Coordination and activation of diazoalkanes in the presence of Rh/Ru and Rh/Os metal combinations

Article abstract of DOI:10.1021/om700756q

The substituted diazoalkanes-ethyl diazoacetate (EDA), diethyl diazomalonate (DEDM), and trimethylsilyl diazomethane (TMSDM)-react readily with the methylene-bridged tricarbonyl species [RhM(CO)₃(μ-CH ₂)(dppm)₂][CF₃SO₃] (M = Os (5), Ru (6); dppm = μ-Ph₂PCH₂PPh₂) to produce the corresponding substituted olefin (through loss of N₂ and subsequent coupling of the diazoalkane-generated alkylidene fragment and the methylene unit), the known tetracarbonyl species [RhM(CO)₄(dppm)₂] [CF₃SO₃] (M = Os (1), Ru (2)), and uncharacterized decomposition products. In the reaction of 5 with EDA at -20 °C the intermediate olefin adduct, [RhOs(CO)₃(η²-CH ₂-CHR)(dppm)₂][CF₃SO₃] (7; R = CO₂Et) is observed, which decomposes with release of the olefin upon warming. The methylene-bridged tetracarbonyl species [RhM(CO) ₄(μ-CH₂)(dppm)₂] [CF₃SO ₃] (M = Os(3), Ru (4)) also react with these diazoalkanes to yield the same products, only at a much slower rate. Attempts to generate the alkylidene-bridged products, analogous to 3 and 4, by reaction of 1 or 2 with EDA, DEDM, and TMSDM accompanied by loss of N₂ instead gave complexes containing the diazoalkane unit. In the reaction with EDA condensation of this unit with a carbonyl ligand occurs accompanied by migration of the diazoalkane hydrogen to the carbonyl oxygen, yielding [RhM(CO)₃(μ- η¹:η¹ -N₂C(CO₂Et) ₂)(dppm)₂][CF₃SO₃] (M = Os (8), Ru (9)). The new diazoalkane-like group bridges the metals via the terminal nitrogen, while also binding to the group 8 metal via the enol carbon that originated from a carbonyl group. In the related reaction involving DEDM, substitution of a carbonyl yields [RhM(CO)₃(μ-η¹: η¹-N₂C(CO₂Et)₂)(dppm) ₂][CF₃SO₃] (M = Os (10), Ru (11)), in which the intact diazoalkane ligand again bridges via the terminal nitrogen. However, in this case one of the ester carbonyls binds to the group 8 metal, replacing the carbonyl group that was displaced. Although the RhRu compound 2 does not react with TMSDM, the analogous RhOs compound (1) does react, yielding [RhOs(CO) ₃(η ¹-N₂CHSiMe₃)(dppm) ₂][CF₃SO₃]] (12), in which the TMSDM ligand binds terminally on Os opposite the Rh-Os bond.

Full text of DOI:10.1021/om700756q

3070
Organometallics 2008, 27, 3070–3081
Coordination and Activation of Diazoalkanes in the Presence of
Rh/Ru and Rh/Os Metal Combinations
Rahul G. Samant, Todd W. Graham, Bryan D. Rowsell, Robert McDonald,‡ and
Martin Cowie*
Department of Chemistry, UniVersity of Alberta, Edmonton, AB, Canada T6G 2G2
ReceiVed July 25, 2007
The substituted diazoalkanessethyl diazoacetate (EDA), diethyl diazomalonate (DEDM), and trimethylsilyl
diazomethane (TMSDM)sreact readily with the methylene-bridged tricarbonyl species [RhM(CO)3(µ-
CH2)(dppm)2][CF3SO3] (M ) Os (5), Ru (6); dppm ) µ-Ph2PCH2PPh2) to produce the corresponding substituted
olefin (through loss of N2 and subsequent coupling of the diazoalkane-generated alkylidene fragment and the
methylene unit), the known tetracarbonyl species [RhM(CO)4(dppm)2][CF3SO3] (M ) Os (1), Ru (2)), and
uncharacterized decomposition products. In the reaction of 5 with EDA at -20 °C the intermediate olefin
adduct, [RhOs(CO)3(η2-CH2-CHR)(dppm)2][CF3SO3] (7; R ) CO2Et) is observed, which decomposes with
release of the olefin upon warming. The methylene-bridged tetracarbonyl species [RhM(CO)4(µ-
CH2)(dppm)2][CF3SO3] (M ) Os(3), Ru (4)) also react with these diazoalkanes to yield the same products,
only at a much slower rate. Attempts to generate the alkylidene-bridged products, analogous to 3 and 4, by
reaction of 1 or 2 with EDA, DEDM, and TMSDM accompanied by loss of N2 instead gave complexes
containing the diazoalkane unit. In the reaction with EDA condensation of this unit with a carbonyl ligand
occurs accompanied by migration of the diazoalkane hydrogen to the carbonyl oxygen, yielding [RhM(CO)3(µ-
η1:η1-N2C(COH)CO2Et)(dppm)2][CF3SO3] (M ) Os (8), Ru (9)). The new diazoalkane-like group bridges
the metals via the terminal nitrogen, while also binding to the group 8 metal via the enol carbon that originated
from a carbonyl group. In the related reaction involving DEDM, substitution of a carbonyl yields [RhM(CO)3(µ-
η1:η1-N2C(CO2Et)2)(dppm)2][CF3SO3] (M ) Os (10), Ru (11)), in which the intact diazoalkane ligand again
bridges via the terminal nitrogen. However, in this case one of the ester carbonyls binds to the group 8 metal,
replacing the carbonyl group that was displaced. Although the RhRu compound 2 does not react with TMSDM,
the analogous RhOs compound (1) does react, yielding [RhOs(CO)3(η1-Ν2CHSiMe3)(dppm)2][CF3SO3]] (12),
in which the TMSDM ligand binds terminally on Os opposite the Rh-Os bond.
Chart 1
Introduction
Diazoalkanes are important reagents in both cyclopropanation
reactions1–5 andforthegenerationofmetallacarbenecomplexes.2,5–7
In addition to their role as carbene sources, the coordination
chemistry of diazoalkanes is equally rich,5,6,8,9 displaying a
variety of coordination modes, as shown in Chart 1.6 When
bound to a single metal center two coordination modes have
commonly been observed, involving either η1, end-on coordina-
tion (type I),10–32 which can assume a number of valence-bond
formulations and associated geometries, or the η2-NN side-on
coordination (type II).20,33–39 In addition, three other coordina-
tion modes can also be envisioned, although they have yet to
be observed: an η2-NC side-on coordination (type III), a four-
membered “MCNN” metallacycle (type IV), and a η1-C-bound
* To whom correspondence should be addressed. E-mail: martin.cowie@
ualberta.ca.
‡ X-ray Crystallography Laboratory.
coordination (type V). In binuclear complexes containing two
metal centers, diazoalkane coordination at either one of the
metals, as illustrated above in structures I-V, can again occur;
however, additional modes are also possible in which the
diazoalkane bridges the pair of metals. Two bridging modes
have been observed: one in which the diazoalkane ligand bridges
both metals through the terminal nitrogen (type VI)40–43 and a
related mode in which the diazoalkane again bridges via the
terminal nitrogen while also binding in an η2-fashion to one
metal via the pair of nitrogen atoms (type VII).44–54
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10.1021/om700756q CCC: $40.75
2008 American Chemical Society
Publication on Web 05/28/2008

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3071
Our primary interest in diazoalkanes has stemmed from their
utility as synthons for the generation of alkylidene-bridged
bimetallic compounds and in the subsequent carbon-carbon
bond formation involving these alkylidene units.55–65 In a
previous study we reported facile methylene-group incorporation
and coupling by reaction of [RhOs(CO)4(dppm)2][X] (1) (X )
CF3SO3; BF4) with diazomethane to give either [RhOs(CO)4(µ-
CH2)(dppm)2][X] (3), [RhOs(C3H5)(CH3)(CO)3(dppm)2][X], or
[RhOs(C4H8)(CO)3(dppm)2][X], depending on reaction tempera-
ture.56,63 On the basis of labeling studies we proposed a reaction
sequence in which the coupling of methylene groups occurred
via C2H4- and C3H6-bridged intermediates. Surprisingly, the
analogous Rh/Ru species (2) did not promote the coupling of
methylene groups, but instead incorporated only a single
methylene group to give [RhRu(CO)4(µ-CH2)(dppm)2][X] (4).58
In order to gain a better understanding of the above methylene-
coupling sequence, we set out to generate stable C2- and C3-
bridged analogues of the putative C2H4- and C3H6-bridged
intermediates noted above. Two strategies were adopted, involv-
ing either the reaction of substituted diazoalkanes with the
methylene-bridged Rh/Os species 3 and 5, in attempts to
generate substituted C2 or higher fragments, or through the
generation of the alkylidene-bridged products [RhOs(CO)n(µ-
CRR′)(dppm)2][X], the chemistry of which, with regard to C-C
bond formation, could subsequently be pursued. In addition,
we also investigated the related chemistry of the Rh/Ru metal
combination, in hopes of learning more about the subtle
reactivity differences that can arise from different metal
combinations. This chemistry is described herein.
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General Comments. All solvents were dried (using appropriate
drying agents), distilled before use, and stored under nitrogen.
Reactions were performed under an argon atmosphere using
standard Schlenk techniques. Triosmium dodecacarbonyl was
purchased from Colonial Metals Inc., while rhodium trichloride
hydrate and triruthenium dodecacarbonyl were purchased from
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(ENNG), all of which were obtained from Aldrich, as were ethyl
diazoacetate (EDA) and trimethylsilyl diazomethane (TMSDM; 2
M, Et2O solution). The compounds [RhOs(CO)4(dppm)2][CF3SO3]
(1),66 [RhRu(CO)4(dppm)2][CF3SO3] (2),58 [RhOs(CO)4(µ-CH2)-
(dppm)2][CF3SO3] (3),56 [RhRu(CO)4(µ-CH2)(dppm)2][CF3SO3]
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3072 Organometallics, Vol. 27, No. 13, 2008
Samant et al.
(4),58 [RhOs(CO)3(µ-CH2)(dppm)2][CF3SO3] (5),56 and [RhRu-
(CO)3(µ-CH2)(dppm)2][CF3SO3] (6)58 were prepared by literature
procedures. Diethyl diazomalonate (DEDM) was prepared by a
standard route.67
decomposition products (ca. 20%), and the corresponding olefin,
as observed in part b.
(d) Reaction of Compound 5 with Diazoalkanes at Ambient
Temperature. One equivalent of either EDA, DEDM, or TMSDM
was added to 15 mg (0.011 mmol) of [RhOs(CO)3(µ-
CH2)(dppm)2][CF3SO3] (5) in 0.7 mL of CD2Cl2 in an NMR tube
at 22 °C. After 1 min the solution became lighter in color, and
after 20 min the complete disappearance of 5 and quantitative
generation of the substituted olefin accompanied by compound 1
(ca. 10%) and numerous decomposition products (ca. 90%) was
1
The H NMR and 31P{1H} NMR spectra were recorded on a
Varian Inova 400 spectrometer operating at 399.8 and 161.8 MHz,
respectively. 13C{1H} NMR and all variable-temperature NMR
spectra were recorded on a Varian Unity spectrometer operating at
1
100.6 MHz for 13C, 161.9 MHz for 31P, and 399.8 MHz for H.
Infrared spectra were recorded in CH2Cl2 solution on a Nicolet
Avatar 370 DTGS spectrometer unless otherwise noted. Elemental
analyses were performed by the microanalytical services within the
department. Mass spectrometry measurements were performed by
the electrospray ionization technique on a Micromass ZabSpec TOF
spectrometer in the mass spectrometry facility of the department.
Spectroscopic data for all new compounds are given in Table 1.
Photolysis experiments were conducted in NMR tubes that were
irradiated using a Hanovia 450 W high-pressure mercury lamp
placed six inches from the reaction vessel, all of which waSup-
portings enclosed in a photolysis chamber. Experimental procedures
for reactions that gave only unidentified decomposition products
appear in the Information.
1
confirmed by 31P and H NMR spectroscopy.
(e) Reaction of Compound 6 with Diazoalkanes at Ambient
Temperature. [RhRu(CO)3(µ-CH2)(dppm)2][CF3SO3] (6) (15 mg,
0.012 mmol) was dissolved in 0.7 mL of CD2Cl2 in an NMR tube
at 22 °C. The resulting yellow solution was treated with 1 equiv of
each of the above diazoalkanes, causing the solution to lighten in
color within seconds. After 20 min 31P and 1H NMR spectroscopy
indicated the presence of compound 2 (ca. 10%), the corresponding
substituted olefin, and numerous uncharacterized decomposition
products (ca. 90%).
(f) Reactions d and e at Low Temperature. Procedures d and
e were repeated, but in this instance the solutions were cooled to
-78 °C prior to the addition of the diazoalkane. After treatment
with the diazoalkane at -78 °C, the sample was inserted into an
NMR probe precooled to -80 °C. The reaction was monitored by
NMR while warming in 10 °C intervals. In all cases, formation of
a short-lived species spectroscopically similar to 7 (ca. 10% by
integration) was observed at -20 °C along with numerous unidenti-
fied decomposition products. At temperature above -20 °C, only
decomposition products were observed.
Preparation of Compounds. (a) [RhOs(CO)3(η2-CH2d
C(H)CO2CH2CH3)(dppm)2][CF3SO3] (7). Method i: To a solution
of compound 5 (19.2 mg, 0.015 mmol), dissolved in 0.7 mL of
CD2Cl2 in an NMR tube, cooled to -78 °C, was added 1.7 µL
(0.016 mmol) of N2C(H)CO2Et via a gastight syringe. The NMR
tube was then transferred to the NMR probe, which had been
precooled to -80 °C, and the reaction monitored by NMR
spectroscopy while warming in 10 °C intervals. No change was
observed below -20 °C. Holding the sample at -20 °C for 1.7 h
resulted in the formation of the product (7) in a 3:2 ratio with
unreacted compound 5. After 1.7 h, decomposition of 7 into
numerous unidentified products began to occur. Method ii: To a
stirring deep red solution of compound 5 (30 mg, 0.023 mmol) in
1.5 mL of CH2Cl2 at 20 °C was added 2.4 µL of N2C(H)CO2Et
(0.023 mmol). The solution instantly turned light brown and was
left to stir for 5 min, then cooled to -80 °C to prevent rapid
decomposition of the product. A beige solid was precipitated by
the slow addition of 20 mL of pentane (precooled to -80 °C), the
solvent was decanted by canula, and the solid was rinsed twice
with 5 mL of Et2O and finally dried in Vacuo (yield 50% as
determined by integration of the 31P NMR spectra relative to
residual bis(triphenylphosphoranylidene) ammonium chloride; we
were unable to separate compound 7 from the numerous unidentified
decomposition products). HRMS: m/z calcd for C60H52O5P4RhOs
(M+ - CF3SO3) 1247.1426, found 1247.1437.
(g) [RhOs(CO)3(µ-η1:η1-N2C(CO2CH2CH3)COH)(dppm)2]-
[CF3SO3] (8). To a yellow solution of 1 (45 mg, 0.034 mmol) in
5 mL of CH2Cl2 at ambient temperature was added 40 µL of ethyl
diazoacetate (EDA) as a 1 M ethereal solution. This mixture was
allowed to stir for 24 h, during which time no color change was
noted. However, the 31P NMR spectrum indicated the complete
conversion to the product (8). Slow addition of 20 mL of Et2O
afforded a yellow powder, which, after decanting the clear
supernatant, was rinsed with two 5 mL washings of Et2O before
being dried under a stream of argon followed by drying in Vacuo
(yield 88%). HRMS: m/z calcd for C58H50O6N2P4RhOs (M+
CF3SO3) 1289.1280, found 1289.1289.
-
(h) [RhRu(CO)3(µ-η1:η1-N2C(CO2CH2CH3)COH)(dppm)2]-
[CF3SO3] (9). To a stirring yellow solution of 25 mg (0.020 mmol)
of 2 in 15 mL of CH2Cl2 at ambient temperature was added 55 µL
(0.053 mmol) of N2CH(CO2Et). Over the course of 1 h the yellow
solution turned bright orange, at which point it was concentrated
to 2 mL followed by the slow addition of Et2O to afford an orange
powder. After removal of the clear supernatant, the product was
recrystallized from CH2Cl2/Et2O (1 mL/10 mL), yielding bright
orange microcrystals, which were isolated by filtration and dried
(b) Reaction of Compound 3 with Diazoalkanes at Ambient
Temperature. [RhOs(CO)4(µ-CH2)(dppm)2][CF3SO3] (3) (10 mg,
0.008 mmol) was placed in an NMR tube and dissolved in 0.7 mL
of CD2Cl2 at ambient temperature to produce a yellow solution.
To this was added 1 equiv of diazoalkane either neat (EDA and
DEDM) or as a 2 M ethereal solution (TMSDM). After 60 h the
in Vacuo (yield 75%). Anal. Calcd for
C62.1H55.2Cl0.2F3-
N2O11P4RhRuS: C, 52.33; H 3.73; N, 2.07; Cl, 0.52. Found: C,
52.16; H, 3.74; N, 1.98; Cl, 0.54. The presence of 0.1 equiv of
CH2Cl2 was confirmed by 1H NMR spectroscopy in THF-d8.
HRMS: m/z calcd for C58H50O6N2P4RhRu (M+ - CF3SO3)
1199.0718, found 1199.0718.
1
31P and H NMR spectra were recorded. In all cases the products
detected were unreacted starting material (ca. 80%), compound 1
(ca. <10%), unidentified decomposition products (ca. 10%), and
the corresponding olefin (H2CdCRR′; R ) H, R′ ) CO2Et, SiMe3;
R ) R′ ) CO2Et).
(i) [RhOs(CO)3(µ-η1:η1-N2C(CO2CH2CH3)2)(dppm)2][CF3SO3]-
(10). Compound 1 (53 mg, 0.040 mmol) was dissolved in 5 mL of
CH2Cl2 at ambient temperature, yielding a yellow solution. To this
solution was added 0.05 mL (0.24 mmol) of N2C(CO2CH2CH3)2,
causing the solution to darken slightly. The solution was allowed
to stir for 24 h, at which time the solution was dark orange, and
only the product, compound 10, was observed by 31P NMR
spectroscopy. The solvent volume was reduced to 2 mL, and a bright
yellow powder was precipitated by the addition of 20 mL of Et2O.
After removing the clear supernatant, the yellow product was rinsed
(c) Reaction of Compound 4 with Diazoalkanes at Ambient
Temperature. The reaction of 10 mg of [RhRu(CO)4(µ-
CH2)(dppm)2][CF3SO3] (4) (0.008 mmol) with either EDA, DEDM,
or TMSDM was carried out exactly as described in part b. After
1
60 h 31P and H NMR spectroscopy revealed the products to be
starting material (ca. 70%), compound 2 (ca. 10%), uncharacterized
(67) Searle, N. E. Org. Synth. 1956, 36, 25.

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3073

3074 Organometallics, Vol. 27, No. 13, 2008
Samant et al.
twice with 5 mL of Et2O and the product dried under an argon
refinement of the setting angles of 5554 reflections from the data
collection. The space group was determined to be P1(No. 2). The
j
stream and then in Vacuo (yield 85%). Anal. Calcd for
C61.1H54.2N2O10F3SP4Cl0.2RhOs: C, 49.19; H, 3.66; N 1.89. Found:
data were corrected for absorption through use of Gaussian
integration (involving face-indexing of the crystal). See Table 2
for a summary of crystal data and X-ray data collection information.
Structure Solution and Refinement. The structure of 8 was
solved using the Patterson location of heavy atoms and structure
expansion routines as implemented in the DIRDIF-9669 program
system. Refinement was completed using the program SHELXL-
97. Hydrogen atoms were assigned positions based on the geom-
etries of their attached carbon atoms and were given thermal
parameters 20% greater than those of the attached carbons, with
the exception of hydrogen H(40), which was located and freely
refined. The final model for 6 was refined to values of R1(F) )
C, 48.70; H, 4.02; N, 1.73. The presence of 0.1 equiv of CH2Cl2was
confirmed by 1H NMR spectroscopy in THF-d8. HRMS: m/z calcd
for C60H54O7N2P4RhOs (M+ - CF3SO3) 1333.1542, found
1333.1526.
(j) [RhRu(CO)3(µ-η1:η1-N2C(CO2CH2CH3)2)(dppm)2][CF3SO3]
(11). A 36 mg (0.028 mmol) amount of 2 was dissolved in 20 mL
of CH2Cl2 at ambient temperature, and 24 µL of N2C(CO2Et)2(0.14
mmol) was added. The resulting yellow solution darkened slightly
within 15 min. After 2 h, only compound 11 remained, as confirmed
by 31P NMR spectroscopy of the deep red solution, from which a
salmon-red product was precipitated by the slow addition of 30
mL of Et2O. After removing the solvent, the precipitate was rinsed
with two 5 mL portions of Et2O and dried under a stream of argon
then in Vacuo (yield 82%). Anal. Calcd for C61H54F3N2O10SP4RhRu:
C, 52.63; H, 3.91; N, 2.01. Found: C, 52,26; H, 3.93; N, 1.91.
HRMS: m/z calcd for C60H54O7N2P4RhRu (M+ - CF3SO3)
1243.0980, found 1243.0980.
2
0.0281 (for 11 411 data with Fo2 g 2σ(Fo )) and wR2(F2) ) 0.0765
(for all 12 323 independent data).
Results and Compound Characterization
(a) Treatment of Methylene-Bridged Complexes with
Diazoalkanes. Reaction of the methylene-bridged tetracarbonyl
species [RhM(CO)4(µ-CH2)(dppm)2][CF3SO3] (M ) Os (3), Ru
(4)) with either ethyl diazoacetate (EDA), trimethylsilyldiaz-
omethane (TMSDM), or diethyldiazomalonate (DEDM) results
in the generation of the respective free olefins over the period
of several days, as verified by comparison of their NMR spectra
to those reported in the literature (H2CdCRR′; R ) H, R′ )
CO2Et, SiMe3; R ) R′ ) CO2Et).70 The formation of these
olefins corresponds to expected coupling of the diazoalkane-
generated alkylidene unit and the methylene group. In all cases,
the phosphorus-containing compounds remaining were identified
by 31P{1H} NMR spectroscopy as unreacted compound 3 or 4,
the respective byproduct 1 or 2, and numerous uncharacterized
decomposition products all in varying proportions depending
on the diazoalkane. In contrast, the tricarbonyl analogues
[RhM(CO)3(µ-CH2)(dppm)2][CF3SO3] (M ) Os (5), Ru (6))
react instantly with these diazoalkanes at ambient temperature,
resulting in the rapid evolution of the free olefin, together with
the respective compound 1 or 2, again accompanied by
numerous uncharacterized decomposition products as determined
by 31P NMR spectroscopy. This reactivity is summarized in
Scheme 1. We assume that formation of the tetracarbonyl
species 1 and 2, from the tricarbonyl precursors 5 and 6,
respectively, results from carbonyl scavenging from the de-
composition products.
Quenching the reaction involving compound 5 and EDA by
cooling the sample to -20 °C allows the isolation of a new
species, [RhOs(η2-H2CdCHCO2Et)(CO)3(dppm)2][CF3SO3] (7),
as also shown in Scheme 1. In the 31P{1H} NMR spectrum,
compound 7 gives rise to three complex multiplets at δ 25.3,
-4.1, and -10.5 in a 2:1:1 intensity ratio. The two high-field
signals appear as the C and D portion of an ABCDX spin system
(X ) 103Rh) and clearly correspond to the inequivalent
phosphorus nuclei at the Os-bound ends of the dppm ligands,
on the basis of the higher-field chemical shifts66 and the absence
of Rh coupling. The low-field resonance of double intensity
corresponds to the two accidentally degenerate A and B
resonances for the Rh-bound ends of the dppm ligand and
appears as an approximate doublet of multiplets with strong
(k) [RhOs(CO)3(η1-N2C(H)Si(CH3)3)(dppm)2][CF3SO3] (12).
Method i: A yellow solution of compound 1 (55 mg, 0.042 mmol)
in 5 mL of CH2Cl2 at ambient temperature was treated with 0.30
mL of a 2 M solution of N2C(H)Si(CH3)3 in Et2O. After 16 h
31P{1H} NMR spectroscopy revealed 25% conversion to compound
12, and after 4 days 31P{1H} NMR spectroscopy showed compound
1
12 as the only detectable compound; however H-NMR spectros-
copy revealed numerous decomposition products. The solvent was
removed in Vacuo and the crude brown residue recrystallized from
CH2Cl2/Et2O to afford oily brown microcrystals of the product
which decomposed in the solid within hours. The solid was rinsed
several times with Et2O before being dried in Vacuo (yield 72%).
Method ii: If the same procedure noted in the first method was
repeated, except a slow stream of Ar was also passed over the
solution, the reaction proceeded to completion in 16 h. Anal. Calcd
for C58H54N2F3O6P4SSiRhOs: C, 49.43; H, 3.86; N, 1.99; S, 2.28.
Found: C, 49.59; H, 4.03; N, 1.23; S, 2.42. MS: m/z ) 1147. The
elemental analysis was performed within 1 h of isolation of the
solid sample to minimize decomposition.
(l) [RhOs(CO)2(PMe3)(µ-η1:η1-N2C(CO2CH2CH3)2)(dppm)2]-
[CF3SO3] (13). In an NMR tube at ambient temperature, 10 mg of
compound 10 (0.0068 mmol) was dissolved in 0.7 mL of CD2Cl2,
to which was added 7 µL of a 1 M solution of PMe3 (in toluene)
(0.007 mmol). After 2.5 days quantitative conversion to the product
(13) was confirmed by 31P NMR spectroscopy. The solution was
left to stand for a period of 2 weeks, over which time no further
transformations were observed by 31P NMR spectroscopy. Anal.
Calcd for C63H63F3N2O10P5RhOsS: C, 48.97; H 4.11; N, 1.81; S,
2.08. Found: C, 49.18; H, 4.53; N, 1.74; S, 2.97.
(m) Photolysis Experiments. Compounds 8, 9, 10, and 11 (ca.
15 mg) were each dissolved in 0.7 mL of CD2Cl2 in separate NMR
tubes. The samples were irradiated in the photolysis chamber
described above for 16 h and monitored by 31P NMR spectroscopy.
No changes were observed.
(n) Thermolysis Experiments. Method i: Compounds 8, 9, 10,
and 11 (ca. 25 mg) were each dissolved in 2 mL of freshly distilled
THF. The solutions were left to reflux and the reactions observed
by 31P{1H} NMR spectroscopy. No new species were noted after
16 h. Method ii: The reaction described in method i was repeated
using freshly distilled toluene instead of THF. Again, no new
species were observed.
X-ray Data Collection. Yellow-orange crystals of [RhOs-
(CO)3(µ-η1,η1-NdNC(COH)CO2Et)(dppm)2][CF3SO3](8) · C2H4Cl2
were obtained by slow evaporation of a 1,2-dichloroethane solution
of the compound. Data were collected at -80 °C on a Bruker
PLATFORM/SMART 1000 CCD diffractometer68 using Mo KR
radiation. Unit cell parameters were obtained from a least-squares
(68) Programs for diffractometer operation, data collection, data reduc-
tion, and absorption correction were those supplied by Bruker.
(69) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;
Garcia Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF-
96 program system; Crystallography Laboratory, University of Nijmegen:
The Netherlands, 1996.
(70) http://www.aist.go.jp/RIODB/SDBS/cgi-bin/cre_index.cgi.

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3075
Table 2. Crystallographic Experimental Details for Compound 8
A. Crystal Data
C61H54Cl2F3N2O9OsP4RhS
formula
fw
1536.01
cryst dimens (mm)
cryst syst
space group
unit cell paramsa
a (Å)
0.63 × 0.37 × 0.28
triclinic
j
P1(No. 2)
11.3899(7)
15.0631(9)
18.4369(11)
87.7008 (12)
76.4194(11)
88.4252(11)
3071.7(3)
2
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Fcalcd (g cm-3
)
1.661
2.627
µ (mm-1
)
B. Data Collection and Refinement Conditions
diffractometer
Bruker PLATFORM/SMART 1000 CCD68
radiation (λ [Å])
graphite-monochromated Mo KR (0.71073)
temperature (°C)
-80
scan type
ω scans (0.2°) (20 s exposures)
52.76
data collection 2θ limit (deg)
total no. of data collected
no. of indep reflns
no. of observed reflns
structure solution method
refinement method
absorp corr method
range of transmn factors
no. of data/restraints/params
goodness-of-fit (S)c
final R indicesd
17 696 (-14 e h e 14, -17 e k e 18, -23 e l e 23)
12 323 (Rint ) 0.0162)
2
11 411 [Fo g 2σ(Fo2)]
Patterson search/structure expansion (DIRDIF-9969
full-matrix least-squares on F2 (SHELXL-97b)
Gaussian integration (face-indexed)
0.5205-0.2696
)
2
12 323 [Fo g -3σ(Fo2)]/0/761
2
1.038 [Fo g -3σ(Fo2)]
2
R1 [Fo g 2σ(Fo2)]
0.0281
0.0765
1.749 and -1.048
2
wR2 [Fo g -3σ(Fo2)]
largest diff peak and hole (e Å-3
)
a Obtained from least-squares refinement of 5554 centered reflections. b Sheldrick, G. M. SHELXL-97, Program for crystal structure determination;
2
2 2
University of Go¨ttingen: Germany, 1997. c S ) [∑w(Fo - Fc ) /(n - p)]1/2 (n ) number of data; p ) number of parameters varied; w ) [σ2(Fo2) +
2
2
2 2
(0.0454P)2 + 3.7359P]-1 where P ) [Max(Fo2, 0) + 2Fc ]/3). d R1 ) ∑||Fo| - |Fc||/Σ|Fo|; wR2 ) [∑w(Fo - Fc ) /∑w(Fo4)]1/2
.
olefin, is observed that shows no coupling to any of the 31P
nuclei or to Rh. The 1H NMR spectrum of 7 shows two
resonances at δ 1.08 and 1.72, both of which display coupling
of approximately 8 Hz to an additional proton resonance at δ
2.77. These three protons correspond to the three olefinic protons
of the coordinated olefin. Although one might expect different
cis/trans coupling constants for the olefinic protons, rehybrid-
ization of the olefin upon coordination presumably results in
equivalent coupling involving the pseudocis and pseudotrans
protons. In the 13CH2-enriched sample of compound 7 the two
high-field protons display coupling to the 13C nucleus of 144
and 157 Hz coupling, respectively, indicating that they cor-
respond to this CH2 group. These one-bond C-H coupling
constants lie between those typical for an olefin (ca. 160 Hz)
and that of an alkane (ca. 130 Hz),71 reflecting some degree of
rehybridization, as noted above. The dppm methylene protons
appear as four separate broad multiplets at δ 4.47, 4.27, 4.10,
and 3.12, indicating a lack of front-back and top-bottom
symmetry, rendering each proton chemically unique. This
suggests that olefin rotation is slow on the NMR time scale.
Finally, the ethyl fragment of the ethyl acrylate ligand resonates
at δ 1.32 (CH3) and 4.23 (CH2), similar to that of free ethyl
acrylate.70 We have ruled out an olefin-bridged structure for
compound 7 since in such an arrangement the olefin could be
considered a 1,2-dimetalated ethane unit, with essentially
complete rehybridization of the olefinic carbons to sp3. This is
Scheme 1
coupling to Rh (approximately 113 Hz). The strong coupling
between the pair of signals corresponding to the 31P nuclei
bound to Os (2JPcPd ) 259 Hz) indicates a mutually trans
phosphine arrangement at this metal. In the 13C{1H} NMR
spectrum of 7 three carbonyl resonances appear: two triplets at
δ 182.8 and 185.5, displaying no Rh coupling, correspond to
two terminally bound carbonyls on Os, and a doublet of triplets
at δ 182.5, showing strong Rh coupling (1JRhC ) 77.6 Hz),
corresponds to the carbonyl that is terminally bound to Rh. If
13CH2-enriched compound 5 is used in the above reaction, an
additional 13C{1H} resonance at δ 3.80 in the 13C{1H} NMR
spectrum, corresponding to the CH2 end of the coordinated
(71) Silverstein, R. M.; Webster, F. X. Spectroscopic Identification of
Organic Compounds; John Wiley and Sons, Inc.: New York, 1998.

3076 Organometallics, Vol. 27, No. 13, 2008
Samant et al.
Chart 2
Scheme 2
inconsistent with the large C-H coupling constants noted above.
In addition, the absence of spin-spin coupling involving either
end of the olefin and the Rh nucleus establishes that the ethyl
acrylate ligand is bound solely to Os. Compound 7 can be
compared to two related monoethylene adducts, [RhOs(C2H4)-
(CO)3(dppm)2][BF4] (A) and [RhOs(CO)3(C2H4)(dppm)2][BF4]
(B), shown in Chart 256b (dppm ligands omitted). A structure
like B can be immediately ruled out, since as established above,
the olefin is bound to Os while one carbonyl is on Rh. In
addition, the spectral data do not fit structure A. It has been
established that terminal carbonyls that lie adjacent to another
metal, and therefore having weak interactions with it, usually
resonate downfield of those that lie remote from the adjacent
metal;72 this is clearly seen in compound A, in which the
chemical shifts of the two Os-bound carbonyls differ by 10 ppm.
In contrast, the pair of Os-bound carbonyls in 7 are very similar,
having a chemical shift difference of only 2.7 ppm. Furthermore,
the relatively low-field shift for the Os-bound carbonyls suggests
that they are adjacent to the Rh center, possibly interacting
weakly with this second metal (generally, terminal carbonyls
on third-row metals appear significantly upfield from those of
the second-row analogues). On the basis of the above NMR
data, the structure shown in Scheme 1 and Chart 2 is proposed
in which an ethyl acrylate ligand is bound to Os in an η2 fashion
opposite the metal-metal bond with both Os-bound carbonyls
adjacent to the metal-metal bond. The IR spectrum of 7 shows
a stretch for the CO2Et group at 1683 cm-1 and three terminally
bound carbonyl ligands (2047, 1981, and 1942 cm-1). In
addition, a weak band at 1558 cm-1 can be assigned to the
olefinic C-C stretch shifted significantly from that of free ethyl
acrylate (1638 cm-1).70
unusual products [RhM(CO)3(µ-η1:η1-NdNC(CO2Et)COH)-
(dppm)2][CF3SO3] (M ) Os (8), Ru (9)), diagrammed in
Scheme 2 (dppm ligands omitted). High-resolution mass
spectrometry for both products verifies that dinitrogen loss has
not occurred from the diazoalkane molecule, and this is
confirmed by the X-ray structure of 8 shown in Figure 1. The
X-ray study also demonstrates that although N2 loss has not
occurred, the diazoalkane molecule has transformed into a new
diazoalkane-like moiety via condensation with a carbonyl ligand,
accompanied by hydrogen migration from the diazoalkane
carbon to the carbonyl oxygen to give and metalla-enol
functionality. Migratory insertions, involving carbonyl ligands,
Compound 7 is labile, decomposing in solution above -20
°C into free ethyl acrylate, compound 1, and numerous
unidentified phosphorus-containing products. Attempts to dis-
place the ethyl acrylate ligand in 7 by ethylene, under an
ethylene atmosphere, gave no evidence of an ethylene adduct
over the temperature range from 22 to -80 °C. Again, only
decomposition of 7 resulted over time or upon warming.
Although we have not been able to isolate analogous olefin
adducts in the reactions of 5 with the other substituted
diazoalkanes, we have observed such species in small quantities
(<10% by integration of all phosphorus-containing species) in
the 31P{1H} NMR spectra at -20 °C, having very similar spectra
to those of 7. We assume that the greater lability of these
intermediates results from destabilizing repulsions involving the
additional ester substituent in the case of the DEDM reaction
or the larger trimethylsilyl substituent in the case of TMSDM.
In the case of the Rh/Ru complex 6, no olefin adduct analogous
to 7 was observed in any of the diazoalkane reactions, although
the same olefins were ultimately obtained.
(b) Diazoalkane Complexes. In attempts to generate alky-
lidene-bridged complexes analogous to compounds 3 and 4 via
N2 loss from the parent diazoalkane, the reactions of 1 and 2
with a number of substituted diazoalkanes were attempted.
Treatment of both compounds 1 and 2 with EDA yields the
Figure 1. Perspective view of the [RhOs(CO)3{µ:η1,η1-
NdNC(COH)CO2Et}(dppm)2]+ complex cation of compound 8,
showing the atom-labelling scheme. Non-hydrogen atoms are
represented by Gaussian ellipsoids at the 20% probability level.
Hydrogen atoms are shown with arbitrarily small thermal parameters
except for phenyl hydrogens, which are not shown.
(72) George, D. S. A.; McDonald, R.; Cowie, M. Organometallics 1998,
17, 2553.

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3077
Table 3. Selected Distances and Angles for Compound 8
obvious bonding distance of the enol oxygen (O(4)-H(40) )
0.87(6) Å), but also in a position to be hydrogen-bonded to the
carbonyl oxygen of the CO2Et group (O(5)-H(40) ) 1.85(6)
Å), with this separation being significantly less than the sum of
the van der Waals radii of 2.6 Å. The presence of hydrogen
bonding is further substantiated by the planar arrangement of
the six-membered ring that results (see Figure 1), involving the
enol and carbonyl functionalities; a repulsive interaction between
H(40) and O(5) would lead to rotation of the OH group out of
the enol plane.
Distances (Å)
atom 1 atom 2 distance
atom 1 atom 2 distance
Os
Os
Os
Os
Os
Rh
Rh
O(1)
O(2)
O(3)
O(4)
Rh
3.3206(3)
O(4) H(40) 0.87(6)
O(5) C(6) 1.227(5)
O(5) H(40) 1.85(6)a
O(6) C(6) 1.326(4)
O(6) C(7) 1.464(5)
N(1) N(2) 1.270(4)
N(2) C(5) 1.402(4)
C(4) C(5) 1.391(4)
C(5) C(6) 1.455(5)
C(7) C(8) 1.473(7)
N(1) 2.088(2)
C(1) 1.976(3)
C(2) 1.904(3)
C(4) 2.055(3)
N(1) 2.023(3)
C(3) 1.842(4)
C(1) 1.139(4)
C(2) 1.145(4)
C(3) 1.143(5)
C(4) 1.328(4)
The diazo-containing ligand in compounds 8 and 9 functions
as a dianionic 6 e- donor to the Rh(I)/M(II) (M ) Os, Ru)
centers. In keeping with these oxidation-state formulations, the
coordination geometry at Rh is square planar, while that of Os
is octahedral. This formulation differs from that shown in the
compounds displaying structure VI (Chart 1) owing to the
additional enolate functionality, which donates an additional pair
of electrons.
Angles (deg)
angle atom 1 atom 2 atom 3
atom 1 atom 2 atom 3
angle
N(1)
N(1)
N(1)
C(1)
C(1)
C(2)
N(1)
C(4)
C(6)
Os
Os
Os
Os
Os
Os
Os
Rh
C(1)
C(2)
C(4)
C(2)
C(4)
C(4)
C(3)
98.9(1)
166.8(1) Os
75.6(1)
94.2(1)
174.5(1) Os
91.3(1)
176.5(1) N(2) C(5) C(4)
114(4) N(2) C(5) C(6)
115.9(3) C(4) C(5) C(6)
107.8(1) O(5) C(6) O(6)
119.9(2) O(5) C(6) C(5)
132.3(2) O(6) C(6) C(5)
113.3(3) O(6) C(7) C(8)
Os
C(1) O(1)
C(2) O(2)
C(3) O(3)
C(4) O(4)
C(4) C(5)
173.3(3)
174.3(3)
178.9(4)
125.0(2)
114.1(2)
120.9(3)
117.1(3)
121.3(3)
121.6(3)
122.3(3)
121.8(3)
115.9(3)
108.0(2)
Rh
Os
O(4) C(4) C(5)
The transformation of compounds 1 and 2 into the products
8 and 9, respectively, is irreversible; addition of CO to either
of the products gives no further reaction and does not regenerate
1 or 2, most likely a result of the condensation and hydrogen
migration steps, which in this case appear irreversible. For
compounds 8 and 9 the spectral parameters are closely
comparable (see Table 1), so compound 9 is assumed to have
O(4) H(4O)
O(6) C(7)
N(1) Rh
N(1) N(2)
N(1) N(2)
N(2) C(5)
Os
Rh
N(1)
1
a Nonbonded distance.
the same structure as that established for 8. In the H NMR
spectrum of 8 the dppm methylene and the ethyl group
resonances are as expected, while the hydroxyl proton appears
as a singlet at δ 11.04, as is characteristic of a hydrogen-bonded
hydroxyl proton.71 13C{1H} NMR data show only three terminal
carbonyl resonances; two triplets at δ 177.9 and 179.6 cor-
respond to the Os-bound groups, while a doublet of triplets at
δ 195.5, with 63.2 Hz coupling to the Rh nucleus, corresponds
to the Rh-bound CO (surprisingly, in this case the Os-bound
carbonyl that is adjacent to Rh cannot be differentiated from
the one that occupies the remote position). The enol carbon is
observed as a triplet at δ 231.6 (2JCP ) 8.3 Hz) with coupling
to the pair of Os-bound 31P nuclei. A solution IR of a sample
of 8 shows two terminal carbonyl stretches (2020 cm-1 (s), 1970
cm-1 (s, br)), a stretch at 1633 cm-1, attributed to the CO2Et
group, and a stretch at 3058 cm-1, which is assigned to the OH
functionality.
are very common73 and have been observed with diazoalkane-
generated74 or metal-bound75 alkylidene groups, and with nitrido
groups that have resulted from N-N bond cleavage in diaz-
oalkanes.76 However, we are unaware of any report involving
condensation of a carbonyl ligand with an intact diazoalkane
group within a metal complex. The new diazo-containing group
in 8 binds in a bridging arrangement bound to both metals
through the terminal nitrogen (not unlike structure VI shown
in Chart 1) while chelating to Os via the enol carbon to give a
five-membered Os-N-N-C-C metallacycle. Within this met-
allacycle, the bond lengths and angles, given in Table 3, are
consistent with the valence-bond formulation shown in Scheme
2. Therefore, the N(1)-N(2) distance (1.270(4) Å) is that of a
typical NdN double bond, while the N(2)-C(5) distance
(1.402(4) Å) is as expected for a single bond between sp2-
hybridized N and C atoms.77 The newly formed C(4)-C(5) bond
(1.391(4) Å) is typical of an enolic double bond,77 while the
C(5)-C(6) and C(4)-O(4) distances (1.455(5), 1.328(4) Å,
respectively) are consistent with single bonds within such
groups. Within the N2-containing metallacycle, the angles are
all somewhat smaller (range 113.3(3)-119.9(2)°) than the
idealized value for sp2 hybridization, reflecting a degree of strain
within this group, as a result of the acute N(1)-Os-C(4) bite
angle (75.6(1)°). The diazo-containing fragment bridges in a
close-to-symmetrical manner in which the Os-N(1) distance
is only slightly longer than Rh-N(1) (2.088(2) vs 2.023(3) Å).
The X-ray study has also allowed the unambiguous location
and successful refinement of the enol hydrogen, placing it within
The corresponding reactions of compounds 1 and 2 with
DEDM yield the respective diazoalkane-bridged complexes
[RhM(CO)3(µ-η1:η1-NdNC(CO2Et)2)(dppm)2][CF3SO3] (M )
Os (10), Ru (11)) (Scheme 2), in which the DEDM ligand is
again proposed to bridge via the terminal nitrogen. Although
binding of DEDM is accompanied by the loss of one carbonyl,
the reactions cannot be reversed by addition of CO. The presence
of an N2 moiety within these complexes is verified by HRMS
and elemental analysis, while the loss of a carbonyl is seen in
the 13C{1H} NMR spectra of 13CO-enriched samples, in which
only three equal intensity resonances are observed. For com-
pound 11, the pair of Ru-bound carbonyls appear at δ 197.0
and 195.6, whereas the one bound to Rh appears at δ 195.4,
with typical coupling (63.7 Hz) to Rh. All resonances display
typical patterns consistent with coupling to the adjacent pair of
31P nuclei. The ester carbonyls on the DEDM ligand appear at
δ 165.7 and 155.3, demonstrating the inequivalence of these
two ester groups. This inequivalence is also evident in the ethyl
(73) (a) Collman, J. P.; Hegedus, L. S.;. Norton, J. S.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry; Univer-
sity Science Books: Mill Valley, CA, 1987; Chapter 12. (b) Niu, S.; Hall,
M. B. Chem. ReV 2000, 100, 353.
(74) Herrmann, W. A.; Plank, J.; Ziegler, M. L.; Weidenhammer, K.
J. Am. Chem. Soc. 1979, 101, 3133.
1
(75) Proulx, G.; Bergman, R. G. Science 1993, 259, 661.
(76) Li, B.; Tan, X.; Xu, S.; Song, H.; Wang, B. J. Organomet. Chem.
2008, 693, 667.
group resonances in the H NMR spectrum with one group
appearing at δ 1.91 (CH2) and 0.62 (CH3) while the other
appears at δ 4.35 (CH2) and 1.40 (CH3). The NMR spectral
parameters for the Os analogue very much mirror those given
(77) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.

3078 Organometallics, Vol. 27, No. 13, 2008
Samant et al.
Chart 3
occurred. A solution of compound 13 was monitored over a
period of two weeks by 31P NMR spectroscopy, and no further
transformations were observed. Again, attempts to obtain X-ray
quality crystals of 13 failed. Interestingly, the IR spectrum of
13 shows ester stretches at 1752 and 1686 cm-1, more in line
with structure C and with that of the Rh/Ru species 11.
above, except that the Os-bound carbonyls appear upfield to
those bound to Ru, as is typically the case.
On the basis of the significant inequivalence of the pair of
ester groups in each complex and the presence of only two Ru-
or Os-bound carbonyls, as was observed in compounds 8 and
9, we propose that coordination of one of the ester groups to
Ru or Os occurs, as diagrammed for one valence-bond
representation in Scheme 2. Similar binding of DEDM, via both
nitrogen and an ester carbonyl, has been proposed.36 The
valence-bond representation shown in Scheme 2 for compounds
10 and 11 would be expected to give rise to two very different
ester carbonyl stretches in the IR spectrum: one at a value close
to that of free DEDM and another (corresponding to the
coordinated ester) at a significanty lower frequency. In the case
of the Ru species (11) these stretches are observed at 1725 and
1641 cm-1, in support of the formulation given. This lowering
of the ester carbonyl stretches has also been observed by
Eisenberg et al.78 However, for the Os analogue (10) the ester
stretches both appear at the intermediate values, 1699 and 1690
cm-1, suggesting a structure in which there is significant
delocalization over the pair of ester groups. Two limiting
valence-bond formulations for the coordinated DEDM ligand
in compounds 10 and 11 are shown in Chart 3 (dppm groups
omitted). Structure C corresponds to the bonding extreme
proposed in the Rh/Ru species (11), while the structure proposed
for the Rh/Os species (10) is presumably a hybrid of C and D.
In either case, the ligand functions as a dianionic 6 e- donor to
the Rh(I)/M(II) centers (M ) Os, Ru), with the square-planar
and octahedral geometries about the respective metals. Unfor-
tunately, attempts to obtain X-ray quality crystals of 10 and 11
in different solvents and by exchanging the triflate anion for
Treatment of compound 1 with N2CHSiMe3 (TMSDM) also
leads to a carbonyl-substitution product, [RhOs(CO)3(N2CHSi-
(CH3)3)(dppm)2][CF3SO3] (12), again containing the intact
diazoalkane group, as confirmed by elemental analysis. In
addition, the mass spectrometry fragmentation pattern observed
strongly suggests a product in which the intact diazoalkane group
is present, showing peaks associated with the loss of both the
CHSiMe3 and N2CHSiMe3 fragments. The formation of com-
pound 12 is accompanied by the formation of numerous
unknown silane products, presumably from the decomposition
of TMSDM. Furthermore, this product is unstable, even in the
solid state, decomposing over several hours into an oily residue
composed of a complex mix of unknown products, limiting our
characterization of 12. In the 13C{1H} NMR spectrum two
carbonyl resonances are observed in a 2:1 intensity ratio. The
latter appears as a doublet of multiplets at δ 181.2 with coupling
to the pair of Rh-bound 31P nuclei and to 103Rh (1JRhC ) 80.3
Hz), establishing that it is terminally bound to this metal, while
the other resonance, at δ 194.6, appears as a doublet of
multiplets of double intensity and displays coupling to the Os-
bound phosphines, as well as weak coupling to the 103Rh nucleus
of 3.3 Hz. The lower-field resonance for the Os-bound carbonyls
is consistent with these groups being bound primarily to Os
but having a weak semibridging interaction with Rh,66,72
a
formulation that is supported by the small Rh-C coupling
observed. However, the IR spectrum, which shows two terminal
(ν(CO) ) 2059, 1977 cm-1) and one bridging CO stretch (1804
cm-1), suggests a slightly different interpretation in which one
Os-bound carbonyl is terminally bound while the other is
semibridging, as diagrammed earlier in Scheme 2 (a classical
bridging carbonyl is ruled out on the basis that the averaged
1JRhC would be higher than the 3.3 Hz observed). Facile
exchange of these carbonyls between the terminal and semibridg-
ing positions is rapid on the NMR time scale and probably
involves the movement of only a few tenths of an angstrom.
As a consequence, only the average signal is seen in the 13C{1H}
NMR spectrum, even at -80 °C, whereas the faster time scale
of the IR experiment allows both the bridging and terminal
stretches to be observed. Similar dynamics have been reported
previously in related species.56b,79 The four dppm methylene
protons are equivalent on the NMR time scale, appearing as a
single multiplet at δ 4.03, even at -80 °C, indicating the same
average chemical environment on each side of the RhOsP4 plane.
This suggests that the diazoalkane is bound in a linear or close-
to-linear manner in the site opposite the Rh-Os bond; any other
site would result in the loss of symmetry about the RhOsP4
plane, resulting in the chemical inequivalence of the two pairs
of dppm methylene protons. In addition, occupation of any other
-
-
BF4 and BPh4 were unsuccessful.
Addition of PMe3 to compound 10 in attempts to displace
the Os-bound ester group instead gives the product [RhOs(CO)2-
(PMe3)(µ-η1:η1-N2C(CO2Et)2)(dppm)2][CF3SO3] (13), the struc-
ture of which is diagrammed below, via substitution of a
carbonyl. The 31P NMR spectrum of this product exhibits three
multiplets of 2:2:1 intensity ratio at δ 28.4, 12.1, and -18.5.
The low-field signal, being the AA′ portion of an AA′BB′CX
spin system, corresponds to the Rh-bound phosphorus nuclei
of the dppm ligand, clearly evident by its strong coupling to
the Rh nucleus (1JRhP ) 162 Hz) and additional coupling to the
other phosphines of the dppm bridge as well as to the PMe3
ligand, whereas the signal at δ 12.1 (the BB′ portion) corre-
sponds to the Os-bound phosphines of the dppm ligand on the
basis of its higher-field chemical shift and absence of Rh
coupling. Finally, a doublet of triplets at δ -18.5 is assigned
to the PMe3 ligand and shows strong coupling to both the Rh-
bound phosphines of the dppm ligands (2JPP ) 42 Hz) and the
Rh nucleus (1JRhP ) 129 Hz) to which it is bound. This clearly
establishes that coordination of PMe3 to Rh and not to Os has
(79) Ristic-Petrovic, D.; Anderson, D. J.; Torkelson, J. R.; Ferguson,
M. J.; McDonald, R.; Cowie, M. Organometallics 2005, 24, 3711.
(78) Woodcock, C.; Eisenberg, R. Organometallics 1985, 4, 4.

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3079
Scheme 3
site on the Os center by the TMSDM ligand would not allow
the facile carbonyl exchange noted above. Linearly bound
diazoalkane ligands are well documented.10,12 The structure
proposed is similar to those reported previously, for the
symmetric isomer of the monoethylene and bis-ethylene com-
pounds[RhOs(CO)3(η2-C2H4)(dppm)2][BF4]and[RhOs(CO)2(η2-C2H4)2-
(dppm)2][BF4], respectively, which have similar symmetries with
respect to the pair of Os-bound carbonyls. Consistent with the
proposal of similar structures, the spectral parameters for these
ethylene complexes are closely comparable to those of 12.56b
In particular, the Os-bound carbonyls for these two ethylene
adducts appear at δ 198.0 (1JRhC ) 6 Hz) and 195.5 (1JRhC
)
6 Hz), respectively, in the 13C{1H} NMR spectra, very close to
the low-field resonances of 12.
Attempts to generate the Rh/Ru analogue of 12, namely,
[RhRu(CO)3(N2CHSiMe3)(dppm)2][CF3SO3], failed, giving ei-
ther no reaction or uncharacterized decomposition products,
depending on the reaction conditions.
Having the series of metal-bound diazoalkanes in which these
ligands were either bridging the pairs of metals (compounds
8-11) or terminally bound to one (compound 12), we were
interested in determining whether these coordination modes
activated these ligands toward N2 loss. However, photolysis or
subjecting compounds 8-11 to reflux in a number of solvents,
even for extended periods, did not induce N2 loss, leaving the
complexes unchanged. Compound 12 decomposed to unidenti-
fied products under these conditions, as described earlier.
even less stable than 7, appearing in only small quantities
relative to decomposition products at temperatures below -20
°C. In the case of the Rh/Ru analogues, no olefin adduct is
observed down to -80 °C, even for ethyl acrylate, presumably
owing to the weaker olefin binding to Ru compared to Os.80
(b) Diazoalkane and Related Complexes. Attempts to
generate substituted alkylidene-bridged analogues of compounds
3 and 4, by the reactions of 1 and 2, respectively, with a series
of diazoalkanes, also did not yield the targeted products. Instead
an interesting series of complexes have been generated in which
the “NNC” moiety of the diazoalkane precursors has remained
intact. Of the three different diazoalkanes investigated, the most
surprising result involves EDA, in which coordination in the
bridging site is accompanied by condensation of the diazoalkane
carbon and an adjacent carbonyl group with subsequent
hydrogen transfer from this diazoalkane to the carbonyl oxygen,
yielding an enolate moiety. Formation of a metallacyclic-enol
product from a metal-carbonyl precursor suggests the involve-
ment of a keto-enol tautomerization81 step. This proposal is
outlined in Scheme 3 (ancillary ligands omitted), in which
reaction of ethyl diazoacetate with the “RhM(CO)4” fragment
is proposed to first yield the diazoalkane-bridged intermediate
(E), similar to other diazoalkane-bridged complexes.40–43 Nu-
cleophilic attack of the diazoalkane carbon at the carbonyl ligand
would yield the metallacycloketone (F), which could then
tautomerize to the metallacyclic enol (G). This enol product is
presumably stabilized by conjugation between the olefinic
double bond and both the diazo and the ester carbonyl
functionalities and also by the hydrogen bond between the enol
and ester carbonyl group. Reactions of the very analogous
homobinuclearcomplexes[Rh2(CO)2(µ-H)2(dppm)2]and[Ru2(CO)4(µ-
CO)(dppm)2] with EDA did not yield metallacyclic products
like 8 and 9, but instead gave simple diazoalkane-bridged
products, depicted by structure VI.43,78 Although, as noted
earlier, condensations involving diazoalkanes and carbonyl
ligands have not been observed on a metal template, the
transformations reported for 8 and 9 are reminiscent of the
Discussion
(a) Methylene and Alkylidene Coupling. Although the
reaction of [RhOs(CO)4(µ-CH2)(dppm)2][X] (3) with diaz-
omethane is facile at temperatures above -50 °C, yielding C3
or C4 fragments coordinated to the metals,56,63 the analogous
reactions of 3 with the substituted diazoalkanes N2CRR′ (R )
H, R′ ) CO2Et, SiMe3; R ) R′ ) CO2Et) are extremely slow
even at ambient temperature. Furthermore, in these latter
reactions no metal-bound hydrocarbyl fragments are observed
and after several days only the olefins that correspond to
coupling of the bridging methylene group and the diazoalkane-
generated alkylidene fragment, together with compound 1 and
decomposition products, are observed. Carrying out the reactions
at low temperature only serves to slow the reactions further,
without allowing the observation of reaction intermediates. The
Rh/Ru analogue (4) reacts similarly. However, the same
reactions using the tricarbonyl complexes [RhM(CO)3(µ-
CH2)(dppm)2][X] (M ) Os (5), Ru (6)) are almost immediate
at ambient temperature, again yielding the free olefins and
(where identified) the same metal-containing products. If the
reaction involving 5 and EDA (N2CHCO2Et) is carried out at
-20 °C, an olefin-containing species, [RhOs(CO)3(η2-
H2CdC(H)CO2Et)(dppm)2][X] (7), is observed as diagrammed
in Scheme 1. This product has resulted from coupling of the
methylene and alkylidene fragments at the adjacent metals. This
olefin complex does not have a targeted olefin-bridged structure;
instead this group appears to be terminally bound to Os.
Furthermore, unlike the ethylene analogue prepared by the
reaction of 5 with diazomethane, which is stable upon warming
to ambient temperature,56b compound 7 readily undergoes loss
of ethyl acrylate at temperatures above -20 °C. We assume
that the greater lability of 7 compared to its ethylene analogue
results from unfavorable steric interactions involving the dppm
phenyl groups and the bulky CO2Et group. The olefin adducts,
analogous to 7, involving the larger SiMe3 substituents (from
TMSDM) and the two CO2Et substituents (from DEDM) are
(80) (a) Li, J.; Schreckenbach, G.; Ziegler, T. Inorg. Chem. 1995, 34,
3245. (b) Burke, M. R.; Takats, J.; Grevels, F.-W.; Reuvers, J. G. A. J. Am.
Chem. Soc. 1983, 105, 4092.
(81) Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic
Chemistry; Oxford University Press: Oxford, 2001.

3080 Organometallics, Vol. 27, No. 13, 2008
Samant et al.
coupling reactions involving ethyl diazoacetate and aldehydes,
using a variety of catalysts..82
Whereas the methylene-bridged Rh/Ru complex (4)58 had
behaved very differently than its Rh/Os analogue56 in failing
to react further with diazomethane, this complex did react with
the substituted diazoalkanes, yielding olefins, paralleling the
chemistry observed with Rh/Os. In both cases, the methylene-
bridged tricarbonyls were much more reactive than the tetra-
carbonyl analogues.
In an attempt to observe and characterize a species that would
model either the diazoalkane-bridged species E or the metal-
lacyclic ketone species F, we undertook the reactions of 1 and
2 with diethyl diazomalonate (DEDM), which lacks a hydrogen
substituent on the diazoalkane carbon, ruling out the possibility
of keto-enol tautomerism. This strategy has succeeded in
generating a diazoalkane-bridged product, with both the Rh/Os
and the Rh/Ru systems. However, in both of these cases one of
the ester carbonyl groups also displaces a carbonyl ligand and
binds to the group 8 metal as a dianionic, 6 e- donor diazoalkane
ligand. The failure of DEDM to yield a metallacyclic ketone
similar to structure F (Scheme 3), which would result from
coupling of the bridging diazoalkane ligand and an adjacent
carbonyl ligand, may be due to the increased steric crowding
in the disubstituted diazoalkane (DEDM) ligand, which inhibits
approach of the diazoalkane carbon to the carbonyl ligand. The
entropy increase that results from the carbonyl displacement in
generating compounds 10 and 11 also cannot be overlooked as
a driving force for the observed transformation.
The singly substituted TMSDM group, having hydrogen and
trimethylsilyl substituents on the diazoalkane carbon, has no
carbonyl oxygen with which to chelate to the group 8 metal,
ruling out products analogous to compounds 10 and 11.
However it has the potential to yield a condensation product
such as F and in principle can also undergo a tautomerization
step yielding a metallacyclic enol similar to G. In this case,
however, the absence of an ester carbonyl group means that
such an enol product would have neither the stabilization of
the hydrogen bond nor the stabilization resulting from conjuga-
tion involving the olefin and carbonyl functionalities that exist
in 8 and 9. In this case, however, no product containing a
bridging diazoalkane group is observed. Instead the unstable
product (12) contains a terminally bound diazoalkane ligand,
as shown in Scheme 2.
In order to minimize the steric bulk of the diazoalkane, we
also investigated reactions involving diazoethane (N2C(H)CH3)
and compounds 1 and 2. Surprisingly, no reaction involving
this diazoalkane and these compounds was observed between
-80 °C and ambient temperature. We certainly observed the
formation of both cis- and trans-butene, under the conditions
investigated; however, these olefins were also obtained under
the same conditions in the absence of compounds 1 and 2, so
their formation was not metal-mediated.
(c) Comparisons of N2CH2 and N2CRR′. The reactions of
the mixed-metal complexes described above with substituted
diazoalkanes and with diazomethane differ significantly. In the
reactions involving the methylene-bridged Rh/Os complexes 3
and 5, all diazoalkanes (including N2CH2) initially behave
similarly, resulting in coupling of the bridging methylene group
with the alkylidene fragment of the diazoalkane. However,
whereas diazomethane reacted with 3 to give multiple methylene
couplings, the substituted diazoalkanes give rise to olefins arising
from coupling of an alkylidene fragment with the metal-bound
methylene group. We suggest that this may be a consequence
of steric factors that labilize the substituted olefins, first forcing
them out of the bridging site, where methylene coupling is
proposed to occur in this system,56b,63 and subsequently leading
to olefin loss.
In the reactions of diazoalkanes with the tetracarbonyl
complexes [RhM(CO)4(dppm)2][X] (M ) Ru (2), Os (1)), the
difference between the prototype (N2CH2) and the substituted
diazoalkanes studied is even more pronounced. With diaz-
omethane, no species was ever observed containing the intact
N2CH2 ligand; instead N2 extrusion was extremely facile
(occurring even at -80 °C), generating methylene groups and
other hydrocarbyl ligands formed from methylene-group con-
densations. In contrast, the substituted diazoalkanes (with the
exception of diazoethane, which failed to react) formed only
diazoalkane complexes or related species and could not be
induced to undergo N2 loss when heated or photolyzed. Milstein,
Martin, and co-workers have argued convincingly that “. . .an
important requirement for the carbene formation is the viability
of and η1-C coordinated diazo complex” and have shown that
the viability of this key intermediate is largely determined by
steric factors;13 the greater the steric demand of the diazoalkane
substituents, the less favorable will be this pivotal intermediate.
In support of these ideas, our only observation of carbene
formation is with unsubstituted diazomethane. Coordination of
the diazoalkane or related moiety through the terminal nitrogen,
in a bridging (compounds 8-11) or terminal η1-N mode
(compound 12), does not lead to carbene formation, even under
forcing conditions.
Finally, it seems puzzling that coordination of diazoalkane
is observed in reaction of 1 and 2, with no alkylidene formation,
yet alkylidene formation clearly occurs at some stage in reactions
involving the more crowded methylene-bridged complexes 3-6,
since the corresponding olefins (CH2dCRR′) are obtained,
through coupling of the methylene fragment with the diazoal-
kane-generated alkylidene fragment. It appears unlikely that in
these reactions the requisite η1-C coordinated diazoalkane
complex can be accessed to allow alkylidene formation for
subsequent coupling with the bridging methylene group, since
such reactivity was not observed for the less crowded complexes
1 and 2. For this reason, we propose that coupling of the intact
diazoalkane ligand and the methylene group may occur prior
to N2 extrusion, instead of the opposite pathway, although we
have no evidence of diazoalkane coordination prior to olefin
formation.
Conclusions
Although our efforts to generate substituted C1- and C2-
bridged species through the use of substituted diazoalkanes were
not successful, we were able to characterize an interesting,
unstable ethyl acrylate complex, resulting in the coupling of an
ethyl diazoacetate-generated alkylidene and the metal-bridging
methylene group. It appears that our failure to generate the
targeted species is largely a result of steric influences of the
diazoalkane substituents; generation of the η1-C coordinated
diazoalkanes as prerequisites to the targeted bridging alkylidene
species appears to be inhibited by the bulk of the diazoalkane
substituents, and the olefin-bridged targets appear to be unstable,
again for steric reasons. However, in our unsuccessful attempts
to generate alkylidene-bridged products we were able to generate
three interesting and rather different complexes that involve
incorporation of the intact “NNC” part of diazoalkane molecules,
(82) (a) Evans, D. A.; Truesdale, L. K.; Grimm, K. G. J. Org. Chem.
1976, 41, 3335. (b) Yao, W.; Wang, J. J. Org. Chem. 2003, 5, 1527. (c)
Xiao, F.; Liu, Y.; Wang, J. Tetrahedron Lett. 2007, 48, 1147. (d) Kantam,
M. L.; Balasurbrahmanyam, V.; Kumar, K. B. S.; Venkanna, G. T.; Figueras,
F. AdV. Synth. Catal. 2007, 349, 1887.

Coordination and ActiVation of Diazoalkanes
Organometallics, Vol. 27, No. 13, 2008 3081
adding further to the interesting coordination chemistry of these
species. Unlike our previous studies involving diazomethane,
for which the Rh/Ru and Rh/Os metal combinations displayed
very different reactivities, both metal combinations reacted quite
analogously with the substituted diazoalkanes studied.
NSERC for funding of the Bruker PLATFORM/SMART
1000 CCD X-ray diffractometer and the Nicolet Avatar IR
spectrometer.
Supporting Information Available: CIF file giving crystal-
lographic details for compound 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
University of Alberta for funding of this research and
OM700756Q