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Water-Soluble Organometallic Compounds. 7. Further Studies of 1,3,5-Triaza-7-Phosphaadamantane Derivatives of Group 10 Metals, Including Metal Carbonyls and Hydrides

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posted on 21.04.1999, 00:00 by Donald J. Darensbourg, Jeffrey B. Robertson, David L. Larkins, Joseph H. Reibenspies
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.