[(tBu2PCH2SiMe2) 2N]RuCH3: The origin of extremely facile, double H-C(sp3) activation generating a "hydrido-carbene" complex

Article abstract of DOI:10.1021/ja052973a

The four-coordinate compound [(^tBu₂PCH₂SiMe₂)₂N]RuCH₃ undergoes rapid double H-C(sp³) activation at -78 °C to generate a "hydrido-carbene" complex. DFT calculations suggest that the origin of the low barrier to methane elimination is an α-agostic interaction in the low-lying singlet state of the highly unsaturated (PNP)RuMe. The hydrido-carbene complex can be viewed as a "masked" resting state of the four-coordinate cyclometalated alkyl complex, [(^tBu₂PCH₂SiMe₂)N(Me₂SiCH₂P(^tBu)(C(CH₃)₂CH₂)]Ru, where hydride migration from metal to carbon occurs before any subsequent reactivity. Copyright

Full text of DOI:10.1021/ja052973a

Published on Web 07/14/2005
[(tBu2PCH2SiMe2)2N]RuCH3: The Origin of Extremely Facile, Double H-C(sp3)
Activation Generating a “Hydrido-Carbene” Complex
Michael J. Ingleson, Xiaofan Yang, Maren Pink, and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana
Received May 6, 2005; E-mail: caulton@indiana.edu
The reactivity of the Ru(II) complex (PNP)RuCl (PNP )
(tBu2PCH2SiMe2)2N-) is of significant interest due to its formal
14-electron configuration, low-coordination number (4), and triplet
(S ) 1) ground state.1 Recent work has demonstrated that despite
the electron deficiency of the (PNP)RuCl moiety,1,2 the Ru(II) center
can act as a reducing agent, with halide for azide salt metathesis
resulting in rapid metal oxidation and nitride formation (eq 1).3
This seemingly paradoxical behavior can be attributed to the absence
Figure 1. (Left) Formation of hydrido-carbene 1, from (PNP)RuCl. (Right)
Related ruthenium pincer carbene complex, i.
of any π acid ligands, the presence of the π basic amide, and the
electron-rich nature of the two phosphine donors.
planarity (N-Ru-CH3 angles, singlet ) 145° and triplet ) 173.8°).
Of particular note in the singlet structure is a ‘tilted’ methyl group
due to one agostic Ru-H contact (2.15 Å to H84) and a number
of close methyl C to tBu proton distances (shortest at 2.75 Å). The
calculated structure is approaching that previously found for the
transition state of a concerted reductive elimination process,14,15
where asymmetry in the M-CH3 interaction enables a CH3 valence
orbital to have an improved orientation for forming a bonding
interaction with a proximal group. In this case, the agostic
interaction to H84 aligns the fourth CH3 orbital away from Ru and
more suitable for the exceptionally low barrier to H abstraction
+TMSN3
(PNP)Ru(II)Cl -TMSCl8 (PNP)Ru(IV)N + N2
(1)
We are currently interested in extending our studies to examine
the effects that installing a strong σ donor [X]- ligand has on the
ground-state electronic configuration (triplet versus singlet), on
molecular structure (planar versus “cis divacant” octahedral) and
on subsequent reactivity of the four-coordinate (PNP)RuX. Herein
we report preliminary experimental and theoretical findings for the
methylation of (PNP)RuCl.
Treating (PNP)RuCl with a stoichiometric equivalent of MeLi
at -78 °C in THF ultimately affords the hydrido-carbene complex
1 via the double intramolecular C-H activation of a single Bu
methyl group (Figure 1). Important NMR spectroscopic features
(d8-THF, 233 K) include the carbene dCH resonances at 13.7 ppm
(1H, d of d) and 276 ppm (13C{1H} s), a hydride signal as a “pseudo”
triplet at -5.1 ppm along with the 31P{1H} spectrum exhibiting an
AX pattern with a large 2JPP coupling of 338 Hz. Attempts to more
fully characterize 1 were frustrated by its instability in solution
above 273 K, resulting in intractable mixtures. The double activation
of a single pendant CH3 group of a pincer ligand by a Ru(II) center
to generate a carbene complex has precedence (Figure 1 (i)), albeit
t
from Bu; the view in Figure 2 is nearly down the C3 axis of the
agostic methyl on Ru. No methyl “canting” or R-agostic interactions
are observed in the triplet state due to the necessary metal orbital
now being singly occupied. With a triplet/singlet energy difference
for (PNP)RuMe of only 3.4 kcal mol-1, a low minimum energy
crossing point between the potential energy surfaces of the two
spin isomers would be expected.16 This would lead to a kinetically
competent concentration of singlet, “pre-organized” (with respect
to CH4 loss) (PNP)RuMe. The conversion of A to B is calculated
to be essentially thermoneutral (i.e. less than (2.0 kcal mol-1) and
entropically driven by irreversible CH4 loss. This mechanism is
supported by the formation of perprotio 1 and CD3H on use of
d3-MeLi. At no point was there any spectroscopic indication for
the formation of (PNP)Ru(dCH2)(H), the product from the more
conventional outcome of an alpha agostic interaction (namely C-H
cleavage of the R-agostic bond). The calculated endoergic electronic
t
t
requiring more severe conditions (boiling BuOH in the presence
of base) than for formation of 1.4 Furthermore, double C-H
activation, leading to hydrido-carbenes, is well-known and has
widespread literature precedent.5-12
The reaction proceeds via an observed red, transient complex
which within minutes at -78 °C completely converts to 1. We
assign this intermediate as (PNP)RuMe, A (Scheme 1), but attempts
at gaining any direct spectroscopic evidence have failed due to its
short lifetime even at low temperatures. However, the immediate
addition of benzonitrile successfully “traps” A as (PNP)Ru(PhCN)-
Me, 2. A mechanism to 1 can be proposed, involving stepwise C-H
cleavage of a proximal CH3 group, via an unobserved cyclometa-
lated complex B (Scheme 1). To probe this facile reaction13 and
the relative energies of the spin isomers, DFT calculations (using
the full PNP ligand) were undertaken.
energy for conversion to (PNP)Ru(dCH2)(H) (+10.4 kcal mol-1
)
shows that this C-H activation pathway is unfavorable; therefore,
an undetected isomerization between A and (PNP)Ru(dCH2)H is
unlikely. The “thermoneutral” nature of the formation of B along
with the orbital pre-organization in the singlet state of A could
account for the absence of spin-blocking effects in the rapid
formation of diamagnetic 1 on methylation of paramagnetic (PNP)-
RuCl.
Analogous calculations performed for B produced a singlet
ground state (3.0 kcal mol-1 below its triplet) and a geometry-
optimized structure that is significantly closer to a “cis-divacant”
octahedron (N-Ru-C angle of 114.8°) than that for singlet (PNP)-
RuMe.17 We attribute this to the constraints imposed by the
asymmetrical four-membered metallacycle destabilizing the square
planar conformer. An R-agostic interaction is also observed in B,
For (PNP)RuMe, the triplet/singlet spin isomer energy difference
is small (the triplet isomer is more stable by 3.4 kcal mol-1), in
contrast to the parent chloride complex (∆E ) 10.0 kcal mol-1).
The singlet isomer (Figure 2) shows considerable deviation from
9
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J. AM. CHEM. SOC. 2005, 127, 10846-10847
10.1021/ja052973a CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1 Formation and Reactivity of the Hydrido-Carbene, 1; Inset, ORTEP View of the Non-hydrogen Atoms of 4 (50% Probability
Ellipsoids, Disorder in 4 Omitted for Clarity)a
a Selected bond lengths (Å) and angles (deg): Ru-N1, 2.129(2); Ru-C23, 1.881(4); Ru-C24, 2.075(5); C23-O23, 1.101(4); C24-O24, 1.190(6);
N1-Ru-C24, 100.02; N1-Ru-C23 177.47(12); P1-Ru-P2, 173.25(3); Ru-C24-C17, 118.3(3).
Ru(η2-NC5H4) 5, where C-H activation at the ortho position
effected the opening of the metallacycle of B.
In summary, the unusually low barrier observed here for CH4
loss is a consequence of the agostic interaction, which is in turn
made possible by the high degree of unsaturation of (PNP)RuMe
in its very low-lying singlet state. The “R-agostic-facilitated” CH4
elimination mechanism (supported by the lack of any observed
R-agostic C-D cleavage) suggests a different consequence in A
of this interaction from that more often considered (and proposed
Figure 2. DFT geometry optimized structures (with Ru eclipsing the amide
N in each case) of: (Left) Singlet (PNP)RuMe, selected bond lengths (Å)
and angles (deg) Ru1-C2, 2.02; Ru-H84, 2.15; N7-Ru1-C2, 145.0.
(Right) 1, selected bond lengths (Å) and angles (deg): Ru1-C27, 1.84;
H79-Ru1-N6 150.4; N6-Ru1-C27, 112.2; H79-Ru1-C27, 97.0.
here to connect 1 and B), the conversion to a hydrido-carbene.
Acknowledgment. This work was supported by the National
Science Foundation. Professor Daniel Mindiola is thanked for
helpful discussions and the reviewers for useful comments.
Supporting Information Available: Full synthetic and spectro-
scopic details for all compounds, along with crystallographic details
(CIF) for compound 4, together with DFT-optimized geometries on
molecules described in the text (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
which could facilitate the negligibly (3.4 kcal mol-1) endothermic
second C-H bond activation. The transformation of B to 1 results,
apart from a reduction in the Ru-C bond length, in only minor
geometrical changes in the metallacycle (in A, a Ru-C distance
of 2.08 Å decreases to 1.84 Å in 1, similar to that previously
reported for structurally characterized (i), RudC 1.868(4) Å).4 The
optimized geometry (Figure 2) for 1 is a distorted square-based
pyramid with the hydride approximately trans to the amide nitrogen,
in agreement with the observed chemical shift of the hydride in 1.
The facile formation of 1 from A is related to the behavior of [(Cy2-
PCH2SiMe2)2N]RuH3 on dehydrogenation, that also undergoes
multiple ligand sp3 C-H activation, generating the allyl complex,
[Cy2PCH2SiMe2NSiMe2CH2PCy(C6H8)]Ru.2
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