Functional Metal–Organic Frameworks via Ligand Doping: Influences of Ligand Charge and Steric Demand

Doping a functional ligand into a
known crystalline system built from ligands of similar shape and length
provides a powerful strategy to construct functional metal–organic
frameworks (MOFs) with desired functionality and structural topology.
This mix-and-match approach mimics the widely applied metal ion doping
(or solid solution formation) in traditional inorganic materials,
such as metal oxides, wherein maintaining charge balance of the doped
lattice and ensuring size match between doped metal ions and the parent
lattice are key to successful doping. In this work, we prepared three
sterically demanding dicarboxylate ligands based on Ir/Ru-phosphors
with similar structures and variable charges (−2 to 0), [Ir­(ppy)3]-dicarboxylate (L1,
ppy is 2-phenylpyridine), [Ir­(bpy)­(ppy)2]+-dicarboxylate
(L2, bpy is 2,2′-bipyridine),
and Ru­(bpy)3]2+-dicarboxylate (L3), and successfully doped them into the
known IRMOF-9/-10 structures by taking advantage of matching length
between 4,4′-biphenyl dicarboxylate (BPDC) and L1–L3. We systematically investigated the effects of size and charge
of the doping ligand on the MOF structures and the ligand doping levels
in these MOFs. L1 carries a −2
charge to satisfy the charge requirement of the parent Zn4O­(BPDC)3 framework and can be mixed into the IRMOF-9/-10
structure in the whole range of H2L1/H2BPDC ratios from 0 to 1. The steric bulk
of L1 induces a phase transition
from the interpenetrated IRMOF-9 structure to the non-interpenetrated
IRMOF-10 counterpart. L2 and L3 do not match the dinegative charge
of BPDC in order to maintain the charge balance for a neutral IRMOF-9/-10
framework and can only be doped into the IRMOF-9 structure to a certain
degree. L2 and L3 form a charge-balanced new phase with a
neutral framework structure at higher doping levels (>8% For L2 and >6% For L3). This systematic investigation reveals the
influences of steric demand and charge balance on ligand doping in
MOFs, a phenomenon that has been well-established in metal ion doping
in traditional inorganic materials.