A New Linear Tricobalt Compound with Di(2-pyridyl)amide (dpa) Ligands: Two-Step Spin Crossover of [Co3(dpa)4Cl2][BF4]

Influence of the Counterion and the Solvent Molecules in the Spin Crossover System [Co(4-terpyridone)2]Xp·nH2O

A series of new complexes belonging to the [Co(4-terpyridone)2]X(p) x nS family (4-terpyridone = 2,6-bis(2-pyridyl)-4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4]- (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4]- (p = 1) and [SiF6]2- (p = 0.5) and S = CH(3)OH for 3, X = [SiF6]2- (p = 1) and S = 3CH3OH and H2O for 4, X = [Co(NCS)4]2- (p = 1) and S = 0.5CH3OH for 5, X = I- (p = 2) and S = 5H2O for 6, X = [PF6]- (p = 1) for 7, and X = [NO3]- (p = 2) for 8. Compounds 1-7 can be grouped in three sets according to the space group in which they crystallize: (i) P1 triclinic (1, 3), (ii) P2(1) monoclinic (2), and (iii) P2(1)/c monoclinic (4-7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co-N axial bonds involving the pyridone rings are markedly shorter than the Co-N equatorial bonds collectively denoted as Co-N(central) and Co-N(distal), respectively. The differences in the average Co-N(central) or Co-N(distal) distances observed for 1-7 reflect the different spin states of Co(II). Complexes 7 and 4' are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)4]2- in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400-100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to DeltaH = 3.04 kJ mol(-1), DeltaS = 20 J K(-1) mol(-1), and Gamma = 0.95 kJ mol(-1).

Structure and Magnetism of [M3]6/7+ Metal Chain Complexes from Density Functional Theory: Analysis for Copper and Predictions for Silver

The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.

Weak antiferromagnetic coupling for novel linear hexanuclear nickel(II) string complexes (Ni6 12+) and partial metal-metal bonds in their one-electron reduction products (Ni6 11+)

The preparation, crystal structures, magnetic properties and electrochemistry of novel linear hexanuclear nickel string complexes (Ni6(12+)) and their corresponding 1-e(-) reduction products (Ni6(11+)) are reported. In these complexes, the hexanickel chain is in a symmetrical arrangement (approximately D(4) symmetry) and is helically supported by four bpyany(2-) ligands [bpyany(2-) = the dianion of 2,7-bis(alpha-pyridylamino)-1,8-naphthyridine]. The Ni6(12+) complexes show that the two terminal nickel ions have high-spin states (S = 1) and the four inner ones have low-spin states (S = 0). The two terminal nickel ions exhibit weak antiferromagnetic coupling of ca.-5 cm(-1). All of Ni6(12+) complexes display three reversible redox couples at about -0.70, -0.20 and +1.10 V (vs. Ag/AgCl). The first reduction wave at about -0.20 V suggests facility of 1-e(-) reduction for the Ni(6)(12+) compounds. The reaction of Ni(6)(12+) complexes with hydrazine afforded the 1-e(-) reduction products (Ni6(11+)). As far as we are aware, the shortest bond distance of 2.202 A with a partial metal-metal bond was observed in Ni6(11+) compounds. The magnetic results of these Ni6(11+) compounds are in agreement with a localized model, in which the two terminal nickel ions are in a spin state of S = 1 whereas the central Ni3-Ni4 pair in a spin state of S = 1/2. The N6(11+) compounds show relatively strong antiferromagnetic coupling of about 60 cm(-1) between the terminal and the central dinickel ions.

Novel linear hexanuclear cobalt string complexes (Co6(12+)) and one-electron reduction products (Co6(11+)) supported by four bpyany2- ligands

The new ligand, 2,7-bis(alpha-pyridylamino)-1,8-naphthyridine (H2bpyany), was synthesized by the reaction of 2,7-dichloro-1,8-naphthyridine with 2-aminopyridine in the presence of t-BuOK under palladium(0)-catalyzed conditions. The preparation and characterization of novel hexacobalt string complexes, [Co6(mu6-bpyany)4(NCS)2](PF6)n (n=1 (1); n=2 (2)) and [Co6(mu6-bpyany)4(OTf)2](OTf)n (n = 2 (3); n = 1 (4)) are presented. The crystal structures for compounds have been determined by X-ray crystallography. Compounds 1 and 4 have the Co6 11+ configurations and are air-stable. Compounds 2 and 3 with Co6 12+ configurations are structurally similar to 1 and 4, respectively. The electrochemistry of 1 displays four redox couples at E1/2= -0.55, +0.38, +0.91, and +1.18 V (vs. Ag/AgCl). The magnetic data show that compounds 1 and 4 are in a spin state of S = 1/2, and 2 and 3 in a spin state of S = 1. The results of the EHMO calculations on compounds 1 and 2 are in agreement with their magnetic measurements.

A trimetal chain cocooned by two heptadentate polypyridylamide ligands

Compounds of nickel in which a chain of three metal atoms is closely embraced by two interlocking heptadentate dianions derived from a chain of five pyridyl groups linked at the 2, or 2 and 6, positions by four amide nitrogen atoms are reported. This new type of extended metal atom chain (EMAC) compound differs from earlier ones in that ligand exchange at the axial positions cannot occur, because the axial ligands are part of the entire ligand. Four such compounds, all crystallographically characterized, are reported. This work is a proof-of-concept project that will be extended to other metals with these and other homologous ligands.

Synthesis and properties of highly fluorescent indolizino[3,4,5-ab]isoindoles

We report here the synthesis, X-ray structures, optical and electrochemical properties, fabrication of light-emitting devices, and density functional calculations for indolizino[3,4,5-ab]isoindole (INI) derivatives. Strongly luminescent heterocycles based on the INI unit were synthesized by 1,3-dipolar cycloaddition reactions between pyrido[2,1-a]isoindole (PIS) and acetylene or ethylene derivatives. They are indolizino[3,4,5-ab]isoindoles 2-9 and 14-15, benzo[1',2'-1,2]indolizino[3,4,5-ab]isoindoles 10, pyridazino[4',5':1,2]indolizino[3,4,5-ab]isoindoles 12-13, and 2,3-hydropyridazino[4',5':1,2]indolizino[3,4,5-ab]isoindole-1,4-dione 11. The relative luminescence quantum yield can be as high as 90%. Their reduction and oxidation potentials and high luminescence can make these heterocycles possible alternatives to tris(8-hydroxyquinolinato)aluminum (Alq(3)). The brightness of the light-emitting device reached as high as 10(4) cd/m(2) and indolizino[3,4,5-ab]isoindole 3 emits beautifully blue light. The X-ray crystal structures of INI derivatives were obtained for the first time. The geometries obtained from X-ray data and density functional theory calculations shed more light on an interesting formally antiaromatic 16pi system, which is divided into 10pi and 6pi aromatic systems. We also report a relatively easy protonation of INI, which occurs at a carbon, rather than nitrogen atom.

Linear and cyclic tetranuclear copper(i) complexes containing anions of N,N′-bis(pyrimidine-2-yl)formamidine

The reaction of Kpmf (pmf = anion of N,N[prime or minute]-bis(pyrimidyl-2-yl)formamidine, Hpmf) with CuSCN afforded the complexes K[Cu4(pmF)3(SCN)2], 1, and Cu(4)(pmf)4, 2. Reaction of 1 with [(n-Bu)4N]PF6 in THF gave the complex [(n-Bu)4N][Cu4(pmf)3(SCN)2], 3. Their structures were characterized by X-ray crystallography. Complexes 1 and 3 are the first linear tetranuclear complexes containing only Cu(I) atoms, while complex 2 is cyclic. The four Cu(I) atoms of complexes 1 and 3 are helically bridged by three tetradentate pmf- ligands. The [Cu4(pmf)3(SCN)2]- anions of 1 show weak interactions with adjacent [K(THF)5]+ cations through the sulfur atoms, forming infinite chains which are subjected to a series of intermolecular pi-pi interactions. In complex 2, the pmf- ligands are coordinated to the copper atoms in bidentate fashion through the two central amine nitrogen atoms, leaving the pyrimidine nitrogen atoms uncoordinated. Unexpected fluxional behaviors were observed for complexes 1 and 3 in solution. By the DNMR analysis, the free energy of activation (DeltaGc(not equal)) for the exchange is 12.8 kcal mol(-1) at 278 K (T(c)), and the rate constant of exchange (K(c)) is 470 s(-1) for 1. The DeltaGc(not equal) and Kc are 12.6 kcal mol(-1) at 273 K and 433 s(-1), respectively, for 3.

Unique Oxidative Metal−Metal Bond Formation of Linearly Aligned Tetranuclear Rh−Mo−Mo−Rh Clusters

Reaction of Mo2(pyphos)4 (1) with [RhCl(CO)2]2 followed by treatment of excess amounts of tBuNC resulted in the clean formation of [Mo2Rh2(tBuNC)4(pyphos)4](X)2 (4a; X = Cl). The X-ray diffraction study as well as spectroscopic analyses of 4c (X = BPh4) implied that there is no direct sigma-bonding interaction between each Rh(I) atom and the Mo2 core. Each Rh(I) atom in 4 can be oxidized concurrently by 2 equiv of [Cp2Fe]PF6 to afford [Mo2Rh2(Cl)2(tBuNC)4(pyphos)4](PF6)2 (5) along with the formation of two Mo-Rh(II) single bonds and the reduction of the bond orders of the Mo-Mo moiety.

A phosphorus supported multisite coordinating tris hydrazone P(S)[N(Me)N=CH-C6H4-o-OH]3 as an efficient ligand for the assembly of trinuclear metal complexes: synthesis, structure, and magnetism

A phosphorus supported multisite coordinating ligand P(S)[N(Me)N=CH-C(6)H(4)-o-OH](3) (2) was prepared by the condensation of the phosphorus tris hydrazide P(S)[N(Me)NH(2)](3) (1) with o-hydroxybenzaldehyde. The reaction of 2 with M(OAc)(2).xH(2)O (M = Mn, Co, Ni, x = 4; M = Zn, x = 2) afforded neutral trinuclear complexes [P(S)[N(Me)N=CH-C(6)H(4)-o-O](3)](2)M(3) [M = Mn (3), Co (4), Ni (5), and Zn (6)]. The X-ray crystal structures of compounds 2-6 have been determined. The structures of 3-6 reveal that the trinculear metal assemblies are nearly linear. The two terminal metal ions in a given assembly have an N(3)O(3) ligand environment in a distorted octahedral geometry while the central metal ion has an O(6) ligand environment also in a slightly distorted octahedral geometry. In all the complexes, ligand 2 coordinates to the metal ions through three imino nitrogens and three phenolate oxygens; the latter act as bridging ligands to connect the terminal and central metal ions. The compounds 2-6 also show intermolecular C-H...S=P contacts in the solid-state which lead to the formation of polymeric supramolecular architectures. The observed magnetic data for the (s = 5/2)3 L(2)(Mn(II))(3) derivative, 3, show an antiferromagnetic nearest- and next-nearest-neighbor exchange (J = -4.0 K and J' = -0.15 K; using the spin Hamiltonian H(HDvV) = -2J(S(1)S(2) + S(2)S(3)) - 2J'S(1)S(3)). In contrast, the (s = 1)(3) L(2)(Ni(II))(3) derivative, 5, displays ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions (J = 4.43 K and J' = -0.28 K; H = H(HDvV)+ S(1)DS(1) + S(2)DS(2)+ S(3)DS(3)). The magnetic behavior of the L(2)(Co(II))(3) derivative, 4, reveals only antiferromagnetic exchange analogous to 3 (J = -4.5, J' = -1.4; same Hamiltonian as for 3).

Increasing solubility and stability of linear tricobalt(II) chains with depa (diethyldipyridylamide) ligands

A new and more basic ligand has been used to make two new tricobalt extended metal atom chains (EMACs), namely, Co(3)(depa)(4)Cl(2), 3, and Co(3)(depa)(4)(CN)(2), 4. The depa ligand is di(4-ethyl-2-pyridyl)amide, a useful modification of the well-known di(2-pyridyl)amide, dpa, ligand. The new compounds have been thoroughly characterized, and informative comparisons are made of 3 with Co(3)(dpa)(4)Cl(2), 1, and 4 with Co(3)(dpa)(4)(CN)(2), 2, which were described several years ago. Introduction of the eight ethyl groups into the trimetal compounds improves solubility, makes oxidations easier, and allows for a more complete examination of the magnetic susceptibility as a function of temperature for 3 than was possible for 1. The data clearly indicate that the upper state for 3 has S = (3)/(2) and suggest that this is the case for 1 as well.

Oxidation of Ni3(dpa)4Cl2 and Cu3(dpa)4Cl2: nickel-nickel bonding interaction, but no copper-copper bonds

Of the known trinuclear dipyridylamido complexes of the first-row transition metals, M(3)(dpa)(4)Cl(2) (dpa is the anion of di(2-pyridyl)amine, M = Cr, Co, Ni, Cu), the one-electron-oxidation products of only Cr(3)(dpa)(4)Cl(2) and Co(3)(dpa)(4)Cl(2) have been isolated previously. Here we report one-electron-oxidation products of Ni(3)(dpa)(4)Cl(2) (1) and Cu(3)(dpa)(4)Cl(2) (3): Ni(3)(dpa)(4)(PF(6))(3) (2) and [Cu(3)(dpa)(4)Cl(2)]SbCl(6) (4). While there are no Ni-Ni bonds in 1, the Ni-Ni distances in 2 are 0.15 A shorter than those in 1, very suggestive of metal-metal bonding interactions. In contrast, the oxidation of 3 to 4 is accompanied by a lengthening of the Cu-Cu distances, as expected for an increase in electrostatic charge between positively charged nonbonded metal ions, which is further evidence against Cu-Cu bonding in either 3 or 4. A qualitative model of the electronic structures of all [M(3)(dpa)(4)Cl(2)](n+) (n = 0, 1) compounds is presented and discussed.

Unsymmetrical linear pentanuclear nickel string complexes: [Ni(5)(tpda)(4)(H(2)O)(BF(4))](BF(4))(2) and [Ni(5)(tpda)(4)(SO(3)CF(3))(2)](SO(3)CF(3))

The one-electron oxidized linear pentanuclear nickel complexes [Ni(5)(tpda)(4)(H(2)O)(BF(4))](BF(4))(2) (1) and [Ni(5)(tpda)(4)(SO(3)CF(3))(2)](SO(3)CF(3)) (2) have been synthesized by reacting the neutral compound [Ni(5)(tpda)(4)Cl(2)] with the corresponding silver salts. These compounds have been characterized by various spectroscopic techniques. Compound 1 crystallizes in the monoclinic space group P2(1)/n with a = 15.3022(1) A, b = 31.0705(3) A, c = 15.8109(2) A, beta = 92.2425(4) degrees, V = 7511.49(13) A(3), Z = 4, and compound 2 crystallizes in the monoclinic space group C2/c with a = 42.1894(7) A, b = 17.0770(3) A, c = 21.2117(4) A, beta = 102.5688(8) degrees, V = 14916.1(5) A(3), Z = 8. X-ray structural studies reveal an unsymmetrical Ni(5) unit for both compounds 1 and 2. Compounds 1 and 2 show stronger Ni-Ni interactions as compared to those of the neutral compounds.

A trinickel dipyridylamido complex with metal-metal bonding interaction: prelude to polynickel molecular wires and devices?

The molecule Ni(3)(dpa)(4)Cl(2) (1) can be oxidized by AgPF(6) to give crystalline Ni(3)(dpa)(4)(PF(6))(3) (2) (dpa is the anion of di(2-pyridyl)amine). This reversible oxidation occurs at a potential of 0.908 V vs Ag/AgCl electrochemically. The X-ray structure of 2 shows that the oxidation causes a major structural change (even though it is reversible), namely, a contraction of the Ni-Ni distances from ca. 2.43 A to 2.284[1] A. In addition, the electronic structure changes so that from four unpaired electrons in 1 there is only one in 2. From these remarkable results, it is inferred that while 1, and all higher homologues with 5, 7, 9,... nickel atoms are poor electronic conductors, the cations obtainable from them may be much better ones. This in turn means that by controlling the oxidation state electrochemically, these molecules may be able to function as nanoscale diodes.

Metal-metal bond length variability in Co(3)(dipyridylamide)(4)Cl(2): bond-stretch isomerism, crystal field effects, or spin transition process? A DFT study

The unprecedented structural behavior of Co(3)(dipyridylamide)(4)Cl(2), characterized in two crystalline forms in which the tricobalt framework is either symmetric or highly nonsymmetric at room temperature is investigated by means of gradient-corrected DFT calculations. The isolated molecule is assigned a single energy minimum associated with a low-spin (doublet) electronic configuration. The optimal geometry closely reproduces the X-ray structure observed for the isomer displaying equivalent metal-metal distances. However, the ground-state potential energy surface is extremely shallow with respect to a distortion of the Co(3) framework. A "weak" distortion, similar to that observed for the unsymmetrical complex at low temperature (Deltad(Co-Co) = 0.08 A at 110 K) induces a destabilization of 1.1 kcal.mol(-1) only. The distortion observed at room temperature (Deltad(Co-Co) = 0.17 A) destabilizes the isolated complex by 4.2 kcal.mol(-1). These results are rationalized in terms of the "three-electron three-center" concept applied to the sigma-bonding electrons of the cobalt framework. A phenomenological model based upon the Heisenberg Hamiltonian successfully reproduces the calculated potential energy curve and assigns the relative stability of the symmetric structure to local forces (Pauli repulsion, ligand bite, etc.) distinct from delocalized sigma bonding. In view of these results, the two structures characterized from X-rays cannot be termed "bond-stretch isomers" according to the strict definition given by Parkin. To investigate the origin of the distorted form, an electric field was applied to the isolated molecule, but it did not shift the equilibrium position toward asymmetry, despite a strong polarization of the electron density. Finally, the quartet state of lowest energy ((4)A state) has an optimal structure that is distorted and that reproduces most of the distinctive features observed in the nonsymmetric structure. Despite the high relative energy calculated for this quartet state, we assign the occurrence of the nonsymmetric form and its extreme variability with temperature to a progressive population of this excited state as temperature increases.

Compounds with symmetrical tricobalt chains wrapped by dipyridylamide ligands and cyanide or isothiocyanate ions as terminal ligands

Three new linear compounds of the type Co(3)(dpa)(4)X(2), where dpa is the anion of di(2-pyridyl)amine and X is NCS(-) (5), CN(-) (6), and N(CN)(2)(-) (7), have been prepared, and their structures and magnetic behavior have been studied. In all of them, including three different solvates of 5, the Co(3) chains are symmetrical with Co-Co distances of ca. 2.31-2.32 A. The appearance of four lines in the (1)H NMR spectra of the three compounds is also consistent with a symmetrical structure in solution. For all compounds, the magnetic behavior is quite similar with mu(eff) of ca. 1.9-2.0 micro(B) at temperatures between 1.8 and 200 K. As the temperature increases, the effective moments increase gradually, but since saturation is not reached, even at 400 K, the high-spin state cannot be assigned.

Linear trichromium complexes with the anion of 2,6-di(phenylimino)piperidine

The anion of 2,6-di(phenylimino)piperidine (DPhIP) has been found to support linear chains of three metal atoms. Three new compounds, [Cr3(DPhIP)4Cl]Cl.(1).5CH2Cl2.0.5H2O (1.1.5CH2Cl2.0.5H2O), [Cr3(DPhIP)4(CH3CN)]- (PF6)2.H2O.4CH3CN (2.H2O.4CH3CN), and [Cr3(DPhIP)4(F)(CH3CN)](BF4)2.5CH3CN (3.5CH3CN), have been synthesized and characterized by X-ray crystallography. Compound 1 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion, with two of them forming a quadruply bonded unit (Cr-Cr distance 1.932(2) A) and the third being a non-metal-metal-bound 5-coordinate unit (Cr...Cr distance 2.659(2) A). The fifth coordination site is occupied by a chloride ion, and another chloride ion is located in the interstices of the crystal. The trimetal unit in compound 2 is structurally similar to that in compound 1 except that the axial ligand in 2 is a CH3CN molecule. Compound 3 is an oxidation product prepared by reaction of 1 with AgBF4. Here, a square pyramidal CrIII unit, FCrN4, and a Cr-Cr quadruply bonded (Cr-Cr distance 1.968(2) A) unit, with an axially coordinated acetonitrile molecule, form the trichromium chain. The CrIII...CrII separation of 2.594(2) A in 3 is too long to be considered a bonding interaction.

New linear tricobalt complex of di(2-pyridyl)amide (dpa), [Co3(dpa)4(CH3CN)2][PF6]2

Reaction of the linear tricobalt compound Co3(dpa)4Cl2 (1) (dpa = di(2-pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co3(dpa)4(CH3CN)2][PF6]2 (2). Two crystalline forms are obtained from the same solution, namely, a monoclinic (P2(1)) form 2xCH3CNx2Et2O and a triclinic (P1) form, 2x3CH3CN. The tricobalt units in both crystals are essentially symmetrical, though this is not required by crystal symmetry, with Co-Co distances in the range 2298-2304 A. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule with Co-N distances in the range 2068-2111 A at 213 K. The spiral arrangement of ligands gives an overall idealized D4 point group symmetry for the cation [Co3(dpa)4(CH3CN)2]2+ . Chiral crystals of both delta and lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the 1H NMR spectrum. Magnetic susceptibility measurements in the solid state indicates that 2 has a doublet ground state and exhibits an increase of the effective moment at high temperature (approximately 160 K) due to a spin crossover process.