A catalytically active, charge-neutral Rh(I) zwitterion featuring a P,N-substituted "naked" indenide ligand

Article abstract of DOI:10.1021/ja034543v

The synthesis and characterization of a new class of cationic and zwitterionic Rh(I) complexes, which feature multidentate ligands comprised of donor-functionalized indene or indenide units, are reported. This unusual new ligation strategy provides access

Full text of DOI:10.1021/ja034543v

Published on Web 04/18/2003
A Catalytically Active, Charge-Neutral Rh(I) Zwitterion Featuring a
P,N-Substituted “Naked” Indenide Ligand
Mark Stradiotto,*,† Judy Cipot,† and Robert McDonald‡
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, and X-Ray
Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received February 6, 2003; E-mail: mark.stradiotto@dal.ca
Scheme 1. The Synthesis of Compounds 1-4a
Cationic complexes of Rh(I) figure prominently in the field of
homogeneous catalysis, as they are capable of mediating substrate
transformations that are often difficult to achieve by use of neutral
catalysts.1 In particular, cationic Rh(I) complexes supported by
hemilabile phosphine-amine (P,N) bidentate ligands have attracted
considerable attention.2,3 These species often exhibit reactivity
properties that are superior to the corresponding diphosphine and
diamine complexes.2-4 Unfortunately, the polar nature of Rh(I) salts
can necessitate the use of high-dielectric reaction media, possibly
leading to attenuated catalytic activity resulting from unfavorable
competition between substrate molecules and the counteranion or
solvent for the metal active site. The use of charge-neutral
zwitterionic complexes represents a means of circumventing such
problems.5 Zwitterionic Rh(I) complexes have been shown to
catalyze a wide range of reactions under mild conditions and with
high selectivity.6 To date, the construction of Rh(I) zwitterions has
relied almost exclusively on borato-type ligands,7 and, surprisingly,
while both “N,N” and “P,P” bidentate zwitterions of Rh(I) have
been explored, the corresponding “P,N” zwitterions are unknown.
In the pursuit of new and synthetically useful metal-mediated
reaction chemistry, we sought to prepare alternative classes of
platinum-group zwitterions that do not rely on borate ligation. In
this context, we envisaged that the incorporation of an extended
Hu¨ckel aromatic carbanion within the periphery of an ancillary
ligand might provide an efficient means of inducing formal charge
separation. Although main group functionalized indenyl compounds
have been widely used as carbocyclic π-ligands, the sequestering
of an anionic charge in the form of a 10-π indenide unit built into
the backbone of a bidentate ligand has thus far been overlooked.8
Herein we report that this unusual new ligation strategy provides
access to the first charge-neutral [κ2-P,N]Rh(I) zwitterion, 4, a
complex that functions as a catalyst for the dehydrogenative
coupling of C-H and Si-H fragments.
a Reagents: (i) n-BuLi, ClPiPr2; (ii) 0.5[L2RhCl]2, L2 ) COD, 2a;
or NBD, 2b; (iii) AgBF4; (iv) using 3a, NaNTMS2; (v) n-BuLi,
0.5[CODRhCl]2.
Figure 1. The crystallographically determined structure of 3b, shown with
40% displacement ellipsoids. Selected hydrogen atoms, as well as the THF
-
solvate and BF4 counterion, have been omitted for clarity. Selected
interatomic distances (Å): Rh-P 2.2798(7); Rh-N 2.179(2); P-C3
1.806(3); N-C2 1.455(3); C1-C2 1.509(4); C2-C3 1.346(4); C3-C3a
1.477(4); C3a-C7a 1.404(4); C1-C7a 1.499(4).
The new indene-supported P,N ligand, 1, can be prepared in 93%
Chloride abstraction (AgBF4) from rhodium in either 2a or 2b
generates the cationic chelate complexes, 3a and 3b, respectively,
in which the indenyl framework has undergone a structural
yield via low-temperature lithiation of 2-dimethylaminoindene,
i
followed by the addition of Pr2PCl (Scheme 1). The formulation
of 1 as the allylic (C1) rather than the vinylic (C3) isomer is
consistent with data obtained from NMR spectroscopic studies.9a
Treatment of 1 with 0.5 equiv of either [CODRhCl]2 (COD ) 1,5-
cyclooctadiene) or [NBDRhCl]2 (NBD ) 2,5-norbornadiene)
produces the corresponding mononuclear phosphine complexes, 2a
and 2b, respectively. These and the other Rh(I) species depicted in
Scheme 1 are formed quantitatively (based on 31P NMR data
obtained from the crude reaction mixtures), and in all cases the
desired complexes can be isolated as analytically pure solids in
high yield. An X-ray crystallographic study of 2a confirms that
the coordinated indenyl ligand (1) is bound exclusively through
the phosphorus donor.9b
i
rearrangement that places the Pr2P fragment at the C3 position.
The crystallographically determined structure of 3b is shown in
Figure 19c and is comparable to other [(κ2-P,N)Rh(olefin)2]+X- salts
reported in the literature.10
In contrast, the alkane-soluble zwitterionic Rh(I) species, 4, is
without precedent. This novel complex can be cleanly generated
either by deprotonation of 3a or via lithiation of 1 followed by
treatment with 0.5 equiv of [CODRhCl]2. The preference of the
[CODRh+] fragment in 4 to coordinate at the P,N site rather than
binding in an η5-fashion to the indenyl ring is highly unusual,
especially in light of the numerous (η5-indenyl)Rh(olefin)2 com-
plexes that are known.11 The crystallographically determined
structure of 4 is shown in Figure 2.9d Particularly striking are the
differences between the interatomic distances within the C5 ring
† Dalhousie University.
‡ University of Alberta.
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10.1021/ja034543v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
characteristics brought about by this new ligand set will engender
reactivity in a variety of late transition metal fragments.
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada and Dalhousie University for
financial support, Mr. Juergen Mueller (Dalhousie) for the prepara-
tion of custom Schlenk glassware, and Dr. James Pincock (Dal-
housie) and Dr. Robert Singer (St. Mary’s University) for assistance
in the acquisition of GC data.
Supporting Information Available: Experimental details and
characterization data for 1-4 including tabulated crystallographic data
for 2a, 3b, and 4 (PDF), as well as X-ray crystallographic information
files (CIF) for 2a, 3b, and 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. The crystallographically determined structure of 4, shown with
40% displacement ellipsoids. Nonindenide hydrogen atoms have been
omitted for clarity. Selected interatomic distances (Å): Rh-P 2.3173(6);
Rh-N 2.242(2); P-C3 1.758(2); N-C2 1.479(3); C1-C2 1.386(3); C2-
C3 1.419(3); C3-C3a 1.446(3); C3a-C7a 1.442(4); C1-C7a 1.430(4).
References
of 3b (e.g., C1-C2 ) 1.509(4) Å; C2-C3 ) 1.346(4) Å) and those
found in 4 (e.g., C1-C2 ) 1.386(3) Å; C2-C3 ) 1.419(3) Å).
Whereas the cation exhibits bond length alternation consistent with
an indene framework, the indenide unit in 4 possesses a highly
delocalized structure and is appropriately described as a “naked”
indenide anion.12
Given our interest in the development of catalysts for E-H bond
activation (E ) main group element), we selected the dehydro-
genative silylation (DS) of styrene as an initial means of bench-
marking the catalytic performance of 4 (eq 1). The reaction of 3
equiv of styrene (functioning both as a substrate and as an H2
acceptor) with 1 equiv of Et3SiH in the presence of 1 mol % 4 at
80 °C for 2.5 h in toluene results in the consumption of the silane,
along with the production of 5, 6 (∼83:17), and ethylbenzene.9a,e
Notably, the selectivity of 4 for DS over hydrosilylation (HS) in
this reaction (i.e., the preferred formation of 5 over 6) approaches
that of the most effective Rh(I) catalyst systems for DS.13
(1) Dickson, R. S. Homogeneous Catalysis with Compounds of Rhodium and
Iridium; Reidel: Dordrecht, 1985.
(2) Sloan, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem. 1999,
48, 233.
(3) Gavrilov, K. N.; Polosukhin, A. I. Russ. Chem. ReV. 2000, 69, 661.
(4) For selected reactivity studies see: (a) Anderson, M. P.; Casalnuovo, A.
L.; Johnson, B. J.; Mattson, B. M.; Mueting, A. M.; Pignolet, L. H. Inorg.
Chem. 1988, 27, 1649. (b) Faller, J. W.; Chase, K. J. Organometallics
1994, 13, 989. (c) Alonso, M. A.; Casares, J. A.; Espinet, P.; Valle´s, E.;
Soulantica, K. Tetrahedron Lett. 2001, 42, 5697.
(5) Chauvin, R. Eur. J. Inorg. Chem. 2000, 577.
(6) For selected examples see: (a) Westcott, S. A.; Blom, H. P.; Marder, T.
B.; Baker, R. T. J. Am. Chem. Soc. 1992, 114, 8863. (b) Kishimoto, Y.;
Itou, M.; Miyatake, T.; Ikariya, T.; Noyori, R. Macromolecules 1995, 28,
6662. (c) Dai, C.; Robins, E. G.; Scott, A. J.; Clegg, W.; Yufit, D. S.;
Howard, J. A. K.; Marder, T. B. Chem. Commun. 1998, 1983. (d) Van
den Hoven, B. G.; Alper, H. J. Am. Chem. Soc. 2001, 123, 10214.
(7) For selected examples, see: (a) Trofimenko, S. J. Am. Chem. Soc. 1967,
89, 6288. (b) Schrock, R. R.; Osborn, J. A. Inorg. Chem. 1970, 9, 2339.
(c) Westcott, S. A.; Taylor, N. J.; Marder, T. B.; Baker, R. T.; Jones, N.
L.; Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1991, 304. (d) Betley,
T. A.; Peters, J. C. Inorg. Chem. 2002, 41, 6541.
(8) (a) Stradiotto, M.; McGlinchey, M. J. Coord. Chem. ReV. 2001, 219-
221, 311. (b) Allen, A. D.; Tidwell, T. T. Chem. ReV. 2001, 101, 1333.
(9) (a) Full experimental details, including spectroscopic characterization data
for all of the compounds reported herein, are provided in the Supporting
Information. (b) Crystal data for 2a‚CH2Cl2: space group ) P21/n; a )
15.773(1) Å; b ) 8.2243(6) Å; c ) 20.994(2) Å; â ) 91.688(1)°; V )
2722.2(3) Å3; data/restraints/parameters ) 5548/0/289; GOF ) 1.019;
R1 ) 0.0328; wR2 ) 0.0874. Largest difference peak and hole ) 1.362
and -0.861 e Å-3. (c) Crystal data for 3b‚C4H8O: space group ) P-1;
a ) 9.4626(5) Å; b ) 12.2347(6) Å; c ) 12.9609(6) Å; R ) 77.6417(9)°;
â ) 73.6000(9)°; γ ) 82.7989(9)°; V ) 1402.7(1) Å3; data/restraints/
parameters ) 5714/0/334; GOF ) 1.047; R1 ) 0.0355; wR2 ) 0.0871.
Largest difference peak and hole ) 0.813 and -0.451 e Å-3. (d) Crystal
data for 4: space group ) P21/n; a ) 9.7054(6) Å; b ) 16.985(1) Å; c
) 14.5743(9) Å; â ) 109.392(1)°; V ) 2266.2(2) Å3; data/restraints/
parameters ) 4611/0/253; GOF ) 1.083; R1 ) 0.0329; wR2 ) 0.0861.
Largest difference peak and hole ) 1.335 and -0.694 e Å-3. (e) GC-
MS/GC-FID. (f) THF used to ensure complete dissolution of 3a.
(10) For selected examples see: (a) Cullen, W. R.; Einstein, F. W. B.; Huang,
C.-H.; Willis, A. C.; Yeh, E.-S. J. Am. Chem. Soc. 1980, 102, 988. (b)
Yang, H.; Lugan, N.; Mathieu, R. Organometallics 1997, 16, 2089. (c)
Dahlenburg, L.; Go¨tz, R. J. Organomet. Chem. 2001, 619, 88.
(11) For two crystallographically characterized (η5-indenyl)Rh(olefin)2 species,
see: Kakkar, A. K.; Jones, S. F.; Taylor, N. J.; Collins, S.; Marder, T. B.
J. Chem. Soc., Chem. Commun. 1989, 1454.
A head-to-head comparison of the cation, 3a, and its charge-
neutral analogue, 4, was also conducted in THF at 50 °C.9f After
22 h using 1 mol % 4, Et3SiH is consumed, and the silanes 5 and
6 are formed (∼82:18). In contrast, when 1 mol % 3a is employed
under these conditions, complete consumption of Et3SiH is not
observed, and the selectivity for DS over HS is greatly diminished
(5:6 ≈ 59:41). These preliminary data suggest that the zwitterion,
4, may provide reactivity advantages over related Rh(I) cations
under appropriate conditions. A more thorough survey of the
stoichiometric and catalytic reactivity of 4 and its derivatives is
currently underway and will be the subject of future reports.
In conclusion, a modular and high-yielding strategy for the
construction of new classes of cationic and zwitterionic Rh(I)
complexes has been introduced. This methodology provides access
to the first [κ2-P,N]Rh(I) zwitterion, 4, a unique complex that
mediates the cross-coupling of C-H and Si-H fragments. We
anticipate that the confluence of hemilabile and zwitterionic
(12) For crystallographically characterized salts containing “naked” indenide
anions, see: (a) Marder, T. B.; Williams, I. D. J. Chem. Soc., Chem.
Commun. 1987, 1478. (b) Boche, G.; Ledig, B.; Marsch, M.; Harms, K.
Acta Crystallogr. 2001, E57, m570.
(13) Takeuchi, R.; Yasue, H. Organometallics 1996, 15, 2098. Reactions
employing styrene (6 mmol), Et3SiH (2 mmol), [COD2Rh]+BF4 (0.02
-
mmol), PPh3 (0.04 mmol) in diethyl ketone (3 mL): 70 °C, 1 h, 92%
conversion (5:6 ≈ 87:13); 102 °C, 1 h, 92% conversion (5:6 ≈ 92:8).
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