A structural, theoretical and coordinative evaluation of the bicyclic guanidinate derived from 1,4,6-triazabicyclo[3.3.0]oct-4-ene

Abstract

Partial deprotonation of the bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) is achieved using ⁿBuLi. Isolation of the resulting lithium salts has resulted in the structural characterization of the mixed anion complex {[Li(tbo)(VIII)(tboH)]₂}_∞1a (where VIII-H = 1-(2-aminoethyl)-2-imidazolidinethione) and the partially deprotonated salt Li₆(tbo)₆(Htbo)₃, 1b. The neutral guanidine Htbo reacts cleanly with AlMe₃ and ZnMe₂ to afford the organometallic complexes [Al(tbo)Me₂]₂ [2]₂, and Zn₃(tbo)₄Me₂ (3). Structural characterization of these compounds enables comparison between the {5:5}-bicyclic system, [tbo]⁻, and the previously reported {6:6}-bicyclic system, [hpp]⁻ (where hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). Results indicate that delocalization within the [tbo]⁻ anion is restricted to the CN₂ amidinate component, with retention of electron density in the non-bonding nitrogen lone-pair. These conclusions are supported by a DFT analysis of the neutral guanidines, Htbo and hppH.