Synthesis, characterization and Schlenk equilibrium studies of methylmagnesium compounds with O- and N-donor ligands - the unexpected behavior of [MgMeBr(pmdta)] (pmdta = N,N,N′,N″,N″-pentamethyldiethylenetriamine)

MgMe₂ (1) was found to react with 1,4-diazabicyclo[2.2.2]octane (dabco) in tetrahydrofuran (thf) yielding a binuclear complex [{MgMe₂(thf)}₂(μ-dabco)] (2). Furthermore, from reactions of MgMeBr with diglyme (diethylene glycol dimethyl ether), NEt₃, and tmeda (N,N,N′,N′-tetramethylethylenediamine) in etheral solvents compounds MgMeBr(L), (L = diglyme (5); NEt₃ (6); tmeda (7)) were obtained as highly air- and moisture-sensitive white powders. From a thf solution of 7 crystals of [MgMeBr(thf)(tmeda)] (8) were obtained. Reactions of MgMeBr with pmdta (N,N,N′,N″,N″-pentamethyldiethylenetriamine) in thf resulted in formation of [MgMeBr(pmdta)] (9) in nearly quantitative yield. On the other hand, the same reaction in diethyl ether gave MgMeBr(pmdta) · MgBr₂(pmdta) (10) and [{MgMe₂(pmdta)}₇{MgMeBr(pmdta)}] (11) in 24% and 2% yield, respectively, as well as [MgMe₂(pmdta)] (12) as colorless needle-like crystals in about 26% yield. The synthesized methylmagnesium compounds were characterized by microanalysis and ¹H and ¹³C NMR spectroscopy. The coordination-induced shifts of the ¹H and ¹³C nuclei of the ligands are small; the largest ones were found in the tmeda and pmdta complexes. Single-crystal X-ray diffraction analyses revealed in 2 a tetrahedral environment of the Mg atoms with a bridging dabco ligand and in 8 a trigonal-bipyramidal coordination of the Mg atom. The single-crystal X-ray diffraction analyses of [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) showed them to be monomeric with five-coordinate Mg atoms. The square-pyramidal coordination polyhedra are built up of three N and two C atoms in 12 and three N and two Br atoms in 13. The apical positions are occupied by methyl and bromo ligands, respectively. Temperature-dependent ¹H NMR spectroscopic measurements (from 27 to −80 °C) of methylmagnesium bromide complexes MgMeBr(L) (L = thf (4); diglyme (5); NEt₃ (6); tmeda (7)) in thf-d₈ solutions indicated that the deeper the temperature the more the Schlenk equilibria are shifted to the dimethylmagnesium/dibromomagnesium species. Furthermore, at −80 °C the dimethylmagnesium compounds are predominant in the solutions of Grignard compounds 4-6 whereas in the case of the tmeda complex7 the equilibrium constant was roughly estimated to be 0.25. In contrast, [MgMeBr(pmdta)] (9) in thf-d₈ revealed no dismutation into [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) even up to −100 °C. In accordance with this unexpected behavior, 1:1 mixtures of 12 and 13 were found to react in thf at room temperature yielding quantitatively the corresponding Grignard compound 9. Moreover, the structures of [MgMeBr(pmdta)] (9c), [MgMe₂(pmdta)] (12c), and [MgBr₂(pmdta)] (13c) were calculated on the DFT level of theory. The calculated structures 12c and 13c are in a good agreement with the experimentally observed structures 12 and 13. The equilibrium constant of the Schlenk equilibrium (2 9c ? 12c + 13c) was calculated to be K_gas = 2.0 × 10⁻³ (298 K) in the gas phase. Considering the solvent effects of both thf and diethyl ether using a polarized continuum model (PCM) the corresponding equilibrium constants were calculated to be K_thf = 1.2 × 10⁻³ and K_ether = 3.2 × 10⁻³ (298 K), respectively.     

Structure of N,C,N-chelated organotin(IV) fluorides

The set of starting tri-, di- and monoorganotin(IV) halides containing N,C,N-chelating ligand (L^NCN = {1,3-[(CH₃)₂NCH₂]₂C₆H₃}⁻) has been prepared (1-5) and two compounds structurally characterized ([L^NCNPh₂Sn]⁺I₃⁻ (1c), L^NCNSnBr₃ (5)) in the solid state. These compounds were reacted with KF with 18-crown-6, NH₄F or L^CNnBu₂SnF to give derivatives containing fluorine atom(s). Triorganotin(IV) fluorides L^NCNMe₂SnF (2a) and L^NCNnBu₂SnF (3a) revealed monomeric structural arrangement with covalent Sn-F bond both in the coordinating and non-coordinating solvents, except the behaviour of 3a that was ionized in the methanol solution at low temperature. The products of fluorination of L^NCNSnPhCl₂ (4) and 5 were described by NMR in solution as the ionic hypervalent fluorostannates or the oligomeric species reacting with chloroform, methanol or moisture to zwitterionic monomeric stannate L^NCN(H)⁺SnF₄⁻ (5c), which was confirmed by XRD analysis in the solid state.     

Complexes derived from the copper(II)/succinamic acid/N,N′,N″-chelate tertiary reaction systems: Synthesis,structural and spectroscopic studies

The use of succinamic acid (H₂sucm) in Cu^II/N,N′,N″-donor [2,2′:6′,2″-terpyridine (terpy), 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine (dmbppy)] reaction mixtures yielded compounds [Cu(Hsucm)(terpy)]_n(ClO₄)_n (1), [Cu(Hsucm)(terpy)(MeOH)](ClO₄) (2), [Cu₂(Hsucm)₂(terpy)₂](ClO₄)₂ (3), [Cu(ClO₄)₂(terpy)(MeOH)] (4), [Cu(Hsucm)(dmbppy)]_n(NO₃)_n·3nH₂O (5^.3nH₂O), and [CuCl₂(dmbppy)]·H₂O (6·H₂O). The succinamate(−1) ligand exists in four different coordination modes in the structures of 1–3 and 5, i.e., the μ₂-κO:κO′:κO″ in 1 and 5 which involves asymmetric chelating coordination of the carboxylato group and ligation of the amide O-atom leading to 1D coordination polymers, the μ₂-κ²O:κO′ in 3 which involves asymmetric chelating and bridging coordination of the carboxylato group, and the asymmetric chelating mode in 2. The primary amide group, either coordinated in 1 and 5, or uncoordinated in 2 and 3, participate in hydrogen bonding interactions, leading to interesting crystal structures. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the Hsucm⁻ ligands. The thermal decomposition of complex 5·3nH₂O was monitored by TG/DTG and DTA measurements.     

Coordination compounds of N,N′-olefin functionalized imidazolin-2-ylidenes

N-Heterocyclic carbene ligands (NHC) were metalated with Pd(OAc)₂ or [Ni(CH₃CN)₆](BF₄)₂ by in situ deprotonation of imidazolium salts to give the N-olefin functionalized biscarbene complexes [MX₂(NHC)₂] 3-7 (3: M = Pd, X = Br, NHC = 1,3-di(3-butenyl)imidazolin-2-ylidene; 4: M = Pd, X = Br, NHC = 1,3-di(4-pentenyl)imidazolin-2-ylidene; 5: M = Pd, X = I, NHC = 1,3-diallylimidazolin-2-ylidene; 6: M = Ni, X = I, NHC = 1,3-diallylimidazolin-2-ylidene; 7: M = Ni, X = I, NHC = 1-methyl-3-allylimidazolin-2-ylidene). Molecular structure determinations for 4-7 revealed that square-planar complexes with cis (5) or trans (4, 6, 7) coordination geometry at the metal center had been obtained. Reaction of nickelocene with imidazolium bromides afforded the η⁵-cyclopentadienyl (η⁵-Cp) monocarbene nickel complexes [NiBr(η⁵-Cp)(NHC)] 8 and 9 (8: NHC = 1-methyl-3-allylimidazolin-2-ylidene; 9: NHC = 1,3-diallylimidazolin-2-ylidene). The bromine abstraction in complexes 8 and 9 with silver tetrafluoroborate gave complexes [NiBr(η⁵-Cp)(η³-NHC)] 10 and 11. The X-ray structure analysis of 10 and 11 showed a trigonal-pyramidal coordination geometry at the nickel(II) center and coordination of one N-allyl substituent.     

Dioxomolybdenum(VI) complexes containing N-heterocyclic carbenes

Compound MoO₂Cl₂(THF)₂ reacts with two equivalents of 1,3-dialkyl substituted 4,5-dimethylimidazol-2-ylidenes to give the dioxomolybdenum(VI) complexes MoO₂Cl₂(L^R)₂ [R = Me (1), i-Pr (2)]. Treatment of MoO₂Cl₂(THF)₂ with one equivalent of the N-heterocyclic carbenes L^Me, L^i-Pr and ^C1L^n-Bu (L^Me = 1,3,4,5-tetramethylimidazol-2-ylidene, L^i-Pr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, and ^C1L^n-Bu = 1,3-dibutyl-4,5-dichloroimidazol-2-ylidene) affords the monocarbene adducts MoO₂Cl₂(L^R) [R = Me (3), i-Pr (4)] and MoO₂Cl₂(^C1L^n-Bu) (5), respectively. Decomposition of complexes 1-5 affords a molybdenum oxychloride anion [Mo₂O₅Cl₄]²⁻ as an imidazolium salt.     

A “sodium trap” based on benzo-15-crown-5 with an exocyclic N-(thiophosphoryl)thiourea moiety

For N-(thio)phosphorylthioureas of the common formula RC(S)NHP(X)(OiPr)₂HL^I (R = N-(4′-aminobenzo-15-crown-5), X = S), HL^II (R = N-(4′-aminobenzo-15-crown-5), X = O), HL^III (R = PhNH, X = S), HL^IV (R = PhNH, X = O), and (N,N′-bis-[C(S)NHP(S)(OiPr)₂]₂-1,10-diaza-18-crown-6) H₂L^V, salts LiL^I,III,IV, NaL^I–IV, KL^I–IVM₂L^V (M = Li⁺, Na⁺, K⁺), Ba(L^I,III,IV)₂, and BaL^V have been synthesized and investigated. Compounds NaL^I,II quantitatively drop out as a deposit in ethanol medium, allowing the separation of Na⁺ and K⁺ cations. This effect is not displayed for the other compounds. The crystal structures of HL^III and the solvate of the composition [K(Me₂CO)L^III] have been investigated by X-ray crystallography.     

Dimolybdenum complexes containing anions of N,N′-bis(pyrimidine-2-yl)formamidine and N-(2-pyrimidinyl)formamide

The reactions of Mo₂(O₂CCH₃)₄ with different equivalents of N,N′-bis(pyrimidine-2-yl)formamidine (HL1) and N-(2-pyrimidinyl)formamide (HL2) afforded dimolybdenum complexes of the types Mo₂(O₂CCH₃)(L1)₂(L2) (1) trans-Mo₂(L1)₂(L2)₂ (2) cis-Mo₂(L1)₂(L2)₂ (3) and Mo₂(L2)₄ (4). Their UV–Vis and NMR spectra have been recorded and their structures determined by X-ray crystallography. Complexes 2 and 3 establish the first pair of trans and cis forms of dimolybdenum complexes containing formamidinate ligands. The L1 ligands in 1–3 are bridged to the metal centers through two central amine nitrogen atoms, while the L2 ligands in 1–4 are bridged to the metal centers via one pyrimidyl nitrogen atom and the amine nitrogen atom. The Mo–Mo distances of complexes 1 [2.0951(17) Å], 2 [2.103(1) Å] and 3 [2.1017(3) Å], which contain both Mo?N and Mo?O axial interactions, are slightly longer than those of complex 4 [2.0826(12)–2.0866(10) Å] which has only Mo?O interactions.     

Further studies of tetrakis(N,N′-dialkylbenzamidinato)diruthenium(III) chloro and alkynyl compounds: molecular engineering of metallayne monomers

Three diruthenium(III) compounds Ru₂(L)₄Cl₂, where L is mMeODMBA (N,N′-dimethyl-3-methoxybenzamidinate, 1a), DiMeODMBA (N,N′-dimethyl-3,5-dimethoxy benzamidinate, 1b), or DEBA (N,N′-diethylbenzamidinate, 1c), were prepared from the reactions between Ru₂(OAc)₄Cl and respective HL under reflux conditions. Metathesis reactions between 1 and LiC₂Y resulted in bis-alkynyl derivatives Ru₂(L)₄(C₂Y)₂ [Y=Ph (2), SiMe₃ (3), SiⁱPr₃ (4) and C₂SiMe₃ (5)]. The parent compounds 1 are paramagnetic (S=1), while bis-alkynyl derivatives 2-5 are diamagnetic and display well-solved ¹H- and ¹³C-NMR spectra. Molecular structures of compounds 1b, 1c, 2c, 3c and 4b were established through single crystal X-ray diffraction studies, which revealed RuRu bond lengths of ca. 2.32 Å for parent compounds 1 and 2.45 Å for bis-alkynyl derivatives. Cyclic voltammograms of all compounds feature three one-electron couples: an oxidation and two reductions, while the reversibility of observed couples depends on the nature of axial ligands.     

Au(I) and Au(III) complexes of a sterically bulky benzimidazole-derived N-heterocyclic carbene

The reaction of [AuCl(SMe₂)] with in situ generated [AgCl(ⁱPr₂-bimy)] (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene), which in turn was obtained by the reaction of Ag₂O with 1,3-diisopropylbenzimidazolium bromide (ⁱPr₂-bimyH⁺Br⁻, A), afforded the monocarbene Au(I) complex [AuCl(ⁱPr₂-bimy)] (1). Subsequent reaction of 1 and the ligand precursor ⁱPr₂-bimyH⁺BF₄⁻, (B) in acetone in the presence of K₂CO₃ yielded the bis(carbene) complex [Au(ⁱPr₂-bimy)₂]BF₄ (2) as a white powder in 80% yield. The oxidative addition of elemental iodine to complex 2 gave the bis(carbene) Au(III) complex trans-[AuI₂(ⁱPr₂-bimy)₂]BF₄ (3) as an orange-red powder in 92% yield. All complexes 1-3 have been fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry, elemental analysis, and X-ray single crystal diffraction. Complexes 1 and 2 adopt a linear geometry around metal centers as expected for d¹⁰ metals. The geometry around the Au(III) metal center in 3 is essentially square-planar with two carbene ligands in trans-position to each other. Complex 3 shows absorption and photoluminescence properties owing to a ligand to metal charge transfer.     

Reactivity of M(en)Cl2 (M = Pd/Pt,en = 1,2-diaminoethane) with N,N′-bis(salicylidene)-p-phenylenediamine: Binding with hexafluorobenzene

Schiff base N,N′-bis(salicylidene)-p-phenylenediamine (LH₂) complexed with Pt(en)Cl₂ and Pd(en)Cl₂ provided [Pt(en)L]₂ · 4PF₆ (1) and Pd(Salen) (2) (Salen = N,N′-bis(salicylidene)-ethylenediamine), respectively, which were characterized by their elemental analysis, spectroscopic data and X-ray data. A solid complex obtained by the reaction of hexafluorobenzene (hfb) with the representative complex 1 has been isolated and characterized as 3 (1 · hfb) using UV–Vis, NMR (¹H, ¹³C and ¹⁹F) data. A solid complex of hfb with a reported Zn-cyclophane 4 has also been prepared and characterized 5 (4 · hfb) for comparison with complex 3. The association of hfb with 1 and 4 has also been monitored using UV–Vis and luminescence data.     

Synthesis and characterization of N-(2-pyridyl)benzamide-based nickel complexes and their activity for ethylene oligomerization

A series of N-(2-pyridyl)benzamides (1)-(11) and their nickel complexes, [N-(2-pyridyl)benzamide]dinickel(II) di-μ-bromide dibromide (12)-(16) and (aryl)[N-(2-pyridyl)benzamido](triphenylphosphine)nickel(II) (17)-(24), were synthesized and characterized. The single-crystal X-ray analysis revealed that 12 and 14 are binuclear nickel complexes bridged by bromine atoms and each nickel atom adopts a distorted trigonal bipyramidal geometry. The key feature of the complexes 17, 19 and 23 is each has a six-membered nickel chelate ring including a deprotonated secondary nitrogen atom and an O-donor atom. The nickel complexes show moderate to high catalytic activity for ethylene oligomerization with methylaluminoxane (MAO) as cocatalyst. The activity of 12-16/MAO systems is up to 3.3 × 10⁴ g mol⁻¹ h⁻¹ whereas for 17-24/MAO systems it is up to 4.94 × 10⁵ g mol⁻¹ atm⁻¹ h⁻¹. The influence of Al/Ni molar ratio, reaction temperature, reaction period and PPh₃/Ni molar ratio on catalytic activity was investigated.     

ESR and pH-potentiometric study of the mixed-ligand complex formation in the copper(II)-4-fluorosalicylic acid-N,N-diethylnicotinamide system: Structure and spectral properties of [Cu(4-fluorosalicylate)2(N,N-diethylnicotinamide)2(H2O)2] complex

EPR simulation method together with pH-potentiometry combined with UV-Vis spectrophotometry were used for the study of the ternary system 4-fuorosalicylic acid (HA)-N,N-diethylnicotinamide (B)-copper(II) in aqueous solution. The N,N-diethylnicotinamide ligand is a weak donor, its mixed-ligand complexes with 4-fluorosalicylate anions are more favoured. The number of coordinated N,N-diethylnicotinamide molecules increases with decreasing temperature: up to four ones were detected in the coordination sphere of copper(II) in frozen solutions. The formation of [CuH₋₁AB₂] and [CuH₋₁A] was detected by all methods at neutral pH. At lower pH values, [CuA₂B₂] and [CuB] become dominant, and this fact is in good agreement with [CuA₂B₂(H₂O)₂] crystals obtained from similar solutions. The structural unit of the [CuA₂B₂(H₂O)₂] complex consists of a copper(II) ion, which is monodentately coordinated by a pair of 4-fluorosalicylate anions and by a pair of N,N-diethylnicotinamide in trans positions in the basal plane, and by two water molecules in the axial positions of a tetragonal bipyramid.     

A new sulfonamide derivative as magnesium ion receptor: N-tosyl-2,6-diisopropyl-4-(2,3-dimethoxylbenzoylamide)aniline

N-Tosyl-2,6-diisopropyl-4-(2,3-dimethoxylbenzoylamide)aniline (1) has been synthesized and its metal ion (Na⁺, K⁺, Ca²⁺, Mg²⁺) coordinating properties investigated by FT-IR, ESI-MS, and ¹H NMR methods. Among the tested metal ions, the overall stability constant (log K) for Mg²⁺ (6.89) is the highest (Na⁺, 5.64; K⁺, 5.43; Ca²⁺, 5.51) in 10% water/THF at 25.0 ± 0.5 °C determined by UV-vis spectroscopy, indicating that 1 is a potent ionophore for Mg²⁺ ion.     

N,N-polymethylene-bisadenine complexes with d metal ions

The reaction of N⁹,N^9′-(tri or tetramethylene)-bisadenines (Ade₂C_x; x = 3 or 4) in HCl 2 M at 50 °C with MCl₂ · 2H₂O [M = Zn(II), Cd(II)] yields outer sphere compounds like the previously described [(H-Ade)₂C₃][ZnCl₄] · H₂O (3) and [(H-Ade)₂C₃]₂[Cd₂Cl₈(H₂O)₂] · 4H₂O (4) for Ade₂C₃ and the new {[(H-Ade)₂C₄][Cd₂Cl₆(H₂O)₂] · 2H₂O}_n (5) for Ade₂C₄. On the other hand, only in case of Zn(II) complexes by changing [HCl] to 0.1 M, the inner sphere compounds [H-(Ade)₂C₃(ZnCl₃)] (6) and [H-(Ade)₂C₄(ZnCl₃)] · 1.5H₂O (7) are obtained. X-ray diffraction study of compound 6, which represents the first inner sphere complex with a N⁹,N^9′-bisadenine, shows a zwitterionic form with one adenine ring protonated at N(1) while the other ring is coordinated via N(7) to a ZnCl₃ moiety as in other alkyl-adenine derivatives. In addition, with Ade₂C₄, is also possible to obtain another inner sphere complex: [(H-Ade)₂C₄(ZnCl₃)₂] · 3H₂O (8).     

Experimental and theoretical study of N-(2-hydroxyethyl)morpholine betaine

N-(2-hydroxyethyl)morpholine betaine (HEMB) has been characterized by a single crystal X-ray analysis, FTIR spectroscopy and DFT calculations. The crystals are monoclinic, space group P2₁/c with a=10.273(2), b=9.360(2), c=9.447(2) Å and β=104.72(3)Å. Two molecules of HEMB form a centrosymmetric dimer (X2) connected by a pair of hydrogen bonds between the CH₂CH₂OH and COO⁻ groups, with the O?O distance of 2.672(2) Å. The morpholine ring adopts a chair conformation with the CH₂CH₂OH group in the axial and the CH₂COO⁻ group in the equatorial position. The structures of the dimer, B2, and two monomers, B1a and B1b, have been optimized by the B3LYP approach using the 6-31G(d,p) basis set. The computed structure of B2, agrees well with the experimental X2. From two stable monomeric conformers the more favored is B1a, with the intramolecular hydrogen bond with the O-H?O distance of 2.566 Å. The effects of hydrogen bonding and electrostatic interactions on the conformation of the molecules investigated have been discussed. The FTIR spectrum shows a broad absorption in the 3300-2600 cm⁻¹ region, typical of moderate O-H?O hydrogen bonds.     

DNA binding and cleavage behaviors of copper(II) complexes with amidino-O-methylurea and N-methylphenyl-amidino-O-methylurea,and their antibacterial activities

The two designed copper(II) complexes, [Cu(L^1m)₂]Cl₂ (1) (L^1m = amidino-O-methylurea) and [Cu(L^2m)₂]Cl₂ (2) (L^2m = N-methylphenyl-amidino-O-methylurea), have been investigated for their interaction with calf thymus DNA by utilizing the absorption titration method, viscometric studies and thermal denaturation. The cleavage reaction on pBR322 DNA has been monitored by agarose gel electrophoresis. The results suggest that the two complexes can bind to DNA by non-intercalative modes and exhibit nuclease activities in which supercoiled plasmid DNA is converted to the linear form. Complex 2, with an intrinsic binding constant (K_b) of 1.16 × 10⁵ M⁻¹, shows a higher binding efficiency and a better nuclease activity than complex 1, with a K_b value of 5.67 × 10⁴ M⁻¹. Their DNA cleavage potential can be significantly enhanced by hydrogen peroxide, indicating an oxidative cleavage process. Further examination of the antibacterial activities against Campylobacter has revealed inhibition zones of 9.0 (for 1) and 14.5 mm (for 2), which are in agreement with their minimum inhibitory concentration (MIC) values of 1.56 and 0.78 mg mL⁻¹, respectively. The substantially better reactivity of 2 results from the aromatic moieties on the side chain of the L^2m ligand which act as an additional binding site.     

New ICT probes: synthesis and photophysical studies of N-phenylaza-15-crown-5 aryl/heteroaryl oxadiazoles under acidic condition and in the presence of selected metal ions

Molecular probes 6 and 7, incorporating N-phenylaza-15-crown-5 and aryl/heteroaryl oxadiazole have been designed to function as the new intramolecular charge transfer (ICT) probes. Photophysical properties have been studied under acidic condition as well as in the presence of selected metal ions, Ca²⁺, Ba²⁺, Mg²⁺, Na⁺, K⁺, and Li⁺. The changes in the ICT character of the probes, following the addition of trifluoroacetic acid, were interpreted in terms of site and degree of protonations. Based on the cation affinity, the ICT bands in both UV-vis and emission spectra experienced varying degrees of blue shifts due to removal of the aza-crown ether nitrogen from conjugation. The cation-induced spectral shifts and the stability constants revealed binding strength in the order Ca²⁺>Ba²⁺?Li⁺>Na⁺>K⁺>Mg²⁺. Competitive experiments performed in a matrix of ions also indicated superior interaction of 6 and 7 with Ca²⁺. The excited state decay profiles remained largely unperturbed in the presence of metal ions. The studied probes displayed positive solvatochromism and the Stokes shifts and excited state lifetimes increased with increasing solvent polarity. These findings can be rationalized by invoking highly polar nature of the emittive states. The chemoionophores 6 and 7 constitute potentially interesting Ca²⁺ sensitive probes due to their relatively high binding interaction for Ca²⁺ (log K_s=3.55-3.10) vis-a-vis that of biologically interfering Mg²⁺ (log K_s=1.67-1.30).     

Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C₂-chiral bis(oxazoline) ligand (^R2,R3BOX: R₂ = (4S)-tBu, R₃ = H (a); R₂ = (4S)-Ph, R₃ = H (b); R₂ = (4R)-Ph, R₃ = (5S)-Ph (c)) with Zn(R₁)₂ (R₁ = Et (1), Me (2)) led to the heteroleptic three-coordinate complexes (^R2,R3BOX)ZnR₁, 1a-c and 2a, which were isolated in 92-96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed and synthesized: the 1,3-bis(2-pyridylmethyl)imidazolinium salt (d) and the protected NHC adduct 2-(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine (e). The reaction of ligands d and e with ZnEt₂ led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC-C₆F₅(H))ZnEt₂4e, respectively, in high yield. In situ combinations of selected complexes 1a-c, 3d and 4e with B(C₆F₅)₃ (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3-5.9 10⁶ g_polym. mol_Zn⁻¹ h⁻¹) and high molecular weight (M_n up to 132 10³ g mol⁻¹) for CHO polymerization. Although the in situ B(C₆F₅)₃-activated zinc species were not isolated, the sterically demanding BOX ligands (1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42-88%).     

The structures of (phenylato)(N-2-thiophenecarboxamido-meso-tetraphenylporphyrinato)mercury(II) and bisphenylmercury(II) complex of 21-(4-tert-butyl-benzenesulfonamido)-5,10,15,20-tetraphenylporphyrin

The reaction of PhHgOAc with N-NHCO-2-C₄H₃S-Htpp (5) and N-p-HNSO2C6H4^tBu-Htpp (4) gave a mercury (II) complex of (phenylato) (N-2-thiophenecarboxamido-meso-tetra phenylporphyrinato)mercury(II) 1.5 methylene chloride solvate [HgPh(N-NHCO-2-C₄H₃S-tpp) · CH₂Cl₂ · 0.5C₆H₁₄;  6 · CH₂Cl₂ · 0.5C₆H₁₄] and a bismercury complex of bisphenylmercury(II) complex of 21-(4-tert-butyl-benzenesulfonamido)-5,10,15,20-tetraphenylporphyrin, [(HgPh)2(N-p-NSO₂C₆H₄^tBu-tpp); 7], respectively. The crystal structures of 6 · CH₂Cl₂ · 0.5C₆H₁₄ and 7 were determined. The coordination sphere around Hg(1) in 6 · CH₂Cl₂ · 0.5C₆H₁₄ and Hg(2) in 7 is a sitting-atop derivative with a seesaw geometry, whereas for the Hg(1) in 7, it is a linear coordination geometry. Both Hg(1) in 6 · CH₂Cl₂ · 0.5C₆H₁₄ and Hg(2) in 7 acquire 4-coordination with four strong bonds [Hg(1)–N(1) = 2.586(3) Å, Hg(1)–N(2) = 2.118(3) Å, Hg(1)–N(3) = 2.625(3) Å, and Hg(1)–C(50) = 2.049(4) Å for 6 · CH₂Cl₂ · 0.5C₆H₁₄; Hg(2)–N(1) = 2.566(6) Å, Hg(2)–N(2) = 2.155(6) Å, Hg(2)–N() = 2.583(6) Å, and Hg(2)–C(61) = 2.064(7) Å for 7]. The plane of the three pyrrole nitrogen atoms [i.e., N(1)–N(3)] strongly bonded to Hg(1) in 6 · CH₂Cl₂ · 0.5C₆H₁₄ and to Hg(2) in 7 is adopted as a reference plane 3N. For the Hg²⁺ complex in 6 · CH₂Cl₂ · 0.5C₆H₁₄, the pyrrole nitrogen bonded to the 2-thiophenecarboxamido ligand lies in a plane with a dihedral angle of 33.4° with respect to the 3N plane, but for the bismercury(II) complex in 7, the corresponding dihedral angle for the pyrrole nitrogen bonded to the NSO₂C₆H₄^tBu group is found to be 42.9°. In the former complex, Hg(1)²⁺ and N(5) are located on different sides at 1.47 and −1.29 Å from its 3N plane, and in the latter one, Hg(2)²⁺ and N(5) are also located on different sides at −1.49 and 1.36 Å form its 3N plane. The Hg(1)?Hg(2) distance in 7 is 3.622(6) Å. Hence, no metallophilic Hg(II)?Hg(II) interaction may be anticipated. NOE difference spectroscopy, HMQC and HMBC were employed to unambiguous assignment for the ¹H and ¹³C NMR resonances of 6 · CH₂Cl₂ ·  0.5C₆H₁₄ in CD₂Cl₂ and 7 in CDCl₃ at 20 °C. The ¹⁹⁹Hg chemical shift δ for a 0.05 M solution of 7 in CDCl₃ solution is observed at −1074 ppm for Hg(2) nucleus with a coordination number of four and at −1191 ppm for Hg(1) nucleus with a coordination number of two. The former resonance is consistent with that chemical shift for a 0.01 M solution of 6 in CD₂Cl₂ having observed at −1108 ppm for Hg(1) nucleus with a coordination number of four.     

Novel Ag(I), Pd(II), Ni(II) complexes of N,N′-bis-(2,2-diethoxyethyl)imidazole-2-ylidene: Synthesis, structures, and their catalytic activity towards Heck reaction

Tetra-ether substituted imidazolium salts, LHX (where LH = N,N′-bis(2,2-diethoxyethyl)imidazolium cation and X = Br, BF₄, PF₆, BPh₄, NO₃ and NTf₂ anions) were derived from imidazole. Attempts to produce aldehyde functionalized imidazolium salt through acid hydrolysis of LHBr resulted an unexpected tetra-hydroxy compound L_AHBr and the dialdehyde compound L_BHBr. Reaction of LHBr with Ag₂O afforded [L₂Ag][AgBr₂] (1). Mononuclear Pd-complex trans-[L₂PdCl₂] (2) and dinuclear Pd-complex [(LPdCl₂)₂] (3) were obtained by 1:1 and 1:2 reaction of in situ generated Ag-carbene with Pd(CH₃CN)₂Cl₂. cis-[LPdPPh₃Cl₂] (4) was synthesized from reaction of PPh₃ with dinuclear complex 3. Hydrolysis of 3 under acidic conditions also generates a hydroxy derivative 3A and the aldehyde derivative 3B. Direct heating of LHBr with Ni(OAc)₂ · 4H₂O at 120 °C under vacuum generated trans-[L₂NiBr₂] (5). These complexes were characterized by NMR, mass, elemental analysis, and X-ray single crystal diffraction analysis. Pd--Pd interaction was observed in 3. All the Pd complexes exhibited excellent catalytic activity in Heck reaction.