Influences of N-heterocyclic carbene and PMe3 ligands on the tautomerism between methylene/hydride and bridging methyl complexes of Rh/Os and unusual examples of ligand deprotonation by the NHC group

Article abstract of DOI:10.1021/om1010066

The complexes [RhOsL(CO)₃(dppm)₂][CF ₃SO₃] (L = IMe₄ (1,3,4,5-tetramethylimidazol-2- ylidene) (6), PMe₃ (8); dppm = μ-Ph₂PCH ₂PPh₂) were prepared by substitution of a carbonyl ligand in [RhOs(CO)₄(dppm)₂][CF₃SO₃]. Reaction of 6 with additional IMe₄ resulted in deprotonation of a dppm ligand, yielding [RhOs(IMe₄)(CO)₃(μ- κ¹:κ¹-Ph₂PCHPPh₂)(dppm)] (7). Although reaction of 8 with diazomethane at -78 °C yielded the known methylene-bridged [RhOs(PMe₃)(CO)₃(μ-CH ₂)(dppm)₂][CF₃SO₃] (3), compound 6 was unreactive toward diazomethane over a wide temperature range. The methylene-bridged species [RhOs(IMe₄)(CO)₂(μ-CH ₂)(dppm)₂][CF₃SO₃] (9) was obtained by reaction of [RhOs(CO)₃(μ-CH₂)(dppm) ₂][CF₃SO₃] with IMe₄, although [RhOs(CO)₃(μ-CH₂)(μ-κ¹: η²-Ph₂PCHPPh₂)(dppm)] (10) was also obtained by competing dppm deprotonation by IMe₄. Protonation of [RhOsL(CO)₂(μ-CH₂)(dppm)₂][CF ₃SO₃] (L = IMe₄ (9), PMe₃ (11)) with triflic acid at -78 °C yielded two isomers in each case. The more abundant isomer, [RhOsL(CO)₂(μ-CH₃)(dppm)₂][CF ₃SO₃]₂, has a bridging agostic methyl group, while the minor isomer has a terminal, Os-bound methyl group. Upon warming, both isomers transformed to species having an Os-bound methyl group and a coordinated triflate ion, subsequently rearranging to the thermodynamic products [RhOsL(CO)₂(μ-H)(μ-CH₂)(dppm)₂][CF ₃SO₃]₂ near ambient temperature. Attempts to prepare an IMe₄-containing methyl species directly via triflate ion substitution in [RhOs(CH₃)(OSO₂CF₃)(CO) ₃(dppm)₂][CF₃SO₃] by IMe₄ instead resulted in deprotonation of the methyl group to give the known product [RhOs(CO)₃(μ-CH₂)(dppm)₂][CF ₃SO₃]. Addition of methyl triflate to 6 gave no reaction, but protonation of 6 with triflic acid at -78 °C yielded the kinetic isomer of [RhOsH(IMe₄)(CO)₃(dppm)₂][CF ₃SO₃]₂, in which the hydride is terminally bound to Os, and warming this product to ambient temperature resulted in rearrangement to the hydride-bridged, thermodynamic isomer.

Full text of DOI:10.1021/om1010066

ARTICLE
pubs.acs.org/Organometallics
Influences of N-Heterocyclic Carbene and PMe3 Ligands on the
Tautomerism between Methylene/Hydride and Bridging Methyl
Complexes of Rh/Os and Unusual Examples of Ligand Deprotonation
by the NHC Group
Kyle D. Wells,† Robert McDonald,‡ Michael J. Ferguson,‡ and Martin Cowie*,†
†Department of Chemistry and ‡X-ray Crystallography Laboratory, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S Supporting Information
b
ABSTRACT:
The complexes [RhOsL(CO)3(dppm)2][CF3SO3] (L = IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) (6), PMe3 (8); dppm =
μ-Ph2PCH2PPh2) were prepared by substitution of a carbonyl ligand in [RhOs(CO)4(dppm)2][CF3SO3]. Reaction of 6 with
additional IMe4 resulted in deprotonation of a dppm ligand, yielding [RhOs(IMe4)(CO)3(μ-κ1:κ1-Ph2PCHPPh2)(dppm)] (7).
Although reaction of 8 with diazomethane at ꢀ78 °C yielded the known methylene-bridged [RhOs(PMe3)(CO)3(μ-CH2)-
(dppm)2][CF3SO3] (3), compound 6 was unreactive toward diazomethane over a wide temperature range. The methylene-bridged
species [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] (9) was obtained by reaction of [RhOs(CO)3(μ-CH2)(dppm)2]-
[CF3SO3] with IMe4, although [RhOs(CO)3(μ-CH2)(μ-κ1:η2-Ph2PCHPPh2)(dppm)] (10) was also obtained by competing
dppm deprotonation by IMe4. Protonation of [RhOsL(CO)2(μ-CH2)(dppm)2][CF3SO3] (L = IMe4 (9), PMe3 (11)) with triflic
acid at ꢀ78 °C yielded two isomers in each case. The more abundant isomer, [RhOsL(CO)2(μ-CH3)(dppm)2][CF3SO3]2, has a
bridging agostic methyl group, while the minor isomer has a terminal, Os-bound methyl group. Upon warming, both isomers
transformed to species having an Os-bound methyl group and a coordinated triflate ion, subsequently rearranging to the
thermodynamic products [RhOsL(CO)2(μ-H)(μ-CH2)(dppm)2][CF3SO3]2 near ambient temperature. Attempts to prepare an
IMe4-containing methyl species directly via triflate ion substitution in [RhOs(CH3)(OSO2CF3)(CO)3(dppm)2][CF3SO3] by
IMe4 instead resulted in deprotonation of the methyl group to give the known product [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3].
Addition of methyl triflate to 6 gave no reaction, but protonation of 6 with triflic acid at ꢀ78 °C yielded the kinetic isomer of
[RhOsH(IMe4)(CO)3(dppm)2][CF3SO3]2, in which the hydride is terminally bound to Os, and warming this product to ambient
temperature resulted in rearrangement to the hydride-bridged, thermodynamic isomer.
’ INTRODUCTION
metal. The potential involvement of such a structure in the facile
CꢀH bond activation of surface-bound methyl groups5 is obvious,
particularly for late metals for which R-hydrogen elimination at a
single metal is not common. Such facile CꢀH bond activation of
methyl groups, presumably through an intermediate such as A, has
Bridging methyl groups can play an important role in a number of
chemical transformations such as alkyl transfers,1 carbonꢀhydrogen
bond activation,1c,2 and olefin polymerization.3 The most common
arrangement for methyl groups that bridge a pair of late metals is the
unsymmetric bridging mode,4 shown as structure A in Chart 1, in
which the methyl ligand is σ-bound through carbon to one metal
while also being involved in an agostic interaction with an adjacent
Received: October 22, 2010
Published: April 20, 2011
r
2011 American Chemical Society
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Chart 1
prepared using their respective published procedures, and the labeled
13CH2 and 13CO analogues were prepared by substituting Diazald-
N-methyl-13C or 13CO in the preparations.
All reagents were purchased from Sigma-Aldrich, unless otherwise noted,
and used as is. [H(OEt2)2][BArF ] (BArF4 = B(3,5-(CF3)2C6H3)4] was
4
prepared according to the published procedure.17 PMe3 (1 M solution in
THF) and neat triflic acid (CF3SO3H = HOTf) were transferred into
Teflon-capped Schlenk flasks and stored in the refrigerator under argon.
Diazomethane was generated in situ from a methanolic slurry containing
Diazald, by the slow dropwise addition of KOH(aq) (5.3 M). The
generated gas was transferred directly via cannula to the reaction flask
without further drying. Carbon monoxide was purchased from Praxair, and
the 13CO (99%) was obtained from Cambridge Isotope Laboratories.
The NMR spectra were recorded using a Varian iNova-400 spectrometer
been observed in late-metal clusters,2b,4a,6 which can serve as
valuable models for related surface-promoted transformations.7
A common approach for generating unsymmetrically bridged
methyl groups is by protonation of methylene-bridged precur-
sors,4d,e,8,9 although in some cases the favored product is instead
the methylene/hydride tautomer (B)2b,6c,10 and not a species
containing an intact methyl group. Shapley and co-workers have
demonstrated that the equilibrium between tautomers A and B in
a triosmium cluster could be shifted toward the methylene-
bridged hydride species (B) through substitution of a carbonyl
ligand by a more basic phosphine ligand.11
We have previously investigated the protonation reactions of a
series of methylene-bridged complexes [MM0(CO)4(μ-CH2)-
(dppm)2]þ (M = Rh, M0 = Ru,8b Os;8a M = Ir, M0 = Ru8d) and
have found in all cases that the apparent kinetic product is an
unsymmetrically bridged methyl species, which is stable only at
low temperature, converting upon slight warming to a product
containing a conventional, terminally bound methyl group on the
group 9 metal. Unlike the case for the Shapley study,11 substitut-
ing a carbonyl in the Rh/Os system by a number of different
phosphine ligands did not yield the methylene/hydride products,
although this substitution did stabilize the unsymmetrically
bridged methyl group, allowing these products to be isolated at
ambient temperature.8c
operating at 399.8 MHz for 1H, 161.8 MHz for 31P, and 100.6 MHz for 13
C
nuclei. Infrared spectra were obtained on solutions using a Nicolet Magna
760 spectrometer or on solid samples using a NicPlan FTIR spectrometer.
Elemental analyses were performed by the microanalytical service within the
department using a CE 1108 CHNS-O analyzer. Electrospray ionization
mass spectra (EIMS) were acquired on a Micromass ZabSpec spectrometer
by the staff in the departmental mass spectrometry laboratory. In all cases
the distribution of isotopic peaks matched the calculated distribution for the
appropriate parent ion very closely. Spectroscopic data for all compounds
are given in Table 1.
Preparation of Compounds. (a). [RhOs(IMe4)(CO)3(dppm)2]-
[CF3SO3] (6). One equivalent of IMe4 (0.71 mL, 0.2133 M THF)
was added via syringe to a solution containing compound 1 (200 mg,
0.15 mmol) in 10 mL of THF. After the mixture was stirred for 30 min,
Et2O (25 mL) was added to precipitate the product. The resulting
yellow solid (200 mg, 94%) was dried in vacuo. HRMS: m/z calcd for
C60H56N2O3P4RhOs (Mþ ꢀ CF3SO3) 1271.1911, found 1271.1918
(Mþ ꢀ CF3SO3). Anal. Calcd for C61H56N2F3O6P4RhOsS: C, 51.62; H,
3.98; N, 1.97. Found: C, 51.23; H, 4.06; N, 2.23.
(b). [RhOs(IMe4)(CO)3(Ph2PCHPPh2)(dppm)] (7)
Method i. Excess IMe 4 (1.78 mL, 0.2133 M THF, 2.5 equiv) was added
via cannula to 1 (200 mg, 0.15 mmol) dissolved in 15 mL of THF. After
30 min, the resulting orange solution was concentrated to an oily residue
before 5 mL of pentane was added, giving a bright yellow solid (approxi-
mately 74%). This procedure led to contamination of the product by the
resulting [HIMe4][OTf] salt, which remained in an approximate 1:10 ratio
with the product. To purify the product, a concentrated dichloromethane
solution containing the crude solid was layered with pentane, affording
platelike crystals of 7 overnight (60 mg, 28%).
On the basis of the well-documented analogies between phos-
phine and N-heterocyclic carbene (NHC) ligands,12,13 we sought to
extend the above studies on the [RhOsL(CO)3(μ-CH2)(dppm)2]þ
systems (L = CO, PR3) by substituting L by the NHC ligand
1,3,4,5-tetramethylimidazol-2-ylidene (IMe4), in order to deter-
mine if the superior donor strength of IMe4 would favor the
methylene/hydride species (B), in which the metals can be
considered to be in a higher formal oxidation state than in the
agostic methyl tautomer (A). In this paper our attempts to
compare the effects of the IMe4 and PMe3 ligands on the methyl/
methylene hydride tautomerism, shown in Chart 1, are reported,
together with some unanticipated reactivity involving the NHC
ligand.
Method ii. The addition of IMe4 (0.20 mL, 2.133 ꢁ 10ꢀ1 M THF,
0.043 mmol) to compound 6 (50 mg, 0.04 mmol), dissolved in 3 mL of
CH3CN, instantly yielded a yellow precipitate from the resulting orange
solution. This proved to be a more favorable route for the synthesis of 7,
since both the imidazolium salt and any remaining free carbene
remained soluble in the mother liquor, allowing the solid sample to be
readily separated. The resulting solid was isolated and further rinsed with
2 ꢁ 1 mL of CH3CN before drying in vacuo (yield 84%). Anal. Calcd for
’ EXPERIMENTAL SECTION
C60H55N2O3P4RhOs 1.5CH2Cl2: C, 52.89; H, 4.19; N, 2.01. Found: C,
General Comments. All solvents were dried using the appropriate
desiccants, distilled before use, and stored under nitrogen. Reactions
were performed under an argon atmosphere using standard Schlenk
techniques. The 1,3,4,5-tetramethylimidazol-2-ylidene ligand (IMe4)
was prepared using a published procedure14 and then recrystallized
from toluene to afford colorless crystals, which were stored at ꢀ30 °C.
For reactions that did not require its isolation, a standardized solution
containing IMe4 in THF (0.2133 M) was stored over potassium metal in
the freezer and used without further purification.
3
52.85; H, 4.40; N, 2.06.
(c). [RhOs(PMe3)(CO)3(dppm)2][CF3SO3] (8). To a stirred solution
containing 1 (100 mg, 0.076 mmol) in 10 mL of THF was added 80 μL
of 1 M PMe3 (0.08 mmol, THF solution), resulting in the darkening of
the light yellow solution. A 20 mL portion of Et2O was used to
precipitate a yellow solid, which was rinsed with a further 10 mL portion
of pentane before being allowed to dry in vacuo (yield 94 mg, 90%).
HRMS: m/z calcd for C56H53O3P5RhOs (Mþ ꢀ CF3SO3) 1223.1347,
found 1223.1346 (Mþ ꢀ CF3SO3). Anal. Calcd for C57H53F3O6P5-
RhOsS: C, 49.93; H, 3.90. Found: C, 49.52; H, 3.84.
The compounds [RhOs(CO)4(dppm)2][CF3SO3] (1),15 [RhOs-
(CO)3(μ-CH2)(dppm)2][CF3SO3] (2),16 [RhOs(PMe3)(CO)3(μ-CH2)-
(dppm)2][CF3SO3] (3),8c [RhOs(CO)4(μ-CH2)(dppm)2][CF3SO3]
(4),16 and [RhOs(Me)(OSO2CF3)(CO)3(dppm)2][CF3SO3] (5)8c were
(d). [RhOs(PMe3)(CO)3(μ-CH2)(dppm)2][CF3SO3] (3). Compound 8
(100 mg, 0.073 mmol) was dissolved in 15 mL of dichloromethane and
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the resulting solution cooled to ꢀ78 °C in a dry ice/acetone bath. Gaseous
diazomethane (18 equiv), generated from 300 mg (1.4 mmol) of
Diazald, was passed through the solution for 1 h. The solution was kept
at ꢀ78 °C for another 1 h while being purged with argon gas. Once it was
warmed to ambient temperature, the resulting solution was concentrated to
approximately 5 mL in vacuo before 20 mL of Et2O was used to precipitate
the yellow solid (yield 94%). LRMS: m/z calcd for C57H55O3P5RhOs
turn orange. In both cases the resulting solutions were stirred for 1 h and
then 1 mL of pentane wasadded tofully precipitate yellowsolids containing
the respective products [RhOs(L)(CO)2(μ-CH2)(μ-H)(dppm)2][CF3-
SO3]2 (L=IMe4 (16), PMe3 (17)), which were dried in vacuo. Data for 16:
HRMS m/z calcd for C5713C3H59N2O2OsP4Rh (M2þ ꢀ CF3SO3), found
630.1148. Data for 17: HRMS m/z found 630.1136 (M2þ ꢀ CF3SO3).
Anal. Calcd for C58H56F6O8OsP5RhS2: C, 46.22; H, 3.75; S, 4.25. Found:
C, 46.45; H, 3.88; S, 4.01. Compounds 16 and 17 decomposed when left in
chlorinated solvents for extended periods of time.
(i). Low-Temperature Protonation of Compounds 9 and 11 by HOTf.
At ꢀ78 °C, 1 equiv of HOTf was added to dark solutions containing either
9 (50 mg, 0.036 mmol) or 11 (50 mg, 0.037 mmol) in 0.7 mL of CD2Cl2,
causing both solutions to immediately turn orange. Kept at this temperature,
the NMR spectra indicated that only two species were present in solution;
the major products were [RhOs(L)(CO)2(μ-CH3)(dppm)2][CF3SO3]2
(L = IMe4 (12a), PMe3 (13a)), containing the bridged agostic methyl groups,
while the minor products were the isomers 12b (L = IMe4) and 13b
(L = PMe3), containing terminal, Os-bound methyl groups. These isomers
were observed in an approximate 3:1 ratio in the case of PMe3 and a 10:1
ratio in the case of IMe4.
When the temperature was raised to 0 °C, the low-temperature products
containing the IMe4 ligand (12a,b) transformed into [RhOs(CH3)(CF3-
SO3)(L)(CO)(μ-CO)(dppm)2][CF3SO3] (L = IMe4 (14)) via triflate
ion coordination, while for the PMe3-containing species the formation of
the analogous triflate-coordinated species (15) occurred at ꢀ40 °C. Upon
further warming to ambient temperature compound 14 transformed to
[RhOs(L)(CO)2(μ-CH2)(μ-H)(dppm)2][CF3SO3]2 (L = IMe4 (16)),
while the PMe3-containing species (15) transformed to an analogous
methylene/hydride species (17) at 0 °C.
1
(Mþ ꢀ CF3SO3) 1237.2, found 1237.2. H, 13C{1H}, and 31P{1H}
NMR spectra of this product were identical with those of a previously
characterized sample, prepared by another route.8c
(e). [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] (9) þ [RhOs(CO)3-
(μ-CH2)(Ph2PCHPPh2)(dppm)] (10). A deep red solution containing
compound 2 (300 mg, 0.23 mmol) in 5 mL of THF was cooled to
ꢀ78 °C using a dry ice/acetone bath. A heavy argon purge was
employed before 1.1 equiv of IMe4 (1.2 mL, 0.213 M THF) was added
dropwise via syringe. The resulting dark red solution was stirred for 2 h
before being warmed to ambient temperature. An aliquot of the reaction
mixture showed a 6:2:1 mixture of the desired product 9, the deproto-
nated species 10 (vide supra), and the tetracarbonyl species
[RhOs(CO)4(μ-CH2)(dppm)2][OTf] (4) (formed from CO scaven-
ging by 2), respectively, as determined by 31P NMR spectroscopy.
Compounds 9 and 4, along with [HIMe4][OTf], were precipitated as a
dark red oil by the addition of 10 mL of pentane to the reaction mixture.
The resulting bright yellow supernatant containing 10 was separated
from the oil and evaporated to dryness in vacuo. The remaining crude
product, containing 4 and 9, was dissolved in a mixture of 4 mL of
benzene and 1 mL of THF, leaving behind a yellow precipitate,
determined by 31P NMR spectroscopy to be compound 4. The dark
red supernatant was transferred to a new flask, and 10 mL of pentane was
added to fully precipitate a dark oily solid which was rinsed with 5 mL of
Et2O and dried in vacuo, yielding approximately 40% of 9. Failure to
completely separate 9 from residual [HIMe4][OTf] gave unsatisfactory
elemental analyses. NMR spectra for 9 are available as Supporting
Information. HRMS: m/z calcd for C60H58N2O2P4RhOs (Mþ ꢀ CF3-
SO3) 1257.2113, found 1257.2112 (Mþ ꢀ CF3SO3).
(j). [RhOs(PMe3)(CO)2(μ-CH2)(dppm)2][BPh4] (11-BPh4). To a slur-
ry containing compound 2 (200 mg, 0.15 mmol) in 5 mL of THF was
added 103 mg of NaBPh4 (0.30 mmol). The solution was stirred for
30 min, and then the THF was removed in vacuo and the residue allowed
to dry overnight under reduced pressure. A 5 mL portion of CH2Cl2 was
added to the resulting residue, generating an orange solution and a white
precipitate, which were separated by filtration through Celite. Com-
pound 4 (as the BPh4ꢀ salt) was precipitated from the orange solution
by the addition of 15 mL of pentane. 19F NMR confirmed the absence of
(f). Reaction of [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] (9)
with CO. A dark red solution containing compound 9 (30 mg, 0.021
mmol) in 0.7 mL of C6D6 was gently purged with CO gas, causing the
solution to immediately turn orange. After 5 min a yellow precipitate
formed, which was separated from the mother liquor and dissolved in
CD2Cl2. 31P NMR spectroscopy (in CD2Cl2) established that the
precipitate was 6. When wet C6D6 was employed for the same
procedure, the mother liquor showed the formation of acetic acid, as
determined from 13C and 1H NMR spectroscopy.
(g). [RhOs(PMe3)(CO)2(μ-CH2)(dppm)2][CF3SO3] (11). A solution
consisting of 50 mg (0.036 mmol) of 3 in 0.7 mL of CD2Cl2 was added to a
NMR tube containing 6 equiv of trimethylamine oxide (TMNO;
16 mg, 0.22 mmol) via cannula with no immediate reaction. The solution
was warmed to 39 °C in the NMR instrument and the reaction monitored
to completion using 31P NMR (taking approximately 30 min). Once the
reaction was complete, the dark green solution was immediately cooled in a
dry ice/acetone bath and 1 mL of pentane was added to precipitate a dark
green oil. The yellow mother liquor left behind contained the suspected
impurities, [RhOsCl(CO)2(μ-CH2)(dppm)2] and OPMe3, which were
decanted to waste. A 1 mL portion of Et2O was then employed to triturate
the remaining oil. The light green ethereal solution was removed from the
resulting dark green solid, which was dissolved in 0.2 mL of CH2Cl2 and
then precipitated fully using a 1.5 mL portion of Et2O, isolated, and allowed
to dry further in vacuo (yield 77%). Anal. Calcd for C57H55F3O5OsP5RhS:
C, 50.45; H, 4.08; S, 2.36. Found: C, 50.38; H, 3.99; S, 2.22.
1
triflate ion. The 31P, 13C, and H NMR spectra of this species were
closely comparable to those of the compound having the triflate anion.16
This compound was converted to 3-BPh4 by addition of PMe3, as reported
for the triflate salt.8c To a 50 mg sample of 3-BPh4 (0.032 mmol) in 0.7 mL
of CD2Cl2 in an NMR tube was added 9.6 mg of TMNO (4 equiv). The
solution was warmed to 39 °C in the NMR probe with constant monitoring.
After 30 min the reaction was complete; leaving the mixture for longer
periods resulted in decomposition. The resulting dark green solution was
cooled in a dry ice/acetone bath followed by the addition of 1 mL of
pentane to give a dark green oil. The yellow mother liquor was decanted to
waste, and 1 mL of Et2O was used to triturate the remaining oil. The light
green ethereal solution was removed from the dark green solid, which was
dissolved in 0.2 mL of CH2Cl2 and then precipitated using a 1.5 mL portion
of Et2O. The NMR parameters of 11-BPh4 were in agreement with those of
the triflate salt and are given in the Supporting Information.
(k). Low-Temperature Protonation of 11-BPh4 by [H(OEt2)2][BArF4].
To a 50 mg portion (0.033 mmol) of 11-BPh4 dissolved in 0.8 mL of
CD2Cl2 was added 1 equiv of [H(OEt2)2][BArF ] at ꢀ80 °C. Monitoring
4
the reaction by 31P NMR spectroscopy showed no reaction at this
temperature. Warming to ꢀ60 °C gererated the 13b cation, and warming
to ꢀ50 °C showed the appearance of the cation of 13a, in an approximate
1:2.5 ratio. After 14 h at this temperature the new species 18 appeared,
giving a 1:4:1 mix of 18, 13a, and 13b. Warming to ꢀ30 °C resulted in the
complete conversion of 13a,b to 18, which was stable upon warming to
ambient temperature. The solvent was removed from the solution of 18
under reduced pressure, generating an orange oil. Dissolving this oil in 1 mL
(h). Protonation of Compounds 9 and 11 by HOTf at Ambient
Temperature. One equivalent of HOTf was added to a dark red solution
of 9 (50 mg, 0.036 mmol) or a dark green solution of 11 (50 mg,
0.037 mmol) in 0.7 mL of CD2Cl2, causing each solution to immediately
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of diethyl ether and adding 0.5 mL of pentane again yielded an orange oil,
which was separated from the mother liquor via cannula and dried in vacuo.
NMR characterization of this species showed large amounts of ether
present, which resulted in unsatisfactory elemental analyses. HRMS: m/z
calcd for C56H56O2P5RhOs (M2þ ꢀ BBh4 ꢀ BArF4 ꢀ OEt2) 605.0814,
found 605.0813.
(l). Reaction of [RhOs(Me)(OSO2CF3)(CO)3(dppm)2][CF3SO3] (5) with
IMe4. To a rapidly stirred yellow solution consisting of 20 mg (0.01 mmol)
of 5 in 0.7 mL of d8-THF was added slightly less than 1 equiv of IMe4 as a
standardized THF solution, resulting in an instantaneous color change to
dark red. Both 1H and 31P NMR spectra confirm the conversion to 2 with
concomitant formation of [HIMe4][OTf].
(m). [RhOs(H)(IMe4)(CO)(μ-CO)2(dppm)2][CF3SO3]2 (19a). At ꢀ78 °C,
an excess of triflic acid (15 μL, 0.17 mmol) was added by syringe to a
Schlenk flask containing compound 6 (200 mg, 0.14 mmol) in 5 mL of
CH2Cl2. The orange solution immediately turned yellow, and 10 mL
of ꢀ78 °C pentane was added via cannula to precipitate a yellow solid,
which was removed from the mother liquor by decantation and dried in
vacuo (203 mg, 92%). HRMS for 19a: m/z calcd for C61H57N2O6F3P4S-
RhOs (M2þ ꢀ CF3SO3) 1421.1510, found 1421.1513 (M2þ ꢀ CF3SO3).
(n). [RhOs(IMe4)(CO)2(μ-H)(μ-CO)(dppm)2][CF3SO3]2 (19b)
Method i. A 100 mg portion of 19a (0.06 mmol) dissolved in 5 mL of
CH2Cl2 was stirred for a 3 h period at ambient temperature, converting it
fully to 19b.
Method ii. An excess of triflic acid (15 μL, 0.17 mmol) was added by
syringe to a Schlenk flask containing compound 6 (100 mg, 0.07 mmol)
in 10 mL of CH2Cl2. The orange solution immediately turned pale
yellow, and after 10 min of stirring 20 mL of Et2O was added to
precipitate a pale yellow solid, which was dried in vacuo (203 mg, 92%).
Dissolving a portion of the crude product in CD2Cl2 revealed a 50:50
mixture of both bridging (19b) and terminal (19a) hydride isomers.
Over a 3 h period at ambient temperature, 19a converted fully to 19b.
Anal. Calcd for 19b, C62H57N2O9F6P4S2RhOs: C, 47.45; H, 3.66;
N, 1.79. Found: C, 47.48; H, 3.73; N, 1.88.
X-ray Structure Determinations. (a). General Considerations.
All data sets were collected using a Bruker SMART 1000 CCD detector/
PLATFORM diffractometer18 using Mo KR radiation, with the crystals
cooled to ꢀ80 °C. The data were corrected for absorption through use
of a multiscan model (SADABS18 (6) or TWINABS18 (8)) or through
Gaussian integration from indexing of the crystal faces18 (19b). Struc-
tures were solved using direct methods (SHELXS-9719 (6, 19b)) or
direct methods/structure expansion (SIR9720 (8)). Refinements were
completed using the program SHELXL-97.19 Hydrogen atoms were
assigned positions on the basis of the idealized sp2 or sp3 geometries of
their attached carbon atoms and were given thermal parameters 20%
greater than those of the attached carbons. See the Supporting Informa-
tion for a listing of crystallographic experimental data.
(b). Data Collection and Structure Solution. (i) Yellow-orange
crystals of compound 6 were grown via slow diffusion of CH2Cl2 into
an Et2O solution of the compound. Attempts to refine peaks of residual
electron density as solvent Et2O oxygen or carbon atoms were un-
successful. The data were corrected for disordered electron density
through use of the SQUEEZE procedure21 as implemented in
PLATON.22 A total solvent-accessible void volume of 1174.0 Å3 with
a total electron count of 347 (consistent with eight molecules of solvent
diethyl ether or two molecules per formula unit of the metal complex
ion) was found in the unit cell.
reciprocal space. Integrated intensities for the reflections from the two
components were written into a SHELXL-97 HKLF 5 reflection file with
the data integration program SAINT (v. 7.06A),18 using all reflection
data (exactly overlapped, partially overlapped and nonoverlapped). The
CꢀCl and Cl Cl distances of the disordered and/or partially
3 3 3
occupied solvent dichloromethane molecules (distributed over three
different sites) were restrained to be 1.80(1) and 2.77(2) Å, respectively.
(iii) Yellow crystals of compound 19b were grown via diffusion of
n-pentane into a CH2Cl2 solution of the compound. Attempts to refine
peaks of residual electron density as solvent CH2Cl2 chlorine or carbon
atoms were unsuccessful. The data were corrected for disordered
electron density through use of the SQUEEZE procedure21 as imple-
mented in PLATON.22 A total solvent-accessible void volume of
661.5 Å3 with a total electron count of 102 was found in the unit cell,
consistent with four molecules of solvent CH2Cl2 in the cell or two
molecules of CH2Cl2 per formula unit of the complex dication. For further
details of X-ray data collection and results for all structure determinations
the reader is referred to the Supporting Information.
’ RESULTS AND COMPOUND CHARACTERIZATION
As noted in the Introduction, we set out to study the influence
of the N-heterocyclic carbene, tetramethylimidazol-2-ylidene
(L = IMe4), on the equilibrium shown in eq 1 and in particular,
to compare it to the case in which L = PMe3. Our goal
synthetically was to approach this problem from both sides of
eq 1, first by protonation of the methylene-bridged precursors,
namely [RhOs(L)(CO)3(μ-CH2)(dppm)2]þ, and second by
direct synthesis of the methyl products (D), either from methyl-
containing precursors or by the introduction of methyl ligands
into L-containing precursors.
2þ
½RhOsðHÞLðCOÞ3ðμ-CH2ÞðdppmÞ2ꢂ
C
2þ
h ½RhOsLðCOÞ3ðμ-CH3ÞðdppmÞ2ꢂ
ð1Þ
D
Two routes seemed obvious for accessing the methylene-
bridged species [RhOs(L)(CO)3(μ-CH2)(dppm)2]þ (L = IMe4,
PMe3; dppm = Ph2PCH2PPh2), containing either the monopho-
sphine or N-heterocyclic carbene as an ancillary ligand: (1) intro-
duction of the bridging methylene group into the respective
precursor complexes [RhOs(L)(CO)3(dppm)2]þ (L = IMe4,
PMe3) by reaction with diazomethane and (2) introduction of
the ancillary ligands, IMe4 or PMe3, into either of the known
methylene-bridged complexes, [RhOs(CO)3(μ-CH2)(dppm)2]þ
or [RhOs(CO)4(μ-CH2)(dppm)2]þ.16
IMe4 and PMe3 Precursors. Starting with the first route, the
IMe4 precursor [RhOs(IMe4)(CO)3(dppm)2][OTf] (6) is
readily accessed in near-quantitative yield by the careful addition
of 1 equiv of IMe4 to [RhOs(CO)4(dppm)2][OTf] (1), result-
ing in substitution of a Rh-bound carbonyl by the NHC ligand, as
outlined in Scheme 1 (in this and all subsequent schemes, the
phenyl rings on the dppm and derived groups are omitted in the
drawings).
The 31P{1H} NMR spectrum of 6 displays a pattern char-
acteristic of an AA0BB0X spin system, with one resonance at
δ 29.4 appearing as a doublet of multiplets, having 132 Hz
coupling to Rh, and the other as a multiplet at δ ꢀ3.5. The
magnitude of the Rh coupling unambiguously establishes the
low-field signal as that due to the pair of 31P nuclei bound to this
metal, leaving the upfield resonance, which displays no Rh
coupling, as due to those bound to Os. In the 1H NMR spectrum
the resonance for the dppm methylene groups, at δ 3.58, appears
(ii) Orange-red crystals of compound 8 were grown via diffusion of
diethyl ether into a dichloromethane solution of the compound. The
crystal used for data collection was found to display nonmerohedral
twinning. Both components of the twin were indexed with the program
CELL_NOW18 (Bruker AXS Inc., Madison, WI, 2004). The second
twin component can be related to the first component by 179.8° rotation
1
about the [ꢀ /4,1,0] axis in real space and about the [0,1,0] axis in
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Scheme 1
as an apparent quintet displaying essentially equal coupling to all
31P nuclei. The appearance of a single dppm methylene resonance
suggests the presence of front/back symmetry on either side of the
RhOsP4 plane, consistent with the IMe4 ligand binding opposite the
metalꢀmetal bond, as shown. Selective decoupling at each indivi-
dual phosphorus resonance in turn simplifies the methylene reso-
nance to a triplet having coupling to the remaining phosphorus
nuclei of 4.3 Hz in each case, while broad-band 31P decoupling
simplifies the resonance to a singlet. Further evidence for the front/
back symmetry in the complex comes from the appearance of only
two sharp singlets (integrating as six protons each) at δ 2.71 and
1.61, for the N-bound and C-bound methyl groups, respectively, of
the imidazol-2-ylidene ring.
In the 13C{1H} NMR spectrum, the carbene carbon, at
δ 176.4, appears as a doublet of triplets, showing typical coupling
to rhodium23,24 of 50 Hz and cis coupling of 16 Hz to the pair of
adjacent Rh-bound phosphorus nuclei. The carbonyl resonances
appear in a 2:1 intensity ratio at δ 216.2 and 183.7, respectively,
with the former displaying 10 Hz coupling to Rh, suggesting a
weak semibridging interaction with this metal for this pair of
carbonyls, while the absence of Rh coupling for the high-field
signal indicates an Os-bound carbonyl. However, a slightly
different interpretation emerges upon consideration of the IR
spectra, in which three carbonyl bands are observed in both
solution and the solid state. Two stretches, at approximately 1967
and 1914 cmꢀ1, are assigned to two terminally bound carbonyls,
while the lowest frequency band at 1785 cmꢀ1 is attributed to a
bridging carbonyl. The appearance of two terminal carbonyl
stretches and only one bridging band in the IR spectra contra-
dicts the above interpretation of the 13C NMR experiments,
which suggest that two carbonyls are bridging, leading us to
propose a fluxional process that rapidly exchanges two of the
carbonyls on the NMR time scale, while the faster time scale of
the IR experiment allows the differentiation of these groups. We
therefore suggest the static structure shown in Scheme 1, for
which rapid exchange on the NMR time scale occurs. In this
fluxional process the semibridging carbonyl, on one side of the
RhOsP4 plane, moves out of the bridge to a terminal position on
Os while the terminal Os-bound carbonyl that lies on the
opposite face of the RhOsP4 plane moves to a semibridging
position. Since a terminal Os-bound carbonyl is not expected to
show resolvable coupling to 103Rh, the average value (10 Hz)
Figure 1. Perspective view of the complex cation of [RhOs(IMe4)-
(CO)3(dppm)2][CF3SO3] (6), showing the atom-labeling scheme.
Phenyl carbon atoms are numbered sequentially around the ring,
starting from the ipso carbon such that the first digit represents the ring
number. Non-hydrogen atoms are represented by Gaussian ellipsoids at
the 20% probability level, while hydrogen atoms are shown with
arbitrarily small thermal parameters. Phenyl hydrogens are omitted for
clarity. Relevant parameters (distances in Å and angles in deg): Osꢀ
Rh = 2.8303(4), OsꢀC(1) = 1.929(5), OsꢀC(2) = 1.889(5), OsꢀC(3)
= 1.982(5), RhꢀC(1) = 3.335(5), RhꢀC(3) = 2.130(5), RhꢀC(4) =
2.049(5); C(1)ꢀOsꢀC(2) = 114.2(2), C(1)ꢀOsꢀC(3) = 135.5(2),
C(2)ꢀOsꢀC(3) = 110.3(2), OsꢀC(3)ꢀO(3) = 154.7(4), RhꢀC-
(3)ꢀO(3) = 118.0(4).
observed in the 13C{1H} NMR spectrum suggests a 20 Hz
coupling for this carbonyl while bridging. This coupling to Rh is
less than the 30 Hz expected for a symmetrically bridged CO,16
and so it is attributed to a semibridging group.
The X-ray crystal structure determination of 6 has been carried
out, and the representation of the complex cation, shown in
Figure 1, confirms the IR spectral assignment, displaying one
semibridging and two terminal carbonyls. It should be noted that
the reversible conversion of the terminally bound C(1)O(1) to a
semibridging group accompanied by conversion of C(3)O(3) to
terminal, as proposed above for the fluxional process, requires
only slight movement of each carbonyl through a twisting of the
“Os(CO)3” fragment about the approximate P(1)ꢀOsꢀP(3)
axis. This structure also confirms that the NHC ring is bound to
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Rh opposite the RhꢀOs bond and shows that it lies in the
equatorial plane defined by the metals and carbonyls, allowing
the four NHC methyl substituents to avoid unfavorable interac-
tions with the dppm phenyl rings.
purely terminally bound group and probably results from a weak
interaction with the adjacent metal.
As a comparison with the above IMe4 compound (6), we have
also prepared its PMe3 analogue. Unlike the IMe4 case, the
addition of excess PMe3 to 1 results in the exclusive formation of
[RhOs(PMe3)(CO)3(dppm)2][OTf] (8) with no evidence of
deprotonation products. The 31P{1H} NMR spectrum of 8
displays three resonances in a 2:2:1 ratio for the two ends of
the pair of diphosphines and the PMe3 ligand, respectively. The
dppm resonances are very similar to those of the IMe4 analogue
(see Table 1), while the PMe3 signal at δ ꢀ23.3 also appears,
displaying typical coupling to Rh (144 Hz) and to the adjacent
pair of Rh-bound 31P nuclei of 40 Hz.
The metalꢀmetal separation of 2.8303(4) Å is consistent with
the presence of a single bond, showing significant contraction
compared to the intraligand PꢀP separations of 3.029(2) and
3.040(2) Å, while the RhꢀC(4) distance of 2.049(5) Å is normal
for a single bond, as shown for other NHC complexes,23a,24 and is
significantly longer than the metalꢀcarbonyl distances. Carbonyl
C(3)O(3) is identified as semibridging on the basis of the large
asymmetry in the RhꢀC(3)ꢀO(3) and OsꢀC(3)ꢀO(3) angles
(118.0(4) and 154.7(4)°, respectively) and in the RhꢀC(3) and
OsꢀC(3) distances (2.130(5) and 1.982(5) Å, respectively),
showing that this group is primarily bonded to Os while being
involved in a weaker semibridging interaction with Rh. The
As was observed with compounds 6 and 7, the 13C{1H} NMR
spectrum of 8 shows two carbonyl resonances in a 2:1 ratio, while
its IR spectrum displays carbonyl stretches at 1987, 1933, and
1771 cmꢀ1. On the basis of the spectral similarities of these
compounds (particularly between 6 and 8), a fluxional process
that exchanges the semibridging and terminal Os-bound carbo-
nyls on the NMR time scale is again proposed. Notably, the IR
stretches for the terminal carbonyls in 8 are at higher frequency
than for 6, while that of the semibridging carbonyl is at slightly
lower frequency. The first observation is consistent with IMe4
being a better donor than PMe3, while the reason for the lower
frequency for the bridging carbonyl in 8 is not immediately
obvious, although it should be noted that in our experience the
carbonyl stretching frequency is not very diagnostic for identify-
ing different types of bridging carbonyl modes (e.g., symmetrical,
unsymmetrical, semibridging).
An X-ray structure determination of compound 8 was carried
out, and the complex cation is represented in Figure 2, confirm-
ing the overall structural similarities with 6 while also pointing
out the subtle differences. At a glance, the similarities in structure
are obvious with the IMe4 and PMe3 groups bound to Rh,
essentially opposite the RhꢀOs bond. In addition, the structures
of both confirm the single-carbonyl-bridged structure that was
proposed on the basis of IR data.
1
observation of only two methyl resonances in the H NMR
spectrum of 6 is readily rationalized by the carbonyl fluxionality
noted above but could also result from rotation about the
RhꢀIMe4 bond. However, facile rotation about the Rhꢀcarbene
bond seems unlikely, owing to the severe repulsions that would
appear to occur with the dppm phenyl groups.
The addition of another 1 equiv of IMe4, in attempts to
obtain a di-NHC complex (possibly having the second IMe4
group on Os), instead gives a very different product (see
Scheme 1), formulated as [RhOs(IMe4)(CO)3(dppm-H)-
(dppm)] (7; dppm-H = bis(diphenylphosphino)methanide),
resulting from deprotonation of one of the dppm methylene
groups yielding [HIMe4][OTf] as the other product, which is
observed in the 1H NMR spectrum. The methylene groups of
coordinated dppm are known to be acidic, and dppm-H species
similar to 7 have previously been observed by us25aꢀd and
others.25eꢀk Furthermore, the strong basicity of NHC ligands is
also well recognized.26 The 31P{1H} NMR spectrum of 7 shows
four equal-intensity resonances at δ 34.2, 29.9, ꢀ4.6, and ꢀ6.2, for
the four chemically unique phosphorus nuclei. Again the low-field
resonances correspond to those bound to Rh, displaying 123 and
130 Hz coupling, respectively, to this metal. Simulation of this
spectrum using SpinWorks27 generates values of 345 and 188 Hz for
The RhꢀOs bonds in both compounds are also very similar
(2.8303(4) Å (6), 2.8316(5) Å (8)). However, the very different
shapes of the IMe4 and PMe3 ligands, with the former exerting a
greater steric influence in the equatorial plane, perpendicular to
the metalꢀphosphine vectors, and the probable electronic
differences12c in the two, have noticeable effects on the bonding
and positioning of the carbonyl ligands.
31
the trans 31Pꢀ P couplings observed across the Rh and Os centers,
respectively, consistent with the values previously reported for a
similar species, [RhOs(CO)4(dppm-H)(dppm)] (2JPP = 321,
117 Hz).25b The derived 145 Hz intraligand-coupling (2JPP) within
the dppm-H bridge is greater than the 87 Hz observed within the
unaltered dppm group, consistent with π-delocalization over the
deprotonated fragment of the former and in close agreement with
previous observations.25bꢀd The 1H NMR spectrum of 7 indicates
that the symmetry about the RhOsP4 plane is maintained, with only
one resonance observed, at δ 3.26, for dppm methylene protons. In
addition, the methanide dppm-H resonance, appearing at δ 2.51, is
consistent with previous observations.25aꢀc The carbonyl region of
the 13C{1H} NMR spectrum for 7 is similar to that observed for the
parent complex 6, displaying two resonances in a 2:1 intensity ratio
As noted earlier, on the basis of IR data, the IMe4 ligand appears to
be a better donor than PMe3. This is manifested in a much stronger
interaction of Rh with the semibridging carbonyl in compound 6
than in 8 (RhꢀC(3) = 2.130(5), 2.281(5) Å, respectively), as a
consequence of the more electron-rich Rh center in the former. The
stronger attraction of C(3)O(3) to Rh in the IMe4 complex (6)
results in a twist of the trigonal arrangements of carbonyls about the
PꢀOsꢀP vector in that direction, resulting in an increase in the
nonbonding RhꢀC(1) distance (3.114(6) Å in 8, 3.335(5) Å in 6)
and an alignment of the axial carbonyl (C(2)O(2)) off the RhꢀOs
axis (RhꢀOsꢀC(2) = 159.0(2)°) in compound 6. This greater
attraction of the semibridging CO for Rh in 6 occurs in spite of the
greater steric demands of the adjacent IMe4 group. It seems reason-
able to expect that the greater size of IMe4 in the plane of the
carbonyls would result in it bending away from the semibridging CO.
However, the IMe4 group in 6 is aligned almost along the OsꢀRh
bond (OsꢀRhꢀC(4) = 178.4(2)°), while the PMe3 group in 8 is
bent away from the bridging CO (OsꢀRhꢀP(5) = 166.18(4)°). A
at δ 218.2 (1JCRh = 7 Hz, 2JCP(Os) = 9 Hz) and δ 188.1 (2JCP(Os)
=
13 Hz), respectively. As was shown for 6, the IR spectrum of 7 also
displays two terminal and one bridging carbonyl stretch (1960,
1906, and 1744 cmꢀ1), again suggesting a rapid exchange process of
the two carbonyls on either side of the RhOsP4 plane, as noted for 6.
The slightly lower frequencies of these stretches compared to those
of the cationic precursor is consistent with increased electron density
at the metal nuclei in the case of this neutral product. In all related
compounds, the intermediate carbonyl stretch is somewhat low for a
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Scheme 2
The second possible route for generating an IMe4-containing,
methylene-bridged target, namely the introduction of the NHC to a
pre-existing methylene-bridged species, as noted above for PMe3
and reported for other phosphines,8c was also attempted for IMe4.
Addition of IMe4 to the tricarbonyl, methylene-bridged species 2, as
shown in Scheme 2, does yield a methylene-bridged, IMe4-contain-
ing species, [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][OTf] (9), but
unlike the case for the PMe3 analogue, the simple adduct is not
formed. Instead, substitution of one carbonyl has occurred, pre-
sumably due to the significantly increased steric bulk of IMe4 in the
equatorial plane that labilizes the Rh-bound carbonyl. Furthermore,
in this case the methylene-bridged product (9) is obtained as
the minor product together with the unexpected coproduct [RhOs-
(CO)3(μ-CH2)(μ-κ1:η2-Ph2PCHPPh2)(dppm)] (10) in a 1:4
ratio—the latter species resulting from competing Brønstedꢀ
Lowry acid/base chemistry through deprotonation of a dppm
methylene group by IMe4. Repeating the reaction at ꢀ78 °C
reverses the relative abundance of these species, giving an approx-
imate 3:1 ratio of 9 and 10, and separation of these species is
described in the Experimental Section. The deprotonation product
10 can be generated quantitatively by the reaction of 2 with the
strong base KN(Si(CH3)3)2 but is not discussed further here, since
its chemistry is the topic of another paper.25d
Figure 2. Perspective view of the complex cation of [RhOs(PMe3)-
(CO)3(dppm)2][CF3SO3] (8), showing the atom-labeling scheme.
Atom numbering and thermal parameters are as described for Figure 1.
Relevant parameters (distances in Å and angles in deg): OsꢀRh =
2.8316(5), OsꢀC(1) = 1.919(6), OsꢀC(2) = 1.876(6), OsꢀC(3) =
1.950(5), RhꢀP(5) = 2.3260(15), RhꢀC(1) = 3.114(6), RhꢀC(3) =
2.281(5); C(1)ꢀOsꢀC(2) = 107.3(3), C(1)ꢀOsꢀC(3) = 132.6(2),
C(2)ꢀOsꢀC(3) = 120.1(3), OsꢀC(3)ꢀO(3) = 163.8(5), Rhꢀ
C(3)ꢀO(3) = 112.5(4).
A satisfactory elemental analysis for complex 9 could not be
obtained, as attempts to fully remove residual imidizolium salt
were unsuccessful; however, high-resolution mass spectrometry
and multinuclear NMR spectroscopy (see the Supporting In-
formation) agree with the assigned formulation. The 31P{1H}
NMR spectrum displays two resonances, in which the Rh-bound
ends of the diphosphines appear at δ 25.0 displaying 132 Hz
coupling to rhodium, while the Os-bound ends appear at
δ ꢀ2.66. The front/back asymmetry across the approximate
RhOsP4 plane, a consequence of the bridging CH2 group, gives
rise to four chemically unique methyl resonances for the IMe4
ligand (δ 3.14, 2.90, 2.63, and 1.47) in the 1H NMR spectrum,
while the dppm methylene groups appear as two signals at δ 4.55
and 3.97. The methylene group bridging the metals appears
consideration of the structures reveals that these orientations are
related more to the dppm phenyl group orientations than to the
equatorial ligand positions. In 8 the more conical shape of PMe3
interacts strongly with phenyl rings 4 and 8, forcing the phosphine
off the RhꢀOs axis, while for 6 the more planar IMe4 group allows
the corresponding phenyl rings to lie more horizontally, creating
less repulsion.
Methylene-Bridged Complexes. As noted earlier, the next
step in generating methylene-bridged species was to react the IMe4
and PMe3 precursors (6 and 8) with diazomethane. Although the
related tetracarbonyl complex 1 has a rich chemistry with dia-
zomethane,16 displaying reactivity even at ꢀ80 °C, the NHC
analogue 6 is unreactive toward diazomethane, even at elevated
temperatures, failing to yield the methylene-bridged, IMe4-contain-
ing target by this route. The PMe3 analogue 8, on the other hand,
reacts readily with diazomethane, although a mixture of unidenti-
fied products is obtained at ambient temperature, as determined by
31P NMR spectroscopy. However, the methylene-bridged target,
[RhOs(PMe3)(CO)3(μ-CH2)(dppm)2][OTf] (3), can be ob-
tained quantitatively by carrying out the reaction at ꢀ78 °C (see
Scheme 1); this product was previously prepared by PMe3 addition
to the methylene-bridged precursors [RhOs(CO)x(μ-CH2)-
(dppm)2]þ (x = 3, 4).8c The products observed in the ambient-
temperature reaction of 8 with diazomethane were not investigated
further. We suggest that the failure of 6 to react with diazomethane
is due to the steric dominance of the IMe4 group in the equatorial
plane, preventing prior coordination of CH2N2.
1
downfield in the H NMR spectrum at δ 8.71 as a triplet of
triplets and displays unsymmetrical coupling to the Rh- (5.5 Hz)
and Os-bound (15.4 Hz) 31P nuclei, as confirmed by selective 31
P
decoupling experiments. This coupling pattern suggests that the
μ-CH2 group is more strongly bound to osmium, possibly a
result of steric repulsion involving the bulky Rh-bound IMe4
group. In the 13C NMR spectrum, this bridging methylene group
appears at δ 121.3, displaying typical coupling to rhodium of
19 Hz8c,16 and a one-bond CꢀH coupling of 138 Hz. Interest-
1
ingly, both the H and 13C chemical shifts for the bridging
methylene group in compound 9 are significantly downfield from
those observed in the closely related, but more coordinatively
saturated, methylene-bridged compounds, in which the NHC
group is replaced by a pair of carbonyls or a monophosphine and
a carbonyl.8c The two carbonyls, observed at δ 188.5 and 184.3 in
the 13C{1H} NMR spectrum, are terminally bound to osmium,
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Scheme 3
Scheme 4
as shown by the absence of Rh coupling and the observed
coupling to the adjacent Os-bound ends of the diphosphines.
(IMe4)(CO)2(dppm)2][CF3SO3]2 (12a,b), in an approximate
10:1 ratio, while protonation of the PMe3 analogue 11 yields the
corresponding pair of isomers 13a,3b in a 2:1 ratio. The
protonations of compounds 9 and 11 proceed in parallel, as
shown in Scheme 5, and the spectral parameters for the IMe4 and
PMe3 series of compounds are quite comparable (in all cases the
resonances for the dppm, IMe4, and PMe3 groups are as expected
and so are not discussed here). The reactivities of compounds 9
and 11 differ only in the relative concentrations of the pairs of
isomers produced at ꢀ78 °C (12a,b or 13a,b) and in the
temperatures at which conversions to subsequent products occur.
In both systems the differences in the a and b isomers relate
mainly to the bonding of the methyl and carbonyl groups.
Unfortunately, the methyl resonances, in both the 13C and
1H NMR spectra, are broad over the accessible temperature
range, obscuring much of the spinꢀspin coupling information.
Nevertheless, the significant differences in these isomers are
obvious from the available data. For the a isomers, the 13C{1H}
NMR resonances for the methyl groups appear at δ 7.9 (12a)
13
The 13Cꢀ C coupling constant of 11 Hz, observed between the
downfield carbonyl and the bridging methylene group, in the
13CO/13CH2-enriched sample, suggests that these groups are
mutually trans. The carbene carbon of the NHC group, observed
at δ 179.7, displays coupling to rhodium of 48 Hz, while the IMe4
methyl groups appear as singlets at δ 34.6, 33.7, 8.2, and 8.1,
further demonstrating their chemical inequivalence.
Methyl Complexes. (a). Protonation of Methylene-Bridged
Species. Having in hand methylene-bridged complexes of IMe4
(9) and PMe3 (3), we set out to compare their protonations.
However, the IMe4-containing, methylene-bridged product 9 has
one less carbonyl ligand than the known PMe3 complex 3,
complicating any direct comparison of their chemistries. At-
tempts to generate the carbonyl adduct of 9, which would be
directly comparable to 3, did not succeed, resulting instead in loss
of the methylene group to give 6 (see Scheme 3). In this
transformation the methylene group is presumably lost as ketene,
although this product is not observed. Its involvement is pro-
posed on the basis of the observation of acetic acid in solution,
which presumably results by the reaction of ketene with adven-
titious water. When the reaction was carried out using 13CO,
partial 13C incorporation was observed in the acetic acid formed.
Having been unable to generate the IMe4 analogue of 3, we
instead generated the PMe3 analogue of 9, namely [RhOs-
(PMe3)(CO)2(μ-CH2)(dppm)2][CF3SO3] (11), by reaction
of 3 with trimethylamine N-oxide (TMNO), as outlined in
Scheme 4. The 31P{1H} NMR spectrum of 11 displays three
signals in a 2:2:1 ratio, in which the Rh-bound ends of the
diphosphines appear at δ 25.7, displaying 130 Hz coupling to
rhodium and 38 Hz coupling to the PMe3 group, the Os-bound
ends appear at δ 2.02, and the PMe3 signal appears at δ ꢀ24.4,
displaying typical coupling to Rh (122 Hz) and to the adjacent
pair of Rh-bound 31P nuclei, noted above. The methylene group
1
and ꢀ6.0 (13a) with the H resonances appearing at δ 0.08
(12a) and ꢀ0.74 (13a). In the proton-coupled 13C NMR spectra
of 13CH3-enriched samples, one-bond CꢀH coupling of 111 Hz
(12a) and 117 Hz (13a) is observed. This coupling is much lower
than is typical for a terminal methyl group28 (for example, see
1
compounds 14 and 15, for which JCH = 133 Hz) and instead
results from an unsymmetrically bridged methyl group in which
the three hydrogens are exchanging rapidly. The values observed
correspond to the weighted average of two terminal CꢀH bonds
and the bridging (agostic) CꢀH interaction. In unsymetrically
bridged methyl groups the coupling constants (1JCH) involving
the nonbridged bonds are often observed near 140 Hz.8c Using
this value for terminal CꢀH coupling, the agostic interaction can
be approximated at 53 Hz in the case of IMe4 and 71 Hz in the
case of PMe3—values that represent a substantial weakening of
this CꢀH bond owing to a strong interaction with the adjacent
metal. These values also suggest that the IMe4 ligand provides a
more electron-rich Rh center in comparison to PMe3 with
concomitant increased back-donation to the CꢀH σ* orbital.
Protonation of the PMe3 compound 11, using a sample of
DOTf that was contaminated by HOTf, gave rise to two
1
bridging the metals, which appears downfield in the H NMR
spectrum at δ 7.09 as a triplet of triplets of doublets, displays
unsymmetrical coupling to the Rh-bound (7.2 Hz) and Os-
bound (14.5 Hz) 31P nuclei, as confirmed by 31P decoupling
experiments, and coupling to 103Rh of 2.6 Hz. As was the case for
9, this coupling pattern suggests that the μ-CH2 group is more
strongly bound to osmium. In the 13C NMR spectrum, this
bridging methylene group is also downfield, at δ 105.7, display-
ing coupling to rhodium of 17 Hz8c,16 and one-bond CꢀH
coupling of 133 Hz. The two carbonyls, observed at δ 187.3 and
180.2 in the 13C{1H} NMR spectrum, are terminally bound to
osmium, as shown by the absence of Rh coupling and their
coupling to the Os-bound ends of the diphosphines. For the low-
field carbonyl an additional 12 Hz coupling is observed to the
μ-13CH2 group, suggesting a mutually trans arrangement of these
groups.
1
resonances in the H NMR spectrum at δ ꢀ0.74 and ꢀ0.95
for the CH3 and CH2D groups, respectively, of 13a; the observed
isotope shift is consistent with the presence of the IPR phenom-
enon, as has been well documented,1c,4a,4e,8a,8c,28 supporting the
bridged, agostic structure.
In the 13C{1H} NMR spectra of 12a and 13a, both species
display two terminal Os-bound carbonyls, at δ 183.4 and 169.6 for
12a and at δ182.0 and 169.0 for 13a, each displaying coupling to the
pair of adjacent 31P nuclei; in addition, two-bond coupling (11 Hz
(12a) and 12 Hz (13a)) between the low-field carbonyl and the
Os-bound methyl group is also observed, suggesting a mutually
As outlined in Scheme 5, protonation of the IMe4 complex 9
by HOTf at ꢀ78 °C yields a pair of isomers, [RhOs(CH3)-
2
trans arrangement. Similar JCC values, involving the μ-CH2 and
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Scheme 5
low-field carbonyl, were observed in the parent species 9 and 11,
suggesting little change in geometry upon protonation. A dative
RhfOs(II) bond is proposed for isomers 12a and 13a in order for
Os to achieve its favored 18e configuration. The structures of these
isomers very much resemble that of the kinetic product in the
protonation of [RhOs(CO)3(μ-CH2)(dppm)2]þ, with the excep-
tion that triflate ion coordination was observed for this tricarbonyl
product.8c In contrast, the 19F NMR spectra of the 12a,b and 13a,b
mixtures indicate that triflate ion coordination has not occurred in
any of these species, with only free triflate ion at δ ꢀ78.8 in the 19F
NMR spectrum being observed.
PMe3 (15)). Now coordination of a CF3SO3ꢀ anion is clear, as
seen by the appearance of coordinated triflate, at δ ꢀ77.3 (14)
and δ ꢀ76.7 (15), together with free triflate ion in the 19F NMR
spectrum. In both compounds the methyl ligand appears in the
1H NMR spectrum as a triplet, at δ 0.18 (14) and at δ 0.06 (15),
displaying 6.3 and 6.8 Hz coupling, respectively, to the Os-bound
31P nuclei. Notably, in 13CH3-enriched samples an additional
133 Hz coupling to carbon is observed in both cases; this value is
normal for a terminal methyl group.28 In the 13C NMR spectra,
the bridging carbonyl groups, at δ 205.9 (14) and 199.5 (15),
show strong 39 Hz (14) and 40 Hz (15) coupling to 103Rh,
together with an additional trans coupling of 40 Hz to the 31PMe3
ligand in compound 15. The terminally bound carbonyls, at δ
177.6 (14) and 176.3 (15), show the expected coupling to the
Os-bound ends of the dppm groups and 4 Hz coupling to
the 13CH3 group consistent with a mutually cis arrangement.
The 13CH3 resonance appears as a quartet at δ 13.8 (14) and
14.5 (15) in the 13C NMR spectra.
As noted, the major differences between isomers a and b are
seen in the parameters for the methyl and carbonyl groups. For
isomers 12b and 13b one carbonyl on each is terminal (δ 178.8
(12b) and 179.5 (13b)), displaying coupling to only the Os-
bound 31P nuclei. However, for these isomers the other carbonyl
is bridging, as seen by its downfield shift (δ 203.6 (12b) and
193.6 (13b)) and its coupling to 103Rh (23 and 18 Hz,
respectively). In the proton-coupled 13C NMR spectra, the
13CH3 groups are observed at δ ꢀ28.8 (12b) and ꢀ29.2 (13b),
displaying significantly larger one-bond CꢀH coupling of
125 Hz (12b) and 126 Hz (13b) than for the a isomers. This
coupling is within the “normal” range for terminally bound methyl
groups (125ꢀ140 Hz)28 and in addition, in the experiment
described earlier in which the protonation of 11 was carried out
using a HOTf/DOTf mixture, the small isotope shift for 13b-CH3
(δ 0.93) compared to that for 13b-CH2D (δ 0.90) is typical for
geminal substitution of H by D29 and is too small for the IPR
effect, arguing against any significant bridging arrangement for
this group. Although the protonation of 9 using DOTf was not
carried out, the 1H NMR resonance for the methyl group in 12b
(δ 1.60) is typical for a terminal group. For isomer 13b, an
additional 34 Hz coupling between the methyl carbon and the
Further warming of compound 14 to ambient temperature or of
compound 15 to 0 °C yields the final products, [RhOs(L)(CO)2-
(μ-CH2)(μ-H)(dppm)2][CF3SO3]2 (L = IMe4 (16), PMe3 (17)),
containing bridging methylene and hydride groups; these are the
only products observed on protonation of 9 or 11 at ambient
temperature. In the 1H NMR spectra the bridging methylene groups
appear as pseudoquintets with coincidental 8.5 Hz (16) and 6.0 Hz
(17) coupling to the inequivalent sets of Rh- and Os-bound 31P
nuclei, confirming their bridging arrangement, and in 13CH2-labeled
samples an additional 133.7 (16) and 138.7 Hz (17) one-bond
coupling to the 13C nucleus is observed. As is often the case, two-
1
bond coupling to 103Rh is not observed in these H signals. The
resonances assigned to the bridging hydride of 16 (δ ꢀ11.47) and
of 17 (δ ꢀ10.90) appear as complex multiplets, due to coupling to
the four (16) or five (17) 31P nuclei, to 103Rh, and (in a 13CO-
enriched sample) to both Os-bound carbonyls. Upon broad-band
31P decoupling, these hydride resonances simplify to a doublet of
doublets with 14.8 Hz (16) and 15.0 Hz (17) coupling to 103Rh and
7 Hz coupling to the trans carbonyl appearing at δ 173.2 (16) and
176.4 (17) in the 13C{1H} NMR spectra. In the case of 17, 60 Hz
two-bond coupling of the hydride resonance to the PMe3 group is
also observed, suggesting a trans arrangement of these groups across
Rh. For both compounds (16 and 17) two Os-bound carbonyl
resonances are observed, one of which displays 14ꢀ15 Hz trans
31
31P nucleus of PMe3 is observed. Although three-bond 13Cꢀ P
coupling is typically not this large,30 particularly when these
groups occupy different metals, we have previously observed
a number of cases in which such coupling through the metalꢀ
metal bond has been observed.8c,31
As shown in Scheme 5, warming solutions of 12a,b to 0 °C or
of 13a,b to ꢀ40 °C results in the conversion of both sets of
isomers to the terminally bound methyl species [RhOs(CH3)-
(OSO2CF3)(L)(CO)(μ-CO)(dppm)2][CF3SO3] (L = IMe4(14),
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coupling to the bridging methylene groups, which appear at δ 99.6
(16) and 79.4 (17) and display 32 and 36 Hz coupling to Rh,
respectively.
Scheme 6
We have also investigated the protonation of [RhOs(PMe3)-
(CO)2(μ-CH2)(dppm)2][BPh4] (11-BPh4), in which the trꢀi-
flate ion in 11 has been replaced by the noncoordinating BPh4
anion and in which the acid [H(OEt2)2][BArF ], also containing
4
a noncoordinating anion, was used. In this case, no reaction is
observed at ꢀ80 °C, but warming to ꢀ60 °C yields the complex
cation of 13b, having NMR spectral parameters very similar to
those of the triflate salt (see the Supporting Information for
spectra). Although protonation of 11 by triflic acid yielded both
13a and 13b at ꢀ80 °C, 13a does not appear together with 13b
species was observed, even in an excess of methyl triflate.
Attempts to generate the known PMe3 analogue [RhOs(Me)-
(PMe3)(CO)3(dppm)2]2þ by reaction of [RhOs(PMe3)(CO)3-
(dppm)2]þ (8) with methyl triflate also failed, with no reaction
observed. Presumably the methyl triflate is too weak an electro-
phile to react with these cationic species.
when [H(OEt2)2][BArF ] is used but appears on subsequent
4
warming to ꢀ50 °C. After 14 h at this temperature species 13a,b
have apparently reached their equilibrium concentrations, while
a new species (18) has also emerged, giving a 13a:13b:18 ratio of
4:1:1. Further warming to ꢀ30 °C results in the complete
transformation of 13a,b into 18, which is stable upon warming
to ambient temperature. Compound 18 is formulated as
Hydride Products. Although compound 6 did not react with
methyl triflate, it is readily protonated by triflic acid, yielding the
kinetic product 19a when carried out at ꢀ78 °C. This species
slowly converts to the thermodynamic product 19b over the
course of several hours at ambient temperature. Compounds
19a,b are formulated as isomers of the monoprotonation pro-
duct, [RhOs(H)(IMe4)(CO)3(dppm)2][CF3SO3]2, as shown in
Scheme 7. Their 31P{1H} NMR spectra are as expected for the
proposed structures (see Table 1). In the 1H NMR spectrum, the
hydride resonance for 19a, located at δ ꢀ5.94, shows coupling to
only the adjacent Os-bound 31P nuclei, indicating that it is
terminally bound to this metal. The 13C{1H} NMR spectrum
shows three unique carbonyl environments; two are found to
be bridging, while one (a triplet at δ 173.8 (2JCP = 9 Hz)) is
terminally bound to Os. One of the bridging carbonyls, at
δ 211.4, displays coupling of 24 Hz to Rh, while the other, at
δ 198.6, appears as a doublet of triplets, having 18 Hz coupling to
103Rh and 3.5 Hz coupling to the Os-bound ends of the dipho-
sphines. On the basis of the coupling constants, both appear to be
semibridging, although with the former interacting somewhat
more strongly with Rh. The 13C{1H} NMR spectrum also shows
the carbene carbon at δ 161.7 with coupling to both Rh (63 Hz)
and the Rh-bound 31P nuclei (19 Hz).
[RhOs(CH3)(PMe3)(CO)2(OEt2)(dppm)2]2þ, having BPh4
ꢀ
and BArF ꢀ counterions.
4
The transformation of the 13a,b mix to 18 has been accom-
panied by the migration of PMe3 to Os and of a carbonyl to Rh. In
addition, the methyl ligand is terminally bound to Os, as seen by its
1
lack of coupling to Rh in the H and 13C NMR spectra and its
coupling to the Os-bound31P nuclei. Inthe 31P{1H} NMR spectra
five resonances are observed for the chemically inequivalent 31P
nuclei; the pair on Rh are shown by their mutual coupling of 46 Hz
to be mutually cis, while the Os-bound ends of the dppm groups
display trans coupling of 219 Hz. The inequivalence of the 31P
nuclei on each metal is presumed to arise from a twisting about the
RhꢀOsbond, which places them in inequivalentenvironments. At
ambient temperature only one set of broad resonances for ether is
observed, however, upon cooling to ꢀ80 °C resonances for the
coordinated ether appear at δ 3.98 and 1.33, together with free
ether. On the basis of the spectroscopy summarized in Table 1,
compound 18 is assigned the structure shown. Although the exact
geometry is not known, the connectivity seems clear and the
binding of the methyl group terminally to Os is unambiguous. No
evidence was seen for a methylene/hydride species, as was
observed in the triflic acid protonation.
From 1H NMR spectroscopy it is clear that the thermodynamic
product 19b is a hydride-bridged species, displaying the hydride
resonance in the 1H NMR spectrum at δ ꢀ10.91 as a doublet of
triplet of triplets, having coupling to 103Rh of 18.8 Hz, and to the Rh-
and Os-bound 31P nuclei of 9.3 and 7.3 Hz, respectively. In the
13C{1H} NMR spectrum, the two terminal carbonyls bound to
osmium appear as triplets at δ 176.7 and 171.6, with coupling to the
Os-bound phosphorus nuclei, while the third carbonyl appears as a
complex multiplet, at δ 223.5, displaying coupling to all phosphorus
nuclei and to Rh. Broad-band decoupling (13C{31P, 1H}) simplifies
this resonance to a doublet displaying 29 Hz coupling to rhodium,
typical of a symmetrically bridged carbonyl. Compounds 19a,b are
readily deprotonated by addition of triethylamine, regenerating 6.
The structure for the complex dication 19b is shown in Figure 3
and unambiguously confirms the spectroscopic assignments. Pro-
tonation of 6 has resulted in lengthening of the metalꢀ
metal separation from 2.8303(4) Å to 2.9030(3) Å, consistent
with the conversion of a two-center, two-electron RhꢀOs bond
to a three-center, two-electron RhꢀHꢀOs bond. The resulting
geometry at Os is best described as octahedral, while that at Rh
appears as a tetragonal pyramid having the bridging carbonyl
(C(3)O(3)) at the apical site. Protonation at the metalꢀmetal
(b). Other Possible Routes to Methyl Species. Two additional
direct routes to the target IMe4/methyl species were also attempted.
In the first we attempted to replace the labile Os-bound triflate anion
in the known species [RhOs(Me)(OSO2CF3)(CO)3(dppm)2]-
[CF3SO3] (5)8c by IMe4. However, in parallel to the case
observed earlier, the BrønstedꢀLowry basicity of IMe4 again
dominates, leading instead to deprotonation of the methyl
ligand to yield the known species 2 and [HIMe4][OTf], as
outlined in Scheme 6.
Another potential route to an IMe4/methyl complex of the
type targeted is the reaction of [RhOs(IMe4)(CO)3(dppm)2]þ
(6) with methyl triflate. However, no reaction between these
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Scheme 7
’ DISCUSSION
The similarities between the two ligand classes (PR3 and NHC)
notwithstanding, there remain significant differences between the
two, some of which are demonstrated in this study. The steric
differences between the two ligand types12d are widely recognized,
and in this study the pseudoplanar IMe4 ligand exerts a significantly
greater steric influence in the equatorial plane of the binuclear
complexes (perpendicular to the μ-dppm groups) than the more
conical PMe3 group. This greater “equatorial presence” is presum-
ably the reason that the IMe4 analogue of [RhOsL(CO)3(μ-CH2)-
(dppm)2]þ (L = CO, PMe3) could not be prepared, owing to the
labilization of the third carbonyl ligand by the larger IMe4 group.
Furthermore, the failure of diazomethane to react with [RhOs-
(IMe4)(CO)3(dppm)2]þ, even at above-ambient temperatures, is
in marked contrast to the facile reactivity of the PMe3 and CO
analogues at ꢀ80 °C and is rationalized on the basis that the larger
IMe4 group prevents diazomethane access to the metals.
However, the most significant difference between IMe4 and
PMe3 to emerge from this study is the significantly greater
tendency of the former to function as a Brønsted base, as demon-
strated by its facile deprotonation of either dppm or methyl
ligands. NHC ligands are known to deprotonate acidic hydro-
carbons,26c,32 and so the deprotonation of the acidic dppm
methylene group,25 although not anticipated, was not surprising.
On the other hand, deprotonation of a methyl ligand was surprising,
and in particular its deprotonation in preference to the dppm
groups was unexpected. Certainly, however, dppm deprotonation
by IMe4 has been a recurring theme throughout our recent
studies on this and related dppm-bridged systems.25c,d
Figure 3. Perspective view of the complex dication of [RhOs(IMe4)-
(CO)2(μ-H)(μ-CO)(dppm)2][CF3SO3]2 (19b), showing the atom-
labeling scheme. Atom numbering and thermal parameters are as
described for Figure 1. Relevant parameters (distances in Å and angles
in deg): OsꢀRh = 2.9030(3), OsꢀC(3) = 2.082(3), OsꢀC(2) =
1.923(3), OsꢀC(1) = 1.934(3), OsꢀH(1) = 1.85(4), RhꢀC(4) =
2.043(3), RhꢀC(3) = 2.026(3), RhꢀH(1) = 1.79(4); C(3)ꢀOsꢀC(2) =
93.82(13), C(2)ꢀOsꢀC(1) = 92.73(14), C(1)ꢀOsꢀH(1) = 92.8(12),
H(1)ꢀOsꢀC(3) = 80.6(12), C(4)ꢀRhꢀC(3) = 118.67(11), C(3)ꢀ
RhꢀH(1) = 83.4(13), C(4)ꢀRhꢀH(1) = 157.8(13), OsꢀC(3)ꢀO(3) =
143.1(2), RhꢀC(3)ꢀO(3) = 126.8(2).
Given this tendency for IMe4 to function as a Brønsted
base, we investigated the protonation of [RhOs(IMe4)(CO)3-
(dppmþ)2]þ (6) in order to determine whether elimination of
HIMe4 from the metals would occur. Reductive elimination
of NHC groups with either hydrido or alkyl ligands has been
observed previously,33 and in at least one case, reductive elim-
ination of the imidizolium salt was induced by protonation of the
NHC complex.33b The kinetic product of protonation, in our
study [RhOsH(IMe4)(CO)3(dppm)2]2þ (19a), has the hydrido
and NHC ligands on different metals, where they are not
appropriately placed for a reductive elimination, while the
thermodynamic isomer (19b) has the hydrido ligand bridging
the metals. Although in this product both the hydrido and NHC
groups are bound to Rh, their mutually trans arrangement is
again not conducive to reductive elimination; consequently, no
elimination is observed.
bond has also forced the IMe4 and CO ligands away from this
protonation site such that the OsꢀRhꢀC(4) angle observed for
6 (178.4(2)°) has decreased to 164.45(8)°, while the RhꢀOsꢀ
C(1) angle has opened up from 86.8(1)° to 129.3(1)° upon
protonation. Protonation has also resulted in a compression of
the intercarbonyl angles to near 93°, reflecting the nearly
octahedral geometry at Os. Other structural changes upon
protonation of 6 reflect the changes in orientation of the dppm
phenyl rings (particularly those on P(1) and P(2)) in order to
minimize nonbonding contacts with the carbonyl and IMe4
ligands in the equatorial plane. Interestingly, the close-to-trans
arrangement of the bridging hydride and the IMe4 group in 19b is
the geometry suggested earlier, on the basis of NMR evidence,
for the PMe3 and hydride groups in 17.
An alternate potential route to methyl-containing products
involving the reaction of the hydride-bridged species 19b with
diazomethane, in hope of bringing about insertion of the
diazomethane-generated methylene group into one of the
metal-hydride bonds, gave no reaction, even after extended
periods. Presumably, the saturation at Os and steric crowding
at Rh, preventing diazomethane attack, are two factors inhibiting
reaction.
Our original goal in this study was to determine the influence
of the IMe4 ligand on the position of the tautomeric equilibrium
between the unsymmetrically bridged methyl species D and the
methylene/hydride C, shown in eq 1. In an earlier study the
anticipation that a basic ligand (PR3) would favor the methylene/
hydride species, as had been observed by Shapley and co-workers
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in Os3 clusters,11 had not materialized in our Rh/Os series.
Nevertheless, we considered that the greater basicity of IMe4
might favor the methylene/hydride species. One approach to
such products was through protonation of the corresponding
methylene-bridged complexes, and although the initially targeted
tricarbonyl precursor [RhOs(IMe4)(CO)3(μ-CH2)(dppm)2]þ
could not be synthesized, the dicarbonyl analogues [RhOsL-
(CO)2(μ-CH2)(dppm)2][OTf] (L = IMe4 (9), PMe3 (11))
were obtained and their protonations by HOTf carried out and
compared to that of the carbonyl analogue (L = CO). In all cases
(L = CO, PMe3, IMe4) the kinetic products, observed at ꢀ80 °C,
are the unsymmetrically bridged (agostic) methyl species,
although for L = PMe3, IMe4 minor isomers having terminally
bound methyl groups were also observed. In the methyl-bridged
products the average CꢀH coupling constants for these groups
decreased in the order L = CO (120 Hz) > PMe3 (117 Hz) >
IMe4 (111 Hz), consistent with weakening of the agostic CꢀH
bond by increased back-donation as ligand (L) basicity increased.
Consistent with our proposal and with Shapley’s findings,11 the
bridging methyl products [RhOsL(CO)2(μ-CH3)(dppm)2]-
[OTf]2, containing the more basic PMe3 and IMe4 ligands,
transformed to the respective methylene/hydride tautomers
upon warming to ambient temperature. However, this transforma-
tion did not occur directly but instead proceeded via an intermedi-
ate, [RhOs(CH3)(OTf)(L)(CO)2(dppm)2][OTf] (L = IMe4
(14), PMe3 (15)), having a coordinated triflate ion and the methyl
group terminally bound to Os (see Scheme 5). We assume that the
dicationic charge of the methyl-bridged species 12a and 13a
disfavors CꢀH bond cleavage and that this transformation occurs
more readily from a more electron-rich monocationic (presumably
also methyl-bridged) species derived from 14 and 15.
spectral data for the intermediates in the protonation of 11-BPh4
by [H(OEt2)2][BArF ] and CIF files giving atomic coordinates,
4
interatomic distances and angles, anisotropic thermal para-
meters, and hydrogen parameters for compounds 6, 8, and
19b. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: martin.cowie@ualberta.ca. Fax: 01 7804928231. Tel: 01
7804925581.
’ ACKNOWLEDGMENT
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of Alberta for
financial support of this research and the NSERC for funding for
the Bruker PLATFORM/SMART 1000 CCD diffractometer,
and the Nicolet Avatar IR spectrometer. We thank the Depart-
ment’s Analytical and Instrumentation Laboratory and NMR
Spectroscopy Laboratory for outstanding support and M. E.
Slaney for supplying the sample of [H(OEt2)2][BArF ].
4
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reversible, with the slower CꢀH bond cleavage step still occur-
ring from the methyl-bridged species 12a and 13a. In order to
establish the role of anion coordination in the CꢀH activation
process, we generated [RhOs(PMe3)(CO)2(μ-CH2)(dppm)2]-
[BPh4] (11-BPh4), having a noncoordinating anion, and inves-
tigated its protonation by [H(OEt2)2][BArF ], also having a
4
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methyl products analogous to 13a,b, as obtained with HOTf.
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hydride species was obtained in the absence of coordinating anions.
This experiment appears to rule out the direct transformation of 13
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It is interesting that in a previous study the methyl-bridged PMe3-
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’ ASSOCIATED CONTENT
S
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