A systematic approach to the generation of long-lived metal alkane complexes: Combined IR and NMR study of (Tp)Re(CO)2(cyclopentane)

Article abstract of DOI:10.1039/b819671k

Short wavelength photolysis of (Tp)Re(CO)₃ (Tp = tris(pyrazol-1-yl)borate) at low-temperature in cyclopentane yielded (Tp)Re(CO)₂(cyclopentane), an alkane complex with three nitrogen ligands that was characterised by NMR spectroscopy

Full text of DOI:10.1039/b819671k

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A systematic approach to the generation of long-lived metal alkane
complexes: combined IR and NMR study of
(Tp)Re(CO)2(cyclopentane)w
Simon B. Duckett,*a Michael W. George,*b Omar S. Jina,b Steven L. Matthews,a
Robin N. Perutz,*a Xue-Zhong Sunb and Khuong Q. Vuongb
Received (in Cambridge, UK) 5th November 2008, Accepted 17th December 2008
First published as an Advance Article on the web 21st January 2009
DOI: 10.1039/b819671k
Short wavelength photolysis of (Tp)Re(CO)3 (Tp = tris(pyrazol-
1-yl)borate) at low-temperature in cyclopentane yielded
(Tp)Re(CO)2(cyclopentane), an alkane complex with three
nitrogen ligands that was characterised by NMR spectroscopy.
essential to the characterisation of stable s-complexes
(E = H, Si, B) and agostic complexes.1
In a milestone paper that built on TRIR evidence for the
exceptionally long lifetime of [(Z5-C5H5)Re(CO)2(alkane)],9
Geftakis and Ball10a demonstrated that these species can be
observed by low-temperature NMR spectroscopy with in situ
photolysis in neat alkanes.10 So far, [(Z5-C5H4R)Re(CO)3]
In transition-metal s-complexes, a non-metal s-bond E–H
acts as the donor to a transition-metal centre. Of the four
principal classes of intermolecular s-complexes, dihydrogen,
silane, borane and alkane complexes, the last group is the most
elusive because of the weakness of the interaction,1 and has
therefore needed specialist detection methods. Alkane com-
plexes are distinct from their better known relations, agostic
alkyl complexes, that are stabilised by a M–C s-bond and a 3
centre M–C–H interaction.1i Alkane complexes play a role as
intermediates in many C–H bond activation reactions and are
implicated in photosubstitution reactions in alkane solvents
and isotope exchange reactions.2
i
(R = H, Pr) has been studied in cyclopentane, pentane and
isobutane–cyclohexane mixtures. The only study that combined
evidence from TRIR spectroscopy and NMR spectroscopy,
characterised the analogue [(Z5-C5H4 Pr)Re(CO)(PF3)(alkane)].11
i
The principal findings of these NMR studies were: (a) that spectra
were consistent with Re(Z2-C–H) coordination with fast exchange
between the coordinated C–H bond and the uncoordinated
geminal C–H bond(s); (b) a small preference for the different
alkane positions, favouring CH2 over CH3 coordination in
n-pentane, and a preference for axial over equatorial C–H bonds
in cyclohexane. The NMR studies provided evidence of alkane
bonding not accessible previously.
Organometallic alkane complexes were first observed in
alkane matrices as the photoproducts of metal carbonyls in
1975.2,3 They have also been detected as short-lived inter-
mediates by time-resolved spectroscopy of metal carbonyls in
alkane solution,4 and by matrix IR spectroscopy of reac-
tion products of metal atoms with alkanes.5 Numerous
computational studies report their structures, energies and
roles as intermediates in an even wider range of reactions
than indicated by current experiments.6 While time-resolved
infrared (TRIR) spectroscopy has demonstrated the existence
of alkane complexes in solution, and provides lifetimes and
reaction kinetics,7 the structural detail concerning bonding of
the alkane is indirect and limited. Many researchers have
devised routes to isolable agostic complexes1i but their
attempts at isolating alkane complexes have failed. Crystal
structures where alkanes interact with a metal centre8 concern
metal–ligand systems that are not representative of the systems
studied by other means. NMR spectroscopy has proved
In spite of these examples, further progress in characterising
alkane complexes remains a formidable challenge. We were inter-
ested in developing a more systematic approach for selecting
systems for study by NMR spectroscopy with improved methods
for in situ irradiation. This methodology should allow us to
discover whether (Z5-C5H4R)Re(CO)(L)(alkane) (L = CO, PF3)
complexes were unique in being observable by conventional NMR
spectroscopy. With this improved equipment, we report the
observation of the first alkane complexes with nitrogen ligands
in place of cyclopentadienyl, [(Tp)Re(CO)2(Z2-C,H-CypH)]
(CypH = cyclopentane). We compare its reactivity and spectro-
scopy to those of its cyclopentadienyl analogues.
Our strategy optimises the combination of IR and NMR
spectroscopy. In addition to determination of IR spectra and
activation parameters by TRIR spectroscopy, we have used
conventional FTIR spectroscopy to measure precise lifetimes
at 200 K in alkane solution. This method avoids error-prone
extrapolations of kinetics from much higher temperature and
allows us to predict the optimum conditions and substrate for
NMR experiments. Our previous set-up for in situ laser
photolysis within an NMR spectrometer12 utilised a con-
tinuous wave photolysis laser operating at a different wave-
length from the TRIR experiments. The TRIR and NMR
photochemistry are now based on a common pulsed laser
operating at 266 nm. We have constructed a new system
using a 600 MHz widebore spectrometer irradiating a sample
within the probehead. The shorter wavelength required a
a Department of Chemistry, University of York, Heslington, York,
UK YO10 5DD. E-mail: rnp1@york.ac.uk;
Fax: +44 (0)1904 432516; Tel: +44 (0)1904 432549
b School of Chemistry, University of Nottingham, University Park,
Nottingham, UK NG7 2RD.
E-mail: mike.george@nottingham.ac.uk; Fax: +44 (0)115 9513563;
Tel: +44 (0)115 9513512
w Electronic supplementary information (ESI) available: Sample
synthesis and handling, plots of kobs vs. [CO], Arrhenius and Eyring
plots, high resolution NMR spectra of (Tp)Re(CO)2(CypH) and
(Cp)Re(CO)2(C5H9D). See DOI: 10.1039/b819671k
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new compound for laser alignment with para-hydrogen
amplification in real time;12a irradiation of [Ru(H)2(CO)-
(Ph2PCH2CH2CH2PPh2)(PPh3)] at high dilution in CD2Cl2
proved effective. The new spectrometer offers increased
sensitivity and better solvent suppression, while the laser
allows irradiation in a region of high absorption with suffi-
cient flux to achieve high conversion in a shorter time than
previously possible.
ca. 15–20 min at 200 K, the approximate temperature of NMR
measurement. These values showed promise for further inves-
tigation, whereas the very similar (Tp*)Re(CO)2(alkane) was
dramatically shorter-lived. More precise measurements of the
stability of (Tp)Re(CO)2(CypH) by FTIR spectroscopy at
200 K gave a lifetime of 19 ꢂ 1 min. Under similar conditions
(Cp)Re(CO)2(CypH) had a lifetime of ca. 25 min.14
The lifetime measurements obtained by FTIR spectroscopy
established that (Tp)Re(CO)2(CypH) should be suitable for
1
NMR study. H NMR spectra were measured in neat cyclo-
We investigated several systems by TRIR spectroscopy in-
cluding W(CO)6 and (Tp*)Re(CO)3 (Tp* = tris(3,5-dimethyl-
pyrazol-1-yl)borate) that yielded alkane complexes successfully
but their lifetimes were too short for NMR detection. The
alkane complexes derived from (Tp)Re(CO)3 proved much
longer-lived but the solubility of this complex in alkanes was
much lower than that of its Cp analogue, hampering NMR
spectroscopy.
pentane using optimised solvent suppression techniques at
190 K. The three proton environments of the pyrazolyl ligands
of (Tp)Re(CO)3 are observed at d 7.74 (d), 7.50 (d) and
6.10 (t). On laser irradiation at 266 nm, these resonances are
reduced in intensity to be replaced with six new resonances
(Fig. 2).z In the high-field region normally associated with
hydrides and agostic hydrogens, a new resonance at d ꢁ2.70 of
integration 2H is observed on photolysis as a broad multiplet
(Fig. 3). This resonance broadens with increasing temperature
and cannot be observed above 195 K. No other changes to the
The TRIR spectrum13 of (Tp)Re(CO)3 in cyclopentane solu-
tion obtained at ambient temperature 100 ms after irradiation
clearly shows that the parent bands are bleached and two new
transient bands are formed at 1940 and 1862 cmꢁ1 (Fig. 1).
These bands can be assigned to (Tp)Re(CO)2(CypH) by com-
parison with previous TRIR studies on (Cp)Re(CO)2(CypH)
(Table 1).9,14 Table 1 also compares the kinetics of recombina-
tion with CO of (Tp)Re(CO)2(CypH) with those of
1
spectrum are observed on photolysis. The H NMR spectra
indicate that the ligand environment has changed from C3v to
Cs symmetry.
The NMR spectra in Fig. 2 appear to suggest that the
conversion of (Tp)Re(CO)3 to (Tp)Re(CO)2(CypH) exceeds
90%. However, integration against a standard indicates con-
version of ca. 75%, implying that some (Tp)Re(CO)3 pre-
cipitates during the experiment. Nevertheless, the high
conversion suggests that convection currents cause mixing of
the sample during irradiation. The lifetime of the alkane
complex obtained from the NMR results is ca. 2 h at 200 K.y
It is important to determine whether the peak at d ꢁ2.70
originates from the solvent or the Tp ligand. We therefore
conducted a similar experiment using cyclo-C5H9D. As a
benchmark, we investigated the photolysis of (Cp)Re(CO)3
in cyclo-C5H9D and observed two resonances in the hydride
region separated by 1.80 ppm with an 8 : 1 intensity pattern
(see ESIw). Photolysis of (Tp)Re(CO)3 in cyclo-C5H9D reveals
a spectrum in the hydride region with a similar intensity
pattern, and an isotope shift of 1.26 ppm (Fig. 3), confirming
(Cp)Re(CO)2(CypH). The activation parameters (DHz
=
45 ꢂ 2 kJ molꢁ1, DSz = ꢁ49 ꢂ 10 J Kꢁ1 molꢁ1) for the
reaction of (Tp)Re(CO)2(CypH) with CO predict a lifetime of
Fig. 1 (a) TRIR difference spectrum obtained 100 ms after irradiation
(266 nm) of (Tp)Re(CO)3 in cyclopentane in the presence of CO (2 atm)
at room temperature; (b) FTIR difference spectrum after photolysis of
(Tp)Re(CO)3 in cyclopentane at 200 K; insets showing the decay of
(Tp)Re(CO)2(CypH) at (c) room temperature and at (d) 200 K.
Table 1 Comparative IR and NMR data of Re–CypH complexes
(Cp)Re(CO)2(CypH)
(Tp)Re(CO)2(CypH)
n
n
sym, shift/cmꢁ1
asym, shift/cmꢁ1
1948, ꢁ82a
1882, ꢁ55a
1.1 ꢃ 103
32
1940, ꢁ89b
1862, ꢁ57b
4.2 ꢃ 103
45
kCO/dm3 molꢁ1 sꢁ1
DHz/kJ molꢁ1
d
ReCH/ppm
ꢁ2.35
ꢁ2.70
Fig. 2 1H NMR spectra after 266 nm laser photolysis of (Tp)Re(CO)3
in cyclopentane at 190 K showing the Tp resonances. The photolysis
times 0, 9, 18, 27 and 36 min are shown at right.
Dd(IPE)/ppm
a
1.80
1.26
b
Shift relative to (Cp)Re(CO)3. Shift relative to TpRe(CO)3.
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y This is significantly longer than those obtained from FTIR, although
in both experiments the stability of such complexes is generally
governed by impurities in the system rather than the intrinsic lifetime
of the complex.
z The use of cyclo-C5H9D proved to avoid some of the extra isotope
perturbation encountered with cyclo-C5H8D2.11
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Fig. 3 Below: high-field region of 1H NMR spectrum obtained
following 266 nm laser irradiation of (Tp)Re(CO)3 in cyclopentane
(C5H10) at 190 K with expansion shown in inset. Above: corresponding
spectrum after irradiation in cyclopentane-d1 (C5H9D).
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d ꢁ2.70.z
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The isotope shifts can be understood if there is one coordinated
C–H bond that undergoes a rapid exchange with the unbound
geminal proton of the alkane. This exchange results in a large
isotopic perturbation of equilibrium (IPE)15 that moves this
resonance upfield by 1.26 ppm for (Tp)Re(CO)2(CypH) and by
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shows that it is more favourable for the metal to be coordinated
by a C–H bond than by a C–D bond in the geminal CHD
isotopomer as a result of the change in zero-point energy
on isotope incorporation. The more intense resonance is at the
same position as observed with C5H10 and corresponds to the
isotopomers with D in the a or b positions (see Fig. 3).
We have utilised both IR and NMR data to characterise a
new alkane complex at a common temperature and photolysis
wavelength in high conversion. This species represents a new
class of alkane complexes with three N-donor ligands. The
n(CO) band positions determined by IR spectroscopy allow
characterisation of the product as an alkane complex rather
than an alkyl hydride, and these measurements also provide
the kinetic information required to optimise the conditions for
NMR detection of these highly reactive molecules. The NMR
results provide structural and dynamic information that
indicate the formation of a s-complex with an Z2-C–H bond.
The Tp resonances of (Tp)Re(CO)2(CypH) are consistent with
k3-coordination of the Tp ligand, but we are not able to
specify the hapticity of the Tp ligand definitively. The new
NMR equipment will be capable of detecting even more
reactive species. In current experiments, we are probing the
limits on the lifetimes needed for characterising such species.
We thank the EPSRC (EP/D058031 and EP/D055768),
Universities of Nottingham and York for funding.
MWG gratefully acknowledges a Royal Society Wolfson
Merit Award.
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Notes and references
1H, 600 MHz: d(J/Hz) 7.71 (d, 1.8, 2H), 7.58 (d, 2.2, 2H), 7.48
(d, 2.2, 1H), 7.40 (d, 2.3, 1H), 6.15 (t, 2.1, 2H), 5.93 (t, 2.3, 1H).
z
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1401–1403 | 1403