Variations in the coordination environment of Co2+, Cu2+ and Zn2+ complexes prepared from a tridentate (imino)pyridine ligand and their structural comparisons

Article abstract of DOI:10.1016/j.ica.2007.03.002

The variations in the coordination environment of Co(II), Cu(II) and Zn(II) complexes with the neutral, tridentate ligand bis[1-(cyclohexylimino)ethyl]pyridine (BCIP) are reported. Analogous syntheses were carried out utilizing either the M(BF₄)₂ · xH₂O or MCl₂ · xH₂O metal salts (where M = Co(II), Cu(II) or Zn(II)) with one equivalent of BCIP. When the hydrated BF₄^- metal starting material was used, cationic, octahedral complexes of the type [M(BCIP)₂]²⁺ were isolated as the tetrafluoroborate salt (4, 5). Conversely, when the hydrated chloride metal salt was used as the starting material, only neutral, pentacoordinate [M(BCIP)Cl₂] complexes (1-3) formed. All complexes were characterized by X-ray diffraction studies. The three complexes that are five coordinate have distortions due mainly to the pyridine di-imine bite angle. The [Cu(BCIP)Cl₂] (2) also exhibits deviations in the Cu(II)-Cl bond distances with values of 2.4242(9) and 2.2505(9) A?, which are not seen in the analogous Zn(II) and Co(II) structures. Similarly, the two six coordinate complexes (5, 6) are also altered by the ligand frame bite angle giving rise to distorted octahedral geometries in each complex. The [Cu(BCIP)₂](BF₄)₂ (6) also exhibits Cu(II)-N_imine bond lengths that are on average 0.14 A? longer than those found in the analogous 5 coordinate complex, [Cu(BCIP)Cl₂]. In addition to X-ray analysis, all complexes were also characterized by UV/Vis and IR spectroscopy with ¹H NMR spectroscopy being used for the analysis of the Zn(II) analogue (3).

Full text of DOI:10.1016/j.ica.2007.03.002

Inorganica Chimica Acta 360 (2007) 3061–3068
www.elsevier.com/locate/ica
Variations in the coordination environment of Co2+, Cu2+
and Zn2+ complexes prepared from a tridentate (imino)pyridine
ligand and their structural comparisons
a
b
a
a,
*
´
Renee N. Davis , Joseph M. Tanski , James C. Adrian Jr. , Laurie A. Tyler
a
Department of Chemistry and Biochemistry, Union College, Schenectady, NY 12308, USA
b
Department of Chemistry, Vassar College, Poughkeepsie, NY 12604, USA
Received 5 December 2006; received in revised form 28 February 2007; accepted 2 March 2007
Available online 12 March 2007
Abstract
The variations in the coordination environment of Co(II), Cu(II) and Zn(II) complexes with the neutral, tridentate ligand bis[1-
(cyclohexylimino)ethyl]pyridine (BCIP) are reported. Analogous syntheses were carried out utilizing either the M(BF4)2 Æ xH2O or
À
MCl2 Æ xH2O metal salts (where M @ Co(II), Cu(II) or Zn(II)) with one equivalent of BCIP. When the hydrated BF4 metal starting
material was used, cationic, octahedral complexes of the type [M(BCIP)2]2+ were isolated as the tetrafluoroborate salt (4, 5).
Conversely, when the hydrated chloride metal salt was used as the starting material, only neutral, pentacoordinate [M(BCIP)Cl2]
complexes (1–3) formed. All complexes were characterized by X-ray diffraction studies. The three complexes that are five coordinate
have distortions due mainly to the pyridine di-imine bite angle. The [Cu(BCIP)Cl2] (2) also exhibits deviations in the Cu(II)–Cl
˚
bond distances with values of 2.4242(9) and 2.2505(9) A, which are not seen in the analogous Zn(II) and Co(II) structures.
Similarly, the two six coordinate complexes (5, 6) are also altered by the ligand frame bite angle giving rise to distorted octahedral
˚
geometries in each complex. The [Cu(BCIP)2](BF4)2 (6) also exhibits Cu(II)–Nimine bond lengths that are on average 0.14 A
longer than those found in the analogous 5 coordinate complex, [Cu(BCIP)Cl2]. In addition to X-ray analysis, all complexes were
also characterized by UV/Vis and IR spectroscopy with 1H NMR spectroscopy being used for the analysis of the Zn(II) analogue
(3).
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: (Imino)pyridine; Di-imine ligands; Copper (II); Cobalt (II); Zinc (II)
1. Introduction
Metal complexes containing substituted (imino)pyridine
ligands in their coordination sphere have been of interest
for many years [1–3]. This interest has arisen, in part, due
to the fact that complexes formed with this motif in the
ligand frame are stable and also because the substituents
on the ligand periphery can be varied (R1 and R2) provid-
ing a platform for comparison.
R2
R1
R2
R1
N
N
N
Indeed, (imino)pyridine ligands have been used in the
synthesis of some initial model complexes of blue copper
proteins [4] and it has been pointed out that many model
studies are improved by altering the substituents of the
*
Corresponding author.
E-mail address: tylerl@union.edu (L.A. Tyler).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.03.002

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R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
ligand frame in order to better replicate the spectroscopic
properties of the metalloenzyme. These ligand frames
provide an accessible route to structural modifications
[5,6].
2. Experimental
2.1. Materials
(Imino)pyridine ligands have also been employed in
the synthesis of metal complexes capable of catalyzing
ethylene polymerization. Initial studies performed by
Brookhart and Gibson revealed that Co(II) and Fe(II)
complexes containing (imino)pyridine ligands with
bulky aryl substituents at the imine position (R1) are
highly active and long lived ethylene polymerization
catalysts [7,8]. Subsequent studies have shown that
Fe(III) [9] and Cr(III) [10,11] complexes with similar
coordination environments also exhibit catalytic behavior
and that the catalytic activity is directly related to the
identity and steric bulk of the substituents on the
periphery of the (imino)pyridine ligand frame [12–15].
These catalysts are all generated from the corresponding
Cyclohexylamine, 2,6-diacetylpyridine, Co(BF4)2 Æ 6H2O,
CoCl2 Æ 6H2O, ZnCl2, Zn(BF4)2 Æ xH2O, CuCl2 Æ 2H2O and
Cu(BF4)2 Æ xH2O were purchased from Aldrich Chemical
Co. and used without further purification. All solvents
were dried and distilled before use.
2.2. Physical measurements
Infrared spectra were obtained with a Thermoelectron,
Avatar 330 FT-IR spectrophotometer equipped with a
Smart Orbit reflectance insert, diamond window. Absorp-
tion spectra were measured on a Hewlett-Packard 8453
1
diode array spectrophotometer. H and 13C NMR spectra
were recorded on a Varian 200 MHz spectrometer.
[Mn+LXn] precursor, where
L
is the substituted
2.3. Preparation of compounds
(imino)pyridine ligand and X is either chloride or
bromide.
The ligand Bis[1-(cyclohexylimino)ethyl]pyridine (BCIP)
was prepared by following the published procedure [17].
More recently ruthenium complexes of an (imino)pyri-
dine ligand with mesityl groups at the R1 position and
methyl groups at the R2 position were utilized to activate
silicon hydride and chloride bonds of tetravalent silanes.
The product of activation was then trapped forming a
Ru(0) complex which contained the ligated silylene group
in a pocket surrounded by the pendant mesityl groups from
the (imino)pyridine ligand backbone. This report was the
first example of formation of a metal silylene complex via
a-chloride abstraction [16].
The continued interest in (imino)pyridine ligands and
their utility in the synthesis and study of metal complexes
with varied applications has prompted us to study coordi-
nation reactions with bis[1-(cyclohexylimino)ethyl]pyridine
(BCIP).
2.3.1. Synthesis of [Co(BCIP)Cl2] (1)
A batch of 304 mg (0.934 mmol) of BCIP was suspended
in 30 mL of CH3OH and 222 mg (0.934 mmol) of
CoCl2 Æ 6H2O was added to it. After 10 min, a blue-brown
color developed. The reaction was stirred for 8 h and then
filtered. Green microcrystalline 1 was obtained by slow
evaporation of the mother liquor. The solid was collected
via gravity filtration and dried under high vacuum. Yield:
0.20 g (47%). Selected IR bands: (cmÀ1) 1629 (m, tNC),
1590 (m), 808 (s). Electronic absorption spectrum in
CH3CN: kmax (nm) (e, MÀ1 cmÀ1) 305 (403).
2.3.2. Synthesis of [Cu(BCIP)Cl2] (2)
A batch of 270 mg (0.829 mmol) of BCIP was dissolved
in 30 mL of CH3CN. In a separate flask, 140 mg
(0.829 mmol) of CuCl2 Æ 2H2O was dissolved in 20 mL of
CH3CN. The copper solution was then added drop wise
to the ligand which resulted in immediate precipitate for-
mation. The mixture was stirred for 3 h after which the
green precipitate was collected via vacuum filtration and
washed with 10 mL of CH3CN. The product was further
purified by dissolving crude 2 in 20 mL of chloroform
and allowing the solution to slowly evaporate in the hood.
The green crystals were collected via gravity filtration and
dried under high vacuum for 5 h. Yield: 0.38 g (74%).
Selected IR bands: (cmÀ1) 1582 (m, tNC), 1450 (m), 740
(s). Electronic absorption spectrum in DMF: kmax (nm)
(e, MÀ1 cmÀ1) 800 (129).
N
N
N
BCIP
These studies are aimed to (i) provide additional under-
standing of the coordination behavior of ligands with an
(imino)pyridine backbone and (ii) provide a structural
comparison of the complexes synthesized from them. In
this report we describe the preparation of the Co(II), Cu(II)
and Zn(II) complexes of BCIP, their spectroscopic charac-
terization and crystal structures.
2.3.3. Synthesis of [Zn(BCIP)Cl2] (3)
A batch of 308 mg (0.945 mmol) of BCIP was dissolved
in 30 mL of CH3CN and 129 mg (0.945 mmol) of ZnCl2,
dissolved in 20 mL of CH3CN was added to it. Upon addi-

R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
3063
tion of the metal salt to the ligand, an off-white precipitate
formed. The mixture was stirred for 3 h and the solid col-
lected via gravity filtration. The precipitate was then re-dis-
solved in CHCl3 and slow evaporation of this solution
yielded 3 as a microcrystalline solid. Yield: 0.30 g (76%).
1H NMR (CDCl3, 200 MHz, 25 ꢁC, d from TMS): 1.33
(m, 8H), 1.94 (m, 8H), 2.39 (m, 4H), 2.48 (s, 6H, CH3),
8.00 (d, 2H, py), 8.23 (d, 2H, py), 8.27 (d, 2H, py). Selected
IR bands: (cmÀ1) 1633 (m, tNC), 1593 (m), 1466 (m), 809
(s). Electronic absorption spectrum in DMF: no absorp-
tion between 300 and 900 nm.
resulted in an immediate color change to dark green-blue
and the solution became homogenous within 5 min. The
solution was stirred for 3 h and then concentrated to
7 mL and cooled to 0 ꢁC. Microcrystalline 5 was collected
after 2 h via gravity filtration and dried under high vac-
uum. Yield: 0.15 g (66%). Selected IR bands: (cmÀ1) 1588
(m, tNC), 1051 (s, tBF). Electronic absorption spectrum in
CH3CN: kmax (nm) (e, MÀ1 cmÀ1): 756 (169).
2.4. X-ray data collection and structure solution and
refinement
2.3.4. Synthesis of [Co(BCIP)2](BF4)2 (4)
Crystals suitable for X-ray analysis were obtained using
the following procedures: Purple plates of [Co(BCIP)Cl2]
(1) were obtained by slow diffusion of Et2O into a methan-
olic solution of 1. Slow evaporation of a separate solution
of [Cu(BCIP)Cl2] (2) in CH3OH and [Zn(BCIP)Cl2] (3) in
CHCl3 yielded yellow-green and colorless blocks of 2 and
3, respectively. Red blocks of [Co(BCIP)2](BF4)2 Æ CH3OH
(4 Æ CH3OH) and blue-green blocks of [Cu(BCIP)2]
(BF4)2 Æ CH3OH (5 Æ CH3OH) were obtained by slow evap-
oration of separate solutions of 4 and 5 in methanol. X-ray
diffraction data were collected on a Bruker APEX 2 CCD
A batch of 290 mg (0.895 mmol) of BCIP was suspended
in 30 mL of methanol and 305 mg (0.894 mmol) of
Co(BF4)2 Æ 6H2O was added to it. Upon addition of the
metal salt, the solution immediately turned a dark red-
orange color and became homogeneous. The solution was
stirred for 20 min and then gravity filtered. Slow evapora-
tion of the filtrate over a period of 24 h afforded pure 4
as red-brown crystals. Yield: 0.30 g (68%). Selected IR
bands: (cmÀ1) 1583 (m, tNC), 1451, 1046 (s, tBF). Electronic
absorption spectrum in CH3CN: kmax (nm) (e, MÀ1 cmÀ1):
478 (303), 558 (192).
˚
platform diffractometer (Mo Ka (k = 0.71073 A)) equipped
with an Oxford liquid nitrogen cryostream. Crystals were
mounted in a nylon loop with Paratone-N cryoprotectant
oil. The structures were solved using direct methods and
standard difference map techniques, and were refined by
full-matrix least-squares procedures on F2 with SHELXTL
(Version 6.14) [18]. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms on carbon were included
2.3.5. Synthesis of [Cu(BCIP)2](BF4)2(5)
The synthesis of 5 was carried out in a similar procedure
used to obtain complex 4. BCIP (166 mg, 0.51 mmol) was
dissolved in 20 mL of CH3OH. In a separate vial, 163 mg
of Cu(BF4)2 Æ xH2O (19–22% Cu) was dissolved in 10 mL
of CH3OH and added to the ligand solution. The addition
Table 1
Summary of crystal data and intensity collection and structure refinement parameters for complexes 1–3
[Co(BCIP)Cl2] (1) [Cu(BCIP)Cl2] (2)
21H31Cl2CoN3
[Zn(BCIP)Cl2] (3)
Empirical formula
Molecular weight
Crystal color, habit
Crystal size (mm)
Temperature (K)
Crystal system
C
C21H31Cl2CuN3
459.93
C21H31Cl2N3Zn
461.76
colorless, block
0.35 · 0.18 · 0.18
115(2)
455.32
purple, plate
0.32 · 0.26 · 0.10
125(2)
yellow-green, block
0.13 · 0.10 · 0.06
115(2)
orthorhombic
Pbca
monoclinic
P21/c
orthorhombic
Pbca
Space group
Unit cell dimensions
˚
a (A)
10.2293(4)
16.8746(7)
24.539(1)
90
90
90
4235.9(3), 8
1.428
1.074
1.66–28.12
97.7
12.9820(8)
9.9698(6)
17.046(1)
90
98.485(1)
90
2182.1(2), 4
1.400
1.257
10.2180(5)
16.8895(8)
24.674(1)
90
90
90
4258.2(3), 8
1.441
1.416
1.65–28.29
96.4
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A ), Z
Dcalc (mg mÀ3
)
Absorption coefficient (l, mmÀ1
U Range collected (ꢁ)
)
1.59–28.25
93.8
Completeness to Umax (%)
Reflections collected/unique (Rint
Data/restraints/parameters
R1, wR2 (I > 2rI)
R1, wR2 (all data)
Goodness-of-fit on F2
)
46871/5063 (0.042)
5063/0/244
0.0273, 0.0663
0.0355, 0.0709
1.021
23870/5075 (0.0578)
5075/0/246
0.0480, 0.1237
0.0734, 0.1364
1.050
45518/5102 (0.0259)
5102/0/246
0.0218, 0.0565
0.0257, 0.0584
1.044
3
˚
Largest difference in peak/hole (e/A )
0.114, À0.217
1.835, À0.536
0.461, À0.222

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R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
Table 2
Table 3
˚
Summary of crystal data and intensity collection and structure refinement
Selected bond distances (A) of complexes 1–5
parameters for complexes 4 and 5
Bond distances
[Co(BCIP)Cl2] (1)
[Co(BCIP)2](BF4)2 Æ CH3OH [Cu(BCIP)2](BF4)2
(4 Æ CH3OH)
43H66B2CoF8N6O
915.57
(5 Æ CH3OH)
Co–N(1)
Co–N(3)
Co–Cl(1)
2.022(1)
2.196(1)
2.3070(4)
Co–N(2)
Co–Cl(2)
N(2)–C(13)
2.261(1)
2.2790(4)
1.281(2)
Empirical formula
Molecular weight
C
C43H66B2CuF8N6O
920.18
Crystal color, habit red, block
blue-green, block
0.17 · 0.17 · 0.12
125(2)
[Cu(BCIP)Cl2] (2)
Cu–N(1)
Cu–N(3)
Crystal size (mm)
Temperature
0.20 · 0.20 · 0.20
115(2)
1.954(3)
2.132(3)
2.4242(9)
Cu–N(2)
Cu–Cl(2)
N(2)–C(13)
2.120(3)
2.2505(9)
1.285(4)
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/n
monoclinic
P21/n
Cu–Cl(1)
[Zn(BCIP)Cl2] (3)
Zn–N(1)
Zn–N(3)
2.063(1)
2.323(1)
2.2828(3)
Zn–N(2)
Zn–Cl(2)
N(2)–C(13)
2.230(1)
2.2542(3)
1.280(2)
˚
a (A)
11.4820(6)
18.0443(9)
22.752(1)
90
103.693(1)
90
4579.9(4), 4
1.328
0.447
11.4925(9)
18.103(2)
22.427(2)
90
102.111(1)
90
4562.2(6), 4
1.340
0.552
˚
b (A)
Zn–Cl(1)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
[Co(BCIP)2](BF4)2 (4)
Co–N(11)
Co–N(12)
Co–N(13)
N(13)–C(16)
2.008(2)
Co–N(21)
Co–N(22)
Co–N(23)
N(23)–C(26)
2.008(2)
2.219(2)
2.250(2)
1.283(3)
2.221(2)
2.273(2)
1.280(3)
3
˚
V (A ), Z
Dcalc (mg mÀ3
Absorption
coefficient
)
[Cu(BCIP)2](BF4)2 (5)
(l, mmÀ1
)
Cu–N(11)
Cu–N(12)
Cu–N(13)
N(13)–C(16)
1.940(3)
Cu–N(21)
Cu–N(22)
Cu–N(23)
N(23)–C(26)
1.937(3)
2.266(3)
2.266(3)
1.279(5)
2.243(3)
2.300(3)
1.269(5)
U Range collected
(ꢁ)
Completeness to
Umax (%)
1.84–28.30
1.46–23.81
100.0
93.8
Reflections are
collected/unique
51073/10683 (0.0338)
41276/7005
(0.0549)
Table 4
(Rint
)
Selected bond angles for complexes 1–3 with the average Npy–M(II)–
Data/restraints/
parameters
R1, wR2 (I > 2rI)
R1, wR2 (all data)
Goodness-of-fit on
F2a
10683/0/555
7005/0/536
Nimine reported
[Co(BCIP)Cl2] [Cu(BCIP)Cl2] [Zn(BCIP)Cl2]
0.0526, 0.1502
0.0701, 0.1643
1.020
0.0553, 0.1449
0.0776, 0.1554
1.099
(1)
(2)
(3)
Nimine–M(II)–Nimine 151.86(4)
155.53(1)
78.07(1)
115.50(3)
149.35(4)
74.68(4)
126.28(1)
Npy–M(II)–Nimine
Cl(2)–M(II)–Cl(1)
75.95(5)
128.09(2)
Largest difference in 0.965, À0.772
0.512, À0.405
3
˚
peak/hole (e/A )
Table 5
in calculated positions and were refined using a riding
model. The methanol solvent of crystallization in
(5 Æ CH3OH) is disordered. The atoms of the solvent were
included in the refinement as a diffuse contribution to the
scattering using the program SQUEEZE in the PLATON suite
of programs [19]. Crystal data and refinement details are
presented in Table 1 for complexes 1–3 and Table 2 for
complexes 4 and 5. Selected bond distances for complexes
1 – 5 are listed in Table 3. Bond angles are given in Table
4 for complexes 1–3 and Table 5 for complexes 4 and 5.
Selected bond angles and average Npy–M(II)–Nimine and Nimine–M(II)–
Nimine bond angles for complexes 4 and 5
[Co(BCIP)2](BF4)2 [Cu(BCIP)2](BF4)2
(4)
(5)
Npy–M(II)–Nimine
76.22(8)
77.38(1)
(intraligand)
Npy–M(II)–Nimine
(interligand)
103.83(8)
93.27(8)
102.61(1)
92.74(1)
Nimine–M(II)–Nimine
(interligand)
Nimine–M(II)–Nimine (trans)
152.31(8)
176.24(8)
154.74(1)
178.04(1)
Npy–M(II)–Npy
3. Results and discussion
synthesis of both complexes 4 and 5 were carried out using
the tetrafluoroborate salt of the hydrated metal(II) starting
material. Only the bis complexes were isolated from these
reactions even though in both cases a 1:1 metal to ligand
ratio was used. The yields for these reactions were: 4 –
68% and 5 – 64%, which confirm that each is the major
product for the reactions. In no case was a 5 coordinate
hydrated complex formed when the starting material con-
In many cases the synthesis of metal imine complexes
involves the use of in situ condensation of the amine and
aldehyde in the presence of the metal starting material
[20–22]. However, in this study, the ligand was synthesized
and characterized before addition to the metal salt. We
employed the use of either M(BF4)2 Æ xH2O or MCl2 Æ
xH2O (where M = Cu(II), Co(II) or Zn(II)), and found
that the ligand to metal ratio in the isolated product was
dependent on the identity of the starting metal salt. The
À
tained the non-coordinating BF4 anion. This latter result

R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
3065
could be due to the use of dry CH3OH as solvent for these
reactions as 5-coordinate M(II) complexes with coordi-
nated water have been reported [23,24]. These findings sug-
gest that, under the reaction conditions reported here,
coordination of other N-type donor atoms is preferable
over water when the coordination sphere of the metal con-
tains other nitrogen atoms. A similar Cu(II) complex has
been reported (substitution of methyl imine group for
hydrogen) however, attempts to isolate the corresponding
[Cu(L)2]2+ complex starting with the tetrafluoroborate salt
were unsuccessful [25]. These attempts resulted in brown
solutions from which no pure product was obtained. The
authors suggested that perhaps the hydrated Cu(BF4)2 salt
was undergoing spontaneous reduction to the Cu(I) species
which was confirmed by their investigation of the reactivity
of the ligand with the corresponding Cu(I) starting mate-
rial. We however did not see similar reactivity in the syn-
similar yield as reported for the 1:1 metal-to-ligand synthe-
sis. The isolation of 2 as the only product may be a result of
the insolubility of the product in CH3CN and it preferen-
tially precipitating out of solution before the bis complex
can form. To confirm that the Cu(II) bis complex can also
form in the presence of chloride ion, [Cu(BCIP)Cl2] was
dissolved in CH3OH and excess BCIP was added to it.
After 5 min the color of the solution changed from light
green to blue-green in color and a shift in kmax from
800 nm to 756 nm confirmed that [Cu(BCIP)2]2+ also forms
under high ligand to metal ratios.
3.1. Structure of complexes
3.1.1. Structure of [Co(BCIP)Cl2] (1)
The structure of [Co(BCIP)Cl2] is shown in Fig. 1. The
five coordinate Co(II) complex is comprised of one BCIP
ligand coordinated in a mer fashion, with the Npy and
the two chloride ions positioned in the equatorial plane
and the imine nitrogen donors in the axial positions. The
distortion of the complex arises mainly from the 2,6-pyri-
dine di-imine bite angle, with an N(2)–Co–N(3) bond angle
of 151.86(4)ꢁ. The N(1)–Co–Cl(2) bond angle is 120.52(4)ꢁ
while the Cl(2)–Co–Cl(1) bond angle is 128.09(2)ꢁ, with the
difference most likely due to the larger size of the chloride
thesis and isolation of
5 and only obtained blue
solutions. Attempts were made to isolate the analogous
Zn(II) complex using Zn(BF4)2 Æ xH2O, however no pure
product was isolated from these attempts.
Unlike the syntheses that resulted in the formation of bis
complexes 4 and 5, no bis complexes were isolated from
analogous syntheses utilizing one equiv. of BCIP and start-
ing metal salts that contain chloride ion, MCl2 Æ xH2O,
(where M = Cu, Zn, or Co). These syntheses resulted in
coordination of the two chloride ions and the formation
of one of the five coordinate complexes 1–3 as the major
product in each reaction. None of the syntheses resulted
in the formation of the corresponding aqua complex which
suggests that under these conditions, ClÀ is a better ligand
than water in these complexes when the coordination
sphere contains N-donor atoms. Two syntheses for com-
plex 1 have been reported previously however, one method
was not detailed [26] and no crystallographic analysis was
included in either report [27]. In the first report, the com-
plex was isolated as a grey-green solid which showed no
propensity to air oxidize. We report complex 1 as a green
solid which is air stable but does undergo air oxidation
over the course of 24 h in MeOH to give an orange-brown
solution.
˚
ions. The Co(II)–Npy bond distance is 2.022(1) A which is
shorter than the corresponding Co(II)–Nimine average bond
˚
distance of 2.2286(12) A. The average Co(II)–Cl bond dis-
˚
tance is 2.2930(4) A. The reported bond distances are sim-
ilar to other Co(II)–N bond lengths [28].
3.1.2. Structure of [Cu(BCIP)Cl2] (2)
The structure of [Cu(BCIP)Cl2] is shown in Fig. 2. The
distortion in this complex can be seen not only in the bite
We were interested to know if bis complexes could be
obtained from reactions with 1:2 metal-to-ligand ratios in
the presence of chloride ion. In the case of using 2.1 equiv.
of BCIP with CoCl2 Æ 6H2O, the reaction yielded a solution
which had a similar color (dark red-brown) to the solution
which yielded the bis complex 4. However upon crystalliza-
tion, only complex 1 was isolated, albeit in lower yield.
This result, combined with the lower yield, suggested that
at high ligand-to-metal ratios the Co(II) bis complex can
form in the presence of chloride ion. In order to confirm
this hypothesis, a reaction was carried out using a 1:5
metal-to-ligand ratio. These reactions produced only red-
orange solutions from which [Co(BCIP)2]2+ was identified
using UV/Vis spectroscopy. Similar reactions were carried
out with CuCl2 Æ 2H2O, however only complex 2 was iso-
lated. The reaction produced a light green precipitate in
Fig. 1. Thermal ellipsoid plot (30% probability level) of complex 1
showing the numbering scheme. H atoms have been omitted for clarity.

3066
R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
angle of the tridentate ligand, but also in the Cu(II)–Cl
bond lengths. The Cu(II)–Cl bond distances are
3.1.4. Structure of [Co(BCIP)2](BF4)2 Æ CH3OH
(4 Æ CH3OH)
˚
˚
2.2505(9) A and 2.4242(9) A, a difference of approxi-
The monoclinic crystals of [Co(BCIP)2](BF4)2 co-crys-
tallized with one methanol molecule. The structure of the
cation, [Co(BCIP)2]2+, is shown in Fig. 3. The octahedral
geometry around the Co(II) center is comprised of 2 BCIP
ligands coordinated in a mer fashion with the Npy groups
positioned trans to each other. The average Co–Npy bond
˚
mately 0.17 A. Comparatively, Co(II)–Cl bonds have
˚
nearly identical lengths with a difference of 0.028 A. Over-
all, the Cu(II)–N bond lengths are slightly shorter than
the analogous Co(II)–N distances. The Cu–Npy and aver-
˚
age Cu–Nimine bond lengths in 2 are 1.954(3) A and
˚
˚
2.126(3) A, respectively. The N(2)–Cu–N(3) angle is
distance is 2.008(2) A, a value that is slightly shorter than
155.5(1)ꢁ which is somewhat larger than the analogous
angle in complex 1. These differences are most likely
due to Cu(II) having a slightly smaller radius than Co(II)
[29]. The bond distances reported are within the range of
previously reported Cu(II)–N and Cu(II)–Cl bond dis-
tances [30,31].
the corresponding bond lengths in complex 1. All the
˚
Co(II)–Nimine bond distances are within 0.05 A with an
˚
average distance of 2.239(2) A. The N(22)–Co–N(23) bond
angle is 152.05(8)ꢁ while the N(11)–Co–N(21) bond angle is
176.24(8)ꢁ which suggests that the distortion is due to the
BCIP bite angle. All the Co–N bond distances are similar
to those previously reported [28].
3.1.3. Structure of [Zn(BCIP)Cl2] (3)
The structure of [Zn(BCIP)Cl2] has been submitted as
supporting information. Unlike [Cu(BCIP)Cl2], the Zn
analogue exhibits little distortion and is remarkably simi-
lar to complex 1, with the same orthorhombic crystal sys-
tem and space group (Pbca) as the Co(II) complex. The
3.1.5. Structure of [Cu(BCIP)2](BF4)2(5 Æ CH3OH)
Like complex 4, complex 5 also co-crystallized with one
solvent methanol in the same crystal system (monoclinic)
and space group as complex 4. The structure of the cation
is shown in Fig. 4. The average Cu(II)–Npy bond length is
˚
˚
Zn(II)–Npy bond length is 2.063(1) A while the average
1.939(3) A which is slightly longer than the analogous bond
Zn(II)–Nimine and Zn(II)–Cl bond distances are
2.2761(1) and 2.2685(3) A, respectively. All distances are
length in complex 2 and slightly shorter than the corre-
sponding Co(II)–Npy bond distance in complex 4. The
average Cu(II)–N(12)imine and Cu(II)–N(13)imine bond dis-
˚
within the range of previously reported Zn(II)–N and
Zn–Cl bond distances [32]. The bond lengths in complex
3 are somewhat longer than analogous Co(II) bond
lengths and the deviation is most likely due to the slightly
larger size of the Zn(II) ion [28]. This is supported by the
slight decrease in the N(2)–M(II)–N(3) bond angle from
151.86(4)ꢁ in the Co(II) complex to 149.35(4)ꢁ in the
Zn(II) species as well.
˚
tance is 2.272(3) A while the average Cu(II)–N(22)imine and
˚
Cu(II)–N(23)imine is 2.266(3) A. The combined average of
˚
the Cu(II)–Nimine distances is 0.143 A longer than the aver-
age Cu(II)–Nimine distance found in complex 2. The N(11)–
Cu–N(21) is 178.0(1)ꢁ which is approximately 2ꢁ larger
than the analogous Co(II) bond angle while the N(22)–
Cu–N(23) is 3ꢁ larger at 155.1(1)ꢁ. Similar to complex 2,
all Cu(II)–N bond lengths are comparable to other
reported values [30,31].
Fig. 2. Thermal ellipsoid plot (30% probability level) of complex 2
Fig. 3. Thermal ellipsoid plot (30% probability level) of the cation of 4
showing the numbering scheme. H atoms have been omitted for clarity.
showing the numbering scheme. H atoms have been omitted for clarity.

R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
3067
The differences between similar bonds lengths within
each complex 1–5 are given in Table 6.
combine to afford distorted complexes which minimize
the strain energy [33].
Both Cu(II) and Co(II) (high-spin) complexes are
expected to exhibit Jahn–Teller distortion, however the dis-
tinction between the degeneracy in the eg set (Cu(II)) versus
the t2g set (Co(II)) would expectedly give rise to greater dis-
tortion in octahedral Cu complexes. Complex 4 is slightly
compressed along the Npy–M–Npy axis with a decrease in
The differences in bond lengths for the pentacoordi-
nate [M(II)(BCIP)Cl2] complexes, 1–3, are not as
straightforward. The two M(II)–Cl bond distances are
nearly identical to each other in the Co complex 1 and
˚
Zn complex 3 structures (2.2790(4) and 2.3070(4) A and
2.2542(3) and 2.2828(3), respectively) while the Cu
˚
˚
the average Co(II)–Npy bond length of 0.0138 A and a
analogue (2) has greater than a 0.17 A difference between
lengthening of the Co(II)–Nimine average bond length by
the two. There are two possibilities that may explain the
deviation in the Cu(II)–Cl bond lengths in complex 2
which arise from its assignment as either a distorted
square pyramid (SP) or a distorted trigonal bipyramid
(TBP) [34]. The assignment to these geometries can be
made using the continuous symmetry approach developed
by Pinsky and Avnir [33] or by using the parameter s,
introduced by Addison, Reedijk and co-workers [35],
both of which provide a measure of the degree of SP
versus TBP geometry. However, neither approach gives
a clear assignment of the Cu(II) structure, with the
calculated values of the parameters falling between the
two geometries. Therefore, if the structure is assigned
as distorted SP, then one of the chloride groups occupies
the axial site which would result in a natural lengthening
of this bond compared to the Cu(II)–Cl bond in the
equatorial plane. However, if the assignment is made as
distorted TBP, then the results suggest that the distortion
is due to the electron configuration of the Cu(II) center
indicating that it is Jahn-Teller in nature. Additionally,
there is no clear trend for the M(II)–Nimine bond dis-
tances. The two Cu(II)–Nimine bond distances are nearly
˚
0.040 A compared to complex 1. A similar comparison
˚
between complexes 2 and 5 reveals a variance of 0.015 A
in the of the Cu(II)–Npy bond lengths, a value that is com-
parable to the difference between the cobalt complexes.
However, the average Cu(II)–Nimine distance in complex
˚
5 is 0.143 A longer than those in complex 2. There is very
little difference between the average of the two sets of axial
M(II)–Nimine bond lengths (from the two BCIP ligands L1
and L2) within complexes 4 and 5 with values of only
˚
˚
0.0055 A in complex 5 and 0.0125(2) A in complex 4. These
results suggest that the distortion in these complexes is dic-
tated by the limited flexibility of the ligand frame and by
the shorter M(II)–Npy bonds lengths compared to M(II)–
Nimine distances in these complexes. Both of these factors
˚
identical to each other (2.120(3) and 2.132(3) A) while
˚
the two bonds differ by greater 0.06 A in the Co analogue
˚
and nearly 0.1 A in the Zn analogue.
4. Concluding remarks
This work has described the synthesis and character-
ization of five imino(pyridine) metal complexes and their
crystallographic analysis. The product isolated from each
synthesis is directly related to the starting metal salt that
was used. The structural analysis of the octahedral
complexes showed both complexes 4 and 5 exhibit distor-
tion. The structural analysis of the five coordinate
complexes showed no clear trend in metal-ligand bond
lengths however the Cu(II) analogue (2) showed the
greatest distortion. Work on obtaining the corresponding
tris complexes with the analogous bidentate ligand is cur-
rently underway.
Fig. 4. Thermal ellipsoid plot (20% probability level) of the cation of 5
showing the numbering scheme. H atoms have been omitted for clarity.
Table 6
Acknowledgements
Differences in the M–Cl and M–Nimine bond lengths within each complex
1–5
Union College is thanked for its generous financial
support and start up grant (LAT). We also thank the
National Science Foundation (NSF 0521237) for sup-
porting the X-ray diffraction facility at Vassar College
(JMT). TCH is thanked for helpful discussions.
1 (Co)
2 (Cu)
3 (Zn)
4 (Co)
5 (Cu)
DM(II)–Cl
DM(II)–N(L1)imine 0.065(1) 0.012(3)
DM(II)–N(L2)imine
0.028(4) 0.1737(9) 0.0286(3)
0.093(1)
0.052(2) 0.057(3)
0.031(2)
0

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R.N. Davis et al. / Inorganica Chimica Acta 360 (2007) 3061–3068
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Appendix A. Supplementary material
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[16] H. Yoo, P.G. Carroll, D.H. Berry, J. Am. Chem. Soc. 128 (2006) 6038.
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[18] G.M. Sheldrick, SHELXTL, An Integrated System for Solving, Refining
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Chem. 39 (2000) 5787.
CCDC 629360, 629359, 629358, 629356 and 629357 con-
tain the supplementary crystallographic data for 1, 2, 3, 4
and 5. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2007.03.002.
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