One-dimensional bromo-bridged Ni III complexes [Ni(S,S-bn)2Br]Br2 (S,S-bn=2S,3S-diaminobutane): synthesis, physical properties, and electrostatic carrier doping

A new bromo-bridged Ni III compound has been synthesized. This compound displayed a strong antiferromagnetic interaction between spins located on Ni III species (J=(2350+/-500) K) that result from the strong covalency of the Ni--Br bond and the spin-Peierls transition below 150 K. This was shown by the results of magnetic susceptibility and 81Br nuclear quadrupole resonance spectroscopy analysis. We succeeded in the electrostatic carrier doping of a single crystalline sample by using a field-effect transistor device. This compound also showed n-type semiconductor behavior, which can be reasonably rationalized by the existence of a small amount of Ni II impurities.     

The structural investigation of SbBr3· (POBr3)2 BY 81Br and 121SbNQR

⁸¹Br and ¹²¹Sb NQR have been observed for SbBr₃·(POBr₃)₂, and the Zeeman effect on ⁸¹Br NQR lines examined at 77 K. It is considered from these results that this complex consists of a dimer of two SbBr₃·(POBr₃)₂ molecules bridged by two Br atoms.     

Nuclear quadrupole resonances of α-(CH3)2 TeX2 (X=Cl, Br, I) and (CH3)2 TeI4

NQR spectra were observed for α-(CH₃)₂ TeX₂ (X=Cl, Br, I) and (CH₃)₂ TeI₄ at various temperatures. The two ⁸¹Br NQR lines were observed above 110 K in α-(CH₃)₂TeBr₂. The characteristic temperature dependence of the ¹²⁷I NQR line in α-(CH₃)₂ TeI. can be explained by the 3c—4e bond of the linear I---Te---I group. The positive temperatures dependence of the lowest ¹²⁷I NQR line in (CH₃)₂TeI₄ is discussed on the basis of the electron population calculated from Townes—Dailey treatment.     

Molecular metamagnet [Ni(4ImNNH)(2)(NO(3))(2)] (4ImNNH = 4-imidazolyl nitronyl nitroxide) and the related compounds showing supramolecular H-bonding interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)(2)X(2)], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO(3); M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P2(1)/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/k(B) = +85 K, where the spin Hamiltonian is defined as H = -2J(S(1).S(2) + S(2).S(3)) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical pi and metal dsigma orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)(2)X(2)] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)(2)Cl(2)], and [Cu(4ImNNH)(2)Br(2)] always have intermolecular hydrogen bonds of H(imidazole).X(axial ligand)-M, where X = NO(3), Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.     

Effects of electron configuration and coordination number on the vibrational circular dichroism spectra of metal complexes of trans-1,2-diaminocyclohexane

Transition metal complexes of ethylenediamine have attracted significant interest as prototype systems for a range of studies related to their chiroptical properties. In order to better elucidate the effects of different central metal ions and also different coordination numbers on the vibrational circular dichroism (VCD) spectra, trans-1,2-diamino cyclohexane (chxn) was chosen as the chiral ligands in the current report. In this case the conformation of the diamino ligand is predetermined by its absolute configuration and the transition from the λ- to the δ-form that can occur in the case of ethylenediamine is no longer possible. The fingerprint region of the vibrational absorption and VCD spectra of three transition metal complexes of chxn have been analysed in detail. For the tris chelate complexes Ni(chxn)(3)(2+) and Cu(chxn)(3)(2+), selective enhancement of some VCD bands in the otherwise almost identical spectra has been observed and explained in terms of a ring current mechanism and of a different number of unpaired electrons of the metal centers. The comparison of the VCD spectra of Cu(chxn)(3)(2+) and Cu(chxn)(2)(2+) reveals the effects of coordination number that manifest as an inversion of the strong bisignate VCD pattern of the NH(2) scissor vibrational modes. This leads to the conclusion that this region can be used to extract information about the ligand environment and the chirality of the metal center.     

Electronic structure of Co(III) doped bromo-bridged Ni complexes, [Ni1-xCox(chxn)2Br]Br2

This article describes the electronic structure of the Co(III) doped Br bridged Ni(III) complexes, [Ni(1-x)Cox(chxn)2Br]Br2 (x = 0.01, 0.02, 0.05, and 0.11) by using a optical spectroscopy, scanning tunneling microscopy (STM), and electron spin resonance spectroscopy. In the optical reflectivity spectrum, the new band was formed at about 0.5 eV, which is reasonably recognized as the d(z2) band of doped Co(III) ions. In the STM images of [Ni(1-x)Cox(chxn)2Br]Br2, the bright spots attributable to the tunnel current from the Fermi level of the STM tip to the conduction band of the sample were observed. In addition, some brighter spots were also observed. Because the number of the brighter spots is in good agreement with that of doped Co species, the brighter spots can be assigned to doped Co(III) sites. These are reasonably explained by the tunnel current from the Fermi level of the tip to the d(z2) band of Co(III). The Curie spin concentration was gradually increased with increasing Co(III) ions, which is explained by the scissions of the S = 1/2 1D antiferromagnetic chains.     

Biferrocene-M(mnt)2 charge-transfer complexes (M = Ni, Co; mnt = maleonitriledithiolate). structure, valence states, and magnetic properties

Charge-transfer salts of branched-alkyl biferrocenes, (1',1' '-R2-1,1' '-biferrocene)[Ni(mnt)2] (1a, R = isopropyl; 2a, R = dineopentyl) and (1',1' '-R2-1,1' '-biferrocene)2[Co(mnt)2]2 (1b, R = isopropyl; 2b, R = dineopentyl), were prepared. Their valence states were investigated using X-ray crystallography and M?ssbauer spectroscopy. Complexes 1a and 1b show segregated-stack crystal structures that contain columns of acceptors, whereas structures of 2a and 2b, which contain bulky donors, are rather discrete. All of the complexes contain mixed-valent biferrocenium monocations. A two-step valence transition was found in complex 1a. The crystal contains two crystallographically independent cations: one undergoes valence localization below room temperature; the other undergoes valence localization below ca. 130 K. The former transition is derived from asymmetry of the crystal environment around the cation, whereas the latter one is caused by symmetry lowering coupled with a spin-Peierls transition (T(C) = 133.2 K) associated with the dimerization of the acceptors. This compound was found to exhibit a dielectric response based on valence tautomerization. Other complexes (1b, 2a, and 2b) show a valence-trapped state. In all complexes, charge localization was found to occur through local electrostatic interactions between the donor's cationic moiety and the acceptor's electronegative moieties.     

35Cl NQR and 1H NMR studies of [C(NH2)3]AuCl4, [C(NH2)3]2PtCl4, and [C(NH2)3]2PdCl4

The temperature dependence measurements of ³⁵Cl NQR frequencies and ¹H NMR spin-lattice relaxation time T₁ were carried out for guanidinium tetrachloro-aurate(III), -platinate(II), and -palladate(II). The gold(III) complex showed four NQR lines at various temperatures between 77 and 344 K, while the platinum-(II) and palladium(II) complexes gave two NQR lines in the temperature ranges 77–169 K and 77–220 K, respectively. An unusual phase transition was located at 363 K for the gold(III) complex. The high-temperature phase was easily supercooled. All the complexes studied yielded a T₁ minimum attributable to the reorientation of the planar cation about its C₃ axis. The motional parameters were evaluated. The Zeeman-quadrupole cross relaxation between protons and chlorine nuclei was observed for the platinum(II) and palladium(II) complexes at various temperatures below room temperature, while it was also detected for the high-temperature phase of the gold(III) complex.     

NQR and phase transitions in mixed A2BX6 compounds

³⁵Cl NQR frequency and phase transition temperature were investigated in the mixed crystals (A_xK_1?x) SnCl₆ (A = Rb, NH₄) and K₂ (Re_x Sn_1?x) Cl₆. The frequency and the intensity of the NQR signal were measured in the temperature range 100 K to 300 K for different chemical compositions. Phase transition temperatures were determined from the modifications in the line pattern. The observed change of the transition temperature as a function of concentration x can be fitted by the relation T_c(x) = T_c(O) (1 + a x)/(1 + b x). This formula is explained in the context of the virtual crystal approximation and the Landau theory.     

Tuning of electronic structures of quasi-one-dimensional bromo-bridged Ni(III) complexes with strong electron-correlation by doping of Co(III) ions, [Ni(1-x)Co(x)(Chxn)(2)Br]Br(2)

We have succeeded in synthesizing the Ni(III) complexes doped by Co(III) ions, [Ni(1-x)Co(x)(chxn)(2)Br]Br(2) (x = 0, 0.043, 0.093, and 0.118) by using an electrochemical oxidation method. The single-crystal reflectance spectrum of x = 0.118 shows an intense CT band about 0.5 eV, which is lower than that of [Ni(chxn)(2)Br]Br(2) (1.3 eV). The single-crystal electrical conductivities at room temperature of these compounds increase with increase of the amounts of doping of Co(III) ions. In the ESR spectra, peak-to-peak line widths DeltaH(pp) at room temperature change about 600 G in [Ni(chxn)(2)Br]Br(2) to 200 G in x = 0.118. Such a large x dependence of DeltaH(pp) seems to be ascribed to the increasing contribution from the increasing Curie spins which have smaller line width. Therefore, we have tuned the electronic structures of quasi-one-dimensional bromo-bridged Ni(III) complexes with strong electron correlations by doping of Co(III) ions.     

Single crystal NQR and NMR study of (NH4)2InBr6·H2O

The ⁸¹Br NQR tensors of (NH₄)₂InBr₅·H₂O have been studied on single crystals by combining a 4π Zeeman goniometer with Fourier transform NQR. AT T=293K the coupling constants are 136.27 e²φ_ZZQh^?(su81Br)/Mhz ≦ 146.14 and the asymmetry parameter 0.013 ≦ η (⁸¹Br) ≦ 0.133. The EFG axes φ_zz(⁸¹Br) are almost parallel to the bond directions InBr. The strong changes of φ_yy(¹¹⁵In) and φ_zz(¹¹⁵In) with temperature can be explained by assuming small changes of the In(OH₂) bond distance in the complex anion (InBr₅·H₂Cl^2?. The D₂O molecule within the deuterated complexes flips freely in the range studied (100 ≦ T/K ≦ 298). Small EFG's at the ¹⁴N site in the cation ND^?₄ are due to the EFG of the surrounding lattice charges.     

Br2 elimination in 248-nm photolysis of CF2Br2 probed by using cavity ring-down absorption spectroscopy

By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br2 molecular elimination following photodissociation of CF2Br2 at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Piou+-X1Sigmag+ transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br2(v=1)/Br2(v=0) is determined to be 0.4+/-0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br2 elimination reaction is also measured to be 0.04+/-0.01. This work provides direct evidence to support molecular elimination occurring in the CF2Br2 photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF2Br2 is excited probably to the 1B1 and 3B2 states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the 1A'(1B1) state to the high vibrational levels of the ground state X 1A'(1A1) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF2Br2 feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF2Br+Br(CF2Br...Br) and the products CF2+Br2. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br2 elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br2 fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br2(v=1)Br2(v=0) agrees with the proposed mechanism.     

Surface Ohmic Conductivity on a Mott Insulator Based on a One-dimensional Bromide-bridged Nickel(III) Complex

Ohmic contacts are of critical importance to improve the performance of semiconductor devices as well as those of low-dimensional materials. Halogen-bridged metal complexes (MX-Chains) are fascinating quasi-one-dimensional (1D) electronic materials. However, reports on the electrical characteristics of MX-Chains remain elusive. Herein, we report the electrical characteristics of single crystals of [Ni(chxn)₂Br]Br₂ (chxn: (1R,2R)-(−)-1,2-cyclohexanediamine), which is a 1D Mott insulator, using different probe arrangements. [Ni(chxn)₂Br]Br₂ was shown for the first time to exhibit Ohmic characteristics on the crystal surface, albeit that the conductivity behavior of the interior of the crystal is nonlinear due to the Joule heating effect. The current flow on the surface is shallow (a few micrometers in depth) despite the millimeter-scale crystal size; this phenomenon is strongly connected to its structural anisotropy. The Ohmic contact revealed in this work should be valuable for the application of MX-Chains as electronic devices in the near future.     

Weakly coordinating anions: crystallographic and NQR studies of halogen-metal bonding in silver, thallium, sodium, and potassium halomethanesulfonates

35Cl, (79,81)Br, and (127)I NQR (nuclear quadrupole resonance) spectroscopy in conjunction with X-ray crystallography is potentially one of the best ways of characterizing secondary bonding of metal cations such as Ag(+) to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O(3)SCH(2)Cl) (a = 13.241(3) A; b = 7.544(2) A; c = 4.925(2) A; orthorhombic; space group Pnma; Z = 4) and compared it with the known structure of Ag(O(3)SCH(2)Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598-3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. (127)I NQR studies on Ag(O(3)SCH(2)I) show the expected NQR consequences of three-coordination of iodine: substantially reduced NQR frequencies nu(1) and nu(2) and a fairly small NQR asymmetry parameter eta. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O(3)SCH(2)X) becomes more substantial in the series X = Cl < Br < I, indicating that the coordination to Ag(+) strengthens in this series, as expected from hard-soft acid-base principles. The numbers of electrons donated by the organic iodine atom to Ag(+) have been estimated; these indicate that the bonding to the cation is weak but not insignificant. We have not found any evidence for the bonding of these organohalogen atoms to another soft-acid metal ion, thallium. A scheme for recycling of thallium halide wastes is included.     

Electronic Effects in X(CH2)nY Homologous Series by NMR and Quantum Chemistry

The ^79,81Br NQR spectra of compounds of the Br(CH₂) _n COOH (n = 1-5) and Br(CH₂) _n COOSi·(CH₃)₃ (n = 1, 2, 4, 5) homologous series were measured at 77 K. The NQR frequencies of compounds of the first series as n increases to 3 and then oscillate. The ³⁵Cl NQR frequencies of Cl(CH₂) _n Cl series molecules, estimated from the Cl3p populations resulting from RHF/6-31G(d) calculations, steadily decrease as n increases from 1 to 10. No oscillation of calculated ³⁵Cl frequencies was revealed for compounds of the latter series (by contrast to experimental observations). The calculation results give no way to deducing the mechanism of the oscillation effect.     

Carrier doping effect into quasi-one-dimensional bromo-bridged Ni(III) complexes with strong electron-correlation by Cu(II) ions, [Ni1-xCux(chxn)2Br]Br2-x

This communication will describe the electron doping effect into Ni(III) complexes by Cu(II) ions, [Ni(1-x)Cu(x)(chxn)(2)Br]Br(2-x) (x = 0.038 and 0.101) by using an electrochemical oxidation method. A drastic increase of electrical conductivity as well as a new absorption band around 0.5 eV in single crystal reflectance spectra was observed by doping Cu(II) ions, indicating the electron doping was successfully made. An ESR result shows unpaired electrons locate in the d(x2-y2) orbitals of Cu(II) and have almost no interaction with those of other ions.     

Gas phase SN2 reactions of halide ions with trifluoromethyl halides: front- and back-side attack vs. complex formation

Density functional theory computations and pulsed-ionization high-pressure mass spectrometry experiments have been used to explore the potential energy surfaces for gas-phase S(N)2 reactions between halide ions and trifluoromethyl halides, X(-) + CF(3)Y --> Y(-) + CF(3)X. Structures of neutrals, ion-molecule complexes, and transition states show the possibility of two mechanisms: back- and front-side attack. From pulsed-ionization high-pressure mass spectrometry, enthalpy and entropy changes for the equilibrium clustering reactions for the formation of Cl(-)(BrCF(3)) (-16.5 +/- 0.2 kcal mol(-1) and -24.5 +/- 1 cal mol(-1) K(-1)), Cl(-)(ICF(3)) (-23.6 +/- 0.2 kcal mol(-1)), and Br(-)(BrCF(3)) (-13.9 +/- 0.2 kcal mol(-1) and -22.2 +/- 1 cal mol(-1) K(-1)) have been determined. These are in good to excellent agreement with computations at the B3LYP/6-311+G(3df)//B3LYP/6-311+G(d) level of theory. It is shown that complex formation takes place by a front-side attack complex, while the lowest energy S(N)2 reaction proceeds through a back-side attack transition state. This latter mechanism involves a potential energy profile which closely resembles a condensed phase S(N)2 reaction energy profile. It is also shown that the Cl(-) + CF(3)Br --> Br(-) + CF(3)Cl S(N)2 reaction can be interpreted using Marcus theory, in which case the reaction is described as being initiated by electron transfer. A potential energy surface at the B3LYP/6-311+G(d) level of theory confirms that the F(-) + CF(3)Br --> Br(-) + CF(4) S(N)2 reaction proceeds through a Walden inversion transition state.     

Structural phase transition driven by spin-lattice interaction in a quasi-one-dimensional spin system of [1-(4'-iodobenzyl)pyridinium][Ni(mnt)2

Crystal structures and magnetic properties were determined for two novel compounds, [1-(4'-iodobenzyl)pyridinium][M(mnt)2] (mnt2- = maleonitriledithiolate; M = Ni (1) or Cu (2)). At room temperature, single crystals of 1 and 2 were isostructural, featuring the formation of segregated columnar structures with regular stacks of cations and anions. For crystal 1, a magnetic transition was observed at approximately 120 K; furthermore, its magnetic behavior was consistent with that of a regular Heisenberg antiferromagnetic (AFM) chain of S = 1/2 in the high-temperature phase (HT phase) and that of a spin-gap system in the low-temperature phase (LT phase). Such a phenomenon is similar to the spin-Peierls transition. However, the crystal structure of 1 in the LT phase at 100 K revealed that its structural transition is associated with the magnetic transition. Because crystal 2 (S = 0) did not exhibit a structural transition, the structural transition of 1 is driven by spin-lattice interaction.     

Kinetics of the reactions of CH2I, CH2Br, and CHBrCl radicals with NO2 in the temperature range 220-360 K

The kinetics of the CH2I + NO2, CH2Br + NO2, and CHBrCl + NO2 reactions have been studied at temperatures between 220 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time-resolved measurements to obtain reaction rate coefficients under pseudo-first-order conditions. The bimolecular rate coefficients of all three reactions are independent of the bath gas (He or N2) and pressure within the experimental range (2-6 Torr) and are found to depend on temperature as follows: k(CH2I + NO2) = (2.18 +/- 0.07) x 10(-11) (T / 300 K)(-1.45) (+/- 0.22) cm3 molecule(-1) s(-1) (220-363 K), k(CH2Br + NO2) = (1.76 +/- 0.03) x 10(-11) (T/300 K)(-0.86) (+/- 0.09) cm3 molecule(-1) s(-1) (221-363 K), and k(CHBrCl + NO2) = (8.81 +/- 0.28) x 10(-12) (T/300 K)(-1.55) (+/- 0.34) cm3 molecule(-1) s(-1) (267-363 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about +/-25%. In the CH2I + NO2 and CH2Br + NO2 reactions, the observed product is formaldehyde. For the CHBrCl + NO2 reaction, the product observed is CHClO. In addition, I atom and iodonitromethane (CH2INO2) or iodomethyl nitrite (CH2IONO) formations have been detected for the CH2I + NO2 reaction.