Structure-dependent spectroscopic and redox properties of copper(I) complexes with bidentate iminopyridine ligands

Article abstract of DOI:10.1016/j.ica.2009.01.008

A series of iminopyridine ligands; cyclopropylpyridin-2-ylmethyleneamine (A), cyclopentylpyridin-2-ylmethyleneamine (B), cyclohexylpyridin-2-ylmethyleneamine (C), and cycloheptylpyridin-2-ylmethyleneamine, (D) and their copper(I) complexes, [Cu(L)₂

Full text of DOI:10.1016/j.ica.2009.01.008

Inorganica Chimica Acta 362 (2009) 2872–2878
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Structure-dependent spectroscopic and redox properties of copper(I)
complexes with bidentate iminopyridine ligands
b
Werner Massa a, Saeed Dehghanpour b, , Khadijeh Jahani
*
a Department of Chemistry, Philipps-University, D-35032 Marburg, Germany
b Department of Chemistry, Alzahra University, P.O. Box 1993891176, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of iminopyridine ligands; cyclopropylpyridin-2-ylmethyleneamine (A), cyclopentylpyridin-2-
ylmethyleneamine (B), cyclohexylpyridin-2-ylmethyleneamine (C), and cycloheptylpyridin-2-ylmethyl-
eneamine, (D) and their copper(I) complexes, [Cu(L)2]+ (1a–1d) and [Cu(L)(PPh3)2]+ (2a–2d) have been
synthesized and characterized by CHN analyses, 1H NMR and IR and UV–Vis spectroscopy. Structures
of 1a, 1b, 1c and 2a were determined by X-ray crystallography. The coordination polyhedron about
the CuI center in the complexes is best described as a distorted tetrahedron. The dihedral angles between
the least-squares planes of the chelate ligands show considerable variation from 86.1° in 1a to 68.3° in
1b, indicating the importance of packing forces in the crystalline environment. The UV–Vis spectra of
the complexes are characterized by first metal to ligand charge transfer bands increasing in wavelength
with increasing size of the ring substituents in the ligands, except for the cyclopropyl compounds (1a and
2a), in good agreement with the variation of the dihedral angles between the ligand planes. Cyclic vol-
tammetry of the complexes indicates a quasireversible redox behavior for the complexes. The bulkier
ligands (PPh3) inhibit the geometric distortion within the oxidized form and the redox potentials of com-
plexes 2a–2d are shifted to more positive values, therefore.
Received 24 September 2008
Received in revised form 29 December 2008
Accepted 9 January 2009
Available online 14 January 2009
Keywords:
Copper(I) complexes
2-Pyridinecarbaldehyde
Diimine ligands
X-ray structure analyses
Distorted tetrahedral structure
Spectroscopy
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
trochemical properties of Cu(I) complexes of phenylpyridin-2-ylm-
ethyleneamine derivatives, but no considerable change has been
Copper(I) complexes with various N-donor ligands are of grow-
ing interest as most of these complexes combine remarkable fea-
tures like ease of preparation, reversible electrochemical
behavior, light absorption in the visible spectral region; character-
istic structural flexibility, supramolecular architecture, long-lived
electronically excited states, and intense luminescence [1–6].
Developments in these fields are of great interest under the point
of view of various applications like solar energy conversion or cat-
alytic activity in photo-redox reactions [7–9]. A variety of struc-
tures have been described for the Cu(I) diimine system with
symmetrical and unsymmetrical chelating ligands and most of
the studies have been on four-coordinated tetrahedral CuI com-
plexes of the type [Cu(NN)2]+ or [Cu(NN)(PPh3)2]+ where NN is a
diimine and P is a phosphine. Factors such as the steric, electronic,
and conformational interactions influence the redox potential of
these complexes and modify their spectroscopic properties which
are important in practical applications [10–12].
observed on such properties upon variation of the ligands with dif-
ferent steric and electronic properties [15,16]. However, alkyl sub-
stituents in the iminopyridine ligands may modify the steric and
electronic interactions and tune the physical and chemical proper-
ties of copper complexes [17–21]. To isolate the steric effect more
effectively, there is clearly a need to develop iminopyridine deriv-
atives with systematic variation of sterically active substituents in
the ligands.
The following report deals with a series of diimine ligands with
simple alkyl substituents where the size of an aliphatic ring varies
between three and seven members. We describe the synthesis and
structural characterization of Cu(I) complexes of the type
[Cu(NN)2]+ and [Cu(NN)(PR3)2]+ where NN is one of these unsym-
metrical diimine ligands (A–D) (Fig. 1). We have studied the rela-
tion between the structural variation and spectroscopic changes
of the complexes.
Iminopyridine ligands stabilizing low valent metal redox-states
seem to be good candidates for such studies and were used for the
synthesis of Cu(I), Re(I) and Ru(II) complexes in this field [13,14]. In
a previous work we have studied the spectral, structural and elec-
2. Experimental
2.1. General
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive and should be handled with
care.
* Corresponding author. Tel./fax: +98 021 88041344.
E-mail address: Dehganpour_farasha@yahoo.com (S. Dehghanpour).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.01.008

W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
2873
H3
H1
H4
H1), 8.17(s, 1H, H5). Anal. Calc. for C11H14N2: C, 75.82; H, 8.10; N,
16.08. Found: C, 75.81; H, 8.12; N, 16.9%.
H5
N
H
H22
N
R
2.2.3. Cyclohexylpyridin-2-ylmethyleneamine (C)
This ligand was prepared by a procedure similar to A using
99 mg (1 mmol) of cyclohexylamine. Yield: 88%. IR, (KBr):
1626 cmꢀ1 (C@N). 1H NMR (CDCl3; d): 0.98 (m, 1H, amine ring),
m
,
,
,
R:
1.41 (m, 4H, amine ring), 1.81 (m, 5H, amine ring), 3.38 (m, 1H,
amine ring), 7.28 (d, 1H, JH3,4 = 12.8 Hz, H4), 7.36 (dd, 1H,
JH3,4 = 10.85, JH3,2 = 8.4 Hz, H3), 7.77 (t, 1H, JH2,1 = 12.9,
JH2,3 = 12.8 Hz, H2), 8.12 (d, 1H, JH2,1 = 7.4 Hz, H1), 8.28 (s, 1H, H5).
Anal. Calc. for C12H16N2: C, 75.55; H, 8.57; N, 14.88. Found: C,
75.55; H, 8.59; N, 14.87%.
(A)
(B)
(C)
(D)
[CuL2]+
[CuL(PPh3)2]+
Complex Ligand (L)
Complex Ligand (L)
2.2.4. Cycloheptylpyridin-2-ylmethyleneamine (D)
1a
1b
1c
1d
A
B
C
D
2a
2b
2c
2d
A
B
C
D
This ligand was prepared by a procedure similar to A using
113 mg (1 mmol) of cycloheptylamine. Yield: 95%. IR, (KBr):
1630 cmꢀ1 (C@N). 1H NMR (CDCl3; d): 1.3–1.8 (m, 12H, amine
m
ring), 3.7 (m, 1H, amine ring), 7.25 (d, 1H, JH3,4 = 12.8 Hz, H4), 7.4
(dd, 1H, JH3,4 = 8.81, JH3,2 = 12.1 Hz, H3), 7.74 (t, 1H, JH2,1 = 12.4,
JH2,3 = 12.6 Hz, H2), 7.91 (d, 1H, JH2,1 = 7.35 Hz, H1), 8.09 (S, 1H,
H5). Anal. Calc. for C13H18N2: C, 77.18; H, 8.97; N, 13.83. Found:
C, 77.16; H, 8.98; N, 13.84%.
Fig. 1. Chemical formula of ligands (A–D), and Cu(I) complexes 1a–1d and 2a–2d.
2.2.5. [CuI(A)2]ClO4 (1a)
All chemicals used were reagent grade and used as received.
Solvents used for the reactions were purified by literature methods
[22]. [Cu(CH3CN)4]ClO4 and [Cu(CH3CN)4]BPh4 were freshly pre-
pared according to the literature procedures [23].
Elemental analyses were performed by using a Heraeus CHN-O-
RAPID elemental analyzer. Infrared spectra were recorded on a
Bruker Tensor 27 instrument. Electronic absorption spectra were
To a stirring solution of cyclopropylpyridin-2-ylmethylene-
amine, A (14.6 mg, 0.1 mmol) in 5 ml acetonitrile was added
[Cu(CH3CN)4]ClO4 (16.4 mg, 0.05 mmol) in 5 ml acetonitrile and
stirred for 10 min. The solution turned dark-red rapidly. The volume
of the solvent was reduced under vacuum to about 4 ml. Diffusion of
diethyl ether vapor into the concentrated solution gave dark-red
crystals. The resulting crystals were filtered off and washed with a
mixture of diethylether-acetonitrile (9:1 v/v), and dried under vac-
recorded on a JASCO V-570 spectrophotometer; kmax (loge). NMR
spectra were obtained on a BRUKER AVANCE DRX500 (500 MHz)
spectrometer. Proton chemical shifts are reported in part per mil-
lion (ppm) relative to an internal standard of Me4Si. All voltammo-
grams were recorded with a three electrodes system consisting of
an Ag/AgCl reference electrode, a platinum wire counter electrode,
and Au as working electrode. A Metrohm multipurpose instrument
model 693 VA processor with 694A Va stand was used. In all elec-
trochemical experiments the test solution was purged with argon
gas for at least 5 min.
uum. Yield: 90%. IR, (KBr): 1586 cmꢀ1 (C@N). 1H NMR (CDCl3; d):
m
0.42–0.92 (m, 8H, amine ring), 3.15 (m, 2H, amine ring), 7.21 (d,
2H, JH3,4 = 12.85 Hz, H4), 7.33 (dd, 2H, JH3,4 = 9.05, JH3,2 = 11.5 Hz,
H3), 7.70(t, 2H, JH2,1 = 12.8, JH2,3 = 11.35 Hz, H2), 7.92(s, 2H, H5),
8.16(d, 2H, JH1,2 = 7.7 Hz, H1). Anal. Calc. for C18H20ClCuN4O4: C,
74.48; H, 4.43; N, 12.30. Found: C, 74.45; H, 4.45; N, 12.31%.
2.2.6. [CuI(B)2] BPh4 (1b)
This complex was prepared by a procedure similar to 1a using
17.4 mg (0.1 mmol) of cyclopentylpyridin-2-ylmethyleneamine,
B. Dark-red crystals were collected by filtration and dried in vacuo.
2.2. Syntheses
Yield: 93%. IR, (KBr): 1586 cmꢀ1 (C@N). 1H NMR (CDCl3; d): 1.45
m
2.2.1. Cyclopropylpyridin-2-ylmethyleneamine (A)
Although these types of ligands were synthesized before
[24,14], here, we report a simpler method for their synthesis. To
a solution of pyridine-2-carbaldehyde (107 mg, 1 mmol) in 10 ml
diethylether was added a solution of cyclopropylamine (57 mg,
1 mmol) in 10 ml diethylether and stirred for 2 h. The ligand, cyclo-
propylpyridin-2-ylmethyleneamine was obtained as a pale yellow
(m, 8H, amine ring), 1.95 (m, 8H, amine ring), 3.95 (m, 2H, amine
ring), 6.86(t, 4H, para H of BPh4), 7.03 (m, 8H, meta H of BPh4),
7.16 (d, 2H, JH3,4 = 12.8 Hz, H4), 7.43 (dd, 2H, JH3,4 = 10.45,
JH3,2 = 8.8 Hz, H3), 7.53(b, 8H, ortho
H of BPh4), 7.69(t, 2H,
JH2,1 = 12.8, JH2,3 = 12.58 Hz, H2), 7.80(s, 2H, H5), 8.20(d, 2H,
JH2,1 = 8 Hz, H1). Anal. Calc. for C46H48BCuN4: C, 75.55; H, 6.62; N,
7.66. Found: C, 75.56; H, 6.61; N, 7.65%. A crystal taken for X-ray
investigations before drying proved to contain one molecule of
acetonitrile per formula unit.
oil. Yield: 90%. IR, (KBr): 1628 cmꢀ1 (C@N). 1H NMR (CDCl3, ppm):
m
d 0.51–0.99 (m, 4H, amine ring), 3.12 (m, 1H, amine ring), 7.18 (d,
1H, JH3,4 = 12.84 Hz, H4), 7.31 (dd, 1H, JH3,4 = 9.15, JH3,2 = 11.5 Hz,
H3), 7.76(t, 1H, JH2,1 = 12.7, JH2,3 = 11.35 Hz, H2), 8.01(d, 1H,
JH1,2 = 7.7 Hz, H1), 8.21(s, 1H, H5). Anal. Calc. for C9H10N2: C,
73.94; H, 6.89; N, 19.16. Found: C, 73.96; H, 6.89; N, 19.17%.
2.2.7. [CuI(C)2] BPh4 (1c)
This complex was prepared by a procedure similar to 1a using
18.8 mg (0.1 mmol) of cyclohexylpyridin-2-ylmethyleneamine, C.
Dark-red crystals were collected by filtration and dried in vacuo.
2.2.2. Cyclopentylpyridin-2-ylmethyleneamine (B)
Yield: 86%. IR, (KBr): 1585 cmꢀ1 (C@N). 1H NMR (CDCl3; d): 0.98
m
This ligand was prepared by a procedure similar to A using
85 mg (1 mmol) of cyclopentylamine. Yield: 90%. IR (KBr):
(m, 2H, amine ring), 1.30 (m, 8H, amine ring), 1.63 (m, 10H, amine
ring), 3.43 (m, 2H, amine ring), 6.58 (t, 4H, Para H of BPh4), 7.02 (m,
8H, Meta H of BPh4), 7.27 (d, 2H, JH3,4 = 12.75 Hz, H4), 7.37 (dd, 2H,
JH3,4 = 10.85, JH3,2 = 12.5 Hz, H3), 7.52 (b, 8H, Ortho H of BPh4), 7.74
(t, 2H, JH2,1 = 12.85, JH2,3 = 12.8 Hz, H2), 7.89 (s, 2H, H5), 8.19 (d, 2H,
1621 cmꢀ1 (C@N). 1H NMR (CDCl3; d): 1.40 (m, 3H, amine ring),
m
2.01 (m, 2H, amine ring), 3.89 (m, 1H, amine ring), 7.17 (d, 1H,
JH3,4 = 12.9 Hz, H4), 7.39 (dd, 1H, JH3,4 = 10.48, JH3,2 = 8.8 Hz, H3),
7.56(t, 1H, JH2,1 = 12.6, JH2,3 = 12.58 Hz, H2), 7.91(d, 1H, JH2,1 = 8 Hz,

2874
W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
JH2,1 = 7.65 Hz, H1). Anal. Calc. for C48H52BCuN4: C, 75.93; H, 6.90;
2.3. X-ray analyses
N, 7.38. Found: C, 75.95; H, 6.91; N, 7.39%.
Crystals of 1a, 1b, 1c and 2a suitable for X-ray diffraction were
obtained as described above. Single crystals were mounted on Stoe
2.2.8. [CuI(D)2]BPh4 (1d)
This complex was prepared by a procedure similar to 1a using
20.2 mg (0.1 mmol) of cycloheptylpyridin-2-ylmethyleneamine, D.
Dark-red crystals were collected by filtration and dried in vacuo.
IPDS area detector systems (Mo Ka k = 0.71073 Å). The crystal data
and refinement details are summarized in Table 1. The structures
were solved by direct methods [25] and refined against all F2 data
using full-matrix least-squares techniques [26]. For all heavier
atoms anisotropic displacement parameters were used. Though
the H atoms could be located in difference Fourier maps, most of
them were kept riding on idealized positions with isotropic dis-
placement parameters taken as 1.2Ueq of their bonding partners.
Only were significant deviations from the observed positions were
found (H6, H7 in 1a, H1a, H2a, H2b, H3a, H3b in 2a) the positions
were refined.
Yield: 89%. IR, (KBr): 1590 cmꢀ1
m
(C@N). 1H NMR (CDCl3; d):
1.28–1.75 (m, 24H, amine ring), 3.61 (m, 2H, amine ring), 6.86
(t, 4H, Para of BPh4), 7.02 (t, 8H, Meta of Bph4), 7.20 (d, 2H,
JH3,4 = 12.8 Hz, H4), 7.35 (dd, 2H, JH3,4 = 8.75, JH3,2 = 12.1 Hz, H3),
7.52 (b, 8H, Ortho H of BPh4), 7.70 (t, 2H, JH2,1 = 12, JH2,3 = 12.65 Hz,
H2), 7.79 (S, 2H, H5), 8.19 (d, 2H, JH2,1 = 7.35 Hz, H1). Anal. Calc. for
C50H56BCuN4: C, 76.27; H, 7.17; N, 7.12. Found: C, 76.29; H, 7.18;
N, 7.11%.
2.2.9. [CuI(A)(PPh3)2]BPh4 (2a)
3. Results and discussion
To a 3 ml acetonitrile solution of [Cu(CH3CN)4]BPh4 (54.8 mg,
0.1 mmol), 2 equiv. of Ph3P (52.2 mg, 0.2 mmol) were added,
and the solution was stirred for 15 min. The solvent was evapo-
rated under vacuum at room temperature. The dry product
[Cu(CH3CN)2(PPh3)2]BPh4, was added to a stirring solution of
14.6 mg (0.1 mmol) cyclopropylpyridin-2-ylmethyleneamine, A,
in 3 ml acetonitrile. The solution rapidly turned yellow, and it
was stirred for 20 min at room temperature. The reaction med-
ium was concentrated under vacuum, until the first crystals ap-
peared in the liquid phase. Bright-yellow crystals were
obtained by diffusion of diethylether vapor into the concentrated
3.1. Crystal structures
The crystallographic data of compounds 1a, 1b, 1c and 2a are
summarized in Table 1 and selected bond distances and angles
are given in Table 2.
3.1.1. [Cu(A)2]ClO4 (1a)
Complex 1a crystallizes in space group Fddd with Z = 16. Two
independent perchlorate anions are disordered on two positions
with site symmetry 222 (D2). One of them showed orientational
and positional disorder. As refinements of split atom models were
not very satisfying, its contribution to the diffraction data was sub-
tracted by the back-Fourier-transform method [27]. The cation that
shows C2 symmetry is shown in Fig. 2 along with the atom-num-
bering scheme. While a tetrahedral geometry might be expected
for a four-coordinated copper(I) center, the coordination sphere
around the metal ion in this complex is distorted by the restricted
bite angles of the chelating ligand. According to a bite size N1ꢁ ꢁ ꢁN2
of 2.664(4) Å, the angle N2–Cu1–N1 (81.36(9)°) is much less than
that in a regular tetrahedron. In contrast, the intraligand angle
N2–Cu1–N20 is much larger than 109.5°, being 134.43(15)°. The
two chelate ligands that are equivalent by a 2-fold axis are almost
planar (Table 3). The dihedral angle between them is with 86.1(4)°
close to 90°, indicating weak sterical influence of the cyclopropyl
ring.
solution. Yield: 91%. IR, (KBr): 1581 cmꢀ1
m
(C@N), 1H NMR
(CDCl3; d): 0.36 (m, 2H, amine ring), 0.78 (m, 2H, amine ring),
3.02 (m, 1H, amine ring), 6.82–7.52 (m, 53H, pyridine, Bph4,
PPh3), 7.60 (s, 1H, H5), 7.65 (d, 1H, JH2,1 = 7.1 Hz, H1), Anal. Calc.
for C69H60BCuN2P2: C, 78.66; H, 5.74; N, 2.66. Found: C, 78.68;
H, 5.75; N, 2.65%.
2.2.10. [CuI(B)(PPh3)2]BPh4 (2b)
This complex was prepared by a procedure similar to 2a using
17.4 mg (1 mmol) of cyclopentylpyridin-2-ylmethyleneamine, B.
Bright-yellow crystals were collected by filtration and dried in va-
cou. Yield: 79%. IR, (KBr): 1583 cmꢀ1 (C@N). 1H NMR (CDCl3; d):
m
1.43 (m, 8H, amine ring), 3.76 (s, 1H, amine ring), 6.88–7.55 (m,
53H, pyridine, Bph4, PPh3), 7.75 (s, 1H, H5), 7.83 (d, 1H, JH2,1 = 7.2,
H1). Anal. Calc. for C69H60BCuN2P2: C, 78.66; H, 5.74; N, 2.66.
Found: C, 78.68; H, 5.75; N, 2.65%.
3.1.2. [Cu(B)2]BPh4 ꢁ CH3CN (1b ꢁ CH3CN)
2.2.11. [CuI(C)(PPh3)2]BPh4 (2c)
Compound 1b crystallizes together with one molecule of aceto-
nitrile in the monoclinic space group P21/n. A view of the symme-
tryless cation including the atom-numbering scheme is illustrated
in Fig. 3.
This complex was prepared by a procedure similar to 2a using
18.8 mg (0.1 mmol) of cyclohexylpyridin-2-ylmethyleneamine, C.
Bright-yellow crystals were collected by filtration and dried in va-
cou. Yield: 90%. IR, (KBr): 1582 cmꢀ1 (C@N).1H NMR (CDCl3; d):
m
As in complex 1a, the coordination environment around Cu(I) of
these complex is approximately tetrahedral, since the average of
six angles involving Cu(I) is 110.2°. However, the coordination is
clearly distorted, arising from the restricting bite angles of the che-
lating ligand. For 1b, the N2–Cu1–N1 and N3–Cu1–N4 angles
(81.18(5)°, 81.50(6)°) are narrower than the ideal tetrahedral angle
of 109.5, whereas the opposite N2–Cu1–N4 angle (132.32(5)°) is
wider (Table 2).
0.72 (m, 1H, amine ring), 1.09 (m, 4H, amine ring), 1.42 (m, 5H,
amine ring), 3.25 (m, 1H, amine ring), 6.87–7.54 (m, 53H, pyridine,
Bph4, PPh3), 7.66 (s, 1H, H5), 7.91 (d, 1H, JH2,1 = 6.95, H1). Anal. Calc.
for C72H66BCuN2P2: C, 78.93; H, 6.07; N, 2.56. Found: C, 78.95; H,
6.05; N, 2.57%.
2.2.12. [CuI(D)(PPh3)2]BPh4 (2d)
This complex was prepared by a procedure similar to 2a using
20.2 mg (0.1 mmol) of cycloheptylpyridin-2-ylmethyleneamine,
D. Bright-yellow crystals were collected by filtration and dried in
The five-membered rings in 1b both show envelope conforma-
tion in different orientations. In the first ring the four atoms
C10, C11, C12 and C13 are nearly planar and the atom C19
(bound to N) is at the flap by 0.615(2) Å, the folding angle at the
C10ꢁ ꢁ ꢁC13 axis is 143.6(1)°. In the second one, C18, C20, C21, and
C22 are coplanar and C19 (not bound to N) is 0.555(2) Å above this
plane, resulting in a folding angle of 140.4(2)° at the C18ꢁ ꢁ ꢁC20
axis.
vacuo. Yield: 84%. IR, (KBr): 1582 cmꢀ1 (C@N). 1H NMR (CDCl3;
m
d): 1.23–1.50 (m, 12H, amine ring), 3.42 (m, 1H, amine ring),
6.88–7.56 (m, 53H, pyridine, Bph4, PPh3), 7.67 (s, 1H, H5), 7.92 (d,
1H, JH2,1 = 7.45, H1). Anal. Calc. for C73H68BCuN2P2: C, 79.01; H,
6.18; N, 2.52. Found: C, 79.02; H, 6.17; N, 2.50%.

W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
2875
Table 1
Crystal data and single crystal X-ray diffraction refinement details for compounds 1a, 1b ꢁ CH3CN, 1c and 2a.
1a
1b ꢁ CH3CN
1c
2a
Formula
C18H20ClCuN4O4
455.37
193(2)
0.71073
orthorhombic
Fddd
13.5839(15)
24.114(2)
24.7107(19)
90
C48H51BCuN5
772.29
123(2)
0.71073
monoclinic
P21/n
10.6308(3)
27.3111(8)
14.6340(4)
90
C48 H52 B Cu N4
759.29
150(2)
0.71073
monoclinic
P21/n
27.737(3)
11.0628(6)
C69H60BCuN2P2
1053.48
193(2)
0.71073
monoclinic
C2/c
35.306(3)
17.2777(15)
18.2337(14)
90
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
27.711(2)
90
a
(°)
b (°)
90
90
109.596(2)
9
4002.7(2)
4
1.282
0.586
110.153(11)
90
7982.4(12)
8
1.264
0.586
92.885(9)
90
11108.6(16)
8
1.260
0.495
c
(°)
V (A3)
Z
8094.3(13)
16
1.495
D (g cmꢀ3
)
l
(mmꢀ1
)
1.243
F(000)
3744
1632
3216
4416
Crystal size
h Range for data collection (°)
Index ranges
0.50 ꢂ 0.14 ꢂ 0.03
2.36–29.26
ꢀ18 6 h 6 18, ꢀ32 6 k 6 33,
ꢀ32 6 l 6 32
15677
0.46 ꢂ 0.16 ꢂ 0.13
1.65–29.18
ꢀ14 6 h 6 14, ꢀ37 6 k 6 36,
ꢀ20 6 l 6 20
37779
0.55 ꢂ 0.30 ꢂ 0.30
0.50 ꢂ 0.26 ꢂ 0.20
2.37–26.80
2.24–25.88
ꢀ35 6 h 6 35, ꢀ11 6 k 6 14,
ꢀ35 6 l 6 35
ꢀ43 6 h 6 43, ꢀ21 6 k 6 21,
ꢀ22 6 l 6 22
Reflections collected
43560
47649
Independent reflections [Rint
Absorption correction
]
2718 [0.0763]
numerical
10785[0.0456]
numerical
16243 [0.0614]
semi-empirical from equivalents
10629 [0.0957]
semi-empirical from
equivalents
Maximum and minimum
transmission
Refinement method
0.959 and 0.709
0.929 and 0.824
0.714 and 0.699
0.955 and 0.703
full-matrix least-squares on F2
full-matrix least-squares on F2
full-matrix least-squares on F2 as
full-matrix least-squares on F2
ꢀ
(101) twin
Data/restraints/parameters
2718/0/134
0.819
10785/0/497
1.033
16243/0/968
0.623
10629/6/694
0.897
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0430, wR2 = 0.0906
R1 = 0.1076, wR2 = 0.1018
0.334 and ꢀ0.243
R1 = 0.0423, wR2 = 0.0905
R1 = 0.0606, wR2 = 0.0951
0.630 and ꢀ0.305
R1 = 0.0393, wR2 = 0.0518
R1 = 0.0968, wR2 = 0.0572
1.072 and ꢀ0.630
R1 = 0.0498, wR2 = 0.0908
R1 = 0.1050, wR2 = 0.0978
0.567 and ꢀ0.417
Largest difference in peak and
hole (e Åꢀ3)
Table 2
Selected bond lengths (Å) and bond angles (°) for 1b ꢁ CH3CN, 1c and 2a.
1a
1b ꢁ CH3CN
1c
2a
I
II
Cu1–N1
Cu1–N1#1
Cu1–N2
Cu1–N2#1
2.072(2)
2.072(2)
2.015(2)
2.015(2)
134.43(15)
122.33(9)
81.36(9)
81.36(9)
122.33(9)
120.46(13)
117.2(3)
131.90(19)
110.83(19)
120.1(3)
112.1(2)
127.6(2)
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
2.0078(13)
2.0212(14)
2.0761(13)
2.0831(14)
129.61(6)
81.18(5)
127.93(5)
132.32(5)
81.50(6)
108.92(5)
117.68(14)
130.36(11)
110.35(10)
118.27(13)
113.68(10)
128.02(11)
Cu1–N1
Cu1–N3
Cu1–N4
Cu1–N2
1.943(4)
2.016(3)
2.054(4)
2.078(3)
137.34(13)
123.86(14)
80.81(15)
82.60(15)
117.27(15)
119.63(14)
116.9(4)
111.8(3)
131.1(3)
117.8(4)
132.1(3)
109.8(3)
Cu2–N8
Cu2–N6
Cu2–N5
Cu2–N7
1.954(3)
2.021(3)
2.046(4)
2.079(3)
137.86(13)
124.68(15)
82.40(15)
82.17(14)
113.98(14)
119.77(14)
116.3(4)
133.3(3)
110.4(3)
119.4(4)
111.3(3)
128.7(3)
Cu1–N1
Cu1–N2
Cu1–P1
Cu1–P2
2.092(3)
2.104(3)
2.2500(10)
2.2521(10)
79.12(12)
114.21(8)
109.26(8)
118.78(8)
106.35(8)
120.04(3)
117.8(3)
112.0(2)
130.2(3)
117.4(3)
112.8(3)
129.6(3)
N2–Cu1–N2#1
N2–Cu1–N1#1
N2#1–Cu1–N1#1
N2–Cu1–N1
N2#1–Cu1–N1
N1#1–Cu1–N1
C1–N1–C5
C1–N1–Cu1
C5–N1–Cu1
C6–N2–C7
N2–Cu1–N3
N2–Cu1–N1
N3–Cu1–N1
N2–Cu1–N4
N3–Cu1–N4
N1–Cu1–N4
C8–N1–C1
C8–N1–Cu1
C1–N1–Cu1
C2–N2–C9
C2–N2–Cu1
C9–N2–Cu1
N1–Cu1–N3
N1–Cu1–N4
N3–Cu1–N4
N1–Cu1–N2
N3–Cu1–N2
N4–Cu1–N2
C18–N1–C19
C18–N1–Cu1
C19–N1–Cu1
C13–N2–C17
C13–N2–Cu1
C17–N2–Cu1
N8–Cu2–N6
N8–Cu2–N5
N6–Cu2–N5
N8–Cu2–N7
N6–Cu2–N7
N5–Cu2–N7
C49–N5–C53
C49–N5–Cu2
C53–N5–Cu2
C54–N6–C55
C54–N6–Cu2
C55–N6–Cu2
N1–Cu1–N2
N1–Cu1–P2
N2–Cu1–P2
N1–Cu1–P1
N2–Cu1–P1
P2–Cu1–P1
C4–N1–C1
C4–N1–Cu1
C1–N1–Cu1
C5–N2–C9
C5–N2–Cu1
C9–N2–Cu1
C6–N2–Cu1
C7–N2–Cu1
3.1.3. [Cu(C)2]BPh4 (1c)
3.1.4. Influence of ring size on the [CuL2] chelate complex geometry
The Cu–N bond distances (1a 2.044, 1b 2.047, 1c1 2.028, 1c2
2.022 Å) are similar to that found in other pseudotetrahedral
Cu(I) diimine complexes [13,28–30] (typical Cu–Nav = 2.055 Å
[30]) like the [Cu(dpdmp)2]+ cation (2.047 Å [29]). In all three com-
pounds, the iminopyridine units are almost planar (maximum
deviation from best planes 0.088 Å).
When comparing the chelate bond geometry depending on the
ring substituents (Table 3), it becomes clear that the cyclopentane
derivative shows the strongest distortion. In contrast to the least
Compound 1c crystallizes in the monoclinic space group P21/n,
too, with two molecules per asymmetric unit probably due to
packing reasons, resulting in some small conformational differ-
ꢀ
ences. The crystal proved to be a pseudomerohedral 101 reflection
twin with a twin ratio of 60.49(5):39.51(5). Fig. 4 shows one of the
cations along with the atom-numbering scheme. The spread of
‘‘tetrahedral angles” is similar to that in 1a and 1b (range from
80.8(2)° to 137.9(1)°, see Table 2). All four independent cyclohexyl
rings in 1c adopt an almost regular chair conformation.

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W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
C2
C5
C3
C6
C7
C1
C1
C2
N2
C4
N1'
N2'
C9
C13
N1
Cu1
C8
C5
C6
N1
C8
C21
C12
N2
C10
C11
C22
Cu1
C20
C19
N4
N3
C7
C14
C15
C18
C9
C4
C3
Fig. 2. Structure of the [Cu(A)2]+ cation of 1a in the crystal, showing the atom
labeling scheme. Thermal ellipsoids with 50% probability. Hydrogen atoms are
omitted for clarity.
C16
C17
Fig. 3. Structure of the [Cu(B)2]+ cation of 1b ꢁ CH3CN in the crystal, showing the
atom labeling scheme. Thermal ellipsoids with 50% probability. Hydrogen atoms are
omitted for clarity.
distorted cyclopropane and the cyclohexane compound, the Cu
atom is significantly (up to 0.32 Å) displaced out of the planes of
the ligands. As well, the dihedral angle between the planes of the
chelate ligands (90° in an undistorted tetrahedral complex) has a
clear minimum for the cyclopentane compound 1b (68.3°) pointing
to remarkable steric interference between the bulky chelate li-
gands. Obviously, this effect is less for the cyclohexyl compound
1c (dihedral angle 79.0° in average), and smallest with the cyclo-
propyl rest (86.1°, small distortion is also observed in 2a, Section
3.1.5). Accordingly, the shortest contact distances between C-
atoms of the ring substituents at both chelate ligands are smallest
for 1b (3.68 Å), 4.09 Å for 1a, and 4.31/4.29 Å for 1c. This sequence
suggests that the influence of packing is crucial. The exceptional
structural behavior of the cyclopropyl compound correlates with
a red-shift of the first band in the optical absorption spectra as
compared with the other [CuL2] complexes (vide infra).
C62
C63
C61
C50
C51
C49
C64
N7
C52
C65
Cu2
C66
N5
C53
N6
C54
N8
C72
C56
C55
C67
C71
C57
C68
C70
C60
3.1.5. [Cu(A)(PPh3)2] BPh4
The cation of complex 2a, along with the atom-numbering
scheme, is shown in Fig. 5. The coordination environment around
the metal ion in this complex is pseudotetrahedral with large
angular distortion arising from the low intraligand N1–Cu1–N2
chelate angle, 79.12(12)°. However, the P2–Cu1–P1, 120.04(3)° an-
gle has opened up due to the steric effects from the bulky Ph3P li-
gands. The average Cu–N and Cu–P bond distances are 2.098 and
2.251 Å, respectively, and are comparable to those reported for
[Cu(dmp)(PPh3)2]NO3 (2.117(6) and 2.294(2) Å) and other copper
complexes [31,32]. The dihedral angle between the best plane of
the chelate ligand (N1, C4–C9, N2, maximum deviation
0.021(4) Å) and the plane defined by P1ꢀCu1ꢀP2 is 88.4(5)°. How-
ever, this dihedral angle is considerably larger in comparison to the
similar complexes [13,31,32]. Again, the Cu atom is displaced by
0.1374(4) Å out of the plane of the chelate ligand.
C69
C58
C59
Fig. 4. Structure of the [Cu(C)2]+ cation of 1c in the crystal, showing the atom
labeling scheme. Thermal ellipsoids with 50% probability. Hydrogen atoms are
omitted for clarity.
3.2. Spectroscopic characterization
The IR spectra of the free ligands exhibit
1630 cmꢀ1. In complexes, (C@N) appears at 1581–1587 cmꢀ1
and is red-shifted by 40–43 cmꢀ1. This has been attributed to the
presence of d(Cu) ?
*(ligand) back bonding [33,34].
m(C@N) at 1621–
m
p
Table 3
Geometry of the [CuL2] chelate coordination in compounds 1a, 1b ꢁ CH3CN, and 1c (two independent molecules). Best planes calculated for the eight atoms of the py-CH@N-
fragments.
1a
1b ꢁ CH3CN
1c1
0.026(3)
1c2
0.074(5)
0.018(4)
ꢀ0.0346(6)
ꢀ0.1605(6)
8.2(7)
Deviation (Å) from a best plane of ligand 1
Deviation (Å) from a best plane of ligand 2
Distance (Å) of Cu to the plane of ligand 1
Distance (Å) of Cu to the plane of ligand 2
Torsion angle (°) N–C@CꢀN (ligand 1)
Torsion angle (°) N–C@C–N (ligand 2)
Dihedral angle (°) between planes 1 and 2
0.011(4)
same by symmetry
0.0681(4)
ꢀ0.087(1)
ꢀ0.088(2)
ꢀ0.3162(2)
ꢀ0.1981(2)
8.2(2)
0.055(4)
ꢀ0.1082(6)
ꢀ0.0211(6)
ꢀ3.0(6)
1.1(5)
5.5(2)
68.31(4)
ꢀ6.9(7)
2.3(6)
78.1(1)
86.1(4)
79.85(9)

W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
2877
The 1H NMR spectra and peak assignments are presented in the
experimental section. These peaks are assigned based on the split-
ting of the resonance signals, spin coupling constants and the liter-
ature, and are clearly in accordance with the molecular structure
determined by X-ray crystal structure analysis. The spectra of the
ligand are clearly divided into two portions; the down-field part
is due to pyridine and imine protons (H1–H5) and the upfield sig-
nals refer to alkyl protons. Aside from the aromatic H-atoms, which
appear at 7.00–8.15 ppm in the ligands, the imine protons appear
as a singlet at 8.60–9.50 ppm in the ligands. The multiplet peak
at about 3.7 ppm in the ligands is assigned to the proton in vicinity
of imine nitrogen of the alkyl group. The other alkyl protons appear
in 1–4 ppm as multiplet. The 1H resonances of the coordinated li-
gands are commonly observed in complexes 1a–2d. In complexes
2a–2d, however, the aromatic H atoms of the coordinated Ph3P li-
gands and BPh4 anion overlap to some extent with those of the
phenyl H atoms of ligands A–D. The down-field shift of the iminic
protons in complexes relative to the free ligands can be attributed
to the deshielding effect resulting from the coordination of the li-
gands [38,39].
Fig. 5. Structure of the [Cu(A)(PPh3)2]+ cation of 2a in the crystal, showing the atom
labeling scheme. Thermal ellipsoids with 50% probability.
3.3. Electrochemistry
The electronic spectra of the complexes were recorded in chlo-
roform solution in the range 700–200 nm. The spectral data are gi-
ven in Table 4. The visible range of the spectrum is dominated by a
metal-to-ligand charge transfer (MLCT) transition which is a char-
acteristic feature of copper(I) complexes when bonded to a conju-
gated organic chromophore [35]. The shape of the absorption
The redox behavior of the complexes in CH2Cl2 solution was
examined by cyclic voltammetry. The four ligands are electroinac-
tive in the working potential region. The complexes (1a–1d) un-
dergo a quasireversible oxidation–reduction reaction (Table 4)
[40]. The response is attributed to the copper(II)/copper(I) couple
([Cu(L)]2+ + eꢀ [Cu(L)]1+). The complexes show a quasireversible
CuII/I couple (Table 4) and the ratio of the anodic and cathodic peak
currents, (ipa/ipc), approaches 1 as the scan rate increases. The
peak-to-peak separation increases as the scan rate is changed from
50 mV/s to 500 mV/s. The CuII/I potential in a CuIN4 chromophore is
believed to increase with increasing the electron-donating or -
withdrawing properties of the ligands and the resistance to
tetrahedral distortion occurring in the corresponding CuIIN4 chro-
mophore [13,34]. Generally, assuming there are no extreme
changes in the electron-donating or -withdrawing properties of
the ligands, this redox couple can be used to indicate the resistance
to tetrahedral distortion the ligands impart on the complex by
observing the shift in the Cu2+/+ redox potentials. Based on the
electrochemical studies in dichloromethane (Table 4), the com-
plexes are ranked as follows: 1b < 1a < 1c ꢃ 1d. This trend is
approximately similar to the trend obtained from analysis of the
absorption spectra and the dihedral angle between the planes of
the chelate ligands in complexes.
+
spectrum of a Cu(L)2 complex can provide some insight into the
solution structure in that systems exhibiting an intense charge
transfer band in the visible region [36,37]. The absorption spec-
trum of the [Cu(L)2]+ complexes show a band in the visible region
around 470 nm. The band migrates to high wavelengths as the size
of the ring substituents on the ligand increases, except for the
cyclopropyl compound 1a, the band of which appears at 483 nm.
The wavelength of the first MLCT band correlates with the dihedral
angle between the planes of the chelate ligands. With decreasing
dihedral angle u decreases the wavelength, for example for
u = 86.1°, kmax = 483 nm in 1a; for u = 68.31°, kmax = 467 nm in 1b
(vide supra).
[Cu(A)(PPh3)2]BPh4 (2a) shows a band at 373 nm, which is
shifted considerably relative to complex 1a. A similar shift has
been reported in going from [Cu(dmp)2]+ (kMLCT = 454 nm) to
[Cu(dmp)(PPh3)2]+ (kMLCT = 365 nm) [28,31,32] and also similar
shifts were observed in the other [Cu(L)(PPh3)2]+ complexes, 2b–
2d relative to the [Cu(L)2]+ complexes 1a–1d (Table 4). An increase
in the wavelength of this band is also observed with increasing of
the ring size in the 2b–2d complexes. In contrast, the higher energy
band in 2a is red-shifted and appears at 385 nm, similar to the shift
observed for 1a. Additional absorption bands are also observed in
the spectra of 1a–2d in chloroform in the UV region (Table 4).
An observable deviation is found for 2a–2d where the Cu(II)/
Cu(I) couple appears at a higher potential than 1a–1d. Although
a higher degree of conjugation exists in 1a–1d relative to 2a–2d,
the existence of bulkier ligands in 2a–2d which prevent the in-
ner-sphere reorganization to flattened tetrahedral, more appropri-
ate to Cu(II) oxidation state, play a key role in shifting the oxidation
potential to higher values for complexes 2a–2d relative to 1a–1d.
The intensities of these bands are consistent with being assigned
*
as ligand-centered
p ? p and/or charge transfer transitions.
4. Conclusion
Table 4
IR, UV–Vis spectral data and cyclic voltammetric data of ligands and complexes.
A series of [Cu(L)2]+ and [Cu(L)(PPh3)2]+ complexes with sys-
tematic variation of aliphatic rings in iminopyridine ligands L has
been synthesized for examining the influence of structural varia-
tion on the spectroscopic and redox properties of the complexes.
Structural studies show less distorted geometries in the cyclopro-
pyl compounds, most distortion at the cyclopentyl derivative.
Comparison of the series of [Cu(L)2]+ complexes shows a clear cor-
relation between geometric distortion upon variation of the li-
gands and the spectroscopic and redox properties. Red shift of
(C@N) (cmꢀ1
)
kmax (nm) (log /Mꢀ1 cmꢀ1
)
Ep
Ep
a
c
Compound
m
1a
1b
1c
1d
2a
2b
2c
2d
1586
1586
1585
1587
1581
1583
1585
1583
254 (4.45), 279 (4.25), 483 (3.49)
259 (4.43), 285 (4.33), 467 (3.56)
251 (4.31), 278 (4.39), 475 (3.47)
251 (4.44), 279 (4.36), 484 (3.51)
253 (4.19), 275 (4.33), 385 (3.45)
259 (4.25), 270 (4.41), 371 (3.49)
250 (4.30), 279 (4.39), 377 (3.44)
255 (4.29), 277(4.36), 381 (3.42)
0.54
0.50
0.57
0.57
0.88
0.85
0.88
0.89
0.41
0.35
0.45
0.46
0.76
0.72
0.77
0.76

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W. Massa et al. / Inorganica Chimica Acta 362 (2009) 2872–2878
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5. Supplementary material
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