COMMUNICATION Molecular Structure of [Me 2 Al(l-OPh)] 2 : A Crystallographic and Ab initio Study Douglas Ogrin Æ Simon G. Bott Æ Andrew R. Barron Received: 8 February 2008 / Accepted: 12 February 2008 / Published online: 4 March 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The molecular structure of [Me 2 Al(l-OPh)] 2 has been determined. The phenoxide ring is parallel to the Al 2 O 2 ring rather than the energetically favored perpen- dicular configuration determined by ab initio calculations. Ab initio calculations successfully predict the structures of sterically demanding analogs. The adoption of the parallel configuration allows for an estimation of the magnitude of crystal packing forces. A discussion of the parameters controlling the structures of dialkylaluminum phenoxides is presented. Crystal data: group Pbca, a = 12.127(2), b = 8.491(2), c = 17.299(4) A ˚ , V = 1781.3(6) A ˚ 3 , Z = 4, R = 0.0695, wR 2 = 0.1390. Keywords Aluminum Á Aryloxide Á Calculations Á Steric interaction Introduction Since the report by Oliver et al. [1] of the first structurally characterized mono-phenoxide of a Group 13 metal, [R 2 M(l-OC 6 H 5-x R 0 x )] 2 , there have been an increasing number of crystallographic studies, in particular those of aluminum [2–6]. Irrespective of the identity of the sub- stituents on the metal or the aryl ring, the structures share two specific characteristics: a dimeric structure and the orientation of the phenoxide ring perpendicular to the M 2 O 2 core. The dimeric structure is expected based on the steric bulk of the phenoxide [7] while the perpendicular conformation is the global minimum of steric interactions between the phenyl ring and the Group 13 metal substitu- ents. The only exceptions reported have involved sterically hindered tert-butyl derivatives [8–10], or the unusual pentafluoro derivative, [Me 2 Al(l-OC 6 F 5 )] 2 [2]. This latter compound has one ring perpendicular and one parallel to the Al 2 O 2 cycle. The phenyl rings in the tert-butyl deriv- atives are in neither of the expected configurations, but are canted 48.9(1)°–55.3(1)° from the M 2 O 2 plane. A consid- eration of a space-filling representation showed that the arene rings are situated so as to minimize their steric interaction with the tert-butyl groups. As part of a study on the reactivity of di-alkylaluminum phenoxides bidentate bridging Lewis bases, we have structurally characterized [Me 2 Al(l-OPh)] 2 . Although it is expected to have the archetypal structure, surprisingly the phenyl ring is coplanar with the Al 2 O 2 core. This result prompted a study of inter-molecular distances and an ab initio study to confirm the relative stability of the ring configuration. These results are discussed herein along with a summary of the factors that control the structural parameters of di-alkylaluminum phenoxides. Experimental Crystal Data A sample of [Me 2 Al(l-OPh] 2 was prepared by a variation of the method of Rogers et al. [7]. A crystal (0.95 9 0.85 9 0.052 mm) was sealed in a glass capillary in a dry box and mounted on the goniometer of a Bruker D. Ogrin Á A. R. Barron (&) Department of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, USA e-mail: arb@rice.edu URL: www.rice.edu/barron S. G. Bott Department of Chemistry, University of Houston, Houston, TX 77204, USA 123 J Chem Crystallogr (2008) 38:397–401 DOI 10.1007/s10870-008-9329-7