Evidence for Non-Innocence of a β-Diketonate Ligand

Bis(acyl)phosphide Complexes of U(III)/U(IV): A Case of a Hidden Redox-Active Ligand

The recently reported tris(bis(2,4,6-triisopropylbenzoyl)-phosphide)uranium (UIII(trippBAP)3, 2) complex (Inorg. Chem. 2022, 61 (32), 12508-12517) demonstrated a silent 31P NMR spectrum. This complex was described as a U(III) complex with an organic radical ligand fragment. Moreover, the EPR spectrum of 2 was indicative of an organic radical in the ligand framework complexed to uranium, in contrast to that of UIV(mesBAP)4, 1. Herein, with the help of relativistic density functional theory (DFT) calculations, the electronic structures of 1, 2, and U(mesBAP)3 (4) are examined in an effort to understand the unusual 31P NMR spectrum of 2. Results indicate the reduction of the carbonyl bonds and delocalization of the electrons over the ligands, indicative of U → L backbonding. Additionally, the reduced acyl carbons are found to exist as ketyl radicals [O═C• -] that are responsible for the silent 31P NMR spectra of 2. These findings demonstrate the redox noninnocent nature of BAP- in 2 and 4, causing uranium to exist in a formal oxidation state of +4.

Homoleptic Uranium-Bis(acyl)phosphide Complexes

The first uranium bis(acyl)phosphide (BAP) complexes were synthesized from the reaction between sodium bis(mesitoyl)phosphide (Na(^mesBAP)) or sodium bis(2,4,6-triisopropylbenzoyl)phosphide (Na(^trippBAP)) and UI₃(1,4-dioxane)_1.5. Thermally stable, homoleptic BAP complexes were characterized by single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy, when appropriate, for the elucidation of the electronic structure and bonding of these complexes. EPR spectroscopy revealed that the BAP ligands on the uranium center retain a significant amount of electron density. The EPR spectrum of the trivalent U(^trippBAP)₃ has a rhombic signal near g = 2 (g₁ = 2.03; g₂ = 2.01; and g₃ = 1.98) that is consistent with the EPR-observed unpaired electron being located in a molecular orbital that appears ligand-derived. However, upon warming the complex to room temperature, no resonance was observed, indicating the presence of uranium character.

Redox-Induced Intramolecular C-C Coupling of Acyclic Bis(2-pyridylmethylene)ethylenediamine on a Ru(acac)2 Platform

The paper documents redox-triggered C-C coupling of acyclic N,N'-bis(2-pyridylmethylene)ethylenediamine (BPE) to yield 2,3-bis(2-pyridyl)pyrazine (DPP) upon coordination to an electron-rich {Ru(acac)₂} (acac = acetylacetonate) unit. This led to DPP-bridged [{Ru(acac)₂}₂(DPP)]^0/+ (2 and [2]ClO₄) along with the unperturbed BPE-bridged [{Ru(acac)₂}₂(BPE)] (1). On the contrary, electron-poor {Ru(Cl)(H)(CO)(PPh₃)₃} yielded BPE-bridged [3](ClO₄)₂ as an exclusive product. Synergistic metal (Ru)-ligand (BPE) redox participation toward chemical noninnocence of the Schiff base ligand and DPP-mediated electronic communication in Ru^IIRu^III-derived [2]ClO₄ are addressed.

Rich redox-activity and solvatochromism in a family of heteroleptic cobalt complexes

The combination of redox-active metals with redox-active ligands can lead to interesting charge transfer behaviours, including valence tautomerism and solvatochromism. With the aim of investigating a relatively underexplored redox-active metal/redox-active ligand combination, complexes [Co^II(acac)₂(X-BIAN)] (acac^- = acetylacetonate; X-BIAN = bis(4-X-phenyl)iminoacenaphthene; 1: X = -CF₃, 2: X = -Cl, 3: X = -H, 4: X = -Me) and [Co^III(acac)₂(Me-BIAN)]⁺ (5+) have been synthesised and characterised. At all temperatures investigated, and in both the solid and solution state, complexes 1-4 exist in a Co^II-BIAN⁰ charge distribution, while 5+ adopts a Co^III-BIAN⁰ charge distribution. In the case of 1-4, the potential Co^III-BIAN˙^- valence tautomer is inaccesible; the energy ordering between the ground Co^II-BIAN⁰ state and the excited Co^III-BIAN˙^- state must be reversed in order for an entropically driven interconversion to be possible. The energy gap between the states can be monitored via metal-to-ligand charge transfer bands in the visible region. We demonstrate tuning of this energy gap by varying the electronic properties of the BIAN ligand, as well as by controlling the molecular environment through solvent choice. Solvatochromic analysis, in combination with crystallographic evidence, allows elucidation of the specific solvent-solute interactions that govern the molecular behaviour of 1-4, affording insights that can inform potential future applications in sensing and switching.

Redox induced oxidative C-C coupling of non-innocent bis(heterocyclo)methanides

Redox driven C-C bond formation has gained recent attention over the traditional sequence of oxidative addition, insertion and reductive elimination reactions. In this regard, the transient radical mediated diverse reactivity profile of bis(heterocyclo)methanes (H-BHM: HL1-HL4) has been demonstrated as a function of varying metal ions and ligand backbones. It highlighted the following events: (a) redox induced homocoupling of deprotonated HL1 and HL4 on coordination to M(OAc)₂ precursors (M = Cu^II, Zn^II, Pd^II, Ag^I), including the effective role of molecular oxygen in the transformation process; (b) steric inhibition of C-C coupling of HL1 or HL4 on inserting the substituent at the bridged methylene centre (Ph in HL2 or CH₃ in HL3); (c) competitive C-C coupling versus oxygenation of free HL1 with varying concentrations of Pd^II(OAc)₂ as the ease of oxygenation over dimerisation of the deprotonated HL1 was corroborated by the DFT calculated lower activation barrier and greater thermodynamic stability of the former; and (d) redox non-innocence of BHMs on a coordinatively inert ruthenium platform, which in turn favored the involvement of a radical pathway for the aforestated coupling or oxygenation process. A combined structural, spectroscopic and DFT calculated transition state analysis demonstrated the mechanistic outline for the metal assisted oxidative coupling of BHMs.

Thiolate-Thione Redox-Active Ligand with a Six-Membered Chelate Ring via Template Condensation and Its Pt(II) Complexes

A new template condensation reaction has been discovered in a mixture of Pt(II), thiobenzamide, and base. Four complexes of the general form [Pt(ctaPh^R)₂], R = CH₃ (1a), H (1b), F (1c), Cl (1d), cta = condensed thioamide, have been prepared under similar conditions and thoroughly characterized by ¹H NMR and UV-vis-NIR spectroscopy, (spectro)electrochemistry, elemental analysis, and single-crystal X-ray diffraction. The ligand is redox active and can be reduced from the initial monoanion to a dianionic and then trianionic state. Chemical reduction of 1a with [Cp₂Co] yielded [Cp₂Co]₂[Pt(ctaPh^CH3)₂], [Cp₂Co]₂[1a], which has been similarly characterized with the addition of EPR spectroscopy and SQUID magnetization. The singly reduced form containing [1a]^1-, (nBu₄N)[Pt(ctaPh^CH3)₂], has been generated in situ and characterized by UV-vis and EPR spectroscopies. DFT studies of 1b, [1b]^1-, and [1b]^2- confirm the location of additional electrons in exclusively ligand-based orbitals. A detailed analysis of this redox-active ligand, with emphasis on the characteristics that favor noninnocent behavior in six-membered chelate rings, is included.

Ligand field-actuated redox-activity of acetylacetonate

The quest for simple ligands that enable multi-electron metal-ligand redox chemistry is driven by a desire to replace noble metals in catalysis and to discover novel chemical reactivity. The vast majority of simple ligand systems display electrochemical potentials impractical for catalytic cycles, illustrating the importance of creating new strategies towards energetically aligned ligand frontier and transition metal d orbitals. We herein demonstrate the ability to chemically control the redox-activity of the ubiquitous acetylacetonate (acac) ligand. By employing the ligand field of high-spin Cr(ii) as a switch, we were able to chemically tailor the occurrence of metal-ligand redox events via simple coordination or decoordination of the labile auxiliary ligands. The mechanism of ligand field actuation can be viewed as a destabilization of the d z 2 orbital relative to the π* LUMO of acac, which proffers a generalizable strategy to synthetically engineer redox-activity with seemingly redox-inactive ligands.

A heteroleptic diradical Cr(iii) complex with extended spin delocalization and large intramolecular magnetic exchange

Successive chemical reductions of the heteroleptic complex [(tpy)Cr^III(tphz)]³⁺ (tpy = terpyridine; tphz = tetrapyridophenazine) give rise to the mono- and di-radical redox isomers, [(tpy)Cr^III(tphz˙^-)]²⁺ and [(tpy˙^-)Cr^III(tphz˙^-)]⁺, respectively. As designed, the optimized overlap of the involved magnetic orbitals leads to extremely strong magnetic interactions between the S = 3/2 metal ion and S = 1/2 radical spins, affording well isolated S_T = 1 and S_T = 1/2 ground states at room temperature.