Kinetics and thermodynamics of the reversible reaction between carbon monoxide and palladium(I) dimers containing bis(diphenylphosphino)methane

Abstract

The kinetic and thermodynamic parameters for the reversible insertion of carbon monoxide into the palladium-palladium bond of Pd2(dpm)2X2 complexes (dpm = Ph2PCH2PPh2; X = Cl, Br, I, NCO) in iVJV-dimethylacetamide solution have been determined by spectrophotometric methods. The forward reaction is first order in both Pd2 and CO and occurs on the stopped-flow time scale at around ambient temperatures, the rate constants being similar for the four systems (3.15-6.60) X 103 M"1 11s-1 at 24 °C. The equilibrium constants, estimated by kinetic and/or static methods, decrease in the order X = NCO > Cl > Br > I (from 3.7 X 105 to 1.3 X 103 M"1 at 24 °C) and are governed largely by the first-order off-rates, which probably depend more on a weakening of the Pd-CO bonds throughout the series than changes in the metal-metal bond strength. A correlation between In for the off-rates and the energy of a high intensity UV band of the Pd2(dpm)2X2 complexes is noted. The CO addition to give Pd2(dpm)2(/u-CO)X2 occurs because of the exothermicity of the reaction that is unfavorable in terms of AS; an approximate value of 90-120 kJ mol"1 is estimated for the Pd-Pd bond strength. The systems show high selectivity for binding of CO in the presence of H2, C2H2, C2H4, C02, 02, or air.

Mechanistic aspects of the reaction of hydrogen sulfide with dinuclear palladium(I) complexes containing bis(diphenylphosphino)methane and related ligands

“…Reaction [I] is readily monitored by solution electronic spectroscopy; species such as 1 are reddish-orange with two to three absorption maxima in the 340-590 nm region (2,15,16), while the p-S products (2) are brown and have absorption maxima in the 325-350 and 470-485 nm regions (2,17). Figure 3 shows spectral changes on treating a CH2C12 solution of the bromide l b with H2S at 25°C; rates for the bromide system were followed by observing the decreasing absorption at 428 nm as a function of time, and the observed isosbestic point showed a well-behaved system.…”

“…Within 1, the Pd-Pd bond strength is expected to increase in the order I < Br < C1 (16), the reverse of the trans effect of the halides (32), and thus the reactivity trend is not dominated by differences in the metal-metal bond strength. Reactivity of 1 toward CO is also C1> Br > I in terms of equilibrium constants, and this is governed by the rate of CO loss from the p-carbonyl product that decreases in the reverse order (I > Br > Cl), the forward carbonylation reaction rates being essentially the same for the three halide species (15); it has been suggested that the strength of the metal-carbonyl bond in the product (I < Br < C1) governs the reactivity trend (15). Such could be the case in I the H2S systems, if the k-' step of the mechanism outlined in eq.…”

Dinuclear Pd(i) complexes—solely precatalysts? Demonstration of direct reactivity of a Pd(i) dimer with an aryl iodide

“…24,25 This is in line with previously reported experimental bond energy data of a related Pd(I) dimer, determined to be between 21 and 29 kcal mol À1 . 26 For the alternative dinuclear pathway we were able to locate three distinctly different transition states for halide exchange at iodobenzene (see Fig. 2), featuring (i) direct oxidative addition by Pd-Pd (TS A ), in which the coordination number at Pd is Scheme 3 Attempted reductive elimination from Pd(II).…”

Catalysis with Palladium(I) Dimers

“…By contrast, odd oxidation state palladium is comparatively scarce in catalysis. Due to their unpaired electrons and the potential formation of a rather strong Pd−Pd bond (usually about 25 kcal mol −1 ) [2] these oxidation states tend to form dinuclear complexes. A wide variety of Pd I dimers has been synthesized and has been known for decades.…”