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, C 8H8) with the aryl chromium(II) halide [AriPr 4Cr(-Cl)]2 (AriPr4 = C6H 3-2,6-(C6H3-2,6-iPr 2)2) gave (CrAriPr4) 2(2-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 Cr 2+ 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