Selective formation of formamidines, carbodiimides and formimidates from isocyanide complexes of Mn(i) mediated by Ag₂O

Abstract

The isocyanide ligands in complexes fac-[Mn(CNR)(bipy)(CO)3]+ are selectively transformed into formamidines, carbodiimides and formimidates upon nucleophilic addition of NH2Me or alkoxides and subsequent treatment with Ag2O, which promotes tautomerization or oxidation processes of the former carbene ligands.

Ag2O versus Cu2O in the Catalytic Isomerization of Coordinated Diaminocarbenes to Formamidines: A Theoretical Study

“…On the contrary, protic diaminocarbene complexes are very stable [16,17] and, as shown in the present case, can be transformed into the corresponding formamidine complexes by addition of a catalyst (Scheme 1). Interestingly, we have similarly demonstrated that coordinated NHCs isomerize to imidazoles under the same conditions [13,14]. In order to shed light on the mechanism of this intriguing reaction and to make a contribution to the rather scarcely developed field of mechanistic studies of organic and organometallic transformations assisted by Ag 2 O, we have now carried out DFT theoretical calculations supporting the finding that the exclusive behavior of Ag 2 O in the catalytic isomerization of diaminocarbenes to formamidines is essentially based on an ideal combination of the basic character of the oxide ion and the Lewis acid character of the Ag + ion.…”

“…The detailed experimental procedure for the synthesis and spectroscopic structural characterization (FTIR and NMR) of 1, 2, and 3 complexes has been described previously [13,14]. Selected crystallographic data and structure refinement parameters for 3•ClO − 4 are given here for the first time in Table S1 of the Supplementary Material, while Figure 1 shows the experimental molecular structure together with the atomic numbering scheme and selected bond lengths and angles.…”

“…In spite of that, mechanistic studies of reactions involving Ag 2 O are really scarce, with some notable exceptions being those related to the ethylene epoxidation reaction [9][10][11] and the generation of NHC-Ag(I) complexes used in further transmetalation processes [12]. We have recently described an efficient experimental procedure to transform isocyanides into formamidines by reaction with primary amines [13,14], involving a key step tautomerization of diaminocarbenes coordinated to Mn(I), mediated by Ag 2 O, into formamidines. This behavior is apparently exclusive to Ag 2 O, as isostructural Cu 2 O does not show any activity in the same reaction, nor do other metallic oxides of variable nature.…”

Manganese-N-heterocyclic carbene (NHC) complexes – An overview

“…These ligands were then used as carbene transfer agents (see section 6.4). More recently, they published a report on the reversible tautomerization of this type of complexes mediated by Ag 2 O [51].…”

“…They also showed that the same procedure does not work for the synthesis of silver(I) complexes. More recently, this tautomerization phenomenon has been used by Ruiz and co-workers to transform isocyanides into diaminocarbenes, formamidines, carbodiimides, alkoxyaminocarbenes and formimidates [51].…”