Search

Only Si/P-based, Si/N-based, and Si/As-based FLPs can facilitate the [2+5] cycloaddition reaction with benzaldehyde both kinetically and thermodynamically.

Our computational investigations suggest that all NNG15-Rea and G15G15P-Rea molecules can be energetically feasible to undergo 1,3-dipolar cycloaddition reactions with cyclic alkynes, except for only the NNN-Rea 1,3-dipole molecule.

The nature of the group 15 analogs of the five-membered NHCs (G15-Rea (G15 = N, P, As, Sb, and Bi)) was studied employing five methodologies: calculations of the NICS (the nucleus-independent chemical shifts), the ACID (anisotropy of the current-induced density), the PA (proton affinity), the electrophilicity, and the Fukui functions using density functional theory (DFT). The deep analysis of the results indicates the reactivity of heavy G15′-Rea (G15′ = P, As, Sb, and Bi) is governed by its lowest unoccupied molecular orbital, a vacant p-π orbital. Conversely, the highest occupied molecular orbital, a nonbonding sp2 lone pair orbital, determines the reactivity of N-Rea. To probe the origin of activation barriers of chemical reactions for group 15 analogs of G15-Rea, two types of model reactions (C–H bond insertion of methane and alkene cycloaddition) were theoretically examined with the help of the activation strain model (ASM). Our theoretical evidences reveal that the origin of the activation energies of the model reactions can be explained in terms of the atomic radii of the pivotal group 15 elements and electronic factors of the five-membered G15-Rea species. Interestingly, our theoretical findings strongly suggest that both aromaticity and singlet-triplet energy splitting ΔEst of the G15-Rea molecules can be used as a diagnostic tool for the prediction of the reactivity of diverse five-membered monocationic group 15 NHCs with saturated and saturated hydrocarbons.

The N-phosphinoamidinato digermyne [LG̈e–G̈eL] (L = tBu2PNC(Ph)NDipp, Dipp = 2,6-iPr2C6H3) demonstrated transition-metal–like reactivity in undergoing reversible oxidative addition and reductive elimination with CO2 to form [LG̈eOC(O)G̈eL].

The reactivities of

The trapping reactions of

This work describes a straightforward method to synthesize a borylborylene without proceeding via the rearrangement of a diborene. An amidinato amidosilylene [LSiNMe2] (L = PhC(NtBu)2) and PMe3 were reacted with an N-phosphinoamidinato diborane 1 and KC8 to form a stable silylene-borylborylene 2 and a persistent phosphine-borylborylene 3, respectively. Compound 2 is stable as the borylene center is well stabilized by the silylene donor and boryl substituent, whereas compound 3 is unstable in solution due to labile PMe3. The latter was illustrated by reacting compound 3 with Ar'NC (Ar' = 2,6-Me2C6H3), where Ar'NC displaced PMe3 and inserted into the N-phosphinoamidinate ligand and B-B bond to form compound 4. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) C.-W.S. thanks the Ministry of Education Singapore, AcRF Tier 2 (MOE2019-T2-2-129) and A*STAR MTC Individual Research Grant (M21K2c0117).

The mechanisms of C-H bond insertion and alkene cycloaddition were investigated theoretically using five model systems: group 13 analogues of the four-membered nucleophilic N-heterocyclic carbenes (NHCs) (1E; E = group 13 element). The theoretical findings indicate that, except for 1B with H2C=CH2, these four-membered NHCs undergo insertion and [1 + 2] cycloaddition reactions with difficulty because their activation barriers are quite high (31 kcal/mol). The theoretically confirmed chemical inertness of the four-membered NHCs 1Ga and 1In might explain why they have been experimentally detected at room temperature. Additionally, our theoretical observations indicate that the reactivity of these four-membered NHCs featuring a central group 13 element follows the order 1B ≫ 1Al > 1Ga > 1In > 1Tl. The theoretical examination suggests that the smaller the atomic radius of the central group 13 element in the four-membered NHC analogue is, the larger the aromaticity of this carbenic molecule is, the higher the basicity of this carbenic molecule in nature is, the larger its nucleophilic attack on other oncoming molecules is, the smaller the barrier heights of its C-H bond insertion and [1 + 2] cycloaddition reactions will be, the higher its exothermicities for these products will be, and thus, the greater its reactivity will be. Moreover, the present theoretical findings reveal that the reactivity of 1B is governed by its highest occupied molecular orbital, a nonbonding sp2 lone pair orbital. In contrast, the reactivity of the four heavier 1E' (E' = Al, Ga, In, and Tl) molecules is mainly determined by their lowest unoccupied molecular orbital, a vacant p-π orbital. The conclusions gained from this study allow many predictions to be made.

The reaction of the bis(thiophosphinoyl) geminal dianion [C(PPh2S)2]2- with B2Br4(SMe2)2 in toluene afforded the diboron-carbene complex [(SPh2P)2CB]2 (2). The lability of the boron-carbene bond was found in the reaction of the sterically hindered thiophosphinoyl iminophosphoranyl geminal dianion [C(PPh2NAr)(PPh2S)]2- (Ar = 2,6-iPr2C6H3) with B2Br4(SMe2)2 in toluene to form the diboron-carbene complex 4, where one boron center bonds with two methanediides, leading to an allyl anionic-like C═B←C2- electronic structure, while another boron center is stabilized by two imino substituents. Complex 4 underwent rearrangement via a 1,2-methanediide shift in toluene at 60 °C to give the diboron-carbene complex [(SPh2P)(ArNPh2P)CB]2 (5). Ministry of Education (MOE) This work was supported by the Ministry of Education Singapore, AcRF Tier 1 (RG121/17) (C.-W.S.). M.-C.Y. and M.-D.S. are grateful to the Ministry of Science and Technology of Taiwan for financial support.

Three fundamental concepts (aromaticity/basicity/electrophilicity), being heavily used in modern chemistry, have been applied in this work to study the chemical reactivity of six-membered-ring group 13 N-heterocyclic carbenes (G13-6-Rea; G13 = group 13 elements) using density functional theory (BP86-D3(BJ)/def2-TZVP). G13-6-Rea is isolobal to benzene. Two model reactions have been used in the present study: the insertion reaction of G13-6-Rea with methane and the [1 + 2] cycloaddition reaction of G13-6-Rea with ethene. Our theoretical analysis reveals that the chemical reactivity of B-6-Rea, Al-6-Rea, and Ga-6-Rea is governed by their HOMO (the sp2-σ lone pair orbital on the G13 element), and thus they can be considered nucleophiles. Conversely, the chemical behavior of In-6-Rea and Tl-6-Rea is determined by their LUMO (the vacant p-π orbital on the G13 element), and thus they can be considered electrophiles. On the basis of the VBSCD (valence bond state correlation diagram) model and ASM (activation strain model), this theoretical evidence demonstrates that the origin of activation barriers for the above model reactions is due to the atomic radius of the pivotal group 13 element in the six-membered-ring of G13-6-Rea. Accordingly, our theoretical conclusions suggest that the lower the atomic number and the smaller the atomic radius of the G13 atom, the higher the aromaticity of the six-membered-ring of G13-6-Rea and the smaller the singlet–triplet energy splitting ΔEst of this N-heterocyclic carbene analogue, which will facilitate its chemical reactions. The theoretical findings originating from this study allow many predictions in experiments to be made.

0.5 mol% of the amidinato disilyne [LSi:]2 (1, L = PhC(NtBu)2) and 1 mol% of B(C6F5)3 cooperatively polymerized methyl methacrylate (MMA) to form poly(MMA) with the H and CH2C(=CH2)COMe end groups (P1, Mn = 3.1 ×103 gmol-1; repeating unit, n = 31; polydispersity index, Đ: 1.55). The catalytic mechanism is proposed, where compound 1 could react with MMA and B(C6F5)3 to form a zwitterionic active species [LSi{CH2C(Me)=C(OMe)O}B(C6F5)3]2. The latter could activate MMA molecules affording poly(MMA) chains on the silicon centers. The methyl group of the last enolate of a poly(MMA) chain on a silicon center could then react with the Si-C bond of the adjacent poly(MMA) chain to form P1, as well as regenerate compound 1 and B(C6F5)3. When compound 1 was replaced by the amidinato amidosilylene [LSiN(SiMe3)2] (2), 1 mol% of compound 2 and 1 mol% of B(C6F5)3 mediated living MMA polymerization in toluene to form the poly(MMA) P2 (Mn = 1.04 ×104 gmol-1; Đ: 1.97). National Research Foundation (NRF) Submitted/Accepted version This work is supported by the National Research Foundation Singapore NRF-ANR (NRF2018-NRF-ANR026 Si-POP). M.-C. Yang and M.-D. Su are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time, and the Ministry of Science and Technology of Taiwan for the financial support.

A boron analogue of vinyl cation, pyridine-stabilized N-phosphinoamidinato N-heterocyclic carbene (NHC)-diboravinyl cation 2+ , was synthesized by displacement of bromide in diborene 1 with excess pyridine. Experimental and computational studies showed that the positive charge is mainly at the B-B skeleton with delocalization to the pyridine ligand. One of the main modes of reactivity is through the B=B double bond alongside activation of the pyridine substituent, where the Bpyridine center is the predominant nucleophilic center and the predominant electrophilic center is either the activated pyridine para position or the BNHC center, illustrating the presence of diborene cation A, borylene-borenium cation B and diborene-pyridinium cation C resonance structures in cation 2+ . Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This work is supported by the Ministry of Education Singapore, AcRF Tier2 (MOE2019-T2-2-129) and A*STAR MTC Individual Research Grants (M21K2c0117) for the financial support (C.-W.S)

The

The amidinato amidosilylene [LSiNMe2] (1, L = PhC(NtBu)2) was reacted with B2Br4(SMe2)2 in toluene at room temperature to form the bis(silylene)-tetrabromodiborane [L{Me2N}Si]2B2Br4 (2). It was then reacted with excess KC8 in THF at room temperature to afford the bis(silylene)-dibromodiborene [L{Me2N}Si]2B2Br2 (3). Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version C.-W.S. thanks the National Research Foundation Singapore, NRF-ANR (NRF2018-NRF-ANR026 Si-POP), and Ministry of Education Singapore, AcRF Tier 2 (MOE2019-T2-2-129), for financial support. M.-C.Y. and M.-D.S. acknowledge the National Center for High-Performance Computing of Taiwan for generous amounts of computing time and the Ministry of Science and Technology of Taiwan for financial support.

The reactions of carbenic cations (PtBu3)2M+ (M = B, Al, Ga, In, and Tl) with methane and ethene are studied using density functional theory. The activation energies and reaction enthalpies are analyzed by the energy decomposition analysis (EDA) to understand the reactivity of the reactions. For the reactions with methane, the activation energies and reaction enthalpies increase with the atomic numbers of the group 13 elements (M). For the reactions with ethene, the activation energies and reaction enthalpies also increase with the atomic numbers of M but with a notable exception. The activation energy of the reaction of (PtBu3)2B+ with ethene is abnormally higher than those of (PtBu3)2Al+ and (PtBu3)2Ga+. The activation energy abnormality is due to the excessive strain energy associated with the tremendous structural deformation, in which (PtBu3)2B+ has to reform in order to reach the prepared conformation of the transition state. The present theoretical evidence reveals that the atomic radius of the central group 13 element M of the carbenic (PtBu3)2M+ species plays a key role in both insertion and [1+2] cycloaddition reactions. In other words, this can be traced back to the fundamental reason that the 2s and 2p orbitals are more amenable to hybridize than the other ns and np orbitals for n ≥ 3.

In this study, the reactivity influenced by group 14 elements (E = C, Si, Ge, Sn, and Pb), which are used as substituents in heterocyclic five-membered rings, was theoretically examined by using density functional theory (B3PW91/def2-SVP). For this purpose, five group 14 heterocycles containing a selenium atom, Rea-E, have been chosen as model reactants in this work. Theoretical evidence revealed that the electronic effects of group 14 elements played a crucial role in forming new optoelectronic materials with a heteroatomic five-membered ring, which could be an interesting synthetic target for the next generation. Moreover, the addition reactions for Rea-E with dihydrogen as well as acetylene along with the reaction coordinates were quantitatively analyzed by using the EDA–NOCV (energy decomposition analysis–natural orbitals for chemical valence) method in this study. Theoretical examinations demonstrated that both the activation barrier and reaction energy were strongly dependent on the atomic radius of the E atom due to the Pauli repulsion. Since the carbon element possessed the smallest atomic radius in the group 14 family, the addition reactions of Rea-C with H2 as well as acetylene can lead to the largest activation barrier and an endothermic reaction.

The reactivity of a reported hyper-coordinated alkene (Rea-E; Rea = reactant; E = group 14 element) featuring a central E[double bond, length as m-dash]E moiety was theoretically analyzed using DFT (density functional theory) and the EDA-NOCV (energy decomposition analysis-natural orbitals for chemical valence) method. M06-2X/def2-SVP and B3LYP-D3/def2-SVP results demonstrate that five Rea-E molecules have an energy minimum as their structures have no imaginary frequency. Theoretical examinations based on three types of bond order calculations (Wiberg, Mayer, and Fuzzy), the LOL (localized orbital locator) analyses, Lewis structures and the NBO (natural bond orbital) analyses suggest that a very weak central Si-Si single bond and an extremely weak central Ge-Ge single bond, rather than a double bond, are present in the Rea-Si and Rea-Ge molecules, respectively. On the other hand, no bond is found between the two central group 14 atoms in Rea-C, Rea-Sn, and Rea-Pb. The theoretical investigation demonstrates that the reactivity of the Rea-E compound decreases in the order Rea-SiRea-GeRea-C, a trend that results from the differences in the atomic radii of the group 14 elements. Carbon has the smallest atomic radius in the group 14 family, causing steric crowding between Rea-C and other attacking species. This circumstance, in turn, increases the activation energies of its addition reactions and renders these reactions energetically infeasible. For the cyclic product of Rea-Ge, the theoretical evidence reveals that the comparatively large atomic radius of Ge induces the weakest Pauli repulsions and the smallest overlap integrals between Rea-Ge and the other doubly bonded molecules. This situation, in turn, makes the overall cyclization reaction of Rea-Ge endothermic. As a result, only the silicon-centered molecule, Rea-Si, can undergo the [2 + 2] cycloaddition reactions with doubly bonded molecules without kinetic or thermodynamic difficulty, which agrees well with the available experimental findings.

The heterolytic cleavage of H2 by multiply bonded phosphorus-bridged G13-P-P-Rea (G13 = B, Al, Ga, In, and Tl) and G15-P-Ga-Rea (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) has been theoretically investigated using density functional theory calculations. For the above nine FLP-type molecules, our theoretical findings suggest that only Al-P-P-Rea, Ga-P-P-Rea, and In-P-P-Rea can undergo the energetically feasible H2 activation reaction from kinetic and thermodynamic viewpoints. Our study based on the activation strain model (ASM) reveals that gaining a better orbital overlap between G13-P-P-Rea and G15-P-Ga-Rea molecules and H2 affected the reaction barriers through the atomic radius of G13 and G15. According to our energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) results, the bonding of these H2 activation reactions involving G13-P-P-Rea and G15-P-Ga-Rea is dominated by the donor-acceptor interaction (singlet-singlet interaction) rather than the electron-sharing interaction (triplet-triplet interaction). Moreover, our EDA-NOCV evidence reveals that the best description for the above bonding situations is the lone pair(G15) → σ*(H2) interaction rather than the empty p-π-orbital(G13) ← σ(H2) interaction. In particular, the findings in this work based on theoretically calculated geometries and the corresponding relative free energies of the stationary points combined with the results from the above sophisticated methods nicely agree with the famous Hammond postulate.

The reactions of chalcogen N-heterocyclic carbenic (NHC) dications [(Dipp2DAB)M2+, M = O, S, Se, and Te, and Dipp2DAB = 1,4-(2,6-diisopropyl)phenyl-1,4-diaza-1,3-butadiene] with methane and ethene are investigated by the density functional theory. The activation energies and reaction enthalpies are analyzed by the energy decomposition analysis (EDA) to understand the reactivity of the reactions. Calculations show that the oxide carbenic dication (Dipp2DAB)O2+ reactant has a pair of rather unbalanced O–N bonds due to the combined effects of the smallness of the central O atom and the bulkiness of the two Dipp ligands surrounding the DAB ring, suggesting the oxide reactant is much less stable than the chalcogenide ones. Because of its unique characteristics, the oxide carbenic dication distinguishes itself from the rest of the chalcogen family. The differences between oxides and chalcogenides of the chalcogen family complexes well known in inorganic chemistry are also observed in this theoretical investigation. This work further reveals that the reasons for the distinctive characteristics between oxide and chalcogenide complexes are due to the fundamental fact that the 2s and 2p atomic orbitals are more amenable to hybridization than those of higher valences, and the mixing of the former valence orbitals results in much smaller atoms than the latter.

The use of the

The use of the N-phosphinoamidinato NHC-diborene catalyst 2 for hydroboration is described. The N-phosphinoamidine tBu2PN(H)C(Ph)═N(2,6-iPr2C6H3) was reacted with nBuLi in Et2O to afford the lithium derivative, which was then treated with B2Br4(SMe2)2 in toluene to form the N-phosphinoamidinate-bridged diborane 1. It was reacted with the N-heterocyclic carbene IMe (:C{N(CH3)C(CH3)}2) and excess potassium graphite at room temperature in toluene to give the N-phosphinoamidinato NHC-diborene compound 2. It can stoichiometrically activate ammonia-borane and carbon dioxide. It also showed catalytic capability. A 2 mol % portion of 2 catalyzed the hydroboration of carbon dioxide (CO2) with pinacolborane (HBpin) in deuterated benzene (C6D6) at 110 °C (conversion >99%), which afforded the methoxyborane [pinBOMe] (yield 97.8%, TOF 33.3 h-1) and the bis(boryl) oxide [(pinB)2O]. In addition, 5 mol % of 2 catalyzed the N-formylation of secondary and primary amines by carbon dioxide and pinacolborane to yield the N-formamides (average yield 91.6%, TOF 25.9 h-1). Moreover, 2 showed chemoselectivity toward catalytic hydroboration of carbonyl compounds. In mechanistic studies, the B═B double bond in compound 2 activated the substrates, the intermediates of which then underwent hydroboration with pinacolborane to yield the products and regenerate catalyst 2. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version C.-W.S. thanks the Ministry of Education Singapore, AcRF Tier 2 (MOE2019-T2-2-129), and the National Research Foundation Singapore, NRF-ANR (NRF2018-NRF-ANR026 Si-POP), for the financial support. M.-C.Y. and M.-D.S. acknowledge the National Center for High-Performance Computing of Taiwan for generous amounts of computing time and the Ministry of Science and Technology of Taiwan for financial support.

The mechanisms for the photochemical CO-dissociation and the oxidative addition reactions are studied theoretically using three model systems: M(CO)5 (M = Fe, Ru, and Os) and the CASSCF/Def2-SVP (fourteen-electron/ten-orbital active space) and MP2-CAS/Def2-SVP//CASSCF/Def2-SVP methods. The structures of the intersystem crossings and the conical intersections, which play a decisive role in these CO photo-extrusion reactions, are determined. The intermediates and the transition structures in either the singlet or triplet states are also computed, in order to explain the reaction routes. These model studies suggest that after the irradiation of Fe(CO)5 with UV light, it quickly loses one CO molecule to generate a 16-electron iron tetracarbonyl, in either the singlet or the triplet states. It is found that the triplet Fe(CO)4 plays a vital role in the formation of the final oxidative addition product, Fe(CO)4(H)(SiMe3), but the singlet Fe(CO)4 plays a relatively minor role in the formation of the final product. However, its vacant coordination site interacts weakly with solvent molecules ((Me3)SiH) to yield the alkyl-solvated iron complexes, which are detectable experimentally. The theoretical observations show that Ru(CO)5 and Os(CO)5 have similar photochemical and thermal potential energy profiles. In particular, this study demonstrates that the oxidative addition yield for Fe is much greater than those for its Ru and Os counterparts, under the same chemical conditions.