The reaction of a dinucleating bis(iminopyridine) ligand L bearing a xanthene linker (L = N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni₂(COD)₂(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene) leads to the formation of a new dinuclear complex Ni₂(L)(DPA). Ni₂(L)(DPA) can also be obtained in a one-pot reaction involving Ni(COD)₂, DPA and L. The X-ray structure of Ni₂(L)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features Ni^I centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni₂(L)(DPA) with one equivalent of ethyl propiolate (HCCCO₂Et) forms the Ni₂(L)(HCCCO₂Et) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerised products by ¹H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni₂(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5 : 1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerisation products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.

The synthesis of a heterodinucleating ligand LH₂ (LH₂ = (E)-3-(((2,7-di-tert-butyl-9,9-dimethyl-5-((pyridin-2-ylmethylene)amino)-9H-xanthen-4-yl)amino)methyl)benzene-1,2-diol) was undertaken toward a functional model of the bimetallic active site found in Mo–Cu carbon monoxide dehydrogenase (Mo–Cu CODH), and to understand the origins of heterobimetallic cooperativity exhibited by the enzyme. LH₂ features a hard potentially dianionic catechol chelate for binding Mo(VI) and a soft iminopyridine chelate for binding Cu(I). Treatment of LH₂ with either Cu(I) or M(VI) (M = Mo, W) sources leads to the anticipated site-selective incorporation of the respective metals. While both [Cu^I(LH₂)]⁺ and [M^VIO₃(L)]²⁻ complexes are stable in the solid state, [M^VIO₃(L)]²⁻ complexes disproportionate in solution to give [M^VIO₂(L)₂](NEt₄)₂ complexes, with [M^VIO₄]²⁻ as the by-product. The incorporation of BOTH Mo(VI) and Cu(I) into L forms a highly reactive heterobimetallic complex [Mo^VIO₃Cu^I(L)](NEt₄)₂, whose formation and reactivity was interrogated via¹H NMR/UV-vis spectroscopy and DFT calculations. These studies reveal that the combination of the two metals triggers oxidation reactivity, in which a nucleophilic Mo(VI) trioxo attacks Cu(I)-bound imine. The major product of the reaction is a crystallographically characterized molybdenum(VI) complex [Mo(L′)O₂](NEt₄) coordinated by a modified ligand L′ that contains a new C–O bond in place of the imine functionality. This observed hydroxylation reactivity is consistent with the postulated first step of Mo–Cu CODH (nucleophilic attack of the Mo(VI)–oxo on the Cu(I)-bound electrophilic CO) and xanthine oxidoreductase (nucleophilic attack of Mo(VI)–oxo on the electrophilic xanthine carbon).