Multiple One-Electron Transfers in Bipyridine Complexes of Bis

Article pubs.acs.org/Organometallics

Multiple One-Electron Transfers in Bipyridine Complexes of Bis(phospholyl) Thulium Léa Jacquot, Mathieu Xémard, Carine Clavaguéra,* and Grégory Nocton* Laboratoire de Chimie Moléculaire, CNRS, Ecole Polytechnique, Route de Saclay, Palaiseau 91128, France S Supporting Information *

ABSTRACT: The synthesis of original neutral bis(phospholyl) thulium complexes, Dtp2Tm(L), where L is tetramethylbiphosphinine (tmbp) and bipyridine (bipy), is reported. The electronic structures of these complexes have been investigated and it appears that, in both cases, an electron transfer occurs from the divalent metal to the ligand, a consequence of the strong reduction potential of the bis(phospholyl) thulium fragment, Dtp2Tm. When 1 equiv of bipyridine is added to the Dtp2Tm(tmbp) complex, another electron transfer occurs to form the Dtp2Tm(bipy) complex along with free tmbp ligand. Astonishingly, despite the apparent trivalent nature of the thulium center, the Dtp2Tm(bipy) complex is still reactive toward neutral bipyridine to form a new complex in which one phospholyl ligand is replaced by a bipyridine radical anion. An experimental kinetic analysis is reported to rationalize this unprecedented redox reaction with thulium and reveals an associative type of mechanism.

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INTRODUCTION The use of divalent lanthanide halides as reductive sources of single electrons is a current topic of interest in organic chemistry.1 While the use of SmI2 has been known for more than 30 years, the reason for new recent developments lies in the discovery of easy access to nonclassical divalent lanthanides such as TmI2, DyI2, and NdI2,2−7 as opposed to the classical SmI2 and YbI2.8 The first report of a Tm(II) organometallic complex in 20029 opened a new area of synthetic organometallic chemistry10 and electron transfer reactivity, among them the reports of small-molecule activationincluding N2 pyridine reductive coupling, and strong bond cleavage.11−14 A strategy to prevent reductive side reactions in the synthesis of divalent thulium complexes is to suit the reactive metal ion with bulky ligands, and cyclopentadienyl or phospholyl ligands bearing bulky tert-butyl or trismethylsilyl groups proved to be efficient.9,15,16 The fact that the steric hindrance around the reactive metal center plays a direct role in the eventuality of an electron transfer to occur or not is not in doubt17−21 and makes chemical sense. However, in contradiction to this, in organolanthanides, the same increased bulk around a trivalent metal center−that is supposed to be inert to redox chemistryis known to induce a reductive chemistry similar to that observed with divalent lanthanide complexes.22 This type of reactivity discovered and substantiated over the past decade by Evans and his group is named “sterically induced reduction” and remains an attractive research area.23,24 In these reactions the electron is provided from a bulky ligand that inclines to reduce a substrate, although in the absence of the lanthanide metal center no redox reaction would occur and therefore these singular reactions prompt the question of the way organolanthanides manipulate electrons within the overall edificemetal, ligand, and substrate. © 2014 American Chemical Society

Additionally, it constitutes another topic of broad interest, since there are reports of lanthanide complexes containing radical ligands that behave as single-molecule magnets with record barriers.25,26 Several recent reports address the question of an electronic contribution to the redox and magnetic behavior of organolanthanides,27−30 and yet a separation of both steric and electronic effects remains an important challenge.21 We are interested in studying in depth single-electron transfer (SET) in lanthanide complexes with the aim of relating their geometry and their electronic structure to their chemical reactivity. We took advantage of having bulky phospholyl ligands (Dtp−) stabilizing divalent thulium to study the SET toward tetramethyl-2,2′-biphosphinine (tmbp) and bipyridine (bipy) ligands and demonstrate in the present work the first reduction of a bipyridine ligand induced by a trivalent thulium complex (Scheme 1). Scheme 1

Received: June 7, 2014 Published: July 29, 2014 4100

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RESULTS AND DISCUSSION Synthesis and Physical Properties. The syntheses of Dtp2Tm(tmbp) (2) and Dtp2Tm(bipy) (3) were performed at room temperature from the base-free Dtp2Tm (1)31 in toluene with stoichiometric amounts of previously sublimed tetramethyl-2,2′-biphosphinine and bipyridine ligands, respectively. Both solutions turned dark brown instantaneously. Crystallization of dark black-red crystals of 2 occurred in toluene at −30 °C, while dark brown crystals of 3 deposited at room temperature from a concentrated n-pentane solution. Complex 3 may also be obtained from 2 by addition of 1 equiv of bipyridine in toluene (Scheme 1). The displacement of the tetramethyl-2,2′biphosphinine ligand is fast and is checked by 31P NMR with the appearance of one singlet at 184 ppm,32 which is a characteristic chemical shift of the free tmbp ligand. Both 2 and 3 show a symmetrical solution structure in 1H NMR with the presence of two signals in a 36:12 ratio for the Dtp ligands and four different signals in a 6:6:2:2 ratio for the tetramethyl-2,2′biphosphinine ligand in 2 (Figure 1), while in 3, only three

3 were analyzed by X-ray diffraction experiments, and ORTEP drawings and crystallographic parameters are shown in Figure 2 and Table S1 (Supporting Information). The main distances and angles are given in the caption of Figure 2. Complexes 2 and 3 crystallize in a monoclinic system with Z = 4 and have essentially the same arrangement in the cell. The Tm−P(Dtp) distances are 2.84(2) and 2.843(2) Å in 2 and 3, respectively, and are only 0.04 Å shorter than that of Dtp2Tm (2.88(1) Å) and 0.11 Å shorter than that of Dtp2Tm(thf) (2.95(2) Å).15 This shrinkage of the Tm−Dtp distancesthat can be associated with the oxidation of the metal centeris also noted in the Ct(Dtp)−Tm(1) distances of 2.44 and 2.47 Å in 2 and 3, respectively, in comparison to 2.53 Å in Dtp2Tm(thf).31 In contradiction to this, a shorter Tm−Dtp distance of 2.42 Å is found in the Dtp2Tm complex. The basefree nature of divalent thulium fragment, viz. its less saturated coordination environment, is responsible for this singularity. On the other hand, the Ct(Dtp)−Tm(1) and Tm−P(Dtp) distances found in Dtp2TmI5a similar trivalent thulium complex of 2.38 and 2.815(3) Å, respectively, are longer than those of 2 and 3. The intermediate nature of the distance found in 2 and 3, between pure divalent thulium complexes and pure trivalent complexes, is somewhat intriguing but was also noticed in the Dtp2Tm(Ph2NN) complex reported by Nief et al. a decade ago.31 The bending angle Ct(Dtp)−Tm(1)− Ct(Dtp) of 2 is 4° larger than that of 3, in agreement with the bulkier tmbp ligand in comparison to bipy. Another aspect that has to be noted on these crystallographic data is the C−C distance that links the two aromatic rings of the tmbp and bipy ligands, C(35)−C(36) in 2 and C(33)− C(34) in 3. In bipy and substituted bipy complexes of the Cp*2Yb fragment, this distance is symptomatic of the reduction of the bipy and is of 1.49 Å when the bipy is neutral and varies between 1.41 and 1.46 Å when the bipyridine is reduced.33,34 A similar trend is noticeable in the tmbp ligand. A distance of 1.490(8) Å is reported for the free ligand32,35 and 1.440(2) Å for the radical anion.35 The C(35)−C(36) and C(33)−C(34) distances of 1.44(1) and 1.41(1) Å in 2 and 3, respectively, are therefore in agreement with reduced ligands, L•−. This was confirmed by the presence of the typical transition bands of the reduced ligands36 around 900 nm in the visible−near-IR

Figure 1. 1H NMR of 2 recorded at room temperature (293 K) in toluene-d8. The asterisk corresponds to signals of the solvent residue.

signals in a 2:2:2 ratio could be detected for the bipyridine ligand. Nevertheless, these 1H NMR spectra are in accordance with the presence of an averaged C2v symmetry in solution. When the temperature is lowered, the signals are broadened and tend to disappear below −70 °C, but no coalescence temperature could be measured in either case. Crystals of 2 and

Figure 2. ORTEP drawings of 2 and 3. Thermal ellipsoids are at 50% probability level, and hydrogen atoms have been removed for clarity. Bond lengths (Å) and angles (deg) for 2: Tm(1)−P(1), 2.862(2); Tm(1)−P(2), 2.825(2); Ct(Dtp)−Tm(1), 2.44; Tm(1)−P(3), 2.857(2); Tm(1)−P(4), 2.847(2); C(35)−C(36), 1.44(1); P(1)−Tm(1)−P(2), 92.57(5), Ct(Dtp)−Tm(1)−Ct(Dtp), 144.5. Bond lengths (Å) and angles (deg) for 3: Tm(1)−P(1), 2.843(2); Tm(1)−P(2), 2.844(2); Ct(Dtp)−Tm(1), 2.47; Tm(1)−N(1), 2.331(5); Tm(1)−N(2), 2.310(6); C(33)−C(34), 1.41(1); P(1)−Tm(1)−P(2), 98.39(4); Ct(Dtp)−Tm(1)−-Ct(Dtp), 140.6. 4101

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Addition of 1 equiv of 4,4′-Me2bipy to 3 did not change the 1H NMR spectrum of 3 before it degraded, which tends to indicate that there is no exchange between coordinated and free ligand on the NMR time scale. Therefore, the observed rate of the reaction at 50 °C was measured with different concentrations of bipyridine and a direct proportionality was found between both (Figure 4), implying an associative type of mechanism.

spectrum (see the Supporting Information), although in a recent article Holland and coauthors indicated that their interpretation and quantification should be cautious.37 The fact that 3 can be obtained from 2 is very significant because it means that the electron transferred from the metal to the tetramethyl-2,2′-biphosphinine ligand is still available for further reactivity. Reversible electron transfer is already known in organolanthanides, since Cp*2Yb(4,5-diazafluorene) already showed this behavior in pyridine.38 However, it still constitutes a very important piece of information because it implies that, once transferred, the electron is stored in the ligand and may be used for further reaction. The question that arises at this stage is whether the two ligands are in exchange in solution or if the process implies a coordination of the second ligand. When another 1 equiv of neutral bipyridine is added to a dark brown solution of 3, the color changes gradually with time to dark green to yield dark green crystals which were analyzed by X-ray diffraction. The X-ray structure consists of a thulium complex that has lost one Dtp radical ligand in favor of a bipyridine (Scheme 1). The net chemical balance of this reaction is provided with the loss of 1/2 equiv of (Dtp)2 that was observed in 31P NMR (δ, −6 ppm). The molecular structure of DtpTm(bipy)2 (4) is shown in Figure 3.

Figure 4. Observed reaction rate as a function of the concentration of bipyridine.

With this knowledge, the concentration of 3 was modified and the observed rate was measured in a large excess of bipyridine at 50 °C. No change of observed rate occurs, which is indicative of a monomolecular, pseudo-first-order process in 3. Therefore, assuming a monomolecular process, the rate law can be written using a steady-state approximation for I (Scheme 2) and an Eyring analysis was performed over a small range of Scheme 2

Figure 3. ORTEP drawing of 4. Thermal ellipsoids are at the 50% probability level, and hydrogen atoms have been removed for clarity. Bond distances Åa) and angles (deg): Tm(1)−P(1), 2.841(2); Ct(Dtp)−Tm(1), 2.41; Tm(1)−N(1), 2.346(5); Tm(1)−N(2), 2.343(5); Tm(1)−N(3), 2.333(5); Tm(1)−N(4), 2.355(5); C(19)− C(20), 1.419(8); C(29)−C(30), 1.409(9); P(1)−Tm(1)−N(1), 92.42(16); N(1)−Tm(1)−N(4), 128.66(16); N(1)−Tm(1)−N(3), 86.04(16).

The Tm−P(Dtp) and Ct(Dtp)−Tm distances are 2.841(2) and 2.409 Å, respectively, and are close to those found in 3. The average Tm−N distance of 2.34(1) Å is comparable to that of 3 as well (2.32(1) Å). In both bipyridine ligands, the C−C distance between the two rings is short (1.419(8) and 1.409(9) Å) and indicates that both bipyridine ligands are reduced. The increase in intensity of the band located at 900 nm in the visible−near-IR spectrum also confirms this statement (see the Supporting Information). As an additional note, when 4 is let stand with more free bipyridine, no further reaction occurs. Kinetic Analysis of the Reaction 3 → 4. The curious nature of the reaction that transforms 3 to 4 and its slow rate at room temperature encouraged us to perform an elaborate kinetic analysis following the 1H NMR resonances. 3 and 4 are paramagnetic, and integration of the NMR signals requires care.

temperature in a large excess of bipyridine (pseudo first order). The enthalpy and entropy thermal parameters are 26.0(4) kcal mol−1 (ΔrH⧧) and 11(2) cal mol−1 K−1 (ΔrS⧧), respectively; the enthalpy barrier is in very good agreement with the observation of an accelerated rate at temperatures higher than 50 °C. The small and positive entropy is to be taken cautiously because of the small temperature range of the experiment, but the excellent correlation of the Eyring plot allows several conclusions. The very modest positive value attests to the fact that the rate-determining step (RDS) is not a dissociative process but at the same time it is not in agreement with a purely associative RDS. This indicates that the RDS is likely to involve the elimination of a Dtp radical ligand and the geometry and electronic structure of the intermediate (I) is 4102

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crucial to understand how the electron is traveling from a Dtp− ligand to the bipyridine π* orbitals. However, the absence of structural information on I on the NMR time scale does not allow its study with this experimental approach, and we therefore turned to theoretical calculations. Computational Analysis. A theoretical study was carried out to give insights into the electronic structure of the complexes. DFT calculations indicate an intermediate state of 2 and 3 with totals of 12.3 and 12.4 f electrons on the thulium center that can be compared to a pure Tm(II) complex for 1 with 13.0 f electrons. Analysis of the binding energy between the thulium center and the ligands shows a progressive increase of the total interaction energy from 1 to 4, in line with an increase of the electrostatic interactions and stable orbital interactions (see Table S4, Supporting Information). The spin density and molecular orbitals analysis highlight an important role of 4f orbitals as they behave as the HOMO in 1 and they mix with bipyridine orbitals in 3 (Figure 5 and the Supporting

Figure 6. Calculated structure of I: average Tm−P distance, 3.05(2) Å; average Tm−N distance, 2.58(4) Å; N−N−N−N, 34°.

free energy than the products 4 and Dtp radicals, which explains their easy formation from I. Magnetism. Solid-state magnetic data recorded between 5 and 300 K (Figure 7) of 4 are in agreement with the presence

Figure 5. Spin density distributions of the optimized structures of 3 and 4.

Information). For 1, the electronic structure is similar at both CASSCF and DFT levels with one single f electron as the HOMO in a monoconfigurational electronic structure (see the Supporting Information). For 3, the spin density plot obtained from DFT points out an electron transfer from Tm to the bipyridine ligands, in line with the X-ray structure and UV−vis studies. However, multireference calculations are very hard to carry out and do not provide a clear picture. A definitive answer on the nature of the ground state should be provided by the inclusion of dynamic correlation as a further step to describe the electronic structure, but these calculations are not straightforward. In the case of 4, DFT results reveal an electronic structure with one single electron on each bipy and two open-shell f electrons on Tm (Figure 5). For this case again, DFT results are in agreement with CASSCF calculations and point out two isoenergetic states composed by a multiconfigurational open-shell singlet and a monoconfigurational triplet with 4f−bipy mixing (see the Supporting Information). Using DFT calculations, the structure of the associated intermediate I is obtained and features a complex with two Dtp ligands in an η1 interaction and the two bipy ligands in the equatorial plane with a twist angle of 34° (Figure 6). The change in the interaction from η5 to η1 is necessary to avoid steric repulsion with the bipy and is likely to be the key step of this reaction. Examples of η1-phospholide complexes are known with lanthanides,39,40 and the Tm−P distance of 3.05(2) Å in I is shorter than that of (η1-1,1′-dibenzophospholyl)2Sm(thf)4 at 3.1908(6) Å,39 in good agreement with lanthanide contraction along the series. Additionally, I is found 26 kcal mol−1 higher in

Figure 7. Temperature-dependent magnetic data of 2 (filled circles), 3 (empty circles), and 4 (crosses). The straight line at χT = 2.54 cm3 mol−1 K serves as a guide for the expected value of a complex with a 2 F7/2 ground state.

of a Tm(III) ion (3H6 ground state) and two radical anions that are not correlated at room temperature (χT = 7.73 cm3 mol−1 K at 300 K and χT = 4.71 cm3 mol−1 K at 8 K). The gentle decrease of χT with decreasing temperature in the lowtemperature range is due to the thermal depopulation of lowlying crystal field states and is typical of the magnetism of organolanthanides.26 Below 8 K, the magnetism drops very quickly. This could be due to the presence of a nonmagnetic singlet ground state, viz. a correlation of the unpaired electrons, but intermolecular interactions cannot be ruled out, especially with compounds containing radicals. The magnetic behavior of 4 is in good agreement with the theoretical calculation results. On the other hand, solid-state magnetic data of 2 and 3 are far from this ideal situation. In the case of an electronic transfer a χT value of 7.55 cm3 mol−1 K is expected at 300 K for the 3H6 ground state of the Tm(III) and the ligand radical, but this value is not reached in either of these cases (see Figure 7). In addition, the gradual decrease of χT vs T when the temperature decreases is not typical of a thermal depopulation of the lowlying crystal field states, nor is the inflection point of χT located at 30 K for 2 and 20 K for 3 (Figure S11, Supporting Information). In the case of Tm(II) complexes (2F7/2), the 4103

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containment and other experimental details have been described previously.43 Diamagnetic corrections were made using Pascal’s constants. The ligands bipyridine (bipy) and tetramethyl-2,2′-biphosphinine (tmbp) were sublimed before use. The kinetic analyses were performed in J. Young NMR tubes at variable temperatures. Because of the high reactivity of 3, we found that at higher temperature (above 40 °C), when the concentration decreases, an overall mass loss (about 10%) was noticed. Therefore, only the first t1/2 valuewas used and reported in the Eyring plot, although the reactions were followed until 3t1/2. Single crystals of the compounds 2−4 were mounted on a Kapton loop using Paratone oil and cooled to 150 K under a nitrogen stream for X-ray structure determination. The loop was transferred to a Nonius Kappa diffractometer using Mo Kα (λ = 0.71073 Å) X-ray source, a graphite monochromator, and a Bruker APEX-II detector. Preliminary orientation matrixes and cell constants were determined by collection of 10 s frames, followed by spot integration and leastsquares refinement. Data were integrated and corrected for Lorentz and polarization effects. The crystal structures were solved in SIR9744 and refined in SHELXL-97 and SHELXL-201345 by full-matrix least squares using anisotropic thermal displacement parameters for all nonhydrogen atoms. All hydrogen atoms were placed at geometrically calculated positions. Details of crystal data and structure refinements are summarized in Table S1 (Supporting Information). ORTEP drawings were produced using Mercury. CIF files were deposited at the Cambridge Data Base Centre under the reference CCDC numbers 998820 (2), 998821 (3), and 998822 (4). Syntheses. Synthesis of 2. A clear toluene solution (1.5 mL) of previously sublimed tmbp ligand (28 mg, 0.114 mmol) was added to a toluene solution (1.5 mL) of 1 (60 mg, 0.114 mmol), resulting instantaneously in a color change from dark green to dark brown. This solution was stirred for 2 h at room temperature and was let stand at −30 °C. Large black-red crystals formed overnight, which were separated by filtration and dried under reduced pressure to yied 74 mg (85%) of X-ray-quality crystals of 2. 1H NMR (δ, 293 K): 301.48 (2H, tmbp), 102.24 (36H, tBu, Dtp), 57.76 (6H, Me, tmbp), 13.91 (6H, Me, tmbp), −72.81 (2H, tmbp), −82.35 (12H, Me, Dtp). Anal. Calcd for C42H64P4Tm: C, 58.54; H, 7.49. Found: C, 58.39; H, 7.62. Synthesis of 3. A pentane suspension (2.0 mL) of previously sublimed bipy ligand (48 mg, 0.288 mmol) was added to a pentane solution (2.0 mL) of 1 (150 mg, 0.287 mmol), resulting instantaneously in a color change from dark green to dark brown along with the dissolution of the bipy ligand. The latter solution was stirred for 2 h at room temperature and was concentrated to 2.0 mL under reduced pressure; the resulting suspension was warmed to 40 °C to solubilize the compound and was let stand at room temperature. Brown crystals formed over 3 days, which were separated by filtration and dried under reduced pressure to yied 170 mg (88%) of X-rayquality crystals of 3. 1H NMR (δ, 293 K): 41.95 (36H, tBu, Dtp), −22.31 (12H, Me, Dtp), −31.12 (2H), −84.50 (2H), −157.03 (2H). Anal. Calcd for C38H56P2N2Tm: C, 59.14; H, 7.31; N, 3.63. Found: C, 58.83; H 7.41; N, 3.54. Synthesis of 4. To a brown solution of 3 in toluene (50 mg, 0.074 mmol) was added 1 equiv of bipyrdine (12 mg, 0.077 mmol), and the resulting solution was warmed to 70 °C over a period of 3 days, after which the solution turned dark green. When the solution was cooled to room temperature, a dark precipitate formed, which was collected by filtration and recrystallized in toluene at −20 °C (46 mg, 81%). 1H NMR (δ, 293 K): 114.95 (12H, Me, Dtp), −106.60 (36H, tBu, Dtp). Anal. Calcd for C34H40N4PTm: C, 57.96; H, 5.72; N, 7.95. Found: C, 57.84; H, 5.45; N, 8.28. Calculations. The geometries of the complexes were optimized with the ADF2013 package46,47 at the PBE-D3 density functional level of theory (Perdew−Burke−Ernzerhof functional48 complemented by dispersion effects in the D3 framework proposed by Grimme49). Scalar relativistic effects were taken into account using the zero-order regular approximation (ZORA).50 Double-ζ (DZ) Slater basis sets were employed, and the frozen-core approximation was applied to Tm, resulting in 23 explicit valence electrons. Using the PBE-D3 optimized

magnetic data would be equivalent to that of Yb(III) complexes and the χT value would not exceed 2.54 cm3 mol−1 K (light straight line in Figure 7). Therefore, the situation in 2 and 3 is intermediate between both situations and several explanations can be hypothesized to explain these results: (i) one electron is transferred to the ligand and correlates with a single f electron in an antiferromagnetic manner and an open-shell singlet forms between one f electron and the bipyridine radical anion but the net valency of the thulium center remains +III and/or (ii) the system adopts a multiconfigurational ground state and its valence is therefore intermediate, a situation comparable to that found for Cp*2Yb(bipy).41 In both 2 and 3, the exchange coupling must be important42 but because of the electronic structure of the Tm(III), i.e. f12, the quantification of the exchange coupling is not trivial and additional spectroscopic techniques are still needed to perform this deed, a strategy that we intend to embrace in future studies. Along the same line and as the magnetic data pointed out, the electronic structure of 3 is likely to be difficult to rationalize with the only information yet in our hand, and a deeper theoretical study is currently in progress.

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CONCLUSION In summary, we report the synthesis and the detailed characterization of two new complexes of DtpTm(L), where L stands for the tmbp (2) or bipyridine (3) ligand. Their electronic structures are in agreement with the presence of an oxidized thulium center and a radical ligand, although strong electron correlation within these complexes masks their net valency. Additionally, 2 is, to the best of our knowledge, the first example of a lanthanide complex bearing a biphosphinine ligand. An electron transfer occurs from tmbp to bipy to form 3 when bipyridine is added to a solution of 2. This interesting result stresses the possible use of such ligands as electron reservoirs in organolanthanides. Furthermore, the reactivity of 3 in the presence of free bipyridine shows a remarkable bipyridine reduction with the concomitant departure of a Dpt radical ligand. A kinetic analysis of the latter reaction is reported and is in agreement with an associative mechanism in which the nature of the intermediate was computed. Our preliminary studies on the similar Dtp2Sm(bipy) complex indicate no reactivity with additional free bipyridine ligand. Work is currently in progress to prepare the complexes and solve this riddle. If the reaction that transforms 3 to 4 falls in the category of sterically induced reductions, it seems that the singular electronic structure of 3 could also be the trigger for the reductive reactivity to occur and that was described in this work.

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EXPERIMENTAL SECTION

General Considerations. All reactions were performed using standard Schlenk-line techniques or in a drybox (MBraun). All glassware was dried at 130 °C for at least 12 h prior to use. Toluene, pentane, and diethyl ether were dried over sodium. Toluene-d8 was dried over sodium. All of the solvents were degassed prior to use. 1H NMR spectra were recorded on Bruker Avance II and III 300 MHz instruments. 1H chemical shifts are in δ units relative to TMS. Samples for UV−vis−NIR spectroscopy were contained in a Schlenk-adapted quartz cuvette and obtained on a Agilent Cary 60 spectrometer. Solvent background was corrected. Elemental analyses were obtained from the London Metropolitan University Science Centre. Magnetic susceptibility measurements were made for all samples at 1, 5, and 40 kOe in a 7 T Cryogenic SX600 SQUID magnetometer. Sample 4104

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geometries, energy decomposition analysis (EDA) and bonding properties were obtained with the B3LYP-D3 hybrid functional associated with all-electron TZ2P (core double-ζ, valence triple-ζ, doubly polarized) relativistic basis sets for all atoms including Tm. The EDA is a way to divide the binding energy into energy contributions such as Pauli repulsion, total steric interaction, and orbital interaction terms for a quantitative interpretation of chemical bonds.51−53 Furthermore, the nature of the intermediate compound I was inspected at the DFT level with both B3LYP and M0654 functionals using the Gaussian09 program package.55 Geometry optimizations and frequency calculations were performed from different starting points to determine the most plausible geometry. A small-core relativistic pseudopotential combined with the associated basis set was used for Tm,56,57 and 6-31G(d,p) basis sets were used for C, N, P, and H atoms. The final geometry was used for all-electron B3LYP-D3 relativistic calculations to perform bonding analysis with the ADF package. To have full confidence in the nature of the electronic structures of the complexes, complete active space SCF (CASSCF) calculations were carried out using the SCF orbitals. For 1, the full system was considered and the results were compared to those for the simplified complex in which tBu groups have been substituted by methyl groups in order to check that this substitution has no large effect on the electronic structure. Various active spaces were used for each complex to distribute 4f electrons in 4f orbitals and π* orbitals of the ligands. For 3 and 4, the calculations were performed on the simplified model. CASSCF calculations were performed using the MOLPRO 2012.1 package.58,59

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving information concerning magnetic susceptibility, vis−NIR spectroscopy, kinetic analysis, X-ray crystallography, and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for C.C.: [email protected]. *E-mail for G.N.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is dedicated to Dr. François Nief on the occasion of his retirement. We thank the CNRS and Ecole Polytechnique for financial support and Prof. Dr. Richard A. Andersen and Dr. Emmanuel Nicolas for many scientific discussions and help. Dr. Xavier F. Le Goff is thanked for preliminary work on the X-ray structures of 2−4. The tetramethyl-2,2′-biphosphinine (tmbp) ligand was a gift from Dr. Nicolas Mézailles. This work was granted access to the HPC resources of [CCRT/CINES/ IDRIS] under the allocation c2013086830 made by GENCI (Grand Equipement National de Calcul Intensif).

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REFERENCES

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