Water-Soluble Organometallic Compounds. 7.¹ Further Studies of 1,3,5-Triaza-7-Phosphaadamantane Derivatives of Group 10 Metals, Including Metal Carbonyls and Hydrides

The syntheses of Ni(PTA)₄ (5), Pd(PTA)₄ (6), and Pt(PTA)₄ (8) are accomplished through the reduction of the M^IICl₂ 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 pK_a between 7.44 and 9.25. The bound PTA ligands on complex 8 have a pK_a 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.