2P-126

DFT study of colorimetric fluoride ion sensing by the heterodinuclear maingroup/transition-metal complexes through allosteric coordination(Fukui Institute for Fundamental Chemistry, Kyoto University) Wei Guan, Shinichi
Yamabe, Shigeyoshi Sakaki*[Introduction]
Fluoride anion plays a crucial role in biological process and daily life. However, excessive intake
will lead to dental and skeletal fluorosis and even osteosarcoma. Taking into account theseadverse effects of fluoride anion, the recognition and sensing of fluoride anions are necessary. Fora potential fluoride chemosensor, suchfactors as large ion affinity, highselectivity,visible
photophysicalresponse, and hydrolytic stability
must be satisfied. Recently, Wade etal. synthesized and characterized adinuclear stibine–palladium complex[(o-(Ph2P)C6H4)3SbPdCl]+ with highselectivity and visual recognition for
F− anion.1 Although the colorimetricfluoride sensing properties have been
investigated by X-ray diffraction,
NMR spectroscopy, and UV/vis
spectra, there are several importantquestions: What is the origin of highselectivity for F− anion? What role
does the transition-metal play insensing F− anion? What is the sensing
mechanism that really controls theobserved color change process? With
these questions, we will present
systematic DFT calculations on aFigure 1. Optimized structures and selected geometricseries of selected and designed
parameters of 2, 2F, 3 and 3F calculated atheterodinuclear main-group/transition
PCM(CH2Cl2)/PBE0/BS-I level (bond lengths and-metal complexes [(o-(Ph2P)C6H4)3M1
angles in angstroms and degrees, respectively).
M2Cl]+ (M1 = As, Sb Bi; M2 = Pd andHydrogen atoms are omitted for clarity.
Pt); see Figure 1.
[Computational methods]
All calculations were performed with the Gaussian09 program package at the DFT level in bothgas phase and dichloromethane solvent, where the PBE0 hybrid functional was used throughout.
The nature of each stationary point was verified by harmonic vibrational frequency calculation.
Two kinds of basis set system were employed here. In basis set I (BS-I), (541/541/211/1) and(21/21/1) basis sets were employed with Los Alamos ECPs for transition metals (Pd and Pt) andmain group heavy atoms (As, Sb and Bi), respectively. For C, H, O atoms, 6-31G(d) wereemployed, and 6-31+G(d) were used for P and halogen (F, Cl, Br) atoms. All of the TDDFT
calculations were carried out with a larger basis set system (BS-II). In BS-II, (5311/5311/211/1)
basis set was employed for Pd and Pt with the same Los Alamos ECPs, while aug-cc-pVTZ wasused for main group atoms (As, Sb, Bi) with relativistic ECPs. cc-pVDZ basis sets were used for C,H, and O atoms, and aug-cc-pVDZ basis sets were employed for P and halogen atoms.
[Results and Discussion]
In complexes 2 and 3, the Pd center takes a four-coordinate planar structure, in which the threecoordinate Sb center interacts with the Pd center. In the fluoride ion-capturing process, the Sbcenter changes to a five-coordinate system, indicating that the Sb takes hypervalent electronicstructure (2F and 3F in Figure 1). In addition, the Pd center of 3F becomes five-coordinatetrigonal-bipyramidal structure by one more coordination of phosphine P3. Complex 3 presents astrong capturing ability for F− anion; ∆E = −15.5 kcal mol−1, where ∆E represents the potential
energy change by the F− adduct formation (Table 1). It is noted that this kind of hetero-dinuclear
compound possesses higher selectivity for F− anion than such other anions as Cl−, Br−, CN−, andSCN− (Table 1). Cooperation between the intramolecular acid-base interaction (M1···P3) andcoordination interaction (M2···P3) controls the change of coordination structure at the M2 center,
which plays a crucial role in the colorimetric response for F− anion (Figure 2). For instance, theconversion of 2 to 2F decreases the absorption band around 380 nm but provides a new peakaround 330 nm, as experimentally observed1 (Figure 2). The conversion of 3 to 3F quenches theabsorption band around 400 nm, but provides a new absorption peak around 475 nm. This is
assigned as Pd-based ligand-field transition (LFT: ݀௫௬ → ݀௭ మ and ݀௫మି௬మ → ݀௭ మ ); in ourcalculations, these two excitations occur at nearly the same energy. It is noted that a fivecoordinate 3F induces intense LFT, which is important for hyperchromic effect. Besides, theintroduction of heavy-main-group element Bi causes bathochromic shift in the absorption spectra,
while the replacement by late-transition-metal Pt makes the spectra blue shift.

Figure 2. Comparison of experimental and calculated absorption spectra of systems 2 and 3 uponbinding F− anion by PCM(CH2Cl2)/TD-PBE0/BS-II level. Experimental data see ref. 1.
We tried to understand theoretically the sensing performance of such complexes for hydrated
fluoride at different extents [F···(H2O)n]− (n = 1-3) instead of a naked F− anion. In the presence[NMe4]+ and the phenyl ligands around F− anion, the maximum number of water moleculesinteracting with F− anion is 2. The strong interaction between water molecules and F− anion
makes the F− anion-capturing process of 3 gradually difficult. However, the F− adduct formation isexothermic, indicating that 3 also possesses good ability for F−-capturing from aqueous solution.
The presence of two water molecules leads to the formation of four-coordinate complex without P3
coordination, which makes colorimetric sensing of 3 for hydrated fluoride become not obvious.
Table 1. Proposed anionic capturing reactions and the reaction energies (∆E, in kcal mol−1)
calculated at PCM/PBE0/BS-II//PCM/PBE0/BS-I level (CH2Cl2 as solvent).
(∆E)
M1 M2 reaction
(1)
Sb
Pd
[3]BMe4 + [NMe4]F → 3F + [NMe4]BMe4−15.5(2)Sb
Pd[3]BMe4 + [NMe4]F → 3F'a + [NMe4]BMe4−13.86.0(3)Sb
Pd[3]BMe4 + [NMe4]Cl → 3Cl'a + [NMe4]BMe4
4.3(4)Sb
Pd[3]BMe4 + [NMe4]Br → 3Br'a + [NMe4]BMe4−9.9(5)SbPd[3]BMe4 + [NMe4]CN → 3CN'a + [NMe4]BMe4
3.1(6)Sb
Pd[3]BMe4 + [NMe4]SCN → 3SCN'a + [NMe4]BMe4−14.0(7)SbPd[3]BMe4 + [NMe4]F·H2O → 3F·H2O + [NMe4]BMe4−11.5(8)Sb
Pd[3]BMe4 + [NMe4]F·2H2O → 3F'·2H2Oa + [NMe4]BMe4−11.6(9)Sb
Pd[3]BMe4 + [NMe4]F·3H2O → 3F'·3H2Oa + [NMe4]BMe4
a Four-coordinate structure around Pd.
[Reference]
1. Wade, C. R., Ke, I.-S., Gabbai, F. P. Angew. Chem. Int. Ed. 2012, 51, 478−481.