Since our discovery of the catalytic reduction of dinitrogen to ammonia at a single molybdenum center, we have embarked on a variety of studies designed to further understand this complex reaction cycle. These include studies of both individual reaction steps and of ligand variations. An important step in the reaction sequence is exchange of ammonia for dinitrogen in neutral molybdenum(III) compounds. We have found that this exchange reaction is first order in dinitrogen and relatively fast (complete in <1 h) at 1 atm of dinitrogen. Variations of the terphenyl substituents in the triamidoamine ligand demonstrate that the original ligand is not unique in its ability to yield successful catalysts. However, complexes that contain sterically less demanding ligands fail to catalyze formation of ammonia from dinitrogen; it is proposed as a consequence of a base-catalyzed decomposition of a diazenido (Mo-NANH) intermediate.

A series of compounds of the form [M(2)L(4)] and [[((t)()BuCO(2))(3)M(2)](2)(mu-L')] have been made where M = Mo or W, L = a thienyl, bithienyl, or terthienyl carboxylate, and L' = a corresponding thienyl dicarboxylate. The electronic absorption spectra are reported and the electronic structures discussed. Intense metal-to-ligand charge transfer bands traverse the visible and near-IR regions of the electronic absorption spectrum. The compounds show reversible metal-based oxidations and quasireversible ligand-based reductions. The molecular structure of Mo(2)(O(2)C-2-Th)(4).2THF is reported, on the basis of a single crystal X-ray diffraction study. These compounds provide insight into the expected properties of related dimetalated polythiophenes incorporating MM quadruple bonds.

The photophysical properties of the series of quadruply bonded M(2)(O(2)C-Ar)(4) [M = Mo, Ar = phenyl (ph), 1-naphthalene (1-nap), 2-naphthalene (2-nap), 9-anthracene (9-an), 1-pyrene (1-py), and 2-pyrene (2-py); M = W, Ar = ph, 2-nap] complexes were investigated. The lowest energy absorption of the complexes is attributed to a metal-to-ligand charge transfer (1)MLCT transition from the metal-based delta HOMO to the pi* O(2)C-Ar LUMO. The Mo(2)(O(2)C-Ar)(4) complexes exhibit weak short-lived emission (<10 ns) and a nonemissive, long-lived (40-76 mus) excited state detected by transient absorption spectroscopy. The short- and long-lived species are attributed to the (1)MLCT and (3)MLCT excited states, respectively, based on the large Stokes shift, vibronic progression in the low-temperature emission spectrum, and solvent dependence. Comparisons are made to the W(2)(O(2)C-Ar)(4) complexes, which are easier to oxidize and exhibit greater spin-orbit coupling than the Mo(2) systems. From the excited-state energy of the emissive (1)MLCT state and the electrochemical properties of the complexes, it is predicted that this excited state should be a powerful reducing agent. The crystal and molecular structure of Mo(2)(O(2)C-9-an)(4) is also reported together with electronic structure calculations employing density functional theory. To our knowledge, this is the first observation of MLCT excited states in quadruply bonded complexes. In addition, the photophysical properties of the present systems parallel those of organic aromatic molecules and may be viewed as metal-mediated organics. The introduction of the M(2) delta orbital in the complexes in conjugation with the organic pi-system of the ligands affords the opportunity to tune the excited-state energies and redox potentials.

Zinc dust reduction of tris(acetylacetonato)ruthenium(III), [Ru(acac)3], in hot aqueous THF in the presence of ethene gives cis-[Ru(acac)2(η2-C2H4)2] (5), isolated as an orange solid in ca. 60% yield. A single-crystal X-ray crystallographic study shows the metal atom to be coordinated octahedrally by a pair of bidentate O-bonded acac groups and two mutually cis orthogonal ethene ligands. In the NMR spectra of 5, the ethene carbon atoms remain equivalent, while the protons are inequivalent in pairs, down to −95 °C; intermolecular exchange with ethene is slow on the NMR time scale at room temperature but becomes fast above ca. 80 °C. One of the ethene molecules of 5 is replaced by ligands at room temperature to give [Ru(acac)2(η2-C2H4)(L)] [L = SbPh3 (6), MeCN (7), NH3 (8), and C5H5N (9)]. Complexes 6 and 7 are isolated as cis-isomers, 9 as a cis − trans mixture, and 8 in the form of the trans-isomer which transforms into the cis-isomer when heated or chromatographed on alumina. The structures of cis-8 and trans-9 have been confirmed by X-ray crystallography. The results indicate that ligand substitutions on 5 may occur via a square pyramidal intermediate [Ru(acac)2(η2-C2H4)] in which ethene occupies the apical site. All the complexes display either a quasi-reversible or a fully reversible one-electron oxidation by cyclic voltammetry at −50 °C, with E 1/2 values in the range +0.37−0.59 V for 6−9 and +0.95 V for 5 (in 0.5 M [Bu4N]PF6/CH2Cl2 vs Ag/AgCl/MeCN). The blue oxidized species 5 + , 6 + , and 8 + can be generated by bulk electrolysis and are stable for hours at −50 °C. Their UV−vis spectra and the ESR spectra of 6 + and 8 + indicate that the oxidized species are ruthenium(III)-ethene cations. Oxidation of the known cyclooctene complex cis-[Ru(acac)2(η2-C8H14)(SbPh3)] (2) with AgPF6 at −50 °C gives an isolable, deep blue ruthenium(III) salt [2]+[PF6]-, which is stable at room temperature.