The catalytic fixation of N by molecular Fe compounds is a rapidly developing field, yet thus far few complexes can effect this transformation, and none are selective for NH production. Herein we report that the simple Fe(0) complex Fe(EtPCHCHPEt)(N) (1) is an efficient catalyst for the selective conversion of N (>25 molecules N fixed) into NH, attendant with the production of ca. one molecule of NH. Notably, the reductant (CoCp*) and acid (PhNHOTf) used are considerably weaker than conventional chemical H and e sources used in previous demonstrations of N turnover by synthetic Fe compounds. These results show that the direct catalytic conversion of N to the hydrazine oxidation state on molecular Fe complexes is viable and that the mechanism of NH formation by such systems may proceed via Fe-NH intermediates.

Catalytic reduction of N2 to NH3 by a Ti complex has been achieved, thus now adding an early d‐block metal to the small group of mid‐ and late‐d‐block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH3 by N2 reduction and protonolysis under homogeneous, abiological conditions. Reduction of [TiIV(TrenTMS)X] (X=Cl, 1A; I, 1B; TrenTMS=N(CH2CH2NSiMe3)3) with KC8 affords [TiIII(TrenTMS)] (2). Addition of N2 affords [{(TrenTMS)TiIII}2(μ‐η1:η1‐N2)] (3); further reduction with KC8 gives [{(TrenTMS)TiIV}2(μ‐η1:η1:η2:η2‐N2K2)] (4). Addition of benzo‐15‐crown‐5 ether (B15C5) to 4 affords [{(TrenTMS)TiIV}2(μ‐η1:η1‐N2)][K(B15C5)2]2 (5). Complexes 3–5 treated under N2 with KC8 and [R3PH][I], (the weakest H+ source yet used in N2 reduction) produce up to 18 equiv of NH3 with only trace N2H4. When only acid is present, N2H4 is the dominant product, suggesting successive protonation produces [{(TrenTMS)TiIV}2(μ‐η1:η1‐N2H4)][I]2, and that extruded N2H4 reacts further with [R3PH][I]/KC8 to form NH3.

Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5–8), cyclobutadienyl (C4) complexes remain exceedingly rare. Here, we report that reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use of a borohydride and preformed C4‐unit circumvents difficulties in product isolation and closing a C4‐ring at uranium. Complex 3 is an unprecedented example of an f‐element half‐sandwich cyclobutadienyl complex, and it is only the second example of an actinide‐cyclobutadienyl complex, the other being an inverse‐sandwich. The U−C distances are short (av. 2.513 Å), reflecting the formal 2− charge of the C4‐unit, and the SiMe3 groups are displaced from the C4‐plane, which we propose maximises U−C4 orbital overlap. DFT calculations identify two quasi‐degenerate U−C4 π‐bonds utilising the ψ2 and ψ3 molecular orbitals of the C4‐unit, but the potential δ‐bond using the ψ4 orbital is vacant.

The organometallic first-row transition-metal complexes [M(2,2'-bipy)(mes)2] (M = Cr (1), Mn (2), Co (4), Ni (5); 2,2'-bipy = 2,2'-bipyridine; mes = 2,4,6-Me3C6H2) were reacted with potassium and a suitable alkali-metal sequestering agent to yield salts of the anionic species [M(2,2'-bipy)(mes)2](-). The neutral parent compounds and their corresponding anionic congeners were characterized by single-crystal X-ray diffraction in [Cr(2,2'-bipy)(mes)2]·1.5C6H6, [Mn(2,2'-bipy)(mes)2], [Co(2,2'-bipy)(mes)2]·THF, [Ni(2,2'-bipy)(mes)2], [K(dibenzo-18-crown-6)·THF][Cr(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·0.67py·0.67tol, [K(2,2,2-crypt)][Co(2,2'-bipy)(mes)2], and [K(2,2,2-crypt)][Ni(2,2'-bipy)(mes)2]. These species, along with the previously reported neutral and anionic iron complexes [Fe(2,2'-bipy)(mes)2](0/-) (3/3(-)), form a homologous series of compounds which allow for an in-depth study of the interactions between metals and ligands. Single-crystal X-ray diffraction data, DFT calculations, and various spectroscopic and magnetic measurements indicate that the anionic complexes (1(-)-5(-)) can be best formulated as M(II) complexes of the 2,2'-bipyridyl radical anion. These findings complement recent studies which indicate that bond metric data from single-crystal X-ray diffraction may be employed as an important diagnostic tool in determining the oxidation states of bipyridyl ligands in transition-metal complexes.

The thermally robust silylium complex [iPr3Si-PtBu3](+)[B(C6F5)4](-) (1) activates H2/D2 at 90 °C (PhCl); no evidence for dissociation into the separated Lewis pair is found. DFT calculations show H2 cleavage proceeds via Si-P bond elongation to form an encounter complex directly from the adduct, thus avoiding the non-isolable iPr3Si(+)-PtBu3 FLP.