Heterocyclophane Complexes of Transition Metals. 1. Synthesis and Crystal Structure of Both the (η⁵-[2, 2](2, 5)Pyrroloparacyclophane)tricarbonylchromium and the (η⁶-[2.2](2, 5)Pyrroloparacyclophane)tricarbonylchromium

Reaction of [2.2](2, 5)pyrroloparacyclophane with (MeCN)₃Cr(CO)₃in a dioxane/hexane mixture gives both (η⁵-[2.2](2, 5)pyrroloparacyclophane)tricarbonylchromium, 1, and (η⁶-[2.2](2, 5)pyrroloparacyclophane)tricarbonylchromium, 2, after appropriate workup. The mode of linkage of the Cr(CO)₃unit to the heterocyclophane has been determined in each instance by X-ray diffraction techniques. 1 crystallizes in the orthorhombic space group Pnma with a = 13.782 (6) Å, b = 12.442 (6) Å, c = 8.617 (4) Å, and p_calcd- 1.50 g cm^-1for Z = 4. The structure was refined to R=0.035 with 484 observed reflections. 2 belongs to the rhombohedral space group R3 with a = 13.312 (6) Å, α = 98.59 (3)°, and ρ_calcd= 1.46 g cm^-3for Z = 6. The structure was refined to R = 0.024 for 769 observed reflections. 1 exhibits a crystallographic mirror plane which contains the chromium and nitrogen atoms and one carbonyl and bisects C-C bonds of the C₆unit. The nitrogen atom of the heterocyclic ring is closest to the chromium atom at Cr-N = 2.102 (7) A. The electron-rich nitrogen atom is positioned exactly anti to an electron-withdrawing carbonyl group. 2 contains no crystallographically imposed symmetry and the Cr-C(π) bond lengths range from 2.188 (5) to 2.423 (6) A.