Unique Group 1 cations stabilised by homoleptic neutral phosphine coordination

Neutral phosphine ligands, PR3 (R = alkyl, aryl), are ubiquitous in transition metal chemistry, owing to their capacity to tune the electronic and steric properties, and hence the reactivity, of the complexes, and to the strong s-donor properties of the soft phosphine donor functions. This has led to wide utilisation of phosphine co-ligands in many transition metal reagents and catalysts. Phosphine complexes of many p-block acceptors have also developed substantially in recent years. However, complexes involving coordination of neutral phosphine ligands towards the strongly electropositive s-block elements, particularly the Group 1 cations, have remained extremely elusive, and there are no reported examples with exclusively PR3 coordination. This is no doubt in part due to the high affinity of the alkali metal and alkaline earth cations for hard, electronegative Lewis bases such as water, alkoxide, amide etc., their high lability, as well as the high lattice energies often associated with many Group 1 and 2 salts, which severely limit their solubilities in non-competitive organic media. Thus, to-date there has been only one reported example of a neutral phosphine co-ligand coordinated to an alkali metal cation, the organometallic silylamide dimer [Li{N(Ar)CC(R)Si(R)2NAr}(m-Me2PCH2CH2PMe2)]2 (mean d(Li P) = 2.650(3) Å), and two structurally authenticated species with PR3 coordination to alkaline earth ions; [BeCl2(k -Ph2PCH2PPh2)2] 4 and the dinuclear [Be2Cl2(m-Cl)2(PCy3)2], 5 both containing distorted tetrahedral Be(II). A small number of anionic ligands bearing phosphine functions have been coordinated to s-block cations, including [Mg{C6H3-2,6-(CH2PMe2)2}2], in which the Mg–P bonds are also stabilised by the anionic pincer ligand framework, [{Li(2-PPh2-C6H4)}2(OEt2)2] (d(Li–P) = 2.69–2.75 Å), 7 hindered alkoxy–phosphine complexes, including [Li(m-OCBu2CH2PR2)]2 (R = Me or Ph) and [Li(m-OCBu2CH2PPh2)2Li(OC Bu2)] (d(Li–P) = 2.50–2.65 Å), [Na(H2Al{P(SiMe3)2}2)(dme)2], 9 [Li(solvent)x{Ph2B(CH2P Pr2)2}] (solvent = thf, x = 2; Et2O, x = 1) (d(Li–P) = 2.596(3), 2.608(3) Å), as well as a small number of Li complexes with (phosphinomethyl)aluminate ligands. The negative charge on the anionic ligands in these species brings a significant electrostatic component to the bonding, and contrasts the covalent metal– phosphine bonding present in the dand p-block acceptor ions. In recent work we reported that complexes of Na with polyamines and aza macrocycles, including the [Na(Me3-tacn)2] + sandwich cation and the distorted five-coordinate [Na(thf)(Me4-cyclam)] +

Neutral phosphine ligands, PR 3 (R = alkyl, aryl), are ubiquitous in transition metal chemistry, owing to their capacity to tune the electronic and steric properties, and hence the reactivity, of the complexes, and to the strong s-donor properties of the soft phosphine donor functions. This has led to wide utilisation of phosphine co-ligands in many transition metal reagents and catalysts. 1 Phosphine complexes of many p-block acceptors have also developed substantially in recent years. 2 However, complexes involving coordination of neutral phosphine ligands towards the strongly electropositive s-block elements, particularly the Group 1 cations, have remained extremely elusive, and there are no reported examples with exclusively PR 3 coordination. This is no doubt in part due to the high affinity of the alkali metal and alkaline earth cations for hard, electronegative Lewis bases such as water, alkoxide, amide etc., their high lability, as well as the high lattice energies often associated with many Group 1 and 2 salts, which severely limit their solubilities in non-competitive organic media. Thus, to-date there has been only one reported example of a neutral phosphine co-ligand coordinated to an alkali metal cation, the organometallic silylamide dimer [Li{N(Ar)CC(R)-Si(R) 2 NAr}(m-Me 2 PCH 2 CH 2 PMe 2 )] 2 (mean d(LiÀP) = 2.650(3) Å), 3 and two structurally authenticated species with PR 3 coordination to alkaline earth ions; [BeCl 2 (k 1 -Ph 2 PCH 2 PPh 2 ) 2 ] 4 10 as well as a small number of Li + complexes with (phosphinomethyl)aluminate ligands. 11 The negative charge on the anionic ligands in these species brings a significant electrostatic component to the bonding, and contrasts the covalent metalphosphine bonding present in the d-and p-block acceptor ions.
In recent work we reported 12 that complexes of Na + with polyamines and aza macrocycles, including the [Na(Me 3 -tacn) 2 14 To develop this chemistry further we sought to establish whether it would be possible to induce coordination of softer, neutral phosphine ligands towards Group 1 cations without the additional stability offered by the macrocyclic frameworks employed in the aza and thioether chemistry. To achieve this we have used both the [BAr F ] À15 and [Al{OC(CF 3 ) 3 } 4 ] À weakly coordinating anions. [16][17][18][19] We describe here the first series of Group 1 cations coordinated only to neutral phosphine ligands, in the form of distorted octahedral Li + and Na + cations containing tris-diphosphine coordination.
Reaction ] ratio. Attempts to prepare the analogous K + complexes by reaction of the diphosphine with K[BAr F ] in a 3 : 1 molar ratio failed, while the weaker donor and sterically bulkier o-C 6 H 4 (PPh 2 ) 2 , and the diarsine, o-C 6 H 4 (AsMe 2 ) 2 (the direct analogue of diphos), did not coordinate to Li + or Na + under similar conditions.
The coordination environments present in the new complexes were established unambiguously from X-ray crystallographic studies on three examples. The structure § of [Na(dmpe) 3 ][BAr F ] contains discrete Na + cations coordinated to three chelating dmpe ligands, in a distorted octahedral environment ( Fig. 1), with discrete [BAr F ] À anions providing charge balance. The Na-P bond distances lie in the range 2.95-3.03 Å, suggesting relatively weak coordination, and the P-Na-P angles within the five-membered chelate rings are very acute (69.8-73.41). A similar structure is present in [Na(diphos) 3 ][BAr F ], § with coordination at Na + through six P-donor atoms from three chelating diphos ligands, with d(Na-P) = 2.92-3.07 Å (Fig. 2). As in the dmpe complex, these are considerably longer than the sum of the ionic radius for Na (1.02 Å) and the covalent radius for P (1.06 Å). They compare with [Na(H 2 Al{P(SiMe 3 ) 2 } 2 )(dme) 2 ] (d(Na-P) = 3.052(1), 3.092(1) Å). 9 The P-Na-P angles within the chelate rings are even more acute, ca. 651, reflecting the smaller bite angle associated with the rigid o-phenylene diphosphine cf. the dimethylene-linked dmpe. The large [BAr F ] À anions remain discrete, but interleave between the cations (Fig. S1, ESI †).
The structure § of the lithium-diphosphine complex, [Li(diphos) 3 ][Al{OC(CF 3 ) 3 } 4 ] was also determined from a small, weakly diffracting crystal. While the weak diffraction data mean that detailed geometric comparisons require caution, the presence of three chelating neutral diphos ligands at Li + , giving homoleptic P 6 -coordination, is unequivocal (Fig. 3). The aluminate anion provides charge balance, but does not interact with the cation. The Li-P bond distances are considerably shorter (by ca. 0.4 Å) than d(Na-P) in these systems, while the P-Li-P angles within the chelate rings are correspondingly larger (ca. 751), as expected due to the smaller ionic radius.
To investigate the properties of these unusual complexes further, we obtained the MAS NMR spectroscopic data ( 31 P, 23 Na and 7 Li) from the powdered solids. The NMR data are summarised in Table 1. The spectra for [Li(dmpe) 3 ] + are shown in Fig. 4 (the other spectra are provided as ESI, † Fig. S2-S4). ¶ The 31 P NMR data from direct excitation ( Fig. 4(a)) exhibits two peaks, the main one at À54.5 ppm is attributed to the six equivalent P-donor atoms in the complex cation; the minor peak at À48.5 ppm is consistent with the chemical shift for 'free' dmpe   in solution (À48 ppm). 20 This is further confirmed by 31 P crosspolarization (CP) MAS 21 data (Fig. 4(b)), where the second peak is absent, in accord with the highly mobile nature of 'free' dmpe.
The chemical shift differences between 'free' and coordinated diphosphine (D) are small and negative for all four complexes; (L = dmpe: D = À6 ppm for Li + ; À9.4 ppm for Na + ; L = diphos: D = À5 ppm for Li + ; ca. À6 ppm for Na +À ). These contrast with the large, positive D values typically observed in transition metal phosphine complexes which contain five-membered chelate rings. 22 No 7 Li-31 P/ 23 Na-31 P couplings are evident in the spectra, presumably due to the small magnitude of the J values, which fall within the line width. Fig. 4 Solution 1 H and 31 P{ 1 H} NMR spectra (d 8 -toluene or CD 2 Cl 2 ) on the four compounds also show very small coordination shifts, although in these spectra the resonances are closer to the respective 'free' ligand. These, as well as the 7 Li and 23 Na solution spectra, are essentially unchanged upon cooling to 183 K (it seems likely that the low temperature-limiting spectrum is not reached at the freezing point of the solvent). These observations may indicate that in solution the complexes are partially dissociated, leading to chemical shifts closer to the free ligand values. Sharp singlets are observed by 7 Li and 23 Na NMR spectroscopy, with chemical shifts similar to those from the solid state spectra (Table 1).
These results demonstrate that unusual homoleptic neutral phosphine complexes of the Group 1 cations can be readily accessed in (non-polar) organic media through the use of the strong s-donating bidentate ligands with 'naked' metal cation sources.
We thank the EPSRC for support through a Programme Grant (EP/I033394/1). The SCFED Project (www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids. MCa also thanks the Royal Society for a University Research Fellowship.
Notes and references ‡ Synthetic procedure. Schlenk techniques and a glove-box were used for all manipulations, which were conducted under anhydrous and anaerobic conditions.
[   20 and À55 (diphos) 20 ) were also observed in the solid state spectra in some of the samples, arising from some sample degradation during spectral acquisition. b Li complexes recorded in d 8 -toluene solution (298 K); Na complexes recorded in CD 2 Cl 2 solution (298 K). c d 23 Na measured for Na[BAr F ] = À35.5 (s) ppm.   The crystals were held at 100 K in a nitrogen gas stream. Structure solution and refinement on the Na complexes were mostly straightforward, 23,24 except for some disorder in the CF 3 groups of the [BAr F ] À anions which was modelled satisfactorily. For [Li(diphos) 3 ][Al{OC(CF 3 ) 3 } 4 ], despite several attempts, only very small crystals could be obtained. This led to weak diffraction, particularly at high angle, and hence higher R-factors and a less well-defined structure. The H atoms were placed in calculated positions and refined using a riding model. The H atoms on the disordered CH 2 and CH 3 group were not located. CCDC 1044099-1044101.
¶ Solid state NMR experiments. All measurements were performed on a Bruker 9.4 T magnet with a Chemagnetics Infinity console using a double-resonance 4 mm APEX probe. Solid powdered samples were transferred into 4 mm zirconium oxide thin wall rotors within the glovebox, using special end caps with o-rings to exclude air. Magic angle spinning (MAS) conditions have been applied with a spinning speed of 7.1 kHz at room temperature, using N 2 gas flow for bearing and drive. The chemical shift scales were referenced by setting at 0 ppm the signals of LiCl 1 M, NaCl 1 M and 85% H 3 PO 4 , respectively for 7 Li, 23 Na and 31 P. Spectra were recorded with direct excitation using a 901 pulse followed by acquisition, without proton decoupling. For 31 P NMR, additional measurements were also performed with ramped CP methods with 3 ms contact time.