Unsymmetrically bridging aryls of iridium

Article abstract of DOI:10.1021/om0304816

The iridium(I) complex [(cod)IrCl]₂ (1; cod = 1,5-cyclooctadiene) reacts with 2,6-disubstituted aryl Grignard reagents to give air-stable diiridium complexes (cod)IrBr(μ,k¹:η⁶-Ar)Ir-(cod) (Ar = 2,4,6-Me₃C₆H₂ (2), 2,6-Me₂C₆H₃ (3)). The aryl groups in these compounds are σ-bonded to one square-planar iridium(I) center and π-bonded to another (cyclooctadiene)- iridium(I) fragment. Spectroscopic and structural data indicate that the bonding is best described as a resonance hybrid between a zwitterionic σ-aryl/η⁶-arene structure and a neutral carbene/η⁵-cyclohexadienyl structure, with the former making a greater contribution. The resonance appears to stabilize both the iridium(I)-aryl σ- and π-bonds, with the result that the bridging aryl moiety is extremely chemically robust. The complexes react selectively at the square-planar iridium, undergoing ligand substitution of the cyclooctadiene or halide and oxidative addition of I₂.

Full text of DOI:10.1021/om0304816

4480
Organometallics 2003, 22, 4480-4489
Un sym m etr ica lly Br id gin g Ar yls of Ir id iu m
J ohn Muldoon and Seth N. Brown*
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall,
University of Notre Dame, Notre Dame, Indiana 46556-5670
Received J une 23, 2003
The iridium(I) complex [(cod)IrCl]2 (1; cod ) 1,5-cyclooctadiene) reacts with 2,6-disubsti-
tuted aryl Grignard reagents to give air-stable diiridium complexes (cod)IrBr(µ,κ1:η6-Ar)Ir-
(cod) (Ar ) 2,4,6-Me3C6H2 (2), 2,6-Me2C6H3 (3)). The aryl groups in these compounds are
σ-bonded to one square-planar iridium(I) center and π-bonded to another (cyclooctadiene)-
iridium(I) fragment. Spectroscopic and structural data indicate that the bonding is best
described as a resonance hybrid between a zwitterionic σ-aryl/η6-arene structure and a
neutral carbene/η5-cyclohexadienyl structure, with the former making a greater contribution.
The resonance appears to stabilize both the iridium(I)-aryl σ- and π-bonds, with the result
that the bridging aryl moiety is extremely chemically robust. The complexes react selectively
at the square-planar iridium, undergoing ligand substitution of the cyclooctadiene or halide
and oxidative addition of I2.
In tr od u ction
These compounds prove to be extraordinarily stable,
with both the σ-linkage to the one iridium and the η6-
linkage to the second iridium showing much greater
chemical stability than is typically shown by unbridged
aryl or arene complexes. A variety of ligand substitution
and oxidation or reduction reactions can be carried out
at the σ-bonded iridium center without disrupting the
iridium-carbon σ-bond or the π-bonded iridium frag-
ment. The unexpected stability of this diiridium frag-
ment is rationalized on the basis of resonance stabili-
zation by a carbene/η5-cyclohexadienyl canonical form,
which is supported by spectroscopic and structural
studies.
Both σ-bonded aryl groups and π-bonded η6-arenes
have played pivotal roles in the history of organometallic
chemistry. Indeed, these two classes of compounds have
been intertwined since the beginning of this century,
since Hein’s “polyphenylchromium” species discovered
in the early part of the 20th century proved to be η6-
arene complexes formed via the intermediacy of σ-bond-
ed aryls.1 However, compounds in which an aryl group
is simultaneously bonded both to one transition metal
atom in a σ sense and to a second transition metal in a
π sense generally require multistep procedures to
prepare. The π-arene complex can be formed first, with
the σ-bond being formed subsequently (e.g., eq 1).
Methods for forming the metal-carbon σ-bond include
lithiation followed by reaction with a transition metal
halide,2 directed metalation,3 and oxidative addition of
a carbon-halogen4 or carbon-sulfur5 bond. Less com-
monly, the σ/π-bridging mode can be attained by initial
formation of a σ-arylmetal complex followed by its
π-complexation (e.g., eq 2).6
Exp er im en ta l Section
Unless otherwise noted, all procedures were carried out on
the benchtop without special precautions to exclude air or
(2) (a) van Rooyen, P. H.; Schindehutte, M.; Lotz, S. Organometallics
1992, 11, 1104-1111. (b) Lotz, S.; Schindehutte, M.; van Rooyen, P.
H. Organometallics 1992, 11, 629-639. (c) Elschenbroich, C.; Schmidt,
E.; Metz, B.; Harms, K. Organometallics 1995, 14, 4043-4045. (d)
Meyer, R.; Wessels, P. L.; van Rooyen, P. H.; Lotz, S. Inorg. Chim.
Acta 1999, 284, 127-132.
(3) (a) Clark, G. R.; Metzler, M. R.; Whitaker, G.; Woodgate, P. D.
J . Organomet. Chem. 1996, 513, 109-134. (b) Djukic, J .-P.; Maisse,
A.; Pfeffer, M.; de Cian, A.; Fischer, J . Organometallics 1997, 16, 657-
667.
(4) (a) Heppert, J . A.; Morgenstern, M. A.; Scherubel, D. M.;
Takusagawa, F.; Shaker, M. R. Organometallics 1988, 7, 1715-1723.
(b) Richter-Addo, G. B.; Hunter, A. D.; Wichrowska, N. Can. J . Chem.
1990, 68, 41-48. (c) Dufaud, V.; Thivolle-Cazat, J .; Basset, J .-M.;
Mathieu, R.; J aud, J .; Waissermann, J . Organometallics 1991, 10,
4005-4015. (d) Aoki, T.; Ishii, Y.; Mizobe, Y.; Hidai, M. Chem. Lett.
1991, 615-618.
(5) (a) Dullaghan, C. A.; Sun, S.; Carpenter, G. B.; Weldon, B.;
Sweigart, D. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 212-214. (b)
Dullaghan, C. A.; Zhang, X.; Walther, D.; Carpenter, G. B.; Sweigart,
D. A.; Meng, Q. Organometallics 1997, 16, 5604-5606. (c) Zhang, X.;
Dullaghan, C. A.; Carpenter, G. B.; Sweigart, D. A.; Meng, Q. Chem.
Commun. 1998, 93-94. (d) Li, H.; Carpenter, G. B.; Sweigart, D. A.
Organometallics 2000, 19, 1823-1825.
Here we describe the efficient preparation of diiridium
complexes with σ/π-bridging 2,6-disubstituted aryl groups
directly from an iridium halide and Grignard reagents.
(6) (a) Hunter, A. D. Organometallics 1989, 8, 1118-1120. (b) Butler,
I. R.; Gill, U.; LePage, Y.; Lindsell, W. E.; Preston, P. N. Organome-
tallics 1990, 9, 1964-1970. (c) Hunter, A. D.; Ristic-Petrovic, D.;
McLernon, J . L. Organometallics 1992, 11, 864-871.
* Corresponding author. E-mail: Seth.N.Brown.114@nd.edu.
(1) (a) Seyferth, D. Organometallics 2002, 21, 1520-1530. (b)
Seyferth, D. Organometallics 2002, 21, 2800-2820.
10.1021/om0304816 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/30/2003

Unsymmetrically Bridging Aryls of Iridium
Organometallics, Vol. 22, No. 22, 2003 4481
moisture. When dry THF or Et2O was required, it was vacuum
transferred from sodium benzophenone ketyl. Benzene was
dried over sodium. All other reagents were commercially
available and used without further purification. 1H and 13C-
{1H} NMR spectra were measured on a Varian-300 FT-NMR
and are reported in parts per million downfield of TMS.
Infrared spectra were recorded as evaporated films on KBr
plates (unless otherwise noted) on a Perkin-Elmer Paragon
1000 FT-IR spectrometer. Elemental analyses were performed
by M-H-W Laboratories (Phoenix, AZ).
(I)Ir(cod) CHH′), 2.11 (m, 4H, (π-Ar)Ir(cod) CHH′), 2.00
(pseudo-q, 8 Hz, 4H, (π-Ar)Ir(cod) CHH′), 1.64 (m, 2H, (Ar)-
(I)Ir(cod) CHH′), 1.27 (m, 2H, (Ar)(I)Ir(cod) CHH′). IR (cm-1):
2976 (s), 2908 (s), 2870 (s), 2822 (s), 1470 (w), 1438 (m), 1441
(m), 1371 (m), 1323 (m), 1298 (w), 1204 (w), 1152 (w), 1140
(w), 1014 (m), 966 (w), 908 (w), 836 (w). Anal. Calcd for
Ir2C24H33I: C, 34.64; H, 3.96. Found: C, 34.52; H, 4.06.
cis-Ir (CO)2(Br )(µ-2,4,6-Me3C6H2)Ir (cod ) (5). Complex 2
(300 mg, 0.372 mmol) was dissolved in 10 mL of CH2Cl2.
Carbon monoxide was bubbled for 10 min through the solution.
The reaction mixture turned dark red within a few seconds,
then faded to light yellow after 5 min. After the solvent was
removed in vacuo, the residue was slurried in hexanes (50 mL)
and the insoluble product was collected by suction filtration.
The yellow crystals were washed with ether and air-dried to
furnish 263.5 mg of 5 (94%). 1H NMR (CD2Cl2): δ 5.88 (s, 2H,
Ar-H), 3.97 (m, 4H, alkene C-H), 2.44 (s, 3H, para-CH3), 2.38
(s, 6H, ortho-CH3), 2.24 (m, 4H, cod CHH′), 2.06 (pseudo-q, 9
Hz, 4H, cod CHH′). 13C{1H} NMR (CD2Cl2): δ 184.6 (CO),
171.8 (CO), 146.0 (ipso C), 116.8, 113.5, 97.9, 63.2, 33.3, 24.6,
19.1. IR (cm-1): 2943 (m), 2837 (m), 2040 (vs, νCO), 1957 (vs,
Ir (cod )(Br )(µ-2,4,6-Me3C6H2)Ir (cod ) (2). In the drybox, a
solution of [(cod)Ir(µ-Cl)]2 (1; Strem, 300 mg, 0.447 mmol; cod
) 1,5-cyclooctadiene) in dried THF (11 mL) in a 20 mL screw-
cap vial was treated with 1 equiv of a 1.0 M THF solution of
mesitylmagnesium bromide (Aldrich, 450 µL, 0.447 mmol). The
vial was immediately capped and shaken rapidly, and the
solution turned yellow. After standing for 1 h, the solution was
opened to the air. The solution was transferred to a 50 mL
round-bottom flask and diluted with fresh THF to a volume
of 20 mL. Tetrabutylammonium bromide (Aldrich, 1.5 g, 4.65
mmol) was added and the mixture stirred for 22 h. The
insoluble material was removed by suction filtration. After the
solvent was removed in vacuo, the residue was slurried in
methanol (50 mL) and the insoluble product was collected by
suction filtration. The yellow crystals were washed with ether
and air-dried to furnish 322.3 mg of analytically pure Ir(cod)-
(Br)(µ-2,4,6-Me3C6H2)Ir(cod) (2, 90%). 1H NMR (C6D6): δ 5.09
(m, 2H, alkene C-H trans to mes), 4.63 (s, 2H, Ar-H), 3.77 (m,
4H, (π-Ar)Ir(cod) alkene C-H), 2.48 (m, 2H, alkene C-H trans
to Br), 2.34 (s, 6H, ortho-CH3), 2.31 (m, 8H, cod CHH′), 1.83
(pseudo-q, J ) 8 Hz, 4H, (η6-Ar)Ir(cod) CHH′), 1.78 (pseudo-t,
J ) 8 Hz, 2 H, (Ar)(Br)Ir(cod) CHH′), 1.55 (pseudo-t, J ) 8
Hz, 2H, (Ar)(Br)Ir(cod) CHH′), 1.48 (s, 3H, para-CH3). 13C{1H}
NMR (CD2Cl2): δ 171.6 (ipso C), 112.6, 110.5, 95.9, 78.5, 62.6,
53.9, 33.8, 33.2, 30.6, 23.2, 18.7. IR (cm-1): 2974 (s), 2946 (s),
2912 (s), 2872 (s), 2825 (s), 1441 (m), 1373 (m), 1324 (m) 1298
(w), 1262 (w), 1241 (w), 1207 (w), 1157 (w), 1076 (w), 1018
(m), 996 (m), 967 (w), 933 (w), 918 (w), 881 (w), 864 (w), 839
(w), 809 (w), 783 (w), 744 (m), 702 (w), 673 (w). Anal. Calcd
for Ir2C25H35Br: C, 37.54; H, 4.40. Found: C, 37.54; H, 4.59.
Ir (cod )(Br )(µ-2,6-Me2C6H3)Ir (cod ) (3). The 2,6-dimeth-
ylphenyl complex 3 was prepared as described above, substi-
tuting 2,6-dimethylphenylmagnesium bromide (Aldrich) for the
mesitylmagnesium bromide. The yield was 310.3 mg (88%).
1H NMR (C6D6): δ 5.17 (t, 6 Hz, 1H, para), 5.08 (m, 2H, alkene
C-H trans to Ar), 4.57 (d, 6 Hz, 2H, meta), 3.84 (m, 4H, (π-
Ar)Ir(cod) alkene C-H) 2.43 (m, 2H, alkene C-H trans to Br),
2.28 (s, 6H, CH3), 2.25 (m, 8H, cod CHH′), 1.83 (pseudo-q, 8
Hz, 4H, (π-Ar)Ir(cod) CHH′), 1.77 (pseudo-t, 8 Hz, 2 H, (Ar)-
(Br)Ir(cod) CHH′), 1.53 (pseudo-t, 8 Hz, 2H, (Ar)(Br)Ir(cod)
CHH′). 13C{1H} NMR (CD2Cl2): δ 174.9 (ipso C), 113.4, 94.8,
93.4, 78.8, 61.6, 54.0, 33.7, 33.2, 30.5, 23.5. IR (cm-1): 3060
(w), 3013 (w), 2971 (s), 2910 (s), 2871 (s), 2821 (s), 1472 (w),
1439 (m), 1401 (m), 1372 (s), 1324 (m), 1299 (w), 1263 (w),
1233 (w), 1206 (w), 1141 (w), 1015 (s), 993 (w), 967 (w), 910
(w), 839 (w), 820 (w), 807 (w). Anal. Calcd for Ir2C24H33Br: C,
36.73; H, 4.24. Found: C, 36.64; H, 4.40.
νCO), 1443 (s), 1373 (s), 1328 (m), 1160 (m), 1020 (s), 954 (w),
915 (m), 875 (w), 841 (w), 711 (w), 676 (w), 608 (s), 562 (w),
495 (m). Anal. Calcd for Ir2C19H23O2Br: C, 30.52; H, 3.10.
Found: C, 30.56; H, 2.99.
cis-Ir (CO)2(Br )(µ-2,6-Me2C6H3)Ir (cod ) (6) was prepared
analogously to 5. The yield from 300 mg of 3 was 266 mg (95%).
1H NMR (CD2Cl2): δ 6.57 (t, 6 Hz, 1H, para), 5.85 (d, 6 Hz,
2H, meta), 4.05 (m, 4H, alkene C-H), 2.35 (s, 6H, CH3), 2.21
(m, 4H, cod CHH′), 2.01 (pseudo-q, 9 Hz, 4H, cod CHH′). 13C-
{1H} NMR (CD2Cl2): δ 184.5 (CO), 171.8 (CO), 148.3 (ipso C),
117.8, 97.0, 96.6, 62.2, 33.2, 24.8. IR (cm-1): 2038 (vs, νCO),
1961 (vs, νCO), 1442 (s), 1411 (w), 1373 (s), 1326 (m), 1022 (m),
920 (w), 876 (w), 841 (w). Anal. Calcd for Ir2C18H21O2Br: C,
29.47; H, 2.88. Found: C, 29.38; H, 2.70.
cis-Ir (CO)2(I)(µ-2,6-Me2C6H3)Ir (cod ) (7) was prepared
analogously to 5. The yield from 300 mg of 4 was 259.6 mg
1
(92%). H NMR (CD2Cl2): δ 6.55 (t, 6 Hz, 1H, para), 5.83 (d,
6 Hz, 2H, meta); 4.03 (m, 4H, alkene C-H), 2.27 (s, 6H, CH3),
2.20 (m, 4H, cod CHH′), 2.02 (pseudo-q, 9 Hz, 4H, cod CHH′).
IR (cm-1): 2978 (m), 2940 (m), 2904 (m), 2037 (vs, νCO), 1951
(vs, νCO), 1476 (w), 1440 (s), 1400 (m), 1374 (m), 1329 (w), 1302
(w), 1264 (w), 1028 (m), 921 (w), 858 (w), 788 (w), 739 (w), 764
(w). Anal. Calcd for Ir2C18H21O2I: C, 27.68; H, 2.69. Found:
C, 27.63; H, 2.46.
cis-Ir (CO)2(2,6-Me2C6H3)(µ-2,6-Me2C6H3)Ir (cod ) (8). Cis-
Ir(CO)2(I)(µ-2,6-Me2C6H3)Ir(cod) (7, 200 mg, 0.256 mmol) was
dissolved in 12 mL of dry THF in a 20 mL screw-cap vial in
the drybox. The solution was treated with 5 equiv of a 1.0 M
THF solution of 2,6-dimethylphenylmagnesium bromide (Al-
drich, 1.28 mL, 1.28 mmol), and the vial was capped and
allowed to stand at room temperature in the drybox for a week.
The yellow crystals of 8 that deposited were collected by
suction filtration in the air, washed with methanol followed
by CH2Cl2, and air-dried to give 171.5 mg of 8 (88%). The
1
compound was too insoluble to obtain H or 13C NMR spectra.
IR (Nujol mull, cm-1): 2011 (vs, νCO), 1935 (vs, νCO), 1208 (w),
1156 (s), 1071 (w), 1005 (w), 972 (w), 892 (w), 768 (m). Anal.
Calcd for Ir2C26H30O2: C, 41.13; H, 3.96. Found: C, 41.30; H,
4.01.
Ir (cod )(I)(µ-2,6-Me2C6H3)Ir (cod ) (4). The iodo complex
was prepared as described for the bromo complex 3 through
the point where the crude reaction mixture was exposed to
the air. At this point, the solution was diluted with acetone to
a volume of 20 mL. Sodium iodide (Aldrich, 524.6 mg, 3.5
mmol, 7.8 equiv) was added and the mixture stirred 6 h in
the dark. After filtration and evaporation of the solvent, the
residue was slurried in methanol (50 mL) and the yellow
crystalline product collected by suction filtration, washed with
ether, and air-dried to furnish 316.2 mg of analytically pure
cis-Ir (CO)2(2,4,6-Me3C6H2)(µ-2,6-Me2C6H3)Ir (cod) (9). The
mixed diaryl complex 9 was prepared as described above,
substituting mesitylmagnesium bromide for 2,6-dimethyl-
phenylmagnesium bromide. The yield from 200 mg of 7 was
1
162.5 mg (82%). H NMR (CD2Cl2): δ 6.62 (s, 2H, Me3C6H2),
6.44 (t, 6.3 Hz, 1H, para, µ-2,6-Me2C6H3), 5.76 (d, 5.7 Hz, 2H,
meta, µ-2,6-Me2C6H3), 4.06 (m, 4H, alkene C-H), 2.38 (s, 3H,
para, Me3C6H2), 2.34 (s, 6H, µ-2,6-(CH3)2C6H3), 2.23 (m, 4H,
cod CHH′), 2.13 (s, 6H, ortho, Me3C6H2), 2.03 (pseudo-q, 8.1
Hz, 4H, cod CHH′). IR (Nujol mull, cm-1): 2010 (vs, νCO), 1934
(vs, νCO), 1208 (w), 1156 (m), 1078 (w), 1014 (w), 973 (m), 892
1
iodo complex 4 (85%). H NMR (CD2Cl2): δ 6.29 (t, 6 Hz, 1H,
para), 5.59 (d, 6 Hz, 2H, meta), 4.40 (m, 2H, alkene C-H trans
to Ar), 4.05 (m, 4H, (π-Ar)Ir(cod) alkene C-H), 2.32 (s, 6H,
CH3), 2.30 (m, 2H, alkene C-H trans to I), 2.26 (m, 4H, (Ar)-

4482 Organometallics, Vol. 22, No. 22, 2003
Muldoon and Brown
(w), 849 (m). Anal. Calcd for Ir2C27H32O2: C, 41.95; H, 4.14.
Found: C, 41.77; H, 4.27. The compound was too insoluble to
obtain 13C NMR spectra.
above. The crystal was monoclinic (space group P21/n). The
unit cell was determined based on 25 reflections with 15.07°
< θ < 15.93°. A total of 4199 reflections with 2θ < 50° were
collected. Crystal quality was monitored by recording three
standard reflections approximately every 160 reflections mea-
sured; decay was negligible. An empirical absorption correction
was applied (µ ) 13.764 cm-1, transmission factors 0.4266-
0.9678). The iridium atoms were located on a Patterson map,
and remaining non-hydrogen atoms were found on difference
Fourier syntheses and refined anisotropically. Hydrogens were
placed in calculated positions. Final full-matrix least-squares
refinement on F2 converged at R ) 0.0415 for 3262 reflections
with Fo > 2σ(Fo), R ) 0.0582 for all data (wR2 ) 0.1076,
0.1163, respectively.).
X-r a y Str u ctu r e Deter m in a tion s of cis-Ir (CO)2(Br )(µ-
2,4,6-Me3C6H2)Ir (cod ) (5), cis-Ir (CO)2(2,6-Me2C6H3)(µ-2,6-
Me2C6H3)Ir (cod) (8), an d tr a n s-Ir (CO)2I2Br (µ-2,6-Me2C6H3)-
Ir (cod ) (12). Crystals of complex 5 were formed by slow
diffusion of ether into a solution of the complex in 1,2-
dichloroethane. A 0.5 × 0.5 × 0.3 mm irregular fragment was
coated with an inert oil and attached to the tip of a glass fiber
in the cold N2 stream of a Bruker Apex CCD diffractometer.
An empirical absorption correction was applied using
SADABS; decay was negligible. The crystals were orthorhom-
bic, and the space group was determined to be Pbca based on
systematic absences. The iridium atoms were located on a
Patterson map, and remaining non-hydrogen atoms were
found on difference Fourier syntheses and refined anisotropi-
cally. Hydrogens were placed in calculated positions. Final full-
matrix least-squares refinement on F2 converged at R ) 0.0366
for 4160 reflections with Fo > 2σ(Fo), R ) 0.0421 for all data
(wR2 ) 0.0886, 0.0905, respectively).
tr a n s-Ir I3(CO)2(µ-2,6-Me2C6H3)Ir (cod ) (10). cis-Ir(CO)2-
(I)(µ-2,6-Me2C6H3)Ir(cod) (7, 200 mg, 0.256 mmol) was dis-
solved in 7 mL of CH2Cl2 in a 20 mL screw-cap vial. To it was
added iodine (65 mg, 0.256 mmol, 1.00 equiv) in 3 mL of CH2-
Cl2. The vial was immediately capped and shaken rapidly; the
solution turned light orange. The reaction mixture was layered
with ether, and the orange crystals formed were filtered,
washed with ether, and air-dried to furnish 185.4 mg of 10
1
(70%). H NMR (CD2Cl2): δ 6.52 (t, 6 Hz, 1H, para), 5.92 (d,
6 Hz, 2H, meta), 4.11 (m, 4H, alkene C-H), 2.65 (s, 6H, CH3),
2.31 (m, 4H, cod CHH′), 2.07 (pseudo-q, 9 Hz, 4H, cod CHH′).
13C{1H} NMR (CD2Cl2): δ 161.1 (CO), 114.6 (ipso C), 100.54,
100.47, 95.2, 65.9, 33.5, 30.95. IR (Nujol mull, cm-1): 2053
(vs, νCO), 1158 (m), 992 (w), 917 (w), 838 (w). Anal. Calcd for
Ir2C18H21O2I3: C, 20.89; H, 2.03. Found: C, 21.00; H, 2.00.
[Ir (cod )(m es)2]MgBr 0.5Cl0.5‚6THF (11). In the drybox, the
iridium(I) chloride dimer 1 (200 mg, 0.298 mmol), dry ether
(10 mL), and dry benzene (2 mL) were placed in a 20 mL vial.
The vial was securely capped and vigorously shaken to yield
an orange solution of 1. To the vial was added 5 equiv of 1.0
M mesitylmagnesium bromide in THF (1.5 mL, 1.49 mmol).
The vial was immediately capped and shaken rapidly, and the
solution turned dark red. Dark red needles of the diaryl anion
11 as a magnesium bromide salt were deposited overnight.
The red needles of 11 that deposited were collected by suction
filtration, washed with ether, and air-dried to give 476.5 mg
of 11 (76%, assuming the composition of the counterion is that
1
stated above). H NMR (C6D6): δ 6.90 (s, 4H, Ar-H), 3.61 (br,
24H, THF), 3.44 (br, 4H, alkene C-H), 3.30 (br, 6H, ortho-CH3),
2.48 (m, 4H, cod CHH′), 2.35 (s, 3H, para-CH3), 1.92 (m, 4H,
cod CHH′), 1.13 (br, 24H, THF).
Orange prisms of the diaryl complex 8 were deposited
overnight directly from the reaction mixture. A 0.27 × 0.28 ×
0.35 mm crystal was mounted and examined as described
above. The crystal was orthorhombic, and its space group was
determined to be Pbca based on systematic absences. The
structure was solved and refined as described above. Final full-
matrix least-squares refinement on F2 converged at R ) 0.0232
for 6330 reflections with Fo > 2σ(Fo), R ) 0.0260 for all data
(wR2 ) 0.0617, and 0.0630, respectively.).
X-r a y Str u ctu r e Deter m in a tion s of Ir (cod )(Br )(µ-2,4,6-
Me3C6H2)Ir (cod ) (2) a n d Ir (cod )(Br )(µ-2,6-Me2C6H3)Ir -
(cod ) (3). Yellow obelisks of the complex 2 were deposited after
slow diffusion of ether into a solution of the complex in
benzene. A 0.42 × 0.18 × 0.08 mm crystal was glued to the
tip of a glass fiber in the air and examined at 20 °C on an
Enraf-Nonius CAD4 diffractometer using Mo KR radiation
with a graphite monochromator (λ ) 0.71037 Å). The crystal
was monoclinic, and its unit cell was determined based on 25
reflections with 13.1° < θ < 13.9°. A total of 8482 reflections
in two octants (hkl, hkhl) with 2θ < 50° were collected. Crystal
quality was monitored by recording three standard reflections
approximately every 220 reflections measured; decay was 8%.
An empirical absorption correction was applied (µ ) 13.310
cm-1, transmission factors 0.242-0.770). After corrections for
absorption and for Lorentz and polarization effects, 8171
unique reflections were obtained (Rint ) 0.0249), of which one
was suppressed during least-squares refinement due to highly
Complex 7 (25 mg, 0.032 mmol) was dissolved in 0.7 mL of
CD2Cl2 in an NMR tube and treated with 1 equiv of iodine
1
(8.1 mg, 0.032 mmol). The H NMR spectrum of the reaction
mixture showed a mixture of complexes similar to 10, at-
tributed to various isomers and halide compositions with an
average composition of trans-Ir(CO)2I2Br(µ-2,6-Me2C6H3)Ir(cod)
(12). Orange needles of 12 were deposited overnight. A 0.15
× 0.08 × 0.02 mm crystal was mounted and examined as
described above. The crystal was monoclinic, and its space
group was determined to be P21/n based on systematic
absences. The structure was solved and refined as described
above. The halide atoms were disordered and were modeled
by refining the composition of each atom independently,
subject to the global constraint of an overall 2:1 I:Br composi-
tion. Attempts to refine the atoms as iodine and bromine atoms
in separate positions failed, and so each mixed-composition
atom was given a single set of positional and thermal param-
eters. Final full-matrix least-squares refinement on F2 con-
verged at R ) 0.0376 for 4197 reflections with Fo > 2σ(Fo), R
) 0.0564 for all data (wR2 ) 0.0873, 0.1037, respectively.).
negative apparent values of Fobs
.
The space group was determined to be P21/c based on
systematic absences. The asymmetric unit contained two
molecules of the complex. The four independent iridium atoms
were located on a Patterson map, and remaining non-hydrogen
atoms were found on difference Fourier syntheses. Hydrogens
were placed in calculated positions. Final full-matrix least-
squares refinement on F2 converged at R ) 0.0516 for 5391
reflections with Fo > 2σ(Fo), R ) 0.1117 for all data (wR2 )
0.1187, 0.1442, respectively.). All calculations used SHELXTL
(Bruker Analytical X-ray Systems) with scattering factors and
anomalous dispersion terms taken from the literature.7
Yellow blocks of the complex 3 were deposited after slow
diffusion of ether into a solution of the complex in 1,2-
dichloroethane. A 0.35 × 0.32 × 0.25 mm crystal was glued to
the tip of a glass fiber in the air and examined as described
Resu lts
P r ep a r a tion a n d Ch a r a cter iza tion of Br id gin g
Ar yl Com p lexes. Treatment of the iridium chloride
dimer [(cod)IrCl]2 (1; cod ) 1,5-cyclooctadiene) with 1
mol of mesitylmagnesium bromide (mesityl ) 2,4,6-
trimethylphenyl) per mole of dimer results in the
formation of the yellow, diamagnetic, air-stable complex
(7) International Tables of Crystallography; Kluwer Academic Pub-
lishers: Dordrecht, The Netherlands, 1992; Vol. C.

Unsymmetrically Bridging Aryls of Iridium
Organometallics, Vol. 22, No. 22, 2003 4483
2. The 1H NMR spectrum of 2 displays an unusual
upfield shift of the aromatic protons of the mesityl group
(δ 4.63 ppm in C6D6; upfield shifts are not as marked
in nonaromatic solvents). This suggests that the aro-
matic ring is coordinated in a π fashion to an iridium
atom. The NMR spectrum also shows two chemically
inequivalent 1,5-cyclooctadiene ligands per mesityl
group. One of these cyclooctadiene ligands is evidently
bound in an unsymmetrical fashion (alkene protons at
δ 5.09 and 2.48 ppm), while the other cyclooctadiene
1
shows only a single set of alkene resonances in the H
NMR. The 13C{1H} NMR spectrum shows 12 reso-
nances, including a downfield resonance at 171.6 ppm
assigned to the ipso carbon of the mesityl group. These
data are consistent with a formulation of 2 as a
diiridium complex containing one square-planar irid-
ium(I) center σ-bonded to the mesityl group and a
second iridium(I) center π-bonded to the same mesityl
group (eq 3). The cyclooctadiene bound to the square-
planar iridium shows inequivalent alkene resonances
because of the distinct trans ligands, while the cyclo-
octadiene bound to the π-bonded iridium is fluxional.
The fact that 2 contains a single halide can be inferred
from the presence in the crude reaction mixture of a
minor amount of a compound spectroscopically similar
to 2, assigned as its chloride analogue. The mixture is
easily converted to pure bromo complex 2 upon treat-
ment with tetrabutylammonium bromide. No signals
attributable to a mixed chloride/bromide complex are
observed.
F igu r e 1. SHELXTL plot (30% thermal ellipsoids) of one
of the two independent molecules of Ir(cod)(Br)(µ-2,4,6-
Me3C6H2)Ir(cod) (2).
F igu r e 2. SHELXTL plot (30% thermal ellipsoids) of Ir-
(cod)(Br)(µ-2,6-Me2C6H3)Ir(cod) (3).
crystal X-ray diffraction. Crystallographic details are
listed in Table 1, and selected bond distances and angles
in Table 2. The structures of both crystallographically
independent molecules of 2 are very similar to each
other and to the structure of 3. The average Ir1-C1
bond length of 2.059(19) Å is significantly shorter than
what is seen in the crystallographically characterized
iridium(I) mesityl complexes Ir(mes)(CO)(dppe) (2.12
Å)9,10 and Ir(mes)(CO)(PPh3)2 (2.14 Å).11 The π-bonded
cyclooctadieneiridium(I) fragment is coordinated strongly
to only five carbons, with the mean Ir2-C(2-6) ) 2.26-
(4) Å (average over all three structures), while the mean
Ir2-C1 bond distance is 2.42(3) Å. The ipso carbon atom
is pulled significantly out of the plane formed by the
other five carbon atoms, with C1 deviating from the C2-
C3-C4-C5-C6 mean plane by 0.139 ( 0.026 Å. Distor-
An analogous bridging aryl complex 3 is formed from
2,6-dimethylphenylmagnesium bromide, with the aryl
signals at δ 5.17 (para) and 4.57 (meta) again serving
as a signature of the unusual σ,π-bridging mode. The
3J HH coupling constant between the meta and para
hydrogens of 6 Hz is somewhat reduced from the usual
7-9 Hz coupling constant typically observed between
ortho hydrogens in aromatics. This reduced coupling
constant has been seen in other π-bonded areneiridium-
(I) complexes.8 Reactions with less hindered aryl Grig-
nard reagents such as phenyl- or o-tolylmagnesium
bromide do not produce analogous bridged compounds,
judging from the absence of any resonances in the δ
4-5.5 ppm region of the 1H NMR spectra of these
reaction mixtures.
The structures of bromocyclooctadiene complexes 2
(Figure 1) and 3 (Figure 2) were determined by single-
(9) Cleary, B. P.; Eisenberg, R. Organometallics 1992, 11, 2335-
2337.
(8) (a) Grundy, S. L.; Smith, A. J .; Adams, H.; Maitlis, P. M. J . Chem.
Soc., Dalton Trans. 1984, 1749-1754. (b) Torres, F.; Sola, E.; Mart´ın.;
Lo´pez, J . A.; Lahoz, F. J .; Oro, L. A. J . Am. Chem. Soc. 1999, 121,
10632-10633.
(10) Cleary, B. P.; Eisenberg, R. J . Am. Chem. Soc. 1995, 117, 3510-
3521.
(11) Dahlenburg, L.; Von Deuten, K.; Kopt, J . J . Organomet. Chem.
1981, 216, 113-127.

4484 Organometallics, Vol. 22, No. 22, 2003
Muldoon and Brown
Ta ble 1. Su m m a r y of Deta ils of X-r a y Cr ysta llogr a p h y
Ir(CO)2(2,6-
Ir(cod)Br(µ-2,4,6-
Me3C6H2)Ir(cod) (2)
Ir(cod)Br(µ-2,6-
Me2C6H3)Ir(cod) (3)
Ir(CO)2Br(µ-2,4,6-
Me3C6H2)Ir(cod) (5)
Me2C6H3)(µ-2,6-
Me2C6H3)Ir(cod) (8)
Ir(CO)2BrI2(µ-2,6-
Me2C6H3)Ir(cod) (12)
empirical formula
fw
C
25H35BrIr2
C
24H33BrIr2
C
19H23BrIr2O2
C
26H30Ir2O2
C18H21BrI2Ir2O2
987.46
799.84
785.81
747.68
758.90
temperature (K)
λ
space group
total data
collected
no. of indep reflns
a (Å)
293(2)
0.71073 Å (Mo KR)
P21/c
293(2)
0.71073 Å (Mo KR)
P21/n
173(2)
0.71073 Å (Mo KR)
Pbca
170(2)
0.71073 Å (Mo KR)
Pbca
170(2)
0.71073 Å (Mo KR)
P21/n
8482
4199
32944
54340
23765
8171
3902
4719
6802
5409
15.383(2)
21.473(2)
14.5907(10)
90
102.942(8)
90
4697.1(9)
8
2.262
7.815(2)
20.328(3)
14.534(3)
90
105.67(2)
90
2223.2(7)
4
2.348
14.1633(11)
14.7640(11)
18.7081(14)
90
90
90
3912.0(5)
8
2.539
14.2533(6)
14.6197(6)
21.3585(8)
90
90
90
4450.7(3)
8
2.265
9.1441(3)
15.8388(6)
15.0766(6)
90
92.5990(10)
90
2181.32(14)
4
3.007
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
calc F (g/cm3)
cryst size (mm)
0.42 × 0.18 × 0.08
0.35 × 0.32 × 0.25
0.5 × 0.5 × 0.3
0.27 × 0.28 × 0.35
0.02 × 0.08 × 0.15
µ (mm-1
)
13.031
13.764
15.644
11.968
16.860
R indices
[I>2σ(I)]a
R indices
(all data)a
R1 ) 0.0516,
wR2 ) 0.1187
R1 ) 0.1117,
wR2 ) 0.1442
1.039
R1 ) 0.0415,
wR2 ) 0.1076
R1 ) 0.0582,
wR2 ) 0.1163
1.052
R1 ) 0.0366,
wR2 ) 0.0886
R1 ) 0.0421,
wR2 ) 0.0905
1.117
R1 ) 0.0232,
wR2 ) 0.0617
R1 ) 0.0260,
wR2 ) 0.0630
1.083
R1 ) 0.0376,
wR2 ) 0.0873
R1 ) 0.0564,
wR2 ) 0.1037
1.084
GOF (all data)
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo - Fc2)2]/∑w(Fo2)2)1/2
.
a
2
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for Cr ysta llogr a p h ica lly Ch a r a cter ized Com p ou n d s
Ir(CO)2(2,6-
Ir(cod)Br(µ-2,4,6- Ir(cod)Br(µ-2,4,6-
Ir(cod)Br(µ-2,6-
Me2C6H3)Ir(cod)
(3)
Ir(CO)2Br(µ-2,4,6- Me2C6H3)(µ-2,6- Ir(CO)2BrI2(µ-2,6-
Me3C6H2)Ir(cod)
Me3C6H2)Ir(cod)
Me3C6H2)Ir(cod)
Me2C6H3)Ir(cod)
Me2C6H3)Ir(cod)
(2), molecule 1
(2), molecule 2
(5)
(8)
(12)
Ir1-C1
2.082(15)
2.10(2)
2.04(2)
2.171(15)
2.19(2)
2.489(2)
2.047(13)
2.09(2)
2.12(2)
2.17(2)
2.16(2)
2.481(2)
2.049(9)
2.088(6)
2.124(3)
2.114(9)
Ir1-C21
2.113(11)
2.100(11)
2.182(11)
2.176(11)
2.5000(12)
Ir1-C22
Ir1-C25
Ir1-C26
Ir1-Br
2.4909(8)
1.849(7)
1.897(7)
Ir1-C31
1.878(4)
1.856(4)
2.142(4)
2.411(3)
2.247(3)
2.276(4)
2.292(4)
2.210(4)
2.306(3)
2.131(4)
2.123(4)
2.132(4)
2.126(4)
1.919(11)
1.924(11)
Ir1-C32
Ir1-C51
Ir2-C1
2.39(2)
2.320(15)
2.232(14)
2.30(2)
2.25(2)
2.25(2)
2.11(2)
2.17(2)
2.14(2)
2.11(2)
88.6(7)
90.1(7)
80.6(8)
94.1(9)
93.3(4)
89.5(5)
91.1(6)
2.443(12)
2.304(15)
2.17(2)
2.33(2)
2.26(2)
2.227(15)
2.13(2)
2.12(2)
2.153(15)
2.129(14)
87.5(6)
89.5(7)
82.9(7)
94.0(8)
93.3(4)
92.5(5)
88.9(6)
2.441(10)
2.290(9)
2.200(10)
2.280(6)
2.256(6)
2.244(6)
2.122(11)
2.117(12)
2.125(12)
2.148(12)
88.4(4)
88.5(4)
81.0(5)
95.6(5)
94.3(3)
92.1(4)
90.0(4)
2.385(6)
2.319(6)
2.213(6)
2.309(6)
2.292(4)
2.248(4)
2.147(7)
2.119(6)
2.132(6)
2.141(7)
2.383(9)
2.315(9)
Ir2-C2
Ir2-C3
2.198(10)
2.272(11)
2.283(10)
2.241(10)
2.115(11)
2.122(10)
2.143(9)
Ir2-C4
Ir2-C5
Ir2-C6
Ir2-C11
Ir2-C12
Ir2-C15
Ir2-C16
2.161(10)
C1-Ir1-C21
C1-Ir1-C22
C21-Ir1-C26
C22-Ir1-C26
C1-Ir1-Br
C25-Ir1-Br
C26-Ir1-Br
C1-Ir1-C31
C1-Ir1-C32
C1-Ir1-C51
C31-Ir1-C32
C31-Ir1-Br
C32-Ir1-Br
C31-Ir1-C51
C32-Ir1-C51
85.2 (2)
92.2(3)
175.7(3)
91.86(15)
174.88(14)
90.55(13)
93.3(2)
95.5(4)
95.6(4)
91.9(3)
177.1(2)
90.6(2)
168.8(4)
177.51(15)
84.33(14)
tion from planarity is also shown by the “fold angle”
between the planes C2-C1-C3 and the C2-C6 mean
plane, which averages 9.9 ( 1.5° in the three structures.
The cyclooctadiene ligand on Ir1 is bound somewhat
unsymmetrically, with longer Ir-C distances trans to
the aryl ligand (2.17 vs 2.10 Å) due to the structural
trans influence of the aryl group.
In the presence of excess mesitylmagnesium bromide,
dimeric complexes such as 2 or 3 are not formed.
Treatment of iridium chloride dimer 1 with excess
mesitylmagnesium bromide (g4 equiv per dimer) in
Et2O/benzene results in immediate formation of a red
solution, from which red crystals of a dimesityl anion
(11) as a salt with a THF-coordinated halomagnesium

Unsymmetrically Bridging Aryls of Iridium
Organometallics, Vol. 22, No. 22, 2003 4485
cation are deposited overnight (eq 4). The NMR spec-
trum of 11 shows two equivalent mesityl groups for each
cyclooctadiene ligand, which is symmetrical. Further
confirmation of the connectivity of the anionic complex
was given by a low-quality X-ray crystal structure,
where extensive disorder of the cationic component and
lattice solvent prevented satisfactory refinement. The
isolated bridging aryl complex 2 does not react readily
with added mesityl Grignard, with reactions taking
several days and producing unidentified products, but
no anion 11. The isolated dimesityl anion 11 does not
react with the iridium(I) chloride dimer 1 over several
days at room temperature.
Liga n d Su bstitu tion Rea ction s of Br id gin g Ar yl
Com p lexes. The bridging aryl complexes 2 and 3
undergo reaction selectively at the square-planar iri-
dium center. For example, treatment of the bromo
cyclooctadiene complex 3 with NaI results in the forma-
tion of the iodo cyclooctadiene complex 4. This is
analogous to the treatment of the crude reaction mixture
of 2 and 3 with tetrabutylammonium bromide to convert
any chloro complex to the bromo complex.
When carbon monoxide is bubbled through a solution
of 2 in dichloromethane, the solution gives a transient
dark red color that fades to pale yellow within a few
seconds. The NMR spectrum of the final dicarbonyl
complex 5 shows only one symmetrical set of cyclo-
octadiene resonances, suggesting that the cyclooctadiene
bound to the square-planar iridium(I) center is com-
pletely displaced. This is confirmed by the detection of
free cyclooctadiene by NMR in the crude reaction
mixture (eq 5). The bromo- and iodocyclooctadiene
complexes with bridging 2,6-dimethylphenyl groups (3,
4) react similarly with carbon monoxide to form the
corresponding bromo- and iododicarbonyl complexes 6
and 7.
F igu r e 3. SHELXTL plot (30% thermal ellipsoids) of cis-
Ir(CO)2(Br)(µ-2,4,6-Me3C6H2)Ir(cod) (5).
for the smaller ligand to approach the bulky (cyclo-
octadiene)iridium(I) fragment that is π-bonded to the
arene. The Ir1-C1 bond distance of 2.088(6) Å is
significantly longer, while the Ir2-C1 bond distance of
2.385(6) Å is significantly shorter, than the correspond-
ing distances in the cyclooctadiene complexes 2 and 3.
The phenomenon of C1 being pulled out of the plane of
the mesityl group is still apparent in 5, but it is much
less marked than in 2 and 3 (distance of Ir2 to C1 is
0.09 Å longer than the average distance of 2.28(4) Å to
C2-C6, deviation of C1 from the mean plane ) 0.071
Å, fold angle ) 5.2°).
The bromo cyclooctadiene complex 2 undergoes no
reaction with PPh3 (60 °C, 3 days). It reacts very slowly
at room temperature with 1 equiv of 1,2-bis(diphen-
ylphosphino)ethane to give a mixture of products.
The iodo dicarbonyl complex 7 undergoes halide
displacement over the course of several days at room
temperature when treated with excess aryl Grignard
reagents in THF (eq 6). Yellow air-stable crystals of aryl
dicarbonyl complex 8 or 9 precipitate over the course of
the reaction of 7 with 2,6-dimethylphenylmagnesium
bromide or mesitylmagnesium bromide, respectively.
The infrared spectra of the products display two termi-
nal CO bands (e.g., 2011 and 1935 cm-1 for 8). The shift
of ∼25 cm-1 to low frequency relative to the halo
complexes 5-7 (e.g., 2038, 1961 cm-1 for 6) is consistent
with substitiution of the more electron-donating aryl
ligand for a halide. The iodo cyclooctadiene complex 4
reacts very slowly with aryl Grignard reagents to give
a mixture of products.
NMR and IR spectra of the products 5-7 indicate that
the complexes have maintained the bridging aryl group
but that the cyclooctadiene ligand of the square-planar
iridium center has been replaced with a cis-dicarbonyl
unit. This has been confirmed by an X-ray structure of
the mesityl-bridged bromo dicarbonyl complex 5 (Figure
3). The Ir-CO bond distances of 1.849(7) and 1.897(7)
Å are typical of iridium(I) carbonyl distances.9-12 The
orientation of the square-planar iridium differs from
what is seen in the dicyclooctadiene complexes 2 and 3,
in that CO, rather than bromide, is syn to the second
iridium. Presumably this is due to a steric preference
The structure of the complex 8, with both terminal
and bridging 2,6-dimethylphenyl groups, was confirmed
(12) Schumann, H.; Cielusek, G.; Pickardt, J .; Bruncks, N. J .
Organomet. Chem. 1979, 172, 359-365.

4486 Organometallics, Vol. 22, No. 22, 2003
Muldoon and Brown
F igu r e 5. SHELXTL plot (30% thermal ellipsoids) of
trans-Ir(CO)2I2Br(µ-2,6-Me2C6H3)Ir(cod) (12). The halide
atoms X are disordered and were modeled by refining the
composition of each atom independently, subject to the
global constraint of an overall 2:1 I:Br composition.
F igu r e 4. SHELXTL plot (30% thermal ellipsoids) of cis-
Ir(CO)2(2,6-Me2C6H3)(µ-2,6-Me2C6H3)Ir(cod) (8).
(L ) sulfides, phosphines).13 Further reaction with
iodine results in iridium-carbon bond cleavage and
formation of iodomesitylene.
by crystallography (Figure 4). The Ir1-C1 bond distance
of the bridging aryl group (2.124(3) Å) is significantly
shorter than the corresponding distance in the terminal
aryl group (2.142(4) Å), although the difference is small.
The distortions in the bridging aryl group (Ir2-C1 bond
distance of 2.411(3) Å vs average distance from Ir2 to
C2-C6 of 2.27(3) Å, deviation of C1 from the mean
plane of 0.1129 Å, and fold angle of 8.1°) are all quite
similar to those seen in the cyclooctadiene complexes 2
and 3 and show noticeably more distortion than is seen
in the bromo dicarbonyl complex 5.
In contrast, the halo dicarbonyl complexes 6 and 7
undergo clean oxidative addition of molecular iodine at
the square-planar iridium(I) center. For example, ad-
dition of 1 equiv of a dichloromethane solution of I2 to
a dichloromethane solution of iodo dicarbonyl complex
7 results in triiodo dicarbonyl complex 10 (eq 8). The
infrared spectrum displays a single terminal CO band
(at 2050 cm-1), suggesting that the carbonyls have
isomerized from cis to trans upon oxidation. The spectral
features of the (cod)Ir(I) fragment are largely unper-
turbed, indicating that this is a formally mixed-valence
Ir(III)/Ir(I) complex.14 The bromo dicarbonyl complex 6
reacts with iodine in a similar fashion, forming a
mixture of tri(halo) dicarbonyl compounds containing
both bromides and iodides (12). A crystal from the latter
reaction was analyzed by X-ray diffraction (Figure 5).
The X-ray structure confirms that net oxidative addition
of iodine to form an octahedral iridium(III) center with
trans carbonyls (C31-Ir1-C32 ) 168.8°) has taken
place. The plane of the bridging aryl groups is staggered
with respect to the four equatorial ligands on Ir1. Atom
type disorder among the halides (refined as mixed atom
types X) precludes obtaining accurate metrical data for
the Ir-halogen bond lengths in this crystal.15 The Ir1-
C1 bond distance of 2.114(9) Å is significantly longer
than in the bromo dicarbonyl complex 5. The average
Ir-CO distance of 1.922 Å is substantially longer than
the Ir(I)-CO distances in 5 (1.87 Å) and 8 (1.86 Å), as
Treatment of the 2,6-dimethylphenyl-bridged iodo
dicarbonyl complex 6 with mesitylmagnesium bromide
leads to a single isomer of the mixed aryl complex 9. In
the product the 2,6-dimethylphenyl group retains its
bridging position (δ 5.76 for the meta protons in CD2-
Cl2), and the incoming mesityl group is σ-bonded (δ 6.62
1
for the aromatic protons). The H NMR spectrum of the
mixed aryl complex 9 is unaffected by heating at 60 °C
for 3 days, indicating that the π-bonded (cyclooctadiene)-
Ir(I) fragment cannot flip between the 2,6-dimethyl-
phenyl group and the mesityl group (eq 7).
Oxid a tion of Br id gin g Ar yl Com p lexes. Oxidation
of iridium aryls often induces reactivity at the iridium-
carbon bond. For example, the iridium anion [(cod)-
Ir(mes)2]- (11) reacts initially with iodine to form a
green NMR-silent species that we presume to be the
neutral iridium(II) complex (cod)Ir(mes)2 analogous to
other Ir(mes)2L2 complexes prepared by Wilkinson
(13) Danopoulos, A. A.; Wilkinson, G.; Hussain-Bates, B.; Hurst-
house, M. B. J . Chem. Soc., Dalton Trans. 1992, 3165-3170.
(14) Examples of other Ir(III)/Ir(I) complexes: (a) Schenck, T. G.;
Milne, C. R. C.; Sawyer, J . F.; Bosnich, B. Inorg. Chem. 1985, 24, 2338-
2344. (b) Kolel-Veetil, M. K.; Rheingold, A. L.; Ahmed, K. J . Organo-
metallics 1993, 12, 3439-3446. (c) Tejel, C.; Ciriano, M. A.; Lopez, J .
A.; Lahoz, F. J .; Oro, L. A. Organometallics 2000, 19, 4977-4984.
(15) Parkin, G. Chem. Rev. 1993, 93, 887-911.

Unsymmetrically Bridging Aryls of Iridium
Organometallics, Vol. 22, No. 22, 2003 4487
expected due to decreased back-bonding in the Ir(III)
complex. The deviation of C1 from the mean plane of
the aryl group is 0.0901 Å, and the fold angle is 6.8°.
to air and moisture both as solids and in solution.
Iridium(I) and rhodium(I) π-arene complexes are gener-
ally quite labile, often undergoing facile displacement
even by weak ligands.20 We have never observed dis-
placement of the bridging aryl from the π-bonded (cod)-
IrI fragment under any conditions. Ligand displacement
of the cyclooctadiene of the σ-bonded iridium in 2 and
3 takes place rapidly with CO, but the bridging aryl is
unaffected. Furthermore, the cyclooctadiene of the 18-
electron (cod)Ir(η6-arene) fragment is not displaced
under these conditions, which suggests that even a
hapticity change of the π-bonded arene is not taking
place. The dicarbonyl complex 5 is inert to triphen-
ylphosphine, and even intramolecular arene exchange,
as between the terminal and bridging aryls of (CO)2Ir-
(mes)(µ-2,6-Me2C6H3)Ir(cod) (9) is not observed. Like-
wise, the σ-bond to the bridging aryl group appears to
be more resistant to chemical attack than the σ-bond
of typical hindered aryl ligands bonded to iridium. For
example, iodine oxidation of the diaryl anion [(mes)2-
Ir(cod)]- leads eventually to iodinolysis of the Ir-C
bond, while aryl-to-carbonyl migration is induced by
iodine oxidation of (dppe)Ir(mes)(CO).10 In contrast, the
bridging aryl complex 7 reacts with iodine by simple
oxidative addition, without any modification of the
bridging 2,6-dimethylphenyl ligand.
Rearrangement of carbonyls from cis to trans is rather
unusual, as carbonyls generally prefer to occupy cis
positions to minimize their competition for back-bond-
ing. We see no sign of a cis isomer in the Ir(III) complex,
but reduction to iridium(I), for example by reaction with
2,6-dimethylphenyl Grignard, restores the cis arrange-
ment of the two CO groups (eq 9). In the presence of
excess Grignard reagent, reduction to the iridium(I) iodo
dicarbonyl complex 6 is immediate and is followed by
much slower displacement of iodide to form the σ-bond-
ed aryl complex 7 (eq 6).
The stability of the unsymmetrically bridging aryls
can be rationalized based on considering the bonding
in these species as a resonance hybrid of a zwitterionic
σ-aryl/η6-arene structure and a neutral carbene/η5-
cyclohexadienyl structure (eq 11). Such resonance sta-
bilization would explain strengthening of both the σ and
the π interactions relative to unbridged analogues.
Discu ssion
Str u ctu r e, Rea ctivity, a n d Bon d in g in σ/π-Br id g-
in g Ar yl Com p lexes. The chloro-bridged diiridium(I)
complex [(cod)IrCl]2 (1) reacts with 2,6-disubstituted
aryl Grignard reagents to give air-stable diiridium
complexes 2 and 3. The aryl groups in these compounds
show an unusual bridging mode, being σ-bonded to one
square-planar iridium(I) center and π-bonded to another
(cyclooctadiene)iridium(I) fragment. This bonding mode
contrasts with the σ-bridged structures typical of main
group elements such as Li,16 Al, 17 and Mg,18 where the
ipso carbons participate in relatively symmetrical three-
center, two-electron bonds. The σ/π-bridged aryl most
similar to the iridium complexes discussed here is the
dirhodium complex (CO)2Rh(µ,σ,η6-C6H2Me3)(µ,η1(O),η1-
(C)-C6H2Me3CO)Rh(CO), formed on carbonylation of Rh-
(mes)3 (eq 10).19 Our observations indicate that this
motif is readily accessible in iridium chemistry and that
the stability of the bridging aryl group does not require
the presence of an additional bridging ligand.
Spectroscopic and structural data support this view
of the bonding. The 13C NMR data for bromo cyclo-
octadiene complexes 2 and 3 are especially informative,
showing downfield resonances for the ipso carbon at
171.6 and 174.9 ppm, respectively. The extreme low field
of this resonance is highly suggestive of some degree of
carbene character for the ipso carbon (δ 195-325 for
iridium(I) carbene complexes).21 Structurally, distortion
(19) Hay-Motherwell, R. S.; Koschmieder, S. U.; Wilkinson, G.;
Hussain-Bates, B.; Hursthouse, M. B. J . Chem. Soc., Dalton. Trans.
1991, 2821-2830.
(20) (a) Torres, F.; Sola, E.; Mart´ın, M.; Ochs, C.; Picazo, G.; Lo´pez,
J . A.; Lahoz, F. J .; Oro, L. A. Organometallics 2001, 20, 2716-2724.
(b) Torres, F.; Sola, E.; Mart´ın, M.; Lo´pez, J . A.; Lahoz, F. J .; Oro, L.
A. J . Am. Chem. Soc. 1999, 121, 10632-10633. (c) Singewald, E. T.;
Shi, X.; Mirkin, C. A.; Schofer, S. J .; Stern, C. L. Organometallics 1996,
15, 3062-3069. (d) Dixon, F. M.; Masar, M. S., III; Doan, P. E.; Farrell,
J . R.; Arnold, F. P., J r.; Mirkin, C. A.; Incarvito, C. D.; Zakharov, L.
N.; Rheingold, A. L. Inorg. Chem. 2003, 42, 3245-3255. (e) Canepa,
G.; Sola, E.; Mart´ın, M.; Lahoz, F. J .; Oro, L. A.; Werner, H.
Organometallics 2003, 22, 2151-2160.
Indeed, the most noteworthy feature of the σ/π-
bridging aryl groups is their remarkable chemical
stability. All aryl-bridged diiridium complexes are stable
(21) (a) Werner, H.; Schulz, M.; Windmu¨ller, B. Organometallics
1995, 14, 3659-3668. (b) Werner, H.; Lass, R. W.; Gevert, O.; Wolf, J .
Organometallics 1997, 40777-4088. (c) Ortmann, D. A.; Weberndo¨rfer,
B.; Ilg, K.; Laubender, M.; Werner, H. Organometallics 2002, 21, 2369-
2381. (d) Ilg, K.; Werner, H. Chem. Eur. J . 2001, 7, 4633-4639. (e)
Fryzuk, M. D.; Gao, X. L.; J oshi, K.; MacNeil, P. A.; Massey, R. L. J .
Am. Chem. Soc. 1993, 115, 10581-10590. (f) Grotjahn, D. B.; Bikzha-
nova, G. A.; Collins, L. S. B.; Concolino, T.; Lam, K.-C.; Rheingold, A.
L. J . Am. Chem. Soc. 2000, 122, 5222-5223.
(16) (a) Van Koten, G.; J astrzebski, J . T. B. H.; Noltes, J . G. J .
Organomet. Chem. 1977, 140, C23-C27. (b) Thoennes, D.; Weiss, E.
Chem. Ber. 1978, 111, 3157-3161.
(17) Malone, J . F.; McDonald, W. S. J . Chem. Soc., Dalton Trans.
1972, 2646-2648.
(18) Markies, P. R.; Schat, G, Akkermann, O. S.; Bickelhaupt, F.;
Smeets, W. J . J .; van der Sluis, P.; Spek, A. L. J . Organomet. Chem.
1990, 393, 315-331.

4488 Organometallics, Vol. 22, No. 22, 2003
Muldoon and Brown
Ta ble 3. Cor r ela tion betw een Electr on Den sity a t th e Ir id iu m Cen ter σ-Bon d ed to th e Ar yl Gr ou p a n d
Str u ctu r a l a n d Sp ectr oscop ic P r op er ties of th e Br id gin g Ar yl Com p lexes
most electron-rich
8
most electron-poor
groups bonded to σ-bonded
Ir in LnIr(µ-Ar)Ir(cod)
Br(cod) (av 2a , 2b, 3)
(2,6-Me2C6H3)(CO)2 (8)
Br(CO)2 (5)
BrI2(CO)2 (12)
Ir1-C1 (Å)
2.059(19)
2.425(30)
0.139
2.124(3)
2.411(3)
0.113
8.1
2011, 1935
b
2.088(6)
2.385(6)
0.071
5.2
2040, 1957
146.0
2.114(9)
2.383(9)
0.090
Ir2-C1 (Å)
deviation of C1 from mean plane (Å)
fold angle (deg)
9.9
6.8
νCO (cm-1
)
2053a
114.6a
δ
13C, Cipso (ppm)
173.2
a
b
Measured for the triiodide complex 10. Not measured due to low solubility.
toward an η5 structure is supported by the substantial
elongation of the Ir2-C1 distance relative to the dis-
tance to the other five aromatic carbons (2.42 vs 2.26 Å
in 2 and 3) and by the fact that C1 is pulled significantly
out of the plane formed by the other five carbon atoms
(deviation of C1 from the mean plane is 0.139 ( 0.026
Å, and the “fold angle” between the planes C2-C1-C3
and the mean plane is 9.9° ( 1.5° in the three struc-
tures). The relatively short σ-bonded Ir1-C1 distance
of 2.06 Å is also consistent with some multiple bonding
to this carbon.
As the iridium center σ-bonded to the aryl group
becomes more electron-deficient, the η6 canonical form
should become increasingly important relative to the
η5/carbene form. This is generally supported by the
structural and spectroscopic trends exhibited by the
various compounds (Table 3). As the σ-bonded iridium
fragments become increasingly electron-poor in the
order (cod)BrIrI < (CO)2(2,6-Me2C6H3)IrI < (CO)2BrIrI
< (CO)2X3IrIII, there is a general structural trend toward
an η6 structure (shortening of the Ir2-C1 distance,
decreasing deviation of C1 from the mean plane) and
away from any carbene contribution (lengthening σ-bond-
ed Ir1-C1 distance and upfield shift in the 13C NMR of
the ipso carbon). While the structural trends are not
completely monotonic, they are generally consistent
with this trend. The changes in the 13C NMR are
particularly dramatic, with a >50 ppm upfield shift from
the cyclooctadieneiridium(I) complexes 2 and 3 to the
iridium(III) complex 10, a trend opposed to the simple
expectation that chemical shifts in the more electron-
poor complex should shift downfield, not upfield.
An alternative way of formulating these bridging aryl
complexes is to consider the aryl group with its π-bonded
(cyclooctadiene)iridium(I) fragment as a neutral (for-
mally zwitterionic) ligand bound to square-planar iri-
dium(I) or octahedral iridium(III). When viewed in this
way, the analogy of σ-Ar(Ir[cod]) to N-heterocyclic
(Arduengo-type) carbenes is striking. Given the increas-
ing importance of heterocyclic carbenes as supporting
ligands in catalysis,28 it is intriguing to consider the
possibility of using neutral π-metalated/σ-bonded aryl
groups, with their potential for planar chirality, as
ancillary ligands in catalytic systems.
Mech a n ism of F or m a tion of Br id gin g Ar yls. In
contrast to the formation of bridging aryl complexes 2
and 3 on treatment of the iridium(I) dimer [(cod)Ir(µ-
Cl)]2 (1) with 1 mol of aryl Grignard per mole of dimer,
reaction of excess mesitylmagnesium bromide leads to
exclusive formation of the dimesityl anion [(cod)Ir(mes)2]-
(11). Importantly, the isolated bridging mesityl cyclo-
octadiene complex 2 does not react readily with added
mesityl Grignard, with reactions taking several days to
give unidentified products which do not include anion
11. Likewise, the isolated dimesityl anion 11 does not
react with added iridium(I) chloride dimer 1 to form
bridging aryl complex 2. These observations indicate
that neither the bridging aryl complex 2 nor the
This view of the bonding treats the σ-bonded iridium
center as a π-donor to the ipso carbon of the bridging
aryl. This is borne out by the structural similarity of
the bridging aryl complexes to known iridium(I) com-
plexes of arenes substituted with π-donors such as
hydroxyl, as in [(cod)Ir(η5-HOC6H3-2,6-Ph2)]+,22 or diphen-
ylamino, in [Ir(TFB)(η5-PhNPh2)]+ (TFB ) tetrafluo-
robenzobarrelene).23 The Ir2-C1 distance of 2.42 Å in
2 and 3 is close to that observed in the triphenylamine
complex (2.457[11] Å), but about 0.1 Å shorter than that
observed in the phenol complex (Ir-Cipso ) 2.53 Å). At
least by this criterion, then, the (cod)BrIrI fragment
appears to be similar in its donor ability to Ph2N, though
somewhat weaker than OH. Structural distortions in
(arene)Cr(CO)3 complexes containing transition metals
σ-bonded to the arene have been attributed to analogous
π-donation from the σ-bonded metal center,24 although
inductive effects due to the electropositive metal sub-
stituent can also lead to distortions.25 However, the
chromium tricarbonyl complexes seem to show only
moderate modification of their chemical properties due
to σ-complexation of a second metal.26 This contrast with
the diiridium complexes described here may be due to
the fact that in the chromium complexes it is the
carbene/cyclohexadienyl resonance structure that is
zwitterionic, thus decreasing its importance and making
the complexes resemble the unperturbed σ-aryl/η6-arene
complexes. Little structural distortion toward a carbene/
cyclohexadienyl structure was noted in anionic [η6-
({(CO)5W}C6H5)Cr(CO)3]- (where both canonical forms
would contain one negative charge).27
(22) Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P. Polyhedron. 1990
9, 969-974.
(23) Uso´n, R.; Oro, L. A.; Carmona, D.; Esteruelas, M. A.; Foces-
Foces, C.; Cano, F. H.; Garc´ıa-Blanco, S.; Va´zquez de Miguel, A. J .
Organomet. Chem. 1984, 273, 111-128.
(24) Li, J .; Hunter, A. D.; McDonald, R.; Santarsiero, B. D.; Bott, S.
G.; Atwood, J . L. Organometallics 1992, 11, 3050-3055.
(25) Meyer, R.; Schindehutte, M.; van Rooyen, P. H.; Lotz, S. Inorg.
Chem. 1994, 33, 3605-3608.
(26) Richter-Addo, G. B.; Hunter, A. D. Inorg. Chem. 1989, 28,
4063-4065.
(27) Heppert, J . A.; Thomas-Miller, M. E.; Scherubel, D. M.; Takusa-
gawa, F.; Morgenstern, M. A.; Shaker, M. R. Organometallics 1989, 8,
1199-1206.
(28) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291-1309.

Unsymmetrically Bridging Aryls of Iridium
Organometallics, Vol. 22, No. 22, 2003 4489
Sch em e 1. P r op osed Mech a n ism of Rea ction of
Mesitylm a gn esiu m Br om id e w ith
[(cod )Ir (µ-Cl)]2 (1)
either 2 or 11, depending on the amount of added
Grignard, strongly suggests the presence of a common
intermediate in the two pathways. The notion that one
branch of the mechanism involves a further attack by
Grignard on a complex that already contains one aryl
group provides a reasonable explanation for why less
hindered aryl groups, such as phenyl or o-tolyl, lead only
to anions and not bridging complexes. A monoaryl
intermediate with the less hindered aryl groups should
have more accessible iridium(I) sites and hence would
undergo associative displacement more rapidly.
Con clu sion s
The iridium(I) chloride dimer (1) reacts with 2,6-
disubstituted aryl Grignard reagents (1 mol per dimer)
to yield air-stable diiridium complexes (2 and 3). The
aryl groups in these complexes are σ-bonded to one
iridium and π-bonded to a second. While this bridging
mode is precedented, this appears to be the first
example where both σ and π complexation take place
in a single chemical step. Spectroscopic and structural
data indicate that the bonding is best described as a
resonance hybrid between a zwitterionic σ-aryl/η6-arene
structure and a neutral carbene/η5-cyclohexadienyl
structure, and this resonance stabilization appears to
contribute to the enhanced chemical stability of both the
σ- and π-bonds relative to unbridged analogues. The
complexes react selectively at the square-planar iridium
without disturbing the π-bonded iridium center. In the
presence of excess mesityl Grignard, the iridium(I)
chloride dimer (1) reacts to form the monomeric diaryl-
iridium(I) anion [(cod)Ir(mes)2]- (11), but this anion and
the bridging aryl complexes cannot be readily intercon-
verted.
dimesityl anion 11 is an intermediate on the pathway
leading to the other.
A plausible scenario to account for both of the mesityl
products (Scheme 1) involves initial displacement of a
chloride by mesityl Grignard to give an intermediate
13. This intermediate has two possible fates. In the
presence of excess Grignard, another chloride is dis-
placed, leading to the formation of anion (11). In the
absence of Grignard, unimolecular displacement of the
second chloride by the σ-bonded mesityl group can take
place, resulting in the formation of the bridging aryl
complex 2. While Scheme 1 depicts the crucial inter-
mediate that precedes this branch point as having a
bridging chloride ion and a nonbridging mesityl, other
structures, such as one involving a σ-bridging mesityl
group, are certainly possible. Indeed, while it is intu-
itively appealing that the dimeric structure of the
starting material 1 is retained through the entire
pathway to dimeric 2, even that is by no means certain.
The formation of the dirhodium complex (CO)2Rh(µ,σ,η6-
C6H2Me3)(µ,η1(O),η1(C)-C6H2Me3CO)Rh(CO) from mon-
omeric Rh(mes)3 (eq 10)19 indicates that monomeric
intermediates may be reasonably invoked in the reac-
tion. However, the fact that the reaction can cleanly give
Ack n ow led gm en t. We thank Dr. Maoyu Shang and
Dr. Alicia Beatty for assistance with the X-ray crystal
structures. Financial support from a DuPont Young
Professor Award, from the Camille and Henry Dreyfus
Foundation (New Professor Award), and from the Dow
Chemical Company (Innovation Recognition Program)
is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for 2, 3, 5, 8, and 12 in tabular form and in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0304816