Structures of the Copper(I) and Copper(II) Complexes of 2,9-Diphenyl-1,10-phenanthroline: Implications for Excited-State Structural Distortion

Article abstract of DOI:10.1021/ic971164s

The syntheses, crystal structures, and electronic absorption spectra of the copper(I) and copper(II) complexes of 2,9-diphenyl-1,10-phenanthroline (dpp) are reported. The complex [Cu(dpp)₂](PF₆) (1) crystallizes in space group P2₁lc with a = 11.081(4) ?, b = 25.491(8) ?, c = 14.263(5) ?, β = 92.84(3)°, Z = 4, and V = 4024(2) ?³. For 4813 unique data with F > 4.0σ(F), R = 5.41% and R_w = 6.43%. The coordination geometry about the copper-(I) center in [Cu(dpp)₂]⁺ is best described as distorted tetrahedral with approximate C₂ symmetry. The structure of [Cu(dpp)₂]⁺ is largely determined by interligand π-stacking interactions that occur between the phenyl groups of one ligand and the phenanthroline moiety of the other ligand. Solution-state absorption and ¹H NMR spectra indicate that the [Cu(dpp)₂]⁺ complex is fluxional in solution, rocking between two enantiomeric structures of C₂ molecular symmetry through an intermediate of C_s symmetry. The complex [Cu(dpp)₂](C1C₄)₂ (2) crystallizes in space group P1? with a = 7.809(3) ?, b = 13.027(6) ?, c = 20.344(10) ?, α = 87.68(4)°, β = 89.16(4)°, γ = 79.26(4)°, Z = 2, and V = 2032(1) ?³. For 4943 unique data with F > 4.0σ(F), R = 5.22% and R_w = 5.37%. The coordination geometry about the copper(II) center in [Cu(dpp)₂]²⁺ is best described as flattened tetrahedral with approximate D₂ symmetry. There are no interligand π-stacking interactions in the structure of [Cu(dpp)₂]²⁺. The four-coordinate geometry in [Cu(dpp)₂]²⁺ persists in solution on the basis of solution-state and solid-state absorption spectroscopy. Structural distortion in the metal-to-ligand charge-transfer excited state of [Cu(dpp)₂]⁺ is discussed on the basis of the structures of 1 and 2.

Full text of DOI:10.1021/ic971164s

Inorg. Chem. 1998, 37, 2285-2290
2285
Structures of the Copper(I) and Copper(II) Complexes of
2
,9-Diphenyl-1,10-phenanthroline: Implications for Excited-State Structural Distortion
Mark T. Miller, Peter K. Gantzel, and Timothy B. Karpishin*
Department of Chemistry and Biochemistry, University of California, San Diego,
La Jolla, California 92093-0358
ReceiVed September 10, 1997
The syntheses, crystal structures, and electronic absorption spectra of the copper(I) and copper(II) complexes of
,9-diphenyl-1,10-phenanthroline (dpp) are reported. The complex [Cu(dpp)2](PF6) (1) crystallizes in space group
2
3
P21/c with a ) 11.081(4) Å, b ) 25.491(8) Å, c ) 14.263(5) Å, â ) 92.84(3)°, Z ) 4, and V ) 4024(2) Å . For
4
813 unique data with F > 4.0σ(F), R ) 5.41% and Rw ) 6.43%. The coordination geometry about the copper-
+
(I) center in [Cu(dpp)2] is best described as distorted tetrahedral with approximate C2 symmetry. The structure
+
of [Cu(dpp)2] is largely determined by interligand π-stacking interactions that occur between the phenyl groups
of one ligand and the phenanthroline moiety of the other ligand. Solution-state absorption and H NMR spectra
indicate that the [Cu(dpp)2] complex is fluxional in solution, rocking between two enantiomeric structures of C2
1
+
molecular symmetry through an intermediate of Cs symmetry. The complex [Cu(dpp)2](ClO4)2 (2) crystallizes in
space group P 1h with a ) 7.809(3) Å, b ) 13.027(6) Å, c ) 20.344(10) Å, R ) 87.68(4)°, â ) 89.16(4)°, γ )
3
7
9.26(4)°, Z ) 2, and V ) 2032(1) Å . For 4943 unique data with F > 4.0σ(F), R ) 5.22% and Rw ) 5.37%.
2
+
The coordination geometry about the copper(II) center in [Cu(dpp)2] is best described as flattened tetrahedral
with approximate D2 symmetry. There are no interligand π-stacking interactions in the structure of [Cu(dpp)2] .
The four-coordinate geometry in [Cu(dpp)2] persists in solution on the basis of solution-state and solid-state
absorption spectroscopy. Structural distortion in the metal-to-ligand charge-transfer excited state of [Cu(dpp)2]+
is discussed on the basis of the structures of 1 and 2.
2+
2
+
Introduction
Thus, the majority of copper(I) photochemical research has
focused on the complexes of 2,9-disubstituted phenanthrolines.4
-31
In the past two decades, the field of inorganic photochemistry
has focused on molecular systems that possess low-lying metal-
to-ligand charge-transfer (MLCT) excited states capable of
electron and energy transfer. Although complexes of ruthenium-
(8) Gamache, R. E., Jr.; Rader, R. A.; McMillin, D. R. J. Am. Chem.
Soc. 1985, 107, 1141-1146.
(
9) Palmer, C. E. A.; McMillin, D. R.; Kirmaier, C.; Holten, D. Inorg.
(II), osmium(II), and rhenium(I) have been the most studied in
Chem. 1987, 26, 3167-3170.
this area, certain copper(I) polypyridine complexes also display
these photophysical characteristics.
McMillin first demonstrated the room-temperature luminescence
of [Cu(dmp)2] (dmp ) 2,9-dimethyl-1,10-phenanthroline) in
dichloromethane upon excitation into the visible MLCT band.
After that study, the structural requirements for emission in
homoleptic copper(I) polypyridine complexes were investigated.
(
10) Berger, R. M.; McMillin, D. R.; Dallinger, R. F. Inorg. Chem. 1987,
1
-3
In 1980, Blaskie and
26, 3802-3805.
(
11) Goodwin, K. V.; McMillin, D. R. Inorg. Chem. 1987, 26, 875-877.
12) Crane, D. R.; DiBenedetto, J.; Palmer, C. E. A.; McMillin, D. R.;
Ford, P. C. Inorg. Chem. 1988, 27, 3698-3700.
(
+
4
(13) Gushurst, A. K. I.; McMillin, D. R.; Dietrich-Buchecker, C. O.;
Sauvage, J.-P. Inorg. Chem. 1989, 28, 4070-4072.
(
14) Everly, R. M.; McMillin, D. R. Photochem. Photobiol. 1989, 50, 711-
716.
+
Since the complex [Cu(phen)2] (phen ) 1,10-phenanthroline)
shows no detectable emission even at 77 K, substituents at the
(15) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393-396.
(16) Klemens, F. K.; Palmer, C. E. A.; Rolland, S. M.; Fanwick, P. E.;
McMillin, D. R.; Sauvage, J.-P. New J. Chem. 1990, 14, 129-133.
2
- and 9-positions of the phenanthroline have been shown to
(
(
17) Everly, R. M.; McMillin, D. R. J. Phys. Chem. 1991, 95, 9071-9075.
18) Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R. Inorg. Chem.
1991, 30, 559-561.
5
be necessary for luminescence. The complex of 4,4′,6,6′-
tetramethyl-2,2′-bipyridine (tmbp) is emissive; however the
lifetime in CH2Cl2 is significantly shorter than that of the dmp
(19) Cunningham, K. L.; Hecker, C. R.; McMillin, D. R. Inorg. Chim. Acta
1
996, 242, 143-147.
+
+
6
complex: [Cu(tmbp)2] , τ ) 18 ns; [Cu(dmp)2] , τ ) 90 ns.
(
20) Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J.
Inorg. Chem. 1997, 36, 172-176.
(
1) McMillin, D. R.; Kirchhoff, J. R.; Goodwin, K. V. Coord. Chem. ReV.
(21) Crane, D. R.; Ford, P. C. J. Am. Chem. Soc. 1991, 113, 8510-8516.
(22) Edel, A.; Marnot, P. A.; Sauvage, J.-P. NouV. J. Chim. 1984, 8, 495-
498.
1985, 64, 83-92.
(
(
(
(
2) Kutal, C. Coord. Chem. ReV. 1990, 99, 213-252.
3) Horv a´ th, O. Coord. Chem. ReV. 1994, 135/136, 303-324.
4) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519-3522.
5) Ichinaga, A. K.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker,
C. O.; Marnot, P. A.; Sauvage, J.-P. Inorg. Chem. 1987, 26, 4290-
(23) Federlin, P.; Kern, J.-M.; Rastegar, A.; Dietrich-Buchecker, C.; Marnot,
P. A.; Sauvage, J.-P. New J. Chem. 1990, 14, 9-12.
(24) Armaroli, N.; Balzani, V.; Barigelletti, F.; De Cola, L.; Sauvage, J.-
P.; Hemmert, C. J. Am. Chem. Soc. 1991, 113, 4033-4035.
(25) Armaroli, N.; De Cola, L.; Balzani, V.; Sauvage, J.-P.; Dietrich-
Buchecker, C. O.; Kern, J.-M.; Bailal, A. J. Chem. Soc., Dalton Trans.
1993, 3241-3247.
(26) Dietrich-Buchecker, C. O.; Nierengarten, J.-F.; Sauvage, J.-P.; Ar-
maroli, N.; Balzani, V.; De Cola, L. J. Am. Chem. Soc. 1993, 115,
11237-11244.
4292.
(6) Kirchhoff, J. R.; Gamache, R. E., Jr.; Blaskie, M. W.; Del Paggio, A.
A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380-
2384.
(7) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J.-P.; Kirchhoff,
J. R.; McMillin, D. R. J. Chem. Soc., Chem. Commun. 1983, 513-515.
S0020-1669(97)01164-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/14/1998

2286 Inorganic Chemistry, Vol. 37, No. 9, 1998
Miller et al.
One area of emphasis in inorganic photochemistry has been
Although a structure of the copper(I) complex of dpp has been
1
6
the investigation of the ligand structural requirements that are
necessary for long-lived excited states in fluid solution. Since
complexes with longer luminescent lifetimes are more useful
for photoinduced electron and energy transfer, the factors that
control lifetimes are of interest. In this context, a significant
achievement in the study of copper(I) polypyridine complexes
came in 1983 when Sauvage and McMillin showed that the
reported, we have determined the crystal structure of [Cu-
+
(dpp)2] with a different anion and present that structure. The
+
2+
analysis and comparison of the [Cu(dpp)2] and [Cu(dpp)2]
complexes provide insight into the structural distortion that
occurs in the excited states of copper(I) polypyridine complexes.
Since excited-state structural information is relevant to energy-
transfer and electron-transfer quenching processes,1
9,31
it is
+
complex of 2,9-diphenyl-1,10-phenanthroline ([Cu(dpp)2] ) has
important to understand the stereochemical preferences of the
copper(I) and copper(II) ions.
7
a lifetime in CH2Cl2 at room temperature of 310 ns. This
lifetime has subsequently been reported to be 250 ns in CH2-
Cl2,5,13,31
14
Experimental Section
and is slightly concentration dependent. Relative
+
to [Cu(dmp)2] , there is an increase in lifetime by about a factor
of 3. There is also an increase in quantum yield by about a
factor of 4 for the dpp complex over the dmp complex (CH2-
General Procedures. All chemicals used were reagent grade unless
otherwise specified. Acetonitrile and dichloromethane were Burdick
and Jackson high-purity grade and were used as received. The
37
5
,13
compounds [Cu(CH
(
3 4 6
CN) ](PF ) and 2,9-diphenyl-1,10-phenanthroline
Cl2).
(
Sauvage and McMillin also showed that, unlike [Cu-
dpp)38 were synthesized as previously reported. Elemental analyses
+
+
dmp)2] , [Cu(dpp)2] is emissive in donor solvents such as
were performed by Desert Analytics, Tucson, AZ. Proton NMR spectra
were recorded with a General Electric QE300 spectrometer. Absorption
spectra were recorded with a Hewlett-Packard 8452A diode array
spectrophotometer.
5,7
methanol (τ ) 180 ns). For the dmp complex, quenching of
4
the excited state occurs in methanol, ethanol, and acetonitrile.
The quenching of *[Cu(dmp)2]+ by these solvents has been
explained by McMillin and co-workers in terms of an exciplex
model. This model depicts the formation of a five-coordinate
[
Cu(dpp)
2
](PF
6
) (1). This procedure is based on one previously
published. A solution of dpp (500 mg; 1.50 mmol) in 100 mL of
degassed CH CN was added with stirring to a flask charged with [Cu-
(CH CN) ](PF ) (280 mg; 0.75 mmol) under Ar. The red solution was
stirred for 10 min and taken to dryness, and the residue was redissolved
in CH Cl . Layering of the solution with Et O yielded large, air-stable
red-brown crystals. UV-vis [λmax (nm) (ꢀ (M cm ))]: CH
3800); CH CN, 440 (3300); CH CN/H O (50/50), 440 (3550).
NMR (dmso-d ), δ: 6.52 (t, 8H, phenyl meta), 6.78 (t, 4H, phenyl
para), 7.43 (d, 8H, phenyl ortho), 8.05 (d, 4H, H3,8), 8.20 (s, 4H, H5,6),
.74 (d, 4H, H4,7). Anal. Calc (found) for CuC48 PF : C, 66.02
65.47); H, 3.69 (3.67); N, 6.42 (6.30).
Cu(dpp) ](ClO (2). To a suspension of dpp (42 mg; 0.13 mmol)
in 15 mL of MeOH was added [Cu(ClO ]‚6H O (25 mg; 0.067 mmol).
31
1
,12,15
adduct in the excited state that decays very rapidly.
In
3
the localized extreme, the MLCT excited state can be viewed
3
4
6
II •-
32,33
as a copper(II) species: [Cu (L )(L)]
(recent results indicate
2
2
2
that the excited electron may be delocalized over both phenan-
-
1
-1
34
2 2
Cl , 440
throlines ). Therefore, the exciplex model is consistent with
1
(
3
3
2
H
3
5
the tendency for copper(II) to be five- (or six-) coordinate.
6
+
Since [Cu(dpp)2] is emissive in donor solvents, the phenyl
groups are thought to protect the metal center from the solution
environment and prevent the formation of a five-coordinate
8
(
32
H N
4
6
7
exciplex.
[
2
4 2
)
With these systems, excited-state structural information can
be obtained from crystal structures of the copper(II) complexes.
In the case of the copper(II) complex of dmp, a five-coordinate
4
)
2
2
The blue solution was stirred at room temperature for 2 h, resulting in
the deposition of a blue-black crystalline material. The solution was
placed in a freezer overnight, and the air-stable crystals were collected.
structure has been observed in the solid state ([Cu(dmp)2-
+
Frozen-solution EPR spectra of 2 in CH
) 2.07, g ) 2.37, A ) 177 G. UV-vis [λmax (nm) (ꢀ (M cm ))]:
CH Cl , 582 (620); CH CN, 572 (610); CH CN/H O (50/50), 572
580). Anal. Calc (found) for CuC48 Cl : C, 62.18 (61.43);
2 2
Cl (158 K) were collected:
(
ONO2)] ), demonstrating the tendency for the copper(II) ion
-
1
-1
g
⊥
|
|
36
to bind a fifth ligand. Although considerable research has
2
2
3
3
2
+
focused on the photochemistry of the [Cu(dpp)2] com-
(
H
32
N
4
2 8
O
5
,8,12,13,19,21,30,31
plex
and copper(I) bis(phenanthroline) complexes
H, 3.48 (3.39); N, 6.04 (5.78). Caution! Perchlorate salts of metal
complexes with organic ligands are potentially explosive.
with 2,9-diaryl substituents,2
4-28
no structural information about
39
the copper(II) complex of dpp or related ligands has been
published.
Crystal Structure Determinations. For the structure determinations
of 1 and 2, a Siemens R3m/V four-circle diffractometer was used to
collect the data using Mo KR radiation. Data collection and crystal
parameters are reported in Table 1. No absorption corrections were
applied. Each of the structures was solved by direct methods
This paper describes our investigation of the solid-state and
2+
solution-state structure of [Cu(dpp)2] . The crystal structure
of this molecule represents the first definitive example of a four-
coordinate copper(II) phenanthroline complex in the solid state.
(SHELXTL PLUS). All non-hydrogen atoms were refined anisotro-
pically, while the hydrogens were calculated and fixed in idealized
positions (d(C-H) ) 0.96 Å). Tables of positional parameters, bond
lengths, bond angles, and anisotropic thermal parameters and unit-cell
packing diagrams are available in the Supporting Information.
(
27) Armaroli, N.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.;
Sauvage, J. P.; Hemmert, C. J. Am. Chem. Soc. 1994, 116, 5211-
5217.
(
28) Armaroli, N.; Rodgers, M. A. J.; Ceroni, P.; Balzani, V.; Dietrich-
Buchecker, C. O.; Kern, J.-M.; Bailal, A.; Sauvage, J.-P. Chem. Phys.
Lett. 1995, 241, 555-558.
Results and Discussion
+
Solid-State Structure of [Cu(dpp)2] . The structure of the
(
(
(
(
(
(
29) Yao, Y.; Perkovic, M. W.; Rillema, D. P.; Woods, C. Inorg. Chem.
PF6- salt determined here (1) is very similar to the structure
1
992, 31, 3956-3962.
30) Castellano, F. N.; Ruthkosky, M.; Meyer, G. J. Inorg. Chem. 1995,
4, 3-4.
31) Ruthkosky, M.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1996,
5, 6406-6412.
32) McGarvey, J. J.; Bell, S. E. J.; Bechara, J. N. Inorg. Chem. 1986, 25,
325-4327.
33) McGarvey, J. J.; Bell, S. E. J.; Gordon, K. C. Inorg. Chem. 1988, 27,
003-4006.
34) Turr o´ , C.; Chung, Y. C.; Leventis, N.; Kuchenmeister, M. E.; Wagner,
P. J.; Leroi, G. E. Inorg. Chem. 1996, 35, 5104-5106.
35) Murphy, B. P. Coord. Chem. ReV. 1993, 124, 63-105.
36) van Meerssche, M.; Germain, G.; Declercq, J. P.; Wilputte-Steinert,
L. Cryst. Struct. Commun. 1981, 10, 47.
-
previously reported by McMillin and Sauvage for the CuCl2
3
16
salt; however, the present structure has a higher degree of
accuracy (Tables 1 and 2). A stereoview of the complex cation
is shown in Figure 1, and selected bond angles and distances
are presented in Table 2. Although the coordination geometry
3
4
+
-
of [Cu(dpp)2] in the CuCl2 salt was described as trigonal
4
(37) Kubas, G. J. Inorg. Synth. 1979, 19, 90-91.
(38) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J.-P. Tetrahedron
Lett. 1982, 23, 5291-5294.
(
(
(39) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.

Cu Complexes of 2,9-Diphenyl-1,10-phenanthroline
Inorganic Chemistry, Vol. 37, No. 9, 1998 2287
Table 1. Crystallographic Data for 1 and 2
[Cu(dpp)
2 6
](PF ) (1)
2 4 2
[Cu(dpp) ](ClO ) (2)
formula
CuC48
H
32
F
6
N
4
P
2 4 8
CuC48H32Cl N O
color; habit
crystal size (mm)
crystal system
space group
a (Å)
b (Å)
c (Å)
red-brown prisms
0.5 × 0.7 × 0.2
monoclinic
blue-black blocks
0.3 × 0.3 × 0.4
triclinic
P1h
7.809(3)
13.027(6)
20.344(10)
87.68(4)
89.16(4)
79.26(4)
2032(1)
2
1
P2 /c
11.081(4)
25.491(8)
14.263(5)
R (deg)
â (deg)
92.84(3)
γ (deg)
3
V (Å )
4024(2)
4
Z
fw
temp (K)
873.3
295
927.2
188
2
θ range (deg)
3.0-50.0
10580
7099
3.0-50.0
no. of reflns collected
no. of indep reflns
7400
7177
no. of obsd reflns (F > 4.0σ(F))
4813
4943
final R indices (obs data) (%): R, R
w
5.41, 6.43
8.01, 10.72
1.31
5.22, 5.37
7.92, 7.27
1.66
R indices (all data) (%): R, R
goodness of fit
w
data-to-parameter ratio
8.9:1
8.9:1
Table 2. Selected Structural Data for 1 and 2
2 4 2
Cu(dpp) ](PF ) (1) [Cu(dpp) ](ClO ) (2)
Copper(I) bis(phenanthroline) complexes generally display
distorted tetrahedral geometries. The distortion from D2d
symmetry can be described in terms of the angles θx, θy, and θz
[
2
6
Distances (Å)
2.112(3)
2.019(3)
2.082(3)
2.032(3)
40
(Table 2). These θ values describe the interligand angles based
Cu-N1A
Cu-N2A
Cu-N1B
Cu-N2B
1.993(4)
1.997(4)
1.980(4)
1.994(4)
on the CuN4 core of the complex. For a molecule that possesses
4
0
D2d symmetry, θx ) θy ) θz ) 90°. The θz value is similar
to the dihedral angle between the ligand planes. Deviation of
θz from 90° indicates a flattening distortion of the molecule
that lowers the symmetry to D2. The θx and θy values indicate
the degree of a “rocking” distortion. The observed θz of 100.2°
in 1 is similar to that observed for related copper(I) com-
Angles (deg)
82.5
121.4
98.9
142.4
124.7
82.8
104.6
69.7
N1A-Cu-N2A
N2B-Cu-N1A
N1B-Cu-N1A
N2B-Cu-N2A
N1B-Cu-N2A
N1B-Cu-N2B
85.0
110.5
139.5
140.2
107.5
84.6
91.2
88.8
1
6,40
pounds.
The large deviations from 90° for the θx and θy
values are due to the π-stacking interactions which cause
considerable distortion from D2d symmetry.
θ
x
θ
y
θ
z
The dihedral angles between the phenanthroline moieties and
their phenyl substituents in 1 range from 43 to 61° (Table 2).
The phenyl groups (C and E) that engage in interligand
π-stacking are observed to have the largest dihedral angles.
Thus, it appears that these dihedral angles are larger to allow
π-stacking interactions; however, in the absence of π-stacking,
the angles are lower to increase π-conjugation in the ligand.
100.2
118.9
Phenanthroline-Phenyl Dihedral Angles (deg)
A-C
A-D
B-E
B-F
61.4
50.2
55.1
42.6
39.9
40.1
43.3
39.9
Angles between Planes Involved in π-Stacking (deg)
2+
A-E
B-C
12.5
7.5
Solid-State Structure of [Cu(dpp)2] . A stereoview of the
four-coordinate complex cation is shown in Figure 1, and
selected bond angles and distances are presented in Table 2.
The coordination geometry around the copper in 2 is best
described as flattened tetrahedral, and the molecular symmetry
is almost perfectly D2. Unlike what is seen in the copper(I)
complex, there are no intermolecular or intramolecular π-stack-
ing interactions observed in the copper(II) complex. As
expected, the Cu-N bond lengths decrease with the increase
in oxidation state (Table 2). Although these shorter bonds may
lead to the loss of the π-stacking interactions, it is more likely
that the angular requirements of the copper(II) ion do not allow
interligand π-stacking. The lack of π-stacking interactions
results in phenanthroline-phenyl dihedral angles of 40-43° that
allow for increased π-conjugation in the ligands (Table 2).
The value of θz increases with the increase in oxidation
state: 100.2° in 1 to 118.9° in 2 (Table 2). The value of θz
will approach 180° (or 0°) as the coordination geometry
pyramidal, the geometry around the copper in 1 is best described
as distorted tetrahedral with approximate C2 symmetry. The
C2 symmetry is indicated by the six N-Cu-N bond angles and
the fact that there are two long Cu-N bonds (to N1A and N1B)
and two short Cu-N bonds (to N2A and N2B) (Table 2). The
C2 axis bisects the N1B-Cu-N1A angle, thus relating the two
phenanthroline ligands and resulting in approximate molecular
C2 symmetry. A significant feature of the structure is the
π-stacking interactions at 3.4 Å between phenanthrolines A and
B and phenyl groups E and C, respectively (Figure 1). These
-
16
interactions were also observed in the CuCl2 structure. The
remaining phenyl groups (D and F, Figure 1) are involved in
an edge-to-face interaction. The two longer Cu-N bonds are
adjacent to the phenyl groups that are involved in π-stacking.
In addition, intermolecular phenanthroline-phenanthroline
π-stacking interactions at 3.4 Å are present, resulting in phenyl-
phenanthroline-phenanthroline-phenyl π-stacks in the unit cell
(
40) Dobson, J. F.; Green, B. E.; Healy, P. C.; Kennard, C. H. L.;
(see the Supporting Information).
Pakawatchai, C.; White, A. H. Aust. J. Chem. 1984, 37, 649-659.

2288 Inorganic Chemistry, Vol. 37, No. 9, 1998
Miller et al.
Figure 1. ORTEP stereoviews of [Cu(dpp)
2
]+ (top) and [Cu(dpp)
2
]2+ (bottom). Ellipsoids are shown at the 50% probability level.
approaches “square planar” with D2h symmetry. Thus, relative
to those of 1, the phenanthroline ligands in 2 are flattened toward
a D2h geometry. This is expected since copper(II) prefers a
square planar geometry with neutral N ligands, as shown by
2
+
2+
the structures of [Cu(en)2] and [Cu(pn)2] (pn ) 1,3-
41-44
diaminopropane).
Also, the θx and θy values in 2 are much
closer to 90° (Table 2), reflective of the D2 symmetry of [Cu-
2
+
(
dpp)2] .
Four-coordinate structures of copper(II) are not typical.
Copper(II) complexes with diimine ligands almost exclusively
form five- or six-coordinate complexes in the solid state (and
in solution; vide infra). Of the 62 structures in the Cambridge
II
Structural Database containing the [Cu (phen)2] moiety (with
or without ligand substituents), only [Cu(phen)2](PF6)2 is four-
4
5
coordinate. Coordination at the fifth and/or sixth position
-
-
-
usually occurs from H2O, CH3COO , ClO4 , or BF4 . In the
structure of [Cu(phen)2](PF6)2, a Cu-F distance of 2.75 Å is
observed, indicating a secondary interaction.45 The tendency
for copper(II) bis(diimine) complexes to be five-coordinate is
further demonstrated by the structures of [Cu(tmbp)2](ClO4)2
and [Cu(tmbp)2](ClO4)2‚2H2O (tmbp ) 4,4′,6,6′-tetramethyl-
]2 and the two ClO
+
-
Figure 2. Space-filling view of [Cu(dpp)
counterions.
2
4
-
2
,2′-bipyridine), both of which are five-coordinate with ClO4
46
and H2O coordination, respectively. The methyl groups at
the 6- and 6′-positions of the bipyridine ligands in these
complexes do not provide sufficient steric shielding of the
copper center to prevent coordination by solvent or counterions.
In the structure of 2, however, the shortest Cu-OClO3 distance
is 6.19 Å, clearly indicating no interaction. The phenyl groups
(
41) Brown, D. S.; Lee, J. D.; Melsom, B. G. A. Acta Crystallogr. 1968,
B24, 730-734.
(
42) Koman, M.; Macaskova, L.; Ondrejovic, G.; Koren, B.; Battaglia, L.;
Corradi, A. Acta Crystallogr. 1988, C44, 245-246.
43) Morosin, B.; Howatson, J. Acta Crystallogr. 1970, B26, 2062-2068.
(
(
2
+
2+
44) The structures of [Cu(en)2] and [Cu(pn)2] are almost perfectly
square planar with axial anion coordination, resulting in tetragonally
distorted octahedral geometries.
2
+
in [Cu(dpp)2] are very effective at shielding the copper(II)
(
45) Amournjarusiri, K.; Hathaway, B. J. Acta Crystallogr. 1991, C47,
(46) Burke, P. J.; Henrick, K.; McMillin, D. R. Inorg. Chem. 1982, 21,
1881-1886.
1383-1385.

Cu Complexes of 2,9-Diphenyl-1,10-phenanthroline
Inorganic Chemistry, Vol. 37, No. 9, 1998 2289
Figure 3. Electronic absorption spectra of [Cu(dpp)
2
](PF
6
) (left) and [Cu(dpp)
2
](ClO
4
)
2
(right) at room temperature.
center. A space-filling view of [Cu(dpp)2]2+ and the perchlorate
anions is shown in Figure 2. It is also apparent from Figure 2
that the orientation of the phenyl groups relative to the
phenanthroline planes (i.e., with approximate 40° dihedral
angles) results in more effective shielding of the metal center
than if the phenyl groups were perpendicular to the phenan-
throline planes.
to be highly sensitive to the coordination environment. In
different coordinating solvents, the energies and extinction
2
+
coefficients of the visible bands of [Cu(dmp)2] vary signifi-
cantly, since this cation is five-coordinate in solution.49 In CH2-
Cl2 and in the solid state (KBr pellet; data not shown), the
2
+
energies of the visible bands of [Cu(dpp)2] do not change. In
2+
addition, the visible spectra of [Cu(dpp)2] are nearly identical
in CH2Cl2, CH3CN, and CH3CN/H2O (50/50) (Figure 3). Thus,
the cation remains four-coordinate in each of these cases.
+
Solution-State Structure of [Cu(dpp)2] . An interesting
+
aspect of the visible spectrum of [Cu(dpp)2] is the large spread
of the MLCT bands over the range 400-650 nm.5 This is in
,13
contrast to what is observed in the visible spectrum of [Cu-
Conclusions
+
(
dmp)2] , which has a sharper band centered at 454 nm (CH2-
1
3
Cl2). McMillin and Sauvage have shown that the solid-state
absorption spectrum of [Cu(dpp)2] is essentially the same as
As outlined in the Introduction, the principal impetus for this
work was to determine the structural changes that occur in the
+
+
the solution-state spectrum, and these authors have interpreted
[Cu(dpp)2] complex upon oxidation to copper(II) and relate
+
+
the data to reflect a low-symmetry structure for [Cu(dpp)2] in
these changes to the MLCT excited state of [Cu(dpp) ] . The
2
1
3
1
solution. The room-temperature H NMR spectrum of [Cu-
(
structure of 2 demonstrates that the shielding effects of the
+
47
dpp)2] , however, demonstrates that the four phenyl groups
phenyl groups result in a rarely seen four-coordinate copper(II)
2+
are equivalent (see Experimental Section). These results indicate
that the molecule is fluxional in solution. On the basis of the
solid-state structure of 1, the molecular motion is assigned to a
rocking between two enantiomeric C2-symmetric structures via
an intermediate of Cs symmetry (neglecting the phenyl substit-
uents). The structure shown in Figure 1 represents one of the
C2 enantiomers. Two π-stacking interactions occur in the C2
structures. One of these interactions is not possible in the
intermediate, and thus the Cs structure would likely be of higher
energy. An averaging mechanism through an intermediate of
D2 or D2d symmetry is also possible; however, in these cases,
no π-stacking interactions would occur in the intermediate (vide
species for [Cu(dpp)2] in the solid state that is shown to persist
+
in solution. This is consistent with the fact that [Cu(dpp)2]
emits at room temperature in donating solvents such as methanol
and acetonitrile. Thus, exciplex quenching does not readily
+
occur in the MLCT excited state of [Cu(dpp) ] .
2
Although there is no change in coordination number, there
are substantial structural differences between the coordination
+
2+
geometries of [Cu(dpp)2] and [Cu(dpp) ] . An overlay of
2
the cores of the complexes is shown in Figure 4. Immediately
+
after excitation into the MLCT state of [Cu(dpp) ] , the copper
2
ion will retain the geometry of the ground-state copper(I)
5
0
complex. Subsequent evolution to the lowest energy excited
state will lead to a geometry that closely resembles the structure
+
supra). The absorption spectra of [Cu(dpp)2] in Figure 3
2+
demonstrate that the solution-state structure is not solvent
dependent.
of [Cu(dpp) ] . The structures shown in Figure 4 indicate that
2
a significant rotation of the phenanthroline ligands is expected
2+
Solution-State Structure of [Cu(dpp)2] . As noted above,
there is a great tendency for [Cu(NN)2]2 (where NN represents
a chelating diimine ligand) species to bind a fifth ligand. It
in the lowest energy excited state. Such a large distortion would
be expected to lead to fast nonradiative decay from the excited
+
5
1
state and may play an important role in limiting the excited-
state lifetimes in these systems.
2+
was therefore of interest to investigate whether the [Cu(dpp)2]
species retains its four-coordinate structure in solution and in
An additional point of interest concerns the stereochemical
preferences of the copper(I) and copper(II) ions in the structures
2
+
donating solvents. The absorption spectra of [Cu(dpp)2] in
three solvent systems are shown in Figure 3. The low-energy
shoulder at 750 nm is assigned to a d-d transition, and the 575
nm band, to an LMCT transition.48 These bands are expected
(48) Lever, A. B. P. Inorganic Electronic Spectroscopy; 2nd ed.; Elsevier:
Amsterdam, 1984.
(
(
49) Karpishin, T. B.; Miller, M. T. Unpublished results.
50) Roundhill, D. M. Photochemistry and Photophysics of Metal Com-
plexes; Plenum Press: New York, 1994.
(
47) The 1H NMR spectrum of [Cu(dpp)2]+ has been previously mea-
sured: Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J. P.;
Kintzinger, J. P.; Maltese, P. NouV. J. Chim. 1984, 8, 573-582.
(51) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene, F. R.;
Meyer, T. J. Inorg. Chem. 1996, 35, 2242-2246.

2290 Inorganic Chemistry, Vol. 37, No. 9, 1998
Miller et al.
An important goal in inorganic photochemistry is to determine
ways to maximize excited-state lifetimes since complexes with
longer lifetimes are more useful for photoinduced electron and
energy transfer. Reducing the amount of structural distortion
in the excited state has been shown to significantly extend
II
II
52-55
lifetimes in Ru and Os polypyridine complexes.
To
reduce excited-state distortion in copper(I) photochemistry will
necessitate the design of systems in which the geometries around
the copper(I) and copper(II) ions are similar. These complexes
will thus require significant steric constraint to prevent the
excited-state flattening distortion.
Acknowledgment. Partial support for this work was pro-
vided through a Hellman Faculty Fellowship (to T.B.K.). The
authors thank Tommaso Vannelli for acquiring the EPR data
2
+
for [Cu(dpp)2] .
Figure 4. View of the cores of [Cu(dpp)
2
]+ and [Cu(dpp)
2
]2+. One
Supporting Information Available: Tables of positional param-
eters, bond lengths, bond angles, and anisotropic thermal parameters
and unit-cell packing diagrams for 1 and 2 (14 pages). Ordering
information is given on any current masthead page.
phenanthroline moiety of each of the complex cations was overlaid to
show the relative orientation of the other ligand. The phenyl groups at
the 2- and 9-positions of the ligands have been omitted for clarity.
of 1 and 2. As discussed above, the structure of [Cu(dpp)2]+
is largely determined by π-stacking interactions, whereas these
IC971164S
2+
(52) Boyde, S.; Strouse, G. F.; Jones, W. E., Jr.; Meyer, T. J. J. Am. Chem.
Soc. 1990, 112, 7395-7396.
interactions are not observed in [Cu(dpp)2] . Solution-state
1
0
data indicate that significant fluxionality occurs with the d
(53) Benniston, A. C.; Grosshenny, V.; Harriman, A.; Ziessel, R. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1884-1885.
ion, which permits the phenanthroline ligands to “rock” back
9
(54) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W.
E., Jr.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487.
and forth to maximize π-stacking. In contrast, the d ion
constrains the phenanthroline ligands closer to a square planar
(55) Grosshenny, V.; Harriman, A.; Romero, F. M.; Ziessel, R. J. Phys.
geometry in which π-stacking interactions are not possible.
Chem. 1996, 100, 17472-17484.