Stable dimeric aromatic cation-radicals. Structural and spectral characterization of through-space charge delocalization

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).     

Complexes of a hexa-nitrile dianion with neutral, chelating co-ligands: self-assembly, structure and magnetism

Several new first-row transition-metal complexes have been synthesised by combining the polynitrile dianion HCTMCP(2-) (hexacyanotrimethylenecyclopropandiide) with neutral, chelating co-ligands; 2,2'-bipyridine, 1,10-phenanthroline and 3-(2-pyridyl)pyrazole. The products cover a remarkable range of species including mononuclear complexes, dimers, charge-separated species and coordination polymers. Complexes containing 2,2'-bipyridine take the form [Mn(2,2'-bipy)(2)(HCTMCP)](2)·2MeOH (1) or [M(2,2'-bipy)(3)](HCTMCP) (2Fe and 2Co) which are dimeric and charge-separated products, respectively. The products obtained using 1,10-phenanthroline were the discrete complex [Co(HCTMCP)(1,10-phen)(2)(H(2)O)]·H(2)O·MeCN (3) and the 1D coordination polymer [Mn(HCTMCP)(1,10-phen)(H(2)O)(MeOH)] (4). Complexes using the 3-(2-pyridyl)pyrazole co-ligand (pypzH) form similar 1D complexes to 4, namely [Mn(pypzH)(HCTMCP)(MeOH)(H(2)O)] (5) and [M(pypzH)(HCTMCP)(MeOH)(2)] (6Co and 6Fe), albeit with different hydrogen-bonding motifs between the chains. The polymeric HCTMCP complexes show weak to zero antiferromagnetic coupling between metal centres and thus no long-range ordering.     

Square planar vs tetrahedral coordination in diamagnetic complexes of nickel(II) containing two bidentate pi-radical monoanions

The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H(2)[L(1-6)], with Ni(OAc)(2).4H(2)O in ethanol in the presence of air yields six four-coordinate species [Ni(L(1-6)(*))(2)] (1-6) where (L(1-6)(*))(1-) represent the monoanionic pi-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L(1)(-3)(*))(2)], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L(5)(*))(2)] (5) and [Ni(L(6)(*))(2)] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1-3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand pi-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4-6 consist of a paramagnetic high-spin Ni(II) ion (S(Ni) = 1), which is strongly antiferromagnetically coupled to two ligand pi-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1-6 can be reduced to form stable, paramagnetic monoanions [1-6](-) (S = (1)/(2)). The anions [1-3](-) are square planar Ni(II) (d,(8) S(Ni) = 0) species where the excess electron is delocalized over both ligands (class III, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = (1)/(2)). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [Ni(II)(L(4-6)(*))(L(4-6))](-) where (L(4-6))(2-) is the closed shell dianion of the ligands H(2)[L(4-6)] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [Ni(II)(L(1,ox))(2)I(2)] (7), [Ni(II)(L(1,ox))(3)](I(3))(2) (8), and trans-[Ni(II)(L(5,ox))(2)(I(3))(2)] (9), which have been characterized by X-ray crystallography; (L(1-)(6,ox)) represent the neutral, two-electron oxidized forms of the corresponding dianions (L(1-6))(2-). The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1-5.     

Metal-intercalator-mediated self-association and one-dimensional aggregation in the structure of the excised major DNA adduct of a platinum-acridine agent

The crystal structure of the excised major DNA monoadduct, [Pt(en)(ACRAMTU-S)(dGuo-N7)]3+ ("dGuo*"; en = ethane-1,2-diamine; ACRAMTU = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, acridinium cation; dGuo = 2'-deoxyguanosine), of a platinum-acridine cytotoxic agent is reported. The adduct dGuo*, previously identified in enzymatic digests of native DNA treated with this drug, is partially deprotonated and dimerizes through formation of a rare GG- mismatch base pair, which is sandwiched between the planar chromophores of the acridine nonleaving groups linked to platinum. NMR evidence exists that indicates that the dimeric form persists in neutral aqueous solution. The one-dimensional pi-stack produced by the dimers in the solid state is reminiscent of a coordinative-intercalative DNA binding mode.     

Effect of the cooling rate on dimerization of C60(•-) in fullerene salt (DMI+)2·(C60(•-))·{Cd(Et2NCS2)2I-}

The salt (DMI(+))(2)·(C(60)(?-))·{Cd(Et(2)NCS(2))(2)I(-)} (1) containing fullerene radical anions, the anions of cadmium diethyldithiocarbamate iodide, and N,N'-dimethylimidazolium cations was obtained. Fullerenes are monomeric in 1 at 250 K and form three-dimensional packing in which each fullerene has nearly tetrahedral surroundings from neighboring fullerenes. Fullerenes with a shorter interfullerene center-to-center distance of 10.031(2) ? form spiral chains arranged along the lattice c axis. The convolution consists of four fullerene molecules. Dimerization realized in 1 within the spiral chains below 135 K manifests a strong dependence on the cooling rate. The "frozen" monomeric phase was obtained upon instant quenching of 1. This phase is stable below 95 K for a long time but slowly converted to the dimeric phase at T > 95 K. It exhibits a weak antiferromagnetic interaction of spins below 95 K (the Weiss temperature is -4 K), which results in the splitting of the electron paramagnetic resonance (EPR) signal into two components below 10 K. A disordered phase containing both C(60)(?-) monomers and singly bonded (C(60)(-))(2) dimers with approximately 0.5/0.5 occupancies is formed at an intermediate cooling rate (for 20 min). The position of each fullerene in this phase is split into three positions slightly shifted relative to each other. The central position corresponds to nonbonded fullerenes with interfullerene center-to-center distances of 9.94-10.00 ?. Two other positions are coincided to dimeric fullerenes formed with the right and left fullerene neighbors within the spiral chain. This intermediate phase is paramagnetic with nearly zero Weiss temperature due to isolation of C(60)(?-) by diamagnetic species and exhibits a strongly asymmetric EPR signal below 20 K. A diamagnetic phase containing ordered singly bonded (C(60)(-))(2) dimers can be obtained only upon slow cooling of the crystal for 6 h.     

Ionic fullerene compounds containing negatively charged dimers and coordinatively bound anions

The review summarizes the results of investigations of ionic fullerene compounds containing negatively charged dimers and fullerene anions coordinated to metalloporphyrins. Fullerene radical anions were found to form diamagnetic singly bonded (C₆₀ ⁻)₂ and (C₇₀ ⁻)₂ dimers. Dimerization is reversible and leads to paramagnetic—diamagnetic phase transitions or a decrease in the magnetic moment of the complexes. The temperature, at which dissociation of the (C₆₀ ⁻)₂ dimers begins, varies from 140 to 320 K; the corresponding temperature for the (C₇₀ ⁻)₂ dimers varies from 260 to 360 K and higher. We prepared the first doubly bonded (C₆₀ ⁻)₂ dimer. At 300 K, this dimer exists as a biradical. The fullerene radical anions form Co—C coordination bonds with cobalt(II) porphyrinates. The resulting anions are diamagnetic. In some cases, Co—C coordination bonds undergo reversible dissociation, resulting in magnetic transitions from the diamagnetic to the paramagnetic state. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 361–381, March, 2007.     

The nature of the [TTF]˙+···[TTF]˙+ interactions in the [TTF]2(2+) dimers embedded in charged [3]catenanes: room-temperature multicenter long bonds

The properties of tetrathiafulvalene dimers ([TTF](2)(2+)) and the functionalized ring-shaped bispropargyl (BPP)-functionalized TTF dimers, [BPP-TTF](2)(2+), found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are energetically unstable towards dissociation. When enclosed in the 4(+)-charged central cyclophane ring of charged [3]catenanes (CBPQT(4+)), [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are also energetically unstable with respect to leaving the CBPQT(4+) ring; since the barrier for the exiting process is only about 3 kcal mol(-1), that is, within the reach of thermal energies at room temperature (neutral [TTF](2)(0) dimers are stable within the CBPQT(4+) ring). However, the [BPP-TTF](2)(2+) dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP-TTF(+) macrocycle. Finally, it was shown that the [TTF](2)(2+), [BPP-TTF](2)(2+) dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation-anion interactions. In these environmental conditions at room temperature the TTF units of the [BPP-TTF](2)(2+) dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room-temperature multicenter long bond is formed, similar to those previously found in other [TTF](2)(2+) salts and their solutions.     

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.     

From an S(T) = 3 single-molecule magnet to diamagnetic ground state depending on the molecular packing of Mn(III)salen-type dimers decorated by N,N'-dicyano-1,4-naphthoquinonediiminate radicals

Two manganese(III) tetradentate Schiff-base dimers to which N,N'-dicyano-1,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn2(5-MeOsaltmen)2(DCNNQI)2].MeOH (1) and [Mn2(5-MeOsaltmen)(2)(DCNNQI)(2)] x 2CH2Cl2.2CH3CN (2) [5-MeOsaltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI.-)-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)], where -(O)2- is a biphenolate bridge in the out-of-plane dimerized [Mn(2)(5-MeOsaltmen)2]2+ moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties (with the nearest contact being approximately 3.0 A) is observed in 2, forming a one-dimensional chain of [-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)2-](infinity). The magnetic susceptibility of 1 can be modeled with an [S = 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic Mn(III)/DCNNQI radical coupling (J(Mn/rad)/kB = -23 K) and ferromagnetic Mn(III)/Mn(III) coupling through the biphenolate bridge (J(Mn/Mn)/kB = +2.0 K). These interactions lead to an ST = 3 ground state that possesses significant uniaxial anisotropy (D(S=3)/kB = -2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating Mn(III)-Mn(III) antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or pi-pi contact along the b axis.     

High-resolution IR spectroscopy of dimers of HDO with H2O in helium nanodroplets

We report on rotationally resolved IR spectra of dimers of HDO as a deuterium (d) donor with H(2)O, HDO, and D(2)O embedded in superfluid Helium nanodroplets in the 2650-2660 and 2725-2740 cm(-1) regions of the O-D donor stretch and symmetric acceptor stretch vibrations, respectively. By comparing spectra at different levels of deuteration we were able to unambiguously assign the donor stretch signals of H(2)O···DOH, HDO···DOH, and D(2)O···DOH. For H(2)O···DOH, three ΔK(a) = 0 sub-bands were found that were assigned to transitions from the lower and upper acceptor switching states of K(a) = 0 and the lower acceptor switching state of K(a) = 1. In addition, b- and c-type transitions in the acceptor stretch region of HDO···DOH were observed that allowed us to determine the acceptor switching splitting of Δv? = 5.68 cm(-1) in the HDO···DOH vibrational ground state. We suggest that the dominating broadening mechanism is intervibrational relaxation due to coupling of the rovibrational levels of the chromophore via internal droplet excitations.     

Cyanil](2)(2-) dimers possess long, two-electron ten-center (2e(-)/10c) multicenter bonding

A long, two-electron ten-center (2e(-)/10c) [8 carbon plus 2 oxygen] bond in diamagnetic dimers of radical-anion tetracyano-1,4-benzoquinoneide (cyanil, [Q] (-)), [Q](2)(2-), is described by B3LYP and CASSCF(2,2)/MCQDPT calculations.     

Coulombic aggregations of Mn(III) salen-type complexes and Keggin-type polyoxometalates: isolation of Mn2 single-molecule magnets

The reaction of Mn(III) salen-type complexes with di- and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing Mn(III) out-of-plane dimeric units (abbreviated as [Mn(2)](2+)) that are potentially single-molecule magnets (SMMs) with an S(T) = 4 ground state were synthesized: [Mn(2)(5-MeOsaltmen)(2)(acetone)(2)][SW(12)O(40)] (1), [Mn(2)(salen)(2)(H(2)O)(2)](2)[SiW(12)O(40)] (2), and [Mn(5-Brsaltmen)(H(2)O)(acetone)](2)[{Mn(2)(5-Brsaltmen)(2)}(SiW(12)O(40))] (3), where 5-Rsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen(2-) = N,N'-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW(12)O(40)](2-), is composed of a 1:1 aggregating set of [Mn(2)](2+)/POM, and 2, with a tetraanionic POM, [SiW(12)O(40)](4-), is a 2:1 set. Compound 3 with [SiW(12)O(40)](4-) forms a unique 1D coordinating chain with a [-{Mn(2)}-POM-](2-) repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H(2)O)(acetone)](2)(2+)) is present as a countercation. Independent of the formula ratio of [Mn(2)](2+)/POM, Mn(III) dimers and POM units in 1-3 form respective segregated columns along a direction of the unit cell, which make an alternate packing to separate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn(2)](2+) dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magnetic interaction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of Mn(III) dimers has been characterized in this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic order competitively affects slow relaxation of the magnetization at low ac frequencies.     

The maximin principle of pi-radical packings

A two-term model is proposed for hydrocarbon and N-containing pi-radicals which are in close contact with one another. The first term is attractive (due to partially occupied frontier pi-orbitals), and the second, repulsive (due to hard-core repulsion between close-lying atoms). This model is applied to dimers where intermolecular contacts are closer than <0.95 x the sum of the atomic van der Waals radii. The maximin principle is proposed. The maximin principle states that the lowest energy conformation maximizes overlap of the frontier orbitals while simultaneously minimizing intermolecular contacts. A Hückel Hamiltonian, the mu(2)-Hamiltonian, which contains the above attractive and repulsive terms, is applied. The interaction surfaces of two pi-hydrocarbon radical cations were calculated for the three systems known crystallographically to contain cations in close contact: naphthalene, fluoranthene, and pyrene. The global minima of these surfaces correspond to the experimentally determined structures. The mu(2)-Hamiltonian energy surfaces of the naphthalene cation dimer are qualitatively similar to those calculated at the RHF/6-311G(d,p) and MP2/6-311G(d,p) levels. The maximin principle is applied to N-containing pi-radicals. Except in the case of tetracyanoethene, the maximin principle correctly predicts the most common dimer crystal packing. (MgPc)(NO(3)).0.5THF and (MgPc)(ReO(4)).1.5THF (Pc = phthalocyanine) were prepared: both new crystal structures follow the maximin principle. The maximin principle is used to suggest the dimer cation ground state of oligoacenes, cations important as organic hole-based semiconductors.     

Chemically induced cyclometalation of 2-(arylazo)phenols. Synthesis,characterization, and redox properties of a family of organoosmium complexes

Reaction of 2-(arylazo)phenols (H(2)ap-R; R = OCH(3), CH(3), H, Cl, and NO(2)) with [Os(PPh(3))(2)(CO)(2)(HCOO)(2)] affords a family of organometallic complexes of osmium(II) of type [Os(PPh(3))(2)(CO)(ap-R)] where the 2-(arylazo)phenolate ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh(3))(2)(CO)(ap-H)] complex has been determined by X-ray crystallography. All the [Os(PPh(3))(2)(CO)(ap-R)] complexes are diamagnetic and show characteristic (1)H NMR signals and intense MLCT transitions in the visible region. They also show emission in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh(3))(2)(CO)(ap-R)] complexes shows a reversible Os(II)-Os(III) oxidation within 0.39-0.73 V vs SCE, followed by a reversible Os(III)-Os(IV) oxidation within 1.06-1.61 V vs SCE. Coulometric oxidation of the [Os(PPh(3))(2)(CO)(ap-R)] complexes generates the [Os(III)(PPh(3))(2)(CO)(ap-R)](+) complexes, which have been isolated as the hexafluorophosphate salts. The [Os(III)(PPh(3))(2)(CO)(ap-R)]PF(6) complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as 1:1 electrolytes and show intense LMCT transitions in the visible region. The [Os(III)(PPh(3))(2)(CO)(ap-R)]PF(6) complexes have been observed to serve as mild one-electron oxidants in a nonaqueous medium.     

Reactivity of aqua coordinated monoporphyrinate lanthanide complexes: synthetic, structural and photoluminescent studies of lanthanide porphyrinate dimers

Dimerization of monoporphyrinate lanthanide complexes [Yb(Por)(H(2)O)(3)]Cl, (Por = TTP(2-), TMPP(2-) and TPP(2-)) in the presence of sterically hindered tripodal ligand, zinc Schiff-base, dilute HCl, K(2)CO(3) solution, 4,4'-bipyridine (bipy), and basic 8-hydroxyquinaldine (HQ) solution was observed in CH(2)Cl(2) at room temperature. Six neutral dimeric lanthanide porphyrinate complexes, [Yb(TTP)(mu-OH)](2)(mu-THF) (1), [Yb(TMPP)(mu-OH)(H(2)O)](2) (2), [Yb(TPP)(mu-OH)(mu-H(2)O)](2) (4), [Yb(TMPP)(mu-Cl)(H(2)O)](2) (5), [Yb(TMPP)(mu-OH)](2)(THF) (6) and [Yb(TPP)](2)(mu-OH)(mu-Q) (7), were obtained. X-Ray diffraction studies showed that for the dimers, the two lanthanide ions were bridged by OH(-), Cl(-) or H(2)O. Photoluminescent studies showed that the porphyrinate dianion acted as an antenna, transferred its absorbed visible energy to the lanthanide ion and enabled the latter emitting in the near-infrared (NIR) region. In general, the NIR emission is more intense for the dimers than for the monomers, and the NIR emission intensity decreases as the number of O-H oscillators present in the molecule increases.     

Characterization of the diradical *NSNSC-CNSSN* and [NSNSC-CNSSN][MF6]n (n=1, 2). The first observation of an excited triplet state in dimers of 7pi -CNSSN* radicals

Preparation and full characterization of the main-group diradical *NSNSC-CNSSN*, 8, the MF6- salt (As, Sb) of radical cation +NSNSC-CNSSN*, 8*+, and the AsF6- salt of the dication +NSNSC-CNSSN+, 82+, are presented. 8, a=6.717 (4), b=11.701(2), c=8.269(3) A, alpha=gamma=90, beta=106.69(3) degrees, monoclinic, space group P21/n, Z=4, T=203 K; 8SbF6, a=6.523(2), b=7.780(2), c=12.012(4) A, alpha=91.994(4), beta=96.716(4), gamma=09.177(4) degrees, triclinic, space group P, Z=2, T=198 K; 8[AsF6]2, a=12.7919(14), b=9.5760(11), c=18.532(2) A, alpha=gamma=90, beta=104.034(2) degrees, monoclinic, space group Pn, Z=6, T=198 K. Preparation of 8MF6 was carried out via a reduction of [CNSNS]2[MF6]2 (M=As, Sb) with either ferrocene or a SbPh3-NBu4Cl mixture. In the solid state, diamagnetic 8SbF6 contains centrosymmetric dimers [8*+]2 linked via two-electron four-centered pi*-pi* interactions with a thermally excited triplet state as detected by electron paramagnetic resonance (EPR). This is the first observation of a triplet excited state for a 7pi 1,2,3,5-dithiadiazolyl radical dimer. The singlet-triplet gap of the [-CNSSN*]2 radical pair was -1800+/-100 cm(-1) (-22+/-1 kJ/mol) with the ZFS components |D|=0.0267(6) cm(-1) and |E|=0.0012(1) cm(-1), corresponding to an in situ dimerization energy of ca. -11 kJ/mol. Cyclic voltammetry measurements of 8[AsF6]2 showed two reversible waves associated with a stepwise reduction of the two isomeric rings [E1/2 (+2/+1)=1.03 V; E1/2 (+1/0)=0.47 V, respectively]. 8MF6 (M=As, Sb) was further reduced to afford the mixed main-group diradical 8, containing two isomeric radical rings. In solution, 8 is thermodynamically unstable with respect to *NSSNC-CNSSN*, but is isolable in the solid state because of its low solubility in SO2. Likewise, 8SbF6, 8 is dimeric, with pi*-pi* interactions between different isomeric rings, and consequently diamagnetic; however, a slight increase in paramagnetism was observed upon grinding [from C=6.5(3)x10(-4) emu.K/mol and temperature-independent paramagnetism (TIP)=1.3(1)x10(-4) emu/mol to C=3.2(1)x10(-3) emu.K/mol and TIP=9.0(1)x10(-4) emu/mol], accompanied by an increase in the lattice-defect S=1/2 sites [from 0.087(1) to 0.43(1)%]. Computational analysis using the multiconfigurational approach [CASSCF(6,6)/6-31G*] indicated that the two-electron multicentered pi*-pi* bonds in [8*+]2 and [8]2 have substantial diradical characters, implying that their ground states are diradicaloid in nature. Our results suggest that the electronic structure of organic-radical ion pairs, for example, [TTF*+]2, [TCNE*-]2, [TCNQ*-]2, [DDQ*-]2, and related pi dimers, can be described in a similar way.     

Stepwise neutral-ionic phase transitions in a covalently bonded donor/acceptor chain compound

Neutral (N)-ionic (I) transitions in organic donor (D)/acceptor (A) charge-transfer complexes are intriguing because a 'reservoir of functions' is available. For systematically controlling N-I transitions, tuning the ionization potential of D and the electron affinity of A is extremely important. However, the effect of Coulomb interactions, which likely causes a number of charge-gap states at once in a system bringing about stepwise transitions, is a long-standing mystery. Here, we show definite evidence for stepwise N-I transitions caused by contributions from anisotropic interchain Coulomb interactions in a metal-complex-based covalently bonded DA chain compound, [Ru(2)(2,3,5,6-F(4)PhCO(2))(4)(DMDCNQI)]·2(p-xylene) (1; 2,3,5,6-F(4)PhCO(2)(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine), where the [Ru(2)(II,II)(2,3,5,6-F(4)PhCO(2))(4)] moiety has a paddlewheel diruthenium(II,II) motif with a Ru-Ru bond. An intermediate-temperature phase involving self-organized N and I chains was observed in the temperature range between 210 K (= T(2)) and 270 K (= T(1)) with N phase at T > T(1) and I phase at T < T(2). Accompanying the charge transitions, the spin-ground states as well as the ferrimagnetic ordering in the I phase vary. The stepwise feature of the N-I transition with a highly sensitive magnetic response should bring about new dynamical functionalities associated with charge, spin, and lattice.     

Mechanism of the interconversions between C- and N-bound transition metal alpha-cyanocarbanions

The first presentation of the intra- and intermolecular mechanisms of the C-N interconversions of transition metal alpha-cyanocarbanions is described. A pair of N- and C-bound isomers of isonitrile complex Ru+Cp(NCCH-SO2Ph)(PPh3)(CN-t-Bu) (1) and RuCp[CH(CN)SO2Ph](PPh3)(CN-t-Bu) (2) was synthesized for the mechanistic studies on the N-to-C isomerizations. Structural characterization by X-ray diffractions of 1 and 2 indicated their typical zwitterionic and alpha-metalated structures. The kinetic studies on the irreversible isomerization of 1 to 2 in benzene-d6 at 333-348 K were carried out using 1H NMR spectroscopy, affording the first-order rate constants k1 and the activation parameters DeltaH = 107 +/- 2 kJ.mol-1 and DeltaS = -22 +/- 5 J.K-1.mol-1. The almost identical values of k1 were obtained upon similar treatment of 1 with 4 equiv of external ligands such as PPh3, CH3CN, and t-BuNC at 333 K, indicating that the N-to-C isomerization proceeds in an intramolecular manner without dissociation of a ligand. As a model system for the C-to-N isomerization, the irreversible transformation of RuCp[CH(CN)SO2Ph](PPh3)2 (3) to Ru+Cp(NCCH-SO2Ph)(PPh3)2 (4) was investigated under various reaction conditions. The reaction of 3 at room temperature in THF affords the coordination dimers (RRu*,SC*,RRu*,SC*)-{RuCp[CH(CN)SO2Ph](PPh3)}2 (5) stereoselectively, and its distorted mu2-C,N-bound structure was determined by X-ray analysis. The reaction profiles for the isomerization of 3 includes the generation- and temperature-dependent decays of dimeric species 5 and its diastereomer 6, which strongly suggests that the intra- and intermolecular pathways are included in the C-to-N isomerization. The intramolecular process of the C-to-N isomerization of 3 has been confirmed by the kinetic studies on the isomerization of 3 with excess amount of PPh3 in benzene-d6 at 333-348 K which afford the first-order kinetics with the activation parameters of DeltaH = 121 +/- 1 kJ.mol-1 and DeltaS = 42 +/- 4 J.K-1.mol-1. Treatment of 5 with PPh3 in boiling benzene gives rise to the quantitative formation of N-bound complex 4. The controlled kinetic experiments on the cleavage of 5 with PPh3 have concluded that the cleavage of 5 with PPh3 proceeds via simultaneous C-Ru and N-Ru bond scissions, indicating the temperature-dependent participation of intermolecular process in the C-to-N isomerization of 3.     

Monomeric and dimeric copper(II) complexes of a novel tripodal peptide ligand: structures stabilized via hydrogen bonding or ligand sharing

The novel tripodal ligand N-(bis(2-pyridyl)methyl)-2-pyridinecarboxamide (Py3AH) affords monomeric and dimeric copper(II) complexes with coordinated carboxamido nitrogens. Although many chloro-bridged dimeric copper(II) complexes are known, [Cu(Py3A)(Cl)] (1) remains monomeric and planar with a pendant pyridine and does not form either a chloro-bridged dimer or the ligand-shared dimeric complex [Cu(Py3A)(Cl)]2 (4) in solvents such as CH3CN. When 1 is dissolved in alcohols, square pyramidal alcohol adducts [Cu(Py3A)(Cl)(CH3OH)] (2) and [Cu(Py3A)(Cl)(C2H5OH)] (3) are readily formed. In 2 and 3, the ROH molecules are bound at axial site of copper(II) and the weak axial binding of the ROH molecule is strengthened by intramolecular hydrogen bonding between ROH and the pendant pyridine nitrogen. Two ligand-shared dimeric species [Cu(Py3A)(Cl)]2 (4) and [Cu(Py3A)]2(ClO4)2 (5) have also been synthesized in which the pendant pyridine of one [Cu(Py3A)] unit completes the coordination sphere of the other [Cu(Py3A)] neighbor. These ligand-shared dimers are obtained in aqueous solutions or in complete absence of chloride in the reaction mixtures.     

Factors affecting metal-metal bonding in the face-shared d(3)d(3) bioctahedral dimer systems,MM'Cl(9)(5-) (M,M' = V,Nb, Ta)

Density functional theory (DFT) calculations have been used to investigate the d(3)d(3) bioctahedral complexes, MM'Cl(9)(5-), of the vanadium triad. Broken-symmetry calculations upon these species indicate that the V-containing complexes have optimized metal-metal separations of 3.4-3.5 A, corresponding to essentially localized magnetic electrons. The metal-metal separations in these weakly coupled dimers are elongated as a consequence of Coulombic repulsion, which profoundly influences (and destabilizes) the gas-phase structures for such dimers; nevertheless, the intermetallic interactions in the V-containing dimers involve significantly greater metal-metal bonding character than in the analogous Cr-containing dimers. These observations all show good agreement with existing experimental (solid state) results for the chloride-bridged, face-shared dimers V(2)Cl(9)(5-) and V(2)Cl(3)(thf)(6)(+). In contrast to the V-containing dimers, complexes featuring only Nb and Ta have much shorter intermetallic distances (approximately 2.4 A) consistent with d-electron delocalization and formal metal-metal triple bond formation; again, good agreement is found with available experimental data. Calculations on the complexes V(2)(mu-Cl)(3)(dme)(6)(+), Nb(2)(mu-dms)(3)Cl(6)(2-), and Ta(2)(mu-dms)(3)Cl(6)(2-), which are closely related to compounds for which crystallographic structural data exist, have been pursued and provide an insight into the intermetallic interactions in the experimentally characterized complexes. Analysis of the contributions from d-orbital overlap (E(ovlp)) stabilization, as well as spin polarization (exchange) stabilization of localized d electrons (E(spe)), has also been attempted for the MM'Cl(9)(5-) dimers. While E(ovlp) clearly dominates over E(spe) as a stabilizing factor in those dimers containing only Nb and Ta metal atoms, detailed assessment of the competition between E(ovlp) and E(spe) for V-containing dimers is obstructed by the instability of triply bonded V-containing dimers against Coulombic explosion. On the basis of the periodic trends in E(ovlp) versus E(spe), the V-triad dimers have a greater propensity for metal-metal bonding than do their Cr-triad or Mn-triad counterparts.