeta), 123.9 (Cortho), 57.5 (CH2CH3), 49.7
(C′H2CH3), 15.2 (CH2CH3), 14.1 (CH2C′H3).
(Cpara), 132.6 (Cortho), 130.4 (Cmeta), 125.7 (s, Cipso), 57.1 (CH2CH3),
49.3 (s, C′H2CH3), 12.5 (s, CH2CH3 + CH2C′H3).
Computational Methods. The DFT studies were performed
with the M06 functional,43,44 as implemented in the Gaussian09
program package.45 The 6-31+G(d) basis set was used for C, O, S, Cl,
N, and H,46,47 and LANL2TZ(f) for Cu, Ag, A, and W48,49 (basis set
I). Solvation was introduced in all optimizations, and frequency
calculations and potential energy refinement were carried out through
the SMD model, where we applied the experimental solvent,
acetonitrile, as the solvent (ε = 27.9). All structure optimizations
were carried out in the solvent phase with no symmetry restrictions.
Free energy corrections were calculated at 298.15 K and 105 Pa
pressure, including zero-point energy corrections (ZPE), and the
energies were converted to 1 M standard state in solution (adding/
subtracting 1.89 kcal mol−1 for nonunimolecular processes). Vibra-
tional frequency calculations were performed in order to confirm that
the stationary points were minima (without imaginary frequencies) or
transition states (with one imaginary frequency). The connectivity of
the transition state structure was confirmed by relaxing the transition
state geometry toward both the reactant and the product. Final
potential energies were refined by performing additional single-point
energy calculations (also in solution); Cu, Ag, Au, and W were still
described with the LANL2TZ(f) basis set, and the remaining atoms
were treated with the 6-311++G(d,p) basis set (basis set II). All
energies presented correspond to free energies in solution, obtained
Characterization of 3b and 4b. A vial was charged with AgSbF6
(0.03 g, 0.087 mmol), and a solution of 2b (0.02 g, 0.057 mmol) in
0.6 mL of CD3CN was added under an atmosphere of nitrogen at 243
K and stirred for 10 min. The yellowish solution was transferred to a 5
mm NMR tube and the mixture (3b:4b = 10:1 mol ratio)
characterized by NMR at 243 K. Data for 3b are as follows. 1H
NMR (500 MHz, δ, CD3CN, 243 K): 8.31 (m, 2H, Hortho), 7.92 (m,
1H, Hpara), 7.64 (m, 2H, Hmeta), 4.94 (s, 3H, OCH3). 13C{1H} NMR
(125.76 MHz, δ, CD3CN, 243 K): 264.6 (Ccarbene), 141.5 (Cipso),
140.2 (Cpara), 135.9 (Cortho), 130.2 (Cmeta), 73.4 (OCH3). Data for 4b
are as follows. 1H NMR (500 MHz, δ, CD3CN, 243 K): 8.42 (m, 2H,
Hortho), 7.93 (m, 1H, Hpara), 7.70 (m, 2H, Hmeta), 5.10 (s, 3H, OCH3).
13C{1H} NMR (125.76 MHz, δ, CD3CN, 243 K): 287.4 (Ccarbene),
142.2 (Cipso), 140 (Cpara), 135.4 (Cortho), 130.2 (Cmeta), 72.2 (OCH3).
44
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̈
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00675.
■
sı
Additional experimental data, selected spectra, and
computational details (PDF)
Cartesian coordinates of the calculated structures (XYZ)
Accession Codes
CCDC 2038426 and 2038428 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
́
Ana C. Albeniz − IU CINQUIMA/Química Inorgánica,
Universidad de Valladolid, 47071 Valladolid, Spain;
orcid.org/0000-0002-4134-1333; Email: albeniz@
qi.uva.es
Authors
Alberto Toledo − IU CINQUIMA/Química Inorgánica,
Universidad de Valladolid, 47071 Valladolid, Spain
Vanesa Salamanca − IU CINQUIMA/Química Inorgánica,
Universidad de Valladolid, 47071 Valladolid, Spain;
orcid.org/0000-0001-6470-3934
́
(6) Diaz-Requejo, M. M.; Perez, P. J. Coinage Metal Catalyzed C-H
Bond Functionalization of Hydrocarbons. Chem. Rev. 2008, 108,
́
3379−3394. (b) Caballero, A.; Perez, P. J. Catalyst design in the
alkane C-H bond functionalization of alkanes by carbene insertion
with TpxM complexes (Tpx = hydrotrispyrazolylborate ligand; M =
Cu, Ag). J. Organomet. Chem. 2015, 793, 108−113. (c) Fructos, M.;
́
Teresa Perez-Moro − IU CINQUIMA/Química Inorgánica,
Universidad de Valladolid, 47071 Valladolid, Spain
́
Diaz-Requejo, M. M.; Perez, P. J. Gold and diazo reagents: a fruitful
tool for developing molecular complexity. Chem. Commun. 2016, 52,
7326−7335.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00675
(7) (a) Wang, C.; Ye, F.; Wu, C.; Zhang, Y.; Wang, J. Construction
of All-Carbon Quaternary Centers through Cu- Catalyzed Sequential
Carbene Migratory Insertion and Nucleophilic Substitution/Michael
Addition. J. Org. Chem. 2015, 80, 8748−8757. (b) Che, J.; Xing, D.;
Hu, W. Metal-Catalyzed Cross-Coupling of Terminal Alkynes with
Different Carbene Precursors. Curr. Org. Chem. 2015, 20, 41−60.
(8) Julian, R. R.; May, J. A.; Stoltz, B. M.; Beauchamp, J. L. Gas-
Phase Synthesis of Charged Copper and Silver Fischer Carbenes from
Diazomalonates: Mechanistic and Conformational Considerations in
Metal-Mediated Wolff Rearrangements. J. Am. Chem. Soc. 2003, 125,
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Author Contributions
The manuscript was written through contributions of all
authors./All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the financial support of the Spanish MINECO
(SGPI, grants CTQ2016-80913-P, PID2019-111406GB-I00
and BES-2014-067770 fellowship to A.T.) and the Junta de
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M.; Perez, P. J.; Maseras, F. Mechanism of Side Reactions in Alkane C
H Bond Functionalization by Diazo Compounds Catalyzed by Ag and
Cu Homoscorpionate ComplexesA DFT Study. ChemCatChem
2011, 3, 1646−1652. (b) Besora, M.; Braga, A. A. C.; Sameera, W. M.
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Castilla y Leon (grant VA062G18).
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