A General Concept for the Electronic and Steric Modification of 1-Metallacyclobuta-2,3-dienes: A Case Study of Group 4 Metallocene Complexes

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe₃ substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(η⁵-tetrahydroindenyl)). The organolithium compounds [Li₂(RC₃Ph)] (1 b: R=Ph, 1 c: R=SiMe₃) as well as the metallacycles of the general formula [rac-(ebthi)M(R₁C₃R₂)] (2 b: M=Ti, R₁=R₂=Ph, 2 c: M=Ti, R₁=Ph, R₂=SiMe₃; 3 b: M=Zr, R₁=R₂=Ph; 3 c: M=Zr, R₁=Ph, R₂=SiMe₃) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me₃SiC₃SiMe₃)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. ¹³C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the β−C−Ti interaction.