Remarkably facile and reversible Ru-C(sp3) bond cleavage to give a reactive 16-electron Cp*Ru(κ2-P,carbene) zwitterion

Article abstract of DOI:10.1021/ja071684e

An 18-electron Cp*Ru(κ³-P,C,C′) complex provides access to a 16-electron Cp*Ru(κ²-P,C) zwitterion by way of an unprecedented Ru-C(sp³) bond cleavage process that is both facile and reversible. Notably, this reactive zwitterion has proven capable of facilitating the Ru-mediated double geminal Si-H bond activation of Ph₂SiH₂. Copyright

Full text of DOI:10.1021/ja071684e

Published on Web 04/26/2007
Remarkably Facile and Reversible Ru-C(sp3) Bond Cleavage to Give a
Reactive 16-Electron Cp*Ru(K2-P,Carbene) Zwitterion
Matthew A. Rankin,† Gabriele Schatte,‡ Robert McDonald,§ and Mark Stradiotto*,†
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, the Saskatchewan
Structural Sciences Centre, UniVersity of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9, and the X-Ray
Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received March 9, 2007; E-mail: mark.stradiotto@dal.ca
Coordinatively and electronically unsaturated metal complexes
represent key reactive intermediates in a diversity of stoichiometric
and catalytic reactions, including transformations involving the
activation of otherwise robust bonds in both organic and inorganic
substrates. Although there is considerable interest in preparing
isolable complexes of this type, the synthesis of reactive, low-
coordinate species can pose a significant challenge: the use of
sterically demanding ancillary ligands designed to stabilize such
unsaturated species can inhibit access of substrates to the reactive
metal center, while the generation of low-coordinate complexes in
the absence of bulky stabilizing ligands commonly results in the
irreversible degradation of the target complex. An alternative
approach involves the use of an ancillary ligand whose strategic
design affords stability to the resting state of the complex, while
also providing access to the desired unsaturated metal species by
way of a reversible ligand binding/reorganization process.1 Indeed,
cyclometalation,1a,b â-hydride elimination,1b,c hemilabile chelation,1d
ligand-promoted dimer-to-monomer rearrangements,1e and other
reversible processes have been exploited successfully as a means
of unveiling unsaturated species in situ. In the pursuit of new classes
of highly reactive complexes that are capable of mediating difficult
bond activation reactions, the rational development of novel
ancillary ligands whose dynamic coordination behavior renders the
Figure 1. Reagents: (a) CH2Cl2; (b) NaN(SiMe3)2; (c) DMAP (DMAP )
associated metal fragments operationally unsaturated2 represents
4-dimethylaminopyridine, for 5) or CO (for 6); (d) Ph3SiH (for 7) or Ph2-
SiH2 (for 8). ORTEP diagrams shown with 50% ellipsoids; selected C and
an important goal. Particularly appealing are functional ancillary
ligands of this type that are capable of re-establishing coordinative
unsaturation at a reactive metal center following an initial bond
activation step and/or that enable substrate bond activation pro-
cesses.3 Herein we report the synthesis and characterization of the
18-electron Cp*Ru(κ3-P,C,C′) species 3sa complex that provides
access to the 16-electron Cp*Ru(κ2-P,C) zwitterion 4 by way of
an unprecedented Ru-C(sp3) bond cleavage process that is both
facile and reversible. Notably, the anionic phosphine-carbene
ancillary ligand in 4 facilitates the Ru-mediated double geminal
Si-H bond activation of Ph2SiH2.
In the quest to establish new metal-mediated E-H bond
activation chemistry (E is a main group element), and inspired by
reports documenting the utility of unsaturated Cp*RuLn+ complexes
in a range of challenging stoichiometric and catalytic E-H bond
activation processes,4,5 we have initiated a research program
targeting cationic and formally zwitterionic complexes of this type
derived from 1-PiPr2-2-NMe2-indene and related ancillary ligands.6
In this context, we have reported previously the isomerization of a
16-electron zwitterionic Cp*Ru(κ2-P,N) complex to the 18-electron
hydridocarbene 1 (Figure 1) by way of double geminal C-H
bond activation.6a,b We have observed subsequently that complex
H atoms have been omitted for clarity. Bond lengths (Å) for 3: Ru-P
2.2158(5); Ru-C1 2.250(2); Ru-C27 1.941(2); P-C1 1.805(2); N-C2
1.397(2); N-C27 1.346(2). Bond lengths (Å) for 5: Ru-P 2.3011(9); Ru-
C27 1.949(3); Ru‚‚‚C3 (shortest Ru-indenide distance) 3.403; P-C3 1.779-
(3); N-C2 1.425(4); N-C27 1.329(4). Bond lengths (Å) for 10: Ru-P
2.295(2); Ru-Si 2.391(2); Ru-H 1.48(9); Ru-C27 2.169(8); P-C1 1.889-
(7); Si-C1 1.958(8); Si-H 1.90(8); N-C2 1.37(1); N-C27 1.47(1).
1 is transformed cleanly into the chlorocarbene 2 upon exposure
to CH2Cl2. In viewing this chlorocarbene as a potential precursor
to new 16-electron Cp*RuLn+ species via dehydrohalogenation, 2
was treated with NaN(SiMe3)2 and the progress of the reaction was
monitored by use of 31P NMR methods. Over the course of 45 min,
complete conversion to a new phosphorus-containing product 3 was
observed. While elemental analysis data for 3 were found to be
consistent with the anticipated net removal of HCl from 2, NMR
spectroscopic and X-ray crystallographic data for 3 confirmed the
identity of this complex as the C1-symmetric, 18-electron Cp*Ru-
(κ3-P,C,C′) species depicted in Figure 1.7 Surprisingly, however,
data from 1D and 2D EXSY 1H NMR experiments provided
definitive spectroscopic evidence for the operation of an unprec-
edented and reversible Ru-C(sp3) bond cleavage process involving
3. In the case of 1H EXSY experiments, irradiation of either of the
diastereotopic P(CHMe2)2 signals in 3 resulted in significant
positively phased enhancement of the other methine resonance,
indicating that these two sites undergo chemical exchange. Simi-
† Dalhousie University.
‡ University of Saskatchewan.
§ University of Alberta.
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10.1021/ja071684e CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
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larly, the H-1H EXSY spectrum of 3 exhibits positively phased
method of unveiling a diversity of reactive low-coordinate metal
species in situ by way of carbanion hemilability. We are examining
the utility of 4 and related unsaturated metal complexes in mediating
a range of demanding stoichiometric and catalytic substrate
transformations and will report the results of such studies in due
course.
off-diagonal exchange cross-peaks that connect the two diaste-
reotopic P(CHMe2)2 environments, as well as pairs of P(CHMe2)2
resonances. These observations can be rationalized in terms of the
reversible breaking and reforming of the Ru-C1 linkage in 3,
thereby providing access to the unsaturated Cs-symmetric interme-
diate 4. Given that the Ru-C1 distance in 3 (2.250(2) Å) is not
unusually long,8 it is likely that the remarkable ease with which
the Ru-C1 bond undergoes formal heterolytic cleavage in the
reversible transformation of 3 into 4 can be attributed in part to
the relief of ring strain in 3, as well as to the energetic favorability
of generating a 10π-electron indenide unit within the zwitterion 4
upon ring opening. Indirect support for the reversible generation
of 4 from 3 was obtained through the isolation of 4‚L complexes
(L ) 4-dimethylaminopyridine (DMAP), 5; L ) CO, 6) upon
treatment of 3 with DMAP or CO; the delocalized bonding that is
observed within the indenide unit of 5 (Figure 1) is consistent with
a 10π-electron framework, in keeping with related zwitterionic
complexes featuring donor-substituted indenide ligands.6a,9 Interest-
ingly, the facile Ru-C(sp3) bond cleavage observed in the course
of transforming the 18-electron Cp*Ru(κ3-P,C,C′) complex 3 into
6 upon addition of CO contrasts the chemistry exhibited by Cp*Ru-
(κ3-HC(PPh2NPh)2), whereby a Cp*Ru(CO)(κ2-N,C,N) adduct aris-
ing from net substitution of an ancillary ligand N-donor arm is
produced.8b
Having established that 3 behaves as a masked source of the
16-electron Cp*Ru(κ2-P,C) zwitterion 4 in reacting with L donors,
we turned our attention to examining the reactivity of 3 with E-H-
containing substrates. Exposure of 3 to H2 (∼1 atm) in C6D6
afforded 1 in the absence of observable (31P NMR) intermediates.10a
In contrast, the first-formed product (7, Figure 1) that was observed
spectroscopically upon combination of Ph3SiH and 3 can be
rationalized as arising from Si-H addition to the reactive inter-
mediate 4;10b after extended reaction times (weeks), 7 rearranged
to the isolable, thermodynamic product 9. An analogous intermedi-
ate 8 was identified as the kinetic product upon treatment of 3 with
Ph2SiH2.10b,c However, in contrast to the net transfer of H+ from
Ru to the carbocyclic backbone that was observed in the slow
transformation of 7 into 9, compound 8 rearranged cleanly to the
crystallographically characterized 10 (Figure 1) over the course of
24 h; experimentation directed toward elucidating the mechanistic
pathway linking 8 and 10 is ongoing.10d Although we are hesitant
to comment definitively regarding the location of the hydride in
10, the final refined metrical parameters point to an unsymmetrical
Ru-H‚‚‚Si bridging interaction in this complex.11 Notably, related
Ru-mediated double geminal Si-H bond activation processes have
been implicated in a novel alkene hydrosilylation mechanism
reported recently by Glaser and Tilley.5
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada (including a Discovery Grant
for M.S. and a Postgraduate Scholarship for M.A.R.), the Canada
Foundation for Innovation, the Nova Scotia Research and Innova-
tion Trust Fund, and Dalhousie University for their generous support
of this work. We also thank Dr. Michael Lumsden (Atlantic Region
Magnetic Resonance Center, Dalhousie) for assistance in the
acquisition of NMR data.
Supporting Information Available: Experimental details and
characterization data, including EXSY spectra for 3 and X-ray
crystallographic information files (CIF) for 3, 5, and 10. The material
is available free of charge via the Internet at http://pubs.acs.org.
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In summary, the 18-electron η1-indenyl complex 3 provides
access to the reactive 16-electron “η0-indenide” zwitterion 4 by
way of a remarkably facile and reversible Ru-C(sp3) bond cleavage
process, as evinced by data obtained from dynamic NMR investiga-
tions and reactivity studies. We propose that this unusual rear-
rangement is promoted by the incipient formation of a Hu¨ckel
aromatic anion within the ligand backbone of 4 upon heterolytic
cleavage of the Ru-Cindenyl bond in 3, and we anticipate that this
unusual ancillary ligand design strategy will provide a general
2
1H, RudCH), 5.72 (d, J ) 4.1 Hz, 1H, C1-H), -9.21 (d, JPH ) 24.3
Hz, 1H, Ru-H); 31P{1H} NMR (C6D6) δ 54.1. Diagnostic NMR data for
8: 1H NMR (C6D6) δ 9.68 (s, 1H, RudCH), 5.96 (d, J ) 3.7 Hz, 1H,
C1-H), 5.10 (s, 1H, Si-H), -9.90 (d, 2JPH ) 25.2 Hz, 1H, Ru-H); 31P-
{1H} NMR (C6D6) δ 54.8. (c) Treatment of 3 with PhSiH3 was observed
to generate an intractable mixture of products. (d) A possible mechanism
is provided in Scheme S1 of the Supporting Information.
(11) (a) Lachaize, S.; Sabo-Etienne, S. Eur. J. Inorg. Chem. 2006, 2115. (b)
Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
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