Metal-induced Enhancement of Tetrel Bonding. The Case of C···X-IrIII (X = Cl, Br) Tetrel Bond Involving a Methyl Group.

In X-ray structures of the isomorphic mer-[IrX3(THT)(CNXyl)2] (X = Cl 1, Br 2; THT = tetrahydrothiophene; Xyl = 2,6-Me2C6H3-) complexes, we revealed short intermolecular contacts between the C-atom of an isocyanide methyl group and halide ligands of another molecule. Geometrical consideration of the X-ray data and analysis of appropriate DFT studies allowed the attribution of these contacts to CMe···X-IrIII (X = Cl, Br) tetrel bond. Specifically, through the application of DFT calculations and various theoretical models, the presence of tetrel bonding interactions was validated, and the contribution of the CMe···X-IrIII interaction was assessed. The reinforcement of the tetrel bond upon the isocyanide coordination to iridium(III) is substantiated by molecular electrostatic potential (MEP) surface calculations. To distinguish the tetrel bonding characteristics of CMe···X-IrIII (X = Cl, Br) interactions from conventional hydrogen bonding, we employed multiple computational methodologies, including Natural Bond Orbital (NBO) analysis and Electron Localization Function (ELF) analysis. Additionally, Energy Decomposition Analysis (EDA) was applied to selected model systems to explore the underlying physical nature of these interactions.

Conformation-Associated C···dz2-PtII Tetrel Bonding: The Case of Cyclometallated Platinum(II) Complex with 4-Cyanopyridyl Urea Ligand.

The nucleophilic addition of 3-(4-cyanopyridin-2-yl)-1,1-dimethylurea (1) to cis-[Pt(CNXyl)2Cl2] (2) gave a new cyclometallated compound 3. It was characterized by NMR spectroscopy (1H, 13C, 195Pt) and high-resolution mass spectrometry, as well as crystallized to obtain two crystalline forms (3 and 3·2MeCN), whose structures were determined by X-ray diffraction. In the crystalline structure of 3, two conformers (3A and 3B) were identified, while the structure 3·2MeCN had only one conformer 3A. The conformers differed by orientation of the N,N-dimethylcarbamoyl moiety relative to the metallacycle plane. In both crystals 3 and 3·2MeCN, the molecules of the Pt(II) complex are associated into supramolecular dimers, either {3A}2 or {3B}2, via stacking interactions between the planes of two metal centers, which are additionally supported by hydrogen bonding. The theoretical consideration, utilizing a number of computational approaches, demonstrates that the C···dz2(Pt) interaction makes a significant contribution in the total stacking forces in the geometrically optimized dimer [3A]2 and reveals the dz2(Pt)→π*(PyCN) charge transfer (CT). The presence of such CT process allowed for marking the C···Pt contact as a new example of a rare studied phenomenon, namely, tetrel bonding, in which the metal site acts as a Lewis base (an acceptor of noncovalent interaction).

Triiodide-Based Chair-Like Copper Complex Assembled by Halogen Bonding.

Cocrystallization of the dimeric [Cu2(µ-I)2(CNXyl)4] (Xyl = 2,6-Me2C6H3, 1) and polymeric catena-[Cu(µ-I)(CNC6H3-2-Cl-6-Me)2] (2) complexes with I2 at different molar ratios between the reactants resulted in a series of (RNC)2CuI-based crystal polyiodides formed along with gradual accumulation of iodine, namely the cocrystals [1·I2]·[Cu(µ1,1-I3)(CNXyl)2]2 followed by the generation of [Cu(µ1,3-I3)(CNXyl)2]2·2I2 (5·2I2) or [Cu(µ1,1-I3)(CNC6H3-2-Cl-6-Me)2]2 and then [Cu(µ1,3-I3)(CNC6H3-2-Cl-6-Me)2]n·n/2I2. The polyiodide 5·2I2 exhibits a novel supramolecular motif─a purely inorganic halogen-bonded Cu2(µ1,3-I3)2 core in the chair conformation. The X-ray structure of 5·2I2 featuring I···I contacts was analyzed by a set of theoretical methods and attributed to moderately strong halogen bonding (from -3.2 to -3.9 kcal/mol); these interactions determine the supramolecular architecture of 5·2I2.

Halogen Bonding Involving Isomeric Isocyanide/Nitrile Groups.

2,3,5,6-Tetramethyl-1,4-diisocyanobenzene (1), 1,4-diisocyanobenzene (2), and 1,4-dicyanobenzene (3) were co-crystallized with 1,3,5-triiodotrifluorobenzene (1,3,5-FIB) to give three cocrystals, 1·1,3,5-FIB, 2·2(1,3,5-FIB), and 3·2(1,3,5-FIB), which were studied by X-ray diffraction. A common feature of the three structures is the presence of I···Cisocyanide or I···Nnitrile halogen bonds (HaBs), which occurs between an iodine σ-hole and the isocyanide C-(or the nitrile N-) atom. The diisocyanide and dinitrile cocrystals 2·2(1,3,5-FIB) and 3·2(1,3,5-FIB) are isostructural, thus providing a basis for accurate comparison of the two types of noncovalent linkages of C≡N/N≡C groups in the composition of structurally similar entities and in one crystal environment. The bonding situation was studied by a set of theoretical methods. Diisocyanides are more nucleophilic than the dinitrile and they exhibit stronger binding to 1,3,5-FIB. In all structures, the HaBs are mostly determined by the electrostatic interactions, but the dispersion and induction components also provide a noticeable contribution and make the HaBs attractive. Charge transfer has a small contribution (<5%) to the HaB and it is higher for the diisocyanide than for the dinitrile systems. At the same time, diisocyanide and dinitrile structures exhibit typical electron-donor and π-acceptor properties in relation to the HaB donor.

Halogen Bond-Involving Supramolecular Assembly Utilizing Carbon as a Nucleophilic Partner of I⋠⋠⋠C Non-covalent Interaction.

Co-crystallization of 180°-orienting σ-hole-accepting tectons, namely, 1,4-diisocyanobenzene (1) and 1,4-diisocyanotetramethylbenzene (2), with such homoditopic halogen bond donors as 1,4-diiodotetrafluorobenzene (1,4-FIB) and 4,4'-diiodoperfluorobiphenyl (4,4'-FIBP) afforded co-crystals 1 ⋅ 1,4-FIB, 1 ⋅ 4,4'-FIBP, and 2 ⋅ 1,4-FIB. Their solid-state structures exhibit 1D-supramolecular arrangements, which are based on poorly explored I⋅⋅⋅C halogen bonding; this study is the first in which the supramolecular assembly utilizing halogen bonding with a terminal C atom was performed. The use of the potentially tetrafunctional σ-hole accepting tetraiodoethylene (TIE) leads to supramolecular architecture of a higher dimension, 3D-framework, observed in the structure of 1 ⋅ TIE. DFT calculations, used to characterize the halogen bonding situation, revealed that the I⋅⋅⋅C non-covalent interactions are moderately strong, ranging from -4.07 in 1 ⋅ TIE to -5.45 kcal/mol in 2 ⋅ 1,4-FIB. The NBO analysis disclosed that LP(C)→σ* charge transfer effects are relevant in all co-crystals.

Structure-Directing Interplay between Tetrel and Halogen Bonding in Co-Crystal of Lead(II) Diethyldithiocarbamate with Tetraiodoethylene.

The co-crystallization of the lead(II) complex [Pb(S2CNEt2)2] with tetraiodoethylene (C2I4) gave the co-crystal, [Pb(S2CNEt2)2]∙½C2I4, whose X-ray structure exhibits only a small change of the crystal parameters than those in the parent [Pb(S2CNEt2)2]. The supramolecular organization of the co-crystal is largely determined by an interplay between Pb⋯S tetrel bonding (TeB) and I⋯S halogen bonding (HaB) with comparable contributions from these non-covalent contacts; the TeBs observed in the parent complex, [Pb(S2CNEt2)2], remain unchanged in the co-crystal. An analysis of the theoretical calculation data, performed for the crystal and cluster models of [Pb(S2CNEt2)2]∙½C2I4, revealed the non-covalent nature of the Pb⋯S TeB (-5.41 and -7.78 kcal/mol) and I⋯S HaB (-7.26 and -11.37 kcal/mol) interactions and indicate that in the co-crystal these non-covalent forces are similar in energy.

Polymorph-Dependent Phosphorescence of Cyclometalated Platinum(II) Complexes and Its Relation to Non-covalent Interactions.

Cyclometalated platinum(II) complexes [Pt(ppy)Cl(CNAr)] (ppy = 2-phenylpyridinato-C2,N; Ar = C6H4-2-I 1, C6H4-4-I 2, C6H3-2-F-4-I 3, and C6H3-2,4-I2 4) bearing ancillary isocyanide ligands were obtained by the bridge-splitting reaction between the dimer [Pt(ppy)(µ-Cl)]2 and 2 equiv any one of the corresponding CNAr. Complex 2 was crystallized in two polymorphic forms, namely, 2 I and 2 II, exhibiting green (emission quantum yield of 0.5%) and orange (emission quantum yield of 12%) phosphorescence, respectively. Structure-directing non-covalent contacts in these polymorphs were verified by a combination of experimental (X-ray diffraction) and theoretical methods (NCIplot analysis, combined electron localization function (ELF), and Bader quantum theory of atoms in molecules (QTAIM analysis)). A noticeable difference in the spectrum of non-covalent interactions of 2 I and 2 II is seen in the Pt···Pt interactions in 2 II and absence of these metallophilic contacts in 2 I. The other solid luminophores, namely, 1, 3 I-II, 4, and 4·CHCl3, exhibit green luminescence; their structures include intermolecular C-I···Cl-Pt halogen bonds as the structure-directing interactions. Crystals of 1, 2 I, 3 I, 3 II, 4, and 4·CHCl3 demonstrated a reversible mechanochromic color change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering.

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d-orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron-rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d-[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d-[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Copper(II)-Mediated Iodination of 1-Nitroso-2-naphthol.

The 3-Iodo-1-nitrosonaphthalene-2-ol (I-NON) was obtained by the copper(II)-mediated iodination of 1-nitroso-2-naphthol (NON). The suitable reactants and optimized reaction conditions, providing 94% NMR yield of I-NON, included the usage of Cu(OAc)2·H2O and 1:2:8 CuII/NON/I2 molar ratio between the reactants. The obtained I-NON was characterized by elemental analyses (C, H, N), high-resolution ESI+-MS, 1H and 13C{1H} NMR, FTIR, UV-vis spectroscopy, TGA, and X-ray crystallography (XRD). The copper(II) complexes bearing deprotonated I-NON were prepared as follows: cis-[Cu(I-NON-H)(I-NON)](I3) (1) was obtained by the reaction between Cu(NON-H)2 and I2 in CHCl3/MeOH, while trans-[Cu(I-NON-H)2] (2) was synthesized from I-NON and Cu(OAc)2 in MeOH. Crystals of trans-[Cu(I-NON-H)2(THF)2] (3) and trans-[Cu(I-NON-H)2(Py)2] (4) were precipitated from solutions of 2 in CHCl3/THF and Py/CHCl3/MeOH mixtures, respectively. The structures of 1 and 3-4 were additionally verified by X-ray crystallography. The characteristic feature of the structures of 1 and 3 is the presence of intermolecular halogen bonds with the involvement of the iodine center of the metal-bound deprotonated I-NON. The nature of the I···I and I···O contacts in the structures of 1 and 3, correspondingly, were studied theoretically at the DFT (PBE0-D3BJ) level using the QTAIM, ESP, ELF, NBO, and IGM methods.

Metal-Involving Chalcogen Bond. The Case of Platinum(II) Interaction with Se/Te-Based σ-Hole Donors.

Platinum(II) complexes exhibiting an expressed dz2-nucleophilicity of the positively charged metal centers, namely, [Pt(ppy)(acac)] (1; acacH is acetylacetone; ppyH is 2-Ph-pyridine) and [Pt(ppy)(tmhd)] (2; tmhdH is 2,2,6,6-tetramethylheptanedione-3,5), were cocrystallized with the chalcogen bond donors (4-NC5F4)2Ch (Ch = Se, Te) to form two isostructural cocrystals 1·1/2(4-NC5F4)2Ch, and 2·2/3(4-NC5F4)2Se and 2·(4-NC5F4)2Te. The X-ray data for these cocrystals and appropriate theoretical DFT calculations (PBE0-D3BJ) allowed the recognition of the metal-involving chalcogen bond, namely, Ch···dz2-PtII (its energy spans from -7 to -12 kcal/mol). In 1·1/2(4-NC5F4)2Ch, Ch···dz2-PtII bonding is accompanied by the C···dz2-PtII interaction, representing a three-center bifurcate, whereas in 2·(4-NC5F4)2Te the chalcogen bond Te···dz2-PtII is purely two-centered and is stronger than that in 1·1/2(4-NC5F4)2Ch because of more efficient orbital overlap. The association of 2 with (4-NC5F4)2Te and the structure of the formed adduct in CDCl3 solutions was studied by using 1H, 13C, 19F, 195Pt, 125Te NMR, 19F-1H HOESY, and diffusion NMR methods. The 195Pt and 125Te NMR titration and the isothermal titration calorimetry results revealed a 1:1 association of 2 with (4-NC5F4)2Te.

Comparative Structural Study of Three Tetrahalophthalic Anhydrides: Recognition of X···O(anhydride) Halogen Bond and πh···O(anhydride) Interaction.

Structures of three tetrahalophthalic anhydrides (TXPA: halogen = Cl (TCPA), Br (TBPA), I (TIPA)) were studied by X-ray diffraction, and several types of halogen bonds (HaB) and lone pair···π-hole (lp···πh) contacts were revealed in their structures. HaBs involving the central oxygen atom of anhydride group (further X···O(anhydride) were recognized in the structures of TCPA and TBPA. In contrast, for the O(anhydride) atom of TIPA, only interactions with the π system (π-hole) of the anhydride ring (further lp(O)···πh) were observed. Computational studies by a number of theoretical methods (molecular electrostatic potentials, the quantum theory of atoms in molecules, the independent gradient model, natural bond orbital analyses, the electron density difference, and symmetry-adapted perturbation theory) demonstrated that the X···O(anhydride) contacts in TCPA and TBPA and lp(O)···πh in TIPA are caused by the packing effect. The supramolecular architecture of isostructural TCPA and TBPA was mainly affected by X···O(acyl) and X···X HaBs, and, for TIPA, the main contribution provided I···I HaBs.

Structure-directing sulfur...metal noncovalent semicoordination bonding.

The abundance and geometric features of nonbonding contacts between metal centers and `soft' sulfur atoms bound to a non-metal substituent R were analyzed by processing data from the Cambridge Structural Database. The angular arrangement of M, S and R atoms with ∠(R-S...M) down to 150° was a common feature of the late transition metal complexes exhibiting shortened R-S...M contacts. Several model nickel(II), palladium(II), platinum(II) and gold(I) complexes were chosen for a theoretical analysis of R-S...M interactions using the DFT method applied to (equilibrium) isolated systems. A combination of the real-space approaches, such as Quantum Theory of Atoms in Molecules (QTAIM), noncovalent interaction index (NCI), electron localization function (ELF) and Interacting Quantum Atoms (IQA), and orbital (Natural Bond Orbitals, NBO) methods was used to provide insights into the nature and energetics of R-S...M interactions with respect to the metal atom identity and its coordination environment. The explored features of the R-S...M interactions support the trends observed by inspecting the CSD statistics, and indicate a predominant contribution of semicoordination bonds between nucleophilic sites of the sulfur atom and electrophilic sites of the metal. A contribution of chalcogen bonding (that is formally opposite to semicoordination) was also recognized, although it was significantly smaller in magnitude. The analysis of R-S...M interaction strengths was performed and the structure-directing role of the intramolecular R-S...M interactions in stabilizing certain conformations of metal complexes was revealed.

The (Dioximate)NiII/I2 System: Ligand Oxidation and Binding Modes of Triiodide Species.

Reinvestigation of (o-benzoquinonedioximate)2Ni/I2 systems demonstrated that the reaction itself and also the crystallization conditions dramatically affect the identity of generated species. Crystallization (CHCl3, 20-25 °C) of the nickel(II) dioximate complex [Ni(bqoxH)2] (bqoxH2 = o-benzoquinonedioxime) with I2 in the 1:(1-10) molar ratios of the reactants led to several (o-benzoquinonedioximate)2Ni derivatives and/or iodine adducts [Ni(I)(bqoxH)(bqoxH2)]·3/2I2, [Ni(I3)(bqoxH)(bqoxH2)]·[Ni(bqoxH)2], and [Ni(I3)(bqox•-)(bqoxH2)]·I2; the latter one, featuring the anion-radical bqox•- ligand, is derived from the formal (-2H+/1e-)-oxidation of bqoxH2. In these three adducts, various types of noncovalent interactions were identified experimentally and their existence was supported theoretically. The [Ni(I3)(bqox•-)(bqoxH2)]·I2 adduct exhibits simultaneous semicoordination and coordination patterns of the triiodide ligand; this is the first recognition of the semicoordination of any polyiodide ligand to a metal center. The semicoordination noncovalent contact Ni···I3 (3.7011(10) Å) is substantially longer that the Ni-I3 coordination bond (2.8476(9) Å), and the difference in energies between these two types of linkages is 8-12 kcal/mol.

Four-Center Nodes: Supramolecular Synthons Based on Cyclic Halogen Bonding.

The isocyanide trans-[PdBr2 (CNC6 H4 -4-X')2 ] (X'=Br, I) and nitrile trans-[PtX2 (NCC6 H4 -4-X')2 ] (X/X'=Cl/Cl, Cl/Br, Br/Cl, Br/Br) complexes exhibit similar structural motif in the solid state, which is determined by hitherto unreported four-center nodes formed by cyclic halogen bonding. Each node is built up by four Type II C-X'⋅⋅⋅X-M halogen-bonding contacts and include one Type I M-X⋅⋅⋅X-M interaction, thus giving the rhombic-like structure. These nodes serve as supramolecular synthons to form 2D layers or double chains of molecules linked by a halogen bond. Results of DFT calculations indicate that all contacts within the nodes are typical noncovalent interactions with the estimated strengths in the range 0.6-2.9 kcal mol-1 .

Electrophilic-Nucleophilic Dualism of Nickel(II) toward Ni···I Noncovalent Interactions: Semicoordination of Iodine Centers via Electron Belt and Halogen Bonding via σ-Hole.

The nitrosoguanidinate complex [Ni{NH═C(NMe2)NN(O)}2] (1) was cocrystallized with I2 and sym-trifluorotriiodobenzene (FIB) to give associates 1·2I2 and 1·2FIB. Structures of these solid species were studied by XRD followed by topological analysis of the electron density distribution within the framework of Bader's approach (QTAIM) at the M06/DZP-DKH level of theory and Hirshfeld surface analysis. Our results along with inspection of XRD (CCDC) data, accompanied by the theoretical calculations, allowed the identification of three types of Ni···I contacts. The Ni···I semicoordination of the electrophilic nickel(II) center with electron belt of I2 was observed in 1·2I2, the metal-involving halogen bonding between the nucleophilic nickel(II)-dz2 center and σ-hole of iodine center was recognized and confirmed theoretically in the structure of [FeNi(CN)4(IPz)(H2O)]n (IPz = 4-N-coordinated 2-I-pyrazine), whereas the arrangement of FIB in 1·2FIB provides a boundary case between the semicoordination and the halogen Ni···I bondings. In 1·2I2 and 1·2FIB, noncovalent interactions were studied by variable temperature XRD detecting the expansion of noncovalent contacts with preservation of covalent bond lengths upon the temperature increase from 100 to 300 K. The nature and energies of all identified types of the Ni···I noncovalent interactions in the obtained (1·2I2 and 1·2FIB) and in the previously reported ([FeNi(CN)4(IPz)(H2O)]n, [NiL2](I3)2·2I2 (L = o-phenylene-bis(dimethylphosphine), [NiL]I2 (L = 1,4,8,11-tetra-azacyclotetradecane), Ni(en)2]n[AgI2]2n (en = ethylenediamine), and [NiL](ClO4) (L = 4-iodo-2-((2-(2-(2-pyridyl)ethylsulfanyl)ethylimino)methyl)-phenolate)) structures were studied theoretically. The estimated strengths of these Ni···I noncovalent contacts vary from 1.6 to 4.1 kcal/mol and, as expected, become weaker on heating. This work is the first emphasizing electrophilic-nucleophilic dualism of any metal center toward noncovalent interactions.

Metal-Involving Synthesis and Reactions of Oximes.

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Metal-mediated reactions between dialkylcyanamides and acetamidoxime generate unusual (nitrosoguanidinate)nickel(ii) complexes.

The nitrosoguanidinate complexes [Ni{NH[double bond, length as m-dash]C(NR2)NN(O)}2] (R2 = Me21, (CH2)4O 2, (CH2)43, (CH2)54, (Me)Ph 5, Ph26, (p-MeC6H4)27) were obtained in low-to-moderate (12-26%) yields but reproducible yields in an unexpected metal-mediated reaction in MeOH between the nickel salt NiCl2·2H2O, N,N-disubstituted cyanamides NCNR2, and the amidoxime MeC([double bond, length as m-dash]NOH)NH2. These complexes were formed along with a spectrum of cyanamide-oxime coupling products. The IR and X-ray data indicate the delocalization within the NNO and NCN systems of the nitrosoguanidinate ligand. This delocalization was additionally confirmed by inspection of Wiberg bond indices for the selected bonds. In the X-ray structure of 5, the rare metallophilic contacts NiNi between stacks of the square-planar complexes were detected and these non-covalent interactions were studied by non-relativistic and relativistic DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method). The estimated strength of the NiNi interactions is 1.3-1.9 kcal mol-1 and they are mostly determined by crystal packing effects and weak attractive interactions between adjacent metal centers due to the overlapping of their dz2 and pz orbitals.

Copper(I)-Catalyzed 1,3-Dipolar Cycloaddition of Ketonitrones to Dialkylcyanamides: A Step toward Sustainable Generation of 2,3-Dihydro-1,2,4-oxadiazoles.

CuI-catalyzed cycloaddition (CA) of the ketonitrones, Ph2C=N+(R')O- (R' = Me, CH2Ph), to the disubstituted cyanamides, NCNR2 (R = Me2, Et2, (CH2)4, (CH2)5, (CH2)4O, C9H10, (CH2Ph)2, Ph(Me)), gives the corresponding 5-amino-substituted 2,3-dihydro-1,2,4-oxadiazoles (15 examples) in good to moderate yields. The reaction proceeds under mild conditions (CH2Cl2, RT or 45 °C) and requires 10 mol % of [Cu(NCMe)4](BF4) as the catalyst. The somewhat reduced yields are due to the individual properties of 2,3-dihydro-1,2,4-oxadiazoles, which easily undergo ring opening via N-O bond splitting. Results of density functional theory calculations reveal that the CA of ketonitrones to CuI-bound cyanamides is a concerted process, and the copper-catalyzed reaction is controlled by the predominant contribution of the HOMOdipole-LUMOdipolarophile interaction (group I by Sustmann's classification). The metal-involving process is much more asynchronous and profitable from both kinetic and thermodynamic viewpoints than the hypothetical metal-free reaction.

Alkenylation of Arenes and Heteroarenes with Alkynes.

This review is focused on the analysis of current data on new methods of alkenylation of arenes and heteroarenes with alkynes by transition metal catalyzed reactions, Bronsted/Lewis acid promoted transformations, and others. The synthetic potential, scope, limitations, and mechanistic problems of the alkenylation reactions are discussed. The insertion of an alkenyl group into aromatic and heteroaromatic rings by inter- or intramolecular ways provides a synthetic route to derivatives of styrene, stilbene, chalcone, cinnamic acid, various fused carbo- and heterocycles, etc.

Regio- and Stereoselective 1,3-Dipolar Cycloaddition of Cyclic Azomethine Imines to Platinum(IV)-Bound Nitriles Giving Δ(2)-1,2,4-Triazoline Species.

The complex trans-[PtCl4(EtCN)2] (14) reacts smoothly at 25 °C with the stable cyclic azomethine imines R(1)CH═N(a)NC(O)CH(NHC(O)C6H4R(3))C(b)H(C6H4R(2))((a-b)) [R(1)/R(2)/R(3) = p-Me/H/H (8); p-Me/p-Me/H (9); p-Me/p-MeO/H (10); p-Me/p-Cl/p-Cl (11); p-MeO/p-Me/H (12); p-MeO/p-Cl/m-Me (13)], and the reaction proceeds as stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl4(EtCN){N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1)}])((a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (15); p-Me/p-Me/H (16); p-Me/p-MeO/H (17); p-Me/p-Cl/p-Cl (18); p-MeO/p-Me/H (19); p-MeO/p-Cl/m-Me (20)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C≡N bonds of HCN and of Pt(IV)-bound EtCN has different regioselectivity leading to Δ(2)-1,2,3-triazolines and Δ(2)-1,2,4-triazolines, respectively. Platinum(II) species trans-[PtCl2(EtCN){N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1)}]((a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (21); p-Me/p-Me/H (22); p-Me/p-MeO/H (23); p-Me/p-Cl/p-Cl (24); p-MeO/p-Me/H (25); p-MeO/p-Cl/m-Me (26)] were obtained by a one-pot procedure from 14 and 8-13 followed by addition of the phosphorus ylide Ph3P═CHCO2Me. Δ(2)-1,2,4-Triazolines N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1(a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/p-Cl/p-Cl (30); p-MeO/p-Me/H (31); p-MeO/p-Cl/m-Me (32)] were liberated from 21-26 by the treatment with bis(diphenylphosphyno)ethane (dppe). Platinum(II) complexes 21-26 were characterized by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), and IR and (1)H and (13)C{(1)H} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25·CH3OH, 26·(CHCl3)0.84. The structure of 26 was also determined by COSY-90 and NOESY NMR methods in solution. Quantitative evaluation of several pairs of interproton distances obtained by NMR and X-ray diffraction agrees well with each other and with those obtained by the MM+ calculation method. Platinum(IV) complexes 15-20 were characterized by (1)H NMR spectroscopy. Metal-free 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles (27-32) were characterized by high-resolution ESI-MS and IR and (1)H and (13)C{(1)H} NMR spectroscopies and single crystal X-ray diffraction for 29·CDCl3. Theoretical density functional theory calculations were carried out for the investigation of the reaction mechanism, interpretation of the reactivity of Pt-bound and free nitriles toward azomethine imines and analysis of the regio- and stereoselectivity origin.