Monosubstituted Doublet Sn(I) Radical Featuring Substantial Unquenched Orbital Angular Momentum

Reaction of a Potassium Aluminyl with Sn[N(SiMe3)2]2 - Isolation of a Stable, Trimetallic Sn(I) Radical Anion

The reaction of the potassium aluminyl K[Al(NON)] ([NON]2-=[O(SiMe2NDipp)2]2-, Dipp=2,6-iPr2C6H3) with the stannylene Sn[N(SiMe3)2]2 in benzene afforded K3[(Sn4){Al(NON)}2{N(SiMe3)2}], containing a distorted tetrahedral Sn4-cluster. Computational analysis indicates that four of the edges in this unit are composed of Sn-Sn bonds, with the remaining two that are spanned by aluminium involved in three centre two electron (3c2e) Sn-Al-Sn bonds. The formation of Al(II) species during this reaction is indicated by the isolation of the dialuminated cyclohexadiene 1,4-[Al(NON)]2(μ-C6H6). Repeating the reaction in methylcyclohexane generated a thermally stable, trimetallic Sn(I) radical anion in K[Sn{Al(NON)}2]. Compared to all other reported Sn(I) radicals, its EPR spectrum is unique; the main turning points of its spectrum appear at g values above 2 and the Sn hyperfine coupling is substantially smaller in magnitude. These data, together with ENDOR measurements and DFT calculations show that the SOMO is entirely localised in an unhybridised 5p orbital, such that spin-orbit contributions to the g and Sn hyperfine tensors are quenched.

Overcoming Challenges in Density Functional Theory-Based Calculations of Hyperfine Coupling Constants for Heavy Heteroatom Radicals

This study assesses density functional theory (DFT) methods for their accuracy in calculating hyperfine coupling constants (HFCCs) of heavy heteroatom radicals with heteroatoms including Sb, Bi, In, Tl, and Sn. Given the essential role of electron paramagnetic resonance spectroscopy in characterization of these species, it is crucial that theoretical models can predict HFCCs accurately for heavy elements. This work presents a computational approach that addresses crucial factors: selection of basis set, hybrid exchange-correlation functional, higher Hartree-Fock (HF) exchange, and the Gaussian description of nuclear charge. The relativistic effects are introduced using one-component linear response theory with the second-order Douglas-Kroll-Hess formalism and the fully relativistic four-component Dirac-Kohn-Sham method. The findings show that, while one-component DFT is accurate for the 4th-row elements, the four-component method is more precise for the 5th-row radicals and the one-component approach fails for the 6th-row congeners. Increasing HF exchange significantly improves HFCC predictions. The developed framework for accurate HFCC calculations will enhance the understanding of electronic and magnetic properties of heavy element radicals and can be used by computational chemists and experimentalists alike.

Crystalline Arylstibinidene Chalcogenides: Heavier Congeners of Aromatic Nitroso Compounds

Nitroso compounds, R-N═O, containing N═O double bonds are ubiquitous and widely utilized in organic synthesis. In contrast, heavier congeners of nitroso compounds, namely pnictinidene chalcogenides R-Pn = E (Pn = P, As, Sb, Bi; E = O, S, Se, Te), are highly reactive and scarce. They have been stabilized in the coordination sphere of Lewis acid/base or by pronounced contribution from resonance structures, whereas free species with unperturbed pnictogen-chalcogen double bonds remains elusive. In this work, we report the isolation and characterization of arylstibinidene chalcogenides, which are the first heavier congeners of aromatic nitroso compounds. They are facilely synthesized through the salt metathesis reactions of aryldichlorostibane and dilithium chalcogenides. They bear unperturbed Sb═E (E = S, Se and Te) double bonds due to poor orbital overlap between the C 2p orbitals of the phenyl ring of the substituent and the Sb 5p orbitals. Moreover, they show versatile reactivity, including acting as chalcogen atom transfer reagents and reacting with small molecules via (cyclo)addition.

A Spectroscopic Criterion for Identifying the Degree of Ground-Level Near-Degeneracy Derived from Effective Hamiltonian Analyses of Three-Coordinate Iron Complexes

The fascinating magnetic and catalytic properties of coordinatively unsaturated 3d metal complexes are a manifestation of their electronic structures, in particular their nearly doubly or triply degenerate orbital ground levels. Here, we propose a criterion to determine the degree of degeneracy of this class of complexes based on their experimentally accessible magnetic anisotropy (parametrized by the electron spin g- and zero-field splitting (ZFS)-tensors). The criterion is derived from a comprehensive spectroscopic and theoretical study in the trigonal planar iron(0) complex, [(IMes)Fe(dvtms)] (IMes = 1,3-di(2',4',6'-trimethylphenyl)imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane, 1). Accurate ZFS-values (D = +33.54 cm-1, E/D = 0.09) and g-values (g ∥ = 1.96, g ⊥ = 2.45) of the triplet (S = 1) ground level of complex 1 were determined by complementary THz-EPR spectroscopy and SQUID magnetometry. In-depth effective Hamiltonian (EH) analyses coupled to wave-function-based ab initio calculations show that 1 features a ground level with three energetically close-lying orbital states with a "two-above-one" energy pattern. The observed magnetic anisotropy results from mixing of the two excited electronic states with the ground state by spin-orbit coupling (SOC). EH investigations on 1 and related complexes allowed us to generalize this finding and establish the anisotropy of the g - and ZFS-tensors as spectroscopic markers for assigning two- or three-fold orbital near-degeneracy.

Isolation and characterization of a triplet nitrene

Nitrene radical compounds are short-lived intermediates in a variety of nitrogen-involved transformations. They feature either a singlet or a triplet ground state, depending on the electronic properties of the substituents. Triplet nitrenes are highly reactive and their isolation in the condensed phase under ambient conditions is challenging. Here we report the synthesis and isolation of a triplet arylnitrene supported by a bulky hydrindacene ligand. The arylnitrene is fully characterized by various spectroscopic and structural techniques including electron paramagnetic resonance spectroscopy and single-crystal X-ray diffraction. Its high stability is largely attributed to the steric hindrance and effective electron delocalization provided by the supporting ligand. Electron paramagnetic resonance spectroscopy in conjunction with highly correlated wavefunction-based ab initio calculations provides support for a triplet ground state nitrene with axial zero-field splitting D = 0.92 cm-1 and vanishing rhombicity E/D = 0.002.

Silicon Analogues of Cyclopropyl Radical Derived from a Highly Stable Cyclic Disilene Compound Featuring a Si-Br Bond

A halogen-substituted cyclic disilene compound, bromocyclotrisilene, Si3Br(Eind)3 (3a), bearing fused-ring bulky Eind (a: R1 = R2 = Et) groups, has been synthesized as an extraordinarily air-stable compound by the reduction of 1,2-dibromodisilene, (Eind)BrSi═SiBr(Eind) (2a), or tribromosilane, (Eind)SiBr3 (1a), with the Mg or Li metal. The X-ray diffraction analysis of 3a showed that the disilene moiety has an almost planar, but slightly trans-bent structure. Even though 3a is quite air-stable both in solutions and in the solid state, its Si-Br bond is reactive under reducing conditions. The further treatment of 3a with the Li metal leads to the formation of room-temperature thermally stable silicon homologues of the cyclopropyl radical, i.e., the cyclotrisilanyl radicals (6a) [6a(syn) and 6a(anti)], via intramolecular C-H bond activation in a transient silicon homologue of the cyclopropenyl radical, i.e., the cyclotrisilenyl radical, [Si3(Eind)3]• (5a). The formation mechanism of 6a from 5a is discussed based on the theoretical calculations. The unique structural and electronic properties of these Si3 three-membered ring species incorporating the Eind groups have been experimentally and theoretically investigated.

Molecular-strain induced phosphinidene reactivity of a phosphanorcaradiene

Phosphanorcaradienes are an appealing class of phosphorus compounds that can serve as synthons of transient phosphinidenes. However, the synthesis of such species is a formidable task owing to their intrinsic high reactivity. Herein we report straightforward synthesis, characterization and reactivity studies of a phosphanorcaradiene, in which one of the benzene rings in the flanking fluorenyl substituents is intramolecularly dearomatized through attachment to the phosphorus atom. It is facilely obtained by the reduction of phosphorus(III) dichloride precursor with potassium graphite. Despite being thermally robust, it acts as a synthetic equivalent of a transient phosphinidene. It reacts with trimethylphosphine and isonitrile to yield phosphanylidene-phosphorane and 1-phospha-3-azaallene, respectively. When it is treated with one and two molar equivalents of azide, iminophosphane and bis(imino)phosphane are isolated, respectively. Moreover, it is capable of activating ethylene and alkyne to afford [1 + 2] cycloaddition products, as well as oxidative cleavage of Si-H and N-H bonds to yield secondary phosphines. All the reactions proceed smoothly at room temperature without the presence of transition metals. The driving force for these reactions is most likely the high ring-constraint of the three-membered PC2 ring and recovery of the aromaticity of the benzene ring.

An Isolable One-Coordinate Lead(I) Radical with Strong g-Factor Anisotropy

Lead-based radicals in the oxidation state of +1 are elusive species and are highly challenging to isolate in the condensed phase. In this study, we present the synthesis and characterization of the first isolable free plumbylyne radical 2 bearing a one-coordinate Pb(I) atom. It reacts with an N-heterocyclic carbene (NHC) to afford a two-coordinate NHC-ligated Pb(I) radical 3. 2 and 3 represent the first isolable Pb(I)-based radicals. Theoretical calculations and electron paramagnetic resonance analysis revealed that the unpaired electron mainly resides at the Pb 6p orbital in both radicals. Owing to the unique one-coordinate nature of the Pb atom in 2, it possesses two-fold orbital pseudo-degeneracy and substantial unquenched orbital angular momentum, and exhibits hitherto strongest g-factor anisotropy (gx,y,z=1.496, 1.166, 0.683) amongst main group radicals. Preliminary investigations into the reactivity of 2 unveiled its Pb-centered radical nature, and plumbylenes were isolated as products.

Approach to the "Missing" Diarylsilylene: Formation, Characterization, and Intramolecular C-H Bond Activation of Blue Diarylsilylenes with Bulky Rind Groups

The treatment of the bulky Rind-based dibromosilanes, (Rind)2SiBr2 (2) [Rind = 1,1,7,7-tetra-R1-3,3,5,5-tetra-R2-s-hydrindacen-4-yl: EMind (a: R1 = Et, R2 = Me) and Eind (b: R1 = R2 = Et)], with two equivalents of tBuLi in Et2O at low temperatures resulted in the formation of blue solutions derived from the diarylsilylenes, (Rind)2Si: (3). Upon warming the solutions above -20 °C, the blue color gradually faded, accompanying the decomposition of 3 and yielding cyclic hydrosilanes (4) via intramolecular C-H bond insertion at the Si(II) center. The molecular structures of the bulky Eind-based 3b and 4b were confirmed by X-ray crystallography. Thus, at -20 °C, blue crystals were formed (Crystal-A), which were identified as mixed crystals of 3b and 4b. Additionally, colorless crystals of 4b as a singular component were isolated (Crystal-B), whose structure was also determined by an X-ray diffraction analysis. Although the isolation of 3 was difficult due to their thermally labile nature, their structural characteristics and electronic properties were discussed based on the experimental findings complemented by computational results. We also examined the hydrolysis of 3b to afford the silanol, (Eind)2SiH(OH) (5b).

One-Electron (2c/1e) Tin···Tin Bond Stabilized by ortho-Phenylenediamido Ligands

Odd-electron bonds, i.e., the two-center, three-electron (2c/3e), or one-electron (2c/1e) bonds, have attracted tremendous interest owing to their novel bonding nature and radical properties. Herein, complex [K(THF)6][LSn:···Sn:L] (1), featuring the first and unsupported 2c/1e Sn···Sn σ-bond with a long distance (3.2155(9) Å), was synthesized by reduction of stannylene [LSn:] (L = N,N-dpp-o-phenylene diamide) with KC8. The one-electron Sn-Sn bond in 1 was confirmed by the crystal structure, DFT calculations, EPR spectroscopy, and reactivity studies. This compound can be viewed as a stabilized radical by delocalizing to two metal centers and can readily mediate radical reactions such as C-C coupling of benzaldehyde.

Pinacol Cross-Coupling Promoted by an Aluminyl Anion

A simple sequential addition protocol for the reductive coupling of ketones and aldehydes by a potassium aluminyl grants access to unsymmetrical pinacolate derivatives. Isolation of an aluminium ketyl complex presents evidence for the accessibility of radical species. Product release from the aluminium centre was achieved using an iodosilane, forming the disilylated 1,2-diol and a neutral aluminium iodide, thereby demonstrating the steps required to generate a closed synthetic cycle for pinacol (cross) coupling at an aluminyl anion.

Synthesis and Reactivity of N-Heterocyclic Carbene Coordinated Formal Germanimidoyl-Phosphinidenes

Treatment of N-heterocyclic carbene (NHC) ligated germylidenylphosphinidene MsFluidtBu-GeP(NHCiPr) (where MsFluidtBu is a bulky hydrindacene substituent, and NHCiPr is 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene) with mesityl azide and 4-tertbutylphenyl azide afforded NHC coordinated formal germanimidoyl-phosphinidenes, which represent the first compounds bearing both Ge═N double bond and phosphinidene functionalities. Studies of the chemical properties revealed that the reactions preferred to occur at the Ge═N double bond, which underwent [2 + 2] cycloadditions with CO2 and ethyl isocyanate, and coordinated with coinage metals through the nitrogen atom.

A silylene-stabilized ditin(0) complex and its conversion to methylditin cation and distannavinylidene

Due to their intrinsic high reactivity, isolation of tin(0) complexes remains challenging. Herein, we report the synthesis of a silylene-stabilized ditin(0) complex (2) by reduction of a silylene-supported dibromostannylene (1) with 1 equivalent of magnesium (I) dimer in toluene. The structure of 2 was established by single crystal X-ray diffraction analysis. Density Functional Theory calculations revealed that complex 2 bears a Sn=Sn double bond and one lone pair of electrons on each of the Sn(0) atoms. Remarkably, complex 2 is readily methylated to give a mixed-valent methylditin cation (4), which undergoes topomerization in solution though a reversible 1,2-Me migration along a Sn=Sn bond. Computational studies showed that the three-coordinate Sn atom in 4 is the dominant electrophilic center, and allows for facile reaction with KHBBus3 furnishing an unprecedented N-heterocyclic silylenes-stabilized distannavinylidene (5). The synthesis of 2, 4 and 5 demonstrates the exceptional ability of N-heterocyclic silylenes to stabilize low valent tin complexes.

Triplet bismuthinidenes featuring unprecedented giant and positive zero field splittings

Isolation of triplet pnictinidenes, which bear two unpaired electrons at the pnictogen centers, has long been a great challenge due to their intrinsic high reactivity. Herein, we report the syntheses and characterizations of two bismuthinidenes MsFluindtBu-Bi (3) and MsFluind*-Bi (4) stabilized by sterically encumbered hydrindacene ligands. They were facilely prepared through reductions of the corresponding dichloride precursors with 2 molar equivalents of potassium graphite. The structural analyses revealed that 3 and 4 contain a one-coordinate bismuth atom supported by a Bi-C single σ bond. As a consequence, the remaining two Bi 6p orbitals are nearly degenerate, and 3 and 4 possess triplet ground states. Experimental characterizations with multinuclear magnetic resonance, magnetometry and near infrared spectroscopy coupled to wavefunction based ab initio calculations concurred to evidence that there exist giant and positive zero field splittings (>4300 cm-1) in their S = 1 ground states. Hence even at room temperature the systems almost exclusively populate the lowest-energy nonmagnetic Ms = 0 level, which renders them seemingly diamagnetic.

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

Open-Shell Germylene Stabilized by a Phenalenyl-Based Ligand

An open-shell germylene 1 stabilized by a phenalenyl-based bidentate ligand was synthesized and characterized. Because of the high thermal stability originating from spin delocalization over the phenalenyl moiety, it was possible to isolate compound 1 in crystalline form by sublimation at ca. 300 °C. Electron spin resonance (ESR) spectra, crystallographic analysis, theoretical calculations, and reactivities with carbon radicals suggest that the spin of 1 is distributed on the phenalenyl moiety, while 1 reacted with C₂Cl₆, PhSSPh, and p-benzoquinone at the germanium center to form Ge-E (E = Cl, S, O) bonds. Furthermore, compound 1 is featured by its reactivity as a "formal germylyne", which allows for the formation of three new σ-bonds or one σ-bond with metal complexation on the germanium center.

Disproportionation of Sn(II){CH(SiMe3)2}2 to ˙Sn(III){CH(SiMe3)2}3 and ˙Sn(I){CH(SiMe3)2}: characterization of the Sn(I) product

Half a century after the photolytic disproportionation of Lappert's dialkyl stannylene SnR₂, R = CH(SiMe₃)₂ (1) gave the persistent trivalent radical [˙SnR₃], the characterization of the corresponding Sn(I) product, ˙SnR is now described. It was isolated as the hexastannaprismane Sn₆R₆ (2), from the reduction of 1 by the Mg(I)-reagent, Mg(BDI^Dip)₂ (BDI = (DipNCMe)₂CH, Dip = 2,6-diisopropylphenyl).