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The synthesis of diindolylamines via the palladium-catalyzed cross-coupling of aminoindoles and bromoindoles has been investigated, and efficient coupling conditions using BrettPhos, Pd(OAc) 2 , K 2 CO 3 , and tBuOH have been identified. The diindolylamines were found to be unstable in ambient conditions. Blocking the reactive 3-position of the bromoindole coupling partner with a tert -butyl group results in a diindolylamine with improved air stability. NMR, CV, and UV-vis studies on an asymmetrically substituted 3- tert -butyl-3'H-diindolylamine indicate that the instability of the diindolylamine substrates is likely due to oxidative oligomerization. Literature conditions used for the preparation of 3- tert -butylindoles afforded only the indole tetramer. The presence of water during the alkylation reaction was identified as the cause of the formation of the tetramer. Replacing hygroscopic t BuOH with nonhygroscopic t BuCl as the alkylating reagent provided access to 7-bromo-3- tert -butyl indole., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Reactions of indigo with quinones in the presence of sodium hydride leads unexpectedly to products containing two indigo subunits; one indigo is featured in a cis configuration and fused via its indole nitrogen atoms to a second indigo at the central C-C bond of the latter. Structural, optical, and redox properties of the new compounds are reported.

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH 3 CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH 3 CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH 3 CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of the thioindigo-N,N'-diphenyldiimine chelating ligand 3 have been prepared. The thioindigo diimine ligand binds to ruthenium in a bidentate fashion in the mononuclear compound 2 and serves as a bidentate chelating bridging ligand in the diruthenium complexes 1a and 1b. Compound 2 was isolated as a racemic mixture while the diruthenium complexes were isolated as the meso (ΔΛ) 1a and rac (ΔΔ and ΛΛ) 1b diastereomers. In-depth structural characterization of the compounds was performed, including X-ray crystallography, 1 H, 13 C, and 19 F nuclear magnetic resonance (NMR) spectroscopy, and 2D NMR correlation experiments. Electrochemical properties were evaluated utilizing cyclic voltammetry. Ground state optical properties of the complexes were examined using UV-visible spectroscopy and spectroelectrochemistry. The excited state dynamics of the series were investigated by ultrafast transient absorption spectroscopy. Variable temperature NMR experiments demonstrated that the rac diruthenium compound 1b undergoes conformational exchange with a rate constant of 8700 s -1 at 298 K, a behavior that is not observed in the meso diastereomer 1a. The series of complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λ max 689-783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes for 1a, 1b, and 2, and spectroelectrochemical experiments confirmed assignment of the transient species as arising from MLCT transitions. Unexpectedly, the transient absorption measurements revealed disparate time constants for the excited state decay of diastereomers 1a and 1b.

The synthesis and ligand-centered redox chemistry of palladium complexes bearing two potentially bidentate verdazyl ligands is explored. Reaction of 1,5-diisopropyl-3-pyridin-2-yl-6-oxoverdazyl radical 1 with Pd(NCMe) 4 ·2BF 4 gives a complex containing two coordinated verdazyl radicals. The structure of this complex consists of one verdazyl bound to Pd in a bidentate mode and the second verdazyl bound in a monodentate fashion through the pyridine substituent; the fourth coordination site is occupied by a solvent molecule (acetonitrile (3) or dimethyl sulfoxide (4)). Two-electron reduction of this complex with decamethylferrocene affords a bis(verdazyl) palladium complex (5) in which both verdazyls have been reduced to their anionic state and are both bound to Pd in bidentate manner. Complex 5 can be independently synthesized by a redox reaction between 1 and Pd 2 (dba) 3 . Reduced complex 5 can be re-oxidized to 3 or 4 with AgBF 4 ; in contrast, oxidation with PhICl 2 leads to ligand dissociation, ultimately giving radical 1 and a mono(verdazyl)dichloropalladium complex 2. One-electron oxidation using PhICl 2 produces a formally "mixed valent" (in ligand) bis(verdazyl)chloropalladium complex (6) with one bidentate verdazyl anion ligand and one monodentate (pyridine-bound) verdazyl radical. Attempted protonation of the verdazyl ligands in complex 5 leads to complete ligand dissociation and protonation of both the tetrazine and pyridine moieties; deprotonation regenerates 5. Subsequent air oxidation of the tetrazane/pyridinium cation (formed as a tetrachloropalladate salt) leads to re-coordination of the verdazyl ligands to give 6 initially, but ultimately produces a combination of free radical 1 and 2.

The protoisomerization (isomerization induced by protonation) mechanisms of indigo as well as indigo di- and monoimine derivatives were investigated using computational chemistry. Both density functional theory (M06-2X) and wave function theory (GMC-QDPT) methods were used to obtain reliable results. A solvation model (C-PCM with CH2Cl2 solvent) was employed to mimic the actual environment of the isomerization. The calculations reveal that the protoisomerizations of both the indigo and its imine derivatives are thermodynamically favorable. However, the energy barriers for rotating the double bonds in the derivatives are found to be lower than the one for indigo. More importantly, the imine derivatives undergo one-step isomerization, whereas a two-step process is predicted for indigo itself. The computational results provide insightful explanation for the different protoisomerization propensities of the parent indigo and its imine derivatives observed in experiment.

Indigo di- and monoimines can be protonated to form stable salts in which the central C=C bond has isomerized from a trans to cis configuration. Deprotonation of these salts regenerates the neutral trans species. The protonation chemistry of indigo is also explored.

Remote coordination of a Ru(hfac)2 moiety to a chelating N-benzyl tetrazine lowers the C-N homolytic bond dissociation enthalpy by approximately 20 kJ mol(-1). The significant bond strength perturbation is believed to arise as a consequence of metal-ligand redox non-innocence.

Reactivity studies of a Pd(II)-verdazyl complex reveal novel ligand-centred reduction processes which trigger pseudo-reductive elimination at Pd. Reaction of the complex with water induces a ligand-centred redox disproportionation. The reduced verdazyl ligands can also be reversibly protonated.

The synthesis and characterization of two new N,N'-diarylverdazyl radical ligands and their corresponding PdCl2 complexes are described. One of the two radicals is of the "Kuhn verdazyl" structure type and was made by adaptation of standard synthetic procedures for this class of verdazyl. The N,N'-diphenyl-6-oxoverdazyl was prepared by hydrolysis of a related tetrazane; the resulting N,N'-diphenylcarbohydrazide was condensed with pyridinecarboxaldehyde and then oxidized to the verdazyl according to standard protocols. Square planar PdCl2 complexes of both verdazyls were prepared by reactions of the radicals with PdCl2 in acetonitrile solution. The structural, spectroscopic, and electrochemical properties of the new verdazyl ligands and their Pd complexes are reported; generally the distinct ligand-centred properties associated with each verdazyl type carry over into the properties of the complexes. The electrochemical studies reveal ligand-centred oxidation and reduction processes; despite the minimal extent of spin delocalization onto Pd in the metal complexes, large shifts in oxidation and reduction potentials (relative to those of the free verdazyl ligands) are discussed.

Reactions of Nindigo-BF2 complexes with Pd(hfac)2 produced mixed complexes with Nindigo binding to both a BF2 and a Pd(hfac) unit. These complexes are the first in which the Nindigo ligand binds two different substrates, and provide a conceptual link between previously reported bis(BF2) and bis(Pd(hfac)) complexes. The new Pd/B complexes have intense near IR absorption near 820 nm, and they undergo multiple reversible oxidations and reductions as probed by cyclic voltammetry experiments. The spectral, redox, and structural properties of these complexes are compared against those of the corresponding B2 and Pd2 complexes with the aid of time-dependent density functional calculations. In all cases the low-energy electronic transitions are ligand-centered π-π* transitions, but the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies--and hence the absorption wavelength as well as the oxidation and reduction potentials--are significantly modulated by the moieties bound to the Nindigo ligand.

The electronic structures of (LX)2Ru(Vd)Ru(LX)2 complexes (Vd = 1,5-diisopropyl-3-(4,6-dimethyl-2-pyrimidinyl)-6-oxoverdazyl radical; LX = acac (acetylacetonate) or hfac (hexafluoroacetylacetonate)) in multiple charge states have been investigated experimentally and computationally. The main focus was to probe the consequences of the interplay between the ruthenium ions and the redox-active verdazyl ligand for possible mixed-valent behavior. Cyclic voltammetry studies reveal one reversible reduction and one reversible oxidation process for both complexes; in addition the acac-based derivative possesses a second reversible oxidation. Analysis of a collection of experimental (X-ray structures, EPR, electronic spectra) and computational (TD-DFT (PCM)) data reveal that the ruthenium ancillary ligands (acac vs hfac) have dramatic consequences for the electronic structures of the complexes in all charge states studied. In the hfac series, the neutral complex is best regarded as a binuclear Ru(II) species bridged by a neutral radical ligand. Reduction to give the anionic complex takes place on the verdazyl ligand, whereas oxidation to the cation (a closed shell species) is shared between Vd and ruthenium. For the acac-based complexes, the neutral species is most accurately represented as a Ru(II)/Ru(III) mixed valent complex containing a bridging verdazyl anion, though some bis(Ru(II))-neutral radical character remains. The monocation complex contains a significant contribution from a "broken symmetry" singlet diradical structure, best represented as a bis-Ru(III) system with an anionic ligand, with significant spin coupling of the two Ru(III) centers via the Vd(-1) ligand (calculated J = -218 cm(-1)). The dication, a spin doublet, consists of two Ru(III) ions linked (and antiferromagnetically coupled) to the neutral radical ligand. Computed net σ- and π-back-donation, spin densities, and orbital populations are provided. Time dependent DFT is used to predict the optical spectra and assign experimental data.

Reactions of indigo with a variety of substituted anilines produce the corresponding indigo diimines ("Nindigos") in good yields. Nindigo coordination complexes are subsequently prepared by reactions of the Nindigo ligands with Pd(hfac)(2). In most cases, binuclear complexes are obtained in which the deprotonated Nindigo bridges two Pd(hfac) moieties in the expected bis-bidentate binding mode. When the Nindigo possesses bulky substituents on the imine (mesityl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl, etc.), mononuclear Pf(hfac) complexes are obtained in which the Nindigo core has isomerized from a trans- to a cis-alkene; in these structures, the palladium is bound to the cis-Nindigo ligand at the two indole nitrogen atoms; the remaining proton is bound between the imine nitrogen atoms. The palladium complexes possess intense electronic absorption bands [near 920 nm for the binuclear complexes and 820 nm for the mononuclear cis-Nindigo complexes; extinction coefficients are (1.0-2.0) × 10(4) M(-1) cm(-1)] that are ligand-centered (π-π*) transitions. Cyclic voltammetry investigations reveal multiple redox events that are also ligand-centered in origin. All of the palladium complexes can be reversibly oxidized in two sequential one-electron steps; the binuclear complexes are reduced in a two-electron process whose reversibility depends on the Nindigo ligand substituent; the mononuclear palladium species show two one-electron reductions, only the first of which is quasi-reversible.

The electronic structures of (Vd)Ru(LX)(2) complexes (Vd = 1,5-diisopropyl-3-(2-pyridyl)-6-oxoverdazyl radical; LX = acac or hfac) as neutral, cationic, and anionic species have been investigated experimentally and computationally to probe the interplay between the ruthenium ion and the redox-active verdazyl ligand. The cationic complexes were prepared by oxidation of the corresponding neutral species with silver(I) salts. The hfac-based anionic complex was synthesized by reduction of the neutral species with cobaltocene, but the anionic acac analogue could not be prepared. Experimental (X-ray structures, electronic spectra) and computational (TD-DFT (PCM)) studies reveal that the expression of redox activity of the ligand and metal moieties is highly sensitive to the nature of the ancillary ligands on ruthenium. In the hfac series, the cationic, neutral, and anionic complexes can, respectively, be adequately described as Ru(II) complexes of a coordinated verdazyl cation, neutral radical, and anion. However, the more electron-donating acac coligands facilitate a stronger interaction between ruthenium and verdazyl via Ru(d) to Vd(π*) backbonding which is dependent on the overall charge of the complex and has the net effect of creating a high degree of metal-ligand covalency. Studies on the two cationic complexes reveal further distinctions between the acac- and hfac-containing systems: whereas the former has a significant open-shell singlet contribution to the complex ground state, this open-shell formulation is a minor component of the latter.

Reactions of indigo with anilines provide a simple route to indigo N,N'-diaryldiimines ("Nindigo"), a new binucleating ligand with two beta-diketiminate-type metal binding sites. Bis-palladium complexes have interesting ligand-centred properties such as redox activity and intense near infrared absorption.

Spectroscopic, structural, and computational studies of verdazyl radical-ruthenium complexes reveal the verdazyl to be a redox-active and non-innocent ligand. The ensuing delocalization of charge is highly sensitive to the nature of the beta-diketonate co-ligands.

Copper(I) complexes of a diketiminate featuring CF(3) groups on the backbone and dimethylphenyl substituents (4) and a nitroformazan (5) were synthesized and shown by spectroscopy, X-ray crystallography, cyclic voltammetry, and theory to contain copper(I) sites electron-deficient relative to those supported by previously studied diketiminate complexes comprising alkyl or aryl backbone substituents. Despite their electron-poor nature, oxygenation of LCu(CH(3)CN) (L = 4 or 5) at room temperature yielded bis(hydroxo)dicopper(II) compounds and at -80 degrees C yielded bis(mu-oxo)dicopper complexes that were identified on the basis of UV-vis and resonance Raman spectroscopy, spectrophotometric titration results (2:1 Cu/O(2) ratio), electron paramagnetic resonance spectroscopy (silent), and density functional theory calculations. The bis(mu-oxo)dicopper complex supported by 5 exhibited unusual spectroscopic properties and decayed via a novel intermediate proposed to be a metallaverdazyl radical complex, findings that highlight the potential for the formazan ligand to exhibit "noninnocent" behavior.

The synthesis and characterization of several formazans containing strong electron-withdrawing substituents (cyano and nitro) in the 3 position of the ligand backbone are described. Reactions of aryldiazonium cations with the conjugated bases of either cyanoacetic acid or nitromethane lead to 1,5-diaryl-3-cyano- or 3-nitroformazans, respectively. When these reactions are carried out in aqueous conditions, the range of aromatic groups is limited by the stability of the diazonium salt. However, 3-nitroformazans containing bulky substituents on the nitrogen atoms (2,6-dimethylphenyl, 2,4,6-trimetyhlphenyl, 2,6-diisopropylphenyl, and 3,5-ditert-butylphenyl) could be made by performing the reactions under nonaqueous and anhydrous conditions. NMR and electronic spectroscopic studies indicate that the 3-nitroformazans exist exclusively as closed ( trans-syn, s-cis) isomers whereas the 3-cyanoformazans exist as mixtures of isomers which are substrate-dependent. The crystal structures of five of the formazans are presented: two 3-nitroformazans, both of which are closed, and three 3-cyanoformazans, two of which are closed and one of which adopts an open ( trans-syn, s-trans) structure. Solid state (diffuse reflectance) spectroscopy has been employed to ascertain the isomeric preferences of the other formazans which could not be crystallographically characterized.

The synthesis and characterization of six transition metal complexes of 3-cyano- and 3-nitroformazans are described. Three different formazans were reacted with nickel(II) to produce complexes with bidentate formazan ligands. Mononuclear NiL2 (L = deprotonated formazan) or binuclear hydroxo-bridged (LNi)2(mu-OH) 2 species were produced depending on the steric bulk on the formazan N-aromatic substituents. 1,5-Bis(2-methoxyphenyl)-3-cyanoformazan acts as a tetradentate monoanionic ligand in a copper(II) complex, whereas the analogous 1,5-bis(2-hydroxyphenyl)-3-cyanoformazan binds as a trianion in a tetradentate manner to Fe(III) and Co(III). Crystal structures-the first examples of metal complexes of cyano- or nitroformazans-as well as the electronic spectra of the complexes are discussed in relation to each other as well as that of the uncoordinated formazans.

The redox properties of verdazyl radicals are presented using cyclic voltammetry techniques. These radicals can be reversibly reduced as well as oxidized. Electron-donating and -withdrawing substituents have significant effects on the oxidation and reduction potentials as well as the cell potential (E(cell) = | E(ox) degrees - E(red) degrees |) for these radicals; a correlation between the electron spin distribution and redox properties is developed.

Two benzene-bridged N,N'-bis(isopropyl)6-oxoverdazyl diradicals 7a (1,4-benzene-bridged) and 7b (1,3-benzene-bridged) were prepared and studied by an array of physicochemical techniques aimed at elucidating the intramolecular electronic and magnetic coupling between verdazyl chromophores. The very high stability of these diradicals permits comprehensive investigations of their properties for the first time. The UV-vis spectra suggest negligible direct conjugative overlap involving the radical SOMOs, although significant differences in higher energy absorptions suggest that radical orbitals other than the SOMO are likely communicating via the central p-phenylene bridge in 7a. The electrochemical features of the two diradicals are nearly identical; in each derivative, both radicals are oxidized essentially independently while the reductions occur in a stepwise manner with differences of approximately 100 mV between the two reductions. EPR and magnetic susceptibility collectively indicate that the para-bridged diradical is weakly antiferromagnetically coupled while the meta analogue is weakly ferromagnetically coupled, in accord with the topology of the substitution pattern on the central benzene ring.

Many kinds of radicals are stable enough to isolate, handle, and store without any special precautions. The diversity in molecular architectures of these stable radicals is sufficiently large that the common factors governing radical stability/persistence, geometric and electronic structure, association/dimerization preferences, and reactivity have generally not been well articulated or appreciated. This review provides a survey of the major classes of stable or persistent organic/organomain group radicals with a view to presenting a unified description of the interdependencies between radical molecular structure and properties.

For over two decades there have been intense efforts aimed at the development of alternatives to conventional magnets, particularly materials comprised in part or wholly of molecular components. Such alternatives offer the prospect of realizing magnets fabricated through controlled, low-temperature, solution-based chemistry, as opposed to high-temperature metallurgical routes, and also the possibility of tuning magnetic properties through synthesis. However, examples of magnetically ordered molecular materials at or near room temperature are extremely rare, and the properties of these materials are often capricious and difficult to reproduce. Here we present a versatile solution-based route to a new class of metal-organic materials exhibiting magnetic order well above room temperature. Reactions of the metal (M) precursor complex bis(1,5-cyclooctadiene)nickel with three different organics A-TCNE (tetracyanoethylene), TCNQ (7,7,8,8-tetracyanoquinodimethane) or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)--proceed via electron transfer from nickel to A and lead to materials containing Ni(II) ions and reduced forms of A in a 2:1 Ni:A ratio--that is, opposite to that of conventional (low Curie temperature) MA(2)-type magnets. These materials also contain oxygen-based species within their architectures. Magnetic characterization of the three compounds reveals spontaneous field-dependent magnetization and hysteresis at room temperature, with ordering temperatures well above ambient. The unusual stoichiometry and striking magnetic properties highlight these three compounds as members of a class of stable magnets that are at the interface between conventional inorganic magnets and genuine molecule-based magnets.

Formazans react with boron triacetate to produce boratatetrazines, which can be reduced to yield borataverdazyl radical anions--the first boron containing verdazyl radicals.

The mechanism of the magnetic interaction in the pyridyl-verdazyl radical:hydroquinone (pyvd:hq) molecular co-crystal is important as it has been suggested to originate by a unique "mediated through-space" magnetic interaction. This interaction was proposed to magnetically connect two nonadjacent pyridyl-verdazyl radicals within a pi stack, where adjacent radicals pile up in a head-over-tail orientation. The connection is made through a third radical sitting between the previous two mediated radicals. Given the relevance of this proposal, we decided to reinvestigate the magnetic properties of this co-crystal by using our recently proposed first-principles "bottom-up" procedure. Based on B3LYP/6-31+G(d) and CASSCF(6,6)/6-31+G(d) calculations (the results of which are identical to those provided by CASSCF(10,10)/6-31+G(d) calculations), we have computed the microscopic J(AB) values for all direct through-space magnetic interactions between nearby pyridyl-verdazyl radicals. The magnetic interactions give rise to two dominant values of similar strength: -56 and -54 cm(-1) at the B3LYP level, which are calculated as -38 and -31 cm(-1) at the CASSCF(6,6) and CAS(10,10) levels (all other interactions being smaller than |1| cm(-1)). The dominant interactions correspond to the direct through-space interaction between two adjacent radicals of a pi stack. The crystal also exhibits a radical-mediated through-space interaction of -0.31 cm(-1) between two nonadjacent radicals of a pi stack. The direct through-space magnetic interactions are two orders of magnitude larger than the mediated through-space interaction. Thus, first-principles calculations do not support a mediated through-space mechanism to explain the magnetism of the pyvd:hq co-crystal. The magnetic topology generated by the two dominant antiferromagnetic interactions in the pyvd:hq co-crystal consists of one-dimensional (1D) alternating chains (interacting very weakly along the b and c axes). By using this topology, the computed macroscopic magnetic susceptibility curve reproduces the experimental one properly. This first-principles bottom-up description of the magnetism in the pyvd:hq co-crystal differs in some fundamental aspects from that previously proposed in the literature.

The solid state structure of 1,1'-bis(verdazyl)ferrocene reveals the two radical moieties associated intramolecularly to give the first example of a pi-dimer of a stable verdazyl radical. The pi-dimer structure is not maintained in solution. Magnetic characterization indicates that the radicals are sufficiently strongly antiferromagnetically coupled so as to render the molecule diamagnetic at room temperature.

A series of alpha,omega-bis(mesitylthio)oligothiophenes of various chain lengths and with different side substitution patterns have been studied in their oxidized states by means of electron absorption and Raman spectroscopies in combination with theory in the framework of the density functional theory. Upon chemical oxidation, stable radical cations, dications, and even radical trications are generated. Longer chain lengths better stabilize higher oxidation states. The tetramer can be easily converted to the dication, and a trication can be obtained for the ethylenedioxy derivative. The alpha,omega-sulfur atoms are actively involved in the formation of the charged species and exert a favorable tuning of their electronic structure. Raman spectra provide experimental evidence of the attainment of quinoidal structures within the conjugated path, initially heteroaromatic, with different extension as a function of the p-doping level.

[structure: see text] The syntheses of several verdazyl radical and diradicals containing pyridine-based multitopic coordination sites are described. These compounds were designed to be paramagnetic analogues of oligopyridine metallosupramolecular building blocks.

The verdazyl radical 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)(2)(2+).2X(-) (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = ClO(4), PF(6)) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows: 5, monoclinic space group Pna2(1), a = 18.755(4) A, b = 11.154(3) A, c = 16.594(4) A, alpha = 90.00 degrees, beta = 90.00 degrees, gamma = 90.00 degrees, V = 3471.4(13) A(3), and Z = 4; 7, triclinic space group Ponedblac;, a = 9.4638(18) A, b = 9.8442(19) A, c = 18.769(4) A, alpha = 103.746(3) degrees, beta = 92.925(3) degrees, gamma = 94.869(3) degrees, V = 1687.8(6) A(3), and Z = 2; 8, triclinic space group Ponedblac;, a = 9.4858(14) A, b = 9.7919(14) A, c = 18.889(3) A, alpha = 104.196(3) degrees, beta = 92.855(3) degrees, gamma = 94.216(3) degrees, V = 1692.1(4) A(3), and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseudooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal-verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (J(Ni-vd) = +240 cm(-1)), and strongly antiferromagnetic in 5 (J(Mn-vd) = -93 cm(-1)). Complex 7 exhibits weak ferromagnetic coupling (J(Cu-vd) = -4.5 cm(-1)). Intramolecular radical-radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (J(vd-vd) = -8 cm(-1)). Intramolecular radical-radical exchange was generally weak in the four metal complexes, ranging from -10 cm(-1) (for 5) to +2 cm(-1) (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions.

The synthesis of a verdazyl radical with a carboxylate substituent renders the radical highly soluble in water, thereby permitting the aqueous synthesis of Ni(II) and Co(II) verdazyl complexes which have been structurally and magnetically characterized.

Two binuclear metal-radical complexes, formed by the reaction of M(hfac)(2) x 2H(2)O (M = Mn or Ni; hfac = hexafluoroacetylacetonate) with the 1,5-dimethyl-3-(4,6-dimethylpyrimidin-2-yl)-6-oxoverdazyl radical (3), were synthesized. The binuclear Mn complex 5 (i.e., 3[Mn(hfac)(2)](2)) crystallizes in the monoclinic space group C2/c: C(30)H(17)N(6)O(9)F(24)Mn(2), a = 29.947(3), b = 17.143(3), c = 16.276(3) A, beta = 123.748(3)*, Z = 4. The compound consists of two pseudo-octahedral Mn(II) ions, both bearing two hfac ancillary ligands, bridged by the bis(bidentate) radical 3. The temperature dependence of the magnetic susceptibility of 5 reveals moderate antiferromagnetic exchange between each of the Mn(II) ions and the verdazyl radical (J = -48 cm(-1)). The S = 9/2 ground spin state of the complex was corroborated by low-temperature magnetization versus field measurements. In contrast, the magnetic susceptibility versus temperature behavior of 6 (whose molecular structure is presumed to be analogous to that of 5) indicates that the two Ni(II) ions are strongly ferromagnetically coupled to the verdazyl radical (J = +220 cm(-1)). The magnetization versus field behavior of 5 is consistent with an S = 5/2 ground-state species.

The X-ray crystal structure and magnetic properties of a molecular crystal consisting of 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl radical and hydroquinone (pyvd:hq) are presented. The structure contains a two-dimensional network of hydrogen bonds involving the hydroquinones and the pyridine ring of the pyvd radical. The radicals adopt an unusual head-over-tail (antiparallel) pi-stacked array perpendicular to the hydrogen-bonded planes. The variable-temperature magnetic susceptibility data can be modeled using a one-dimensional antiferromagnetic chain model, with J = -58 cm(-1). The strength of the magnetic coupling is very unusual because there are no close intermolecular radical-radical contacts to provide conventional pathways for magnetic interactions. A pathway for coupling is proposed involving the mediation of magnetic exchange interactions between radical centers by the pyridine rings. Density functional calculations on the pyvd radical, as well as aggregates thereof based on the X-ray structure, have been employed in attempts to understand the possible mechanisms by which the strong magnetic interactions are achieved.

Reaction of P-dimethylaminophosphonic acid bis(1-methylhydrazide) (6) with trimethyl orthobenzoate gave 1,2,5,6-tetrahydro-1,5-dimethyl-6-(N,N-dimethylamino)-6-phenyl-1,2,4,5,6-tetrazaphosphorine-6-oxide (7), which was subsequently oxidized to the corresponding P-diemthylamino-6-phosphaverdazyl (5) as a persistent radical. Analysis of the electron paramagnetic resonance spectrum of 5 revealed significant spin density on the exocyclic nitrogen but very little spin density on the phosphorus, in contrast to the previously reported P-phenyl-6-phosphaverdazyl (4). Density functional theory calculations on simplified models of 4, 5, and related radicals were performed and revealed that spin polarization effects and the nature of the substituents on phosphorus have significant effects on the structures and spin distributions of these radicals. The spin transfer to the dimethylamino group in 5 was revealed to arise from spiroconjugation-type overlap between the nitrogen 2p orbital with the verdazyl radical singly occupied molecular orbital.