Synthesis, X-ray Crystal Structure Determination, and Low-Temperature n-Hexane Glass Luminescence Studies of [Cu26(hfac)12(C⋮CR)14] (R = n-C4H9, n-C5H11, n-C6H13)

By employment of monoanionic hfac(1,1,1,5,5,5-hexafluoroacetylacetonate) as a “capping ligand”, it is possible to modulate the facile accretion of [{Cu(C⋮CR)} n ] moieties to yield discrete high-nuclearity Cu(I) clusters of the type [Cu x + y (hfac) x (C⋮CR) y ]. Three more examples of this type of system have been synthesized for alkynyl ligands possessing linear carbon chains (−C4H9 n to −C6H13 n ), all with similar formula numbers: x + y = 26, x = 12, and y = 14. These clusters have broad structural similarities to other members of this class of systems, possessing a Cu6 |Cu6 |Cu12 ( 1 *) -triannular, doubled-layered Cu structure, assembled about a single interplanar linear (η σ 1-RC⋮C)2Cu(I) “fulcrum” unit. These systems also possess novel structural features in that the “26th” outermost Cu(I)* ion, required to maintain the charge neutrality of the cluster, is positionally disordered over the periphery of the molecule, inserted into the third Cu-annulus, and therefore effectively acting as a “outersphere” cation. This ion insertion, in two cases, causes significant distortion of the molecule, thus disrupting the complicated mesh of alkynyl-Cu bridging modes within the inner Cu-annuli. The room-temperature (293 K) emission spectrum of [Cu26(hfac)12(C⋮CC6H13 n )14] in n-hexane solution exhibits two features, a high-intensity envelope at 344 nm and a lower energy, low-intensity, vibronically structured emission [366, 382, and 399(sh) nm]. Lifetime measurements on these two emissions indicate that they both originate from singlet−singlet transitions. For [Cu26(hfac)12(C⋮CC6H13 n )14] at low temperature, in an n-hexane solvent glass, excitation at 280 nm results in two emission bands: one at 350 nm, the other at lower energy, which possesses considerable vibronic fine structure, exhibiting clearly defined maxima at 406, 419, 425, 433, and 448 nm and shoulders at 464 and 482 nm. The absence of the latter feature at 293 K indicates that it is phosphorescent emission, as confirmed by a measured lifetime of 170 ms.