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 2 to η 1 rearrangement of the dipnictene ligands in titanocene complexes of the type Cp 2 Ti(Pn 2 Ar 2 ) (Pn=P, As; Ar=2,4,6-Me 3 -C 6 H 2 , Mes; 2,6-iPr 2 -C 6 H 3 , Dip; 2,4,6-iPr 3 -C 6 H 2 , Tip), initiated by Lewis basic ligands (L=MeCN, PMe 3 , AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η 2 to η 1 and complexes of the general form Cp 2 Ti(L)(Pn 2 Ar 2 ) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [Cp 2 Ti III ]⋅ + and [Pn 2 Ar 2 ]⋅ - fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe 3 /MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).