Structure and reactivity of acetylene complexes of bis(imino)pyridine ruthenium(0)

Article abstract of DOI:10.1021/om100953g

Reaction of the low-valent ruthenium complex [κ²-N ₃]Ru(η⁶-MeC₆H₅) (1), [N ₃] = 2,6-(MesN=CMe)₂C₅H₃N, with acetylene leads to the displacement of toluene and formation of the monoacetylene adduct, [N₃]Ru(C₂H₂) (2). The short alkyne-metal distances in 2 are consistent with 4 e^- donation to the metal; that is, there is some degree of overlap of the perpendicular alkyne π- and metal d_xz-orbitals. The NMR data and DFT calculations suggest π_⊥ donation is weaker in 2 than in many 4 e^- donor alkyne complexes. Further reaction of 2 with acetylene leads to catalytic cyclotrimerization and release of benzene, although the catalyst is short-lived. In the case of diphenylacetylene, two molecules of C₂Ph₂ react per molecule of 1 to generate the metallacyclic [N₃]Ru(C₄Ph₄) (3), which is best described as a ruthenacyclopentadiene, or ruthenole. Compound 3 does not react further with diphenylacetylene, but does react with terminal alkynes by addition of the acetylenic C-H bond across a ruthenole Ru-C bond. The new complexes, [N₃]Ru(C=C-R)(cis,cis-1,2,3,4-tetraphenylbutadienyl-μ-H) (R = H, 5; Ph, 6), contain terminal acetylide and cis,cis-tetraphenylbutadienyl ligands (Ru-(CPh)₄H), where the vinylic C-H bond is weakly bound to the metal through an agostic interaction. This type of ruthenole cleavage by terminal acetylenes may explain the short life of 2 as a catalyst for cyclotrimerization. The order in which HCCH and PhCCPh are introduced into the coordination sphere alters the course of the reaction: whereas isolated Ru(C₄Ph ₄) metallacycle 3 is cleaved by acetylene to give 5, preformed acetylene complex 2 reacts with diphenylacetylene to produce the free cyclization product 1,2,3,4-tetraphenylbenzene (and 3). These observations highlight the key role of five-membered metallacycles in alkyne cyclotrimerization, as well as the importance of steric factors in these reactions. Cyclization is observed in cases where the π-system of the ruthenacyclic intermediate is accessible to an incoming alkyne, but not in cases where steric bulk hinders access.

Full text of DOI:10.1021/om100953g

ARTICLE
pubs.acs.org/Organometallics
Structure and Reactivity of Acetylene Complexes of Bis(imino)pyridine
Ruthenium(0)
Noah L. Wieder, Patrick J. Carroll, and Donald H. Berry*
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States
S Supporting Information
b
ABSTRACT: Reaction of the low-valent ruthenium complex
[κ2-N3]Ru(η6-MeC6H5) (1), [N3] = 2,6-(MesNdCMe)2-
C5H3N, with acetylene leads to the displacement of toluene
and formation of the monoacetylene adduct, [N3]Ru(C2H2)
(2). The short alkyne-metal distances in 2 are consistent with
4 e- donation to the metal; that is, there is some degree of
overlap of the perpendicular alkyne π- and metal dxz-orbitals.
The NMR data and DFT calculations suggest π^ donation is
weaker in 2 than in many 4 e- donor alkyne complexes. Further
reaction of 2 with acetylene leads to catalytic cyclotrimerization
and release of benzene, although the catalyst is short-lived. In
the case of diphenylacetylene, two molecules of C2Ph2 react per
molecule of 1 to generate the metallacyclic [N3]Ru(C4Ph4) (3), which is best described as a ruthenacyclopentadiene, or ruthenole.
Compound 3 does not react further with diphenylacetylene, but does react with terminal alkynes by addition of the acetylenic C-H
bond across a ruthenole Ru-C bond. The new complexes, [N3]Ru(CtC-R)(cis,cis-1,2,3,4-tetraphenylbutadienyl-μ-H) (R = H, 5;
Ph, 6), contain terminal acetylide and cis,cis-tetraphenylbutadienyl ligands (Ru-(CPh)4H), where the vinylic C-H bond is weakly
bound to the metal through an agostic interaction. This type of ruthenole cleavage by terminal acetylenes may explain the short life
of 2 as a catalyst for cyclotrimerization. The order in which HCCH and PhCCPh are introduced into the coordination sphere alters
the course of the reaction: whereas isolated Ru(C4Ph4) metallacycle 3 is cleaved by acetylene to give 5, preformed acetylene complex
2 reacts with diphenylacetylene to produce the free cyclization product 1,2,3,4-tetraphenylbenzene (and 3). These observations
highlight the key role of five-membered metallacycles in alkyne cyclotrimerization, as well as the importance of steric factors in these
reactions. Cyclization is observed in cases where the π-system of the ruthenacyclic intermediate is accessible to an incoming alkyne,
but not in cases where steric bulk hinders access.
’ INTRODUCTION
to starting materials in vacuo. In this contribution we report the
reactions of 1 with alkynes to form ruthenium(0) complexes
[N3]Ru(η2-HCCR) (R = H, Me) and the ruthenium(II) me-
tallacyclopentadiene complex [N3]Ru(C4Ph4). Structural stud-
ies and the reactivity of these complexes, including catalytic
cyclotrimerization of alkynes, are described.
Transition metal catalyzed formal [2þ2þ2] cyclotrimeriza-
tion of alkynes is a reaction of considerable importance both
industrially and in the laboratory.1 Efforts to better understand
this process have been well underway for at least the past three
decades, and the topic has been reviewed thoroughly.2
The low-valent ruthenium complex [κ2-N3Ar]Ru(η6-
MeC6H5) (1) and the bridging dinitrogen complex {[N3Ar]-
Ru}2(μ-N2) ([N3Ar] = 2,6-(ArNdCMe)2C5H3N, Ar = xylyl or
mesityl) were recently described by our group.3 Although 1 and
{[N3]Ru}2(μ-N2) are formally Ru(0) complexes, formal oxida-
tion states often provide an incomplete description of these and
other low-valent complexes bearing [N3]-type ligands. In some
instances, low-valent [N3]M complexes are better described by
canonical forms representing full transfer of one or more
electrons from the metal to relatively remote ligand orbitals,
e.g., [N3]-/Mþ or [N3]2-/M2þ. Previously, 1 was shown to react
with various small donor ligands L (L = PMe3, CO, C2H4) to
yield 18-electron [N3]RuL2 complexes3 and with H2 to form an
unusual paramagnetic hydride complex.4 In the case of ethylene,
the resultant [N3]Ru(C2H4)2 complex was not isolable, reverting
’ RESULTS
Synthesis and Structure of [N3]Ru(C2H2), 2. Treatment of
the Ru(0) arene complex [κ2-N3]Ru(η6-MeC6H5) (1) or dini-
trogen complex {[N3]Ru}2(μ-N2) with 1.3 equiv of acetylene
leads to formation of the bright green acetylene complex
[N3]Ru(C2H2) (2) within minutes at room temperature (63%
isolated yield, eq 1). Use of excess acetylene results in decreased
yields of 2 due to byproducts generated during catalytic cyclo-
trimerization (vide infra). The 1H NMR spectrum of 2 is
consistent with C2v symmetry, with a peak at δ 6.68 assigned
Received: October 1, 2010
Published: March 31, 2011
r
2011 American Chemical Society
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ARTICLE
to the acetylene ligand, and no change in symmetry was observed
1
in the H NMR down to -68 ꢀC. The imine methyl signal (δ
1.51) is shifted somewhat upfield compared to the free ligand (δ
2.61). A similar, albeit much greater, perturbation of the imine
methyl shift (δ -0.06) is observed in the dinitrogen complex
{[N3]Ru}2(μ-N2).3 Chirik and co-workers previously suggested
that the upfield shift of the [N3] imine methyl resonances in a
variety of iron(0) complexes is due to temperature-independent
paramagnetism (TIP) and reflects mixing of higher spin excited
states with the ground state.5 In the 13C NMR spectrum, the
acetylene ligand is observed at δ 131.0 (1JCH = 226.4 Hz), as
assigned on the basis of a 2D-NMR experiment.
The geometry of 2 in the solid state was determined by single-
crystal X-ray diffraction to be square planar, with the acetylene
ligand lying within the [N3]Ru plane and coordinated trans to
the pyridyl nitrogen atom (Figure 1). The coordinated CtC
bond length (1.239(4) Å) is longer by 0.06 Å compared to free
acetylene (1.181(7) Å),6 and the acetylene ligand of 2 is bound in
an essentially symmetrical manner (D(Ru-C) = 2.032(2) and
2.040(2) Å). Only two other structures of mononuclear ruthe-
nium η2-HCCH complexes7 were found in the Cambridge
Structural Database,8 and both can be unambiguously assigned
as 2 e- donor acetylene coordinated to Ru(II) centers. Whereas
the CtC bond length of 2 is similar to these known Ru(II)
acetylene complexes, the Ru-C bond lengths are ca. 0.16 Å
shorter. Possible origins of the short Ru-C distances in 2 will be
addressed below.
Complex 2 is stable in benzene solution indefinitely at room
temperature in the absence of air and water; unlike the binuclear
(μ-N2) complex, 2 does not revert to [κ2-N3]Ru(η6-C6H6).
However, dissociation of acetylene can be induced by treatment
of 2 with excess CO, which leads to formation of the dicarbonyl
complex [N3]Ru(CO)2 (eq 2).3
Figure 1. ORTEP drawing of [N3]Ru(C2H2), 2, with 30% probability
thermal ellipsoids. Selected bond lengths (Å): C28-C29, 1.239(4);
Ru1-C28, 2.032(2); Ru1-C29, 2.040(2).
with 10 equiv of acetylene at room temperature in THF-d8 leads
to formation of benzene by cyclotrimerization. Not surprisingly,
cyclotrimerization of acetylene is also observed in the reaction of
[η2-N3]Ru(η6-MeC6H5) (1) with excess acetylene, which gen-
erates 2 in situ. Although coordinated arene complexes may be
intermediates during cyclotrimerization, the resting state of the
catalyst appears to be adduct 2; the η6-benzene complex
analogous to 1 is not observed during the catalytic reaction in
THF. This is consistent with the observation that benzene
solutions of 2 are not in equilibrium with significant quantities
of [κ2-N3]Ru(η6-C6H6).
The cyclotrimerization reaction is catalytic, but activity is
maintained only briefly, as 2 decomposes during the course of
the reaction. In a typical run with 10 equiv of C2H2, 2.7 equiv of
benzene was generated over a period of ca. 4 h (∼ 8 equiv of
C2H2 consumed), after which catalytic activity ceases and an
intractable black solid precipitates from the brown-black solu-
tion. Only benzene and residual acetylene were observed by 1H
NMR of the volatile components of the reaction mixture; organic
products such as olefins and ene-ynes were not detected by NMR
and GC-MS. The number of catalytic turnovers achieved before
catalyst decomposition was not significantly affected by increas-
ing the initial acetylene ratio from 10 to 20 equiv.
The reaction of 1 with excess propyne also leads to trimeriza-
tion, generating 1,3,5-trimethylbenzene (mesitylene) as the only
organic product detected by NMR and GC-MS. Approximately 1
Cyclotrimerization of Alkynes Mediated by [N3]Ru(C2H2)
and [η2-N3]Ru(η6-MeC6H5). Reaction of [N3]Ru(C2H2) (2)
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equiv of mesitylene is produced before trimerization activity
ceases. An intermediate ruthenium complex can be observed
1
by H NMR in the reaction of 1 with 0.9 equiv of propyne
and is tentatively identified as the propyne analogue of 2,
[N3]Ru(HCtCMe). However, this complex decomposes with-
in minutes at room temperature, much faster than 2 during
the trimerization of acetylene. Attempts to isolate the propyne
adduct at low temperatures and in the absence of excess
propyne were not successful. The 1H NMR spectrum of
[N3]Ru(HCtCMe) in situ indicates Cs symmetry. The imine
methyl groups (δ 1.32, 1.26) and para mesityl methyls (δ 2.29,
2.26) are each inequivalent pairs, appearing as four singlets (3H
each), in addition to two resonances for the meta mesityl
methyls (6H each). This overall pattern of six methyl resonances
(3:3:3:3:6:6) is consistent with the alkyne ligand lying in the
[N3] plane as in 2. In contrast, the perpendicular orientation of
the propyne ligand, although also of Cs symmetry, would give rise
to a 1H spectrum with a total of four methyl resonances
(3 mesityl, 1 imine), each integrating for six protons (6:6:6:6).
A doublet at δ 1.85 in the 1H spectrum of [N3]Ru(HCtCMe)
corresponds to the propynyl methyl group, with a measur-
able four-bond coupling to the propynyl proton at δ 6.78
(4JHH = 2.8 Hz).
Reaction of 1 with Diphenylacetylene: Synthesis and
Structure of [N3]Ru(C4Ph4), 3. Treatment of the Ru(0) arene
complex 1 with excess diphenylacetylene at room temperature
leads to toluene loss and coupling of two alkynes to generate the
metallacyclopentadienyl complex [N3]Ru(C4Ph4) (3), isolated
as a blue crystalline solid in 93% yield (eq 3).
Figure 2. ORTEP drawing of [N3]Ru(C4Ph4), 3, with 30% probability
thermal ellipsoids. Selected bond lengths (Å) and angles (deg): Ru1-
C28, 2.028(2); Ru1-C31, 2.040(2); C28-C29, 1.368(3); C29-C30,
1.465(3), C30-C31, 1.369(3); N2-Ru1-C28, 172.48(8); N2-Ru1-
C31, 94.93(8).
relatively short Ru-C (1.969(4) Å), long CR-Cβ, and short
CβdCβ bonding motif.10b
No intermediate with a single coordinated diphenylacetylene
comparable to 2 is observed during the reaction between 1
and diphenylacetylene, prior to the formation of 3. The diphe-
nylacetylene analogue of 2, with the alkyne ligand lying in the
[N3]Ru plane, is not likely due to the severe steric interactions
that would result between the bulky mesityl groups on the
[N3] ligand and the alkyne. A more plausible intermediate
would be the diphenylacetylene adduct with the alkyne perpen-
dicular to the [N3]Ru plane. Although a mono-C2Ph2 complex
was not observed in the present case, Chirik and co-
workers reported the analogous [N3Ar]Fe(C2Ph2) complex (Ar =
2,6-iPr2C6H3),11a and a number of earlier examples of d8
The 1H NMR spectrum of 3 is consistent with a five-
coordinate, square-pyramidal geometry with an empty site trans
to the apical position and indicates loss of the mirror plane
coincident with the [N3] plane. Treatment of 3 with excess
phenylacetylene at temperatures up to 70 ꢀC does not produce
hexaphenylbenzene, and prolonged heating at >100 ꢀC leads to
slow decomposition.
The solid-state structure of 3 was determined crystallographi-
cally (Figure 2), revealing a distorted square-pyramidal geome-
try. One R-carbon of the ruthenacyclopentadienyl moiety is in
the basal site, approximately trans to the pyridyl nitrogen atom
(N2-Ru1-C28 172.48(8)ꢀ), and the other is apical. The
ruthenacyclopentadienyl unit is approximately perpendicular to
the [N3] plane, and the Ru-Cbasal (2.028(2) Å) and Ru-Capical
(2.040(2) Å) bond distances are similar and are in the range for
typical Ru-C(sp2) single bonds in ruthenoles.9 Notably, the
ruthenacyclopentadienyl Cβ-Cβ bond (1.465(3) Å) and
CRdCβ bonds (1.369(3), 1.368(3) Å) exhibit clear bond length
alternation. The Ru-C and C-C ring distances are consistent
with a 2,4-diene formulation, as opposed to the dicarbenoid
metallacyclopentatriene description attributed to MC4 systems
in some instances.10 For example, Dinjus and co-workers re-
ported the RuC4 ring in Cp*RuCl(2,5-Ph2C4) exhibits a
L3M(C2Ph2) complexes have been described as well.11b,c The
Ar
greater bulk of the diisopropylphenyl-substituted [N3
]
ligand employed by Chirik leads to a distorted tetrahedral
geometry in the iron complex with the alkyne oriented perpen-
dicular to the [N3]Fe plane, compared with the square-planar
coordination in 2. Greater steric hindrance also prevents the
addition of a second C2Ph2 molecule to form the ferrole
analogous to 3.
Metallacycle 3 is also formed in the reaction of 2 with excess
diphenylacetylene (eq 4). Interestingly, unlike the reaction of 2
with CO, the C2H2 ligand is not displaced as free acetylene,
but rather is incorporated with 2 equiv of phenylacety-
lene into 1,2,3,4-tetraphenylbenzene. The identity of 1,2,3,4-
tetraphenylbenzene was confirmed by comparison of the
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Figure 4. Selected chemical shifts and coupling constants for
[N3]Ru(CtC-H)(cis,cis-1,2,3,4-tetraphenylbutadienyl-μ-H).
(107.00(10)ꢀ). Both metallacyclic Ru-C bonds are longer in 4
than 3. The Ru-C bond trans to the carbonyl ligand in 4
(2.158(2) Å) is longer than the Ru-C bond trans to the pyridine
(2.097(2) Å), reflecting the strong trans influence of the carbonyl
ligand.
1
The H NMR spectrum of 3 in the presence of excess
trimethylphosphine at room temperature exhibits broad peaks
consistent with weak and reversible coordination. The instability
of the phosphine adduct is due primarily to steric congestion.
Attempts to isolate the adduct were unsuccessful due to loss of
PMe3 in vacuo.
Reaction of Metallacycle 3 with Terminal Alkynes. Treat-
ment of [N3]Ru(C4Ph4) (3) with excess acetylene at room
temperature leads to addition of an acetylenic C-H bond to the
ruthenium, generating a new purple complex bearing a terminal
acetylide ligand and a cis,cis-tetraphenylbutadienyl ligand. The
new C-H bond on the 4-position of the butadienyl group is
agostic, interacting weakly with the metal center (eq 6).
Figure 3. ORTEP drawing of [N3]Ru(C4Ph4)(CO), 4, with 30%
probability thermal ellipsoids. Selected bond lengths (Å): Ru1-
C56, 1.910(3); Ru1-C28, 2.097(2); Ru1-C31, 2.158(2); C28-C29,
1.359(4); C29-C30, 1.481(3); C30-C31, 1.357(3); C56-O1,
1.143(3).
1H NMR spectrum to the literature values12 and GC-MS
analysis.
Two singlets integrating for one proton each appear in the 1H
NMR spectrum of 5 (δ 2.77 and 0.93). On the basis of a HMQC
experiment, the peak in the 1H spectrum at δ 0.93 is correlated
with a 13C resonance at δ 100.85 (1JCH = 220 Hz). The large C-
H coupling constant and chemical shift are strongly diagnostic of
a proton attached to an sp-hybridized carbon atom; thus the
resonance at δ 0.93 (1H NMR) is assigned to the terminal
acetylide proton. The proton resonance at δ 2.77 correlates
with a 13C signal at δ 111.3 (1JCH = 122 Hz) and is assigned as
the agostic proton attached to the terminal butenyl carbon
(Figure 4). Assignment of the alkynyl carbon resonances was
established from spectra of isotopically labeled 5-13C2, generated
in situ from enriched 13C2H2 and 3.
Tetraphenyl ruthenole 3 rapidly coordinates carbon monoxide
to generate the orange [N3]Ru(C4Ph4)(CO) (4, 77% isolated,
eq 5). The 1H and 13C NMR spectra of 4 are quite similar to that
of the 16 e- parent compound 3, and a band in the IR spectrum
of 4 at 1926 cm-1 is assigned to the carbonyl ligand.
The C-H coupling constant for the agostic proton is reduced
compared to typical JCH values associated with an sp2-hybri-
1
dized carbon atom (148-159 Hz),13 consistent with an agostic
C-H Ru interaction. Further evidence for the agostic C-H
A single-crystal X-ray diffraction study confirms the structure
of 4 (Figure 3). The geometry around the metal is a distorted
octahedron, with the carbonyl ligand occupying the formerly
vacant site of 3 and bent away from the pyridyl nitrogen atom
3 3 3
interaction is found in the solid-state structure (vide infra).
Surprisingly, formation of 5 from 3 and acetylene is readily
reversible in solution, reverting to 3 within ca. 1 d at room
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Figure 5. ORTEP drawing of [N3]Ru(CtC-H(cis,cis-1,2,3,4-tetra-
phenylbutadienyl-μ-H), 5, with 30% probability thermal ellipsoids. R1 =
14.25%. Selected distances (Å): Ru1-C28, 2.433(7).
Figure 6. ORTEP drawing of [N3]Ru(CtC-Ph)(cis,cis-1,2,3,4-tetra-
phenylbutadienyl-μ-H), 6, with 30% probability thermal ellipsoids.
Selected distances (Å): Ru1-H39, 1.90(3); Ru1-C39, 2.452(2);
Ru1-C36, 2.101(2), C36-C37, 1.363(3); C37-C38, 1.469(3);
C38-C39, 1.369(3).
temperature in the absence of excess acetylene. The reversibility
was further confirmed by treatment of 5 with 13C-labeled
acetylene (13C2 H2). Significant incorporation of the 13C label
1
into the acetylide ligand of 5 is observed within 1 h by 1H and
13C NMR spectrum at δ 110.29 (1JCH = 121.2 Hz) for the
butenyl terminus. This relatively small C-H coupling constant is
similar to that seen in 5 and is also consistent with a C-H
13C NMR.
Although the reversibility of the reaction prevented isolation
of 5 uncontaminated with 3, single crystals of 5 were grown from
a concentrated solution under acetylene, and an X-ray structure
determination was performed (Figure 5). The diffraction data
collected were not of high quality, and the structure refined to a
fairly high R-factor (R1 = 14.25%). This is sufficient, however, to
establish the overall connectivity, including the linear terminal
acetylide ligand and long distance (2.433(7) Å) between Ru and
terminal carbon of the cis,cis-butadienyl ligand. A peak was found in
the difference Fourier map between the metal and terminal carbon
of the C4Ph4 ligand, in a location consistent with the agostic
C-H-Ru linkage suggested by the NMR data. Fortunately, more
reliable evidence for the bridging C-H-Ru moiety is found in
the structure of the phenylacetylide derivative described next.
Treatment of 3 with phenylacetylene results in a rapid reaction
to generate 6 (eq 7), the phenylacetylide analogue of 5. Forma-
tion of 6 does not appear to be readily reversible, and the
compound was isolated as a dark solid in 80% yield.
Ru interaction.
3 3 3
A single-crystal X-ray structure determination was performed
for 6 (Figure 6). The geometry for 6 is distorted octahedral, with
the agostic butenyl C-H coordinated trans to the pyridine. The
bridging hydrogen was located and refined isotropically. Both the
agostic Ru H bond distance (1.90(3) Å) and the Ru H-C
3 3 3
3 3 3
angle (109(2)ꢀ) are well within the ranges reported for an agostic
H-C interaction (1.8-2.3 Å, 90-140ꢀ).14
M
3 3 3
Synthesis and Structure of [N3]RuCl2(C2H2). The acetylene
dichloride [N3]RuCl2(C2H2) (7) was prepared as a Ru(II)
analogue of 2. Generation of the known THF complex
[N3]RuCl2(THF),15 followed by in situ treatment with acety-
lene, afforded 7 as a purple solid (81%) (eq 8). The acetylenic
protons appear in the 1H NMR spectrum of 7 at δ 5.04, which
correlates to a 13C NMR peak at δ 84.99.
The solid-state structure of 7 exhibits an octahedral geometry
(Figure 7), with mutually trans chlorides and the acetylene ligand
lying within the [N3]Ru plane, coordinated trans to pyridine.
The Ru-C bond distances (2.179(3), 2.182(4) Å) are not
statistically different and are similar to the other structurally
characterized ruthenium η2-acetylene complexes reported in the
A singlet assigned to the agostic hydride is observed at δ 3.30
in the 1H NMR spectrum of 6, which correlates to a peak in the
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literature.7 The CtC bond length (1.198(8) Å) is slightly
elongated compared to that of free acetylene (1.181(7) Å).6
DFT Calculations on [N3]Ru(C2H2). The electronic structure
of 2 was investigated computationally using the Gaussian0916
package of programs at the B3LYP17 level of theory employing
the 6-31G(d,p) basis set18 for C, H, and N atoms and the
quasirelativistic small-core SDD pseudopotential and [6s5p3d]
contracted valence basis set for Ru,19 supplemented with two 4f-
type and one 5g-type function as described by Martin and
Sundermann.20 Because of the similar chemical reactivity pre-
viously observed in [N3]Ru systems containing mesityl and 2,6-
xylyl groups,3 the latter ligand was used for these studies.
The experimental solid-state and calculated geometries for 2
are shown in Figure 8, and metrical parameters listed in Table 1.
Whereas the former exhibits approximately C2v symmetry, the
lowest energy calculated geometry has the acetylene ligand
displaced slightly out of the plane of the [N3]Ru core; the angle
formed by the midpoint of the alkyne C-C bond, metal, and
pyridine N is 170.6ꢀ. In contrast, the planar geometry optimized
to a first-order saddle point with a single imaginary vibrational
mode corresponding to the out-of-plane bend of the HCCH
ligand (i.e., distorting toward the ground-state Cs structure).
However, the energy difference between the calculated Cs ground
state and C2v “transition state” is negligible (ΔEelect ca. 0.13 kcal
mol-1) and is easily within the error associated with this DFT
calculation.
Interestingly, closer examination of the crystal structure
reveals distinct elongation of the ruthenium and acetylenic
carbon thermal ellipsoids along the axis perpendicular to the
[N3]Ru plane (Figure 7), suggesting either thermal motion or an
unresolved crystallographic disorder. It is possible that observed
anisotropy results from inversion of the nonplanar geometry as in
the calculated structure, but the two required alkyne sites were
too close to be refined separately at half-occupancy. In short, the
calculated and experimental geometries are essentially the same
within the errors of the respective methods.
As mentioned above, the [N3] ligand is potentially noninno-
cent, and some formally zerovalent metal complexes are more
accurately described as [N3]-/M(I) or [N3]2-/M(II). Alkyne
complex 2 is diamagnetic; thus this type of [N3]-/Ru(I) valence
tautomer would correspond to a broken symmetry, open-shell
singlet (i.e., antiferromagnetically coupled unpaired electrons on
the metal and ligand anion). However, multiple attempts to use
unrestricted DFT methods either converged to the closed-shell
singlet during the SCF or yielded open-shell singlets that were
found to be unstable. In contrast, the closed-shell singlet
state used for the geometry optimization was confirmed to be
stable21 (i.e., exhibited no restricted-unrestricted or internal
instabilities). It has been shown that the degree of noninnocence
in [N3]Fe complexes is most pronounced with weak field ligands
Table 1. Comparison of Selected Experimental and Calcu-
lated Bond Distances in [N3]Ru(C2H2) (2)
distance (Å)
X-ray crystal structurea
DFT
Ru-C
2.032(2)
2.040(2)
1.239(4)
1.961(2)
2.057(2)
2.058(2)
1.332(3)
1.326(3)
1.425(3)
1.428(3)
2.023
2.023
1.280
1.950
2.098
2.098
1.331
1.331
1.427
1.427
CtC
Ru-Npyr
Ru-Nim
CdNim
Cipso-Cim
Figure 7. ORTEP drawing of [N3]RuCl2(C2H2), 7, with 30% prob-
ability thermal ellipsoids. Selected bond lengths (Å): Ru1-C29,
2.179(3), Ru1-C28, 2.182(4), C28-C29, 1.198(6).
a Numbers in parentheses are estimated standard deviations in the least
significant digits.
Figure 8. Comparison of experimental geometry of 2 (left) and optimized geometry from DFT calculations (right) showing ca. 10ꢀ bend of HCCH out
of the [N3]Ru plane.
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Table 2. NMR Data for Selected η2-Acetylene Complexes
Scheme 1. Mechanism for Cyclotrimerization of Acetylene
complex
1H δ (C2H2) 13C δ (C2H2)
4-electron donorsa
W(C2H2)Cl4(Et2O)b
W(C2H2)CO(S2CNEt2)c
17.5
215.5
12.5, 13.5 206, 207
Mo(tBuNS)2(tBuNC)2(C2H2)d
Mo(dppe)2(C2H4)e
[OsH(=C=CH2)(C2H2)(PiPr3)2]BF4
10.43
10.18
10.17
171.7
171
f
137.3
2-electron donorsa
[Os(en)2(C2H2)(C2H4)]2þg
7.3
103.4
h
[Re(C2H2)(NO)(PPh3)(Cp)]BF4
6.72, 6.83 84.5, 89.7
i
Ni(C2H2)(PPh3)2
6.41
122
Re2Cl4(μ-dppm)2(μ:η2,η2-C2H2)(η2-C2H2)j 6.31
96.3
64.5
60.4
66.14
62.4
Re(C2H2)(CO)2(Cp)k
5.61
5.57
4.38
4.98
l
[Ru(C2H2)(PMe2Ph)2(Cp)]BF4
Scheme 2. [4þ2] and [2þ2] Mechanisms for Reaction of
m
[Cp*Ru(C2H2)(PEt3)2]BPh4
Metallacyclopentadienes with Acetylene
n
[Ru(C2H2)(PMe3)2(Cp)]BF4
a Number of electrons formally donated to the metal by the acetylene
ligand. b Ref 25d. c Ref 25c. d Ref 25a. e Ref 25b. f Ref 25e. g Ref.25i. h Ref
25k. i Ref 25h. j Ref 25h.25h k Ref 25g. l Ref 7b. m Ref 7a. n Ref 25j.
such as N2 and p-dimethylaminopyridine,5 and it is likely the
acetylene in 2 is sufficiently strong field to favor the closed-shell
ground state.
’ DISCUSSION
Bonding in [N3]Ruthenium Acetylene Complexes. Bond-
ing between the metal and the acetylene ligand in 2 and 7 can be
described in part by the Dewar-Chatt-Duncanson22 model,
where the primary interaction occurs through orbitals in the
M(C2) plane (alkyne π to dσ and d to alkyne π *). In addition,
the orthogonal C-C π-orbital can donate up to two electrons to
a vacant metal d-orbital (alkyne π^ to d). Although allowed by
orbital symmetry, back-bonding from a metal d-orbital into the
alkyne π^* LUMO is generally insignificant due to poor overlap.
Whereas the dichloride 7 is an 18 e- ruthenium(II) complex
with an unambiguous 2 e- donor alkyne ligand, the bonding in 2
warrants closer scrutiny. The 14 e- [N3]Ru fragment can
potentially accept 4 e- from the alkyne to achieve saturation.
The molecular structures and spectroscopic properties of 2 e-
and 4 e- alkyne complexes are usually diagnostic; thus compar-
ison of 2 and 7 is instructive.
Although the short alkyne-metal distances in 2 are consistent
with 4 e- donation to the metal, the NMR parameters are less
conclusive. The 1H resonance for the alkyne in 2 (δH = 6.68) is in
the range expected for 2 e- donors, whereas the 13C resonance
(δC = 131.0) lies downfield of the 2 e- donor range, almost into
the range observed for 4 e- donors. However, the proximity of
the alkyne protons to the mesityl rings of the [N3] ligand could
lead to an unusually upfield shift of the 1H (and 13C) resonances
than typically found for 4 e- donor alkynes. Overall, the chemical
shift trends for 2 are consistent with a greater degree of electron
donation from the acetylene to the formally Ru(0), 14 e- center
than in the Ru(II) dichloride.
Mechanism of Cyclotrimerization. Metal-mediated formal
[2þ2þ2] cyclotrimerization of alkynes has been studied
extensively,2a and consensus has developed that metallacyclo-
pentadienes are key intermediates in both cyclization and some
cases of linear alkyne coupling.26 Although no metallacyclic
intermediates were observed during the coupling of acetylene
to benzene by 2, the formation of ruthenole 3 from PhCCPh
supports the role of an analogous species in the catalytic cycle, as
depicted in the mechanism proposed in Scheme 1. Reaction of 2
with a second equivalent of acetylene generates ruthenacyclo-
pentadienyl intermediate B, possibly via a discrete adduct such as
A. There are at least two precedented paths by which B could
react with acetylene to yield benzene, and these are considered in
greater detail below.
In general, metal-carbon bond distances decrease with great-
er electron donation from the alkyne.23 More specifically, Carbꢀo
and co-workers have noted that Os-C distances in osmium-
alkyne complexes are substantially shorter (ca. 0.15 Å) for four-
electron -donor alkynes than for two-electron donors, whereas the
alkyne triple-bond lengths are relatively unaffected. Significantly,
the Ru-C distance in 2 is ca. 0.14 Å shorter than in 7, although the
alkyne C-C distances are similar (ca. 0.04 Å longer for 2).
Another diagnostic for 4 e- donor ligands is the position of the
alkyne resonances in the 1H and 13C NMR, which are generally
1
found significantly downfield of 2 e- donors.24 Acetylenic H
NMR shifts in structurally characterized η2-acetylene complexes
range from about δ 4.4 to 7.3 for 2 e- donors and from δ 10.2 to
13.5 for 4 e- donor complexes (Table 2),7,25 and 13C shifts range
from δ 60 to 103 for 2 e- donors and δ 137 to 207 for 4 e-
donors. As expected, 7 exhibits alkyne shifts (δH = 5.04, δC =
85.0) well within the 2 e- donor ranges.
The details of the reaction between metallacycles such as B
and a third alkyne have been a subject of considerable interest.
Consistent with the bulk of previous studies, recent work by
Gandon and co-workers27 indicates that formal [4þ2] cycliza-
tion between the alkyne and the metallacyclopentadiene fur-
nishes a 7-metallanorbornadiene (C) (Scheme 2, path I).
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Reductive elimination would then give benzene. The obvious
alternative mechanism—alkyne insertion into the metallacyclo-
pentadiene to yield a seven-membered ring, followed by reduc-
tive elimination—has been considered less likely than the [4þ2]
addition. However, recently the groups of Schmid28 and
Yamamoto29 have made a strong case for the intermediacy of a
seven-membered metallacycle in alkyne trimerization catalyzed
by certain complexes. Interestingly, these authors suggest alkyne
insertion does not proceed directly, but rather involves the
formal [2þ2] cycloaddition of an alkyne to a MdC bond of
the metallacyclopentatriene resonance structure, which is sig-
nificant for some “metallacyclopentadiene” complexes. The
resultant metallabicyclo[3.2.0]heptatriene intermediate (D)
(Scheme 2, path II) rearranges to the seven-membered metalla-
cycle (E), from which reductive elimination generates benzene.
The geometric parameters for ruthenole 3 clearly suggest the
ruthenacyclopentadiene depiction is preferred over ruthenacy-
clopentatriene, and this would seem to favor path I over path II.
Note, however, that 3 does not react with alkynes to yield
trimerization products and that path II cannot be ruled out in
the trimerization of parent acetylene or in the reaction of 2 with
diphenylacetylene to give tetraphenylbenzene. In either case,
steric bulk of the metallacycle—combined with the bulky mesityl
groups on the [N3] ligand—would likely hinder access to the
MC4 π system by an incoming alkyne. The observation that 3
does not participate in alkyne cyclization reactions is consistent
with steric inaccessibility of the ruthenacyclic π system.
Scheme 3. Comparison of Reactivity of 3 with Diphenylace-
tylene or Terminal Alkynes versus the Reactivity of 2 with
Diphenylacetylene
Comparing the reactivity of 3 with diphenylacetylene or terminal
alkynes versus the reactivity of 2 with diphenylacetylene further
illustrates the role of ruthenacycle sterics in cyclization (Scheme 3).
The steric bulk of 3 prevents cyclization with diphenylacetylene,
although the less hindered end-on approach of the C-H bond of
terminal alkynes permits formation of agostic complexes 5 and 6. In
contrast, coupling of diphenylacetylene and acetylene can be
accomplished if the sequence of alkyne addition is reversed: C2H2
complex 2 reacts with excess diphenylacetylene to yield tetraphe-
nylbenzene. The intermediate ruthenacycle for this cyclization
Figure 9. Molecular orbitals involving the acetylene π-orbitals perpendicular to the [N3]Ru plane in 2: Acetylene π^ to Ru dxz donation (left, HOMO-
11) and weak Ru dyz to π^* back-donation (right, HOMO-2). Top and bottom figures are of each MO viewed from different perspectives.
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(A, Scheme 3), although not observed, would be diphenyl- rather
than tetraphenyl-substituted and thus less hindered than 3, and
apparently permits reaction with a second equivalent of PhCCPh.
The agostic complexes 5 and 6 suggest at least one explanation
for the short lifetime of 2 under catalytic conditions. In analogy
with the formation of 5, a secondary path for reaction of
acetylene with the parent metallacyclopentadiene by addition
of the acetylenic C-H bond would produce the acyclic butadie-
nyl species, possibly stabilized by an agostic C-H interaction.
Subsequent insertion(s) of acetylene into either the acetylide or
butadienyl Ru-C bonds could lead to a distribution of polyene
oligomers that could be difficult to characterize and might retain
the ruthenium complex.
Propyne (Aldrich) was degassed prior to use. Triethylsilane (Aldrich)
was dried over Na prior to use. Ruthenium complex [κ2-N3]Ru(η6-
MeC6H5) (1) was synthesized according to the literature procedure.3
Abbreviations used: Mes = 1,3,5-trimethylphenyl; [N3] = 2,6-(MesNd
CMe)2C5H3N.
DFT Calculations. All calculations were performed using the
Gaussian0916 package of programs at the B3LYP17 level of theory
employing the 6-31G(d,p) basis set18 for C, H, and N atoms and the
quasirelativistic small-core SDD pseudopotential and [6s5p3d] con-
tracted valence basis set19 for Ru, supplemented with two 4f-type and
one 5g-type function as described by Martin and Sundermann.20
Calculated energy minima for optimized geometries were confirmed
as a stationary point by the absence of imaginary vibrational modes in a
subsequent frequency calculation. All wave functions derived from single-
point energy calculations were confirmed to exhibit no restricted-
unrestricted or internal instabilities.21
’ CONCLUSIONS
The reaction of [κ2-N3]Ru(η6-MeC6H5) (1) with acetylene
generates the formally Ru(0) acetylene complex [N3]Ru(C2H2)
(2). Spectroscopic, structural, and computational evidence sug-
gest significant participation of the acetylene π-orbitals ortho-
gonal to the RuC2 plane (π^), which is consistent with the 14 e-
count of the parent [N3]Ru fragment. However, the NMR data
suggest π^ donation is weaker in 2 than in many 4 e- donor
alkyne complexes. DFT calculations also indicate 2 is appro-
priately described by a closed-shell singlet wave function and that
2 is adequately described by the Ru(0) formal oxidation state.
The behavior of [N3]Ru(C2H2) (2) as an alkyne cyclotrimer-
ization catalyst and the lack of trimerization activity for
[N3]Ru(C4Ph4) (3) both shed light on the trimerization me-
chanism. In particular, the sterics of the intermediate metalla-
cycle plays a key role in determining the outcome of the reaction.
For example, the isolated ruthenole 3 is a model for a likely
intermediate. However, trimerization requires the interaction of
an alkyne with the metallacyclic π system, and this is prevented
by the steric bulk of the MC4 moiety. In contrast, acetylene
complex 2 reacts with either acetylene or diphenylacetylene to
generate trimerization products. In these cases, the likely—
although unobserved—intermediate ruthenoles are less bulky
and allow access of a third alkyne.
Synthesis of [N3]Ru(C2H2), 2. A round-bottomed flask was
charged with [κ2-N3]Ru(η6-MeC6H5) (1) (200 mg, 0.339 mol), and
diethyl ether (ca. 15 mL) was vacuum transferred into the reaction
vessel. The solution was cooled to -196 ꢀC, and acetylene (0.44 mmol,
1.3 equiv) was measured out with a calibrated gas bulb and condensed
into the vessel. The reaction mixture was warmed to room temperature
and stirred 10 min, upon which the color of the solution turned green.
Recrystallization from diethyl ether afforded 112 mg (63% yield) of 2 as
green crystals. 1H NMR (C6D6; δ): 7.92 (d, 2H, Py-Hm), 7.81 (t, 1H,
Py-Hp), 6.91 (s, 4H, Mes-Hm), 6.68 (s, 2H, C2H2), 2.26 (s, 6H, Mes-
Mep), 1.99 (s, 12H, Mes-Meo), 1.28 (s, 6H, Im-Me). 1H NMR (THF-d8;
δ): 8.26 (d, 2H, 3JHH = 7.7 Hz, Py-Hm), 7.98 (t, 1H, 3JHH = 7.7 Hz, Py-
Hp), 7.05 (s, 4H, Mes-Hm), 6.10 (s, 2H, Ru(C2H2), 2.41 (s, 6H, Mes-
Mep), 1.87 (s, 12H, Mes-Meo), 1.51 (s, 6H, Im-Me). 13C{1H} NMR
(THF-d8; δ): 153.86, 153.81, 145.90, 135.11, 130.91 (quaternary),
131.00 (1JCH = 226.4 Hz, C2H2), 129.25 (Mes-Cm), 118.60 (Py-Cm),
117.34 (Py-Cp), 21.30, 19.36, 18.53 (Mes-Mep and Im-Me).
Reaction of [K2-N3]Ru(η6-MeC6H5) (1) with CO. An NMR
tube equipped with a PTFE needle valve was loaded with a benzene-d6
solution (0.3 mL) of 1 (5 mg, 0.0095 mmol). The solution was degassed,
and 1 atm of CO was backfilled into the NMR tube. Only [N3]Ru(CO)2
and free acetylene were observed by 1H NMR. 1H NMR of
[N3]Ru(CO)2 (C6D6; δ): 7.11 (d, 2H, Py-Hm), 6.93 (t, 1H, Py-Hp),
6.80 (s, 4H, Mes-Hm), 2.13 (s, 6H, Mes-Mep or Im-Me), 2.11 (s, 6H,
Mes-Mep or Im-Me), 2.02 (s, 12H, Mes-Meo).
Reaction of [N3]Ru(C2H2) (2) with Excess Acetylene. An
NMR tube equipped with a PTFE needle valve was loaded with a
cyclohexane-d12 solution (0.3 mL) of 2 (2 mg, 0.00381 mmol) with 1 μL
of hexamethyldisiloxane added as an internal integration standard. The
solution was degassed, and excess acetylene (0.04 mmol, ca. 10.5 equiv)
was measured out with a calibrated gas bulb and condensed into the
NMR tube. The reaction was monitored by H NMR until all 2 was
consumed, at which time the reaction mixture contained 2.7 equiv of
benzene compared to 2 originally present.
’ EXPERIMENTAL SECTION
General Methods. All manipulations were performed in Schlenk-
type glassware on a dual-manifold Schlenk line or in a nitrogen-filled
Vacuum Atmospheres glovebox.30 All glassware was oven-dried prior to
use. Infrared spectra were recorded on a Perkin-Elmer Spectrum 100
FT-IR spectrometer. GC-MS analyses were performed with an Agilent
6890 GC system coupled to an Agilent 5973 mass-selective detector.
NMR spectra were obtained at 300, 360, and 500 MHz on Bruker DMX-
300, AM-360, and AMX-500 FT NMR spectrometers, respectively.
13C{1H} NMR spectra were recorded with broadband 1H decoupling on
a Bruker DMX-360 spectrometer, and proton-coupled 13C spectra were
recorded using a gated decoupling sequence. All NMR spectra were recorded
at 300 K unless stated otherwise. Chemical shifts are reported relative to
tetramethylsilane for 1Hand13C spectra. The temperature of the NMR probe
for all variable-temperature studies was calibrated against methanol.
Materials. Hydrocarbon solvents were dried over Na/K alloy-
benzophenone. Benzene-d6, toluene-d8, and tetrahydrofuran-d8 were
dried over Na/K alloy. Chloroform-d was dried over activated 4 Å
molecular sieves. Acetylene (Airco) was purified according to literature
procedures.31 Acetylene-13C2 (Cambridge Isotope Laboratories) was
degassed prior to use. Ethylene and CO (Airco) were used as received.
Trimethylphosphine was prepared according to literature procedures.32
1
Reaction of [K2-N3]Ru(η6-MeC6H5) (1) with Propyne. An
NMR tube equipped with a PTFE needle valve was loaded with a C6D6
solution (0.3 mL) of 1 (10 mg, 0.0169 mmol). The solution was
degassed, and a slight deficit of propyne (0.015 mmol, ca. 0.9 equiv)
was measured out with a calibrated gas bulb and condensed into the
NMR tube. Following thawing, the solution was monitored by 1H NMR
and was found to contain ca. 10% 1,3,5-trimethylbenzene (mesitylene)
and an unstable complex tentatively assigned as [N3]Ru(HCtCMe).
1H NMR of mesitylene (C6D6; δ): 6.71 (s, 3H, Ar-H), 2.15 (s, 9H, Ar-
Me). 1H NMR of [N3]Ru(HCtCMe) (C6D6; δ): 8.01 (d, 1H, Py-Hm),
7.97 (d, 1H, Py-Hm), 7.80 (t, 1H, Py-Hp), 6.93 (s, 2H, Mes-Hm), 6.91 (s,
2H, Mes-Hm), 6.78 (br, 1H, HCtC), 2.29, 2.26 (both s, 3H, Mes-Mep),
4
2.02, 1.98 (both s, 6H, Mes-Meo), 1.85 (d, 3H, JHH = 2.8 Hz, -Ct
CMe), 1.32, 1.26 (both s, 3H, Im-Me).
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Synthesis of [N3]Ru(C4Ph4), 3. A round-bottomed flask was
charged with [η2-N3]Ru(η6-MeC6H5) (1) (150 mg, 0.254 mmol) and
diphenylacetylene (100 mg, 0.561 mmol, 2.2 equiv). The reactants were
dissolved in toluene (4 mL) and stirred for 1 h, whereupon the solution
turned bright blue. Recrystallization from toluene yielded 200 mg of 3 as
a crystalline blue solid (92% yield). 1H NMR (C6D6; δ): 7.29 (d, 2H,
aryl-H), 7.13-6.98 (m, aryl-H), 6.94 (t, aryl-H), 6.87-6.69 (m, 6H,
aryl-H), 6.46 (s, 2H, Mes-Hm), 6.40 (t, 2H), 5.64 (d, 2H), 5.70 (d, 2H),
2.72, 2.10, 1.48, 1.13 (all s, 6H, Im-Me and Mes-Me). 1H NMR (THF-d8;
δ): 7.68 (d, 2H, Py-Hm), 7.44 (t, 1H, Py-Hp), 6.93 (s, 2H, Mes-Hm), 6.78
(s, 2H, Mes-Hm), 6.57-6.48 (m, aryl-H), 6.38 (t, 2H, aryl-H), 6.30 (t,
2H, aryl-H), 5.46 (d, 2H, aryl-H), 5.20 (d, 2H, aryl-H), 2.51, 2.25, 1.85,
1.12 (all s, 6H, Im-Me and Mes-Me). 13C{1H} NMR (THF-d8; δ):
215.76, 169.24, 158.48, 156.26, 153.44, 151.00, 147.45, 144.61, 144.18,
141.49, 140.88, 135.26 (all quaternary), 133.43 (aryl-CH), 132.43
(quaternary), 131.98 (aryl-CH), 130.07 (quaternary), 129.84, 129.76,
129.27, 129.07, 127.84, 127.73, 126.95, 126.79, 126.73, 126.95, 126.79,
126.72, 126.21, 123.96, 123.26, 122.17, 121.18 (all aryl-CH), 21.08,
20.16, 17.96, 17.84 (Im-Me and Mes-Me).
Reaction of [N3]Ru(C2H2) (2) with Diphenylacetylene. An
NMR tube was loaded with a benzene-d6 solution (0.4 mL) of
[N3]Ru(C2H2) (2) (5 mg, 0.0095 mmol) and diphenylacetylene (7
mg, 4.2 equiv). The NMR tube was flame-sealed, and the reaction was
monitored by 1H NMR. All of the [N3]Ru(C2H2) (2) initially present
was consumed within 1.5 h, and both 3 and 1,2,3,4-tetraphenylbenzene
were observed by 1H NMR. The reaction mixture was eluted through a
silica gel column to separate the 1,2,3,4-tetraphenylbenzene. 1H NMR of
tetraphenylbenzene (CDCl3; δ): 7.50 (s, 2H), 7.12 (m, 10H), 6.92 (m,
10H), 6.80 (m, 4H). MS (CI): calcd 383.1704 [M þ H]þ, found 383,
calcd 411.2107 [M þ C2H5]þ 411
(all quaternary), 133.44, 132.58, 131.45 (all Ar-CH), 131.31
(quaternary), 130.41, 129.96, 129.85, 129.08, 127.32, 127.03, 126.96,
126.52, 126.22, 126.10, 125.01, 124.92, 124.27, 124,05, 122.36, 121.64
(all Ar-CH), 118.77 (1JCC = 106.2 Hz, Ru-CC), 111.13 (1JCH = 122 Hz,
Ru H-C), 100.85 (1JCC = 106.2 Hz, 1JCH = 220 Hz, Ru-CC-H), 22.44,
3 3 3
21.02, 20.89, 17.56 (Im-Me and Mes-Me).
Reversibility of Formation of 5: Reaction of 5 with 13C2H2.
An NMR tube equipped with a PTFE needle valve was loaded with a
THF-d8 (0.3 mL) solution of [N3]Ru(C4Ph4) (3) (6 mg, 0.007 mmol),
and excess acetylene (0.01 mmol, 1.4 equiv) was condensed into the
tube. The reaction was monitored by 1H NMR until formation of 5 was
complete, whereupon excess acetylene-13C2 (0.014 mmol, 2 equiv) was
condensed into the NMR tube. The reaction was monitored by 1H and
13C NMR. Incorporation of the 13C label into 5 was observed within 0.5 h.
13C NMR (THF-d8; δ): 118.77 (1JCC = 106.1 Hz, Ru-CC 111.13
(1JCH = 122 Hz, 1JCC = 106.1 Hz, Ru H-C).
3 3 3
Synthesis of [N3]Ru(CtC-Ph)(cis,cis-1,2,3,4-tetraphenyl-
butadienyl-μ-H), 6. A round-bottomed flask was charged with 4
(150 mg, 0.0175 mmol), and toluene was added (5 mL). A slight excess
of phenylacetylene (23 μL, 0.0209 mmol, 1.2 equiv) was added, effecting
an immediate color change from blue to brown. Recrystallization from
1
toluene afforded 135 mg of 6 (80% yield) as a brown solid. H NMR
(C6D6; δ): 7.22 (d, 2H, Ar-H), 7.05 (t, 2H, Ar-H), 6.98-6.95 (m, Ar-
H), 6.91-6.89 (m, Ar-H), 6.85-6.71 (m, Ar-H), 6.63 (m, 2H, Ar-H),
6.50 (t, 2H, Ar-H), 6.38 (d, 2H, Ar-H), 5.94 (d, 2H, Ar-H), 3.30 (s, 1H,
Ru HC), 2.70, 2.15, 2.14, 1.92 (all s, 6H, Im-Me and Mes-Me). 1H
3 3 3
NMR (CDCl3; δ): 7.49 (d, 2H, Py-Hm), 7.26 (t, 1H, Py-Ho), 6.88 (m,
Ar-H), 6.74 (s, 2H, Mes-Hm), 6.68-6.49 (m, Ar-H), 6.39 (t, 2H, Ar-H),
5.87 (d, 2H, Ar-H), 5.75 (d, 2H, Ar-H), 2.83 (s, 1H, Ru H), 2.55,
3 3 3
2.34, 2.30, 1.94 (all s, 6H, Im-Me and Mes-Me). 13C NMR (CDCl3 δ):
166.24, 164.67, 154.97, 147.84, 142.15, 142. 06, 141.47, 141.31, 139.80,
135.14, 133.27 (all quaternary), 131.98, 131.60, 131.10, 130.57 (all Ar-
CH), 130.57 (quaternary), 129.77, 129.38, 127.12, 126.73, 126.43,
126.01, 125.62, 124.13, 123.95, 123.79, 123.75, 123.63, 123.59,
Synthesis of [N3]Ru(C4Ph4)(CO), 4. A 10 mL round-bottomed
flask was charged with 3 (30 mg, 0.035 mmol), and toluene (4 mL) was
vacuum transferred into the reaction vessel. CO (200 Torr) was
admitted, and the resultant slurry was stirred for 0.5 h. The reaction
mixture was filtered and dried in vacuo to afford 24 mg of 4 as an orange
solid (77% yield). 1H NMR (C6D6; δ): 7.20 (d, 2H), 7.08-6.89 (m),
7.72-6.88 (m), 6.48 (t, 2H), 6.07 (d, 2H), 5.92 (d, 2H), 2.99, 2.18, 1.72,
1.66 (all s, 6H, Im-Me and Mes-Me). 1H NMR (THF-d8; δ): 7.61 (d, 2H,
Py-Hm), 7.51 (t, 1H, Py-Hp), 7.00 (s, 2H, Mes-Hm), 6.81 (s, 2H, Mes-
Hm), 6.63-6.45 (m), 6.36 (d), 5.97 (d, 2H, Ar-H), 5.42 (d, 2H, Ar-H),
2.79, 2.33, 2.28, 1.72 (all s, 6H, Im-Me and Mes-Me). 13C{1H} NMR
(THF-d8; δ): 194.06, 191.58, 184.49, 170.87, 158.98, 154.87, 154.06,
150.34, 147.68, 145.98, 145.29, 136.13, 132.67 (all quaternary), 132.20,
130.87, 130.46, 129.83, 129.74, (all Ar-CH), 128.82 (quaternary),
127.40, 127.04, 126.80, 126.57, 126.50, 125.33, 123.63, 123.06,
121.93, 121.41 (all Ar-CH), 21.12, 21.02, 18.59, 17.77 (Im-Me and
Mes-Me). Ir (Nujol): ν(CO) 1926 cm-1. HRMS (ES): calcd 883.3076
(M(102Ru))þ, found 883.3103.
Observation of [N3]Ru(CtC-H)(cis,cis-1,2,3,4-tetraphenyl-
butadienyl-μ-H), 5. An NMR tube equipped with a PTFE needle
valve was loaded with a THF-d8 solution (0.3 mL) of [N3]Ru(C4Ph4)
(3) (10 mg, 0.012 mmol). Excess acetylene (0.06 mmol, 5 equiv) was
measured out with a calibrated gas bulb and condensed into the NMR
tube. Following thawing, the solution changes from blue to purple over a
period of 5 min. Attempts to isolate pure 5 were thwarted by reversion to
3 in vacuo. A small number of crystals of 5 were fortuitously grown under
excess acetylene and used for the X-ray study. 1H NMR (THF-d8; δ):
7.69 (d, 2H, 3JHH = 7.9 Hz, Py-Hm), 7.35 (d, 1H, 3JHH = 7.9 Hz, Py-Hp),
7.16 (m, 2H, Ar-H), 6.91 (s, 2H (Mes-Hm), 6.85 (m, 2H, Ar-H), 6.76
(m, 2H, Ar-H), 6.68 (s, 2H, Mes-Hm), 6.57-6.41 (m, 10H, Ar-H), 5.94
(d, JHH = 7.4 Hz, Ar-H), 5.74 (d, JHH = 7.4 Hz, Ar-H), 2.77 (s, 1H,
122.89, 121.73, (all Ar-CH), 120.87 (quaternary), 110.29 (1JCH
=
121.2 Hz, Ru H-C), 20.98, 20.64, 20.36, 17.59 (Im-Me and Mes-Me).
3 3 3
Synthesis of [N3]RuCl2(C2H2), 7. A thick-walled, tubular reaction
vessel was charged with 200 mg (0.0503 mmol) of the [N3] ligand,
2,6-(MesNdCMe)2C5H3N, and 154 mg of [(p-cymene)RuCl2]2
(0.0252 mmol). THF (15 mL) was vacuum transferred into the reaction
vessel, which was then heated in a 150 ꢀC oil bath until all of the reactants
were consumed (5 d), as evidenced by 1H NMR. The resultant solution
was transferred into a swivel frit assembly, and the volume of THF was
reduced to ca. 5 mL. Excess acetylene (2.5 mmol, ca. 5 equiv) was
measured out with a calibrated gas bulb and condensed into the reaction
vessel, and the mixture was stirred for 1 h at room temperature.
Recrystallization from THF under an acetylene atmosphere afforded
243 mg of 7 as a crystalline, purple solid (81%). 1H NMR (C6D6; δ):
7.01 (d, 2H, Py-Hm), 6.77 (t, 1H, Py-Hp), 6.68 (s, 4H, Mes-Hm), 5.44 (s,
2H, C2H2), 2.39 (s, 12H, Mes-Meo), 2.08, 2.05 (s, 6H, Mes-Mep and Im-
Me). 1H NMR (CDCl3; δ): 8.05 (d, 2H, Py-Hm), 7.79 (t, 1H, Py-Hp),
6.92 (s, 4H, Mes-Hm), 5.04 (s, 2H, Ru(C2H2), 2.73, 2.31 (s, 6H, Mes-
Mep and Im-Me) 2.15 (s, 12H, Mes-Meo). 13C{1H} NMR (CDCl3; δ):
174.62, 157.02, 145.33, 136.36, 131.28 (all quaternary), 129.88 (Mes-
Cm), 118.60 (Py-Cp), 117.34 (Py-Cm), 84.99 (C2H2), 19.36 (Mes-Meo),
20.91, 18.93 (Mes-Mep and Im-Me).
Single-Crystal X-ray Diffraction Analysis: General Proce-
dures. X-ray intensity data were collected on a Rigaku Mercury CCD
area detector employing graphite-monochromated Mo KR radiation
(λ = 0.71069 Å) at a temperature of 143 K. Preliminary indexing was
performed from a series of 12 0.5ꢀ rotation images with exposures of
30 s. Oscillation images were processed using CrystalClear,33 producing a
listing of unaveraged F2 and σ(F2) values, which were then passed to the
CrystalStructure34 program package for further processing and structure
Ru HC), 2.51, 2.32, 2.25, 1.87 (all s, 6H, Im-Me and Mes-Me), 0.93
3 3 3
(s, 1H, Ru-CCH). 13C NMR (THF-d8; δ): 212.38, 167.29, 167.37,
156.00, 148.95, 148.91, 148.07, 143.22, 143.05, 140.97, 135.59, 134.48
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Organometallics
ARTICLE
solution. The intensity data were corrected for Lorentz and polarization
effects and for absorption using REQAB. The structures were solved by
direct methods (SIR97).35 Refinement was by full-matrix least-squares
based on F2 using SHELXL-97.36 All reflections were used during
refinement (F2’s that were experimentally negative were replaced by
F2 = 0). Non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were refined using a “riding” model, except hydride hydrogen
atoms, which were refined isotropically, unless otherwise noted.
[N3]Ru(C2H2) (2). Suitable X-ray quality crystals of 2 were grown by
slow evaporation of a diethyl ether solution.
[N3]Ru(C4Ph4) (3). Suitable X-ray quality crystals of 3 were grown
by slow diffusion of cyclohexane into a concentrated toluene solution.
[N3]Ru(C4Ph4)(CO) (4). Suitable X-ray quality crystals of 4 were
grown by slow evaporation of a cyclohexane solution. There were
two areas of disordered solvent (cyclohexane). The X-ray data were
corrected for the presence of disordered solvent using SQUEEZE.37
[N3]Ru(CtC-H)(cis,cis-1,2,3,4-tetraphenylbutadienyl-μ-H)
(5). Marginal quality crystals of 5 were grown by slow evaporation of a
diethyl ether solution under an acetylene atmosphere. Non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined
using a “riding” model, except H28 (the agostic Ru-H-C), which was
not refined but was included as a constant contribution to the structure
factors. The diffraction data collected were not of high quality, and the
structure refined to a fairly high R-factor (R1 = 14.25%), but is sufficient
to establish the general connectivity.
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data
b
1
(in CIF format) and H NMR spectra of purified samples are
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: berry@sas.upenn.edu.
€
Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
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’ ACKNOWLEDGMENT
We are grateful to the National Science Foundation for
support of this work under grants CHE-9904798 and CHE-
0131132.
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