Highly Symmetric Group 13 Metal−Phosphinato Complexes: Multinuclear NMR (²⁷Al, ³¹P, ⁷¹Ga) Determination of Stability Constants at Low pH

An N4O3 tripodal tren-based (aminomethylene)phosphinato ligand tris(4-(phenylphosphinato)-3-methyl-3-azabutyl)amine (H3ppma) has been synthesized, and its complexation properties with the group 13 metals Al, Ga, and In have been investigated. The molecular structure of the indium complex [In(H3ppma)2](NO3)3·3H2O (C60H96InN11O24P6) has been solved by X-ray methods; the complex crystallizes in the trigonal space group R3̄c, with a = 18.984(3) Å, c = 36.256(5) Å, and Z = 6. The structure was solved by Patterson methods and was refined by full-matrix least-squares procedures to R = 0.040 (Rw = 0.039) for 1415 reflections with I > 3σ(I). The structure of the bis-complex showed the ligand to coordinate in a tridentate manner through the three phosphinate oxygens, resulting in a bicapped octahedral structure of exact S6 symmetry. The solved structure was of the RRRSSS diastereomer, where half of the molecule contained phosphorus atoms of R chirality and the other half contained phosphorus atoms of S chirality. The highly symmetric environment about the metal atoms produces a low electric field gradient at the metal nucleus leading to unusually narrow line widths in the 27Al, 71Ga, and 115In NMR spectra. The aluminum complex [Al(H3ppma)2](NO3)3·2H2O exhibited an extremely rare example of aluminum−phosphorus coupling in both the 31P and 27Al NMR spectra, where 2JAlP was shown from both spectra to be 6.7 Hz. The narrow line widths made the complexes amenable to stability constant studies via a combination of 27Al, 71Ga, and 31P NMR spectroscopies (25 °C). The formation constants for In3+ (log β2 ≥ 5.4), Ga3+ (log β2 = 4.24), and Al3+ (log β1 = 0.93, log β2 = 3.45) decrease by an order of magnitude as the group is ascended, consistent with increasing steric interactions of the phenyl groups as the two trisphosphinate ligands are crowded together in order to coordinate the smaller metal ions. Variable temperature 27Al and 31P NMR spectroscopic studies indicated the RRRSSS diastereomer to be rigid up to 55 °C in CD3OD.