Anion⋯anion self-assembly under the control of σ- and π-hole bonds

The electrostatic attraction between charges of opposite signs and the repulsion between charges of the same sign are ubiquitous and influential phenomena in recognition and self-assembly processes. However, it has been recently revealed that specific attractive forces between ions with the same sign are relatively common. These forces can be strong enough to overcome the Coulomb repulsion between ions with the same sign, leading to the formation of stable anion⋯anion and cation⋯cation adducts. Hydroden bonds (HBs) are probably the best-known interaction that can effectively direct these counterintuitive assembly processes. In this review we discuss how σ-hole and π-hole bonds can break the paradigm of electrostatic repulsion between like-charges and effectively drive the self-assembly of anions into discrete as well as one-, two-, or three-dimensional adducts. σ-Hole and π-hole bonds are the attractive forces between regions of excess electron density in molecular entities (e.g., lone pairs or π bond orbitals) and regions of depleted electron density that are localized at the outer surface of bonded atoms opposite to the σ covalent bonds formed by atoms (σ-holes) and above and below the planar portions of molecular entities (π-holes). σ- and π-holes can be present on many different elements of the p and d block of the periodic table and the self-assembly processes driven by their presence can thus involve a wide diversity of mono- and di-anions. The formed homomeric and heteromeric adducts are typically stable in the solid phase and in polar solvents but metastable or unstable in the gas phase. The pivotal role of σ- and π-hole bonds in controlling anion⋯anion self-assembly is described in key biopharmacological systems and in molecular materials endowed with useful functional properties.

Expanding the dimensionality of bis(tetrazolyl)alkane-based Fe(II) coordination polymers by the application of dinitrile coligands

Reactions between 1,2-di(tetrazol-2-yl)ethane (ebtz), 1,6-di(tetrazol-2-yl)hexane (hbtz) or 1,1'-di(tetrazol-1-yl)methane (1ditz) and Fe(BF4)2 in the presence of adiponitrile (ADN), glutaronitrile (GLN) or suberonitrile (SUN) resulted in the formation of coordination polymers [Fe(μ-ebtz)2(μ-ADN)](BF4)2 (1), [Fe(μ-hbtz)2(μ-ADN)](BF4)2 (2), [Fe(μ-1ditz)2(GLN)2](BF4)2·GLN (3) and [Fe(μ-1ditz)2(μ-SUN)](BF4)2·SUN (4). It was established that the application of dinitriles allows an increase in the dimensionality of the ebtz and hbtz based systems while maintaining the structure of the polymeric units characteristic of previously studied mononitrile based analogues. In 3 and 4, regardless of the type of dinitrile coligand, the motif of 2D polymeric layers constituted by 1ditz molecules remains preserved. However, the dimensionality of 1ditz based networks is governed by the coordination modes of dinitriles. 3, based on a shorter molecule of glutaronitrile, crystallizes as a two-dimensional (2D) coordination polymer. In this compound, dinitriles coordinate monodentately or play the role of guest molecules. The substitution of glutaronitrile with suberonitrile enables the bridging of neighboring polymeric layers, resulting in a 3D network. The intentional selection of bis(tetrazoles) and dinitriles as building blocks has led, as expected, to obtaining systems with the structure of the first coordination sphere consisting of four tetrazole rings and two axially coordinated nitrile molecules. It created the conditions required for the occurrence of thermally induced spin crossover. Magnetic measurements and single crystal X-ray diffraction studies were used for the characterization of the spin crossover properties of 1-4.

Self-assembly of three-dimensional oxalate-bridged alkali(i)–lanthanide(iii) heterometal–organic frameworks

Three isostructural 3D oxalate bridged alkali(i)–lanthanide(iii) MOFs with a pcu net based on cubane-like [Ln₄O₄] clusters and their magnetic, CO₂ adsorption, and photoluminescence sensing properties are presented.

Spin crossover Fe(ii) complexes of a cross-bridged cyclam derivative

A cross-bridged cyclam derivative containing two 2-pyridylmethyl pendant arms (L = 4,11-bis((pyridin-2-yl)methyl)-1,4,8,11-tetraaza-bicyclo[6.6.2]hexadecane) was synthesized by dialkylation of the cross-bridged cyclam with 2-chloromethylpyridine. A series of Fe(ii) complexes with L and different counter-anions of the formulas [Fe(L)][FeCl4]·H2O (1·H2O), [Fe(L)]Cl2·4H2O (2·4H2O), [Fe(L)](BF4)2·0.5CH3CN (3·0.5CH3CN) and [Fe(L)](BPh4)2·CH3OH (4·CH3OH) was prepared and thoroughly characterized. In all the cases, the [Fe(L)]2+ cation adopts a cis-V configuration with a distorted octahedral geometry, and with the FeN6 donor set. The magnetic measurements within the temperature interval of 5-400 K revealed the spin crossover (SCO) behaviour of all the complexes with the transition temperature T1/2 increasing with the counter anion in the order BF4- (3) < [FeCl4]2- (1) < BPh4- (4). However, the SCO process was complete in the case of compound 3 only, with T1/2 = 177 K, which proceeded after removal of co-crystallized CH3CN molecules accompanied by a change of the crystallographic phase. The SCO behaviour of 3 was also confirmed by a single crystal X-ray analysis providing the average Fe-N distances of 2.086 Å at 120 K and 2.197 Å at 293 K typical of low-spin, and high-spin FeII complexes, respectively. The obtained results clearly showed that the nature of the counter anion and the presence/absence of co-crystallized solvent molecule(s) significantly affected the temperature as well as the abruptness of the spin transition. This is the first report of SCO behaviour observed for iron complexes containing a cross-bridged cyclam derivative.

(E)-1,1′-(Diazene-1,2-diyl)bis(cyclohexane-1-carbonitrile)

The whole molecule of the title compound, C14H20N4, is generated by inversion symmetry. The mid-point of the N=N bond is situated on the inversion centre. The conformation about this central N=N bond is E. The carbonitrile groups occupy axial positions on the cyclohexane rings. In the crystal, there are no significant intermolecular interactions present.

Distortion Pathways of Transition Metal Coordination Polyhedra Induced by Chelating Topology

A continuous shape measures analysis of the coordination polyhedra of a host of transition metal complexes with bi- and multidentate ligands discloses the distortion pathway associated with each particular topology of the chelate rings formed. The basic parameter that controls the degree of distortion is the metal-donor atom bond distance that induces nonideal bond angles due to the rigidity of the ligands. Thus, the degree of distortion within each family of complexes depends on the atomic size, on which the high- or low-spin state has a large effect. Special attention is therefore paid to several spin-crossover systems and to the enhanced distortions that go along with the transition from low- to high-spin state affected by temperature, light, or pressure. Several families of complexes show deviations from the expected distortion pathways in the high-spin state that can be associated to the onset of intermolecular interactions such as secondary coordination of counterions or solvent molecules. Also, significant displacement of counterions in an extended solid may result from the changes in metal-ligand bond distances when ligands are involved in intermolecular hydrogen bonding.

Influence of conformational changes on spin crossover properties and superstructure formation in 2D coordination polymers [Fe(hbtz)2(RCN)2](ClO4)2

Spin-crossover, governed by conformational changes of nitrile molecule, triggers corrugation of polymeric skeleton and formation of superstructure in [Fe(hbtz)₂(allyl cyanide)₂](ClO₄)₂.

Role of Fe-N-C geometry flip-flop in bistability in Fe(tetrazol-2-yl)4(C2H5CN)2-type core based coordination network

[Fe(ebtz)(2)(C(2)H(5)CN)(2)](ClO(4))(2) was prepared in the reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(ClO(4))(2)·6H(2)O in propionitrile. The compound crystallizes as a one-dimensional (1D) network, where bridging of neighboring iron(II) ions by two ebtz ligand molecules results in formation of a [Fe(ebtz)(2)](∞) polymeric skeleton. The 1D chains are assembled into supramolecular layers with axially coordinated nitrile molecules directed outward. The complex in the high spin (HS) form reveals a very rare feature, namely, a bent geometry of the Fe-N-C(propionitrile) fragment (149.1(3)° at 250 K). The HS to low spin (LS) HS→LS transition triggers reorientation of the propionitrile molecule resulting in accommodation of a typical linear geometry of the Fe-N-C(nitrile) fragment. The switching of the propionitrile molecule orientation in relation to the coordination octahedron is associated with increase of the distance between the supramolecular layers. When the crystal is in the LS phase, raising the temperature does not cause reduction of the distance between supramolecular layers, which contributes to further stabilization of the more linear geometry of Fe-N-C(C(2)H(5)) and the LS form of the complex. Thus, a combination of Fe-N-C(C(2)H(5)) geometry lability and lattice effects contributes to the appearance of hysteretic behavior (T(1/2)(↓) ≈ 112 K, T(1/2)(↑) ≈ 141 K).

Poly[[diaquatetrakis(μ2-benzene-1,4-dicarbonitrile-κ2N:N′)iron(II)] bis[tetrachloridoferrate(III)] nitromethane tetrasolvate]

In the title compound, {[Fe^II(C₈H₄N₂)₂(H₂O)₂][Fe^IIICl₄]₂·4CH₃NO₂}_n, the Fe^II and Fe^III ions are hexa- and tetra­coordinated, respectively. Each unique benzene-1,4-dicarbonitrile mol­ecule lies across a crystallographic inversion centre and bridges two Fe^II ions (each situated on an inversion centre), generating two-dimensional (4,4) square grid layers. The tetra­chloridoferrate(III) anions and nitro­methane solvent mol­ecules lie between the square grid layers and are further link to the adjacent layers into a three-dimensional supra­molecular structure through O—H⋯Cl and O—H⋯O hydrogen bonds.