Abstract
For the preparation of the compound, as olution of 3-aminophenylboronic acid monohydrate (0.250 g, 1.61 mmol) and catechol (0.177 g, 1.61 mmol) in 20 mL of asolvent mixture of methanol and benzene (1:4) was refluxed for 1hinthe presence of aD ean-Stark trap. Ap recipitate formed upon cooling was recrystallized from as olvent mixture of methanol, dichloromethane and water to give crystals suitable for X-ray diffraction analysis (m.p. 448 K).
Hatoms were positioned geometrically and constrained using the riding-model approximationw ith d(C-H aryl )=0 .93 Å and U iso (H aryl )=1 .2 U eq (C). Hydrogen atoms bonded to O( H3, H31A, H31B, H31C, H32A, H32B and H32C) and N(H1A, H1B and H1C) were located in difference Fourier maps. One of the two hydrogen atoms in each water molecule is disordered over two positions with an occupancy factor of 0.50 (H31B and H31C for O31; H32B and H32C for O32). The disorder arises from the presence of crystallographic inversion centers in the O31···O31 and O32···O32 H-bonds. As aconsequence disorder is observed also in the O31···O32 hydrogen bonds. This is the only remaining site for disorder, since the remaining coordination sites in each water molecule are already involved in hydrogen-bonds with an occupancy of 1.0 (for O31: O3-H···O31 and O31-H···p;for O32: N1-H···O32 and O32-H···O1). The coordinates of the O-Hand N-Hh ydrogen atoms were refined with restraints:
During the past few years al arge series of substituted phenylboronic acid derivatives have been prepared, mainly for applications in organic synthesis [1,2] and for molecular recognition of biochemically active molecules [3] . More recently, such boronic acid derivatives have attracted attention also as building blocks for the self-assembly of macrocyclic and polymeric assemblies. For this purpose, the boronic acid is generally converted to an ester (boronate) via condensation with an aliphatic or aromatic diol, which is then assembled to amacromolecular structure via reaction of the additional functional group attached to the B-phenyl ring [4] [5] [6] . In this context, 3-aminophenylboronic acid has been employed for the generation of macrocycles and cages [7] [8] [9] [10] [11] . In contrast to the analogous ester derived from pinacol [12] , in this case the water adduct has been obtained, which has converted to the corresponding zwitterion and crystallized as dihydrate. A search of the Cambridge Structural Database (CSD, version 5.31) [13] , revealed that there is no entry so far for an aqua complex of a boronic acid ester. Until now, solid-state structures of aqua-boron complexes have been reported only for triphenylborane derivatives, in particular for tris(pentafluorophenyl)borane [14, 15] . The asymmetric unit of the aqua-catecholboronate contains one aqua-boronate and two uncoordinated water molecules (figure, top). Upon coordination to the boron atom the coordinated water molecule is deprotonated to form the corresponding zwitterion. 
Discussion
During the past few years al arge series of substituted phenylboronic acid derivatives have been prepared, mainly for applications in organic synthesis [1, 2] and for molecular recognition of biochemically active molecules [3] . More recently, such boronic acid derivatives have attracted attention also as building blocks for the self-assembly of macrocyclic and polymeric assemblies. For this purpose, the boronic acid is generally converted to an ester (boronate) via condensation with an aliphatic or aromatic diol, which is then assembled to amacromolecular structure via reaction of the additional functional group attached to the B-phenyl ring [4] [5] [6] . In this context, 3-aminophenylboronic acid has been employed for the generation of macrocycles and cages [7] [8] [9] [10] [11] . In contrast to the analogous ester derived from pinacol [12] , in this case the water adduct has been obtained, which has converted to the corresponding zwitterion and crystallized as dihydrate. A search of the Cambridge Structural Database (CSD, version 5.31) [13] , revealed that there is no entry so far for an aqua complex of a boronic acid ester. Until now, solid-state structures of aqua-boron complexes have been reported only for triphenylborane derivatives, in particular for tris(pentafluorophenyl)borane [14, 15] . The asymmetric unit of the aqua-catecholboronate contains one aqua-boronate and two uncoordinated water molecules (figure, top). Upon coordination to the boron atom the coordinated water molecule is deprotonated to form the corresponding zwitterion. confirmed by the tetrahedral character of the boron atom (78 %) [18] . In the title crystal structure, the boronate molecules are linked through N-H···O(H)B and N-H···O(B) hydrogen bonds to chains along [100] (figure, middle). These hydrogen bonding interactions give rise to two different hydrogen-bonded rings of the composition {H 4B2N2O4}( motif A) [ , bottom) . Recently, such water-aggregates have received attention because of their importance in biological systems [20] . Additionally, in the crystal packing there are also p-facial hydrogen bonds between water molecules (O31) and phenyl rings C1-C6 (Cg1 i )(symmetry code i: 1-x,1-y,1-z) [21, 22] . The distance between O31 and the benzene centroid (Cg1) is 3.190(2) Å.Therefore, in this crystal structure each crystal lattice water molecule is involved in atotal of four hydrogen bonds. 

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