Intra- and intermolecular C-H activation by bis(phenolate) pyridineiridium(III) complexes

Article abstract of DOI:10.1021/om201069k

A bis(phenolate)pyridine pincer ligand (henceforth abbreviated as ONO) has been employed to support a variety of iridium complexes in oxidation states I, III, and IV. Complexes (ONO)IrL₂Me (L = PPh₃, PEt ₃) react with I₂ to cleave the Ir-C bond and liberate MeI, apparently via a mechanism beginning with electron transfer to generate an intermediate Ir(IV) complex, which can be isolated and characterized for the case L = PEt₃. The PPh₃ complex is transformed in benzene at 65 °C to the corresponding phenyl complex, with loss of methane, and subsequently to a species resulting from metalation of a PPh₃ ligand. Labeling and kinetics studies indicate that PPh₃ is the initial site of C-H activation, even though the first observed product is that resulting from intermolecular benzene activation. C-H activation of acetonitrile has also been observed.

Full text of DOI:10.1021/om201069k

Article
pubs.acs.org/Organometallics
Intra- and Intermolecular C−H Activation by
Bis(phenolate)pyridineiridium(III) Complexes
Ross Fu, John E. Bercaw,* and Jay A. Labinger*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: A bis(phenolate)pyridine pincer ligand (henceforth abbreviated as ONO) has been employed to support a variety
of iridium complexes in oxidation states I, III, and IV. Complexes (ONO)IrL2Me (L = PPh3, PEt3) react with I2 to cleave the
Ir−C bond and liberate MeI, apparently via a mechanism beginning with electron transfer to generate an intermediate Ir(IV)
complex, which can be isolated and characterized for the case L = PEt3. The PPh3 complex is transformed in benzene at 65 °C
to the corresponding phenyl complex, with loss of methane, and subsequently to a species resulting from metalation of a
PPh3 ligand. Labeling and kinetics studies indicate that PPh3 is the initial site of C−H activation, even though the first
observed product is that resulting from intermolecular benzene activation. C−H activation of acetonitrile has also been
observed.
involves Pt(II) and Pt(IV). A separate program in our group
has examined dianionic tricoordinate (“LX2”) pincer ligands for
reactions involving high-valent early transition metal centers,7
and it occurred to us that such strongly electron donating, hard,
and robust ligands might also be useful here, to provide more
facile access to the relatively high Ir(V) oxidation state.
We previously reported some chemistry of an bis-
(phenolate)-N-heterocyclic carbene ligand (OCO) that sup-
ports complexes in the oxidation states Ir(I), Ir(III), and Ir(IV),
but no C−H activation chemistry was demonstrated.8 Here we
report on two related bis(phenolate)pyridine ligands (ONO =
pyridine-2,6-bis[2-(4,6-di-tert-butylphenolate)]; ONOtBu = 4-
tert-butylpyridine-2,6-bis[2-(4,6-di-tert-butylphenolate)]),
which also enable synthesis of a wide variety of Ir(I), Ir(III),
and Ir(IV) complexes. In addition, the complex (ONOtBu)-
IrIII(PPh3)2Me readily activates C−H bonds of benzene and
acetonitrile, as well as of coordinated triphenylphosphine.
INTRODUCTION
■
Functionalization of alkanes, such as the selective oxidation of
methane to methanol, has long been a desired goal of both
academic and industrial research.1 Much of the focus has been
on routes involving C−H activation at a transition metal center
followed by oxidative cleavage of the resulting C−M bond,
which offer greater possibilities for good selectivity by avoiding
radical-based pathways. (Yields of methanol from the direct
reaction of methane with dioxygen, where radical pathways
predominate, appear to be limited to about 5%, because
methanol is more rapidly oxidized than methane, a con-
sequence of its weaker C−H bond.1d) Platinum complexes have
received a great deal of attention,2 starting with Shilov’s seminal
work on the PtCl42−/PtCl6 system, which can oxidize
2−
methane to methanol with some selectivity,3 and including
the platinum bipyrimidine system, which has achieved the most
impressive performance to date, converting methane to methyl
bisulfate in up to 70% yield.4
RESULTS AND DISCUSSION
■
However, none of this platinum-based chemistry has yet
been shown to lead to a practical process, owing in part to low
reactivity, and a good deal of research activity has turned to
other metals. Iridium in particular has been a popular choice;
many (primarily low-valent) Ir complexes have been reported
to show good activity for C−H activation,5 and examples of
oxidative functionalization with Ir systems are known as well.6
Interconversions between Ir(III) and Ir(V) might be important
in this chemistry, by analogy to the Shilov system, which
Synthesis and Characterization of Ir(I) Complexes. Be-
cause Ir(III) is usually highly substitutionally inert, we decided
to enter the Ir(ONO) system via Ir(I). H2(ONO)9 was
doubly deprotonated with NaH, followed by metalation with
[Ir(cod)Cl]2, giving Na[Ir(cod)(ONO)], 1, which was
Received: November 2, 2011
Published: November 29, 2011
© 2011 American Chemical Society
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characterized by NMR and mass spectrometry. The symmetry
1
of the H NMR (only two tert-butyl peaks, for example) is
consistent with either a five-coordinate structure or a fluxional
four-coordinate structure, with the phenolates coordinated to
Na+ and Ir rapidly interchanging (1a and 1b, respectively, in
Scheme 1). The latter appears more likely on two grounds.
First, the related OCO complex of Ir(I) was crystallized (as the
[K(18-crown-6)]+ salt 2) and shown to have a structure
analogous to that of 1b.8 Second, attempted crystallization of 1
in the presence of 15-crown-5 (attempts to crystallize 1 without
15-crown-5 were unsuccessful) instead yielded 3, the result of
C−H activation at the 3-position of the central pyridine ring
(Figure 1).
Figure 1. X-ray structure of 3. The nearest Na(15-crown-5) is located
above the plane of the ligand and has been omitted for clarity. Because
of crystal sensitivity, the structure obtained was only of sufficient
quality to establish connectivity.
C−H activation probably results from complexation of the
sodium ion by 15-crown-5, making it too sterically bulky to
coordinate to the pendant phenolate in structure 1b and thus
labilizing the pyridine, which can flip over to allow the oxidative
addition of the 3-C−H bond; the resulting iridium(III) hydride
is deprotonated by the pendant phenolate to regenerate square-
planar Ir(I). This reactivity appears to require that a solution of
1 contains structure 1b, either as the sole species present or, at
least, in equilibrium with 1a. C−H activation in preference to
N-coordination of a pyridine ring has been observed previously
for an NHC-pyridine ligand;10 in that case (and probably here
as well) N-coordination is disfavored sterically.
the ligand, which was straightforward and analogous to that of
H2(ONO), is shown in Scheme 2. Deprotonation of
H2(ONOtBu) with NaH, followed by metalation with half an
equivalent (i.e., one Ir per ligand) of [Ir(cod)Cl]2, gave deep
red [NaIr(cod)(ONOtBu)]2 (4, Scheme 3), shown crystallo-
graphically to have a dimeric structure (Figure 2). Unlike ONO
complex 1, 4 is readily soluble in pentane, presumably because
the hydrophilic Na2O2 core is surrounded by a lipophilic shell
1
of tert-butyl-substituted phenyl rings. As with 1, the H NMR
spectrum indicates either a more symmetric structure in
solution or a fluxional process; on cooling to −100 °C, the
spectrum exhibited some line broadening, but no decoalescence
In order to prevent this unwanted ligand C−H activation, we
prepared H2(ONOtBu), with an additional tert-butyl group to
inhibit C−H activation of the pyridine linker. The synthesis of
Scheme 1
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Scheme 2
Scheme 3
of peaks. 4 reacts with CO to give Ir4(CO)12 and is reduced by
H2 to metallic Ir.
they behave as strong electrophiles, polymerizing THF and
slowly decomposing in other coordinating solvents.
If instead a full equivalent of [Ir(cod)Cl]2 is added to one
equivalent of Na2(ONOtBu), the orange diiridium complex
[Ir(cod)]2(ONOtBu) (5) is obtained. 5 may also be prepared
from 4 plus half an equivalent of [Ir(cod)Cl]2, but 4 is not
formed from 5 and additional Na2(ONOtBu) (Scheme 3). The
two iridium atoms in 5 are inequivalent: Ir1 is five-coordinate,
bound to both oxygens and the nitrogen of the ligand, while Ir2
is square planar and coordinated only to the two phenolate
oxygens (Figure 3). (The Ir1−N and Ir2−N distances are 2.07
and 3.00 Å, respectively.)
Synthesis and Characterization of Ir(III) Complexes.
Complex 4 is oxidized by AgPF6 or [FeCp2][PF6] in a mixture
of THF and acetonitrile to [(ONOtBu)Ir(cod)(MeCN)][PF6]
(6, Scheme 4), which was isolated as a crystalline product,
although it decomposes over time in the presence of excess
acetonitrile. Oxidation of 4 with AgOTf in benzene gave the
neutral triflate complex (ONOtBu)Ir(cod)OTf (7). Both com-
plexes exhibit approximately octahedral geometry;11 in solution
Treatment of 7 with two equivalents of triphenylphosphine
yielded yellow trans-(ONOtBu)Ir(PPh3)2OTf (8, eq 1). Attempts
to displace cyclooctadiene from 7 with triethylphosphine or tri-
cyclohexylphosphine led only to decomposition, while the
reaction of 7 with tri(o-tolyl)phosphine resulted in protonated
ligand, perhaps by benzylic C−H activation followed by
reductive elimination of phenol.
Exposure of 8 to water followed by addition of proton
sponge at room temperature gave trans-(ONOtBu)Ir(PPh3)2OH
(9, eq 2). Both 8 and 9 were converted to the corresponding
chloride (10) by heating at 90 °C in dichloromethane for
several hours or several days, respectively (eq 3). Formation of
10 from 8 or 9 is much faster in the presence of added
tetrabutylammonium chloride.
All three complexes (8, 9, and 10) react with proton sponge
to give the hydride (ONOtBu)Ir(PPh3)2H (11). The reaction
of 8 takes place over several days at room temperature or in
an hour at 90 °C, while 9 and 10 react more slowly at 90 °C.
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with I2 yields the iodide (ONOtBu)Ir(PPh3)2I (13) and methyl
iodide (Scheme 6).
Figure 2. X-ray structure of 4. The tert-butyl groups and a disordered
pentane solvent molecule have been removed for clarity. Selected
bond lengths (Å) and angles (deg): Ir−O1, 2.0990(1); Ir−N1,
2.1274(1); O1−Na1, 2.2145(1); O2−Na1, 2.1926(1); O2′−Na1,
2.2841(1); O1−Ir−N1, 81.268(3); O1−Na1−O2, 104.799(3); O1−
Na1−O2′, 106.989(3); O2−Na1−O2′, 100.484(2).
Figure 3. X-ray structure of 5. The tert-butyl groups have been
removed for clarity. Selected bond lengths (Å) and angles (deg): Ir1−
O1, 2.2701(1); Ir1−O2, 2.2962(1); Ir1−N1, 2.0818(1); Ir2−O1,
2.0808(1); Ir2−O2, 2.0771(1); Ir2−N1, 2.9953(1); O1−Ir1−O2,
68.864(1); N1−Ir1−O1, 76.957(2); N1−Ir1−O2, 75.747(1); O1−
Ir2−O2, 76.775(2); Ir1−O1−Ir2, 103.134(2); Ir1−O2−Ir2,
102.367(2).
The proposed mechanism for this transformation is single-
electron transfer followed by H-atom abstraction (Scheme 5);
the oxidized proton sponge byproduct shown was observed
(quantitatively) by NMR. A related reaction of proton sponge
with an Ir(III) complex has previously been reported.12 In
further support of this proposal, 11 was also obtained by
treatment of 8 with cobaltocene (a one-electron donor) and the
H atom donor 9,10-dihydroanthracene (DHA) or by heating 8
with sodium hydride. 11 can also be obtained from the reaction
of 8 with lithium methoxide or diethylzinc, presumably via
β-hydride elimination from an iridium-methoxy13 or iridium-
ethyl intermediate, respectively. The two latter routes give 11
most cleanly; in the reactions with proton sponge, we were
unable to separate 11 from the byproducts.
Crystal structures for complexes 8, 9, 11, 12, and 13 all
exhibit closely analogous octahedral geometries.11 The
structure of 12 is shown in Figure 4.
Treatment of 8 with dimethylzinc yielded the water-stable
Unlike 8, 12 reacts cleanly with triethylphosphine to give the
analogous methyl complex (14, Scheme 7). 14 appears to be
methyl complex (ONOtBu)Ir(PPh3)2Me (12). Oxidation of 12
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Scheme 4
Scheme 5
Scheme 6
more thermally stable than 12, slowly decomposing at
temperatures above 120 °C. It reacts similarly with I2, giving
methyl iodide and (ONOtBu)Ir(PEt3)2I (15). Both 14 and 15
were characterized crystallographically.11 Iodide complex 15 is
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MeI and some 15. All of this suggests that the reaction of 14 (or 12)
with I2 begins with a single electron transfer to generate an
intermediate cationic Ir(IV)-Me species, which undergoes SN2
attack by I− at the Ir−carbon bond. The CV data suggest that
further oxidization of 16 might be possible, perhaps even to
an Ir(V)-methyl complex, but no such species has yet been
isolated or otherwise identified. (The close similarity of the
second potentials, in contrast to the first, may indicate that
the second wave corresponds to oxidation of the ONO
ligand rather than at Ir.)
C−H Activation by Ir(III)-(ONOtBu) Complexes. In
benzene solution, (ONOtBu)Ir(PPh3)2Me converts over the
course of several hours at 65 °C to the analogous phenyl
complex (ONOtBu)Ir(PPh3)2Ph (17), with evolution of
methane. The benzene solution of 17 reacts further over
the course of several weeks at 65 °C to give a cyclometalated
product, (ONOtBu)Ir(PPh3)[κ2-PPh2(o-C6H4)] (18, eq 4).
Figure 4. Crystal structure of 12. Only ipso carbon atoms of the phenyl
groups on phosphorus are shown. Selected atom distances (Å) and
angles (deg): Ir−O1, 2.0901(1); Ir−O2, 2.0700(1); Ir−C74,
2.0952(1); Ir−N, 2.1042(1); Ir−P1, 2.3449(1); Ir−P2, 2.3895(1);
O1−Ir−O2, 176.495(3); P1−Ir−P2, 172.872(2); N−Ir−C74,
175.671(3); C74−Ir−O1, 95.588(2); C74−Ir−O2, 87.799(2); C74−
Ir−P1, 85.584(2); C74−Ir−P2, 87.309(2); O1−Ir−P1, 82.692(2);
O1−Ir−P2, 98.559(2); O2−Ir−P1, 96.698(2); O2−Ir−P2, 82.473(2);
N−Ir−O1, 88.429(2); N−Ir−O2, 88.212(2); N−Ir−P1, 96.553(2);
N−Ir−P2, 90.503(2).
unstable in methylene chloride, converting to the analogous
chloride over several days at room temperature. Such behavior
was not observed with the PPh3 analogue 13.
In both I2 oxidations a dark blue color is observed over much of
the course of the reaction, even though the starting complexes and
the products are all yellow. Cyclic voltammetry experiments show
quasi-reversible oxidations at 20 and 780 mV for 12 and reversible
oxidations at −100 and 840 mV for 14 (all versus ferrocene). The
reaction of 14 with AgPF6 gave dark blue [(ONOtBu)Ir(PEt3)2Me]-
PF6 (16, Scheme 7), characterized by a crystal structure and an
EPR spectrum (Figure 5) that are very similar to those of the
(relatively few) organometallic Ir(IV) species reported.14 16 could
be reduced back to 14 with either cobaltocene or methyllithium;
furthermore, treatment of 16 with tetrabutylammonium iodide gave
When 17 is isolated, redissolved in a solvent other than
benzene (such as p-xylene), and heated to 65 °C, conversion to
18 is complete within a few days. Both 1711 and 18 (Figure 6)
were characterized crystallographically.
Scheme 7
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Figure 5. EPR spectrum of [(ONOtBu)Ir(PEt3)2Me]PF6, 16, in dichloromethane at 20 K.
at 120 °C to give (ONOtBu)Ir(PEt3)2Ph and CH4 or CH3D,
respectively.
Thermolysis of 12 at 65 °C in methylene chloride, THF,
ether, or p-xylene leads directly to 18 (eq 6). These
transformations require less than one day for complete
conversion (in methylene chloride or p-xylene), comparable
to the time needed for conversion of 12 to 17 and much faster
than the conversion of 17 to 18 in benzene. Reaction of 12 in
toluene at 65 °C proceeds similarly to that in benzene;
although a single pure compound could not be isolated, the
mass spectrum of the mixture is consistent with formation of an
Ir(III) tolyl compound, (ONOtBu)Ir(PPh3)2(C6H4CH3). The
31P NMR shows two strong signals of roughly equal intensity
with shifts close to that of phenyl compound 17, suggesting the
formation of two isomers, presumably meta and para (since
p-xylene is unreactive); a much weaker nearby signal may
indicate a small amount of the ortho isomer.
Although 12 is insoluble in acetonitrile, thermolysis in
methylene chloride containing a small amount of acetonitrile
affords a mixture of 18 and a product resulting from C−H
activation of acetonitrile, [(ONOtBu)Ir(PPh3)(CH2CN)]2 (19);
with CD3CN, CH4 is the only methane isotopolog produced
(eq 7). 19 has the dimeric structure shown in Figure 7, where
Figure 6. Crystal structure of (ONOtBu)Ir(PPh3)[κ2-PPh2(o-C6H4)],
18. A dodecane solvent molecule and parts of several phenyl and tert-
butyl groups have been removed for clarity. Selected atom distances
(Å) and angles (deg): Ir−O1, 2.0696(1); Ir−O2, 2.0612(1); Ir−C46,
2.0669(1); Ir−N, 2.1156(1); Ir−P4, 2.3462(1); Ir−P5, 2.3483(1);
O1−Ir−O2, 174.874(3); P4−Ir−P5, 164.008(2); N−Ir−C46,
167.055(4); C46−Ir−O1, 100.779(3); C46−Ir−O2, 80.655(3);
C46−Ir−P4, 95.708(2); C46−Ir−P5, 68.499(2); O1−Ir−P4,
85.316(3); O1−Ir−P5, 94.944(2); O2−Ir−P4, 99.475(2); O2−Ir−P5,
80.959(3); N−Ir−O1, 88.656(3); N−Ir−O2, 89.198(3); N−Ir−P4,
93.867(3); N−Ir−P5, 102.125(3).
Thermolysis of 12 in C6D6 yields CH4 as the only detectable
methane isotopolog, and mass spectrometry of the resulting isolated
phenyl product 17 shows incorporation of six D atoms (eq 5).
In contrast, heating the (ONOtBu)Ir[P(C6D5)3]2(CH3) (12-d30)
in C6D6 yields only CH3D; this reaction is slower than that of
all-protio-12, exhibiting a KIE of 4 (vide infra). The PEt3 analog
14 reacts very slowly in C6H6 or C6D6, requiring several weeks
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product. Subsequent conversion to the thermodynamic product
18 is much slower.
C−H activation is shown in Scheme 8 as proceeding via
oxidative addition to give an Ir(V) intermediate, a mechanism
that has been supported by both experiment17 and
calculations18 for the above-mentioned16 Cp* system. It
seems reasonable that similar considerations would apply
here as well; but an alternative pathway, such as σ-bond
metathesis, cannot be excluded. In any case, conversion of A to
C must be irreversible, as no multiple H/D exchange is
observed, whereas conversion of C to F must be reversible,
since 17 eventually reverts to 18.
The fact that products are obtained from solvent activation
with toluene but not p-xylene suggests that C−H activation
ortho to a methyl substituent is sterically inhibited. As a
consequence, in the latter solvent C (or a π-arene adduct
analogous to D) has no alternative but to bind PPh3 and 18
forms directly, as it does in solvents (dichloromethane, ether,
THF) that are not susceptible to C−H activation at all. In
acetonitrile the two pathways are competitive. The reaction of
14 is much slower, presumably because PEt3 dissociates much
less readily, and leads only to a phenyl product, indicating
cyclometalation of PEt3 does not take place.
Kinetics of C−H Activation. The transformation of 12 to
17 and/or 18 is readily followed by 31P NMR, as each of the
species has a distinct spectrum. In early experiments results
were not always highly reproducible; most probably this is due
to oxidation of small amounts of PPh3 by adventitious
impurities, which could effect significant acceleration (vide
infra); indeed, deliberate addition of oxidants such as chloranil
and dioxygen did speed up conversion of 12 considerably.
However, reasonably reproducible rates (±15%) could be
achieved by careful attention to purification of starting complex
and solvent as well as exclusion of air.
Figure 7. Crystal structure of [(ONOtBu)Ir(PPh3)(μ-CH2CN)]2, 19.15
A benzene solvent molecule, phosphine phenyl groups, and (ONO)
tert-butyl groups have been removed for clarity. Selected atom
distances (Å) and angles (deg): Ir1−O1, 2.0190(2); Ir1−O2,
2.0883(2); Ir1−C104, 2.1472(2); Ir1−N1, 2.0758(2); Ir1−N4,
2.0882(2); Ir1−P1, 2.2654(2); N3−C103, 1.1271(1); C103−C104,
1.4380(1); N4−C102, 1.1278(1); C102−C101, 1.4482(2); Ir2−O3,
2.0912(2); Ir2−O4, 2.0396(2); Ir2−C101, 2.1740(3); Ir2−N2,
2.0854(3); Ir2−N3, 2.1290(2); Ir2−P2, 2.2574(2); O1−Ir1−O2,
173.203(10); N4−Ir1−P1, 173.670(9); N1−Ir1−C104, 170.591(10);
C104−Ir1−O1, 81.941(8); C104−Ir1−O2, 98.236(8); C104−Ir1−
N4, 86.476(8); C104−Ir1−P1, 90.036(7); O1−Ir1−N4, 88.426(8);
O1−Ir1−P1, 96.321(8); O2−Ir1−N4, 84.807(8); O2−Ir1−P1,
90.475(8); N1−Ir1−O1, 89.793(8); N1−Ir1−O2, 89.439(8); N1−
Ir1−N4, 88.823(8); N1−Ir1−P1, 95.360(8); Ir1−N4−C102,
156.881(14); N4−C102−C101, 171.704(16); C102−C101−Ir2,
104.11(1); Ir2−N3−C103, 156.797(13); N3−C103−C104,
172.115(16); C103−C104−Ir1, 105.731(10); O3−Ir2−O4,
174.306(10); N3−Ir2−P2, 171.891(9); N2−Ir2−C101, 172.156(10);
C101−Ir2−O3, 96.853(8); C101−Ir2−O4, 84.460(8); C101−Ir2−
N3, 85.543(7); C101−Ir2−P2, 88.633(7); O3−Ir2−N3, 85.175(8);
O3−Ir2−P2, 89.930(7); O4−Ir2−N3, 89.412(8); O4−Ir2−P2,
The somewhat simplified mechanism shown in Scheme 9
may be used for analysis; it leaves out the (unobserved)
intermediates B (which will rapidly convert to C) and D and E
(which are presumed to be in rapid equilibrium with C and F);
none of these should have any kinetic consequences. Three
transformations will be considered in turn: (1) conversion of
12 to 17 in benzene; (2) conversion of 12 to 18 in xylene; and
(3) conversion of 17 to 18 in benzene or xylene.
coordination of N from the (CH2CN) group bonded to each Ir
1
The first of these may be analyzed in terms of reversible
dissociation of PPh3 (henceforth abbreviated as L) followed by
rate-determining C−H activation (the k2 step). Since the
conversion of A to C is irreversible (vide supra), no buildup of
any intermediate is observed, and formation of 17 is essentially
complete before any 18 is observed. In this simplified
framework the rate law would be −d[12]/dt = d[17]/dt =
(k2K1[12])/[L], and the rate should be cleanly first-order in 12
and inverse-first-order in L. There is a potential complication if
no extra L is added: [L] will be governed by both K1 and K4, so
if these two equilibrium constants were significantly different,
[L] would vary in a complex manner as conversion of 12 to 17
proceeds, and the disappearance of 12/appearance of 17 might
not follow clean first-order kinetics. Nonetheless, it does: a
good exponential fit is achieved (Figure 8), with kobs = 1.8(3) ×
10−4 s−1 (equivalent to a half-life of about an hour), so any
difference in K values is small enough to neglect. The 12-d30
isotopolog behaves similarly, yielding kobs = 4.4(6) × 10−5 s−1
and hence KIE = 4(1),11 consistent with the assignment of C−H
activation of L as rate-determining.
has displaced a PPh3 ligand from the other. Although two H
NMR signals would be expected for the (CH2CN) methylene
protons, which are diastereotopic in the solid-state structure,
only one is observed; this could result from either accidental
degeneracy or dissociation of the dimeric structure in solution.
A plausible mechanism for the chemistry of 12 in benzene is
shown in Scheme 8. The sequence of product formation17
and 18 being preferred kinetically and thermodynamically,
respectively16would seem to suggest that the first C−H
activation is the intermolecular reaction with benzene. But that
is excluded by the isotopic labeling results. The H atom that
departs with methane clearly comes from a PPh3 ligand, not
solvent benzene, so PPh3 must be the site of the first C−H
activation, even though the corresponding product 18 does not
appear at all until much later. We propose therefore that
dissociation of one PPh3 ligand gives coordinatively unsaturated
A, followed by intramolecular C−H activation and loss of
methane to generate cyclometalated intermediate C. In
benzene solution coordination of PPh3 to C is not competitive;
instead C reacts with benzene to give 17 as initial observed
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Scheme 8
As expected, the addition of extra L slows the reaction
significantly; a plot of 1/kobs is linear in [L] (Figure 9). In
principle, at sufficiently high [L] some 18 should form directly
in competition with 17; in practice, such conditions are
probably not reachable (and conversion of 12 would become
extremely slow anyway).
by Scheme 9: the concentration of C, the precursor to 18, will
be inversely related to the concentration of benzene. The rate
law derived11 for that mechanism is −d[17]/dt = d[18]/dt =
(k‑4k5[17])/(k5 + k4K3[PhH]). In benzene solution the reaction
should be first-order in 17 and independent of [L]; both were
observed. In xylene, on the other hand, [PhH] is continuously
increasing as 17 is consumed, so first-order kineticswhich are
observedwould not be expected unless k5 is large compared
to k4K3[PhH] over the course of the reaction.
We can test this for self-consistency with the observed rate
constants, again assuming that there is little difference in
parameters between benzene and xylene reactions. For
conversion of 17 to 18 in benzene, kobs = 4.2(6) × 10−7 s−1.
Since no direct formation of 18 from 12 was observed, k5 must
be small compared to k4K3[PhH], so that value of kobs is
approximately equal to k‑4k5/k4K3[PhH]. For the reaction in
p-xylene, kobs = 2.9(4) × 10−6 s−1; if our assumption that k5 is
large compared to k4K3[PhH] is correct, that value of kobs = k−4.
Substituting this value into the above expression, we get
k5/k4K3[PhH] = 0.13, so k5 is not completely negligible com-
pared to k4K3[PhH], but small enough (within the limits
of reproducibility as well as the assumption of solvent-
independent parameters) to validate the approximation for kobs
in benzene. Furthermore, since for neat benzene [PhH] ≈ 11
M, k5/k4K3 = 1.4 M. For the reaction in p-xylene the starting
[17] = the maximum [PhH] = 8.8 mM and the minimum value
The rate law for direct conversion of 12 to 18 in p-xylene
should be identical to the above, and the reaction does exhibit
clean first-order kinetics,11 at a rate (kobs = 8(1) × 10−5 s−1)
about a factor of 2 slower than that for 12 to 17. There are
several possible explanations for the discrepancy. First, the
values of K1 and k2 need not be identical to those in benzene
(they would not be expected to differ very much). Second,
intermediate C may be complexed by xylene to give an analog
of intermediate D, which in this case (in contrast to benzene)
would not lead to any productive reaction, but would reduce
the concentration of C and hence the rate of formation of 18.
However, any substantial (i.e., sufficient to account for the
entire factor of 2) concentration of a new intermediate should
have been detectable by NMR. Finally (and perhaps most
likely), it may be that agreement within a factor of 2 (especially
on changing solvent) is as good as can be expected, given the
reproducibility considerations discussed earlier.
As noted above, the conversion of 17 to 18 is much slower in
benzene than in xylene, taking place over weeks or days,
respectively (Figure 10). This observation is readily explained
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Scheme 9
Figure 9. Dependence of the reciprocal of the rate of conversion of 12
to 17 on the amount of added triphenylphosphine.11
Figure 8. Kinetics of conversion of 12 to 17 in C6D6 at 65 °C.
of I2; there is evidence that the reaction proceeds via one-
electron oxidation to an (isolable) Ir(IV) methyl intermediate.
Triphenylphosphine complex 12 is able to activate C−H bonds
in benzene (and toluene) under quite mild conditions; the
mechanism follows an unusual pathway in which the first C−H
activation is intramolecular, but the product of that process,
although thermodynamically preferred, is kinetically disfavored
with respect to the intermolecular benzene activation product.
Unfortunately the (ONO)Ir(III) system is not sufficiently
reactive to activate nonaromatic C−H bonds (except for
acetonitrile).
of k5/k4K3[PhH] = 160, so the assumption required to account
for first-order kinetics is valid as well. Accordingly, the kinetics
appear completely consistent with the proposed mechanism.
CONCLUSIONS
■
The ONO ligand provides a versatile framework for synthesis
of a wide variety of Ir(I) and Ir(III) complexes. Using this
framework, two key steps in a scheme for functionalization of
hydrocarbons have been demonstrated. Conversion of the Ir−Me
bond in complexes 12 and 14 to MeI is effected by addition
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Figure 10. Kinetics for conversion of 17 to 18 in C6D6 (left) and p-xylene-d10 (right) at 65 °C.
sodium carbonate. The organic layer was extracted with methylene
chloride, filtered, and evaporated to dryness to yield yellow crystals
EXPERIMENTAL SECTION
■
General Methods. All compounds were prepared in a nitrogen-
filled glovebox unless otherwise specified. Reactions taking place in
dichloromethane at 90 °C were conducted in either sealed glass bombs
or J-Young NMR tubes. Solvents in the glovebox were dried using the
Grubbs method.19 All NMR solvents were purchased from Cambridge
Isotope Laboratories, Inc., (except for CDCl3) filtered through
alumina, and stored over 4 Å molecular sieves. POCl3 was purchased
from Acros Organics. n-BuLi, t-BuLi, 4-tert-butylpyridine, [FeCp2][PF6],
indene, N,N-dimethylaminoethanol, PPh3, propylene, proton sponge,
AgPF6, AgOTf, 15-crown-5, and NaH were purchased from Aldrich.
H2O2 was purchased from EMD. [Ir(cod)Cl]2 and Pd(PPh3)4 were
purchased from Strem. The ligand H2(ONO) was prepared as
previously described.9 1H, 13C, 19F, and 31P NMR spectra were
recorded on Varian Mercury 300 MHz NMR spectrometers. X-ray
data were collected using a Bruker KAPPA APEX II diffractometer.
Synthesis of 4-tert-Butylpyridine-N-oxide. Following a pub-
lished procedure,20 4-tert-butylpyridine (18.0 mL, 16.6 g, 123 mmol)
was mixed with 135 mL of glacial acetic acid and 100 mL of hydrogen
peroxide (30% in water) in a 1 L round-bottom flask under air. After
refluxing for 4 h, 100 mL of additional hydrogen peroxide (30% in
water) was added, and the mixture was refluxed overnight. Then 200
mL of the solvent was removed by distillation, and 100 mL water was
added and then removed by distillation as well. The remaining
solution was neutralized with sodium carbonate. The organic layer was
extracted with methylene chloride, filtered, and evaporated to dryness
to yield white crystals (which gave a yellow solution in CH2Cl2).
Isolated yield: 15.3 g, 101 mmol, 82%. 1H NMR (300 MHz, CDCl3): δ
1.31 (s, 9H, C(CH3)3), 7.26 (d, 2H, 3J = 7.2 Hz, aryl-H), 8.15 (d, 2H,
3J = 7.5 Hz, aryl-H). 13C{1H} NMR (75.4 MHz, CDCl3): δ 30.5
(C(CH3)3), 34.6 (C(CH3)3), 123.1, 138.6, 151.2 (aryl).
1
(10.3 g, 55.3 mmol, 70% yield). H NMR (300 MHz, CDCl3): δ 1.31
(s, 9H, C(CH3)3), 7.19 (dd, 1H, 3J = 6.6 Hz, 4J = 3.0 Hz, aryl-H), 7.44
4
3
5
(d, 1H, J = 2.7 Hz, aryl-H), 8.27 (dd, 1H, J = 6.6 Hz, J = 0.6 Hz,
aryl-H). 13C{1H} NMR (75.4 MHz, CDCl3): δ 30.4 (C(CH3)3), 34.7
(C(CH3)3), 121.3, 124.1, 139.8, 151.3 (aryl).
Synthesis of 2,6-Dichloro-4-tert-butylpyridine. In a 250 mL
round-bottom flask open to the air, 2-chloro-4-tert-butylpyridine-N-
oxide (15.3 g, 101 mmol) was dissolved in 40 mL of neat POCl3. The
mixture was allowed to reflux under argon for 12−16 h. Excess POCl3
was removed by distillation, and the remaining liquid was washed with
a saturated aqueous solution of sodium carbonate. The organic layer
was extracted with ether, filtered, and evaporated to dryness, yielding a
thick black oil. This oil was subjected to column chromatography over
silica (95:5 hexanes/ethyl acetate), affording brown crystals after
evaporation of solvent. The crystals were washed with hexanes, giving
white 2,6-dichloro-4-tert-butylpyridine (5.57 g, 27.3 mmol, 49% yield;
1
yield over four steps from 4-tert-butylpyridine: 22%). H NMR (300
MHz, CDCl3): δ 1.30 (s, 9H, C(CH3)3), 7.23 (s, 2H, aryl-H). 13C{1H}
NMR (75.5 MHz, CDCl3): δ 30.3 (C(CH3)3), 35.4 (C(CH3)3), 120.2,
150.6, 166.3 (aryl).
Synthesis of MOM2(ONOtBu). O-Methoxymethyl-2-bromo-4,6-di-
tert-butylphenoxide, S111 (6.15 g, 18.7 mmol), was dissolved in 60 mL
of THF. tert-butyllithium (2.52 g, 39.4 mmol) was dissolved in 27 mL
of pentane and added to the solution of S1 at −78 °C. The mixture
was allowed to warm to room temperature and stirred for 1 h. A
suspension of ZnCl2 (1.80 g, 13.2 mmol) in THF was then added, and
the mixture stirred for another 30 min. A solution of 2,6-dichloro-4-
tert-butylpyridine (1.72 g, 8.42 mmol) and Pd(PPh3)4 (218 mg, 189
μmol) in THF was added, the reaction vessel was transferred to a
preheated 75 °C oil bath, and the mixture was stirred for 16 h. The
mixture was quenched with 10 mL of water, and the volatiles were
removed by rotary evaporation. Another 70 mL of water was then
added, and the organic layer was extracted three times with ether,
dried with MgSO4, and evaporated to dryness, giving a white powder,
which was purified by washing with cold methanol (2.67 g, 4.23 mmol,
50.3% yield). 1H NMR (300 MHz, CDCl3): δ 1.35 (s, 18H,
C(CH3)3), 1.37 (s, 9H, C(CH3)3), 1.49 (s, 18H, C(CH3)3), 3.38 (s,
Synthesis of 2-Chloro-4-tert-butylpyridine. In a 250 mL
round-bottom flask open to the air, 4-tert-butylpyridine-N-oxide
(15.3 g, 101 mmol) was dissolved in 61 mL of neat POCl3. The
mixture was allowed to reflux under argon for 12−16 h (refluxing can
also be done under air, but the yield is somewhat lower). Excess POCl3
was removed by distillation, and the remaining liquid was washed with
a saturated aqueous solution of sodium carbonate. The organic layer
was extracted with ether, filtered, and evaporated to dryness, yielding a
4
6H, OCH3) 4.60 (s, 4H, OCH2O), 7.42 (d, 2H, J = 2.6 Hz, aryl-H),
1
7.56 (d, 2H, 4J = 2.6 Hz, aryl-H), 7.66 (s, 2H, NC5H2). 13C{1H} NMR
(75.4 MHz, CDCl3): δ 30.6, 30.9, 31.4, 34.6, 34.9, 35.4 (C(CH3)3),
57.3 (OCH3), 99.3 (OCH2O), 120.4, 124.8, 126.6, 134.3, 142.2, 145.8,
151.3, 158.0, 159.7 (aryl).
dark red oil (13.5 g, 79.3 mmol, 79% yield). H NMR (300 MHz,
3
4
CDCl3): δ 1.26 (s, 9H, C(CH3)3), 7.16 (dd, 1H, J = 5.4 Hz, J = 1.8
4
3
Hz, aryl-H), 7.25 (d, 1H, J = 1.8 Hz, aryl-H), 8.23 (dd, 1H, J = 5.7
5
Hz, J = 0.6 Hz, aryl-H). 13C{1H} NMR (75.4 MHz, CDCl3): δ 30.3
Synthesis of H2(ONOtBu). MOM2(ONOtBu) (2.67 g, 4.23 mmol)
was suspended in a mixture of methanol (40 mL) and concentrated
HCl (40 mL) and heated at 80 °C for 6 h. The suspension was then
cooled to 0 °C and filtered, and the precipitate, a white powder, was
washed with cold MeOH to afford H2(ONOtBu) (1.71 g, 3.14 mmol,
74% yield). 1H NMR (300 MHz, d8-THF): δ 1.36 (s, 18H, C(CH3)3),
(C(CH3)3), 35.0 (C(CH3)3), 119.7, 121.3, 149.4, 151.7, 163.5 (aryl).
Synthesis of 2-Chloro-4-tert-butylpyridine-N-oxide. In a 1 L
round-bottom flask, 2-chloro-4-tert-butylpyridine (13.5 g, 79.3 mmol)
was mixed with 100 mL of glacial acetic acid and 80 mL of hydrogen
peroxide (30% in water). After refluxing for 4 h under air, 80 mL of
additional hydrogen peroxide (30% in water) was added and the
mixture was refluxed overnight. Then 200 mL of the solvent was
removed by distillation, and 100 mL of water was added and then
distilled off as well. The remaining solution was neutralized with
4
1.46 (br, 27H, C(CH3)3), 7.42 (d, 2H, J = 2.4 Hz, aryl-H), 7.53 (d,
2H, 4J = 2.7 Hz, aryl-H), 7.81 (s, 2H, NC5H2), 10 (br s, concentration
dependent, 2H, OH). 13C{1H} NMR (75.4 MHz, CDCl3): δ 29.6,
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30.6, 31.6, 34.4, 35.3, 35.5 (C(CH3)3), 117.7, 121.7, 122.6, 126.0,
137.2, 141.2, 153.0, 157.2, 164.1 (aryl). MS FAB+: 543.4052 (M+).
Anal. Calcd for C37H53NO2: C, 81.72; H, 9.82; N, 2.58. Found: C,
81.57; H, 10.06; N, 2.55.
introduced into the headspace. The solution was then brought back to
room temperature and allowed to stir overnight. Afterward, the
solution was pumped down, yielding a yellow-brown solid with IR
stretching frequencies at 2057, 2030, and 1977 cm−1. This solid was
reintroduced into the glovebox, redissolved in pentane, and filtered.
Allowing the pentane solution to slowly evaporate yielded brown
crystals. X-ray diffraction of the crystals revealed their identity to be
Ir4(CO)12, consistent with the aforementioned IR stretching
frequencies.
Synthesis of Na[Ir(cod)ONO], 1. H2(ONO) (91.2 mg, 187
μmol) was dissolved in 10 mL of THF, and an excess of NaH (24.6
mg, 1.03 mmol) was added. The yellow suspension first turned orange
and then yellow again, with vigorous evolution of H2. The suspension
was allowed to stir for one hour, after which it was filtered through
Celite and added to a stirring solution of [Ir(cod)Cl]2 (60.0 mg, 89.3
μmol) in 5 mL of THF. After 30 min of stirring the solution was
filtered and pumped down, giving 1 as a red powder. Residual solvent
was removed by trituration with benzene followed by overnight
evacuation under high vacuum. Isolated yield was 148.3 mg (183.3
Reaction of 4 with H2. [NaIr(cod)(ONOtBu)]2 (10.2 mg, 5.89
μmol) was dissolved in 0.7 mL of d8-THF in a J-Young NMR tube.
Two drops of benzene were added as an internal standard. The
headspace was then charged with 2.9 atm H2, and the NMR tube
continuously inverted for mixing. After 11 h, the solution color had
lightened considerably, and an iridium mirror had deposited. 1H NMR
showed quantitative conversion to the protonated ligand H2(ONOtBu).
Synthesis of [Ir(cod)]2(ONOtBu), 5. H2(ONOtBu) (57.6 mg, 106
μmol) was dissolved in 5 mL of THF, and an excess of NaH (30.2 mg,
1.26 mmol) was added. The yellow suspension first turned orange and
then yellow again, with vigorous evolution of H2. The suspension was
allowed to stir for an hour, after which it was filtered through Celite.
A 5 mL THF solution of [Ir(cod)Cl]2 (71.1 mg, 107 μmol) was then
added while stirring. After 1 h of stirring, the solvent was removed in
vacuo; and the orange compound was redissolved in pentane and
filtered. The solvent was then removed in vacuo again, giving 5 as an
orange powder. Crystals were obtained by allowing a pentane solution
1
μmol, 98.0%); the yield was quantitative by NMR. H NMR (300
MHz, d8-THF): δ 0.84 (m, 4H, sp3 on cod), 1.32 (s, 18H, 2 t-Bu on
ligand phenolate), 1.49 (s, 18H, 2 t-butyl on ligand phenolate), 1.68
(br m, 4H, sp3 on cod), 2.99 (br d, 4H, J = 2.7 Hz, sp2 on cod), 7.19
(d, 2H, 4J = 2.7 Hz, ligand aryl), 7.24 (d, 2H, 4J = 2.6 Hz, ligand aryl),
7.26 (br s, 2H, ligand aryl), 7.66 (t, 1H, 3J = 8.0 Hz, ligand pyridine 4-
position). 13C{1H} NMR (75.4 MHz, d8-THF): δ 14.6, 23.4, 31.5,
32.5, 32.8, 34.7, 36.2, 49.7, 121.3, 123.8, 125.0, 131.9, 135.7, 137.2,
139.2, 159.4, 166.9. MS FAB+: 786.3856 (M+ − Na).
Synthesis of Pyridine-Activated Complex 3. Na[Ir(cod)-
(ONO)] (1) (85.1 mg, 105 μmol) was dissolved in 1 of mL of
C6H6. 15-Crown-5 (21 μL, 106 μmol) was mixed in, and the solution
was allowed to sit undisturbed for several days. Orange crystals formed
and were shown crystallographically to consist of 3. A pure (by NMR)
sample was obtained by washing with C6H6, redissolving in THF, and
drying under vacuum. Isolated yield: 82.7 mg (79.8 μmol, 75.9%).
1H NMR (500 MHz, d8-THF): δ 1.35 (s, 9H, t-Bu on ligand phenolate),
1.42 (s, 9H, t-Bu on ligand phenolate), 1.51 (br s, 18H, 2 t-Bu on
ligand phenolate), 1.77 (br, 4H, sp3 on cod), 2.18 (br, 4H, sp3 on cod),
3.29 (br, 2H, sp2 on cod), 3.37 (s, 20H, crown ether), 4.46 (br, 2H, sp2
on cod), 7.24 (d, 1H, 4J = 2.6 Hz, ligand aryl), 7.25 (d, 1H, 4J = 2.4 Hz,
ligand aryl), 7.48 (d, 1H, 3J = 8.4 Hz, ligand aryl), 7.78 (d, 1H, 4J = 2.4
Hz, ligand aryl), 7.91 (d, 1H, 3J = 8.2 Hz, ligand aryl), 8.45 (d, 1H, 4J =
2.6 Hz, ligand aryl), 18.14 (s, 1H, ligand phenol). 13C{1H } NMR (126
MHz, d8-THF): δ 30.4, 31.1, 31.6, 32.4, 32.9, 33.9, 35.1, 35.4, 36.1,
36.4, 45.1, 70.2, 74.4, 112.5, 120.2, 120.8, 123.0, 124.3, 124.4, 126.9,
135.0, 137.4, 138.5, 138.8, 146.5, 149.7, 152.6, 153.2, 159.7, 162.7.
Synthesis of [NaIr(cod)(ONOtBu)]2, 4. H2(ONOtBu) (246.8 mg,
453.8 μmol) was dissolved in 10 mL of THF, and an excess of NaH
(64.8 mg, 2.70 mmol) was added. The light yellow solution first
turned orange and then yellow again, with vigorous evolution of H2.
The suspension was allowed to stir for an hour, after which it was
filtered through Celite. [Ir(cod)Cl]2 (145.2 mg, 216.2 μmol) was
dissolved in THF and slowly added to the stirring solution of
Na2(ONOtBu) in order to prevent formation of overmetalated
[Ir(cod)]2(ONOtBu), 5. After an hour of stirring the solvent was
removed in vacuo, redissolved in hexanes, and filtered, and the solvent
removed in vacuo again, giving 4·THF as a red solid with a glassy
appearance. The residual THF was removed by adding more hexanes
and evacuating to give 4. 4·THF is very soluble in hexanes, whereas 4
is much less so; consequently impurities in 4 can be removed with a
hexanes wash. Crystals were obtained by allowing a pentane solution
of 4·THF or 4 to evaporate. Isolated yield was 326.4 mg (188.6 μmol,
87.2%). 1H NMR (300 MHz, d8-THF): δ 0.83 (br d, 16H, 2J = 7.5 Hz,
sp3 on 2 cod), 1.33 (s, 36H, 4 t-Bu on ligand phenolate), 1.37 (s, 18H,
2 t-Bu on ligand pyridine), 1.49 (s, 36H, 4 t-Bu on ligand phenolate),
1
to evaporate. Yield was 119.3 mg (104 μmol, 98.6%). H NMR (300
MHz, d8-THF): δ 0.81 (br d, 16H, 2J = 6.9 Hz, sp3 on 2 cod), 1.39 (s,
18H, 2 t-Bu on ligand phenolate), 1.52 (s, 9H, t-Bu on ligand
pyridine), 1.58 (s, 18H, 2 t-Bu on ligand phenolate), 3.08 (br s, 8H,
sp2 on 2 cod), 7.51 (br d, 4H, J = 5.4 Hz, 2 ligand phenolate aryl), 7.82
(s, 2H, ligand pyridine aryl). 13C{1H} NMR (75.4 MHz, d8-THF):
15.2, 24.1, 31.0, 31.6, 32.65, 32.75, 33.4, 35.1, 35.8, 35.9, 36.9, 38.3,
56.1, 121.7, 124.9, 130.0, 134.9, 139.2, 141.8, 159.3, 163.1, 163.6. MS
FAB+: 1141.5018 (M+ with 191Ir), 843.4935 ([Ir(cod)(ONOtBu)]+).
Anal. Calcd for C53H75Ir2NO2: C, 55.71; H, 6.62; N, 1.23. Found: C,
54.81; H, 6.77; N, 1.12.
Alternatively, [NaIr(cod)(ONOtBu)]2 (15 mg, 9 μmol) was
dissolved in 1 mL of THF. [Ir(cod)Cl]2 (6 mg, 9 μmol) was dissolved
in 2 mL of THF and added. After two hours the solution was the
solvent was removed in vacuo, redissolved in pentane, and filtered, and
the solvent removed in vacuo again, giving 5 (20 mg, 18 μmol, 100%
yield) as an orange powder. Characterization data were as reported
above.
Synthesis of [(ONOtBu)Ir(cod)(MeCN)][PF6], 6. [NaIr(cod)-
(ONOtBu)]2 (106.3 mg, 61.4 μmol) was dissolved in 10 mL of
THF. A solution of AgPF6 (59.4 mg, 235 μmol) in acetonitrile was
added, resulting in the mixture immediately turning black. The
solution was allowed to stir overnight. Afterward, the solvent was
removed in vacuo, and the black solid redissolved in benzene, filtered,
and lyophilized to give 6 as a yellow powder. This complex was
recrystallized by layering pentane onto a benzene solution. Yield after
recrystallization was 93.0 mg (90.4 μmol, 73.6%). 1H NMR (300
MHz, CD3CN): δ 1.37 (s, 18H, 2 t-Bu on ligand phenolate), 1.45 (s,
9H, t-Bu on ligand pyridine), 1.53 (s, 18H, 2 t-Bu on ligand
phenolate), 2.26 (m, 4H, sp3 on cod), 2.42 (s, 3H, MeCN ligand), 2.75
(br s, 2H, sp3 on cod), 4.31 (br s, 2H, sp3 on cod), 5.45 (m, 2H, sp2 on
cod), 6.65 (m, 2H, sp2 on cod), 7.41 (two overlapping peaks, s, 4H,
2 ligand phenolate aryl), 7.70 (s, 2H, ligand pyridine aryl). 13C{1H }
NMR (75.4 MHz, C6D6): δ 1.3, 28.0, 29.3, 29.4, 30.3, 31.2, 33.9, 34.4,
35.5, 100.2, 113.9, 119.9, 120.6, 123.2, 125.8, 126.4, 139.4, 140.7,
154.3, 161.8, 162.1. 19F{13C } NMR (282 MHz, C6D6): δ −71.1 (d,
1JPF = 711 Hz). 31P{1H} NMR (122 MHz, C6D6): δ −143.2 (septet,
1JPF = 711 Hz). MS FAB+: 842.4472 ([Ir(cod)(ONOtBu)]+). Anal.
Calcd for C47H66F6IrN2O2P·C6H6: C, 57.54; H, 6.56; N, 2.53. Found:
C, 57.06; H, 6.48; N, 2.42.
4
2.97 (br s, 8H, sp2 on 2 cod), 7.19 (d, 4H, J = 2.7 Hz, 2 ligand
phenolate aryl), 7.23 (d, 4H, 4J = 3.0 Hz, 2 ligand phenolate aryl), 7.25
(s, 4H, 2 ligand pyridine aryl). 13C{1H} NMR (75.4 MHz, d8-THF): δ
31.6, 32.1, 33.1, 33.5, 35.4, 36.2, 36.9, 50.3, 119.2, 124.4, 125.4, 133.0,
136.2, 139.6, 159.6, 161.2, 167.7. MS FAB+: 842.4478 ([Ir(cod)-
(ONOtBu (M+ − Na). Anal. Calcd for C90H126Ir2N2Na2O4: C, 62.47;
H, 7.34; N, 1.62. Found: C, 62.26; H, 7.52; N, 1.35.
Alternatively, [NaIr(cod)(ONOtBu)]2 (103.9 mg, 60.0 μmol) was
dissolved in 10 mL of THF. A suspension of [FeCp2][PF6] (75.6 mg,
228 μmol) in acetonitrile was added, the solution was allowed to stir
overnight, the solvent was removed in vacuo, and the solid redissolved
Reaction of 4 with CO. [NaIr(cod)(ONOtBu)]2 (179.4 mg, 103.7
μmol) was dissolved in THF in a Schlenk flask. The solution was
cooled to −78 °C, and an atmosphere of CO (about 5 mmol) was
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in benzene, filtered, and lyophilized to give 6. Yield after
recrystallization (as above) was 98.7 mg (96.0 μmol, 79.9%).
Characterization data were as reported above.
30H, triphenylphosphines). 13C{1H} NMR (126 MHz, CD2Cl2): δ
29.6, 30.4, 31.9, 34.3, 34.9, 35.7, 120.2, 124.5, 125.1, 126.1, 127.4 (br),
127.8 (br), 128.3 (br), 129.7 (br), 133.7 (br), 136.3 (br), 136.8, 141.0,
155.7, 159.1, 170.1. 31P{1H} NMR (121 MHz, CD2Cl2): δ −25.1. MS
FAB+: 1294.5082 ([Ir(PPh3)2(ONOtBu)Cl]+), 1257.5300 ([Ir-
(PPh3)2(ONOtBu)]+), 1031.3262 ([Ir(PPh3)(ONOtBu)Cl]+). Anal.
Calcd for C73H81ClIrNO2P2: C, 67.75; H, 6.31; N, 1.08. Found: C,
68.60; H, 6.84; N, 1.07.
Synthesis of (ONOtBu)Ir(cod)OTf, 7. [NaIr(cod)(ONOtBu)]2
(310.0 mg, 179.1 μmol) and AgOTf (175.7 mg, 683.8 μmol) were
each dissolved in about 10 mL of benzene. The AgOTf solution was
then added dropwise to the stirring solution of 4. The mixture
immediately turned black and was allowed to stir overnight. It was
then filtered, pumped down, and washed with pentane, yielding
(ONOtBu)Ir(cod)OTf, 7 (259 mg, 261 μmol, 76% yield). Since 7 has
limited solubility in benzene (about 10 mg/mL), it can be purified by
washing with benzene. Crystals were obtained by layering a saturated
benzene solution with pentane. 1H NMR (300 MHz, C6D6): δ 0.93 (s,
9H, t-Bu on ligand pyridine), 1.19 (br s, 8H, sp3 on cod), 1.42 (s, 18H,
2 t-Bu on ligand phenolate), 1.74 (s, 18H, 2 t-Bu on ligand phenolate),
2.42 (br s, 2H, sp2 on cod), 4.62 (m, 2H, sp2 on cod), 7.35 (s, 2H,
Synthesis of (ONOtBu)Ir(PPh3)2H, 11. LiOMe (3.9 mg, 0.10
mmol) was dissolved in methanol, and 8 (67.2 mg, 47.7 μmol) was
added while stirring. A light yellow filtrate formed overnight and was
filtered out. When benzene was added to this filtrate, a black
precipitate immediately formed, which was also filtered out. The
remaining yellow solution was lyophilized, giving (ONOtBu)Ir-
(PPh3)2H, 11 (37.6 mg, 29.9 μmol, 62.5% yield), as a yellow powder.
1H NMR (300 MHz, C6D6): δ −15.4 (t, 1H, 2JP−H = 17.8 Hz, iridium
hydride), 0.79 (s, 18H, 2 t-Bu on ligand phenolate), 1.17 (s, 9H, t-Bu
on ligand pyridine), 1.41 (s, 18H, 2 t-Bu on ligand phenolate), 6.8−7.4
(many broad overlapping peaks, 36H, ligand aryl, triphenylphosphine
aryl). 13C{1H} NMR (75.4 MHz, C6D6): δ 28.4, 30.0, 31.7, 33.7, 34.2,
34.9, 118.6, 123.5, 124.9, 126−128 (may be obscured by C6D6), 128.9
(br), 134.3 (br), 135.2, 154.1, 158.0, 170.3. 31P{1H} NMR (122 MHz,
C6D6): δ 9.8. MS FAB+: 1260.5502 ([Ir(PPh3)2(ONOtBu)H2]+),
997.4542 ([Ir(PPh3)(ONOtBu)H]+).
11 was also obtained from the following reactions, although in most
cases it was observed only by NMR and could not be isolated cleanly:
8 (25.7 mg, 18.3 μmol) and proton sponge (3.8 mg, 17.7 μmol)
were dissolved in benzene and mixed. The solution was heated to
90 °C for 24 h. It was then pumped down, redissolved in pentane,
filtered, and pumped down again. The yield of 11 was nearly
quantitative (by NMR).
8 (11.3 mg, 7.2 μmol) and dihydroanthracene (1.3 mg, 7.2 μmol)
were dissolved in C6D6 and mixed. CoCp2 (1.4 mg, 7.4 μmol)
dissolved in C6D6 was then added, and the entire mixture transferred
to a J-Young NMR tube. After stirring for one hour, NMR revealed the
formation of 11 as the major product.
8 (16.7 mg, 10.7 μmol) was dissolved in approximately 1 mL of
C6D6 in a J-Young NMR tube, and an excess of solid NaH (30 mg,
1 mmol) was added. The NMR tube was heated to 90 °C, and the
reaction was followed by NMR; after 26 h the reaction was complete.
The solution was then removed, filtered, and pumped down. Isolated
yield was 11.8 mg, 9.4 μmol, 88%.
4
ligand pyridine aryl), 7.45 (d, 2H, J = 2.7 Hz, ligand phenolate aryl),
7.58 (d, 2H, 4J = 2.7 Hz, ligand phenolate aryl). 13C{1H } NMR (75.4
MHz, C6D6): δ 27.4, 29.5, 30.0, 30.7, 31.4, 34.0, 34.2, 35.7, 90.7, 112.8,
120.6, 123.3, 125.9, 126.2, 139.1, 139.6, 153.2, 160.9, 161.2. 19F{13C}
NMR (282 MHz, C6D6): δ −77.6. MS FAB+: 991.5552 (M+),
858.5524 ([(ONOtBu)Ir(cod)]+). Anal. Calcd for C46H63F3IrNO5PS:
C, 55.74; H, 6.41; N, 1.41. Found: C, 55.51; H, 6.36; N, 1.38.
Synthesis of (ONOtBu)Ir(PPh3)2OTf, 8. 7(89.4 mg, 90.2 μmol)
and PPh3 (47.3 mg, 180 μmol) were each dissolved in benzene and
mixed together. The mixture was stirred at 90 °C for at least 3 h,
filtered, and then lyophilized. 8 (117.6 mg, 83.5 μmol, 92.6% yield)
was formed as a yellow powder, soluble in benzene and slightly soluble
in pentane (about 1 mg/mL). 8 is stable for several days at 90 °C with
1
little decomposition. H NMR (300 MHz, C6D6): δ 0.94 (s, 9H, t-Bu
on ligand pyridine), 1.25 (s, 18H, 2 t-Bu on ligand phenolate), 1.35 (s,
18H, 2 t-Bu on ligand phenolate), 6.52 (s, 2H, ligand pyridine aryl),
4
6.68 (d, 2H, J = 2.4 Hz, ligand phenolate aryl), 6.87 (br s, 12H,
triphenylphosphines), 7.16 (br s, 6H, triphenylphosphines), 7.18 (d,
2H, 4J = 2.5 Hz, ligand phenolate aryl), 7.63 (br s, 12H,
triphenylphosphines). 13C{1H } NMR (75.4 MHz, C6D6): δ 29.6,
29.7, 29.9, 31.2, 33.6, 35.7, 119.4, 123.2, 125.0, 125.6, 126−128 (may
be obscured by C6D6), 129.6, 134.0, 135.1 (br), 136.7, 140.7, 141.1.
19F{13C } NMR (282 MHz, C6D6): δ −76.4. 31P{1H} NMR (122
MHz, C6 D6 ):
δ −17.2. MS FAB+: 1258.5423 ([Ir-
(PPh3)2(ONOtBu)]+), 1145.3984 ([Ir(PPh3)(ONOtBu)(OTf)]+),
996.4537 ([Ir(PPh3)(ONOtBu)]+). Anal. Calcd for C74H81F3IrNO5P2S:
C, 63.14; H, 5.80; N, 1.00. Found: C, 63.24; H, 5.95; N, 0.87.
Synthesis of (ONOtBu)Ir(PPh3)2OH, 9. 8 (20.8 mg, 14.7 μmol)
was dissolved in benzene, and 2 μL of H2O was added via a microliter
syringe. The mixture was stirred for one hour, after which proton
sponge (3.6 mg, 17 μmol) dissolved in benzene was added. A
precipitate was immediately observed, and the mixture was stirred for
four hours, pumped down, redissolved in benzene, filtered, and
lyophilized, giving 9 (18.8 mg, 14.7 μmol, quantitative yield) as a
yellow powder. 9 is slightly soluble in pentane (about 3 mg/mL) and
was crystallized by letting a pentane solution evaporate. 1H NMR (300
MHz, C6D6): δ −1.63 (s, 1H, iridium hydroxide), 1.05 (br, 27H, 2 t-Bu
on ligand phenolate, t-Bu on ligand pyridine), 1.34 (s, 18H, 2 t-Bu on
ligand phenolate), 6.75 (br, 4H, 2 ligand phenolate aryl), 6.8−7.2
(many broad overlapping peaks, 30H, triphenylphosphines), 7.38 (br
s, 2H, ligand pyridine aryl). 31P{1H} NMR (122 MHz, C6D6):
δ −23.2. MS FAB+: 1275.5450 ([Ir(PPh3)2(ONOtBu)OH]+), 1258.5423
([Ir(PPh3)2(ONOtBu)]+), 1013.4492 ([Ir(PPh3)(ONOtBu)OH]+).
Anal. Calcd for C73H82IrNO3P2: C, 68.73; H, 6.48; N, 1.10. Found:
C, 68.71; H, 6.66; N, 1.10.
8 (38.4 mg, 27.3 μmol) was dissolved in approximately 5 mL of
benzene, and ZnEt2 (1.5 μL, 14 μmol) was added via a microliter
syringe. The stirring solution first turned red and then yellow and was
allowed to stir overnight. The solution was then pumped down,
redissolved in pentane, filtered, and pumped down again to form 11 as
a clean solid.
Synthesis of (ONOtBu)Ir(PPh3)2Me, 12. 8 (203.8 mg, 144.8
μmol) was dissolved in benzene, and ZnMe2 (6.2 μL, 90 μmol) was
added via a microliter syringe. The solution was stirred at room
temperature overnight, filtered, and then lyophilized. 12 (182.6 mg,
143.4 μmol, 99.0% yield) was formed as a yellow powder, soluble in
benzene and slightly soluble in pentane, and could be recrystallized by
layering pentane on a benzene solution. 1H NMR (300 MHz, C6D6): δ
1.13 (s, 9H, t-Bu on ligand pyridine), 1.18 (s, 18H, 2 t-Bu on ligand
phenolate), 1.40 (s, 18H, 2 t-Bu on ligand phenolate), 3.03 (t, 3H,
4
3JP−H = 6.1 Hz, iridium methyl), 6.82 (d, 2H, J = 2.4 Hz, ligand
phenolate aryl), 6.89 (br s, 12H, triphenylphosphines), 6.92 (s, 2H,
4
ligand pyridine aryl), 7.19 (d, 2H, J = 2.6 Hz, ligand phenolate aryl),
7.16 (br s, 6H, triphenylphosphines), 7.46 (br s, 12H, triphenylphos-
phines). 13C{1H } NMR (75.4 MHz, CD2Cl2): δ −31.4 (t, 2JP−C = 8.6
Hz), 29.3, 29.9, 31.3, 33.6, 34.3, 35.1, 119.4, 123.3, 125.9, 126.1, 127.1,
128.7, 134.7 (br), 135.1, 140.3, 153.9, 157.8, 170.4. 31P{1H} NMR
(122 MHz, C6D6): δ −14.6. MS FAB+: 1273.4640 (M+), 1011.4714
([Ir(PPh3)(ONOtBu)(Me)]+), 995.3394 ([Ir(PPh3)(ONOtBu)]+).
Anal. Calcd for C74H84IrNO2P2: C, 69.78; H, 6.65; N, 1.10. Found:
C, 69.81; H, 6.53; N, 1.05.
Synthesis of (ONOtBu)Ir(PPh3)2Cl, 10. 8 (39.0 mg, 27.7 μmol)
was dissolved in 3 mL of dichloromethane and heated to 90 °C for five
hours. The solution was then pumped down, redissolved in benzene,
filtered, and lyophilized, giving 10 (35.4 mg, 27.4 μmol, 98.7% yield)
as a yellow powder. 1H NMR (300 MHz, CD2Cl2): δ 0.79 (s, 18H, 2 t-
Bu on ligand phenolate), 1.12 (s, 9H, t-Bu on ligand pyridine), 1.27 (s,
4
18H, 2 t-Bu on ligand phenolate), 6.50 (d, 2H, J = 2.5 Hz, ligand
phenolate aryl), 6.65 (s, 2H, ligand pyridine aryl), 6.93 (d, 2H, 4J = 2.5
Hz, ligand phenolate aryl), 6.9−7.3 (many broad overlapping peaks,
Synthesis of (ONOtBu)Ir(PPh3)2I, 13. 8 (10.5 mg, 8.24 μmol) and
I2 (2.1 mg, 8.3 μmol) were each dissolved in benzene and mixed
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together. The mixture was stirred at room temperature for at least 3 h,
filtered, and then lyophilized. 13 (10.8 mg, 7.80 μmol, 94.6% yield)
was formed as a yellow, pentane-soluble powder, although impurities
can give it a green color. Crystals were obtained by allowing a pentane
suspension of AgPF6 (8.1 mg, 32 μmol) was added. The mixture was
allowed to stir for one hour before being pumped down. It was then
redissolved in a small amount of benzene and filtered. The solution
was then layered with pentane and allowed to sit overnight. Dark blue
needle-like crystals of 16·2C6H6 (39.1 mg, 30.4 μmol, 94.1% yield)
1
solution to evaporate. H NMR (300 MHz, CD2Cl2): δ 0.90 (s, 18H,
1
2 t-Bu on ligand phenolate), 1.15 (s, 9H, t-Bu on ligand pyridine), 1.28
were recovered. H NMR (300 MHz, CD2Cl2): δ −3.1 (very broad),
4
(s, 18H, 2 t-Bu on ligand phenolate), 6.52 (d, 2H, J = 2.6 Hz, ligand
−2.0 (broad), 0.73, 3.99, 12.0 (very broad). 19F{13C} NMR (282
phenolate aryl), 6.70 (s, 2H, ligand pyridine aryl), 6.96 (d, 2H, 4J = 2.5
Hz, ligand phenolate aryl), 7.08 (br s, 12H, triphenylphosphines), 7.21
(br s, 12H, triphenylphosphines), 8.16 (br s, 6H, triphenylphos-
phines). 13C{1H} NMR (126 MHz, CD2Cl2): δ 30.3, 30.4, 31.8, 34.3,
34.9, 35.7, 120.5, 124.8, 125.1, 126.6, 127.4 (br), 129.8 (br), 133.7
(br), 136.9, 137.3 (br), 141.2, 154.8, 159.2, 170.4. 31P{1H} NMR (122
MHz, C6D6): δ −29.1. MS FAB+: 1386.5627 (M+), 1123.3712
([Ir(PPh3)(ONOtBu)(I)]+).
MHz, CD2Cl2): δ −73.8 (d, JP−F = 710 Hz). 31P{1H} NMR (122
1
1
MHz, CD2Cl2): δ −144.7 (septet, JP−F = 710 Hz). MS FAB+:
985.5561 (M+), 867.4697 ([Ir(PEt3)(ONOtBu)(Me)]+).
Synthesis of (ONOtBu)Ir(PPh3)2Ph, 17. 12 (20.4 mg, 16.0 μmol)
was dissolved in 10 mL of benzene and stirred in a 65 °C oil bath for
24 h. The solution was reintroduced into the glovebox, and the solvent
was removed. The residue was redissolved in pentane and filtered, and
the solution pumped down to obtain 17 (20.9 mg, 15.6 μmol, 98%
yield). Crystals were obtained by allowing a pentane solution to
Synthesis of (ONOtBu)Ir(PEt3)2Me, 14. 12 (79.4 mg, 62.3 μmol)
was dissolved in 5 mL of benzene, and PEt3 (20 μL, 136 μmol) was
added. The solution was allowed to stir for seven hours, after which
the benzene and excess PEt3 were removed by lyophilization. The
yellow powder obtained was redissolved in 5 mL of dichloromethane,
and [Rh(cod)2]OTf (30.5 mg, 62.7 μmol) dissolved in 3 mL of
acetonitrile was added to complex residual PEt3. The solution was
allowed to stir for three hours. Afterward, it was pumped down, and
the resulting 14 was redissolved in hexanes, filtered to remove the
byproducts and impurities, and pumped down again. Isolated yield was
51.4 mg (52.2 μmol, 83.7%). Crystals were obtained by allowing a
pentane solution to evaporate. 1H NMR (300 MHz, C6D6): δ 0.75 (m,
18H, 6 P-CH2-CH3), 1.16 (s, 9H, t-Bu on ligand pyridine), 1.46 (s,
18H, 2 t-Bu on ligand phenolate), 1.65 (m, 33H, 2 t-Bu on ligand
1
evaporate. H NMR (300 MHz, CD2Cl2): δ 0.70 (s, 18H, 2 t-Bu on
ligand phenolate), 1.20 (s, 9H, t-Bu on ligand pyridine), 1.43 (s, 18H,
2 t-Bu on ligand phenolate), 5.83 (br s, 2H, iridium phenyl), 6.54 (br s,
4
1H, iridium phenyl), 6.62 (d, 2H, J = 2.7 Hz, ligand phenolate aryl),
6.78 (s, 2H, ligand pyridine aryl), 6.8−7.1 (multiple broad peaks, 30H,
triphenylphosphines), 7.16 (d, 2H, 4J = 2.6 Hz, ligand phenolate aryl),
7.29 (br s, 2H, iridium phenyl). 13C{1H} NMR (126 MHz, C6D6): δ
30.3, 30.6, 32.2, 34.2, 34.4, 35.9, 120.4, 123.6, 124.1, 125.6, 125.9,
126.9, 127.0, 129.6, 129.7, 133.3 (br), 135.7, 136.5 (br), 141.2, 145.9,
155.3, 157.7, 171.5. 31P{1H} NMR (122 MHz, CD2Cl2): δ −13.2.
Reaction of 12 with Toluene. 12 (7.8 mg, 6.1 μmol) was
dissolved in 700 μL of toluene-d8 in a J-Young NMR tube. The tube
1
was heated at 65 °C and periodically monitored by H NMR until all
4
phenolate, 6 P-CH2-CH3, iridium methyl), 7.34 (d, 2H, J = 2.6 Hz,
starting material signals had disappeared, which took six days; the final
1H NMR spectrum was complex. The mass spectrum of the solid
obtained by evaporation exhibited a signal indicating the presence of
the analogous tolyl complex (1094.5436, [Ir(PPh3)(ONOtBu)-
(C6D4CD3]+, [M − PPh3]+), while the 31P{1H} NMR of the dissolved
solid showed two approximately equal signals at δ −12.2 and −12.3,
tentatively assigned to meta and para isomers. A much weaker signal at
−12.8 may indicate formation of a very small amount of ortho isomer.
Isolation of a pure product was not achieved.
ligand phenolate aryl), 7.43 (s, 2H, ligand pyridine aryl), 7.53 (d, 2H,
4J = 2.6 Hz, ligand phenolate aryl). 13C{1H } NMR (126 MHz, C6D6):
δ −33.4 (t, JC−P = 8.2 Hz), 8.1 (t, JC−P = 1.5 Hz), 13.3 (t, JC−P = 14.3
Hz), 30.6, 31.3, 32.4, 34.4, 35.3, 36.3, 118.7, 124.0, 125.6, 128.1, 136.2,
140.6, 156.3, 159.3, 172.9 (t, J = 2.2 Hz). 31P{1H} NMR (122 MHz,
C6D6): δ −19.2. MS FAB+: 985.5609 (M+), 867.4800 ([Ir(PEt3)-
(ONOtBu)(Me)]+). Anal. Calcd for C50H84IrNO2P2: C, 60.95; H, 8.59;
N, 1.42. Found: C, 61.18; H, 8.32; N, 1.45.
Synthesis of (ONOtBu)Ir(PEt3)2I, 15. 14 (11.8 mg, 12.0 μmol)
and I2 (3.0 mg, 12.0 μmol) were each dissolved in benzene and mixed
together. The mixture was stirred at room temperature for 5 h, filtered,
and then lyophilized. 15 was formed as a yellow, pentane-soluble
powder. Crystals were obtained by allowing a pentane solution to
Synthesis of (ONOtBu)Ir(PPh3)[κ2-(o-C6H4)PPh2], 18. 12 (8.2
mg, 6.4 μmol) was dissolved in 1 mL of CD2Cl2 in a J-Young NMR
tube and submerged in a 65 °C oil bath for 21 h. After NMR
confirmation that the reaction was complete, the solvent was removed
to obtain 18 (7.7 mg, 6.1 μmol, 95% isolated yield). Crystals were
1
1
evaporate. H NMR (300 MHz, C6D6): δ 0.74 (m, 18H, 6 P-CH2-
obtained by allowing a pentane solution to evaporate. H NMR (500
CH3), 1.09 (s, 9H, t-Bu on ligand pyridine), 1.42 (s, 18H, 2 t-Bu on
ligand phenolate), 1.71 (s, 18H, 2 t-Bu on ligand phenolate), 1.96 (m,
12H, 6 P-CH2-CH3), 7.36 (s, 2H, ligand pyridine aryl), 7.37 (d, 2H,
MHz, CD2Cl2): δ 0.47 (s, 18H, 2 t-Bu on ligand phenolate), 1.27 (s,
18H, 2 t-Bu on ligand phenolate), 1.34 (s, 9H, tBu on ligand pyridine),
6.64 (m, 4H, triphenylphosphines), 6.9−7.4 (many multiplets, 23H,
triphenylphosphines), 7.50 (dd, 3J = 4.4 Hz, 3J = 6.6 Hz, 2H,
triphenylphosphines). 13C{1H} NMR (126 MHz, CD2Cl2): δ 29.58,
30.76, 32.11, 34.24, 35.27, 35.32, 119.51, 119.69, 122.48, 122.55,
123.83, 124.00, 125.60, 125.77, 128.32, 128.39, 129.03, 129.54, 133.76,
133.83, 136.03, 139.93, 140.51, 140.61, 152.80, 153.23, 154.77, 159.36,
4
4J = 2.5 Hz, ligand phenolate aryl), 7.46 (d, 2H, J = 2.5 Hz, ligand
phenolate aryl). 13C{1H } NMR (126 MHz, C6D6): δ 8.4 (t, JC−P = 1.6
Hz), 15.0 (t, JC−P = 15.3 Hz), 30.0, 31.0, 31.9, 34.1, 34.9, 35.8, 118.8,
124.5, 124.7, 126.5, 137.2, 141.3, 156.3 (t, JC−P = 1.4 Hz), 159.9, 171.7
(t, JC−P = 2.2 Hz). 31P{1H} NMR (122 MHz, C6D6): δ −26.3.
The reaction of 14 and I2 in dichloromethane gives a new species
along with 15 and MeI; over the course of four days all of the 15 is
converted to the new species. Crystals were obtained after the contents
of the NMR tube were allowed to evaporate. X-ray crystallography,
NMR, and MS support identification of this compound as the
analogous chloride, (ONOtBu)Ir(PEt3)2Cl. 1H NMR (300 MHz,
CD2Cl2): δ 0.74 (m, 18H, 6 P-CH2-CH3), 1.01 (m, 12H, 6 P-CH2-
CH3), 1.33 (s, 18H, 2 t-Bu on ligand phenolate), 1.37 (s, 18H, 2 t-Bu
on ligand phenolate), 1.38 (s, 9H, t-Bu on ligand pyridine), 7.11 (d,
2H, 4J = 2.5 Hz, ligand phenolate aryl), 7.26 (d, 2H, 4J = 2.5 Hz, ligand
phenolate aryl), 7.31 (s, 2H, ligand pyridine aryl). 13C{1H } NMR
(126 MHz, CD2Cl2): δ 7.7, 13.0 (t, JC−P = 15.1 Hz), 30.5, 30.6, 31.9,
34.4, 35.5, 35.9, 119.4, 124.6, 124.8, 126.6, 137.6, 140.3, 157.0, 160.5,
171.7. 31P{1H} NMR (122 MHz, CD2Cl2): δ −20.7. MS FAB+
970.5417 ([Ir(PEt3)2(ONOtBu)]+), 887.3802 ([Ir(PEt3)(ONOtBu)-
(Cl)]+), 852.4370 ([Ir(PEt3)(ONOtBu)]+).
2
170.44. 31P{1H} NMR (122 MHz, CD2Cl2): δ −69.5 (d, JP−P = 448
Hz), −2.5 (d, 2JP−P = 448 Hz). MS FAB+: 1257.5292 (M+), 995.4405
([Ir(PPh3)(ONOtBu)]+).
Synthesis of [(ONOtBu)Ir(PPh3)(CH2CN)]2, 19. 12 (13.1 mg, 10.3
μmol) was dissolved in 1 mL of CD2Cl2 in a J-Young NMR tube, and
40 μL of acetonitrile was added. The tube was then submerged in a 65
°C oil bath for 72 h. After NMR confirmation that the reaction had
completed, [Rh(cod)2]OTf (2.6 mg, 5.3 μmol) dissolved in 1 mL of
acetonitrile was added. The solution was allowed to stir for three
hours. Afterward, it was pumped down, and the resulting 19 was
redissolved in hexanes, filtered to remove the byproducts and
impurities, and pumped down again. Crystals were obtained by
allowing a pentane solution to evaporate. Isolated yield was 91% (9.7
mg, 4.7 μmol). 1H NMR (500 MHz, CD2Cl2): δ 1.23 (s, 9H, 2 t-Bu on
ligand), 1.25 (s, 9H, 2 t-Bu on ligand), 1.43 (s, 9H, 2 t-Bu on ligand),
1.44 (s, 9H, 2 t-Bu on ligand), 1.55 (s, 9H, 2 t-Bu on ligand), 1.87
(s, 2H, Ir-CH2-CN), 6.4−6.5 (m, 3H, triphenylphosphine), 6.7−6.8
(m, 3H, triphenylphosphine), 6.9−7.1 (m, 6H, triphenylphosphine),
Synthesis of [(ONOtBu)Ir(PEt3)2Me]PF6, 16. 14 (31.8 mg, 32.3
μmol) was dissolved in dichloromethane, and a dichloromethane
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7.2−7.5 (m, 6H, triphenylphosphine), 7.6−7.8 (m, 2H, triphenyl-
phosphine), 7.9−8.1 (m, 1H, triphenylphosphine). 13C{1H } NMR
(126 MHz, CD2Cl2): δ 3.4, 30.1, 30.3, 30.5, 31.9, 32.2, 34.2, 34.6, 35.2,
35.7, 35.9, 116.0, 118.0, 120.2, 122.5, 123.5, 125.6, 126.0, 128−130
(br), 135.0, 138.0, 138.7, 141.1 (d, JC−P = 3.8 Hz), 153.9, 154.4, 158.2,
159.8 (d, JC−P = 2.7 Hz), 161.1, 167.6 (d, JC−P = 3.4 Hz), 170.8 (d,
JC−P = 1.0 Hz). 31P{1H} NMR (122 MHz, CD2Cl2): δ −88.3.
Kinetic Experiments. Kinetic experiments were carried out with
8.8 mM solutions of 12 or 14. In a typical example, 7.8 mg (6.1 μmol)
12 was dissolved in 700 μL of C6D6 inside a nitrogen-filled glovebox,
and the solution was transferred to a J-Young NMR tube. The NMR
tube was then removed from the glovebox and fully submerged in a 65
°C ethylene glycol bath. The reactions were periodically halted by
transferring the NMR tube into an ice bath before NMR measure-
ments. The progress of the reactions were measured by 31P NMR
integration, with NOE turned off and the d1 time set to 22.895 s (12
has the longest relaxation time, 4.579 s, of all the relevant iridium
compounds). For PPh3 inhibition experiments, the Ir complex was
mixed with various amounts of a 20 mg/mL stock solution of PPh3
in C6D6, and additional C6D6 was added to bring the volume up to
700 μL.
J. Am. Chem. Soc. 2005, 127, 11372−11389. (d) Tenn, W. J.; Young,
K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. J. Am.
Chem. Soc. 2005, 127, 14172−14173. (e) Bhalla, G.; Periana, R. A.
Angew. Chem., Int. Ed. 2005, 44, 1540−1543. (f) Young, K. J. H.;
Mironov, O. A; Periana, R. A. Organometallics 2007, 26, 2137−2140.
(g) Young, K. J. H.; Oxgaard, J.; Ess, D. H.; Meier, S. K.; Stewart, T.;
Goddard, W. A.; Periana, R. A. Chem. Commun. 2009, 3270−3272.
(7) (a) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008,
27, 6123−6142. (b) Agapie, T.; Henling, L. H.; DiPasquale, A. G.;
Rheingold, A. L.; Bercaw, J. E. Organometallics 2008, 27, 6245−6256.
(8) Weinberg, D. R.; Hazari, N.; Labinger, J. A.; Bercaw, J. E.
Organometallics 2010, 29, 89−100.
(9) The synthesis of H2(ONO) was first reported in: Agapie, T.
Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 2007,
pp 156−159, 214−215. Details are provided in the Supporting
Information.
(10) Danopoulos, A. A.; Winston, S.; Hursthouse, M. B. J. Chem. Soc.,
Dalton Trans. 2002, 3090−3091.
(11) Additional details of synthetic procedures, kinetics results,
derivation of rate laws, crystallographic data, and cyclic voltammetric
experiments are provided in the Supporting Information.
(12) Hughes, R. P.; Kovacik, I.; Lindner, D. C.; Smith, J. M.;
Willemsen, S.; Zhang, D.; Guzei, I. A.; Rheingold, A. L. Organometallics
2001, 20, 3190−3197.
(13) Recent calculations have indicated that β-hydride elimination
from some methoxide complexes may be energetically disfavored:
Theophanis, P. L.; Goddard, W. A. Organometallics 2011, 30, 4941−
4948, , although there are many examples in the literature.
(14) Lu, Z; Jun, C-H; de Gala, S. R.; Sigalas, M. P.; Eisenstein, O;
Crabtree, R. H. Organometallics 1995, 14, 1168−1175.
(15) The crystal structure included a complex solvent region that
could not be satisfactorily modeled and also showed the presence of
Li+ ions, presumably carried along as impurities from an earlier step in
the overall synthetic sequence; the required charge-balancing counter-
anions could not be located, probably because they lie within the
solvent region. However, determination of the molecular structure of
the Ir2 complex 19 was unproblematic; see SI for details.11
(16) A somewhat related situation has been described previously:
Luecke, H. F.; Bergman, R. G. J. Am. Chem. Soc. 1997, 119, 11538−
11539. The cyclometalated Ir(III) complex Cp*(OTf)Ir(κ2-PPh2(o-
C6H4)), obtained from the room-temperature reaction of Cp*(OTf)
Ir(PPh3)Me in CH2Cl2, reacts with benzene to form Cp*(OTf)
Ir(PPh3)Ph. Here however the intermolecular activation product is
thermodynamically preferred; probably the difference between the two
systems is primarily of steric origin, as both include ligands (t-Bu
substituents on ONO; the large Cp* group) that can lead to severe
crowding.
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of procedures for preparing the ligand H2(ONO));
kinetic data for the conversion of 12 to 17; derivation of rate
laws; molecular structures, including selected bond lengths and
angles, that are not shown in the main text (6−9, 11, 13−17);
complete crystallographic data for complexes 3−19, including
CIF files; and details of the cyclic voltammetry experiment.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bercaw@caltech.edu; jal@caltech.edu.
■
ACKNOWLEDGMENTS
■
We thank Dr. Michael W. Day and Lawrence M. Henling
(Caltech) for X-ray crystallographic studies. The Bruker
KAPPA APEXII X-ray diffractometer was purchased via an
NSF CRIF:MU award to the California Institute of Technology,
CHE-0639094. This work was supported by BP in the MC2 and
XC2 programs.
(17) Golden, J. T.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
2001, 123, 5837−5838.
(18) Stour, D. L.; Zaric, S.; Niu, S. Q.; Hall, M. B. J. Am. Chem. Soc.
1996, 118, 6068−6069. Su, M. D.; Chu, S. Y. J. Am. Chem. Soc. 1997,
119, 5373−5383.
(19) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518−1520.
(20) Ko, P.-H.; Chen, T.-Y.; Zhu, J.; Cheng, K.-F.; Peng, S.-M.; Che,
C.-M. J. Chem. Soc., Dalton Trans. 1995, 2215−2220.
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