Inorganic Chemistry
Communication
Treating 2-formylpyridine (24 equiv) with tetrakis(4-
aminophenyl)porphyrinatomanganese(III) (Mn-TAPP; 6
equiv) in the presence of Zn(NTf2)2 (8 equiv) and Bu4NPF6
(6 equiv) resulted in the formation of complex Zn8(L-
Cl)6(NTf2)16 [cubic-(MnIIICl); L = manganese(III)
5,10,15,20-tetrayltetrakis(benzene-4,1-diyl)tetrakis[1-(pyridin-
2-yl)methanimine]porphine; Scheme S2]. Slow diffusion of
diethyl ether into the solution of cubic-(MnIIICl) afforded
crystals suitable for X-ray diffraction analysis. As shown in
Figure 1, cubic-(MnIIICl) crystallized in the monoclinic space
(Figure S14). The association constant calculated from the
ITC titration was 1.6 × 105 M−1. Furthermore, fluorescence
titration experiments were performed to study the guest FMN
binding ability of the cubic-(MnIIICl) cage. Adding cubic-
(MnIIICl) (6.25 μM) to a CH3CN/H2O solution of FMN (10
μM) quenched approximately 70% of the emission intensity of
FMN (Figure 2), suggesting that FMN interacted strongly with
n
Figure 2. (a) Emission spectra of FMN (10 μM) (black line) upon
the addition of cubic-(MnIIICl) up to 6.25 μM. Inset: Hill plot of the
titration curve of fluorescein upon the addition of cubic-(MnIIICl)
showing the associate constant. (b) Luminescence spectra of FMN
(10 μM, black line) in CH3CN/H2O (1:1) upon the addition of
cubic-(MnIIICl) (2.0 μM, red line) and Mn-TAPP (12 μM, blue
line).
Figure 1. Crystal structure of cubic-(MnIIICl) showing the
coordination geometries of Zn(II) and Mn(II) ions and empty
spheres (red balls) for guest encapsulation. Solvent molecules, anions,
and H atoms were omitted for clarity.
cubic-(MnIIICl). A Hill plot with the fitting curve of the profile
indicated the formation of a 1:1 complex with an association
constant of 3.1 × 105 M−1. Both the large association constant
and 1:1 stoichiometric ratio suggested the formation of a stable
FMN@cubic-(MnIIICl) host−guest complex in solution. In
contrast, the association constant of NADH with cubic-
(MnIIICl) (Figure S15) was 2.6 × 104 M−1, which was much
lower than that of FMN, indicating that FMN bound more
strongly with cubic-(MnIIICl) compared with the larger-sized
NADH at the same concentration, while luminescence
titrations of FMN (10 μM) with the addition of Mn-TAPP
(12 μM) quenched about 20% and gave a quenching constant
(Ksv) of about 5.7 × 104 M−1. Meanwhile, adding cubic-
(MnIIICl) (2 μM) to FMN (10 μM) quenched approximately
30% of the intensity (Figure 2b). Obviously, the quenching
efficiency of cubic-(MnIIICl) was higher than that of Mn-
TAPP.
The catalytic activities of most CYPs require one or more
redox partner proteins to sequentially deliver two electrons
from NADH to the heme metal reactive center for dioxygen
activation.19 The possible ET between NADH and the FMN@
cubic-(MnIIICl) supermolecular system was investigated.
Changes in the UV−vis spectra of FMN@cubic-(MnIIICl)
were recorded to study its reduction by NADH. Manganese-
(III) and -(II) porphyrins have distinctive absorption bands at
470 and 440 nm, respectively.13,20 Upon the addition of
NADH to FMN@cubic-(MnIIICl) in N,N-dimethylformamide
(DMF)/acetonitrile (CH3CN) (1:1), the absorption band at
470 nm gradually decreased and a new band at 440 nm was
observed (Figure S7a), reaching equilibrium within 30 min.
The appearance of clear isosbestic points at 418, 458, and 584
nm also demonstrated the direct one-electron reduction of
manganese(III) porphyrin, with no other intermediate
observed. Finally, once exposed to dioxygen, this Mn(II) was
rapidly reoxidized to Mn(III) with the same isosbestic points
group C2/c, with half of the cubic cage in an asymmetric unit.
This arrangement of metal ions and coordination ligands led to
a cubic cage, with the eight vertices occupied by Zn ions and
the six faces occupied by the manganese porphyrin-based
ligand. Meanwhile, the Cl− ion served as the axial ligation to
each manganese porphyrin and was coordinated to manganese
inside/outside (disordered in a 2:1 ratio) of the cage at a
MnIII−Cl distance of 2.38 Å. The average Mn−Mn distance
between opposite faces was approximately 14.5 Å, and the
inner void volume was about 1436 Å3.18 However, because of
the paramagnetic nature of Mn(III), the fine NMR spectra
could not be obtained.
The electrospray ionization mass spectrometry (ESI-MS)
spectrum of cubic-(MnIIICl) in CH3CN exhibited signals at
m/z 891.88, 1022.29, 1185.19, and 1394.49 (Figure S2). A
simple comparison with the simulation results based on the
natural isotopic abundances suggested that the peaks
corresponded to [Zn8L6Cl6(NTf2)n](16−n)+ (n = 6−9). FMN
is a biomolecule produced from riboflavin (vitamin B2) by the
enzyme riboflavin kinase and functions as the prosthetic group
of CPR, with phosphorylated anion groups at its terminal. The
positively charged cubic-(MnIIICl) most likely allowed FMN
encapsulation in its cavity. The ESI-MS spectrum of cubic-
(MnIIICl) in the presence of excess FMN exhibited new
intense peaks at m/z 1253.30, 1472.32, and 1764.46, which
w e r e c l e a r l y a s s i g n e d t o [ Z n 8 L 6 C l 5 ( F M N ) -
(NTf2)n(CH3CN)3](16−n)+ (n = 8−10), indicating the binding
of one FMN molecule in the host (Figure S3), and one of the
chloride ions was dissolved from the Mn(III) center. The
density functional theory (DFT) results also suggested that
FMN could be encapsulated inside the MnIIICl cage. The
strong affinity of Mn(III) to the negatively charged phosphate
moiety accounts for the encapsulation (Figure S17).
An isothermal titration calorimetry (ITC) experiment was
conducted to better understand the guest binding interactions
between cubic-(MnIIICl) and FMN. The observed inclusion
number was 1.0, as determined by an independent model
The reduction potential measured by the cyclic voltammo-
gram experiments for cubic-(MnIIICl) in DMF/CH3CN (1:1,
B
Inorg. Chem. XXXX, XXX, XXX−XXX