Ruthenium vinylidene and carbyne complexes containing a multifunctional tridentate ligand with a PNN donor set

Article abstract of DOI:10.1016/j.jorganchem.2006.06.009

The neutral, octahedral ruthenium vinylidene complexes mer,trans-[(PNN)Cl₂Ru(CCHR)] (PNN = N-(2-diphenylphosphinobenzylidene)-2-(2-pyridyl)ethylamine; R = Ph, 1a; R = ^tBu, 1b) are reported. An X-ray crystallographic study of 1a confirms the tridentate, meridional coordination mode of the PNN ligand. Compounds 1a and 1b undergo regioselective electrophilic addition with HBF₄ · Et₂O at C_β of the vinylidene ligand at low temperatures, and are cleanly and quantitatively converted to the ruthenium carbynes mer,trans-[(PNN)Cl₂Ru(CCH₂R)][BF₄] (R = Ph, 2a; R = ^tBu, 2b). Carbynes 2a and 2b are stable only at low temperatures (<-50 °C). Complex 1a undergoes ligand substitution with L to yield mer,trans-[(PNN)Cl₂Ru(L)] (L = MeCN, 3a; L = CO, 3b).

Full text of DOI:10.1016/j.jorganchem.2006.06.009

Journal of Organometallic Chemistry 691 (2006) 4147–4152
www.elsevier.com/locate/jorganchem
Note
Ruthenium vinylidene and carbyne complexes containing
a multifunctional tridentate ligand with a PNN donor set
a
a
b
a,
*
Nicholas J. Beach , Jesse M. Walker , Hilary A. Jenkins , Gregory J. Spivak
a
Department of Chemistry, Lakehead University, Thunder Bay, Ont., Canada P7B 5E1
Department of Chemistry, Saint Mary’s University, Halifax, NS, Canada B3H 3C3
b
Received 23 May 2006; accepted 6 June 2006
Available online 15 June 2006
Abstract
The neutral, octahedral ruthenium vinylidene complexes mer,trans-[(PNN)Cl2Ru(CCHR)] (PNN = N-(2-diphenylphosphinobenzylid-
ene)-2-(2-pyridyl)ethylamine; R = Ph, 1a; R = tBu, 1b) are reported. An X-ray crystallographic study of 1a confirms the tridentate,
meridional coordination mode of the PNN ligand. Compounds 1a and 1b undergo regioselective electrophilic addition with HBF4 Æ Et2O
at Cb of the vinylidene ligand at low temperatures, and are cleanly and quantitatively converted to the ruthenium carbynes mer,trans-
[(PNN)Cl2Ru(CCH2R)][BF4] (R = Ph, 2a; R = tBu, 2b). Carbynes 2a and 2b are stable only at low temperatures (<À50 ꢀC). Complex 1a
undergoes ligand substitution with L to yield mer,trans-[(PNN)Cl2Ru(L)] (L = MeCN, 3a; L = CO, 3b).
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium; Vinylidene; Carbyne; Electrophilic addition; Hemilabile
1. Introduction
patterns and substantial promise as alkyne metathesis
catalysts, much like their ruthenium carbenoid
counterparts.
Over the past decade, the explosive development of well-
defined and highly efficient catalysts for olefin metathesis
has been accompanied by a rapid expansion in applications
of these catalysts in a variety of C–C bond forming reac-
tions in organic and polymer synthesis [1]. Many of the
greatest advances have involved the Grubbs’ class of ruthe-
nium carbene catalysts [2]. Despite the tremendous interest
in refining and applying these catalysts, considerably less
attention has been given to the development of well-defined
catalysts capable of mediating alkyne metathesis [3]. Cur-
rently, alkyne metathesis is limited to catalysts based on
early metals, primarily molybdenum and tungsten. Alter-
natively, late metal ruthenium carbyne complexes [4] may
possess similar properties, and demonstrate novel reactivity
In our continuing quest to develop alkyne metathesis-
active ruthenium carbyne complexes [4b,4f], we wish to
report some results which illustrate the capacity and extent
of the potentially tridentate ligand N-(2-diphenylphosphin-
obenzylidene)-2-(2-pyridyl)ethylamine [5] (PNN, Scheme
1) to serve as a supporting ligand in ruthenium carbyne
chemistry. Our decision to consider this ligand was moti-
vated by the expectation that the diphenylphosphino and
imino groups of the PNN ligand might form a substitution-
ally inert anchor to the metal, while the pyridyl group
could be reversibly displaced by substrate (i.e., hemilabile
behaviour) under catalytic conditions [4a,5a,5d]. In this
way, the hemilabile PNN ligand could facilitate metal car-
byne-mediated alkyne metathesis by furnishing a vacant
site for alkyne substrate to coordinate to the carbyne
catalyst, but also stabilize the catalyst in its resting state
through re-attachment of the pyridyl group to the
metal.
*
Corresponding author. Tel.: +1 807 343 8297; fax: +1 807 346 7775.
E-mail address: greg.spivak@lakeheadu.ca (G.J. Spivak).
0022-328X/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.06.009

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N.J. Beach et al. / Journal of Organometallic Chemistry 691 (2006) 4147–4152
Scheme 1.
2. Experimental
2.1. Synthesis of mer,trans-[(PNN)Cl2Ru(CCHR)
(1: R = Ph, 1a; R = tBu, 1b)
All experiments and manipulations were conducted
under an inert atmosphere of prepurified N2 using standard
Schlenk techniques. Bulk solvents used in large-scale prep-
arations were rigorously dried, and either distilled under
nitrogen immediately prior to use, or stored over activated
4A molecular sieves in bulbs with Teflon taps and purged
with nitrogen prior to use: CH2Cl2 (CaH2); benzene, tolu-
ene, diethyl ether and hexanes (sodium metal/benzophe-
none); acetonitrile (4A molecular sieves). NMR solvents
used in solution structure elucidations were dried with
appropriate drying agents, vacuum distilled, freeze–
pump–thaw degassed three times, and stored in bulbs with
Teflon taps: CDCl3 (anhydrous CaCl2); CD2Cl2 (CaH2).
NMR spectra (1H, 13C{1H} and 31P{1H}) were obtained
using a Varian Unity INOVA 500 MHz spectrometer, with
chemical shifts (in ppm) referenced to solvent peaks (1H
and 13C) or external 85% H3PO4(31P). Elemental analyses
were performed on a CEC 240XA analyzer by the Lake-
head University Instrumentation Laboratory. The ruthe-
nium vinylidene precursors [(Cy3P)2Cl2Ru(CCHR)] (R =
A typical procedure involved the following. The com-
plex [(Cy3P)2Cl2Ru(CCHR)] (0.599 mmol) and a slight
excess of the ligand PNN (0.719 mmol) were combined
and dissolved in benzene (20 mL). The mixture was stirred
under nitrogen for 24 h (for R = Ph, the mixture requires
an additional 60 min at reflux to ensure completion). The
solvent was then removed under reduced pressure. The
remaining residue was stirred in hexanes (60 mL) for ca.
10 min and then filtered. The solid was dried under reduced
pressure. Analytically pure samples were obtained by
recrystallizing the products from CH2Cl2/diethyl ether via
slow diffusion. Data for complex 1a: Yield: 75%. Anal.
Calc. for C34H29Cl2N2PRu: C, 61.07; H, 4.38; N, 4.19.
Found: C, 61.10; H, 4.77; N, 3.97%. 1H NMR
(499.9 MHz, CDCl3, 22 ꢀC): 9.97 (m, 1H, o-H of py),
8.65 (s, 1H, imino H), 7.74–6.97 (23H, benzylidene, Ph
4
and py), 4.69 (d, 1H, CCHPh, JPH = 5 Hz), 4.94 (br m,
2H, NCH2CH2py), 3.60 (br m, 2H, NCH2CH2py).
13C{1H} (125.7 MHz, CDCl3, 22 ꢀC): 367.4 (d, RuC,
2JPC = 22.1 Hz), 165.6 (d, imino C), 162.1 (s, py), 155.5
(s, py), 137.6 (d, benzylidene), 137.4 (s, py), 137.0 (d, ben-
zylidene), 135.5 (s, benzylidene), 134.5 (d, Ph), 132.7 (d,
t
Ph or Bu) [6] and the ligand N-(2-diphenylphosphino-
benzylidene)-2-(2-pyridyl)ethylamine (PNN) [5a] were
prepared as described in the literature.

N.J. Beach et al. / Journal of Organometallic Chemistry 691 (2006) 4147–4152
4149
benzylidene), 131.8 (d, benzylidene), 131.5 (s, py), 130.2 (s,
Ph), 128.7 (m, Ph), 127.9 (d, Ph), 127.8 (d, Ph), 126.1 (s,
Ph), 124.9 (s, benzylidene), 123.2 (s, py), 113.3 (s, RuCC),
62.4 (s, NCH2CH2py), 36.9 (s, NCH2CH2py). 31P{1H}
(202.3 MHz, CDCl3, 22 ꢀC): 50.2 (s, PPh2). Data for com-
plex 1b: Yield: 75%. Anal. Calc. for C32H33Cl2N2PRu: C,
59.26; H, 5.13; N, 4.32. Found: C, 59.35; H, 5.26; N,
(125.7 MHz, CD2Cl2, À20 ꢀC): 346.9 (d, RuC,
2JPC = 18.2 Hz), 169.5 (d, imino C), 162.3 (d, py), 157.9
(d, py), 140.6 (s, py), 138.5 (d, benzylidene), 135.6 (d, ben-
zylidene), 135.3 (d, benzylidene), 134.7 (d, Ph), 134.5 (d,
benzylidene), 134.0 (d, py), 133.2 (d, Ph), 129.5 (d, Ph),
128.8 (d, benzylidene), 125.6 (d, Ph), 124.6 (s, py), 122.7
t
(d, benzylidene), 69.8 (s, RuCCH2 Bu), 59.5 (s,
1
4.22%. H NMR (499.9 MHz, CDCl3, 22 ꢀC): 10.39 (m,
NCH2CH2py), 39.3 (s, NCH2CH2py), 35.9 (s, CMe3),
31.4 (s, CMe3).31P{1H} NMR (202.3 MHz, CD2Cl2,
À20 ꢀC): 30.9 (s, PPh2).
1H, o-H of py), 8.64 (s, 1H, imino H), 7.71–7.03 (18H, ben-
zylidene, Ph and py), 4.47 (br m, 2H, NCH2CH2py), 3.64
4
(d, 1H, CCH tBu, JPH = 5 Hz), 3.60 (br m, 2H,
t
NCH2CH2py), 0.95 (s, 9H, Bu). 13C{1H} (125.7 MHz,
2.3. Synthesis of mer,trans-[(PNN)Cl2Ru(MeCN)] (3a)
2
CDCl3, 22 ꢀC): 364.3 (d, RuC, JPC = 21.6 Hz), 165.8 (d,
imino C), 162.6 (s, py), 154.8 (s, py), 137.5 (d, benzylidene),
137.3 (s, py), 137.2 (d, benzylidene), 135.4 (s, benzylidene),
134.5 (d, Ph), 132.6 (d, benzylidene), 132.5 (d, benzyli-
dene), 131.3 (d, py), 130.0 (d, Ph), 128.8 (d, Ph), 127.9 (d,
Ph), 125.9 (d, benzylidene), 122.4 (s, py), 119.5 (s, RuCC),
62.4 (s, NCH2CH2py), 37.0 (s, NCH2CH2py), 32.6 (s,
CMe3), 32.2 (s, CMe). 31P{1H} (202.3 MHz, CDCl3,
22 ꢀC): 51.3 (s, PPh2).
Complex 1a (0.210 g, 0.315 mmol) was dissolved in ben-
zene (10 mL). Excess acetonitrile (1 mL) was added via syr-
inge and the mixture was refluxed for 48 h. During this
time, a red solid had deposited. Once the mixture had
cooled, the solid was filtered off and washed with diethyl
ether (2 · 10 mL) before drying under reduced pressure.
Complex 3a was found to be insoluble in benzene and tol-
uene, and only sparingly soluble in CHCl3 and CH2Cl2.
Yield: 87%. Anal. Calc. for C28H26Cl2N3PRu: C, 55.58;
1
2.2. Low temperature observation of mer,trans-
[(PNN)Cl2Ru(CCH2R)][BF4] (2: R = Ph, 2a; R = tBu,
2b)
H, 4.33; N, 6.95. Found: C, 56.07; H, 4.58; N, 6.82%. H
NMR (499.9 MHz, CDCl3, 22 ꢀC): 9.45 (br m, 1H, o-H
of py), 8.77 (br s, 1H, imino H), 7.70–7.09 (18H, benzyli-
dene, Ph and py), 4.42 (br m, 2H, NCH2CH2py), 3.51 (br
m, 2H, NCH2CH2py), 1.92 (s, MeCN). 31P{1H} NMR
(202.3 MHz, CD2Cl2, 22 ꢀC): 60.7 (s, PPh2).
A typical procedure involved the following. Complex 1
(0.0484 mmol) was added to a 5 mm NMR tube which
was then fitted with a rubber septum and finally evacu-
ated/purged with N2. CD2Cl2 (0.6 mL) was added via syr-
inge and the solution was cooled to À78 ꢀC. Once cool, the
deep red solution was treated with a slight excess of
HBF4 Æ Et2O (0.0532 mmol, 8.7 lL of a 54% solution in
Et2O). An instant colour change from deep red to orange
was observed. The sample was then transferred quickly to
a precooled (À50 ꢀC) NMR probe and data acquisition
was performed immediately. The quantitative formation
2.4. Synthesis of mer,trans-[(PNN)Cl2Ru(CO)] (3b)
Complex 1a (0.189 g, 0.283 mmol) was dissolved in tol-
uene (20 mL). A gentle stream of CO was passed through
the solution while it was maintained at 80 ꢀC for 72 h. Dur-
ing this time, a yellow solid had deposited. The mixture was
allowed to cool to room temperature and the volatiles were
removed under reduced pressure. The yellow solid that
remained was washed with diethyl ether (3 · 20 mL) and
dried under reduced pressure. Yield: 77%. An analytically
pure sample was obtained by recrystallizing the product
from CH2Cl2/diethyl ether via slow diffusion. Anal. Calc.
for C27H23Cl2N2OPRu: C, 54.64; H, 3.91; N, 4.72. Found:
C, 54.21; H, 4.18; N, 4.50%. IR (Nujol): m(CO) = 1945 (s)
1
of 2 was confirmed by H, 31P{1H} and 13C{1H} NMR
spectroscopy. Data for complex 2a: 1H NMR
(499.9 MHz, CD2Cl2, À50 ꢀC): 8.83 (m, 1H, o-H of py),
8.63 (s, 1H, imino H), 7.98–6.83 (23H, benzylidene, Ph
and py), 4.13 (br m, 2H, NCH2CH2py), 3.86 (s, 2H,
RuCCH2Ph), 3.35 (br m, 2H, NCH2CH2py). 13C{1H}
NMR (125.7 MHz, CD2Cl2, À50 ꢀC): 334.5 (d, RuC,
2JPC = 17.7 Hz), 168.5 (d, imino C), 161.7 (d, py), 157.5
(s, py), 140.3 (s, py), 138.3 (d, benzylidene), 135.2 (d, ben-
zylidene), 134.8 (m, benzylidene), 134.7 (d, Ph), 134.0 (s,
py), 133.3 (s, Ph), 131.4–129.9 (m, Ph), 129.5 (d, Ph),
129.0 (d, benzylidene), 126.0 (d, Ph), 125.0 (s, py), 122.9
(s, Ph), 122.1 (d, benzylidene), 70.4 (s, RuCCH2Ph), 59.7
(s, NCH2CH2py), 36.0 (s, NCH2CH2py). 31P{1H} NMR
(202.3 MHz, CD2Cl2, À50 ꢀC): 32.2 (s, PPh2). Data for
complex 2b: 1H NMR (499.9 MHz, CD2Cl2, À20 ꢀC):
9.15 (m, 1H, o-H of py), 8.69 (s, 1H, imino C), 8.00–7.05
(18H, benzylidene, Ph and py), 4.12 (br m, 2H,
NCH2CH2py), 3.53 (br m, 2H, NCH2CH2py), 2.51 (s,
cmÀ1 1H NMR (499.9 MHz, CDCl3, 22 ꢀC): 9.43 (m,
.
1H, o-H of py), 8.74 (s, 1H, imino H), 7.75–7.05 (18H, ben-
zylidene, Ph and py), 4.44 (br m, 2H, NCH2CH2py), 3.55
(br m, 2H, NCH2CH2py). 31P{1H} NMR (202.3 MHz,
CD2Cl2, 22 ꢀC): 53.3 (s, PPh2).
2.5. X-ray crystallographic study
Suitable crystals of complex 1a were grown from a
CH2Cl2/diethyl ether solution at room temperature under
nitrogen over a period of several weeks. A crystal was
mounted on a glass fibre and the diffraction data were col-
lected on
a
Siemens SMART/CCD diffractometer
t
2H, RuCCH2 Bu), 1.07 (s, 9H, tBu). 13C{1H} NMR
equipped with an LT-II low-temperature device. The data

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N.J. Beach et al. / Journal of Organometallic Chemistry 691 (2006) 4147–4152
were collected at À50 ꢀC using Mo Ka radiation; Lorentz
and polarization corrections were applied. Diffraction data
were corrected for absorption using SADABS. Direct meth-
ods and Fourier techniques were used to solve the
structure; refinement was conducted using full-matrix
least-squares calculations and SHELXTL PC V 5.03. All non-
hydrogen atoms were refined anisotropically, and hydro-
gen atoms were treated as riding models and updated after
each refinement. Two molecules of recrystallization solvent
were located in the lattice, each at approximately 0.5 occu-
pancy and each positioned on an inversion centre. Diethyl
ether was disordered; CH2Cl2 was not. Crystal data, condi-
tions for the intensity collection and features of the struc-
tural refinement: C36.5H35Cl3N2O0.5PRu, MW = 748.05,
hydrogens of the ruthenium–iminoethylpyridyl ring are
observed as broadened multiplets of equal intensity at
room temperature. Considering the conformational flexi-
bility of the six-membered ring, this broadening most likely
can be attributed to a wagging motion of the ethylene
bridge [9]. This motion could not be frozen out on the
NMR time scale, even at À50 ꢀC. While this complicated
efforts to establish unambiguously the overall symmetry
of the complex using NMR spectroscopy, the number of
signals observed for the ethylene bridge is consistent with
Cs symmetry, and suggests a meridionally coordinated
PNN ligand and trans chlorides.
Confirmation of the coordination mode of the PNN
ligand was obtained through a single-crystal X-ray crystal-
lographic study on complex 1a. Single crystals were grown
by slow diffusion of diethyl ether into a saturated CH2Cl2
solution of 1a at room temperature. A labelled view of 1a
is provided in Fig. 1, and selected bond distances and
angles are provided in the caption.
A number of notable features of the structure of 1a war-
rant additional comments. Although several structures of
1a could be envisioned, the solid state structure confirms
that the PNN ligand does adopt a meridional coordination
mode, with trans chloride ligands and the vinylidene ligand
trans to the imino nitrogen. The chelate rings are not
strained i.e., \P(1)–Ru(1)–N(1) = 88.84(7)ꢀ and \N(1)–
Ru(1)–N(2) = 90.81(9)ꢀ, as one might expect for six-mem-
bered rings. The Ru–C(1) (Ca of the vinylidene ligand)
distance of 1.809(3) A compares well with other neutral,
octahedral ruthenium vinylidene complexes [7], but is
larger compared to five-coordinate ruthenium vinylidene
complexes in which the site trans to the vinylidene is vacant
˚
k = 0.71073 A, monoclinic, space group C2/c, a =
˚
˚
˚
17.3346(11) A, b = 12.0982(7) A, c = 33.166(2) A, a = 90ꢀ,
3
˚
b = 101.4890(10)ꢀ, c = 90ꢀ, V = 6816.1(7) A , Z = 8, d
(calcd.) = 1.458 Mg/m3, F(000) = 3056, T = 223(2) K,
l = 0.772 mmÀ1, crystal size = 0.4 · 0.48 · 0.5 mm, data
collection range 1.25 6 h 6 25.00ꢀ, À20 6 h 6 20, À14 6
k 6 14, À38 6 l 6 39, R1 = 0.0391, wR2 (all data) =
0.0766.
3. Results and discussion
One essential goal of this work was to establish first a
convenient route towards the synthesis of neutral ruthe-
nium vinylidene complexes containing the PNN ligand.
We reasoned that such complexes presumably would serve
as suitable precursors to ruthenium carbyne complexes
[4b,4f]. Within this context, we found that the ruthenium
vinylidene complexes [(Cy3P)2Cl2Ru(CCHR)] (R = Ph or
tBu) [6] undergo a clean ligand exchange reaction with a
slight excess of the ligand PNN to yield the complexes
mer,trans-[(PNN)Cl2Ru(CCHR)] (1: R = Ph, 1a; R =
tBu, 1b) in good isolated yields (typically ca. 70–80%) as
air-stable light red (1a) or orange-brown (1b) solids
(Scheme 1).
Preservation of the vinylidene ligand, and the tridentate
coordination mode of the PNN ligand in each of 1a and 1b
were established using multinuclear NMR spectroscopy.
For example, the Ca atom of the vinylidene ligand in each
complex gives rise to a doublet (1a, d = 367.4; 1b,
2
d = 364.3) in the 13C{1H} NMR spectrum, with JPC cou-
pling constants (22.1 Hz and 21.6 Hz for 1a and 1b, respec-
tively) in the range typically observed for neutral,
octahedral ruthenium vinylidene complexes [7]. The Cb
vinylidene hydrogen atom in each of complexes 1a and
1
1b also appears as a doublet in the H NMR spectrum
4
4
(1a, d = 4.69, JPH = 5 Hz; 1b, d = 3.64, JPH = 5 Hz).
Confirmation that the pyridyl group of the PNN ligand
is coordinated to the metal in 1a and 1b is obtained from
1
Fig. 1. Molecular structure of 1a (the hydrogen atoms have been omitted
for clarity). Selected bond lengths (A) and angles (ꢀ): Ru(1)–C(1) 1.809(3),
Ru(1)–P(1) 2.2798(8), Ru(1)–Cl(1) 2.3833(7), Ru(1)–Cl(2) 2.3980(7),
Ru(1)–N(1) 2.208(2), Ru(1)–N(2) 2.208(2), C(1)–C(2) 1.318(4), P(1)–
Ru(1)–N(1) 88.84(7), N(1)–Ru(1)–N(2) 90.81(9), Cl(1)–Ru(1)–Cl(2)
165.36(3), P(1)–Ru(1)–N(2) 178.60(6), Ru(1)–C(1)–C(2) 175.5(2).
the considerable downfield shift observed in the H NMR
spectrum for the o-H atom on the pyridine ring [8]
(d = 9.97 for 1a, d = 10.39 for 1b) compared to that
observed for the free ligand (d = 8.49) [5a]. Two 1H
NMR signals corresponding to the two sets of methylene

N.J. Beach et al. / Journal of Organometallic Chemistry 691 (2006) 4147–4152
4151
(d(Ru–Ca) ꢀ 1.75 A) [6]. The relatively large trans influence
exerted by the vinylidene ligand on the imino moiety can be
seen in the Ru–N(1) distance of 2.208(2) A, which is
slightly longer than that observed in other ruthenium–imi-
this work (see Section 2 for details). A dissociative mecha-
nism is likely operating here, however it may not necessar-
ily involve the pyridyl group. There are several reports
describing the displacement of vinylidene ligands on ruthe-
nium by nucleophiles [12]. In the case of complex 1, this
presumably requires isomerization of the vinylidene ligand
to the (unobserved) g2-HCCR ligand prior to dissociation.
This might be assisted by the cis chloride co-ligands and/or
the p-accepting properties of the imino and pyridyl moie-
ties of the PNN ligand which reduce both the r- and p-
bonding properties of the ruthenium fragment and weaken
the Ru–vinylidene bond [12c].
˚
nophosphine chelate complexes (d(Ru–Nimine) ꢀ 2.10 A)
[10].
The original intention of this work was to investigate the
ability of the PNN ligand to support ruthenium carbyne
complexes. We next turned our attention towards explor-
ing regioselective electrophilic addition to the vinylidene
ligand of 1 (Scheme 1) [4b,4f]. When cold (À78 ꢀC) CD2Cl2
solutions of either 1a or 1b were treated with a slight excess
of HBF4 Æ Et2O, an immediate colour change from red to
orange was observed, and the carbyne complexes mer,
trans-[(PNN)Cl2Ru(CCH2R)][BF4] (2: R = Ph, 2a;
R = tBu, 2b) formed immediately and quantitatively, as
4. Summary
The ruthenium vinylidene complexes 1 serve as conve-
nient precursors to the ruthenium carbyne complexes 2.
The poor thermal stability observed for complexes 2 would
seem to suggest the PNN ligand may be insufficiently Lewis
basic to stabilize the oxidized ruthenium and p-acidic car-
byne ligand. In addition, the vinylidene ligand of 1, rather
than the pyridyl moiety of the PNN ligand, can be dis-
placed by small molecules under forcing conditions.
1
determined by low temperature H, 31P{1H} and 13C{1H}
NMR spectroscopy. At low temperatures, new doublet res-
onances attributed to Ca of the carbyne ligands of 2 are
observed in the 13C{1H} NMR spectrum (d = 334.5, 2a;
d = 346.9, 2b). These signals are shifted upfield, and display
2
smaller (by ca. 4 Hz) JPC coupling constants compared to
the parent vinylidenes 1, suggesting oxidation of the ruthe-
nium centre upon protonation [4b,4f]. The pyridyl moiety
remains bound to the ruthenium centres in 2, as evidenced
by the downfield shifts of the o-H atoms on the pyridine
ring (d = 8.83 for 2a and d = 9.15 for 2b). Two broad sig-
nals are observed for the methylene hydrogens of the ethyl-
ene bridge of the ruthenium–iminoethylpyridyl ring at
d = 4.13 and 3.35 for 2a, and d = 4.12 and 3.53 for 2b, con-
sistent with a meridional PNN ligand and trans chlorides
(vide supra). Disappointingly, the carbynes 2 proved to
be stable only at low temperatures (<À50 ꢀC for 2a,
<À20 ꢀC for 2b), and decomposed within minutes to yield
unappealing mixtures of products (NMR) at higher tem-
peratures. It is possible the observed instability is linked
to the moderate donating abilities of the imino, pyridyl
and PPh2 moieties [5a,11], which may mitigate the ability
of the PNN ligand to stabilize sufficiently the formally
Ru(IV) centre of the carbynes [4f].
Acknowledgement
We gratefully acknowledge financial support from the
Natural Sciences and Engineering Research Council of
Canada.
Appendix A. Supplementary data
Crystallographic data for compound 1a have been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC No. 607401. Copies of this information may be
obtained free of charge from: The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: int. code
+44 1223 336 033; email: deposit@ccdc.cam.ac.uk; www:
http://www.ccdc.cam.ac.uk).
The thermal instability of carbynes 2a and 2b precluded
efforts to investigate further the substitutional lability of
the pyridyl moiety of the PNN ligand in these complexes.
The labile behaviour of the pyridyl moiety of the PNN
ligand has been observed [5a] or speculated [5d] in several
ligand displacement reactions involving small molecules.
Alternatively, we attempted to investigate the hemilability
of the PNN ligand in the vinylidene complexes 1. What
we observed was substitution of the vinylidene ligand in
the final products, rather than the pyridyl group (Scheme
1). For example, when complex 1a is heated with either
excess MeCN or CO in benzene or toluene over 48–72 h,
the substitution products mer,trans-[(PNN)Cl2Ru(L)] (3:
L = MeCN, 3a; L = CO, 3b) deposit as either bright red
(3a) or yellow (3b) solids. The 1H NMR spectra of 3a
and 3b are very similar, and suggest structures analogous
to those observed for other complexes reported as part of
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