Controlling Structure and Reactivity in Cationic Solid-State Molecular Organometallic Systems Using Anion Templating

Article abstract of DOI:10.1021/acs.organomet.8b00215

The role that the supporting anion has on the stability, structure, and catalytic performance, in solid-state molecular organometallic systems (SMOM) based upon [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBD)][BAr^X₄], [1-NBD][BAr^X₄], is reported (X = Cl, F, H; NBD = norbornadiene). The tetra-aryl borate anion is systematically varied at the 3,5-position, Ar^X= 3,5-X₂C₆H₃, and the stability and structure in the solid-state compared with the previously reported [1-NBD][BAr^CF3₄] complex. Single-crystal X-ray crystallography shows that the three complexes have different packing motifs, in which the cation sits on the shared face of two parallelepipeds for [1-NBD][BAr^Cl₄], is surrounded by eight anions in a gyrobifastigium arrangement for [1-NBD][BAr^F₄], or the six anions show an octahedral cage arrangement in [1-NBD][BAr^H₄], similar to that of [1-NBD][BAr^CF3₄]. C-X···X-C contacts, commonly encountered in crystal-engineering, are suggested to be important in determining structure. Addition of H₂ in a solid/gas reaction affords the resulting σ-alkane complexes, [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^X₄] [1-NBA][BAr^X₄] (NBA = norbornane), which can then proceed to lose the alkane and form the zwitterionic, anion-coordinated, complexes. The relative rates at which hydrogenation and then decomposition of σ-alkane complexes proceed are shown to be anion dependent. [BAr^CF3₄]^- promotes fast hydrogenation and an indefinitely stable σ-alkane complex. With [BAr^H₄]^- hydrogenation is slow and the σ-alkane complex so unstable it is not observed. [BAr^Cl₄]^- and [BAr^F₄]^- promote intermediate reactivity profiles, and for [BAr^Cl₄]^-, a single-crystal to single-crystal hydrogenation results in [1-NBA][BAr^Cl₄]. The molecular structure derived from X-ray diffraction reveals a σ-alkane complex in which the NBA fragment is bound through two exo Rh···H-C interactions-different from the endo selective binding observed with [1-NBA][BAr^CF3₄]. Periodic DFT calculations demonstrate that this selectivity is driven by the microenvironment dictated by the surrounding anions. [1-NBA][BAr^X₄] are catalysts for gas/solid 1-butene isomerization (298 K, 1 atm), and their activity can be directly correlated to the stability of the σ-alkane complex compared to the anion-coordinated decomposition products.

Full text of DOI:10.1021/acs.organomet.8b00215

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Controlling Structure and Reactivity in Cationic Solid-State Molecular
Organometallic Systems Using Anion Templating
Alasdair I. McKay,† Antonio J. Martínez-Martínez,† Hannah J. Griffiths,† Nicholas H. Rees,†
Jordan B. Waters,† Andrew S. Weller,*,† Tobias Kramer,‡,§ and Stuart A. Macgregor*,‡
̈
†Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom
‡Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
S
* Supporting Information
ABSTRACT: The role that the supporting anion has on the
stability, structure, and catalytic performance, in solid-state
molecular organometallic systems (SMOM) based upon [Rh-
(Cy2PCH2CH2PCy2)(η2η2-NBD)][BArX ], [1-NBD][BArX ], is
4
4
reported (X = Cl, F, H; NBD = norbornadiene). The tetra-aryl
borate anion is systematically varied at the 3,5-position, ArX= 3,5-
X2C6H3, and the stability and structure in the solid-state
compared with the previously reported [1-NBD][BArCF3
]
4
complex. Single-crystal X-ray crystallography shows that the
three complexes have different packing motifs, in which the
cation sits on the shared face of two parallelepipeds for [1-
NBD][BArCl4], is surrounded by eight anions in a gyrobifastigium arrangement for [1-NBD][BArF ], or the six anions show an
4
octahedral cage arrangement in [1-NBD][BArH ], similar to that of [1-NBD][BArCF34]. C−X···X−C contacts, commonly
4
encountered in crystal-engineering, are suggested to be important in determining structure. Addition of H2 in a solid/gas reaction
affords the resulting σ-alkane complexes, [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArX ] [1-NBA][BArX ] (NBA = norbornane),
4
4
which can then proceed to lose the alkane and form the zwitterionic, anion-coordinated, complexes. The relative rates at which
hydrogenation and then decomposition of σ-alkane complexes proceed are shown to be anion dependent. [BArCF34]− promotes
fast hydrogenation and an indefinitely stable σ-alkane complex. With [BArH ]− hydrogenation is slow and the σ-alkane complex
4
so unstable it is not observed. [BArCl4]− and [BArF ]− promote intermediate reactivity profiles, and for [BArCl4]−, a single-crystal
4
to single-crystal hydrogenation results in [1-NBA][BArCl4]. The molecular structure derived from X-ray diffraction reveals a σ-
alkane complex in which the NBA fragment is bound through two exo Rh···H−C interactions-different from the endo selective
binding observed with [1-NBA][BArCF34]. Periodic DFT calculations demonstrate that this selectivity is driven by the
microenvironment dictated by the surrounding anions. [1-NBA][BArX ] are catalysts for gas/solid 1-butene isomerization (298
4
K, 1 atm), and their activity can be directly correlated to the stability of the σ-alkane complex compared to the anion-coordinated
decomposition products.
proligand15−18 or protonation of a metal−alkyl complex with a
1. INTRODUCTION
strong acid,19,20 for example, [Re(HEB)(CO)2(CypH)][Al-
The functionalization of alkanes via C−H activation to produce
{OC(CF3)3}4] (HEB = η6-hexaethylbenzene, Cyp = cyclo-
commodity and fine chemicals or new materials is of
pentyl)21 or [Rh(PONOP)(H4C)][BArCF34] (PONOP =
(2,6-tBu2PO)2C5H3N, ArCF3 = 3,5-(CF3)2(C6H3)), respectively.
Although these elegant techniques allow for the direct
observation of σ-alkane complexes by NMR spectroscopy,
these are generated in low absolute quantities, e.g., 1−20 mg,
and are relatively short-lived, e.g., half-lives of less than 90 min
at 212 K. This makes such routes less than optimal for the
generation of materials suitable for structural elucidation by
single-crystal X-ray diffraction techniques or onward reactivity
studies. We recently reported an alternative route to form σ-
considerable contemporary interest.1−4 σ-Alkane complexes,
in which the C−H bond engages in a 3-center 2-electron
interaction with the metal center,5 have long been recognized as
key intermediates in C−H activation processes that occur by an
inner-sphere mechansim.6−12 σ-Alkane complexes are also of
interest in terms of their challenging, but fundamentally
important, coordination chemistry. The strong nonpolar C−
H bond in alkanes, combined with the steric hindrance of
proximal groups, mean that alkanes are very poor ligands,
typically binding to metal centers with bond enthalpies of ∼60
kJ mol−1 or less.9,13,14 The characterization of σ-alkane
complexes has been generally limited to very low-temperature,
in situ, solution-based spectroscopic studies, e.g., TRIR or
NMR spectroscopy, either by photogeneration of a coordina-
tively unsaturated metal center in a solvent that also acts as a
Special Issue: In Honor of the Career of Ernesto Carmona
Received: April 11, 2018
© XXXX American Chemical Society
A
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alkane complexes directly in the solid-state by solid/gas single-
crystal to single-crystal transformations,22,23 thereby enabling
the structure of the product to be directly determined by single-
crystal X-ray diffraction and solid-state NMR techniques. It also
allows for structures to be further interrogated by periodic DFT
computational analysis. Exemplifying this approach, addition of
H2 to the diene precursor [Rh(Cy2PCH2CH2PCy2)(η2η2-
NBD)][BArCF34], [1-NBD][BArCF34] (NBD = norborna-
ligand are not fully resolved, although we have noted that the
steric profile appears to be important,25 while a number of weak
C−H···F−C hydrogen bonds in the extended lattice may
contribute to kinetic and/or thermodynamic stability.24 In this
contribution we concentrate on the role that the supporting
anion has on the stability, structure, and catalytic performance
of SMOM-catalysts based upon [1-NBA]+, in which the anion
is systematically varied (Scheme 1) using various tetra-aryl
diene), affords [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArCF3
]
4
([1-NBA][BArCF34], NBA = norbornane) in essentially
Scheme 1. SMOM Systems Studied in This Contribution
quantitative yield (Figure 1A),24 in which a saturated NBA
borates in which substitution at the 3,5-position is changed.
Although anion effects are well-documented in homogeneous
organometallic chemistry and catalysis,32,33 their influence in
the solid-state with regard to single-crystal to single-crystal
synthesis and catalysis is considerably less well developed.34−37
Figure 1. (A) Synthesis of σ-alkane complexes in the solid-state. (B)
Solid-state structure of [1-NBA][BArCF34]. (C) Packing of anions
around a single cation (van der Waals radii).
fragment interacts with the Rh-center through two endo-C−H···
Rh 3c−2e interactions. This complex is remarkably stable in the
solid-state (months at 298 K under Ar) with respect to NBA
loss and formation of the [BArCF34]− coordinated zwitterion,
[Rh(Cy2PCH2CH2PCy2){(η6-C6H3(CF3)2)BArCF33}], [1-
BArCF34]. With other substituents on phosphorus, stability is
significantly reduced. For example, when R = cyclopentyl
2. RESULTS AND DISCUSSION
2.1. Variation of the Borate Anion in the 3,5 Positions:
Effects on Structure. New precursor complexes [1-NBD]-
[BArCl4] (ArCl = 3,5−Cl2C6H3), [1-NBD][BArF ] (ArF = 3,5−
4
F2C6H3), and [1-NBD][BArH ] (ArH = C6H5) were synthe-
4
(Cyp) a lifetime of 96 h is measured,25 while when R = Bu,
i
sized by addition of the respective sodium borate salt to a
dichloromethane solution of [RhCl(Cy2PCH2CH2PCy2)]2 in
the presence of excess NBD (Scheme 2).
stability at 298 K is limited to only a few hours.26 Stability is
enhanced at lower temperatures, allowing for characterization
by single-crystal X-ray diffraction techniques at 150 K. We
speculate that the general stability in the solid-state of σ-alkane
Scheme 2. Synthesis of NBD Precursor Complexes
complexes such as [1-NBA][BArCF34] and its analogues
−
originates from an octahedral cavity defined by the [BArCF3
]
4
anions (Figure 1B,C), which permits the hydrogenation of the
alkene to occur without loss of crystallinity, trapping the
resultant alkane. These anions thus provide a microenviron-
ment that allows for transformations within a crystalline
“molecular flask”.27 We describe these exceptionally well-
defined systems as solid-state molecular organometallics
(SMOM) in reference to related and well-developed supported
organometallic catalysts (SOMC)28 and single-site heteroge-
neous catalysts (SSHC),29 and like these systems, they support
transformations such as alkane C−H activation30 and catalysis,
e.g., solid/gas alkene isomerization.31
Solution NMR data are fully consistent with their
formulation (Supporting Information) being similar to
previously reported examples.24,38 Orange single-crystals
suitable for single-crystal X-ray diffraction could be grown
from CH2Cl2/pentane solvent mixtures, and the resulting solid-
state structures determined are shown in Figure 2. The effect of
the anions on the solid-state structure is signaled by the three
complexes crystallizing in different space groups: [1-NBD]-
The precise factors governing the variation of relative
stabilities of these complexes on changing the phosphine
[BArCl ] P1 (Z′ = 1), [1-NBD][BArF ] Fdd2 (Z′ = 0.5), and
̅
4
4
[1-NBD][BArH4] Pbca (Z′ = 1). The cations show
B
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a
Figure 2. Extended solid-state structures of (A) [1-NBD][BArCl4]; (B) [1-NBD][BArF ]; (C) [1-NBD][BArH4]. ArX groups omitted. Solvent
4
accessible voids calculated by PLATON,39 expressed as a percentage of the unit cell volume. No solvent accessible voids were identified.
b
unremarkable structural metrics, being very similar to that of
the parent complex [1-NBD][BArCF34]. However, consider-
ation of the packing of the anions in the lattice shows a
dramatic difference between all three. For [1-NBD][BArCl4],
the cation sits on the shared face of two parallelepipeds, but
there is no crystallographically imposed symmetry. In [1-
NBD][BArF ] the cation is surrounded by eight anions which
4
adopt a gyrobifastigium arrangement (two face-sharing trigonal
prisms rotated 90° to one another) and sits on a special
position generating C2 symmetry. For [1-NBD][BArH ], each
4
cation is surrounded by six anions in an octahedral cage
arrangement, similar to that in [1-NBD][BArCF3 ] (P1, Z =
̅
4
2).24 However, unlike in [1-NBD][BArCF34] where the
Figure 3. Interanion interactions in the solid-state structures of (A)
[1-NBD][BArCl4] and (B) [1-NBD][BArF ].
{Rh(NBD)} points toward the apical borate and is flanked
4
by two of its ArCF3 groups, the NBD ligand in [1-
NBD][BArH ] points to an edge of the octahedron (Figures
4
(F4···F8 2.913(3) Å, F17···F220 2.924(6) Å), which lie within
error of the combined van der Waals radii (2.92 Å).40 There are
1B and 2C). This is a consequence of the different local
orientation of the [BArH ]− anions compared to [BArCF34]− in
no interanion contacts in [1-NBD][BArH ] that lie within the
4
4
which aryl groups of the former penetrate the cavity of the
sum of van der Waals radii.
octahedron (Supporting Information) and for which the
These new NBD complexes have been studied by solid-state
NMR (SSNMR) techniques at 294 K. The 31P{1H} SSNMR
spectrum of [1-NBD][BArCl4] features two distinct but closely
spaced phosphine environments [δ 64.7, J(RhP) = 145 Hz; δ
volume is also smaller (average cross-cage B···B distances of
16.9 and 18.8 Å, respectively).
The extended solid-state structures of both [1-NBD][BArF ]
4
and [1-NBD][BArCl4] display interanion contacts. In [1-
63.0, J(RhP) = 147 Hz]. Similarly, for [1-NBD][BArH ], two
4
NBD][BArF ], the F1···H27 distance of 2.465(2) Å (Figure
4
phosphine environments are observed [δ 75.8, J(RhP) = 134
Hz; δ 64.8, J(RhP) = 132 Hz]. The observation of two 31P
environments is consistent with the solid-state structures that
have crystallographically nonequivalent phosphines. In contrast,
3B) lies well within the combined van der Waals radii of F and
H (2.66 Å)40 being comparable to that reported in the solid-
state structure of fluorobenzene (2.47 Å).41 The C27−H27···F1
angle is essentially linear, 174.4(2)°. For [1-NBD][BArCl4], a
number of interanion chlorine···chlorine contacts (Cl1···Cl6
3.412(1) Å, Cl1···Cl8 3.575(1) Å, Figure 3A) also lie within the
combined van der Waals radii (3.64 Å)40 and are arranged close
to 90° [100.97(9) and 80.35(8)°, respectively]. These are best
described as Type II C−X···X−C contacts,42 which are similar
to those observed in the solid-state structure of 1,3-
dichlorobenzene.43 These types of halogen···hydrogen44 and
halogen···halogen45 contacts have been widely utilized in crystal
engineering.46 Given that these two anions are essentially
isosteric, these interactions, albeit weak, clearly influence the
extended packing motif in the lattice. Long interanion contacts
the 31P{1H} SSNMR spectrum of [1-NBD][BArF ] displays a
4
single, broad 31P environment at δ 70.9, consistent with the C2
symmetry observed in its solid-state structure, although
coupling to 103Rh was not resolved. These features are mirrored
in the corresponding 13C{1H} SSNMR spectra. [1-NBD]-
[BArCl4] and [1-NBD][BArH ] each feature six signals47
4
between δ 90 and 50, which are assigned to the NBD fragment,
while the 13C{1H} SSNMR spectrum of [1-NBD][BArF ]
4
displays four distinct signals assigned to the NBD fragment
under crystallographic C2 symmetry.
2.2. Reactivity of [1-NBD][BArX ] (X = Cl, F, H) with H2
4
in the Solid-State as Studied by NMR Spectroscopy.
are observed in the solid-state structure of [1-NBD][BArCF3
]
With the complexes [1-NBD][BArX ] in hand their solid/gas
4
4
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reactivity with H2 was studied with respect to changing the
disappearance of diagnostic signals at δ 88.4, 87.4, 80.3, and
79.4 assigned to alkene in the starting material. These data
signal the formation of a σ-alkane complex [1-NBA][BArCl4].
The 31P{1H} SSNMR resonances were accompanied by a broad
signal centered at δ 86 (relative ratio 25:75), assigned to the
amorphous decomposition product [1-BArCl4] which has been
independently synthesized and crystallographically character-
ized and is similar to that reported for [Rh-
(iBu2PCH2CH2PiBu2){(η6-C6H3Cl2)BArCl3}] (see the Support-
anion-identity, and compared against [1-NBD][BArCF34].
Finely powdered single crystals of [1-NBD][BArX ] were
4
exposed to H2 (1 bar, 298 K) for various periods of time, H2
was then removed under vacuum, dichloromethane was added
by vacuum transfer and the sample warmed to 298 K. The
extent of hydrogenation was determined by comparing the
relative integrals in the 31P{1H} NMR spectrum of the starting
material and zwitterionic [1-BArX ] (Figure 4A). Complexes
4
ing Information).38 We have not observed [1-NBA][BArCl
]
4
free of zwitterion product; however, low temperatures halt
onward decomposition. Complete decomposition of [1-NBA]-
[BArCl4] occurs in 7 h at 294 K. This stability of [1-
NBA][BArCl4] is notable considering that the room temper-
ature hydrogenation of microcrystalline [Rh-
(iBu2PCH2CH2PiBu2)(η2η2-NBD)][BArCl4] affords [Rh-
(iBu2PCH2CH2PiBu2){(η6-C6H3Cl2)BArCl3}] quantitatively in
10 min and no intermediate alkane σ complex is observed,38
reflecting the influence of the phosphine steric-profile on σ-
complex stability.
Although hydrogenation of [1-NBD][BArH ] occurs at the
4
same rate as [1-NBA][BArCl4] (Figure 4A) an intermediate
alkane complex is not observed. The 31P{1H} SSNMR
spectrum (158 K, Figure 5D) of hydrogenated microcrystalline
Figure 4. (A) Temporal evolution of H2 addition (1 bar, 298 K, first 2
h) to [1-NBD][BArX ] (X = Cl, F, H, CF3) as measured by the
formation of [1-BArX 4] in CD2Cl2 solution.48 (B) Selectivity of [1-
4
NBA][BArX ] formation after H2 addition (2 bar, 298 K, 1 h) as
4
measured by 31P{1H} SSNMR at 158 K.
[1-BArX ] have been independently prepared and characterized
4
(Supporting Information).48 This methodology only indirectly
signals the formation of a σ-alkane complex, i.e., [1-NBA]-
[BArX ], as the alkane would be rapidly displaced by the anion
4
on solvation.24 However, it does allow for the expedient
measurement of the relative rates of hydrogenation in the solid-
state.
Figure 5. 31P{1H} SSNMR spectra (158 K) of H2 addition (2 bar, 298
K, 1 h) to powdered crystalline [1-NBD][BArX ]. (A) [1-NBD]-
4
These data reveal the rate of NBD hydrogenation to be
anion-dependent, with replacement of the 3,5-(CF3)2 sub-
stituents on the borate resulting in significantly slower
[BArCF34]; (B) [1-NBD][BArCl4]; (C) [1-NBD][BArF ]; (D) [1-
4
NBD][BArH4].
reactions. For [1-NBD][BArF ], hydrogenation of the NBD
4
ligand is only complete after ∼24 h. By contrast, [1-
[1-NBD][BArH ] (2 bar, 1 h, 298 K), features a single very
4
NBD][BArCl4] and [1-NBD][BArH ] are both hydrogenated
broad resonance centered at δ 89.0, which is assigned to the
4
in 2 h, although this is still considerably slower than for [1-
NBD][BArCF34] for which a maximum time of 15 min is
obtained (first measured point). These different rates of
hydrogenation are also signaled by color changes to the
crystalline material. While orange microcrystalline [1-NBD]-
amorphous zwitterionic decomposition product [1-BArH ],
4
which has been independently prepared (Supporting Informa-
tion). Zwitterionic complexes such as Rh(PR3)2{(η6-C6H5)-
BArH } are well-known.50−54 No signals were observed that
3
could be assigned to a σ-alkane complex. For [1-NBD][BArF ],
4
[BArCl4] and [1-NBD][BArF ] undergo little immediate color
after hydrogenation for 1 h, two broad closely spaced downfield
doublets where observed [δ 105.1, J(RhP) ≈ 180 Hz; δ 102.2,
J(RhP) ≈ 190 Hz] that are assigned to σ-alkane complex [1-
4
change, over longer times (3 h) the color of the powder
changed to yellow which is characteristic of zwitterion
formation. For [1-NBD][BArH ], this color change was more
NBA][BArF ]. However, a very broad resonance [δ 89.3, ca.
4
4
rapid (10 min).
69%] also dominates, assigned to an amorphous phase of
Following the hydrogenation in the solid-state (2 bar, 1 h,
298 K)49 of [1-NBD][BArCl4] by 31P{1H} SSNMR spectros-
copy (measured at 158 K to slow decomposition − vide infra)
revealed complete consumption of the starting material to form
an intermediate species [δ 102.7, J(RhP) ≈ 180 Hz; δ 97.2,
J(RhP) ≈ 198 Hz]. These data are similar to those reported for
[1-NBA][BArCF34] [δ ∼110, J(RhP) ≈ 207 and 216 Hz]
(Figure 4B).24 The 13C{1H} SSNMR spectrum confirmed that
hydrogenation of the alkene had occurred, with the
zwitterionic [1-BArF ], in addition to starting material (14%).
4
[1-BArF ] has been independently synthesized and crystallo-
4
graphically characterized (Figure 6 and Supporting Informa-
tion) and presents a rare example of this anion coordinating
with a metal center in an η6-motif. Structures showing κ1− and
κ2− F···M interactions55,56 with the [B(C6F5)4]− are known.
These data are consistent with not only the slower rate of
hydrogenation in the solid-state for [1-NBD][BArF ] but also
4
that the stability of [1-NBA][BArF ] with respect to zwitterion
4
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[BArH ], the alkane complex is persistent and can be observed
4
alongside amorphous [1-BArCl4]. These observations indicate
that the fluorous −CF3 groups may facilitate H2 transport
through the nonporous crystal35,59−62 and/or permit rearrange-
ment of the NBD fragment necessary to promote hydro-
genation and formation of a σ-alkane complex, while retaining
the same microenvironment of [BArCF34]− anions to allow for
subsequent observation of the σ-alkane complex.
2.3. Characterization of a σ-Alkane Complex by
Single-Crystal X-ray Crystallography. The temporal profile
of hydrogenation coupled with the relative lifetime of the σ-
alkane complex encouraged a single-crystal to single-crystal
study of the hydrogenation of [1-NBD][BArCl4]. H2 was added
to crystalline material (2 bar, 298 K) and a selected single-
crystal rapidly transferred to the cryostream (150 K) of an X-
ray diffractometer. This procedure was repeated a number of
times to optimize the hydrogenation time (30 min) to obtain
the highest-quality structure. Under these temporal conditions
(cf. Figure 4), the best solution still had ∼7% [1-NBD]-
[BArCl4] in the lattice that could be adequately accounted for at
this partial occupancy. Longer reaction times (>1 h) afforded
solids that had lost long-range order, i.e., Figure 5B. The solid-
state structure of [1-NBA][BArCl4] is shown in Figure 7A. This
single-crystal to single-crystal transformation proceeds without
Figure 6. Solid-state structure of [1-BArF ]. Displacement ellipsoids
4
are shown at 50% probability level, second molecule in the asymmetric
unit and hydrogen atoms omitted for clarity. Selected bond distances
(Å): Rh1−P1 2.2631(5), Rh1−P2 2.2449(5), Rh1−C27 2.4604(18),
Rh1−C28 2.3423(18), Rh1−C29 2.2802(18), Rh1−C30 2.3908(18),
Rh1−C31 2.3194(18), Rh1−C32 2.3185(18).
formation must be sufficient to allow for its observation, albeit
not as the major species after 1 h. The broad signals observed in
the 31P{1H} SSNMR that are assigned to amorphous phases of
these zwitterionic complexes have been noted previously for
[Rh(iPr2PCH2CH2PiPr2){(η6-C6H3(CF3)2)BArCF33}] which,
interestingly, undergoes a phase change at 323 K to give
crystalline material, resulting in sharper signals.24 We have not
a change in space group (P1) or the overall structural motif of
̅
investigated whether this occurs for [1-BArX ] (X = F, Cl, H).
4
1
We have previously used H/13C FSLG-HETCOR SSNMR
experiments57 to indirectly detect Rh···H−C interactions in σ-
alkane complexes.24,25,58 The HETCOR spectrum (158 K) of
[1-NBA][BArCl4] (as prepared as a mixture with [1-BArCl4])
features a high-field 1H cross-peak (δ −3.1) which correlates to
a partially obscured 13C signal at δ 25 assigned to the NBA
carbon atoms involved in the 3c−2e interaction (Supporting
Information). We did not observe a distinctive cross peak for
the bridge methylene group in NBA to high-field shifted proton
signals. This is different from that in [1-NBA][BArCF34], in
which a 13C signal at δ ∼44 correlates to a high-field shifted
1
peak in the H NMR projection that is due to these protons
experiencing ring current shielding from the proximal 3,5-
(CF3)2C6H3 groups in the lattice.24,58 This suggests a different
structure for [1-NBA][BArCl4] (vide infra). We were not
successful in obtaining a meaningful HETCOR spectrum of [1-
NBA][BArF ].
4
−
These data combined show that the [BArCF3
]
anion
4
promotes fast hydrogenation of the NBD precursor to form a
stable alkane complex as already reported.24 In comparison,
although [1-NBD][BArH ] shares a similar solid-state packing
4
environment, hydrogenation is significantly slower, while anion
coordination and alkane loss occurs at a comparable, likely
faster rate, which means that the intermediate σ-alkane complex
is not seen. For [1-NBD][BArCl4] and [1-NBD][BArF ] the
4
Figure 7. (A) Solid-state structures of the cations in exo-[1-
NBA][BArCl4]. Displacement ellipsoids are shown at 50% probability
level. Hydrogen atoms excepting those on the alkane fragment omitted
for clarity. Selected bond lengths (Å): Rh1−P1 2.2095(11), Rh1−P2
2.2059(12), Rh1−C27 2.387(5), Rh1−C28 2.378(5), C27−C28
situation is more finely balanced. The motif present in [1-
NBD][BArF ], with C−H···F interactions, clearly inhibits the
4
hydrogenation of the coordinated NBD, while the relative lack
of stability of the corresponding σ-alkane complex, [1-
NBA][BArF ], with respect to zwitterion [1-BArF ] means
4
4
1.561(7), C30−C31 1.548(9). (B) Relationship between the [1-
−
NBA]+ cation and the proximal [BArCl
]
anion (van der Waals
that this complex is never observed as the major species. For
[1-NBA][BArCl4], this situation is balanced more in favor of
the alkane complex over starting material. Although the rate of
hydrogenation is essentially the same as that for [1-NBD]-
4
surface shown). (C) Molecular structure of the cation in [1-
NBA][BArCF34] demonstrating the endo-coordination motif of the
cation.24
E
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the anion-packing. The structural model is consistent with the
coordination of norbornane with the metal center through two,
η2, 3c−2e Rh···H−C bonds. The Rh−C27/C28 distances
[2.387(5) and 2.378(5) Å, respectively] are the same within
error as those reported for [1-NBA][BArCF34] [2.389(3) and
2.400(3) Å].24 The C27−C28 and C30−C31 distances
[1.561(7) and 1.548(9) Å] confirm the hydrogenation of the
diene to afford an alkane. The Rh−P distances are shorter than
in the precursor NBD complex, e.g., Rh−P1, 2.2095(11), cf.
2.3008(6) Å, and reflect the low trans influence of the σ alkane
interaction.
these solid-state structures. Interestingly, the exo-coordination
of C−H···Rh interactions observed in [1-NBA][BArCl4] reflect
the intermediate implicated in selective solid/gas H/D
exchange processes in [1-NBA][BArCF34].30
2.4. Selectivity of the NBD Hydrogenation and
Subsequent C−H Activation in the Solid-State. Reaction
of [1-NBD][BArCF34] with D2 in a solid/gas reaction (5 min)
proceeds with ∼95% endo selectivity, forming endo,endo-D4-[1-
1
NBA][BArCF34] as determined by GC-MS and H/2H NMR
spectroscopy of the liberated NBA and single-crystal neutron
diffraction.30 When [1-NBA][BArCF34] is exposed to D2 (16 h)
C−H activation occurs at the remote, exo-C−H bond, to give
exo-D4-[1-NBA][BArCF34]. No H/D exchange was observed at
the endo-sites despite it being these that interact with the Rh
center. This latter experiment shows that the exo sites are both
accessible (Figure 8) and kinetically competent for C−H
activation. That the D4 isotopologue is selectively formed also
demonstrates a fluxional process is occurring in the solid-state
that brings both sets of exo-C−H bonds to the Rh-center. This
was shown experimentally and computationally to involve an
initial low energy C2 rotation of the NBA fragment in endo-[1-
NBA][BArCF34]. Interestingly, when [Rh(iBu2PCH2CH2-
PiBu2)(η2η2-NBD)][BArCF34] is exposed briefly to D2 it results
in endo-D2-exo-D2-NBA, the different selectivity demonstrating
reorganization in the solid-state via an intermediate NBE
complex.26
The hydrogen atoms associated with C27 and C28, i.e., the
3c−2e interactions, were located and their positions refined in
the final model. These, and the relative orientation of the NBA
fragment, show it is the exo-hydrogens of the NBA methylene
groups C27/C28 that are interacting with Rh-center. This is in
contrast to the identical cation in [1-NBA][BArCF34] (Figure
7C) in which it is the endo-CH groups that interact with the
metal center.24 This different orientation is thus due to the
different anion packing in the solid-state. Closer inspection of
the relationship between cation and proximal anion in [1-
NBA][BArCl4] (Figure 7B) reveals that the NBA bridge
methylene group (C33) does not sit in the cleft of two aryl
groups, as found in [1-NBA][BArCF34] (Figure 1B), but instead
the anion sits off to one side. This is consistent with the 1H/13C
HETCOR studies that indicates that there are no ring-current
shielded methylene groups in the solid-state.
The selectivity of H2 addition to [1-NBD][BArX ] (X = Cl,
Previous calculations on the isolated cation [1-NBA]+ have
shown that the exo- and endo-forms are essentially isoenergetic
and can be interconverted by a low-energy rocking motion
(TSrock, 9.1 kcal/mol, Figure 8A). However, when placed in the
4
F, H) was similarly studied. Longer reaction times (40 h) and
powdering of precursor employed due to the significantly
slower rates of hydrogenation exhibited by these complexes
2
relative to [1-NBD][BArCF34]. For [1-NBA][BArCl4], the H
NMR spectrum of the liberated alkane (vacuum transferred
using CD2Cl2) shows deuterium incorporation in both the
endo- (δ 1.16) and exo-sites (δ 1.46) [Lit. d4-methanol: 1.21
and 1.52, respectively].63 Relative integrals, calibrated to the
1
bridgehead methine resonance, from the corresponding H
NMR spectrum show deuterium incorporation at the endo
positions to be ∼90 with ∼25% at the exo positions (Scheme
3). GC-MS indicated the major ion at m/z = 100.1185 which
corresponds to D4-NBA (calcd C7H8D4 = 100.1190). Addi-
tional ions were observed at m/z = 102.1310 (calcd C7H6D6 =
a
Scheme 3. D2 Addition to [1-NBD][BArX ]
4
Figure 8. (A) Relative energies of endo- and exo-NBA isomers in [1-
NBA]+ as a function of anion and their barriers for interconversion via
TSrock. (B) Detail of the structure of TSrock computed in the solid-state
for [1-NBA][BArCl4]. Method: [CP2K]PBE-D3/DZVP-MOLOPT-
SR-GTH/GTH-PBE (cutoff 500 Ry).
extended crystal environment periodic DFT calculations on [1-
NBA][BArCF34] revealed that the endo-isomer is now clearly
favored by 9.7 kcal/mol, while the exo-isomer was kinetically
accessible with a barrier to interconversion of 13.9 kcal/mol,
albeit at very low equilibrium populations.30 For [1-NBA]-
[BArCl4], this situation is reversed. The most stable isomer is
now exo-[1-NBA][BArCl4], with the endo-isomer at 2.5 kcal/
mol being kinetically accessible via TSrock and a slightly smaller
barrier of 10.1 kcal/mol (see Figure 8B and Supporting
Information). These calculations are fully consistent with the
observed, crystallographically determined structures and high-
light the importance of considering the extended environment
rather than isolated metal−ligand complexes when discussing
a
The signal at m/z = 1 is lower than the parent ion, i.e., m/z = 99.1,
which reflects the fragmentation pattern of NBA: NBA−[H]•.
F
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Article
102.1315) and m/z = 104.1437 (calcd C7H4D8 = 104.1441) all
with fragmentation patterns very similar to those of NBA,
consistent with the incorporation of six and eight deuterium
atoms, respectively. These data are consistent with reductive
deuteration of the alkenes to form D4-[1-NBA][BArCl4] and
subsequent C−H activation of the alkane in a solid/gas
reaction. Similar NMR and mass spectral data were obtained for
the liberated alkane obtained via reaction of powdered [1-
NBD][BArF ] with D2. In contrast, GC−MS of NBA liberated
4
from the analogous reaction with [1-NBD][BArH ], features a
4
single ion at m/z = 100.1181 (calcd C7H8D4 = 100.1190)
consistent with deuteration of the double bonds only and that
no H/D exchange is occurring. This is consistent with the poor
stability of [1-NBA][BArH ] relative to those of [1-NBA]-
4
[BArCl4] and [1-NBA][BArF ]. The relative integrals in the 1H
4
NMR spectrum of the liberated NBA shows the deuterium
addition is selective (∼90%) for the endo-faces.
The slow hydrogenation and poor stability of all the new σ-
alkane complexes reported here meant that delineation of the
selectivities of the initial hydrogenation and the H/D exchange
reactions was not possible, unlike that for [1-NBA][BArCF34].
Nevertheless, the observation of 90% endo/25% exo deuterium
incorporation for [1-NBD][BArF ] and [1-NBD][BArCl
]
4
4
suggests that H/D exchange occurs at the alkane as well as
D2 addition to the alkene (which would only produce 100%
total endo/exo D incorporation). Further evidence for C−H
activation at the bound alkane ligand in the solid-state comes
from addition of D2 to [1-NBD][BArCl4] or [1-NBD][BArF ]
4
in CD2Cl2 solution, where the NBA ligand is lost to solvent
coordination,24,26 and only deuteration of the double bonds is
observed.
Figure 9. (A) Comparison of catalysts in the isomerization of 1-butene
to 2-butene as measured by gas phase H NMR spectroscopy. All
1
2.5. Catalytic 1-Butene Isomerization. We have
previously shown31 [1-NBA][BArCF34] to be effective in
solid-state molecular organometallic catalysis (SMOM-cat) for
the isomerization of 1-butene to a mixture of cis-and trans-2-
butene at 298 K and 1 atm. Studies suggest catalysis is likely
dominated by active species near to the surface of the crystal,
although the NBA ligand is readily displaced by alkene in the
catalysts = ∼2.0 mg finely powdered sample. Complexes [1-
NBA][BArX ] (X = Cl, F, H) were prepared in situ. 1-Butene = 1
4
atm (86 μmol at 298 K). Dashed line indicates 90% conversion. (B)
Relative ratio of 1-butene, trans-2-butene, and cis-2-butene as
measured by GC.
whole crystal. In situ prepared [1-NBA][BArX ] (X = Cl, F, H)
NBA][BArF ], consistent with the higher proportion of NBA
4
4
enabled the effect of the anion on the catalytic isomerization of
1-butene to be studied. Catalysts were prepared by hydro-
genation of ∼2 mg of finely powdered NBD precursors in an
NMR tube for 1 h to form the corresponding NBA−alkane
complexes in the proportions shown in Figure 4B. These in situ
prepared catalysts were then exposed to 1-butene gas, and the
progress of the resulting solid/gas isomerization catalysis
followed by gas-phase NMR spectroscopy and gas chromatog-
raphy (Figure 9A). As shown previously, [1-NBA][BArCF34] is
a very effective catalyst, and the first measured point (1 min)
shows that the thermodynamic equilibrium64 between 1-butene
and 2-butene is almost reached at 94% conversion (ToN = 54,
ToF(min) = 3170 h−1). Complexes [1-NBA][BArCl4] and [1-
alkane complex present. All eventually reach close to the
thermodynamic equilibrium for the 1-butene/2-butene ensem-
ble.64 Precatalyst [1-NBD][BArH ] is essentially inactive post
4
activation with H2, as the alkane complex is not persistent.
Without H2 activation, all the NBD precatalysts are inactive for
isomerization. These observations regarding catalyst activity
and active-site structure are similar to those made by Bianchini
and co-workers on the role of active-site isolation in solid/gas
ethene hydrogenation using [Ir(triphos)(H)2(C2H4)][X].37
When X = [BArH ]− dimerization occurs at temperatures
4
above 70 °C to form a much less active hydride bridged species.
In contrast, when the much larger polyoxometalate
[PW12O40]3− is used dimerization is avoided as the active
sites are isolated.
Monitoring the 2-butene cis/trans ratio over time shows at
the early stages of catalysis (2 min) a cis/trans ratio of ∼1:1 for
all three active catalysts (Figure 9B). When left for an extended
time (60 min) the ratio changes to be very close to the
thermodynamic position of ∼1:3 at 298 K. This demonstrates
that cis-2-butene is the kinetic product, and we suggest this is
driven by the relative stabilities of cis and trans isomers when
bound to the Rh catalyst.31 The observed resting state in the
[1-NBA][BArCF34] system is the cis-isomer of [1-(2-butene)]-
[BArCF34], an observation supported by periodic DFT
NBA][BArF ] were both found to be active catalysts but were
4
slower than [1-NBA][BArCF34]. These systems are also active
on recharging with 1-butene (Supporting Information). These
recharging experiments showed slightly reduced ToF which we
have previously commented upon as being due to the
formation of [1-butadiene]+, by a transfer dehydrogenation
of 1-butene, which is a poorer catalyst.31
The relative rates of isomerization track the relative
proportions of alkane complex formed after 1 h of hydro-
genation (Figure 4B). Figure 9B compares relative conversion
after 2 and 60 min, and [1-NBA][BArCl4] is faster than [1-
G
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Organometallics
Article
calculations that show the cis-coordinated isomer to be 11.4
kcal/mol more stable than the trans, reflecting the micro-
environment imposed by the crystalline lattice, and 5.7 kcal/
mol more stable than the 1-butene complex. The initial 1:1
ratio observed here and then equilibration to the thermody-
namic limit suggest reversible binding of the isomers of 2-
butene post double-bond isomerization. Compared with the cis-
2-butene resting state, isomerization, and then loss of trans-2-
butene at the metal center presents the overall highest energy
span within the system. Similar selectivity for cis- over trans-2-
butene has been noted for Pt-nanoparticle catalysts65 and is
suggested to be due to the increased stability of the surface-
bound cis-isomer. Homogeneous66 and surface supported
organometallic systems (under propene metathesis conditions)
have also been shown to show selectively at early stage of
catalysis for cis-butene.67,68
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: andrew.weller@chem.ox.ac.uk (A.S.W.).
*E-mail: s.a.macgregor@hw.ac.uk (S.A.M.).
■
ORCID
Alasdair I. McKay: 0000-0002-6859-172X
Antonio J. Martínez-Martínez: 0000-0002-0684-1244
Andrew S. Weller: 0000-0003-1646-8081
Tobias Kramer: 0000-0001-5842-9553
̈
Stuart A. Macgregor: 0000-0003-3454-6776
Present Address
§T.K.: Department of Chemistry, Maynooth University,
Maynooth, County Kildare, Ireland.
3. CONCLUSIONS
Notes
The interplay between reactive cationic metal center and
weakly coordinating anion is a well-established way of tuning
structure, reactivity, and stability in homogeneous organo-
metallic catalytic systems. We have extended this concept to
solid-state molecular organometallic systems (SMOM) by using
solid/gas reactivity to synthesize sometimes transient σ-alkane
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank SCG Chemicals Co., Ltd, Thailand (A.S.W.) and the
EPSRC (A.S.W., S.A.M.; EP/M024210, EP/K035908, EP/
K035681) for funding. This work used the ARCHER UK
National Supercomputing Service (http://www.archer.ac.uk).
complexes and then benchmarked their reactivity for alkene
−
isomerization. Although of the anions studied only [BArCF3
]
4
affords indefinitely stable σ-alkane complexes, there is a strong
correlation between the lifetime of the complexes and catalytic
activity in 1-butene double bond isomerization. The relative
rates of hydrogenation of the NBD precursors are less
straightforward to rationalize, but it is clear that the −CF3
groups on the [BArCF34] anion play a significant role in both
facilitating the movement of H2 through the essentially
nonporous crystalline framework and stabilizing the lattice
toward collapse. More generally, the ability for the anion
framework to control the spatial orientation and stereo-
chemistry of the bonding mode of the alkane ligand with the
metal center, e.g., exo-[1-NBA][BArCl4], suggests future
opportunities to harness the local microenvironment in
SMOM systems to control selectivity in catalytic processes.60
This is similar to that recently reported in catalysts
encapsulated in metal−organic frameworks that also offer a
local spatial environment that promotes selectivity for one
pathway over another.69
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* Supporting Information
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I
DOI: 10.1021/acs.organomet.8b00215
Organometallics XXXX, XXX, XXX−XXX