Reports: DNI356081-DNI3: Isolation and Studies of Oxoheme Complexes in Metal-Organic Frameworks

T. David Harris, PhD, Northwestern University

Part 1. With a decade of exploration, postsynthetic metal exchange has become a powerful tool to construct metal-organic frameworks (MOFs) that are not amenable to direct synthesis. MOFs synthesized via postsynthetic metal exchange have shown exotic coordination geometries for activating small gas molecules. However, one frequently-observed limitation is their slow reaction kinetics. Some of the metal exchange reactions require several weeks to a year to reach thermodynamic equilibrium, and only a fraction of the native metal was replaced in many cases. Therefore, developing methods to accelerate metal-exchange and to boost completeness are highly-desirable.

We developed a method that can accelerate metal exchange by over 2000-fold through harnessing the structural dynamics of a 2D manganese-benzoquinoid MOF. This represents the most effective method to accelerated metal exchange to date. The second best method to accelerate metal exchange is to use a different solvent, which can accelerate metal exchange by 6 times.

We first investigated the metal exchange using fully-solvated crystals of (Et4N)2[Mn2(L)3] (2), which was prepared using solvothermal method . Soaking 2 in a DMF solution of Co2+ or Zn2+ leads to complete (for Co) or partial (57% for Zn) metal–exchanged MOFs featuring [MII(DMF)6]2+ (M = Co, Zn) as oppose to (Et4N)+ counterion to balance the charge . To achieve 50% metal exchange, the reaction takes 98 (for Co) and 79 (for Zn) hours. This [MII(DMF)6]2+ counterion can be readily exchanged with (Et4N)+ by soaking in its chloride salt solution.

According to the single-crystal structures, the bulkiness of the [MII(DMF)6]2+ relative to the diameter of the 1D channel within the MOFs suggests the metal exchange might be kinetically hindered caused by the diffusion barrier along this channel. To verify this hypothesis, we prepared a partially metal-exchanged MOF for EDS analysis. A fully solvated sample of 1 was soaked Co2+ solution for 120 hours followed by counterion exchange with (Et4N)+. This MOF has a formula of (Et4N)2[Mn1.38Co0.62(L)3]. EDS mapping and a line scan along c direction indicate gradual radial distributions of the two metals to form a sandwich-like structure. A line scan within ab plane suggests a homogenous distribution of the two metals.

We discovered that the 1, upon desolvation or resolvation, undergoes a fully-reversible phase change. A 20% volume change is associated with this structure dynamics. Since the volume expansion upon resolvation must incur rapid adsorption of solvent molecules towards the interior, we sought to utilize this dynamic structural behavior to accelerate metal exchange.

Indeed, when we soak partially-desolvated crystals of 1 directly to a metal solution, we observed a drastic acceleration by 120 times and over 2000 times for Co and Zn, respectively. Moreover, it boosted the degree of metal exchange for zinc from 57% to 89%. In addition, this method produced the phase-pure [Co(DMF)6][Co2L3], which is otherwise produced with an impurity from fully-solvated 1.

Part 2. During this reporting period, we have also focused efforts on isolating high-valent oxomanganese porphyrin complexes and using them to carry out the hydroxylation of C-H bonds. As a first step, we have prepared a four-coordinate porphyrin manganese(II) complex within the metal-organic framework PCN-224. Metalation of the porphyrin with MnII was carried out by heating single crystals of PCN-224 under N2 in a DMF solution containing excess MnBr2 and 2,6-lutidine, followed by evacuation at 150 °C for 12 h, to give the compound PCN-224MnII (5). Complete metalation of the porphyrin within the bulk crystalline material was confirmed by solid-state diffuse reflectance UV/Visible spectroscopy and trace metals analysis. Furthermore, N2 adsorption data collected for a desolvated sample of 5 at 77 K provided a Brunauer-Emmett-Teller surface area of 2455 m2/g, close to the accessible surface reported for other metalated variants of PCN-224, thereby confirming the retention of porosity upon metalation and the successful removal of solvent molecules from the pores. While several manganese porphyrin-containing MOFs have been reported, to our knowledge, 1 represents the first example of a MOF that features a four-coordinate MnII porphyrin complex.

The diffuse reflectance spectrum obtained for an activated sample of 1 exhibits similar peak maxima to the absorption spectrum of 1 suspended in toluene, but nevertheless features key differences. Notably, the spectrum reported for 5 in toluene displays a Soret band at 448 nm, while spectrum of activated 5 features a Soret band at 417 nm. These differences can possibly be attributed to the rigorous four-coordinate nature of the Mn center in 1, compared to a slight distortion from local D4h symmetry at Mn in the toluene solution imposed by Mn–toluene interactions.

The structure of 5 exhibits a four-coordinate MnII center, residing in a square planar coordination environment. No significant residual electron density was located in the difference Fourier map, confirming the absence of axial ligation at Mn. The Mn–N distance of 1.998(5) Å is notably shorter than those of 2.082(2)–2.085(2) Å previously reported for the toluene-solvated compound (TPP)Mn·2C7H8. While this molecular compound features a pseudo four-coordinate MnII center, weak contacts between Mn and a toluene molecule, with a closest Mn–Ctoluene distance of 3.04 Å, lead to a 0.19 Å displacement of Mn from the N4 plane and thus slightly longer Mn–N bonds relative to 5. As such, the structure of 5 provides the first example of a four-coordinate Mn porphyrin species. The isolation of a rigorously four-coordinate Mn center within a porphyrin ligand is remarkable given the relatively large ionic radius of MnII, and thus its propensity to displace out of the N4 plane to form five-coordinate complexes.