Dimer-formation in the bis(arene)chromium fulleride Cr(C7H8)2 C60

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3 N@Ih -C80 in Neutral, Radical Anion, and Dimeric Anionic Forms

A series of compounds with Sc₃ N@I_h -C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃ N@I_h -C₈₀ ⋅3 C₆ H₄ Cl₂ (1) and (Sc₃ N@I_h -C₈₀ )₃ (TPC)₂ ⋅5 C₆ H₄ Cl₂ (2, TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃ N@I_h -C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃ N@I_h -C₈₀ ^- )₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2.5 C₆ H₄ Cl₂ (3) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (4) and {[2.2.2]cryptand- (Cs⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (5) in which only one major dimer orientation was found. The thermal stability of the (Sc₃ N@I_h -C₈₀ ^- )₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400-460 K range producing monomeric Sc₃ N@I_h -C₈₀ ^.- radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C-C bond was estimated to be 234±7 kJ mol^-1 , the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃ MeP⁺ )₃ (Sc₃ N@I_h -C₈₀ ^.- )₃ ⋅C₆ H₄ Cl₂ (6) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃ N@I_h -C₈₀ ^.- . Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃ N cluster. Both the monomeric and dimeric Sc₃ N@I_h -C₈₀ ^- anions show an intrinsic shift of the IR bands attributed to the Sc₃ N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.

Negatively charged singly-bonded dimers of C1-[C70(CF3)10] and bare C70 fullerene

Singly-bonded {C₇₀(CF₃)₁₀⁻}₂ and (C₇₀⁻)₂ dimers are obtained as crystalline salts allowing their molecular structure, and optical and magnetic properties to be studied.

Synthesis, Structure, and Properties of the Fullerene C60 Salt of Crystal Violet, (CV(+) )(C60 (.-) )⋅0.5 C6 H4 Cl2 , which Contained Closely Packed Zigzagged C60 (.-) Chains

The reduction of fullerene C60 by zinc dust in the presence of crystal violet cations (CV(+) ) yielded a deep-blue solution, from which crystals of (CV(+) )(C60 (.-) )⋅0.5 C6 H4 Cl2 (1) were obtained by slow mixing with n-hexane. The salt contained isolated, closely packed zigzagged chains that were composed of C60 (.-) radical anions with a uniform interfullerene center-to-center distance of 9.98 Å. In spite of the close proximity of the fullerenes, they did not dimerize, owing to spatial separation by the phenyl substituents of CV(+) . The room-temperature conductivity of compound 1 was 3×10(-2)  S cm(-1) along the fullerene chains. The salt exhibited semiconducting behavior, with an activation energy of Ea =167 meV. Spins localized on C60 (.-) were antiferromagnetically coupled within the fullerene chains, with a Weiss temperature of -19 K without long-range magnetic ordering down to 1.9 K.

Strong antiferromagnetic coupling of spins in the (MDABCO+)(C60·-) salt with 3D close packing of the C60·- radical anions (MDABCO+: N-methyldiazabicyclooctanium cation)

A new salt, (MDABCO(+))(C60(·-)) (1; MDABCO(+) = N-methyldiazabicyclooctanium cation), was obtained as single crystals. The crystal structure of 1 determined at 250 and 100 K showed 3D close packing of fullerenes with eight fullerene neighbors for each C60(·-). These neighbors are located at 10.01-10.11 Å center-to-center distances (250 K) and van der Waals interfullerene C⋅⋅⋅C contacts are formed with four fullerene neighbors arranged in the bc plane. Fullerene ordering observed below 160 K is accompanied by the appearance of one and a half independent C60(·-) and trebling of the unit cell along the b axis. Fullerenes are packed closer to each other at 100 K. As a result, fullerenes are located in the three-dimensional packing at 9.91-10.12 Å center-to-center distances and 18 short interfullerene C⋅⋅⋅C contacts are formed for each C60(·-). Although they are closed packed, fullerenes are not dimerized down to 1.9 K. Magnetic data indicate strong antiferromagnetic coupling of spins in the 70-300 K range with a Weiss temperature of Θ = -118 K. Magnetic susceptibility shows a round maximum at 46 K. Such behavior can be described well by the Heisenberg model for square two-dimensional antiferromagnetic coupling of spins with an exchange interaction of J/kB = -25.3 K. This magnetic coupling is one of the strongest observed for C60(·-) salts.

Synthesis and structure analysis of (K[DB18 C6])4(C60)5·12THF containing C60 in three different bonding states

A new fulleride, (K[DB18C6])(4)(C(60))(5)·12THF, was prepared in solution using the "break-and-seal" approach by reacting potassium, fullerene, and dibenzo[18]crown-6 in tetrahydrofuran. Single crystals were grown from solution by the modified "temperature difference method". X-ray analysis was performed revealing a reversible phase transition occurring on cooling. Three different crystal structures of the title compound at different temperatures of data acquisition are addressed in detail: the "high-temperature phase" at 225 K (C2, Z=2, a=49.055(1), b=15.075(3), c=18.312(4) Å, β=97.89(3)°), the "transitional phase" at 175 K (C2 m, Z=2, a=48.436(5), b=15.128(1), c=18.280(2) Å, β=97.90(1)°), and the "low-temperature phase" at 125 K (Cc, Z=4, a=56.239(1), b=15.112(3), c=36.425(7) Å, β=121.99(1)°). On cooling, partial radical recombination of C(60)(·-) into the (C(60))(2)(2-) dimeric dianion occurs; this is first time that the fully ordered dimer has been observed. Further cooling leads to formation of a superstructure with doubled cell volume in a different space group. Below 125 K, C(60) exists in the structure in three different bonding states: in the form of C(60)(·-) radical ions, (C(60))(2)(2-) dianions, and neutral C(60), this being without precedent in the fullerene chemistry, as well. Experimental observations of one conformation exclusively of the fullerene dimer in the crystal structure are further explained on the basis of DFT calculations considering charge distribution patterns. Temperature-dependent measurements of magnetic susceptibility at different magnetic fields confirm the phase transition occurring at about 220 K as observed crystallographically, and enable for unambiguous charge assignment to the different C(60) species in the title fulleride.

Effect of the Co-C(C60(-)) bond formation on magnetic properties of the ionic complex {cryptand[2,2,2] x (Na+)} x {Co(II)TPP x (C60(-))} x (C6H4Cl2)2

A new ionic complex {cryptand[2,2,2] x (Na(+))} x {Co(II)TPP x (C(60)(-))} x (C(6)H(4)Cl(2))(2) has been obtained as single crystals by a diffusion technique. It involves coordination {Co(II)TPP x u(C(60)(-))} units and bulky {cryptand[2,2,2] x (Na(+))} cations. The Co-C(C(60)(-)) coordination bond is short (2.262(2) A at 100 K) which provides ordering in the C(60)(-) anions. This is the first ordered structure of {Co(II)TPP x (C(60)(-))}. The coordination bonds weaken upon heating to be 2.316(4) A at 250 K. As a result, the C(60)(-) anions begin to rotate at 250 K about the Co-C bond between two orientations. The complex is diamagnetic from 4 up to 320 K. The Co-C bonds dissociate above 320 K only to form unbound paramagnetic Co(II)TPP and C(60)(*-) species. The process is accompanied by the reversible increase in magnetic susceptibility of the complex and the appearance of a new broad EPR signal with g = 2.1187 and a linewidth of approximately 120 mT (350 K). The signal was attributed to both paramagnetic Co(II)TPP and C(60)(*-), which showed strong exchange interaction.

Ionic and Neutral C60 Complexes with Coordination Assemblies of Metal Tetraphenylporphyrins, MIITPP2·DMP (M = Mn, Zn). Coexistence of (C60-)2 Dimers Bonded by One and Two Single Bonds in the Same Compound

Coordination assemblies of metal tetraphenylporphyrins, MIITPP2.DMP (M=Mn, Zn) were shown to form ionic multicomponent and neutral complexes with fullerene, {(MnIITPP)2.DMP}.(C60-)2.(DMETEP+)2.(C6H4Cl2)5 (1) and {(ZnTPP)2.DMP}.(C60)2.(C6H5Cl)4 (2), where DMP=N,N'-dimethylpiperazine and DMETEP+=the cation of N,N'-dimethyl-N'-ethylthioethylpiperazine. The crystal structure of 1 contains zigzag chains of the (C60-)2 dimers alternating with the DMETEP+ cations in the channels formed by the (MnIITPP)2.DMP units, whereas in 2 zigzag chains of the C60 molecules are separated by the (ZnTPP)2.DMP units and C6H5Cl molecules. The (MIITPP)2.DMP assemblies (M=Mn, Zn) have axial M-N(DMP) bonds of 2.315(2) and 2.250(2) A length, average equatorial M-N(DMP) bonds elongated to 2.141(3) and 2.077(2) A, and MII atoms displaced from the porphyrin plane toward the ligand by 0.677 and 0.485 A, respectively. The single-bonded sigma-(C60-)2 dimer coexists in 1 with the (C60-)2 dimer bonded by two single bonds with 86/14 occupancy factors. The sigma-(C60-)2 dimers are unusually stable and begin to dissociate only above a temperature of 320-330 K that results in the increase of the magnetic moment of 1 from 8.33 microB (320 K) to 8.66 microB (360 K). The electron paramagnetic resonance (EPR) signal of the dimeric phase (T<320 K) with the features spread over the range of 0-0.7 T was attributed to the interacting Mn2+ centers in the (MnIITPP)2.DMP units. The dissociation of the sigma-(C60-)2 dimers to the EPR-active C60*- radical anions manifests a new broad Lorenz signal above 320 K with g=2.0179 and DeltaH=65.5 mT. This signal can appear due to the exchange coupling between paramagnetic (MnIITPP)2.DMP and C60*- species. The vis-NIR spectrum of the sigma-(C60-)2 dimers is discussed.

Negatively Charged π-(C60-)2 Dimer with Biradical State at Room Temperature

A negatively charged pi-(C60-)2 dimer bonded by two single bonds was found in the ionic multicomponent complex {(MDABCO+).CoIITMPP}2.(C60-)2.(C6H4Cl2)2.5.(C6H5CN)1.5 (1). In contrast to the previously described diamagnetic sigma-(C60-)2 dimer, the negatively charged pi-dimer has a biradical state at room temperature: (C60*-)2 (S = 1). The behavior of spins in this dimer can be described by a model with a singlet ground state (S = 0) and a close lying excited triplet (S = 1) state with the energy gap of 2|JAF| = 70 +/- 2 cm-1. On the whole, complex 1 shows a strong antiferromagnetic interaction of spins with a Weiss constant of -34 K.

Structural aspects of two-stage dimerization in an ionic C60complex with bis(benzene)chromium: Cr(C6H6)2·C60·C6H4Cl2

Single crystals of the ionic C60 complex with bis(benzene)chromium: {Cr(I)(C6H6)2(.+)}.(C60.-)).C6H4Cl2 (1) were obtained. The crystal structure of 1 shows the presence of monomeric C60.- radical anions at 250 K and the formation of single-bonded (C60-)2 dimers at 90 K. The dimerization is realized in two types of the C60.-) pairs with different interfullerene center-to-center distances of 10.052 and 10.279 A arranged in zigzag chains along the a-direction. As a result, two symmetrically independent (C60-)2 dimers found in 1 at 90 K have different environments, intercage C-C bond lengths and C60- center-to-center distances. Such differences should provide different thermal stability of these dimers and result in the appearance of two stages at the dimerization. Indeed, according to SQUID measurements, the magnetic moment of 1 decreases stepwise at the dimerization in two temperature ranges at 240-200 and 200-160 K.

Neutral and Ionic Complexes of C60 with Metal Dibenzyldithiocarbamates. Reversible Dimerization of C60•- in Ionic Multicomponent Complex [CrI(C6H6)2•+]·(C60•-)·0.5[Pd(dbdtc)2]

New molecular complexes of C60 with metal(II) dibenzyldithiocarbamates, M(dbdtc)2.C60.0.5(C6H5Cl), where M=Cu(II), Ni(II), Pd(II), and Pt(II) and an ionic multicomponent complex [Cr(I)(C6H6)2*+].(C60*-).0.5[Pd(dbdtc)2] (Cr(C6H6)2: bis(benzene)chromium) were obtained. According to IR, UV-visible-NIR, and EPR spectra, involve neutral components, whereas 5 comprises neutral Pd(dbdtc)2 and C60*- and Cr(I)(C6H6)2*+ radical ions. The crystal structure of at 90 K reveals strongly puckered fullerene layers alternating with those composed of Pd(dbdtc)2. The Cr(I)(C6H6)2*+ radical cations are arranged between the layers. Fullerene radical anions form pairs within the layer with an interfullerene C...C contact of 3.092(2) A, indicating their monomeric state at 90 K. This contact is essentially shorter than the sum of van der Waals radii of two carbon atoms, and consequently, C60*- can dimerize. According to SQUID and EPR, single-bonded diamagnetic (C60-)2 dimers form in below 150-130 K on slow cooling and dissociate above 150-170 K on heating. The hysteresis was estimated to be 20 K. For the (C60-)2 dimers in, the dissociation temperature is the lowest among those for ionic complexes of C60 (160-250 K). Fast cooling of the crystals within 10 min from room temperature down to 100 K shifts dimerization temperatures to lower than 60 K. This shift is responsible for the retention of a monomeric phase of at 90 K in the X-ray diffraction experiment.

Solid-State 93Nb and 13C NMR Investigations of Half-Sandwich Niobium(I) and Niobium(V) Cyclopentadienyl Complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.

Formation of single-bonded (C60-)2 and (C70-)2 dimers in crystalline ionic complexes of fullerenes

New ionic complexes of fullerenes C(60) and C(70) with decamethylchromocene Cp*(2)Cr.C60.(C(6)H(4)Cl(2))(2) (1), Cp*(2)Cr.C60.(C(6)H(6))(2) (2); the multicomponent complex of (Cs(+))(C70-) with cyclotriveratrylene CTV.(Cs)(2).(C70)(2).(DMF)(7).(C(6)H(6))(0.75) (3); bis(benzene)chromium Cr(C(6)H(6))(2).C60.(C(6)H(4)Cl(2))(0.7) (4), Cr(C(6)H(6))(2).C60.C(6)H(5)CN (5), Cr(C(6)H(6))(2).C70.C(6)H(4)Cl(2) (6), Cr(C(6)H(6))(2).C60 (7); cobaltocene Cp(2)Co.C60.C(6)H(4)Cl(2) (8), Cp(2)Co.C70.(C(6)H(4)Cl(2))(0.5) (9); and cesium Cs.C70.(DMF)(5) (10) have been obtained. The complexes have been characterized by the elemental analysis, IR-, UV-vis-NIR spectroscopy, EPR and SQUID measurements. It is shown that C(60)(.-) exists as a single-bonded diamagnetic (C60-)2 dimer in 1, 2, 4, 5, and 8 at low temperatures (1.9-250 K). The dimers dissociate above 160-250 K depending on donor and solvent molecules involved in the complex. C60(.-) dimerizes reversibly and shows a small hysteresis (<2 K) at slow cooling and heating rates. The single-bonded diamagnetic (C70-)2 dimers are also formed in 6, 9, and 10 and begin to dissociate only above 250-360 K. The IR and UV-vis-NIR spectra of sigma-bonded negatively charged fullerenes are presented.

The reversible formation of a single-bonded (C(60)(-))(2) dimer in ionic charge transfer complex: Cp*(2)Cr.C(60)(C(6)H(4)Cl(2))(2). The molecular structure of (C(60)(-))(2)

A new ionic complex of C60 with decamethylchromocene, Cp*2Cr.C60(C6H4Cl2)2 (1), has been obtained. The fullerides are monomeric in 1 at room temperature, whereas they form a single-bonded (C60-)2 dimer at low temperatures, the structure of which has been studied by the X-ray diffraction on a single crystal at 100 K. The length of the intercage C-C bond is 1.597(7) A and the interfullerene distance is equal to 9.28 A. A phase transition attributed to the reversible C60*- dimerization is observed in the 220-200 K range. The transition is accompanied by changes in the unit cell parameters, the decrease of the magnetic moment from 4.20 muB (S = 3/2, 1/2) to 3.88 muB (S = 3/2) and the appearance of EPR signal from Cp*2Cr+, simultaneously.