Cooperative spin transition in the two-dimensional coordination polymer [Fe(4,4'-bipyridine)2(NCX)2]·4CHCl3 (X = S, Se)

Two new isostructural two-dimensional (2D) coordination polymers exhibiting spin crossover (SCO) behavior of formulation [Fe(4,4'-bipy)(2)(NCX)(2)]·4CHCl(3) (4,4'-bipy = 4,4'-bipyridine; X = S [1·4CHCl(3)], Se [2·4CHCl(3)]) have been synthesized and characterized, and both undergo cooperative spin transitions (ST). For 1·4CHCl(3) the ST takes place in two steps with critical temperatures of T(c1)(down) = 143.1 K, T(c2)(down) = 91.2 K, T(c1)(up) = 150.7 K, and T(c2)(up) = 112.2 K. 2·4CHCl(3) displays half ST characterized by T(c)(down) = 161.7 K and T(c)(up) = 168.3 K. The average enthalpy and entropy variations and cooperativity parameters associated with the ST have been estimated to be ΔH(1)(av) = 5.18 kJ mol(-1), ΔS(1)(av) = 35 J K(-1) mol(-1), and Γ(1) = 2.8 kJ mol(-1) and ΔH(2)(av) = 3.55 kJ mol(-1), ΔS(2)(av) = 35 J K(-1) mol(-1), and Γ(2) = 2.6 kJ mol(-1) for 1·4CHCl(3), and ΔH(av) = 6.25 kJ mol(-1), ΔS(av) = 38.1 J K(-1) mol(-1), and Γ = 3.2 kJ mol(-1) for 2·4CHCl(3). At T > [T(c1) (1·4CHCl(3)); T(c) (2·4CHCl(3))], both compounds are in the space group P2/c while at T < [T(c1) (1·4CHCl(3)); T(c) (2·4CHCl(3))] they change to the C2/c space group and display an ordered checkerboard-like arrangement of iron(II) sites where the high- and low-spin states coexist at 50%.     

Single-molecule magnets: preparation and properties of mixed-carboxylate complexes

Methods are reported for the preparation of mixed-carboxylate versions of the [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] family of single-molecule magnets (SMMs). [Mn(12)O(12)(O(2)CCHCl(2))(8)(O(2)CCH(2)Bu(t))(8)(H(2)O)(3)] (5) and [Mn(12)O(12)(O(2)CHCl(2))(8)(O(2)CEt)(8)(H(2)O)(3)] (6) have been obtained from the 1:1 reaction of the corresponding homocarboxylate species. Complex 5.CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1 with, at -165 degrees C, a = 15.762(1), b = 16.246(1), c = 23.822(1) A, alpha = 103.92(1), beta = 104.50(1), gamma = 94.23(1) degrees, Z = 2, and V = 5674(2) A(3). Complex 6.CH(2)Cl(2) crystallizes in the triclinic space group P1 with, at -158 degrees C, a = 13.4635(3), b = 13.5162(3), c = 23.2609(5) A, alpha = 84.9796(6), beta = 89.0063(8), gamma = 86.2375(6) degrees, Z = 2, and V = 4207.3(3) A(3). Complexes 5 and 6 both contain a [Mn(12)O(12)] core with the CHCl(2)CO(2-) ligands ordered in the axial positions and the RCO(2-) ligands (R = CH(2)Bu(t) (5) or Et (6)) in equatorial positions. There is, thus, a preference for the CHCl(2)CO(2-) to occupy the sites lying on the Mn(III) Jahn-Teller axes, and this is rationalized on the basis of the relative basicities of the carboxylate groups. Direct current magnetic susceptibility studies in a 10.0 kG field in the 2.00-300 K range indicate a large ground-state spin, and fitting of magnetization data collected in the 10.0-70.0 kG field and 1.80-4.00 K temperature range gave S = 10, g = 1.89, and D = -0.65 K for 5, and S = 10, g = 1.83, and D = -0.60 K for 6. These values are typical of [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] complexes. Alternating current susceptibility studies show the out-of-phase susceptibility (chi(M)' ') signals characteristic of the slow relaxation in the millisecond time scale of single-molecule magnets. Arrhenius plots obtained from chi(M)' ' versus T data gave effective barriers to relaxation (U(eff)) of 71 and 72 K for 5 and 6, respectively. (1)H NMR spectra in CD(2)Cl(2) show that 5 and 6 are the main species present on dissolution, but there is evidence for some ligand distribution between axial and equatorial sites, by intra- and/or intermolecular exchange processes.     

Construction of segregated arrays of multiple donor and acceptor units using a dendritic scaffold: remarkable dendrimer effects on photoinduced charge separation

Dendritic molecules appended with multiple zinc porphyrin units (DPm, m [number of zinc porphyrin units] = 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py2Fn, n [number of C60 units] = 1-3), affording coordination complexes DPm superset Py2Fn having a photoactive layer consisting of spatially segregated donor and acceptor arrays on their surface. Complexes DPm superset Py2Fn are stable enough (K [average binding affinity] = 1.1 x 10(6)-4.4 x 10(6) M(-1) in CHCl3 at 25 degrees C) to be isolated by gel permeation chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP12 superset Py2F3. Photoexcitation of the zinc porphyrin units in DPm superset Py2Fn results in a zinc porphyrin-to-fullerene electron transfer to generate a charge separation. The charge-separation rate constant (kCS) in CH2Cl2 at 20 degrees C increases from 0.26 x 10(10) to 2.3 x 10(10) s(-1) upon increment of m and n, whereas the charge-recombination rate constant (kCR) remains almost unchanged at 4.5 x 10(6)-6.7 x 10(6) s(-1). Consequently, DP24 supersetPy2F3 furnishes the largest ratio of kCS/kCR (3400) among the family.     

Atmospheric chemistry of t-CF3CH=CHCl: products and mechanisms of the gas-phase reactions with chlorine atoms and hydroxyl radicals

FTIR-smog chamber techniques were used to study the products and mechanisms of the Cl atom and OH radical initiated oxidation of trans-3,3,3-trifluoro-1-chloro-propene, t-CF(3)CH=CHCl, in 700 Torr of air or N(2)/O(2) diluent at 296 ± 2 K. The reactions of Cl atoms and OH radicals with t-CF(3)CH=CHCl occur via addition to the >C=C< double bond; chlorine atoms add 15 ± 5% at the terminal carbon and 85 ± 5% at the central carbon, OH radicals add approximately 40% at the terminal carbon and 60% at the central carbon. The major products in the Cl atom initiated oxidation of t-CF(3)CH=CHCl were CF(3)CHClCHO and CF(3)C(O)CHCl(2), minor products were CF(3)CHO, HCOCl and CF(3)COCl. The yields of CF(3)C(O)CHCl(2), CF(3)CHClCOCl and CF(3)COCl increased at the expense of CF(3)CHO, HCOCl and CF(3)CHClCHO as the O(2) partial pressure was increased over the range 10-700 Torr. Chemical activation plays a significant role in the fate of CF(3)CH(O)CHCl(2) and CF(3)CClHCHClO radicals. In addition to reaction with O(2) to yield CF(3)COCl and HO(2) the major competing fate of CF(3)CHClO is Cl elimination to give CF(3)CHO (not C-C bond scission as previously thought). As part of this study k(Cl + CF(3)C(O)CHCl(2)) = (2.3 ± 0.3) × 10(-14) and k(Cl + CF(3)CHClCHO) = (7.5 ± 2.0) × 10(-12) cm(3) molecule(-1) s(-1) were determined using relative rate techniques. Reaction with OH radicals is the major atmospheric sink for t-CF(3)CH=CHCl. Chlorine atom elimination giving the enol CF(3)CH=CHOH appears to be the sole atmospheric fate of the CF(3)CHCHClOH radicals. The yield of CF(3)COOH in the atmospheric oxidation of t-CF(3)CH=CHCl will be negligible (<2%). The results are discussed with respect to the atmospheric chemistry and environmental impact of t-CF(3)CH=CHCl.     

First example of the solid-state thermal cyclometalation of ligated benzophenone imine giving novel luminescent platinum(II) species

The (benzophenone imine)platinum(II) compounds trans-[PtCl2(Ph2C=NH)(RR'SO)] [R, R'=Me, Me (2); n-Pr, n-Pr (3); (CH2)4 (4); Me, Ph (5); Me, p-MeC6H4 (6)] were prepared by the reaction of Ph2C=NH with K[PtCl3(RR'SO)], obtained in situ from K2[PtCl4] and the corresponding sulfoxide, giving 2-6 as well as cis-[PtCl2(Ph2C=NH)2] (1) as a minor product. The complexes were characterized by 1H, 13C, and 195Pt NMR and IR spectroscopy, electrospray ionization mass spectrometry, and C, H, and N elemental analysis. The X-ray crystallography of 1 enables confirmation of the cis configuration of the complex, while in 2 and 4.1/2CHCl3, the imine and sulfoxide ligands are mutually trans. The solid-state structure of 4.1/2CHCl3 consists of two dimeric Pt moieties representing a rather weak Pt...Pt interaction. The dimeric architecture of 4.1/2CHCl3 is enhanced by the hydrogen bonding between imine H atoms and O atoms. The orthometalation of 1 and 2-6 proceeds both in the solid phase and in a toluene suspension, leading to the formation of [PtCl{Ph(C6H4)C=NH}(Ph2C=NH)] (7) and [PtCl{Ph(C6H4)C=NH}(RR'SO)] (8-12), respectively, isolated in nearly quantitative yields. Complexes 8-12 are emissive at room temperature both in solution (lambdaemmax approximately 535 nm) and in the solid state (lambdaemmax 560-610 nm), with excited-state lifetimes of ca. 300-600 ns, representing a new family of PtII-based luminescent complexes. Compounds 8 and 10 have been characterized by X-ray analysis, confirming the square-planar coordination geometry of the metal center with the almost planar platinacycles. In 8, the asymmetric unit contains two independent Pt molecules, while in 10, it includes four Pt molecules linked by the intermolecular hydrogen-bonding network between the NH group and Cl atoms.     

Kinetics of the CH2Cl + CH3 and CHCl2 + CH3 radical-radical reactions

The CH2Cl + CH3 (1) and CHCl2 + CH3 (2) cross-radical reactions were studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 301-800 K and bath gas (helium) density (6-12) x 10(16) atom cm(-3). The observed rate constant of reaction 1 can be represented by an Arrhenius expression k1 = 3.93 x 10(-11) exp(91 K/T) cm3 molecule(-1) s(-1) (+/-25%) or as an average temperature-independent value of k1= (4.8 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1). The rate constant of reaction 2 can be expressed as k2= 1.66 x 10(-11) exp(359 K/T) cm3 molecule(-1) s(-1) (+/-25%). C2H4 and C2H3Cl were detected as the primary products of reactions 1 and 2, respectively. The experimental values of the rate constant are in reasonable agreement with the prediction based on the "geometric mean rule." A separate experimental attempt to determine the rate constants of the high-temperature CH2Cl + O2 (10) and CHCl2 + O2 (11) reaction resulted in an upper limit of 1.2 x 10(-16) cm(3) molecule(-1) s(-1) for k10 and k11 at 800 K.     

Syntheses and structural characterization of the (OCnOPor) capped porphyrins: Co(OC2OPor).CH2Cl2, Co(OC2OPor)(NO)out.0.46CHCl3, Co(OC3OPor).CHCl3, and Co(OC3OPor)(MeIm).3C7H8

The compounds Co(OC2OPor).CH2Cl2 (1), Co(OC2OPor)(NO)out.0.46CHCl3 (2), Co(OC3OPor).CHCl3 (3), and Co(OC3OPor)(MeIm).3C7H8 (4) (OC2OPor = 5,10,15,20-(benzene-1,2,4,5- tetrakis(2-phenyloxy)ethoxy)-2',2",2"',2"-tetraylporphyrinato dianion; OC3OPor = 5,10,15,20-(benzene-1,2,4,5-tetrakis(2- phenyloxy)propoxy)-2',2",2"',2"-tetraylporphyrinato dianion; MeIm = 1-methylimidazole), have been synthesized, and their structures have been determined by single-crystal X-ray diffraction methods at T = -120 degrees C: 1, a = 8.824(1) A, b = 16.674(1) A, c = 16.836(1) A, alpha = 104.453(1) degrees, beta = 92.752(1) degrees, gamma = 90.983(1) degrees, P1, Z = 2; 2, a = 9.019(1) A, b = 16.588(2) A, c = 16.909(2) A, alpha = 103.923(2) degrees, beta = 92.082(2) degrees, gamma = 93.583(2) degrees, P1, Z = 2; 3, a = 13.484(3) A, b = 14.404(3) A, c = 14.570(3) A, alpha = 105.508(3) degrees, beta = 100.678(3) degrees, gamma = 93.509(4) degrees, P1, Z = 2; 4, a = 16.490(1) A, b = 22.324(2) A, c = 17.257(1) A, b = 92.437(1) degrees, P2(1)/n, Z = 4. These compounds are the first structurally characterized Co-bound members of the OCnOPor ligand system. The NO ligand in 2 and the MeIm ligand in 4 bind asymmetrically and lead to several metrical changes in these porphyrins, e.g., variations in average porphyrin deviations and Co atom displacements relative to the porphyrinato N atoms and the mean porphyrin planes.     

Synthesis, redox, and magnetic properties of a neutral, mixed-valent heptanuclear manganese wheel with S = 27/2 high-spin ground state

Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads to the formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {MnII subset[MnII2MnIII4Cl6(L3)6]} (4). The manganese wheel crystallizes in the triclinic space group P as 4.2CHCl3 or 4.3THF when either diethyl ether or n-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of each manganese ion in 4.2CHCl3 or 4.3THF were assigned on the basis of detailed symmetry, bond length, and charge considerations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and were further supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4.2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagnetic exchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higher than 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements on single crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and by high-frequency electron spin resonance measurements on a polycrystalline powder of 4.2CHCl3 (D = -0.068 K, E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U approximately 10 K. Finally, 2DEG Hall magnetization measurements revealed that 4.2CHCl3 shows single-molecule magnet behavior up to the blocking temperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of the magnetization.     

Kinetics of the reactions of chlorinated methyl radicals (CH2Cl, CHCl2, and CCl3) with NO2 in the temperature range 220-360 K

The kinetics of the reactions of chlorinated methyl radicals (CH2Cl, CHCl2, and CCl3) with NO2 have been studied in direct measurements at temperatures between 220 and 360 K using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated at 193 or 248 nm by pulsed laser photolysis of appropriate precursors. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of NO2 being in large excess over radical concentrations. The bimolecular rate coefficients of all three reactions are independent of the bath gas (He or N2) and pressure within the experimental range (1-6 Torr) and are found to depend on temperature as follows: k(CH2Cl + NO2) = (2.16 +/- 0.08) x 10(-11) (T/300 K)(-1.12+/-0.24) cm3 molecule(-1) s(-1) (220-363 K), k(CHCl2 + NO2) = (8.90 +/- 0.16) x 10(-12) (T/300 K)(-1.48+/-0.13) cm3 molecule(-1) s(-1) (220-363 K), and k(CCl3 + NO2) = (3.35 +/- 0.10) x 10(-12) (T/300 K)(-2.2+/-0.4) cm3 molecule(-1) s(-1) (298-363 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about +/-25%. In the reactions CH2Cl + NO2, CHCl2 + NO2, and CCl3 + NO2, the products observed are formaldehyde, CHClO, and phosgene (CCl2O), respectively. In addition, a weak signal for the HCl formation has been detected for the CHCl2 + NO2 reaction.     

Binary organogelators which show light and temperature responsiveness

The gelation ability of 10 alkylammonium (CnH(2n+1)NH3+ where n=4-11, 12 and 16) anthracene-9-carboxylates (1n) has been evaluated. In cyclohexane, 1(4), 1(5), 1(6) and 1(7) only provided precipitates whereas 1(11), 1(12) and 1(16) provided very viscous solutions. In contrast, 1(8) 1(9) and 1(10) resulted in gels. The critical gelation concentration of 1(10) was very low (5.0 x 10(-4) mol dm(-3)). SEM observations showed that in the gel phase the morphology changes from straight fibrils to frizzy fibrils with the increase in n, whereas in the sol phase the formation of the sheet-like, two-dimensional aggregate is recognized. When the cyclohexane 1(10) gel was photoirradiated (lambda > 300 nm), the UV-VIS absorption bands assignable to monomeric anthracene were decreased and the gel was changed into the sol. It was confirmed by dark-field optical microscopy that the fibrillar bundles supporting the gel formation gradually disappear with photoirradiation time. When this sol was warmed at 30 degrees C in the dark, the gel was not regenerated but the precipitation of 1(10) resulted. When this sol was heated once at the bp of cyclohexane and cooled to 15 degrees C, the solution was changed into the gel again. This finding indicates that the fibrillar structure required for the gel formation is not reconstructed at 30 degrees C but obtained only when the hot cyclohexane solution is cooled.     

Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended pi-conjugated cyclometalated ligands

Reacting K2PtCl4 with the tridentate R-C(wedge)N(wedge)C-H2 ligands 2,6-di-(2'-naphthyl)-4-R-pyridine (R = H, 1a; Ph, 1b; 4-BrC6H4, 1c; 3,5-F2C6H3, 1d) in glacial acetic acid, followed by heating in dimethyl sulfoxide (DMSO), gave complexes [(R-C(wedge)N(wedge)C)Pt(DMSO)] (2a-d). In the crystal structures of 2a-c, the molecules are paired in a head-to-tail orientation with Pt...Pt separations >6.3 A, and there are extensive close C-H...pi (d = 2.656-2.891 A), pi...pi (d = 3.322-3.399 A), and C-H...O=S (d = 2.265-2.643 A) contacts. [(Ph-C(wedge)N(wedge)C)Pt(PPh3)] (3) was prepared by reacting 2b with PPh3. Reactions of 2a-d with bis(diphenylphosphino)methane (dppm) gave [(R-C(wedge)N(wedge)C)2Pt2(mu-dppm)] (4a-d). Both head-to-head (syn) and head-to-tail (anti) conformations were found for 4a.6CHCl3.C5H12, whereas only one conformation was observed for 4b.2CHCl3 (syn), 4c.3CH2Cl2 (syn), and 4d.2CHCl3 (anti). In the crystal structures of 4a-d, there are close intramolecular Pt...Pt contacts of 3.272-3.441 A in the syn conformers, and long intramolecular Pt...Pt separations of 5.681-5.714 A in the anti conformers. There are weak C-H...X (d = 2.497-3.134 A) and X...X (X = Cl or Br; d = 2.973-3.655 A) interactions between molecules 4a-d and occluded CHCl3/CH2Cl2 molecules, and their solvent channels are of varying diameters (approximately 9-28 A). Complexes 2a-d, 3, and 4a-d are photoluminescent in the solid state, with emission maxima at 602-643 nm. Upon exposure to volatile organic compounds, 4a shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents (except CCl4). Powder X-ray diffraction analysis of desolvated 4a revealed a more condensed molecular packing of syn and anti complexes than crystal 4a.6CHCl3.C5H12.     

Chiral Pyridine-Based Macrobicyclic Clefts: Synthesis and Enantiomeric Recognition of Ammonium Salts

An achiral (3) and two chiral pyridine-based macrobicyclic clefts (4 and 5) have been prepared by treating 2,6-bis[[2',6'-bis(bromomethyl)-4'-methylphenoxy]methyl]pyridine (2) with the appropriate achiral and chiral glycols. Starting 2 was prepared by first treating 2,6-bis(hydroxymethyl)-4-methylphenol with 2,6-[(tosyloxy)methyl]pyridine followed by phosphorus tribromide. Achiral macrobicyclic cleft 3 formed a complex at 25 degrees C in 50% CH(3)OH/50% CHCl(3) (v/v) with a primary ammonium salt (log K = 3.15) as evidenced by a significant change in the (1)H NMR spectrum. Highly organized (S,S,S,S)-4, prepared by treating 2 with (1S,5S)-3-oxapentane-1,5-diol, exhibited recognition at 25 degrees C in 20% C(2)H(5)OH/80% 1,2-C(2)H(4)Cl(2) (v/v) for the (S)-enantiomer of alpha-(1-naphthyl)ethylammonium perchlorate (NapEt) over its (R)-form (Delta log K = 0.85). This high recognition factor probably reflects an increase in molecular rigidity by the introduction of a second macro ring on the monocyclic pyridinocrown ligand.     

Synthesis and structure of tetranuclear orthometallated Pd(II) complexes derived from bis-iminophosphoranes

The reaction of Pd(OAc)2 with bis-iminophosphoranes Ph3P=NCH2CH2CH2N=PPh3 (1a), [C6H4(C(O)N=PPh3)2-1,3] (1b) and [C6H4(C(O)N=PPh3)2-1,2] (1c), gives the orthopalladated tetranuclear complexes [{Pd(mu-Cl){C6H4(PPh2=NCH2-kappa-C,N)-2}}2CH2]2 (2a) [{Pd(mu-OAc){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',3']2 (2b) and [{Pd(mu-OAc){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',2']2 (2c). The reaction takes place in CH2Cl2 for 1a, but must be performed in glacial acetic acid for 1b and 1c. The process implies in all cases the activation of a C-H bond on a Ph ring of the phosphonium group, with concomitant formation of endo complexes. This is the expected behaviour for 1a, but for 1b and 1c reverses the exo orientation observed in other ketostabilized iminophosphoranes. The influence of the solvent in the orientation of the reaction is discussed. The dinuclear acetylacetonate complexes [{Pd(acac-O,O'){C6H4(PPh2=NCH2-kappa-C,N)-2}}2CH2] (3a), [{Pd(acac-O,O'){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',3'] (3b) and [{Pd(acac-O,O'){C6H4(PPh2=NC(O)-kappa-C,N)-2}}2C6H4-1',2'] (3c) have been obtained from the halide-bridging tetranuclear derivatives. The X-ray crystal structure of [3c.4CHCl3] is also reported.     

Spin transition at the mesophase transition temperature in a cobalt(II) compound with branched alkyl chains

A cobalt(II) compound, [Co(C5C12C10-terpy)2](BF4)2 [C5C12C10-terpy = 4',5' '-decyl-1' '-(heptadecyloxy)-2,2':6',2' '-terpyridine] with branched alkyl chains, based on a terpyridine frame, was synthesized. The cobalt(II) compound exhibits a spin transition between low-spin and high-spin with a thermal hysteresis loop (T(1/2) upward arrow = 288 K and T(1/2) downward arrow = 284 K) at the liquid-crystal transition temperature. It is the first example in the cobalt(II) compounds in which the spin transition occurs at the crystal-liquid crystal transition temperature.     

Wheel-shaped Mn16 single-molecule magnets

The syntheses, structures, and magnetic properties of two new single-stranded hexadecanuclear manganese wheels [Mn16(CH3COO)8(CH3CH2CH2COO)8(teaH)12] x 10 MeCN (1 x 10 MeCN) and [Mn16((CH3)2CHCOO)16(teaH)12] x 4 CHCl3 (2 x 4 CHCl3), where teaH(2-) is the dianion of triethanolamine, are reported. 1 crystallizes in the tetragonal I4(1)/a space group [a = b = 33.519(4) A and c = 16.659(2) A]. 2 crystallizes in the monoclinic C2/c space group [a = 21.473(5), b = 26.819(6), c = 35.186(7), and beta = 93.447(5) degrees]. Both complexes consist of 8 Mn(II) and 8 Mn(III) ions alternating in a wheel-shaped topology with 12 monoprotonated triethanolamine ligands. Variable-temperature direct current (DC) magnetic susceptibility data were collected in 1 T, 0.1 and 0.01 T fields, and in the 1.8-300 K temperature range for 1 and 2. Variable-temperature variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges and least-squares fitting of these reduced magnetization versus H/T data indicates a S = 13 ground-state for 1 and 2. Single-crystal magnetization hysteresis measurements were performed in a 0.04-1 K temperature range for complex 2. Hysteresis loops were observed that showed a temperature dependence, which indicates that 2 exhibits magnetization relaxation and is a SMM. Both 1 and 2 show frequency-dependent out-of-phase signals in the AC susceptibility measurements, collected in a temperature range of 1.8-5 K and in the frequency range of 50-10,000 Hz. Extrapolation of the in-phase component of the AC susceptibility data to 0 K indicates an S = 12 ground state for 1 and an S = 11 ground-state for 2. Complex 1 has the highest-spin ground state reported to date for a single-stranded manganese wheel and is likely to be an SMM based on a frequency-dependent out-of-phase signal in the AC susceptibility. The AC susceptibility as well as magnetization hysteresis data for 2 confirm that this species is an SMM.     

Chiroptical nanofibers generated from achiral metallophthalocyanines induced by diamine homochirality

Optically active cofacial π-π phthalocyanine (Pc) stacked supramolecules were spontaneously and successfully generated from a mixture of four possible geometric isomers (C(4h), D(2h), C(2v), C(s)) of achiral 4(5),4'(5'),4'(5'),4'(5')-tetracarboxymetallophthalocyanine (tcPcM; M=Ni, Cu) induced by an equimolar amount of inexpensive chiral diamine molecular sources in DMSO/CHCl(3) mixed solvent under optimized conditions of the volume ratio of poor/good cosolvents, the molar ratio of chiral molecular diamine to tcPcM, the cavity metal of phthalocyanine, and the addition order of the amines. The sign and amplitude of circular dichroism spectra due to the supramolecular chirality and structure of the diamine molecules are impossible to remove by additions of the antipode diamine and trifluoroacetic acid. The chiral diamine was partly contained in nanofibers, and that retained in the solution can be recycled and reused to induce optically active tcPcM supramolecules.     

Cation-independent electron transfer between ferricyanide and ferrocyanide ions in aqueous solution

Electron transfer between Fe(CN)(6)(3-) and Fe(CN)(6)(4-) in homogeneous aqueous solution with K(+) as the counterion normally proceeds almost exclusively by a K(+)-catalyzed pathway, but this can be suppressed, and the direct Fe(CN)(6)(3)(-)-Fe(CN)(6)(4-) electron transfer path exposed, by complexing the K(+) with crypt-2.2.2 or 18-crown-6. Fe((13)CN)(6)(4-)-NMR line broadening measurements using either crypt-2.2.2 or (with extrapolation to zero uncomplexed [K(+)]) 18-crown-6 gave consistent values for the rate constant and activation volume (k(0) = (2.4 +/- 0.1) x 10(2) L mol(-1) s(-1) and Delta V(0) = -11.3 +/- 0.3 cm(3) mol(-1), respectively, at 25 degrees C and ionic strength I = 0.2 mol L(-1)) for the uncatalyzed electron transfer path. These values conform well to predictions based on Marcus theory. When [K(+)] was controlled with 18-crown-6, the observed rate constant k(ex) was a linear function of uncomplexed [K(+)], giving k(K) = (4.3 +/- 0.1) x 10(4) L(2) mol(-2) s(-1) at 25 degrees C and I = 0.26 mol L(-1) for the K(+)-catalyzed pathway. When no complexing agent was present, k(ex) was roughly proportional to [K(+)](total), but the corresponding rate constant k(K)' (=k(ex)/[K(+)](total)) was about 60% larger than k(K), evidently because ion pairing by hydrated K(+) lowered the anion-anion repulsions. Ionic strength as such had only a small effect on k(0), k(K), and k(K)'. The rate constants commonly cited in the literature for the Fe(CN)(6)(3-/4-) self-exchange reaction are in fact k(K)'[K(+)](total) values for typical experimental [K(+)](total) levels.     

Kinetics of Dissociation of Iron(III) Complexes of Tiron in Aqueous Acid

The kinetics of dissociation of the mono, bis, and tris complexes of Tiron (1,2-dihydroxy-3,5-benzenedisulfonate) have been studied in acidic aqueous solutions in 1.0 M HClO(4)/NaClO(4), as a function of [H(+)] and temperature. In general, the kinetics can be explained by two reactions, (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) + H(+) (k(n), k(-n)) and (HO)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) (k(n)', k(-n)'), a rapid equilibrium, (H(2)O)Fe(L(n)H) right arrow over left arrow (H(2)O)Fe(L)(n) + H(+) (K(cn)), and the formation constant (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L)(n) + 2H(+). For n = 1, the reaction was observed at 670 nm, and at [H(+)] of 0.05-0.5 M at temperatures of 2.0, 14.0, 25.0, and 36.7 degrees C. For n = 2, the analogous conditions are 562 nm, at [H(+)] of 1.5 x 10(-3) to 1.4 x 10(-2) M at temperatures of 2.0, 9.0, and 14.0 degrees C. For n = 3, the conditions are 482 nm, at pH 4.5-5.7 in 0.02 M acetate buffer at temperatures of 1.8, 8.0, and 14.5 degrees C. The rate or equilibrium constants (25 degrees C) with DeltaH or DeltaH degrees (kcal mol(-1)) and DeltaS or DeltaS degrees (cal mol(-1) K(-1)) in brackets are as follows: for n = 1, k(1) = 2.3 M(-1) s(-1) (8.9, -27.1), k(-1) = 1.18 M(-1) s(-1) (4.04, -44.8), K(c1) = 0.96 M (-9.99, -33.6), K(f1) = 2.01 M (-5.14, -15.85); for n = 2, k(-2)/K(c2) = 1.9 x 10(7) (19.9, 41.5) and k(-2)'/K(c2) = 1.85 x 10(3) (1.4, -38.8) and a lower limit of K(c2) > 0.015 M; for n = 3, k(3) = 7.7 x 10(3) (15.8, 12.3), k(-3) = 1.7 x 10(7) (16.2, 28.9), K(c3) = 7.4 x 10(-5) M (4.1, -5.1), and K(f3) = 3.35 x 10(-8) (3.7, -21.7). From the variations in rate constants and activation parameters, it is suggested that the Fe(L)(2) and Fe(L)(3) complexes undergo substitution by dissociative activation, promoted by the catecholate ligands.     

Kinetics of the reactions of O(3P) with CCl2=CH2, (Z)-CHCl=CHCl, and CCl2=CCl2: a temperature dependence study

Absolute rate coefficients for the gas-phase reactions of ground-state oxygen atoms with CCl(2)=CH(2) (1), (Z)-CHCl=CHCl (2) and CCl(2)=CCl(2) (3) have been measured directly using the fast flow discharge technique. The experiments were carried out under pseudo-first-order conditions with [O((3)P)](0) < [chloroethene](0). The temperature dependences of the reactions of O((3)P) with CCl(2)=CH(2), (Z)-CHCl=CHCl and CCl(2)=CCl(2) were studied in the range 298-359 K. The kinetic data obtained were used to derive the following Arrhenius expressions (in units of cm(3) molecule(-1) s(-1)): k(1) = (1.82 +/- 1.29) x 10(-11) exp[-(12.63 +/- 0.97) x 10(3)/RT], k(2) = (1.56 +/- 0.92) x 10(-11) exp[-(16.68 +/- 1.54) x 10(3)/RT], k(3) = (4.63 +/- 1.38) x 10(-11) exp[-(19.59 +/- 3.21) x 10(3)/RT]. This is the first temperature dependence study of the reactions of O((3)P) atoms with (Z)-CHCl=CHCl and CCl(2)=CCl(2). All the rate coefficients display a positive temperature dependence and pressure independence, which points to the importance of the irreversibility of the addition mechanism for these reactions. The obtained rate coefficients are compared with previous studies carried out mainly at room temperature. The rates of addition of O atoms and OH radicals to the double bond of alkenes at 298 K are related by the expression: log k(OH) = 0.57278 log k(O(3P)) - 4.095. A correlation is presented between the reactivity of chloroethenes toward O atoms and the second-order perturbational term of the frontier molecular orbital theory which carries the contribution of the different atomic orbitals to the HOMO of the chloroethene. To a first approximation, this correlation allows room-temperature rate coefficients to be predicted within +/-25-30% of the measured values.     

An EPR and 1H NMR active mixed-valence manganese (III/II/III) trinuclear compound

A mixed-valence Mn(III)-Mn(II)-Mn(III) trinuclear complex of stoichiometry MnIIIMnIIMnIII(Hsaladhp)2(Sal)4.2CH3CN (1), where H3saladhp is a tridentate Schiff-base ligand, has been structurally characterized with X-ray crystallography. The Mn(III)Mn(II)Mn(III) angles are strictly 180 degrees as required by crystallographic inversion symmetry. The complex is valence-trapped with two terminal Mn(III) ions in a distorted square pyramidal geometry. The Mn(III)...Mn(II) separation is 3.495 A. The trinuclear complex shows small antiferromagnetic exchange J coupling. The magnetic parameters obtained from the fitting procedure in the temperature range 10-300 K are J1 = -5.7 cm-1, g = 2.02, zJ = -0.19 cm-1, and R = 0.004. The EPR spectrum was obtained at 4 K in CHCl3 and in tetrahydrofuran glasses. The low-field EPR signal is a superposition of two signals, one centered around g = 3.6 and the other, for which hyperfine structure is observed, centered around g = 4.1 indicating an S = 3/2 state. In addition, there is a 19-line signal at g = 2.0. The multiline signal compares well with that observed for the S2 or S0* states of the oxygen-evolving complex. 1H NMR data reveal that the trinuclear compound keeps its integrity into the CHCl3 solution. Crystal data for complex 1: [C54H52N4O18Mn3], M = 1209.82, triclinic, space group P1, a = 10.367(6) A, b = 11.369(6) A, c = 13.967(8) A; alpha = 112.56(1) degree, beta = 93.42(2) degrees, gamma = 115.43(1) degree, Z = 1.