Two-Coordinate, Quasi-Two-Coordinate, and Distorted Three Coordinate, T-Shaped Chromium(II) Amido Complexes: Unusual Effects of Coordination Geometry on the Lowering of Ground State Magnetic Moments.

The synthesis and characterization of the mononuclear chromium(II) terphenyl substituted primary amido-complexes Cr{N(H)ArPri6}2 (ArPri6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2 (1), Cr{N(H)ArPri4}2 (ArPri4 = C6H3-2,6-(C6H3-2,6-iPr2)2 (2), Cr{N(H)ArMe6}2 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2 (4), and the Lewis base adduct Cr{N(H)ArMe6}2(THF) (3) are described. Reaction of the terphenyl primary amido lithium derivatives Li{N(H)ArPri6} and Li{N(H)ArPri4} with CrCl2(THF)2 in a 2:1 ratio afforded complexes 1 and 2, which are extremely rare examples of two coordinate chromium and the first stable chromium amides to have linear coordinated high-spin Cr2+. The reaction of the less crowded terphenyl primary amido lithium salt Li{N(H)ArMe6} with CrCl2(THF)2 gave the tetrahydrofuran (THF) complex 3, which has a distorted T-shaped metal coordination. Desolvation of 3 at about 70 °C gave 4 which has a formally two-coordinate chromous ion with a very strongly bent core geometry (N-Cr-N= 121.49(13)°) with secondary Cr--C(aryl ring) interactions of 2.338(4) Å to the ligand. Magnetometry studies showed that the two linear chromium species 1 and 2 have ambient temperature magnetic moments of about 4.20 μB and 4.33 μB which are lower than the spin-only value of 4.90 μB typically observed for six coordinate Cr2+. The bent complex 4 has a similar room temperature magnetic moment of about 4.36 μB. These studies suggest that the two-coordinate chromium complexes have significant spin-orbit coupling effects which lead to moments lower than the spin only value of 4.90 μB because λ (the spin orbit coupling parameter) is positive. The three-coordinated complex 3 had a magnetic moment of 3.79 μB. [ABSTRACT FROM AUTHOR]