Double activation of the geminal carbon-hydrogen bonds in 1,3-butadiene by a diiridium complex

Article abstract of DOI:10.1021/om0006054

The binuclear complex [Ir₂(CH₃)(CO)₂(dppm)₂] -[CF₃SO₃] (1) (dppm = Ph₂PCH₂PPh₂) reacts with 1,3-butadiene at ambient temperature over a 48 h period to give the vinylvinylidene-bridged product [Ir₂ (CH₃)(H)-(CO)₂(μ-H)(μ-CC(H)C(H)CH₂) (dppm)₂][CF₃SO₃] (2). At -55°C the same reactants yield the 1,3-butadiene adduct [Ir₂(CH₃)(CO)₂(μ-η²:η²- H₂C=C(H)C(H)=CH₂)-(dppm)₂][CF₃SO₃] (3), in which the diolefin binds in an s-trans geometry on one face of the complex. A proposal is advanced rationalizing the conversion of 3 to 2 upon warming.

Full text of DOI:10.1021/om0006054

4432
Organometallics 2000, 19, 4432-4434
Dou ble Activa tion of th e Gem in a l Ca r bon -Hyd r ogen
Bon d s in 1,3-Bu ta d ien e by a Diir id iu m Com p lex
Dusan Ristic-Petrovic, J effrey R. Torkelson, Robert W. Hilts,†
Robert McDonald,‡ and Martin Cowie§,*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received J uly 17, 2000
Summary: The binuclear complex [Ir2(CH3)(CO)2(dppm)2]-
[CF3SO3] (1) (dppm ) Ph2PCH2PPh2) reacts with 1,3-
butadiene at ambient temperature over a 48 h period to
give the vinylvinylidene-bridged product [Ir2(CH3)(H)-
(CO)2(µ-H)(µ-CC(H)C(H)CH2)(dppm)2][CF3SO3] (2). At
-55 °C the same reactants yield the 1,3-butadiene
adduct [Ir2(CH3)(CO)2(µ-η2:η2-H2CdC(H)C(H)dCH2)-
(dppm)2][CF3SO3] (3), in which the diolefin binds in an
s-trans geometry on one face of the complex. A proposal
is advanced rationalizing the conversion of 3 to 2 upon
warming.
report the double C-H activation of one set of geminal
C-H bonds in 1,3-butadiene by a diiridium complex
under ambient conditions, in which prior activation of
the complex by ligand loss is not required.
Our previous studies have shown that the reaction
of [Ir2(CH3)(CO)2(dppm)2][CF3SO3] (1; dppm ) Ph2-
PCH2PPh2) with a variety of substrates resulted in C-H
bond cleavage of the methyl ligand by the adjacent
metal, yielding the methylene-bridged hydride products
[Ir2H(L)(CO)2(µ-CH2)(dppm)2][CF3SO3] (L ) CO, SO2,
PR3, CNR).8 In addition, reaction with unsaturated
organic substrates yielded vinyl carbenes, and these
have also been proposed to proceed via methylene-
bridged intermediates.9 The present study was initiated
as part of a continuation of the reactivity studies of 1,
in which olefins were investigated as the substrates.
The activation of vinylic carbon-hydrogen bonds by
transition-metal complexes has attracted considerable
interest.1,2 In the majority of cases the activating
complex is a highly reactive, coordinatively unsaturated
species that has been generated by ligand loss through
photolysis1,2d,3 or thermolysis,2c,d,3b,4 or by prior substrate
hydrogenation by a polyhydride precursor complex.5 In
most cases the activation of only a single olefinic C-H
bond occurs. However, there have been a few reports of
double C-H activation of an olefin to give either
dimetallaolefin or vinylidene products.6 Double C-H
activation of ethers to give Fischer carbenes7 and the
activation of a single C-H bond in two separate olefin
molecules5b,d have also been reported. In this paper we
Although compound 1 reacts with the monoolefins
ethylene and tetrafluoroethylene to give simple olefin-
adducts in which the olefin was either bound to one
metal or bridging the two,10 it reacts very differently
with 1,3-butadiene. At ambient temperature, over a 48
h period compound 1 reacts with a butadiene-saturated
CH2Cl2 solution to give the dihydrido vinylvinylidene-
bridged product [Ir2(CH3)(H)(CO)2(µ-H)(µ-CC(H)C(H)-
CH2)(dppm)2][CF3SO3] (2),11 as shown in Scheme 1. In
the 13C{1H} NMR spectrum three resonances are ob-
served for the vinylvinylidene fragment; the two at δ
145.8 and 112.0 are typical of olefinic moieties,12 and
the resonance at δ 196.0 shows the usual downfield shift
characteristic of the R-carbon of a bridging vinylidene
group.13 The signal for the â-carbon is not observed and
* To whom correspondence should be directed.
† Present address: Grant McEwan College, Edmonton, AB.
‡ X-Ray Crystallography Laboratory.
§ McCalla Research Professor 1999-2000.
(1) (a) Stoutland, P. O.; Bergman, R. G. J . Am. Chem. Soc. 1988,
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121, 3666.
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1998, 120, 4047. (b) Torkelson, J . R.; McDonald, R.; Cowie, M.
Organometallics 1999, 18, 4134. (c) Ristic-Petrovic, D.; Torkelson, J .
R.; McDonald, R.; Cowie, M. Unpublished results.
(5) (a) Fryzuk, M. D.; J ones, T.; Einstein, F. W. B. Organometallics
1984, 3, 185. (b) Suzuki, H.; Omori, H.; Moro-Oka, Y. Organometallics
1988, 7, 2579. (c) J ia, G.; Meek, D. W.; Gallucci, J . C. Organometallics
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Chem. Soc., Chem. Commun. 1973, 277. (d) Deeming, A. J .; Hasso, S.;
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J .; Matheson, T. W. J . Chem. Soc., Chem. Commun. 1974, 807. (e)
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(10) Torkelson, J . R. Ph.D. Thesis, University of Alberta, 1998,
Chapter 4.
(11) Spectroscopic data for 2: 1H NMR (400 MHz, CD2Cl2) δ 6.79
3
3
(d, J HH ) 10.0 Hz, 1H), 5.28 (ddd, J HH ) 16.5, 10.0, 10.0 Hz, 1H),
3
3
4.91 (d, J HH ) 16.5 Hz, 1H), 4.38 (d, J HH ) 10.0 Hz, 1H), 3.75 (m,
2H), 2.63 (m, 2H), -0.51 (t, 3H), -12.64 (t, 2J PH ) 17 Hz, 1H), -13.90
2
(q, J PH ) 6 Hz, 1H); 31P{1H} NMR δ -4.9 (m), 13.1 (m); 13C{1H}
(natural abundance) δ 196.0 (q, 2J PC ) 9.5 Hz, µ-CdCHCHCH2), 173.6
2
2
(t, J PC ) 9.8 Hz, CO), 173.2 (t, J PC ) 9.2 Hz, CO), 145.8 (s, µ-Cd
CHCHdCH2), 112.0 (s, µ-CdCHCHdCH2), -9.9 (b, CH3); IR 2014, 2003
cm-1
.
(12) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-
copy; VCH Publishers: New York and Weinheim, Germany, 1991.
10.1021/om0006054 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 09/30/2000

Communications
Organometallics, Vol. 19, No. 22, 2000 4433
Sch em e 1
F igu r e 1. Perspective view of the cation of 2 in which only
the ipso carbons of the phenyl groups are shown. Important
bond lengths (Å) and angles (deg): Ir(1)-Ir(2) ) 2.9339-
(4); Ir(1)-C(4) ) 2.057(7); Ir(2)-C(4) ) 2.070(7); Ir(2)-C(3)
) 2.261(9); C(4)-C(5) ) 1.34(1); C(5)-C(6) ) 1.57(1); C(6)-
C(7) ) 1.33(2); Ir(1)-C(4)-Ir(2) ) 90.6(3); C(4)-C(5)-C(6)
) 123.2(8); C(5)-C(6)-C(7) ) 122.9(11). Hydride positions
are idealized.
is presumably obscured by the phenyl resonances.
Similarly, the hydrogen resonances for the vinylvi-
nylidene group are as expected. The hydrido ligands
appear in their typical high-field positions, and their
couplings to the phosphines indicate that one is termi-
nally bound while the other is bridging. Confirmation
of this structural proposal comes from the X-ray deter-
mination,14 the results of which are shown in Figure 1.
The vinylvinylidene group bridges the metals sym-
metrically, and all parameters within this group are
consistent with the 1,3-diolefin formulation. The Ir-Ir
bond length (2.9339(4) Å) shows the lengthening ex-
pected when involved in a three-center, Ir(µ-H)Ir inter-
action (compare: 2.7775(5) Å in the precursor 1).8
In an attempt to discover how this unusual double
oxidative addition occurred and to determine the effects
of the adjacent metals in the C-H activation steps, the
reaction was carried out at low temperature, reacting
about 30% excess of the diene with 1, in an effort to
observe intermediates in the transformation. At tem-
peratures between -50 and -95 °C the diolefin adduct
[Ir2(CH3)(CO)2(µ-η2:η2-H2CdCH-CHdCH2)(dppm)2][CF3-
SO3] (3) was observed15 in equilibrium with compound
1 and free butadiene. At -55 °C the equilibrium
constant for formation of the butadiene adduct (3) is
approximately 105 M-1, as determined by NMR. At
ambient temperature the equilibrium favors the starting
materials and 3 is not observed. The 31P{1H} NMR
spectrum of 3 indicates that all phosphorus atoms are
chemically inequivalent. Although the appropriate num-
Ch a r t 1
ber of resonances for the coordinated butadiene molecule
1
are seen in the H NMR spectrum, their unambiguous
identification, together with the identification of the
butadiene carbon resonances and the orientation of this
ligand, required a series of 2D NMR experiments
(COSY, TOCSY, ROESY, HMQC). From these experi-
ments the structure shown in Scheme 1 is proposed, in
which the butadiene molecule sits atop one face of the
Ir2P4 unit with each olefinic group bound to a different
metal. A number of butadiene-bridged complexes have
been reported,17,18 including examples of s-trans-buta-
diene coordination18 similar to that proposed for 3. The
identification of the CAHaHb moiety (see Chart 1) as
being adjacent to the methyl ligand is seen from the
NOE observed between Hb and the methyl protons
(13) (a) Wang, L.-S.; Cowie, M. Organometallics 1995, 14, 3040. (b)
Torkelson, J . R.; McDonald, R.; Cowie, M. Organometallics 1999, 18,
4134.
(14) Crystal data for 2: monoclinic, P21/c; a ) 10.0835(3) Å, b )
20.6894(4) Å, c ) 27.3273(5) Å, â ) 100.636(2)°; V ) 5603.1(2) Å3; Z )
4; Dcalc ) 1.692 g cm-3; µ(Mo KR) ) 49.54 cm-1; T ) 26 °C; λ(Mo KR)
) 0.71073 Å; 12 777 independent reflections measured, 9213 observed
(I > 2σ(I)); 698 variables; R1(F) ) 0.0520 (observed data), wR2(F2) )
0.0926 (all data). Data were collected on a Bruker P4/RA/SMART 1K
CCD system using φ rotations and ω scans (0.3° frames, 40 s exposures)
to 2θ ) 55.0°. Hydrogen atom positions were idealized. Atoms H(1),
C(3), C(6), and C(7) are disordered about the pseudo 2-fold axis along
C(4)-C(5) in a 2/3:1/3 ratio. The molecule having the major occupancy
is shown in Figure 1.
(16) Mann, B. E. Adv. Organomet. Chem. 1974, 12, 135.
(17) (a) King, J . A.; Vollhardt, K. P. C. Organometallics 1983, 2,
684. (b) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J . J . Am. Chem. Soc.
1985, 107, 8259. (c) Kaneko, Y.; Suzuki, T.; Isobe, K. Organometallics
1998, 17, 996.
(18) (a) Adam, V. C.; J arvis, J . A. J .; Kilbourn, B. T.; Owston, P. G.
Chem. Commun. 1971, 467. (b) Sasse, Von H. E.; Ziegler, M. L. Z.
Anorg. Allg. Chem. 1972, 392, 167. (c) Tachikawa, M.; Shapley, J . R.;
Haltiwanger, R. C.; Pierpont, C. G. J . Am. Chem. Soc. 1976, 98, 4651.
(d) Pierpont, C. G. Inorg. Chem. 1978, 17, 1976. (e) Franzreb, K.-H.;
Kreiter, C. G. Z. Naturforsch. 1982, 37B, 1058.
(15) Spectroscopic data for 3: 1H NMR δ 1.28 (m, Ha), 1.62 (dm,
3
3
3J HH ) 9 Hz, Hb), 2.40 (dm, J HH ) 9 Hz, Hc), 2.54 (ddd, J HH ) 9, 8,
6 Hz, Hd), 2.67 (d, 3J HH ) 6 Hz, He), 1.24 (d, 3J HH ) 8 Hz, Hf), 4.89 (m,
1H, PCH2P), 4.51 (m, 1H, PCH2P), 3.59 (m, 1H, PCH2P), 3.27 (m, 1H,
PCH2P); 13C{1H} NMR δ -3.50 (CA), 64.51 (CB), 72.14 (CC), 47.88 (CD),
-20.25 (CH3), 186.9 (CO), 184.7 (CO); 31P{1H} NMR δ 3.22(m), 9.42-
(m), 22.66(m), 25.64(m).

4434 Organometallics, Vol. 19, No. 22, 2000
Communications
Sch em e 2
fragment, as observed in the structure of the final
product 2. This “hydride inversion” brings the vinyl
group into proximity with the adjacent metal, leading
to activation of the second C-H bond to give 2.
The possibility of initial C-H activation of the methyl
group, as previously observed or proposed in reactions
of 1 with various substrates,8,9 has been tentatively
ruled out by experiments involving CD3-labeled 1, in
which no deuterium incorporation into the butadiene
ligand of 3 or into the hydride or vinylvinylidene ligands
of 2 was observed.
Support for our proposal, that precoordination of
butadiene at both metals precedes the double C-H
activation, comes from attempted reactions of 1 with
monosubstituted butadienes. Therefore, compound 1 in
the presence of 2-methyl-1,3-butadiene or either cis- or
trans-1,3-pentadiene results in no reaction after several
days at ambient temperature or after 24 h in refluxing
CH2Cl2 or THF. The failure of the pentadiene com-
pounds to give a C-H activation product strongly
suggests that coordination of both olefin functionalities
is required; otherwise substitution at one end of the
butadiene unit would not be expected to inhibit C-H
activation at the other end. Although 2-methyl-1,3-
butadiene does not form an adduct at temperatures
down to -95 °C, both pentadiene isomers form weak
adducts at this temperature. Unfortunately the ex-
change occurring at this temperature has not allowed
us to carry out the necessary 2D NMR studies in order
to determine the orientations of the pentadiene mol-
ecules; these studies are continuing. Additional support
that prior coordination of both π bonds of butadiene
facilitates the C-H activation observed comes from the
reaction with ethylene, which does not give C-H
activation products, yielding only a π complex.10
The activation of both geminal C-H bonds of buta-
diene is unusual enough; however, activation by a
binuclear complex without prior activation of the com-
plex by ligand loss is quite remarkable. Currently we
are carrying out DFT calculations on the butadiene-
bridged adduct (3) in attempts to obtain additional
insights into the proposed transformation of this species
into the doubly C-H activated product (2) and are
investigating related systems in an effort to obtain
additional support for this proposed transformation.
(protons exhibiting NOE are indicated by the double-
headed arrows). NOE was also observed between each
of Hc and Hd and one hydrogen of each of the dppm
methylene groups, consistent with the structure shown.
The 13C chemical shifts for CB, CC, and CD (between δ
47.88 and 72.14) and the C-H coupling constants
involving the attached hydrogens (between 147 and 161
Hz) appear typical for a coordinated olefin;16 however,
the 13C chemical shift for CA (δ -3.50) is at unusually
high field and together with the associated C-H cou-
pling constants (1J CH ) 139 Hz) suggests a substantial
rehybridization of this carbon toward sp3.
Although the direct transformation of compound 3 to
2 cannot be observed, since 3 is only detectable at low
temperatures, at which C-H activation of the butadiene
ligand does not occur, we propose that 3 is an interme-
diate in the formation of 2. We suggest that C-H
activation occurs stepwise, with the first activation
occurring at the iridium having the methyl group
attached. This metal should be more electron rich and
more susceptible to oxidative addition owing to the
electron-donor properties of the methyl ligand; the
adjacent metal should be less prone to oxidative addition
owing to its positive charge. These suggestions are
consistent with the proposed rehybridization noted
earlier for CA, indicating greater back-donation to this
end of the diolefin. Although structure 3 has a carbonyl
on each metal, C-H activation might be accompanied
by carbonyl transfer from one metal to the other,
generating the necessary unsaturation. Oxidative ad-
dition of the first C-H bond would yield a vinyl hydride
as shown in structure A of Scheme 2 (dppm ligands,
perpendicular to the plane of the drawing, are not
shown). Activation of a single terminal vinylic C-H
bond of 1,3-butadiene by a binuclear species to give a
bridging butadienyl moiety has previously been ob-
served.19 Inversion of the “Ir2(µ-H)” core, as dia-
grammed, is common20 and would yield a species such
as B in which the hydride ligand is now on the opposite
face of the Ir2P4 plane from the resulting hydrocarbyl
Ack n ow led gm en t. The authors thank the Natural
Sciences and Engineering Research Council of Canada
(NSERC) and the University of Alberta for financial
support, the University of Alberta for a McCalla Profes-
sorship (to M.C.), and Dr. J ames F. Britten (McMaster
University) for X-ray data collection on compound 2. Dr.
T. T. Nakashima is acknowledged for assistance with
the 2D NMR studies.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
information (PDF) is available free of charge via the Internet
at http://pubs.acs.org.
(19) Fryzuk, M. D.; J ones, T.; Einstein, F. W. B. J . Chem. Soc.,
Chem. Commun. 1984, 1556.
(20) Antonelli, D. M.; Cowie, M. Inorg. Chem. 1990, 17, 2553, and
references therein.
OM0006054