Mechanism of the Competition between Phenyl Insertion and Ligand Reductive Elimination on a Hindered Platinum(IV) Cyclometalated Complex

Abstract

Careful tuning of the reaction conditions has been proved to be essential for the operation of two disjunctive reaction mechanisms occurring on the ortho-metalated platinum compounds [Pt IV Br(Ph) 2 -(C 5 CH 4 CHNZ)(SMe 2 )] (Z ) Me, Bzl, CH 2 (2,4,6-Me 3 C 6 H 2 )). In dilute solutions, where the already established lability of the SMe 2 ligand favors the existence of the pentacoordinated {Pt IV Br(Ph) 2 (C 4 CH 4 -CHNZ)} species, the complex evolves to produce the insertion of one of the phenyl ligands into the cyclometalated Pt IV -C bond to yield the complexes [Pt II Br(C 4 CH 3 C 6 H 4 CHNZ)(SMe 2 )], which contain a seven-membered cyclometalated ring. The process involves the formation of an hydride intermediate, which has been detected via low-temperature proton NMR for Z ) Bzl, prior to the reductive elimination of benzene. In more concentrated solutions, or in the presence of large amounts (200-500 fold) of SMe 2 , the existence of pentacoordinated species is reduced to a minimum and a reductive C-C coupling takes place between the metalated imine carbon and one of the phenyl ligands, yielding the coordination complexes [Pt II Br(Ph)(C 6 H 3 C 6 H 4 CHNZ)(SMe 2 )], which evolve rapidly to trans-[Pt II Br(Ph)(SMe 2 ) 2 ] and free C 5 H 3 C 6 H 4 CHNZ. The validity of the mechanisms proposed has been proved via stoichiometric and reactivity studies carried out under carefully controlled conditions, both on initial Pt IV complex and on the final inserted complex, [Pt II Br(C 4 CH 3 C 6 H 4 CHNZ)(SMe 2 )]. The overall reactivity is rather surprising, given the generally accepted dissociative processes involved in the preliminary steps of reductive elimination reactions on Pt IV complexes. DFT calculations have been carried out in order to check the energetic validity of the proposed disjunctive reaction mechanisms. From the data obtained, it is clear that the formation of the pentacoordinated species {Pt IV Br(Ph) 2 (C 5 CH 4 CHNZ)} could effectively lead to the standard C-C reductive coupling, but in our case the existence of the parallel insertion process is highly favored. As a consequence the observed reductive elimination reaction can only occur via the otherwise less favored direct C-C coupling on the octahedral [Pt IV Br(Ph) 2 (C 4 CH 4 CHNZ)(SMe 2 )] starting material.

Kinetico-mechanistic studies on the formation of seven-membered [C,N]-platinacycles: the effect of methyl or fluoro substituents on the aryl ancillary ligands

“…4a) are very different, which is not surprising in view of the results indicated in the previous paragraphs and in the second column of the table. 14,15 In this case it is the reductive elimination reaction step that has been quantified, showing rather high values of ΔH ‡ , practically zero values of ΔS ‡ and positive volumes of activation (Fig. 4b).…”

“…The different steps involved in the full preparative process indicated have been often studied from a kinetico-mechanistic perspective. 14,15,19 The behaviour is well-established for the initial oxidative addition reactions (4 → 5),10,20-24 as well as for the reductive elimination/oxidative addition follow up processes (5 → 7).16-18 From the data collected so far in the literature, it is clear that, despite the relative formal simplicity of the reactivity observed, the general mechanism is a compendium of multistep processes that include fast coordination/decoordination equilibria21,23,24 and isomerization reactions.16,17 Scheme 3 collects the kineticomechanistic sequential steps expected for the process.…”

“…S1, ESI †) was observed for the time span used, as expected from the above NMR data. 14,15 The data collected in Table 1 correspond to the rate constants of the rate limiting process of formation of compound 7 from complex 6 in a cis-N imine /C form, as measured by 1 H NMR spectroscopy. The values determined for ΔH ‡ , ΔS ‡ and ΔV ‡ are the first ones determined for the oxidative addition reaction leading to the formation of seven-membered metallacycles with monodentate ligands indicated in Scheme 3 (Fig.…”

“…9,10 In recent years, a novel class of seven-membered terdentate [C,N imine ,N amine ] or bidentate [C,N imine ] platinacycles containing biaryl moieties has been reported. [11][12][13][14][15] The mechanism of formation of this type of terdentate [C,N imine ,N amine ] compounds has been carefully studied, including the isolation of several intermediates. 16 As shown in Scheme 1, the initial step of the complete process corresponds to the formation of cyclometallated platinum(IV) compounds through intramolecular C-Br or C-Cl bond activation of the ligand at the platinum precursor.…”

Diarylplatinum(II) Compounds as Versatile Metallating Agents in the Synthesis of Cyclometallated Platinum Compounds with N-Donor Ligands

“…Initial work was based on the reaction of [PtPh 2 (SMe 2 ) 2 ] with imine 2-BrC 6 H 4 CH=NCH 2 C 6 H 5 [64,65] and further work was carried out using cis-[Pt(4-MeC 6 H 4 ) 2 (μ-SEt 2 )] 2 and imines 2-BrC 6 H 4 CH=NCH 2 (4-ClC 6 H 4 ) and 2,6-Cl 2 C 6 H 3 CH=NCH 2 (4-ClC 6 H 4 ) [62]. In these reactions, both intramolecular activation of C-Br or C-Cl bonds lead eventually to formation of seven-membered platinacycles.…”