Orange and Yellow Crystals of Copper(I) Complexes Bearing 8-(Diphenylphosphino)quinoline: A Pair of Distortion Isomers of an Intrinsic Tetrahedral Complex

Article abstract of DOI:10.1021/ic1024636

The tetrafluoroborate salt of bis{8-(diphenylphosphino)quinoline}copper(I), [Cu(Ph₂Pqn)₂]BF₄, afforded orange prismatic (2O) or yellow columnar (2Y) crystals, dependent on the solvent and concentration of the recrystallization solution used. X-ray analysis revealed that crystals of 2O and 2Y had the same composition and exhibited different crystal systems: 2O was triclinic, with space group P1 and Z = 2, and 2Y was monoclinic with space group P2₁/c and Z = 4. In these crystals, the tetrahedral copper(I) complex exhibited a strong "rocking distortion" toward a trigonal pyramidal coordination geometry (by a slide translation of one of the unsymmetrical bidentate chelating ligands along the tetrahedral edge). In addition, both the 2O and 2Y complexes showed a "flattening distortion", meaning that the dihedral angle between the two chelate planes were off-perpendicular and oriented toward opposite directions, which resulted in a pair of distortion isomers: syn clinal (sc: 2O) and anti clinal (ac: 2Y). ³¹P CP-MAS NMR spectroscopy indicated that 2O and 2Y could be distinguished. Both isomers exhibited inequivalent P atoms, but a larger difference in chemical shift was observed in 2Y. TD-DFT calculations reproduced the difference in spectra between the orange- and yellow-colored complexes, which originated from metal-to-ligand charge-transfer transitions.

Full text of DOI:10.1021/ic1024636

ARTICLE
pubs.acs.org/IC
Orange and Yellow Crystals of Copper(I) Complexes Bearing
8-(Diphenylphosphino)quinoline: A Pair of Distortion Isomers of an
Intrinsic Tetrahedral Complex
Takayoshi Suzuki,*,† Hiroshi Yamaguchi,‡ Akira Hashimoto,‡ Koichi Nozaki,§ Mototsugu Doi,#
Naoya Inazumi,# Noriaki Ikeda,^ Satoshi Kawata,z Masaaki Kojima,† and Hideo D. Takagi‡
†Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530, Japan
‡Graduate School of Science and Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan
§Department of Chemistry, Graduate School of Science and Technology, University of Toyama, Toyama 930-8555, Japan
#Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
^Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Kyoto 606-8585, Japan
zDepartment of Chemistry, Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan
S Supporting Information
b
ABSTRACT: The tetrafluoroborate salt of bis{8-(diphenylphos-
phino)quinoline}copper(I), [Cu(Ph2Pqn)2]BF4, afforded or-
ange prismatic (2O) or yellow columnar (2Y) crystals, depen-
dent on the solvent and concentration of the recrystallization
solution used. X-ray analysis revealed that crystals of 2O and 2Y
had the same composition and exhibited different crystal
systems: 2O was triclinic, with space group P1 and Z = 2, and
2Y was monoclinic with space group P21/c and Z = 4. In these
crystals, the tetrahedral copper(I) complex exhibited a strong “rocking distortion” toward a trigonal pyramidal coordination
geometry (by a slide translation of one of the unsymmetrical bidentate chelating ligands along the tetrahedral edge). In addition,
both the 2O and 2Y complexes showed a “flattening distortion”, meaning that the dihedral angle between the two chelate planes
were off-perpendicular and oriented toward opposite directions, which resulted in a pair of distortion isomers: syn clinal (sc: 2O) and
anti clinal (ac: 2Y). 31P CP-MAS NMR spectroscopy indicated that 2O and 2Y could be distinguished. Both isomers exhibited
inequivalent P atoms, but a larger difference in chemical shift was observed in 2Y. TD-DFT calculations reproduced the difference in
spectra between the orange- and yellow-colored complexes, which originated from metal-to-ligand charge-transfer transitions.
’ INTRODUCTION
give P and M enantiomers). In previous studies,3,4 we have prepared
and characterized square-planar NiII and PdII complexes containing
8-quinolylphosphines (R2Pqn, where R2 = Ph2, Me2, or MePh). At
first glance, these complexes should assume a square-planar coordi-
nation geometry with a cis(P,P) configuration. However, the co-
ordination plane shows a skewed distortion to some extent, which
affords chiral sp structures, because of a steric interaction between the
ortho-H atoms of mutually cis-positioned quinolyl groups. In this
study, we have investigated the related CuI complexes with 8-qui-
nolylphosphines, which were envisioned to have an idealized pseudo-
tetrahedral coordination geometry. However, in the case of the
Ph2Pqn complex, we obtained orange- and yellow-colored crystals
consisting of one or the other flattened distortion isomers, i.e., the sc-
or ac-isomer. The crystal structure determinations and spectroscopic
characterizations of both isomers are reported.
Stereochemical isomerism is the most fundamental and most
interesting phenomenon that occurs in coordination chemistry, and a
large number of geometrical isomeric pairs of transition-metal
complexes have been prepared.1 The differences in the structure
and properties between such isomers have been compared. In the
case of four-coordinate square-planar complexes with an unsymme-
trical bidentate ligand, (SP-4)-[M(AꢀB)2], a pair of geometrical (cis
and trans) isomers can occur, but both isomers would be achiral (see
Scheme 1). In contrast, tetrahedral complexes of (T-4)-[M(AꢀB)2]
would be chiral (i.e., P and M enantiomers would be formed). When
(T-4)-[M(AꢀB)2] is distorted toward a planar geometry (i.e., a
flattening distortion occurs), there would be two different modes,
affording a pair of “distortion isomers”. These would be termed syn
clinal (sc) and anti clinal (ac) isomers, using the nomenclature used
for organic compounds2 (see Scheme 1). This situation is similar to
that of square-planar complexes with cis (syn periplanar, sp) or trans
(anti periplanar, ap) geometry, where a tetrahedral (skewing)
distortion of the coordination plane makes the complex chiral (to
Received: December 9, 2010
Published: March 29, 2011
r
2011 American Chemical Society
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Scheme 1. Nomenclature Used for Distorted Square-Planar
and Tetrahedral Complexes
the N1 and C2ꢀC10 atoms) of only 0.116(4) and 0.038(4) Å,
respectively, and those of the Cu1 and P2 atoms from the plane[qn2]
(defined by atoms N21 and C22ꢀC30) of 0.095(4) and 0.094(4) Å,
respectively. The dihedral angle between the plane [Cu1,P1,N1]
(defined by atoms Cu1, P1, and N1) and the plane [qn1] was only
4.77(6)ꢀ, and that between the plane [Cu1,P2,N21] and the plane
[qn2] was 2.60(7)ꢀ. The CuꢀP and CuꢀN bond lengths are in the
range 2.226(2)ꢀ2.238(2) and 2.097(3)ꢀ2.105(3) Å, respectively,
and the PꢀCuꢀN chelate bite angles are 87.3(1)ꢀ and 86.9(1)ꢀ,
which are typically found in other Me2Pqn complexes.5 The pseudo-
tetrahedral coordination geometry around the Cu atom is not
flattened, as the dihedral angle (θCu) between plane[Cu1,P1,N1]
and plane [Cu1,P2,N21] is 86.29ꢀ (the dihedral angle (θqn) between
the two quinoline planes, plane [qn1] vs plane [qn2], is 88.56(9)ꢀ),
but it exhibits a significant rocking distortion,6 as emphasized in
Figure 1b. The angle between the Cu1ꢀP2 bond and plane[Cu1,P1,
N1] (154.3ꢀ) is much larger than that between the CuꢀN21 bond
and plane [Cu1,P1,N1] (119.2ꢀ).7 In addition, the P1ꢀCu1ꢀP2
bond angle (138.21(6)ꢀ) is much larger than the ideal tetrahedral
angle, while the N1ꢀCu1ꢀN21 bond angle (103.4(1)ꢀ) is smaller
than the ideal tetrahedral angle. This rocking distortion is caused by
the intramolecular steric interaction between the ꢀPMe2 groups. In
the case of bis(1,10-phenanthroline)-type copper(I) complexes,6
such a rocking distortion leads to an approximately trigonal pyrami-
dal coordination geometry with an exceptionally long CuꢀN(apical)
bond. However, in complex 1, the Cu1ꢀN21(apical) bond length is
comparable to that of the Cu1ꢀN1 bond length (see Table S1 in the
Supporting Information).
Synthesis and Crystal Structure of [Cu(Ph2Pqn)2]BF4 (2).
Recently, Tsukuda et al. have reported on the Ph2Pqn analogue of
complex 1, [Cu(Ph2Pqn)2]PF6 CH2Cl2 (20 CH2Cl2),8 indepen-
3
3
dently to our study. They prepared the complex 20 in a 66% yield
from [Cu(CH3CN)4]PF6 and Ph2Pqn and characterized its photo-
physical properties. They also analyzed the crystal structure of
20 CH2Cl2, but did not discuss any distortion of the CuI coordina-
3
tion geometry in detail. Similar to complex 1, no flattening distortion
of the idealized pseudo-tetrahedron was observed (θCu = 89.46ꢀ),
but a rocking distortion was shown by the larger angle between the
CuꢀP(1) bond and plane[Cu,P(2),N(1)] (148.3ꢀ) than that
between the CuꢀN(2) bond and the same plane (124.3ꢀ). The
P(1)ꢀCuꢀP(2) bond angle was 131.12(5)ꢀ. These angles indicate
that the degree of rocking distortion was smaller in this Ph2Pqn
complex 20 than that in the corresponding Me2Pqn complex 1.
In this study, we investigated the corresponding tetrafluoro-
borate salt, [Cu(Ph2Pqn)2]BF4 (2). This compound was simi-
larly obtained from the stoichiometric reaction of Ph2Pqn and
[Cu(CH3CN)4]BF4 in a near-quantitative yield. In addition, it
could be prepared from the reaction of Cu(BF4)2 6H2O and
3
Figure 1. (a) ORTEP view of [Cu(Me2Pqn)2]PF6 (1); the thermal
ellipsoids are drawn at the 50% probability level, and the hydrogen atoms
have been omitted for clarity. (b) Schematic drawing of the cation in 1,
emphasizing the rocking distortion of the tetrahedron.
Ph2Pqn in ethanol, albeit with a yield of only ca. 40% after
purification by reprecipitation from an ethanol solution after the
addition of hexane. Recently, Qin et al. reported on an interesting
mixed ligand complex, [Cu(Ph2Pqn)(DPEphos)]BF4 (where
DPEphos = bis[2-(diphenylphosphino)phenyl]ether), which
was prepared from a 1:1:1 mixture of [Cu(CH3CN)4]BF4,
Ph2Pqn, and DPEphos.7
Orange prismatic crystals (2O) were deposited when com-
pound 2 was recrystallized from a nitromethane solution or a
saturated acetonitrile solution by vapor diffusion of diethyl ether.
However, if the concentration of the acetonitrile solution was less
than half the saturation value, yellow columnar crystals (2Y) were
formed (see Figure S1 in the Supporting Information). We
could easily separate crystals of 2O and 2Y manually under a
’ RESULTS AND DISCUSSION
Synthesis and Crystal Structure of [Cu(Me2Pqn)2]PF6 (1).
The reaction of [Cu(CH3CN)4]PF6 and 8-(dimethylphosphino)-
quinoline in acetonitrile in a molar ratio of 1:2 afforded pale
yellow crystals of [Cu(Me2Pqn)2]PF6 (1) in a near-quantitative
yield. Single-crystal X-ray analysis revealed that the molecular
structure of 1 is that shown in Figure 1. The ligand, Me2Pqn, acts
as a planar five-membered chelate ligand, with deviations of the
Cu1 and P1 atoms from the least-squares plane[qn1] (defined by
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Figure 2. (a) An ORTEP drawing (at the 50% probability level, H atoms omitted) of the cationic complex in the orange prismatic crystals (2O) of
[Cu(Ph2Pqn)2]BF4. (b and c) Schematic views of the cation in 2O.
microscope. Both crystals had no solvent of crystallization, which
was confirmed by elemental analysis, 1H NMR spectroscopy (see
Figure S2 in the Supporting Information), and X-ray crystal-
lography (vide infra). The UVꢀvis absorption spectra of aceto-
nitrile solutions of 2O and 2Y were identical to each other (see
Figure S3 in the Supporting Information). Furthermore, solu-
tions of dissolved 2O or 2Y both reproduced crystals of 2O and
2Y on recrystallization under suitable conditions (i.e., appro-
priate solvent and concentration). This observation suggests that
crystals of 2O and 2Y are polymorphs9 of [Cu(Ph2Pqn)2]BF4,
and we succeeded in characterizing both crystal structures using
X-ray diffraction.
In the orange crystals of 2O, the compound crystallizes with a
triclinic space group P1 and Z = ꢀ2, consisting of a complex cation
of [Cu(Ph2Pqn)2]þ and a BF4 anion in an asymmetric unit.
The molecular structure of the complex cation in 2O is shown in
Figure 2. The chelate coordination of Ph2Pqn is normal, and the
P1ꢀCu1ꢀN1 and P2ꢀCu1ꢀN21 bite angles are 86.90(5)ꢀ and
87.03(5)ꢀ, respectively. The five-membered chelate ring is almost
planar; the deviations of the Cu1 and P1 atoms from plane[qn1] are
0.256(3) and 0.139(3) Å, respectively; and the deviation of the Cu1
and P2 atoms from plane[qn2] are 0.144(3) and 0.005(3) Å,
respectively. Similar to complexes 1 and 20, a significant rocking
distortion of the pseudo-tetrahedral coordination geometry was
observed (see Figure 2b). The angle between the Cu1ꢀP1 bond
and plane [Cu,P2,N21] was 154.83ꢀ, while the angle between the
Co1ꢀN1 bond and the same plane was 125.62ꢀ. In addition, the
P1ꢀCu1ꢀP2 bond angle (124.59(2)ꢀ) was larger than the ideal
tetrahedral angle, while the N1ꢀCu1ꢀN21 bond angle (97.79(7)ꢀ)
was much smaller than the tetrahedral angle. The slight elongation of
the Cu1ꢀN1 bond, 2.078(2) Å, versus the length of the Cu1ꢀN21
bond, 2.049(2) Å, may be related to this rocking distortion.
Compared with the molecular structure of the complex cation
in 20, there was a marked additional flattening distortion of the
CuI coordination geometry in 2O. The dihedral angle of θCu
(plane[Cu1,P1,N1] vs plane [Cu1,P2,N21]) was 69.88ꢀ, and the
angle of θqn (plane [qn1] vs plane [qn2]) was 63.31(6)ꢀ.
According to the notation shown in Scheme 1, this complex
was assigned as the sc-isomer, which is well recognizable when
the complex is viewed from the direction parallel to the two
quinoline planes (see Figure 2c).
also characterized the crystal structure of the corresponding NiII
complex, cis(P,P)-[Ni(Ph2Pqn)2](BF4)2, where the dihedral angle of
θNi was 19.7ꢀ (the dihedral angle of θqn was 35.0(2)ꢀ).3 Therefore,
the NiII complex was assigned as the sp (syn periplanar) isomer (see
Scheme 1).
In the yellow crystals of 2Y, the compound crystallizes with
monoclinic space group P21/c and Z = 4. Similar to 2O, crystals
of 2Y also consisted of a complex cation of [Cu(Ph2Pqn)2]þ and
a BF4ꢀ anion in an asymmetric unit. The molecular structure of
the complex cation in 2Y is shown in Figure 3. Similar to the
complex in 2O discussed above, a rocking distortion of the pseudo-
tetrahedron of CuI coordination geometry was observed (see
Figure 3b). The angle between the Cu1ꢀP2 bond and plane[Cu,
P1,N1] was 150.57ꢀ, and the angle between the CoꢀN21 bond and
the same plane was 126.35ꢀ. This observation indicates that a rocking
distortion is common for CuI complexes with 8-quinolylphosphines
(1, 20, 2O, and 2Y). Presumably, this arises from the large intramo-
lecular steric interaction between the methyl or phenyl substituents
on the P atoms.
The P1ꢀCu1ꢀP2 bond angle was 137.60(2)ꢀ, which is com-
parable to that observed in complex 1. However, unlike the other
complexes of 1, 20, and 2O, the N1ꢀCu1ꢀN21 bond angle was
markedly large, at 120.72(6)ꢀ. A flattening distortion was ob-
served, but the direction of the distortion was opposite to that
observed in 2O. In2Y, the two phosphino donor groups were twisted
away from each other (see Figure 3c). The dihedral angle of θCu was
101.99ꢀ and the dihedral angle of θqn was 105.23(4)ꢀ (the angles of
θCu and θqn were defined as 0ꢀ and 180ꢀ for the cis (sp) and trans
(ap) isomers, respectively). Thus, the complex cation in 2Y was
assigned as the ac-isomer (see Scheme 1), which is the counterpart of
the complex observed in 2O as the pair of “flattening distortion
isomers”of the intrinsic tetrahedral complex. The CuꢀPbondlength
in 2Y was comparable to that in 2O, while the CuꢀN bond lengths,
Cu1ꢀN1 (2.064(2) Å) and Cu1ꢀN21 (2.101(2) Å), are slightly
longer than those observed in 2O (by 0.02 Å).
The CuI complexes in 2O and 2Y are a pair of distortion
isomers, but the degrees of their flattening distortion (from an
idealized tetrahedron with θ = 90ꢀ) are not similar. The distor-
tion of the sc-isomer in 2O (θCu = 69.88ꢀ) is almost twice that
observed for the ac-isomer in 2Y (θCu = 101.99ꢀ). This fact may
be a key factor for understanding the optical properties of these
crystals, which will be discussed later on. Moreover, in the
crystals of 2Y, the chelate coordination of one of the Ph2Pqn
ligands is unusual. The Cu1 atom is displaced out of the plane
[qn2] by 0.413(2) Å, while the deviation of the P2 atom is only
Note that the compound in 2O is isomorphous with
[Ni(Ph2Pqn)2]BF4.3 The NiI complex was also assigned as the
sc-isomer, because the dihedral angle of θNi was 57.43ꢀ (the
dihedral angle of θqn was 61.06(9)ꢀ). In a previous study, we have
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Figure 3. (a) ORTEP drawing (at the 50% probability level, H atoms omitted) of the cationic complex in the yellow columnar crystals (2Y) of
[Cu(Ph2Pqn)2]BF4. (b and c) Schematic views of the cation in 2Y.
different bonding environments for the two Ph2Pqn ligands. Our
X-ray analysis showed that the degree of rocking distortion in 2O and
2Y was similar (vide supra). In contrast, one of the two Ph2Pqn
chelates in 2Y showed a remarkable deviation from planarity. In other
words, the inequivalency between the two Ph2Pqn chelate coordina-
tion modes is more pronounced in 2Y. Thus, the observed inequi-
valency in the 31P NMR chemical shift can be taken into account by
the different crystal structures of 2O and 2Y.
Diffuse Reflectance Spectra and TD-DFT Calculations of
[Cu(Ph2Pqn)2]BF4 (2). The colorimetric difference between 2O
and 2Y is obvious (see Figure S1 in the Supporting Information),
Figure 4. 31P CP-MAS NMR spectra (242.64 MHz, spinning frequency
of 25 kHz, 30 ꢀC) of (a) 2O and (b) 2Y.
and the diffuse reflectance spectra of powdered samples are com-
pared in Figure 5. The spectrum of 2Y exhibited an absorption
shoulder occurring at ∼360 nm, while the spectrum of 2O exhibited
a stronger skirt on the absorption band, occurring ∼470 nm. We
carried out time-dependent density functional theory (TD-DFT)
calculations to characterize the differences in the spectra of 2O and
2Y. The energy levels of the molecular orbitals used for the complex
cations found in 2O and 2Y were calculated using the X-ray struc-
tures at the density functional theory (DFT) level. The absorption
spectra of the complexes were simulated (up to 350 nm, for simpli-
city), using the excitation energies and oscillator strengths calculated
at the TD-DFT level. The absorption bands were assumed to have a
Gaussian line-shape function having a full width at half-maximum of
5000 cmꢀ1. The calculated difference in the spectra agreed well with
the observed difference (see Figure 5).
In the calculations on both distortion isomers in 2O and 2Y
(Figure 6), the LUMO (164) and LUMOþ1 (165) orbitals were
the localized π* orbital of the quinoline moiety of the Ph2Pqn
ligands, which are, in principle, independent of the distortion of
the tetrahedral coordination geometry of CuI. In contrast, the
HOMO (163), HOMOꢀ1 (162), and HOMOꢀ2 (161) orbi-
tals, which are three MꢀL σ antibonding orbitals and have their
main contribution from the CuI d orbitals (the t2 set in an
idealized tetrahedral system), showed a considerable depen-
dence on the flattening distortion. Figure 6b illustrates the
exchange of HOMO and HOMOꢀ1 orbitals between the
sc- and ac-isomers (in 2O and 2Y, respectively). More impor-
tantly, the difference in energy between the HOMO and
HOMOꢀ1 orbitals of the sc-isomer in 2O is markedly larger than
the difference in energy of the ac-isomer in 2Y. The larger flattening
distortion in the sc isomer in 2O would lead to a larger difference in
energy between these orbitals. Thus, the HOMO-to-LUMO
0.006(2) Å, forming an envelope-type five-membered chelate
ring. The other Ph2Pqn chelate in 2Y is more planar. The
deviation of the Cu1 and P1 atoms from the plane[qn1] is
0.247(2) and 0.047(2) Å, respectively. The dihedral angle
between plane[Cu1,P2,N21] and plane[qn2] is 15.26(4)ꢀ, while
the dihedral angle between plane[Cu1,P1,N1] and plane[qn1] is
8.40(4)ꢀ. This nonplanarity of one of the Ph2Pqn chelates may
have a significant effect on the inequivalency of the two P nuclei
observed using 31P CP-MAS NMR spectroscopy (vide infra).
31P CP-MAS NMR Spectra of [Cu(Ph2Pqn)2]BF4 (2). The
31P CP-MAS NMR spectra of solid samples of 2O and 2Y were
measured. In Figure 4, it can be clearly seen (and in Figure S4 in
the Supporting Information) that the spectra of 2O and 2Y are
different from each other. We measured several spectra for each
sample, using crystals from different batches, and the results were the
same as those shown in Figure 4. In addition to the coupling to a 63Cu
or 65Cu nucleus (I = 3/2), both spectra showed inequivalency of the
two P atoms, which was confirmed using two different spectrometers
with different magnetic fields (see Figure S4 in the Supporting Infor-
mation). Sample 2O exhibited two quartet resonances at δ = ꢀ24.3
(JCuꢀP = 1190 Hz) and ꢀ22.5 (JCuꢀP = 1120 Hz), while 2Y
exhibited two quartet resonances at δ = ꢀ25.2 (JCuꢀP = 1250 Hz)
and ꢀ16.2 (JCuꢀP = 1100 Hz). Our X-ray crystal structure analysis
indicated that the CuI complexes in 2O and 2Y were the flattening
distortion isomers. However, the flattening distortion itself cannot
cause the two P nuclei in each complex to be inequivalent. For the
chemical inequivalency of the two P nuclei, it is necessary to have
either a rocking distortion of the CuI coordination geometry, or
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Distortion Isomers and Polymorphs of [Cu(Ph2Pqn)2]BF4 (2).
The CuI complexes in the crystals of 2O and 2Y form a pair of
distortion isomers, and no interconversion between these isomers
was observed in the solid state (at least at ambient temperature).
However, as mentioned in the above section, acetonitrile solutions
of 2O and 2Y exhibited identical UVꢀvis absorption spectra (see
Figure S3 in the Supporting Information), showing two distinct
absorption peaks occurring at ∼450 nm and ∼360 nm. Our
1H NMR measurements on CD3CN solutions of 2O and 2Y also
exhibited identical spectra, corresponding to a single species,
which also exhibited an equivalency of the two Ph2Pqn ligands in
solution. These observations can be taken into account by a rapid
equilibrium that occurs between the distorted isomers, relative to
the NMR time scale. This fact may indicate that crystals of 2O
and 2Y are conformational polymorphs9 of complex 2.
Polymorphism, or pseudo-polymorphism, associated with
color changes is often observed in transition-metal complexes.9,10
Polymorphism is found in almost every substance, and it is typically
based on the packing forms of the constituent molecules or ions,
where weak intermolecular interactions, such as hydrogen bonds,
πꢀπ stacks, and CꢀH/π interactions often play an important role.
In typical polymorphs of transition-metal complexes, differences in
color between polymorphs are not commonly observed, because the
constituent metal complexes have the same geometrical structure.11
When crystals contain different types and/or numbers of solvent
molecules of crystallization, then they are categorized as pseudo-
polymorphs, and often exhibit different colors, because the solvent
molecules may have an interaction with the metal center or the
ligands, inducing a change in the coordination geometry.4 In the
case of a compound having the same composition, but with different
constituent units (exhibiting another type of pseudo-polymor-
phism), the crystals of metal complexes often show a distinct change
in color. For example, Reedijk et al. reported that dichlorocopper(II)
complexes with long-chain bidentate NꢀN ligands afforded red
crystals consisting of a monomeric [CuCl2(NꢀN)] complex and
green crystals consisting of a polymeric [CuCl2(μ-NꢀN)]n one-
dimensional chain-type compound.12 Green and orange crystals
of monomeric and polymeric oxovanadium(IV) complexes con-
taining a tetradentate Schiff base ligand represent another
example.13 Supramolecular isomerism in network solids14 is also
categorized as pseudo-polymorphism.
Figure 5. (a) Observed diffuse reflectance spectra in the solid state, and
(b) TD-DFT-derived absorption spectra of the orange crystals (2O:dashed
line) and the yellow crystals (2Y: solid line) of [Cu(Ph2Pqn)2]BF4.
A notable precedent for conformational polymorphism in
transition-metal complexes is a sulfimide copper(II) complex,
[CuCl2(NHSPh2)2], exhibiting blue and green crystals.15 In the
blue crystals, the CuII ion exhibits a perfect square-planar geometry,
while a pseudo-tetrahedral coordination geometry is observed in
the green crystals. The (trifluoroacetato)(tripyrrinato)palladium(II)
complex also shows two different molecular structures in poly-
morphic crystals, i.e., a helical distortion of the tripyrrin ligand or a
pronounced tetrahedral deviation of the PdII coordination plane
(although the colors of these polymorphic crystals are indistingui-
shable).16 The present case is another example of a conformational
polymorph that consists of a pair of flattening distortion isomers of
the intrinsic tetrahedral complex.
In summary, we have isolated a pair of sc- and ac-isomers of
[CuI(AꢀB)2]þ-type complexes, [Cu(Ph2Pqn)2]BF4, and their
crystal structures have been confirmed from X-ray analyses.
Distinguishable properties between the solid-state isomers have
been observed in diffuse reflectance and 31P CP-MAS NMR
spectra. In solution, these complexes immediately reach equilib-
rium. However, if a suitable molecular design is provided by the
ligand, then each isomer can be produced selectively.
Figure 6. (a) Time-dependent density functional theory (TD-DFT)-derived
molecular orbital energy, and (b) illustration of the HOMOꢀ1, HOMO,
LUMO, and LUMOþ1 orbitals of the cationic complexes in 2O and 2Y.
(or LUMOþ1) and (HOMOꢀ1)-to-LUMO (or LUMOþ1)
transitions (and therefore, the metal-to-ligand charge-transfer
transitions) of 2O would occur at ∼470 and ∼380 nm, while
the same transitions in 2Y were observed to be almost degenerate
at ∼390 nm (see Figure 5b).
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ARTICLE
Table 1. Crystallographic Data
abbreviation
1
2O
2Y
compound
formula
[Cu(Me2Pqn)2]PF6
C22H24CuF6N2P3
586.88
[Cu(Ph2Pqn)2]BF4
C42H32BCuF4N2P2
776.99
[Cu(Ph2Pqn)2]BF4
C42H32BCuF4N2P2
776.99
formula weight, FW
temperature, T (K)
color, shape
crystal size (mm)
crystal system
space group, Z
a (Å)
200(2)
200(2)
200(2)
pale yellow, block
0.20 ꢁ 0.20 ꢁ 0.12
monoclinic
P21/c, 4
orange, prism
0.40 ꢁ 0.30 ꢁ 0.22
triclinic
yellow, column
0.30 ꢁ 0.12 ꢁ 0.1
monoclinic
P21/c, 4
P1, 2
9.762(7)
27.08(2)
9.651(8)
90
8.6882(5)
12.2070(6)
17.3712(9)
88.981(1)
80.862(2)
79.761(1)
1789.9(2)
1.442
10.8254(11)
14.8647(17)
22.025(3)
90
b (Å)
c (Å)
R (deg)
β (deg)
92.42(7)
90
99.646(6)
90
γ (deg)
V (Å3)
Dx (Mg mꢀ3
2548(4)
3494.1(7)
1.477
)
1.530
F(000)
1192
796
1592
μ(Mo KR) (mmꢀ1
Tmin, Tmax
)
1.103
0.755
0.773
0.810, 0.898
5855/308
0.059
0.752, 0.851
8156/470
0.039
0.647, 0.927
8050/469
0.035
Refln/param ratio
R1 [F02 > 2σ(F02)]
wR2 (all refln)
0.118
0.160
0.062
goodness of fit, GoF
1.001
1.113
0.998
’ EXPERIMENTAL SECTION
2Y. Anal. Found for 2O: C 64.8, H 3.95, N 3.59%. Found for 2Y: C 63.9,
H 4.00, N 3.63%. Calc. for C42H32BCuF4N2P2: C 64.9, H 4.15, N 3.61%.
Measurements. The diffuse reflection spectra of solid samples
were recorded at room temperature, using a Jasco Model V-550
spectrophotometer using a Jasco Model ISN-470 integration sphere.
The solution 1H NMR spectra were measured using a JEOL Model EX-
270 spectrometer at 30 ꢀC. The chemical shifts were referenced to the
residual signal of CD3CN, and are reported vs TMS. The 31P CP-MAS
NMR spectra were acquired using two spectrometers: (1) a Chemag-
netics Model CMX 300 system (121.64 MHz for 31P) at 25 ꢀC with a
spinning frequency of 9 kHz, and (2) a Varian Model VNS 600 system
(242.64 MHz for 31P) at 30 ꢀC with a spinning frequency of 25 kHz.
Ammonium monohydrogenphosphate, (NH4)2HPO4, was used as the
external standard for the 31P chemical shifts.
Crystallography. The X-ray diffraction data were obtained at
ꢀ73(2) ꢀC using a Rigaku R-axis rapid imaging plate detector with graphite-
monochromated Mo KR radiation (λ = 0.71073 Å). A suitable crystal was
mounted with a cryoloop and flash-cooled using a cold nitrogen gas stream.
The data were processed using the Process-Auto software package,19 and the
absorption corrections were applied using the empirical method.20 The
structures were solved using the direct method employing the SIR92 software
package21 and refined on F2 (with all independent reflections) using the
SHELXL97 software package.22 All the nonhydrogen atoms were refined
anisotropically, and the hydrogen atoms were introduced at the positions
calculated theoretically and treated with riding models. All the calculations
were performed using the CrystalStructure software package.23 The crystal
data are shown in Table 1.
The ligands Me2Pqn5b and Ph2Pqn17 were prepared according to
literature methods. The phosphines were handled under an atmosphere
of nitrogen using the standard Schlenk technique until such time that the
air-stable copper(I) complexes were formed. The copper(I) acetonitrile
complexes, [Cu(CH3CN)4]BF4 and [Cu(CH3CN)4]PF6, were synthe-
sized using literature methods.18
Preparation of the Complexes. [Cu(Me2Pqn)2]PF6 (1). To
an acetonitrile solution of [Cu(CH3CN)4]PF6 (0.52 g, 1.4 mmol), 8-
(dimethylphosphino)quinoline (0.56 g, 3.0 mmol) was added drop-
wise with stirring. After stirring the mixture at room temperature for
ca. 1 h, the solvent was removed to dryness under reduced pressure.
The resulting yellow residue was recrystallized from an acetonitrile
solution by diffusion of diethyl ether vapor, affording pale yellow
crystals of 1. Yield: 0.68 g (83%). Anal. Found: C 44.5, H 4.23, N
4.58%. Calcd. for C22H24CuF6N2P3: C 45.0, H 4.12, N 4.77%.
[Cu(Ph2Pqn)2]BF4 (2). To a refluxing solution of Ph2Pqn (0.51 g, 1.6
mmol) in ethanol (20 cm3), an ethanol solution (5 cm3) ofCu(BF4)2 6H2O
3
(0.276 g, 0.80 mmol) was added. The color of the mixture immediately
turned dark green and then gradually changed to a yellow-orange color upon
refluxing for 2 h. After cooling the mixture to ambient temperature, the
solution was concentrated (to ca. 5 cm3) under reduced pressure. The result-
ing pale yellow precipitate was collected by filtration and recrystallized from a
mixture of ethanol and hexane. Yield: 0.229 g (37%). Anal. Found: C 64.5, H
4.06, N 3.57%. Calcd. for C42H32BCuF4N2P2: C 64.9, H 4.15, N 3.61%.
Crystallization of 2O and 2Y. When the above product (2) was
recrystallized from a saturated acetonitrile solution by vapor diffusion of
Et2O, orange prismatic crystals (2O) were deposited. Recrystallization
from nitromethane by vapor diffusion of Et2O also gave orange crystals
of 2O. Yellow columnar crystals (2Y) were afforded, together with 2O,
when Et2O vapor was diffused into an acetonitrile solution of 2 with a
concentration less than half that of saturation. Recrystallization from
dichloromethane by vapor diffusion of Et2O gave only yellow needles of
DFT Calculations. The energy levels of the molecular orbitals of the
copper(I) complexes were calculated using the X-ray structures at the DFT
level. The basis functions used in the computations were the DunningꢀHay
split-valence double-ζ functions for the C, H, and N atoms (D95) and the
HayꢀWadt double-ζ functions with the Los Alamos relativistic effective core
potential for heavy atoms (LANL2DZ).24 The hybrid functional of Perdew,
Burke, and Ernzerhof (PBE1PBE) was employed.25 The absorption spectra
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Inorganic Chemistry
ARTICLE
of the complexes were simulated using the excitation energies and oscillator
strengths calculated at the TD-DFT level. The absorption bands were
assumed to have a Gaussian line-shape function, having a full width at half-
maximum of 5000 cmꢀ1. All the DFT calculations were performed using the
Gaussian 03.D suite.26 Drawings of the molecular orbitals were made using
the Molekel software package.27
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic infor-
b
mation for all the compounds analyzed in this study (CIF
format); tables of selected bond lengths and angles of the
complexes, a photograph of crystals 2O and 2Y, UVꢀvis and
1H NMR spectra of complex 2 in solution, and the 31P CP-MAS
NMR spectra of 2O and 2Y taken using the other spectrometer
(PDF) are available. This material is available free of charge via
the Internet at http://pubs.acs.org.
(18) Kubas, G. J. Inorg. Synth. 1990, 28, 68–70.
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for Imaging Plate Diffractometer; Rigaku Co., Ltd.: Akishima, Tokyo,
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2008, A64, 112–122.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: suzuki@cc.okayama-u.ac.jp.
’ ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research (No. 20550064) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
(23) CrystalStructure ver. 3.6.0. Crystal Structure Analysis Package;
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