SelfAssembly of Triphenylboroxine and the Phenylboronic ...rtqsupramolecular.com.mx/wp-content/uploads/2017/... · DOI: 10.1002/ejic.201501121 Full Paper Boron–Nitrogen Adducts

DOI: 10.1002/ejic.201501121 Full Paper

Boron–Nitrogen Adducts

Self-Assembly of Triphenylboroxine and the PhenylboronicEster of Pentaerythritol with Piperazine, trans-1,4-Diaminocyclohexane, and 4-AminopyridineJorge Cruz-Huerta,[a] Gonzalo Campillo-Alvarado,[a] Herbert Höpfl*,[a]

Patricia Rodríguez-Cuamatzi,[a] Viviana Reyes-Márquez,[a] Jorge Guerrero-Álvarez,[a]

Domingo Salazar-Mendoza,*[b] and Norberto Farfán-García[c]

Abstract: The dinuclear phenylboronic ester derived frompentaerythritol and trinuclear triphenylboroxine were com-bined with three diamine tectons, that is, 1,4-diazacyclohexane(pz), trans-1,4-diaminocyclohexane (1,4-chda), and 4-amino-pyridine (4-apy), to generate supramolecular N→B bound as-semblies and to enhance the knowledge concerning the factorsgoverning the formation of such aggregates. From thesereactions, three novel complexes of composition{(PhBO)3(pz)}n·nDMF (2), {[(PhBO)3]2(1,4-chda)}·1,4-chda (3), and{[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O (4) were achieved andcharacterized by elemental analysis, IR and NMR spectroscopy,

IntroductionBoronic acids are organoboron derivatives of boric acid of com-position RB(OH)2 (with R = alkyl, aryl) with applications in or-ganic synthesis,[1] ion and molecular recognition,[2] pharmaceu-tical sciences,[3] and chromatography.[4] More recently, theyhave also been applied as building blocks for crystal engineer-ing[5] and the generation of supramolecular architectures in-cluding macrocycles,[6] cages,[7] rotaxanes,[8] and polymers (1D,2D, and 3D)[9] with potential utility in diverse fields such asseparation, storage, sensing, sorption, and catalysis. Boronicacids undergo two key reactions: one, dehydration to boroxineswith six-membered B3O3 rings; two, condensation with alco-hols, particularly diols, to give boronic esters.[10]

Boroxines have applications in organic synthesis,[11] electro-chemistry,[12] polymer chemistry,[13] and materials science.[14] Inthe absence of electron-donating substituents, boroxines are

[a] Centro de Investigaciones Químicas, Instituto de Investigación en CienciasBásicas e Aplicadas, Universidad Autónoma del Estado de Morelos,Av. Universidad 1001, Chamilpa, Cuernavaca 62209, Morelos, MéxicoE-mail: [email protected]

[b] Universidad Tecnológica de la Mixteca,Carretera a Acatlima Km 2.5, Huajuapan de León 69000, Oaxaca, MéxicoE-mail: [email protected]

[c] Facultad de Química, Departamento Química Orgánica,Universidad Nacional Autónoma de México, Cd. Universitaria,Coyoacán, México 04510, MéxicoSupporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/ejic.201501121.

Eur. J. Inorg. Chem. 2016, 355–365 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim355

and single-crystal X-ray diffraction analysis. Structural character-ization in the solid state revealed that all three products exhibitN→B bonds but have different compositions concerning the Band N tectons to give a 1:1 adduct for 2, a 2:1 adduct for 3, anda 1:2 adduct for 4. In the solid state, compound 2 comprises a1D coordination polymer, whereas compounds 3 and 4 havediscrete molecular structures. Owing to the presence of N–Hhydrogen-bonding sites, in all cases overall 2D or 3D hydrogen-bonding networks are formed. In solution, the N→B aggregatesare mostly dissociated at room temperature, as shown by11B NMR spectroscopy.

enthalpically disfavored, as shown by NMR spectroscopy analy-sis and theoretical calculations.[15] However, N-donor ligandsforming 1:1 adducts with arylboroxines stabilize the B3O3 entitytowards ring opening and hydrolysis to the correspondingboronic acid and enable the formation of boroxines even undermild reaction conditions.[10f,10j,15b,15c] This stabilization has beenattributed to the relief of ring strain upon the geometry changeof one boron atom from trigonal planar to tetrahedral, whichat the same time diminishes the reactivity of the boroxine ringtowards hydrolysis.[15b,15c] Thus, in combination with N-donorligands boroxines are attractive building blocks for supra-molecular chemistry.[16]

Owing to the stabilization of the trigonal boron atomsthrough Bπ–Oπ bonding, boroxine rings tend to adopt a planarconformation unless steric factors induce distortions.[10f,10j] Con-cerning adducts with Lewis bases, the predominant compoundsin the present literature are 1:1 complexes formed from arylbor-oxines and N-donor ligands having 1:1 stoichiometry.[17–26]

On the contrary, structures exhibiting 1:2 boroxine/ligand stoi-chiometry are rare.[21,25,27,28] Additionally, there is a limitednumber of ortho-substituted phenylboroxines, in which up tothree intramolecular N→B bonds are formed through chelatering formation.[29–33]

Boronic esters are easily accessible condensation productsderived from diols and alkyl- or arylboronic acids that readilyform dative bonds with N-donor ligands. Starting from relativelysimple reagents, bridging through N→B bond formation be-

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Scheme 1. Synthesis of coordination and hydrogen-bonded polymers by using triphenylboroxine (1a) and the phenylboronic ester of pentaerythritol (1b) asbuilding blocks in combination with representative dinuclear N-containing ligands.

tween boronic esters has successfully been used for the con-struction of tweezer-type complexes,[34] macrocycles,[35]

cages,[36] rotaxanes,[37] polymers,[38] gels,[39] and covalent or-ganic framework type structures.[40]

In both boroxines and boronic esters, N→B bond formationresults in a structural change in the geometry of boron fromtrigonal-planar to tetrahedral.[41] The strength of the N→B inter-action depends on the steric and electronic characteristics ofthe reaction partners and in solution additionally on the sol-vent.[38c]

In recent publications, we showed that intermolecular N→Bbond formation with boronic esters and boroxines can achievetweezer-type, macrocyclic, and cage-like assemblies as well as1D, 2D, and 3D molecular networks.[16,34b,38e,40b] Herein, we re-port on the formation of three supramolecular polymers thatwere achieved from two different boron-containing buildingblocks, that is, triphenylboroxine {(PhBO)3} (1a) and the phenyl-boronic ester of pentaerythritol, {(PhBO2)2(C5H8)} (1b), in combi-nation with the dinuclear N-containing ligands 1,4-diazacyclo-hexane (pz), trans-1,4-diaminocyclohexane (1,4-chda), and 4-aminopyridine (4-apy) (Scheme 1).

Results and Discussion

1D Coordination Polymer Derived from Triphenylboroxine(1a) and Piperazine

Compound 2 was prepared by heating a 3:1 stoichiometric so-lution of phenylboronic acid and 1,4-diazacyclohexane (piper-azine, pz) in dimethylformamide under reflux with the use of aDean Stark trap to remove the water formed during boroxineformation. Cooling the reaction mixture slowly to room temper-

Eur. J. Inorg. Chem. 2016, 355–365 www.eurjic.org © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim356

ature afforded colorless crystals of the composition{(PhBO)3(pz)}n·nDMF in 44 % yield. The same product was iso-lated upon using a 3:1.5 stoichiometric ratio.

Compound 2 crystallizes in the monoclinic space group P21/c, and single-crystal X-ray diffraction analysis revealed an inter-esting 1D polymeric assembly, in which each triphenylboroxine(1a) is bound to two piperazine nitrogen atoms through coordi-nate covalent N→B bonds (Figure 1, a, Table 1). In other words,two of the three boroxine boron atoms are involved in coordi-

Figure 1. As shown by single-crystal X-ray diffraction (SXRD) analysis, in com-bination with piperazine, the triphenylboroxine (1a) tecton results in a N→Bbound adduct of the composition {(PhBO)3(pz)}n·nDMF (2) (a) to give 1D poly-mer chains along the a axis (b). For clarity, in panel b the hydrogen atomsare omitted. Thermal ellipsoids are drawn at the 50 % probability level.

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Table 1. Crystallographic data for compounds 1b, 2, 3, and 4.

1b 2 3 4

Formula C17H18B2O4 C22H25B3N2O3·DMF C42H44B6N2O6·C6H14N2 4(C27H30B2N4O4)·4CHCl3·5H2OFW 307.93 470.97 851.84 2552.23Crystal system trigonal monoclinic triclinic triclinicSpace group P3221 P21/c P1̄ P1̄T [K] 100(2) 100(2) 100(2) 100(2)a [Å] 6.2237(6) 11.9873(15) 9.3976(14) 10.2620(13)b [Å] 6.2237(6) 11.9543(14) 10.5853(16) 11.5619(14)c [Å] 34.411(5) 17.153(2) 13.427(2) 27.233(3)α [°] 90 90 107.184(3) 81.225(2)� [°] 90 94.939(2) 100.850(3) 79.717(2)γ [°] 120 90 103.247(3) 75.102(2)V [Å3] 1154.3(2) 2448.9(5) 1194.1(3) 3052.9(7)Z 3 4 1 1μ [mm–1] 0.091 0.084 0.075 0.346ρcalcd. [g cm–3] 1.329 1.277 1.185 1.388Reflns. collected 12836 23784 8909 29450Reflns. ind. (Rint) 1051 (0.045) 4346 (0.054) 4174 (0.041) 10688 (0.049)Reflns. observed 1024 3522 3398 8674R1 [I > 2σ(I)] 0.0516 0.0675 0.0853 0.0757wR2 (all data) 0.1152 0.1688 0.1561 0.1717

nation with the nitrogen donor ligand, which introduces chiralboron atoms into the system. Previous theoretical calculationsshowed that the Lewis acidity of the boron atoms in boroxinering systems decreases with an increasing number of coordina-tive ligands.[15b,15c] This is in agreement with the still relativelylow number of 1:2 boroxine/nitrogen donor ligand adductsknown so far, of which the majority are ortho-substitutedphenylboroxines with intramolecular N→B bonds owing to che-late ring formation.[21,25,27–33] The computational study furthershowed that 1:2 adducts having anti configuration are morestable than those with syn configuration.[15b] Therefore, it wasinteresting to find that compound 2 exhibits the less stable syn-oriented coordination of the N ligand in the solid state (Fig-ure 1, a). However, the energy required for stabilization of thisconfiguration might be compensated by energy gain resultingfrom more compact packing of the polymeric chains.

In 2, the four-coordinate boron atoms have distorted tetra-hedral geometries with bond angles ranging from 101.5(2) to115.3(3)°. Of these, the largest values correspond to the O–B–Oangles within the boroxine ring, and as a consequence, the O–B–N bond angles are reduced and adopt the smallest values(Table 2). The N→B bond lengths are 1.681(4) and 1.688(4) Å,and the values for the tetrahedral character (THC)[41] of B1 andB2 are 70 and 72 %, respectively. The long N→B distances andthe relatively small THC values provide experimental evidencefor the decreased interaction energy between the boron andnitrogen atoms. For comparison, the THC values and N→B bondlengths for monoadducts of composition {(ArBO)3L} with L =aliphatic or aromatic N-donor ligand range from 72 to 82 %and from 1.61 to 1.67 Å, respectively.[17–26] For the previouslydescribed 1:2 adducts {[PhBO]3·2L} with L = hexamethylene-tetramine[25] and 2L = CpRu(1,3,5-triaza-7-phosphaadamant-ane)Cl (Cp = cyclopentadienyl),[27] the corresponding THC anddN→B values are 70 % and 1.72–1.74 Å, respectively.

Despite the twofold N→B syn coordination of the boroxine,the B3O3 ring is only slightly distorted from planarity, as seen


from the BOBO torsion angles having values in the range of–7.2(4) to +7.2(4)° (Table 2). Additionally, the Bsp2 phenyl ring issignificantly rotated out of the B3O3 mean plane (Figure 1, a),as indicated by the angle of 13.8° formed between the meanplanes of B3O3 and the respective B-phenyl group. N→B adductformation also alternates the B–O bonding scheme, which wasanalyzed in a comparative manner in the section below on thestructural description of compound 3.

The 1D polymer chains in 2 running parallel to the a axishave a 21 helical conformation (Figure 1, b) with B···B separa-tions of 6.2 Å. Interestingly, between neighboring chains sec-ondary intermolecular interactions other than van der Waalscontacts were not detected. However, as a result of the tweezer-type structure of the central boroxine complex, the crystalstructure exhibits cavities formed between neighboring

Figure 2. In the crystal structure of {(PhBO)3(pz)}n·nDMF (2), neighboring{(PhBO)3pz} moieties form cavities that are filled by DMF solvate molecules.For clarity, some of the hydrogen atoms are omitted. Symmetry operator:2 – x, 2 – y, 1 – z.

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{(PhBO)3pz} moieties, which are occupied by DMF solvate mol-ecules. The inclusion of DMF stabilizes the crystal structurethrough bifurcated N–H···O=C interactions formed between theNH hydrogen atoms of neighboring piperazine molecules andthe DMF carbonyl group. Additionally, the DMF solvates exhibita number of weaker C–H···π and C–H···O contacts with nearbyboroxines (Figure 2; see also Table S1, Supporting Information).

Table 2. Selected geometric parameters for compounds 2 and 3.

2 3

Bond lengths [Å]

B1–O1 1.429(4) 1.464(4)B1–O3 1.466(4) 1.478(4)B2–O1 1.423(4) 1.346(4)B2–O2 1.467(4) 1.393(4)B3–O2 1.357(4) 1.397(4)B3–O3 1.355(4) 1.338(4)B1–C1 1.617(5) 1.620(4)B2–C7 1.623(5) 1.565(5)B3–C13 1.585(5) 1.567(5)B1–N1 1.681(4) 1.621(4)B2–N2 1.688(4) –

Bond angles [°]

B1–O1–B2 124.0(3) 121.8(2)B2–O2–B3 121.0(3) 119.4(3)B1–O3–B3 121.2(3) 121.5(3)O1–B1–O3 115.1(3) 112.4(3)O1–B1–N1 104.1(2) 107.5(2)O1–B1–C1 114.9(3) 112.2(3)O3–B1–N1 102.5(2) 103.5(2)O3–B1–C1 111.2(3) 110.7(2)N1–B1–C1 107.7(2) 110.1(3)O1–B2–O2 115.3(3) 121.0(3)O1–B2–N2 104.5(2) –O1–B2–C7 113.5(3) 121.4(3)O2–B2–N2 101.5(2) –O2–B2–C7 111.4(3) 117.6(3)N2–B2–C7 109.4(2) –O2–B3–O3 123.0(3) 121.1(3)O2–B3–C13 118.7(3) 117.0(3)O3–B3–C13 118.3(3) 121.8(3)

Torsion angles [°]

B1–O1–B2–O2 –7.2(4) +10.1(4)B2–O1–B1–O3 +7.2(4) –17.8(4)B2–O2–B3–O3 –2.9(5) +2.0(4)B3–O2–B2–O1 +4.7(4) –1.3(4)B1–O3–B3–O2 +3.0(5) –11.4(4)B3–O3–B1–O1 –4.8(4) +18.5(4)

Other

B···B [Å] 6.2 8.1THC BT [%] 70 83

72

2D Hydrogen-Bonded Polymer Derived fromTriphenylboroxine (1a) and trans-1,4-Cyclohexanediamine

Compound 3 was obtained by heating a 3:1 stoichiometric solu-tion of phenylboronic acid and trans-1,4-cyclohexanediamine(1,4-chda) in a solvent mixture of benzene/absolute ethanol(3:1 v/v) to reflux with the use of a Dean–Stark trap. After cool-ing the reaction mixture slowly to room temperature, colorless


crystals of the composition {[(PhBO)3]2(1,4-chda)}·1,4-chda (3)were afforded in 56 % yield.

Compound 3 crystallizes in the triclinic space group P1̄(Table 1). Although the boroxine and diamine ligand in the solidstate exhibit the same 1:1 stoichiometry as that in compound2, the structural organization is quite different. Contrary to ad-duct 2, in which the boroxine and diamine tectons provide a1D coordination polymer through N→B bond formation involv-ing two of the three boroxine boron atoms (Figure 1, b), incompound 3 only half the diamine ligands participate in com-plex formation with the boroxine tectons, whereas the secondhalf remains noncoordinated. Thus, discrete complex moleculesconsisting of two boroxine units and one diamine ligand areformed (Figure 3). Structurally related 2:1 boroxine/diamine li-gand adducts of the composition {[PhBO]3L[PhBO]3} were previ-ously reported with L = p-phenylenediamine,[42] 1,4-diazabi-cyclo[2.2.2]octane,[42] and hexamethylenetetramine.[25]

In the crystal structure, the 2:1 adducts are located aroundcrystallographic inversion centers and adopt a molecular struc-ture with anti configuration (Figure 3). The four-coordinateboron atoms have distorted tetrahedral coordination geome-tries with a THC[41] of 83 % and a N→B bond length of1.621(4) Å. The bond angles around the tetrahedral boronatoms range from 103.5(2) to 112.4(3)°. The B···B distance withinthe 2:1 adduct is 8.1 Å. As a result of the approximate trigonal-planar coordination environment of the remaining two boronatoms, the six-membered B3O3 boroxine ring is distorted signifi-cantly from planarity, as seen from the BOBO torsion angleswith values in the range of –17.8(4) to +18.5(4)° (Table 2), andit adopts an overall envelope conformation, in which the tetra-hedral boron atom is displaced by 0.21 Å from the mean planeformed by the sp2-hybridized boron and oxygen atoms. As pre-viously observed for other monoadducts of triphenylboroxine(1a) with amine ligands,[17–26] N→B bond formation increasesthe O–Bsp3 bond lengths in compound 3, 1.464(4) and1.478(4) Å, and the O–Bsp3–O bond angle deviates significantlyfrom the ideal value of 120°, that is, 112.4(3)°. In addition to thedistortion of the boroxine ring from an ideal planar hexagon,the pendant Bsp2 aryl rings are not coplanar with the B3O3 skele-ton, as seen from the angles formed between the boroxine andB-phenyl ring planes (15.5 and 24.4°). Despite this distortion,the corresponding Bsp2–C bonds still exhibit significant pπ–pπ

bond character, as seen from a comparison with the B–C bondsformed to the sp3-hybridized boron atoms: 1.565(5)–1.567(5) Åfor Bsp2–C versus 1.620(4) Å for Bsp3–C. Nevertheless, relativeto noncoordinated triphenylboroxine with a mean distance of1.546 Å,[43] the Bsp2–C bonds in 3 are significantly elongated.Moreover, the Bsp2–O bonds adjacent to the four-coordinateboron atoms are significantly shorter, 1.338(4) and 1.346(4) Å,than the remaining Bsp2–O bonds having values of 1.393(4) and1.397(4) Å. This indicates a stronger Bπ–Oπ bonding interactionwith the oxygen atoms bound to Bsp3, which exhibit only σ

bonds with the tetrahedral boron, as evidenced by the Bsp3–Obond lengths of 1.464(4) and 1.478(4) Å. For compound 2, inwhich only one boroxine boron atom is trigonal planar, theBsp3–O bonds are shorter than those in 3 and the Bsp2–C bondis elongated, probably in response to the weaker N→B coordi-

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Figure 3. As shown by SXRD analysis, in {[(PhBO)3]2(1,4-chda)}·1,4-chda (3) the triphenylboroxine (1a) and trans-1,4-cyclohexanediamine (1,4-chda) tectonsform discrete 2:1 adducts. Thermal ellipsoids are drawn at the 50 % probability level.

nation: 1.423(4)–1.467(4) Å for Bsp3–O and 1.585(5) Å for Bsp2–C.On the contrary, the Bsp2–O and Bsp3–C bonds show less signifi-cant variations (Table 2).

The analysis of the supramolecular context shows that the[(PhBO)3][1,4-chda][(PhBO)3] adducts are further assembledthrough N–H···N and N–H···O hydrogen bonds with noncoordi-nated 1,4-chda molecules to give 1D hydrogen bonded zigzagchains running parallel to the c axis (Figure 4, a). Neighboringchains are connected through additional N–H···O hydrogen

Figure 4. The crystal structure of {[(PhBO)3]2(1,4-chda)}·1,4-chda (3) contains noncoordinated 1,4-chda ligand molecules that are linked to the 2:1 adductthrough N–H···N and N–H···O hydrogen-bonding interactions to give 1D chains along the c axis (a). Additional N–H···O hydrogen-bonding and N–H···πinteractions provide overall 2D hydrogen-bonded layers running parallel to the bc plane. For clarity, some of the hydrogen atoms are omitted. Symmetryoperators: (I) 1 + x, y, z; (II) 2 – x, 1 – y, 1 – z; (III) 1 – x, 1 – y, –z; (IV) 2 – x, 2 – y, 1 – z.


bonds and N–H···π contacts to provide 2D layers parallel to thebc plane (Figure 4, b). In the third dimension, the structure isstabilized by π···π and van der Waals contacts (Table S1).

3D Hydrogen-Bonded Polymer Derived from 1b and 4-Aminopyridine

Compound 4 is a 1:2 adduct composed of the dinuclear phenyl-boronic ester of pentaerythritol (1b) and 4-aminopyridine (4-

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apy); it was first obtained by reaction of 1b with the ligand(1 equiv.). Further experiments following previously establishedprotocols for multicomponent reactions among arylboronicacids, a diol, and a nitrogen-containing ligand showed that thecomplex could also be formed by a three-component assemblyreaction directly from the starting materials.[6g,36b,36d,38b] Thus,upon heating a 2:1:2 stoichiometric solution of phenylboronicacid, pentaerythritol, and 4-aminopyridine in a solvent mixtureof toluene/absolute ethanol (3:1 v/v) under reflux with the useof a Dean Stark trap, a colorless solid was afforded, which afterrecrystallization from a solvent mixture of chloroform/hexane(1:1 v/v) provided crystals suitable for single-crystal X-ray dif-fraction analysis (Table 1).

Structure analysis revealed that compound 4 crystallizes inthe triclinic space group P1̄ as a chloroform/water solvate ofcomposition {[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O. Theasymmetric unit comprises two crystallographically independ-ent molecules of the 1:2 adduct [PhBO2C5H8O2BPh][4-apy]2, twochloroform molecules, and three water molecules, one of whichis located at a special position (occupancy: 0.5). In the boronadduct, the ligands are coordinated through N→B bonds bythe more Lewis basic pyridine nitrogen atom (Figure 5). Thefour-coordinate boron atoms have distorted tetrahedral coordi-nation geometries with bond angles varying from 105.5(3) to115.3(3)° to give tetrahedral characters in the range of 83 to86 %. Of these, the largest bond angles correspond to the O–B–O fragment of the boronic ester and the smallest to the O–B–N bond angles (Table 3). The N→B bond lengths vary from1.634(5) to 1.665(5) Å. The two crystallographically independentmolecules have anti configuration, and the intramolecular B···Bdistances are 5.44 and 5.45 Å. For the sake of comparison, themolecular structure of starting tecton 1b was also analyzed bysingle-crystal X-ray diffraction analysis (Table 1). Comparativeanalysis showed that the angle formed between the centroids

Figure 5. As shown by SXRD analysis, the assembly reaction of dinucleartecton 1b and 4-aminopyridine provided a 1:2 adduct of the composition{[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O (4) with two crystallographically in-dependent molecules in the asymmetric unit. Thermal ellipsoids are drawnat the 50 % probability level.


of the B-phenyl rings and the central spiro C2 atom changesfrom 127° (in 1b) to 121° (in 4) upon N→B coordination (Fig-ure 6, Table 3). Interestingly, with exception of the macrocyclic

Table 3. Selected geometric parameters for compounds 1b and 4.

1b 4Molecule A Molecule B

Bond lengths [Å]

B–O 1.356(4) 1.442(5) 1.448(5)1.364(4) 1.449(5) 1.459(5)

1.459(5) 1.462(5)1.460(5) 1.469(5)

B–C 1.569(4) 1.612(6) 1.610(6)1.615(6) 1.616(6)

B–N – 1.658(5) 1.634(5)1.665(5) 1.650(5)

Bond angles [°]

O–B–O 123.6(3) 115.0(3) 114.0(3)115.3(3) 114.1(3)

O–B–C 117.8(3) 108.5(3) 108.4(3)118.5(2) 109.8(3) 109.5(3)

110.8(3) 111.2(3)111.2(3) 111.5(3)

O–B–N – 105.5(3) 105.6(3)106.2(3) 106.0(3)106.5(3) 106.6(3)106.9(3) 107.5(3)

N–B–C – 108.6(3) 109.3(3)108.8(3) 109.6(3)

Other [Å (°),%]

B···B [Å] 5.16 5.44 5.45THC BT [%] – 83 85

84 86Phcent–C2–Phcent

[a] [°] 127 121.5 120.8

[a] cent = centroid.

Figure 6. Comparison of the molecular structures of dinuclear tecton 1b ifnoncoordinated (a) and within the 1:2 adduct formed with 4-aminopyridine(b). For clarity, in compound 4 part of the atoms of the N-tectons are omitted.Thermal ellipsoids are drawn at the 50 % probability level.

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and polymeric assemblies reported by the Severingroup,[36c,36d,37,38b,38d,40a] to date there are only a few reports onintermolecular adducts of catecholboronic esters with N-donorligands.[36h,44]

Although N→B bond formation through both nitrogen at-oms of the ditopic 4-aminopyridine ligand could not beachieved, the complex molecules assembled by means of N–H···O hydrogen bonds to 24-membered tetranuclear macro-cyclic dimers within the crystal structure involving the noncoor-dinated dimethylamino groups of the ligand and part of theoxygen atoms of the boronic ester. Interestingly, these dimersare formed between pairs of the crystallographically independ-ent adduct molecules (AA and BB). Within these macrocycles,the B···B distances of 5.4 and 8.8 Å are identical, but there areslight differences in the values of the B···B···B angles (65.7 and114.3° for AA; 66.8 and 113.2° for BB), which indicate parallelo-gram-shaped polygons. Additional N–H···O hydrogen bondsformed between AA and BB pairs of macrocycles give rise to1D chains. Interestingly, one of the four NH2 groups (N2) partici-pates additionally in a N–H···π interaction (Figure 7, a). Neigh-boring chains are connected to each other through hydrogenbonds formed with the crystal lattice water molecules to pro-vide in a first instance 2D hydrogen-bonded layers parallel to(024). The water molecules exhibit various types of hydrogen-bonding interactions, including N–H···Ow, Ow–H···Ow, Ow–H···OB, and Ow–H···π interactions (Figure 7, b). The 2D hydrogenbonded layers are further interconnected by N–H···Ow hydro-gen bonds and a series of C–H···O and C–H···π contacts to givean overall 3D hydrogen-bonded network (Table S1).

The cavities within the 2D layers given in Figure 7 (b) arefilled by the chloroform solvate, which further stabilizes thecrystal structure through a series of interesting intermolecularinteractions. As seen from Figure 8, one CHCl3 molecule (C91)

Figure 8. The crystal structure of {[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O exhibits interesting secondary interactions of the C–H···Cl, C–H···π, and Cl···π typethat are formed between the 1:2 adducts and the chloroform solvate molecules, which stabilizes the structure in the third dimension. Symmetry operators:(I) –x, 1 – y, 1 – z; (II) x, 1 + y, z; (III) 1 + x, 1 + y, z; (IV) 1 + x, y, z.


is embedded in two C–H···Cl interactions and C–H···π contactswith a neighboring B-phenyl group (CH···centroid 3.32 Å). Thesecond CHCl3 molecule (C92) exhibits a single C–H···Cl interac-tion and C–H···π contacts comparable to that of the first CHCl3

Figure 7. In the crystal structure of {[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O(4), the 1:2 adducts form 24-membered N–H···O hydrogen-bonded macro-cycles to give 1D chains (a) that are connected to each other through hydro-gen bonds formed with the crystal lattice water molecules to provide 2Dhydrogen-bonded layers parallel to (024) (b). For clarity, some of the hydro-gen atoms are omitted. Symmetry operators: (I) x, –1 + y, z; (II) –x, 1 – y, –z;(III) 1 – x, –y, 1 – z.

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molecule (CH···centroid 3.26 Å), and additionally, two of thethree chlorine atoms of the chloroform molecule are involvedin Cl···π interactions with a B-phenyl group and a pyridyl ringof one of the ligands. The corresponding Cl···centroid distancesare 3.27 and 3.80 Å.

Solution NMR Spectroscopy Analysis

Previous variable-temperature NMR spectroscopy studies on 1:1adducts of arylboroxines and amines have evidenced fast N-donor ligand-exchange processes at room temperature; this fre-quently gives only a single signal by 11B NMR spectroscopyanalysis, which is sensitive to the coordination environment ofboron atoms.[15b,17,19,25,42] Boronic esters and boroxines withsp2-hybridized boron atoms give signals at approximately δ =30 ppm, whereas N→B bound complexes with sp3-hybridizedboron atoms are observed in the range of δ = 0 to 10 ppm.[45]

To analyze if the N→B bonds in the boron adducts examinedherein are stable in solution, the 1H NMR and 11B NMR spectrawere recorded for solutions of compounds 2, 3, and 4 in CDCl3.However, compounds 2–4 exhibited only relatively low solubil-ity, which inhibited low-temperature NMR spectroscopy studies.In the 1H NMR and 11B NMR spectra, only a single set of signalswas observed, and the only small highfield shifts in the 11B NMRspectra (2, δ = 29 ppm; 3, δ = 29 ppm; 4, δ = 27 ppm) withrespect to tectons 1a (δ = 30 ppm) and 1b (δ = 29 ppm) indi-cate that the adducts are mostly dissociated in solution. In com-parison to the abovementioned previous studies, the chemicalshift displacements are smaller than expected, which might beattributed to the dilute solution conditions for the NMR spec-troscopy analysis of compounds 2–4.

Conclusions

The reactions examined herein with the use of di- and trinuclearboron compounds and representative dinuclear amineswidened the horizon of boronic acid self-assembly throughN→B bonds and provided a total of three novel adducts exhib-iting a variety of different stoichiometric compositions betweenthe assembling B and N tectons (i.e., 3:1, 1:1, and 1:2). Thiscan be attributed to several factors: (1) Although N→B bondcoordination seems to be dominant in all systems, in the caseof triphenylboroxine the maximum number of N→B bonds isnot obtained owing to the close proximity of the boron atoms,which reduces the dimension of the final product. (2) Onlystrong N-Lewis bases such as primary and secondary aliphaticamines and pyridines coordinate to the boronic ester boronatoms. The coordination of amino groups attached to aryl ringsdoes not seem to be strong enough to achieve N→B bondformation for the systems explored herein. (3) Amine tectonsderived from primary amines are strongly susceptible to formhydrogen-bonding interactions, which thus play an importantrole in the resulting supramolecular structure. Given that theseinteractions can compete with N→B bond formation, supramo-lecular structures containing both coordinated and noncoordi-nated N tectons are feasible, as observed for {[(PhBO)3]2(1,4-chda)}·1,4-chda (1,4-chda = trans-1,4-diaminocyclohexane).


Nevertheless, these hydrogen-bonding interactions enhancethe dimension of the intermolecular connectivity, which pro-vides opportunities for crystal engineering.

Experimental SectionInstrumentation: Elemental analyses were performed on samplesdried previously in an Abderhalden apparatus by using FisonsInstruments EA-1108 and Elementar Vario ELIII equipment. IR spec-tra were recorded with Bruker Vector 22 and Nicolet 6700 FT spec-trophotometers and were measured in the range of 4000 to400 cm–1 by using the KBr pellet technique. NMR spectra were re-corded with Varian Gemini 200 and Varian Inova 400 spectrometers.Chemical shifts are reported in ppm relative to tetramethylsilane(1H, 13C) and BF3·OEt2 (11B).

Preparative Methods: Phenylboronic acid, pentaerythritol, 1,4-di-azacyclohexane (piperazine), trans-1,4-cyclohexanediamine, and 4-aminopyridine were commercially available from Sigma–Aldrich. Allstarting materials and solvents were used as received without fur-ther purification. All preparative methods were performed in airwithout the use of an inert atmosphere.

{(PhBO2)2(C5H8)} (1b): Phenylboronic acid (0.100 g, 0.820 mmol)and pentaerythritol (0.055 g, 0.41 mmol) were dissolved in ethanol/toluene (1:3 v/v, 20 mL). After heating the mixture under reflux withthe use of a Dean Stark trap for 3 h, the resulting solution wasconcentrated to a volume of approximately 5 mL. Cooling the reac-tion mixture slowly to room temperature afforded a colorless pre-cipitate, which was washed with water and acetone, yield 0.086 g(68 %), m.p. 222–223 °C. 1H NMR (200 MHz, CDCl3, 20 °C): δ = 3.98(s, 8 H, -CH2-), 7.36 (m, 6 H, BC6H5-Hmeta, BC6H5-Hpara), 7.75 (m, 4 H,BC6H5-Hortho) ppm. 11B NMR (128 MHz, CDCl3, 20 °C): δ = 29 ppm.IR (KBr): ν̃ = 3075 (w), 3059 (w), 2967 (w), 2928 (w), 2902 (w), 1600(m), 1482 (s), 1443 (m), 1413 (m), 1320 (s), 1269 (s), 1189 (m), 1151(w), 1097 (m), 1024 (w), 931 (w), 765 (m), 702 (m), 650 (m), 536 (w),453 (w) cm–1. MS (FAB+): m/z (%) = 309 (20) [M + H]+, 154 (100).C17H18B2O4 (307.94): calcd. C 66.30, H 5.89; found C 67.18, H 5.86.

{(PhBO)3(pz)}n·nDMF (2): Phenylboronic acid (0.050 g, 0.41 mmol)and 1,4-diazacyclohexane (0.012 g, 0.14 mmol) were dissolved inDMF (25 mL). After heating the mixture under reflux with the useof a Dean Stark trap for 3 h, the resulting solution was concentratedto a volume of approximately 10 mL. Cooling the reaction mixtureslowly to room temperature afforded 2 as a colorless crystallinesolid, yield 0.028 g (44 %), m.p. 147–154 °C (decomp.). 1H NMR(200 MHz, CDCl3, 20 °C): δ = 3.02 (s, 8 H, -NCH2-), 7.40 (m, 9 H,BC6H5-Hmeta, BC6H5-Hpara), 8.00 (d, 6 H, BC6H5-Hortho) ppm. 11B NMR(128 MHz, CDCl3, 20 °C): δ = 29 ppm (h1/2 = 170 Hz). IR (KBr): ν̃ =3156 (m), 1643 (s), 1441 (m), 1362 (s), 1310 (m), 1224 (m), 1169 (m),1082 (m), 971 (w), 899 (w), 817 (m), 729 (m), 703 (m), 618 (m)cm–1. C22H25B3N2O3 (397.88): calcd. C 66.41, H 6.33, N 7.04; foundC 66.13, H 6.31, N 7.07.

{[(PhBO)3]2(1,4-chda)}·1,4-chda (3): Phenylboronic acid (0.100 g,0.82 mmol) and trans-1,4-diaminocyclohexane (0.031 g, 0.27 mmol)were dissolved in benzene/absolute ethanol (3:1 v/v, 30 mL). Afterheating the mixture under reflux with the use of a Dean–Stark trapfor 3 h, the resulting solution was concentrated to a volume ofapproximately 5 mL. Cooling the reaction mixture slowly to roomtemperature afforded 3 as a colorless crystalline solid, yield 0.065 g(56 %), m.p. 129–132 °C (decomp.). 1H NMR (400 MHz, CDCl3, 20 °C):δ = 1.18 (m, 4 H, -CH2-), 1.97 (m, 4 H, -CH2-), 2.78 (d, 2 H, -CH-), 7.38(m, 9 H, m-Ph, p-Ph), 7.90 (d, 6 H, o-Ph) ppm. 11B NMR (128 MHz,CDCl3, 20 °C): δ = 29 ppm (h1/2 = 150 Hz). IR (KBr): ν̃ = 3438 (w),

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2928 (w), 2856 (w), 1652 (w), 1573 (m), 1500 (m), 1460 (m), 1357(s), 1245 (m), 1115 (m), 1013 (m), 913 (m), 815 (m), 768 (m), 701 (s),605 (m) cm–1. C48H58B6N4O6 (851.86): calcd. C 67.68, H 6.86, N 6.58;found C 67.21, H 6.81, N 6.69.

{[(PhBO2)2(C5H8)][4-apy]2}·CHCl3·1.25H2O (4): Phenylboronic acid(0.050 g, 0.41 mmol), pentaerythritol (0.028 g, 0.21 mmol), and 4-aminopyridine (0.039 g, 0.41 mmol) were dissolved in toluene/abso-lute ethanol (3:1 v/v, 20 mL). After heating the mixture under refluxwith the use of a Dean Stark trap for 3 h, the resulting solutionwas concentrated to a volume of approximately 5 mL. Cooling thereaction mixture slowly to room temperature afforded a colorlessprecipitate, which was washed with water and acetone. Crystalssuitable for single-crystal X-ray diffraction analysis were grown fromchloroform/hexane (1:1 v/v), yield 0.062 g (61 %), m.p. 170–175 °C.1H NMR (200 MHz, CDCl3, 20 °C): δ = 4.05 (s, 8 H, -CH2-), 6.52 (d, 4H, 4-apy-H3), 7.38 (m, 6 H, BC6H5-Hmeta, BC6H5-Hpara), 7.77 (d, 4 H,BC6H5-Hortho), 8.21 (d, 4 H, 4-apy-H2) ppm. 11B NMR (128 MHz,CDCl3, 20 °C): δ = 27 ppm (h1/2 = 420 Hz). IR (KBr): ν̃ = 3336 (w),3143 (m), 2928 (m), 1637 (s), 1528 (m), 1482 (m), 1440 (m), 1313 (s),1200 (s), 1094 (s), 1064 (s), 1038 (s), 992 (m), 783 (m), 741 (m), 726(m), 700 (m), 645 (m) cm–1. MS (FAB+): m/z (%) = 403 (24) [M + H –4-apy]+, 325 (100). C27H30B2N4O4 (496.17): calcd. C 65.36, H 6.09, N11.29; found C 65.08, H 6.16, N 11.52.

X-ray Crystallography: Single-crystal X-ray diffraction studies wereperformed at T = 100 K with a Bruker-APEX diffractometer equippedwith a CCD area detector (λΜ�Κα = 0.71073 Å, monochromator:graphite). Frames were collected by ω/φ rotation at 10 s per frame(Bruker-SMART).[46a] The measured intensities were reduced to F2

and were corrected for absorption with SADABS (Bruker-SAINT).[46b]

Corrections were made for Lorentz and polarization effects. Struc-ture solution, refinement, and data output were performed with theBruker SHELXTL-NT program package.[46c,46d] Non-hydrogen atomswere refined anisotropically. Hydrogen atoms attached to carbonwere positioned geometrically and were constrained by using theriding model approximation, whereas N–H and O–H hydrogenatoms were located in difference Fourier maps. The O–H and N–Hdistances were fixed at 0.840(1) and 0.860(1) Å, respectively, withUiso(H) = 1.5Ueq(O,N), and the coordinates were refined with theserestraints. Crystals of 1b crystallized in a noncentrosymmetric spacegroup; however, the data were merged owing to the absence ofheavy atoms. The crystal of compound 2, for which data were col-lected, exhibited racemic twinning, which was treated as suggestedby Platon with the TWIN –1 0 0 0 –1 0 0.246 0 1 instruction (BASF =0.0093). The asymmetric unit of compound 4 comprises two crystal-lographically independent molecules of the boron compound, twochloroform molecules, and three water molecules. Of the latter, onewater molecule is located at a crystallographic inversion center andthe hydrogen atoms are disordered over three positions, whichwere refined with occupancies of 0.50, 0.25, and 0.25. Figures werecreated with Diamond.[47] Hydrogen-bonding interactions in thecrystal lattice were calculated with the WINGX program package.[48]

CCDC 1428596 (for 1b), 1428597 (for 2), 1428598 (for 3), 1428599(for 4) contain the supplementary crystallographic data for this pa-per. These data can be obtained free of charge from The CambridgeCrystallographic Data Centre.

Supporting Information (see footnote on the first page of thisarticle): Table with intermolecular interactions and short contactsfor compounds 2–4.


Acknowledgments

This work was supported by Consejo Nacional de Ciencia y Tec-nologia (CONACyT) through project numbers CIAM-59213 andCB-154115. The authors gratefully acknowledge access to Labo-ratorio Nacional de Estructura de Macromoléculas (LANEM).

Keywords: Supramolecular chemistry · Self-assembly ·Boron–nitrogen adducts · Boron

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Received: September 30, 2015Published Online: December 21, 2015

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