Heterometal substitution in the dimensional reduction of cluster frameworks: synthesis of soluble [Re(6-n)Os(n)Se(8)Cl(6)](4-n)- (n = 1-3) cluster-containing solids.

Abstract

A general method for deconstructing cluster frameworks via heterometal substitution is demonstrated with application to the two-dimensional phase Re(6)Se(8)Cl(2). Solid-state reactions intended to replace two of the Re(III) centers in the parent compound with Os(IV)Cl units indeed yield Re(4)Os(2)Se(8)Cl(4). The crystal structure of this new phase reveals a less tightly connected two-dimensional framework, wherein face-capped octahedral [Re(4)Os(2)Se(8)](4+) cluster cores are linked through mu(2)-chloride bridges in one dimension and unsupported metal-selenium bonds in the other. Use of CsCl as a standard dimensional reduction agent in conjunction with heterometal substitution affords Cs(3)Re(5)OsSe(8)Cl(6) and Cs(2)Re(4)Os(2)Se(8)Cl(6), soluble salts containing discrete molecular clusters. Reactions employing KCl and targeting a triosmium cluster further produce the soluble mixed-cluster salt K(2)[Re(3)Os(3)Se(8)Cl(6)][Re(4)Os(2)Se(7)Cl(7)]. FT-ICR mass spectra confirm the presence of [Re(6-n)Os(n)Se(8)Cl(6)](4-n)- (n = 1-3) clusters in solutions of these solids. Metathesis reactions supply (Bu(4)N)(3)[Re(5)OsSe(8)Cl(6)] and (Bu(4)N)(2)[Re(4)Os(2)Se(8)Cl(6)], which then react with PEt(3) under forcing conditions to give [Re(5)OsSe(8)(PEt(3))(6)](3+) and [Re(4)Os(2)Se(8)(PEt(3))(6)](4+) in high yield. Analysis of the latter diosmium species by (31)P NMR spectroscopy indicates a mixture of isomers, in which 55% of the clusters have the osmium atoms disposed trans to each other, while the remainder adopt the alternative cis configuration. Cyclic voltammetry measurements reveal a reversible one-electron reduction for [Re(5)OsSe(8)Cl(6)](3-) and multiple one-electron reductions for the mono- and diosmium hexaphosphine clusters; in the latter case, the trans isomer is observed to be more easily reduced than the cis isomer. Electronic structure calculations utilizing density functional theory show the trans isomer of [Re(4)Os(2)Se(8)Cl(6)](2-) to be 4.5 kcal/mol more stable than the cis isomer, in approximate agreement with the observed ratio of reaction products. Moreover, the calculations expose significant differences in the contributions of rhenium and osmium to the frontier orbitals of the clusters, suggesting the possibility of observing metal-selective reactivity. An initial example of such behavior is provided with the synthesis of trans,trans-[Re(4)Os(2)Se(8)(PEt(3))(2)Cl(4)], wherein PEt(3) ligands preferentially bind the osmium centers.