Di- and trinuclear complexes with the mono- and dianion of 2,6-bis(phenylamino)pyridine: High-field displacement of chemical shifts due to the magnetic anisotropy of quadruple bonds

The monoanion of 2,6-bis(phenylamino)pyridine (HBPAP^-) has been found to support quadruply bonded Cr₂⁴⁺ and Mo₂⁴⁺ units in Cr₂(HBPAP)₄ (1) and Mo₂(HBPAP)₄ (2). The corresponding dianion BPAP^2- was able to stabilize the trinuclear complexes, (TBA)₂Cr₃(BPAP)₄ (3) and (TBA)₂Ni₃(BPAP)₄ (4), where TBA is the tetrabutylammonium cation. The dinuclear complexes have the typical paddlewheel configuration with Cr-Cr distances of about 1.87 Å and a Mo-Mo distance of 2.0813(5) Å and exhibit a high-field displacement of the corresponding N-H signals caused by the magnetic anisotropy of the quadruple bonds. For the trinuclear complexes, 3 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion with two chromium atoms paired to give a quadruply bonded unit (Cr-Cr distance: 1.904(3) Å and an isolated, square planar Cr^II unit at 2.589(3) Å from the dimetal unit. On the other hand, the three nickel atoms in 4 are evenly spaced, having Ni···Ni distances of 2.3682(8) Å. The trinuclear compounds show a twisted conformation with an overall torsion angle of about 30°.