Synthesis, Electronic Spectra and Solvent-Induced Reversible Dissociation of Diphosphine(hexafluoroacetylacetonato)copper(I) Complexes

Article abstract of DOI:10.1515/znb-2003-1003

The complexes Cu^I(P-P)(hfac) with P-P = 1,2- bis(diphenylphosphino)ethane (diphos), 1,3-bis-(diphenylphosphino)propane (prophos), 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (binap) and hfac = hexafluoroacetylacetonate were synthesized and spectroscopically characterized. In the solid state or in solutions of non-polar solvents these compounds are yellow owing to a long-wavelength (hfac^- →P-P) ligand-to-ligand charge transfer absorption. In coordinating solvents such as CH₃CN the complexes undergo a reversible dissociation according to the equation: Cu(P-P)(hfac) + n CH₃CN ? [Cu(P-P)(CH ₃CN)_n]⁺hfac^-. While the complexes are not luminescent the ion pairs [Cu(P-P)(CH₃CN)_n] ⁺hfac^- are emissive at 77 K. The cations and the anions show separate emissions as indicated by the excitation spectra.

Full text of DOI:10.1515/znb-2003-1003

Synthesis, Electronic Spectra and Solvent-Induced Reversible
Dissociation of Diphosphine(hexafluoroacetylacetonato)copper(I)
Complexes
Valeri Pawlowski, Andreas Strasser, and Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany
Reprint requests to Prof. Dr. A. Vogler. E-mail: arnd.vogler@chemie.uni-regensburg.de
Z. Naturforsch. 58b, 950 – 954 (2003); received July 11, 2003
The complexes CuI(P-P)(hfac) with P-P = 1,2-bis(diphenylphosphino)ethane (diphos), 1,3-bis-
(diphenylphosphino)propane (prophos), 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (binap) and
hfac = hexafluoroacetylacetonate were synthesized and spectroscopically characterized. In the solid
state or in solutions of non-polar solvents these compounds are yellow owing to a long-wavelength
(hfac− →P-P) ligand-to-ligand charge transfer absorption. In coordinating solvents such as CH3CN
the complexes undergo a reversible dissociation according to the equation:
Cu(P-P)(hfac) + n CH3CN
ꢀ
[Cu(P-P)(CH3CN)n]+hfac−.
While the complexes are not luminescent the ion pairs [Cu(P-P)(CH3CN)n]+hfac− are emissive at
77 K. The cations and the anions show separate emissions as indicated by the excitation spectra.
Key words: Electronic Spectra, Luminescence, Copper Complexes, β-Diketonate Complexes
Introduction
1,3-bis(diphenylphosphino)propane (prophos), 2,2’-
bis(diphenylphosphino)-1,1’-binaphthyl (binap) and
hfac = hexafluoroacetylacetonate for the present inves-
tigation. These complexes should be easily accessible
and suitable targets for the examination of their optical
properties. In this context it is of interest that neutral
phosphine complexes of the type CuI(P-P)BH4 have
been shown to display an intraligand (IL) phosphores-
cence of the coordinated disphosphine at 77 K or even
at r.t. [4, 10 – 13].
Complexes of the type CuILn(β-diketonate) with
L = e. g. phosphine, diolefin and alkyne have attracted
much interest because they are useful precursors for
the chemical vapor deposition of copper metal [1 –
3]. Moreover, copper(I) compounds in general, includ-
ing phosphine complexes, show a luminescence [4, 5]
which might be utilized for various applications in op-
tical devices. Most emissive Cu(I) complexes are ei-
ther cations or polynuclear compounds. Since they are
hardly volatile they cannot be used for copper de-
position from the gas phase as required for diverse
industrial processes. Their limited solubility in non-
polar solvents is a further disadvantage for certain
technical procedures. It follows that neutral mononu-
clear complexes of the general composition CuI(PR3)n
(β-diketonate) with n = 1 and 2 are valuable sub-
jects for further explorations. Several such compounds
have been prepared and structurally characterized [6 –
9]. Surprisingly, bidentate phosphines which may form
more stable complexes have not been included in pre-
vious studies. We explored this possibility and se-
lected compounds of the type CuI(P-P)(hfac) with
P-P = 1,2-bis(diphenylphosphino)ethane (diphos),
Experimental Section
Materials
All solvents used for spectroscopic measurements were
of spectrograde quality. Cu(BTMSA)(hfac) (BTMSA =
bis(trimethylsilyl)acetylene) (Aldrich), diphos (Acros),
prophos (Acros) and binap (Strem) were commercially
available and used without further purification. The com-
plexes Cu(P-P)(hfac) were obtained by the following
procedures.
Cu(prophos)(hfac)
A mixture of Cu(BTMSA)(hfac) (0.54 g, 1.22 mmol) and
prophos (0.51 g, 1.24 mmol) in 15 ml of toluene was refluxed
for 30 min. The mixture was then cooled to r.t. Upon addi-
c
0932–0776 / 03 / 1000–0950 $ 06.00 ꢀ 2003 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Brought to you by | Vanderbilt University
Authenticated
Download Date | 1/1/20 7:06 PM

V. Pawlowski et al. · Diphosphine(hexafluoroacetylacetonato)copper(I) Complexes
951
Table 1. Absorption maxima and extinction coefficients of
tion of hexane a red-brown oily precipitate was formed. It
was removed by filtration. The volume of the filtrate was re- Cu(P-P)(hfac) in dichlormethane at 298 K.
λmax / nm (ε / 1 mol−1 cm−1
Cu(prophos)(hfac) 252 (13800), 288 (14700), ≈ 400 (690)
)
duced by evaporation. Upon addition of ether and standing
at r.t. red-orange crystals separated. They were collected by
filtration and washed with ethanol and ether, and dried over Cu(diphos)(hfac) 253 (12500), 291 (12200), ≈ 400 (670)
Cu(binap)(hfac)
275 (28100) sh, 290 (21400) sh, 329 (9100) sh,
376 (4100)
silica gel.
Yield: 0.35 g (44%). Anal. calcd. C 56.27, H 3.98; found
C 56.46, H 3.92.
Cu(diphos)(hfac)
A mixture of Cu(BTMSA)(hfac) (0.43 g, 1 mmol) and
diphos (0.39 g, 1 mmol) in 20 ml of toluene was refluxed for
20 min. A yellow solution with a yellow solid was obtained.
Precipitation was completed by addition of hexane. The yel-
low precipitate was collected by filtration, washed with hex-
ane and dried over silica gel.
Yield: 0.47 g (72%). Anal. calcd. C 55.65, H 3.77; found
C 56.16, H 4.03.
Cu(binap)(hfac)
A mixture of Cu(BTMSA)(hfac) (0.45 g, 1 mmol) and bi-
nap (0.63 g, 1 mmol) in 20 ml of toluene was refluxed for 1 h
and then cooled to r.t. An orange precipitate formed. It was
collected by filtration, washed with hexane and dried over
silica gel.
Fig. 1. Electronic absorption spectra of Cu(P-P)(hfac) in
dichlormethane at 298 K, 1 cm cell: (a) Cu(binap)(hfac), c =
Yield: 0.74 g (83%). Anal. calcd. C 65.88, H 3.72; found
C 66.05, H 3.63.
3·10−5 mol l−1; (b) Cu(prophos)(hfac), c = 7·10−5 mol l−1
;
(c) Cu(diphos)(hfac), c = 7·10−5 mol l−1
.
Instrumentation
The compounds were obtained as orange or yellow
solids. Their composition was confirmed by elemental
analysis. The complexes dissolve in non-polar solvents
such as CH2Cl2 to yield yellow solutions which are
stable also in the presence of oxygen. The absorption
spectra and maxima of Cu(P-P)(hfac) are presented in
Fig. 1 and Table 1. The compounds are not luminescent
as solids or in non-polar solvents.
Absorption spectra were measured with a Kontron Uvikon
932 double beam spectrophotometer. Emission spectra were
recorded on a Hitachi 850 spectrofluorometer equipped with
a Hamamatsu 928 photomultiplier. The luminescence spec-
tra were corrected for monochromator and photomultiplier
efficiency variations.
Results
When Cu(P-P)(hfac) complexes are dissolved in po-
lar and coordinating solvents such as CH3CN the yel-
low colour disappears and colourless solutions are ob-
tained. This colour change is also observed if CH3CN
is added to the yellow solution of the complexes in
CH2Cl2. At constant complex concentrations and vari-
The complexes Cu(P-P)(hfac) with P-P = diphos,
prophos and binap were prepared by the reaction of
Cu(BTMSA)(hfac) [14 – 16] with P-P in toluene ac-
cording to the equation:
Cu(BTMSA)(hfac) + P-P → Cu(P-P)(hfac) + BTMSA
Brought to you by | Vanderbilt University
Authenticated
Download Date | 1/1/20 7:06 PM

952
V. Pawlowski et al. · Diphosphine(hexafluoroacetylacetonato)copper(I) Complexes
Table 2. Luminescence and excitation band maxima of Cu(P-
P)(hfac) in acetonitrile at 77 K.
Luminescence
λ / nm (λexe = 300 nm)
Cu(prophos) 405, 455, 484, 520 sh
(hfac)
Cu(diphos) 408, 452, 482, 520 sh
(hfac)
Excitation
λmax / nm
306, 315 sh (λem = 450 nm)
274 (λem = 400 nm)
307, 317 sh (λem = 450 nm)
273 (λem = 400 nm)
314 (λem = 460 nm)
320, 353 (λem = 505 nm)
Cu(binap)
(hfac)
457, 506, 544, 586 sh
Fig. 3. Electronic emission and excitation spectra of Cu(pro-
phos)(hfac) in CH3CN at 77 K. Emission (c): λexc = 300 nm.
Excitation (a): λem = 400 nm; (b): λem = 450 nm.
Fig. 2. Changes of the absorption spectrum of Cu-
(diphos)(hfac) in variable CH2Cl2/CH3CN mixtures:
(a) CH2Cl2; (b) CH2Cl2/CH3CN 16:1; (c) CH2Cl2/CH3CN
16:2; (d) CH2Cl2/CH3CN 16:5.
able CH2Cl2/CH3CN mixtures spectral changes in-
cluding an isosbestic point at 321 nm are observed
(Fig. 2). These spectral variations are completely re-
versible. The evaporation of the solvent of a colourless
solution of Cu(P-P)(hfac) in CH3CN regenerates the
yellow solid.
Fig. 4. Electronic emission and excitation spectra of Cu(bi-
nap)(hfac) in CH3CN at 77 K. Emission (c): λexc = 300 nm.
Excitation (a): λem = 460 nm; (b): λem = 505 nm.
At r. t. the solutions of Cu(P-P)(hfac) in CH3CN are
not luminescent, but in a CH3CN matrix at 77 K a
strong emission appears (Figs. 3 – 4 and Table 2). The
excitation spectra (Figs. 3 – 4 and Table 2) consist of
separate excitation bands depending on the emission
wavelength.
at higher energies. The σaπ IL as well as the ππ∗ IL
triplets of CuI(arylphosphine) complexes may be emis-
sive, sometimes even at r.t. The phosphorescence from
the σaπ IL state of coordinated phenylphosphines ap-
pears around 500 nm while the ππ ∗ triplets emit at
shorter wavelength between 400 and 450 nm, depend-
ing on the specific ligand and the solvent.
Discussion
Copper(I) arylphosphine complexes are character-
In general, these arylphosphine complexes are
ized by two low-energy phosphine IL excited states colourless since the σaπ and ππ∗ IL absorptions ap-
[4, 10, 12, 13, 17]. Generally, the lower one is of σaπ pear in the UV spectral region. Simple hfac complexes
type and involves the promotion of an electron from of redox-inert metals as well as CuI(CH3CN)nhfac
the Cu-P σ-bond to the π∗ orbitals of the aromatic sub- are also colourless. These complexes are character-
stituents. The ππ∗ states of the aryl groups are located ized by hfac IL absorptions at λmax ≈300 nm [18, 19].
Brought to you by | Vanderbilt University
Authenticated
Download Date | 1/1/20 7:06 PM

V. Pawlowski et al. · Diphosphine(hexafluoroacetylacetonato)copper(I) Complexes
953
In contrast, our complexes Cu(P-P)(hfac) as well as The release of hfac should be facilitated if the phos-
Cu(PR3)n(hfac) with n = 1 and 2 [6 – 9] are orange phine ligands of Cu(PR3)2(hfac) are sterically demand-
to yellow. This colour is caused by a distinct absorp- ing. In the case of Cu(PCy3)2(hfac) with Cy = cyclo-
tion with a maximum at approximately 400 nm (Fig. 1 hexyl it has been demonstrated that the steric over-
and Table 1). We suggest that this absorption belongs crowding leads to a weakening and lengthening of
to a ligand-to-ligand charge transfer (LLCT) transition the Cu-hfac bonds [9]. The bidentate P-P ligands ap-
from the hfac ligand to the phosphine. Related LLCT parently exert a similar effect. Moreover, the CH3CN
transitions with phosphines and arsines as acceptor lig- as a solvent does not only function as an enter-
ands have been identified in the spectra of other Cu(I) ing ligand but it should also stabilize the ion pair
complexes [20, 21]. The CT donor ability of the hfac [Cu(P-P)(CH3CN)n]+hfac− owing to the high solvent
ligand is well known. Various hfac complexes with ox- polarity.
idizing metal centers display long-wavelength ligand-
The loss of the yellow colour upon formation of the
to-metal charge transfer (LMCT) absorptions [18]. ion pair is now easily explained by the disappearance
Moreover, photo electron spectroscopy and calcula- of the hfac → P-P LLCT absorption. In addition,
tions have shown that the HOMO of Cu(PMe3)(hfac) is strong evidence for the ion pair formation in CH3CN
essentially located at the hfac ligand [22]. Accordingly, is based on luminescence measurement at 77 K. The
it is quite reasonable to assign the longest-wavelength presence of the hfac− anion is indicated by its diagnos-
band of Cu(P-P)(hfac) in CH2Cl2 at λmax ≈400 nm to
tic green phosphorescence [18, 23 – 25] (Figs. 3 – 4,
a LLCT transition.
Table 2) at λmax ≈455 nm. The excitation maxima
at 306, 307 and 314 nm show that hfac− exists as an
What happens if Cu(P-P)(hfac) complexes are dis-
solved in CH3CN? The disappearance of the yellow isolated species. The other emission maxima of Cu(P-
colour is certainly not a solvatochromic effect but is
caused by a reversible chemical reaction as indicated
by the concomitant spectral changes including an isos-
bestic point (Fig. 2). We suggest that our results are
consistent with a ligand substitution:
P)(hfac) in CH3CN (Fig. 3, Table 2) with P-P = diphos,
prophos are attributed to the IL (P-P) phosphorescence
of [Cu(P-P)(CH3CN)n]+. These emissions resemble
those of CuI(P-P)BH4 complexes [4, 10 – 13]. The
long-wavelength emissions of [Cu(binap)(CH3CN)n]+
(λmax = 506, 544 and 586 nm, Fig. 4) is also clearly a
phosphorescence of the binap ligand [26, 27]. Again,
the corresponding excitation spectra cover the region
of the P-P IL absorptions but do not include the hfac
IL band.
Cu(P-P)(hfac) + nCH3CN → [Cu(P-P)(CH3CN)n]+hfac−
(n = 1 or 2).
Previous observations have already shown that the hfac
ligand of Cu(PMe3)2(hfac) is replaced upon addition of
a phosphine excess [8]:
Acknowledgement
Cu(PMe3)2(hfac) + 2PMe3 → [Cu(PMe3)4]+hfac−.
Financial support by BASF is gratefully acknowledged.
[1] K. M. Chi, H.-K. Shin, M. J. Hampden-Smith, T. T. Ko-
das, Inorg. Synth. 31, 289 (1997).
[2] K. M. Chi, H.-K. Shin, M. J. Hampden-Smith, E. N.
Duesler, Polyhedron 10, 2293 (1991).
[3] P. Doppelt, T. H. Baum, J. Organomet. Chem. 517, 53
(1996), and references cited therein.
[4] C. Kutal, Coord. Chem. Rev. 99, 213 (1990).
[8] K. M. Chi, J. Farkas, M. J. Hampden-Smith, T. T. Ko-
das, E. N. Duesler, J. Chem. Soc. Dalton Trans. 3111
(1992).
[9] H.-K. Shin, M. J. Hampden-Smith, E. N. Duesler, T. T.
Kodas, Can. J. Chem. 70, 2954 (1992).
[10] P. A. Grutsch, C. Kutal, J. Am. Chem. Soc. 101, 4228
(1979).
[5] D. M. Roundhill, Photochemistry and Photophysics of [11] S. W. Orchard, C. Kutal, Inorg. Chim. Acta 64, L95
Metal Complexes, Plenum Press, New York (1994).
[6] H.-K. Shin, M. J. Hampden-Smith, E. N. Duesler, T. T.
Kodas, Polyhedron 10, 645 (1991).
[7] H.-K. Shin, K. M. Chi, J. Farkas, M. J. Hampden-
Smith, T. T. Kodas, E. N. Duesler, Inorg. Chem. 31, 424
(1992).
(1982).
[12] D. P. Segers, M. K. De Armond, P. A. Grutsch, C. Ku-
tal, Inorg. Chem. 23, 2874 (1984).
[13] B. Liaw, S. W. Orchard, C. Kutal, Inorg. Chem. 27,
1311 (1988).
Brought to you by | Vanderbilt University
Authenticated
Download Date | 1/1/20 7:06 PM

954
V. Pawlowski et al. · Diphosphine(hexafluoroacetylacetonato)copper(I) Complexes
[14] K. M. Chi, H.-K. Shin, M. J. Hampden-Smith,
T. T. Kodas, E. N. Duesler, Inorg. Chem. 30, 4293
(1991).
[15] P. Doppelt, T. H. Baum, J. Organomet. Chem. 517, 53
(1996).
[16] T. H. Baum, C. E. Larson, J. Electrochem. Soc. 140,
154 (1993).
[17] D. J. Fife, W. M. Moore, K. W. Morse, Inorg. Chem. 23,
1545 (1984).
[20] A. Acosta, J. I. Zink, J. Cheon, Inorg. Chem. 39, 427
(2000).
[21] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 5, 112
(2002).
[22] X. Li, G. M. Bancroft, R. J. Puddephatt, Z. Yuan. K. H.
Tan, Inorg. Chem. 35, 5040 (1996).
[23] D. Wexler, J. I. Zink, L. W. Tutt, S. R. Lunt, J. Phys.
Chem. 97, 13563 (1993).
[24] D. Wexley, J. I. Zink, Inorg. Chem. 35, 4064 (1996).
[25] A. Strasser, A. Vogler, unpublished observations.
[18] R. L. Lintvedt, in A. W. Adamson, P. D. Fleischauer
(eds.): Concepts of Inorganic Photochemistry, chapter [26] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 2, 533
7, p. 299, Wiley, New York (1975).
[19] H. Kunkely, A. Vogler, Z. Naturforsch. 58b, 704
(2003).
(1999).
[27] V. Pawlowski, H. Kunkely, A. Vogler, unpublished ob-
servations.
Brought to you by | Vanderbilt University
Authenticated
Download Date | 1/1/20 7:06 PM