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.

Determining the electronic structure of actinide complexes is intrinsically challenging because inter-electronic repulsion, crystal field, and spin–orbit coupling effects can be of similar magnitude. Moreover, such efforts have been hampered by the lack of structurally analogous families of complexes to study. Here we report an improved method to U≡N triple bonds, and assemble a family of uranium(V) nitrides. Along with an isoelectronic oxo, we quantify the electronic structure of this 5f1 family by magnetometry, optical and electron paramagnetic resonance (EPR) spectroscopies and modelling. Thus, we define the relative importance of the spin–orbit and crystal field interactions, and explain the experimentally observed different ground states. We find optical absorption linewidths give a potential tool to identify spin–orbit coupled states, and show measurement of UV···UV super-exchange coupling in dimers by EPR. We show that observed slow magnetic relaxation occurs via two-phonon processes, with no obvious correlation to the crystal field.

Despite the burgeoning field of uranium-ligand multiple bonds, analogous complexes involving other actinides remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, oxygen, sulfur, selenium and tellurium are reported, and no multiple bonds to phosphorus are known, reflecting a general paucity of synthetic methodologies and also problems associated with stabilising these linkages at the large thorium ion. Here we report structurally authenticated examples of a parent thorium(IV)–phosphanide (Th–PH2), a terminal thorium(IV)–phosphinidene (Th=PH), a parent dithorium(IV)–phosphinidiide (Th–P(H)-Th) and a discrete actinide–phosphido complex under ambient conditions (Th=P=Th). Although thorium is traditionally considered to have dominant 6d-orbital contributions to its bonding, contrasting to majority 5f-orbital character for uranium, computational analyses suggests that the bonding of thorium can be more nuanced, in terms of 5f- versus 6d-orbital composition and also significant involvement of the 7s-orbital and how this affects the balance of 5f- versus 6d-orbital bonding character.

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.

Reaction of [Ln(I)3(THF)4] (Ln = Ce, Pr) or [Ln(I)3(THF)3.5] (Ln = Nd, Sm, Gd, Dy, Er) with three equivalents of [KBz] (Bz = CH2C6H5) at 0 degrees C afforded the corresponding lanthanide tri-benzyl complexes [Ln(Bz)3(THF)3] [Ln = Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Dy (7), Er (8) La (11)] in 48-75% crystalline yields, with the exception of the redox active samarium complex, which was isolated in poor (20%) yield. Complexes 2-8 were found to adopt distorted octahedral geometries, where the Bz and THF groups are bound in a mutually fac manner in the solid state. Although the series is structurally similar, classification of three structural types can be made on the basis of the lanthanide contraction: (i) complexes which exhibit three eta(2) Ln...C(ipso) contacts (1-4, 11); (ii) complexes which show one eta(2) Ln...C(ipso) contact (5); (iii) complexes with no multi-hapto interactions (6-8). For ytterbium, the mixed valence, Yb(II)/Yb(III) complex [Yb(II)(Bz)(THF)5]+[Yb(III)(Bz)4(THF)2]- (9) was reproducibly formed at 0 degrees C and -78 degrees C as a result of partial (50%) Yb(III) --> Yb(II) reduction with concomitant formation of half an equivalent of 1,2-diphenylethane by oxidative coupling. Tri-valent [Yb(Bz)3(THF)3] (10) was apparently not formed. The synthetic utility of tri-benzyl lanthanide complexes 2-8 and 11 were tested in reactions with the bis-(iminophosphorano)methane H2C(PPh2NSiMe3)2 (H2-BIPM), which afforded [Ln(BIPM)(H-BIPM)] [Ln = La (12), Ce (13), Pr (14), Nd (15), Sm (16), Gd (17)] and [Ln(BIPM)(Bz)(THF)] [Ln = Dy (18), Er (19)]. Compounds 2-9 and 12-19 have been variously characterised by X-ray crystallography, multi-nuclear NMR spectroscopy, FTIR spectroscopy, room temperature Evans method solution magnetic moments and CHN micro-analyses.