Inorganic Chemistry, Vol.51, No.5, 3212-3219, 2012
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)Ar-Pr6i}(2) (Ar-Pr6i = C6H3-2,6-(C6H2-2,4,6-Pr-i(3))(2) (1), Cr{N(H)Ar-Pr4i}(2) (Ar-Pr4i = C6H3-2,6-(C6H3-2,6-Pr-i(2))(2) (2), Cr{N(H)Ar-Me6}(2) (Ar-Me6 = C6H3-2,6-(C6H2-2,4,6-Me-3)(2) (4), and the Lewis base adduct Cr{N(H)Ar-Me6}(2)(THF) (3) are described. Reaction of the terphenyl primary, amido lithium derivatives Li{N(H)ArPr6i} and Li{N(H)ArPr4i} 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 highspin Cr2+. The reaction of the less crowded terphenyl primary amido lithium salt Li{N(H)Ar-Me6} with CrCl2(THF)(2) gave the tetrahydrofuran (THF) complex 3, which has a distorted T-shaped metal coordination. Desolvation of 3 at about 70 degrees C gave 4 which has a formally two-coordinate chromous ion with a very strongly bent core geometry (N-Cr-N= 121.49(13)degrees) with secondary Cr-C(aryl ring) interactions of 2.338(4) angstrom to the ligand. Magnetometry studies showed that the two linear chromium species 1 and 2 have ambient temperature magnetic moments of about 4.20 mu(B) and 4.33 mu(B) which are lower than the spin-only value of 4.90 mu(B) typically observed for six coordinate Cr2+. The bent complex 4 has a similar room temperature magnetic moment of about 4.36 mu(B). 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 mu(B) because lambda (the spin orbit coupling parameter) is positive. The three-coordinated complex 3 had a magnetic moment of 3.79 mu(B).