Pyramidal Inversion at Phosphorus Facilitated by the Presence of Proximate Lewis Acids. Coordination Chemistry of Group 13 Elements with the Macrocyclic Bis(amidophosphine) Ligand [P₂N₂] ([P₂N₂] = [PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh])

Abstract

Investigations on the preparation of four- and five-coordinate aluminum and gallium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li(2)(THF)(2)[P(2)N(2)] ([P(2)N(2)] = [PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh]) with AlCl(3) or GaCl(3) in toluene at 25 degrees C leads to the formation of the four-coordinate species anti-MCl[P(2)N(2)] (M = Al (1), Ga (2)). An X-ray diffraction study of anti-GaCl[P(2)N(2)] shows it to be monomeric with a distorted tetrahedral geometry at Ga; only one of the phosphine donors of the [P(2)N(2)] ligand binds to the gallium, resulting in the retention of the anti-configuration. The solution NMR spectra are consistent with C(s)() symmetry. The addition of AlCl(3) or GaCl(3) to the macrocyclic ligand precursor syn-Li(2)(dioxane)[P(2)N(2)] in toluene at 25 degrees C yields the five-coordinate complexes syn-MCl[P(2)N(2)] (M = Al (3), Ga (4)). The X-ray crystal structure of syn-GaCl[P(2)N(2)] reveals a trigonal bipyramidal geometry about the metal atom, necessitating the coordination of both phosphorus atoms. The solution NMR spectra are consistent with a C(2)(v)() symmetric complex. Heating the anti complexes results in the clean conversion to the syn complexes, with pyramidal inversion observed at phosphorus. The kinetics of this inversion were studied by (1)H NMR spectroscopy and found to be first-order. Barriers to pyramidal inversion (DeltaG()) were calculated to be 29.1 and 30.1 kcal mol(-)(1) for the aluminum and gallium complexes, respectively; these barriers are approximately 2-3 kcal mol(-)(1) lower than that determined for the metal-free, protonated compounds anti- and syn-H(2)[P(2)N(2)]. It is suggested that the role that the metals play in this inversion, based on the values of DeltaG(), involves the large negative entropies of activation and thus help organize the transition state.

Coordination chemistry of the main group elements with phosphine, arsine and stibine ligands

“…The macrocyclic bis(amidophosphine) ligand (P 2 N 2 ) is found in two forms syn and anti; the former produces five-coordinate [AlCl{syn-(P 2 N 2 )}] (ClP 2 N 2 coordination), whereas the latter gives four-coordinate [AlCl{anti-(P 2 N 2 )}] (ClPN 2 coordination) [168]. In solution, NMR studies show anti → syn conversion via pyramidal inversion at phosphorus.…”

“…The macrocyclic diamidodiphosphine (P 2 N 2 ) (see Section 6.2) forms syn and anti isomers of [GaCl(P 2 N 2 )] and syn-[GaH(P 2 N 2 )], analogous to the aluminium complexes discussed above. However, for gallium, reduction of syn-[GaCl(P 2 N 2 )] with KC 8 produces the dimer [{Ga(P 2 N 2 )} 2 ], which has an unsupported Ga-Ga bond [168,169]. A variety of coordination modes are present in the gallium(III) complexes of o-C 6 H 4 PPh 2 (SH), o-C 6 H 4 AsPh 2 (SH) and PhP(o-C 6 H 4 SH) 2 , with the majority containing deprotonated thiolates [218,219].…”

Mechanistic Insights into the Palladium‐Catalysed Asymmetric Phosphination of Cyclohexenyl Triflate

“…Therefore, only one diastereomer of the transmetallation adduct 5 having a controlled stereochemistry at phosphorus is expected if the coordination of the phosphide 4 to the metal centre occurs with high stereocontrol. [19] In fact, we observed in the 31 P NMR spectrum two isomers of the transmetallation adduct 5 in a 1:1.5 ratio. [20] The extra isomer 5min appeared to be a conformational isomer, which arose from slow rotation about the Pd-C bond on the NMR time scale.…”

“…Thin-layer chromatography (TLC) was performed on silica gel 60 F-254 plates (0.1 mm) with iodine or UV detection. 1 H, 13 C, 19 F and 31 P NMR spectra were obtained on Bruker DPX 250 or AC 400 spectrometers, 11 B NMR spectra were recorded on a Bruker AC 400 spectrometer.…”

“…31 P, 19 F and 11 B NMR chemical shifts are reported relative to H 3 PO 4 (85 %), CFCl 3 and BF 3 ·Et 2 O used as external references, respectively. Coupling constants are reported in Hertz (Hz).…”