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An adj-dicarbacorrole with CCNN in the core is achieved by replacing the bipyrrole moiety by a simple polycyclic aromatic hydrocarbon, such as biphenyl unit. Spectroscopic studies and structural analyzes confirm the absence of macrocyclic aromatization, thus leading to overall nonaromatic character. The trianionic core is effectively utilized to stabilize a copper(III) ion to form an organocopper complex. [ABSTRACT FROM AUTHOR]

The amount of aromatic stabilizationof C60fullerene(Earoma) and the amount of its destabilizingstrain effect (Estrain) are unknown quantitiesbecause both are intimately connected and difficult to separate. Basedon experimentally known transformation of C60H30to C60and conversion of a polycyclic aromatic hydrocarbonC60H20to the nonaromatic linear conjugatedC60H62, new homodesmotic reaction schemes havebeen proposed to evaluate Earomaand Estrain. The Earomavalues obtained at M06L/6-311G(d,p), M062X/6-311G(d,p), and B3LYP-D3/6-311G(d,p)levels of density functional theory are 122.3, 169.8, and 152.4 kcal/mol,respectively, whereas Estrainvalues atthese levels are 327.3, 382.0, and 381.4 kcal/mol, respectively. Thesedata suggest that a CC bond of C60is destabilized by ∼2.28–2.54kcal/mol compared to that of benzene, and this minor energetic effectexplains the existence of C60as a stable molecule. [ABSTRACT FROM AUTHOR]

The CO2 insertion reactions of [L4Ru(η²-CH2C6H4)] (1), [L4Ru(η²-OC6H3Me)] (2), and [L4Ru(η²-NHC6H4)] (3), where L = PH3 and PMe3, are modeled using density functional theory methods. In 1 and 2, the metal-assisted CO2 insertion occurs because of the favorable initial axial phosphine dissociation mechanism, whereas in 3, the ligand (NHC6H4)-assisted mechanism operates (δG = +19.0 kcal/mol), wherein the nucleophilic affinity of the -NHC6H4 moiety aids the CO2 insertion process. The modeled mechanisms are consistent with the experimental findings by Hartwig et al. (J. Am. Chem. Soc, 1991, 113, 6499), in which the rate of the reactions of 1 and 2 depends on the added phosphine concentration, whereas the rate of the reaction of 3 is independent of the added phosphine concentration. In 1 and 2, CO2 is preferably inserted into the Ru-Caryl bond rather than the competitive Ru-CH2 and Ru-O bonds, respectively. In 1, the π-type orbital interaction of the aryl ring with the metal center is found to stabilize the transition state for Ru-Caryl bond insertion (δG = +25.7 kcal/mol). In 2, the Ru-Caryl insertion (δG = +23.0 kcal/mol) is thermodynamically preferred, while the kinetically preferred Ru-O bond insertion (δG = +17.4 kcal/mol) is highly reversible. The more electron-donating and sterically bulky PMe3 facilitates the CO2 insertion of 1 and 2 because the initial dissociation of axial PMe3 is easier than that of PH3 by ca. +11.0 kcal/mol, whereas in the case of 3, the effect of PMe3 slightly increases the δG value of 3. The increase in the nucleophilic affinity of amido nitrogen in 3 and the increase in the polarity of the solvent decrease the δG value of 3 by 48%. The inclusion of the chelating dimethylphosphinoethane ligand in 3 along with the electron-donating substituent at the -NHC6H4 moiety and the polar solvent further reduces the δG value of 3 by 62%, which demonstrates the role of the chelating ligand, electron-donating substituent, and polar solvent in the ligand-assisted CO2 insertion reactions. [ABSTRACT FROM AUTHOR]

A new family of nickel(II) complexes of the type [Ni(L)(CH3CN)](BPh4)2, where L= N-methyl- N, N′, N′-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl- N, N′, N′-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl- N, N′-bis(pyrid-2-ylmethyl)- N′-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl- N, N′-bis(pyrid-2-ylmethyl)- N′-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl- N, N′-bis(pyrid-2-ylmethyl)- N′-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L3)(CH3CN)](BPh4)2 reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the NiII/NiIII redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH3CN)](BPh4)2 by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH3CN)](BPh4)2 or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH3CN)](BPh4)2 and the quinolylmethyl arms ([Ni(L4)(CH3CN)](BPh4)2, both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)NiIIO.] intermediate (ts1hs, ts2doublet), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [NiII(L5)(OOCOC6H4Cl)]+ ( 5 a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the NiII leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes. [ABSTRACT FROM AUTHOR]

Hydrogen, halogen, and dihydrogenbonds in weak, medium and strongregimes (<1 to ∼60 kcal/mol) have been investigated forseveral intermolecular donor–acceptor (D-A) complexes at ab initio MP4//MP2 methodcoupled with atoms-in-molecules and molecular electrostatic potential(MESP) approaches. Electron density ρ at bond critical pointcorrelates well with interaction energy (Enb) for each homogeneous sample of complexes, but its applicabilityto the entire set of complexes is not satisfactory. Analysis of MESPminimum (Vmin) and MESP at the nuclei(Vn) shows that in all D-A complexes,MESP of A becomes more negative and that of D becomes less negativesuggesting donation of electrons from D to A leading to electron donor–acceptor(eDA) interaction between A and D. MESP based parameter ΔΔVnmeasures donor–acceptor strength ofthe eDA interactions as it shows a good linear correlation with Enbfor all D-A complexes (R2= 0.976) except the strongly bound bridged structures.The bridged structures are classified as donor–acceptor–donorcomplexes. MESP provides a clear evidence for hydrogen, halogen, anddihydrogen bond formation and defines them as eDA interactions inwhich hydrogen acts as electron acceptor in hydrogen and dihydrogenbonds while halogen acts as electron acceptor in halogen bonds. [ABSTRACT FROM AUTHOR]

Designand development of cation receptors represent a fascinatingarea of research, particularly in dealing with chemical and biologicalapplications that require fine-tuning of cation−π interactions.The electronic nature of a substituent is largely responsible fortuning the strength of cation−π interaction, and recentstudies have shown that substituent resonance effect contributes significantlyto such interactions. Using substituent resonance effect as a keyelectronic factor, we have proposed new cation−π receptors(1···M+−4···M+; M+= Li+, Na+, K+, NH4+, and NMe4+). B3LYP/6-311+G(d,p) density functional theory (DFT) calculationsshow that by using a strategy of resonance donation from six nitrogenatoms via three substituted imidazole subunits, more than 4-fold increasein cation−π interaction energy (EM+) can be achieved for a single phenyl ring comparedto benzene. The EM+(M+= NH4+, NMe4+) of 4···M+, wherein M+interacts with only one phenyl ring, is significantly higherthan EM+of a known cationhost with several aromatic rings (abstract figure). Our hypothesison resonance enhancement of cation−π interaction is verifiedusing several π-systems (5–10) containing a lone pair bearing six nitrogens and observed thata nitrogen lone pair attached to a double bond is more effective fordonation than the lone pair that is directly attached to the benzenoidring. Further, a convenient strategy to design electron rich π-systemsis provided on the basis of topographical analysis of molecular electrostaticpotential. [ABSTRACT FROM AUTHOR]

A benchmark study on all possible density functional theory (DFT) methods in Gaussian09 is done to locate functionals that agree well with CCSD/aug-cc-pVTZ geometry and Ave-CCSD(T)/(Q-T) interaction energy ( Eint) for small non-covalently interacting molecular dimers in 'dispersion-dominated' (class 1), 'dipole-induced dipole' (class 2), and 'dipole-dipole' (class 3) classes. A DFT method is recommended acceptable if the geometry showed close agreement to CCSD result (RMSD < 0.045) and Eint was within 80-120% accuracy. Among 382 tested functionals, 1-46% gave good geometry, 13-44% gave good Eint, while 1-33% satisfied geometry and energy criteria. Further screening to locate the best performing functionals for all the three classes was made by counting the acceptable values of energy and geometry given by each functionals. The meta-generalized gradient approximation (GGA) functional M06L was the best performer with total 14 hits; seven acceptable energies and seven acceptable geometries. This was the only functional 'recommended' for at least two dimers in each class. The functionals M05, B2PLYPD, B971, mPW2PLYPD, PBEB95, and CAM-B3LYP gave 11 hits while PBEhB95, PW91B95, Wb97x, BRxVP86, BRxP86, HSE2PBE, HSEh1PBE, PBE1PBE, PBEh1PBE, and PW91TPSS gave 10 hits. Among these, M05, B971, mPW2PLYPD, Wb97x, and PW91TPSS were among the 'recommended' list of at least one dimer from each class. Long-range correction (LC) of Hirao and coworkers to exchange-correlation functionals showed massive improvement in geometry and Eint. The best performing LC-functionals were LC-G96KCIS and LC-PKZBPKZB. Our results predict that M06L is the most trustworthy DFT method in Gaussian09 to study small non-covalently interacting systems. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Mechanism of autotandem catalyticreactions involving EtNH2and EtOH promoted by the complex(PNP)Ru(CO)H (1) has been investigated at the TPSS levelof DFT (PNP = C5H3N(CH2PtBu2)(CHPtBu2), C5H3N(CH2PisoPr2)(CHPisoPr2)).The reaction is identified by three mechanically distinct catalyticcycles. In cycle I, the EtOH adduct of 1undergoes O–Hproton transfer to the PNP ligand to yield the aromatic complex (PN′P)Ru(CO)(OEt)H(3). Subsequent dearomatization of PN′P leadsto the formation of dihydrogen and the complex (PNP)Ru(CO)(OEt) (5). However, an outer-sphere mechanism involving the formationof a dihydrogen intermediate complex between 1and EtOHis more facile for dihydrogen elimination (ΔG⧧= 18.4 kcal/mol) than the ligand aromatization–dearomatizationpathway (ΔG⧧=23.9 kcal/mol). In 5, the migration of a β-hydrogenfrom OEt to the unsaturated P arm of PNP forces removal of MeCHO andformation of the aromatic complex (PN′P)Ru(CO). Regenerationof the catalyst occurs by dearomatization of the PN′P ligandthrough proton migration from the P arm to the metal. In cycle II,aromatization of the amine adduct (PNP)Ru(CO)(EtNH2)H leadsto (PN′P)Ru(CO)(EtNH)H, and subsequent reaction with MeCHO(formed in cycle I) yields the hemiaminal EtNHCH(CH3)OH.The direct reaction between aldehyde and amine can also yield thehemiaminal. In cycle III, the hemiaminal adduct of the catalyst undergoesaromatization at the PNP ligand via N–H proton migration, whichsimultaneously activates C–OH bond to produce the imine (EtNCHMe)and (PN′P)Ru(CO)(OH)H (17). Removal of water from 17leads to dearomatization of PN′P and regenerationof the catalyst. Aromatization steps are characterized by proton migrationfrom the coordinated ligands, viz. the O–H bond of the alcohol,the C–H bond of the alkoxide, the N–H bond of the amine,and the N–H bond of the hemiaminal, to the unsaturated P armof PNP, and the respective ΔG⧧values are 2.4, 15.7, 18.6, and 13.8 kcal/mol. The efficiency ofthis autotandem catalytic reaction is mainly attributed to metal–ligandcooperativity operating through several facile “aromatization–dearomatization”steps. [ABSTRACT FROM AUTHOR]

Trans and eis influences of various X ligands in two isomeric structures of acyclic hypervalent compound Ph[Xl(OH)] and heterocyclic ¿3-iodane Ph[(heterocycle)l(OH)] have been investigated on the basis of I--OH bond distance (d), electron density at I--OH bond critical point (ρ), I--OH stretching frequency (v), and molecular electrostatic potential minimum (Vmin) at the OH lone pair, d, ρ, v, and Vmin are found to be good parameters for quantifying trans and eis influences, and among them, Vmin is the most sensitive parameter. Heterocyclic ƛ³-iodanes showed a smaller trans influence than acyclic ƛ3-iodanes. All systems showed higher trans influence than eis influence while relative order of both is in accordance with the inductive nature of the ligands. Among the heterocyclic ƛ³-iodanes, strong trans/cis influence is observed with N or B in the ring while P gave moderate and S gave weak trans/cis influence. Among the substituents on the cis-positioned phenyl ring in heterocyclic ƛ³-iodanes, electron withdrawing ortho substitution significantly strengthened the hypervalent I--OH bond. The stability of a product resulting from the nucleophilic attack of Cl- on a ƛ³-iodane is directly correlated with the trans/cis influence of the ligands. This relationship is helpful to make good prediction on the interaction energy of a nucleophile in trivalent hypervalent iodine complex and hence useful in designing stable acyclic and heterocyclic hypervalent complexes. [ABSTRACT FROM AUTHOR]

The formation of metallacyclobutadienes (MCBs) from chloro-ligatedalkylidyne complexes of group 4, 5, and 6 transition metals (MCln(C3H3)) has been studiedat the BP86/def2-TZVPP level. All the MCBs showed M–Cβdistances (∼2.1 Å) very close to M–Cαdistances (1.8–2.0 Å), suggesting a bonding interactionbetween the metal and the β-carbon (1,3-MC bond). Energy decompositionanalysis using C2vsymmetricstructures revealed that a b2orbital composed of mainlymetal dπand Cβpπoverlap and an agostic a1orbital contributed to theorbital interaction of the 1,3-MC bond. The bond order of the 1,3-MCbond is a minimum of 0.26 for M = Cr and a maximum of 0.43 for M =Ta. Further, all the MCBs showed a characteristic δ orbitalinteraction through an a2orbital, which contributed tothe double-bond character of M–Cαbonds (bondorder 1.27–1.44). Although the formation of b2anda2orbitals increased the M–C interactions, theysignificantly reduced the π interactions within the C3H3fragment (C–C bond order 1.09–1.18).1,3-MC bonding suggested a planar tetracoordinate configuration forCβ, as the Cα–Cβbonds possessed largely formal Csp2–Csp2single-bond character. Electron density analysis showed a “catastrophic”character of the 1,3-MC bond. In groups 4 and 5, MCBs were more stablethan the isomeric η3-structures (metallatetrahedranes).A mechanistic study on the reaction between acetylene and alkylidynecomplex MClnCH showed that a nearly barrierlessand exothermic pathway exists for MCB formation (exothermic value75–102 kcal/mol for groups 4 and 5; 6–27 kcal/mol forgroup 6). The rich metathesis chemistry associated with Mo and W isattributed mainly to the moderate activation energy required for thealkyne disproportionation step of metathesis. A mechanistic possibilityother than Katz''s is also proposed for alkyne metathesis that showedthat the 1,3-MC bonded MCB complex can act as a metathesis catalystby reacting with alkyne to form a bicyclic intermediate and subsequentlydisproportionating to yield the alkyne and the MCB. For this mechanismto be effective, rearrangement of the bicyclic intermediate to a morestable cyclopentadienyl complex has to be prevented. [ABSTRACT FROM AUTHOR]

Substituent effects on cation−π interactionshave been quantified using a variety of Φ–X···M犉ꦕ where Φ, X, and M檷 the π-system,substituent, and cation, respectively. The cation−π interactionenergy, EM, showed a stronglinear correlation with the molecular electrostatic potential (MESP)based measure of the substituent effect, ΔVmin(the difference between the MESP minimum (Vmin) on the π-region of a substitutedsystem and the corresponding unsubstituted system). This linear relationshipis EM= CM(ΔVmin) EM′ where CM the reaction constant and EM′ is the cation−πinteraction energy of the unsubstituted complex. This relationshipis similar to the Hammett equation and its first term yields the substituentcontribution of the cation−π interaction energy. Further,a linear correlation between CM橷 EM′ has been established, which facilitates the prediction of CM纊 unknown cations. Thus, a predictionof EM纊 any Φ–X···M犉ꦕ is achieved by knowing the values of EM′ and ΔVmin. The generality of the equation is tested for a varietyof cations (Li, Na, K, Mg, BeCl, MgCl, CaCl, TiCl3, CrCl2, NiCl, Cu, ZnCl, NH4, CH3NH3, N(CH3)4, C(NH2)3), substituents(N(CH3)2, NH2, OCH3, CH3, OH, H, SCH3, SH, CCH, F, Cl, COOH, CHO, CF3, CN, NO2), and a large number of π-systems.The tested systems also include multiple substituted π-systems,viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene,indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene,[6]-cyclacene, and corannulene and found that EM纉斣 the additivity of substituent effects.Further, the substituent effects on cationic sandwich complexes ofthe type C6H6···M···C6H5X have been assessedand found that EM熩 be predictedwith 97.7% accuracy using the values of EM′ and ΔVmin. All the Φ–X···M댫⵺showed good agreement between the calculated and predicted EM붩湺, suggestingthat the ΔVminapproach to substituenteffect is accurate and useful for predicting the interactive behaviorof substituted π-systems with cations. [ABSTRACT FROM AUTHOR]

Abstract: Substituent effects in C6H5X…M+ (M+ =Li+, Na+, K+, and ) systems have been characterized using isotropic nuclear magnetic shielding constants (σ). Good linear relationship between σ and the Hammett substituent constant σ p is established which suggests that the substituent effects can be quantified by NMR experiments. Irrespective of the electronic nature of X, meta carbons, and all the hydrogens are always deshielded and this property is useful for detecting cation–π interaction. The σ-scan plots revealed that the deshielding effect of the cation on the aromatic ring is significant even at large cation–π distance (∼4.5Å). [Copyright &y& Elsevier]

The CO2 fixation ability of N-heterocyclic carbenes (NHC) has been assessed on the basis of electronic and steric properties of the N- and C-substituents, measured in terms of molecular electrostatic potential minimum, observed at the carbene lone pair region of NHC (Vmin1) as well as at the carboxylate region of the NHC--CO2 adduct (Vmin2). Both Vmin1 and Vmin2 are found to be simple and efficient descriptors of the stereoelectronic effect of NHCs. The Vmin-based analysis also proved that the stereoelectronic effect of N- and C-substituents is additive. When only C-substituents are present in NHC, its CO2 affinity solely depends on the electronic effect, whereas if the N-center bears the substituents, the steric factor plays a major role in the carboxylation/decarboxylation process. For standard substituents, maximum CO2 binding energy of 18.0 kcal/mol is observed for the most electron-donating combination of NMe2 as the C-substituent and Me as the N-substituent. Introduction of ring strain through five-membered ring fusion at the NC bond slightly increased the electron-rich character of the carbene lone pair and also enhanced the CO2 binding energy to 20.9 kcal/mol. To further improve the CO2 fixing ability of NHCs, we have proposed the use of CH2OH, CH2NHCOMe, and CH2NHPh as N-substituents, as they participate in intramolecular hydrogen bond interaction with the carboxylate. With the new strategy, considerable improvement in the CO2 binding energy (26.5 to 33.0 kcal/mol) is observed. [ABSTRACT FROM AUTHOR]

The trans influence of various X ligands in hypervalent iodine(III) complexes of the type CF3[I(X)Cl] has been quantified using the trans I—Cl bond length (dX), the electron density ρ(r) at the (3,—1) bond critical point of the trans I—Cl bond, and topological features of the molecular electrostatic potential (MESP). The MESP minimum at the Cl lone pair region (Vmin) is a sensitive measure of the trans influence. The trans influence of X ligands in hypervalent iodine(V) complexes is smaller than that in iodine(III) complexes, while the relative ordering of this influence is the same in both complexes. In CF3[I(X)Y] complexes, the mutual trans influence due to the trans disposition of the X and Y ligands is quantified using the energy EXY of the isodesmic reaction CF3[I(X)Cl] + CF3[I(Y)Cl] → CF3[I(Cl)Cl] + CF3[I(X)Y]. EXY is predicted with good accuracy using the trans-influence parameters of X and Y, measured in terms of dX, ρ(r), or Vmin. The bond dissociation energy (Ed) of X or Y in CF3[I(X)Y] is significantly influenced by the trans influence as well as the mutual trans influence. This is confirmed by deriving an empirical equation to predict Ed using one of the trans-influence parameters (dX, ρ(r), or Vmin) and the mutual trans-influence parameter EXYfor a large number of complexes. The quantified values of both the trans influence and the mutual trans-influence parameters may find use in assessing the stability of hypervalent iodine compounds as well as in the design of new stable hypervalent complexes. Knowledge about the I-X bond dissociation energies will be useful for explaining the reactivity of hypervalent iodine complexes and the mechanism of their reactions. [ABSTRACT FROM AUTHOR]

Abstract: Six different pathways for the NHC-mediated CO2 transformation reaction of epoxide to cyclic carbonate are investigated with DFT method at MPWB1K/6-311++G(3df,2p) level which confirms that the most favorable pathway is ter molecular in nature and passes through a regioselective SN2 anti-attack of NHC on the least substituted carbon of the epoxide-CO2 complex. [Copyright &y& Elsevier]