The pi complexation of alkali and alkaline earth ions by the use of meso-octaalkylporphyrinogen and aromatic hydrocarbons

Calix[4]pyrroles as ligands: recent progress with a focus on the emerging p-block element chemistry

Calix[4]pyrroles are readily synthesized in one step from pyrroles and ketones. For several decades, these macrocycles have been exploited as powerful anion receptors or ligands for transition and rare-earth metals. In contrast, calix[4]pyrrolates as ligands for p-block elements were established only in 2018. The present feature article reviews these developments, together with the recent progress on s-, d-, and f-block element complexes of the calix[4]pyrroles. Particular focus is given on the calix[4]pyrrolato aluminate and the corresponding silane, both featuring square planar-coordinated p-block elements in their highest oxidation states. These unique "anti-van't-Hoff-Le-Bel" structures introduce valuable characteristics into main-group element chemistry, such as agostic interactions or ligand-to-metal charge transfer absorptions. The most vital reactivities are highlighted, which rely on properties ranging from amphoterism, redox-activity, and a small HOMO-LUMO gap up to the ability to provide a platform for additional external stimuli. Overall, these developments underscore the beneficial impact of structural constraint of p-block elements and element-ligand cooperativity to enhance the functionality of the most abundant elements in their native oxidation states.

Soft interactions with hard Lewis acids: generation of mono- and dicationic alkaline-earth metal arene-complexes by direct oxidation

The synthesis of the first unsupported dicationic arene complexes of calcium and strontium [(η⁶-HMB)AE(oDFB)₄]²⁺ is reported (HMB = hexamethylbenzene; AE = alkaline earth metal; oDFB = ortho-difluorobenzene). They were prepared by direct oxidation of the elemental metals employing the ligand-forming radical cation salt [HMB][WCA] as an oxidant (WCA = [Al(OR^F)₄] or [μF-{Al(OR^F)₃}₂]; R^F = C(CF₃)₃). In addition, monocationic η⁶-HMB complexes of calcium, strontium and barium supported by coordination of the monodentate anion [F-Al(OR^F)₃]^- are reported. In all examples, almost undistorted η⁶-HMB coordination is observed with rather short M-arene_centroid distances approaching those observed with the isoelectronic but negatively charged pentamethylcyclopentadienyl ligand. The structure and bonding, thermodynamic stability and Lewis acidity (fluoride/hydride ion affinities, FIA/HIA) of the generated complexes were assessed by DFT methods. It followed that the gaseous dications [(η⁶-HMB)AE(oDFB)₄]²⁺ are extremely hard Lewis acids that retain FIAs close to superacidity in solution.

Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of ‘naked’ β-diketiminato magnesium and calcium cations allows the isolation of benzene, toluene, 1,4-difluorobenzene and terminal phosphine adducts.

Phase-transformation-induced twinning of an iron(III) calix[4]pyrrolidine complex

The title compound, tetrachlorido-1κCl;2κ(3)Cl-(2,2,7,7,12,12,17,17-octamethyl-21,22,23,24-tetraazapentacyclo[16.2.1.1(3,6).1(8,11).1(13,16)]tetracosane-1κ(4)N,N',N'',N''')-μ2-oxido-diiron(III), [Fe2Cl4O(C28H52N4)], undergoes a slow phase transformation at ca 173 K from monoclinic space group P2(1)/n, denoted form (I), to the maximal non-isomorphic subgroup, triclinic space group P1, denoted form (II), which is accompanied by nonmerohedral twinning [twin fractions of 0.693 (4) and 0.307 (4)]. The transformation was found to be reversible, as on raising the temperature the crystal reverted to monoclinic form (I). In the asymmetric unit of form (I), Z' = 1, while in form (II), Z' = 2, with a very small reduction (ca 1.8%) in the unit-cell volume. The two independent molecules (A and B) in form (II) are related by a pseudo-twofold screw axis along the b axis. The molecular overlay of molecule A on molecule B has an r.m.s. deviation of 0.353 Å, with the largest distance between two equivalent atoms being 1.202 Å. The reaction of calix[4]pyrrolidine, the fully reduced form of meso-octamethylporphyrinogen, with FeCl3 gave a red-brown solid that was recrystallized from ethanol in air, resulting in the formation of the title compound. In both forms, (I) and (II), the Fe(III) atoms are coordinated to the macrocyclic ligand and have distorted octahedral FeN4OCl coordination spheres. These Fe(III) atoms lie out of the mean plane of the four N atoms, displaced towards the O atom of the [OFeCl3] unit by 0.2265 (5) Å in form (I), and by 0.2210 (14) and 0.2089 (14) Å, respectively, in the two independent molecules (A and B) of form (II). The geometry of the [OFeCl3] units are similar, with each Fe(III) atom having a tetrahedral coordination sphere. The NH H atoms are directed below the planes of the macrocycles and are hydrogen bonded to the coordinated Cl(-) ions. There are also intramolecular C-H···Cl hydrogen bonds present in both (I) and (II). In form (I), there are no significant intermolecular interactions present. In form (II), the individual molecules are arranged in alternate layers parallel to the ac plane. The B molecules are linked by a C-H···Cl hydrogen bond, forming chains along [100].

A theoretical study of the conformation of meso-tetraphenylporphyrin (TPPH2), its anions, cations and metal complexes (Mg2+, Ca2+ and Zn2+)

A theoretical study has been carried out on TPPH2 (both tautomers), its deprotonated and protonated forms, as well as complexes of TPP containing Mg 2+, Ca 2+ and Zn 2+ ions. Two properties have been analyzed. The first one considers the conformation of the meso-phenyl rings and the deformation of the porphyrin macrocycle, the second one relates to the barriers of rotation of the meso-phenyl rings (atropisomerism). In the case of the Zn complex ( ZnTPP ), the coordination effects with the N3 of 1H-imidazole have been calculated.

New Structural Features in Heavy Alkaline Earth-Metal Chemistry — Molecular Heterobimetallic Group 1, Ba Complexes

The preparation of heterobimetallic complexes consisting of alkali and heavy alkaline earth metals remains a challenge due to limited available synthetic strategies. Here we present a new class of group 1, Ba compounds of the type [M(n){Ba(Odpp)(2+n)}] (M=Na(n=1) (1), K(n=1) (2), Cs(n=1) (3), Li(n=2) (4); HOdpp=2,6-diphenylphenol) and the Lewis base adducts [Li(2)(thf)(2){Ba(Odpp)(4)}]PhMe (5) and [K{Ba(Odpp)(3)(diglyme)}] (6) (diglyme=[bis(2-methoxy)ethyl ether]) as the first representatives of heterobimetallic group 1, Ba species of low nuclearity. The compounds display a significant degree of metal-arene interaction, believed to be a key factor in stabilizing these highly reactive species. Obtained by solid-state direct metalation, the target compounds are available without further work-up.

Multielectron Redox Chemistry of Iron Porphyrinogens

Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E(1/2) = -0.57 V vs NHE in CH(3)CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E(p) = +0.77 V vs NHE in CH(3)CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C(alpha)-C(alpha) bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker sigma-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.

Thermally stable potassium N-heterocyclic carbene complexes with alkoxide ligands, and a polymeric crystal structure with distorted, bridging carbenes

Alkoxide-functionalized N-heterocyclic carbenes (NHCs) stabilise the potassium-imidazole-2-ylidene fragment against 1,2 alkyl rearrangement reactions; this allows the first structural characterisation of a potassium NHC complex, which reveals distorted and unusual bonding interactions between the imidazole ring and the potassium cation.

Multielectron Chemistry of Zinc Porphyrinogen: A Ligand-Based Platform for Two-Electron Mixed Valency

The synthesis, electronic structure, and oxidation-reduction chemistry of a homologous series of Zn(II) porphyrinogens are presented. The fully reduced member of the series, [LZn](2-), was prepared in two steps from pyrrole and acetone. The compound undergoes consecutive two-electron, ligand-based, oxidations at +0.21 and +0.63 V vs NHE to yield [L(Delta)Zn] and [L(Delta Delta)Zn](2+), which also have been independently prepared by chemical means. X-ray diffraction analysis of the redox intermediary, [L(Delta)Zn], shows that the partly oxidized macrocycle is composed of a methylene-bridged dipyrrole that is doubly strapped to a two-electron oxidized dipyrrole bridged by a cyclopropane ring (L(Delta)). The localization of two hole equivalents on the oxidized side of the porphyrinogen framework is consistent with a two-electron mixed valency formulation for the [L(Delta)Zn] species. Electronic structure calculations and electronic spectroscopy support this formalism. Density functional theory computations identify the HOMO to be localized on the reduced half of the macrocycle and the LUMO to be localized on its oxidized half. As implicated by the energy level diagram, the lowest energy transition in the absorption spectrum of [L(Delta)Zn] exhibits charge-transfer character. Taken together, these results establish the viability of using a ligand framework as a two- and four-electron/hole reservoir in the design of multielectron redox schemes.

Tetrakis(imidazolyl)borate-based coordination polymers: group II network solids, M[B(Im)4]2(H2O)2 (M = Mg, Ca, Sr)

We are using the coordinating anion tetrakis(imidazolyl)borate to construct new metal-organic framework structures. In this report, we present three alkaline earth metal network solids incorporating this anion. All three compounds have the same formula, M[B(Im)(4)](2)(H(2)O)(2) (M = Mg, Ca, Sr), and the same coordination environment about the metal. However, the three compounds have different network structures with different degrees of hydrogen bonding; the Mg material forms a two-dimensional network and the Ca and Sr compounds form one-dimensional chains. In addition, we present the structure of the protonated anion B(HIm)(Im)(3) as a model for the default structure of this anion and discuss how the conformation of tetrakis(imidazolyl)borate can affect the structure of network solids.