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The electronic properties of the family of pyridyl− and pyrimidyl− ferrocenyl complexes [Fe(η⁵-C5H4-4-C5H4N)(η⁵-C5H5)] (1), [Fe(η⁵-C5H4-4-C5H4N)2] (2), [Fe(η⁵-C5H4-4-C5H4N)(η⁵-C5H4-5-C4H3N2)] (3), [Fe(η⁵-C5H4-5-C4H3N2)2] (4), [Fe(η⁵-C5H4−C6H4-4-C5H4N)(η⁵-C5H4-5-C5H4N)] (5), and [Fe(η⁵-C5H4−C6H4-4-C5H4N)2] (6), prepared by Suzuki coupling starting from ferrocenyl diboronic acid, have been investigated by electrochemical, spectroscopic, and quantum-chemical methods. All complexes display photo- and electrochemical stability and strong electronic interactions in the ground state between the ferrocenyl moiety and pyridine/pyrimidine rings. The efficient modulation of the potentials of ferrocenyl-based oxidation is shown to be strongly dependent on the nature of the hetero ring. These features, together with their high chelating ability, are potentially useful in the preparation of redox-active complexes of complexes such as mixed-metal metallamacrocycles.

Supramolecular adducts of formula [N(CH2CH2)3N]−H−[OOC(CH2)nCOOH] (n = 1−7) have been obtained by mechanochemical reaction of solid dicarboxylic acids of variable chain length HOOC(CH2)nCOOH (n = 1−7) with solid 1,4-diazabicyclo[2.2.2]octane (dabco). ¹H MAS and ¹⁵N CPMAS spectra have been measured to investigate the presence of intermolecular hydrogen bonds between acid and base. Proton and nitrogen chemical shifts allow a distinction to be made between N⁺−H···O^- interactions (with proton transfer) and N···H−O interactions (without proton transfer) and between strong and weak hydrogen bonds. Correlations among isotropic ¹H, ¹⁵N chemical shift data and the N−O distances of the atoms involved in the hydrogen bond interaction are found. Density functional theory, applied to explore changes upon hydrogen bonding in the ¹H and ¹⁵N shielding parameters, is in agreement with the experimental values found by solid-state NMR spectroscopy. Hydrogen/deuterium H/D isotope effects on the ¹⁵N NMR chemical shifts have been also investigated.

The bidentate ferrocenyl sandwich [Fe(η⁵-C5H4-1-C5H4N)2] (1) has been used as a coordination ligand in reactions with AgNO3, Cd(NO3)2, Cu(CH3COO)2, Zn(CH3COO)2, and ZnCl2 to obtain the heterobimetallic metallamacrocyclic derivatives [Fe(η⁵-C5H4-1-C5H4N)2]2Ag2(NO3)2·1.5H2O, [Fe(η⁵-C5H4-1-C5H4N)2]2 Cu2(CH3COO)4·3H2O, [Fe(η⁵-C5H4-1-C5H4N)2]2Cd2(NO3)4·CH3OH·0.5C6H6, [Fe(η⁵-C5H4-1-C5H4N)2]2Zn2(CH3COO)4, and [Fe(η⁵-C5H4-1-C5H4N)2]2Zn2Cl4 (abbreviated in the following as (1Ag^I)2²⁺, (1Cd^II)2⁴⁺, (1Cu^II)2⁴⁺, (1Zn^IIacetate)2⁴⁺, and (1Zn^IIchloride)2⁴⁺). The reaction with the ferrocenedicarboxylic acid complex [Fe(η⁵-C5H4COOH)2] has led to the supramolecular adduct [Fe(η⁵-C5H4-1-C5H4N)2][Fe(η⁵-C5H4COOH)2] (1[Fe(η⁵-C5H4COOH)2]). All compounds have been structurally characterized by single-crystal X-ray diffraction.

The synthesis and structural characterization of the ferrocenyl diboronic acid complex [Fe(η⁵-C5H4-B(OH)2)2] (1) and of its products of monosubstitution, [Fe(η⁵-C5H4-4-C5H4N)(η⁵-C5H4-B(OH)2)] (2 in three polymorphic modifications, 2a−c), and of disubstitution, [Fe(η⁵-C5H4-4-C5H4N)2] (4), [Fe(η⁵-C5H4-C6H4-4-C5H4N)2] (6), and [Fe(η⁵-C5H4-5-C4H3N2)2] (7), are reported together with an investigation of the mode of supramolecular bonding in the solid state. The competition between the hydrogen-bonding interactions of the (B)O−H···O(B) and (B)O−H···N types in the cases of crystalline 1 and 2 has been investigated. The B(OH)2 group provides two hydrogen bonding donor groups and two acceptors, forming mainly cyclic hydrogen-bonded systems in topological analogy with a primary amido group. Compounds 4, 6, and 7 are examples of neutral disubstituted pyridyl and pyrimidyl ferrocenyl complexes with potentials as supramolecular ligands. The compounds [Fe(η⁵-C5H4-4-C5H4NH)(η⁵-C5H4-B(OH)2)][NO3] (3a), [Fe(η⁵-C5H4-4-C5H4NH)(η⁵-C5H4-B(OH)2)][SO4]·3H2O (3b), and [Fe(η⁵-C5H4-4-C5H4NH)2][Cl]2·4H2O (5) have been obtained by treatment with acids of compounds 2 and 4, respectively. The interionic hydrogen bonds have also been investigated.

Exposure of the solid zwitterion [(η⁵-C5H4COOH)(η⁵-C5H4COO)Co^III] (1) to vapors of CF3COOH and HBF4 quantitatively produces the corresponding salts of the cation [(η⁵-C5H4COOH)2Co^III]⁺ (2), namely 2[CF3COO] and 2[BF4]. Both heterogeneous reactions are fully reversible; the acids can be quantitatively removed by mild thermal treatment, which regenerates the starting material. Single crystals of 2[CF3COO] and 2[BF4] for X-ray diffraction can be obtained if the same reaction is carried out in solution. They possess the same structures as those produced in the gas−solid reactions.

The μ-1-azavinylidene−μ-alkenyl diruthenium complex [Ru2(μ-η¹:η¹-N&dbd;CPh2)(μ-η¹:η²-PhC&dbd;CHPh)(CO)6] (1) reacts with excess 1-phenyl-1-propyne or 3-hexyne in 1,2-dichloroethane at reflux temperature to give cis-stilbene and the metallacyclic derivatives [Ru2{μ-η³:η⁴-R¹C&dbd;CR²CR¹&dbd;CR²N&dbd;CPh(C6H4)}(μ-CO)(CO)4] (2, R¹ = Ph, R² = Me; 3, R¹ = R² = Et). These compounds contain ligands that result from the ortho metalation of a phenyl group of the original 1-azavinylidene ligand, the release of cis-stilbene, the insertion of the corresponding alkyne molecule into a Ru−N bond, and the insertion of a second alkyne molecule into a Ru−C bond. The reaction of compound 1 with phenylacetylene in 1,2-dichloroethane at reflux temperature gives cis-stilbene, [Ru2{μ-η¹:η²-N&dbd;CPh(C6H4)}(μ-η²:η⁴-PhC&dbd;CHCH&dbd;CPh)(CO)5] (4), and [Ru{η⁵-PhC&dbd;CHCPh&dbd;CHCPh&dbd;CHN&dbd;CPh(C6H4)}(CO)2] (5). The hydrocarbyl ligand of 4, which can be described as a 1,4-diphenylbuta-1,3-diene-1,4-diyl ligand, arises from a head-to-head dimerization of phenylacetylene, whereas the organic ligand of 5 results from an unprecedented insertion of phenylacetylene into a Ru−N bond in addition to a head-to-tail trimerization of phenylacetylene. Thermolysis of compound 1 in refluxing 1,2-dichloroethane gives the tetranuclear derivative [C48H30N2O8Ru4] (6) and the binuclear product [Ru2(μ-η¹:η¹-N&dbd;CPh2)(μ-η³:η⁴-PhCH&dbd;CPhCPh&dbd;CPh)(CO)4] (7). The molecular structure of complex 6 remains unknown. Compound 7 contains a bridging dienyl ligand (with a phenyl substituent η²-coordinated to a Ru atom) that arises from the coupling of diphenylacetylene with the diphenylalkenyl ligand of 1. Mechanistic proposals that account for the formation of complexes 2−7 from compound 1 are given. The new C−N bonds of compounds 2, 3, and 5 are remarkable because they have been formed by migratory insertion of weakly electrophilic alkynes into Ru−N bonds, a process that has hitherto been very seldom observed only with internal alkynes and never with terminal alkynes.

The synthesis and structural characterization of the neutral diamagnetic organometallic dicarboxylic acid [Cr⁰(η⁶-C6H5COOH)2] (1) is reported. Oxidation of 1 by oxygen leads to formation of the paramagnetic dicarboxylic acid cation [Cr^I(η⁶-C6H5COOH)2]⁺ (2), which has been isolated as the [PF6]^- salt in two crystalline polymorphic modifications, a monoclinic form α and a triclinic form β. The neutral zwitterionic form [Cr^I(η⁶-C6H5COOH)(η⁶-C6H5COO)] (3) has been obtained in a cocrystal with [NH4][PF6] upon treatment of the acid salt [Cr^I(η⁶-C6H5COOH)2]⁺[PF6]^-with ammonia. The hydrogen bonding interactions established by the neutral and cationic acids are discussed and compared with those observed in the zwitterionic form and in the related complexes [Fe^II(η⁵-C5H4COOH)2] and [Co^III(η⁵-C5H4COOH)2]⁺[PF6]^-.