What's the smallest Hoberman sphere possible?
Hoberman sphere shares the same topological features being an icosidodecahedron with a family of polyoxometalates called Keplerates $\ce{\{(\color{teal}{Mo|W})_72\color{orange}{M}_30\}}$ ($\ce{\color{orange}{M} = Fe,Cr,V}$, anionic residues and alkali metal cations are removed for clarity) ([1, 2]):



Also, there is a cluster $\ce{[\color{blue}{La}_20\color{green}{Ni}_30(IDA)30(CO3)6(NO3)6(OH)30(H2O)12](CO3)6 * 72H2O}$ (IDA - iminodiacetate $\ce{C4H5NO4}$) consisting of an icosidodecahedron of Ni(II) Ions
encapsulating an inner La(III)-dodecahedron [3]:



The problem is, these structures only resemble the symmetry, but not the expandability of Hoberman sphere. Currently there are many attempts of creating nanomachines with mechanically active molecular assemblies, but I didn't find any working models close to Hoberman structures (yet). 

Also, some "spherical" viruses such as Cowpea chlorotic mottle virus (CCMV) with a structure of truncated icosahedron, which is significantly larger than Keplerates ($\pu{28 nm}$ (CCMV) vs. $\pu{3.2 nm}$ (average Keplerate)), can undergo swelling (pH &lt; 6.5) and expansion (pH > 6.5), and show pH and metal ion dependent polymorphism [4, 5]


  as a result of deprotonation of carboxyl moieties at the inter-capsomere contacts, leading to electrostatic repulsion that opens out the structure but falls short of complete disassembly through preservation of interwoven
  carboxyl/protein links between capsomeres.




Don't forget that "there's plenty of room at the bottom", so Hoberman sphere can possibly be a real working entity at molecular scale.

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