Transition metal complexes pincer

Another application of hyperbranched polymers as supports for catalysts is their use as backbones for the covalent attachment of organometallic fragments. NCN-pincer complexes (NCN-pincer = 2,6-bis[(dimethylamino)-methyl] phenyl anion) are attractive building blocks for catalytic reactions [20,21], Covalent introduction of the transition-metal complexes can also be of interest for visualization and imaging of dendritic polymers by transmission electron microscopy (TEM). 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

H. Lang, R. Packheiser and B. Walfort (2006) Organometal-lics, vol. 25, p. 1836 - Organometallic 7r-tweezers, NCN pincers, and ferrocenes as molecular Tinkertoys in the S5mthesis of multiheterometallic transition-metal complexes . 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

Boro, B.J. (2009) Investigations of Pincer-Iigated Transition Metal Complexes for the Activation of Molecular Oxygen. PhD Dissertation. University of New Mexico. 

Pincer organometallic compounds are reported mainly with regard to two types of compounds, PCP and NCN transition metal complexes [28, 34]. However, ruthenium pincer CNN compounds have also been applied to hydrogen-transfer reductions of ketones. 

Takaya H, Iwaya T, Ogata K, Isozaki K, Yokoi T, Yoshida R, Yasuda N, Seike H, Takenaka T, Nakamura M (2013) Synthesis, structure, and function of PCP pincer transition-metal-com-plex-bound norvahne derivatives PCP-Pincer transition-metal-complex-bound norvaline derivatives synlett accepted... 

Bis-NHC CNC-type pincer ligands were first used with Pd and, later on, coordinated onto early-transition metal complexes. The titanium (III) complex 35 was readily prepared from (THF)3TiCl3 and the free bis(carbene)pyridine, whereas the imido Ti complex 36 was obtained from Ti(NtBu)Cl2(pyridine)3 (Scheme 14.19) [66,67]. The pincer complex 35 was tested in ethylene polymerization. With 500 equivalents of MAO cocatalyst, an activity of 791kgmor bar h was observed. 

Rapid hydrogen releases can be achieved under mild conditions by using transition metal complexes as catalysts. Some noble metal complexes, including Pt, Pd, Rh, Ir, and Ru, were proven to be very effective for promoting hydrogen release from AB. The Rh, Pd, and Ru complexes could catalyze AB to dehydrogenate at room temperature with suppressing the release of volatile cyclotriborazane, borazine, and poly(iminoborane) [107]. The iridium pincer... 

Reactivity of Nickel Pincer Complexes with CO2. There is great interest involving the activation of CO2 and its conversion into value-added chemicals. One strategy utilizes late transition-metal complexes where the key step involves metal insertion to form metal carboxylates. Nickel pincer complexes were prepared and the subsequent reactivities of these complexes with CO2 were studied. The complex [(PCP)Ni(Cl)] was refluxed in THF with excess NaNH2 for 24 h to form [(PCP)Ni(NH2)] (eq 38). Further refluxing with 1 equiv of oxazole for 3 day via protonolysis, liberated ammonia to form [(PCP)Ni(oxazole)]. It is worth noting that the protonolysis is air and moisture sensitive and that if rigorously dry conditions are not employed, [(PCP)Ni(OH)] will form instead. 

Lang, H. Packheiser, R., and WaUbrt, B. (2006). Organometallic Jt-Tweezers, NCN Pincers, and Ferrocenes as Molecular Tinkertoys in the Synthesis of Multiheterometallic Transition-Metal Complexes. Organometallics, 25, 1836-1851. 

Chapter 4 - The chemistry of mono- and bis(alkynyl) transition metal complexes, modified ferrocenes, functionalized alkynyls, diaminoaryl NCN pincer molecules (NCN = [C6H2(CH2NMe2-2,6)2] ), and 1,4- and 1,3,5-substituted benzene derivatives towards diverse metal fragments will be discussed and serves to understand the manifold and sometimes unexpected reaction behavior of such species. Interesting novel (hetero)bi- to undecametallic compounds with often uncommon structural motifs are formed in which the respective... 

Bezuidenhout DI, Kleinhans G, Guisado-Barrios G, Liles DC, Ung G, Bertrand G. Isolation of a potassium bis(l,2,3-triazol-5-yhdene)carbazolide a stabilizing pincer Hgand for reactive late transition metal complexes. Chem Commun. 2014 50 2431-2433. 

Silylene-based pincer ligands offer exciting reactivities in terms of transition metal complex formation and their applications in catalytic systems. The pincer complex [SiCSi)Ni(II) can be synthesized by oxidative addition of C—H bond of the corresponding [SiC(H)Si] ligand. [SiCSi]Ni(II) complex has been employed as catalyst for Ni-catalyzed Sonogashira reactions (8). Moreover bis(silylene) pincer complexes of iridium and rhodium reveal strong 5-donating ability of divalent silicon and have demonstrated selectivity in catalytic C—H borylation reactions with arenes (9). 

Multidentate donor ligands specifically the pincer-type motif [EDE] (D = pyridyl, phenyl E = PR2, NR2, SR) with tridentate coordinating ligand framework have the advantage that they can simultaneously increase the electronic density and stabilize the coordination sphere of a transition metal. This can have suhstantial effects on the catalytic activity of the transition metal complexes. As an example, iron-hased pincer-type complexes present high catalytic activity for different reactions, such as hydrogenation (1—3), hydrosilylation (4,5), and hydroho-ration (6). 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

Note The wingtip groups in transition metal pincer carbene complexes point away from the metal centre. This limits the steric influence of the wingtip groups on the metal s coordination sphere. In particular, aryl ligands can align themselves perpendicular to the pincer plane creating two walls on either side of the fourth coordination site. 

Figure 3.147 Steric factors within transition metal pincer carbene complexes.

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