Precursors to Water-Soluble Dinitrogen Carriers. Synthesis of Water-Soluble Complexes of Iron(II) Containing Water-Soluble Chelating Phosphine Ligands of the Type 1,2-Bis(bis(hydroxyalkyl)phosphino)ethane

The reactions of the water-soluble chelating phosphines 1,2-bis(bis(hydroxyalkyl)phosphino)ethane (alkyl = n-propyl, DHPrPE; n-butyl, DHBuPE; n-pentyl, DHPePE) with FeCl₂·4H₂O and FeSO₄·7H₂O were studied as routes to water-soluble complexes that will bind small molecules, dinitrogen in particular. The products that form and their stereochemistry depend on the solvent, the counteranion, and the alkyl chain length on the phosphine. In alcoholic solvents, the reaction of FeCl₂·4H₂O with 2 equiv of DHBuPE or DHPePE gave trans-Fe(L₂)₂Cl₂. The analogous reactions in water with DHBuPE and DHPePE gave only cis products, and the reaction of FeSO₄·7H₂O with any of the phosphines gave only cis-Fe(L₂)₂SO₄. These results are interpreted as follows. The trans stereochemistry of the products from the reactions of FeCl₂·4H₂O in alcohols is suggested to be the consequence of the trans geometry of the Fe(H₂O)₄Cl₂ complex, i.e., substitution of the water molecules by the phosphines retains the geometry of the starting material. The formation of cis-Fe(DHPrPE)₂Cl₂ is an exception to this result because the coordination of two −OH groups forms two six-membered rings, as shown in the X-ray structure of the molecule. DHBuPE and DHPePE reacted with FeSO₄·7H₂O in water to initially yield cis-Fe(P₂)₂SO₄ compounds, but subsequent substitution reactions occurred over several hours to give sequentially trans-Fe(DHBuPE)₂(H₂O)(SO₄) and then trans-[Fe(DHBuPE)₂(H₂O)₂]SO₄. The rate constants and activation reactions for these aquation reactions were determined and are consistent with dissociatively activated mechanisms. The cis- and trans-Fe(L₂)₂X (X = (Cl)₂ or SO₄) complexes react with N₂, CO, and CH₃CN to yield trans complexes with bound N₂, CO, or CH₃CN. The crystal structures of the cis-Fe(DHPrPE)₂SO₄, trans-Fe(DHPrPE)₂(CO)SO₄, trans-Fe(DHBuPE)₂Cl₂, trans-[Fe(DHBuPE)₂(CO)(Cl)][B(C₆H₅)₄], trans-Fe(DMeOPrPE)₂Cl₂, trans-Fe(DMeOPrPE)₂Br₂, and trans-[Fe(DHBuPE)₂Cl₂]Cl complexes are reported. As expected from using water-soluble phosphines, the complexes reported herein are water soluble (generally greater than 0.5 M at 23 °C).