A Comparison of the Stability and Reactivity of Diamido‐ and Diaminocarbene Copper Alkoxide and Hydride Complexes

Abstract

The mononuclear N-heterocyclic carbene (NHC) copper alkoxide complexes [(6-NHC)CuOtBu] (6-NHC = 6-MesDAC (1), 6-Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert-butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6-NHC)CuMes] (6-NHC = 6-MesDAC (3), 6-Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring-opening reactions, the latter affording [(6-MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6-MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6-MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6-MesDAC-H)Cu(P(p-tolyl)3)] (9) upon trapping by P(p-tolyl)3. Migratory insertion was also observed during attempts to prepare [(6-Mes)CuH], with [(6-Mes-H)Cu(6-Mes)] (10) isolated, following a reaction that was significantly slower than in the 6-MesDAC case. The longer lifetime of [(6-Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi-reduction of alkynes and hydrosilylation of ketones.

Experimental and Computational Studies of the Copper Borate Complexes [(NHC)Cu(HBEt3)] and [(NHC)Cu(HB(C6F5)3)]

“…[6] This MI process thwarted efforts to spectroscopically characterize any putative Cu-H intermediates (e.g. [6] This MI process thwarted efforts to spectroscopically characterize any putative Cu-H intermediates (e.g.…”

“…These indicate asignificantly stronger H-B(C 6 F 5 ) 3 bond in 6 (DG 1 =+38.2 kcal mol À1 )compared to the H-BEt 3 bond in 5 (DG 1 =+10.8 kcal mol À1 ). [6] The 11 BNMR spectrum showed resonances at d = 60, 52 and À14 ppm suggestive of multiple boron-containing species being produced. TheB H 3 moiety in III is also strongly bound (DG 1 = + 36.9 kcal mol À1 ), consistent with the isolation of the borohydride complex.…”

“…In recent work attempting to prepare [LCuH] species stabilized by large six-membered ring carbenes (e.g.from the reaction of 1 with tertiary silanes,Scheme 2), we showed that migratory insertion (MI) products such as 3 formed under mild conditions. [6] This MI process thwarted efforts to spectroscopically characterize any putative Cu-H intermediates (e.g. 2); however,C u-H formation was implied through trapping with alkyne to give 4.…”

“…However,n one of the signals matched those of 4. [6] The 11 BNMR spectrum showed resonances at d = 60, 52 and À14 ppm suggestive of multiple boron-containing species being produced. There was no reaction between 6 and the alkyne over days at room temperature,w ith only the transformation to the homoleptic cationic bis-carbene complex 7 apparent from the 11 BNMR spectrum.…”

Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold‐ ‐ ‐Copper Hydride Complexes

“…[1][2][3][4][5][6][7][8] Them ajority of catalytic studies have focused on the in situ generation of terminal metal hydride complexes stabilized by N-heterocyclic carbene (NHC) ligands. [1][2][3][4][5][6][7][8] Them ajority of catalytic studies have focused on the in situ generation of terminal metal hydride complexes stabilized by N-heterocyclic carbene (NHC) ligands.…”