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

Abstract 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 SiMe 3 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(η 5 ‐tetrahydroindenyl)). The organolithium compounds Li 2 (RC 3 Ph) ( 1 b : R=Ph, 1 c : R=SiMe 3 ) as well as the metallacycles of the general formula rac ‐(ebthi)M(R 1 C 3 R 2 ) ( 2 b : M=Ti, R 1 =R 2 =Ph, 2 c : M=Ti, R 1 =Ph, R 2 =SiMe 3 ; 3 b : M=Zr, R 1 =R 2 =Ph; 3 c : M=Zr, R 1 =Ph, R 2 =SiMe 3 ) 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(Me 3 SiC 3 SiMe 3 ) ( 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. 13 C 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.