Mono(Lewis Base)-Stabilized Gallium Iodide: An Unexplored Class of Promising Ligands.

Quantum-chemical (DFT) calculations on hitherto unknown base(carbene)-stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p-orbital - both centered at the central gallium atom - and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor-acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non-innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations.

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Cooperative Activation of Small Molecules by Base-Stabilized Borylenes.

Computational investigations were carried out using density functional theory (ωB97XD(toluene)/6-311+G*) on a series of base-stabilized borylenes to understand their ligand properties and potential toward the activation of enthalpically strong E-H bonds (E = H, NH2, SiH2Ph, and CH3) as well as binding with small molecules such as CO and CNMe. The calculated reaction free energies and activation barriers suggest the ability of hitherto unexplored carbene-stabilized borylenes to not only split such bonds but also bind with CO and CNMe. A detailed mechanistic study of these bond activation processes reveals the noninnocent behavior of the carbene moiety attached to the boron center, thereby leading to cooperative splitting of the bonds of interest. The binding of CO and CNMe to the base-stabilized borylenes was investigated by energy decomposition analysis (EDA) with natural orbitals for chemical valence (NOCV), which gave appreciable interaction energy (ΔEint) values, thereby indicating strong binding of CO and CNMe to these borylenes.

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding.

Invited for the cover of this issue are Felipe Fantuzzi, Holger Braunschweig, Ashwini K. Phukan and co-workers at the University of Kent, Julius-Maximilians-Universität Würzburg and Tezpur University. The image depicts a borylene "fishing" for a molecule of nitrogen. Read the full text of the article at 10.1002/chem.20210123.

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding.

Computational investigations were carried out to probe the potential of several dicoordinate, singly base-stabilized borylenes of the form [L→BR] (L=neutral Lewis base) in dinitrogen binding. The calculated reaction free energies and activation barriers associated with the formation of mono- and diborylene-N2 adducts suggest the presence of thermally surmountable kinetic barriers towards their possible isolation. Our results show that the exergonicity of dinitrogen activation and fixation is linearly dependent on the natural charge at the boron center, which can be tuned to design novel boron-based compounds with potential applications to small-molecule activation. EDA-NOCV analysis reveals strong binding of dinitrogen to these base-stabilized borylenes.

Unravelling the Potential of Ylides in Stabilizing Low-Valent Group 13 Compounds: Theoretical Predictions of Stable, Five-Membered Group 13 (Aluminum and Gallium) Carbenoids Capable of Small-Molecule Activation.

Computational investigations provide evidence toward the remarkable ability of strongly electron-donating ylidic functionalities in stabilizing singlet group 13 carbenoids with promising ligand properties. All of the proposed carbenoids are found to be considerably nucleophilic and possess significant singlet-triplet energy separation values. The calculated activation barriers and reaction free energies obtained for the cleavage of different enthalpically strong bonds by these carbenoids are found to be either comparable to or lower than those of the experimentally evaluated aluminum and gallium carbenoids, thereby indicating their potential in small-molecule activation.

Hybrid Inorganic-Organic Cross-Metathesis between Diborenes and Acetylene.

The ruthenium-catalyzed cross-metathesis of alkenes and alkynes, which splits the alkene C═C double bond and couples one-half to each carbon of the alkyne C≡C triple bond, is one of the most efficient tools for the synthesis of 1,3-dienes, with wide-ranging applications, including pharmaceutical and polymer chemistry. In contrast, inorganic main-group metathesis reactions are restricted to a handful of examples of heavier p-block multiple bonds (P═P, Ge═Ge, and E≡E, E = Ge, Sn, Pb). We now report the first examples of thermally induced, transition-metal-free cross-metathesis between an organic alkyne and inorganic cyclic alkyl(amino)carbene (CAAC)-stabilized B═B double bonds, which yield fully planar, π-delocalized 1,8-diaza-3,6-diboraoctatetraenes. Density functional theory studies show that these compounds have an open-shell singlet biradical ground state with a thermally accessible closed-shell state. In-depth computational mechanistic analyses show that they are formed via a biradical cycloaddition-cycloreversion mechanism. Finally, unlike their organic counterparts, these B,N-analogues of octatetraene can undergo two-electron chemical reduction to form diamagnetic dianions.

Understanding, Modulating, and Leveraging Transannular M → Z Interactions.

Density functional theory calculations have been performed on metallatranes featuring a group 13 elements at the bridgehead position to understand the factors that influence the nature of the M···Z (M = Fe, Co, Ni; Z = Al, Ga, In) interaction present in these complexes and the resultant reactivity at the metal center. The strength of the M···Z interaction increases with the increase in the size and polarizability of the bridgehead group 13 elements. The calculated reaction free energies (ΔG° values) for binding of different Lewis bases to the metallatranes are found to be significantly more exergonic for the larger In(III) ions. Quantum theory of atoms in molecules calculations reveal the covalent nature of the M···Z interactions, while the EDA-NOCV analysis indicates the strong binding ability of these metallatranes not only to different σ-donor and π-acceptor ligands but also to relatively inert species, such as N2.

Chalcogen Stabilized bis-Hydridoborate Complexes of Cobalt: Analogues of Tetracyclo[4.3.0.02,4 .03,5 ]nonane.

Treatment of Li[BH3 ER] (E=Se or Te, R=Ph; E=S, R=CH2 Ph) with [Cp*CoCl]2 led to the formation of hydridoborate complexes, [{CoCp*Ph}{Cp*Co}{µ-EPh}{µ-κ2 -E,H-EBH3 }], 1a and 1 b (1 a: E=Se; 1 b: E=Te) and a bis-hydridoborate species [Cp*Co{µ-κ2 -Se,H-SeBH3 }]2 , 2. All the complexes, 1 a, 1 b and 2 are stabilized by ß-agostic type interaction in which 1 b represents a novel bimetallic borate complex with a rare B-Te bond. QTAIM analysis furnished direct proof for the existence of a shared and dative B-chalcogen and Co-chalcogen interactions, respectively. In parallel to the formation of the hydridoborate complexes, the reactions also yielded tetracyclic species, [Cp*Co{κ3 -E,H,H-E(BH2 )2 -C5 Me5 H3 }], 3 a and 3 b (3 a: E=Se and 3 b: E=S), wherein the bridgehead boron atoms are surrounded by one chalcogen, one cobalt and two carbon atoms of a cyclopentane ring. Molecules 3 a and 3 b are best described as the structural mimic of tetracyclo[4.3.0.02,4 .03,5 ]nonane having identical structure and similar valence electron counts.

First Bis(σ)-borane Complexes of Group†6 Transition Metals: Experimental and Theoretical Studies.

A series of bis(σ)-borane complexes of Group 6 transition metals were prepared by direct dihydroborane coordination to the metal center. Reaction of [M(CO)3 (PCy3 )2 ] and two dihydroboranes [DurBH2 ] and [(Me3 Si)2 NBH2 ] (Dur=2,3,5,6-Me4 C6 H) yielded bis(σ)-borane complexes fac-[M(CO)3 (PCy3 ){η2 -(H2 BR)}] (R=Dur; 1: M=Cr, 2: M=W; R=N(SiMe3 )2 ; 3: M=Cr, 4: M=W). In the case of molybdenum, we have isolated an arene complex (5) with [DurBH2 ] in which the Dur group acts as a η6 -bound ligand, and with [(Me3 Si)2 NBH2 ] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO)2 (PCy3 )2 {η2 -(H2 BN(SiMe3 )2 }] (6), with a different pattern of auxiliary ligands. The complexes were investigated by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and computational methods. Quantum theory of atoms in molecules (QTAIM) calculations demonstrated that the borane complexes may be described as pure bis(σ)-borane complexes rather than elongated or stretched examples given that the calculations do not show the presence of a ring-critical point (RCP) at the ring formed by the interactions of the B-H with metal center.

Introducing N-Heterocyclic Borylenes: Theoretical Prediction of Stable, Neutral, Monomeric Boron(I) Carbenoids.

Quantum-chemical calculations predict that synthetically accessible cyclic four-membered, four-π-electron ylides could be used as building blocks for the realization of hitherto unknown N-heterocyclic boron(I) carbenoids. The boron(I) carbenoids proposed in this work possess the largest computed singlet-triplet separations known to date, which are comparable to those of the corresponding aluminum(I) analogue computed at the same level of theory. Furthermore, they owe their stability not only to the substantial transfer of electron density from nitrogen to boron atoms but also to the presence of thermodynamically robust ylidic bonds. On the basis of their computed proton affinity and carbonyl stretching frequencies, they may be considered as promising ligands for transition-metal complexes.

Hydroboration of Alkynes: η4-Alkene-Borane versus η4-E-Boratabutadiene.

A series of hydroborated η4-σ,π-alkene-borane complexes have been synthesized from the reaction of ruthenium-bis(σ)borate complex [Cp*Ru(µ-H)2BH(S-CH═S)] (1) and terminal as well as internal alkynes. Likewise, the reactions of manganese-bis(σ)borate complexes [Mn(CO)3(µ-H)2BHNCSC6H4(NL)] (L = NCSC6H4 or H) were explored with terminal alkynes that yielded boratabutadiene complexes [Mn(CO)3{(NCSC6H4)2N}{(R1MeC)═B(HC═CHR1)}] [R1 = phenyl (4a) or p-tolyl (4b)] via triple hydroboration of alkynes. These complexes feature a boratabutadiene ligand that is coordinated to a metal through the η4-CBCC mode. To the best of our knowledge, these are the first examples of η4-E-boratabutadiene-coordinated manganese complexes generated by the trans-hydroboration of alkynes. The steric and electronic effects of the borate ligands have been demonstrated using a less sterically hindered manganese-bis(σ)borate complex, [Mn(CO)3(µ-H)2BH(HN2CSC6H4)], that generated monohydroborated complexes [(CO)3Mn(µ-H)2(HN2CSC6H4)B(R1C═CHR2)] (for 6, R1 = Ph and R2 = H; for 7, R1 = p-Tol and R2 = H; for 8, R1 = R2 = Ph). Theoretical studies using density functional theory methods and chemical bonding analyses established the bonding and stability of these species.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group†VI Terminal Borylene Complexes.

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.

Strongly Phosphorescent Transition Metal π-Complexes of Boron-Boron Triple Bonds.

Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case CuI. Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metalation of the B-B triple bonds causes lengthening of the B-B and B-CNHC bonds, as well as large upfield shifts of the 11B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne/alkali metal cation complexes. In contrast to previously reported fluorescent copper(I) π-complexes of boron-boron double bonds, the Cun-π-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. While the Cu diborene bond is dominated by electrostatic interactions, giving rise to S1 and T1 states of pure IL(π-π*) nature, DFT studies show that the CuI π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S1 and T1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S1 → Tn and phosphorescence T1 → S0.

Electronic and Ligand Properties of Skeletally Substituted Cyclic (Alkyl)(Amino)Carbenes (CAACs) and Their Reactivity towards Small Molecule Activation: A Theoretical Study.

Quantum chemical calculations have been carried out to understand the ligand properties of skeletally modified cyclic alkyl amino carbenes. The stability of these carbenes has been assessed from an evaluation of their singlet-triplet and stabilization energy values. Ylide substituted carbenes are found to be more stable than non-ylidic ones in their optimized singlet state. Nucleophilicity and electrophilicity indices values were evaluated in order to further investigate the reactivity of these carbenes. The calculated values of proton affinities and the degree of gallium pyramidalization in the carbene-GaCl3 adducts correlate well with the σ-basicity of these carbenes. The reactivity of non-ylidic carbenes toward the activation of both H2 and NH3 are calculated to be more favourable compared to that of parent CAAC. On the other hand, ylide anchored carbenes are found to be unsuccessful toward the activation of both H2 and NH3 .

Synthesis and Trapping of Iminoboranes by M=B/C=N Bond Metathesis.

Although the metathesis of metal-boron double bonds with elemental chalcogenides is an established process, no similar reactivity has been observed with element-nitrogen bonds. Such a reaction would provide a new route to iminoborane compounds (RB≡NR'), which have recently experienced renewed synthetic interest. Herein, we present the first observation of M=B/C=N metathesis reactions, which led to the isolation of a stable iminoborane in addition to further iminoborane cycloaddition products.

Theoretical strategies toward stabilization of singlet remote N-heterocyclic carbenes.

Theoretical investigations predict that the singlet states of ylide-substituted remote carbenes are significantly stable and comparable to those of experimentally known NHCs. They are also found to be strongly σ-donating in nature as evident from an evaluation of the carbonyl stretching frequencies (νCO ) of their complexes with the [Rh(CO)2 Cl] fragment. NICS and QTAIM based bond magnetizability calculations indicate the presence of cyclic electron delocalization in majority of the molecules. © 2016 Wiley Periodicals, Inc.

Moving toward Ylide-Stabilized Carbenes.

The effect of ylide substitution at the α position to the carbene carbon (Cc ) atom on the stability and σ-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet-triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the σ-symmetric lone-pair orbital at the Cc atom increases as a result of the introduction of ylide centers near to the Cc atom. This indicates an enhanced σ-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the σ basicity of the carbenes.

New Routes to a Series of σ-Borane/Borate Complexes of Molybdenum and Ruthenium.

A series of agostic σ-borane/borate complexes have been synthesized and structurally characterized from simple borane adducts. A room-temperature reaction of [Cp*Mo(CO)3 Me], 1 with Li[BH3 (EPh)] (Cp*=pentamethylcyclopentadienyl, E=S, Se, Te) yielded hydroborate complexes [Cp*Mo(CO)2 (µ-H)BH2 EPh] in good yields. With 2-mercapto-benzothiazole, an N,S-carbene-anchored σ-borate complex [Cp*Mo(CO)2 BH3 (1-benzothiazol-2-ylidene)] (5) was isolated. Further, a transmetalation of the B-agostic ruthenium complex [Cp*Ru(µ-H)BHL2 ] (6, L=C7 H4 NS2 ) with [Mn2 (CO)10 ] affords a new B-agostic complex, [Mn(CO)3 (µ-H)BHL2 ] (7) with the same structural motif in which the central metal is replaced by an isolobal and isoelectronic [Mn(CO)3 ] unit. Natural-bond-orbital analyses of 5-7 indicate significant delocalization of the electron density from the filled σBH orbital to the vacant metal orbital.

Spectroscopic Distinction of Different Carbon Bases: An Insight from Theory.

Spectroscopic differentiation based on the (13)C NMR chemical shift of the parent and protonated derivatives of carbon(II) and carbon(0) bases has been proposed. The (13)C chemical shift of the central carbon atom of carbenes in their parent and protonated forms will experience more downfield shift, whereas the central carbon atom of carbones will experience a lesser downfield shift; such shifts for compounds that possess "hidden" carbon(0) characteristics will lie between these two extremes. The (13)C chemical shifts of the protonated derivatives are solely dependent on the out-of-plane pπ occupancies of the central carbon atom. This difference arises due to their unique difference in bonding and may provide an easier distinction between these two classes of compounds.