Ground-State Charge-Density Distribution in a Crystal of the

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A: Molecular Structure, Quantum Chemistry, and General Theory

Ground-State Charge-Density Distribution in a Crystal of the Luminescent Ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline Katarzyna N. Jarzembska, Rados#aw Kami#ski, Krzysztof Durka, and Krzysztof Wozniak J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00832 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Ground-State Charge-Density Distribution in a Crystal of the Luminescent Ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline Katarzyna N. Jarzembska,a* Radosław Kamiński,a Krzysztof Durka,b Krzysztof Woźniak c

a

Department of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland

b

Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

c

Biological and Chemical Research Centre, Department of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland

* Corresponding author: Katarzyna N. Jarzembska ([email protected])

Abstract: This contribution is devoted to the first electron density studies of a luminescent oxyquinolinato boron complex in the solid state. Ortho-phenylenediboronic acid mixed with 8-hydroxyquinoline in dioxane forms high quality single crystals via slow solvent evaporation, which allows successful high resolution data collection (sin / = 1.2 Å−1) and charge density distribution modelling. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and to the solvent species exhibiting anharmonic thermal motion. The experiment and theory compared rather well in terms of atomic charges and volumes, except for the boron centres. Boron atoms, as expected, constitute the most electron-deficient species in the complex molecule, whereas the neighbouring oxygen and carbon atoms are the most significantly negatively charged ones. This part of the molecule appears to be very much involved in the charge transfer occurring between the acid fragment and oxyquinoline moiety leading to the observed fluorescence, as supported by the TDDFT results and the generated transition density maps. TDDFT calculations indicated that p-type atomic orbitals contributing to the HOMO-1, HOMO and LUMO play the major role in the lowest energy transitions, and enabled further comparison with the charge density features, which is discussed in details. Furthermore, the results confirmed the known fact the Q ligand character is most important for the spectroscopic properties of this class of complexes.

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1. Introduction Since the early 1980s arylboronic acids and their derivatives have been attracting increasing attention thank to wide applications in organic synthesis and medicine.1 Nowadays, these compounds are also of particular interest in crystal engineering due to their abilities to form complex hydrogen-bonded networks.2-17 Additionally, they exhibit promising properties regarding supramolecular18-21 and materials chemistry.22-25 For instance, diarylborinic compounds, described by a general formula Ar BX (X = Cl, OH, OR), can be easily complexed with various (N,O), (N,N), or (N,C) rigid π-conjugated ligands, leading to luminescent 4-coordinated organoboron systems.26-30 This enables their wide use in the construction of optical and optoelectronic devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and photoresponsive materials.31-36 Specifically, 8-oxyquinolinato complexes are among the most promising compounds in this context.37-43 This is presumably due to their high stability and good photophysical properties, such as high quantum yields and chargetransport properties. Modification of the chelating ligand with electron donating or electron withdrawing group is a straightforward method for fine tuning of the optical properties.37-38 On the other hand, the diversification of organoboron cores is usually limited to a difluoro- or diarylboron-based substituent.44 In our recent works we have demonstrated that some representative arylboronic acids, namely ortho-phenylenediboronic acids (odbas), are also able to bind the 8-oxyquinolinato ligand forming complex 1 (Scheme 1).45 This is especially interesting in view of the fact that boronic acids exhibit moderate Lewis acidity, and thus the corresponding chelate complexes are characterised by a relatively low stability. In the case of odba, the boron Lewis acidity is strongly enhanced by the synergetic effect of two neighbouring boronic groups and their ability to form semianhydrides based on a cyclic oxadiborole scaffold. In addition, depending on electronic properties of the substituents attached to the aromatic ring, one or both boron centres can be effectively saturated. Furthermore, different solvatomorphs can be easily obtained – most of them being isostructural with slightly different solvent cavity sizes.46 We have shown that the preferably formed complex network contains cavities, in which 1,4-dioxane fits very well. Consequently, the crystal structure of 1 with dioxane (1diox), which does not exhibit any structural disorder, appeared to be of sufficient quality for high-resolution X-ray diffraction studies. Since the ground-state electron-density


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distribution is one of the properties which to some extent may explain the observed spectroscopic bahaviour, this contribution has been dedicated to charge density studies of 1diox. To our best knowledge, this is the first work devoted to the electron density studies in oxyquinolinato boron complexes. The experimental results are compared with theoretical electron density distribution features, and supported by energetic investigations. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and also to the solvent species exhibiting anharmonic thermal motion.

Scheme 1. Schematic representation of the studied crystal structure, 1diox, containing molecules of the title complex and a half of the 1,4-dioxane molecule in the asymmetric unit.

2. Experimental section 2.1. Crystal synthesis. Synthesis of 1 was conducted according to our previously published procedure45 from freshly-prepared odba8 and commercially-available (Sigma-Aldrich Co.) 8-hydroxyquinoline. Charge-density-quality single crystals of 1diox were prepared by slow solvent evaporation from a concentrated solution at room temperature. 2.2. Data collection and processing. A single crystal of a suitable size was chosen for the purpose of the current study. The data set was collected using a Bruker AXS Kappa APEX II Ultra single-crystal diffractometer equipped with a CCD-type APEX II area detector, molybdenum TXS rotating anode (Mo-Kα radiation, = 0.71073 Å), 4circle goniometer, multi-layer optics, and a low-temperature nitrogen gas-flow device by Oxford Cryosystems (700 Series Cryostream). The determination of the unit cell



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parameters and the integration of raw diffraction images were performed with the

APEX3 program package.47 The data set was corrected for Lorentz, polarization, and oblique incidence effects. The multi-scan absorption correction, frame-to-frame scaling and merging of reflections were carried out with the SORTAV program.48-50 The collected data set exhibited high resolution limit of sin / ≈ 1.2 Å−1, and overall completeness of more than 94%. Final data collection and reduction parameters are summarised in Table 1. Furthermore, the selected sets of raw diffraction frames and associated data are available online under the following DOI: 10.18150/repod.6759064 (Repository

for

Open

Data,

Interdisciplinary

Centre

for

Mathematical

and

Computational Modelling, University of Warsaw, Warsaw, Poland). 2.3. Structure solution and refinement. The crystal structure was solved by the charge-flipping method51-53 with the SUPERFLIP program.54 Initial independent atom model (IAM) refinements were performed with the JANA program.55 Multipole refinements for all data sets were carried out using the MOPRO suite56-57 combined with the current version of the University at Buffalo Data Bank (UBDB),58 which employs the Hansen-Coppens multipole model.59 Full details are present in the Supporting Information.58, 60-81 Despite the relatively low extreme values in the residual density and quite satisfactory -factors, inspection of the residual density distribution revealed some systematic effects in the solvent molecule fragment, namely around the O4 oxygen atom (Figure 1a). Careful examination indicated that mostly the high-angle data are responsible for the presence of these spurious features suggesting that the observed model deficiencies are related to some inappropriate thermal motion description of this atom. Indeed, we were able to take these effects successfully into account by refining the 3rd-order Gram-Charlier parameters (Figure 1b).82-84 All the final refinement statistics are summarized in Table 2, whereas more details are available from the Supporting Information.85-92 The CIF file is present in the Supporting Information, or can be retrieved from the Cambridge Structural Database (CSD)93-94 (deposition number: CCDC 1814107).


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(a)

(b)

Figure 1. Residual density maps in the vicinity of the O4 oxygen atom in the dioxane molecule (all maps, computed for all data, are drawn at the solvent mirror plane): (a) harmonic model; (b) anharmonic model. Blue solid lines – positive values; red dashed lines – negative values; olive green dashed lines – zero value; contours drawn at 0.05 e·Å−3.

Table 2. Parameters characterising the studied crystal, X-ray data collection and refinement.

Compound & crystal parameters Formula Formula weight, / g·mol−1 Crystal system Space group ! "### Crystal colour and habit Crystal size Data parameters Temperature, $ / K %/Å &/Å /Å '/° Volume, ( / Å3 )*+* / g·cm−3 Absorption coefficient, , / mm−1 range

sin / / Å−1 Index ranges

No. of reflections collected / unique

C H B NO + ½ C H O 318.93 monoclinic 2 / (no. 14) 4 664 yellow block 0.07 × 0.16 × 0.30 90 K 12.6209(8) 10.7943(7) 12.6367(8) 113.0062(9) 1584.6(2) 1.337 0.093 2.57° − 58.96° 1.20 −18 ≤ - ≤ 30 −25 ≤ . ≤ 21 −29 ≤ / ≤ 15 93013 / 21837



Completeness 0 ¶ No. of reflections with 1 ≥ 33 1 Refinement parameters No. of parameters / restraints 4"5 (1 ≥ 33 1) / (all data) 64" 5 (1 ≥ 33 1) / (all data) 74" 5 (1 ≥ 33 1) / (all data)

Ground-State Charge-Density Distribution in a Crystal of the

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