Extremely bent cyanide coordination at a preorganized dinickel site and assembly of a starlike nonanuclear complex from the constrained dinickel building blocks

An extremely bent cyanide coordination at a dinickel scaffold is reported. Preorganization of two nickel ions is achieved by means of a compartmental dinucleating pyrazolate ligand L(-), setting up a bimetallic coordination pocket with constrained metal-metal separation. The mixed-spin dinickel(II) complex [LNi2(CN)(MeCN)](ClO4)2 (1) has been characterized by X-ray diffraction. The MeCN bound to the high-spin nickel(II) ion can be removed or replaced by other ligands, e.g., by the cyanide ligand of a tetracyanonickelate(II) moiety to give the starlike nonanuclear complex ([LNi2(CN)]4[Ni(CN)4])(ClO4)6 (2) that contains four of the constrained pyrazolate-based dinickel(II) fragments grouped around a central tetracyanonickelate(II) unit, as revealed by X-ray crystallography. Spectral and electrochemical properties of 1 and 2 are reported, and the formation of reduced mixed-valent Ni(I)Ni(II) species is investigated by IR and UV/vis spectroelectrochemistry.     

Mechanism of formation of iron(II) complexes with pentadentate ligands via C-C bond formation between trans-[Fe(2,4-bis(2-pyridylmethylimino)pentane)(MeCN)2]ClO4]2.MeCN and various nitriles

The order of relative reactivity of nitriles for the formation of Fe(II) complexes (2s) with 3-(1-alkyl(or aryl)methyl)-1-imino-2,4-bis(2-pyridylmethylimine)(L(2)s) from that with 2,4-bis(2-pyridylmethylimono)pentane (L1), trans-[FeL(1)(MeCN)(2)][ClO(4)](2).MeCN (1), and various nitriles has been determined based on the following order: C(6)F(5)CN > 3,4-difluorobenzonitrile > 4-fluorobenzonitrile > C(6)H(5)CN > C(6)H(5)CH(2)CN > C(2)H(5)CN > MeCN > Me(2)CHCN >Me(3)CN. An iron(II) complex with L1 in a cis-configuration was prepared as the ternary complex [FeL(1)(bpy)][ClO(4)](2).1.5MeNO(2).0.5H(2)O, 3a (bpy = bipyridine). Compounds 2s and 3a undergo enantiomeric interconversion with an activation energy of ca. 60 kJ mol(-1).     

Hexanuclear Fe(III)2Co(III)2M(II)2 (M = Cu, Ni, Mn) clusters based on Kläui's tripodal ligand and tricyanometalates: syntheses, structures and magnetic properties

With the use of Kl?ui's tripodal ligand, [(Cp)Co(P(O)(OEt)(2))(3)](-) (L(CoEt), Cp = cyclopentadiene) as the auxiliary ligand to react with different metal salts and tricyanometalate building blocks, five neutral trimetallic hexanuclear complexes: [(Tp)(2)Fe(2)(CN)(6)Cu(2)(L(CoEt))(2)]·6H(2)O (1, Tp = hydridotris(pyrazolyl)borate), [(Tp*)(2)Fe(2)(CN)(6)Cu(2)(L(CoEt))(2)]·2H(2)O (2, Tp* = hydridotris(3,5-dimethyl-pyrazolyl)borate), [(pzTp)(2)Fe(2)(CN)(6)Cu(2)(L(CoEt))(2)]·H(2)O·3MeOH (3, pzTp = tetra(pyrazolyl)borate), [(Tp)(2)Fe(2)(CN)(6)Ni(2)(L(CoEt))(2)(MeCN)(2)]·2MeCN·2H(2)O (4) and [(Tp)(2)Fe(2)(CN)(6)Mn(2)(L(CoEt))(2)(MeCN)(2)]·2MeCN (5), have been obtained and structurally characterized. Magnetic measurements confirm that there are ferromagnetic couplings between the cyano-bridged Fe and Cu/or Ni ions and antiferromagnetic interaction between the cyano-bridged Fe and Mn ions. Slow relaxation of the magnetization is observed in complexes 1 and 4, while complex 3 exhibits metamagnetic behavior with a critical field of 17.5 kOe.     

Ligand effects on luminescence of new type blue light-emitting mono(2-phenylpyridinato)iridium(III) complexes

Newly prepared hydrido iridium(III) complexes [Ir(ppy)(PPh3)2(H)L](0,+) (ppy = bidentate 2-phenylpyridinato anionic ligand; L = MeCN (1b), CO (1c), CN(-) (1d); H being trans to the nitrogen of ppy ligand) emit blue light at the emission lambda(max) (452-457, 483-487 nm) significantly shorter than those (468, 495 nm) of the chloro complex Ir(ppy)(PPh3)2(H)(Cl) (1a). Replacing ppy of 1a-d with F2ppy (2,4-difluoro-2-phenylpyridinato anion) and F2Meppy (2,4-difluoro-2-phenyl-m-methylpyridinato anion) brings further blue-shifts down to the emission lambda(max) at 439-441 and 465-467 nm with CIE color coordinates being x = 0.16 and y = 0.18-0.20 to display a deep-blue photoemission. No significant blue shift is observed by replacing PPh3 of 1a with PPh2Me to produce Ir(ppy)(PPh2Me)2(H)(Cl) (1aPPh 2Me), which displays emission lambda max at 467 and 494 nm. The chloro complexes, [Ir(ppy)(PPh3)2(Cl)(L)](0,+) (L = MeCN (2b), CO (2c), CN(-) (2d)) having a chlorine ligand trans to the nitrogen of ppy also emit deep-blue light at emission lambda(max) 452-457 and 482-487 nm.     

First structural example of a metal uncoordinated mesoionic imidazo[1,5-a]pyridine and its precursor intermediate copper complex: an insight to the catalytic cycle

Four copper complexes of a tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl) methyleneimine, L(1) have been synthesized. All theses species, namely, [L(1)Cu(2)(SCN)(3)](n) (1), [Cu(SCN)(CH(3)CN)](n) (3), [(L(1))Cu(N(3))(Cl)] (4) and [(L(1))Cu(N(3))(SCN)] (5) have been structurally characterized. Complex 1 in acetonitrile promotes cycloaddition of a Cu(II) bound SCN(-) ion to L(1) that exclusively and stoichiometrically forms a mesoionic imidazo[1,5-a]pyridine, namely, 3-(imino-N'-2-methylthiophenyl)imidazo[1,5-a]pyridinium-1-thiolate (2) and a thiocyanato bridged Cu(i) complex, [Cu(SCN)(CH(3)CN)](n) (3). The X-ray crystal structure of 1 confirms the presence of square-pyramidal Cu(II) and tetrahedral Cu(I) ions in N(3)S(2) and N(2)S(2) coordination environments, respectively, bridged to each other via thiocyanate anion. The Cu(II) ions are bonded to a tridentate ligand L(1) and two SCN(-) ions occupy the remaining equatorial and an axial coordination site to adopt a square-pyramidal coordination geometry. To investigate which SCN(-) ion, axially or equatorially bound to Cu(II) center, underwent cycloaddition to L(1) to form 2, two mononuclear Cu(II) complexes 4 and 5 have been synthesized and their reactivity towards externally added KSCN was studied. The molecular structures of 4 and 5 feature a meridionally bound L(1) and an azide ion (N(3)(-)) in the square plane, while a Cl(-) or SCN(-) ion are occupying the axial site, respectively, to fulfill square-pyramidal coordination geometry. Complex 4 reacts with SCN(-) ion to form 5. That an MeCN solution of 5 itself, or of 5 in the presence of KSCN, does not produce 2, supports that possibly the Cu(II) bound equatorial SCN(-) ion is responsible for cycloaddition to L(1). Dark purple solid 2 has also been prepared (turnover number ~4 or 41% yield) efficiently following an alternative and easier one-pot synthesis procedure, that is from a mixture of KSCN and L(1) in the presence of a catalytic amount of anhydrous CuCl(2) (10 mol%) in MeCN in air. The X-ray crystal structure, (1)H NMR spectrum and solution conductivity measurements strongly support that 2 is mesoionic. An MeCN solution of 2 fluoresces at room temperature upon excitation at 240 nm with an emission maximum λ(em) at 470 nm, associated with a quantum yield of 0.16 with respect to a standard Rhodamine-6G fluorophore.     

A new family of spin-crossover complexes based on a FeII(tetrazolyl)4(MeCN)2-type core

A bitopic ligand 2-hydroxy-1-(tetrazol-1-yl)-3-(tetrazol-2-yl)propane (12pbtzOH) was synthesized and reacted with Fe(ClO4)(2).6H2O, giving a 1D coordination polymer {[Fe(12pbtzOH)2(CH3CN)2](ClO4)(2).2CH3CN} infinity that exhibits a high-spin to low-spin transition (T1/2(downward arrow)=T1/2(upward arrow) congruent with 104 K). This is an unprecedented example of an iron(II) complex containing Fe(tetrazolyl) 4(MeCN)2 cores.     

Architectural formation of a conjugated bimetallic Pd(II) complex via oxidative complexation and a tetracyclic Pd(II) complex via self-assembling complexation

The conjugated homobimetallic palladium(II) complex [(L1)Pd(qd)Pd(L1)] (qd = quinonediimine) was obtained in a one-pot reaction by the in-situ oxidative complexation of 1,4-phenylenediamine with the palladium(II) complex [(L1)Pd(MeCN)] (H2L1 = N,N'-bis(2-phenylethyl)-2,6-pyridinedicarboxamide) while in the absence of an additional ligand [(L1)Pd(MeCN)] was converted to the amide-bridged macrocyclic tetramer [Pd(L1)]4.     

Coordination chemistry of a new cofacial binucleating macropolycycle derived from 1,4,7-triazacyclononane

We have prepared and characterized a new phenol-based compartmental ligand (H(2)L) incorporating 1,4,7-triazacyclononane ([9]aneN(3)), and we have investigated its coordination behavior with Cu(II), Zn(II), Cd(II), and Pb(II). The protonation constants of the ligand and the thermodynamic stabilities of the 1:1 and 2:1 (metal/ligand) complexes with these metal ions have been investigated by means of potentiometric measurements in aqueous solutions. The mononuclear [M(L)] complexes show remarkably high stability suggesting that, along with the large number of nitrogen donors available for metal binding, deprotonated phenolic functions are also involved in binding the metal ion. The mononuclear complexes [M(L)] show a marked tendency to add a second metal ion to afford binuclear species. The formation of complexes [M(2)(H(2)L)](4+) occurs at neutral or slightly acidic pH and is generally followed by metal-assisted deprotonation of the phenolic groups to give [M(2)(HL)](3+) and [M(2)(L)](2+) in weakly basic solutions. The complexation properties of H(2)L have also been investigated in the solid state. Crystals suitable for X-ray structural analysis were obtained for the binuclear complexes [Cu(2)(L)](BF(4))(2).(1)/(2)MeCN (1), [Zn(2)(HL)](ClO(4))(3).(1)/(2)MeCN (2), and [Pb(2)(L)](ClO(4))(2).2MeCN (4). In 1 and 2, the phenolate O-donors do not bridge the two metal centers, which are, therefore, segregated each within an N(5)O-donor compartment. However, in the case of the binuclear complex [Pb(2)(L)](ClO(4))(2).2MeCN (4), the two Pb(II) centers are bridged by the phenolate oxygen atoms with each metal ion sited within an N(5)O(2)-donor compartment of L(2)(-), with a Pb.Pb distance of 3.9427(5) A.     

Probing the excited states of Ru(II) complexes with dipyrido[2,3-a:3',2'-c]phenazine: a transient resonance raman spectroscopy and computational study

The lifetimes and transient resonance Raman spectra for Ru(II) complexes with the dipyrido[2,3-a:3',2'-c]phenazine (ppb) ligand and substituted analogues have been measured. The effect of altering the Ru(II) center ([Ru(CN)4]2- versus [Ru(bpy)2]2+), of the complex, on the excited-state lifetimes and spectra has been considered. For [Ru(bpy)2L]2+ complexes the excited-state lifetimes range from 124 to 600 ns in MeCN depending on the substituents on the ppb ligand. For the [Ru(CN)4L]2- complexes the lifetimes in H2O are approximately 5 ns. The transient resonance Raman spectra for the MLCT excited states of these complexes have been measured. The data are analyzed by comparison with the resonance Raman spectra of the electrochemically reduced [(PPh3)2Cu(mu-L*-)Cu(PPh3)2]+ complexes. The vibrational spectra of the complexes have been modeled using DFT methods. For experimental ground-state vibrational spectra of the complexes the data may be compared to calculated spectra of the ligand or metal complex. It is found that the mean absolute deviation between experimental and calculated frequencies is less for the calculation on the respective metal complexes than for the ligand. For the transient resonance Raman spectra of the complexes the observed vibrational bands may be compared with those of the calculated ligand radical anion, the reduced complex [Ru(CN)4L*-]3-, or the triplet state of the complex. In terms of a correlation with the observed transient RR spectra, calculations on the metal complex models offered no significant improvement compared to those based on the ligand radical anion alone. In all cases small structural changes are predicted on going from the ground to excited state.     

Synthesis, structure and reactivity of palladium(II) complexes of chiral N-heterocyclic carbene-imine and -amine hybrid ligands

The synthesis and structures of chiral N-heterocyclic carbene (NHC)-N-donor complexes of silver(I) and palladium(II) are reported. The X-ray structure of an NHC-imine silver(I) complex [((nPr)CN(CHPh))AgBr](2) exhibits an Ag(2)Br(2) dimer motif where the imine group is not coordinated to the silver atom. Reaction between 2 and [PdCl(2)(MeCN)(2)] gives the palladium(II) complex [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) that contains a chelating NHC-imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) and [(kappa(2)-((Ph)(2)CH)CN(CHPh))PdCl(2)](4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC-amino palladium(II) complexes [(kappa(2)-(nPr)CN(H(2)))PdCl(2)](6) and [(kappa(2)-((Ph)(2)CH)CN(H(2)))PdCl(2)](7), respectively. In the solid state, complexes 6 and 7 both exhibit intermolecular hydrogen bonding between chlorine and an amino-hydrogen atom resulting in an infinite chain structure. Substitution of an amino hydrogen for an ethyl group gives the soluble complex [(kappa(2)-(iPr)CN((H)Et))PdCl(2)](12). Reaction between two equivalents of 2 and [PdCl(2)(MeCN)(2)] gives the di-NHC complex [(kappa(1)-(nPr)CN(CHPh))(2)PdCl(2)](5) that does not contain a coordinated imine as shown by single crystal X-ray diffraction. Conproportionation between 5 and an equivalent of [PdCl(2)(MeCN)(2)] to does not occur at temperatures up to 100 degrees C in CD(3)CN.     

Synthesis and structural characterization of a tetracopper(I) complex of 2,6-(bis(3- dimethylamino)-propyliminomethyl)-4- methylthiophenolate

The compound 2,6-diformyl-4-methylphenyl (1) has been made by hydrolysis of S-(2,6- diformyl-4-methylphenyl)dimethylthiocarbamate followed by oxidation with I₂. Condensation of 1 with 4 equiv. of 3-dimethylaminopropylamine and subsequent reduction with Na/benzophenone results in the sodium salt of the Schiff base binucleating ligand 2,6-(bis(3-dimethylamino)-propyliminomethyl)-4-methyl-thiophenolate (2, NaL). The ligand (L) binds 2 eq of Cu(I) and crystallizes from MeCN solution to give an air sensitive tetracopper(I) complex with the formula [Cu₄L₂(MeCN)](PF₆)₂ (3)·0.5 MeCN·0.25 MeOH as determined by X-ray crystallography. The complex contains three distorted trigonal planar and one distorted tetrahedral Cu¹ centers with Cu---S bond lengths short for their respective coordination geometries. In DMSO solution, ¹H NMR spectra of 3 reflect free ligand symmetry suggesting dissociation into dimers. Cyclic voltammetry of 3 in MeCN reveals one irreversible oxidation at 110 mV (vs SCE).     

Spontaneous reduction of a low-spin Fe(III) complex of a neutral pentadentate N(5) Schiff base ligand to the corresponding Fe(II) species in acetonitrile

The iron complexes of a designed pentadentate Schiff base ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy(3)) have been synthesized. The low-spin mononuclear Fe(III) complex [(SBPy(3))Fe(DMF)](ClO(4))(3) (2), though stable in the solid state, is spontaneously reduced to the corresponding Fe(II) species [(SBPy(3))Fe(MeCN)](2+) in MeCN. Fe(II) complex [(SBPy(3))Fe(MeCN)](BF(4))(2) (3) has been isolated independently and characterized by crystallography. Electrochemical studies indicate that SBPy(3), like other pentadentate polypyridine ligands, stabilizes the Fe(II) center to a great extent (E(1/2) = 1.01 V vs SCE in MeCN). This fact is responsible for the ready reduction of 2. It is evident that such reactivity has brought complications in the syntheses of iron complexes of polypyridine ligands reported in previous accounts. Very low solubility of 2 in MeOH has allowed isolation of analytically pure 2 in the present work. Storage of dilute methanolic solution of 2 results in the formation of the mu-oxo Fe(III) dimer [(SBPy(3))FeOFe(SBPy(3))](ClO(4))(4) (5), the structure of which has also been determined. Fe(II) complex 3 reacts with CN(-) to afford cyanide adduct [(SBPy(3))Fe(CN)](BF(4)) (4) but does not exhibit any reactivity toward NO. The azomethine moiety (CH=N-py) of 2 is rapidly oxidized by H(2)O(2) to a pyridine-2-carboxamido (C(=O)-N-py) unit and affords [(PaPy(3))Fe(MeCN)](ClO(4))(2) (1), a complex previously reported by us.     

Luminescent complexes of Re(I) and Ru(II) with appended macrocycle groups derived from 5,6-dihydroxyphenanthroline: cation and anion binding

A range of ligands in which a macrocyclic unit is fused to a 1,10-phenanthroline unit has been prepared starting from 5,6-dihydroxyphenanthroline. The ligands are L1 in which the pendant ligand is 18-crown-6; L2, in which the pendant ligand is benzo-24-crown-8; and L(3), in which the macrocycle contains two carboxamide units. Ligands L1 and L2 can bind Group 1 and 2 metal cations in their crown-ether cavities; L3 contains two H-bond (amide) donors and is suitable for anion-binding. Luminescent complexes of the form [Ru(bipy)2L]2+, [ReL(CO)3Cl] and [RuL(CN)4]2- were prepared and some were structurally characterised; their interactions with various guest species were investigated by luminescence and NMR spectroscopy. For complexes with the crown ethers (L1 and L2), binding of K+ was rather weak, but the electrostatic effect due to the charge on the host complex was clear with [RuL1(CN)4]2- binding K+ more strongly than [Ru(bipy)2L1]2+. Binding to the pendant crown ethers was much stronger with Ba2+, and both [ReL1(CO)3Cl] and [ReL2(CO)3Cl] showed substantial luminescence quenching in MeCN on addition of Ba2+ ions, with binding constants of 4.5 x 10(4) M(-1) for [ReL1(CO)3Cl]/Ba2+ and 1.3 x 10(5) M(-1) for [ReL2(CO)3Cl]/Ba2+. Complexes [Ru(bipy)2L3]2+ and [ReL3(CO)3Cl], due to their H-bond donor sites, showed binding of dihydrogenphosphate to the macrocycle. Whereas [ReL3(CO)3Cl] showed 1 : 1 binding with (H2PO4)- in dmso with a binding constant of 65 M(-1), [Ru(bipy)2L3]2+ showed 1 : 2 binding, with microscopic association constants of ca. 1 x 10(6) and 1.6 x 10(6) M(-1) in MeCN. The fact that K2 > K1 suggests a cooperative interaction whereby binding of the first anion makes binding of the second one easier to an extent which overcomes electrostatic effects, and a model for this is proposed which also accounts for the substantial increase in luminescence from [Ru(bipy)2L3]2+ (5-fold enhancement) when the second (H2PO4)- anion binds. Both [Ru(bipy)2L3]2+ and [ReL3(CO)3Cl] undergo complete luminescence quenching and a change in colour to near-black in the presence of (anhydrous) fluoride in MeCN, probably due to deprotonation of the carboxamide group. These changes are however irreversible on a long timescale and lead to slow decomposition.     

Synthesis,crystal structure,and magnetic properties of mu-hydroxo-bis[pentakis(acetonitrile)chromium(III)] tetrafluoroborate: an acetonitrile analogue to "acid rhodo"

The reaction of [Cr(NCCH(3))(6)](2+) with dioxygen in acetonitrile (MeCN) solution acidified with HBF(4) gave red crystals of the binuclear complex [(CH(3)CN)(5)Cr(OH)Cr(NCCH(3))(5)](BF(4))(5) (1). From the X-ray crystal structure of 1, the Cr-O-Cr angle was found to be 147.5(2) degrees. Magnetic susceptibility measurements of 1 showed an antiferromagnetic coupling between the two chromium(III) centers with a triplet energy J = 35.9(1) cm(-1). On redissolution of 1 in MeCN, the hydroxo bridge was deprotonated, and a green solution of the complex [(CH(3)CN)(5)CrOCr(NCCH(3))(5)](4+) formed. The electronic absorption spectrum of this solution is very similar to the spectrum of the classical complex [(H(3)N)(5)CrOCr(NH(3))(5)](4+) with intense bands in the UV and near-UV region. From the temperature dependence of the absorption spectrum near 12900 cm(-1), the triplet energy J was found to be 1067(19) cm(-1). The acidity of the hydroxo bridge in 1 is very high with an acid dissociation constant K(a) > 1 M.     

Structural variation, dynamics, and catalytic application of palladium(II) complexes of di-N-heterocyclic carbene-amine ligands

A series of palladium(II) complexes incorporating di-NHC-amine ligands has been prepared and their structural, dynamic and catalytic behaviour investigated. The complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))PdCl(2)] (12) and [trans-(kappa(2)-(Mes)CN(H)C(Mes))PdCl(2)] (13) do not exhibit interaction between the amine nitrogen and palladium atom respectively. NMR spectroscopy between -40 and 25 degrees C shows that the di-NHC-amine ligand is flexible expressing C(s) symmetry and for 13 rotation of the mesityl groups is prevented. In the related C(1) complex [(kappa(3)-(tBu)CN(H)C(tBu))PdCl][Cl] (14) coordination of NHC moieties and amine nitrogen atom is observed between -40 and 25 degrees C. Reaction between 12-14 and two equivalents of AgBF(4) in acetonitrile gives the analogous complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))Pd(MeCN)(2)][BF(4)](2) (15), [trans-(kappa(2)-(Mes)CN(H)C(Mes))Pd(MeCN)(2)][BF(4)](2) (16) and [(kappa(3)-(tBu)CN(H)C(tBu))Pd(MeCN)][BF(4)](2) (17) indicating that ligand structure determines amine coordination. The single crystal X-ray structures of 12, 17 and two ligand imidazolium salt precursors (tBu)C(H)N(Bn)C(H)(tBu)][Cl](2) (2) and [(tBu)C(H)N(H)C(H)(tBu)][BPh(4)](2) (4) have been determined. Complexes 12-14 and 15-17 have been shown to be active precatalysts for Heck and hydroamination reactions respectively.     

Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand

A novel nickel(II) complex [Ni(L)₂Cl]Cl with a bidentate phosphinopyridyl ligand 6‐((diphenylphosphino)methyl)pyridin‐2‐amine (L) was synthesized as a metal‐complex catalyst for hydrogen production from protons. The ligand can stabilize a low Ni oxidation state and has an amine base as a proton transfer site. The X‐ray structure analysis revealed a distorted square‐pyramidal Ni^II complex with two bidentate L ligands in a trans arrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the Ni^II complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400 s^?1 in 0.1 m [NBu₄][ClO₄]/MeCN in the presence of acetic acid (290 equiv) at an overpotential of circa 590 mV.     

Control of molecular architecture by the degree of deprotonation: self-assembled di- and tetranuclear copper(II) complexes of N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide

In the absence of added base, a deep navy-blue dimeric copper complex [CuII(H2L)(MeCN)]2(BF4)4 (1) of the non-deprotonated bis-terdentate diamide ligand H2L self-assembles whereas in the presence of base a grass-green [2 x 2] grid complex [CuII(HL)]4(BF4)4 (2) of the monodeprotonated ligand HL-, a rare example of a discrete grid of pyrazine-bridged metal ions, is formed.     

Electrochemical reduction of a bridging imide: generation of ammonia at a dimolybdenum tris(mu-thiolate) site

The electrochemical reduction of the imide complex [Mo2(cp)2(mu-SMe)3(mu-NH)]+ (1+) has been investigated in THF and MeCN electrolytes by cyclic voltammetry, controlled-potential electrolysis and coulometry. In the absence of free protons, the electrochemical reduction produces the amide derivative [Mo2(cp)2(mu-SMe)3(mu-NH2)] (2) after consumption of 1 Fmol(-1) of 1+. In THF in the presence of acid, the reduction of 1+ occurs through a two-electron process. The presence of acid also results in the shift of the equilibrium between 1+ and amide dication 2(2+) (MeCN electrolyte) or induces an isomerisation of the imide ligand (THF electrolyte). This allows the electrolysis to be conducted at a potential 600 mV less negative than the reduction potential of 1+. Controlled-potential electrolyses in the presence of acid (2 equiv HTsO) produce the ammine derivative. Ammonia is released from these compounds either by coordination of the solvent (MeCN electrolyte) or by the binding of chloride to the ammine-tosylate complex (electrolyses in THF in the presence of acid and chloride). The final products, isolated almost quantitatively (>95%), are [Mo2(cp)2(mu-SMe)3(MeCN)2]+ and [Mo2(cp)2(mu-SMe)3(mu-Cl)], respectively.     

New cyanometalate building units: synthesis and characterization of [Re(CN)(7)]3- and [Re(CN)(8)]3-

Two new cyanorhenate complexes of potential utility in constructing magnetic and photomagnetic materials are reported. Reaction of (Bu4N)CN with [ReCl6]2- in acetonitrile affords yellow (Bu4N)3[Re(CN)7] (1), featuring the pentagonal bipyramidal complex [Re(CN)7]3-. The spectral and magnetic properties of 1 indicate that the complex has an S = 1/2 ground state with considerable anisotropy in the g tensor. In aqueous solution, 1 reacts with Mn2+ ions to generate the three-dimensional cyano-bridged solid [fac-Mn(H2O)3][cis-Mn(H2O)2][Re(CN)7].3H2O (2) containing diamagnetic [Re(CN)7]4-. Addition of KIO4 to the reaction solution, originally intended to prevent reduction of the rhenium during solid formation, instead yields white (Bu4N)3[Re(CN)8] (3). As crystallized in K3[Re(CN)8].2MeCN (4.2MeCN), the diamagnetic [Re(CN)8]3- complex adopts a nearly perfect square antiprismatic coordination geometry. In solution, this species behaves analogously to the isoelectronic [M(CN)8]4- (M = Mo, W) complexes, apparently converting to a dodecahedral geometry and photooxidizing under UV radiation to give paramagnetic [Re(CN)8]2-.     

An Integrated approach (thermodynamic, structural, and computational) to the study of complexation of alkali-metal cations by a lower-rim calix[4]arene amide derivative in acetonitrile

The calix[4]arene secondary-amide derivative L was synthesized, and its complexation with alkali-metal cations in acetonitrile (MeCN) was studied by means of spectrophotometric, NMR, conductometric, and microcalorimetric titrations at 25 °C. The stability constants of the 1:1 (metal/ligand) complexes determined by different methods were in excellent agreement. For the complexation of M(+) (M = Li, Na, K) with L, both enthalpic and entropic contributions were favorable, with their values and mutual relations being quite strongly dependent on the cation. The enthalpic and overall stability was the largest in the case of the sodium complex. Molecular and crystal structures of free L, its methanol and MeCN solvates, the sodium complex, and its MeCN solvate were determined by single-crystal X-ray diffraction. The inclusion of a MeCN molecule in the calixarene hydrophobic cavity was observed both in solution and in the solid state. This specific interaction was found to be stronger in the case of metal complexes compared to the free ligand because of the better preorganization of the hydrophobic cone to accept the solvent molecule. Density functional theory calculations showed that the flattened cone conformation (C(2) point group) of L was generally more favorable than the square cone conformation (C(4) point group). In the complex with Na(+), L was in square cone conformation, whereas in its adduct with MeCN, the conformation was slightly distorted from the full symmetry. These conformations were in agreement with those observed in the solid state. The classical molecular dynamics simulations indicated that the MeCN molecule enters the L hydrophobic cavity of both the free ligand and its alkali-metal complexes. The inclusion of MeCN in the cone of free L was accompanied by the conformational change from C(2) to C(4) symmetry. As in solution studies, in the case of ML(+) complexes, an allosteric effect was observed: the ligand was already in the appropriate square cone conformation to bind the solvent molecule, allowing it to more easily and faster enter the calixarene cavity.