Bifurcated and Monocentric Halogen Bonds in Cocrystals of Metal(II

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Bifurcated and Monocentric Halogen Bonds in Cocrystals of Metal(II) Acetylacetonates with p‑Dihalotetrafluorobenzenes Vladimir Stilinovic,́ * Toni Grguric,́ † Tomislav Piteša,† Vinko Nemec, and Dominik Cincǐ c*́ Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10002 Zagreb, Croatia

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S Supporting Information *

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 1,4dibromotetrafluorobenzene (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.

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INTRODUCTION After its resurgence in the field of supramolecular chemistry during the early 1990s, the halogen bond has gradually been taking its place as a supramolecular interaction on par with the hydrogen bond.1−12 This is due to its strength and directionality, which are comparable, and even superior, to those of the hydrogen bond.13−21 The higher directionality of a halogen bond is a consequence of the charge distribution on the halogen atomthe relatively small positive region on the halogen (σ hole) is surrounded by negative charge, which forces binding of the electron donor at an angle as close as possible to 180° to the covalent bond.22−26 This extreme tendency toward linearity implies another significant difference between halogen and hydrogen bonds: the hydrogen atom, being positive over the entire contact surface, is likely to bind to two (or more) electron donors, thus forming bifurcated hydrogen bonds, which are relatively common. However, bifurcation of halogen bonds, where a halogen atom would act as a donor to two (or more) acceptors, appears to be less probable and to this day remains a somewhat vexing question. In the seminal paper by Laurence et al.,27 bifurcated halogen bonds are found to be unlikely for any strong (inorganic) halogen donor, while it may be possible for weaker halogen bond donors (which will form less covalent bonds) to form symmetric halogen bonds. Indeed, numerous experimental (in particular with heterocycles28−32 and nitro groups33−42 as acceptors) and theoretical studies43−46 have shown the © XXXX American Chemical Society

existence of bifurcated halogen bonds. However, in the vast majority of cases the bifurcated halogen bonds show pronounced asymmetry, one contact being shorter and straighter and the other longer and more bent. This has been shown to be the energetically preferred arrangement in the case of o-dimethoxybenzene acceptors, as the symmetric bond does not correspond to a minimum of potential energy but rather to a first-order saddle point.44 On the other hand, symmetrical bifurcated halogen bonds have been detected28,33,35,37−39 and, although they do remain very few in number, are apparently more of an exception than a rule. A recent series of papers has demonstrated the potential of the oxygen atoms of the o-methoxyhydroxybenzene group in imines derived from o-vanillin to be able to act as halogen bond acceptors.47−51 As the acceptor group in these compounds is itself asymmetric, the halogen bonding contacts toward the two oxygen atoms are never equivalent. In spite of this, these results indicate that a pair of oxygen atoms ca. 2.55− 2.65 Å apart might be a suitable binding site for a halogen atom. A similar binding site to the ortho-disubstituted benzene is also present in metal complexes of β-diketones, particularly in square-planar bis(β-diketonates) of divalent cations. The oxygen atoms belonging to (anionic) diketonate ligands are Received: November 5, 2018 Revised: December 5, 2018 Published: December 12, 2018 A

DOI: 10.1021/acs.cgd.8b01659 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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also more negatively charged than those in the orthodisubstituted benzene derivatives; therefore, they should present particularly good halogen acceptors (the presence of a formal negative charge on the acceptor may increase interaction energy by 1 order of magnitude52), and halogenbonded contacts with ligating diketonate oxygen atoms have been observed on several earlier occasions.53−60 We have therefore decided to attempt the preparation of halogenbonded cocrystals based on square-planar bis(β-diketonate) complexes. We have selected a series of acetylacetonates of divalent cations as potential halogen bond acceptors: 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) (Scheme 1 and Figure 1). Acetylacetonate complexes were selected as

acceptors, since acetylacetonate ligand side chains (methyl) should not cause steric hindrance for the approach of the halogen to the (pairs) of chelating oxygen atoms. The chosen series of potential halogen bond acceptors contains complexes where there is no possible competition for the chelating oxygen atom as halogen bond acceptors (1 and 2) and those with additional potential acceptors, the oxo group in 3 and coordinated water molecules in 4−6, the last three also having additional potential for competition between the halogen and hydrogen bonds. These were cocrystallized with two “classic” halogen bond donors, 1,4-diiodotetrafluorobenzene (tfib, a) and 1,4-dibromotetrafluorobenzene (tfbb, b) which differ in the expected halogen bond energies, while having identical molecular geometries, and would, expectedly, form isostructural cocrystals with the respective acceptors.

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Scheme 1. Halogen Bond Donors and Acceptors Used in This Study

RESULTS AND DISCUSSION All syntheses have been performed both by mechanochemical liquid-assisted grinding (LAG)61−68 of a mixture of donors and acceptors in a ball mill and by crystallization from solution in order to obtain single crystals. Complexes 1 and 2 formed cocrystals with both donors used (Table 1) mechanochemically, and three of the cocrystals were also obtained by crystallization from solution (1a,b and 2a). All four cocrystals were found to be isostructural (the powder diffraction pattern of mechanochemically obtained 2b is nearly identical with those of the remaining three; see Figures S9, S10, S15, and S16 in the Supporting Information). The donor molecules are bonded through the expected bifurcated halogen bond onto pairs of acac oxygen atoms (Table 1 and Figure 2) interconnecting the molecules into chains with each donor molecule binding to two acceptor molecules and vice versa. As the chains lie along the crystallographic 2-fold axis (b axis in C2/c space group), the bifurcated halogen bonds are symmetrical. However, a common feature in all three singlecrystal structures (1a,b and 2a) is that the displacement ellipsoid of the I (or Br) atom is somewhat elongated in the molecular plane and is perpendicular to the C−X bond, while a similar deformation of the displacement ellipsoids (albeit to a lesser extent) is also observable for the fluorine atoms of the donor molecules (Figure 3). While this may be attributable to thermal motion (rotation-like vibration of the whole donor molecule around its symmetric equilibrium position about the axis perpendicular to the plane of the molecule), it could also

Table 1. Halogen Bond Geometry in the Crystal Structures of the Cocrystals C−X···O 1a 1b 2a 3a 4a 5a 6a 6b

C4−I1···O1 C4−I1···O1a C4−Br1···O1 C4−Br1···O1a C4−I1···O1 C4−I1···O1a C4−I1···O1 C4−I1···O2 C4−I1···O3 C4−I1···O3 C4−I1···O1 C4−I1···O2 C4−Br1···O1 C4−Br1···O2

d(X···O)/Å (RS/%) 3.405 3.405 3.356 3.356 3.304 3.304 3.285 3.352 3.057 3.050 3.080 3.271 3.116 3.169

(2.7) (2.7) (0.4) (0.4) (5.6) (5.6) (6.1) (4.2) (12.7) (12.9) (12.0) (6.5) (7.5) (5.9) B

α(C−X···O)/deg

β(O···X···O)/deg

158.0 158.0 157.1 157.1 156.1 156.1 161.8 150.1 176.2 176.1 163.2 143.1 156.1 149.8

44.0 45.9 47.9 48.0

51.8 52.8



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Figure 1. Optimized molecular structures and electron density isosurfaces (ρiso = 0.002 au) with the mapped ESP of halogen bond donors a and b and acceptors 1−5. The ESP of 6 was analyzed on a slightly modified geometry of 6 as found in the crystal structure of 6a (see the Supporting Information for details). Maximum and minimum values of ESP in the vicinity of significant molecular sites are noted and given in kJ mol−1 e−1.

to another acac oxygen atom (possibly with a low energy barrier between the two positions, allowing the donor molecule to oscilate and change the its binding oxygen atom), which would lead to an average crystallographic 2-fold symmetry. The latter supposition was given additional credence by the fact that in 3a and 6a,b, where the bifurcated halogen bond was also found (see Table 1, Scheme 2, and the discussion below), it was not symmetric, the lengths of the two X···O contacts differing by as much as 0.2 Å. Scheme 2. Definition of the Halogen Bond Parameters Given in Table 1

In order to ascertain whether the bifurcated halogen bonds in cocrystals 1a,b and 2a are indeed symmetrical, we have examined the electrostatic potential (ESP) of 1 to determine whether it is of a single-well (minimum on the central line of the O−Cu−O angle) or double-well (two minima near oxygen atoms and one maximum between them) configuration in the

Figure 2. Halogen-bonded chains in (a) 1a, (b) 1b, and (c) 2a.

indicate that the donor molecules are structurally disordered, half of them (randomly distributed throughout the cocrystal) being asymmetrically halogen bonded to one and the other half

Figure 3. ORTEP plots of the donor and acceptor molecules in (a) 2a, showing elongation of the displacement ellipsoid of the iodine atom (I1) in the tfib molecule and (b) 1b, showing elongation of the displacement ellipsoids of the fluorine atoms (F1) in the tfbb molecule. C



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Figure 4. (a) Distribution of ESP in the O−Cu−O plane of 1 mapped on different electron density isosurfaces. (b) ESP mapped on the Hirshfeld surface of a molecule of 1 in 1a cocrystal. ESP values in two symmetric minima and in maximum between them are noted.

Table 2. Geometric Parameters and Energies of Halogen Bonds in Potential Energy (in Vacuo) Minimaa of Acceptor···Donor Halogen-Bonded Complexesa 1···a 1···b 2···a 2···b 3···a (bifurcated) 3···a (monocentric) 3···b (bifurcated) 3···b (monocentric) 4···a (bifurcated) 4···a (monocentric) 4···b (bifurcated) 4···b (monocentric) 5···a (bifurcated) 5···a (monocentric) 5···b (bifurcated) 5···b (monocentric) 6···a (bifurcated)a 6···b (bifurcated)a

C−X···O

d(X···O)/Å

α(C−X···O)/deg

β(O···X···O)/deg

E/(kJ mol−1)

C−I···O1 C−I···O2 C−Br···O1 C−Br···O2 C−I···O1 C−I···O2 C−Br···O1 C−Br···O2 C−I···O1(acac) C−I···O2(acac) C−I···O3(oxo) C−Br···O1(acac) C−Br···O2(acac) C−Br···O3(oxo) C−I···O1(acac) C−I···O2(acac) C−I···O3(water) C−Br···O1(acac) C−Br···O2(acac) C−Br···O3(water) C−I···O1(acac) C−I···O2(acac) C−I···O3(water) C−Br···O1(acac) C−Br···O2(acac) C−Br···O3(water) C−I···O1(acac) C−I···O2(acac) C−Br···O1(acac) C−Br···O2(acac)

3.138 3.138 3.132 3.132 3.143 3.143 3.092 3.092 3.175 3.227 2.864 3.135 3.160 2.998 2.882 3.405 2.987 2.968 3.242 3.051 3.044 3.196 2.932 3.002 3.155 2.946

155.1 155.1 155.1 155.1 154.5 154.5 154.1 154.1 159.5 152.2 176.6 157.7 153.1 159.4 170.7 133.7 179.6 158.8 147.1 151.5 158.4 151.2 177.8 159.0 150.0 164.2

49.8

30.67

49.8

21.85

51.1

29.38

51.8

20.35

48.3

29.19

49.1

30.89 20.68

51.2

20.45 36.69

52.8

28.28 25.00

49.0

21.37 37.58

49.7

23.42 24.20 17.70 31.7 20.7

a For acceptors with multiple binding sites (3−5) halogen-bonded complexes with both bifurcated and monocentric modes have been studied. The geometry of complexes with 6 was not optimized as the pentacoordinated molecule of 6, as present in the solid state, does not correspond to a minimum of energy in the gas phase (see Figure 1). Instead, geometries of 6···a and 6···b as found in the crystal structures of 6a,b were used for computation of halogen bond energies. Therefore, in these systems only a bifurcated halogen bond could be studied.

O−Cu−O plane. This should indicate whether the σ hole of the halogen atom should preferentially approach the molecule of 1 along the central line of the O−Cu−O angle (corresponding to the crystallographic 2-fold axis in the crystal structure), leading to a symmetric bifurcated bond or rather

approach a single oxygen atom of 1, leading to an asymmetric bifurcated halogen bond. The distribution of ESP in the O− Cu−O plane was found to significantly vary with the electron density isosurface on which ESP was examined, having the single-well configuration for 0.001 ≤ ρiso/au ≤ 0.004 and D



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double-well configuration for 0.005 ≤ ρiso/au ≤ 0.007 (Figure 4a). However, as isosurfaces with ρiso ≥ 0.002 au are generally positioned too close to the nuclei to correspond to intermolecular contact distances (and the ESP on isosurfaces with 0.001 ≤ ρiso/au ≤ 0.002 is generally studied as being the most indicative of molecular binding preferences), it is reasonable to expect that the halogen donor molecule “feels” the ESP distribution of a molecule of 1 as a single-well configuration. An alternative to examination of the ESP on an (arbitrary) isosurface of electron density calculated in vacuo for a specific intermolecular contact (in a particular crystal) is to plot the ESP on the molecular Hirshfeld surface of a molecule, the latter being a closer approximation of the “shape” of a molecule in a particular crystal. When this was calculated for a molecule of 1 in the cocrystal 1a, the obtained ESP on the Hirshfeld surface was found to be in fact of a double-well configuration, with two minima in the points of closest I···O contacts and a maximum (albeit with a quite small onemerely 6% above the minimum) between the two minima (Figure 4b). As the two approaches gave apparently contradictory results, it was not possible to reach any definite conclusion whether the halogen bond in 1a is indeed symmetric or asymmetric. We have therefore performed geometry optimizations of a 1···a complex (in the gas phase) in order to examine the potential energy surface of the complex. The optimized geometry (corresponding to a minimum in potential energy) was found to be a structure with a symmetric bifurcated halogen bond (Table 2 and Figure 5a), and this was obtained regardless of starting geometry (symmetric or asymmetric). On this basis, we are inclined to conclude that the symmetry of the halogen bond in the cocrystal 1a is real and not an artifact imposed on the model by the crystallographic space group symmetry. It should be noted that during the preparation of this paper a paper was published by the Kukushkin group,69 covering the structures of 1a and 2a as well as their platinum analogue (Pt(acac)2·tfib cocrystal), which was also found to belong to the same isostructural series. Their results are in agreement with our conclusion on the symmetry of the halogen bonds in this series and include measurements at 100 K (reducing the thermal motion of the atoms and the displacement ellipsoids, while retaining the C2/c space group symmetry of the structure) as well as a QTAIM study of the bifurcated I···O2 halogen bond with a pair of (equivalent) bond paths and bond critical points in the electron density topology. Similarly to 2, 3 has also yielded cocrystals with both halogen bond donors, of which only that with tfib (3a) could be obtained by crystallization from solution, while the mechanochemically obtained 3b was found to be isostructural with 3a by means of powder diffraction (cf. Figures S11 and S17 in the Supporting Information). The formed cocrystals are again of 1:1 stoichiometry, with halogen bond donors bridging between pairs of molecules of 3 and vice versa through bifurcated halogen bonds (Figure 6), although here they were no longer symmetricin the structure of 3a the two I···O contacts differed by ca. 0.07 Å, with the shorter contact corresponding to the more linear halogen bond (ca. 11° higher C−I···O angle). The general geometry of the halogen bonding was also quite different from that in the cocrystals of 1 and 2. While in the former structures the donor and acceptor molecules were approximately coplanar (dihedral angles between mean planes of the molecules between 0.2° in 1b and ca. 3° in 2a) and the halogen atom approached the acac

Figure 5. Five types of optimized geometries of halogen-bonded molecular complexes in vacuo: (a) symmetrical bifurcated halogen bond with acac oxygen atoms as acceptors (shown for 1···a, identical type of bonding as in 1···b, 2···a, and 2···b, found in crystal structures of 1a,b and 2a); (b) asymmetric bifurcated halogen bond with acac oxygen atoms as acceptors (shown for 3···a, identical type of bonding as in 3···b, found in crystal structure of 3a and similar to that in 6a,b); (c) monocentric halogen bond with the oxovanadium oxygen as acceptor (shown for 3···a, identical type of bonding as in 3···b, not found in crystal structures); (d) monocentric halogen bond with water oxygen as acceptor (shown for 4···a, identical type of bonding as in 4···b, 5···a, and 5···b, found in crystal structures of 4a and 5a); (e) asymmetric bifurcated halogen bond with acac oxygen atoms of 4 and 5 as acceptors (shown for 4···a, identical type of bonding as in 4··· b, 5···a, and 5···b, not found in crystal structures).

Figure 6. Crystal structure of 3a: (a) halogen-bonded chains; (b) interconnection of linear coordination polymers of VO(acac)2 units through halogen bonding with tfib molecules.

oxygen atoms along the mean plane of the four oxygen atoms, in 3a the molecule 3 was not planar (the vanadium atom was displaced ca. 0.4 Å above the mean plane of the acac oxygen atoms and the mean planes of the two acac chelate rings were at an angle of ca. 51.9°), the tfib ring lay out of the mean plane of the oxygen atoms, and the iodine atom approached the mean plane of the oxygen atoms at an angle of ca. 16.6° (Figure 6b). E



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Figure 7. Crystal structures of 4a and 5a: (a) halogen-bonded chains and (b) interconnection of hydrogen bonded chains of Ni(acac)2(H2O)2 units through halogen bonding with tfib molecules in 4a; (c) halogen-bonded chains and (d) interconnection of hydrogen-bonded chains of Co(acac)2(H2O)2 units through halogen bonding with tfib molecules in 5a.

vacuo (Table 2) for a hypothetical a···3 complex with an I··· Ooxo halogen bond is larger than that for a bifurcated I···Oacac bond as found in the solid state (Table 2 and Figure 5b,c), but the difference is only 1.7 kJ mol−1well within the energy range of dispersion forces. While the addition of a competing halogen acceptor site to the diketonate has not caused a significant change in the halogen bonding pattern, the introduction of coordinated water molecules as hydrogen bond donors, capable of competition with the halogen donor, has been shown to have a considerable effect. Thus, 4 and 5, both of which contain two coordinated water molecules in the molecule, form isostructural 1:1 cocrystals with tfib (4a and 5a), both by liquid-assisted grinding and by crystallization from solution. In this case, however, the coordinated water molecules form hydrogen bonds with the acac oxygen atoms of two neighboring molecules of 4 (or 5) (Figure 7). As each molecule acts as a donor and an acceptor of four such hydrogen bonds, no acac oxygen atoms are left for forming halogen bonds, and the halogen bond is established with the only remaining oxygen atomthat of the coordinated water molecule. The result is a structure comprised of hydrogenbonded chains along the crystallographic c axis, with molecules of 4 (5) bridged by halogen bonding with molecules of tfib along the crystallographic [101] direction into a 2D network perpendicular to b. The halogen bond geometry (Table 1) of the monocentric bond in 4a and 5a is more favorable (shorter distance and angle closer to 180°) than any of the bifurcated bonds, apparently indicating that the coordinated water is in fact the preferred halogen bond acceptor. However, the ESP on a molecule of 4 (and 5) is far more negative in the region of the acac oxygen atoms than on the oxygen atom of water molecule (ca. 23%; cf. Figure 1) and remains negative over a large portion of the isosurface, making it possible for an energetically more favorable contact with the σ hole of the halogen atom. This favorability of the bifurcated halogen bond with the acac oxygen atoms, in contrast to the monocentric bond with the coordinated water, has been confirmed by computing the halogen bond energy between donor and acceptor molecules bonded by monocentric bonds as found in the crystal structure, as well as the bonding energies in hypothetical a···4 and a···5 complexes with bifurcated halogen bonds formed with the acac oxygen atoms as present in the structures of cocrystals of 1−3 (Table 2 and Figure 5d,e). In

Introduction of an additional potential halogen acceptor site (oxovanadium oxygen in 3) could be expected to significantly disrupt the halogen bonding, particularly as the ESP on the oxovanadium oxygen is slightly more negative than that on the acac oxygen atoms (Figure 1), which should make it a preferred halogen bond acceptor site. However, the fivecoordination of the vanadium atom in 3 provides an additional site with a positive ESP in the structurethe vanadium atomto which the oxovanadium of a neighboring molecule binds through an extremely short V−O bond of 2.433 Å interconnecting the molecules of 3 into a linear coordination polymer along the crystallographic b axis. As the oxovanadium oxygen is involved in this contact, it is no longer “available” for halogen bonding, and the halogen bond is achieved through acac oxygen atoms, leading to a 2D structure interconnected by orthogonal V−O and bifurcated I···O2 halogen bonds (Figure 4b). It is interesting to note that, although binding of Lewis bases (e.g., pyridine derivatives) to vanadium in oxovanadium(IV) diketonates is commonplace,70−73 there are only a handful of reported structures where an oxovanadium oxygen bridges to another vanadium atom in oxovanadium(IV) complexes to form coordination polymers.74−76 The preference of oxovanadium oxygen to bind to the vanadium atom and acac oxygen atoms to the iodine of tfib is an apparent aberration of the general rule that the most positive donor (strongest Lewis acid) will bind to the most negative acceptor (strongest Lewis base). As the tfib iodine exhibits a significantly more positive ESP (167.28 kJ mol−1 e−1) in comparison to the vanadium atom in 3 (124.67 kJ mol−1 e−1), it should be expected to bind to the more negative site of 3, the oxovanadium oxygen (ESP of −178.28 kJ mol−1 e−1), rather than acac oxygen atoms (−170.23 kJ mol−1 e−1). However, as on one hand the approach of the vanadium atom to the acac oxygen atoms is much more sterically hindered than its approach to the oxovanadium oxygen and on the other hand the preference of iodine to form a halogen bond with more diffuse negative regions has been documented,50 it is possible that the crystal-packing effects have in this case prevailed over the formation of the strongest possible intermolecular interactions. This seems particularly likely as the ESPs on the two negative sites of 3 are in fact quite similar (less than 5% difference) and therefore are not so dissimilar Lewis bases. Indeed, the calculated halogen bond energy in F



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Figure 8. Crystal structures of 6a,b: (a) halogen- and hydrogen-bonded chains and (b) interconnection of hydrogen-bonded chains of Zn(acac)2(H2O) units through halogen bonding with tfib molecules in 6a; (c) halogen- and hydrogen-bonded chains and (d) interconnection of hydrogen-bonded chains of linear coordination polymers of Zn(acac)2(H2O) units through halogen bonding with tfbb molecules in 6b.

cocrystals with tfib and tfbb as halogen donors, 6a,b (Figure 8), which are the only cocrystals covered by this study with a stoichiometry other than 1:1, having crystallized with two complex molecules per halogen bond donor. Here also the coordinated water binds to the acac oxygen atoms to form hydrogen-bonded chains; however, the hydrogen-bonding scheme involves only one pair of acac oxygen atoms per molecule of 6, leaving the other pair free for halogen bonding. There is therefore a possible competition between two acceptor sites capable of forming two different type halogen bondsa pair of acac oxygen atoms as an acceptor of a bifurcated halogen bond and a coordinated water oxygen atom as an acceptor of a linear monocentric bond. Both halogen bond donors were found to bind to the pair of acac oxygen atoms, forming an asymmetric bifurcated halogen bond, thus corroborating the previous conclusion of the favorability of the bifurcated bond. It is interesting to note that the halogen bond is in fact more nearly symmetric with the bromine donor in 6b than with iodine in 6a, where one I···O contact is almost 0.2 Å shorter (and ca. 20° more closely linear) than the other. Although generally such a (relatively minor) difference in bond geometries would hardly present a strong argument in comparing the proclivity of iodine and bromine toward bifurcated halogen bonds, in this case, as the comparison is made between isostructural compounds where other supramolecular interactions and packing effects are (more or less) identical, it can be justified to ascribe the difference in bond geometry to the difference in contact atoms. It would therefore appear that bromine is more likely than iodine to form bifurcated halogen bonds. The different modes of halogen bonding in the studied series of cocrytals also, to an extent, affect their thermal stability (Figure 9). The melting points of 1a−3a fall in the range 178− 216 °Chigher than the melting point of tfib (106−107 °C) but lower than those of the respective complexes 1−3 (243− 257 °C). It is interesting to note, however, that in the case of 1a and 2a the melting point of the complexes exceeds the boiling point of tfib (ca. 180−185 °C), and their melting proceeds by simultaneous evaporation of tfib. The melting points of their tfbb analogues (1b−3b) are considerably lower, in the range 109−149 °C, which is still higher than the melting

both cases the energies of the bifurcated halogen bonds were found to be higher than those for the monocentric bonds with the water oxygen atomby ca. 8 kJ mol−1 in the case of a···4 and 14 kJ mol−1 in the case of a···5 (Table 2). It therefore follows that the bifurcated halogen bond in these structures is not absent due to an inherent unfavorability but is rather due to specific structural features of the structures of 4a and 5a, in particular the hydrogen bond interconnecting the complex molecules. The ESP on molecules of 4 and 5 in the vicinity of the water hydrogen atoms shows considerable positive maxima, 204.32 and 229.56 kJ mol−1 e−1, respectively, much higher than the maxima on the σ holes of tfib (167.28 kJ mol−1 e−1, Figure 1). It therefore appears that the most favorable interactions are the (pair of) hydrogen bonds between acac oxygen atoms (the strongest Lewis base) and the hydrogen atoms of a water molecule (the strongest Lewis acid), leaving the remaining weaker Lewis acid and base to form monocentric I···O halogen bonds. Therefore, in spite of allowing a favorable linear geometry and close approach of the halogen atom to the acceptor oxygen, the coordinated water molecule is a poorer halogen bond acceptor than the (pair of) acac oxygen atoms, and the monocentric linear halogen bond in 4a and 5a is in fact weaker than the bifurcated halogen bond in other cocrystals. This is further confirmed by the attempted syntheses of cocrystals with tfbb as a poorer halogen bond donor (ESP on the σ holes of 129.88 kJ mol−1 e−1) which should expectedly form weaker halogen bonds. While a series of cocrystals isostructural with the corresponding tfib analogues were obtained in all cases where the halogen bond donor binds through a bifurcated halogen bond, 4 and 5 were not found to react with tfbb, leaving them as the only two halogen bond acceptors covered by this study which in a LAG experiment with tfbb yielded only a mixture of reactants. Therefore, bifurcated Br···Oacac halogen bonds with tfbb were sufficiently strong to allow supramolecular synthesis, while nonbifurcated bonds Br···Owater (equivalent to I···Owater in 4a and 5a) apparently were not. A good demonstration of the preferential formation of the bifurcated halogen bond is provided by the cocrystals of 6, a molecule of which contains only a single coordinated water molecule per complex molecule. It formed two isostructural G



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them to (below) room temperature, which would make it most unlikely for 4b and 5b to be obtainable either from solution or by mechanochemical experiments at room temperature (ca. 20 °C).

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CONCLUSION The studied series of halogen-bonded complexes indicated that in this case the bifurcated halogen bond is a preferable mode for binding of molecules into cocrystals. The computed energies of bifurcated halogen bonds in all cases are larger than those for possible monocentric bonds, even for complexes 4···tfib and 5···tfib, which were found to bind via monocentric bonds in the crystal structure. This is particularly supported by the crystal structures of 6a,b, where there is a possible competition of the bifurcated and monocentric sites for the halogen donor, with only the bifurcated bond being present. The bifurcated bond was found to be symmetric in the case of square-planar acceptors (1 and 2) but asymmetric when the acceptors have additional ligands. The bifurcated bonds form equally readily with the bromine as with the iodine donor (in spite of a narrower σ hole on the bromine atom), and tfbb in all cases forms solids isostructural with the tfib equivalents, although with ca. 30% lower halogen bond energies (bifurcated halogen bonds with tfbb usually ca. 20 kJ mol−1 and those with tfib ca. 30 kJ mol−1). The favorability of the bifurcated halogen bond in cocrystals of metal(II) acetylacetonates with pdihalotetrafluorobenzenes also reflects their considerable thermal stabilitythe cocrystals of metal(II) acetylacetonates which do not contain coordinated water molecules (1−3) show surprisingly high thermal stabilitiesin all cases above the melting points of the corresponding halogen bond donor and in the case of 1a and 2a even above its boiling point.

Figure 9. Melting/decomposition temperatures (measured at ambient pressure) of the obtained cocrystals plotted against the computed halogen bond energy (in vacuo). Data points corresponding to cocrystals of tfib are depicted by circles and those corresponding to cocrystals of tfbb by squares. For comparison also depicted are melting points (mp) and boiling points (bp) of tfib and tfbb, as well as the boiling point of water (dashed line).

point of tfbb (78−80 °C), although systematically lower than the boiling point of tfbb (ca. 157 °C). The cocrystals of tfbb have ca. 50−75 °C lower melting points than their tfib counterparts; the difference in thermal stability can primarily be attributed to the difference in halogen bond strengths (as in all cases the tfib and tfbb cocrystals of a given complex are isostructural, the only significant difference in the structures is the halogen bond and the contributions of the remaining interactions in the pairs of structures should be approximately equal). The largest difference between the melting points was indeed found in the case of cocrystals of 2 (the melting point of 2a was 216 °C and of 2b was 142 °C), which also exhibit the largest difference between the I···Oacac and Br···O halogen bonds (9.1 kJ mol−1). The cocrystals of complexes containing water molecules, on the other hand, all start to melt below 100 °C, and the temperatures of melting generally correspond to water loss in pure complexes 4−6, with corresponding decreases in mass of the sample due to water evaporation. This leads to the conclusion that the melting of these cocrystals is in fact thermal decomposition due to the loss of coordinated water. The presence of halogen bonding does not seem to lead to any particular stabilization of these structuresin fact 6a was found to have a somewhat lower decomposition temperature (ca. 81 °C) than 6b (ca. 90 °C), in spite of it containing a stronger halogen bond donor. The low melting/decomposition temperatures of these cocrystals also provide a possible explanation for the failure of the attempted syntheses of cocrystals of 4 and 5 with tfbb. From the point of view of interaction energies in vacuo (Table 2) this failure is somewhat surprising, as the decrease in interaction energy from I···Owater to Br···Owater is on average only ca. 7 kJ mol−1 (as opposed to an average decrease of ca. 9 kJ mol−1 for I···Oacac to Br···Oacac), with the Br···Owater interaction energies of 21.37 and 17.70 kJ mol−1 for 4···tfbb and 5···tfbb, respectively (note that the former interaction energy is in fact higher than in the successfully obtained 1b−3b). However, the low melting/ decomposition temperatures of 4a and 5a (80 and 42 °C, respectively) indicate possibly even lower decomposition temperatures of 4 and 5 cocrystals with tfbb, possibly reducing

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EXPERIMENTAL SECTION

Synthesis of Complexes. Complexes 1−6 were prepared according to previously published procedures.77−82 1 and 3−6 were prepared by adding acetylacetone (Hacac) to an ethanol solution of the corresponding metal salt in a 2:1 molar ratio, with addition of a small amount of trimethylamine. The precipitated product was filtered under reduced pressure. Complex 278 was prepared from aqueous solution by dissolving palladium(II) chloride in an aqueous solution of NaCl and adding acetylacetone in a double molar ratio and a small amount of triethylamine to the solution. The obtained yellow precipitate was filtered and dried under suction. Mechanochemical Synthesis. In a typical milling experiment, either tfib or tfbb was placed in a stainless steel grinding jar (10 mL) with an equimolar amount of the halogen bond acceptors along with addition of a small amount of acetonitrile (MeCN); a pair of two stainless steel balls of 7 mm radius was used for milling. The milling experiments were performed for 20 min on a Retsch MM200 grinder mill. Cocrystallization from Solution. The single crystals were prepared by evaporation of a methanol or methanol/ethanol solution (1/1 v/v) of a mixture of the corresponding acetylacetonate and tfib or tfbb in a 1:1 molar ratio at room temperature (ca. 20 °C). X-ray Crystallography. The diffraction data for molecular and crystal structure determination were collected at 295 K for all crystals. Diffraction measurements were made on an Oxford Diffraction Xcalibur Kappa CCD X-ray diffractometer83 with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Structures were solved by direct methods and refined using SHELXS and SHELXL programs.84 The structural refinement was performed on F2 using all data. All calculations were performed and the figures prepared using the WINGX crystallographic suite of the programs. Hydrogen atoms involved in hydrogen bonding were located in the electron difference map, while those which do not participate in such H



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interactions were placed on calculated positions. All hydrogen atoms were isotropically refined. Powder X-ray Diffraction Experiments. PXRD experiments on the samples were performed on a PHILIPS PW 1840 X-ray diffractometer with Cu Kα1 (1.54056 Å) radiation at 40 mA and 40 kV. The scattered intensities were measured with a scintillation counter. The angular range was from 5 to 40° (2θ) with steps of 0.02−0.03°, and the measuring time was 0.2−0.5 s per step. Data collection and analysis were performed using the program package Philips X’Pert.85 Thermal Analysis. Thermal analysis was carried out on Mettler Toledo TGA/SDTA 851 and DSC823 modules in sealed aluminum pans (40 μL), heated under flowing nitrogen (200 mL min−1) at 10 °C min−1. Data collection and analysis were performed by the program package STARe Software 14.00.86 The nonisothermal crystallization experiments for 1 and 2 were carried out under a nitrogen atmosphere with a flow rate of 60 mL min−1. The samples were heated from 25 to 400 °C, at a heating rate of 10 °C min−1 Computational Details. All quantum chemical calculations were performed with the Gaussian 09 (Rev D.01) program,87 employing the M06 functional88 with ultrafine integration grid (99 radial shells and 590 points per shell). The def2-TZVP basis set89 was used with effective core potentials for iodine atoms, taken from the EMSL basis set Web site.90,91 The calculations of harmonic frequencies were performed to prove the success of each geometry optimization. Energies of all halogen bonds between acceptor and donor molecules were calculated as EXB = −Eint + Erelax(A) + Erelax(D), where Eint = AD AD A A EAD AD(AD) − EAD(A) − EAD(D), Erelax(A) = EA(A) − EAD(A), and Erelax(D) = EDD(D) − EDAD(D). Here, each term noted as EZY(X) stands for the energy of the species X calculated on the optimized geometry of species Y using the basis set of species Z, while A stands for the acceptor, D for the donor, and AD for the donor−acceptor halogenbonded complex. Thus, Eint is the interaction energy between donor and acceptor molecules, calculated on the optimized geometry of the complex and corrected for the basis set superposition error by the scheme of Boys and Bernardi,92 while Erelax(X) is the difference in the energy of the X molecule (donor or acceptor) in its optimized geometry and in the optimized geometry of the complex (the socalled monomer relaxation energy). The Erelax terms were included because the geometries of some acceptor molecules in the complexes were in some cases significantly different from those corresponding to their own minima. ESP mapping to the electron density isosurfaces and search for the lowest/highest ESP values was done by our own code, while the data were generated by the cubegen utility of Gaussian on a standard rectangular grid, having 12 points/bohr.

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Article

AUTHOR INFORMATION

Corresponding Authors

*V.S.: tel, +385 1 4606371; fax, +385 1 4606341; e-mail, [email protected]. *D.C.: tel, +385 1 4606362; fax, +385 1 4606341; e-mail, [email protected] ORCID

Vladimir Stilinović: 0000-0002-4383-5898 Vinko Nemec: 0000-0001-7862-6888 Dominik Cinčić: 0000-0002-4081-2420 Present Address †

T.G. and T.P.: Ruđer Bošković Institute, Bijenička cesta 54, Zagreb, Croatia. Funding

This research was supported by the Croatian Science Foundation under the project IP-2014-09-7367. Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01659. Experimental details, crystallographic data for all compounds, halogen bond geometries, ORTEP plots of asymmetric units with atom labels, powder diffraction patterns for all the obtained solids, TG and DSC curves, and atomic coordinates and electron energies for the computed structures (PDF) Accession Codes

CCDC 1858529−1858536 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. I



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