ELSEVIER Inorganica Chimica Acta 266 (1997) I-3

Preliminary Communication

Metal complexes of quadridentate Schiff bases which can form a four-membered chelate ring

Alessandro Pasini ~'*, Rosa Pia Ferrari b Sarah Lanfranconi ¢ Andrea Pozzi ~ Enzo Laurenti b Maurizio Moroni a

a Dipartbnento CIMA and CNR Centre, Universitfi di Milano. Via Venezian 21, 20133 Milan, Italy b Dipartimento di Chhnica IFM. Universitd di Torino. Via Giuria 7. !0125 Turin, Italy c istimto di Scienze MM FF CC, Oniversitt'~ di Milano, Via Lucini 3. 22100 Come, Italy

Received 17 April 1997

Abstract

The metal complexes of deprotonated N,N'-bis(2-hydroxybenzylidene)arylmethanediimines show a strong preference for tetrahedral structures. Oxidation of the Co(ll) derivatives gives compounds with S ~- !. © 1997 Elsevier Science S.A.

Keywords: Metal complexes; Schiff base complexes

1. Introduction

The anionic quadridentate ligands, obtained by deproton~ ation el' L.tH:=i)H~ III (see Scheme I), resenable the well known s~den, except that tile N~CHAr~-N moiety of the fl)r- met could Ibrm a fouromembered chelate ring. We reasoned that thi~ could produce distorted coordination geometries and give rise to metal complexes with novel and interesting prop= erties. To our knowledge the only reported complex wilh this type of ligand is an Mo(VI) derivative of stoichiometry Mo~L 4 (L 4, R ~ H), in which L '~ bridges two molybdenum atoms 121. We report here a preliminary account on 1/i complexes of L I-L "~ with Co (II), Ni (11), Cu (I1) and Zn (II), and an interesting result obtained by oxidation of the Co(ll) derivatives.

2. Experimental

LtH~-L3H2 were prepared according to Refs. I I 1. ML~-ML 3 were obtained by refluxing, under a nitrogen

atmosphere, 40 ml of an ethanol solution of 2 mmol of the metal acetates with an equimolar amount of LH2, in the pres- ence of 0.5 ml of Et3N. The compounds were recrystailized from DMFtdiethyl ether.

* Corresponding author. Tel.: + 39-2-26 680 673; fax: + 39-2-23 62 748.

0020-1693/97 / $17.00 © 1997 Elsevier Science S.A. All rights reserved Pil S0020-1693 ( 97 ) 05 7 30-7

L 1, R = COH5; L 2, R -~ p=CH3oCoH4; L3, pNO2.CoH 4 Schel~e I,

3, Results and discussion

Elemental analyses (:1:0.3%) are in agreement with a 1 / 1 stoichiometry of the complexes. Relevant data for some selected compounds are given in Table I. The FAB mass spectra (nitrobenzylalcohol) gave peaks corresponding to both dimeric and monomeric structures (all 100% abun., dance), however the monomers prevail in the spectra taken from DMF solutions.

The effective magnetic moments of the Co(ll) derivatives (4.2=4.5 BM at room temperature) are in the lower limit of the range expected for tetrahedral Co(II) complexes (4,2- 5 BM [3]), and are similar to the values reported for the (tetrahedral) bis-N-alkylsalicylaldimine Co(II) complexes (planar [Co(salen) ] has/X~rf = 2.1=2.6 BM [4] ). The elec° tronic spectra are also very similar [3]. CoL "~ Iollows the Curie law down to 5 K.

2 A. Pusini et al./ l , org,mica Chimica Acta 266 (1997) 1-3

Table I Magnetic and Sl~ ' l~copic data for sonm selected compounds

Compound FAB mass spectra" / ~ . b (BM)

Electronic spectra, v ( 10 -~ cm- ') (e)

Reflectance dmf solution

CoL t D: 388 [M+ 1] * 4.52 8.47, 1 !.83s, 14.81 18.87s, 25

CoL ~ D: 432 [M] ÷ 4.45 c 8.33, 10.53s, 14.71s 18.18s, 20.66, 26.32

[CoL t ] I D: 387 [ M - I] * 2.90 8.51, 11.49, 14.60, 585, [M+ DMF] " t7.73s, 25.0, 27.03

[CoL~I I D: 432, [ M - 1 ] * 3.00 ,s 8.26, ! i.63, 15.15s, 505, [M= I + DMF] " 17.54s, 24.94, 27.17

NiL ~ ~' N: 387, [M~ I] " 0 19.61s, 24.27, 25.77

NiL "~r N:401, [M+ I ]" 3.15 9.09, 14.81s 800. [M.d * 18.90s, 25.32

NiL ~ r 3.10 9.43, 14.81s, 16.95 25,51

CuL* D: 392, [M + l l * !.89 10.20, 14.29s N,D:783, [M: + I ] * 20.83s, 25.64

CuL: D: 406. [M+ ! J * 1.80 9.09s. 15.38 813, [M~+ ! ] * 21.05

CuL* D: 437, [ 14+ I ] * !.73 ~ 9.52, 14.60s, 20.41S, 25.32

ZaL ~ N: 406, [M] * 19.53, 24,88 813, [M~+ I]*

7.99 (40), iOA6 (20s), 15.82 ( 1 i ) 18.05 (60s), 26.60 ( 1.25 lip) 8.0 (45), 10.53 (25) 17.73 (60s) 26.60 (2.8 liP) 8.29 (22), 10.75 (13), 15.53 (92s), 17.54 (148s), 25.13 (6400) 7.96 (31), I 1.16 (20), 15.43 ( l l0s) , 25.12 (I.8 lip) 18.18 (500), 21.74 (3000s) 24.57 (6800), 31.06 ( 1.2 104) 9.56, 14.45, 18.52 (100), 21.28 (569), 23.81 (6400), 26.32 ( I.i 10 ~) 9.63 (12), 13.15 (125), 14.77 (15) 16.64 (205), 18.31 (335), 26.32 (2.9 104) 10.0 (10), 14.12 (112), 21.10 (707), 26.32 t5630) I0.0 (15), 15.0 (120), 21.05 (700), 26.30 (7500) 10.0 (s), 13.89, 19.23, 23.81s, 25.77 ~

~ ~/~ values and assignment, all peaks gave correct cluster abundance; matrix: N. nitrobenzylalcohol; D. dimethyiformamide solution dispersed in nitmbcn~lal¢ohol, "At room temperature, ~ valu¢~ in BM,

Thi~ compound follows the Cude law down to 5 K, Tht~ ~om~und follow~ Iho Curte=Wet.~ I~w down to 5 K: 0~ ~ - 8 K

f Gf~n,

e Q.~li t~tivo ~pectfum, du~ lotho low ~ol~lhility of tho ¢ompoul~d

I !

~4S0 ~850 3~50 ~650

Magnetk' field (GaussI

ICuL~I at 77 K. Simulated spe~'tr~m (b) | ' ~ l ~ n c d with ~n SIM 14 QCPB

As for the Cu derivatives, well resolved EPR spectra were obtained for [CuL ~ ] (frozen methanol solution, see Fig. I; the spectrum in DMF solution is almost identical):

g, ~ 2.240, g ~ ~ 2.06, A~ ~ 180 G. The g, and A~ values are

respectively higher and Io~cr than those of planar [Cu(salen) ] and suggest a tetrahedral distortion tbr ICuL~I (see Ref, 15 ] ), The ligand field hands at low energy are also in agreement with such an assumption | 5,6l. The EPR spec- tra of the other compounds are very similar, although less well resolved, presumably because of some association of these latter compounds also in DMF solutions (see the FAB data).

It therefore appears that, compared to salen, these iigands tend to favor tetrahedrally distorted coordination geometries. However, electronic factors play an, as yet, unknown role. Thus [ NiL t ] is red and diamagnetic (probably planar), while [NiL ~ ] and [NiL "~ ] are green and paramagnetic both in the solid state and in DMF solutions, However, different degrees of polymerization may also account for the observed isomerisms,

An unexpected result was obtained upon oxidation of the Co([D complexes, Treatment of [COL] with one equivalent of iodine gives I / I electrolytes (DMF or acetonitrile solu- tions) of stoichiometry [CaLl L These compounds have #,~. around 3 BM and [CoL:~] I follows the Curie-Weiss law to 5

A. Pasini et al, / b~organica Chimica Actu 2~6 ~ i99D !-3 3

K, with O= ~ - 8 K. The near-IR ligand field spectra of [Col. ] I are similar to those of the parent Co(ll) compounds, making the description of [ CoL ] I as Co(liD derivatives in the (rare [7] ) triplet ground state unlikely. An alternative description would be that of Co(II) complexes of stable rad- icals, with antiferromagnetic cobalt-radical coupling accounting lbr the observed magnetic moments. Since we have confirmed that (planar) [Co(salen)I] (which has a donor set analogous to that of our compounds) is diamagnetic [ 8], the stabilization of the triplet state, or of the radical cation, must arise from the distortion of the complexes imposed by the strain of the ligands, but clearly more work is needed to fully characterize these compounds.

Acknowledgements

We thank the MURST for financial help.

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