The life, death, and ROMP activity of ruthenium complexes containing the basic, chelating diphosphine bis(dicyclohexyl)-1,4-phosphinobutane

Article abstract of DOI:10.1139/v00-208

Reaction of RuCl₂(PPh₃)₃ with bis(dicyclohexyl)-1,4-phosphinobutane (dcypb) under N₂ affords access to a formerly elusive family of dcypb complexes based on the RuCl₂(PP) core. Under Ar or vacuum atmosphere, decomposition occurs via Ru-promoted dehydrogenation of the dcypb ligand. While the N₂-stabilized species [RuCl₂(dcypb)]₂(N₂) (4a) is easily handled under N₂ in nonchlorinated solvents, reaction with chlorinated solvents rapidly yields paramagnetic Ru₂Cl₅(dcypb)₂ (5). The N₂ ligand within 4a is readily displaced under H₂ or CO atmosphere, yielding Ru₂Cl₄(dcypb)₂(H₂) (6) or RuCl₂(dcypb)(CO)₂, the latter as a mixture of ccc-(7) and tcc-(8) isomers. Benzylidene derivative RuCl₂(dcypb)(CHPh) (1a), prepared in situ by reaction of 4a with PhCHN₂, proves exceptionally active in ring-opening metathesis polymerization (ROMP) of norbornene. The X-ray crystal structure of 5 is reported: triclinic, a = 13.390(2), b = 15.726(2), c = 19.524(2) A, α = 77.325(2), β = 70.964(2), γ = 73.478(2)°, with space group P1 and Z = 2.

Full text of DOI:10.1139/v00-208

958
The life, death, and ROMP activity of ruthenium
complexes containing the basic, chelating
diphosphine bis(dicyclohexyl)-1,4-phosphinobutane
Dino Amoroso, Glenn P.A. Yap, and Deryn E. Fogg
Abstract: Reaction of RuCl2(PPh3)3 with bis(dicyclohexyl)-1,4-phosphinobutane (dcypb) under N2 affords access to a for-
merly elusive family of dcypb complexes based on the RuCl2(PP) core. Under Ar or vacuum atmosphere, decomposition
occurs via Ru-promoted dehydrogenation of the dcypb ligand. While the N2-stabilized species [RuCl2(dcypb)]2(N2) (4a) is
easily handled under N2 in nonchlorinated solvents, reaction with chlorinated solvents rapidly yields paramagnetic
Ru2Cl5(dcypb)2 (5). The N2 ligand within 4a is readily displaced under H2 or CO atmosphere, yielding
Ru2Cl4(dcypb)2(H2) (6) or RuCl2(dcypb)(CO)2, the latter as a mixture of ccc-(7) and tcc-(8) isomers. Benzylidene deriva-
tive RuCl2(dcypb)(CHPh) (1a), prepared in situ by reaction of 4a with PhCHN2, proves exceptionally active in ring-
opening metathesis polymerization (ROMP) of norbornene. The X-ray crystal structure of 5 is reported: triclinic, a =
13.390(2), b = 15.726(2), c = 19.524(2) Å, α = 77.325(2), β = 70.964(2), γ = 73.478(2)°, with space group P1 and Z = 2.
Key words: ruthenium, alkylphosphine, dehydrogenation, carbene, metathesis, crystal structure.
Résumé : La réaction du RuCl2(PPh3)3 avec le bis(dicyclohexyl)-1,4-phosphinobutane (dcypb), sous atmosphère d’azote,
permet d’accéder à la famille antérieurement élusive de complexes de dcypb comportant un noyau RuCl2(PP). Sous at-
mosphère d’argon ou sous vide, il se produit une décomposition qui se produit par le biais d’une déshydrogénation du li-
gand dcypb provoquée par le Ru. Alors que l’on peut facilement manipuler l’espèce stabilisée par le N2
[RuCl2(dcypb)]2(N2), 4a, sous atmosphère d’azote, dans des solvants non chlorés, la réaction dans des solvants chlorés
conduit à la formation de l’espèce paramagnétique Ru2Cl5(dcypb)2, 5. On peut facilement déplacer le ligand N2 du com-
posé 4a sous atmosphère de H2 ou de CO avec formation de Ru2Cl4(dcypb)2(H2), 6, ou de RuCl2(dcypb)(CO)2 qui existe
sous la forme de mélange d’isomères ccc-(7) et tcc-(8). Le dérivé benzylidène, RuCl2(dcypb)(CHPh), 1a, préparé in situ
par réaction du composé 4a avec PhCHN2, est exceptionnellement actif dans la polymérisation par ouverture de cycle et
métathèse du norbornène. On a déterminé la structure cristalline par diffraction des rayons X du composé 5: triclinique, a =
13,390(2), b = 15,726(2) et c = 19,524(2)Å, α = 77,325(2), β = 70,964(2) et γ = 73,478(2)°, groupe d’espèce P1 et Z = 2.
Mots clés : ruthénium, alkylphosphine, déshydrogénation, carbène, métathèse, structure cristalline.
[Traduit par la Rédaction] Amoroso et al. 963
Introduction
RuCl2(PP)(CHPh) (PP = R2P(CH2)4PR2; R = Cy (dcypb,
1,4-bis(dicyclohexylphosphino)butane) (1a), R = Ph (dppb,
1,4-bis(diphenylphosphino)butane) (1b)) were generated in
situ by reaction of mixed-phosphine species RuCl2(PP)(PPh3)
(2) or dimeric [RuCl2(dppb)]2 with phenyldiazomethane.
The inhibiting effect of free PPh3 was explored within the
RuCl2(dppb)(PPh3)–[RuCl2(dppb)]2 pair, but no route to the
corresponding [RuCl2(dcypb)]2 precursor existed. Here we
report a new route to PPh3-free Ru complexes containing the
chelating, basic diphosphine ligand dcypb, with insights into
decomposition pathways that have until now hindered the
broader deployment of such systems. The ROMP activity of
the PPh3-free system is described.
Enhanced catalytic activity is associated with use of elec-
tron-rich phosphine ligands in a range of reactions promoted
by late transition metal complexes, including metathesis (1,
2) and hydrogenation (3, 4) catalysis. Complexes of chelat-
ing alkyldiphosphines remain little explored in such chemis-
try, relative to the ubiquitous aryldiphosphine derivatives.
We recently described the first Ru–diphosphine catalysts to
exhibit high activity in ring-opening metathesis polymeriza-
tion (ROMP) without ligand loss (2). Benzylidene catalysts
Received September 5, 2000. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on July 11, 2001.
Dedicated to Professor Brian R. James on the occasion of his
retirement, in honour of his outstanding contributions to
catalysis and chemistry.
Experimental
Unless otherwise stated, all operations were performed
under N2 using standard Schlenk or drybox techniques. Dry,
oxygen-free solvents were obtained using an Anhydrous En-
gineering solvent purification system, and stored over Linde
4 Å molecular sieves. CDCl3 and C6D6 were dried over acti-
vated sieves (Linde 4 Å) and degassed by consecutive
D. Amoroso, G.P.A. Yap, and D.E. Fogg.1 Center for
Catalysis Innovation and Research, Department of Chemistry,
University of Ottawa, Ottawa, ON K1N 6N5, Canada.
1Corresponding author (telephone: (613) 562-5800 ext. 6057;
fax: (613) 562-5170; e-mail: dfogg@science.uottawa.ca).
Can. J. Chem. 79: 958–963 (2001)
DOI: 10.1139/cjc-79-5/6-958
© 2001 NRC Canada

Amoroso et al.
959
freeze-pump-thaw cycles. Phenyldiazomethane (5), dcypb
(6), RuCl2(PPh3)3 (3) (7), and t-RuCl2(dppe)2 (8) were pre-
pared according to literature procedures. RuCl3·3H2O was
purchased from Strem Chemicals. Norbornene was pur-
chased from Aldrich and distilled from sodium under N2.
Hydrogen (Praxair, UHP grade) was passed through a
Deoxo cartridge and Drierite column in series; CO (Praxair)
was used as received.
was sustained over three further cycles. (ii) Large-scale. A
suspension of 4a (20 mg, 0.016 mmol Ru2) in benzene
(10 mL) was freeze-pump-thaw degassed until no signals
were evident by NMR. Concentration and addition of cold
hexanes afforded a dark grey-green solid, which was filtered
off, washed with cold hexanes, and dried under a flow of Ar.
Yield: 16 mg. IR (Nujol) (cm–1): 1944 (Ru-H) (m-w), 1629
(C=C) (m-w). (iii) Prolonged exposure of solid 4a to vac-
uum at 50°C resulted in a mixture of diamagnetic products
(multiple 31P NMR signals in the region from 75 to –15 ppm).
31P and 13C NMR spectra were recorded on a Varian XL-
1
300 spectrometer, H NMR spectra on a Varian Gemini 200
or Bruker AMX-500 spectrometer. Peaks are reported in
ppm, relative to 85% aq. H3PO4 (31P) or the deuterated sol-
vent (1H, 13C). Solid state NMR data were recorded on a
Bruker ASX-200 MHz spectrometer, infrared spectra on a
Bomem MB100 IR spectrometer. Microanalytical data were
obtained using a PerkinElmer Series II CHNS/O instrument.
Gel permeation chromatography (GPC) data were obtained
with CH2Cl2 as eluent (flow rate 1.0 mL min–1; samples
1–2 mg mL–1) using a Wyatt DAWN light-scattering instru-
ment equipped with Optilab DSP refractometer, HPLC sys-
tem with a Waters model 515 pump, Rheodyne model 7725i
injector with a 200 µL injection loop, and Waters Styragel
HR3 column.
NMR-scale preparation of Ru2Cl4(dcypb)2(H2) (6)
Stirring a suspension of 4a (4 mg, 3 µmol Ru2) in C6D6
(0.5 mL) under H2 at room temperature afforded a homoge-
neous orange solution within 15 min. No starting material
was spectroscopically observable after 30 min. 1H NMR
(C6D6) δ: 0.6–3.0 (br, CH2, Cy of dcypb), –11.8 (br s, H2).
31P{1H} NMR (C6D6) δ: 65.1 (d, 2JP,P = 25 Hz), 59.2 (d, 2JP,P
=
2
2
40 Hz), 45.9 (d, JP,P = 25 Hz), 43.9 (d, JP,P = 40 Hz). Hy-
dride T1 (min) = 26 msec (300 K, 500 MHz).
Preparation of RuCl2(dcypb)(CO)2 (ccc-(7), tcc-(8))
A suspension of 4a (27 mg, 39 µmol Ru2) in THF (5 mL)
was stirred under CO for 24 h, affording a clear yellow solu-
tion. The solution was concentrated and hexanes added to
coprecipitate 7 and 8 as a yellow powder (1:4). Yield: 23 mg
(88%). IR (Nujol) (cm–1): 2052 (CO) (s, 7), 2038 (s, 8),
Preparation of Ru2Cl4(dcypb)2(N2) (4a)
A green solution of 3 (500 mg, 0.52 mmol) and dcypb
(259 mg, 0.57 mmol) in C6H6 (5 mL) was stirred at 21°C
under N2. Orange 4a began to precipitate from solution
within 1 h. The orange suspension was diluted with hexanes
after 18 h and the product was filtered off, washed with hex-
anes (3 × 10 mL), and dried under a steady flow of N2 for
24 h. Yield: 0.303 g (91%). FAB-MS (m/z): 1245 ([M –
1976 (s, 8), 1962 (s, 7). 1H NMR (C6D6) δ: 0.8–2.4 (br, CH2,
2
Cy of dcypb). 13C{1H} NMR (C6D6) δ: 198.7 (t, JP,P(cis)
=
2
2
12 Hz, 7), 193.5 (dd, JP,P(trans) = 113 Hz, JP,P(cis) = 11 Hz,
2
2
7), 193.8 (dd, JP,P(trans) = 109 Hz, JP,P(cis) = 23 Hz, 8).
31P{1H} NMR (C6D6) δ: 39.4 (d, JP,P = 22 Hz, 7), 17.1 (d,
2
2JP,P
= 22 Hz, 7), 13.8 (s, 8). Anal. calcd. for
1
N2]+). IR (Nujol) (cm–1): 2124 (N2) (m). H NMR (CDCl3)
C30H52Cl2O2P2Ru: C 53.09, H 7.72; found: C 53.39, H 7.93.
δ: 0.7–3.1 (br, CH2, Cy of dcypb). 31P{1H} NMR (CDCl3) δ:
57.5 (d, 2JP,P = 39 Hz), 45.0 (d, 2JP,P = 27 Hz), 44.6 (d, 2JP,P
=
=
In situ polymerization of norbornene by 1a
2
2
39 Hz), 42.3 (d, JP,P = 27 Hz); (C6D6) δ: 60.1 (d, JP,P
Optimization of the procedure has been described (2).
Blank polymerization experiments carried out without added
Ru catalyst showed no ROMP over 24 h. In a representative
catalytic procedure, PhCHN2 (1.1 µL, 10.6 µmol) was added
to an orange suspension of 4a (6.6 mg, 10.6 µmol) in C6D6
(2 mL). Vigorous bubbling ensued, and all of the solids dis-
solved, forming a deep orange solution, which was immedi-
ately added to a stirred solution of norbornene (200 mg,
2.1 mmol) in CDCl3 or C6D6 (5 mL). The progress of the reac-
tion was monitored by removing aliquots for NMR analysis.
(i) ROMP via 4a + 2 PhCHN2 in CDCl3–C6D6: Mn 2.4 × 106,
Mw/Mn = 1.4; (ii) via 2a (prepared in situ by addition of 2
equiv PPh3 to 4a) + PhCHN2 in CDCl3–C6D6: Mn 2.4 × 106,
Mw/Mn = 1.2; (iii) via 4a + 2 PhCHN2 in neat benzene: bi-
modal, Mn 1.9 × 106, Mw/Mn = 2.4; Mn 20 000, Mw/Mn = 3.8.
2
2
39 Hz), 49.1 (d, JP,P = 26 Hz), 45.3 (d, JP,P = 39 Hz), 37.4
(d, JP,P = 26 Hz). Solid-state 31P{1H} CP-MAS NMR
2
(80.9 MHz) δ: 45–57 (br, unresolved), 43.6 (br), 41.2 (br).
Anal. calcd. for C56H104Cl4N2P4Ru2: C 52.81, H 8.25,
N 2.20; found: C 52.90, H 8.19, N 1.90.
Preparation of Ru2Cl5(dcypb)2 (5)
Complex 4a (40 mg, 0.04 mmol) was dissolved in CDCl3
(0.5 mL) and the solution was monitored by 31P{1H} NMR.
Resolution degraded perceptibly over ~30 min, and a broad
singlet emerged at 50 ppm. On further reaction, S/N values
decreased, and many new peaks could be discerned (46–63,
15–32 ppm). Slow evaporation deposited red crystals of 5.
Decomposition of 4a under vacuum
(i) NMR-scale. A solution of 4a (3.0 mg, 4.7 µmol Ru) in
C6D6, with t-RuCl2(dppe)2 (2.3 mg, 2.35 µmol) as an inter-
nal standard, was freeze-pump-thaw degassed in a round-
bottom Schlenk flask. 31P{1H} NMR spectra were measured
after every second cycle of degassing. No NMR signals were
discerned after six degassing cycles, though gas evolution
X-ray crystallographic analysis of 5
Crystal and data collection parameters for 5 are provided
in Table 1.2 Suitable crystals were selected, mounted on thin
glass fibres using viscous oil, and cooled to the data collec-
tion temperature. Data were collected on a Bruker AX
SMART 1k CCD diffractometer using 0.3° ω-scans at 0, 90,
2 Copies of materials on deposit may be purchased from The Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council of Canada, Ottawa, ON K1A OS2, Canada. Tables of hydrogen atom coordinates and bond lengths and angles have also been de-
posited with the Cambridge Crystallographic Database, and can be obtained on request from the Director, Cambridge Crystallographic Data
Centre, University Chemical Laboratory, 12 Union Road, Cambridge CB2 1EZ, U.K.
© 2001 NRC Canada

960
Can. J. Chem. Vol. 79, 2001
Scheme 1.
Fig. 1. ORTEP drawing of Ru2Cl5(dcypb)2 (5). Thermal ellipsoids
depicted at 30% probability level; cyclohexyl rings, hydrogen at-
oms, and solvate molecules omitted for clarity. A detailed struc-
tural representation is included in the Supplementary Material.
RuCl2(PPh3)3 + dcypb
3
Ar
N2
– PPh3
RuCl2(dcypb)(PPh3) 2a
[(N2)(dcypb)Ru( -Cl)3RuCl(dcypb)] 4a
N2 H2
CDCl3
[RuCl2(dcypb)] 2(H2)
Ru2Cl5(dcypb)2 5
6
+ other products
and 180° in φ. Unit-cell parameters were determined from
60 data frames collected at different sections of the Ewald
sphere. Semi-empirical absorption corrections based on
equivalent reflections were applied (9). No symmetry higher
than triclinic was evident from the diffraction data. Solution
in P1 yielded chemically reasonable and computationally
stable results of refinement. The structure was solved by di-
rect methods, completed with difference Fourier syntheses
and refined with full-matrix least-squares procedures based
on F2. A carbon atom, C(14), in one of the ligands was lo-
cated conformationally disordered in two positions with
roughly 50/50 site occupation distribution. The large
anisotropic displacement parameters on C(13) suggest simi-
lar disorder. Attempts to model this disorder were unsuc-
cessful, however, owing to insufficient resolution between
the contributing atomic positions. A molecule of benzene
and two molecules of chloroform were located in the asym-
metric unit. The benzene molecule of crystallization was
constrained as flat hexagon. All nonhydrogen atoms were re-
fined with anisotropic displacement parameters. All hydro-
gen atoms were treated as idealized contributions. All
scattering factors and anomalous dispersion factors are con-
tained in the SHEXTL 5.10 program library (10).
dcypb backbone. A strong IR band for ν(NϵN) is evident at
2124 cm–1.
The solubility of 4a is slightly improved in halogenated
solvents such as CH2Cl2 or CDCl3, but decomposition in
these solvents yields the mixed-valence dimer Ru2Cl5(dcypb)2
5.3 James and co-workers (12, 13) have reported extensively
on the synthesis, reactivity, and catalytic chemistry of such
species, obtained via reaction of chelating diphosphines with
RuCl3(PR3)2 (R = Ph, p-tolyl). Attempts to prepare clean 5
on large scale by the reaction of 4a with CDCl3 or ethereal
HCl, resulted in product mixtures, as judged by in situ
31P NMR analysis prior to complete loss of the diamagnetic
signals. Slow evaporation of CDCl3 solutions under N2 per-
mitted isolation of crystals suitable for X-ray analysis. The
molecular structure of 5 is shown in Fig. 1; crystal data and
selected structural parameters appear in Tables 1 and 2, re-
spectively.
Complex 5 adopts a triply chloride-bridged diruthenium
structure, in which the coordination geometry at each metal
center is distorted octahedral. The structure is unsymmetri-
cal, with one of the octahedra being rotated by 120° about
the Ru–Ru vector. A similar structure was earlier reported
for Ru2Cl5(P(n-Bu)3)4 (14), whereas the only previous crys-
Results and discussion
Treatment of 3 with dcypb in benzene under Ar yields the
mixed-phosphine complex 2a (11), which differs from its
dppb analogue 2b in undergoing no dimerization (with loss
of PPh3) in solution. The very high solubility of 2a has so
far prevented its isolation. In strong contrast is the reaction
of 3 with dcypb under N2 atmosphere, which affords a high-
yield route to sparingly soluble [(N2)Ru(dcypb)(µ-
Cl)3RuCl(dcypb)] (4a) (Scheme 1). This net displacement of
triphenylphosphine ligand by N2 has no precedent in the cor-
responding aryldiphosphine chemistry. The identity of 4a is
confirmed by NMR and IR spectroscopy and microanalysis.
31P NMR analysis reveals four doublets for the inequivalent
phosphine groups within 4a, in a pattern similar to that ear-
tal structure of
a
chelating diphosphine derivative,
Ru2Cl5(chiraphos)2, exhibits a symmetrical ligand arrange-
ment (13). Ruthenium—phosphorus and —chloride distances
fall within the ranges reported, as do angles within the Ru-P-
Cl skeleton. Chloride atoms trans to phosphorus display dis-
tinctly longer Ru—Cl bond distances (av. 2.49 Å) compared
to those trans to Cl (av. 2.38 Å), as expected from the stron-
ger trans influence of phosphine. The average bridging angle
in 5 (83.7°) is nearly 15° larger than the ideal value of 70.5°
for two regular face-sharing octahedra (14), indicating that
the Ru centers are further apart than expected for a regular
cofacial bioctahedron. The Ru—Ru distance of 3.278 Å is
considerably longer than the upper limit associated with the
presence of a metal–metal bond (2.95 Å) (13). As with the
chiraphos and the P(n-Bu)3 complexes, the crystallographic
data do not permit assignment of formal oxidation states to
the ruthenium atoms.
1
lier reported (12) for dppb analogue 4b. The H NMR spec-
trum of 4a is less informative, consisting only of a series of
overlapping multiplets between 0.7–3.1 ppm arising from
the cyclohexyl protons and the methylene protons of the
3 Note added in proof: complex 5 was originally prepared via this route, and partially transformed into 6 by treatment with H2 in DMA or
CH2Cl2. Private communication from B.R. James, based on data contained in ref. 6.
© 2001 NRC Canada

Amoroso et al.
961
Table 1. Crystal data and structure refinement for 5.
Empirical formula
FW
C64H112Cl11P4Ru2
1597.51
T (K)
203(2)
Wavelength (Å)
0.71073
Crystal system, space group
Unit cell dimensions
a, b, c (Å)
Triclinic, P1
13.390(2), 15.726(2), 19.524(2)
α, β, γ (°)
77.325(2), 70.964(2), 73.478(2)
3689.3(8)
2, 1.438
0.932
Volume (Å3)
Z, densitycalcd. (mg m–3)
Absorption coefficient (mm–1)
F(000)
1662
Crystal size (mm)
Theta range for data collection (°)
Limiting indices
0.10 × 0.10 × 0.03
1.11–20.81
–12 ≤ h ≤ 13, –15 ≤ k ≤ 15, 0 ≤ l ≤ 19
Reflections collected/unique (R(int) = 0.1237)
Completeness to θ = 28.83
Absorption correction
Max. and min. transmission
Refinement method on F2
Data/restraints/parameters
GoF on F2
29133/7712
99.7
Semi-empirical from equivalents
0.928077 and 0.637019
Full-matrix least-squares
7712/0/722
1.025
0.0533
0.1009
Ra
b
Rw
aR = Σ||Fo | – |Fc||/Σ|Fo|.
2
bRw = [Σwδ2/ΣwFo ]1/2
.
Table 2. Selected bond lengths (Å) and angles (°) for (5).
Bond lengths (Å)
for bound H2 at –11.8 ppm. The T1 (min) value of 26 msec
(300 K, 500 MHz) corresponds to an H—H distance of
0.89 Å, assuming rapid rotation of the dihydrogen ligand;
this compares well with the value of 0.86 Å reported for 4b
(15). H2-coordination is readily reversed under N2 atmo-
sphere. Under CO, mononuclear RuCl2(dcypb)(CO)2 is
formed as a mixture of isomers, 7 and 8 (Scheme 2). The
two are readily distinguished by 31P{1H} NMR; the phos-
phine groups of all-cis-7 appear as a pair of doublets,
whereas a singlet is found for tcc-8. Consistent with the pro-
posed structures is the carbonyl region of the 13C{1H} NMR
spectrum, as well as the IR data. The former shows a triplet
and a doublet of doublets for 7, but only a doublet of dou-
blets for 8. Two IR ν(CO) bands are seen for each complex.
Ru(1)—P(1)
Ru(1)—P(2)
Ru(1)—Cl(5)
Ru(1)—Cl(1)
Ru(1)—Cl(3)
Ru(1)—Cl(4)
Bond angles (°)
2.309(3) Ru(2)—P(3)
2.325(3) Ru(2)—P(4)
2.395(2) Ru(2)—Cl(2)
2.387(2) Ru(2)—Cl(3)
2.495(2) Ru(2)—Cl(4)
2.504(2) Ru(2)—Cl(5)
2.323(2)
2.348(3)
2.360(3)
2.373(2)
2.466(2)
2.505(2)
P(1)-Ru(1)-P(2)
95.14(9)
P(3)-Ru(2)-P(4)
93.75(9)
Cl(5)-Ru(1)-Cl(1) 168.81(8)
Cl(2)-Ru(2)-Cl(3) 168.58(9)
P(1)-Ru(1)-Cl(3)
P(2)-Ru(1)-Cl(4)
169.32(9)
172.19(9)
P(3)-Ru(2)-Cl(5)
P(4)-Ru(2)-Cl(4)
168.60(9)
173.97(9)
Surprisingly, in view of the observed lability of the
dinitrogen ligand in 4a, no peaks for a dimer of type 9 were
evident even under Ar atmosphere, or following freeze-
pump-thaw (FPT) experiments carried out to shift the pre-
sumed equilibrium (eq. [1]) to the right. Sustained gas evo-
lution was observed in NMR tubes subjected to successive
FPT cycles (>20), but no new peaks were evident by
31P{1H} NMR, even at low temperature (183 K). Degassing
experiments using trans-RuCl2(dppe)2 as internal standard
were carried out in Schlenk vessels, in which the higher sur-
face area permitted more efficient removal of dissolved gas.
A steady decrease in concentration of 4a is measured with
increasing number of FPT cycles, signifying conversion of
4a to a paramagnetic Ru product. Gas evolution is sustained
after complete loss of the NMR signals, probably owing to
paramagnetic broadening of signals for remaining 4a. Obser-
vation of a large 1H NMR singlet for dissolved H2 at 4.2 ppm
after each FPT cycle is consistent with Ru-promoted
In aromatic solvents, 4a is stable for weeks in solution un-
der N2. In contrast with the dppb systems, which exist in
equilibrium with the “naked” dimers [RuCl2(PP)]2 9 under
N2 (eq. [1], (12)), no peaks for [RuCl2(dcypb)]2 are ob-
served.
[1]
The lability of the dinitrogen ligand is indicated, however,
by its facile replacement under H2 or CO atmosphere. Dis-
placement of N2 by dihydrogen is complete within 10 min at
1 atm H2, as indicated by a pronounced shift in the location
of the 31P NMR doublets associated with the “L-end” of the
1
complex (Table 3). The H NMR spectrum reveals a singlet
© 2001 NRC Canada

962
Can. J. Chem. Vol. 79, 2001
Table 3. 31P{1H} NMR data for Ru2Cl4(PP)2(L) complexes.
Complex
Chemical shifts (δ, ppm)
2JP,P (Hz)
Ru2Cl4(dcypb)2(N2) (4a)
Ru2Cl4(dppb)2(N2) (4b) (12)
Ru2Cl4(dcypb)2(H2) (6)
60.1 (d), 45.3 (d); 49.1 (d), 37.4 (d)
54.4 (d), 53.5 (d); 46.6 (d), 36.8 (d)
59.2 (d), 43.9 (d); 65.1 (d), 45.9 (d)
40; 26
45; 32
40; 25
Note: Measured at room temperature in C6D6, 121 MHz.
Fig. 2. ROMP activity of dcypb complexes. (a) 4a + 2 PhCHN2,
CDCl3-C6D6; (b) 2a + 2 PhCHN2, CDCl3-C6D6; and (c) 4a + 2
PhCHN2, C6D6.
Scheme 2.
CO
Ru
Cl
P
P
P
P
Cl
CO
CO
CO
+
Ru
4a
CO
Cl
Cl
8
7
PhCHN2
n
[RuCl2(dcypb)(CHPh)]
n
1a
dehydrogenation of the cyclohexyl rings and (or) the dcypb
backbone (catalytic dehydrogenation of the solvent itself is
excluded by observation of this behaviour in benzene sol-
vent). A <(C=C) stretching band appears at 1629 cm–1 in the
IR spectrum, as well as a signal at 1944 cm–1 assigned to
Ru–H. No HCl was detected by GC-MS, and no precipitate
formed on forcing the effluent gas through concentrated am-
monia or NH4PF6 solutions, suggesting that HCl is not
evolved in the reaction.
molecular attack opens up a potentially rich catalytic and
coordination chemistry.
We recently described polymerization via RuCl2(PP)(CHPh)
systems derived from 2, in which high ROMP activity in
norbornene polymerization was observed despite the pres-
ence of the catalyst poison PPh3 (2). PPh3-free 1a, generated
in situ by addition of PhCHN2 to 4a (Scheme 2), functions
as a much more avid catalyst (Fig. 2). Thus, while ROMP of
norbornene by the 2a–PhCHN2 system requires nearly an
hour at substrate:catalyst ratios of 200:1, polymerization via
4a is complete before the first NMR measurement can be
taken (5 min, lower limit for the turnover frequency =
2400 h–1). Despite the susceptibility of 4a to decomposition
by chlorinated solvent, mixed solvent systems, in which the
catalyst was generated in C6D6 and subsequently added to a
CDCl3 solution of norbornene, showed much higher activity
than ROMP in neat C6D6. Decreased metathesis activity in
nonpolar solvents is characteristic of these Ru systems (1).
The slow rate of polymerization in neat benzene permits
competing decomposition via extrusion of the carbene as
stilbene (a process characteristic of Ru–carbene complexes).
Polymer polydispersities for reactions in benzene are conse-
quently higher than those observed in the mixed solvent sys-
tems (PDI 3.8 vs. 1.2–1.4), and a bimodal molecular weight
distribution is observed, indicating the presence of more
than one ROMP-active species. The very rapid rate of poly-
merization in CDCl3–C6D6 (Fig. 2a) suggests that decompo-
sition of 1a cannot compete with norbornene metathesis in
this solvent system, and this is supported by the low poly-
dispersity obtained.
The utility of closely related Ru complexes in catalytic
dehydrogenation of sp3 C–H bonds has been established by
Leitner and co-workers (16, 17). While forcing conditions
were required for activation of cyclooctane, dehydrogenation
of the cyclohexyl rings within complexes containing chelat-
ing Cy2P(CH2)nPCy2 (n = 3, 4) was induced at 50°C, result-
ing in diamagnetic η3-cyclohexenyl derivatives. Chaudret
and co-workers (18–20) have likewise described intra-
molecular dehydrogenation of cyclohexyl rings within Ru–
PCy3 complexes at room temperature, affording Ru(II) com-
plexes containing η3- and (or) η2-cyclohexenyl rings. A re-
lated process is almost certainly involved in dehydrogenation
of 4a, in which retention of both chloride ligands may be re-
sponsible for formation of a Ru(III) product. We do not ex-
clude the possibility of concomitant attack on the four-carbon
backbone; agostic interactions between Ru and bound dppb,
culminating in deuterium incorporation into the backbone
methylene groups, have been noted (21), while we have ob-
served C–H bond activation within the dcypb backbone in
related silylene chemistry (22). Confirmation of the identity
of the dehydrogenation product(s) is hampered by paramag-
netism and poor crystallinity. Attempts to probe the reaction
by thermal gravimetric analysis are complicated by the ob-
servation of other diamagnetic products in solid-state degas-
sing experiments. The high activity of these species toward
activation of saturated C–H bonds under exceptionally mild
conditions is notable; however, facile intramolecular attack
may be attributed to the combination of steric bulk and high
basicity in the dcypb ligand. The ease of such reactions in
the absence of stabilizing donor ligands may account for
prior difficulties in gaining access to this chemistry under Ar
atmosphere. Use of N2 as a labile donor to inhibit intra-
Conclusions
The foregoing describes the first general route into the
chemistry of chlororuthenium–phosphine complexes con-
© 2001 NRC Canada

Amoroso et al.
963
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taining the basic, bulky diphosphine, dcypb. Steric pressure,
allied with high electron density, renders these species sus-
ceptible to decomposition by attack on solvent or on the
dcypb ligands. Where such processes can be restrained, this
heightened reactivity can be redirected and exploited in ca-
talysis, as exemplified in the potent ROMP reactivity of the
carbene derivative.
Acknowledgments
This work was supported by the Natural Sciences and En-
gineering Research Council of Canada (NSERC), the Can-
ada Foundation for Innovation, and the Ontario Innovation
Trust.
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© 2001 NRC Canada