Syntheses, characterization and crystal structures of [Cu(PCHO)2(2,2′-bipy)][BF4], [Cu2(PCHO)4(4,4′-bipy)][BF4]2 and [Cu2(PCHO)4(bptz)] [BF4]2 (PCHO = o-(diphenylphosphino)benzaldehyde and bptz = 3,6-bis(2′-pyridyl)-1,2,4,5-tetrazine)