Stable mononuclear organometallic Pd(III) complexes and their C-C bond formation reactivity

Article abstract of DOI:10.1021/ja103001g

Organometallic Pd(III) complexes have been implicated as intermediates in a number of catalytic and stoichiometric transformations. While a few dinuclear organometallic Pd(III) complexes have been characterized, no mononuclear organometallic Pd(III) complexes have been isolated to date. Reported herein is the synthesis and characterization of a series of Pd(III) complexes supported by the tetradentate ligand N,N′-di-tert-butyl-2,11-diaza[3.3](2,6) pyridinophane (N4). Chemical or electrochemical oxidation of the Pd(II) complexes (N4)Pd^II(R)(X) (R = Me, X = Cl: 1; R = Ph, X = Cl: 2; R = X = Me: 3) generates [(N4)Pd^IIIMeCl]⁺ (1⁺), [(N4)Pd^IIIPhCl]⁺ (2⁺), and [(N4)Pd ^IIIMe2]⁺ (3⁺). These stable Pd(III) complexes were isolated and characterized by X-ray diffraction, cyclic voltammetry, UV-vis, EPR, magnetic moment measurements, and DFT to confirm the presence of paramagnetic d⁷ Pd(III) centers. Moreover, these Pd(III) complexes undergo light-induced C-C bond formation to give the corresponding homocoupled products ethane or biphenyl. Particularly remarkable is the observation for the first time of ethane formation from monomethyl Pd complexes.

Full text of DOI:10.1021/ja103001g

Published on Web 05/12/2010
Stable Mononuclear Organometallic Pd(III) Complexes and Their C-C Bond
Formation Reactivity
Julia R. Khusnutdinova,† Nigam P. Rath,‡ and Liviu M. Mirica*,†
Department of Chemistry, Washington UniVersity, One Brookings DriVe, St. Louis, Missouri 63130-4899 and
Department of Chemistry and Biochemistry, UniVersity of MissourisSt. Louis, One UniVersity BouleVard,
St. Louis, Missouri 63121-4400
Received April 15, 2010; E-mail: mirica@wustl.edu
Scheme 1. Synthesis of (N4)PdIII Complexes
Palladium is one of the most catalytically versatile transition
metals, its complexes being efficient catalysts for a wide range of
C-C coupling, C-H functionalization, and hydrocarbon oxidation
reactions.1 While Pd(0), Pd(II),1b and Pd(IV)2 complexes have been
extensively characterized, much less is known about organometallic
Pd(III) complexes. Ritter et al. and Sanford et al. have proposed
recently that dinuclear Pd(III) complexes are the active intermediates
in the oxidative functionalization of C-H bonds.3,4 Moreover,
mononuclear Pd(III) complexes have been proposed as transient
intermediates in oxidatively induced reductive elimination of ethane
from a PdIIMe2 complex,5 the insertion of dioxygen into a Pd-Me
bond,6 and Kumada coupling.7
While a few dinuclear organometallic Pd(III) complexes have
been structurally characterized,3,8 no mononuclear organometallic
Pd(III) complexes have been isolated to date. The only two
While electrochemical oxidation of 3 did not give the corresponding
Pd(III) product, chemical oxidation with 1 equiv of thianthrenyl
examples of mononuclear Pd(III) coordination compounds are the
hexafluoroantimonate (Th•+SbF6-) or ferrocenium hexafluorophos-
homoleptic 1:2 adducts with the tridentate macrocyclic ligands
phate (Fc+PF6-) yields a stable product identified as [(N4)PdIIIMe2]+
triazacyclononane and trithiacyclononane, respectively.9,10 Con-
(3+, Scheme 1).12 Compounds [1+]PF6 and [3+]ClO4 are to the best
sidering the preferred distorted octahedral geometry for Pd(III)
complexes,9 we proposed that the tetradentate ligand N,N′-di-tert-
of our knowledge the first isolated group 10 metal(III)-methyl
complexes reported to date.
butyl-2,11-diaza[3.3](2,6)pyridinophane (N4) can sufficiently sta-
bilize a Pd(III) species, while leaving two coordination sites
available for various exogenous ligands. Reported herein is the
isolation and characterization of a series of stable mononuclear
organometallic (N4)PdIII complexes. Interestingly, the monomethyl
and monophenyl Pd(III) complexes 1+ and 2+ undergo a remarkable
light-induced elimination of the homocoupled products ethane and
biphenyl, respectively, the observed formation of ethane from
monomethyl Pd complexes being unprecedented.
The Pd(II) precursors (N4)PdII(R)(X) (R ) Me, X ) Cl: 1; R )
Ph, X ) Cl: 2; R ) X ) Me: 3) were synthesized through the
reaction of N4 with common Pd starting materials.11,12 Cyclic
Figure 1. ORTEP representation (50% probability ellipsoids) of 1+ (left),
2+ (middle), and 3+ (right). Selected bond distances (Å), 1+: Pd1-C1
2.0921(18), Pd1-Cl1 2.3403(5), Pd1-N1 2.0966(16), Pd1-N2 2.0204(16),
Pd1-N3 2.4271(16), Pd1-N4 2.4265(16); 2+: Pd1-C(Ph) 2.0713(13),
Pd1-Cl1 2.3478(4), Pd1-N1 2.0927(11), Pd1-N2 2.0281(11), Pd1-N3
2.4241(12), Pd1-N4 2.4135(12); 3+: Pd1-C1 2.0739(15), Pd1-C2
2.0476(16), Pd1-N1 2.0931(13), Pd1-N2 2.1099(12), Pd1-N3 2.4844(13),
Pd1-N4 2.4589(13).
voltammetry of the Pd(II) complexes in MeCN, THF, or CH2Cl2
n
n
(0.1 M Bu4NClO4 or Bu4NPF6) reveals two oxidation waves for
both 1 and 2, an oxidation peak at 150-370 mV, and a quasire-
versible wave at 580-710 mV vs Fc+/Fc.13 Similarly, complex 3
shows two oxidation waves at more negative potentials, -326 mV
and 64 mV vs Fc+/Fc.12 Since the N4 ligand is not redox active
within this potential range, the two oxidations are assigned to Pd(II)/
Pd(III) and Pd(III)/Pd(IV) couples, respectively.14
X-ray quality crystals for [1+]PF6, [2+]ClO4, and [3+]ClO4 were
obtained from MeCN/Et2O solutions and all three structures show
a tetragonally distorted octahedral geometry at the metal center, as
expected for a Jahn-Teller distorted d7 Pd(III) center (Figure 1).9
The axial Pd-amine nitrogen bond distances (2.41-2.48 Å) are
elongated compared to the equatorial Pd-pyridyl bond distances
(2.02-2.11 Å) and also longer than the axial Pd-N bonds in the
[Pd(tacn)2]3+ complex (Pd-Nax ) 2.180 Å).9a
Controlled potential electrolysis (CPE) of 1 and 2 at potentials
that are ∼200 mV higher than the corresponding first anodic wave
were performed to generate intensely colored dark-green complexes
1+ and 2+ that can be isolated in good yields (Scheme 1). These
complexes are indefinitely stable in the solid state at -20 °C and
are stable for a few weeks in solution at RT in the absence of light.12
Complexes 1+, 2+, and 3+ are all paramagnetic and show a
magnetic moment of 1.61-1.80 µB at RT,12 corresponding to one
† Washington University.
‡ University of MissourisSt. Louis.
9
10.1021/ja103001g  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 7303–7305 7303

C O M M U N I C A T I O N S
Table 1. Spectroscopic Properties of PdIII Complexes
Scheme 2. Reactivity of 1+ (4 is [(N4)PdIICl(MeCN)]+)
c
EPRb
gx, gy, gz
µeff
II/III
EpcIII/II, Epa
,
UV-vis (MeCN)
E1/2III/IV(∆Ep), mVa
λ, nm (ꢀ, M-1 cm-1
)
(µB)
1+
2+
3+
-426, 150,
723 (1100), 545 (sh, 490),
368 (3300), 263 (12 000)
2.183,
2.137,
2.059
2.204,
2.123,
2.026
2.175,
2.160,
2.064
1.80d
585 (68)
-229, 368,
708 (65)
732 (1100), 386 (2500),
258 (20 000)
1.65d
1.72e
-882, -326,
64 (62)
741 (360), 350 (2300),
263 (10 300)
1.61e
a Measured vs Fc+/Fc by CV in 0.1 M Bu4NPF6/MeCN, scan rate
100 mV/s; ∆Ep is the peak potential separation for the Pd(III)/Pd(IV)
couple (ref 12). b MeCN glass, 120 K. c Evans method, 298 K. d In
CD3CN. e In DMSO-d6.
Scheme 3. Additional Reactivity of PdIII Complexes
Scheme 4. Proposed Mechanisms for Ethane Formation from 1+
Figure 2. (a) EPR spectra of 1+, 2+, and 3+ (see Table 1 for g values). (b)
DFT (B3LYP/CEP-31G(d)) calculated SOMO of 1+.
unpaired electron (Table 1). The EPR spectra (120 K, MeCN glass)
exhibit rhombic signals with different anisotropies for the three
complexes and gave ) 2.118-2.133, suggesting a Pd(III) center with
a dz ground state (Table 1 and Figure 2a).12,15 Such an assignment
2
is supported by the DFT-calculated singly occupied molecular
Formation of ethane from 1+ is suppressed completely in the
presence of effective alkyl radical scavengers such as TEMPO
(forming Me-TEMPO quantitatively)25 and O2 (forming MeOH,
formaldehyde, and formic acid),26 which suggests a homolytic
cleavage of the Pd(III)-Me bond (Scheme 2, b-c).27 Deuterium
abstraction from CDCl3 to give CH3D in 21 ( 1% yield further
supports formation of Me• (Scheme 2, d).28 Ethane may thus form
by a subsequent fast reaction of Me• with another molecule of 1+
(Scheme 4, path A).29 Similar fast bimolecular reactions (k
∼107-108 M-1 s-1) of Me• with Co(III)-Me and Ni(III)-Me
complexes to yield ethane were reported previously and proposed
to involve a direct attack of a Me radical at the carbon atom of the
methyl ligand.30 An alternate mechanism may involve binding of
Me• to the Pd(III) center to afford an [(η3-N4)PdIVMe2Cl]+
intermediate that can undergo reductive elimination to form
ethane.31 Interestingly, while photolysis of 1+ in the presence of
the H• donor 1,4-cyclohexadiene (CHD) generates methane in
appreciable yields (Scheme 2, e),32 the yield of ethane (22 ( 1%)
is only slightly decreased vs the uninhibited reaction (25 ( 1%),
suggesting that either another mechanism is operative or CHD is
not an efficient alkyl radical trap under these conditions.33
+
2
orbital (SOMO) of 1 , which reveals its metal-based dz character
(Figure 2b).16 Additionally, the UV-vis spectra of 1+, 2+, and 3+
in MeCN show two strong absorption bands at 723-741 nm and
350-386 nm that are assigned tentatively as LMCT bands (Table
1).17 Overall, the spectroscopic and magnetic properties are
consistent with the formation of paramagnetic Pd(III) species for
1+, 2+, and 3+.
The isolation of these organometallic Pd(III) complexes allowed
the investigation of their reactivity, in light of the recently proposed
role of transient Pd(III) intermediates in reductive elimination
reactions.5 Complexes 1+, 2+, and 3+ are stable in MeCN solutions
in the absence of light,18 yet when a solution of 1+ in MeCN is
exposed to light19 elimination of ethane (25 ( 1% yield), methyl
chloride (8 ( 1%), and methane (9 ( 2%) occurs along with
formation of [(N4)PdIICl(MeCN)]+ (4) in 80 ( 4% yield (Scheme
2, a).12,20 While the reductive elimination of the biaryl products
from Pd-aryl complexes is precedented,21 formation of ethane from
monomethyl Pd complexes has not been reported to date.22 Such
reactivity is currently of great interest due to its relevance to
catalytic methane oligomerization.5,23
A similar reactivity is observed for the monophenyl complex
2+ that undergoes photoinduced elimination of biphenyl and 4
(Scheme 3, I), while irradiation of an equimolar mixture of 1+ and
2+ gives toluene in 19 ( 2% yield along with ethane, methyl
chloride, biphenyl, and 4, suggestive of crossover reactivity (Scheme
3, II).24 Additionally, irradiation of the dimethyl complex 3+
generates ethane and [(N4)PdIIMe(MeCN)]+ in 41 ( 4% and 83
( 8% yield, respectively (Scheme 3, III),20a a reactivity similar to
that observed recently during the oxidation of (tBu2bipy)PdIIMe2.5
A nonradical pathway to form [(N4)PdIVMe2Cl]+ via a methyl
group transfer between two molecules of 1+ cannot be excluded
unambigously (Scheme 4, path B).34 A similar nonradical
disproportionation mechanism was proposed by Sanford and
Mayer et al. and Tilset et al. for the reactivity of dimethyl
complexes observed during 1e- chemical or electrochemical
oxidation of (tBu2bipy)PdIIMe2 or (diimine)PtIIMe2, respectively.5,35
Although the [(N4)PdIVMe2Cl]+ intermediate was not detected
during photolysis at 0 °C, 1 reacts with strong 2e- oxidants such
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C O M M U N I C A T I O N S
A.; Khairoun, S.; Dance, J. M.; Hagenmuller, P. Z. Anorg. Allg. Chem.
1984, 517, 43.
as PhI(OAc)2 and Bz2O2 to yield MeCl and methyl esters,
suggesting that the Pd(IV) oxidation state is accessible and
undergoes reductive elimination.36 Additionally, 3 readily reacts
with MeI to yield ethane in 20% yield and (N4)PdIIMeI.37
Overall, our radical scavenging studies suggest that homolysis
of a Pd(III)-C bond is involved in the observed photoreactivity of
the Pd(III) complexes. Formation of ethane may proceed via the
initial homolytic cleavage of the Pd(III)-Me bond, although
nonradical pathways cannot be excluded.38 While the observed
reactivity for the Pd(III) complexes requires the presence of light,
suggestive of a radical mechanism, the photoactivation of octahedral
Pd(III) complexes may lead to dissociation of an amine donor and
creation of an empty coordination site required for a subsequent
Me or Ph group transfer.39
(11) Meneghetti, S. P.; Lutz, P. J.; Kress, J. Organometallics 2001, 20, 5050.
(12) See Supporting Information.
(13) Coulometry measurements confirm that both oxidations correspond to one
electron processes.
(14) The Pd(III)/Pd(II) reduction wave is observed at potentials ∼500 mV lower
than the corresponding Pd(II)/Pd(III) oxidation wave. The large separation
between the anodic and cathodic waves of the Pd(III)/Pd(II) couple in 1,
2, and 3 is attributed to a significant ligand conformational change from
the syn chair-chair conformation (η2) in Pd(II) complexes (ref 11) to a
syn boat-boat conformation (η4) needed to stabilize the distorted octahedral
geometry of Pd(III).
(15) Spin integration of the EPR spectra vs a Cu standard confirms the formation
of the Pd(III) species in more than 95% yield.
(16) Similar SOMOs were calculated for 2+ and 3+, using the B3LYP/CEP-
31G(d) functional/basis set combination (see Supporting Information).
(17) Detailed DFT and TD-DFT computational studies are ongoing.
(18) Less than 1% of MeCl and no ethane were observed when 1+ was heated
in the dark at 51 °C for 65 h.
(19) Samples were irradiated with either two 100 W halogen lamps or a 450 W
medium pressure mercury lamp.
In summary, we have been able to isolate and characterize a
series of mononuclear organometallic Pd(III) complexes that exhibit
interesting reactivity profiles. The stability of these Pd(III) com-
plexes vs Pd(II) or Pd(IV) species is presumably due to the steric
properties of the N4 ligand. While the axial nitrogens of this
tetradentate ligand can coordinate and stabilize the Jahn-Teller
distorted Pd(III) center vs a square planar Pd(II) center,9 the rigidity
of the macrocycle cannot accommodate a more symmetric octa-
hedral geometry preferred by a d6 Pd(IV) center.40 Moreover, the
systems described herein can allow for a direct investigation of
Pd(III) chemistry by taking advantage of their light-triggered
reactivity. Particularly remarkable is the obserVation for the first
time of ethane formation from monomethyl Pd complexes. This
transformation has direct implications in the development of
catalysts for oxidative oligomerization of methane in particular5,23
and oxidatively induced Pd-catalyzed C-C bond formation reac-
tions in general.2 Our current research efforts are aimed at
understanding in more detail the properties and reactivity of these
Pd(III) complexes.
(20) (a) The theoretical yield of ethane is 50% (ref 12). (b) Complex 4 was
independently synthesized and characterized by NMR, ESI-MS, and
elemental analysis (ref 12). (c) A small amount of the protonated ligand
[N4*H]+ (5 ( 1%) was also formed.
(21) (a) Yagyu, T.; Hamada, M.; Osakada, K.; Yamamoto, T. Organometallics
2001, 20, 1087. (b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc.
2009, 131, 918. (c) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger,
J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884.
(22) Remy, M. S.; Cundari, T. R.; Sanford, M. S. Organometallics 2010, 29,
1522.
(23) (a) Martin, G. A.; Mirodatos, C. Fuel Process. Technol. 1995, 42, 179. (b)
Stoukides, M. Res. Chem. Intermed. 2006, 32, 187. (c) Olah, G. A. Acc.
Chem. Res. 1987, 20, 422.
(24) (a) Perkins, D. C. L.; Puddephatt, R. J.; Tipper, C. F. H. J. Organomet.
Chem. 1979, 166, 261. (b) Hill, R. H.; Puddephatt, R. J. Organometallics
1983, 2, 1472.
(25) Kim, J. S.; Sen, A.; Guzei, I. A.; Siable-Sand, L. M.; Rheingold, A. L.
J. Chem. Soc., Dalton Trans. 2002, 4726.
(26) Aerobic homolysis of Co-Me in methylcobalamin gives oxygenated
products arising from MeOO reactivity, whereas anaerobic photolysis yields
ethane and methane. (a) Hogenkamp, H. P. C. Biochemistry 1966, 5, 417.
(b) Schrauzer, G. N.; Sibert, J. W.; Windgassen, R. J. J. Am. Chem. Soc.
1968, 90, 6681.
(27) TEMPO and O2 were reported to be efficient radical traps with reaction
rates of ∼108-109 M-1 s-1. (a) Van Leeuwen, P. W. N. M.; et al. J.
Organomet. Chem. 1981, 209, 169. (b) Bowry, V. W.; Ingold, K. U. J. Am.
Chem. Soc. 1992, 114, 4992. (c) Thomas, J. K. J. Phys. Chem. 1967, 71,
1919. (d) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data
1996, 25, 709.
(28) (a) Van Leeuwen, P. W. N. M.; Roobeek, C. F.; Huis, R. J. Organomet.
Chem. 1977, 142, 233. (b) Hux, J. E.; Puddephatt, R. J. J. Organomet.
Chem. 1992, 437, 251.
(29) Dimerization of two Me radicals to yield ethane is unlikely given the
appreciable yields of ethane formed even in the presence of H-donors (ref
5).
Acknowledgment. We thank Prof. Tien-Sung Tom Lin for his
help with the EPR experiments and Sophia White for GC/MS
analysis. We thank the Department of Chemistry at Washington
University for startup funds and American Chemical Society
Petroleum Research Fund (49914-DNI3) for support.
(30) (a) Shaham, N.; Masarwa, A.; Matana, Y.; Cohen, H.; Meyerstein, D. Eur.
J. Inorg. Chem. 2002, 87. (b) Sauer, A.; Cohen, H.; Meyerstein, D. Inorg.
Chem. 1988, 27, 4578.
(31) The fast reactions of alkyl radicals with transition metal complexes (k
∼107-108 M-1 s-1) have been extensively studied. (a) Kochi, J. K. Acc.
Chem. Res. 1974, 7, 351. (b) Espenson, J. H. Acc. Chem. Res. 1992, 25,
222. (c) Masarwa, A.; Meyerstein, D. AdV. Inorg. Chem. 2004, 55, 271,
and ref 30.
Supporting Information Available: Experimental details, synthesis
of Pd complexes, spectroscopic characterization, reactivity studies,
computational details, and X-ray crystallographic data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(32) A higher yield of [N4*H]+ and a black precipitate were also observed,
suggesting a subsequent reductive reaction at a Pd(II) center.
(33) The rate constants of the reaction of CHD with alkyl radicals are ∼105
M-1 s-1(Hawari, J. A.; Engel, P. S.; Griller, D. Int. J. Chem. Kinet. 1985,
17, 1215), slower than the reaction of methyl radical with transition metal
complexes to yield ethane (∼107-108 M-1 s-1, ref 30).
(34) An alternate mechanism involving dinuclear complexes (ref 3) is unlikely
due to steric hindrance imposed by the bulky N4 ligand.
(35) Johansson, L.; Ryan, O. B.; Romming, C.; Tilset, M. Organometallics 1998,
17, 3957.
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