Trapping of Oligomeric Cyclopentadienyllithium Cationic and Anionic Fragments by a VV‐Bonded Ligand

Abstract

Main-group cyclopentadienyl compounds are typically far more ionic than their transition-metal relatives. [1][2][3] This difference is apparent not only in their reactivity patterns but also, in particular, in the frequent formation of electrostatically bonded polymeric structures for many main-group cyclopentadienyl species. Owing to the similar ionic sizes and charges of the metals, analogous infinite polymeric structures [M(h 5 -Cp)] 1 are found for the parent cylopentadienyl compounds of the alkali metals (M = Li, Na, K, Rb, Cs) [4] and for the heavier Group 13 metals (M = In, Tl).[5] As a consequence of Cp···M or M···M interactions, these polymers can be either linear or bent at the metal atoms (Scheme 1). The structures of the alkaline earth complexes and those of the heaviest Group 14 metal Pb can also be regarded as similarly related. [2] An important focus of research in this area in recent years has been the investigation of multidecker cations and anions. [3,6] These species can be conceived of as representing fragments of the parent cyclopentadienyl lattices and are produced by a number of routes. As outlined in Scheme 2 for Group 1 and Group 13 metals, these methods include formal+ cations, [7] and restricted solvation in the presence of bulky Cp ligands to give salts of the type[8]Our recent interests in this field have concerned attempts to extend the ionic chemistry of main-group cyclopentadienyl compounds to the transition metals, with a particular aim of obtaining new magnetic molecules and lattices. [9,10] We have shown that manganocene [Cp 2 Mn], the most ionic transitionelement metallocene, exhibits similar chemistry to maingroup metallocenes. Herein, we report for the first time that vanadocene [Cp 2 V] displays similar reactivity with the lithiated amide (hpp)Li (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine); the reaction results in substitution of the Cp ligands (as CpLi) and formation of the known VV-bonded compound [V 2 (hpp) 4 ] (L).[11f] Remarkably, the product crystallizes with five units of CpLi to form the ionic species ions are paired. [11,12] The spectrum shows a series of poorly resolved multiplets for the hpp ligand in the region d = 3.57-1.74 ppm and a singlet C 5 H 5 resonance (d = 5.70 ppm). The complicated appearance of the spectrum and its temperatureand concentration-dependent behavior can be attributed to the presence of dynamic equilibria in solution. On changing from 300 to 190 K, the 1 H NMR spectrum exhibits several changes, including the appearance of an additional Cp resonance (d = 6.20 ppm). The temperature-dependent 7 Li NMR Scheme 1. Schematic representation of the structures of [CpM] 1 (M = Li, Na, K, Rb, Cs, In, Tl).Scheme 2.