Bond activation, substrate addition and catalysis by an isolable two-coordinate Pd(0) bis-isocyanide monomer

Article abstract of DOI:10.1021/ja905338x

(Chemical Equation Presented) Mg metal reduction of the divalent precursor PdCl₂(CNAr^Dipp2)₂ (Dipp = 2,6- diisopropylphenyl) provides the isolable, two-coordinate Pd(0) bis-isocyanide, Pd(CNAr^Dipp2)₂,

Full text of DOI:10.1021/ja905338x

Published on Web 07/24/2009
Bond Activation, Substrate Addition and Catalysis by an Isolable
Two-Coordinate Pd(0) Bis-Isocyanide Monomer
Liezel A. Labios, Matthew D. Millard, Arnold L. Rheingold, and Joshua S. Figueroa*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe,
Mail Code 0358, La Jolla, California 92093-0358
Received June 30, 2009; E-mail: jsfig@ucsd.edu
In analogy to binary Pd(0) carbonyls,1 monomeric homoleptic
isocyanide complexes of Pd(0) have remained elusive species.
Indeed, when studied in conjunction with isocyanides such as
CNXyl, CNt-Bu, and CNCy (Cy ) cyclohexyl), [Pd(CNR)n] species
are observed invariably to aggregate into higher nuclearity clusters.2
With respect to purported bis-isocyanide “[Pd(CNR)2]” species,
early preparations3 did not conclusively establish their monomeric
nature, and subsequent reports4 strongly favored the trimeric
formulation [Pd3(CNR)6]. These latter studies culminated in Francis’
structural determination of triangulo-[Pd(µ2-CNCy)(CNCy)]3, which
was the first binary Pd(0) isocyanide complex to be definitively
characterized.5 Presumably, the proclivity of unencumbered iso-
cyanides to bridge metal centers facilitates the aggregation of these
reduced Pd species. Accordingly, herein we report that the
encumbering m-terphenyl isocyanide, CNArDipp2 (Dipp ) 2,6-
(i-Pr)2C6H3), can successfully stabilize the highly reactive two-
coordinate bis-isocyanide monomer Pd(CNArDipp2)2. Because of the
functionality, lending credence to its zerovalent formulation.
Crystallographic characterization of Pd(CNArDipp2)2 revealed a two-
coordinate monomer which diverges slightly from an ideal linear
geometry (∠(C1-Pd-C2) ) 169.8(2)°, Figure 1a). Isocyanide
bending is observed for one CNArDipp2 ligand (∠C1-N1-C3 )
163.6(4)°), while the other remains comparatively unperturbed
(∠C2-N2-C4 ) 174.1(4)°). Whereas this lack of bending may
be a reflection of only moderate π-back-donation to the isocyanide
ligands, it is important to note that Pd(CNArDipp2)2 gives rise to
νCN stretches (2073 and 2011 cm-1, KBr), that are considerably
lower in energy than found for divalent PdCl2(CNArDipp2)2 (νCN
)
2202 cm-1, KBr). Furthermore, Pd(CNArDipp2)2 exhibits average
Pd-Ciso bond distances which are shorter relative to those in
PdCl2(CNArDipp2)2 (1.930(3) Å av vs 1.976(2) Å av, respectively).
These structural data are consistent with appreciable π back-
donation in Pd(CNArDipp2)2, as zerovalent centers may be reasonably
expected to exhibit longer M-L bond distances than their divalent
counterparts when only σ-donor ligands are present. Significant π
back-donation in Pd(CNArDipp2)2 is also indicated by DFT calcula-
tions, which clearly reveal two orthogonal π-back-bonding interac-
tions (see the Supporting Information).
strong π-acidic nature of the isocyanide function, Pd(CNArDipp2
)
2
serves as an intriguing counterpoint to two-coordinate Pd0L2
complexes featuring strongly σ-donating phosphine6 (PR3) or
NHC7,8 ligands.
Access to orange Pd(CNArDipp2)2 was achieved by Mg0 reduction
of the dichloride PdCl2(CNArDipp2)2 in a 4:1 Et2O/THF mixture.
Generation of Pd(CNArDipp2)2 by straightforward reduction of a
divalent precursor is notable in that similar protocols have been
reported to yield exclusively trimeric [Pd(µ2-CNR)(CNR)]3
The encumbering ArDipp2 units provide Pd(CNArDipp2)2 with a
substantial degree of thermal and kinetic stability in solution. As
indicated by 1H NMR spectroscopy, Pd(CNArDipp2
) does not
2
decompose in C6D6 when heated to 80 °C for up to 5 d.
Furthermore, while the CNArDipp2 ligands effectively stabilize a
monomeric Pd(0) complex, they also enforce a homoleptic bis-
species.4d Both the H NMR (C6D6) and FTIR (KBr) spectra of
1
Pd(CNArDipp2)2 are devoid of features characteristic of a hydride
isocyanide formulation. Thus, as assayed by both H NMR and
1
Figure 1. (A) Reaction pinwheel for Pd(CNArDipp2
)
and molecular structures of [TlPd(CNArDipp2)2]OTf (left), Pd(CNArDipp2 (center), and Pd(κ1-N-
)
2
2
PhNO)2(CNArDipp2)2 (right). (B) HOMO, LUMO, and qualitative MO diagram for Pd(κ1-N-PhNO)2(CNArPh2)2 based on restricted S ) 0 DFT calculations.
9
11318 J. AM. CHEM. SOC. 2009, 131, 11318–11319
10.1021/ja905338x CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
FTIR spectroscopies, addition of another equivalent of CNArDipp2
to Pd(CNArDipp2)2 in C6D6 results in rapid isocyanide exchange
rather than formation of a tris-isocyanide species. Variable tem-
perature studies in toluene-d8 indicate that isocyanide exchange
It is tempting to suggest that ligation to Pd(CNArDipp2)2 results in a
one-electron reduction of each κ1-N-PhNO unit to its O-centered
nitroxyl radical. Coupled with the observed diamagnetism of
Pd(κ1-N-PhNO)2(CNArDipp2)2, such a valence bond picture suggests
that a singlet diradical form18 may be a significant resonance
contribution to its electronic structure. However, an alternative, MO
description featuring a (σ)4(π)4(π*)2 singlet ground state with nonde-
generate π* components (ag and au in Ci symmetry) may also accurately
describe the electronic structure of the NO units in Pd(κ1-N-
PhNO)2(CNArDipp2)2. Indeed, restricted DFT calculations on the S )
0 state of the model Pd(κ1-N-PhNO)2(CNArPh2)2 correspond well with
this latter view (Figure 1b). Notably, both foregoing bonding descrip-
tions correspond to a formal NO bond order of 1.5 for each κ1-N-
PhNO ligand, which to our knowledge is unprecedented in the
coordination chemistry of nitroso compounds.16 Accordingly, detailed
investigations into Pd(κ1-N-PhNO)2(CNArDipp2)2 and the chemistry
accessible to zerovalent Pd(CNArDipp2)2 are in progress.
1
remains fast on the H NMR time scale down to -80 °C.
In accord with its reduced nature, Pd(CNArDipp2)2 is competent
for the oxidative addition of σ-bonds. For instance, Pd(CNArDipp2
)
2
readily forms the benzyl chlorido complex PdCl(Bz)(CNArDipp2
)
2
upon reaction with PhCH2Cl. Similarly, Pd(CNArDipp2)2 also adds
across the carbon-bromine bond of mesityl bromide (MesBr) to
generate PdBr(Mes)(CNArDipp2)2 (Figure 1a). Remarkably, despite
the additional presence of the encumbering Mes substituent,
PdBr(Mes)(CNArDipp2)2 retains its integrity in C6D6 solution at 80
°C for several days. Such behavior is notable since LnM(R)(CNR′)
species, especially those featuring sterically congested coordination
environments, are well-known to form iminoacyl complexes (i.e.,
LnM(C(dNR′)R)) via migratory insertion.9
The resistance of PdBr(Mes)(CNArDipp2)2 toward migratory insertion
processes suggested that a CNArDipp2-supported Pd system may effect
Suzuki-Miyaura C-C bond formation.10 Indeed, Pd(0) complexes
of the type Pd(PR3)2 and Pd(NHC)2 are well-known to be chemically
competent for catalytic Caryl-Caryl and Caryl-N bond coupling.6,8,11
However, π-acidic ligands have received limited attention as ancillary
groups in Pd-based cross-coupling chemistry. This is surprising given
that electron-rich, monoligated Pd0L species are proposed6 as the
catalytically active protagonists in cross-coupling schemes and may
be further stabilized by a π-acidic ligand. Accordingly, in preliminary
unoptimized screens, 5 mol % Pd(CNArDipp2)2 was found to readily
cross-couple MesBr with phenyl boronic acid (PhB(OH)2) in 94%
isolated yield in THF solution at room-temperature. Furthermore, the
less hindered substrate, 2-MeC6H4Br, is similarly coupled with
PhB(OH)2 in 95% isolated yield.
Acknowledgment. We are grateful to UCSD and the Camille
and Henry Dreyfus Foundation for support. Profs. Karl Wieghardt
and Peter T. Wolczanski are thanked for stimulating discussions.
Supporting Information Available: Synthetic procedures, results
of DFT, NMR, FTIR and crystallographic studies (PDF and CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Tripathi, S. C.; Srivastava, S. C.; Mani, R. P.; Shrimal, A. K. Inorg. Chim.
Acta 1976, 17, 257–290.
(2) Yamamoto, Y. Coord. Chem. ReV. 1980, 32, 193–233.
(3) (a) Malatesta, L. J. Chem. Soc. 1955, 3924–3926. (b) Fischer, E. O.; Werner,
H. Chem. Ber. 1962, 95, 703–708. (c) Otsuka, S.; Nakamura, A.; Tatsuno,
Y. J. Am. Chem. Soc. 1969, 91, 6994–6999.
(4) (a) Day, V. M.; Day, R. O.; Kristoff, J. S.; Hirsekorn, F. J.; Muetterties,
E. L. J. Am. Chem. Soc. 1975, 97, 2571–2573. (b) Thomas, M. G.; Pretzer,
W. R.; Beier, B. F.; Hirsekorn, F. J.; Muetterties, E. L. J. Am. Chem. Soc.
1977, 99, 743–748. (c) Christofides, A. J. Organomet. Chem. 1983, 259,
355–365. (d) Yamamoto, Y.; Yamazaki, H. J. Chem. Soc., Dalton Trans.
1989, 2161–2166.
(5) Francis, C. G.; Khan, S. I.; Morton, P. M. Inorg. Chem. 1984, 23, 3680–3681.
(6) For examples of structurally characterized Pd(PR3)2 complexes, see: Barder,
T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc.
2005, 127, 4685–4696. and references therein.
(7) NHC ligands are recognized as primarily strong σ-donors with weak and
variable π-acidity properties; see: (a) Kelly, R. A. III.; Clavier, H.; Giudice,
S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, B. I.; Hoff, C. D.;
Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202–210. (b) Nemcsok,
D.; Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640–3646.
(8) For examples of structurally characterized Pd(NHC)2 complexes, see: (a)
Arnold, P. L.; Cloke, F. G. N.; Geldbach, T.; Hitchcock, P. B. Organo-
metallics 1999, 18, 3228–3233. (b) Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.;
Weskamp, T.; Herrmann, W. A. J. Organomet. Chem. 2000, 595, 186–
190. (c) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. Angew.
Chem., Int. Ed. 2003, 42, 3690–3693.
(9) Durfee, L. D.; Rothwell, I. P. Chem. ReV. 1988, 88, 1059–1079.
(10) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483.
(11) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G. R. J.
Angew. Chem., Int. Ed. 1995, 34, 2371–2374. (b) Herrmann, W. A.;
Reisinger, C.-P.; Spiegler, M. J. Organomet. Chem. 1998, 557, 93–96.
(12) For a Pd(0)-Tl(I) interaction featuring two-coordinate Tl, see: Catalano,
V. J.; Bennett, B. L.; Yson, R. L.; Noll, B. C. J. Am. Chem. Soc. 2000,
122, 10056–10062.
The low-coordinate, electron-rich nature of Pd(CNArDipp2
)
2
renders it active toward Lewis acidic substrates. Thus, treatment
of Pd(CNArDipp2)2 with TlOTf forms the Lewis acid-base adduct
[TlPd(CNArDipp2)2]OTf, which contains a one-coordinate Tl(I) center
directly bound to Pd (Figure 1a).12 Interestingly, Tl(I) acetate is
known13 to accelerate Pd-catalyzed C-C bond formation, and
further investigations of [TlPd(CNArDipp2)2]OTf in conjunction with
the coupling chemistry outlined above may potentially elucidate
the elementary steps governing this process.
Bis-isocyanide Pd(CNArDipp2)2 also reacts smoothly with elec-
tronically unsaturated substrates. Addition of 1 equiv of dioxygen
to Pd(CNArDipp2
) proceeds smoothly to the peroxo complex
2
(η2-O2)Pd(CNArDipp2)2, which serves as a structurally characterized
complement to (O2)Pd(CNt-Bu)2 prepared by Otsuka (Figures 1a
and S4.6).3c Most remarkably however, Pd(CNArDipp2)2 reacts with
2 equiv of nitrosobenzene (PhNO) to form the dark red, diamagnetic
complex Pd(κ1-N-PhNO)2(CNArDipp2)2. Structural characterization
of the latter revealed a distinctly square planar coordination
geometry about Pd, thus strongly indicating the presence of a
divalent metal center (Figure 1a). Metrical parameters supporting
this claim include a d(Pd-Ciso) of 2.004(2) Å,14 which is markedly
longer than those of Pd(CNArDipp2)2, and near linear Ciso-N-Cipso
angles (174.6(2)°) reflective of decreased π-back-donation to the
isocyanide ligands.15 Furthermore, the N-O bond length of
1.291(2) Å for Pd(κ1-N-PhNO)2(CNArDipp2)2 is longer than typically
found in monomeric nitrosoarene compounds but shorter than
standard N-O single bonds.16 However, it is in fact considerably
longer than the N-O bond length in divalent PdCl2(κ1-N-PhNO)2
(d(NO) ) 1.209(3) Å).17
(13) (a) Grigg, R.; Kennewell, P.; Teasdale, A. J. Tetrahedron Lett. 1992, 33, 7789–
7792. (b) Grigg, R.; Sridharan, V. Tetrahedron Lett. 1993, 34, 7471–7474.
(14) Pd(κ1-N-PhNO)2(CNArDipp2)2 exhibits crystallographic inversion symmetry.
(15) The lowest energy νCN stretch for Pd(κ1-N-PhNO)2(CNArDipp2
) is 2113
2
cm-1, which is considerably higher in energy relative to Pd(CNArDipp2
and thereby indicates a less electron-rich Pd center.
)
2
(16) Lee, J.; Chen, L.; West, A. H.; Richter-Addo, G. B. Chem. ReV 2002, 102,
1019–1066.
(17) Little, R. G.; Doedens, R. J. Inorg. Chem. 1973, 12, 537–540.
(18) For example, see: Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg.
Chem. 2002, 41, 4179–4193.
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