Reductive Cleavage of Acyl Halides to Carboxylato Alkylidyne Complexes from Their Reactions with the Quintuply Bonded Dimolybdenum Amidinate

Mediated by Lewis acids, a Friedel–Crafts type of addition of acyl chlorides to alkynes gives β-chloro α,β-unsaturated ketones, which are important precursors to the preparation of heterocycles. A similar principle had been applied to the quintuply bonded dimolybdenum amidinate [Mo2(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (1). Addition of acyl halides to 1 resulted in the formation of the β-halo α,β-quadruply bonded dimolybdenum acyl complexes anti-Mo2(X)­(μ-κ2-OCR)­[μ-κ2-HC­(N-2,6-iPr2C6H3)2]2 (R = Me, X = Cl (3); R = C6H5, X = Cl (4), Br (5); R = 4-FC6H4, X = Cl (6); R = 2-MeC6H4, X = Cl (7)). DFT calculations on Mo2(Cl)­(μ-κ2-OCMe)­[μ-κ2-HC­(N-2,6-iPr2C6H3)2]2 suggested that it equilibrates with the oxo alkylidyne Mo2(O)­(μ-Cl)­(μ-CMe)­[μ-κ2-HC­(N-2,6-iPr2C6H3)2]2 in a ratio of about 100:1. The nucleophilic MoO functionality turned out to be reactive toward various electrophiles. Complexes 3–7 were subsequently treated with acyl halides RC­(O)­X to undergo haloacylation and yield carbyne-bridged dimolybdenum species [Mo2(X)­(μ-X)­(μ-CR)­(κ2-O2CR)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (R = Me, X = Cl (10); R = C6H5, X = Cl (11), Br (12); R = 4-FC6H4, X = Cl (13); R = 2-MeC6H4, X = Cl (14)). However, when 1 was treated with the less reactive benzoyl fluoride, the two products [Mo2(F)­(μ-F)­(μ-CC6H5)­(κ2-O2CC6H5)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (15) and [Mo2(μ-η2-1,2:η2-4,5-C6H5COF)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (16) were isolated and the latter predominated. The oxo alkylidyne intermediates were further corroborated by reacting 4 with N-methylbenzimidoyl chloride and diphenylphosphinic chloride to give the two benzylidyne species [Mo2(Cl)­(μ-Cl)­(μ-CC6H5)­(κ2-ONCH3C6H5)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (17) and [Mo2(Cl)­(μ-Cl)­(μ-CC6H5)­(κ2-O2PPh2)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (18). Additionally, by swapping the addition order of benzoyl chloride and o-toluoyl chloride, we isolated the pair of benzylidyne isomers [Mo2(Cl)­(μ-Cl)­(μ-CC6H5)­(κ2-O2C-2-MeC6H4)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (19) and [Mo2(Cl)­(μ-Cl)­(μ-C-2-MeC6H4)­(κ2-O2CPh)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (20). All of these results revealed that the alkylidyne groups were derived from the first equivalent of the acyl group and the second acyl group served as an oxygen acceptor to give the carboxylato group. Interestingly, the Mo–C­(alkylidyne) bond lengths did not change as the valency of Mo atoms decreased from IV to III, exemplified by KC8 reduction of 13 to give the halo-free alkylidyne complex [Mo2(μ-C-4-FC6H4)­(κ2-O2C-4-FC6H4)­(μ-κ2-HC­(N-2,6-iPr2C6H3)2)2] (23).