Olefin binding in a binuclear iridium complex as a function of fluorine substitution: Ethylene to tetrafluoroethylene

Article abstract of DOI:10.1021/om030328b

The reactions of the diiridium methyl complex [Ir₂(CH₃)(CO)(μ-CO)(dppm)₂][CF₃SO ₃] (1) with ethylene, fluoroethylene, Z-1,2-difluoroethylene, 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroet

Full text of DOI:10.1021/om030328b

Organometallics 2003, 22, 4647-4657
4647
Olefin Bin d in g in a Bin u clea r Ir id iu m Com p lex a s a
F u n ction of F lu or in e Su bstitu tion : Eth ylen e to
Tetr a flu or oeth ylen e
Dusan Ristic-Petrovic, D. J ason Anderson, J effrey R. Torkelson,
Robert McDonald,‡ and Martin Cowie*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received May 5, 2003
The reactions of the diiridium methyl complex [Ir2(CH3)(CO)(µ-CO)(dppm)2][CF3SO3] (1)
with ethylene, fluoroethylene, Z-1,2-difluoroethylene, 1,1-difluoroethylene, trifluoroethylene,
and tetrafluoroethylene have been investigated. Reaction of 1 with ethylene at -78 °C yields
[Ir2H(η2-C2H4)(CO)2(µ-CH2)(dppm)2][CF3SO3] (2a ), resulting from C-H activation of the
methyl group induced by ethylene coordination, whereas reaction at higher temperatures
yields the simple ethylene adduct [Ir2(CH3)(CO)(η2-C2H4)(µ-CO)(dppm)2][CF3SO3] (2b).
Reactions of 1 with fluoroethylene and Z-1,2-difluoroethylene yield only the olefin adducts
analogous to 2b. At -78 °C reaction with 1,1-difluoroethylene yields the methylene-bridged
hydride product [Ir2H(η2-C2F2H2)(CO)2(µ-CH2)(dppm)2][CF3SO3] (5a ), which upon warming,
yields first the olefin adduct [Ir2(CH3)(CO)(η2-C2F2H2)(µ-CO)(dppm)2][CF3SO3] (5b) followed
by the olefin-bridged product [Ir2(CH3)(CO)2(µ-C2F2H2)(dppm)2][CF3SO3] (5c). Trifluoro- and
tetrafluoroethylene yield only the olefin-bridged products [Ir2(CH3)(CO)2(µ-olefin)(dppm)2]-
[CF3SO3] (olefin ) C2F3H (6), C2F4 (7)). The structure of the tetrafluoroethylene-bridged,
tricarbonyl species [Ir2(CH3)(CO)3(µ-C2F4)(dppm)2][CF3SO3] (8), determined by X-ray tech-
niques, is reported.
In tr od u ction
The chemistry demonstrated by us in which the
methyl ligand in [Ir2(CH3)(CO)(µ-CO)(dppm)2][CF3SO3]
(1) (dppm ) µ-Ph2PCH2PPh2) was transformed into
methylene and hydride fragments upon addition of
ligands (L), to yield [Ir2H(L)(CO)2(µ-CH2)(dppm)2]-
[CF3SO3],7 represents a rare example of cooperative
C-H activation of a methyl ligand by adjacent metals.8
This transformation suggested additional routes to
carbon-carbon bond formation in binuclear methyl
complexes, either via coupling of an unsaturated organic
substrate and the resulting methylene group or by prior
insertion of the substrate into the metal-hydride bond
followed by coupling of the resulting hydrocarbyl unit
with the methylene group. The former transformation
has been observed with olefins,9 allenes,10 and alkynes,11
and although the latter sequence of events has appar-
The formation of carbon-carbon bonds, promoted by
metal complexes, is a fundamental process in a range
of important catalytic reactions such as olefin polym-
erization, methanol carbonylation, olefin hydroformy-
lation, and Fischer-Tropsch chemistry.1 A key step in
each of these processes involves the migration of an
alkyl group or other hydrocarbyl fragment to either a
bound olefin, a carbonyl ligand, or a methylene group,
and examples of these processes have been documented
for the prototype methyl complexes.2-4 Related trans-
formations involving methyl migration to alkyne5 or
other unsaturated hydrocarbyl ligands6 have also been
reported.
‡ X-ray Crystallography Laboratory.
* Corresponding author. E-mail: martin.cowie@ualberta.ca.
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10.1021/om030328b CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/04/2003

4648 Organometallics, Vol. 22, No. 23, 2003
Ristic-Petrovic et al.
and were also used as received (unless otherwise stated). The
compound [Ir2(CH3)(CO)(µ-CO)(dppm)2][CF3SO3] (1) was pre-
pared as previously reported,7 and [Ir2(CD3)(CO)(µ-CO)(dppm)2]-
[CF3SO3] (1-CD3) was prepared identically except using per-
deuteriomethyl triflate.
Proton NMR spectra were recorded on Varian Unity 400,
500, or 600 spectrometers or on a Bruker AM400 spectrometer.
Carbon-13 NMR spectra were recorded on Varian Unity 400
or Bruker AM300 spectrometers, on samples that were 13CO-
or 13CH3-enriched. Phosphorus-31 and fluorine-19 spectra were
recorded on Varian Unity 400 or Bruker AM400 spectrometers.
Two-dimensional NMR experiments (COSY, ROESY, and
TOCSY) were obtained on Varian Unity 400 or 500 spectrom-
eters. All elemental analyses were performed by the microana-
lytical service within the department. Spectroscopic data for
all compounds are given in Table 1.
ently not been demonstrated, the individual steps are
well documented.12,13
In this paper we report our investigations into the
chemistry of ethylene and selected fluoroolefins with
compound 1 in attempts to gain further information
about the reactivity of compound 1 with unsaturated
substrates and the effects of sequential fluorine substi-
tution in reactions of fluoroolefins. Apart from tetrafluo-
roethylene, the coordination chemistry of which is well
documented,14 little has been published on partially
fluorinated ethylene.14k-m,15-17
Exp er im en ta l Section
Gen er a l Com m en ts. All solvents were dried (using ap-
propriate drying agents), distilled before use, and stored under
argon. Deuterated solvents used for NMR experiments were
freeze-pump-thaw degassed (three cycles) and stored under
nitrogen or argon over molecular sieves. Reactions were carried
out under argon using standard Schlenk techniques, and
compounds that were used as solids were purified by recrys-
tallization. Prepurified argon and nitrogen were purchased
from Linde, carbon-13-enriched CO (99%) was supplied by
Isotec Inc, ethylene was supplied by Praxair, fluoroethylene,
Z-1,2-difluoroethylene, and 1,1-difluoroethylene were supplied
by Lancaster, trifluoroethylene was supplied by PCR or
prepared by a literature method,18 and tetrafluoroethylene was
prepared by a literature method.19 All purchased gases were
used as received. Other reagents were obtained from Aldrich
P r ep a r a tion of Com p ou n d s. (a ) [Ir 2(CH3)(η2-C2H4)-
(CO)2(d p p m )2][CF 3SO3] (2b). This product was obtained by
passing ethylene through a solution of 1 (20 mg, 0.015 mmol)
in 0.7 mL of CD2Cl2 in an NMR tube for 2 min at ambient
temperature, followed by allowing the solution to stand for 1
h. The NMR spectra were found to be identical with those of
the previously characterized compound 2b.20 These data are
given in Table 1 for completeness.
(b) [Ir 2(H)(η2-C2H4)(CO)2(µ-CH2)(d p p m )2][CF 3SO3] (2a ).
In a typical experiment 3 mL (5 equiv) of ethylene was added
slowly by means of a gastight syringe to a solution of 30 mg
(0.02 mmol) of 1 in 0.7 mL of CD2Cl2 in an NMR tube cooled
to -78 °C in a solid CO2/acetone bath, resulting in a change
of color of the solution from red to orange. Before addition of
the ethylene, the syringe needle was cooled in the chilled
solution of 1 to minimize warming of the solution upon addition
of ethylene. One-dimensional NMR spectra (1H, 31P, 13C) were
recorded at -80 °C. Warming to above -60 °C resulted in
irreversible conversion of 2a to 2b.
(c) [Ir 2(CH3)(η2-C2H3F )(CO)2(d p p m )2][CF 3SO3] (3). The
reaction of 1 (30 mg in 0.7 mL of CH2Cl2) with 6 mL of
fluoroethylene (10 equiv) at -78 °C was carried out as
described in part b, again resulting in an orange solution.
Between -80 and -50 °C only one product (3) was observed
by NMR. Above -50 °C compound 3 decomposed to 1 and free
fluoroethylene.
(d ) [Ir 2(CH3)(η2-CHF dCHF )(CO)2(d p p m )2][CF 3SO3] (4).
This reaction was carried out as described in part c except that
ca. 10 equiv of Z-1,2-difluoroethylene was added to compound
1, yielding an orange solution of compound 4 at temperatures
below -40 °C. Above this temperature decomposition to 1 and
Z-1,2-difluoroethylene occurred.
(e) Rea ction of 1 w ith 1,1-Diflu or oeth ylen e. In an NMR
experiment, as described in parts c and d, the reaction of 1
with ca. 5 equiv of 1,1-difluoroethylene at -78 °C gave un-
reacted 1, [Ir2(H)(η2-F2CCH2)(CO)2(µ-CH2)(dppm)2][CF3SO3]
(5a ), and [Ir2(CH3)(η2-F2CCH2)(CO)2(dppm)2][CF3SO3] (5b) in
an approximate 2:1:2 ratio, together with minor amounts of
unidentified species. Warming the orange solution resulted in
a transformation of 5a into 5b, such that at -30 °C this con-
version was complete and compounds 1 and 5b were the two
major products. Leaving this solution at -20 °C overnight
resulted in an approximately 50% conversion to [Ir2(CH3)-
(CO)2(µ-CF2CH2)(dppm)2][CF2SO3] (5c). Above 0 °C decomposi-
tion to unidentified products resulted.
(f) [Ir 2(CH3)(CO)2(µ-CHF CF 2)(d p p m )2][CF 3SO3] (6). The
reaction between 1 and ca. 5 equiv of trifluoroethylene was
carried out at -78 °C as described in part c, causing the
solution to change from red to orange. At -80 °C 1 disappeared
slowly, giving way to 6; however, warming to -50 °C resulted
in rapid conversion to only 6. At 20 °C this species decomposed
to unidentified products over a period of several hours.
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Olefin Binding in a Binuclear Iridium Complex
Organometallics, Vol. 22, No. 23, 2003 4649
Ta ble 1. Sp ectr a l Da ta a ,b for Rea ction s of Com p ou n d 1 w ith Eth ylen e a n d F lu or oolefin s
NMRa,b
productc
IRd
δ(31P{1H})e
δ(1H)f,g
5.02 (b, 2H),
4.91 (b, 2H),
3.16 (b, 2H),
1.16 (b, 4H),
-12.3 (b, 1H)
3.25 (m, 4H),
1.05 (t, 3H),
0.70 (t, 4H)
δ(13C)f
δ(19F)h
[Ir2(H)(η2-C2H4)(CO)2(µ-CH2)(dppm)2]+ (2a )
-2.0 (m),
-3.7 (m)
189.1 (b),
2
178.4 (t, J PC
)
10 Hz)
[Ir2(CH3)(η2-C2H4)(CO)2(dppm)2]+ (2b)20
[Ir2(CH3)(CO)2(η2-CHFdCH2)(dppm)2]+ (3)
1971 (vs),
1787 (s),d
1978 (s),
1790 (m)i
19.0 (m),
5.7 (m)
204.0 (t)
18.8 (m),
6.1 (m)
3.50 (b, 2H), 3.10 (b,
1H), 3.00 (b, 1H), 2.2
(b, 1H), 1.13 (t, 3H,
3J PC ) 8.5 Hz), 0.96
208.3 (b), 209.8 (b)
-172.1 (b)
(b, 2H)
1.18 (t, 3H, J PH
[Ir2(CH3)(CO)2(η2-CHFdCHF)(dppm)2]+ (4)
[Ir2(H)(µ-CH2)(CO)2(η2CF2dCH2)(dppm)2]+ (5a )
17.2 (m),
2.6 (m)
-2.8 (m),
-4.3 (m)
)
207.4 (b)
191.3 (b)
-211.6 (b)
-80.4 (b)
3
9.0 Hz)
5.37 (b, 2H), 5.11 (b,
2H), 3.28 (b, 2H),
0.08 (b, 2H), -12.4
(b, 1H)
[Ir2(CH3)(CO)2(η2-CH2dCF2)(dppm)2]+ (5b)
[Ir2(CH3)(CO)2(µ-CH2dCF2)(dppm)2]+ (5c)
16.1 (m),
6.4 (m)
3.80 (m, 2H), 3.02
(m, 2H), 1.15 (t, 3H,
3J PH ) 8.8 Hz), 0.37
(m, 2H)
4.44 (m, 2H), 3.60
(m, 2H), 2.47 (m,
2H), 0.18 (t, 3H,
3J PH ) 4.8 Hz)
210.7 (b), 202.3 (b)
-83.7 (b)
16.1 (m),
5.9 (m)
196.6 (b), 185.6 (b)j -45.8 (m)
[Ir2(CH3)(CO)2(µ-CHFdCF2)(dppm)2]+ (6)
14.9 (m),
5.1 (m)
3.97 (m, 1H), 3.92
(m, 2H), 3.56 (m,
1H), 0.36 (t, 3H,
3J PH ) 6.0 Hz)
196.9 (t, J PC
)
-52.6 (d,
2
8.6 Hz), 184.5 (b)j
2J FF ) 253 Hz),
2
-81.6 (d, J FF
253 Hz), -193.8 (b)
-79.7 (m)
)
[Ir2(CH3)(CO)2(µ-C2F4))(dppm)2]+ (7)
[Ir2(CH3)(CO)3(µ-C2F4)(dppm)2]+ (8)
2020 (vs),
2001 (s)
16.8 (m),
5.2 (m)
3.89 (m, 4H), 0.41
191.9 (m), 181.6
3
(t, 3H, J PH ) 5.0 Hz) (m), -7.4 (bt)
-86.4 (m)
2062 (sh), -7.61 (m),
4.61 (m, 4H), 0.79
178.8 (m), 177.8
-73.6 (t),
3
2018
-15.5 (m)
(t, 3H, J PH ) 6.0 Hz) (m), 156.5 (bs),
-84.7 (b)
(br, vs)
2006 (vs),
1990 (s)
-13.2 (bs)
[Ir2(CH3)(PMe3)(CO)2(µC2F4))(dppm)2]+ (9)
-18.2 (m),
-19.0
5.23 (m, 2H), 4.99
(m, 2H), 0.92 (bt,
185.2 (m), 180.1
-70.0 (dt,
2
(t, J PC)13 Hz),
3J PF ) 55 Hz),
-83.3 (m)
3
3
(m), -68.8
3H, J PH ) 6.5 Hz),
-18.9 (dm, J PC
)
3
(t, J PF
0.80 (d, 9H)
14 Hz)
) 55 Hz)
a
b
NMR abbreviations: s ) singlet, d ) doublet, t ) triplet, q ) quintet, m ) multiplet, b ) broad. NMR data in CD2Cl2 unless
otherwise stated. c The anion in all cases is trifluoromethane sulfonate. All samples as solids unless otherwise noted. IR abbreviations
d
(ν(CO) values given): s ) strong, vs ) very strong, sh ) shoulder, br ) broad. e 31P{1H} chemical shifts are referenced vs external 85%
H3PO4. f 1H and 13C chemical shifts are referenced vs external TMS. Chemical shifts for the phenyl hydrogens are not given in the 1H
g
data. 19F chemical shifts are referenced vs external CFCl3. Recorded in CH2Cl2. Recorded in acetone-d6.
h
i
j
(g) [Ir 2(CH3)(CO)2(µ-C2F 4)(d p p m )2][CF 3SO3] (7). Com-
pound 1 (40 mg, 0.029 mmol) was dissolved in 5 mL of
CH2Cl2, and tetrafluoroethylene was passed over the solution
for 1 min. The solution was then stirred under a static atmos-
phere of the gas for ca. 72 h, during which the color changed
from red to dark red-brown. No species other than 1 and 7
was observed by NMR at intermediate times, over a range of
temperatures from -80 °C to ambient. The solution of com-
pound 7 was evaporated to ca. 2 mL in vacuo, and a brown
powder was precipitated and washed with pentane (3 × 10
mL) and dried under vacuum. Yield: 93%. Anal. Calcd for
Ir2SP4F7O5C56H47: C, 45.65; H, 3.22. Found: C, 45.38; H, 3.02.
X-r a y Da ta Collection . Pale yellow crystals of [Ir2(CH3)-
(CO)3(µ-C2F4)(dppm)2][CF3SO3]‚CH2Cl2 (8), suitable for X-ray
diffraction, were grown via slow diffusion of diethyl ether into
a concentrated CH2Cl2 solution of the compound under a CO
atmosphere. A suitable crystal was mounted and flame-sealed
in a glass capillary under solvent vapor to minimize decom-
position or deterioration due to solvent loss. Data were
collected at -60 °C on a Siemens P4/RA diffractometer using
graphite-monochromated Cu KR radiation,21 and unit cell
parameters were obtained from a least-squares refinement of
50 reflections in the range 56.7° < 2θ < 59.2°. The ortho-
rhombic cell, the diffraction symmetry, and the systematic
absences defined the space group as Fdd2. Three reflections
were chosen as intensity standards and were remeasured every
120 min of X-ray exposure time; in no case was decay evident.
Crystal parameters and details of data collection are sum-
marized in Table 2. The crystal faces were indexed and
measured, with absorption corrections being carried out using
Gaussian integration.
(h ) [Ir 2(CH3)(CO)3(µ-C2F 4)(d p p m )2][CF 3SO3] (8). Com-
pound 7 (30 mg, 0.020 mmol) was slurried in 5 mL of CH2Cl2.
Carbon monoxide was passed over the solution, causing a color
change from dark red-brown to bright yellow. An elemental
analysis was not obtained since carbonyl loss, yielding 7, occurs
upon flushing the solution with N2 or upon applying vacuum.
(i) [Ir 2(CH3)(P Me3)(CO)2(µ-C2F 4)(d p p m )2][CF 3SO3] (9).
Compound 7 (30 mg, 0.020 mmol) was slurried in 5 mL of CH2-
Cl2, and 1 equiv of trimethylphosphine (2.1 µL, 0.020 mmol)
was added, causing the solution to turn yellow. After 10 min
the solvent was evaporated to ca. 2 mL in vacuo, and a pale
yellow powder was precipitated. The solid was isolated, washed
with pentane (3 × 10 mL), and dried under vacuum. Yield:
Str u ctu r e Solu tion a n d Refin em en t. The positions of the
iridium and phosphorus atoms were found using the direct-
methods program SHELXS-86;22 the remaining atoms were
found using a succession of least-squares and difference
Fourier maps. Refinement proceeded using the program
(21) Programs for diffractometer operation and data collection were
those of the XSCANS systems supplied by Siemens.
(22) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
76%. Anal. Calcd for Ir2SP5F7O5C59H56
Found: C, 45.70; H, 3.42.
: C, 45.73; H, 3.65.

4650 Organometallics, Vol. 22, No. 23, 2003
Ristic-Petrovic et al.
Ta ble 2. Cr ysta llogr a p h ic Da ta for Com p ou n d 8
Sch em e 1
A. Crystal Data
[Ir2(CH3)(CO)3(µ-C2F4)(dppm)2]-
[CF3SO3] (8)
formula
C58H49Cl2F7Ir2O6P4S
fw
1586.21
cryst dimens, mm
color
0.46 × 0.10 × 0.10
yellow
cryst syst
orthorhombic
Fdd2 (No. 43)
space group
unit cell paramsa
a, Å
b, Å
c, Å
V, Å3
Z
37.872(2)
53.711(3)
11.7901(6)
23983(2)
16
d
calcd, g cm-3
1.757
11.230
µ, mm-1
B. Data Collection and Refinement Conditions
diffractometer
radiation (λ, Å)
Siemens P4/RAb
graphite-monochromated
Cu KR (1.54178)
-60
T, °C
scan type
2θ (max), deg
total data collected
Ch a r t 1
θ-2θ
113.5
8494 (-40 e h e 40,
-58 e k e 58, -12 e l e 12)c
8025
no. of unique reflns
no. of observations (NO)
no. of variables (NV)
range of transm factors
Flack absolute structure
paramd
2
7082 (Fo g 2σ(Fo2))
683
0.3997-0.0788
0.00 (2)
residual density, e Å-3
2.182 and -1.284
0.0577
0.1583
to yield [Ir2(CH3)(η2-C2H4)(µ-CO)2(dppm)2][CF3SO3] (2b),
in which the methyl ligand is bound to one metal while
the ethylene ligand is bound to the other.20 The geom-
etry shown in Scheme 1, which is slightly different than
originally proposed, was established by IR spectroscopy
and by a series of 1H and 13C NMR experiments in
which selective 31P decoupling established that the
methyl and carbonyl ligands all coupled to the pair of
31P nuclei on one metal, thereby establishing that these
ligands were all bound to this metal, while the ethylene
ligand was bound to the adjacent metal. In the 13C{1H}
NMR spectrum of a 13CO-enriched sample of 2b only
one carbonyl resonance was observed at δ 204.0, indi-
cating that both carbonyls are chemically equivalent on
the NMR time scale. The low-field chemical shift for
these carbonyls (compare, compound 2a ) suggests that
they are engaged in a semibridging interaction with the
adjacent metal, and this is supported by selective 31P
decoupling experiments which show a resolvable coup-
ling (giving a triplet) to one pair of 31P nuclei and
unresolved coupling to the other pair that results in
peak broadening. However, the IR spectrum of 2b
suggests a somewhat different interpretation. The solu-
tion IR spectrum displays one stretch for a terminal
carbonyl at 1978 cm-1 and another due to a bridging
carbonyl at 1790 cm-1. Taken together with the NMR
results this suggests the fluxional process shown in
Chart 1, in which exchange between a terminal and a
bridging carbonyl is occurring. We would expect this
exchange to be facile since it involves only very slight
movement of the two exchanging carbonyls. Even at -80
°C this exchange is rapid on the NMR time scale,
resulting in the observation of only one carbonyl reso-
nance. However, the much faster IR time scale24 allows
2
R1(Fo g 2σ(Fo2))e
2
wR2(Fo g -3σ(Fo2))e
goodness-of-fit (S)f
1.062
a
Obtained from least-squares refinement of 50 reflections with
b
56.7° < 2θ < 59.2°. Programs for diffractometer operation and
data collection were those of the XSCANS system supplied by
Siemens. c Data were collected in Friedel-opposite octants with
d
indices of the form +h+k+l and -h-k-l. Flack, H. D. Acta
Crystallogr. 1983, A39, 876-881. The Flack parameter will refine
to a value near 0 if the structure is in the correct configuration
and will refine to a value near 1 for the inverted configuration.
e R1 ) ∑||Fo| - |Fc||/∑|Fo; wR2 ) [∑w(Fo - Fc2)2/∑w(Fo4)]1/2
.
f S )
2
[∑w(Fo - Fc2)2/(n - p]1/2 (n ) number of data; p ) number of
parameters varied; w ) [δ2(Fo2) + (a0P)2 + a1P]-1 where P )
[Max(Fo2, 0) + 2Fc2]/3; a0 ) 0.1016, a1 ) 455.6115).
2
SHELXL-93.23 Hydrogen atom positions were calculated by
assuming idealized sp2 or sp3 geometries about their attached
carbon atoms (as appropriate) and were given thermal pa-
rameters 120% of the equivalent isotropic displacement pa-
rameters of their attached carbons. Further details of structure
refinement (other than described below) and final residual
indices may be found in Table 2.
Location of all atoms in the complex cation proceeded
smoothly; however the triflate anion was not well behaved, so
the bond lengths were constrained to enforce an idealized
geometry as follows: d(S-C(91)) ) 1.80 Å; d(S-O(91)) )
d(S-O(92))
) d(S-O(93)) ) 1.45 Å; d(F(91)-C(91)) )
d(F(92)-C(91)) ) d(F(93)-C(91)) ) 1.35 Å; d(F(91)-F(92)) )
d(F(91)-F(93)) ) d(F(92)-F(93)) ) 2.20 Å; d(O(91)-O(92)) )
d(O(91)-O(93)) ) d(O(92)-O(93)) ) 2.37 Å; d(F(91)-O(92))
) d(F(91)-O(93)) ) d(F(92)-O(91)) ) d(F(92)-O(93)) )
d(F(93)-O(91)) ) d(F(93)-O(92)) ) 3.04 Å.
Resu lts a n d Com p ou n d Ch a r a cter iza tion
1. Rea ction s w ith Eth ylen e. At ambient tempera-
ture the reaction of 1 with ethylene has been reported
(23) Sheldrick, G. M. SHELXL-93, Program for crystal structure
determination; University of Go¨ttingen: Germany, 1993.
(24) Drago, R. S. Physical Methods in Chemistry, W.B. Saunders
Co.: Philadelphia, PA, 1977; Chapter 4.

Olefin Binding in a Binuclear Iridium Complex
Organometallics, Vol. 22, No. 23, 2003 4651
Sch em e 2
both carbonyl binding modes to be identified. The
greater coupling of the carbonyl ligands to the 31P nuclei
adjacent to the methyl group, in the 13C NMR spectrum,
is consistent with the process shown in Chart 1, which
gives rise to an average structure in which the carbonyls
are more strongly bound to the iridium center bearing
the methyl ligand.
When the reaction of 1 with ethylene is carried out
at -78 °C, an additional product, [Ir2H(CO)2(η2-C2H4)-
(µ-CH2)(dppm)2][CF3SO3] (2a ), is observed. This species
can be obtained as the major product, together with 2b
(approximate 4:1 ratio, respectively), upon slow addition
of a prechilled sample of ethylene. However, warming
the sample to -60 °C or above results in irreversible
conversion of 2a to 2b. Compound 2a shows the normal
31P{1H} NMR spectrum typical of an AA′BB′ spin
system, consistent with two chemically inequivalent
phosphorus environments. In the 13C NMR spectrum,
two resonances for terminal carbonyls are observed at
δ 189.1 and 178.4; the former is a broad singlet and the
latter is a triplet. The 1H NMR spectrum displays a
broad hydride resonance at δ -12.3, an approximate
quintet at δ 5.02, corresponding to a methylene group,
and a very broad resonance at δ 1.16 for the four
ethylene protons. The structure proposed for 2a is
analogous to that established for the PMe3 adduct [Ir2H-
(CO)2(PMe3)(µ-CH2)(dppm)2][CF3SO3],7 in which ligand
attack (C2H4 or PMe3) at one metal results in C-H
activation of the methyl ligand by the adjacent metal.
in complex 3. The dppm methylene protons are also
broad and appear at δ 3.50 (2H), 3.10 (1H), and 3.00
(1H), while the iridium-bound methyl group appears as
a triplet at δ 1.13 (3J PH ) 8.5 Hz), with equal coupling
to two adjacent phosphorus nuclei. This resonance is
shifted downfield from the δ 0.58 seen for unreacted 1.
Coordination of fluoroethylene, shown in Scheme 2, does
not induce activation of the iridium-bound methyl group
at the temperatures studied; this methyl group remains
intact over the temperature range from -80 to -50 °C,
above which the complex is not stable. The facile
exchange of free and coordinated fluoroethylene is
consistent with it remaining intact upon coordination,
but the possibility of reversible C-H or C-F activation
cannot be unambiguously ruled out. The 13C{1H} NMR
spectrum of a 13CO-enriched sample of 3 shows two
broad carbonyl resonances at δ 209.8 and 208.3, com-
parable to the chemical shift of the time-averaged
carbonyls in [Ir2(CH3)(CO)2(η2-C2H4)(dppm)2][CF3SO3]
(2b), at δ 204.0.20 These resonances are intermediate
between the values usually observed in related com-
plexes for terminal and bridging carbonyls25 and are
more in line with what we have observed for the
semibridging geometry.26 Although we were unable to
acquire the IR spectrum of 3 (or other low-temperature
intermediates) at the required temperature, we assume
a ground-state structure like that of 2b in which
exchange between terminal and bridging carbonyls gives
average signals resembling an intermediate semibridg-
ing geometry. In this case two signals result because of
the inequivalence of both sides of the Ir2P4 plane. The
19F NMR spectrum shows an upfield shift for the
coordinated ligand, from δ -116.4 in the free olefin to
δ -172.1 in complex 3.
Reaction of the perdeuteriomethyl analogue of 1,
[Ir2(CD3)(CO)(µ-CO)(dppm)2][CF3SO3], with ethylene at
-78 °C followed by slow warming of the solution to 25
°C yielded [Ir2(CD3)(C2H4)(µ-CO)2(dppm)2][CF3SO3], in
which no detectable incorporation of deuterium into the
ethylene ligand was seen.
2. Rea ction s w ith F lu or oolefin s. (a ) F lu or oeth -
ylen e. Addition of a 3-fold excess of fluoroethylene to a
CD2Cl2 solution of [Ir2(CH3)(CO)2(dppm)2][CF3SO3] (1)
in an NMR tube at -78 °C gives ca. 90% conversion to
[Ir2(CH3)(CO)2(η2-CH2dCHF)(dppm)2][CF3SO3] (3). As
the sample is warmed in the NMR spectrometer from
-80 °C, the amount of adduct decreases until above -50
°C none of the product is observed, having reverted to
starting material (1). Complex 3 appears in the 31P{1H}
NMR spectrum as two equal intensity multiplets at δ
18.8 and 6.1, with the higher field multiplet having a
more complex appearance, suggesting that it is due to
the partial overlap of the resonances of two inequivalent
phosphorus nuclei, while the lower field multiplet is due
to two almost equivalent phosphorus nuclei. Overall, the
pattern is characteristic of an ABCD spin system, in
which two nuclei have accidentally equivalent chemical
shifts.
(b) Z-1,2-Diflu or oeth ylen e. Z-1,2-Difluoroethylene
binds reversibly to 1, forming the adduct [Ir2(CH3)(CO)2-
(η2-Z-CHFdCHF)(dppm)2][CF3SO3] (4), visible in the
NMR spectra between -80 and -40 °C. At the lower
Exchange between free and coordinated fluoroethyl-
1
ene was demonstrated by a 500 MHz H-TOCSY experi-
ment at -60 °C which showed exchange between the
protons giving rise to the two signals at δ 4.78 and 4.52,
due to the “dCH2” end of the free fluoroethylene
molecule, and the equivalent protons of the coordinated
ligand, which resonate at δ 0.96. By comparison, the
protons of the ethylene ligand in [Ir2(CH3)(CO)2(η2-
C2H4)(dppm)2][CF3SO3] (2b) appear as a triplet at δ 0.70
at room temperature.20 A very broad resonance at ca. δ
2.2 is assigned to the vinylic proton, geminal to fluorine,
(25) See for example: (a) Torkelson, J . R.; Oke, O.; Muritu, J .;
McDonald, R.; Cowie, M. Organometallics 2000, 19, 854. (b) Ristic-
Petrovic, D.; Torkelson, J . R.; Hilts, R. W.; McDonald, R.; Cowie, M.
Organometallics 2000, 19, 4432. (c) Dell’Anna, M. M.; Trepanier, S.
J .; McDonald, R.; Cowie, M. Organometallics 2001, 20, 88.
(26) See for example: (a) Xiao, J .; Cowie, M. Organometallics 1993,
12, 463. (b) Antonelli, D. M.; Cowie, M. Organometallics 1991, 10, 2550.
(c) Hilts, R. W.; Franchuk, R. A.; Cowie, M. Organometallics 1991, 10,
1297.

4652 Organometallics, Vol. 22, No. 23, 2003
Ristic-Petrovic et al.
Ch a r t 2
temperature there is only about 60% conversion when
5 equiv of olefin are employed, by -40 °C conversion to
the adduct is reduced to about 20%, and above this
temperature only starting material remains.
The 31P{1H} NMR spectrum of 4 shows a pair of
complex multiplets at δ 17.2 and 2.6, each of which is
due to two overlapping signals, consistent with an
ABCD spin system resulting from four chemically
inequivalent phosphorus nuclei. The 1H NMR spectrum
shows the methyl resonance as a triplet at δ 1.18 (3J PH
) 9.0 Hz), indicating that it is terminally bound to one
metal. Unfortunately, it is not possible to assign the
remaining resonances because of the poor quality of the
spectra resulting from the presence of a large excess of
the free olefin, impurities in the olefin, and broad
signals. The 13C NMR spectrum shows a single broad
peak at δ 207.4, consistent with the proposed structure
shown in Scheme 2 and a fluxional process as described
earlier for compounds 2b and 3. The 19F NMR spectrum
shows a single broad peak at δ -211.6 due to the two
equivalent fluorine nuclei in the η2 complex. An olefin-
bridged complex is ruled out on the basis that two
inequivalent fluorines would have been expected owing
to the inequivalence of the metals (one having a methyl
ligand attached), and similarly, two terminal 13CO
resonances at higher field than those observed would
have been expected. The NMR spectral parameters of
the η2 adducts 3 and 4 resemble those of the ethylene
adduct [Ir2(CH3)(CO)2(η2-C2H4)(dppm)2][CF3SO3] (2b).20
a single broad peak is observed at δ -80.4, shifted only
slightly downfield from the free olefin at δ -82.2.
Although two isomers of 5a might be expected, as
diagrammed in Chart 2, only one species is observed in
the NMR spectra. Whether this corresponds to only one
of the isomers shown or whether the one observed
species results from facile exchange between both
isomers is not known, although no evidence of fluxion-
ality was observed over the temperature range inves-
tigated.
Warming a solution of 5a from -80 to -30 °C causes
the transformation of 5a into the second product,
[Ir2(CH3)(CO)2(η2-CF2dCH2)(dppm)2][CF3SO3] (5b), and
by -30 °C only 5b and a small quantity of 1 remain.
Compound 5b displays equal intensity multiplets at δ
16.1 and 6.4 in the 31P{1H} NMR spectrum, consistent
with an AA′BB′ spin system. Because this species
predominates in solution at -30 °C, its NMR charac-
terization was carried out at this temperature. Its dppm
methylene resonances, at δ 3.80 and 3.02 in the 1H NMR
spectrum, are sharper than those of 3, and the meth-
ylene signal of the coordinated difluoroethylene ligand
shows resolved coupling (this signal is a broad envelope
for the fluoroethylene complex, 3). This methylene
signal in 5b is a virtual quintet at δ 0.37, collapsing to
a triplet (3J HF ) 9.2 Hz) upon broadband 31P decoupling,
indicating that this group is coupled approximately
equally to two phosphorus nuclei, and also to two
fluorines of the difluoromethylene group. The iridium-
bound methyl group appears as a triplet at δ 1.15, with
a coupling of 8.8 Hz to two equivalent phosphorus
nuclei. In the 19F NMR spectrum, the difluoromethylene
resonance appears as a broad peak at δ -83.7. Decoup-
(c) 1,1-Diflu or oeth ylen e. When an excess (>5 equiv)
of 1,1-difluoroethylene is added to a solution of 1 at -78
°C, the 31P NMR spectrum acquired at this temperature
typically shows three compounds, 1, 5a , and 5b, in a
ratio of ca. 2:1:2, together with small amounts of several
uncharacterized species.
The adduct, [Ir2(H)(CO)2(η2-CF2dCH2)(µ-CH2)-
(dppm)2][CF3SO3] (5a ), in which the iridium-bound
methyl of the starting complex 1 has undergone in-
tramolecular C-H activation to form a hydride and a
methylene ligand bridging the two metals, is character-
ized in the 31P{1H} NMR spectrum by equal intensity
resonances at δ -2.8 and -4.3, which are broad with
some multiplet structure visible. These chemical shifts
are very similar to those of the ethylene adduct (2a ),
proposed to have a similar structure. The proton NMR
spectrum of 5a displays a broad hydride signal at δ
-12.40 and a broad resonance at δ 5.37 due to the
bridging methylene group. In addition, the dppm meth-
ylene protons appear as broad peaks at δ 5.11 and 3.28
with some partially resolved coupling to the 31P nuclei,
while the methylene group of the coordinated 1,1-
difluoroethylene appears as a broad resonance at δ 0.08,
displaying the upfield shift expected of the protons of a
coordinated olefin.27 Only a tentative assignment of the
13C NMR spectrum could be made because of the
difficulty encountered in producing sufficient quantities
of 5a in solution and the presence of other products.
Only a single very broad carbonyl resonance at δ 191.3
could be attributed to 5a , whereas a pair of resonances
would normally be expected, unless the carbonyl carbons
were accidentally isochronous. In the 19F NMR spectrum
1
ling experiments (31P or H decoupling) did not succeed
in resolving this resonance into a multiplet. The 13C
NMR spectrum of a 13CO-enriched sample reveals two
inequivalent carbonyls at δ 210.7 and 202.3. Again, we
assume a structure as shown in Scheme 3 accompanied
by carbonyl exchange as proposed for 2b, 3, and 4.
Warming a solution of 5b above -20 °C results in its
disappearance with concomitant appearance of 1, until
by 10 °C only starting material is observed. After 24 h
at ambient temperature, a mixture of 1 and the olefin
gives decomposition products. However, maintaining a
solution of 5b at ca. -20 °C overnight results in
approximately 50% conversion to a third species,
[Ir2(CH3)(CO)2(µ-CF2dCH2)(dppm)2][CF3SO3] (5c). This
third isomer appears in the 31P NMR spectrum as
multiplets at δ 16.1 and 5.9; the lower field multiplet is
coincident with the lower field multiplet of compound
5b. As 5c forms, the integral ratio of the δ 16.1 multiplet
to the sum of the δ 5.9 and 6.4 resonances remains
constant, indicating that this low-field resonance is due
1
to both compounds. In the H NMR spectrum the dppm
methylenes for 5c appear at δ 4.44 and 3.60, the olefin
protons appear as a multiplet at δ 2.47, and the intact
(27) Elsenbroich, Ch.; Salzer, A. Organometallics: A Concise Intro-
duction; VCH Publishers: New York, 1989; p 298.

Olefin Binding in a Binuclear Iridium Complex
Organometallics, Vol. 22, No. 23, 2003 4653
Sch em e 3
Sch em e 4
age coordination shift is 45.9 ppm for this pair of
geminal nuclei, consistent with rehybridization toward
sp3 for the carbon bearing two fluorines due to substan-
tial back-donation from the metals. This shift is similar
to that observed for the analogous tetrafluoroethylene
complex described below. As noted, the coordination
shift of the remaining fluorine, while not as pronounced,
is also downfield.
iridium-bound methyl group appears as a triplet at δ
0.18, with a 4.8 Hz coupling to two equivalent phospho-
rus nuclei. The olefinic methylene resonance collapses
to a triplet (3J HF ) 20 Hz) upon broadband 31P irradia-
tion, and this coupling is also found in the resonance
for the difluoromethylene group in the 19F{31P} spec-
trum, which appears as a triplet at δ -45.8. The olefinic
methylene 1H resonance appears at lower field than that
in the η2-coordinated complexes 5a and 5b, where it
appears at δ 0.08 and 0.37, respectively, and is consis-
tent with the proposed dimetallacyclobutane formula-
tion. The 13C NMR spectrum of a 13CO-enriched sample
shows two inequivalent terminal carbonyls at δ 196.6
and 185.6.
The olefinic fluorine signal of the 1,1-difluoroethylene
ligand is shifted downfield in the 19F NMR spectrum
by 36.4 ppm from the free olefin, comparable to the
coordination shift displayed in the formation of the
tetrafluoroethylene-bridged complex [Ir2(CH3)(CO)2(µ-
C2F4)(dppm)2][CF3SO3] (7), where an average downfield
shift of ca. 52 ppm is observed (vide infra). Unfortu-
nately, we were unable to confirm the orientation of the
bridging difluoroethylene group in 5c, but it is shown
in Scheme 3 in the orientation having the larger fluorine
atoms closer to the less crowded metal.
The 31P{1H} NMR spectrum reveals the presence of
a pair of complex multiplets at δ 14.9 and 5.1, in which
each multiplet is actually comprised of two closely
spaced resonances, consistent with four inequivalent
1
phosphorus nuclei. The H NMR spectrum shows the
dppm protons as multiplets at δ 3.97 (1H), 3.92 (2H),
and 3.56 (1H), while the iridium-bound methyl appears
as a triplet at δ 0.36 (3J PH ) 6.0 Hz), indicating a
terminally bound methyl group, the protons of which
couple approximately equally to an adjacent pair of
phosphorus nuclei. We were unable to locate the single
vinylic proton of the coordinated trifluoroethylene moi-
ety. The 13C NMR spectrum of a 13CO-enriched sample
of 6 shows two inequivalent terminal carbonyl reso-
nances at δ 196.9 and 184.5, which are very similar to
the terminal carbonyls of the 1,1-difluoroethylene-
bridged compound 5c (δ 196.6 and 185.6).
The product 6 persists in CD2Cl2 solution upon
warming to room temperature, but decomposes after
several hours to a mixture of unidentified products.
(e) Tetr a flu or oeth ylen e. Unlike the other fluoroole-
fins, which react rapidly with 1, even at -80 °C, the
reaction of 1 with tetrafluoroethylene occurs very slowly
over a several day period at ambient temperature. No
evidence for any species containing a terminally bound
η2-tetrafluoroethylene unit was observed at any tem-
perature between -80 °C and ambient. The 31P{1H}
NMR spectrum of the product [Ir2(CH3)(CO)2(µ-C2F4)-
(dppm)2][CF3SO3] (7) shows two resonances at δ 5.2 and
16.8, having patterns typical of an AA′BB′ spin system.
A 13C{1H} NMR spectrum of a 13CO- and 13CH3-enriched
sample of 7 shows the terminal carbonyls at δ 191.9 and
181.6 as multiplets and the methyl group at δ -7.4 as
a broad triplet. Selective 31P decoupling indicates that
the methyl and the high-field carbonyl are coupled to
the same pair of phosphorus nuclei, so are probably on
the same metal, while the other carbonyl couples to the
phosphorus nuclei on the other metal. The methyl
(d ) Tr iflu or oeth ylen e. Trifluoroethylene reacts with
1 at -78 °C, forming [Ir2(CH3)(CO)2(µ-CHFdCF2)-
(dppm)2][CF3SO3] (6), as shown in Scheme 4. The 19F
NMR spectrum shows three broad signals, at δ -52.6,
-81.6, and -193.8, all of which are shifted to low field
from the free ligand, the corresponding resonances of
which appear at δ -100.0, -125.9, and -205.0, respec-
tively. The two lower field resonances show mutual
coupling of 253 Hz, which is consistent with an sp3-
hybridized “CF2” group.28 Coupling to the high-field
fluorine at δ -193.8 is not discernible. The broadness
of the peaks in the 19F NMR spectrum is not due to
unresolved coupling to phosphorus, as revealed by the
failure of the resonances to sharpen upon broadband
31P decoupling.
All three fluorine nuclei of trifluoroethylene have
moved to lower field upon complexation, with the
geminal fluorines having moved the farthest. The aver-
(28) Dungan, C. H.; VanWazer, J . R. Compilation of Reported 19F
NMR Chemical Shifts; Wiley-Interscience: New York, 1970.

4654 Organometallics, Vol. 22, No. 23, 2003
Ristic-Petrovic et al.
Ta ble 3. Selected In ter a tom ic Dista n ces a n d
An gles for Com p ou n d 8
(a) Distances (Å)
atom 1
atom 2
distance
atom 1
atom 2
distance
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(2)
Ir(2)
Ir(2)
Ir(1)
Ir(1)
Ir(2)
Ir(2)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
P(1)
P(3)
P(2)
2.8967(9)
1.95(2)
1.93(2)
2.10(2)
2.10(2)
1.84(2)
2.16(2)
2.376(5)
2.393(4)
2.344(5)
Ir(2)
F(1)
F(2)
F(3)
F(4)
O(1)
O(2)
O(5)
C(3)
P(4)
C(3)
C(3)
C(4)
C(4)
C(1)
C(2)
C(5)
C(4)
2.346(5)
1.40(2)
1.46(2)
1.40(2)
1.37(2)
1.11(2)
1.10(2)
1.19(2)
1.54(3)
F igu r e 1. Perspective view of the complex cation of [Ir2-
(CH3)(CO)3(µ-C2F4)(dppm)2][CF3SO3] (8) showing the atom-
labeling scheme. Only the ipso carbons of the phenyl rings
are shown. Non-hydrogen atoms are represented by Gauss-
ian ellipsoids at the 20% probability level, except for
hydrogen atoms, which are drawn arbitrarily small.
(b) Angles (deg)
atom 1 atom 2 atom 3 angle atom 1 atom 2 atom 3 angle
Ir(2)
Ir(2)
Ir(2)
P(1)
C(1)
C(1)
C(2)
Ir(1)
Ir(1)
Ir(1)
P(2)
C(4)
C(4)
C(5)
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(1)
Ir(2)
Ir(2)
Ir(2)
Ir(2)
Ir(2)
Ir(2)
Ir(2)
C(1)
C(1) 168.5(5) Ir(1)
C(2)
C(3)
C(3)
C(3)
C(3)
C(3)
C(3)
C(4)
C(4)
C(4)
C(4)
C(4)
C(4)
C(5)
O(2) 178(2)
F(1) 116(1)
F(2) 111(1)
C(4) 109(1)
F(2) 100(1)
C(4) 112(1)
C(4) 108(1)
F(3) 114(1)
F(4) 115(1)
C(3) 109(1)
F(4) 102(1)
C(3) 107(1)
C(3) 111(1)
O(5) 172(2)
C(2)
C(3)
82.7(5) Ir(1)
71.0(5) Ir(1)
P(3) 174.2(2) Ir(1)
C(2) 108.8(7) F(1)
1
resonance in the H NMR spectrum appears at δ 0.41
and displays 139 Hz coupling in the 13CH3-enriched
sample. As expected, two fluorine resonances for the
different ends of the bridging tetrafluoroethylene ligand
appear at δ -86.4 and -79.7, and each is coupled to
different pairs of 31P nuclei, as shown by selective
decoupling experiments, confirming the bridging ar-
rangement of this olefin.
C(3)
C(3) 153.7(7) F(2)
C(4) 71.2(5) Ir(2)
97.5(7) F(1)
C(5) 118.6(5) Ir(2)
C(6) 154.9(6) Ir(2)
P(4) 177.1(2) F(3)
C(5) 170.1(7) F(3)
83.7(7) F(4)
86.5(8) Ir(2)
O(1) 179(2)
Reaction of 7 with CO yields the labile tricarbonyl
complex [Ir2(CH3)(CO)3(µ-C2F4)(dppm)2][CF3SO3] (8).
Spectroscopic data for this species, given in Table 1, are
consistent with the structure diagrammed in Scheme
4, in which CO addition to the unsaturated metal in 7
has occurred. In the 13C NMR spectrum the three
carbonyl resonances appear as multiplets at δ 178.8,
177.8, and 156.5. Selective 31P decoupling suggests that
the low-field carbonyl (δ 178.8) and the methyl group
are bound to one metal, with the other two carbonyls
on the other. The two low-field carbonyl resonances are
proposed to correspond to the adjacent pair that are cis
to the metal-metal bond, while the high-field signal
corresponds to the carbonyl lying opposite the metal-
metal bond. The low-field chemical shift of terminal
carbonyls that are adjacent to another metal, compared
to terminal carbonyls that are remote from another
metal, has been noted.29 Reaction of a 13CO-enriched
sample of 7 with 12CO gives a 13C NMR spectrum in
which the two low-field carbonyl resonances dominate.
A very weak resonance (ca. 10% of the intensity of each
of the others) appears at δ 156.5. This indicates that
CO attack occurs at the unsaturated metal, predomi-
nantly at the site opposite the Ir-Ir bond; the small
amount of 13CO at the high-field site suggests that some
12CO attack at the site adjacent to the Ir-Ir bond might
occur. Leaving the sample solution overnight causes no
change in the intensities of the 13CO resonances,
indicating that if exchange of the carbonyls is occurring,
it is very slow.
C(6)
C(6)
tions are clear from the Ir-Ir-C(olefin) angles of 71.2-
(5)° and 71.0(5)°. However, the origins of other subtle
distortions that give rise to differences in geometry at
each metal are not as obvious. For example, carbonyl
C(5)O(5) is almost opposite the one olefinic carbon
C(4) (170.1(7)°) and bent away from Ir(1) with an
Ir(1)-Ir(2)-C(5) angle of 118.6(5)°, whereas the equiva-
lent carbonyl on Ir(1) (C(2)O(2)) is bent toward Ir(2),
yielding an Ir(2)-Ir(1)-C(2) angle of 82.7(5)° and, as
seen from Figure 1, is not opposite the olefinic carbon
C(3). The tilting of C(2)O(2) toward Ir(2) gives rise to
angles at Ir(1) between the carbonyls and the olefin of
greater than 97°, whereas the opposite tilt of C(5)O(5)
compresses the equatorial angles at Ir(2) to less than
85°. The metal-metal separation, of 2.8967(9) Å, is at
the long end of typical single bonds in such com-
plexes,7,25a,30,31 but is shorter than the nonbonded in-
traligand P-P separations (3.011(6) and 3.019(6) Å),
showing mutual attraction of the metals.
Binding of the tetrafluoroethylene molecule in the
bridging site has clearly resulted in a rehybridization
of the olefinic carbons such that this unit is best
described as a dimetallacyclobutane moiety. As a result,
all angles around the olefinic carbons are now close to
the tetrahedral value and the C(3)-C(4) bond (1.54(3)
Å) is that expected for a single bond.32 The Ir-C bonds
to the bridging olefin (2.10(2), 2.10(2) Å) are slightly
shorter than that involving the methyl ligand (2.16(2)
Å) as might be expected for stronger bonds involving
fluoroalkyl groups.33 Despite the slight asymmetry at
The structure proposed for 8 has been confirmed by
an X-ray determination, and a view of the complex
cation is shown in Figure 1, with important bond
lengths and angles given in Table 3. The coordination
geometry at each metal is distorted octahedral, in which
the constraints induced by the bridging olefin give rise
to distortions from idealized geometries. These distor-
(30) Torkelson, J . R.; McDonald, R.; Cowie, M. Organometallics
1999, 18, 4134.
(31) Ristic-Petrovic, D.; Wang, M.; McDonald, R.; Cowie, M. Orga-
nometallics 2002, 21, 5172.
(29) George, D. S. A.; McDonald, R.; Cowie, M. Organometallics
1998, 17, 2553.
(32) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 1987, 51.

Olefin Binding in a Binuclear Iridium Complex
Organometallics, Vol. 22, No. 23, 2003 4655
each metal, the tetrafluoroethylene is symmetrically
bridging, with identical bond lengths and angles within
the “Ir2C2” framework.
is observed. For difluoro substitution, two cases are
observed; for Z-1,2-difluoroethylene only the terminal
mode is observed, whereas for 1,1-difluoroethylene the
terminal mode is observed at low temperatures with
slow rearrangement to bridging at higher temperatures.
We find that only fluoroolefins, having two fluorines on
at least one of the olefin carbons, display the bridging
coordination mode. On the basis that lower fluorine
substitution gives rise to terminal coordination whereas
greater substitution favors the bridging mode, it is
tempting to conclude that the bridging site is more
accessible since it is able to coordinate the bulkier (more
substituted) olefins. However, this is inconsistent with
the kinetic product in the case of 1,1-difluoroethylene
being an η2-adduct. Furthermore, the electronic effects
of substituting hydrogens by the much more electrone-
gative fluorines must also be considered. In the bridging
mode, the olefin can be viewed as a dimetallaalkane in
which the organic fragment is now saturated, with
rehybridization of the olefin carbons to sp3, as was
clearly demonstrated in the structure of the tetrafluo-
roethylene complex 8. According to Bent’s rule,36 the
electronegative fluorine substituents should favor re-
hybridization of the olefinic carbons, rendering them less
electronegative (less s character in the sp3 compared to
sp2 hybrids). On the basis of this argument, the obser-
vation that greater fluorine substitution favors the
bridging mode is not surprising. In the case of the
difluorinated cases (Z-1,2- and 1,1-difluoroethylene) it
appears that only the latter olefin, having two fluorines
on the same carbon, yields a bridged adduct. From this
we assume that more is gained from rehybridization of
a CF2 and a CH2 carbon than is gained from rehybrid-
ization of two CHF carbons. The tendency of gem-
difluoroolefins to form saturated compounds is well
documented.37 Of course, steric repulsions must also be
considered since it may also be that the relative repul-
sions between the terminal and bridging sites differ
between these two olefins.
That no terminal η2-adduct is observed with either
trifluoro- or tetrafluoroethylene, even at low tempera-
ture, suggests that this bonding mode is sterically
unfavorable. This is especially true for the tetrafluoro-
ethylene adduct 7, which is formed only slowly, but for
which no terminal adduct is observed. Certainly there
seems to be no obvious electronic rationale for the
absence of η2-tetrafluoroethylene adducts since there is
plenty of documentation in which binding of tetrafluo-
roethylene to a single low-valent, late transition metal
has been shown to be favorable.14
We have found that the 19F chemical shift in these
fluoroolefin complexes can be diagnostic for the olefin
coordination mode. Thus, in the η2-complexes 3, 4, and
5b the coordination shift of the fluorine nuclei in the
19F NMR spectrum, compared to the free olefin, is
toward higher field, whereas in the bridged com-
plexes 5c and 6-9 the shift is to lower field. The
changes in chemical shift (∆δ) upon complexation are
summarized in Table 4. Although for the olefins
HFCdCH2, Z-CFHdCFH, HFCdCF2, and C2F4 and for
Reaction of 7 with 1 equiv of PMe3 yields the PMe3
adduct [Ir2(CH3)(PMe3)(CO)2(µ-C2F4)(dppm)2][CF3SO3]
(9). The 31P{1H} NMR spectrum of this product shows
the dppm resonances at δ -18.2 and -19.0 and the
PMe3 resonance at δ -68.8. This latter signal is a
triplet, showing 55 Hz coupling to one end of the
tetrafluoroethylene ligand. No coupling between the
PMe3 group and the dppm 31P nuclei is resolved. In the
13C NMR spectrum of a 13CO- and 13CH3-enriched
sample of 9 the terminal carbonyls appear at δ 185.2
and 180.1 and the methyl group appears at δ -18.9 and
displays 14 Hz coupling to the PMe3 phosphorus nucleus.
1
The H NMR spectrum shows the expected resonances
for the phosphines and displays the methyl ligand signal
at δ 0.92. Two resonances appear in the 19F NMR
spectrum, at δ -70.0 and -83.3, with the former
displaying the above-noted 55 Hz coupling to PMe3.
On the basis of the reaction of 7 with CO, in which
the added carbonyl occupies the axial site opposite the
metal-metal bond, we had anticipated that PMe3 would
react likewise. This proposal is supported by the ob-
served 14 Hz coupling in the 13C NMR spectrum
between the 13CH3 and the PMe3 groups. Such coupling
through the metal-metal bond has been observed when
the ligands occupied the axial sites.25c,34 However, the
very large coupling (55 Hz) between the PMe3 group and
the fluorines on one end of the tetrafluoroethylene
ligand strongly suggests that these groups are mutually
trans at one metal; this would mean that PMe3 attack
occurred at the unsaturated metal in the site between
the metals. By comparison, three-bond P-F coupling
when a phosphine ligand is cis to a CF3 ligand is
normally in the range 4-25 Hz,35 whereas trans coup-
ling in the range 40-50 Hz has been reported.35c The
location of the PMe3 group (i.e., trans to or cis to the
Ir-Ir bond) at this stage remains uncertain, and we
have not succeeded in growing suitable crystals of the
complex to unambiguously determine this.
Discu ssion
In comparing the binding of ethylene and a series of
fluoroethylenes to the binuclear complex [Ir2(CH3)(CO)-
(µ-CO)(dppm)2][CF3SO3] (1), we observe two olefin bind-
ing modessterminal and bridgingsdepending upon the
degree and nature of the fluorine substitution. For
olefins containing three or four fluorines (i.e., tri- or
tetrafluoroethylene) only the bridging mode is observed,
whereas with zero or one fluorine substituents (ethylene
or fluoroethylene) only the terminal, η2-binding mode
(33) (a) Kiplinger, J . L.; Richmond, T. G.; Osterberg, C. E. Chem.
Rev. 1994, 94, 373. (b) Brothers, P. J .; Roper, W. R. Chem. Rev. 1988,
88, 1293.
(34) (a) Oke, O.; McDonald, R.; Cowie, M. Organometallics 1999,
18, 1629. (b) Vaartstra, B. A.; Xiao, J .; J enkins, J . A.; Verhagen, R.;
Cowie, M. Organometallics 1991, 10, 2708. (c) Antwi-Nsiah, F. H.;
Torkelson, J . R.; Cowie, M. Inorg. Chim. Acta 1997, 259, 213. (d)
Mague, J . T. Organometallics 1986, 5, 918. (e) Brown, M. P.; Fisher,
J . R.; Hill, R. H.; Puddephatt, R. J .; Seddon, R. R. Inorg. Chem. 1981,
20, 2516.
(35) (a) Albietz, P. J ., J r.; Cleary, B. P.; Paw, W.; Eisenberg, R. J .
Am. Chem. Soc. 2001, 123, 12091. (b) Burrell, A. K.; Clark, G. R.;
J effrey, J . G.; Richard, C. E. F.; Roper, W. R. J . Organomet. Chem.
1990, 388, 391. (c) Albietz, P. J ., J r.; Clearly, B. P.; Paw, W.; Eisenberg,
R. Inorg. Chem.
(36) (a) Bent, H. A. Chem. Educ. 1960, 37, 616. (b) Bent, H. A. Chem.
Rev. 1961, 61, 275.
(37) Smart, B. E. In The Chemistry of Functional Groups, Supple-
ment D; Patai, S., Rappoport, Z., Eds.; J ohn Wiley & Sons: New York,
1983; Chapter 14, p 603.

4656 Organometallics, Vol. 22, No. 23, 2003
Ristic-Petrovic et al.
Ta ble 4. Ch em ica l Sh ift Ch a n ge u p on Coor d in a tion for F lu or in a ted Eth ylen es
change in chemical shift upon complex formation (ppm),
compounda
∆δ ) δ(complex) - δ(ligand)
[Ir2(CH3)(CO)2(η2-CHFdCH2)(dppm)2]+ (3)
[Ir2(CH3)(CO)2(η2-CHFdCHF)(dppm)2]+ (4)
[Ir2(H)(µ-CH2)(CO)2(η2-CH2dCF2)(dppm)2]+ (5a )
[Ir2(CH3)(CO)2(η2-CH2dCF2)(dppm)2]+ (5b)
[Ir2(CH3)(CO)2(µ-CH2dCF2)(dppm)2]+ (5c)
[Ir2(CH3)(CO)2(µ-CHFdCF2)(dppm)2]+ (6)
[Ir2(CH3)(CO)2(µ-CF2dCF2)(dppm)2]+ (7)
[Ir2(CH3)(CO)3(µ-C2F4)(dppm)2]+ (8)
-56
-47
2
-2
36
47, 44, 11
55, 49
61, 50
65, 52
[Ir2(CH3)(PMe3)(CO)2(µ-C2F4)(dppm)2]+ (9)
a
-
In all cases the anion is CF3SO3
.
Sch em e 5
metal-metal bond is symmetry forbidden and that an
unsymmetrical intermediate is involved.38
Attack at sites 1 and 2 is clearly reversible since
warming either product type results in regeneration of
the free olefin and compound 1. Furthermore, exchange
between free and coordinated fluoroethylene in the
olefin adduct 3 was established by 1H TOCSY NMR
experiments. In contrast, attack at site 3 leading to the
bridged adducts is apparently irreversible. Although the
bridged adducts 5c and 6 are not stable at ambient
temperature, their decomposition leads not to compound
1 and free olefin but to mixtures of unidentified prod-
ucts. We have preliminary evidence suggesting that this
results from C-F bond activation via HF loss, which is
a subject of ongoing studies.39 Labilization of a C-F
bond in a tetrafluoroethylene-bridged, diiron complex
has previously been observed.14e
It appears that the methylene hydride species are the
least stable products and are not observed at all for the
weakly coordinating fluoroethylene and Z-1,2-difluoro-
ethylene. Although not observed for these olefins at
temperatures down to -80 °C, we assume that this
structural type would be observed at lower tempera-
tures. At temperatures above -80 °C the methylene
hydride species transform into the more stable olefin
adducts [Ir2(CH3)(olefin)(CO)(µ-CO)(dppm)2][CF3SO3],
which are observed for all olefins in this study except
trifluoroethylene and tetrafluoroethylene.
As demonstrated for 1,1-difluoroethylene, the most
stable adduct for olefins containing geminal fluorines
is the olefin-bridged species. For trifluoro- and tetrafluo-
roethylene this structural type was the only one ob-
served. Even at temperatures down to -80 °C no η2-
olefin adduct was observed with these olefins. We
assume that steric factors inhibit attack at sites 1 and
2 for these highly substituted ethylenes. As noted
earlier, the rehybridization of the olefin carbons that
accompanies the bridged-olefin bonding mode appears
to require at least one CF2 group within the olefin.
Consequently only 1,1-difluoroethylene, trifluoroethyl-
ene, and tetrafluoroethylene achieve the bridging con-
figuration.
The transformations from one isomeric olefin adduct
to another at increasing temperature presumably in-
volve olefin loss from one site and recoordination at the
next most favorable site. Within the series of olefins
investigated only 1,1-difluoroethylene displays the com-
plete set of three isomeric structures, presumably hav-
the bridging CF2dCH2 group these shifts are unam-
biguous, the η2-CF2dCH2 complexes 5a and 5b undergo
shifts of only (2 ppm from the free olefin. Even here,
however, the chemical shift differences between termi-
nal and bridging (i.e., 5a and 5b vs 5c) are substantial
(34 and 38 ppm, respectively).
As described earlier, the products in the reactions of
1 with ethylene and selected fluoroolefins can be grouped
into three structural types. With 1,1-difluoroethylene
all three products are observed at different tempera-
tures, while for the other olefins only one or two of these
structure types are observed. These three product types
are shown in Scheme 5 (dppm groups above and below
the plane of the drawing are omitted and no substitu-
ents are shown on the olefin) and can be viewed as
resulting from substrate attack at the three different
sites on compound 1. Attack at site 1 forces the methyl
ligand toward the adjacent metal, where it undergoes
C-H bond cleavage and formation of the methylene-
bridged hydride species as observed in 2a (olefin )
ethylene) and 5a (olefin ) 1,1-difluoroethylene). Sub-
strate attack at site 2, between the carbonyls, yields the
species in which the methyl and olefin groups lie
opposite the metal-metal bond, flanked by the carbo-
nyls. This species type was observed for ethylene (2b),
fluoroethylene (3), Z-1,2-difluoroethylene (4), and 1,1-
difluoroethylene (5b). Finally, attack at site 3, between
the metals, yields the olefin-bridged products as ob-
served for 1,1-difluoroethylene (5c), trifluoroethylene
(6), and tetrafluoroethylene (7). We assume that olefin
attack in this last case occurs at one metal, followed by
movement of the olefin into the bridging site with
concomitant movement of the bridging carbonyl on the
opposite face of the complex to a terminal position.
Extended Hu¨ckel calculations on related dppm-bridged
complexes have suggested that direct attack at the
(38) Hoffman, D. M.; Hoffmann, R. Inorg. Chem. 1981, 20, 3543.
(39) Ristic-Petrovic, D.; Anderson, D. J .; Cowie, M., unpublished
data.

Olefin Binding in a Binuclear Iridium Complex
Organometallics, Vol. 22, No. 23, 2003 4657
ing the appropriate combination of steric factors to allow
favorable access to all three sites and electronic factors
to allow observation of the least stable adduct (type 1)
at low temperature while also favoring the bridging
mode at higher temperature.
rich metal,14k as anticipated from the Dewar-Chatt-
Duncanson model.40 We find that the binding affinities
of fluoroethylene, Z-1,2-difluoroethylene, and 1,1-dif-
luoroethylene, when bound in an η2-mode to one metal,
are less than that of ethylene. This study also demon-
strates the greater diversity observed for a binuclear
system compared to a similar mononuclear one in which
the η2-olefin binding mode is the only one anticipated.
Although such an adduct could display a number of
geometric isomers, we would not expect significant
chemical differences to arise as a consequence. By
contrast, the binuclear species 1 displays three struc-
tural types with different olefins. In the product in
which C-H activation of the methyl group has occurred
one might anticipate coupling of the olefin and the
bridging methylene group, although in this study no
evidence of such reactivity was observed. Similarly, in
the olefin-bridged products migratory insertion of the
olefin and the methyl ligand could occur, although again
this was not observed; even ligand addition (CO, PMe3)
to the tetrafluoroethylene adduct 7 does not induce a
migratory-insertion reaction. Nevertheless, the facile
decomposition of the olefin-bridged adducts of 1,1-
difluoroethylene and trifluoroethylene has suggested
that these fluoroolefins could be susceptible to fluoride
ion loss in this coordination mode, a reactivity that has
precedent in a diiron tetrafluoroethylene-bridged com-
plex.14e Studies are underway in pursuit of the idea that
binding of fluoroolefins in a bridging arrangement could
lead to unusual reactivity of these groups.
The olefin-metal bond in an η2-olefin complex is
commonly described in terms of the Dewar-Chatt-
Duncanson (DCD) model,40 in which the two synergic
components of the bonding involve σ-donation from the
filled olefin π orbital to the metal and π back-donation
from the metal into the ligand π* orbital. On the basis
of this model we would anticipate that for a low-valent,
late transition metal complex increased substitution by
electronegative fluorines on the olefin should strengthen
the metal-olefin bond through enhanced π back-dona-
tion from the electron-rich metal. This is clearly not the
case in the series of olefins studied. Although ethylene
forms a stable η2-adduct at ambient temperature, the
mono- and difluoroolefins yield labile adducts only at
low temperatures. This decrease in metal-olefin bond
strength upon increasing fluorine substitution has
previously been observed14k and has been ascribed to a
combination of steric repulsions involving the fluorine
substituents and the greater energy required to deform
the coordinated olefinic carbons toward the sp3 limit,
as occurs with increasing fluorine substitution.41 Only
at tetrafluoroethylene does the olefin binding energy
match that of ethylene, with both giving isolable com-
plexes at ambient temperature, although in this study
these olefins display different binding modes. It is
difficult to establish how trifluoroethylene compares
since its decomposition at ambient temperature seems
not to be a function of olefin binding affinity, but
appears to result from other factors (vide supra).
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada (NSERC)
and the University of Alberta for financial support of
this research and NSERC for funding the P4/RA dif-
fractometer, and acknowledge Professor Eric Weitz
(Northwestern University) for helpful discussions.
Con clu sion s
This study has reconfirmed that increasing the fluo-
rine substitution in fluoroolefin ligands does not neces-
sarily lead to increased binding affinity to an electron-
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
experimental details, atomic coordinates, interatomic distances
and angles, anisotropic thermal parameters, and hydrogen
parameters for compound 8. This material is available free of
charge via the Internet at http://pubs.acs.org.
(40) (a) Dewar, M. J . S. Bull. Chem. Soc. Fr. 1951, 18, C79. (b) Chatt,
J .; Duncanson, L. A. J . Chem. Soc. 1953, 2939.
(41) (a) Ceden˜o, D. L.; Weitz, E.; Be´rces, A. J . Phys. Chem. A 2001,
105, 8077. (b) Ceden˜o, D. L.; Weitz, E. J . Am. Chem. Soc. 2001, 123,
12857.
OM030328B