Zinc and cadmium complexes with an achiral symmetric helicand. Crystal structure of an enantiomerically pure K-Zn(II) monohelicate Manuel R. Bermejo,* a Miguel Va ´zquez, a Jesu ´s Sanmartı ´n, a Ana M. Garcı ´a-Deibe, a Matilde Fondo a and Carlos Lodeiro b a Dpto. de Quı ´mica Inorga ´ nica, Facultade de Quı ´mica, Universidade de Santiago de Compostela, Campus Sur, Santiago de Compostela, 15782, Spain. E-mail: qisuso@usc.es b Dpto de Quı ´mica, C.Q.F.B. Facultade de Cie ˆncias e Tecnologı ´a, Universidade Nova de Lisboa, Monte de Caparica, 2859-516, Portugal Received (in Montpellier, France) 4th February 2002, Accepted 17th June 2002 First published as an Advance Article on the web 12th September 2002 Zn(II) and Cd(II) complexes with an N-tosyl substituted N 4 -donor Schiff base, containing a 2-propanol residue as spacer, have been prepared. The X-ray crystal structure of the monohelicate L-Zn(OHPTs)H 2 O [H 2 OHPTs: N,N 0 -bis(2-tosylaminobenzylidene)-1,3-diamino-2-propanol] has been solved. The Zn(II) ion assumes a distorted tetrahedral coordination geometry, involving the four donor N atoms of the bisdeprotonated ligand. Strong (O–HO) hydrogen bonds between neighbouring complex and lattice water molecules lead to intricate intermolecular interactions that seem to drive the crystal packing. This Zn(II) complex shows an intense blue fluorescence in solution (l ¼ 430 nm, f ¼ 0.14), which is also observed in the solid state (l ¼ 490 nm). Cd(OHPTs)4H 2 O, although at a lower level (f ¼ 0.08), is also luminescent (l ¼ 430 nm) in acetonitrile solution. Enantiopure compounds are of central importance in many domains of chemistry. 1 The obtention of helical complexes, 2,3 which are intrinsically chiral, has significantly contributed to their development in the last years. Enantiopure ligands 4 are employed to induce stereoselectivity, or even stereospecificity, and so provide enantiomeric excesses of the P or M metallohe- licates. 3 Achiral ligands yield racemic mixtures of self- assembled P and M helixes, although in exceptional cases, crystallisation allows their spontaneous resolution. 5 Chroma- tographic techniques 6 or resolving agents 7 can be also employed to resolve racemates. General construction principles involved in the preparation of helical complexes have been widely studied. 2–4,8 Now, it is known that the spacer group between the metal binding domains is a key element in the formation and even in the microarchitecture of metallo-helicates. Moreover, hydrogen bonds and other weak interactions, 9 such as face-to-face or edge-to-face p-stacking, 10 are frequently responsible for the construction of supramolecular motifs. In recent years, we have been working with N-tosyl substi- tuted N 4 -donor diimines containing two- or three-membered spacers. 11,12 We have found that a ligand of this type contain- ing a (CH 2 ) 3 alkyl spacer can act as a N 4 -tetradentate ligand with Cd 2+ and as a bis(N 2 -bidentate) one with Zn 2+ ions, yielding both mono- and double helical metal complexes, respectively. 11a Here, we would like to explore how the ability of a CH 2 – CH(OH)–CH 2 spacer to form hydrogen bonds, can influence the spatial arrangement of N,N 0 -bis(2-tosylaminobenzyli- dene)-1,3-diamino-2-propanol (H 2 OHPTs, Scheme 1) in its zinc and cadmium complexes. Additionally, the study of some photophysical properties of the ligand and its complexes has also attracted our attention, since another Zn(II) complex con- taining this type of diimine, but with a 1,2-diaminocyclohexane residue as spacer, was fluorescent. 12a In this case, the presence of a hydrogen bonding site, such as the OH group, could enhance the fluorescence emission. 13 Experimental Materials and methods Chemicals of the highest commercial grade available (Aldrich) were used as received. Zinc and cadmium metal plates (Aldrich) were washed with a dilute hydrochloric acid solution prior to electrolysis. Elemental analyses were performed on a Carlo Erba EA 1108 analyser. NMR spectra were recorded on Bruker DPX- 250 and DRX-500 spectrometers, using acetonitrile-d 3 as sol- vent. Infrared spectra were recorded as KBr pellets on a Bio- Rad FTS 135 spectrophotometer in the range 4000–600 cm 1 . Mass spectra (FAB) were recorded on a Micromass Autospec spectrometer, employing m-nitrobenzyl alcohol as matrix. UV absorption spectra were recorded on a Perkin– Elmer Lambda 6 spectrophotometer, and fluorescence emis- sion spectra on a SPEX F111 Fluorolog spectrofluorimeter. All measurements were carried out at room temperature and in the presence of air. Scheme 1 Schematic representation of H 2 OHPTs and labelling scheme for 1 H NMR studies. DOI: 10.1039/b201433p New J. Chem., 2002, 26, 1365–1370 1365 This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002 Downloaded by Universidade de Santiago de Compostela on 04/04/2013 15:49:10. Published on 12 September 2002 on http://pubs.rsc.org | doi:10.1039/B201433P View Article Online / Journal Homepage / Table of Contents for this issue