Rhodium(0) metalloradicals in binuclear C-H activation

Article abstract of DOI:10.1021/ja909022p

(Chemical Equation Presented) A reactive rhodium(0) metalloradical capable of binuclear activation of an aromatic C-H bond of PPh₃ is disclosed. Kinetic measurements and density functional theory calculations reveal a binuclear mechanism: two metalloradicals add to a 'double bond' of the aromatic substrate while approaching the rate limiting C-H activation step (TS). Such aromatic C-H bond activation with Rh⁰ metalloradicals potentially produces kinetically labile Rh^I-aryl and RhI-H species, and thus, this could become a viable new approach to hydrocarbon functionalization.

Full text of DOI:10.1021/ja909022p

Published on Web 12/15/2009
Rhodium(0) Metalloradicals in Binuclear C-H Activation
Florian F. Puschmann,‡ Hansjo¨rg Gru¨tzmacher,*,‡ and Bas de Bruin*,†
Van’t Hoff Institute for Molecular Sciences (Homogeneous and Supramolecular Catalysis Group) and Department of
Chemistry, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and
Department of Chemistry and Applied Biology, ETH-Ho¨nggerberg, CH-8093 Zu¨rich, Switzerland
Received October 30, 2009; E-mail: gruetzmacher@inorg.chem.ethz.ch; b.debruin@uva.nl
Scheme 1. Binuclear Metalloradical C-H Bond Activation
The development of effective strategies for selective hydrocarbon
activation and functionalization continues to be one of the major
synthetic challenges. Generally applicable methods do not exist,
and each of the known methods has its limitations or disadvantages.1
Less conventional approaches are thus welcome. An interesting
one consists of the use of two metalloradicals, as depicted in
Scheme 1.
Scheme 2. Intermolecular C-H Bond Activation for 1L•/1M•
This concept has been scarcely explored. Wayland and co-
workers reported the use of porphyrinatorhodium(II) complexes,
[RhII(por)], which are capable of breaking C-H bonds of unacti-
vated substrates, even methane.2 These metalloradicals are proposed
to activate the substrates via a binuclear transition state formed in
a termolecular reaction in which one [RhII(por)] radical accepts the
hydrogen atom and another the activated carbon fragment (Scheme
1).2d–g These systems have a preference for breaking aliphatic C-H
bonds and are not reactive toward aromatic C-H bonds.2a–c
Although these are interesting from an academic and mechanistic
point of view, the thus-obtained “activated” hydrocarbons in the
form of organometallic [R-RhIII(por)] species lack reactivity
because of the strong Rh-C bond. Furthermore, the only available
site for further substrate binding is in the position trans to the
hydrocarbon fragment R. Similar approaches using MII(N-ligand)
metalloradicals (M ) Rh, Ir) with nonplanar N-ligands have not
led to useful C-H bond transformations either,3 as the MIII-con-
taining products are coordinatively saturated and kinetically inert.
On the contrary, the use of low-valent metalloradicals (e.g., Rh0 or
Ir0) in binuclear C-H activations should become synthetically more
useful because kinetically labile and thus reactive RhI-alkyl and
RhI-H species are the expected primary products. However, “true
rhodium(0) complexes” are rare, as most of them (on close
inspection) actually prove to be highly delocalized radicals or
“ligand radicals” having only a low spin density at the metal.4
Consequently, most of them are not reactive toward C-H bonds,
although there are some interesting (but poorly understood)
exceptions.5,6
Here we show that PPh3 dissociation from 1L•/1M• produces
the reactive species 15e, which reacts with the metalloradical 1M•
to activate an aromatic C-H bond.
THF solutions containing a mixture of the radicals 1L• and 1M•
(obtained cleanly upon one-electron reduction of the diamagnetic
[RhI(trop2PPh)(PPh3)]+ precursor) in the presence of 10 additional
equiv of PPh3 are fully stable for up to several days at room
temperature.7 In absence of additional PPh3, we observed quantita-
tive and selective conversion of the radical species to the diamag-
netic RhI complexes 2 and 3 in a 1:1 ratio within 10 h at room
temperature (Scheme 2).8 The benzo-1-rhoda-2-phosphacyclobutane
complex 2 is ortho-metalated at one of the aromatic rings of PPh3,
while the axial hydride complex 3 contains an unmodified PPh3
ligand.
We recently reported the first well-documented example of a
metal-centered Rh metalloradical: [Rh0(trop2PPh)(PPh3)] (1M•).7
This metalloradical exists as a clearly detectable species in rapid
equilibrium with its “ligand radical” electromeric form 1L• (i.e.,
1L• and 1M• are “redox isomers”).7
The X-ray structure of compound 2 is shown in Figure 1.9 The
structure of 2 is unique in the sense that all of the Rh complexes
with ortho-metalated R2P-Ph ligands reported to date in the
Cambride Structural Database (CSD) have higher oxidation states.10
Complex 2 is the first example that contains RhI.
1L• and 1M• differ structurally only in their Ptrop-Rh-PPPh3
angles (Scheme 2) but have very different electronic structures.
The spin density of the thermodynamically slightly preferred
electromer 1L• (Erel ) -2.0 kcal mol-1) is strongly delocalized,
and only ∼30% of the spin density is located at the metal. The
other electromer 1M• shows 86% of the spin density in the Rh-Ptrop
bond, with ∼60% at the metal. Therefore, this radical can be
regarded as a “Rh(0) complex”.
The reaction of 1L•/1M• to form 2 and 3 could proceed via
several possible mechanisms, so we decided to investigate the
mechanism of this reaction in detail. The kinetics of the formation
1
of 2 and 3 from the radicals 1L• and 1M• were measured by H
NMR spectroscopy. Experimental restrictions prevented us from
following the decay of 1L•/1M• directly in a reliable way. However,
1
the characteristic paramagnetically broadened H NMR signals of
1L•/1M• at 10.90, 9.85, and 4.73 ppm (as well as their characteristic
EPR signals) disappeared simultaneously with the appearance and
buildup of the H NMR signals of the diamagnetic products, so
† Universiteit van Amsterdam.
‡ ETH-Ho¨nggerberg.
1
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10.1021/ja909022p  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 73–75 73

C O M M U N I C A T I O N S
Figure 1. ORTEP plot (30% ellipsoid probability) showing the X-ray
structure of 2·1/2Et2O. The diethyl ether solvent molecule and hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Rh1-P1, 2.275(1); Rh1-P2, 2.382(1); Rh1-C38, 2.076(3); Rh1-C4,
2.185(3); Rh1-C5, 2.201(3); Rh1-C19, 2.171(3); Rh1-C20, 2.187(3);
C4dC5, 1.414(3); C19dC20, 1.424(4); Rh1-ct1, 2.076(3); Rh1-ct2,
2.059(3); C38-Rh1-P1, 178.9(1); C38-Rh1-P2, 67.2(1); P1-Rh-P2,
113.9(1); ct1-Rh1-ct2, 132.9(1); ∑∠(C-P1-C), 310.9(4); ∑∠(C-P2-C),
319.7(4).
Figure 3. Computed minimum-energy reaction pathway (MERP) for
formation of 2 and 3. Relative free energies (DFT, bp86, TZVP, T ) 298
K, P ) 1 bar, corrected for the condensed-phase reference volume) in kcal
mol-1. Selected bond lengths (Å) of the TS are shown in the inset.
(2) reaction of this 15-electron radical 15e with 1M• in the rate-
limiting H-transfer step to give rhodaheterocycle 2 and the
unsaturated hydride trans-[RhH(trop2PPh)] (16e-H) (k2; this is the
rate-limiting step); and (3) capture of free PPh3 by the latter to
give 3 (k3, k-3, and Keq3 ) k3/k-3). The experimentally observed
rate equation can be given as rate ) -d[1L•]/dt ) kobs[1L•]2, with
kobs ≈ k2Keq1[PPh3]-1
.
We performed additional density functional theory (DFT)
calculations to shed some light on the nature of the rate-limiting
step (see the Supporting Information for details), and the computed
reaction path (with full molecules 1L•, 1M•, 2, and 3) at T ) 298
K is shown in Figure 3.11 The transition-state search for the
hydrogen-transfer pathway was approached from the diamagnetic
products side, causing the transition state (TS) to be connected with
the diamagnetic high-energy intermediate A, but this species is
likely the result of a computational artifact.12 The potential energy
surface around A is flat, and its fragmentation into the radicals 1M•
and 15e is essentially barrierless. In that respect, the conversion of
1M• and 15e via TS to 2 and 16e-H should be considered as a
one-step process. Nonetheless, the addition of 15e across a double
bond of the PPh3 fragment coordinated to 1M• to produce species
A with two Rh(I) centers and disruption of the arene structure of
the bridging phenyl group as 1M• and 15e approach TS is strikingly
similar to the formation of MIII-CH2-CH2-MIII species through
radical-radical coupling of MII metalloradicals (M ) Rh, Ir), as
reported for [RhII(por)] and [IrII(Me3tpa)] species upon reaction with
ethene.13 The C-H activation step via TS (see the inset in Figure
3) can be described as a ꢀ-hydrogen elimination reaction from A,
producing species 2 and the 16-electron hydride species 16e-H.
Capture of PPh3 completes the overall exothermic (-13.2 kcal
mol-1) reaction sequence. The computed mechanism agrees very
well with the observed rate equation and conveniently explains the
experimental KIE [kH/kD ) 3.0 (exptl) and 3.1 (DFT)]. The
calculated activation parameters (∆Gq ) 20.8 kcal mol-1, ∆Hq )
8.9 kcal mol-1, ∆Sq ) -39.6 cal mol-1 K-1 with respect to 1L•)
are in good agreement with the experimental data (which must in
part be fortuitous), suggesting that the actual H-transfer process
might indeed proceed as shown in Figure 3.
Figure 2. (A) Second-order kinetic plot for the reaction of 1• to give species
2 and 3. (B) Inverse-first-order behavior of the reaction rate with respect to
the [PPh3] concentration.
the kinetics could be conveniently followed by monitoring the
appearance of species 2 and 3 with time ([1•]t ) 2[2]final - [2]t )
2[3]final - [3]t), providing the following mechanistic information:
(a) The rate of decay of 1• is proportional to [1•]2, with a second-
order rate constant kobs ) 0.0067 M-1 s-1 at 293 K (Figure 2A).
(b) The lower relative decay rate of the radical
[Rh(trop2PPh)([D15]PPh3)] with the perdeuterated phosphane ligand
reveals a kinetic isotope effect (KIE) of kH/kD ) 3.0, thus indicating
rate-limiting C-H bond activation. This reaction selectively
produces D14-2 and D16-3, the latter containing a deuteride (Rh-D)
ligand. (c) When 0.25-2.0 molar equiv of PPh3 is added, the
reaction rate decreases and is inversely proportional to [PPh3]
(Figure 2B). (d) The activation parameters for the reaction 1• f 2
+ 3 were experimentally determined: ∆Gq ) 25.8 kcal mol-1, ∆Hq
) 12.6 kcal mol-1, and ∆Sq ) -45.1 cal mol-1 K-1 (the Eyring
plot is shown in Figure S2 in the Supporting Information). The
large negative activation entropy points to an ordered transition
state, consistent with the second-order kinetics with respect to 1•.
On the basis of these observations, we propose the mechanism
shown in Figure 3, which includes: (1) dissociation of PPh3 from
1L•/1M• to give [Rh(trop2PPh)] (15e) (k1, k-1, and Keq1 ) k1/k-1);
In conclusion, the results reported here indicate that the concerted
action of two Rh0 radicals with appropriate structures may very
well be of use in bond activation chemistry. This reactivity is
somewhat similar to bond activation processes observed for RhII
9
74 J. AM. CHEM. SOC. VOL. 132, NO. 1, 2010

C O M M U N I C A T I O N S
and IrII metalloradicals.14 However, while activation of CdC or
C-H bonds with MII radicals typically produces kinetically inert
MIII species, aromatic C-H bond activation with Rh0 metalloradi-
cals produces RhI-aryl and RhI-H species. The kinetic lability of
2 and 3 needs to be confirmed, but this is generally the case for
RhI species. Therefore, binuclear oxidative addition of C-H bonds
between two Rh0 radicals may create interesting opportunities for
follow-up reactivity. Current research is thus focused along these
lines, aiming at the detection and use of 15-electron low-valent
Rh0 radicals for C-H functionalization.
(5) (a) Sofranko, J. A.; Eisenberg, R.; Kampmeier, J. A. J. Am. Chem. Soc.
1980, 102, 1163. (b) Pilloni, G.; Zotti, G.; Zecchini, S. J. Organomet. Chem.
1986, 317, 357. (c) Mueller, K. T.; Kunin, A. J.; Greiner, S.; Henderson,
T.; Kreilick, R. W.; Eisenberg, R. J. Am. Chem. Soc. 1987, 109, 6313.
(6) Abstraction of “hydrogen atoms” from MeOH (fast) and CH3CN (slow)
reported for [Rh0(dppe)2] proceeds via disproportionation of Rh0 to Rh+I
and Rh-I followed by protonation of Rh-I to give Rh+I -H: Kunin, A. J.;
Nanni, E. J.; Eisenberg, R. Inorg. Chem. 1985, 24, 1852.
(7) Puschmann, F. F.; Harmer, J.; Stein, D.; Ru¨egger, H.; de Bruin, B.;
Gru¨tzmacher, H.; Angew. Chem., Int. Ed. [Online early access]. DOI:
10.1002/anie.200903201. Published Online: Dec 2, 2009.
(8) A small amount (5-10%) of the equatorial hydride [Rh(Heq)(trop2PPh)
(PPh3)ax] (3′), probably resulting from a side reaction with traces of H2O,
was also detected (see the Supporting Information).
(9) Crystal structure of [Rh((C6H4)PPh2)(trop2PPh)]·1/2Et2O (2): Yellow single
crystals were obtained from slow evaporation of a diethyl ether solution
Acknowledgment. The work was supported by the Swiss
National Science Foundation (SNF), The Netherlands Organization
for Scientific ResearchsChemical Sciences (NWO-CW VIDI
Project 700.55.426), the European Research Council (Grant Agree-
ment 202886), the University of Amsterdam, and ETH Zu¨rich.
j
of 2; C56H46O0.5P2Rh; triclinic; space group P1; a ) 12.245(3) Å, b )
14.106(3) Å, c ) 15.104(3) Å, R ) 63.58(3)°, ꢀ ) 78.33(3)°, γ ) 66.59(3)°;
V ) 2143.1(8) Å3; Z ) 2; Fcalcd ) 1.382 g m-3; crystal dimensions 0.12 ×
0.10 × 0.05 mm3; Bruker SMART K1 diffractometer with CCD area
detector; Mo KR radiation (λ ) 0.71073 Å), 200 K, 2Θmax ) 56.63°; 10598
reflections, 8995 independent (Rint ) 0.0421); direct methods; empirical
absorption correction using SADABS version 2.03; refinement against full
matrix (vs F2) with SHELXTL version. 6.12 and SHELXL-97; 543
parameters; R1 ) 0.0392, wR2 (all data) ) 0.0857; max/min residual
electron density 0.647/-0.457 e Å-3. All non-hydrogen atoms except those
assigned to the disordered diethyl ether solvent molecule were refined
anisotropically. The contribution of the hydrogen atoms, in their calculated
positions, was included in the refinement using a riding model. The strongly
disordered diethyl ether solvent molecule was described by 6.6 carbon atoms
(C55-C61), corresponding to 39.6 electrons (one diethyl ether molecule
contains 42 electrons). The crystallographic data for 2 (excluding structure
factors) has been deposited with the Cambridge Crystallographic Data
Centre (CCDC) as supplementary publication no. CCDC-706732. Copies
of the data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. [fax: (+44) 1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk).
Supporting Information Available: Experimental and computa-
tional details, coordinates of optimized geometries (PDB, XYZ),
computed energies (XLS), and crystallographic data for 2 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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