Noninnocent behavior of bidentate amidophosphido [NP]2- ligands upon coordination to copper

Article abstract of DOI:10.1021/ic402257z

The synthesis and preliminary coordination chemistry of two new redox-active bidentate ligands containing amido and phosphido donors are described. Treatment of the [^RNP]^2- (R = Ph, 2,4,6-trimethylphenyl) ligands with CuCl₂ and PMe₃ results in a dimeric copper(I) P-P coupled product via ligand oxidation. The intermediate of this reaction is proposed to involve a ligand radical generated via oxidation of the [^RNP]^2- ligand by copper(II), and the existence of such an intermediate is probed using computational methods. Significant radical character on the phosphorus atoms of the alleged [ ^RNP]^?-/copper(I) intermediate leads to P-P radical coupling.

Full text of DOI:10.1021/ic402257z

Communication
pubs.acs.org/IC
Noninnocent Behavior of Bidentate Amidophosphido [NP]2− Ligands
upon Coordination to Copper
Mark W. Bezpalko, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States
S
* Supporting Information
with phosphorus donors. Herein, we turn our attention
ABSTRACT: The synthesis and preliminary coordination
chemistry of two new redox-active bidentate ligands
containing amido and phosphido donors are described.
Treatment of the [RNP]2− (R = Ph, 2,4,6-trimethylphenyl)
ligands with CuCl2 and PMe3 results in a dimeric
copper(I) P−P coupled product via ligand oxidation.
The intermediate of this reaction is proposed to involve a
ligand radical generated via oxidation of the [RNP]2−
ligand by copper(II), and the existence of such an
intermediate is probed using computational methods.
Significant radical character on the phosphorus atoms of
the alleged [RNP]•−/copper(I) intermediate leads to P−P
radical coupling.
specifically to the arene-linked mixed-donor amidophosphido
ligands [NP]2− (Scheme 1) and examine their noninnocent
behavior upon coordination to copper.
Scheme 1
The target ligands were synthesized from commercially
available starting materials via a relatively high-yielding (50−
67% overall yield) three-step synthetic route (Scheme 2).
he study of redox-active ligands has become increasingly
1
Twidespread in recent years for both catalytic applications
and biomimetic studies.2 The ultimate purpose of these ligands
(in both biology and catalysis) is to work in tandem with
transition-metal centers to facilitate the multielectron redox
transformations required for the activation and functionalization
of small-molecule substrates. Redox-active ligands are typically
comprised of an aromatic or unsaturated backbone and provide
stability for ligand radicals via delocalization throughout the π
system. Most redox-active ligands to date are derived from either
diimines or 1,2-disubstituted benzene rings and involve
combinations of nitrogen, oxygen, and sulfur donors.1,3−8
Perhaps surprisingly, there have been limited reports of similar
arene-based redox-active ligand frameworks containing phos-
phorus donor atoms. The exploration of phosphorus-containing
redox-active ligands has been mostly confined to metallocenes
substituted with phosphine donors (i.e., dppf and derivatives
thereof)9,10 and metal clusters appended to phosphine donors.11
By analogy to reported noninnocent ligands with aryl-linked
nitrogen, oxygen, or sulfur donors, it might also be expected that
aryl phosphide ligands might be found to display redox activity.
Indeed, the bridging phosphido donor of a tridentate pincer
ligand in a series of Cu2P2 diamond-core complexes was shown to
be the center of multielectron redox activity.12,13 Within our own
group, we have investigated pincer ligands with N-heterocyclic
phosphido ligands and demonstrated their ability to participate
in redox processes via geometrical conformation changes.14
While tridentate pincer ligands with phosphido donors have
shown interesting coordination chemistry with both early and
late transition metals15−23 and bidentate phosphide24,25 and
amidophosphine ligands26 have been investigated, there are no
systematic investigations of the redox activity of bidentate ligands
Scheme 2
Palladium-catalyzed Buchwald−Hartwig cross-coupling of the
corresponding aniline and aryl halide precursors generated the o-
fluorodiarylamines 1a and 1b. Treatment of the aryl fluorides
with KPPh2 installed the o-diphenylphosphine functionality in 2a
and 2b. The desired secondary phosphine precursors 3a and 3b
could then be formed in modest yield via reductive cleavage of
one P−C bond using 3.5 equiv of Li0 metal in tetrahydrofuran
(THF), followed by hydrolysis.27 The 31P NMR spectra of 3a
and 3b showed singlets at δ −56.9 and −59.6, respectively, and
1
the H NMR spectra of these compounds revealed diagnostic
doublets at δ 5.19 and 5.33 with large coupling to 31P [1JP−H
=
223 (3a), 221 Hz (3b)] for the phosphorus-bound protons. The
doubly deprotonated amidophosphido ligands [RNP]2− [R = Ph
Received: September 4, 2013
Published: October 21, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
(4a), Mes (4b)] can be formed quantitatively upon treatment
with KH in THF and were isolated as orange solids. The 31P
NMR resonance for 4a was shifted downfield to δ −29.3 upon
deprotonation.
dimeric complexes linked by Ar2P−PAr2 functionalities (2.15−
2.38 Å).28 The ring linking the amide and phosphine donor
atoms maintains its aromatic character, with an average C−C
distance of 1.40 Å (see Table S1 in the SI).
While many transition-metal (typically multimetallic) diphos-
phine R2P−PR2 complexes have been reported, they are typically
formed directly from R2P−PR2, and in many cases, the P−P
bond is cleaved upon addition to transition metals to form
phosphido-bridged complexes. Group IV metals have been
shown to mediate P−P bond formation via dehydrocou-
pling,29−31 and in a few instances, the coupling of two metal-
bound phosphido units has been suggested to proceed via a
radical mechanism.32−34
The addition of either 4a or 4b to CuCl2 in the presence of
excess PMe3 in THF at −78 °C afforded an orange/red solution
from which the diamagnetic copper complexes [(RNP)Cu-
(PMe3)2]2 [R = Ph (5a), Mes (5b)] could be isolated. The 31
P
NMR spectrum of complex 5a features two resonances in a 2:1
integral ratio at δ −46.0 and −22.6 (δ −46.6 and −28.2 for 5b),
corresponding to bound PMe3 and PhNP ligands, respectively.
Notably, no P−P coupling is observed by 31P NMR, and only one
broad signal for the bound PMe3 ligands is observed in the 31
P
We speculate that, in the present case, P−P coupling occurs via
oxidation of the [RNP]2− ligand by copper(II) upon coordination
to form a transient [RNP]•− CuI complex (A) featuring a ligand-
based radical (Scheme 3). Intermolecular P−P coupling then
leads to complexes 5a and 5b. Although no intermediates were
observed spectroscopically, computational studies using density
functional theory (DFT) were used to better elucidate the
electronic structure of the purported ligand radical formed en
route to 5a and 5b. A geometry optimization was performed on
the hypothetical monomer [RNP]Cu(PMe3)2 (A). The geom-
etry about copper in intermediate A is pseudotetrahedral, with a
relatively planar geometry about the amido nitrogen atom (ΣN =
358.9°). The geometry about phosphorus, however, is pyramidal
(ΣP = 320.1°) and indicative of stereochemically active electron
density. Visualization of the singly occupied molecular orbital
(SOMO; Figure 2) reveals significant electron density on the
NMR spectrum of 5a at all temperatures investigated (228−293
K), but at low temperature (<253 K), the resonances associated
with the methyl groups of PMe3 decoalesce into two separate
signals (see Supporting Information, SI). At first, the apparent
diamagnetism of complexes 5a and 5b suggested a dimeric
copper(II) antiferromagnetically coupled product; however, the
X-ray structure determination revealed both products to be
dimeric copper(I) complexes linked via P−P coupling of the
ligated NP ligands (Scheme 3).
Scheme 3
Figure 2. Pictorial representation of the calculated SOMO of
intermediate A.
phosphorus atom (Mulliken spin density = 0.40), with very little
unpaired spin on the copper center (Mulliken spin density =
0.09). The remainder of the electron density on the SOMO of A
resides on the amide nitrogen atom (Mulliken spin density =
0.16) and throughout the aromatic π system of the ligand. This
delocalization results in a lengthening of the average C−C bond
in the N-aryl−P ring to 1.43 Å (see Table S1 in the SI). Thus,
DFT predicts that, upon treatment with dianionic ligand 4a or
4b, copper(II) is reduced by the ligand to form a monomeric
[RNP]•−/copper(I) intermediate featuring a ligand radical rather
than the simple [RNP]2−/copper(II) metathesis product.
Because the majority of the unpaired electron density resides
on the phosphorus atom, intermediate A can easily dimerize via
P−P coupling to form 5a or 5b. Similar phenomena have been
observed with catecholate analogues bearing one or two sulfur
donors: while phenoxyl radicals are typically delocalized
throughout the aromatic π system, phenylthiyl radicals tend to
have significantly more radical character on the sulfur atom, and
examples of S−S radical dimerization have been reported.35−38
Further studies have shown that treatment of 4a with
stoichiometric CuCl in the presence of PMe3 does not lead to
The solid-state structures of complexes 5a and 5b reveal that
each copper atom adopts a distorted tetrahedral geometry and
that the two copper(I) units are oriented in a staggered
conformation (Figure 1 and S4 in the SI). The P(1)−P(1′)
bond distance of 2.2338(9) Å is similar to that reported for other
Figure 1. Displacement ellipsoid (50%) representation of complex 5a
with all hydrogen atoms omitted for clarity. Relevant interatomic
distances (Å): Cu1−P1, 2.2876(6); Cu1−P2, 2.2584(6); Cu−P3,
2.2773(6); Cu1−N1, 2.0520(16); P1−P1′, 2.2338(9).
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Inorganic Chemistry
Communication
5a but rather a mixture of CuI−PMe3 adducts and free ligand 3a;
however, the addition of AgPF6 to a mixture of 4a, CuCl, and
PMe3 produced compound 5a quantitatively (by 31P NMR
spectroscopy). Thus, an outer-sphere oxidant is also capable of
oxidizing the [RNP]2− ligand to eventually form the P−P coupled
product. In fact, compound 5a can also be produced by the
addition of 2 equiv of CuCl to 4a in the presence of PMe3 (albeit
in low yield), presumably via the sacrificial reduction of 1 equiv of
copper(I) to copper(0).
Cyclic voltammetry was used to explore the further redox
chemistry of 5a and 5b. The cyclic voltammograms (CVs) of 5a
and 5b in THF display irreversible oxidative waves at −0.27 and
−0.38 V, respectively (vs FeCp20/+), presumably assigned to the
copper(II)/copper(I) couple. The lower oxidation potential for
complex 5b can be explained by the more electron-releasing
nature of the N-mesityl rings, and the CV of this complex has a
second irreversible feature at −0.11 V, which may be attributed to
ligand-based redox activity. Nonetheless, the irreversibility of
these oxidative features may imply that oxidation of these
complexes to copper(II) involves further ligand-based chemical
rearrangement.
In summary, we have reported the syntheses of new bidentate
amidophosphido mixed-donor ligands that, based on analogy to
similar oxygen-, nitrogen, and sulfur-donor ligands, have the
potential to participate in ligand-based redox activity. Preliminary
investigations into their coordination chemistry with CuCl2
result in a reduced dicopper(I) complex in which P−P coupling
has occurred. This result suggests that copper(II) is capable of
oxidizing the ligand, leading to a ligand-based radical with most
of its electron density on phosphorus, allowing radical P−P
coupling to occur.
Investigations into the coordination chemistry of this series of
ligands with other transition metals are currently underway.
Additionally, modified ligands with more sterically encumbering
phosphorus-donor substituents are under development to
discourage intermolecular coupling processes.
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ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, experimental and
spectroscopic details for all compounds, CVs of complexes 5a
and 5b, additional crystallographic data and refinement details
for complexes 5a and 5b, and computational details. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: thomasc@brandeis.edu.
Notes
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The authors declare no competing financial interest.
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Tsipis, C. A. Angew. Chem., Int. Ed. 2005, 44, 2407−2410.
ACKNOWLEDGMENTS
(35) Kimura, S.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K. J.
■
̈
Am. Chem. Soc. 2001, 123, 6025−6039.
The authors thank the American Chemical Society Petroleum
Research Fund under Award DNI-50974 for financial support of
this project.
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Weyhermuller, T.; Wieghardt, K. Angew. Chem., Int. Ed. 2003, 42, 563−
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