A series of cobalt POCOP pincer complexes with the formulas {2,6-(iPr2PO)2-4-R′-C6H2}Co(CO)2 (R′ = H (1a), NMe2 (1b), OMe (1c), CO2Me (1d)), {2,6-(Ph2PO)2C6H3}Co(CO)2 (1e), and {2,6-(tBu2PO)2C6H3}Co(CO) (2f) have been synthesized through C–H bond activation of the corresponding pincer ligands with Co2(CO)8. These complexes have been demonstrated to catalyze the hydrosilylation of PhCHO with (EtO)3SiH, which exhibits an induction period and the decreasing reactivity order 1b > 1c > 1a > 1d > 1e. The catalytic protocol can be applied to various aldehydes with turnover numbers of up to 300. The CO ligands in the dicarbonyl complexes have been shown to exchange with 13CO at room temperature and partially dissociate from cobalt at high temperatures. Substitution of CO by tert-butyl isocyanide has been accomplished with 1a at 50–80 °C, resulting in the formation of {2,6-(iPr2PO)2C6H3}Co(CNtBu)(CO) (3a) and {2,6-(iPr2PO)2C6H3}Co(CNtBu)2 (4a). The catalytic reactions are more efficient when they are carried out in an open system or if the catalysts are preactivated by the aldehydes. The structures of 1a–e, 3a, and 4a have been studied by X-ray crystallography.
2 . Complexes 1−3 have been established as catalysts for the hydrosilylation of aldehydes bearing various functional groups. According to the mechanistic studies, the silyl hydride species exists in the catalytic cycle, whereas the bis(trimethylphosphine) species sits outside the catalytic cycle. Dissociation of PMe 3 is required prior to aldehyde insertion into the silyl hydride species, which is the turnover-limiting step of the catalytic cycle. Consequently, 3 outperforms 1 in catalyzing the hydrosilylation reaction due to the presence of only one PMe 3 ligand. The structures of 1−4 have been studied by Xray crystallography.
A nickel hydride complex, {2,6-( i Pr2PO)2C6H3}NiH, has been shown to catalyze the coupling of RCHO and R′OH to yield RCO2R′ and RCH2OH, where the aldehyde also acts as a hydrogen acceptor and the alcohol also serves as the solvent. Functional groups tolerated by this catalytic system include CF3, NO2, Cl, Br, NHCOMe, and NMe2, whereas phenol-containing compounds are not viable substrates or solvents. The dehydrogenative coupling reaction can alternatively be catalyzed by an air-stable nickel chloride complex, {2,6-( i Pr2PO)2C6H3}NiCl, in conjunction with NaOMe. Acids in unpurified aldehydes react with the hydride to form nickel carboxylate complexes, which are catalytically inactive. Water, if present in a significant quantity, decreases the catalytic efficiency by forming {2,6-( i Pr2PO)2C6H3}NiOH, which causes catalyst degradation. On the other hand, in the presence of a drying agent, {2,6-( i Pr2PO)2C6H3}NiOH generated in situ from {2,6-( i Pr2PO)2C6H3}NiCl and NaOH can be converted to an alkoxide species, becoming catalytically competent. The proposed catalytic mechanism features aldehyde insertion into the nickel hydride as well as into a nickel alkoxide intermediate, both of which have been experimentally observed. Several mechanistically relevant nickel species including {2,6-( i Pr2PO)2C6H3}NiOC(O)Ph, {2,6-( i Pr2PO)2C6H3}NiOPh, and {2,6-( i Pr2PO)2C6H3}NiOPh·HOPh have been independently synthesized, crystallographically characterized, and tested for the catalytic reaction. While phenol-containing molecules cannot be used as substrates or solvents, both {2,6-( i Pr2PO)2C6H3}NiOPh and {2,6-( i Pr2PO)2C6H3}NiOPh·HOPh are efficient in catalyzing the dehydrogenative coupling of PhCHO with EtOH.
The reaction of 1,3,5-(iPr2PO)3C6H3 with Co2(CO)8 leads to the isolation of a POCOP-type mononuclear pincer complex {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2 (1) or a tetranuclear species {κP-{κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2}2Co2(CO)6 (2), depending on the ligand to cobalt ratio employed. The latter compound can be an impurity during the synthesis of {2,6-(iPr2PO)2-4-Me2N-C6H2}Co(CO)2, when the ligand precursor 5-(dimethylamino)resorcinol is contaminated with phloroglucinol due to incomplete monoamination. Similarly, the reaction of 1,3,5-(iPr2PO)3C6H3 with NiCl2 in the presence of 4-dimethylaminopyridine provides {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}NiCl (3) bearing an appended phosphinite group. Structures 1–3 have been studied by X-ray crystallography.
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