Hexaazaoctadecahydrocoronene.^† Structural and Physical Properties of [HOC]ⁿ (n = 0, 1+, 2+, 3+, 4+)

Hexaazaoctadecahydrocoronene and its mono-, di-, tri-, and tetracations, [HOC]ⁿ (n = 0, 1+, 2+, 3+, 4+), have been prepared and characterized by X-ray diffraction, EPR, magnetic susceptibility, and electronic and vibrational spectroscopies. HOC, [HOC]^*+[BF₄]^-, [HOC]^*+[TCNE]^*- (TCNE = tetracyanoethylene), [HOC]^*+[F₃CSO₃]^-, [HOC]²⁺{[PF₆]^-}₂, [HOC]²⁺{[BF₄]^-}₂, [HOC]²⁺{[C(CN)₃]^-}₂, [HOC]²⁺{C [C(CN)₂]₃}^2-, [HOC]²⁺{Ni[S₂C₂(CF₃)₂]₂ ^*-}₂, [HOC]^*3+{[PF₆]^-}₃, [HOC]^*3+{[SbF₆]^-}₃, and [HOC]⁴⁺{[SbF₆]^-}₄. MeCN have been studied by single-crystal X-ray diffraction. HOC and [HOC]⁴⁺ have essentially equivalent C₆-ring C-C bond distances averaging 1.397 and 1.436 Å, respectively. Their average C₆-ring C-N distances are 1.416 and 1.318 Å, respectively. The dication, [HOC]²⁺, has a distorted (Jahn-Teller) structure corresponding to coupled cyanine fragments with distinctly different short (1.395- Å) and long (1.471- Å) C₆-ring C-C and C-N (1.337- and 1.405- Å) bond distances. The 1+ and 3+ radical ions also show distorted structures, although the distortions are smaller than observed for the 2+ structure. The short C₆-ring C-C bond distances are 1.382 and 1.417 Å, and the long C₆-ring C-C bond distances are 1.444 and 1.439 Å, while the short C₆-ring C-N bond distances are 1.371 and 1.326 Å and the long C₆-ring C-N bond distances are 1.422 and 1.339 Å, respectively. Solid-state magnetic susceptibility measurements show that the n = 0, 2+, and 4+ compounds are diamagnetic and that the n = 1+ and 3+ salts are S = 1/2 paramagnets that obey the Curie-Weiss expression between 2 and 320 K. The effective moments, µ_eff, and Curie-Weiss constants, θ, are 1.77 and 1.75 µ_B and θ = -3.3 and -0.9 K for the mono- and trications, respectively. The n = 1+ and 3+ radical cations are fluxional in solution. The EPR spectrum of [HOC]^*3+ in the fast-exchange limit (25 °C) shows that all protons and N's have the same hyperfine splitting (2.56 G, g = 2.003 15). In the slow-exchange limit (-90 °C), there are 2 sets of 12 equivalent protons (0.974 and 4.222 G) and 6 equivalent N's (2.595 G). This precludes the observation of a Jahn-Teller distortion in solution. The [HOC]^*3+ ion is more fluxional in solution as it exhibits fast-exchange behavior at -60 °C [a(24H) = a(6 N) = 2.81 G; g = 2.0310]. EPR spectra of polycrystalline [HOC]²⁺ salts show the presence of thermally accessible triplet species whose zero-field splitting parameters are |D| = 0.0550 ± 0.0008 cm^-1 and |E| = 0.0024 ±0.0005 cm^-1 and are appropriate for triplet states of less than 3-fold symmetry. The temperature dependence of the ∆M = ±2 EPR absorption afforded the separation between the ground singlet state and the excited triplet state of the dication for a variety of [HOC]²⁺ salts. For most counteranions, these separations are greater than 3 kcal/mol (1050 cm^-1, 0.13 eV). Solid-state magnetic susceptibility measurements for those [HOC]²⁺ with singlet-triplet gaps less than 4.0 kcal/mol show increases in susceptibilities at higher temperatures, consistent with the presence of thermally populated triplet excited states. Magnetic susceptibility and single-crystal X-ray structures of the dication in the crystalline materials of this study are consistent with a singlet ground state. Molecular orbital (ab initio) calculations are in general agreement with the above results.