Electron Transfer Activation in the Oxidative Addition of Iodine to Rhodium(I) Complexes

Abstract

The kinetics of the oxidative addition of iodine to the following isocyaniderhodium(I) monomers and dimers in MeCN has been examined; [Rh(RNC)4]+, [Rh(RNC)2(PPh3)2]+, [Rh2(dppm)2(RNC)4]2+, and [Rh2(dicp)4]2+ (R=alkyl, aryl; dppm=bis(diphenylphosphino)methane; dicp=1,3-diisocyanopropane). The reaction proceeds via two consecutive steps which consist of the formation of the initial adduct followed by the intramolecular isomerization to yield the final trans-adduct. The kinetic results for the first step of the oxidative addition reactions suggest that there exist three kinds of intermediates formed between the Rh(I) complex and iodine with 1:1, 1:2, and 2:1 stoichiometry prior to the formation of the initial adduct. This mechanism coincides with that for the electron transfer reactions between the Rh(I) complexes and inorganic oxidants such as [Fe(N-N)3]3+ (N-N=1,10-phenanthroline and 2,2′-bipyridine). The values of the rate constants for the first step of the oxidative addition of iodine to dppm- or dicp-bridged Rh(I) dimers are found to agree with those for the electron transfer reactions between the same reactants by using linear free energy relationships. The possibility of electron transfer activation as the rate determining step in the oxidative addition to Rh(I) complexes is discussed.

Effects of base on oxidation of an NADH model compound by iron(III) complexes and tetracyanoethylene

“…The linear correlation between k (= kB(')/[TCNE]) and the base concentration in Figure 6 accords with equation (18) under the condition k-l kz + k,'[B] + kl[TCNE], when equation ( 18) is reduced to equation (19), where K (= k,/k-') is the formation constant of the radical ion pair [BNAH+' TCNE-'I. With higher concentration of base, equation (18) under the condition k-l 9 k2 + kl[TCNE] may be rewritten as equation (20), where k(') represents the pseudofirst-order rate constant in the absence of a base and the k(')/[TCNE] value accords with the rate constant k in the absence of a base [equation ( 6)], 2.1 x 10' mol-' dm3 s-'.…”

“…The formation constant K for the radical ion pair [BNAH+' TCNE-'1 can be obtained from a linear plot between l/kB(') and l/[TCNE] with a fixed base concentration, based on equation (22), which is derived from equation ( 18 Thus, the kinetics of the reduction of TCNE by BNAH in the presence of a base can be fully explained by equations ( 10)- (12). By using the K value evaluated from Figure 8, the proton transfer rate constants kg' for various bases may be determined from the slopes in Figure 6, using equation (19). The results are listed in the Table, together with the rate constants k, obtained independently from the electron-transfer reactions of BNAH with [Fe(N-N)3]3+ in the presence of various bases.…”

Ligand Effects on the Oxidative Addition of Halogens to (dpp-nacnacR)Rh(phdi)

“…Isomerization often complicates mechanistic understanding in these reactions, with mechanisms proposed in which the cis isomer forms first, followed by isomerization to the trans isomer, and vice versa. , No models for analogous η 1 -adducts exist for other catalytically significant metals such as rhodium, though I 2 has been shown to form a bridge between bimetallic rhodium(II) complexes . The mechanism of X 2 addition to square planar Rh(I) complexes has also been shown to depend on the relative concentrations of the reactants …”

“…24 The mechanism of X 2 addition to square planar Rh(I) complexes has also been shown to depend on the relative concentrations of the reactants. 25 Previously, we reported the synthesis and redox properties of (dpp-nacnac R )Rh (N-i Pr 2 C 6 H 3 )] 2 − , phdi =9,10-phenanthrenediimine} complexes. 26 These complexes showed noninnocent electronic behavior owing to the juxtaposition of a low-valent rhodium(I) center and a reducible, redox-active, α-diimine ligand.…”

Rhodium and Iridium Complexes of Lutidine-Based NHC/Amino Pincer Ligands: From Monodentate to Tetradentate Coordination Enabled by C–H Activation

“…All in all, this suggests an S N 2-like mechanism with the metal oxidation from M­(I) to M­(III), giving a cationic pentacoordinated monoiodido species [MI­{κ 4 ­ C , C ′, N , N ′-(CH 2 ­CMe 2 ImCH 2 PyCH 2 NMe 2 )}] + ( D1 ), which in turn evolves to the octahedral product of the reaction E (see the Supporting Information). As for the reaction of D with I 2 , neither intermediates nor TS have been located on the potential energy surface, although the formation of η 1 -I 2 has been previously reported to be relevant in the oxidative addition of diiodine to rhodium or iridium complexes. , …”