Molecular Structure of Hg-Bridged Tetramethyl[2]ferrocenophane Salt ([{C5H4(CH3)2}2Fe–Hg–Fe{C5H4(CH3)2}2]2+ (BF4−)2) and Related Salts

<jats:title>Abstract</jats:title> <jats:p>The crystal form of the salt [{C5H4(CH3)2}2Fe–Hg–Fe{C5H4(CH3)2}2Hg(BF4)2, 1], prepared by a reaction of tetramethyl[2]ferrocenophane and Hg(CN)2 in ethanol containing HBF4, is triclinic, space group , a = 9.315(3), b = 12.823(4), c = 14.108(5) Å, α = 106.055(2), β = 100.253(2), γ = 94.460(2)°, Z = 2 and the final R = 0.059 and Rw = 0.072. Two ferrocene moieties are coordinated to the Hg atom from the opposite side, and the previously proposed Fe–Hg–Fe bridged structure has been verified. The structural feature of the cation is found in the conformation of the ferrocene moiety around the Fe–Hg–Fe bond. In order to reduce the steric hindrance between the Hg and ferrocene moieties, each moiety rotates largely with respect to the other (the torsion angle of the Cpcnt–Fe(1)···Fe(2)–Cpcnt is 75(1)° and Cpcnt is defined as the centroid of the Cp-ring). The mean Fe–Hg distance is 2.598(8) Å and the Fe–Hg–Fe bond angle is 176.4(1)°. The two C5H4 rings are greatly tilted, the mean tilting angle between them being 33.72°. Analogous Hg(II) salts of ruthenocene (2) and osmocene (3) were prepared by a similar method. Crystal 2 is monoclinic, space group P21/a, a = 9.633(1), b = 18.864(3), c = 13.388(1) Å, β = 94.161(9)°, Z = 4 and the final R = 0.058 and Rw = 0.072. The mean Ru–Hg distance is 2.661(2) Å and the Ru–Hg–Ru bond angle is 174.4(1)°; the mean tilting angle between the C5H4-planes is 27.39°. The structure of the cation is very similar to that of 1 (the torsion angle of the Cpcnt–Ru(1)···Ru(2)–Cpcnt is 70(1)°).</jats:p> <jats:p>The crystal form of 3 is triclinic, space group , a = 9.823(1), b = 11.194(1), c = 12.241(1) Å, α = 113.822(8), β = 95.828(8), γ = 90.104(8)°, Z = 2 and the final R = 0.030 and Rw = 0.032. The mean Os–Hg distance is 2.69(2) Å and the Os–Hg–Os angle is 170.1(3)°. The most interesting structural difference of 3 compared with 1, 2 is found to be the conformation of the osmocene moieties around the Os–Hg–Os bond. The two moieties are coordinated to Hg in parallel, and the torsion angle of Cpcnt–Os(1)···Os(2)–Cpcnt is negligible (2(1)°) so as to reduce the steric hindrance between the cations and the PF6− anions.</jats:p>