Formation of a bridging-imido d6 rhodium compound by nitrene capture. Insertion and cycloaddition reactions

Article abstract of DOI:10.1021/ic801703d

The first d⁶ rhodium imido complex, [{(C₅Me ₅)Rh(μ-NSO₂C₆H₄Me-p)} ₂] (2), has been obtained from the reaction of [(C₅Me ₅)Rh(C₂H₄)₂] with chloramine-T. Carbon monoxide inserts into the N-Rh bonds in 2 to give the dinuclear ureylene complex [(C₅Me₅)Rh(μ-{(Ts)N-CO-N(Ts)}Rh(CO)(C ₅Me₅)], while the azide C₆F₅N ₃ adds to 2 to give the mononuclear tetrazene complex [(C ₅Me₅)Rh{(p-MeC₆H₄SO ₂)N₄(C₆F₅)}].

Full text of DOI:10.1021/ic801703d

Inorg. Chem. 2008, 47, 10220-10222
Formation of a Bridging-Imido d6 Rhodium Compound by Nitrene
Capture. Insertion and Cycloaddition Reactions
Cristina Tejel,* Miguel A. Ciriano, Sonia Jime´nez, Vincenzo Passarelli, and Jose´ A. Lo´pez
Instituto de Ciencia de Materiales de Arago´n (ICMA), Departamento de Qu´ımica Inorga´nica,
CSIC-UniVersidad de Zaragoza, E-50009 Zaragoza, Spain
Received September 4, 2008
The first d6 rhodium imido complex, [{(C5Me5)Rh(µ-NSO2C6H4Me-
p)}2] (2), has been obtained from the reaction of [(C5Me5)Rh(C2H4)2]
with chloramine-T. Carbon monoxide inserts into the N-Rh bonds
in 2 to give the dinuclear ureylene complex [(C5Me5)Rh(µ-{(Ts)N-
CO-N(Ts)}Rh(CO)(C5Me5)], while the azide C6F5N3 adds to 2 to
give the mononuclear tetrazene complex [(C5Me5)Rh{(p-MeC6-
H4SO2)N4(C6F5)}].
rhodium has been developed rather more slowly. At present,
just two families of d8-rhodium imido complexes are known,
namely, dinuclear dppm-bridged amido/imido tautomers7 and
polynuclear complexes based on triply bridging “µ3-
NC6H4Me” imido ligands,8 along with an early compound
reported by McGlinchey and Stone,9 and a novel dinuclear
d7-rhodium complex just reported.10 For a long time, d6
rhodium imido compounds have been obstinately elusive,
although evidence for the complex [(C5Me5)RhNR] (R )
2,6-iPr2C6H3) as a transient intermediate species was given
by Danopoulos and Wilkinson.11 Here we report our
preliminary results on the successful synthesis and full
characterization of the first d6 rhodium imido complex and
some exploratory reactions that open this chemistry, inac-
cessible up to now.
Imido complexes of the late transition metals (groups 9
and 10) are rare. Such complexes are of interest as
intermediates in imido group transfer reactions, and they may
play relevant roles in catalytic C-N bond formation reac-
tions.1 This practical and fundamental significance has
encouraged several groups to search for suitable and
reproducible synthetic methods for this class of compounds.
While iridium and ruthenium imides, such as [(C5Me5)-
IrtNtBu]2 and [(η6-benzene)RudNAr] (Ar ) 2,6-iPr2C6H3),3
remained for some time as isolated examples of late-
transition-metal terminal imides, several groups have recently
described imido complexes of the first-row metals cobalt4
and nickel,5 which show an interesting reactivity.6 In marked
contrast, the imido chemistry of the second-row metal
Treatment of the yellow complex [(C5Me5)Rh(C2H4)2] (1)
with chloramine-T (p-MeC6H4SO2NCl-Na+) in dichlo-
(4) (a) Cowley, R. E.; Bontchev, R. P.; Sorrell, J.; Sarracino, O.; Feng,
Y.; Wang, H.; Smith, J. M. J. Am. Chem. Soc. 2007, 129, 2424–2425.
(b) Shay, D. T.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.;
Theopold, K. H. Angew. Chem., Int. Ed. 2005, 44, 1508–1510. (c)
Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322–16323. (d)
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* To whom the correspondence should be adressed. E-mail: ctejel@
unizar.es.
(6) Bai, G.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 1856–1859,
and references cited therein.
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Mehn, M. P., Jr. Angew. Chem., Int. Ed. 2003, 42, 4357–4360. (d)
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10220 Inorganic Chemistry, Vol. 47, No. 22, 2008
10.1021/ic801703d CCC: $40.75  2008 American Chemical Society
Published on Web 10/15/2008

COMMUNICATION
Figure 1. Molecular structure of 2 (only the C(ipso) of the p-tolyl groups
is shown). Selected bond distances (Å): Rh(1)-N(1), 2.006(3); Rh(1)-N(2),
2.006(3); Rh(2)-N(1), 1.992(3); Rh(2)-N(2), 2.017(3); S(1)-N(1), 1.584(3),
S(2)-N(2), 1.581(3). Sum of the bond angles around N (deg): N(1), 354.6;
N(2), 344.8.
Figure 2. Molecular structure of 5 (only the C(ipso) of the p-tolyl groups
is shown). Selected bond distances (Å): Rh(1)-N(1), 2.135(6); Rh(1)-N(2),
2.141(6); Rh(2)-N(2), 2.116(6); N(1)-C(28), 1.371(9); N(2)-C(28),
1.455(9); C(28)-O(3), 1.208(8); S(1)-N(1), 1.619(6); S(2)-N(2), 1.654(6).
Sum of the bond angles around N (deg): N(1), 354.9; N(2), 336.9 (Rh(2)
is excluded).
romethane renders sodium chloride and a navy-blue solution,
from which the dinuclear imido compound [{(C5Me5)Rh(µ-
NTs)}2] (2; Ts ) p-MeC6H4SO2) was easily isolated as a
navy-blue solid in good yield. Chloramine-T has been used
recently as a nitrene source in green chemistry for alkene
aziridination and alkane tosylamidation,12 but it is a very
unusual reagent in organometallic chemistry. In this area,
(sulfonylimido) metal species remain sparse.13
Quite surprisingly, complex 2 was found to be an air- and
moisture-stable compound, and single crystals were grown
by slow evaporation of a hexane solution of 2 in air without
special precautions. Apparently, the electron-withdrawing
sulfonyl group rather than alkyl or aryl on the imido nitrogen
in 2 could be one of the reasons for the unusual stability of
this complex. In the structure of 2 (Figure 1), two p-
tosylimido ligands bridge two (C5Me5)Rh moieties, making
a folded four-membered “Rh2N2” ring with a relatively short
Rh-Rh separation, 2.7992(4) Å. The metric parameters in
2 fall in the range of those found in the related bis(imido)-
bridged diiridium complexes, and the structure was found
to be quite similar to that of the recently reported iridium
counterpart [{(C5Me5)Ir(µ-NTs)}2].14
compound. Attempts to obtain mononuclear imido complexes
by bridge splitting reactions of 2 with conventional two-
electron donors, such pyridine, triphenylphosphane, or ac-
etonitrile, were unsuccessful.
Preliminary studies on the reactivity of the novel com-
pound 2 indicate an ambivalent character, acting as a mono-
or dinuclear compound depending on the substrate. Thus,
treatment of 2 in dichloromethane with carbon monoxide
under atmospheric pressure results in a color change from
navy blue to deep green with the formation of the complex
[(C5Me5)Rh(µ-(TsN-CO-NTs)Rh(CO)(C5Me5)] (5) in 10
min. Complex 5 was isolated as a dark-green crystalline solid
in good yield, and it was fully characterized. Its structure is
shown in Figure 2.
Complex 5 is a result of the incorporation of two molecules
of carbon monoxide into the dinuclear complex 2. One of
them becomes coordinated to Rh(2) as a terminal ligand,
while the other develops two N-C bonds with the former
imido nitrogen atoms to build the ureylene moiety. The newly
formed dianionic carbonylbis(p-tosylazanido) ligand is bonded
to the metals in a nonsymmetrical way, chelating one of the
rhodium atoms [Rh(1)] through both nitrogen atoms and
bridging both metals via N(2). Electron counting of 5 (34
v.e.) and the quite short Rh-Rh distance [2.6919(8) Å] are
consistent with a Rh-Rh bond in the molecule.
Prolonged exposure of a green solution of 5 to carbon
monoxide produces cleavage of the complex into the orange
rhodium(III) compound [(C5Me5)Rh(CO)(TsN-CO-NTs)] (6)
and the well-known complex [(C5Me5)Rh(CO)2]. Complex
6 retains the chelating symmetrically bonded N,N′-bis(tosyl)-
ureato ligand plus a carbonyl ligand, as shown in the structure
of 6 (Figure 3).
A rational sequence of reactions yielding complexes 5 and
6 is shown in Scheme 1. Most probably, the reaction of 2
with carbon monoxide starts with the coordination of this
molecule to one of the metals to give the intermediate A. In
A, the carbon monoxide ligand inserts into one Rh-N(imido)
bond to form B, which contains a carbonyl(p-tosyl)azananido
ligand bridging the two rhodium atoms.
Because steric factors, modulated by thermodynamic
considerations,15 can control the mononuclear/dinuclear
nature of imido complexes, as proven by Burrell and
Steedman in ruthenium chemistry,3 we tried to obtain an
imido mononuclear d6 rhodium complex by using the
t
complex [(C5H3 Bu2)Rh(C2H4)2] (3) with a bulky Cp ring.
A new imidorhodium(III) complex (4) resulted from the
reaction of 3 with chloramine-T, although it was further
t
identified as the dinuclear complex [{(C5H3 Bu2)Rh(µ-
NTs)}2] (4) with a structure similar to that of 2 (see the
Supporting Information). The preparation of 4 provides, on
the other hand, a second example of a d6 rhodium imido
(12) For example, see: (a) Fructos, M. R.; Trofimenko, S.; D´ıaz-Requejo,
M. M.; Pe´rez, P. J. J. Am. Chem. Soc. 2006, 128, 11784–11791. (b)
Sureshkumar, D.; Maity, S.; Chandrasekaran, S. J. Org. Chem. 2006,
71, 1653–1657. (c) Nicolas, C.; Lacour, J. Org. Lett. 2006, 8, 4343–
4346. (d) Kano, D.; Minakata, S.; Komatsu, M. J. Chem. Soc., Perkin
Trans. 2001, 3186–3188. (e) Liang, C.; Robert-Peillard, F.; Fruit, C.;
Mu¨ller, P.; Dodd, R. H.; Dauban, P. Angew. Chem., Int. Ed. 2006,
45, 4641–4644.
(13) Lorber, C.; Choukroun, R.; Vendier, L. Inorg. Chem. 2007, 46, 3192–
3202, and references cited therein.
(14) Ishiwata, K.; Kuwata, S.; Ikariya, T. Organometallics 2006, 25, 5847–
5849.
This ligand could be reductively eliminated as a neutral
isocyanate, but it is not. Instead, coordination of a second
(15) Dobbs, D. A.; Bergman, R. G. Organometallics 1994, 13, 4594–4605.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10221

COMMUNICATION
Scheme 2. Formation of the Unsymmetrical Tetrazene Complex 7
to dimetallacycloimide complexes.17 However, the few
reported ways to ureato complexes from imido complexes
are their reactions with isocyanates,11,18 which could explain
the formation of ureas in the reductively catalyzed carbo-
nylation of nitroaromatics.19
Figure 3. Molecular structure of 6 (only the C(ipso) of the p-tolyl groups
is shown). Selected bond distances (Å): Rh-N, 2.113(2); N-C(8), 1.398(4);
C(8)-O(3), 1.212(5); S-N, 1.628(3).
On the other hand, complex 2 reacts easily with the organic
azide C6F5N3 in toluene to give the red complex [(C5Me5)-
Rh{(p-MeC6H4SO2)N4(C6F5)}] (7), which contains a chelat-
ing and unsymmetrical N,N′-diaryltetraazenediido ligand. The
tetrazenide complex 7 was isolated as a crystalline red solid
in good yield, and it was fully characterized in solution and
in the solid state (see Supporting Information). Because a
single isomer was observed in the product, the tetrazenide
ligand results from a regioselective addition of pentafluo-
rophenylazide to the p-tosylimido ligand in such a way that
the substituted nitrogen atoms are bonded to the metal
(Scheme 2). This is a typical result of a [3 + 2] cycloaddition
reaction of organic azides with mononuclear imido com-
plexes, previously reported for osmium,18a iridium,11 and
zirconium complexes,20 which would indicate that 2 behaves
as a mononuclear imido complex for cycloaddition reactions.
In conclusion, imidorhodium complexes, other than poly-
nuclear ones, with a d6 metal configuration can be prepared.
The Rh-N bond is highly reactive in insertion and cycload-
dition reactions with appropriate substrates. These findings
provide an opening for the chemistry of these long elusive
compounds to be studied further.
Scheme 1. Proposed Pathway for the Reaction of 2 with Carbon
Monoxide To Give Complex 5 ([Rh] ) (C5Me5)Rh, R )
p-MeC6H4SO2)
carbon monoxide molecule to the intermediate B promotes
migration of the C(O)-Rh bond to the close imido nitrogen
to form the ureylene fragment, as found in the isolated
complex 5. This last step represents a reductive elimination
resulting in the product, the rhodium(II) complex. In this
context, formation of the ureylene fragment is the result of
a dinuclear reactivity in which both metals cooperate to
coordinate and insert carbon monoxide into two N(imido)-Rh
bonds. The formation of these two new N-C bonds is
facilitated by the bis(imido) molecular core, which makes
the dinuclear elimination step feasible. The slow addition
of carbon monoxide to 5 simply cleaves the metal-metal
bond, splitting the molecule unsymmetrically into the rhod-
ium(III) complex 6, which keeps the N,N′-bis(tosyl)ureato
ligand, and a rhodium(I) complex.
Acknowledgment. The generous financial support from
MEC/FEDER (Project CTQ2005-06807) and Gobierno de
Arago´n (GA, Group E70-Inorganic Molecular Architecture)
is gratefully acknowledged. S.J. thanks GA for a fellowship,
and V.P. thanks CSIC for an I3P postdoctoral contract.
Supporting Information Available: X-ray crystallographic data
in CIF format, experimental procedures for the preparation and
characterization of the new complexes (elemental analyses, NMR
spectra, and mass spectra), crystal data, and full ORTEP representa-
tions for 2 and 4-7. This material is available free of charge via
the Internet at http://pubs.acs.org.
Isolation of complexes with a ureato ligand from reactions
of imido complexes with carbon monoxide is unprecedented
in imido metal chemistry. Typically, these reactions lead to
free or coordinated isocyanates2,16,17 and, in some instances,
IC801703D
(18) (a) Michelman, R. L.; Bergman, R. G.; Andersen, R. A. Organome-
tallics 1993, 12, 2741. (b) Hogart, G.; Richards, I. Dalton Trans. 2005,
760–773.
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117, 974–985.
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Organometallics 2005, 24, 2251–2254. (b) Mindiola, D. J.; Hillhouse,
G. L. Chem. Commun. 2002, 1840–1841.
(17) Ge, Y. W.; Sharp, P. R. Inorg. Chem. 1992, 31, 379–384.
10222 Inorganic Chemistry, Vol. 47, No. 22, 2008