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).