Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes †

Reduction of the heteroleptic Ln( III ) precursors [Ln(Tp) 2 (OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl( I ) anion or KC 8 yielded the adduct-free homoleptic Ln( II ) complexes dimeric 1-Eu [{Eu(Tp)( l - j 1 : g 5 -Tp)} 2 ] and monomeric 1-Yb [Yb(Tp) 2 ]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph 3 PO yielded 1-Ln(OPPh 3 ) 2 . All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f–5d electronic transitions in Ln( II ). Single-crystal X-ray diffraction data reveal first l - j 1 : g 5 -coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.

The formal reduction potentials (E 0 ) of Ln 3+ /Ln 2+ for Ln = Eu À0.35 V and Yb À1.15 V in aqueous solution vs. the Normal Hydrogen Electrode (NHE) make them the most accessible Ln(III) candidates for reduction. 14 Upon reduction of the parent Ln(III), the stability of these classical Ln(II) is achieved by either the attainment of half-filled 4f orbitals in Eu(II) or full 4f orbitals in Yb(II). It is of note that the redox potentials of Ln(III) depend on the ancillary ligand environment, for example [Ln(C 5 H 4 SiMe 3 ) 3 ] (Ln = Eu, Yb) were recently reported to be ca. 1 V more difficult to reduce than their formal reduction potentials. 15 There are no data in the literature for the E 0 of Ln 3+ /Ln 2+ in a Tp ligand environment, but the reduction potentials are expected to be intermediate between the above examples. We therefore explored the reduction chemistry of our Ln(III) (Ln = Eu, Yb) precursors with various reductants.
Complex 1 Our previous work on the metathesis chemistry of [Ln(Tp) 2 (OTf)] 13 had shown that elimination of K(OTf) in noncoordinating solvents was most effective, and this is also the case for the reduction chemistry. Neither clean reduction nor K(OTf) elimination could be achieved in ethereal solvents (see Fig. S68 for the crystal structure of 1-Yb(DME) co-crystallised with KOTf, DME = 1,2-dimethoxyethane, ESI †). While metallic K does reduce [Ln(Tp) 2 (OTf)] the reaction conditions could not be adequately controlled to achieve clean formation of Ln(II). Therefore, the adduct-free divalent lanthanide complexes 1-Ln were synthesised by the reduction of [Ln(Tp) 2 (OTf)] with excess KC 8 (Scheme 1) at ambient temperature in toluene with stirring, followed by filtration and subsequent removal of volatiles Complexes 1-Ln have good solubility and stability in both coordinating and non-coordinating solvents. Furthermore, reaction of 1-Ln with two equivalents of Ph 3 PO in toluene resulted in the isolation of the bis-adduct complexes 1-Ln(OPPh 3 ) 2 (1-Eu(OPPh 3 ) 2 91%, 1-Yb(OPPh 3 ) 2 87%). Elemental analyses of 1-Ln and 1-Ln(OPPh 3 ) 2 are consistent with their respective formulations.
No resonances were observed by NMR spectroscopy for the paramagnetic complexes 1-Eu and 1-Eu(OPPh 3 ) 2 , consistent with the half-filled 4f 7 electronic configuration of Eu(II). The corrected Evans' method magnetic moments (m eff ) for 1-Eu and 1-Eu(OPPh 3 ) 2 in d 6 -benzene at room temperature were found to be in the ranges of 7.62-7.78 m B and 7.19-7.46 m B , respectively. These data are consistent with the calculated spin-only magnetic moment (m so ) of 7.94 m B for 4f 7 Gd(III), 18  Diffusion-ordered NMR spectroscopy ( 1 H 2D-DOSY) of 1-Yb, and 1-Yb with 1 or 2 equivalents of Ph 3 PO show that 1-Yb is monomeric, and that in solution there is an equilibrium between Ph 3 PO and 1-Yb(OPPh 3 ) x (see ESI, † S1.6). The resonances assigned to 1-Yb(OPPh 3 ) 2 in the 1 H and 31 P NMR data are therefore an equilibrium average. The phenyl protons of Ph 3 PO, appear at d = 6.92, 7.02 and 7.49 ppm in the expected 12 : 6 : 12 ratio for two equivalents of Ph 3 PO, slightly shifted from free Ph 3 PO (d = 6.97-7.08 and 7.72-7.82 ppm in a 9 : 6 ratio). By 31 P NMR the singlet resonance at d = 27.63 ppm assigned to 1-Yb(OPPh 3 ) 2 , is shifted from free Ph 3 PO (d = 24.72 ppm). The Tp-borohydrides of both 1-Yb and 1-Yb(OPPh 3 ) 2 were observed as doublet resonances at d = À1.85 ppm by 11 B NMR. The pyrazolyl carbon resonances of 1-Yb and 1-Yb(OPPh 3 ) 2 were assigned via 2D 1 H-13 C HSQC NMR experiments (see ESI †). The IR spectra of 1-Ln are near-identical, with weak absorptions between 2350-2560 cm À1 assigned to the n BH of the Tp ligands. 6d,f,13 The IR data for 1-Ln(OPPh 3 ) 2 are consistent with 1-Ln but additionally display strong n PQO at 1117 cm À1 . The n PQO is shifted from 1184 cm À1 in free Ph 3 PO to lower wavenumbers upon coordination to the Ln(II). 21 Complexes 1-Ln and 1-Ln(OPPh 3 ) 2 are intensely coloured due to Laporte allowed 4f nÀ1 -5d 1 transitions characteristic of Ln(II). 22 The electronic absorption spectra of 1-Ln in both noncoordinating (a) and coordinating (b) solvents, and 1-Ln(OPPh 3 ) 2 in MeCN (c) are shown in Fig. 1. In toluene (Fig. 1a) or hexane ( Fig. S53 and S54 in ESI †) complexes 1-Ln display broad and strong absorptions (Ln = Eu e = 1.45-3.67 Â 10 3 M À1 cm À1 ; Ln = Yb e = 0.68-1.93 Â 10 3 M À1 cm À1 ) in the near UV and visible, with l max of 395 nm for 1-Eu, and l max of 341 nm and 520 nm for 1-Yb.
Photoluminescence (PL) of 1-Eu is also shown in Fig. 1, in the solid-state under a UV lamp (d) and the excitation and emission spectra in toluene solution (e). Complex 1-Eu shows emission at 590 nm (l Ex of 389 nm), and the Excitation-Emission Matrix data (EEM, Fig. S61, ESI †) demonstrate that the emission of 1-Eu originates only from the parity-allowed 4f 6 5d 1 to 4f 7 transition of Eu(II  22a In acetonitrile, the transition envelopes for 1-Ln (Fig. 1b)  change in magnitude of molar extinction co-efficient, however, in 1-Eu(MeCN) e decreases significantly. The increased solvent cut-off in MeCN allows for the observation of additional absorptions in the UV for 1-Ln. No spectral features in these data originate from the Tp ligand. The data for 1-Ln(OPPh 3 ) 2 in acetonitrile are near-identical to 1-Ln, with the addition of the p-p* transitions arising of Ph 3 PO (l max = 265-272, e = 2.6-5.3 Â 10 3 M À1 cm À1 ). 23 The solid-state molecular structures of 1-Eu (a), 1-Yb (b), 1-Yb(THF) (c), and 1-Eu(OPPh 3 ) 2 (d) are shown in Fig. 2. The structures of 1-Eu(THF) 2 are shown in Fig. S63 and S64 and 1-Yb(OPPh 3 ) in Fig. S69 (ESI †). Important structural metrics are tabulated in the ESI † (see S4), data comparison in Table S4 and crystallographic information in Table S5. The Ln-N(k 3 -Tp), 6a,d,f,10 Eu-(Z 5 -Tp), 24 Ln-O(THF), 1a,b and Ln-O(OPPh 3 ) 21c,23 bond distances in 1-Ln, 1-Ln(THF) x and 1-Ln(OPPh 3 ) x (Ln = Eu, x = 2; Yb, x = 1) fall within the expected ranges. All data are consistent with the lanthanide contraction 25 and the increase in ionic radius upon reduction from Ln(III) to Ln(II). 13 In the absence of coordinating solvents, complex 1-Eu (a) is an unusual example of a dimeric structure, whereas complex 1-Yb (b) is monomeric as expected. 6a,c,d,f In 1-Eu (a) each Eu(II) is bound by a k 3 -Tp ligand and an m-k 1 :Z 5 Tp ligand. The m-k 1 :Z 5 binding mode has been seen in Eu(II) pyrazolyl complexes, 24 but this is the first example of m-k 1 :Z 5 Tp binding with bridging of lanthanide metal centres. 6f-h,d,10 In 1-Eu a significantly longer bond distance is observed for Eu-N(m-k 1 :Z 5 -Tp). Either dissolution in THF of 1-Ln or the addition of 2 Ph 3 PO to 1-Ln in toluene, and recrystallisation resulted in single-crystals of the Lewis base adduct complexes 1-Ln(THF) x (c) or 1-Ln(OPPh 3 ) x (d) (Ln = Eu, x = 2; Yb, x = 1). Complexes 1-Ln(THF) x and 1-Ln(OPPh 3 ) x are all monomeric, with two axial k 3 -coordinated Tp ligands bound to each Ln metal centre, and either one (Yb) or two (Eu) adduct molecules bound in the equatorial plane. In the single-crystal of 1-Yb(OPPh 3 ) the binding of one Ph 3 PO ligand is attributed to the specific crystallisation conditions. Fig. 1 Overlay of the electronic absorption spectra of 1-Ln in either toluene (a) or in MeCN (b) and overlay of the electronic absorption spectra of 1-Ln(OPPh 3 ) 2 (c) in MeCN, all spectra recorded at room temperature. The traces are coloured in the colour of the complex. Data (l max and e) are tabulated in ESI, † Table S3. Photoluminescence of a solid sample of 1-Eu under a UV lamp (d). Excitation (LHS, l Em = 590 nm) and emission (RHS, l Ex = 389 nm) spectra of 1-Eu, recorded in toluene (e).  (d). Hydrogen atoms and lattice solvent molecules omitted for clarity and pyrazolyl carbon atoms of Tp, backbone carbon atoms of THF, phenyl carbon atoms of Ph 3 PO displayed in wireframe. Displacement ellipsoids drawn at 50% probability. Data and crystallographic information can be found in the ESI, † S4 and Tables S4, S5.
Reduction of Ln(III) in [Ln(Tp) 2 (OTf)] has been demonstrated to be an excellent route to Ln(II) in [Ln(Tp) 2 ] (Ln = Eu, Yb). The binding strength in combination with ability of the unsubstituted Tp ligand to bridge, bend and flex, results in very stable and soluble Ln(II) complexes. Enabling in turn the collection of both solid-state and solution-state spectroscopic data. Highlights include the direct observation of the effect of solvent on 4f-5d electronic transitions in Ln(II), PL from Eu(II) and first example of m-k 1 :Z 5 Tp binding to lanthanides. Exploration of the chemistry of [Ln(Tp) 2 ] is ongoing in our laboratory, and we anticipate future study of the chemical and physical properties of these Ln(II) complexes.
The authors acknowledge the University of Glasgow for funding and the EPSRC ECR Capital Award Scheme (EP/S017984/1). WJP acknowledges the Royal Society (RGS\R2\192190) for funding. The authors also acknowledge the awards of a College of Science and Engineering PhD Scholarship to TC, and a Victoria University of Wellington PhD Scholarship to MJE.

Conflicts of interest
There are no conflicts to declare.