Transformation of a Norbornadiene Unit to Ethylenylcyclopentene Requiring Cooperation between Boron and Rhodium Centers

Article abstract of DOI:10.1021/acs.organomet.0c00155

The synthesis of the lithium salt of a bis-substituted borohydride anion containing a phenyl substituent and a 2-mercaptopyridyl unit (mp) is reported herein. This salt has been used as a pro-ligand for the synthesis of rhodium(I) complex, [Rh{κ³-H,H,S-H₂B(Ph)(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene). The new boron-based ligand coordinates to the rhodium center via the thione donor and the two B-H bonds of the BPhH₂ unit, with a dihydroborate motif. Reaction of complex 1 with two equivalents of triphenylphosphine leads to an unprecedented rearrangement and transfer of the former norbornadiene ligand to the boron center. The transformation occurs via an initial hydride migration from boron to rhodium center followed by a hydroboration of one of the double bonds. Finally, a ring-opening process occurs involving both boron and rhodium centers leading to an unusual boron-bound ethylenylcyclopentene unit. The product of this reaction was confirmed as [Rh{η¹-S,η²-B,C-B(Ph)(CHCH₂(C₅H₇)(mp)}(PPh₃)₂] (2). The net result of these transformations is the incorporation of the two hydrogen atoms from the secondary borohydride ligand [BPhH₂(mp)]^- into the former norbornadiene unit. The end point positions of these hydrogen atoms were confirmed by deuterium labeling experiments. Complex 2 was further reacted with carbon monoxide to generate [Rh{η¹-S,η²-B,C-B(Ph)(CHCH₂(C₅H₇)(mp)}(CO)(PPh₃)] (3) via ligand substitution. The new ligand and the three complexes, 1, 2, and 3, have been characterized by spectroscopic techniques as well as by X-ray crystallography. Detailed characterization of 2 and 3 revealed an unusual η²-B,C coordination mode within these complexes. Further studies have demonstrated that complexes 2 and 3 react with hydrogen gas (or dimethylamine borane as a source of H₂) to generate the hydrogen addition products involving the unprecedented activation of the Rh-η²-B,C motif. Complexes 2 and 3 were further found to be active catalysts for the hydrogenation of olefins and the dehydrogenation of dimethylamine borane.

Full text of DOI:10.1021/acs.organomet.0c00155

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Article
Transformation of a Norbornadiene Unit to Ethylenylcyclopentene
Requiring Cooperation between Boron and Rhodium Centers
Angelo Iannetelli, Rosenildo C. Da Costa, Alan J. Guwy, Graham J. Tizzard, Simon J. Coles,
and Gareth R. Owen*
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ABSTRACT: The synthesis of the lithium salt of a bis-substituted
borohydride anion containing a phenyl substituent and a 2-
mercaptopyridyl unit (mp) is reported herein. This salt has been
used as a pro-ligand for the synthesis of rhodium(I) complex, [Rh{κ3-
H,H,S-H2B(Ph)(mp)}(NBD)] (1) (where NBD = 2,5-norborna-
diene). The new boron-based ligand coordinates to the rhodium
center via the thione donor and the two B−H bonds of the BPhH2
unit, with a dihydroborate motif. Reaction of complex 1 with two
equivalents of triphenylphosphine leads to an unprecedented
rearrangement and transfer of the former norbornadiene ligand to
the boron center. The transformation occurs via an initial hydride
migration from boron to rhodium center followed by a hydroboration of one of the double bonds. Finally, a ring-opening process
occurs involving both boron and rhodium centers leading to an unusual boron-bound ethylenylcyclopentene unit. The product of
this reaction was confirmed as [Rh{η1-S,η2-B,C-B(Ph)(CHCH2(C5H7)(mp)}(PPh3)2] (2). The net result of these transformations is
the incorporation of the two hydrogen atoms from the secondary borohydride ligand [BPhH2(mp)]− into the former norbornadiene
unit. The end point positions of these hydrogen atoms were confirmed by deuterium labeling experiments. Complex 2 was further
reacted with carbon monoxide to generate [Rh{η1-S,η2-B,C-B(Ph)(CHCH2(C5H7)(mp)}(CO)(PPh3)] (3) via ligand substitution.
The new ligand and the three complexes, 1, 2, and 3, have been characterized by spectroscopic techniques as well as by X-ray
crystallography. Detailed characterization of 2 and 3 revealed an unusual η2-B,C coordination mode within these complexes. Further
studies have demonstrated that complexes 2 and 3 react with hydrogen gas (or dimethylamine borane as a source of H2) to generate
the hydrogen addition products involving the unprecedented activation of the Rh−η2-B,C motif. Complexes 2 and 3 were further
found to be active catalysts for the hydrogenation of olefins and the dehydrogenation of dimethylamine borane.
example published by our group, we reported a system
exhibiting multiple migrations between boron and transition
metal centers.5 This presented the activation of a norbornadiene
moiety within a rhodium transition metal complex containing a
monosubstituted borohydride ligand. In this case, the
norbornadiene unit was transferred to the boron center via a
formal hydroboration step (Scheme 1). We found that this
transformation did not occur via an intermolecular process. A
mechanism was proposed in which hydride migration from
boron to the rhodium center had occurred, followed by
migratory insertion of the double bond into the resulting
rhodium−hydride bond. In the final step, a migration of the
INTRODUCTION
■
“Ligand cooperation” can be defined as systems in which both
metal and ligand centers work together to facilitate trans-
formations that would not occur by using just one of these
centers. Transformations involving cooperation between ligands
and transition metal centers have become a major focus of
research over the past decade or so.1−3 In particular, those
involving the transfer and storage of hydrogen atoms have been a
major feature, where either a “protic” species or “hydridic”
species is transferred between a metal center and a position
within a precoordinated ligand. One strategy for the mobility of
hydrogen within the coordination sphere of transition metal
complexes has employed borane or borohydride functional
groups. The latter acts as a metal−hydride source and leads to
the formation of Z-type borane complexes.2 There are a range of
examples exhibiting low energy2f and reversible hydride
migrations3 between boron and metal centers. There are, to a
much lesser extent, examples of transfers involving other
functional groups such as methyl or phenyl.4,5 In a recent
Received: March 3, 2020
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[H2B(Ph)(mp)]− and the excess [H3B(Ph)]− reagent used in
the reaction. A similar intermediate species was observed in the
synthesis of Na[H3B(mp)] which was characterized as the
isomer containing a boron−sulfur bond.5 Following its
completion, isolation of the targeted product from the reaction
mixture was readily achieved by removal of all volatiles and
washing the residue with toluene. The pure solid product was
isolated as a fine white powder with a yield of 72%. It should be
noted that the solid was found to be hygroscopic.
Scheme 1. Formal Hydroboration of One of the Double
Bonds of the Norbornadiene Leading to a Norbornenyl
Group at Boron via a Series of Migrations between Boron and
a
Rhodium Centers
The isolated salt, Li[H2B(Ph)(mp)], was characterized by
NMR spectroscopy using CD3CN as solvent. The 11B NMR
spectrum confirmed the presence of a [BH2]− unit as a triplet
resonance centered at −6.3 ppm (1JBH = 92 Hz). This signal
appeared as a singlet in the corresponding 11B{1H} NMR
experiment. Confirmation of the presence of both phenyl and 2-
mercaptopyridyl units was obtained in the 1H NMR spectrum.
This exhibited a total of seven aromatic signals; three accounting
for the five protons of the phenyl group and four corresponding
to the four environments of the 2-mercaptopyridyl ring. Also
present within the 1H NMR spectrum was a very broad quartet
resonance centered at 2.22 ppm, typical of protons attached to a
quadrupolar boron center. This became more clear in the
corresponding 1H{11B} NMR experiment in which this
resonance appeared as a sharp singlet integrating for 2 protons
with respect to the aromatic signals. This further confirmed the
presence of the BH2 unit within the ligand. Full assignment of
a
L/L = CO/CO or CO/PR3 where the PR3 ligand is trans to the BH2
unit
newly formed norbornenyl (nbe) unit from the rhodium to the
boron center occurred. These multiple migration steps appeared
to transpire as a result of cooperation between the rhodium and
boron centers. This also provided the first reported example of
hydride migration from boron to metal center in the case of a
monosubstituted borohydride ligand.
In order to understand the processes involved in these
migrations further, we developed new derivative compounds
and tested their reactivity in a similar manner. It was of interest
to us to explore the impact of a higher substitution pattern
around the boron center. Herein, we wish to report our latest
results in which we have found an unprecedented ring-opening
transformation on norbornadiene involving direct cooperation
between boron and rhodium centers. These newly formed
products contain a highly novel borata-alkene unit which binds
to the rhodium centers via a η1-S,η2-B,C coordination mode. We
further demonstrate the ability of this unit to activate molecular
hydrogen across the BC bond. Finally, we also report the
catalytic application of these complexes for hydrogenation and
dehydrogenation transformations.
1
each signal in the H NMR and the corresponding 13C{1H}
NMR spectra was possible via a series of correlation experiments
(see Experimental Section). Finally, the [BH2] unit was also
observable by ATR-FT-IR spectroscopy where the B−H bonds
exhibited a broad absorption band at 2263 cm−1.
With the new ligand in hand, we set out to synthesize new
group nine complexes. Rhodium complex, [Rh{κ3-S,H,H-
H2B(Ph)(mp)}(NBD)] (1), was readily prepared from the
reaction between the Li[H2B(Ph)(mp)] and one-half of an
equivalent of the rhodium dimer precursor, [RhCl(NBD)]2,
according to established procedures (Scheme 3).6 The resulting
RESULTS AND DISCUSSION
■
We reported the synthesis of monosubstituted borohydride
ligand, Na[H3B(mp)] (where mp = 2-mercaptopyridyl) in a
recent publication.5 A straightforward adaptation of this
procedure was utilized in the synthesis of new ligand,
Li[H2B(Ph)(mp)] (Scheme 2). The solid precursors Li[H3B-
Scheme 3. Synthesis of [Rh{κ3-H,H,S-
H2B(Ph)(mp)}(NBD)] (1)
Scheme 2. Synthesis of Li[H2B(Ph)(mp)]
product was isolated as a spectroscopically pure orange solid in
excellent yield. The 11B NMR spectrum of 1 revealed a single
peak centered at 1.0 ppm as a triplet resonance (1JBH = 62 Hz)
confirming the presence of the BH2 unit in the product. The
downfield chemical shift with respect to free ligand is consistent
with its coordination to the rhodium center.7 The reduction in
the 1JBH coupling constant in the complex is also suggestive of a
significant degree of interaction between the BH2 units and the
metal center (cf. 92 Hz for the ligand with 62 Hz for 1). This
strong interaction is also evident from the solid-state infrared
spectrum of 1 where a band at 1827 cm−1 is observed for the BH2
unit coordinated to the rhodium center.8 The 1H, 1H{11B} and
13C{1H} NMR spectra were also fully consistent with the
expected product. Of particular note was the presence of a
(Ph)] and 2-mercaptopyridine were added to a Schlenk flask in a
2:1 molar ratio (an excess of the phenylborohydride). An
immediate reaction occurred upon addition of THF solvent as
observed by the release of hydrogen gas from the mixture. After
approximately 5 min, the effervescence subsided. The reaction
mixture was monitored by 11B NMR spectroscopy. After 1 h,
three boron-containing species were observed at −26.7 ppm
(quartet) corresponding to unreacted [H3B(Ph)]−, at −15.5
ppm (broad triplet) corresponding to a [H2B(Ph)]− unit of an
intermediate species and at −7.4 ppm (triplet) identified as the
targeted [H2B(Ph)(mp)]− product. A 11B NMR spectrum of the
reaction mixture after 48 h confirmed the consumption of the
intermediate species and a spectrum which revealed only
1
doublet signal at −1.59 ppm in the H{11B} NMR spectrum,
which revealed a small coupling constant to rhodium center
B
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(1JRhH = 2.0 Hz). This corresponds to the BH2 protons which are
shifted upfield upon coordination. The data confirmed a
symmetrical and strong coordination of the BH2 unit in
H3B(mp)}(NBD)] led to the formation of [Rh{κ3-H,H,S-
H2B(nbe)(mp)}(CO)2] as highlighted above. The same
reaction was carried out with complex 1 in a Young’s NMR
tube using C6D6 solvent. Upon addition of an atmosphere of
CO, a color change from yellow to orange was observed. The 11B
NMR spectrum of the reaction mixture after 20 min revealed
partial conversion of the starting material to a new product (ca.
30%). On the basis of the NMR spectroscopic characterization
and the reactivity observed in similar complexes, we believe that
the product formed in situ has the formula, [Rh{κ3-H,H,S-
H2B(Ph)(mp)}(CO)n]x.10 This is the result of a ligand
substitution reaction resulting in the elimination of the diene
from the rhodium coordination sphere. Unfortunately, this
reaction mixture showed signs of significant decomposition
when left for longer periods, and after 24 h, almost all signs of
any boron-containing species were absent from the NMR
solution. It was clear that the complex containing the
[H2B(Ph)(mp)]− ligand reacted differently to that containing
the [H3B(mp)]− ligand.
1
solution. The magnitude of the JRhH coupling constant,
however, was notable in comparison to the related complex
[Rh{κ3-H,H,S-H3B(mp)}(NBD)].5 In this previously reported
complex, a much stronger ligand coordination was observed
where the 1JRhH coupling constant was found to be 19.5 Hz.
Single crystals of 1 were obtained by allowing a saturated
diethyl ether solution stand at room temperature for 48 h. The
molecular structure, as determined by X-ray crystallography, is
shown in Figure 1. The structure is fully consistent with the
Complex 1 was next tested for its reactivity toward two
equivalents of triphenylphosphine. A potential product of this
again could have been straightforward ligand substitution of the
norbornadiene ligand with the two monodentate phosphine
ligands. In this case, however, we observed an unusual
transformation. Complex 1 was fully dissolved in C6D6 in the
presence of two equivalents of PPh3 in a Young’s NMR tube.
Upon addition of the solvent, the mixture quickly changed color
from orange to red. The 11B NMR spectrum after 10 min
showed the complete consumption of the starting material. The
spectra contained a significantly broad signal centered at 34.7
ppm (h.h.w. of 1550 Hz) as the only resonance. This substantial
change in the chemical shift and peak width was indicative of
major change in the chemical environment of the boron center.
The corresponding 31P{1H} NMR spectrum was also consistent
with a single new product. The spectrum revealed two doublet of
doublet signals indicating the coordination of two triphenyl-
phosphine ligands cis to each other at the rhodium center.
Accordingly, the reaction was carried out on a preparative scale
(see Experimental Section).11 A pure brick-red solid, complex 2,
was isolated via a standard workup in very good yield and was
fully characterized by spectroscopic and analytical methods. The
two resonances in the 31P{1H} NMR spectrum were found at
31.9 ppm (dd, 1JRhP = 134 Hz, 2JPP = 33 Hz) and 47.8 ppm (dd,
1JRhP = 176 Hz, 2JPP = 33 Hz). The coupling constant in the latter
signal is consistent with a triphenylphosphine ligand trans to a
sulfur-based ligand.5 The smaller 1JRhP constant in the signal at
31.9 ppm indicated that the other PPh3 ligand was trans to a
ligand with a strong trans influence. The 1H NMR spectrum was
consistent with the coordination of two PPh3 ligands to the
metal center (integrating for 30 H). The spectrum also contains
four new environments for a 2-mercaptopyridinyl unit and three
environments for the phenyl group on boron. Comparisons of
Figure 1. Molecular structure of complex 1. Thermal ellipsoids of all
atoms with the exception of hydrogen are drawn at 50% level. Selected
distances (Å) and angles (deg) Rh−B 2.239(2), Rh−S 2.2981(5), B−N
1.567(3), B−C 1.590(3), Rh−H(1A) 1.84(2), Rh−H(1B) 1.83(2), B−
H(1A) 1.20(2), B−H(1B) 1.21(2), CS 1.729(2), B−Rh−S
83.68(6).
spectroscopic data and supports the solution-state assignment
that the BH2 unit was bound to the rhodium center via a bridging
BH2Rh mode. It also confirms the pseudo square planar
geometry of the rhodium(I) metal center. The diene is
coordinated to the rhodium center via a typical η2 coordination
mode. One of the double bonds is trans to the thione group
while the other is trans to the anionic BH2 unit. The distances
between the centroid of each CC bond and the rhodium
center is 2.0379(14) Å for the double bond trans to thione donor
and 1.9919(14) Å for the double bond trans to the BH2 unit.
This indicates a weak trans influence of the anionic unit.5,9 The
sum of the metal’s cis interangles for 1 is 360.03(14)° when
calculated using the sulfur and boron atoms along with the
centroid positions of the alkene bonds of the norbornadiene
ligand. The phenyl substituent at boron is oriented so that it is
close to being perpendicular to the mercaptopyridine ring, as
evidenced by a N1−B1−C6−C11 angle of 70.9(3)°.
As highlighted in the Introduction, we were interested in
further probing the reactivity of the borohydride unit with the
coordinated norbornadiene ligand. Our previous results have
shown that by disturbing the coordination sphere of the related
complex [Rh{κ3-H,H,S-H3B(mp)}(NBD)], the norbornadiene
ligand undergoes a formal hydroboration.5 This leads to the
formation of a secondary borohydride functional group (as
indicated in Scheme 1). We aimed to explore similar conditions
with the newly formed complex. Accordingly, solutions of
complex 1 were subjected to various ligands. In our previous
investigations, addition of carbon monoxide to [Rh{κ3-H,H,S-
1
this spectrum and the corresponding H{11B} NMR spectrum
indicated that there were no hydrogen substituents at the boron
center. Furthermore, there were no signals in the hydride regions
of these spectra. It was therefore apparent that the two hydrogen
substituents at boron in complex 1 had undergone a rather
intriguing transformation during the reaction. Curiously, the
signals corresponding to the NBD moiety, either as coordinated
or uncoordinated diene in solution, were also absent. There
were, however, two additional resonances at 5.58 and 5.67 ppm
each integrating for 1 proton and a number of new signals in the
aliphatic region of the spectrum with a total integration of eight
C
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protons. On the basis of this evidence, it was clear that the two
former hydrogen substituents at boron had been incorporated
into the norbornadiene (C7H8) unit to provide a novel C7H10
organic fragment containing one double bond. The 13C{1H}
NMR spectrum also confirmed the presence of a hydrocarbon
fragment with seven carbon environments.12
With this evidence, we were confident that a transformation
had occurred involving the former norbornadiene ligand and the
two hydrogen atoms originally bound to the boron center. Single
crystals of complex 2 were obtained by allowing a saturated
acetone solution stand at room temperature for several days.
These were of suitable quality for single crystal X-ray diffraction
and allowed for the unambiguous structural determination of
complex 2 (Scheme 4 and Figure 2). The crystal structure
in a η2-B,C coordination mode. This rare functional group has
only be observed in a handful of cases,13 although some similar
borataalkenes motifs have been reported.14,15 The unique
bonding features of the boron-based ligand motif are outlined
below.
Close examination of the structure reveals a square planar
rhodium(I) center where two triphenylphosphine ligands are cis
to each other. The structure also reveals the coordination of the
B(Ph)(CHCH2(C5H7)(mp) ligand, which binds to the
rhodium center via the thione and via both the α-carbon and
boron centers on the BCHCH2(C5H7) group. The sulfur atom
of the mercaptopyridine unit and the carbon atom α to the
boron are located on the plane. The sum of the metal’s cis-inter
ligand angles is 359.92(12)° [ranging between 82.62(2)° and
96.01(2)°]. The boron center is located at a distance 1.385(3) Å
below the plane defined by the atoms Rh1, C6, P1, P2, and S1.
The C6−Rh(1)−B(1) angle is 37.79(8)° and the Rh(1)−C(6)
and Rh(1)−B(1) distances are 2.232(2) Å and 2.352(3) Å,
respectively. The Rh−B distance is longer than previously
reported for mercaptopyridine−borane-based ligands. A survey
of the Cambridge Structural Database revealed a range of
compounds featuring a tricoordinate borane ligand which
appears to be interacting with a rhodium center. In these
cases, the Rh−B distances ranged from 2.632 Å16 to 2.053 Å.17
The positioning of the C(6) and B(1) atoms is of interest. The
Rh(1)−C(6) distance is typical of a σ-alkyl fragment
coordinated trans to a triphenylphosphine ligand at a
monovalent rhodium center.18 The corresponding Rh(1)−
P(2) distance is 2.3293(6) Å is consistent with a phosphine trans
to an alkyl unit. The labilization or influence of the boron
functionality at an angle of 140.30(7)° for B(1)−Rh(1)−P(2) is
not apparent or reflected in the Rh(1)−C(6) distance.19 The
B(1)−C(6) distance is 1.489(3) Å. This is shorter than a typical
boron carbon single bond [cf. 1.584(3) Å as the B(1)−C(13)
distance in this same structure], and it is therefore suggestive of
some multiple bond character within the rhodium bound η2-B,C
unit. Furthermore, the angles around the boron center are,
N(1)−B(1)−C(13) 112.35(18)°, C(6)−B(1)−N(1)
117.9(2)° and C(6)−B(1)−C(13) 126.3(2)°. The sum of
these three angles is 356.6(6)° indicating a significant degree of
planarity of the boron center. The above parameters appear to
suggest that the boron is only weakly interacting with the
rhodium center, most likely due to its position with respect to
the square plane.
Scheme 4. Synthesis of [Rh{η1-S,η2-B,C-
B(Ph)(CHCH2(C5H7)(mp)}(PPh3)2] (2)
The reactivity of complex 2 with carbon monoxide was
investigated. This led to the formation of [Rh{η1-S,η2-B,C-
Figure 2. Molecular structure of complex 2. Thermal ellipsoids drawn
at 50% level. Hydrogen atoms apart from those on the ethyl-
enylcyclopentene have been omitted for clarity. Selected distances
(Å) and angles (deg) Rh−B 2.352(3), Rh−S 2.3449(6), Rh−C(6)
2.232(2), Rh−P(1) 2.2716(6), Rh−P(2) 2.3293(6) B−N 1.577(3),
B−C(13) 1.584(3), B−C(6) 1.489(3), CS 1.711(2), N−B−C(6)
117.9(2), N−B−C(13) 112.35(18), C(6)−B−C(13) 126.3(2), B−
Rh−S 79.95(6), B−Rh−P(1) 105.67(6), C(6)−Rh−S 91.86(6),
C(6)−Rh−P(1) 89.43(6), S−Rh−P(2) 82.62(2), P(1)−Rh−P(2)
96.01(2), ∑ of angles at Rh 359.92(12).
B(Ph)(CHCH2(C5H7)(mp)}(CO)(PPh3)] (3). Complex 3
was prepared via a straightforward ligand substitution step
involving the addition of carbon monoxide to a solution of
complex 2 (Scheme 5). Alternatively, complex 3 could be also be
prepared in a “one-pot” procedure directly from complex 1. Both
methods provided the product as a spectroscopically pure solid
in good yield. The spectroscopic data for 3 were comparable to
those for complex 2. A minor difference was related to the
chemical equivalence of alkene and adjacent methylene groups
within the cyclopentenyl ring. This is presumably due to the
reduction of steric bulk upon going from two triphenylphos-
phine ligands to one, which facilitates unhindered rotation of
this unit within the complex. The 1JRhP coupling constant (133
Hz) observed in the 31P{1H} NMR spectrum confirmed that the
remaining phosphine ligand was located trans to the BC unit
(essentially trans to the carbon atom). Confirmation for the
indeed confirmed that a remarkable and unprecedented
transformation had occurred. The boron had lost both hydrogen
substituents, and a ring-opening transformation had occurred on
the former norbornadiene ligand to form an ethylene-yl bridged
cyclopentene unit. Furthermore, this new organic unit resided at
the boron center and was coordinated to the rhodium center via
one of the carbons of the ethylene-yl bridge and the boron atom
D
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Rh(1)−P(1) distance is 2.2993(4) Å. The B(1)−C(6) distance
is 1.471(3) Å. The angles around the boron center are N(1)−
B(1)−C(6) 114.78(17)°, C(13)−B(1)−N(1) 113.61(16)°,
and C(13)−B(1)−C(6) 129.17(17)°. The sum of these three
angles is 357.5(5)°. All of these parameters are similar to those
found for complex 2.
Scheme 5. Synthesis of [Rh{η1-S,η2-B,C-
B(Ph)(CHCH2(C5H7)(mp)}(CO)(PPh3)] (3)
Transformation of Complex 1 to Complexes 2 and 3.
The ring strain within norbornadiene means that it readily
undergoes transformations at transition metal centers such as
ring-opening polymerizations, other types of ring opening, and
additions across the double bonds. In particular, addition of
dihydrogen across the double bonds in norbornadiene is well
established. To the best of our knowledge, the transformation of
a norbornadiene unit to form an ethylene-yl bridged cyclo-
pentene unit is without precedent. The mechanism of this
transformation was not immediately obvious given the position
of the double bond in the final product. It was clear that both
hydrogen substituents at boron had been incorporated into the
former norbornadiene unit. The way in which the bicyclic ring
opened was not clear, at first. In order to gain a more detailed
insight, we carried out a number of deuterium labeling studies to
work out the end point positions of the hydrogen substituents
originating from the boron center. The method utilized to
determine the end point positions of these two hydrogen is
presented below and has led us to postulate a mechanism for the
transformation (vide infra).
coordination of a carbonyl to the rhodium center came from the
13C{1H} NMR spectrum with a doublet of doublet signal at
193.3 ppm with 1JRhC and 2JCP coupling constants 72 and 15 Hz,
respectively. This is further evidenced in the powder IR
spectrum of 3, which gave a very strong absorption band at
1945 cm−1 characteristic of a coordinated carbonyl stretching.
Complex 3 was also structurally characterized. Single crystals
suitable for X-ray crystallography where obtained by allowing a
saturated diethyl ether solution of the complex stand at −40 °C
for several days (Figure 3). The structure obtained confirmed
First, the deuterium labeled ligand, Li[D2B(Ph)(mp)] was
prepared by the same methodology used for the preparation of
Li[H2B(Ph)(mp)] (see Experimental Section). The corre-
sponding labeled complex, [Rh{κ3-D,D,S-D2B(Ph)(mp)}-
(NBD)] 1-d2 was subsequently prepared by the same method
used for 1. The spectroscopic data for this labeled complex was
as expected and were comparable to those for 1. The pertinent
features are outlined herein. Whereas the BH2 unit appeared as a
triplet in the 11B NMR spectrum for 1, an unresolved broader
1
signal was observed for 1-d2 where the JBD coupling was not
observable. As indicated above, the BH2 unit was observed as a
1
doublet signal at −1.59 ppm in the H{11B} NMR of 1. The
2
corresponding BD2 unit was located at −1.58 ppm in the D
NMR spectrum of 1-d2. With this deuterium labeled complex in
hand, we subjected it to the same reaction conditions used in the
synthesis of 2, that is, two equivalents of triphenylphosphine
were added to a benzene solution of 1-d2. The resulting product
was carefully analyzed by NMR spectroscopy in order to confirm
the product and the final location of the two deuterium atoms.
No deuterium scrambling was apparent over time. The chemical
shifts of all carbon and proton environments within the
ethylene-yl bridged cyclopentene unit were determined by
two-dimensional correlation experiments. Interestingly and
fortuitously, there appeared to be no rotation of the organic
moiety. This aided our assignment and helped build a picture of
the mechanism for this transformation. When samples were
heated up to 80 °C, they showed no indication of rotation of the
Figure 3. Molecular structure of complex 3. Thermal ellipsoids drawn
at 50% level. Hydrogen atoms apart from those on the ethyl-
enylcyclopentene have been omitted for clarity. Selected distances
(Å) and angles (deg) Rh−B 2.369(2), Rh−S 2.3446(4), Rh−C(6)
2.2728(18), Rh−C(19) 1.8345(19), Rh−P(1) 2.2993(4), B−N
1.570(2), B−C(6) 1.471(3), B−C(13) 1.576(3), CS 1.7260(18),
N−B−C(6) 114.78(17), N−B−C(13) 113.61(16), C(6)−B−C(13)
129.17(17), B−Rh−S 78.46(5), B−Rh−C(19) 100.53(7), C(6)−Rh−
S 100.42(5), C(6)−Rh−C(19) 81.39(7), S−Rh−P(1) 88.622(16),
C(19)−Rh−P(1) 90.27(6), ∑ of angles at Rh 360.7(2).
the coordination of the PPh3 and CO ligands coordinated cis to
each other at the rhodium center and that the CO is trans to the
thione unit while the phosphine is trans to the BC unit. As with
complex 2, the carbon center of the BC unit is positioned close
to one of the idealized position of a square planar complex. The
sum of the cis-inter ligand angles is 360.7(2)° [ranging between
81.39(7)° and 100.42(5)°]. The boron center was located at a
distance 1.063(3) Å below the plane defined by the atoms Rh1,
C6, P1, C19, and S1. The C6−Rh(1)−B(1) angle is 36.89(7)°,
and the Rh(1)−C(6) and Rh(1)−B(1) distances are
2.2728(18) Å and 2.369(2) Å, respectively. The corresponding
cyclopentenyl ring.20 Complex 2-d2 was confirmed as [Rh{η1-
S,η2-B,C-B(Ph)(CHC(H)D(C5H6D)(mp)}(PPh3)2] where
one deuterium atom was located at the carbon β to the boron
and the other at one of the allylic positions within the
cyclopentene ring (as shown in Scheme 6). The deuterium
was located on only one side of the cyclopentene ring which
confirms no rotation of the cyclopentenyl ring in solution. This
also suggests that the conformation observed in the crystal
structure for this compound is the same as that found in solution.
E
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Scheme 6. Postulated Mechanism for Synthesis of [Rh{η1-S,η2-B,C-B(Ph)(CHCH(D)(C5H6D)(mp)}(PPh3)2] (2-d2)
Locating the end point positions of the two deuterium atoms
was relatively straightforward. A comparison of the aliphatic
region of the 1H NMR spectra of complexes 2 and 2-d2 is shown
in Figure 4. As can be seen, there are two signals which are absent
stereochemistry of deuterium incorporation into 2-d2, a series of
selective 1-D NOESY NMR experiments were performed
(details of these experiments can be found in the Supporting
Information). These, in combination with analysis of the COSY
NMR experiment of 2, allowed the confirmation of the exact
locations of the deuterium atoms. Both deuterium atoms added
to the endo face of the former norbornadiene ligand in the
positions indicated in Scheme 6.
With conformation of the positions of the deuterium atoms
within 2-d2, it was possible to work out a consistent mechanism
for this transformation (Scheme 6). Our rationale for this
mechanism is outlined herein. A number of studies have been
carried out looking at the hydrogenation of the norbornadiene
ligand at late transition metal centers.21 In several cases, the
norbornadiene undergoes migration into a transition metal−
hydride species to form norbornenyl or its valence isomer,
nortricyclyl unit.21b,22 The formation of a nortricyclyl unit was
ruled out in this case since there is no C−C bond breaking route
which would lead to a product containing a five membered ring.
Next, we considered a fully intermolecular route. In order to rule
this out, we investigated the reactivity of the complex [Rh{κ3-
H,H,S-H2B(Ph)(mp)}(PPh3)2] (4)23 with free norbornadiene
(Scheme 7). If an intermolecular transformation was occurring
Figure 4. Comparison of the aliphatic region of the 1H NMR spectra for
complexes 2 (top) and 2-d2 (bottom) indicating the positions of the
deuterium atoms by their absence in the spectrum for 2-d2 (see Scheme
6 for numbering scheme).
in the spectrum for 2-d2. The signal at 2.84 ppm in the spectrum
of 2 is missing in the spectrum for 2-d2. This corresponds to one
of the two methylene protons on the carbon β to the boron
atom. The second missing signal is a multiplet signal centered at
2.03 ppm. In the spectrum of 2, this consists of 2 overlapping
signals integrating for 2 protons. In 2-d2 this only integrates for 1
proton. As a result of the lack of rotation, all four of the
methylene signals were found at different chemical shifts. Two
overlapping signals at 2.04 ppm were confirmed as the proton
attached to carbon γ to the boron and one of the four methylene
protons within the cyclopentenyl ring. We were able to confirm
that the signal that remained at this chemical shift in 2-d2
corresponded to a CH unit (as opposed to a CH2 unit) via a
HSQC experiment (see Figures S11.1 and S11.2 in the
Supporting Information). This was identified as the CH in the
position γ to the boron center. As such, we confirmed that the
deuterium substituents were attached to carbon-2 and carbon-7
as indicated in the numbering scheme for the organic unit in
Scheme 6. Further support for our assignments was obtained
from the corresponding 2D NMR spectrum of 2-d2, which gave
broad signals at the same chemical shifts as the missing signals in
Scheme 7. Attempted Reaction to Rule out an Intermolecular
Mechanism to Form Complex 2
in the formation of complex 2, then complex 4 would be an
intermediate. The norbornadiene species would undergo
consecutive, or perhaps concerted, additions across both double
bonds. This would be a hydroboration at the boron center and a
migratory insertion into a newly formed rhodium−hydride
species. Prior to both these transformations occurring, a hydride
migration from the borohydride species to the rhodium center
would need to occur (since a tricoordinate boron is a
prerequisite for hydroboration). We can confirm that there
was no evidence of any reactivity between complex 4 and free
norbornadiene over several days.
1
the H NMR spectrum of 2. In an attempt to ascertain the
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The fact that complete conversion of 1 into 2 occurs within 10
min suggests a series of rapid intramolecular reaction steps as
outlined in Scheme 6. We do have precedent for the earlier steps
leading to species D. As highlighted above, our previous results
from the related complex, [Rh{κ3-H,H,S-H3B(mp)}(NBD)]
leads to the formation of [Rh{κ3-H,H,S-H2B(nbe)(mp)}(L)2]
(where L/L = CO/CO or CO/PPh3) following a similar ligand-
substitution-based activation (Scheme 1).5 On the basis of these
observations and the number of examples involving hydride
migration from borohydride species,2,3 we believe that the steps
leading up to the formation of [Rh{κ2-H,S-H2B(Ph)(nbe)-
(mp)}(PPh3)2], labeled species D in Scheme 6, are analogous. It
is therefore likely that the transformation from 1 entails a change
in the coordination mode of the norbornadiene ligand from η4 to
η2, induced by coordination of the first phosphine ligand (1-d2
to A). This is followed by a migration of hydride (or deuteride in
the case of 1-d2) from the borohydride to the rhodium center (A
to B). The next step is a migratory insertion step of the diene
into the rhodium−hydride leading to the formation of the
norbornenyl (nbe) unit (B to C) which migrates to the borane
group to form species D. The location of the deuterium atom in
2-d2 confirms syn-addition of the rhodium deuteride across the
double bond.
For our previously reported system, containing the parent
BH3 ligand, it appears that the transformation stops at this point
and the stable complex of the type [Rh{κ3-H,H,S-H2B(nbe)-
(mp)}(L)2] remains as the final product. For the phenyl-
substituted ligand reported herein, the transformation appears
to proceed further. There are a number of differences between
species D and those complexes [Rh{κ3-H,H,S-H2B(nbe)(mp)}-
(L)2]. First, as a consequence of the substitution at boron, the
coordination mode of the ligand is κ2-H,S in D, whereas in the
previously reported complexes, the coordination mode is κ3-
H,H,S. Clearly, the B−H---Rh interaction in D would not be
expected to be as strong as the BH2Rh dihydroborate
interactions (κ1−H vs κ2−H,H).8,9,24 Thus, it is likely to be
more reactive. The change in steric bulk and the electron density
at boron might also be responsible for further reactivity. Finally,
at least one of the coligands in the previously reported
complexes is a CO ligand. In species D, both coligands are
triphenylphosphine. This might also have a bearing on whether
subsequent transformations occur.25
The conversion of 1 to 2 was complete within 15 min, and it
was not possible to observe any intermediate species
spectroscopically. The postulated mechanism is based on the
location of the deuterium atoms within 2-d2. The location of the
deuterium atom at the position labeled 2 in Scheme 6 is
consistent with the aforementioned steps leading to the
formation of intermediate species D. This is consistent with a
straightforward addition across the double bond (positions 1
and 2). The subsequent steps are a little less clear since the
double bond in the product 2 (2-d2) is in a different position
than in species D (cf. positions 5 and 6 in the product 2 (or 2-d2)
versus 6 and 7 in D). Furthermore, the C−C bond between
positions 1 and 5 is broken in the product. With the norbornenyl
group attached to the boron center, the double bond in this
organic unit can be held in a specific conformation in close
proximity to the rhodium center for further reactivity (as
demonstrated in Scheme 6). For this to occur, the remaining
hydrogen (or deuterium) from boron must be transferred to the
rhodium center (D to E). A migratory insertion of the double
bond would occur leading to species F, the product that would
be formed as a result of a formal hydroboration on one double
bond of norbornadiene and a migration insertion into the other
double bond. This would lead to the addition of the deuterium
atom on the endo face which is consistent with our assignment
of the location of the deuterium atoms. In this sense, the
rhodium and boron centers are working “in cooperation” to
perform these transformations. The final step, F to 2 (or 2-d2),
involves an unprecedented concerted ring-opening step where
the newly formed Rh−C bond is broken, a double bond between
the carbons 5 and 6 is formed and a rhodium−carbon bond is
formed with carbon 1. In summary, these steps involve a
concerted ring-opening process on the norbornenyl unit
between the rhodium and boron centers.
Cleavage of the H−H Bond across the η2-B,C Unit. As
outlined above, there are limited examples of η2-B,C
coordination modes or related compounds reported within the
chemical literature.12−14 This unit has been incorporated into
palladium catalysts with applications for trans hydroboration of
1,3-enynes.12 A recent computational study by Shi focusing on
the involvement of the η2-B,C unit within the mechanism of
hydroboration confirmed that it plays an instrumental role
within the catalytic reaction.12 This role is related to its tight
interaction with the metal center and steric impact rather than
any specific transformations involving this functional group,
however. Their calculations ruled out any ligand-assisted
activation of the borane substrate within the catalytic reaction.
Thus, a borohydride intermediate was not involved in the
transformation within these hydroborations. This evidence
suggested that it might not be possible to carry any further
transformation on the η2-B,C unit within complexes 2 and 3.
However, there has been a number of examples in which ligand-
assisted activations have indeed occurred across Z-type
transition metal borane complexes.2 The rhodium−borane
distances in complexes 2 and 3 are 2.352(3) Å and 2.367(2) Å,
respectively. These distances appear to suggest that it might be
possible for the boron center to accommodate a hydride species
without significant rearrangement (cf. the Rh---B distance of
2.240(2) Å in complex 1 which features a BH2Rh bridging unit).
We therefore set out to explore the reactivity of 2 and 3 with H2
since this could open up a new strategy for the activation of
element−element bonds.2,26
Solutions of 2 and 3 in C6D6 were placed under a hydrogen
atmosphere in a Young’s NMR tubes and investigated by NMR
spectroscopy. An immediate change in color was evident in the
reaction involving 2 where the mixture became lighter in color.
The 11B NMR spectrum confirmed the complete conversion to a
new compound, complex 5, within 10 min (Scheme 8). The very
broad resonance at 34.7 ppm for the starting material was
replaced by a singlet at 5.6 ppm (h.h.w. = 700 Hz). This large
upfield change in chemical shift is indicative of the formation of a
tetra−coordinated borate species. The corresponding 11B{1H}
Scheme 8. Reactivity of 2 and 3 with H2
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NMR spectrum did not provide any clear indication of the
formation of a B−H bond in this case because the signal was too
broad to distinguish between the coupled and decoupled
spectra. Furthermore, the corresponding 1H and 1H{11B} NMR
spectra gave clear evidence for the addition of a hydrogen
substituent at the boron center. A well-resolved doublet of
doublet of doublets signal was located in the hydride region of
the spectrum at −6.88 ppm (1JRhH = 46 Hz, 2JPHtrans = 20 Hz and
2JPHcis = 9 Hz). This signal was broad in the corresponding 1H
NMR spectrum which indicates that this hydrogen is located at
the quadrupolar boron center. The chemical shift is also typical
for a single B−H unit interacting with the rhodium metal center.
The coupling pattern for the BH signal confirms that the two
PPh3 ligands remain coordinated to the metal center cis to each
other. This is also supported by the presence of two doublet of
doublet signals in the 31P{1H} NMR spectrum. The first is
Hydrogen-Based Catalysis. We and others have previously
demonstrated the catalytic activation of hydrogen involving
boron-based ligands.26 Given the reactivity of complexes 2 and 3
with molecular hydrogen (either by direct addition or utilizing
dimethylamine borane as a H2 source) and its formal addition
across the η2-B,C unit, it prompted us to explore their potential
role as catalysts in hydrogenation and dehydrogenation
transformations. Both complexes 2 and 3 were found to be
active catalysts for the dehydrogenation of dimethylamine
borane. Reactions were attempted with 5 mol % catalytic loading
of either 2 or 3 in benzene as solvent. Conversions were
recorded after 20 h. For complex 2, a 95% conversion was
recorded while a conversion of <5% was recorded for 3. While
these results demonstrate promise, there are other systems
within the chemical literature which show greater catalytic
activity for such dehydrogenation reactions.28
2
centered at 38.6 ppm (1JRhP = 168 Hz, JPP = 45 Hz), which
Complexes 2 and 3 were also tested for their application as
hydrogenation catalysts. The results of these are highlighted in
Table 1. Preliminary investigations reveal that these complexes
corresponds to the PPh3 ligand trans to the BH unit, and a
second at 45.5 ppm (1JRhP = 183 Hz and 2JPP = 45 Hz), which
corresponds to the PPh3 ligand trans to the thione unit. With one
hydrogen located at the boron center, it was necessary to work
out the location of the second hydrogen atom. Eleven protons
Table 1. Catalytic Hydrogenation of Olefins with Complexes
a
b
2 and 3 Using H2 Gas or Dimethylamine Borane (AB) as
Source of H2
1
were identified in the H NMR spectrum for the ethylenyl-1-
cyclopentenyl group, confirming that an additional hydrogen
had indeed been incorporated into this fragment. It was,
however, difficult to identify its specific location since these
signals corresponded to a large number of multiplets in the
spectrum. In order to confirm the location, a further deuterium
labeling experiment was performed to make the analogous
complex 5-d2, via addition of D2 to complex 2. A comparison of
the spectra for 5 and 5-d2 revealed that the H2 (or D2) had been
added to the boron center (as discussed above) and to a
methylene group adjacent to the boron (signal at 0.56 ppm).
These assignments were confirmed by 13C{1H} NMR spectros-
copy and a series of correlation experiments. These results
indeed confirm the activation of H2 via formal addition across
the η2−B,C bond. To the best of our knowledge, this is an
unprecedented transformation. Removal of the hydrogen
atmosphere led to decomposition of the complex, unfortunately.
Furthermore, leaving a diethyl ether solution of 5 under a
hydrogen atmosphere for several days led to the isolation of
single crystals identified as [RhH2(κ2-S,N-mp)(PPh3)2] (com-
plex 7; see Supporting Information for details).27
Addition of H2 to a C6D6 solution of complex 3 exhibited the
same reactivity. In this case, the reactivity was significantly
slower, and it took 6 days for the reaction to go to completion.
Nevertheless, the analogous complex, [Rh{κ2-H,S-HB(Ph)-
(CH2CH2C5H7)(mp)}(CO)(PPh3)] (6) was characterized
via NMR spectroscopy (see Experimental Section). This
compound also degraded slowly in solution when left for
periods of over a week. As with 5, it was not possible to isolate a
solid sample of 6 because it decomposed in the absence of a
hydrogen atmosphere.
cat. loading
c
source of
H2
conversion
d
complex
(mol %)
substrate
(%)
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
5.0
5.0
1.0
1.0
0.1
0.1
5.0
5.0
1.0
1.0
0.1
0.1
5.0
5.0
1.0
1.0
0.1
5.0
5.0
1.0
1.0
0.1
H2 gas
AB
H2 gas
AB
H2 gas
AB
H2 gas
AB
H2 gas
AB
H2 gas
AB
H2 gas
AB
H2 gas
AB
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
styrene
styrene
styrene
styrene
styrene
>99
45
70
25
7
19
>99
>99
>99
96
77
47
69
30
11
10
7
styrene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
styrene
styrene
styrene
styrene
styrene
H2 gas
H2 gas
AB
H2 gas
AB
95
95
59
25
10
H2 gas
a
b
c
2.0 bar H2, 4.0 equiv of BH3·NH(CH3)2, Catalysis conducted in 1
d
mL C6D6, conversion measure by NMR integration relative to
internal standard after 18 h at 80 °C.
are active catalysts for the hydrogenation of the substrates
styrene and cyclooctene utilizing both H2 gas or dimethylamine
borane (AB) as sources of hydrogen. Direct use of H2 gas was
most efficient in almost all cases, as expected. The two substrates
were fully converted to ethylbenzene and cyclooctane,
respectively, at 2.0 bar H2 at 80 °C over a period of 18 h,
utilizing 5 mol % catalytic loading of complex 2. Under all
conditions tested, complex 2 demonstrated higher activities than
complex 3. This is likely due to the ligand dissociation step
required for the alkene substrate to coordinate to the metal
center. Ligand dissociation is mostly likely to be more favorable
Both complexes 5 and 6 could also be synthesized by utilizing
dimethylamine borane as source of hydrogen.28 In a typical
reaction, 10 equivalents of the amine borane was added to a
C6D6 solution of complexes 2 or 3, respectively. The complete
conversion of 2 to 5 occurred within 1 h at room temperature
where the main boron-containing byproducts were identified as
H2BNMe2 and NMe2B(H)NMe2.28,29 Dehydrogenation of
the remaining equivalents of amine-borane occurred when left
for 24 h resulting in the formation the cyclic dimer product,
[H2BNMe2]2.
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filtrate was reduced to a few milliliters under reduced pressure. The
solid product was obtained as a fine off-white powder upon addition of
hexane. The supernatant liquid was removed via cannula filtration, and
the solid was dried under dynamic vacuum. Yield: 873.1 mg, 3.25 mmol,
72%. 1H NMR δ (CD3CN): 6.49 (t, 3JHH = 6.6 Hz, 1H, mp5CH), 6.94 (t,
3JHH = 7.2 Hz, 1H, p‑PhCH), 7.06 (overlapping m, 3H, m‑PhCH and
mp4CH), 7.17 (d, 3JHH = 7.5 Hz, 2H, o‑PhCH), 7.41 (d, 3JHH = 8.4 Hz,
in the bis-triphenylphosphine complex. At lower catalyst
loadings (1.0 mol % and 0.1 mol %), these substrates were
reduced to their corresponding alkanes with lower conversions.
Further investigations and optimizations are currently being
carried out to explore the role of the η2-B,C unit within the
catalytic cycle of these hydrogenations.
3
1
1H, mp3CH), 7.98 (d, JHH = 6.3 Hz, 1H, mp6CH). H{11B} NMR δ
CONCLUSIONS
(CD3CN): 3.22 (s, 2H, BH2). 11B NMR δ (CD3CN): −6.26 (t, 1JBH
=
■
92 Hz, BH2). 11B{1H} NMR δ (CD3CN): −6.26 (s, h.h.w. = 55 Hz,
BH2). 13C{1H} NMR δ (CD3CN): 112.9 (s, mp5C), 124.3 (s, p‑PhC),
127.5 (s, m‑PhC), 133.4 (s, mp4C), 134.8 (s, o‑PhC), 135.4 (s, mp3C), 147.6
(s, mp6C), 182.4 (s, mp2C). IR (cm−1, ATR powder film) 2263w (B−H).
FTMS −p NSI: m/z 200.0715 [M-Li]−. Accurate Mass:
(C11H11N110B1S1) mcalc = 199.0747 Da, mexp = 199.0751 Da.
We have introduced an additional boron-based ligand system
which exhibits some interesting reactivity facilitated by the
boron center. The intimate cooperation between the boron and
metal centers provides a means of carrying out an
unprecedented novel ring opening of norbornadiene. This
interesting transformation requires a series of migration steps
and the positioning of the boron and metal centers in close
proximity to each other. The newly formed boron−ethylenyl
cyclopentene unit exhibits an unusual η2−B,C coordination
motif in which the boron center is weakly interacting with the
rhodium center. We have demonstrated for the first time that
this functional motif is capable of reacting with dihydrogen to
undergo its formal addition across the boron−carbon bond.
This could open up a new strategy for the activation of element−
element bonds. We have further demonstrated the dehydrogen-
ation and hydrogenation reactivity of the resulting complexes as
well as their catalytic application.
Synthesis of Li[D2B(Ph)(mp)]. The deuterated analogue of the
ligand was prepared through an identical synthesis to that described for
the Li[H2B(Ph)(mp)], with Li[D3B(Ph)] used in place of Li[H3B-
(Ph)]. 11B NMR δ (C6D6): −6.82 (bs, h.h.w. = 90 Hz, BD2).
Synthesis of [Rh{κ3-S,H,H-H2B(Ph)(mp)}(NBD)] (1). A Schlenk
flask was charged with [RhCl(NBD)]2 (100 mg, 0.217 mmol), which
was fully dissolved in 20 mL of DCM. Li[H2B(Ph)(mp)]·0.85THF
(116.4 mg, 0.434 mmol) was added to the solution. The reaction
mixture was continuously stirred and allowed to react for 1 h before
being filtered into another Schlenk flask. The solvent was removed
under reduced pressure, and the residue was extracted with hexane (20
mL). The hexane was removed under reduced pressure and the solid
product was dried under dynamic vacuum. Yield: 142.1 mg, 0.360
1
mmol, 83%. H NMR δ (C6D6): 1.07 (s, 2H, NBDCH2), 3.28 (s, 2H,
NBDCH), 3.94 (s, 4H, NBDCH), 5.80 (τd, 3JHH = 6.7 Hz, 4JHH = 1.4 Hz,
1H, mp5CH), 6.38 (τd, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 1H, mp4CH), 7.24
(t, 3JHH = 7.4 Hz, 1H, p‑PhCH), 7.35 (t, 3JHH = 7.4 Hz, 2H, m‑PhCH), 7.58
(dt, 3JHH = 8.7 Hz, 4JHH = 1.1 Hz, 1H, mp3CH), 7.61 (d, 3JHH = 7.4 Hz,
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were conducted under inert
atmosphere using standard Schleck line techniques. Solvents were
supplied extra dry from Acros Organics and were stored over 4 Å
molecular sieves. CD3CN and C6D6 NMR solvents were stored in a
Young’s ampule under N2, over 4 Å molecular sieves and had be
degassed through three freeze−pump−thaw cycles prior to use. All
reagents were purchased from commercial sources. The rhodium
bimetallic precursor [RhCl(NBD)]2 was synthesized according to a
published procedure.30 All NMR experiments were conducted on a
Bruker 400 MHz Ascend 400. Infrared spectra where recorded on a
PerkinElmer Spectrum Two ATR FT-IR. Mass spectra were recorded
by the EPSRC NMSF at Swansea University. Proton (1H) and carbon
(13C) assignments (Figure 5) were supported by HSQC, HMBC, and
COSY NMR experiments.
3
1
2H, o‑PhCH), 7.87 (d, JHH = 6.5 Hz, 1H, mp6CH). H{11B} NMR δ
(C6D6): −1.59 (d, 1JRhH = 2 Hz, BH2). 11B NMR δ (C6D6): 0.95 (t, 1JBH
= 62 Hz). 11B{1H} NMR δ (C6D6): 0.95 (s, h.h.w. = 70 Hz). 13C{1H}
NMR δ (C6D6): 49.0 (s, NBDCH), 62.0 (d, 3JRhC = 5.3 Hz, NBDCH2),
114.7 (s, mp5C), 127.2 (s, p‑PhCH), 128.5 (s, m‑PhCH), 128.9 (d, 3JRhC
=
1.5 Hz, mp3C), 132.3 (s, mp4C), 136.6 (s, o‑PhCH), 143.9 (s, mp6C), 175.1
(d, 2JRhC = 1.8, mp2C). IR (cm−1, ATR powder film) 1827w (BH2Rh). +
p EI MS: m/z 395.0 [M]+. Accurate Mass: (C18H19N110B1Rh1S1) mcalc
394.0417 Da, mexp = 394.0419 Da.
=
Synthesis of [Rh{κ3-S,D,D-D2B(Ph)(mp)}(NBD)] (1-d2). This
complex was synthesized in a similar manner to the nondeuterated
analogue, complex 1, by the addition of 2 equiv of Li[D2B(Ph)(mp)] to
[RhCl(NBD)]2. D NMR δ (C6D6): −1.58 (bs, BD2). 11B NMR δ
2
(C6D6): 0.44 (s, h.h.w. = 90 Hz, BD2).
Synthesis of [Rh{κ2-S,B-B(Ph)(mp)(CHCH2C5H7)}(PPh3)2] (2). A
Schlenk flask was charged with [Rh{κ3-S,H,H-H2B(Ph)(mp)}(NBD)]
(50 mg, 0.127 mmol) and PPh3 (66.4 mg, 0.253 mmol). Benzene (10
mL) was added to the flask, and the reaction mixture was continuously
stirred for 1 h. The solvent was thereafter minimized to approximately 2
mL under reduced pressure. Hexane (20 mL) was subsequently added
to precipitate out an orange/brown solid. The solid was separated from
the supernatant via cannula filtration before being dried under dynamic
vacuum. Yield: 88.5 mg, 0.096 mmol, 76%. 1H NMR δ (C6D6): 1.83 (m,
1H, ecp4aCH2), 2.03 (m, 1H, ecp7bCH2), 2.05 (m, 1H, ecp3CH), 2.29 (m,
1H, ecp4bCH2), 2.42 (m, 1H, ecp7aCH2), 2.56 (m, 1H, ecp2bCH2), 2.84
(m, 1H, ecp2aCH2), 3.13 (m, 1H, ecp1CH), 5.58 (m, 1H, ecp6CH), 5.67
(m, 1H, ecp5CH), 5.68 (τd, 3JHH = 6.7 Hz, 4JHH = 1.2 Hz, 1H, mp5CH),
6.19 (τd, 3JHH = 6.9 Hz, 4JHH = 1.6 Hz, 1H, mp4CH), 6.79 (d, 3JHH = 8.6
Hz, 1H, mp3CH), 6.89 (m, 9H, PPhCH), 6.93 (m, 9H, PPhCH), 7.16 (d,
3JHH = 5.6 Hz, 1H, mp6CH), 7.36 (unresolved, 1H, PhCH), 7.39
Figure 5. Numbering scheme used for assignments of NMR spectra
(see Figure S12.1 in Supporting Information for assignment of
stereochemistry for complex 2).
Synthesis of Li[H2B(Ph)(mp)]·0.85(THF). A Schlenk flask was
charged with Li[H3B(Ph)] (881.7 mg, 9.01 mmol) and 2-
mercaptopyridine (500.7 mg, 4.5 mmol). Dry degassed THF (50
mL) was added, and the reaction mixture was stirred continuously for
72 h. After this time, the reaction mixture was filtered into a new
Schlenk flask, the solvent volume was reduced to approximately 10 mL,
and hexanes (10 mL) were added to precipitate a small amount of a
white solid. The reaction mixture was again filtered and the resulting
3
(unresolved, 2H, PhCH), 7.54 (t, JHH = 8.4 Hz, 6H, PPhCH), 7.66
(unresolved, 2H, PhCH), 7.66 (t, 6H, PPhCH). 11B NMR δ (C6D6): 34.7
(bs, h.h.w. = 1550 Hz, BC). 11B{1H} NMR δ (C6D6): 34.7 (bs, h.h.w.
= 1550 Hz, BC). 31P{1H} NMR δ (C6D6): 31.90 (dd, 1JRhP = 134.2
2
1
2
Hz, JPP = 32.6 Hz). 47.75 (dd, JRhP = 175.8 Hz, JPP = 33.2 Hz).
13C{1H} NMR δ (C6D6): 39.36 (s, ecp4CH2), 39.38 (s, ecp7CH2), 41.4
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(d, ecp2CH2), 42.9 (d, ecp3CH), 58.1 (Assigned through HSQC,
ecp1CH), 113.9 (s, mp7CH), 126.7 (s, PhC), 127.3 [d, JPC = 9.1 Hz,,
P(C6H5)3], 127.8 (unresolved, PhC), 128.6 (unresolved, mp3CH), 128.6
[unresolved, P(C6H5)3], 128.8 [unresolved, P(C6H5)3], 130.3 (s,
ecp6CH), 130.6(s, ecp5CH), 134.4 (s, PhC), 135.4 [d, JPC = 12.4 Hz,
P(C6H5)3], 135.5 (s, mp4CH), 135.8 [d, JPC = 11.7 Hz, P(C6H5)3],
136.8 [d, JPC = 32.5 Hz, i-P(C6H5)3], 137.8 [d, JPC = 36.5 Hz, i-
P(C6H5)3], 142.8 (s, mp6CH), 147.7 (bs, assigned through HMBC,
i‑PhC), 177.4 (dd, 2JRhC = 15.1 Hz, 3JPC = 2.3 Hz, mp2C). IR (cm−1, ATR
powder film) 3050, 1426. + p EI MS: m/z 737.1 [M − B(Ph)(C7H10)]+,
720.1 [M − B(Ph)(mp)]+.
Synthesis of [Rh{κ2-S,B-B(Ph)(mp)(CHCHDC5H6D)}(PPh3)2] (2-
d2). This complex was synthesized in a similar manner to the
nondeuterated analogue, complex 2, by the addition of 2 equiv of PPh3
to complex 1-d2 in benzene. 2D NMR δ (C6D6): 1.98 (bs, ecp7bCHD),
2.79 (bs, ecp2aCHD).
13C{1H} NMR δ (C6D6): 23.1 (bs, ecp1CH2), 35.0 (s,, ecp2CH2), 39.2 (s,
ecp4,7CH2), 39.9 (s, ecp4,7CH2), 41.1 (s, ecp3CH), 114.6 (s, mp5CH), 125.5
(s, PhC), 127.2 (s, PhC), 127.5 [d, JPC = 12.6 Hz, P(C6H5)3], 128.9 [d,
J
PC = 12.5 Hz, P(C6H5)3], 130.3 (s, ecp5,6C), 130.9 (d, 3JRhC = 3.4 Hz,
mp3CH), 133.1 (s, mp4CH), 134.8 (s, PhC), 134.9 [d, JPC = 12.4,
P(C6H5)3], 135.4 [d, JPC = 11.3 Hz, P(C6H5)3], 138.2 [d, 1JPC = 37.0
Hz, P(C6H5)3], 143.2 (s, mp6CH), 145.8 (bs, assigned through HMBC,
i‑PhC), 179.1 (dd, 2JRhC = 11.1 Hz, mp2C).
Synthesis of [Rh{κ2-S,D-HB(Ph)(mp)(CHDCH2C5H7)}-(PPh3)]
(5-d2). This complex was synthesized in a similar manner to the
nondeuterated analogue, complex 4, by the addition of D2 to complex 2
in benzene. 2D NMR δ (C6D6): −6.90 (bs, BD) 0.57 (bs, ecp1CHD).
Synthesis of [Rh{κ2-S,H-HB(Ph)(mp)(CH2CH2C5H7)}(CO)-
(PPh3)] (6). A Young’s NMR tube was charged with [Rh{κ2-S,H-
HB(Ph)(mp)(CH2CH2C5H7)}(CO)(PPh3)] (3 mg, 4.38 × 10−3
mmol) and dissolved in 0.8 mL of benzene-d6. The resulting solution
was degassed through three consecutive freeze−pump−thaw cycles
before the headspace was subsequently filled with H2. The reaction
mixture was left to react for 6 days before the NMR of the product was
Synthesis of [Rh{κ2-S,B-B(Ph)(mp)(CHCH2C5H7)}(CO)(PPh3)]
(3). A Schlenk flask was charged with [Rh{κ3-S,H,H-H2B(Ph)(mp)}
(NBD)] (50 mg, 0.127 mmol) and PPh3 (66.4 mg, 0.253 mmol).
Benzene (10 mL) was added to the flask, and the reaction mixture was
continuously stirred for 1 h. Once this time had elapsed the reaction
mixture was freeze−pump−thaw degassed, and the headspace was filled
with CO gas. The reaction mixture was left stirring for 24 h before the
solvent was removed under reduced pressure. The pure product was
obtained as a yellow solid after recrystallization from diethyl ether.
1
recorded. H NMR δ (C6D6): 1.58 (m, 1H, ecp1CH2), 1.65 (m, 1H,
ecp2CH2), 2.09 (m, 1H, ecp2CH2), 2.15 (m, 2H, epc4,7CH2), 2.54 (m, 1H,
ecp3CH), 2.63 (m, 2H, epc4,7CH2), 5.72 (s, 2H, ecp5,6CH), 5.95 (τd, 3JHH
= 6.8 Hz, 1H, mp5CH), 6.30 (τd, 3JHH = 6.8 Hz, 1H, mp4CH), 7.02 (m,
9H, PPhCH), 7.24 (d, 1H, mp3CH), 7.55 (m, 6H, PPhCH), 7.65 (d, 1H,
mp6CH), 1H{11B} NMR δ (C6D6): −3.57 (dd, 1JRhH = 45.5 Hz, 3JPH
=
20.6 Hz). 11B NMR δ (C6D6): 3.7 (bs, h.h.w. = 440 Hz, BH). 11B{1H}
NMR δ (C6D6): 3.7 (bs, h.h.w. = 400 Hz, BH). 31P{1H} NMR δ
(C6D6): 41.8 (d, 1JRhP = 160.4 Hz).13C{1H} NMR δ (C6D6): 24.6 (bs,
ecp1CH2), 34.2 (s, ecp2CH2), 38.4 (s, ecp4,7CH2), 38.5 (s, ecp4,7CH2), 47.8
(s, ecp3CH), 114.9 (s, mp5CH), 127.3 [unresolved, P(C6H5)3], 129.0 [s,
P(C6H5)3], 129.3 (s, ecp5,6C), 126.3 (d, mp3CH), 133.1 (s, mp4CH),
133.5 [s, P(C6H5)3], 142.2 (s, mp6CH), 177.8 (s, mp2C).
1
Yield: 53.4 mg, 0.078 mmol, 61%. H NMR δ (C6D6): 2.17 (m, 1H,
ecpCH2), 2.23 (m, 1H, ecpCH2), 2.28 (m, 1H, ecpCH2), 2.62 (m, 1H,
ecpCH2), 2.68 (m, 1H, ecpCH2), 2.69 (m, 1H, ecpCH2), 2.84 (m, 1H,
ecp3CH), 4.82 (m, 1H, ecp1CH), 5.71 (s, 2H, ecp5,6CH), 5.82 (τd, 3JHH
=
3
4
6.7 Hz, 1H, mp5CH), 6.29 (τd, JHH = 7.2 Hz, JHH = 1.6 Hz, 1H,
mp4CH), 6.84 (d, 3JHH = 8.5 Hz, 1H, mp3CH), 7.01 (m, 9H, m/p‑PPhCH),
7.17 (d, 1H, mp6CH), 7.21 (t, 3JHH = 7.3 Hz, 1H, p‑PhCH), 7.31 (t, 3JHH
=
B
7.5 Hz, 2H, m‑PhCH), 7.66 (m, 6H, o‑PPhCH), 7.68 (d, 2H, o‑PhCH). 11
NMR δ (C6D6): 33.8 (bs, h.h.w. = 600 Hz, BPh). 11B{1H} NMR δ
(C6D6): 33.8 (bs, h.h.w. = 600 Hz, BPh). 31P{1H} NMR δ (C6D6):
31.95 (d, 1JRhP = 132.9 Hz). 13C{1H} NMR δ (C6D6): 39.4 (s, ecpCH),
40.0 (s, ecpCH), 40.3 (s, ecpCH), 41.3 (s, ecp3CH), 61.9 (bs, assigned
CATALYTIC INVESTIGATIONS
■
General procedure for the catalytic hydrogenation of cyclo-
octene and styrene by complexes 2 and 3. For 5 mol % catalytic
loading using 2 and 3: A Young’s NMR tube was charged with
complex 2 (3.0 mg, 3.26 × 10−3 mmol) or complex 3 (3.0 mg,
4.38 × 10−3 mmol) in addition to 1,3,5-trimethoxybenzene
(10.0 mg, 5.95 × 10−2 mmol) as an internal standard. To
complex 2 was added 0.8 mL of C6D6 containing cyclooctene
(8.5 μL) or styrene (7.3 μL) and to complex 3, 1.0 mL of C6D6
containing cyclooctene (11.4 μL) or styrene (9.8 μL). The
reaction mixture was degassed through three consecutive
freeze−pump−thaw cycles before the headspace was pressur-
ized to 2.0 bar with hydrogen. The NMR tube was heated to 80
°C for 18 h. The conversion from the olefin to the hydrogenated
product was determined by peak integrals relative to the internal
standard (1,3,5-trimethoxybenzene). A similar approach was
adopted for 1 mol % catalytic loading using 2 and 3: Complex 2
(0.6 mg, 6.52 × 10−4 mmol) or complex 3 (3.0 mg, 8.76 × 10−4
mmol) in addition to 1,3,5-trimethoxybenzene (10.0 mg, 5.95 ×
10−2 mmol) was added to a Young’s NMR tube. To complex 2
was added 0.8 mL of C6D6 containing cyclooctene (8.5 μL) or
styrene (7.3 μL) and to complex 3, 1.0 mL of C6D6 containing
cyclooctene (11.4 μL) or styrene (9.8 μL). For 0.1 mol %
catalytic loading of 2 and 3: complex 2 (3.0 mg, 3.26 × 10−3
mmol) or complex 3 (3.0 mg, 4.38 × 10−3 mmol) in addition to
1,3,5-trimethoxybenzene (10.0 mg, 5.95 × 10−2 mmol) was
added to a Young’s NMR tube. To complex 2 was added 0.8 mL
of C6D6 containing cyclooctene (85 μL) or styrene (73 μL) and
to complex 3, 1.0 mL of C6D6 containing cyclooctene (114 μL)
or styrene (98 μL). Hydrogen, at 2.0 bar pressure, was added
after degassing, and the NMR tube was heated to 80 °C for 18 h.
The conversion from the olefin to the hydrogenated product was
through HSQC, ecp1CH), 115.9 (s, mp5CH), 127.2 (s, p‑PhC), 128.2
3
(unresolved, m‑PhC), 128.4 [unresolved, P(C6H5)3], 129.4 (d, JRhC
=
2.2 Hz, mp3CH), 129.9 [d, JPC = 2.0 Hz, P(C6H5)3], 130.4 (d, JRhC = 35.6
Hz, ecp5,6C), 133.0 (s, o‑PhC), 134.5 [d, 1JPC = 40.4, i-P(C6H5)3], 134.9
[d, 2JPC = 12.7 Hz, o-P(C6H5)3], 136.8 (s, mp4CH), 143.0 (s, mp6CH),
144.3 (bs, assigned through HMBC, i‑PhC), 177.4 (dd, 2JRhC = 17.0 Hz,
mp2C), 193.3 (dd, 1JRhC = 72.2 Hz, 2JPC = 14.9 Hz, CO). IR (cm−1, ATR
powder film) 1945 (CO). + p EI MS: m/z 685.0 [M]+, 657.0 [M −
(CO)]+, 563.0 [M − (CO) − (C7H10)]+. Accurate Mass:
(C37H34O1N110B1P1Rh1S1) mcalc = 684.1278 Da, mexp = 684.1278 Da;
(C36H34N110B1P1Rh1S1) mcalc = 656.1328 Da, mexp = 656.1329 Da.
Synthesis of [Rh{κ2-S,H-HB(Ph)(mp)(CH2CH2C5H7)}(PPh3)2]
(5). A Young’s NMR tube was charged with [Rh{κ2-S,H-HB(Ph)-
(mp)(CH2CH2C5H7)}(PPh3)2] (3 mg, 3.26 × 10−3 mmol) and
dissolved in 0.8 mL of benzene-d6. The resulting solution was degassed
through three consecutive freeze−pump−thaw cycles before the
headspace was subsequently filled with H2. The reaction mixture was
left to react for 30 min before the NMR of the product was recorded. 1H
NMR δ (C6D6): −6.88 (bd, h.h.w. = 90 Hz, 1H, BH), 0.56 (td, 3JHH
=
13.6 Hz, 1H, ecp1CH2), 0.70 (td, 3JHH = 13.6 Hz, 1H, ecp1CH2), 1.12 (m,
1H, ecp2CH2), 1.56 (m, 1H, ecp2CH2), 1.89 (m, 2H, epc4,7CH2), 2.10 (m,
1H, ecp3CH), 2.47 (m, 2H, epc4,7CH2), 5.69 (s, 2H, ecp5,6CH), 5.81 (τd,
3JHH = 6.7 Hz, 4JHH = 1.2 Hz, 1H, mp5CH), 6.19 (τd, 3JHH = 6.9 Hz, 4JHH
= 1.5 Hz, 1H, mp4CH), 6.91 (m, 18H, PPhCH), 7.27 (d, 1H, mp3CH),
7.29 (m, 5H, PPhCH), 7.50 (d, 3JHH = 6.4 Hz, 1H, mp6CH), 7.59 (m, 6H,
PPhCH), 7.65 (m, 6H, PPhCH). 1H{11B} NMR δ (C6D6): −6.88 (ddd,
1JRhH = 45.5 Hz, 3JPH = 20.6 Hz, 3JPH = 8.9 Hz, 1H, BH). 1H{31P} NMR
δ (C6D6): −6.88 (bd, 1JRhH = 20.8 Hz, 1H, BH). 11B NMR δ (C6D6):
5.5 (bs, h.h.w. = 720 Hz, BH). 11B{1H} NMR δ (C6D6): 5.5 (bs, h.h.w.
= 690 Hz, BH). 31P{1H} NMR δ (C6D6): 38.6 (dd, 1JRhP = 168.0 Hz,
2JPP = 45.3 Hz, PPh3), 45.5 (dd, 1JRhP = 182.7 Hz, 2JPP = 45.1 Hz, PPh3).
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determined by peak integrals relative to the internal standard
(1,3,5-trimethoxybenzene).
AUTHOR INFORMATION
Corresponding Author
■
Gareth R. Owen − School of Applied Sciences, University of South
Wales, Pontypridd CF37 4AT, U.K.; orcid.org/0000-0002-
8695-757X; Email: gareth.owen@southwales.ac.uk
CRYSTALLOGRAPHY
■
Single-crystal X-ray diffraction studies of complexes 1, 2, 3, and 7
were carried out at the UK National Crystallography Service at
the University of Southampton.31 Single crystals of the
complexes 1 and 3 were obtained by allowing a saturated
ether solution stand at room temperature, while those of 2 were
prepared from a saturated solution of the complex dissolved in
acetone. Single crystals of 7 were obtained by allowing a diethyl
ether solution of [Rh(κ2-H,S-HB(Ph)(CH2CH2C5H7)(mp)}-
(PPh3)2] (5) for several days. A single crystal from each sample
was mounted on a MITIGEN holder in perfluoroether oil on a
Rigaku FRE+ Mo anode equipped with HF Varimax confocal
mirrors and an AFC12 goniometer and HG Saturn 724+
detector (1, 2) or HyPix 6000 hybrid pixel detector (3) or a
Rigaku 007HF Cu anode equipped with Varimax confocal
mirrors and an AFC11 goniometer and HyPix 6000 hybrid pixel
detector (7). The crystals were kept at T = 100(2) K during data
collection. Data were collected and processed and empirical
absorption corrections were carried out using CrysAlisPro.32
The structures were solved by Intrinsic Phasing using the
ShelXT structure solution program33 and refined on Fo2 by full-
matrix least-squares refinement with version 2017/1 of ShelXL
for complexes 1 and 2 and version 2018/3 for complex 334 as
implemented in Olex2.35 All hydrogen atom positions were
calculated geometrically and refined using the riding model with
the following exceptions. In the structures of 1 and 7, H1A and
H1B bonded to B1 were located in the difference map and were
refined with the riding model and their positions allowed to
freely refine. In structure 2, one of the triphenylphosphine
ligands is disordered over two positions (ca. 50:50). Also, one of
the solvent acetone molecules is disordered over two positions
(ca. 50:50). Structure 7 solved as a nonmerohedral twin with the
second component rotated by 179.9117° about the b-axis. A
summary of the crystallographic data collection parameters and
refinement details for all complexes are presented in the table
below. Anisotropic parameters, bond lengths and (torsion)
angles for these structures are available from the CIF files which
have been deposited with the Cambridge Crystallographic Data
Centre and given the following deposition numbers, 1955411
(1), 1955412 (2), 1955413 (3) and 1955414 (7). These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Authors
Angelo Iannetelli − School of Applied Sciences, University of
South Wales, Pontypridd CF37 4AT, U.K.
Rosenildo C. Da Costa − School of Applied Sciences, University
of South Wales, Pontypridd CF37 4AT, U.K.; orcid.org/
0000-0002-1005-8455
Alan J. Guwy − Sustainable Environment Research Centre,
University of South Wales, Pontypridd CF37 4AT, U.K.
Graham J. Tizzard − UK National Crystallography Service,
University of Southampton, Highfield, Southampton SO17 1BJ,
U.K.
Simon J. Coles − UK National Crystallography Service,
University of Southampton, Highfield, Southampton SO17 1BJ,
U.K.; orcid.org/0000-0001-8414-9272
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00155
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
■
The authors would like to thank the Leverhulme Trust (PRG-
2015-097) for funding (RCDC, GRO). The project is also part
funded by the FLEXIS research project (WEFO 80835). We are
also very grateful to the EPSRC UK National Mass
Spectrometry Facility (NMSF) at Swansea University and to
Johnson Matthey for the loan of the platinum group metal salts.
REFERENCES
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00155.
Crystallographic parameters for all crystal structures in
addition to selected spectroscopic data (PDF)
Accession Codes
CCDC 1955411−1955414 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
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(27) The crystal structure of the corresponding iridium complexes has
previously been reported see: Mura, P.; Robinson, S. D. Structure of cis-
M
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Organometallics XXXX, XXX, XXX−XXX