Oxidative addition and reductive elimination reactions of trans-[Ir(PPh3)2(CO)(NC4H4)] and trans,cis-[Ir(PPh3)2(H)2(CO)(NC 4H4)], including N-H bond-forming reductive elimination of pyrrole

Article abstract of DOI:10.1021/om971049p

The complex trans-(PPh₃)₂(CO)Ir(NC₄H₄) (1) has been synthesized and is an analogue of metal-aryl complexes, but with a nitrogen of the heteroaromatic group covalently bonded to the transition metal. Compound 1 readily undergoes initial reaction with a variety of substrates at the metal center rather than at the pyrrolyl nitrogen, allowing for the study of reactions between the pyrrolyl group and accompanying covalent ligands. These reactions ultimately produce N-substituted pyrroles, X-NC₄H₄ (X = C(O)CH₃, C(O)C₆H₄CH₃, H, SnMe₃, SiMe₃, SiEt₃, Bcat). Compound 1 undergoes oxidative addition of H₂ to form the stable Ir-(III) product (PPh₃)₂(CO)Ir(H)₂(NC₄H₄) (2). When pure 2 is heated, it undergoes simple elimination of H₂ to regenerate 1; however, if 2 is heated for longer times under H₂ in the presence of PPh₃, it undergoes reductive elimination of pyrrole and forms (PPh₃)₃(CO)Ir(H). Qualitative analysis of the mechanism of this reaction suggests that it occurs by either direct reductive elimination from the octahedral complex or rate-determining ligand dissociation, followed by rapid reductive elimination of pyrrole. Reductive elimination of pyrrole from 2 was also observed to occur photochemically by initial irreversible dissociation of a dative ligand.

Full text of DOI:10.1021/om971049p

1134
Organometallics 1998, 17, 1134-1143
Oxid a tive Ad d ition a n d Red u ctive Elim in a tion Rea ction s
of tr a n s-[Ir (P P h 3)2(CO)(NC4H4)] a n d
tr a n s,cis-[Ir (P P h 3)2(H)2(CO)(NC4H4)], In clu d in g N-H
Bon d -F or m in g Red u ctive Elim in a tion of P yr r ole
Michael S. Driver and J ohn F. Hartwig*
Department of Chemistry, Yale University, P.O. Box 208107,
New Haven, Connecticut 06520-8107
Received November 28, 1997
The complex trans-(PPh3)2(CO)Ir(NC4H4) (1) has been synthesized and is an analogue of
metal-aryl complexes, but with a nitrogen of the heteroaromatic group covalently bonded
to the transition metal. Compound 1 readily undergoes initial reaction with a variety of
substrates at the metal center rather than at the pyrrolyl nitrogen, allowing for the study
of reactions between the pyrrolyl group and accompanying covalent ligands. These reactions
ultimately produce N-substituted pyrroles, X-NC4H4 (X ) C(O)CH3, C(O)C6H4CH3, H, SnMe3,
SiMe3, SiEt3, Bcat). Compound 1 undergoes oxidative addition of H2 to form the stable Ir-
(III) product (PPh3)2(CO)Ir(H)2(NC4H4) (2). When pure 2 is heated, it undergoes simple
elimination of H2 to regenerate 1; however, if 2 is heated for longer times under H2 in the
presence of PPh3, it undergoes reductive elimination of pyrrole and forms (PPh3)3(CO)Ir(H).
Qualitative analysis of the mechanism of this reaction suggests that it occurs by either direct
reductive elimination from the octahedral complex or rate-determining ligand dissociation,
followed by rapid reductive elimination of pyrrole. Reductive elimination of pyrrole from 2
was also observed to occur photochemically by initial irreversible dissociation of a dative
ligand.
In tr od u ction
proceed via metal-alkoxo and metal-amido intermedi-
ates. The proposed mechanistic cycle for these hydro-
genation reactions invokes O-H and N-H bond-
forming reductive eliminations to form the final alcohol
and amine products; likewise, the reverse reaction,
Oxidative addition and reductive elimination reac-
tions involving the C-C and C-H bonds of organic
substrates are among the fundamental transformations
of organometallic chemistry. The mechanisms of these
transformations have been studied extensively and have
provided a detailed understanding of how these pro-
cesses occur. Such mechanistic information has been
instrumental in the development of homogeneous cata-
lytic reactions resulting in C-H and C-C bond forma-
tion.1 Recently, there has been an increasing interest
in parallel oxidative addition and reductive elimination
reactions involving the H-heteroatom (N, O, and S) and
C-heteroatom bonds of organic compounds. It is impor-
tant to understand how these transformations occur in
order to apply them to the development of catalytic
reactions involving the cleavage and formation of H-X
and C-X bonds. The catalytic hydrogenation of
ketones,2-11 imines,2,12-29 and nitriles30,31 is thought to
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S0276-7333(97)01049-2 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/24/1998

Reductive Elimination of Pyrrole
Organometallics, Vol. 17, No. 6, 1998 1135
dehydrogenation of alcohols, may involve the oxidative
addition of an O-H bond.4,32 In a similar manner, the
alcoholysis of organohalides33 is believed to proceed
through metal-alkoxide complexes to form the ether
products. The hydration of olefins has also been ob-
served to occur by olefin insertion into a metal-
hydroxide bond.34 Another catalytic process which has
recently been reported leads to the addition of aniline
across an olefin via oxidative addition of the aniline to
form a metal-amido intermediate.35,36
Recently, there has been a significant amount of
research conducted on C-N bond-forming reductive
eliminations of amines. Much of this work has been
conducted in combination with the development of
palladium-catalyzed aryl halide amination chemistry.37-41
Mechanistic analysis of the original amination reaction
involving aminostannanes42,43 and analysis of C-N
bond-forming reductive-elimination reactions to form
amines44,45 provided insights that led to substantial
improvements in the catalytic amination chemistry.
These improvements included the replacement of toxic,
air-sensitive aminostannanes with free amines and an
alkoxide base46,47 and the use of catalytic species
containing chelating phosphines. These developments
have dramatically increased substrate specificity and
compatibility.48-50 N-H bond cleavage chemistry is
crucial to the amination using amines as substrates.
The reactivity of N-H bonds with late-transition-
metal complexes remains scarce. Although there have
been reports of oxidative addition30,35,51-64 and reductive
elimination65,66 reactions of N-H bonds with late-
transition-metal complexes, no clear mechanistic un-
derstanding of these transformations has emerged.
Earlier studies conducted on the reductive elimination
of amine and alcohol from (C5Me5)(PPh3)Ir(X)(H) (X )
NHPh, OEt)66 showed the formation of an intermediate
but did not directly probe the O-H and N-H bond-
forming steps. The introduction of a ligand (L ) CO,
C2H4, CNBut, PPh3, PPh2Me) to a solution of (C5Me5)-
(PPh3)Ir(X)(H) induced reductive elimination of either
aniline or ethanol. Results from a detailed mechanistic
analysis of these reactions ruled out pathways including
dative ligand association, dative ligand dissociation,
ligand homolysis to form radicals, dissociation of the
anionic ligand, ligand migration to the Cp* ring, forma-
tion of an ethanol or aniline σ-complex, as well as direct
elimination. A pathway involving a ring slip of the Cp*
ligand from an η5- to η3-bonding mode followed by
trapping of this intermediate by PPh3 was the most
probable mechanism. Further information on O-H and
N-H bond-forming reductive eliminations in other
systems is needed to provide a general view on how this
class of reaction occurs.
We report here the preparation and reactivity of an
iridium-pyrrolyl complex. Little organometallic reac-
tion chemistry of η1-pyrrolyl or indolyl ligands has been
published. The synthesis of a related Ir(I) η1-indolyl
from Vaska’s complex was reported during the course
of the work described here, but the only reaction
chemistry included in this paper was protonation of the
indolyl ligand with triflic acid.67 We report that the
pyrrolyl complex trans-(PPh3)2(CO)Ir(NC4H4) undergoes
oxidative addition and reductive elimination reactions
with a variety of substrates to ultimately produce
N-substituted pyrroles. In one of these reactions, the
pyrrolyl complex oxidatively adds H2 to form a stable
Ir(III)-pyrrolyl dihydride. When heated in the presence
of free PPh3 and H2, the Ir(III) complex undergoes
reductive elimination of pyrrole. The reductive elimina-
tion of pyrrole was also induced photochemically. The
mechanism of these reductive eliminations is not clearly
defined, but it appears to involve irreversible ligand
dissociation.
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1136 Organometallics, Vol. 17, No. 6, 1998
Ta ble 1. In fr a r ed Sp ectr a of Selected Ir id iu m Com p lexes
Driver and Hartwig
Ir(I) complexes
ν
CO, cm-1
Ir(III) complexes
ν
CO, cm-1
ν
IrH/D, cm-1
(PPh3)2(CO)Ir(NC4H4)
(PPh3)2(CO)IrCl
(PPh3)2(CO)IrBr
(PPh3)2(CO)IrI
1964a
1950b
1955b
1975b
(PPh3)2(CO)Ir(H)2(NC4H4)
(PPh3)2(CO)Ir(H)2Cl
(PPh3)2(CO)Ir(H)2Br
(PPh3)2(CO)Ir(H)2I
(PPh3)2(CO)Ir(D)2(NC4H4)
(PPh3)2(CO)Ir(D)2Cl
2075a
1975b
1982b
1995b
2017a
1995b
2142, 2183a
2100, 2190b
2105, 2200b
2100, 2225b
1553a
1572b
a
b
KBr pellet. Reference 64.
Ta ble 2. Da ta Collection a n d Refin em en t
P a r a m eter s for th e Str u ctu r e of 2
A. Crystal Data
empirical formula
fw
C43.50H34NOP2Ir
840.92
cryst color, habit
cryst dimens
cryst syst
yellow, prismatic
0.19 × 0.25 × 0.40 mm
triclinic
lattice type
lattice params
primitive
a ) 9.470(1)Å
b ) 12.123(1) Å
c ) 17.531(1) Å
R ) 79.208(6)o
â ) 74.742(5)o
γ ) 71.834(8)o
V ) 1832.7(3) Å3
P1h (No. 2)
space group
Z value
Dcalc
2
1.524 g/cm3
F000
834.00
F igu r e 1. ORTEP drawing of 2. Hydrogen atoms were
omitted for clarity. The two iridium hydrides were not
located.
µ(Mo KR)
no. of reflns measd
37.74 cm-1
total 7682
unique 7428 (Rint ) 0.024)
Lorentz-polarization abs
(transmission factors
0.7283 - 1.0000)
6426
corrections
Sch em e 1
no. of observations
(I > 3.00σ(I))
no. of variables
residuals: R; Rw
427
0.034; 0.042
Ta ble 3. Selected Bon d Len gth s (Å) for 2
Ir(NC4H4) (1) and (PPh3)2(CO)Ir(H)2(NC4H4) (2) are
summarized in Scheme 1. Addition of NaNC4H4 to a
THF solution of trans-(PPh3)2(CO)IrCl (3) led to the
formation of 1 in 87% isolated yield. Subsequently, 2
was prepared in 86% isolated yield by addition of ∼2
atm of H2 to 1 at room temperature for 36 h. Complexes
1 and 2 were characterized by 1H, 13C{1H}, and 31P{1H}
NMR and IR spectroscopy, as well as elemental analy-
sis. Complex 2 was also characterized by X-ray diffrac-
tion, as discussed below. The 31P{1H} NMR spectrum
of 1 consisted of a singlet at δ 20.7, upfield from the
resonance for Vaska’s complex 3 (δ 24.8), while the 31P-
{1H} NMR signal for the pyrrolyl dihydride 2 was
located even further upfield at δ 7.0. The hydride
resonances for 2 appeared as two doublets of triplets (δ
-15.84, -6.89, J ) 5.0, 16.7 Hz), indicating that the
hydrides are cis to one another, inequivalent, and
located cis to the two equivalent phosphine ligands, as
shown in Scheme 1. In the IR spectrum, the νCO for 1
appeared at 1964 cm-1, a higher stretching frequency
than the νCO for Vaska’s complex (1950 cm-1). The CO
ligand of 2 vibrates at a slightly higher frequency, 1967
cm-1 (see Table 1).
Ir(1)-P(1)
Ir(1)-N(1)
O(1)-C(1)
N(1)-C(5)
C(3)-C(4)
2.323(1)
2.154(4)
1.147(7)
1.344(6)
1.399(9)
Ir(1)-P(2)
Ir(1)-C(1)
N(1)-C(2)
C(2)-C(3)
C(4)-C(5)
2.338(1)
1.916(6)
1.354(6)
1.366(8)
1.373(8)
Ta ble 4. Selected Bon d An gles (d eg) for 2
P(1)-Ir(1)-P(2)
P(1)-Ir(1)-C(1)
P(2)-Ir(1)-C(1)
Ir(1)-N(1)-C(2)
C(2)-N(1)-C(5)
N(1)-C(2)-C(3)
C(3)-C(4)-C(5)
169.76(4)
92.7(2)
97.3(2)
126.2(3)
107.7(4)
110.1(5)
107.3(5)
P(1)-Ir(1)-N(1)
P(2)-Ir(1)-N(1)
N(1)-Ir(1)-C(1)
Ir(1)-N(1)-C(5)
Ir(1)-C(1)-O(1)
C(2)-C(3)-C(4)
N(1)-C(5)-C(4)
90.8(1)
90.1(1)
96.5(2)
126.1(3)
174.6(5)
105.8(5)
109.1(5)
in Tables 3 and 4. The structure of 2 showed a pseudo-
octahedral geometry. The two metal hydrides were not
located, but are clearly present in the two vacant cis
sites. The pyrrolyl ligand maintains planarity; the sum
of the three bond angles at nitrogen is exactly 360°. The
plane of the pyrrolyl ligand is slightly skewed from the
plane of the two hydrides, the CO, and the iridium. The
C(1)-Ir-N-C(5) torsional angle is 26.8(4)°. Most no-
tably, the iridium-nitrogen distance is 0.074 Å longer
than the Ir-indolyl distance in a PPh3-ligated Ir(I)
indolyl complex67 but only 0.02 Å longer than that in a
related tris-PMe3 Ir(III) indolyl complex.55
X-r a y Str u ctu r e of 2. Crystals of 2 suitable for an
X-ray diffraction study were obtained by layering a
toluene solution of 2 with pentane. An ORTEP drawing
of 2 is provided in Figure 1; data on crystal parameters,
data collection, and refinement are provided in Table
2, while selected bond distances and angles are given
Oxid a tive Ad d ition Rea ction s of H2 a n d MeI to
1. Complex 1 undergoes oxidative addition reactions
similar to those observed for Vaska’s complex 3 and the

Reductive Elimination of Pyrrole
Organometallics, Vol. 17, No. 6, 1998 1137
Sch em e 2
falls at the end of the spectrum of rates, I > Br > Cl >
NC4H4 (Scheme 2). However, the νCO values do not
seem to follow the same trend. The pyrrolyl complex
falls in the middle of the halides: Cl > Br > NC4H4 >
I. The origins of the subtle σ and π effects on the νCO
are complex and are still evolving.77,78 Perhaps the
trends in oxidative addition reaction rates are a better
measure of the electron density of the metal centers
than νCO in this case.
Oxid a tive Ad d ition a n d Red u ctive Elim in a tion
Rea ction s of 1 Resu ltin g in N-Su bstitu ted P yr -
r oles. Complex 1 undergoes reaction with a variety of
substrates to produce N-substituted pyrroles, as shown
in Scheme 3. The formation of an N-substituted pyrrole
is indicative of a pathway initiated by oxidative addition
of the substrate to the Ir center followed by reductive
elimination of the corresponding pyrrole. A pathway
of external nucleophilic attack would result in a 2-sub-
stituted pyrrole, since the R-carbon is the most nucleo-
philic site on the pyrrolyl ligand. The formation of a
2-substituted pyrrole has been observed previously in
our laboratory from the addition of CD3I to (DPPE)Pd-
(CH3)(NC4H4)79 to give 2-methyl-d3-pyrrole,75 and the
indolyl complex reported by Rakowski DuBois67 is
protonated at the 3-position.
other halide derivatives of 3.68 As discussed above, 1
oxidatively adds H2 to form 2. Qualitative studies of
the addition of H2 to 1 indicated that the reaction is not
inhibited by the addition of free phosphine, disfavoring
a mechanism involving initial, reversible dissociation
of PPh3. As is the case for Vaska’s complex, the addition
of H2 to 1 is reversible. When a benzene solution of 2
was heated under vacuum at 95 °C, H2 was eliminated
to regenerate 1. Due to these similarities in reactivity
and in the geometries of the two addition products, the
addition of H2 to 1 most likely proceeds through the
same mechanism of concerted, direct cis addition as
observed for Vaska’s complex.68 In qualitative compari-
sons of the rates of H2 addition, pyrrolyl complex 1
reacted much slower than did Vaska’s complex.
When the methyl iodide oxidative-addition product 4
was heated at 95 °C for 18 h, no N-methylpyrrole was
observed; only the formation of pyrrole was detected.
No other organic products were identified, and no CH4
or C2H6 was observed. The proton source is unknown.
However, the addition of MeI to trans-(PPh3)2(CO)Ir-
(OMe)80 gave a similar formation of MeOH with no
Me2O.
Complex 1 also underwent oxidative addition of MeI
to form (PPh3)2(CO)Ir(CH3)(I)(NC4H4) (4), as shown in
Scheme 2. The formation of 4 was indicated by a large
upfield shift in the 31P{1H} NMR spectrum from δ 20.7
1
for 1 to δ -14.8. The Me resonance appeared in the H
NMR spectrum as a triplet at δ 1.26 (J ) 4.9 Hz),
indicating that the Me group is located cis to the two
equivalent phosphine ligands. This reactivity of MeI
at the metal center contrasts that of late-transition-
metal amido complexes which undergo N-alkylation of
the amido ligand.69-75 In a competition study, equimo-
lar amounts of 1 and Vaska’s complex were mixed with
0.1 equiv of MeI. The ratio of 4: (PPh3)2(CO)Ir(CH3)-
(I)(Cl) was 2:1. Since the addition of MeI is thought to
be an SN2 mechanism,68 the preferential reactivity of 1
with MeI indicates that 1 possesses a more nucleophilic
Ir center than does Vaska’s complex.
Complex 1 underwent oxidative addition with acetyl
chloride to form the new Ir(III) species (PPh3)2(CO)Ir-
[C(O)CH3](Cl)(NC4H4) (5). Again, the 31P{1H} NMR
spectrum showed a large upfield shift for the Ir(III)
species (δ -15.4), and a new acetyl peak appeared at δ
1
1.39 in the H NMR spectrum. Attempts to isolate this
Ir(III) complex were unsuccessful due to decomposition
of this acetyl halide adduct. However, when the reac-
tion of 2 equiv of acetyl chloride with 1 was monitored
by NMR spectrometry in benzene-d6, it was observed
that all of 1 was converted to the Ir(III) species 5.
Heating the complex generated in situ at 70 °C resulted
in the formation of N-acetyl pyrrole (12%) and pyrrole
(60%).81 The only metal product observed by 31P{1H}
NMR spectroscopy was (PPh3)2(CO)Ir[C(O)CH3](Cl)2 (δ
-18.8). Similar reactivity was observed upon addition
of p-toluoyl chloride to 1. The addition of 2 equiv of
p-toluoyl chloride to a toluene-d8 solution of 1, however,
did not result in the formation of an Ir(III) complex at
room temperature. Instead, addition of p-toluoyl chlo-
ride followed by heating of the sample at 80 °C led to
the formation of N-p-toluoylpyrrole (17%) and pyrrole
(59%). The identities of the N-substituted pyrroles were
The relative rates for H2 and MeI additions to 1 can
be interpreted along with the relative rates for addition
of these substrates to (PPh3)2(CO)IrX (X ) Cl, Br, I).
For example, the relative rates for MeI addition are
NC4H4 > Cl > Br > I, as shown in Scheme 2.68 This
trend suggests that the Ir center is the most nucleophilic
in the case of pyrrolyl complex 1. It is well-known that
the relative rates for addition of H2 to the various halide
versions of Vaska’s complex are the opposite of those
for the addition of MeI.68,76 Again, pyrrolyl complex 1
(68) Chock, P. B.; Halpern, J . J . Am. Chem. Soc. 1966, 88, 3511.
(69) Bryndza, H. A.; Fultz, W. C.; Tam, W. Organometallics 1985,
4, 939.
(70) Bryndza, H. E.; Tam, W. Chem. Rev. 1988, 88, 1163 and
references therein.
(71) Cowan, R. L.; Trogler, W. C. J . Am. Chem. Soc. 1989, 111, 4750.
(72) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. Rev. 1989, 95,
1-40 and references therein.
(73) Dewey, M. A.; Bakke, J . M.; Gladysz, J . A. Organometallics
1990, 9, 1349.
(77) Poulton, J . T.; Sigalas, M. P.; Folting, K.; Streib, W. E.;
Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1994, 33, 1476.
(78) Abu-Hasanayn, F.; Goldman, A. S.; Krogh-J espersen, K. Inorg.
Chem. 1994, 33, 5122.
(74) Dewey, M. A.; Knight, A.; Arif, A.; Gladysz, J . A. Chem. Ber.
1992, 125, 815.
(75) Driver, M. S. Ph.D. Thesis, Yale University, 1997.
(76) Strohmeier, W. J . Organomet. Chem. 1971, 32, 137.
(79) DPPE ) 1,2-bis(diphenylphospino)ethane.
(80) Bernard, K. A.; Atwood, J . D. Organometallics 1987, 6, 1133.
(81) Yields measured by 1H NMR spectroscopy using an internal
standard.

1138 Organometallics, Vol. 17, No. 6, 1998
Driver and Hartwig
Sch em e 3
confirmed by generation of an authentic sample by
addition of acetyl chloride or p-toluoyl chloride to sodium
pyrrolyl.
too rapidly to observe an Ir(III) oxidative addition
product. The identity of the N-substituted pyrroles was
confirmed by generation of an authentic sample by
addition of either Me3SiCl, Me3SnCl, or ClBcat to
sodium pyrrolyl.
Electrophilic substrates containing Sn, Si, and B were
reacted with 1 to explore the scope of its oxidative
addition/reductive elimination reactions with main-
group reagents. Heating a sample of Me3SnCl with 1
in benzene-d6 at 80 °C for 14 h led to the formation of
N-(trimethylstannyl)pyrrole in 69% yield. Vaska’s com-
plex 3 was the only Ir product observed by 31P{1H} NMR
spectroscopy. None of the Ir(III) oxidative addition
product was detected either at room temperature or
throughout the course of the reaction. Silanes and
chlorosilanes also reacted with 1. Heating a benzene-
d6 solution of Me3SiCl with 1 at 80 °C for 14 h gave
N-(trimethylsilyl)pyrrole in 43% yield. Again, 3 was the
only Ir product observed by 31P{1H} NMR spectroscopy.
When a benzene-d6 solution of Et3SiH and 1 was heated
at 80 °C for 18 h, N-(triethylsilyl)pyrrole was formed in
64% yield. The iridium product was the known (PPh3)2-
Free pyrrole reacts with electrophiles at the carbons
of the aromatic ring. The addition of alkyl halides leads
to the formation of a mixture of 2- and 3-alkylpyrroles,
while the addition of acyl halides gives 2-acylpyrrole.86
As mentioned above, palladium pyrrolyl complexes that
do not undergo oxidative additions give reactions at the
pyrrolyl carbons, and the indolyl complex reported by
Rakowski DuBois67 is protonated at the 3-position. The
key to formation of N-substituted pyrroles from 1 is the
initial reaction of the substrate with the Ir(I) center to
give the Ir(III) oxidative addition intermediate with
subsequent N-E (E ) B, Si, Sn) reductive elimination
to form the respective pyrrole products.
Red u ctive Elim in a tion of P yr r ole fr om 2. As
discussed previously, 1 oxidatively adds H2 to form 2.
When a solution of 2 was heated at 95 °C, it reductively
eliminated H2 to regenerate 1. Thus, an equilibrium
between 2 and 1 + H2 exists, and the equilibrium
constant Keq between 2 and 1 + H2 (eq 1) at 95 °C, which
is the temperature of the mechanistic studies presented
below, was measured to be 500 M-1 at 95 °C. The ratio
82
(CO)Ir(SiEt3)(H)2 (66%). Again, no Ir(III) pyrrolyl
species was observed either at room temperature or
throughout the course of the reactions with silanes. The
reactions of 1 with chloroboronate esters occurred
rapidly, even below room temperature. Addition of
ClBcat83 to a room-temperature solution of 1 led to the
rapid formation of 2-pyrrolyl-1,3,2-benzodioxaborole in
78% yield and 3 in 89% yield. Similarly, addition of 1
equiv of HBcat84 to a solution of 1 with added PPh3 gave
2-pyrrolyl-1,3,2-benzodioxaborole in 82% yield. (PPh3)3-
(CO)Ir(H)85 (6) was the dominant Ir product (ca. 90%)
observed by 31P{1H} NMR spectroscopy. When an
excess amount of HBcat was used, the product 6 was
observed to react with HBcat to form an unidentified
Ir complex. With both boronate esters, formation of the
N-substituted pyrrole occurred directly upon mixing of
the samples at room temperature and no Ir(III) species
was detected. Even addition of the boronate esters at
-80 °C led to the formation of the N-substituted pyrrole
of 2:1 was 6.2:1 at the approximately 2 atm of H2
employed during the studies of pyrrole elimination. At
room temperature, only 2 existed in measurable quanti-
ties under 2 atm of H2.
(82) Chalk, J . J . Chem. Soc., Chem. Commun. 1969, 1207.
(83) ClBcat ) 2-chloro-1,3,2-benzodioxaborole.
(84) HBcat ) 1,3,2-benzodioxaborole.
(86) J ones, R. A.; Bean, G. P. The Chemistry of Pyrroles; Academic
Press: London, 1977; Vol. 34.
(85) Yagupsky, G.; Wilkinson, G. J . Chem. Soc. A 1969, 725.

Reductive Elimination of Pyrrole
Organometallics, Vol. 17, No. 6, 1998 1139
In contrast to this result, heating a solution of 2 with
added PPh3 under 2 atm of H2 for several days led to
N-H bond-forming reductive elimination of pyrrole
(91%) and the formation of 6, as shown in eq 1. N-H
bond-forming reductive elimination has also been ob-
served in a few other systems. The elimination of
aniline has been observed from (C5Me5)(PPh3)Ir(H)-
(NHPh),66 while the addition of π-acids to succinimido-
hydride complexes of Ni, Pd, and Pt induced the elimi-
nation of succinimide.65 By understanding the N-H
bond-forming reductive elimination of amines, insight
can be also be gained on the reverse reaction, the oxida-
tive addition (activation) of the N-H bond of amines.
The activation of amine N-H bonds has been observed
in a handful of systems. Some early-transition-metal
systems have been observed to activate ammonia51 and
pyrroles.52 Although often accompanied by further
chemistry, products resulting from some type of oxida-
tive addition of amines to late-transition-metal com-
plexes have been isolated from reactions with ammo-
nia,53 pyrroles,54-56,57 aryl and alkylamines,35,59,60,63 as
well as imides.65,87
Mech a n ism of P yr r ole Red u ctive Elim in a tion .
The activation of amines to form reactive metal-amido
complexes is a key step in catalytic pathways for the
functionalization of amines. A better understanding of
N-H reductive elimination and oxidative addition reac-
tions is necessary for the further development of such
catalysis. We studied the N-H bond-forming reductive
elimination of pyrrole from 2 in order to elucidate the
mechanism of this transformation. These studies were
hampered by difficulties in obtaining reproducible rate
constants, but we were able to show that the usual
mechanism for C-H bond-forming reductive elimina-
tions from octahedral compounds that involves revers-
ible predissociation of a phosphine ligand is an unlikely
pathway.
F igu r e 2. Linear plot of kobs vs [H2O] (R ) 0.97) with a
y-intercept of 8.9 × 10-7 ( 3.7, demonstrating the presence
of two concurrent pathways for formation of pyrrole
from 2.
partial formation of 1 under the reaction conditions, the
reactions were allowed to stand at room temperature
for a minimum of 3 h before obtaining 1H NMR spectra
to allow all of the iridium-pyrrolyl species to return to
2 and to, therefore, allow [Iridium]total to simply equal
[2]. Quantitative studies were hampered by difficulties
in reproducing the rate constants for these reactions,
and the half-lives for reactions varied by as much as a
factor of 2. However, qualitative studies involving
added phosphine, CO, H2, and pyrrole showed that
reaction rates were independent of the concentration of
all these species. Further, no noticeable difference in
the reaction rates was observed between reactions in
THF and reactions in toluene.
Reversible dissociation of phosphine, followed by
reductive elimination of pyrrole, would lead to reaction
rates that are retarded by increasing concentrations of
phosphine. The lack of rate dependence on [PPh3] rules
out this path. Similarly, the absence of an effect of
added CO rules out reversible CO dissociation. The
absence of an effect of pyrrole concentration on reaction
rates also shows that the lack of ligand dependence does
not result from reversible ligand dissociation that is
followed by reversible reductive elimination and ir-
reversible ligand association. A mechanism involving
dissociation of a pyrrolyl group to generate a cationic
hydride, which is deprotonated by the pyrrolyl anion,
would be related to the ionic mechanism for oxidative
addition of alkyl halides to Vaska’s complex. This
mechanism was ruled out by the absence of a significant
solvent effect on the rates of the reaction.
A mechanism involving reductive elimination cata-
lyzed by adventitious water could occur by protonation
of the pyrrolyl ligand to form pyrrole and an Ir(III)
hydrido hydroxide intermediate. Indeed, we did find
that water catalyzed the reductive elimination. A plot
of kobs vs [H2O] for reactions conducted with varying
amounts of added water (Figure 2) was linear and had
a positive slope. However, this plot also had a nonzero
y-intercept, and this y-intercept corresponds to the rate
constant with [H2O] equal to zero. The value of this
y-intercept was within the range of rate constants we
obtained in the absence of added water. Thus, a
mechanism catalyzed by adventitious water could be
ruled out, at least as the dominant pathway for reac-
tions without purposefully added water.
Previous studies conducted on other Ir(III) complexes
revealed C-H bond-forming reductive eliminations that
preceded by initial, reversible ligand dissociation. The
88
carborane complex Ir(H)(Cl)(σ-carb)(CO)(PPh3)2 re-
ductively eliminated carborane after initial PPh3 dis-
sociation.89 In a similar manner, the complex Ir(H)-
(σ-CHCH2C(O)OC(O)](σ-carb)(CO)(PhCN)(PPh3) first dis-
sociated PhCN to generate a five-coordinate intermedi-
ate, which underwent C-H bond-forming reductive
elimination to form succinic anhydride.90 The Rh(III)
complex (PMe3)3Rh(H)(Cl)[CH2C(O)CH3] reductively
eliminated acetone after dissociation of a PMe3 ligand.91
The O-H bond-forming reductive elimination of (PEt3)3-
Ir(H)(OMe)Cl to form MeOH was reported to proceed
by direct elimination from the six-coordinate Ir(III)
complex;92 yet, a mechanism involving irreversible
phosphine loss is also consistent with the kinetic data
presented.
1
Reaction rates were measured by H NMR spectrom-
etry under H2 with excess PPh3 added. Because of the
(87) Roundhill, D. M. Inorg. Chem. 1970, 9, 254.
(88) carb ) 7-C6H5-1,7-C2B10H10
.
(89) Basato, M.; Morandini, F.; Longato, B.; Bresadola, S. Inorg.
Chem. 1984, 23, 649.
(90) Basato, M.; Longato, B.; Morandini, F.; Bresadola, S. Inorg.
Chem. 1984, 23, 3972.
(91) Milstein, D. Acc. Chem. Res. 1984, 17, 221.
(92) Blum, O.; Milstein, D. Angew. Chem., Int. Ed. Engl. 1995, 34,
229.
Instead, mechanisms involving irreversible phosphine
dissociation or direct reductive elimination are more
likely. Isotope effects, which would distinguish direct

1140 Organometallics, Vol. 17, No. 6, 1998
Driver and Hartwig
Sch em e 4
dissociation of CO rather than PPh3 from the photo-
chemically excited state of 2. These data, in conjunction
with our qualitative kinetic data on the thermal chem-
istry of 2, suggest that the thermal reductive elimina-
tion occurs after irreversible dissociation of ligand.
Considering that phosphine dissociation is typically
more rapid than CO dissociation under thermal condi-
tions with these systems, we tentatively conclude that
thermal reductive elimination occurs after irreversible
PPh3 dissociation. These mechanistic conclusions are
consistent with the previously determined mechanisms
for C-H bond-forming reductive elimination from Ir-
(III), as described above.
reductive elimination from elimination after irreversible
phosphine dissociation, were not large enough to evalu-
ate confidently from our data. However, the photo-
chemical experiments described below suggest that
elimination occurs after irreversible dissociation of a
ligand, as shown in Scheme 4.
Con clu sion s
P h otoch em ica lly In d u ced Red u ctive Elim in a -
tion of P yr r ole fr om 2. We investigated potential
reductive elimination from 2 after photochemically
induced ligand dissociation. As shown in eq 2, complex
2 underwent reductive elimination of pyrrole in 82%
yield upon irradiation in benzene solution with a
medium-pressure Hg arc lamp. The hydride complex
The Ir(I) pyrrolyl complex (PPh3)2(CO)Ir(NC4H4) (1)
undergoes oxidative addition reactions with substrates
such as MeI and H2 in a manner similar to Vaska’s
complex, (PPh3)2(CO)IrCl. Complex 1 also undergoes
initial oxidative addition and reductive elimination re-
actions with a variety of substrates to form N-substi-
tuted pyrroles, X-NC4H4 (X ) C(O)CH3, C(O)C6H4CH3,
H, SnMe3, SiMe3, SiEt3, Bcat). This reactivity differs
from that seen for free pyrrole and strongly suggests
that the Ir center is directly involved in the formation
of the N-substituted pyrroles. Complex 1 reversibly
adds H2 to form the Ir(III) dihydride (PPh3)2(CO)Ir(H2)-
(NC4H4) (2). When 2 is heated in the presence of PPh3
under an atmosphere of H2, it undergoes N-H bond-
forming reductive elimination to give pyrrole and the
Ir(I) hydride 6. The pyrrole is formed through a
mechanism that does not involve reversible phosphine
dissociation before reductive elimination. Instead, re-
ductive elimination more likely occurs by irreversible
dissociation of CO or PPh3 or without any ligand
dissociation. The observation of photochemically in-
duced reductive elimination, which does not show any
isotope effect, suggests that elimination of pyrrole can
proceed after irreversible ligand dissociation and that
this pathway may be occurring in both the thermal and
photochemically induced reductive elimination.
1
6 was the only Ir species observed by either H or 31P-
{1H} NMR spectroscopy. Irradiation of a solution of 2
with free PPh3 under a pressure of CO again led to the
formation of pyrrole in 88% yield. In this case, the Ir
product observed by 1H and 31P{1H} NMR spectroscopy
was (PPh3)2(CO)2Ir(H).85
Qualitative studies indicated that the conversion of
2 to pyrrole at various reaction times was unaffected
by added PPh3 or CO. Also, side-by-side reactions of
2-d 2 and 2 showed no apparent isotope effect, suggesting
that the Ir-H bond is broken after an irreversible
photochemical step. This irreversible step is likely to
be ligand dissociation. Thus, reductive elimination of
pyrrole is likely to occur rapidly after irreversible
photochemical dissociation of PPh3 or CO (Scheme 4).
Because the rate of elimination was unaffected by either
added PPh3 or CO, our data does not reveal which ligand
is lost before elimination occurs; however, it is well-
known that irradiation of Ir(III) derivatives of Vaska-
type complexes leads to facile CO dissociation. Previous
studies conducted on the photochemically induced elimi-
nation of H2 from Ir(III) dihydrides93 (PPh3)2(CO)Ir(H)2-
Cl and (PPh3)3Ir(H)2Cl occurred by formation of a
common intermediate (PPh3)2IrCl, which was not the
result of secondary photolysis of the expected square-
planar Ir(I) products. Thus, CO was preferentially lost
from the Ir(III) starting complex (PPh3)2(CO)Ir(H)2Cl,
which is analogous to 2, to give (PPh3)2Ir(H)2Cl. The
pentacoordinate Ir(III) complex (PPh3)2Ir(H)2Cl, which
is analogous to the potential intermediate (PPh3)2Ir(H)2-
(NC4H4), then undergoes elimination of H2 to give
(PPh3)2IrCl.
Exp er im en ta l Section
Gen er a l Meth od s. Unless otherwise noted, all reactions
and manipulations were performed in an inert atmosphere
glovebox or by using standard Schlenk techniques. All 31P-
{1H} NMR chemical shifts are reported in parts per million
relative to an 85% H3PO4 external standard. Shifts downfield
of the standard are reported as positive. Benzene, toluene,
THF, ether, and pentane solvents were distilled from sodium/
benzophenone prior to use. Methylene chloride was degassed
and dried over calcium hydride. Acetyl chloride was purified
by literature procedures.94 Triphenylphosphine was sublimed
prior to use in kinetic reactions. Vaska’s complex was
prepared according to published methods.95 Sodium pyrrolyl
was prepared by the addition of sodium hydride to a pentane
solution of pyrrole and isolated as a white powder. Cat-
echolborane was vacuum distilled prior to use. All other
chemicals were used as received from commercial suppliers.
P r ep a r a tion of tr a n s-(P P h 3)2(CO)Ir (NC4H4). Into a 20
mL vial was weighed 88.6 mg (0.114 mmol) of trans-(PPh3)2-
Thus, we suggest that the photochemically induced
reductive elimination of pyrrole occurs via irreversible
(94) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(95) Collman, J . P.; Sears, C. T.; Varieze, K.; Kubota, M. Inorg.
Synth. 1990, 28, 92.
(93) Wink, D. A.; Ford, P. C. J . Am. Chem. Soc. 1986, 108, 4838.

Reductive Elimination of Pyrrole
Organometallics, Vol. 17, No. 6, 1998 1141
(CO)IrCl. The solid was dissolved in 8 mL of THF with
stirring. NaNC4H4 (10.7 mg, 0.120 mmol) was added as a
solid. The reaction was stirred at room temperature for 2 h,
after which time the solvent was removed under vacuum. The
residue was dissolved in 5 mL of CH2Cl2 and filtered through
Celite. The resulting solution was concentrated, layered with
Et2O, and cooled at -35 °C for 12 h to obtain 80.1 mg (87%) of
yellow crystalline material. 1H NMR (C6D6): δ 6.41 (t, 1.6 Hz,
2H), 6.44 (t, 1.6 Hz, 2H), 6.97-7.03 (m, 18H), 7.51-7.54 (m,
12H). 13C{1H} NMR (CD2Cl2): δ 107.3 (s), 127.6 (s), 128.2 (s),
130.3 (s), 132.3 (br), 134.4 (s), 179.7 (s). 31P{1H} NMR (C6H6):
δ 20.6 (s);.IR (cm-1), (KBr): 3059 (m), 2969 (w), 2923 (w), 2859
(w), 1964 (s, νCO), 1919 (w), 1668 (w), 1586 (m), 1567 (m), 1479
(m), 1434 (s), 1311 (w), 1261 (w), 1183 (m), 1167 (m), 1091 (s),
1048 (s), 1039 (m), 1027 (m), 999 (m), 812 (m), 749 (s), 722 (s),
693 (s), 621 (w), 605 (m), 569 (w), 520 (s). Anal. Calcd for
C41H34NIrOP2: C, 60.65; H; 4.22; N, 1.73. Found: C, 60.51;
H, 4.29; N, 1.61.
taken. Integration of the spectrum gave a ratio of products
(PPh3)2(CO)Ir(Me)(NC4H4)I:(PPh3)2(CO)Ir(Me)(Cl)I of 2:1.
Th er m olysis of (P P h 3)2(CO)Ir (H)2(NC4H4). (PPh3)2(CO)-
Ir(H)2(NC4H4) (7.4 mg, 0.0091 mmol) was weighed into a small
vial and dissolved in 0.6 mL of THF. The solution was
transferred to an NMR tube. The NMR tube was then
equipped with a vacuum adapter, frozen in N2(l), evacuated,
and flame-sealed. The solution was thawed, and the sample
was heated at 95 °C for 4 h. A 31P{1H} NMR spectrum of the
reaction solution showed complete conversion of the starting
material to (PPh3)2(CO)Ir(NC4H4).
Th er m olysis of (P P h 3)2(CO)Ir (Me)(NC4H4)I. (PPh3)2-
(CO)Ir(Me)(NC4H4)I (10.1 mg, 0.0106 mmol) was weighed into
a small vial and dissolved in 0.6 mL of THF. The solution
was transferred to an NMR tube. The NMR tube was then
equipped with a vacuum adapter, frozen in N2(l), evacuated,
and flame-sealed. The solution was thawed, and then the
reaction was heated at 130 °C for 12 h.
A
31P{1H} NMR
P r epar ation of (P P h 3)2(CO)Ir (H)2(NC4H4). trans-(PPh3)2-
(CO)Ir(NC4H4) (260 mg, 0.32 mmol) was dissolved in 40 mL
of benzene and transferred to a flask fused to a Kontes vacuum
adapter. The solution was frozen in N2(l), and the flask was
evacuated. A pressure of 400 Torr of H2 was added to the
cooled flask to provide a H2 pressure of ∼2 atm at room
temperature. The flask was sealed, and the solution was
thawed. The reaction was stirred at room temperature for 36
h, over which time the solution turned pale yellow. The final
reaction solution was transferred to a 50 mL round-bottomed
flask, and the benzene was removed under vacuum. The
product was recrystallized from THF/Et2O to give 223 mg
(86%) of (PPh3)2(CO)Ir(H)2(NC4H4). 1H NMR (C6D6): δ -15.84
(dt, 4.9, 15.3 Hz, 1H), -6.89 (dt, 4.9, 17.6 Hz, 1H), 6.37 (t, 1.5
Hz, 2H), 6.63 (t, 1.5 Hz, 2H), 6.95-7.10 (m, 18H), 7.52-7.60
(m, 12H). 13C{1H} NMR (C6D6): δ 108.9 (s), 128.4 (t, 5.2 Hz),
130.3 (s), 134.3 (t, 5.5 Hz), 134.4 (t, 28.7 Hz), 134.5 (s), 177.6
(t, 7.1 Hz). 31P{1H} NMR (C6H6): δ 7.0. IR (cm-1, KBr): 3051
(m), 2917 (w), 2183 (m, νIr-H), 2142 (m, νIr-H), 2075 (s, νCO),
1978 (m), 1966 (s), 1482 (s), 1464 (w), 1433 (s), 1390 (w), 1334
(w), 1308 (w), 1266 (w), 1190 (m),1169 (m), 1161 (m), 1095 (s),
1071 (w), 1046 (m), 1033 (m), 999 (m), 970 (w), 928 (w), 894
(m), 854 (m), 836 (s), 820 (m), 805 (m), 748 (s), 715 (s), 706 (s),
691 (s), 649 (m), 618 (w), 544 (w), 523 (s), 512 (s). Anal. Calcd
for C41H36NIrOP2: C, 60.58; H, 4.46; N, 1.72. Found: C, 60.50;
H, 4.79; N, 1.62.
Ad d ition of MeI to tr a n s-(P P h 3)2(CO)Ir (NC4H4). trans-
(PPh3)2(CO)Ir(NC4H4) (100 mg, 0.123 mmol) was weighed into
a 20 mL vial and dissolved in 12 mL of THF with stirring.
MeI (8.4 µL, 0.135 mmol) was added to the vial by syringe.
The vial was capped, and the reaction was allowed to stir at
room temperature for 10 h. The THF was concentrated and
layered with Et2O. The vial was cooled at -35 °C for 12 h to
obtain 84 mg (72%) of a white crystalline material of (PPh3)2-
(CO)Ir(Me)(NC4H4)I. 1H NMR (C6D6): δ 1.26 (t, 4.9 Hz, 3H),
6.47 (m, 2H), 6.91-7.10 (m, 20H), 7.36 (m, 12H). 31P{1H} NMR
(C6H6): δ -14.8 (s). IR (cm-1, KBr): 3140 (w), 3125 (w), 3076
(w), 3053 (w), 2919 (w), 2849 (w), 2015 (s, νCO), 1589 (w), 1572
(w), 1483 (m), 1454 (w), 1429 (m), 1393 (w), 1317 (w), 1247
(m), 1189 (m), 1162 (m), 1093 (s), 1032 (m), 1000 (m), 822 (m),
744 (s), 728 (s), 691 (s). Anal. Calcd for C42H37NIrOP2I: C,
52.94; H, 3.91; N, 1.47. Found: C, 53.17; H, 4.22; N, 1.44.
Ad d it ion of MeI t o tr a n s-(P P h 3)2(CO)Ir (NC4H 4) a n d
tr a n s-(P P h 3)2(CO)Ir Cl. trans-(PPh3)2(CO)Ir(NC4H4) (9.9 mg,
0.012 mmol) was weighed into vial and dissolved in 1.0 mL of
THF. trans-(PPh3)2(CO)Ir(NC4H4) (9.5 mg, 0.012 mmol) was
weighed into another vial and dissolved in 1.0 mL of THF.
The two THF solutions were combined in a 5 mL vial. A 10
µL amount of MeI was diluted with 2.0 mL of THF to make a
0.080 M solution. An aliquot of the MeI solution (15.2 µL,
0.0012 mmol) was added to the combined Ir solutions. The
reaction vial was capped and stirred at room temperature for
12 h. A 31P{1H} NMR spectrum of the reaction solution was
spectrum of the reaction solution showed complete conversion
of the starting material to (PPh3)2(CO)IrI (δ 25.1). A GC/MS
of the reaction solution indicated that free pyrrole was the only
organic product formed. Upon repeating the reaction in
benzene-d6, no formation of methane or ethane was observed
1
by H NMR spectroscopy.
Rea ction of tr a n s-(P P h 3)2(CO)Ir (NC4H4) w ith Acetyl
Ch lor id e. trans-(PPh3)2(CO)Ir(NC4H4) (7.2 mg, 0.0089 mmol)
was weighed into a vial containing a small amount (<2 mg)
of trimethoxybenzene. The solids were dissolved in benzene-
d6 and transferred to an NMR tube. Initial 1H and 31P{1H}
NMR spectra were taken. The NMR tube was then equipped
with a vacuum adapter, frozen in N2(l), and evacuated. Acetyl
chloride (1.4 mg, 0.018 mmol) was transferred into the tube
as a gas using a bulb of known volume. The tube was flame-
sealed and thawed. The reaction mixture was left at room
temperature for 1 h, and a 31P{1H} NMR spectrum was taken.
All of the starting complex had been converted to the oxidative-
addition product (PPh3)2(CO)Ir(NC4H4)[C(O)CH3](Cl) (δ )
-15.4). The reaction was heated at 70 °C for 3 h. The 1H
NMR spectrum indicated the formation of pyrrole (60%) and
N-acetylpyrrole (11.7%). (PPh3)2(CO)Ir[C(O)CH3](Cl)2 (δ )
-18.8) was the only phosphorus-containing iridium product
observed by 31P{1H} NMR spectroscopy.
Rea ction of tr a n s-(P P h 3)2(CO)Ir (NC4H4) w ith p-Tolu -
oyl Ch lor id e. trans-(PPh3)2(CO)Ir(NC4H4) (5.2 mg, 0.0064
mmol) was weighed into a vial and dissolved in toluene-d8
containing some pentane, which was used as an internal
standard. The solution was transferred to an NMR tube. An
1
initial H NMR spectrum was taken. p-Toluoyl chloride (2.1
mg, 0.014 mmol) was added to the NMR tube. The tube was
heated at 80 °C for 15 h. The 1H spectrum indicated the
formation of pyrrole (59%) and N-p-toluoylpyrrole (16.7%).
Gen er a l Rea ction of tr a n s-(P P h 3)2(CO)Ir (NC4H4) w ith
Sn - a n d Si-Con t a in in g Su bst r a t es. trans-(PPh3)2(CO)Ir-
(NC4H4) was weighed into a vial and dissolved in benzene-d6
containing a small amount of Et2O, which was used as an
1
internal standard. Initial H and 31P{1H} NMR spectra were
taken. The main-group reagent was then added to the NMR
tube. The reaction was left at room temperature for 2 h. The
31P{1H} NMR spectrum indicated no reaction. The tube was
1
heated at 80 °C for 14 h. A H NMR spectrum was taken at
the end of the reaction to determine the organic products
formed. The yields of the products were measured relative to
an internal Et2O standard. A combination of 1H and 31P{1H}
NMR spectra were used to identify the final Ir product.
Rea ction of 2 w ith Tr im eth yltin Ch lor id e. The general
procedure using 7.1 mg (0.0088 mmol) of 2 and 0.012 mmol of
trimethyltin chloride gave N-(trimethylstannyl)pyrrole in 69%
yield. (PPh3)2(CO)Ir(Cl) was the only metal product observed.
Rea ction of 2 w ith Tr im eth ylsilyl Ch lor id e. The gen-
eral procedure using 7.6 mg (0.0094 mmol) of 2 and 3.0 mg of

1142 Organometallics, Vol. 17, No. 6, 1998
Driver and Hartwig
trimethylsilyl chloride (0.028 mmol) gave N-(trimethylsilyl)-
pyrrole (43%) along with (PPh3)2(CO)Ir(Cl) (47%).
in N2(l), evacuated, and charged with 500 Torr of H2. The tube
was flame-sealed, and the sample was thawed. The samples
were immersed in a 95 °C constant-temperature water bath.
The progress of the reaction was followed by 31P{1H} NMR
spectroscopy. Qualitative comparison of the NMR spectra
indicated no significant difference in the rates of reaction.
Qu a lita tive Com p a r ison of Rea ction Ra tes of P yr r ole
Elim in a tion fr om (P P h 3)2(CO)Ir (H)2(NC4H4) w ith Ad d ed
CO. Into a vial was weighed 26.1 mg (0.032 mmol) of (PPh3)2-
(CO)Ir(H)2(NC4H4) and 42.2 mg (0.161 mmol) of PPh3. The
solids were dissolved in 2.3 mL of toluene to make a stock
solution. Aliquots (0.5 mL, 0.007 mmol of 2) of the stock
solution were transferred by syringe to two medium-walled
NMR tubes. The tubes were equipped with vacuum adapters,
frozen in N2(l), and evacuated. One tube was charged with
250 Torr of H2 and flamed-sealed. A second tube was charged
with a mixture of 250 Torr of H2 and 250 Torr of CO (gases
mixed in the vacuum line) and flame-sealed. The tubes were
thawed, and 31P {1H} NMR spectra were taken. The tubes
were heated simultaneously at 95 °C in a constant-tempera-
ture water bath, and 31P {1H} NMR spectra were obtained at
approximately 10 h intervals.
Rea ction of 2 w ith Tr ieth ylsila n e. The general proce-
dure using 10.7 mg (0.0132 mmol) of 2 and 3.1 mg (0.026
mmol) of triethylsilane gave N-(triethylsilyl)pyrrole in 64%
yield along with (PPh3)2(CO)Ir(H)2(SiEt3) in 66% yield.
Reaction of tr a n s-(P P h 3)2(CO)Ir (NC4H4) with 2-Ch lor o-
1,3,2-ben zod ioxa bor ole. trans-(PPh3)2(CO)Ir(NC4H4) (6.0
mg, 0.0074 mmol) was weighed into a vial and dissolved in
benzene-d6 containing some toluene, which was used as an
internal standard. The solution was transferred into an NMR
tube. Initial 1H and 31P{1H} NMR spectra were taken.
2-Chloro-1,3,2-benzodioxaborole (2.6 mg, 0.017 mmol) was
added to the NMR tube. The tube was left at room temper-
1
ature for 1 h. The H NMR spectrum indicated the formation
of 2-pyrrolyl-1,3,2-benzodioxaborole (78%) (11B NMR δ ) 24)
with (PPh3)2(CO)Ir(Cl) (89%). No other metal products were
observed in the 31P{1H} NMR spectrum.
Rea ction of tr a n s-(P P h 3)2(CO)Ir (NC4H4) w ith 1,3,2-
Ben zod ioxa bor ole. trans-(PPh3)2(CO)Ir(NC4H4) (5.6 mg,
0.0069 mmol) and PPh3 (2.8 mg, 0.011 mmol) were weighed
into a vial and dissolved in benzene-d6 containing some
toluene, which was used as an internal standard. The solution
was transferred into an NMR tube. Initial 1H and 31P{1H}
NMR spectra were taken. 1,3,2-Benzodioxaborole (1.1 mg,
0.0094 mmol) was added to the NMR tube. The tube was left
at room temperature for 1 h. The 1H NMR spectrum indicated
the formation of 2-pyrrolyl-1,3,2-benzodioxaborole (82%) (11B
NMR δ ) 24). (PPh3)3(CO)Ir(H) was the only metal product
observed in the 31P{1H} NMR spectrum.
Th er m a l R ed u ct ive E lim in a t ion of P yr r ole fr om
(P P h 3)2(CO)Ir (H)2(NC4H4). (PPh3)2(CO)Ir(H)2(NC4H4) (7.8
mg, 0.0096 mmol) was weighed into a vial. PPh3 (12 mg, 0.041
mmol) was weighed into another vial and dissolved in 0.6 mL
of THF. The phosphine solution was transferred into the vial
containing the Ir complex. Upon dissolution, the solution was
transferred to a medium-walled NMR tube. The NMR tube
was equipped with a vacuum adapter in the drybox, frozen in
N2(l), evacuated, and charged with 500 Torr of H2. The tube
was flame-sealed, and then the reaction solution was thawed.
The reaction was heated at 130 °C until all of the starting
material had been converted to (PPh3)3(CO)Ir(H) by 31P {1H}
NMR spectroscopy (approximately 3 days). A GC/MS of the
reaction mixture indicated the formation of free pyrrole. Upon
repeating the reaction in toluene-d8, 1H NMR spectroscopy
using dimethoxyethane as an internal standard indicated the
formation of pyrrole in 91% yield.
Mea su r em en t of Keq for th e Loss of H2 fr om (P P h 3)2-
(CO)Ir (H)2(NC4H4) a t 95 °C. (PPh3)2(CO)Ir(H)2(NC4H4) (8.4
mg, 0.010 mmol) was weighed into a small vial and dissolved
in 0.4 mL of toluene-d8. The solution was transferred to a
medium-walled NMR tube. The NMR tube was equipped with
a vacuum adapter, frozen in N2(l), evacuated, and charged with
500 Torr of H2. The tube was flame-sealed, and the sample
1
was thawed. A H NMR spectrum was obtained. The NMR
tube was heated at 95 °C in the NMR probe for 1 h.
A
1H
NMR spectrum was obtained at 95 °C. The concentrations of
(PPh3)2(CO)Ir(H)2(NC4H4), (PPh3)2(CO)Ir(H)2(NC4H4), and H2
were determined relative to an internal Et2O standard. The
Keq was calculated to be 2.0 × 10-3 M.
R ea ct ion of (P P h 3)2(CO)Ir (H )2(NC4H 4) w it h H 2O.
(PPh3)2(CO)Ir(H)2(NC4H4) (26.6 mg, 0.0328 mmol) was weighed
into a vial along with 60.1 mg (0.229 mmol) of PPh3. The solids
were dissolved in 1.8 mL of toluene-d8 to make a stock solution.
A small amount of dimethoxyethane was added to the solution
for use as an internal standard. Aliquots (0.4 mL, 0.00728
mmol of 2) of the stock solution were transferred to each of
four medium-walled NMR tubes by syringe. The tubes were
sealed with rubber septa and removed from the drybox.
Degassed water (0.6 µL, 0.036 mmol; 1.2 µL, 0.073 mmol; 1.8
µL, 0.10 mmol; 2.4 µL, 0.13 mmol) was added to the NMR
tubes. The tubes were equipped with a vacuum-line adapter,
frozen in N2(l), and evacuated. Each of the NMR tubes was
charged with 500 Torr of H2 and flame-sealed. The reaction
was conducted at 95 °C in a constant-temperature water bath.
1H NMR spectra were obtained after heating the samples for
4 h and then standing at room temperature for 2 h. The
concentration of 2 was monitored throughout the course of the
reaction by integration of the pyrrolyl resonance relative to
the dimethoxyethane internal standard.
Kin etic An a lysis of th e Th er m a l Red u ctive Elim in a -
tion of (P P h 3)2(CO)Ir (H)2(NC4H4). (PPh3)2(CO)Ir(H)2(NC4H4)
(23.5 mg, 0.0289 mmol) was weighed into a vial and dissolved
in 1.4 mL of toluene-d8. Dimethoxyethane (2.3 µL, 0.0066
mmol) was added to the solution as an internal standard. PPh3
was weighed into a vial, and a 0.3 mL aliquot (0.0062 mmol)
of the Ir solution was added to the vial. When used, the
pyrrole (0.8 µL, 0.01 mmol) was added by syringe at this point.
The solution was then transferred into a medium-walled NMR
tube. The tubes were frozen in N2(l), evacuated, and charged
with 500 Torr of H2. The tubes were then flame-sealed. The
sample was immersed in a 95 °C constant-temperature water
P h otoch em ica l Red u ctive Elim in a tion of (P P h 3)2(CO)-
Ir (H)2(NC4H4). Rea ction in th e P r esen ce of Excess P P h 3.
(PPh3)2(CO)Ir(H)2(NC4H4) (6.8 mg, 0.0084 mmol) and PPh3 (6.1
mg, 0.023 mmol) were weighed into a vial and dissolved in
0.5 mL of benzene-d6 containing some toluene, which was used
as an internal standard. An initial 1H NMR spectrum was
taken. The sample was irradiated with a 450 W Hg arc lamp
for 1 h. The 1H NMR spectrum indicated the formation of
pyrrole in 85% yield. (PPh3)3(CO)Ir(H) was the only metal
product observed in the 31P{1H} NMR spectrum.
1
bath, and H NMR spectra were obtained after heating for 10
h and standing at room temperature for a minimum of 3 h to
convert all of the Ir material from 1 to 2. The concentration
of 2 was monitored throughout the course of the reaction by
integration of a pyrrolyl resonance relative to the dimethoxy-
ethane internal standard.
Solven t Effect. Two samples of (PPh3)2(CO)Ir(H)2(NC4H4)
(7.2 mg, 0.0089 mmol) were weighed into separate vials. PPh3
(11 mg, 0.042 mmol) was weighed into each vial. The solids
were dissolved in 0.3 mL of toluene and THF, respectively.
The solutions were transferred to medium-walled NMR tubes.
The NMR tubes were equipped with a vacuum adapter, frozen
Rea ction in th e P r esen ce of Excess CO a n d P P h 3.
(PPh3)2(CO)Ir(H)2(NC4H4) (8.1 mg, 0.010 mmol) and PPh3 (4.2
mg, 0.016 mmol) were weighed into a vial and dissolved in
0.5 mL of benzene-d6 containing some toluene, which was used
as an internal standard. The solution was transferred into
an NMR tube, which was frozen in N2(l), evacuated, and

Reductive Elimination of Pyrrole
Organometallics, Vol. 17, No. 6, 1998 1143
charged with 300 Torr of CO. The tube was flame-sealed and
thawed. An initial 1H NMR spectrum was taken. The sample
was irradiated with a 450 W Hg arc lamp for 1 h. The 1H
NMR spectrum indicated the formation of pyrrole in 87% yield.
(PPh3)2(CO)2Ir(H) was the only metal product observed in the
31P{1H} NMR spectrum.
stants and an orientation matrix for data collection, obtained
from a least-squares refinement using the setting angles of
25 carefully centered reflections in the range 10.27 < 2θ <
17.98°, corresponded to a primitive triclinic cell. For Z ) 2
and fw ) 840.92, the calculated density was 1.52 g/cm3. On
the basis of a statistical analysis of the intensity distribution
and the successful solution and refinement of the structure,
the space group was determined to be P1h (No. 2).
The data were collected at a temperature of -90 ( 1 °C.
Of the 7682 reflections that were collected, 7428 were unique
(Rint ) 0.024). No decay correction was applied. An empirical
absorption correction based on azimuthal scans of several
reflections was applied, which resulted in transmission factors
ranging from 0.73 to 1.00. The data were corrected for Lorentz
and polarization effects.
The structure was solved by heavy-atom Patterson methods
and expanded using Fourier techniques. Most non-hydrogen
atoms were refined anisotropically, while the disordered
solvent molecule atoms were refined isotropically. The pen-
tane molecule was located on a crystallographic inversion
center (refining in P1 did not remove this disorder) with the
terminal carbon refined at 50% occupancy in either position.
Hydrogen atoms were included but not refined. The final cycle
of full-matrix least-squares refinement was based on 6426
observed reflections (I > 3.00σ(I)) and 427 variable parameters
and converged (largest parameter shift was 0.00 times its esd)
with unweighted and weighted agreement factors of R ) σ||Fo|
- |Fc||/σ|Fo| ) 0.034, Rw ) [(σw(|Fo| - |Fc|)2/σwFo2)]1/2 ) 0.042.
The standard deviation of an observation of unit weight was
2.32. The maximum and minimum peaks on the final differ-
ence Fourier map corresponded to 2.31 and -1.45 e-/Å3,
respectively. All calculations were performed using the teXsan
crystallographic software package of Molecular Structure Corp.
Dep en d en ce of Con ver sion on [P P h 3]. (PPh3)2(CO)Ir-
(H)2(NC4H4) (14.6 mg, 0.0180 mmol) was weighed into a vial
and dissolved in 1.3 mL of benzene-d6. Into two vials was
weighed PPh3 (9.1 mg, 0.035 mmol or 27.2 mg, 0.104 mmol).
The phosphine was dissolved in a 0.5 mL aliquot of the reaction
solution. The solutions were transferred into two NMR tubes.
The samples were irradiated with a 450 W Hg arc lamp with
1H NMR spectra obtained periodically. The concentration of
the starting complex was monitored by the integration of a
pyrrolyl resonance relative to a pentane internal standard.
Dep en d en ce of Con ver sion on CO. (PPh3)2(CO)Ir(H)2-
(NC4H4) (15.1 mg, 0.0186 mmol) was weighed into a vial and
dissolved in 1.3 mL of benzene-d6. A 0.5 mL aliquot of the
solution was transferred into two NMR tubes. The samples
were frozen in N2(l), and CO was added (200 or 500 Torr at 77
K). The tubes were flame-sealed and thawed. The samples
were irradiated with a 450 W Hg arc lamp with 1H NMR
spectra obtained periodically. The concentration of the start-
ing complex was monitored by the integration of a pyrrolyl
resonance relative to a pentane internal standard.
Isotop e Effect Mea su r em en t. (PPh3)2(CO)Ir(H)2(NC4H4)
(6.4 mg, 0.0079 mmol) or (PPh3)2(CO)Ir(D)2(NC4H4) (6.2 mg,
0.0076 mmol) was weighed into a vial and dissolved in 0.5 mL
of toluene-d8. PPh3 (10.3 mg, 0.039 mmol) was weighed into
a separate vial and dissolved in each of the reaction solutions.
The solution was transferred into an NMR tube. The samples
were irradiated with a 450 W Hg arc lamp with 1H NMR
spectra obtained periodically. The concentration of starting
complex was monitored by the integration of a pyrrolyl
resonance relative to a pentane internal standard.
Su p p or tin g In for m a tion Ava ila ble: Text giving the full
experimental details for the structure solution of 2 and tables
of experimental details, atomic coordinates, anisotropic dis-
placement parameters, bond lengths, bond angles, tension
angles, and nonbonded contacts (16 pages). Ordering informa-
tion is given on any current masthead page.
X-r a y Str u ctu r e of 2. The structure of 2 was solved by
methods described previously.45 A yellow prismatic crystal of
C
41H34NOP2Ir‚C2.5 having approximate dimensions of 0.19 ×
0.25 × 0.40 mm was mounted on a glass fiber. All measure-
ments were made on an Enraf-Nonius CAD4 diffractometer
with graphite-monochromated Mo KR radiation. Cell con-
OM971049P