Neutral zinc, lower-order zincate and higher-order zincate derivatives of pyrrole: synthesis and structural characterisation of zinc complexes with one, two, three or four pyrrolyl ligands

Abstract

Towards a systematic development of the zinc chemistry of the important five-membered nitrogen heterocycle pyrrole, this work reports the synthesis and characterisation of five crystalline zinc-pyrrolyl complexes. Pyrrolyl in this context means where conversion of the N-H bond to an N-zinc bond has occurred. Two neutral complexes, [(t)BuZn(NC(4)H(4))(TMEDA)·HNC(4)H(4)] 1 and [Zn(NC(4)H(4))(2)(TMEDA)] 2, containing one and two pyrrolyl ligands, respectively, were synthesised by reacting di-t-butylzinc with different amounts of pyrrole in the presence of TMEDA (TMEDA is N,N,N',N'-tetramethylethylenediamine). X-ray crystallographic studies established that both adopt mononuclear structures with the salient feature of the former the presence of an additional parent protonated pyrrole molecule which engages its anionic counterpart in N-H…πC-C interactions. Employing a similar synthetic approach but adding n-butylsodium to the reaction mixture in attempts to form ate derivatives delivered three distinct sodium zincate (anionic zinc) compounds in [{(THF)(2)·NaZn(THF)(NC(4)H(4))(3)}(∞)] 3, [{(TMEDA)·Na}(2)Zn(NC(4)H(4))(4)] 4, and [{(PMDETA)·Na}(2)Zn(NC(4)H(4))(4)] 5 (PMDETA is N,N,N',N'',N''-pentamethyldiethylenetriamine). From their crystal structures, the 1 : 1, Na:Zn complex 3 can be classified as a lower-order zincate having three pyrrolyl ligands bound to zinc in a polymeric chain arrangement, while the 2 : 1, Na:Zn complexes 4 and 5 are molecular higher-order zincates having Zn centres fully saturated by four pyrrolyl ligands. Discussion of the structures of 1-5 focuses on the interplay of σ-bonding and π-bonding between the pyrrolyl ligands and the metal centres. Revealingly, the zinc-free sodiopyrrole complex [{(PMDETA)·Na(NC(4)H(4))}(2)] 6, made and characterised for comparison, shows that on its own sodium prefers the former type of bonding, but is forced to switch to the latter type when combined with the stronger Lewis acid zinc in the zincate compositions. Complexes 1-6 have also been characterised in solution by NMR spectroscopy.