Tertiary phosphine oxide complexes of lanthanide diiodides and dibromides

The reaction of OPR 3 (R = Me, Ph) with YbI 2 , EuI 2 and EuBr 2 in rigorously anhydrous MeCN under N 2 produces the divalent lanthanide complexes [LnX 2 (OPR 3 ) 4 ] (Ln = Yb, Eu, X = I; Ln = Eu X= Br) in moderate to good yield, whilst [SmI 2 (OPR 3 ) 4 ] were obtained from SmI 2 and OPR 3 in dry, degassed thf. These are the first examples involving divalent lanthanide ions and the complexes have been characterised by microanalysis, IR, UV/visible and 31 P{ 1 H} NMR spectroscopy. The X-ray crystal structure of [EuI 2 (OPPh 3 ) 4 ] ⋅ MeCN confirmed the six-coordinate Eu(II) species with a cis -octahedral geometry, which IR spectroscopy suggests is present in all of the OPPh 3 complexes. In contrast the [LnX 2 (OPMe 3 ) 4 ] complexes appear to be trans isomers. The OPPh 3 complexes are readily oxidised by dry O 2 or I 2 , yielding the corresponding trivalent cations, [LnX 2 (OPPh 3 ) 4 ] + ; a crystal structure of the product formed by oxidation of [EuI 2 (OPPh 3 ) 4 ] in MeCN solution confirms this to be [EuI 2 (OPPh 3 ) 4 ]I 3 ⋅ 1.5MeCN, containing a trans -octahedral cation.

ligands for trivalent lanthanides, and many examples with a range of R groups are known as oxoanion salts (nitrate, triflate, etc.), diketonates and halides, as described in a very recent comprehensive review [9]. Taking OPPh3 (the most thoroughly investigated ligand) as an example, all the LnCl3 complexes are six-coordinate, with both mer-[LnCl3(OPR3) 3
Here we report the preparations and properties of complexes of OPPh3 and OPMe3 with the divalent LnI2 (Ln = Yb, Eu and Sm) and EuBr2.

Experimental
Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer Spectrum 100 spectrometer over the range 4000−200 cm −1 . UV/visible spectra were recorded from sealed PTFE cell with silica window on neat samples, using the diffuse reflectance attachment, in a Perkin Elmer 750S spectrometer. 31 P{ 1 H} NMR spectra were recorded using a Bruker AV 400 spectrometer and are referenced to external 85% H3PO4. Microanalyses were undertaken by London Metropolitan University.
Lanthanide dihalides, solvents and other reagents were obtained from Sigma-Aldrich and used as received. Trimethylphosphine oxide was dried by sublimation in vacuo, and triphenylphosphine oxide melted under vacuum before use. MeCN was dried by distillation from CaH2 and thf from sodium benzophenone-ketyl. Syntheses were routinely carried out under a dry dinitrogen atmosphere, and all solids and spectroscopic samples were handled in a dry dinitrogen filled glove box. Since the complexes have limited stability in dilute solution, all NMR samples were freshly prepared immediately before recording data.

[YbI2(OPPh3)4]
YbI2 (0.05 g, 0.12 mmol) was dissolved in anhydrous MeCN (10 mL), a solution of OPPh3 (0.13 g, 0.47 mmol) in MeCN (5 mL) added, and the mixture stirred from 20 min. during which the orange colour of the solution deepened. The solution was concentrated to a small volume when a bright yellow precipitate formed. This was filtered off, and dried in vacuo to yield a bright yellow powder. Yield 0.14 g, 79%.
Required for C72H60I2O4P4Yb (1539. The MeCN solution of [YbI2(OPPh3)4] exposed to air or O2 resulted in rapid diminution of the 31 P{ 1 H} NMR resonance and development of a new resonance at δ = -39.0, assigned as a Yb(III) species, often with a weak feature at +29.6 ppm (OPPh3). The same species was generated by addition of I2 to an MeCN solution of the divalent complex.

X-ray experimental:
Details of the crystallographic data collection and refinement parameters are given in Table 1.
Crystals suitable for single crystal X-ray analysis were obtained as described above. Data collections

Results and discussion
The reaction of YbI2 with OPR3 (R = Ph or Me) in anhydrous and deoxygenated MeCN produced   [20][21][22][23] and are quite different to the f-f spectra seen in the Ln(III) complexes. For phosphine oxide complexes, the near UV region is dominated by the π→π* transitions of the P=O unit [24] (and for OPPh3 π→π* transitions of the aryl groups), but at lower energy the expected Ln(II)-centred transitions are seen (Section 2) and support the attribution as divalent metal centres.
Although most lanthanide ions are paramagnetic, 31 P{ 1 H} NMR spectra for Ln(III) phosphine oxide complexes are mostly obtainable and show characteristic shifts correlating with the 4f n configuration present [10,13]. For the diamagnetic divalent complex, [YbI2(OPR3)4], the complexes show sharp singlets at +36.1 (R = Ph) and +50.9 (R = Me) shifted from the values in the parent phosphine oxides of +29 and +36 respectively [10,16]. In contrast, the [EuX2(OPR3)4] failed to exhibit any 31 P resonance, presumably due to the effect of the f 7 Eu(II) centre. Gadolinium(III) complexes, which are also f 7 , were the only Ln(III) complexes that did not exhibit a 31 P NMR resonance from their   (19).

Conclusions
The first examples of divalent lanthanide halide complexes bearing phosphine oxide ligands have been isolated and characterised. Unexpectedly, spectroscopic and crystallographic data confirm that with OPPh3 these six-coordinate species adopt a cis-octahedral geometry, [LnX2(OPPh3)4], in the solid state, while the OPMe3 complexes appear to be the trans isomers. As might be expected, the divalent species are very readily oxidised to form [LnX2(OPR3)4] + and crystallographic analysis shows that for Eu(III), the [EuI2(OPPh3)4] + cation is the trans isomer, similar to other known trivalent analogues. The extension of the ligand types known to form Ln(II) species to OPR3, indicates that Ln(II) complexes with other ligand types merit investigation, including OAsR3, O2SR2 and possibly PR3. Although not observed in the present work, it seems possible that under appropriate (different) conditions, SmI2 especially, may be able to reduce P=O bonds since it can reduce OSR2 or O2SR2 to SR2 [8].