Interaction of a bulky N-heterocyclic carbene ligand with Rh(I) and Ir(I). Double C-H activation and isolation of bare 14-electron Rh(III) and Ir(III) complexes

Article abstract of DOI:10.1021/ja043249f

Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C-H bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers are stabilized by π-donation of the NHC ligands.

Full text of DOI:10.1021/ja043249f

Published on Web 02/19/2005
Interaction of a Bulky N-Heterocyclic Carbene Ligand with
Rh(I) and Ir(I). Double C-H Activation and Isolation of Bare
14-Electron Rh(III) and Ir(III) Complexes
†
†
†
‡
Natalie M. Scott, Reto Dorta, Edwin D. Stevens, Andrea Correa,
‡
,†
Luigi Cavallo, and Steven P. Nolan*
Contribution from the Department of Chemistry, UniVersity of New Orleans,
New Orleans, Louisiana 70148, and Dipartimento di Chimica, UniVersit a` di Salerno,
Baronissi (SA) 84081, Italy
Received November 9, 2004; E-mail: snolan@uno.edu
Abstract: Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-
heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C-H
bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes
can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-
carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon
monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers
are stabilized by π-donation of the NHC ligands.
limited to two Ir(III) and three Ru(II) compounds.4a,5,6 Fourteen-
electron Rh(III) compounds have so far eluded isolation. Crucial
Introduction
7
The wide interest in the synthesis of coordinatively unsatur-
ated metal complexes arises from their central role as intermedi-
ates in many stoichiometric and catalytic processes. In the
to the stability of these highly unsaturated metal centers is the
additional σ-donation offered through agostic interactions with
C-H bonds of the bulky phosphine ligands surrounding the
1
context of catalytic C-H bond activation and functionalization,
8
,9
8
metal center. These and analogous results on 14-electron d -
6
d 14-electron metal complexes are of special importance. Such
ML3 compounds have led to the concept that bare 14-electron
6
four-coordinate, 14-electron d -ML4 complexes are generally
transient species that are formed in situ by ligand dissociation.
Due to the exceptionally high Lewis acidity inherent in these
1
0
complexes will not be stable or isolable as such.
The current research effort in the area of N-heterocyclic
carbene (NHC) complexes of late transition metals originated
from the desire to develop highly active, phosphine-free
1
4-electron metal complexes, they tend to interact with even
the weakest of nucleophiles and the empty coordination sites
are most commonly filled by counterions,2 or by weakly
coordinating solvent molecules such as alcohols, THF, acetone,
or methylene chloride.3,4 Recent efforts, notably by Caulton et
al., have shown that, by using bulky phosphine ligands and non-
coordinating anions, fully characterized 14-electron d -ML4
compounds can indeed be synthesized. Examples of crystallo-
graphically characterized compounds are very rare and are
1
1
catalysts. While the use of NHCs has proven advantageous
in a number of catalytic processes, these ligand systems are
only starting to emerge as possible candidates for stabilizing
12
unusual metal complexes. Recent theoretical and experimental
6
(
5) (a) Cooper, A. C.; Streib, W. E.; Eisenstein, O.; Caulton, K. G. J. Am.
Chem. Soc. 1997, 119, 9069-9070. (b) Cooper, A. C.; Clot, E.; Huffman,
J. C.; Streib, W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am.
Chem. Soc. 1999, 121, 97-106.
(6) (a) Baratta, W.; Herdtweck, E.; Rigo, P. Angew. Chem., Int. Ed. 1999, 38,
†
University of New Orleans.
Universit a` di Salerno.
1
629-1631. (b) Watson, L. A.; Ozerov, O. V.; Pink, M.; Caulton, K. G.
‡
J. Am. Chem. Soc. 2003, 125, 8426-8427.
(
(
(
1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In Principles
and Applications of Organotransition Metal Chemistry; University Sci-
ence: Mill Valley, CA, 1987.
2) For a very recent example, see: (a) Rifat, A.; Kociok-K o¨ hn, G.; Steed, J.
W.; Weller, A. S. Organometallics 2004, 23, 428-432. For a review, see:
(7) Recent publications by Werner et al. and Milstein et al. show that while
neutral compounds of this type are dimeric, cationic complexes are stabilized
by counterions or solvent molecules: (a) Canepa, G.; Brandt, C. D.; Werner,
H. J. Am. Chem. Soc. 2002, 124, 9666-9667. (b) Gandelman, M.;
Konstantinovski, L.; Rozenberg, H.; Milstein, D. Chem. Eur. J. 2003, 9,
2595-2602.
(8) For reviews on agostic interactions, see: (a) Brookhart, M.; Green, M. L.
H.; Wong, L. Prog. Inorg. Chem. 1988, 36, 1-124. (b) Crabtree, R. H.
Angew. Chem., Int. Ed. Engl. 1993, 32, 789-805.
(9) σ-Donation through agostic interactions might be comparable in strength
to σ-donation by acetone; see: Dorta, R.; Goikhman, R.; Milstein, D.
Organometallics 2003, 22, 2806-2809.
(10) (a) Strauss, S. H. Chemtracts: Inorg. Chem. 1994, 6, 1-13. (b) Casares,
J. A.; Espinet, P.; Salas, G. Chem. Eur. J. 2002, 8, 4843-4853.
(11) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309. (b)
Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39-91. (c) Arduengo, A. J., III.; Harlow R. L.; Kline, M. J. Am.
Chem. Soc. 1991, 113, 361-363.
(
b) Strauss, S. H. Chem. ReV. 1993, 93, 927-942.
3) An important series of LTM complexes stabilized by O-bound ligands
+
2 3 2 2
comprises the catalytically active [M(H) (PR ) (S) ] (M ) Rh, Ir; S )
2
alcohol, THF, H O, acetone) species. Two such compounds have been
characterized crystallographically; see: (a) Crabtree, R. H.; Hlatky, G. G.;
Parnell, C. P.; Segm u¨ ller, B. E.; Uriarte, R. J. Inorg. Chem. 1984, 23, 354-
3
2
58. (b) Luo, X.-L.; Schulte, G. K.; Crabtree, R. H. Inorg. Chem. 1990,
9, 682-686.
2
(
4) For η -coordination of CH
2
Cl
2
to a 14-electron Ru(II) complex, see: (a)
Huang, D.; Huffman, J. C.; Streib, W. E.; Bollinger, J. C.; Eisenstein, O.;
Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 7398-7399. For a review on
6
halocarbon coordination to unsaturated d complexes, see: (b) Kulawiec,
R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89-115.
3516
9
J. AM. CHEM. SOC. 2005, 127, 3516-3526
10.1021/ja043249f CCC: $30.25 © 2005 American Chemical Society

Interaction of a Carbene Ligand with Rh(I) and Ir(I)
A R T I C L E S
t
work in our group has shown that NHC ligands are more
electron-donating than even basic alkylphosphines. More im-
Scheme 1. Reactivity of I Bu with [M(COE)2Cl]2
t
portantly, both IAd and I Bu ligands (IAd ) N,N-di(adamantyl)-
t
imidazol-2-ylidene, I Bu ) N,N-di(tert-butyl)imidazol-2-ylidene)
appear to be substantially more bulky than sterically demanding
t
13
phosphine ligands such as P Bu3. We reasoned that reactivity
t
studies involving the very bulky and basic I Bu with appropriate
Rh(I) and Ir(I) precursors might allow for the isolation of stable,
unsaturated, and high-valent compounds via intramolecular
C-H activation.14
t
Here, we report on the interaction of I Bu with [M(COE)2Cl]2
(M ) Rh, Ir), which results in a unique double cyclometalation
process of the NHC ligand system at room temperature and
ultimately leads to the synthesis and structural characterization
6
of unprecedented examples of bare, 14-electron d -RhL4 and
6
d -IrL4 metal complexes. DFT calculations on these complexes
provide an explanation for their stability and uncover a
surprising electronic aspect of the N-heterocyclic carbene ligand
class. Furthermore, reactivity studies show that while these
complexes do not coordinate ligands such as THF, acetone, or
dichloromethane, they react cleanly with carbon monoxide to
give rare examples of all-carbon bound d6-ML6 compounds.
Some experimental data on the Rh system described here have
been published as a communication.15
Results and Discussion
t
Interaction of I Bu with [Rh(COE)2Cl]2. To avoid any
unwanted reactivity between the solvent and the metal precursors
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
RhClH(I Bu)(I Bu′) (2) and IrClH(I Bu)(I Bu′) (3)
t
t
t
t
t
and/or the NHC ligands, reactivity studies between I Bu and
complex 2
complex 3
[
M(COE)2Cl]2 were carried out in hydrocarbon solvents only.
As a starting point, we investigated the reactivity of a hexane
slurry containing [Rh(COE)2Cl]2 with a slight excess of I Bu
Rh1-C1
2.081
2.058
2.074
2.530
1.430
2.704
2.073
2.653
Ir1-C1
Ir1-C12
Ir1-C5
Ir1-Cl
2.029
2.048
2.112
2.513
1.390
2.700
2.018
2.669
t
Rh1-C12
Rh1-C5
(4.16 equiv). Stirring this slurry at room temperature for 4 h
Rh1-Cl
Rh1-H1
Rh1‚‚‚C16
Rh1‚‚‚H16A
Rh1‚‚‚H16B
afforded an almost limpid yellow solution. Concentration and
subsequent addition of pentane led to the precipitation of
Ir1-H1
Ir1‚‚‚C16
t
t
Ir1‚‚‚H16A
Ir1‚‚‚H16B
RhClH(I Bu)(I Bu′) (2; Scheme 1) as a yellow solid in high yield.
X-ray-quality crystals of 2 were obtained by slow evaporation
of a saturated hexane solution. Its structure is represented in
Figure 1, and selected bond distances and angles can be found
in Table 1. As anticipated, one of the NHC ligands is
C1-Rh1-C12
C1-Rh1-C5
C1-Rh1-Cl
C1-Rh1-H1
169.499
79.002
104.927
88.723
C1-Ir1-C12
C1-Ir1-C5
C1-Ir1-Cl
C1-Ir1-H1
169.961
79.210
104.075
86.793
t
cyclometalated (I Bu′), with the metalated CH2 group cis to the
t
hydride ligand and trans to the chloride and the second I Bu.
Most significantly, the empty coordination site in this 16-
electron complex is taken up by a rather strong agostic
t
interaction from one of the Bu groups of the non-metalated
t
I Bu ligand. The agostic interaction of the tert-butyl groups with
the rhodium metal center shows distances of 2.073 Å [Rh(1)-
H(16A)] and 2.704 Å [Rh(1)-C(16)], with a second hydrogen
(
(
(
12) (a) Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.;
Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944-4945. (b) Clement,
N. D.; Cavell, K. J.; Jones, C.; Elsevier: C. J. Angew. Chem., Int. Ed.
t
t
Figure 1. Ball-and-stick representations of complexes RhClH(I Bu)(I Bu′)
2
004, 43, 1277-1279. (c) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.;
t
t
31
(2; left) and IrClH(I Bu)(I Bu′) (3; right).
Lewis, A. K. de K. Angew. Chem., Int. Ed. 2004, 43, 5824-5827.
13) (a) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Peterson, J. L.; Cavallo, L.;
Nolan, S. P. Organometallics 2003, 22, 4322-4326. (b) Dorta, R.; Stevens,
E. D.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 10490-
atom at 2.653 Å [Rh(1)-H(16B)].16 The nature of the agostic
1
1
0491. (c) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo,
interaction in 2 was studied by variable-temperature H NMR
L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485-2495.
14) Whereas such cyclometalation processes are well-documented and rather
common for bulky phosphine and nitrogen-donor ligands, they have been
rarely observed in NHCs. An example of intramolecular C-H activation
of an NHC on Rh (using the less bulky and less basic IMes ligand) has
appeared: Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics 2000,
spectroscopy in toluene-d8. Data recorded at room temperature
show that all signals for the cyclometalated I Bu′ ligand give
rise to sharp resonances. Interestingly, both the hydride peak
t
1
9, 1194-1197.
15) Dorta, R.; Stevens, E. D.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5054-
055.
(16) A discussion on nonclassical vs classical agostic interactions can be found
in the following reference: Baratta, W.; Mealli, C.; Hertweck, E.; Ienco,
A.; Mason, S. A.; Rigo, P. J. Am. Chem. Soc. 2004, 126, 5549-5562.
(
5
J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005 3517

A R T I C L E S
Scott et al.
(
at -22.93 ppm) and the resonances associated with the agostic
t
I Bu ligand are broad, indicating a fluxional exchange of the
17
tert-butyl groups. The existence of this agostic bond in solution
seems also to be reflected in the chemical shift value of the
hydride ligand trans to it. In fact, signals of hydrides trans to
an empty site for Rh(III) compounds are normally found at
significantly higher field than the value here.1
8,19
When the
temperature is gradually lowered, a sharpening of the peaks
t
associated with I Bu is observed. At 243 K, the spectrum of the
t
Figure 2. Ball-and-stick representations of complexes RhCl(I Bu′)2 (4; left)
and IrCl(I Bu′)2 (5; right).
agostically bound ligand shows two separate signals for the
imidazole backbone protons, indicating a different influence of
t
t
the two Bu groups (agostic and non-agostic, respectively) on
Table 2. Relevant Bond Lengths (Å) and Angles (deg) for
RhCl(I Bu′)2 (4), IrCl(I Bu′)2 (5), RhCl(I Bu′)2(CO) (8), and
t
t
t
t
the imidazole ring. The protons of the agostic and pendant Bu
t
IrCl(I Bu′)2(CO) (9)
groups display two separate singlet resonances in the aliphatic
region, each integrating for nine protons. All three CH3 groups
4
5
8
9
t
1
M1-C1
M1-C5
M1-C12
M1-C16
M1-Cl1
M1-C23
2.008(1)
2.032(1)
2.111(2)
2.044(1)
2.521(2)
1.981(16)
2.210(20)
2.103(16)
2.081(15)
2.509(4)
2.085(11)
2.123(12)
2.087(11)
2.076(11)
2.551(3)
2.075(8)
2.128(9)
2.075(8)
2.090(8)
2.513(3)
2.013(10)
of the agostic Bu remain equivalent on the H NMR time scale,
implying a very small rotational barrier for these methyl groups.
t
To establish whether coalescence of pendant and agostic Bu
groups was observable, the sample was heated to 323 K. At
this temperature, the pale yellow color of 2 slowly changes to
orange. More importantly, formation of a new set of resonances
is observed alongside extensive H/D scrambling between the
deuterated solvent and complex 2.
1.950(13)
C1-M1-C5
C1-M1-C16
C1-M1-C12
C1-M1-Cl1
C16-M1-Cl1 167.10(4)
C12-M1-Cl1 101.40(4)
C23-M1-C1
C23-M1-C5
C23-M1-C12
81.06(6)
88.71(6)
167.43(5)
91.03(4)
76.8(7)
90.9(6)
171.0(6)
83.6(4)
174.3(4)
105.4(4)
80.4(5)
88.9(5)
163.7(4)
83.8(3)
172.7(3)
106.5(3)
105.3(5)
173.6(5)
88.1(5)
80.1(3)
88.7(3)
162.5(3)
84.3(2)
172.6(2)
106.0(2)
105.4(3)
174.4(3)
88.7(3)
This latter result indicated that a different outcome might be
t
observed when the reaction between I Bu and [Rh(COE)2Cl]2
is carried out in benzene. Indeed, stirring a benzene solution of
t
[
Rh(COE)2Cl]2 with a slight excess of I Bu (4.16 equiv) at room
temperature for 4 h and subsequent workup of the reaction
1
mixture gave a dark yellow solid in 88% yield. The H NMR
To establish whether complex 4 is obtained through initial
formation of 2, we monitored the reaction between [Rh(COE)2-
Cl]2 and I Bu (4.16 equiv) by H NMR in C6D6. Indeed, during
the first hour, the formation of 2 as indicated by a strong signal
for the hydride at -23 ppm is observed. This was followed, after
ca. 1 h, by the appearance of a characteristic peak at 4.74 ppm
data are consistent with formation of the unusual, doubly
t
cyclometalated complex RhCl(I Bu′)2 (4). The proton spectrum
t
1
features two doublets at 6.61 and 6.42 ppm for the imidazole
backbone protons. The cyclometalated CH2 groups give rise to
two separate signals, a doublet at 2.21 ppm and a doublet of
doublets at 2.92 ppm. This additional doublet signal is due to
JRh-H coupling of one proton of each CH2 group with the metal
center, while the other doublets arise from geminal coupling
(
solubilized H2) and concomitant formation of the resonances
21
for 4. In addition, the doubly cyclometalated complex 4 is
also obtained from a benzene solution containing isolated 2.
22
(
JH-H ) 10 Hz). The 13C NMR supports these data and shows
one resonance at low field for the carbenic carbons (JRh-C )
3.1 Hz) and one signal at 28.97 ppm for the CH2 groups (JRh-C
34.7 Hz). To ascertain the exact structure of complex 4,
t
The solvent-dependent reactivity of [Rh(COE)2Cl]2 with I Bu
described above seems to stem from different solubilities of
complexes 2 and 4 and from different polarities associated with
the solvents used. This prompted us to investigate the reaction
4
)
X-ray-quality crystals were grown from a hexane solution. The
unsaturated complex 4 displays a distorted square pyramidal
structure around the rhodium center (Figure 2, bond lengths and
angles in Table 2). Analogous to 2, the two carbenic carbon
atoms are disposed trans to each other at an angle of 167.43°
with their corresponding cyclometalated CH2 groups mutually
t
between [Rh(COE)2Cl]2 and I Bu (4.16 equiv) in pentane. To
our delight, we were able to isolate the mixed dimer [Rh(COE)-
t
(
I Bu)Cl]2 (1) selectively by appropriately choosing the reaction
conditions. When a dilute pentane slurry of [Rh(COE)2Cl]2
t
(
1 mL/4 mg) is treated with I Bu (4.16 equiv), initial dissolution
is observed (10 min), and after 30 min, a heavy pale-yellow
t
cis (90.58°). One of the Bu groups of the ligand approaches
23
precipitate containing 1 is formed. Decanting the supernatant
the empty coordination site, but the distances to the metal center
indicate no efficient agostic interaction in this case [2.557 Å
for Rh(1)-H(9C) and 3.165 Å for [Rh(1)-C(9)].20
solution and washing the solid with pentane gives pure 1 in
high yield. Instead, if the pentane suspension containing 1 is
treated in situ with additional benzene and stirred overnight,
(
17) Similarly fluctional behavior has recently been observed in a Ru-NHC
complex; see: Abdur-Rashid, K.; Fedorkiw, T.; Lough, A. J.; Morris, R.
H. Organometallics 2004, 23, 86-94.
(20) These distances might indicate a very weak agostic interaction; for
comparison, see: Neve, F.; Ghedini, M.; Crispini, A. Organometallics 1992,
11, 3324-3327.
(
18) (a) Masters, C.; Shaw, B. L. J. Chem. Soc. A 1971, 3680-3686. (b)
Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020-
(21) Complex 2 can be isolated on a preparative scale by decreasing the reaction
time in benzene to 1 h, but contamination by already formed 4 is usually
observed.
1
024. (c) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1977, 2285-2291.
(22) Reaction times are longer than for the in situ reaction and allowed
1
3
(
19) The value found in 2 is similar to values for hydrides trans to O-bound
characterization by C NMR of 2 in benzene.
ligands; see: (a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1976,
(23) Dilution is important for the outcome of this reaction. When the reaction
is carried out in a more concentrated pentane slurry/solution, the yellow
precipitate contains various amounts of 2 (up to 30%).
9
8, 8, 2134-2142. (b) Rybtchinski, B.; Konstantinovsky, L.; Shimon, L.
J. W.; Vigalok, A.; Milstein, D. Chem. Eur. J. 2000, 6, 3287-3292.
3
518 J. AM. CHEM. SOC. VOL. 127, NO. 10, 2005
9

Interaction of a Carbene Ligand with Rh(I) and Ir(I)
A R T I C L E S
dissolution of 1 and formation of 4 are observed. On the other
hand, isolated 1 (without the presence of excess I Bu) is stable
The structure is entirely consistent with the solution spectrum
t
of this compound and closely resembles complex 2. Again, one
t
t
in a benzene solution. These control experiments convincingly
demonstrate that complex 1 represents a precursor on the way
to 2 and 4. Furthermore, they show that the presence of only
of the Bu groups of the non-metalated I Bu ligand is involved
in an agostic interaction from the C-H bond to the unsaturated
iridium center. This strong agostic interaction is characterized
by an Ir‚‚‚C(16) distance of 2.700 Å, with one of the located
and refined protons pointing toward the metal center [Ir‚‚‚
H(16C) ) 2.018 Å] and a second proton [H(16B)] at 2.669 Å.
Complex 3 undergoes a second cyclometalation involving the
t
one electron-donating I Bu ligand is not sufficient to favor
intramolecular C-H activation and that the first cyclometalation
process probably proceeds through initial formation of Rh-
t
(
I Bu)2Cl. Complex 1 was fully characterized by NMR spec-
t
troscopy and elemental analysis. The proton spectrum shows
agostically bonded I Bu ligand. In fact, when a benzene solution
t
resonances at 6.53 and 2.17 ppm (both singlets) for the I Bu
containing 3 is stirred at room temperature for 4 days, workup
t
ligand and a broad signal at 2.61 ppm for the olefinic protons
yields the doubly cyclometalated complex IrCl(I Bu′) (5) in
2
13
of COE. The C NMR gives doublets for the carbenic (at 178.30
ppm) and the olefinic (at 58.51 ppm) carbon atoms. Crystals of
quantitative yield as a dark red solid (Scheme 1). It is worth
noting that the same complex can also be synthesized in high
t
1
were subjected to an X-ray analysis, and although the data
yield directly from [Ir(COE) Cl] and 4.16 equiv of I Bu (C H ,
2
2
6
6
set was somewhat poor and precluded a detailed discussion of
its solid-state structure, the connectivity of 1 could be established
and confirmed its dimeric nature.
room temperature, 5 days). X-ray-quality crystals of complex
5 were obtained by slow evaporation of a dichloromethane
solution, and its structure is shown in Figure 2. Bond lengths
and angles are given in Table 2. The 16-electron complex 5
displays a distorted square pyramidal structure around the
iridium center, and its overall structure is similar to the rhodium
analogue 4. One clear difference is that, in the solid-state
structure of 5, the sixth coordination site is taken up by an
agostic interaction, showing bond distances of 2.785 Å [Ir(1)-
C(9)] and of 2.039 Å [Ir(1)-H(9C)] to the metal center. Con-
t
Interaction of I Bu with [Ir(COE)2Cl]2. The reactivity
described above prompted us to investigate the interaction
t
between the analogous iridium precursor and I Bu. Reaction of
t
[
Ir(COE)2Cl]2 with 4.16 equiv of I Bu in benzene affords the
t
t
metalated hydride complex IrClH(I Bu)(I Bu′) (3) after 20 h at
room temperature (Scheme 1). Complex 3 was isolated as a
yellow-green solid in nearly quantitative yield. Proton NMR
1
(C6D6) data at room temperature essentially replicate the
trary to both hydride complexes 2 and 3, the H NMR spectrum
t
spectrum of the rhodium complex at low temperatures and
indicate the presence of a somewhat stronger agostic interaction
of 5 does not give any indication for the agostic Bu group and
t
26
displays only one resonance for both Bu groups (1.72 ppm).
t
t
in 3. The cyclometalated Bu arm shows two distinct doublet
The reaction between [Ir(COE)2Cl]2 and I Bu (4.16 equiv)
in hexanes and pentane was also investigated. Unfortunately,
several attempts to synthesize the mixed dimer [Ir(COE)(I Bu)-
signals for the CH2 group and two resonances for the C(CH3)2-
CH2 protons. The protons of the two imidazole rings give rise
to four separate signals at 6.78, 6.69, 6.49, and 6.41 ppm,
t
Cl]2 were unsuccessful. To gain a better understanding of the
differences between the Rh and Ir systems, we prepared C6D6
t
indicating a different influence of the two Bu groups on the
t
t
imidazole ring of the non-metalated I Bu ligand. More impor-
solutions of [M(COE)2Cl]2 with only 2 equiv of I Bu (i.e., L:M
tantly, three different resonances are detected for the three non-
) 1:1) and monitored the progress of these reactions by NMR
spectroscopy. In the Ir case, the only products that could be
detected were 3 and unreacted [Ir(COE)2Cl]2. The same reaction
with Rh, while substantially more rapid (as seen by the faster
disappearance of the signals for free I Bu), gave a mixture of
[Rh(COE)2Cl]2, 1, and 2 f 4. These results indicate that the
t
cyclometalated Bu groups, again reflecting the chemical
t
t
inequivalence of the two Bu groups of the non-metalated I Bu
ligand. The presence of the agostic interaction in 3 and its ability
to alter the electronic properties via σ-donation from its C-H
bond are especially apparent when looking at the hydride signal.
Whereas signals of hydrides trans to an empty site in Ir(III)
complexes are found at -43 ppm or lower,24 the spectrum of 3
gives a singlet resonance for the hydride ligand at -32.83 ppm.
This chemical shift value clearly suggests that the hydride is
trans to another ligand (i.e., the agostic interaction) and parallels
the electronic situation found for iridium(III) hydrides that are
t
relative rates for the individual steps leading to the cyclometa-
t
t
lated product MClH(I Bu)(I Bu′) are different for Rh and Ir, and
t
that displacement of the first COE by I Bu in [Ir(COE)2Cl]2 is
far slower than the subsequent reactions, precluding isolation
t
of the mixed dimer [Ir(COE)(I Bu)Cl]2.
The reactivity displayed here between I Bu and Rh(I)/Ir(I) is
t
2
5
trans to O-bound ligands such as THF, acetone, or alcohols.
highly unusual. For instance, double cyclometalations of other
ligand systems such as bulky phosphines or nitrogen-based
systems have very rarely been observed for iridium,27 and are
unknown altogether for rhodium. To the best of our knowledge,
this is also the first time that activation of a C-H bond by Rh-
Well-formed single crystals were grown by direct reaction
t
of [Ir(COE)2Cl]2 with 4.16 equiv of I Bu in a more concentrated
benzene solution that was kept under constant shaking (!). A
representation of complex 3 is displayed in Figure 1, and
important bond lengths and angles can be found in Table 1.
2
8
(
III) is directly observed. It is also worth noting that, while
agostic interactions between C-H bonds and metal centers are
(
24) See ref 9 and the following references: (a) Masters, C. J.; Shaw, B. L.;
Stainbank, R. E. J. Chem. Soc., Chem. Commun. 1971, 209. (b) Moulton,
C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020-1024. (c)
Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jesperson, K.; Goldman,
A. S. J. Am. Chem. Soc. 2000, 122, 11017-11018. (d) Ben-Ari, E.;
Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. J. Am.
Chem. Soc. 2003, 125, 4716-4717.
8
(26) VT NMR studies (tol-d ) show that the spectrum of 5 remains virtually
unchanged in the range 297-233 K.
(27) We are aware of one such example, involving double cyclometalation on
an Ir-PCP system; see: Mohammad, H. A. Y.; Grimm, J. C.; Eichele, K.;
Mack, H.-G.; Speiser, B.; Novak, F.; Quintanilla, M. G.; Kaska, W. C.;
Mayer, H. A. Organometallics 2002, 21, 5775-5784
(28) Activation of aryl C-H bonds by Rh(III) has been proposed in the catalytic
transfer dehydrocoupling of arenes and silanes: Ezbiansky, K.; Djurovich,
P. I.; LaForest, M.; Sinning, D. J.; Zayes, R.; Berry, D. H. Organometallics
1998, 17, 1455-1457 and references therein.
(
25) (a) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am.
Chem. Soc. 1982, 104, 107-113. (b) Crabtree, R. H.; Demou, P. C.; Eden,
D.; Mihelcic, J. M.; Parnell, C. A.; Quirk, J. M.; Morris, G. E. J. Am.
Chem. Soc. 1982, 104, 6994-7001.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005 3519

A R T I C L E S
Scott et al.
t
Figure 4. Ball-and-stick representations of complexes [Rh(I Bu′)2]PF6 (6;
t
-
left) and [Ir(I Bu′)2]PF6 (7; right). Hydrogen atoms and PF6 omitted for
t
t
t
clarity.
Figure 3. Partial views of RhClH(I Bu)(I Bu′) (2; left) and IrClH(I Bu)-
(
t
I Bu′) (3; right), showing hydridic-to-protonic interactions.
from the reaction of 3 with AgPF6 in C6H5F, meaning that the
commonly postulated intermediates en route to C-H bond
activation, such species are difficult to detect. We are not aware
of any examples where both the immediate precursors (i.e., the
agostic complexes 2 and 3) and the products of a C-H
activation process (4 and 5) have been characterized by X-ray
crystallography.2 While the results here show that both rhodium
and iridium undergo unique double cyclometalation processes,
the ease with which these C-H activation processes take place
is markedly different. Comparing reaction times for the trans-
formation in benzene illustrates this point: 1 h (first cyclo-
metalation) and 4 h (first and second) for rhodium, 20 h (first
cyclometalation) and 120 h (first and second) for iridium. On
the basis of available data on C-H reactivity of other Ir(III)
compounds and the lack thereof for Rh(III) complexes, the
results for the second cyclometalation step described here appear
counterintuitive. In situ NMR experiments for both rhodium
and iridium systems did not show buildup of intermediates
during the second cyclometalation and preclude any meaningful
discussion on possibly different mechanisms at work for the
two metals (i.e., oxidative addition vs metathesis). On the
contrary, analysis of the solid-state structures of 2 and 3 shows
that there are close hydridic-protonic interactions between the
hydride ligand in 2 and 3 and two of the hydrogen atoms of the
t
t
initially formed cationic complex [IrH(I Bu)(I Bu′)]PF6 under-
goes a rapid second cyclometalation step to give 7 (overall
32
reaction time 2 h). This result is not entirely unexpected, as it
III
is known from studies employing Cp*Ir complexes that
cationic species are substantially more prone to undergo C-H
9
33
activation processes than their neutral analogues.
The 14-electron compounds 6 and 7 were fully characterized
by elemental analysis, spectroscopic methods, and X-ray
31
19
analyses. The P and F NMR spectra of 6 and 7 give
-
1
resonances typical for the free PF6 counterion. The H NMR
spectrum shows characteristic signals for the metalated CH2
groups [at 3.06 (d) and 2.39 (dd) ppm for 6, at 4.10 (d) and
3
.86 (d) ppm for 7]. Three distinct resonances for the free CH3
groups (intensity 3:1:1) and two sets of signals for the protons
of the imidazole rings gave no indication of agostic interactions
t
of the free Bu groups. The composition of 6 and 7 was
confirmed by elemental analyses, and to unambiguously ascer-
tain the exact structure of these complexes, X-ray-quality
crystals were obtained by slow evaporation of a CH2Cl2/C6H5F
solution (for 6) and by slow diffusion of diethyl ether into CH2-
Cl2 (for 7). Their structures are shown in Figure 4, and important
bond lengths and angles can be found in Table 3. Analogous to
those of their neutral 16-electron counterparts 4 and 5, the two
carbenic carbon atoms are disposed in a mutually trans arrange-
ment with their corresponding cyclometalated CH2 groups cis
with respect to each other. The geometry of these unique com-
pounds is best described as highly distorted, divacant octahedral
t
t
second, nonagostic Bu group of I Bu (Figure 3). The resulting
M-H‚‚‚H-C contacts with distances between 1.9 and 2.1 Å
are considerably shorter than the sums of the van der Waals
radii for H of 2.4 Å expected in the absence of attractive
interactions.30 Although unlikely, we therefore cannot exclude
(Table 3). We note that although these highly electrophilic com-
t
the possibility that the non-agostic Bu group is activated
plexes have two empty coordination sites, they are stabilized
neither by agostic interactions nor by a change in spin state as
(through a mechanism involving initial proton transfer) and that
the agostic bonds in 2 and 3 are merely rendering the metal
center more susceptible to the second C-H activation process.
Isolation and Characterization of Bare 14-Electron Com-
plexes. The addition of 1 equiv of AgPF6 to 4 and 5 in CH2Cl2
leads to abstraction of the chloride ligand and formation of 14-
6b
observed very recently by Caulton et al. for a Ru(II) compound.
In fact, the distances between the metal centers and the free
t
Bu groups are outside the range for even weak agostic inter-
8,34
actions. DFT calculations below give a convincing explana-
tion for the stability of these bare 14-electron compounds.
t
t
electron complexes [Rh(I Bu′)2]PF6 (6) and [Ir(I Bu′)2]PF6 (7)
(
29) For an example of an agostic Ir complex showing reversible metalation,
according to eq 1. Workup provides the cationic complexes as
see: Albeniz, A. C.; Schulte, G.; Crabtree, R. H. Organometallics 1992,
11, 242-249.
(
30) Related examples of proton-hydride interactions involve close contacts
between iridium(III) hydrides and a hydrogen atom coordinated to an
electronegative heteroatom (O, N). Proton transfer from these acidic
hydrogens to Ir-H and formation of iridium dihydrogen complexes have
been observed; see: (a) Lough, A. J.; Park, S.; Ramachandran, R.; Morris,
R. H. J. Am. Chem. Soc. 1994, 116, 8356-8357. (b) Lee, J. C. Jr.; Peris,
E.; Rheingold, A. L.; Crabtree, R. H. J. Am. Chem. Soc. 1994, 116, 11014-
11019. For a review on proton-hydride interactions, see: (c) Custelcean,
R.; Jackson, J. E. Chem. ReV. 2001, 101, 1963-1980.
31) Several X-ray structures shown here appear as mirror images of each other.
This is due to the fact that they are chiral at the metal center and that an
inversion center is imposed upon refinement of their structures.
32) In the rhodium case, we observe decomposition to several unidentified
products.
(
(
(
yellow-orange (6) and green-red (7) solids in high yield.
Interestingly, the iridium complex 7 is also obtained directly
33) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman,
R. G. J. Am. Chem. Soc. 2002, 124, 1400-1410.
3520 J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005

Interaction of a Carbene Ligand with Rh(I) and Ir(I)
A R T I C L E S
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
t
t
t
[
Rh(I Bu′)2]PF6 (6), [Ir(I Bu′)2]PF6 (7), [Rh(I Bu′)2(CO)2]PF6 (10),
t
and [Ir(I Bu′)2(CO)2]PF6 (11)
6
7
10
11
M1-C1
M1-C5
M1-C12
M1-C16
M1-C23
M1-C24
2.049(6)
2.014(6)
2.015(6)
2.014(5)
2.028(11)
2.038(10)
2.028(11)
2.038(10)
2.079(5)
2.092(6)
2.079(5)
2.106(6)
1.956(7)
2.017(8)
2.000(17)
2.025(16)
1.991(18)
2.128(18)
1.993(19)
2.070(20)
t
C1-M1-C5
C1-M1-C16
C1-M1-C12
C5-M1-C12
C5-M1-C24
C16-M1-C23
80.4(2)
96.5(2)
173.14(8)
95.7(3)
79.4(4)
97.8(4)
175.0(5)
97.8(4)
80.4(2)
87.9(2
163.4(2)
87.2(2)
171.6(3)
175.0(2)
78.1(8)
89.3(7)
160.2(8)
85.0(8)
173.7(9)
176.1(8)
Figure 5. Ball-and-stick representations of complexes RhCl(I Bu′)2(CO)
t
(8; left) and IrCl(I Bu′)2(CO) (9; right). Hydrogen atoms are omitted for
clarity.
Reactivity Studies of Complexes 4-7 with CO. The
coordinatively unsaturated nature of the cationic 14-electron
complexes 6 and 7, and of their neutral 16-electron analogues
4
and 5, offers an opportunity to examine whether a small linear
molecule such as CO can simply add to these to generate six-
coordinate saturated complexes and confirm, by analysis of
structural differences, the unsaturated nature of the “precursor”
complexes 4-7. Indeed, complexes 4-7 react instantaneously
t
Figure 6. Ball-and-stick representations of complexes [Rh(I Bu′)2(CO)2]-
t
-
PF6 (10; left) and [Ir(I Bu′)2(CO)2]PF6 (11; right). Hydrogen atoms and PF6
omitted for clarity.
with carbon monoxide. Treating a benzene solution of MCl-
of complexes 4 or 5 (see Table 2). The metal bond distances of
the cyclometalated carbons (C5, C12) are marginally longer for
the same reason. While the Ir-Cl(1) distance of 9 is similar to
that of 5, the Rh-Cl1 bond length for 8 is considerably longer
than the corresponding distance in 4.
t
(
I Bu′)2 (4 or 5) with 1 atm of CO results in rapid formation of
t
t
the saturated RhCl(I Bu′)2(CO) (8) and IrCl(I Bu′)2(CO) (9)
complexes that precipitate from solution as white solids in
almost quantitative yield (eq 2).
t
The reactivity of the 14-electron complexes [M(I Bu′)2]PF6
(6, 7) toward CO showed the two vacant sites to be readily
accessible, and the reaction proceeds by CO coordination of
the empty sites only. Treating 6 or 7 with 1 atm of CO in CH2-
t
Cl2 afforded the colorless 18-electron complexes [Rh(I Bu′)2-
t
(
CO)2]PF6 (10) and [Ir(I Bu′)2(CO)2]PF6 (11) in high yield
according to eq 3.
Satisfactory elemental analysis confirmed the compositions
of 8 and 9. Infrared spectroscopy showed the presence of one
-
1
-1
terminal ν(CO) band at 2015 cm (8) and 1980 cm (9),
1
13
indicating efficient back-bonding to the carbonyls. H and
C
NMR spectra of 8 and 9 in CD2Cl2 showed resonances
t
characteristic of the cyclometalated I Bu′ ligands (i.e., two
signals for the metalated CH2 groups, two resonances for the
free CH3 groups, and two sets of signals for the protons of the
imidazole rings). Coordination of CO in 8 and 9 was confirmed
by an additional low-field signal in 13C NMR near 180 ppm.
Colorless crystals of 8 and 9 suitable for X-ray diffraction were
obtained by slow diffusion of hexane into a concentrated CH2-
Cl2 solution. The geometry around each metal center is that of
a distorted octahedron (Figure 5). Analogous to those of
complexes 4 and 5, the two carbenic carbon atoms are disposed
trans to each other with their corresponding cyclometalated CH2
group cis with respect to each other but trans to either the
carbonyl or the chloride group. The incoming CO occupies the
previously vacant site. Consistent with an increase in steric
congestion around the metal center, rationalized by simply
having one more ligand present, the trans carbenic carbon atoms
in 8 and 9 are perturbed from linearity when compared to those
IR data for 10 and 11 indicate that a dicarbonyl species is
13
present, while the C NMR data show that the two carbonyl
1
ligands are equivalent. H NMR data for 10 and 11 give
t
resonances characteristic of cyclometalated I Bu′ ligands. To
establish the structure of these complexes and to permit a
comparison with 6 and 7, single-crystal diffraction studies were
performed on 10 and 11 from colorless crystals obtained by
slow diffusion of Et2O over saturated CH2Cl2 solutions. These
structures are shown in Figure 6, and bond lengths and angles
are given in Table 3. Each molecule is monomeric with a six-
coordinate octahedral metal center. The arrangement of the two
t
cyclometalated I Bu′ ligands is very similar to that of 6 and 7
with two CO molecules now occupying the previously empty
t
axial and equatorial positions. Although the I Bu′ environment
remains essentially unperturbed with respect to that of 6 and 7,
marginal wing contractions in the butterfly-like structures are
(
34) These do not give solvent-ligand adducts when dissolved in potentially
coordinating solvents such as dichloromethane, THF, or acetone.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005 3521

A R T I C L E S
Scott et al.
Figure 7. Overlap of the X-ray structures of [Rh(I Bu′)2]+ (6) with [Rh(I Bu′)2(CO)2] (10; left) and [Ir(I Bu′)2] (7) with [Ir(I Bu′)2(CO)2] (11; right).
t
t
+
t
+
t
+
observed [C1-M-C12 ) (6) 173.14(8)°, (7) 175.0(5)°, (10)
1
63.4(2)°, and (11) 160.2(8)°] due to the steric requirement of
adding two ligands around the now six-coordinate complexes
see the structure-overlapping diagram in Figure 7). The small
(
variations observed in the overlays lend further support to the
unique nature of complexes 6 and 7, and no further reactivity,
t
such as CO insertion via migration of the metalated I Bu′ arms,
is observed for complexes 10 and 11. These compounds are
rare examples of all-carbon-based octahedral Rh(III) and Ir-
(III) compounds. To the best of our knowledge, reported
complexes of this type are limited to the homoleptic hexamethyl
3-
species [MMe6] (M ) Rh, Ir) and the hexacarbonyl compound
3
+ 35
[
Ir(CO)6] .
t
t
DFT Calculations on MCl(I Bu′)2 and [M(I Bu′)2]PF6
Compounds. On a qualitative level, the unusual stability of
complexes 6 and 7 might be attributed to the electron-donating
nature of the all-carbon-based NHC ligands surrounding the
metals, which renders them less electrophilic than the previously
6
36
reported phosphine-containing 14-electron d -ML4 compounds.
To paint a more detailed picture of the electronic situation in
complexes 6 and 7, we performed density functional theory
t
t
+
(
DFT) calculations on the MCl(I Bu′)2 and [M(I Bu′)2] (M )
Rh, Ir) fragments. For the sake of simplicity, we will limit our
discussion to the Ir systems, but the calculations on the Rh
systems lead to analogous conclusions.
Figure 8. Most representative MOs of 5 and of 7. Hydrogen atoms are
omitted for clarity.
First, we note that both the X-ray and the DFT-optimized
structures of 5 present rather different Ir-C(carbene) bond
lengths, the Ir-C(1) distance (X-ray, 1.982 Å; DFT, 1.988 Å)
being roughly 0.1 Å shorter than the Ir-C(12) distance (X-ray,
The MO situation in complex 7 is quite different. This
complex is characterized by a local C2 symmetry axis that bisects
the two empty coordination positions and the two Ir-CH2 bonds.
The highest occupied molecular orbital (HOMO), shown in
Figure 8, is oriented along the local C2 symmetry axis, and has
2
.103 Å; DFT, 2.108 Å). Since the two Ir-C(carbene) bonds
are trans to each other, this asymmetry suggests a different
bonding scheme between the two NHC ligands and the Ir atom.
Natural bond order (NBO) analysis indicates that the bond order
of the Ir-C(1) bond (0.62) is 0.11 greater than that of the Ir-
C(12) bond (0.51). Molecular Orbital (MO) analysis suggests
that the higher bond order of the shorter Ir-C(1) bond is due
to a partial π f d donation from the filled π MO highest in
energy of the NHC ligand to empty d MOs of the metal. The
lowest unoccupied molecular orbital (LUMO) of 5, shown in
2
a strong dz character. The HOMO - 1 and HOMO - 2 are a
combination of dxz and dyz orbitals mainly, and are also shown
in Figure 8. Finally, the HOMO - 3 and HOMO - 4 show a
strong interaction between the filled π MO highest in energy
of the NHC ligands and empty d orbitals of the metal.
To obtain further insights into the NHC-M bonding, we
analyzed the model system 12 within C2 symmetry. The
optimized geometry of 12 is reported in Figure 9. Replacing
t
the σ-bonding alkyl arm of I Bu′ with a simple CH3 group allows
2
Figure 8, has a dz character and is oriented along the empty
the first BDE of the NHC to the Ir atom in 12 to be estimated,
coordination position of the octahedral Ir atom.
6
5.4 kcal/mol. This value is comparable the NHC BDEs
calculated by Meyer and co-workers for related cationic Cu,
Ag, and Au complexes, 81.5, 55.2, and 76.2 kcal/mol, respec-
tively. The C2 symmetry of 12 does not allow easy separation
(
35) (a) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain, B.; Hursthouse, M. B.
J. Chem. Soc., Chem. Commun. 1989, 1436-1437. (b) von Ahsen, B.;
Berkei, M.; Henkel, G.; Willner, H.; Aubke, F. J. Am. Chem. Soc. 2002,
37
1
24, 8371-8379.
(36) NHCs are better donor ligands than alkylphosphines; see ref 13.
of σ and π contributions to the bonding energy. Nevertheless,
3522 J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005

Interaction of a Carbene Ligand with Rh(I) and Ir(I)
A R T I C L E S
Scheme 2. Mixing of Empty NHC Molecular Orbitals
Figure 9. DFT-optimized geometry of 12.
the bonding to the metal. For instance, recent results by Meyer
and co-workers nicely indicated that NHC can accept electron
*
density from electron-rich metal atoms, through a d f π back-
3
7
donation scheme. Our analysis broadens further the bonding
ability of NHC ligands, since it suggests that they can also
donate electron density to electron-poor metal atoms, through
a π f d donation scheme. Stabilization of this kind might have
important implications in catalysis. Slower decomposition of
the catalytically active, highly unsaturated species normally
involved in catalysis could be the reason LTM-NHC com-
pounds often display higher thermal stability when compared
to analogous LTM-PR3 systems. As an example, the active
species in ruthenium-catalyzed metathesis reactions, RuCl2(L)-
(dCHPh) (L ) PR3 or NHC), has the same electronic
configuration as complexes 6 and 7 and might be stabilized more
efficiently through similar π(NHC)-d(LTM) donations.
Figure 10. Qualitative representation of the interactions between π MOs
+
of the (NHC)2 fragment and the d MO of the [Ir(Me)2] fragment of 12.
the bonding between the two NHC fragments and the metal
can be rationalized with qualitative MO analysis. Linear
combination of the filled and highest in energy π MOs of the
NHC fragments results in the π MOs 12A and 12B of Figure
Conclusions
We have described the interaction of the bulky N-heterocyclic
38
t
1
0. The filled π 12A MO strongly interacts with the dxy MO
carbene ligand I Bu with the neutral Rh(I) and Ir(I) precursors
t
of the metal fragment. The 12A-dxy interaction is further
stabilized by mixing of empty π* MOs of the NHC fragments.
Mixing of this empty MO, depicted in Scheme 2 for a single
NHC ligand, substantially localizes electron density on the
carbene C atom of the NHC ligand, enhancing the π donor
properties of the NHC ligands. The composition of the resulting
MO of the complex is 64% 12A and 28% dxy, while the above-
mentioned π* MO of the NHC fragments contributes 3%.
Conversely, the filled π 12B MO interacts less efficiently with
the mainly dxz MO of the metal fragment; see Figure 10. The
less efficient B symmetric 12B-dxy interaction results in an MO
of the complex with the following composition: 80% 12B and
[M(COE) Cl] using 2 equiv of I Bu per metal center. Initial
2
2
replacement of the cyclooctene ligands by the NHCs is followed
t
by a unique double cyclometalation process of both I Bu ligands.
We were able to isolate and fully characterize the resulting
t
M(III) complexes of formula MCl(I Bu′) . By a simple change
2
of solvent (for Rh) or of reaction times (for Ir), we obtained
several precursors, most notably the hydride complexes arising
from the first intramolecular C-H activation step, namely,
t
t
MClH(I Bu)(I Bu′). Abstraction of the chloride ligand from neu-
t
tral and unsaturated complexes MCl(I Bu′) enabled the isolation
2
of the first examples of “bare” 14-electron complexes [Rh-
t
t
(I Bu′) ]PF and [Ir(I Bu′) ]PF . Not only are stabilizing agostic
2
6
2
6
9
% dxz.
interactions absent in these homoleptic complexes, but they also
do not form metal-ligand adducts with σ-donor ligands such
as acetone or THF. However, they do interact readily with CO
The MO analysis indicates that the ability of NHC ligands
to act as π-electron donors is key in understanding the unusual
stability of the 14-electron complexes 6 and 7. In fact, there
are two reservoirs of electron density that can be donated to
alleviate the electron deficiency on the metal: the π orbital of
t
to give the saturated complexes [M(I Bu′) (CO) ]PF , which
2
2
6
represent rare examples of octahedral, all-carbon-bound orga-
nometallic species. Finally, DFT calculations on both the 16-
electron and 14-electron complexes reveal an important new
aspect in the bonding interaction of the N-heterocyclic carbene
ligand class with metal centers. Whereas NHCs have normally
been considered as pure σ-donor ligands, they can also donate
electron density through π f d interactions, and we show here
that the π-donor ability of NHCs is crucial in understanding
the remarkable stability as well as the lack of agostic interactions
t
the NHCs and the σ orbitals of the C-H bonds of the Bu
groups. Our analysis shows clearly that, in the present case,
the π orbital of the NHC ligand is preferred over the σ orbital
of the C-H bond, and this explains both the absence of agostic
interactions in 6 and 7 and their remarkable stability.
This analysis illustrates that NHC ligands are electronically
much more flexible than commonly thought (simple σ donors),
since the π orbitals on the NHC ring can be deeply involved in
t
in the 14-electron [M(I Bu′)2]PF6 compounds.
Whether the reactivity profile seen here is limited to I Bu is
t
(
37) Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004,
3, 755-764.
38) The MO numbering follows the ADF output.
presently being examined in our laboratories. The ease of
intramolecular C-H activation as well as the new bonding
2
(
J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005 3523

A R T I C L E S
Scott et al.
aspects of NHCs described here might also open new prospects
for the activation and functionalization of external substrates.
These exciting possibilities are currently being explored.
CH-olefin), 33.55 (s, C(CH
olefin), 27.34 (s, CH -olefin). Anal. Calcd for C38
C, 53.21; H, 7.99; N, 6.53. Found: C, 53.12; H, 8.29; N, 6.29.
3
)
3
), 31.23 (s, CH
2
-olefin), 29.99 (s, CH
2
-
2
H68Cl
2
N
4
Rh (857.70):
2
t
t
Synthesis of RhClH(I Bu)(I Bu′) (2). A hexane solution (10 mL)
Experimental Section
t
of I Bu (315 mg, 1.742 mmol) was added dropwise under stirring to a
2 2
yellow hexane slurry (60 mL) of [Rh(COE) Cl] (300 mg, 0.418 mmol).
General Considerations. All reactions were carried out using
standard Schlenk techniques under an atmosphere of dry argon or in
an MBraun glovebox containing dry argon. Solvents were distilled from
appropriate drying agents or were passed through an alumina column
in an MBraun solvent purification system. Other anhydrous solvents
were purchased from Aldrich, degassed prior to use by purging with
dry argon, and kept over molecular sieves. Solvents for NMR
spectroscopy were degassed with argon and dried over molecular sieves.
Experiments involving silver salts were carried out in the dark. NMR
spectra were collected on 300, 400, or 500 MHz Varian Gemini
spectrometers. Elemental analyses were performed by Robertson
The resulting yellow solution was stirred at room temperature for 4 h
and then concentrated to ca. 1/3 of its volume.43 The partially formed
yellow precipitate was treated with pentane (40 mL), leading to further
precipitation. The supernatant solution was decanted and the solid
washed with pentane (3 × 5 mL) and dried in vacuo. Yield: 379 mg
1
(
91%). H NMR (C
6 6
D ): δ ) 6.74 (d, J ) 1.8 Hz, 1H, CH-imidazole),
6
.59 (br, 2H, CH-imidazole), 6.46 (d, J ) 1.8 Hz, 1H, CH-imidazole),
ca. 2.1 (overlapped, 2H, C(CH CH ), 2.03 (s, 9H, C(CH ), 1.64 (br,
8H, C(CH ), 1.36 (s, 6H, C(CH CH ), -22.93 (br, 1H, RhH).
NMR (C ): δ ) 186.14 (d, JRh-C ) 44.9 Hz, C-carbene), 179.91
(d, JRh-C ) 39.6 Hz, C-carbene), 118.59 (s, CH-imidazole), 114.48 (s,
)
3 2
2
3 3
)
1
3
1
3
)
D
6
3
3
)
2
2
C
6
3
9
Microlit Laboratories, Madison, NJ. [Rh(COE)
2
Cl]
were synthesized following literature procedures. The free carbene
I Bu (N,N-di(tert-butyl)imidazol-2-ylidene) was synthesized according
2
2
and [Ir(COE) -
t
t
40
CH-imidazole), 64.84 (s, C Bu), 58.89 (s, C Bu), 32.01 (s, C(CH
31.73 (s, C(CH CH ), 28.26 (d, JRh-C ) 28.9 Hz, C(CH CH ). Anal.
Calcd for C22 40ClN Rh (498.93): C, 52.96; H, 8.08; N, 11.23.
Found: C, 53.12; H, 8.23; N, 10.83.
3 3
) ),
Cl]
2
t
)
3 2
2
)
3 2
2
4
1
H
4
to an adaptation of the literature procedure.
t
Synthesis of I Bu‚HBF
4
. To a stirring solution of toluene (200 mL)
t
t
Synthesis of IrClH(I Bu)(I Bu′) (3). A benzene solution (3 mL) of
and paraformaldehyde (6.00 g, 0.20 mol) in a two-necked round-bottom
flask at 0 °C was added dropwise tert-butylamine (29.26 g, 0.40 mol)
over 20 min. The reaction mixture was stirred for a further 5 min, and
then 50 mL of HCl (4 N in dioxane) was added slowly. After the
reaction reached completion (no longer exothermic), the mixture was
allowed to warm to room temperature, and glyoxal (40 wt %, 29.02 g,
t
I Bu (330 mg, 1.830 mmol) was added dropwise under stirring to an
orange benzene slurry (5 mL) of [Ir(COE) Cl] (395 mg, 0.440 mmol).
2
2
The resulting yellow-orange solution was stirred at room temperature
for 20 h and then concentrated to ca. 1/3 of its volume. The partially
formed yellow precipitate was treated with pentane (10 mL), leading
to further precipitation. The supernatant solution was decanted and the
0
.20 mol) was slowly added. The resulting mixture was stirred at 25
solid washed with pentane (3 × 5 mL) and dried in vacuo. Yield: 471
°
C for 12 h. The solution was then degassed with argon (15 min), and
1
mg (91%). H NMR (C
6 6
D ): δ ) 6.78 (s, 1H, CH-imidazole), 6.69 (s,
under argon the solvent volume was concentrated almost to dryness
via slow distillation by heating the reaction mixture to 105 °C for 4 h.
The reaction mixture was cooled to 50 °C, and the remaining solvent
1
H, CH-imidazole), 6.49 (s, 1H, CH-imidazole), 6.41 (s, 1H, CH-
imidazole), 2.55 (d, J ) 11.6 Hz, 1H, C(CH CH ), 2.42 (d, J ) 10.8
Hz, 1H, C(CH ) CH ), 2.01 (s, 9H, C(CH ) ), 1.82 (s, 9H, C(CH ) ),
3
)
2
2
t
was removed under vacuum. The resulting brown solid (I Bu‚HCl) was
3
2
2
3
3
3 3
1
.42 (s, 3H, C(CH
3 2 2 3 2 2
) CH ), 1.32 (s, 3H, C(CH ) CH ), 1.15 (s, 9H,
4
dissolved in water (150 mL) and filtered, and HBF (0.20 mol) was
1
3
t
C(CH ) ), -32.83 (s, 1H, Ir-H). C NMR (C D ): δ ) 177.11 (s,
added to the solution to afford a white precipitate (I Bu‚HBF
4
). The
3
3
6
6
C-carbene), 167.78 (s, C-carbene), 118.23 (s, CH-imidazole), 118.13
(s, CH-imidazole), 115.82 (s, CH-imidazole), 114.43 (s, CH-imidazole),
white solid was filtered, washed with water (3 × 20 mL), and then
1
dried under vacuum. Yield: 39.60 g (65%). H NMR (CDCl
3
): δ )
t
t
t
t
t
65.07 (s, C Bu), 59.55 (s, C Bu), 58.83 (s, C Bu), 57.79 (s, C Bu), 31.83
8
4
.83 (s, 1H, I Bu‚HBF ), 7.46 (s, 2H, CH-imidazole), 1.72 (s, 18H,
t
(s, C(CH ) ), 31.56 (s, C(CH ) ), 31.41 (s, C(CH ) CH ), 30.70 (s,
C Bu).
Synthesis of I Bu. In a drybox I Bu‚HBF
1.38 g, 58.0 mmol), and a catalytic amount of KO Bu (100 mg) were
3
3
3
3
3
2
2
t
t
C(CH
for C22
H, 6.83; N, 9.35.
Synthesis of RhCl(I Bu′)
(315 mg, 1.742 mmol) was added dropwise under stirring to a yellow
benzene solution (10 mL) of [Rh(COE) Cl] (300 mg, 0.418 mmol).
3 2
)
2
CH ), 29.96 (s, C(CH
3 3
) ), 9.49 (s, C(CH
3
)
2
CH
2
). Anal. Calcd
4
(8.65 g, 29.0 mmol), NaH
t
4
H40ClN Ir (588.26): C, 44.92; H, 6.85; N, 9.52. Found: C, 45.29;
(
loaded into a flask. Tetrahydrofuran (125 mL) was added to the flask,
and the resulting mixture was stirred for 12 h. The solvent was removed
under vacuum, and the resulting solid was transferred to a sublimation
t
t
2
(4). A benzene solution (10 mL) of I Bu
t
apparatus. I Bu was sublimed under static vacuum with slight heating
2
2
1
to afford a white crystalline product. Yield: 2.52 g, (41%). H NMR
The resulting solution was stirred at room temperature for 4 h, giving
a dark orange solution which was subsequently concentrated to ca. 3
mL. Addition of pentane (15 mL) led to the precipitation of a yellow
solid which was washed with pentane (3 × 5 mL) and dried in vacuo.
t
(C
6
D
6
): δ ) 7.56 (s, 2H, CH-imidazole), 1.89 (s, 18H, C Bu).
t
Synthesis of [Rh(COE)(I Bu)Cl]
2
(1). To a yellow pentane slurry
(
2 2
130 mL) of [Rh(COE) Cl] (600 mg, 0.836 mmol) was added dropwise
t
Yield: 364 mg (88%). H NMR (C D ): δ ) 6.61 (d, J ) 2.0 Hz, 2H,
1
a pentane solution (20 mL) of I Bu (627 mg, 3.478 mmol). Stirring at
room temperature for 10 min afforded a limpid yellow solution. Con-
tinued stirring led to the precipitation of a pale yellow solid after 30
min. After being stirring for another 30 min, the suspension was de-
6
6
CH-imidazole), 6.42 (d, J ) 2.0 Hz, 2H, CH-imidazole), 2.92 (dd, J
) 9 Hz and 4 Hz, 2H, C(CH CH ), 2.21 (d, J ) 9.5 Hz, 2H,
C(CH CH ), 1.77 (s, 18H, C(CH ), 1.40 (s, 6H, C(CH CH ), 1.28
(s, 6H, C(CH ) CH ). 13C NMR (C D ): δ ) 185.43 (d, J
) 43.1
)
3 2
2
)
3 2
2
3
)
3
3
)
2
2
4
2
canted and the solid washed with additional pentane (3 × 10 mL)
3
2
2
6
6
Rh-C
1
Hz, C-carbenes), 117.46 (s, CH-imidazole), 114.82 (s, CH-imidazole),
and dried in vacuo. Yield: 595 mg (83%). H NMR (C
6
D
6
): δ ) 6.53
t
t
(
s, 2H, CH-imidazole), 2.61 (br, 2H, CH-olefin), 2.20-2.43 (2 br, 4H,
CH -olefin), 2.17 (s, 18H, C(CH ), 1.40-1.78 (several br, 8H, CH
olefin). C NMR (C
64.87 (s, C Bu), 58.12 (s, C Bu), 32.45 (s, C(CH
C(CH CH ), 31.46 (s, C(CH ), 28.97 (d, JRh-C ) 34.7 Hz,
C(CH ) CH ). Anal. Calcd for C H ClN Rh (496.92): C, 53.17; H,
3 2 2
) CH ), 31.53 (s,
)
2
2
3 3
)
2
3
)
3
2
-
3
13
D
6 6
): δ ) 178.30 (d, JRh-C ) 56.9 Hz, C-carbene),
19.89 (s, CH-imidazole), 59.45 (s, C Bu), 58.51 (d, JRh-C ) 18.1 Hz,
3
2
2
22 38
4
t
7.71; N, 11.27. Found: C, 53.48; H, 8.14; N, 11.13.
1
t
Synthesis of IrCl(I Bu′)
2
(5) from 3. A benzene solution (18 mL)
(
39) Hofmann, P.; Meier, C.; Englert, U.; Schmidt, U. Chem. Ber. 1992, 125,
t
t
of IrClH(I Bu)(I Bu′) (3) (180 mg, 0.306 mmol) was stirred vigorously
at room temperature for 4 days. During this period, the reaction flask
was opened to argon twice a day to evacuate the dihydrogen formed.
3
53-359.
(
40) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 19-20.
41) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. K., III; Sommer,
W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P. Organometallics
(
2
004, 23, 1629-1635.
(42) Evaporation and analysis of the mother liquor show formation of 2 (in
(43) Leaving the solution at room temperature for 3 days and subsequent workup
addition to free ligand and traces of 1).
result in a mixture of 2 (major) and 4 (minor).
3524 J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005

Interaction of a Carbene Ligand with Rh(I) and Ir(I)
A R T I C L E S
The resulting, dark red solution was subsequently concentrated to ca.
brown solid was decanted, washed with pentane (2 × 5 mL), and dried
4
mL. Addition of pentane (10 mL) led to the precipitation of a deep
in vacuo, giving a red-green solid. Yield: 205 mg (86%).
t
red solid which was washed with pentane (3 × 5 mL) and dried in
Synthesis of RhCl(I Bu′) (CO) (8). A 50 mL flask was charged
2
1
t
vacuo. Yield: 172 mg (96%). H NMR (C
6 6
D ): δ ) 6.69 (s, 2H, CH-
with 200 mg (0.402 mmol) of RhCl(I Bu′) and 20 mL of benzene in
2
imidazole), 6.44 (s, 2H, CH-imidazole), 3.43 (d, J ) 10.5 Hz, 2H,
the glovebox. The flask was taken out of the box and connected to a
Schlenk line. The clear orange solution was purged with CO at room
temperature for 5 min, after which the solvent was removed under
vacuum. The residue was washed with pentane (2 × 5 mL), filtered,
and dried in vacuo to afford a white powder. Yield: 180 mg (85%).
C(CH
3
)
2
CH
), 1.40 (s, 6H, C(CH
): δ ) 179.88 (s, C-carbenes), 117.97 (s, CH-imidazole),
2
), 3.03 (d, J ) 10.0 Hz, 2H, C(CH
3 2 2
) CH ), 1.72 (s, 18H,
13
C(CH
3
)
3
)
3 2
CH ), 1.31 (s, 6H, C(CH
2
3 2
)
2
CH ).
C
NMR (C
6 6
D
t
t
1
15.07 (s, CH-imidazole), 65.74 (s, C Bu), 58.63 (s, C Bu), 32.85 (s,
1
C(CH
3
)
)
2
CH
2
), 32.04 (s, C(CH
). Anal. Calcd for C22
3
)
2
CH
2
), 31.60 (s, C(CH
3
)
3
), 8.92 (s,
H NMR (CD Cl ): δ ) 7.17 (s, 1H, CH-imidazole), 7.10 (s, 1H, CH-
2
2
C(CH
3
2
CH
2
H38ClN
4
Ir (586.24): C, 45.07; H,
imidazole), 6.88 (s, 1H, CH-imidazole), 6.82 (s, 1H, CH-imidazole),
6
.53; N, 9.56. Found: C, 45.18; H, 6.68; N, 9.22.
Direct Synthesis of 5. A benzene solution (10 mL) of I Bu (600
mg, 3.328 mmol) was added dropwise under stirring to an orange
benzene solution (50 mL) of [Ir(COE) Cl] (774 mg, 0.800 mmol).
The resulting solution was vigorously stirred at room temperature for
days, giving a dark red solution. During this time, the reaction flask
was open to argon from time to time to evacuate the H formed. The
solution was subsequently concentrated to ca. 8 mL. Addition of pentane
20 mL) led to the precipitation of a deep red solid which was washed
with pentane (3 × 10 mL) and dried in vacuo. Yield: 825 mg
88%).
Synthesis of [Rh(I Bu′)
solution (20 mL) containing 4 (200 mg, 0.404 mmol) was added
dropwise a dichloromethane solution (20 mL) of AgPF (102 mg, 0.404
2.60 (d, J ) 10.0 Hz, 2H, C(CH ) CH ), 1.93 (dd, J ) 11.5, 2H,
3
2
2
t
C(CH
3
)
2
CH
2
), 1.86 (s, 9H, C(CH
3
)
3
), 1.83 (s, 9H, C(CH
CH ), 1.27 (3H, C(CH
): δ ) 192.07 (d, JRh-C ) 43.5 Hz, CO), 177.26 (d,
3
)
3
), 1.77 (s,
6H, C(CH
3
)
2
2
CH ), 1.51 (3H, C(CH
3
)
2
2
3
)
2
CH ).
2
13
2
2
C NMR (CD
2
Cl
2
JRh-C ) 42 Hz, C-carbene), 173.50 (d, JRh-C ) 41 Hz, C-carbene),
5
119.70 (s, CH-imidazole), 118.66 (s, CH-imidazole), 116.04 (s, CH-
t
2
imidazole), 115.67 (s, CH-imidazole), 65.32 (s, C Bu), 64.40 (s,
t
C(CH
C(CH
3
)
)
2
CH
CH
2
), 59.21 (s, C Bu), 58.14 (s, C(CH
3
)
2
CH
), 31.10 (s, C(CH
CH
ORh (524.93): C, 52.63; H,
2
), 34.52 (s,
(
3
2
2
), 32.15 (s, C(CH
)
3 2
CH
2
3
3
) ), 30.55 (s,
Cl ): ν(CO)
C(CH
3
)
3
), 29.95 (d, JRh-C ) 32.5 Hz, C(CH
38C1N
)
3 2
2
). IR(CH
2
2
(
2015 cm-1. Anal. Calcd for C23
H
4
t
7.30; N, 10.67. Found: C, 52.31; H, 7.33; N, 10.23.
2
]PF
6
(6). To a yellow dichloromethane
t
Synthesis of IrCl(I Bu′)
2
(CO) (9). A 50 mL flask was charged with
t
6
2
86 mg (0.150 mmol) of IrCl(I Bu′) and 20 mL of benzene in the
mmol). Stirring at room temperature for 35 min was followed by
filtration of the AgCl salt over cotton/Celite. The filtered solution was
concentrated to ca. 4 mL and precipitated with benzene (4 mL) and
glovebox. The flask was taken out of the box and connected to a
Schlenk line. The clear red solution was purged with CO at room
temperature for 5 min, after which the solvent was removed under
vacuum. The residue was washed with pentane (2 × 5 mL), filtered,
pentane (12 mL). The resulting yellow-orange solid was decanted and
1
1
dried in vacuo. Yield: 227 mg (93%). H NMR (CD
2
Cl
2
): δ ) 7.26
and dried in vacuo to afford a white powder. Yield: 75 mg (83%). H
(
d, J ) 2.0 Hz, 2H, CH-imidazole), 7.12 (s, J ) 2.0 Hz, 2H, CH-
imidazole), 3.07 (d, J ) 10.0 Hz, 2H, C(CH CH ), 2.39 (dd, J ) 9.6
and 4.8 Hz, 2H, C(CH CH ), 1.62 (s, 18H, C(CH ), 1.51 (s, 6H,
C(CH CH ), 1.40 (s, 6H, C(CH ). P NMR (CD Cl ): δ )
138.44 (septet, J ) 711.5 Hz). F NMR (CD Cl ): δ ) 25.7 and
): δ ) 181.82 (d, JRh-C
4.9 Hz, C-carbenes), 119.33 (s, CH-imidazole), 117.27 (s, CH-
2 2
NMR (CD Cl ): δ ) 7.21 (d, J ) 2.0 Hz, 1H, CH-imidazole),7.07 (d,
J ) 1.6 Hz, 1H, CH-imidazole), 6.86 (d, J ) 2.0 Hz, 1H, CH-
imidazole), 6.78 (d, J ) 1.6 Hz, 1H, CH-imidazole), 2.47 (d, J ) 11.5
3
)
2
2
3
)
2
2
3 3
)
3
1
3
)
2
2
3
)
2
CH
2
2
2
Hz, 2H, C(CH
9H, C(CH ), 1.83 (s, 9H, C(CH
(3H, C(CH CH , 1.45 (s, 3H, C(CH
C NMR (CD Cl ): δ ) 179.29 (s, CO), 162.20 (s, C-carbene), 154.77
3
)
2
CH
2
), 2.20 (d, J ) 10.5, 2H, C(CH
), 1.55 (3H, C(CH
CH ), 1.22 (3H, C(CH
3
)
2
CH
2
), 1.86 (s,
), 1.49
CH ).
1
9
-
2
4
2
2
3
)
3
3
)
3
)
3 2
CH
2
1
3
3.9 (J ) 711.2 Hz). C NMR (CD
2
Cl
2
)
3
)
2
2
3
)
2
2
3
)
2
2
13
2
2
t
t
imidazole), 67.51 (s, C Bu), 58.54 (s, C Bu), 32.12 (s), 32.01 (s,
C(CH ), 29.92 (s), 29.62 (d, JRh-C ) 39.5 Hz, C(CH CH ), 28.58
s). Anal. Calcd for C22 PRh (606.43): C, 43.57; H, 6.32; N,
(s, C-carbene), 119.63 (s, CH-imidazole), 118.51 (s, CH-imidazole),
t
3
)
3
3
)
2
2
116.62 (s, CH-imidazole), 115.70 (s, CH-imidazole), 65.06 (s, C Bu),
t
(
9
H
38
F N
6 4
64.74 (s, C(CH
(s, C(CH CH
C(CH ), 13.99 (s, C(CH
Anal. Calcd for C23
3
)
2
CH
), 35.14 (s, C(CH
CH
2
), 59.45 (s, C Bu), 58.45 (s, C(CH
3
)
2
CH
2
), 35.51
.24. Found: C, 43.28; H, 6.43; N, 9.15.
3
)
2
2
3
)
2
CH
2
), 31.90 (s, C(CH
3
)
3
), 31.50 (s,
t
-1
Synthesis of [Ir(I Bu′)
2
]PF
6
(7) from Complex 3. To a yellow
3
)
3
3
)
2
2
). IR (CH
2
2
Cl ): ν(CO) 1980 cm .
fluorobenzene solution (8 mL) containing 3 (80 mg, 0.136 mmol) was
added dropwise a fluorobenzene solution (8 mL) of AgPF (34 mg,
.341 mmol). Stirring at room temperature for 90 min was followed
H38C1N
4
OIr (614.24): C, 44.97; H, 6.23; N, 9.12.
Found: C, 45.18; H, 6.69; N, 9.22.
6
t
0
Synthesis of [Rh(I Bu′)
2
(CO)
2
]PF
6
t
(10). A 50 mL flask was charged
by filtration of the AgCl salt over cotton/Celite. The filtered solution
was concentrated to ca. 2 mL and precipitated with pentane (10 mL).
The resulting red-brown solid was decanted, washed with pentane (2
with 100 mg (0.165 mmol) of [Rh(I Bu′)
2
]PF
6
and 20 mL of dichlo-
romethane in a glovebox. The flask was taken out of the box and
connected to a Schlenk line. The clear red solution was purged with
CO at room temperature for 5 min, after which the solvent was removed
under vacuum. The residue was washed with pentane (2 × 5 mL),
filtered, and dried in vacuo to afford a white powder. Yield: 89 mg
×
3 mL), and dried in vacuo, giving a red-green solid. Yield: 75 mg
1
(
79%). H NMR (CD
imidazole), 7.19 (s, J ) 1.8 Hz, 2H, CH-imidazole), 4.10 (d, J ) 11.4
Hz, 2H, C(CH CH ), 3.86 (d, J ) 11.1, 2H, C(CH CH ), 1.58 (s,
), 1.50 (s, 6H, C(CH CH ), 1.33 (s, 6H, C(CH CH ).
P NMR (CD Cl ): δ ) -143.40 (septet, J ) 708.6 Hz). F NMR
CD Cl
): δ ) -72.55 and -75.06 (J ) 708.2 Hz). 13C NMR (CD
): δ ) 184.85 (s, C-carbenes), 119.97 (s, CH-imidazole), 116.80
2 2
Cl ): δ ) 7.42 (d, J ) 1.8 Hz, 2H, CH-
)
2
2
(82%). 1H NMR (CD
imidazole), 7.06 (d, J ) 2.0 Hz, 2H, CH-imidazole), 2.30 (d, J ) 10.9
Hz, 2H, C(CH CH ), 2.00 (d, J ) 10.5 Hz, 2H, C(CH CH ), 1.79
(s, 18H, C(CH ), 1.63 (s, 6H, C(CH CH ), 1.41 (s, 6H, C(CH
). P NMR (CD Cl ): δ ) -138.13 (septet, J ) 711.0 Hz).
NMR (CD Cl ): δ ) 25.89 and 24.00 (J ) 710.4 Hz). C NMR (CD
2 2
Cl ): δ ) 7.32 (d, J ) 2.0 Hz, 2H, CH-
3
)
2
2
3
1
8H, C(CH
3
)
3
3
)
2
2
3
19
)
2
2
31
2
2
)
3 2
2
3
)
2
2
(
Cl
2
2
2
-
3
)
3
)
3 2
2
3
19
)
2
-
F
31
2
CH
2
2
2
t
t
13
(
(
C
s, CH-imidazole), 68.79 (s, C Bu), 58.88 (s, C Bu), 32.86 (s), 32.05
s, C(CH ), 25.32 (s), 8.22 (s, C(CH CH ). Anal. Calcd for
PIr (695.75): C, 37.98; H, 5.51; N, 8.05. Found: C, 37.73;
2
2
2
-
)
3 3
3
)
2
2
Cl
2
): δ ) 185.57 (br, CO), 162.71 (d, JRh-C ) 40.0 Hz, C-carbenes),
t
22
H F N
38 6 4
121.69 (s, CH-imidazole), 118.18 (s, CH-imidazole), 65.14 (s, C Bu),
H, 5.46; N, 7.70.
58.83 (s, C(CH
(s, C(CH ), 29.95 (s, C(CH
38 6 4 2
cm . Anal. Calcd for C24H F N O
3
)
2
CH
2
), 33.24 (d, JRh-C ) 28.0 Hz, C(CH
CH ). IR (CH Cl ): ν(CO) 2093, 2058
PRh (662.45): C, 43.51; H, 5.78;
3 2 2
) CH ), 31.26
Synthesis of 7 from Complex 5. To a yellow dichloromethane
solution (6 mL) containing 5 (200 mg, 0.341 mmol) was added dropwise
3
)
3
3
)
2
2
2
2
-
1
a dichloromethane solution (6 mL) of AgPF
6
(86 mg, 0.341 mmol).
N, 8.46. Found: C, 43.50; H, 5.73; N, 8.14.
t
Stirring at room temperature for 40 min was followed by filtration of
the AgCl salt over cotton/Celite. The filtered solution was concentrated
to ca. 1 mL and precipitated with pentane (12 mL). The resulting red-
Synthesis of [Ir(I Bu′)
with 100 mg (0.144 mmol) of [Ir(I Bu′)
2
(CO)
2
]PF
6
t
(11). A 50 mL flask was charged
2
]PF
6
and 20 mL of dichlo-
romethane in the glovebox. The flask was taken out of the box and
J. AM. CHEM. SOC. VOL. 127, NO. 10, 2005 3525
9

A R T I C L E S
Scott et al.
5
2
connected to a Schlenk line. The clear red solution was purged with
CO at room temperature for 5 min, after which the solvent was removed
under vacuum. The residue was washed with pentane (2 × 5 mL),
Molecular orbitals were plotted with the Molekel package. Natural
bond order analysis53 was performed with the Gaussian 03 program.
The ADF-optimized geometries were used for single-point calculations
with the BP86 functional as implemented in the Gaussian 03 package.
54
filtered, and dried in vacuo to afford a white powder. Yield: 86 mg
1
55
(
79%). H NMR (CD
2
Cl
2
): δ ) 7.32 (d, J ) 2.0 Hz, 2H, CH-
The SDD/ECP triple-ú basis set was used for rhodium and iridium,
imidazole), 7.04 (d, J ) 2.0 Hz, 2H, CH-imidazole), 2.33 (d, J ) 12.4
Hz, 4H, C(CH CH ), 1.82 (s, 18H, C(CH ), 1.69 (s, 6H, C(CH
CH ), 1.64 (s, 3H, C(CH CH ), 1.36 (s, 3H, C(CH
CD Cl ): δ ) -143.40 (septet, J ) 709.6 Hz). F NMR (CD
δ ) -72.46 and -74.97 (J ) 707.9 Hz). 13C NMR (CD
Cl ): δ )
84.84 (s, CO), 168.33 (s, C-carbenes), 121.56 (s, CH-imidazole),
while the SVP double-ú basis set with a polarization function was used
)
2
-
for main group atoms.56
3
)
2
2
3
)
3
3
3
1
2
)
3 2
2
3
)
2
CH
2
). P NMR
Cl ):
Acknowledgment. The National Science Foundation is grate-
fully acknowledged for financial support of this work. L.C.
thanks the University of Salerno for financial support (Grant
Piccole Attrezzature di Ateneo 2002).
1
9
(
2
2
2
2
2
2
1
1
t
18.44 (s, CH-imidazole), 65.23 (s, C Bu), 59.21 (s, C(CH
), 29.98 (s), 25.42 (s, C(CH CH
): ν(CO) 2082, 2034 cm . Anal. Calcd for C24
3
)
2
CH
). IR (CH
PIr
2
), 33.24
Supporting Information Available: Crystallographic infor-
mation files (CIFs) of the complexes. This material is available
free of charge via the Internet at http://pubs.acs.org. CCDC-
254678-254684 (compounds 3, 5, and 7-11) contain the
supplementary crystallographic data for this paper. These data
can be downloaded free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.hmtl (or obtained from the Cambridge Crystal-
lographic Data Centre, 12 Union Rd., Cambridge CB21EZ,
U.K., fax (+44) 1223-336-033, e-mail deposit@ccdc.cam.ac.uk).
(
Cl
s), 31.26 (s, C(CH
)
3 3
3
)
2
2
2
-
-
1
2
H
38
F
6
N
4
O
2
(
7
751.77): C, 38.34; H, 5.09; N, 7.45. Found: C, 38.51; H, 4.97; N,
.35.
Computational Details. The Amsterdam Density Functional (ADF)
program was used to obtain all the geometries we discussed.4 The
electronic configuration of the molecular systems was described by a
triple-ú STO basis set for iridium (4f, 5s, 5p, 5d, 6s, 6p), augmented
with single p and f functions. Triple-ú STO basis sets were also used
for chlorine (3s, 3p), nitrogen and carbon (2s, 2p), augmented with
single d and f functions, and hydrogen (1s) augmented with single d
and p functions. All these basis sets correspond to ADF basis set
TZ2P. The inner shells on rhodium and iridium (including 4d),
chlorine (including 2p), and nitrogen and carbon (1s) were treated within
the frozen core approximation. Energies and geometries were evaluated
using the local exchange-correlation potential by Vosko et al.,
augmented in a self-consistent manner with Becke’s exchange gradient
correction and Perdew’s
effects were included self-consistently with the zero-order relativistic
approximation (ZORA).
4,45
4
3
JA043249F
4
3
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8943-8953.
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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.; 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,
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Gaussian, Inc.: Pittsburgh, PA, 2003.
46
(
47
48,49
correlation gradient correction. Relativistic
50,51
Since relativistic effects were included, the
basis sets used were the relativistic basis sets (ZORA basis sets in ADF).
(
44) ADF 2000, Users Manual; Vrije Universiteit: Amsterdam, The Netherlands,
2
000.
(45) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; Van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
2
2, 931-967.
(
(
(
(
(
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3526 J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005