Incorporation of Hexanuclear Mn(II,III) Carboxylate Clusters with a {Mn6O2} Core in Polymeric Structures

A new series of hexanuclear mixed-valent carboxylate coordination clusters of the type [Mn6O2(O2CR)10L4] (R = CMe3; CHMe2) featuring a {MnII4MnIII2(μ4-O)2} core of composition [Mn6O2(O2CCMe3)10(Me3CCO2H)3(EtOH)]•(Me3CCO2H) (1), [Mn6O2(O2CCMe3)10(Me3CCO2H)2 (EtOH)2]•2(EtOH) (2) and [Mn6O2(O2CCMe3)10(Me3CCO2H)2(MeOH)2]•2(MeOH)•H2O (3), and coordination polymers which incorporate such clusters, namely [Mn6O2(O2CCHMe2)10(pyz)(MeOH)2]n (4), {[Mn6O2(O2CCHMe2)10(pyz)1.5(H2O)]•0.5(H2O)}n (5), and [Mn6O2(O2CCMe3)10(HO2CCMe3)2(en)]n (6), have been synthesized (where pyz = pyrazine, en = ethyl nicotinate). The modification of the cluster surface by a diverse combination of capped or bridging ligands attached to peripheral MnII atoms results in discrete clusters with a closed hydrophobic exterior shell in 1 and 2, supramolecular chains built through hydrogen bonded solvent molecule clusters in 3, linear coordination polymers in 4 and 6 or a ladder-like coordination polymer in 5. The H-bonded coordination polymers 4 and 5 form supramolecular layers in crystals.


Introduction
Hexanuclear Mn(II,III) carboxylate clusters with a mixed valent {Mn II 4 Mn III 2 (µ 4 -O) 2 } core have been successfully used as building blocks for the synthesis of cluster-based coordination polymers (CCPs) [1] starting from the first prepared CCP by Yamashita et al. in 2002 [2]. This type of cluster has a common formula [Mn 6 O 2 (O 2 CR) 10 L 4 ] (where L = monodentate neutral ligand) is stable and able to resist decomposition during the reaction and may introduce its inherent physical properties in newly formed structures. Moreover, in the hexanuclear oxo-carboxylate Mn(II,III) clusters, various capped monodentate neutral ligands L may be obtained and completely or partially replaced by bridging ligands to give coordination polymers. Furthermore, under specific conditions, the role of carboxylate ligands can be changed from intracluster bridges to intercluster bridges with the formation of polymeric structures. Such structures are characterized by short distances between nearest {Mn 6 [4] CCPs, respectively. Notably, the choice of exo-polydentate spacer ligands, selection of precursors and reaction conditions make it possible to obtain CCPs that incorporate hexanuclear Mn(II,III) clusters with different dimensionality-1D polymeric chains [2][3][4][5][6], 2D layers [7,8] and 3D networks [9]. The spacer length and spacer flexibility both have an important effect on the complex structure and physical properties of the end-product. For example, the replacement of one pivalic acid molecule in [Mn 6 O 2 (O 2 CCMe 3 ) 10 (Me 3 CCO 2 H) 4 ] by a long 4,4 -bipyridine ligand leads to the organization of a 1D zigzag polymeric chain with Mn···Mn intercluster distances of 11.68 Å [2]. Nearly linear chains are also formed by bridged nicotinamide molecules which are coordinated through an amid O atom and a pyridine N atom (intercluster Mn···Mn distance is 8.690 Å) [5]. Unique meander-type chains are formed in {[Mn 6 O 2 (O 2 CCHMe 2 ) 10 (Me 2 CHCO 2 )(EtOH)(bpe)]·Me 2 CHCO 2 } n where the hexanuclear {Mn 6 } clusters are linked by 1,2-bis(4-pyridyl)ethane (bpe) (intercluster Mn···Mn distance is 9.239 Å) [5]. The shorter rigid pyrazine (pyz) ligand connects neighboring clusters in {[Mn 6 O 2 (O 2 CCHMe 2 ) 10 (pyz) 3 ]·H 2 O} n into a zigzag chain with Mn···Mn distances of 7.222 Å [5]. Pyrazine molecules occupy four sites in peripheral Mn II centers, but two of them serve as exo-bidentate ligands, while two others are monodentate. As can be seen from the given examples, despite the potential for cross-linking into 2D or 3D networks, the employed spacer ligands resulted exclusively in 1D polymer chains that are packed in parallel or both parallel and perpendicular propagation directions in the crystal.
The first reported 2D coordination polymers based on hexanuclear {Mn 6 } carboxylate clusters were prepared using isonicotinamid (ina) as an N,O-donor bridging ligand [7]. In this case, all four capped molecules in the initial {Mn 6 } cluster have been substituted by ina that led to the formation of two-dimensional (4,4) 10 (ina) 2 ] n 2D coordination polymer has also been obtained [8].
Two  2 ]·(thf)·3(EtOH)} n were synthesized utilizing the angled semi-rigid N,N'-donor aldrithiol spacer (adt) [8]. In the first of them, each Mn 6 cluster is connected via adt ligands to four neighbors in 2D networks with a (4,4) topology similar to the above-mentioned CCP with isonicotinamide ligand, whereas in the second case an unprecedented 2D bilayer motif is formed.
It should be noted that hexanuclear Mn(II,III) carboxylate clusters based on pivalate or isobutyrate ligands represent globe-shaped nanosized particles with ca. 2.0 × 1.8 × 1.5 nm dimension with a hydrophobic surface and which are malleable for modification. The methylene groups of the ligands on the surface of such clusters are often disordered and crystal solvent molecules may be incorporated. The disorder is manifested as being essentially enhanced compared with the cluster core thermal motion or resolved with two different positions of disordered fragments. Such a labile cluster surface allows to accommodate different combinations of capped or bridging ligands and solvent molecules; however, this complicates the prediction of a final cluster composition and cluster-based polymer topology. In continuation of our study on the design and preparation of cluster-based coordination polymeric networks based on Mn (II, III) (6), where {Mn 6 } clusters are arranged by pyrazine (pyz) or ethyl nicotinate (en) into linear (4 and 6) and ladder-like (5) chains.

Materials and Methods
All manipulations were performed under aerobic conditions using chemicals and solvents as received without further purification. The precursors Mn(Me 3 CCO 2 ) 2 , Mn(Me 2 CHCO 2 ) 2 have been prepared by using methods described elsewhere [10,11]. Cluster [Mn 6 O 2 (O 2 CCMe 3 ) 10 (Me 3 CCO 2 H) 4 ] has been prepared with a slight modification [11] by the reaction of manganese(II) acetate with an excess of pivalic acid followed by adding MeCN solution to the reaction mixture. Recrystallization of the precipitated solid from hot tetrahydrofuran yields [Mn 6 (O 2 CCMe 3 ) 10 (thf) 4 ] [12]. IR spectra were recorded in the solid state on a Perkin-Elmer Spectrum 100 FT-IR (Perkin-Elmer, Waltham, MA, USA) spectrometer in the 650-4000 cm −1 range.

X-ray Crystallography
Diffraction datasets were collected on a SuperNova diffractometer (for 1, 3, and 6, Rigaku, Tokyo, Japan), Bruker APEX II (for 5, Bruker AXS Inc., Madison, WI, USA), and Xcalibur E (for 2 and 4, Oxford diffraction LTD., Yarnton, UK ) CCD area-detector diffractometers using mirror optics or graphite monochromated Mo-Kα radiation. The crystallographic data and summary of the data collection and refinement are listed in Table 1. Data were corrected for Lorentz, polarization effects, and absorption. The structures were solved by direct methods and refined by full-matrix least squares on weighted F 2 using the SHELX suite of programs [13]. In all structures, tert-butyl and isopropyl groups show enhanced thermal motion. The hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective atoms. The hydrogen atoms of disordered solvent molecules have not been localized or included in the model of refinement.

Synthetic Aspects
The use of some amount of lanthanide salts in the synthesis of 2 and 3 was mandatory and afforded crystals 2-3 suitable for single-crystal X-ray measurements.

X-ray Structure Study
Compounds 1-6 comprise virtually identical {Mn II 4 Mn III 2 O 2 } 10+ cores consisting of two edge-shared distorted Mn 4 tetrahedra with two µ 4 -O 2− ions lying inside of each tetrahedron and which are similar to the earlier described compounds [1][2][3][4][5][6][7][8][9]. The common edge is formed by two Mn III ions with the shortest intracluster Mn···Mn distance being 2.812(2)-2.824(1) Å. All other Mn III ···Mn II and Mn II ···Mn II bond distances in Mn 4 tetrahedra are longer (~3.2 and 3.7 Å, respectively). Peripheral ligation is provided by 10 bridging pivalate (1, 2, 3, 6 Table S1. In crystal structure 2, {Mn6} cluster resides on two-fold axis passing through both μ4-O atoms, thus presuming C2 molecular symmetry of the cluster and only three Mn atoms are symmetry independent. However, it should be noted, that two-fold axis is passed also through two pivalate ligands, which bridge symmetry related Mn2, Mn2' and Mn3 and Mn3' atoms ( Figure 2a). The pivalate ligands do not possess the two-fold symmetry due to the presence of tert-butyl group and In crystal structure 2, {Mn 6 } cluster resides on two-fold axis passing through both µ 4 -O atoms, thus presuming C 2 molecular symmetry of the cluster and only three Mn atoms are symmetry independent. However, it should be noted, that two-fold axis is passed also through two pivalate ligands, which bridge symmetry related Mn2, Mn2' and Mn3 and Mn3' atoms ( Figure 2a). The pivalate ligands do not possess the two-fold symmetry due to the presence of tert-butyl group and that implies the disorder of tert-butyl groups over two positions due to the crystal symmetry. This disorder also introduces the disorder in positions of both two pivalic acid molecules which cap the Mn3 and Mn3' atoms and solvent ethanol molecules associated with the cluster via H-bonds. In 3, the tert-butyl group of three bridging pivalate ligands and one methanol solvent molecule were found to be disordered over two positions. The sixth position in the Mn II atoms environment in each Mn4 tetrahedron is filled by coordinated pivalic acid or methanol molecules. Thus, the mutual arrangement of pivalic acid/alcohol molecules that cap peripheral Mn II atoms differs from their arrangement in cluster 2, (Figure 3a In the crystal structure 4, the peripheral ligation of metal atoms is provided by 10 bridging isobutyrate ligands instead of pivalate ones in 1-3. Three isobutyrate ligands revealed disorder of the isopropyl group over two positions. Two methanol molecules trans-positioned across the cluster center and two pyrazine ligands trans-positioned and symmetry related by translation b cap the Mn II atoms. The bridging exo-bidentate pyrazine molecules link {Mn6} cluster moieties into linear coordination polymers which propagate along the crystallographic b axis (Figure 4).  Table S2. The disordered solvate ethanol molecule is hydrogen bonded with the cluster via coordinated ethanol molecules and similar to 1, closes the hydrophobic shell of the cluster (Figure 2b).
In 3, the tert-butyl group of three bridging pivalate ligands and one methanol solvent molecule were found to be disordered over two positions. The sixth position in the Mn II atoms environment in each Mn 4 tetrahedron is filled by coordinated pivalic acid or methanol molecules. Thus, the mutual arrangement of pivalic acid/alcohol molecules that cap peripheral Mn II atoms differs from their arrangement in cluster 2, (Figure 3a). Similar to 1 and 2, each of the pivalic acid molecules forms strong intramolecular hydrogen bonds (2.587(3) and 2.630(5) Å), which stabilize their orientation. The peculiarity of this cluster is related with solvent water molecules attached to the cluster by two O-H···O hydrogen bonds (2.882(3) and 2.838(4) Å). The lone pairs of this interstitial water molecule are pointing outside the cluster and create the hydrophilic region on the cluster surface predisposed for an involvement in hydrogen bonds, (Figure 3b). The solvent methanol molecules are H-bonded with water molecules (O-H···O = 2.691(4) Å) forming water-methanol solvent molecule associates and are also involved in the hydrogen bonds with coordinated methanol molecules of neighboring clusters (O-H···O = 2.642(4) Å). As a result, clusters are joined in zigzag-like H-bonded polymeric chains extended along the b crystallographic axis, (Figure 3c). The second symmetry independent methanol molecule is disordered over two positions and H-bonded to another coordinated methanol molecule in the cluster, thus it decorates the chain (O-H···O = 2.63(1)/2.74(2) Å).
In the crystal structure 4, the peripheral ligation of metal atoms is provided by 10 bridging isobutyrate ligands instead of pivalate ones in 1-3. Three isobutyrate ligands revealed disorder of the isopropyl group over two positions. Two methanol molecules trans-positioned across the cluster center and two pyrazine ligands transpositioned and symmetry related by translation b cap the Mn II atoms.
The bridging exo-bidentate pyrazine molecules link {Mn 6 } cluster moieties into linear coordination polymers which propagate along the crystallographic b axis (Figure 4).  The intercluster Mn···Mn separation through the pyrazine molecule is equal to 7.443(2) Å. This distance is slightly longer than the corresponding distance of 7.288(3) Å in the related zigzag-like coordination polymer of composition {[Mn6O2(O2CCHMe2)10(pyz)3]·2H2O}n [5], where unlike 4, bridging pyrazine molecules are cis-situated and attached to Mn II atoms of the same Mn4 tetrahedron of the cluster. The adjacent, inversion symmetry related parallel polymeric chains, are connected via O-H···O (2.739(7) and 2.721(7) Å) hydrogen bonds between methanol molecules and pivalate ligands, which results in a supramolecular layer parallel to the (−1 0 1) plane. The shortest      The crystal structures of 1-6 reveal a close packing motif, mostly due to hydrophobic-lipophilic interactions. Upon removing solvent molecules and disordered fragments/molecules of the second positions, the total solvent-accessible volume obtained using PLATON [14] was estimated to be only 10.7% in 1, 13.8% in 2, 5.0% in 3, 4.4% in 4, 4.9% in 5 and 1.4% in 6.

Conclusions
Based on the crystal structure of six new compounds, we demonstrated how the nanosized hexanuclear mixed-valent carboxylate manganese coordination clusters with a hydrophobic surface may be incorporated in hydrogen bonded or polymeric supramolecular extended structures through the modification of the cluster surface by a diverse combination of capping or bridging ligands attached to peripheral Mn atoms. In contrast to the secondary building unit (SBU) approach, we use cluster as supermolecular building block (SBB) [15] which may be isolated to produce the supramolecular extended structure without risking cluster decomposition or rearrangement. The relative size of coordination clusters vs. metal ions may afford unprecedented advances in terms of scale to develop extended supramolecular structures.
Supplementary Materials: The following are available online at www.mdpi.com/link, Figure S1: View of clusters 1-3 illustrates the diverse combination of capped ligands attached to peripheral Mn II atoms. CIF files of the solved structure. Table S1: Selected bond distances (Å) in 1-6, Table S2: The parameters of O-H···O hydrogen bonds in the crystal structure 1-6. The crystal structures of 1-6 reveal a close packing motif, mostly due to hydrophobic-lipophilic interactions. Upon removing solvent molecules and disordered fragments/molecules of the second positions, the total solvent-accessible volume obtained using PLATON [14] was estimated to be only 10.7% in 1, 13.8% in 2, 5.0% in 3, 4.4% in 4, 4.9% in 5 and 1.4% in 6.

Conclusions
Based on the crystal structure of six new compounds, we demonstrated how the nanosized hexanuclear mixed-valent carboxylate manganese coordination clusters with a hydrophobic surface may be incorporated in hydrogen bonded or polymeric supramolecular extended structures through the modification of the cluster surface by a diverse combination of capping or bridging ligands attached to peripheral Mn atoms. In contrast to the secondary building unit (SBU) approach, we use cluster as supermolecular building block (SBB) [15] which may be isolated to produce the supramolecular extended structure without risking cluster decomposition or rearrangement. The relative size of coordination clusters vs. metal ions may afford unprecedented advances in terms of scale to develop extended supramolecular structures.