Oxidative Addition of a Hypervalent Iodine Compound to Cycloplatinated(II) Complexes for the C–O Bond Construction: Effect of Cyclometalated Ligands

Abstract

The complex [PtMe­(Obpy)­(OAc)2(H2O)], 2a, Obpy = 2,2′-bipyridine N-oxide, is prepared through the reaction of [PtMe­(Obpy)­(SMe2)], 1a, by 1 equiv of PhI­(OAc)2 via an oxidative addition (OA) reaction. Pt­(IV) complex 2a attends the process of C–O bond reductive elimination (RE) reaction to form methyl acetate and corresponding Pt­(II) complex [Pt­(Obpy)­(OAc)­(H2O)], 3a. The kinetic of OA and RE reactions are investigated by means of different spectroscopies. The obtained results show that the reaction rates of OA step of 1a are faster than its analogous complex [PtMe­(ppy)­(SMe2)], 1b, ppy = 2-phenylpyridine. The density functional theory (DFT) calculations signify that the OA reaction initiated by a nucleophilic attack of the platinum­(II) central atom of 1b on the iodine­(III) atom while it had commenced by a nucleophilic substitution reaction of coordinated SMe2 in 1a with a carbonyl oxygen atom of PhI­(OAc)2. Our calculation revealed that the key step for 1a is an acetate transfer from the I­(III) to Pt­(II) through a formation of square pyramidal iodonium complex. This can be attributed to the more electron-withdrawing character of Obpy ligand than to ppy which reduces the nucleophilicity of Pt atom in 1a. Furthermore, 2a with electron-withdrawing Obpy ligand prone to C–O bond formation faster than complex [PtMe­(ppy)­(OAc)2(H2O)], 2b, with an electron-rich ppy ligand which conforms to the anticipation that REs occur faster on electron-poor metal centers.

High Selectivity in Csp2–Csp2 versus Csp3–O Reductive Elimination from Cycloplatinated(IV) Complexes

“…In the aromatic region, the H atom of the C−H group adjacent to the ligating N atom of the bhq ligand, i.e., H 2 , emerged as a doublet of doublets at 9.26 ppm, coupled to the Pt atom with a typical value of 3 J PtH 2 = 12.9 Hz for the cycloplatinated(IV) complexes. 27,42 S6 and S12).…”

“…Transition metal-catalyzed selective C–H functionalization of hydrocarbons is one of the major challenges of organometallic chemistry. − This transformation involves a number of fundamental steps such as oxidative addition (OA), β-hydride elimination, migratory insertion, and reductive elimination (RE). ,, RE reactions are critical for the final product-forming step. − To reduce deleterious side reactions and enhance overall catalyst efficiency, the metal must undergo the appropriate series of fundamental reactions with great selectivity . Understanding the parameters that influence the selectivity of these reactions is key to designing and improving novel transition metal-catalyzed transformations. − High-valency octahedral metal complexes have more donor ligands at the metal center than their more common square planar counterparts, which may indicate greater competitiveness in the formation of C–C or C–X (X = heteroatom such as O, N, and S) bonds and thus require greater control over the reaction selectivity. , For instance, as different research groups have reported, competitive carbon–carbon and/or carbon–heteroatom bond RE can be seen at Pt­(IV) and Pd­(IV) centers in a single complex. ,− Notably, high-valency d 6 metal systems typically prefer the C–C over the C–X coupling reaction, though exceptions to this trend have been noted. ,− On the other hand, the formation of new C–C or C–X (X = heteroatom) bonds through RE from a M­(IV) center (group 10) constitutes the product-forming step in various catalytic oxidative transformations that have garnered considerable attention over the past 2 decades. ,− Despite some noteworthy advancements in this field, mechanistic studies in selective C–X vs C–C bond RE reactivity have not been extensively investigated. ,, Furthermore, the chemoselectivity of C–X bond RE is poorly known in systems where different competing coupling reactions may occur . As a result of their kinetic inertness, Pt­(IV) compounds can be considered excellent models for studying these coupling processes. ,,…”

Synthesis and Crystallographic Characterization of Heteroleptic Ir(III) Complexes Containing the N-oxide Functional Group and Crystallographic Characterization of Ir(III) N-oxide Precursors

“…The tendency to coordinate the oxygen atom of a heterocyclic N-oxide group is generally favored in most transition metal complexes [20][21][22][23], largely due to the hardness of the N-oxide as a donor. Strategies for orienting the oxygen atom of the N-oxide group externally on a metal complex (that is, not coordinated to the metal) may include utilizing ligands with favorable coordination sites away from the N-oxide or oxidizing the exposed external nitrogen atom of a coordinated ligand [24][25][26][27][28][29][30][31][32]. For the former, ligands such as 5-methypyrazine-2-carboxylate (5 mpca) and its corresponding N-oxide (Acipimox, 5 mp-caO, Figure 1) may be promising [33].…”

Cyclometalated platinum(ii) complex as a selective light switch for G-quadruplex DNA

“…17 Cyclometalated platinum(II) complexes bearing π-conjugated organic ligands have been much less widely studied in the context of G quadruplex binding than N-coordinated analogues. [18][19][20][21][22][23][24] Although for tridentate ligand coordinated complexes they have advantageous photophysical properties that have been exploited across a range of domains, [25][26][27][28] including as probes for biomolecules 29 and bioimaging. [30][31][32][33] Given their structural analogy to N,N,N Pt terpy complexes we rationalised they might be expected to show similar affinity but with superior addressability due to their photophysical properties in terms of both luminescent quantum yield and red emission wavelength.…”