Inorganica Chimica Acta 357 (2004) 83–88

www.elsevier.com/locate/ica

Chiral bimetallic complexes from chiral salen metal complexesand mercury (II) halides and acetates: the anionic groups interact

with Cu(II) in apical position q

Marisabel Lebron Colon a, Steven Y. Qian b, Donald Vanderveer c, Xiu R. Bu a,*

a Department of Chemistry, Clark Atlanta University, Atlanta, GA 30314, USAb Free Radical Metabolite Section, Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences,

Research Triangle Park, NC 27709, USAc School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA

Received 15 April 2003; accepted 20 June 2003

Abstract

A series of chiral bimetallic complexes have been prepared containing both Cu(II) and Hg(II) metal centers. The complexes

possess chiral salen ligands which host Cu(II) in the center of the cis-N2O2 chromophore and Hg(II) via two oxygen atoms of the

chromophore. Halogen and acetate groups from mercury salts interact with the Cu(II) center. The X-ray crystallographic data of

11 reveals a short distance of Cl� � �Cu (3.22–3.26 �AA). EPR study also discloses a strong interaction, in particular, of acetate group

with Cu.

� 2003 Published by Elsevier B.V.

Keywords: Chiral salen complexes; Chiral bimetallic complexes; Copper complexes; Mercury complexes; EPR

1. Introduction

Chiral salen metal complexes have been subject to

extensive investigation due to their excellent catalyticproperties in asymmetrical reactions [1–5]. This has

prompted much increasing effort in the development of

new chiral complexes. Apart from conventional mono-

nuclear complexes [6,7], our recent focus has been on

chiral bimetallic salen complexes possessing simple chiral

salen metal complexes. Salen ligands are known to be

capable to chelate many metal ions [8–13]. Most signifi-

cantly, salen complexes themselves can act as ligands forfurther coordination as a result of the interaction of co-

ordination chromophore with another metal substrate

[14–19]. Usually, the oxygen atoms serve as donor atoms

further coordinated to second metal ions. Such interac-

qSupplementary data associated with this article can be found, in the

online version, at doi:10.1016/S0020-1693(03)00426-2.* Corresponding author. Tel.: +1-404-880-6897; fax: +1-404-880-

6890.

E-mail address: xbu@cau.edu (X.R. Bu).

0020-1693/$ - see front matter � 2003 Published by Elsevier B.V.

doi:10.1016/S0020-1693(03)00426-2

tion can be also realized in the presence of additional

donor groups at 3,30-positions [20–22]. The resultant

geometry from such bimetallic complexes is significantly

different from bi-compartmental bimetallic complexes[23–27]. This chelating capability can be potentially used

to help the removal of other metals, and to construct new

classes of chiral reagents when chiral ligands are used.

Here, we report that a new series of chiral bimetallic salen

complexes that possess chiral salen copper (II) and nickel

(II) complexes which are coordinated to mercury (II).

2. Experimental

2.1. Synthesis and instrumental

All chemicals and solvents were purchased from Al-

drich or Fisher Scientific and were used as received. 1H

NMR data were acquired on a Bruker 400 MHz spec-

trometer with the use of TMS as internal reference. El-emental analyses were performed by Atlantic Microlab,

84 M. Lebron Colon et al. / Inorganica Chimica Acta 357 (2004) 83–88

Inc., Norcross, Georgia. Solvents for EPR measurement

are spectroscopic grade and purged with nitrogen before

use. EPR measurements were taken on a Bruker ESP300

EPR spectrometer with the following settings: modula-

tion amplitude, 1G; modulation frequency, 100 kHz;time constant 163.84 ms; scan time, 83.886 s; receiver

gain, 6.32� 103 unless otherwise noted. The frequency is

calibrated using frequency meter. The solvents were

purged with nitrogen before use.

2.2. Crystal structure determination

Crystals suitable for X-ray determination were ob-tained by the recrystallization from 1,4-dioxane.

C24H28Cl2CuHgN2O4: 11 �C4H8O2, M ¼ 743:51, tri-

clinic, space group P1, a ¼ 12:7011ð14Þ, b ¼ 13:8700ð15Þ,c ¼ 14:6910ð17Þ �AA, b ¼ 108:201ð2Þ�, V ¼ 2458:5ð5Þ,Z ¼ 4, Dc ¼ 2:009, crystal size 0.27� 0.17� 0.14 mm,

kðMo KaÞ 0.71073 �AA, T ¼ 173ð2Þ K, F ð000Þ ¼ 1444,

lðMo KaÞ ¼ 7:354 cm�1.

2.2.1. Data collection

Absorption correction was calculated using SADABS

with maximum and minimum transmission factors of

0.2396 and 0.4346, respectively; intensity data were

collected on a Bruker 1K CCD platform diffractometer;

A total of 13 078 reflections were collected, of which

8465 independent reflections (Rint ¼ 0:0372) were ob-

served with 1:46 < h < 25� and used in all the calcula-tions. The unit cell dimensions were determined by a

least-square fit of 2h angles for 7102 reflections.

2.2.2. Structure analysis and refinement

The structure was solved by direct methods and sub-

sequent Fourier difference techniques, and refined an-

isotropically, by full-matrix least squares, on F 2 using

SHELXTL 5.1. Hydrogen atoms were calculated fromideal geometry with coordinates fixed and temperature

factors varied. Weights based on counting-statistics were

used (based on w�1 ¼ r2ðF 2o Þ þ ð0:0844PÞ2 þ 0:34P ,

where P ¼ ½maxðF 2o ; 0Þ þ 2F 2

c Þ=3�). The last least square

cycle was calculated with 647 parameters and 7102 re-

flections. For reflections with I > 2rðIÞ, RðF Þwas 0.0354,and RwðF 2Þ 0.0764 with goodness-of-fit 0.938 where Rw is

defined by RwðF 2Þ ¼ ½P

ðxðF 2o � F 2

c ÞÞ2= ½

PðxðF 2

o ÞÞ2�1=2

and GoF defined by GoF ¼ ½P

ðxðFo� FcÞÞ2=13807�1=2.For all reflections, RðF Þ was 0.0413, and RwðF 2Þ 0.0773.The maximum D=r ¼ 0:001 while the mean D=r ¼ 0:000.The deepest hole is )0.425 e/�AA3 and the highest peak is

0.446 e/�AA3. Full crystallographic data are given in the

supporting materials.

2.3. Bimetallic complex synthesis. General procedure

The ligands were prepared by the condensation

of salicylaldehyde or 5-t-butylsalicylaldehyde with

(1R; 2R)-())1,2-diaminocyclohexane in ethanol [5].

Cu(salen) complexes were prepared by the treatment of

ligands with Cu(OAc)2 �H2O in ethanol [30,31]. Cu(sa-

len) (0.25 mmol) was dissolved in hot 1,4-dioxane (25

ml), and to this solution was added a methanol solution(10 ml) of mercury bromide (0.25 mmol) with stirring.

The mixture was heated at 120 �C for 5 h. Upon cooling,

the solution was concentrated and green dark crystals

were obtained. 5: Anal. Calc. for C20H20N2NiO2HgBr2:

C, 32.48; H, 2.73; N, 3.79. Found: C, 32.94; H, 2.87; N,

3.67%. 6: Anal. Calc. for C28H36N2NiO2HgBr2 �3.5C4H8O2: C, 43.46; H, 5.56; N, 2.41. Found: C, 43.84;

H, 5.31; N, 2.93%. 7: Anal. Calc. for C20H20N2NiO2-HgCl2 � 5/8C4H8O2: C, 38.30; H, 3.57; N, 3.97. Found:

C, 38.53; H, 3.26; N, 4.41%. 8: Anal. Calc. for C24H26-

N2NiO6Hg: C, 41.32; H, 3.76; N, 4.01. Found: C, 41.58;

H, 3.58; N, 4.22%. 9: Anal. Calc. for C20H20N2-

CuO2HgBr2: C, 32.27; H, 2.71; N, 3.76. Found: C,

32.77; H, 2.74; N, 3.83%. 10: Anal. Calc. for C28H36N2-

CuO2HgBr2 � 2C4H8O2: C, 41.87; H, 5.08; N, 2.71.

Found: C, 41.52; H, 4.65; N, 3.07%. 11: Anal. C24H28-Cl2N2O4CuHg for C20H20N2CuO2HgCl2 � C4H8O2

from X-ray crystallographic determination. 12: Anal.

Calc. for C24H26N2CuO6Hg � 3/5C4H8O2: C, 41.97; H,

4.10. Found: C, 42.03; H, 3.67%.

3. Results and discussion

The synthesis of new chiral bimetallic complexes 5–12

involves the use of a chiral salen metal complex and a

mercury salt (Scheme 1). For example, treatment of

ðR;RÞ-N ;N 0-bis(salicylidene)-1,2-cyclohexanediamino-

copper 3 with mercury (II) chloride in dioxane afforded

the corresponding bimetallic complex 11. The reaction

was achieved at elevated temperature. Upon cooling and

reducing volume of the solvent, the product precipitatesas a solid. Although the reaction is straightforward in

mild condition, the substituent groups in chiral salen

metal complexes have significant effect on the reaction.

The reaction gives isolated products when the sub-

stituent group is the para-position (5-) of a salen

complex as evident in using ðR;RÞ-N ;N 0-bis(5-t-butyl-salicylidene)-1,2-cyclohexanediaminonickel (II) 2 or its

copper analogue 4. However, when the substituentgroup is in ortho-position (3-), the reaction fails to give

any isolated product. For example, the use of ðR;RÞ-N ;N 0-bis(3-t-butylsalicylidene)-1,2-cyclohexanediami-

nocopper to react with mercury bromide did not

produce any bimetallic complexes, but only led to the

recovery of starting salen metal complexes. Even the

presence of smaller substituent groups such as methyl is

still unfavorable to the reaction. This observation is alsoapplied to Ni(II) metal center.

Several mercury (II) salts are found effective in the

reaction, including chloride, bromide, and acetate. The

Scheme 1.

M. Lebron Colon et al. / Inorganica Chimica Acta 357 (2004) 83–88 85

reaction gives generally high yields (85–99%) except for

8 with 65% yield (Table 1). Solubility of new bimetallic

complexes is dramatically different from that of startingsalen complexes. Although most of the salen complexes

are soluble in chloroform or methylene chloride, none of

new bimetallic complexes is. The best solvents for sale-

nNiHgX2 are those much stronger polar solvents such

as DMSO and pyridine. Moreover, salenCuHgX2

complexes are only soluble in pyridine.

Proton signals in 1H NMR show no significant effect

of mercury addition. All the Ni-containing bimetalliccomplexes give almost identical 1H NMR spectra to

those from the corresponding salen Ni complexes with

little chemical shift. For instance, 6 in d6-DMSO has

aromatic proton signals at 7.33 (d, J ¼ 2:5 Hz), 7.25 (dd,

J ¼ 8:9 Hz, J ¼ 2:5 Hz), and 6.64 ppm (d, J ¼ 8:9 Hz),

comparable to those at 7.33 (d, J ¼ 2:6 Hz), 7.25 (dd,

Table 1

Yields of bimetallic complexes from mercury salt-chiral salen metal

complex

Bimetallic

complex

M in salen

complex

X R Y (%)

5 Ni(II) Br H 85

6 Ni(II) Br t-Bu 95

7 Ni(II) Cl H 85

8 Ni(II) OAc H 65

9 Cu(II) Br H 99

10 Cu(II) Br t-Bu 98

11 Cu(II) Cl H 85

12 Cu(II) OAc H 99

Table 2

Bond distances and angles of coordination chromophores of the complexes

Molecule A (11) Molecule B (11

Cu1–O1 1.902 (6) Cu10–O10

Cu1–O2 1.896 (7) Cu10–O20

Cu1–N1 1.930 (8) Cu10–N10

Cu1–N2 1.916 (9) Cu10–N20

O1–Cu1–O2 86.0 (3) O10–Cu10–O20

O1–Cu1–N1 94.7 (3) O10–Cu10–N10

O1–Cu1–N2 171.4 (3) O10–Cu10–N20

O2–Cu1–N1 169.9 (3) O20–Cu10–N10

O2–Cu1–N2 95.2 (3) O20–Cu10–N20

N1–Cu1–N2 85.6 (3) N10–Cu10–N20

J ¼ 8:9 Hz, J ¼ 2:6 Hz), and 6.640 ppm (d, J ¼ 8:9 Hz)

of 2 in d6-DMSO. 5 in d6-DMSO has aromatic signals at

7.36 (d, J ¼ 7:8 Hz), 7.16 (dd, J ¼ 7:8 Hz, J ¼ 7:3 Hz),6.69 (d, J ¼ 8:5 Hz), 6.50 ppm (J ¼ 8:5, J ¼ 7:3 Hz),

comparable to those at 7.363 (J ¼ 7:7 Hz), 7.16 (J ¼ 7:7Hz, J ¼ 7:3 Hz), 6.691 (J ¼ 8:5 Hz), 6.503 ppm (dd,

J ¼ 8:5 Hz, J ¼ 7:3 Hz) of 1 in d6-DMSO.

Molecular structure of 11 determined by X-ray crys-

tallography revealed that there are two molecules (de-

fined as molecule A with normal atom labeling and

molecule B with atom labeling with prime sign) in a unitalthough there is little difference between them. Tables 2

and 3 list bond distances and angles around copper and

mercury, respectively. The distance between Cu and Hg

is 3.2715(4) �AA in molecule A. The core feature of the

bimetallic structure possesses the average Cu–O bond of

1.899(7) �AA and Hg–O bond of 2.6201(7) �AA, accompa-

nied by the average Hg–Cl bond of 2.312(3) �AA and Cu–

N bond of 1.923(9) �AA. Most significantly, HgCl2 unit isno longer linear upon the addition to the chiral complex.

With \Cl–Hg–Cl of 162.99(11)�, it apparently bends

about 17� from the original linear geometry. The present

Hg–Cl distance, which is close to 2.28(4) �AA of HgCl2 in

gaseous state, resembles covalent bond character of

HgCl2 [28]. The bimetallic core (Hg1, O1, O2, Cu1) has

a folded-envelop geometry. For this reason, Hg is dis-

placed 1.8779(85) �AA from a plane defined by O1, Cu1,and O2. For the chiral salen Cu complex moiety, bond

distances and angles around the coordination chromo-

phore are almost same as those in Cu(salen) [29]. The

before and after mercury addition

) Cu(salen)29

1.910 (7) 1.901 (2)

1.910 (7) 1.896 (3)

1.936 (2) 1.918 (3)

1.932 (3) 1.927 (3)

86.1 (3) 89.1 (1)

95.2 (3) 94.6 (1)

176.6 (3) 170.3 (2)

173.1 (3) 169.2 (1)

94.0 (3) 93.4 (1)

85.1 (4) 84.6 (1)

Table 3

Bond distances and angles around mercury center in 11

Molecule A Molecule B

Hg1–O1 2.652 (7) Hg10–O10 2.570 (7)

Hg1–O2 2.589 (7) Hg10–O20 2.574 (7)

Hg1–Cl1 2.323 (2) Hg10–Cl10 2.327 (2)

Hg1–Cl2 2.300 (3) Hg10–Cl20 2.297 (3)

Hg1� � �Cu1 3.2715 (4) Hg10� � �Cu10 3.2750 (14)

O1–Hg1–O2 59.2 (2) O10–Hg10–O10 60.9 (2)

O1–Hg1–Cl1 89.83 (15) O10–Hg10–Cl10 89.92 (15)

O1–Hg1–Cl2 96.53 (15) O10–Hg10–Cl20 107.98 (15)

O2–Hg1–Cl1 87.72 (15) O20–Hg10–Cl10 87.18 (15)

O2–Hg1–Cl2 109.09 (15) O20–Hg10–Cl20 99.99 (15)

Hg1–O1–Cu1 90.3 (2) Hg10–O10–Cu10 92.8 (3)

Hg1–O2–Cu1 92.4 (3) Hg10–O20–Cu10 92.7 (3)

Hg1–O2–C13 131.3 (6) Hg10–O20–C130 116.2 (6)

Hg1–O1–C14 114.2 (6) Hg10–O10–C140 126.2 (6)

Fig. 2. EPR spectra of 3, 9, 11, and 12 at low temperature (77 K).

86 M. Lebron Colon et al. / Inorganica Chimica Acta 357 (2004) 83–88

most significant difference is the \O–Cu–O angle. The

addition of mercury onto the copper complex appar-

ently made the angle smaller than that in original

Cu(salen) by about 3� [86.0 (3) versus 89.1 (1)]. The cis-

N2O2 donor atoms adopt a tetrahedrally distorted

square planar geometry [30,31] in which Cu is centered

in the mean plane with little deviation [0.0103 (36) �AA].

Both O1 and N2 are 0.15675 (36) �AA above the meanplane, and both O2 and N1 0.15670 9 (36) �AA below.

Chiral diamine effect on chelating rings brings signifi-

cant but different deviations on their adjacent phenyl

rings [30,31]. For example, the chelating plane {Cu1–

N2–C7–C8–C13–O2} is twisted 5.82 (0.20)� from the

phenyl ring {C8 to C13}. In marked contrast, the ring

{Cu–N1–C20–C19–C14–O1} is almost perfectly copla-

nar with its neighboring phenyl {C14–C19}, withmarginal deviation of 0.19 (0.33)�. Corresponding de-

viations in molecule B were found to be 5.18 (0.22)� and2.92 (0.32)�, respectively. It should add that the cis-

N2O2 plane is much less tetrahedrally distorted. Both

O10 and N20 are only 0.0856 (34) �AA above the mean

plane, and both O20 and N10 0.08565 (56) �AA below

(Fig. 1).

Fig. 1. ORTEP drawings of mole

The anisotropic Ak values measured at )78 �C (in

pyridine) are 183–185 Gs for the bimetallic Cu com-

plexes, slightly lower than those found in monomeric Cu

complexes (Fig. 2, Table 4). The gk values obtained at)78 �C are around 2.22 for the bimetallic Cu complexes,

close to those of monomeric Cu complexes. The present

anisotropic g values with gk � g? > 2:0 eliminate a

possibility of dz2 ground state in square planar geometry

[32]. Thus, the unpaired electron is in the copper dx2�y2

or dx�y orbital, which undergoes the formation of rmetal–donor bonds with two nitrogen and two oxygen

donor atoms.The room temperature spectra consist of four well-

resolved copper resonance lines belonging to mI ¼ 3=2,1/2, )3/2, )1/2 (Fig. 3). The A0 values for mercury

bromide-based bimetallic complexes 9 and 10 are varied

according to the substituents in the phenyl rings. The

presence of t-butyl group at the 5-position of the salen

ligand increases the in-plane interaction of Cu and

chelating atoms since the alkyl group is considered as aweak electron-donating group with ortho/para-directing

cules A (left) and B (right).

Table 4

ESR data of bimetallic complexes containing Cu(II)

A0 g0 Ak A? gk g?

9 82.89 2.123 184.33 32.18 2.219 2.075

10 84.72 2.124 184.75 34.70 2.221 2.075

11 82.63 2.124 184.17 31.86 2.218 2.076

12 80.16 2.121 183.58 28.45 2.221 2.072

3 83.15 2.123 185.34 32.06 2.218 2.076

4 82.63 2.121 185.93 30.98 2.217 2.073

A0 ¼ 1=3ðAk þ 2A?Þ and g0 ¼ 1=3ðgk þ 2g?Þ.All the A values are in Gs.

Fig. 3. EPR spectra of 3, 9, 11, and 12 at ambient temperature.

Fig. 4. The apical interaction of halide (a) and acetate (b) with Cu(II).

M. Lebron Colon et al. / Inorganica Chimica Acta 357 (2004) 83–88 87

reactivity [33]. The A0 value of 10 higher than that of 9,

indicates the substituent effect. 11 possessing no sub-

stituent in the 5-position has the A0 value very close to

that of 9.There is a different mechanism by which the A0 value

is affected in 12, which has mercury acetate. The ligand

in it is no different from that used in 9 and 11, suggesting

that there will be no substituent effect. Thus, the lower

A0 value can be no longer accounted for by the elec-

tronic perturbation described above due to the lack of

the substituent effect from the 5-position.

Another factor playing a role in the lower A0 value in12 is the pseudo John–Teller distortion of the metal

center by the acetate group. A close re-examination of

the crystal structure of 11 revealed a short distance of

Cl� � �Cu (3.2297 �AA) in one molecule. This short distance

between Cl and Cu is also found in the other molecule

(3.2637 �AA) in the same unit. These consistent short dis-

tances indicate a possible interaction between Cl and the

metal center. Halogen atoms in several Cu complexeshave been reported to interact with the metal center,

leading to the John–Teller distortion. For example,

the bromo substituent in the phenyl ring induced a

configuration change around the Cu(II) at a distance of

3.66 �AA [34]. The Cu and Br interaction has also been

reported at the distance of 3.427 (1) �AA in a dimeric

complex [35]. The covalent radii of Br and Cl are 1.14and 0.99 �AA, respectively [36]. The Cl� � �Cu distance in 11

is shorter than the reported Br� � �Cu distances accord-

ingly due to the slight difference between these radii.

Thus, the Cl is positioned to have the John–Teller effect

acting as an apical atom to interact with the Cu. To ac-

commodate this, Hg is displaced 1.8779 (85) �AA above the

{O1–Cu1–O2} plane instead of being in the plane, an-

other sign that the heteroatom from one center interactswith the other [37].

The interactions of Cu with Cl and acetate are sig-

nificantly different. HgCl2 tends to be in linear geometry,

but bends to ca. 163� in the adduct. The interaction be-

tween Cl and Cu puts Cl in a pseudo-three-membered

ring system. However, the interaction between acetate

and Cu is in a pseudo-five-membered ring system (Fig. 4).

The geometry is more relaxed due to the ring size, and theinteraction of acetate with Cu is much stronger. The

apical interaction of oxygen from acetate with Cu will

naturally weaken the in-plane interaction strength of Cu

with chelating atoms [38]. The present lower A0 value in

12 is accounted for by such strong interaction of car-

bonyl oxygen with the central metal [39].

The significant difference between the interactions of

Cu(II) with chloride and acetate groups has also beenobserved in hyperfine structure. In the mI ¼ �3=2 band,

there are several superfine lines registered for 14N and

imine 1H nuclei [40]. These lines are clearly resolved for 11

(Fig. 5) and identical to those of 3, indicating that the weak

interaction of Cl with Cu does not affect the hyperfine

Fig. 5. The hyperfine structures at mI ¼ �3=2 of 11 and 12 at ambient

temperature.

88 M. Lebron Colon et al. / Inorganica Chimica Acta 357 (2004) 83–88

structure. However, those lines are less structural in 12

(Fig. 5), clearly indicating that a strong interaction of ac-

etate group generates an impact on the hyperfine structure.

This type of interaction is considered as a stabilizingfactor, which helps the formation of bimetallic com-

plexes. Otherwise, it will be impossible to generate bi-

metallic complexes of this kind. In instance, similar

bimetallic vanadyl analogues cannot be prepared upon

treatment of salen vanadyl complex (similar to 1 with

M¼VO) with any mercury salt. Most of the salen

vanadyl complexes adopt pyramidal square geometry

with vanadium deviated from the basal N2O2 plane,which makes it difficult to have an access to the sixth

position [41]. The fifth position is occupied by oxygen of

the vanadyl. Thus, there is no apical position available

for the interaction.

In summary, a series of chiral bimetallic complexes

possessing chiral salen metal complexes has been pre-

pared. Salen metal complexes are copper (II) and nickel

(II) complexes. Mercury (II) binds to two oxygen of thesalen metal complexes. The choice of mercury is based

on its toxicity and reactivity. The current work indicates

that mercury can be potentially removed by salen metal

complexes for environmental remediation, and the re-

sultant bimetallic complexes may be used as chiral

mercury reagents.

Acknowledgements

Financial supports in part by NIH/NIGMS

(S06GM08247 and R25GM60414) by US Army

(DAAD19-01-0746) and by DOE (Cooperative Agree-ment No. DE-FC02-02EW15254) are gratefully ac-

knowledged.

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