Ancillary ligand effects on C-H bond activation reactions promoted by β-diiminate iridium complexes

Article abstract of DOI:10.1021/om050705f

A series of β-diiminate iridium(I) olefin, diolefin, and hydride complexes have been synthesized and evaluated in carbon-hydrogen bond activation reactions. Treatment of [Ir-(COE)₂Cl]₂ (COE = cyclooctene) with the lithio β-diiminate anions [Li(OEt₂)] [BDI] (BDI = ArNC(Me)CH(Me)CNAr; Ar = 2,6-Me₂C₆H₃, 2,6-Et₂C₆H₃, 2,6-ⁱPr ₂C₆H₃) under an N₂ atmosphere furnished the corresponding iridium(I) cycloctene dinitrogen complexes. Using a similar procedure, the analogous β-diiminate iridium(I) cyclooctadiene compounds have also been prepared and characterized. Addition of the sterically demanding β-diiminate anion (2,6-Me₂C₆H ₃)NC(CMe₃)CH(CMe₃)CN(2,6-Me₂C ₆H₃) to [Ir(COD)Cl]₂ (COD = 1,5-cyclooctadiene) yielded an unusual η⁵-arene complex that is stabilized by a significant contribution from an iminocyclohexadienyl resonance form. The relative electronic influence of each β-diiminate ligand has been evaluated by preparation of the corresponding iridium dicarbonyl complexes and reveals little electronic perturbation among alkyl substituents on the aryl rings. With respect to C-H bond activation, warming the β-diiminate iridium(I) cyclooctene dinitrogen compounds to 50°C resulted in intramolecular dehydrogenation chemistry, the outcome of which is dependent on the β-diiminate aryl substituents. For the 2,6-dimethyl-substituted complex, transfer dehydrogenation of the cyclooctene ligand is observed, while for the larger diethyl- and diisopropyl-substituted variants, dehydrogenation of the aryl substituents occurs.

Full text of DOI:10.1021/om050705f

6250
Organometallics 2005, 24, 6250-6259
Ancillary Ligand Effects on C-H Bond Activation
Reactions Promoted by â-Diiminate Iridium Complexes
Wesley H. Bernskoetter, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell University, Ithaca, New York 14853
Received August 15, 2005
A series of â-diiminate iridium(I) olefin, diolefin, and hydride complexes have been
synthesized and evaluated in carbon-hydrogen bond activation reactions. Treatment of [Ir-
(COE)2Cl]2 (COE ) cyclooctene) with the lithio â-diiminate anions [Li(OEt2)][BDI] (BDI )
ArNC(Me)CH(Me)CNAr; Ar ) 2,6-Me2C6H3, 2,6-Et2C6H3, 2,6-iPr2C6H3) under an N2 atmo-
sphere furnished the corresponding iridium(I) cycloctene dinitrogen complexes. Using a
similar procedure, the analogous â-diiminate iridium(I) cyclooctadiene compounds have also
been prepared and characterized. Addition of the sterically demanding â-diiminate anion
(2,6-Me2C6H3)NC(CMe3)CH(CMe3)CN(2,6-Me2C6H3) to [Ir(COD)Cl]2 (COD ) 1,5-cycloocta-
diene) yielded an unusual η5-arene complex that is stabilized by a significant contribution
from an iminocyclohexadienyl resonance form. The relative electronic influence of each
â-diiminate ligand has been evaluated by preparation of the corresponding iridium dicarbonyl
complexes and reveals little electronic perturbation among alkyl substituents on the aryl
rings. With respect to C-H bond activation, warming the â-diiminate iridium(I) cyclooctene
dinitrogen compounds to 50 °C resulted in intramolecular dehydrogenation chemistry, the
outcome of which is dependent on the â-diiminate aryl substituents. For the 2,6-dimethyl-
substituted complex, transfer dehydrogenation of the cyclooctene ligand is observed, while
for the larger diethyl- and diisopropyl-substituted variants, dehydrogenation of the aryl
substituents occurs.
Introduction
type8 ligands such as cyclopentadienyl derivatives,9 tris-
(pyrazolyl)borate,10 and pincer-type anions.11-13
The activation and functionalization of saturated
carbon-hydrogen bonds by soluble transition-metal
compounds continues to be an area of active interest,
given the potential of these reactions to selectively
convert abundant feedstocks into value-added fine and
commodity chemicals.1,2 Iridium complexes have emerged
as attractive candidates in hydrocarbon functionaliza-
tion processes, and examples of both stoichiometric3 and
catalytic reactions such as isotopic exchange,4 arene
borylation,5 alkane dehydrogenation,6 and olefin hy-
droarylation7 have been described. In general, most of
these transformations have been dominated by “L2X”-
Introduction of lower electron count, “LX-type” â-di-
iminate anions14 offers the possibility of generating
more reactive transition-metal centers, owing to coor-
dinative unsaturation imparted by a formally three-
electron donor. Indeed, Goldberg and co-workers have
reported the first examples of alkane dehydrogenation
with platinum using â-diiminates as supporting ligands.
(5) (a) Iverson, C. N.; Smith, M. R., III. J. Am. Chem. Soc. 1999,
121, 7696. (b) Cho, J.-Y.; Iverson, C. N.; Smith, M. R., III. J. Am. Chem.
Soc. 2000, 122, 12868. (c) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura,
N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390.
(d) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith, M.
R., III. Science 2002, 295, 305. (e) Ishiyama, T.; Takagi, J.; Hartwig,
J. F.; Miyaura, N. Angew. Chem., Int. Ed. 2002, 41, 3056. (f) Chotana,
G. A.; Rak, M. A.; Smith, M. R. J. Am. Chem. Soc. 2005, 127, 10539.
(6) Jensen, C. M. Chem. Commun. 1999, 2443.
* To whom correspondence should be addressed. E-mail:
pc92@cornell.edu.
(1) For general reviews on hydrocarbon activation, particularly with
precious metal complexes, see: (a) Labinger, J. A.; Bercaw, J. E. Nature
2002, 417, 507. (b) Crabtree, R. H. Dalton Trans. 2001, 2437. (c) Fekl,
U.; Goldberg, K. I. Adv. Inorg. Chem. 2003, 54, 259. (d) Arndtsen, B.
A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res.
1995, 28, 154.
(2) Goldberg, K. I.; Goldman, A. S. Activation and Functionalization
of C-H Bonds; American Chemical Society: Washington, DC, 2004.
(3) For recent examples see: (a) Bhalla, G.; Liu, X. Y.; Oxgaard, J.;
Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 11372.
(b) Rodriquez, P.; Diaz-Requejo, M. M.; Belderrain, T. R.; Trofimenko,
S.; Nicasio, M. C. Organometallics 2004, 23, 2162 and references
therein.
(4) (a) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J.
Am. Chem. Soc. 2002, 124, 2092. (b) Yung, C. M.; Skaddan, M. B.;
Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 13033. (c) Slugovc, C.;
Perner, B. Inorg. Chim. Acta 2004, 357, 3104. (d) Skaddan, M. B.;
Yung, C. M.; Bergman, R. G. Org. Lett. 2004, 6, 11. (e) Kruger, J.;
Manmontri, B.; Fels, G. Eur. J. Org. Chem. 2005, 7, 1402.
(7) (a) Bhalla, G.; Oxgard, J.; Goddard, W. A.; Periana, R. A.
Organometallics 2005, 24, 3229. (b) Periana, R. A.; Lui, X. Y.; Bhalla,
G. Chem. Commun. 2002, 3000. (c) Matsumoto, T.; Taube, D. J.;
Periana, R. A.; Taube, H.; Yoshida, H. J. Am. Chem. Soc. 2000, 122,
7414.
(8) For a description of the “L” and “X” formalism see: Green, M.
L. H. J. Organomet. Chem. 1995, 500, 127.
(9) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
(10) Slugovc, C.; Padilla-Martinez, I.; Sirol, S.; Carmona, E. Coord.
Chem. Rev. 2001, 213, 129.
(11) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan,
H.-J.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3596.
(12) (a) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103,
1759. (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999,
38, 870.
(13) Weng, W.; Guo, C.; Moura, C.; Yang, L.; Foxman, B. M.; Ozerov,
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10.1021/om050705f CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/05/2005

â-Diiminate Iridium Complexes
Organometallics, Vol. 24, No. 25, 2005 6251
Figure 1. â-Diiminate ligands and their shorthand designations.
Both stoichiometric dehydrogenation chemistry15 and
H/D exchange in pentane16 have been observed. On the
basis of these results and the high activity of iridium
pincer complexes in catalytic dehydrogenation,6 we
became interested in exploring the corresponding â-di-
iminate iridium chemistry. This area has been pio-
neered by Budzelaar and co-workers, who described the
synthesis of (BDI-1)Ir(η3-C8H13)H (BDI-1 ) ArNC(Me)-
CH(Me)CNAr, Ar ) 2,6-Me2C6H3), arising from allylic
C-H activation of the putative cyclooctene compound.17
Addition of dihydrogen to (BDI-1)Ir(η3-C8H13)H fur-
nished the â-diiminate iridium(III) olefin dihydride
(BDI-1)Ir(COE)H2, a molecule that is unreactive toward
olefins and excess H2.18
Our laboratory has discovered that replacing the aryl
methyl substituents with more sterically demanding
isopropyl groups on the â-diiminate ligand dramatically
changes the reactivity of the iridium center. Metalation
of the lithio â-diiminate precursor BDI-3 (BDI-3 )
ArNC(Me)CH(Me)CNAr,Ar)2,6-iPr2C6H3),with[Ir(COE)2-
Cl]2 furnished the olefin dinitrogen complex (BDI-3)Ir-
(COE)N2, with no evidence of allylic C-H activation to
form the corresponding iridium octenyl hydride com-
pound. Addition of dihydrogen to (BDI-3)Ir(COE)N2
resulted in rapid olefin hydrogenation, yielding cyclooc-
tane and the trigonal-prismatic â-diiminate iridium(V)
tetrahydride (BDI-3)IrH4.19 This compound and two
phosphine dihydride adducts, (BDI-3)Ir(PR3)H2 (R )
iPr, Cy), promote the C-H activation and isotopic
exchange in arene substrates such as benzene, toluene,
and mesitylene.20 In both cases mechanistic studies
implicate oxidative addition of an aromatic C-H bond
to a putative â-diiminate iridium(III) dihydride, in
contrast with the preferred pathway for most saturated
precursors such as (η5-C5Me5)Ir(PMe3)H2, where Ir(I)-
Ir(III) redox couples are generally favored.1d
found to promote intramolecular dehydrogenation chem-
istry, the outcome of which is dependent on aryl group
substitution.
Results and Discussion
Synthesis of â-Diiminate Iridium(I) Olefin Com-
plexes. The family of â-diiminate ligands employed in
this study is presented in Figure 1. Initial experiments
focused on understanding the effect of aryl group
substitution on the course of â-diiminate metalation.
Budzelaar and co-workers17 reported that addition of
the THF solvate of the lithio salt of the smallest member
of the series, BDI-1, to [Ir(COE)Cl]2 furnished the
â-diiminate iridium(III) cyclooctenyl hydride (BDI-1)-
Ir(η3-C8H13)H, arising from allylic activation of the
coordinated olefin. Our laboratory has reported that
metalation of the â-diiminate ligand with isopropyl aryl
substituents afforded the corresponding iridium(I) cy-
clooctene dinitrogen complex (BDI-3)Ir(COE)N2.19 In an
attempt to reconcile this apparent discrepancy, several
additional metalation experiments were performed.
One likely possibility for the difference in metalation
outcomes is a deficiency of dinitrogen, resulting in allylic
C-H activation of the coordinated olefin and leading
to isolation of the iridium(III) cyclooctenyl hydride (BDI-
1)Ir(η3-C8H13)H. To explore this possibility, 1 equiv of
[Li(OEt2)][BDI-1] was added to a diethyl ether solution
of [Ir(COE)2Cl]2 under 1 atm of dinitrogen. Following
filtration and recrystallization from pentane at -35 °C,
a yellow powder identified as the desired cyclooctene
dinitrogen complex, (BDI-1)Ir(COE)N2, was isolated in
moderate yield (eq 1). Using a similar synthetic proce-
Intrigued by the profound reactivity differences as-
sociated with slight alterations in the aryl substituents,
a more systematic study of ancillary ligand effects in
â-diiminate iridium chemistry seemed warranted. Here
we describe the synthesis, structural characterization,
and electronic properties of a series of â-diiminate
iridium(I) compounds. Several olefin complexes were
(15) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286.
(16) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804.
(17) Budzelaar, P. H. M.; de Gelder, R.; Gal, A. Organometallics
1998, 17, 4121.
(18) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits, J.
M. M.; Gal, A. Eur. J. Inorg. Chem. 2000, 753.
(19) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem.
Commun. 2004, 764.
(20) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organome-
tallics 2005, 24, 4367.
dure, a â-diiminate iridium(I) cyclooctene dinitrogen
complex containing 2,6-diethyl aryl substituents, (BDI-
2)Ir(COE)N2, was also prepared (eq 1). Both complexes
have been characterized by NMR and IR spectroscopy

6252 Organometallics, Vol. 24, No. 25, 2005
Bernskoetter et al.
and combustion analysis. Diagnostic N-N stretches
centered at 2120 and 2121 cm-1 are observed in the
benzene solution infrared spectra of (BDI-1)Ir(COE)N2
and (BDI-2)Ir(COE)N2, respectively. These peaks are in
agreement with the value of 2122 cm-1 observed for
19
(BDI-3)Ir(COE)N2 and suggest only weak dinitrogen
activation by the d8 iridium center.
One difference between our synthetic protocol and
that described by Budzelaar is the solvate of the lithio
â-diiminate precursor. To determine whether this influ-
ences the product distribution, the metalation of BDI-1
with [Ir(COE)2Cl]2 was carried out in THF. As with our
other procedures, the reaction was performed in the
presence of 1 atm of dinitrogen and, as a result, (BDI-
1)Ir(COE)N2 was again isolated. Accordingly, perform-
ing the metalation under vacuum furnished (BDI-
1)Ir(η3-C8H13)H. Thus, the apparent differences in
metalation are a result of dinitrogen availability rather
than substituent or solvent effects.
Figure 2. Partially labeled view of the molecular structure
of (BDI-5)Ir(COD) with 30% probability ellipsoids. Hydro-
gen atoms are omitted for clarity.
Attempts to prepare other â-diiminate iridium cy-
clooctene dinitrogen complexes by addition of BDI-4 or
BDI-5 to [Ir(COE)2Cl]2 have been unsuccessful. In the
case of the fluorinated ligand BDI-4, no reaction was
observed at ambient temperature and heating ulti-
mately resulted in an unidentified mixture of iridium
products and recovery of protio â-diiminate ligand. For
the tert-butyl-substituted BDI-5, decomposition was
observed upon addition of the lithio salt to [Ir(COE)2Cl]2.
Table 1. Selected Bond Distances (Å) for
(BDI-5)Ir(COD)
Ir(1)-C(9)
Ir(1)-C(10)
Ir(1)-C(11)
Ir(1)-C(12)
Ir(1)-C(13)
Ir(1)-C(14)
C(1)-C(2)
C(5)-C(6)
2.373(4)
2.231(4)
2.241(4)
2.245(4)
2.292(4)
2.704(4)
1.431(6)
1.426(6)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
N(1)-C(14)
N(2)-C(17)
1.413(6)
1.404(6)
1.399(6)
1.434(5)
1.479(5)
1.295(4)
1.288(4)
The inability to prepare clean olefin dinitrogen com-
plexes with the fluorinated and tert-butyl-substituted
â-diiminate ligands prompted investigation into other
iridium(I) olefin precursors as potential entry points for
C-H bond activation chemistry. Addition of a stoichio-
metric quantity of the lithio salts of each of the following
â-diiminate ligands, BDI-1, BDI-3, and BDI-4, to [Ir-
(COD)Cl]2 (COD ) 1,5-cyclooctadiene) furnished the
desired â-diiminate iridium cyclooctadiene complexes,
albeit in modest yields (eq 2). Each compound was
cally demanding tert-butyl-substituted ligand BDI-5
produced unexpected results. Treatment of [Ir(COD)-
Cl]2 with a stoichiometric quantity of [Li(OEt2)][BDI-
5] furnished a yellow compound that exhibits the
number of 1H and 13C NMR peaks anticipated for a
molecule with Cs rather than C2v symmetry. In addition,
a doublet and a triplet appear upfield at 4.64 and 5.04
ppm, respectively, and are assigned to the meta and
para protons on one of the aryl rings, suggesting that
the â-diiminate is bound to the iridium through one of
the arene rings rather than the nitrogen atoms (eq 3).21
The solid-state structure (Figure 2) of (BDI-5)Ir(COD)
was confirmed by X-ray diffraction, and selected metri-
cal parameters are reported in Table 1. In agreement
with the solution spectroscopic data, the iridium center
is ligated by a cyclooctadiene ligand and one of the aryl
substituents of the â-diiminate ligand. Inspection of the
bond distances and the geometry of the arene reveal-
sthat only five of the six carbon atoms are engaged in
bonding with the iridium center, suggesting the imi-
characterized by 1H and 13C NMR spectroscopy and
exhibits the number of resonances expected for a C2v
symmetric diolefin complex. For (BDI-4)Ir(COD), a 19
F
NMR resonance was observed at 108.10 ppm in benzene-
d6 for the equivalent CF3 groups on the â-diiminate
backbone. (BDI-3)Ir(COD) was also characterized by
X-ray diffraction; a view of the solid structure and
metrical parameters for this compound can be found in
the Supporting Information.
(21) Uson, R.; Oro, L. A.; Carmona, D.; Esteruelas, M. A.; Foces-
Foces, C.; Cano, F. H.; Garcia-Blanco, S. J. Organomet. Chem. 1983,
254, 249.
Attempts to synthesize the corresponding â-diketimi-
nate iridium(I) cyclooctadiene complex with the steri-

â-Diiminate Iridium Complexes
Organometallics, Vol. 24, No. 25, 2005 6253
Table 2. Infrared CO Stretching Frequencies
nocyclohexadienyl resonance form as an important
contributor to the overall hybrid. For the five carbons
bound to the iridium, metal-carbon bond lengths range
between 2.231(4) and 2.373(4) Å and are significantly
shorter than the Ir(1)-C(14) distance of 2.704(4) Å. Also
consistent with this bonding description is the pucker-
ing observed in the coordinated arene. A dihedral angle
of 31.4(3)° is formed between the idealized planes
defined by C(9)-C(14)-C(13) and C(9)-C(10)-C(11)-
C(12)-C(13). Accordingly, the C(9)-C(14) and C(13)-
C(14) distances are elongated to 1.468(5) and 1.479(5)
Å, respectively, and are indicative of iminocyclohexa-
dienyl character. It should be noted that the solid-state
structure of (BDI-5)Ir(COD) lacks the symmetry ob-
served by 1H and 13C NMR spectroscopy. In solution fast
rotation about any of the C-C or C-N bonds along the
ligand backbone would render both sides of the aryl
groups equivalent and result in a time-averaged struc-
ture.
(cm-1) of Various Iridium(I) Dicarbonyl Complexes
compound
ν(CO)
ν(CO)
ν(CO)av
ref
a
Cp*Ir(CO)2
1953
1960
1986
1985
1985
2005
1930
1949
2020
2039
2054
2054
2053
2069
1986.5
1999.5
2020
25
25
a
Tp*Ir(CO)2
(BDI-1)Ir(CO)2
(BDI-2)Ir(CO)2
(BDI-3)Ir(CO)2
(BDI-4)Ir(CO)2
this work
this work
this work
this work
26
2019.5
2019
b
b
2037
(PCPtBu2)Ir(CO)b,c
1930
(PCOPtBu2)Ir(CO)b,d
1949
26
a Recorded in hexanes. b Recorded in pentane. c PCPtBu2
)
(C6H3)-1,3-(CH2PtBu2)2. d PCOPtBu2 ) (C6H3)-1,3-(OPtBu2)2
(CO)2, concomitant with loss of 1,5-cyclooctadiene (eq
5). Treatment of (BDI-5)Ir(COD) with 1 atm of carbon
There are several examples22 of iridium complexes
with η5-cyclohexadienyl ligands, and these are generally
stabilized by either an iminocyclohexadienyl or oxocy-
clohexadienyl resonance form. Of particular relevance
to (BDI-5)Ir(COD) is the iridium diphenylaniline com-
plex (η5-PhNPh2)Ir(TFB) (TFB ) tetraflourobarrelene),
reported by Esteruelas and co-workers.23 In this case,
the dihedral puckering in the arene is 16.8(9)° and is
stabilized by the iminocyclohexadienyl resonance con-
tribution.
Comparison of the Electronic Structures of
â-Diiminate Iridium(I) Complexes. With a family of
â-diiminate iridium(I) complexes in hand, the relative
electronic properties of each metal center were inves-
tigated. Because metal-carbonyl infrared stretching
frequencies have proven useful in evaluating the rela-
tive electronic environments imparted by ancillary
ligation,24 the corresponding â-diiminate iridium(I) di-
carbonyl complexes were synthesized. Treatment of
(BDI-1)Ir(COE)N2 or (BDI-2)Ir(COE)N2 with 1 atm of
carbon monoxide resulted in displacement of the olefin
and dinitrogen ligands to furnish (BDI-1)Ir(CO)2 and
(BDI-2)Ir(CO)2, respectively (eq 4).
monoxide produced an intractable mixture of pro-
ducts.
The solution CO stretching frequencies of each dicar-
bonyl derivative were determined in pentane and are
reported in Table 2. Also included are the CO bands of
other iridium carbonyl complexes supported by monoan-
ionic ancillary ligands. Inspection of the data establishes
several salient trends. As expected, manipulation of the
alkyl substituents in the ortho positions of the aryl rings
has little effect on the CO stretching frequencies. Thus,
the electronic environments imparted by BDI-1, BDI-
2, and BDI-3 to the iridium center are very similar.
Replacement of the methyl groups with CF3 substit-
uents on the â-diiminate backbone has a pronounced
electronic effect, resulting in an increase in the average
carbonyl stretching frequency of 18 cm-1, consistent
with a more electron deficient metal center in (BDI-4)-
(22) (a) Grundy, S. L.; Smith, A. J.; Adams, H.; Maitlis, P. M. J.
Chem. Soc., Dalton Trans. 1984, 1747. (b) Chebi, D. E.; Fanwick, P.
E.; Rothwell, I. P. Polyhedron 1990, 9, 969. (c) Le Bras, J.; Amouri,
H.; Vaissermann, J. Organometallics 1996, 15, 5706. (d) Le Bras, J.;
Rager, M. N.; Besace, Y.; Amouri, H.; Vaissermann, J. Organometallics
1997, 16, 1765. (e) Le Bras, J.; Amouri, H.; Vaissermann, J. Organo-
metallics 1998, 17, 1116. (f) Le Bras, J.; Amouri, H.; Vaissermann, J.
Organometallics 1998, 17, 5850. (g) Le Bras, J.; Amouri, H.; Vaisser-
mann, J. J. Organomet. Chem. 1998, 567, 57. (h) Amouri, H.;
Vaissermann, J.; Rager, M. N.; Besace, Y. Inorg. Chem. 1999, 38, 1211.
(i) Mousi, J.; Guyard-Duhayon, C.; Herson, P.; Amouri, H.; Rager, M.
N.; Jutand, A. Organometallics 2004, 23, 6231.
(23) Uson, R.; Oro, L. A.; Carmona, D.; Esteruelas, M. A.; Foces-
Foces, C.; Cano, F. H.; Garcia-Blanco, S.; De Miguel, A. V. J.
Organomet. Chem. 1984, 273, 111.
(24) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313. (b) Zachmanoglou,
C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G. E.; Brandow, C. G.;
Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.; Kiester, J. B. J.
Am. Chem. Soc. 2002, 124, 9525. (c) Bradley, C. A.; Flores-Torres, S.;
Lobkovsky, E.; Abrun˜a, H. D.; Chirik, P. J. Organometallics 2004, 23,
5332.
Similarly, addition of 1 atm of CO to (BDI-3)Ir(COD)
and (BDI-4)Ir(COD) also afforded the corresponding
dicarbonyl complexes, (BDI-3)Ir(CO)2 and (BDI-4)Ir-

6254 Organometallics, Vol. 24, No. 25, 2005
Bernskoetter et al.
Figure 3. Cyclooctene dehydrogenation with (BDI)Ir(COE)N2.
Ir(CO)2. Comparison of the â-diiminate iridium dicar-
bonyl complexes to (C5Me5)Ir(CO)2, tris(pyrazolyl)-
borate,25 or the pincer complexes26 demonstrates that
the â-diiminates engender the most electron deficient
metal centers in the series, consistent with a three-
electron, LX-type ligand as compared to five-electron,
L2X-type donors.8
3). The C-H activation process is reversible, as addition
of excess dinitrogen to (BDI-1)Ir(η3-C8H13)H regenerates
(BDI-1)Ir(COE)N2. These results again demonstrate
that the isolation of the cyclooctenyl hydride from
metalation of [Li(THF)][(BDI-1)] with [Ir(COE)2Cl]2 is
a kinetic rather than thermodynamic phenomenon
resulting from an N2 deficiency.
Continuing the thermolysis resulted in consumption
of (BDI-1)Ir(COE)N2 and (BDI-1)Ir(η3-C8H13)H and
produced two new major iridium products, arising from
transfer dehydrogenation of the cyclooctene ligand
(Figure 3). In addition, a trace product formed in
approximately 5% yield has been identified as (BDI-1)-
Ir(COD). The two major products are formed in near-
equimolar quantities. The first has been identified as
the â-diiminate iridium(III) cyclooctene dihydride (BDI-
1)Ir(COE)H2, originally reported by Budzelaar and co-
workers from hydrogenation of (BDI-1)Ir(η3-C8H13)H,18
while the second exhibits the number of 1H and 13C
NMR resonances expected for a C2v-symmetric iridium
diolefin complex. Treatment of the latter product with
1 atm of carbon monoxide afforded (BDI-1)Ir(CO)2 and
a stoichiometric quantity of 1,4-cyclooctadiene, identi-
fied by comparison to the spectrum of authentic mate-
rial.27 Attempts to perform the dehydrogenation with a
traditional organic hydrogen acceptor such as tert-
butylethylene produced no change in the product dis-
tribution.
The observed products derived from the thermolysis
of (BDI-1)Ir(COE)N2 are a result of transfer dehydro-
genation of the cyclooctene ligand. Unfortunately, 1
equiv of the iridium starting material, (BDI-1)Ir(COE)-
N2, serves as the stoichiometric hydrogen acceptor,
forming (BDI-1)Ir(COE)H2. Under the experimental
conditions employed, this product may be a result of
dinitrogen loss and transfer hydrogenation of (BDI-1)-
Ir(η3-C8H13)H (vide infra). The isolation and robust
C-H Activation with â-Diiminate Iridium(I)
Olefin Complexes. Each of the â-diiminate iridium(I)
olefin complexes was evaluated for alkane dehydroge-
nation activity as well as other C-H bond activation
chemistry. The cyclooctadiene complexes (BDI-1)Ir-
(COD), (BDI-3)Ir(COD), and (BDI-4)Ir(COD) are robust
1
in solution, as no change in the H NMR spectrum is
observed upon heating to 110 °C in alkane or arene
solvent for several days. Addition of dihydrogen to each
compound also produced no reaction, even at elevated
temperatures. In contrast, the analogous cyclooctene
dinitrogen complexes participate in a range of C-H
bond activation reactions, depending on the â-diiminate
ligand.
Thermolysis of the least sterically hindered member
of the series, (BDI-1)Ir(COE)N2, at 50 °C in cyclohexane-
d12 or benzene-d6 for 3 h under vacuum produced several
new iridium products. It is important that the ther-
molysis reaction be carried out under vacuum, as
performing the experiment under a dinitrogen atmo-
sphere inhibits product formation. The initial kinetic
product of the thermolysis procedure was identified as
the â-diiminate iridium(III) cyclooctenyl hydride (BDI-
1)Ir(η3-C8H13)H, previously reported by Budzelaar and
co-workers.17 This compound results from N2 loss and
allylic C-H activation of the cyclooctene ligand (Figure
(25) Tellers, D. M.; Skoog, S. J.; Bergman, R. G.; Gunnoe, T. B.;
Harman, W. D. Organometallics 2000, 19, 2428.
(26) Go¨ttker-Schnetmann, I.; White, P. S.; Brookhart, M. Organo-
metallics 2004, 23, 1766.
(27) Moon, S.; Ganz, C. R. J. Org. Chem. 1969, 34, 465.

â-Diiminate Iridium Complexes
Organometallics, Vol. 24, No. 25, 2005 6255
nature of (BDI-1)Ir(1,5-COD) demonstrates that the
observed major dehydrogenation product, (BDI-1)Ir(1,4-
COD), is a result of selective C-H activation rather than
diolefin isomerization. The preference for 1,4-diolefin
formation is most likely due to the conformation of the
COE ligand, which renders the C-H bonds in the
4-position more susceptible to oxidative addition than
those in the 5-position of the olefin.
Thermolysis of the more sterically hindered â-diimi-
nate iridium(I) precursor (BDI-3)Ir(COE)N2 also pro-
duced intramolecular dehydrogenation chemistry but
with a different C-H bond preference. Heating a
cyclohexane-d12 or benzene-d6 solution of (BDI-3)Ir-
(COE)N2 resulted in loss of cyclooctane and formation
of a single new iridium compound identified as 2, arising
from dehydrogenation of an isopropyl aryl substituent
(eq 6). Similar chemistry has been reported by Fekl and
Figure 4. Partially labeled views of 2 with 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles
(deg) for 2
Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-C(27)
2.0431(12) Ir(1)-C(28)
2.0528(12) C(27)-C(28)
2.1627(15)
2.1260(15)
1.416(2)
Goldberg during the thermolysis of (BDI-3)PtMe3, af-
fording the analogous â-diiminate platinum olefin hy-
dride.16 In the present case, the dehydrogenation reac-
tion occurs at much lower temperature.
N(1)-Ir(1)-N(2)
C(28)-C(27)-C(29) 120.93(16) C(4)-N(1)-Ir(1) 125.64(10)
89.44(5)
C(4)-N(2)-Ir(1) 123.11(9)
Accordingly, the 1H NMR spectrum of 2 exhibits
diagnostic, upfield-shifted resonances centered at 2.73
and 3.14 ppm for the methylene protons of the coordi-
nated olefin. These values are similar to the spectral
features reported for the corresponding platinum hy-
dride compound. The iridium complex 2 was also
characterized by X-ray diffraction. The solid-state struc-
ture, presented in Figure 4, reveals a pseudo-three-
coordinate intramolecular olefin complex. Selected bond
distances and angles are reported in Table 3. The
iridium atom is positionally disordered and was suc-
cessfully modeled. Unlike other iridium complexes
containing BDI-3 ligation, the iridium atom is signifi-
cantly displaced from the â-diiminate plane and the
activated aryl substituent is rotated from its normal
position orthogonal to the ligand plane. The C(27)-C(28)
bond distance of 1.416(2) Å is elongated from that
expected for a typical carbon-carbon double bond, a
result of back-bonding from the d8 iridium(I) center.
This value is similar to that reported for the corre-
sponding platinum hydride complex.16 Interestingly,
there is no evidence for N2 coordination to 2 either in
the solid state or in solution. Because (BDI-3)Ir(COE)-
N2 is thermodynamically favored compared to its N2-
loss product, the lack of N2 coordination by 2 is most
likely steric in origin, arising from the orientation of the
â-diiminate aryl substituents, a consequence of in-
tramolecular olefin coordination.
Several additional experiments were conducted to
gain insight into the intramolecular alkane dehydroge-
nation observed with (BDI-1)Ir(COE)N2. Performing the
thermolysis experiment in the presence of dinitrogen
produced no reaction, demonstrating that N2 loss is a
prerequisite for C-H bond activation. Additionally,
treatment of 2 with cyclooctane under an atmosphere
of N2 regenerated (BDI-3)Ir(COE)N2, demonstrating the
ability of â-diiminate iridium compounds to promote a
stoichiometric dehydrogenation reaction. In this case,
the coordinated olefin serves as the dihydrogen acceptor.
Attempts to extend the dehydrogenation activity to
other alkanes such as heptane and cyclohexane pro-
duced no reaction, even after heating to 65 °C for 12 h.
We believe that the origin of this reactivity difference
is due to the relative dehydrogenation enthalpy of COA
as compared to other alkanes.26

6256 Organometallics, Vol. 24, No. 25, 2005
Bernskoetter et al.
Figure 5. Proposed mechanism for the formation of 2.
On the basis of the limited experimental data avail-
able, a mechanism to account for the observed dehy-
drogenation chemistry with (BDI-3)Ir(COE)N2 is pro-
posed in Figure 5. Following initial dissociation of
dinitrogen, C-H activation of an isopropyl methyl
substituent affords the â-diiminate iridium(III) olefin
alkyl hydride complex. Olefin insertion into the iridium
hydride followed by â-hydrogen elimination and reduc-
tive elimination of cyclooctane yields the observed
products. Other sequences of these transformations are
possible and cannot be ruled out on the basis of our
experimental observations.
and furnished the intramolecular dehydrogenation com-
pound 3 as the sole iridium product (eq 7). No evidence
Importantly, manipulation of the aryl substituents on
the â-diiminate ligand from methyl to isopropyl changes
the preferred substrate for transfer dehydrogenation.
For the less hindered compound (BDI-1)Ir(COE)N2,
C-H activation and dehydrogenation of the cyclooctene
ligand is observed, whereas in the more hindered
analogue (BDI-3)Ir(COE)N2, cyclooctane formation and
activation of an isopropyl aryl substituent occurs. The
presence of aryl substituents with â-hydrogens offers
the possibility for intramolecular dehydrogenation un-
available in the methyl-substituted variant. As will be
demonstrated in a later section, the methyl groups on
the BDI-1 ligand can participate in reversible cyclom-
etalation reactions; however, the lack of â-hydrogens
prevents dehydrogenation. In addition, the more open
metal “pocket” offered by the less hindered â-diiminate
ligand may also account for the differences by accom-
modating the formation of the cyclooctadiene derivatives
that are observed from the thermolysis of (BDI-1)Ir-
(COE)N2.
for â-diiminate iridium cyclooctadiene complexes was
obtained from the H NMR spectrum. To confirm this
1
assignment, 3 was treated with 1 atm of carbon mon-
oxide to furnish the dicarbonyl complex 4 (eq 7).
Significantly, no free cyclooctadiene was detected by 1H
NMR spectroscopy. It should be noted that both 3 and
4 have been isolated in very low yield and small
quantities, thereby preventing combustion analysis.
The observation of aryl group dehydrogenation from
the thermolysis of (BDI-2)Ir(COE)N2 demonstrates the
preference for â-diiminate iridium complexes with â-hy-
drogen-containing aryl substituents to C-H activating
these groups over cyclooctene. While the steric environ-
ment associated with BDI-2 is larger than that imposed
by BDI-1, the presence of â-hydrogen-containing aryl
substituents allows for a productive pathway for in-
tramolecular olefin complex formation. Only when
â-hydrogens are absent is cyclooctene dehydrogenation
observed.
In an attempt to determine if smaller aryl substitu-
ents containing â-hydrogens would accommodate cy-
clooctadiene formation, the thermolysis of (BDI-2)Ir-
(COE)N2 was studied. Warming a benzene-d6 solution
of the iridium cyclooctene dinitrogen complex to 50 °C
for 3 h liberated a stoichiometric quantity of cyclooctane
Ancillary Ligand Effects on the Hydrogenation
of â-Diiminate Iridium Olefin Complexes. Because

â-Diiminate Iridium Complexes
Organometallics, Vol. 24, No. 25, 2005 6257
(BDI-1)Ir(COE)N2 serves as the stoichiometric dihydro-
gen acceptor during the dehydrogenation of coordinated
cyclooctene, its reactivity with H2 was independently
investigated. As with C-H activation, literature reports
suggest that â-diiminate aryl substitution has a pro-
found effect on the outcome of olefin hydrogenation
reactions. Budzelaar and co-workers reported that ad-
dition of H2 to (BDI-1)Ir(η3-C8H13)H afforded (BDI-1)-
Ir(COE)H2 with no evidence for free alkane.17 In con-
trast, introduction of larger isopropyl aryl substituents
in the case of (BDI-3)Ir(COE)N2 resulted in facile olefin
hydrogenation of the cyclooctene ligand to afford the
trigonal-prismatic iridium(V) tetrahydride (BDI-3)-
IrH4.19
One possible reason for the difference in hydrogena-
tion reactivity is the nature of the iridium starting
material. The synthesis and isolation of (BDI-1)Ir(COE)-
N2 allows direct comparison of its hydrogenation chem-
istry with that of (BDI-3)Ir(COE)N2. Addition of 4 atm
of H2 to a benzene-d6 solution of (BDI-1)Ir(COE)N2 at
23 °C afforded an approximate 6:1 ratio of two new
â-diiminate iridium hydrides along with free cyclooctane
(eq 8). Quantitation of the free alkane by 1H NMR
Figure 6. Proposed pathways for the formation of (BDI-
1)Ir(COE)H2 and (BDI-1)Ir(N2)H2 from (BDI-1)Ir(COE)N2.
In an attempt to determine the origin of the two
â-diiminate iridium hydrides, the effect of H2 pressure
on the product distribution was studied. Each experi-
ment was conducted at low conversion to minimize
complications from decomposition reactions. Addition of
approximately 0.25 atm of H2 to an NMR tube contain-
ing a 0.01 M solution of (BDI-1)Ir(COE)N2 furnished a
2:1 mixture of (BDI-1)Ir(COE)H2 to (BDI-1)Ir(N2)H2.
Performing similar hydrogenations with 1 and 4 atm
increased the amount of the dinitrogen dihydride com-
plex (BDI-1)Ir(N2)H2 to 1:1.2 and 1:5.7, respectively.
Therefore, as the pressure of H2 increases, the amount
of (BDI-1)Ir(N2)H2 also increases (eq 8).
To determine whether the two hydride products can
be interconverted, N2 and H2 were added to (BDI-1)Ir-
(COE)H2. In agreement with previous reports,17 no
change was observed in the 1H NMR spectrum, indicat-
ing that dinitrogen does not trigger the conversion of
(BDI-1)Ir(COE)H2 to (BDI-1)Ir(N2)H2 with loss of cy-
cloctane. The lack of hydrogenation activity of (BDI-1)-
Ir(COE)H2 was further investigated with an isotopic
labeling experiment. Facile isotopic exchange occurs
within the iridium hydride positions upon addition of 1
atm of D2 to (BDI-1)Ir(COE)H2, allowing synthesis of
(BDI-1)Ir(COE)D2. Monitoring benzene-d6 solutions of
the olefin dideuteride complex by 1H NMR spectroscopy
over the course of 4 days at ambient temperature
resulted in no incorporation of the isotopic label into the
cyclooctene ligand, demonstrating that olefin insertion
and alkyl migration do not readily occur under these
conditions.
The experimental results are consistent with the
observed mixture of â-diiminate iridium dihydrides
arising from two independent pathways (Figure 6). On
the basis of the limited experimental data available, we
tentatively conclude that (BDI-1)Ir(N2)H2 arises from
hydrogenation of (BDI-1)Ir(COE)N2, while (BDI-1)Ir-
(COE)H2 is derived from H2 addition to (BDI-1)Ir(η3-
C8H13)H. During the thermolysis experiments described
earlier, the equilibrium between (BDI-1)Ir(COE)N2 and
(BDI-1)Ir(η3-C8H13)H and free N2 was directly observed
at 50 °C in benzene-d6. To account for the observed
spectroscopy indicated that it is derived from the
formation of the major product. A combination of NMR
and IR spectroscopy has been used to characterize the
major product as the iridium dihydride dinitrogen
complex (BDI-1)Ir(N2)H2. The minor product has been
identified as the previously reported cyclooctene dihy-
dride (BDI-1)Ir(COE)H2.17 It should be noted that when
the reaction is carried out under these conditions and
run to complete conversion of starting material, signifi-
cant decomposition (∼15%) to free ligand and iridium
black accompanies the hydrogenation. Isolation of small
quantities of pure (BDI-1)Ir(N2)H2 has been accom-
plished by successive recrystallizations from pentane at
-35 °C. (BDI-1)IrH2N2 is a relatively rare example of
an Ir(III) dinitrogen complex. Bergman and co-workers
have reported a cationic iridium(III) N2 compound
supported by a hydridotris(pyrazolyl)borate ligand.28
The N2 ligand in this compound can be displaced by
dative ligands and H2 and also promotes C-H activation
in benzene and aldehydes.
(28) (a) Tellers, D. M.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122,
954. (b) Tellers, D. M.; Bergman, R. G. Organometallics 2001, 20, 4819.

6258 Organometallics, Vol. 24, No. 25, 2005
Bernskoetter et al.
Concluding Remarks
ratios of products as a function of H2 and N2 pressure,
it appears that the absolute rate constant for the
hydrogenation of (BDI-1)Ir(COE)N2 is smaller than that
for (BDI-1)Ir(η3-C8H13)H. Support for this hypothesis is
the observation of (BDI-1)Ir(COE)H2 as the sole product
from stoichiometric transfer hydrogenation. However,
the observation of increased quantities of (BDI-1)Ir(N2)-
H2 as a function of increased H2 pressure suggests a
higher order in H2 for the hydrogenation of (BDI-1)Ir-
(COE)N2 compared to (BDI-1)Ir(η3-C8H13)H. This could
be a consequence of cyclooctene hydrogenation for (BDI-
1)Ir(COE)N2 requiring additional equivalents of dihy-
drogen or a rate-determining reductive coupling process
in (BDI-1)Ir(η3-C8H13)H that precedes H2 addition.
Another possibility is that the equilibration between
(BDI-1)Ir(η3-C8H13)H and (BDI-1)Ir(COE)N2 is rela-
tively slow, occurring on a time scale competitive with
hydrogenation. If (BDI-1)Ir(η3-C8H13)H undergoes faster
hydrogenation but is present at a lower concentration,
then the equilibrium between the two iridium starting
materials would be continually reestablished as the
hydrogenation proceeds, favoring (BDI-1)Ir(COE)H2 at
low dihydrogen pressures. At higher H2 pressures, the
equilibration could not compete and more (BDI-1)Ir(N2)-
H2 would result. Unfortunately the presence of parallel
reaction pathways in conjunction with a decomposition
reaction prohibits a meaningful kinetic study.
This study highlights the importance of aryl group
substitution on the reactivity of â-diiminate iridium
complexes. When smaller methyl substituents are
present, intramolecular transfer dehydrogenation of
cyclooctene from the corresponding iridium(I) olefin
dinitrogen complex affords two isomeric cyclooctadiene
compounds along with an iridium cyclooctene dihydride
complex. Increasing the size of the aryl substituents to
ethyl or isopropyl groups changes the substrate for
dehydrogenation, resulting in intramolecular olefin
complexes derived from the aryl substituents. Hydro-
genation is also subject to aryl group substituent effects,
as addition of H2 to the less hindered precursor (BDI-
1)Ir(COE)N2 yielded two products, depending on the
dihydrogen pressure. One product, (BDI-1)Ir(N2)H2, is
favored at higher H2 pressures (4 atm) and is active for
aromatic C-H bond cleavage, promoting H/D exchange
with benzene-d6. Significantly, deuterium incorporation
is observed in the aryl methyl groups, demonstrating
that all of the â-diiminate ligands undergo cyclometa-
lation. Overall, this study highlights the significant
impact subtle changes in aryl group substitution can
have on C-H activation reactions promoted by â-diimi-
nate iridium complexes.
Experimental Section29
Isolation of (BDI-1)Ir(N2)H2 from the addition of H2
to (BDI-1)Ir(COE)N2 is in contrast with the hydrogena-
tion chemistry of (BDI-3)Ir(COE)N2, where formation
of the â-diiminate iridium(V) tetrahydride is observed.
Because the electronic properties of the metal complexes
with the two different â-diiminates are nearly identical,
the observed difference in reactivity is most likely a
result of steric effects. The smaller methyl substituents
allow tighter N2 binding, and as a result, oxidative
addition of a second equivalent of dihydrogen to form
the corresponding iridium(V) tetrahydride derivative is
not observed. Accordingly, addition of 1 atm of dinitro-
gen to (BDI-3)IrH4 does not induce reductive elimination
of dihydrogen. In analogy to the case for (BDI-1)Ir(PR3)-
H2 complexes,20 (BDI-1)Ir(N2)H2 promotes H/D ex-
change in benzene-d6 at ambient temperature. Analysis
of the iridium complex by 2H NMR spectroscopy follow-
ing isotopic exchange revealed deuterium incorporation
into the metal hydride position and the methyl substit-
uents of the â-diiminate ligand, demonstrating revers-
ible cyclometalation of the ligand.
The ability of the â-diiminate iridium dinitrogen
dihydride (BDI-1)Ir(N2)H2 to undergo insertion chem-
istry was also explored. Addition of 2 equiv of cy-
clooctene to a benzene-d6 solution of (BDI-1)Ir(N2)H2
produced 1 equiv of cyclooctane along with (BDI-1)Ir-
(COE)N2 (Figure 6). Performing the same experiment
with a larger excess of COE also furnished (BDI-1)Ir-
(COE)N2 and cyclooctane along with small quantitites
of (BDI-1)Ir(COE)H2. Because conversion of (BDI-1)Ir-
(COE)N2 to (BDI-1)Ir(COE)H2 requires 1 equiv of di-
hydrogen, the observation of (BDI-1)Ir(COE)H2 from
(BDI-1)Ir(N2)H2 in the absence of H2 suggests that the
olefin dihydride is obtained from simple displacement
of the dinitrogen ligand by cyclooctene. This side
pathway is only detectable at higher concentrations of
free olefin.
Preparation of (BDI-1)Ir(COE)N2. A 20 mL scintillation
vial was charged with 0.178 g (0.199 mmol) of [Ir(COE)2Cl]2
and approximately 12 mL of diethyl ether. The resulting
orange solution was chilled to -35 °C, and 0.151 g (0.398
mmol) of Li[BDI-1]‚Et2O in approximately 5 mL of diethyl
ether was added in small portions. The resulting reaction
mixture was stirred for 2 days and the solvent was removed
in vacuo, leaving a yellow solid. Extraction into pentane and
filtration through a pad of Celite followed by solvent removal
produced a yellow foam. Recrystallization from pentane at -35
°C afforded 0.132 g (52%) of yellow powder identified as (BDI-
1)Ir(COE)N2. Anal. Calcd for C29H39IrN4: C, 54.78; H, 6.18;
N, 8.81. Found: C, 54.50; H, 6.13; N, 8,69. 1H NMR (benzene-
d6): δ 1.07-1.44 (m, 10H, COE CH2), 1.36 (s, 6H, Ar Me), 1.58
(s, 6H, Ar Me), 1.67 (br s, 2H, COE CH2), 2.15 (s, 3H, CH3),
2.48 (s, 3H, CH3), 2.64 (br d, 2H, COE CH), 5.26 (s, 1H, CH),
6.89-7.12 (Ar H). 13C NMR (benzene-d6): δ 18.75, 19.24 (Ar
Me), 23.28, 25.73, 26.78 (COE CH2), 28.45, 31.40 (CMe), 59.12
(CH COE), 101.74 (CH), 126.16, 126.31 (p-Ar), 128.05, 128.19
(m-Ar), 132.29, 133.47 (o-Ar), N-C quaternary signals not
located. IR (benzene-d6): νN-N 2120 cm-1
.
Preparation of 2. A thick-walled glass vessel was charged
with 0.058 g (0.091 mmol) of (BDI-3)Ir(COE)N2 and ap-
proximately 20 mL of heptane. On the high-vacuum line, the
vessel was submerged in liquid nitrogen and evacuated. The
solution was thawed and the resulting yellow solution heated
to 65 °C for 4-6 h. Periodically the vessel was again sub-
merged in liquid nitrogen and the atmosphere removed in
vacuo to remove any liberated dinitrogen. This process was
repeated four times, after which the solvent was removed in
vacuo. Pentane was vacuum-transferred onto the resulting
solid and removed in vacuo. This process was repeated several
times to remove residual cyclooctane. The solid was then
recrystallized from pentane at -35 °C, yielding 0.019 g (36%)
of yellow-orange crystals identified as 2. Anal. Calcd for C29H39-
IrN2: C, 57.30; H, 6.78; N, 4.61. Found: C, 57.60; H, 7.36; N,
4.51. 1H NMR (cyclohexane-d12): δ 0.71 (d, 3H, 7 Hz, CHCH3),
(29) General considerations and additional experimental details can
be found in the Supporting Information.

â-Diiminate Iridium Complexes
Organometallics, Vol. 24, No. 25, 2005 6259
1.10 (d, 3H, 7 Hz, CHCH3), 1.16 (d, 3H, 7 Hz, CHCH3), 1.23
(d, 3H, 7 Hz, CHCH3), 1.25 (d, 3H, 7 Hz, CHCH3), 1.35 (d, 3H,
7 Hz, CHCH3), 1.74 (s, 3H, CH3), 2.03 (s, 3H, CH3), 2.16 (s,
3H, CH3), 2.73 (s, 1H, olefinic CH), 2.85 (sept, 1H, 7 Hz,
CHCH3), 2.98 (sept, 1H, 7 Hz, CHCH3), 3.14 (s, 1H, olefinic
CH), 3.30 (sept, 1H, 7 Hz, CHCH3), 5.13 (s, 1H, CH), 6.81-
7.12 (Ar H). 13C NMR (benzene-d6): δ 20.29, 22.23, 22.50,
23.56, 23.68, 28.78, 29.18, 33.23, 34.95 (CH3), 24.69, 25.22,
25.61 (CH), 47.06 (CdCH2), 51.41 (CdCH2), 103.12 (CH),
123.43, 123.85, 124.04, 124.20, 124.49, 126.45 (o-, p-, m-Ar),
N-C quaternary signals not located.
CHMe2), 1.47 (d, 12H, 7 Hz, CHMe2), 1.64 (s, 6H, CH3), 3.30
(sept, 4H, 7 Hz, CHMe2), 5.24 (s, 1H, CH), 7.10 (br s, 4H, Ar
m-H), 7.11 (br s, 2H, Ar p-H). 13C NMR (benzene-d6): δ 23.44,
24.49 (CHMe2), 24.57 (CMe), 28.05 (CHMe2), 100.97 (CH),
124.14 (p-Ar), 127.70 (m-Ar), 141.35 (o-Ar), 154.62 (NC), 160.47
(CMe), 198.50 (CO). IR (pentane): νCO 1985, 2053 cm-1
.
Preparation of (BDI-1)IrH2N2. A thick-walled glass vessel
was charged with 0.098 g (0.15 mmol) of (BDI-1)Ir(COE)N2
and approximately 10 mL of pentane. On the vacuum line,
the vessel was submerged in liquid nitrogen and evacuated,
and 1 atm of H2 was admitted. The contents of the vessel were
then thawed, and the resulting yellow solution was stirred at
ambient temperature for 8 h. The resulting dark purple
solution was transferrd into the drybox and filtered through
a glass frit. The filtrate was collected and the pentane removed
in vacuo, yielding a dark foam. Recrystallization from pentane
at -35 °C yielded 0.024 g (30%) of a black solid, identified as
Preparation of (BDI-1)Ir(COD). A 20 mL scintillation
vial was charged with 0.093 g (0.14 mmol) of [Ir(COD)Cl]2 in
approximately 5 mL of diethyl ether. The resulting orange
slurry was chilled in a drybox freezer to -35 °C for about 20
min, after which time 0.105 g (0.280 mmol) Li[BDI-1]‚Et2O in
approximately 5 mL of diethyl ether was added. The orange
mixture was stirred for 16 h at ambient temperature. Filtra-
tion through Celite followed by solvent removal in vacuo and
subsequent recrystallization from pentane at -35 °C affords
0.053 g (37%) of a yellow solid identified as (BDI-1)Ir(COD).
Anal. Calcd for C29H37IrN2: C, 57.49; H, 6.16; N, 4.62. Found:
C, 57.40; H, 5.84; N, 4.36. 1H NMR (benzene-d6): δ 1.42 (br d,
4H, COD), 1.56 (s, 6H, CH3), 2.08 (br s, 4H, COD), 2.28 (s,
12H, ArMe), 3.04 (br s, 4H, COD), 5.39 (s, 1H, CH), 6.95-
7.04 (Ar H). 13C NMR (benzene-d6): δ 19.28 (Ar Me), 25.77
(CMe), 31.89, 63.22 (COD), 102.21 (CH), 126.07 (p-Ar), 129.00
(m-Ar), 132.97 (o-Ar), N-C quaternary signals not located.
Preparation of (BDI-3)Ir(CO)2. A thick-walled glass
vessel was charged with 0.012 g (0.020 mmol) of (BDI-3)IrH4
and approximately 15 mL of pentane. On the vacuum line,
the vessel was submerged in liquid nitrogen and evacuated,
and 1 atm of CO was added. The contents of the vessel were
thawed, and the resulting solution was stirred for 24 h at
ambient temperature. The pentane was removed in vacuo and
the resulting yellow solid recrystallized from pentane at -35
°C, yielding 0.009 g (71%) of (BDI-3)Ir(CO)2. Anal. Calcd for
C31H41IrN2O2: C, 55.92; H, 6.21; N, 4.21. Found: C, 55.44; H,
5.69; N, 3.94. 1H NMR (benzene-d6): δ 1.08 (d, 12H, 7 Hz,
1
(BDI-1)IrH2N2. H NMR (cyclohexane-d12): δ 1.59 (s, 6H, Ar
Me), 1.80 (s, 6H, Ar Me), 2.18 (s, 3H, CH3), 2.26 (s, 3H, CH3),
5.36 (s, 1H, CH), 6.93-7.08 (Ar H), -20.45 (s, 2H, Ir-H),
-20.09 (t, 3.6 Hz, 1H, Ir-HD in benzene-d6). 13C NMR
(cyclohexane-d12): δ 19.03, 19.27 (Ar Me), 22.29, 22.46 (CMe),
101.97 (CH), 124.76, 125.85 (p-Ar), 128.41, 128.43 (m-Ar),
130.91, 131.51 (o-Ar), N-C quaternary signals not located. IR
(benzene-d6): νN-N 2167 cm-1. IR (KBr): νN-N 2170 cm-1
.
Acknowledgment. We thank Cornell University
and the National Science Foundation (CAREER Award)
for financial support. Christopher A. Bradley is also
acknowledged for the initial preparation and crystal-
lographic characterization of (BDI-3)Ir(COD).
Supporting Information Available: Text giving ad-
ditional experimental procedures, figures giving additional
NMR spectra, and CIF files giving crystal structure data for
(BDI-3)Ir(COD), (BDI-5)Ir(COD), and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM050705F