Synthesis of the First Stable Palladium Allenylidene Complexes

Abstract

Oxidative addition of BrCtCC(dO)NR 2 to [Pd(PPh 3) 4 ] affords the trans-alkynylbromopalladium complexes trans-[Br(PPh 3) 2 Pd-CtCC(dO)NR 2 ] (NR 2) NMe 2 (2a), N(CH 2) 4) (2b)). Subsequent reaction of 2a,b with P i Pr 3 in excess gives trans-[Br(P i Pr 3) 2 Pd-CtCC(dO)NR 2 ] (5a,b). The analogous reaction of 2b with P(C 6 H 4 OMe-4) 3 gives trans-[Br(P{C 6 H 4 OMe-4} 3) 2 Pd-CtCC(dO)NR 2 ] (7b), and that of 2a with trifluoroacetate gives trans-[(F 3 CCOO)(PPh 3) 2 Pd-CtCC(dO)NMe 2 ] (9a). Methylation of 2a,b, 7b, and 9a with either MeOTf or [Me 3 O]BF 4 and ethylation of 2a,b with [Et 3 O]BF 4 yield the first cationic allenylidene complexes of palladium, trans-[R*(PR′ 3) 2 Pd-CtCC(OMe)NR 2 ] + X-(R*) Br, CF 3 COO; R′) Ph, C 6 H 4 OMe-4, i Pr; X) OTf, BF 4).

Nickel alkynyl and allenylidene complexes: Synthesis and properties

“…However, a considerable p interaction of the lone electron pair at nitrogen with the allenylidene chain can be deduced from the appearance of two NeMe signals in the NMR spectra of complexes 6a and 6b. Similar results have recently been obtained with related Pd(II) complexes [10].…”

“…These structural features and the spectroscopic data indicate an important contribution of resonance form III and, to a significant smaller degree, of resonance form IV to the overall bond description (Scheme 4). A comparison of the spectroscopic data of these cationic nickel allenylidene complexes with those of related cationic palladium(II) allenylidene complexes [10] and of neutral pentacarbonyl chromium and tungsten complexes [(CO) 5 M]C]C]C(NMe 2 )OMe] (M ¼ Cr, W) [18] reveals that in the nickel complexes the alkynyl character (see III/IV in Scheme 4) is significantly more pronounced than in the corresponding chromium and tungsten complexes, as evidenced by the n(CC) vibration at higher energy by about 70e90 cm À1 , but a little less than in the palladium complexes.…”

“…Although allenylidene complexes of several transition metals are now known for quite some time, the synthesis of the first stable palladium allenylidene complexes has been reported only recently [10,11]. Analogously, until now only one nickel allenylidene complex has been described by Butenschön et al [12].…”

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

“…[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. Moreover, we describe the synthesis of a silver complex and subsequent transmetalation reactions with palladium and ruthenium.…”

“…The palladium-carbon bond length (1.986(2) ) lies at the shorter end of the range of PdÀC bond lengths found in NHC-palladium complexes [28] and is similar to that reported for the only known allenylidene Pd complex (1.925 ). [17] Next, we selected ruthenium as a metal center because of the considerable interest of allenylidene ruthenium complexes in homogeneous catalysis. [13] In this case, tris(triphenylphosphine)ruthenium dichloride was mixed with…”