A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand

Article abstract of DOI:10.1021/ja043092r

A Cu₂N₂ diamond core structure, {(PNP)Cu^I}₂ (2), supported by a [PNP]^- ligand (1) ([PNP]^- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (τ > 10 μs) and an unusually high quantum yield (φ > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]^- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc⁺/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations. Copyright

Full text of DOI:10.1021/ja043092r

Published on Web 01/27/2005
A Highly Emissive Cu2N2 Diamond Core Complex Supported by a [PNP]-
Ligand
Seth B. Harkins and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received November 16, 2004; E-mail: jpeters@caltech.edu
Photoluminescent complexes have garnered much attention
because of their possible utilization in electrochemical devices, as
sensors and biological imaging agents, and in solar energy con-
version schemes.1 In this context, polypyridine-supported Cu(I)
systems show promising features, including the availability of low-
lying charge-transfer (CT) excited states and the relatively low cost
of copper in comparison to other transition metal luminophores.
However, their tendency to display weak emission and short-lived
excited states is problematic.2 McMillin has underscored these
Figure 1. Left: Molecular representation of 2. Center: Space-filling
collective points,3 and his group recently reported a fascinating
representation of 2 from crystal coordinates. Right: Displacement ellipsoid
representations (50%) of the core atoms of 2. See Supporting Information
for complete details.
mononuclear complex, [Cu(dmp)(POP)]+ (dmp ) 2,9-dimethyl-
1,10-phenanthroline; POP ) bis[2-(diphenylphosphino)phenyl]-
ether), that exhibits both a relatively high quantum yield and long-
lived excited state as compared to other polypyridine-Cu(I) sys-
Complex 2 is characterized by a single resonance in its 31P NMR
tems.4 Incorporation of a tightly bound bis(phosphine) chelate
appears to create a rigid environment around the copper center that
(i) suppresses solvent-induced exciplex formation due to steric
factors and (ii) limits problematic ligand dissociation from the
excited state.
spectrum (-33.9 ppm), and XRD analysis establishes the dimeric
structure represented in Figure 1. A short Cu‚‚‚Cu bond distance
of 2.6245(8) Å is observed that is similar in length to the Cu‚‚‚Cu
distance reported for {(SNS)CuI}2.5 The P-Cu-P angles of
132.93(5)° and 137.69(5)° are significantly smaller than the
S-Cu-S angles in {(SNS)CuI}2 (av 153°). As a result, the copper
centers are less severely distorted from a tetrahedral geometry than
in {(SNS)CuI}2, which more closely approximates a cis-divacant
octahedron at each copper site. These geometrical differences
presumably arise from the different steric requirements of the
thioether [SNS]- and diisobutylphosphine [PNP]- ligands.
Herein we describe an amido-bridged bimetallic copper system,
{(PNP)CuI}2 (2), derived from a chelating bis(phosphine)amide
ligand ([PNP]- ) bis(2-(diisobutylphosphino)phenyl)amide). This
species is an exceptional luminophore in its own right. Aside from
the absence of supporting polypyridine ligands, its combined
quantum yield (φ > 0.65) and lifetime (τ > 10 µs), in combination
with its dinuclear structure and redox behavior, are without prece-
dent. The synthesis of 2 was motivated by our recent elucidation
of the dinuclear, thioether-supported Cu2N2 complex {(SNS)CuI}2
([SNS]- ) bis(2-tert-butylsulfanylphenyl)amide).5 {(SNS)CuI}2
exhibits a reversible 1e- oxidation to form a mixed-valence [{-
(SNS)Cu1.5}2]+ complex, and electrochemical and XRD studies
establish minimal structural reorganization between these two redox
partners. While {(SNS)CuI}2 is negligibly emissive at 298 K, its
phosphine congener 2 emits strongly in solution and in the solid
state when irradiated by visible light.
The required tridentate PNP-H ligand, 1, was synthesized by
addition of nBuLi to bis(2,2′-difluorophenyl)amine in THF followed
by the addition of lithium diisobutylphosphide.6 Heating this mixture
at 45 °C for 4 days and subsequent passage of the crude product
through silica gel afforded amine 1 as a spectroscopically pure
viscous oil (75% yield). Deprotonation with nBuLi affords [PNP]-
[Li], [1]Li, in good yield. Related bis(phosphino)amido ligands were
first introduced by Fryzuk and have received the attention of several
groups more recently.6-8 [1]Li reacts rapidly with CuBr‚Me2S in
diethyl ether to generate a luminescent yellow solution. The neutral,
diamagnetic copper complex 2 can be subsequently isolated in pure
form by crystallization (92%). Alternatively, complex 2 can be
generated in good yield by the addition of {(2,4,6-Me3C6H2)Cu}9
to free amine 1, producing mesitylene upon metalation.
Electrochemical analysis of 2 in CH2Cl2 (Fc+/Fc, 0.3 M [nBu4N]-
[PF6], 250 mV/s, Fc ) ferrocene) reveals two reversible waves,
one centered at -550 mV and the other at 300 mV. An irreversible
wave is encountered at higher potential (Epa ) 860 mV).10 The
event at -550 mV is assigned to a reversible Cu1.5Cu1.5/Cu1Cu1
redox process by analogy to the {(SNS)CuI}2 system. The Cu1.5
-
Cu1.5/Cu1Cu1 event is cathodically shifted for 2 in comparison to
{(SNS)CuI}2 (by ca. 160 mV) because of its stronger phosphine
donors. The second reVersible wave observed for 2 (E1/2 ) 300
mV) is noteworthy and is distinct from {(SNS)CuI}2, for which
only an irreVersible redox process is observed at similar potential
(Epa ) 560 mV).5 Incorporation of the phosphine donors appears
to stabilize an unusual second oxidation event in 2, at least on the
time scale of the electrochemical experiment (250 mV/s). While it
is tempting to assign this second wave to a Cu2Cu2/Cu1.5Cu1.5 redox
process, at this stage it is equally plausible to suggest that a ligand-
centered oxidation process is operative.11,12
The absorption spectra for ligand 1 and complex 2 are shown in
Figure 2A, and the corrected emission and excitation spectrum for
2 (298 K in cyclohexane) is also shown in part B.13 The optical
spectrum of 2 is typical for Cu(I). MLCT bands at 23 800 and
22 300 cm-1 give rise to its yellow color. Its emission spectrum,
collected by excitation into its lowest energy absorption band (λex
) 440 nm, ∼22 700 cm-1), shows a λmax at 20 000 cm-1. Its
9
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10.1021/ja043092r CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
excited-state complex resistant to donor solvent ligation. Each of
these factors can otherwise contribute to undesirable exciplex
quenching. Lewis acidic Cu(I) cations, such as Cu(phen)2+ systems,
suffer from exciplex quenching because of solvent and/or coun-
teranion binding in the excited state.3 The space-filling model of 2
shown in Figure 1 reveals just how effectively the copper sites are
shrouded by the surrounding phosphine ligand framework.
As a final point of interest, we note that a value for E00 ) 2.6
eV can be estimated from the intersection of the emission and
excitation profiles of 2. Subtracting this value from the reversible
Cu1.5Cu1.5/Cu1Cu1 redox couple provides an estimated value of -3.2
V (vs Fc+/Fc) for the excited-state reduction potential of *2. It is
possible that *2 will prove to be a potent photoreductant/
photosensitizer,18 and given the presence of two reversible redox
couples within this bimetallic copper system, there may be an
opportunity to photochemically drive multielectron reaction pro-
cesses.19
Figure 2. (A) Absorption spectra for 1 and 2 in cyclohexane. (B) Corrected
emission spectrum (green, λex ) 440 nm) and excitation spectrum (black,
λem ) 560 nm) of 2 in cyclohexane. (C) Luminescence decay measurement
of 2 in cyclohexane (laser pulse at t ) 0 µs, λex ) 440 nm, λem ) 510 nm).
corresponding excitation profile is also shown (λem ) 560 nm,
∼17 900 cm-1).
Acknowledgment. This work was supported with funds pro-
vided by the NSF (CHE-01232216)) and the MC2 program in
collaboration with BP. We acknowledge Dr. Jennifer C. Lee for
technical assistance with the lifetime measurements, and Larry
Henling for crystallographic assistance. Dr. Jay R. Winkler provided
numerous insightful discussions.
The intensity of the emission from the excited state of 2, *2, is
quite striking to the eye, even at room temperature in relatively
polar donor solvents such as tetrahydrofuran (THF). This property
is consistent with the unusually high quantum yield we measured
for 2 at 298 K: φ ) 0.68(2) in cyclohexane and φ ) 0.67(4) in
THF. These quantum yields were determined by established
methods using a fluorescein standard (φ ) 0.90 in 0.1 N NaOH).14
It was also of interest to determine the excited-state lifetime of *2,
measured as 10.2(2) µs in cyclohexane (see inset in Figure 2) and
10.9(4) µs in THF. These lifetimes were determined by a mono-
exponential fit to raw decay data collected at 510 nm upon
excitation at 460 nm.13 Complex 2 is thus a highly efficient
luminophore, with a lifetime similar to that McMillin reported for
mononuclear [Cu(dmp)(POP)]+ and a quantum yield that is ca. four
times greater. We also note that diffusion-limited excited-state
electron transfer (kQ ) 1.2 × 1010 M-1 s-1) has been demonstrated
by time-resolved quenching experiments using 2,6-dichloroquinone
(see Supporting Information).15
Supporting Information Available: X-ray crystallographic files
(CIF), complete synthetic details, electrochemistry, spectroscopy, and
tables of experimental data (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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+
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