Paramagnetic cp/dithiolene complexes as molecular hinges: interplay of metal/ligand electronic delocalization and solid-state magnetic behavior

Design of a One-Dimensional Stacked Spin Peierls System with Room-Temperature Switching from Quantum Mechanical Predictions

Planar bis-1,2-dithiolene complex anions of a transition metal (denoted as [M(dithiolato)₂]^- and M = Ni, Pd, or Pt ion) favor forming columnar stacks of anions in the crystal that feature S = 1/2 spin-chains, and such a spin-chain compound often undergoes a spin-Peierls-type transition, making this a promising material for conducting and magnetic switching. However, current examples show the transition temperatures are far too low for most applications. We use quantum mechanics to predict that changing the cation arrangement from the boat-type to the chair-type packing configuration in a spin-Peierls-type [Ni(dithiolato)₂]^- complex will substantially stabilize the antiferromagnetic coupling, dramatically increasing the transition temperature. We estimate that the [Ni(mnt)₂]-based complexes (mnt = maleonitriledithiolate) with chair-type packing of cations will lead to critical temperatures of ∼170, ∼252, and ∼310 K for S-, Se-, and Te-based mnt, respectively. We also suggest how to stabilize the chair-type configurations of these systems.

A computational investigation into the catalytic activity of a diselenolene sulfite oxidase biomimetic complex

Molybdenum is the only 4d metal found in almost all life. One such molybdenum-containing enzyme is sulfite oxidase, which also contains the dithiolene-molybdopterin ligand. Sulfite oxidase is essential in the degradation of sulfur-containing compounds such as cysteine and methionine. Past work has shown parallels in the chemistry of dithiolene–metal and diselenolene–metal complexes. Thus, in this present work, the oxygen atom transfer mechanism for a diselenolene sulfite oxidase biomimetic complex was investigated using computational tools, the results of which were compared to the analogous dithiolene biomimetic complex. From the results obtained, the molybdenum-diselenolene sulfite oxidase biomimetic complex is able to catalyse the oxygen atom transfer and does so with a marginally lower value of ΔrG‡ than that for the analogous dithiolene complex. In particular, it was found that on average, the diselenolene complex had an activation energy 1.2 kJ mol–1 lower in energy than the analogous dithiolene complex. However, the calculated value of ΔrG suggests that the oxidation of sulfite is more favourable for the dithiolene complex where the average difference in reaction aqueous Gibbs reaction energy was –9.4 kJ mol–1 relative to the diselenolene complex. It is noted that with the use of D3 and D3BJ corrections in combination with the B3LYP functional, the barrier for oxygen atom transfer is lowered by more than 30.0 kJ mol–1 for both the diselenolene and dithiolene complexes. Such results suggest that to study such oxo-transfer reactions, the proper treatment of dispersion interaction is necessary.

Observation of hysteretic magnetic phase transitions coupled with orientation motion of ions and dielectric relaxation in a one-dimensional nickel-bis-dithiolene molecule solid

The second polymorph, the β-crystal, of the nickel-bis-dithiolene compound [4'-CF3bzPy][Ni(mnt)2], where 4'-CF3bzPy = 1-(4'-trifluoromethylbenzyl)pyridinium and mnt(2-) = maleonitriledithiolate, was obtained. The variable-temperature single crystal structures, magnetic behavior in 1.8-300 K and dielectric nature in 123-373 K have been investigated for the β-crystal. This polymorph experiences two hysteretic magnetic phase transitions in a narrow temperature region (190-217 K) with the thermal hysteresis loops ca. 6 K and ca. 11 K. The two hysteretic magnetic phase transitions are coupled with two isostructural phase transitions (IPTs), respectively, which are driven by the novel step-wise dynamic orientation motion of the anion and cation in the β-crystal. There is an absence of a dielectric anomaly in the structural transformation temperature interval. However, a dielectric relaxation, related to the dipole motion of polar CF3 groups in the cations under an ac electrical field, emerges in the high-temperature phase.

Influence of the molecular structures of dithiolate ligands on crystal packing modes and magnetic properties in salts (Bz-Et3N)[Ni(dmit)x(mnt)2−x] (x = 0–2)

Nickel-bis-dithiolene salts, (Bz-Et₃N)[Ni(dmit)_x(mnt)_2−x] (x = 0–2 for 1–3 and mnt²⁻ = maleonitriledithiolate, dmit²⁻ = 2-thioxo-1,3-dithiole-4,5-dithiolate), have been prepared and characterized.

An organometallic dithiolene complex exhibiting electrochemically initiated hydrogen generation

We clarify the electrochemical behavior of an organometallic cobalt dithiolene complex (1-NH) with a secondary sulfonyl amide (–NHSO(2)–) substituted Cp ligand involving proton dynamics. The reductions of 1-NH and its deprotonated derivative (1-N(−)) are centered on the CpCoS(2) moiety. The 2e(−) reduction of 1-NH quantitatively gives 1-N(−) with hydrogen generation.

Observation of metal ion dependent packing structures and magnetic behaviors of metal-bis-1, 2-dithiolene complexes

The crystal structures and magnetic properties were investigated experimentally and theoretically for two S = ½ spin chain complexes, which consist of [M(mnt)(2)](-) (M = Pt for 1 or Pd for 2) with 1-(4'-bromo-2'-flurobenzyl)-4-aminopyridinium (1-BrFBz-4-NH(2)Py(+)). The 1-BrFBz-4-NH(2)Py(+) cations exhibit different molecular conformations and arrangements in 1 and 2; the [M(mnt)(2)](-) anions form regular stacks in 1, whereas they form irregular stacks in 2. In addition, the intermolecular interactions between the [M(mnt)(2)](-) anions and cations are also different from each other in the crystals of 1 and 2. Complex 1 shows the magnetic characteristics of a low-dimensional antiferromagnetic coupling spin system with a spin-Peierls-type transition around 7 K, and complex 2 exhibits diamagnetism over the temperature range of 5-300 K. Theoretical analyses, based on the calculations for the charge density distributions of [Pt(mnt)(2)](-) and [Pd(mnt)(2)](-) anions and the magnetic exchange constants within the anion spin chains, addressed the diverse molecular alignments in the crystals of 1 and 2 and distinct magnetic behaviors between 1 and 2.

Synthesis, structure and redox properties of bis(cyclopentadienyl)dithiolene complexes of molybdenum and tungsten

The compounds [Cp(2)M(S(2)C(2)(H)R)] (M = Mo or W; R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl) and [Cp(2)Mo(S(2)C(2)(Me)(pyridin-2-yl)] have been prepared by a facile and general route for the synthesis of dithiolene complexes, viz. the reaction of [Cp(2)MCl(2)] (M = Mo or W) with the dithiolene pro-ligand generated by reacting the corresponding 4-(R)-1,3-dithiol-2-one with CsOH. These Mo compounds were reported previously (Hsu et al., Inorg. Chem. 1996, 35, 4743); however, the preparative method employed herein is more versatile and generates the compounds in good yield and all of the W compounds are new. Electrochemical investigations have shown that each compound undergoes a diffusion controlled one-electron oxidation (OX(I)) and a one-electron reduction (RED(I)) process; each redox change occurs at a more positive potential for a Mo compound than for its W counterpart. The mono-cations generated by chemical or electrochemical oxidation are stable and the structures of both components of the [Cp(2)Mo(S(2)C(2)(H)R)](+)/[Cp(2)Mo(S(2)C(2)(H)R)] (R = Ph or pyridin-3-yl) redox couples have been determined by X-ray crystallography. For each redox related pair, the changes in the Mo-S, S-C and C-C bond lengths of the {MoSCCS} moiety are generally consistent with OX(I) involving the loss of an electron from a π-orbital that is Mo-S and C-S antibonding and C-C bonding in character. These results have been interpreted successfully within the framework provided by DFT calculations accomplished for [Cp(2)M(S(2)C(2)(H)Ph)](n) (M = Mo or W; n = +1, 0 or -1). The HOMO of the neutral compounds is derived mainly from the dithiolene π(3) orbital (65%); therefore, OX(I) is essentially a dithiolene-based process. The similarity of the potentials for OX(I) (ca. 30 mV) for analogous Mo and W compounds is consistent with this interpretation and the EPR spectra of each of the Mo cations show that the unpaired electron is coupled to the dithiolene proton but relatively weakly to (95,97)Mo. The DFT calculations indicate that the unpaired electron is more localised on the metal in the mono-anions than in the mono-cations. In agreement with this, the EPR spectrum of each of the Mo-containing mono-anions manifests a larger (95,97)Mo coupling (A(iso)) than observed for the corresponding mono-cation and RED(I) for a W compound is significantly (ca. 300 mV) more negative than that of its Mo counterpart. [Cp(2)W(S(2)C(2)(H)(quinoxalin-2-yl))] is anomalous; RED(I) occurs at a potential ca. 230 mV more positive than expected from that of its Mo counterpart and the EPR spectrum of the mono-anion is typical of an organic radical. DFT calculations indicate that these properties arise because the electron is added to a quinoxalin-2-yl π-orbital.

A radical pathway to synthesise Mo and W dithiolene complexes

A new method has been developed to synthesise bis η(5)-cyclopentadienyl dithiolene complexes of molybdenum and tungsten. This procedure involves the in situ thermolysis of the azo compounds, 2,2'-azobisisobutyronitrile (AIBN) or 1,1'-azobiscyclohexanecarbonitrile (ACCN) (R'(2)N(2), R' = CMe(2)CN or C(6)H(10)CN, respectively), which initiates a reaction between [Cp(2)M(S(4))] (M = Mo or W) and an alkyne (HC(2)R, R = Ph, 2-pyridyl or 2-quinoxalinyl) and produce the corresponding [Cp(2)M(S(2)C(2)RR')] compound.

Structural distortions upon oxidation in heteroleptic [Cp(2)W(dmit)] tungsten dithiolene complex: combined structural, spectroscopic, and magnetic studies

Four different cation radical salts are obtained upon electrocrystallization of [Cp(2)W(dmit)] (dmit = 1,3-dithiole-2-thione-4,5-dithiolato) in the presence of the BF(4)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions. In these formally d(1) cations, the WS(2)C(2) metallacycle is folded along the S···S hinge to different extents in the four salts, an illustration of the noninnocent character of the dithiolate ligand. Structural characteristics and the charge distribution on atoms, for neutral and ionized complexes with various folding angles, were calculated using DFT methods, together with the normal vibrational modes and theoretical Raman spectra. Raman spectra of neutral complex [Cp(2)W(dmit)] and its salts formed with BF(4)(-), AsF(6)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions were measured using the red excitation (λ = 632.8 nm). A correlation between the folding angle of the metallacycle and the Raman spectroscopic properties is analyzed. The bands attributed to the C═C and C-S stretching modes shift toward higher and lower frequencies by about 0.3-0.4 cm(-1) deg(-1), respectively. The solid state structural and magnetic properties of the three salts are analyzed and compared with those of the corresponding molybdenum complexes. Temperature dependence of the magnetic susceptibility shows the presence of one-dimensional antiferromagnetic interactions in the BF(4)(-), PF(6)(-), and [Au(CN)(2)](-) salts, while an antiferromagnetic ground state is identified in the Br(-) salt below T(Néel) = 7 K. Interactions are systematically weaker in the tungsten salts than in the isostructural molybdenum analogs, a consequence of the decreased spin density on the dithiolene ligand in the tungsten complexes.

Efficient transfer of either one or two dithiolene ligands from nickel to ruthenium: synthesis and crystal structures of [Ru(SCR=CPhS)(2)(PPh(3))] and [RuCl(2)(SCR=CPhS)(PPh(3))(2)] (R = Ph, H)

High yields of two different types of ruthenium dithiolene complex have been obtained by reactions that involve transfer of the dithiolene ligands from the nickel complexes [Ni(SCR=CPhS)(2)] (R = Ph, H) to [RuCl(2)(PPh(3))(3)]. At room temperature one dithiolene is rapidly transferred to yield [RuCl(2)(SCR=CPhS)(PPh(3))(2)], whereas under thermal conditions (refluxing toluene) two dithiolene ligands are incorporated to give [Ru(SCR=CPhS)(2)(PPh(3))]. The crystal structures of the ruthenium bis(dithiolene) complexes indicate that the dithiolene ligands are bonded in the monoanionic form, whereas in the monodithiolene complexes the dithioketone canonical form of the dithiolene ligand is more in evidence, as shown by the average C-S and C=C bond distances in the ligands. This is consistent with both complexes containing Ru(II) centers. The synthesis of the mixed-ligand bis(dithiolene) complex [Ru(SCH=CPhS)(S(2)C(2)Ph(2))(PPh(3))] has been achieved and it is shown that the bis(dithiolene) complexes undergo relatively slow scrambling of the dithiolene ligands in solution. The complex [Ru(SCH=CFcS)(2)(PPh(3))], containing two ferrocenyl-substituted dithiolene ligands, was also prepared, but attempts to establish the degree of electrochemical communication between them were hampered by instability and the irreversible nature of the redox processes.