Structural snapshots of a flexible Cu2P2 core that accommodates the oxidation states CuICuI, Cu 1.5Cu1.5, and CuIICuII

Article abstract of DOI:10.1021/ja056071l

The phosphido-bridged dicopper(I) complex {(PPP)Cu}₂ has been synthesized and structurally characterized ([PPP]^- = bis(2-di-iso-propylphosphinophenyl)phosphide). Cyclic voltammetry of {(PPP)Cu}₂ in THF shows fully reversib

Full text of DOI:10.1021/ja056071l

Published on Web 10/28/2005
Structural Snapshots of a Flexible Cu2P2 Core that Accommodates the
Oxidation States CuICuI, Cu1.5Cu1.5, and CuIICuII
Neal P. Mankad, Eric Rivard, 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 September 2, 2005; E-mail: jpeters@caltech.edu
Scheme 1
Understanding how nature controls electron transfer (ET) rates
in copper-containing proteins, such as the “blue copper” protein
family, has provided impetus for theoretical, biochemical, and small
molecule model studies.1 It is generally accepted that structural
reorganization as a function of ET needs to be minimized if rapid
rates are to be achieved,2 and that this can be difficult to accomplish
in synthetic copper systems due to ligand rearrangement, loss, and
exchange processes.3 Recently, we described an amido-bridged
Cu2N2 system, {(SNS)Cu}2 (1) ([SNS]- ) bis(2-tert-butylsulfa-
nylphenyl)amide),4 that shares the diamond core structural motif
of the CuA site5 and reversibly accommodates rapid ET via low
overall structural reorganization.6 Because the copper centers in 1
possess geometries intermediate between tetrahedral and square
planar (coined pseudotetrahedral herein), we thought ligand
modification might provide reversible access to a third, formally
CuIICuII species. Furthermore, we have described a highly lumi-
nescent Cu2N2 system, {(PNP)Cu}2 (2) ([PNP]- ) bis(2-di-iso-
butylphosphinophenyl)amide), that mediated two fully reversible
redox events according to CV data.7,8 We thus sought to replace
the amide bridging ligands in 2 with more electron-releasing
phosphides to further stabilize the doubly oxidized species and to
afford us the possibility of isolating and structurally analyzing the
system across the oxidation states CuICuI, Cu1.5Cu1.5, and CuIICuII.
Here we describe the characterization of a phosphido-bridged
dicopper system, {(PPP)Cu}2 (3) ([PPP]- ) bis(2-di-iso-propylphos-
phinophenyl)phosphide), in which each Cu center maintains a highly
distorted tetrahedral geometry across these three oxidation states.
While the overall topology of each structure is very similar to those
Cu2N2 systems we have described previously, the Cu2P2 system
displays remarkable flexibility at the phosphide hinge to allow for
dramatic changes in both the Cu‚‚‚Cu distance and the Cu-Pµ-Cu
angle as a function of oxidation state. Dicopper systems that can
reversibly access three discrete oxidation states were essentially
unknown prior to this study. Indeed, a complete set of structural
data for any such transition metal system is rare. Most relevant to
the current study is an aryloxide-bridged Fe2O2 system that was
structurally characterized in the oxidation states FeIIFeII, FeIIFeIII,
FeIIIFeIII.9 The structural analyses of systems supported by redox-
active ligands across three oxidation states have also been de-
scribed.10
systems4,7 is its much longer Cu‚‚‚Cu distance (3.307(2) Å for
3 vs 2.5989(3) Å for 1 and 2.6245(8) Å for 2) resulting from
the larger phosphide bridging groups. The Cu-X-Cu angles
are likewise expanded (Cu-Pµ-Cu (avg) ) 90.63° for 3 vs
Cu-N-Cu (avg) ) 75.72° for 1 and 74.07° for 2).
Figure 2a compares the cyclic voltammetry of 3 and its Cu2N2
relative 2. Two fully reversible redox events are observed for each
system. There is a large cathodic shift of approximately 600 mV
for each redox couple of the phosphide-bridged system 3. Its
Figure 1. Pertinent X-ray data for complexes 3, 4, and 5.
To prepare the dicopper system of interest to us (Scheme 1), a
chlorophosphine precursor of (PPP)H was synthesized by ortho-
lithiation of 2-di-iso-propylphosphinophenyl bromide with n-
butyllithium, followed by addition of 0.5 equiv of PCl3. Subsequent
reduction of the resulting diarylchlorophosphine by LiAlH4 gave
(PPP)H as a colorless oil. The reaction of (PPP)H with mesityl-
copper(I) gave diamagnetic red-orange 3 (31P NMR (ppm): 34.0
(br, 4P), -21.0 (br, 2P)) with concomitant formation of mesitylene.
XRD analysis of suitable crystals of 3 confirmed its dimeric
diamond-core structure (Figure 1). The most striking difference
between the structure of 3 and previously characterized Cu2N2
Figure 2. (a) Cyclic voltammograms of 2 and 3 (THF, 0.35 M [nBu4N]-
[PF6], 200 mV/s) vs Fc+/Fc. (b) Optical spectra for 3 (solid), 4 (dashed),
and 5 (dotted).
9
16032
J. AM. CHEM. SOC. 2005, 127, 16032-16033
10.1021/ja056071l CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Cu1.5Cu1.5/CuICuI couple is assigned at E°′ ) -1.02 V, and its
CuIICuII/Cu1.5Cu1.5 couple is assigned at E°′ ) -0.42 V.
is known to have an extremely high quantum yield (φ ) 0.67(4))
and a long excited-state lifetime (τ ) 10.9(4) µs) in THF at 298
Chemical oxidation of 3 with 1 equiv of [FeCp2][BPh4] yielded
the red-purple paramagnetic species [{(PPP)Cu}2][BPh4] (4). The
X-band EPR spectrum of 4 at 10 K showed an isotropic S ) 1/2
signal. The complex hyperfine splitting pattern features components
from the two Cu centers and the phosphide bridges (see Supporting
Information). These data, along with a signature low-energy inter-
K,7
corresponding emission measurements for 3 reveal significantly
lower values (φ ) 0.013(3) and τ ) 0.6(3) µs). It seems most likely
that this attenuation in emission is related to a greater degree of
structural reorganization upon MLCT in 3 than in 2.
To summarize, we have shown that synthetic dicopper centers
bridged in the diamond-core structural motif become unusually
redox active, in this case sampling the oxidation states CuICuI,
Cu1.5Cu1.5, and CuIICuII. The auxiliary ligand preserves a geometry
that is midway between square planar and tetrahedral about each
copper center, yet is flexible enough to accommodate pronounced
changes in the Cu‚‚‚Cu distance as a function of ET.
Acknowledgment. We thank the BP MC2 program for financial
support. N.P.M. was supported by an NSF graduate research
fellowship, and E.R. by an NSERC postdoctoral fellowship. Larry
Henling, Ted Betley, Brian Leigh, and Dr. Angelo Di Bilio provided
technical assistance. The Zewail group provided access to a near-
IR spectrometer. Luminescence measurements were acquired at the
Beckman Institute Laser Resource Center.
valence charge-transfer band at 1095 nm (ꢀ ) 2800 M-1 cm-1
,
Figure 2b), suggest that 4 be described as a delocalized mixed-
valence system.11
This electronic structure for 4 was at first surprising to us given
the unusually long Cu‚‚‚Cu distance present in 3, but XRD analysis
revealed that one-electron oxidation of 3 also resulted in a huge
Cu-Cu contraction (Cu‚‚‚Cu ) 2.7694(5) Å in 4, see Figure 1).
This 0.538 Å contraction is accompanied by a dramatic compression
of the average Cu-Pµ-Cu angle from 90.63° in 3 to 75.02° in 4.
While modest distortions of a similar nature are observed upon
one-electron oxidation of 1,4 the Cu2P2 system is unexpectedly
flexible. Despite the structural compression that occurs upon
oxidation, there is complete retention of the pseudotetrahedral Cu
centers and the dimeric topology.
Supporting Information Available: Experimental and character-
ization data; crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Two-electron oxidation of 3 with 2 equiv of [FeCp2][BArF ] (ArF
4
) 3,5-(CF3)2C6H3) yielded [{(PPP)Cu}2][BArF ] (5), a deep blue-
4 2
purple compound with an intense LMCT absorption at 683 nm (ꢀ
) 10 600 M-1 cm-1, Figure 2b). 1H, 13C, 19F, and 31P NMR
resonances were observed for 5, consistent with population of an
S ) 0 ground state at room temperature. 31P{1H} NMR resonances
occur at 266.1 (2P) and 37.3 ppm (4P) in CD2Cl2. The extreme
downfield chemical shift of the bridging phosphide resonance of 5
is likely the result of the large change in bond angles at the bridging
phosphides.12,13
References
(1) (a) Gray, H. B.; Malmstrom, B. G.; Williams, R. J. P. J. Biol. Inorg.
Chem. 2000, 5, 551. (b) Rorabacher, D. B. Chem. ReV. 2004, 104, 651.
(2) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(3) Azacryptand ligands can overcome the need for large structural reorga-
nization and ligand addition/loss/exchange. See: Nelson J.; McKee, V.;
Morgan, G. G. Prog. Inorg. Chem. 1998, 47, 167.
(4) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 2885.
(5) (a) Williams, P. A.; Blackburn, N. J.; Sanders, D.; Bellamy, H.; Stura, E.
A.; Fee, J. A.; McRee, D. E. Nat. Struct. Biol. 1999, 6, 509. (b) DeBeer
George, S.; Metz, M.; Szilagyi, R. K.; Wang, H.; Cramer, S. P.; Lu, Y.;
Tolman, W. B.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem.
Soc. 2001, 123, 5757. (c) Gamelin, D. R.; Randall, D. W.; Hay, M. T.;
Houser, R. P.; Mulder, T. C.; Canters, G. W.; de Vries, S.; Tolman, W.
B.; Lu, Y.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 5246.
(6) For other examples of dicopper class III mixed-valence species, see: (a)
Houser, R. P.; Young, V. G., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1996,
118, 2101. (b) He, C.; Lippard, S. J. Inorg. Chem. 2000, 39, 5225.
(7) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030. Our
efforts to further characterize this Cu2N2 system will be reported in due
course.
(8) A dicopper system has been recently described for which CV data indicate
two pseudo-reversible redox events. Uptake of an additional ligand occurs
upon oxidation of the CuICuI to the Cu1.5Cu1.5 state: Jiang, X.; Bollinger,
J. C.; Baik, M.-H.; Lee, D. Chem. Commun. 2005, 1043.
(9) Snyder, B. S.; Patterson, G. S.; Abrahamson, A. J.; Holm, R. H. J. Am.
Chem. Soc. 1989, 111, 5214.
(10) For example: (a) Sellmann, D.; Binder, H.; Ha¨ussinger, D.; Heinemann,
F. W.; Sutter, J. Inorg. Chim. Acta 2000, 300-302, 829. (b) Huynh, M.
H. V.; El-Samanody, E.-S.; White, P. S.; Meyer, T. J. Inorg. Chem. 1999,
38, 3760.
(11) Robin, M.; Day, P. AdV. Inorg. Radiochem. 1967, 10, 247.
(12) For discussions on bridging phosphide 31P NMR resonances relating to
metal-metal bonding, see: (a) Targos, T. S.; Geoffroy, G. L.; Rheingold,
A. L. Organometallics 1986, 5, 12 and references therein. (b) Cartwright,
S. J.; Dixon, K. R.; Rattray, A. D. Inorg. Chem. 1980, 19, 1120.
(13) For a discussion on the dependence of 31P chemical shifts on bond angle,
see: Garrou, P. E. Chem. ReV. 1981, 81, 229.
Though suitable single crystals of 5 proved elusive, using AgSbF6
instead of [FeCp2][BArF ] afforded the more readily crystallized
4
salt [{(PPP)Cu}2][SbF6]2. Its molecular structure reveals two SbF6
anions per dimeric unit, and it is gratifying to observe that the Cu2P2
diamond core is maintained along with retention of pseudotetra-
hedral geometries at each copper center (Figure 1). Close examina-
tion of the bond parameters reveals a further contraction of the
Cu‚‚‚Cu distance (2.596(2) Å) and further compression of the
average Cu-Pµ-Cu angle (70.71°). Although the onset of a direct
bonding interaction between the two 17-electron centers upon
oxidation cannot be dismissed, it is plausible that the Cu‚‚‚Cu
contraction is a consequence of an optimized bridging angle that
facilitates exchange coupling between the Cu centers.14 Indeed, the
simplest oxidation state description for 5 involves two strongly
antiferromagnetically coupled CuII centers. The prototypical Cu2Cl62-
dianion, whose structure is very similar to that of 5, also features
two CuII centers that are either ferromagnetically or antiferromag-
netically coupled depending on the choice of counterion and the
resulting perturbations in bond angles.14c The structure of 5 is
distinctive because small-molecule CuII ions residing in pseudo-
tetrahedral coordination environments are rare.15 Moreover, few
examples of CuII phosphine complexes are known,16 and only one
previous example has been structurally characterized.16a
(14) For discussions on the effect of bridging angle on coupling between d9
centers in dicopper systems, see: (a) Charlot, M. F.; Jeannin, S.; Jeannin,
Y.; Kahn, O.; Lucrece-Abaul, J.; Martin-Frere, J. Inorg. Chem. 1979, 18,
1675. (b) Crawford, V. H.; Richardson, H. W.; Wasson, J. R.; Hodgson,
D. J.; Hatfield, W. E. Inorg. Chem. 1976, 15, 2107. (c) Hay, P. J.;
Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1975, 97, 4884.
(15) For examples, see: (a) Baumann, F.; Livoreil, A.; Kaim, W.; Sauvage,
J.-P. Chem. Commun. 1997, 35. (b) Tolman, W. B. Inorg. Chem. 1991,
30, 4877. (c) Knapp, S.; Keenan, T. P.; Zhang, X.; Fikar, R.; Potenza, J.
A.; Schugar, H. J. J. Am. Chem. Soc. 1990, 112, 3452. (c) Kitajima, N.;
Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990, 112, 3210 and
references therein.
A ramification of the elasticity of the Cu2P2 core manifests itself
in the luminescence behavior of 3. Excitation into the low-energy
absorptions of 3 (λex ) 500 nm) afforded an emission spectrum
with λmax ) 687 nm. This emission band is much broader and quite
red-shifted compared to that of the amide derivative 2.7 The
reorganizational energy upon excitation, which can be estimated
using the width of an emission band,17 is accordingly higher for 3
(ca. 3250 cm-1) relative to 2 (ca. 2600 cm-1). In addition, while 2
(16) (a) Pilloni, G.; Bandoli, G.; Tisato, F.; Corain, B. Chem. Commun. 1996,
433. (b) Lobana, T. S.; Bhatia, P. K. J. Chem. Soc., Dalton Trans. 1992,
1407. (c) Zelonka, R. A.; Baird, M. C. Chem. Commun. 1971, 780.
(17) See Supporting Information for details.
JA056071L
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16033