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

An N 4 O 3 tripodal tren-based (aminomethylene)phosphinato ligand tris(4-(phenylphosphinato)-3-methyl-3-azabutyl)amine (H 3 ppma) 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(H 3 ppma) 2 ](NO 3 ) 3 ·3H 2 O (C 60 H 96 InN 11 O 24 P 6 ) has been solved by X-ray methods; the complex crystallizes in the trigonal space group R 3̄ c, 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 ( R w = 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 S 6 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 27 Al, 71 Ga, and 115 In NMR spectra. The aluminum complex [Al(H 3 ppma) 2 ](NO 3 ) 3 ·2H 2 O exhibited an extremely rare example of aluminum−phosphorus coupling in both the 31 P and 27 Al NMR spectra, where 2 J AlP 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 27 Al, 71 Ga, and 31 P NMR spectroscopies (25 °C). The formation constants for In 3+ (log β 2 ≥ 5.4), Ga 3+ (log β 2 = 4.24), and Al 3+ (log β 1 = 0.93, log β 2 = 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 27 Al and 31 P NMR spectroscopic studies indicated the RRRSSS diastereomer to be rigid up to 55 °C in CD 3 OD.