359 Heteroatom Chemistry Volume 9, Number 4, 1998 X-ray Crystallographic Study of Neutral Boron Chelates with b-Diketones and Tropolone Herbert Ho ¨ pfl, 1 Nury Pe ´rez Herna ´ ndez, 2 Susana Rojas Lı´ma, 2 Rosa Santillan, 2 and Norberto Farfa ´n 2 1 Centro de Investigaciones Quı ´micas, Universidad Auto ´noma del Estado de Morelos, Avenida Universidad No. 1001, Col. Chamilpa, 62210 Cuernavaca, Morelos, Me ´xico; E-mail: hhopfl@servin.fc.uaem.mx; Fax: (73) 297040 2 Departamentode Quı´mica, Centro de Investigacio ´n y de Estudios Avanzados del Instituto Polite ´cnico Nacional, Apdo. Postal 14-740, 07000 Me ´xico, D.F., Me ´xico; Fax: (5) 747 7113; E-mail: jfarfan@mail.red.cinvestav.mx Received 31 July 1997; revised 7 October 1997 ABSTRACT: A series of six new boron b-diketonates and tropolonates has been synthesized, each com- pound being crystallized and characterized by spectro- scopic methods as well as by X-ray crystallography. A comparative study of the NMR and X-ray data with similar structures from the literature permitted us to develop a detailed evaluation of the bonding in these chelates that lead to several interesting correlations. The present compounds have important physical prop- erties that justify this study.  1998 John Wiley & Sons, Inc. Heteroatom Chem 9:359–368, 1998 INTRODUCTION Neutral chelates can be obtained from boric or bor- inic esters in their reaction with b-diketones, tropo- lones [1], 8-quinolinol [2], 2-pyridinealcohols [2b,3], 2-pyridinecarboxylic acids [2b], and salicylaldehyde azine [4]. The tropolone and b-diketone chelates hav- ing a five- or six-membered chelate ring, in particu- Dedicated to Prof. Dr. Heinrich No ¨ th on the occasion of his seventieth birthday. Correspondence to: Herbert Ho ¨ pfl.  1998 John Wiley & Sons, Inc. CCC 1042-7163/98/040359-10 lar, have high dipole moments [5]. Thus, an almost complete electronic delocalization in the ligand [1a,6] and efficient fluorescence from the excited state [7] bring out interesting physical properties such as barrier crossing [8], photoconductive charge transfer [9], electron acceptance in the photoexcited state [7], and second-harmonic generation [10]. Therefore, on the basis of these molecules, barrier- crossing reactions [8], the deactivation mechanism of excited optical systems [1b], the function of ac- ceptor-type sensitizers for polymeric photoconduc- tors of the donor type used in electrophotography [9], ground- and excited-state electron donor–accep- tor complexes with substituted benzenes [7], and nonlinear optical properties [10] have been studied. To reveal more information about the electronic delocalization in the ligands of these b-diketone and tropolone chelates, which should be responsible for their physical properties, herein is presented a com- parative analysis of their molecular structures. It would be of interest to investigate how complete is the delocalization, and if there exists a dependence on the Lewis acidity and steric requirements of the moiety, that can be considered as a pseudome-  BR 2 tal cation [1d]. Concerning potential applications as nonlinear optically active materials, it is necessary, for physical reasons, that the molecules crystallize in a noncen-