I am currently collecting information for an abstract and therefore I wanted to discuss different $\ce{[BO3]^3-}$ and $\ce{[BO4]^5-}$ connectivities. I also found a book on crystallography and the following passage

(Summarized translation of the original German text in [1])

When a finite connection of $\ce{[BO3]^3-}$ units is considered group borates form. [...] $\ce{[B2O5]^4-}$ (via vertices), $\ce{[B2O4]^2-}$ (via edges) or for example $\ce{[B4O8]^{4-}}$ (four-membered ring). [...]

It's about the last example that I cannot really find any other mention in literature. Although it looks okay if you consider vertex-connected oxoborate units forming a $\ce{[B4O8]^{4-}}$ anion, I cannot seem to find any example in literature. Even the book doesn't give any example.

I found some minerals for example $\ce{Ca2B4O8}$ but it's also described as $\ce{CaB2(OH)8}$. There are examples with $\ce{[B4O8]^{4-}}$ but they always contain additional $\ce{(OH)-}$ ligands.

The any other explanation I could think of would be $\ce{[B4O7]^{2-}}$. That would have a different structure (they also sketched their molecule anion and the got the charges correct so I don't think $\ce{[B4O8]^{4-}}$ is typing mistake). So, did anyone else find any example on a $\ce{[B4O8]^{4-}}$? I would usually check the Gmelin but due to the virus spread our library is currently closed.

[1] W. Kleber, H.-J. Bautsch, J. Bohm, Einführung in die Kristallographie, 19th edition, Oldenbourg Verlag, Munich, 2010.

Edit: I also did a quick DFT, hybrid functional B3LYP with Def2-SVP on the anion and according to my bond analysis (don't know if you can trust that for a tetra-anion here) all $\ce{B-O}$ bonds inside the ring have a bond order of 1.x while the external $\ce{B-O}$ bonds have a double bond character (I guess it's treating them as $\ce{B=O}$ rather than with charges here). But in theory that thing should be stable to me.


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