Combined Low Temperature Rapid Scan and¹H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

Abstract

A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)Pt II diphenyl complex (denoted as (N-N)PtPh 2 ) has been undertaken in dichloromethane solution with and without acetonitrile as a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the temperature range -80 to +27 °C allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh 2 with HBF 4 3 Et 2 O in CH 2 Cl 2 /MeCN occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces (N-N)PtPh 2 H(NCMe) + (ΔH q = 29 ( 1 kJ mol -1 , ΔS q = -47 ( 4 J K -1 mol -1 ). At higher temperatures, (N-N)PtPh 2 H(NCMe) + eliminates benzene to furnish (N-N)PtPh(NCMe) + . This reaction proceeds by rate-limiting MeCN dissociation (ΔH q = 88 ( 2 kJ mol -1 , ΔS q = +62 ( 6 J K -1 mol -1 , ΔV q = +16 ( 2 cm 3 mol -1 ). Protonation of (N-N)PtPh 2 in dichloromethane in the absence of MeCN cleanly produces the Pt(II) π-benzene complex (N-N)PtPh(η 2 -C 6 H 6 ) + at low temperatures. Addition of MeCN to a solution of the π-benzene complex causes an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by 1 H NMR (ΔH q = 39 ( 2 kJ mol -1 , ΔS q = -126 ( 11 J K -1 mol -1 ). When the stronger triflic acid is employed in dichloromethane/acetonitrile, a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe) + produces (N-N)Pt(NCMe) 2 2+ and benzene with no detectable intermediates (ΔH q = 69 ( 1 kJ mol -1 , ΔS q = -43 ( 3 J K -1 mol -1 ). The mechanisms for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(η 2 -C 6 H 6 ) + . It appears that the exchange occurs by a direct σ-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling sequence that was proposed.

Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

“…Similar behavior involving protonation at the metal and subsequent hydrocarbon elimination has been reported for (Ar-DAB)PtMe 2 [16][17][18]55] and (Ar-DAB)PtPh 2 [15,32] complexes. The formation of 2a-g is fully consistent with protonation at Pt to give a coordinately unsaturated, five-coordinate Pt IV hydride intermediate that is trapped by acetonitrile, in agreement with mechanistic studies on the mentioned diimine-Pt dimethyl and diphenyl complexes.…”

“…The formation of 2a-g is fully consistent with protonation at Pt to give a coordinately unsaturated, five-coordinate Pt IV hydride intermediate that is trapped by acetonitrile, in agreement with mechanistic studies on the mentioned diimine-Pt dimethyl and diphenyl complexes. [16,32] Low-Temperature Protonation of (N-N)PtPh 2 (4a-g, Scheme 4) at sub-ambient temperatures. The 1 H NMR spectra of these complexes exhibit a characteristic singlet arising from the η 2 -C 6 H 6 ligand at approximately δ = 7.1 (4a-f) or 6.9 (4g); the lower chemical shift value of the latter may again reflect the better donor capacity of the BICAT system when compared to BIAN.…”

“…On the other hand, the η 2 -benzene complexes will be stabilized by the 2,6-dimethyl groups because the displacement of benzene (and other hydrocarbons) from these and related diimine-Pt complexes has been demonstrated to be associative reactions. [32,59,60] Scheme 4. It has been demonstrated that the kinetically preferred site of protonation is the Pt center for (Ar-DAB)PtMe 2 complexes. [17,18] We have recently presented experimental evidence that this is also the case for (Ar-DAB)PtPh 2 analogues, [32] a scenario that had already been predicted by DFT calculations.…”

“…the protonation of Pt II aryl complexes. [15,32] Of particular relevance for this contribution, we have recently [32] conducted detailed kinetic and mechanistic studies of a protonation-induced sequence of reactions that occur from (N-N)PtPh 2 with N-N representing the DAB-type ligand Ar-N=CMe-CMe=N-Ar; Ar = 2,6-Me 2 C 6 H 3 . Here, low-temperature protonation in CH 2 Cl 2 /MeCN cleanly yielded the Pt IV hydride (N-N)-PtPh 2 H(NCMe) + which upon heating eliminated benzene to furnish benzene and (N-N)PtPh(NCMe) + in a reaction for which initial MeCN dissociation was rate limiting.…”

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

“…Journal of the American Chemical Society ARTICLE (NÀN = ArNd CMeÀCMedNAr, Ar = 2,6-Me 2 C 6 H 3 ) occurs by a σ-bond metathesis pathway with a calculated activation energy ∼5.0 kcal/mol lower than that calculated for oxidative addition. 60 The calculated transition state is similar to that proposed for σ-bond metathesis-type transition states in which the metal appears to interact with the activated hydrogen, which have been coined "σ-complex assisted metathesis" (σ-CAM) or oxidative hydrogen migration.…”