Bis carbene complexes

The dicationic ditriflate salt of 1,2,4-trimethyltriazolium with silver acetate gives the bis-carbene complex 185 (00JOM(600)l 12). In excess silver acetate, the one-dimensional polymeric species with alternating silver ions and 1,2,4-triazol-3,5-diylidene carbenes result, where both carbon atoms of each heteroring are engaged in coordination. 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

Mono or bis-carbene complexes are possible depending on the carbene/ metal precursor ratio and the steric bulk of the carbene. Most of the metal precursors and bases used for the synthesis of chiral complexes are presented below Metal precursors ... 

Linear coordination complexes are also observed with carbenes rather than carbonyl ligands. However, recent structural work carried out on bis(carbene) complexes indicates that coordination of the counteranion can lead to significant deviation of the Ccalbene-Hg-Gcalbene angle from linearity. In 70, for example, the two chloride anions are... 

Like I+ (vide infra), H+ can form bis(carbene) complexes, 9 (13). These homoleptic complexes, which feature C-H-C interactions, were formed by treatment of 4 (R = Mes R = H) with the corresponding... 

It is also possible to isolate bis(carbene) complexes involving the heavier alkaline earth metals. Thus, the reaction of two equivalents of 4 (R = Me or (Bu, R = H) with calcium, strontium and barium bis(trimethylsilyl)amides [M N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) resulted in the displacement of two thf molecules to afford the corresponding biscarbene species, 19 (19). The solubilities and stabilities of these complexes were found to decrease from calcium to barium. 

A mononuclear nickel hydride complex with three N-heterocyclic carbene ligands has been reported the compound was formed by oxidative addition of an imidazolium salt to the Ni(0) bis(carbene) complex [19]. The hydride signal of this nickel(II) complex appears at -15 ppm. 

G. Pelli, B. Traldi, P. FAB Induced Decomposition Pattern of die Gold(III) Bis(Carbene) Complex [[(/>-... 

The bis(carbene) complex Hg C=CCsCC(NMc2)=W(CO)5 2 shows a maximum wavelength absorption at 444 nm in toluene. ... 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

Bis(butadiene) complexes, with tantalum, 5, 173 Bis(z-butanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(calixarene) complexes, as organic molecule hosts, 12, 799 Bis(carbene) complexes with gold(I), 2, 287-288 with manganese, 5, 780, 5, 826 with mercury, 2, 429 with palladium, 8, 230 with silver , 2, 206... 

The binaphthohydroquinone skeleton is accessible by a double benzannulation of a bi-phenylbis(carbene) complex using two equivalents of alkyne. Application of one alkyne equivalent affords the mono-benzannulated product the second benzannulation step proceeds with distinctly lower yield [68a]. A complementary approach to biaryls is based on 1,3-dialkynes (for example, 64) which undergo a stepwise benzannulation sequence (for example, yielding 65 and 66) (Scheme 26) [68c]. A one-step protocol starting from the ethylene-bridged bis-carbene complex 67 affords the conformationally less flexible biaryl 68, albeit in only moderate yield. 

A bidirectional benzannulation strategy allows the extension of an existing biaryl skeleton. The BINOL-derived bis-carbene complex 69, accessible through a sequence of regioselective double ortfio-lithiation/Fischer carbene complex synthesis, undergoes bidirectional benzannulation to give the dinuclear biphenanthrene complex 70. The optical induction exerted by the binaphthyl core is, however, only moderate a mixture of C2- and Cj-symmetrical bis(phenanthrohydroquinone) bis-chromium complexes is formed, in which the C2-... 

The reactivity of dioxygen with nitrogen-coordinated copper(I) complexes has received extensive attention over the past two decades [53,54]. To date, analogous reactivity has not been realized for NHC-coordinated Cu(I). Ster-ically unhindered bis-carbene complexes of Cu(I) undergo rapid conversion to the corresponding ureas upon exposure to air in CH2CI2 solution (Eq. 4) [55]. This result suggests NHCs may not be universally applicable to metal-mediated oxidation chemistry. 

Scheme 8.3 Dimerisation with Nickel bis-carbene complexes...

The intramolecular carbene ligand dimerization of chromium bis-carbene complex 206 furnished the benzo [ /][l,4]dioxocines 207 substituted in the dioxocine ring (Equation 45) <2001JA851>. 

The intramolecular dimerization of chromium bis-carbene complex allowed the preparation of 1,4-dioxonine 271 (Equation 35) <2001JA851>. 

Reaction of a monopyridyl functionalised imidazoUum salt with AgBF in the presence of a base (NaOH), a variant of the Ag O method since Ag O is usually synthesised from AgNOj and NaOH, yields a silver(I) bis-carbene complex with two pendant pyridyl functional groups [15] (see Figure 3.3). 

Note The trigonal bipyramidal rhodium(I) complex in Figure 3.43 is a rare example of a trigonal bipyramidal bis-carbene complex where the carbene ligands are in the right, axial, position. Normally, steric constraints force the (bulky) carbene ligands in the electronically less favoured equatorial positions [131 ]. 

Synthesis of transition metal carbene complexes is achieved by the silver(I) oxide method and subsequent transfer to the next transition metal (in this case gold) (see Figure 3.62). Two different types of silver(l) carbene complexes are realised, the monocarbene and bis-carbene complexes. Transfer of the carbene to a gold(l) centre does not always occur with retention of structure as only the gold(l) monocarbene complexes are observed. 

Figure 3.102 Bridging and chelating alkyl tethered rhodium (I) bis-carbene complexes derived from electron-rich olefins.

The determining factor in these group 13 carbene complexes of whether the bridging or chelating bis-carbene complex is formed is the electronic requirements (ultimately the... 

---