Iron−Rhodium and Iron−Iridium Mixed-Metal Nitrido−Carbonyl Clusters. Synthesis, Characterization, Redox Properties, and Solid-State Structure of the Octahedral Clusters [Fe5RhN(CO)15]2-, [Fe5IrN(CO)15]2-, and [Fe4Rh2N(CO)15]-. Infrared and Nuclear Magnetic Resonance Spectroscopic Studies on the Interstitial Nitride

The cluster [Fe5RhN(CO)15]2- was synthesized in 40% yield from [Fe4N(CO)12]- and [Rh(CO)4]- in refluxing tetrahydrofuran, whereas the analogous anion [Fe5IrN(CO)15]2- was prepared in CH3CN at room temperature from [Fe6N(CO)15]3- and [Ir(C8H14)2Cl]2; the yields are higher than 60%. The monoanion [Fe4Rh2N(CO)15]- was obtained in 70% yield from [Fe5RhN(CO)15]2- and hydrated RhCl3. The solid-state structures of the three anions were determined on their [PPh4]+ salts:  the six metal atoms are arranged in octahedral cages and are coordinated to 3 edge-bridging and 12 terminal carbonyl ligands and to a μ6-N ligand. The Rh and Ir atoms have less terminal COs than Fe, in order to equalize the excess electrons at the d9 metal centers. The two rhodium atoms in [Fe4Rh2N(CO)15]- are directly bound. The 15N NMR spectra of the three compounds have been recorded; the signals of the nitride ligands were found at δ = 514 ppm for the dianions and 470 ppm for [Fe4Rh2N(CO)15]-; any group 9 atom shifts the resonance of nitrogen to higher fields. The coupling constants J(15N−103Rh) are 8−9 Hz. The vibrational patterns of the metal cores have been interpreted on the basis of an idealized M6 octahedral arrangement, subsequently modified by the perturbations given by different atomic masses and M−M stretching force constants. The motions of the nitrogen are related to the idealized symmetry of the cage; the M−N force constant values depend on the type of metal and on the charge of the anion. The dianions [Fe5MN(CO)15]2- can be electrochemically oxidized at −20 °C to their short-lived monoanions, which can be characterized by EPR spectroscopy. In contrast, the cluster [Fe4Rh2N(CO)15]- undergoes a single-step 2-electron reduction to the partially stable trianion [Fe4Rh2N(CO)15]3-, which was also characterized by EPR spectroscopy. The Fe−Rh nitride clusters are active catalysts for the hydroformylation of 1-pentene, but display low selectivity (35−65%) in n-hexanal and are demolished under catalytic conditions.