Rh-POP pincer Xantphos complexes for C-S and C-H activation. Implications for carbothiolation catalysis

Article abstract of DOI:10.1021/om500984y

The neutral Rh(I)-Xantphos complex [Rh(κ³-_P,O,P-Xantphos)Cl]_n, 4, and cationic Rh(III) [Rh(κ³-_P,O,P-Xantphos)(H)₂][BAr^F₄], 2a, and [Rh(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)(H)₂][BAr^F₄], 2b, are described [Ar^F = 3,5-(CF₃)₂C₆H₃; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C₆H₃(CF₃)₂ = 9,9-dimethylxanthene-4,5-bis(bis(3,5-bis(trifluoromethyl)phenyl)phosphine]. A solid-state structure of 2b isolated from C₆H₅Cl solution shows a κ¹-chlorobenzene adduct, [Rh(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)(H)₂(κ¹-ClC₆H₅)][BAr^F₄], 3. Addition of H₂ to 4 affords, crystallographically characterized, [Rh(κ³-_P,O,P-Xantphos)(H)₂Cl], 5. Addition of diphenyl acetylene to 2a results in the formation of the C-H activated metallacyclopentadiene [Rh(κ³-_P,O,P-Xantphos)(ClCH₂Cl)(σ,σ-(C₆H₄)C(H)=CPh)][BAr^F₄], 7, a rare example of a crystallographically characterized Rh-dichloromethane complex, alongside the Rh(I) complex mer-[Rh(κ³-_P,O,P-Xantphos)(η²-PhCCPh)][BAr^F₄], 6. Halide abstraction from [Rh(κ³-_P,O,P-Xantphos)Cl]_n in the presence of diphenylacetylene affords 6 as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ³-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh(κ³-_P,O,P-Xantphos)]⁺ fragment and facilitates ortho-directed C-S activation in a number of 2-arylsulfides to give mer-[Rh(κ³-_P,O,P-Xantphos)(σ,κ¹-Ar)(SMe)][BAr^F₄] (Ar = C₆H₄COMe, 8; C₆H₄(CO)OMe, 9; C₆H₄NO₂, 10; C₆H₄CNCH₂CH₂O, 11; C₆H₄C₅H₄N, 12). Similar C-S bond cleavage is observed with allyl sulfide, to give fac-[Rh(κ³-_P,O,P-Xantphos)( η³-C₃H₅)(SPh)][BAr^F₄], 13. These products of C-S activation have been crystallographically characterized. For 8 in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of fac-κ³-8, which then proceeds to isomerize to the mer-isomer. With the para-ketone aryl sulfide, 4-SMeC ₆H₄COMe, C-H activation ortho to the ketone occurs to give mer-[Rh(κ³-_P,O,P-Xantphos)(σ,κ¹-4-(COMe)C₆H₃SMe)(H)][BAr^F₄], 14. The temporal evolution of carbothiolation catalysis using mer-κ³-8, and phenyl acetylene and 2-(methylthio)acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed fac-isomer of the C-S activation product is considerably more active than the mer-isomer (i.e., mer-8), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data.

Full text of DOI:10.1021/om500984y

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Rh−POP Pincer Xantphos Complexes for C−S and C−H Activation.
Implications for Carbothiolation Catalysis
†
Peng Ren, Sebastian D. Pike, Indrek Pernik, Andrew S. Weller,* and Michael C. Willis
Department of Chemistry, Chemistry Research Laboratories, Mansfield Road, University of Oxford, Oxford, OX1 3TA, U.K.
*
S Supporting Information
ABSTRACT: The neutral Rh(I)−Xantphos complex [Rh-
3
3
(
κ - -Xantphos)Cl] , 4, and cationic Rh(III) [Rh(κ -
-
P,O,P
P,O,P
n
F
3
Xantphos)(H) ][BAr ], 2a, and [Rh(κ - -Xantphos-3,5-
C H (CF ) )(H) ][BAr ], 2b, are described [Ar = 3,5-
6
2
4
P,O,P
F
F
3
3 2
2
4
(
CF ) C H ; Xantphos = 4,5-bis(diphenylphosphino)-9,9-
3 2 6 3
dimethylxanthene; Xantphos-3,5-C H (CF ) = 9,9-dimethyl-
6
3
3 2
xanthene-4,5-bis(bis(3,5-bis(trifluoromethyl)phenyl)-
phosphine]. A solid-state structure of 2b isolated from C H Cl
6
5
1
3
solution shows a κ -chlorobenzene adduct, [Rh(κ -P,O,P-
Xantphos-3,5-C H (CF ) )(H) (κ -ClC H )][BAr ], 3. Ad-
1
F
6
3
3 2
2
6
5
4
3
dition of H to 4 affords, crystallographically characterized, [Rh(κ - -Xantphos)(H) Cl], 5. Addition of diphenyl acetylene to
2
P,O,P
2
3
2
a results in the formation of the C−H activated metallacyclopentadiene [Rh(κ - -Xantphos)(ClCH Cl)(σ,σ-(C H )C(H)
P,O,P
2
6
4
F
CPh)][BAr ], 7, a rare example of a crystallographically characterized Rh−dichloromethane complex, alongside the Rh(I)
4
3
2
F
3
complex mer-[Rh(κ - -Xantphos)(η -PhCCPh)][BAr ], 6. Halide abstraction from [Rh(κ - -Xantphos)Cl] in the
P,O,P
4
P,O,P
n
presence of diphenylacetylene affords 6 as the only product, which in the solid state shows that the alkyne binds perpendicular to
3
3
+
the κ -POP Xantphos ligand plane. This complex acts as a latent source of the [Rh(κ - -Xantphos)] fragment and facilitates
P,O,P
3
1
F
ortho-directed C−S activation in a number of 2-arylsulfides to give mer-[Rh(κ - -Xantphos)(σ,κ -Ar)(SMe)][BAr ] (Ar =
P,O,P
4
C H COMe, 8; C H (CO)OMe, 9; C H NO , 10; C H CNCH CH O, 11; C H C H N, 12). Similar C−S bond cleavage is
6
4
6
4
6
4
2
6
4
2
2
6
4
5
4
3
3
F
observed with allyl sulfide, to give fac-[Rh(κ - -Xantphos)(η -C H )(SPh)][BAr ], 13. These products of C−S activation
P,O,P
3
5
4
have been crystallographically characterized. For 8 in situ monitoring of the reaction by NMR spectroscopy reveals the initial
3
formation of fac-κ -8, which then proceeds to isomerize to the mer-isomer. With the para-ketone aryl sulfide, 4-SMeC H COMe,
C−H activation ortho to the ketone occurs to give mer-[Rh(κ - -Xantphos)(σ,κ -4-(COMe)C H SMe)(H)][BAr ], 14. The
temporal evolution of carbothiolation catalysis using mer-κ -8, and phenyl acetylene and 2-(methylthio)acetophenone substrates
shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed fac-isomer
of the C−S activation product is considerably more active than the mer-isomer (i.e., mer-8), the latter of which is formed rapidly
by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data.
6
4
3
1
F
P,O,P
6
3
4
3
1. INTRODUCTION
The transition-metal chemistry associated with diphosphine,
and related, pincer ligands is significant to many areas of
organometallic chemistry, catalysis, and materials science. This
is due to the attractive properties that such ligands give the
resulting metal complexes, for example: stability, electronic and
steric variability, and, in some cases, metal−ligand coopera-
1
−5
tivity.
based around “PCP” and “PNP”-type ligand scaffolds (Figure
). These provide relatively rigid ligand environments, that
Central to many of these studies have been systems
Figure 1. M(PCP), M(PNP), and M(POP) metal−ligand motifs.
1
3
9
often result in mer-κ -coordination geometries at the metal
center. A less explored class of pincer ligand comes from POP-
frameworks based upon the Xantphos-motif (Xantphos = 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene, Figure 1),
that although originally developed to be cis-chelating bidentate
example in carbonylation (A), activation of H
/alkenes (B−
2
10−12
13
14
D),
silylation (E), hydroamination (F), hydroacylation
(G), and amine−borane dehydrocoupling and hydroboration
15
16−18
6
,7
processes (H).
We have recently reported the use of the [Rh(Xantphos)]+
catalyst fragment in carbothiolation reactions between alkynes
3
8
phosphine ligands can also act as mer-κ or, less commonly,
3
9
fac-κ POP-pincer ligands. Recently there has been significant
interest in the chemistry associated with Xantphos acting as a
POP-pincer ligand to the group 9 metals (Figure 2), for
Received: September 29, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/om500984y
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Article
elevated temperatures, e.g. coupling phenyl acetylene and 2-
(
methylthio)acetophenone at 373 K in chlorobenzene or
dichloroethane solvent. In addition to this atom-efficient C−
C and C−S bond formation process, the resulting alkenyl
sulfide can be used for further functionalization, i.e. functional
group recycling. This Xantphos system delivers improved yields
and conversion times compared to our previously reported
20
DPEphos system [cf. Xantphos 99%, 2 h; DPEphos 90%, 24
h, Scheme 1, DPEphos = oxidi-2,1-phenylene-bis-
(
diphenylphosphine)]. For the DPEphos catalyst, in situ
NMR spectroscopic studies identified the C−S activated
3
3
intermediates fac-κ -I and mer-κ -J to be in equilibrium with
3
one another at 298 K, while at 333 K mer-κ -J was the observed
20
resting-state during catalysis.
Transition-metal-catalyzed C−S activation processes are
becoming increasingly important in synthesis, especially
21
22−26
their use in cross coupling reactions.
In this contribution
we describe a study of the C−S activation processes occurring
F
in the [Rh(Xantphos)][BAr ] system. We also report a new,
4
readily accessible, Rh(I)−Xantphos precursor complex which
has a labile alkyne ligand; report C−H activation processes
mediated by this fragment; and also make comment upon the
likely active species in carbothiolation catalysis.
3
2. RESULTS AND DISCUSSION
Figure 2. Representative examples of κ -Xantphos complexes.
2
.1. Synthesis of Rh(I) Precursor Complexes. A suitable
and ketone-bearing aryl sulfides to produce alkenyl sulfide
products (Scheme 1). These catalyst systems operate best at
entry point into the C−S activation chemistry would be a Rh(I)
precursor that can promote oxidative addition of the ketosulfide
substrate. Previously in the DPEphos system we used the ortho-
xylene-ligated [Rh(κ - -DPEphos)(o-xylene)][BAr ] as a
suitable precursor. This was generated by addition of H to
19
2
F
Scheme 1. Carbothiolation Using Rh−DPEphos and
P,P
4
a
Xantphos Catalysts
2
the corresponding NBD complex in o-xylene (NBD =
20
norbornadiene). Noting that our recent report of the alkyne
+
carbothiolation catalyzed by [Rh(Xantphos)] used chloroben-
zene as a solvent, we targeted an appropriate precursor that
involved this as a ligand. Addition of H to previously reported
2
2
F
15
cis-[Rh(κ - -Xantphos)(NBD)][BAr ], 1a, in C H Cl
P,P
4
6
5
solvent resulted in the quantitative formation of a Rh(III)
3
F
dihydride complex [Rh(κ - -Xantphos)(H) ][BAr ], 2a, as
P,O,P
2
4
1
31
1
characterized by in situ H and P{ H} NMR spectroscopy
Scheme 2), which is analogous to recently reported variants
(
t
11
i
11
Xantphos- Bu or Xantphos- Pr . Complex 2a could not be
isolated as a solid. However, treatment of the 3,5-C H (CF )
6
3
3 2
P-substituted Xantphos analog of 1a (complex 1b) with H in
2
3
C H Cl solution led to the isolation of [Rh(κ -P,O,P-Xantphos-
,5-C H (CF ) )(H) (κ -ClC H )][BAr ], 3, as a white
6
5
1
F
3
6 3 3 2 2 6 5 4
crystalline solid in 79% yield. The solid-state structure of the
cation as determined by single crystal X-ray diffraction is shown
in Figure 3.
a
F

4
[
BAr ] anions are not shown.
a
Scheme 2. Synthesis of Rh(III) Dihydride Xantphos Complexes
a
F

4
[
BAr ] anions are not shown.
B
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Article
Figure 3. Solid-state structure of the cations in complexes 3 (left) and 5 (right). Displacement ellipsoids are shown at the 50% probability level. Most
hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg): (3): Rh1−P1, 2.2673(12); Rh1−P2, 2.2707(13); Rh1−Cl1,
2
2
.5207(12); Rh1−O1, 2.249(3); P1−Rh1−P2, 161.03(4); O1−Rh1−C1, 90.16(8). (5): Rh1−P1, 2.2591(10); Rh1−P2, 2.2779(11); Rh1−Cl1,
.4625(11); Rh1−O1, 2.248(3); P1−Rh1−P2, 163.85(4); O1−Rh1−C1, 86.87(8).
The solid-state structure of 3 shows a cis-dihydride species
As noted recently by Goldman, the formation of a Rh(III)
1
(
the hydrides were located in the difference map), with a κ -
dihydride complex over a Rh(I) species on addition of H could
2
3
chlorobenzene [Rh−Cl 2.5207(12) Å] and a mer-κ -Xantphos
well be due to the relatively weak trans-influence of the POP−
ligand. Complex 3 is closely related to similar compounds in
ether ligating group in Xantphos that favors the product of
1
5
16
11
which acetone, MeCN, or H B·NMe
replace the
3
3
oxidative addition of H2. In addition to this, wide-bite angle
ligands such as Xantphos will tend to favor Rh(III)
chlorobenzene ligand, and the gross structural metrics are
similar to these adducts. In the H NMR spectrum of complex
1
28
(
dihydrides) over Rh(I) species. Thus, avoiding the use of
3
in C D Cl solution, a single environment, indicated by a
6 5
H in the production of the precursor for C−S activation would
2
broad signal at δ −17.7, is observed for the hydride ligand, and
thus be advantageous in generating a Rh(I) precursor. To
accomplish this, addition of Xantphos to [Rh(COE) Cl]
2
a single Xantphos-methyl signal (6 H) is observed. A single
1
03
2
environment showing coupling to Rh [δ 45.3, J(RhP) = 118
(
COE = cyclooctene) in C H6 solution resulted in the
3
1
1
6
Hz] is also observed in the P{ H} NMR spectrum. These
data suggest a time-averaged C2v symmetry for the metal
fragment, likely approximated to a 5-coordinate “Y-shaped”
formation of an insoluble (benzene) material that we
tentatively characterize as the Rh(I) complex [Rh(Xantphos)-
3
F
Cl] , 4, presumably formed as a coordination polymer with
n
[
Rh(κ - -Xantphos-3,5-C H (CF ) )(H) ][BAr ], i.e. 2b.
P,O,P 6 3 3 2 2 4
bridging halide ligands (Scheme 3), based upon its insolubility.
This material analyzed correctly for composition by elemental
When complex 3 is dissolved in CD Cl , a signal assigned to
2
2
free C H Cl is observed, in addition to very similar NMR
6
5
analysis. Dissolution in CH Cl2 resulted in a mixture of,
spectroscopic data to 2a in CH Cl solution. Complexes 2a/b
2
2
2
uncharacterized, products. Interestingly, the analogous com-
in solution, or 3 in the solid state, do not lose H on application
2
−3
of a vacuum (∼10 Torr). Cooling of 3 (183 K, CD Cl ) did
not result in a H NMR spectrum that reflects the solid-state
2
2
1
Scheme 3. Scheme Relating the Synthesis of Complexes 4, 5,
a
6
, and 7
structure (i.e., inequivalent hydrides were not observed). This is
3
F
15
in contrast to [Rh(κ - -Xantphos)(H) (acetone)][BAr ],
P,O,P
2
4
in which the hydrides are equivalent at 298 K but inequivalent
at low temperature, which is suggested to be due to reversible
decoordination of acetone. Given these data, we suggest that
complex 3 exists in C H Cl or CD Cl solution predominantly
6
5
2
2
3
F
as [Rh(κ -
-Xantphos-3,5-C H (CF ) )(H) ][BAr ], 2b,
P,O,P
6 3 3 2 2 4
but can be recrystallized from C H Cl solvent to give the
6
5
solvent adduct 3. We discount a time-averaged six-coordinate
complex with trans-hydrides as this would likely have a
chemical shift for the hydride ligands at higher field than is
3
t
observed [e.g., [Rh(κ - -Xantphos- Bu)(H) (OH )][SbF ] δ
P,O,P
2
2
6
1
1
−
9]. Complex 3 adds to the very small number of
crystallographically characterized κ -chlorobenzene com-
plexes. We cannot comment on whether a similar complex
would be formed with 2a, as we have been unable to produce
crystalline material suitable for analysis.
1
2
7
a
F

4
[BAr ] anions are not shown.
C
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Article
Figure 4. Solid-state structure of complexes 6 (left) and 7 (right) showing the cations only. Displacement ellipsoids are shown at the 50% probability
level. Most hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg): (6): Rh1−P1, 2.2825(12); Rh1−P2, 2.2896(12); Rh1−
C46, 2.114(4); Rh1−C47, 2.094(4); Rh1−O1, 2.138(3); C46−C47, 1.256(6); P1−Rh1−P2, 166.65(4); O1−Rh1−C46, 166.73(15); O1−Rh1−
C47, 158.46(15). (7): Rh1−P1, 2.3036(11); Rh1−P2, 2.2969(10); Rh1−C40, 2.010(4); Rh1−C47, 2.020(4); Rh1−O1, 2.223(3); C45−C56,
1
9
.447(7); C46−47, 1.348(7); Rh1−Cl200, 2.393(7); Cl200−C201, 1.788(7); C201−Cl202, 1.781(7); P1−Rh1−P2, 163.83(4); O1−Rh1−C40,
4.81(14); O1−Rh1−C47, 176.11(15). Only the major disordered competent of the CH Cl ligand is shown.
2
2
t
11
i
12
plexes with Xantphos− Bu or Xantphos− Pr are soluble in
benzene and shown to be monomers in the solid state. This
presumably reflects the differing steric demands of the P-
substituents. Suspension of complex 4 in toluene and addition
C−S activation chemistry, which is described in the next
section.
An alternative methodology for generating complex 6 is
addition of H (1 atm) to complex 1a in CH Cl solution to
2
2
2
of H resulted in the formation of the neutral, soluble, Rh(III)
form complex 2a, evacuation, followed by addition of a CH Cl
2 2
solution of diphenylacetylene (Scheme 5). Under these
2
3
cis-dihydride complex Rh(κ -
-Xantphos)(H) Cl, 5, ob-
P,O,P
2
tained in 79% yield (Scheme 3) as colorless crystals. The
conditions of excess alkyne, cis-stilbene is also observed to be
formed by H NMR spectroscopy [δ 6.57]. Also produced is a
parallel product, identified by NMR spectroscopy and single
1
solid-state structure of 5 is shown in Figure 3 and is very closely
t
11
related to recently reported Rh−Xantphos− Bu and Ir−
i
12
3
Xantphos− Pr analogues (i.e., B Figure 2). The NMR
spectroscopic data for 5 are in full accord with the solid-state
structure and show inequivalent hydride environments in the
crystal X-ray diffraction as [Rh(κ - -Xantphos)(ClCH Cl)-
P,O,P
2
F
(σ,σ-(C H )C(H)CPh)][BAr ], 7. Complex 7 is formed as
6
4
4
a major product relative to 6 under these conditions (relative
ratio 2:1 of 7 to 6). Although single crystals of complex 7 could
be produced in this manner, in bulk form it was always
contaminated with some 6.
1
H NMR spectrum, in contrast to complex 3. The formation of
5
in good yield from simple addition of H to 4 further
2
supports the latter’s formulation.
Removal of the chloride ligand in 4 is easily achieved by
The solid-state structure of 7 (Figure 4) shows a trans-
stilbene-derived ligand that has additionally undergone C−H
activation to give a metallacyclopentadiene [Rh−C 2.010(4)
and 2.020(4) Å]. Similar activation of diphenylactylene has
treatment of a suspension in 1,2-C H F solvent with
6
4 2
F
Na[BAr ] in the presence of a slight excess of diphenylactylene
to yield the Rh(I) complex mer-[Rh(κ - -Xantphos)(η -
PhCCPh)][BAr ], 6, as orange crystalline material in good
4
3
2
P,O,P
31,32
F
been reported at Rh and Ir centers,
and the structural
4
(
82%) yield on work-up. The solid-state structure of the cation
metrics previously reported for these metallacyclopentadienes
are similar to 7. The coordination sphere of the metal is
in complex 6 is shown in Figure 4, and this shows the alkyne to
be bound perpendicular to the Rh−POP plane (angle between
planes = 90.7°). The C−C distance [1.256(6) Å] is lengthened
completed by a coordinated CH
Å]. Complexes in which CH Cl
established, and a handful of examples with Rh have been
Cl
ligand [Rh−Cl, 2.393(7)
2
2
acts as ligand are now well−
2
2
2
9
33
4−37
compared to free ligand [1.206(2) Å ]. The NMR
spectroscopic and microanalytical data support this formula-
tion; for example, a single environment is observed at δ 20.4 [d,
3
1
31
1
reported.
(CD Cl
The H and P{ H} NMR data for complex 7
solution) are in full accord with the solid-state
2
2
3
1
1
1
J(RhP) = 124 Hz] in the P{ H} NMR spectrum, while one,
relative integral 6 H, signal is observed for the Xantphos methyl
groups. There are surprisingly few pseudo square planar
complexes reported which have pincer ligands and bound
alkynes or alkenes. One example is (PCP)Ir(trans-1,4-phenyl-
structure. In particular, the H NMR spectrum shows
inequivalent Xantphos methyl groups, and signals that can be
attributed to the metallocycle unit. Signals due to the
coordinated CH Cl ligand were not observed, but it is likely
2 2
33
that this ligand is in rapid exchange with the bulk solvent. The
3
t
31
1
but-3-ene-1-yne) [PCP = (κ -C H -2,6-(CH P Bu ) )], which
P{ H} NMR spectrum displays a single environment as a
6
3
2
2 2
doublet, showing coupling to 1 Rh consistent with coordina-
tion with a Rh(III) center [J(RhP) = 119 Hz] and a mirror−
plane of symmetry in solution.
03
also adopts a similar coordination mode with the metal−pincer
fragment to 6 and shows a similar C−C distance [1.270(4)
3
0
Å]. Complex 6 represents an excellent starting point into the
D
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4
0
The observation that complex 7 is preferentially formed over
complex 6 when complex 2a is formed in situ suggests that this
latter Rh(III) complex is central to the C−H activation process
to form 7. Complex 6 does not undergo C−H activation to
form 7 (Scheme 4), supporting this hypothesis. The C−H
(sigma−complex assisted metathesis), loss of H , and
2
coordination of CH Cl to form 7.
2
2
2.2. C−S Activation Chemistry. With a suitable Rh(I)
precursor to C−S activation in hand in complex 6, reactivity
with a variety of arylsulfides was explored (Figure 5). Given the
a
Scheme 4. Reactivity of Complex 7
Figure 5. Arylsulfides used in this study.
a
F

4
[
BAr ] anions are not shown.
use of the substrate I [2-(methylthio)acetophenone] in our
recently reported carbothiolation chemistry,
1
9,20
initial studies
activation process can be reversed. Complex 7 reacts with H (1
atm) slowly (12 h, CD Cl ) to form complex 2a as determined
by P{ H} NMR spectroscopy. No cis-stilbene was observed to
be formed [δ 6.57, CDCl ] under these conditions, and when
the reaction was performed with D , signals at δ 7.25 and 7.57
were observed in the H NMR spectrum, consistent with
2
focused upon using this starting material. Addition of I to a
toluene solution of 6 resulted, at 298 K, in the formation of a
mixture of two isomers, identified spectroscopically as fac- and
2
2
31
1
3
3
1
F
mer-[Rh(κ -
-Xantphos)(σ,κ -C H COMe)(SMe)][BAr ],
P,O,P
6 4 4
2
2
8, Scheme 6. These isomers were initially formed in a 20:1 ratio
respectively after 20 min and equilibrated over time to give κ -
mer-8 as the thermodynamic product after 32 h. This process
3
1
formation of trans-PhCHC(D)(C H D) [lit. H δ 7.19, 7.60
6
4
3
8
CDCl3 ]. d -2a was also observed to be formed by a broad
2
2
may be accelerated by heating to 373 K for 40 min, after which
signal at ∼δ −18 in the H NMR spectrum. This shows that
under conditions exogenous of H , complex 7 can be recycled
back to form 2a, but only slowly.
3
time κ -mer-8 can be isolated in good (70%) yield as the only
2
product. In both cases free diphenylacetylene is also observed
to be formed. The 31P{ H} NMR spectrum of κ -fac-8 shows
two environments, each with a distinctive doublet of doublet
coupling pattern, that show two very different magnitudes of
coupling to 1 Rh: δ 34.8 [J(RhP) = 150, J(PP) = 17 Hz] and δ
1
3
Scheme 5. Suggested Mechanism for the Formation of
Complex 7 .
a
03
2
2.6 [J(RhP) = 104, J(PP) = 17 Hz]. Such a pattern is
31
characteristic of P environments in which one sits opposite to
a high trans influence ligand (such a SMe) while one sits
3
9,41
1
opposite a weaker ligand, i.e. the ketone.
The H NMR
spectrum shows two signals at δ 1.56 and 1.43 (both of relative
integral 3 H) as doublets [J = 7.6 and 1.2 Hz, respectively].
These are assigned to the SMe and ketone−methyl group,
respectively, and the larger coupling to 31P in the former
(
confirmed by selective decoupling experiments) is fully
20,39
consistent with its trans disposition to a phosphine.
Although we have not obtained a solid-state structure for κ -
fac-8, similar complexes have been crystallographically charac-
3
a
F

[
BAr ] anions are not shown. Xantphos is abbreviated POP.
4
1
0,39,41
3
terized with DPEphos.
The NMR data for κ -mer-8 are
also fully consistent with its formulation; notably, a single
environment is observed in the P{ H} NMR spectrum
31
1
A suggested mechanism for the formation of complex 7 is
shown in Scheme 5. Addition of H to 1a forms the dihydride
[J(RhP) = 106 Hz], and the SMe [δ 0.87] and ketone−methyl
2
1
complex 2a, which then undergoes coordination and hydride
insertion with diphenylacetylene to give the corresponding cis-
diphenylvinyl intermediate. The pathway then bifurcates:
Elimination of cis-stilbene (observed) and subsequent coordi-
nation of diphenylacetylene eventually give 6. Alternatively a
cis−trans isomerization (possibly via a metallacyclopropene
groups [δ 1.99] are observed as singlets in the H NMR
spectrum; consistent with their now cis-relationship with the
phosphines. Two methyl environments are observed for the
3
Xantphos ligand. The solid-state structure of κ -mer-8 is shown
in Figure 6. In the unit cell there are two crystallographically
distinct cations, for which the structural metrics are very similar.
3
1,39
3
intermediate)
followed by C−H activation at the ortho-
The structure demonstrates the κ -mer geometry of the final
position of arene occurs, likely via a sigma−CAM process
product, in which the thiomethyl and aryl groups sit mutually
E
DOI: 10.1021/om500984y
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Article
a
Scheme 6. Formation of fac-8 and mer-8
a
F

4
[
BAr ] anions are not shown.
Scheme 7. Products of Reaction with the 2-
a,b
Arylmethylsulfides Used in This Study
Figure 6. Solid-state structure of complex mer-8 showing the cation
only. Only one of the crystallographically independent cations is
shown. Displacement ellipsoids are shown at the 50% probability level.
Hydrogen atoms are not shown. Selected bond lengths (Å) and angles
(
2
1
deg): Rh1−P1, 2.2983(16); Rh1−P2, 2.3068(16); Rh1−O1,
.102(3); Rh1−O2, 2.215(3); Rh1−S1, 2.3373(14); Rh1−C40,
.975(5); P1−Rh−P2, 165.22(5); O1−Rh1−P1, 90.08(10); O1−
Rh1−S1, 173.00(10).
cis with one another [Rh1−C40, 1.975(5); Rh1−S1,
a
b
F
4

2
.3373(14) Å]. The remaining coordination site on the Rh(III)
CH Cl at 298 K. [BAr ] anions are not shown.
2 2
center is occupied by a dative bond from the ortho substituent
on the arene [Rh1−O1, 2.102(3) Å]. The thiomethyl group sits
syn to one phosphine and anti to the other, and it is likely that
low energy libration around the Rh−S bond gives the cation
spectroscopy, ESI−MS, microanalysis, and X-ray crystallog-
raphy (apart from complex 9, in which extensive disorder of the
cation precluded a reliable analysis). Solid-state structures of
the cations in complexes 10 and 11 are shown in Figure 7
(complex 12 is presented in the Supporting Materials) and are
closely related to that of mer-8. For the structures 10−12, the
activated thio−aryl ligand is equally disordered between two
orientations in which the SMe and ortho-coordinating groups
have swapped positions, giving the overlaid disordered
structures C2v symmetry in the solid state. Although this
disorder could be modeled satisfactorily, this does mean that
caution should be exercised in the interpretation of the fine
details of the structural metrics around the metal. Nevertheless,
the structures confirm the formulation.
time-averaged C symmetry in solution.
s
Exchange between isomers of Xantphos, in which the
phosphines adopt a cis or trans geometry, has been noted
1
0,42,43
previously, and in some cases this is reversible.
For the
3
F
system fac/mer-[Rh(κ -
-Xantphos)(PCyp )(H )][BAr ],
P,O,P
3 2 4
equilibrium measurements indicate that the fac isomer is
preferred and that although enthalpically there is little
difference between the isomers, entropic considerations
determine the final position of the equilibrium (ΔS is positive
10
for the generation of the fac isomer). On heating mer-8 (373
31
K, C D CD ), we saw no evidence, to the detection limit of P
6
5
3
NMR spectroscopy, for fac-8. However, we cannot discount its
formation at a very low equilibrium concentration under these
conditions.
Using the protocol for the isolation of pure C−S activated
product developed above, the 2-arylmethylsulfides II−V were
combined with complex 6 to afford complexes 9−12 in good
yields (66%−78%), all as the mer-isomers, Scheme 7. These
red/brown complexes have been characterized by NMR
When allyl phenyl sulfide VI is combined with complex 6,
3
3
the C−S activated product fac-[Rh(κ -
-Xantphos)(η -
P,O,P
F
C H )(SPh)][BAr ], 13, is isolated as a green crystalline
3
5
4
solid in 60% yield, Scheme 8. The solid-state structure of 13 is
3
shown in Figure 8 and demonstrates the κ -fac geometry of the
Xantphos ligand [Rh1−O1, 2.200(2) Å, P−Rh−P 109.54(3)°]
3
and the η coordination mode of the allyl group at the Rh(III)
F
DOI: 10.1021/om500984y
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Organometallics
Article
Figure 7. Solid-state structure of complexes 10 (left) and 11 (right) showing the cations only. Only one disordered component of each is shown.
Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg): (10):
Rh1−P1, 2.3143(7); Rh1−O1, 2.200(2); Rh1−O2, 2.133(3); Rh1−S1, 2.3074(10); Rh1−C10, 1.967(4); P1−Rh−P1′, 165.07(3); O1−Rh1−P1,
8
2.690(17); O2−Rh1−S1, 175.13(8). (11): Rh1−P1, 2.3060(9); Rh1−O1, 2.209(4); Rh1−N3, 2.173(5); Rh1−S2, 2.296(4); N3−C48, 1.397(7);
C48−C47, 1.441(6); C47−O3, 1.463(7); O3−C46, 1.339(6); C46−N3, 1.323(5); P1−Rh−P1′, 165.42(5); O1−Rh1−P1, 82.72(2); N3−Rh1−S1,
1
77.54(18).
a
Scheme 8. Synthesis of Complexes 13 and 14
in these C−S activations, Scheme 8. The product of this
3
1
reaction is mer-[Rh(κ -
−Xantphos)(σ,κ -C H (COMe)-
P,O,P
6
3
F
SMe)(H)][BAr ], 14, isolated in moderate yield (38%) as
4
colorless crystalline material, for which NMR spectroscopy and
a single crystal X-ray diffraction study showed it to be the
product of C−H activation ortho to the directing ketone group.
1
The H NMR data for 14 show a single hydride environment at
δ −14.99 (relative integral 1 H) as a doublet of triplets and the
absence of a methyl resonance below δ 1 that would be
31
1
indicative of a Rh−SMe group. The P{ H} NMR spectrum
displays a single environment showing coupling to a Rh(III)
center [δ 39.5, J(RhP) = 114 Hz]. These data suggest C−H
activation has occurred rather than C−S activation, and the
solid-state structure derived from a single crystal X-ray analysis
confirms this, Figure 8. This shows that the 4-(methylthio)-
acetophenone has undergone C−H activation ortho to the
ketone to afford a cis-hydridoaryl motif, with the hydride ligand
located in the final Fourier difference map. The rhodium’s
coordination sphere is completed by coordination of the ketone
through a dative bond, similar to that observed for the C−S
activated products. Such reactivity is directly related to the
directed activation of ortho-C−H bonds first reported by
a
F

4
[
BAr ] anions are not shown.
center. The molecule has, noncrystallographically imposed,
pseudomirror symmetry. The solution NMR data for 13 are
47
Murai, in which arylketones underdo hydroarylation with
48
3
1
consistent with this, with one P environment observed
J(RhP) = 132 Hz], while the allyl ligand is revealed by three
signals in a 1:2:2 ratio [multiplets centered at δ 4.80, 3.99,
alkenes. Complex 14 does not react further with alkynes, for
[
example phenyl acetylene.
2.3. Alkyne Trimerization. Complex 6 does not react
further with diphenylacetylene. However, it does react slowly
(20 h, 298 K) with phenylacetylene to afford the products of
2
.64]. Oxidative addition of allyl sulfides to late transition
44,45
metals is known.
coordination of the alkene followed by oxidative cleavage of the
C−S bond. Very few examples of crystallographically
characterized Xantphos (or DPEphos) complexes that also
contain allyl ligands have been reported, and all have a κ -P,P
geometry of the chelate ligand, different from the κ -POP
observed for complex 13.
Presumably this process occurs via initial
cyclotrimerization, 1,2,3-Ph C H and 1,2,4-Ph C H , as
2 6 3 3 6 3
4
4
1
identified by H NMR spectroscopy and GC−MS. These are
formed in a 2:1 ratio (Scheme 9). Complex mer-8 will also
promote this process, but only at elevated (373 K) temperature,
and there is no reaction at 298 K. This latter reaction
presumably occurs via initial carbothiolation to release the
2
46
3
1
The reaction of (4−methylthio)acetophenone VII with
alkenyl sulfide product (observed by H NMR spectroscopy),
+
complex 6 probes the requirement for a ortho-directing group
and the resulting [Rh(Xantphos)] fragment is then free to
G
DOI: 10.1021/om500984y
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Article
Figure 8. Solid-state structure of complexes 13 (left) and 14 (right) showing the cations only. Displacement ellipsoids are shown at the 50%
probability level. Most hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg), (13): Rh1−P1, 2.2847(7); Rh1−P2, 2.3337(7);
Rh1−O1, 2.200(2); Rh1−S1, 2.3393(8); Rh1−C1, 2.190(3); Rh1−C2, 2.197(3); Rh1−C3, 2.249(3); C1−C2, 1.408(5); C2−C3, 1.388(5); P1−
Rh1−P2, 109.54(3); O1−Rh1−S1 166.44(6). (14): Rh1−P1, 2.2699(6); Rh1−P2, 2.2729(6); Rh1−O1, 2.1861(17); Rh1−O2, 2.1609(18); Rh1−
C1, 1.968(3); P1−Rh1−P2, 166.45(2); O1−Rh1−P1, 83.53(5); O1−Rh1−C1, 175.74(9).
Scheme 9. Phenyl Acetylene Trimerization As Catalyzed by
Complexes 6 and mer-8
mediate cyclotrimerization. These trimerization products are
also observed during carbothiolation catalysis but in low relative
yield (less than 20%), meaning that although this process is
competitive with carbothiolation in this system, use of a slight
excess of alkyne (2 equiv) means that full conversion of the
ketone is achieved.
Figure 9. Time conversion plot for the catalytic reaction of I (0.1 M)
and phenylactylene (0.2 M). [cat.] = 6 or mer-8 0.005 M, solvent =
C D CD , Temperature = 373 K. Both reactions reach completion.
6
5
3
Lines of best fit are for visual comparison only.
2
.4. Catalysis. Under room temperature conditions (298
consistent with the initial formation of a very active catalyst that
turns over the carbothiolation rapidly but quickly forms a less
active catalyst. We suggest, based on our stoichiometric
experiments, that this can be explained by the initial formation
of fac-8. In principle this could decoordinate its POP ether
linkage relatively easily to allow access to the alkyne to the now
16-electron metal center. Such potentially hemilabile behavior
has been noted for related Xantphos complexes on reaction
with MeCN. Isomerization to the presumably less active mer-
8, shown to be rapid at 373 K, results in a slower rate of
catalysis. Consistent with this, mer-8 is the only observed
organometallic species during catalysis, as the resting state,
while starting from mer-8 results in no rapid initial burst of
activity, but only slow turnover at a rate broadly similar to that
for the slow regime when starting from 6. Whether catalysis in
K) complex 6 will only slowly catalyze the carbothiolation of I
with phenyl acetylene (55% conversion at 24 h). However,
heating to 373 K results in faster turnover (6 h) with full
conversion to the alkenyl sulfide product. Complex mer-8 was
the only observed organometallic product during this catalysis
1
[
by 31P{ H} NMR spectroscopy]. Starting from mer-8, the
reaction is considerably slower (16 h, 373 K), and again, mer-8
is the only observed species during catalysis. Following these
reactions by in situ NMR spectroscopy during the early stages
of catalysis using 1,3-dimethoxybenzene as internal standard
Figure 9) revealed the temporal profiles for the formation of
the product for both starting pre-catalysts. For catalysts using
10
(
complex 6, initial very rapid catalysis (∼15% conversion after
0 min) is then followed by a much slower rate. These data are
2
H
DOI: 10.1021/om500984y
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Article
the slow regime occurs by a small (undetectable by 31P NMR
spectroscopy) equilibrium concentration of fac-8 or an
alternative active species is not clear. A suggested pathway for
catalysis is shown in Scheme 10. This has close similarities for
synthetic and catalytic studies. Demonstrating the utility of 6 in
this regard, we have shown that it readily undergoes ketone-
directed C−S and C−H activation processes with aryl−
ketosulfides. Additionally we have also reported rare examples
of Rh−chlorobenzene and Rh−dichloromethane complexes.
The temporal evolution of catalysis using 6 shows initial fast
catalysis and then and then much slower turnover, and we
suggest that the fac-isomer of the C−S activation product (i.e.,
fac-8) is considerably more active than the mer-isomer (i.e.,
mer-8), the latter being formed rapidly by isomerization. A
future strategy for optimizing this catalyzed carbothiolation
reaction might well be to limit the flexibility of the ligand set so
that this isomerization does not occur, and studies are currently
ongoing to study the effect of incorporating such ligands into
the catalyst structure.
Scheme 10. Suggested Catalytic Cycle for the Homogenous
Carbothiolation Catalysis Using the [Rh(Xantphos)]+
Catalyst
4
. EXPERIMENTAL SECTION
General Experimental Procedures. All manipulations, unless
otherwise stated, were performed under an argon atmosphere using
standard Schlenk and glovebox techniques. Glassware was oven-dried
at 130 °C overnight and flamed under vacuum prior to use. Pentane,
dichloromethane, benzene, and toluene were dried using a Grubbs
type solvent purification system (MBraun SPS-800) and degassed by
5
2
successive freeze−pump−thaw cycles. Benzene, toluene, C H F, and
6
5
1
,2-F C H were, where appropriate, further dried over CaH , vacuum
2
6
4
2
distilled, and stored over 3 Å molecular sieves. Chlorobenzene (less
than 0.005 H O) was purchased from Aldrich and degassed by
2
successive freeze−pump−thaw cycles and stored over 3 Å molecular
F
53
54
55
2
sieves. Na[BAr ], [Rh(COE) Cl] , [Rh(NBD)Cl] , [Rh(κ -
-
4
2
2
2
P,P
F
15
2
0
Xantphos)(NBD)][BAr ] (1a), (9,9-dimethyl-9H-xanthene-4,5-
4
56
those suggested for alkyne carbothiolation and alkyne
diyl)bis(3,5-bis(trifluoromethyl)phenyl)phosphine, 2-(methylthio)-
3
9
+
hydroacylation using the [Rh(DPEphos)] system. We have
observed related intermediates previously in alkyne hydro-
20
20
acetophenone, 4-(methylthio)acetophenone, 2-(2-(methylthio)-
phenyl)-4,5-dihydrooxazole,
pyridine were prepared by literature methods. NMR spectra were
recorded on a Bruker Avance III HD nanobay 400 MHz (Hg400) and
Bruker DPX200 spectrometer at room temperature, unless otherwise
57
and 2-(2-(methylthio)phenyl)-
+
39,41
5
8
acylation systems with [Rh(DPEphos)] fragments.
Given the scatter present in the data (Figure 9), we are
reluctant to speculate further the likely rate-determining steps
or the order of reaction, and although the broad temporal
profile was reproducible (e.g., mer-8 versus 6), there was
significant variation between repeat runs with regard to the
approximate rate in the slow regime. Tests for homogeneous/
1
31
stated. H NMR spectra are referenced to residual solvent signals.
P
NMR spectra were referenced against 85% H PO (external).
Chemical shifts (δ) are quoted in ppm and coupling constants (J)
3
4
in Hz. ESI-MS were recorded on a Bruker micrOTOF instrument
5
9
49
interfaced with a glovebox. Microanalysis was performed at London
Metropolitan University. Reverse phase HPLC analysis was performed
on a Ascentis Express C18, 2.7 μm column (4.6 mm × 100 mm),
heterogeneous behavior indicated a homogeneous catalyst
Hg-test, sub-stoichiometric phosphine, microfiltration tests; a
(
clear red solution is observed during catalysis). However, we
cannot completely rule out the presence of a small amount of
colloidal Rh-catalyst, or decomposition to a different active
species. Notably the Hg-test still showed an initial burst of
reaction rate followed by slower turnover, consistent with our
mechanistic hypothesis. It is also interesting to note that when
catalysis is performed in dichloroethane solvent at 353 K
eluted by gradient MeCM/H O. GC-MS was performed on a Waters
GCT ToF mass spectrometer.
Synthesis of 2-Nitrothioanisole. To a stirred solution of 1-
fluoro-2-nitrobenzene (0.268 mg, 1.9 mmol) in DMF (10 mL) was
added a sodium thiomethoxide 21% aqueous solution (2.2 mmol).
After the addition was complete, the solution was stirred at room
2
temperature for 2 days. Then 100 mL H O was added to precipitate
2
the product. The solvent was decanted off and the resulting solid was
recrystallized (dichloromethane/petrol) to give the sulfide as pure
(
rather than toluene at 373 K), a very different profile is
observed characteristic of a greater than zero order process: the
1
product (167 mg, 52%). The H NMR data is consistent with the
reactions are much faster and the reaction solutions are
60
reported values.
19
observed to be black. Whether this indicates a change from
homogeneous to heterogeneous catalysis under these different
conditions is as yet unclear, but it is notable that nanoparticle
formation has been shown to be strongly dependent on solvent
1
H NMR (200 MHz, CDCl ). δ 8.26 (dd, J = 8.2 Hz, J = 1.4 Hz, 1H),
3
7
.59 (ddd, J = 8.2 Hz, J = 7.0 Hz, J = 1.4 Hz, 1H), 7.37 (d, J = 7.6 Hz,
1H), 7.25 (ddd, J = 8.2 Hz, J = 7.2 Hz, J = 1.4 Hz 1H), 2.50 (s, 3H).
2
Synthesis of New Complexes: cis-[Rh(κ - -Xantphos-3,5-
P,P
5
0,51
F
C H (CF ) )(NBD)][BAr ] (1b). Xantphos-3,5-C H (CF ) (243 mg,
polarity.
6
3
3 2
4
6
3
3 2
0
.217 mmol) in CH Cl2 (13 mL) was added dropwise to
2
[
Rh(NBD)Cl]2 (50 mg, 0.108 mmol) in CH Cl2 (1 mL). The
3
. CONCLUSIONS
2
F
resulting reaction mixture was then added dropwise to Na[BAr ]
(
for 2 h. The solution was filtered, and the filtrate solvent was reduced
to ∼5 mL. Then the solution was layered with pentane (20 mL). The
solution was stored overnight at −18 °C to give the recrystallization
product. Yield = 80%, 378 mg.
3
-Xantphos)(η2-
4
We report the synthesis of mer-[Rh(κ -
PhCCPh)][BAr ], 6, which acts as a latent source of the
P,O,P
192 mg, 0.217 mmol) in CH Cl (1 mL) and the mixture was stirred
F
2 2
4
3
+
[
Rh(κ - -Xantphos)] fragment. That this complex comes
P,O,P
3
from simple halide abstraction from [Rh(κ -P,O,P-Xantphos)Cl]n
suggests that this might well be a useful methodology for future
I
DOI: 10.1021/om500984y
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Article
1
H NMR (400 MHz, CD Cl ). δ 8.01−7.97 (m, 4H, Ar−H), 7.91 (d, J
diffraction were obtained by layering the benzene solution of the title
compound with hexane at room temperature.
2
2
F
=
7.6 Hz, 2H, Ar−H), 7.76−7.69 (m, 8H, BAr ), 7.64−7.54 (m, 8H,
4
F
4
1
Ar−H), 7.54 (s, 4H, BAr ), 7.47 (t, J = 8.0 Hz, 2H, Ar−H), 6.98−
H NMR (400 MHz, C D ). δ 8.34−8.22 (m, 4H, Ar−H), 7.88−7.76
6 6
6
.92 (m, 2H, Ar−H), 4.04 (s, 4H, NBD CHCH), 3.79 (s, 2H, NBD
(m, 4H, Ar−H), 7.44−7.33 (m, 2H, Ar−H), 7.11−6.97 (m, 8H, Ar−
H), 6.95−6.85 (m, 6H, Ar−H), 6.76 (t, J = 7.6 Hz, 2H Ar−H), 1.25 (s,
CH), 1.87 (s, 6H, Xantphos Me), 1.48 (s, 2H, NBD CH ).
2
3
1
1
1
P{ H} NMR (160 MHz, CD Cl ). δ 17.5 [d, J = 148 Hz].
3H, Xantphos CH ), 1.15 (s, 3H, Xantphos CH ), −15.06 - −15.47
3 3
2
2
RhP
+
ESI-MS (Acetone) Positive Ion. [M] m/z = 1317.06 (calc.
317.06).
Microanalysis. C H BF OP Rh (2180.88) requires: C 47.36, H
.03; found: C 47.50, H 1.95.
(m, 1H, Rh−H), −18.60 to −18.99 (m, 1H, Rh−H).
3
1
1
1
1
2
P{ H} NMR (160 MHz, C
Microanalysis. C39 34ClOP
found: C 65.33, H 4.66.
6
D
6
). δ 37.3 [d, JRhP = 117 Hz].
Rh (719.00) requires: C 65.15, H 4.77;
2
H
86
44
48
2
3
F
3
2
F
[
Rh(κ -
-Xantphos)(H) ][BAr ] (2a). The title compound was
mer-[Rh(κ -P,O,P-Xantphos)(η -PhCCPh)][BAr
containing 4 (175 mg), Na[BAr ] (212 mg, 0.24 mmol, 1 equiv.
4
] 6. To a flask
P,O,P
2
4
F
2
prepared in situ by placing a solution of [Rh(κ -P,P-Xantphos)(NBD)]-
BAr ] (10 mg, 6.1 × 10 mol) in CD Cl (0.3 mL) under 4 atm of
4
F
−6
relative to Rh), diphenyl acetylene (51 mg, 0.29 mmol, 1.2 equiv.
relative to Rh) was added 15 mL 1,2-difluorobenzene, the resulting
solution was stirred for 3 h at room temperature. The solution was
filtered, and the filtrate was concentrated to dryness under vacuum, the
resulting residue washed with pentane, then recrystallized by diffusion
of pentane into a dichloromethane solution of the crude product at
[
4
2
2
hydrogen. The compound was characterized in situ by NMR
spectroscopy and ESI-MS. There was no change to the NMR
spectrum on removal of the solvent to dryness and redissolving in
CD Cl .
2
2
1
F
H NMR (300 MHz, CD Cl ). δ 7.84−7.38 [m, 38H, Ar−H + BAr ;
2
2
4
F
4
F
4
−18 °C to yield orange crystals. Yield = 82%, 340 mg.
7
2
.72 (8H, BAr ), 7.55 (4H, BAr ), 1.73 (br s, 6H, CH ), −18.28 (dt,
3
1
H NMR (400 MHz, CD Cl ). δ 7.82 (d, J = 7.6 Hz, 2H, Ar−H),
H, J(RhH) = 33, J(PH) = 12, Rh−H).
2
2
F
F
3
1
1
7.74−7.71 (m, 8H BAr ), 7.55 (br, 4H BAr ), 7.45 (d, J = 7.6 Hz, 4H
P{ H} NMR (122 MHz, CD Cl ). δ 41.50 (d, J(RhP) = 117).
4
4
2
2
+
Ar−H), 7.40 (t, J = 8.0 Hz, 2H Ar−H), 7.33−7.24 (m, 14H Ar−H),
.15 (t, J = 7.2 Hz, 10H Ar−H), 7.02 (t, J = 7.6 Hz, 4H Ar−H), 1.86
(s, 6H, Xantphos CH ).
ESI-MS (fluorobenzene, 60 °C, 4.5 kV) positive ion. [M] m/z =
83.1250 (calc. 683.1134).
7
6
3
1
mer-[Rh(κ -
-Xantphos-3,5-C H (CF ) )(H) (κ -ClC H )]-
3
P,O,P
6
3
3 2
2
6
5
31
1
1
F
P{ H} NMR (160 MHz, CD Cl ). δ 20.4 [d, J = 124 Hz].
[
BAr ] (2b/3). 1b (50 mg, 0.023 mmol) was dissolved in
2 2 RhP
4
+
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 859.18 (calc. 859.18).
chlorobenzene (5 mL), then the solution was degassed and reacted
with hydrogen for 30 min under 4 atm. Then the solution was
degassed again and layered with hexane (10 mL). The solution was
stored overnight at −18 °C to give white solid. Yield = 79%, 40 mg.
Crystals of 3 suitable for X-ray diffraction were obtained from
chlorobenzene solution in situ.
2
2
Microanalysis. C H BF OP Rh (1722.99) requires: C 59.25, H
85 54
24
2
3
.16; found: C 59.17, H 3.05.
3
mer-[Rh(κ -
-Xantphos)(ClCH Cl)(σ,σ-(C H )C(H)CPh)]-
BAr ] (7). 1a (246 mg, 0.15 mmol) was dissolved in dichloro-
4
P,O,P
2
6
4
F
[
methane (5 mL), and the solution was exposed to H for 30 min (1
2
1
atm). Then the solution was degassed by evaporation to dryness,
diphenylacetylene (107 mg, 0.60 mmol) in CH Cl (5 mL) added, and
H NMR (400 MHz, CD Cl ). δ 8.21 (s, 4H), 8.10−7.98 (br, 5H,
2
2
2
2
C H Cl), 7.96 (dd, J = 7.6 Hz, J = 1.2 Hz, 2H), 7.74−7.66 (m, 8H,
6
5
F
F
the mixture was stirred for 1h. The solution was evaporated and
washed with hexane, The solid product was recrystallized with
dichloromethane and hexane at −18 °C. Total yield =70%, 180 mg.
BAr ), 7.62 (t, J = 7.6 Hz, 2H), 7.53 (s, 4H, BAr ), 7.50−7.42 (m,
4
4
2
2
H), 7.37−7.22 (m, 8H), 1.80 (s, 6H, Xantphos Me), −17.89 (br. s,
H).
3
The two products 6 and [Rh(κ -POP-Xantphos)(ClCH Cl)(σ,σ-
3
1
1
1
2
P{ H} NMR (160 MHz, CD Cl ). δ 44.0 [d, J = 119 Hz].
F
2
2
RhP
(
C H )C(H)CPh)][BAr ] 7 were observed by NMR spectrocopy,
6
4
4
The following NMR spectroscopic data in C D Cl was obtained by
placing a solution of 1b (8 mg) in D-chlorobenzene (0.4 mL) under 4
6
5
the ratio being 1:2 respectively. Crystals of 7 suitable for X-ray
diffraction were obtained from dichloromethane solution with pentane
at −18 °C.
atm of hydrogen, and characterized in situ by NMR spectroscopy.
1
F
H NMR (500 MHz, C D Cl). δ 8.23−8.17 (m, 8H, BAr ), 8.15−
1
6
5
4
H NMR (400 MHz, CD Cl ). δ 8.00 (d, J = 7.6 Hz, 2H), 7.77−7.69
F
2
2
8
7
.00 (m, 8H), 7.85 (s, 4H), 7.56 (s, 4H, BAr ), 7.43−7.40 (m, 2H),
F
4
4
(
6
m, 8H BAr ), 7.60−7.13 (m, 25H), 7.00 (t, J = 7.6 Hz, 4H), 6.79−
.38−7.32 (m, 2H), 7.09 (t, J = 7.5 Hz, 2H), 1.33 (s, 6H, Xantphos
.70 (m, 4H), 6.48 (t, J = 7.6 Hz, 1H), 6.38 (d, J = 2.8 Hz,, 1H), 6.21
d, J = 7.2 Hz, 1H), 6.13 (t, J = 7.6 Hz, 1H), 6.05−6.01 (m, 1H), 2.11
s, 3H, Xantphos CH ), 1.70 (s, 3H, Xantphos CH ). (coordinated
Me), −17.74 (br. s, 2H). (coordinated D-chlorobenzene is not
(
(
observed).
3
1
1
1
3
3
P{ H} NMR (200 MHz, C D Cl). δ 45.3 [d, J = 118 Hz].
6
5
RhP
dichloromethane was not observed.)
The 31P{1H} and H NMR spectra of the product in CH Cl at 183
K are essentially unchanged from that in CD Cl at 298 K.
Microanalysis. C H BClF OP Rh (2180.88) requires: C 46.34,
1
31
1
1
2
2
P{ H} NMR (160 MHz, CD Cl ). δ 27.2 [d, J = 119 Hz].
2
2
RhP
+
2
2
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 859.17 (calc. 859.18).
2 2
3
1
F
85
43
48
2
[Rh(κ -
-Xantphos)(σ,κ -C H COMe)(SMe)][BAr ] (8). A
P,O,P 6 4 4
H 1.97; found: C 46.47, H 1.91.
Rh(Xantphos)Cl]n (4). To a J. Youngs tube containing [Rh-
COE) Cl] (305 mg, 0.425 mmol) and Xantphos (492 mg, 0.85
−3
solution of 6 (8 mg, 4.6 × 10 ) and 2-(methylthio)acetophenone
in D-toluene (0.5 mL) was prepared in a J. Young NMR tube and
analyzed in situ by NMR spectroscopy at room temperature. A mixture
[
(
2
2
mmol) was added 10 mL benzene, sealed, and heated to 80 °C for 4 h.
The solution changed color to dark red, followed by product
precipitation. The resulting precipitate was filtered and washed with
pentane. The precipitate was dried under vacuum to yield a brick-red
powder. Yield = 91%, 555 mg. Complex 4 was insoluble in benzene,
acetone, and reacts with dichloromethane to give an unidentified
mixture. Toluene can be replaced for benzene in the procedure with
no significant change in yield.
Microanalysis. C H ClOP Rh requires: C 65.33, H 4.50; found:
C 65.41, H 4.37.
mer-[Rh(κ -
formed by placing a solution of 4 (50 mg, 7.0 × 10 mmol) in toluene
5 mL) under an atmosphere of H (1 atm). The solution was stirred
for 16 h at room temperature. The solution was degassed and was
reduced to ∼1 mL, 10 mL hexane was added to the solution to
precipitate the product, the solid product washed with hexane and
dried in vacuo. Yield = 79%, 40 mg. The crystals suitable for X-ray
3
1
F
of mer-[Rh(κ -POP-Xantphos)(σ,κ -C H COMe)(SMe)][BAr ]
6
4
4
3
1
(
[
mer-8) and fac-[Rh(κ -POP-Xantphos)(σ,κ -C H COMe)(SMe)]-
BAr ] (fac-8) was formed in the ratio of 1:20 after 20 min. The
6 4
F
4
ratio of fac-8 increased continually, so that after 32 h fac-8 was the only
observed Rh complex.
mer-8 can be isolated in pure form with the following procedure:
To a J. Youngs tube containing 6 (258 mg, 0.15 mmol), 1-(2-
(methylthio)phenyl)ethanone (55 mg, 0.33 mmol) was added 5 mL
toluene, sealed and the resulting solution was stirred for 40 min at 100
°C. The solution was concentrated to dryness under vacuum, the
resulting residue washed with pentane, then recrystallized by diffusion
of pentane into a 1,2-difluorobenzene solution of the crude product at
−18 °C to yield orange crystals. Yield = 70%, 180 mg.
39
32
2
3
-Xantphos)(H) Cl] (5). The title compound was
2
P,O,P
−2
(
2
1
Data for mer-8. H NMR (400 MHz, CD Cl ). δ 8.06 (d, J = 8.0 Hz,
2
2
1H, Ar−H), 7.93 (dd, J = 7.6 Hz, J = 2.0 Hz 2H, Ar−H), 7.82−7.76
F
(m, 4H Ph), 7.74−7.70 (m, 8H BAr ), 7.62−7.47 (m, 10H Ar−H +
4
F
BAr ), 7.47−7.39 (m, 5H Ar−H), 7.33 (t, J = 7.6 Hz, 2H Ph′), 7.26
4
J
DOI: 10.1021/om500984y
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
6
1
dd, J = 8.0 Hz, J = 1.6 Hz, 1H, Ar−H), 7.18−7.11 (m, 5H), 6.71−
mL chlorobenzene, sealed and the resulting solution was stirred for 3 h
at 80 °C. The solution was concentrated to dryness under vacuum, the
resulting residue washed with pentane, then recrystallized by diffusion
of pentane into a dichloromethane and toluene mixture solution of the
crude product at −18 °C to yield red crystals. Yield = 69%, 18 mg.
.64 (m, 4H, Ph′) 1.99 (s, COMe 3H), 1.78 (s, Xantphos CH 3H),
3
.32 (s, Xantphos CH 3H), 0.87 (s, SMe 3H).
3
3
1
1
1
P{ H} NMR (160 MHz, CD Cl ). δ 23.8 [d, J = 106 Hz].
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 847.14 (calc. 847.14).
2
2
RhP
+
2
2
Microanalysis. C H BF O P RhS (1710.99) requires: C 56.16, H
1
80
54
24
2
2
H NMR (400 MHz, CD Cl ). δ 8.05 (d, J = 7.6 Hz, 1H), 7.96 (dd, J
2
2
3
.18; found: C 55.93, H 3.16.
=
7.2 Hz, J = 1.6 Hz 2H), 7.91−7.82 (m, 4H), 7.76−7.70 (m, 8H
4
1
Data for fac-8. H NMR (400 MHz, D-Toluene). δ 8.37−6.09 (m,
F
BAr ), 7.64−7.12 (m, 20H), 7.12−7.03 (m, 3H), 6.82 (t, J = 7.6 Hz,
4
1
2H), 1.56 (d, J = 7.6 Hz, 3H, SMe), 1.48 (s, Xantphos CH , 3H),
3
4
H), 6.43−6.34 (m, 4H, Ph), 1.92 (s, Xantphos Me 3H), 1.90 (s,
.43 (d, J = 1.2 Hz, 3H), 1.39 (s, Xantphos CH , 3H).
3
3
1
1
1
= 150 Hz, 2J
Xantphos Me 3H), 0.88 (s, SMe 3H).
P{ H} NMR (160 MHz, D-Toluene). δ 34.8 [dd, J
RhP
PP
31
1
1
1
2
P{ H} NMR (160 MHz, CD
Cl
2
2
). δ 20.1 [d, JRhP = 105 Hz].
=
17 Hz], 22.6 [dd, J = 104 Hz, J = 17 Hz].
RhP PP
+
3
1
F
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 882.16 (calc. 882.16).
2
2
mer-[Rh(κ -
-Xantphos)(σ,κ -C H COOMe)(SMe)][BAr ]
P,O,P
6
4
4
Microanalysis. C H BF NOP RhS (1746.04) requires: C 57.10,
(9). To a J. Youngs tube containing 6 (100 mg, 0.06 mmol), methyl 2-
83 55
24
2
H 3.19, N 0.80; found: C 57.20, H 3.16, N 0.87.
(
methylsulfanyl)benzoate (13 mg, 0.07 mmol) was added 10 mL
3
3
F
chlorobenzene, sealed and the resulting solution was stirred for 30 min
at 80 °C. The solution was concentrated to dryness under vacuum, the
resulting residue washed with pentane, then recrystallized by diffusion
of pentane into a dichloromethane solution of the crude product at
fac-[Rh(κ -P,O,P-Xantphos)(η -C H )(SPh)][BAr ] (13). To a J.
3 5 4
Youngs tube containing 6 (100 mg, 0.06 mmol), allyl phenyl sulfide
(19 mg, 0.12 mmol) was added 10 mL chlorobenzene, sealed and the
resulting solution was stirred for 2 h at 80 °C. The solution was
concentrated to dryness under vacuum, the resulting residue washed
with pentane, then recrystallized by diffusion of pentane into a
dichloromethane solution of the crude product at −18 °C to yield
green crystals. Yield = 60%, 61 mg.
−
18 °C to yield brown crystals. Yield =77%, 80 mg.
1
H NMR (400 MHz, CD Cl ). δ 8.00 (d, J = 8.0 Hz, 1H, Ar−H), 7.94
2
2
(
d, J = 7.2 Hz, 2H, Ar−H), 7.90−7.81 (m, 4H Ph), 7.74−7.71 (m, 8H
F
F
BAr ), 7.64−7.51 (m, 10H Ar−H + BAr ), 7.49−7.41 (m, 5H Ar−
4
4
H), 7.36 (t, J = 6.8 Hz, 3H Ar−H), 7.26−7.11 (m, 5H Ar−H), 6.77−
1
H NMR (400 MHz, CD Cl ). δ 7.88−7.78 (m, 4H), 7.77−7.62 (m,
2
2
6
1
.69 (m, 4H, Ph′) 2.82 (s, COOMe 3H), 1.96 (s, Xantphos Me 3H),
1
6
0H), 7.66−7.48 (m, 14H), 7.23−7.12 (m, 3H), 7.08−6.99 (m, 4H),
.92−6.88 (m, 4H), 6.82 (t, J = 7.6 Hz, 2H), 6.66 (d, J = 7.6 Hz, 2H),
.82 (s, Xantphos Me 3H), 0.85 (s, SMe 3H).
3
1
1
1
P{ H} NMR (160 MHz, CD Cl ). δ 22.2 [d, J = 106 Hz].
2
2
RhP
+
4.80 (quintet, J = 10.0 Hz, 1H CH C
C H ), 2.71−2.57 (m, 2H CH C H ), 1.95 (s, Xantphos Me 3H),
1
3
H
5
), 4.05−3.93 (m, 2H CH
2
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 863.14 (calc. 863.14).
2
2
3
5
2
3
5
Microanalysis. C H BF O P RhS (1726.99) requires: C 55.64, H
80
54
24
3 2
.30 (s, Xantphos Me 3H).
3
.15; found: C 55.55, H 3.07.
31
1
1
3
1
F
P{ H} NMR (160 MHz, CD Cl ). δ 22.7 [d, J = 132 Hz].
mer-[Rh(κ -
-Xantphos)(σ,κ -C H NO )(SMe)][BAr ] (10).
2 2 RhP
P,O,P
6
4
2
4
+
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 831.15 (calc. 831.15).
To a flask containing 6 (100 mg, 0.06 mmol), 2-nitrothioanisole (10
mg, 0.06 mmol) was added 15 mL dichloromethane, the resulting
solution was stirred for 24 h room temperature. The solution was
concentrated to dryness under vacuum, the resulting residue washed
with pentane, then recrystallized by diffusion of pentane into a 1,2-
difluorobenzene solution of the crude product at −18 °C to yield
brown crystals. Yield = 66%, 68 mg.
2
2
Microanalysis. C H BF OP RhS (1695.00) requires: C 56.69, H
80 54
24
2
3
.21; found: C 56.84, H 3.17.
3
1
H ]
SMe)(H)][BArF
3 4
mer-[Rh(κ -P,O,P-Xantphos)(σ,κ -(COMe)C
14). To a J. Youngs tube containing 6 (80 mg, 0.046 mmol), 4-
6
(
(methylthio)acetophenone (11.5 mg, 0.069 mmol) was added 10 mL
toluene, the resulting solution was stirred for 2 days at room
temperature. The solution was filtered, and then concentrated to
dryness under vacuum, the resulting residue washed with pentane,
then recrystallized by diffusion of pentane into a chlorobenzene
solution of the crude product at −18 °C to yield colorless crystals.
Yield =38%, 30 mg.
1
H NMR (400 MHz, CD Cl ). δ 8.14 (d, J = 8.0 Hz, 1H, Ar−H),
2
2
F
7
.96−7.94 (m, 2H), 7.79−7.68 (m, 12H, Ar−H + BAr ), 7.65−7.54
4
F
(
(
m, 11H Ar−H + BAr ), 7.47 (t, J = 7.6 Hz, 4H Ar−H), 7.39−7.34
4
m, 3H Ar−H), 7.21 (t, J = 7.6 Hz, 1H Ar−H), 7.15 (t, J = 7.6 Hz, 4H
Ar−H), 6.81−6.75 (m, 4H, Ph′), 1.98 (s, Xantphos Me 3H), 1.77 (s,
Xantphos Me 3H), 0.88 (s, SMe 3H).
1
H NMR (400 MHz, CD Cl ). δ 8.02−7.94 (m, 4H), 7.79 (dd, J = 7.6
3
1
1
1
2
2
P{ H} NMR (160 MHz, CD Cl ). δ 23.1 [d, J = 102 Hz].
2
2
RhP
Hz, J = 2.0 Hz 3H), 7.77−7.68 (m, 9H), 7.63−7.56 (m, 10H), 7.47−
+
ESI-MS (CH Cl ) Positive Ion. [M] m/z = 850.12 (calc. 850.12).
2
2
7
1
.35 (m, 6H), 7.24−7.17 (m, 7H), 6.78 (dd, J = 8.4 Hz, J = 1.6 Hz,
H), 6.69−64 (m, 3H), 2.33 (s, 3H), 1.86 (s, 3H), 1.77 (s, 3H), 1.43
Microanalysis. C H BF NO P RhS (1713.95) requires: C 54.66,
78
51
24
3 2
H 3.00, N 0.82; found: C 54.57, H 2.87, N 0.86.
3
1
(s, 3H), −14.99 (dt, JRhH = 29.2 Hz, JPH = 10.8 Hz, 1H)
mer−[Rh(κ -
-Xantphos)(σ,κ -2-(2-oxazolinyl)phenyl)-
P,O,P
3
1
1
1
F
P{ H} NMR (160 MHz, CD Cl ). δ 39.5 [d, J = 114 Hz].
2 2 RhP
(
SMe)][BAr ] (11). To a J. Youngs tube containing 6 (100 mg, 0.06
4
+
ESI-MS (Acetone) Positive Ion. [M] m/z = 847.15 (calc. 847.15).
Microanalysis. C H BF O P RhS (1710.99) requires: C 56.16, H
mmol), 2-(2-(methylthio)phenyl-4,5-dihydrooxazole (14 mg, 0.07
mmol) was added 10 mL chlorobenzene, sealed and the resulting
solution was stirred for 60 min at 100 °C. The solution was
concentrated to dryness under vacuum, the resulting residue washed
with pentane, then recrystallized by diffusion of pentane into a
dichloromethane solution of the crude product at −18 °C to yield
80 54
24
2 2
3.18; found: C 56.38, H 3.01.
Crystallography. X-ray crystallography data were collected on an
Agilent SuperNova diffractometer using graphite monochromated Cu
Kα radiation (λ = 1.54180 Å), or an Enraf Nonius Kappa CCD
diffractometer using Mo Kα radiation (λ = 0.71073 Å) with the use of
orange crystals. Yield =78%, 81 mg.
1
61
H NMR (400 MHz, CD Cl ). δ 7.99−7.86 (m, 7H), 7.76−7.69 (m,
2
2
low-temperature devices [150(2) K]. Data were reduced using the
F
F
6
2,63
8
H, BAr ), 7.67−7.55 (m, 8H Ar−H + BAr ), 7.51 (t, J = 7.6 Hz, 2H
4
4
instrument manufacturer software, DENZO-SMN,
and Crystal-
Ar−H), 7.44−7.30 (m, 8H Ar−H), 7.29−7.20 (m, 5H Ar−H), 6.72−
6
9
64
66
Clear. All structures were solved ab initio using SIR92, or
SuperFlip, and the structures were refined using CRYSTALS or
SHELX. All non-hydrogen atoms were refined with anisotropic
.63 (m, 4H, Ph), 3.32 (t, J = 9.6 Hz, 2H oxazoline CH ), 2.35 (t, J =
65
2
.6 Hz, 2H oxazoline CH ), 1.94 (s, Xantphos Me 3H), 1.72 (s,
67
2
Xantphos Me 3H), 0.84 (s, SMe 3H).
displacement parameters. All hydrogen atoms were placed in
calculated positions using the riding model unless otherwise stated.
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Center under CCDC 1025603−1025612.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/
cif.
3
1
1
1
P{ H} NMR (160 MHz, CD Cl ). δ 17.8 [d, J = 103 Hz].
2
2
RhP
+
ESI-MS (CH Cl ) positive ion. [M] m/z = 874.16 (calc. 874.15).
2
2
Microanalysis. C H BF NO P RhS (1738.02) requires: C 55.98,
81
55
24
2 2
H 3.19, N 0.81; found: C 56.04, H 3.09, N 0.78.
3
1
mer-[Rh(κ -
-Xantphos)(σ,κ -2-(2-pyridyl)phenyl)(SMe)]-
P,O,P
F
[BAr ] (12). To a J. Youngs tube containing 6 (25 mg, 0.015 mmol),
4
2
-(2-(methylthio)phenylpyridine (4 mg, 0.02 mmol) was added 10
K
DOI: 10.1021/om500984y
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
20) Hooper, J. F.; Chaplin, A. B.; Gonzal
́
ez-Rodríguez, C.;
ASSOCIATED CONTENT
Supporting Information
Details of the crystallographic refinement, with CIF files giving
positional and displacement parameters, crystallographic data,
and bond lengths and angles. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
Thompson, A. L.; Weller, A. S.; Willis, M. C. J. Am. Chem. Soc.
*
S
2
(
(
(
012, 134, 2906−2909.
21) Jones, W. D. Inorg. Chem. 2005, 44, 4475−4484.
22) Wang, L.; He, W.; Yu, Z. Chem. Soc. Rev. 2012, 42, 599−621.
23) Barbero, N.; Martin, R. Org. Lett. 2012, 14, 796−799.
(24) Pan, F.; Shi, Z.-J. ACS Catal. 2013, 4, 280−288.
(
25) Hooper, J. F.; Young, R. D.; Pernik, I.; Weller, A. S.; Willis, M.
C. Chem. Sci. 2013, 4, 1568−1572.
26) Modha, S. G.; Mehta, V. P.; Van der Eycken, E. V. Chem. Soc.
Rev. 2013, 42, 5042−5055.
27) Liu, X.-Y.; Venkatesan, K.; Schmalle, H. W.; Berke, H.
Organometallics 2004, 23, 3153−3163.
28) Wilson, A. D.; Miller, A. J. M.; DuBois, D. L.; Labinger, J. A.;
AUTHOR INFORMATION
Corresponding Author
E-mail: andrew.weller@chem.ox.ac.uk.
Present Address
■
*
(
(
†
(
P.R.) Department of Chemistry, Princeton, NJ 08544, USA.
(
Notes
Bercaw, J. E. Inorg. Chem. 2010, 49, 3918−3926.
(29) Thomas, R.; Lakshmi, S.; Pati, S. K.; Kulkarni, G. U. J. Phys.
Chem. B 2006, 110, 24674−24677.
The authors declare no competing financial interest.
(
30) Ghosh, R.; Zhang, X.; Achord, P.; Emge, T. J.; Krogh-Jespersen,
K.; Goldman, A. S. J. Am. Chem. Soc. 2007, 129, 853−866.
31) Werner, H.; Wolf, J.; Schubert, U.; Ackermann, K. J. Organomet.
Chem. 1986, 317, 327−356.
32) Choudhury, J.; Pratihar, S.; Maity, A. K.; Roy, S. Can. J. Chem.
ACKNOWLEDGMENTS
■
The Swiss National Science Foundation (postdoctoral fellow-
ship for P.R.); the EPSRC (DTG award to S.D.P., EP/
K505031/1); the Archimedes Foundation (Estonia) for a Ph.D.
Scholarship (I.P.); Amit Kumar for assistance with ESI−MS
measurements; E. J. Emmett and R. J. Pawley for preliminary
synthetic studies.
(
(
2009, 87, 183−187.
(33) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chem.
1999, 38, 115−124.
(34) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J.; Bergman,
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