Pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiSC(6)H(4)Z (R = Ph and i-Pr; Z = p-OCH(3), p-CH(3), H, p-Cl, and p-CF(3)) have been synthesized from [2,6-(R(2)PO)(2)C(6)H(3)]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni-S bond length (approx. 2.20 Å) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with "[2,6-(Ph(2)PO)(2)C(6)H(3)]Ni" rather than "[2,6-(i-Pr(2)PO)(2)C(6)H(3)]Ni", and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH(2)Br, CH(3)I, CCl(4), and Ph(3)CCl have been examined and several mechanistic pathways have been explored.

Pe = cyclopentyl) have been synthesized from [2,6-(R 2 PO) 2 C 6 H 3 ]PdCl (1a-c) and LiAlH 4 or LiBEt 3 H. These hydride complexes react with phenylacetylene to afford H 2 , [2,6-(R 2 PO) 2 C 6 H 3 ]PdCuCPh (3a-c) and a small amount of styrene. When the R groups are isopropyl groups, a second palladium species is generated, and it has been identified as an alkenyl complex (E)-[2,6-( i Pr 2 PO) 2 C 6 H 3 ]PdCHvCHPh (4b). Mechanistic studies have shown that decomposition of these palladium pincer complexes and related palladium methyl complexes [2,6-(R 2 PO) 2 C 6 H 3 ]PdCH 3 (5a-c) occurs at room temperature in the presence of H 2 (1 atm or lower), resulting in the leaching of palladium particles. These particles have been shown to catalyze the hydrogenation of phenylacetylene and diphenylacetylene to their alkene and alkane products. A mechanism for the formation of palladium particles has been proposed. The structures of 1a, 1c, 2a, 2c, 3a, 4b and 5b have been studied by X-ray crystallography.Scheme 1 † Electronic supplementary information (ESI) available: Details of DLS experiments. CCDC 965919-965925 for 1a, 1c, 2a, 2c, 3a, 4b and 5b. For ESI and crystallographic data in CIF or other electronic format see

A series of nickel pincer complexes of the type [4-Z-2,6-(R 2 PO) 2 C 6 H 2 ]NiX (R = t Bu, i Pr, Ph; Z = H, CO 2 Me; X = NCS, N 3 ) have been synthesized from the reactions of the corresponding nickel chloride complexes [4-Z-2,6-(R 2 PO) 2 C 6 H 2 ]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A comparable Ni-N bond length (approx. 1.87 Å for the isothiocyanate complexes and 1.91 Å for the azide complexes) and an almost identical Ni-C ipso bond length (approx. 1.89 Å) have been observed for these complexes. Metathesis reactivity of [4-Z-2,6-(R 2 PO) 2 C 6 H 2 ]NiCl and ligand exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center. The thermodynamic stability of these nickel complexes has been rationalized using the hard-soft acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 experimental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in SCNand N 3 -.

The reaction of the formate complex {2,6-(R2PO)2C6H3}­Ni­(OCHO) (R = t Bu, 5; R = i Pr, 6) with CS2 shows first-order kinetics in nickel concentration and zero-order in [CS2] when CS2 is used in large excess. Rate measurement at different temperatures gives activation parameters ΔH ⧧ = 22.6 ± 0.9 kcal/mol and ΔS ⧧ = −5.2 ± 3.0 eu for the decarboxylation of 5 and ΔH ⧧ = 22.6 ± 1.0 kcal/mol and ΔS ⧧ = −4.3 ± 3.2 eu for the decarboxylation of 6. Comparing the decarboxylation rate constants for 6 and {2,6-( i Pr2PO)2C6H3}­Ni­(OCDO) (6- d ) yields KIE values of 1.67–1.90 within the temperature range 30–45 °C. On the basis of these experimental results and DFT calculations, an ion pair mechanism has been proposed for the decarboxylation process. The CS2 insertion products {2,6-(R2PO)2C6H3}­Ni­(SCHS) (R = t Bu, 3; R = i Pr, 4) have been characterized by X-ray crystallography.

Phosphinite-based metallacycles are important intermediates, catalyst precursors, or catalysts for a variety of chemical transformations. Facile, reversible formation of P−O bonds allows phosphinites to act as catalytic directing groups that lead to more efficient and selective metal-catalyzed processes. Relatively low costs and convenient syntheses make metallacyclic phosphinite complexes attractive compounds for catalytic studies. Given the high interest in developing the related phosphine-and phosphite-based catalysts, this Perspective focuses specifically on the comparisons between these different metallacyclic complexes in catalytic reactions. In a number of examples, phosphinite-based metallacycles are more efficient catalysts due to better-matched ligand properties for rate-limiting steps or faster conversion of the complexes to catalytically active species.

The P-stereogenic nickel complex {2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiCl (2) has been synthesized via cyclometalation of the POCOP-pincer ligand 1,3-[(t-Bu)­(Ph)­PO]2C6H4 (1) with NiCl2. The initially isolated 2 consists of a 1:1 mixture of racemic and meso isomers that are separable through repeated crystallization and is configurationally stable even at 110 °C. Upon mixing with t-BuOK, the meso isomer (2- meso ) displays a higher ligand substitution rate than the racemic isomer (2- rac ), likely because its nickel center is sterically more accessible. Complex 2, as either pure 2- rac or a 2- rac /2- meso mixture, can be converted to the nickel triflate complex {2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiOTf (3) or the nickel formate complex {2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiOCHO (7) without epimerization at the phosphorus centers. Under a dynamic vacuum at 90 °C, decarboxylation of 7- meso is faster than that of 7- rac , suggesting that in the transition state the formato hydrogen approaches the nickel center from the axial site rather than the equatorial site. The structure of 2- rac has been studied by X-ray crystallography.

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