Hydrothermal synthesis and crystal structure of poly[bis(μ3-3,4-diaminobenzoato)manganese], a layered coordination polymer

The title compound is a two-dimensional coordination polymer with inter-layer connectivity provided by N—H⋯N hydrogen bonds.


Chemical context
The benzoate anion, C 7 H 5 O 2 À , is a classic ligand in coordination chemistry, with over 1500 crystal structures reported in the Cambridge Structural Database (Groom et al., 2016) for complexes of first-row transition metals, which include monodentate (O), chelating ( 2 O,O 0 ) and bridging ( 2 -O,O 0 ) modes for the ligand [for ligand bonding-mode notation, see Janicki et al. (2017)]. Functionalized benzoate derivatives add further structural variety: for example, -NH 2 substituents at the ortho, meta and/or para positions of the benzene ring can form or accept hydrogen bonds with respect to nearby acceptor or donor groups and/or bond to another metal ion (i.e., as a possible 2 -N,O or 3 -N,O,O 0 bridging ligand). ISSN 2056-9890 As part of our ongoing studies in this area (Khosa et al., 2015), we now describe the hydrothermal synthesis and crystal structure of the title compound (I), where C 7 H 7 N 2 O 2 À is the 3,4-diaminobenzoate (dbz À ) anion.

Structural commentary
The title complex (I) consists of an Mn 2+ cation located on a crystallographic inversion centre and one deprotonated dbz À ligand with its atoms lying on general positions (Fig. 1), which of course generates the overall 1:2 metal to ligand ratio and ensures charge balance. The C7/O1/O2 carboxylate group of the ligand is rotated from the plane of the C1-C6 aromatic ring by 13.5 (2) and the C-O bonds [C7-O1 = 1.261 (3) Å and C7-O2 = 1.277 (3) Å ] are of similar length, indicating substantial electronic delocalization. The C2-C3 and C5-C6 bonds (mean = 1.383 Å ) are marginally shorter than the other bonds in the benzene ring (mean = 1.396 Å ), which can be related to resonance of the para-N atom lone pair with the carboxylate group (Mukombiwa & Harrison, 2020). This is also presumably reflected in the fact that the C4-N2 bond length [1.413 (3) Å ] is slightly shorter than C3-N1 [1.432 (3) Å ]. Even so, it is noteworthy that the bond-angle sums about atoms N1 and N2 of 329.8 and 335.8 , respectively, are indicative of a significant tendency towards sp 3 hybridization of the N atoms, i.e., localization of the lone pairs. In terms of the non-hydrogen atoms attached to the benzene ring, N1 and N2 deviate slightly from the mean plane of the ring in opposite directions by À0.023 (3) and 0.024 (3) Å , respectively, whereas atom C7 shows a larger deviation of À0.077 (3) Å .
The dbz À ligand bonds to three different metal ions from both of its carboxylate O atoms and also from its meta-N atom ( Fig. 1), i.e., 3 -N,O,O 0 mode. The preference for the meta-N atom to bond to the metal ion (rather than the para-N atom) can be related to the resonance effect noted in the previous paragraph. With respect to the carboxylate group, the metal ion bonded to O1 is displaced in an 'upwards' sense by 1.046 (7) Å and the other (bonded to O2) is displaced 'downwards' by À1.651 (6) Å .
Crystal symmetry generates a centrosymmetric trans-MnN 2 O 4 elongated octahedron for the metal ion, in which the Mn1-N1 bond length of 2.3065 (19) Å is distinctly longer than the Mn1-O1 [2.1591 (14) Å ] and Mn1-O2 [2.2062 (15) Å ] bonds. The manganese ion presumably has a high-spin 3d 5 configuration, thus the distortion of the octahedron cannot be electronic in nature (i.e.: a Jahn-Teller effect) and might arise for steric reasons. The angular variance (Robinson et al., 1971) of the cis X-Mn-Y (X, Y = N, O) bond angles is (7.7 ) 2 , indicating relatively little angular distortion from the ideal values of 90 ; the minimum and maximum angles are 86.75 (6) and 93.25 (6) , respectively. The trans bond angles are constrained by symmetry to be 180 . The bond-valence sum (in valence units) (Brown & Altermatt, 1985) for the metal ion is 1.97, in very good agreement with the expected value of 2.00 for Mn 2+ .
In the extended structure of (I), the 3 bridging ligand links the metal ions into infinite (101) sheets (Fig. 2). These sheets can be decomposed into [010] chains of octahedra linked by the bridging C7/O1/O2 carboxylate groups [shortest MnÁ Á ÁMn(x, y + 1, z) separation = 4.4212 (2) Å ], with connectivity in the [101] direction achieved via the benzene ring of the ligands and their meta-N atoms [shortest MnÁ Á ÁMn(Àx + 1 2 , y + 1 2 , Àz + 1 2 ) = 8.1520 (4) Å ]. Hydrogen bonding helps to consolidate the structure of (I): the para -N2H 2 group forms an intra-sheet N2-H4NÁ Á ÁO2 iii bond ( Fig. 1 and Table 1, where symmetry codes are defined) but also participates in an inter-sheet N2-H3NÁ Á ÁN2 ii link, i.e., N2 'accepts its own hydrogen bond' from an adjacent     Table 1; additionally: (iv) Àx + 3 2 , y + 1 2 , Àz + 1 2 .] symmetry related -N2H 2 group and C(2) infinite chains propagating in the [010] direction arise in the crystal (Figs. 1 and 2) with adjacent N atoms related by the 2 1 screw axis; it is notable that the -NH 2 groups in adjacent layers are aligned opposite to each other to facilitate the formation of this intersheet hydrogen bond. As well as forming a coordinate bond to the metal ion from N1, the meta -N1H 2 group forms an N1-H1NÁ Á ÁO1 i intra-sheet hydrogen bond while the N1-H2N group does not participate in a hydrogen bond, perhaps due to steric crowding. There are no significant aromaticstacking interactions in the crystal of (I), the shortest centroid-centroid separation between C1-C6 rings being 4.4211 (13) Å .

Hirshfeld surface analysis
In order to gain further insight into non-covalent interactions in the crystal of (I), the Hirshfeld surface and two-dimensional fingerprint plots were calculated using CrystalExplorer (Turner et al., 2017) following the approach recently described by Tan et al. (2019). The Hirshfeld surface of the dbz À anion in (I) (see supporting information) largely shows the expected red spots of varying intensity corresponding to close contacts resulting from the O-Mn and N-Mn coordinate bonds and N-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonds described above. The Hirshfeld surface mapped onto d norm for the manganese cation in (I) (Fig. 3) is a distinctive 'dimpled cube', with the intense red spots (short interactions) corresponding to its coordinate bonds, which correlates nicely with its octahedral coordination geometry. The most important outward (i.e., non-reciprocal) percentage contributions of the different type of contacts for the anion and the cation are listed in Table 2. It may be seen that HÁ Á ÁH (van der Waals) contacts are by far the most significant contributor for the anion followed by CÁ Á ÁH and HÁ Á ÁC contacts (total contribution = 59.9%). The contacts associated with the hydrogen bonds, i.e., HÁ Á ÁN (donor), HÁ Á ÁO (donor), NÁ Á ÁH (acceptor) and OÁ Á ÁH (acceptor) collectively account for some 24.2% of the surface. Finally, the coordinate bonds to the metal ion (O-Mn and N-Mn), despite their presumed importance in establishing the crystal structure, account for a modest 7.2% of the anion's surface.

Figure 4
Two-dimensional Hirshfeld fingerprint plot for the Mn 2+ cation in (I).
The wing-like two-dimensional fingerprint plot for the manganese cation in (I) (Fig. 4) shows two prominent features: the spike ending at (d i , d e ) = ($1.14, $1.06 Å ) and extending backwards corresponds to the Mn-O coordinate bonds and the (1.18, 1.14 Å ) feature just separated from it equates with the Mn-N bonds. The MnÁ Á ÁH contacts are overlapped with the Mn-O and Mn-N contacts in the main body of the 'wing' with the shortest MnÁ Á ÁH contact at about (1.40, 1.25 Å ), which correlates well with the short MnÁ Á ÁH contacts noted in the previous paragraph.

Database survey
The structure of (I) may firstly be compared with the isostructural M(C 8 H 8 NO 2 ) 2 family [M = Mn (CCDC refcode ULEZUI) (II), Co (ULIBAU), Ni (ULIBEY) and Zn (ULIBIC)], where C 8 H 8 NO 2 À is the 3-amino-4-methylbenzoate anion (Khosa et al., 2015). These phases contain 3 -N,O,O 0 ligands and centrosymmetric MN 2 O 4 octahedra, which generate very similar polymeric layers to those found in (I). In (II), the sheets propagate parallel to the (100) plane of the monoclinic cell due to a different choice of unit-cell setting (see Refinement section). The major difference arises with respect to the para-substituent: in (II) the methyl groups in adjacent (100) layers are laterally shifted with respect to each other (Fig. 5) to avoid an unfavourable close steric contact and there are no directional inter-sheet interactions beyond normal van der Waals' contacts, which compares to the interlayer N-HÁ Á ÁN links already mentioned for (I).
Despite its potential as a polyfunctional bridging ligand, just five crystal structures of complexes of the 3,4-diamino-  , 1994). Key data for (I) and these structures are summarized in Table 3, which indicates a wide variety of metal coordination polyhedra, ligand bonding modes and topologies. Unlike the isostructural manganese and zinc phases in the M(C 8 H 8 NO 2 ) 2 family, MIWSES features a water molecule bonded to the trigonalbipyramidally coordinated zinc ion and has a quite different overall structure to (I). It may finally be noted that (I)  Packing diagram (redrawn from Khosa et al., 2015) for (II) viewed down [010], with the MnN 2 O 4 octahedra shown in polyhedral representation and N-HÁ Á ÁX hydrogen bonds shown as yellow lines. Note how the layers, which propagate parallel to the (100) plane in this setting of the space group, are laterally shifted compared to those in Fig. 2: the shortest inter-layer NÁ Á ÁN separation in (I) is 3.106 (3) Å (via the N1-H1NÁ Á ÁN1 hydrogen bond) compared to the shortest inter-layer CÁ Á ÁC separation of 3.948 (2) Å in (II).  represents the first reported example of a 3 -N,O,O 0 bonding mode for the dbz À anion.

Synthesis and crystallization
A mixture of 99 mg (0.50 mmol) of MnCl 2 Á4H 2 O and 152 mg (1.00 mmol) of 3,4-diaminobenzoic acid were added to 1.0 ml of 1 M KOH under stirring. The resulting mixture was heated to 423 K in a 23 ml Teflon-lined autoclave for 10 h. The autoclave was then removed from the oven and cooled to room temperature over several hours and opened. Colourless plates of (I) were recovered by vacuum filtration and rinsed with acetone and dried.

Poly[bis(µ 3 -3,4-diaminobenzoato-κ 3 N 3 ,O,O′)manganese(II)]
Crystal data [Mn(C 7 (2) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.