Selective modification of the metal coordination environment in heavy alkaline-earth iodide complexes

A series of heavy alkaline-earth (Ae) iodide coordination compounds is reported that contains various neutral donor ligands, including triphenylphosphine oxide (OPPh₃), tri-n-propylphosphine oxide (OPPr₃), tetraethyl urea (TEU), dimethylpropylene urea (DMPU), and the nitrosobenzene dimer ([ONPh]₂). Phosphine oxides can displace iodide from calcium or barium in coordination complexes, sometimes displaying cooperative binding and/or generating cationic species of the general formula [AeI_m(OPR₃)_6-(m+p)(THF)_p]^(2-m)+. In particular, despite the nonstoichiometric ratio of the reagents, 2 equiv. of OPPh₃ react with CaI₂ in THF to generate CaI₂(OPPh₃)₃(THF) (1), although the use of 4 equiv. will produce the expected CaI₂(OPPh₃)₄ (2). With CaI₂ and 5 equiv. of tri(n-propyl)phosphine oxide, the cationic species [CaI(OPPr₃)₅]I (3) is formed. With 4 equiv. of OPPh₃ and BaI₂ in THF, the cation [BaI(OPPh₃)₅]I (4) is generated. For comparative purposes, the ureas tetraethylurea (TEU) and N,N′-dimethylpropylene urea (DMPU) were used to form the complexes CaI₂(TEU)₄ (5) and CaI₂(DMPU)₆ (6). Nitrosobenzene reacts with CaI₂ in THF to form CaI₂(O₂N₂Ph₂)₂(THF)₂, which in the presence of trace amounts of water is converted to the hydrogen-bonded [Ca(O₂N₂Ph₂)(H₂O)₂(THF)₃]I₂ (7). Crystal structures are reported for 3-5 and 7; the latter is the first for the nitrosobenzene dimer in a main group complex.