Solid-State Structure of a Bromo(difluoromethylenephosphonato)cuprate - a Key Intermediate for the Synthesis of α,α-Difluoromethylenephosphonates: Solid-State Structure of a Bromo(difluoromethylenephosphonato)cuprate - a Key Intermediate for the Synthesis of α,α-Difluoromethylenephosphonates