Trigonal bipyramidal or square planar? Density functional theory calculations of iron bis(dithiolene) N-heterocyclic carbene complexes

Density functional theory (DFT) calculations of 57 iron bis(dithiolene)-N-heterocyclic carbene adducts were conducted to determine what parameters predict, and possibly influence, the coordination of these aforementioned adducts. The parameters considered herein include three different types of nuclear magnetic resonance (C-NMR, Se-NMR, and P-NMR) isotropic chemical shifts, the Tolman Electronic Parameter (TEP), the Huynh Electronic Parameter (HEP), and the percent buried volume (%Vbur) of the different N-heterocyclic carbenes (NHCs) calculated from DFT. These parameters were selected based upon prior literature connection to σ-donor ability, π-acidity, and steric effects. The computed values of the properties were compared via multivariable linear regression models to see which properties best predict the Addison τ parameter-a measure of whether the complex would assume the square pyramidal or trigonal bipyramidal geometry. It was determined that a combination of TEP and %Vbur are the best predictors of the τ value, for the parameters considered herein. The inclusion of additional parameters yields mild improvement to the statistical models for the prediction of the τ value.

Revisiting Isomerism in Square Planar Palladium NHC Complexes with NacAc Chelators

Quantifying the donor ability of bidentate ligands in inorganic chemistry is not straightforward, and the Huynh electronic parameter for chelators (HEP2) presents a rare solution to the task. Aiming to extend the ligand scope in this work, the soundness and practicality of HEP2 was further scrutinized with seven stereoelectronically diverse β-ketiminato ("NacAc") chelators using palladium complexes of the type [PdBr(iPr2-bimy)(ArNacAc)] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene). Notably, the unsymmetrical nature of this κ2-N,O ligand family allowed for an intriguing exploration into isomerism in the square planar PdII products, which depends on the N-aryl ortho- and para-substituents based on experimental and theoretical findings. The HEP2 values of both isomeric forms correlate well with the widely used σp Hammett constants, which reinforces the reliability of this modern methodology.

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

Reactivity umpolung (reversal) of ligands in transition metal complexes

The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.

Donor Strength Determination of Anionic Ligands

14 new gold(I) NHC complexes of the type [AuX(iPr2-bimy)] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been prepared and fully characterized. These complexes and their reported analogues were used to systematically compare and rank the donating abilities of overall 34 anionic X-type donors by 13C NMR spectroscopy. Specifically, the carbene chemical shift of the iPr2-bimy ligand was found to be responsive to the ligand X spanning an overall range Δδ > 37 ppm between the strongest and weakest donor in this study.

Stereoelectronic Profiling of Neutral and Monoanionic Biimidazoles and Mixed Diimines

14 mono-, di-, and tetranuclear palladium complexes were prepared to study the coordination chemistry of symmetrical and unsymmetrical azole-derived diimines and their anions. The diverse range of complexes obtained highlights the structural and electronic diversities imposed by these ligands. Using the monopalladium species, the electronic properties of selected bidentate ligands were determined, ranked, and compared by ¹³C NMR spectroscopy, extending the scope of the HEP2 (Huynh electronic parameter 2) scale, which can detect even subtle differences. Moreover, the %V_bur (percentage volume buried) values as an estimate for the steric bulk of some ligands were determined using the solid-state molecular structures of their complexes, and a preliminary stereoelectronic map was established.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO₂ reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.

Versatile halogenation via a CNHC^Csp3 palladacycle intermediate

Stable cyclopalladated complexes containing an (sp³)C-Pd bond were synthesized via α-CH₂ deprotonation and palladation of N-alkyl groups of carbene ligands bearing electron-withdrawing substituents. The strong electron donating strengths of the resulting C_NHC^C_sp3 chelators were experimentally identified, and the palladacycle underwent template-directed, versatile C-halogenation with X₂.

Controlled Access to Four- and Six-Membered Palladacycles via Modifying Donor Abilities of β-Ketiminato Ligands ("NacAcs")

The synthesis of Pd complexes of the type [PdBr(ⁱPr₂-bimy)(NacAc)] (NacAc = β-ketiminate, ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was attempted, in a continuing effort to quantify donor abilities of chelating β-ketiminate ligands using the Huynh electronic parameter for bidentate donors (HEP2). Subtle variation of N-substituents on the NacAc backbone was discovered to induce a drastic change in the preferred chelating mode, in that the commonly encountered κ²-N,O-six-membered palladacycles were observed with R = Me and Et, while the unusual κ²-C,N-four-membered palladacycles were isolated with R = ⁱPr, Cy, and ^tBu. Computational studies subsequently corroborated these findings, in the form of an overall exergonic six-to-four-membered ring contraction process and a lower associated activation energy for the three more electron-donating alkyl moieties. This trend in the established energy profiles can be attributed to a reduced HOMO-LUMO gap in the corresponding optimized structures of the six-membered ring complexes.

Gauging the donor strength of iron(0) complexes via their N-heterocyclic carbene gold(i) adducts

We isolate and characterize the gold(i)–iron(0) adducts [(iPr2-bimy)Au–Fe(CO)3(PMe3)2][BArF4] and [Au–{Fe(CO)3(PMe3)2}2][BArF4] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, BArF4 = tetrakis(pentafluorophenyl)borate).

Mixed NHC-thiolato complexes of palladium: understanding the formation of di-versus mononuclear complexes

The preference for the formation of mono- versus dinuclear mixed carbene/thiolato complexes of Pd^II has been studied with three types of N-heterocyclic carbenes derived from benzimidazole, imidazole and 1,2,4-triazole. The complexes were prepared by treatment of halido/NHC precursors with sodium isopropylthiolate in a salt metathesis reaction. Mononuclear complexes are formed when the sulfur and carbon donors are exclusively cis to each other, while their trans arrangement preferably leads to dinuclear complexes with μ₂-bridging thiolato ligands. The increased electron density in the latter case cannot be sufficiently compensated by one Pd^II center alone and leads to the formation dinuclear species with bridging thiolato ligands.