Bifurcated and Monocentric Halogen Bonds in Cocrystals of Metal(II) Acetylacetonates with p-Dihalotetrafluorobenzenes

Abstract

In order to test metal diketonates as potential acceptors of bifurcated halogen bonds, the series of acetylacetonates (acac) of divalent cations Cu­(acac)2 (1), Pd­(acac)2 (2), VO­(acac)2 (3), Ni­(acac)2(H2O)2 (4), Co­(acac)2(H2O)2 (5), and Zn (acac)2(H2O) (6) were cocrystallized with 1,4-diiodotetrafluorobenzene (tfib) and 1,4-dibromotetrafluorobenzene (tfbb) as halogen donors. This has yielded a series of 10 cocrystals, tfib having formed cocrystals with all six acceptors and tfbb with all except for 4 and 5. In eight cocrystals a pair of acac oxygen atoms acts as a bifurcated halogen bond acceptor, the bond being symmetric in cocrystals of 1 and 2 and asymmetric in cocrystals of 3 and 6. The only cocrystals in which a halogen bond was formed with alternative acceptor sites were cocrystals of tfib with 4 and 5, where coordinated water molecules form hydrogen bonds with all available acac oxygen atoms, leaving only the water molecules themselves as halogen bond acceptors. The favorability of the bifurcated halogen bond was also confirmed by QM computations, which have shown the bifurcated bonds to be the most favorable interactions in vacuo, with bond energies in the range of 29–37 kJ mol–1 for tfib and 20–25 kJ mol–1 for tfbb. This also reflects on the thermal stability of the cocrystals of 1–3 (which do not contain coordinated water) with tfib, which melt/decompose between ca. 180 and 220 °C.

The halogen bond with isocyano carbon reduces isocyanide odor

“…The former XB (d XB = 3.029(5)/3.043(4) Å; 82% of ∑BvdW) is even shorter than that in (CNMes)•IPFB, whereas ∠(C-I•••C) is also in the 170-180°range. In the case of the bifurcated contact 12,30,31 ; these values agree well with those for other reported bifurcated XBs with η 2 -(N,N) 30 , -(O,O) 31 , and -(Halogen,Metal) 11,12 linkages.…”

“…1c), the crystallographically independent part consists of two pairs of CNMes and 1,2-FIB molecules forming two slightly asymmetrical units joined by XBs. Each 1,2-FIB provides two XB patterns, namely between one I and the isocyano C atoms (shown by yellow dotted line) and also bifurcated XB 11,12,30,31 involving the other I atom and simultaneously the isocyano C and, on the other hand, one I of the neighboring 1,2-FIB (orange dotted lines). The former XB (d XB = 3.029(5)/3.043(4) Å; 82% of ∑BvdW) is even shorter than that in (CNMes)•IPFB, whereas ∠(C-I•••C) is also in the 170-180°range.…”

“…At the same time, the identification of XB by IR/Raman methods requires the registration of usually hard-to-reach far-IR area. On the other hand, the spectroscopic approaches for XB identification can give more information by utilization of XB acceptors featuring NMR-active nuclei (e.g., 31 P, 77 Se) 42,43 and also those multiply bonds which are well distinguished in IR/Raman. Notably, the solid-state 13 C NMR and FTIR have never been used for recognition of XBinvolving carbon centers.…”

Cobaloximes as Building Blocks in Halogen-Bonded Cocrystals

“…Since the revival of research into halogen bonding [1][2][3], there have been two main avenues of research: the fundamental study of the nature of halogen bonding, and the application of halogen bonding in the design of new materials. The results of the former (such as the predominantly electrostatic [4] and occasionally covalent [5][6][7][8] nature of the interaction, comparison with hydrogen bonding [9][10][11], the possibility of bifurcation [12][13][14], and metal and metalloid atoms as acceptors [15,16]) have contributed to a considerable extension of the possibilities for preparing various halogen-bonded functional materials [17,18], such as liquid crystals [19,20], anion receptors [21,22], photoluminescent [23,24] and photoreactive materials, and gelators [25][26][27].…”

“…Particularly scarce are studies involving cocrystals of neutral metal-organic components acting as halogen bond acceptors [42][43][44][45][46][47]. The most promising building blocks to date have been metal coordination complexes comprised of pendant acceptor groups such as halogenide ligands [46,47], chelating ligands such as imines [43], β-dicarbonyl compounds [14] and ligands with additional (non-coordinating) electron rich atoms or groups such as morpholine or thiomorpholine [48,49], which can form cocrystals with halogen bond donors, such as perfluorinated iodobenzenes.…”