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 (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.
Greer
Melinda L.
ML
Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-0336.
Blackstock
Silas C.
SC