Bis(Diphenylphosphino)Methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy †

: Bis(diphenylphosphino)methane dioxide (dppmO 2 ) forms eight-coordinate cations [M(dppmO 2 ) 4 ]Cl 3 (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl 3 in ethanol. Similar reaction in a 3:1 ratio produced seven-coordinate [M(dppmO 2 ) 3 Cl]Cl 2 (M = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), whilst LuCl 3 alone produced six-coordinate [Lu(dppmO 2 ) 2 Cl 2 ]Cl. The complexes have been characterised by IR, 1 H and 31 P{ 1 H}-NMR spectroscopy. X-ray structures show that [M(dppmO 2 ) 4 ]Cl 3 (M = Ce, Sm, Gd) contain square antiprismatic cations, whilst [M(dppmO 2 ) 3 Cl]Cl 2 (M = Yb, Dy, Lu) have distorted pentagonal bipyramidal structures with apical Cl. The [Lu(dppmO 2 ) 2 Cl 2 ]Cl has a cis octahedral cation. The structure of [Yb(dppmO 2 ) 3 (H 2 O)]Cl 3 · dppmO 2 is also reported. The change in coordination numbers and geometry along the series is driven by the decreasing lanthanide cation radii, but the chloride counter anions also play a role.


Introduction
Early work viewed the chemistry of the lanthanides (Ln) (Ln = La-Lu, Pm unless otherwise indicated) in oxidation state III as very similar and often only two or three elements were examined, and the results were assumed to apply to all. More recent work has shown this to be a very unreliable approach and detailed studies of all fourteen elements (excluding only the radioactive Pm) are required to establish properties and trends [1,2]. Sometimes yttrium is also included since it is similar in size to holmium. The main changes along the series are due to the lanthanide contraction, the reduction in the radius of the M 3+ ions between La (1.22 Å) and Lu (0.85 Å), and at some point a reduction in coordination number may be driven by steric effects, especially with bulky ligands. However, the decrease in radius also results in an increase in the charge/radius ratio along the series and this can lead to significant electronic effects on the ligand preferences. This interplay of steric and electronic effects means that changes in coordination number or ligand donor set can occur at different points along the series with different ligands. The effects are very nicely demonstrated in a recent article, which examined the changes which occurred in the series of lanthanide nitrates with complexes of 2,2 -bipyridyl, 2,4,6-tri-α-pyridyl-1,3,5-triazine and 2,2 ; 6 ,2"-terpyridine [2]. Tertiary phosphine oxides have proved popular ligands to explore lanthanide chemistry and the area has been the subject of a comprehensive review [3], and several detailed studies of trends along the series La-Lu have been reported [4][5][6][7]. We reported bis(diphenylphosphino)methane dioxide (dppmO 2 ) formed square-antiprismatic  6 ] counter ions, but lutetium gave only octahedral [Lu(dppmO 2 ) 2 X 2 ] + (X = Cl, I) and [Lu(dppmO 2 ) 2 Cl(H 2 O)] 2+ [8]. Other dppmO 2 complexes reported include several types with Ln(NO 3 ) 3 [4], [Dy(dppmO 2 ) 4 ][CF 3 SO 3 ] 3 [9], [Eu(dppmO 2 ) 4 ][ClO 4 ] 3 [10], [La(dppmO 2 ) 4 ][CF 3 SO 3 ] 3 3 SO 3 ] 3 [11]. Here, we report a systematic study of the systems LnCl 3 -dppmO 2 for all fourteen accessible lanthanides.

Materials and Methods
Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer Spectrum 100 spectrometer over the range 4000-200 cm −1 . 1 H and 31 P{ 1 H}-NMR spectra were recorded using a Bruker AV-II 400 spectrometer and are referenced to the protio resonance of the solvent and 85% H 3 PO 4 , respectively. Microanalyses were undertaken by London Metropolitan University or Medac. Hydrated lanthanide trichlorides and anhydrous LnCl 3 (Ln-Nd, Pr, Gd, Ho) were from Sigma-Aldrich and used as received. The Ph 2 PCH 2 PPh 2 (Sigma-Aldrich) in anhydrous CH 2 Cl 2 was converted to Ph 2 P(O)CH 2 P(O)Ph 2 by air oxidation catalysed by SnI 4 [12].
X-Ray Experimental. Details of the crystallographic data collection and refinement parameters are given in Table 1. Many attempts were made to grow crystals for X-ray examination from a variety of solvents including EtOH and CH 2 Cl 2 , either by slow evaporation or layering with hexane or pentane. The crystal quality was often rather poor, and all of the structures have disordered co-solvent, either water or ethanol. No attempt was made to locate the protons on the co-solvent. Several showed disorder in one or more of the phenyl rings. Good-quality crystals used for single crystal X-ray analysis were grown from [Lu(dppmO 2 ) 4  Data collections used a Rigaku AFC12 goniometer equipped with a HyPix-600HE detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 µm focus) with the crystal held at 100 K (N 2 cryostream). Structure solution and refinements were performed with either SHELX(S/L)97 or SHELX(S/L)2013 [13,14]. The crystallographic data in cif format have been deposited as CCDC 2033611-2033618.
All samples were dried in high vacuum at room temperature for several hours, but this treatment does not remove lattice water or alcohol. Heating the samples in vacuo is likely to cause some decomposition of the complexes [7] and was not applied.

Results
The reaction of LnCl 3 ·nH 2 O (Ln = La [8], Ce, Pr, Nd, Sm, Eu or Gd; n = 6 or 7) with four mol. equivalents of dppmO 2 in ethanol gave good yields of tetrakis-dppmO 2 complexes, [Ln(dppmO 2 ) 4 ]Cl 3 . The IR and 1 H-NMR spectra show the the isolated complexes retain significant amounts of lattice water, and sometimes EtOH, which is not removed by prolonged drying of the bulk powders in vacuo. The high molecular weights make the microanalyses rather insensitive to the amount of water, but are generally consistent with a formulation [Ln(dppmO 2 ) 4 ]Cl 3 ·nH 2 O (n = 6: Ce, Pr; n = 4: Nd, Sm, Eu, Gd), although the amount of lattice solvent probably varies with the sample and is unlikely to be stoichiometric. The presence of significant amounts of lattice solvent is common in lanthanide phosphine oxide systems [7][8][9][10], and although evident in X-ray crystal structures, it is often disordered and difficult to model. Obtaining good quality crystals of the complexes proved difficult, but crystals of the Ce, Sm and Gd salts were obtained from various organic solvents and the compositions are shown in Table 1. The crystals contain different amounts of solvent of crystallisation to the bulk samples as they were grown from different media (and crystals were not dried in vacuo). The IR spectra (Table 2) show that the υ(PO) stretch in dppmO 2 at 1187 cm −1 has been lost and replaced by a new very strong and broad band~1160 cm −1 and a second band at~1100 cm −1 , which are due to the coordinated phosphine oxide groups. The frequencies appear invariant with the lanthanide present, which may be due to small differences being obscurred by the width of the bands. In [LnCl 3 (OPPh 3 ) 3 ] and [LnCl 2 (OPPh 3 ) 4 ] + the frequency of the υ(PO) stretch increases by~10 cm −1 between La and Lu [7]. The 31 P{ 1 H}-NMR chemical shift of dppmO 2 at δ = +25.3 shows a high frequency shift to +33.1 in [La(dppmO 2 ) 4 ]Cl 3 , whilst the corresponding spectra of the Ce, Pr, Nd and Sm complexes show larger shifts due to the presence of the paramagnetic lanthanide ion (Table 2). In contrast, although the solid [Eu(dppmO 2 ) 4 ]Cl 3 complex was isolated without difficulty, the 31 P{ 1 H}-NMR spectrum shows a strong feature at δ~+25 ("free" dppmO 2 ), along with a second resonance at δ = −13.4, which may be assigned to [Eu(dppmO 2 ) 3 Cl]Cl 2 (see below), indicating substantial dissociation of one dppmO 2 in solution; the broad resonance of the free dppmO 2 is indicative of exchange on the NMR timescale.
[Gd(dppmO 2 ) 4 ]Cl 3 was isolated, and its constitution confirmed by its X-ray crystal structure, but no 1 H or 31 P{ 1 H}-NMR resonances were observed, an effect seen in other gadolinium systems [6,7] and ascribed to fast relaxation by the f 7 configuration of the metal. Attempts to isolate [Ln(dppmO 2 ) 4 ]Cl 3 complexes for Ln = Dy-Lu were unsuccessful. We note that [Dy(dppmO 2 ) 4 ][CF 3 SO 3 ] 3 [9] was isolated with triflate counter ions, but with chloride only [Dy(dppmO 2 ) 3 Cl]Cl 2 was produced (below). An in situ 31 P{ 1 H}-NMR spectrum of CeCl 3 ·7H 2 O + 2 dppmO 2 in CH 2 Cl 2 showed a single resonance at δ = +48, which is consistent with formation of [Ce(dppmO 2 ) 4 ] 3+ , confirming the preference for formation of the tetrakis complexes early in the series, even when there is a deficit of ligand.   [16], which has a distorted dodecahedral geometry with a CeO 8 donor set.
[La(dppmO2)4][PF6]3 [8] and [Nd(dppmO2)4]Cl3 [15]. The average Ln-O distances in this series are: La = 2.514 Å, Ce = 2.486 Å, Nd = 2.465 Å, Sm = 2.429 Å and Gd = 2.420 Å, correlating well with the decreasing Ln 3+ radii (La = 1.216 Å, Ce =1.196 Å, Nd = 1.163 Å, Sm = 1.132 Å, Gd = 1.107 Å). The P = O bond lengths and the O-Ln-O chelate angles do not vary significantly along the series. The Ce-O(P) distances in [Ce(dppmO2)4]Cl3 are markedly longer than those in [Ce(Me3PO)4(H2O)4]Cl3 (2.372(2)-2.423(2) Å) [16], which has a distorted dodecahedral geometry with a CeO8 donor set.       The reaction of LnCl 3 ·6H 2 O (Ln =Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) with 3 mol. equivalents of dppmO 2 in EtOH, followed by concentration of the solution or precipitation with hexane, afforded [Ln(dppmO 2 ) 3 Cl]Cl 2 complexes. Examination of the IR and 1 H-NMR spectra indicated these incorporated less water or ethanol lattice solvent molecules than the [Ln(dppmO 2 ) 4 ]Cl 3 , and this was confirmed by the microanalyses. The Sm and Eu complexes appear largely free of solvent of crystallisation, whilst the Tb, Ho and Yb approximate to [Ln(dppmO 2 ) 3 Cl]Cl 2 ·H 2 O, and the Gd, Er and Tm complexes are [Ln(dppmO 2 ) 3 Cl]Cl 2 ·3H 2 O; again, this is likely to vary from sample to sample and with the isolation method. The IR spectra (Table 2) show the two υ(PO) bands as in the tetrakis complexes, but the higher energy bands of the tris complexes are~5-10 cm −1 lower in frequency than in the former. We were unable to identify υ(Ln-Cl) vibrations in the far IR spectra. The 31 P{ 1 H}-NMR spectra of the [Ln(dppmO 2 ) 3 Cl]Cl 2 show single resonances to high or low frequency of dppmO 2 depending on the f n configuration of the Ln ion present ( Table 2) and are generally similar to those found in other systems [5][6][7], although the magnitude of the shifts varies widely with the specific f n configuration. The line broadening is also highly variable between complexes of different Ln ions. The addition of dppmO 2 to a solution of [Ln(dppmO 2 ) 3 Cl]Cl 2 (Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) in CH 2 Cl 2 showed 31 P{ 1 H}-NMR resonances assignable to "free" dppmO 2 and [Ln(dppmO 2 ) 3 3 Cl]Cl 2 were also obtained and show the same cation type, but during refinement, several of the phenyl rings exhibited severe disorder and the data are therefore not included here ( Figure S43). of ligand.
A different crystal isolated from the YbCl 3 -dppmO 2 reaction proved, on structure solution, to be [Yb(dppmO 2 ) 3 (H 2 O)]Cl 3 ·dppmO 2 ·12H 2 O (Figure 7), which contains a seven-coordinate Yb centre coordinated to three dppmO 2 and a water molecule, with the Lu-coordinated water hydrogen-bonded to an adjacent uncoordinated dppmO 2 molecule. The geometry is best described as a very distorted pentagonal bipyramid with the water occupying an equatorial position and is similar to the geometry found in [Lu(dppmO 2 ) 3 (H 2 O)][CF 3 SO 3 ] 3 [11]. The Yb-OH 2 distance of 2.3263(14) Å is~0.05 Å longer than the Yb-O(P).
A large number of disordered solvate water molecules were also present, which proved very difficult to model, but the geometry of the ytterbium cation is clearly defined. slightly longer than in the six-coordinate cation, but are shorter than the corresponding bonds in [Yb(dppmO2)3Cl]Cl2, showing that the expected contraction continues along the series. The complex, [Lu(dppmO2)3(H2O)][CF3SO3]3, is known and its X-ray crystal structure showed seven-coordinate lutetium [11]. Although not confirmed by an X-ray structure, yttrium is reported to form a six-coordinate complex, [Y(dppmO2)2Cl2]Cl [18]. A different crystal isolated from the YbCl3-dppmO2 reaction proved, on structure solution, to be [Yb(dppmO2)3(H2O)]Cl3⋅dppmO2⋅12H2O (Figure 7), which contains a seven-coordinate Yb centre coordinated to three dppmO2 and a water molecule, with the Lu-coordinated water hydrogen-bonded to an adjacent uncoordinated dppmO2 molecule. The geometry is best described as a very distorted pentagonal bipyramid with the water occupying an equatorial position and is similar to the geometry found in [Lu(dppmO2)3(H2O)][CF3SO3]3 [11]. The Yb-OH2 distance of 2.3263(14) Å is ~ 0.05 Å longer than the Yb-O(P).

Discussion
The chemistry of dppmO 2 with lanthanides described in the previous section proves to be very systematic along the series La-Lu. For La-Gd, it was possible to isolate [Ln(dppmO 2 ) 4 ]Cl 3 . Although it could be isolated in the solid state, the solution 31 P-NMR spectroscopic data indicate that [Eu(dppmO 2 ) 4 ]Cl 3 was largely dissociated in CH 2 Cl 2 solution into [Eu(dppmO 2 ) 3 Cl] 2+ and dppmO 2 ; the isolation of the tetrakis-dppmO 2 complex no doubt resulting from it being the least soluble species in an exchanging mixture in solution, although present in very minor amounts. The case of [Gd(dppmO 2 ) 4 ]Cl 3 is likely to be similar, although the fast relaxation of the f 7 ion precluded 31 P-NMR study. For the elements Sm-Yb, the complexes [Ln(dppmO 2 ) 3 Cl]Cl 2 were readily isolated, but only for samarium was it possible to convert [Ln(dppmO 2 ) 3 Cl]Cl 2 to [Ln(dppmO 2 ) 4 ]Cl 3 in CH 2 Cl 2 solution by treatment with more dppmO 2 . Similarly, at the end of the series, the complex isolated was [Lu(dppmO 2 ) 2 Cl 2 ]Cl, for which treatment with dppmO 2 afforded a new species in solution, identified as [Lu(dppmO 2 ) 3 Cl]Cl 2 by a structure determination from a few crystals obtained in the presence of excess dppmO 2 , although a bulk sample could not be isolated [8]. The change from eight-coordination in [Ln(dppmO 2 ) 4 ]Cl 3 at the beginning of the series, to seven-coordination from Sm onwards, and finally to six-coordination at Lu, parallels the reduction in Ln 3+ radii. Isolation of both the eight-and seven-coordinate complexes was possible only for Sm, Eu and Gd. However, one should note that the chloride counter ions also have some role, in that whilst in the LnCl 3 /dppmO 2 series tetrakis-dppmO 2 species did not form beyond Gd, the complex [Dy(dppmO 2 ) 4 ][CF 3 SO 3 ] 3 [9] has been isolated from dmf solution with triflate counter ions. The role that anions and solvents play in lanthanide chemistry is often overlooked [2], but can be critical in determining which complex is isolated from solution. For example, the reaction of LnCl 3 with Ph 3 PO results in isolation of [Ln(Ph 3 PO) 3 Cl 3 ] from acetone, but [Ln(Ph 3 PO) 4 Cl 2 ]Cl from ethanol [7]. On further examination by 31 P-NMR spectroscopy, both species were found to be present in either solvent (in varying amounts), and the form isolated reflected the least soluble complex in the particular solvent, which then precipitated from the mixture of rapidly interconverting species.

Conclusions
Through this synthetic, structural and spectroscopic study of the coordination of dppmO 2 to the lanthanide trichlorides, we have established where the switch from eight-, to seven-, to six-coordination at the Ln(III) centre occurs along the lanthanide series, with X-ray crystallographic authentication for representative examples. The data also reveal subtle, but systematic, variations in the spectroscopic (e.g., ν(PO)) and structural parameters across the series, reflecting the change in ionic radii, the charge:radius ratio and also the influence of the presence of the competitive chloride ions.