Diverse coordination modes and transformations of allenes at adjacent iridium/osmium centers

Article abstract of DOI:10.1021/om300316h

The methylene-bridged complex, [IrOs(CO)₃(μ-CH ₂)(dppm)₂][BF₄] (dppm = μ-Ph ₂PCH₂PPh₂) (2), reacts with allene, resulting in C-C bond formation, to yield an equilibrium mix of two isomers of [IrOs(CO)₃(μ-η³:κ¹-C(CH ₂)₃)(dppm)₂][BF₄] (3/3a), in which the hapticity of the trimethylenemethane ligand with respect to the two metals, as well as the carbonyl ligand arrangement, is different in each isomer. Reaction of 2, as the triflate salt (2-CF₃SO₃), with methylallene also yields two isomers, [IrOs(CO)₃(μ- η³:κ¹-C(CHCH₃)(CH₂) ₂)(dppm)₂][CF₃SO₃] (4/4a); however, in this case, the binding mode of the substituted trimethylenemethane moiety is the same in each isomer and differs only in the position of the methyl group on the allylic moiety. The addition of 1,1-dimethylallene to 2-CF ₃SO₃ results in loss of 4-methyl-1,3-pentadiene and subsequent reaction of the remaining [IrOs(CO)₃(dppm) ₂]⁺ species with excess 1,1-dimethylallene to give [IrOs(CO)₃(μ-η³:κ¹-CH ₂CCMe₂)(dppm)₂][CF₃SO₃] (5), in which the dimethylallene moiety is κ¹-bound to Os through the central carbon and η³-bound to Ir. Both allene and methylallene react with the tetracarbonyl complex, [IrOs(CO)₄(dppm) ₂][BF₄] (6), to generate analogous products, [IrOs(CO)₃(μ-η³:κ¹-CH ₂CCHR)(dppm)₂][BF₄] (R = H (7), CH₃ (8), respectively). Reaction of 6-CF₃SO₃ with 1,1-dimethylallene yields [IrOs(CO)₄(μ-C=C(H)C(CH ₃)=CH₂)(dppm)₂][CF₃SO₃] (9), the result of activation of the geminal C-H bonds of the unsubstituted end of the allene, and additional activation of a methyl C-H bond. The addition of 1,1-difluoroallene to 6-CF₃SO₃ yields [IrOs(CO) ₄(μ-κ¹:κ¹-F₂C=C= CH₂)(dppm)₂][CF₃SO₃] (10), in which this cumulene bridges both metal centers through the central carbon and the CH₂ end of the substrate. These reactivities are compared to those of related Ir₂, Rh/Ru, Rh/Os, and Ir/Ru complexes.

Full text of DOI:10.1021/om300316h

Article
pubs.acs.org/Organometallics
Diverse Coordination Modes and Transformations of Allenes at
Adjacent Iridium/Osmium Centers
Tiffany J. MacDougall,† Michael J. Ferguson,†,‡ Robert McDonald,†,‡ and Martin Cowie*,†
‡
†Department of Chemistry and X-ray Crystallography Laboratory, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2
S
* Supporting Information
ABSTRACT: The methylene-bridged complex, [IrOs(CO)3(μ-CH2)(dppm)2][BF4] (dppm = μ-Ph2PCH2PPh2) (2), reacts
with allene, resulting in C−C bond formation, to yield an equilibrium mix of two isomers of [IrOs(CO)3(μ-η3:κ1-
C(CH2)3)(dppm)2][BF4] (3/3a), in which the hapticity of the trimethylenemethane ligand with respect to the two metals, as
well as the carbonyl ligand arrangement, is different in each isomer. Reaction of 2, as the triflate salt (2-CF3SO3), with
methylallene also yields two isomers, [IrOs(CO)3(μ-η3:κ1-C(CHCH3)(CH2)2)(dppm)2][CF3SO3] (4/4a); however, in this case,
the binding mode of the substituted trimethylenemethane moiety is the same in each isomer and differs only in the position of
the methyl group on the allylic moiety. The addition of 1,1-dimethylallene to 2-CF3SO3 results in loss of 4-methyl-1,3-
pentadiene and subsequent reaction of the remaining “[IrOs(CO)3(dppm)2]+” species with excess 1,1-dimethylallene to give
[IrOs(CO)3(μ-η3:κ1-CH2CCMe2)(dppm)2][CF3SO3] (5), in which the dimethylallene moiety is κ1-bound to Os through the
central carbon and η3-bound to Ir. Both allene and methylallene react with the tetracarbonyl complex, [IrOs(CO)4(dppm)2]-
[BF4] (6), to generate analogous products, [IrOs(CO)3(μ-η3:κ1-CH2CCHR)(dppm)2][BF4] (R = H (7), CH3 (8), respectively).
Reaction of 6-CF3SO3 with 1,1-dimethylallene yields [IrOs(CO)4(μ-CC(H)C(CH3)CH2)(dppm)2][CF3SO3] (9), the
result of activation of the geminal C−H bonds of the unsubstituted end of the allene, and additional activation of a methyl C−H
bond. The addition of 1,1-difluoroallene to 6-CF3SO3 yields [IrOs(CO)4(μ-κ1:κ1-F2CCCH2)(dppm)2][CF3SO3] (10), in
which this cumulene bridges both metal centers through the central carbon and the CH2 end of the substrate. These reactivities
are compared to those of related Ir2, Rh/Ru, Rh/Os, and Ir/Ru complexes.
Chart 1.
INTRODUCTION
■
Allenes have proven to be useful substrates for the synthesis of
complex target molecules that have applications as pharma-
ceuticals, dyes, and polymers,1 and their reactions with
nucleophiles, often catalyzed by palladium, provide facile routes
to the formation of carbon−carbon and carbon−heteroatom
bonds,1,2 via π-allyl intermediates.3 Allenes also exhibit a diverse
coordination chemistry. Whereas these groups bind to a single
metal almost exclusively in the η2-olefinic mode via one of the π
bonds,4 the presence of a pair of adjacent metals introduces a
number of additional modes, as outlined in Chart 1, for allene
itself,5 in which either one or both sites of unsaturation can be
involved in the bonding. In structure A, the allene is bound in a
μ-κ1:κ1 binding mode through one of the π bonds to form a
dimetallacyclobutane moiety having exocyclic unsaturation.6 In
the related structure B, the unsaturated moiety of A can
additionally coordinate to the distal metal (in the allyl position)
to give the κ1:η3 binding mode.6,7 Allene can also bind in an
η2:η2 fashion in which each π bond interacts with a different
metal, either in an oblique manner (C), in which the allene
ligand is oblique to the metal−metal axis, or in a parallel mode
(D), in which it lies parallel to the metals.7f,j,8 Each of these
Received: April 18, 2012
Published: June 4, 2012
© 2012 American Chemical Society
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binding modes may or may not be accompanied by a metal−
metal bond. Even in the presence of adjacent metals, the allene
ligand can still interact with only one of the metals, much as
observed in mononuclear complexes.4,9
Allenes can also undergo a number of insertion reactions at
transition-metal centers.10 Of primary interest to our group is
the insertion reaction that results in the coupling of allenes and
bridging methylene groups, which can yield a number of
hydrocarbyl fragments, including bridging trimethylenemethane
groups.11 These insertion reactions can be envisioned to occur
by initial allene coordination to one metal center, in one of two
orientations (structures E and G, Scheme 1). Insertion of the
interaction at the group 9 metal.11d In the case of 1,1-
dimethylallene addition to [RhOs(CO)3(μ-CH2)(dppm)2][X]
(X = BF4, CF3SO3; dppm = Ph2PCH2PPh2), the resulting
product was 4-methyl-1,3-pentadiene, presumably via structure
F, followed by a sequence of β-H elimination and reductive
elimination steps.11d The addition of allene to the analogous
methylene-bridged Ir/Ru complex also resulted in β-H
elimination and reductive elimination to generate 1,3-
butadiene, while the addition of substituted allenes yielded
iridacycle products, presumably through an intermediate, such
as F, in which migration of the Ru-bound end of the bridging
hydrocarbyl unit to Ir occurred,13 a probable consequence of
strain within the C3-bridged unit in F, and favored by the
stronger Ir−C bonds. Surprisingly, the addition of a second
equivalent of cumulene to the chelated complexes resulted in
multiple C−H bond activation reactions,13 which were also
observed upon addition of allenes to the related tetracarbonyl
complex, [IrRu(CO)4(dppm)2]+.14 The propensity for Ir to
undergo facile C−H bond activation in this Ir/Ru system led us
to investigate the related Ir/Os complexes in order to
determine the influence of the adjacent Os center.
Scheme 1.
RESULTS AND COMPOUND CHARACTERIZATION
■
Reactivity of [IrOs(CO)3(μ-CH2)(dppm)2]+ with Allenes.
Allene. The methylene-bridged tetracarbonyl complex, [IrOs-
(CO)4(μ-CH2)(dppm)2][BF4] (1), reacts over a 20 h period
with propadiene (allene) to generate an equilibrium mix of two
isomers of the trimethylenemethane-bridged product, [IrOs-
(CO)3(μ-η3:κ1-C(CH2)3)(dppm)2][BF4] (3/3a), in an approx-
imate 4:3 ratio accompanied by CO loss. The same reaction,
carried out using the tricarbonyl analogue, [IrOs(CO)3(μ-
CH2)(dppm)2][BF4] (2), yields the identical isomer mix within
10 h (Scheme 2). To take advantage of the shorter reaction
times, the methylene-bridged tricarbonyl complex (2) is used in
subsequent reactions, in preference to the tetracarbonyl
analogue (1). Differentiating between the two isomers, 3 and
3a, is challenging since neither metal displays coupling to the
allene into the M−CH2 bond can then generate C3-bridged
species (structures F or H, Scheme 1), depending on the allene
orientation prior to insertion. Propanediyl species, such as
structure F, have a tendency to undergo β-H elimination from
the central bridging carbon;12 however, structure H, a
trimethylenemethane-like unit, lacks β-hydrogens, and is not
susceptible to β-H elimination.
Previous work in our group, involving coupling of allenes
with methylene-bridged, mixed-metal complexes of groups 8
and 9 yielded a variety of products depending on the metal
combination and the cumulene. These Rh−M (M = Ru, Os)
systems often generated trimethylenemethane-bridged com-
plexes, presumably a result of rearrangement of the κ1-allylic
moiety in H to a κ1:η3 arrangement, invariably having the η3
1
NMR-active nuclei in the compounds (31P, 13C, H), and the
chemical shifts for the respective nuclei bound to these metals
are in approximately the same regions of the spectra.
Each isomer displays two resonances in the 31P{1H} NMR
spectrum, consistent with an AA’BB’ spin system, in which the
ends of the diphosphines bound to the different metals are
inequivalent, (3: δ −9.5, −27.2; 3a: δ −7.3, −19.2). Assignment
of the observed NMR resonances to 3/3a is accomplished by
Scheme 2.
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Figure 1. (a) Perspective view of the complex cation of [IrOs(CO)3(μ-η3:κ1-C(CH2)3)(dppm)2][CF3SO3] (3/3a-CF3SO3). Thermal ellipsoids are
shown at the 20% probability level except for hydrogens, which are omitted for the phenyl groups but are shown arbitrarily small for the others. The
atoms Ir(A) and Os(A) are refined as a combination of 50% Ir and 50% Os, so the alternate conformation (3a), composed of Ir(B) and Os(B), is
equally abundant. (b) Alternate view of the cation of 3/3a-CF3SO3 with only the ipso-carbons of the phenyl rings shown. Selected parameters
(distances in Å and angles in deg): Ir(A)−Os(A) = 2.9162(4), Ir(A)−C(4) = 2.278(6), Os(A)−C(7) = 2.202(6), C(4)−C(5) = 1.437(8), C(4)−
C(6) = 1.449(8), C(4)−C(7) = 1.456(9), P(1)−Ir(A)−P(3) = 112.69(5); P(2)−Os(A)−P(4) = 174.91(5), C(5)−C(4)−C(6) = 110.0(6). C(6)−
C(4)−C(7) = 124.1(6).
NMR experiments involving heteronuclear decoupling (31P,
13C, 1H), and on literature precedent involving analogous
mononuclear Ir and Os complexes, as outlined in what follows.
1H NMR experiments using selective 31P decoupling establish
that the two downfield signals represent the 31P nuclei bound
to the metal having the κ1-bound methylene unit in both
isomers, while the upfield signals represent the 31P nuclei of the
interaction has one carbonyl attached, while the other metal
has two, consistent with the changing hapticity of the
trimethylenemethane ligand with respect to the two
metals being accompanied by migration of a carbonyl from
metal to metal. In this way, each metal maintains an
18e− configuration.
An X-ray structure determination of 3/3a, in which the
tetrafluoroborate anion has been exchanged by triflate, is shown
for the complex cation in Figure 1, and confirms the μ-η3:κ1-
bonding of the trimethylenemethane moiety and the
accompanying carbonyl arrangement suggested above. Tables
of bond lengths and angles for 3/3a-CF3SO3 appear in the
Supporting Information. Differentiating these two metals using
crystallographic methods is challenging, owing to their similar
X-ray scattering amplitudes; however, the best refinement
results were obtained using a 1:1 disorder of the metal
positions, as described in the Experimental Section. Dissolving
the crystals used for X-ray analysis in CD2Cl2 shows the
presence of both isomers (by 31P{1H} NMR) in the same
equilibrium 4:3 mix observed in earlier spectroscopy.
Furthermore, as noted, both metals have saturated 18e−
configurations in both isomers, so the metals cannot be
distinguished on the basis of their electronic preferences.
The Ir−Os separation of 2.9162(4) Å is somewhat shorter
than the intraligand P−P distances of ca. 3.09 Å, suggesting a
metal−metal bonding interaction resulting in compression
along the axis of the metals. However, the nature of the metal−
metal bond and the corresponding metal oxidation states are
uncertain. In the formulations shown in Scheme 2, an Ir(I)/
Os(II) formulation is accompanied by a dative Ir → Os bond,
although an alternate formulation has an Ir(II)/Os(I)
formulation and a conventional Ir−Os bond. We favor the
former description since it corresponds to the favored oxidation
states of the metals.
1
metal having the η3-allyl-type interaction. In the H NMR
spectrum, the CH2 protons of the dppm ligands appear at δ
5.04 and 4.46 (3) and δ 5.62 and 4.94 (3a), with an average
mutual two-bond coupling of 15.5 Hz; however, these
resonances give no significant structural insight. The protons
from the η3-allyl-type interaction appear at δ 2.72 and 1.29 in
3a, and δ 3.55 and 1.94 in 3. These data can be compared to
known allyl complexes of Os15 and Ir3c,9 in which the Ir-bound
allyl protons are often observed at lower field than those of Os-
bound groups. Interestingly, the CH2 protons of the κ1-bound
methylene group appear at very different chemical shifts in 3 (δ
1.74, 3JHP(δ−9.5) = 10.7 Hz) and 3a (δ 4.35, 3JHP(δ−7.3) = 4.8 Hz)
and aid in the identification of each isomer, on the basis that
Os-bound κ1-CH2 protons have been shown to appear upfield
of Ir-bound CH2 protons in related bridging hydrocarbyl
complexes.16 Proton spin saturation transfer experiments also
support the equilibrium proposed above, and irradiating the
proton resonance at δ 1.74 in 3 results in a decrease in intensity
of the resonance at δ 4.35, and vice versa. In a 13CH2-enriched
sample of 3/3a, obtained from [IrOs(CO)3(μ-13CH2)-
(dppm)2][BF4] (2-13C), the 13C{1H} NMR spectrum has
one broad signal at δ −9.5 for the labeled carbon of the κ1-
methylene in both isomers; however, scrambling of the 13CH2
label is also observed over the other two equivalent methylene
groups of the trimethylenemethane unit in both isomers, the
broad signal for which appears at δ 64.4, consistent with the
exchange process suggested above. 13C NMR experiments on
The geometry about the monocarbonyl end, Ir(A)/Os(B),
can be viewed as slightly distorted trigonal-bipyramidal if the
η3-allyl moiety is viewed as occupying a single coordination site
a
13CO-enriched sample, with selective 1H and 31P
decoupling, confirm that the metal with the η3-allyl-type
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Scheme 3.
Scheme 4.
or as distorted octahedral if occupying two sites, while the
dicarbonyl end, (Os(A)/Ir(B)) is clearly octahedral. Within the
trimethylenemethane moiety, the C(4)−C(7) bond length
(1.456(9) Å) is shorter than expected for a single bond, while
the C(4)−C(5) and C(4)−C(6) bond lengths (1.437(8) and
1.449(8) Å, respectively) are intermediate between double and
single bonds, as expected for an η3-allyl group.17 These bond
lengths are consistent with those reported for other metal-
bound trimethylenemethane units.11c,18 The bridging diphos-
phines have a cis arrangement at the metal that is bound to the
η3-allylic fragment (P(1)−Ir(A)−P(3) = 112.69(5)°) and a
trans arrangement at the other (P(2)−Os(A)−P(4) =
174.91(5)°; see Figure 1b), as seen in a number of such
compounds.9,11d
Attempts to observe intermediates in the formation of 3/3a
by the addition of allene to a CD2Cl2 solution of 2 at −78 °C
resulted in the observation of several compounds at this
temperature, for which the peaks in the 31P{1H} NMR
spectrum were broad and overlapping, preventing their
characterization. Upon warming to +10 °C, the formation of
3/3a is observed in small amounts (ca. 10%) along with
the aforementioned unidentified species, and warming to
ambient temperature results in complete conversion to 3/3a
within 10 h.
Methylallene. Compound 2-CF3SO3 also reacts with 1,2-
butadiene (methylallene) over a 10 h period, generating two
isomers of [IrOs(CO)3(μ-η3:κ1-C(CHCH3)(CH2)2)(dppm)2]-
[CF3SO3] (4/4a) in a 1:1 ratio, as shown in Scheme 3. In this
case, isomers 4 and 4a have the same orientation of the
hydrocarbyl fragment with respect to the different metals and
identical orientations of CO ligands; the substituted trimethy-
lenemethane unit differs only in the orientation of the methyl
group of the η3-allyl-type moiety, being either syn or anti to the
Os-bound CH2 group.
The 31P{1H} NMR spectrum of compounds 4 and 4a
contains eight resonances, representing the four chemically
inequivalent 31P nuclei for each isomer. These resonances are
often overlapping, making the differentiation of 4 and 4a
1
difficult. The H NMR spectrum for 4/4a consists of eight
resonances for the dppm CH2 protons and three signals for the
four CH2 protons of the η3-allyl moiety of both isomers (δ 5.20,
3.79, 2.90) in a 1:1:2 ratio, the last resulting from overlap of the
two olefinic anti proton resonances of the two isomers. The
methyl groups of the two isomers appear at δ 1.60 and 1.10 and
their adjacent protons at δ 3.12 and 4.69, respectively, while
there are two closely spaced signals for the four protons of the
Os-bound CH2 groups at δ 1.84 and 1.80. The chemical shift of
these CH2 protons supports the orientation of the η3-allyl-type
moiety (bound to Ir as opposed to Os), as shown in Scheme 3;
for the reverse orientation, the signal for the κ1-CH2 protons
would be expected near δ 4.16 Previous work on 1-methyl-
substituted allyl groups bound to rhodium19 reports syn
protons adjacent to methyl groups appearing between δ 4.06
and 5.19 and anti protons adjacent to methyl groups appearing
upfield between δ 3.25 and 3.65. Related iridium complexes are
reported to have upfield chemical shifts for methyl protons
when the methyl group is anti, as opposed to syn.20 This
information suggests that the methyl resonance at δ 1.60
corresponds to the methyl group being syn, and the resonance
at δ 1.10 corresponds to the anti isomer with respect to the κ1-
CH2 group. The 13C{1H} NMR spectrum displays five CO
resonances (δ 185.4, 177.9, 177.0, 171.0, and 162.1) in a
2:1:1:1:1 ratio, consistent with each isomer having three
carbonyls; however, the overlap of the 31P NMR signals
prevents the use of selective 31P-decoupling experiments to
accurately assign the carbonyls to a specific isomer or metal. A
sample of compounds 4/4a, prepared from 13CH2-enriched 2-
CF3SO3 displays only one signal for this methylene group at δ
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Figure 2. Simulated and observed 31P{1H} NMR spectra of 5 (in CD2Cl2); labeling scheme shown as inset.
−9.5 in the 13C{1H} NMR spectrum, consistent with it
remaining on Os.
resonances for the three carbonyls (δ 186.9, 178.0, 173.9). A
crystal structure determination of an analogous complex
generated from allene (vide infra) has the η3-allyl interaction
at Ir and has similar allylic proton shifts in the H NMR
spectrum, also supporting the orientation of 5, as shown in
1,1-Dimethylallene. The methylene-bridged compound, 2-
CF3SO3, also reacts with excess 1,1-dimethylallene at ambient
temperature over an 8 h period to form an unexpected μ-η3:κ1-
product, [IrOs(CO)3(μ-η3:κ1-CH2CCMe2)(dppm)2]-
[CF3SO3] (5), as shown in Scheme 4. The 31P{1H} NMR
spectrum of 5 displays four resonances characteristic of an
ABCD spin system in which all four 31P nuclei are chemically
inequivalent (see Figure 2 for labeling). The two-bond PA−PC
coupling constant is 254.9 Hz, typical for 31P nuclei in a
mutually trans arrangement, whereas the smaller PB−PD
coupling constant of 23.7 Hz suggests a cis arrangement.
Intraligand two-bond PA−PB coupling (2JPP = 64.6 Hz) and
PC−PD coupling (2JPP = 53.1 Hz) are also observed, as well as
PA−PD coupling (JPP = 14.0 Hz) and PB−PC coupling (JPP = 7.6
Hz). A simulation of the 31P{1H} NMR spectrum using the
above coupling constants supports the structure proposed (see
1
Scheme 4.
Surprisingly, this product contains the dimethylallene moiety
without the original metal-bridged “CH2” group of compound
2. A clue regarding the fate of this “CH2” group is the presence
of 4-methyl-1,3-pentadiene, the additional product of this
reaction, which has resulted from coupling of the cumulene
with the methylene group of 2, as has previously been
reported.11d,13 This diene compound was characterized by H
1
NMR spectroscopy. In the reaction of a 13CH2-enriched sample
of 2 with dimethylallene, the terminal CH2 group in this
pentadiene product appears in the 13C{1H} NMR spectrum at
δ 115.1. We propose that the formation of 4-methyl-1,3-
pentadiene results from a sequence of cumulene insertion, β-
hydride elimination, and reductive elimination steps, similar to
a proposal in our previous studies for the elimination of 1,3-
butadiene in the reaction of [IrRu(CO)4(μ-CH2)(dppm)2]+
with allene.13
1
Figure 2). The H NMR spectrum is as expected for the
formulation shown, with four signals for the dppm CH2
protons (δ 4.70, 4.65, 3.81, and 3.24), two signals for the
methyl groups (δ 1.68 and 1.63), and two signals for the two
CH2 protons of the η3-allyl group (δ 3.97 (syn), 2.17 (anti)).
The chemical shifts of the allylic protons are indicative of an η3-
allyl-type interaction with Ir, being downfield from those of Os-
bound η3-allylic protons (ca. δ 2 and 1, for syn and anti,
respectively).3c,9,15 The 13C{1H} NMR spectrum displays three
The reaction of 2 with 1,1-difluoroallene yielded several
unidentified products, even when carried out at low temper-
ature. This result is puzzling, as this substrate reacted cleanly
with the analogous methylene-bridged Ir/Ru complex,13
yielding an Ir chelate similar to the alkyl-substituted allenes.
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Reactivity of [IrOs(CO)4(dppm)2][BF4] with Allenes.
The proposal above that the μ-η3:κ1-1,1-dimethylallene adduct
(5) results from dimethylallene attack on the tricarbonyl
species, [IrOs(CO)3(dppm)2]+, suggested that such products
could also be obtained with other allenes. Since this proposed
tricarbonyl complex is unstable and decomposes rapidly in
solution, investigations instead focused on the reactions
involving [IrOs(CO)4(dppm)2]+ (6).
Allene and Methylallene. The tetracarbonyl complex,
[IrOs(CO)4(dppm)2][BF4] (6), reacts with allene and
methylallene at ambient temperature over 4 h to generate the
corresponding products, [IrOs(CO)3(μ-η3:κ1-CH2CCH2)-
(dppm)2][BF4] (7), and [IrOs(CO)3(μ-η3:κ1-CH2CCHCH3)-
(dppm)2][BF4] (8), respectively, accompanied by carbonyl
loss. As proposed for 5, the cumulene in 7 and 8 is bound κ1 to
Os by the central carbon, and η3 to Ir, as shown in Scheme 5.
substituted trimethylenemethane complexes 4 and 4a, only one
isomer of 8 is observed. As was observed earlier, both 7 and 8
display a cis/trans arrangement of disphosphines at the
1
different metals. The H NMR spectrum of 7 displays two
resonances for the dppm CH2 protons at δ 5.61 and 4.96 (2JHH
=
15.1 Hz), while the resonances for the metalaallyl protons
appear at δ 4.35 (syn) and 3.55 (anti) and are similar to the
resonances observed in a crystallographically characterized
9
1
diiridium complex. The H NMR spectrum of 8 has four
dppm CH2 proton signals (δ 5.64, 5.57, 5.05, and 4.84), two
signals for the syn and anti CH2 protons of the allene (δ 5.20
and 3.79, respectively), one signal for the proton adjacent to
the methyl group (δ 4.61), and one signal for the methyl
protons (δ 1.09), with the latter two showing mutual three-
bond coupling of 6.1 Hz. These data suggest that the methyl
group is anti to the Os−C bond,9,11d,19 and the chemical shifts
of the olefinic CH2 protons of the allene suggest an η3-binding
mode to Ir.3c,9 The 13C{1H} NMR spectrum of a 13CO-
enriched sample of 7 confirms the loss of a carbonyl ligand with
only three equal-intensity signals observed (δ 185.9, 178.0, and
171.7), in which the two downfield signals couple to the set of
31P nuclei at δ −7.4, and the upfield signal couples to the other
set of 31P nuclei at δ −19.3. In a 13CO-enriched sample of 8, the
carbonyl resonances appear at δ 185.9, 178.3, and 171.5; here,
the two downfield carbonyls show coupling to the 31P nuclei at
δ −7.3, while the upfield carbonyl couples to the 31P nuclei at δ
−16.8 and −19.9.
Scheme 5.
An X-ray structure determination of 7 verifies the geometry
proposed above, in which the hydrocarbyl fragment is bound κ1
to Os and η3 to Ir. The complex cation of compound 7 is
shown in Figure 3. Tables of selected bond lengths and angles
for 7 are given in the Supporting Information. Within the
bridging allene group, the C(4)−C(5) and C(5)−C(6) bond
lengths (1.431(6) and 1.430(6) Å, respectively) and C(4)−
C(5)−C(6) angle of 106.7(4)° are typical of an η3-bound
allyl.9,17,21 Although not common, similar bridging allene
coordination has been reported.7a−c,f,i,j,9,22 The central atom
Removal of a carbonyl from [IrOs(CO)4(dppm)2][BF4] (6)
using trimethylamine N-oxide (TMNO), in the presence of
allene or methylallene, instantly generates the same products.
The 31P{1H} NMR spectrum of 7 displays two resonances at
δ −7.4 and −19.3, consistent with the chemical inequivalence
of the Ir- and Os-bound ends of the diphosphines, whereas 8
displays three resonances at δ −7.3, −16.8, and −19.9, in a
2:1:1 ratio, with the higher-intensity signal resulting from
coincidental overlap of the resonances of two of the four
inequivalent 31P nuclei. Unlike the earlier study of the methyl-
Figure 3. (a) Perspective view of the complex cation of [IrOs(CO)3(μ-η3:κ1-H2CCCH2)(dppm)2][BF4] (7). Thermal ellipsoids are as in Figure 1.
(b) Alternate view of the cation of 7 with only the ipso-carbons of the phenyl rings shown. Selected parameters (distances in Å and angles in deg):
Ir−Os = 2.8407(2), Ir−C(5) = 2.115(4), Os−C(5) = 2.120(4), C(4)−C(5) = 1.431(6), C(5)−C(6) = 1.430(6); C(4)−C(5)−C(6) = 106.7(4),
P(1)−Ir−P(3) = 111.80(4), P(2)−Os−P(4) = 170.34(4).
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of the allyl fragment is essentially equidistant from both metals
(Ir−C(5) = 2.115(4) Å, Os−C(5) = 2.120(4) Å), and the Ir−
Os separation of 2.8407(2) Å indicates compression along the
Ir−Os axis compared with the intraligand P−P separations of
ca. 3.07 Å. The P(1)−Ir−P(3) angle of 111.80(4)° is acute,
being bent away from the η3-allyl moiety in a cis arrangement,
while the almost linear P(2)−Os−P(4) angle (170.34(4)°)
indicates a trans arrangement at the Os, as clearly shown in
Figure 3b.
1,1-Dimethylallene. Compound 6-CF3SO3 also reacts with
excess 1,1-dimethylallene at ambient temperature over 12 h, in
this case, giving a very different product, [IrOs(CO)4(μ-C
C(H)C(CH3)CH2)(dppm)2][CF3SO3] (9), having a bridg-
ing 3-methyl-1,3-butadienylidene fragment, as shown in
Scheme 6, resulting from the activation of the two geminal
when carried out at slightly elevated temperatures. The Ir/Ru
analogue also did not react with this substrate.14
If the reaction of 6 with 1,1-dimethylallene is attempted in
the presence of the decarbonylating agent, TMNO, the
resultant product obtained immediately is the cumulene adduct,
[IrOs(CO)3(μ-η3:κ1-CH2CCMe2)(dppm)2][CF3SO3] (5), the
same product observed earlier in the reaction of the methylene-
bridged complex (2) with excess 1,1-dimethylallene.
1,1-Difluoroallene. Compound 6-CF3SO3 also reacts with
1,1-difluoroallene within 1 h at ambient temperature to form
the adduct, [IrOs(CO)4(μ-κ1:κ1-F2CCCH2)(dppm)2]-
[CF3SO3] (10), in which the fluoroallene bridges both metals
(Scheme 7). The 31P{1H} NMR spectrum displays the
Scheme 7.
Scheme 6.
1
expected two resonances, at δ −12.0 and −19.5. In the H
NMR spectrum, the dppm CH2 protons appear at δ 4.79 and
4.46, while the CH2 protons of the fluoroallene ligand appear as
a triplet at δ 1.55, coupling to the 31P nuclei at δ −12.0 (3JHP
=
11.8 Hz). The observation of one allene CH2 resonance is
consistent with binding through this end of the cumulene, as is
typically observed.8j,9,14,24 The upfield chemical shift of the CH2
protons also suggests that they are adjacent to Os and not Ir,16
the latter of which characteristically resonate downfield at ca. δ
4. Binding of the 1,1-difluoroallene through the “CCH2” end
is confirmed in the 19F NMR spectrum by the pair of
resonances at δ −74.3 and −92.2 for the two chemically
inequivalent fluorines, having a mutual two-bond coupling of
C−H bonds of the cumulene, and one C−H bond of a methyl
group, with the concomitant evolution of H2. This reactivity is
reminiscent of that observed in previous work with the related
[IrRu(CO)4(dppm)2]+ system.14
The 31P{1H} NMR spectrum of 9 displays two signals, at δ
−5.2 and −9.2, ruling out the formation of a μ-η3:κ1 binding
mode, as observed for allene and methylallene, which is
expected, in this case, to give rise to four different 31P
environments. Four signals for the vinylvinylidene fragment
appear in the 1H NMR spectrum: the methyl group is observed
as a multiplet at δ 0.41, the lone vinylidene proton resonates
characteristically downfield as a broad multiplet at δ 8.01, and
the two geminal protons can be observed as doublets of multi-
plets at δ 4.69 and 4.46, with a mutual two-bond coupling of
1.6 Hz. The data obtained from the 13C{1H} NMR spectrum
support the carbonyl arrangement shown in Scheme 6, in
which three carbonyls (δ 194.9, 175.5, 171.8) are bound to
one metal (Os) and one carbonyl (δ 179.9) is bound to the
other (Ir). Although the most downfield chemical shift is sug-
gestive of a semibridging interaction, there is no resolvable
coupling observed between it and the Ir-bound 31P nuclei.
This carbonyl arrangement is not surprising, given the lower
tendency for third-row metals to support bridging carbonyls.23
Two of the Os-bound carbonyls (δ 194.9 and 171.8) show
mutual two-bond coupling of 26.2 Hz in a 13CO-enriched
sample, suggesting a mutually trans arrangement, as shown in
Scheme 6.
60.4 Hz, which is typical for such an arrangement.9,25
A
13CO-
enriched sample of 10 displays four resonances in the 13C{1H}
NMR spectrum at δ 183.4, 180.2, 177.5, and 154.2, and
selective 31P-decoupling experiments determine that two
carbonyls are bound terminally to each metal.
An X-ray structure determination of 10 confirms the
proposed connectivity in which the protons of the difluor-
oallene ligand are adjacent to Os, as shown in Figure 4. Tables
of bond lengths and angles for 10 are given in the Supporting
Information. Within the bridging difluoroallene unit, the C(5)−
C(6) bond length of 1.511(4) Å is typical of a single bond
between sp2 and sp3 carbons.17 Both metals have a distorted
octahedral coordination in which the sixth site is the metal−
metal bond.
Rearrangement of the bridging difluoroallene to a μ-η3:κ1
hydrocarbyl fragment, as observed with allene and methylallene,
is never observed, with the κ1:κ1-difluoroallene arrangement
being retained upon heating to 40 °C for 8 h, after which time,
decomposition occurs. The η3-allyl binding mode has
previously been reported for 1,1-difluoroallene in only two
multimetallic systems.8j,24,26
Compound 9 can be viewed as resulting from the geminal
activation of both C−H bonds at the end of 2-methyl-1,3-
butadiene that is γ to the methyl substituent, with the
accompanying loss of H2. However, attempts to generate 9
by reaction of 6 with this substrate does not occur in the
presence of 100 equiv of the substrate over several days, even
DISCUSSION
■
The products obtained in the reactions of the methylene-
bridged compound [IrOs(CO)3(μ-CH2)(dppm)2]+ (2) and the
tetracarbonyl complex [IrOs(CO)4(dppm)2]+ (6) with a
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the group 9 metal. The bridging carbonyl in these latter systems
provides incipient unsaturation at the group 9 metal, by facile
movement to a terminal position on the group 8 metal with
accompanying cleavage of the metal−metal bond upon the
introduction of substrates at the other metal. Without this
incipient unsaturation, the reactions of [IrOs(CO)4(μ-CH2)-
(dppm)2]+ (1) with allenes (and also with alkynes16) are much
slower than with the other metal combinations. We, therefore,
investigated the reactivity of the tricarbonyl analogue, [IrOs-
(CO)3(μ-CH2)(dppm)2]+ (2), which reacts much more readily.
As part of our strategy to generate bridging hydrocarbyl
fragments, we anticipated that allene insertion into the Ir−CH2
bond could occur in one of two ways, as shown earlier in
Scheme 1, to give either structure F or H, depending on the
allene orientation upon coordination at one metal, which we
propose is dictated by steric effects. Neither insertion product F
nor H is observed in this study, although a compound having a
structure like F was observed in an earlier study involving the
Rh/Os metal combination.11d Despite our failure to observe
these species, the subsequent rearranged products provide
insight into the initial coordination modes and insertion
pathways.
Reaction of the methylene-bridged tricarbonyl, [IrOs-
(CO)3(μ-CH2)(dppm)2]+ (2), with allene and methylallene
results in the formation of the μ-η3:κ1-complexes, [IrOs-
(CO)3(μ-η3:κ1-C(CH2)3)(dppm)2][BF4] (3/3a) and [IrOs-
(CO)3(μ-η3:κ1-C(CHCH3)(CH2)2)(dppm)2][CF3SO3] (4/
4a), respectively (see Schemes 2 and 3). The generation of
trimethylenemethane complexes (as in 3/3a) has been documented
in both mononuclear18,27 and binuclear systems.11a,c−e,28
We propose that the μ-η3:κ1-binding mode of this group
proceeds via intermediate H, having the unsaturated “C
CHR” fragment in the allyl position with respect to each metal.
Facile conversion of the κ1- to η3-allyl binding is well-
documented,29 so conversion of intermediate H to the μ-
η3:κ1- binding mode is hardly surprising, representing the
transformation of a κ1-allylic moiety (relative to both metal
centers), to an η3-allyl bridging mode at one end of the
trimethylenemethane unit. The equilibrium between isomers 3
and 3a, in which the η3:κ1-hapticity of the trimethylenemethane
group alternates from metal to metal, does not appear to be
unusual, having been shown to occur in other bimetallic
systems.11c,e This rearrangement has been confirmed by labeling
Figure 4. Perspective view of the complex cation of [IrOs(CO)4(μ-
κ1:κ1-F2CCCH2)(dppm)2][CF3SO3] (10). Thermal ellipsoids are
as shown in Figure 1. Selected parameters (distances in Å and angles in
deg): Ir−Os = 2.9095(2), Ir−C(5) = 2.111(3), Os−C(6) = 2.219(3),
C(5)−C(6) = 1.511(4), C(5)−C(7) = 1.298(4), Ir−C(5)−C(6) =
112.1(2); Ir−C(5)−C(7) = 126.4(2), Os−C(6)−C(5) = 105.7(2),
C(6)−C(5)−C(7) = 121.5(3).
number of allenes, as described in this paper, display a
surprising structural diversity in comparison to those of the
other closely related group 8/group 9 metal combinations
previously studied in our group (Rh/Ru,11d Rh/Os,11d and Ir/
Ru13,14). Although the Rh-based systems display significant
differences in reactivity compared with the Ir/Ru system, the
current Ir/Os system demonstrates essentially all of the diverse
reactions observed in the previous three metal combinations,
depending on both the allene and the metal complex
investigated.
Reactivity of [IrOs(CO)3(μ-CH2)(dppm)2]+ with Allenes.
As discussed in an earlier paper,16 [IrOs(CO)4(μ-CH2)-
(dppm)2]+ (1) has a different structure than the Rh/Ru, Rh/
Os, and Ir/Ru combinations of metals, having all carbonyls
terminal and symmetrically distributed, two on each metal. In
contrast, all the other metal combinations have two terminal
carbonyls on the group 8 metal, one bridging, and only one on
Scheme 8.
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studies in which the 13CH2 group of the 13CH2-labeled precursor
(2) becomes incorporated into all three methylene positions of
the trimethylenemethane ligand upon allene insertion. This
scrambling process can be rationalized by a series of η3:κ1 to κ1:κ1
to η3:κ1 transformations, as shown (in abbreviated form) in
Scheme 8 (dppm groups omitted for clarity).
via β-H elimination and reductive elimination steps. Again, it
appears that cumulene coordination as in E is favored in a more
sterically congested environment, in this case, a result of double
substitution on the cumulene. In the Rh/Os case, the
unsaturated “[RhOs(CO)3(dppm)2]+” fragment that results
from loss of the pentadiene unit decomposed immediately.
However, in the current study, the resulting “[IrOs-
(CO)3(dppm)2]+” fragment reacts with excess 1,1-dimethylal-
lene to generate a μ-η3:κ1 complex, [IrOs(CO)3(μ-η3:κ1-
CH2CCMe2)(dppm)2][CF3SO3] (5). In the case of [IrRu-
(CO)4(μ-CH2)(dppm)2]+, reaction with 1,1-dimethylallene
again appeared to proceed through intermediates E and F,
although, in this case, migration of the Ru-end of the bridging
hydrocarbyl unit to Ir was favored instead of β-H elimination,
giving an Ir chelate.13
In the previously reported reactions of the methylene-
bridged Ir/Ru complex, [IrRu(CO)4(μ-CH2)(dppm)2]+, with
methyl- and dimethylallene, incorporation of a second
equivalent of cumulene13 was observed, accompanied by
subsequent C−H bond activations of the second substrate.
No evidence of incorporation of a second cumulene or of C−H
activation is observed with [IrOs(CO)3(μ-CH2)(dppm)2]+ (2),
even in the presence of a large excess of substrate. The related
tetracarbonyl complex [IrRu(CO)4(dppm)2]+14 was also shown
to effect the activation of multiple C−H bonds in allenes, so we
sought to investigate the analogous Ir/Os compound to test its
propensity for C−H activation.
Unlike isomers 3 and 3a, the isomers, [IrOs(CO)3(μ-η3:κ1-
C(CHCH3)(CH2)2)(dppm)2][CF3SO3] (4/4a), resulting from
methylallene addition to 2, are static, displaying only η3-binding
of the substituted trimethylenemethane unit to Ir. As a
consequence, no 13CH2 scrambling is observed in this case,
and this label remains κ1-bound to Os. The two isomers in this
case result from syn/anti positioning of the methyl substituent
on the allylic portion of the ligand. The failure of 4/4a to
reverse hapticity from metal to metal, as observed for 3/3a, is
somewhat puzzling, since the introduction of a single methyl
substituent would not appear to introduce unusual steric
demands. However, it may be that the hapticity exchange,
combined with carbonyl migration from metal to metal and the
concomitant realignment of the diphosphines from mutually cis
at one metal to mutually trans, is inhibited by the additional
substituent on the trimethylenemethane ligand.
The generation of a μ-η3:κ1-trimethylenemethane fragment
was also observed upon reaction of allene with the methylene-
bridged Rh/Ru and Rh/Os analogues. However, in these
systems, additional facile carbonyl loss occurred from Rh,
generating a coordinatively unsaturated “(η3-allyl)RhP2” frag-
ment, consistent with the greater lability of Rh compared with
that of Ir. The failure of this Rh-based system to undergo the
κ1/η3 isomerization observed for 3 and 3a can be rationalized
on the basis that the κ1:κ1 intermediate, analogous to that
shown in Scheme 8, introduces additional coordinative
unsaturation to an already unsaturated system, which is
presumably unfavorable.
The reactivity observed upon the addition of allene to
[IrOs(CO)4(μ-CH2)(dppm)2]+ (1) or [IrOs(CO)3(μ-CH2)-
(dppm)2]+ (2), in which the bridging η3:κ1-trimethylene-
methane unit is generated, is markedly different from that
observed for the related Ir/Ru complex, [IrRu(CO)4(μ-
CH2)(dppm)2]+, which generates 1,3-butadiene through
allene−methylene coupling.13 The loss of this diene fragment
for Ir/Ru suggests that allene coordination and subsequent
insertion into the Ir−CH2 bond occurs via structures similar to
E and F (Scheme 1), instead of through intermediates G and
H, possibly attributable to a more crowded environment in the
Ir/Ru complexes due to retention of a fourth carbonyl ligand
throughout the reaction. The β-hydrogens present in this
binding mode provide a route to the common decomposition
pathway12 through β-H elimination, to yield 1,3-butadiene. The
apparent influence of greater steric crowding in [IrRu(CO)4(μ-
CH2)(dppm)2]+ was also evident in its reaction with
methylallene,13 which again inserts via intermediates E and F,
although, in this case, the hydrocarbyl fragment migrates to a
chelating position on Ir, instead of undergoing the β-hydrogen
elimination. The formation of the μ-η3:κ1-trimethylenemethane
fragments was also observed in the reactions of allene and
methylallene with the less crowded tricarbonyl Rh-based
complexes, [RhM(CO)3(μ-CH2)(dppm)2]+ (M = Os, Ru).11d
The reaction of [IrOs(CO)3(μ-CH2)(dppm)2]+ (2) with 1,1-
dimethylallene appears to proceed much as it does with
[RhOs(CO)3(μ-CH2)(dppm)2]+,11d in which the dimethylal-
lene couples with the methylene group (structures E and F,
Scheme 1) and is lost as 4-methyl-1,3-pentadiene, presumably
Reactivity of [IrOs(CO)4(dppm)2]+ with Allenes. The
addition of allene and methylallene to the tetracarbonyl
complex [IrOs(CO)4(dppm)2]+ (6) results in the formation
of the η3:κ1-cumulene-bridged complexes, [IrOs(CO)3(μ-η3:κ1-
C(CH2)2)(dppm)2][BF4] (7) and [IrOs(CO)3(μ-η3:κ1-C-
(CH2)(CHCH3)(dppm)2][BF4] (8), respectively, analogous
to 5, accompanied by loss of a carbonyl ligand. This
coordination mode of allene and methylallene has been
documented in related Ir2 complexes,9,30 and other multi-
metallic systems.7a−c,f,i,j,22 We assume that cumulene coordina-
tion at Ir leads to migration to the bridging position, as shown
for structure I in Chart 2, followed by κ1-to-η3 coordination at
Chart 2.
Ir, not unlike that proposed in the transition from structure H
to the μ-η3:κ1 binding of the trimethylenemethane described
earlier.
This reactivity with allene and methylallene, observed with
Ir/Os, is completely different than that observed with the
analogous [IrRu(CO)4(dppm)2]+ complex,14 in which the
activation of two geminal C−H bonds occurs with accompany-
ing H migrations to generate the corresponding vinylidene-
bridged complexes, [IrRu(CO)4(μ-CCHR)(dppm)2]+ (R =
CH3, C2H5, respectively). We assume that the strong M−C
bonds in the Ir/Os species favor retention of the bridging
hydrocarbyl unit and does not allow the activation of C−H
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modification of the published procedures,9,33 in which a solution of
100 μL of 2-bromo-3,3,3-trifluoro-1-propene (purchased from
Aldrich) in 3 mL of diethyl ether was cooled to −90 °C in an
acetone/liquid N2 bath. To this was added dropwise 385 μL of a 2.5 M
solution of n-butyllithium in hexanes (purchased from Aldrich),
followed by stirring of the solution at this temperature for 30 min. The
−90 °C bath was replaced by an acetonitrile/CO2(s) bath (ca. −40
°C), and the solution was stirred for 15 min. The solution was then
slowly warmed over 15 min to −20 °C, resulting in the evolution of
1,1-difluoroallene gas.
bonds. It appears that, in these cases, the bridging modes do
not result in C−H bond activation.
Surprisingly, the only case of multiple C−H bond activation
with the Ir/Os metal combination occurs in the reaction of
[IrOs(CO)4(dppm)2]+ (6) with 1,1-dimethylallene. As with the
Ir/Ru system,14 double, geminal C−H activation of the
cumulene is accompanied by C−H activation of a methyl
substituent. We propose that this sole example of C−H bond
activation in this Ir/Os system is a result of steric crowding,
which we assume does not favor the otherwise unreactive
bridging modes, favoring instead C−H activation. The
influence of steric crowding in dictating the reactivity of the
allenes is clearly seen in the differing reactivities of [IrOs-
(CO)4(dppm)2]+ (6) and “[IrOs(CO)3(dppm)2]+” (generated
in situ as shown in Scheme 4) with 1,1-dimethylallene. While
the tetracarbonyl complex results in C−H activation, the less
sterically crowded, coordinatively unsaturated tricarbonyl
complex yields the bridged η3:κ1 adduct, much as observed
for allene and methylallene.
The 1H and 31P{1H} NMR spectra were recorded on a Varian Inova
400 spectrometer operating at 399.9 and 161.8 MHz, respectively. All
low-temperature spectra and the heteronuclear decoupling experi-
ments (13C{1H} and 13C{1H,31P}) were recorded on a Varian Unity
spectrometer operating at 161.9 MHz for 13C, 202.3 MHz for 31P, and
499.8 MHz for 1H. Mass spectrometry was performed on a Micromass
ZabSpec TOF spectrometer by the mass spectrometry facility of this
department, and elemental analyses were also carried out in the
departmental facility.
Preparation of Compounds. Spectroscopic data for the
compounds are presented in Table 1.
[IrOs(CO)3(μ-η3:κ1-C(CH2)3)(dppm)2][BF4] (3/3a). Under an argon
atmosphere, allene was passed through a stirred red solution of 2 (42
mg, 0.031 mmol) in 10 mL of CH2Cl2 for 1 min at a rate of
approximately 0.1 mL/s, resulting in an immediate color change of the
solution to yellow. The solution was left to stir under an allene
atmosphere for 10 h, after which time 30 mL of diethyl ether and 10
mL of pentane were added to a yellow/orange solution to precipitate a
light yellow solid. This solid was recrystallized from CH2Cl2/diethyl
ether, washed with 2 × 5 mL of diethyl ether, and dried in vacuo (85%
yield). HRMS: m/z calcd for C58H50IrO3OsP4 (M+), 1291.1949;
found, 1291.1941 (M+). Anal. Calcd for C58F3H50IrO6OsP4S (3/3a-
CF3SO3): C, 48.43; H, 3.50. Found: C, 48.14; H, 3.61.
Interestingly, the 1,1-difluorallene adduct of [IrOs-
(CO)4(dppm)2]+ (6), namely, [IrOs(CO)4(μ-κ1:κ1-F2CC
CH2)(dppm)2][CF3SO3] (10), displays the μ-κ1:κ1-binding
mode as proposed above in Chart 2, except that the binding is
reversed, with the central carbon of the fluoroallene bound to Ir
instead of to Os. In this case, however, rearrangement to the μ-
η3:κ1-mode is not observed and the 18e−/18e− count is
maintained by retention of all four carbonyls. Presumably, the
failure of this complex to transform into the μ-η3:κ1-binding
mode is related to the poor donor ability of the η3-allyl moiety
having electronegative fluorines attached, and its consequent
inability to displace a carbonyl. In all cases,9,14 when the
fluoroallene bridges the metals, no C−H activation is observed,
suggesting that the μ-κ1-κ1 arrangement, like the other bridging
modes, is not conducive to C−H activation.
Although the chemistry of the Ir/Os compounds with
allenes, described herein, is analogous to that described
previously in closely related Ir2,9 Rh/Ru,11d Rh/Os,11d and
Ir/Ru13,14 systems, the chemistry of this Ir/Os system is more
diverse than that of any one of the above metal combinations,
demonstrating almost the full spectrum of reactivities reported
for the other four groups. In the current Ir/Os series, the
comparable Ir−C and Os−C bond strengths appear to favor a
bridging coordination mode for the hydrocarbyl fragment,
which appears to be unreactive to subsequent transformations,
such as C−H bond cleavage.
[IrOs(CO)3(μ-η3:κ1-C(CHCH3)(CH2)2)(dppm)2][CF3SO3] (4/4a).
Under an argon atmosphere, methylallene was passed through a
stirred red solution of 2-CF3SO3 (33 mg, 0.024 mmol) in 10 mL of
CH2Cl2 for 1 min at a rate of approximately 0.1 mL/s, resulting in an
immediate color change of the solution to yellow. The solution was
left to stir under a methylallene atmosphere for 10 h, after which time
30 mL of diethyl ether and 10 mL of pentane were added to the
yellow/orange solution to precipitate a light yellow solid. This solid
was recrystallized from CH2Cl2/diethyl ether, washed with 2 × 5 mL
of diethyl ether, and dried in vacuo (83% yield). HRMS: m/z calcd for
C59H52IrO3OsP4 (M+), 1215.1493; found, 1215.1534 (M+). Anal.
Calcd for C59F3H52IrO6OsP4S: C, 48.79; H, 3.61. Found: C, 48.58; H,
3.96.
[IrOs(CO)3(μ-η3:κ1-CH2CCMe2)(dppm)2][CF3SO3] (5). Compound 2-
CF3SO3 (47 mg, 0.034 mmol) was dissolved in 10 mL of CH2Cl2 to
afford a red solution to which was added 1,1-dimethylallene (1.2 equiv,
0.042 mmol, 4.1 μL). The solution turned light yellow immediately
and then darkened to yellow-orange within 5 min. The solution was
left to stir under an argon atmosphere for 8 h, after which time 30 mL
of diethyl ether and 10 mL of pentane were added to the orange
solution to precipitate a light orange solid. This solid was recrystallized
from CH2Cl2/diethyl ether, washed with 2 × 5 mL of diethyl ether,
and dried in vacuo (86% yield). HRMS: m/z calcd for
C58H52IrO3OsP4 (M+), 1305.2105; found, 1305.2085 (M+). Anal.
Calcd for C59F3H52IrO6OsP4S (5·0.61CH2Cl2): C, 47.60; H, 3.57.
Found: C, 47.31; H, 3.70. 1H NMR spectroscopy in CDCl3 confirmed
the presence of dichloromethane of crystallization. The gas formed
from the reaction was condensed into another NMR tube and
characterized by 1H NMR. 1H NMR data for 4-methyl-1,3-pentadiene
EXPERIMENTAL SECTION
■
General Comments. All solvents were dried using appropriate
desiccants, distilled before use, and stored under a nitrogen
atmosphere. Reactions were performed under an argon atmosphere
using standard Schlenk techniques unless otherwise noted. Diazo-
methane was generated from Diazald, which was purchased from
Aldrich, as was the 13C-enriched Diazald. 13C-enriched N-methyl-N-
nitroso-p-toluenesulfonamide was also prepared by using a modified
version of the procedure for synthesizing the 14C-enriched radio-
labeled analogue.31 The trimethylamine N-oxide dihydrate was also
purchased from Aldrich and was dried according to the literature
procedure,32 and the 13CO was purchased from Cambridge Isotopes.
The methylene-bridged complex [IrOs(CO)3(μ-CH2)(dppm)2][BF4]
(2) and the tetracarbonyl complex [IrOs(CO)4(dppm)2][BF4] (6)
were prepared according to the published procedures.16
(+27 °C in CD2Cl2): 1.77 (b, 6H), 4.94 (dd, 1H, 3JHH = 9.9 Hz, 2JHH
=
1.9 Hz), 5.09 (dd, 1H, 3JHH = 16.5 Hz, 2JHH = 1.9 Hz), 5.92 (m, 1H),
3
3
6.44 (dd, 1H, JHH = 16.5 Hz, JHH = 9.9 Hz).
[IrOs(CO)3(μ-η3:κ1-CH2CCH2)(dppm)2][BF4] (7). Under an argon
atmosphere, allene was passed through a stirred yellow/orange
solution of compound 6 (39 mg, 0.029 mmol) in 10 mL of CH2Cl2
for 1 min at a rate of approximately 0.1 mL/s. The solution was left to
stir under an allene atmosphere for 4 h at ambient temperature, during
The methylallene was purchased from Organic Technologies; allene
from Matrix Scientific; 1,1-dimethylallene from Aldrich; and 2-methyl-
1,3-butadiene from Alfa Aesar. 1,1-Difluoroallene was prepared using a
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Article
which time the solution lightened to yellow. After stirring, the solution
volume was reduced to 3 mL, and to this was added 30 mL of diethyl
ether to precipitate a yellow solid that was washed with 2 × 5 mL of
diethyl ether and dried in vacuo (88% yield). HRMS: m/z calcd for
C56H48IrO3OsP4 (M+), 1277.1792; found, 1277.1770 (M+). Anal.
Calcd for BC56F4H48IrO3OsP4: C, 49.38; H, 3.55. Found: C, 48.98; H,
3.64.
Special Refinement Conditions. (i) 3/3a: The exact identities of
the metals at each position could not be unambiguously assigned from
the structure refinement alone. Values of R1, wR2, and the goodness-of-
fit were extremely similar (to the fourth decimal place) regardless of
whether the iridium atom was singly or triply coordinated to the
trimethylenemethane ligand. Refinement of the relative occupancies of
each metal was attempted, but this was unsuccessful, as the refinement
became unstable during these calculation cycles. Satisfactory results
were obtained by setting the occupancy factors to be 50% Ir and 50%
Os at each metal atom position, suggesting a disordered mix of isomers
in the crystals, which would be consistent with the solution-phase
spectroscopic data for 3/3a. The Cl−C distances (d(Cl(1S)−C(1S),
d(Cl(2S)−C(1S)) within the disordered solvent CH2Cl2 molecule
were restrained to be 1.75(1) Å during refinement. (ii) 10: The Cl−C
distances within the minor (30%) component of the disordered
solvent CH2Cl2 molecule were restrained during refinement: d(Cl-
(3S)−C(2S)) = d(Cl(4S)−C(2S)) = 1.75(1) Å. (iii) For each of the
complexes in which the metals have dissimilar coordination environ-
ments, the final models were tested by switching the identities of the
metal atom centers (Ir for Os and vice versa) and refining these to
completion. In each case, the final indices became subtly, but
[IrOs(CO)3(μ-η3:κ1-CH2CCHCH3)(dppm)2][BF4] (8). Under an argon
atmosphere, methylallene gas was passed through a stirring yellow/
orange solution of compound 6 (33 mg, 0.024 mmol) in 5 mL of
CH2Cl2 for 1 min at a rate of approximately 0.1 mL/s. The solution
was left to stir under a methylallene atmosphere for 4 h at ambient
temperature, during which time the solution lightened to yellow. After
stirring, the solution volume was reduced to 2 mL, and to this was
added 30 mL of diethyl ether to precipitate a yellow solid that was
washed with 2 × 5 mL of diethyl ether and dried in vacuo (85% yield).
HRMS: m/z calcd for C57H50IrO3OsP4 (M+), 1291.1949; found,
1291.1929 (M+). Anal. Calcd for BC57F4H50IrO3OsP4: C, 49.75; H,
3.66. Found: C, 49.52; H, 3.74.
[IrOs(CO)4(μ-CC(H)C(CH3)CH2)(dppm)2][CF3SO3] (9). Com-
pound 6-CF3SO3 (39 mg, 0.028 mmol) was dissolved in 5 mL of
CH2Cl2 to afford a yellow/orange solution to which was added 1,1-
dimethylallene (10 equiv, 0.29 mmol, 28.4 μL) by microsyringe. The
solution was left to stir at ambient temperature for 12 h, after which
time the solution had lightened to yellow. The solvent volume was
reduced to 3 mL, and 30 mL of diethyl ether was added to precipitate
a light yellow solid that was washed with 2 × 5 mL of diethyl ether and
dried in vacuo (82% yield). HRMS: m/z calcd for C59H50IrO4OsP4
(M+), 1331.1898; found, 1331.1883 (M+). Anal. Calcd for
BC59F4H50IrO4OsP4: C, 50.04; H, 3.56. Found: C, 50.02; H, 3.67.
[IrOs(CO)4(μ-κ1:κ1-CF2CCH2)(dppm)2][CF3SO3] (10). Com-
pound 6-CF3SO3 (35 mg, 0.025 mmol) was dissolved in 0.7 mL of
CD2Cl2 in an NMR tube fitted with a J. Young valve to afford an
orange/yellow solution. The solution was cooled to −78 °C, and the
headspace was evacuated and replaced with 1,1-difluoroallene,
generated as described above. The NMR tube was warmed over 30
min to ambient temperature and left to react at ambient temperature
for 1 h, after which time 20 mL of diethyl ether and 5 mL of pentane
were added to the yellow solution to precipitate a light yellow solid.
This solid was recrystallized from CH2Cl2/diethyl ether, washed with 2
× 5 mL of diethyl ether, and dried in vacuo (90% yield). 19F NMR of
noticeably, worse for the “reversed-metals” models (7: from R1
=
0.0289, wR2 = 0.0619 to R1 = 0.0293, wR2 = 0.0639; 10: from R1 =
0.0204, wR2 = 0.0564 to R1 = 0.0207, wR2 = 0.0577).
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables of crystallographic experimental details and selected
bond lengths and angles for compounds 3/3a-CF3SO3, 7, and
10 in pdf format and atomic coordinates, interatomic distances
and angles, anisotropic thermal parameters, and hydrogen
parameters for these compounds in a CIF file. 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.
■
Notes
The authors declare no competing financial interest.
2
10 (+27 °C in CD2Cl2): δ −74.3 (bd, 1F, JFF = 60.4 Hz), δ −92.2
2
(bd, 1F, JFF = 60.4 Hz). HRMS: m/z calcd for C57F2H46IrO4OsP4
ACKNOWLEDGMENTS
(M+), 1341.1553; found, 1341.1560 (M+). Anal. Calcd for
C58F5H46IrO7OsP4S: C, 46.80; H, 3.12. Found: C, 46.82; H, 3.22.
Attempted Reaction of [IrOs(CO)4(dppm)2][CF3SO3] (6-CF3SO3)
with 2-Methyl-1,3-butadiene. To a stirring yellow/orange solution of
6-CF3SO3 (24 mg, 0.018 mmol) was added 2-methyl-1,3-butadiene
(100 equiv, 0.18 mmol, 180 μL) by microsyringe. The solution was left
at ambient temperature for 1 week, and after this time, the 31P{1H}
NMR spectrum revealed compound 6-CF3SO3 as the only
phosphorus-containing product. Slightly elevated temperatures did
not result in a reaction.
■
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of Alberta for
financial support for this research and NSERC for funding the
Bruker D8/APEX II CCD diffractometer, and the Nicolet
Avatar IR spectrometer. We thank the Department’s Mass
Spectrometry Facility and the NMR Spectroscopy and
Analytical and Instrumentation Laboratories. Finally, we
thank NSERC, the University of Alberta, and the Government
of the Province of Alberta for funding support for T.J.M.
X-ray Structure Determinations. General. Crystals were grown
via layered-solvent slow diffusion using the following solvent systems:
CH2Cl2/pentane (3/3a), acetonitrile/Et2O (7), and CH2Cl2/Et2O
(10). Data were collected using a Bruker APEX II CCD detector/
PLATFORM diffactometer34 (3/3a, 10) or with a Bruker APEX II
CCD detector/D8 diffractometer34 (7); all data were collected using
Mo Kα radiation (λ = 0.71073 Å) and with the crystals cooled to −100
°C. The data were corrected for absorption via a multiscan method
(3/3a) or through Gaussian integration from indexing of the crystal
faces (7, 10). Structures were solved using Patterson search/structure
expansion (DIRDIF-200835) (3/3a, 10) or direct methods (SHELXS-
9736) (7), and least-squares refinements were completed using the
program SHELXL-97.36 Hydrogen atoms attached to carbons were
assigned positions based on the sp2- or sp3-hybridization geometries of
their attached carbons and were given thermal parameters 20% greater
than those of their parent atoms.
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