Photochemical Oxidation of Pt(IV)Me₃(1,2-diimine) Thiolates to Luminescent Pt(IV) Sulfinates

Abstract

We report the formation of dinuclear complexes from, and photochemical oxidation of, (CH3)3-Pt­(IV)­(N^N) (N^N = 1,2-diimine derivatives) complexes of thiophenolate ligands to the analogous sulfinates (CH3)3Pt­(N^N)­(SO2Ph) and structural, spectroscopic, and theoretical studies of the latter revealing tunable photophysics depending upon the 1,2-diimine ligands. Electron-rich thiolate and conjugated 1,2-diimines encourage formation of thiolate-bridged dinuclear complexes; smaller 1,2-diimines or electron-poor thiolates favor mononuclear complexes. Photooxidation of the thiolate ligand yields hitherto unreported Pt­(IV)-SO2R complexes, promoted by electron-deficient thiolates such as 4-nitrothiophenol, which exclusively forms the sulfinate complex. Such complexes exhibit expected absorptions due to π-π* ligand transitions of the 1,2-diimines mixed with spin-allowed singlet MLCT (d-π*) at relatively high energy (270–290 nm), as well as unexpected broad, lower energy absorptions between 360 and 490 nm. DFT data indicate that these low energy absorption bands result from excitation of Pt–S and Pt–C σ-bonding electrons to π* orbitals on sulfinate and 1,2-diimine, the latter of which gives rise to emission in the visible range.

Cross-Coupling versus Homo-Coupling at a Pt(IV) Center: Computational and Experimental Approaches

“…Interestingly, the Pt­(IV) complexes [Pt­( i PrDAB)­(Me) 4 ] and [Pt­( i Pr-DAB)­(Me) 2 (SnPh 3 ) 2 ] are photoreactive, cleaving the axial ligands as · CH 3 or · SnPh 3 radicals from an excited SBLCT (sigma-bond-to-ligand charge transfer state (frequently also called ligand­(axial)-to-ligand­(N^N) charge transfer (L’LCT) state) upon irradiation. , The halido Pt­(IV) complexes of the type [Pt­(N^N)­X­(Me) 3 ] (N^N = diimines such as i Pr-DAB, bpy, or phen) are more photostable. They have the same type of excited states (here called XLCT) but at markedly lower energy, and these complexes might be emissive alternatively to showing photochemistry. − For the corresponding [Pt­(N^N)­X 2 (Me) 2 ] complexes, these SBLCT (or XLCT) excited states are not photoreactive anymore. − …”

Sulfur-center Reactivity toward Oxygenation Mediated by Ruthenium: Effective Bioactive Compounds (A Review)

“…Some bizarre photo-induced S-center oxygenation of metal-thiolato to the sulfonated compound is also present in the literature. 34 Photooxidation reactions of sulfur compounds like thioanisoles, thiophene and many other diverse nonaromatic organosulfur compounds, were published, 42,69 in which, most of the photooxidation involving 1 ∆O 2 mediated by H 2 O 2 70 or by irradiation of solution of organosulfur compounds saturated by air in presence of a sensitizer. 71 Toma and Hanan reported a photooxidation of a Ru-thiolate, [Ru(Hmctpy)(dmbpy)(κSSpyH)] 2+ (13) (Hmctpy= carboxy-substituted terpyridine, dmbpy=methyl-substituted bipyridine) to the corresponding partial S-oxygenated compound, [Ru(mctpy)(dmbpy) (κS-SO 2 py)] ( 14) involving 1 ∆O 2 under exposure of UV-Vis light in acetonitrile.…”

“…36,37 Nonetheless, there are some peroxo and activated oxygen donating sources (H 2 O 2 , dimethyldioxirane etc), [38][39][40] can effectively oxygenated the thiolato sulfur center similar to the molecular oxygen. However, analogous scrutiny of S-centered oxidation of aryl thiolates with platinum-group-metals for ruthenium, rhodium, palladium, platinum and gold are less common [41][42][43] and only a few reports for osmium 44 and iridium 45 have so far been made. It is worth of mentioning that such type of platinum group metal thiolato is very much prone to alike oxidation producing metallosulfones under aerobic condition and strictly O 2 atmosphere is not required.…”

Singlet Oxygen Generation, Quenching and Reactivity with Metal Thiolates†

“…Metal thiolate complexes are found in many different areas of chemistry, ranging from metalloproteins to luminescent materials (3,4). The thiolate ligand may be susceptible to oxidation by different oxygen-containing oxidants ranging from triplet oxygen to hydrogen peroxide to singlet oxygen (Type-II photooxidation), as well as Type-I photooxidation processes (5)(6)(7), leading to a variety of different products including sulfenates (S-bound sulfoxide), sulfinates (S-bound sulfones), sulfonates and rearranged products. Hence, formation of these products is not necessarily indicative of singlet oxygen being the oxidant; involvement of singlet oxygen is usually demonstrated by either direct observation of the near-infrared luminescence of the singlet oxygen molecule and/or kinetic studies involving deuterated solvents (where singlet oxygen has a longer lifetime) or addition of specific singlet oxygen quenchers such as sodium azide or 1,4diazabicyclo[2.2.2]octane (DABCO).…”

“…These pathways are summarized in Scheme 10 below. It is interesting to note that the very recent report of a Type-I photooxidation of a series of Pt(IV)(CH 3 ) 3 (1,2-diimine)(thiophenolate) complexes also yields the corresponding sulfinate complexes; the reaction was suggested to proceed via formation of a sulfur radical cation (7). This illustrates that formation of sulfinate products alone is not necessarily indicative of a Type-II mechanism and that further tests to differentiate between Type-I and Type-II photooxidation (solvent isotope effects or addition of singlet oxygen quenchers or direct observation of singlet oxygen luminescence) must be performed to establish the involvement of singlet oxygen in the oxidation of such metal thiolate complexes.…”