UC Santa Barbara
Expanding the Scope and Reactivity of Transition Metal Nanoclusters
- Author(s): Cook, Andrew Warren
- Advisor(s): Hayton, Trevor W
- et al.
The group 11 hydride clusters [Ag6H4(dppm)4(OAc)2] and [Cu3H(dppm)3(OAc)2] (dppm = 1,1-bis(diphenylphosphino)methane) were synthesized from the reaction of M(OAc) (M = Ag, Cu) with Ph2SiH2, in the presence of the bidentante phosphine ligand, dppm. [Ag6H4(dppm)4(OAc)2] is the first structurally characterized homometallic polyhydrido silver cluster to be isolated. Both clusters catalyze the hydrosilylation of (,–unsaturated) ketones. Notably, this represents the first example of hydrosilylation with an authentic silver hydride complex. The larger copper hydride clusters [Cu10H10(DBEphos)4] (DBEphos = oxydi-2,1-phenylene)bis(diphenylphosphino)) and [Cu14H14(DBFphos)5] (DBFphos = 4,6-bis(diphenylphosphino)dibenzofuran) have also been isolated. Preliminary data suggest that there is a positive correlation between the bite angle of the bidentate phosphine and the size of the cluster generated. Finally, the copper selenoate cluster [Cu13H10(SePh)3(PPh3)7] was generated from the in situ reduction of Ph2Se2 by [CuH(PPh3)]6 and represents a facile method of selenium incorporation into copper clusters.
Further studies into Cu cluster formation is examined through the reactivity of RSH (R = CH2CH2Ph, n-Bu, n-C12H25) with Cu(II) under anhydrous conditions, which results in the formation of “Atlas-sphere”-type copper thiolate nanoclusters, including [Cu12(SR’)6Cl12][(Cu(R’SH))6] (R’ = nBu) and [H(THF)2]2[Cu17(SR”)6Cl13(THF)2(R”SH)3] (R” = CH2CH2Ph). Consistent with the X-ray crystallographic data, the XANES edge energies of these clusters suggest they are constructed exclusively with Cu(I) ions. Given these results, as well as past work on Cu(II)/thiol reactivity, it is argued that Cu(0) cannot be accessed by reaction of Cu(II) with a thiol, and that previous reports of Cu(0)-containing clusters synthesized in this manner are likely erroneous.
The generation of a partially metallic Cu cluster was realized through the synthesis of the mixed-valent organometallic cluster, [Cu20(CCPh)12(OAc)6)], which was isolated from the reduction of Cu(OAc) with Ph2SiH2 in the presence of phenylacetylene. This cluster is a rare example of a two-electron copper superatom, and the first to feature a tetrahedral [Cu4]2+ core. [Cu20(CCPh)12(OAc)6)] can be readily immobilized on dry, partially-dehydroxylated silica. Both materials are effective catalysts for [3+2] cycloaddition reactions between alkynes and azides (i.e., “Click” reactions) at room temperature. Significantly, neither material requires any pre-treatment for activation toward catalysis.
To further expand the scope of first row transition metal clusters with partial metallic character, a re-examination of the synthesis and of monolayer-protected Cox(SCH2CH2Ph)m nanoclusters is addressed. These clusters were reportedly formed by the reaction of CoCl2 with NaBH4 in the presence of HSCH2CH2Ph, and were suggested to contain between 25 and 30 Co atoms. However, there is no evidence to support the existence of these large clusters in the reaction mixture. Instead, this reaction results in the relatively clean formation of the Co(II) coordination complex [Co10(SCH2CH2Ph)16Cl4]. This complex represents the first example of a thiolate-protected Co(II) T3 supertetrahedral cluster.
The ketimide ligand has been shown to stabilize high oxidation states of the transition metals, though it has recently been hypothesized that this ligand may be useful in the generation of low-valent species as well. The isolation of the Pd(0) containing, ketimide-stabilized cluster Pd7(N=CtBu2)6 reinforces this hypothesis. Similar reactivity studies with Pt results in the isolation of the Pt(II) complex, Pt(N=CtBu2)2. Additionally, the low-valent Fe-ketimide cluster, [Fe4(N=CPh2)6], is isolated. These complexes have been previously reported by former Hayton group members, though their syntheses and characterization have been significantly improved. Pt(N=CtBu2)2 exhibits exceptionally short Pt-N distances (av. Pt-N = 1.815 Å) and an unusually deshielded 195Pt chemcial shift (δPt = -629 ppm) with a large 1JPtN coupling constant (537 Hz). Pd7(N=CtBu2)6 features a mixed-valent, hexagonal planar [Pd7]6+ core stabilized by six ketimide ligands. Finally, [Fe4(N=CPh2)6] displays fully delocalized metal bonding electrons, which gives rise to a ground spin-state of S = 7 and single molecule magnet behavior.