This study deals with two striking
phenomena: the complete protection
against decomposition of hypothetically monocoordinated AuI intermediates [AuL]Y (L = strongly coordinating ligand; Y– = poorly coordinating anion) by addition of small substoichiometric
amounts (5 mol % relative to Au) of not strongly coordinating ligands
(e.g., AsPh3) and the fact that, in contrast, strongly
coordinating ligands cannot provide this substoichiometric protection.
The two phenomena are explained considering that (i) the existence
of real monocoordinated [AuL]Y is negligible in condensed phases and
the kinetically efficient existing species are dicoordinated [AuL(W)]Y
(W = any very weakly coordinating ligand existing in solution, including
OH2, the solvent, or the Y– anion) and
(ii) these [AuL(W)]Y intermediates give rise to decomposition by a
disproportionation mechanism, via polynuclear intermediates formed
by associative oligomerization with release of some W ligands. It
is also shown that very small concentrations of [AuL(W)]Y are still
catalytically efficient and can be stabilized by overstoichiometric
adventitious water, so that full decomposition of the catalyst is
hardly reached, although eventually the stabilized concentration can
be kinetically inefficient for the catalysis. These results suggest
that, in cases of gold catalysis requiring the use of a significant
quantity of gold catalyst, the turnover numbers can be increased or
the concentration of gold catalyst widely reduced, using substoichiometric
protection properly tuned to the case.
X-Ray and DFT studies support that the red-shift of luminescence from [AuAr(CNPy-4)] (Ar = C6F5, C6F3Cl2-3,5) to [Ag[AuAr(CNPy-4)]2](BF4) is not due to non-existent Au⋯Ag interactions but to adoption of structures with shorter Au⋯Au distances.
Experiments mixing the stable 16e 5-coordinate complexes [RhCp*Ar] (Cp* = CMe; Ar = CF, CFCl-3,5) uncover fast aryl transmetalations. Unexpectedly, as supported computationally, these exchanges are not spontaneous, but catalyzed by minute amounts of 18e (μ-OH)[RhCp*Ar] as a source of 16e [RhCp*Ar(OH)]. The OH group is an amazingly efficient bridging partner to diminish the activation barrier of transmetalation.
By combining kinetic experiments,t heoretical calculations,a nd microkinetic modeling,w es how that Pf/Rf (C 6 F 5 /C 6 Cl 2 F 3 )e xchange between [AuPf(AsPh 3 )] and trans-[RhRf(CO)(AsPh 3 ) 2 ]d oes not occur by typical concerted Pf/ Rf transmetalation via electron-deficient double bridges. Instead, it involves asymmetric oxidative insertion of the Rh I complex into the (Ph 3 As)Au À Pf bond to produce a[ (Ph 3 As)Au À RhPfRf(CO)(AsPh 3 ) 2 ]i ntermediate,f ollowed by isomerization and reductive elimination of [AuRf(AsPh 3 )]. Interesting differences were found between the LAuÀAr asymmetric oxidative insertion and the classical oxidative addition process of H 2 to Vaska complexes.
Aryl rearrangements triggered by Cl extraction from starting from trans-[AuIII(Rf)2Cl2]– (Rf = C6F3Cl2-3,5), led fastly to a mixture of [Au(Rf)3(solv)], [Au(Rf)2Cl(solv)], and [Au(Rf)Cl2(solv)] (solv = OEt2, OH2). 19F NMR and...
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