Monohelical Metal Complexes of a Bis-Bidentate Schiff Base with a ShortRigid Spacer. The Spontaneous Resolution of P-[Ni(FTs)]·CH3CN

Miguel Vazquez, Manuel R. Bermejo*, Matilde Fondo, Ana M. Garcıa-Deibe, Ana M. Gonzalez,Rosa Pedrido, and Jesus Sanmartın

Santiago de Compostela/Spain, Departamento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela

Received December 13th, 2001.

Dedicated to Professor Joachim Strähle in the Occasion of his 65th Birthday

Abstract. The electrochemical reaction of the bis-bidentate Schiffbase H2FTs [N,N�-bis(2-tosylaminobenzylidene)-1,2-diaminoben-cene] with cobalt, nickel, copper, zinc and cadmium, lead to theisolation of neutral [M(FTs)] complexes. All of them were charac-terized by elemental analyses, mass spectrometry, IR and 1H NMRspectroscopy and magnetic measurements, where appropriate.Recrystallization of the nickel complex yields single crystals of[Ni(FTs)]·CH3CN (1). The x-ray characterization shows a distorted

Monohelikale Metallkomplexe einer bis-zweizähnigen Schiff ’schen Base mit kurzenAbstandshaltern. Spontane Enantiomerentrennung von P-[Ni(FTs)] · CH3CN

Inhaltsübersicht. Die elektrochemische Reaktion der bis-zweizähni-gen Schiff ’schen Base H2FTs [N,N�-bis(2-tosylaminobenzyliden)-1,2-Diaminbenzol] mit Cobalt, Nickel, Kupfer, Zink und Cadmiumführt zu den neutralen Komplexen [M(FTs)]. Alle Verbindungen,so weit möglich, wurden durch Elementaranalysen, Massenspek-tren, IR- und 1H-NMR Spektren charakterisiert. Umkristallisation

Introduction

Interest in inorganic helical complexes derives from theirintrinsic chirality and their potential for yielding materialswith novel chemical, electronic, biological or topologicalproperties [1�5]. In the recent years, much effort has beendevoted to the design and synthesis of ligands capable ofwrapping around one or more metal centres in order toobtain helical compounds. In particular, a number of at-tractive bis-bidentate Schiff base ligands have been recentlyreported [6�16]. Most of these ligands contain long rigidand/or semirigid spacers between the binding domains [10,11, 13, 15�16] that predispose the Schiff base to be par-titioned off and to co-ordinate to two or more metal atoms.However, short semirigid or flexible spacers can also giverise to dinuclear complexes. In this way, van Koten described

* Prof. M. R. BermejoDepartamento de Quımica Inorganica, Facultade de QuımicaUniversidade de Santiago de Compostela15706 Santiago de Compostela / SpainFax: �34-981-597525E-mail: quimb45@1sc.es

1068 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044�2313/02/628/1068�1074 $ 20.00�.50/0 Z. Anorg. Allg. Chem. 2002, 628, 1068�1074

square-planar environment for the nickel atom, with the Schiff baseacting as a tetradentate N4 donor. Complex 1 can be described as amononuclear single-stranded helical compound, with spontaneousresolution of the P enantiomer upon crystallisation.

Keywords: Cobalt; Nickel; Copper; Zinc; Cadmium; Schiff bases;Helical complexes; Electrochemical synthesis

des Nickelkomplexes ergibt Einkristalle von [Ni(FTs)]·CH3CN (1).Die röntgenographische Charakterisierung zeigt verzerrt-quadra-tisch-planare Koordination für das Nickelatom mit der Schiff ’-schen Base als vierzähnigen N4-Donator. 1 kann als einsträngige,helikale Verbindung beschrieben werden mit spontaner Enantiome-rentrennung bei der Kristallisation.

that potential tetradentate Schiff bases, containing cyclo-hexane or ethane between the N-imine atoms, yield di-nuclear boxes with ZnII, CuII, and AgI [6]. These short spa-cers, with two carbon atoms separating the N-imine donors,were then reintroduced by Wild [7] and Fabbrizzi [9], withsimilar results. In addition, some helical complexes wereisolated with rigid spacers that set the imine moieties sepa-rated by a benzene group [16] but not many studies werereported with this type of ligand.

We have recently designed some bis-bidentate Schiff baseligands with bulky tosyl substituents on the bidentate do-mains [17], that should induce torsion of the ligand and,

Monohelical Metal Complexes of a Bis-Bidentate Schiff Base with a Short Rigid Spacer

Table 1 Elemental analysis and some selected data for the complexes

Compound Yield(%) Colour Analysis (%)a) FAB (amu)b) µ (BM)

%C %H %N %S

H2FTs 85 yellow 65.6(65.6) 4.9(4.9) 9.1(9.0) 10.4(10.3) 623.3Co(FTs) 73 garnet 60.2(60.1) 4.0(4.1) 8.3(8.2) 9.3(9.4) 680.1 4.5Ni(FTs) 80 red 60.1(60.1) 4.0(4.1) 8.4(8.3) 9.3(9.4) 679.1 �Cu(FTs) 73 green 59.2(59.7) 4.1(4.1) 8.4(8.2) 9.0(9.4) 684.0 2.2Zn(FTs) 87 yellow 59.1(59.5) 3.9(4.1) 8.0(8.2) 9.3(9.3) 685.1 �Cd(FTs) 77 yellow 55.4(55.6) 3.9(3.8) 7.6(7.6) 8.2(8.7) 734.0 �

a) Found (calculated); b) Peaks corresponding to [ML]�

Table 2 Selected IR bands (in cm�1) for the ligands and com-plexes

Compound ν(N-H) ν(C�N) ν(C-N) νas(SO2) νs(SO2)

H2FTs 3487 br 1615 s 1338 s 1291 s 1160 sCo(FTs) 1605 s 1281 s 1242 m 1134 sNi(FTs) 1605 s 1281 s 1249 m 1138 sCu(FTs) 1605 s 1281 s 1248 m 1134 sZn(FTs) 1613 s 1281 s 1239 m 1138 sCd(FTs) 1613 s 1288 s 1258 m 1130 s

therefore, a helical disposition of the Schiff base. In fact,these ligands, which contain flexible or semi-rigid spacers,always generate mononuclear and/or dinuclear helical com-plexes [17].

As an extension of this work and by merging our resultswith those previously reported, we have prepared the newH2FTs ligand (Scheme 1), with a short rigid benzene group(two carbon atoms) between the two imine moieties. Itscrystal structure [18] shows an unusual torsion of this typeof rigid ligand that could do it useful for isolating helicalcomplexes upon co-ordination to a metal centre. Therefore,the interaction of H2FTs with cobalt, nickel, copper, zinc,and cadmium under an electrical current was investigatedand the results achieved are described herein.

Results and Discussion

Characterization of the ligand

The Schiff base H2FTs [IUPAC name: N,N�-bis(2-tosylami-nobenzylidene)-1,2-diaminobencene] gave satisfactorily el-emental analysis (Table 1) and was further characterised byIR (Table 2), 1H NMR spectroscopy (Table 3) and FABmass spectrometry (Table 1). Besides, the structure of theligand was known by x-ray diffraction studies [18], showinga helical disposition. The space group and the analysis ofthe crystal data indicate the formation of a racemic mixture,with both P and M enantiomers present in the unit cell in50 % yield.

Synthesis and characterization of the metal complexes

All the complexes were prepared by an electrochemicalmethod previously described [17]. The electrochemical ef-

Z. Anorg. Allg. Chem. 2002, 628, 1068�1074 1069

ficiency of the cell Ef was close to 0.5 mol·F�1 in all cases,which is compatible with the reaction mechanism shown inscheme 2.

Elemental analyses (Table 1) show that all metals reactwith the ligand in molar ratio 1:1 to afford complexes ofthe bis-deprotonated ligand [FTs]2� with high purity. Allthese neutral metal complexes are obtained in good yieldsand appear to be stable the solid state and in solution.

Solid state studies1. X-ray studies

An ORTEP diagram of [Ni(FTs)]·CH3CN (1) is shown inFigure 1. Experimental details are recorded in Table 4 andmain distances and angles are contained in Table 5.

The x-ray structure shows that complex 1 consists ofmononuclear neutral Ni(FTs) molecules with acetonitrileas solvate.

The ligand uses its four nitrogen atoms (two imine andtwo amide) to link the nickel atom. Furthermore, one oxy-gen atom of each tosyl group is weakly interacting with themetal centre [distances O(1)···Ni(1) and O(3)···Ni(1) of ca.2.8 A]. These distances are too long to be considered astrue co-ordinated bonds and should be best described assecondary intramolecular interactions. Therefore, the metalcentre is in a tetraco-ordinated unsatured environment, withthe tosyl groups playing an important role in the adoptedco-ordination number.

The Ni(1)-N distances are in the range of those expectedfor tetraco-ordinated nickel complexes [14, 16�17], cor-roborating the non-coordination of the tosyl groups. Thedihedral angle θ between the two NiNN terminal planes(ca. 20.8°) indicates that the NiII ion displays a tetrahedrallydistorted square planar co-ordination. This co-ordination isalso reflected by the bond angles (Table 5). The four anglessubtended at nickel by adjacent nitrogen atoms range from85.6(2) to 94.9(2) and the two angles between the nickelcentre and opposite N-donors range from 163.3(2) to165.4(2), showing a slightly distorted square planar situ-ation.

M. Vazquez, M. R. Bermejo, M. Fondo, A. M. Garcıa-Deibe, A. M. Gonzalez, R. Pedrido, J. Sanmartın

Table 3 Selected 1H NMR data (in ppm)

H2�H2� H3 H1�H H6 H7 H4 H8 H5 H9 CH3

H2FTs1 7.21 (m, 2H) 8.71 (s, 2H) 7.41 (m, 4H) 7.32 (d, 2H) 7.83 (d, 2H) 7.47 (d, 4H) 7.15 (t, 2H) 7.07 (d, 4H) 2.21 (s, 6H)Ni(FTs) 7.48 (m, 2H) 9.32 (s, 2H) 7.84 (m, 2H) 7.34 (t, 2H) 6.95 (d, 2H) 7.67 (m, 6H) 7.04 (m, 6H) 2.24 (s, 6H)Zn(FTs) 7.54 (m, 2H) 9.01 (s, 2H) 7.90 (m, 2H) 7.25 (m, 4H) 7.63 (d, 2H) 7.67 (d, 4H) 6.99 (t, 2H) 7.11 (d, 4H) 2.27 (s, 6H)Cd(FTs) 7.50 (m, 2H) 8.93 (s, 2H) 7.71 (m, 2H) 7.24 (m, 4H) 7.65 (d, 2H) 7.79(d, 4H) 6.95 (t, 2H) 7.18 (d, 4H) 2.29 (s, 6H)

1 NH: 12.34 (s, 2H)

Table 4 Crystal data and structure refinement[Ni(FTs)]·CH3CN (1)a)

Empirical formula C36H31N5O4NiS2

Formula weight 720.49Crystal system orthorhombica /A 12.1187(6)b /A 13.3008(6)c /A 21.2539(10)Volume /A3 3425.9(3)Temperature /K 298(2)Space group P212121

Z 4Absorption coefficient /cm�1 0.735Reflections collected 12390Independent reflections 6055[Rint � 0.0732]Final R indices [I>2σ(I)] R1 � 0.0499

wR2 � 0.0835R indices (all data) R1 � 0.1121

wR2 � 0.1010

a) CCDC 175799 contains the supplementary crystallographic data for thispaper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/const/retrieving.html (or from the Cambridge Crystallographic Data Centre,12, Union Road, Cambridge CB2 1EZ, UK; fax: �44 1223 336033; or de-posit@ccdc.cam.ac.uk)

Table 5 Selected bond lengths/A and angles/° for[Ni(FTs)]·CH3CN (1)

Ni(1)-N(1) 1.909(4) N(1)-Ni(1)-N(2) 91.8(2)Ni(1)-N(2) 1.872(4) N(1)-Ni(1)-N(3) 165.4(2)Ni(1)-N(3) 1.862(4) N(1)-Ni(1)-N(4) 94.9(2)Ni(1)-N(4) 1.905(4) N(2)-Ni(1)-N(3) 85.6(2)

N(2)-Ni(1)-N(4) 163.3(2)N(3)-Ni(1)-N(4) 91.6(2)

The deviations from the mean square N(1)N(2)N(3)N(4)calculated plane corroborate the proposed structure (maxi-mum deviation of any atom of �0.259 A, with the nickelatom practically in the plane). Two of the N-atoms [N(1)and N(3)] are displaced above and two [N(2) and N(4)] be-low the plane, showing the tetrahedral distortion. This dis-tortion implies a high strain of the ligand, which should bemore planar due to conjugation. The degree of torsion isalso inferred from the angles between chelate rings: the li-gand forms three chelate rings [N(1)C(8)C(13)C(14)N(2),N(2)C(15)C(20)N(3) and N(3)C(21)C(22)C(27)N(4)], witha maximum half-step distance of any atom from the mean-square calculated planes of �0.071 A. The angles betweenadjacent planes are 23.09° and 25.15°, respectively, andshow the screwed disposition of the Schiff base. The twist-

Z. Anorg. Allg. Chem. 2002, 628, 1068�10741070

Figure 1 An ORTEP view of complex 1 (thermal ellipsoids aredrawn at 30 % of probability level).

Figure 2 Space-filling representation of 1 showing the helical dis-position of the ligand.

ing of the ligand can be attributed to an attempt of theadditional peripheral tosyl groups to minimise the sterichindrance, that are orientated in different directions, withan angle between their benzylidene moieties of ca. 39.20°.This situation leads to a helical wrap of the ligand aroundthe metal centre (Fig. 2) and, therefore, complex 1 can bedescribed as a mononuclear single-stranded distortedsquare planar helical complex, with a wrapping angle ofca. 270°.

It is remarkable that only the right handed (P) helix [3]is present in the unit cell, while both P and M enantiomerscan be observed for the free ligand in solid state [18]. Thus,the crystallisation of 1 occurs with spontaneous resolution,giving rise to an enantiomeric pure P helix. Refinement of

Monohelical Metal Complexes of a Bis-Bidentate Schiff Base with a Short Rigid Spacer

Table 6 Comparison of some selected distances/A and angles/° in 1 and similar related complexes

[Ni(PTs)]2·CH3CNa) [Ni(CyTs)]·CH3CNb) [Ni(FTs)]·CH3CN (1)Ni-Nimine 1.999(6); 2.017(5) 1.883(3); 1.872(3) 1.872(4); 1.862(4)

2.016(6); 2.018(6)Ni-Namide 1.991(5); 1.985(5) 1.925(3); 1.918(3) 1.909(4); 1.905(4)

1.977(5); 1.971(5)Ni···Otosyl 2.460, 2.472; 2.451, 2.504 2.867; 2.857 2.862; 2.827α(NiNN)/(NiNN) 95.2; 96.7 15.9 20.8Nimine-Ni-Nimine 101.0(2); 103.5(2) 85.85(14) 85.6(2)Namide-Ni-amide 151.7(2); 149.8(2) 95.45(15) 94.9(2)Nimine-Ni-Namide 91.7(2), 91.5(2); 91.9(2), 91.7(2) 91.11(14); 89.81(14) 91.8(2); 91.6(2)a

103.9(2), 108.8(2); 107.6(2), 106.2(2) 167.87(15); 167.54(15) 165.4(2); 163.3(2)References [17a] [17c] This work

a) H2PTs � N,N�-bis(2-tosylaminobenzylidene)-1,3-diaminopropane; b) H2CyTs � N,N�-bis(2-tosylaminobenzylidene)-1,2-diaminocyclohexane

the Flack enantiopole parameter lead to a value of ca. zero,confirming the enantiomeric purity of the crystals. The unitcell of some other crystals was checked, giving the sameparameters as 1.

The isolation of chiral helical complexes where a givenhelicity is not induced by the presence of chiral groups orconfigured ligands is still scarcely described in literature[19�24] and complexes usually crystallise as racemates. Inour case the reaction of H2FTs with nickel proceeds withchiral self-recognition and complex 1 is isolated as the en-antiomerically pure P conformer. The chirality in this casestudy seems to be a consequence of the attempt of the tosylgroups to avoid steric hindrance. In addition, edge-to-faceπ stacking interactions are observed between the centralaromatic spacer and two binding bencylidene moieties oftwo different neighbouring units.

It should be noted that some nickel complexes with thiskind of ligand, containing tosyl groups, were previously de-scribed [17] and that spontaneous resolution does not pro-ceed. This could be attributed to the different rigidity of thespacer: all the other ligands contain flexible or semi-rigidspacers that can easily twist in order to avoid the steric hin-drance. In this case study the spacer is rigid and its twistingis hindered by the conjugation of the ligand.

Finally, the comparison of 1 with the mentioned relatednickel complexes [17] (Table 6) allows drawing some con-clusions:

1. Nickel forms dinuclear complexes with tosylatedSchiff bases containing aliphatic chains of 3 carbon atomsbetween the imine groups, while give rise to mononuclearhelixes when the imine moieties are connected via two car-bons atoms belonging to semirigid (cyclohexane) or rigid(benzene) groups. Only the rigid ligand allows the isolationof an enantiomerically pure complex (1).

2. The presence of the bulky tosyl groups imposes a co-ordination number of 4 for the nickel atom, independentlyof the length and rigidity of the ligand, as well as of thenuclearity of the helix.

3. The structure of the complex seems to be dictated bythe length and flexibility of the spacer. 1 and[Ni(CyTs)]·CH3CN are square planar (θ � 20.8 and 15.9°,respectively) while [Ni(PTs)]2·CH3CN is tetrahedral (θ �95.2 and 96.7°).

Z. Anorg. Allg. Chem. 2002, 628, 1068�1074 1071

4. The different structure of the three complexes isclearly reflected in the bond lengths and angles (Table 6):for 1 and [Ni(CyTs)]·CH3CN the Ni-Nimine bonds areshorter than the Ni-Namide ones while this does not occursin [Ni(PTs)]2·CH3CN. Furthermore, complex 1 and[Ni(CyTs)]·CH3CN show shorter Ni-Nimine and Ni-Nam-ide bonds than [Ni(PTs)]2·CH3CN. In addition, the second-ary intramolecular interactions with the tosyl groups seemto be stronger for tetrahedral complexes.

2. IR spectroscopy

The IR spectra of all complexes show a strong band at ca.1600 cm�1, attributed to ν(C�N) (Table 2). This supposesa negative shift of 2�10 cm�1 respect to the free ligand.Besides, the ν(C-N) stretching frequency is shifted to lowerwavenumbers in all the complexes. This behaviour is com-patible with the participation of both imine an amide nitro-gen atoms in the co-ordination to metal centres. In ad-dition, two bands in the ranges 1239�1258 cm�1 and1130�1138 cm�1 are assigned to the asymmetric and sym-metric vibration modes of the SO2 group [25�26]. Finally,no bands above 2500 cm�1 were detected for any of thecomplexes, indicating the deprotonation of the NH groupsand, therefore, the dianionic character of the ligand.

3. Magnetic Characterization

Magnetic measurements at room temperature of the para-magnetic cobalt and copper complexes were performed.The values of the magnetic moments are close to those ex-pected for magnetically diluted MII ions, confirming theoxidation state �II for the central atom and, therefore, thedeprotonation of the Schiff base ligand. The value for thecopper complex does not allow drawing any other con-clusion. However, the magnetic moment of Co(FTs) is inagreement with a high spin d7 system in a tetrahedral en-vironment.

Besides, the magnetic moment of 1 at room temperaturewas checked. Despite the tetrahedral distortion from thesquare-planar environment for the nickel atom shown by x-ray studies, 1 is found to be diamagnetic in nature in solid

M. Vazquez, M. R. Bermejo, M. Fondo, A. M. Garcıa-Deibe, A. M. Gonzalez, R. Pedrido, J. Sanmartın

state and, therefore, low-spin. This also occurs in solution,as the 1H NMR studies (below) supports.

Solution studies1. FAB mass spectrometry

FAB mass spectra show peaks assigned to [ML]� fragments(Table 1) for all the complexes, corroborating the co-ordi-nation of the ligand to the metal. However, no peaks as-signed to [ML]2� fragments could be detected for any ofthe complexes, suggesting a mononuclear nature of all ofthem in solution.

2. 1H NMR spectroscopy

The 1H NMR spectra of the free ligand, nickel, zinc andcadmium complexes were recorded in DMSO-d6 as solventat room temperature (Table 3) and assigned by COSYexperiments. The diamagnetic nature of the nickel complexconfirms that it retains the square-planar geometry in solu-tion. The spectrum of 1, recorded at different times, showno co-ordination of the DMSO-d6 solvent, suggesting thatthe secondary interactions present in solid state should re-main in solution.

The 1H NMR spectrum of the free ligand shows con-siderable temperature and time dependence. When the spec-trum is recorded in DMSO-d6 at room temperature immedi-ately after solution of the sample, a single set of resonances(a) is shown, indicating the presence of a single species andsymmetry of the two bis-bidentate domains. If the solutionis left to stand, a second set of resonances (b) appears andthe intensity of these signals depends upon time. After 12 h.standing, the ratio of both ligand species is 2:1 (b:a), basedon the imine hydrogen atom. However, when the ligand isin solution for a long time (i.e 2 days), it hydrolyses. Thedynamic equilibrium and hydrolysis of the ligand are fav-oured by temperature: the spectrum recorded immediatelyat 60 °C, shows two different set of signals of relativelyintensities 2:1 (b:a). Besides, the intensity of the carbonylhydrogen atom indicates that hydrolysis is also faster onraising the temperature.

The spectrum of the major species at room temperatureafter solution of the sample (a) was fully assigned (Table 3)and it is of note that the hydrogen atoms of the phenylspacer (H1 and H1�, H2 and H2�) are not equivalent, asthey appear as multiplets. This result could indicate that ahelical isomer of the ligand remains in solution, retainingthe solid state structure [27]. However, when the solution isleft to stand, a second diastereoisomer (b) exists as themajor species in equilibrium with the first one (a). It hasnot been possible to assign all of the signals of the secondisomer due to the overlap of peaks between the two dia-stereoisomers and with the initial products, as a result ofthe partial hydrolysis of the ligand. Therefore, the spectrumat low temperature (�20 °C) in CD3CN was recorded. Thisexperiment shows that the hydrolysis process is avoided onlowering the temperature but the severe overlapping of the

Z. Anorg. Allg. Chem. 2002, 628, 1068�10741072

peaks still prevents accurate assignments of all the signalsof the second isomer (b).

The spectra of the complexes show just a singlet set ofproton resonances and they do not change with time, indi-cating a high symmetry and a single species in solution.

The comparison of the 1H NMR spectrum of the “a”isomer of the free ligand with those of the complexes atroom temperature, show some interesting aspects (Table 3):

1. In all cases, it is observed the disappearance of the NHprotons present in the free ligand, in agreement with thebis-deprotonation of the Schiff base.

2. The imine hydrogen atom (H3) shows a shift to lowerfield for all the complexes, indicating a withdrawal ofcharge from the imine group upon co-ordination. Thiseffect is more pronounced for the nickel complex (ca.0.6 ppm).The imine protons of the cadmium complex are flankedby satellites arising from spin-spin coupling to 111/113Cd,indicating that the complex is kinetically inert on theNMR timescale.

3. The aromatic protons of the phenyl spacer are the mostaffected ones, except H3, on co-ordination and they areshifted to lower field. This shows a drain of charge fromthe phenyl ring. These protons appear as two multipletsignals, each peak being the result of the overlapping oftwo doublets. Therefore, these protons are not moreequivalent, and this observation could be a hint of a heli-cal conformation of the complexes, as reported elsewhere[27], which is the only diastereoisomer present in solu-tion.

4. The protons of the central binding moiety are displacedupfield. The higher displacements are observed for H4and H5. In fact, H5 is the aromatic proton at higherfield in the complexes except for 1, where H7 appears atlower frequency. Therefore, it seems that the co-ordi-nation of the amide nitrogen atom induce a greater drainof charge of the ring towards this nitrogen atom in 1.

5. The aromatic proton of the tosyl groups H8 is signifi-cantly displaced to higher field (0.32�0.2) with respectto the free ligand. This could be attributed to some kindof interaction between the tosyl group and the metal cen-tre, as the x-ray structure of 1 confirms in solid state.

As a result of all the characterisation techniques em-ployed in this work, some general considerations could bemade. All complexes seem to be mononuclear, as the FABmass spectrometry suggests. In addition, 1H NMR studiesindicate that the solid state structure of 1 is retained in solu-tion. Besides, the 1H NMR splitting of the aromatic protonsof the phenyl spacer seems to indicate a helical nature forthe diamagnetic compounds, with just one diastereoisomerin solution. The magnetic studies and the similar IR pat-terns, as well as comparison with other well characterisedcompounds [17], agree with the non-coordination of the to-syl groups and, therefore, with a co-ordination number of4 for all the metal centres.

Monohelical Metal Complexes of a Bis-Bidentate Schiff Base with a Short Rigid Spacer

In summary, it seems that Co, Ni, Cu, Zn and Cd formsquare-planar (Ni) or tetrahedral (Co, Cu, Zn and Cd)mononuclear helical complexes with the Schiff base H2FTS,which contains a rigid phenyl spacer between the biden-tate moieties.

Conclusions

The racemic mixture of the helical Schiff base H2FTS reactswith Ni, Co, Cu, Zn and Cd to give tetraco-ordinated com-plexes. They seem to be mononuclear and helical in solidstate and in solution, with spontaneous resolution of the Penantiomer of the nickel complex upon crystallisation. Themetal complexes exist as a single diastereoisomer in solu-tion, in spite of the equilibrium between two isomers in thefree ligand.

Experimental Section

General: Elemental analysis was performed on a Carlo Erba EA1108 analyser. The NMR spectra were recorded on a BruckerAMX-500 spectrometer using DMSO and CD3CN as solvents. In-frared spectra were recorded as KBr pellets on a Bio-Rad FTS 135spectrophotometer in the range of 4000�600 cm�1. Mass spec-trometry (FAB) was performed on a Micromass Autospec massspectrometer, employing m-nitrobenzyl alcohol as a matrix. Mag-netic measurements at room temperature were performed using aSherwood Scientific Magnetic Susceptibility Balance, calibratedwith mercury tetrakis(isothiocyanato)cobaltate(II).

Syntheses: All the starting materials were purchase from Aldrichand used without further purification, except 1,2-diaminobencenethat was recrystallised from methanol. 2-tosylaminobenzaldehydewas prepared according to a method previously reported in litera-ture [28�29].

N,N�-bis(2-tosylaminobenzylidene)-1,2-diaminobencene (H2FTs): Toa solution of 2-tosylaminobenzaldehyde (5.0 g, 18.2 mmol) inchloroform (125 mL) 1,2-diaminobencene (0.98 g, 9.1 mmol) wasadded and the mixture was refluxed. The volume of the solutionwas reduced over a 4 h. period to ca. 50 mL, using a Dean-Starktrap, and was then allowed to cool. The resultant yellow solutionwas concentrated to ca. 10 mL. The solid, which precipitated, wascollected by filtration, washed with diethyl ether (3 mL), dried invacuo and recrystallised from chloroform [18].

Synthesis of the Metal Complexes: All complexes were prepared ina similar way, that can be exemplified by the isolation of[Ni(FTs)].CH3CN 1: A solution of H2FTs (0.1 g, 0.16 mmol) inacetonitrile (80 mL), containing ca. 10 mg of tetramethylam-monium perchlorate as supporting electrolyte, was electrolysed un-der a nitrogen stream for 1 h. 43 min. using a current of 5 mA.Concentration of the resultant solution to a third of its initial vol-ume yielded a red-brown solid that was washed with diethyl etherand dried in vacuo. Recrystallisation of the obtained powder inacetonitrile yields single crystals of [Ni(FTs)]·CH3CN 1, suitablefor x-ray diffraction studies.

The other complexes were obtained by the same method, using theappropriate metal anode.

Crystallographic measurements. Crystal data and refinement detailsof 1 are given in Table 4.

Z. Anorg. Allg. Chem. 2002, 628, 1068�1074 1073

Crystals of 1 suitable for single x-ray diffraction studies were ob-tained as previously described. Data were collected at room tem-perature using a Siemens CCD diffractometer, employing graphite-monochromated Mo-Kα (λ � 0,71073 A) radiation. The structureswere solved by direct methods [30�31] and refined by full matrixleast squares on F2. An absorption correction (Sadabs) was ap-plied. All hydrogen atoms were included in calculated positions.

Acknowledgements: The authors thank Xunta de Galicia(PGIDT99PXI20903B) and Ministerio de Educacion y Ciencia(Spain) (PB98-0600) for financial support. M.V. thanks FundacionSegundo Gil Davila for a grant.

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