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Five new Pb(II) complexes of hydrazone-based and Schiff-based ligands and three different anionic co-ligands (azide, thiocyanate and bromide) have been synthesized and characterized by structural, analytical and spectroscopic methods. This variety of ligands can coordinate to the Pb(II) metal center in a tridentate or tetradentate fashion via a different combination of any of nitrogen, oxygen and sulphur donor atoms. Moreover, the organic ligands can be in mono-deprotonated or neutral forms. By using single-crystal X-ray crystallography, we show that the synthesized complexes aggregate into larger supramolecular entities due to the formation of noncovalent tetrel bonding interactions. The Pb(II) center is hemidirectionally coordinated even in those complexes where the coordination number is six. Consequently, this is sterically ideal for establishing tetrel bonding interactions with electron-rich nitrogen, bromide or sulphur atoms. These contacts are significantly larger than the sums of the covalent radii. Hence, they can be described as non-covalent tetrel bonding interactions. They interconnect the covalently bonded units into supramolecular assemblies (dimers or tetramers). The contribution of contacts involving the Pb atom has been studied using Hirshfeld surface analysis and fingerprint plots. We have analysed the supramolecular assemblies observed in the solid state by means of DFT calculations and characterized them using Bader's theory of atoms-in-molecules. 20 34 5033 5044

Formation of adducts between oxovanadium(IV) diketonates and Lewis bases is well documented [1]. The Lewis bases coordinate on sixth coordination site of the vanadium atom either in trans- or cis- position relative to the oxygen of vanadyl . The position in which the Lewis base is coordinated depends on the basicity and steric effect of the Lewis base [2]. In this study we have investigated formation of adducts between bis(pentane-2, 4-dionato)oxovanadium(IV) (VO(acac)2) and a series of pyridine derivatives in the solid state and solution. The stability constants of the adducts were determined by means of spectrophotometric titrations. The corresponding stability constants were correlated with the Hammett substituent coefficients and pKa values of the pyridine derivatives. Crystal structures of eight adducts were determined and it was found that in the solid state the pyridine, 3-methylpyridine and 4-methylpyridine formed trans- adducts with VO(acac)2, whereas in the case of 4-benzoylpyridine adduct the pyridine derivative is in cis-position, 2-amino-3-methylpyridine and 2-amino-5-methylpyridine also formed trans- adducts in addition of creating N-H...O hydrogen bond via pyridine amino group and chelate ring of complex. In case of 2, 4, 6-trimethylpiridine and 2-amino-6-methylpyridine, molecule of water was coordinated on VO(acac)2 and formed hydrogen bonds with 2 molecules of neighbouring pyridine molecules. It was found that the stereochemistry of the coordinated species (cis/trans) present in the solid state depends on the basicity of the Lewis base and it is reflected in the characteristic UV-spectra of the adducts.

Semiquinones are a class of stable organic radical anions ; especially stable are ones with four electronegative substituents, which enhance delocalisation of the unpaired electrons. Therefore, they are potential candidates for design of functional materials with fine-tuned magnetic properties. Semiquinoid anions in crystals are usually stacked by π-interactions, and their magnetic properties depend on their interplanar distance. Most often, the radicals form closely bound dimers with coupled spins [1, 2], which result in diamagnetic properties. However, in smartly designed crystals the distance can be increased, so stacks with equidistant rings will appear, which may be antiferromagnetic [3] or paramagnetic. Therefore, we varied substituents on the semiquinone ring - tetrachloro- (CA), tetrabromo- (BA) and 2, 3-dicyano-5, 6-dichlorosemiquinone (DDQ) were used as anions ; aromatic, approximately planar organic cations (pyridinium, N-methylpyridinium and diquat) were used to check for possible π-interactions between cations and anions. The goal of our research was design novel stable (in air, at room temperature) semiquinone systems with fine-tuned magnetic properties by crystal engineering: we varied size of substituents on the quinoid ring and aromatic organic cations. A combination of crystallographic, magnetic (EPR, SQUID), electrical (impedance spectroscopy) and thermal (TG/DTA, DSC) methods was used to obtain data on stability and magnetic properties of the crystalline compounds. To gain further insight into spin coupling/decoupling, impedance spectroscopy was used to measure band gap between radicals (or dimers of radicals) ; measurement was done on single crystals with electrical contacts applied in the direction of stacking. A pair of analogous salts, N-MePy∙CA and N-MePy∙BA is especially interesting, and be described in detail. In triclinic polymorphh of CA salt closely bound radical dimers occur, similar to previously known alkali salts of semiquinones [1, 2], and the crystals are diamagnetic due to spin pairing. In its triclinic polymorph and isostructural BA salt radicals are equdistant leading to different spin ordering, similar to high-temperature polymorph of K∙CA∙MeCOEt [3]. All crystals were found to be good semiconductors and stable in air at room temperature. [1] K. Molčanov, B. Kojić-Prodić, D. Babić, D. Žilić, B. Rakvin, CrystEngComm, 13 (2011), 5170-5178 ; [2] K. Molčanov, D. Babić, B. Kojić-Prodić, J. Stare, N. Maltar-Strmečki, L. Androš, Acta Crystallogr. B, 70 (2014), 181-190 ; [3] K. Molčanov, B. Kojić-Prodić, D. Babić, D. Pajić, N. Novosel, K. Zadro, CrystEngComm, 14 (2012), 7958-7964.

Over the past decades of ever more intensive work in the field of crystal engineering hydrogen bonds have always been and remained the most typical tool for design of crystal structures. This is due to their strength and directionality which makes them quite reliable, and allows a certain degree of predictability of the supramolecular motifs which will form by hydrogen bonding of given hydrogen donors and acceptors. More recently, halogen interactions have also been recognised as useful interactions in crystal engineering, since they can be comparable in strength to hydrogen bonds. As halogen bonds are by nature electrostatic interactions between a (partial) negative charge on a “halogen acceptor” atom and a partial positive charge on a halogen atom which is present due to the anisotropy of electron density, they are also highly directional. As the partial positive charge on a halogen atom will be better defined as the size of the halogen increases, the most uses of halogen bonds in crystal engineering have involved iodine and bromine derivatives. Here we will present the results of our study of trichloroacetates of different organic bases. Trichloroacetate ion has three chlorine atoms, which are relatively poor halogen bond donors, but at the same time the halogen acceptor (oxygen) carries a net negative charge, rendering it a strong acceptor capable of forming charge assisted halogen bonds. Out of 7 structures studied thus far three indeed show short (< sum of van der Waals radii) O•••Cl contacts. In other structures both carboxylate oxygen atoms participate in strong hydrogen bonds with hydrogen donors, usually from the organic base molecules. It appears therefore that there is a competition between halogen (chlorine) and hydrogen bond donors for the same (carboxylate oxygen) acceptor. As halogen bonding is present only when there is an oxygen atom which participates in no hydrogen bonds, it would seem that hydrogen bond tends to be victorious in this competition.

Hexacyanoferrate anions are, being potential acceptors (or donors, if protonated) of six or more (bifurcated) hydrogen bonds, interesting as useful building blocks in creating multidimensional hydrogen bonded networks. In our ongoing research on hexacyanoferrate(II) salts of organic bases o-phenylenediamine was chosen as a possible bridging molecule between hexacyanoferrate units. Thus, a number of o-phenylenediamine hexacyanoferrates(II) were prepared and characterized by single crystal X-ray diffraction methods. As expected, in the structures of all analysed compounds hexacyanoferrate(II) units are bridged by protonated o-phenylenediamine and solvent molecules whereby complex hydrogen bonded networks are formed. Also, many interesting structural motifs such as multiple bridging (Figure 1. a), as well as pseudosimmetry or multiple periodicities along the same axis (Fiure 1. b) were found in the structures of analysed compounds. Starting from the first crystallization experiments, it was interesting to found that there was always more than one crystalline product of a single crystallization. In order to find out how many o-phenylenediamine hexacyanoferrates(II) really exist, and how the solvents and crystallisation conditions affect the outcome of preparations, series of crystallization experiments were performed, followed by X-ray powder diffraction analysis of bulk products. Results show that there is always the same dominant compound and usually several others, not all of them found as crystals suitable for diffraction analysis.

Among the first crystal structures a student is acquainted to, are those of ionic compounds. The basic rule such a student is supposed to learn at that point is that ions of opposite charges are placed next to each other, so that every ion is surrounded by neighbours having the opposite charge. However, when simple inorganic ions are replaced by organic ones, in particularly such which are capable of hydrogen bonding, this rule may no longer apply, since in such crystals there are interactions other then the electrostatic ones, and these codetermine the crystal structure. To study the interplay of electrostatic and other interactions (with emphasis on the hydrogen bond) we have selected picrates of various pyridine derivatives. 18 such compounds were synthesised, crystallised and their crystal structures determined. Since the picrate anion can only act as a hydrogen acceptor, the hydrogen bonding will be determined by hydrogen donor abilities of the substituents on the pyridinium cation. The supramolecular synthon (I) is to be expected in the majority of structures since it combines N—H∙∙∙O hydrogen bonding with electrostatic interactions bringing the positively charged pyridinium nitrogen in proximity to the negatively charged picrate oxygen. Synthon I was indeed encountered in the structures of such pyridine derivatives which posses no hydrogen donors other than the positively charged nitrogen. However, when other hydrogen donors are present, hydrogen bonding often changes drastically. These changes include bonding of cations directly to other cations, different protonation sites on the cation and formation of hydrates, which occurs when other hydrogen donors are present, but not in their absence. Generally, the alkoxide oxygen atom of the picrate anion, being the best hydrogen acceptor, will form hydrogen bonds with the best donor, even at the cost of increasing the distance between the positive and the negative charge. It therefore seems that hydrogen bonding plays a greater role in the determination of crystal structures of pyridinium picrates than electrostatic interactions do.

Hexacyanoferrates(II) and (III) (mostly their potassium salts) are often used as analytical reagents due to their easily conducted redox reactions and high stabilities under various conditions. In structural chemistry, though, they are of much greater importance and interest not only due to their ability to form extensive hydrogen bond networks, wherein hexacyanoferrate anions are usually hydrogen bond acceptors, but also because of their potential in making new solid electrolytes and metal-organic frameworks. In present research 4, 4'-bipyridinium hexacyanoferrate(II) tetrahydrate (I) and 4, 4'-bipyridinium 4-(pyrid-4-yl)pyridinium hexacyanoferrate(III) trihydrate (II) are prepared and their crystal structures determined by single crystal X-ray diffraction analysis. Compound I crystallizes as dark blue triclinic platelets (space group P , Z = 1) with iron atoms in inversion centers. Three structure determining hydrogen bond motifs are present (shown in Figure 1), chains of hexacyanoferrate(II) anions bridged by two water molecules per anion along the b axis, and chains of same anions, bridged by two 4, 4'-bipyridinium cations in two different directions, along the a axis and the [0 1 1] direction. Thus a 3D hydrogen bond network is formed. Compound II crystallizes as light yellow monoclinic needles (space group P21/c, Z = 4). In it's structure 4, 4'-bipyridine is present in two different forms, as double protonated 4, 4'-bipyridinium cations that, together with water molecules and hexacyanoferrate(III) anions form a complex 3D hydrogen bond network where N―H∙∙∙O, O―H∙∙∙N and O―H∙∙∙O hydrogen bonds are present, and wherein, along the a axis, channels are situated, and as single protonated 4-(pyrid-4-yl)pyridinium cations that form long N―H∙∙∙N hydrogen bonded chains inside the channels.

As a part of our ongoing study of bis(benzoylacetonato)manganese(II) adducts with various N and O donors, a morpholine adduct, bis(benzoylacetonato)bis(morpholine-κ N)manganese(II) was prepared. The compound was obtained as large (mostly single) crystals by overlayering a saturated solution of bis(benzoylacetonato)manganese(II) in morpholine with propan-2-ol. It was shown that the adduct crystallised as a morpholine solvate with two morpholine molecules per a [Mn(bzac)_2(morph)_2] unit. The title compound crystallises in the space group P2_1/n, with two formula units per unit cell. Manganese atoms are located at inversion centres (2c special position of the space group) and are coordinated by two benzoylacetonate anions and two molecules of morpholine, in a distorted octahedral geometry. This coordination moiety is hydrogen bonded to two morpholine molecules by a N– H...N bond of 3.146 A, and a N– H...O bond of 3.270 A. The shortest intermolecular contacts between these molecular aggregates is a C– H...π contact of 3.715 A between a solvent morpholine carbon atom and a phenyl ring of the neighbouring coordination moiety. These interactions interconnect the molecular aggregates to form long chains along the a axis. When the structure is viewed along the direction, the solvate morpholine molecules appear to be placed in channels protruding in that direction, surrounded by molecules of complex. However, the structure cannot be described as one of host-guest type since there are no significant interactions between molecules of complex which would hold them in such loose array. In fact, hydrogen bonding with solvate morpholine molecules is the main reason of the formation of channels in the crystal structure. When removed from the solution, or replaced into another solvent (other than morpholine), the crystals immediately start losing the solvent morpholine, upon which they crumble. The rate of the loss of morpholine was studied at several temperatures by measuring the isothermal weight loss on a thermogravimetric balance.