Magnetic Properties Diagnostic for the Existence of Iron(II)-Iron(II) Bonds in Dinuclear Complexes Which Derive from Stepwise Insertion Reactions on Unsupported Iron-Aryl Bonds

Several iron-aryl complexes have been considered for studying iron-aryl carbon bonds: [Fe(2)Mes(4)] (1, Mes = 2,4,6-Me(3)C(6)H2); [Fe-2(2,4,6-(Pr3C6H2)-C-i)(4)] (2), and [Py(2)FeMes(2)] (3, Py = pyridine). The first one, however, is the most accessible on a large scale and thus has been used in our exploratory work on insertion reactions. The reaction of 1 with Bu(t)NC led to the dimeric homoleptic iminoacyl complex [{eta(2)-C(Mes)=NBu(t)}Fe-2(2){mu-C(Mes)=NBu(t)}(2)] (4), containing a very short Fe-Fe distance [2.371(4) Angstrom]. The iron-carbon bonds present in complex 4 reacted further, inserting CO2 and CyN=C=NCy [Cy = C6H11] and leading to the monomeric complexes [{OC(O)C(Mes)=NBu(t)}(2)-Fe] (5) and [{CyNC(=NCy)C(Mes)=NBu(t)}Fe-2] (6). For complexes 5 and 6, respectively, the original iron-aryl carbon band has been functionalized twice by a successive insertion of Bu(t)NC and a cumulene. In the reaction with PhCN, only the bridging mesityl inserted the nitrile functionality to give [(PhCN)(2)(Mes)(2)Fe-2{mu-N=C(Mes)(Ph)}(2)] (7). Complex 7 contains a bridging imino group and has a rather long Fe-Fe distance [2.859(2) Angstrom]. The unreacted terminal mesityl group in 7 has been engaged in other insertion reactions, i.e. with CyN=C=NCy and PhNCO. In the resulting dimeric compounds [{CyNC(Mes)=NCy}Fe-2(2){mu-N=C(Mes)(Ph)}(2)] (8), and [{OC(Mes)=NPh)(2)}Fe-2-{mu-N=C(Mes)(Ph)}(2)] (9), the [Fe-2{mu-N=C(Mes)(Ph)}(2)] bimetallic core is conserved and the magnetic properties are very close to those of 7. In complex 10, [{(Mes)(Ph)C=N}Fe-2(2){mu-C(Mes)=NBu(t)}(2)], derived from the reaction of 7 with Bu(t)NC, the iminoacyl replaces the imino group as bridging ligand. This conclusion is strongly supported by the magnetic analysis. All of the iron(II)-iron(II) dimers reported here have strongly reduced magnetic moments. A detailed analysis of the magnetic susceptibility as a function of the temperature allowed us to make a dear distinction between antiferromagnetic couplings and the presence of a metal-metal bond. Crystallographic details are as follows: 2 is monoclinic; space group C2; a = 26.154(7), b = 12.868(6), c = 19.476(6) Angstrom; beta = 108.06(4)degrees; Z = 4; and R = 0.057. 3 is monoclinic; space group P2(1)/n; a = 8.190(1), b = 22.141(3), c = 14.674(3) Angstrom; beta = 102.52(4)degrees; Z = 4; and R = 0.061. 4 is monoclinic; space group P2/n; a = 15.879(2), b = 10.865(1), c = 18.126(3) Angstrom; beta = 108.52(2)degrees; Z = 2; and R = 0.038. 6 is triclinic; space group P (1) over bar; a = 14.476(2), b = 16.378(2), c = 12.122(1) Angstrom; alpha = 90.31(1)degrees, beta = 111.74(2)degrees, gamma = 93.12(1)degrees; Z = 2; and R = 0.051. 7 is triclinic; space group P (1) over bar; a = 12.680(3), b = 13.510(3),c = 10.870(5) Angstrom; alpha = 107.63(2)degrees, beta = 104.30(3)degrees, gamma = 67.00(2)degrees; Z = 1; and R = 0.060.