Nitrogen Fixation by Manganese Complexes - Waiting for the Rush?

Manganese is currently experiencing a great deal of attention in homogeneous catalysis as a sustainable alternative to platinum group metals due to its abundance, affordable price and low toxicity. While homogeneous nitrogen fixation employing well-defined transition metal complexes has been an important part of coordination chemistry, manganese derivatives have been only sporadically used in this research area. In this contribution, the authors systematically cover manganese organometallic chemistry related to N2 activation spanning almost 60 years, identify apparent pitfalls and outline encouraging perspectives for its future development.

Four-Coordinate Co(III) Imide with an Unusually Tilted Terminal Imido Ligand

We report herein the synthesis and characterization of a terminal Co(III) imido complex supported by an intermediate field N,N,C heteroscorpionate. This chemistry is enabled through the development of an additional member of this ligand type featuring Ph2(CH3)C- substituents, one of which weakly binds and stabilizes Co in the corresponding Co(I) precursor. The Co(III) imide is low-spin with no evidence for thermal population of open-shell excited states. Unusually, the imido ligand in this molecule tilts markedly toward the Calkyl donor. DFT calculations suggest this structural feature to be largely a result of strong Co-C covalency, underscoring the importance of M-C bonding in determining the (electronic) structure of metal centers supported by this class of ligand.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )]

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Direct Transformation of SiH4 to a Molecular L(H)2Co═Si═Co(H)2L Silicide Complex

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si-H bond activations in SiH₄ at the metal centers of 14-electron LCo^I fragments (L = Tp″, HB(3,5-diisopropylpyrazolyl)₃^-; [BP₂^tBuPz], PhB(CH₂P^tBu₂)₂(pyrazolyl)). Upon exposure of (Tp″Co)₂(μ-N₂) (1) to SiH₄, a mixture of (Tp″Co)₂(μ-H) (2) and (Tp″Co)₂(μ-H)₂ (3) was formed and no evidence for Si-H oxidative addition products was observed. In contrast, [BP₂^tBuPz]-supported Co complexes led to Si-H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP₂^tBuPz]Co)₂(μ-N₂) (5) with SiH₄ gave the dicobalt silicide complex [BP₂^tBuPz](H)₂Co═Si═Co(H)₂[BP₂^tBuPz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP₂^tBuPz] ligand on Co-Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP₂^tBuPz]Co(L)_x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH₄, despite the lack of significant Si-H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of Me_xSiH_4-x (x = 1-3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.

Four-Coordinate Fe N2 and Imido Complexes Supported by a Hemilabile NNC Heteroscorpionate Ligand

Inspired by mechanistic proposals for N2 reduction at the nitrogenase FeMo cofactor, we report herein a new, strongly σ-donating heteroscorpionate ligand featuring two weak-field pyrazoles and an alkyl donor. This ligand supports four-coordinate Fe(I)-N2, Fe(II)-Cl, and Fe(III)-imido complexes, which we have characterized using a variety of spectroscopic and computational methods. Structural and quantum mechanical analysis reveal the nature of the Fe-C bonds to be essentially invariant between the complexes, with conversion between the (formally) low-valent Fe-N2 and high-valent Fe-imido complexes mediated by pyrazole hemilability. This presents a useful strategy for substrate reduction at such low-coordinate centers and suggests a mechanism by which FeMoco might accommodate the binding of both π-acidic and π-basic nitrogenous substrates.

Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V3N4 Double-Diamond Core

Transmetallation of [VCl₃(THF)₃] and [TlTp^tBu,Me] afforded [(Tp^tBu,Me)VCl₂] (1, Tp^tBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC₈ to form a C_3v symmetric V^II complex, [(Tp^tBu,Me)VCl] (2). Complex 1 has a high-spin (S = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an S = 3/2 ⁴A₂ ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN₃ to form the V^V nitride-azide complex [(Tp^tBu,Me)V≡N(N₃)] (3). A likely V^III azide intermediate en route to 3, [(Tp^tBu,Me)VCl(N₃)] (4), was isolated by reacting 1 with N₃SiMe₃. Complex 4 is thermally stable but reacts with NaN₃ to form 3, implying a bis-azide intermediate, [(Tp^tBu,Me)V(N₃)₂] (A), leading to 3. Reduction of 3 with KC₈ furnishes a trinuclear and mixed-valent nitride, [{(Tp^tBu,Me)V}₂(μ₄-VN₄)] (5), conforming to a Robin-Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN₃ to produce an azide-bridged dimer, [{(Tp^tBu,Me)V}₂(1,3-μ₂-N₃)₂] (6), with two antiferromagnetically coupled high-spin V^II ions. Complex 5 could be independently produced along with [(κ₂-Tp^tBu,Me)₂V] upon photolysis of 6 in arene solvents. The putative {V^IV≡N} intermediate, [(Tp^tBu,Me)V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp^tBu,Me)V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [(κ₂-Tp^tBu,Me)₂V]. A more stable adduct (B-OPPh₃) was prepared by reacting B-THF with OPPh₃. These adducts of B are the first neutral and mononuclear V^IV nitride complexes to be isolated.

Paramagnetic Pyrazolylborate Complexes Tp2M and Tp*2M: 1H, 13C, 11B, and 14N NMR Spectra and First-Principles Studies of Chemical Shifts

The paramagnetic pyrazolylborates Tp₂M and Tp*₂M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp₂M]⁺ and [Tp*₂M]⁺ (M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The ¹H signal shift ranges vary from ∼30 ppm (Cu(II) and V(III)) to ∼220 ppm (Co(II)), and the ¹³C signal shift ranges from ∼180 ppm (Fe(III)) to ∼1150 ppm (Cr(II)). The ¹¹B and ¹⁴N shifts are ∼360 and ∼730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved ¹³C,¹H couplings. All chemical shifts have been calculated from first-principles on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splitting, and g tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology.

Activation of Dinitrogen by Polynuclear Metal Complexes

Activation of dinitrogen plays an important role in daily anthropogenic life, and the processes by which this fixation occurs have been a longstanding and significant research focus within the community. One of the major fields of dinitrogen activation research is the use of multimetallic compounds to reduce and/or activate N2 into a more useful nitrogen-atom source, such as ammonia. Here we report a comprehensive review of multimetallic-dinitrogen complexes and their utility toward N2 activation, beginning with the d-block metals from Group 4 to Group 11, then extending to Group 13 (which is exclusively populated by B complexes), and finally the rare-earth and actinide species. The review considers all polynuclear metal aggregates containing two or more metal centers in which dinitrogen is coordinated or activated (i.e., partial or complete cleavage of the N2 triple bond in the observed product). Our survey includes complexes in which mononuclear N2 complexes are used as building blocks to generate homo- or heteromultimetallic dinitrogen species, which allow one to evaluate the potential of heterometallic species for dinitrogen activation. We highlight some of the common trends throughout the periodic table, such as the differences between coordination modes as it relates to N2 activation and potential functionalization and the effect of polarizing the bridging N2 ligand by employing different metal ions of differing Lewis acidities. By providing this comprehensive treatment of polynuclear metal dinitrogen species, this Review aims to outline the past and provide potential future directions for continued research in this area.

New binding modes for CSe: coinage metal coordination to a tungsten selenocarbonyl complex

The reaction of the new tungsten selenocarbonylate [Et₄N][W(CSe)(CO)₂(Tp*)] (Tp* = hydrotris(dimethyl-pyrazolyl)borate) with coinage metal based electrophiles provides access to a range of new bridging modes for the selenocarbonyl ligand.