The modular synthesis of rare earth-transition metal heterobimetallic complexes utilizing a redox-active ligand †

We report a robust and modular synthetic route to heterometallic rare earth-transition metal complexes. We have used the redox-active bridging ligand 1,10-phenathroline-5,6-dione (pd), which has selective N,N’ or O,O’ binding sites as the template for this synthetic route. The coordination complexes [Ln(hfac)3(N,N’-pd)] (Ln = Y [1], Gd [2]; hfac = hexafluoroacetylacetonate) were synthesised in high yield. These complexes have been fully characterised using a range of spectroscopic techniques. Solid state molecular structures of 1 and 2 have been determined by X-ray crystallography and display different pd binding modes in coordinating and non-coordinating solvents. Complexes 1 and 2 are unusually highly coloured in coordinating solvents, for example the vis-NIR spectrum of 1 in acetonitrile displays an electronic transition centred at 587 nm with an extinction coefficient consistent with significant charge transfer. The reaction between 1 and 2 and VCp2 or VCp t 2 (Cp t = tetramethylcyclopentadienyl) resulted in the isolation of the heterobimetallic complexes, [Ln(hfac)3(N,N’-O,O’-pd)VCp2] (Ln = Y [3], Gd [4]) or [Ln(hfac)3(N,N’-O,O’-pd)VCp t 2] (Ln = Y [5], Gd [6]). The solid state molecular structures of 3, 5 and 6 have been determined by X-ray crystallography. The spectroscopic data on 3–6 are consistent with oxidation of V(II) to V(IV) and reduction of pd to pd in the heterobimetallic complexes. The spin-Hamiltonian parameters from low temperature X-band EPR spectroscopy of 3 and 5 describe a A1 ground state, with a V(IV) centre. DFT calculations on 3 are in good agreement with experimental data and confirm the SOMO as the dx2−y2 orbital localised on vanadium.


Introduction
The cooperativity between metal centres in multimetallic complexes, clusters and polymeric species gives rise to very remarkable chemistry in a diverse range of fields. 1 The potential applications of complexes containing both transition metals and lanthanides are significant.In particular there is research interest in the design of rare earth-transition metal singlemolecule magnets (SMM) and catalysts. 2,3he chemistry of multimetallic complexes containing f-elements is significantly less well developed than that of transition metals.This is attributable to the fact that the directed synthesis of f-d complexes is non-trivial.The inspiration for our approach was the Tb analogue of the [{(SiMe 3 ) 2 N}Ln] 2 N 2 K system.This complex was remarkable both for the (N 2 ) 3•- anion and the SMM behaviour. 4It generated significant subsequent interest in multimetallic lanthanide radical bridged complexes. 5The very recent reports of magnetic hysteresis at 60 K in a dysprosocenium cation are a testament to the power of the organometallic molecular design approach. 6,7ur aim was to identify and use a redox-active bridging ligand with multiple and selective binding sites as the template for a general and modular synthetic route to heterometallic d-f or f-f′ complexes.The ligand used in this work 1,10phenathroline-5,6-dione ( pd) has been predominantly studied for its physical organic, biological and bio-inorganic chemistry, particularly as a precursor to DNA intercalation agents and photocatalysts. 8,9][12] There is transition metal literature precedent for using pd as a bridging ligand to synthesise multi-metallic complexes.The first transition metal complexes of pd were synthesised by Balch by the addition of Pt(PPh 3 ) 4 to pd to form [(O,O'-pd)Pt (PPh 3 ) 2 ]. 13 The electrochemical and electrocatalytic properties of these complexes were subsequently investigated by Abruna. 10Pierpont demonstrated that [(O,O'-pd)Pt(PPh 3 ) 2 ] could be used as a bipyridine equivalent to bind a second metal. 14This paper also reported the selective N,N'-pd binding of PdCl 2 and the first crystal structures of transition metal pd complexes (Chart 1).Eisenberg combined pd and bipyridine (bipy) to form the homobimetallic Pt complexes, [PtCl 2 (N,N′-O,O'-pd)Pt(ditert-butyl-bipy)].These were then used as precursors to synthesise [Pt(L)(N,N′-O,O′-pd)Pt(di-tert-butyl-bipy)] (where L = dithiolate, dicatecholate or pd) and probe electronic structure. 15irectly relevant to this work, is the first report of the coordination and redox chemistry of group 4 and 5 metals with pd, by Pampaloni and Calderazzo. 168][19] Prior to this work there are just two reports of structurally characterised molecular lanthanide complexes of pd, both by Faulkner and Ward, which describe the sensitised NIR emission of [Ln(tta) 3 (N,N'-O,O'-pd)Pt(PPh 3 ) 2 ] (Ln = La, Nd, Gd, Er, Yb; tta = 1-thenoyl-5,5,5-trifluoroacetylacetonate). 12,20e note that there are reports of the biological activity of combinations of Ln and pd.
Here we report a robust and modular synthesis and first structural characterisation of [Ln(hfac) 3 (N,N'-pd)] (Ln = Y, Gd) complexes.We also report the reduction of these coordination chemistry complexes using vanadocenes to form heterobimetallic rare earth-transition metal complexes.The [Ln(hfac) 3 (N,N′-O,O′-pd)VCp R 2 ] complexes have been fully characterised by a range of spectroscopic techniques.These complexes are spectroscopically rich and represent the first building-blocks in our studies of rare earth-transition metal complexes.

Results and discussion
Synthesis of [Ln(hfac) 3 (N,N'-pd)] Metal precursor choice is particularly important in f-element chemistry.In this work, we chose to use the Ln(OTf ) 3 (Ln = Y, Gd, OTf = CF 3 SO 3 ) more commonly applied in organic synthesis as Lewis acid catalysts. 21This was due to the ease of preparation from inexpensive Ln 2 O 3 starting materials, their stability in H 2 O and facile dehydration by comparison to commercially available LnCl 3 •(H 2 O) x .The hfac (hfac = 1,1,1-5,5,5hexafluoroacetylacetonate) ligand was chosen because [Ln(hfac) 3 (glyme)] are well-defined molecular species. 22,23The hfac ancillary ligand environment has good solubility and stability, with 19 F NMR as an additional probe of solution behaviour.We note that while you can access [Ln(hfac) 3 (glyme)] directly from Ln 2 O 3 , the synthesis requires the addition of chelating polyethers to prevent aggregation. 24owever, using a salt elimination reaction enables this synthetic route to be modular and applicable to other ligand environments.The coordination complexes [Ln(hfac) 3 (N,N′pd)] were synthesised in high yield.Ln(OTf ) 3 (Ln = Y, Gd), pd and 3 equivalents of K(hfac) were combined dry, and THF added at room temperature with stirring, giving a colour change from yellow to deep green.The reaction was stirred at room temperature for 16 h, and THF removed in vacuo.Extraction into toluene, filtration to exclude K(OTf ) and removal of toluene in vacuo gave the neutral complexes [Ln(hfac) 3 (N,N′-pd)] as pastel green solids in excellent yields (Ln = Y: 82% [1], Gd: 79% [2]) (Scheme 1).Elemental analyses of 1 and 2 crystallised from toluene at −35 °C support the [Ln(hfac) 3 (N,N′-pd)] formulation, with one toluene of crystallisation associated. 1 and 2 are soluble in both coordinating and non-coordinating solvents, to give green and yellow solutions, respectively.

Spectroscopy of [Ln(hfac) 3 (N,N'-pd)]
The room temperature 1 H NMR spectrum of 1 in d 6 -benzene is consistent with complexation to form the 1 : 1 adduct.The pd resonances in 1 at δ = 6.37, 7.53, and 9.16 ppm are shifted downfield relative to the pd free ligand and the aromatic splitting pattern (dd) is no longer observed for the protons neighbouring the N,N′-pocket.The expected ratio of 2 : 2 : 2 : 3 is observed between the pd resonances and the hfac singlet at δ = 6.23 ppm.The 19 F NMR spectrum displays a singlet at δ = −76.9ppm.The X-band EPR spectrum of 2 at room temperature (see ESI †) is common for Gd(III) and simulation gave g iso = 1.989, close to that of the free electron, which is typical for Gd(III). 25The 8 S 0 ground state of Gd(III) has no angular orbital momentum, and so unlike its f-block counterparts, Gd(III) gives an EPR response at room temperature.The Evans Method was used to determine the magnetic moment of 2 in d 8 -thf at room temperature.The corrected magnetic moments are in the range of 7.56-7.71μB for 2, consistent with Gd(III).

Reduction chemistry of 1 and 2
The pd ligand can be sequentially reduced by two electrons, first to the semiquinone radical anion pd •− and then to the fully reduced and diamagnetic catecholate pd 2-; the reduction potentials are −0.85 and −1.71 V vs. Fc + /Fc, respectively. 10,32lmost all published metal complexes of the reduced pd ligand contain pd 2− and the few examples of the radical anion of pd •− were generated in situ, usually under electrochemical conditions and characterised by electronic spectroscopies such as EPR and UV-vis-NIR.This is in spite of the ease of reduction of pd to pd •− relative to the structurally related 1,10-phenanthroline 10 and 2,2′-bipyridine. 33omplex 1 is unstable under standard electrochemical conditions.Therefore, we used the reduction potentials of the free pd ligand to rationalise the choice of organometallic reducing agent. 34The first reversible oxidation process of V(II) to V(III) in VCp 2 to VCp 2 + is a good match at −0.93 V vs. Fc + /Fc. 35We note that VCp 2 has been previously shown to reduce pd, with the product formulated as [(O,O′-pd)VCp 2 ] and in our hands this reaction formed intractable dark green solids. 16Additionally, we used a substituted vanadocene, VCp t 2 (Cp t = tetramethylcyclopentadienyl) to test whether the added steric bulk would promote the formation of an ion pair.The substituted Cp t ligand environment is also a good model for extending this synthetic route to incorporate f-element organometallics. 36he data for 3 and 5 are consistent with a single unpaired electron i.e. 3d 1 V(IV), resulting from the oxidative addition of V(II) to V(IV) and the concomitant reduction of pd by two electrons to pd 2− .The data for 6 are consistent with the value predicted by the spin only formula for Gd(III) and V(IV).This is chemical behaviour similar to the oxidative addition of VCp 2 to acetylene to form the metallocyclopropane. 37he IR spectra of complexes 3-6, are consistent with pd reduction.The ν CO at 1699 and 1697 cm −1 assigned to neutral N,N′-pd and at 1668 and 1661 cm −1 assigned to N,N′-O-pd are no longer visible.Instead, complexes 3-6, display ν CO in the range 1370-1381 cm −1 , consistent with the reported examples of pd 2-complexes. 14,16n overlay of the electronic spectra of 3 and 4 collected in MeCN are contrasted with the electronic spectra of 5 and 6 collected in THF and presented in Fig. 4. The d-d transitions at 740 and 640 nm usually present in pseudo-tetrahedral vanadocenes 38,39 are not directly observed as they are hidden underneath the charge transfer transitions.The transitions are assigned with the aid of computational analysis (vide infra).][44][45] The metrics that are important in assessing the reduction of the pd ligand are C5-C6, C5-O1 and C6-O2.The C5-C6 bond distances in 3 of 1.The room temperature X-band EPR spectra of 3 and 5 shown in Fig. 7 are typical for V(IV) with an 8-line pattern from coupling of the electron spin (S = 1/2) to the I = 7/2 nuclear spin of the 51 V (99.8% abundance).42]44 In contrast, the spectrum of 5 exhibits the two overlapping 8-line signals; simulations yielded an equal mix of one entity with g iso = 1.9819,A iso = 68.4× 10 −4 cm −1 , and the other g iso = 1.9779A iso = 64.9× 10 −4 cm −1 (Table 1).

Synthesis of [Ln
The corresponding frozen solution spectra recorded in toluene at 130 K clearly depicts a signal indicative of a single chemical species of 3 and 5 (Fig. 8).The simulation parameters are nearly identical, with g = (1.9657,1.9776, 1.9978) and A = (106.6,78.1, 11.0) × 10 −4 cm −1 for 3, and g = (1.9654,1.9794, 1.9978) and A = (104.1,78.0, 9.5) × 10 −4 cm −1 for 5.The existence of one signal in the frozen matrix suggests two conformations of 5 are present in fluid solution with subtle differences in their spin-Hamiltonian parameters.The spectral profile for 3 and 5 describes a 2 A 1 ground state (d x 2 −y 2 orbital in C 2v symmetry) with archetypal g x ≈ g y < g z < g e and A x ≈ A y > A z splitting patterns. 38The narrow lines neatly show a slight rhombicity in the spectrum with a notable inequivalence of g x and g y , and A x and A y in this ligand field.This is in contrast to three-fold symmetric tris(chelate)vanadium complexes, which have the same 2 A 1 ground state but overall three-fold symmetry, such that g x = g y and A x = A y . 38,41,42he X-band EPR spectrum of 4 is shown in Fig. 9 and displays a superposition of the single broad signal for Gd(III) entwined with the 8-line pattern of V(IV).The spectrum was simulated with three subspectra, two of which arise from different conformations of the complex which are distinguished by the g-values of the V(IV) centre because of the narrow linewidth as seen for 5.In 4 the major (10.5%) and minor (1.8%) conformer sum to a seventh of proportion of the intensity as expected for a heterobimetallic species with one Gd(III) S = 7/2 ion and one V(IV) S = 1/2 ion.In addition we observe no manifestation of exchange coupling between the Gd(III) and V(IV) centres such that J ≈ 0 (i.e. two uncoupled spins).

DFT calculations on 3
Density functional theoretical (DFT) calculations were carried out on 3 (S = 1/2).The optimised structure is in excellent agreement with the crystallographic structures of both 3 and 5 (Table S2 †).The computed C-C and average C-O bond distances of 1.391 and 1.336 Å, respectively, are representative of a bridging pd ligand in its dianionic (catecholate) form.The highest occupied molecule orbital (HOMO) is singly-occupied (SOMO) with 91% vanadium d character that depicts the d x 2 −y 2 orbital which is a 1 in C 2v symmetry as shown by EPR (vide supra).The Mulliken spin population analysis shows + 1.35 spins (α-spin) located on the V(IV) centre with matching β-spin on the first coordination sphere atoms (Fig. 10).This is the result of bond polarisation inherent to the relatively Lewis acidic V(IV) ion. 41ime-dependent (TD) DFT calculations were carried out on 3 in order to assign the electronic spectra of this series of heterobimetallic compounds.Each complex (3-6) displays two     low-energy bands at ca. 800 and 600 nm (Fig. 4).These signature spectral features were well-reproduced in the TD-DFT analysis (Fig. 11).Both bands are ligand-to-metal charge transfer (LMCT) transitions between the same donor and acceptor orbitals.These are the b 2 symmetric HOMO−1, which is localised on the O,O′ end of the pd bridging ligand, and the LUMO, which is the vanadium d xy orbital (Fig. 11).The computed spectrum reveals the spin-up (α-spin) excitation at 767 nm which perfectly matches the experimental peak at 800 nm in 3. The corresponding spin-down (β-spin) excitation appears at 489 nm which is ca. 100 nm blue shifted from the experimental peak.This aforementioned polarisation inherent to Lewis acidic V(IV) sees the α-spin manifold is exchange stabilised relative to the β-spin manifold by virtue of the unpaired electron in the d x 2 −y 2 orbital.This is the same phenomenon that results in >1 spins located at the V(IV) centre in the spin population analysis (Fig. 10).The experimental polarisation is 0.54 eV is overestimated by the calculations at 0.92 eV.This is typical for DFT as energies of virtual (unoccupied) orbitals are less reliable especially given the TD-DFT computed spectrum is derived from energy differences between donor and acceptor orbitals based on the ground state calculation.This neglects additional factors that can (de) stabilise excited states, and therein fail to accurately reproduce electronic spectra of paramagnetic compounds.Nevertheless, this approximation is sufficient to assign the observed spectral features.The prominent band at 410 nm is tentatively assigned as a π → π* transition from the pd 2-ligand.

Conclusions
We have demonstrated the synthesis and first structural characterisation of [Ln(hfac) 3 (N,N′-pd)] for both Y and Gd complexes.The synthetic route works in exactly the same way for both diamagnetic Y and paramagnetic Gd.Considering the size of Gd and Y, this synthetic route would be expected to be directly applicable to all the Ln elements from Sm-Lu.As the route is modular, a range of ancillary ligand environments would also be expected to be synthetically accessible.We have also shown the reduction of these coordination chemistry complexes using vanadocenes to form heterobimetallic rare earth-transition metal complexes.The spectroscopic data for [Ln(hfac) 3 (N,N′-O,O′-pd)VCp R 2 ] are consistent with oxidation of V(II) to V(IV) and reduction of pd to pd 2-in these complexes.This redox chemistry is driven by the O,O′-pd binding of the VCp R 2 unit.Work is ongoing to isolate rare-earth transition metal complexes containing the radical anion of pd bridging the metal centres and to synthesise heterobimetallic f-f′ complexes using 4f and 5f elements.

General details
All air-sensitive manipulations were carried out in an MBraun glovebox (O 2 and H 2 O < 1 ppm) or by using standard Schlenk techniques under N 2 .All glassware was dried at 130 °C overnight prior to use.Filter cannulas were prepared using Whatman 25 mm glass microfiber filters and were pre-dried at 130 °C overnight.Dry solvents were obtained using an Innovative Technology Inc. Pure Solv 400-5-MD solvent purification system (activated alumina columns).Solvents were sparged with N 2 and stored in ampoules over activated molecular sieves under N 2 .Deuterated benzene and THF were dried by refluxing over K. Deuterated acetonitrile was dried by refluxing over CaH 2 .Dry deuterated solvents were degassed by three freeze-thaw cycles, vacuum distilled, and kept in ampoules in the glovebox under N 2 .The following starting materials were prepared according to literature procedures: VCp 2 , VCp t 2 . 46Hexafluoroacetylacetone (Hhfac) was purchased from Aldrich and degassed by three freeze-thaw cycles before use.Potassium hydride in mineral oil was purchased from Aldrich, the mineral oil removed by washing with anhydrous hexanes, and stored under N 2 .Ln 2 O 3 (Ln = Y, Gd), trifluoromethanesulfonic acid and 1,10-phenanthroline were purchased from Aldrich and used without further purification.

Physical methods
1 H NMR data were recorded on an AVIII 400 MHz instrument and were referenced internally to the appropriate residual protio-solvent and are reported relative to tetramethylsilane (δ = 0 ppm). 19F and 19 F{ 1 H} NMR spectra were recorded on a Bruker AVIII 400 MHz spectrometer and were referenced to CFCl 3 (δ = 0 ppm).All spectra were recorded at a constant temperature of 300 K. Coupling constants ( J) are reported in hertz (Hz).Standard abbreviations indicating multiplicity were used as follows: m = multiplet, d = doublet, t = triplet, s = singlet.UV/vis/NIR spectra were collected using a Shimadzu UV-3600 UV/vis/NIR spectrometer using anhydrous solvents, which were filtered through Celite® prior to use.ATR-IR spectra were collected using either a Shimadzu IRAffinity-1S or (5) were collected by the EPSRC UK National Crystallography Service using a Rigaku AFC12 goniometer, mounted at the window of a FR-E+ SuperBright molybdenum rotating anode generator, and equipped with a (HG) Saturn724+ detector.CCDC numbers 1832429-1832434 and 1841191 † contain the crystallographic information for this paper.
X-band EPR spectra were collected on a Bruker ELEXSYS E500 spectrometer and simulations were performed using Bruker's Xsophe software package. 47nthesis of [Y(hfac) 3 (N,N′-pd)] (1) ‡ THF (15 mL) was added to a Schlenk flask charged with a dry mixture of Y(OTf ) 3 (1.16g, 2.17 mmol), pd (0.456 g, 2.17 mmol) and K(hfac) (1.60 g, 6.51 mmol) at room temperature with stirring.The yellow suspension was stirred at room temperature for 1 h, whereupon the solution became deep green.The green solution was stirred for 16 h, THF was removed in vacuo and the green solids were extracted into toluene (3 × 5 mL), giving a yellow solution.Removal of toluene in vacuo gave green solids, which were washed with hexane (2 × 5 mL) to give [Y(hfac) 3 (N,N′-pd)] as a pale green solid (1.70 g, 1.84 mmol, 85% yield).Single crystals suitable for X-ray diffraction were grown from both a saturated Et

Conflicts of interest
There are no conflicts to declare.
Crystallography of [Ln(hfac) 3 (N,N'-pd)] Crystals of 1 suitable for X-ray diffraction were grown from either a saturated Et 2 O solution at −35 °C over 7 days (1a) or a saturated toluene solution at −15 °C overnight (1b).Similarly crystals of 2 suitable for X-Ray diffraction were grown from either a saturated Et 2 O solution at −35 °C overnight (2a) or a saturated toluene solution at −15 °C overnight (2b).The solid state molecular structures and selected parameters are shown in Fig. 1-3 or in the ESI.† All structures adopt a distorted square antiprismatic geometry, and confirm N,N′-pd coordination.As expected the pd C-O distances in 1a-2b are unchanged from the free ligand (1.213(

Fig. 3
Fig. 3 Molecular structure of 2b, showing the polymeric structure.All atoms depicted as wireframe or ball and stick.

Fig. 4
Fig. 4 Overlay of the electronic spectra of 3 and 4 in MeCN (top) and 5 and 6 in THF (bottom) recorded at room temperature.

Fig. 7 X
Fig. 7 X-band EPR spectra of 3 and 5 recorded in toluene solution at 293 K (experimental conditions: frequency, 9.8634 GHz; power, 0.63 mW; modulation, 0.2 mT).Experimental data are represented by the black line; simulation is depicted by the red trace.

Fig. 8 X
Fig. 8 X-band EPR spectra of 3 and 5 recorded in toluene solution at 130 K (experimental conditions: frequency, 9.4244 GHz; power, 0.2 mW; modulation, 0.1 mT).Experimental data are represented by the black line; simulation is depicted by the red trace.

Fig. 9 X
Fig. 9 X-band EPR spectrum of 4 recorded in toluene solution at 293 K (experimental conditions: frequency, 9.8717 GHz; power, 0.63 mW; modulation, 0.3 mT).Experimental data are represented by the black line; simulation is depicted by the red trace.

Fig. 11
Fig. 11 Overlay of the experimental (solid line) and calculated (dashed line) electronic absorption spectra for 3.The vertical bars show the individual calculated transitions.Inset orbitals depict the HOMO−1 → LUMO transition that constitutes the two lowest energy bands designated the α-spin and β-spin excitations, respectively.