Synthesis and structural characterization of Cu(I) and Ni(II) complexes that contain the bis[2-(diphenylphosphino)phenyl]ether ligand. Novel emission properties for the Cu(I) species

Article abstract of DOI:10.1021/ic0201809

The pseudotetrahedral complexes [Cu(NN)(DPEphos)]BF₄, where DPEphos = bis[2-(diphenylphosphino)phenyl]-ether and NN = 1,10-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-di-n-butylphenanthroline (3), or two dimethylcyanamides (4), and NiCl₂(DPEphos) (5) have been synthesized and structurally characterized by X-ray crystallography and their solution properties examined by use of a combination of cyclic voltammetry, NMR spectroscopy, and electronic absorption spectroscopy. Complexes 1-4 possess a reversible Cu(II)/Cu(I) couple at potentials upward of +1.2 V versus Ag/AgCl. Compounds 1-3 exhibit extraordinary photophysical properties. In room-temperature dichloromethane solution, the charge-transfer excited state of the dmp (dbp) derivative exhibits an emission quantum yield of 0.15 (0.16) and an excited-state lifetime of 14.3 μs (16.1 μs). Coordinating solvents quench the charge-transfer emission to a degree, but the photoexcited dmp complex 2 retains a lifetime of over a microsecond in acetone, methanol, and acetonitrile.

Full text of DOI:10.1021/ic0201809

Inorg. Chem. 2002, 41, 3313−3322
Synthesis and Structural Characterization of Cu(I) and Ni(II) Complexes
that Contain the Bis[2-(diphenylphosphino)phenyl]ether Ligand. Novel
Emission Properties for the Cu(I) Species
Shan-Ming Kuang, Douglas G. Cuttell, David R. McMillin,* Phillip E. Fanwick, and Richard A. Walton*
Department of Chemistry, Purdue UniVersity, 1393 Brown Building,
West Lafayette, Indiana 47907-1393
Received March 7, 2002
The pseudotetrahedral complexes [Cu(NN)(DPEphos)]BF , where DPEphos ) bis[2-(diphenylphosphino)phenyl]-
4
ether and NN ) 1,10-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-di-n-butylphenanthroline (3), or
two dimethylcyanamides (4), and NiCl2(DPEphos) (5) have been synthesized and structurally characterized by
X-ray crystallography and their solution properties examined by use of a combination of cyclic voltammetry, NMR
spectroscopy, and electronic absorption spectroscopy. Complexes 1−4 possess a reversible Cu(II)/Cu(I) couple at
potentials upward of +1.2 V versus Ag/AgCl. Compounds 1−3 exhibit extraordinary photophysical properties. In
room-temperature dichloromethane solution, the charge-transfer excited state of the dmp (dbp) derivative exhibits
an emission quantum yield of 0.15 (0.16) and an excited-state lifetime of 14.3 µs (16.1 µs). Coordinating solvents
quench the charge-transfer emission to a degree, but the photoexcited dmp complex 2 retains a lifetime of over
a microsecond in acetone, methanol, and acetonitrile.
Introduction
groups.8-10 However, these systems have some limitations.
One is that the lowest energy CT excitation inevitably stems
from a metal-ligand dσ* orbital.8 An important consequence
is that the excited state typically prefers a tetragonally
flattened geometry whereas the ground state usually adopts
a more tetrahedral-like coordination geometry appropriate
for a closed-shell ion.8,11 Aside from reducing the energy
content, the geometric relaxation that occurs in the excited
state facilitates relaxation back to the ground state.12,13 Donor
media also tend to quench the lifetime. Blaskie and McMil-
lin14 first reported this type of exciplex quenching, and by
now, many other studies have confirmed the mecha-
nism.8,9,15,16 Introduction of sterically demanding ligands can
impede geometric relaxation as well as solvent attack. For
The relative ease of synthesis and the rich visible absorp-
tion spectra of transition metal complexes have continued
to foster interest in the use of many of them as platforms on
which to examine and develop photochemical applications.
Areas of recent interest include their role in solar energy
conversion,1 luminescence-based sensing,2,3 display devices,4
probes of biological systems,5,6 and phototherapy,7 where,
for the sake of brevity, the literature citations focus only on
some relatively recent contributions. Because of their ready
availability and the prevalence of low-lying charge-transfer
(CT) excited states, copper(I) complexes of polypyridine
ligands have attracted the attention of several research
* To whom all correspondence should be addressed. E-mail: mcmillin@
purdue.edu (D.R.M.); rawalton@purdue.edu (R.A.W.).
(1) Bignozzi, C. A.; Argazzi, R.; Kleverlaan, C. J. Chem. Soc. ReV. 2000,
29, 87.
(2) de Silva, A. P.; Fox, D. B.; Moody, T. S.; Weir, S. M. Pure Appl.
Chem. 2001, 73, 503.
(8) McMillin, D. R.; McNett, K. M. Chem. ReV. 1998, 98, 1201.
(9) Armaroli, N. Chem. Soc. ReV. 2001, 30, 113.
(10) Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J.
Coord. Chem. ReV. 2000, 208, 243.
(11) Eggleston, M. K.; Fanwick, P. E.; Pallenberg, A. J.; McMillin, D. R.
Inorg. Chem. 1997, 36, 4007.
(3) Rudzinski, C. M.; Nocera, D. G. Mol. Supramol. Photochem. 2001,
7, 1.
(4) Elliott, C. M.; Pichot, F.; Bloom, C. J.; Rider, L. S. J. Am. Chem.
Soc. 1998, 120, 6781.
(12) Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J.
Inorg. Chem. 1997, 36, 172.
(13) Cunningham, C. T.; Cunningham, K. L. H.; Michalec, J. F.; McMillin,
D. R. Inorg. Chem. 1999, 38, 4388.
(5) Zhao, Y. D.; Richman, A.; Storey, C.; Radford, N. B.; Pantano, P.
Anal. Chem. 1999, 71, 3887.
(6) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. ReV. 1999, 99, 2777.
(7) Ali, H.; van Lier, J. E. Chem. ReV. 1999, 99, 2379.
(14) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519.
(15) McMillin, D. R.; Kirchhoff, J. R.; Goodwin, K. V. Coord. Chem. ReV.
1985, 64, 83.
(16) Horvath, A. Coord. Chem. ReV. 1997, 159, 41.
10.1021/ic0201809 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/24/2002
Inorganic Chemistry, Vol. 41, No. 12, 2002 3313

Kuang et al.
example, [Cu(phen)(PPh3)2]+, where phen denotes 1,10-
phenanthroline, is virtually nonemissive in methanol, while
[Cu(dmp)(PPh3)2]+, where dmp denotes 2,9-dimethyl-1,10-
phenanthroline, has a lifetime of 330 ns in deoxygenated
solution.17 However, even with two bulky triphenylphos-
phines and a sterically active dmp ligand in the coordination
sphere, temperature-dependent emission studies suggest that
photoexcited [Cu(dmp)(PPh3)2]+ is still subject to exciplex
quenching in methanol.18
bite angle24 which is likely to influence both ground- and
the excited-state properties of resulting copper(I) complexes.
In fact, results described in this work show that [Cu(dmp)-
(DPEphos)]+ and related systems exhibit unusually long-
lived CT excited states that are relatively resistant to solvent-
induced exciplex quenching. Some of these results have been
the subject of a preliminary communication.25 Note that in
this earlier report25 we abbreviated bis[2-(diphenylphosphi-
no)phenyl]ether as POP, we but now use the alternative of
DPEphos.24
Our interest in exploring the photochemistry of mixed-
ligand copper(I) complexes that incorporated bidentate
phosphines in the coordination sphere has evolved from our
recent studies19 that were focused on the ability of the ligand
bis[2-(diphenylphosphino)phenyl]ether (abbreviated DPE-
phos), and other closely related tridentate ligands containing
P,O,P or P,N,P donor sets, to stabilize novel unsymmetrical
multiply bonded dirhenium(IV,II) and dirhenium(III,II)
complexes, in which these donors were bound in a tridentate
fashion. Interestingly, although DPEphos has previously been
used as a component of several catalytic palladium(0) and
palladium(II) systems for amination and cross coupling
reactions,20 few fully characterized transition metal com-
plexes of this ligand have been isolated. While mononuclear
compounds of nickel(0) and (II),21 palladium(0) and (II),20d,20g,22
platinum(II),20d and rhodium(I)23 have been described, only
one of these, namely the palladium(0) complex Pd(DPE-
phos)(TCNE) (TCNE ) tetracyanoethylene), has been
structurally characterized.21 We set out to address this dearth
of structural information by isolating and fully characterizing
a series of mixed-ligand copper(I) complexes, that contain
this ligand in combination with dimethylcyanamide and
various 1,10-phenanthroline ligands, along with the simple
pseudotetrahedral nickel(II) complex NiCl2(DPEphos). Dur-
ing the course of this work, we discovered some novel
photochemical properties of the copper(I) species, an ex-
amination of which became the major focus of this study.
While the conformational requirements imposed by the
ether linkage present in DPEphos do not prevent the ligand
from acting as a bidentate phosphine, the ligand has a wide
Experimental Section
A. Starting Materials. The DPEphos ligand was prepared by
the literature procedure,23 as were the phenanthroline ligands 2,9-
di-n-butyl-1,10-phenthroline (dbp) and 2,9-diphenyl-1,10-phenan-
throline (dpp).26 Samples of dimethylcyanamide, 2,9-dimethyl-1,10-
phenanthroline monohydrate (dmp‚H2O), and triphenylphosphine
were purchased from Aldrich Chemical Co., while 1,10-phenan-
throline monohydrate (phen‚H2O) was obtained from Fisher
Scientific Co. The copper(I) complex [Cu(NCCH3)4]BF4 was
prepared by the reaction of Cu(BF4)2 with Cu metal in refluxing
acetonitrile,27 while NiCl2‚3H2O was supplied by J. T. Baker
Chemical Co. and ferrocene was obtained from Aldrich Chemical
Co. The emission standard [Ru(bpy)3]Cl2‚6H2O was acquired from
the G. F. S. Smith Chemical Co. Potassium ferrioxalate28 and [Cu-
(dmp)(dppe)]PF6,29 where dppe is 1,2-bis(diphenylphosphino)-
ethane, were available from previous studies, as were [Cu(phen)-
(PPh3)2]BF418 and [Cu(dmp)(PPh3)2]BF4.18 Spectral grade solvents
came from standard supply houses.
B. Synthesis of Complexes of the Type [Cu(NN)(DPEphos)]-
BF4, Where NN ) 1,10-Phenanthroline or a Substituted
Phenanthroline Ligand, or Two Dimethylcyanamide Ligands.
(i) [Cu(phen)(DPEphos)]BF4 (1). A typical procedure is as follows.
A mixture of [Cu(NCCH3)4]BF4 (31 mg, 0.10 mmol) and bis[2-
(diphenylphosphino)phenyl]ether (54 mg, 0.10 mmol) in 20 mL of
dichloromethane was stirred at 25 °C for 2 h and then treated with
a solution of 1,10-phenanthroline monohydrate (20 mg, 0.10 mmol)
in 5 mL of dichloromethane. This reaction mixture was stirred for
an additional 1 h and filtered and the clear yellow filtrate
concentrated to ca. 5 mL. Approximately 5 mL of acetonitrile was
added, and the vapor diffusion of diethyl ether into the resulting
solution afforded yellow crystals of the complex, [Cu(phen)-
(DPEphos)]BF4 (1), that were washed with diethyl ether (2 × 5
mL); yield 61 mg (70%). The identity of this product was
established by X-ray crystallography. 1H NMR spectroscopy showed
the presence of lattice acetonitrile and diethyl ether, and this was
confirmed by the structure determination.
(17) Rader, R. A.; McMillin, D. R.; Buckner, M. T.; Matthews, T. G.;
Casadonte, D. J.; Lengel, R. K.; Whittaker, S. B.; Darmon, L. M.;
Lytle, F. E. J. Am. Chem. Soc. 1981, 103, 5906.
(18) Palmer, C. E. A.; McMillin, D. R. Inorg. Chem. 1987, 26, 3837.
(19) Kuang, S.-M.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. Commun.
2001, 4, 745. (b) Kuang, S.-M.; Fanwick, P. E.; Walton, R. A. Inorg.
Chem. 2002, 41, 405.
(20) Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1998, 120, 3694.
(b) Shaughnessy, K. H.; Kim, P.; Hartwig, J. F. J. Am. Chem. Soc.
1999, 121, 2123. (c) Kranenburg, M.; Kamer, P. C. J.; van Leeuwen,
P. W. N. M. Eur. J. Inorg. Chem. 1998, 25. (d) Kranenburg, M.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M. Eur. J. Inorg. Chem.
1998, 155. (e) Sadighi, J. P.; Harris, M. C.; Buchwald, S. L.
Tetrahedron Lett. 1998, 39, 5327. (f) Singer, R. A.; Buchwald, S. L.
Tetrahedron Lett. 1999, 40, 1095. (g) van Haaren, R. J.; Oevering,
H.; Coussens, B. B.; van Strijdonck, G. P. F.; Reek, J. N. H.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M. Eur. J. Inorg. Chem. 1999, 1237.
(21) Kranenburg, M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Vogt,
D.; Keim, W. J. Chem. Soc., Chem. Commun. 1995, 2177.
(22) Kranenburg, M.; Delis, J. G. P.; Kamer, P. C. J.; van Leeuwen, P. W.
N. M.; Vrieze, K.; Veldman, N.; Spek, A. L.; Goubitz, K.; Fraanje, J.
J. Chem. Soc., Dalton Trans. 1997, 1839.
The procedures for the synthesis of 2-5 were essentially identical
to that described in (i). Only the quantities of ligand that were used,
the product yields, and the elemental microanalyses are given.
(ii) Synthesis of [Cu(dmp)(DPEphos)]BF4 (2). 2,9-Dimethyl-
1,10-phenanthroline (21 mg, 0.10 mmol). Yield: 57 mg (63%).
(24) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Acc. Chem.
Res. 2001, 34, 895.
(25) Cuttell, D. G.; Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.; Walton,
R. A. J. Am. Chem. Soc. 2002, 124, 6.
(26) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J. P. Tetrahedron
Lett. 1982, 23, 5291.
(27) Kubas, G. J. Inorg. Synth. 1979, 19, 90.
(28) Ahn, B. T.; McMillin, D. R. Inorg. Chem. 1978, 17, 2253.
(29) Palmer, C. E. A. Ph.D. Dissertation, Purdue University, West Lafayette,
IN, 1980.
(23) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics, 1995,
14, 308.
3314 Inorganic Chemistry, Vol. 41, No. 12, 2002

NoWel Emission Properties for Cu(I) Species
Table 1. Crystallographic Data for the Complexes of Composition [Cu(phen)(DPEphos)]BF4·1.5Et2O·CH3CN (1), [Cu(dmp)(DPEphos)]BF4·CH2Cl2 (2),
[Cu(dbp)(DPEphos)]BF4·CH3CN (3), [Cu(NCNMe2)2(DPEphos)]BF4·0.5CH2Cl2·0.5H2O (4), and NiCl2(DPEphos) (5)
1
2
3
4
5
empirical formula
fw
C56H54BCuF4N3O2.50P2
1021.37
C51H42BCl2CuF4N2OP2
982.11
C58H55BCuF4N3OP2
1022.40
C
42.50H42BClCuF4N4O1.50P2
C36H28Cl2NiOP2
668.19
880.58
space group
a, Å
b, Å
P1h (No. 2)
12.9326(3)
14.3210(3)
15.1443(4)
105.8830(13)
99.3086(13)
106.9454(12)
2490.1(2)
2
1.362
0.561
0.061
0.154
P21/c (No. 14)
10.7133(2)
14.7971(2)
28.8529(5)
90
98.1590(7)
90
4527.6(2)
4
1.441
0.727
0.062
0.161
P1h (No. 2)
11.6779(2)
14.0495(2)
17.6505(3)
77.3452(6)
71.4646(7)
66.7766(6)
2508.57(10)
2
1.353
0.555
0.049
0.125
P1h (No. 2)
14.9521(3)
16.9940(3)
18.2098(4)
69.2158(8)
82.2185(8)
77.5514(16)
4215.4(2)
4
1.387
0.712
0.060
0.145
P1h (No. 2)
9.9627(2)
11.2002(3)
17.6107(4)
79.8739(13)
76.6783(13)
76.7340(9)
1845.43(11)
2
1.202
0.782
0.044
0.122
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcd, g/cm-3
µ, mm-1
R (Fo)a
Rw (Fo )b
2
GOF
1.030
1.050
1.037
1.048
0.930
2
2
2
2
2 2
a R ) ∑||Fo| - |Fc||/∑|Fo| with Fo > 2σ(Fo ). b Rw ) [∑w(|Fo | - |Fc |)2/∑w|Fo | ]1/2
.
Anal. Calcd for C50.5H41BClCuF4N2OP2 (i.e., 2‚0.5CH2Cl2): C,
64.32; H, 4.43. Found: C, 64.32; H, 4.42.
(iii) Synthesis of [Cu(dbp)(DPEphos)]BF4 (3). 2,9-Di-n-butyl-
1,10-phenanthroline (29 mg, 0.10 mmol). Yield: 70 mg (71%).
Anal. Calcd for C58H55BCuF4N3OP2 (i.e., 3‚CH3CN): C, 68.14;
H, 5.42. Found: C, 68.36; H, 5.29.
of the structure solution program PATTY in DIRDIF 92.32 The
remaining atoms were located in succeeding difference Fourier
syntheses. Hydrogen atoms were placed in calculated positions
according to idealized geometries with C-H ) 0.95 Å and U(H)
) 1.3Ueq(C). They were included in the refinement but constrained
to ride on the atom to which they are bonded. An empirical
absorption correction using SCALEPACK33 was applied. The final
refinements were performed by the use of the program SHELXL-
97.34
(iv) Synthesis of [Cu(NCNMe2)2(DPEphos)]BF4 (4). Dimeth-
ylcyanamide (0.5 mL). Yield: 37 mg (45%). Anal. Calcd for
C42.5H42BClCuF4N4O1.5P2 (i.e., 4‚0.5CH2Cl2‚0.5H2O): C, 60.84; H,
4.86. Found: C, 60.23; H, 4.76.
In all structures except that of 5, the crystals were found to
contain identifiable solvent molecules (see Table 1), the atoms of
which refined satisfactorily with anisotropic thermal parameters with
the exception of the diethyl ether molecules present in the crystal
of 1. In this case, only the O atom of a well behaved (C2H5)2O
molecule was refined anisotropically, the C atoms isotropically;
isotropic thermal parameters were used for the O and C atoms of
a second (disordered) (C2H5)2O molecule that was located about a
center of inversion. During the structure refinement of 5, an
unidentifiable and badly disordered solvent molecule (probably CH2-
Cl2) was removed with the squeeze option in PLATON.35
The largest peaks remaining in the final difference maps of 1-5
were 1.33, 0.73, 1.33, 1.10, and 0.68 e/Å3, respectively.
F. Physical Measurements. Infrared spectra were recorded as
C. Synthesis of NiCl2(DPEphos) (5). A mixture of NiCl2‚3H2O
(236 mg, 1.0 mmol) and DPEphos (540 mg, 1.0 mmol) in 50 mL
of n-butanol was refluxed for 3 h to obtain a purple solid, that was
filtered off and washed with ethanol (2 × 10 mL) and diethyl ether
(2 × 10 mL). This product was recrystallized from CH2Cl2/Et2O;
yield, 580 mg (87%). Anal. Calcd for C36H28Cl2NiOP2: C, 64.71;
H, 4.22. Found: C, 64.19; H, 4.43.
D. Synthesis of [Cu(dpp)2]BF4 (6). A solution of 2,9-diphenyl-
1,10-phenanthroline monohydrate (33 mg, 0.10 mmol) in CH2Cl2
(10 mL) was added to a solution of [Cu(NCCH3)4]BF4 (15.5 mg,
0.05 mmol) in CH2Cl2 (10 mL), and the mixture was stirred at
room temperature for 30 min to produce a clear red solution that
was filtered and the filtrate concentrated to about 5 mL. Vapor
diffusion of diethyl ether into the resulting solution gave red crystals
that were filtered off and washed with diethyl ether (2 × 5 mL);
yield, 31 mg (76%). This complex was identified on the basis of
its known spectroscopic properties.30
All attempts to prepare [Cu(dpp)(DPEphos)]BF4, with the use
of the procedure described in section B(i), afforded complex 6.
E. X-ray Crystallography. Single crystals of complexes 1-4
were obtained by vapor diffusion of diethyl ether into solutions of
the complexes in mixtures of dichloromethane and acetonitrile (ca.
1:1). Crystals of 5 were obtained through the slow evaporation of
a solution of this complex in a 1:1 mixture of 1,2-dichloroethane
and benzene that had been exposed to di-isopropyl ether vapor.
Data collections were carried out at 150(()K with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å) on a Nonius
KappaCCD diffractometer. Lorentz and polarization corrections
were applied to the data sets. The key crystallographic data are
given in Table 1.
1
KBr pellets on a Perkin-Elmer 2000 FT-IR spectrometer. H and
31P{1H} NMR spectra were obtained with the use of a Varian
INOVA 300 spectrometer. Proton resonances were referenced
internally to the residual protons in the incompletely deuterated
solvent. The 31P{1H} spectra were recorded at 121.6 MHz, with
85% H3PO4 as an external standard. Electrochemical measurements
were carried out with use of a BAS Inc. model CV-27 instrument
in conjunction with a BAS model RXY recorder and were recorded
on dichloromethane solutions that contained 0.1 M tetra-n-butyl-
ammonium hexafluorophosphate (TBAH) as supporting electrolyte.
E0 values, determined as (Ep,a + Ep,c)/2, were referenced to the
silver/silver chloride (Ag/AgCl) electrode at 25 °C and were
(31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(32) Beurskens, P. T.; Admirall, G.; Beurskens, G.; Bosman, W. P.; Garcia-
Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF92
Program System; technical report; Crystallography Laboratory, Uni-
versity of Nijmegen: Netherlands, 1992.
(33) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307.
(34) Sheldrick, G. M. SHELXL97. A Program for Crystal Structure
Refinement; University of Gottingen: Gottingen, Germany, 1997.
(35) Sluis, P. V. D.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194.
The structures 1, 4, and 5 were solved by direct methods using
SIR9731 while the structures of 2 and 3 were solved with the use
(30) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J.-P.; Kirchhoff,
J. R.; McMillin, D. R. J. Chem. Soc., Chem. Commun. 1983, 513.
Inorganic Chemistry, Vol. 41, No. 12, 2002 3315

Kuang et al.
uncorrected for junction potentials. Under our experimental condi-
tions, E0 ) +0.47 V versus Ag/AgCl for the ferrocenium/ferrocene
couple.
Electronic absorption spectra were obtained with use of a Varian
Cary 100 Bio instrument, while the fluorometer was a SLM Aminco
SPF 500C unit, and the cryostat was an Oxford Instruments DN
1704 with a ITC4 temperature controller. The pulsed laser used
for the lifetime studies was a Laser Science VSL-337ND-S unit
with a DLMS-220 pumped dye attachment. The digitizing oscil-
loscope was a Tektronix 520.
Elemental microanalyses were performed by Dr. H. D. Lee of
the Purdue University Microanalytical Laboratory.
G. Methods Used in Studies of the Electronic Absorption
Spectra and Emission Properties of the Copper(I) Complexes.
Absorbance versus concentration plots yielded estimated molar
extinction coefficients for the copper complexes. For the lumines-
cence studies, repeated freeze-pump-thaw cycles removed dis-
solved dioxygen from solution. A user-written program TAURES
provided satisfactory fits of the emission decay on the assumption
of a single-exponential process. A long-necked cell facilitated
temperature-dependent, steady-state emission measurements in the
cryostat. With [Ru(bpy)3]2+ in water as the standard (Φ ) 0.042),36
the method of Parker and Rees37 yielded the emission quantum
yields. The origin of the instrumental correction factors has been
described.12 For quenching studies with Lewis bases, Stern-Volmer
analyses of emission intensities versus quencher concentration
yielded quenching rate constants.38 The fluorometer lamp was the
source for the photostability measurements, and the actinometer
was ferrioxalate.39
Figure 1. ORTEP40 representation of the structure of the cation [Cu(phen)-
(DPEphos)]+ present in 1 showing the pairwise stacking of phen ligands
between neighboring cations. Thermal ellipsoids are drawn at the 50%
probability level except for the phenyl carbon atoms of the Ph2P groups
which are circles of arbitrary radius.
Results
(a) Synthesis and Structural Characterization of Cop-
per(I) and Nickel(II) Complexes. The pale yellow, dia-
magnetic, mixed-ligand complexes of the type [Cu(NN)-
(DPEphos)]BF4, where DPEphos ) bis[2-(diphenylphosphino)-
phenyl]ether and NN ) 1,10-phenanthroline (phen) (1), 2,9-
dimethyl-1,10-phenanthroline (dmp) (2), 2,9-di-n-butyl-1,-
10-phenanthroline (dbp) (3), or two dimethylcyanamides (4),
were prepared by the reaction of [Cu(NCCH3)4]BF4 with
stoichiometric quantities of the ligands DPEphos and NN in
dichloromethane at 25 °C. For the synthesis of paramagnetic
NiCl2(DPEphos) (5), a mixture of NiCl2‚3H2O and DPEphos
was refluxed in n-butanol. Yields of these products ranged
from 45% to 87%. Attempts to prepare the complex [Cu-
(dpp)(DPEphos)]BF4, where dpp ) 2,9-diphenyl-1,10-
phenanthroline, gave only red crystalline [Cu(dpp)2]BF4.30
In all instances, single-crystal X-ray structure determinations
of 1-5 showed the presence of pseudotetrahedral metal-
containing species in which the DPEphos ligand was bound
only through its pair of P donor atoms.
Figure 2. ORTEP40 representation of the structure of the cation [Cu(dmp)-
(DPEphos)]+ present in 2. Thermal ellipsoids are drawn at the 50%
probability level except for the phenyl carbon atoms of the Ph2P groups
which are circles of arbitrary radius.
Table 2, we note that the dihedral angles between the NCuN
and PCuP planes of complexes 1-4 are 91.6°, 97.7°, 101.2°,
and 88.3°, respectively, while for nickel(II) complex 5 the
dihedral angle between the ClNiCl and PNiP planes is 91.9°.
As shown in Table 2, the ether O atom of DPEphos is at a
nonbonding distance (>3.1 Å) to the metal center in all cases.
The resulting eight-membered CsCsPsMsPsCsCsO
rings assume a tublike conformation in the solid state, with
a dihedral angle between the phenyl rings of the [C6H4s
OsC6H4] unit having values of 77.9°, 72.6°, 74.4°, 93.9°,
and 89.3° for 1-5, respectively. In the structure of 4, the
CusN bond lengths involving the Me2NCtN ligands (for
molecule 1) are 1.999(4) and 2.023(4) Å, and the NtC
distances are 1.143(6) and 1.146(5) Å, while the CusNtC
angles are 151.1(4)° and 167.3(4)°. The CusN and NtC
distances for molecule 2 in the asymmetric unit are es-
sentially the same as for molecule 1 (vide supra), but the
CusNtC angles again show a significant variation from
one another (165.8(4)° and 173.8(4)°) and from those of
The ORTEP40 representations of these structures are shown
in Figures 1-5 while important bond length and bond angles
are given in Table 2. In addition to the data summarized in
(36) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853.
(37) Parker, C. A.; Rees, W. T. Analyst (Cambridge, U.K.) 1960, 85, 587.
(38) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings: Menlo Park, 1978.
(39) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518.
(40) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National
Laboratory, TN, 1976.
3316 Inorganic Chemistry, Vol. 41, No. 12, 2002

NoWel Emission Properties for Cu(I) Species
Figure 5. ORTEP40 representation of the structure of the complex NiCl2-
(DPEphos) (5). Thermal ellipsoids are drawn at the 50% probability level
except for the phenyl carbon atoms of the Ph2P groups which are circles of
arbitrary radius.
rings; the closest intermolecular C‚‚‚C and C‚‚‚N contacts
are in the range 3.59-3.64 Å.
Figure 3. ORTEP40 representation of the structure of the cation [Cu(dbp)-
(DPEphos)]+ present in 3. Thermal ellipsoids are drawn at the 50%
probability level except for the phenyl carbon atoms of the Ph2P groups
which are circles of arbitrary radius.
Pseudotetrahedral nickel(II) complex 5 is of a type
encountered previously, of which NiCl2(PP), where PP )
4,6-bis(diphenylphosphino)-2,8-dimethylphenoxathiine (which
is a more rigid variant of DPEphos)42 or bis(diphenylphos-
phinoethyl)ether,43 are close structural analogues. The com-
plex of (Ph2PCH2CH2)2O has Cl-Ni-Cl and P-Ni-P
angles of 127.1(2)° and 107.1(1)°, which are similar to the
corresponding angles in 5 of 123.10(3)° and 101.22(2)°,
respectively.
(b) Spectroscopic, Electrochemical, and Magnetic Prop-
erties of Copper(I) and Nickel(II) Complexes. Diamagnetic
copper(I) complexes 1-4 display very similar spectroscopic
and electrochemical properties in accord with their close
structural similarity. 1H NMR spectroscopy (in CDCl3)
confirmed the presence of lattice solvent molecules as
established by X-ray crystallography. The aromatic DPEphos
and phenanthroline ligand protons of 1-3 appear as complex
sets of multiplets (some overlapping) in the region δ ) +8.9
to +6.8, with the 2,9 protons of the phen ligand of 1 assigned
to a multiplet centered at δ ) +8.90. For complex 2, the
methyl resonances of the dmp ligand are at δ ) +2.47, while
the n-butyl resonances of the dbp ligand in 3 consist of
multiplets at δ ) +2.86, +1.28, +0.78, and +0.65. The 1H
NMR spectrum of 4 (in CDCl3) shows resonances for the
DPEphos ligand as overlapping multiplets at δ ) ∼+7.4,
∼+7.0, and +6.78, while the methyl groups of the Me2-
NCN ligands are characterized by a singlet at δ ) +2.72.
The 31P{1H} NMR spectra of solutions of 1-4 in CDCl3
show singlets at δ ) -9.3, -11.9, -12.4, and -13.6,
respectively, for the coordinated phosphine ligand DPEphos;
for the free ligand, δ ) -18.4 in CDCl3. The presence of
the [BF4]- anion was seen in the IR spectra of 1-4 (recorded
as KBr pellets) with the ν(BsF) mode at 1058 ((5) (vs)
cm-1. In addition, the IR spectrum of 4 showed a split band
at 2216(s) cm-1 assigned to ν(CtN) of the Me2NCN ligands.
Figure 4. ORTEP40 representation of the structure of the cation [Cu-
(NCNMe2)2(DPEphos)]+ present in 4. Thermal ellipsoids are drawn at the
50% probability level except for the phenyl carbon atoms of the Ph2P groups
which are circles of arbitrary radius.
molecule 1. This large spread in the CusNtC angles (range
151.1(4)° to 173.8(4)°) presumably reflects a shallow energy
minimum for the Cusnitrile bond and the sensitivity of these
angles to crystal packing forces. By way of comparison, the
structure of the homoleptic nitrile complex [Cu(NCCH3)4]-
BF4 has CusNtC angles that range from 169.9(5)° to 179.0-
(7)°.41 The solution properties of 4 show the presence of a
single copper(I) species. The only complex for which a
significant intermolecular interaction was present in the solid
state was compound 1, wherein the phen ligands of the pairs
of symmetry related cations partially overlay one another.
The components of this pair are related to one another
through a center of inversion that is located between the phen
(42) Goertz, W.; Keim, W.; Vogt, D.; Englert, U.; Boele, M. D. K.; van
der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem.
Soc., Dalton Trans. 1998, 2981.
(41) Jones, P. G.; Crespo. O. Acta Crystallogr. 1998, C54, 18.
(43) Greene, P. T.; Sacconi, L. J. Chem. Soc. A 1970, 866.
Inorganic Chemistry, Vol. 41, No. 12, 2002 3317

Kuang et al.
Table 2. Comparison of the Important Bond Lengths (Å) and Bond Angles (deg) for Complexes 1-5a
1
2
3
4b
5
Cu-N(1)
Cu-N(10)
Cu-P(1)
Cu-P(2)
Cu‚‚‚O(12)
2.071(3)
2.064(3)
2.2314(8)
2.2614(9)
3.205
Cu-N(1)
Cu-N(12)
Cu-P(1)
Cu-P(2)
Cu‚‚‚O(12)
2.104(3)
2.084(3)
2.2691(11)
2.2728(11)
3.151
Cu-N(1)
Cu-N(12)
Cu-P(1)
Cu-P(2)
Cu‚‚‚O(12)
2.097(2)
2.109(2)
2.2712(7)
2.2793(6)
3.205
Cu(1)-N(11)
Cu(1)-N(21)
Cu(1)-P(1)
Cu(1)-P(2)
2.023(4)
1.999(4)
2.2767(13)
2.2718(13)
3.128
Ni-Cl(1)
Ni-Cl(2)
Ni-P(1)
Ni-P(2)
Ni‚‚‚O(12)
2.2080(7)
2.2177(7)
2.3308(7)
2.3053(7)
3.206
Cu(1)‚‚‚O(12)
N(1)-Cu-N(10) 80.83(11) N(1)-Cu-N(12) 80.88(13) N(1)-Cu-N(12) 80.51(8) N(11)-Cu(1)-N(21) 105.79(15) Cl(1)-Ni-Cl(2) 123.10(3)
P(1)-Cu-P(2) 110.81(3) P(1)-Cu-P(2) 116.44(4) P(1)-Cu-P(2) 112.91(2) P(1)-Cu(1)-P(2) 111.51(5) P(1)-Ni-P(2) 101.22(2)
a Numbers in parentheses are estimated standard deviations in the least significant digits. b Data are given for one of the two molecules present in the
asymmetric unit (molecule 1); both molecules are essentially identical.
Table 3. Physical Data for Solutions of Copper(I) Complexes in
Dichloromethane at Room Temperature
absorbance corrected emission
b
E0,a V vs
Ag/AgCl
λmax
,
λmax,
nm
complex
nm
Φc τ,d µs
[Cu(phen)(DPEphos)]BF4 +1.22 (90) 391 (3000) 700 0.0018 0.19
[Cu(dmp)(DPEphos)]BF4 +1.35 (130) 383 (3100) 565 0.15
[Cu(dbp)(DPEphos)]BF4 +1.40 (90) 378 (2900) 560 0.16
14.3
16.1
e
[Cu(NCNMe2)-
+1.29 (120)
e
e e
(DPEphos)]BF4
[Cu(phen)(PPh3)2]BF4
[Cu(dmp)(PPh3)2]BF4
[Cu(dmp)(dppe)]PF6
f
+1.34g
370 (3900) 680 0.0007 0.22
365 (2500) 560 0.0014 0.33
h
e
+1.17 (80) 400 (3200) 630 0.010
1.33
a Anodic-cathodic peak separation (in mV) given in parentheses. b Molar
absorptivity (M-1 cm-1) given in parentheses. c Error (10%. d Error (5%.
e Not recorded. f Spectroscopic data taken from ref 18. g Anodic maximum
(Ep,a value) corresponds to a multielectron process. h Spectroscopic data
recorded in methanol and taken from ref 17.
Figure 7. Absorption and emission spectra in dichloromethane at room
temperature: absorbance of [Cu(dmp)(DPEphos)]+ (thick) and [Cu(phen)-
(DPEphos)]+ (thin) (left axis); corrected emission spectra with same
conventions (right axis). Areas reflect the relative quantum yields.
work,17,44-46 the absorption band that occurs in the vicinity
of 360 nm (ꢀ ) 3-4000 M-1 cm-1) corresponds to a metal-
to-ligand charge-transfer transition. More intense intraligand
transitions appear at shorter wavelengths. The MLCT
absorption maxima occur at longer wavelengths for the [Cu-
(NN)(DPEphos)]+ species as opposed to the analogous [Cu-
(NN)(PPh3)2]+ series, and in each series, the phen complex
exhibits the longest wavelength CT absorption. Within the
series of DPEphos complexes, the absorption maximum shifts
to longer wavelength as the average Cu-P bond distance
decreases.
At least two types of solvent effects are evident in the
absorption data. First, studies of dmp-containing complex 2
reveal that the MLCT maximum shifts slightly toward shorter
wavelengths in more polar solvents. More specifically, the
maximum shifts from 383 nm in DCM to 377 nm in
methanol and to 375 nm in acetone or acetonitrile. On simple
polarity grounds, the observed negative solvatochromism
implies that the complex has a smaller dipole moment in
the excited state than the ground state. As previously
observed for [Cu(dmp)(PPh3)2]+,45,47 the other effect is that
[Cu(dmp)(DPEphos)]+ undergoes solvent-dependent, ligand
redistribution reactions. Figure 8 shows how addition of
excess DPEphos alters the absorbance of 2 in acetonitrile.
At a copper concentration of 0.1 mM, addition of 0.5-1.0
Figure 6. Single scan cyclic voltammograms of [Cu(phen)(PPh3)2]BF4
(top) and [Cu(dmp)(dppe)]PF6 (bottom) in dichloromethane at room
temperature. Scan rate 50 mV s-1 in 0.1 M TBAH.
The cyclic voltammetric properties of solutions of 1-4
in 0.1 M TBAH-CH2Cl2 are similar, each showing a
reversible process (ip,a ) ip,c) that corresponds to a net one-
electron oxidation of the bulk complex. These data are given
in Table 3, along with related information for salts of the
[Cu(phen)(PPh3)2]+ and [Cu(dmp)(dppe)]+ cations. The CV
of [Cu(dmp)(dppe)]PF6 resembles those of 1-4; its Eo value
is the most accessible of this group of five complexes.
However, in the case of [Cu(phen)(PPh3)2]BF4, the oxidation
is irreversible and appears to be a multielectron process as
a consequence of the rapid decomposition of the Cu(II)
species. A comparison of the single scan CVs of [Cu(dmp)-
(dppe)]PF6 and [Cu(phen)(PPh3)2]BF4 is shown in Figure 6.
The electronic absorption spectral data for the copper(I)
complexes 1-4 are presented in Table 3, and the spectral
traces for solutions of compounds 1 and 2 in dichloromethane
(DCM) are compared in Figure 7. By analogy with previous
(44) Buckner, M. T.; McMillin, D. R. J. Chem. Soc., Chem. Commun. 1978,
759.
(45) Kirchhoff, J. R.; McMillin, D. R.; Robinson, W. R.; Powell, D. R.;
McKenzie, A. T.; Chen, S. Inorg. Chem. 1985, 24, 3928.
(46) Casadonte, D. J.; McMillin, D. R. Inorg. Chem. 1987, 26, 3950.
(47) Rader, R. A. Ph.D. Dissertation, Purdue University, West Lafayette,
IN, 1980.
3318 Inorganic Chemistry, Vol. 41, No. 12, 2002

NoWel Emission Properties for Cu(I) Species
Figure 8. Absorbance of [Cu(dmp)(DPEphos)]+ in acetonitrile: trace of
a 0.1 mM solution (A); traces with 0.1 and 0.5 mole excess of DPEphos
ligand added (B and C, respectively).
Figure 9. Stern-Volmer quenching of photoexcited [Cu(dmp)(DPEp-
hos)]+ in dichloromethane at room temperature; quenching by DMF (9)
and DMSO (b).
mol DPEphos/mol Cu suppresses the growth of an absorption
band at ca. 450 nm that arises from the formation of [Cu-
(dmp)2]+ in solution. Similar equilibria occur in DMF
solution. The redistribution equilibrium in eq 1 could account
for the influence that excess DPEphos has on the appearance
of [Cu(dmp)2]+ in solution:
2[Cu(dmp)(DPEphos)]+ )
[Cu(dmp)2]+ + [Cu(DPEphos)]+ + DPEphos (1)
where [Cu(DPEphos)]+ represents a copper(I) species un-
derstood to have solvent molecules completing its coordina-
tion sphere. A quantitative analysis would be very difficult
because multiple species absorb in the vicinity of 375 nm,
including [Cu(dmp)]+ and [Cu(dmp)2]+.48 Fortunately, DPE-
phos dissociation is not significant in DCM, and a small
excess of the phosphine effectively suppresses formation of
other dmp-containing complexes in alternative solvents.
Some of the most dramatic findings of this work are the
exceptionally high emission yields the [Cu(dmp)(DPEphos)]+
and [Cu(dbp)(DPEphos)]+ systems exhibit in DCM solution
compared to previously studied systems (Table 3). In
addition, data in the same table reveal that the complexes
also exhibit extremely long excited-state lifetimes. In con-
trast, the emission yield and the lifetime of photoexcited [Cu-
(phen)(DPEphos)]+ are very similar to those of the [Cu-
(phen)(PPh3)2]+ analogue. Both phen complexes are also
alike in exhibiting a comparatively large shift between the
absorption and emission maxima.
Solvent effects on the emission from [Cu(dmp)(DPE-
phos)]+ vary. Because of the relatively small dipole moment
of the excited state, the corrected emission maximum is
relatively constant. Thus, in DCM, the emission maximizes
at ca. 565 nm, whereas in three more basic solvents (acetone,
MeCN, and MeOH) the emission maximum falls at ca. 560
nm. However, the excited-state lifetime is shorter in basic
media, that is, 3.8 µs in acetone, 1.1 µs in MeCN, and 2.4
µs in MeOH. Quenching of the emission is complete in the
more strongly donating solvents DMF and DMSO, almost
certainly because of associative (exciplex) quenching, vide
Figure 10. Increase in emission from [Cu(dmp)(DPEphos)]+ in methanol
with an increase in temperature.
infra. Stern-Volmer studies, carried out in MeOH, reveal
quenching rate constants of 4.5 × 106 M-1 s-1 for DMF
and 1.4 × 107 M-1 s-1 for DMSO. The quenching data are
presented in Figure 9.
In MeOH and DCM, the emission from [Cu(dmp)-
(DPEphos)]+ shows the distinctive temperature dependence
often found for copper(I) complexes containing phenanthro-
line ligands, at least in DCM solution.18,49 In particular,
Figure 10 shows that the emission intensity increases at
higher temperatures in MeOH. At higher temperatures, the
emission maximum also shifts to shorter wavelengths.
The complexes have good photostability in solution.
Irradiation of deoxygenated solutions of [Cu(dmp)(DPE-
phos)]BF4 at 380 nm (6.1 × 1014 photons/sec) in dichlo-
romethane at room temperature produced a negligible ab-
sorbance change after 2 h. The estimated quantum yield for
photoinduced decomposition is φ j0.001.
The spectroscopic and magnetic properties of nickel(II)
complex 5 are in accord with its pseudotetrahedral structure
as established by X-ray crystallography.50,51 The magnetic
moment of 5 is 3.22 µâ at 300 K, similar to the value of
3.26 µâ reported for NiCl2[(Ph2PCH2CH2)2O].52 The elec-
tronic absorption spectrum of a solution in dichloromethane
(49) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.;
Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380.
(50) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier Publish-
ing: Amsterdam, The Netherlands, 1968; pp 341-342.
(51) Figgis, B. N.; Lewis, J. Prog. Inorg. Chem. 1964, 6, pp 207-209.
(52) Sacconi, L.; Gelsomini, J. Inorg. Chem. 1968, 7, 291.
(48) Atkins, C. E.; Park, S. E.; Blaszak. J. A.; McMillin, D. R. Inorg. Chem.
1984, 23, 569.
Inorganic Chemistry, Vol. 41, No. 12, 2002 3319

Kuang et al.
shows the characteristic 3T1(P) r 3T1 transition (ν3) at 18150
cm-1 (ꢀ ) 150), with a weak broad shoulder at ∼15600. A
rising absorption at the limit of our measurements (∼12000
copper, as indicated by the nonorthogonality of the CuNN
and CuPP planes. Although the [Cu(phen)(DPEphos)]+
species shows minimal flattening in the solid state, the
distortion is undoubtedly possible in solution because the
phen ligand has no substituents in the 2,9 positions. That
would explain the relative ease of oxidation of [Cu(phen)-
(DPEphos)]+, ca. 0.15 V less positive than either the dmp
or dbp analogue (Table 3), even though the presence of
electron-donating alkyl substituents might be expected to
stabilize the copper(II) form of the couple. Actually,
extensive studies of Cu(II)/Cu(I) couples in related complexes
have shown that sterically active alkyl groups raise the
potential when they inhibit flattening motions that stabilize
the copper(II) oxidation state.12,57,58 Thus, phen complex 1
is likely to adopt the most flattened structure upon oxidation.
Still another indication of the flexibility of the [Cu(phen)-
(DPEphos)]+ species comes from spectral data. Data in Table
3 reveal that [Cu(phen)(DPEphos)]+ shows by far the largest
shift between the emission and the CT absorption maximum
in the series. As Riesgo et al.59 have emphasized, a large
Stokes-like shift occurs when the copper system undergoes
a substantial structural relaxation in the excited state. In
accordance with the electrochemical results, photoexcited
phen complex 1 is likely to undergo a flattening distortion
because CT excitation entails formal oxidation of the copper
center.
(b) Excited-State Energetics. The [Cu(NN)(DPEphos)]+
systems exhibit higher energy CT states and more positive
Cu(II)/Cu(I) potentials than the [Cu(NN)2]+ analogues. In
the latter complexes, narrow, ca. 82°, N-Cu-N bite angles
help drive down the energy of CT excitation because the
highest filled dxz and dyz orbitals of the metal have dσ*
character with respect to the phenanthroline lone pairs.60
Wider P-Cu-P bite angles may ease dσ* interactions in
[Cu(NN)(DPEphos)]+ and related systems and in so doing
enhance the energy required for CT excitation. At the same
time, the larger, more diffuse donor orbitals of phosphine
ligands probably stabilize copper(I) versus copper(II) forms.
Substituent effects vary. In the case of [Cu(phen)2]+,
introduction of 2,9 methyl substituents induces a shift of the
CT absorption to longer wavelength,61 but [Cu(phen)-
(DPEphos)]+ has the longest wavelength CT absorption
maximum within the DPEphos series. Here, the interligand
steric repulsions that are responsible for elongation of the
Cu-P bonds in [Cu(dmp)(DPEphos)]+ and [Cu(dbp)(DPE-
phos)]+ probably also account for destabilization of the CT
excited state.
3
3
cm-1) signals the appearance of the expected A2 r T1
transition (ν2).50 The corresponding spectrum of NiCl2[(Ph2-
PCH2CH2)2O] shows these transitions at 18700 and 11700
cm-1, respectively.52
Discussion
The present study has provided copper(I) and nickel(II)
complexes that establish the propensity of the DPEphos
ligand to coordinate in a bidentate P,P fashion to afford
mononuclear pseudotetrahedral structures. These compounds
(1-5) provide a useful structural benchmark for the η2-
bonding mode of this ligand. This behavior contrasts with
the η3-P,O,P bonding mode encountered in the multiply
bonded dirhenium species we isolated previously.19 Of
special significance are the unusually long-lived, high
quantum yield emissions that several of the copper(I)
complexes exhibit in fluid solution. This behavior was not
observed in the case of dimethylcyanamide complex 4. The
following discussion relates to the unusual spectroscopic
properties of compounds 1-3.
One advantage of the DPEphos systems is that ligand
redistribution equilibria pose few problems in the case of
copper(I). For example, only a small excess of the chelating
phosphine is necessary to ensure that the [Cu(dmp)(DPE-
phos)]+ species is the predominant copper-dmp complex
in solution. This is not the case with [Cu(dmp)(PPh3)2]+
which is in equilibrium with a number of species in DCM
and in MeCN.17,45,47
(a) Flexibility within [Cu(NN)(DPEphos)]+ Systems. A
variety of observations reveal that the mixed-ligand frame-
work is much more flexible when 1,10-phenanthroline is the
coligand with DPEphos. Even solid state measurements
reveal the flexibility of the phen derivative because only in
the [Cu(phen)(DPEphos)]+ structure does one observe a
canting of the DPEphos ligand toward the NN ligand. As a
result of the displacement within the P-Cu-P plane, the
coordination environment about copper approaches trigonal
pyramidal with P(2) apical and P(1) equatorial. This so-called
rocking distortion, which has precedent in structures of
various [Cu(NN)2]+ systems,53-56 results in elongation of the
Cu-P(2) bond relative to Cu-P(1). As with other copper-
(I) systems, the driving force for the distortion probably
derives from the fact that exposing a face of the phenan-
throline ligand opens the way for intermolecular stacking
interactions to occur in the lattice.53,54 In the structures of
[Cu(dmp)(DPEphos)]+ and [Cu(dbp)(DPEphos)]+ (Figures
2 and 3), where there is no sign of intermolecular stacking,
the most prominent distortion is a tetragonal flattening about
As is the norm for [Cu(NN)2]+ and related mixed-ligand
systems,49 the temperature dependence of the emission from
[Cu(dmp)(DPEphos)]+ as depicted in Figure 10 indicates that
there are at least two emitting states, the higher energy of
(53) Cesario, M.; Dietrichbuchecker, C. O.; Guilhem, J.; Pascard, C.;
Sauvage, J. P. J. Chem. Soc., Chem. Commun. 1985, 244.
(54) Klemens, F. K.; Palmer, C. E. A.; Rolland, S. M.; Fanwick, P. E.;
McMillin, D. R.; Sauvage, J. P. New J. Chem. 1990, 14, 129.
(55) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem. 1998,
37, 2285.
(57) Hawkins, C. J.; Perrin, D. D. J. Am. Chem. Soc. 1962, 84, 1351.
(58) Federlin, P.; Kern, J. M.; Rastegar, A.; Dietrichbuchecker, C.; Marnot,
P. A.; Sauvage, J. P. New J. Chem. 1990, 14, 9.
(59) Riesgo, E. C.; Hu, Y.-Z.; Bouvier, F.; Thummel, R. P.; Scaltrito, D.
V.; Meyer, G. J. Inorg. Chem. 2001, 40, 3413.
(60) Sakaki, S.; Mizutani, H.; Kase, Y. Inorg. Chem. 1992, 31, 4575.
(61) Schilt, A. A. Analytical Applications of 1,10-Phenanthroline and
Related Compounds; Pergamon: New York, 1969.
(56) Cunningham, C. T.; Moore, J. J.; Cunningham, K. L. H.; Fanwick, P.
E.; McMillin, D. R. Inorg. Chem. 2000, 39, 3638.
3320 Inorganic Chemistry, Vol. 41, No. 12, 2002

NoWel Emission Properties for Cu(I) Species
which exhibits a distinctly greater emission yield. The effect
arises because the energy gap between the singlet (1CT) and
triplet (3CT) excited states is small and because the radiative
rate constant of a 3CT state is not very favorable for a first-
row transition metal ion. At higher temperatures, increased
participation of the 1CT excited state also causes the emission
to shift to shorter wavelength. However, the overall shift in
Figure 10 is small, a possible indication that other excited
states also participate.62
(c) Photophysics. One of the important factors influencing
excited-state dynamics is the energy of the excited state;
indeed, the often-invoked energy gap law predicts that the
lifetime increases roughly as an exponential function of the
energy.63-65 However, [Cu(phen)(DPEphos)]+ has a high-
energy CT absorption but a relatively low energy emission
maximum. The reason for this is the geometric relaxation
that occurs in the excited state because of a d-orbital vacancy
that promotes a tetragonal flattening. In effect, the distortion
narrows the gap between the ground and excited states,
enhances the coupling between the two states, and shortens
the excited-state lifetime. As in [Cu(NN)2]+ systems with
relatively bulky ligands,12,58 the excited states of [Cu(dmp)-
(DPEphos)]+ and [Cu(dbp)(DPEphos)]+ experience less
flattening because of the steric requirements of the dmp and
dbp ligands. The upshot is that these systems have relatively
small shifts between the absorption and emission maxima
and much longer excited-state lifetimes.
As implied by Riesgo et al.,59 it is also possible to foster
nested energy surfaces by building interligand strain into the
ground state so as to bias the structure in favor of the excited
state. It is interesting that X-ray structural data indicate that
both [Cu(dmp)(DPEphos)]+ and [Cu(dbp)(DPEphos)]+ ex-
hibit somewhat flattened structures. The distortion, which
is larger in the dbp complex, may well reflect a steric
interaction involving the ether linkage of DPEphos and one
of the alkyl substituents of the phenanthroline ligand. If so,
the flattening distortion is intrinsic and will moderate the
structural reorganization that attends formation of the CT
excited state in solution. A related effect occurs in some [Cu-
(NN)2]+ systems containing 2,9 aryl substituents. Like most
bis-phenanthroline complexes of copper(I), photoexcited [Cu-
(dpp)2]+ has a flattened geometry, whereas cooperative
intraligand substituent effects enforce a pseudotetrahedral
coordination geometry on the dptmp analogue (dptmp ) 2,9-
diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline).56 Never-
theless, the emission energies of the two complexes are
surprisingly similar. The explanation is that limited structural
relaxation occurs with excitation of the [Cu(dpp)2]+ species
because intra- and/or interligand interactions also impose a
flattened geometry on the ground state.56
Figure 11. Schematic potential energy surfaces for ligand addition to a
copper complex. The copper-ligand distance decreases from right to left:
repulsive interaction for the pseudotetrahedral d10 ground state (A); exciplex
formation with a CT excited state (B); same excited state as in B but with
a severely crowded coordination sphere (C).
way to exciplex quenching via attack by Lewis bases.12,14,15,58,66
Curves A and B in Figure 11 show schematically how
formation of a new metal-ligand bond in the excited state
can compress the energy separation between the ground and
excited states and thereby promote relaxation. In the case of
[Cu(dmp)2]+, quenching is very efficient even in weakly
donating solvents such as acetonitrile and methanol. For
example, at room-temperature, photoexcited [Cu(dmp)2]+ has
a lifetime of 95 ns in deoxygenated DCM as compared with
only 2 ns in acetonitrile.67 For better donors such as DMF
and DMSO, the rate constants for quenching the excited state
are of the order of 108 M-1 s-1 according to Stern-Volmer
quenching studies carried out in DCM.66 In contrast, quench-
ing of the CT excited state of [Cu(dmp)(DPEphos)]+ occurs
with a rate constant approximately 2 orders of magnitude
smaller, such that the excited state retains a microsecond
lifetime in weakly donating solvents such as acetone and
methanol. As with other systems involving bulky ligands,12,53
steric crowding undoubtedly inhibits an increase in the
coordination number of the metal center of the [Cu(dmp)-
(DPEphos)]+ system.
According to lifetime data with methanol as the solvent,
exciplex quenching is also about an order of magnitude
slower for [Cu(dmp)(DPEphos)]+ than [Cu(dmp)(PPh3)2]+.
This result is surprising because one would expect two PPh3
groups to occupy a larger volume than DPEphos because of
interligand repulsions and the absence of the ether linkage.
Consistent with this expectation, the Cu-N and Cu-P
distances are, in fact, about 0.02 Å shorter in the mixed-
ligand DPEphos complex as compared with [Cu(dmp)-
(PPh3)2]+.45 The quenching results suggest that, aside from
introducing steric bulk, ligands can impose conformational
restrictions that hamper expansion of the coordination
number. The fact that exciplex quenching is not important
for [Cu(dmp)(DPEphos)]+ in methanol also accounts for
another important contrast with the [Cu(dmp)(PPh3)2]+
species. Thus, Palmer and McMillin18 showed that the [Cu-
(dmp)(PPh3)2]+ species is unusual in that the emission
(d) Exciplex Quenching. In copper complexes with low-
lying MLCT states, the formal increase in oxidation state of
the metal center that occurs in the excited state paves the
(62) Everly, R. M.; McMillin, D. R. J. Phys. Chem. 1991, 95, 9071.
(63) Jortner, J.; Rice, S. A.; Hochstrasser, R. M. AdV. Photochem. 1969,
7, 149.
(64) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.
(65) Stufkens, D. J.; Aarnts, M. P.; Nijhoff, J.; Rossenaar, B. D.; Vlcek,
A. Coord. Chem. ReV. 1998, 171, 93.
(66) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393.
(67) Palmer, C. E. A.; McMillin, D. R.; Kirmaier, C.; Holten, D. Inorg.
Chem. 1987, 26, 3167.
Inorganic Chemistry, Vol. 41, No. 12, 2002 3321

Kuang et al.
intensity increases at lower temperatures in methanol. They
concluded that in this case the temperature dependence of
solvent-induced exciplex quenching outweighs the effect of
thermal equilibration between the emitting CT and CT
excited states, vide supra.
of [Cu(NCNMe2)(DPEphos)]BF4 and NiCl2(DPEphos), each
reveal a metal complex with a pseudotetrahedral coordination
geometry. The copper-phenanthroline complexes exhibit
particularly well-behaved properties in solution. For example,
by comparison with the [Cu(dmp)(PPh3)2]BF4 system, spe-
ciation of the DPEphos analogue is easy to control because
the addition of a slight excess of phosphine ensures that [Cu-
(dmp)(DPEphos)]+ is the dominant copper-containing com-
plex in solution. In addition, the Cu(II)/Cu(I) couples of the
[Cu(NN)(DPEphos)]+ species are chemically reversible on
the cyclic voltammetry time scale. Although photoexcited
[Cu(phen)(DPEphos)]+ is comparatively short-lived, the [Cu-
(dmp)(DPEphos)]+ system exhibits an extremely long life-
time of 14.3 µs in dichloromethane solution and an impres-
sive quantum efficiency of 15%. Donor solvents usually
quench charge-transfer emission from copper(I) systems, but
[Cu(dmp)(DPEphos)]+ exhibits an excited-state lifetime of
3.8 µs in acetone and 2.4 µs in methanol. However, DMF
and DMSO are very effective quenchers. The emission from
[Cu(dmp)(DPEphos)]+ occurs at a significantly higher energy
than that of [Cu(phen)(DPEphos)]+ in dichloromethane
solution because of interligand steric interactions that inhibit
an excited-state flattening distortion. Although DPEphos is
a flexible ligand capable of chelating to a metal with a
P-M-P angle that can range from 101° (this work) to as
much as 150° in the η3-bound form,19 all evidence suggests
that a four-coordinate pseudotetrahedral geometry is remark-
ably stable for systems such as [Cu(dmp)(DPEphos)]+. The
fact that expansion of the coordination number also tends to
be slow in the excited state enhances the potential the mixed-
ligand DPEphos complexes have for practical, luminescence-
based applications because (Lewis) base-induced quenching
is usually a severe limitation with copper systems.
1
3
In addition to constraining the ligand arrangement within
the coordination sphere of copper, the ether oxygen of
DPEphos can influence exciplex quenching in other ways
as well. One possibility is via intramolecular exciplex
quenching. In principle, nucleophilic attack by oxygen at the
copper center is feasible because DPEphos is capable of
functioning as a tridentate ligand.19 There is precedent for
intramolecular exciplex quenching in a [Cu(NN)2]+ system
with N-(p-toluidino)methyl substituents in the 2,9 positions
of the phenanthroline ligand.68 However, formation of η3-
DPEphos is at best a slow process in [Cu(dmp)(DPEphos)]+
and [Cu(dbp)(DPEphos)]+ in view of the lifetimes. Finally,
the ether group can influence exciplex quenching by virtue
of being a π-donating substituent. Sakaki et al.69 have argued
that electron-rich phosphines suppress exciplex quenching
by reducing the effective charge on the metal center. A single
substituent would probably not have much impact, but Sakaki
et al.69 find adding p-methoxy substituents to the phenyl
groups significantly enhances the excited-state lifetime in a
series of complexes related to [Cu(dmp)(PPh3)2]+. In par-
ticular, the lifetime increases from 0.7 µs for the PPh3
complex to 5.5 µs for the P(C6H4OMe-p)3 derivative in
EtOH-water (60:40). A similar effect occurs in substituted
terpyridine complexes of platinum(II) where delocalization
of the “hole” onto the substituent diminishes platinum(III)
character and suppresses exciplex quenching of the CT
excited state.70,71
Concluding Remarks. Crystallographic analyses of three
structures of the type [Cu(NN)(DPEphos)]BF4, where NN
denotes a 1,10-phenanthroline ligand, as well as the structures
Acknowledgment. The National Science Foundation
provided funding for this research through Grant CHE 01-
08902 (to D.R.M.). One of us (R.A.W.) thanks the John A.
Leighty Endowment Fund for support.
(68) Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R. Inorg. Chem.
1991, 30, 559.
(69) Sakaki, S.; Mizutani, H.; Kase, Y.; Inokuchi, K. J.; Arai, T.; Hamada,
T. J. Chem. Soc., Dalton Trans. 1996, 1909.
(70) Tears, D. K. C.; McMillin, D. R. Coord. Chem. ReV. 2001, 211, 195.
(71) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193.
Supporting Information Available: X-ray crystallographic files
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0201809
3322 Inorganic Chemistry, Vol. 41, No. 12, 2002