On Haptotropic Rearrangements of Diphosphene and Diarsene Ligands in Titanium Complexes.

Dipnictenes of the type RE=ER (E=P, As, Sb, Bi) are the isovalence electronic heavier analogs of alkenes. Although diphosphenes and dipnictenes in general show a variety of binding modes in metal complexes, little is known about haptotropic shift reactions involving these ligands. Herein, we report an unprecedented η to η rearrangement of the dipnictene ligands in titanocene complexes of the type CpTi(PnAr) (Pn=P, As; Ar=2,4,6-Me-CH, Mes; 2,6-iPr-CH, Dip; 2,4,6-iPr-CH, Tip), initiated by Lewis basic ligands (L=MeCN, PMe, AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η to η and complexes of the general form CpTi(L)(PnAr) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [CpTi]⋅ and [PnAr]⋅ fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe/MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).