Recent progress in transition metal complexes featuring silylene as ligands

Silylenes, divalent silicon(II) compounds, once considered highly reactive and transient species, are now widely employed as stable synthons in main-group and coordination chemistry for myriad applications. The synthesis of stable silylenes represents a major breakthrough, which led to extensive exploration of silylenes in stabilizing low-valent main-group elements and as versatile ligands in coordination chemistry and catalysis. In recent years, the exploration of transition metal complexes stabilized with silylene ligands has captivated significant research attention. This is due to their robust σ-donor characteristics and capacity to stabilize transition metals in low valent states. It has also been demonstrated that the transition metal complexes of silylenes are effective catalysts for hydroboration, hydrosilylation, hydrogenation, hydrogen isotope exchange reactions, and small molecule activation chemistry. This review article focuses on the recent progress in the synthesis and catalytic application of transition metal complexes of silylenes.

Pushing the Limits of Organometallic Redox Chemistry with an Isolable Mn(-I) Dianion

We report an incredibly reducing and redox-active Mn-I dianion, [Mn(CO)3(Ph2B(tBuNHC)2)]2- (NHC = N-heterocyclic carbene), furnished via 2e- reduction of the parent 16e- MnI complex with Na0 or K0. Cyclic voltammograms show a Mn0/-I redox couple at -3.13 V vs Fc+/0 in tetrahydrofuran (THF), -3.06 V in 1,2-dimethoxyethane, and -2.85 V in acetonitrile. The diamagnetic Mn-I dianion is stable in solution and solid-state at room temperature, tolerating a wide range of countercations ([M(2.2.2)crypt]+, [M(18-crown-6)]+, [nBu4N]+; M = Na, K). Countercation identity does not significantly alter 13C NMR spectral signatures with [nBu4N]+ and Na+, suggesting minimal ion pairing in solution. IR spectroscopy reveals a significant decrease in CO stretching frequencies from MnI to Mn-I (ca. 240 cm-1), consistent with a drastic increase in electron density at Mn. State-of-the-art DFT calculations are in excellent agreement with the observed IR spectral data. Moreover, the Mn-I dianion behaves as a chemical reductant, smoothly releasing 1e- or 2e- to regenerate the oxidized Mn0 or MnI species in solution. The reducing potential of [Mn(CO)3(Ph2B(tBuNHC)2)]2- surpasses the naphthalenide anion in THF (-3.09 V) and represents one of the strongest isolable chemical redox agents.

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Phenylimido complexes of rhenium: fluorine substituents provide protection, reactivity, and solubility

Reactions of [Re(NPhF)Cl₃(PPh₃)₂] ({NPhF}^2- = p-fluorophenylimide) with a variety of alkyl and aryl isocyanides have been studied. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1 : 1 and 1 : 2 exchange products of Re(V) with the general formulae mer-[Re(NPhF)Cl₃(PPh₃)(isocyanide)] and cis- or trans-[Re(NPhF)Cl₃(isocyanide)₂]. The stability of the obtained products is correlated with the substitution pattern of the isocyanide ligands. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy as well as mass spectrometry. The use of partially fluorinated starting materials and ligands allows the modulation of the solubilities of the starting materials and the products as well as the monitoring of the reactions by means of ¹⁹F NMR. The attachment of the CF₃ or F substituent on the isocyanides gives control over the steric bulk and the electronic properties of the ligands and, thus, their reactivity.

Na[Tc(CO)(CNp-F-ArDArF2)4]: an isocyanide analogue of the elusive Na[Tc(CO)5]

The first crystalline technetium complex in a negative oxidation state, [Tc^-I(CO)(CNp-F-Ar^DArF2)₄]^-, was isolated and structurally characterized as its [Na(Crypt-2.2.2)]⁺ salt. It mirrors the properties of the textbook organometallic compound Na[Tc(CO)₅], which has eluded isolation and structural characterization until today. [Na(Crypt-2.2.2)][Tc^-I(CO)(CNp-F-Ar^DArF2)₄] reacts expectedly as a nucleophile, which is demonstrated by reactions with HCl and ClSnMe₃. They give the unprecedented monohydrido and trimethylstannyl complexes of technetium.

A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0) complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical

Using the potentially tridentate N,N′-bis(N-heterocyclic silylene)pyridine [SiNSi] pincer-type ligand, 2,6-N,N′-diethyl-bis[N,N′-di-tert-butyl(phenylamidinato)silylene] diaminopyridine, led to the first isolable bis(silylene)pyridine-stabilized manganese(0) complex, {κ³-[SiNSi]Mn(dmpe)} 4 (dmpe = (Me₂P)₂C₂H₄), which represents an isolobal 17 VE analogue of the elusive Mn(CO)₅ radical. The compound is accessible through the reductive dehalogenation of the corresponding dihalido (SiNSi)Mn(ii) complexes 1 (Cl) and 2 (Br) with potassium graphite. Exposing 4 towards the stronger π-acceptor ligands CO and 2,6-dimethylphenyl isocyanide afforded the related Mn(0) complexes κ²-[SiNSi]Mn(CO)₃ (5) and κ³-[SiNSi]Mn(CNXylyl)₂(κ¹-dmpe) (6), respectively. Remarkably, the stabilization of Mn(0) in the coordination sphere of the [SiNSi] ligand favors the d⁷ low-spin electronic configuration, as suggested by EPR spectroscopy, SQUID measurements and DFT calculations. The suitability of 4 acting as a superior pre-catalyst in regioselective hydroboration of quinolines has also been demonstrated.

An isolable bis(silylene)pyridine stabilized manganese(0) complex {κ³-[SiNSi]Mn(dmpe)}, isolobal to elusive Mn(CO)₅ radical has been synthesized and fully characterised.

A redox-active Mn(0) dicarbene metalloradical

A rare redox-active Mn(0) dicarbene anion with solvent-dependent electrochemical behaviour has been synthesized and thoroughly characterized.

Direct observation of reversible bond homolysis by 2D EXSY NMR

Bond homolysis is one of the most fundamental bond cleavage mechanisms. Thus, understanding of bond homolysis influences the development of a wide range of chemistry. Photolytic bond homolysis and its reverse process have been observed directly using time-resolved spectroscopy. However, direct observation of reversible bond homolysis remains elusive. Here, we report the direct observation of reversible Co–Co bond homolysis using two-dimensional nuclear magnetic resonance exchange spectroscopy (2D EXSY NMR). The characterization of species involved in this homolysis is firmly supported by diffusion ordered NMR spectroscopy (DOSY NMR). The unambiguous characterization of the Co–Co bond homolysis process enabled us to study ligand steric and electronic factors that influence the strength of the Co–Co bond. Understanding of these factors will contribute to rational design of multimetallic complexes with desired physical properties or catalytic activity.

We report the first direct observation of reversible bond homolysis using EXSY NMR. This study revealed that electron donating groups weaken the Co–Co bond.

A closed-shell monomeric rhenium(1-) anion provided by m-terphenyl isocyanide ligation

The mixed isocyanide/carbonyl complexes cis- and trans-[Re(CO)3Br(CNArDipp2)2] (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) can be synthesized from reactions of [Re(CO)5Br] and CNArDipp2 depending on the conditions applied. Reduction of the neutral Re(i) species gives the monoanionic complex [Re(CO)3(CNArDipp2)2]- or the neutral [Re(CO)3(CNArDipp2)2], which contain rhenium in the formal oxidation states "-1" and "0", respectively.

Three-Coordinate Formal Cobalt(0), Iron(0), and Manganese(0) Complexes with Persistent Carbene and Alkene Ligation

Low-coordinate transition-metal species, i.e., metal species with coordination numbers of less than 4, represent a category of ubiquitous reactive intermediates in metal-catalyzed reactions that take place in solution, in metalloenzymes, on supported nanomaterials and single-atom catalysts, and so on. While reactive intermediates are usually transient and hard to isolate, which makes detailed investigation challenging, molecular representatives of low-coordinate transition-metal intermediates can be synthesized by the judicious use of supporting ligands, allowing detailed study of their inherent chemical and physical properties. By the use of bulky nitrogen- and oxygen-based anionic ligands, plenty of three- and two-coordinate group 4-10 metal complexes with the oxidation states of the metal centers being +3, + 2, and +1 have been prepared and subjected to extensive study. Much less known are low-coordinate zero-valent metal complexes, and knowledge about them had been restricted to group 10 metal complexes until very recently. In this Account, we summarize the studies of the synthesis, spectroscopic features, electronic structures, and reactivities of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes with persistent carbene and alkene ligation. The introduction of the π-accepting alkene ligands 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (dvtms) and vinyltrimethylsilane (vtms) into the reduction reactions of MCl₂ (M = Co, Fe, Mn) with persistent carbenes and alkaline metals effectively suppresses ligand C-H bond activation reactions, leading to the successful preparation of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes LM(η²:η²-dvtms), LM(η²-vtms)₂, and L₂M(η²-vtms) (M = Co, Fe, Mn; L = N-heterocyclic carbene (NHC), cyclic (alkyl)(amino)carbene (cAAC)). These three-coordinate metal complexes feature pronounced back-donation from the filled metal 3d orbitals to the alkene π* orbital(s), resulting in electronic configurations of (d_xy+π_alkene^*)²(d_x²-y²+π'_alkene^*)²(d_z²,d_xz,d_yz)ⁿ (the coordination plane was chosen as the xy plane; n = 5, 4, and 3 for Co, Fe, and Mn, respectively) for the bis(alkene) complexes LM(η²:η²-dvtms) and LM(η²-vtms)₂. The alkene ligands in the low-coordinate formal zero-valent metal complexes are amenable to undergo ligand-exchange reactions with better π-accepting ligands. In reactions with organic azides, hydrosilanes, nitrosoarenes, alkynes, etc., the alkene ligands dissociate from the metal coordination sphere, and three-coordinate formal zero-valent metal complexes function as synthons of L_nM⁰ (L = NHC, cAAC; n = 1, 2; M = Co, Fe, Mn) to perform redox reactions with these substrates, affording divalent and tetravalent cobalt, iron, or manganese complexes. These electronic structure and reactivity features hint at the potential of low-coordinate zero-valent group 7-9 metal complexes for the development of new 3d metal catalysts and magnetic materials.

Distibanes and Distibenes from Reduction of Sb(NONR )Cl by using MgI Reagents

The bis(amidodimethyl)disiloxane antimony chlorides Sb(NON^R )Cl (NON^R =[O(SiMe₂ NR)₂ ]^2- ; R=tBu, Ph, 2,6-Me₂ C₆ H₃ =Dmp, 2,6-iPr₂ C₆ H₃ =Dipp, 2,6-(CHPh₂ )₂ -4-tBuC₆ H₂ =tBu-Bhp) are reduced to Sb^II and Sb^I species by using Mg^I reagents, [Mg(BDI^R' )]₂ (BDI=[HC{C(Me)NR'}₂ ]^- ; R'=2,4,6-Me₃ C₆ H₂ =Mes, Dipp). Stoichiometric reactions with Sb(NON^R )Cl (R=tBu, Ph) form dimeric Sb^II stibanes [Sb(NON^R )]₂ , shown crystallographically to contain Sb-Sb single bonds. The analogous distibane with R=Dmp substituents has an exceptionally long Sb-Sb interaction and exhibits spectroscopic and reactivity properties consistent with radical character in solution. When R=Dipp, reductions with Mg^I reagents directly give distibenes [Sb(μ-NON^Dipp )Mg(BDI^R' )(THF)_n ]₂ (R'=Mes, n=1; R'=Dipp, n=0). Crystallographic analysis shows a trans-substitution of the Sb=Sb double bond, with bridging NON^Dipp -ligands between the Sb^I and Mg^II centres. An attempt to access the NON^Ph -analogue using the same protocol afforded the polystibide cluster Sb₈ [μ₄ ,η^2:2:2:2 -Mg(BDI^Mes )]₄ , which co-crystallized with the ligand transfer product, [Mg(BDI^Mes )]₂ (μ-NON^Ph ).

Side-On Coordination of Nitrous Oxide to a Mononuclear Cobalt Center

Despite its utility as an oxygen-atom transfer reagent for transition metals, nitrous oxide (N₂O) is a notoriously poor ligand, and its coordination chemistry has been limited to a few terminal, end-on κ¹-N complexes. Here, the synthesis of a mononuclear cobalt complex possessing a side-on-bound N₂O molecule is reported. Structural characterization, IR spectroscopy, and DFT calculations support an η²-N,N binding mode for binding of N₂O to the cobalt center.

Substrate Redox Non-innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on Low-Coordinate Cobalt(0) Species

The reactions of nitrosoarenes with transition-metal species are fundamentally important for their relevance to metal-catalyzed transformations of organo-nitrogen compounds in organic synthesis and also the metabolization of nitroarenes and anilines in biology. In addition to the well-known reactivity of metal-mediated N-O bond activation and cleavage of nitrosoarenes, we present herein the first observation of a nitrosoarene C-N bond oxidative addition reaction upon the interaction of a three-coordinate cobalt(0) species [(IPr)Co(vtms)₂] with 2,4,6-tri( tert-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces a cobalt nitrosyl aryl complex, [(IPr)Co(Ar*)(NO)] (1), with a bis(nitrosoarene)cobalt complex, [(IPr)Co(η²-ONAr)(κ¹- O-ONAr)] (2), as an intermediate. Spectroscopic characterizations, DFT calculations, and kinetic studies revealed that the redox non-innocence of nitrosoarene induces a stepwise pathway for the C-N bond oxidative addition reaction.

Zero-valent isocyanides of nickel, palladium and platinum as transition metal σ-type Lewis bases

Transition metal complexes that contain metal-to-ligand retrodative σ-bonds have become the subject of increasing studies over the last decade. Lewis acidic "Z-type ligands" can modulate the electronic structure of their resultant complexes in a manner distinct from 2e^- donor ligands, and can also engage in cooperative reactivity with a Lewis basic transition metal. In this Feature article, we summarize our work with transition metal isocyanide complexes of group 10 metals that have exploited metal-based σ-type Lewis basicity. While the complexes Ni(CNAr^Mes2)₃, Pd(CNAr^Dipp2)₂ and Pt(CNAr^Dipp2)₂ were initially targeted as analogues to unstable, low-coordinate metal carbonyls, it soon became apparent that these zero-valent metal centers bore appreciable Lewis basic qualities due largely to the enhanced σ-donor/π-acid ratio of isocyanides compared to CO. Detailed spectroscopic and structural studies of metal-only Lewis pairs (MOLPs) formed from these complexes have furthered our understanding of the electronic structure perturbations effected by Z-type ligand binding. In addition, the platinum (boryl)iminomethane (BIM) complex Pt(κ²-N,B-^Cy₂BIM)(CNAr^Dipp2) has illuminated a general ligand design strategy that can engender significant reverse-dative interactions with buttressed Lewis acids, and also has expanded the known scope of cooperative reactivity that can be realized at a transition metal-borane linkage.

Crystalline Coordination Networks of Zero-Valent Metal Centers: Formation of a 3-Dimensional Ni(0) Framework with m-Terphenyl Diisocyanides

A permanently porous, three-dimensional metal-organic material formed from zero-valent metal nodes is presented. Combination of ditopic m-terphenyl diisocyanide, [CNAr^Mes2]₂, and the d¹⁰ Ni(0) precursor Ni(COD)₂, produces a porous metal-organic material featuring tetrahedral [Ni(CNAr^Mes2)₄]_n structural sites. X-ray absorption spectroscopy provides firm evidence for the presence of Ni(0) centers, whereas gas-sorption and thermogravimetric analysis reveal the characteristics of a robust network with a microdomain N₂-adsorption profile.

Comparison of nucleophilic- and radical-based routes to the formation of manganese-main group element single bonds

One-electron activation of main-group substrates by a stable manganese metalloradical provides a facile pathway to Mn-element single bonds.

Electrochemical Properties and CO2-Reduction Ability of m-Terphenyl Isocyanide Supported Manganese Tricarbonyl Complexes

To circumvent complications with redox-active ligands commonly encountered in the study of manganese electrocatalysts for CO₂ reduction, we have studied the electrochemistry of the manganese mixed carbonyl/isocyanide complexes XMn(CO)₃(CNAr^Dipp2)₂ (X = counteranion), to evaluate the pairing effects of the counteranion and their influence over the potential necessary for metal-based reduction. The complexes described herein have been shown to act as functional analogues to the known homoleptic carbonyl manganese complexes [Mn(CO)₅]ⁿ (n = 1-, 0, 1+). The m-terphenyl isocyanide ligand CNAr^Dipp2 improves the kinetic stability of the resulting mixed carbonyl/isocyanide systems, such that conversion among all three oxidation states is easily effected by chemical reagents. Here, we have utilized an electrochemical study to fully understand the redox chemistry of this system and its ability to facilitate CO₂ reduction and to provide comparison to known manganese-based CO₂ electrocatalysts. Two complexes, BrMn(CO)₃(CNAr^Dipp2)₂ and [Mn(THF)(CO)₃(CNAr^Dipp2)₂]OTf, have been studied using infrared spectroelectrochemistry (IR-SEC) to spectroscopically characterize the redox states of these complexes during the course of electrochemical reactions. A striking difference in the necessary potential leading to the first one-electron reduction has been found for the halide and triflate species, respectively. Complete selectivity for the formation of CO and CO₃^2- is observed in the reactivity of [Mn(CO)₃(CNAr^Dipp2)₂]^- with CO₂, which is deduced via the trapping and incorporation of liberated CO into the zerovalent species Mn(CO)₃(CNAr^Dipp2)₂ to form the dimers Mn₂(CO)₇(CNAr^Dipp2)₃ and Mn₂(CO)₈(CNAr^Dipp2)₂.

Insertion of phenyl isothiocyanate into a P-P bond of a nickel-substituted bicyclo[1.1.0]tetraphosphabutane

A new reaction mode for bicyclo[1.1.0]tetraphosphabutanes is reported. The C[double bond, length as m-dash]S and C[double bond, length as m-dash]N bonds of phenyl isothiocyanate reversibly insert into a P-P bond of [{CpNi(IMes)}2(μ-η(1):η(1)-P4)] (, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), forming isomers and . X-ray crystallography and (31)P{(1)H} NMR spectroscopy revealed similar bicyclo[3.1.0]heterohexane structures for these compounds.

Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride

This review summarizes recent research on novel photochemical methods for the initiation and control of the polymerization of main chain fluorinated monomers as exemplified by vinylidene fluoride (VDF) and for the synthesis of their block copolymers. Such reactions can be carried out at ambient temperature in glass tubes using visible light. Novel, original protocols include the use of hypervalent iodide carboxylates alone or in conjunction with molecular iodine, as well as the use of photoactive transition metal carbonyls in the presence of alkyl, fluoroalkyl, and perfluoroalkyl halides. An in-depth study of the reaction parameters highlights the use of dimethyl carbonate as a preferred polymerization solvent and outlines the structure-property relationship for hypervalent iodide carboxylates and halide initiators in both the free radical and iodine degenerative transfer controlled radical polymerization (IDT-CRP) of VDF. Finally, the rational selection of metal carbonyls that are successful not only as IDT mediators but, more importantly, in the quantitative activation of both PVDF-CH2-CF2-I and PVDF-CF2-CH2-I chain ends toward the synthesis of well-defined PVDF block copolymers is presented.

Persistent four-coordinate iron-centered radical stabilized by π-donation

Dinuclear iron carbonyl complex 2, which contains an elongated unsupported Fe-Fe bond, was synthesized by the reaction between Fe₂(CO)₉ and phosphinyl radical 1. Thermal Fe-Fe bond homolysis led to the generation of a four-coordinate carbonyl-based iron-centered radical, 3, which is stabilized by π-donation. Complex 3 exhibited high reactivity toward organic radicals to form diamagnetic five-coordinate Fe(ii) complexes.

Dinitrogen binding, P4-activation and aza-Büchner ring expansions mediated by an isocyano analogue of the CpCo(CO) fragment

Synthetic studies targeting an m-terphenyl isocyanide analogue of the unstable 16e⁻, S = 1 complex CpCo(CO) are reported (Cp = η⁵-C₅H₅).