Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris(aryloxide) U(ODtbp)(3), U(OC(6)H(3)-Bu(t)(2)-2,6)(3), and the tri-tert-butyl analogue U(OTtbp)(3), U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3), in the form of bridging, side-on dinitrogen complexes [U(OAr)(3)](2)(μ-η(2):η(2)-N(2)), for which the tri-tert-butyl N(2) complex is the most robust U(2)(N(2)) complex isolated to date. Attempted reduction of the tris(aryloxide) complex under N(2) gave only the potassium salt of the uranium(III) tetra(aryloxide) anion, K[U(OAr)(4)], as a result of ligand redistribution. The solid-state structure is a polymeric chain formed by each potassium cation bridging two arenes of adjacent anions in an η(6) fashion. The same uranium tris(aryloxides) were also found to couple carbon monoxide under ambient conditions to give exclusively the ynediolate [OCCO](2-) dianion in [U(OAr)(3)](2)(μ-η(1):η(1)-C(2)O(2)), in direct analogy with the reductive coupling recently shown to afford [U{N(SiMe(3))(2)}(3)](2)(μ-η(1):η(1)-C(2)O(2)). The related U(III) complexes U{N(SiPhMe(2))(2)}(3) and U{CH(SiMe(3))(2)}(3) however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3) reacts with CO(2) to give an insertion product containing bridging oxo and aryl carbonate moieties, U(2)(OTtbp)(4)(μ-O)(μ-η(1):η(1)-O(2)COC(6)H(2)-Bu(t)(3)-2,4,6)(2), which has been structurally characterized. The presence of coordinated N(2) in [U(OTtbp)(3)](2)(N(2)) prevents the occurrence of any reaction with CO(2), underscoring the remarkable stability of the N(2) complex. The di-tert-butyl aryloxide does not insert CO(2), and only U(ODtbp)(4) was isolated. The silylamide also reacts with carbon dioxide to afford U(OSiMe(3))(4) as the only uranium-containing material. GGA and hybrid DFT calculations, in conjunction with topological analysis of the electron density, suggest that the U-N(2) bond is strongly polar, and that the only covalent U→N(2) interaction is π backbonding, leading to a formal (U(IV))(2)(N(2))(2-) description of the electronic structure. The N-N stretching wavenumber is preferred as a metric of N(2) reduction to the N-N bond length, as there is excellent agreement between theory and experiment for the former but poorer agreement for the latter due to X-ray crystallographic underestimation of r(N-N). Possible intermediates on the CO coupling pathway to [U(OAr)(3)](2)(μ-C(2)O(2)) are identified, and potential energy surface scans indicate that the ynediolate fragment is more weakly bound than the ancillary ligands, which may have implications in the development of low-temperature and pressure catalytic CO chemistry.

Transition-metal-arene complexes such as bis(benzene)chromium Cr(η(6)-C(6)H(6))(2) are historically important to d-orbital bonding theory and have modern importance in organic synthesis, catalysis and organic spintronics. In investigations of f-block chemistry, however, arenes are invariably used as solvents rather than ligands. Here, we show that simple uranium complexes UX(3) (X = aryloxide, amide) spontaneously disproportionate, transferring an electron and X-ligand, allowing the resulting UX(2) to bind and reduce arenes, forming inverse sandwich molecules [X(2)U(µ-η(6):η(6)-arene)UX(2)] and a UX(4) by-product. Calculations and kinetic studies suggest a 'cooperative small-molecule activation' mechanism involving spontaneous arene reduction as an X-ligand is transferred. These mild reaction conditions allow functionalized arenes such as arylsilanes to be incorporated. The bulky UX(3) are also inert to reagents such as boranes that would react with the traditional harsh reaction conditions, allowing the development of a new in situ arene C-H bond functionalization methodology converting C-H to C-B bonds.

Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.

a b s t r a c tIt is shown that the deprotonation of bulky amides such as HN(SiMe 2 Ph) 2 may be accelerated by the use of catalytic quantities of an alkali metal tert-butoxide salt, affording, for example, overnight syntheses of NaN(SiMe 2 Ph) 2 . The new uranium(IV) and uranium(III) complexes [U{N(SiMe 2 H) 2 } 4 ] and [U{N (SiMe 2 Ph) 2 } 3 ] are both accessible from the Group 1 salts of the amides and UI 3 (thf) 4 in thf. The choice of sodium or potassium salt made no difference to the reaction outcome. Both exhibit Weak interactions between uranium and with silyl-H or silyl-Ph groups in the solid-state.

The synthesis and X-ray crystal structures of five N-heterocyclic stannylenes are reported. These compounds, containing a variety of backbones, were prepared by the salt metathesis of the appropriate dilithiated diamide with SnCl(2) and show a high degree of thermal stability compared to the corresponding species with unsaturated backbones. If bulky diisopropylphenyl groups are attached to the nitrogen centers then the structures are monomeric, but when the less bulky mesityl groups are employed the solid-state structure was shown to be dimeric.

Cyclic diamino plumbylenes derived from saturated heterocycles are obtained from deprotonation of diamines and subsequent reaction with PbCl(2), or by reaction of a suitable diamine with Pb[N(SiMe(3))(2)](2). Single crystal X-ray studies have been used to probe the solid state structures of a range of these complexes and have shown the fine balance between monomer and dimer formation which is related to the bulk of the organic group attached to the nitrogen atoms. Dimerisation is also shown to effect structural changes within the core of the heterocyclic plumbylene.

Diphosphorus ligands connected by a single atom (RPEPR; E = CR, C[double bond, length as m-dash]CR and NR) give chelating ligands with very small bite-angles (natural bite-angle of 72° for dppm) as well as enable access to other properties such as bridging modes and hemilability. Their use in catalysis has been growing over the last two decades as researchers have sought to apply the properties of small bite-angle ligands to a wide number of catalytic reactions, often complementing the well-established applications of wide bite-angle ligands in catalysis. This Perspective reviews the properties of diphosphorus ligands featuring a single-atom linker and their use in several catalytic transformations of alkenes, including selective ethene oligomerisation, ethene polymerisation and co-polymerisation with CO, hydroacylation and hydrogenation, as well as their use in transfer hydrogenation and hydrogen-borrowing reactions.

Ligand backbone alkylation of the complex [Cr(CO) 4 (dppm)] (dppm = bis(diphenylphosphino)methane) with alkyl iodides yields the C-substituted dppm ligand complexes [Cr(CO) 4 {Ph 2 PCH(R)PPh 2 }] (R = methyl, n-hexyl, benzyl). Activation of these complexes via one-electron oxidation with Ag[(Al(OC 4 F 9 ) 4 ] and CO removal with triethylaluminium, or (in the case of R=methyl) by in situ treatment of the free ligand with a chromium salt and modified methyl alumoxane (MMAO), leads to catalysts showing some selectivity for ethylene trimerization and tetramerization. NMR spectroscopic studies of the parent dppm or [Cr(CO) 4 (dppm)] compounds suggest that ligand deprotonation and decomplexation may be the cause of the surprisingly poor catalytic performance of these specific derivatives.