Pincer compounds

Based on these results, it became clear that there was indeed a need for new, more robust complexes capable of withstanding the higher temperatures at which it was beheved the alkane dehydrogenation process would occur more efficiently. Consequently, the chemistry of pincer compounds appeared on the scene. In 1976,... 

In spite of the success that Ir—PCP pincer compounds have enjoyed in the development of more efficient and promising alkane dehydrogenation systems, one caveat of using these complexes has been the sometimes difficult or tedious synthesis of the pincer ligands and complexes. An answer to this problem was... 

Pincer compounds with a general structure like 20 are common in organo-metallic chemistry [832]. They are useful as precursors to catalysts and as components of hybrid organometallic polymers [84-86]. In most cases, they have high stability to oxygen and water in addition to high thermal stability. 

The independent control of the association kinetics relative to thermodynamics is therefore desirable, and Yount and others showed that N,CJ -pincei metal-ligand coordination motifs such as 1 2 (Fig. 3.5) are well suited to that goal. Pincer compounds 1 and analogs have been synthesized and smdied extensively (Rietveld et al. 1997 Albrecht and van Koten 2001 Rodriguez et al. 2002 Slagt et al. 

These characteristics are illustrated by studies on SP networks formed through the same van Koten type pincer coordination complexes described above. We focus the discussion on SPs formed from poly(4-vinylpyridine) (PVP) that is cross-linked in DMSO by bis(M(ll)-pincer) compounds 4a-d (Fig. 3.5 Yount et al. 2005a, 2005b). The same simple steric effect in the pincer alkylamino ligands is used. 

In the case of dense, surface-grafted polymer brush thin hlms, cross-links are introduced by the simple addition of solutions containing bis(Pd -pincer) compounds 4a or 4b to grafted PVP brushes (Loveless et al. 2006). Because the association constants for pyridine coordination are similar, the uptake of 4a and 4b from equimolar solutions into the PVP brushes (at constant grafting density and molecular weight) is effectively equivalent, producing samples with comparable structure (number and placement of cross-links). 

Yount et al. (2005) reported the formation of organogels using poly(vinylpyridine) (PVP) and ditopic metallopincer cross-linkers. This study provided particularly pertinent information on the dynamic elements of MSPs and elegantly demonstrated how they control the material s properties. PVP dissolved in dimethylsulfoxide (DMSO) is cross-linked with either bis-Pd - (18a) or Pt -pincer compounds (18b, Fig. 7.13 Yount et al. 2005). Addition of 5% 18a -Pd to a PVP solution results in a viscous material (77 = 6.7 Pa s), whereas the corresponding PVP 18b Pd is a gel (77 = 550 Pa s). Changing R from methyl to ethyl does not affect the thermodynamics of the pyridine/Pd interaction however, the rate of exchange decreases by approximately 2 orders of magnitude. Further studies on these materials and their... 

Figure 7.13 Schematic representation of the structure of a cross-linked MSP gel prepared by Loveless and coworkers (Yount et al. 2005 Loveless et al. 2007) from poly(vinyl pyridine) (PVP) cross-linked with bis-Pd - or Pt -pincer compounds (18 and 19).

For a recently-published book containing numerous examples of C-H and C-H activation as well as several other organometallic reactions involving pincer ligands, see D. Morales-Morales, The Chemistry of Pincer Compounds, Elsevier Amsterdam, 2007. 

D. Morales-Morales and C. M. Jensen, The Chemistry of 35. Pincer Compounds, 2007, Elsevier. 

Morales-Morales, D. and Jensen, C.M. (2007) The Chemistry of Pincer Compounds, Elsevier. 

These cyclometalation reactions are regarded as involving almost all transition metal compounds, especially palladium compounds [2, 6, 8, 10, 16-18, 20, 26-34, 60,85,86,89,90,102-111], Many reviews on pincer compounds have also reported on transition metal compounds [21-25, 65, 102, 112-116]. Few reviews have reported on both transition metal and main group metal compounds [9,13,15] or on main group metal compounds alone [14], except the reviews published by the author. 

Morales-Morales D, Jensen CM (eds) (2007) The chemistry of pincer compounds. Elsevier, Amsterdam... 

Panzner MJ, Tessier CA, Youngs WJ (2007) Pincer complexes of N-heterocyclic carbenes. Potential uses as pharmaceuticals. In Morales-Morales D, Jensen CM (eds) The chemistry of pincer compounds. Elsevier, Amsterdam, p 139... 

Since cyclometalation reactions proceed extremely easily, many reactions have been tried with many substrates, and the number of reaction substrates employed has increased rapidly as a result. Many representative substrates are shown in Fig. 5.11 [58]. These substrates are derived from bidentate compounds or tridentate compounds. The tridentate substrates are usually called pincer compounds. Tridentate pincer compounds are used as catalysts, with an phasis on their stability as catalysts. 

For example, 6-(4 -methylphenyl)-2-pyridylmethylamine ruthenium pincer compounds 8.110 are highly efficient catalysts in transfer hydrogenation involving 2-propanol to perform the reduction of ketone quantitatively with very low loading and in a short time, as shown in Eq. (8.30) [151]. 

Equation (116)). Complex 290 features a six-coordinate octahedral geometry, but other reported examples of Ni(iii) pincer compounds are five coordinate, square pyramidal. A four-coordinated, square-planar homoleptic Ni(iii) complex, 291, has been prepared by oxidation of the corresponding Ni(ii) precursor with CI2 at low temperature (Equation (117)). This complex has low thermal stability and slowly decomposes at room temperature, with the production of perchlorobiphenyl. Organometallic Ni(iii) compounds have low spin electronic configurations, and display well-resolved EPR spectra (Table 12). 

The chemistry of pincer complexes of the group 9 has been dominated by the study of the derivatives with rhodium and iridium. To date, the reactivity of cobalt pincer compounds with carbon monoxide has not been reported, mainly due to the fact that the number of pincer complexes derived from cobalt is reduced [17]. Thus, the interest in the Rh and Ir PCP pincer complexes is mainly owing to the fact that they have been proved efficient in the catalytic dehydrogenation of aliphatic C—H bonds this is particularly true in the case of the iridium PCP pincer complexes [3j. 

It was not until 1997 that the first study involving a direct reaction of carbon monoxide with a ruthenium pincer compound was reported. Jia and coworkers reported [47] the synthesis of (PCP)Ru(II) (118) and examined its reactivity toward carbon monoxide. They found that the reaction of (118) with CO gas leads to the replacement of the PPh3 ligand, giving place to the white compound (PCP)RuCl(CO)2 (119) (Scheme 2.53). 

The only study on the direct reaction of carbon monoxide with osmium pincer complexes was reported by Milstein and coworkers in 2001 [52]. In this case, the pincer (PCP)Os(II) complex (131) was reacted with CO in benzene at room temperature to afford quantitatively the carbon monoxide adduct (132). This species further reacted with an additional equivalent of CO to yield the bis-carbonyl (PCP)Os(II) pincer complex (133) with the elimination of PPhs. Additionally, complex (133) can be directly generated by pressurizing the pincer compound (131) with 3 bar of CO or from the reaction of complex (134) with CO under mild conditions. The structure of... 

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