C(sp3)-C(sp3) Coupling with a Pd(II) Complex Bearing a Structurally Responsive Ligand

Article abstract of DOI:10.1021/acs.organomet.9b00014

A dimer with two Pd^II-Me fragments is stabilized by the bridging coordination of the supporting 1-azaallyl/phosphine ligand. Heating the complex results in C(sp³)-C(sp³) bond formation and the release of ethane. Two different palladium products are generated, each with distinct coordination modes of the 1-azaallyl/phosphine ligand.

Full text of DOI:10.1021/acs.organomet.9b00014

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
C(sp3)−C(sp3) Coupling with a Pd(II) Complex Bearing a Structurally
Responsive Ligand
†
†
†
‡
́
Kyle M. K. Jackman, Benjamin J. Bridge, Ethan R. Sauve, Christopher N. Rowley,
Cameron H. M. Zheng,† James M. Stubbs,† Paul D. Boyle,† and Johanna M. Blacquiere*,†
†Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7
‡Department of Chemistry, Memorial University of Newfoundland, St. John’s, Canada A1B 3X7
S
* Supporting Information
ABSTRACT: A dimer with two PdII−Me fragments is stabilized by the
bridging coordination of the supporting 1-azaallyl/phosphine ligand.
Heating the complex results in C(sp3)−C(sp3) bond formation and the
release of ethane. Two different palladium products are generated, each
with distinct coordination modes of the 1-azaallyl/phosphine ligand.
igands that exhibit a diversity of coordination modes are
designed phosphine imine ligand 1H (Scheme 1, left), a
Lwell-known in coordination chemistry and transition- proligand to a phosphine 1-azaallyl ligand (1), by deprotona-
metal catalysis.1−4 A change in ligand denticity or hapticity to a
lower coordination number can promote binding of an
exogenous ligand (i.e., a substrate for catalysis). Thus, a
mode with a higher coordination number can stabilize latent
low-coordinate complexes. This is particularly attractive in
catalysis, in which such intermediates are often both key active
catalysts and precursors to decomposition pathways. Structural
changes in ligand coordination can also promote reactivity
distinct from substitution reactions. Palladium-catalyzed direct
arylation reactions depend on the isomerization of a
carboxylate ligand from κ2-O,O to κ1-O.5,6 The released
oxygen atom acts as an intramolecular base, which lowers the
activation barrier for Ar−H bond activation. Alternatively,
isomerization of multidentate N-donor ligands to a higher
coordination number (i.e., κ2-N,N to κ3-N,N,N) promotes
oxidation of Ni(II).7−9 The higher coordination number
stabilizes the uncommon Ni(III) or Ni(IV) intermediates,
which can subsequently undergo reductive elimination to give
new C−E bonds (E = C, O, N). Despite these prominent
cases, examples in which structurally responsive ligands
promote reactivity beyond substitution reactions remains
limited.
Scheme 1. Synthesis of Phosphine-Imine Complex 2 (Left)
and Displacement Ellipsoid (50% Probability) Plot of 2
(Right)
a
a
All hydrogen atoms have been omitted for clarity, except those on
C1A, C2A, and C11A.
tion at C2.12 Previously, we showed that an octahedral
ruthenium complex with 1 exhibited only a κ2-P,N
coordination mode. Herein, we report a palladium complex
with ligand 1 that exhibits both changes in ligand coordination
and an unusual C(sp3)−C(sp3) bond-forming reaction.
The proligand 1H12 reacts rapidly at room temperature with
PdCl(Me)(COD) to give 2, the structure of which was
confirmed by single-crystal X-ray crystallography (Scheme 1).
The 1-azaallyl functionality can coordinate to metals through
a diversity of modes, in which η3-N,C,C binding is the most
common among late transition metals.10,11 A reversible change
between the η3-N,C,C and κ1-N coordination modes was
proposed for a bis-1-azaallyl Ni(II) complex.11 We hypothe-
sized that a structurally responsive 1-azaallyl ligand could
engender unusual and unexpected reactivity. To this end, we
Received: January 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00014
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
The complex has a slightly distorted square planar geometry, as
evidenced by a τ4 value13 of 0.10 (for a perfect square plane, τ4
= 0). A weak14 hydrogen bond (X···A = 3.5498(24) Å, X−H−
A bonding angle 134.6°) is observed between the chloride
ligand (Cl1A) and the methine proton (on C2A). This is also
observed in solution, where the diamagnetic anisotropy of the
chloride lone pairs15,16 shifts the H2 resonance downfield by ca.
2 ppm relative to the analogous signal in 1H.
Deprotonation of 2 with Li[HMDS] in noncoordinating
fluorobenzene gave rapid and quantitative formation of 3,
observed as a singlet at 43.7 ppm in the 31P{1H} NMR
spectrum (Scheme 2, route A). Scale-up of 3 via this route
that of a related dimer (5, vide infra) and slower than that of a
monomer (4, vide infra). This supports the assignment of 3 as
a dimer in solution.
DFT calculations were conducted to compare the stability of
a dimer to that of a monomeric complex, in which the fourth
coordination site is occupied by a δ-agostic interaction with the
ligand methyl substituent. The dimer is more stable than the
agostic monomer (ΔG° = −8.8 kcal mol−1), which further
supports the structural assignment of 3. Structures of dimer 3
in which the H3′ protons bind agostically to one or both of the
axial palladium sites are only 1.4 and 2.7 kcal mol−1 higher in
Gibbs energy, respectively, than the structure without agostic
interactions. The close energies are in line with the broad
1
signal for H3′ in the room-temperature H NMR spectrum.
Scheme 2. Formation of 3 via Deprotonation of 2 or
a
Single crystals were obtained by slow evaporation of a
solution of 3 in a mixture of toluene and Et2O (Scheme 2).
Two different dimer structures were found in the unit cell that
differ in the position of the carbon framework of the 1-azaallyl
unit, due to rotation about the N−C bond. Only one of the
dimers is depicted in Scheme 2 and discussed here, but the
other has very similar bond parameters. The dimeric structure
shows two slightly distorted square planar Pd environments
(Pd1A, τ4 = 0.14; Pd2A, τ4 = 0.11) that are bridged by the
anionic nitrogen atoms of ligand 1. The Pd1A−N1A−Pd2A
and Pd1A−N2A−Pd2A angles are 78.02(7) and 78.14(7)°,
respectively, which gives a folded Pd2N2 cycle and an angle
between the square planes of 107.85°. The carbon frameworks
of both of the 1-azaallyl groups are found in axial positions
with respect to this ring. The Pd1A−Pd2A distance (2.7635(7)
Å) is within the van der Waals radius (3.26 Å)22 and is in a
range consistent with a d8−d8 interaction.23,24 However, the
large angle between the square planes is expected to minimize
Pd−Pd bonding.25
Metalation of K[1] (Top) and Displacement Ellipsoid
b
(50% Probability) Plot of 3 (Bottom)
a
Conditions: (i) Li[N(SiMe3)2], C6H5F, room temperature, 10 min
Compound 3 readily reacts with pyridine to give adduct 4
(Scheme 3, left). Coordination of pyridine was evident from
(*the yield decreases for scales >50 mg); (ii) (a) 2 equiv of KH, THF,
room temperature, 2 h, 86% isolated yield of K[1], (b) PdClMe-
b
(COD), C6H5F, room temperature, 30 min. All hydrogen atoms
Scheme 3. Lewis Acidic and Reductive Elimination
Reactivity of 3
have been omitted.
consistently gave low yields, and Pd black was observed during
the workup, likely due to the instability of 3 to excess base or
the conjugate acid H[HMDS]. Instead, 3 was obtained cleanly
in very good yield (>80%) from metalation of the
deprotonated ligand salt K[1] (Scheme 2, route B). The
identity of 3 was assigned as a dimer in which ligand 1 bridges
the two metal centers through a μ-(κ1-P,κ2-N) coordination
mode. Only a single set of ligand resonances is observed by 1H
NMR spectroscopy, which indicates that 3 has C2 symmetry in
solution. Unequivocal evidence for the cis disposition of the
phosphine and methyl ligands is provided by the character-
istically small coupling constant (3JHP = 1.3 Hz)17−19 for the
methyl doublet at δH 0.51. The absence of a signal for the
methine proton (H2) is consistent with ligand deprotonation
to give the anionic 1-azaallyl10,11 moiety. The two methyl
groups (H3 and H3′) of ligand 1 are no longer equivalent, and
both are observed as singlets. The signal for H3′ is broad at
room temperature (ω1/2 = 169 Hz), which is likely due to a
1
weak agostic bonding to the axial site of palladium. A H−13C
the 1H NMR spectrum of isolated 4 in which signals for bound
pyridine are observed. The ligand in 4 adopts a κ2-P,N
coordination mode according to the chemical shift for C2 (δC
118.5). Single crystals of 4 were obtained; while the structure
was not of sufficient quality to refine the data, it did confirm
the indicated connectivity. The facile formation of 4 indicates
that the stabilizing bridging interaction of 3 is easily displaced,
HSQC NMR experiment reveals a correlation from each
methyl to the quaternary carbon C2 at 117.2 ppm (see Scheme
1 for numbering). The location of this signal suggests that the
carbon atoms of the 1-azaallyl moiety are not bound to the
metal center.11,12,20,21 A 1H DOSY NMR analysis (Figure S24)
of 3 revealed that the compound diffuses at a rate similar to
B
DOI: 10.1021/acs.organomet.9b00014
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
and this is confirmed by DFT, in which 4 is calculated to be
more stable than 3 (ΔG° = −1.2 kcal mol−1).
coupling is improbable, since the Pd products 5 and 6 are
likewise afforded on heating 3 in the presence of 3 equiv of
TEMPO• as a radical trap33,34 (5, 20%; 6, 40%) or in the dark
(5, 27%; 6, 50%). The second pathway is even more unlikely,
given that no oxidant is present. No intermediates are observed
in the conversion of 3 to 5 and 6, suggesting that a bimolecular
mechanism35,36 is followed for reductive elimination. This
could involve methyl transfer36,37 between Pd centers and
subsequent reductive elimination38,39 from a PdIIMe2 fragment
as part of a dimer or as a monomeric intermediate. Regardless
of the exact reaction path, we propose that the capacity of
ligand 1 to coordinate through three distinct coordination
modes is important for the observed reactivity. Recently,
reductive elimination of ethane was observed as a side reaction
from a PdII−Me complex with a diphosphazane monoxide
ligand. On reductive elimination the ligand isomerized from a
κ2-P,O to a μ-(κ2-P,O,κ1-O′) mode in the PdI−PdI dimer
product.40 This suggests that the reductive elimination
reactivity observed here is general to other complexes with
structurally responsive ligands.
At room temperature in benzene 3 slowly evolves ethane,
which is observed both as a dissolved gas by 1H NMR
spectroscopy and in the gas phase by GC-MS analysis of the
headspace. Heating 3 at 70 °C for 2.5 h and analysis by
calibrated GC-FID gives a yield of ca. 76% ethane. The Pd-
based products from ethane release are two compounds, 5 and
6, generated in a similar overall yield of 85% (35 and 50%
yields, respectively, based on starting ligand 1; Scheme 3,
1
right). H NMR analysis of 5 and 6 confirms that neither has
retained the Pd−Me functionality, a requirement for ethane
release. NMR analysis of 5 is consistent with a PdI−PdI dimer;
such compounds are well-known,26−29 and one-electron
reduction of palladium is consistent with a net bimolecular
reductive elimination of ethane. The structural assignment of 5
was confirmed crystallographically, in which a bridging, μ-(κ2-
P,N,η2-C,C), coordination mode for ligand 1 is evident (Figure
1). The C(1)−C(2) bond length (1.396(4) Å) is ca. 0.07 Å
In summary, an anionic phosphine 1-azaallyl ligand, K[1],
reacted with PdCl(Me)(COD) to give a dimer, 3, in which
ligand 1 is bound in a bridging μ-(κ1-P,κ2-N) mode. Heating 3
produces ethane via Csp3-Csp3 bond formation along with two
Pd products 5 and 6. Complex 5 is a Pd(I)−Pd(I) dimer with
μ-(κ2-P,N,η2-C,C) binding of 1 and product 6 is a Pd(II)
complex with two κ2-P,N-bound ligands. The coordination
chemistry of ligand 1 is diverse and responsive. The observed
chemistry is suggestive that the capacity for 1 to alter its
coordination mode promotes the unusual C(sp3)−C(sp3)
bond forming reaction. Ongoing studies are aimed at
elucidating the mechanism for ethane formation and the role
of the ligand in the process.
Figure 1. Displacement ellipsoid (50% probability) plots of 5 (left)
and 6 (right). Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å): 5, Pd(1)−Pd(1) 2.6109(7), Pd(1)−N(1)
2.061(3), Pd(1)−P(1) 2.3136(9), Pd(1)−C(1) 2.308(3), Pd(1)−
C(2) 2.205(3), C(1)−C(2) 1.396(4); 6, Pd(1)−P(1) 2.2383(10),
Pd(1)−P(2) 2.2437(10), Pd(1)−N(1) 2.088(3), Pd(1)−N(2)
2.091(3), N(1)−C(7) 1.408(4), N(2)−C(29) 1.389(4).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00014.
longer than the analogous bond in 3 (C1A−C2A = 1.333(4)
Å) due to the π back-bonding from the Pd(I) center. Expected
Pd−Pd distances can be estimated from Pd−Me bond lengths
of 3.27 The experimental Pd−Pd length for 5 of 2.6109(7) Å is
nearly 0.1 Å longer than the predicted length. This is likely due
to the geometric constraints imposed by the bridging 1-azaallyl
group. The π coordination of the 1-azaallyl group is confirmed
by 13C{1H} NMR spectroscopy, which showed a signal for C2
at 74.5 ppm, significantly upfield of the expected location for a
κ1-N coordination mode of the 1-azaallyl fragment.
Experimental details, NMR, MALDI, and IR spectra, and
GC-FID calibration data (PDF)
Accession Codes
CCDC 1839629−1839631 and 1873060 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
An independent reaction of PdCl2(COD) with 2 equiv of
K[1] confirmed that product 6 is Pd(1)2. The generation of
Pd(II) from reductive elimination of 3 is accompanied by the
1
formation of Pd black. H−13C HMBC NMR analysis of 6
AUTHOR INFORMATION
reveals that C2 has δC 118.4, which is consistent with a κ1-N
coordination mode for the 1-azaallyl group. The attempted
crystallization of 3 over several days afforded X-ray-quality
crystals of 6 (Figure 1). The phosphine groups are cis-
disposed, and the geometry is distorted square planar (τ4 =
0.16) due to the steric repulsion of the 1-azaallyl groups.
A few examples of C−C bond formation from Pd(II)
monomethyl or -alkyl complexes have been reported. Light-
induced homolytic cleavage of Pd−alkyl bonds leads to radical
C−C coupling.30 Alternatively, oxidation of PdII−Me induces
methyl transfer and reductive elimination from a high-valent
PdIVMe2 intermediate.31,32 In the present case, radical C−C
■
Corresponding Author
*E-mail for J.M.B.: johanna.blacquiere@uwo.ca.
ORCID
Johanna M. Blacquiere: 0000-0001-6371-6119
Funding
The NSERC (Canada) is thanked for financial support. B.J.B.
and C.Z. thank the NSERC for USRAs, and J.M.B. thanks
Petro-Canada for a Young Innovator Award.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.organomet.9b00014
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Organometallics
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Phosphine Sulfonate Palladium Complexes. J. Am. Chem. Soc. 2007,
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ACKNOWLEDGMENTS
■
Glenn Facey and Daniel do Nascimento of the University of
Ottawa are thanked for their assistance in the acquisition of the
DOSY NMR spectra.
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