π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO₂ to Ketenyl Transformation

Abstract

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF) 2 ][Ta(C t Bu)-(CH 2 t Bu) 3 ], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH 2 t Bu) 3 (μ-C t Bu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta�C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CH t Bu)-(CH 2t Bu) 2 (μ-CH t Bu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO 2 resulting in rare examples of coinage metal ketenyl species, ( t BuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO 2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

CO2 cleavage by tantalum/M (M = iridium, osmium) heterobimetallic complexes

“…The design of synthetic bimetallic complexes associating different metals with complementary Lewis acidic/Lewis basic behaviour has raised interest for cooperative reactivity, [1][2][3][4][5][6][7][8][9][10] including CO 2 activation. [11][12][13][14][15][16][17] In many instances, these bifunctional complexes lead to CO 2 adducts or insertion products, in which a bent CO 2 fragment binds across the two metals. [18][19][20][21][22][23][24][25][26][27] In contrast, only a few heterobimetallic complexes have clearly exhibited the capability to cleave the C-O bond within CO 2 .…”