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CHAPTER I

CHAPTER - I

I. Introduction

Copper 1s an essential trace element and the third most abundant transition

metal in the human body following iron and zlnc. It IS second only to iron in its

prevalence in redox active metalloproteins. Copper Ions, as centers of the active slte

of a large number of biologically important metalloproteins play an essential role In

b~ological processes [ l ] i.e. electron transfer, oxldat~on and dioxygen transport.

Based on the function of the protein, the structure of the active site and/or the

spectroscopic behaviour of the copper ions, the copper protelns are classified Into

four types. They are:

Blue copper proteins

Normal copper prote~na

Dinuclear copper proteins

Multlcopper oxidase

From a structural and spectroscopic polnt of view, the three main types of

biologically active copper centers found in the copper proteins may be distinguished

according to a generally accepted convention deriving mainly from their electron

paramagnetic resonance spectra.

Type 1 (TI), have 'blue' copper centers, consist of a large family of electron

transferring proteins w ~ t h a trigonal planar N2S' chromophore, together with one or

two weakly coordinating axial ligands (S and/or 0). The typical spectroscopic

features (e.g. small 41 value and intense blue colour due to a LMCT Cu(II) -t S') are

dominated by the unique, short copper-thiolate bond (2.13 A) [2]. This Cu-S bond

has unusually high 'covalent' character, interpreted as substantial delocalisation of

the S @) rr - orbital with the copper orbitals: the half - filled redox active orbital of

the blue copper site has only 40% d.2.; character [3].

Type 2 (T2), have 'non-blue' copper centers, and similar to common Cu(I1)

coordination complexes containing an N, 0 chromophore with tetragonal symmetry.

T h ~ s class of copper proteins consists of several mononuclear copper proteins, albeit

with very different biochemical functions. Examples include copper-zlnc

superoxide dismutase (Cu-Zn SOD), dopamine-phydroxylase (Dm), phenylalanine

hydroxylase (PhH) and galactose oxidase (GaOx).

Type 3 /T3), have copper dimers. The nitrogens come from histidine groups, the

sulphurs from methionine and cysteine, and the oxygens from a carboxylic acid In

the proteln. Water, hydroxide and alkoxide oxygens are also used.

Both Type I and Type 2 copper proteins are not focus of our interest since

they contain monocopper at their active sites. Hence it is worth to discuss in detail

on Type 3 copper proteins that contain coupled bicopper center at their actwe site.

1.1. Dinuclear Copper Proteins containing the Type-3 site

Three well known coupled copper proteins are hernocyanin, tyrosinase and

catechol oxidase. Due to strong anti-femomagnetic coupling between both Cu(1I)

centers, the oxy-form of a type-3 is EPR silent. (-23 > 600 cm.').

Hemocyanin (Greek for 'blue bloods') function 1s the dioxygen carners for

the mollusks and arthropods [4] The crystal structures of both the oxy and

deoxyfonn of the L~mulus subunlt II hemocyamn have been determined [5, 61 In

the deoxygenated form, the two Cu(1) Ions are ca 4 6 A apart and are both

coordlnated by three ~m~dazole nltrogens from h~stidine res~dues In a tr~gonal

envlronment

F I ~ I 1 Schemat~c draw~ng of the actlve site of hemocyanln and its reactlon w ~ t h 02

In the oxygenated fonn (Fig I 11, the d~oxygen is bound as peroxo (02 ' ) 111 a

4-4 geometry between the two copper Ions, the Cu(I1)-Cu(I1) d~stance 1s reported

to be about 3 6 A The copper Ions are both coordlnated 1n a square pyram~dal

envlronment In the equatorial plane each copper ion is bound to two h~s t~dlne

tutrogens and two-peroxo oxygen atoms At the axlal posltlon each copper Ion is

weakly cwrdlnated by hlstidlne nitrogen Oxyhemocyanin exhlblts two Intense

absorption In the visible reglon, at ca 350 (E = ca 20000 dm3 moll cm ') and at 580

nrn (E = ca. 1000 dm3 m01.'cxn.~), both attributable to a peroxo to Cu(II), ligand to

metal charge-transfer (LMCT) band. The extremely low v(o.0, frequency (ca.750

cm") observed in the resonance Raman spectrum revealed that the 0-0 band is

significantly we&ened by the coordination of the oxygen to the copper ions [7].

The oxy form of the protein is EPR silent, due to a strong antiferromagnetic

interaction between the two copper(I1) ions (-2J > 600 cm").

Tyrosinase

Tyroslnase is a mono-oxygenase found in microorganisms, plants and

animals and catalyses the o-hydroxylation of monophenols to o-diphenols and

further oxidat~on to o-quinones [g]. It is responsible for the chemistry involved in

skin tanning and for the browning reaction observed when mushrooms, potatoes and

fru~ts are injured and exposed to dioxygen. The actlve site of tyrosinase is believed

to be similar to that of hemocyanin, but tyrosinase has the additional feature that the

slte is h~ghly accessible to substrates, which bind directly to the copper center [g]. A

proposed mechanism of tyrosinase catalysis is depicted in Fig. 1.2.

Fig. 1.2. Proposed mechanism of tyrosinase catalysis

Catch01 oxidases are ub~quitous plant enzymes containing a dlnuclea

copper center Without act~ng on tyrosine, they only catalyze the oxidation of a

broad range of o-diphenols to the corresponding o-qumones [lo] coupled w ~ t h the

reduct~on of oxygen to water T h ~ s reactlon is of great importance In rned~cal

diagnosis for the determlnat~on of the hornonally active catecholamines adrenaline,

nonadrenallne and dopa [ l l ] Secondary reactions (melanin formation) follow after

oxidation of the substrate in the presence of polyphenol oxidases, which cause the

brown color of Injured plant [12] The copper In the lsolated catechol oxidases is

found to be EPR s~lent and has been asslgned to an ant~ferrornagnetically spln

coupled Cu(L1)-Cu(I1) pair [13] The proteln part of the W - V i s spectrum of the oxy

catechol ox~dase from Ipomoea batatas exhib~ts an intense absorption band at 343

nm and a weaker band at 580 nrn, corresponding to the peroxo complexes of

hernocyanin and tyrosinase They are asslgned to peroxo + Cu(I1) charge transfer

transitions [14] w ~ t h an 0-0 stretching vtbrat~on band at 749 crn lndicatlng a

posslble p - 4 - $ - bndging mode of the peroxo group XAS lnvestlgatlons on the

nat~ve met forms of catechol oxidases from Lycopus europueus and I Batatus have

revealed that the actlve site conslsts of a dlcopper(I1) center, In which the metal

ztorns are coordinated by four N/O donor l~gands Multiple scanenng EXAFS

calculations have shown hlgh significance for one or two coordlnat~ng hlstldine

residues [15] The short metal-metal dlstance of 2 9 A and the results of EPR

~nvestigations indlcate a p - hydroxo bndged dicopper(I1) active site in the met

forms of the proteins [16]

1.2. Need for Synthetic Models

Over the past decade, much progress has been made in the structural

characterization of copper proteins by spectroscopic techniques, X-ray analysis and

site-directed mutagenesis [17]. However, specific reaction pathways and

mechanisms of reactlon often remain unclear. In order to understand their catalytic

mechanisms and their unusual spectroscopic characteristics, the development of

relatively simple, synthetic models of the active site structures of copper proteins

therefore remains of considerable interest [18].

1.3. Significance of Synthetic Models

Low molecular we~ght model compounds can be examined easler than the

metalloenzyme itself, active site analogues can help to elucidate details of the

active site of many metalloenzymes.

Once a structural model is obtained, this may lead to the development of a

coordination complex with reactivity properties similar to that of the native

metalloprotelns: a functional model.

Low molecular weight synthetic model compounds can be obtained relatively

easy in large amounts, which is often not the case for native metalloproteins.

Moreover, synthetic models are relatively easy to modify, allowing optlmizat~on

of ligand-design in terms of reactivity and selectivity, in contrast to the

metalloenzyme itself.

1.4. Literature Overview

Model complexes for dinuclear copper proteins can be prepared using

dinucleating ligands, designed to accommodate two copper ions in close proximity.

The term "dinucleating ligand" is introduced in 1970 by Robson [I91 to describe the

class of polydentate chelating ligands, able to bind simultaneously two metal ions.

Since then, a very large number of such ligands are designed and their coordination

compounds are thoroughly investigated. The possible applications of the complexes

with this type of ligands vary from modeling the active sites of many metallo

enzymes [20-221 to hosting and canying small molecules [23-251 or catalysis [26,

271.

Among many different types of dinucleating ligands, the phenol based

compartmental ligands attracted particularly w ~ d e attention of scientists. The term

"compartmental" is introduced to indicate a ligand containing two adjacent, similar

or dissimilar coordination sites [20]. Particular interest in the ligands having distinct

donor sets has resulted from the recent recognition of the asymmetric nature of a

number of dimetallic hiosites [28, 291.

Understanding the ability of individual metal ions to play possibly different

functions in dinuclear sites In metalloenzymes lead to the design of a great number

of symmetric and asymmetric dinucleating ligands, where two compartments would

provide a same or different coordination sumounding for the two metal ions. An

extraordinary number of multidentate dinucleating phenoxide ligands have been

studied, and a representative collection of these types of ligands is listed in Table.

1.1-3.

Table I. 1 . Syrnmctncal Saturated Dinucleahng L~gands

L~gands Reference Ligands Reference

(:I ""0 [31, 341

I I R- CN Br

v

)$, [361 h f ~

R R

VI1

R = CI, R'=CH, R - CH,. R'= CI

IW a:. ".:D

Table 1.1 Symmetrical Saturated Dinucleating Ligands (Continued)

Ligands Reference Ligands Reference

XI11 XIV

Table 1.2 Unsymmetrical Saturated Dinucleating Ligands

Ligands Reference Ligands Reference

XIX XX

A 0 OH Q 1 aNAa [1&591

XXI

(=j N b

XXIl

Xxlll XXIV

Table 1.3 Unsymmemcal Unsaturated Dinucleating Ligands

L~gands Reference Ligands Reference

xxv XXVI

A, "2

XXIX

Y

Y - OK NH,

XXVIIl

XXX

1651 N, OH d

I

1.5. Dicopper(II) Complexes as Structural and Functional Models for Type3

Copper Proteins

T.N. Sorrel1 et al. [67], one of the pioneers in the study of copper proteins,

have attempted to make a model for the active site of oxidized hemocyanin

derivatives. They synthesized and X-ray crystallographically characterized both p-

o-acetato and p-1,3-azido copper(I1) complexes (A) with a Cu-Cu separation >3.5 A.

X= OAc, N,

(A)

Variable temperature magnetic susceptibility measurements demonstrate strong

antiferromagnetic coupling (2J = -1800 cm.') for the azido derivative but negligible

magnetic interaction between the copper ions in the acetato bridged complex.

K.D. Karlin et al. [68] reported on the synthesis, structural and spectroscopic

comparisons of analogous phenolate and X (where X = OH, C1', Br', OBz', OAc-)

loubly bridged dinuclear copper (11) complexes (B), which act as a suitable model

For met-hernocyanin derivatives. Stmctural comparison reveal the presence of a

larger X atom results in distortion away from pure square pyramidal geometry with

an opening of the Cu-0-Cu bridging angle resulting in a greater Cu-Cu separation.

Temperature dependent magnetic measurements of the structurally characterized

complexes reveal that the halide bridged complexes are the least (25 = -335 cm")

strongly coupled and the OK bridged complex is the most strongly coupled (21

-600 em.'), while the azide complex falls in between (25 = -440 cm-').

X = N;, CI., Br, OBz, OAc-

(B)

On the basis of the C U - O ~ ~ ~ ~ ~ ~ ~ ~ - C U angles, the coupling should increase in the

d~rection OH. C: p-l,l-N; < Bi but the opposite trend is actually observed and is

attnbuted to the modulating effect of the exogenous bridges.

Rajendiran et al. [69] have studied the synthesis, spectral and

electrochemical behaviour of dicopper(I1) complex ( C ) derived from pentadentate

dinucleating ligand. Two acetates bridge the two coppers through basal and apical

pos~tions to provide the square pyramidal geometry around the distorted square base.

The variable temperature magnetic studies reveal weak antiferromagnetic interaction

(2J = -93 cm") between the two metal centers. The cyclic voltammetric behaviour

of the complexes shows the quasireversible nature of the process.

Belle et al. [70] have studied the PH induced changes of redox, specboscopic,

structural properties and catecholase activity of the dicopper(I1) complexes (D) and

(E). They also correlated the PH dependent catalytic abilities of the complexes with

changes in the coordination sphere of the metal centers. p-hydroxo bridged

complexes ffom HLC~], HLF and bH3 exhibit a catecholase activity.

R- F, CF,, OCH,

Modification of R-substituent induces a drastic effect on the catecholase activity; the

presence of an electron-donating group on the ligand increases this activity, whereas

the reverse effect is observed with an electron withdrawing group.

J.D. Crane et al. [71] have reported the systematic design, synthesis and

characterization of symmetrical and unsymmetrical dinuclear copper(I1) complexes

(F, G and H) that can be viewed as first generation models for the dinuclear copper

centers in metalloproteins and enzymes.

J. Reim and B. Krcbs [72] have studied the synthesis of a series of

symmetrical and unsymmetrical dinuclear copper(II) complexes (I and J) as a

ptential structural and functional models for the active site of catechol oxidase.

The cyclic voltammogram reveal irreversible nature of the redox process and are

attributed to changes in the coordination geometry or coordination number upon

change of the oxidation state even to the expulsion of metal ions from the

coordination sphere. Investigation of the catecholase activity of compounds shows

that these complexes have significant catalytic activity with respect to the aerial

oxidation of 3, 5-DTBC to its corresponding o-quinone. But no clear relationship

between the electrochemical properties of the complexes and the catecholase activ~ty

exists.

The symmetrical dicopper(I1) complexes (K) with exogenous bridging motifs

like OAc, OH and Br have been synthesized and studied by Kandaswamy et al. [73].

They reported that the complexes undergo quasireversible reduction steps at

negative potential. The reduction potential is exogenous donor dependent and

follows the order O M e OH> Br. The -2J value ofthe complexes calculated !?om

variable temperature magnetic moment also follows the same trend as the

:lactnx.hernical data.

R = CH,, R' = CI, CH, R = CI, CH, R = H, CH,

R"= CH,, C,Hl, R' = CH,, C,Hll X = OMe, OH, Br X = OH, Br, OAc

(K) (L) (M)

Eventhough, the study on unsymmetrical dicopper(l1) complexes is sparse,

Kandaswamy et al. also succeeded in synthesizing the complexes (L) of the same

with OH, Br and OAc as the bridging units [74]. The exogenous dependent

reduction potential is observed and it follows the order OH > Br > OAc. The lower

-21 value observed in these complexes reveal the reduction in electron density and

the distorted geometry around the copper center. The interesting feature noticed in

the complex (M) [9] is that the CufJI) ion present in the oxime compartment reduces

at a lesser potential compared to the other Cu(I1) ion present in the piperazinc

compartment because of the oxime compartment has three imino nitrogen

coordinating sites.

Fenton and coworkers [76] have studied the crystal structure of the

complexes (N and 0) . The coordination environment at each Cu atom is a square

pyramidal. The magnetic moment of complex (N) is 1.85 PB at room temperature

and the moment is practically independent of temperature down to liquid N2

temperature.

On the other hand, the magnetic moment of ( 0 ) is subnormal at room temperature

(1.19 p8 per Cu) and decreases with decreasing temperature to 0.63 peat 82 K. The

results suggest the operation of an antiferromagnetic interaction between the two

Cu(1I) ions. The complexes are also differenttated by cyclic voltammetic studies.

The facile reduction of (N) relative to ( 0 ) certainly relates to the square pyramidal

geometry about the copper atoms in (N) and the involvement of soft Br- anion. The

reduction of the Cu atoms of (0) is more difficult because the square planar

environment about the metals in an unfavorable geometric environment for Cu(1).

1.6. Scope of thia Thesis

In o rda to understand the structure and functions of the actlve site m proteln

units, it is necessary to synthesize discrete molecular systems whose metal

environmmt is similar to that of the active sites of proteins. Binuclear copper

complexes arc found to mimic the active site of the type-3 copper proteins. Besides

m~m~cking the active site, these binuclear copper(I1) complexes are found interesting

due to its significance and application in the fields of magneto chemistry, catalysis,

matenal science, superconductivity and redox chemistry.

One way to mimic the dinuclear slte of type-3 copper proteins with model

compound is to make use of ligand systems, which can bind two metal Ions and

bring them together. Our aim is to understand the influence of ligand on the

structure, spectral, electrochemical, magnetic and catalytic propemes. Hence we

have focused on the building block for the ligand and their corresponding

complexes. End-off compartmental hgands arc generally used slnce they provlde

d~stlnct coordination environments. Many of this subgroup binucleating ligands are

derived from a 2, 6-disubstituted phenol. Ligands of this type strongly favour the

formation of dimetallic species because of the enforced ideal distance between the

donor sets and the presence of the endogenous bridging phenolate group. Moreover,

they also provide space for the coordination of one or two exogenous bndgiilg

llgand to the metal center.

The goal of research descnbed in this thesis is the synthesis and

characterization of symmehical and unsymmetrical binucleating ligands and thelr

dicopper(I1) complexes with exogenous bridging motifs.

The research approach described in this thesis can be divided into 6 chapters.

A literature overview of a number of multidentate symmemcal and unsymmetrical

monophmoxide dinucleating tigands and their dicopper(I1) complexes which act as

a potential structural and functional models for the active site of type-3 copper

proteins are discussed in chapterl. In chapter 2, various instmentation techniques,

which are employed to characterize the ligands and to study the properties of the

complexes, are discussed. Chapter 3 describes the synthesis and studies of

unsymmetrical saturated binucleating ligands and their dicopper (II) complexes.

Synthesis and studies of unsymmetrical saturated binucleating ligands and their

dicopper(I1) complexes arc discussed in chapter 4. In chapter 5, synthesis, spectral,

electrochemical, magnetic and catecholase activity of dicopper(I1) complexes

derived from unsymmetrical unsaturated ligands are discussed. Chapter 6 contains

general conclusions and suggestions for further research and ends with the list of

publications that are resulted from this thesis.

1. E.I. Solomon, B.L. Hemming and D.E. Root, in Bioinorganic Chemistry of

copper, K.D. Karlii and Z. Tyeklar (Eds.), Chapman & Hall, London, 1993,

Metal ions in Biological systems, H . Sigel (Ed.), Marcel Dekker Inc., New

York, 1981, 13.

2. (a) E.I. Solomon and M.D. Lowery, Sclence, 1993,259, 1575.

(b) J.A. Guckert, M.D. Lowery and E.I. Solomon, ' Am. Chem. Soc., 1995,

117,2817.

3. S. Larson, A. Broo and L.J. Sjolin, Phys. Chem., 1995, 99, 4860.

4. E.I. Solomon, M. J. Baldwin and M.D. Lowery, Chem. Rev , 1992,92,521.

5. K.A. Magnus, H. Ton-that and J. Carpenter, Chem. Rev., 1994, 94, 727.

6. N. Kitajima and Y. Moro-oka, Chem. Rev., 1994, 94, 737.

7. E.I. Solomon, M.J. Baldwin and M.D. Lowery, Chem. Rev., 1992,92,521.

8. N . Kitajima and Y. Moro-oka, Chem. Rev., 1994, 94, 737.

9 N. Kitajima and Y. Moro-oka, Chem. Rev., 1994,94, 737.

10. M. Tremolicres and J.B. Bieth, Phylochem~st~ . 1984, 23, 501.

11. D. Meiwes, B. Ross, M. Kiesshauer, K. Cammann, H. Witzel, M. Knoll, M.

Borchardt and C. Sandermaier, Lab. Med., 1992, 15, 24

12. K. Lerch, Enrym. Brown. Prev., 1995, 600,64.

13. A. Rompel, H. Fischer, K. Buldt-Karentzopoulos, D. Meiwes, F. Zippel,

H.H. Nolting, C. Hermes, B. Krebs and H. Witzel, J. Inorg. Biochem., 1995,

59, 715.

14. (a) N.C. Eickman, R.S. Himrncrwright and E.I. Solomon, Proc. Acad. Sci.

USA. 1979,76,2094.

@) E.I. Solomon, M.J. Baldwin and M.D. Lowny, Chem. Rev., 1992, 92,

521.

15. F. Zippel, F. Ahlcrs, B. Krebs, S. Behning, K. Buldt-Karcnt~~poulos, H.

Witzel and M. Oversluizen, Daresbury Annual Report, D a r e s b q

Laboratory, Daresbury, UK, 1994,95, 102.

16 B. Krebs, K. Buldt-Karentzopoulos, C E~cken, A. Rompel, H. Wltzel, A.

Feldrnann, R. Kruth, J. Reim, W. Stemforth, S. Te~pel, F. Z~ppel, S.

Schindler and F. Wiesemann, in: DFG Deutsche Forschungsgemeinschaft

(Ed.), Bioinorganzc Chemistry: Transztzon Metals m Biology and thezr

Coordination Chemistry, D.14, VCH, Welnheim, 1997, p. 616.

17. W. Kaim and J. Rall, Angew. Chem. Int, Ed Engl., 1996, 35,43.

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20 D.E. Fenton, Inorg. Chem. Commun, 2002,5,537.

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Hutchinson and J. Zubieta, J. Am. Chem. Soc., 1984,106,2121.

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Chem. Soc., 1997,119,9424.

23. N.N. Murthy, M. Mahroof-Tahir and K.D. Karlin, Inorg. Chem., 2001, 40,

628.

24. F. Meyer and P. Rutsch, Chem. Commun., 1998, 1037.

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26. P. Gamez, J. Von Harras, 0 . Roubeau, W.L. driesscn and 1. Reedjik, Inorg.

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28. T. Klabunde, C. Eicken, J.C. Sacchcttini and B. b b s , Nut. struct. Biol. 5

(dec), 1998, 1084.

29. E.I. Solomon, U.M. Sundaram and T.E. Machonkin, Chem. Rev., 1996, 96,

2563.

30. R, Gupta, S. Mukherjee and R. Mukherjee, J. Chem. Soc. Dalton Trans.,

1999,4025.

31. J. Reim and B. Krebs, J. Chem. Soc. Dalton Trans., 1997, 3793.

32. (a) C.K. Williams, N.R. Brooks, M.A. Hillmycr and W.B Tolman, J. Chem.

Soc Chem. Commun., 2002,2132.

(b) N.V. Kaminskaia, B. Spingler, and S.J. Lippard, J Am. Chem. Soc. ,

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6728.

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34 T.M. Rajendiran, R. Venkatesan, P.S. Rao and M. Kandaswamy,

Polyhedron, 1998,17,3427.

35. M.S. Mashuta, R.J. Webb, J.K. Mccushker, E.A. Schmit, K.J. Oberhausen,

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36. P. Amudha, M. Kandaswamy, L. Govindaswamy and D. Velmumgan, Inorg.

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37. (a) M.A. de Brito, A.J. Bortoluzzi, A. Gnatti, A.S. Ceccato, A.C. Joussef

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38. T.N. Sorrell, C.0' Conner, O.P. Anderson and J.H. Reibenspies, J. Am.

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39. (a) S. Uhlenbrock and B. Krcbs, Angew Chem. Int. Ed. Engl., 1992, 31,

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40. A.J. Bortoluzzi, A. Neves, I. Vencato, C. Zucco and M. Homer, Acta

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41. T. Ookubo, H. Sug~moto, T. Nagayama, H. Masuda, T. Sato, K. Tanka,

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43 K. Schepers, B. Bremer, B. Krebs, G. Henkel, E. Althaus, B. Mosel and W.

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44. A.S. Borovik, M.P. Hendrich, T.R. Holman, E. Munch, V. Papaefiymiou

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46. S. Torclli, C. Belle, I. Gauticr-Luneau, S. Hamman and J.L. Pierre, Inorg.

Chrm. Acta. 2002,333, 144.

47. H.-R.Chang, H. Diril, M.J. Nilges, X. Zhang, J.A. Potenza, H.J. Schugar,

D.N. Hmdrikson, S.S. Isied, J, Am. Chem. Soc., 1988, 110, 625.

48. J. Reim and B. Krebs J. Chem. Soc.. Dalton Trans., 1997,3793.

49. M. Lubben and B.L. Feringa, J. Org. Chem., 1994,59,2227.

50. J.D. Crane, D.E. Fenton, J.M. Latour, A.J. Smith, J. Chem. Soc. Dalton

Trans., 1991,2979.

51. M. Lubben, R. Hage, A. Meetsma, K. Byma and B.L. Feringa, Inorg. Chem.,

1995,34,2217.

52. P. Kamaras, M.C. Cajulis, M. Rapta, G.A. Brewer and G.B. Jameson, J Am.

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