Soft diphosphine and diarsine complexes of niobium(v) and tantalum(v) fluorides: synthesis, properties, structures and comparisons with the corresponding chlorides

Abstract

The reactions of the soft diphosphines o-C6H4(PMe2)2, Me2P(CH2)2PMe2, Et2P(CH2)2PEt2 or o-C6H4(PPh2)2 with NbF5 or TaF5 in anhydrous MeCN solution produce [MF4(diphosphine)2][MF6] (M = Nb or Ta), which have been characterised by microanalysis, IR, (1)H, (19)F{(1)H}, (31)P{(1)H} and (93)Nb NMR spectroscopy. X-ray crystal structures are reported for the isomorphous [MF4{o-C6H4(PMe2)2}2][MF6], which confirm the presence of eight-coordinate (distorted dodecahedral) cations. The corresponding reactions using o-C6H4(AsMe2)2 produced [MF4{o-C6H4(AsMe2)2}2][MF6] which were similarly characterised, including by the X-ray structure of [NbF4{o-C6H4(AsMe2)2}2][NbF6]. These are very rare examples of arsine complexes of high valent metal fluorides. The chloro complexes [NbCl4{o-C6H4(PMe2)2}2]Cl, [TaCl4{o-C6H4(PMe2)2}2][TaCl6], [NbCl4{Me2P(CH2)2PMe2}2][NbCl6] and [MCl4{o-C6H4(AsMe2)2}2][MCl6] were prepared and their structural and spectroscopic properties compared with the fluoride analogues. Attempts to prepare diphosphine complexes of NbOF3 were unsuccessful, but the NbOCl3 complexes, [{{Me2P(CH2)2PMe2}NbOCl3}2{μ-Me2P(CH2)2PMe2}] and [{o-C6H4(PMe2)2}NbOCl3(μ-O)NbCl3(CH3CN){o-C6H4(PMe2)2}] were obtained. X-Ray structures are also reported for [NbCl4{o-C6H4(PMe2)2}2]Cl, [NbCl4{o-C6H4(AsMe2)2}2][NbCl5(OEt)], [NbCl4{o-C6H4(PMe2)2}2][NbOCl4(CH3CN)], [{{Me2P(CH2)2PMe2}NbOCl3}2{μ-Me2P(CH2)2PMe2}] and [{o-C6H4(PMe2)2}NbOCl3(μ-O)NbCl3(CH3CN){o-C6H4(PMe2)2}].

Niobium tetrahalide complexes with neutral diphosphine ligands

“…In the solid state (diffuse reflectance) UV-visible spectra (Experimental Section), for the chlorocomplexes, there are intense bands at ~ 23 000 and ~ 30 000 cm -1 which may be assigned at P(σ) → Nb and Cl(π) → Nb, which are consistent with charge transfer energies in the Nb(V) analogues, 4 and with the six-…”

“…4 Whilst the complexes of niobium(V) chloride and bromide are mononuclear and based upon six-or eight-coordinate metal centres, those in lower oxidation states are more varied. For Nb(IV) with tertiary phosphines, there are well-characterised mononuclear examples with six-e.g.…”

“…The structure of [TaCl4(Me2PCH2CH2PMe2)2] 18 has been determined, but curiously the tantalum is coordinated in a square antiprismatic geometry, whilst the Nb(V) or Ta(V) complexes are dodecahedral. 4,18 Several diarsine complexes of NbCl4 have been briefly described, [19][20][21] and the structure of [NbCl4{o-C6H4(AsMe2)2}2]…”

“…For Nb(IV) with tertiary phosphines, there are well-characterised mononuclear examples with six-e.g. 4], which appears to dissociate into a six-coordinate monomer in solution. 8,9 There is also spectroscopic evidence for the existence of an eight-coordinate [NbCl4(PMe3) 4] monomer in solution.…”

“…4], which appears to dissociate into a six-coordinate monomer in solution. 8,9 There is also spectroscopic evidence for the existence of an eight-coordinate [NbCl4(PMe3) 4] monomer in solution. 6 As pointed out by Cotton,5,6,8 the speciation often differs between the solids and solutions in thf or CH2Cl2, and the solution speciation is only partially understood.…”