Improved cooperativity of spin-labile iron(III) centers by self-assembly in solution

Supramolecular principles have been applied for improving the spin crossover activity of metal centers due to cooperative effects in solution. Thus, incorporation of alkyloxy tails at the phenyl group of Fe(sal2trien) 2a provides amphiphilic complexes Fe(sal-OR2trien) 2b-d (b, R = C6H13; c, R = C8H17; d, R = C18H37) comprising an apolar group for supramolecular organization and a polar headgroup with potential spin crossover activity due to the presence of a spin-labile iron(III) center. Self-assembly of these complexes in solution resulted in the formation of microsize and submicrosize particles when the alkyl chain was long enough (2d) but not with shorter chains (2a-c). Solutions of 2d showed enhanced spin crossover activity as compared to complexes 2a-c, both in terms of transition temperature and steepness of the transition. This observation has been correlated to an improved cooperativity of the metal centers in 2d due to self-assembly, thus facilitating a tandem spin transition.     

Insertion of a spin crossover Fe(III) complex into an oxalate-based layered material: coexistence of spin canting and spin crossover in a hybrid magnet

The syntheses, structures, and magnetic properties of the compounds of formula [Fe (III)(sal 2trien)] 2[Mn (II) 2(ox) 3].4H 2O.C 3H 7NO ( 1) and [In (III)(sal 2trien)] 2[Mn (II) 2(ox) 3].3H 2O.CH 3OH (2) are reported. The structure presents a homometallic 2D honeycomb anionic layer formed by Mn (II) ions linked through oxalate ligands and a cationic double layer of [Fe(sal 2trien)] (+) or [In(sal 2trien)] (+) complexes intercalated between the 2D oxalate network. The magnetic properties and M?ssbauer spectroscopy of 1 indicate the coexistence of a magnetic ordering of the Mn(II) oxalate network that behaves as a weak ferromagnet and a gradual spin crossover of the intercalated [Fe(sal 2trien)] (+) complexes.     

Hydrogencyanamide-bridged one-dimensional polymers built on Mn(III)-Schiff base fragments: synthesis, structure, and magnetism

The ability of NCNH(-) to construct transition metal coordination polymers and to transmit magnetic coupling was investigated. By introduction of various tetradentate Schiff base ligands (L) and different solvents (S), nine NCNH(-)-bridged manganese(III) coordination complexes were obtained. Their structures can be divided into three types: I) NCNH-bridged chains built on mononuclear [Mn(III)(L)] units, [Mn(III)(L)(mu(1,3)-NCNH)](n) (L=5-Brsalen (1), 5-Clsalen (2)); II) NCNH-bridged chains built on dinuclear [Mn(III) (2)(L)(2)] units, complexes 3-8, [Mn(III) (2)(L)(2)(mu(1,3)-NCNH)]ClO(4)S (L=salen, 5-Fsalen, 5-Clsalen, 5-OCH(3)salen; S=CH(3)OH or C(2)H(5)OH); III) NCNH-bridged Mn(III) dimers linked by hydrogen bonds into a 1D polymer, {[Mn(III)(3-OCH(3)salen)(H(2)O)](2)(mu(1,3)-NCNH)}ClO(4) x 0.5 H(2)O (9, salen=N,N'-bis(salicylidene)-1,2-diaminoethane). In these complexes, the N[triple chemical bond]C--NH(-) resonance structure dominates the bonding mode of the NCNH(-) ligand adopting the mu(1,3)-bridging mode. Magnetic characterization shows that the asymmetric NCNH(-) bridge transmits antiferromagnetic interaction between Mn(III) ions and often favors the weak ferromagnetism caused by spin canting in these one-dimensional chains. However, these complexes exhibit different magnetic behaviors at low temperatures.     

A magnetic iron(III) switch with controlled and adjustable thermal response for solution processing

Spin crossover requires cooperative behavior of the metal centers in order to become useful for devices. While cooperativity is barely predictable in solids, we show here that solution processing and the covalent introduction of molecular recognition sites allows the spin crossover of iron(III) sal(2)trien complexes to be rationally tuned. A simple correlation between the number of molecular recognition sites and the spin crossover temperature enabled the fabrication of materials that are magnetically bistable at room temperature. The predictable behavior relies on combining function (spin switching) and structure (supramolecular assembly) through covalent interactions in a single molecular building block.     

Synthesis and Characterization of New Chiral Schiff Base Complexes with Diiminobinaphthyl or Diiminocyclohexyl Moieties as Potential Enantioselective Epoxidation Catalysts

New chiral Schiff base complexes have been obtained by condensation of 2,2'-diamino-1,1'-binaphthalene or 1,2-diaminocyclohexane and various salicylaldehydes and by subsequent metalation with manganese, iron, cobalt, nickel, copper, or zinc. The complete (1)H and (13)C NMR characterization of the ligands is reported, as are the X-ray crystal structures of (1R,2R)-(-)-N,N'-bis[3-(N,N-dimethylamino)salicylidene]-trans-1,2-cyclohexanediimine and [(1R,2R)-(-)-N,N'-bis(salicylidene)-trans-1,2-cyclohexanediiminato]copper(II). The new chiral manganese complexes have been evaluated in the oxygenation of prochiral olefins and sulfides using sodium hypochlorite, hydrogen peroxide, or N-methylmorpholine N-oxide/m-chloroperbenzoic acid as oxidant.     

X-ray crystallographic approach to the design of phenolic schiff base reagents for the mutual separation of lanthanoids.

A novel mutual selectivity of lanthanoids in the N,N'-bis(5-nitrosalicylidene)ethylenediamine (H2Nsalen)-KCl and N,N'-bis(5-nitrosalicylidene)-o-phenylenediamine (H2Nsaloph)-KCl extraction systems was evaluated by the X-ray analysis of similar model complexes for extracted species. Cerium(IV) complexes with N,N'-bis(5-chlorosalicylidene)ethylenediamine (H2Clsalen), N,N'-bis(salicylidene)-o-phenylenediamine (H2saloph) and N,N'-bis(3,5-dibromosalicylidene)-o-phenylenediamine (H2Br2saloph) were selected as the models. The result of the X-ray analysis suggested that [Ln(III)(Nsalen)2]- and [Ln(III)(Nsaloph)2]- are meridional type (two ligands are oriented perpendicular to each other) and sandwich type (two ligands are oriented parallel to each other), respectively. It was suggested that the selectivity of the meridional structure is superior to that of the sandwich structure in these extraction systems.     

End-to-end azide-bridged manganese(III) chain compounds: field-induced magnetic phase transitions and variation of T(C) to 38 K depending on the side groups of the Schiff bases

Three one-dimensional coordination polymers [Mn(L)(N(3))](n) [L = L1 (1), L2 (2), L3 (3); L1H(2) = N,N'-bis(5-chlorosalicylideneiminato)-1,3-diaminopentane, L2H(2) = N,N'-bis(5-bromosalicylideneiminato)-1,3-diaminopentane, L3H(2) = N,N'-bis(5-bromosalicylideneiminato)-1,3-diamino-2-dimethylpropane] bridged by end-to-end azides were prepared. The crystal systems differ according to the Schiff bases used. Each Mn atom adopts a typical Jahn-Teller distortion. The helicity of the chains occurs in a racemic manner only for 2. No noncovalent forces are relevant in 2, while π-π contacts are visible in 1 and 3. Magnetic measurements show the presence of apparent spin canting. Complexes 1 and 3 exhibit a field-induced metamagnetic transition from an antiferromagnetic state to a weak ferromagnetic phase, whereas 2 embraces a field-induced two-step magnetic phase transition. The critical temperature is observed at 38 K for 2, which is relatively higher than those for 1 (11 K) and 3 (10 K). The pronounced long-range order may contribute from intrachain exchange couplings and through-space dipolar interactions between adjacent chains.     

General synthesis of (salen)ruthenium(III) complexes via N...N coupling of (salen)ruthenium(VI) nitrides

Reaction of [Ru (VI)(N)(L (1))(MeOH)] (+) (L (1) = N, N'-bis(salicylidene)- o-cyclohexylenediamine dianion) with excess pyridine in CH 3CN produces [Ru (III)(L (1))(py) 2] (+) and N 2. The proposed mechanism involves initial equilibrium formation of [Ru (VI)(N)(L (1))(py)] (+), which undergoes rapid N...N coupling to produce [(py)(L (1))Ru (III) N N-Ru (III)(L (1))(py)] (2+); this is followed by pyridine substituion to give the final product. This ligand-induced N...N coupling of Ru (VI)N is utilized in the preparation of a series of new ruthenium(III) salen complexes, [Ru (III)(L)(X) 2] (+/-) (L = salen ligand; X = H 2O, 1-MeIm, py, Me 2SO, PhNH 2, ( t )BuNH 2, Cl (-) or CN (-)). The structures of [Ru (III)(L (1))(NH 2Ph) 2](PF 6) ( 6), K[Ru (III)(L (1))(CN) 2] ( 9), [Ru (III)(L (2))(NCCH 3) 2][Au (I)(CN) 2] ( 11) (L (2) = N, N'-bis(salicylidene)- o-phenylenediamine dianion) and [N ( n )Bu 4][Ru (III)(L (3))Cl 2] ( 12) (L (3) = N, N'-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.     

Iron(II) complexes with tetradentate bis(aminophenolate) ligands: synthesis and characterization, solution behavior, and reactivity with O(2)

Tetradentate bis(aminophenolate) ligands H(2)salan(X) and H(2)bapen(X) (where X refers to the para-phenolate substituent = H, Me, F, Cl) react with [Fe{N(SiMe(3))(2)}(2)] to form iron(II) complexes, which in the presence of suitable donor ligands L (L = pyridine or THF) can be isolated as the complexes [Fe(salan(X))(L)(2)] and [Fe(bapen(X))(L)(2)]. In the absence of donor ligands, either mononuclear complexes, for example, [Fe(salan(tBu,tBu))], or dinuclear complexes of the type [Fe(salan(X))](2) are obtained. The dynamic coordination behavior in solution of the complexes [Fe(salan(F))(L)(2)] and [Fe(bapen(F))(L)(2)] has been investigated by VT (1)H and (19)F NMR spectroscopy, which has revealed equilibria between isomers with different ligand coordination topologies cis-α, cis-β and trans. Exposure of the iron(II) salan(X) complexes to O(2) results in the formation of oxo-bridged iron(III) complexes of the type [{Fe(salan(X))}(2)(μ-O)] or [{Fe(salan(X))(L)}(2)(μ-O)]. The lack of catalytic activity of the iron(II) salan and bapen complexes in the oxidation of cyclohexane with H(2)O(2) as the oxidant is attributed to the rapid formation of stable and catalytically inactive oxo-bridged iron(III) complexes.     

Synthesis, characterization, and photoresponsive properties of a series of Mo(IV)-Cu(II) complexes

Six Mo(IV)-Cu(II) complexes, [Cu(tpa)](2)[Mo(CN)(8)]·15H(2)O (1, tpa = tris(2-pyridylmethyl)amine), [Cu(tren)](2)[Mo(CN)(8)]·5.25H(2)O (2, tren = tris(2-aminoethyl)amine), [Cu(en)(2)][Cu(0.5)(en)][Cu(0.5)(en)(H(2)O)][Mo(CN)(8)]·4H(2)O (3, en = ethylenediamine), [Cu(bapa)](3)[Mo(CN)(8)](1.5)·12.5H(2)O (4, bapa = bis(3-aminopropyl)amine), [Cu(bapen)](2)[Mo(CN)(8)]·4H(2)O (5, bapen = N,N'-bis(3-aminopropyl)ethylenediamine), and [Cu(pn)(2)][Cu(pn)][Mo(CN)(8)]·3.5H(2)O (6, pn = 1,3-diaminopropane), were synthesized and characterized. Single-crystal X-ray diffraction analyses show that 1-6 have different structures varying from trinuclear clusters (1-2), a one-dimensional belt (3), two-dimensional grids (4-5), to a three-dimensional structure (6). Magnetic and ESR measurements suggest that 1-6 exhibit thermally reversible photoresponsive properties on UV light irradiation through a Mo(IV)-to-Cu(II) charge transfer mechanism. A trinuclear compound [Cu(II)(tpa)](2)[Mo(V)(CN)(8)](ClO(4)) (7) was synthesized as a model of the photoinduced intermediate.     

Intramolecular Gas-Phase Reactions of Synthetic Nonheme Oxoiron(IV) Ions: Proximity and Spin-State Reactivity Rules

The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV) (O)(L)(OTf)](+) (OTf=trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8) Py(2) ) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV) (O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV) (O)(L)](2+) cations. DFT calculations on [Fe(IV) (O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C?H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ(CH) -σ*?z?2 overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa- and pentacoordinate complexes.     

Influence of electron-attractor substituents on the magnetic properties of Ni(II) string complexes

Density functional theory calculations carried out on the anionic Ni(II) chain complexes {trinickeltetrakis[2,6-bis(amido)pyridine]}2- and {trinickeltetrakis[N,N'-bis(sulfonyl)pyridyldiamido]}2- show that the electron-attractor character of the sulfonyl substituent modifies the basicity of the amido ligand ends so as to raise the electronic state of the underlying metal atom from low spin (S = 0) to high spin (S = 1). The computed elongation of the outermost Ni-N bonds with the grafting of SO2H substituents is in agreement with this interpretation.     

Synthesis, characterization and coordination properties of bis(alkyl)selenosalen ligands

New bis (alkyl) selenosalen podand ligands having Se2N2 donor sites have been synthesized by the condensation of unsymmetrical o-formylphenyl alkyl selenide (1-3) with ethylenediamine. The reaction of bis(alkyl)selenosalen podands with Pd(II) and Pt(II) afforded selenoether-selenolate coordination complexes 7-10via cleavage of one of the two Se-C(alkyl) bonds of bis(alkyl)selenosalen podands upon complexation. DFT calculations revealed that the cleavage of Se-C(alkyl) bonds occurred possibly via S(N)2 mechanism instead of a sequence of oxidative addition and reductive elimination reactions. The spectral data and elemental analyses confirmed the formation of selenoether-selenolate complexes. The structures of the podands N,N'-bis[(2-methylseleno)phenylmethylene]-1,2-ethanediamine (4), N,N'-bis[(2-decylseleno)phenylmethylene]-1,2-ethanediamine (5) and the selenoether-selenolate complex 8 have been determined by single crystal X-ray diffraction analysis. The crystal structure of 5 showed SeH interaction with a ladder like 3D supramolecular arrangement via interdigitation of long alkyl chains. Comparison of crystal packing of podands 4 and 5 indicates that the alkyl chain length has significant impact on the crystal packing. The platinum selenolate complex 8 shows a square planar arrangement around the Pt centre, where the Se atoms in the selenolate and the selenoether have nearly equal Pt-Se bond length.     

Iron-catalyzed oxidation of thioethers by iodosylarenes: stereoselectivity and reaction mechanism

Catalytic properties of a series of iron(III)-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) and related complexes in asymmetric sulfoxidation reactions, with iodosylarenes as terminal oxidants, have been explored. These catalysts have been found to efficiently catalyze oxidation of alkyl aryl sulfides to sulfoxides with high chemoselectivity (up to 100 %) and moderate-to-high enantioselectivity (up to 84 % with isopropylthiobenzene and iodosylmesitylene), the TON (TON=turnover number) approaching 500. The influence of the ligand (electronic and steric effects of the substituents), oxidant, and substrate structures on the oxidation stereoselectivity has been investigated systematically. The structure of the reactive intermediates (complexes of the type [Fe(III)(ArIO)(salen)] and the reaction mechanism have been revealed by both mechanistic studies with different iodosylarenes and direct in situ (1)H NMR observation of the formation of the reactive species and its reaction with the substrate.     

Unique spin transition and wide thermal hysteresis loop for a cobalt(II) compound with long alkyl chain

The cobalt(II) compounds with long alkyl chains, [Co(C12-terpy)(2)](BF(4))(2)·EtOH·0.5H(2)O(1·EtOH·0.5H(2)O) and [Co(C12-terpy)(2)](BF(4))(2) (1) was synthesized and characterized. The compound 1·EtOH·0.5H(2)O exhibits a "re-entrant spin crossover". The compound 1 exhibits the reentrant spin crossover and multi phase transitions with a wide thermal hysteresis loop.     

Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes

Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymerization (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH(2)=N,N'-bis(salicylidene)ethylene-1,2-diamine; homosalenH(2)=N,N'-bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepared in situ are pure enough to apply to the polymerizations without purification. The crystal structures of the key complexes are elucidated by X-ray diffraction, which confirms that they are chiral. However, analysis of the (1)H and (13)C NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in solution at 25 degrees C and that the complexes are achiral under the polymerization conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-dimethyl substituents in the backbone (ArCH==NCH(2)CMe(2)CH(2)N==CHAr), whereas tBuMe(2)Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (P(meso)=0.9(8); T(m)=210 degrees C). The ratio of the rate constants in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymerization, which is the most attractive in industry, although with some loss of stereoselectivity at high temperature, and the afforded polymer shows a higher melting temperature (P(meso)=0.9(2), T(m) up to 189 degrees C) than that of homochiral poly(L-lactide) (T(m)=162-180 degrees C). The "livingness" of the bulk polymerization at 130 degrees C is maintained even at a high conversion (97-98 %) and for an extended polymerization time (1-2 h).     

Tuning of the emission efficiency and HOMO-LUMO band gap for ester-functionalized {Al(salophen)(H2O)2}+ blue luminophors

A series of [AlL(H(2)O)(2)(NO(3))] complexes, with L standing for an ester substituted salophen-type ligand, has been synthesized, and the luminescence properties have been investigated. These derivatives differ by the nature of the ester-R group introduced at the C5 position of their salicylidene rings (i.e., phenyl, 7a,a'; naphthyl, 7b,b'; pentafluorophenyl, 7c,c'; and p-nitrophenyl, 7d) and by the bis-imino bridge (i.e., 1,2- phenylene, 7a-d; and 1,2-naphthalene, 7a'-c'). All the complexes are characterized by luminescence in the blue range, the chemical diversity having no effect on the emission wavelength (480-485 nm). However, the emission efficiency was found to be strongly dependent on the Schiff-base ligand with quantum yields ranging from ? = 22% to 44%, the highest values being for the salophen derivatives with the electron-withdrawing ester-R groups (7a, 34%; 7a', 23%; 7b, 31%; 7b', 22%; 7c, 40%; 7c', 29%, and 7d, 44%). Both the electrochemical data and DFT calculations show that the HOMO-LUMO band gap is modified as a function of the ester R group (from 2.92 to 3.16 eV, based on the redox potentials). The crystal structures for the N,N'-bis(5-(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine and the N,N'-bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phenylenediamine aluminum complexes (7a and 7d) are reported.     

Cobalt(II) complexes of new biomimetic polydentate amide: spectroscopic, kinetics of thermal decomposition and XRPD studies

Complexes of Co(II) with new ligands N',N'-bis(3-carboxy-1-oxoprop-2-enyl) 2-amino-N-arylbenzamidine (C(21)H(17)N(3)O(6)), N',N'-bis(3-carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (C(21)H(21)N(3)O(6)) and N',N'-bis(3-carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine (C(29)H(21)N(3)O(6)) have been synthesized and characterized by elemental analyses, vibrational spectra, electronic spectra, TOF-mass spectra, magnetic susceptibility measurements, thermal studies and X-ray powder diffraction studies. Vibrational spectra indicate coordination of amide and carboxylate oxygen of the ligands along with two water molecules giving a MO(6) weak field octahedral chromophore. Electronic spectra and magnetic susceptibility measurements reveal octahedral geometry for Co(II) complexes. The elemental analyses and mass spectral data have justified the ML complexes. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first-order kinetics. Powder diffraction determines the cell parameters of the complexes.     

Homochiral nickel coordination polymers based on salen(Ni) metalloligands: synthesis, structure, and catalytic alkene epoxidation

One-dimensional (1D) homochiral nickel coordination polymers [Ni(3)(bpdc)(RR-L)(2)·(DMF)](n) (2R, RR-L = (R,R)-(-)-1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene), bpdc = 4,4'-biphenyldicarboxylic acid) and [Ni(3)(bpdc)(SS-L)(2)·(DMF)](n) (2S, SS-L = (S,S)-(-)-1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene) based on enantiopure pyridyl-functionalized salen(Ni) metalloligand units NiL ((1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene))Ni(II)) have been synthesized and characterized by microanalysis, IR spectroscopy, solid-state UV-vis spectroscopy, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, cyclic voltammetric measurement, and powder and single crystal X-ray diffraction. Each NiL as unbridging pendant metalloligand uses one terminal pyridyl group to coordinate achiral unit (nickel and bpdc(2-)) building a helical chain, while the other pyridyl group remains uncoordinated. Both 2R and 2S contain left- and right-handed helical chains made of the achiral building blocks, while the NiL as remote external chiral source is perpendicular to the backbone of the helices. The nickel coordination polymers 2R and 2S containing unsaturated active nickel center in metalloligand NiL can be used as self-supported heterogeneous catalysts. They show catalytic activity comparable with their homogeneous counterpart in alkene epoxidation and exhibit great potential as recyclable catalysts.