Silyl-Silylene Interplay in Cationic PSiP Pincer Complexes of Platinum

Article abstract of DOI:10.1021/acs.organomet.5b00596

Herein we report the synthesis and spectroscopic observation of a Pt complex featuring a P₂Si= ligand containing a central silylene donor connected to phosphines via two o-arylene linkers. Species of the type (P₂Si=)PtR undergo a net 1,2-migration of R from Pt to Si, which leads to highly unsaturated Pt cations displaying only a very distant interaction with the weakly coordinating carborane anion. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00596

Communication
pubs.acs.org/Organometallics
Silyl−Silylene Interplay in Cationic PSiP Pincer Complexes of
Platinum
Jessica C. DeMott,†,∥ Weixing Gu,†,∥,# Billy J. McCulloch,† David E. Herbert,†,⊥ Mitchell D. Goshert,‡
Justin R. Walensky,‡ Jia Zhou,§ and Oleg V. Ozerov*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
‡Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States
§Department of Chemistry, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
S
* Supporting Information
ABSTRACT: Herein we report the synthesis and spectroscopic
observation of a Pt complex featuring a P2Si ligand containing a
central silylene donor connected to phosphines via two o-arylene
linkers. Species of the type (P2Si)PtR undergo a net 1,2-
migration of R from Pt to Si, which leads to highly unsaturated Pt
cations displaying only a very distant interaction with the weakly
coordinating carborane anion.
he proliferation of pincer-style ligands has led to a number
of discoveries in the fundamental reactivity of transition
(2), we surmised9 that hydride abstraction15,16 from the HSi−Pt
group could provide access to cationic silylene/bis(phosphine)
complexes of Pt. Herein we report that, while our efforts were
successful, the silylene complexes proved to be rather unstable
and displayed behavior indicative of substantial silylium character
of these species.
T
metals and their catalytic applications. The success of pincer
ligands is due in part to the feasibility of incorporating different
combinations of donor atoms into these tridentate, meridionally
binding frameworks.1 Among the many options, incorporation of
silylene donors (R2Si) remains uncommon. Recent work by
Hartwig and Driess explored N-heterocyclic silylenes as side
arms (Chart 1, A),2 while the appearance of silylene in a central
Metalation of PSiH2P (1) with Pt(1,5-COD)Cl2 in the
presence of 2,6-lutidine produced a mixture of 2 and (PSiClP)-
PtCl, but it was possible to isolate pure 2 in 85% yield after
recrystallization. Milstein’s synthesis of 2 using (Me2S)2Pt(Me)-
Cl gave 62% yield following chromatographic purification.9
Simple salt metathesis reactions allowed replacement of chloride
in 2 with other anionic ligands to give 3−6 (Scheme 1).
Chart 1. Pincer Complexes with Silylene and Carbene Donors
Scheme 1. Synthesis of (PSiHP)PtR Compounds
donor position of a pincer has only been documented in a 1993
report by Zybill et al. (B).3 We were especially interested in
accessing a silylene housed within a structure where it is
connected to phosphine side arms by o-arylene linkers. This
connectivity provides for a very robust framework prearranged
for meridional binding to averagely sized transition metals.
Topologically similar ligands with central C,4 N,5 O,6 P,7 and Sb8
as well as silyl9−12 donors have been broadly explored. Recent
work by Whited et al. implicated a silylene intermediate in some
reactions of (PSiP)Rh complexes, but silylene was not directly
observed (C).13
Compounds 2−6 were characterized by multinuclear NMR
spectroscopy and 2 by a solid-state diffraction study. The
presence of a central silyl donor with an sp3-hybridized Si atom
carrying a hydrogen was evident from the NMR data: the 29Si
NMR resonances ranged from −1.5 to 43.1 ppm (JPt−Si = 608−
1
1192 Hz), while in the H NMR spectrum, a signal showing
coupling to 29Si (1JSi−H between 166 and 193 Hz) and 195Pt
(2JPt−H between 20 and 60 Hz) was observed in the 5.5−6.5 ppm
range.
Our group previously reported syntheses of carbene/
bis(phosphine) complexes of Pd that were synthesized by a
combination of C−H activation at PdII followed by hydride
abstraction (D).14 Taking note of the existence of (PSiHP)PtCl
Received: July 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00596
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Organometallics
Communication
Hydride abstraction reactions from the central Si−H were
carried out by the treatment of 2, 3, 5, and 6 with Ph3C+ salts of
[HCB11Cl11]− or [EtCB11Cl11]− anions17,18 in C6H5F (Scheme
2). The two different carborane anions can be viewed as
observed by 31P NMR spectroscopy in C6H5F at 65.5 ppm (1JPt−P
= 2828 Hz), but it was not long-lived enough to isolate or collect
29Si NMR data. We surmised that a bulkier aryl may retard the
rate of migration and prepared the mesityl derivative 7 via
reaction of 2 with Mes2Mg. Treatment of 7 with Ph3C-
[HCB11Cl11] resulted in the development of a deep red color
presumed to correspond to the proposed silylene species 7a. At
room temperature, this species was observed as a broad
resonance at 69.2 ppm in the 31P NMR spectrum. This signal
became sharper at 243 K, and the coupling to 195Pt (1JPt−P = 2526
Hz) was further resolved. The diagnostic15,19 silylene downfield
signal at 323.1 ppm (1JPt−Si = 879 Hz) was observed in the 29Si
NMR spectrum.
Scheme 2. Abstraction Reactions of (PSiHP)PtR
In solution at ambient temperature, the silylene complex 7a
did undergo isomerization to its silyl isomer 7b, whose structure
was confirmed by a single-crystal XRD study (Figure 1). Loss of
the silylene character in 7b arising from formal migration of
mesityl from Pt to Si was apparent from 29Si NMR data, with a
new signal observed at 6.84 ppm (1JPt−Si = 1267 Hz). The rate of
isomerization of 7a to 7b proved to be poorly reproducible: it
had been observed to take place over 16 h or after as little as 30
min at ambient temperature. It is possible that the rate of
isomerization is affected by the presence of varying amounts of
impurities. Unfortunately, this ultimately prevented us from
obtaining X-ray-quality crystals or consistently pure batches of 7a
in the solid form.
equivalent in their coordinating ability and simply offered a way
to alter the solubility and crystallinity of the salts of cations of
interest. Abstraction of the hydride proceeded rapidly in all cases
and resulted in the formation of Ph3CH. In the reaction of 2,
immediate formation of the major Pt-containing product was
evident. The NMR spectroscopic data (29Si NMR, δ 32.7 ppm)
were inconsistent with the silylene formulation, for which a
chemical shift of >200 ppm is expected.15,19 Instead, the NMR
data pointed to the isomeric structure 2b and were corroborated
by an XRD structural study (Figure 1). Isomerization of the
DFT calculations were used to examine the isomerization of
cations of 2a′, 5a′, and 6a′ into 2b′, 5b′, and 6b′ (where ′
indicates the gas-phase cation, Table 1). In accord with the
Table 1. Calculated Energies for Silylene to Silyl
Isomerization
a
Gas-phase calculations for cationic species without [HCB11Cl11]−
(denoted with ′) were performed at the B3LYP level.20 Calculated
energies are given in kcal/mol.
experiment, the migration of Cl/Me/Ph from Pt to Si was
calculated to be favorable by −18.2, −34.5, and −31.4 kcal/mol,
respectively. Relative energies for the isomers 8a′/8b′ and 9a′/
9b′ involving −SPh and −TePh substituents were also
calculated, under the notion that migration of the softer Lewis
bases thiolate and tellurolate from soft Pt to hard Si may be less
favorable. The computed reaction free energies were less
negative (−20.1 and −11.3 kcal/mol, respectively), but because
the isomerization was still predicted to be favorable, the synthesis
of 8 and 9 was not attempted.
Figure 1. ORTEP22 drawings (50% probability ellipsoids) of 2b (top)
and 7b (bottom). H atoms and methyl groups of isopropyl are omitted
for clarity. Selected distances (Å) and angles (deg) are as follows. For
2b: Pt1−Si1, 2.2452(12); Si1−Cl1, 2.0785(15); Pt1−Cl2, 2.9206(13);
P1−Pt1−P2, 163.30(4); Cl2−Pt1−Si1, 177.38(3). For 7b: Pt1−Si1,
2.2756(10); Si1−C13, 1.901(4); Pt1−Cl12, 2.902(1); P1−Pt1−P2,
153.99(4); Si1−Pt1−Cl12, 167.62(4).
The 29Si NMR chemical shifts for 7a and 6b (including a
[HCB11Cl11]− anion in the gas phase, M06/SDD/6-31G(d))20
were calculated to be 302.8 and 53.5 ppm. While these do not
reproduce the experimental values (323 ppm for 7a and 33.0
ppm for 6b) precisely, they are consistent with the highly
downfield chemical shift for the silylene species. The calculated
Pt−Si distances in the silylene complexes are only marginally
(0.02−0.08 Å) shorter than those in their silyl isomers.
putative 2a into 2b can be viewed as a net migration of chloride
from Pt to Si. Similarly, abstraction of hydride from 3 and 5 led to
3b and 5b (29Si NMR, δ 50.1 and 20.7 ppm, respectively). On the
other hand, abstraction of hydride from 6 proceeded to 6b via an
intermediate (6a) that appeared to be associated with the
development and then disappearance of the red color. 6a was
B
DOI: 10.1021/acs.organomet.5b00596
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Communication
Calculations indicate that the LUMO of 7a (Figure 2) is
primarily based on the Si p orbital, with some delocalization into
cationic units linked together by a siloxane bridge (Scheme 3).
Dissolution of crystals from the same batch gave the same 31P
Scheme 3. Formation of 10 and Literature Precedent
NMR resonance at 85.6 ppm, and the formulation of the solid
was supported by elemental analysis. The siloxane-bridged PSiP
ligand has been previously described by Milstein,9 but in a
saturated complex (Scheme 3) in which the Si−O bond had been
formed by Si−H oxidation. Tilley’s disiloxane, also a saturated
system, was derived from a silylene complex by the apparent
addition of O−H across the SiIr bond.24 In contrast, 7b reacts
by hydrolysis of the Si−C(mesityl) bond with retention of
unsaturation at Pt. Protolysis of a Si−C(mesityl) bond under
highly acidic conditions was productively exploited by Siegel et
al. in studies of catalytic C−F activation of aromatic C−F
bonds.25
In the solid-state structures of each of the complexes 2b and 7b
(Figure 1) and 10 (Figure 3), one of the chlorines of the
carborane anion approaches the Pt center trans to the silyl donor.
While the Si−Pt−Cl angles vary from ca. 142 to ca. 178°, the Pt···
Cl distances are quite similar at ca. 2.9 Å.26 They are much longer
than the distances between Pt and the anionic Cl ligand in 2
(2.4524(14) Å) or PSiMeP−Pt−Cl (2.437(2) Å),10 but they are
also considerably longer than the previously reported Pt−Cl
distances in Pt(II) dichloromethane complexes (2.49−2.62 Å)27
or in the AlCl4 adduct of [PSiMeP−Pt]+ (2.5640(9) Å).11,28 The
X-ray structures of 2b, 7b, and 10 are also in good agreement
with the calculated anion-free structures, as can be illustrated by
the similarity of calculated and XRD values for the Pt−Si distance
(2.284 Å for 2b′ vs 2.2452(12) Å for 2b). All in all, the Pt···Cl
distances in 2b, 7b, and 10 are likely indicative of a close cation−
anion approach rather than a proper dative Cl→Pt bond. It is
reflective of the stabilization of the empty site at Pt(II) by a
strongly trans influencing silyl ligand and the steric clash between
the bulky carborane and the PSiXP ligand. The various degrees of
weak stabilization of empty sites in otherwise three-coordinate
Pt(II) complexes have recently been reviewed.29
Figure 2. Depiction of the LUMO (isovalue 0.03) of structure 7a with
[HCB11Cl11]−, calculated by M06/SDD/6-31G(d). Hydrogen atoms
are omitted for clarity.
the aromatic rings and only a modest contribution from a d
orbital of Pt. The Wiberg bond indices for Si−Pt were calculated
to be 0.68 in 6b and 0.74 in 7a. Thus, although the Si center in 7a
is planar (i.e., sp2 hybridized), the modest extent of π bonding
between Pt and Si suggests that 7a may be analyzed as a platinyl-
substituted silylium cation.21 On the other hand, any shortening
of the Si−Pt bond in 7a vs 7b owing to an increase in the Pt−Si
bond order should be attenuated since the silyl Si is trans to an
empty site, while the silylene Si is not. Not surprisingly, the
difference between the Pt−Si bond lengths in silylene vs silyl is
greatest in 2b, where the chloride ligand has a weaker trans
influence than alkyls or aryls. This analysis also explains why the
experimentally observed 1JPt−Si coupling constant in 7a (879 Hz)
is actually smaller than that in its 7b isomer (1267 Hz) and is
closer to that in the neutral complex 7 (608 Hz).
We observed that solutions of 7b upon standing for 26 h
evolved to generate the new product 10, resonating at 85.6 ppm
in the 31P NMR spectrum. An XRD study on a single crystal
(Figure 3) surprisingly revealed that 10 is a product of the
apparent protodesilylation of mesitylene,23 resulting in two Pt
In summary, we have been able to access a silylene-containing
pincer ligand in the coordination sphere of Pt. The silylene has
proven to be an observable but short-lived species that isomerizes
by abstraction of the anionic ligand from the position trans to it.
DFT analysis suggests that the cationic silylene complex has only
a modest degree of π bonding with Pt and may be viewed as a
silylium cation with a platinyl substituent. From this perspective,
it is not surprising that it readily engages in the abstraction of
anionic ligands. The products of such abstraction are effectively
three-coordinate cationic Pt complexes with only distant
approach of the carborane anion to a position trans to the silyl
donor. One of these complexes underwent an unexpected
Figure 3. ORTEP22 drawing (50% probability ellipsoids) of 10. H atoms
and o-difluorobenzene cocrystallized solvent are omitted for clarity.
Selected distances (Å) and angles (deg) are as follows: Pt1−Si1,
2.2499(18); Si1−O1, 1.639(2); Pt1−Cl11, 2.8753(16); P1−Pt1−P2,
162.88(7); Si1−Pt1−Cl11, 141.97(6); O1−Si1−Pt1, 113.91(19); Si1−
O1−Si1#1, 149.8(4).
C
DOI: 10.1021/acs.organomet.5b00596
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AUTHOR INFORMATION
Corresponding Author
*E-mail for O.V.O.: ozerov@chem.tamu.edu.
■
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Present Addresses
⊥Department of Chemistry, University of Manitoba, Winnipeg,
Manitoba R3T 2N2, Canada.
(18) Douvris, C.; Michl, J. Chem. Rev. 2013, 113, PR179−PR233.
(19) Previously reported Pt−silylene complexes possess 29Si NMR
chemical shifts between 298 and 395 ppm. (a) Grumbine, S. D.; Tilley,
T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 7884−
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(20) See the Supporting Information for full details of the DFT
calculations and the relevant references.
#Clariant Chemical (China) Ltd., Shanghai, People’s Republic of
China.
Author Contributions
∥These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(21) Similar characterization has been applied to other late-transition-
metal silylene complexes, especially cationic ones. See ref 15 and also:
(a) Agou, T.; Sasamori, T.; Tokitoh, N. Organometallics 2012, 31,
1150−1154. (b) Hayes, P. G.; Beddie, C.; Hall, M. B.; Waterman, R.;
Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 428−429.
(22) Farrugia, L. J. Appl. Crystallogr. 1997, 30, 565−566.
(23) Allemann, O.; Duttwyler, S.; Romanato, P.; Baldridge, K. K.;
Siegel, J. S. Science 2011, 332, 574−577.
(24) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002,
21, 4065.
We are grateful to the following agencies for support of this
research: U.S. Department of Energy-Office of Basic Energy
Sciences (grant DE-FG02-10ER16201 to O.V.O.), the Welch
Foundation (grant A-1717 to O.V.O.), the Dreyfus Foundation
(Camille Dreyfus Teacher-Scholar Award to O.V.O.), the
Fundamental Research Funds for the Central Universities of
China (Grant No. AUGA5710013115 to J.Z.), and the
Laboratory Directed Research and Development program of
Lawrence Livermore National Laboratory (to J.R.W.). W.G. is
thankful for the support via the Richard W. Schmude, Jr.,
Endowed Graduate Scholarship. We also thank Ms. Linda Redd
for editorial assistance.
(25) Allemann, O.; Duttwyler, S.; Romanato, P.; Baldridge, K. K.;
Siegel, J. S. Science 2011, 332, 574−577.
(26) This distance is similar to an intramolecular Pt···Cl−R contact in a
recently reported Pt(IV) complex: Crosby, S. H.; Deeth, R. J.; Clarkson,
G. J.; Rourke, J. P. Dalton Trans. 2011, 40, 1227−1229.
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D
DOI: 10.1021/acs.organomet.5b00596
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