The dominant form of elemental nitrogen on Earth is dinitrogen, but elemental phosphorus is found predominantly as white phosphorus or other singly bonded allotropes. Thus, there is interest in studying diphosphorus derivatives, most notably trapping between metal ions affording diphosphorus in +2, +1 center dot, 0, 1-center dot, 2-, and 4- charge states. However, the diphosphorus radical trianion form had previously remained elusive due to the instability of main-group diatomics with large, odd negative charges. Here, we disclose a crystalline diuranium diphosphorus radical trianion complex with strong antiferromagnetic uranium-phosphorus magnetic exchange coupling parameters of up to-731 cm-1. This value is over five times greater than that of lanthanide analogues and is comparable to or exceeds d-block metal-metal and metal-ligand exchange couplings, despite being based on a 5f metal, which is typically regarded as possessing contracted valence orbitals compared with d-block ions. This highlights exchange-coupled f-element-p-block radical bridged character that can be engendered in molecular magnetism.

Developing molecular spin technologies requires microscopic knowledge of their spin-dynamics. Calculation of phonon modes, phonon scattering and spin-phonon coupling for a dysprosocenium single-molecule magnet (SMM) give simulations of spin-dynamics that agree with experiment. They show that low-energy phonon scattering is a significant contribution to the high-performance of dysprosocenium SMMs.

The development of technologies exploiting molecular spins, such as single-molecule magnets (SMMs) and molecular qubits, requires detailed knowledge of the microscopic spin-dynamics, which in turn relies on understanding the spin-phonon coupling. Building on our recent work, we perform ab initio calculations of the phonon modes, anharmonic phonon scattering and spin-phonon coupling in a molecular crystal of the dysprosocenium SMM [DyCpttt2][B(C6F5)4]. Spin-dynamics simulations give excellent agreement with experiment and provide further insight into the high-performance of the dysprosocenium class of SMMs. We find that strong phonon scattering in the (pseudo-)acoustic regime leads to very short phonon lifetimes, large phonon linewidths, and, consequently, slower spin relaxation.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D3h-symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR and UV-Vis-NIR spectroscopies, SQUID magnetometry and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, whilst two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M–Si bonds, and dissociation of THF from 1-M in solution to give the locally C3v-symmetric complexes [M{Si(SiMe3)3}3(THF)n] (n = 0 or 1), which show restricted rotation of M–Si bonds on the NMR timescale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution, and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Lanthanides have recently shown magnetic memory at both the atomic and molecular level. The temperatures at which single-molecule magnets (SMMs) show magnetic hysteresis only increased from 4 K to 14 K in nearly 25 years since the discovery of the first SMM in 1993. However, since 2017 energy barriers to magnetic reversal (Ueff) from 1237(28)-1631(25) cm–1 and open magnetic hysteresis loops between 40-80 K have often been delivered by molecules featuring rigid aromatic ligands in axial dysprosium cyclopentadienyl complexes and their derivatives. Here we report a dysprosium bis(amide)-alkene complex, [Dy{N(SiiPr3)[Si(iPr)2C(CH3)=CHCH3]}{N(SiiPr3)(SiiPr2Et)}][Al{OC(CF3)3}4] (1-Dy), which shows Ueff = 1843(11) cm–1 and slow closing of magnetic hysteresis loops up to 100 K. Calculations show that the record Ueff value of 1-Dy arises from the charge-dense bis(silyl)amide ligands, with a pendant alkene taking a structural role to enforce a large N–Dy–N angle while imposing only a weak equatorial interaction. This leads to molecular spin dynamics up to one hundred times slower than the current best SMMs above 100 K.

Treatment of [UIV(N3)(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) with excess Li resulted in the isolation of [{UIV(μ-NLi2)(TrenTIPS)}2] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [UIII(TrenTIPS)] (3), and together with known [{UV(μ-NLi)(TrenTIPS)}2] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{UIV(TrenTIPS)}2(μ-NH)(μ-NLi2)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(IV). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of -4.1 and -3.4 cm-1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the 'diamond-core' dinitride of 2 and matrix-isolated binary U2N2.

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K2{U(OSi(O t Bu)3)3(μ-N)}2], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi(O t Bu)3)3(μ-N)}2], 2 and [K2{U(OSi(O t Bu)3)3}2(μ-N)2(μ-I)], 3 while oxidation with a stronger oxidant ("magic blue") yields the U(vi)/U(vi) complex [{U(OSi(O t Bu)3)3}2(μ-N)2(μ-thf)], 4. The three complexes show very different stability and reactivity, with N2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K2{U(OSi(O t Bu)3)3(μ-NH)}2], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi(O t Bu)3)3}2(μ-NH2)2(μ-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N σ orbital well localised to U[triple bond, length as m-dash]N and π orbitals which partially delocalise to form the U-N single bond with the other uranium.

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)5W-W(CO)5][K(18-crown-6)(THF)2]2 (1), completing the group 6 dianion triad over half a century since the area began. The W-W bond is long [3.2419(8) Å] and, surprisingly, in the solid-state the dianion adopts a D4h eclipsed rather than D4d staggered geometry, the latter of which dominates the structural chemistry of binary homobimetallic carbonyls. Computational studies at levels of theory from DFT to CCSD(T) confirm that the D4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol-1 more stable than the D4h form, since slight destabilisation of the degenerate W-CO π 5dxz and 5dyz orbitals is outweighed by greater stabilisation of the W-W σ-bond orbital. The gas-phase D4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D4h isomer directly to the D4d forms, which are produced by rotation around the W-W bond by ±45°. It is therefore concluded that the gas-phase transition state becomes a minimum on the potential energy surface when subjected to crystal packing in the solid-state.

A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor-acceptor interaction between a metal and a neutral π-acceptor ligand, in which the ligand σ donates to the metal, which π back-bonds to the ligand. This interaction typically involves a metal with an electron-rich, mid-, low- or even negative oxidation state and a ligand with a π* orbital. Here, we report that treatment of a uranium-carbene complex with an organoazide produces a uranium(V)-bis(imido)-dinitrogen complex, stabilized by a lithium counterion. This complex, which was isolated in a crystalline form, involves an electron-poor, high-oxidation-state uranium(V) 5f1 ion that is π back-bonded to the poor π-acceptor ligand dinitrogen. We propose that this is made possible by a combination of cooperative heterobimetallic uranium-lithium effects and the presence of suitable ancillary ligands that render the uranium ion unusually electron rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.

NUF3 is identified as having a N[triple bond, length as m-dash]U triple bond, as has been previously found (Andrews et al., Angew. Chem. Int. Ed., 2008, 47, 5366). By contrast, while previously reported calculations on PUF3 and AsUF3 (Andrews et al., Inorg. Chem., 2009, 48, 6594) gave a E[triple bond, length as m-dash]U triple bond, our calculations suggest a single bond for both molecules, with antibonding π* and non-bonding 5fU orbitals significantly occupied, and highly multiconfigurational wavefunctions. We propose this difference to be due to the smaller [6,6] active space used (σ, π, π* and σ*) in the previous studies. In our calculations, a [6,16] active space was employed in order to include uranium f-orbitals and pnictogen d-orbitals.