Synthesis and characterization of zinc complexes and reactivity with primary phosphines Benjamin A. Vaughan, Eliza M. Arsenault, Stephanie M. Chan, Rory Waterman * Department of Chemistry, University of Vermont, Burlington, VT 05405-0125, USA article info Article history: Received 14 September 2011 Received in revised form 3 October 2011 Accepted 6 October 2011 Keywords: Zinc Phosphine Phosphide Dehydrocoupling X-ray abstract Zinc complexes of chelating monoanionic N-donors and neutral diphosphine ligands were synthesized by reaction of diethylzinc with 1,3-diketimine, diazabutadiene, and diphosphine ligand precursors. These complexes were reacted with primary phosphines in an attempt to solicit phosphine dehydrocoupling reactivity. In most cases, insoluble zinc-containing precipitates were formed and ligands were liberated. For the most sterically encumbered complex, ( Dipp L)ZnEt (3, Dipp L ¼ [(2,6- i PrC 6 H 3 )NC(CH 3 )] 2 CH  ), a product assigned as the zinc-phosphide ( Dipp L)ZnPHPh (6) was observed but could not be isolated as a pure compound. A new, less bulky b-diketiminate complex ( Tol L)ZnEt (2, Tol L ¼ [(p-CH 3 C 6 H 4 )NC(CH 3 )] 2 CH  ) was reacted with primary phosphines to give a precipitate and the bis(b-diketiminate)zinc complex ( Tol L) 2 Zn (5), an apparent product of comproportionation. ( Mes AI)ZnEt (1, Mes AI ¼ MesNC(Me)(Et)C(Me) ¼ NMes) and 2 were structurally characterized. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Dehydrocoupling catalysis is a key transformation in the synthesis of main-group materials [1e8]. However, the catalytic dehydrocoupling of phosphines is less developed than related transformations for elements such as silicon or for Lewis acid-base adducts such as amine-boranes [2,4,6e7,9e10]. For phosphine dehydrocoupling, the catalysts that have appeared in the literature primarily use group 4 metals or rhodium [8]. In developing broad synthetic applications for this transformation, it is attractive to use inexpensive metal complexes as catalysts. Earth-abundant, first-row transition metals are considered promising to displace conventional coinage metal complexes in other catalytic transformations [11]. Zinc complexes have shown excellent reactivity in the copoly- merization of carbon dioxide with epoxides [12e13]. Though zinc complexes would not be expected to demonstrate the same kinds of reactivity as formally d 8 and d 0 metal complexes, which are demonstrated phosphine dehydrocoupling catalysts [8], the recent report of a tin complex, Cp* 2 SnCl 2 , that is a catalyst for dehy- drocoupling prompts the study of alternative metal species [14]. This report outlines efforts to solicit phosphine dehydrocou- pling reactivity from a family of L’ZnEt 2 and LZnEt (L ¼ monoanionic chelating ligand; L 0 ¼ neutral chelating ligand) derivatives. These reactions suggest that the simple zinc complexes reported herein are not viable catalysts due to the formation of insoluble ZneP products, regardless of the nature of the ancillary ligand. 2. Results and discussion 2.1. Synthesis of zinc complexes Simple ligands were selected to support zinc complexes as potential catalysts based on prior observation of related zinc complexes in stoichiometric or catalytic transformations. For example, it was expected that b-diketiminate complexes of zinc might afford reactivity with PeH bonds based on copolymerization catalysis [15e16]. Zinc complexes with a-aminoimine ligands have not seen attention in catalysis since their discovery by van Koten and coworkers [17e21], yet these ligands afford tunable steric and electronic parameters in a way that is similar to b-diketiminate ligands. It was also anticipated that neutral ligands, like chelating diphosphines, might promote different reactivites or stability over anionic counterparts. Reaction of diethylzinc with a mesityl-substitued dia- zabutadiene in toluene solution gave analytically pure, orange crystals of ( Mes AI)ZnEt (1, Mes AI ¼ MesN ¼ C(Me)CEt(Me)NMes) in 81% yield upon crystallization from pentane (Eq. (1)). Complex 1 is the product of a formal 1,2-insertion of one imine into the ZneC bond of an ethyl ligand. Diagnostic spectral features of 1 include the imine and amine resonances of the ligand backbone at d 71.8 and d 191.1, respectively, in the 13 C NMR spectrum. Two methyl * Corresponding author. Tel.: þ1 802 656 0278; fax: þ1 802 656 8705. E-mail address: rory.waterman@uvm.edu (R. Waterman). Contents lists available at SciVerse ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.10.008 Journal of Organometallic Chemistry 696 (2012) 4327e4331