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 Cr2 4+ and Mo2 4+ units in Cr2(HBPAP)4 (1) and Mo2(HBPAP)4 (2). The corresponding dianion BPAP2- 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 CrII 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°.