Reactions of Low-Coordinate Cobalt(0)–N-Heterocyclic Carbene Complexes with Primary Aryl Phosphines

Abstract

Aiming to get knowledge on the reactivity of low-coordinate cobalt(0) species toward primary phosphines, the reactions of [(IPr)­Co­(vtms)2] and [(ICy)2Co­(vtms)] (IPr = 1,3-bis­(2′,6′-diisopropylphenyl)­imidazol-2-ylidene, ICy = 1,3-dicyclohexylimidazol-2-ylidene, and vtms = vinyltrimethylsilane) with several primary aryl phosphines have been examined. The reactions of [(IPr)­Co­(vtms)2] and [(ICy)2Co­(vtms)] with H2PDmp (Dmp = 2,6-dimesitylphenyl) at 80 °C furnish the diamagnetic cobalt­(I) phosphido complexes [(NHC)­Co­(PHDmp)] (NHC = IPr, 1; ICy, 2) that feature the Co–(η6-mesityl) interaction. Complex 1 can coordinate CO to generate the terminal phosphido complex [(IPr)­Co­(CO)3(PHDmp)] (3) and can be oxidized by [Cp2Fe]­[BArF 4] to yield the cobalt­(II) phosphido complex [(IPr)­Co­(PHDmp)]­[BArF 4] (4, BArF 4 = tetrakis­(3,5-di­(trifluoromethyl)­phenyl)­borate). For the reactions with sterically less-hindered primary phosphines, [(IPr)­Co­(vtms)2] is inert toward H2PC6H2-2,4,6-Me3 (H2PMes) at room temperature, whereas [(ICy)2Co­(vtms)] can react with H2PMes at room temperature to produce the cobalt­(II) phosphido alkyl complex trans-[(ICy)2Co­(CH2CH2SiMe3)­(PHMes)] (5). At 80 °C, the cobalt(0) alkene complexes [(IPr)­Co­(vtms)2] and [(ICy)2Co­(vtms)] and also the cobalt phosphido complexes, 1, 2, and 5 can serve as precatalysts for the dehydrocoupling reaction of H2PMes to afford MesHPPHMes. NHC–Co­(I)-phosphido species are proposed as the in-cycle intermediates for these cobalt-catalyzed dehydrocoupling reactions.

Rhodium Complexes in P−H Bond Activation Reactions

“…In these circumstances, the reactivity of the phosphanido ligand is considerably reduced because of the lack of lone electron pairs on the phosphorus atom. This undesirable situation can be avoided by using bulky ligands, as observed in the reaction of [Co(dtbpe)(C 2 H 4 )] (dtbpe=1,2‐bis(di‐ tert ‐butylphosphano)ethane) with 2,6‐dimesitylphenylphosphane (DmpPH 2 ), which gives [Co(dtbpe)(H)(PHDmp)] …”

“…This undesirable situation can be avoided by using bulky ligands, as observed in the reaction of [Co(dtbpe)(C 2 H 4 )] (dtbpe = 1,2-bis(ditert-butylphosphano)ethane) with 2,6-dimesitylphenylphosphane (DmpPH 2 ), which gives [Co(dtbpe)(H)(PHDmp)]. [22] Hydrido-phosphanido complexes of early and middle transition metalsh avea lso been prepared from oxidative addition reactions of the coordinatively and electronicallyu nsaturated complex [Ta(tBu 3 SiO) 3 ], [23] and also from electronically saturated complexes [Mo(Cp*)(Cl)(N 2 )(PMe 3 ) 2 ], [24] and [W(dppe) 2 (N 2 ) 2 ] [25] with HPPh 2 .T he later require light irradiation to dissociate dinitrogen and thus generatethe required coordination vacancy.…”

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

“…Several two-and three-coordinate cobalt catalysts bearing N-heterocyclic carbene (NHC) ligands have been published by Deng and co-workers. [55][56][57][58][59] The three-coordinate Co(I) species 12 was shown to catalyse the hydrosilylation of terminal alkynes, showing broad functional group tolerance and moderate to good selectivity for formation of the E-alkene (Scheme 9a). 56 Subsequently, the Co(II) amide species 13, bearing an asymmetric NHC ligand, catalysed the hydrosilyl- ation of terminal alkenes affording predominantly anti-Markovnikov products (Scheme 9b).…”

“…More recently, the three-coordinate Co(0) complexes 14 and 15 were shown to catalyse the dehydrocoupling of primary arylphosphines to the corresponding diphosphanes (Scheme 10), an unusual example of a cobalt catalyst for such a transformation. 59 The catalysts afforded the coupled products in moderate to good yields (47-73%), but were ineffective with the secondary phosphine Ph 2 PH and primary alkyl phosphine tBuPH 2 (7% and 8% yields, respectively). 59…”

“…59 The catalysts afforded the coupled products in moderate to good yields (47-73%), but were ineffective with the secondary phosphine Ph 2 PH and primary alkyl phosphine tBuPH 2 (7% and 8% yields, respectively). 59…”