Stable Silapyramidanes.

Starting from tetrakis(trimethylsilyl)cyclobutadiene and an amidinate-supported silylene of the Roesky-type, a sequence of addition and reduction cleanly gives the elusive silapyramidane via an isolable cyclobutene intermediate with an exocyclic Si═C bond. The silapyramidane features an unusually shielded 29Si NMR resonance at -448.3 ppm for the apex silicon atom. Treatment with Fe2(CO)9 results in the formation of the corresponding silapyramidane-iron complex. Silapyramidane also reacts with the cyclobutadiene starting material to cleanly afford a fluorescent spirobis(silole).

Cage-size effects on the encapsulation of P2 by fullerenes.

The classic pnictogen dichotomy stands for the great contrast between triply bonding very stable N2 molecules and its heavier congeners, which appear as dimers or oligomers. A banner example involves phosphorus as it occurs in nature as P4 instead of P2 , given its weak π-bonds or strong σ-bonds. The P2 synthetic value has brought Lewis bases and metal coordination stabilization strategies. Herein, we discuss the unrealized encapsulation alternative using the well-known fullerenes' capability to form endohedral and stabilize otherwise unstable molecules. We chose the most stable fullerene structures from Cn (n = 50, 60, 70, 80) and experimentally relevant from Cn (n = 90 and 100) to computationally study the thermodynamics and the geometrical consequences of encapsulating P2 inside the fullerene cages. Given the size differences between P2 and P4 , we show that the fullerenes C70 -C100 are suitable cages to side exclude P4 and host only one molecule of P2 with an intact triple bond. The thermodynamic analysis indicates that the process is favorable, overcoming the dimerization energy. Additionally, we have evaluated the host-guest interaction to explain the origins of their stability using energy decomposition analysis.

σ,π-Conjugated Bis(germylene) Adducts with NHC and CAACs.

Heavier tetrylenes attract attention for their potential in synthesis, catalysis and small molecule activation. The coordination by N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) results in substantial structural and electronic differences although typically only one of these yields stable derivatives for one and the same tetrylene. We now report both NHC- and CAAC-coordination to a bridged bis(germylene) motif. The NHC-coordinated bis(germylene) exhibits pyramidal germanium centers with lone pairs of electrons, while with CAAC an unprecedented stable bis(germene) with two Ge=C bonds is isolated. Spectroscopic and crystallographic evidence as well as DFT calculations confirm the effects of σ,π-conjugation between the two germanium centers in both cases. The coordination of NHC is reversible as the reaction with BPh3 liberates the transient bis(germylene) and thus provides an alternative low-temperature route towards polymers with Ge=Ge bonds.

Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba).

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C5R5)2] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

The first HyDRA challenge for computational vibrational spectroscopy.

Vibrational spectroscopy in supersonic jet expansions is a powerful tool to assess molecular aggregates in close to ideal conditions for the benchmarking of quantum chemical approaches. The low temperatures achieved as well as the absence of environment effects allow for a direct comparison between computed and experimental spectra. This provides potential benchmarking data which can be revisited to hone different computational techniques, and it allows for the critical analysis of procedures under the setting of a blind challenge. In the latter case, the final result is unknown to modellers, providing an unbiased testing opportunity for quantum chemical models. In this work, we present the spectroscopic and computational results for the first HyDRA blind challenge. The latter deals with the prediction of water donor stretching vibrations in monohydrates of organic molecules. This edition features a test set of 10 systems. Experimental water donor OH vibrational wavenumbers for the vacuum-isolated monohydrates of formaldehyde, tetrahydrofuran, pyridine, tetrahydrothiophene, trifluoroethanol, methyl lactate, dimethylimidazolidinone, cyclooctanone, trifluoroacetophenone and 1-phenylcyclohexane-cis-1,2-diol are provided. The results of the challenge show promising predictive properties in both purely quantum mechanical approaches as well as regression and other machine learning strategies.

Crystalline Anions Based on Classical N-Heterocyclic Carbenes.

Herein, the first stable anions K[SIPrBp ] (4 a-K) and K[IPrBp ] (4 b-K) (SIPrBp =BpC{N(Dipp)CH2 }2 , IPrBp =BpC{N(Dipp)CH}2 ; Bp=4-PhC6 H4 ; Dipp=2,6-iPr2 C6 H3 ) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC8 reduction of the neutral radicals [SIPrBp ] (3 a) and [IPrBp ] (3 b), respectively. The radicals 3 a and 3 b as well as [Me-IPrBp ] 3 c (Me-IPrBp =BpC{N(Dipp)CMe}2 ) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPrBp )Br (2 a), (IPrBp )Br (2 b), and (Me-IPrBp )Br (2 c) with KC8 . The cyclic voltammograms of 2 a-2 c exhibit two one-electron reversible redox processes in -0.5 to -2.5 V region that correspond to the radicals 3 a-3 c and the anions (4 a-4 c)- . Computational calculations suggest a closed-shell singlet ground state for (4 a-4 c)- with the singlet-triplet energy gap of 17-24 kcal mol-1 .

Evidence of AlII Radical Addition to Benzene.

Electrophilic AlIII species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent AlI aluminyl anions have showcased oxidative additions towards arenes C-C and/or C-H bonds. Herein, we communicate compelling evidence of an AlII radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC8 in benzene furnishes a double addition to the benzene ring instead of a C-H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable AlII radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.

Diarylpnictogenyldialkylalanes─Synthesis, Structures, Bonding Analysis, and CO2 Capture.

Several new diphenylamino- and diphenylphosphanyldialkylalanes are reported, which were characterized in solution and in the solid state, assisted by in-depth bonding analysis within the DFT framework. In the case of bulky alkyl substituents on the aluminum atom, the species are stable in their monomeric form and were structurally characterized by single crystal X-ray diffraction, expanding the relatively small field of monomeric pnictogenylalanes. In the case of oligomeric diphenylpnictogenyldimethylalanes, their reactivity toward different σ-donor ligands was studied, and several examples of monomeric adducts could be structurally characterized, including the first cyclic(alkyl)(amino)carbene complexes. The reactivity of these CAAC complexes, their oligomeric precursors, and an unstabilized monomeric aminoalane toward CO2 was probed, leading to different insertion products that could be characterized. Additionally, the mechanism was elucidated by DFT calculations.

Path-dependency of energy decomposition analysis & the elusive nature of bonding.

Here, we provide evidence of the path-dependency of the energy components of the energy decomposition analysis scheme, EDA, by studying a set of thirty-one closed-shell model systems with the D2h symmetry point group. For each system, we computed EDA components from nine different pathways and numerically showed that the relative magnitudes of the components differ substantially from one path to the other. Not surprisingly, yet unfortunately, the most significant variations in the relative magnitudes of the EDA components appear in the case of species with bonds within the grey zone of covalency and ionicity. We further discussed that the role of anions and their effect on arbitrary Pauli repulsion energy components affects the nature of bonding defined by EDA. The outcome variation by the selected partitioning scheme of EDA might bring arbitrariness when a careful comparison is overlooked.

Metathesis Reactions of a NHC-Stabilized Phosphaborene.

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P-protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N-heterocyclic carbene (NHC)-stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P-terminal, as a room-temperature-stable crystalline solid accessible via facile NHC-induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P-centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X-ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

Isolation of an Arsenic Diradicaloid with a Cyclic C2 As2 -Core.

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C2 As2 -diradicaloid {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2 ; Dipp = 2,6-iPr2 C6 H3 ). Treatment of (IPr)CH2 (1) with AsCl3 affords the Lewis adduct {(IPr)CH2 }AsCl3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl2 (3) and {(IPr)CAsCl}2 (5 a) or [{(IPr)CAs}2 Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C2 As2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet-triplet energy gap of 8.7 kcal mol-1 . The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe)2 and Fe2 (CO)9 give rise to {(IPr)CAs(SePh)}2 (7) and {(IPr)CAs}2 Fe(CO)4 (8), respectively.

Chemical Bonding in Silicon Carbonyl Complexes.

Although silylene-carbonyl complexes are known for decades, only recently isolable examples have been accomplished. In this work, the bonding situation is re-evaluated to explain the origins of their remarkable stability within the Kohn-Sham molecular orbital theory framework. It is shown that the chemical bond can be understood as CO interaction with the silylene via a donor-acceptor interaction: a σ-donation from the σCO into the empty p-orbital of silicon, and a π-back donation from the sp2 lone pair of silicon into the π*CO antibonding orbitals. Notably, it was established that the driving force behind the surprisingly stable Si-CO compounds, however, is another π-back donation from a perpendicular bonding R-Si σ-orbital into the π*CO antibonding orbitals. Consequently, the pyramidalization of the central silicon atom cannot be associated with the strength of the π-back donation, in sharp contrast to the established chemical bonding model. Considering this additional bonding interaction not only shed light on the bonding situation, but is also an indispensable key for broadening the scope of silylene-carbonyl chemistry.

Isolation of a 16π-Electrons 1,4-Diphosphinine-1,4-diide with a Planar C4 P2 Ring.

Herein, we report the first 1,4-diphosphinine-1,4-diide compound [(ADCPh )P]2 (5-Ph) (ADCPh =PhC{(NDipp)C}2 ; Dipp=2,6-iPr2 C6 H3 ) derived from an anionic dicarbene (ADCPh ) as a red crystalline solid. Compound 5-Ph containing a 16π-electron planar fused-tricyclic ring system was obtained by the 4e reduction of [(ADCPh )PCl2 ]2 (4-Ph) with Mg (or KC8 ) in a quantitative yield. Experimental and computational results imply that the central 8π-electrons C4 P2 ring of 5-Ph, which is fused between two 6π-electrons C3 N2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5-Ph has two electron-lone-pairs, one in a p-type orbital is in conjugation with the C=C bonds of the C4 P2 ring, while the second resides in a σ-symmetric orbital. This can be shown with the gold complex [(ADCPh )P(AuCl)2 ]2 (6-Ph) obtained by reacting 5-Ph with (Me2 S)AuCl. A mixture of 5-Ph and 4-Ph undergoes comproportionation in the presence of MgCl2 to form the intermediate oxidation state compound [(ADCAr )P]2 (MgCl4 ) (7-Ph), which is an aromatic species.

Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands.

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3-E)GaCl4 [(3-E).+ = [{(IPr)C(Ph)E}2 Fe(CO)3 ].+ , E = P or As; IPr = C{(NDipp)CH}2 , Dipp = 2,6-iPr2 C6 H3 ] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}2 (1-E) with Fe2 (CO)9 affords [{(IPr)C(Ph)E}2 Fe(CO)3 ] (2-E), in which 1-E binds to the Fe atom in an allylic (η3 -EECvinyl ) fashion and functions as a 4e donor ligand. Complexes 2-E undergo 1e oxidation with GaCl3 to yield (3-E)GaCl4 . Spin density analysis revealed that the unpaired electron in (3-E).+ is mainly located on the Fe (52-64 %) and vinylic C (30-36 %) atoms. Further 1e oxidation of (3-E)GaCl4 leads to unprecedented η3 -EECvinyl to η3 -ECvinyl CPh coordination shuttling to form the dications (4-E)(GaCl4 )2 .

A Cyclic Iminoborane-NHC Adduct: Synthesis, Reactivity, and Bonding Analysis.

Lewis-base coordinated iminoborane adducts, in contrast to their isoelectronic analogue imines, remain largely unexplored given the lack of efficient synthetic strategies for generating robust compounds. Herein, we report the preparation of a cyclic amino iminoborane carbene complex 2 obtained in quantitative yield by adding NHC to the 1,8-(trimethylsilyl)aminonaphthalene complex of boron 1 to induce the elimination of trimethylsilyl chloride (TMSCl). The iminoborane-NHC adduct 2 shows unprecedented thermal stability both in the solid and solution phases, due to the rigid, pre-established geometry of the 1,8-diaminonaphthalene scaffold. Theoretical calculations reveal an exceptionally strong iminoborane-NHC bond as a consequence of the enhanced boron-center acidity in combination with the lower steric and electronic shielding. We show that the chemical bond can be understood as donor-acceptor interaction, leading to a different kind of electronic situation of the B═N π-bond. The high conjugation between the pz-lone pair of the tricoordinated sp2 hybridized N atom and the B═N π-system results in a particularly long B═N double bond distance. Taking advantage of the pendant lone pair of the dicoordinated sp2 hybridized N atom, the iminoborane-NHC adduct gives access to NHC-stabilized borenium cation 3 through the reaction with trimethylsilyl triflate (Me3SiOTf) or to the gallium adduct 4 by reacting with GaCl3. Incorporating an iminoborane functional group into a π-conjugated system brings a new bonding situation for broadening the scope of BN-containing polyaromatic systems.

Unveiling the Electronic Structure of the Bi(+1)/Bi(+3) Redox Couple on NCN and NNN Pincer Complexes.

Low-valent group 15 compounds stabilized by pincer ligands have gained particular interest, given their direct access to fine-tune their reactivity by the coordination pattern. Recently, bismuth has been employed in a variety of catalytic transformations by taking advantage of the (+1/+3) redox couple. In this work, we present a detailed quantum-chemical study on the electronic structure of bismuth pincer complexes from two different families, namely, bis(ketimine)phenyl (NCN) and triamide bismuthinidene (NNN). The use of the so-called effective oxidation state analysis allows the unambiguous assignation of the bismuth oxidation state. In contrast to previous studies, our calculations suggest a Bi(+1) assignation for NCN pincer ligands, while Bi(+3) character is found for NNN pincer complexes. Notably, regardless of its oxidation state, the central bismuth atom disposes of up to two lone pairs for coordinating Lewis acids, as indicated by very high first and second proton affinity values. Besides, the Bi-NNN systems can also accommodate two Lewis base ligands, indicating also ambiphilic behavior. The effective fragment orbital analysis of Bi and the ligand allows monitoring of the intricate electron flow of these processes, revealing the noninnocent nature of the NNN ligand, in contrast with the NCN one. By the dissection of the electron density into effective fragment orbitals, we are able to quantify and rationalize the Lewis base/acid character.

Beyond the Classical Electron-Sharing and Dative Bond Picture: Case of the Spin-Polarized Bond.

Chemical bonds are traditionally assigned as electron-sharing or donor-acceptor/dative. External criteria such as the nature of the dissociation process, energy partitioning schemes, or quantum chemical topology are invoked to assess the bonding situation. However, for systems with marked multi-reference character, this binary categorization might not be precise enough to render the bonding properties. A third scenario can be foreseen: spin polarized bonds. To illustrate this, the case of a NaBH3 - cluster is presented. According to the analysis NaBH3 - exhibits a strong diradical character and cannot be classified as either electron-sharing or a dative bond. Elaborated upon are the common problems of popular bonding descriptions. Additionally, a simple model, based on the bond order and local spin indicators, which discriminates between all three bonding situations, is provided.

Isolation of 1,4-Diarsinine-1,4-diide and 1,4-Diarsinine Derivatives.

1,4-Diarsinine-1,4-diide compound [(ADCPh )As]2 (5) (ADCPh ={C(DippN)}2 CPh, Dipp=2,6-iPr2 C6 H3 ) with a planar C4 As2 ring fused between two 1,3-imidazole scaffolds has been isolated as a red crystalline solid. Compound 5, formally comprising an 8π-electron C4 As2 ring, is antiaromatic and undergoes 2e-oxidation with AgOTf to form the 6π-electron aromatic system [(ADCPh )As]2 (OTf)2 (6).

Isolation of Elusive Electrophilic Phosphinidene Complexes with π-Donor N-Heterocyclic Vinyl Substituents.

Phosphinidene complexes of the general formula RPM(CO)n (R = an alkyl or aryl group; M = a transition metal) are electrophilic and thermally unstable. Thus, the isolation of these elusive species for structural elucidations remains a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO)4 [NHC = IPr = C{(NDipp)CH}2 for 3; Me-IPr = C{(NDipp)CMe}2 for 4; Dipp = 2,6-iPr2C6H3; NHC = N-heterocyclic carbene] as red crystalline solids containing a π-donor N-heterocyclic vinyl (NHV) substituent at the phosphorus atom. Calculations reveal donor-acceptor type bonding between phosphorus and iron atoms in 3 and 4. The P → Fe donation represents ∼70% of the orbital interaction, whereas the Fe → P π-back-donation corresponds to ∼15% of the orbital interaction. The phosphorus atoms in 3 and 4 carry charges of +0.65e and +0.64e, respectively, indicating the electrophilic character of the phosphinidene {(NHC)C(Ph)}P moiety. Accordingly, 3 reacts with an NHC nucleophile (IMe4) to yield the Lewis adduct [{(NHC)C(Ph)}P(IMe4)]Fe(CO)4 (5) [IMe4 = C(NMeCMe)2]. The coordination of an electron-rich NHC (IMe4) to the phosphorus atom in 5 precludes the π-electron density transfer from the NHV to the phosphorus atom. Thus, the CIPr-Cvinyl and Cvinyl-P bonds of 5 become shorter and longer, respectively, compared to those of 3.

Energy components in energy decomposition analysis (EDA) are path functions; why does it matter?

Here, we discuss that unlike bond dissociation energy (BDE) that is a state function quantity, the energy components of the energy decomposition analysis (EDA), i.e. electrostatic interaction, Pauli repulsion, and orbital interaction, are path (process) function quantities. Being a path function means that EDA energy components are not uniquely defined, i.e. the relative magnitudes of the orbital interaction, Pauli repulsion, and electrostatic components may vary depending on the selected pathway for EDA. Therefore, at best, EDA can define whether closely related chemical bonds are more or less ionic/covalent compared with each other. However, a precise assessment of the nature of a certain type of chemical bond using EDA is a questionable task. Besides, we briefly discuss that the widely used EDA pathway, which is merely an arbitrary choice among infinite possible paths, comes to conclusions not consistent with our widely accepted knowledge of bond formation even for the simplest molecules.