Noncovalent Grafting of Molecular Complexes to Solid Supports by Counterion Confinement

Grafting molecular complexes on solid supports is a facile strategy to synthesize advanced materials. Here, we present a general and simple method for noncovalent grafting on charge-neutral surfaces. Our method is based on the generic principle of counterion confinement in surface micropores. We demonstrate the power of this approach using a set of three platinum complexes: Pt1 (Pt1L4(BF4)2, L = p-picoline), Pt2 (Pt2L4(BF4)4, L = 2,6-bis(pyridine-3-ylethynyl)pyridine), and Pt12 (Pt12L24(BF4)24, L = 4,4′-(5-methoxy-1,3-phenylene)dipyridine). These complexes share the same counterion (BF4–) but differ vastly in their size, charge, and structure. Imaging of the grafted materials by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) and energy-dispersive X-ray (EDX) showed that our method results in a homogeneous distribution of both complexes and counterions. Nitrogen sorption studies indicated a decrease in the available surface area and micropore volume, providing evidence for counterion confinement in the surface micropores. Following the adsorption of the complexes over time showed that this is a two-step process: fast surface adsorption by van der Waals forces was followed by migration over the surface and surface binding by counterion confinement. Regarding the binding of the complexes to the support, we found that the surface–adsorbate binding constant (KS) increases quadratically with the number of anions per complex up to KS = 1.6 × 106 M–1 equaling ΔG°ads = −35 kJ mol–1 for the surface binding of Pt12. Overall, our method has two important advantages: first, it is general, as you can anchor different complexes (with different charges, counterions, and/or sizes); second, it promotes the distribution of the complexes on the support surface, creating well-distributed sites that can be used in various applications across several areas of chemistry.


Additional calculations
Decrease in specific surface area (SSA) The slope, ΔSSA, of the insets in Figure 4a-c have a unit that is described in Eq.S1.
( The surface area that one molecule could cover was estimated based on X-ray structures of Pt 1 1 , Pt 2 1 and Pt 12 2 .The measured diameters are listed in Table S2.In the case of Pt 12 , an X-ray structure of an isostructural cage was used because an X-ray structure of this cage is not reported to date.The diameter of the complexes was used to calculate the area that it could cover as described in Eq.S3.
( 2  -1 ) =  * ( This gives covering area's that are ~5 times lower than the observed decrease in surface area (Table S4).Note: N 2 adsorption of the complexes itself is not considered here as M 2 L 4 cage structures like Pt 2 are known to have negligible affinity for N 2 adsorption themselves.

S13
Decrease in micropore volume (V micro ) The slope, ΔV micro , of the insets in Figure 4d-f have a unit that is described in Eq.S4.
The volume that one BF 4 anion would occupy can be estimated best based on its average diameter while having anion-π interactions, that is ~0.35 nm, 4 using the formula described in Eq.S6.

Figure S11 .Figure S12 .Figure S13 .
Figure S11.UV-Vis spectra of Pt 2 in MeCN in the concentration range 1.49 -17.86 μM measured in a cuvette with a path length of 2 mm.

1 Figure S14 .
Figure S14.Lambert-Beer plot of Pt 12 in MeCN in the concentration range 0.75 -3.01 μM measured in a cuvette with a path length of 2 mm.

Table S2 .
1esorption percentages of Pt 2 /Vulcan based on UV-Vis adsorption studies using the calibration curve in FigureS10and S11 (MeCN) and previous work (DMSO).1

pore with (nm) Complex 0 μmol Pt g Vulcan -1 5.0 μmol Pt g Vulcan -1 12.5 μmol Pt g Vulcan -1
Figure S8.Decrease in specific surface area (SSA) upon the immobilization of one molecule of Pt

1 , Pt 2 or Pt 12
(based on nitrogen sorption data), expressed as a function of BF 4 -ions per complex (see further details on the calculations in section 5 below).
Figure S9.UV-Vis spectra of Pt 1 in MeCN in the concentration range 2.77 -44.32 μM measured in a cuvette with a path length of 10 mm.

Table S4 .
Structural parameters of the complexes.