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.