Examining the role of Rh/Si cooperation in alkene hydrogenation by a pincer-type [P2Si]Rh complex

Article abstract of DOI:10.1039/c6dt00027d

A bis(phosphine)/triflatosilyl pincer-type Rh(i) complex can reversibly store one equivalent of H₂ across the Si-Rh bond upon triflate migration from silicon to rhodium. The triflatosilyl complex serves as an effective precatalyst for norbornene hydrogenation, but Si-OTf bond cleavage is not implicated in the major catalytic pathway. The combined findings suggest possible strategies for M/Si cooperation in catalytic processes.

Full text of DOI:10.1039/c6dt00027d

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DOI: 10.1039/C6DT00027D
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Examining the role of Rh/Si cooperation in alkene hydrogenation
by a pincer-type [P2Si]Rh complex
Matthew T. Whited,*a Alexander M. Deetz,‡a Theodore M. Donnell,‡a and Daron E. Janzenb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A bis(phosphine)/triflatosilyl pincer-type Rh(I) complex can reversibly store preliminary investigations into the role of such bond lability in
one equivalent of H2 across the Si–Rh bond upon triflate migration from catalytic norbornene hydrogenation, showing that these
silicon to rhodium. The triflatosilyl complex serves as an effective rearrangements do not occur on the major hydrogenation
precatalyst for norbornene hydrogenation, but Si–OTf bond cleavage is not pathway and may hinder catalyst performance. However, the
implicated in the major catalytic pathway. The combined findings suggest finding that pincer-supported Si–OTf and Si–H bonds are
possible strategies for M/Si cooperation in catalytic processes.
kinetically labile, including during catalysis, points toward the
possibility of a new suite of cooperative processes at pincer-
stabilized metal silyl and silylene complexes.
Taking inspiration from biological systems, a number of
research groups have recently sought to expand the range of
transformations available to transition metals through the use
of non-innocent ligands. Several promising strategies have
emerged, such as using redox-non-innocent ligands to support
key redox steps1 and coordinatively non-innocent ligands to
stabilize reactive intermediates or control proton
delivery/abstraction.2
As part of a research program focused on developing
metal/silicon cooperative approaches to small-molecule
activation,3 we have recently investigated the coordination
chemistry of bis(phosphine)/dihydrosilyl pincer-type proligands
with Rh(I) precursors.4 Unlike related methylsilyl [P2Si] pincers,5
these ligands readily undergo double Si–H activation and H2 loss
with formation of a new silicon–chloride or –triflate bond. Such
facile bond-breaking and -forming suggests the possibility of
using the electropositive silyl donor together with the electron-
rich Rh(I) center for cooperative catalysis, potentially via
silylene intermediates.6
Scheme 1. Decomposition of Ph1-OTf to a bis(µ-silylene) dimer Ph2 and rendering of the
core of Ph2 determined by X-ray crystallography
We previously reported the formation of a pincer-type
triflatosilyl rhodium(I) complex, [PhP2SiOTf]Rh(nbd) (Ph1-OTf in
Scheme 1, nbd = norbornadiene) via multiple Si–H activations of
a
dihydrosilyl proligand.4 Ph1-OTf is unstable in
dichloromethane for extended periods, and during early
attempts to crystallize Ph1-OTf we noted instead the formation
of red crystals of a new compound. The decomposition product
was obtained in small quantities and proved insoluble in
common solvents but was identified by X-ray crystallography as
the bis(µ-silylene) complex [(PhP2Si)Rh(OTf)]2 (Ph2, Scheme 1).
Ph2 may result from dimerization of monomeric silylene triflate
complexes, though related mechanisms invoking equivalently
reactive species can be envisioned (e.g., nucleophilic attack at
silicon by a 3-coordinate triflatosilyl rhodium(I) center). A
mechanism invoking silylene intermediates is consistent both
with the lability of silicon–triflate bonds7 and with Ozerov’s
recent report of a pincer-type cationic Pt(II) silylene complex
Here we report a bis(phosphine)/silyl pincer-type rhodium
system where kinetically labile Si–OTf and Si–H bonds facilitate
reversible H2 storage across the Rh–Si unit. We present
a Department of Chemistry, Carleton College, Northfield, MN 55057, USA.
b Department of Chemistry and Biochemistry, St. Catherine University, St. Paul, MN
55105, USA.
E-mail: mwhited@carleton.edu
† Electronic Supplementary Information (ESI) available: Synthesis procedures and
characterization data, including NMR spectra for all reported compounds.
Crystallographic data for CyP2SiH2, Ph2 Ph1-OTf, and Cy1-OTf. Computational and
,
catalytic experimental details. CCDC 1444624–1444627. See
DOI: 10.1039/x0xx00000x
‡ These authors contributed equally to the work described.
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that is isoelectronic with the proposed [PhP2Si]Rh(OTf) so we sought to confirm its identity through prepVaierwatAiroticnleoOfnlainne
intermediate and exhibits substantial silylium character.8 analogous complex without a silicon hydride. Beginning with
DOI: 10.1039/C6DT00027D
However, as yet we have not been able to distinguish among the known hydrido chloride complex [CyP2SiMe]Rh(H)(Cl),5a the
possible mechanisms.
related methylsilyl complex [CyP2SiMe]Rh(H)(OTf) (Cy3-Me) was
We were interested in the possibility of trapping a prepared by salt metathesis with silver triflate (Scheme 3). As
[P2Si]Rh(OTf) silylene intermediate or similar species with a expected, Cy3-H and Cy3-Me possess similar spectroscopic
1
reagent such as H2, which could in principle be stored across the signatures. For instance, the Rh–H H NMR signal for Cy3-Me
1
2
Rh=Si bond by a net 1,2-addition. However, Ph1-OTf did not (δ −22.4 (dt, JRhH = 32.0 Hz, JPH = 13 Hz)) exhibits a nearly
react with H2 (1 atm) at ambient temperature and afforded identical chemical shift and coupling constants to the Rh–H of
multiple products at elevated temperatures. We reasoned that Cy3-H, with the distinction that no coupling to a silicon hydride
H2 reactivity could be enhanced by employing bulkier phosphine is observed upon methyl-for-hydride replacement.
substituents to accelerate nbd dissociation and suppress
formation of an insoluble dimeric species. Thus, we prepared
the
dicyclohexylphosphine
analogue
of
Ph1-OTf
,
[CyP2SiOTf]Rh(nbd) (Cy1-OTf) (see ESI for details, including the
synthesis and crystal structure of the CyP2SiH2 proligand).
Comparison of the crystal structures of Ph1-OTf and Cy1-OTf
shows that the complexes exhibit quite similar metrical
parameters (Figure 1). The clearest difference between Ph1-OTf
and Cy1-OTf is in the P–Rh–P angle, which is more obtuse for
Cy1-OTf (122.8°) relative to Ph1-OTf (119.4°) due to the steric
pressure exerted by the cyclohexyl groups. Cy1-OTf also exhibits
a longer Si–OTf bond and slightly larger sum of angles about
Scheme 2. Hydrogenation of Cy1-OTf and H2 transfer from Cy3-H to nbd
By analogy with [CyP2SiMe]Rh(H)(Cl) and related 5-coordinate
d6 complexes,10 we propose that Cy3-H and Cy3-Me exhibit Y
geometries with phosphine donors approximately trans and an
acute Si–Rh–H angle. A Y-type geometry is also supported by
DFT calculations on Me3-H, the dimethylphosphine analogue of
Cy3-H, which is calculated to exhibit an acute Si–Rh–H bond
angle (69°) and approximately trans-disposed hydride and
triflate ligands (156°). Consistent with our experimental
observations, this geometry is also calculated to be 3.5 kcal/mol
more stable than Me4-OTf, the isomer of Cy3-H with triflate
bound to silicon and an η2-dihydrogen ligand on rhodium, which
is the next lowest-energy isomer identified computationally.
silicon excluding the triflate, ∑∠
Si (336.7° for Cy1-OTf versus
335.5° for Ph1-OTf), consistent with a modest increase in silylene
character.7b However, in neither case do the structural
parameters or the 29Si NMR chemical shifts imply a substantial
ground-state silylene contribution (δ 92.2 for Cy1-OTf, 97.8 for
Ph1-OTf compared with δ 200–370 for free silylenes9).
Scheme 3. Synthesis of model complexes Cy1-Me and Cy3-Me
Figure 1. Crystal structures of (a) Ph1-OTf and (b) Cy1-OTf with thermal ellipsoids at the
50% probability level and hydrogen atoms and portions of the Si···P phenylene linkers
omitted for clarity. Selected bond lengths (Å) and angles (°) for Ph1-OTf: Rh–Si, 2.247(1);
Rh–P1, 2.336(1); Rh–P2, 2.326(1); Si–O1, 1.783(3); P1–Rh–P2, 119.38(4). For Cy1-OTf: Rh–
Si, 2.249(1); Rh–P1, 2.351(2); Rh–P2, 2.368(2); Si–O1, 1.796(3); P1–Rh–P2, 122.81(4)
Exposure of Cy3-H to an excess of nbd results in quantitative
regeneration of Cy1-OTf with release of norbornene (nbe)
(Scheme 2), suggesting that Cy3-H can serve as a source of H2. In
contrast, Cy3-Me does not react with nbd. These findings
emphasize the potential importance of incorporating labile
silicon substituents when targeting Rh/Si cooperative reactivity,
since facile rearrangement can occur with Si–OTf and Si–H
bonds, whereas Si–CH3 bonds are more often inert (see ref. 11
for exceptions).
The reactivity shown in Scheme 2 also suggests that Cy1-OTf
may serve as a precatalyst for hydrogenation of nbe, and
Cy1-OTf does hydrogenate nbe to nba at 0.5% loading (complete
reaction in <30 min). However, the finding of efficient catalysis
does not prove the intermediacy of Cy3-H. In fact, several
mechanistic possibilities for hydrogenation can be envisioned,
We were delighted to find that Cy1-OTf reacts with H2 at
ambient temperature to liberate norbornane (nba) and afford
the desired hydrosilyl rhodium(I) hydrido triflate complex, Cy3-
H
(Scheme 2). The 1H NMR spectrum of Cy3-H exhibits a
diagnostic hydride resonance (δ –22.6 (dtd, 1JRhH = 31.5 Hz, 2JPH
3
= 13.4 Hz, JHH(Si) = 4.4 Hz) with coupling to rhodium, the
phosphine ligands, and the silicon hydride (δ 5.66, 1JSiH = 196 Hz,
Δν1/2 = 9 Hz). The presence of an Si–H was also confirmed by a
1H/29Si HMQC experiment (Figure S25).
Complex Cy3-H was unstable under all conditions examined,
precluding microanalytical or crystallographic characterization,
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as outlined in Scheme 4. Cycle A operates via a dihydride-type efficient precatalyst for norbornene hydrogenaVtiieownA,rticthleoOunlginhe
DOI: 10.1039/C6DT00027D
mechanism, where Si–OTf cleavage is not implicated. Cycles B comparative catalytic studies with methyl-substituted
and C both require Si–OTf cleavage but differ in whether the analogues strongly suggest that the Si–OTf bond remains intact
alkane release occurs upon delivery of hydrogen from a second along the major catalytic pathway. Nevertheless, our finding
H2 molecule (B) or from the Si–H (C).
that pincer-supported Si–OTf and Si–H bonds may be easily
We have made preliminary efforts toward understanding exchanged, including during catalysis, indicates that new
the mechanism of nbe hydrogenation by comparing Cy1-OTf stoichiometric and catalytic mechanisms invoking M/Si
with methylsilyl model complexes Cy1-Me and Cy3-Me (Scheme cooperation may be accessible.
3). Hydrogenation by Cy1-Me would be expected to occur via
Acknowledgment is made to the donors of the American
Cycle A, whereas Cy3-Me would operate by Cycle B, and Cy1-OTf Chemical Society Petroleum Research Fund and to Carleton
could access all 3 mechanisms. Under standard conditions at 0.5 College for support of this research. X-ray crystallography was
mol % catalyst loading (see ESI for further details), Cy1-OTf and supported by NSF-MRI Award No. 1125975, “MRI Consortium:
Cy1-Me both fully hydrogenate nbe, though Cy1-OTf is Acquisition of a Single Crystal X-ray Diffractometer for a
considerably slower (complete reaction in 8 min for Cy1-Me Regional PUI Molecular Structure Facility”, and NMR
versus 30 min for Cy1-OTf). Under the same conditions, Cy3-Me spectroscopy was enabled by NSF-MRI Award No. 1428752,
does not hydrogenate nbe (<0.5% conversion in 150 min). These “MRI: Acquisition of a 400 MHz NMR Spectrometer to Support
combined findings suggest that nbe hydrogenation occurs Research and Undergraduate Research Training at Carleton
primarily via Cycle A. The fact that triflatosilyl precatalyst College and St. Olaf College”. A. M. D. thanks the Dreyfus
Cy1-OTf is considerably less efficient than methylsilyl precatalyst Foundation for a research fellowship. The authors thank D.
Cy1-Me may be due to the equilibrium between Cy4-OTf and Gross and L. Yang (Carleton College) for assistance with GC-MS
Cy3-H (Scheme 4), which can siphon catalyst off the primary experiments and B. Noll (Bruker AXS, Inc.) for crystallographic
cycle. The precise role of the Cy4-Cy
/
Cy3-H equilibrium is the assistance with complex Cy1-OTf
.
object of ongoing investigation.
Notes and references
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Scheme 4. Possible catalytic cycles for nbe hydrogenation by precatalyst Cy1-OTf
Although these findings suggest
a simple dihydride
3
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the same H2 molecule, early experiments suggest a more
complicated reality. Hydrogenation of nbe by either Cy1-OTf or
Cy1-Me under 1:1 H2/D2 leads to a significant amount of
norbornane-d1 (see ESI), suggesting that H/D scrambling can
occur at Cy4 and/or that alkane release may occur by hydrogen
transfer from a second H2 molecule rather than direct reductive
elimination. In either case, an intermediate η2-silane may be
implicated, consistent with recent reports of hydrogen delivery
to alkenes from appended silanes at Ru and Pd12 as well as the
important role of reversible Si–H and Si–C bond formation in
[PSiP]Pd-catalyzed allene hydrocarboxylation.13 Experimental
and theoretical efforts are currently directed at understanding
the intimate mechanisms at play.
4
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Goshert, J. R. Walensky, J. Zhou and O. V. Ozerov,
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In conclusion, we have reported a pincer-type triflatosilyl
rhodium complex where facile Si–OTf cleavage allows reversible
H2 storage across the Rh–Si bond. The complex serves as an
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