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Tetranuclear, dinuclear and chain complexes involving some polyfunctional hydrazone and thio-carbohydrazone-based ligands are discussed. Ni(II) and Mn(II) [2 × 2] grids form with μ2-S and μ2-O bridges respectively, and are antiferromagnetically coupled (J = −167(5), −3.59(2) cm−1 respectively). With the Fe(II) based system oxidation to Fe(III) occurs, and a μ2-Ohydrazone bridged dimer results, with antiferromagnetic exchange between the S = 5/2 spin centers (J = −22.5(2) cm−1). In the case of Cu(II) the diazine group acts as a μ2-N–N bridge between Cu(II) centers in two cases involving a tetranuclear and a chain complex. Non-orthogonal bridging through N–N and carboxylate bridges leads to antiferromagnetic exchange in the tetranuclear case (J = −32.7(7), −16.1(7) cm−1 respectively) and ferromagnetic exchange in the chain complex due to orthogonal N–N bridging (J = 3.3(1) cm−1).

The current review provides up-to-date information on the structures, solution and solid state electron paramagnetic resonance (EPR), electronic and magnetic properties, and computational analysis of the novel 1,2,3,4-trithiazolium RCNSSS + radical cation. The stable 7π radical cation RCNSSS + , derived from RCNSSN by isoelectronic substitution of N by S + , is a rare example of a main group radical with ca. 96% of the spin density residing exclusively on the sulfur atoms. In the solid state it forms a great variety of structures, which, depending on the nature of the substituent group, range from dimeric to monomeric. This results in markedly different magnetic properties in which the magnetic coupling varies from −2100 cm −1 to −2 cm −1 , respectively. An important highlight in the chemistry of RCNSSS + is the preparation of (CNSSS + ) 2 2+ , which contains two radical centers directly joined by a single bond and is paramagnetic in all states, making it the only O 2 -like species in main group chemistry. Also important is the observation, by EPR, of a triplet excited state in the [RCNSSS] 2 2+ dimers and estimation of the dimerisation energy of the very weak inter-molecular S⋯S bonds as ca. −11 to −3 kJ/mol. The radical pairs [RCNSSS] 2 2+ also possess a high diradical character, ranging from 45 to 32%, thus lying on the borderline between conventional open shell and closed shell species.

The iron coordination chemistry of some polytopic hydrazone based ligands is examined. The complexes derive from a general self-assembly strategy, where ligand design can be used to devise specific polymetallic [n × n] grid architectures. However, as part of any complex equilibrium process, oligomeric entities can also occur, particularly when ligand tautomeric flexibility is considered, and examples of mononuclear, dinuclear, tetranuclear, and pentanuclear complexes have been observed within a related class of ligands. In addition, ligand site donor composition can lead to coordination spheres that stabilize both high spin Fe(II) and Fe(III) sites, with evidence for Fe(II) spin crossover. Structural and magnetic properties are examined, which reveal the presence of antiferromagnetic exchange in the polynuclear systems.

Polytopic hydrazone-based ligands are discussed in the context of the design attributes of the ligand and the power of self-assembly as a methodology for the synthesis of polymetallic systems with specific and predetermined organization of the metal centers in a closely spaced bridged arrangement. Magnetic exchange coupling occurs as a result of the close proximity of the metal ions. Homometallic, heterometallic, and mixed-spin-state [n x n] (n = 2-5) square grids are highlighted and discussed in terms of their structural and magnetic properties. Antiferromagnetic, ferromagnetic, and ferrimagnetic examples are described.

Reaction of SNSSbF6 and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1)) in SO2 solution in a 2:1 ratio afforded the dicycloaddition product 1(SNS)2(SbF6)2 · SO2 in 85% isolated yield. The dicycloaddition product 1(SNS)2(SbF6)2 · SO2 was fully characterized by IR, Raman, NMR, elemental analysis, and single crystal X-ray crystallography. The product of the site selective dicycloaddition of SNS+ at the nitrile functionalities of 1 represents the first structurally characterized ortho-bis-1,3,2,4-dithiadiazolylium salt which adopts a conformation that utilizes the intramolecular Sδ+–Oδ− electrostatic interactions. The reaction of 1 and SNS[Al(OC(CF3)4] in a 1:1 ratio yielded only the monocycloaddition product 1(SNS)[Al(OC(CF3)3)4] indentified by multinuclear 13C, 14N, 19F, 27Al NMR in SO2 solution. The assignment of the 13C NMR of 1(SNS)+ in SO2 solution, were supported by comparison of the calculated (MPW1PW91/6-31G∗) 13C NMR and experimental chemical shifts, and by comparison of the observed chemical shifts of the related 2(SNS)+ produced on reacting the mono-nitrile CDMQ (2-cyano-5,6-dichloro-3-methoxy-1,4-benzoquinone (2)) with SNS+ (as the SbF 6 - salt). The reaction of SNSSbF6 with TCNE (1,1,2,2-tetracyanoethylene (3)) gave the tri-cycloaddition product 3(SNS)3(SbF6)3 · SO2 (IR, Raman microscopy and X-ray crystallography) in 40% yield. The energetics of the cycloaddition reactions of SNS+ with 1–3 and other multifunctional unsaturated centers (e.g. HC C–CN, NC–CN, C ( CN ) 3 - , o,m,p-(CN)2C6H4, 1,3,5-(NC)3C6H3, 4–10, previously reported, see Chart 1 ), were estimated in the gas phase (MPW1PW91/6-31G∗), solution (PBE0/6-311G∗) and in the solid state by the ‘volume based thermodynamics’ (VBT) approach. The general thermodynamic trends associated with the stepwise cycloaddition of SNS+ with multifunctional nitriles were established. The 1:1 cycloaddition products were calculated to be stable in the gas phase and solution, while in the solid state the larger lattice enthalpy of the 1:2 salt, relative to twice that of the 1:1 salt, favored the 1:2 cycloaddition product.

A series of diradical containing salts CxF2x(CNSSS)(2)(center dot center dot 2+)(AsF6-)2 {X = 2, 1[AsF6](2); x = 3, 3[AsF6](2); x = 4, 2[AsF6](2)} have been prepared. 1[AsF6](2) and 2[AsF6](2) were fully characterized by X-ray, variable-temperature magnetic susceptibility, and solid-state EPR measurements, further allowing us to extend the number of examples of the family of rare 7 pi RCNSSS center dot+ radical cations. 1[AsF6](2): a = 6.5314(7) angstrom, b = 7.5658(9) angstrom, c = 9.6048(11) angstrom, alpha = 100.962(2)degrees, = 96.885(2)degrees, gamma = 107.436(2)degrees, triclinic, space group P (1) over bar, Z = 1, T = 173 K. 2[AsF6](2): a = 10.6398(16) angstrom, b = 7.9680(11) angstrom, c = 12.7468(19) angstrom, beta = 99.758(2)degrees, monoclinic, space group P2(1)/c, Z = 2, T = 173 K. In the solid-state, CxF2x(CNSSS)(2)(center dot center dot 2+) (x = 2, 4) formed one-dimensional polymeric chains of dications containing discrete centrosymmetric radical pairs in which radicals were linked by four centered two-electron pi*- pi* bonds [1(2+), d(S center dot center dot center dot S) = 3.455(1) angstrom; 2(2+), d(S center dot center dot center dot S) = 3.306(2) angstrom]. The exchange interactions in these bonds were determined to be -500 +/- 30 and -900 +/- 90 cm(-1), by variable temperature magnetic susceptibility measurements, respectively, providing rare experimental data on the singlet-triplet gaps in the field of thiazyl radicals. For 2[AsF6](2), the thermally excited triplet state was unambiguously characterized by EPR techniques [vertical bar D vertical bar = 0.0254(8) cm(-1), vertical bar E vertical bar = 0.0013(8) cm(-1)]. These experimental data implied a weakly associated nature of the radical moieties contained in the solids 1[AsF6](2) and 2[AsF6](2). Computational analysis of the dimerization process is presented, and we show that the 2c 4 electron pi*-pi* bonds in 1[AsF6](2) and 2[AsF6](2) have ca. 50% and 40% diradical character, respectively. In contrast, 3[AsF6](2)center dot SO2, containing diradical C3F6(CNSSS)(2)(center dot center dot 2+) with an odd number of CF2 spacers, showed magnetic behavior that was consistent with the presence of monomeric radical centers in the solid state.

Reaction of Cl3CN and F5C2CN with a 1:1 mixture of S4(AsF6)2 and S8(AsF6)2 affords the paramagnetic solids Cl3CNSSSAsF6 (1CCl3AsF6) and F5C2CNSSSAsF6 (1C2F5AsF6). Isotropic electron paramagnetic resonance spectra of 1CCl3AsF6 and 1C2F5AsF6 in SO2 consist of a single line with g = 2.01675 and 2.01580, respectively. The structure of 1CCl3AsF6 contains chains of radical cations with relatively close interchain interactions. In contrast, chains are isolated in 1C2F5AsF6. The magnetic behavior of both compounds was interpreted as that of 1D Heisenberg antiferromagnetic chains (1CCl3AsF6, J = -34 cm(-1), theta = -9 cm(-1), TIP = 0.00082, rho = 0.012; 1C2F5AsF6, J = -21 cm(-1), theta = -4.2 cm(-1), TIP = 0.00092, rho = 0.065). Density functional theory calculated and experimental magnetic coupling constants were in good agreement. The correlation between intermolecular S...S contacts and the strength of magnetic couplings was established.

Reduction of the radical-cation [*NSSNC-CNSNS][AsF(6)] with ferrocene affords the novel biradical *NSNSC-CNSSN* containing both 1,2,3,5- and 1,3,2,4-isomeric dithiadiazolyl rings. Biradicals form centrosymmetric dimers with pi(*)-pi(*) interactions between different isomeric rings. Biradical *NSNSC-CNSSN* is diamagnetic in the solid state (C = 0.00035, TIP = 6.5 x 10(-)(5) emu/Oe.mol); however, an increase in paramagnetism was observed upon grinding (C = 0.003, TIP = 4.2 x 10(-)(4) emu/Oe.mol).

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.

The synthetically useful SNSSbF 6 is prepared, in good yield, from the reactions of S 3 N 2 Cl 2 or S 3 N 3 Cl 3 with stoichiometric amounts of AgSbF 6 and S 8 in liquid SO 2 . SNSSbF 6 crystallizes monoclinic in space group C2/m (a = 9.740(2) A, b = 6.644(2) A, c = 5.334(1) A, β = 90.58(2)°, Z = 2). The crystal structure was determined by standard methods and refined to R 1 = 0.019 and wR 2 = 0.048. The structure consists of discrete linear centrosymmetric SNS + cations [S-N = 1.4871(10) A] and almost octahedral SbF 6 - anions, with weak cation-anion interactions. The lattice energy of SNSSbF 6 was determined from the volume-based method as 525 ′ 32 KJ mol - 1 and the heat of formation of SNSSbF 6 ( s ) has been estimated as -1566 ′ 24 KJ mol - 1 . The FT-IR, Raman, and 1 4 N NMR spectra are reported, as well as an in situ study of the reaction of S 3 N 2 Cl 2 with AsF 5 in SO 2 solution.