New adducts of dirhodium(II) formamidinate complexes with polycyano acceptor molecules. X-ray crystal structure of the tricyanomethanide complex Rh-2(form)(4)[C(CN)(3)] (form equals N,N'-Di-p-tolylformamidinate)

The dirhodium(II) formamidinate complexes Rh-2(form)(2)(O2CCF3)(2)(H2O)(2) (I) and Rh-2(form)(4) (II, form = N,N'-di-p-tolylformamidinate) react with the polycyano acceptor molecules tetracyanoethylene (TCNE), tetracyano-p-quinodimethane (TCNQ), 2,5-dimethyl-N,N'-dicyano-p-quinonediimine (2,5-DMDCNQI), and N,N'-naphthocyano-p-quinonediimine (NCNQI) giving species whose nature is critically dependent on the redox potentials of the two parent complexes. Complex I reacts via axial coordination with negligible charge transfer (CT) from the dimetal unit to the ligand. With TCNE, it gives the labile monoaxial adduct Rh-2(form)(2)(O2CCF3)(2)(TCNE) (1), which easily loses the cyano ligand restoring the parent complex. TCNQ, 2,5-DMDCNQI, and NCNQI react with I giving polymeric materials of composition {[Rh-2(form)(2)(O2CCF3)(2)](2)TCNQ)}(n) (2) and [Rh-2(form)(2)(O2CCF3)(2)X](n) (X = 2,5-DMDCNQI (3), NCNQI (4)). The reaction of II with TCNE, TCNQ, and 2,5-DMDCNQI proceeds via a single electron transfer from the dimetal unit to the cyano ligand to form the CT species [Rh-2(form)(4)X] (X = TCNE (5), TCNQ (6), 2,5-DMDCNQI (7)). Electrochemical and EPR measurements suggest a different extent of coordination between the polycyano fragment and the dirhodium unit, depending upon the polarity of the solvents. Attempts to crystallize complex 5 from acetonitrile unexpectedly led to the formation of the tricyanomethanide complex Rh-2(form)(4)[C(CN)(3)] (5A), arising from the unprecedented transformation of the tetracyanoethylenide ion into the tricyanomethanide anion. The complex crystallizes in the tetragonal P4/ncc space group with a = 14.169(6) Angstrom, c = 29.20(2) Angstrom, V = 5863(5) Angstrom(3), and Z = 4. The molecule consists of a dirhodium unit symmetrically bridged by four formamidinate ligands and one tricyanomethanide anion N-coordinated at the axial position of Rh(2).