Synthesis and Characterisation of Novel Bis(diphenylphosphane oxide)methanidoytterbium(III) Complexes

Reaction of [YbCp2(dme)] (Cp = cyclopentadienyl, dme = 1,2 dimethoxyethane) with bis(diphenylphosphano)methane dioxide (H2dppmO2) leads to deprotonation of the ligand H2dppmO2 and oxidation of ytterbium, forming an extremely air-sensitive product, [YbIII(HdppmO2)3] (1), a six-coordinate complex with three chelating (OPCHPO) HdppmO2 ligands. Complex 1 was also obtained by a redox transmetallation/protolysis synthesis from metallic ytterbium, Hg(C6F5)2, and H2dppmO2. In a further preparation, the reaction of [Yb(C6F5)2] with H2dppmO2, not only yielded compound 1, but also gave a remarkable tetranuclear cage, [Yb4(µ-HdppmO2)6(µ-F)6] (2) containing two [Yb(µ-F)]2 rhombic units linked by two fluoride ligands and the tetranuclear unit is encapsulated by six bridging HdppmO2 donors. The fluoride ligands of the cage result from C-F activation of pentafluorobenzene and concomitant formation of p-H2C6F4 and m-H2C6F4, the last being an unexpected product.


Synthesis and Characterisation of [Yb(HdppmO2)3] 1
Initially  Figure S1a,c) and the 1 H NMR spectrum showed the formation of CpH indicative of deprotonation of H2dppmO2. In addition, the presence of unreacted [Yb Cp2 (dme)] was also detected in the reaction mixture. (See Supplementary Materials, Figure S1b, Table S1).

Scheme 1.
Previous studies (reaction 1a [14]) and our current work (1b and 1c) on rare earth complexes from reactions of H 2 dppmO 2 .  Figure S1a,c) and the 1 H NMR spectrum showed the formation of CpH indicative of deprotonation of H 2 dppmO 2 . In addition, the presence of unreacted [Yb Cp 2 (dme)] was also detected in the reaction mixture. (See Supplementary Materials, Figure S1b, Table S1). To facilitate a rational synthesis and effective utilization of [YbCp 2 (dme)], the reaction stoichiometry was increased to a 3:1 ratio of H 2 dppmO 2 : [YbCp 2 (dme)], resulting in the isolation of crystalline 1 in 65% yield. NMR spectroscopy of the reaction mixture shows complete consumption of the [YbCp 2 (dme)] (See Supplementary Materials, Figure S1f). Thus, both protolysis and redox processes are observed in the reaction of [YbCp 2 (dme)] with H 2 dppmO 2 (Scheme 2, route 2). With the same mole ratio the reaction was carried out in an NMR tube in d 8 -toluene, revealing a resonance at 4.79 ppm attributable to H 2 gas formation [21][22][23] (See Figure S2c). The composition of 1 was established by microanalysis and high-resolution mass spectrometry which revealed the [M+H] + ion (see Experimental Section 3.4).

Results and Discussion
The infrared spectrum of the extremely air-and moisture-sensitive 1 shows very strong ν(P=O, cm −1 ) absorptions at 1154, 1120, 1081 and 1061 ( Figure 1, top), which are very different from those of free ligand (1212, 1197 and 1176 cm −1 ) (Figure 1, bottom). A strong band of the ligand at 1111 cm −1 can be assigned to the X-sensitive mode q, involving P-C stretching [24,25]. This is not unduly shifted or enhanced in intensity on coordination as indicated by the spectra of Ph 3 P=O complexes [24] and presumably contributes to the intensity of the 1120 cm −1 band of the complex. Weaker C-H in-plane deformation modes of the ligand at 1083 and 1027 cm −1 are unlikely to be affected by coordination. The shifts of ν(P=O) are reminiscent of the shift of ν(C=O) of the diketo form of beta-diketones to ν as (C=O) of β-diketonate complexes [26]. The absorption bands of 1 (see Experimental On the other hand, in the complex [Ln(H 2 dppO 2 ) 4 ]3 + , containing neutral H 2 dppmO 2 , the absorptions at 1197 and 1098 cm −1 are assigned to ν(P=O) [15], though the latter may be a slightly shifted q vibration, thereby indicating that the absorptions are little affected when the ligand is neutral. Similarly, [Ga(tBu) 3 (H 2 dppmO 2 )] has strong bands at 1210, 1187, 1170 (ν P=O) and 1110 (q), and medium C-H deformation bands at 1075 and 1028 cm −1 , all near free ligand values, but the complex of the deprotonated ligand [Ga(tBu) 2 (HdppmO 2 )] has strong bands at 1195, 1165, 1110, 1076, 1060 cm −1 [27], which are similar to those of 1, apart from the first value.    Table S4). Although the structure is similar to those of [M(HdppmO) 3 ] (M = La or Y) [14], the structures are not isomorphous, possibly owing to the presence of lattice solvent in 1·MeCN. The Yb-O bond lengths (2.1953 (16) Table S4.

Properties of [Yb(HdppmO2)3] 1 in Solution
The 1 H NMR spectrum of compound 1 in CD2Cl2 (see ESI) is broad owing to the paramagnetic nature of the complex. The two broad singlets observed at −3.26 and 7.45 ppm are assigned to methanide and aromatic protons, respectively. The methanide CHproton is upfield shifted by ~6 ppm relative to the CH2 resonance of the free ligand (3.56 ppm) [31] indicating it is highly shielded in comparison to other [M(HdppmO2)3] complexes (M = Y (δH = 2.26 ppm), M = La (δH = 2.28 ppm)) [14] presumably owing to Yb III paramagnetism. This pronounced shielding of the methanide proton was supported by Natural Bond Orbital (NBO) analysis on complex 1 (see ESI for full details). It was observed that Natural Population Analysis (NPA) revealed substantial negative natural charge of −1.43 at the methanide carbon (see ESI, Table S2), where qC of CHwas significantly more negative than the aromatic carbon atoms attached to the phosphorus  Table S4.

Properties of [Yb(HdppmO 2 ) 3 ] 1 in Solution
The 1 H NMR spectrum of compound 1 in CD 2 Cl 2 (see Supplementary Materials) is broad owing to the paramagnetic nature of the complex. The two broad singlets observed at −3.26 and 7.45 ppm are assigned to methanide and aromatic protons, respectively. The methanide CH − proton is upfield shifted by~6 ppm relative to the CH 2 resonance of the free ligand (3.56 ppm) [31] indicating it is highly shielded in comparison to other [M(HdppmO 2 ) 3 ] complexes (M = Y (δH = 2.26 ppm), M = La (δH = 2.28 ppm)) [14] presumably owing to Yb III paramagnetism. This pronounced shielding of the methanide proton was supported by Natural Bond Orbital (NBO) analysis on complex 1 (see Supplementary Materials for full details). It was observed that Natural Population Analysis (NPA) revealed substantial negative natural charge of −1.43 at the methanide carbon (see Table S2), where qC of CH − was significantly more negative than the aromatic carbon atoms attached to the phosphorus atom (−0.395 to −0.425) in 1, implying high shielding of the CH − proton in the 1 H NMR spectrum, as observed experimentally (See Table S2). The positive natural charge for Yb(III) was calculated to be 1.84 which is in the range as mentioned in the literature [32]. In the series (Ln(HdppmO 2 ) 3 ) Ln = La, Y [14], Yb (this work) the calculated charge decreases 2.30, 2.00, 1.84, respectively, as the size of the ion decreases, reflecting increased ligand to metal charge transfer in the same sequence. The 31 [14], hence the effect of deprotonation outweighs paramagnetic effects in 31 P{ 1 H} NMR spectroscopy (see Figure S5b).
To examine the solution behavior further, 1 H NMR spectra were recorded for complex 1 in CD 2 Cl 2 from 25 • C to −75 • C (Figure 3). On lowering the temperature from 25 • C to 5 • C, coalescence was observed until −15 • C when the peaks were resolved (Figure 3). At −55 • C, sharp peaks are seen with no further changes below. This phenomenon can be rationalised by a conformational equilibrium [33]  To examine the solution behavior further, 1 H NMR spectra were recorded for complex 1 in CD2Cl2 from 25 °C to −75 °C (Figure 3). On lowering the temperature from 25 °C to 5 °C, coalescence was observed until −15 °C when the peaks were resolved ( Figure  3). At −55 °C, sharp peaks are seen with no further changes below. This phenomenon can be rationalised by a conformational equilibrium [33] in 1. At 25 °C the conformers are found to be highly fluxional, while at −55 °C the dynamic equilibrium between the conformers ceases, whereas in case of the reported [M(HdppmO2)3] (M = Y, La) complexes, they are found to be fluxional even at −80 °C [14]. The presence of conformers is further supported by close analysis of the solid-state structure. Three six-membered YbO2P2C metallacycles, present in 1, exhibit a combination of boat and twist-boat conformers within the solid-state (Figures 4 and 5). The torsion angles of the O-P-P-O atoms in the metallacycle are all significantly different for all the three metallacycle rings in both the boat and twist-boat conformations (Table 1). The presence of conformers is further supported by close analysis of the solid-state structure. Three six-membered YbO 2 P 2 C metallacycles, present in 1, exhibit a combination of boat and twist-boat conformers within the solid-state (Figures 4 and 5). The torsion angles of the O-P-P-O atoms in the metallacycle are all significantly different for all the three metallacycle rings in both the boat and twist-boat conformations (Table 1). The presence of conformers is further supported by close analysis of the solid-state structure. Three six-membered YbO2P2C metallacycles, present in 1, exhibit a combination of boat and twist-boat conformers within the solid-state (Figures 4 and 5). The torsion angles of the O-P-P-O atoms in the metallacycle are all significantly different for all the three metallacycle rings in both the boat and twist-boat conformations (Table 1).

O-P-P-O Mean Plane Torsion Angle (°) Conformer
4.18 (11) boat form This indicates that at 25 °C there is a rapid intramolecular dynamic process within complex 1. As the temperature reaches −15 °C, the spectrum indicates the interconversion process slowing down between the possible conformers. At −55 °C the intramolecular dynamic process ceases as resolved peaks are distinctly observed. This indicates the possibility of phenyl groups frozen in the axial and equatorial positions at −55 °C, where there are mirror image peaks (See ESI, Figure S7b). Thereby, we can conclude that at 25 °C, complex 1 is highly fluxional, but at −55 °C the fluxionality is lost. Variable temperature 31 P{ 1 H} NMR spectroscopy did not show much variation within the temperature range of 25 °C to −75 °C other than a slight shift to a higher frequency (See Figure S7c).

Synthesis of 1 by the RTP Method
In order to examine the redox transmetallation protolysis (RTP) protocol [34] an excess of metallic Yb was treated with Hg(C6F5)2 and H2dppmO2, in THF at room temperature for 15 min (Scheme 3). Analysis of the reaction mixture by 19 Table 1. Mean planes and torsion angles of the metallacycle rings in complex 1. This indicates that at 25 • C there is a rapid intramolecular dynamic process within complex 1. As the temperature reaches −15 • C, the spectrum indicates the interconversion process slowing down between the possible conformers. At −55 • C the intramolecular dynamic process ceases as resolved peaks are distinctly observed. This indicates the possibility of phenyl groups frozen in the axial and equatorial positions at −55 • C, where there are mirror image peaks (See Figure S7b). Thereby, we can conclude that at 25 • C, complex 1 is highly fluxional, but at −55 • C the fluxionality is lost. Variable temperature 31 P{ 1 H} NMR spectroscopy did not show much variation within the temperature range of 25 • C to −75 • C other than a slight shift to a higher frequency (See Figure S7c).

Synthesis of 1 by the RTP Method
In order to examine the redox transmetallation protolysis (RTP) protocol [34] an excess of metallic Yb was treated with Hg(C 6 F 5 ) 2 and H 2 dppmO 2 , in THF at room temperature for 15 min (Scheme 3). Analysis of the reaction mixture by 19 [35] (See Figure S8). The 31 P{ 1 H} NMR spectrum showed a single resonance at 39.8 ppm indicative of complex 1. Recrystallisation of this extremely air-and-moisture sensitive compound from dry acetonitrile gave polychromatic blocks of complex 1 in a 71% yield. This simple onepot procedure should be applicable to all rare earth elements, and does not require a prior synthesis of metal precursors, e.g., of tris{bis(trimethylsilyl)amido}-lanthanoid(III) complexes [14]. This is the first synthesis of a methanido-lanthanoid complex by this method, but should be widely applicable, for example to β-diketonates.
A small amount of colourless blocks was deposited from a concentrated reaction mixture aliquot added to d 6 -benzene. The crystals were subjected to X-ray single crystal structure determination which revealed the presence of the C-F activation product [Yb 4 (µ-HdppmO 2 ) 6 (µ-F) 6 ]·6THF·3C 6 D 6 2 ( Figure 5). Compound 2 is obtained in low yield and is insoluble in most deuterated organic solvents (d 6 -benzene, d 8 -thf, CD 3 CN, CD 2 Cl 2 and d 8 -toluene) inhibiting analysis in the solution state. solution colour change from red/orange to colorless over 20 min. The 19 F{ 1 H} NMR spectrum of an aliquot of the reaction mixture revealed seven peaks, identified as corresponding to the major products; pentafluorobenzene (−139.6, −155.59 and −163.5 ppm) [35] and 1,2,4,5-tetrafluorobenzene (−140.7 ppm) [35] and minor product, 1,2,3,5tetrafluorobenzene (−114.2, −133.1 and −167.5 ppm) [38] (See Figure S9a). The corresponding 31 P{ 1 H} NMR spectrum of the reaction mixture shows a sharp peak at 39.1 ppm corresponding to compound 1. (see Figure S9b). A small amount of colourless blocks was deposited from a concentrated reaction mixture aliquot added to d6-benzene. The crystals were subjected to X-ray single crystal structure determination which revealed the presence of the C-F activation product [Yb4(µ-HdppmO2)6(µ-F)6]·6THF·3C6D6 2 ( Figure 5). Compound 2 is obtained in low yield and is  [30] indicating partial double bond character of P-O and P-C bonds. The P-O and P-C bond lengths are very close to those of 1. The Yb-F bond lengths in [Yb(µ-F)] 2 are slightly shorter than those which link two rhombic units ( Figure 5). Overall, the Yb-F bond lengths in compound 2 are in the range of previously reported complexes (c.f. [Yb 4 (p-HC 6 F 4 N(CH 2 ) 2 NMe 2 ) 6 F 6 ], 2.150(2)-2.190(2) Å) [39].
Inhibited by low solubility, 2 was also characterized by elemental analysis (see Experimental Section 3.4). The formation of 2 by C-F activation, utilising an organolanthanoid, is very rare in being accompanied by the formation of 1,2,3,5-tetrafluorobenzene along with 1,2,4,5-tetrafluorobenzene. Ytterbium (and Eu) can activate C 6 F 5 H with the formation of the observed p-C 6 F 4 H 2 , though the reaction is slow [40,41]. However, in the present reaction, the Yb metal is separated before C 6 F 5 H is generated by protolysis. In previous work, 1,2,3,4-tetrafluorobenzene formation has been observed in C-F activation reactions that accompany the redox transmetallation between Sm metal and Hg(C 6 F 5 ) 2 [42], but there was no report of the formation of m-C 6 F 4 H 2 . A possibility source of two tetrafluorobenzenes is that [Yb(HdppmO 2 ) 2 ], the initial product of protolysis of [Yb(C 6 F 5 ) 2 ], reacts unselectively with the m-F and p-F atoms of C 6 F 5 H by single electron transfer to generate [Yb(HdppmO 2 ) 2 F] and isomeric mand p-C 6 H 1 F 4 • radicals which capture hydrogen radicals from the solvent to give mand p-H 2 C 6 F 4 . Complex 2 is then formed by the rearrangement of the [Yb(HdppmO 2 ) 2 F] (Scheme 5).
Inhibited by low solubility, 2 was also characterized by elemental analysis (see experimental). The formation of 2 by C-F activation, utilising an organolanthanoid, is very rare in being accompanied by the formation of 1,2,3,5-tetrafluorobenzene along with 1,2,4,5-tetrafluorobenzene. Ytterbium (and Eu) can activate C6F5H with the formation of the observed p-C6F4H2, though the reaction is slow [40,41]. However, in the present reaction, the Yb metal is separated before C6F5H is generated by protolysis. In previous work, 1,2,3,4-tetrafluorobenzene formation has been observed in C-F activation reactions that accompany the redox transmetallation between Sm metal and Hg(C6F5)2 [42], but there was no report of the formation of m-C6F4H2. A possibility source of two tetrafluorobenzenes is that [Yb(HdppmO2)2], the initial product of protolysis of [Yb(C6F5)2], reacts unselectively with the m-F and p-F atoms of C6F5H by single electron transfer to generate [Yb(HdppmO2)2F] and isomeric m-and p-C6H1F4 • radicals which capture hydrogen radicals from the solvent to give m-and p-H2C6F4. Complex 2 is then formed by the rearrangement of the [Yb(HdppmO2)2F] (Scheme 5).

Scheme 5.
Proposed mechanism for C-F activation and formation of complex 2.

General Procedures
All the lanthanoid metals and lanthanoid(II) and (III) products are highly air-and moisture-sensitive, hence operations were carried out under nitrogen using standard Schlenk-line and glovebox techniques. Ytterbium metal was purchased as metal ingots from Santoku or Eutectix. [YbCp2(dme)] [43], H2dppmO2 [44], Hg(C6F5)2 [45] and [Yb(C6F5)2] [36,37] were synthesised by literature procedures. THF was dried and deoxygenated by refluxing over Na metal and distillation from sodium benzophenone Scheme 5. Proposed mechanism for C-F activation and formation of complex 2.

General Procedures
All the lanthanoid metals and lanthanoid(II) and (III) products are highly air-and moisture-sensitive, hence operations were carried out under nitrogen using standard Schlenk-line and glovebox techniques. Ytterbium metal was purchased as metal in-gots from Santoku or Eutectix. [YbCp 2 (dme)] [43], H 2 dppmO 2 [44], Hg(C 6 F 5 ) 2 [45] and [Yb(C 6 F 5 ) 2 ] [36,37] were synthesised by literature procedures. THF was dried and deoxygenated by refluxing over Na metal and distillation from sodium benzophenone ketyl, whereas MeCN was dried and deoxygenated by refluxing over and distillation from calcium hydride. The dried solvents were stored over 4 Å molecular sieves. Infrared spectra (4000-650) cm −1 were obtained with Nujol mulls between NaCl plates with a Perkin-Elmer 1600 FT-IR spectrometer. Room temperature (25 • C) 1 H and 31 P{ 1 H} NMR spectra were recorded with a Bruker DPX 300 instrument using d 8 -toluene, C 6 D 6 or CD 2 Cl 2 . The solvents were dried over 4 Å molecular sieves for 48 h, and resonances were referenced to residual hydrogen-atom resonances of the deuterated solvent.

Single Crystal X-ray Structure Determination
Crystals for X-ray structure analysis were grown using saturated solutions in acetonitrile (1), or THF-C 6 D 6 (2). Crystals 1 and 2 were immersed in paratone, and were measured on a Rigaku SynergyS diffractometer. The SynergyS operated using microsource Cu-Kα radiation (λ = 1.54184 Å) at 123 K. Data processing was conducted using the CrysAlisPro.55 software suite [46]. Structural solutions were obtained by ShelXT [47] and refined using fullmatrix least-squares methods against F 2 using SHELXL [48], in conjunction with Olex2 [49] graphical user interface. All hydrogen atoms were placed in calculated positions using the riding model.

Computational Studies
All calculations reported were performed using the Gaussian 09 suite of programs. The coordinates for the calculations were directly taken from X-ray structure of compound