STM Study of Au(111) Surface-Grafted Paramagnetic Macrocyclic Complexes [Ni2L(Hmba)](+) via Ambidentate Coligands.

Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni2L(Hmba)](+) were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni2L(Hmba)]ClO4 in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au-S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO4(-) which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures.

Encapsulation of the 4-mercaptobenzoate ligand by macrocyclic metal complexes: conversion of a metallocavitand to a metalloligand.

Complexation of the ambidentate ligand 4-mercaptobenzoate (4-SH-C6H4CO2H, H2mba) by the macrocyclic complex [Ni2L(µ-Cl)]ClO4 (L(2-) represents a 24-membered macrocyclic hexaazadithiophenolate ligand) has been examined. The monodeprotonated Hmba(-) ligand reacts with the Ni2 complex in a selective manner by substitution of the bridging chlorido ligand to produce µ1,3-carboxylato-bridged complex [Ni2L(Hmba)](+) (2(+)), which can be isolated as an air-sensitive perchlorate (2ClO4) or tetraphenylborate (2BPh4) salt. The reactivity of the new mercaptobenzoate complex is reminiscent of that of a "free" thiophenolate ligand. In the presence of air, 2ClO4 dimerizes via a disulfide bond to generate tetranuclear complex [{Ni2L}2(O2CC6H4S)2](2+) (3(2+)). The auration of 2ClO4 with [AuCl(PPh3)], on the other hand, leads to monoaurated complex [Ni(II)2L(mba)Au(I)PPh3](+) (4(+)). The bridging thiolate functions of the N6S2 macrocycle are deeply buried and are unaffected/unreactive under these conditions. The complexes were fully characterized by electrospray ionization mass spectrometry, IR and UV/vis spectroscopy, density functional theory, cyclic voltammetry, and X-ray crystallography [for 3(BPh4)2 and 4BPh4]. Temperature-dependent magnetization and susceptibility measurements reveal an S = 2 ground state that is attained by ferromagnetic coupling between the spins of the Ni(II) ions in 2ClO4 (J = +22.3 cm(-1)) and 4BPh4 (J = +20.8 cm(-1); H = -2JS1S2). Preliminary contact-angle and X-ray photoelectron spectroscopy measurements indicate that 2ClO4 interacts with gold surfaces.

Chemisorption of exchange-coupled [Ni2L(dppba)]+ complexes on gold by using ambidentate 4-(diphenylphosphino)benzoate co-ligands.

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.

2-tert-Butyl-5-(2-pyridyl)-2H-tetrazole as a chelating ligand in the direct synthesis of novel Cu(II) and heterobimetallic Cu(II)/Mn(II) complexes.

For the first time, a representative of the 2,5-disubstituted tetrazoles, namely, 2-tert-butyl-5-(2-pyridyl)-2H-tetrazole (L), has been found to participate in oxidative dissolution of copper powder in homometalic systems Cu0­L­NH4X­DMSO (X = Cl, SCN, ClO4) and heterobimetallic ones Cu0­Mn(OAc)2­L­NH4OAc­Solv (Solv = DMSO, DMF), providing the formation of molecular homometallic complexes [CuL2Cl2] (1), [CuL2(SCN)2] (2), and [CuL2(H2O)](ClO4)2 (3), heterobimetallic complex [Cu2MnL2(OAc)6] (4) from DMF solution and its mixture with complex [Cu2MnL2(OAc)6]·2DMSO (5) from DMSO solution. Free ligand L and complexes 1­4 were characterized by elemental analysis, IR spectroscopy, thermal and X-ray single crystal analyses, whereas complex 5 was characterized by X-ray analysis only. Compounds 1­3 are mononuclear complexes, with chelating coordination mode of L via the tetrazole ring N4 and pyridine ring N7 atoms. Heterobimetallic complexes 4 and 5 possess trinuclear structures, with a linear Cu­Mn­Cu arrangement of the metal atoms, linked by the acetate anions; each copper(II) atom is decorated by a chelating unit of L via the tetrazole ring N1 and pyridine ring N7 atoms in complex 4, and via the N4, N7 atoms in complex 5. Temperature-dependent magnetic susceptibility measurements of complex 4 revealed a weak antiferromagnetic coupling between the paramagnetic copper(II) and manganese(II) ions (J = −2.5 cm(−1), g(Cu) = 2.25 and g(Mn) = 2.01), with magnetic exchange through the acetato bridges.

Binuclear nickel complexes with an edge sharing bis(square-pyramidal) N3Ni(µ-S2)NiN3 core: synthesis, characterization, crystal structure and magnetic properties.

The synthesis of the novel macrocyclic octadentate amino-thiophenolate ligand H(2)L2 (3,7,11,19,23,27-hexaaza-33,34-dithiol-15,31-di(tert-butyl)-tricyclo[27,3,1(13.17)]-tetratriaconta-1(32),13,15,17(34),29,30-hexane) and its ability to support binuclear nickel(II) complexes with dithiolato-bridged square-pyramidal Ni(II) ions are reported. H(2)L2 is obtained as the hexahydrobromide salt from a Schiff-base condensation reaction between 1,2-bis(4-tert-butyl-2,6-diformylphenylthio)ethane and bis(3-aminopropyl)amine followed by two successive reductions with NaBH(4) and Na/NH(3). The ligand forms a green, paramagnetic, binuclear nickel(II) complex dication [Ni(II)(2)L2](2+), which can be isolated as a ClO(4)(-) (4) or BPh(4)(-) salt (5). The binuclear nickel(II) complex contains a central N(3)Ni(µ-S)(2)NiN(3) core with two square-pyramidal coordinated Ni(II) ions. The [Ni(2)L2](2+) dication does not bind further coligands, in striking contrast to the behaviour of the parent [Ni(2)L1](2+) dication supported by the smaller (L1)(2-) macrocycle (containing diethylenetriamine in place of the dipropylenetriamine units) which readily binds a variety of other coligands (L') to form bisoctahedral [Ni(2)L1(L')](+) structures. The unusual behaviour of 4 relates to two different N configurations which leads to a steric shielding of the third bridging position by the CH(2)-groups of the dipropylenetriamine chains. An analysis of the temperature-dependent magnetic susceptibility data of 5 reveals the presence of a weak antiferromagnetic exchange interaction between the spins of the nickel(II) ions with a value for the magnetic exchange coupling constant J of -23.5 cm(-1) (H = -2JS(1)S(2)). These results are further substantiated by DFT calculations.

Stabilization of hypophosphite in the binding pocket of a dinuclear macrocyclic complex: synthesis, structure, and properties of [Ni(2)L(µ-O(2)PH(2))]BPh(4) (L = N(6)S(2) donor ligand).

The dinickel(II) complex [Ni2L(ClO4)]ClO4 (1), where L(2-) represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [nBu4N]H2PO2 to form the hypophosphito-bridged complex [Ni2L(µ-O2PH2)](+), which can be isolated as an air-stable perchlorate [Ni2L(µ-O2PH2)]ClO4 (2) or tetraphenylborate [Ni2L(µ-O2PH2)]BPh4 (3) salt. 3·MeCN crystallizes in the triclinic space group P1̅. The bisoctahedral [Ni2L(µ-O2PH2)](+) cation has a N3Ni(µ1,3-O2PH2)(µ-S)2NiN3 core structure with the hypophosphito ligand attached to the two Ni(II) ions in a µ1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni2L](2+) dication for more basic oxoanions. Treatment of [Ni2L(µ-O2PH2)]ClO4 with H2O2 or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence of ferromagnetic exchange interactions between the Ni(ii) (S = 1) ions with a value for the magnetic exchange coupling constant J of +22 cm(-1) (H = -2JS1S2). These results are additionally supported by DFT (density functional theory) calculations.

Synthesis, structure, and reactivity of dinuclear nickel amino-thiophenolate complexes bearing bridging VO2(OH)2- and VO2(OR)2- coligands.

A series of novel mixed ligand dinickel complexes of the type [Ni(II)(2)L(µ-L')](+), where L' is a tetrahedral oxo-alkoxo vanadate (L' = [O(2)V(V)(OR)(2)](-), R = H or alkyl) and L a macrocyclic N(6)S(2) supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni(2)L(µ-O(2)V(OH)(2))](+) (2), prepared by reaction between [Ni(2)L(µ-Cl)]ClO(4) with Na(3)VO(4) and a phase transfer reagent in CH(3)CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni(2)L(µ-O(2)V(OMe)(2))](+) (3) and [Ni(2)L(µ-O(2)V(OEt)(2))](+) (4). The dialkyl orthovanadate esters in 3 and 4 are readily transesterified with mono- and difunctional alcohols. Complex 3 can also be generated from 4 by transesterification with MeOH. Complexes 3 and 4 react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni(2)L(µ-O(2)V(OH)(OCH(2)CH(2)OH))](+) (5), [Ni(2)L(µ-O(2)V(OCH(2))(2)CH(2))](+) (6), and [Ni(2)L(µ-O(2)V(OCH(2)CH(2))(2)O)] (7). The crystal structures of the tetraphenylborate salts of complexes 3-7 reveal in each case four-coordinate O(2)V(V)(OR)(2)(-) groups bonded in a µ(1,3)-bridging mode to generate trinuclear complexes with a central N(3)Ni(µ-S)(2)(µ(1,3)-O(2)V(OR)(2))NiN(3) core. The stabilization of the four-coordinate V(V)O(2)(OR)(2)(-) moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni(2)L](2+) fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of -2.0 to +2.5 V vs SCE. The spins of the nickel(II) (S(i) = 1 ions) in 3 are weakly ferromagnetically coupled (J = +23 cm(-1), (H = -2JS(1)S(2))) to produce an S = 2 ground state.

Synthesis and mononuclear complexes of the bis-bidentate ligand 2,5-di(2-pyridyl)-1,3,4-thiadiazole (dptd): spin crossover in [Fe(II)(dptd)2(NCSe)2] and [Fe(II)(dptd)2(NCBH3)2]·H2O.

A simple and convenient protocol for the synthesis of the ligand 2,5-di(2-pyridyl)-1,3,4-thiadiazole (dptd) has been developed. Five new 2 : 1-type iron(II), cobalt(II), nickel(II) and copper(II) complexes have been prepared and structurally characterised, all of which feature the mononuclear trans-(N',N(1))(2) coordination mode. Spin crossover behaviour has been found for [Fe(II)(dptd)(2)(NCSe)(2)] (2) and [Fe(II)(dptd)(2)(NCBH(3))(2)]·H(2)O (3·H(2)O) with T(1/2) = 192 and 285 K, respectively, reflecting the increasing ligand field strength of the respective co-ligands and showing that the [Fe(II)(dptd)(2)L(2)] unit is suitable for the investigation of the co-ligand field effects on T(1/2). In addition, the 3 : 1-type low-spin complexes [Fe(II)(dptd)(3)]{B(CN)(4)}(2)·3MeOH (4·3MeOH) and [Fe(II)(dptd)(3)](OTf)(2)·1.25DCM·0.5H(2)O (5·1.25DCM·0.5H(2)O) have been studied by X-ray diffraction and have been found to exist exclusively as the mer or fac stereoisomer, respectively, in the solid state. In MeCN-d(3) solution the latter two complexes, as well as [Fe(II)(dptd)(3)](ClO(4))(2)·H(2)O (6·H(2)O), show counterion-independent stereoisomerisation, each yielding a ca. 3 : 1 mixture of mer and fac stereoisomers as identified by (1)H NMR spectroscopy.

Complexation, computational, magnetic, and structural studies of the Maillard reaction product isomaltol including investigation of an uncommon π interaction with copper(II).

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL(2) are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L(2))(2)]·8H(2)O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato)copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions.

Two-step spin crossover in the mononuclear iron(II) complex [Fe(II)(L)(2)(NCS)(2)] (L = 2,5-di-(2-pyridyl)-1,3,4-thiadiazole).

The first iron(ii) complex of 2,5-di-(2-pyridyl)-1,3,4-thiadiazole (L), namely [Fe(II)(L)(2)(NCS)(2)], has been synthesised and studied by magnetic susceptibility measurements, Mössbauer spectroscopy and X-ray diffraction. It has been found to undergo thermally-induced spin crossover featuring two well-separated, abrupt steps at T(1/2)(1) = 167 K and T(1/2)(2) = 112 K.

Triazolopyridines as ligands: structural diversity in iron(II), cobalt(II), nickel(II) and copper(II) complexes of 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]- pyridine (L¹°) and spin crossover in [Fe(II)(L¹°)2(NCS)2].

The coordination chemistry of the ligand 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L¹°) has been investigated and iron(II), cobalt(II), nickel(II) and copper(II) complexes featuring diverse structural motifs have been prepared. In the 2 : 1-type complexes [Co(II)(L¹°)2(MeOH)2](ClO4)2 (20), [Ni(II)(L¹°)2(MeOH)2](ClO4)2 (21), [Cu(II)(L¹°)2(MeOH)2](ClO4)2 (22), [Co(II)(L¹°)2(H2O)2](ClO4)2 (23) and [Cu(II)(L¹°)2(ClO4)2] (24) the metal centres are N4O2 octahedrally coordinated with two N²,N(pyr) bidentate ligands L¹° in the equatorial positions. In the N6 octahedral 4 : 1-type complex [Co(II)(L¹°)4](ClO4)2·H2O (25) both axially coordinating N¹ unidentate and equatorially bound N²,N(pyr) bidentate ligands L¹° are observed. The N6 octahedral 3 : 1-type complex [Fe(II)(L¹°)3](OTf)2·1.5MeCN·0.13H2O·0.87MeO(t)Bu (27) features three N²,N(pyr) bidentate ligands L¹° in the mer configuration. The two closely related N6 octahedral complexes [Fe(II)(L¹°)2(NCS)2] (29) and [Fe(II)(L¹°)2(dca)2] (30) have fundamentally different structures. While complex 29 features two equatorially bound N²,N(pyr) bidentate ligands L¹° and axial NCS⁻ co-ligands, complex 30 is a one-dimensional doubly µ1,5-dicyanamido-bridged polymer with N¹ unidentate ligands L¹° in the axial positions. Temperature-dependent magnetic susceptibility measurements of the iron(II) complexes 28 and 29 have shown the 3 : 1-type complex [Fe(II)(L¹°)3](ClO4)2·H2O (28) to be in the low-spin state over the range 300-2 K and the 2 : 1-type complex 29 to be a spin crossover compound with T(1/2) = 269 K whereas the dicyanamido-bridged complex 30 remains in the high-spin state even down to 113 K, according to X-ray diffraction data. A single end-to-end bridging NCS⁻ co-ligand is found in the N4S square-pyramidal complex [Cu(II)(L¹°)(NCS)2] (31) which shows Curie-Weiss behaviour over the range 300-2 K. A brief review of the coordination chemistry of triazolopyridines is given.