Various types of non-covalent interactions contributing towards crystal packing of halogenated diphospha-dicarbaborane with an open pentagonal belt
Various types of non-covalent interactions contributing towards crystal packing of halogenated diphospha-dicarbaborane with an open pentagonal belt † Josef Holub, a Zden ˇka Ru ˚z ˇic ˇkova ´,b Pavel Hobza, * c Jindr ˇich Fanfrlı ´k, * c Drahomı ´r Hnyk * a and Ales ˇRu ˚z ˇic ˇka* bWe have prepared 3-Cl-10-I-nido-7,8,9,11-P 2 C 2 B 7 H 7 by the reaction of 3-Cl-nido-7,8,9,11-P 2 C 2 B 7 H 8 with I 2 in the presence of AlCl 3 .The product was obtained in a yield of 49% and was characterized by NMR spectroscopy and X-ray crystallography. Quantum chemical calculations have demonstrated that the crystal structure is stabilized by hydrogen, dihydrogen and pnictogen bonds.
<!>Conflicts of interest
<p>Innerly and/or outerly substituted boron cluster hydrides (heteroboranes) of various architectures are known for various types of non-covalent intermolecular interactions and thus serve as good models for the evaluation and mutual comparison of various types of hydrogen, dihydrogen (diH), halogen (X), chalcogen, pnictogen (Pn) or even tetrel bonds. 1,2 The neutral nido-B 7 P 2 [C(R)] 2 H 7 cage with an open P 2 CBC pentagonal belt can be formally obtained from nido-B 11 H 11 4À (nido-B 11 H 14 À exists) 3 by replacing four BH vertices with two tetrel and Pn vertices. Miscellaneous dicarba dipnictogen nido eleven-vertex heteroboranes have been prepared by reacting arachno-4,5-C 2 B 7 H 13 with PCl 3 , AsCl 3 and SbCl 3 . [4][5][6] The crystal structures of 3-Cl-nido-7,8, (3) compounds are already known. [4][5][6] The crystal packing of 3 is governed by Pn-bonds. 6 In contrast to 3, it has not been possible to record the singlecrystal X-ray data of the parent nido-7,8,9,11-P 2 C 2 B 7 H 9 and nido-7,8,9,11-As 2 C 2 B 7 H 9 in sufficient quality. Interestingly, it is feasible to determine the structure of single-crystal halogenated compounds 1 and 2, revealing a supramolecular architecture mainly resulting from extensive Pn-bonding. [4][5][6] In order to proceed in the examination of the non-covalent interactions of nido-heteroboranes, we attempted to iodinate 1 (Fig. 1) by the reaction with elemental iodine in the presence of aluminum trichloride (details in ESI †). The resulting 3-Cl-10-I-nido-7,8,9,11-P 2 C 2 B 7 H 7 (4) was obtained in a moderate yield of 49% and was characterized in solution in terms of multinuclear NMR spectroscopy (Table 1).</p><p>In the crystal structure of 4 (Fig. 2), four molecules have been found in the unit cell. The structure of the cage of 4 is essentially the same as found for 1 4 except for the very slight shortening of the B(10)-C(9/11) bonds inside the open P 2 CBC pentagonal belt caused by the substitution of the B(10) atom by the heavy iodine. In fact, there are only two examples of a similar structural pattern in the Cambridge Structural Database, i.e. dicarba-closo-dodecaborane-1,2-diyl-1,2-bis(diisopropylamino)diphosphines, 9 but these are heavily distorted with no option of a serious structural comparison.  The discrepancy between the calculated and experimental chemical shift results from the neglect of the effect of spin-orbit (SO) coupling on the overall shielding, which is not taken into account in the GIAO-MP2 scheme. However, it has been shown that due to iodine being terminally bonded to a boron atom within a neutral boron cluster, SO coupling increases the contribution of this B atom towards overall shielding by about 20 ppm, 7,8 which would considerably improve the fit in the present case.</p><p>a Institute of Inorganic Chemistry of the Czech Academy of Sciences,</p><p>To better understand non-covalent interactions of 4, we computed its ESP potential (Fig. 3) and compared it with other recently studied analogous compounds, 1-3. Compound 1 has only one highly positive s-hole located on the P vertices. The other expected s-holes of 1 could not be defined on the P vertices due to the more positive neighboring CH vertices. The Sb and As vertices of 2 and 3 have more positive ESP surfaces than the CH vertices. Both molecules thus have three well-defined s-holes on the Pn atoms. From this point of view, the ESP surface of 4 is thus analogous to 2 and 3. Compound 4 has three s-holes located on P atoms with the same magnitude (V S,max ) of 31.5 kcal mol À1 . The P vertices of 4 have even more positive ESP values than the CH vertices. The ESP surface on top of the CH vertices has the value of 28.2 kcal mol À1 . Besides the s-holes located on P atoms, the 4 compound also has s-holes on the X atoms. While the I atom has a positive s-hole with the V S,max value of 6.8 kcal mol À1 , the Cl atom only has a relative s-hole (i.e. the top of the Cl atom is less negative than its belt but the V S,max value is negative). The 4 compound can thus form Pn-, H-and X-bonds, the last of which are, however, less probable due to the lower V S,max values on the X atoms (Table 2). The most negative ESP surface of 4 is on the negative belt on the I and Cl atoms (the magnitudes of À15.1 and À12.3 kcal mol À1 , respectively).</p><p>Finally, we have studied pairwise interactions in the crystal structure of 4 and compared them with those of 1, 2 and Highly-accurate MP2/CBS interaction energy (DE) values are summarized in Table 3 (see also Fig. 4). The AÁ Á ÁB motif is the most stable motif of 4. The motif can be described as two symmetrical P 2 Á Á ÁI Pn-bonds. Since the Pn-bond is quite long (3.8 Å), it could also be seen as a nonspecific dispersion contact with an optimal dipole-dipole arrangement. The DE value of À9.5 kcal mol À1 is very large, considerably more negative than the most negative bonding motifs of 1 and 2 (the DE of about À6.0 kcal mol À1 ), which also had two Pn-bonds. However, considering that the AÁ Á ÁB motif of 4 has two Pn-bonds, they are weaker than the Pn-bonds of 3. Compound 3 has a binding motif that is exclusively formed by a single Pn-bond and has the DE value of À6.5 kcal mol À1 . 6 The AÁ Á ÁC motif, formed by two symmetrical C-HÁ Á ÁI-B H-bonds, has DE of À6.5 kcal mol À1 . The H-bond is again rather long (3.2 Å). The third most stable motif is AÁ Á ÁD with DE of À6.1 kcal mol À1 . It is formed by the C-HÁ Á ÁH-B dihydrogen bond and the C-HÁ Á ÁI-B H-bond. The C-HÁ Á ÁH-B diH-bond is the only contact in the crystal structure of 4 with the length below the sum of van der Waals radii (2.9 Å between the H and Fig. 2 The molecular structure (ORTEP 50% probability level) and atomic numbering of 4. The selected interatomic distances [Å] are: I1-B10 2.184(2), Cl1-B3 1.786(2), B2-C11 1.730(3), B2-P7 2.053(2), B3-P8 2.1117(19), B3-P7 2.113(2), B4-C9 1.732(2), B4-P8 2.0516(18), B5-C9 1.715(3), B6-C11 1.715(2), C9-B10 1.608(2), C9-P8 1.8681(17), C9-H9 0.91(3), B10-C11 1.604(3), C11-P7 1.8706(19) and P8-P7 2.1970 (6). For all the distances and other structural characteristics, see Table S2 (ESI †).   the center of the B-H bond; HÁ Á ÁH and HÁ Á ÁB distances of 2.3 and 3.0 Å, respectively). The AÁ Á ÁE motif also has all contacts much higher than the sum of the van der Waals radii (e.g. the P 2 Á Á ÁCl Pn bond of 3.9 Å). In summary, compound 4 (3-Cl-10-I-nido-7,8,9,11-P 2 C 2 B 7 H 7 ) was prepared by reacting 3-Cl-nido-7,8,9,11-P 2 C 2 B 7 H 8 with elemental iodine in the presence of aluminum trichloride. Its molecular structure has been characterized by NMR spectroscopy, X-ray crystallography and ab initio computations. Apart from B (10) there is a good accord between the experimental and computed 11 B chemical shifts. It is the spin-orbit coupling that is responsible for such a disagreement related to B (10). The combination of Pn-, diH and H-bonds turned out to be a driving force for the crystal packing as revealed by quantum chemical computations.</p><!><p>There are no conflicts to declare. Fig. 4 The most significant interaction motifs from the crystal structure of 3-Cl-10-I-nido-7,8,9,11-P 2 C 2 B 7 H 7 (4). The distances are in Å. The positions of the H atoms have been optimized at the DFT-D3/BLYP/DZVP level. 10</p>