Structural and Magnetic Studies of a Quasi-Inverse Sandwich Cyclooctatetraene Complex with Two High-Spin Chromium(II) Ions Bound Anti-Facially

Reaction of K2COT (COT = 1,3,5,7-cyclooctatetraene,
C8H8) with the aryl chromium­(II) halide [AriPr4Cr­(μ-Cl)]2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) gave (CrAriPr4)2(μ2-η3:η4-COT) (1), in which a nonplanar
COT ring is complexed between two CrAriPr4 moieties, a configuration previously unknown for chromium
complexes of COT. One Cr2+ ion is bonded primarily to three
COT carbons (Cr–C = 2.22–2.30 Å) as well as an
ipso carbon (Cr–C ≈ 2.47 Å) from a flanking aryl
ring of its terphenyl substituent. The other Cr2+ ion bonds
to an ipso carbon (Cr–C ≈ 2.53 Å) from its terphenyl
substituent as well as four COT carbons (Cr–C = 2.24–2.32
Å). The COT carbon–carbon distances display an alternating
pattern, consistent with the nonplanarity and nonaromatic character
of the ring. The magnetic properties of 1 indicate that
the Cr2+ ions have a high-spin d4 configuration
with S = 2. The temperature dependence of the magnetism
indicates that their behavior is due to zero-field splitting of the S = 2 state. Attempts to prepare 1 by the direct
reaction of quintuple-bonded (CrAriPr4)2 with COT were unsuccessful.