Synthesis and Crystallographic Studies of Disubstituted Carboranyl Alcohol Derivatives: Prevailing Chiral Recognition?

Abstract

The syntheses of new o-carboranyldiols bearing aromatic rings bis-[R(hydroxy)methyl]-1,2-dicarba-closo-dodecaborane (R = 2-pyridyl 1a, 3-pyridyl 1b, 4pyridyl 1c, 2-quinolyl 1d, 4-quinolyl 1e, phenyl 1f) are reported. The compounds are obtained as mixtures of meso (syn) and racemic (anti) stereoisomers with a slight diastereomeric excess (syn:anti ratio of 0.7:1) in all cases but in 1b. The crystal structures of the meso compounds syn-1a•2MeOH, syn-1b, syn-1f•0.25H 2 O and racemic anti-1a•MeOH, anti-1a•EtOH, and anti-1d•2H 2 O are reported. We provide an analysis of these compounds by means of NMR and X-ray crystallography, in the context of crystal engineering and chiral recognition. The results show that the crystal packings for these alcohols are dominated by the supramolecular O−H•••N and/or O− H•••O hydrogen bonds. Supramolecular analysis of all compounds in this work reveals that homochiral self-assembly, that is, formation of homochiral hydrogen bonded complexes, prevails over heterochiral self-assembly (formation of heterochiral hydrogen bonded complexes).

Switchable Surface Hydrophobicity–Hydrophilicity of a Metal–Organic Framework

“…Forinstance,Kitagawa et al reported as uperhydrophobic MOF that they obtained by controlling the corrugation of its crystal surface by using aromatic linkers. Specifically,w eu sed an ortho-carborane functionalized with pyridylmethylalcohol groups at the C-positions [22] (oCB-L, Figure 1a)a sahydrophobic linker in combination with Znbdc (bdc = 1,4-benzenedicarboxylate). [20] Among these hydrophobic molecules,c arboranes are an interesting class of exceptionally stable boron-rich clusters that possess material-favorable properties such as thermal and chemical stability.…”

“…[21] We have exploited this class of molecules to synthesize ar esponsive MOF that exhibits switching between hydrophobicity and superhydrophilicity. Specifically,w eu sed an ortho-carborane functionalized with pyridylmethylalcohol groups at the C-positions [22] (oCB-L, Figure 1a)a sahydrophobic linker in combination with Znbdc (bdc = 1,4-benzenedicarboxylate). This switching behavior is achieved by alternatively exposing the MOF to aNaOH/ DMF solution and as lightly acidic aqueous solution.…”

N,O-Type Carborane-Based Materials

“…Following a similar procedure we [44][45][46][47][48][49] and others [50,51] have prepared an extensive series of new monosubstituted o-, m-and p-carboranylmethylalcohols bearing nitrogenated aromatic rings, by the addition of lithiocarboranes to the corresponding pyridylaldehydes (1-4, Chart 1 and Scheme 1). The addition of dilithiocarboranes to two equivalents of the corresponding aldehydes, under the same reaction conditions, provided a new series of disubstituted o-and m-carboranylmethylalcohols (5-6, Chart 1) [52,53]. This synthetic methodology allows the preparation of the compounds in good yields in gram quantities from one-pot reactions, starting from commercially available materials.…”

“…The enantiopure compounds can be exploited in coordination chemistry, as will be described in the following sections. Separation of the syn-and anti-isomers in the disubstituted series of compounds has been carried out in the case of o-carborane derivatives 5a and 5b [53,55].…”

“…In the case of 2-pyridyl derivatives, 2a, 3a and 4b (both in racemic and enantiopure forms), they all form homochiral helical networks and it has been shown that a correlation exists between the OCCN torsion angles of the molecules in the solid state and the handedness of the supramolecular helices [49]. Regarding the disubstituted derivatives, 5a-f, it was observed that syn and anti stereoisomers crystallized separately from their mixtures and the detailed analysis of their supramolecular structures revealed that homochiral recognition seems to operate also in these molecular systems [53].…”