Synthesis and Reactivity of Four- and Five-Coordinate Low-Spin Cobalt(II) PCP Pincer Complexes and Some Nickel(II) Analogues

Abstract

Anhydrous CoCl2 or [NiCl2(DME)] reacts with the ligand PCPMe-iPr (1) in the presence of nBuLi to afford the 15e and 16e square planar complexes [Co(PCPMe-iPr)Cl] (2) and [Ni(PCPMe-iPr)Cl] (3), respectively. Complex 2 is a paramagnetic d7 low-spin complex, which is a useful precursor for a series of Co(I), Co(II), and Co(III) PCP complexes. Complex 2 reacts readily with CO and pyridine to afford the five-coordinate square-pyramidal 17e complexes [Co(PCPMe-iPr)(CO)Cl] (4) and [Co(PCPMe-iPr)(py)Cl] (5), respectively, while in the presence of Ag+ and CO the cationic complex [Co(PCPMe-iPr)(CO)2]+ (6) is afforded. The effective magnetic moments μeff of all Co(II) complexes were derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements and are in the range 1.9 to 2.4 μB. This is consistent with a d7 low-spin configuration with some degree of spin–orbit coupling. Oxidation of 2 with CuCl2 affords the paramagnetic Co(III) PCP complex [Co(PCPMe-iPr)Cl2] (7), while the synthesis of the diamagnetic Co(I) complex [Co(PCPMe-iPr)(CO)2] (8) was achieved by stirring 2 in toluene with KC8 in the presence of CO. Finally, the cationic 16e Ni(II) PCP complex [Ni(PCPMe-iPr)(CO)]+ (10) was obtained by reacting complex 3 with 1 equiv of AgSbF6 in the presence of CO. The reactivity of CO addition to Co(I), Co(II), and Ni(II) PCP square planar complexes of the type [M(PCPMe-iPr)(CO)]n (n = +1, 0) was investigated by DFT calculations, showing that formation of the Co species, 6 and 8, is thermodynamically favorable, while Ni(II) maintains the 16e configuration since CO addition is unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the apical CO ligand in 4 deviates significantly from linearity, with a Co–C–O angle of 170.0(1)°. The DFT-calculated value is 172°, clearly showing that this is not a packing but an electronic effect.

Electro‐ and Photocatalytic Generation of H2 Using a Distinctive CoII “PN3P” Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source

“…[40] Importantly,amajority of these complexes requiret he presenceo fa cids as the sourceo fh ydrogen in order to electrocatalytically produce H 2 in polar organic solvents. [50][51][52][53] Single crystalso f1 were obtained from CH 2 Cl 2 and X-ray diffraction analysisr evealed two independentm olecules in the structure with distorted square-based pyramidal geometry at Co and with nearly identical bondingp arameters as shown in Scheme 1( Figure S1, Table S2, S3). [8,11,[41][42][43] Cobalt-basedelectrocatalysts that use watero rm ixed organic/water solutions as the hydrogen sourceoperate in the voltage range of À1.5 to À2.0 Vvs.…”

“…[50][51][52][53] Single crystalso f1 were obtained from CH 2 Cl 2 and X-ray diffraction analysisr evealed two independentm olecules in the structure with distorted square-based pyramidal geometry at Co and with nearly identical bondingp arameters as shown in Scheme 1( Figure S1, Table S2, S3). The targeted molecular formula was supported by microanalysis and ar oom temperature magneticm oment of 2.4 m B ,w hich indicatedap aramagnetic low spin Co II center.…”

Activation of Molecular Hydrogen and Oxygen by PSiP Complexes of Cobalt

“…Such examples of square planar Co II –Cl species are relatively rare, with previously known examples including (PNP)CoCl species with varying ligand modifications, as well as a PNCNP‐supported pincer complex featuring N ‐diisopropylphosphino donors . Comparison of the metrical parameters for 1 and such crystallographically characterized PNP‐ and PNCNP‐supported complexes reveals that 1 is significantly more distorted from ideal square‐planar geometry (previously reported Cl–Co–X angles in range of 171° – 180° for X = C or N), and features a longer Co–Cl distance [2.2710(6) Å] than has previously been noted for such complexes {previously reported Co–Cl distances in range of 1.919(2)–2.2291(5) Å, where the shortest distance corresponds to N ‐diisopropylphosphino‐derived (PNCNP)CoCl complex reported by Kirchner and longest distance corresponds to [( t Bu 2 PCH 2 CH 2 ) 2 N]CoCl reported by Schneider[11d]} possibly reflecting the strong trans influence of the silyl donor. The use of CoI 2 under the same reaction conditions was found to be a much more reliable route for the preparation of the iodide analogue (Cy‐PSiP)CoI ( 2 ), which could be reproducibly isolated in >90 % yield via this route.…”

Co(II) PCP Pincer Complexes as Catalysts for the Alkylation of Aromatic Amines with Primary Alcohols

“…16 Inspired by recent discoveries, we describe here the efficient alkylation of amines with alcohols catalyzed by Co(II) complexes which are stabilized by an anionic PCP ligand based on the 1,3-diaminobenzene scaffold ( Figure 2). 17 It has to be noted that Co PCP complexes have as yet not been applied in catalysis. 18 Co complexes 1 and 2 were screened for the alkylation of aniline with benzyl alcohol (1.4 equiv) in toluene (4 mL bearing the strongly basic coligands Me and CH 2 SiMe 3 were studied under base-free conditions with the same substrates but with added 3 Å (0.2 g) molecular sieves (MS) in toluene (4 mL).…”