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Research Area

Research Group of Dr A. Tasiopoulos

The main interests of Tasiopoulos’ research group involve the synthesis and characterization of: i) polynuclear compounds and single – molecule magnets (SMMs) of paramagnetic 3d transition metal ions with aliphatic diols, ii) coordination polymers based on magnetically interesting metal clusters iii) metal – organic frameworks (MOFs) with interesting properties (gas sorption, photoluminescence, magnetic and/or single - crystal to single - crystal transformation). The current intense interest in such metal-organic compounds, stems not only from the fundamental interest in their aesthetically pleasing structures, magnetic interactions, etc, but also from the continuing need for the development of new functional molecule – based materials that could be employed in technological applications. Some selected examples or recent results are summarized below.

Synthesis of Molecule-based Magnets

Polynuclear clusters of paramagnetic metal ions and multidimensional coordination polymers based on magnetically interesting clusters continue to attract significant attention mainly due to their aesthetically pleasing structures and also their interesting and often novel magnetic properties [e.g. ferromagnetic exchange interactions, large ground state spin value, single – molecule magnetism (SMM) or single – chain magnetism (SCM) behavior, etc]. Our group, has been exploring various synthetic strategies for the isolation of new high nuclearity clusters and multidimensional coordination polymers with interesting magnetic properties.

Representation of the molecular structure (left) and the structural core (right) of the hexameric Mn18Na6 aggregate. Color code: Mn; blue, Na; purple, Br; green, O; red, N; dark blue, C; grey. H atoms are omitted for clarity.

The use of the diol 2-hydroxymethyl phenol (hpH2) afforded two nanosized Mn25Na4 and a Mn49 aggregates. Interestingly these complexes are structurally related since they consist of four and eight [MnIII6MnII44-O)4]18+ supertetrahedral sub-units respectively. These compounds have large spin ground-state values (Mn49: S=61/2; Mn25Na4: S=51/2) with the Mn49 cluster displaying SMM behavior and being one of the largest SMMs reported. (Angew. Chem. Int. Ed., 2016, 55, 679-684)

Representations of the Μn19 repeating unit (left) the  3D framework structure (middle) and magnetization (M) versus applied magnetic field (μ0H) hysteresis loops at the indicated temperatures (right). Color scheme: MnIII; blue, Na+; purple, O; red, C; gray. H atoms have been omitted for clarity.

Other multidimensional coordination polymers that have been prepared and characterized based on polynuclear Mn clusters are a family of 1 - and 2 – D coordination polymers consisting of high spin Mn17 octahedral units (Inorg. Chem., 2009, 48, 5049-5051, Polyhedron, 2009, 28, 1814-1817), 1-D polymers based on Mn6 clusters (Bioinorg. Chem. Appl., 2010,Article ID: 367128), two 1-D coordination polymers consisting of triangular [Mn3O(O2CR)6]0/+ units (Inorg. Chim. Acta, 2008, 361, 4100-4106) and an 1-D coordination polymer based on a linear mixed valent [MnIII2MnII] repeating unit which displays single-chain magnet (SCM) behavior with an energy barrier of ~38 K. (Chem. Commun., 2014, 50, 14873-14876)

The ligand hmpib3- connected to three SBUs (left) and the space filling representation of the structure of UCY-1 (right). Color code: Zn; green, O; red, N; blue, C; grey.

Another microporous MOF isolated recently is a Cu2+ complex produced from the initial use of the ligand 5-((pyridin-4-ylmethylene)amino)isophthalic acid (PEIPH2) in 3d metal–organic framework (MOF) chemistry. Complex {[Cu3(PEIP)2(5-NH2-mBDC)(DMF)]·7DMF} denoted as Cu-PEIP·7DMF consists of pillared kgm-a layers containing a hexagonal shaped cavity with a relatively large diameter of ~8–9Å surrounded by six trigonal shaped ones with a smaller diameter of ~4–5Å. Gas sorption studies revealed that Cu-PEIP exhibits a 1785 m2g-1 BET area as well as high CO2 sorption capacity (4.75 mmol g-1 at 273 K) and CO2/CH4 selectivity (8.5 at zero coverage and 273 K). (Inorg. Chem. Front., 2016, 3, 1527-1535.)

Microporous MOFs with interesting single-crystal to single-crystal (SCSC) transformation properties involving exchange of the guest solvent molecules

We recently reported compound {[Co9(INA)18(H2O)6]·11DMF·15H2O} (Co9-INA·11DMF· 15H2O) (INA = the anion of isonicotinic acid) which exhibits a rigid 3D-porous structure. It is based on a Co9 repeating unit and possesses a microporous structure with an appreciable internal surface area (910 m2g-1 BET area) and significant CO2 uptake (4.2 mmol g−1 at 273 K/1 bar) and CO2/CH4 selectivity (6.7 at zero coverage). Furthermore, Co9-INA displays capability for exchange of the guest solvent molecules by various organic molecules in a SCSC fashion. (Cryst. Growth Des., 2015, 15, 185-193)

High nuclearity clusters and SMMs from the use of diols in Mn carboxylate chemistry

A large number of polynuclear Mn clusters have been prepared from the use of diol-type ligands, both homometallic and heterometallic Mn/M (M = a 3d metal ion) some of which also display very large nuclearity and size. One such example is the family of Mn40M4 (M = Na+ or Mn2+) loops of loops aggregates which prepared from the use of the aliphatic diols 1,3-propanediol (pdH2) and 2-methyl-1,3-propanediol (mpdH2) which consist of four Mn10 loops linked through Na+ or Mn2+ ions. The Mn40Na4 clusters do not display SMM behavior possibly because they consist of four weakly interacting Mn10 loops which are connected through diamagnetic metal ions (Inorg. Chem., 2007, 46, 3795-3797). To confirm this assumption we targeted and achieved the synthesis of the homometallic Mn44 loop of loops aggregate analogue which indeed displays SMM behavior (J. Am. Chem. Soc., 2010, 132, 16146-16155).

Modifications in the reaction procedure that resulted in the Mn44 aggregate involved use of a Ni2+ salt and afforded a large heterometallic Mn36Ni4 aggregate with a loop of loops and supertetrahedra structural topology. The Mn36Ni4 cluster displays dominant ferromagnetic exchange interactions and a large spin ground state value ST = 26 ± 1, the highest that has been observed in a heterometallic metal cluster (Chem. Commun, 2012, 48, 5410-5412).

Representation of the molecular structure of Mn36Ni4 cluster (right) and its MnIII6MnII4 supertetrahedral (middle) and MnIII8Ni2 loop (left) subunits. Color code: MnIII, blue; MnII, lavender; NiII, orange;O, red; N, light green; Cl, green; C, gray. H atoms are omitted.

Heterometallic Mn/Ni cluster chemistry also afforded another high nuclearity complex and in particular a Mn24Ni2 aggregate consisting of a [3x4] grid within a Mn12Ni2 loop (Chem. Commun., 2014, 50, 9090-9093).

Representation of the molecular structure of the Mn24Ni2 aggregate. Color code: MnIII, light blue; MnIV, dark blue; Ni, orange; O, red; C, grey. H atoms are omitted for clarity.

A fourth large cluster prepared when diols were combined with various oximes (a project that takes place in collaboration with the group of Professor Euan K. Brechin) is the Mn32 “double – decker” wheel. Although, the diol does not appear in the final product, its presence in the reaction mixture is essential, since reactions in its absence lead to the formation of a known compound. The Mn32 complex displays SMM behavior with an effective barrier for magnetization reorientation Ueff ~ 44.5 K which is the highest value that has been reported for any molecular wheel (Angew. Chem. Int. Ed., 2011, 50, 4441-4444).

Representation of the molecular structure of the Mn32 “double – decker” wheel. MnIII; blue, MnII; pink, O; red. H atoms have been omitted for clarity. Magnetization (M) versus applied magnetic field (μ0H) hysteresis loops for a single crystal of Mn32·3MeCN at the indicated temperatures and a fixed field sweep rate of 0.002 Ts-1 (top) and at the indicated field sweep rates at 0.04 K. M is normalized to it saturation value, Ms at 1.4 T.

The combination of phenolic oximes with diols also afforded a hexameric Mn18Na6 aggregate consisting of oxime-based [Mn3O]7+ triangles linked through Na+ ions possessing a wheel like metal topology. (Chem. Commun., 2016, 52, 12829-12832)

In addition, we have been able to isolate complexes that display entirely ferromagnetic exchange interactions and a high spin ground state value such as the [Mn17Ο8(N3)4(O2CMe)2(pd)10(py)10(MeCN)2(H2O)2](ClO4)3 cluster that displays an  ST = 37 and SMM behavior being the highest spin SMM reported (Inorg. Chem., 2009, 48, 5049-5051).

Representations of the molecular structure of FeIII5 cluster (left), its [Fe53-O)(μ4-O)]11+ structural core (up, right) and its topological motif (bottom, right). The line connecting the Fe3+ ions and the red and yellow colored planes in the bottom, right figure are to emphasize the topological motif of the structural core Color code: Fe; green, O; red, N; blue, C; grey.

Representations of the molecular structure of [Mn17] cluster (left) and its core (right) (the yellow line connecting the Mn ions is to emphasize the octahedral topology). Color scheme: MnIII; blue, MnII; purple, O; red, N; green, C; gray. H atoms have been omitted for clarity.

Other type of alcohol – containing ligands have also been used for the synthesis of 3d metal clusters, such as various aliphatic aminoalcohols. Thus, we have been exploring the use of 3-amino-1-propanol (Hap) and 2-(hydroxymethyl)piperidine (Hhmpip) in Fe3+ carboxylate chemistry. These investigations have resulted so far in various FeIII5, FeIII6 and FeIII7 and a family of  FeIII4CeIV6 clusters (Dalton Trans., 2012, 41, 1544-1552, Polyhedron, 2013, 52, 346-354).

The molecular structure of a representative FeIII4CeIV6 cluster. Color code: FeIII; green, CeIV; pink, N; blue, O; red, C; grey. The hydrogen atoms are omitted for clarity.

Multidimensional coordination polymers based on magnetically interesting polynuclear Mn clusters

We have prepared and characterized several multidimensional coordination polymers based on metal clusters some of which also combine interesting magnetic properties, including a family of 3-D coordination polymers consisting of Mn19 units with a triangular pyramidal frustum topology. The Mn19 units display dominant ferromagnetic exchange interactions, a fairly large spin ground state value S = 23/2 and SMM behavior (Angew. Chem. Int. Ed., 2006, 45, 7722-7725).

Functional Metal Organic Frameworks

Metal organic frameworks (MOFs) are porous materials based on metal ions or clusters and bridging organic ligands. Over the past decade there has been a tremendous research interest

in the construction of such materials mainly because of their intriguing architectures and novel physical properties that lead to potential applications in a series of areas including gas storage and separation, catalysis, magnetism, sensing, etc. We have been interested in the development of new synthetic methods to isolate functional metal organic frameworks and the investigation of their potential applications.

Use of elongated polytopic ligands for the synthesis of highly porous MOFs

This project involves the synthesis and study of the gas sorption properties of highly porous MOFs. For this purpose, we have designed and synthesized novel nanosized ligands, such as the tricarboxylic acid H3hmpib = 4,4′,4″- (1E) - [4,4′,4″-(hydroxymethanetriyl) tris (benzene-4,1-diyl)tris(azan-1-yl-1-ylidene)] tris(methan-1-yl-1-ylidene)tribenzoic acid. We demonstrated the capability of this ligand to stabilize highly porous MOFs through the isolation of a new microporous metal organic framework, namely compound [Zn4O(hmpib)2]∙xDMF.  This material represents a rare example of catenated MOF with quite high internal surface area (~2600 m2 g-1) and also displays high CO2 uptake and selectivity for it over CH4 at near ambient temperature (Inorg. Chem., 2011, 50, 11297-11299).

Another complex with interesting SCSC transformation properties is a new flexible Cd2+ metal organic framework (MOF) [Cd3(CIP)2(DMF)3]·DMF·10H2O [H3CIP = 5-(4-carboxybenzylideneamino)isophthalic acid] that shows a unique (3,3,6)-connected topology. This MOF is based on a unique neutral non-oxo triangular [Cd3(COO)6] secondary building unit and displays significant structural flexibility, capability for exchange of the guest solvents by various organic molecules in a SCSC fashion as well as breathing capacity allowing the incorporation of relatively large amount of benzene into its pores. (CrystEngComm, 2012, 14, 8368-8373)

MOFs that were prepared by employing a combination of polytopic ligands and chelating amino-alcohols

Recently, we have initiated a research program that involves the synthesis of new MOFs using a combination of polytopic organic ligands typically leading to polymeric structures and various amino-alcohols that can act either as structure-directing agents or ligands. These investigations resulted in several new MOFs from reactions involving use of trimesic acid (H3btc) and various amino-alcohol ligands (Metal – Organic Aminoalcohol Frameworks (MOAAF), such as triethanolamine, 2-hydroxy-methyl-piperidine, N-tert-butyl-diethanolamine, N-methyl-diethanolamine, hydroxyl-ethyl-morpholine or 1,4-bis-hydroxyethyl-piperazine. Although the structures of these new MOFs are based on the same polytopic ligand, they exhibit a remarkable diversity and unique structural-topological features that are clearly induced by the amino - alcohols. A new synthetic strategy towards novel MOFs is thus demonstrated. (Cryst. Growth Des., 201212, 5471–5480).

Multifunctional MOFs based on Lanthanide ions

Lanthanide MOFs (LnMOFs) are a very important family of multifunctional materials and additionally, they can be excellent materials for studying coordinating ligand exchange (cle) transformation reactions because of their high stability in air and various solvents as well as their weakly bound solvent ligands that could be easily released.

We prepared a Nd3+ MOF [Nd2(CIP)2(DMF)2.8(H2O)1.2] denoted as UCY- 2 and demonstrated its capability to undergo a plethora of SCSC transformation reactions with some of them being very uncommon. These structural alterations involve the replacement of coordinating solvent molecules of UCY-2 by terminally ligating solvents and organic ligands with multiple functional groups including −OH, −SH, −NH−, and −NH2 or their combinations, chelating ligands, anions, and two different organic compounds. The SCSC coordinating solvent exchange is thus demonstrated as a powerful method for the functionalization of MOFs. (Inorg. Chem.201251, 6308–6314)

SCSC transformations involving the exchange of terminal solvent ligands of UCY-2 by merpdH2 (2-Mercapto-ethanol) and atzH (3-Amino-1H-1,2,4-triazole). For emphasis, the free functional groups are depicted as large balls. Only the SBUs of the exchanged products are shown for clarity.

The other members of this series of Ln-CIP MOFs [Ln2(CIP)2(DMF)4-x(H2O)x] (Ln3+ = La3+, Ce3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+; x = 0 – 2) were synthesized and characterized by thermogravimetric analysis, photoluminescence spectroscopy and magnetic measurements. Single-Crystal to Single-Crystal (SCSC) coordinating solvent exchange experiments with acetone and methanol for the Ce3+ analogue afforded compounds that are isostructural with the pristine material and contain one acetone and one water or 1.25 methanol and 0.75 water ligated molecules per Ce3+ respectively. Liquid methanol sorption experiments indicated a maximum absorption capacity of 96(2) mg g-1 and fast kinetics, while the sorbent is reusable and is also capable of highly selective sorption of MeOH over EtOH. (J. Mater. Chem. A, 2013, 1, 5061-5069)

Representation of the SCSC solvent exchange of UCY-5 with MeOH. For emphasis, the inserted organic molecules are represented with large balls. Representation of the % MeOH–EtOH sorption capacity of UCY-5, which was measured for mixtures of MeOH–EtOH with a ratio of    1 : 1 versus time. The lines are only a guide for the eye.

A series of single-crystal-to-single-crystal (SCSC) transformations for the flexible [Eu2(CIP)2(DMF)2(H2O)2] (UCY-8) and rigid [Eu2(N-BDC)3(DMF)4] (EuNBDC) (H2N-BDC = 2-amino-1,4-benzene dicarboxylic acid) MOFs have been investigated. These studies proved their capability to exchange the coordinating solvent molecules in a SCSC fashion by a series of organic molecules including relatively bulky molecules (such as pyridine, 2-hydroxymethyl-phenol, etc.) as a result of their breathing capacity. Photoluminescence studies of the pristine and the exchanged MOFs revealed a tremendous enhancement of the Eu3+-based photoluminescence (PL) signals, lifetimes and quantum yields (up to 16 times) as a result of the replacement of terminal solvents of the pristine materials by organic ligands being efficient sensitizers for the Eu3+ ion. (J. Mater. Chem. A, 2014, 2, 5258-5266)

SCSC transformation of Eu-NBDC that resulted in the exchange of terminal solvent ligands by m2hmp (top). Observed emission from crystals of EuN-BDC and EuN-BDC/m2hmp crushed on a filter paper and irradiated by a standard laboratory UV lamp (λexc = 365 nm) (bottom).