Fluoro-Ligand Promotion of C-H Activation

Article abstract of DOI:10.1021/om9605488

Ethylene dehydrogenates Ir(H)₂FL₂ (L = J^t-Bu₂Ph) to give the metalated species 1 and 2. These metalated species catalyze the exchange of deuterium from C₆D₆ to ethylene, and to ^tBu and orthophenyl sites of coordinated L. This exchange is proposed to benefit from π-donation from F to Ir.

Full text of DOI:10.1021/om9605488

Organometallics 1997, 16, 505-507
505
F lu or o-Liga n d P r om otion of C-H Activa tion
Alan C. Cooper, Kirsten Folting, J ohn C. Huffman, and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center, Indiana University,
Bloomington, Indiana 47405-4001
Received J uly 8, 1996X
Summary: Ethylene dehydrogenates Ir(H)2FL2 (L ) Pt-
Bu2Ph) to give the metalated species 1 and 2. These
metalated species catalyze the exchange of deuterium
from C6D6 to ethylene, and to tBu and ortho phenyl sites
of coordinated L. This exchange is proposed to benefit
from π-donation from F to Ir.
(time of mixing) reaction with H2 (1 atm) in benzene at
25 °C to give complete conversion to Ir(H)5L2. The
occurrence of this reaction is influenced by the strong
bond in the coproduct, HF. No Ir-X bond cleavage
occurs when Ir(H)2XL2 (X ) Cl, Br, I, O3SCF3) is treated
with H2; only weak adduct formation is observed:
Ir(H)2X(H2)L2.6
If Ir(H)2FL2 is held at 50 °C for 12 h in toluene under
an excess of ethylene, it is dehydrogenated to produce
ethane and the following three products (2 is found as
two diastereomers, due to the chirality at both Ir and
P):7
Fluoride is a ligand which has been only modestly
explored in organometallic and catalytic chemistry.1 In
particular, in addition to its σ-withdrawing (electrone-
gative) effect, we have presented evidence2 that the
π-donating effect of fluoride is also maximal among the
halide group (as it is among BX3 analogs). This means
that fluoride might be very influential in “operational
unsaturation”,3 and in accelerating ligand dissociation
steps (eq 1) and giving higher steady-state concentra-
tions of “unsaturated” intermediates, all by FfM π-do-
nation. This greater π-donation by the most electro-
negative halide is not paradoxical but depends on the
short M-F distance leading to optimal π-overlap. We
report here some observations which extend our under-
standing of the influence of fluoride in the context of
molecular reaction chemistry, where the M-F bond is
retained throughout.
Halide metathesis with Ir(H)2ClL2 (L ) PtBu2Ph)
proceeds rapidly (10 min) at 25 °C in THF in the
presence of excess anhydrous [NMe4]F to give complete
conversion to Ir(H)2FL2.4 The presence of one fluoride
in the product is indicated by doublet structure in the
hydride (J F-H ) 42 Hz) and 31P{1H} (J P-F ) 6 Hz) NMR
signals, as well as by a 19F NMR triplet. An X-ray
structure determination of the non-hydrogen atoms of
Ir(H)2FL2 has shown a T-shaped IrFP2 unit.5
Compounds 1 and 2 show AX 31P{1H} NMR patterns
due to the large displacement of phosphorus chemical
shift resulting from incorporation into a four-membered
ring.8 All show large 2J (P-P) values (340-350 Hz),
indicative of transoid location of the two phosphorus
nuclei. These products are all the result of irreversible
ethylene “scavenging” of hydride ligands from Ir and
rapid trapping of highly unsaturated intermediates by
intramolecular C-H oxidative addition. Ethylene and
t
also BuCHCH2 are particularly effective olefins for
dehydrogenation of Ir(H)2FL2, apparently because they
cannot be competitively isomerized to a (less reactive)
internal (i.e., disubstituted) olefin. Thus, reaction of
Ir(H)2FL2 with an excess of a substituted terminal olefin
(5) Crystallographic data (-171 °C): a ) 15.277(2) Å, b ) 16.424-
(2) Å, c ) 11.357(1) Å, Z ) 4, space group P212121; R(F) ) 0.0361. There
is a 70:30 disorder of two Ir and two F sites, and the hydrides were
therefore not detected. Ir-F ) 2.045(9) Å, and Ir-P ) 2.325(3) and
2.323(3) Å; P-Ir-P ) 176.6(3)°; and P-Ir-F ) 93.3(3) and 89.2(3)°.
(6) Hauger, B. E.; Gusev, D.; Caulton, K. G. J . Am. Chem. Soc. 1994,
116, 208.
The Brønsted basicity of coordinated fluoride, as well
as the unsaturation of Ir(H)2FL2, contributes to its rapid
X Abstract published in Advance ACS Abstracts, February 1, 1997.
(1) Doherty, N. M.; Hoffman, N. W. Chem. Rev. 1991, 91, 553.
(2) Poulton, J . T.; Sigalas, M. P.; Folting, K.; Streib, W. E.;
Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1994, 33, 1476.
(3) J ohnson, T. J .; Huffman, J . C.; Caulton, K. G. J . Am. Chem. Soc.
1992, 114, 2725.
(7) Gen er a tion of Ir H(η2-C6H4P tBu 2)(F )(P tBu 2P h ) a n d Ir H(η2-
CH2CMe2P t Bu P h )F (P t Bu 2P h ) Isom er s. In a 100-mL glass flask
with a Teflon valve was placed a solution of Ir(H)2(F)(PtBu2Ph)2 (100
mg, 0.15 mmol) in 5 mL of toluene. This solution was degassed three
times (freeze-pump-thaw) and 1 atm of ethylene introduced. The
valve was closed tightly and the flask heated to 50 °C for 12 h with
continuous stirring of the solution. During this time, the color of the
solution changed from light orange to dark orange. the toluene was
removed in vacuo and the resulting red oil extracted with pentane (3
× 5 mL). The pentane solution was evaporated to dryness in vacuo to
(4) Ir (H)2F (P tBu 2P h )2. To a flask containing anhydrous tetram-
ethylammonium fluoride (1.2 g, 12.9 mmol) was added a solution of
Ir(H)2Cl(PtBu2Ph)2 (1.0 g, 1.5 mmol) in 35 mL of THF. This hetero-
geneous yellow solution was stirred for 20 min at room temperature
and filtered. The insoluble ammonium salts were washed with THF
(2 × 3 mL) and the THF solutions combined. Removal of the THF in
vacuo yielded a yellow-orange precipitate. This solid was washed with
pentane (3 × 5 mL) to remove dark orange impurities. Recrystalli-
zation by layering of a saturated THF solution with pentane gave very
yield a dark red solid. 31P{1H} NMR (C6D6, 25 °C): δ 53.3 [d, J PP
)
340 Hz, IrH(η2-C6H4PtBu2)(F)(PtBu2Ph) isomer], 52.6 [d, J PP ) 346 Hz,
major diastereomer of IrH(η2-CH2CMe2PtBuPh)F(PtBu2Ph)], 45.7 [d,
J PP ) 348 Hz, minor diastereomer of IrH(η2-CH2CMe2PtBuPh)F(Pt-
Bu2Ph)], 4.4 (d, J PP ) 347 Hz), -11.4 (d, J PP ) 340 Hz), -38.4 (d, J PP′
) 348 Hz). 1H NMR (C6D6, 25 °C, hydride resonances only): δ -34.0
[br s, IrH(η2-CH2CMe2PtBuPh)F(PtBu2Ph) diastereomers], -41.4 [ap-
parent t, J PH ) 11.4 Hz, IrH(η2-C6H4PtBu2)(F)(P′ButPh) isomer]. 19F
NMR (C6D6, 25 °C): δ -242.2 [m, IrH(η2-C6H4PtBu2)(F)(PtBu2Ph)
isomer], -243.6 [m, minor diastereomer of IrH(η2-CH2CMe2PtBuPh)F-
(PtBu2Ph)], -243.9 [m, major diastereomer of IrH(η2-CH2CMe2-
PtBuPh)F(PtBu2Ph)].
large dark yellow crystals (695 mg, 71%). Anal. Calcd for C28H48
-
IrFP2: C, 51.12; H, 7.35. Found: C, 51.02; H, 7.5. 1H NMR (C6D6, 25
°C): δ 8.35 (m), 7.2-7.06 (overlapping m), 1.50 (vt, J PH ) 6.6 Hz), -32.0
(dt, J HF ) 42 Hz, J PH ) 12 Hz). 31P{1H} NMR (C6D6, 25 °C): δ 62.0
(d, J PF ) 6.0 Hz). 19F NMR (C6D6, 25 °C): δ -205 (tt, J HF ) 42 Hz,
J PF ) 6 Hz). 13C{1H} NMR (C6D6, 25 °C): δ 137.2 (s), 134.0 (t, J PC
)
18.9 Hz), 129.7 (s), 127.3 (t, J PC ) 4.4 Hz), 36.4 (t, J PC ) 11.1 Hz), 30.8
(s).
(8) Garrou, P. E. Chem. Rev. 1981, 81, 229.
S0276-7333(96)00548-1 CCC: $14.00 © 1997 American Chemical Society

506 Organometallics, Vol. 16, No. 4, 1997
Communications
(1-hexene or 3-phenylpropene) at 75 °C causes complete
isomerization to more highly substituted olefins (2- and
3-hexene and â-methylstyrene, respectively) within 4 h,
with only very little (<5%) dehydrogenation of the Ir
complex. Extended heating increases the yield of meta-
lated complexes, but the dehydrogenation proceeds at
a very slow rate (e.g., 35 equiv of 1-hexene causes only
50% conversion of Ir(H)2FL2 to 1 and 2 after 48 h at 75
°C). It is also significant that the distribution of
products in the dehydrogenation of Ir(H)2FL2 is depend-
ent on the olefin employed. This suggests kinetic
control, and under conditions which do not subsequently
reach equilibrium among the species 1 and 2.
This dehydrogenation of Ir(H)2XL2 by olefins occurs
only when X ) F. Exposure of the chloride and iodide
analogs to ethylene or 1-hexene under equivalent or
more rigorous conditions (in triglyme up to 200 °C in a
sealed tube) causes no dehydrogenation of the metal
complex.
Moreover, in comparison to the fluoro analog under
comparable conditions, this product is much less deu-
terated (growth of C6D5H is observed), indicating a
slower rate for H/D exchange with 4 and also establish-
ing that OTf and F must be present in the catalytically
active species to influence the rate. Since triflate and
fluoride are at opposite extremes in (σ + π) donor ability,
as judged by ν(CO) values of LnM(CO)X (X ) F, OTf)
species,2,11 we suggest that the rate differences observed
here are influenced by the better donor ability of
fluoride, perhaps in stabilizing a phosphine-loss species
of a metalated phosphine, IrHX(η2-C∼P), which then
reacts with C6D6 (or with olefin).
It was reported recently12 that Cp*Ir(PMe3)(aryl)F
shows “surprisingly easy Ir-F ionization”. The four-
electron destabilization which results from repulsion
between fluorine lone pairs and filled dπ orbitals in this
or any saturated metal complex promotes such ionic loss
of F-.13 The molecules we report here, because they are
unsaturated, lack this propensity, and thus fluoride
actively participates both before and after the adduct
is formed with substrate. These thus represent two
conceptually distinct ways to manipulate reactivity by
incorporation of the lightest halide. Finally, it might
have been generally thought that ortho metalation of a
pendant phenyl ring is a nuisance side reaction. This
is not the case, at least in IrHF(η2-C6H4PtBu2)(PtBu2-
Ph), since the strain in the small ring represents latent
reactivity that can be triggered by an attacking arene
C-H bond.14 Strain provides a facile route to unsatur-
ated Ir(I), via the reductive elimination of the metalated
C-H bond, which is absent in the parent Ir(H)2F-
(PtBu2Ph)2 species.
If the reaction of Ir(H)2FL2 with excess ethylene is
1
monitored in C6D6 by H NMR, one observes, only and
cleanly, the equilibration of deuterium from benzene
t
into the Ir-H, the Bu, the ortho phenyl hydrogens of 1
and 2, and the ethylene sites; the proton signal of
benzene grows proportionately.9 This same H/D ex-
change occurs if a preformed mixture of protio isomers
of 1 and 2 is heated in C6D6 under C2H4, indicating that
the exchange is not simply a property of Ir(H)2FL2 itself
under ethylene (i.e., a more hydrogen-rich system than
1 and 2).
We have established independently that mixtures of
the all-protio metalated species 1 and 2 effect H/D
exchange when heated in C6D6 in the absence of
ethylene, so that ethylene only functions to form the
reactive species but plays no essential role in the attack
on benzene. Particularly because 1 and 2 are Ir(III)
compounds, but are unsaturated, these H/D exchange
reactions could occur by benzene (a) oxidative addition
(implicating Ir(V)), (b) σ-bond metathesis on Ir(III), or
(c) oxidative addition to IrIFL2 (see below). In contrast,
thermolysis of IrH(η2-P(C6H4)tBu2)(X)(PtBu2Ph) (X ) Cl,
Br, I) in C6D6 fails to show any H/D exchange under
comparable conditions.
The triflate (CF3SO3-) analog of Ir(H)2FL2 behaves
very differently.10 If Ir(H)2(OTf)L2 is treated with
ethylene (1 atm, 25 °C) in C6D6, there is immediate
formation of [Ir(H)2(C2H4)2L2]OTf (3), which quickly
precipitates from this solution. While triflate thus
behaves as a good leaving group, heating a slurry of this
salt in C6D6 under 1 atm of ethylene for 2 h at 80 °C
gives conversion to ethane and only one diastereomer
of the molecular (i.e., OTf is coordinated) product 4,
where a tBu C-H bond has oxidatively added to Ir.
Whatever the detailed formula of the transient(s)
involved here, its high reactivity is implicated by its lack
of selectivity: it reacts with aliphatic, olefinic, and aryl
C-H bonds. We have independent experimental ob-
servations for equilibria 2 and 3.15,16 Moreover, phos-
phine loss occurs at a much higher rate (>103) for the
metalated species than for Ir(H)2XL2, thus implicating
eq 3 in the IrH/C6D6 exchange observed here. Reductive
elimination of Ir-C and Ir-D bonds in equilibrium 217
provides a mechanism for deuteration of tBu and ortho
(9) H/D Exch a n ge in C6D6 Solu tion s of Ir (H)2F (P tBu 2P h )2 a n d
C2H4. A solution of Ir(H)2F(PtBu2Ph)2 (15 mg, 0.02 mmol) in 0.6 mL
of C6D6 was placed in an NMR tube. This solution was degassed three
times (freeze-pump-thaw) and 700 Torr of ethylene introduced before
flame-sealing of the tube. The solution was mixed thoroughly for 1 h
at room temperature before 1H assay. The 1H NMR spectrum shows
very little (ca. 2%) conversion to metalated species after this time. The
sample was then heated to 75 °C for 6 h, during which time the color
changed from yellow to light brown. 31P{1H} NMR (C6D6, 25 °C): δ
52.6 [d, J PP ) 347 Hz, major diastereomer of IrH(η2-CH2CMe2Pt-
BuPh)F(PtBu2Ph)], 45.7 [d, J PP′ ) 348 Hz, minor diastereomer of
IrH(η2-CH2CMe2PtBuPh)(PtBu2Ph)], 4.4 (d, J PP′ ) 347 Hz), -38.4 (d,
J PP′ ) 348 Hz). 1H NMR (C6D6, 25 °C): δ 7.15 (s, C6D5H), 0.80 (s,
(11) Vaska, L.; Peone, J . J . Chem. Soc. D 1971, 418. Merrifield, J .
A.; Fernandez, J . M.; Buhro, W. E.; Gladysz, J . A. Inorg. Chem. 1984,
23, 4022.
(12) Veltheer, J . E.; Burger, P.; Bergman, R. G. J . Am. Chem. Soc.
1995, 117, 12478.
(13) Caulton, K. G. New J . Chem. 1994, 18, 25.
(14) Belli, J .; J ensen, C. M. Organometallics 1996, 15, 1532.
(15) Cooper, A. C.; Caulton, K. G. Inorg. Chim. Acta 1996, 251, 41.
(16) Cooper, A. C.; Huffman, J . C.; Caulton, K. G. Organometallics,
submitted for publication.
t
C2H6); Ir-H, Bu, and ortho Ph peaks were absent.
(10) Cooper, A. C.; Huffman, J . C.; Caulton, K. G. Manuscript in
preparation.
(17) Demonstrated for X ) Cl15 and for X ) CCPh.16

Communications
Organometallics, Vol. 16, No. 4, 1997 507
Ack n ow led gm en t. This work was supported by the
National Science Foundation and by a loan of chemicals
from J ohnson Matthey/Aesar.
Su p p or tin g In for m a tion Ava ila ble: Text giving details
of the syntheses and characterization data for the compounds
in this paper and details of the X-ray crystal structure
determination of Ir(H)2F(PtBu2Ph)2 (3 pages). Ordering in-
formation is given on any current masthead page.
phenyl sites. Work is currently underway to further
specify the mechanism of these several C-H activation
processes and the special role played by “hard” ligands
such as fluoride and triflate.
OM9605488