Cyclooctatetraene Dianion Research Articles

Million-fold Relaxation Time Enhancement across a Series of Phosphino-Supported Erbium Single-Molecule Magnets.

Maintaining strong magnetic anisotropy in the presence of collective spin interactions has become a defining challenge in the advancement of single-molecule magnet (SMM) research. Herein we demonstrate effective decoupling of these often competing design goals in a series of new phosphino-supported SMMs containing the anisotropic [Er(COT)]+ (COT2- = cyclooctatetraene dianion) subunit. Across this series, a magnetic nuclearity increase from 1 to 2 and subsequent optimization of the relative local anisotropy axis orientation results in dramatic improvements to the long time scale behavior. Specifically, we observe a 6 orders of magnitude increase in relaxation time at 2 K and a consequent open magnetic hysteresis up to 6 K. This drastic scaling of the magnetic dynamics tracks monotonically with the introduction and approach to parallel of the angle between intramolecular anisotropy axes. These results illustrate the powerful implications of fully controlling direction and magnitude of anisotropy in the design of scalable SMMs.

Enantiopure Benzamidinate/Cyclooctatetraene Complexes of the Rare-Earth Elements: Synthesis, Structure, and Magnetism

Novel chiral rare-earth half sandwich complexes containing the chiral amidinate ligand, (S,S)-N,N′-bis(1-phenylethyl)benzamidinate ((S)-PEBA) or the tBu analogue (S,S)-N,N′-bis(1-phenylethyl) pivalamidinate ((S)-PETA) and the cyclooctatetraene dianion (C8H8)2– (COT) were synthesized. All complexes were fully characterized and the solid state structures were established by single-crystal crystallography. Magnetic property measurements indicated that the complex [{(S)-PETA}Er(COT)(THF)] is a typical field-induced single-molecule magnet (SMM), of which the magnetic properties are in reasonable agreement with high-level quantum chemical calculations. Due to predominantly electrostatic ligand field effects, the relativistic J = 15/2 ground manifold of Er(III) is split into 8 Kramers doublets spanning a range of ∼500 cm–1, representable by means of higher order spin tensor operators. According to the quantum chemical calculations as well as from analysis of the experimental data, the magnetic relaxation pathway...

The dinuclear scandium(III) cyclooctatetraenyl chloride complex di-μ-chlorido-bis[(η8-cyclooctatetraene)(tetrahydrofuran-κO)scandium(III)

Only a few cyclooctatetraene dianion (COT) π-complexes of lanthanides have been crystallographically characterized. This first single-crystal X-ray diffraction characterization of a scandium(III) COT chloride complex, namely di-μ-chlorido-bis[(η8-cyclooctatetraene)(tetrahydrofuran-κO)scandium(III)], [Sc2(C8H8)2Cl2(C4H8O)2] or [Sc(COT)Cl(THF)]2 (THF is tetrahydrofuran), (1), reveals a dimeric molecular structure with symmetric chloride bridges [average Sc-Cl = 2.5972 (7) Å] and a η8-bound COT ligand. The COT ring is planar, with an average C-C bond length of 1.399 (3) Å. The Sc-C bond lengths range from 2.417 (2) to 2.438 (2) Å [average 2.427 (2) Å]. Direct comparison of (1) with the known lanthanide (Ln) analogues (La, Ce, Pr, Nd, and Sm) illustrates the effect of metal-ion (M) size on molecular structure. Overall, the M-Cl, M-O, and M-C bond lengths in (1) are the shortest in the series. In addition, only one THF molecule completes the coordination environment of the small ScIII ion, in contrast to the previously reported dinuclear Ln-COT-Cl complexes, which all have two bound THF molecules per metal atom.

Ferromagnetic coupling in a chloride-bridged erbium single-molecule magnet.

We present the first ferromagnetically-coupled Er3+ complex with linked, high-anisotropy Er-COT (COT2- = cyclooctatetraene dianion) subunits. The dinuclear complex, [Er(μ2-Cl)(COT)(THF)]2, demonstrates single-molecule magnetism with a single, zero-field magnetization relaxation barrier of Ueff = 113 cm-1. This system offers evidence that anisotropy can be preserved in the presence of ferromagnetic linking of the Er-COT subunits, providing a rational means to build strong molecular magnets of tunable dimensionality.

Magnetic Blocking at 10 K and a Dipolar-Mediated Avalanche in Salts of the Bis(η8-cyclooctatetraenide) Complex [Er(COT)2]−

The structures and magnetic properties of [K(18-crown-6)](+) (1) and [K(18-crown-6)(THF)2](+) (2) salts of the η(8)-cyclooctatetraenide sandwich complex [Er(COT)2](-) (COT(2-) = cyclooctatetraene dianion) are reported. Despite slight differences in symmetry, both compounds exhibit slow magnetic relaxation under zero applied dc field with relaxation barriers of ∼150 cm(-1) and waist-restricted magnetic hysteresis. Dc relaxation and dilution studies suggest that the drop in the magnetic hysteresis near zero field is influenced by a bulk magnetic avalanche effect coupled with tunneling of the magnetization. Through dilution with [K(18-crown-6)(THF)2][Y(COT)2] (3), these phenomena are substantially quenched, resulting in an open hysteresis loop to 10 K. Importantly, this represents the highest blocking temperature yet observed for a mononuclear complex and the second highest for any single-molecule magnet. A comprehensive comparative analysis of the magnetism of [K(18-crown-6)][Ln(COT)2] (Ln = Sm, Tb, Dy, Ho, Yb) reveals slow relaxation only for [K(18-crown-6)][Dy(COT)2] (4) with weak temperature dependence. Collectively, these results highlight the utility of an equatorial ligand field for facilitating slow magnetic relaxation in the prolate Er(III) ion.

Free Cyclooctatetraene Dianion: Planarity, Aromaticity, and Theoretical Challenges.

The planarity and 10 π-electron aromaticity of the free cyclooctatetraene dianion (C8H82–, COT2–) have been questioned recently on the basis of conflicting density functional and second-order Moller–Plesset perturbation computations. Rigorous coupled-cluster methods are employed here to establish the structure and properties of COT2–. Like many multiply charged anions, COT2– exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT2– rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT2– but mainly describe COT in a continuum of free electrons. All-electron coupled cluster theory extended through triple excitations [AE-CCSD(T)] yields a planar D8h symmetry COT2– structure. Final focal point analyses place the COT2– resonance state 61.6 kcal mol–1 above neutral COT. Nonetheless, COT2– exhibits structural, magnetic, and energet...

A six-carbon 10π-electron aromatic system supported by group 3 metals

Aromaticity is a fundamental concept with implications spanning all the chemical sciences. Hückel's (4n+2)π-electron rule is the standard criterion to determine aromaticity and it applies well to neutral arenes as well as to charged species such as the cyclopentadienyl anion, the cyclooctatetraene dianion and the cycloheptatrienyl cation (tropylium). In the series of all-carbon aromatic compounds, no example of a benzene tetraanion, which is theoretically a 6C, 10π-electron aromatic system, has been reported although heteroatom analogues of such a system, known as 'electron-rich aromatics', have been studied in detail for a long time. Here we present the isolation of the first tetraanionic-substituted benzene as a ligand coordinated to group 3 metals. The nature of the substituted benzene tetraanion and the aromaticity of the 6C, 10π-electron system were established by X-ray crystallographic studies, multi-nuclei nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy and density functional theory calculations.

Series of Lanthanide Organometallic Single-Ion Magnets

The synthesis, structures, and magnetic properties of a series of lanthanide organometallic mixed sandwich molecules, (Cp*)Ln(COT), are investigated, where Cp* is the pentamethylcyclopentadiene anion and COT is the cyclooctatetraene dianion and Ln represents Tb(III), Dy(III), Ho(III), Er(III), and Tm(III). Among the five complexes, Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex. Both Dy and Ho complexes show obvious quantum tunneling magnetization relaxation in the absence of a static field. The diluted Ho complex behaves two sets of thermally activated relaxation as we reported previously in Er due to the COT ring static disorder. A stair-shaped hysteresis for the Er compound can be observed at 1.6 K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling decoherence relaxation rate increases from Er to Ho to Dy, which may be caused by the relative increase of transverse anisotropy coming from the larger tilting of the two aromatic rings within the molecule. The fine electronic structure is analyzed with ligand-field theory employing the effective Hamiltonian method. The zero-field splitting is determined to be Ising type, and the energy gap between the ground state and the first excited one is consistent with the barrier obtained by Arrhenius analysis.

An experimental estimate of the relative aromaticity of the cyclooctatetraene dianion by fusion to dimethyldihydropyrene

The synthesis of the cyclooctatetraene dianion (COT(2-)) fused at the [e]-position of trans-10b,10c-dimethyl-10b,10c-dihydropyrene (DHP) is described, and by comparison of (1)H NMR properties and NICSAv to the analogous benzene fused DHP, the relative aromaticity of the dianion is found to be at least as great as that of benzene, and substantially larger than that of the cyclopentadienide anion.

Is Free Cyclooctatetraene Dianion an Aromatic System? A Quantum Chemistry Study

AbstractTo investigate whether free cyclooctatetraene dianion (COT2−) is aromatic, quantum chemistry methods were used to optimize its structure. Based on the optimized structures, the natural population analysis (NPA) charge, bond order, delocalization energy, nucleus‐independent chemical shift (NICS), and harmonic oscillator model of aromaticity (HOMA) values were computed by DFT‐B3LYP method with basis set 6‐311++G**, which shows that COT2− is not aromatic as it is not planar and has different bond lengths and bond orders, smallest delocalization energy and positive NICS values. To further confirm the finding, the changes of NICS and energy against ring distortion angle were scanned. The COT2− has positive NICS values all along the angle from 180° to 120° while other aromatic systems always have negative values. The energy scanning suggests that COT2− should have the weakest capability to maintain its planar structure. All the calculations strongly indicate that COT2− is not aromatic. This study also suggests that NICS scan might be a good approach to judge aromaticity.

Calcium, Strontium, Barium, and Ytterbium Complexes with Cyclooctatetraenyl or Cyclononatetraenyl Ligands1

Neutral triple-decker complexes of the heavy alkaline earth metals and ytterbium with tetraisopropylcyclopentadienide anions as terminal ligands and a cyclooctatetraene dianion as a middle deck have been synthesized from tetraisopropylcyclopentadienyl metal halide precursors and disodium cyclooctatetraenide. The pentaisopropylcyclopentadienyl analogue [{(C5iPr5)Yb}2(C8H8)] was prepared from ytterbium metal, cyclooctatetraene, and the free pentaisopropylcyclopentadienyl radical. X-ray crystal structure determinations for the barium and the calcium derivative show an almost linear arrangement of ring centers and metal atoms in both cases with metal-ring center distances of 2.33 A (Ca-Cp), 1.99/1.98 A (Ca-COT) and 2.71 A (Ba-Cp), 2.40 A (Ba-COT). The geometrical features of these molecules could be modeled quite accurately with density functional calculations. With potassium cyclononatetraenide, sparingly soluble bis(cyclononatetraenyl)barium could be prepared and characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. Cyclononatetraenyl(tetraisopropylcyclopentadienyl)barium was obtained from [(C5HiPr4)BaI(THF)2]2 and KC9H9 as a 1:1 mixture with octaisopropylbarocene. Density functional calculations predict sandwich structures with parallel rings and a 2.37 A Ba-ring distance for [Ba(C9H9)2] and a 174 bending with metal-ring distances of 2.72 A (Ba-Cp) and 2.35 A (Ba-CNT) for [(C5HiPr4)Ba(C9H9)]. All alkaline earth sandwich and triple-decker complexes mentioned above have been heated to 250 degrees C without decomposition and have been sublimed in oil pump vacuum.

Chemical applications of topology and group theory: 37. Pentalene as a ligand in transition metal sandwich complexes

AbstractThe η8‐C8H62− pentalene ligands in the recently discovered sandwich compounds (η8‐C8H6)2M (M = Ti, Zr) can be derived either by transannular ‘pinching’ of the cyclooctatetraene dianion, C8H82−, or by edge‐sharing fusion of two cyclopentadienyl ligands C5H5−. This paper depicts the relationship between the molecular orbitals in C8H62−, C8H82−, and 2C5H5−. In addition, this paper shows that the d orbital of the central metal atom is of suitable symmetry for δ bonding in the D2h complexes (η8‐C8H6)2M (M = Ti, Zr) unlike in the D8h complexes (η8‐C8H8)2M derived from cyclooctatetraene. This can account for the fact that the cyclooctatetraene sandwich compounds of stoichiometry (C8H8)2M have unsymmetrical (η8‐C8H8)(η4‐C8H8)M structures in the case of the d‐block metals titanium and zirconium in contrast to perhapto structures (η8‐C8H8)2An in the case of the actinides (An = Th, Pa, U, Np, Pu) where metal f orbitals are available for metal–ring bonding. Copyright © 2003 John Wiley & Sons, Ltd.

Cyclooctatetraene dianion from 1,5-cyclooctadiene. A synthesis in the presence of naphthalene radical anion

A novel synthesis of cyclooctatetraene dianion from 1,5-cyclooctadiene (1,5-COD) is described. The reaction of the potassium–naphthalene adduct with 1,5-COD proceeds at room temperature over 3 days in heptane/tetrahydrofuran to produce the target compound.

Chiral recognition between a substituted cyclooctatetraene dianion and a half crown ether substituted with ibuprofen.

(S)-Verbenol was substituted onto cyclooctatetraene (COT) via an ether linkage. In tetrahydrofuran (THF), Cs(+) or Na(+) counterions are tightly ion associated with the verbenoxy-COT dianion. A cosolvent, consisting of an ibuprofen unit connected to a half crown ether, was added to the verbenoxy-COT(2)(-),M(+)(2) solutions. The intimate interaction between the chiral cosolvent (ibuprofoxymethoxyethoxyethane) and the ion-associated counterion (either Na(+) or Cs(+)) forces a chiral recognition between the verbenoxy moiety and the ibuprofoxy moiety. When a molar excess of the cosolvent is present in the dianion THF solution, separation of the cosolvent associated with the verbenoxy-COT(2)(-),M(+)(2) complex from the uncomplexed cosolvent allows partial resolution of the enantiomers of ibuprofoxymethoxyethoxyethane.

Group 5 Metal Complexes with Cyclooctatetraene: Formation and Characterization of (Butadiene)(cyclooctatetraene)(cyclopentadienyl)tantalum Systems

The [(η4-diene)(η5-RCp)TaCl2] complexes 6a-e (diene = butadiene, isoprene or 2,3-dimethylbutadiene; RCp = pentamethylcyclopentadienyl, methylcyclopentadienyl or cyclopentadienyl) react with cyclooctatetraene dianion (employed as [(cot)Li2]) to yield the (η4-diene)(η3-cyclooctatetraene)(η5-RCp)tantalum complexes 7. The η3-cot coordination was confirmed by an X-ray crystal structure analysis of the compound [(supine-s-cis-η4-C4H6)(η3-cot)(η5-Cp*)Ta] 7a. In solution, the cot ligand of all the complexes 7 is fluxional. Two isomers [supine and proneorientation of the (diene)CpTa subunit] were found for the (η4-butadiene)(η3-cot)CpTa system (7e/7e′ = 95:5). Complex 7e cleanly adds the organometallic Lewis acid B(C6F5)3 to yield the betaine system [(η4-C4H6){C8H8B(C6F5)3}Cp*Ta] 8, which contains a substituted bicyclo[5.1.0]octadienyl ligand.

Group 5 Metal Complexes with Cyclooctatetraene: Formation and Characterization of (Butadiene)(cyclooctatetraene)(cyclopentadienyl)tantalum Systems

The [(η4-diene)(η5-RCp)TaCl2] complexes 6a-e (diene = butadiene, isoprene or 2,3-dimethylbutadiene; RCp = pentamethylcyclopentadienyl, methylcyclopentadienyl or cyclopentadienyl) react with cyclooctatetraene dianion (employed as [(cot)Li2]) to yield the (η4-diene)(η3-cyclooctatetraene)(η5-RCp)tantalum complexes 7. The η3-cot coordination was confirmed by an X-ray crystal structure analysis of the compound [(supine-s-cis-η4-C4H6)(η3-cot)(η5-Cp*)Ta] 7a. In solution, the cot ligand of all the complexes 7 is fluxional. Two isomers [supine and proneorientation of the (diene)CpTa subunit] were found for the (η4-butadiene)(η3-cot)CpTa system (7e/7e′ = 95:5). Complex 7e cleanly adds the organometallic Lewis acid B(C6F5)3 to yield the betaine system [(η4-C4H6){C8H8B(C6F5)3}Cp*Ta] 8, which contains a substituted bicyclo[5.1.0]octadienyl ligand.

Synthesis and structures of paramagnetic binuclear (η 8-1,4-bis(trimethylsilyl)cyclooctatetraenide)titanium(III) chlorides

The reaction between equimolar quantities of TiCl 3(THF) 3 and 1,4-bis(trimethylsilyl)cyclooctatetraene dianion (COT″) yields a mixture of (COT″) 2Ti ( 1), [(COT″)Ti] 2(μ-Cl) 3 ( 2) and [(COT″)Ti(μ-Cl)] 2(THF) ( 3). Crystalline 2 and 3 were isolated on the basis of different solubility in hexane. Their X-ray diffraction analyses revealed dimeric structures linked by chlorine bridges. In 3, only one of the Ti atoms coordinates one molecule of THF. The mixed-valency Ti(III)/Ti(IV) compound 2 shows an ESR spectrum typical of mononuclear (COT″)Ti(III) complexes. The ESR spectra of 3 in toluene or 2-methyltetrahydrofuran glasses prove the triplet state of coupled Ti(III) d 1 electrons.

Does the Aromatic Dianion of 1,2-Di-13C-[8]annulene Automerize As Does 1,2-Di-13C-benzene?

The solid dianion salt of 1,2-di-13C-cyclooctatetraene was synthesized in order to measure the first 13C−13C NMR coupling in an aromatic annulene other than benzene and to find out if this dianion will undergo high-temperature automerizations analogous to those previously observed in the benzene system. The solid potassium salt was heated to over 600 °C for 2 h, and although decomposition with H2 and CH4 evolution takes place, there is no observable scrambling of the 13C's. The positions of the 13C's were monitored via NMR, which necessitated a rather complete understanding of the NMR couplings for the cyclooctatetraene dianion system. Interestingly, the 1JCH and 1JCC couplings for the aromatic COT2- system of 145.9 and 55.8 Hz are all quite comparable to those recently reported for the 1,2-di-13C-benzene system (158.6 and 55.8 Hz, respectively).

The first example of η8 coordination of lithium cations with a cyclooctatetraene dianion: crystal structure of Li 2(dibenzo[a,e]cyclooctatetraene)(TMEDA) 2

The reported crystal structure of Li 2(dibenzo[a,e]cyclooctatetraene)(TMEDA) 2 represents the first crystal structure of a dilithio cyclooctatetraene dianion, and the first example of η 8 coordination of a lithium cation with the cyclooctatetraene system.

Herzberg–Teller coupling and configuration interaction in a metalloporphyrin model: 1,3,5,7-tetramethylcyclo-octatetraene dianion

1,3,5,7-tetramethylcyclo-octatetraene dianion (TMCOT−2) is employed as a metalloporphyrin model. Its UV-VIS absorption spectrum shows two electronic bands, an intense band (Soret or B) at ∼215 nm and a weaker state (Q band) at 382 nm, while cyclo-octatetraene dianion (COT−2) only has a B band at 236 nm. An exact Pariser–Parr–Pople (PPP) calculation of COT−2 predicts a strong Soret band at 231 nm and a vanishing Q band intensity at 366 nm. Resonance Raman spectrum with excitation near the Q and Soret bands of TMCOT−2 shows enhancement patterns remarkably similar to those of metalloporphyrins. Nontotally symmetric modes are enhanced in resonance with the Q band, while the totally symmetric modes are enhanced in resonance with the B state. The cyclic polyene TMCOT−2 spectrum thus provides direct evidence for Herzberg–Teller coupling of the Q band and B state.