Jonathan A. Kitchen

Nano-magnetic materials: spin crossover compounds vs. single molecule magnets vs. single chain magnets

Brief introductions to spin crossover (SCO), single molecule magnetism (SMM) and single chain magnetism (SCM) are provided. Each section is illustrated by selected examples that have contributed significantly to the development of these fields, including recent efforts to produce materials (films, attachment to surfaces etc.). The advantages and disadvantages of each class of magnetically interesting compound are considered, along with the key challenges that remain to be overcome before such compounds can be used commercially as nanocomponents. This invited perspective article is intended to be easily comprehensible to non-specialists in the field.

Mixed Spin‐State [HS‐LS] Pairs in a Dinuclear Spin‐Transition Complex: Confirmation by Variable‐Temperature 57Fe Mössbauer Spectroscopy

Exquisite sensitivity of Mossbauer spectroscopy for tiny local molecular distortion is demonstrated in [FeII2(pmat)2](BF4)4?DMF: high-spin (HS) iron(II) in [HS-HS] and in [LS-HS] (low-spin–high-spin) pairs is clearly distinguished (see picture) for the first time without the need to apply a magnetic field. This dinuclear complex clearly shows that spin crossover via a [LS-HS] species is promoted by the use of a highly constrained bridging ligand (the bis-terdentate pmat).

The btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] binding motif: a new versatile terdentate ligand for supramolecular and coordination chemistry.

Ligands containing the btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] motif have appeared with increasing regularity over the last decade. This class of ligands, formed in a one pot ‘click’ reaction, has been studied for various purposes, such as for generating d and f metal coordination complexes and supramolecular self-assemblies, and in the formation of dendritic and polymeric networks, etc. This review article introduces btp as a novel and highly versatile terdentate building block with huge potential in inorganic supramolecular chemistry. We will focus on the coordination chemistry of btp ligands with a wide range of metals, and how it compares with other classical pyridyl and polypyridyl based ligands, and then present a selection of applications including use in catalysis, enzyme inhibition, photochemistry, molecular logic and materials, e.g. polymers, dendrimers and gels. The photovoltaic potential of triazolium derivatives of btp and its interactions with anions will also be discussed.

Circularly Polarized Lanthanide Luminescence from Langmuir–Blodgett Films Formed from Optically Active and Amphiphilic EuIII‐Based Self‐Assembly Complexes

The development of functional nanomaterials and supramolecular systems is an active area of research, particularly for molecular recognition/sensing, catalysis, optical devices, and magnetically active compounds for switching and data storage. While much attention has been focused on transition-metal-based supramolecular systems, there has been a recent insurgence of lanthanide-based systems. These ions possess rich coordination environments and unique physical properties, such as long-lived and long wavelength emission in the visible or the NIR regions, as well as magnetic properties, which have been exploited for use in the developments of MRI contrast agents. Hence, these properties make them ideal and highly desirable candidates for the formation of functional supramolecular systems. The development of supramolecular assemblies that can be further organized into functional devices is also of great current interest. These assemblies can be achieved by covalently attaching appropriate ligands and complexes to nanoparticles or flat surfaces, through the formation of polymers, or by forming thin films using Langmuir–Blodgett (LB) techniques. Herein we describe our efforts in bridging these two areas of research by employing lanthanide-directed synthesis (using ligands 1 and 2) in the formation of chiral luminescent lanthanide amphiphilic complexes, and their use in the formation of LB films, the properties of which can be probed by using circularly polarized luminescence (CPL). The ligands were designed to include a terdentate coordination pocket with a closely associated sensitizing antenna (i.e. the R- or S-naphthylamine moieties) for the lanthanide ions such as EuIII and TbIII—an approach that has been extremely successful for the development of luminescent supramolecular self-assembly structures, such as chiral “bundles” and dilanthanide triple-stranded helicates. Additionally, a long hydrophobic hexadecyl chain was included to allow the formation of Langmuir–Blodgett films. Ligands 1 and 2 were prepared in yields of 74% and 82%, respectively, by employing EDCI·HCl peptide coupling reactions (EDCI·HCl=1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride) between the R and the S isomers of precursors 3 and 4, respectively, and N-hexadecylamine.

Effect of counteranion X on the spin crossover properties of a family of Diiron(II) Triazole complexes [FeII2(PMAT)2](X)4

Seven diiron(II) complexes, [Fe(II)(2)(PMAT)(2)](X)(4), varying only in the anion X, have been prepared, where PMAT is 4-amino-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole and X = BF(4)(-) (1), Cl(-) (2), PF(6)(-) (3), SbF(6)(-) (4), CF(3)SO(3)(-) (5), B(PhF)(4)(-) (6), and C(16)H(33)SO(3)(-) (7). Most were isolated as solvates, and the microcrystalline ([3], [4]·2H(2)O, [5]·H(2)O, and [6]·½MeCN) or powder ([2]·4H(2)O, and [7]·2H(2)O) samples obtained were studied by variable-temperature magnetic susceptibility and Mössbauer methods. A structure determination on a crystal of [2]·2MeOH·H(2)O, revealed it to be a [LS-HS] mixed low spin (LS)-high spin (HS) state dinuclear complex at 90 K, but fully high spin, [HS-HS], at 293 K. In contrast, structures of both [5]·¾IPA·H(2)O and [7]·1.6MeOH·0.4H(2)O showed them to be [HS-HS] at 90 K, whereas magnetic and Mössbauer studies on [5]·H(2)O and [7]·2H(2)O revealed a different spin state, [LS-HS], at 90 K, presumably because of the difference in solvation. None of these complexes undergo thermal spin crossover (SCO) to the fully LS form, [LS-LS]. The PF(6)(-) and SbF(6)(-) complexes, 3 and [4]·2H(2)O, appear to be a mixture of [HS-LS] and [HS-HS] at low temperature, and undergo gradual SCO to [HS-HS] on warming. The CF(3)SO(3)(-) complex [5]·H(2)O undergoes gradual, partial SCO from [HS-LS] to a mixture of [HS-LS] and [HS-HS] at T(1/2) ≈ 180 K. The B(PhF)(4)(-) and C(16)H(33)SO(3)(-) complexes, [6]·(1)/(2)MeCN and [7]·2H(2)O, are approximately [LS-HS] at all temperatures, with an onset of gradual SCO with T(1/2) > 300 K.

Room-temperature spin crossover and Langmuir-Blodgett film formation of an iron(II) triazole complex featuring a long alkyl chain substituent : the tail that wags the dog

[Fe(II)(C(16)dpt)(2)(NCS)(2)].(2/3)H(2)O displays temperature-mediated spin crossover (SCO) with T((1/2)) = 290 K and the long alkyl chain substituent on the dipyridyltriazole ligand facilitates the formation of a stable Langmuir-Blodgett film at an air-water interface.

High and low spin mononuclear and dinuclear iron(II) complexes of 4-amino and 4-pyrrolyl-3,5-di(2-pyridyl)-4H-1,2,4-triazoles.

The first dinuclear iron(II) complexes of any 4-substituted 3,5-di(2-pyridyl)-4H-1,2,4-triazole ligands, [Fe(II)2(adpt)2(H2O)1.5(CH3CN)2.5](BF4)4 and [Fe(II)2(pldpt)2(H2O)2(CH3CN)2](BF4)4, are presented [where adpt is 4-amino-3,5-di(2-pyridyl)-4H-1,2,4-triazole and pldpt is 4-pyrrolyl-3,5-di(2-pyridyl)-4H-1,2,4-triazole]. Both dinuclear complexes feature doubly triazole bridged iron(II) centers that are found to be [high spin-high spin] at all temperatures, 4-300 K, and to exhibit weak antiferromagnetic coupling. In the analogous monometallic complexes, [Fe(II)(Rdpt)2(X)2](n+), the spin state of the iron(II) center was controlled by appropriate selection of the axial ligands X. Specifically, both of the chloride complexes, [Fe(II)(adpt)2(Cl)2] x 2 MeOH and [Fe(II)(pldpt)2(Cl)2] x 2 MeOH x H2O, were found to be high spin whereas the pyridine adduct [Fe(II)(adpt)2(py)2](BF4)2 was low spin. Attempts to prepare [Fe(II)(pldpt)2(py)2](BF4)2 and the dinuclear analogues [Fe(II)2(Rdpt)2(py)4](BF4)4 failed, illustrating the significant challenges faced in attempts to develop control over the nature of the product obtained from reactions of iron(II) and these bis-bidentate ligands.

Unexpected self-sorting self-assembly formation of a [4:4] sulfate: ligand cage from a preorganized tripodal urea ligand

The design and synthesis of tripodal ligands 1-3 based upon the N-methyl-1,3,5-benzenetricarboxamide platform appended with three aryl urea arms is reported. This ligand platform gives rise to highly preorganized structures and is ideally suited for binding SO4 (2-) and H2 PO4 (-) ions through multiple hydrogen-bonding interactions. The solid-state crystal structures of 1-3 with SO4 (2-) show the encapsulation of a single anion within a cage structure, whereas the crystal structure of 1 with H2 PO4 (-) showed that two anions are encapsulated. We further demonstrate that ligand 4, based on the same platform but consisting of two bis-urea moieties and a single ammonium moiety, also recognizes SO4 (2-) to form a self-assembled capsule with [4:4] SO4 (2-) :4 stoichiometry in which the anions are clustered within a cavity formed by the four ligands. This is the first example of a self-sorting self-assembled capsule where four tetrahedrally arranged SO4 (2-) ions are embedded within a hydrophobic cavity.

Synthesis and structures of 3,5-disubstituted 1,2,4-triazole head units and incorporation of 3,5-dibenzoyl-1,2,4-triazolate into new [2 + 2] Schiff-base macrocyclic complexes

The synthesis and characterization of sodium 3,5-diacetyl-1,2,4-triazolate (4 Me ) and sodium 3,5-dibenzoyl-1,2,4-triazolate (4 Ph ), both of which can be used as head unit building blocks in Schiff-base reactions, are reported. The crystal structures of sodium 3,5-diacetyl-1,2,4-triazolate, as [4 Me (H2O)]∞, and sodium 3,5-dibenzoyl-1,2,4-triazolate, as [4 Ph (CH3OH)2]2, have been determined. The former is a helical polymer whilst the latter is a methanol-bridged dimer. The lead(II) templated cyclization reaction of sodium 3,5-dibenzoyl-1,2,4-triazolate (4 Ph ) with 1,3-diaminopropane or 1,4-diaminobutane, respectively, leads to the formation of two new [2 + 2] Schiff-base macrocycles as their lead(II) complexes, [Pb2 L 3Ph (μ-OH)]ClO4 (5) and [Pb2 L 4Ph (μ-OH)]ClO4 (6), respectively. Transmetallation of 5 with nickel(II) ions yields a novel, structurally characterized, dinickel(II) macrocyclic complex, [Ni2 L 3Ph (NCS)2] (7), which features double triazolate bridging of the two five-coordinate nickel(II) ions.

Fluorescenttren-based 4-amino-1,8-naphthalimide sensor for Cu(II) based on the use of the (fluorophore–spacer–receptor) photoinduced electron transfer (PET) principle

The design, synthesis and photophysical evaluation of a new chemosensor for copper [Cu(II)] is described based on the use of a functionalised 1,8-naphthlimide structure substituted at the 4-position with a tris(2-aminomethyl)amine-based (tren) ligand as a receptor for Cu(II). The sensor 1 functions as a classical photoinduced electron transfer sensor for protons, where the 4-amino-1,8-naphthalimide emission, occurring in the green part of the electromagnetic spectrum, is reversibly switched ‘ON–OFF’. A screen of various groups I and II as well as d-metal ions showed that only in the case of Cu(II), in buffered 100 mM NaCl solution, was the emission of 1 modulated, being quenched by ca. 80% upon addition of one equivalent of CuCl2 in buffered solution. The emission was, however, reversibly switched back ON, by the addition of either EDTA or glutathione. While only small changes occurred in the ground state, analysis of the changes in the emission spectra at various pH values showed that Cu(II) was detected through the formation of either a 2:1 or a 1:1 (ligand to metal) stoichiometry. The overall changes in the emission spectra of 1 as a function of H+ and Cu(II) can also be described as a molecular-level logic gate operation, corresponding to a two-input INHIBIT function.