Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage.

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(µ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((µ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

Assembling diuranium complexes in different states of charge with a bridging redox-active ligand.

Radical-bridged diuranium complexes are desirable for their potential high exchange coupling and single molecule magnet (SMM) behavior, but remain rare. Here we report for the first time radical-bridged diuranium(iv) and diuranium(iii) complexes. Reaction of [U{N(SiMe3)2}3] with 2,2'-bipyrimidine (bpym) resulted in the formation of the bpym-bridged diuranium(iv) complex [{((Me3Si)2N)3UIV}2(µ-bpym2-)], 1. Reduction with 1 equiv. KC8 reduces the complex, affording [K(2.2.2-cryptand)][{((Me3Si)2N)3U}2(µ-bpym)], 2, which is best described as a radical-bridged UIII-bpym˙--UIII complex. Further reduction of 1 with 2 equiv. KC8, affords [K(2.2.2-cryptand)]2[{((Me3Si)2N)3UIII}2(µ-bpym2-)], 3. Addition of AgBPh4 to complex 1 resulted in the oxidation of the ligand, yielding the radical-bridged complex [{((Me3Si)2N)3UIV}2(µ-bpym˙-)][BPh4], 4. X-ray crystallography, electrochemistry, susceptibility data, EPR and DFT/CASSCF calculations are in line with their assignments. In complexes 2 and 4 the presence of the radical-bridge leads to slow magnetic relaxation.

Dianionic States of Trithiadodecaazahexaphyrin Complexes with Homotrinuclear MII3O Clusters (M = Ni and Cu): Crystal Structures, Metal- Or Macrocycle-Centered Reduction, and Doublet-Quartet Transitions in the Dianions.

Metal complexes of trithiadodecaazahexaphyrin (Hhp) that contain MII3O clusters inside a π-extended trianionic (Hhp3-) macrocycle have been prepared. Studies of the magnetic properties of NiII3O(Hhp) and CuII3O(Hhp) reveal a diamagnetic and EPR-silent trianionic (Hhp3-) macrocycle and diamagnetic NiII3(O2-) or paramagnetic CuII3(O2-) tetracations. The positive charge of MII3O(Hhp) is compensated by one acetate anion {MII3O(Hhp)}+(CH3CO2-). The three-electron reduction of {MII3O(Hhp)}+ yields {cryptand(Cs+)}2{NiII2NiIO(Hhp5-)}2-·2C7H8 (1) and {cryptand(Cs+)}2{CuII3O(Hhp•6-)}2-·C7H8 (2) crystalline salts. The magnetic properties of 1 reveal the formation of Hhp5- and the reduction of nickel(II) to the paramagnetic NiI ion (S = 1/2), which is accompanied by the formation of the {NiII2NiIO(Hhp5-)}2- dianion. As a result, the magnetic moment of 1 is 1.68 µB in the 20-220 K range, and a broad EPR signal of NiI was observed. The Hhp5- macrocycle has a singlet ground state, but the increase in the magnitude of the magnetic moment of 1 above 220 K is attributed to the population of the triplet excited state in Hhp5-. The {NiII2NiIO(Hhp5-)}2- dianion is transferred from the doublet excited state to the quartet excited state with an energy gap of 1420 ± 50 K. Salt 1 also shows an unusually strong low-energy NIR absorption, which was observed at 1000-2200 nm. In 2, a highly reduced Hhp•6- radical hexaanion (S = 1/2) coexists with a CuII3(O2-) cluster (S = 1/2) in the {CuII3O(Hhp•6-)}2- dianions. The dianions have a triplet ground state with antiferromagnetic exchange between two S = 1/2 spins with J = -6.4 cm-1. The reduction of Hhp in both salts equalizes the initially alternated C-N bonds, supporting the increase in the Hhp macrocycle electron delocalization.

Effect of reduction on the molecular structure and optical and magnetic properties of fluorinated copper(II) phthalocyanines.

The reduction of copper(ii) octafluoro- {CuII(F8Pc)} and hexadecafluorophthalocyanines {CuII(F16Pc)} by NaCpCo(CO)2 in the presence of cryptand[2.2.2] yields new crystalline {cryptand(Na+)}[CuII(F8Pc)˙3-]-·2C6H4Cl2 (1) and {cryptand(Na+)}2[CuII(F16Pc)4-]2-·C6H14 (2) salts. Together with two previously characterized salts of CuII(FxPc) (x = 8 and 16), this allows the study of the molecular structure and optical and magnetic properties of fluorinated copper phthalocyanines in different reduction states (-1 and -2). The blue shift of the Q-band increases together with the negative charge on the macrocycle, and new weak bands of the anions appear at 820-1013 nm. Alternation of the Nmeso-C bonds manifests itself in reduced macrocycles due to a partial disruption of macrocycle aromaticity. In the case of CuII(F8Pc) this effect is nearly two times stronger for dianions than for monoanions. The alternation of the bonds is less pronounced for perfluorinated CuII(F16Pc)n- (n = 1, 2) anions most probably due to a partial delocalization of the negative charge on fluoro-substituents. The reduction also noticeably elongates the average C-F bonds in CuII(F8Pc). The first reduction centered on the macrocycle leads to the formation of [CuII(F8Pc)˙3-]- in 1 with two S = 1/2 spins positioned on CuII and the radical trianion (F8Pc)˙3- macrocycle. As a result, a broad EPR signal is observed with g = 2.1652 at RT attributable to both paramagnetic species having exchange interactions. The formation of dimerized stacks from [CuII(F8Pc)˙3-]- in 1 results in strong enough magnetic coupling of the (F8Pc)˙3- spins within the dimers (J/kB = -21.8 cm-1), and weaker intramolecular coupling is observed between CuII and (F8Pc)˙3- (J/kB = -10.8 cm-1). Coupling between (F8Pc)˙3- spins from the neighboring dimers is nearly 1.5 times weaker (-14.6 cm-1). Under reduction conditions, a second electron also comes to the macrocycle forming diamagnetic F16Pc4- tetraanions. In this case S = 1/2 spin is preserved on CuII. Magnetic coupling between these centers is weak due to the long distances between them in the [CuII(F8Pc)4-]2- chains of 2. Salt 2 shows an EPR signal with a HF splitting characteristic of CuII with g∥ = 2.1806 (A∥ = 20.11 mT), and g⊥ = 1.9597 at RT.

Strong magnetic coupling of spins in Fe(ii) dimers with differently charged thioindigo ligands.

A first coordination {[2.2.2]cryptand(K+)}2{FeII(TI˙-)(TI2-)}2·2C6H4Cl2 (1) complex of iron(ii) containing radical anions and dianions of thioindigo (TI) was obtained. The complex has two high-spin FeII centers bound by two oxygen atoms, and the TI˙- radical anions are coordinated to each FeII. As a result, the 4-spin system consisting of TI˙- (S = 1/2)-FeII (S = 2)-FeII (S = 2)-TI˙- (S = 1/2) coupled spins is formed within a dimer with strong FeII-FeII (J = -51.1 cm-1) and weaker FeII-TI˙- interactions of (J = -35.4 cm-1).

Effect of One- and Two-Electron Reduction of Terbium(III) Double-Decker Phthalocyanine on Single-Ion Magnet Behavior and NIR Absorption.

Reduction of terbium(III) double-decker phthalocyanine, TbIIIPc2 (1), by sodium fluorenone ketyl in the presence of bis(triphenylphosphoranylidene)ammonium cations yields one-electron-reduced (PPN+){TbIII(Pc2-)2}-·2.5C6H4Cl2 (2) containing the dianionic Pc2- macrocycles, whereas a stronger NaCpCo(CO)2 reductant in the presence of an excess of cryptand yields two-electron-reduced {Cryptand(Na+)}2{(Pc2-)TbIII(Pc•3-)}2- (3) containing the Pc•3- radical trianionic macrocycle. Isolated pairs of the {TbIII(Pc2-)2}- anions are formed in 2, whereas compound 3 has unique 3D packing of the macrocycles with weak π-orbitals overlapping in all three directions. This is the first example of the two-electron-reduced lanthanide double-decker containing Pc•3- radical trianion studied in solid state. Compound 2 manifests single-ion magnet (SIM) behavior with a large effective spin-reversal energy barrier of Ueff = 538 cm-1 in comparison with previously studied (Bu4N+){TbIII(Pc2-)2}- ( Ueff = 230 cm-1). Thus, changes in cation size and shape affect the molecular packing of {TbIII(Pc2-)2}- and increase the spin-reversal energy barrier. At the same time, two-electron-reduced species 3 containing TbIII and Pc•3- show no signs of SIM such as hysteresis loop at 1.9 K, and no peaks are observed on the temperature dependencies of in-phase (χ') and out-of-phase (χ″) signals. In contrast to EPR silent 2, both compounds 1 and 3 manifest broad signals from paramagnetic TbIII ions. Narrower signals attributed to Pc•3- are of high intensity only in 3. In addition to the absorption bands of Pc in the UV and visible spectral range, compound 3 manifests the lowest energy absorption band in solid-state spectra even in the near IR range at 4700 cm-1 (2130 nm), whereas such bands are not observed in the spectrum of 2. These data show that the reduction of the Pc macrocycles in the lanthanide double-deckers leads to the appearance of new very low-energy new transitions associated with Pc•3- whose energy is the lowest among known reduced metal phthalocyanines.

{CpFeII(CO)2SnII(Macrocycle•3-)} Radicals with Intrinsic Charge Transfer from CpFe(CO)2 to Macrocycles (Cp: Cp or Cp*); Effective Magnetic Coupling between Radical Trianionic Macrocycles•3.

Neutral {CpFeII(CO)2[SnII(Pc•3-)]} {Cp is cyclopentadienyl (1, 2) or Cp* is pentamethylcyclopentadienyl (3); Pc: phthalocyanine}, {Cp*FeII(CO)2[SnII(Nc•3-)]} (4, Nc: naphthalocyanine), and {CpFeII(CO)2[SnII(TPP•3-)]} (5, TPP: tetraphenylporphyrin) complexes in which CpFeII(CO)2 fragments (Cp: Cp or Cp*) are coordinated to SnII(macrocycle•3-) have been obtained. The product complexes were obtained at the reaction of charge transfer from CpFeI(CO)2 (Cp: Cp or Cp*) to [SnII(macrocycle2-)] to form the diamagnetic FeII and paramagnetic radical trianionic macrocycles. As a result, these formally neutral complexes contain S = 1/2 spins delocalized over the macrocycles. This provides alternation of the C-Nimine or C-Cmeso bonds in the macrocycles, the appearance of new bands in the near-infrared spectra of the complexes, and blue shift of both Soret and Q-bands. The {CpFeII(CO)2SnII(macrocycle•3-)} units (Cp: Cp or Cp*, macrocycle: Pc or Nc) form closely packed π-stacking dimers in 1 and 3 or one-dimensional chains in 2 and 4 with effective π-π interaction between the macrocycles. Such packing allows strong antiferromagnetic coupling between S = 1/2 spins. Magnetic interaction can be described well by the Heisenberg model for the isolated dimers in 1 and 3 with exchange interaction J/k B = -78 and -85 K, respectively. Magnetic behavior of 2 and 4 is described well by the model that includes contributions from an antiferromagnetically coupled S = 1/2 dimer (J intra) and a Heisenberg S = 1/2 chain with alternating antiferromagnetic spin exchange between the neighbors (J inter). Compound 2 demonstrates large intradimer interaction of J intra/k B = -54 K and essentially weaker interdimer exchange interactions of J inter/k B = -6 K, whereas compound 4 shows strong magnetic coupling of spins within the dimers (J intra/k B = -170 K) as well as between the dimers (J inter/k B = -40 K). Compound {CpFeII(CO)2[SnII(TPP•3-)]} (5) shows no π-π interactions between the porphyrin macrocycles, and magnetic coupling is weak in this case (Weiss temperature is -5 K). Preparation of a similar complex with indium(III) chloride phthalocyanine yields {CpFe(CO)2[In(Pc2-)]} (6). In this complex, indium(III) atoms are reduced instead of the phthalocyanine macrocycles that explains electron paramagnetic resonance silence of 6 in the 4-295 K range.

cis-Thioindigo (TI) - a new ligand with accessible radical anion and dianion states. Strong magnetic coupling in the {[TI-(µ2-O),(µ-O)]Cp*Cr}2 dimers.

Reaction of decamethylchromocene (Cp*2Cr) with thioindigo (TI) yields a coordination complex {[TI-(µ2-O), (µ-O)]Cp*Cr}2·C6H14 (1) in which one Cp* ligand in Cp*2Cr is substituted by TI. TI adopts cis-conformation in 1 allowing the coordination of both carbonyl groups to chromium. Additionally, one oxygen atom of TI becomes a µ2-bridge for two chromium atoms to form {[TI-(µ2-O), (µ-O)]Cp*Cr}2 dimers with a CrCr distance of 3.12 Å. According to magnetic data, diamagnetic TI2- dianions and two Cr3+ atoms with a high S = 3/2 spin state are present in a dimer allowing strong antiferromagnetic coupling between two Cr3+ spins with an exchange interaction of -35.4 K and the decrease of molar magnetic susceptibility below 140 K. Paramagnetic TI˙- radical anions with the S = 1/2 spin state have also been obtained and studied in crystalline {cryptand[2,2,2](Na+)}(TI˙-) (2) salt showing that both radical anion and dianion states are accessible for TI.