Crystal structure of (μ-(bis(diphenylphosphino)ethane dioxide-kO,O′)-bis([2,3-f]-pyrazino-[1,10]phenanthroline-2,3-dicarbonitrile-kN,N′)- bis(dibromido)dicopper(II) — acetonitrile (1:2.5), [Cu2(C26H24P2O2)(C16H6N6)2Br4] · 2.5CH3CN