The metal-carbon multiple bond in iron(I)-and iron(II)-dibenzotetramethyltetra[14]azaannulene: carbene, carbonyl, and isocyanide derivatives

The two parent compounds used for studying the iron-carbon multiple bond interactions are [Fe(tmtaa)] (1), and [Fe(tm-taa)Na(THF)(3)] (2) [tmtaa=dibenzotetramethyltetraaza[14]annulene dianion], the latter being obtained by reduction of 1. The reaction of 1 with CO led to the corresponding monocarbonyl derivative [Fe(tmtaa)(CO)(L)] [L = THF, 3; L = Py, 4], while the reaction with RNC allowed us to isolate mono-isocyanide [Fe(tmtaa)(o-Me3Si-C6H4NC)(THF)] (5), [Fe(tmtaa)((BuNC)-Bu-n)(THF)] (6), and bis-isocyanide [Fe(tmtaa)('BuNC)(2)] (7) derivatives. Reduction of 6 with sodium metal or the reaction of 1 with NaCN led to a monocyano derivative bridged into a dimeric form by sodium cations in [{Fe(tmtaa)(CN)}(2)(mu-NaLn)(2)] [L = THF, n = 3, 8a; L = DME, n = 2, 8b], while the reaction of 1 with (Bu4N+CN-)-Bu-n led to the monomeric form [Fe(tmtaa)(CN)](-)((Bu4N)-Bu-n)(+) (9). A detailed magnetic analysis of 1-10, the last one being the bis-pyridine derivative [Fe(tmtaa)(Py)(2)] (10) showed a variety of low and intermediate spin states, and spin crossovers (with a minor role played by high spin states) as a function of the axial ligands. A remarkable difference was observed with the analogous porphyrin derivatives. The d(7) iron(I) derivative 2 occurs in tight ion-pair form, both iron and sodium being bonded to the tmtaa ligand. The reaction of 2 with carbon monoxide led to a monocarbonyl derivative bridged in a dimeric form by sodium cations bonded to the oxygen atoms in [(Fe(tmtaa)}(2){mu-CONa(THF)(2)}(2)] (11). Both 2 and 11 showed a spin conversion between S=1/2 and S = 3/2, with a small antiferromagnetic coupling in the latter case, due to the dimeric form. The reaction of 1 with diazoalkane RR'CN2 led to the corresponding low-spin diamagnetic carbene derivatives [Fe(tmtaa)(CRR')] [R = R' = Ph, 12; R = Ph, R' = H, 13], the first one being by far more thermally stable, while the second one decomposes at room temperature to 1 and a mixture of cis and trans-stilbene. Both react with O-2 giving Ph2CO and PhCHO and the yore dimer [{Fe(tmtaa)}(mu-O)] (14). The proposed structures are supported by the X-ray analyses on complexes 2, 8b, 9, 11b and 12.