Water-Soluble Organometallic Compounds. 7. Further Studies of 1,3,5-Triaza-7-Phosphaadamantane Derivatives of Group 10 Metals, Including Metal Carbonyls and Hydrides
datasetposted on 21.04.1999 by Donald J. Darensbourg, Jeffrey B. Robertson, David L. Larkins, Joseph H. Reibenspies
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The syntheses of Ni(PTA)4 (5), Pd(PTA)4 (6), and Pt(PTA)4 (8) are accomplished through the reduction of the MIICl2 salts in water with excess 1,3,5-triaza-7-phosphaadamantane (PTA). The products are obtained as partially protonated derivatives with protonation occurring at the nitrogen atoms of the bound PTA ligands. Crystal structures illustrating the monoprotonated and bis-protonated derivatives of 8, [Pt(PTA)3(PTAH)][Cl] (1S) and [Pt(PTA)2(PTAH)2][BF4]2 (2S), are reported. The crystal structure of an intermediate along the reduction pathway, [PdCl(PTA)3][Cl] (7), is also presented, showing a trans influence whereby the Pd−P bond length trans to the Cl- ligand is shorter, 2.238(3) Å, than the average Pd−P bond length cis to the Cl- ligand, 2.334(2) Å. Complex 8 is also protonated at the metal center to form [(H)Pt(PTA)4][X], where X = Cl-, BF4-, HCO3-, and C3H5CO2-, through the addition of weak acids such as CO2/H2O, NH4Cl, PTAH+Cl-, HPy+BF4-, and crotonic acid. Addition of strong acids such as HCl or HBF4 results in protonation at the PTA ligands. Thereby, the Pt metal center is shown to have a pKa between 7.44 and 9.25. The bound PTA ligands on complex 8 have a pKa below 4.69 but above 2.12. Ni(CO)4-n(PTA)n derivatives are also reported for n = 3 (9), 2 (10), and 1 (11). Using IR data of 11, PTA is determined to have an electronic parameter (χ) as defined by Tolman to be 15.3 cm-1, indicating PTA to be slightly less donating than PPh3, where χ = 12.8 cm-1. Complex 10 is crystallized out of a MeOH/ether solution maintained at −15 °C and characterized by X-ray crystallography. It is a distorted tetrahedron and has a crystallographically determined Tolman cone angle of PTA of 103°. Monitored reactions of Ni(PTA)4 with CO in water via in situ IR techniques show that the rate observed for the dissociation of PTA to provide Ni(CO)(PTA)3 is 7.92 × 10-4 s-1 at 20 °C. This rate, along with 31P NMR results, indicates that the M0(PTA)4 complexes exhibit little dissociation and slow exchange of the bound PTA ligands.