Search

Density functional theory calculations have been carried out to investigate the potential of several hitherto unknown low-valent tripodal vanadium complexes towards conversion of dinitrogen to ammonia as a function of different equatorial (P i Pr 2 and S i Pr) and bridgehead groups (B, C and Si). All the newly proposed vanadium complexes were probed towards understanding their efficiency in some of the key steps involved in the dinitrogen fixation process. They were found to be successful in preventing the release of hydrazine during the nitrogen reduction reaction. We have performed a comprehensive mechanistic study by considering all the possible pathways (distal, alternate and hybrid) to understand the efficiency of some of the proposed catalysts towards the dinitrogen reduction process. The exergonic reaction free energies obtained for some of the key steps and the presence of thermally surmountable barrier heights involved in the catalytic cycle indicate that these complexes may be considered as suitable platforms for the functionalization of dinitrogen.

Computational investigations employing Density Functional Theory (SMD(benzene)-M06-2X-D3/6-311+G*) predict that stable metal-free mono (Lewis base)-stabilized borylenes could strongly bind with transition metal (Fe and Ni) complexes. The binding results in increase in the Lewis basicity of the metal centers thus facilitating the formation of metal only Lewis pairs (MOLPs) of the form [L(CO) 4 Fe → GaCl 3 ] and [L(CO) 3 Ni → GaCl 3 ] (L = electron donating ligands). Further, the binding of different ligands with the metals as well as bonding in the MOLPs have been further investigated with the help of QTAIM and EDA-NOCV analyses.

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.

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., (© 2023 Wiley-VCH GmbH.)

Comprehensive computational investigations were carried out to understand the electronic and ligand properties of skeletally substituted β-diketiminate stabilized Al(I) and Ga(I) carbenoids as well as to probe their potential in small molecule activation. All of the proposed group 13 carbenoids possess a stable singlet ground state, and the majority of them have a significantly enhanced electron donation ability compared to the experimentally reported systems. The evaluation of the energetics associated with the splitting of various strong bonds such as H-H, N-H, C-F, and B-H by these carbenoids indicates that many of the proposed Al and Ga carbenoids may be considered as suitable candidates for small molecule activation.

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, NH 2 , SiH 2 Ph, and CH 3 ) 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 (Δ E int ) values, thereby indicating strong binding of CO and CNMe to these borylenes.

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.

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., (© 2022 Wiley-VCH GmbH.)

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.

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 (Δ G ° 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 N 2 .

Treatment of Li[BH 3 ER] (E=Se or Te, R=Ph; E=S, R=CH 2 Ph) with [Cp*CoCl] 2 led to the formation of hydridoborate complexes, [{CoCp*Ph}{Cp*Co}{μ-EPh}{μ-κ 2 -E,H-EBH 3 }], 1a and 1 b (1 a: E=Se; 1 b: E=Te) and a bis-hydridoborate species [Cp*Co{μ-κ 2 -Se,H-SeBH 3 }] 2 , 2. All the complexes, 1 a, 1 b and 2 are stabilized by β-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{κ 3 -E,H,H-E(BH 2 ) 2 -C 5 Me 5 H 3 }], 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[4.3.0.0 2,4 .0 3,5 ]nonane having identical structure and similar valence electron counts., (© 2020 Wiley-VCH GmbH.)

Quantum chemical calculations have been carried out on a series of skeletally modified cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes) to understand their ligand properties and reactivity towards the activation of a variety of small molecules. The installation of boron or silicon atoms into the ring framework of these silylenes/germylenes led to a dramatic increase in their σ-basicity while the incorporation of ylidic moieties resulted in a sharp reduction of their π-acidity although it did help in increasing the electron donation ability. The calculated values of energy barriers for the activation of H-H, N-H, C-H and Si-H bonds by many of the cyclic silylenes considered here are found to be comparable to those for experimentally evaluated systems, indicating the potential of these computationally designed molecules in small molecule activation and calling for synthetic efforts towards their isolation. Furthermore, activations employing CAAGes are found to be more demanding than those with CAASis which may be attributed to the significantly lower Lewis basicity of the former than the latter.

Base-stabilised borylenes that mimic the ability of transition metals to bind and activate inert substrates have attracted significant attention in recent years. However, such species are typically highly reactive and fleeting, and often cannot be isolated at ambient temperature. Herein, we describe a readily accessible trimethylphosphine-stabilised borylborylene which was found to possess a labile P-B bond that reversibly cleaves upon gentle heating. Exchange of the labile phosphine with other nucleophiles (CO, isocyanide, 4-dimethylaminopyridine) was investigated, and the binding strength of a range of potential borylene "ligands" has been evaluated computationally. The room-temperature-stable PMe 3 -bound borylenes were subsequently applied to novel bond activations including [2 + 2] cycloaddition with carbodiimides and the reduction of dichalcogenides, revealing that PMe 3 -stabilised borylenes can effectively behave as stable sources of the analogous fleeting dicoordinate species under mild conditions., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Density functional theory calculations are employed to explore the reactivity of metalla-N-heterocyclic carbenes (MNHCs) towards activation of a variety of small molecules (H2, NH3, PH3, SiH3Ph and CH4). All the MNHCs considered are found to have a stable singlet ground state and possess suitable electronic properties for their application in small molecule activation. The calculated energy barriers of E-H (E = H, C, N, Si, P) activation for the MNHCs are found to be in agreement with those of the experimentally evaluated cyclic alkyl(amino)carbene (CAAC) and diamidocarbenes (DACs), thereby indicating the activating effect of the incorporation of an ancillary metal center within a cyclic NHC, and highlighting a new, underexplored strategy in achieving difficult bond activations with carbenes.

Theoretical studies predict that the stability of the singlet state of metalla-N-heterocyclic carbenes (MNHCs) is strongly influenced not only by the substituents at the α-nitrogen atoms and the nature of ligands at the transition metal center but also by the substituents at the carbenic backbone. All the MNHCs were found to have a stable singlet ground state and the computed ΔE S-T values for some of the MNHCs were found to be significantly large and lie within the range of experimentally known carbenes (31.0-84.0 kcal mol -1 ). Furthermore, they were found to have superior σ-donation ability to conventional NHCs. The calculated proton affinities, gallium pyramidalization, pK a and nucleophilicity index values for the MNHCs were found to be in good agreement with their calculated electron donation ability.

Cyclic alkyl amino carbene (CAAC) anchored [FeFe]-hydrogenase model complex featuring rotated conformation at one of the iron centers are found to be promising candidate for effective production of dihydrogen. A stepwise comparison of the complete mechanism using the CAAC stabilized model complex [1] 0 has been performed with that of an experimentally isolated one ([2] 0 ). Interestingly, the reduction events involved in the catalytic cycles are found to be more favorable than those previously reported for a similar experimentally known system. Furthermore, the computed ΔpK a values indicate that the distal iron center with a vacant coordination site is more basic compared to the amino nitrogen atom of the azadithiolate bridge. We also made an attempt to determine the oxidation states of the iron centers for the intermediates involved in the catalytic cycles on the basis of the computed Mössbauer isomer shift and Mulliken spin density values.

A series of hydroborated η 4 -σ,π-alkene-borane complexes have been synthesized from the reaction of ruthenium-bis(σ)borate complex [Cp*Ru(μ-H) 2 BH(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) 2 BHNCSC 6 H 4 (NL)] (L = NCSC 6 H 4 or H) were explored with terminal alkynes that yielded boratabutadiene complexes [Mn(CO) 3 {(NCSC 6 H 4 ) 2 N}{(R 1 MeC)═B(HC═CHR 1 )}] [R 1 = 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 η 4 -CBCC mode. To the best of our knowledge, these are the first examples of η 4 - 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) 2 BH(HN 2 CSC 6 H 4 )], that generated monohydroborated complexes [(CO) 3 Mn(μ-H) 2 (HN 2 CSC 6 H 4 )B(R 1 C═CHR 2 )] (for 6 , R 1 = Ph and R 2 = H; for 7 , R 1 = p -Tol and R 2 = H; for 8 , R 1 = R 2 = Ph). Theoretical studies using density functional theory methods and chemical bonding analyses established the bonding and stability of these species.

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 (PCy 3 ) 2 ] and two dihydroboranes [DurBH 2 ] and [(Me 3 Si) 2 NBH 2 ] (Dur=2,3,5,6-Me 4 C 6 H) yielded bis(σ)-borane complexes fac-[M(CO) 3 (PCy 3 ){η 2 -(H 2 BR)}] (R=Dur; 1: M=Cr, 2: M=W; R=N(SiMe 3 ) 2 ; 3: M=Cr, 4: M=W). In the case of molybdenum, we have isolated an arene complex (5) with [DurBH 2 ] in which the Dur group acts as a η 6 -bound ligand, and with [(Me 3 Si) 2 NBH 2 ] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO) 2 (PCy 3 ) 2 {η 2 -(H 2 BN(SiMe 3 ) 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.

Pre-pyramidalization of the boron centre in the Lewis acidic groups is a prerequisite for the formation of unsupported donor-acceptor complexes with Lewis basic metal fragments such as [Pt(NHC)2].

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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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-GaCl 3 adducts correlate well with the σ-basicity of these carbenes. The reactivity of non-ylidic carbenes toward the activation of both H 2 and NH 3 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 H 2 and NH 3 ., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case Cu I . 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-C NHC bonds, as well as large upfield shifts of the 11 B 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 Cu n -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 S 1 and T 1 states of pure IL(π-π*) nature, DFT studies show that the Cu I π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S 1 and T 1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S 1 → T n and phosphorescence T 1 → S 0 .

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., (© 2016 Wiley Periodicals, Inc.)

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Theoretical calculations were carried out to understand the effect of annulation on the electronic and ligand properties of boron substituted N-heterocyclic carbenes (B-NHCs). Annulation results in a decrease in stability as indicated by the calculated values of singlet-triplet separations and stabilization energies as well as HOMO-LUMO gaps. Annulated B-NHCs are found to be weaker σ-donors but better π-acceptors than the parent ones. The decrease in σ-donation ability and the increase in π-accepting ability are further supported by the calculated values of proton affinities, nucleophilicity and electrophilicity indices as well as (31)P NMR chemical shifts of the corresponding NHC-PPh adducts. Most of the annulated B-NHCs are found to have significantly enhanced electrophilicity than the other known carbenes.

Cation-π interactions are one of the most important classes of noncovalent bonding, and are seen throughout biology, chemistry, and materials science. However, in almost every documented case, these interactions play only a supporting role to much stronger covalent or dative bonds, thus making examples of exclusive cation-π bonding exceedingly rare. In this study, a neutral diboryne molecule is found to encapsulate the light alkali metal cations Li(+) and Na(+) in the absence of a net charge, covalent bonds, or lone-pair donor groups. The resulting encapsulation complexes are, to our knowledge, the first structurally authenticated species in which a neutral molecule binds the light alkali metals exclusively through cation-π interactions., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker σ donors but better π acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the σ basicities of all of the rNHCs. The (31)P NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the π acidities of these rNHCs.

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of phenylmercuric chloride with an anionic dimanganese borylene (Li(+)[Cp2(CO)4Mn2B](-)) led to the formation of a trimetalloboride featuring the first reported bond between mercury and a non-cluster boron atom. Examination by (199)Hg NMR indicated (11)B-(199)Hg scalar coupling. Theoretical calculations indicated the nature of bonding to be σ-donation from a B-Mn π-orbital to Hg, in conjunction with weak Hgd→π* back-donation.

Quantum chemical calculations have been carried out to investigate the effect of annelation and carbonylation on the electronic and ligand properties of N-heterocyclic silylenes and germylenes. The thermodynamic stability of these ligands has been found to increase with annelation, while the reverse is true for carbonylation. This is in sharp contrast to N-heterocyclic carbenes (NHCs) where annelation leads to a decrease in their thermodynamic stabilities. Compared to nonannelated derivatives, annelated and carbonylated ones are found to be weaker σ donors but better π acceptors. The effect of carbonylation is more pronounced than annelation toward increasing the π acidity of these ligands. Carbonylation at the α-position with respect to the N atom attached to the Si/Ge center has been found to be the most effective way of enhancing the π acidity of these ligands. The computed natural charges reveal that electrophilicity increases upon both annelation and carbonylation. The calculated values of (31)P NMR chemical shifts of corresponding phosphinidene adducts of these ligands have been found to correlate well with the π acidity of these Si/Ge centers.

New metal-only Lewis pairs (MOLPs: Ru→Cr and Os→Cr) are prepared by the insertion of a zerovalent ruthenium or osmium complex into chromium-boron double bonds of borylene complexes. The reaction creates new borylene complexes (the first ever for osmium), and is crystallization-controlled; re-dissolving the complexes results in regeneration of the starting materials. A mechanism is proposed based on DFT calculations, along with a computational study of the unusual MOLPs., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of [Pt(PCy3)2] with Br2B-CH(SiMe3)2 resulted in generation of the first alkylideneboryl complex, trans-[Br(Cy3P)2Pt{B=CH(SiMe3)}], with concomitant elimination of Me3 SiBr. The trans bromide ligand of the alkylideneboryl complex was readily substituted by a methyl group upon treatment with methyllithium, leading to another alkylideneboryl complex, trans-[Me(Cy3P)2Pt{B=CH(SiMe3)}]. Various spectrochemical techniques, single-crystal X-ray crystallography, and quantum chemical calculations confirmed the formulation of a double bond between the boron and the carbon atom. The theoretical studies also provided evidence for the stronger trans influence of the alkylideneboryl ligand over iminoboryl and oxoboryl ligands., (Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO was added to a boron-boron triple bond at room temperature and atmospheric pressure, resulting in a compound into which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized. Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.

The effect of annelation and carbonylation on the electronic and ligating properties of N-heterocyclic carbenes (NHCs) has been studied quantum chemically. The thermodynamic and kinetic stability of these NHCs have been assessed on the basis of their singlet-triplet and HOMO-LUMO gaps respectively. Both annelation and carbonylation have been found to decrease the stability of NHCs. Compared to nonannelated carbenes, annelated and carbonylated carbenes are found to be weaker σ donors but better π acceptors. However, the effect of carbonylation is more pronounced than annelation toward increasing the π acidity of the NHCs. The reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity indices. The calculated values of the relative redox potential and (31)P NMR chemical shifts of corresponding carbene-phosphinidene adducts have been found to correlate well with the π acidity of the NHCs.

Recently, the chemistry of element (0) compounds has attracted the attention of both experimental and theoretical chemists. In this article, some new Si(0) and Ge(0) compounds stabilized by different silylene and germylene ligands are studied theoretically by applying quantum chemical calculations and topography mapping of molecular electrostatic potential (MESP). These compounds are found to have high donor-acceptor bond strengths and are thermodynamically stable. The nature of the molecular orbitals, negative values of MESP at its critical points, as well as proton affinity values suggest that they are very good nucleophiles. Calculated proton affinity values suggest the possible isolation of their diprotonated salts.

The chemistry of nitrogen fixation has been the subject of considerable research with a view to gaining a proper understanding of the mechanistic details. In this article, density functional calculations are performed on all the mechanistic possibilities for dinitrogen reduction mediated by the tripodal iron complexes [(SiP(Me)3)Fe(I)] ([Fe(Si)]) and [(BP(Me)3)Fe(0)] ([Fe(B)]). Dinitrogen addition to the neutral bare complex is found to be thermodynamically more favorable than that to the anionic one. Both symmetric and asymmetric pathways, along with the possible intermediates and transition states, are considered in this study. For both systems, the symmetric path is found to be more likely, although the prospect of the asymmetric path cannot be ignored. Moreover, interconversions between these two pathways are found to be less likely. This study corroborates most experimental observations and provides theoretical insight into the possible existence of some hitherto unknown intermediates such as multiple-bonded Fe-N species in a trigonal bipyramidal geometry. Furthermore, in agreement with experimental observations, this study also highlights the possibility of hydrogen and hydrazine evolution during the complete reduction of dinitrogen., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Recently, donor stabilized divalent carbon(0) compounds have undergone intense experimental and theoretical investigation due to their strong electron rich character. In this Article, some new cyclic and acyclic carbon(0) compounds stabilized by differential coordination modes (such as abnormal, remote and a mixture of both) of N-heterocyclic carbenes are studied theoretically. The cyclic carbon(0) compounds proposed in this study are unusual in the sense that they contain a five membered ring consisting of only carbon atoms with a central carbon atom in the formal oxidation state of zero. All these compounds are found to be very strong nucleophiles which might have wide implications in catalysis. Calculation of first proton affinities of these molecules reveal that they are better σ donors than the carbon(0) compound supported by normal N-heterocyclic carbenes. Quantum chemical calculations indicate that these molecules possess very high donor-acceptor L → C bond strengths and are thermodynamically stable. Calculation of the bond dissociation energies for the complexation of one and two molecules of AuCl indicates the possible isolation of their gem dimetalated derivatives.

Density functional calculations predict that 2,2'-bipyridyl carbenes have some degree of "hidden" carbon(0) character. This is supported by very high second proton affinity values and bond dissociation energies (BDEs) for the dissociation of one AuCl and one Ni(CO)(2) molecule from [(AuCl)(2)(2,2'-bipyridyl carbene)] and [{Ni(CO)(2)}(2)(2,2'-bipyridyl carbene)]. An Arduengo type carbene also showed significant BDE values for gem-dimetallation, thus indicating the possible existence of carbon(0) character. All of the dimetallated derivatives showed metallophilic interactions., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Energetics and mechanistic details for the conversion of dinitrogen to ammonia mediated by vanadium triamidoamine has been studied theoretically involving various mechanistic possibilities. For most of the cases, protonation at the amido nitrogen atom is more favorable compared to the terminal one. Further, the most important steps of the mechanism were compared with the well established chemistry of nitrogen fixation mediated by molybdenum. Such a comparison helps in understanding why vanadium triamidoamine complex performs poorly compared to molybdenum. The main factors responsible for the poor performance of the vanadium complex toward NH(3) production are identified as low exergonic cleavage of the N-N bond and limitation of the ligand exchange step via a dissociative mechanism at the end of the cycle to only one possible pathway. A major aspect of the failure of the vanadium complex to mediate the reduction of N(2) to ammonia is the fact that the protonation steps involve major barriers, which cannot be surmounted thermally. Moreover, unlike molybdenum, the associative mechanism with vanadium triamidoamine complex is not likely to operate during the NH(3)/N(2) exchange step.

Ab initio molecular orbital (MO) calculations at the MP2/6-31+G* level coupled with quantum theory of atoms in molecules (QTAIM) analysis were carried out on group 13 atranes (M = B, Al, Ga) with special emphasis on the nature of the transannular M···N interaction present in these molecules. Substituents at the equatorial position were found to influence the extent of transannular interaction. Boratrane molecules were found to have the strongest M···N interaction and consequently have higher stabilization energies. QTAIM analysis revealed the presence of significant covalent character in the transannular M···N bonds which decreases down the group.

Density functional calculations coupled with quantum theory of atoms in molecules analysis were carried out on Group 4 and 6 metallatranes with special emphasis on the nature of transannular M···N bonds present in these molecules. Substituents at both the apical and equatorial positions are found to influence the extent of transannular interaction. The degree of pyramidalization around the metal and the bridgehead nitrogen atom play a key role in strengthening or weakening the M···N bond. The stability of these molecules are found to depend to a large extent on the strength of M···N bonds with significant contribution coming in from metal-equatorial and metal-apical bonds. Group 6 metallatranes are found to have stronger transannular bonds and, hence, higher stabilization energies than their Group 4 counterparts. Atoms in molecules theory analysis reveals the presence of a considerable amount of covalent character in the M···N bonds which increase from Group 4 to Group 6.

The effect of substituents at the heteroatom on the electronic structures of different N-heterocyclic carbenes (1, 2 and 3), silylene (4) and germylene (5) are examined using Density Functional Theory. The kinetic and thermodynamic stabilities of these molecules are assessed by examining the HOMO-LUMO gap and hydrogenation energies, respectively. The extent of cyclic electron delocalization present in these five-membered ring systems are quantified with the help of NICS calculations. The ligating properties of 1-5 and the recently synthesized free abnormal carbene 6 (Bertrand et al., Science, 2009, 326, 556-559) are examined by looking at the energies of the sigma symmetric electron-donating orbital of the respective molecules. Among the systems considered, 6 is found to have the strongest sigma-donating ability. A comparative study of the ligating properties between the two isomeric carbenes 1 and 6 is performed by calculating the carbonyl stretching frequencies of some iridium carbonyl complexes of these two tautomeric carbenes.

A combined DFT, AIM and ELF study has been carried out on borazine and its heavier analogs containing both the pnictogens and chalcogens as the ring constituent. Compared to the pnictogen substituted rings, chalcogen substituted rings are found to be less aromatic. Except for a few systems, the computed aromatic stabilization energies (ASE) do not correlate with the NICS values. For these ring systems, NICS and bond length equalization are found to be better indicators of aromaticity than ASE. It was found that bulky electronegative substituents at the metal atom dramatically increases the stability and aromaticity of these molecules. AIM and ELF analysis predicts that boron and gallium based heterocycles are moderately aromatic while the aluminium analogs are significantly less aromatic.