Stoichiometric and catalytic solid-gas reactivity of rhodium bis-phosphine complexes

Article abstract of DOI:10.1021/om5013133

The complexes [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)L₂][BAr^F₄] [L₂ = C₄H₆, (C₂H₄)₂, (CO)₂, (NH₃)₂; Ar^F = 3,5-C₆H₃(CF₃)₂] have been synthesized by solid-gas reactivity via ligand exchange reactions with, in some cases, crystallinity retained through single-crystal to single-crystal transformations. The solid-state structures of these complexes have been determined, but in only one case (L₂ = (NH₃)₂) is the cation ordered sufficiently to enable its structural metrics to be determined by single crystal X-ray diffraction. The onward solid-state reactivity of some of these complexes has been probed. The bis-ammonia complex [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(NH₃)₂][BAr^F₄] undergoes H/D exchange at bound NH₃ when exposed to D₂. The bis-ethene complex [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(C₂H₄)₂][BAr^F₄] undergoes a slow dehydrogenative coupling reaction to produce a material containing a 1:1 mixture of the butadiene complex and a postulated mono-ethene complex. The mechanisms of these processes have been probed by DFT calculations on the isolated Rh cations. All the solid materials were tested as heterogeneous catalysts for the hydrogenation of ethene. Complexes with weakly bound ligands (e.g., L₂ = (C₂H₄)₂) are more active catalysts than those with stronger bound ligands (e.g., L = (CO)₂). Surface-passivated crystals, formed through partial reaction with CO, allow for active sites to be probed, either on the surface or the interior of the single crystal.

Full text of DOI:10.1021/om5013133

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Article
pubs.acs.org/Organometallics
Stoichiometric and Catalytic Solid−Gas Reactivity of Rhodium Bis-
phosphine Complexes
Sebastian D. Pike,† Tobias Kramer,‡ Nicholas H. Rees,† Stuart. A. Macgregor,*,‡ and Andrew S. Weller*,†
̈
†Department of Chemistry, Chemistry Research Laboratories, Oxford University, Mansfield Road, Oxford OX1 3TA, U.K.
‡Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, U.K.
S
* Supporting Information
ABSTRACT: The complexes [Rh(iBu2PCH2CH2PiBu2)L2][BArF ]
4
[L2 = C4H6, (C2H4)2, (CO)2, (NH3)2; ArF = 3,5-C6H3(CF3)2] have
been synthesized by solid−gas reactivity via ligand exchange reactions
with, in some cases, crystallinity retained through single-crystal to
single-crystal transformations. The solid-state structures of these
complexes have been determined, but in only one case (L2 = (NH3)2)
is the cation ordered sufficiently to enable its structural metrics to be
determined by single crystal X-ray diffraction. The onward solid-state
reactivity of some of these complexes has been probed. The bis-
ammonia complex [Rh(iBu2PCH2CH2PiBu2)(NH3)2][BArF ] under-
4
goes H/D exchange at bound NH3 when exposed to D2. The bis-ethene complex [Rh(iBu2PCH2CH2PiBu2)(C2H4)2][BArF ]
4
undergoes a slow dehydrogenative coupling reaction to produce a material containing a 1:1 mixture of the butadiene complex
and a postulated mono-ethene complex. The mechanisms of these processes have been probed by DFT calculations on the
isolated Rh cations. All the solid materials were tested as heterogeneous catalysts for the hydrogenation of ethene. Complexes
with weakly bound ligands (e.g., L2 = (C2H4)2) are more active catalysts than those with stronger bound ligands (e.g., L =
(CO)2). Surface-passivated crystals, formed through partial reaction with CO, allow for active sites to be probed, either on the
surface or the interior of the single crystal.
Scheme 1. Solid−Gas Reaction to Form a σ-Alkane
Complex, and the Eventual, Anion-Bound, Thermodymanic
Product (S.S. = Solid-State; SC−SC = Single-Crystal to
Single-Crystal)
1. INTRODUCTION
While the study of organometallic complexes in the solution
phase is widely explored,1 investigation of their solid-state
reactivity is less well-developed.2−7 Potential advantages of
reactivity in the solid-state lead from the constrained local
environment that may influence selectivity and the lack of
possibly deleterious solvent interactions. Indeed, for the
isolation of highly reactive organometallic species, the solid
state might be considered as the “perfect solvent” in that it
offers a potentially noncoordinating environment. Demonstrat-
ing these advantages, we have recently reported the synthesis of
σ-alkane complexes by a solid−gas route, e.g. [Rh-
(iBu2PCH2CH2PiBu2)(NBA)][BArF ] (2) [ArF = 3,5-
4% change in unit-cell volume, although some examples report
larger changes.19−21
4
4
For the transformation of complex 1 to 2, that retains
crystallinity and only has a 1.5% change in unit-cell volume, we
postulated that packing in the lattice is dominated by bulky
C6H3(CF3)2], in which a saturated norbornane (NBA)
fragment coordinates with the metal center through two 3-
center 2-electron interactions.8,9 Complex 2 was generated in
situ in a single-crystal to single-crystal (SC−SC) transformation
by simple addition of H2 to a crystalline norbornadiene (NBD)
[BArF ]− anions which also form a defined octahedral cavity
4
around 1 that is retained almost exactly in 2. The anions thus
have a dual role to play by defining the lattice and also allowing
for structural change in the organometallic cation (i.e., NBD to
NBA). Complex 2 is unstable at room temperature in the solid
state, eventually forming the zwitterion [Rh-
precursor, [Rh(iBu2PCH2CH2PiBu2)(NBD)][BArF ] (1),
4
Scheme 1.
Ligand exchange reactions via solid−gas reactivity of
organometallic complexes have been reported previously,2,4,5,10
and sometimes these can be SC−SC transformations in which
the crystallinity of the sample is retained throughout the
reaction, allowing for characterization of the product directly by
X-ray crystallography.3,5,6,10−18 Typically these transitions occur
with minimal disruption to the gross lattice, with often less than
(iBu2PCH2CH2PiBu2){η6-C6H3(CF3)2}BArF ], 3, and free
3
NBA, the thermodynamic products of the reaction.8 However,
Received: December 22, 2014
Published: March 9, 2015
© 2015 American Chemical Society
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we posited that freshly prepared 2 could act as a suitable
precursor for solid−gas reactivity, as the weakly bound alkane
ligand could be easily displaced by an incoming ligand. In this
contribution, we detail such reactivity with the exogenous
ligands butadiene, ethene, CO, and NH3 to form the
corresponding adducts in the solid state and their onward
reactivity. Some of these reactions are also SC−SC processes.
We also document reactivity (H/D exchange, dehydrocou-
pling) and catalysis (hydrogenation of ethene) in these well-
defined solid materials.
Analytically pure 6 can be generated by recrystallization from
CH2Cl2/pentane while under 1 atm of ethene. Complex 6 is
not stable at room temperature in solution in the absence of an
ethene atmosphere. For example, in CH2Cl2 solution,
decomposition to 3 occurs, whereas in 1,2-C6H4F2 complex 5
forms, making solution-based routes less than ideal. By contrast,
solid 6 does not lose ethene even when placed under a vacuum,
as indicated by elemental microanalysis of a crystalline sample.
That 6 is stable to vacuum in the solid-state suggests that
ethene loss might proceed via an associative mechanism within
the crystal and not by loss of ethene followed by coordination
of an exogenous ligand. Similar observations have been made
previously for the substitution of a dinitrogen ligand at a
{Ir(POCOP)} fragment in the solid-state [POCOP = 2,6-
bis(di-tert-butylphosphinito)benzene].5
2. RESULTS AND DISCUSSION
2.1. Synthesis of Butadiene and Bis-ethene Com-
plexes. The addition of 1,3-butadiene gas (∼4 atm) to freshly
prepared crystalline 2 results in the rapid (∼1 min) formation
of the dark-red/purple butadiene complex [Rh-
1
The H NMR spectrum of complex 6 at 240 K in CD2Cl2
(iBu2PCH2CH2PiBu2)(η4-C4H6)][BArF ], 4, with the concom-
reveals a sharp signal assigned to coordinated ethene (δ 3.98,
fwhm = 7 Hz, relative integral 8 H),24 while the 31P{1H} NMR
spectrum at 240 K shows a doublet signal (δ 54.7, JRhP = 144
Hz). At room temperature under four atmospheres of ethene,
4
itant release of one equivalent of NBA (Scheme 2) as measured
Scheme 2. Synthesis of Complex 4 by Solid-State and
Solution Routes
1
the H NMR spectrum shows a broad signal assigned to free
ethene (δ 5.31, fwhm = 49 Hz), with no separate peak observed
for coordinated ethene, suggesting rapid exchange between
coordinated and free ethene in solution. The 31P{1H} NMR
spectrum displays a single environment essentially unchanged
from that at 240 K. Warming to 298 K in the absence of ethene
results in loss of ethene to form 3. The solid-state 31P{1H}
NMR spectrum shows two very broad environments, consistent
with a disordered cation in the solid state (vide infra).
2.2. Synthesis of a Bis-carbonyl Complex by a Gas−
Solid Reaction. Complexes such as 4 and 6 present ideal
opportunities for solid-state reactivity, as the alkene ligands are
likely to be relatively labile. Reaction with hydrogen in the solid
state rapidly (less than 1 min) releases butane/ethane
(observed by 1H gas-phase NMR spectroscopy, see Supporting
Information) with the concomitant formation of amorphous 3
as determined by 31P{1H} SSNMR spectroscopy, Scheme 4.
1
by H NMR spectroscopy in CH2Cl2 solution. Complex 4
forms as an amorphous solid, i.e., this process is not a SC−SC
transformation but can be conveniently recrystallized from 1,2-
C6H4F2/pentane to afford analytically pure material. Complex 4
can also be formed directly by solution routes by the addition
of 1,3-butadiene gas to a 1,2-C6H4F2 solution of [Rh-
(iBu2PCH2CH2PiBu2)(η6-1,2-C6H4F2)][BArF ], 5. The solu-
4
tion 1H NMR spectrum (CD2Cl2) of 4 prepared by either route
reveals three peaks of equal integration (δ 2.85, 4.27 and 5.44; 2
H each) attributed to the diene, which occur upfield in
comparison to the 1H NMR signals of free 1,3-C4H6 (δ (CCl4)
5.03, 5.14, and 6.27),22 consistent with coordination to a metal
center. The solution 31P{1H} NMR spectrum shows a doublet
(δ 54.6, JRhP = 170 Hz). These data are similar to those
reported for the closely related analogue [Rh-
(iPr2PCH2CH2CH2PiPr2)(η4-C4H6)][CF3SO3].23 The solid-
state 31P{1H} NMR spectrum shows a number of environments
(i.e., more than 2), consistent with a disordered cation (vide
infra and Supporting Information).
Scheme 4. Addition of H2 to Complexes 4 or 6
Presumably, the first formed alkane is lost from the metal
Addition of ethene gas to crystalline 2 forms a red, oily
product assigned to the bis-ethene adduct [Rh-
center to result in [Rh(iBu2PCH2CH2PiBu2)]+ coordinated
with the [BArF ]− anion, with the associated large structural
(iBu2PCH2CH2PiBu2)(η2-C2H4)2][BArF ], 6, and free NBA as
4
4
change leading to the loss of crystallinity. By contrast, addition
measured by solution NMR spectroscopy (Scheme 3).
of CO gas (1 atm) to solid 4 or 6 forms bright yellow
[Rh(iBu2PCH2CH2PiBu2)(CO)2][BArF ], 7, with release of
Scheme 3. Formation of Complex 6 in the Solid-State and
the Decomposition Reactions That Occur in Solution When
Excess Ethene Is Not Present
4
1,3-butadiene or ethene into the head-space of the reaction
vessel (a sealed NMR tube) and no loss of crystallinity (Figure
1). Complex 7 can also be produced by addition of CO gas to 5
in solution (1,2-C6H4F2). The solution 31P{1H} NMR
spectrum of 7 shows a single resonance (δ 56.1, JRhP = 116
Hz). The 31P{1H} SSNMR spectrum also shows one
environment, which is consistent with its solid-state structure
(vide infra). The CO stretching frequencies in the IR spectra of
7 shows the anticipated two CO stretching bands (CH2Cl2,
2093.4 and 2049.1 cm−1; ATR InfraRed, 2099.0 and 2056.9
cm−1). These data are similar to closely related cationic Rh bis-
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2.3. Synthesis of a Bis-ammonia Complex. Addition of
ammonia gas to freshly prepared 2, in the solid state, results in
the formation of bis-ammonia complex [Rh-
(iBu2PCH2CH2PiBu2)(NH3)2][BArF ], 8, alongside free NBA
4
(Scheme 5). Crystallinity is lost in this reaction. Complex 8 can
Scheme 5. Synthesis of Complex 8 by Solid/Gas Reactions
be recrystallized from 1,2-C6H4F2/pentane solutions to afford
crystals that were suitable for X-ray diffraction. To avoid traces
of 3 in the final product, the direct addition of a mixture of NH3
and H2 to crystalline 1 was found to be the most effective way
of synthesizing 8. Ammonia complexes similar to 8 have been
previously reported,26−28 in particular the closely related
[Rh(dppe)(NH3)2]+.29 Solution methods using 1,2-C6H4F2
solvent were unsuccessful for the synthesis of 8 with
decomposition to unidentified species occurring, even though
complex 8 is stable in 1,2-C6H4F2 solvent once formed by the
solid-state route. Complex 8 can also be synthesized by the
relatively rapid reaction (1 h) of solid 4 with NH3 gas (with
release of butadiene gas). Neither of these routes are SC−SC
reactions, with the product forming as an amorphous solid, in
contrast to the analogous reaction of 4 with CO. The 31P{1H}
NMR spectrum of 8 solvated in 1,2-C6H4F2 displays a doublet
(δ 65.5, JRhP = 177 Hz), similar to [Rh(dppe)(NH3)2]+.29 The
1H NMR spectrum of 8 contains a sharp peak at δ 1.93 (relative
integral 6 H) assigned to the coordinated ammine ligands. The
proton chemical shift of ammine ligands is dependent upon
hydrogen-bonding interactions with solvent or anion,26 and in
the case of 8 with a weakly coordinating anion in the weakly
coordinating 1,2-C6H4F2 solvent, the chemical shift observed is
upfield of other reported values in similar complexes.26
Complex 8 does not react with ethene in the solid state but
does react with D2, undergoing H/D exchange at the NH3
ligand (Section 2.5).
Figure 1. Reaction of 4 with CO in the solid state to form complex 7.
Optical microscope pictures of single crystals of 4, CO-Passivated-4
and 7 (bottom). Grid size = 0.5 mm.
carbonyl complexes, e.g., [Rh(Cy2PCH2CH2PCy2)(CO)2]-
[BPh4].25
This addition of CO gas (1 atm) to a single crystalline
sample is slow, with full conversion to yellow 7 requiring ∼3
days (for crystals of various sizes up to ∼1 mm3). Interestingly,
the reaction occurs from the surface of the crystal inward, and
an even layer of the yellow carbonyl complex forms on the faces
of the crystal, as shown by optical microscopy (Figure 1). If the
CO atmosphere is removed after 3 h, well-defined crystals
result of CO-Passivated-4 in which surface and near-surface
sites are transformed into 7. The ratio of 4:7 after 3 h, as
determined by 31P{1H} SSNMR spectroscopy, was approx-
imately 1:1, although this is dependent on the crystal size/
surface area from batch to batch. Similar surface passivation
using CO has been reported by Brookhart and co-workers, for
solid−gas reactivity using the [Ir(POCOP)L] (L = N2, CO).5 A
single-crystal X-ray diffraction experiment was conducted on
the final crystalline product of this solid-state reaction of 4 with
CO, i.e., 7. These crystals appeared entirely yellow, retained
their ability to rotate polarized light (which is often indicative of
single crystallinity), and had unit cell parameters very similar to
those of independently prepared 7 (vide infra). However, the
mosaicity determined from the diffraction pattern was large and
we were unable to solve the structure of the material produced
via this solid-state route. The structure of 7, as determined by
crystals obtained by solution routes, is discussed in section 2.4.
The bis-ethene complex 6 also reacts in the solid-state with CO
to form 7. In this case, the reaction takes only 3 h. The product
of this reaction appears crystalline and diffracts with a single
pattern, however, high mosaicity and cation disorder also did
not allow the structure to be solved.
2.4. Solid-State Structures. Single crystals of 4, 6, 7, and 8
were prepared by solution recrystallization (see Supporting
Information). For complexes 4, 6, and 7, all three structures
show significant disorder of the cation, which sits within an
octahedral environment described by six, well-refined, [BArF ]−
4
anions. The cations in these structures were refined by use of
similarity restraints to model the disorder components. In the
case of complex 6, the cation is disordered over two positions
(∼2:1 occupancy ratio) in which the corresponding two RhP2
planes lie approximately perpendicular to each other (angle
between planes 81°), and only the heavy atom positions (Rh
and P) could be reliably resolved. The cation of 7 is also
disordered over two positions (1:1 occupancy ratio), adopting a
structure in which one cation is inverted relative to the other,
and the RhP2 planes are again perpendicular to each other.
Each cation straddles a crystallographic mirror plane, and so the
CO ligands in each cation are crystallographically equivalent.
Figures 2A and B show this in detail. For 4, refinement reveals
four cation positions overlaid, with one rotated, one inverted,
and one rotated and inverted relative to the original. These
structures demonstrate the impact of rigid bulky anions upon
relatively small cations: the anions dominate the packing
structure and the cations may lie in a variety of thermodynami-
It is noteworthy that for a SC−SC transition to occur from 4
(or 6) to 7, not only must CO penetrate the crystal lattice but
the gaseous butadiene (ethene) co-product must be able to
escape without destroying the lattice. This suggests porosity
within the crystalline network, which in some instances can
come from loosely associated solvent that resides in channels in
the crystal.5 In the materials discussed herein, no co-crystallized
solvent is present. It might well be that the CF3 groups of the
[BArF ]− anions undergo a concerted motion that facilitates
4
transport of CO and butadiene through the crystal, as has been
recently suggested for reversible SC−SC transitions in Ag+
coordination polymers which uptake alcohols.21
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often between N−H and an oxygen atom, and N···O distances
vary from 2.84 to 3.19 Å (H···O, 2.00−2.37 Å).26,27 The change
in anion packing structure to accommodate N−H···F hydrogen
bonds could also cause the loss of crystallinity upon transition
from 4 to 8. These hydrogen bonds also result in a cation that
is not disordered, in contrast to, for example, 6, in which the
ethene ligands cannot engage in such interactions.
2.5. H/D Exchange in Complex 8. Microcrystalline
samples of 8 react with D2 in the solid state (48 h), or in
1,2-C6H4F2 solution (24 h), to form d6-8 with H/D exchange
occurring exclusively at the ammine ligands (Scheme 6). When
Scheme 6. H/D Exchange upon Ammine Ligands in Solution
or the Solid State
Figure 2. (A) Detailed view of the anion-cage and disordered
components in complex 7 (ball and stick representation for simplicity,
anions displayed as translucent, hydrogens atoms omitted). (B)
Disordered cation in 7 (displacement ellipsoids at 30%, one disorder
component displayed with pale atoms and bonds, hydrogens omitted).
(C) Simplified packing diagrams of 6, 7, and 4, showing disorder of
the cation position. Cations are represented by {RhP2} only, anions
represented as {BC4} only.
dissolved in 1,2-C6H4F2, the 1H NMR spectrum of d6-8 reveals
that the protio ammine signal observed for 8 [δ 1.93] is absent,
while the H NMR spectra contains a signal corresponding to
2
the ND3 ligands (δ 1.94). No signals indicating H/D exchange
at the phosphine ligands were observed. No degradation of the
crystal was observed during this process, as confirmed by X-ray
crystallography. H/D exchange of bound ammonia has been
previously reported in solution32−34 and, most recently, within
frozen argon matrices using Ir(Cp*)(H)4, however, in this case,
a bound ammonia complex, Ir(Cp*)(H)2(NH3), was only
implied by low temperature (10 K) IR spectroscopic studies.35
To elucidate the mechanism of H/D exchange in 8, DFT
calculations were employed to study H/H exchange between
the ammine ligands of the isolated [(iBu2PCH2CH2PiBu2)Rh-
(NH3)2]+ cation, A, with added H2. The most accessible
computed pathway is outlined in Figure 4, which shows free
energies computed both in vacuo and corrected for 1,2-
C6H4Cl2 solvent (this being used in the absence of data for 1,2-
C6H4F2). Initial H2 oxidative addition in solution proceeds with
a computed barrier of 18.7 kcal/mol to give a RhIII dihydride, B
(Gsolv = +3.3 kcal/mol). The NH3 ligand trans to hydride in B
is labilized (Rh···N = 2.27 Å) and so readily dissociates with a
barrier of only 13.0 kcal/mol to give an isomer C (Gsolv = +9.8
kcal/mol) in which it now H-bonds to the remaining ammine
ligand. Rh−H/N−H exchange then proceeds through TS(C−
D) at +22.8 kcal/mol, in which the outer-sphere NH3
deprotonates the axial hydride (Rh···H1 = 2.31 Å; N2···H1 =
1.08 Å) while simultaneously transferring a proton onto the
equatorial hydride (Ha···H1 = 1.51 Å; N2···Ha = 1.08 Å).
Figure 4 shows the overall computed barrier for H/H
exchange between cation A and H2 is 22.8 kcal/mol, and it is
apparent that the solvent correction reduces the barrier
cally similar positions within the enclosed cavities.30 Although
the anions refine well in these structures the cation disorder
means that comments on bond lengths and angles are not
appropriate.
By contrast, the solid-state structure of 8 is well-ordered.
However, unlike 4, 6, or 7, the [BArF ]− anions do not form a
4
well-defined octahedron around the cation. Instead a distorted
octahedron is observed in which one axial [BArF ]− anion sits
4
closer to the rhodium center than the others. This is
demonstrated by consideration of the Rh−B vectors that lie
approximately along crystallographic axes: ↑b 6.759(7) Å, ↓b
9.622(7) Å; a→ 9.593(1) Å, a← 9.593(1) Å; c→ 10.234(3) Å,
c← 10.234(3) Å, Figure 3. This distortion along the b-axis may
be influenced by hydrogen bonding between the ammine
ligands and CF3 groups on a proximal anion, as indicated by
relatively close N−H····F distances [N(1)···F(190), 3.295(8) Å
(H···F, 2.51 Å)].31 Examples of hydrogen bonding in solid-state
crystal structures of ammonia complexes have been reported,
compared to the result in vacuo (ΔG⧧ = 26.5 kcal/mol).
vac
This suggests increased charge separation in TS(C−D)
compared to A, consistent with the involvement of a near-
+
intact NH4 cation in the former. Experimentally, H/D
exchange occurs more rapidly in solution than in the solid
state. While we have not attempted to model the solid-state
environment here, the proximity of several fluorinated anions
may produce a similar environment to that in solution, with a
resultant reduction in barrier height. Alternative mechanisms
for H/D exchange were also considered, including those based
on initial oxidative addition of ammonia36,37 or heterolytic
Figure 3. (A) Displacement ellipsoid plot (30% probability) for the
cation of 8 in the solid-state. Hydrogen atoms (except those on
ammine ligands) and anion omitted for clarity. (B) Crystal packing
diagram of 8, anion represented by BC4 units (aryl groups omitted),
and hydrogen atoms omitted for clarity. Selected bond length (Å) and
angle (deg) data: Rh(1)−P(1), 2.2028(14); Rh(1)−N(1), 2.181(4);
P(1)−Rh(1)−P(1′), 84.07(6); N(1)−Rh(1)−N(1′), 84.79(16).
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Figure 4. (A) Computed profile for H/H exchange via the reaction of [(iBu2PCH2CH2PiBu2)Rh(NH3)2]+, A, with H2. Relative free energies (kcal/
mol) are reported in vacuo (plain text) and corrected for 1,2-C6H4Cl2 solvent (in italics). H centers derived from the added H2 molecule are
i
highlighted in blue. (B) Computed structure of the Rh−H/N−H exchange transition state TS(C−D) with selected distances in Å. Pr groups and
nonparticipating H atoms on the chelating phosphine are omitted for clarity.
cleavage of H2 over a Rh−NH3 bond. These were both
disfavored, however, the former entailing a barrier of 27.6 kcal/
+
mol, while for the latter no intermediate incorporating an NH4
cation could be located. Full details of these alternative
pathways are given in the Supporting Information (see Figures
S14−S17).
2.6. Solid−State Ethene Dehydrocoupling. Over a
number of weeks at 298 K, orange/red single crystals of
freshly prepared 6 change to a darker red/purple, becoming
similar in color to freshly prepared complex 4. When this
material is dissolved in 1,2-C6H4F2 solvent complex 4 is indeed
observed in the 31P{1H} NMR spectrum, alongside the
difluorobenzene adduct 5 (confirmed by ESI-MS). Complex
5 forms on dissolving 6 in 1,2-C6H4F2 solvent (vide supra) and
Figure 5. Formation of 4 from a solid sample of 6 as measured by
is thus a marker for this unstable bis-ethene complex or closely
integral of the 31P{1H} NMR signals of the products which form upon
related complexes (Scheme 7). The transformation of 6 to 4 is
solvation in 1,2-C6H4F2.
Scheme 7. Solid-State Ethene Dehydrocoupling and
Subsequent Products Observed upon Solvation in 1,2-
C6H4F2 (L = Agostic or Solvent)
liberating one equivalent of hydrogen, which would then react
with unreacted 6 to produce ethane gas (i.e., Scheme 4). It thus
appears 6 converts to 4, accompanied by sacrificial hydro-
genation of one equivalent of ethene from a second molecule of
6, also accounting for the apparent 50% maximum reaction
conversion. Interestingly, crystallinity is retained during this
process, which further suggests the sacrificial 6 does not lose
ethene completely, as the resulting 12-electron fragment would
likely form amorphous-3 (Section 2.1). We suggest the
formation of a 14-electron, 3-coordinate species [Rh-
formally a dehydrocoupling of ethene. This transformation in
the solid state was initially monitored over time by periodic
sampling via dissolution in 1,2-C6H4F2 of a solid sample kept
298 K. This showed a gradual increase in the proportion of
complex 4 present and a decrease in 5, with 40% conversion
after 7 weeks (Figure 5). A sample kept for four months
revealed ∼50% conversion to 4, this likely representing an
upper limit for conversion. The color change of 6 to dark-red/
purple occurs without apparent loss of crystallinity (by
inspection of the sample using a polarizing microscope), and
a single X-ray diffraction pattern could be collected. However,
we were unable to solve the structure fully due to a highly
disordered cation.
(iBu2PCH2CH2PiBu2)(η2-C2H4)][BArF ], 9, in which only
4
one ethene ligand is retained (Scheme 7) to account for the
production of ethane. This species could be stabilized by an
38
i
agostic interaction from the Bu groups, or by an interaction
with the CF3 groups from proximate anions in the lattice, and
would likely form 5 on dissolution in 1,2-C6H4F2. Budzelaar
and co-workers have previously reported a related three-
coordinate rhodium complex with a bidentate β-diiminate
ligand and one alkene ligand.39 Evidence for the formation of
complex 9 comes from solvation (at 200 K, CD2Cl2) of a solid-
sample sample of 6 that has been aged for two months. The
31P{1H} NMR spectrum recorded at this temperature reveals 4
to be the major product but less than 50% of the total integral,
alongside signals for starting material 6 and its solution
decomposition product 3. Two new signals are also observed,
A sealed sample of 6 (NMR tube) releases ethane gas over
several weeks as observed by gas-phase NMR spectroscopy.
This observation is consistent with the conversion of 6 to 4,
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which are assigned to complex 9, which couple to each other
(31P−31P COSY experiment at 200 K), at δ 47.0 (JRhP = 162
Hz, JPP = 28 Hz); δ 71.4 (JRhP = 186 Hz, JPP = 28 Hz), Figure 6.
(iBu2PCH2CH2PiBu2)(η2-C2H4)(L)][BArF ], 9 (L = vacant
4
site, agostic or anion interaction). On exposure to ethene gas,
these additional signals disappear and 6 is reformed, consistent
with the formulation of 9 as having a vacant, or masked, site
(Figure 7E). When a sample of 6 was left at 323 K under an
atmosphere of ethene (sealed NMR tube, 6 days), a small
amount of 2-butene was observed. This could arise from
isomerization of 1-butene, which would be formed by
dimerization of ethene.41 Under these conditions of temper-
ature and excess ethene, no 4 is formed from the sample of 6,
with orange 6 recovered and a trace (∼10%) of a complex
characterized as a hexadiene adduct, 10. Complex 10 was
characterized by an independent synthesis from hexadiene and
complex 5 (Supporting Information). Attempts to make the
reaction turnover in a catalytic sense by increasing the
temperature (e.g., 363 K) led to a partial melting of the
crystalline material and no significant increase in the formation
of coupled products.
The dehydrocoupling of ethene at rhodium centers to form
butadiene complexes has been previously observed and
mechanisms based on initial oxidative coupling or C−H bond
activation have been proposed.42−44 Such processes are also
related to alkene oligomerization reactions.45 Interestingly, the
complex RhTp*(C2H4)2 [Tp* = tris(3,5-dimethyl-1-pyrazol-1-
yl)hydroborate] undergoes dehydrocoupling to give a buta-
diene complex in solution (with the concomitant release of
ethane), but in the solid state, a hydrido-allyl species is
formed.44
Figure 6. Low temperature 31P{1H} NMR spectrum (CD2Cl2, 200 K)
of a solid sample of 6 that has been aged for 2 months.
One signal has a considerably larger JRhP coupling constant,
suggesting a weakly-bound trans-ligand (solvent or agostic
interaction).40 On warming to 250 K, complex 3 grows in at the
expense of complex 9.
SSNMR spectroscopy was also used to follow the solid-state
transformation of 6 in situ. The 13C{1H} and 31P{1H} SSNMR
spectra of freshly prepared complexes 4 and 6 were initially
collected, and in keeping with their highly disordered solid-state
structures, these spectra revealed multiple peaks for the signals
related to the cation. For example, the 31P{1H} SSNMR
spectrum of 4 shows a variety of closely spaced broad peaks,
while that of 6 displays at least three broad phosphorus
environments in the solid state, Figure 7. The 13C{1H} SSNMR
DFT calculations on isolated cations were again used to
assess these different mechanistic possibilities and our favored
pathway is outlined in Figure 8A. Starting from E,
Figure 8. Free energy profiles (kcal/mol) for (A) the dehydrocoupling
of ethene in [(iBu2PCH2CH2PiBu2)Rh(C2H4)2]+, E, and (B) hydro-
genation of E. Several processes involve more than one step and these
are indicated with a double arrow, and only the energy of the highest
lying transition state between the two key minima is indicated. Full
details of these and alternative mechanisms are available in the
Supporting Information (see Figure S18−S24).
Figure 7. SSNMR spectra (13C{1H} left, 31P{1H} right) displaying the
transformation of 6 over time (A−D) followed by the reaction of the
products with ethene gas (E) at 298 K and a comparison with a pure
sample of 6 (F).
spectra of 4 and 6 also show multiple peaks for the butadiene
and ethene environments in their respective spectra, between
60−100 ppm. Monitoring a sample of 6 over the course of 6
weeks revealed the formation of complex 4 in both the 31P{1H}
and 13C{1H} NMR spectra. In addition, a new signal (δ 82.4)
slightly downfield to the ethene resonances in 6 was also
observed in the 13C{1H} NMR spectrum, while the 31P{1H}
SSNMR spectra show additional broad signals centered at δ 77
(Figure 7D). These signals are assigned to [Rh-
[(iBu2PCH2CH2PiBu2)Rh(C2H4)2]+ (i.e., the cation of 6),
oxidative coupling of ethene occurs with a barrier of 22.9 kcal/
mol to give intermediate F (+10.8 kcal/mol). F exhibits a
seesaw structure for which there are precedents with
isoelectronic d6 Ru(II) and Ir(III) centers.46,47 β-H transfer
then generates G from which facile C−H reductive coupling
yields 1-butene complex H (−8.1 kcal/mol). Rearrangement
and γ-H transfer then leads to hydrido-methylallyl intermediate
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I at −11.6 kcal/mol.48 I features a weak Rh···H−C agostic
interaction and so could undergo H-transfer to form a trans
dihydride intermediate (an isomer of J, see below, and
Supporting Information, Figure S20). Instead a lower energy
process involving facile rearrangement of the methylallyl ligand
places the agostic cis to the hydride in isomer I′ (−6.2 kcal/
mol) from which H-transfer then gives J with cis hydrides and a
cis η-C4H6 ligand. Dehydrocoupling is then completed via
reductive elimination of H2 to give K (the cation of 4) at −3.7
kcal/mol.
Figure 8A shows the formation of I is thermodynamically
favored over K, and so the facile, exergonic hydrogenation of E
to N (modeling the cation of species 9 postulated
experimentally) is required to drive the dehydrocoupling to
completion (see Figure 8B: overall barrier = 13.8 kcal/mol; ΔG
= −16.0 kcal/mol). The optimized structure of N features an
Figure 9. Hydrogenation of ethene to ethane using a variety of solid-
state catalysts, 298 K. %Ethane conversion calculated from integrals of
ethane and ethene signals by gas phase NMR spectroscopy.
i
agostic interaction between the Rh center and one of the Bu
methyl groups (see Supporting Information, Figure S11). The
barrier energy span49 of the dehydrocoupling profile (corre-
sponding to I to TS(M-N)) is 25.4 kcal/mol, a value that is
consistent with a slow dehydrocoupling process. The
intermediacy of H is also consistent with the observation of
2-butene formed (after isomerization) in the presence of excess
ethene.
Alternative dehydrocoupling mechanisms based on initial C−
H activation to hydrido-alkenyl intermediates were also
assessed. The C−H activation step (ΔG⧧ = 24.1 kcal/mol;
ΔG = +19.3 kcal/mol) proved only slightly less accessible than
the oxidative coupling; however, the onward reaction, either via
ethene insertion into the Rh−alkenyl bond or insertion into the
Rh−H bond involved transition states at 31.7 or 31.0 kcal/mol.
Thus, these pathways have considerably higher barriers than the
process based on oxidative coupling in Figure 8A. Full details of
all alternative pathways are provided in the Supporting
Information.
2.7. Solid-State Catalysis: Hydrogenation of Ethene.
The complexes 1−8 were tested as solid-state catalysts for the
hydrogenation of ethene. Such solid−gas reactivity has been
reported before for organometallic catalysts in the solid
state.5,50−52 Gas-phase NMR spectroscopy was used to measure
the hydrogenation of ethene in situ by the conversion of ethene
to ethane. T1 relaxation times for ethane and ethene in the gas
phase were found to be similar for both species (ca. 0.6 s), as
were their relative integrals with different acquisition delay
times, suggesting that the relative integration of the two signals
in to the gas-phase species is reliable. In a typical experiment, a
high pressure NMR tube was loaded with (3 0.3) mg (e.g.,
2.2 × 10−3 mmol for 1) of solid catalyst, and to this 1 atm
(∼0.08 mmol) of ethene was added followed by ∼4 atm (∼0.31
mmol) of H2. The progress of the reaction was monitored
directly by integration of the gas-phase 1H NMR spectra at 298
K.
Under these conditions of excess hydrogen, precatalysts
containing alkene ligands (1, 4, and 6) all catalyze the reaction
rapidly; for example, 1 requires less than 2 min to effect
complete conversion (Figure 9). At the end of catalysis, when
the ethene is fully consumed and hydrogen is still in excess,
complex 3 is observed to be formed as characterized by
31P{1H} NMR spectra in CD2Cl2 solution, as there is no
exogenous ligand to stabilize the metal center, consistent with
the stoichiometric studies (section 2.1). This zwitterionic
complex 3 is a very slow catalyst itself, only turning over ∼5%
after 25 min, suggesting that dissociation, or η6- to η2-ring
slippage, of the anion to reveal a vacant site is a disfavored
process in the solid state. This is a solid-state phenomenon, as
in CH2Cl2 solution under an atmosphere of ethene 3 forms 6,
and hydrogenation of the bound alkene is then rapid (upon
mixing). This also suggests that the resting state during catalysis
in the solid state when using precatalysts such as 1 is unlikely to
be 3. Complex 7 is a very slow catalyst, showing essentially no
turnover after 30 min, presumably as the carbonyl ligands are
strongly bound. Complex 8 does show some activity (15%
conversion after 30 min), perhaps suggesting that the ammine
ligands are somewhat labile, consistent with the computational
studies. In both of these cases, the organometallic species
observed after completion of catalysis to the detection limit of
solution 31P{1H} NMR spectroscopy is the same as the starting
material, i.e., unchanged 7 and 8. Interestingly, for precatalysts
2, 5, and 6, the rate of hydrogenation, which is at first rapid,
drops after around 2 min. This similar slower rate for all may be
indicative of transformation of any surface organometallic
species into a common species (possibly related to 3) that turns
over slowly. Also noteworthy is that this simple hydrogenation
reaction occurs rapidly at ambient temperature (∼5 atm total
pressure), while previous reports of the catalytic hydrogenation
of ethene using solid-state organometallic catalysts typically
require longer time scales and/or higher temperatures (albeit
with lower pressure reaction conditions).5,52,53
As the surface area of a solid catalyst is likely to be important
for the rate of catalysis, microcrystalline 1 was ground and
separated into different particle size fractions using microsieves
to give the following particle size regimes: less than 50 μm, 50−
71 μm, and 71−150 μm. These materials were then subjected
to catalytic hydrogenation of ethene using the standard
conditions previously defined (i.e., 3.0 0.3 mg of catalyst).
Figure 10 shows that all the different size particles rapidly
hydrogenate ethene with broadly similar temporal profiles over
the first 10 min, promoting between 75 and 90% conversation.
After a much longer time (16 h), all reached 100% conversion
but at a much slower rate. Interestingly, the smaller particle
sized catalysts appear to operate slightly faster as an ensemble,
suggesting that the surfaces of the crystals are particularly active
compared to the interior, or that smaller particle sizes allow for
more rapid hydrogenation of the precatalyst. An alternative
explanation is that the larger particle sizes of catalyst become
deactivated sooner as there are relatively fewer active sites. As it
might well be that all these factors are operating, possibly with a
different temporal dependence depending on crystal size, and
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Figure 10. Hydrogenation of ethene using various particle size batches
of solid 1 as a catalyst (starting conditions: 295 K, H2 3.9 atm, ethene
1 atm).
Figure 11. Hydrogenation of ethene using crystalline precatalysts.
Ethene added at 1 atm, hydrogen added at less than 1 atm (∼0.3 atm).
Under these conditions, a maximum conversion of ∼30% of ethane is
achievable.
given the differences are relatively small, we are reluctant to
speculate further with the current data.
compared with the 10 min for 4 (Figure 11). After catalysis, the
crystals appear intact and in good condition without significant
cracking or change. A small trace of butane was observed in the
2.8. Solid-State Catalysis: Passivation Studies. The data
presented in the previous section suggest that catalysis might
occur fastest upon the surface sites of the organometallic
crystals. However, it is also likely that the metal centers located
inside the interior of the crystal could be active by gas diffusion
through the crystalline lattice. To probe this, we used CO-
Passivated-4, an approach encouraged by the passivation
techniques used by Brookhart and co-workers in studying
alkene hydrogenation using [Ir(POCOP)(H)2] systems.5 As
complex 7 essentially does not catalyze the hydrogenation of
ethene, while 4 catalyzes the reaction rapidly (Figure 9), the
CO-Passivated-4 probes the ability of these interior metal sites
to take part in catalysis. However, as complex 4 reacts with H2
in the absence of ethene to, deleteriously, form 3, in order to
maintain single crystallinity, and thus the surface passivation of
CO-Passivated-4, ethene is kept in excess with H2 the limiting
substrate in contrast to previously discussed catalysis in which
hydrogen is in excess.
To baseline studies, complexes 4 and 7 were subjected to
these conditions of catalysis in which hydrogen is the limiting
substrate. A high pressure NMR tube containing (3 0.3) mg
of crystalline catalyst 7 was evacuated and refilled with ∼1 atm
of ethene and ∼0.3 atm of H2. As expected, single crystals of
complex 7 are very poor catalysts for the hydrogenation of
ethene under these conditions and only 3% conversion occurs
within 1 h and little further progress is observed over the next
24 h (Figure 11). By contrast, single crystals of 4 hydrogenate
ethene, until all hydrogen is consumed, in only ∼9 min (i.e.,
29% of ethene hydrogenated). These crystals retain their
crystallinity and their ability to rotate polarized light. A second
pressurization of hydrogen can be applied, and catalysis
continues until the hydrogen is used up a second time (42%
of ethene now being hydrogenated). Analysis of the resulting
crystals by low temperature solution 31P{1H}NMR spectrosco-
py (220 K, CD2Cl2) afterward showed that only 4 was present,
and no 6 or 3 were resolved to the detection limit of 31P NMR
spectrocopy (∼5%). This suggests that catalysis in the
crystalline state occurs without significant involvement of the
bulk crystal when ethene is in excess, with presumably only a
small number of sites, likely on the surface, active for catalysis.
Under these conditions, CO-Passivated-4 also catalyzes the
hydrogenation of ethene but at a considerbly slower rate than
pure crystalline 4, with only 14% conversion reached after 33
min and the reaction only reaching completion after 12 h
1
gas phase H NMR spectrum (∼1% relative to the ethene
signal), suggesting that a small proportion of the interior sites
of 4 have been hydrogenated. A second amount of hydrogen
can be subsequently added, and catalysis restarts. Low
temperature 31P{1H} NMR spectroscopic analysis (220 K,
CD2Cl2) of these crystals postcatalysis resolved only 4 and 7 to
be present. This much slower rate of catalysis suggests that
catalysis is mainly a surface process in these systems. However,
that catalysis still does occur suggests that there is some
penetration of ethene and hydrogen to access active interior
sites of a crystal. These sites must be particularly active as no
significant spectroscopic markers for their formation, i.e., 6 or 3
are observed. We cannot discount the possibility of crystal
microcracking occurring (due to reaction with hydrogen and
formation of 3), which may expose the interior of the crystals.54
Unfortunately, the remarkable selective hydrogenation of
ethene in the presence of propene, demonstrated by Brookhart
and co-workers using passivated single crystals, was not able to
be reproduced with crystals of CO-Passivated-4.5 Using a
mixture of ethene, butene, and hydrogen (approximate 2:2:1
ratio), an approximately equal ratio of butane and ethane was
produced in each case with crystalline catalysts of 4, CO-
Passivated-4 and 7, although the reaction rates varied
dramatically as expected (4 > CO-Passivated-4 > 7).
3. CONCLUSIONS
A number of solid-state reactions of well-defined organo-
metallic complexes with gases are reported based upon the
fragment [Rh(iBu2PCH2CH2PiBu2)][BArF ]. In particular, we
4
have shown that the σ-alkane complex 2 is a useful synthon for
such solid-state synthesis, acting via a labile alkane ligand. This
allows for the addition of gases such as ammonia and ethene to
form the corresponding adducts, some of which, for example
with ethene or NH3, are not readily accessible via solution
techniques. The corresponding butadiene complex, 4, acts in a
similar way by loss of alkene. Crystallinity may be maintained
during some of these solid−gas ligand exchange reactions,
when the crystal packing geometry remains similar in the
starting materials and products (e.g., the displacement of
ethene or butadiene ligands by CO). The crystal packing
environments, which incorporate large [BArF ]− anions that
4
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JHH = 7 Hz, 12H], 1.07 [d (iBu CH3), JHH = 7 Hz, 6H], 1.09 [d (iBu
CH3), JHH = 7 Hz, 6H], 1.64 [m (iBu CH or CH2), 4H], 1.74 [m
(PCH2CH2P), 4H], 1.80−2.04 [m, (iBu CH or CH2), 8H], 2.86 [d
(C4H6), JHH = 14 Hz, 2H], 4.27 [d (C4H6), JHH = 7 Hz, 2H], 5.45 [m,
(C4H6), 2H] 7.56 [s (BArF ), 4H], 7.72 [s (BArF ), 8H]. 31P{1H}
provide a pseudo octahedral cavity in which the cations reside,
can thus allow for subtle rearrangements in the solid state
without disruption of the crystallinity or onward reactivity with
the metal cation,55 as we have noted previously.8,9 In the solid
state, these complexes can be used as precatalysts for the
hydrogenation of ethene, show H/D exchange at bound NH3
ligands, and promote ethene dehydrocoupling. Mechanisms for
these last two processes are proposed on the basis of DFT
calculations using molecular model systems, and modeling such
reactivity in the solid state is the focus of current work. For
catalysis, particle size and surface passivation experiments
suggest that species on the surface act as the most active sites
for catalysis rather than within the crystal. These observations
encourage the use of solid-state organometallic chemistry in
both synthesis and catalysis. Of course, in the latter, the
identification of the actual active species/sites will likely be
challenging as these might only represent a relatively small
proportion of the bulk crystal.
4
4
NMR (202 MHz CD2Cl2): δ 54.64 [d, JRhP = 170 Hz, 2P]. 31P{1H}
SSNMR (161 MHz): δ 49.8 (br, major peak), 57.5 (br), 60.1 (br).
13C{1H} SSNMR (101 MHz): δ 22.7−36.2 [m (iBu CH3, CH,
PCH2CH2P), 14C], 38.0−40.6 [m (iBu CH2), 4C], 63.5−64.2 [m
(C4H6), 2C], 99.9−101.9 [m (C4H6), 2C], 116.7 [m (BArF ), 8C],
4
124.6 [br (BArF ), 8C], 130.3 [br (BArF ), 8C], 133.6−134.6 [m
4
4
(BArF ), 4C], 163.8 [br (BArF ), 4C].
4
4
Addition of butadiene (∼4 am) to preformed complex 2 (see below
for insitu synthesis from complex 1) resulted in the formation of
complex 4 as an amorphous solid, which can be recrystallized from 1,2-
C6H4F2/pentane to give analytically pure material. See Supporting
Information for crystallographic studies.
4.3. Synthesis of [Rh(iBu2PCH2CH2PiBu2)(C2H4)2][BArF ] (6).
4
First, 26 mg (0.019 mmol) of solid [Rh(iBu2PCH2CH2PiBu2)(η2η2-
C7H8)][BArF ] (1) was placed in a crystallization tube with a Young’s
4
tap and exposed to hydrogen (1 atm) for 10 min to form
[Rh(iBu2PCH2CH2PiBu2)(η2η2-C7H12)][BArF ] (2). Then the flask
4
4. EXPERIMENTAL SECTION
was evacuated, and ethene (1 atm) was immediately added. With the
flask under a pressure of ethene, CH2Cl2 was added by syringe through
a Suba-Seal to dissolve the compound forming a orange/red solution.
Pentane was also added by syringe to form a layer over the CH2Cl2,
and the flask was sealed under 1.5 atm of ethene. Orange/red crystals
formed over 48 h, which could be isolated and appear vacuum stable
(25% yield). The complex is not stable in solution (or when
suspended in pentane) at room temperature in the absence of an
ethene atmosphere, as decomposition to 3 occurs. Anal. Calcd for
4.1. General Details. All manipulations, unless otherwise stated,
were performed under an atmosphere of argon, using standard
Schlenk-line and glovebox techniques. Glassware was oven-dried at
403 K overnight and flamed under vacuum prior to use. CH2Cl2, Et2O,
and pentane were dried using a Grubbs-type solvent purification
system (MBraun SPS-800) and degassed by successive freeze−pump−
thaw cycles.56 CD2Cl2 and 1,2-C6H4F2 were distilled under vacuum
from CaH2 and stored over 3 Å molecular sieves. Complexes 1, 2, 3,
and 5 were prepared according to previously described methods.8 All
other reagents were used as received from suppliers. Solution and gas-
phase NMR spectra were recorded on Varian Unity 500 MHz, Bruker
AVD 500 MHz, or Varian Mercury 300 MHz spectrometers at room
temperature unless otherwise stated. Nondeuterated solvents were
locked to a standard C6D6 solution. Residual protio solvent was used
1
C54H60BF24P2Rh: C, 48.38; H, 4.51. Found: C, 48.52, H, 4.41. H
NMR (500 MHz CD2Cl2 240 K): δ 0.96 [d (iBu CH3), JHH = 6 Hz,
12H], 1.03 [d (iBu CH3), JHH = 6 Hz, 12H], 1.5−2.0 [br (iBu CH, iBu
CH2 and PCH2CH2P), 16H], 3.98 [s (C2H4), 8H], 7.53 [s (BArF ),
4
4H], 7.71 [s (BArF ), 8H]. 31P{1H} NMR (202 MHz CD2Cl2 240
4
K):δ 54.74 [d, JRhP = 144 Hz, 2P]. 31P{1H} SSNMR (161 MHz): δ
49.3 (br), 55.9 (br). 13C{1H} SSNMR (101 MHz): δ 22.5−25.6 [m
(iBu CH3), 8C], 30.5−39.6 [m (iBu CH, iBu CH2, PCH2CH2P), 10C],
80.7−81.6 [m (C2H4), 4C], 116.5 [m (BArF ), 8C], 124.5 [br (BArF ),
1
2
as reference for H, H, and 13C NMR spectra in deuterated solvent
samples. In 1,2-C6H4F2, 1H NMR spectra were referenced to the
center of the downfield solvent multiplet (δ 7.07). 31P NMR spectra
were externally referenced to 85% H3PO4. All chemical shifts (δ) are
quoted in ppm and coupling constants in Hz. Gas-phase samples were
separately referenced to the reported values for hydrogen, ethane,
ethene, and butadiene (all data were fully consistent with reported
values).52,57 SSNMR spectra were recorded on a Bruker Avance 3HD
400 MHz (University of Oxford) and Varian VNMRS 400 MHz
(EPSRC National SSNMR Service, Durham University), using the
cross-polarization pulse sequence (contact time 3−10 ms and recycle
delay 1−2 s), with Spinal 64 or TPPM decoupling. A magic-angle
spinning rate of 10 kHz was employed (under dry N2). Spectral
referencing is with respect to external neat tetramethylsilane for 13C
and to 85% H3PO4 for 31P. Fourier Transform infrared (IR)
spectroscopy was carried out using a Thermo-Scientific Nicolet iS5
with iD3 ATR and iD1 transmission attachments. Electrospray
ionization mass spectrometry was recorded using a Bruker MicroTOF
instrument directly connected to a modified Innovative Technology
glovebox.58
4
4
8C], 130.1 [br (BArF ), 8C], 134.2 [br (BArF ), 4C], 163.6 [br
4
4
(BArF ), 4C]. See Supporting Information for crystallographic studies.
4.44. Synthesis of [Rh(iBu2PCH2CH2PiBu2)(CO)2][BArF ] (7).
4
[Rh(iBu2PCH2CH2PiBu2)(η6-1,2-C6H4F2)][BArF ] (5) (30 mg,
4
0.022 mmol) was dissolved in CH2Cl2 and the solution frozen and
placed under vacuum. CO gas was used to refill the flask, and upon
warming, a slight color change from yellow/orange to bright-yellow
occurred, and the flask was left sealed for 2 h. The solution was then
degassed and crystallized by adding pentane (23 mg, 80% yield).
Bright-yellow crystalline material formed. Anal. Calcd for
C52H52BO2P2F24Rh: C, 46.59; H, 3.91. Found: C, 46.67; H, 3.82.
ESI-MS [M+] calcd = 477.16, found [M+] m/z = 477.16. IR
spectroscopy (CH2Cl2 solution): CO stretches 2093.4 and 2049.1
1
cm−1; (solid-state) CO stretches 2099.0 and 2056.9 cm−1. H NMR
(300 MHz CD2Cl2): δ 1.12 [d (iBu CH3), JHH = 6 Hz, 12H], 1.16 [d
(iBu CH3), JHH = 6 Hz, 12H], 1.89 [m (iBu CH), 4H], 1.96 [m (iBu
CH2), 8H], 2.10 [d, PCH2CH2P, JPH = 18 Hz, 4H], 7.57 [s (BArF ),
4
4.2. Synthesis of [Rh(iBu2PCH2CH2PiBu2)(η4-C4H6)][BArF ] (4).
4H], 7.72 [s (BArF ), 8H]. 31P{1H} NMR (122 MHz CD2Cl2):δ 56.10
4
4
[Rh(iBu2PCH2CH2PiBu2)(η6-1,2-C6H4F2)][BArF ] (5) (250 mg,
[d, JRhP = 116 Hz, 2P]. 31P{1H} SSNMR (161 MHz): δ 54.1 (br, 2P).
Alternatively, addition of CO gas (1 atm) to a single crystalline
sample of complex 4 (∼30 mg) resulted in slow conversion to yellow 7
as monitored by 31P SSNMR spectroscopy, with full conversion
requiring 3−4 days depending on the crystal size. See Supporting
Information for crystallographic studies on material produced via the
solution route.
4
0.179 mmol) was dissolved in 1,2-C6H4F2 and the solution frozen
and placed under vacuum. Butadiene gas was used to refill the flask
and upon warming a color change to a dark-red/purple solution
occurs, the flask was left sealed for 2 h. Because reducing the pressure
inside the flask drives the backward reaction, releasing butadiene gas
and the formation of 5, complex 4 was recrystallized by adding
pentane via syringe directly to the solution under an atmosphere of
butadiene. Dark-red/purple crystalline material forms, 66% yield. Solid
4 is vacuum stable. Anal. Calcd for C54H58BF24P2Rh: C, 48.45; H, 4.37.
Found: C, 48.57; H, 4.43. ESI-MS [M+] calcd = 475.21, found [M+]
4.5. Synthesis of [Rh(iBu2PCH2CH2PiBu2)(NH3)2][BArF ] (8).
4
Solid [Rh(iBu2PCH2CH2PiBu2)(η2η2-C7H8)][BArF ] (1) (50 mg,
4
0.036 mmol) was placed in a Young’s flask. The flask was degassed
and placed under 1 atm NH3 gas. The flask was then frozen in a liquid
N2 bath and opened to 1 atm pressure of H2 while at low temperature.
1
m/z = 475.21. H NMR (500 MHz CD2Cl2): δ 0.94 [vt (iBu CH3),
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The flask was then sealed and warmed; this generated roughly 1 atm
NH3 and 3 atm H2. The solid quickly turned a bright-yellow color.
The flask was left for 3 h. The product was washed twice with pentane
to remove NBA. Crystals were grown from 1,2-C6H4F2/pentane; 35
mg (0.027 mmol) produced; 73% yield. Anal. Calcd for
C50H58BP2N2F24Rh: C, 45.54; H, 4.43; N, 2.12. Found: C, 45.69; H,
4.43; N, 1.95. 1H NMR (500 MHz 1,2-C6H4F2): δ 1.18 [d (iBu CH3),
JHH = 6 Hz, 12H], 1.33 [d (iBu CH3), JHH = 6 Hz, 12H], 1.51 [d
(PCH2CH2P), JPH = 14 Hz, 4H], 1.57 [m (iBu CH2), 8H], 1.91 [m
AUTHOR INFORMATION
Corresponding Authors
*For A.S.W.: E-mail: andrew.weller@chem.ox.ac.uk.
*For S.A.M.: E-mail: S.A.Macgregor@hw.ac.uk.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(iBu CH), 4H], 1.93 [s, NH3, 6H], 7.68 [s (BArF ), 4H], 8.32 [s
■
4
(BArF ), 8H]. 31P{1H} NMR (202 MHz 1,2-C6H4F2): δ 65.45 [d, JRhP
We thank the EPSRC for funding via a DTP studentship
(S.D.P.) and via grant number EP/K035908/1 (T.K.) as well as
Johnson Matthey for the loan of rhodium salts. The EPSRC
Solid-State NMR Spectroscopy Service at the University of
Durham, UK, is thanked for acquisition of some of the 31P
NMR data.
4
= 177 Hz, 2P]. 31P{1H} SSNMR (161 MHz): δ 58.5 (vt, JRhP = 155−
170 Hz, 2P). See Supporting Information for crystallographic details.
4.6. Synthesis of [Rh(iBu2PCH2CH2PiBu2)(□4-MeCH
CHCHCHMe)][BArF ] (10). First, 0.1 mL of previously degassed
4
2,4-hexadiene (mixture of isomers) was added to 5 mg (0.019 mmol)
of solid [Rh(iBu2PCH2CH2PiBu2)(η6-1,2-C6H4F2)][BArF ] (5) in an
4
NMR tube. To this, 0.4 mL of CH2Cl2 was then added to solvate the
mixture, causing a color change to bright red as 10 formed. 10 was
crystallized by layering the solution with pentane (Yield 3 mg, 61%).
Anal. Calcd for C56H62BF24P2Rh: C, 49.21; H, 4.57. Found: C, 49.35;
DEDICATION
■
In memory of Professor Michael Lappert, a true pioneer and
innovator in organometallic chemistry.
1
H, 4.68. H NMR (500 MHz CD2Cl2 240 K): δ 0.97 [d (iBu CH3),
JHH = 6 Hz, 6H], 1.01 [d (iBu CH3), JHH = 6 Hz, 6H], 1.09 [d (iBu
CH3), JHH = 6 Hz, 6H], 1.15 [d (iBu CH3), JHH = 6 Hz, 6H], 1.70 [m
(iBu CH2), 8H], 1.78−1.95 [br (iBu CH, PCH2CH2P, 2,4-C4H4Me2),
14H], 3.98 [m (2,4-C4H4Me2), 2H], 5.34 [d (2,4-C4H4Me2), 2H]*,
7.56 [s (BArF ), 4H], 7.72 [s (BArF ), 8H] (*partially coincident with
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