A d10 Ni–(H2) Adduct as an Intermediate in H?H Oxidative Addition across a Ni?B Bond

Bifunctional E H activation offers a promising approach for the design of two‐electron‐reduction catalysts with late first‐row metals, such as Ni. To this end, we have been pursuing H₂ activation reactions at late‐metal boratranes and herein describe a diphosphine–borane‐supported Ni—(H₂) complex, [(^PhDPB^iPr)Ni(H₂)], which has been characterized in solution. ¹H NMR spectroscopy confirms the presence of an intact H₂ ligand. A range of data, including electronic‐structure calculations, suggests a d¹⁰ configuration for [(^PhDPB^iPr)Ni(H₂)] as most appropriate. Such a configuration is highly unusual among transition‐metal H₂ adducts. The nonclassical H₂ adduct is an intermediate in the complete activation of H₂ across the Ni B interaction. Reaction‐coordinate analysis suggests synergistic activation of the H₂ ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d¹⁰ Ni—(H₂) interaction and in the H—H cleavage step.     

A d10 Ni–(H2) Adduct as an Intermediate in HH Oxidative Addition across a NiB Bond

Bifunctional E? H activation offers a promising approach for the design of two‐electron‐reduction catalysts with late first‐row metals, such as Ni. To this end, we have been pursuing H₂ activation reactions at late‐metal boratranes and herein describe a diphosphine–borane‐supported Ni—(H₂) complex, [(^PhDPB^iPr)Ni(H₂)], which has been characterized in solution. ¹H NMR spectroscopy confirms the presence of an intact H₂ ligand. A range of data, including electronic‐structure calculations, suggests a d¹⁰ configuration for [(^PhDPB^iPr)Ni(H₂)] as most appropriate. Such a configuration is highly unusual among transition‐metal H₂ adducts. The nonclassical H₂ adduct is an intermediate in the complete activation of H₂ across the Ni? B interaction. Reaction‐coordinate analysis suggests synergistic activation of the H₂ ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d¹⁰ Ni—(H₂) interaction and in the H—H cleavage step.     

The role of macrocyclic ligands in the peroxo/superoxo nature of Ni–O2 biomimetic complexes

The impact of the macrocyclic ligand on the electronic structure of two LNi? O₂ biomimetic adducts, [Ni(12‐TMC)O₂]⁺ (12‐TMC = 1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane) and [Ni(14‐TMC)O₂]⁺ (14‐TMC = 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), has been inspected by means of difference‐dedicated configuration interaction calculations and a valence bond reading of the wavefunction. The system containing the 12‐membered macrocyclic ligand has been experimentally described as a side‐on nickel(III)‐peroxo complex, whereas the 14‐membered one has been characterized as an end‐on nickel(II)‐superoxide. Our results put in evidence the relationship between the steric effect of the macrocyclic ligand, the O₂ coordination mode and the charge transfer extent between the Ni center and the O₂ molecule. The 12‐membered macrocyclic ligand favors a side‐on coordination, a most efficient overlap between Ni 3d and O₂ π* orbitals and, consequently, a larger charge transfer from LNi fragment to O₂ molecule. The analysis of the ground‐state electronic structure shows an enhancement of the peroxide nature of the Ni? O₂ interaction for [Ni(12‐TMC)O₂]⁺, although a dominant superoxide character is found for both systems. © 2012 Wiley Periodicals, Inc.     

Synthesis and spectroscopic studies of diorganotin(IV) adducts based on cyclotriphosphazene scaffolds with exocyclic pyrazolyl substituents

Functionalized cyclotriphosphazenes with four pyrazolyl substituents have been employed for the synthesis of two new organotin complexes. These new compounds have been characterized by elemental analysis and IR, ¹H, ³¹P and ¹¹⁹Sn NMR spectroscopy. On the basis of these data, pyrazolylcyclotriphosphazene is bis-bidentate neutral ligand coordinating to two SnMe₂Cl₂ molecules in the resulting adducts. Coordination occurs only via the pyrazolyl nitrogens; cyclotriphosphazene ring nitrogens are not involved in coordination. The ¹¹⁹Sn NMR data are consistent with increasing of coordination number of tin(IV) in solution.     

Metal complexes with macrocyclic ligands. XVI. Spectrophotometric and thermodynamic studies of solvents and unidentate ligands interaction with the pentacoordinate Co2+-, Ni2+- and Cu2+-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane

The interaction of solvents and of unidentate ligands such as N, SCN^?, OCN^? and OH^? with the Co²⁺-, Ni²⁺ and Cu²⁺-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) have been studied by Spectrophotometric and calorimetric techniques. The spectra in different solvents (Table 2) show that the Ni²⁺- and probably also the Cu²⁺-complex with TMC exist as square planar or pentacoordinate species or as a mixture of both, depending on the donor properties of the solvent. The [Co(TMC)]²⁺-complex is pentacoordinate in all the solvents studied. Ternary complexes [M(TMC)X]ⁿ⁺ are also formed by the unidentate ligands X = N, OCN^?, OH^?, F^? and NH₃ and their stability constants have been determined. Interesting is the high selectivity of [Ni(TMC)]²⁺ towards the addition of a further donor (Table 3). Only small ligands such as those listed above form stable adducts, whereas the larger ones such as imidazole or pyridine do not. This is a consequence of the special structure of the complex and of the trans-I-(RSRS)- conformation of the ligand in these complexes. Since the four methyl groups are all on the side of the macrocycle to which the additional unidentate ligand binds, steric interaction between the four methyl groups and the larger ligands prevents the formation of the adducts. The calorimetric measurements show that the stability of the complexes [M(TMC)X]ⁿ⁺ is due to both an enthalpic and entropic contribution which differ in their magnitude (Table 4). This indicates that several antagonistic factors are important in determining the overall stability.     

Synthesis and structure of mononuclear molecular complexes of molybdenum(VI) and tungsten(VI) oxy- and dioxyhalides

Specific features of the synthesis and structure of mononuclear molecular halide oxide complexes of Group VI d ⁰ metals molybdenum(VI) and tungsten(VI) are surveyed. Various methods of synthesis of adducts based on MOX₄ and MO₂X₂ (M = Mo, W; X = F, Cl, Br) are described, such as direct interaction, ligand exchange, and the method of nascent reagents. The principles of formation of a particular geometric isomer are discussed: according to the self-consistency rule, the coordination of a neutral donor ligand L trans to a multiply bonded oxo ligand is preferable to that of acido ligands X⁻ (anions). Rare exceptions are mentioned.     

Adducts of Niobium (V) and Tantalum (V) Halides. XV. Ligand Exchange of the Adducts with Some Phosphoryl Compounds

The ligand exchange MX₅·L + *L?MX₅·*L + L for the octahedral adducts MX₅·L, in an inert solvent (CH₂Cl₂ or CHCl₃) with neutral ligands, proceeds via a dissociative D mechanism when M = Nb, X = Cl and L = phosphoryl compound. A dissociative interchange I_d mechanism is suggested when M = Nb or Ta, and X = F. A first order rate law and positive values for ΔS* (+4 to +14 cal K^?1 mol^?1) are observed for the exchanges on the pentachloride adducts. However, a second order rate law and large negative values for ΔS* (-15 to -24 cal K^?1 mol^?1) are found for the intermolecular neutral ligand exchange (measured by ¹H-NMR.) and for the intramolecular fluorine exchange (measured by ¹⁹F-NMR.) reactions on the pentafluoride adducts. The fluorine exchange is 2 to 5 times faster than the ligand exchange. The exchanges, on the pentachloride and on the pentafluoride adducts, are slowed down with increasing donor strength of the phosphoryl compound.     

Synthesis of nickel(II)-di(2,3-dichlorophenyl)carbazonate and its adduct formation with heterocyclic nitrogen bases

The nickel(II)-di(2,3-dichlorophenyl)carbazonate [Ni(D2,3DClPC)₂] complex has been prepared and characterized by elemental and spectral studies. The Ni^II chelate forms adducts with heterocyclic nitrogen bases, which were studied spectrophotometrically in monophase CHCl₃ media. Saturated monodentate bases such as pyrrolidine, piperidine, etc., form pentacoordinate adducts of 1:1 stoichiometry, whereas bidentate and unsaturated monodentate bases form hexacoordinated adducts with 1:1 and 1:2 metal–ligand stoichiometries respectively. The results are discussed interms of steric properties, basicity and ring structures of the heterocyclic bases.     

Thermogravimetric and differential scanning calorimetric investigations of manganese–glycine interactions

Thermogravimetric (t.g.) and differential scanning calorimetric (d.s.c.) data have been used to study metal–amino acid interactions in adducts of general formula MnCl₂ · ngly (gly = glycine, n = 0.7, 2.0, 4.0 and 5.0). All the prepared adducts exhibit only a one step mass loss associated with the release of glycine molecules, except for the 0.7gly adduct, which exhibits two glycine mass loss steps. From d.s.c. data, the enthalpy values associated with the glycine mass loss can be calculated: MnCl₂ · 0.7gly = 409 and 399 kJ mol^–1, MnCl₂ · 2.0gly = 216 kJ mol^–1, MnCl₂ · 4.0gly = 326 kJ mol^–1 and MnCl₂ · 5.0gly = 423 kJ mol^–1, respectively. The enthalpy associated with the ligand loss, plotted as function of the number of ligands for the n = 2.0, 4.0 and 5.0 adducts, gave a linear correlation, fitting the equation: H (ligand loss)/kJ mol^–1 = 67 × (number of ligands, n) + 76. A similar result was achieved when the enthalpy associated with the ligand loss was plotted as a function of the _a(COO^–) bands associated with the coordination through the carboxylate group, 1571, 1575 and 1577 cm^–1, respectively, for the n = 2.0, 4.0 and 5.0 adducts, giving the equation H (ligand loss) /kJ mol^–1 = 33.5 × _a(COO^–) /cm^–1 – 52418.5. This simple equation provides evidence for the enthalpy associated with the ligand loss being very closely related to the electronic density associated with the metal–amino acid bonds.     

Copper–Carbene Intermediates in the Copper‐Catalyzed Functionalization of OH Bonds

Copper–carbene [Tp^xCu?C(Ph)(CO₂Et)] and copper–diazo adducts [Tp^xCu{η¹‐N₂C(Ph)(CO₂Et)}] have been detected and characterized in the context of the catalytic functionalization of O?H bonds through carbene insertion by using N₂?C(Ph)(CO₂Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N‐bound diazo adduct is a dormant species and is not en route to the formation of the copper–carbene intermediate.     

Complexes of Copper(II) Aryl Carboxylates with Picoline N-Oxides

Copper(II) aryl carboxylates on interaction with 2-picoline N-Oxide (2-picNO), 3-picoline N-Oxide (3-picNO) and 4-picoline N-Oxide (4-picNO) in acetone or ethyl acetate yield 1:1 adducts. These are regarded as binuclear bridged complexes of composition Cu₂(RC₆H₄COO)₂L₂ where R is o-, m-, p-CH₂; o-, m-Cl: o-NO₂; m-, p-CH₂O; and L is 2-, 3-, and 4-picNO ligand. With 4-picNO some 1:2 adducts have also been obtained.     

Diastereodivergent Asymmetric 1,3‐Dipolar Cycloaddition of Azomethine Ylides and β‐Fluoroalkyl Vinylsulfones: Low Copper(II) Catalyst Loading and Theoretical Studies

A Cu^II‐catalyzed asymmetric 1,3‐dipolar cycloaddition using β‐fluoroalkyl alkenyl arylsulfones as dipolarophiles and glycine/alanine iminoesters as azomethine ylide precursors has been developed. Remarkably, a catalyst loading as low as 0.5 mol % is highly efficient. Accordingly, a wide range of enantioenriched 3‐fluoroalkyl pyrrolidines, as well as Δ²‐pyrroline and pyrrole derivatives, are generated in good to excellent yields with high asymmetric induction. This synthetic approach is diastereodivergent in that exo‐adducts could be converted into the corresponding exo′‐adducts by 1,8‐diazabicyclo[5.4.0]undec‐7‐ene mediated epimerization at C2 of the pyrrolidine core. The free‐energy profiles from DFT calculations suggest the Michael addition of the 1,3‐dipole to be the rate‐ and enantiodetermining step, and the origin of stereoselectivity is studied by means of the noncovalent interaction (NCI) analysis.     

Reactions of 2,4,6-tripyridyl-1,3,5-s-triazine and its Fe(ii) complex with OH· and eaq-

Summary Pulse radiolysis studies indicated that OH^· and e_aq^- formed adducts with 2,4,6-tripyridyl-1,3,5-s-triazine (tptz). The OH-adduct showed an absorption maximum at 290 nm, while the adduct formed from the attack of e_aq^- and subsequent protonation of the anion, has an absorption maximum at 300 nm. In case of the complex, Fe(tptz)₂²⁺, both OH^· and e_aq^- gave ligand centered adducts. All the adducts decayed by second order dimerization or disproportionation, a disproportionation reaction in case of the complex.     

Adducts of niobium(V) and tantalum(V) halides. XIV. Structure and relative stability of the adducts with some phosphoryl compounds

The adducts of niobium(V) and tantalum(V) halides with some phosphoryl compounds have been studied in chloroform solution by ¹H- and ¹⁹F-FT-NMR. spectroscopy. These octahedral adducts of general formula MX₅ · L (M = Nb, Ta; X = F, Cl, Br; L = phosphoryl ligand) are monomeric and neutral. Their relative stability constants have been determined at ?60°. The stabilities are controlled by electronic effects of substituents on the phosphoryl group.     

Electrostatic bonding models: A test on group 1 and 2 metal complexes with H2O,NH3, H2S,PH3, and related ligands

Electrostatic models frequently proposed to describe ion–molecule interactions have been tested on the adducts formed by Group 1 and 2 cations with H₂O, NH₃, H₂S, PH₃, their methyl analogs, and their anions. The results from the model calculations were compared with all-electron calculations (geometry optimized, MP2, TZP basis sets) carried out on adducts formed with Li⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺. The electrostatic potential model was utilized in two ways: The attraction of the point charge was calculated with and without relaxation of the ligand. A third model allowed relaxation of the ligand but treated the cation as a frozen core. The final model was the crude point charge/point dipole approximation. At long range, the models satisfactorily track the effects on energy of gross changes in the ion–ligand interaction (monovalent versus divalent ions, neutral ligands versus anions, parent ligands versus methyl derivatives), but correlation at close range is poor, especially for binding by divalent cations. The hypothesis that the calculated strength of cation–dipole binding is dependent on calculated dipole moment could not be verified. © 1995 by John Wiley & Sons, Inc.     

FAR-INFRARED SPECTRA OF LANGMUIR-BLODGETT FILMS OF CHLOROPHYLL a,CHLOROPHYLL b,PHEOPHYTIN a and THEIR ADDUCTS WITH WATER and DIOXANE*

Fourier transform infrared spectra in the low frequency region (500–150cm^?1) of Langmuir-Blodgett films of chlorophyll a (Chi a), chlorophyll b (Chi b) and pheophytin a have been studied. Correlations between spectral changes in monolayer and multilayers of Chi a and Chi b and their adducts with water and dioxane have been established. Spectroscopic evidence has indicated that, although there are no individual absorption bands that can be assigned to pure Mg-nitrogen and/or Mg-oxygen stretching or bending modes, there are several bands in the400–200 cm^?1 region of the spectra containing considerable contributions from metal-nitrogen and metal-oxygen vibrational modes. These specific vibrations exhibit marked intensity changes and shifts upon water and dioxane interaction. The different states of chlorophyll aggregation in Langmuir-Blodgett mono- and multilayers films resulted in noticeable changes in their far-IR spectra.     

Isonitrosopropiophenone as an ambidentate chelating agent in some transition metal complexes

Summary Isonitrosopropiophenonates of cobalt(III), cobalt(II), nickel(II) and palladium(II) have been synthesised and characterised. Isonitrosopropiophenonatonickel(II) exists as green and brown varieties, the green forming mono- as well as bispyridine adducts. The brown variety is obtained after removal of base from the adducts. I.r. data of the metal chelates suggest atrans-asymmetric structure involving N and O bonding for one form and atrans-symmetric structure involving either N or O bonding of the oximino-group for the other. The former is observed exclusively with isonitrosopropiophenonates of nickel(II) while the latter is common among the other metal chelates. The low magnetic moment of the mono-adduct is ascribed to super-exchange interaction between two metal ions through oxo-bridges in the dimer. Acis-octahedral structure has been assigned to the tris(isonitrosopropiophenonato) cobalt(III) on the basis of¹H n.m.r. data. Isonitrosopropiophenone forms mixed-ligand complexes of cobalt(III) with tetradentate Schiff bases such as SALEN,N,N-ethylenebis(salicylaldimine), and SALPN,N,N-1,3-propylenebis-(salicylaldimine), where the isonitrose group bonds through the nitrogen atom. These complexes have a N,N,N,O,O,O ligand environment. The imino-nickel(II) and -palladium(II) complexes are assigned atrans-symmetric square planar structure. Characterisation of the complexes was based upon elemental analyses, conductivity and magnetic moment, and i.r.,¹H n.m.r., and electronic spectra.     

Adducts of Niobium(V) and Tantalum(V) Halogenides. IX. Relative stability of the adducts of the chlorides and bromides with some dialkylchalcogenides

The relative stability of adducts formed by Nb(V) and Ta(V) pentachlorides and bromides with some dimethylchalcogenides and nitriles has been determined by ¹H-NMR. in dichloromethane at ? 60°. The stabilities are explained in terms of the HSAB principle and Jørgensen's symbiotic effect. A good correlation exists between the ionisation potential of the valence p orbital of the chalcogen atom in the ligand and the logarithm of the relative stability of the adduct formed with a given acid.     

Tin(IV) Chloride Complexes of β-Chlorovinyl Aldehydes: A Multinuclear Nmr Characterization in Solution

Abstract

The reaction of SnCl₄ with β-chlorovinyl aldehydes in anhydrous dichloromethane gave a series of octahedral complexes of the general formula SnCl₄·2L (L = aldehyde). The adducts have been characterized in solution using multinuclear (¹H, ¹³C, and ¹¹⁹Sn) NMR and IR spectroscopy. Solution NMR studies show that the complexes undergo rapid ligand dissociation at ambient temperature. Ligand exchange is slowed significantly at low temperature, such that, in most of the complexes, it is possible to identify both the cis and trans isomers with predominance of the cis form. The magnitude of the metal-ligand interaction was estimated on the basis of ¹¹⁹Sn NMR chemical shifts and used to classify the aldehydes studied according to their Lewis basicity.     

Diastereodivergent Asymmetric Michael Addition of Cyclic Azomethine Ylides to Nitroalkenes: Direct Approach for the Synthesis of 1,7‐Diazaspiro[4.4]nonane Diastereoisomers

The first highly diastereoselective and enantioselective catalytic asymmetric Michael addition of cyclic azomethine ylides with nitroalkenes have been developed to diastereodivergently generate either the syn or anti adducts by employing N,O‐ligand/Cu(OAc)₂ and N,P‐ligand/Cu(OAc)₂ catalytic systems. Both catalytic systems exhibit broad substrate applicability to afford the corresponding Michael adducts in good to excellent yields, with excellent levels of diastereo‐ (up to 99:1 diastereomeric ratio) and enantioselectivities (up to >99 % enantiomeric excess). Importantly, the chiral 1,7‐diazaspiro[4.4]nonane diastereomer derivatives can be easily obtained in good yields through facile NaBH₄ reduction of the Michael adducts.