A-frame complexes of dirhodium bridged by dicarbene and diphosphine ligands

Article abstract of DOI:10.1021/om7007937

The complexes [RhBr(COD)]₂(μ-di-NHC), in which di-NHC represents the di-N-heterocyclic carbenes ^MeCC^meth (1,1′-methylene-3,3′-dimethyldiimidazol-2,2′-diylidene), ^tBuCC^meth (1,1′-methylene-3,3′-di-tert- butyldiimidazol-2,2′-diylidene), and ^MeCC^eth (1,2-ethylene-3,3′-dimethyldiimidazol-2,2′-diylidene), have been prepared from the reactions of the corresponding diimidazolium bromide salts with the acetate-bridged complex [Rh(μ-OAc)(COD)]₂. The analogous complex containing the ^tBuCC^eth group (1,2-ethylene-3, 3′-di-tert-butyldiimidazol-2,2′-diylidene) could not be prepared by this route, but the related chloro complex [RhCl(COD)]₂(μ- ^tBuCC^eth) was prepared by prior double deprotonation of the corresponding diimidazolium salt and reaction of the resulting dicarbene with [RhCl(COD)]₂. Reaction of the above ^MeCC ^eth and ^tBuCC^eth complexes with CO yields the corresponding dicarbene-bridged carbonyl products [RhX(CO)₂] ₂(μ-di-NHC) (X = Cl, Br), while reaction of the ^MeCC^meth and ^tBuCC^meth complexes with CO instead yielded the products [Rh(CO)₂(η¹: η¹-di-NHC)]⁺, containing chelating dicarbenes. The X-ray structure determinations of several of the above dicarbene-bridged species have been carried out and show metal-metal separations of greater than 6.6 A in all cases. Reaction of all of the above COD complexes with dppm (Ph₂PCH₂PPh₂) followed by CO yields the dicarbene- and dppm-bridged A-frame species [Rh₂(CO) ₂(μ-X)(μ-di-NHC)(dppm)]⁺ (X = Cl, Br), in which the larger bite of the dicarbenes, compared to that of dppm, induces strain within the complexes. Introduction of the bridging dppm and bromide groups lowers the Rh-Rh separation to about 3.3 A. Replacement of the bridging bromide by hydroxide in the ^MeCC^eth complex yields [Rh ₂(CO)₂(μ-OH)(μ-^MeCC^eth)-(dppm) ]⁺, in which the bridging hydroxide can be used to deprotonate a monoimidazolium iodide to give [Rh₂Br(CO)₂(IMe)(μ- ^MeCC^eth)(dppm)][I] (IMe = 1,3-dimethylimidazol-2-ylidene). In this species the bromide is on one metal while the monocarbene occupies the other, opening up the dicarbene bite to 5.18 A and the metal-metal separation to 4.07 A. Attempts to better match the larger bite of the dicarbenes by substituting dppm with dppe (Ph₂PCH₂CH ₂PPh₂) did not give a dppe-bridged product and, instead, yielded [(Rh(CO)(dppe))₂(μ-^MeCC^eth)][CF ₃SO₃]₂ in which the dppe groups are chelating.

Full text of DOI:10.1021/om7007937

Organometallics 2008, 27, 691–703
691
A-Frame Complexes of Dirhodium Bridged by Dicarbene and
Diphosphine Ligands
Kyle D. Wells, Michael J. Ferguson,† Robert McDonald,† and Martin Cowie*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
ReceiVed August 3, 2007
The complexes [RhBr(COD)]2(µ-di-NHC), in which di-NHC represents the di-N-heterocyclic carbenes
MeCCmeth (1,1′-methylene-3,3′-dimethyldiimidazol-2,2′-diylidene), tBuCCmeth (1,1′-methylene-3,3′-di-tert-
butyldiimidazol-2,2′-diylidene), and MeCCeth (1,2-ethylene-3,3′-dimethyldiimidazol-2,2′-diylidene), have
been prepared from the reactions of the corresponding diimidazolium bromide salts with the acetate-
bridged complex [Rh(µ-OAc)(COD)]2. The analogous complex containing the tBuCCeth group (1,2-ethylene-
3,3′-di-tert-butyldiimidazol-2,2′-diylidene) could not be prepared by this route, but the related chloro
complex [RhCl(COD)]2(µ-tBuCCeth) was prepared by prior double deprotonation of the corresponding
diimidazolium salt and reaction of the resulting dicarbene with [RhCl(COD)]2. Reaction of the above
MeCCeth and tBuCCeth complexes with CO yields the corresponding dicarbene-bridged carbonyl products
[RhX(CO)2]2(µ-di-NHC) (X ) Cl, Br), while reaction of the MeCCmeth and tBuCCmeth complexes with
CO instead yielded the products [Rh(CO)2(η1:η1-di-NHC)]+, containing chelating dicarbenes. The X-ray
structure determinations of several of the above dicarbene-bridged species have been carried out
and show metal-metal separations of greater than 6.6 Å in all cases. Reaction of all of the above
COD complexes with dppm (Ph2PCH2PPh2) followed by CO yields the dicarbene- and dppm-bridged
A-frame species [Rh2(CO)2(µ-X)(µ-di-NHC)(dppm)]+ (X ) Cl, Br), in which the larger bite of the
dicarbenes, compared to that of dppm, induces strain within the complexes. Introduction of the
bridging dppm and bromide groups lowers the Rh-Rh separation to about 3.3 Å. Replacement of
the bridging bromide by hydroxide in the MeCCeth complex yields [Rh2(CO)2(µ-OH)(µ-MeCCeth)-
(dppm)]+, in which the bridging hydroxide can be used to deprotonate a monoimidazolium iodide
to give [Rh2Br(CO)2(IMe)(µ-MeCCeth)(dppm)][I] (IMe ) 1,3-dimethylimidazol-2-ylidene). In this
species the bromide is on one metal while the monocarbene occupies the other, opening up the
dicarbene bite to 5.18 Å and the metal-metal separation to 4.07 Å. Attempts to better match the
larger bite of the dicarbenes by substituting dppm with dppe (Ph2PCH2CH2PPh2) did not give a
dppe-bridged product and, instead, yielded [(Rh(CO)(dppe))2(µ-MeCCeth)][CF3SO3]2 in which the
dppe groups are chelating.
properties like those of trialkylphosphines3 and may actually
be better donors than phosphines.4 However, NHC’s are not
merely phosphine substitutes; they offer a useful steric alterna-
tive to phosphines in having a very different shape. While
phosphines are often described as conical,5 NHC ligands are
more planar, having a slimmer, less sterically hindered axis
orthogonal to the bulky imidazole ring plane.
Introduction
N-heterocyclic carbenes (NHC’s)1 have proven to be versatile
ligands in organometallic chemistry and catalysis2 and appear
to offer a useful alternative to the ubiquitous phosphines. These
carbene ligands are generally acknowledged to possess bonding
The majority of NHC ligands used are monodentate,1 and
much fewer complexes are known that involve bidentate di-N-
heterocyclic carbene (di-NHC) ligands,2c,d,3a,6–8 in which pairs
of NHC groups are joined by an appropriate linker group. Most
of the complexes containing di-NHC’s employ these groups as
chelating ligands at a single metal,3a,6,7 although some have been
reported in which the di-NHC group bridges pairs of metals.3a,7,8
On the basis of an extensive chemistry of binuclear, late-
metal complexes in which the pair of metals are bridged by
diphosphine ligands,9 we became interested in extending this
chemistry to include bridging di-NHC ligands. In particular, we
* To whom correspondence should be addressed.
† X-ray Crystallography Laboratory.
(1) See for example: (a) Wanzlick, H.-W.; Schönherr, H.-J. Angew.
Chem. Int. Ed. 1968, 7, 141. (b) Öfele, K. J. Organomet. Chem. 1968, 12,
P42. (c) Arduengo, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530. (d) Arduengo, A. J. Acc. Chem. Res. 1999,
32, 913.
(2) See for example: (a) Frenzel, U.; Weskamp, T.; Kohl, F. J.;
Schattenmann, W. C.; Nuyker, O.; Herrmann, W. A. J. Organomet. Chem.
1999, 586, 263. (b) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G.
Chem. ReV. 2000, 100, 39. (c) Döhring, A.; Göhre, J.; Jolly, P. W.; Kryger,
B.; Rust, J.; Verhovnik, G. P. J. Organometallics 2000, 19, 388. (d)
Bielawski, C. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2000, 39, 2903.
(e) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (f) Gsto¨ttmayr,
C. W. K.; Bo¨hm, V. P. W.; Herdtweck, E.; Grosche, M.; Herrmann, W. A.
Angew. Chem., Int. Ed. 2002, 41, 1363. (g) Albrecht, M.; Miecznikowski,
J. R.; Samuel, A.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21,
3596. (h) Poyatos, M.; Uriz, P.; Mata, J. A.; Claver, C.; Fernandez, E.;
Peris, E. Organometallics 2003, 22, 440. (i) Poyatos, M.; Mas-Marzá, E.;
Mata, J. A.; Sanaú, M.; Peris, E. Eur. J. Inorg. Chem. 2003, 1215. (j) Frey,
G. D.; Schutz, J.; Herdtweck, E.; Herrmann, W. A. Organometallics 2005,
24, 4416.
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G. R. J. Chem. Eur. J. 1996, 2, 772. (b) Green, J. C.; Scurr, R. G.; Arnold,
P. L.; Cloke, F. G. N. Chem. Commun. 1997, 1963.
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10.1021/om7007937 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/01/2008

692 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
Chart 1
were interested in generating di-NHC analogues of the well-
known “A-frame” complexes,10 in which pairs of bidentate
diphosphines (usually dppm) bridge a pair of metals. Owing to
the rotational freedom in the linking groups between the
diphosphine or dicarbene ends of these bidentate ligands,
complexes bridged by only one of these groups tend to adopt a
skewed arrangement3a,7,11 in which the metals are quite widely
spaced in order to minimize steric repulsions between the metal
coordination spheres. Unfortunately, this arrangement, with its
concomitant large metal-metal separation, is not entirely
conducive to metal-metal cooperativity effects. In order to bring
the metals together in a bid to optimize metal-metal interactions
and their probability of interacting in a cooperative manner, a
second bridging group is needed. This is exactly the strategy
that has been utilized in “A-frame” complexes.
In this paper we describe our attempts to generate A-frame-like
complexes of rhodium, in which a series of di-NHC ligands,
diagrammed in Chart 1, are used as bridging groups. The
abbreviations used for these di-NHC ligands, also shown in this
chart, in which the substituent on the carbene rings appears first,
followed by the dicarbene notation (CC) and finally an abbreviation
designating a methylene or ethylene linker between the NHC rings,
are those originally suggested by Green and co-workers.6e We are
seeking to determine the influences of the different linker lengths
and of the different steric factors of the NHC substituents.
atmosphere. Deuterated solvents used for NMR experiments were
freeze–pump–thaw degassed and stored under argon over appropri-
ate molecular sieves. Reactions were performed under an inert argon
atmosphere or the reactant gas using standard Schlenk techniques.
Unless otherwise specified, reactions were carried out at ambient
temperature. Bis(diphenylphosphino)methane and bis(diphenylphos-
phino)ethane, sodium tetraphenylborate, sodium borohydride, potas-
sium hydroxide, potassium bis(trimethylsilyl)amide, 1-methylim-
idazole, dibromomethane, and 1,2-dibromoethane were purchased
from Aldrich and used without further purification. 1-tert-Butyl-
imidazole was prepared using a published procedure and purified
by vacuum distillation.12 The preparations of diimidazolium salts
used in this paper have been reported;2g,3a,6b however, a general
synthetic approach has been outlined below. Bis(cycloocta-1,5-
diene)(µ-dichloro)dirhodium was prepared using published proce-
dures and used without further purification,13 Bis(cycloocta-1,5-
diene)(µ-diacetato)dirhodium was prepared as previously reported
and recrystallized from ethyl acetate.14
Experimental Section
General Comments. All solvents were dried (using appropriate
drying agents), distilled before use, and stored under a dinitrogen
The 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on
a Varian iNova-400 spectrometer operating at 399.8 MHz for H,
1
(6) See for example: (a) Herrmann, W. A.; Schwarz, J.; Gardiner, M. G.;
Spiegler, M. J. Organomet. Chem. 1999, 575, 80. (b) Douthwaite, R. E.;
Haüssinger, D.; Green, M. L. H.; Silcock, P. J.; Gomes, P. T.; Martins,
A. M.; Danopoulos, A. A. Organometallics 1999, 18, 4584. (c) Albrecht,
M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem. Commun. 2002, 32. (d)
Danopoulos, A. A.; Winston, S.; Motherwell, W. B. Chem. Commun. 2002,
1376. (e) Douthwaite, R. E.; Green, M. L. H.; Silcock, P. J.; Gomes, P. T.
Dalton Trans. 2002, 1386. (f) Poyatos, M.; Mata, J. A.; Falomir, E.;
Crabtree, R. H.; Peris, E. Organometallics 2003, 22, 1110. (g) Bonnet, L. G.;
Douthwaite, R. E.; Hodgson, R. Organometallics 2003, 22, 4384. (h) Vogt,
M.; Pons, V.; Heinekey, D. M. Organometallics 2005, 24, 1832. (i) Kreisel,
K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics 2006, 25, 4670. (j)
Viciano, M.; Poyatos, M.; Sanaú, M.; Peris, E.; Rossin, A.; Ujaque, G.;
Lledós, A. Organometallics 2006, 25, 1120.
161.8 MHz for 31P, and 100.6 MHz for 13C. Elemental analyses
were performed by the microanalytical service within the Depart-
ment of Chemistry. Likewise, mass spectrometric analyses were
performed by the Mass Spectrometry Laboratory using positive ion
electrospray ionization on a Micromass ZabSpec Hybrid Sector-
TOF. Infrared spectra were obtained using a Nicolet Avatar
370DTGS instrument or as solids using a Nicolet Magna 750 with
a Nic-Plan infrared microscope. Carbonyl stretches reported are
for non isotopically enriched samples. The 31P chemical shifts are
referenced to external 85% H3PO4, while 1H and 13C chemical shifts
are referenced to TMS. The chemical shifts for the phenyl
hydrogens and carbons are not given. Conductivity measurements
on compound 5c as both the BPh4- and Br- salts were carried out
on 1 × 10-3 M solutions of the compounds in nitromethane, using
a Yellow Springs Instrument Model 31 conductivity bridge. For
these species the conductivities obtained were Λ ) 59.3 and 69.0
Ω-1 cm2 mol-1, respectively.
(7) See for example: (a) Poyatos, M.; Sanau´, M.; Peris, E Inorg. Chem.
2003, 42, 2572. (b) Mata, J.; Chianese, A. R.; Miecznikowski, J. R.; Poyatos,
M.; Peris, E.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 1253.
(c) Quezada, C. A.; Garrison, J. C.; Panzner, M. J.; Tessier, C. A.; Youngs,
W. J. Organometallics 2004, 23, 4846. (d) Leung, C. H.; Incarvito, C. D.;
Crabtree, R. H. Organometallics 2006, 25, 6099.
(8) (a) Garrison, J. C.; Simons, R. S.; Talley, J. M.; Wesdemiotis, C.;
Tessier, C. A.; Youngs, W. J. Organometallics 2001, 20, 1276. (b) Khramov,
D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem., Int. Ed. 2006, 45,
6186.
(9) See for example: (a) Puddephatt, R. J. Chem. Soc. ReV. 1983, 12,
99. (b) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. ReV. 1988,
86, 191. (c) Reinking, M. K.; Fanwick, P. E.; Kubiak, C. P. Angew. Chem.,
Int. Ed. Engl. 1989, 28, 1377. (d) Johnson, K. A.; Gladfelter, W. L.
Organometallics 1992, 11, 2534. (e) Jenkins, J. A.; Cowie, M. Organo-
metallics 1992, 11, 2767. (f) Pamplin, C. B.; Rettig, S. J.; Patrick, B. O.;
James, B. R. Inorg. Chem. 2003, 42, 4117. (g) Jones, N. D.; James, B. R.
AdV. Synth. Catal. 2002, 344, 1126. (h) Dennett, J. N. L.; Bierenstiel, M.;
Ferguson, M. J.; McDonald, R.; Cowie, M. Inorg. Chem. 2006, 45, 3705.
(10) See for example: (a) Kubiak, C. P.; Eisenberg, R. J. Am. Chem.
Soc. 1977, 99, 6129. (b) Olmstead, M. M.; Hope, H.; Benner, L. S.; Balch,
A. L. J. Am. Chem. Soc. 1977, 99, 5502. (c) Cowie, M.; Dwight, S. K.
Inorg. Chem. 1979, 18, 2700. (d) Puddephatt, R. J.; Azam, K. A.; Hill,
R. H.; Brown, M. P.; Nelson, C. D.; Moulding, R. P.; Seddon, K. R.;
Grossel, M. C. J. Am. Chem. Soc. 1983, 105, 5642. (e) Ge, Y.-W.; Sharp,
P. R. J. Am. Chem. Soc. 1990, 112, 2632.
Preparation of Compounds. (a) General Synthetic Route to
Diimidazolium Salts (1a-d). A 500 mL round-bottom flask was
charged with 100 mmol of the dibromoalkane and diluted with 200
mL of toluene. An excess (250 mmol) of the 1-alkylimidazole was
then added and the solution refluxed for 24 h. The formed precipitate
was collected by vacuum filtration and recrystallized from boiling
methanol by cooling to -25 °C, whereupon colorless crystals were
obtained with yields ranging from 50 to 90%. The salts were then
dried in vacuo for several days to remove residual solvent and stored
in a desiccator before usage. Compounds 1a-d are the diprotonated
versions of the respective carbenes MeCCmeth (a), tBuCCmeth (b),
MeCCeth (c), and tBuCCeth (d) as dibromide salts, namely 1,1′-
(methylene)-3,3′-dimethyldiimidazolium-2,2′-diylidene dibromide
(11) See for example: (a) Toerien, J. G.; van Rooyen, P. H. J. Chem.
Soc., Dalton Trans. 1991, 2693. (b) Liu, C. W.; Wen, Y.-S.; Liu, L.-K.
Organometallics 1997, 16, 155. (c) Benson, J. W.; Keiter, R. L.; Keiter,
E. A.; Rheingold, A. L.; Yap, G. P. A.; Mainz, V. V. Organometallics
1998, 17, 4275.
(12) (a) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. Synthesis
(Stuttgart) 2003, 2661. (b) Gridnev, A. A.; Mihaltseva, I. M. Synth.
Commun. 1994, 24, 1547.
(13) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218.
(14) Chatt, J.; Venanzi, M. J. Chem. Soc. 1957, 4735.

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 693
1
(1a), 1,1′-(methylene)-3,3′-di-tert-butyldiimidazolium-2,2′-diylidene
dibromide (1b), 1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazolium-
2,2′-diylidene dibromide (1c), and 1,1′-(1,2-ethylene)-3,3′-di-tert-
butyldiimidazolium-2,2′-diylidene dibromide (1d).
1JC-Rh ) 14.5 Hz), 69.1 (d, 2C, JC-Rh ) 14.4 Hz), 33.5 (s, 2C),
32.4 (s, 2C), 30.0 (s, 2C), 28.7 (s, 2C, COD). HRMS: m/z calcd
for C26H38N4BrRh2 (M+ - Br), 691.0390; found:, 691.0385 (M+
- Br). Anal. Calcd for C27H40Br2N4Rh2Cl2 (2c · CH2Cl2): C, 37.83;
H, 4.70; N, 6.54; Cl, 8.27. Found: C, 37.87; H, 4.71; N, 7.08; Cl,
8.51. The presence of 1 equiv of CH2Cl2 was confirmed by 1H
NMR spectroscopy in chloroform.
(b) (µ-1,1′-Methylene-3,3′-dimethyldiimidazoline-2,2′-diylidene)-
bis[bromo(η2:η2-1,5-cyclooctadiene)rhodium(I)] (2a). A 30 mL
portion of THF was added to a equimolar mixture of compound
1a (188 mg, 0.56 mmol) and [Rh2(OAc)2(C8H12)2] (300 mg, 0.56
mmol). The resulting mixture was stirred under reflux for 4 h, after
which the formation of a yellow precipitate was observed. Once it
was cooled to room temperature, the vessel was charged with 60
mL of ether and the supernatant discarded. The remaining solid
was redissolved in 15 mL of CH2Cl2, filtered through Celite, and
precipitated using 40 mL of ether, yielding 0.34 g (80%) of a pale
(e) (µ-1,1′-(1,2-Ethylene)-3,3′-di-tert-butyldiimidazoline-2,2′-
diylidene]bis[chloro(η2:η2-1,5-cyclooctadiene)rhodium(I)) (2d).
Attempts to synthesize 2d using the convenient approach outlined
for compounds 2a-c met with little success. Instead, a 50 mL
portion of THF was added to a mixture of compound 1d (1.33 g,
3.04 mmol, 1 equiv) and excess KN(Si(CH3)3)2 (1.33 g, 6.67 mmol,
2.2 equiv) and the resulting mixture was stirred at room temperature
for 16 h. The next day, a fine white precipitate was removed by
passing the mixture through a bed of Celite contained within a
porous frit. Without further purification, the deep yellow solution
containing the dicarbene was added dropwise to a stirred orange
solution of [Rh2(Cl)2(C8H12)2] (1.5 g, 3.04 mmol, 1 equiv) in 30
mL of THF, resulting in the formation of a yellow precipitate. The
mixture was stirred for 30 min before removal of the volatiles under
reduced pressure afforded an oily residue. The crude product was
extracted with 2 × 15 mL portions of CH2Cl2 and passed through
a filter stick containing a bed of Celite before 50 mL of pentane
was added to precipitate a yellow solid. The product was then dried
1
yellow powder. H NMR (400.4 MHz, CD2Cl2, 27.0 °C): δ 7.80
3
3
(d, 2H, JH-H ) 2.0 Hz), 6.84 (d, 2H, JH-H ) 2.0 Hz, NCH);
7.41 (s, 2H, CH2); 4.04 (s, 6H, NCH3); 5.09 (m, 4H), 3.42 (m,
4H), 2.43 (m, 4H), 2.37 (m, 4H), 2.02 (m, 4H), 1.94 (m, 4H, COD).
13C{1H} NMR (100.7 MHz, CD2Cl2, 27.0 °C): δ 183.5 (d, 2C,
1JC-Rh ) 50.3 Hz, Ccarbene); 123.8 (s, 2C), 121.2 (s, 2C, NCH);
1
62.9 (s, 2C, CH2); 38.0 (s, 2C, NCH3); 99.2 (d, 2C, JC-Rh ) 6.7
1
1
Hz), 98.7 (d, 2C, JC-Rh ) 6.7 Hz), 70.3 (d, 2C, JC-Rh ) 14.4
1
Hz), 69.7 (d, 2C, JC-Rh ) 14.5 Hz), 33.3 (s, 2C), 32.6 (s, 2C),
29.4 (s, 2C), 29.1 (s, 2C, COD). Anal. Calcd for C25H36N4Br2Rh2:
C, 39.60; H, 4.79; N, 7.39. Found: C, 39.56; H, 4.77; N, 7.28.
1
in vacuo to yield 0.48 g (22%) of product. H NMR (300.0 MHz,
(c) (µ-1,1′-Methylene-3,3′-di-tert-butyldiimidazoline-2,2′-diyli-
dene)bis[bromo(η2:η2-1,5-cyclooctadiene)rhodium(I)] (2b). A 50
mL portion of THF was added to a mixture of compound 1b (940
mg, 2.23 mmol) and excess [Rh2(OAc)2(C8H12)2] (1.5 g, 2.78 mmol,
1.25 equiv). The slurry was stirred under reflux overnight, after
which the formation of a yellow precipitate was observed. Once
the mixture had cooled to room temperature, the vessel was charged
with 40 mL of ether and the supernatant recycled. The remaining
yellow solid was rinsed with 2 × 15 mL portions of ether and dried
3
CD2Cl2, 27.0 °C): δ 6.95 (d, 2H, JH-H ) 2.0 Hz), 6.69 (d, 2H,
3JH-H ) 2.0 Hz, NCH); 5.50 (m, 4H, CH2CH2); 1.93 (s, 18H,
NC(CH3)3); 4.92 (m, 4H), 3.26 (m, 4H), 2.52 (m, 4H), 2.40 (m,
4H), 1.98 (m, 4H), 1.88 (m, 4H, COD). 13C{1H} NMR (100.7 MHz,
CD2Cl2, 27.0 °C): δ 179.6 (d, 2C, 1JC-Rh ) 50.4 Hz, Ccarbene); 123.1
(s, 2C), 118.5 (s, 2C, NCH); 51.9 (s, 2C, CH2CH2); 58.5 (s, 2C),
1
32.1 (s, 6C, NC(CH3)3); 96.7 (d, 2C, JC-Rh ) 7.7 Hz), 94.1 (d,
2C, 1JC-Rh ) 7.2 Hz), 70.5 (d, 2C, 1JC-Rh ) 15.3 Hz), 68.9 (d, 2C,
1JC-Rh ) 14.5 Hz), 33.0 (s, 2C), 32.5 (s, 2C), 29.4 (s, 2C), 28.8 (s,
2C, COD). HRMS: m/z Calcd for C32H50N4ClRh2 (M+ - Cl),
1
in vacuo, giving 1.2 g (64%) of product. H NMR (399.8 MHz,
3
CD2Cl2, 27.0 °C): δ 8.35 (s, 2H, CH2); 7.81 (d, 2H, JH-H ) 2.0
731.1829; found, 731.1831 (M+
- Cl). Anal. Calcd for
Hz), 7.05 (d, 2H, 3JH-H ) 2.0 Hz, NCH); 1.97 (s, 18H, NC(CH3)3);
5.04 (m, 4H), 3.45 (m, 4H), 2.51 (m, 4H), 2.30 (m, 4H), 2.05 (m,
4H), 1.82 (m, 4H, COD). 13C{1H} NMR (100.5 MHz, CD2Cl2,
C32H50Cl2N4Rh2: C, 50.08; H, 6.57; N, 7.30. Found: C, 49.28; H,
6.55; N, 7.60. Repeated attempts were always low in the carbon
analysis. A H NMR spectrum of this compound is given in the
1
1
27.0 °C): δ 180.8 (d, 2C, JC-Rh ) 49.8 Hz, Ccarbene); 121.5 (s,
Supporting Information.
2C), 121.0 (s, 2C, NCH); 58.8 (s, 2C), 32.2 (s, 6C, NC(CH3)3);
67.5 (s,1C, CH2); 96.8 (d, 2C, 1JC-Rh ) 7.6 Hz), 95.0 (d, 2C, 1JC-Rh
(f) Methylene[(N-tert-butyl)imidazolium][(N-tert-butyl)im-
idazole-2-ylidene)]bromo(η2:η2-1,5-cyclooctadiene)rhodium(I)
Bromide (3b). A 20 mL portion of CH3CN was added to a solid
mixture containing an excess of compound 1b (582 mg, 1.38 mmol,
2.5 equiv) and [Rh2(OAc)2(C8H12)2] (300 mg, 0.55 mmol, 1 equiv).
The resulting slurry was stirred for 2 h under reflux conditions.
The volatiles were then removed under reduced pressure to give a
yellow solid. The crude product was redissolved in CH2Cl2, 20 mL
of distilled water was added, and the resulting biphasic solution
was stirred for 5 min. After settling, the aqueous phase was decanted
to waste and the remaining organic layer precipitated with pentane.
The light yellow powder was then washed with 2 × 10 mL portions
of ether before being placed under vacuum for several days to
ensure the removal of water: yield 0.62 g (89%). 1H NMR (399.8
1
1
) 7.3 Hz), 72.4 (d, 2C, JC-Rh ) 15.5 Hz), 68.7 (d, 2C, JC-Rh
)
14.2 Hz), 34.2 (s, 2C), 31.4 (s, 2C), 30.2 (s, 2C), 28.7 (s, 2C, COD).
HRMS: m/z calcd for C31H48N4BrRh2 (M+ - Br), 761.1172; found,
761.1172 (M+ - Br). Anal. Calcd for C32H50Br2N4Rh2Cl2
(2b · CH2Cl2): C, 41.45; H, 5.43; N, 6.04; Cl, 7.65. Found: C, 41.57;
H, 5.49; N, 5.96; Cl, 7.56. The presence of 1 equiv of CH2Cl2 was
1
confirmed by H NMR spectroscopy in chloroform.
(d) (µ-1,1′-(1,2-Ethylene)-3,3′-dimethyldiimidazoline-2,2′-diyli-
dene]bis[bromo(η2:η2-1,5-cyclooctadiene)rhodium(I)) (2c). A 50
mL portion of THF was added to a equimolar mixture of compound
1c (0.8 g, 2.2 mmol) and [Rh2(OAc)2(C8H12)2] (1.2 g, 2.2 mmol).
The resulting mixture was stirred under reflux for 3 h, after which
the formation of a yellow precipitate was observed. Once the
mixture had cooled to room temperature, the vessel was charged
with 60 mL of ether and the supernatant discarded. The remaining
flocculent solid was redissolved in 30 mL of CH2Cl2, filtered
through Celite, and precipitated using 40 mL of ether, leaving 1.47 g
4
4
MHz, CD2Cl2, 27.0 °C): δ 10.68 (dd, 1H, JH-H ) 1.6 Hz, JH-H
3
4
) 1.6 Hz, NCHN); 8.84 (dd, 1H, JH-H ) 1.6 Hz, JH-H ) 1.6
3
4
Hz), 7.34 (dd, 1H, JH-H ) 1.6 Hz, JH-H ) 1.6 Hz, NCHimid-H);
1.97 (s, 9H, Nimid-HC(CH3)3); 8.09 (d, 1H, 2JH-H ) 12.0 Hz), 7.64
(d, 1H, 2JH-H ) 12.0 Hz, NCH2N); 8.04 (d, 1H, 3JH-H ) 2.0 Hz),
1
(86%) of a pale yellow powder. H NMR (300.0 MHz, CD2Cl2,
3
3
3
7.16 (d, 1H, JH-H ) 2.0 Hz, NCHimid-Rh); 1.74 (s, 9H, Nimid-Rh
-
27.0 °C): δ 6.89 (d, 2H, JH-H ) 1.8 Hz), 6.59 (d, 2H, JH-H
)
C(CH3)3); 5.04 (m, 2H), 3.42 (m, 2H), 2.40 (m, 4H), 1.90 (m, 4H,
COD). 13C{1H} NMR (100.5 MHz, CD2Cl2, 27.0 °C): δ 182.8 (d,
1C, 1JC-Rh ) 49.9 Hz, Ccarbene); 122.2 (s, 1C), 121.4 (s, 1C, NCHimid-
Rh); 59.3 (s, 1C), 30.0 (s, 3C, Nimid-RhC(CH3)3); 136.5 (s, 1C,
1.8 Hz, NCH); 5.11 (m, 4H, CH2CH2); 4.00 (s, 6H, NCH3); 5.06
(m 4H), 3.38 (m, 4H), 2.46 (m, 4H), 2.34 (m, 4H), 2.06 (m, 4H),
1.93 (m, 4H, COD). 13C{1H} NMR (100.7 MHz, CD2Cl2, 27.0
°C): 181.7 (d, 2C, 1JC-Rh ) 49.7 Hz, Ccarbene); 123.8 (s, 2C), 121.1
(s, 2C, NCH); 50.5 (s, 2C, CH2CH2); 37.9 (s, 2C, NCH3); 98.2 (d,
2C, 1JC-Rh ) 6.7 Hz), 98.0 (d, 2C, 1JC-Rh ) 6.7 Hz), 70.4 (d, 2C,
NCHN); 124.0 (s, 1C), 119.1 (s, 1C, NCHimid-H); 61.0 (s, 1C), 32.0
1
(s, 3C, Nimid-HC(CH3)3); 63.0 (s, 1C, CH2); 97.7 (d, 1C, JC-Rh
)

694 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
7.4 Hz), 95.5 (d, 1C, 1JC-Rh ) 7.0 Hz), 73.1 (d, 1C, 1JC-Rh ) 15.1
1H, 2JH-H) 13.4 Hz, CH2); 6.88 (d, 2H, 3JH-H ) 2.0 Hz), 6.84 (d,
1
3
Hz), 70.0 (d, 1C, JC-Rh ) 13.9 Hz), 33.5 (s, 1C), 31.6 (s, 1C),
2H, JH-H ) 2.0 Hz, NCH); 3.74 (s, 6H, NCH3); 4.18 (dt, 1H,
2
2
29.5 (s, 1C), 28.9 (s, 1C, COD). HRMS: m/z calcd for
C23H37N4BrRh (M+), 551.1251; found, 551.1255 (M+). Satisfactory
analyses for this compound were not possible, owing to weight
gain by the sample during analysis. A 1H NMR spectrum is
available in the Supporting Information.
(g) Conversion of Compound 3b to 2b. To a pale yellow
solution containing 3b (300 mg, 0.48 mmol, 1 equiv) in 20 mL of
THF was added excess [Rh(µ-OAc)(COD)]2 (154 mg, 0.29 mmol,
0.6 equiv) to give an orange solution. The resulting solution was
refluxed overnight, resulting in the appearance of 2b as a yellow
precipitate. A 20 mL portion of ether was added, and the supernatant
was discarded, giving a pale yellow solid, which afforded after
rinsing with pentane and drying in vacuo 0.31 g (76%) of product
2b.
(h) Reactions of 2c and 2d with Excess CO. A 30 min gentle
purge of CO to a stirring solution of either compound 2c or 2d in
THF yielded a pale yellow solution. The conversion to the
respective bis-dicarbonyl complexes 4c or 4d was accompanied
by the facile loss of 1,5-cycloctadiene and was monitored to
completion using 1H NMR spectroscopy. Owing to their high
solubility in nonpolar solvents the products were isolated by removal
of volatiles under reduced pressure. For most reactions the
compounds were used without further purification.
2JH-H ) 13.2 Hz, JH-P ) 9.60 Hz), 3.39 (dt, 1H, JH-H ) 13.2
Hz, 2JH-P ) 12.8 Hz, PCH2P). 13C{1H} NMR (100.6 MHz, CD2Cl2,
1
2
27.0 °C): δ 184.9 (dd, 2C, JC-Rh ) 85.1 Hz, JC-P ) 15.8 Hz,
CO); 177.8 (dd, 2C, 2JC-P ) 116.9 Hz, 1JC-Rh ) 43.1 Hz, Ccarbene);
125.1 (s, 2C), 121.3 (s, 2C, NCH); 64.0 (s, 1C, CH2); 38.7 (s, 2C,
CH3); 24.3 (t, JC-P ) 19.1 Hz, PCH2P). 31P NMR (161.8 MHz,
1
CD2Cl2, 27.0 °C): δ 24.5 (symmetric multiplet with two principal
lines separated by 107.8 Hz, dppm). IR (solid, cm-1): 1969 broad
(CO). HRMS: m/z calcd for C36H34N4O2P2BrRh2 (M+ - BPh4),
900.9445; found, 900.9443 (M+ - BPh4). Anal. Calcd for
C60H54BN4O2P2BrRh2: C, 58.99; H. 4.46; N, 4.59. Found: C, 59.17;
H, 4.61; N, 4.37.
(j) [(µ-1,1′-methylene-3,3′-di-tert-butyldiimidazoline-2,2′-diyl-
idene)(µ-dppm)(µ-Br)(bis[((carbonyl)rhodium(I))[BPh4] (5b).
One equivalent of dppm (0.69 g, 1.78 mmol) was dissolved in 15
mL of THF and the resulting solution added via cannula to a
solution containing 2b (1.5 g, 1.78 mmol) in 15 mL of THF. The
mixture was stirred for 15 min before CO was added, resulting in
a deep orange solution. An anion exchange was formed by the
addition of one equivalent of sodium tetraphenylborate (0.61 g, 1.78
mmol) dissolved in 15 mL of acetone. The mixture was then filtered
through Celite and the volatiles removed under reduced pressure.
The resulting oily residue was washed with 2 × 15 mL portions of
ether, giving 1.7 g of the yellow solid 4b, which was dried in vacuo
[Rh2(CO)4Br2(µ-C10H14N4)] (4c). 1H NMR (399.8 MHz, C4D8O,
27.0 °C): δ 7.06 (br s, 2H), 6.82 (br s, 2H, NCH); 4.96 (br m, 2H),
4.65 (br m, 2H, CH2CH2); 3.86 (s, 6H, NCH3). 13C{1H} NMR
(100.7 MHz, CD2Cl2, 27.0 °C): δ 186.6 (d, 2C, 1JC-Rh ) 53.5 Hz),
182.5 (d, 2C, 1JC-Rh ) 76.1 Hz, CO); 173.8 (d, 2C, 1JC-Rh ) 42.9
Hz, Ccarbene); 124.0 (s, 2C), 123.0 (s, 2C, NCH); 51.3 (s, 2C,
CH2CH2); 39.0 (s, 2C, NCH3). IR (solid, cm-1): 2071 (CO), 2044
(CO), 2024 (CO), 1997 (CO). HRMS: m/z calcd for
1
(72%). H NMR (399.8 MHz, CD2Cl2, 27.0 °C): δ 8.82 (d, 1H,
2
2JH-H ) 13.2 Hz), 6.50 (d, 1H, JH-H ) 13.2 Hz, CH2); 7.96 (d,
2H, 3JH-H ) 2.0 Hz), 7.32 (d, 2H, 3JH-H ) 2.0 Hz, NCH); 1.61 (s,
18H, NC(CH3)3); 4.50 (dt, 1H, 2JH-H ) 13.2 Hz, 2JH-P ) 9.6 Hz),
3.28 (dt, 1H, 2JH-H ) 13.2 Hz, 2JH-P ) 9.6 Hz, PCH2P). 13C{1H}
1
NMR (100.6 MHz, CD2Cl2, 27.0 °C): δ 186.0 (dd, 2C, JC-Rh
)
86.3 Hz, 2JC-P ) 16.4 Hz, CO); 175.1 (dd, 2C, 2JC-P ) 115.8 Hz,
1JC-Rh ) 42.9 Hz, Ccarbene); 122.4 (s, 2C), 121.4 (s, 2C, NCH);
67.5 (s, 1C, CH2); 59.6 (s, 2C), 32.1 (s, 6C, NC(CH3)3); 24.0 (t,
1JC-P ) 18.3 Hz, PCH2P). 31P NMR (161.8 MHz, CD2Cl2, 27.0
°C): δ 26.1 (symmetric multiplet with two principle lines separated
by 109.7 Hz, dppm). IR (solid, cm-1): 1974 (CO), 1962 (CO). Anal.
Calcd for C43H46N4O2P2BrRh2SO3F3(elemental analyses carried out
on the triflate salt of 5b): C, 45.48; H, 4.08; N, 4.93. Found: C,
45.77; H, 4.24; N, 4.71. HRMS: m/z calcd for C42H46N4O2P2BrRh2
(M+ - BPh4), 985.0384; found, 985.0382 (M+ - BPh4).
C14H14N4O4BrRh2 (M+ - Br): 586.8303; found, 586.8301 (M+
-
Br). Anal. Calcd for C14H14N4O4Br2Rh2: C, 25.18; H, 2.11; N, 8.39.
Found: C, 25.31; H, 2.18; N, 8.22.
[Rh2(CO)4Cl2(µ-C16H26N4)] (4d). 1H NMR (499.8 MHz, C4D8O,
3
3
27.0 °C): δ 7.05 (d, 2H, JH-H ) 1.7 Hz), 7.01 (d, 2H, JH-H
)
1.7 Hz, NCH); 5.25 (m, 2H), 4.99 (m, 2H, CH2CH2); 1.82 (s, 18H,
NC(CH3)3). 13C{1H} NMR (100.7 MHz, CD2Cl2, 27.0 °C): δ 186.2
1
1
(d, 2C, JC-Rh ) 54.8 Hz), 182.9 (d, 2C, JC-Rh ) 75.5 Hz, CO);
171.9 (d, 2C, JC-Rh ) 42.6 Hz, Ccarbene); 122.8 (s, 2C), 119.6 (s,
1
2C, NCH); 52.1 (s, 2C, CH2CH2); 59.1 (s, 2C), 32.2 (s, 6C,
NC(CH3)3). IR (film cast, cm-1): 2077 (CO), 1997 (CO). HRMS:
m/z calcd for C20H26N4O4ClRh2 (M+ - Cl), 626.9747; found,
626.9753 (M+ - Cl). Anal. Calcd for C20H26Cl2N4O4Rh2: C, 36.22;
H, 3.95; N, 8.45. Found: C, 36.36; H, 4.08; N, 8.52.
(k) [(µ-1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diyli-
dene)(µ-dppm)(µ-Br)bis[(carbonyl)rhodium(I)][BPh4] (5c). A
slurry of 30 mL of CH3CN and 2c (1.47 g, 1.94 mmol) was stirred
under a gentle purge of CO. After 30 min, the formation of 4c was
observed as a pale yellow solution. A solution of dppm (0.75 g,
1.94 mmol, 1 equiv) in 20 mL of THF was added dropwise, via
cannula, to the solution containing 4c, giving a deep orange solution.
Continued stirring under a CO atmosphere caused the formation
of a yellow precipitate; addition of 50 mL of ether yielded 5c,
containing the bromide counterion. Alternatively, a THF solution
containing equimolar amounts of dppm and 2c followed by the
addition of CO led to the formation of the desired product in similar
yield. The tetraphenylborate salt of 5c was obtained by redissolving
the bromide product in a minimal amount of CH2Cl2 followed by
the slow addition of a concentrated solution of sodium tetraphe-
nylborate (0.66 g, 1.94 mmol, 1 equiv) in acetone. After it was
stirred for 15 min, the mixture was filtered through Celite and 1.20 g
of product 5c was recovered by the addition of excess pentane and
subsequent drying in vacuo (50%). 1H NMR (399.8 MHz, CD2Cl2,
(i) [(µ-1,1′-methylene-3,3′-dimethyldiimidazoline-2,2′-diylidene)-
(µ-dppm)(µ-Br)bis((carbonyl)rhodium(I))][BPh4] (5a). One equiva-
lent of dppm (0.30 g, 0.78 mmol) was dissolved in 10 mL of THF,
and the resulting solution added via cannula to a suspension
containing 2a (0.61 g, 0.78 mmol, 1 equiv) in 15 mL of THF. The
mixture was stirred for 15 min before CO was added, resulting in
a deep orange solution. After several minutes of CO purging, the
formation of a yellow precipitate was observed and a 50 mL portion
of ether was employed to isolate the product. An exchange of anions
was achieved by redissolving the solid in a minimal volume of
THF, followed by the addition of sodium tetraphenylborate (0.27
g, 0.78 mmol, 1 equiv) as a single portion. The immediate formation
of a precipitate was observed and the resulting mixture filtered
through a filter stick containing Celite. Excess pentane was then
added, precipitating a yellow solid, which was dried in vacuo to
give 7.61 g (80%) of product.
3
3
27.0 °C): δ 7.71 (d, 2H, JH-H ) 1.6 Hz), 6.94 (d, 2H, JH-H
)
1.6 Hz, NCH); 6.07 (m, 2H), 5.01 (m, 2H, CH2CH2); 3.71 (s, 6H,
Attempts to synthesize 5a via the addition of CO prior to dppm
NCH3); 4.22 (dt, 1H, 2JH-H ) 13.2 Hz, 2JH-P ) 9.6 Hz), 3.51 (dt,
2
2
addition met with little success, leading only to the exclusive
1H, JH-H ) 13.2 Hz, JH-P ) 9.8 Hz, PCH2P). 13C{1H} NMR
1
1
formation of [Rh2(CO)2(µ-Br)(µ-CO)(dppm)2]+. H NMR (399.8
(100.6 MHz, CD2Cl2, 27.0 °C): δ 185.6 (dd, 2C, JC-Rh ) 86.0
MHz, CD2Cl2, 27.0 °C): δ 7.80 (d, 1H, 2JH-H ) 13.4 Hz), 5.61 (d,
Hz, 2JC-P ) 15.7 Hz, CO); 175.1 (dd, 2C, 2JC-P ) 117.1 Hz, 1JC-Rh

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 695
) 43.1 Hz, Ccarbene); 123.9 (s, 2C), 123.4 (s, 2C, NCH); 49.9 (s,
(n) [(µ-1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-
diylidene)(µ-dppm)(1,3-dimethylimidazol-2-ylidene)(carbonyl)rhod-
ium(I)(iodo)(carbonyl)rhodium(I)][BPh4] (7c). To a yellow solution
containing 1 equiv of 6c (0.1 g, 0.085 mmol) in 10 mL of CH2Cl2
was added excess 1,3-dimethylimidazolium-2-ylidene iodide (38.1
mg, 0.17 mmol, 2 equiv). The resulting suspension was stirred
overnight where 31P NMR spectroscopy revealed a clean conversion
to product 7c. Solvent removal in vacuo gave a residue, and the
addition of 15 mL of THF resulted in a suspension. Once this
suspension was filtered through Celite, 0.098 g of product 6c was
recovered by the addition of excess ether and subsequent drying in
vacuo (84%). Attempts to obtain crystals of compound 7c suitable
for an X-ray study failed. However, suitable crystals of the closely
related [Rh2Br(CO)2(IMe)(µ-MeCCeth)(dppm)][I] (7c′; IMe ) 1,3-
dimethylimidazol-2-ylidene) were obtained by the reaction of 6c,
as the bromide salt, with excess 1,3-dimethylimidazolium-2-ylidene
iodide. The structure determination was therefore carried out on
1
2C, CH2CH2); 39.1 (s, 2C, NCH3); 26.8 (t, JC-P ) 18.6 Hz,
PCH2P). 31P NMR (161.8 MHz, CD2Cl2, 27.0 °C): δ 23.7
(symmetric multiplet with two principal lines separated by 107.8
Hz, dppm). IR (solid, cm-1): 1986 (CO), 1974 (CO). Anal. Calcd
for C61H56N4O2P2BrRh2B: C, 59.30; H, 4.57; N, 4.53. Found: C,
59.24; H, 4.62; N, 4.63. HRMS: m/z calcd for C37H36N4O2P2BrRh2
1
(M+ - BPh4), 914.9607; found, 914.9609 (M+ - BPh4). The H
and 31P{1H} NMR spectra of compound 5c are available in the
Supporting Information.
(l) [µ-(1,1′-(1,2-ethylene)-3,3′-di-tert-butyldiimidazoline-2,2′-
diylidene)(µ-dppm)(µ-Cl)bis((carbonyl)rhodium(I))][BPh4]
(5d)). To a flask containing an equimolar mixture of 2d (0.60 g,
0.78 mmol) and dppm (0.30 g, 0.78 mmol) was added 20 mL of
THF, followed immediately by a CO purge through the slurry until
the mixture dissolved, giving an orange solution. The reaction
mixture was stirred for an additional 30 min before sodium
tetraphenylborate (0.29 g, 0.86 mmol, 1.1 equiv) was added to the
reaction vessel in a single portion. The mixture was filtered through
a bed of Celite and the solvent evaporated under reduced pressure
to give a dark oily solid. To extract the compound from a mixture
containing various species, the crude product was redissolved in
15 mL of CH2Cl2 and 20 mL of ether was then added, causing an
orange oil to settle. The remaining ethereal supernatant was
transferred to a new flask, and the addition of 30 mL of pentane
precipitated a bright yellow oil that was isolated and placed under
1
7c′ (vide infra). H NMR (399.8 MHz, CD2Cl2, 27.0 °C): δ 8.20,
8.08, 7.20, 7.06 (d, 1H, 3JH-H ) 1.6 Hz, NCH); 6.61, 6.19 (d, 1H,
2
3JH-H ) 2.0 Hz, NCH); 6.56, 4.98, 4.79, 4.31 (ddd, 1H, JH-H
)
14 Hz, 3JH-H ) 12, 6 Hz, CH2CH2); 3.76, 3.69, 3.62, 3.21 (s, 3H,
NCH3); 4.68 (dt, 1H, 2JH-H ) 13.6 Hz, 2JH-P ) 9.6 Hz), 3.69 (dt,
1H, JH-H ) 13.6 Hz, JH-P ) 13.6 Hz, PCH2P). 13C{1H} NMR
2
2
1
(100.5 MHz, CD2Cl2, 27.0 °C): δ 185.4 (dd, 1C, JC-Rh ) 81.4
2
1
2
Hz, JC-P ) 16.1 Hz), 181.1 (dd, 1C, JC-Rh ) 45.2 Hz, JC-P
)
2
1
16.1 Hz, CO); 177.7 (dd, 1C, JC-P ) 114.6 Hz, JC-Rh ) 43.2
2
1
1
Hz), 177.5 (dd, 1C, JC-P ) 43.2 Hz, JC-Rh ) 43.2 Hz) 176.7
vacuum to give 0.20 g (20%) of an oily solid product. H NMR
2
1
3
(dd, 1C, JC-P ) 99.5 Hz, JC-Rh ) 46.2 Hz, Ccarbene); 123.3 (s,
1C), 122.9 (s, 1C), 122.6 (s, 1C), 122.5 (s, 2C), 122.2 (s, 1C, NCH);
50.5, 49.8 (s, 1C, CH2CH2); 39.4, 39.3, 38.6, 38.5 (s, 1C, NCH3);
34.5 (t, 1JC-P ) 16.1 Hz, PCH2P). 31P NMR (161.8 MHz, CD2Cl2,
27.0 °C): 24.3 (dd, 1P, 1JP-Rh ) 124.6 Hz, 2JP-P ) 34.0 Hz), 21.8
(dd, 1P, 1JP-Rh ) 113.3 Hz, 2JP-P ) 34.0 Hz). IR (film cast, cm-1):
1975 (CO). HRMS: m/z calcd for C42H44N6O2P2IRh2 (M+ - BPh4),
1059.0150; found, 1059.0154 (M+ - BPh4). Anal. Calcd for
C66H64BIN6O2P2Rh2: C, 57.50; H, 4.68; N, 6.10. Found: C, 57.87,
H, 4.92; N, 6.22.
(399.8 MHz, CD2Cl2, 27.0 °C): δ 7.05 (d, 2H, JH-H ) 2.0 Hz),
3
6.78 (d, 2H, JH-H ) 2.0 Hz, NCH); 6.44 (m, 2H), 4.36 (m, 2H,
2
CH2CH2); 1.61 (s, 18H, NC(CH3)3); 4.22 (dt, 1H, JH-H ) 12.8
2
2
2
Hz, JH-P ) 10.0 Hz), 3.20 (dt, 1H, JH-H ) 12.8 Hz, JH-P
)
10.0 Hz, PCH2P). 13C{1H} NMR (100.5 MHz, CD2Cl2, 27.0 °C):
1
2
δ 185.5 (dd, 2C, JC-Rh ) 84.6 Hz, JC-P ) 16.4 Hz, CO); 175.0
2
1
(dd, 2C, JC-P ) 116.4 Hz, JC-Rh ) 43.5 Hz, Ccarbene); 121.6 (s,
2C), 120.7 (s, 2C, NCH); 48.8 (s, 2C, CH2CH2); 59.0 (s, 2C), 32.2
(s, 6C, NC(CH3)3); 23.8 (t, JC-P ) 16.8 Hz, PCH2P). 31P NMR
1
(161.8 MHz, CD2Cl2, 27.0 °C): δ 26.4 (symmetric multiplet with
two principal lines separated by 113.1 Hz, dppm). IR (solid, cm-1):
1973 broad (CO). HRMS: m/z calcd for C43H48N4O2P2ClRh2 (M+
- BPh4), 955.1046; found, 955.1046 (M+ - BPh4). Anal. Calcd
for C67H68BN4O2P2ClRh2: C, 63.10; H, 5.37; N, 4.39. Found: C,
63.39, H, 5.61; N, 4.55.
(o) [(µ-1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diyli-
dene)bis((carbonyl)(η1:η1-dppe)rhodium(I))][CF3SO3]2 (8c). To
a yellow solution containing 2c (0.4 g, 0.60 mmol) in 15 mL of
CH2Cl2 was added dropwise via cannula 15 mL of CH2Cl2
containing dppe (0.46 g, 1.20 mmol). The flask was then placed
under a CO atmosphere, the mixture was stirred for 30 min, and
then a 1.20 mol portion of sodium triflate was added, resulting in
the formation of a white precipitate. The mixture was filtered
through Celite before a 50 mL portion of pentane was added, giving
a yellow precipitate. The solid was isolated, and 31P NMR
spectroscopy revealed a mixture of two species: 8c and a doublet
at 58.6 (1JP-Rh ) 133.0 Hz), assigned to [Rh(dppe)2][CF3SO3]. The
dicationic product was separated on the basis of its low solubility
in THF. The addition of 20 mL of THF to the mixture resulted in
a slurry, which was stirred for 2 h and then allowed to stand for
several minutes, yielding a fine yellow precipitate. The yellow
supernatant was discarded. The product was allowed to dry in vacuo,
giving 0.11 g (14%) of product, and was shown by 31P NMR
spectroscopy to contain only compound 8c. 1H NMR (400.4 MHz,
(m) [(µ-1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diyli-
dene)(µ-dppm)(µ-OH)bis((carbonyl)rhodium(I))][BPh4] (6c). To
an orange solution containing 5c (0.50 g, 0.41 mmol) in 15 mL of
THF was added an excess of concentrated NaOH(aq) (2 mL, 6
M). After it was stirred for 1 h, the light yellow biphasic solution
was brought to dryness in vacuo and the crude mixture dehydrated
for 3 days under reduced pressure. Once dry, a 30 mL portion of
THF was added and the resulting suspension filtered through Celite,
from which 0.42 g of product 6c was recovered by the addition of
1
excess pentane and subsequent drying in vacuo (87%). H NMR
(400.4 MHz, OC4D8, 27.0 °C; the olefinic protons within the NHC
rings were not located): δ 6.13 (m, 2H), 4.44 (m, 2H, CH2CH2);
3.62 (s, 6H, NCH3); 3.81 (dt, 1H, 2JH-H ) 12.8 Hz, 2JH-P ) 10.8
2
2
Hz), 3.26 (dt, 1H, JH-H ) 12.8 Hz, JH-P ) 12.8 Hz, PCH2P);
3
0.81 (t, 1H, JH-Rh ) 4.4 Hz, OH). 13C{1H} NMR (100.7 MHz,
2
CD2Cl2, 27.0 °C): δ 6.83 (d, 2H, JH-H ) 1.6 Hz), 6.15 (d, 2H,
3JH-H ) 1.6 Hz, NCH); 3.95 (m, 2H), 2.67 (m, 2H, CH2CH2); 3.48
(s, 6H, NCH3); 2.95 (m, 2H), 2.91 (m, 2H), 2.50 (m, 2H), 1.98 (m,
2H, dppe). 13C{1H} NMR (100.5 MHz, CD2Cl2, 27.0 °C): δ 188.5
OC4D8, 27.0 °C): δ 189.7(dd, 2C, 1JC-Rh ) 70 Hz, 2JC-P ) 17 Hz,
2
1
CO); 179.8 (dd, 2C, JC-P ) 117 Hz, JC-Rh ) 47 Hz, Ccarbene);
123.0 (s, 2C), 122.2 (s, 2C, NCH); 49.8 (s, 2C, CH2CH2); 38.0 (s,
1
2
2
2C, NCH3); 19.0 (t, JC-P ) 15.1 Hz, PCH2P). 31P NMR (162.1
1
(ddd, 2C, JC-Rh ) 65.4 Hz, JC-P ) 100.5 Hz, JC-P ) 14.1 Hz,
CO); 175.7 (ddd, 2C, 1JC-Rh ) 44.2 Hz, 2JC-P ) 87.5 Hz, 2JC-P
14.1 Hz, Ccarbene); 124.5 (s, 2C), 123.1 (s, 2C, NCH); 50.8 (s, 2C,
)
MHz, OC4D8, 27.0 °C): δ 28.9 (symmetric multiplet with two
principal lines separated by 120.0 Hz, dppm). IR (film cast, cm-1):
1958 (CO). HRMS: m/z calcd for C37H37N4O3P2Rh2 (M+ - BPh4),
853.0446; found, 853.0442 (M+ - BPh4). Anal. Calcd for
C61H57BN4O3P2Rh2: C, 62.48; H, 4.90; N, 4.78. Found: C, 62.64,
H, 5.19; N, 4.73.
1
CH2CH2); 38.1 (s, 2C, NCH3); 28.7 (dd, 2C, JC-P ) 29.7 Hz,
2JC-P ) 16.7 Hz), 27.1 (dd, 2C, JC-P ) 29.7 Hz, JC-P ) 15.6
Hz, PCH2CH2P). 31P NMR (162.0 MHz, CD2Cl2, 27.0 °C): 62.4
(dd, 2P, 1JP-Rh ) 110.7 Hz, 2JP-P ) 29.7 Hz), 56.1 (dd, 2P, 1JP-Rh
1
2

696 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
2
) 126.4 Hz, JP-P ) 29.7 Hz, dppe). IR (solid, cm-1): 2009.5
Scheme 1
broad (CO). HRMS: m/z calcd for C64H62N4O2P4Rh2 (M2+),
624.0961; found, 624.0957 (M2+). We were unable to separate
compound 8c from a minor impurity; therefore, an elemental
analysis was not possible. The H and 31P{1H} NMR spectra of
1
8c are given in the Supporting Information.
(p) Attempted Reduction of Compounds 4a-d. An NMR tube
containing 20 mg of 4(a-d)dissolved in 0.7 mL of d8-THF was
charged with either an excess of NaBH4 or concentrated KOH (aq)
followed by a gentle CO purge. The reaction was then monitored
using 31P NMR spectroscopy; in all cases, reduction attempts let
to the formation of [Rh2(CO)3(dppm)2] as indicated by the rise of
a symmetric multiplet at 19.4 with two principle lines separated
by 144.3 Hz.
X-ray Structure Determinations. (a) General Considerations.
Crystals were grown via slow diffusion of diethyl ether (2b,d, 4c,
5c, 7c′, 8c) or pentane (5b) into a CH2Cl2 solution of the compound.
Data were collected using a Bruker SMART 1000 CCD detector/
PLATFORM diffractometer15 using Mo KR radiation, with the
crystals cooled to -80 °C. The data were corrected for absorption
through use of a multiscan model (SADABS (2a,d, 4c, 8c) or
TWINABS (5b)) or through Gaussian integration from indexing
of the crystal faces (5c, 7c′). Structures were solved using the direct
methods program SHELXS-9716 (2b,d, 4c, 5c, 8c) or using the
Patterson search/structure expansion facilities within the DIRDIF-
9917 program system (5b, 7c′). Refinements were completed using
the program SHELXL-97.18 Hydrogen atoms were assigned posi-
tions on the basis of the geometries of their attached carbon atoms
and were given thermal parameters 20% greater than those of the
attached carbons. See the CIF files, given as Supporting Information,
for the crystallographic experimental data.
previously reported,2g,3a,6b by the reaction of the appropriate
1-alkylimidazole with either methylene dibromide or 1,2-
dibromoethane. Two methods were used to generate the di-N-
heterocyclic carbene-bridged dirhodium complexes from these
diimidazolium salts, as outlined in Scheme 1. In the first method,
the diimidazolium bromide salts 1a-c were reacted with the
acetate-bridged species [Rh(µ-OAc)(COD)]2 in refluxing THF
to yield the respective di-N-heterocyclic carbene-bridged species
[RhBr(COD)]2(µ-di-NHC) (2a-c). All attempts to obtain the
corresponding product by this method using the diimidazolium
salt 1d failed; only starting materials were observed after
refluxing in a number of solvents for periods of up to 2 days.
The missing member of this series, as the chloro complex (2d)
was instead synthesized through initial double deprotonation
of the diimidazolium salt 1d using K[N(SiMe3)2], followed by
reaction of the resulting dicarbene with [Rh(µ-Cl)(COD)]2. This
product had been previously synthesized by a transmetalation
reaction, using the appropriate Ag2(di-NHC)Br2 species as the
carbene-transfer agent.7d
(b) Special Refinement Conditions. (i) 2d. The C(11A)-C(12A)
distance was restrained to be 1.400(2) Å. The geometry of the minor
(40%) forms of the disordered tert-butyl group and the disordered
cyclooctadiene group were restrained to be the same as that of the
major (60%) form by use of the SHELXL-97 SAME instruction
during refinement.
Although a number of analogous dirhodium complexes have
(ii) 6b. The crystal used for data collection was found to display
nonmerohedral twinning. Both components of the twin were indexed
with the program CELL_NOW. The first twin component can be
related to the second component by 180° rotation about the [-1/4
-1/4 1] axis in real space and about the [0 0 1] axis in reciprocal
space. Integrated intensities for the reflections from the two
components were written into a SHELXL-97 HKLF 5 reflection
file with the data integration program SAINT (version 7.06A), using
all reflection data (exactly overlapped, partially overlapped and
nonoverlapped). Distances within the solvent dichloromethane
molecule were given fixed idealized values: d(Cl(S)-C(1S)) )
d(C1(1S)-C(1S)) ) 1.80 Å; d(Cl(1S) · · · Cl(2S)) ) 2.95 Å. .
(iii) 7c′. The P-C distances involving the ipso carbons of the
disordered dppm phenyl ring were constrained to be equal (within
0.001 Å): d(P(2)-C(51A)) ) d(P(2)-C(51B)).
been reported incorporating the MeCCmeth MeCCeth, and tBuCCeth
,
bridging dicarbenes used in this paper (see Chart 1 for the
abbreviations used), compound 2b appears to represent the first
report of a species containing the bridging tBuCCmeth dicarbene.
In a previous study Crabtree and co-workers had generated a
series of binuclear species incorporating C2-, C3-, and C4-linked,
tert-butyl-substituted di-NHC ligands, using the above trans-
metalation route,7d but had not observed the CH2-linked
analogue, instead obtaining only a mononuclear species in which
the tBuCCmeth group functioned as a chelating ligand. Very
analogous species incorporating the nBuCCmeth and nBuCCeth
groups,7b as well as C2-, C3-, and C4-linked dicarbenes having
isopropyl substituents,7d have also been reported.
The spectral parameters for compounds 2a,c,d are closely
comparable to those of related species which have previously
been reported,2a,3a,7a,d and so will not be discussed here.
However, since compound 2b has not previously been reported,
its spectral parameters are briefly discussed here. The 1H NMR
spectrum of 2b shows the typical resonances for the coordinated
COD ligands (between δ 1.82 and 5.04) as given in the
Experimental Section. In addition, a single resonance appears
at δ 1.97 for all 18 protons of the two tert-butyl groups. The
inequivalent vinylic protons within the unsaturated carbene rings
appear as two doublets at δ 7.81 and 7.05, displaying mutual
coupling of 2.0 Hz, while the protons in the linking methylene
group appear as a singlet at δ 8.35. In the 13C{1H} NMR
spectrum the carbene carbons appear as a doublet at δ 180.8,
displaying 49.8 Hz coupling to Rh; both the chemical shift and
the coupling constant are typical for related NHC complexes.
Results and Compound Characterization
The methylene- or ethylene-linked diimidazolium salts
(1a-d) that were subsequently used to generate the series of
dicarbene-bridged binuclear complexes were synthesized, as
(15) Programs for diffractometer operation, unit cell indexing, data
collection, data reduction, and absorption correction were those supplied
by Bruker.
(16) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(17) Sheldrick, G. M. SHELXL-97. Program for Crystal Structure
Determination; University of Göttingen, Gottingen, Germany, 1997.
(18) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granada,
S.; Israel, R.; Gould, R. O.; Smits, J. M. M. The DIRDIF-99 program system;
Crystallography Laboratory, University of Nijmegen, Nijmegen, The
Netherlands, 1999.

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 697
Figure 1. Three-dimensional representation of [Rh2Br2(COD)2(µ-
tBuCCmeth)] (2b) showing the numbering scheme. Primed atoms
are related to unprimed ones by a crystallographic C2 axis passing
through C(10). Thermal ellipsoids are shown at the 20% probability
level, except for the linker CH2 and the olefinic protons, which are
Figure 2. Representation of [Rh2Cl2(COD)2(µ-tBuCCeth)] (2d)
showing the numbering scheme. Primed atoms are related to
unprimed ones by a crystallographic C2 axis bisecting the
C(10)-C(10)′ bond. Thermal parameters for the hydrogens of the
C2H4 linker are shown artificially small; all others are as described
in Figure 1. Relevant parameters (distances in Å): Rh-Rh′ )
7.2627(9), Rh-C(1) ) 2.048(5).
t
shown artificially small. Hydrogen atoms on the COD and Bu
groups are not shown. Relevant parameters (distances in Å and
angles in deg): Rh-Rh′ ) 6.6575(5), Rh-Br ) 2.5158(3),
Rh-C(1) ) 2.051(2), Rh-C(11) ) 2.207(3), Rh-C(12) )
2.205(3), Rh-C(15) ) Rh-C(16) ) 2.115(3); N(1)-C(10)-N(1)′
) 112.5(3)°.
compared to C(15)-C(16) (1.399(4) Å), consistent with less π
The 13C resonance for the methylene linker appears at δ 67.5,
while the olefinic carbons of the NHC groups appear at δ 121.5
and 121.0, and the quaternary carbon and the methyl carbons
of the tert-butyl groups appear at δ 58.8 and 32.2, respectively.
As has previously been noted for related species,7b two
diastereomers of compound 2b are possiblesthe meso form, in
which the two RhBr(COD) units are related by mirror symmetry,
and the dl form, in which these units are related by a C2 rotation
axis. The appearance of only one set of resonances for the
complex indicates that only one diastereomer is present, and
the appearance of the resonance for the protons of the methylene
linker as a singlet indicates that the dl form is present; the meso
form would be expected to display an AB quartet for these
protons. This is also the diastereomer found in the solid state
by X-ray crystallography, in which each molecule has crystal-
lographically imposed C2 symmetry. A representation of
complex 2b is shown in Figure 1. A comparison of this structure
back-donation in this case.
As is typical for square-planar NHC complexes, the carbene
plane lies essentially perpendicular to the metal coordination
plane. Thus, for 2b the dihedral angle between the five-
membered carbene ring and the plane defined by Rh, Br, and
C(1) is 87.75(8)°.
We have also carried out an X-ray structural study of the
C2H4-linked analogue [RhCl(COD)]2(µ-tBuCCeth) (2d) as a
comparison, and a representation of this species is given in
Figure 2. This structure compares very well with that previously
determined for [RhCl(COD)]2(µ-MeCCeth),3a with both adopting
a synclinal conformation about the H2C-CH2 bond of the linker;
in compound 2d the torsion angle about this C-C bond is
36.7(10)°, compared to 55.3 and 57.6° in the two independent
molecules of the related MeCCeth complex. Both species have
similar Rh-Rh separations (7.2627(9) Å in 2d and 7.221, 7.258
Å in the MeCCeth complex), which are significantly larger than
the values for 2b (6.658 Å) and a related CH2-linked species
(6.638 Å),7a consistent with the additional methylene group
linking the two carbene units in 2d. Again, the Rh-carbene
distance (2.048(5) Å) is close to those in previous determinations
and the carbene plane is close to orthogonal to the metal
coordination plane (dihedral angle 85.3(2)°).
with that of the closely related [RhI(COD)]2(µ-MeCCmeth 7a
)
species shows that the two have very similar geometries. The
Rh-Rh separation in 2b (6.6575(5) Å) is somewhat longer than
that observed in the iodo analogue (6.6376 Å), presumably as
a consequence of the bulkier tert-butyl substituents in our case.
Similarly, the Rh-carbene distance in 2b (2.051(2) Å) is only
slightly longer than the related distances (2.023, 2.014 Å) in
the iodo species. A glance at Figure 1 shows that this latter
elongation in 2b may also result from repulsions between the
tert-butyl substituents on the dicarbene and the COD ligands.
Further indications of the steric repulsions involving the bulky
tert-butyl substituents are the unsymmetrical angles at the
carbene carbon (Rh-C(1)-N(1) ) 122.3(2)° and Rh-C(1)-N(2)
) 133.8(2)°), with the larger angle corresponding to that
adjacent to the tert-butyl group.
Interestingly, the Rh-C(11) and Rh-C(12) separations
(2.207(3), 2.205(3) Å) involving the COD ligand are longer than
those to the other olefin moiety (Rh-C(15) ) Rh-C(16) )
2.115(3) Å). This difference is probably a consequence of both
steric repulsions between half of the COD ligand and the
adjacent bromo ligand and the larger trans influence of the
carbene ligand.3a,6c,f,7b The weaker Rh-olefin interaction is
paralleled by a shorter C(11)-C(12) distance (1.360(5) Å)
On one occasion, the overnight reflux in THF, required to
generate 2b, was interrupted after only 4 h. The resulting
solution was found to contain a number of products, one of
which was isolated in low yield and found to correspond to the
mononuclear species 3b, shown in Scheme 2, in which
deprotonation of only one end of the diimidazolium salt has
occurred to give a monocarbene complex having a pendant
imidazolium group. This species was subsequently obtained in
high yield after 3 h of reflux in acetonitrile of a 1:0.5 mixture
of 1b and the acetate-bridged [Rh(µ-OAc)(COD)]2. This
product, [RhBr(tBuC(H)-η1-Cmeth)(COD)][Br] (3b), is analogous
to an iridium complex, reported by Albrecht et al.,2g and is found
to have spectral parameters very comparable to those of this
species. In particular, the acidic proton of the pendant imid-
1
azolium group appears in the H NMR spectrum at δ 10.68 in
3b, compared to δ 10.44 in the Ir analogue, and the protons of

698 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
Scheme 2
the methylene linker appear as an AB quartet at δ 8.09 and
7.64 (2JH-H ) 12.0 Hz) compared to the resonances at δ 7.80
and 7.41 (2JH-H ) 12.9 Hz) for the Ir species. The appearance
of the methylene protons as an AB quartet suggests that the
plane of the NHC unit is bound perpendicular to the square
plane of the metal, as has been observed in all such cases. In
agreement with our formulation, four separate resonances appear
for the vinylic protons within the NHC and the imidazolium
rings, appearing at δ 8.04 and 7.16 (3JH-H ) 2.0 Hz) for the
former and at δ 8.84 and 7.34 (3JH-H ) 1.6 Hz) for the latter.
These latter resonances also display a coincidental 1.6 Hz
coupling to the acidic hydrogen of this imidazolium group and
so appear as pseudotriplets. In the 13C{1H} NMR spectrum the
carbene carbon appears at δ 182.8 and displays 49.9 Hz coupling
to Rh, while the carbonium carbon of the imidazolium group
appears at δ 136.5; by comparison, the carbene carbon in the Ir
analogue was reported at δ 180.5. The other spectral parameters,
given in the Experimental Section, offer additional support for
this formulation, including the mass spectral data, which show
the incorporation of only a single RhBr(COD) unit. Two related
monocarbene complexes of Pd, having a pendant imidazolium
group, have also been reported.6e,19
Figure 3. Representation of [Rh2Br2(CO)4(µ-MeCCeth)] (4c). Ther-
mal ellipsoids are as in Figure 1. All hydrogens are shown
artificially small. Relevant parameters (distances in Å): Rh(1)-Rh(2)
) 7.0052(8), Rh(1)-C(12) ) 2.054(3), Rh(2)-C(22) ) 2.055(3).
Scheme 3
Not surprisingly, compound 3b can be deprotonated by
additional [Rh(µ-OAc)(COD)]2, adding a second RhBr(COD)
unit to give the originally targeted 2b. No analogous intermedi-
ate was observed at intermediate times in any of the other
reactions generating compounds 2a,c,d, even when attempted
under conditions that successfully yielded 3b.
shorter than for the COD complex 2d (vide infra), for which
the larger COD and tert-butyl groups presumably force the
metals further apart. The Rh-carbene bonds (2.054(3), 2.055(3)
Å) are also as expected. There is a significant shortening of the
Rh-carbonyl bonds opposite the bromo ligands (1.832(3),
1.836(3) Å) compared to those opposite the carbenes (1.906(3),
1.898(3) Å), resulting from the π-donor properties of the former
and the trans influence of the latter. This shortening is
accompanied by a slight but noticeable lengthening of the
corresponding C-O bonds (1.137(3), 1.134(3) Å compared to
1.124(3), 1.129(3) Å).
Reaction of compounds 2a-d with 1 equiv of dppm
(Ph2PCH2PPh2), followed by the addition of CO and NaBPh4,
yieldsthedicarbene-anddiphosphine-bridgedproducts[Rh2(CO)2(µ-
Br)(µ-di-NHC)(µ-dppm)][BPh4] (5a-d), shown in Scheme 3.
Compound 5c (di-NHC ) MeCCeth) can also be generated from
the reaction of 4c with 1 equiv of dppm or by reacting 2c with
CO and dppm in the reverse order to that given above. However,
reaction of compounds 2a or 2b first with CO then with dppm
does not give the targeted dppm-bridged products but instead
gives mixtures of products, presumably related to their reactions
with CO noted earlier, in which fragmentation of the binuclear
framework occurred. In the case of 2d, reaction with CO,
followed by addition of dppm, or alternatively by reaction of
4d with dppm does yield 5d, but only in very low yield, together
with a complex mixture of products which appear to be
oligomeric in which dppm groups presumably connect adjacent
dicarbene-bridged Rh2 fragments.
Compounds 2a-d react readily with CO with displacement
of the COD ligands. However only 2c,d give the clean
generation of the binuclear products [RhBr(CO)2]2(µ-MeCCeth
)
(4c) and [RhCl(CO)2]2(µ-tBuCCeth) (4d), respectively, and the
structure of the former is shown in Figure 3. Under CO,
compounds 2a,b yield the mononuclear products [Rh(CO)2η1:
η1-di-NHC)][Br], containing chelating dicarbenes, together with
[Rh(µ-Br)(CO)2]2. The former species were identified by their
mass spectra, which showed mononuclear species, and by a
comparison of their NMR data with the known compounds,7b,20
while the latter product was identified by its reaction with dppm
to give the known species [Rh2(CO)2(µ-Br)(µ-CO)(dppm)2]-
[Br].21
As has been the case for the other binuclear, C2H4-linked
compounds, the dl diastereomer is observed in the crystal of
4c, in which the synclinal conformation is again observed,
displaying a N(11)-C(10)-C(20)-N(21) torsion angle of
61.9(3)° about the H2C-CH2 bond of the linking group. All
structural parameters within this product are basically as
anticipated, in which the Rh-Rh separation of 7.0052(8) Å is
(19) Herrmann, W. A.; Schwarz, J.; Gardiner, M. G. Organometallics
1999, 18, 4082.
(20) Burling, S.; Field, L. D.; Li, H. L.; Messerle, B. A.; Turner, P.
Eur. J. Inorg. Chem. 2003, 3179.
(21) Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2500.

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 699
Figure 5. Representation of the complex cation of [Rh2(CO)2(µ-
Br)(µ-MeCCeth)(dppm)][BPh4] (5c). Thermal parameters are as
described in Figure 1. Phenyl hydrogens are not shown. Relevant
parameters (distances in Å and angles in deg): Rh(1)-Rh(2) )
3.3370(4), Rh(1)-C(10) ) 2.048(4), Rh(2)-C(16) ) 2.051(3),
P(1)-P(2) ) 3.041(1), C(10)-C(16) ) 3.845(5); Rh(1)-Br-Rh(2)
) 82.28(1), P(1)-Rh(1)-C(10) ) 165.9(1), P(2)-Rh(2)-C(16)
) 176.3(1).
Figure 4. Representation of the complex cation of [Rh2(CO)2(µ-
Br)(µ-tBuCCmeth)(dppm)][BPh4] (5b). Thermal parameters are as
described in Figure 1. Hydrogen atoms on the phenyl and tert-
butyl groups are not shown. Relevant parameters (distances in Å
and angles in deg): Rh(1)-Rh(2) ) 3.3022(5), Rh(1)-C(10) )
2.083(3), Rh(2)-C(15) ) 2.057(4), P(1)-P(2) ) 3.048(1),
C(10)-C(15) ) 3.838(5); Rh(1)-Br-Rh(2) ) 80.94(2).
Compounds 5a-d all display very similar spectral parameters,
particularly for ligands that all products have in common. For
example, all display 31P{1H} NMR spectra in which the dppm
resonance appears at ca. δ 25 as a second-order pattern,
characteristic of an AA′XX′ spin system, in which the separation
between the two major peaks is ca. 110 Hz. The methylene
protons of the dppm group appear as two doublets of triplets at
ca. δ 4.2 and 3.3, having approximately 10 Hz coupling to both
certainly no bond is expected on the basis of electron counting,
in which both metals have a favorable square-planar geometry,
typical of Rh(I). The geometry observed for 5b is very
reminiscent of the classical “A-frame” complexes involving two
mutually trans dppm groups,10 with one of the dppm groups
being replaced by the di-NHC ligand. Clearly, the larger bite
of the dicarbene ligand compared to dppm gives rise to some
distortion within the complex in which the C(10)-C(15)
separation of the two carbene centers (3.838(5) Å) is much
greater than the P(1)-P(2) separation (3.048(1) Å), with the
Rh-Rh separation being intermediate.
1
31P nuclei and 13 Hz coupling to each other. Sample H and
31P{1H} NMR spectra for compound 5c are shown in the
Supporting Information.
The IR spectra for all species are consistent with the presence
of terminal carbonyls and display either a single broad carbonyl
stretch at ca. 1970 cm-1 (5a,d) or two closely spaced stretches
(1974, 1963 cm-1 (5b); 1986, 1974 cm-1 (5c)). Furthermore,
all display a single carbonyl resonance at ca. δ 185 in their
13C{1H} NMR spectra. The carbonyl resonances for these
compounds display approximately 85 Hz coupling to Rh and
16 Hz coupling to a single adjacent 31P nucleus. Also in the
13C{1H} NMR spectra, the carbene carbons appear as doublets
of doublets at ca. δ 175; in this case, however, the coupling to
phosphorus (ca. 116 Hz) is larger than that to Rh (ca. 43 Hz),
indicating that the carbene and phosphine groups are mutually
trans at each metal. The larger Rh coupling involving the
carbonyl ligands is consistent with the carbonyl carbon having
greater “s” character than the carbene carbon.
The Rh-carbene bonds (2.057(4), 2.083(3) Å) are normal
and are significantly longer than the metal–carbonyl distances
(1.799(4), 1.802(4) Å), consistent with the accepted notion that
there is little π-back-donation to these carbenes. As is also
typical for square-planar complexes, involving NHCs, the
carbene planes lie essentially perpendicular to the coordination
planes of the metals (dihedral angles at Rh(1) and Rh(2): 85.9(1),
87.8(1)°). One consequence of this arrangement is that the tert-
butyl groups are directed into the open sites above the Rh square
planes, remote from the adjacent metal and away from the
A-frame pocket. In concert with this dicarbene arrangement,
one phenyl group on each dppm ligand (rings containing C(22)
and C(56)), on the sides adjacent to the tert-butyl groups, lie
close to horizontal, thereby minimizing nonbonded contacts
between the phenyl and tert-butyl groups, while on the opposite
face the phenyl rings take a close-to-vertical arrangement,
aiming into the relatively open pocket between the metals.
In the 1H NMR spectra the methylene protons of the RCCmeth
ligands appear as a pair of doublets, displaying mutual coupling
of about 13 Hz, while the resonances for the ethylene linker in
the RCCeth ligands appear as two multiplets. The methyl and
We have also carried out an X-ray structural determination
of the MeCCeth-bridged species 5c, in which the two NHC rings
are linked by a longer C2H4 unit. Although the overall geometry
of 5c is very similar to that of 5b (see Figure 5), the additional
CH2 group in the linker has resulted in some noticeable
distortions in the latter compound. These distortions result from
the relatively fixed geometry of the dppm fragment in which
the P-P separation (3.041(1) Å) remains essentially unchanged
from that in 5b, while the separation between the pair of NHC
rings has increased by the addition of a CH2 group. Surprisingly
t
tert-butyl resonances (R ) Me, Bu) appear as expected.
An X-ray structure determination for [Rh2(CO)2(µ-Br)(µ-
tBuCCmeth)(dppm)][BPh4] (5b) confirms the tribridged geometry
(bridging dicarbene, diphosphine, and bromide), as shown for
the complex cation in Figure 4. The most notable effect of
adding the diphosphine is the dramatic compression of the
metal-metal separation from 6.66 Å in 2b to 3.3022(5) Å in
5b. This latter metal-metal separation is still well beyond what
one would usually consider appropriate for a Rh-Rh bond, and

700 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
perhaps, the increased length of the linker unit has given rise
to only a very slight increase in carbene-carbene separation
from the 3.838(5) Å noted above in 5b to 3.845(5) Å in 5c; at
the same time, the metal-metal separation has increased by
only 0.035 Å to 3.3370(4) Å in 5c. Although the additional
CH2 group in the linker has not given rise to the expected
increase in carbene-carbene separation, it is instead manifest
inabendingbackofthecarbeneunitatRh(1)(C(10)-Rh(1)-P(1)
) 165.9(1)°), significantly distorting this square plane (compare
C(16)-Rh(2)-P(2) ) 176.3(1)°), and a skewing of this carbene
plane with regard to the Rh(1) coordination plane. The dihedral
angle between this carbene plane and the Rh(1) coordination
plane (68.8(1)°) is twisted significantly from the favored
orthogonal arrangement, as observed at Rh(2) (87.2(1)°). It
appears that both of these distortions could be avoided by a
slight increase in the Rh(1)-Rh(2) separation. However, it also
appears that the bridging bromo ligand plays a role in inhibiting
the necessary expansion of the metal-metal separation that
would minimize the distortions noted above (more about this
later). As was the case for compound 5b, the orientation of the
NHC planes is such that the substituents on the NHC rings are
aimed on the opposite faces from the A-frame pocket. As with
the other NHC complexes, reported herein, the Rh-carbene
distances (2.048(4), 2.051(3) Å) are normal, as are most other
parameters in the complex.
Figure 6. Representation of the complex cation of [Rh2Br(CO)2-
(IMe)(µ-MeCCeth)(dppm)][I] (7c′). Thermal parameters are as in
Figure 1. Phenyl hydrogens are not shown. Relevant parameters
(distances in Å and angles in deg): Rh(1)-Rh(2) ) 4.0745(3),
Rh(1)-C(11) ) 2.063(3), Rh(1)-C(3) ) 2.091(3), Rh(2)-C(16)
) 2.042(3), P(1)-P(2) ) 3.202(1), C(11)-C(16) ) 5.180(4);
Rh(1)-P(1)-C(8) ) 119.5(1), Rh(2)-P(2)-C(8) ) 117.3(1),
P(1)-C(8)-P(2) ) 120.9(2).
In all cases, the complexes 5a-d were ultimately prepared
-
as the BPh4 salts, owing to their superior crystallinity. These
salts were derived from the original products, having bromide
counterions, by anion exchange. Although we have not exten-
sively characterized these bromide salts, their 31P{1H} NMR
spectra are almost superimposable on those of the BPh4- salts.
Furthermore, we have ruled out bromide ion coordination to
give neutral dibromo compounds, through conductivity studies
coupling to the pair of Rh atoms. Otherwise, the spectral
parameters resemble those of the other A-frame species (5a-d).
We had anticipated that reaction of 6c with carbon monoxide
would yield a hydride-bridged product through elimination of
CO2, mimicking water-gas-shift chemistry; however, as with
the other dicarbene-bridged A-frames, compound 6c is unre-
active to CO at atmospheric pressure and its reactivity under
higher pressures was not investigated. An X-ray structural
determination of 6c was carried out, but this structure could
not be refined well, owing to disorder, and so is not reported
here. Nevertheless, the A-frame core structure is obvious,
confirming our formulation.
Having synthesized the hydroxide-bridged complex [Rh2-
(CO)2(µ-OH)(µ-MeCCeth)(dppm)][BPh4] (6c), we questioned
whether the hydroxide ligand could function as a base in the
deprotonation of additional imidazolium salts, much as was
accomplished earlier in the use of the acetate-bridged dimer
[Rh(µ-OAc)(COD)]2 for generating compounds 2a-c. Reacting
6c with the imidazolium salt 1,3-dimethylimidazolium iodide
instantly yields [Rh2I(CO)2(IMe)(µ-MeCCeth)(dppm)][BPh4] (7c;
IMe ) 1,3-dimethylimidazol-2-ylidene). Attempts to obtain
suitable crystals of 7c failed. However, we did obtain crystals
of the closely related [Rh2Br(CO)2(IMe)(µ-MeCCeth)(dppm)][I]
(7c′) by the analogous reaction of the bromide salt of 6c, namely
[Rh2(CO)3(µ-OH)(µ-MeCCeth)(dppm)][Br], with the imidazolium
iodide. This compound (7c′), containing a bromo ligand, was
actually of more interest to us, since a more direct structural
comparison to the bromide-bridged 5c could be made. The
ORTEP diagram of the complex cation of 7c′ is shown in Figure
6. A glance at this figure confirms that the dicarbene- and
diphosphine-bridged framework has remained intact, but now
this pair of bidentate ligands are the only bridging groups; the
anionic bromo ligand, which was bridging for compounds 5a-d,
is now terminally bound to one Rh center while the new
monocarbene unit is bound to the other, giving two independent
square-planar Rh centers. Also obvious from Figure 6, is the
-
which show both 5c (BPh4 anion) and its bromo salt have
conductivities characteristic of 1:1 electrolytes (see the Experi-
mental Section).
In attempts to generate the dicarbene-bridged analogues of
the formally M(0)/M′(0) or M2(0) complexes [MM′(CO)3-
(dppm)2] (M, M′ ) Rh, Ir)22–24 or [M2(CO)3(dmpm)2] (M )
Rh, Ir; dmpm ) Me2PCH2PMe2),25 respectively, compounds
5b,c were reacted with either NaBH4 or NaOH under an
atmosphere of carbon monoxidesconditions that had previously
been used for the reduction of the bis-diphosphine-bridged halide
precursors. Although compounds 5b,c both react instantly under
these conditions, the dicarbene- and disphosphine-bridged cores
do not remain intact and instead disproportionate, generating
the known bis-dppm species [Rh2(CO)3(dppm)2]22 as the only
phosphorus-containing product. The fates of the dicarbene
ligands and the other 1 equiv of “Rh2” in these reactions were
not determined, although presumably mononuclear species
containing chelating dicarbenes were obtained as described
earlier.
Reaction of the bromo-bridged A-frame 5c with excess NaOH
in the absence of CO results in replacement of the bromide
ligand by hydroxide, yielding the hydroxide-bridged analogue,
-
which was isolated as the BPh4 salt [Rh2(CO)2(µ-OH)(µ-
MeCCeth)(dppm)][BPh4] (6c). The hydroxide proton appears as
a triplet in the 1H NMR spectrum at δ 0.81, displaying 4.4 Hz
(22) Kubiak, C. P.; Woodcock, C.; Eisenberg, R. Inorg. Chem. 1982,
21, 2119.
(23) Sutherland, B. R.; Cowie, M. Organometallics 1985, 4, 1637.
(24) McDonald, R.; Cowie, M. Inorg. Chem. 1990, 29, 1564.
(25) Reinking, M. K.; Ni, J.; Fanwick, P. E.; Kubiak, C. P. J. Am. Chem.
Soc. 1989, 111, 6459.
(26) Jenkins, J. A.; Cowie, M. Organometallics 1992, 11, 2767.

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 701
substantial opening up of the dicarbene part of the complex, in
which the separation between the carbene centers (C(11)-C(16))
has widened to 5.180(4) Åsan increase of more than 1.3 Å
from that observed in the C2H4-linked 5c. Two factors appear
to be responsible for this opening up of the A-frame structure:
(1) the absence of the bridging bromo ligand allows the metals
to increase their separation from that observed in 5c (to
4.0745(3) from 3.3370(4) Å), and (2) the large monocarbene
unit on Rh(1) and adjacent bromo ligand on Rh(2) appear to
force the metal centers apart even further than what might have
been achieved in 5c, even in the absence of the bridging
bromide. The strain within complex 7c′, resulting from the
increased bite of the dicarbene, is evident in the geometry within
the dppm group. First, the P(1)-P(2) separation (3.202(1) Å)
is much larger than is usually observed, where values near 3.05
Å, as observed for 5b,c, are more typical. This larger P-P
separation also gives rise to significantly enlarged angles at
phosphorus (Rh(1)-P(1)-C(8) ) 119.5(1), Rh(2)-P(2)-C(8)
) 117.3(1)°) and at C(8) (P(1)-C(8)-P(2) ) 120.9(2)°). By
comparison, in compounds 5b,c, the former angles range from
114.4 to 116.9° while the latter are approximately 112°.
The opening up of the carbene-carbene separation has
relieved some of the strain that was evident in 5c, so that now
both NHC planes of the dicarbene are symmetrically oriented
with respect to the metal coordination planes, being skewed from
these planes by similar amounts (76.8(1), 75.48(8)°), although
both still deviate from the favored 90°. Similarly, the mono-
carbene unit is also skewed from the Rh(1) plane by 74.0(1)°.
All Rh-carbene distances (2.063(3), 2.091(3), 2.042(3) Å) are
again typical for such species.
Figure 7. Representation of the complex dication of [Rh2(CO)2-
(dppe)2(µ-MeCCeth)][CF2SO3]2 (8c). Only the ipso carbons of the
dppe phenyl rings are shown. Relevant parameters (distances in
Å):Rh(1)-Rh(2))5.8605(6),Rh(1)-C(10))2.053(3),Rh(2)-C(16)
) 2.069(3), Rh(1)-P(1) ) 2.3185(7), Rh(1)-P(2) ) 2.2796(7),
Rh(2)-P(3) ) 2.3246(7), Rh(2)-P(4) ) 2.2763(7).
The fate of the dicarbene unit lost in the formation of
[Rh(dppe)2]+ was not established.
The 31P{1H} NMR spectrum of 8c (as the triflate salt), which
is given in the Supporting Information, displays two doublets
of doublets at δ 62.4 and 56.1; the former resonance shows
110.7 Hz coupling to Rh and 56.1 Hz coupling to the other P
nucleus, while the Rh coupling in the latter is 126.4 Hz. In the
13C{1H} NMR spectrum the carbonyl resonance appears as a
doublet of doublets of doublets at δ 188.5, displaying coupling
to the trans phosphine (100.5 Hz), Rh (65.4 Hz), and the cis
phosphine (14.1 Hz). The carbene carbon at δ 175.7 shows a
similar pattern, also having coupling to the trans phosphine (87.5
Hz), rhodium (44.2 Hz), and the cis phosphine (14.1 Hz).
In an attempt to decrease the strain within these mixed-bridge,
A-frame species that has resulted from the differing bite
distances of the dicarbene and dppm ligands (particularly for
R
the C2H4-linked CCeth groups), we attempted to generate the
bis(diphenylphosphino)ethane (dppe) analogue of 5c, in which
dppe bridged the metals instead of dppm. The bite of dppe,
having the additional CH2 group linking the two ends of the
diphosphine, should be a better match to the longer separation
between the carbene centers. Although dppe is commonly used
as a chelating group, it is also known to bridge pairs of metals.27
Using reaction conditions similar to those used to generate
the dppm-bridged 5c did not give the targeted dppe-bridged
analogue but instead yielded a 2:1 mixture (from 31P{1H} NMR)
of the known species [Rh(dppe)2]+28 and the new product
[Rh2(CO)2(dppe)2(µ-MeCCeth)]2+ (8c), together with unreacted
starting material. The yield of both dppe products could be
increased, accompanied by the disappearance of 2c, by the
addition of excess dppe. Attempts to generate a dppe-bridged
analogue of 5c by slow addition of dppe, even at low
temperatures, only yielded the chelated product 8c, which was
subsequently isolated as the triflate salt after ion replacement.
Crystals of 8c were obtained by recrystallization, and an X-ray
structural determination was carried out. Figure 7 gives a
representation of the complex dication, which clearly shows that
the dppe groups have adopted a chelating arrangement at both
metals (only the ipso carbons of the dppe phenyl rings are
shown). The resulting Rh(1)-Rh(2) separation (5.8605(6) Å)
is significantly shorter than the 7 Å or greater that is observed
in the COD and dicarbonyl precursors 2c and 4c, respectively,
but is still indicative of two independent, noninteracting Rh
centers. For the most part, the structural parameters are as
expected for a di-NHC-bridged species. Both Rh-carbene bonds
(2.053(3), 2.069(3) Å) are normal, and the NHC planes are
twisted out of the coordination planes of the respective metals
(dihedral angles 76.62(9) and 75.76(9)°), although still deviating
significantly from the idealized orientation perpindicular to the
metal plane. The dicarbene unit again adopts a synclinal
arrangement about the C(14)-C(15) bond of the ethylene linker,
such that the N(11)-C(14)-C(15)-N(13) torsion angle is
72.9(3)°. Binding of each end of the diphosphine ligands
opposite either the carbene or the carbonyl units results in
inequivalent sets of Rh-P distances; the distances opposite the
carbonyls (Rh(1)-P(1) ) 2.3185(7), Rh(2)-P(3) ) 2.3246(7)
Å) are longer than those opposite the carbenes (Rh(1)-P(2) )
2.2796(7), Rh(2)-P(4) ) 2.2763(7) Å), demonstrating a larger
trans influence of the former.
(27) See for example: (a) Xiao, J.; Cowie, M. Can. J. Chem. 1993, 71,
726. (b) Di Vaira, M.; Costantini, S. S.; Mani, F.; Peruzzini, M.; Stoppioni,
P. J. Organomet. Chem. 2004, 689, 1757. (c) Khimyak, T.; Johnson, B. F. G.
J. Cluster Sci. 2004, 15, 543. (d) Yamazaki, S.; Taira, Z.; Yanemura, T.;
Deeming, A. J. Organometallics 2005, 24, 20. (e) Adrio, L. A.; Amoedo,
A.; Antelo, J. M.; Ferna´ndez, J. J.; Martínez, J.; Ortigueira, J. M.; Pereira,
M. T.; Vila, J. M. Z. Anorg. Allg. Chem. 2005, 631, 2204. (f) Zhu, W.;
Marr, A. C.; Wang, Q.; Neese, F.; Spencer, D. J. E.; Blake, A. J.; Cooke,
P. A.; Wilson, C.; Schro¨der, M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102,
18280. (g) Pe´tillon, F. Y.; Robin-Le Guen, F.; Rumin, R.; Schollhammer,
P.; Talarmin, J.; Muir, K. W. J. Organomet. Chem. 2006, 691, 2853. (h)
Martinez, J.; Adrio, L. A.; Antelo, J. M.; Ortigueira, J. M.; Pereira, M. T.;
Ferna´ndez, J. J.; Ferna´ndez, A.; Vila, J. M. J. Organomet. Chem. 2006,
691, 2721.
(28) Kunin, A. J.; Nanni, E. J.; Eisenberg, R. Inorg. Chem. 1985, 24,
1852.

702 Organometallics, Vol. 27, No. 4, 2008
Wells et al.
of greater than 6.5 Å. However, taking a page out of A-frame
chemistry, the addition of 1 equiv of dppm brings the metals
to within 3.34 Å apart through the generation of new A-frame
species in which the metals are simultaneously bridged by
mutually trans dicarbene and diphosphine groups and by a
halide ligand (see Scheme 3). Although the differing bites
of these dicarbene and diphosphine groups, in which the
carbene-carbene separation is greater than the phosphine-
phosphine separation, results in some distortions within the
A-frame, there is surprisingly little difference in metal-metal
and carbene-carbene separations on proceeding from a CH2
to a C2H4 linker between the carbenes. It appears that in these
A-frame species the bridging halide plays a strong role in
defining the metal-metal separation and appears unable to
accommodate additional separation. The addition of a mono-
carbene unit, which forces the bromide ligand from its
bridging site to a terminal site, clearly demonstrates the
restrictive role played by the bridging bromide through a
substantial opening up of the metal-metal separation from
ca. 3.34 to 4.07 Å, accompanied by an even greater increase
in carbene-carbene separation from 3.85 to 5.18 Å.
On the basis of the strain inherent in this system, we were
surprised that the products [Rh2(CO)2(µ-Br)(µ-di-NHC)(dpp-
m)][Br], having a bromide counterion, were ionic rather than
having both bromide ions coordinated, one to each metal, much
as was observed for the monocarbene and bromide ligands in
compound 7c′. In such an arrangement the metals would have
been able to separate, allowing the bridging dicarbene to achieve
a less strained arrangement. We assume that the bromide-bridged
geometries observed for 5b,c are a compromise in which
the strain within the dicarbene unit is tolerated, owing to the
absence of such strain within the dppm framework. Furthermore,
the basic dicarbene ligands may also play a role in favoring the
cationic complex obtained upon bromide loss.
Discussion
A number of strategies have been reported for the generation
of NHC complexes, the most common of which are (1) prior
deprotonation of the imidazolium ion precursor, using a variety
of bases, and subsequent direct reaction of the carbene with an
appropriate metal complex, (2) transmetalation, in which the
carbene is transferred from a metal-carbene precursor (usually
silver) to the metal complex of choice, (3) direct metalation in
which the complex and the imidazolium salt are reacted in the
presence of a weak base (in this case a transition state in which
simultaneous coordination of the base and the imidazolium salt
to the metal has been proposed2g), and (4) deprotonation of the
imidazolium salt by a basic ligand bound to the precursor
complex. In this last route the basic ligands used include acetate,
acetylacetonate, and alkoxide ions. This route has been the
method of choice in much of the chemistry reported herein,
yielding the dicarbene-bridged complexes [RhBr(COD)]2(µ-di-
NHC) (2a-c) from the acetate-bridged dimer [Rh(µ-OAc)-
(COD)]2 and the bromo salts of the appropriate imidazolium
ions. One advantage of this route is that the stoichiometry is
limited by the number of basic ligands in the precursor; thus,
the use of the above diacetate dimer results in the double
deprotonation of only one diimidazolium dication to yield a
single dicarbene unit.
We have also demonstrated in one case that the deprotonation
can proceed stepwise at a rate such that the monocarbene product
[RhBr(COD)(MeC(H)-η1-Ceth)][Br] (3c), resulting from depro-
tonation of one end of the diimidazolium salt, can be isolated,
as shown earlier in Scheme 2. The pendant imidazolium
functionality that remains can be subsequently deprotonated by
additional [Rh(µ-OAc)(COD)]2 to yield the dicarbene-bridged
dirhodium species (2c), suggesting the potential for this and
related pendant compounds to be used for the generation of
dicarbene-bridged, mixed-metal complexes via reaction of the
pendant intermediate with an appropriate metal source. Such
studies are currently underway in our group.
An additional consequence of the above-noted strain
appears to be the inability of compounds 5a-d to react with
CO. We had anticipated that the uptake of 1 equiv of CO
would yield the carbonyl-bridged adducts [Rh2(CO)2(µ-Br)(µ-
CO)(µ-di-NHC)(dppm)]+, much as is well-known in bis-
dppm-bridged A-frames. We assume that the inability of our
dicarbene-bridged species to react with CO results from their
inability to form these carbonyl-bridged adducts with an
accompanying metal-metal bond, owing to the increased
strain within the dicarbene framework that would result by
facing the metals closer together. Similarly, attempts to
reduce the Rh(I) A-frames 5a-d, to formally Rh(0),
metal-metal-bonded species, as has been achieved for the
bis-dppm and bis-dmpm analogues, failed for our dicarbenes.
Again we assume that such products cannot be attained,
owing to the contraction in metal-metal separation that
would be required with concomitant Rh-Rh bond formation.
Although the reactions of the methyl-substituted imidazolium
salts 1a,c with the acetate-bridged dimer occur readily (in 3–4
h), that of the tert-butyl-substituted 1b is very sluggish, requiring
overnight reflux. Therefore, it is possibly not surprising that no
reaction is observed with the tert-butyl-substituted 1d and the
acetate-bridged dimer, even after prolonged reflux. Nevertheless,
the targeted dicarbene-bridged product 2d (as the chloride) was
readily obtained by prior deprotonation of 1d and reaction of
the resulting dicarbene with [RhCl(COD)]2.
Although it would appear to be a trivial transformation to
convert compounds 2a-d into the tetracarbonyl analogues
[RhX(CO)2]2(µ-di-NHC) via COD displacement by CO, this
reaction only worked cleanly for compounds 2c,d. The reactions
of 2a,b gave rise to the mononuclear products [Rh(CO)2(η1:
η1-di-NHC)][Br], containing chelating di-NHC groups, and
[Rh(CO)2(µ-Br)]2. The bridging dicarbenes in these latter
precursors were surprisingly labile, converting to chelating at
ambient temperature. It is noteworthy that the CH2-bridged
Conclusions
t
dicarbenes RCCmeth (R ) Me, Bu) are less prone to remain
Although the targeted A-frame complexes [Rh2(CO)2(µ-
Br)(µ-di-NHC)(dppm)]+ have been successfully prepared, their
apparent inability to support metal-metal bonds, owing to the
strain imposed by the relatively large bite within the dicarbenes,
threatens to limit the scope of reactivity for these species; many
of the transformations usually observed in binuclear species are
accompanied by metal-metal bond making and breaking.
Nevertheless, the reactivity of the monobridged ([RhXL2]2(µ-
di-NHC)) and tribridged ([Rh2(CO)2(µ-X)(µ-di-NHC)(dppm)]+)
compounds will be investigated and compared to that of related
bridging than the C2H4-linked analogues RCCeth. This is in
keeping with a previous conclusion6b that the C2H4-linked
dicarbenes are under a greater degree of steric strain than the
CH2-linked analogues when chelating. An extension of this
argument would suggest that the C2H4-linked dicarbenes prefer
a bridging coordination mode.
In all of the above compounds, bridged by a single
dicarbene unit, the skewing of the two carbene rings about
the CH2 or C2H4 linkers gives rise to large Rh-Rh separations

A-Frame Complexes of Dirhodium
Organometallics, Vol. 27, No. 4, 2008 703
mononuclear analogues to probe whether such systems can
display metal-metal cooperativity.
preparation and purification and in preparing the Supporting
Information.
1
Supporting Information Available: Figures giving H NMR
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
University of Alberta for financial support of this research,
the NSERC for funding the Bruker PLATFORM/SMART
1000 CCD diffractometer and the Nicolet Avator IR spec-
trometer, and Matt Zamora for technical assistance in sample
spectra for compounds 2d and 3b, and 1H and 31P{1H} NMR
spectra for compounds 5c and 8c and CIF files giving crystal data
for compounds 2b,d, 4c, 5b,c, 7c′, and 8c. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM7007937