Unprecedented Allenylidene Transfer from Chromium to Tungsten

Abstract

Cartesian coordinates and energies (at 298 K) of the starting and product complexes, optimized minima, transition states, and fragments (CO) 5 Cr=C=C=CMe 2 (Aa) Total Gibbs free energy, Gtot (Htot-T∆S):

Synthesis of the First Stable Palladium Allenylidene Complexes

“…3 The Pd-C bond (1.925(3) Å) in the allenylidene complex 10a-BF 4 is close to the shorter limit of observed Pd-C bond lengths and is shorter than the Pd-C bond in the related cationic N-heterocyclic carbene (NHC) complexes trans-[L(PR 3 ) 2 Pd-(NHC)] + (1.97-2.01 Å) 16 or in the neutral alkynyl complexes trans-[L(PR 3 ) 2 Pd-CtCR] (1.947(3) Å in 5b; range usually observed 1.95-2.07 Å 14d,17 ). Similarly to other neutral π-donorsubstituted allenylidene complexes of chromium and tungsten [11][12][13]18 the C(1)-C(2) bond is very short (1.217(4) Å) and only slightly longer than in the alkynyl complex 5b (1.209 5Å). Conversely, the C(2)-C(3) bond in 10a-BF 4 (1.420 4) Åis rather long and is even longer than that in [(CO) 5 CrdCd CdC(O-adamantyl)NMe 2 ] (1.366(7) Å) 18 but, as expected, is shorter than in 5b (1.454(4) Å).…”

“…Similar trends have been observed on alkylation of alkynylpentacarbonylchromate complexes to give neutral allenylidene complexes. 12 The extent of these shifts and the observation of two resonances for the dimethylamino substituent in the 1 H and 13 C NMR spectra demonstrate the importance of the zwitterionic resonance forms II and III for the overall bond description of these cationic allenylidene complexes (Scheme 4). 12 As in 2a, 5a, 7b, and 9a, both phosphine ligands are mutually trans, as indicated by the presence of only one signal for both phosphorus nuclei in the 31 P NMR spectra.…”

“…11 The analogous transmetalation of several allenylidene ligands from chromium to tungsten likewise proceeded quickly in yields ranging from 83 to 97%. 12 However, all attempts to transfer allenylidene ligands from chromium to palladium met with failure. Therefore, the strategy had to be changed and an approach starting from alkynyl complexes was investigated.…”

Bis(allenylidene) Complexes of Palladium and Platinum

“…In contrast to C α and C β , the resonances of the C γ atom and of the N substituents are almost unaffected on alkylation. Similar trends have earlier been observed on alkylation of (alkynyl)pentacarbonylchromate complexes to give neutral allenylidene complexes …”

Synthesis of Allenylidene Lithium and Silver Complexes, and Subsequent Transmetalation Reactions

“…[14] They are generally generated in the coordination sphere of the metal, often using propargylic alcohols as sources of the allenylidene C 3 fragment, a strategy introduced by Selegue. [15] In 2006, Fischer and co-workers reported the first example of allenylidene transfer from chromium to tungsten, [16] but later stated that this exchange process could not be extrapolated to other metals, such as palladium. [17] Herein we report our attempts to isolate a free (diamino)allenylidene C, which resulted in the characterization of the first allenylidene lithium adduct.…”