A new type of luminescent alkynyl Au4Cu2 cluster

Article abstract of DOI:10.1021/ic051503e

Changes in the optical properties of an alkynyldigold(I) complex upon reaction with Cu(I) are associated with a complicated structural change to form an unusual Au₄Cu₂ cluster with metallophilic interactions as well as π-alkyne coordination.

Full text of DOI:10.1021/ic051503e

Inorg. Chem. 2006, 45, 1418−1420
A New Type of Luminescent Alkynyl Au4Cu2 Cluster†
He´ctor de la Riva,‡ Mark Nieuwhuyzen,‡ Claudio Mendicute Fierro,‡ Paul R. Raithby,§
Louise Male,§ and M. Cristina Lagunas*,‡
School of Chemistry, Queen’s UniVersity Belfast, Belfast BT9 5AG, U.K., and Department of
Chemistry, UniVersity of Bath, ClaVerton Down, Bath BA2 7AY, U.K.
Received September 2, 2005
Scheme 1. Example of a Digold Compound Proposed as a Cation
Changes in the optical properties of an alkynyldigold(I) complex
upon reaction with Cu(I) are associated with a complicated
structural change to form an unusual Au4Cu2 cluster with
Probe1c
metallophilic interactions as well as
π-alkyne coordination.
Scheme 2. Synthesis of Complexes 1 and 2
Extensive research on Au(I)‚‚‚Au(I) (aurophilic) interac-
tions has been stimulated by the interesting photophysical
properties they confer on Au(I) complexes. The possibility
of switching “on” and “off” the emission of Au(I) compounds
by favoring or restricting aurophilicity can be exploited in
the development of molecular sensors or optical devices. For
example, Yam et al.1 have undertaken pioneering work on
the use of dinuclear Au(I) complexes containing crown ether
fragments or alkynyl ligands (Scheme 1) that are capable of
trapping an additional metal ion and concomitantly alter the
emission of the compounds. However, the structures of the
final products in these “switches” are unknown, and it is
therefore difficult to establish to what extent variable
Au‚‚‚Au distances are actually responsible for the observed
changes in the emission. To get a better insight into this type
of “switch”, we have prepared a digold(I) alkynyl complex
containing the rigid diphosphine 4,6-bis(diphenylphosphino)-
dibenzofuran (dbfphos), in contrast to the more flexible
phosphines of the previous examples, and studied its reactiv-
ity toward Cu(I). The crystal structures and optical behavior
of both the precursor and final compounds are described.
As expected, the optical properties are perturbed in the
presence of Cu(I), but instead of π-alkynyl coordination
of a single Cu(I) ion, a dimeric species with an unprece-
dented Au4Cu2 core, partly held through Au(I)‚‚‚Cu(I) and
Cu(I)‚‚‚Cu(I) interactions, is found in the solid state.
PhCtCH in the presence of a base (Scheme 2). Its structure
(Figure 1) exhibits a very weak intramolecular Au‚‚‚Au
contact of 3.401(1) Å.3 The gold alkynyl fragments are
almost perpendicular to each other, and the geometries
around the metal atoms are slightly distorted from linearity,
with P1-Au1-C13 and P2-Au2-C33 angles of 166.4(2)°
and 171.1(2)°, respectively.
Addition of [Cu(MeCN)4][PF6] to a solution of 1 resulted
in an immediate color change from off-white to orange as
complex 2 formed. It crystallized as the dimer [(µ-dbfphos)-
Complex [Au2{(CtC(C6H5)}2(µ-dbfphos)] (1) was readily
obtained by the reaction of [Au2Cl2(µ-dbfphos)]2 with
† Dedicated to Dr. Jose´-Antonio Abad on the occasion of his retirement.
* To whom correspondence should be addressed. E-mail: c.lagunas@
qub.ac.uk.
(2) Pintado-Alba, A.; de la Riva, H.; Nieuwhuyzen, M.; Bautista, D.;
Raithby, P. R.; Sparkes, H. A.; Teat, S. J.; Lo´pez-de-Luzuriaga, J.
M.; Lagunas, M. C. Dalton Trans. 2004, 3459-3467.
‡ Queen’s University Belfast.
(3) Crystal data for C52H36Au2OP2 (1): M ) 1132.68, orthorhombic, space
group P212121, a ) 11.5165(1) Å, b ) 18.0618(2) Å, c ) 19.6211(2)
§ University of Bath.
(1) (a) Yam, V. W.-W.; Li, C.-K.; Chan, C.-L. Angew. Chem., Int. Ed.
1998, 37, 2857-2859. (b) Li, C.-K.; Lu, X.-X.; Wong, K. M.-C.; Chan,
C.-L.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2004, 43, 7421-7430.
(c) Yam, V. W.-W.; Cheung, K.-L.; Cheng, E. C.-C.; Zhu, N.; Cheung,
K.-K. Dalton Trans. 2003, 1830-1835.
Å, U ) 4081.36(7) Å3, T ) 150 K, Z ) 4, µ ) 7.299 mm-1, Rint
)
0.0871. A total of 72 223 reflections were measured for the angle range
2.95 < 2θ < 27.43°, and 9294 independent reflections were used in
the refinement. The final residuals were wR2 ) 0.0757 and R1 )
0.0310 [I > 2σ(I)].
1418 Inorganic Chemistry, Vol. 45, No. 4, 2006
10.1021/ic051503e CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/28/2006

COMMUNICATION
in 1), there are two Cu‚‚‚Au contacts of 2.8524(16) Å
(Cu1‚‚‚Au1). This distance is shorter than the sum of the
Au and Cu van der Waals radii (3.06 Å) and lies within the
range found in other Au(I)/Cu(I) complexes (2.6-3.0 Å).6-8
The distance between Cu1 and Au2 [3.2847(16) Å] is
significantly longer, but it could still contribute to the overall
stability of the cluster. In addition, a Cu‚‚‚Cu contact of
2.898(3) Å, slightly above the sum of the van der Waals
radii of two Cu atoms (2.8 Å), is found. Although other
mixed-metal alkynyl clusters have been reported,6 including
the pentanuclear Au3Cu2 alkyne derivative [NnBu4][Au3-
Cu2(C2Ph)6] [Au‚‚‚Cu, 2.783-3.016(3) Å],6a the nuclearity
of 2 is unique. Moreover, in contrast with the abundance of
crystallographic evidence on Au(I)‚‚‚Au(I) and, to a much
lesser extent, Au(I)‚‚‚Ag(I) contacts, data on Au(I)‚‚‚Cu(I)
interactions are still very rare.9
Figure 1. Crystal structure of 1 (H atoms omitted for clarity).4
The Cu atoms in 2 can be considered as pentacoordinated,
each interacting with the other Cu, one Au atom, and three
alkynyl groups. One alkyne (C9tC10) of each [(µ-dbfphos)-
Au2{η2-CtC(C6H5)}2] fragment bridges between the two Cu
atoms, whereas the other (C1tC2) coordinates only to one
Cu. The Cu-alkyne bonds with C9tC10 are highly asym-
metric, with the Cu1-C10 distance [2.420(12) Å] being
significantly longer than that of Cu1-C9 [2.182(12) Å]. In
each case, the Cu-C distance for the C bonded to Au
(2.016-2.200 Å) is shorter than that for the C attached to
the phenyl group. Analogous asymmetric Cu-alkyne π
bonding has been found in [NnBu][Au3Cu2(C2Ph)6].6a The
Au-C, Au-P, and CtC distances are not affected by
Cu(I) π coordination because they are similar in both 1 and
2. The coordination around each Au atom in 2 is distorted
from linearity with angles of 171.2(4)° (P1-Au1-C9) and
Figure 2. Crystal structure of the cation in 2 (H atoms omitted for clarity).4
Au2{η2-CtC(C6H5)}2Cu]2[PF6]2‚2Cl2CH2 (Scheme 2 and
Figure 2). Its room-temperature 1H and 31P{1H} NMR show
broad peaks, indicating a dynamic process in solution. Thus,
in addition to the [PF6]- septuplet at -143 ppm, the room-
temperature 31P{1H} NMR spectrum of 2 contains a broad
signal at ca. 24 ppm, which becomes sharper at 213 K. In
the crystal structure of 2 (Figure 2), the two P atoms of each
diphosphine are inequivalent. If complex 2 adopts the same
structure in solution as it does in the crystal, the P centers
must be in fast exchange, even at low temperature, to give
rise to only one singlet. In this case, the broadness of the
room-temperature signal would have to be due to a different
dynamic process (e.g., different species in exchange). After
several hours, additional species are observed in the 31P-
{1H} NMR spectra. A full investigation of these solution
processes is currently underway and will be included in a
full-length paper following this Communication.
The solid-state IR spectrum of 2 shows the ν(CtC)
stretching at 2031 cm-1, shifted to lower frequency in relation
to that of 1 (2116 cm-1), as expected from the weakening
of the CtC bond upon π coordination.1c
The crystal structure of 2 (Figure 2) contains a dimeric
cation of C2 symmetry with four Au(I) and two Cu(I) atoms.5
There are also two PF6 anions and two molecules of CH2-
Cl2 per dication. Instead of a weak Au‚‚‚Au interaction (as
(5) Crystal data for C106H76Au4Cl4Cu2F12O2P6 (2): M ) 2852.23,
monoclinic, space group P2(1)/c, a ) 14.0349(10) Å, b ) 18.0306-
(14) Å, c ) 19.9673(15) Å, R ) 90°, â ) 107.5050(10)°, γ ) 90°,
U ) 4818.9(6) Å3, Z ) 2, µ ) 6.785 mm-1, Rint ) 0.0942. A total of
41 379 reflections were measured for the angle range 1.52 < 2θ <
28.34°, and 11 259 independent reflections were used in the refinement.
The final parameters were wR2 ) 0.1374 and R1 ) 0.0657 [I > 2σ-
(I)].
(6) (a) Abu-Salah, O. M.; Al-Ohaly, A. R.; Knobler, C. B. J. Chem. Soc.,
Chem. Commun. 1985, 1502-1503. (b) Hussain, M. S.; Ul-Haque,
M.; Abu-Salah, O. M. J. Cluster Sci. 1996, 7, 167-177. (c) Abu-
Salah, O. M. J. Organomet. Chem. 1998, 565, 211-216.
(7) (a) Hao, L.; Mansour, M. A.; Lachicotte, R. J.; Gysling, H. J.;
Eisenberg, R. Inorg. Chem. 2000, 39, 5520-5529. (b) Copley, R. C.
B.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 1996, 491-500.
(c) Chan, C.-K.; Guo, C.-X.; Cheung, K.-K.; Li, D.; Che, C.-M. J.
Chem. Soc., Dalton Trans. 1994, 3677-3692.
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Longoni, G.; Monari, M.; Zacchini, S. J. Cluster Sci. 2001, 12, 75-
87. (b) Krogstad, D. A.; Young, V. G., Jr.; Pignolet, L. H. Inorg.
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T. J. Chem. Soc., Dalton Trans. 1993, 559-566. (d) Haupt, H.-J.;
Seewald, O.; Florke, U.; Buss, V.; Weyhermuller, T. J. Chem. Soc.,
Dalton Trans. 2001, 3329-3336. (e) Kappen, T. G. M. M.; Schlebos,
P. P. J.; Bour, J. J.; Bosman, W. P.; Smits, J. M. M.; Beurskens, P.
T.; Steggerda, J. J. Inorg. Chem. 1995, 34, 2133-2142. (f) Kappen,
T. G. M. M.; Schlebos, P. P. J.; Bour, J. J.; Bosman, W. P.; Smits, J.
M. M.; Buerskens, P. T.; Steggerda, J. J. J. Am. Chem. Soc. 1995,
117, 8327-8334. (g) Salter, I. D.; Sik, V.; Williams, S. A.; Adatia,
T. J. Chem. Soc., Dalton Trans. 1996, 643-652.
(9) A recent search in the Cambridge Data Base resulted in ca. 800 and
ca. 50 hits for complexes with Au‚‚‚Au and Au‚‚‚Ag contacts,
respectively, whereas only 16 complexes with Au(I)‚‚‚Cu(I) contacts
were found (see refs 6-8). Most of these are trimetallic clusters of
the type AunCumMl (M ) Fe, Pt, Ru, or Re and n + m + l ) 4-12;
see ref 8).
(4) ORTEP representations have been done using ORTEP-3 for Windows
(Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565). Ellipsoids are
drawn at 40% (Figure 1) or 30% (Figure 2) probability.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1419

COMMUNICATION
The solid-state spectra of both compounds consist of broad
bands, with that of 2 (ca. 530 nm) red-shifted relative to the
emission of 1 (508 nm). The lifetime of 1 measured in the
solid state at 77 K (λexc ) 325 nm) is 41.6 µs, and it is
consistent with an excited state of triplet parentage. The
emission of other phosphinylalkynylgold complexes has
previously been assigned to σ(Au-P) f π*(CtCPh) and/
or σ(Au-P) f π*(aryl-phosphine).10 In the present case,
a σ(Au-P) f π*(CtCPh) origin, where the π* orbital of
the acetylide decreases in energy when coordinated to
Cu(I), is a plausible assignment. However, some metal
character in the excited states of both 1 and 2 should also
be considered. The excitation spectrum of 1 shows a
maximum at ca. 320 nm, whereas that of 2 appears
featureless and consists of broad continuous excitations
extending to ca. 420 nm. These features correspond with the
LE shoulders observed in the absorption spectra of 1 and 2
and support Au- and cluster-centered origins for their
respective emissions. Although relatively long metallophilic
contacts are found in 1 (3.401 Å) and 2 [e.g., Cu1‚‚‚Au2,
3.2847(16) Å], shortening of the metal‚‚‚metal distances in
the excited state has been shown to be responsible for intense
luminescence in multimetallic complexes.11
The excitation and emission of 1 in dichloromethane at
77 K are similar to those of the solid, whereas at room
temperature, an additional band is observed at 465 nm. This
could be related to IL [π f π*(CtCPh and/or phosphine)]
transitions. In glassy dichloromethane, the emission of 2 is
similar to that of the solid when measured immediately upon
dissolution, indicating that the same structure remains in these
conditions.
In conclusion, whereas the coordination of Cu(I) to Au-
alkyne moieties significantly perturbs the optical properties
of the parent compound, the resulting product has a complex
solid-state structure involving weak Cu‚‚‚Cu/Au interactions
and π-alkyne coordination. Analogous processes described
before1 may also involve complicated structural changes.
Figure 3. Emission spectra (intensity in arbitrary units) of 1 (λexc ) 320
nm) and 2 (λexc) 350-395 nm).
176.9(4)° (P2-Au2-C1). The larger distortion in the former
corresponds to the P-Au-C strand supporting the Au‚‚‚Cu
interaction. The Au1-C9tC10 and Au2-C1tC2 angles are
162.6(11)° and 164.5(12)°, respectively. The torsion angle
between the two Au-alkynyl axes of each diphosphine is
of ca. 41°.
The UV-vis spectrum of 1 in dichloromethane contains
a broad, intense high-energy (HE) absorption between 230
and 300 nm with local maxima at 265 and 284 nm (ꢀ )
70 695 and 72 794 dm3 mol-1 cm-1, respectively). In
addition, there is a low-energy (LE) shoulder at ca. 307 nm
(ꢀ ) 20 832 dm3 mol-1 cm-1) with a tail extending to ca.
330 nm. The absorption spectrum of 2 in dichloromethane
also has a HE broad band at 250-320 nm and a LE tail
reaching to longer wavelengths (ca. 350 nm) than that of 1.
The broad HE absorptions are likely to originate from
intraligand (IL) π f π*(CtCAr) transitions of the alkynyls
(i.e., the free alkynes also have intense bands at 250-300
nm). However, π* orbitals associated with the aromatic
groups of the phosphine can also be involved in such
transitions because the electronic absorption spectra of both
the free dbfphos and [Au2Cl2(µ-dbfphos)] also have broad
bands in the same region.2 The LE shoulders/tails may arise
from σ f π* absorptions involving the promotion of an
electron from the σ(Au-P) bond orbital to an empty
antibonding orbital of π origin (CtCPh or phosphine), but
they could also correspond to metal- or cluster-centered
transitions. In addition, the lowest-energy absorption in 2
(330-350 nm) could also be associated with the Cu-π-
(alkynyl) core (i.e., the π*(CtC) orbital decreases in energy
upon coordination to Cu).
Acknowledgment. We are indebted to the EPSRC (HdlR;
Grant GR/R95005/01) for financial support, to Johnson-
Matthey PLC for the loan of NaAuCl4, and to Dr. J. M.
Lo´pez de Luzuriaga for the measurement of the lifetime of
1 and for very fruitful discussions.
Supporting Information Available: Experimental section;
absorption, excitation, and emission spectra; lifetime of the solid-
state emission of 1; and CIF files for 1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
Luminescence was observed for both compounds in
solution and in the solid state at 77 K. Room-temperature
emission was measured for 1 in solution and for 2 in the
solid state (Figure 3). The emission of 2 was recorded at
various wavelengths, between 320 and 395 nm, and no
significant changes were observed (see the Supporting
Information). Compound 1 did not exhibit significant emis-
sion in the solid state at room temperature, whereas complex
2 showed room-temperature solution emission, but the
spectra were not reproducible, presumably because of the
formation of different species with time (as observed by
NMR, see above). A full account of the solution behavior
of this and analogous compounds will be included in a full-
length paper.
IC051503E
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1420 Inorganic Chemistry, Vol. 45, No. 4, 2006