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The reaction of new 2,6-bis(pyrazol-3-yl)pyridine ligand (L) containing a redox-active ferrocenyl substituent with cobalt(II) salt gave the cobalt(II) complex [Co(L)2](ClO4)2 (I), which was isolated in a pure state and characterized by elemental analysis, NMR spectroscopy, cyclic voltammetry, and X-ray diffraction. According to X-ray diffraction data (CIF file CCDC no. 2049714) and the Evans method, which allows determining the spin state of paramagnetic compounds in solution on the basis of NMR spectra, the cobalt(II) ion in complex I occurs in the high-spin state and does not undergo temperature-induced spin transition in the temperature range of 120–370 K.

The design of new redox-switchable molecules requires the development of simple and efficient synthetic approaches. This study demonstrates the possibility of ortho-lithiation of ferrocenecarboxaldehyde (aryl)imines (Ia–Ic) followed by the reaction with ketones to give 1,2-disubstituted ferrocenes (IIa–IIc). These products, in turn, can be cyclized by treatment with trimethylsilyl triflate to give the cationic precursors of ferrocene-containing N-heterocyclic carbenes (IIIa–IIIc), in which the heterocycle is annulated to one of the ferrocene cyclopentadienyl ring. Treatment of IIIa–IIIc with a base in the presence of a source of rhodium afforded rhodium carbene complexes (IVa, IVb) in which the carbene ligand resembled cyclic alkylaminocarbenes in the electron-donor properties. Compounds Ib and IVa were studied by X-ray diffraction (CIF file CCDC nos. 2000413 and 2000414, respectively).

The reactions of 2,6-bis(1,4-diphenyl-5-hydroxy-1H-pyrazol-3-yl)pyridine (L), which is the first representative of a series of 2,6-bis(pyrazol-3-yl)pyridine ligands bearing substituents in positions 1 and 4 of the pyrazol-3-yl ring, with the divalent iron and cobalt salts afford new Со(II) and Fe(II) complexes: [Co(L)2](ClO4)2 (I) and [Fe(L)2](ClO4)2 (II). The compounds are isolated in the individual state and characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction analysis (CIF files CCDC nos. 1967892 (I) and 1967893 (II)). According to the data obtained, the transition metal ion in complexes I and II exists in the high-spin state (S = 3/2 for Co(II) and S = 2 for Fe(II)) in a range of 120–345 K, and the proposed modification of the ligand does not lead to the temperature-induced spin transition in both the solution and crystalline state.

The cobalt(II) and iron(II) complexes with 2,6-di(5-amino-1H-pyrazol-3-yl)pyridine (L), CoL2(ClO4)2 (I) and FeL2(ClO4)2 (II), are synthesized by the template reaction, isolated in the individual form, and characterized by elemental analysis, NMR spectroscopy, and thermogravimetry. The structures of complexes I and II (including the iron(II) complex obtained as two new solvate forms (FeL2(ClO4)2 ∙ 2(C2H5)2O ∙ H3CN (IIa) and FeL2(ClO4)2 ∙ (C2H5)2O ∙ CH3CN ∙ 0.75H2O (IIb)) are confirmed by X-ray diffraction analysis. The data obtained in the crystal (by the X-ray diffraction method) and in solutions (using the proposed approach to an analysis of paramagnetic shifts in 1H NMR spectra) indicate that the metal ion in the complexes exists in the high-spin state (S = 3/2 for Co(II) and S = 2 for Fe(II)) and undergoes no temperature-induced spin transition in a range of 120–300 K.

The direct template reactions afford the cobalt(II) complexes (I–III) with the hydroxy- and acetyl-substituted 2,6-bis(pyrazol-3-yl)pyridine ligands containing the phenyl and 4-tert-butylphenyl group in position 1. All compounds are isolated in the individual state and characterized by elemental analysis and NMR spectroscopy. The X-ray diffraction data obtained for crystals of compound I (СIF file CCDC 1577238) and the data for solutions (using the proposed approach to analysis of paramagnetic NMR shifts on the basis of quantum chemical calculations) show that in the complexes the metal ion exists in the high-spin state (S = 3/2) and undergoes no temperature-induced spin transition in a temperature range of 120–300 K.

Metal-organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal-organic vanadium - 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O2. The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O2 causes an increase in V oxidation state from VII to VIV, resulting in a single strongly bonded (DPTZ2-)VIVO product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal-ligand complexes at surfaces compared to metal nanoislands.

A pyridyl pincer ligand was developed to incorporate steric bulk, through a PtBu2 arm, and proton responsivity, through a pyrazole pincer ligand arm, together with reactivity at benzylic hydrogen and redox activity within a 1,4 diazabutadiene moiety. Binding it to CoCl2 yielded square-pyramidal [(PNNH)CoCl2 ], which was deprotonated by Li[N(SiMe3 )2 ] to form [{Li(THF)2 PNN}CoCl2 ]. Reduction of this LiCl adduct with KC8 under CO atmosphere led to formation of CoI mono- and dicarbonyl complexes, which can be protonated but also further deprotonated at the benzylic CH group to give a dearomatized pyridyl group. The ligand was characterized in its neutral, monoanionic, and dianionic forms, and the anions were shown to exist as intimate ion pairs with Li+ bound to pyrazolate N and chloride bound to Lewis acidic cobalt. X-ray photoelectron spectroscopy was used to assay both Li content and cobalt oxidation states. The general character of binding of LiCl to a metal complex acidic at metal and nucleophilic at ligand (pyrazolate Nβ) is discussed, as are potential catalytic applications of the concept.

Here we report a combined use of THz-EPR and NMR spectroscopy for obtaining a detailed electronic structure of a long-known high-spin complex, cobalt(II) bis[tris(pyrazolyl)borate]. The lowest inter-Kramers transition was directly measured by THz-EPR spectroscopy, while the energies of higher Kramers doublets were estimated by a recently proposed NMR-based approach. Together, they produced magnetic parameters for a full model that explicitly includes spin–orbit coupling. This approach is applicable to all transition metal ions for which the spin–orbit coupling cannot be treated perturbatively.

Rational, systematic tuning of single-site metal centers on surfaces offers a new approach to increase selectivity in heterogeneous catalysis reactions. Although such metal centers of uniform oxidation states have been achieved, the ability to control their oxidation states through the use of carefully designed ligands had not been shown. To this end, tetrazine ligands functionalized by two pyridinyl or pyrimidinyl substituents were deposited, along with vanadium metal, on the Au(100) surface. The greater oxidizing power of the bis-pyrimidinyltetrazine facilitates the on-surface redox formation of V(3+), compared to V(2+) when paired with the bis-pyridinyltetrazine, as determined by X-ray photoelectron spectroscopy. This demonstrates the ability to control metal oxidation states in surface coordination architectures by altering the redox properties of organic ligands. The metal-ligand complexes take the form of one-dimensional polymeric chains, resolved by scanning tunneling microscopy. The chain structures in the first layer are very uniform and are based on the same quasi-square-planar coordination geometry around single-site V with either ligand. Formation of a different, dimer structure is observed in the early stages of the second layer formation. These systems offer new opportunities in controlling the oxidation state of single-site transition metal atoms at a surface for new advances in heterogeneous catalysts.

Pincer complexes are widely used in catalysis and activation of small molecules. We present here the synthesis and spectroscopic study of rhodium dihydrogen and dinitrogen complexes with metallocene-based P,C,P pincer ligands. Relative electron donor ability of ferrocene-, ruthenocene- and benzene-derived pincer ligands, as well as H H distance were elucidated based on JHD coupling constants and T1 relaxation time for dihydrogen complexes.

The alkylation of imidazole and 5-benzyloxycarbonyl-3,4-diethylpyrrole with 1,3-bis-(hydroxymethyl)ferrocene (1) afforded bis-imidazole (4) and bis-pyrrole (7) derivatives of ferrocene, respectively. The reaction of diol 1 with trifluoroacetic acid gave the dicarbocationic complex [{1,3-(CH2)2C5H3}Fe(C5H5)]2+ (2) characterized by 1H NMR spectroscopy.

Cationic ruthenium hydrido-carbonyls {RuH(CO)[tBuP,CH,PM]}BArF4 (M = Fe, 3; M = Ru, 4) (ArF = 3,5-(CF3)2C6H3) obtained in the reaction of H2 with RuCl(CO)[tBuP,C,PM] (M = Fe, 1; M = Ru, 2) in the presence of NaBArF4 add CO smoothly, giving the corresponding dicarbonyl complexes {RuH(CO)2[tBuP,CH,PM]}BArF4 (M = Fe, 11; M = Ru, 12). According to X-ray analysis of 11 and 12, addition of extra CO to complexes 3 and 4 leads to strengthening of the C(1)−H(1)···Ru(1) agostic interaction. Simultaneously, CO addition to 3 and 4 triggers a sequence (up to three steps) of unprecedented intramolecular rearrangements including migration of H atoms. First metallocenylidene complexes {Ru(CO)2[tBuP,C,PM]}BArF4 (M = Fe, 15; M = Ru, 16) were obtained in the course of these rearrangements accompanied by H2 evolution.

Rhodium hydrido chloride pincer complex RhH(Cl)[2,6-(Buptb2PO)b2Cb6Hb3] was synthesized and used for the preparation of new complexes with labile two-electron ligands Rh(L)[2,6-(Buptb2PO)b2Cb6Hb3] (L = MeCN or S(CHb2)b4) and complexes with small molecules, such as CO, Ob2, Hb2, and Nb2.

Resorcinol-based ruthenium bis(phosphinite) complexes were synthesized. Complexes RuCl(CO)[2,6-(But 2PO)2C6H3] (9) and RuH(CO)[2,6-(But 2PO)2C6H3] (10) were obtained by cyclometallation of 1,3-(But 2PO)2C6H3 with RuCl2(DMSO)4 in 2-methoxyethanol in the presence of Hunig’s base. The interconversion of complexes 9 and 10 was studied. The addition of carbon monoxide to complexes 9 and 10 yielded 18e adducts, RuCl(CO)2[2,6-(But 2PO)2C6H3] (11) and RuH(CO)2[2,6-(But 2PO)2C6H3] (12), respectively. In the case of complex 9, this reaction is reversible. Reaction of complex 10 with trifluoroacetic acid resulted in complex Ru(CF3COO)(CO)[2,6-(But 2PO)2C6H3] (13), which reacted with carbon monoxide to give complex Ru(CF3COO)(CO)2[2,6-(But 2PO)2C6H3] (14). Based on the IR spectral data, the TFA ligand in complexes 13 and 14 is bound in a bi- and monodentate fashion, respectively. The structure of compound 9 was determined by X-ray diffraction analysis.