Dynamic NMR Study of Ethene Exchange in Cationic CNN-Type Platinum(II) Complexes

Abstract

Cationic ethylene platinum(II) complexes of the type [Pt(CNN)(C(2)H(4))](+), containing a methyl fragment and different diimines (NN), or terdentate (kappaC-kappa(2)NN') anionic ligands, were synthesized and fully characterized both as solids and in solution [NN = 2,2'-dipyridylamine, 1; 2,2'-dipyridylsulfide, 2; 1,10-phenanthroline, 3; 4,7-diphenyl-1,10-phenanthroline, 4; 3,4,7,8-tetramethyl-1,10-phenanthroline, 5; 2,2'-bipyridine, 6; HC-NN = 6-tert-butyl-2,2'-bipyridine, 7; 6-neo-pentyl-2,2'-bipyridine, 8; 6-phenyl-2,2'-bipyridine, 9; 6-(alpha-methyl)benzyl-2,2'-bipyridine, 10; 6-(alpha-ethyl)benzyl-2,2'-bipyridine, 11; 6-(alpha,alpha-dimethyl)benzyl-2,2'-bipyridine, 12]. Crystals suitable for X-ray analysis of complexes 5 and 7 were obtained. Ethene exchange at the cyclometalated platinum(II) complexes 7, 8, and 10-12 was studied by (1)H NMR line-broadening experiments in chloroform-d, as a function of both temperature and olefin concentrations. For the other prepared complexes the process was too fast to be monitored on the NMR time scale even at the lowest temperature. The ethylene exchange rates show a linear dependence on the concentration of the free ligand, with a negligible k(1) term indicating that either a solvolytic or a dissociative pathway to the products is absent or negligible. The values of the second-order rate constants k(exc), as obtained by linear regression analysis of the experimental data at 298 K, are in a range of ca. 10(4)-10(5) s(-1) m(-1). The activation entropies are negative, ranging between -129 and -112 J K(-1) mol(-1), as expected for associative processes. The activation process is largely entropy controlled: the TDeltaS()() contribution to the free energy of activation is extremely large, amounting to more than 80% for all complexes, with a smaller enthalpy contribution. All the experimental findings evidence that the mechanism takes place via an associative attack by the entering olefin, through a well-ordered, stable pentacoordinated transition state with the two ethene molecules on the trigonal plane. The reactivity of [Pt(CNN)(C(2)H(4))](+) complexes is strongly dependent on the choice of coordinated 6-substituted-2,2'-bipyridines, especially when the terdentate anionic fragment is capable of generating steric crowding and congestion on the coordination plane.

Low‐Temperature Kinetic NMR Studies on the Insertion of a Single Olefin Molecule into a ZrC Bond: Assessing the Counterion–Solvent Interplay

“…A smaller amount of energy is required to displace the B(C 6 F 5 ) 4 À anion from the first coordination sphere but, because of the fact that the degrees of mobility of B(C 6 F 5 ) 4 À are similar in ISIPs and OSIPs, the recovery of entropy is smaller and approaches the typical value of DS°of an associative bimolecular process occurring in low polar solvents (around À50 cal mol À1 K À1 ). [36][37][38][39] Notably, almost the same recovery of entropy (DS°= À44 AE 5 cal mol À1 K À1 ) was observed for the sole olefin polymerization catalyst with the MeB(C 6 F 5 ) 3 À counterion in the second coordination sphere. [29] The insertion of 2 in 1a and 1b is around 3-5 and 50-100 times slower, respectively, in [D 8 ]toluene/[D 5 ]chlorobenzene than in [D 8 ]toluene (DDG°= 0.5-1.0 and 2.2-2.5 kcal mol À1 for 1a and 1b, respectively) and the anion has no effect on the insertion rate (Table 1, entries 9-16).…”

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

“…ΔG q values for related systems ranged from ∼14.5 kcal/mol to <9.5 kcal/mol. 85 We attempted to calculate the energetics of a second insertion of ethylene from [ …”

Synthesis and Reactivity of Square‐Planar PtII–η1‐Hydrocarbyl Complexes Containing cis‐Coordinated Olefin or Alkyne

“…As a consequence, only one stereoisomer was observed in the case of α-olefins in the temperature range 203-328 K (actually only one couple of enantiomers), namely I I of Figure 3, in which the steric contacts are minimized by orienting the alkene substituent RЈЈ toward the Pt-Me bond. These qualitative studies were recently [12] confirmed and implemented by preparing other complexes of the same general type (I-38 to I-42), along with related cyclometalated platinum(II) species (I-54 to I-59). These compounds were analogously synthesized by substituting the halides of neutral [PtCl(N,NЈ,C)] precursors with ethylene in the presence of a silver or sodium salt.…”