Preparation, Characterization, and Structural Systematics of Diphosphane and Diarsane Complexes of Gallium(III) Halides

Abstract

The diphosphane o-C6H4(PMe2)2 reacts with GaX3 (X = Cl, Br, or I) in a 1:1 molar ratio in dry toluene to give trans-[GaX2{o-C6H4(PMe2)2}2][GaX4], the cations of which contain the first examples of six-coordinate gallium in a phosphane complex. The use of a 1:2 ligand/GaCl3 ratio produced [GaCl2{o-C6H4(PMe2)2}][GaCl4], containing a pseudotetrahedral cation, and similar pseudotetrahedral [GaX2{o-C6H4(PPh2)2}][GaX4] complexes are the only products isolated with the bulkier o-C6H4(PPh2)2. On the other hand, Et2P(CH2)2PEt2, which has a flexible aliphatic backbone, formed [(X3Ga)2{mu-Et2P(CH2)2PEt2}], in which the ligand bridges two pseudotetrahedral gallium centers. The diarsane, o-C6H4(AsMe2)2, formed [GaX2{o-C6H4(AsMe2)2}][GaX4], also containing pseudotetrahedral cations, and in marked contrast to the diphosphane analogue, no six-coordinate complexes form; a very rare example where these two much studied ligands behave differently towards a common metal acceptor. The complexes [(I3Ga)2{mu-Ph2As(CH2)2AsPh2}] and [GaX3(AsMe3)] are also described. The X-ray structures of trans-[GaX2{o-C6H4(PMe2)2}2][GaX4] (X = Cl, Br or I), [GaCl2{o-C6H4(PPh2)2}][GaCl4], [GaX2{o-C6H4(AsMe2)2}][GaX4] (X = Cl or I), [(I3Ga)2{mu-Ph2As(CH2)2AsPh2}], and [GaX3(AsMe3)] (X = Cl, Br or I) are reported, and the structural trends are discussed. The solution behavior of the complexes has been explored using a combination of 31P{1H} and 71Ga NMR spectroscopy.

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

“…However, there seem to be only small effects on the Ga-E bond lengths from changing the R group, although in analogous complexes with R = aryl the Ga-E seem to be marginally longer than R = alkyl [170][171][172][173][174][175][176][177], which is in keeping both with the Ga-X bonds being dominant compared to Ga-E, and also with the absence of significant steric effects in these complexes. There appear to be no established examples of gallium(III) halide complexes of stibines; early studies suggested fragmentation of the stibine or X/R exchange [7].…”

“…There is limited modelling work on the Lewis acidity of gallium systems, but it is likely that the key factors identified for boron and aluminium are also operating in the gallium systems. As discussed for aluminium (Section 6.2), other factors sometimes complicate interpretation of Lewis acidity based upon bond length data, but it seems clear in the case of gallium(III) that, apart from a few anomalies, the data indicate an order with halide Cl > Br > I [131,170,171]. Analogous to the aluminium system, GaF 3 is an inert polymer and no soft donor complexes are known [130].…”

“…As is clear from [177], and this reference contains a thorough discussion of the various contributions to the observed chemical shifts. Solution NMR studies in chlorocarbon solvents also reveal that some [GaX 3 (ER 3 )] undergo fast ligand exchange in solution in non-coordinating solvents at room temperature or above (depending upon the specific combination of ER 3 /X present), and cooling the solutions may be necessary to observe resonances and spin-spin couplings [170,171,176]. In solution in chlorocarbon solvents, 31 [205].…”

“…[186]. there is no evidence for the formation of [GaX 2 {Et 2 P(CH 2 ) 2 PEt 2 }] + [170,176,180]. The diphosphinoalkene cis-Ph 2 PCH = CHPPh 2 reacts with GaX 3 (X = Br or I) in hot toluene to form [X 3 Ga(-transPh 2 PCH = CHPPh 2 )GaX 3 ], the cis → trans isomerisation being confirmed spectroscopically and by the X-ray structure of the bromide complex [181].…”

“…28) which are the only forms isolated in the solid state, although in situ 31 P{ 1 H} NMR studies suggested other species may be present in solution in the presence of excess diphosphine. The structures of the cations are distorted by the small chelate bite of the diphosphine (<P-Ga-P ∼85 • ), and the distortions cause loss of the 71 Ga NMR resonances of the cations as a consequence of the increased electric field gradients and resulting fast relaxation [170,179] (Fig. 29) [170].…”