Expanded‐Ring N‐Heterocyclic Carbenes for the Stabilization of Highly Electrophilic Gold(I) Cations

Abstract

Strategies for the synthesis of highly electrophilic Au(I) complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr2 C6 H3 ). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C6 F5 )3 , by contrast, generates a species (at low temperatures) featuring a [HB(C6 F5 )3 ](-) fragment with spectroscopic signatures similar to the "free" borate anion. Subsequent rearrangement involves BC bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr(f) 4 ] (Ar(f) =C6 H3 (CF3 )2 -3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}2 (μ-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr(f) 4 ](-) counterion.

Heterolysis of Dihydrogen by Silver Alkoxides and Fluorides

“…Expanded‐ring NHCs, including those derived from six‐ and seven‐membered cyclic amidinium salts, are stronger σ‐donors than imidazolinylidenes,27 and their bonds to silver(I) are less labile than those of five‐membered NHCs 28. Six‐membered NHCs support the formation of rhodium(I) fluorides,29 and of μ‐hydrido‐ and μ‐chlorodigold cations formed via strongly electrophilic [(NHC)Au] + equivalents 30. We hoped that a more inert NHCAg bond would suppress the formation of homoleptic [L 2 Ag] + from the [(LAg) 2 (μ‐H)] + cation.…”

“…In principle, the latter scenario is plausible, but the free energy for dissociation of H 2 from [(6 Me)Au(η 2 ‐H 2 )] + (6 Me is the N,N′ ‐dimethyl analogue of 6Dipp) has been calculated at +9.5 kcal mol −1 30. Because goldligand bonds are generally stronger than the corresponding silverligand bonds, we regard this figure as a rough upper boundary on the barrier for dissociation of H 2 from [LAg(η 2 ‐H 2 )] + 40.…”

Experimental and Computational Studies of the Copper Borate Complexes [(NHC)Cu(HBEt3)] and [(NHC)Cu(HB(C6F5)3)]

“…[15] Use of PhMe 2 SiH rather than Et 3 SiH gave only 6.A nX -ray crystal structure ( Figure 3) showed that 6 was mononuclear like 5,but that, in addition to aCu-H interaction, there was also one short Cu-C ipso contact (2.2183 (17) )tothe C 6 F 5 ring based on C29. [16] Then ature of this interaction differs noticeably from those found with more electropositive metals,w hich invariably involve ortho-C-F bond contacts.I np articular, the Cu-C ipso interaction in 6 was seen to be concomitant with lengthening of the B(1)-C(29) distance to 1.657(3) ,relative to the B(1)-C(23) and B1-C(35) distances of 1.624(3) and 1.628(3) , respectively.A dditionally,t he C 6Mes -Cu-H angle of 146.5(8)8 8 in 6 is notably more acute than that observed in 5 (162.8(14) 8 8).…”

“…[15] Use of PhMe 2 SiH rather than Et 3 SiH gave only 6.A nX -ray crystal structure ( Figure 3) showed that 6 was mononuclear like 5,but that, in addition to aCu-H interaction, there was also one short Cu-C ipso contact (2.2183 (17) )tothe C 6 F 5 ring based on C29. [16] Then ature of this interaction differs noticeably from those found with more electropositive metals,w hich invariably involve ortho-C-F bond contacts.I np articular, the Cu-C ipso interaction in 6 was seen to be concomitant with lengthening of the B(1)-C(29) distance to 1.657 (3) Weaker donation to Cu in 6 is also manifest in ah igher computed positive charge at Cu (Figure 4b)and the reduced s B-H !Cu interaction which the NBO 2 nd order perturbation analysis quantifies as 42.9 kcal mol À1 .T he similar charges at Cu, Band Hin6 and the free [(6Mes)Cu] + and [HB(C 6 F 5 ) 3 ] À ions again suggest 6 is ab orate complex. An additional feature,a gain consistent with am ore electron deficient Cu center, is the presence of aC u-C ipso bond path which entails aring critical point associated with the {CuHBC ipso }unit.…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…The[ LAu] + cation can be trapped by an equivalent of the NHCAuXp recursor to form Scheme19. [282] Theclosest to an isolable 4 ]. [395] Angewandte Chemie Reviews [{(NHC)Au} 2 (m-X)] + cations (where NHC = 6Dipp or 7Dipp; X = Ho rC l) when trying to remove the hydride by protonlysis with [H(OEt 2 ) 2 ][B(Ar CF3 ) 4 ]and chloride by abstraction with Na[B(Ar CF3 ) 4 ].…”