Complexes of oxygenated trans-azoalkanes and tetracyanoethylene. The interaction of pi oxygen dipoles with an electron poor alkene

The N-oxide and N,N'-dioxide (lao and 1ado) of trans-1,1'-azonorbornane (1) associate with TCNE in solution and in the solid state. The solution complexes are characterized by charge-transfer optical absorptions at lambda(m) 364 and 540 nm in CH2Cl2 for lao/TCNE and lado/TCNE, respectively. These complexes are weakly bound, with K-f values of 0.5-3.0 M(-1), Crystals of (1ao)(2)/TCNE and lado/TCNE are isolable, and their structures have been determined by X-ray diffraction. Local donor-acceptor (DA) interactions between the pi dipolar donors and the electron poor TCNE are found, Crystals of (1ao)(2)/TCNE are composed of discrete D-A-D triads in which the 1ao oxygen approaches one of the olefinic C atoms (C-1, 2.86 Angstrom) of TCNE more closely than the other (C-2, 3.07 Angstrom). The O-C-1-C-2 angle is 87 degrees, and the azooxide molecular plane lies perpendicular to the molecular plane of TCNE. 1ado/TCNE crystals are composed of extended -D-A-D-A- strands in which both oxygen poles of the azodioxide simultaneously interact with alternate TCNE accepters, The D-A geometry here is structurally analogous to that in the (1ao)(2)/TCNE crystal. PM3 calculations of 1:1 and 1:2 trans HN(O)NH(O)/TCNE complexes constrained to have D-A (O-C) distances of 2.88 Angstrom, but which are otherwise geometry optimized, reproduce the DA topology observed in the crystalline samples.