Iridium(I) and iridium (III) complexes supported by a diphenolate imidazolyl-carbene ligand

Article abstract of DOI:10.1021/om900803r

Deprotonation of l,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (la) followed by reaction with chloro-l,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)] (2: OCO = l,3-di(2-hydroxy-5-tert- butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits squareplanar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv of [FeCp₂][PF₆] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF₆] (3). Reaction of 3 with H₂ results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in acetonitrile to form [{OCO}Ir(MeCN)₃][PF₆] (4) or by reaction with either PMe₃ or PCy₃ to generate [{OCO}Ir(PMe ₃)₃][PF₆] (5) or [{OCO}Ir(PCy₃) ₂(MeCN)][PF₆] (6), respectively. 6 reacts with CO in acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy ₃)₂(CO)][PF₆] (7) and with chloride to generate [{OCO}Ir(PCy₃)₂Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolylcarbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding interaction between the phenolates and the metal center in the highest occupied molecular orbital (HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH₂Cl₂ at -0.22 and at 0.58 V (vs ferrocene/ ferrocenium); EPR spectra, mass spectroscopy, and DFT calculations suggest that the product of the first oxidation is [{OCO}Ir(PCy₃) ₂Cl]⁺ (8⁺), with the unpaired electron occupying a molecular orbital that is delocalized over both the metal center and the diphenolate imidazolyl-carbene ligand.

Full text of DOI:10.1021/om900803r

Organometallics 2010, 29, 89–100 89
DOI: 10.1021/om900803r
Iridium(I) and Iridium(III) Complexes Supported by a Diphenolate
Imidazolyl-Carbene Ligand
David R. Weinberg, Nilay Hazari,† Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125. † Current address: Chemistry Department, Yale University, P.O. Box 208107,
New Haven, CT 06520.
Received September 14, 2009
Deprotonation of 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (1a) followed by reac-
tion with chloro-1,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)]
(2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir
complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits square-
planar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv
of [FeCp2][PF6] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF6] (3). Reaction of 3 with H2
results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of
cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in
acetonitrile to form [{OCO}Ir(MeCN)3][PF6] (4) or by reaction with either PMe3 or PCy3 to generate
[{OCO}Ir(PMe3)3][PF6] (5) or [{OCO}Ir(PCy3)2(MeCN)][PF6] (6), respectively. 6 reacts with CO in
acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy3)2(CO)][PF6] (7) and with chloride to
generate [{OCO}Ir(PCy3)2Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolyl-
carbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding
interaction between the phenolates and the metal center in the highest occupied molecular orbital
(HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH2Cl2 at
-0.22 and at 0.58 V (vs ferrocene/ferrocenium); EPR spectra, mass spectroscopy, and DFT calculations
suggest that the product of the first oxidation is [{OCO}Ir(PCy3)2Cl]þ (8þ), with the unpaired electron
occupying a molecular orbital that is delocalized over both the metal center and the diphenolate
imidazolyl-carbene ligand.
Introduction
hydrosilylations,6-8 hydroaminations,9 borations,10-12
alkylations,1,13 cycloadditions,14 cyclopropanations,15 hy-
drogen isotope exchanges,16 and polymerizations.17,18 The
strong σ-donation of NHC ligands often imparts stability,
whereas their large trans effects tend to increase the rates at
which other ligands in the complex exchange.19 Increased
rates of ligand exchange may be important for catalysis
because Ir(III) complexes are often relatively substitution-
ally inert.20
Both Ir(I) and Ir(III) complexes containing N-heterocyclic
carbene (NHC) ligands have proven capable of catalyzing a
variety of transformations, including hydrogenations,1-5
*Corresponding author. E-mail: bercaw@caltech.edu.
(1) Chang, Y.-H.; Fu, C.-F.; Liu, Y.-H.; Peng, S.-M.; Chen, J.-T.;
Liu, S.-T. Dalton Trans. 2009, 861–867.
(2) Zhu, Y.; Burgess, K. Adv. Synth. Catal. 2008, 350, 979–983.
(3) Baskakov, D.; Herrmann, W. A.; Herdtweck, E.; Hoffmann, S. D.
Organometallics 2007, 26, 626–632.
NHC ligands can be readily synthesized with a wide
variety of different substituents on the nitrogen atoms,
(4) Vazquez-Serrano, L. D.; Owens, B. T.; Buriak, J. M. Inorg. Chim.
Acta 2006, 359, 2786–2797.
(5) Nanchen, S.; Pfaltz, A. Helv. Chim. Acta 2006, 1559–1573.
(6) Chen, T.; Liu, X.-G.; Shi, M. Tetrahedron 2007, 63, 4874–4880.
(7) Chianese, A. R.; Mo, A.; Datta, D. Organometallics 2009, 28, 465–
472.
(8) Chianese, A. R.; Crabtree, R. H. Organometallics 2005, 24, 4432–
4436.
(13) Gnanamgari, D.; Sauer, E. L. O.; Schley, N. D.; Butler, C.;
Incarvito, C. D.; Crabtree, R. H. Organometallics 2009, 28, 321–325.
(14) Peng, H. M.; Webster, R. D.; Li, X. Organometallics 2008, 27,
4484–4493.
(15) Winkelmann, O.; Nather, C.; Luning, U. J. Organomet. Chem.
2008, 693, 923–932.
€
€
(9) Bauer, E. B.; Andavan, G. T. S.; Hollis, T. K.; Rubio, R. J.; Cho,
J.; Kuchenbeiser, G. R.; Helgert, T. R.; Letko, C. S.; Tham, F. S. Org.
Lett. 2008, 10, 1175–1178.
(10) Corberan, R.; Lillo, V.; Mata, J. A.; Fernandez, E.; Peris, E.
Organometallics 2007, 26, 4350–4353.
(11) Frey, G. D.; Rentzsch, C. F.; von Preysing, D.; Scherg, T.;
(16) Brown, J. A.; Irvine, S.; Kennedy, A. R.; Kerr, W. J. Chem.
Commun. 2008, 1115–1117.
(17) Jin, G.-X.; Xiao, X.-Q. J. Organomet. Chem. 2008, 693, 3363–
3368.
(18) Zhang, Y.; Wang, D.; Wurst, K.; Buchmeiser, M. R. J. Organo-
ꢀ
met. Chem. 2005, 690, 5728–5735.
€
Muhlhofer, M.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem.
(19) Lee, M.-T.; Hu, C.-H. Organometallics 2004, 23, 976–983.
(20) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley & Sons: New York, 1999; pp
1039-1063.
2006, 691, 5725–5738.
(12) Grasa, G. A.; Moore, Z.; Martin, K. L.; Stevens, E. D.; Nolan,
S. P.; Paquet, V.; Lebel, H. J. Organomet. Chem. 2002, 658, 126–131.
r
2009 American Chemical Society
Published on Web 12/07/2009
pubs.acs.org/Organometallics

90 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
allowing modulation of reactivity. Chelating carbene ligands
have been utilized in some Ir catalysts to further modify
reactivity and increase stability.3,5,7-10,12
with the crown ether-encapsulated Kþ ion, and there is
normal (η2)2-cod coordination.
We have chosen to examine diphenolate imidazolyl-
carbene ligands (1), which have recently been shown to
support a variety of transition metal complexes and cat-
alysts,21-28 although none involve group 9 transition metals.
We envisioned that the combination of the “soft” carbene
and the “hard” phenolate donors could stabilize Ir in
more than one oxidation state;29 furthermore, the redox
chemistry could be particularly interesting, as phenolate
ligands may be oxidized to generate stable ligand-centered
radicals.30,31
The 1H NMR spectrum for 2 in THF-d8 (without
18-crown-6 ether) is shown in Figure 2; the spectra in
CD3CN and DMSO-d6, as well as those in THF-d8 and in
C6D6 in the presence of 18-crown-6 ether, are all very similar.
There is clearly fluxional behavior: from the solid-state
structure we would expect two sets of phenolate aromatic
protons and tert-butyl groups, at least (depending on the rate
of rotation about the dangling phenolate N-C bond) two
environments each for the cod -CH and imidazolyl -CH2
groups, and at least four environments for the cod -CH2
protons;33 however, there is only one set of peaks for the
phenolates and the cod -CH protons and two signals each
for both the cod and imidazolyl -CH2 groups. The 13C
NMR spectrum exhibits only one set of peaks for the -CH2
groups in the imidazolyl ring, the aromatic carbons, and the
tert-butyl groups.34
Results and Discussion
Synthesis and Characterization of an Ir(I) Complex. The
NHC precursor 1,3-bis(2-hydroxy-5-tert-butylphenyl)imid-
azolium chloride (1a) was prepared by a procedure similar to
those used for analogous species.32 It was then deprotonated
with 3 equiv of potassium hexamethyldisilazide and treated
with [IrCl(cod)]2 to cleanly generate [K][{OCO}Ir(cod)]
(2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl;
cod = 1,5-cyclooctadiene; eq 1). 2 was crystallized from
benzene as the 18-crown-6 etherate, and the structure deter-
mined crystallographically (Figure 1). The geometry about
the Ir(I) center is square planar: the NHC ligand is κ2(C,O)-
coordinated with one “dangling” phenoxide O interacting
1H NMR spectra were obtained at various temperatures
between -70 and 60 ꢀC. All the peaks broaden significantly
below -40 ꢀC; between -40 and -70 ꢀC, the two imidazo-
lyl -CH2 peaks coalesce, and the cyclooctadiene -CH peak
splits. The latter observation suggests that exchange between
the coordinated and dangling phenolates (as depicted in eq 1)
is freezing out; in that interpretation, the two peaks observed
at room temperature in the 1H (but not the 13C) NMR
spectrum for the imidazolyl -CH2 groups would be ascribed
to slow rotation about the nitrogen-phenolate bonds.35 That
would also imply further nonequivalence of the cod protons,
but all those signals are relatively broad; any additional split-
ting may be thus masked. There is no obvious reason why the
two imidazolyl -CH2 peaks should converge at low tempera-
ture; possibly this is an accidental degeneracy associated with
differential temperature dependence of the chemical shifts.
Synthesis and Chemistry of Ir(III) Complexes. Oxidation of
2 is effected by ferrocenium in acetonitrile; the resulting 1H
NMR spectrum, showing one set of phenolate signals, two
for the imidazolyl -CH2 groups, and six peaks for coordi-
nated cod (the latter are shifted significantly downfield from
their positions in 2), is consistent with the octahedral Ir(III)
complex[{mer-OCO}Ir(cod)(MeCN)][PF6] (3, Scheme 1). A
molecular ion corresponding to 3 was observed in the mass
spectrum.
(21) Bellemin-Laponnaz, S.; Welter, R.; Brelot, L.; Dagorne, S.
J. Organomet. Chem. 2009, 694, 604–606.
(22) Zhang, D.; Liu, N. Organometallics 2009, 28, 499–505.
(23) Manna, C. M.; Shavit, M.; Tshuva, E. Y. J. Organomet. Chem.
2008, 693, 3947–3950.
(24) Zhang, D. Eur. J. Inorg. Chem. 2007, 4839–4845.
(25) Zhang, D.; Aihara, H.; Watanabe, T.; Matsuo, T.; Kawaguchi,
H. J. Organomet. Chem. 2007, 692, 234–242.
(26) Xu, X.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46,
3743–3751.
(27) Yagyu, T.; Oya, S.; Maeda, M.; Jitsukawa, K. Chem. Lett. 2006,
35, 154–155.
(28) Aihara, H.; Matsuo, T.; Kawaguchi, H. Chem. Commun. 2003,
2204–2205.
(29) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev.
2007, 36, 1732–1744.
(30) Kurahashi, T.; Kikuchi, A.; Tosha, T.; Shiro, Y.; Kitagawa, T.;
Fujii, H. Inorg. Chem. 2008, 47, 1674–1686.
(31) Mukherjee, A.; Lloret, F.; Mukherjee, R. Inorg. Chem. 2008, 47,
4471–4480.
(32) Bellemin-Laponnaz, S.; Welter, R.; Brelot, L.; Dagorne, S.
J. Organomet. Chem. 2009, 694, 604–606.
Simply heating 3 at 90 ꢀC in acetonitrile results in the
displacement of cod (observed by H NMR spectroscopy
(33) Rodman, G. S.; Mann, K. R. Inorg. Chem. 1988, 27, 3338–
3346.
1
(34) The 13C NMR assignments of the imidazolyl -CH2 peak and the
when the reaction is performed in CD3CN) to generate
[{mer-OCO}Ir(MeCN)3][PF6] (4, Scheme 1), confirmed by
mass spectrometry. When 4 is generated in CH3CN, isolated,
cod -CH peak were made from a 1H-13C HMQC spectrum of 2.
€
(35) Karabiyik, H.; Kilinc-arslan, R.; Aygun, M.; C-etinkaya, B.;
€
€ €
€
Buyukgungor, O. Z. Naturforsch., B: Chem. Sci. 2005, 60, 837–842.

Article
Organometallics, Vol. 29, No. 1, 2010 91
Figure 1. Structural drawing of 2 with 18-crown-6 ether coordinated to potassium. Displacement ellipsoids are drawn at the 50%
probability level. A molecule of benzene in the unit cell is not shown. (a) Perspective taken perpendicular to the square plane of the
˚
complex. (b) Perspective taken facing down the C15-Ir bond; the 18-crown-6 ether was removed for clarity. Selected bond lengths (A)
and angles (deg): Ir-C15 2.041(3); Ir-O1 2.0423(19); Ir-C(24) 2.093(3); Ir-C25 2.145(3); Ir-C28 2.159(3); Ir-C29 2.174(3); N1-C15
1.369(4); N2-C15 1.348(4); O2-K 2.576(2); C15-Ir-O1 87.56(10); C15-Ir-C24 93.31(12); O1-Ir-C24 153.01(11); C15-Ir-C25
100.71(10); C15-Ir-C25 39.06(12); C15-Ir-O1 87.56(10); C15-Ir-C24 93.31(12); O1-Ir-C24 153.01(11); C15-Ir-C25
100.71(12); O1-Ir-C25 165.77(10); C24-Ir-C25 39.06(12); C15-Ir-C28 164.27(12); O1-Ir-C28 88.10(10); C24-Ir-C28
97.40(12); C25-Ir-C28 80.84(11); C15-Ir-C29 156.85(12); O1-Ir-C29 86.99(10); C24-Ir-C29 81.68(12); C25-Ir-C29
89.62(12); C28-Ir-C29 37.61(12).
Figure 2. 1H NMR spectrum of 2 in THF-d8 at 298 K. Note some benzene was isolated with 2, and it is present in solution. Also, the
“-CH3” peak is truncated, as it has a much greater intensity than the other peaks.
and then redissolved in CD3CN, two peaks for Ir-coordi-
nated CH3CN appear in the 1H NMR spectrum, at 2.53 and
2.68 ppm with an approximate ratio of 2:1. 4 was kept in
CD3CN at room temperature for two weeks; during this time
the smaller peak, probably corresponding to CH3CN
coordinated trans to the carbene donor, disappeared com-
decreased in size, but only slightly, while the free CH3CN
peak grew. The peaks at 2.53 and 2.68 ppm are not present
when 4 is generated in CD3CN. The displacement of cod by
acetonitrile indicates relatively weak binding to Ir(III), not
unexpected for a high-valent olefin complex; the facile
exchange of the acetonitrile trans to the carbene is more
noteworthy for Ir(III).
1
pletely from the H NMR spectrum. The larger peak also

92 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
Scheme 1
Treatment of 3 with 1 atm of H2 in THF at room
temperature results in the formation of cyclooctane; the 1H
NMR spectrum showed only trace amounts of cyclooctene,
no Ir-hydride signals, and broad peaks attributable to the
diphenolate carbene ligand. The Ir-containing product can
catalyze the hydrogenation of cyclohexene to cyclohexane.
Removing the solvent and other volatiles under reduced
pressure and heating the resulting solid in CD3CN at 90 ꢀC
affords 4, suggesting that the diphenolate imidazolyl-
carbene ligand remains bound to Ir, but the identity of the
catalyst is otherwise unclear. Although no Ir hydride signals
were observed, they may be present in small amounts; most
Ir-catalyzed hydrogenations involve either Ir(I) or
hydridoiridium(III) as catalyst precursors.36-43
Scheme 2
Reaction of 3 with an excess (greater than 3 equiv) of PMe3
or PCy3 proceeds slowly at room temperature, but goes to
completion after 12 h at 90 ꢀC, via displacement of cod to
give [{OCO}Ir(PMe3)3][PF6] (5) and [{OCO}Ir(PCy3)2-
(MeCN)][PF6] (6), respectively (Scheme 2). The structure
of 5 was assigned on the basis of NMR data, including the
observation of doublet and triplet 31P signals in 2:1 intensity
ratio, along with HRMS. In contrast, the 31P NMR spectrum
of 6 shows only one singlet for coordinated PCy3, and only
one set of phenolate peaks is observed in the 1H NMR
spectrum, indicating the mer, trans configuration shown.
Steric factors likely cause the PCy3 ligands to bind trans to
each other and prevent a third PCy3 ligand from binding to
the metal center.
other cationic diphosphine Ir(III) carbonyl complexes,44-46
indicates relatively weak π-donation from Ir to the carbonyl
π*-orbital, consistent with the fact that 7 and 6 equilibrate.
Complex 6 reacts reversibly with CO in acetonitrile; an
equilibrium mixture of 6 and [{OCO}Ir(PCy3)2(CO)][PF6]
(7) is established over 3 days at 90 ꢀC under 1.3 atm CO, with
the ratio of 6 to 7 approximately 2:1 (eq 2). 7 is converted
back to 6 when the solution is degassed and reheated to
90 ꢀC. 7 exhibits an infrared CO-stretching signal at
2064 cm-1, which shifts to 2016 cm-1 (2018 cm-1 calculated)
when 13CO is used; 7-13CO also displays the expected triplet
in the 13C NMR spectrum and doublet in the 31P NMR
spectrum. The relatively high νCO, while within the range of
Determination of a precise equilibrium constant is pre-
cluded by the formation of yellow crystals above the surface
of the solution during the reaction; surprisingly, these result
from cocrystallization of 6 and 7 as a 4:1 mixture (Figure 3).
The two complexes have indistinguishable structural para-
meters aside from the identity of the sixth ligand. While this
type of substitutional disorder appears to be rare, similar
(36) Vazquez-Serrano, L. D.; Owens, B. T.; Buriak, J. M. Inorg.
Chim. Acta 2006, 359, 2786–2797.
(37) Nagaraja, C. M.; Nethaji, M.; Jagirdar, B. R. Organometallics
2007, 26, 6307–6311.
(38) Roseblade, S. J.; Pfaltz, A. Acc. Chem. Res. 2007, 40, 1402–1411.
(39) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 3rd ed.; Wiley & Sons: New York, 2001; pp 80-114, 222-258.
(40) Lee, D.-H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R. H.
Chem. Commun. 1999, 297–298.
(41) Ogo, S.; Kabe, R.; Hayashi, H.; Harada, R.; Fukuzumi, S.
Dalton Trans. 2006, 4657–4663.
(44) O’Connor, J. M.; Pu, L.; Rheingold, A. L. J. Am. Chem. Soc.
1990, 112, 6232–6247.
(42) Sablong, R.; Osborn, J. A. Tetrahedron: Asymmetry 1996, 7,
3059–3062.
(43) Chan, Y. N. C.; Meyer, D.; Osborn, J. A. J. Chem. Soc., Chem.
(45) Clark, H. C.; Reimer, K. J. Inorg. Chem. 1975, 14, 2133–2140.
(46) Thorn, D. L.; Fultz, W. C. J. Phys. Chem. 1989, 93, 1234–1243.
(47) Rotar, A.; Varga, R. A.; Silvestru, C. Acta Crystallogr. 2008,
E64, m45.
Commun. 1990, 869–871.

Article
Organometallics, Vol. 29, No. 1, 2010 93
Figure 3. Three perspectives of the structure of 6/7, with displacement ellipsoids at the 50% probability level; [PF6]- is not shown. In
the unit cell, there is a 78% occupancy of positions N3A, C60A, and C61A of 6 and a 22% occupancy of positions C3B and O3B of 7.
˚
The Cipso-O1-O2-Cipso torsion angle is 2.9ꢀ. Selected bond lengths (A) and angles (deg): Ir1-C1 1.9694(19); Ir1-O2 2.0331(14);
Ir1-O1 2.0390(13); Ir1-N3A 2.055(8); Ir1-C3B 2.14(4); C3B-O3B 0.95(4); Ir1-P2 2.4348(5); Ir1-P1 2.4437(5); C1-Ir1-O2
91.57(8); C1-Ir1-O1 92.21(7); O2-Ir1-O1 176.10(5); C1-Ir1-N3A 178.8(2); O2-Ir1-N3A 87.8(2); O1-Ir1-N3A 88.5(2);
C1-Ir1-C3B 174.1(10); O2-Ir1-C3B 94.4(10); O1-Ir1-C3B 81.9(10); N3A-Ir1-C3B 6.7(12); C1-Ir1-P2 92.79(6); O2-Ir1-P2
91.00(5); O1-Ir1-P2 89.77(4); N3A-Ir1-P2 86.2(2); C3B-Ir1-P2 87.5(11); C1-Ir1-P1 94.97(6); O2-Ir1-P1 89.17(5);
O1-Ir1-P1 89.56(4); N3A-Ir1-P1 86.0(2); C3B-Ir1-P1 84.8(11); P2-Ir1-P1 172.229(16); Ir1-C3B-O3B 178(4);
Ir1-N3A-C60A 175.1(7); N3A-C60A-C61A 177.1(7); Cipso-O1-O2-Cipso 2.71(22).
examples do exist.47-51 The diphenolate carbene ligand
in 6/7 is relatively planar; the Cipso-O1-O2-Cipso tors-
ion angle, a useful measure of the degree of distortion, is
only 2.9ꢀ.
80 and 35.19(26)ꢀ for 800, a considerably greater distortion
for the latter.
Displacement of acetonitrile from 6 by chloride generates
air-stable [{OCO}Ir(PCy3)2Cl] (8, eq 3), confirmed by single-
crystal X-ray diffraction (Figure 4). Again, the solid-state
structure is not straightforward: the unit cell contains two
conformers of 8 (80 and 800), along with two molecules of
CH2Cl2. In both conformers the metal, chloride, two oxy-
gens, and the carbene carbon are nearly coplanar, but the
rings of the diphenolate imidazolyl ligand are significantly
distorted out of this plane, from C2v-symmetry toward
C2-symmetry, to an extent that differs for the two molecules.
The Cipso-O1-O2-Cipso torsion angle is 15.72(26)ꢀ for
The chloro-Ir(III) complex 8 can be further oxidized; the
cyclic voltammogram in CH2Cl2 (Figure 5; similar results are
obtained in THF and dimethylformamide) displays two
reversible oxidations (ΔEp=98 mV for both), at -0.22 and
0.58 V. Coulometric oxidation at 0.25 V resulted in the
passage of 0.92 faraday per mole, indicating that the wave
at -0.22 V corresponds to a one-electron oxidation of 8;
further coulometric oxidation at 0.96 V resulted in the
passage of 0.99 faraday per mole, indicating that the wave
at 0.58 V also corresponds to a one-electron oxidation.
(48) Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Robins, E. G. Acta
Crystallogr. 2000, C56, 50–52.
(49) Laungani, A. C.; Keller, M.; Breit, B. Acta Crystallogr. 2008,
E64, m24–m25.
(50) Marimuthu, T.; Bala, M. D.; Friedrich, H. B. Acta Crystallogr.
2008, E64, 6772.
(51) de Silva, N.; Nichiporuk, R. V.; Dahl, L. F. Dalton Trans. 2006,
2291–2300.

94 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
Figure 4. Structural drawing of the two conformers in the unit cell of 8 with displacement ellipsoids drawn at the 50% probability level;
two molecules of CH2Cl2, not shown, are also in the unit cell. (a) Conformer 80; the perspective on the left is directed approximately
bisecting the P2A-Ir1-O1A angle; that on the right is directed down the C15A-Ir1 bond. The Cipso-O1-O2-Cipso torsion angle is
˚
15.72(26)ꢀ. Selected bond lengths (A) and angles (deg): Ir(1)-C(15A) 1.944(2); Ir(1)-O(2A) 2.0257(17); Ir(1)-O(1A) 2.0343(16);
Ir(1)-P(2A) 2.4194(7); Ir(1)-P(1A) 2.4365(7); Ir(1)-Cl(1) 2.4584(5); C(15A); Ir(1)-O(2A) 91.66(8); C(15A)-Ir(1)-O(1A) 93.06(8);
O(2A)-Ir(1)-O(1A) 175.24(6); C(15A)-Ir(1)-P(2A) 94.16(7); O(2A)-Ir(1)-P(2A) 90.58(5); O(1A)-Ir(1)-P(2A) 88.45(5); C-
(15A)-Ir(1)-P(1A) 93.67(7); O(2A)-Ir(1)-P(1A) 92.81(5); O(1A)-Ir(1)-P(1A) 87.51(5); P(2A)-Ir(1)-P(1A) 171.367(19); C-
(15A)-Ir(1)-Cl(1) 179.44(8); O(2A)-Ir(1)-Cl(1) 87.99(4); O(1A)-Ir(1)-Cl(1) 87.29(4); P(2A)-Ir(1)-Cl(1) 85.40(2);
P(1A)-Ir(1)-Cl(1) 86.79(2). (b) Conformer 800 with approximately the same perspectives. The Cipso-O1-O2-Cipso torsion angle is
˚
35.19(26)ꢀ. Selected bond lengths (A) and angles (deg): Ir(2)-C(15B) 1.944(2); Ir(2)-O(1B) 2.0307(17); Ir(2)-O(2B) 2.0380(18);
Ir(2)-P(1B) 2.4102(8); Ir(2)-P(2B) 2.4260(8); Ir(2)-Cl(2) 2.4529(5); C(15B)-Ir(2)-O(1B) 92.75(9); C(15B)-Ir(2)-O(2B) 90.14(9);
O(1B)-Ir(2)-O(2B) 177.00(6); C(15B)-Ir(2)-P(1B) 93.81(8); O(1B)-Ir(2)-P(1B) 92.32(5); O(2B)-Ir(2)-P(1B) 86.70(5); C-
(15B)-Ir(2)-P(2B) 94.60(8); O(1B)-Ir(2)-P(2B) 88.27(5); O(2B)-Ir(2)-P(2B) 92.29(6); P(1B)-Ir(2)-P(2B) 171.53(2); C(15B)-Ir-
(2)-Cl(2) 177.33(8); O(1B)-Ir(2)-Cl(2) 85.42(4); O(2B)-Ir(2)-Cl(2) 91.66(4); P(1B)-Ir(2)-Cl(2) 84.32(2); P(2B)-Ir(2)-Cl(2)
87.30(2).
EPR spectra were recorded for the solutions generated by
the above controlled potential electrolysis of 8. The EPR
spectrum obtained after the first oxidation of 8 (Figure 6) is
indicative of an S=1/2 species with g||=2.19 and g^ ≈ 1.94;
even at temperatures as low as 7 K, no hyperfine splitting was
resolved. Mass spectroscopy of this sample showed the
expected molecular ion for 8. The same EPR spectrum was
obtained from oxidation of 8 with less than 1 equiv of
[FeCp2][PF6]. Characterization of products generated in
the second oxidation was complicated by product instability;
a mixture of species was suggested by both EPR and NMR
spectroscopies, while a molecular ion for 8 was still detected
in the mass spectrum.
The EPR signal in Figure 6 most probably corresponds to
[{OCO}Ir(PCy3)2(Cl)]þ (8þ), generated by chemical or
(reversible) electrochemical one-electron oxidation of 8. The
electronic configuration of radical-cation 8þ is not obvious:
oxidation of Ir(III) complexes to Ir(IV) and oxidation
(52) Benisvy, L.; Bill, E.; Blake, A. J.; Collison, D.; Davies, E. S.;
Garner, C. D.; McArdle, G.; McInnes, E. J. L.; McMaster, J.; Ross,
S. H. K.; Wilson, C. Dalton Trans. 2006, 258–267.
€
€
(53) Sokolowski, A.; Muller, J.; Weihermuller, T.; Schnepf, R.;
Hildebrandt, P.; Hildenbrand, K.; Bothe, E.; Wieghardt, K. J. Am.
Chem. Soc. 1997, 119, 8889–8900.
(54) Diversi, P.; de Biani, F. F.; Igrosso, G.; Laschi, F.; Lucherini, A.;
Pinzino, C.; Zanello, P. J. Organomet. Chem. 1999, 584, 73–86.

Article
Organometallics, Vol. 29, No. 1, 2010 95
Figure 5. Cyclic voltammogram of a 3 mM CH2Cl2 solution of 8 at a scan rate of 100 mV/s with 0.3 M [NBu4][BF4] as the supporting
electrolyte.
Figure 6. EPR spectrum at 7 K of the solution resulting from controlled potential oxidation of 8 at 0.25 V (0.92 faraday per mole).
of phenolates to phenoxyl radicals are both well preced-
ented,52-63 and while both typically occur at potentials greater
than that for the oxidation of 8 (-0.22 V), the latter value does
fall within the range observed for both types of oxidation. The
EPR spectrum might be expected to exhibit hyperfine coupling
to Ir (191Ir, 37%; 193Ir, 63%:bothI=3/2) and P for Ir(IV), or to
H and N for a phenoxyl radical;46,64 there is precedent, though,
for Ir(IV) complexes for which hyperfine splitting cannot be
resolved even at low temperatures, and highly delocalized
singly occupied orbitals can also fail to show coupling.56
The g-values differ significantly from the free electron value
of 2.0023 and from values for previously characterized phe-
noxyl radicals,46,53,58-66 suggesting either a metal-centered
radical or a delocalized orbital with metal character.
(55) Panda, M.; Das, C.; Lee, G.-H.; Peng, S.-M.; Goswami, S.
Dalton Trans. 2004, 2655–2661.
(56) Acharyya, R.; Basuli, F.; Peng, S.-M.; Lee, G.-H.; Wang, R.-Z.;
Mak, T. C. W.; Bhattacharya, S. J. Organomet. Chem. 2005, 690, 3908–
3917.
(57) Li, X.; Chen, Z.; Zhao, Q.; Shen, L.; Li, F.; Yi, T.; Cao, Y.;
Huang, C. Inorg. Chem. 2007, 46, 5518–5527.
(58) Wu, F.-I.; Su, H.-J.; Shu, C.-F.; Luo, L.; Diau, W.-G.; Cheng,
C.-H.; Duan, J.-P.; Lee, G.-H. J. Mater. Chem. 2005, 15, 1035–1042.
(59) Hoganson, C. W.; Babcock, G. T. Biochemistry 1992, 31, 11874–
11880.
The reversibility of the second one-electron oxidation of 8
suggests that [{OCO}Ir(PCy3)2Cl]2þ (82þ) is the initial pro-
duct; either an Ir(V) species or an Ir(IV) species with a ligand-
centered radical could be consistent with oxidation poten-
tials observed previously for Ir(IV) complexes and pheno-
lates.52,53,59,60,62,63,67 Unfortunately, the appearance of
multiple signals in both the EPR and NMR spectra indicates
that 82þ is unstable, precluding characterization or a detailed
(60) Halfen, J. A.; Jazdzewski, B. A.; Mahapatra, S.; Berreau, L. M.;
Wilkinson, E. C.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1997,
119, 8217–8227.
electronic description of 82þ
.
€
(61) Chaudhuri, P.; Hess, M.; Muller, J.; Hildenbrand, K.; Bill, E.;
€
Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599–
9610.
(65) Hulsebosch, R. J.; van den Brink, J. S.; Nieuwenhuis, S. A. M.;
Gast, P.; Raap, J.; Lugtenburg, J.; Hoff, A. J. J. Am. Chem. Soc. 1997,
119, 8685–8694.
(66) Kim, S. H.; Aznar, C.; Brynda, M.; Silks, L. A. “P.”; Michalczyk,
R.; Unkefer, C. J.; Woodruff, W. H.; Britt, R. D. J. Am. Chem. Soc. 2004,
126, 2328–2338.
(62) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1992, 3477–3482.
(63) Larsen, S. K.; Pierpont, C. G. J. Am. Chem. Soc. 1988, 110, 1827–
1832.
(64) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders:
Philadelphia, PA, 1977; pp 467-509.
(67) Castillo-Blum, S. E.; Richens, D. T.; Sykes, A. G. Inorg. Chem.
1989, 28, 954–960.

96 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
Figure 9. (a) Depiction of the HOMO for the fully optimized
gas-phase structure of 9. (b) Depiction of the SOMO for the fully
optimized gas-phase structure of 9þ.
An intriguing structural feature of these complexes is the
dependence of the geometry of the OCO ligand on electronic
and steric properties, as has been observed for related ligands
with other metals.68-71 As noted earlier, the ligand is
virtually planar (C2v symmetry) for the mixed carbonyl-
acetonitrile complexes in the structure of 6/7, but is signi-
ficantly distorted in chloro complex 8. Analogous differences
can be readily seen in the side-on views of the calculated
structures for 9 and 9þ (Figures 7 and 8): the ligand is
distorted from planarity to C2 symmetry in the former but
has C2v symmetry in the latter. This is reflected in the
calculated Cipso-O1-O2-Cipso torsion angles, which are
0.3ꢀ in 9þ, similar to that found experimentally for 6/7, and
62.62ꢀ in 9, significantly greater than the two (significantly
different between themselves!) experimentally observed va-
lues in 80 (15.72(26)ꢀ) and 800 (35.19(26)ꢀ).
Figure 7. Illustration (a) and fully optimized gas-phase struc-
tural drawings (b) of 9.
In order to assess the degree to which the difference
between the calculated and experimental values might be
due to the substantially greater steric bulk of the real
molecule compared to the model, we calculated a geometry
for complex 8, with the actual ligands, that was optimized
using QMMM. The molecular orbitals and in particular the
HOMO of 8 (shown in the Supporting Information) are
virtually identical to those of the model compound 9. How-
ever, the calculated structure of 8 (Figure 10) has a much
smaller Cipso-O1-O2-Cipso torsion angle of 9.55ꢀ, closer to
the experimental value for 80, suggesting that the steric bulk
of the PCy3 ligands in 8 does play a significant role.
One might expect the nonplanar C2-geometry to be elec-
tronically favored in 8 and 9 because of the π-antibonding
interaction between the phenolate ligands and the metal
center in the HOMO: increasing the Cipso-O1-O2-Cipso
torsion angle would decrease the overlap and reduce this
unfavorable contribution. This argument is consistent with
the C2v conformation found by calculation for 9þ, where the
orbital containing the unfavorable interaction is only singly
occupied, so that its energetic cost may be outweighed by
increased bonding interactions in lower energy orbitals in a
planar geometry. While we have not carried out any calcula-
tions on 6/7, the fact that they are cationic (and contain an
Figure 8. Illustration (a) and fully optimized gas-phase struc-
tural drawings (b) of 9þ.
Calculational Studies and OCO Geometries. DFT calcula-
tions were performed on [{OCO0}Ir(PH3)2Cl] (9) and
[{OCO0}Ir(PH3)2Cl]þ (9þ), where OCO0=1,3-di(2-hydroxy-
phenyl)imidazolyl, as simpler (and less sterically crowded)
models for 8 and 8þ. The optimized structures of 9 and 9þ
are shown in Figures 7 and 8; the parameters of 9 agree well
with those determined experimentally for 8 (see the Sup-
porting Information for a side-by-side comparison of the
bond lengths and angles in 80, 800, 9, and 9þ), with the
exception of the deviation from planarity of the OCO
ligands (see below). The HOMO of 9 and the SOMO of 9þ
are fairly similar (Figure 9); both have considerable metal
character but are substantially delocalized onto the OCO0
ligand, with significant π-antibonding interactions between
iridium and the phenolate oxygens as well as within the
ligand π-orbitals. The degree of metal character and the
highly delocalized nature of the calculated SOMO appear
consistent with the experimental g-value as well as the
lack of hyperfine splitting in the low-temperature EPR
spectrum of 8þ.
(68) Tonks, I. A.; Henling, L. M.; Day, M. W.; Bercaw, J. E. Inorg.
Chem. 2009, 48, 5096–5105.
(69) Agapie, T.; Henling, L. M.; DiPasquale, A. G.; Rheingold, A. L.;
Bercaw, J. E. Organometallics 2008, 27, 6245–6256.
(70) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27,
6123–6142.
(71) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959.

Article
Organometallics, Vol. 29, No. 1, 2010 97
Figure 10. Optimized gas-phase structure of 8 determined by QMMM calculations. The Cipso-O1-O2-Cipso torsion angle is 9.55ꢀ.
Hydrogen atoms have been removed for clarity.
Figure 12. Relative energies for optimized structures of 9þ
Figure 11. Relative energies for optimized structures of 9 when
the Cipso-O1-O2-Cipso torsion angle is set at various angles
between 0ꢀ and 90ꢀ.
when the Cipso-O1-O2-Cipso torsion angle is set at various
angles between 0ꢀ and 35ꢀ.
electron-withdrawing CO ligand in 7) might similarly attenuate
the antibonding contribution and favor planarity. The drastic
difference in the calculated Cipso-O1-O2-Cipso torsion angles
for 8 and 9 is likely due to the steric bulk of the PCy3 ligands,
which favors the sterically less demanding planar structure
relative to the electronically preferred C2-symmetry.
To assess the overall effect of the Cipso-O1-O2-Cipso torsion
angle on the energy of the molecule, we performed linear transit
calculations on 9 and 9þ with the Cipso-O1-O2-Cipso torsion
angle held constant at various angles between 0ꢀ and 90ꢀ for 9
and 0ꢀ and 35ꢀ for 9þ. The dependence of the relative energy on
torsion angle (Figures 11 and 12) confirms the expected prefer-
ences;planarity for 9þ, distortion for 9;but also indicates that
the potential well is fairly shallow, with only a small energetic
cost for changing the degree of distortion. In this light, it seems
reasonable to account for the dramatic differences in torsion
angle between model 9 and real molecule 8, as well as those
between the two conformers 80 and 800, in terms of steric
crowding and crystal packing, respectively.
techniques or in a glovebox under a nitrogen atmosphere as
described previously.72 (Under standard glovebox conditions
purging was not performed between uses of petroleum ether,
diethyl ether, benzene, toluene, and tetrahydrofuran; thus when
any of these solvents were used, traces of all these solvents were
in the atmosphere and could be found intermixed in the solvent
bottles.) All NMR solvents were purchased from Cambridge
Isotopes Laboratories, Inc. The solvents for air- and moisture-
sensitive reactions were dried by passage through a column
of activated alumina followed by storage under dinitrogen.
All other materials were used as received. 2-Amino-4-tert-
butylphenol, oxalyl chloride, triethylamine, BH3-THF (1 M in
THF), triethyl orthoformate, potassium hexamethyldisilazide
(potassium bis(trimethylsilyl)amide), and [FeCp2][PF6]73 were
purchased from Aldrich. Chloro-1,5-cyclooctadiene Ir(I) dimer
was purchased from Strem Chemicals, Inc. 1H, 13C, 31P, and 19
F
NMR spectra were recorded on Varian Mercury 300 spectro-
meters at room temperature, unless indicated otherwise. Chemical
shifts are reported with respect to residual internal protio solvent
for 1Hand13C{1H} NMR spectra. H3PO4 and CFCl3 were used as
external standards to reference 31P and 19F NMR spectra, respec-
tively, at 0 ppm. High-resolution mass spectra (HRMS) were
obtained at the California Institute of Technology Mass Spectral
Facility. Elemental analyses were obtained by Midwest Microlab,
LLC, located in Indianapolis, IN, and Columbia Analytical
Services (formerly Desert Analytics) located in Tucson, AZ.
All electrochemical measurements were carried out on a
Bioanalytics BAS100B/W. These measurements were performed
under a nitrogen or argon atmosphere in 0.3 M tetrabutylammo-
nium tetrafluoroborate solutions. Ferrocene was used as an
Conclusions
The diphenolate imidazolyl-carbene ligand 1 has been
shown to be capable of stabilizing both Ir(I) and Ir(III)
complexes, including a complex capable of catalyzing olefin
hydrogenations and complexes with interesting structural and
electronic properties. The fact that 1 can support Ir(I) com-
plexes, as well as Ir(III) complexes capable of undergoing two
reversible one-electron oxidations, is promising for the incor-
poration of this ligand into redox-active catalysts.
(72) Burger, B. J.; Bercaw, J. E. In New Developments in the Synthesis,
Manipulation, and Characterization of Organometallic Compounds;
Wayda, A., Darensbourg, M. Y., Eds.; American Chemical Society:
Washington, DC, 1987; Vol. 357.
Experimental Section
(73) Ferrocenium(III) hexafluorophosphate was obtained from the
supplier with a small amount of [BF4]-; hence, all [PF6]- salts synthe-
sized are contaminated with a small amount of the [BF4]- salt.
General Considerations. All air- and/or moisture-sensitive
compounds were manipulated using standard Schlenk

98 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
internal standard, and all potentials are referenced to the ferro-
cene/ferrocenium couple. For cyclic voltammetry experiments,
the working electrode was a glassy carbon disk (2 mm diameter);
the counter electrode was a coiled platinum wire; and a silver wire
was used as a pseudoreference electrode. In the bulk electrolysis
experiments, the glassy carbon disk was replaced with a platinum
wire basket for the working electrode.
The spectrometer used for all EPR experiments was an
X-band Bruker EMX with a standard TE102 cavity. Approxi-
mately 250 μL samples were transferred to an EPR tube, frozen
with liquid nitrogen, and then placed in a modified TE102 cavity
fit with a liquid helium-powered cryostat. This allowed EPR
spectra to be obtained at temperatures between 7 and 77 K.
Single-crystal X-ray diffraction samples were prepared by
decanting or pipetting off any solvent and then coating the
crystals with Paratone N oil. The crystals were then transferred
to a microscope slide. Samples were selected and mounted on a
glass fiber with Paratone N oil. Data collection was carried out
on a Bruker Smart 1000 CCD diffractometer. The structures
were solved by direct methods.
Density functional calculations were carried out using Gauss-
ian 03 revision D.01.74 Calculations on the model systems (with
minimal steric bulk) were performed using the nonlocal ex-
change correction by Becke75,76 and nonlocal correlation
corrections by Perdew,77 as implemented using the bvp86 key-
word in Gaussian. The following basis sets were used:
LANL2DZ78-80 for Ir atoms, Stuttgart-Dresden81,82 for phos-
phorus atoms, and 6-31G** basis set for all other atoms.
Pseudopotentials were utilized for Ir and P atoms, using the
LANL2DZ ECP for Ir and the Stuttgart-Dresden potential
for P, as these gave the best agreement between the calculated
and experimental structure. For calculations on full experi-
mental systems, QMMM calculations were performed using
ONIOM(bvp86:UFF).83 In these calculations the tertiary-butyl
groups on the tridentate diphenolate imidazolyl-carbene ligands
and the cyclohexyl groups on the phosphine ligands were
calculated at the UFF level and the rest of the molecule was
calculated using DFT. Initial geometries were obtained using
the coordinates from X-ray structures, and all optimized struc-
tures were verified using frequency calculations to check that
they did not contain any imaginary frequencies. Isosurface plots
were made using the Molekel program.84
Synthesis of 1,3-Di(2-hydroxy-5-tert-butylphenyl)imidazolium
Chloride (1a). For preparation of the precursor, N,N0-di-
(2-hydroxy-5-tert-butylphenyl)oxalamide, 4.0 mL (5.9 g, 0.046
mol) of oxalyl chloride was combined with 35 mL of CH2Cl2
under argon in a Schlenk flask. Then 100 mL of CH2Cl2 was
used to dissolve 15.0 g (0.091 mol) of 2-amino-4-tert-butylphe-
nol in a second Schlenk flask, and 12.6 mL (9.1 g, 0.090 mol) of
triethylamine was added. Both solutions were then cooled to
273 K. The solution of 2-amino-4-tert-butylphenol and triethy-
lamine was slowly transferred by cannula into the solution of
oxalyl chloride, producing a white gas. Additional CH2Cl2 was
used to wash in any remaining 2-amino-4-tert-butylphenol and
triethylamine. The resulting solution was stirred for 2 h, and
then 150 mL of H2O was added. CH2Cl2 was used to extract the
organic products, the solvent was removed under vacuum, and
the resulting solid was washed with both ethyl acetate and
petroleum ether and dried under vacuum, giving a white
1
solid (9.77 g, 0.0255 mol, 56.2% yield). H NMR (300 MHz,
N,N-dimethylformamide (DMF)-d7): δ 10.61 (s, 2H), 10.04
(s, 2H), 8.45 (d, 4J = 2.34 Hz, 2H), 7.11 (dd, 3J = 8.55 Hz,
3
4J =2.33 Hz, 2H), 6.99 (d, J =8.55 Hz, 2H), 1.31 (s, 18H).
13C{1H} NMR (75 MHz, DMF-d7): δ 157.7, 145.4, 142.5, 125.1,
122.4, 117.0, 114.8, 31.4. HRMS (FABþ): m/z calcd for
C22H28O4N2 þ H, 385.2127; found, 385.2108 (M þ H). Anal.
Calcd for C22H28O4N2: C, 68.73; H, 7.34; N, 7.29. Found: C,
68.53; H, 7.25; N, 7.33.
A 1.00 g (2.60 mmol) sample of N,N0-di-(2-hydroxy-5-tert-
butylphenyl)oxalimide was weighed into an oven-dried Schlenk
flask and dissolved in 26.5 mL of BH3-THF (1 M in THF, 26.5
mmol). The solution turned bright orange, accompanied by
vigorous bubbling; it was heated to 70 ꢀC and stirred for 19 h,
then cooled to room temperature, yielding
a trans-
parent brown solution. While still stirring, methanol was added
until all bubbling ceased; then 5.1 mL of 12 M HCl was added,
and the solvent removed under reduced pressure. The resulting
solid was redissolved in methanol, and solvent was again
removed under reduced pressure; this process was repeated
two more times, resulting in a white solid (the dihydrochloride
salt of the diimine, which was not characterized). In a Schlenk
flask under argon, the solid was suspended in 10.5 mL of
triethylorthoformate and heated to 100 ꢀC with vigorous stir-
ring. The solid still did not dissolve, even when another 10 mL of
triethylorthoformate was added. The suspension was then
heated for 2 h at 120 ꢀC, during which a tan color appeared,
then filtered while still warm, and the resulting solid was washed
with diethyl ether. Further solid deposited from the filtrate upon
cooling. This solid was collected, combined with the first frac-
tion, and dried under reduced pressure to yield 1a as a white
solid (777 mg, 1.93 mmol, 74.2%). 1H NMR (300 MHz,
dimethylsulfoxide (DMSO)-d6): δ 10.74 (br s, 2H), 9.49 (s,
(74) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
4
3
4
1H), 7.38 (d, J=2.09 Hz, 2H), 7.27 (dd, J=8.69 Hz, J=
2.09 Hz, 2H), 7.05 (d, 3J=8.69 Hz), 4.57 (s, 4H), 1.27 (s, 18H).
13C{1H} NMR (75 MHz, DMSO-d6): δ 156.7, 148.0, 142.3,
125.7, 123.0, 120.3, 116.5, 49.8, 34.0, 31.2. HRMS (FABþ): m/z
calcd for C23H31O2N2, 367.2386; found, 367.2390 (Mþ). Anal.
Calcd for C23H31O2N2Cl: C, 68.55; H, 7.75; N, 6.95. Found: C,
68.30; H, 7.78; N, 6.85.
(75) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098–
3100.
(76) Becke, A. D. J. Chem. Phys. 1988, 88, 1053–1062.
Synthesis of [K][{OCO}Ir(cod)] (2). In a glovebox, 1.5 g (4.0
mmol) of 1a and 2.4 g (12 mmol) of potassium hexamethyldi-
silazide were dissolved in a minimal amount of THF in an
Erlenmeyer flask, and the solution was stirred for 30 min. In a
separate flask, chloro-1,5-cyclooctadiene Ir(I) dimer (1.33 g,
1.98 mmol) was dissolved in a minimal amount of THF and
slowly added to the solution of 1a and potassium hexamethyl-
disilazide. The resulting bright orange solution was stirred for
30 min, and the solvent was removed under reduced pressure.
The resulting orange solid was stirred in methanol for 12 h under
argon, solvent was again removed under reduced pressure, the
(77) Perdew, J. P. Phys. Rev. B 1986, 33, 8800–8802.
(78) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283.
(79) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.
(80) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310.
€
(81) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123–141.
€
(82) Bergner, A.; Dolg, M.; Kuchle, W.; Stoll, H.; Preuss, H. Mol.
Phys. 1993, 80, 1431–1441.
(83) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.;
Sieber, S.; Morokuma, K. J. J. Phys. Chem. 1996, 100, 19357–19363.
(84) Flukiger, P.; Portmann, H. P. L.; Weber, J. MOLEKEL; Swiss
Centre for Scientfic Computing: Manno, Switzerland, 2000; Vol. 1.

Article
Organometallics, Vol. 29, No. 1, 2010 99
1
small amounts of cyclooctene were observed after 1.5 h by H
resulting solid was dissolved in THF and stirred for 15 min, the
solvent was removed under reduced pressure. The resulting solid
was dissolved in benzene and filtered, followed by lyophilization
of the filtrate to give a bright orange powder (2.2 g, 3.1 mmol,
NMR, while the cyclooctane peak continued to grow. Another
1H NMR spectrum was taken after 18 h; the spectrum indicated
that the coordinated 1,5-cyclooctadiene of 3 was completely
hydrogenated to cyclooctane. No signals were observed in the
1H NMR upfield of 0 ppm, even after the sample was heated at
90 ꢀC for 12 h. After this period of heating, the solvent was
removed under reduced pressure, and the sample was redis-
solved in CD3CN. Heating the sample for at least 12 h generated
4 as the major product.
1
79%). H NMR (300 MHz, THF-d8): δ 6.81 (m, 4H), 6.43 (d,
3J=8.62 Hz, 2H), 3.90 (app. t, 3J=8.81 Hz, 2H), 3.66 (app. t, 3J=
8.81 Hz, 2H), 3.51 (br s, 4H), 1.84 (br s, 4H), 1.38 (m, 4H), 1.26
(s, 18H). 1H NMR (300 MHz, CD3OD): δ 7.19 (d, 4J=2.58 Hz,
2H), 6.98 (dd, 3J=8.37 Hz, 4J=2.58 Hz, 2H), 6.66 (d, 3J=8.37
Hz, 2H), 4.53 (br s, 2H), 4.27 (br s, 2H), 2.40 (br s, 2H), 3.74 (br s,
2H), 2.92 (br s, 2H), 1.92 (br s, 2H), 1.55 (br s, 2H), 1.33 (br m,
20H). 13C{1H} NMR (75 MHz, THF-d8): δ 196.8, 161.1, 134.3,
132.6, 123.9, 119.6, 51.6, 34.3, 32.4, 32.2. HRMS (FABþ): m/z
calcd for C31H40N2O2Ir, 665.2719; found, 665.2745 (Mþ),
557.1913 (Mþ - cod).
Hydrogenation of Cyclohexene Catalyzed by 3. In a glovebox,
a 1.0 mL THF-d8 solution containing 10 mg (0.012 mmol) of 3
and 22.0 μL (17.8 mg, 0.217 mmol) of cyclohexene was prepared
in a 1.0 mL volumetric flask. Then 500 μL of this solution was
syringed into a 1.5 mL high-pressure, sapphire NMR tube,
which was filled to a pressure of 900 psi with dihydrogen. The
NMR tube was inverted several times to ensure mixing between
Crystallization of 2 with 18-Crown-6 Ether. In a glovebox,
saturated solutions of 2 (40 mg, 0.099 mmol) and 18-crown-6
ether (26.3 mg, 0.0995 mmol) in benzene were prepared.
Another 0.5 mL of benzene was added to each solution, and
the solution of 18-crown-6 ether was slowly added to the
solution of 2 in a 20 mL vial. The vial was sealed and left to
stand; orange crystals formed over the course of approximately
one month.
1
the headspace and the solution. H NMR spectra were recor-
ded at intervals of 608 s, starting about 630 s after mixing
commenced.
In Situ Generation of [{OCO}Ir(PMe3)3][PF6] (5). In a glove-
box, 10 mg (0.012 mmol) of 3 was dissolved in approximately
0.6 mL of THF-d8 and transferred to a J-Young NMR tube.
Then 5.0 μL (0.048 mmol) of PMe3 was added to the solution.
The NMR tube was then sealed, and the solution was mixed.
The NMR tube was then heated at 90 ꢀC in an oil bath for more
than 12 h. Solvent and other volatiles were removed under
Synthesis of [{OCO}Ir(cod)(MeCN)][PF6] (3). In a glovebox,
a saturated acetonitrile solution of 1.18 g (3.55 mmol) of
[FeCp2][PF6] was slowly added to a saturated acetonitrile solu-
tion of 1.00 g (1.42 mmol) of 2. The resulting solution was stirred
for 30 min and filtered. The solvent was removed from the
filtrate under reduced pressure, and the resulting solid was
washed with petroleum ether and diethyl ether and dried under
reduced pressure to give 550 mg (0.65 mmol, 46% yield) of
yellow solid. 1H NMR (300 MHz, CD3CN): δ 7.02 (dd, 3J=8.41
Hz, 4J=2.29 Hz, 2H), 6.95 (d, 3J=2.29 Hz, 2H), 6.87 (d, 3J=8.41
Hz, 2H), 6.35 (m, 2H), 5.12 (m, 2H), 4.51 (m, 2H), 4.29 (m, 2H),
2.59 (br m, 2H), 2.46 (br m, 2H), 2.35 (br m, 2H), 2.17 (br m, 2H),
1.30 (s, 18H). 13C{1H} NMR (75 MHz, THF-d8): δ 152.3, 141.4,
128.3, 127.4, 123.8, 120.3, 115.4, 94.0, 49.3, 35.1, 34.9, 31.8, 26.9.
1
reduced pressure, leaving a yellow solid. H NMR (300 MHz,
THF-d8): δ 6.97 (d, 4J=2.34 Hz, 2H), 6.85 (dd, 3J=8.59 Hz, 4J=
2.34 Hz, 2H), 6.52 (d, 3J=8.59 Hz, 2H), 4.48 (s, 4H), 1.66 (d, 2J=
9.18 Hz, 9H), 1.36 (app. t, J = 3.75 Hz, 18H), 1.28 (s, 18H).
31P{1H} NMR (THF-d8, 121 MHz): δ -32.8 (d, 2J=28.1 Hz,
2P), -44.1 (t, 2J=28.1 Hz, 1P), -144 (septet, 1J=700 Hz, 1P).
HRMS (FABþ): m/z calcd for C32H55IrN2O2P3, 785.3105;
found, 785.3097 (Mþ).
Synthesis of [{OCO}Ir(PCy3)2(MeCN)][PF6] (6). In a glove-
box, 810 mg (0.952 mmol) of 3 and 587 mg (2.09 mmol) of PCy3
were combined in a 500 mL Schlenk bomb with a Teflon stir bar.
CH3CN was added until both solids were fully dissolved. The
bomb was then sealed and heated at 90 ꢀC for 16 h while stirring.
The solvent and volatiles were removed under reduced pressure,
and the solid was washed consecutively with petroleum ether,
benzene, diethyl ether, and a minimal amount of acetonitrile.
Volatiles were again removed under reduced pressure; the solid
was redissolved in acetonitrile and filtered; and the solvent was
removed from the filtrate under reduced pressure, yielding a
1
31P{1H} NMR (CD3CN, 121 MHz): δ -141 (septet, J=700
Hz). 19F{1H} NMR (CD3CN, 282 MHz): δ -72.2 (d), -151.2 (s,
[BF4]-).73 HRMS (FABþ): m/z calcd for C33H43N3O2Ir:
706.2984; found, 706.2987 (Mþ), 665.2721 (Mþ - CH3CN),
557.1781 (Mþ - (CH3CN þ cyclooctadiene)).
In Situ Generation of [{OCO}Ir(MeCN)3][PF6] (4). In a
glovebox, 100 mg (0.118 mmol) of 3 was dissolved in about
25 mL of acetonitrile and transferred into a 50 mL Schlenk
bomb. The bomb was sealed and heated at 90 ꢀC for about 12 h.
It was then cooled to room temperature, and the solvent was
removed under reduced pressure. This solid was extracted with
diethyl ether, and again the solvent was removed under reduced
pressure. A 45 mg (0.054 mmol, 46%) amount of brown solid
1
bright yellow solid (750 mg, 0.57 mmol, 60%). H NMR (300
MHz, CD3CN): δ 6.89 (dd, 3J=8.44 Hz, 4J=2.31 Hz, 2H), 6.84
(d, 4J=2.31 Hz, 2H), 6.39 (d, 3J=8.44 Hz, 2H), 4.27 (s, 4H), 2.14
(m, 6H), 1.90 (br, 12H, overlaps with solvent peak), 1.67 (br m,
24H), 1.26 (s, 18H, overlaps with peak at 1.23 ppm), 1.23 (br m,
24H, overlaps with peak at 1.26 ppm); 13C{1H} NMR (75 MHz,
CD3CN): δ 150.8, 137.6, 127.3, 121.4, 119.3, 113.4, 46.5, 33.0
(apparent t, J=11 Hz), 30.5, 28.0, 27.2 (apparent t, J=3.8 Hz),
25.8. 31P{1H} NMR (CD3CN, 121 MHz): δ -7.3, -143 (septet,
1J=700 Hz). 19F{1H} NMR (CD3CN, 282 MHz): -71.0 (d, 1J=
705 Hz), -151.1. HRMS (FABþ, performed on the MeCN-d3
complex): m/z calcd for C61H94IrN2O2P2D3 - CD3CN,
1
was isolated. The peaks in the H NMR at 2.53 and 2.68 ppm
disappear over time in CD3CN, while the CH3CN peak at 1.97
1
ppm increases by approximately the same amount. H NMR
(300 MHz, CD3CN): δ 6.94 (m, 4H), 6.67 (d, 3J=8.37 Hz, 2H),
4.50 (s, 4H), 2.68 (s, <3H), 2.53 (s, <6H), 1.97 (s, CH3CN), 1.29
(s, 18H). 13C {1H} NMR (75 MHz, CD3CN, taken after peak at
2.68 ppm in 1H NMR disappeared): δ 153.8, 139.4, 128.3, 122.9,
119.9, 115.4, 48.8, 34.5, 31.8, 4.1, 1.8 (CH3CN). 31P{1H} NMR
1
(CD3CN, 121 MHz): δ -144 (septet, J = 700 Hz). 19F{1H}
1117.642; found, 1117.6417 (Mþ - CD3CN), 837.4271 (Mþ
-
NMR (CD3CN, 282 MHz): δ -72.1 (d, 1J=704 Hz), -150.7 (s,
[BF4]-), -150.8 (s, [BF4]-).73 HRMS (FABþ, performed on the
tris(MeCN-d3) complex): m/z calcd for C29H28IrO2N5D9,
689.341; found, 689.3156 (Mþ).
(CD3CN þ PC18H33)). Anal. Calcd for C61H97F6IrN3O2P3: C,
56.20; H, 7.50; N, 3.22; F, 8.74; Ir, 14.75. Found: C, 56.19; H,
7.33; N, 3.12; F, 8.5; Ir, 15.0.
In Situ Generation of [{OCO}Ir(PCy3)2(CO)][PF6] (7). In a
glovebox, 10 mg (0.0077 mmol) of 6 was dissolved in 0.6 mL of
CD3CN and placed in a 3.3 mL J-Young NMR tube. The
solution was degassed by the freeze-pump-thaw method,
and the tube was filled with 1 atm of CO at room temperature.
The tube was heated at 90 ꢀC, and the reaction was monitored by
Hydrogenation of 3. In a glovebox, a solution of 10 mg (0.012
mmol) of 3 in about 0.6 mL of THF-d8 was placed in a 3.3 mL
J-Young NMR tube. The solution was degassed by three
freeze-pump-thaw cycles. The tube was then filled with
1 atm dihydrogen. The solution turned from yellow to orange
over the course of about 15 min, and cyclooctane began to
1
1H NMR spectroscopy. H NMR signals corresponding to 7
1
appear (signal at 1.54 ppm in the H NMR spectrum). Very
appeared over the course of about 3 days, but did not increase

100 Organometallics, Vol. 29, No. 1, 2010
Weinberg et al.
further with continued heating. Yellow crystals appeared in the
J-Young tube slightly above the top of the solution; these were
identified as an approximately 4:1 ratio of 6 to 7 by single-crystal
X-ray diffraction. The 1H NMR spectrum of the solution phase
indicated a ratio of 6 to 7 that was approximately 2:1. When the
solution was degassed and heated for another 2 days, 7 was
converted back to 6, and the ratio of 6:7 increased to greater
than 8:1 based on the 1H NMR spectrum. NMR data for 7: 1H
NMR (300 MHz, CD3CN): δ 6.95 (dd overlapping with d at
6.95, 3J=9.22 Hz, 4J=2.23 Hz, 2H), 6.95 (d overlapping with dd
at 6.95, 4J=2.23 Hz, 2H), 6.48 (d, 3J=9.22 Hz, 2H), 4.46 (s, 4H),
1.1-2.2 (br multiplets for PCy3 overlapping with a water peak,
which was introduced with the CO; the solvent peak; the tert-
butyl peak of 7; and multiple peaks for 12, 66 H), 1.28 (s
overlapping with br multiplets for PCy3 of 6 and 7). 31P{1H}
NMR (CD3CN, 121 MHz): δ 1.43, -143 (septet, 1J=700 Hz).
C59H94IrN2O2P2: C, 61.46; H, 8.22; N, 2.43. Found: C, 61.7; H,
7.9; N, 2.3. CV in CH2Cl2: E1/2, V vs ferrocene at 0.100 V/s
(ΔEp): -0.22 V (98 mV), 0.58 V (98 mV); in THF at 0.050 V/s:
-0.20 V (226 mV), 0.58 V (202 mV); in DMF at 0.100 V/s: -0.13
V (53 mV), 0.59 V (65 mV).
Bulk Electrolysis of 8 with EPR and Mass Spectra of the
Oxidation Products. In a glovebox, a solution of 33 mg (0.029
mmol) of 8 and 1.01 g (3.07 mmol) of tetrabutylammonium
tetrafluoroborate ([NBu4][BF4]) in 10 mL of CD2Cl2 solution
was transferred into the working electrode side of the bulk
electrolysis cell. Then 78 mg (0.24 mmol) of [FeCp2][PF6] and
1.01 g (3.07 mmol) of [NBu4][BF4] were used to make a 10 mL
CD2Cl2 solution, and about 6 mL of this solution was trans-
ferred into the auxiliary side of the bulk electrolysis cell. Coulo-
metric oxidation of 8 was performed at a potential positive of the
first oxidation wave (0.25 V), and 0.92 faraday per mole of 8 was
passed (a plot of charge passed vs time is included in the
Supporting Information). A 250 μL amount of solution was
removed from the working electrode side of the bulk electrolysis
cell and sealed in a J-Young EPR tube, which was frozen at 77 K,
and the EPR spectrum recorded; the EPR sample was subse-
quently analyzed by mass spectroscopy. HRMS (FABþ): m/z
calcd for C59H94ClIrN2O2P2, 1152.61; found, 1152.66 (Mþ).
Coulometric oxidation of 8 was continued at a potential
positive of the second oxidation wave (0.96 V), and 0.99 faraday
per mole of 8 was passed (a plot of charge passed vs time is
included in the Supporting Information). The resulting solution
was analyzed by mass spectroscopy. HRMS (FABþ): m/z calcd
for C59H94ClIrN2O2P2, 1152.61; found, 1152.67 (Mþ).
IR (CD3CN): νCO, 2064 cm-1
.
In Situ Generation of [{OCO}Ir(PCy3)2(13CO)][PF6] (70).
13CO was substituted for ordinary CO in the above procedure.
1H NMR (300 MHz, CD3CN): δ 6.95 (dd overlapping with d at
6.95, 3J=9.22 Hz, 4J=2.23 Hz, 2H), 6.95 (d overlapping with dd
at 6.95, 4J=2.23 Hz, 2H), 6.48 (d, 3J=9.22 Hz, 2H), 4.46 (s, 4H),
1.1-2.2 (br multiplets for PCy3 overlapping with a water peak
(which was introduced with the CO), the solvent peak, the tert-
butyl peak of 70, and multiple peaks for 6, 66 H), 1.28 (s
overlapping with br multiplets for PCy3 of 6 and 70). 13C{1H}
NMR (75 MHz, CD3CN): δ 184.7 (free 13CO), 174.8 (t, 2J=9.7
Hz). 31P{1H} NMR (CD3CN, 121 MHz): δ 1.44 (d, 2J=9.7 Hz),
-143 (septet, 1J=700 Hz). IR (CD3CN): ν13CO, 2015.5 cm-1
.
Synthesis of [{OCO}Ir(PCy3)2Cl] (8). In a glovebox, approxi-
mately 10 mL of CH3CN was used to dissolve 166 mg (0.127
mmol) of 6 in a 20 mL vial. Then 14 mg (0.13 mmol) of
tetramethylammonium chloride was dissolved in about 2 mL
of CH3CN and added to the solution of 6. The resulting solution
was transferred to a 100 mL Schlenk bomb with a Teflon stir
bar. The Schlenk bomb was then sealed, and the solution was
allowed to stir for 3 days, during which time a yellow precipitate
formed. The precipitate was isolated by filtration and washed
with acetonitrile. The yellow solid was then dissolved in ben-
zene, which was then removed under reduced pressure. The
resulting solid was redissolved in CH2Cl2 and filtered. Removal
of solvent and other volatiles under reduced pressure resulted in
a yellow solid (95 mg, 0.082 mmol, 63%). Crystals were grown
by cooling a concentrated CH2Cl2 solution of 8 to -20 ꢀC. 1H
NMR (300 MHz, CD2Cl2): δ 6.81 (dd, 3J=8.56 Hz, 4J=2.41 Hz,
2H), 6.65 (d, 4J=2.41 Hz, 2H), 6.36 (d, 3J=8.56 Hz, 2H), 4.09 (s,
4H), 2.39 (br m, 6H), 2.08 (br m, 12H), 1.61 (br, 24H), 1.26 (s
overlapping with the br multiplet at 1.12, 18H), 1.12 (br multi-
plet overlapping with s at 1.26, 24H). 13C{1H} NMR (75 MHz,
CD2Cl2): δ 153.0, 136.5, 128.6, 121.6, 121.0, 113.0, 47.2, 34.1,
33.4 (apparent t, J=10.7 Hz), 32.0, 30.3, 29.0, 28.5 (apparent t,
J=4.8 Hz), 27.3. 31P{1H} NMR (CD2Cl2, 121 MHz): δ -12.7.
HRMS (FABþ): m/z calcd for C59H94ClIrN2O2P2, 1152.611;
found, 1152.6105 (Mþ); 1117.6425 (Mþ - Cl). Anal. Calcd for
Oxidation of 8 with Ferrocenium(III) Hexafluorophosphate. In
a glovebox, 10 mg (8.7 mmol) of 8 was dissolved in 7 mL of
CH2Cl2 in a vial. In a separate vial, 2 mg (6 mmol) of
[FeCp2][PF6] was dissolved in 1.7 mL of CH2Cl2. The ferro-
cenium solution was added to the solution of 8, while stirring.
Then 200 μL of this solution was transferred to a J-Young EPR
tube. The same EPR spectrum was obtained as for the one-
electron oxidation product in the coulometric oxidation of 8.
Acknowledgment. We thank Larry Henling and Mike
Day for X-ray crystallographic characterization of
complexes, Mona Shahgholi for mass spectroscopic
characterization of complexes, and Angel J. Di Bilio for
assistance with EPR experiments. We are grateful to BP
for financial support through the MC2 program. DFT
calculations were carried out using the Molecular Graphics
and Computation Facility, College of Chemistry, University
of California, Berkeley, California, with equipment support
from NSF grant CHE-0233882.
Supporting Information Available: X-ray crystallographic
files in CIF format for the structure determination of 2, 6/7,
and 8 and details of DFT calculations are available free of
charge via the Internet at http://pubs.acs.org.