New Adducts of Dirhodium(II) Formamidinate Complexes with Polycyano Acceptors. X-Ray Crystal Structure of the Tricyanomethanide Complex Rh2(form)4[C(CN)3] (form = N,N'-di-p-tolylformamidinate).

The dirhodium(II) formamidinate complexes Rh2(form)2(O2CCF3) 2(H2O)2 (I) and Rh2(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 Rh2(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 {[Rh2(form)2(O2CCF3) 2]2TCNQ)}n (2) and [Rh2(form)2(O2CCF3) 2X]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 [Rh2(form)4X] (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 Rh2(form)4[C(CN)3] (SA), 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) Å, c = 29.20(2) Å, V = 5863(5) Å3, 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).