J . Am. Chem. SOC.1987, 109, 2484-2496
2484
Azoalkanes as Electron Donors in Charge-Transfer Complexes. Molecular Structures with Sacrificial (7r*,cr*) and Increvalent (v) Electron Acceptors S. C. Blackstock and J. K. Kochi* Contribution from the Department of Chemistry, University of Houston, University Park, Houston, Texas 77004. Received September 17, I986
Abstract: A novel series of electron donor-acceptor (EDA) complexes of various azoalkanes with the three unique types of electron acceptors are characterized in solution from their charge-transfer (CT) spectra. Among azoalkanes, the electron-rich diazabicyclooctene(DBO) and -heptene (DBH) afford isolable complexes with the prototypical acceptors tetracyanoethylene (T*), carbon tetrabromide (o*), and silver(1) nitrate (v), but the electron-deficient azobisisobutyronitrile (AIBN) does not. Quantitative evaluation of the formation constants K by the Benesi-Hildebrand method indicates that these azoalkane complexes are moderately weak in solution. However, the insolubility of the electron donor-acceptor complexes from DBO/TCNE, DBO/CBr,, and DBO/AgN03 pairs in ethereal solutions allows single crystals to be carefully grown. X-ray crystallography establishes the existence of three unique types of molecular structures for these azo complexes. Thus the 1:l complex with AgNOBin Figure 13 consists of a dimeric six-membered ring in which each silver(1) serves as a bridge between azo groups via the nitrogen lone pairs (n). On the other hand, the 1:l complex with CBr, in Figure 9 consists of an infinite chain with alternating azo units n-bound by a pair of bromine bridges-reminiscent of that previously established for difunctional amines such as in the dabco-CBr, complex. The unusual 2:l complex with TCNE in Figure 8 consists of a sandwich structure in which the central C - C unit is equally bound on each planar face by both nitrogen atoms of the azo moiety. The HOMO-LUMO interactions in these donoracceptor complexes are considered in the context of the anti-bonding (L)and bonding (n+) combination of the azo n-orbitals. The charge-transfer photochemistry of various EDA complexes of azoalkanes is examined briefly. The photochemical efficiency is qualitatively discussed in terms of the stability of the azoalkane cation-radical in the CT excited state, as independently evaluated by ESR studies following their formation by the freon matrix (C1,CF) method.
Azoalkanes are isoelectronic with alkenes. Although both classes of unsaturated comwunds are electron rich. onlv alkenes are known to interact readily with various types of electrophiles and electron acceptors (A) to form molecular complexes reversibly,'V2 i.e.
Chart I. Designation of the n and T Orbitals in Cis and Trans Azo Chromophores According to Descending Energy (Top to B ~ t t o m ) ~
\
+ A e [>C=C
