O,N-Chelated Vanadium(IV) Oxo Aminophenolate Complexes: the Effect of Steric Bulk on the Vanadium Coordination Geometry. Can this Influence be detected Spectroscopically? Henk Hagen a , Santiago Reinoso 1) , Ed J. Reijerse c , Ernst E. van Faassen d , Martin Lutz e , Anthony L. Spek e,2) , and Gerard van Koten a, * Utrecht / The Netherlands, a Debye Institute, Department of Metal-Mediated Synthesis, d Department of Atomic and Interface Physics, and e Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University c Nijmegen / The Netherlands, Department of Molecular Spectroscopy, University of Nijmegen Received July 30 th , 2004. Dedicated to Professor Michael Veith on the Occasion of his 60 th Birthday Abstract. In order to study the effect of steric bulk on the vanadium coordination geometry in O,N-chelated vanadium oxo (bis)phenol- ates, six different ortho-aminophenolate ligands have been used. The ortho-aminophenolate system was changed at three different places, i.e. 1) the second ortho position (C 6 ) of the arene ring (R), 2) the substituents at the amino nitrogen (R' and R), and 3) the benzylic carbon atom (R * ). The phenols were used in the pre- paration of the vanadium oxo (bis)phenolate complexes. In order to study whether it is possible to predict geometrical features of O,N-Chelat Vanadium(IV) Oxo Aminophenolat Komplexe: Der Einfluss der sterischen Hinderung auf die Vanadium Koordinationsgeometrie. Kann dieser Effekt spektroskopisch nachgewiesen werden? Inhaltsübersicht. Zur Untersuchung des sterischen Einflusses auf die Koordinationsverhältnisse von O,N-chelatisierten Vanadium- Oxo(bis)phenolaten wurden sechs verschiedene ortho-Aminophe- nolatliganden angewandt. Das ortho-Aminophenolatsystem wurde auf drei verschiedene Arten variiert: 1) an der zweiten ortho-Posi- tion (C 6 ) des Arenringes (R), 2) mit den Substituenten am Amino- stickstoffatom (R' und R), 3) am benzylischen Kohlenstoffatom (R*). Die Phenole wurden zur Herstellung der Vanadium-Oxo(bis)- phenolat-Komplexe eingesetzt. Um die Möglichkeiten strukturel- * Prof. Dr. G. van Koten Debye Institute, Department of Metal-Mediated Synthesis Utrecht University Padualaan 8, 3584 CH Utrecht / The Netherlands E-mail: g.vankoten@chem.uu.nl Fax: +31 30 2523615 1) Visiting undergraduate student from Universidad del Pais Vasco, Bilbao, Spain 2) To whom correspondence pertaining to crystallographic studies should be addressed. E-mail: a.l.spek@chem.uu.nl Z. Anorg. Allg. Chem. 2004, 630, 2097-2105 DOI: 10.1002/zaac.200400337  2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2097 these vanadium complexes, UV/Vis, solution and frozen state EPR and 14 N ESEEM spectroscopic data was measured and compared to the structural features of four structurally characterized va- nadium oxo (bis)phenolates. Unfortunately, it turned out that is was not possible to correlate the EPR parameters, the UV/Vis HOMO-LUMO transitions or 14 N hyperfine couplings to the structural parameters. Keywords: Vanadium; O,N-ligands; EPR spectroscopy; UV/Vis spectroscopy; 14 N ESEEM spectroscopy ler Merkmale dieser Vanadiumkomplexe vorherzusagen, wurden die UV-vis- sowie die EPR- und 14 N-ESEEM-Spektren in Lösung und im eingefrorenen Zustand gemessen und mit den strukturellen Merkmalen von vier strukturell charakterisierten Vanadium-Oxo- (bis)phenolaten verglichen. Unglücklicherweise stellte sich heraus, daß es nicht möglich ist, die EPR-Parameter, die UV-vis-HOMO- LUMO-Übergänge oder die 14 N-Hyperfein-Kopplungen mit den Strukturparametern zu korrelieren. Introduction Recently we have set out to use vanadium as a catalyst in Ziegler-Natta polymerization of α-olefins. As ancillary li- gands we have chosen ortho-chelating aminophenolates (see Figure 1). This type of ligands has been used previously in our group in aluminium [1], iron [2], zinc [3], tungsten [4], and molybdenum [5] chemistry and they proved to be versa- tile in the stabilization of metal atoms in various oxidation states. Earlier we have reported that substituent variation at the meta or para positions can have a significant electronic influence on the vanadium atom [6].