I think arrays of matrices have the size of the array first, which is why Stan wants something with shape (1,2,2). It is slightly weird as that dimension comes last in the declaration. I think there is a more intuitive ve way of declaring this type/shape, but I have forgotten it.
Edit: This! Index error when using new ODE interface - #2 by rok_cesnovar
I have repeatedly searched for it and failed to find it in the documentation.