Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds

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Metal-Organic Radical Open Frameworks The polytopic organic ligands, generally nitrogen and oxygen

donor ligands, used in metal - organic open frameworks (MOFs), also named as coordination polymers,

connect the 'inorganic' subunits along the space in the appropriate topology to originate connected void

volumes in the resulting structure. As in many other aspects related to the molecular magnetism field, Kahn

and coworkers pioneered the discovery of coordination polymers that combined both porosity characteristics

and magnetic properties. 54 Such coordination polymers were made with closed shell organic multitopic

ligands and isolated magnetically active metallic ions and it can be noted that when the distance between

metal ions increased by enlarging the size of organic ligands, the magnetic couplings between them

decreased exponentially, so limiting obtaining large pores. 55 One strategy to overcome this problem is

the use of stable radicals with rigid and large sizes as polytopic ligands together with magnetically active

transition metal ions. 56 The resulting structures must in principle exhibit larger magnetic couplings and

dimensionalities in comparison with systems made up from diamagnetic polytopic coordinating ligands,

since the organic radicals may become an active part of the magnetic structure as they have their own

spin, rather than 'just' mediate magnetic interactions between adjacent metal ions. 57,58

HOOC

COOH

By using the di and tricarboxylic acid radicals 12 and 13 , a series of metal - organic radical open

frameworks ( MOROF-n ) has been prepared. Interestingly, the crystal structure of Cu 3 ( 13 ) 2 (py) 6

(C 2 H 5 OH) 2 (H 2 O) ( MOROF-1 ) reveals a 2-D honeycomb (6,3) network composed of rings with six metal

units and six 13 radicals connected between them (Figure 2.12). Such planar layers are piled up forming

very large 1-D hexagonal pores, which measure 3.1 and 2.8 nm between opposite vertices. 59 To the best

of our knowledge, these pores are one of the largest reported so far for a metal - organic open framework

structure. Moreover, complex MOROF-1 shows additional channels with square and rectangular shapes

in the perpendicular directions with estimated sizes of 0

3 nm, respectively. The

interconnected pores generate solvent-accessible voids in the crystal structure that amount to 65 % of the

total unit cell volume. This extremely large void volume is responsible for the exotic properties ( vide

infra ) of this material.

Accordingly, with the capability of carboxylic-substituted PTM radicals to transmit efficiently the mag-

netic interactions, each 13 ligand was able to magnetically bridge three copper(II) ions and, therefore, to

extend the magnetic interactions across the infinite layers, giving rise to ferrimagnetic 2-D layers. Indeed,

as shown in Figure 2.13, the smooth decrease of the

5 and 0

T values below 250 K is a clear signature of the

presence of antiferromagnetic couplings between nearest neighboring copper(II) ions and 13 ligands within

the 2-D layers. The minimum of

T corresponds to a short range order state where the spins of adjacent

magnetic centers are antiparallel, provided there is no net compensation due to the 3 : 2 stoichiometry of the

copper(II) ions and radical units. The huge increase of

T at lower temperatures indicates an increase of

the correlation length of antiferromagnetically coupled units of copper(II) and 13 as randomizing thermal

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