Alkylated trimethylene-bridged bis(p-phenylenediamines).

One and two electron oxidation of N,N',N'',N'''-tetramethyl-1,5,12,16-tetraaza[5,5]paracyclophane (Me3C), a bis-trimethylene bridged bis-p-phenylene diamine (PD), and its ethyl and isopropyl analogues are discussed. The monocation and dication are both stable, as demonstrated by optical studies that show they are in equilibrium in solution, with an especially small difference in first and second oxidation potentials for Me3C in MeCN (+23 to -20 mV, measured by simulation of the optical spectrum and of the cyclic voltammogram, respectively). The monocations have charge localized in one PD unit and show a Hush-type mixed valence transition between their PD0 and PD.+ groups. The dications Me3C2+ and Et3C2+ have optical spectra that appear to show large splittings between their PD.+ groups and have a weak ESR spectrum, and 1H NMR data show that the former is a ground-state singlet. iPr3C2+ has a very different optical spectrum and exhibits a triplet ESR spectrum at 120 K. X-ray crystal structures show that for Me3C0 the N(CH2)3N units on each side are in doubly anti (aa) conformations that put the aryl rings as far apart as possible, but Me3C2+ has doubly gg N(CH2)3N trimethylene bridges and both N,N and C,C distances between the PD.+ groups that are significantly below van der Walls contact. In contrast, iPr3C0 is in a doubly ag conformation, and its diradical dication is suggested to be a triplet because it does not attain the doubly gg conformation.

Ionized bicyclo[2.2.2]oct-2-ene: a twisted olefinic radical cation showing vibronic coupling.

The bicyclo[2.2.2]oct-2-ene radical cation (1(.+)) exhibits matrix ESR spectra that have two very different types of gamma-exo hydrogens (those hydrogens formally in a W-plan with the alkene pi bond), a(2H) about 16.9 G and a(2H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C(2v) structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C(2) structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcalmol(-1)). Density functional theory calculations estimate the twist angle at the double bond to be 11-12 degrees and the barrier as 1.2-2.0 kcalmol(-1). Single-configuration restricted Hartree-Fock (RHF) calculations at all levels that were tried give untwisted C(2v) structures for 1(.+), while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn-Teller effect (PJTE). Significantly, twisting does not occur until the sigma-orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene pi bond because they allow the filled alpha and the beta hole of the SOMO to have different geometries. The 2,3-dimethylbicyclo[2.2.2]oct-2-ene radical cation (2(.+)) is twisted significantly less than 1(.+), but has a similar temperature for maximum line broadening. Neither the 2,3-dioxabicyclo[2.2.2]octane radical cation (3(.+)) nor its 2,3-dimethyl-2,3-diaza analogue (5(.+)) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE-active orbitals for 3(.+) is too large for significant PJTE stabilization to occur.

Structural Information from Hydrazine Radical Cation Optical Absorption Spectra.

Transition energies (E(op)) of the nitrogen-centered pi,pi absorption of tetraalkylhydrazine radical cations are quite sensitive to twist at the NN bond, nitrogen pyramidality, and mixing of the sigma orbitals with the pi system. Thirty-one examples for which E(op) varies from 63 to 107.5 kcal/mol are discussed with the aid of calculated values (E(calc)) for the 0,0 transition energy using simple (no configuration interaction) neutral-in-cation-geometry calculations on AM1-UHF geometry-optimized radical-cation structures. Significant changes in the difference between E(op) and E(calc) are observed for bis-N,N'-bicyclic systems, which are syn pyramidalized at nitrogen (twist angles near 0 degrees; E(op) about 23 kcal/mol larger than E(calc)) and for bis-N,N-bicyclic ones, which are anti pyramidalized (twist angles of 180 degrees; difference about 7 kcal/mol when calculations of 180 degrees structures are employed). Within these classes, changes in E(op) caused by changes in pyramidality and sigma,pi interaction are predicted well by the calculations. The tetraisopropylhydrazine radical cation has lambda(max) = 282 nm, but its tetracyclohexyl analogue shows two transitions, at 276 and 386 nm. This surprising difference is attributed to tetracyclohexylhydrazine radical cation having both untwisted and significantly twisted (estimated twist angle approximately 44 degrees ) forms occupied in solution, although the isopropyl compound only has the untwisted form significantly occupied.