Glen T. Cunkle

Comparison of free energy changes for nitrogen inversion and electron loss. 2. 8-Azabicyclo[3.2.1]octyl and 7-azabicyclo[2.2.1]heptyl systems.

Formal potentials for oneand two-electron oxidation of the 2-tetrazenes azo-8-azabicycio[3.2.I]octane (2) and azo-7-azabicyclo[2.2.l]heptane (11) and hydrazine 7,7’-bi-7-azabicyclo[2.2.l]heptane (14) are compared with those of related compounds; 11 and 14 are especially hard to oxidize. The nitrogen inversion barrier of 11 is about 9.2 kcal/mol at -90 OC, and that for 14 is 16.6 kcal/mol at 62 “C. Comparison with literature data verifies the relationship between ease of oxidation and nitrogen inversion barrier for amino nitrogen compounds. Any special destabilization caused by orbital symmetry for the cation radicals, which is significant for cation radical center 7-methyleneand 7-oxanorbornyl units, is too small to be significant for 7-azanorbornyl-containing cation radicals. R2N groups that lower the nitrogen inversion barrier (eq 1) usually make electron loss (eq 2) thermodynamically easier. The reason for this parallel behavior is that a similar geometry change, flattening at the nitrogen atom, takes place during both processes.’ If the resonance interaction between the nitrogen lone pair and X is not large, the nitrogen of neutral R2NX is pyramidal; its lone-pair orbital has some s character. The transition state for nitrogen inversion is planar a t nitrogen,2 where the lone-pair A 0 has pure p character. For quantitative considerationla of the relationship between eq 1 and 2, it is useful to consider Figure 1. Here the free energy of ionization has been artificially separated into two parts, that of flattening at nitrogen (the transition state for R2NX inversion if X is cylindrically symmetrical, and hence labeled AG’i) and that of a nearly vertical ionization of flat R2NX, labeled AG*,. It is experimentally impossible to directly measure AG*,, but we suggested’ that this energy gap should be affected by changing R groups in the same manner as vertical ionization potentials, VIP (which are enthalpies of ionization instead of free energies), which are experimentally available from photoelectron spectroscopy (PES) experiment^.^ A c t i values measured by dynamic N M R experiments were compared with changes in AGO, measured by cyclic voltammetry (CV) for I(X)-III(X) and found to exhibit the parallel behavior expected from Figure 1. AG’i is highest for X = C1 among the X groups studied, and III(C1) has a 6 kcal/mol higher N inversion barrier than I(C1). The range in AGO, is 4.6 kcal/mol for the X = C1 compounds but drops to 2.7 kcal/mol for the 2-tetrazenes [N=N linking two R2N groups], and to 0.5 kcal/mol for the very low AG*i nitroxides [X=O’]. I I V Table I. Cyclic Voltammetry Data“ for Some Bicyclic R2NX Derivatives X compd R2N = IV compd R2N = V CI 1 E: 1.77 13 Epox 1.93 N=NNR2 2 0.51 [0.070] 11 0.83 [0.083] 1.28 [0.065] 1.25 [0.069] NMez not studied 6 0.34 [0.070] NR2 3 0.13 [0.07] 14 0.45 [0.082] 1.01, 1.14* Eno’ 1.35 “Reported numbers are Eo’ (average of oxidation and reduction waves observed, [peak separation, E; E,’d], V for reversible waves. Both first and second oxidation waves are reported for the dimeric hydrazines and the 2-tetrazenes. EpoX values only are reported for compounds showing only irreversible oxidation (no reduction wave observed). Conditions: acetonitrile containing 0.1 M n-Bu,NCIO, as supporting electrolyte, 200 mV/s scan rate, Pt electrode, reported vs SCE. bThe cation radical has syn and anti forms, which oxidize at different potentials; see ref 6 . In this work we extend the range of bicyclic R2N groups examined to include the C N C angle restricted 8-azabicyclo[3.2.l]octyl IV(X) and 7-azabicyclo[2.2.l]heptyl V(X) series4