Heterometallic Modular Metal–Organic 3D Frameworks Assembled via New Tris‐β‐Diketonate Metalloligands: Nanoporous Materials for Anion Exchange and Scaffolding of Selected Anionic Guests

Abstract

The modular engineering of heterometallic nanoporous metal-organic frameworks (MOFs) based on novel tris-chelate metalloligands, prepared using the functionalised β-diketone 1,3-bis(4'-cyanophenyl)-1,3-propanedione (HL), is described. The complexes [M(III)L(3)] (M=Fe(3+), Co(3+)) and [M(II)L(3)](NEt(4)) (M=Mn(2+), Co(2+), Zn(2+), Cd(2+)) have been synthesised and characterised, all of which exhibit a distorted octahedral chiral structure. The presence of six exo-oriented cyano donor groups on each complex makes it a suitable building block for networking through interactions with external metal ions. We have prepared two families of MOFs by reacting the metalloligands [M(III)L(3)] and [M(II)L(3)](-) with many silver salts AgX (X=NO(3)(-), BF(4)(-), PF(6)(-), AsF(6)(-), SbF(6)(-), CF(3)SO(3)(-), tosylate), specifically the [M(III)L(3)Ag(3)]X(3)·Solv and [M(II)L(3)Ag(3)]X(2)·Solv network species. Very interestingly, all of these network species exhibit the same type of 3D structure and crystallise in the same trigonal space group with similar cell parameters, in spite of the different metal ions, ionic charges and X(-) counteranions of the silver salts. We have also succeeded in synthesising trimetallic species such as [Zn(x)Fe(y)L(3)Ag(3)](ClO(4))((2x+3y))·Solv and [Zn(x)Cd(y)L(3)Ag(3)](ClO(4))(2)·Solv (with x+y=1). All of the frameworks can be described as sixfold interpenetrated pcu nets, considering the Ag(+) ions as simple digonal spacers. Each individual net is homochiral, containing only Δ or Λ nodes; the whole array contains three nets of type Δ and three nets of type Λ. Otherwise, taking into account the presence of weak Ag-C σ bonds involving the central carbon atoms of the β-diketonate ligands of adjacent nets, the six interpenetrating pcu networks are joined into a unique non-interpenetrated six-connected frame with the rare acs topology. The networks contain large parallel channels of approximate hexagonal-shaped sections that represent 37-48% of the cell volume and include the anions and many guest solvent molecules. The guest solvent molecules can be reversibly removed by thermal activation with retention of the framework structure, which proved to be stable up to about 270°C, as confirmed by TGA and powder XRD monitoring. The anions could be easily exchanged in single-crystal to single-crystal processes, thereby allowing the insertion of selected anions into the framework channels.

Dipyridyl β-diketonate complexes and their use as metalloligands in the formation of mixed-metal coordination networks

“…We have previously prepared MMOFs based on 3cyanoacetylacetonate metalloligands, 10 whereas Carlucci and co-workers have reported materials based on 1,3-di(4-cyanophenyl)propane-1,3-dionato metalloligands. 11 Pyridylfunctionalised !-diketonates have also attracted some attention. In particular, the Domasevitch 12 and Maverick 13 groups have prepared MMOFs based on 3-(4pyridyl)acetylacetonate, whereas monometallic networks have also been prepared with pyridyl-substituted !-diketonates.…”

Selective anion exchange with nanogated isoreticular positive metal-organic frameworks

“…M etal-organic frameworks (MOFs) have been recognized as an excellent platform for host-guest chemistry, which includes gas storage [1][2][3][4][5] , ion-exchange [6][7][8][9][10] , separation [11][12][13][14][15][16] , and so on [17][18][19][20][21][22][23] . The pore size of MOFs varies from a few angstroms to a few nanometres, making them ideal for incorporating species ranging from small gas molecules [24][25][26] and organic molecules 27,28 to large inorganic and organic species [29][30][31][32] .…”

“…Although ion-exchange of small or bulky guest species into charged frameworks has been observed [6][7][8][9][10]31 , most of the studies were focused on the capture of specific metal ions 6,7 or the enhancement of gas-sorption capability 8 . There are also examples of liquid chromatography for organic dyes using Zn MOFs with neutral framework [33][34][35] , which are less likely to involve ion-exchange.…”

Metal‐organic Frameworks Incorporating Multiple Metal Elements

“…As such, it allows the tuning of the MOF pore environment. Several types of metallo‐ligands have been reported in the construction of multi‐metal MOFs, including β‐diketones, acetylacetonates, Schiff bases, oxamates, pyridines, dipyrrinates, and most notably, metallo‐porphyrin based linkers, among others.…”