  We prove that a semi-direct product of two finite rank free groups $F_k$ and
$F_n$ such that $F_k$ acts on $F_n$ by polynomially growing automorphisms acts
properly isometrically on a finite dimensional CAT(0) cube complex provided
some finite-index unipotent subgroup of $F_k$ satisfies two supplementary
properties. In particular any such group is a-T-menable in the sense of Gromov
(equivalently satisfies the Haagerup property).
