Coordination Chemistry of the Nitronyl and Imino Nitroxides. Linear Chain Adducts with Rhodium(II) Trifluoroacetate Dimer

Article abstract of DOI:10.1021/ja00191a020

The nitronyl and imino nitroxides (2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-oxide (NITPh), 2,4,4,5,5-pentamethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-oxide (NITMe), 2,4,4,5,5-pentamethyl-4,5-dihydro-1H-imidazolyl-1-oxy (IMMe)) react with tetrakis(trifluoroacetato)dirhodium(II) (Rh2(tfac)4) resulting in adducts which are bisnitroxyl complexes or extended linear chains of alternating organic free radicals and metal fragments.Structural data for these adducts are as follows: Rh2(tfac)4*(NITPh)2, 1, space group P<*>, a = imino nitroxides. (1) Angstroem,b = 10.529 (1) Angstroem, c = 14.175 (1) Angstroem, α = 73.71 (1) deg, β = 77.23 (1) deg, γ = 80.23 (1) deg, Z = 1; Rh2(tfac)4*NITMe, 2, space group P2₁/n, a = 11.917 (1) Angstroem, b = 16.220 (2) Angstroem, c = 14.402 (2) Angstroem, β = 100.24 (1) deg, Z = 2; Rh2(tfac)4*IMMe2, 3, space group I4₁/a, a = 23.850 (2) Angstroem, b = 23.850 (2) Angstroem, c = 12.636 (1) Angstroem, Z = 8; Rh2(tfac)4*IMMe, 4, space group P2₁/c, a = 8.614 (1) Angstroem, b = 17.800 (2) Angstroem, c = 18.133 (2) Angstroem, β = 98.65 (1) deg, Z = 4.Both compounds 1 and 3 are centrosymmetric bis-adducts containing respectively axial O-bonded nitronyl nitroxides and N-bonded imino nitroxides.On the other hand compounds 2 and 4 are extended chains of nitroxides bridged by metal fragments.Magnetic data were obtained for the four compounds between 6 and 300 K and were interpreted using the models derived from the structural results.The O-bonded adducts, 1 and 2, exhibit fairly large antiferromagnetic interactions (-167 and -197 cm^-1), while the N-bonded adducts, 3 and 4, display weak (positive or negative, -11 and +4 cm^-1) internitroxide couplings.Extended Hueckel calculations show that this difference is not the consequence of different electronic structures for the nitronyl and the imino nitroxides.Furthemore, the analysis of the short contacts in the X-ray structures demonstrates that the internitroxyl coupling is mediated by the Rh-Rh bond.On the basis of local symmetry considerations, it is proposed that this mediation mainly involves a metal fragment orbital of ? symmetry.It is possible, considering this ? mechanism, to make a quantitative comparison of the magnitude of the coupling constants in two of the complexes.Finally, an EHT calculation on compound 1 shows that the SOMO of each nitroxide does indeed interact mainly with the ? and the ?^* orbitals of the rhodium dimer moiety.