The ditopic molecule 3-(1,3,5-trimethyl-4-1H -pyrazolyl)acetylacetone (HacacMePz) combines an acetylacetone group suitable for deprotonation and O,O' coordination to a Pearson-hard cation with a softer N-donor site. Both binding modes were employed individually: The pyrazolyl moiety was coordinated to Zn II , Cd II , Hg II and Ag I , and with trivalent iron the tris-chelating O,O' complex [Fe(acacMePz) 3 ] was isolated. The Cu II derivative shows shorter O,O' chelation and N coordination in the more distant Jahn-Teller sites and exists in two alternative crystal forms, namely as a tetranuclear discrete complex and as a chain polymer. The dierent Pearson hardnesses of the coordination sites of acacMePz − allow for the design of well-ordered mixed-metal solids. Selective complexation to a hard and a soft cation was achieved in coordination polymers combining hard Fe III and softer Hg II or Ag I . Even slight dierences in Pearson hardness based on dierent oxidation states of the same cation imply sucient selectivity, as shown by the successful synthesis of a mixed-valent Cu II /Cu I chain polymer. A 1 synopsis of all structurally characterized compounds conrms that HacacMePz represents a bridging ligand with restricted conformational freedom. No full rotation about the single bond between the pyrazolyl and acetylacetone fragments occurs, and dihedral angles between these moieties are limited to values of 90°± 17°.

In the hydrochloride of a pyrazolyl-substituted acetylacetone, the chloride anion is hydrogen-bonded to the protonated pyrazolyl moiety. Equimolar co-crystallization with tetrafluorodiiodobenzene (TFDIB) leads to a supramolecular aggregate in which TFDIB is situated on a crystallographic center of inversion. The iodine atom in the asymmetric unit acts as halogen bond donor, and the chloride acceptor approaches the σ-hole of this TFDIB iodine subtending an almost linear halogen bond, with Cl···I = 3.1653(11) Å and Cl···I–C = 179.32(6)°. This contact is roughly orthogonal to the N–H···Cl hydrogen bond. An analysis of the electron density according to Bader’s Quantum Theory of Atoms in Molecules confirms bond critical points (bcps) for both short contacts, with ρbcp = 0.129 for the halogen and 0.321eÅ−3 for the hydrogen bond. Our halogen-bonded adduct represents the prototype for a future class of co-crystals with tunable electron density distribution about the σ-hole contact.

The ditopic molecule 3-(1,3,5-trimethyl-1H-4-pyrazolyl)pentane-2,4-dione (HacacMePz) combines two different Lewis basic sites. It forms a crystalline adduct with the popular halogen bond (XB) donor 2,3,5,6-tetrafluoro-1,4-diiodobenzene (TFDIB) with a HacacMePz:TFDIB ratio of 2 : 3. In a simplified picture, the topology of the adduct corresponds to a hcb net. In addition to the expected acetylacetone keto O and pyrazole N acceptor sites, a third and less common short contact to a TFDIB iodine is observed: The acceptor site is again the most electron-rich site of the pyrazole π-system. This iminic N atom is thus engaged as the acceptor in two orthogonal halogen bonds. Evaluation of the geometric results and of a single-point calculation agree with respect to the strength of the intermolecular contacts: The conventional N···I XB is the shortest (2.909(4) Å) and associated with the highest electron density (0.150 eÅ−3) in the bond critical point (BCP), followed by the O···I contact (2.929(3) Å, 0.109 eÅ−3), and the π contact (3.2157(3) Å, 0.075 eÅ−3). If one accepts the idea of deducing interaction energies from energy densities at the BCP, the short contacts also follow this sequence. Two more criteria identify the short N···I contact as the most relevant: The associated C–I bond is significantly longer than the database average, and it is the only intermolecular interaction with a negative total energy density in the BCP.

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How to screen for k type phase transitions? Pseudo symmetry in reciprocal space represents a successful answer.