Dinuclear Metallacycles with Single M-X-M Bridges (X = Cl-, Br-; M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)): Strong Antiferromagnetic Superexchange Interactions.

A series of monochloride-bridged, dinuclear metallacycles of the general formula [M2(µ-Cl)(µ-L)2](ClO4)3 have been prepared using the third-generation, ditopic bis(pyrazolyl)methane ligands L = m-bis[bis(1-pyrazolyl)methyl]benzene (Lm), M = Cu(II), Zn(II), and L = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*), M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). These complexes were synthesized from the direct reactions of M(ClO4)2·6H2O, MCl2, and the ligand, Lm or Lm*, in the appropriate stoichiometric amounts. Three analogous complexes of the formula [M2(µ-Cl)(µ-L)2](BF4)3, L = Lm, M = Cu(II), and L = Lm*, M = Co(II), Cu(II), were prepared from the reaction of [M2(µ-F)(µ-L)2](BF4)3 and (CH3)3SiCl. The bromide-bridged complex [Cu2(µ-Br)(µ-Lm*)2](ClO4)3 was prepared by the first method. Three acyclic complexes, [Co2(µ-Lm)µ-Cl4], [Co2(µ-Lm*)Cl4], and [Co2(µ-Lm*)Br4], were also prepared. The structures of all [M2(µ-X)(µ-L)2]3+ (X = Cl-, Br-) complexes have two ditopic bis(pyrazolyl)methane ligands bridging two metals in a metallacyclic arrangement. The fifth coordination site of the distorted trigonal bipyramidal metal centers is filled by a bridging halide ligand that has an unusual linear or nearly linear M-X-M angle. The NMR spectra of [Zn2(µ-Cl)(µ-Lm*)2](ClO4)3 and especially [Cd2(µ-Cl)(µ-Lm*)2](ClO4)3 demonstrate that the metallacycle structure is maintained in solution. Solid state magnetic susceptibility data for the copper(II) compounds show very strong antiferromagnetic exchange interactions, with -J values of 536 cm-1 for [Cu2(µ-Cl)(µ-Lm)2](ClO4)3·xCH3CN, 720 cm-1 for [Cu2(µ-Cl)(µ-Lm*)2](ClO4)3, and 945 cm-1 for [Cu2(µ-Br)(µ-Lm*)2](ClO4)3·2CH3CN. Smaller but still substantial antiferromagnetic interactions are observed with other first row transition metals, with -J values of 98 cm-1 for [Ni2(µ-Cl)(µ-Lm*)2](ClO4)3, 55 cm-1 for [Co2(µ-Cl)(µ-Lm*)2](ClO4)3, and 34 cm-1 for [Fe2(µ-Cl)(µ-Lm*)2](ClO4)3. EPR spectra of [Cu2(µ-Cl)(µ-Lm*)2](BF4)3 confirm the dz2 ground state of copper(II). In addition, the sign of the zero-field splitting parameter D was determined to be positive for [Cu2(µ-F)(µ-Lm*)2](BF4)3. Electronic spectra of the copper(II) complexes as well as Mössbauer spectra of the iron(II) complexes were also studied in relation with the EPR spectra and magnetic properties, respectively. Density functional theory calculations were performed using ORCA, and exchange integral values were obtained that parallel but are slightly higher than the experimental values by about 30%.

Silver(I) and rhenium(I) metal complexes of a 2,2'-bipyridine-functionalized third-generation tris(pyrazolyl)methane ligand.

Heterotopic ligands offer the possibility of preparing polynuclear bimetallic complexes. Recent studies of heteroditopic ligands and their metal complexes have focused on novel supramolecular systems, biological activity, and the development of MRI contrast agents. The heteroditopic ligand Bipy-CH2-O-CH2-C(pz)3 (Bipy-L; Bipy is 2,2'-bipyridine and pz is pyrazolyl) reacts with AgBF4 to produce the coordination polymer catena-poly[[silver(I)-(µ-5-{[tris(pyrazol-1-yl)methoxy]methyl-κ2N,N'}-2,2'-bipyridine-κ2N,N')] diethyl ether hemisolvate], {[Ag(C22H20N8O)]BF4·0.5C4H10O}n, and with Re(CO)5Br to form the discrete compound bromidotricarbonyl(5-{[tris(pyrazol-1-yl)methoxy]methyl}-2,2'-bipyridine-κ2N,N')rhenium(I), [ReBr(C22H20N8O)(CO)3]. The silver(I) compound is a one-dimensional coordination polymer, built up by a κ2 coordination mode of the bipyridine group and a κ2-κ0 coordination mode of the -C(pz)3 donor set. In [ReBr(Bipy-L)(CO)3], the ligand coordinates only via the bipyridine end, leaving the -C(pz)3 donor set free for further coordination interactions.

A cadmium(II) coordination polymer formed from a third generation tetratopic tris(pyrazolyl)methane ligand.

The reaction of a third generation tetratopic tris(pyrazolyl)methane ligand, namely 1,2,4,5-{[2,2,2-tris(1H-pyrazol-1-yl)ethoxy]methyl}benzene {1,2,4,5-C6H2[CH2OCH2C(pz)3]4, L4}, and [Cd2(thf)5](BF4)4 (thf is tetrahydrofuran) produces the coordination polymer catena-poly[[[bis[acetonitrilecadmium(II)]-µ4-1,2,4,5-{[2,2,2-tris(1H-pyrazol-1-yl)ethoxy]methyl}benzene] tetrakis(tetrafluoroborate)]-diethyl ether-acetonitrile (1/2/2)], {[Cd2(CH3CN)2(C54H54N24O4)](BF4)4·2C4H10O·2CH3CN}n. The CdII center is coordinated in a κ3-fashion by one tris(pyrazolyl)methane group and in a κ2-κ0 fashion by another, while the sixth coordination site on the CdII cation is occupied by an acetonitrile molecule. This bonding mode of the ligand generates an infinite one-dimensional structure built upon 32-atom metallomacrocycles connected by the C6H2 spacer. This compound is isostructural with the silver(I) analogue of this ligand, i.e. {[Ag2(L4)](BF4)2·4CH3CN}n, thus showing the tendency of this system to form metal-based macrocyclic architectures.

Syntheses, structural, magnetic, and electron paramagnetic resonance studies of monobridged cyanide and azide dinuclear copper(II) complexes: antiferromagnetic superexchange interactions.

The reactions of Cu(ClO4)2 with NaCN and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*) yield [Cu2(µ-CN)(µ-Lm)2](ClO4)3 (1) and [Cu2(µ-CN)(µ-Lm*)2](ClO4)3 (3). In both, the cyanide ligand is linearly bridged (µ-1,2) leading to a separation of the two copper(II) ions of ca. 5 Å. The geometry around copper(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu2(µ-F)(µ-Lm)2](ClO4)3 and (CH3)3SiN3 yields [Cu2(µ-N3)(µ-Lm)2](ClO4)3 (2), where the azide adopts end-on (µ-1,1) coordination with a Cu-N-Cu angle of 138.0° and a distorted square pyramidal geometry about the copper(II) ions. Similar chemistry in the more sterically hindered Lm* system yielded only the coordination polymer [Cu2(µ-Lm*)(µ-N3)2(N3)2]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper(I) complex [Cu5(µ-I4)(µ-Lm*)2]I3. The complexes 1 and 3 show strong antiferromagnetic coupling, -J = 135 and 161 cm(-1), respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the dz(2) and the bridging ligand s and px orbitals. High field EPR studies confirmed the dz(2) ground state of the copper(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of D and E are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex 2 has -J = 223 cm(-1) and DFT calculations indicate a predominantly d(x(2)-y(2)) ground state.

Framework complexes of group 2 metals organized by homochiral rods and π···π stacking forces: a breathing supramolecular MOF.

The reactions of the potassium salts of the ligands (S)-2-(1,8-naphthalimido)propanoate (KL(ala)), (S)-2-(1,8-naphthalimido)-3-hydroxypropanoate (KL(ser)), and (R)-2-(1,8-naphthalimido)propanoate (KL(ala)*), enantiopure carboxylate ligands containing a 1,8-naphthalimide π···π stacking supramolecular tecton, and, in the case of L(ser)(-), an alcohol functional group with calcium or strontium nitrate under solvothermal conditions produce crystalline [Ca(L(ala))2(H2O)]·(H2O) (1); [Ca(L(ser))2]·(H2O)2 (2); [Sr(L(ala))2(H2O)]·(H2O)3 (3); [Sr(L(ala)*)2(H2O)]·(H2O)3 (3*); and [Sr(L(ser))2(H2O)] (5). Placing 3 under vacuum removes the interstitial waters to produce [Sr(L(ala))2(H2O)] (4) in a single-crystal to single-crystal transformation; introduction of water vapor to 4 leads to the reformation of crystalline 3. Each of these new complexes has a solid-state structure based on homochiral rod secondary building unit (SBUs) central cores. Supramolecular π···π stacking interactions between 1,8-naphthalimide rings link adjacent rod SBUs into three-dimensional structures for 1, 3, 4, and 5 and two-dimensional structure for 2. Compounds 1 and 3 have open one-dimensional channels along the crystallographic c axis that are occupied by disordered solvent. For 3, these channels close and open in the reversible single-crystal conversion to 4; the π···π stacking interactions of the naphthalimide rings facilitate this process by rotating and slipping. Infrared spectroscopy demonstrated that the rehydration of 4 with D2O leads to 3d8, and the process of dehydration and rehydration of 3d8 with H2O leads to 3, thus showing exchange of the coordinated water in this process. These forms of 3 and 4 were characterized by (1)H, (2)H, and (13)C solid-state NMR spectroscopy, and thermal and luminescence data are reported on all of the complexes.

Hydroxide-bridged cubane complexes of nickel(II) and cadmium(II): magnetic, EPR, and unusual dynamic properties.

The reactions of M(ClO4)2·xH2O (M = Ni(II) or Cd(II)) and m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M4(µ3-OH)4(µ-Lm)2(solvent)4](ClO4)4, where solvent = dimethylformamide, water, acetone. In the solid state the metal centers are in an octahedral coordination environment, two sites are occupied by pyrazolyl nitrogens from Lm, three sites are occupied by bridging hydroxides, and one site contains a weakly coordinated solvent molecule. A series of multinuclear, two-dimensional and variable-temperature NMR experiments showed that the cadmium(II) compound in acetonitrile-d3 has C2 symmetry and undergoes an unusual dynamic process at higher temperatures (ΔGLm‡ = 15.8 ± 0.8 kcal/mol at 25 °C) that equilibrates the pyrazolyl rings, the hydroxide hydrogens, and cadmium(II) centers. The proposed mechanism for this process combines two motions in the semirigid Lm ligand termed the "Columbia Twist and Flip:" twisting of the pyrazolyl rings along the Cpz­Cmethine bond and 180° ring flip of the phenylene spacer along the CPh­Cmethine bond. This dynamic process was also followed using the spin saturation method, as was the exchange of the hydroxide hydrogens with the trace water present in acetonitrile-d3. The nickel(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an S = 4 ground state, and the nickel(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni­O­Ni angles two types of interactions are observed: J1 = 9.1 cm(­1) (97.9 to 99.5°) and J2 = 2.1 cm(­1) (from 100.3 to 101.5°). "Broken symmetry" density functional theory calculations performed on a model of the nickel(II) compound support these observations.

Dinuclear metallacycles with single M-O(H)-M bridges [M = Fe(II), Co(II), Ni(II), Cu(II)]: effects of large bridging angles on structure and antiferromagnetic superexchange interactions.

The reactions of M(ClO4)2·xH2O and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*) in the presence of triethylamine lead to the formation of monohydroxide-bridged, dinuclear metallacycles of the formula [M2(µ-OH)(µ-Lm)2](ClO4)3 (M = Fe(II), Co(II), Cu(II)) or [M2(µ-OH)(µ-Lm*)2](ClO4)3 (M = Co(II), Ni(II), Cu(II)). With the exception of the complexes where the ligand is Lm and the metal is copper(II), all of these complexes have distorted trigonal bipyramidal geometry around the metal centers and unusual linear (Lm*) or nearly linear (Lm) M-O-M angles. For the two solvates of [Cu2(µ-OH)(µ-Lm)2](ClO4)3, the Cu-O-Cu angles are significantly bent and the geometry about the metal is distorted square pyramidal. All of the copper(II) complexes have structural distortions expected for the pseudo-Jahn-Teller effect. The two cobalt(II) complexes show moderate antiferromagnetic coupling, -J = 48-56 cm(-1), whereas the copper(II) complexes show very strong antiferromagnetic coupling, -J = 555-808 cm(-1). The largest coupling is observed for [Cu2(µ-OH)(µ-Lm*)2](ClO4)3, the complex with a Cu-O-Cu angle of 180°, such that the exchange interaction is transmitted through the dz(2) and the oxygen s and px orbitals. The interaction decreases, but it is still significant, as the Cu-O-Cu angle decreases and the character of the metal orbital becomes increasingly d(x(2)-y(2)). These intermediate geometries and magnetic interactions lead to spin Hamiltonian parameters for the copper(II) complexes in the EPR spectra that have large E/D ratios and one g matrix component very close to 2. Density functional theory calculations were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with the experiments.

NMR investigations of dinuclear, single-anion bridged copper(II) metallacycles: structure and antiferromagnetic behavior in solution.

The nuclear magnetic resonance (NMR) spectra of single-anion bridged, dinuclear copper(II) metallacycles [Cu2(µ-X)(µ-L)2](A)3 (L(m) = m-bis[bis(1-pyrazolyl)methyl]benzene: X = F(-), A = BF4(-); X = Cl(-), OH(-), A = ClO4(-); L(m)* = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene: X = CN(-), F(-), Cl(-), OH(-), Br(-), A = ClO4(-)) have relatively sharp (1)H and (13)C NMR resonances with small hyperfine shifts due to the strong antiferromagnetic superexchange interactions between the two S = 1/2 metal centers. The complete assignments of these spectra, except X = CN(-), have been made through a series of NMR experiments: (1)H-(1)H COSY, (1)H-(13)C HSQC, (1)H-(13)C HMBC, T1 measurements and variable-temperature (1)H NMR. The T1 measurements accurately determine the Cu···H distances in these molecules. In solution, the temperature dependence of the chemical shifts correlate with the population of the paramagnetic triplet (S = 1) and diamagnetic singlet (S = 0) states. This correlation allows the determination of antiferromagnetic exchange coupling constants, -J (H = -JS1S2), in solution for the L(m) compounds 338(F(-)), 460(Cl(-)), 542(OH(-)), for the L(m)* compounds 128(CN(-)), 329(F(-)), 717(Cl(-)), 823(OH(-)), and 944(Br(-)) cm(-1), respectively. These values are of similar magnitudes to those previously measured in the solid state (-Jsolid = 365, 536, 555, 160, 340, 720, 808, and 945 cm(-1), respectively). This method of using NMR to determine -J values in solution is an accurate and convenient method for complexes with strong antiferromagnetic superexchange interactions. In addition, the similarity between the solution and solid-state -J values of these complexes confirms the information gained from the T1 measurements: the structures are similar in the two states.

Tris(pyrazolyl)methane and 1,8-naphthalimide-functionalized dialkynylgold(I) anionic complexes.

The reaction of tetrapropylammonium bis(acetylacetonato)gold(I) with alkyne derivatives of the tris(pyrazolyl)methane and 1,8-naphthalimide functional groups yielded two new compounds, both bridged by the linear C[triple-bond]C-Au-C[triple-bond]C spacer, namely tetrapropylammonium bis{3-[2,2,2-tris(1H-pyrazol-1-yl)ethoxy]prop-1-yn-1-yl}aurate(I), (C16H28N)[Au(C14H13N6O)2], and tetrapropylammonium {η(2)-µ-3-[2,4-dioxo-3-azatricyclo[7.3.1.0(5,13)]trideca-1(12),5,7,9(13),10-pentaen-3-yl]prop-1-yn-yl}bis{3-[2,4-dioxo-3-azatricyclo[7.3.1.0(5,13)]trideca-1(12),5,7,9(13),10-pentaen-3-yl]prop-1-yn-1-yl}digold(I) deuterochloroform disolvate, (C16H28N)[Au2(C15H8NO2)3]·2CDCl3. The alkyne-functionalized scorpionate ligand [Au{C[triple-bond]CCH2OCH2C(pz)3}2](-) features two potentially tridentate tris(pyrazolyl)methane donor groups oriented in a `trans' position relative to the C[triple-bond]C-Au-C[triple-bond]C spacer. The naphthalimide-containing compound comprises a σ-bonded NI-CH2-C[triple-bond]C-Au-C[triple-bond]C-CH2-NI unit (NI is the naphthalimide group) π-coordinated to an NI-CH2-C[triple-bond]C-Au neutral fragment. The crystal packing of this compound is supported by π-π stacking interactions of the NI unit, generating a three-dimensional network containing channels accommodating the tetrapropylammonium cations and deuterated chloroform solvent molecules.

Zinc(II) and cadmium(II) monohydroxide bridged, dinuclear metallacycles: a unique case of concerted double Berry pseudorotation.

The reactions of M(ClO4)2·6H2O [M = Zn(II), Cd(II)] and the ligands m-bis[bis(1-pyrazolyl)methyl]benzene, L(m), or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene, L(m)*, in the presence of a base yield the hydroxide bridged dinuclear metallacycles [M2(µ-OH)(µ-L)2](ClO4)3, L = L(m), M = Zn(II) (1); L = L(m)*, M = Zn(II) (2), Cd(II) (3). In the solid state, the coordination environment of the metals is distorted trigonal bipyramidal with the bridging hydroxide in an equatorial position and M-O-M angles greater than 161°. The observation of two equal intensity resonances for each type of pyrazolyl-ring hydrogen in the (1)H NMR for all three complexes coupled with the determination of the hydrodynamic radius based on the diffusion coefficient of 1 that matches that observed in the crystal structure, demonstrate this structure is retained in solution. Additional proof of the dinuclear structures in solution is given by the (113)Cd NMR spectrum of [Cd2(µ-OH)(µ-L(m)*)2](ClO4)3 showing (111/113)Cd satellites (J(111)(Cd-)(113)(Cd) = 173 Hz). Complex 1 is dynamic in solution, with the resonances for each type of pyrazolyl-ring hydrogen broadening and averaging at higher temperatures. Detailed variable temperature studies show that ΔG(pz)(‡) = 15.2(±0.2) kcal/mol, ΔH(pz)(‡) = 6.6(±0.1) kcal/mol, and ΔS(pz)(‡) = -28.8(±0.4) cal/mol·K at 25 °C for this process. The same ΔG(‡) value for the dynamic process was also determined by saturation transfer experiments. The most plausible mechanism for this dynamic process, which exchanges the axial and equatorial positions of the pyrazolyl rings in the trigonal bipyramidal arrangement, involves Berry pseudorotation at both metal sites using the bridging oxygen atom as the pivot ligand, coupled with the ring flip of the ligand's phenylene spacer by 180°, a rearrangement process we termed the "Columbia Twist and Flip". This process was shown to be influenced by trace amounts of water in the solvent, with a linear relationship between the water concentration and ΔG(pz)(‡); increasing the water concentration lowers ΔG(pz)(‡). Spin saturation transfer experiments demonstrated the exchange of the hydrogens between the water in the solvent and the bridging hydroxide group, with ΔG(OH)(‡) = 16.8(±0.2) kcal/mol at 25 °C, a value larger than the barrier of ΔG(pz)(‡) = 15.2(±0.2) kcal/mol for the "Columbia Twist and Flip". Compounds 2 and 3 do not show dynamic behavior involving the pyrazolyl-rings in solution because of steric crowding caused by the methyl group substitution, but do show the exchange between the water in the solvent and the bridging hydroxide group.

Homochiral helical metal-organic frameworks of group 1 metals.

The reactions of (S)-2-(1,8-naphthalimido)propanoic acid (HL(ala)) and (S)-2-(1,8-naphthalimido)-3-hydroxypropanoic acid (HL(ser)), protonated forms of ligands that contain a carboxylate donor group, an enantiopure chiral center, and a 1,8-naphthalimide π···π stacking supramolecular tecton and in the case of HL(ser) an alcohol functional group, with the appropriate alkali metal hydroxide followed by a variety of crystallization methods leads to the formation of crystalline K(L(ala))(MeOH) (1), K(L(ala))(H2O) (2), Na(L(ala))(H2O) (3), KL(ser) (4), CsL(ser) (5), and CsL(ala) (6). Each of these new complexes has a solid state structure based on six-coordinate metals linked into homochiral helical rod secondary building unit (SBU) central cores. In addition to the bonding of the carboxylate and solvent (in the case of L(ser) the ligand alcohol) to the metals, both oxygens on the 1,8-naphthalimide act as donor groups. One naphthalimide oxygen bonds to the same helical rod SBU as the carboxylate group of that ligand forming a chelate ring. The other naphthalimide oxygen bonds to adjacent SBUs. In complexes 1-3, this inter-rod link has a square arrangement bonding four other rods forming a three-dimensional enantiopure metal-organic framework (MOF) structure, whereas in 4-6 this link has a linear arrangement bonding two other rods forming a two-dimensional, sheet structure. In the latter case, the third dimension is supported exclusively by interdigitated π···π stacking interactions of the naphthalimide supramolecular tecton, forming enantiopure supramolecular MOF solids. Compounds 1-3 lose the coordinated solvent when heating above 100 °C. For 1, the polycrystalline powder reverts to 1 only by recrystallization from methanol, whereas compounds 2 and 3 undergo gas/solid, single-crystal to single-crystal transformations to form dehydrated compounds 2* and 3*, and rehydration occurs when crystals of these new complexes are left out in air. The reversible single-crystal to single-crystal transformation of 2 involves the dissociation/coordination of a terminal water ligand, but the case of 3 is remarkable considering that the water that is lost is the only bridging ligand between the metals in the helical rod SBU and a carboxylate oxygen that is a terminal ligand in 3 moves into a bridging position in 3* to maintain the homochiral helical rods. Both 2* and 3* contain five-coordinate metals. There are no coordinated solvents in compounds 4-6, in two cases by designed ligand modification, which allows them to have high thermal stability. Compounds 1-3 did not exhibit observable Second Harmonic Generation (SHG) efficiency at an incident wavelength of 1064 nm, but compounds 4-6 did exhibit modest SHG efficiency for MOF-like compounds in the range of 30 × α-SiO2.

Dinuclear complexes containing linear M-F-M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] bridges: trends in structures, antiferromagnetic superexchange interactions, and spectroscopic properties.

The reaction of M(BF(4))(2)·xH(2)O, where M is Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II), with the new ditopic ligand m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (L(m)*) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M(2)(µ-F)(µ-L(m)*)(2)](BF(4))(3). The analogous manganese(II) species, [Mn(2)(µ-F)(µ-L(m)*)(2)](ClO(4))(3), was isolated starting with Mn(ClO(4))(2)·6H(2)O using NaBF(4) as the source of the bridging fluoride. In all of these complexes, the geometry around the metal centers is trigonal bipyramidal, and the fluoride bridges are linear. The (1)H, (13)C, and (19)F NMR spectra of the zinc(II) and cadmium(II) compounds and the (113)Cd NMR of the cadmium(II) compound indicate that the metallacycles retain their structure in acetonitrile and acetone solution. The compounds with M = Mn(II), Fe(II), Co(II), Ni(II), and Cu(II) are antiferromagnetically coupled, although the magnitude of the coupling increases dramatically with the metal as one moves to the right across the periodic table: Mn(II) (-6.7 cm(-1)) < Fe(II) (-16.3 cm(-1)) < Co(II) (-24.1 cm(-1)) < Ni(II) (-39.0 cm(-1)) ≪ Cu(II) (-322 cm(-1)). High-field EPR spectra of the copper(II) complexes were interpreted using the coupled-spin Hamiltonian with g(x) = 2.150, g(y) = 2.329, g(z) = 2.010, D = 0.173 cm(-1), and E = 0.089 cm(-1). Interpretation of the EPR spectra of the iron(II) and manganese(II) complexes required the spin Hamiltonian using the noncoupled spin operators of two metal ions. The values g(x) = 2.26, g(y) = 2.29, g(z) = 1.99, J = -16.0 cm(-1), D(1) = -9.89 cm(-1), and D(12) = -0.065 cm(-1) were obtained for the iron(II) complex and g(x) = g(y) = g(z) = 2.00, D(1) = -0.3254 cm(-1), E(1) = -0.0153, J = -6.7 cm(-1), and D(12) = 0.0302 cm(-1) were found for the manganese(II) complex. Density functional theory (DFT) calculations of the exchange integrals and the zero-field splitting on manganese(II) and iron(II) ions were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with experiment.

Homochiral helical metal-organic frameworks of potassium.

Two trifunctional ligands built from enantiopure amino acids and containing a 1,8-naphthalimide group have been used to prepare two new complexes of potassium that have extended structures based on homochiral-rod secondary building units. One structure is a three-dimensional metal-organic framework (MOF), while the other is a two-dimensional solid that is organized into a supramolecular MOF by strong π···π-stacking interactions of the naphthalimide groups in the third dimension.

Halide and hydroxide linearly bridged bimetallic copper(II) complexes: trends in strong antiferromagnetic superexchange interactions.

Centrosymmetric [Cu(2)(µ-X)(µ-L(m)*)(2)](ClO(4))(3) (X = F(-), Cl(-), Br(-), OH(-), L(m)* = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene)], the first example of a series of bimetallic copper(II) complexes linked by a linearly bridging mononuclear anion, have been prepared and structurally characterized. Very strong antiferromagnetic exchange coupling between the copper(II) ions increases along the series F(-) < Cl(-) < OH(-) < Br(-), where -J = 340, 720, 808, and 945 cm(-1). DFT calculations explain this trend by an increase in the energy along this series of the antibonding antisymmetric combination of the p orbital of the bridging anion interacting with the copper(II) d(z(2)) orbital.

Copper(II) carboxylate dimers prepared from ligands designed to form a robust π···π stacking synthon: supramolecular structures and molecular properties.

The reactions of bifunctional carboxylate ligands (1,8-naphthalimido)propanoate, (L(C2)(-)), (1,8-naphthalimido)ethanoate, (L(C1)(-)), and (1,8-naphthalimido)benzoate, (L(C4)(-)) with Cu(2)(O(2)CCH(3))(4)(H(2)O)(2) in methanol or ethanol at room temperature lead to the formation of novel dimeric [Cu(2)(L(C2))(4)(MeOH)(2)] (1), [Cu(2)(L(C1))(4)(MeOH)(2)]·2(CH(2)Cl(2)) (2), [Cu(2)(L(C4))(4)(EtOH)(2)]·2(CH(2)Cl(2)) (3) complexes. When the reaction of L(C1)(-) with Cu(2)(O(2)CCH(3))(4)(H(2)O)(2) was carried out at -20 °C in the presence of pyridine, [Cu(2)(L(C1))(4)(py)(4)]·2(CH(2)Cl(2)) (4) was produced. At the core of complexes 1-3 lies the square Cu(2)(O(2)CR)(4) "paddlewheel" secondary building unit, where the two copper centers have a nearly square pyramidal geometry with methanol or ethanol occupying the axial coordination sites. Complex 4 contains a different type of dimeric core generated by two κ(1)-bridging carboxylate ligands. Additionally, two terminal carboxylates and four trans situated pyridine molecules complete the coordination environment of the five-coordinate copper(II) centers. In all four compounds, robust π···π stacking interactions of the naphthalimide rings organize the dimeric units into two-dimensional sheets. These two-dimensional networks are organized into a three-dimensional architecture by two different noncovalent interactions: strong π···π stacking of the naphthalimide rings (also the pyridine rings for 4) in 1, 3, and 4, and intermolecular hydrogen bonding of the coordinated methanol or ethanol molecules in 1-3. Magnetic measurements show that the copper ions in the paddlewheel complexes 1-3 are strongly antiferromagnetically coupled with -J values ranging from 255 to 325 cm(-1), whereas the copper ions in 4 are only weakly antiferromagnetically coupled. Typical values of the zero-field splitting parameter D were found from EPR studies of 1-3and the related known complexes [Cu(2)(L(C2))(4)(py)(2)]·2(CH(2)Cl(2))·(CH(3)OH), [Cu(2)(L(C3))(4)(py)(2)]·2(CH(2)Cl(2)) and [Cu(2)(L(C3))(4)(bipy)]·(CH(3)OH)(2)·(CH(2)Cl(2))(3.37) (L(C3)(-) = (1,8-naphthalimido)butanoate)), while its abnormal magnitude in [Cu(2)(L(C2))(4)(bipy)] was qualitatively rationalized by structural analysis and DFT calculations.

Zinc paddlewheel dimers containing a strong π···π stacking supramolecular synthon: designed single-crystal to single-crystal phase changes and gas/solid guest exchange.

The ligand 4-(1,8-naphthalimido)benzoate, L(C4)(-), containing a linear link between the strong π···π stacking 1,8-naphthalimide supramolecular synthon and the carboxylate donor group, reacts with Zn(O(2)CCH(3))(2)(H(2)O)(2) in the presence of dimethylsulfoxide (DMSO) to yield [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)). This compound contains the "paddlewheel" Zn(2)(O(2)CR)(4) secondary building unit (SBU) that organizes the rigid phenylene and naphthalimide rings of the carboxylate ligands in a square arrangement. The supramolecular architecture is dominated by π···π stacking interactions between naphthalimide rings of one dimer with four adjacent dimers, essentially at right angles, forming an open three-dimensional network structure. Two symmetry equivalent networks of this type interpenetrate generating overall a densely packed three-dimensional, 2-fold interpenetrated architecture in which the CH(2)Cl(2) solvate molecules are trapped in isolated pockets. Upon cooling, single crystals of [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) undergo two distinct crystallographic phase transitions, as characterized by X-ray diffraction at different temperatures, without loss of crystallinity. These two new phases have supramolecular structures very similar to the room temperature structure, but changes in the ordering of the CH(2)Cl(2) solvate cause shifting of the naphthalimide rings and a lowering of the symmetry. Crystals of [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) undergo a single-crystal to single-crystal gas/solid guest exchange upon exposure to atmospheric moisture, or faster if placed under vacuum or heated under dry gas to 100 °C, followed by atmospheric moisture, to yield [Zn(2)(L(C4))(4)(DMSO)(2)]·3.9(H(2)O). The molecular and supramolecular structures of this new compound are very similar to the dichloromethane adduct, with now the water molecules encapsulated into the framework. The remarkable feature of both the phase changes and exchange of solvates is that this robust network is not porous; local distortions (ring slippage and tilting changes) of the π···π stacking interactions of the naphthalimide rings that organize these structures allow these changes to take place without the loss of crystallinity. The complexes [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) and [Zn(2)(L(C4))(4)(DMSO)(2)]·3.9(H(2)O) show green emission in the solid state.

Syntheses and characterization of copper(II) carboxylate dimers formed from enantiopure ligands containing a strong π···π stacking synthon: enantioselective single-crystal to single-crystal gas/solid-mediated transformations.

Tri- and tetrafunctional enantiopure ligands have been prepared from 1,8-naphthalic anhydride and the amino acids L-alanine, D-phenylglycine, and L-asparagine to produce (S)-2-(1,8-naphthalimido)propanoic acid (HL(ala)), (R)-2-(1,8-naphthalimido)-2-phenylacetic acid (HL(phg)), and (S)-4-amino-2-(1,8 naphthalimido)-4-oxobutanoic acid (HL(asn)), respectively. Reactions of L(ala)(-) with copper(II) acetate under a variety of solvent conditions has led to the formation and characterization by X-ray crystallography of three similar copper(II) paddlewheel complexes with different axial ligands, [Cu(2)(L(ala))(4)(THF)(2)] (1), [Cu(2)(L(ala))(4)(HL(ala))] (2), and [Cu(2)(L(ala))(4)(py)(THF)] (3). A similar reaction using THF and L(phg)(-) leads to the formation of [Cu(2)(L(phg))(4)(THF)(2)] (4). With the exception of a disordered component in the structure of 4, the naphthalimide groups in all of these compounds are arranged on the same side of the square, central paddlewheel unit, forming what is known as the chiral crown configuration. A variety of π···π stacking interactions of the 1,8-naphthalimide groups organize all of these complexes into supramolecular structures. The addition of the amide group functionality in the L(asn)(-) ligand leads to the formation of tetrameric [Cu(4)(L(asn))(8)(py)(MeOH)] (5), where reciprocal axial coordination of one of the amide carbonyl oxygen atoms between two dimers leads to the tetramer. Extensive supramolecular interactions in 5, mainly the π···π stacking interactions of the 1,8-naphthalimide supramolecular synthon, support an open three-dimensional structure containing large pores filled with solvent. When crystals of [Cu(4)(L(asn))(8)(py)(MeOH)] are exposed to (S)-ethyl lactate vapor, the coordinated methanol molecule is replaced by (S)-ethyl lactate, bonding to the copper ion through the carbonyl oxygen, yielding [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)] (6) without a loss of crystallinity. With the exception of the replacement of the one axial ligand, the molecular structures of 5 and 6 are very similar. In a similar experiment of 5 with vapors of (R)-ethyl lactate, again a change occurs without a loss of crystallinity, but in this case the (R)-ethyl lactate displaces only slightly more than half of the axial methanol molecules forming [Cu(4)(L(asn))(8)(py){((R)-ethyl lactate)(0.58)(MeOH)(0.42)}] (7). Importantly, in 7, the (R)-ethyl lactate coordinates through the hydroxyl group. When crystals of [Cu(4)(L(asn))(8)(py)(MeOH)] are exposed to vapors of racemic ethyl lactate, the coordinated methanol molecule is displaced without a loss of crystallinity exclusively by (S)-ethyl lactate, yielding a new form of the tetramer [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)], in which the ethyl lactate in the pocket bonds to the copper(II) ion through the carbonyl oxygen as with 6. Exposure of [Cu(4)(L(asn))(8)(py){((R)-ethyl lactate)(0.58)(MeOH)(0.42)}] to racemic ethyl lactate yields a third form of [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)], where the three forms of [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)] have differences in the number of ordered (S)-ethyl lactate molecules located in the interstitial sites. These results demonstrate enantioselective bonding to a metal center in the chiral pocket of both 5 and 7 during single-crystal to single-crystal gas/solid-mediated exchange reactions.

Copper(II) carboxylate tetramers formed from an enantiopure ligand containing a π-stacking supramolecular synthon: single-crystal to single-crystal enantioselective ligand exchange.

An enantiopure ligand built from connecting the π···π stacking 1,8-naphthalimide supramolecular synthon with L-asparagine, L(asn)(-), forms tetrameric [Cu(4)(L(asn))(8)(py)(MeOH)]. The methanol ligand, located in a chiral pocket, is replaced enantioselectively when exposed to racemic ethyl lactate vapor to yield [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)], in a single-crystal to single-crystal gas/solid transformation.

Homochiral, helical supramolecular metal-organic frameworks organized by strong π · · · π stacking interactions: single-crystal to single-crystal transformations in closely packed solids.

Enantiopure, trifunctional carboxylate ligands synthesized by linking the strong π · · · π stacking 1,8-naphthalimide supramolecular synthon to three naturally occurring amino acids using the azide/alkyne click reaction have been prepared [amino acid = glycine (L(gly)(-)), alanine (L(ala)(-)), and serine (L(ser)(-))]. These ligands have been used to form complexes of the formula [M(L(amino acid))2(4,4'-bipy)(H2O)2] · xH2O (M = Fe, Co, Ni, Cu, Zn; x = 4.25-5.52) when mixed with an appropriate metal salt and 4,4'-bipyridine by layering methods. These complexes are isostructural, with the central metal atom coordinated to two κ(1)-carboxylate ligands, two water molecules, and one end each of two 4,4'-bipyridine ligands in a distorted octahedral environment. Each ligand is oriented in a trans arrangement. These complexes all have homochiral, helical, supramolecular, three-dimensional metal-organic framework structures, with the helical organization of the individual metallic units held together solely by strong, noncovalent π · · · π stacking interactions of the naphthalimide; the other two dimensions are organized mainly by the bipyridine ligands. The helices are extremely large; one turn of the helix travels ∼ 60 Å and has a diameter of ca. 40 Å. For the achiral ligand L(gly)(-), the nickel complex forms two types of homochiral crystals in the same tube, a clear example of spontaneous resolution. Despite the large size of the individual helices, they are tightly interconnected and nestled closely together. Part of the interconnection comes from the interstitial water molecules held inside the framework of the complexes in isolated pockets by hydrogen-bonding interactions. For both [Cu(L(ala))2(4,4'-bipy)(H2O)2] · 4.25H2O and [Co(L(ser))2(4,4'-bipy)(H2O)2] · 4.68H2O, the interstitial water molecules can be removed by placing the crystals under a vacuum for several hours, a process that can be reversed by exposure to atmospheric moisture. This removal/reintroduction of the interstitial water molecules takes place with no loss of crystallinity, representing dramatic examples of single-crystal to single-crystal transformations. The structures undergo little change other than the pockets holding the interstitial water molecules in the hydrated complexes become void spaces in the dehydrated complexes. The removal/reintroduction of the water molecules in these closely packed solids is facilitated by the "soft" π · · · π stacking interactions organizing one dimension of the structures. The observed magnetic and Mössbauer spectral properties are typical of isolated, magnetically dilute, paramagnetic pseudooctahedral divalent transition-metal complexes.

Synthesis, structural and spectroscopic characterization and biomimetic properties of new copper, manganese, zinc complexes: identification of possible superoxide-dismutase mimics bearing hydroxyl radical generating/scavenging abilities.

A series of Cu(II), Zn(II) and Mn(II) coordination compounds has been synthesized by reaction of the corresponding metal salts and pyrazolyl-based ligands, i.e. the neutral 1-(2-(4-((2,2,2-tri(1H-pyrazol-1-yl)ethoxy)methyl)benzyloxy)-1,1-di(1H-pyrazol-1-yl)ethyl)-1H-pyrazole {p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2), (L(1)), and the anionic hydridotris(3-phenyl-5-methylpyrazolyl)borate (L(2))(-), bis(pyrazolyl)acetate (L(3)) and bis(3,5-dimethylpyrazolyl)acetate (L(4))(-): the species [L(1)(CuCl(2))(2)] (1), [L(1)(Cu(OAc)(2))(2)] (2), [L(1)(Zn(OAc)(2))(2)] (3), [(CuCl(L(2))(Hpz(Ph,Me))] (4), [Mn(L(3))(2)].2H(2)O, (5), [ZnCl(L(3))(imH)].MeOH [CuCl(L(4))(imH)].2H(2)O (7) have been obtained (Hpz(Ph,Me)=3-phenyl-5-methylpyrazole, imH=imidazole). Complexes 1 and 4 have been structurally characterized, also using less conventional powder diffraction methods. The superoxide scavenging activity has been characterized by indirect assays including EPR analysis. All complexes exhibit superoxide scavenging activity with IC(50) in the microM range and they protect against the oxidative action of peroxynitrite in different ways. 1, 4 and 7 exert both an anti- and pro-oxidant effect depending on their concentration as evaluated by EPR and fluorescence methods. The pro-oxidative effects are absent in Zn(II) and Mn(II) complexes.