A case of C-H activation (ortho metalation) which is reversible at 25 °C

Article abstract of DOI:10.1021/om9607107

The reaction of LiC₂Ph with Ir(H)₂ClL₂ (L = P^tBu₂Ph) gives Ir(H)₂(C₂Ph)L₂, which readily loses H₂ to form the ortho-metalated species IrH(η²-C₆H₄P^tBu ₂)(C₂Ph)L. This molecule is unique in showing (sp²)C/H reductive elimination/oxidative addition, which is thermally reversible at 25 °C. Line shape analysis of the ³¹P{¹H} NMR spectra yield ΔH^? = 12.3 (±0.4) kcal mol^-1 and ΔS^? = -2.0 (±1.1) cal deg^-1 mol^-1 for this process. This implicates the 14-electron species Ir(C₂Ph)L₂ as highly reactive, yet more easily accessible, than IrClL₂ (which ortho metallates essentially irreversibly). The strained four-membered ring in IrH(η²-C₆H₄P^tBu ₂)(C₂Ph)L reacts readily with PhC₂H to give IrH(C₂Ph)₂L₂, another unsaturated molecule essentially devoid of π-stabilization. The structure of this molecule, a square pyramid with apical hydride (proton NMR chemical shift -44 ppm), minimizes the Lewis acidity of such a species. Crystal data (-181 °C): a = 9.616(1) A?, b = 23.032(2) A?, c = 8.886(1) A?, α = 95.18(1)°, β = 99.03(1), γ = 98.16(1), with Z = 2 in space group P1.

Full text of DOI:10.1021/om9607107

1974
Organometallics 1997, 16, 1974-1978
A Ca se of C-H Activa tion (Or th o Meta la tion ) Wh ich Is
Rever sible a t 25 °C
Alan C. Cooper, J ohn C. Huffman, and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center, Indiana University,
Bloomington, Indiana 47405-4001
Received August 19, 1996X
The reaction of LiC2Ph with Ir(H)2ClL2 (L ) PtBu2Ph) gives Ir(H)2(C2Ph)L2, which readily
loses H2 to form the ortho-metalated species IrH(η2-C6H4PtBu2)(C2Ph)L. This molecule is
unique in showing (sp2)C/H reductive elimination/oxidative addition, which is thermally
reversible at 25 °C. Line shape analysis of the 31P{1H} NMR spectra yield ∆Hq ) 12.3 ((0.4)
kcal mol-1 and ∆Sq ) -2.0 ((1.1) cal deg-1 mol-1 for this process. This implicates the 14-
electron species Ir(C2Ph)L2 as highly reactive, yet more easily accessible, than IrClL2 (which
ortho metallates essentially irreversibly). The strained four-membered ring in IrH(η2-C6H4P-
tBu2)(C2Ph)L reacts readily with PhC2H to give IrH(C2Ph)2L2, another unsaturated molecule
essentially devoid of π-stabilization. The structure of this molecule, a square pyramid with
apical hydride (proton NMR chemical shift -44 ppm), minimizes the Lewis acidity of such
a species. Crystal data (-181 °C): a ) 9.616(1) Å, b ) 23.032(2) Å, c ) 8.886(1) Å, R )
95.18(1)°, â ) 99.03(1), γ ) 98.16(1), with Z ) 2 in space group P1h.
Toluene was dried and deoxygenated over sodium benzophe-
none and distilled under argon. 1-Hexene and decane were
distilled under argon before storage over activated 4 Å mo-
lecular sieves. Deuterated solvents (C6D6 and C7D8) were dried
over sodium metal and vacuum distilled before use in the
glovebox. 1H NMR (referenced to residual solvent impurity)
spectra were collected on a Varian XL-300 spectrometer. 31P
NMR (referenced to external 85% H3PO4) spectra were col-
lected on Nicolet NT-360 and Varian VXR-400 spectrometers
operating at 146 and 162 MHz, respectively. Deuterium NMR
In tr od u ction
Ortho metalation of the aryl ring of a phosphine
attached to a metal (eq 1) is a widely occurring reaction.1
It was one of the earliest examples of metal activation
of (i.e., attack on) a C-H bond. Generally, this reaction
was run on
a Varian Unity 500 spectrometer. Lithium
phenylacetylide was prepared by the reaction of freshly
distilled phenylacetylene with nBuLi (-78 °C, pentane). Ir-
(H)2Cl(PtBu2Ph)2 has been prepared previously.3 The presence
of PtBu2Ph makes Ir(H)2(CCPh)(PtBu2Ph)2 extremely soluble
and unwilling to crystallize, even after column chromatogra-
phy on neutral alumina or silica. The elemental analysis of
IrH(η2-C6H4PtBu2)(CCPh)(PtBu2Ph), because it differs by only
2 H in ∼800 amu, is not as diagnostic of purity as is NMR
spectroscopy. Elemental analysis was therefore not accom-
plished.
Ir (H)2(CCP h )(P tBu 2P h )2. To a flask containing lithium
phenylacetylide (300 mg, 2.8 mmol) was added a solution of
Ir(H)2Cl(PtBu2Ph)2 (300 mg, 0.44 mmol) in 30 mL of toluene
with stirring. This solution was stirred until the color had
changed from orange to pale yellow (ca. 2 h). The solution
was filtered through a 25-micron Teflon filter tip (Centaur
Chemical Company) attached to a cannula, and the toluene
was removed in vacuo to yield a sticky yellow solid (yield:
79%). 1H NMR (C6D6, 25 °C): δ 8.68 (m), 7.25-6.91 (overlap-
ping m), 1.64 (vt, J PH ) 6.3 Hz), -14.0 (t, J PH ) 18 Hz). 13C-
{1H} NMR (C6D6, 25 °C, only sp carbons of acetylide ligand):
δ 139.4 (t, J PC ) 14 Hz), 112.1 (s). 31P{1H} NMR (C6D6, 25
°C): 49.6 (s).
lies to one side: ∆G° is either strongly positive or
negative (depending upon the LnM moiety). Thus, even
the unsaturated molecule RuHCl(PPh3)3 shows no de-
tectable equilibrium population of the metalated alter-
native, although deuterium in RuDCl[P(C6H5)3]3 is
readily exchanged into the phenyl rings by a mechanism
involving an ortho-metalated species.2 Such isotope
exchange is one way to implicate the existence of a
reversible equilibrium involving an otherwise unobserv-
able species.
In the work presented here, we report a situation
where the equilibrium lies toward the ortho-metalated
species, yet the equilibrium is detected by the presence
of two alternative phenyl rings, one on each phosphine.
Because of this, and since this demetalation (C-H
reductive elimination) occurs with an exceptionally low
activation energy, NMR spectroscopy becomes a method
for detecting its occurrence, as well as determination
of activation parameters for the process.
Exp er im en ta l Section
Ir H(η2-C6H4P tBu 2)(CCP h )(P tBu 2P h ). In 15 mL of 1-hex-
ene was dissolved Ir(H)2(CCPh)(PtBu2Ph)2 (150 mg, 0.20
mmol), with stirring. This solution was refluxed under argon
for 12 h, causing a gradual color change of the solution from
pale yellow to dark red. The solvent was removed in vacuo,
yielding a dark red solid (yield: 82%). 1H NMR (C6D6, 25 °C):
δ 7.57 (d), 7.28-6.86 (overlapping m), 1.46 (br apparent d).
Gen er a l P r oced u r es. All manipulations were carried out
using standard Schlenk and glovebox techniques under argon.
X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) Ryabov, A. D. Chem. Rev. 1990, 90, 403. Comprehensive Orga-
nometallic Chemistry II; Pergamon, New York, 1995; Vol. 8, p 205.
DeHand, J .; Pfeffer, M. Coord. Chem. Rev. 1974, 18, 327. Bruce, M. I.
Angew. Chem., Int. Ed. Engl. 1977, 16, 73.
(2) Parshall, G. W.; Knoth, W. H.; Schunn, R. A. J . Am. Chem. Soc.
1969, 91, 4990.
(3) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L. J . Chem.
Soc., Dalton Trans. 1976, 20, 2069.
S0276-7333(96)00710-8 CCC: $14.00 © 1997 American Chemical Society

C-H Activation (Ortho Metalation)
Organometallics, Vol. 16, No. 9, 1997 1975
F igu r e 2. ORTEP view of IrH(CCPh)2(PtBu2Ph)2, showing
selected atom numbering.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Ir H(CCP h )2(P tBu 2P h )2
F igu r e 1. Eyring plot for the reversible ortho metalation,
Ir(1)-P(18)
Ir(1)-P(33)
Ir(1)-C(2)
Ir(1)-C(10)
C(2)-C(3)
2.3524(12)
2.3603(12)
2.017(5)
2.027(5)
1.224(6)
C(3)-C(4)
1.437(6)
1.216(6)
1.443(6)
1.21(4)
from line shape analysis of the 31P NMR spectra.
C(10)-C(11)
C(11)-C(12)
Ir(1)-H(A)
Ta ble 1. Cr ysta llogr a p h ic Da ta for
Ir H(CCP h )2(P tBu 2P h )2
formula: C44H57P2Ir
a ) 9.616(1) Å
b ) 23.032(2) Å
c ) 8.886(1) Å
R ) 95.18(1)°
â ) 99.03(1)°
γ ) 98.16(1)°
V ) 1911.18 Å3
Z ) 2
fw ) 840.10
P(18)-Ir(1)-P(33) 177.24(4)
C(10)-Ir(1)-H(A)
100.2(18)
178.5(5)
space group P1h
T ) -181 °C
P(18)-Ir(1)-C(2)
P(18)-Ir(1)-C(10)
P(33)-Ir(1)-C(2)
P(33)-Ir(1)-C(10)
86.73(13) C(2)-C(3)-C(4)
93.29(13) Ir(1)-C(10)-C(11) 172.0(4)
93.96(13) C(10)-C(11)-C(12) 176.1(5)
λ ) 0.710 69 Åa
Fcalcd ) 1.460 g cm-3
µ ) 35.9 cm-1
R(Fo)b ) 0.0242
Rw(Fo)c ) 0.0215
85.87(13) P(18)-Ir(1)-H(A)
95.4(18)
87.4(18)
83.2(18)
C(2)-Ir(1)-C(10) 176.62(19) P(33)-Ir(1)-H(A)
Ir(1)-C(2)-C(3) 174.1(4) C(2)-Ir(1)-H(A)
brown solid was dissolved in 5 mL of pentane and cooled to
-40 °C overnight to yield dark brown crystals. These were
separated from the solution, washed with cold pentane (0 °C,
2 × 2 mL), and dried in vacuo (yield: 76%). 1H NMR (C6D6,
25 °C): 7.95 (br, s), 7.22-7.10 (overlapping m), 1.56 (CH3, vt,
J PH ) 6.6 Hz), -43.9 (IrH, t, J PH ) 11 Hz). 31P{1H} NMR
(C6D6, 25 °C): 52.7 (s).
Graphite monochromator. R ) ∑||Fo| - |Fc||/∑|Fo|. c Rw
)
a
b
2
[∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2, where w ) 1/σ2(|Fo|).
1H NMR (C7D8, -40 °C): δ -42.6 (apparent t, J PH ) 11 Hz).
13C{1H} NMR (C6D6, 25 °C, only sp carbons of acetylide
ligand): δ 115.3 (apparent t, J PC ) 10 Hz), 108.7 (s). 31P{1H}
NMR (C7D8, -40 °C, with decoupling of non-hydridic pro-
tons): δ 57.6 (dd, J PP ) 328.5 Hz, J PH ) 10.1 Hz), -15.9 (dd,
J PP ) 328.5 Hz, J PH ) 10.5 Hz).
X-r a y Str u ctu r e Deter m in a tion of Ir H(CCP h )2(P tBu 2-
P h )2. A nearly equidimensional crystal was selected and
affixed to the end of a glass fiber using silicone grease. The
mounted sample was then transferred to the goniostat where
it was cooled to -181 °C for characterization (Table 1) and
data collection (6° < 2θ < 45°). Standard inert atmosphere
handling techniques were used throughout the investigation.
A systematic search of a limited hemisphere of reciprocal space
located no symmetry or systematic absences, indicating a
triclinic space group. Subsequent solution and refinement of
the structure confirmed the proper space group to be P1h. Data
were collected using a standard moving crystal, moving
detector technique with fixed background counts at each
extreme of the scan. Data were corrected for Lorentz and
polarization terms and for absorption, based on the measured
distances to the well-defined faces. The structure was solved
by Patterson and Fourier techniques. A difference Fourier
map phased on the non-hydrogen atoms clearly located all
hydrogen atoms, and these were included in the subsequent
least-squares refinement. Examination of the difference Fou-
rier clearly located the hydride as the second largest peak (the
largest is at the metal site). In the final cycles, all atoms were
varied, including the hydride. A final difference Fourier map
was essentially featureless, the largest peak lying at the site
of the Ir atom. Results of the structure determination are
shown in Figure 2 and Table 2.
Va r ia ble-Tem p er a tu r e 31P {1H} NMR Mea su r em en t of
Ir H(η2-C6H4P tBu 2)(CCP h )(P tBu 2P h ) Exch a n ge. The 31P-
{1H} NMR spectra of a sample of IrH(η2-C6H4PtBu2)(CCPh)(P-
tBu2Ph) (ca. 80 mg in 0.7 mL of decane) were recorded on a
Varian VXR-400 spectrometer. Data was collected by increas-
ing the temperature in 5° or 10° intervals from -20 to +130
°C (with an accuracy of (0.1 °C) and obtaining 256 scans at
each temperature. The data was transferred to an Apple
Macintosh computer and processed using an NMR data
processing program (MacFID; Tecmag, Inc.). Approximate line
widths of the decoalesced AX pattern for IrH(η2-C6H4P-
tBu2)(CCPh)(PtBu2Ph) (from -10 to 30 °C) and the coalesced
signal (from 100 to 115 °C) were obtained by a Lorentzian fit
of the appropriate signals. Using an NMR simulation program
(DNMR5) and starting from the calculated line widths, the
direct comparison of actual and simulated NMR spectra was
used to refine the exchange rate at each temperature. The
least-squares linear fit of a plot of 1n k/T vs. 1/T (Eyring plot,
Figure 1) gave the slope and intercept values necessary for
the calculation of ∆Hq and °Sq.
Ir H(C2P h )2(P tBu 2P h ). Freshly distilled PhCCH (14.3 µL,
0.13 mmol) and IrH(η2-C6H4PtBu2)(CCPh)(PtBu2Ph) (80 mg,
0.11 mmol) were stirred together in 10 mL of benzene for 1 h
at 25 °C. After vacuum removal of benzene, the resulting

1976 Organometallics, Vol. 16, No. 9, 1997
Cooper et al.
Resu lts
sharp AX pattern at 25 °C.5 This is consistent with
interconversion of the phosphines by reversible meta-
lation/demetalation of ortho-hydrogens on the two dif-
ferent phosphines (eq 2). These spectra were simulated,
Syn th esis a n d Ch a r a cter iza tion of Ir (H)2(C2P h )-
(P tBu 2P h )2, 1. Reaction of Ir(H)2Cl(PtBu2Ph)2 with
PhC2Li in toluene gives clean conversion to Ir(H)2(C2-
Ph)(PtBu2Ph)2. This molecule shows one 31P NMR
chemical shift and a 1H NMR tBu virtual triplet,
consistent with trans phosphines. The R-carbon of the
acetylide shows triplet splitting by two equivalent
phosphorus nuclei, as does the hydride signal. The
hydride chemical shift (-14 ppm) is inconsistent with
a hydride trans to an open site in a square pyramid (I),
and we therefore assign it the “Y”-shaped structure, II.
and the resulting rate constants yield the Eyring plot
shown in Figure 1. The unusually large temperature
range over which rate constants can be measured is
beneficial to the accuracy of the derived activation
parameters: ∆Hq ) 12.3 ((0.4) kcal mol-1 and ∆Sq )
-2.0 ((1.1) cal deg-1 mol-1
.
Reactivity of Ir H(η2-C6H4P tBu 2)(CCP h )(P tBu 2P h ).
(a ) With H2. Reaction with 1 atm of H2 in C6D6 caused
a rapid color change of the solution from the deep red
of IrH(η2-C6H4PtBu2)(CCPh)(PtBu2Ph) to a lighter red
This molecule is remarkable in being metastable,
losing H2 upon vacuum filtration through Celite or
(oven-dried) filter paper attached to one end of a
cannula. Prolonged exposure of 1 in solution and in the
solid state to a vacuum also causes partial conversion
to the metalated 2. This conversion is, however, difficult
to drive to completion to give pure product. Vacuum
also produces significant amounts of free phosphine and
uncharacterized metal complexes. We have therefore
devised the following conversion.
1
color. 31P and H NMR confirmed a quantitative reac-
tion of the metalated species to give two products in ca.
4:1 ratio. The identity of the minor product is assigned
to the polyhydride complex Ir(H)5(PtBu2Ph)2.6 The
major product has a 31P NMR resonance at 68.3 ppm
1
(s) and a H NMR hydride signal at -26.9 ppm (t, J PH
) 14.1 Hz) with coupling to two equivalent 31P NMR
nuclei. The 1H NMR spectrum also shows that free
phenylacetylene liberated by the reaction of IrH(η2-
C6H4PtBu2)(CCPh)(PtBu2Ph) with H2 is hydrogenated
to styrene.
Syn th esis a n d Ch a r a cter iza tion of Ir H(η2-C6H4-
P tBu 2)(CCP h )(P tBu 2P h ), 2. Generation of Ir(H)2-
(CCPh)(PtBu2Ph)2, followed by pressure filtering with
a cannula fitted with a Teflon filter tip, caused only
limited (5%) conversion of 1. Removal of toluene in
vacuo and refluxing in 1-hexene for 12 h caused a color
change from yellow to the dark red of IrH(η2-C6H4P-
(b ) Wit h H CCP h : Syn t h esis of Ir H (CCP h )2(P -
tBu 2P h )2. Addition of 1.2 equiv of phenylacetylene to
a solution of IrH(η2-PtBu2C6H4)(CCPh)(PtBu2Ph) in
C6H6 did not produce a noticeable color change, yet,
after stirring for 1 h at room temperature, 1H NMR
assay showed the conversion of 2 to one major product
with small (ca. 5%) amounts of uncharacterized impuri-
ties. The benzene was removed in vacuo, and the brown
solid was dissolved in a minimum amount of pentane.
Cooling this solution to -40 °C overnight caused the
formation of dark brown crystals. The crystalline solid
1
tBu2)(CCPh)(PtBu2Ph). Both the H and 31P{1H} NMR
spectra of 2 are broad at 25 °C, so its structure is best
determined at low temperature. At -40 °C, the 31P-
{1H} NMR spectrum shows one resonance within 8 ppm
of that of 1 and a second signal shifted 73.5 ppm to
higher field. Such a large upfield shift is diagnostic of
ortho metalation.4 The P-P coupling constant, 329 Hz,
indicates trans positioning of the phosphorus nuclei. The
hydride chemical shift, -42.6 ppm, is so very far upfield
that it signifies a position trans to no ligand: that is, a
square pyramidal structure (III). Both acetylide car-
1
was characterized as IrH(CCPh)2(PtBu2Ph)2 by H and
31P NMR spectroscopies and X-ray crystal structure
determination.
The structure determination, in which the hydride
was located (Figure 2), shows a square pyramidal
geometry with apical hydride. The trans phosphines
t
adopt a conformation in which one Bu group from each
phosphine lies to the same side of the IrP2C2 plane. This
leaves the opposite side of the coordination plane open,
and it is on this side that the hydride is located. There
are no agostic interactions involving the open side of
Ir; Ir/H distances to methyl hydrogens are all longer
than 2.78 Å. The Ir/C and the acetylide C/C bonds are
bons were detected in the 13C{1H} NMR spectrum, with
the R-carbon showing coupling to two 31P spins. The
spectra simplify at higher temperatures. The hydride
1
signal is too broad to observe at 25 °C. The tBu H NMR
signal simplified to one (broad) apparent doublet at 25
°C. Most diagnostic, however, is the 31P NMR spectrum,
which coalesces from the AX pattern at -40 °C to one
broad line at ca. 60 °C and which further sharpens in
the range 80-115 °C. In contrast, the 31P NMR
spectrum of IrH(η2-C6H4PtBu2)Cl(PtBu2Ph) shows a
(5) Caulton, K. G.; Cooper, A. C. Inorg. Chim. Acta, submitted for
publication.
(6) Ir(H)5(PtBu2Ph)2 has been prepared independently by the reac-
tion of Ir(H)2X(PtBu2Ph)2 (X ) F, OCH2CF3, OH) with excess H2. 31P-
{1H} NMR (C6D6, 25 °C): 67.1 (s). 1H NMR (C6D6, 25 °C): 8.65 (m),
7.22-6.89 (overlapping m), 1.46 (vt, J PH ) 6.3 Hz), -9.75 (t, J PH
11.8 Hz).
)
(4) Garrou, P. E. Chem. Rev. 1981, 81, 229.

C-H Activation (Ortho Metalation)
Organometallics, Vol. 16, No. 9, 1997 1977
both short. A complex with an analogous structure,
RhH(CCC(iPr)2OH)2(PiPr2C2H4OMe)2, was recently re-
ported.7
There are several plausible mechanisms for the reac-
tion in eq 3, and isotope labeling can distinguish certain
of these. The mechanism shown in eq 3a is primary
oxidative addition (forming Ir(V)), followed by H-
C(ortho) reductive elimination. Assuming rapid hydride
the transition state might have a reduced coordination
number and valence electron count (14 e-?). Moreover,
it is the only one with a hydrocarbyl ligand X and, thus,
minimal X f Ir π-donation: in fact, we have ranked
the composite (σ + π) donor ability of C2Ph as compa-
rable to that of Br.9 As such, the electrophilicity of
iridium in the transition state may be exceptionally high
when X ) C2Ph. For comparison, there is no evidence
of exchange among ortho-metalated and pendant phenyl
1
rings in Re(H)2(η2-C6H4PPh2)(PEt3)3 by H NMR spin-
saturation transfer at 85 °C. Other dynamic processes
in the molecule occur with ∆Gq38 °C of 15.1 kcal mol-1 10
.
We have considered three (intramolecular) mecha-
nisms for the site exchange (eq 2). Full oxidative
addition of a new ortho C-H bond prior to reductive
elimination of the former H and C(ortho), via Ir(H)2-
(CCPh)(η2-C6H4PtBu2)2, should have a quite negative
∆Sq and can thus be excluded. Full reductive elimina-
tion of H and C(ortho) to form Ir(CCPh)(PtBu2Ph)2
should have a quite positive ∆Sq and can thus be
excluded.11 An “internal displacement”, wherein an
agostic H-C(ortho) donates to unsaturated Ir(III) and
facilitates the reductive elimination without the neces-
sity of the 14-electron species Ir(CCPh)(PtBu2Ph)2,
should have compensating ordering and disordering and
thus be in accord with the observed ∆Sq of -2 entropy
units. These arguments require that there be no agostic
Ir/H-C(ortho) interaction in the ground state of Ir(H)2-
(CCPh)(η2-C6H4PtBu2)L, where the hydride is trans to
an empty coordination site. This is in accord with the
observed far upfield chemical shift of the hydride ligand.
Both ground state and transition state structures may
in fact be different from IV and V. An alternative to
five-coordinate IV (because of the absence of a more
effective π-donor halide) is a structure with an agostic
ortho-phenyl hydrogen from L. This could immediately
site exchange fluxionality in the seven-coordinate in-
termediate, the isotope will be scrambled between metal
and ortho carbon sites. The mechanism shown in eq
3b involves direct delivery of the acidic acetylene
deuteron to the Ir-C bond and thus to the ortho carbon.
This could involve four center σ-bond metathesis or a
nearly ionic proton transfer mechanism or anything
between these extremes. The mechanism shown in eq
3c involves the incoming acetylene triggering or induc-
ing8 the reductive elimination of the pre-existing Ir-C
and Ir-H bonds. Reductive elimination has been shown
in other systems to be triggered by either nucleophilic
2
attack or oxidation. In reality, the D NMR spectrum
of the product of the reaction of PhC2D with 2 shows
only IrD(C2Ph)2L2, with any deuterium at a phosphine
ortho phenyl site being less than our detection limits
(76.8 MHz, 1024 scans) of 5% of the Ir-D peak height.
1
This same sample was also assayed by H NMR spec-
troscopy; the integration of the ortho hydrogen signal
at 7.95 ppm was undiminished from that expected for
0% D. The mechanism is thus that of eq 3c.
Discu ssion
furnish compensating stability to the species as reduc-
tive elimination of C* and H* begins in IV. Equally
well, species V could be stabilized (thus lowering ∆Gq)
if it retains two agostic ortho C-H bonds, one from each
PtBu2Ph ligand (i.e., trigonal bipyramidal structure). In
each case postulated here, this effect would be more
fully developed for the weakly π-donating acetylide (via
an acetylide filled π-orbital) than for halide or ORf. In
summary, this reaction formally resembles an (intramo-
lecular) displacement (SN2) process and the transition
state thereby avoids full loss of the energy of two Ir-
ligand homolytic bond dissociation energies (compen-
sated by formation of one C-H bond). It is thus
analogous to the mechanism of eq 3c.
Why does Ir(H)2(CCPh)(PtBu2Ph)2 lose H2 so readily?
For comparison, the chloride analog shows no tendency
to lose H2 after 20 days at 65 °C in a solution with high
olefin concentration (1.6 M allylbenzene in C6D6).5 We
suggest that there is a stabilization of the H2 reductive
elimination transition state by formation of a H-C(ortho)
agostic interaction. This would be less likely to occur
in an Ir(H)2X(PtBu2Ph)2 species for a π-donor because
X f Ir π donation makes the agostic interaction less
necessary. In other words, IrX(PtBu2Ph)2 is much more
reactive at Ir when X is acetylide than when it is a
halide or pseudohalide. Although we have also studied
the species IrHX(P-C)L for X ) F, Cl, Br, I and OR and
P-C ) η2-C6H4PtBu2, the case with X ) C2Ph is the only
one to show reversibility of the metalation by NMR
spectroscopy (i.e., phosphorus site exchange as low as
25 °C). What might be the reason for this? Certainly
The reversibility observed here is strong evidence for
the high reactivity of the species of formula Ir(C2Ph)-
(9) Poulton, J . T.; Sigalas, M. P.; Folting, K.; Streib, W. E.;
Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1994, 33, 1476.
(10) J ones, W. D.; Maguire, J . A. Organometallics 1987, 6, 1728.
(11) Solvent coordination to this three-coordinate transition state,
which could make ∆Sq less positive, is unlikely because the coalescence
behavior is similar in toluene and in decane.
(7) Windmu¨ller, B.; Wolf, J .; Werner, H. J . Organomet. Chem. 1995,
502, 147.
(8) Kochi, J . K. Organometallic Mechanisms and Catalyses Academic
Press: New York, 1978.

1978 Organometallics, Vol. 16, No. 9, 1997
Cooper et al.
(PtBu2Ph)2: even when it is produced at a rate of 102
s-1 (i.e., the fluxional process), it does not persist. The
thermodynamically-preferred form is a redox product
(Ir(III)). Dimerization by acetylide bridging,12 which is
the observed form for many M(halide)L2 species, is
apparently precluded by the presence of two bulky
phosphines per metal. In summary, a 14-electron Ir(I)
species, Ir(hydrocarbyl)L2, finds an alternative lower
energy (redox) isomeric form, in spite of the strain
energy involved in forming a four-membered IrPCC
ring.
metastability by adopting a structure (Figure 2) with
the strong σ-donor hydride ligand trans to the empty
coordination site. The very high hydride chemical shift
is diagnostic of such an environment. With an atom of
very low electronegativity (i.e., a strong σ-donor) trans
to the empty site, the unoccupied orbital (LUMO) has
minimum metal character and is thus poorly adapted
to binding another ligand in that site. Thus, this
hydride site, by monopolizing the metal orbitals for its
own binding, minimizes the Lewis acidity (unsaturation)
of the ground state structure. All molecules adopt a
structure which maximizes their “stability”. In the case
of a Lewis acid, this can mean maximizing the HOMO/
LUMO gap and/or making the LUMO the least well-
adapted for binding a Lewis base. In the case at hand,
putting the strongest σ-donor ligand trans to the steri-
cally empty (open) site accomplishes this by minimizing
the spatial extension of the LUMO into the open site.
Con clu sion s
The strain in the ortho-metalated ring is evident in
its willingness to reductively eliminate in the reversible
eq 2. Strain is also evident in its ready “acidolysis” (σ-
bond metathesis?) with the carbon acid PhCC-H, to
form a new Ir-C bond in an otherwise very unsaturated
molecule, IrH(CCPh)2(PtBu2Ph)2. The deuterium iso-
tope study with PhC2D in fact shows that this reaction
is neither acidolysis nor σ-bond metathesis but is
instead C-H reductive elimination triggered by the
interaction of Ir with incoming alkyne, either at its
π-cloud or its C-D bond. IrH(CCPh)2(PtBu2Ph)2, a 16-
electron species, devoid of π-donor ligands, achieves
Ack n ow led gm en t. This work was supported by the
National Science Foundation. We thank J ohnson Mat-
they/Aesar for material support and Marty Pagel for
assistance with certain NMR studies.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, positional parameters, and anisotropic thermal
parameters (6 pages). Ordering information is given on any
current masthead page.
(12) Herres, M.; Lang, H. J . Organomet. Chem. 1994, 480, 235. Lang,
H.; Ko¨hler, K.; Schiemenz, B. J . Organomet. Chem. 1995, 495, 135.
Cirrano, M.; Howard, J . A. K.; Spencer, J . L.; Stone, F. G. A.; Wadepohl,
H. J . Chem. Soc., Dalton Trans. 1979, 1749.
OM9607107