Polyunsaturated Dicarboxylate Tethers Connecting Dimolybdenum Redox and Chromophoric Centers: Syntheses, Structures, and Electrochemistry

Abstract

Compounds with two quadruply bonded Mo₂⁴⁺ units, Mo₂(DAniF)₃ (DAniF = N,N ‘-di-p-anisylformamidinate), linked by unsaturated dicarboxylate dianions of various lengths have been prepared and their spectroscopic and electrochemical properties studied. As identified by the dicarboxylate linkers, these compounds are maleate (7), allene-1,3-dicarboxylate (10), cis,cis-muconate (11), trans,trans-muconate (12), octa-2,4,6-trans,trans,trans-hexatriene-1,8-dioate (tamuate, 13), and deca-2,4,6,8-trans,trans,trans,trans-octatetraene-1,10-dioate (texate, 14). The latter three molecules complete the five-membered (all trans) series [Mo₂(DAniF)₃](O₂C(CHCH)_nCO₂)[Mo₂(DAniF)₃] (n = 0−4). Several unsymmetrical paddlewheel compounds of the type Mo₂(DAniF)₃(O₂CX) (X = C⋮CH (3), CHCH₂ (4), (E)-CHCHCHCH₂ (5)) have also been prepared for comparison to the molecules in which there are linked Mo₂ units. The precursors [Mo₂(DAniF)₃(MeCN)₂](BPh₄), [1]BPh₄, and Mo₂(DAniF)₃Cl(MeCN) (2) have also been isolated and characterized. The structures of all new molecules have been established by X-ray crystallography, including the methyl esters of various carboxylates used as ligands. All of the linked molecules have been examined by cyclic and differential pulse voltammetry, and ΔE_1/2 values, the separation between successive Mo₂⁴⁺/Mo₂⁵⁺ oxidations, have been determined. Those compounds with highly unsaturated, fully conjugated linkers demonstrate electrochemical communication from end-to-end that is more persistent over distance than is accounted for by an electrostatic interaction alone, implying that the π system of these dicarboxylate linkers is mediating communication. In the series [Mo₂(DAniF)₃](O₂C(CHCH)_nCO₂)[Mo₂(DAniF)₃] (n = 0−4), the first oxidation potential shifts progressively to less positive values due to an increasing contribution of the polyolefinic α,ω-dicarboxylate to the molecular orbital undergoing oxidation. This first oxidation potential approaches a limiting value of 63 mV (vs Ag/AgCl) as n becomes infinitely long. Compound 11 can be photoisomerized to 12 in a process that is affected by the presence of the Mo₂⁴⁺ units, as the analogous rearrangement of dimethyl cis,cis-muconate is faster.