REDOX BEHAVIOR OF THE BLACK ROUSSINATE [FE4S3(NO)7]- MONOANION - SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF THE [FE4S3(NO)7]N- (N = 2, 3) ANIONS AND CRYSTAL-STRUCTURES OF THE MONOANIONS AND DIANIONS IN THEIR [NET4]+ SALTS

A reinvestigation of the redox behavior of [Fe4S3(NO)7]- by electrochemical techniques shows the existence of a four-step electron-transfer sequence, in which the overall charge changes from -1 to -4; the reactions are electrochemically reversible, and the electrogenerated [Fe4S3(NO)7]n- (n = 2, 3) species appear to be stable. The redox potentials (V vs SCE) in acetonitrile solution are as follows: E-1/-2 = -0.68, E-2/-3 = -1.26, E-3/-4 = -1.75. Similar results have also been obtained in dichloromethane and tetrahydrofuran solutions. The [Fe4S3(NO)7]2-species has been chemically generated by reduction of the parent monoanion with sodium naphthalenide in tetrahydrofuran, isolated in a crystalline state as the [NEt4]+ salt, and spectroscopically characterized in several solvents; the corresponding [Fe4S3(NO)7]3- Species has been obtained by reduction of the latter in hexamethylphosphortriamide with sodium and has only been spectroscopically investigated 'in situ''. The even-electron [Fe4S3(NO)7]3- is EPR silent, whereas the odd-electron [Fe4S3(NO)7]2- exhibits an EPR spectrum which suggests the presence in solution of a mixture of two paramagnetic species. Their relative abundance is mainly a function of the solvent, counterion, temperature, and time. The first species shows a signal at g = 2.020 consisting of three equally intense lines due to the coupling of the unpaired electron with a unique N-14 nucleus (a(N-14) = 5.0 G). The second displays an EPR signal consisting of a 1:2:3:2:1 quintet at g = 2.026, due to the coupling with two equivalent N-14 nuclei (a(N-14) = 2.7 G). EHMO calculations performed on [Fe4S3(NO)7]- with an idealized C3v symmetry indicate that the LUMO is doubly degenerate; these two molecular orbitals are antibonding with respect to the Fe-Fe, Fe-S, and Fe-NO interactions and receive contribution mainly from the four iron atoms and either one or the remaining two axial nitrosyl groups, respectively. Single occupation of this energy level should result into loss of degeneracy by distortion of the metal framework from C3v to C(s) idealized symmetry along E vibrational modes; depending on the deformation, a triplet or a quintet is to be expected in the EPR signal because of coupling of the unpaired electron with either one or two equivalent nitrogen atoms. An X-ray determination of the structure of [Fe4S3(NO)7]2- as the [NEt4]+ Salt has been achieved. (Crystal data: a = 10.248(3), b = 17.814(8), c = 18.475(4) angstrom; beta = 103.33(3)-degrees; space group P2(1)/c; R = 0.045.) Comparison of its structural parameters with those of [Fe4S3(NO)7]-as the [AsPh4]+ salt failed to confirm unambiguously the presence of the above distortions, probably because of the influence of packing effects. Experimental confirmation of the expected distortions has only been obtained following an X-ray redetermination of the structure of the [Fe4S3(NO)7]- monoanion as its [NEt4]+ salt. (Crystal data: a = 9.161(4), b = 10.254(2), c = 13.480(4) angstrom; alpha = 97.71(2), beta = 109.86(3), gamma = 94.82(2)-degrees; space group P1BAR; R = 0.018.)