Complexes of Oxygenated trans-Azoalkanes and Tetracyanoethylene. The Interaction of π Oxygen Dipoles with an Electron Poor Alkene

The N-oxide and N,N'-dioxide (1ao 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 CH(2)Cl(2) for 1ao/TCNE and 1ado/TCNE, respectively. These complexes are weakly bound, with K(f) values of 0.5-3.0 M(-)(1). Crystals of (1ao)(2)/TCNE and 1ado/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 Å) of TCNE more closely than the other (C(2), 3.07 Å). 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 acceptors. 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 Å, but which are otherwise geometry optimized, reproduce the DA topology observed in the crystalline samples.