A π-basic rhenium center that effects cyclohexene isomerization to a β-agostic carbene ligand

Article abstract of DOI:10.1021/ja035165x

The molecule (PNP^R)Re(H)₄ (PNP^R = (R₂PCH₂SiMe₂)₂N, R = ⁱPr or cyclohexyl) reacts at 20 °C with 2 mol of cyclohexene to form equimolar cyclohexane and (PNP^R)Re(H)₂[=C(CH₂)₅]. This product is characterized by ¹H, ¹³C, and ³¹P NMR and by X-ray diffraction as having one CH₂ hydrogen (from a carbon located β to Re) donating to the metal ( agostic CH ). This interaction occurs in preference to PNP^R amide nitrogen π-donation. DFT calculations confirm this agostic interaction, and show that the (PNP^R)Re(H)₂ fragment indeed reverses the greater stability of free olefin vs free carbene. Copyright

Full text of DOI:10.1021/ja035165x

Published on Web 07/17/2003
A π-Basic Rhenium Center that Effects Cyclohexene Isomerization to a
â-Agostic Carbene Ligand
Oleg V. Ozerov, Lori A. Watson, Maren Pink, and Kenneth G. Caulton*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
Received March 14, 2003; E-mail: caulton@indiana.edu
We have reported earlier1 a series of examples of vinyl ethers
and related electron-rich olefins which are isomerized, upon
coordination to the fragment RuHCl(PiPr3)2, to heteroatom-
substituted carbenes complexed to this metal. Both experiment and
DFT computations revealed that the RuHCl(PiPr3)2 fragment lacks
the ability to isomerize an olefin to a non-heteroatom-substituted
carbene ligand, an effect we have attributed to insufficient ability
of this metal fragment to back-donate to a more π-acidic carbene
ligand. To address this limitation, we proposed to use a more
π-basic metal, Re, and a more powerful π-donor ligand, amide, to
isomerize a coordinated hydrocarbon olefin to a coordinated
carbene. The anionic pincer ligands2 (R2PCH2SiMe2)2N-(“PNPR”)
have already been incorporated3 in the complex (PNPR)Re(H)4,
which can be transformed to lower oxidation states by hydrogen
transfer to olefins. We now report how (PNPR)Re(H)4 converts
cyclohexene into coordinated cyclohexylidene and that this carbene
is â-agostic4 to the metal in (PNPR)Re(H)2[dC(CH2)5].
) 127 Hz, a typical value for (sp3)C-H, while the triplet at 15.6
ppm is characterized by a smaller JCH ) 117 Hz. A diminished
J
CH is one of the indicators of an agostic interaction.5 Assuming a
g127 Hz JCH coupling constant for the nonagostic H, the JCH to
the agostic hydrogen is estimated at ca. e107 Hz. This value is
comparable to other examples of weak agostic CH’s.5 Although
1
the illustrated structure of 2 is chiral (eq 1), the 31P, H, and 13C
NMR spectra reveal mirror symmetry; thus, the agostic interaction
must be fluxional and involve alternately one of the two hydrogens
on one CH2 group to generate a time-averaged mirror plane of
symmetry containing N, Re, and the carbene C. The PNPR signals
in 1H and 13C NMR spectra all show that there is no time-averaged
mirror plane containing N and the two P; therefore, the hydrides
do not migrate past the ReP2N plane.
Performing the reaction of 1a with excess cyclohexene in pentane
leads to the precipitation of 2a in the form of single crystals suitable
for an X-ray diffraction study, which confirmed the proposed
structure of 2a as a â-agostic cyclohexylidene complex (Figure 1)6
The angles Re-C(carbene)-Câ are 100.4(3)° and 147.2(3)°, thus
being distinctly nonequivalent. The Re-C36 distance is 2.635(4)
Å. The agostic interaction is established without perceptible
distortion of the Câ-CR-Câ angle (112.4(3)° while the other five
angles within the C6 ring range from 111.0(3)° to 112.0(4)°) and
the carbene carbon is coplanar with its three attached groups.
i
(PNPR)Re(H)4 (R ) Cy, 1a; R ) Pr, 1b) reacts readily with 2
or more equiv of cyclohexene (22 °C, 1 h) to produce cyclohexane
and 2 (eq 1). The reaction with 1b is essentially quantitative (NMR
evidence), but the reaction of 1a with only 2 equiv of cyclohexene
produces a significant amount of a byproduct resulting from the
cyclometalation of the Cy rings of the PNP ligand. This side reaction
is 99% suppressed by using a 15-fold excess of cyclohexene. The
complex 2b could only be obtained as an orange oil, but 2a could
be isolated in high yield as a crystalline solid of analytical purity.
The collective observations on 2a(b) reveal them to be carbene
complexes with a â-agostic hydrogen. The two hydride ligands
resonate as two broad peaks at -6.4 (-6.7) and -9.9 (-10.0) ppm
at 22 °C, and selective decoupling of the alkyl hydrogens results
in a triplet (JPH ) 12 Hz) 31P NMR signal. The congestion in the
aliphatic region of the 1H NMR spectra of 2 does not allow
assignment of the individual methylene resonances of the cyclo-
hexylidene fragment, but 13C NMR spectra show separate signals
for each of the six carbons of the cyclohexylidene. The R-C
resonates at 262.9 (264.9) ppm, consistent with a multiple rhenium
carbon bond. Three of the CH2 carbons resonate in the 25-30 ppm
range, typical for carbons of a cyclohexyl ring. We assign these
carbons as γ- and δ-CH2. The two remaining CH2 13C signals
resonate at 57.4 (57.4) and 14.9 (15.6) ppm, respectively. Both of
these are outside of the normal cyclohexyl range and we assign
them as the two â-carbons. In the undecoupled 13C NMR spectrum
of 2b both the peak at 57.4 ppm and the peak at 15.6 ppm are
triplets; however, the triplet at 57.4 ppm is characterized by a JCH
Figure 1. ORTEP drawing of (PNPCy)Re(H)2(C6H10) showing selected
atom labeling and hydrogen atoms in idealized positions, to see that the
agostic hydrogen on C36 is equatorial in a chair ring conformation. The
two hydride ligands are expected to be located in the N-Re-C31 plane
and projecting toward the reader. Only one C of each Cy ring on P is
illustrated. Selected structural parameters: Re-P1, 2.4091(9) Å; Re-P2,
2.3715(10) Å; P1-Re-P2, 161.30(4)°; N-Re-C31, 136.90(15)°; P1-Re-
C31, 103.13(11)°; P2-Re-C31, 95.10(12)°; P1-Re-N, 79.88(9)°; P2-
Re-N, 83.98(9)°.
In addition, the two CR-Câ distances are also different to a
statistically significant degree (1.500(6) and 1.526(6) Å), with the
longer distance to CR being the one involved in the agostic inter-
action. While surprising, this inequality is also present in a DFT
optimized geometry (see below). The Re-CR distance, 1.889(4)
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C O M M U N I C A T I O N S
Å, is comparable to those in the Schrock alkylidenes of Re.7 The
Re-N distance, 2.182(3) Å, is longer compared to that in 1a
(2.063(2) Å),3 indicating that in 2a the amide NfRe lone pair
donation is diminished or absent in favor of an agostic interaction
as a means to achieve an 18 valence electron configuration at Re.
It is perhaps surprising that the agostic interaction is preferred
to amide π-donation, given the strong manifestation of the latter in
1.3 It is possible that the strong trans influence of the hydride dis-
courages the amido ligand from taking a position exactly trans to
it, as would be necessary for a competent π-donation from N to Re.
We undertook a DFT computational study of the model com-
pound ((H2PCH2SiH2)2N)Re(H)2(dC(CH2)5) (2H). The optimized
structure of 2H reproduces the relevant features of the experimental
structure of 2a very closely (see Supporting Information). This
supports the conclusion that the origin of the distortions observed
in 2a is of electronic and not steric nature. The agostic Câ-H bond
is slightly longer than the other C-H bonds around the C6 ring,
consistent with the lower JCH observed by NMR for 2. The
conformation of the C6 ring obtained by calculation reproduces the
experimental determination well; thus, one can expect that the
positions of the H atoms on the C6 ring are also accurately predicted
by DFT. The calculated dihedral angle H-C36-C31-Re is 2.7°,
showing that the C-H bond is in the same plane as the p-orbital at
N with which it competes for the coordination site at Re. The
complex is formally d4 at Re (neutral carbene formalism), but the
two filled d-orbitals in a pentagonal bipyramidal geometry are
orthogonal to the N-Re-C plane. As a consequence, (a) the
rotation of the carbene about the Re/C bond has a large barrier and
(b) there is no back-donation into the σ* orbital of C36-H.
Previously reported8,9 cycloalkene to cycloalkylidene rearrange-
ments are effected by d2 Nb and W, while the cyclohexene f
cyclohexylidene rearrangement reported here is effected by a d4
fragment, (PNP)ReH2. While the DFT finding (Scheme 1) that 2H
is essentially thermoneutral with 4H contradicts the experimental
observation of only 2, the PNPR ligand in 2a-b is much more
sterically demanding than PNPH used in the DFT calculations. An
olefin complex should always be sterically disfavored compared
to an isomeric carbene, so that one should expect larger ancillary
ligands to bolster the preference for the carbene isomer.
In summary, we present here an example of a â-agostic carbene
ligand, obtained by transformation of a cyclic olefin on a Re
center.10 It seems likely that this structural motif will be found again
for unsaturated dialkylcarbene complexes, particularly of 5d (vs
4d) metals. There is a precedent4 for a â-agostic carbene in the
cation Tp(OC)2Wd[C(CH3)Ph]+; there, the same bending around
the carbene C and the same reduction of the agostic J(C-H) value
is observed, all parameters suggesting somewhat stronger agostic
donation in that example, despite its being isoelectronic with our
Re example. The existence of â-agostic carbenes is of relevance
to olefin metathesis and C-H activation and most directly to H
migration from a metal to the â-carbon of an η1-vinyl ligand (5) or
a metallocyclopropene (6).11
Acknowledgment. This work was supported by the National
Science Foundation.
Supporting Information Available: Full synthetic, spectroscopic,
computational and crystallographic details, including a CIF file. This
material is available free of charge via the Internet at http://pubs.acs.org.
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