Isolated Ir(V) boryl complexes and their reactions with hydrocarbons [2]

Full text of DOI:10.1021/ja015975d

8422
J. Am. Chem. Soc. 2001, 123, 8422-8423
Scheme 1
Isolated Ir(V) Boryl Complexes and Their Reactions
with Hydrocarbons
Kazumori Kawamura and John F. Hartwig*
Department of Chemistry, Yale UniVersity
P.O. Box 208107, New HaVen, Connecticut 06520-8107
ReceiVed April 9, 2001
ReVised Manuscript ReceiVed July 6, 2001
Transition metal-boryl compounds1,2 are intermediates in a
variety of catalytic processes,3-5 including the regiospecific
functionalization of alkanes.6-8 We recently showed that penta-
methylcyclopentadienyl rhodium and iridium complexes with
labile dative ligands catalyze under thermal conditions the
regiospecific borylation of alkanes at the terminal position (eq
1).6 We proposed that high-valent Rh(V) and Ir(V) boryl
observed for the related silyl compound.10 The dihydrido bisboryl
2 showed a sharp hydride resonance.
Reaction of Li[Cp*IrH3]10 with haloboranes provided an
alternative route to the monoboryl species. This reaction was more
convenient for generation of 1 and allowed for the synthesis of
Ir(V) boryl complexes with different substituents at boron.
Analytically pure 1 was prepared in 87% yield, and catecholboryl
complexes 3a and 3b were prepared in 83-91% yield. Pure
dialkyl boryl 4 was isolated in 85% yield. Compounds 3a and
3b showed NMR features similar to those of 1 and 2: a singlet
11B NMR chemical shift near the corresponding borane and a
1
single hydride signal in the H NMR spectrum at room temper-
ature, which broadened at low temperatures and became resolved
into two signals at -30 °C. Important for the discussion below,
the two hydride signals showed similar line widths at this low
temperature. Compound 4 displayed an 11B NMR chemical shift
of 72.1. This shift was significantly upfield of the typical chemical
shifts for dialkylboryl compounds,14,15 even those of iridium,16
but, like 1-3 was similar in chemical shift to that of the
haloborane.
Several possible binding modes for 1-4 are shown below.
These complexes may exist as dihydrogen complexes (A), as
borane complexes (B), as hydridoborate complexes (C), or as
classical Ir(V) complexes (D). Cp*IrH4 is an Ir(V) tetrahydride10
and Cp*Ir(H)2(SiEt3)2 is an Ir(V) bissilyl dihydride.17,18
Thus, we disfavor formulating 1-4 as Ir(III) dihydrogen
complexes. Unfortunately T1 measurements that have been used
to distinguish classical from nonclassical structures19 would be
affected in this case by the quadrupolar boron and would be
uninformative.20 We and others have previously characterized
complexes of types B-D with different metal centers. These data
provide a spectroscopic and structural platform to interpret our
data for 1-4.
complexes were intermediates in these reactions. The high
reactivity of the rhodium boryl compounds and the high temper-
atures thus far required to form them have prevented isolation of
rhodium intermediates in pure form. Thus, we have targeted
potential intermediates in the iridium-catalyzed process. In
particular, we have sought Ir(V) hydrido boryl complexes.9 We
report our recent success in the generation and the isolation of a
series of Ir(V) boryl polyhydrides, including X-ray structural
characterization of one example. These compounds are the first
fully characterized complexes that react regiospecifically with
alkanes to produce free functionalized products.
Scheme 1 shows our synthetic routes to the Ir(V) boryl
10
complexes. Thermolysis of Cp*IrH4 with a small excess of
pinacolborane (HBpin, 2 equiv) at 80 °C for 50 h in octane formed
the monoboryl trihydride 1 in 81% yield after sublimation.
Reaction of 1 with a large excess (7-20 equiv) of HBpin for 50
h at 100 °C produced bisboryl dihydride 2 in 74% yield after
sublimation. The 11B NMR signals of these materials were singlets
and were located at 33.4 and 33.5 ppm. These chemical shifts
are slightly downfield of those for pinacolborane and chloropina-
colborane and are similar to the chemical shifts of other
dioxaborolanyl complexes of iridium.11-13 Monoboryl 1 displayed
a single hydride resonance at room temperature, but the two
hydrides were observed as a second-order A2B pattern at -40
°C, indicating that site exchange of the hydrides occurred on the
NMR time scale at room temperature. Similar behavior was
(1) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C.
R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV.
1998, 98, 2685.
(2) Braunschweig, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 1786.
(3) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(4) Han, L.-B.; Tanaka, M. Chem. Commun. 1999, 395.
(5) Smith, M. R., III Prog. Organomet. Chem. 1999, 48, 505.
(6) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000,
287, 1995.
(7) Chen, H.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1999, 38, 3391.
(8) Waltz, K. M.; Hartwig, J. F. Science 1997, 277, 211.
(9) For a reaction of phosphine-ligated Ir(III) boryl compounds with arenes
and catalytic borylation of arenes with 3 turnover numbers see: Iverson, C.
N.; Smith, M. R., III J. Am. Chem. Soc. 1999, 121, 7696.
(10) Gilbert, T. M.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1985, 107, 3508.
(11) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.;
Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza,
F. E. S.; Stringer, G.; Whittell, G. R. J. Chem. Soc., Dalton Trans. 1998, 301.
(12) Knorr, J. R.; Merola, J. S. Organometallics 1990, 9, 3008.
(13) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. Can. J.
Chem. 1993, 71, 930.
The spectroscopic features expected for a borane complex
(B)21-23 are not consistent with the data for 1-4. A borane
1
complex would show H-11B coupling in the form of a doublet
(14) Hartwig, J. F.; Huber, S. J. Am. Chem. Soc. 1993, 115, 4908.
(15) Waltz, K. M.; Muhoro, C. N.; Hartwig, J. F. Organometallics 1999,
18, 3383.
(16) Baker, R. T.; Ovenall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor,
N. J.; Williams, I. D.; Marder, T. B. J. Am. Chem. Soc. 1990, 112, 9399.
(17) Ricci, J. S., Jr.; Koetzle, T. F.; Fernandez, M.-J.; Maitlis, P. M.; Green,
J. C. J. Organomet. Chem. 1986, 299, 383.
(18) Fernandez, M.-J.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle, T.
F.; Maitlis, P. M. J. Am. Chem. Soc. 1984, 106, 5458.
(19) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126.
(20) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173.
(21) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435.
(22) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121,
5033.
(23) Muhoro, C. N.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1997, 36,
1510.
10.1021/ja015975d CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/02/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8423
Scheme 2
the boryl complex in these reactions decomposed to bridging oxo
compounds catBOBcat. Likewise, complex 4 formed (Cy2B)2O,
and did not react with either benzene or octane solvent to form
phenyl- or octylborane products.
Heating of an octane solution of 1 at 110 °C for 10 d instead
of 200 °C for 2 d eventually consumed 1, but did not form the
functionalized product. In this case several new iridium boryl
complexes were formed, as indicated by 11B and 1H NMR
spectrometry. We have not been able to isolate these materials
in pure form, but heating of this solution at 200 °C for 140 h
generated octylboronate ester in 47% yield based on starting 1.
Thus, these compounds may be intermediates in the high-
temperature transformation of 1. Alternatively, they may be
formed reversibly by elimination of H2 and may have structures
that are similar to those of dimeric Cp*Ir polyhydrides.28 These
compounds were not observed in the reactions with benzene and
were detected only at early reaction times for the reactions of 1
with octane at 200 °C.
11B NMR signal21 and/or a broad 1H resonance22,23 due to coupling
with the quadrupolar 11B nucleus; hydrides cis and trans to the
boryl group would show different line widths. In addition, Mn
borane complexes have shown increasingly broadened 1H NMR
signals at low temperature due to increased coupling.21 In contrast,
the hydride resonances for 1-4 were all sharp at temperatures
low enough to stop site exchange on the NMR time scale.
A hydridoborate structure (C) would also generate broad
resonances due to coupling to the quadrupolar nucleus and would
display different line widths for the terminal and bridging
hydrides24 when the site exchange is slow on the NMR time scale.
Moreover, this structural class would generate 11B NMR signals
that are upfield of those for related boryl complexes. The 11B
NMR signals of 1 and 3 were located at chemical shifts similar
to those of other iridium dioxaborolanyl compounds. Thus, the
formulation of 1 and 3 as Ir(V) complexes is the most consistent
with our data. Although the upfield boron shift of 4 vs those of
previous dialkylboryl complexes14-16 suggests that this complex
may possess some hydridoborate character, this character was not
reflected in the hydride line widths.
X-ray structural analysis also supports this formulation, even
though the hydride ligands were not located. Ir boryl complexes
show M-B distances of 1.99-2.09 Å, and metal hydroborate
complexes have longer distances than boryl complexes.25-27
Moreover, our Mn-borane complexes also showed longer M-B
distances than those for related boryl complexes. The Ir-B
distance in 1 was 2.047(7) Å (see Supporting Information for
details), which falls in the range of Ir-B distances noted above
for iridium boryl complexes.21
The reactivity of the boryl compounds toward protic reagents,
dihydrogen, and hydrocarbons is summarized in Scheme 2. The
reactivity toward protic reagents varied dramatically. Pinacolboryl
complexes did not react within 12 h with alcohols to form the
tetrahydride, even at 100 °C, but the more electrophilic com-
pounds 3a,b and 4 did form Cp*IrH4 in 84-100% yield at 50-
60 °C and at room temperature, respectively. Complex 1 reacted
with dihydrogen at 90 °C to form Cp*IrH4 in 49% yield after 72
h under 2 atm of H2.
Dioxaborylanes 1-3 reacted with hydrocarbons to give func-
tionalized products, and 1 and 2 reacted with alkanes. For each
reaction with octane, Cp*IrH4 was observed at early reaction
times, but this species decomposes at the temperatures and times
of the reaction (vide infra). Reaction of 1 and 3a in benzene-d6
formed C6D5BR2 in 78 and 79% yield, respectively. Heating of
an octane solution of 1 at 200 °C for 2 d completely consumed
1 and formed the functionalized product in 50% yield. Reaction
of 2 formed 2 equiv of octylboronate ester in 45% yield. In
general, boron oxide pinBOBpin was formed early in the reaction,
presumably from trace amounts of water. After this time, the
yields of octylboronate ester were high. The rate for reaction of
2 with octane was similar to that for reaction of 1. Although it
reacted with benzene, complex 3 did not react with octane. Instead
Reactions of 1 with deuterated hydrocarbons provided mecha-
nistic insight. Reactions in deuterated benzene led to deuterium
incorporation into the Cp* and hydride groups, with faster
incorporation into the hydride. 1H NMR integration of the hydride
and Cp* signals showed a decrease in intensity relative to that of
the boryl group signal. After 16 h at 80 °C, a deuteride signal for
Cp*IrH3-nDn(Bpin) was observed by 2H NMR spectrometry. H/D
exchange with octane also occurred, but was much slower than
exchange with benzene. After 24 h at 110 °C the hydride signal
had decreased in intensity, and a deuteride signal was observed
2
by H NMR spectrometry.
A kinetic isotope effect was observed for the reaction of octane
with 1. Reaction of 1 with a 1:1 mixture of octane and octane-
d18 showed a kH/kD of 2.0. Although qualitative, the reaction of 1
in octane was faster than the separate reaction in octane-d18,
suggesting that the intermediate that reacts with alkane is formed
reversibly and/or that generation of this intermediate involves
cleavage of an Ir-H bond. Reaction of 1 with a 1:1 mixture of
C6H6 and C6D6 was uninformative because of isotope scrambling
in the free arene. With the caveat that H/D exchange can be
mediated by heterogeneous iridium, the H/D exchange of the
iridium hydride along with the primary isotope effect suggests
reversible addition of the hydrocarbon during the reaction.
A comparison of the reactivity of 1 with that of Cp*IrH4 and
Cp*IrH2(SiEt3)2 demonstrates the unusual property of the boryl
group to induce C-H activation and functionalization. The
tetrahydride decomposes before undergoing H/D exchange with
octane. The more thermally stable Cp*IrH2(SiEt3)2 is also inert
toward alkanes, even after 20 h at 200 °C. No DSiEt3 or octyl
triethylsilane was observed. Thus, the boryl ligand generates
compounds that are distinct from both hydride and silyl complexes
in their ability to undergo reactions with alkanes. To date, boryl
complexes are unique in their reactivity toward regiospecific
replacement of a terminal hydrogen in an alkane with a functional
group that is present in the ligand of a transition metal
complex.29-32
Acknowledgment. We thank the NSF for support of this work. K.K.
thanks the JSPS for postdoctoral fellowship support. We thank Huiyuan
Chen for initially observing the clean reaction of Cp*IrH4 with HBpin.
Supporting Information Available: Experimental procedures and
characterization of compounds 1-4 and X-ray structural data for 1 (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
JA015975D
(28) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1994, 116, 198.
(29) Nucleophilic attack on a metal alkyl species is an alternative approach
to alkane functionalization and is described in the following three references.
This reaction has been shown to occur in Shilov processes, but with lower
regioselectivity than the chemistry here.
(24) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L.; Uttley, M. F.
J. Chem. Soc., Dalton Trans. 1976, 2069.
(25) Hartwig, J. F.; De Gala, S. R. J. Am. Chem. Soc. 1994, 116, 3661.
(26) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, M. R., III J. Am.
Chem. Soc. 1994, 116, 10811.
(27) Lantero, D. R.; Miller, S. L.; Cho, J.-Y.; Ward, D. L.; Smith, M. R.,
III Organometallics 1999, 18, 235.
(30) Nelson, A. P.; DiMagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569.
(31) Williams, B. S.; Holland, A. W.; Goldberg, K. I. J. Am. Chem. Soc.
1999, 121, 252.
(32) Luinstra, G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1993, 115, 3004.