Despite the critical role ruthenium and osmium complexes have played in the development of transition metal dinitrogen chemistry, they have not been previously shown to mediate catalytic N2-to-NH3 conversion (N2RR), nor have M-NxHy complexes been derived from protonation of M-N2 precursors (M = Ru, Os). To help delineate key factors for N2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N2RR, and compare their activities with an isostructural iron complex. The Os system is most active, and is demonstrated to liberate more than 120 equiv NH3 per Os center in a single batch experiment using Cp*2Co and [H2NPh2][OTf] as the reductant and acid source. Isostructural Ru and Fe complexes generate very little NH3 under the same conditions. Protonation of an anionic Os-N2− state affords a structurally characterized Os=NNH2+ hydrazido species that itself mediates NH3 generation, suggesting it is a plausible intermediate of the catalysis. Os-hydride species are characterized that form during catalysis as inactive species.

W­(CNAr)6 (CNAr = arylisocyanide) photoreductants catalyze base-promoted homolytic aromatic substitution (BHAS) of 1-(2-iodobenzyl)-pyrrole in deuterated benzene. Moderate to high efficiencies correlate with W­(CNAr)6 excited-state reduction potentials upon one-photon 445 nm excitation, with 10 mol % loading of the most powerful photoreductants W­(CNDipp)6 (CNDipp = 2,6-diisopropyl­phenyl­isocyanide) and W­(CNDippPhOMe3)6 (CNDippPhOMe3 = 4-(3,4,5-trimethoxy­phenyl)-2,6-diisopropyl­phenyl­isocyanide) affording nearly complete conversion. Stern–Volmer quenching experiments indicated that catalysis is triggered by substrate reductive dehalogenation. Taking advantage of the large two-photon absorption (TPA) cross sections of W­(CNAr)6 complexes, we found that photocatalysis can be driven with femtosecond-pulsed 810 nm excitation. For both one- and two-photon excitation, photocatalysis was terminated by the formation of seven-coordinate WII-diiodo [WI2(CNAr)5] complexes. Notably, we discovered that W­(CNDipp)6 can be regenerated by chemical reduction of WI2(CNDipp)5 with excess ligand present in solution.

Homoleptic tungsten(0) arylisocyanides possess photophysical and photochemical properties that rival those of archetypal ruthenium(II) and iridium(III) polypyridine complexes. Previous studies established that extending the π-system of 2,6diisopropylphenylisocyanide (CNDipp) by coupling aryl substituents para to the isocyanide functionality results in W(CNDippAr) 6 oligoarylisocyanide complexes with greatly enhanced metal-to-ligand charge transfer (MLCT) excited-state properties relative to those of W(CNDipp) 6 . Extending electronic modifications to delineate additional design principles for this class of photosensitizers, herein we report a series of W(CNAr) 6 compounds with naphthalene-based fused-ring (CN-1-(2-i Pr)-Naph) and CNDipp-based alkynyl-bridged (CNDipp CC Ar) arylisocyanide ligands. Systematic variation of the secondary aromatic system in the CNDipp CC Ar platform provides a straightforward method to modulate the photophysical properties of W(CNDipp CC Ar) 6 complexes, allowing access to an extended range of absorption/luminescence profiles and highly reducing excited states, while maintaining the high molar absorptivity MLCT absorption bands, high photoluminescence quantum yields, and long excited-state lifetimes of previous W(CNAr) 6 complexes. Notably, W(CN-1-(2-i Pr)-Naph) 6 exhibits the longest excited-state lifetime of all W(CNAr) 6 complexes explored thus far, highlighting the potential benefits of utilizing fused-ring arylisocyanide ligands in the construction of tungsten(0) photoreductants.

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P 3 X ligands (X = B, C, Si)have previously been shown to mediate catalytic N 2 -to-NH 3 conversion (N 2 RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P 3 B ) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and N 2 RR activity, Fe-N 2 complexes supported by analogous group 13 tris(phosphino)alane (P 3 Al ) and tris(phosphino)gallane (P 3 Ga ) ligands are synthesized. The series of P 3 X Fe-N 2 [0/1−] compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N 2 activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12crown-4) 2 ][P 3 X Fe-N 2 ] (X = Al, Ga) with excess acid/reductant in the form of HBAr F 4 /KC 8 generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH 3 per Fe, respectively. Similarly, the use of [H 2 NPh 2 ][OTf]/Cp* 2 Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH 3 . Preliminary reactivity studies confirming P 3 X Fe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N 2 RR may be responsible for the attenuated yields of NH 3 observed for P 3 Al Fe and P 3 Ga Fe relative to P 3 B Fe.

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The perchlorinated carba-closo-dodecaborate anion is typically inert toward B-Cl functionalization. We present here the observation of two competing reactions that occur with this anion at ambient temperature. When this molecule is treated with n-BuLi and subsequently reacted with tosyl azide, a cycloaddition occurs and results in chloride substitution at a B-Cl vertex. The competing and dominant pathway is a substitution reaction to form the azide N3CB11Cl11(-). This rare anionic carboranyl azide reacts with PPh3 in FC6H5 to afford a stable anionic phosphazide. When dissolved in tetrahydrofuran, the phosphazide is in equilibrium with free PPh3 and N3CB11Cl11(-). Both the triazole and phosphazide are characterized by single-crystal X-ray diffraction, NMR and IR spectroscopy, and high-resolution mass spectrometry.

complexes are one-photon visible and two-photon NIR photocatalysts for challenging reductive transformations.