Syntheses and characterization of layered phenyl- and benzylphosphonates of tetravalent vanadium, Ca[(VO)(O3PPh)2]·2H2O, Cd[(VO)(O3PPh)2]·2H2O, Ba[(VO)(O3PCH2Ph)2]·2H2O, Sr[(VO)3(O3PPh)4]·5H2O and Ba[V4O4(OH)2(O3PPh)4]·5H2O