Copper diimine complexes: the synthesis and crystal structures of [Cu(C10H14N2)2(MeOH)][BF4], [Cu(C10H20N2)2]Br, [{(C10H14N2)CuBr(µ-OMe)}2(MeOH)] and [{(C10H20N2)CuBr(µ-OMe)}2]