Ligand Influence on the Redox Chemistry of Organosamarium

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Ligand Influence on the Redox Chemistry of Organosamarium Complexes: Experimental and Theoretical Studies of the Reactions of (C5Me5)2Sm(THF)2 and (C4Me4P)2Sm with Pyridine and Acridine Stéphanie Labouille,† François Nief,*,† Xavier-Frédéric Le Goff,† Laurent Maron,*,‡ Douglas R. Kindra,§ Heidi L. Houghton,§ Joseph W. Ziller,§ and William J. Evans*,§ †

Laboratoire Hétéroéléments et Coordination, CNRS, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau, France University of Toulouse, INSA, UPS, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France, and CNRS, LPCNO UMR 5215, F-31077 Toulouse, France § Department of Chemistry, University of California, Irvine, California 92697-2025, United States ‡

S Supporting Information *

ABSTRACT: The reactions of the samarium(II) complexes Tmp2Sm (Tmp = 2,3,4,5-tetramethyl-1H-phosphol-1-yl) and Cp*2Sm(THF)2 (Cp* = 1,2,3,4,5-tetramethyl-2,4-cyclopentadien-1-yl) with pyridine were found to be different, despite the fact that the Cp* and Tmp π-ligands are similar in size. With Tmp2Sm, a simple adduct, Tmp2Sm(pyridine)2 is isolated, while with Cp*2Sm(THF)2 pyridine is dimerized with concomitant oxidation of samarium to form [Cp*2Sm(C5H 5N)]2 [μ-(NC 5H5−C 5H5N)]. However, reaction of Tmp2Sm with acridine, a better π-acceptor than pyridine, did result in acridine dimerization and the isolation of [Tmp2Sm]2[μ-(NC13H9−C13H9N)]. DFT calculations on the model structures of Tmp2Sm and Cp*2Sm, and on the single electron transfer step from Sm to pyridine and acridine in these ligand environments, confirmed that, even though the Sm−πligand bonds are mostly ionic, the different electronic properties of the Tmp ligand versus that of Cp are responsible for the difference in reactivity of Tmp2Sm and Cp*2Sm.

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spectroscopy.7 We recently gained some insight into this ligand effect when we reported that density functional theory (DFT) calculations could satisfactorily reproduce the different behavior of cyclopentadienyl- and phospholylthulium(II) complexes in electron transfer reactions toward pyridine.8 In the same paper, we predicted that a similar effect should also be present in the case of samarium(II) complexes, but the calculations were less straightforward in this case because we had to use an indirect strategy involving an isodesmic Born−Haber-type thermodynamic cycle in order to describe the electron transfer process of samarium(II) toward pyridine. Hence, we needed experimental verification of the phospholyl/cyclopentadienyl ligand effect in samarium(II) chemistry. We report herein the reactions of Cp*2Sm(THF)2 (Cp* = C5Me5) with pyridine, that of Tmp2Sm (Tmp = C4Me4P) with pyridine and acridine, and the results of theoretical calculations on these reactions together with that of the known reaction of Cp*2Sm(THF)2 with acridine.9

INTRODUCTION Divalent samarium molecular complexes have found many uses as reducing agents in organic chemistry. The vast majority of applications have involved samarium diiodide (SmI2) in a variety of polar solvents, the most frequent being THF.1 SmI2 is also the most commonly used precursor to other inorganic and organometallic complexes of samarium(II).2 It is widely believed that ligand−metal bonding in lanthanide complexes is mostly ionic,3 and so in many respects the samarium(II)− ligand bonding should resemble that involving group 2 metals of similar ionic radii such as strontium(II). Yet the ligand environment does influence the reducing power of samarium(II). It is frequently observed experimentally that various additives, such as hexamethylphosphoric amide (HMPA), are needed in order to increase its reactivity,4 and Flowers et al. have indeed quantified the effect of HMPA on the redox potential of samarium(II) by studying the electrochemical behavior of SmI2/HMPA mixtures in various proportions.5 Apart from this study, surprisingly little is known about the ligand effect on the reactivity of samarium(II) complexes. It is known that strongly donating ligands such as cyclopentadienyl will render the SmII/III redox potential more negative,6 but the reasons for this influence in terms of metal/ligand interaction are still unclear. The previously reported theoretical studies on Cp*2Sm were concerned only with structural aspects and PE © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Computational Details. Calculations were performed with the GAUSSIAN 03 suite of programs.10 DFT was applied by means of the Received: June 22, 2012 Published: July 3, 2012 5196

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Table 1. Crystal Data and Data Collection Parameters for Compounds 1, 2, and 3 molecular formula molecular weight cryst habit cryst dimens (mm) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z d (Mg·m−3) F(000) μ (cm−1) maximum θ reflns measd unique data Rint reflns used wR2 (all data) R1 GoF

1

2

3

C26H34N2P2Sm 586.84 block, black 0.20 × 0.18 × 0.14 monoclinic P21/c 10.991(1) 14.022(1) 17.523(1) 99.068(1) 2666.8(3) 4 1.462 1184 2.337 27.44 19 643 6050 0.0341 5148 0.0897 0.0364 1.123

C60H80N4Sm2·2(C7H8) 1342.25 irregular, red 0.26 × 0.22 × 0.14 monoclinic P21/c 10.1899(6) 14.1166(9) 22.5205(14) 95.9632(7) 3222.0(3) 2 1.384 1384 1.849 27.48 36 130 7349 0.0200 6714 0.0527 0.0206 1.030

C58H66N2P4Sm2 1215.71 needle, brown 0.20 × 0.06 × 0.04 monoclinic C2/c 27.985(1) 16.497(1) 23.894(1) 105.501(1) 10629.9(9) 8 1.519 4896 2.347 25.02 47 236 9323 0.0867 7854 0.2383 0.0874 1.075

B3PW91 hybrid functional.11 Following our previous work, two kinds of relativistic effective core potential (RECPs) have been used to describe the lanthanide center: small-core Stuttgart−Dresden RECP12 (which includes 1s, 2s, 2p, 3s, 3p, and 3d electrons) and large-core Stuttgart−Dresden RECP13 (which includes, in addition, 4s, 4p, 4d, and 4f electrons), depending on the size of the system. The large-core RECP was chosen according to the formal oxidation state of the lanthanide. The RECPs were used in combination with their optimized valence basis sets supplemented by an f polarization function for the large-core RECP. The Stuttgart−Dresden relativistic effective core potential SDD14 was employed for phosphorus centers in association with its valence basis set and a d polarization function, while the 631+G(d) basis set was used for all other atoms. Geometry optimizations were performed on the whole system and without any symmetry constraints. The stationary points were characterized by full vibration frequencies calculations. In order to confirm the validity of our DFT approach, the reactions involving pyridine were further examined by multireference CASSCF calculations based on restricted open-shell canonical SCF (ROHF) orbitals. The six electrons were distributed over six 4f orbitals (singly occupied in the ROHF calculation) and the two π* orbitals of pyridine (LUMO and LUMO +1 in the ROHF calculation), resulting in a (6,8) CAS size. General Procedures. All reactions were performed under an inert atmosphere with purified dry, deoxygenated solvents by using vacuum line, Schlenk, and glovebox techniques. (C 4Me4P)2Sm15 and (C5Me5)2Sm(THF)216 were prepared as previously described. Pyridine (Aldrich) was dried over activated molecular sieves overnight and degassed by three freeze−pump−thaw cycles. All other materials were commercially available and used without further purification. Elemental analyses were performed at the Science Centre, London Metropolitan University, London (UK), or on a Perkin-Elmer 2400 Series II CHNS analyzer at UCI (USA). NMR spectra were recorded with Bruker DRX 500 or Avance 300 instruments. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer. [(C4Me4P)2Sm(C5H5N)2], 1. [(C4Me4P)2Sm] (200 mg, 0.47 mmol) was dissolved in pyridine (5 mL) and stirred for 5 min. The resulting black solution was evaporated to dryness, and the resulting crystalline mass was suspended in hexane (10 mL), filtered, rinsed with hexane (10 mL), and dried under vacuum for 1 h at room temperature to yield 1 as a black solid (170 mg, 62%). Black crystals of 1 suitable for X-ray

analysis were obtained by layering hexane onto a saturated toluene solution at −30 °C. NMR (293 K) 1H (300 MHz, pyridine-d5): δ 5.78 (br s, w1/2 25 Hz, 12H, CH3), 2.66 (br s, w1/2 15 Hz, 12H, CH3); 1H (300 MHz, 9:1 benzene-d6:/THF-d8): δ 10.58 (br s, w1/2 25 Hz, 4H, CH), 7.23 (br s, w1/2 20 Hz, 4H, CH), 6.15 (br s, w1/2 20 Hz, 2H, CH), 5.74 (br s, w1/2 25 Hz, 12H, CH3), 4.69 (br s, w1/2 35 Hz, 12H, CH3); 13C{1H} (75.5 MHz, pyridine-d5): δ 88.90 (br s, w1/2 50 Hz, CH3), 69.36 (br s, w1/2 25 Hz, CH3), 12.92 (br s, w1/2 35 Hz, Cβ), −6.88 (br d, 2JPC ≈ 35 Hz, w1/2 20 Hz, Cα); 31P{1H} (121.5 MHz, pyridine-d5): δ −624 (br s, w1/2 550 Hz). Magnetic moment: μeff = 3.2 μB (Evans’ NMR method, C6D6). Anal. Calcd for C26H34N2P2Sm (588.87): C, 53.21; H, 5.84; N, 4.77. Found: C, 49.28; H, 5.51; N, 4.16. The calculated CHN ratio is 83.37:9.15:7.47 versus the experimental 83.60:9.35:7.06, a possible indication of incomplete combustion.17 [(C5Me5)2Sm(C5H5N)]2[μ-(NC5H5−C5H5N)], 2. Pyridine (79 mg, 1 mmol) dissolved in diethyl ether (10 mL) was added to a stirred solution of (C5Me5)2Sm(THF)2 (225 mg, 0.4 mmol) in diethyl ether (5 mL). The dark purple solution quickly turned dark red upon addition. After 1 h, the solvent was removed under vacuum to yield 2 as a red powder (237 mg, 90%). Crystals suitable for X-ray diffraction were grown from a saturated THF solution or a saturated toluene/ hexane solution at −35 °C. NMR 1H (500 MHz, benzene-d6): δ 6.32 (s, 3H, py), 4.52 (s, 1H, py), 3.85 (s, 2H, py), 3.30 (s, 3H, py), 2.90 (s, 2H, py), 1.45 (s, 2H, py), 1.58 (s, 1H, py), 1.35 (s, 60H, C5Me5), 0.96 (s, 1H, py); 13C NMR (125 MHz, benzene-d6): δ 115.4 (C5Me5), 94.2 (py), 72.9 (py), 17.6 (C5Me5). IR: 3020w, 2964m, 2903s, 2856s, 2753w, 1633s, 1600s, 1561s, 1442s, 1379w, 1276s, 1209m, 1125w, 1009w, 968s, 728m, 706m, 625m, 579w cm−1. Anal. Calcd for 2[C5H5N], C55H75N3Sm2 (1078.92): C, 61.23; H, 7.01; N, 3.89. Found: C, 61.33; H, 7.38; N, 3.88. [(C4Me4P)2Sm]2][μ-(NC13H9−C13H9N)], 3. Toluene (10 mL) was condensed onto a stirred mixture of acridine (84 mg, 0.47 mmol) and [(C4Me4P)2Sm] (200 mg, 0.47 mmol) at −78 °C. This mixture was allowed to warm to room temperature, and after 1 h of stirring, the dark brown solution was evaporated and the resulting solid was washed with pentane and dried under vacuum to yield 3 as a brown solid (134 mg, 46%). Crystals of 3 suitable for X-ray diffraction were grown from a saturated toluene solution in the glovebox at room temperature. NMR (293 K) 1H (300 MHz, benzene-d6): δ 8.60 (d, 5197

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4H, 3JHH = 7.4 Hz, CH), 7.67 (s, 2H, −CH), 6.83 (t, 4H, 3JHH = 7.4 Hz, CH), 3.00 (s, 12H, CH3), 1.75 (d, 4H, 3JHH = 7.4 Hz, CH), 1.61 (s, 12H, CH3), 0.02 (s, 12H, CH3), 0.00 (s, 12H, CH3), −19.23 (br s, w1/2 45 Hz, 4H, CH); 13C{1H} (75.5 MHz, benzene-d6): δ 134.78 (s, CH), 127.83 (s, CH), 117.67 (s, CH), 58.46 (s, −CH), 25.95 (br s, w1/2 15 Hz, CH3), 23.32 (br s, w1/2 15 Hz, CH3), 20.24 (s, CH3), 19.16 (s, CH3), the quaternary carbons and one CH were not observed; 31P{1H} (121.5 MHz, benzene-d6): δ 46.75 (br s, w1/2 80 Hz), 43.24 (br s, w1/2 80 Hz). Anal. Calcd for C58H66N2P4Sm2 (1215.77): C, 57.30; H, 5.47; N, 2.30. Found: C, 57.14; H, 5.39; N, 2.21. X-ray Experimental Data. Crystals for X-ray analysis were obtained as described above. The crystals were manipulated in the drybox and mounted on a fiberglass needle or cryoloop using Paratone-N oil. Data collection was performed on a Bruker SMART APEX II diffractrometer or a Nonius KappaCCD diffractometer with CCD area detector, using graphite-monochromated Mo Kα radiation. Other details are in Table 1 and in the Supporting Information.

pyridine gave a black solution; 31P NMR monitoring showed a broad peak at −624 ppm, in the same region as Tmp2Sm(THF)2 in THF (−580 ppm),15 thus suggesting a pyridine adduct with samarium(II); evaporation to dryness and treatment with hexane produced a black powder. The NMR spectrum in benzene-d6 was not very informative, but adding one drop of THF-d8 produced a spectrum where the pyridine resonances could be integrated, indicating a 1:1 pyridine:Tmp ratio and thus a 2:1 pyridine:Sm stoichiometry The magnetic moment measured by the Evans’ method in C6D6 solution (μeff = 3.2 μB) is fully compatible with samarium(II). Eventually a crystal structure could be obtained, confirming that the product was the samarium(II) compound (Tmp)2Sm(pyridine)2, 1 (Scheme 2 and Figure 1). Scheme 2. Synthesis of Tmp2Sm(pyridine)2, 1, by Reaction of Tmp2Sm with Pyridine

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RESULTS AND DISCUSSION Syntheses and Crystal Structures. We had previously shown that the reductive behavior of thulium(II) toward pyridine was different according to the ligand environment. For instance, Cpttt2Tm [Cpttt = 1,2,4-(tBu)3C5H2] reacted instantaneously with pyridine to give [Cpttt2Tm]2[μ-(NC5H5− C5H5N)] through electron transfer to pyridine followed by dimerization of the intermediate radical. On the other hand, when the “sterically equivalent” phospholyl analogue Dtp2Tm (Dtp = 2,5-tBu2-3,4-Me2C4P) was treated with pyridine, NMR evidence showed that initially no oxidation occurred and pyridine simply coordinated to thulium(II); however, the reaction further evolved into an intractable mixture, presumably through eventual oxidation of thulium(II)18 (Scheme 1). Scheme 1. Reactions of Substituted Cyclopentadienyl- and Phospholylthulium(II) Complexes with Pyridine (after ref 17)

Figure 1. X-ray structure of Tmp2Sm(pyridine)2, 1 (ORTEP plot, 50% ellipsoids, hydrogens omitted). Selected bond lengths (Å) and angles (deg): Sm(1)−N(1) = 2.644(4), Sm(1)−N(2) = 2.688(4), Sm(1)− P(1) = 3.043(1), Sm(1)−P(2) = 3.046(1), Sm(1)−C(av) = 2.93(4), N(1)−Sm(1)−N(2) = 85.8(1), Cnt−Sm(1)−Cnt = 137.9.

In this structure, the Tmp ligand geometry is not significantly different from that of the purely ionic lithium salt,20 and the Sm−N distances (2.644(4)−2.688(4) Å) are similar to those found in SmI2(3,5-dimethylpyridine)4 (Sm−N =2.708(10) Å).19 Thus, no ligand reduction occurred in the interaction of Tmp2Sm with pyridine. On the other hand, when Cp*2Sm(THF)2 in diethyl ether was treated with a slight excess of pyridine, the reaction mixture quickly turned red and a red powder was isolated. X-ray crystallography showed that the isolated product was [Cp*2Sm(C5H5N)]2[μ-(NC5H5−C5H5N)], 2. In this compound, one of the pyridine ligands has been reductively dimerized by the Cp*2Sm moiety, the other staying coordinated to samarium(III) (Scheme 3 and Figure 2). The solid-state structure of 2 is symmetrical around the center of the C28−C28′ bond, thus imposing an anti conformation of the 4,4′-dihydrobis(4,4′-pyridine)-1,1′-diyl bridging ligand, which was also found in the same conformation in [TmI 2 (C 5 H 5 N) 4 ] 2 [μ-(NC 5 H 5 −C 5 H 5 N)] 2 1 and in [Cpttt2Tm]2[μ-(NC5H5−C5H5N)].18 The change in oxidation

As no stable thulium(II)−pyridine adduct could be isolated, we considered that we would have a better chance of isolating stable adducts of pyridine with the less reducing samarium(II), since pyridine adducts of the simple SmI2 are already known.19 In addition, heavily substituted ligands such as Cpttt or Dtp are not required in samarium(II) chemistry, so that it is possible to use simpler ligands such as Cp* or Tmp, with the added benefit that theoretical calculations on real systems are feasible with these ligands. Although the C5Me4H ligand may appear closer to Tmp than Cp* in terms of steric and electronic properties, we nonetheless decided to compare the reactivity of the two well-established samarium(II) compounds Cp*2Sm(THF)2 and Tmp2Sm. Thus, we decided to react Cp*2Sm(THF)2 and Tmp2Sm with pyridine, and we found that the outcome of these reactions was different. Dissolution of green Tmp2Sm in neat 5198

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Scheme 3. Synthesis of [Cp*2Sm(C5H5N)]2[μ-(NC5H5− C5H5N)], 2, by Reaction of Cp*2Sm with Pyridine

Figure 3. X-ray structure of [Tmp2Sm]2[μ-(NC13H9−C13H9N)], 3 (ORTEP plot, 50% ellipsoids, hydrogens omitted). Selected bond lengths (Å) and angles (deg): Sm(1)−N(1) = 2.31(1), Sm(2)−N(2) = 2.30(2), Sm(1)−P(1) = 2.913(5), Sm(1)−P(2) = 2.937(5), Sm(2)−P(3) = 2.889(5), Sm(2)−P(4) = 2.916(5), C(7)−C(20) = 1.59(2), Sm(1)−C(av) = 2.80(4), Sm(2)−C(av) = 2.78(4), Cnt− Sm(1)−Cnt = 135.9, Cnt−Sm(2)−Cnt = 135.2, H(7)−C(7)− C(20)−H(20) = 61.3.

Figure 2. X-ray structure of Cp*2Sm(C5 H5 N)]2 [μ-(NC5H5− C5H5N)], 2 (ORTEP plot, 50% ellipsoids, hydrogens omitted). Selected bond lengths (Å) and angles (deg): Sm(1)−N(1) = 2.547(2), Sm(1)−N(2) = 2.330(2), Sm(1)−C(av) = 2.751(7), C(28)−C(28)′ = 1.559(4), N(1)−Sm(1)−N(2) = 97.53(6), Cnt−Sm(1)−Cnt = 136.1.

The main difference between the solid-state structures of 3 and [Cp*2Sm]2[μ-(NC13H9−C13H9N)], 4, lies in the conformation of the bridging ligand, which is in the gauche conformation in 3 and in the anti conformation in 4. The structure of 3 is not very precise, because of a low-quality crystal and twinning, but on the whole the Sm−N bonds of the bridging ligands in 3, 4, and 2 are similar. All protons could be detected in the spectrum of 3. The most upfield signal at −19.2 ppm (integrating for 4 protons) was attributed to the protons on C2, C12, C15, and C25, which are the closest to samarium (∼2.8 Å in the solid). The conformation of the bridging ligand in solution is unknown, and the observed NMR data are compatible either with an anti conformation, as in the solid-state structures of 2 and 4, or with two equivalent gauche conformations in rapid equilibrium. Computational Studies. The SmII-induced dimerization of pyridine or acridine can start by an inner-sphere electron transfer from SmII to the LUMO of the π-system (pyridine or acridine) in a L2SmII−pyridine or −acridine complex and can be followed by further dimerization of the resulting radical (Scheme 5). We thus undertook a MO study of the L2SmII−

state of samarium in 2 is reflected by the Sm−N(pyridine) distance, shorter in 2 (Sm1−N2 = 2.547(2) Å) than in 1. So in Tmp2Sm electron transfer from samarium(II) to pyridine did not occur presumably because the pyridine LUMO was too high in energy. The energy of this orbital can be lowered either by adding electron-withdrawing substituents on the pyridine ring or by increasing the conjugation of the πsystem (which also raises the HOMO). Acridine seemed a good choice as a test of reactivity with Tmp2Sm for several reasons: the annelation of two benzo rings leaves the p-CH position free for potential dimerization, the molecule has a plane of symmetry so no stereochemical complications are expected, and moreover the reductive dimerization of acridine with Cp*2Sm(THF)2, yielding [Cp*2Sm]2[μ-(NC13H9−C13H9N)], had already been described.9 So when a suspension of Tmp2Sm in toluene was reacted with an equimolar amount of acridine, a 31 P NMR spectrum of the dark brown reaction mixture showed the disappearance of Tmp2Sm and two new broad peaks of equal intensity at 46.7 and 43.2 ppm. The dramatic downfield shift together with the inequivalence of the two phosphorus resonances suggested reduction and dimerization, since in the related [Cp*2Sm]2[μ-(NC13H9−C13H9N)] two different types of Cp* rings are also present in the 1H NMR spectrum. Eventually we were able to obtain a crystal structure confirming that the reduction product was indeed [(C4Me4P)2Sm]2][μ(NC13H9−C13H9N)], 3 (Scheme 4 and Figure 3).

Scheme 5. Proposed Pathway for Dimerization of Pyridinelike Substrates by SmII Complexes

Scheme 4. Synthesis of [Tmp2Sm]2[μ-(NC13H9−C13H9N)], 3, by Reaction of Tmp2Sm with Acridine pyridine or −acridine model complexes (described below), together with a calculation of the electron transfer step, in order to gain insight into the ligand effect in reductive SmII chemistry. In this study, we will first examine the following models: Cp*2SmII (I), Cp*2SmII(Py) (I-Py), Cp*2SmII(Py)2 (I-Py2), and Cp*2SmII(Acridine) (I-Ac) as well as their phospholyl analogues Tmp 2 Sm I I (II), Tmp 2 Sm I I (Py) (II-Py), 5199

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Tmp2SmII(Py)2 (II-Py2), and Tmp2SmII(Acridine) (II-Ac). First of all, as it has been previously found in the related Tm/ pyridine system,8 the single-determinant nature of the groundstate wave function for complexes I-Py and II-Py was verified by performing CASSCF calculations. Two sets of calculations were carried out in order to specifically obtain either the septet ground state or the septet first excited state. In both complexes, the ground state has an f6π*0 electronic configuration and is represented by only one determinant (weight of 1.0 in the wave function development; see Supporting Information). The first septet excited state has an f5π*1 electronic configuration, corresponding to the metal-to-ligand electron transfer. No contribution of the LUMO+1 π*-orbital was found in the ground state or in the first excited state. Since CASSCF has not detected any multiconfigurational nature of I-Py and II-Py in the ground state, we decided that, for our study, it was legitimate to use the more tractable DFT methodology. All complexes were optimized as septuplets (2S+1 = 7) and led to structures with ⟨S2⟩ values ranging between 12.01 and 12.04, very close to that expected for this multiplicity (12.0). All calculated structures represent local minima on the potential energy surfaces. Some of their relevant geometrical parameters (the Sm−Cnt distances, the Cnt−Sm−Cnt angles, and the Sm−N distances) are listed in Table 2.

parameters therefore validates our methodological choice in terms of functional/RECP/basis set. A comparative analysis of the molecular orbitals of I and II reveals an important difference between Cp*- and Tmp-based SmII complexes (Figure 4). In both cases, the six highest

Figure 4. Energy and shape of the highest singly occupied MO for Cp*2Sm (I) and Tmp2Sm (II) derived from B3PW91 DFT calculations.

molecular orbitals have, as expected, a strong f character. But the energy levels of the f manifold are shifted negatively by around −0.6 eV when going from Cp* to Tmp ligands (note that only the highest SOMO of each complex is represented in Figure 4, as the DFT level of theory is not adequate to give reliable crystal field splitting for lanthanide compounds and therefore is not part of the discussion). Since the electron that is removed during the oxidative process comes from one f orbital, this shift is important in understanding the ligand influence on the redox reactivity of SmII complexes. As mentioned above, it is widely believed that the bonding in lanthanide complexes is largely due to electrostatic interactions between the positively charged metal cation and a distribution of negative charges on the ligands.3 In the framework of the crystal field theory, the −0.6 eV energy shift (Figure 4) may be related to a more destabilizing influence of the two (Cp*)− ligands compared to that of the two (Tmp)− ligands toward the orbitals of the Sm2+ cation. This might be due to the greater radial extent of phosphorus orbitals compared to carbon, leading to a more diffuse negative charge distribution on the Tmp ring. A significant result emerging from calculations on samarium−pyridine complexes is that the electronic configuration of the ground state is found to be 4f6π*py0 (the Mulliken spin density on Sm varying between 5.99 and 6.05 electrons depending on the ligands and the number of pyridines coordinated) (Table 2). Therefore reduction of pyridine by Sm(II) is an endoenergetic process and will be associated with an activation energy. On the other hand, examination of Mulliken spin density distributions on the acridine complexes I-Ac and II-Ac shows partial electron transfer from the lanthanide to the π* orbital of acridine. Indeed, the Mulliken spin population on Sm decreases to 5.73 electrons in I-Ac and to 5.87 electrons in II-Ac. In the mean time, the population on the carbon in para position to N increases 0.24 electron in I-Ac and to 0.14 electron in II-Ac. Note that the slightly smaller electron transfer in II-Ac reflects the lower reactivity of Tmp2SmII compared to that of Cp*2SmII. Since the LUMO of acridine is greatly stabilized by delocalization (−1.4 eV vs that of pyridine), it becomes able to mix with a symmetry-adapted samarium-centered orbital, which causes the partial electron transfer mentioned above (Figure 5). Nevertheless, all optimized structures exhibit a

Table 2. Relevant Geometrical Parameters and Mulliken Spin Densities Computed Using a Small-Core RECP for Sm Mulliken spin density

I I-Py I-Py2 I-Ac II II-Py IIPy2 II-Ac

d(Sm−Cnt) (Å)

a(Cnt− Sm−Cnt) (deg)

2.492 2.519 2.573 2.490/2.501 2.549/2.544 2.572/2.586 2.639/2.644

152.1 146.5 138.2 139.9 156.0 143.7 139.0

2.568

140.8

d(Sm−N) (Å)

Sm

2.720 2.722/2.758

6.08 6.00 5.99 5.73 6.07 6.05 6.01

2.604

5.87

2.661 2.720/2.723 2.552

Cpara 0.06 0.03 0.24 0.01 0.01 to 0.03 0.14

First, examination of geometrical parameters of the computed SmII complex I shows a very good agreement with the X-ray data of Cp*2Sm already reported. Indeed, the computed Sm−-Cnt distance is 2.492 Å, while it is 2.524−2.529 Å in the X-ray structure. A larger difference is observed in the Cnt−Sm−Cnt angle, as an angle of 140.1° is found in the crystal structure while the computed angle is 152.1°. However, the presence of a close contact distance between the samarium atom and a methyl group of a second unit in the X-ray structure of Cp*2Sm, as previously mentioned,22 or packing effects in the crystal could be responsible for this discrepancy. Also, a comparison of II-Py2 and the X-ray structure of 1 reveals very similar geometrical parameters. In particular, the Sm−Cnt distance in the DFT-optimized structure ranges between 2.639 and 2.644 Å (II-Py2), while it ranges between 2.644 and 2.646 Å in the crystalline structure (1). The Sm−N distance appears slightly longer in the model (2.74 Å in II-Py2) than in the crystal structure (2.644(4)−2.688(4) Å in 1), yet this difference stays in the range of acceptable errors, considering the fact that it concerns a weakly bonded and labile ligand. The good agreement between experimental and computed geometrical 5200

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Figure 5. Energy and shape of the frontier orbitals for Cp*2Sm(Py) and Cp*2Sm(acridine) derived from B3PW91 DFT calculations.

Figure 6. Energy profiles for the reaction of L2Sm with pyridine (left) and acridine (right). The blue profile refers to Tmp and the red one to Cp* complexes. Values in parentheses correspond to ΔG.

formally 4f6π*py0 ground state, and no complete electron transfer occurred. Next, the energetics of the electron transfer step was addressed. The methodology used to deal with the oxidation of early divalent lanthanides (like samarium) by pyridine-like substrates has been described in a previous publication.8 Following this computational strategy, the redox reactions of L2Sm (L = Cp*, Tmp) with both pyridine and acridine have been examined (Figure 6). In the case of pyridine, the oxidation into I-Py* and II-Py* is computed to be difficult (ΔHSET = 25.1 kcal·mol−1 for I-Py and 31.9 kcal·mol−1 for II-Py). As stated above, preliminary results on the dimerization step indicate that it may be more complicated than just a coupling between two radicals. Nonetheless, our results indicate that the single electron reduction of either pyridine or acridine should be the limiting step or, at least, a fair approximation of it. Returning to the case of pyridine, the results indicate that its reduction by Cp*2Sm is

easier than by Tmp2Sm. The CASSCF oxidation energy is 12.8 kcal·mol−1 for I-Py and 22.8. kcal·mol−1 for II-Py. The value is smaller for I-Py than that calculated with DFT, but the trend is the same, indicating that DFT gives a reasonable representation of the metal-to-ligand electron transfer process. Furthermore, the computed value for the reduction of pyridine by Tmp2Sm is higher than the one commonly allowed for an activation barrier accessible at room temperature, which could be the reason that no reduction is observed experimentally for this system. This is in line with a better stabilization of the +II oxidation state of Sm by phospholyl-based ligands. Moreover, the efficiency of the electron transfer step is associated with the relative energies of the π* orbital of the substrate and of the 4f shell.26 Thus, tuning this energy difference should allow a control of the reaction. As expected, reduction of acridine is computed to be easier than reduction of pyridine for both complexes (ΔHSET = 13.7 kcal·mol−1 for I-Ac and 7.1 kcal·mol−1 for II-Ac). Interestingly, the energetic difference between I-Ac* and II5201

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Ac* is found to be similar to that of I-Py* and II-Py* (ΔΔHSET = 6.6 kcal/mol with acridine vs ΔΔHSET = 6.8 kcal/mol with pyridine). Hence, computation of the electron transfer step gives a qualitative insight into the influence of the ligands on the redox properties of resulting SmII complexes. Finally, the dimerization of I-Py*, II-Py* I-Ac*, and II-Ac* was, as expected, found to be favorable since the energies calculated (for two moles) were respectively −9.7, −8.7, −16.4, and −19.4 kcal·mol−1. Large-core RECPs were used in the calculations, and due to their complexity, no frequency calculations were performed on the last two examples (I-Ac* and II-Ac*).

(3) For a recent balanced view, see: Krinsky, J. L.; Minasian, S. G.; Arnold, J. Inorg. Chem. 2011, 50, 345. Minasian, S. G.; Krinsky, J. L.; Arnold, J. Chem.Eur. J. 2011, 17, 12234−12245. (4) Dahlén, A.; Hilmersson, G. Eur. J. Inorg. Chem. 2004, 3393−3403. Flowers, R. A., II. Synlett 2008, 1427−1439. (5) Choquette, K. A.; Sadasivam, D. V.; Flowers, R. A., II. J. Am. Chem. Soc. 2010, 132, 17396−17398. Shabangi, M.; Kuhlman, M. L.; Flowers, R. A., II. Org. Lett. 1999, 1, 2133−2135. Shabangi, M.; Flowers, R. A., II. Tetrahedron Lett. 1997, 38, 1137−1140. (6) Veauthier, J. M.; Schelter, E. J.; Carlson, C. N.; Scott, B. L.; Da Re, R. E.; Thompson, J. D.; Kiplinger, J. L.; Morris, D. E.; John, K. D. Inorg. Chem. 2008, 47, 5841−5849. Watson, P. L.; Tulip, T. H.; Williams, I. Organometallics 1990, 9, 1999−2009. (7) Andersen, R. A.; Boncella, J. M.; Burns, C. J.; Green, J. C.; Hohl, D.; Rösch, N. J. Chem. Soc., Chem. Commun. 1986, 405−407. Williams, R. A.; Hanusa, T. P.; Huffman, J. C. Organometallics 1990, 9, 1128− 1134. Hollis, T. K.; Burdett, J. K.; Bosnich, B. Organometallics 1993, 12, 3385−3386. Boudreaux, E. A.; Baxter, E. Int. J. Quantum Chem. 1994, 28, 565−569. Timofeeva, T. V.; Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1995, 117, 7452−7459. (8) Labouille, S.; Nief, F.; Maron, L. J. Phys. Chem. A 2011, 115, 8295−8301. (9) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1994, 116, 2600−2608. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (11) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5662. Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244−13249. (12) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730− 1734. Cao, X.; Dolg, M. J. Chem. Phys. 2001, 115, 7348−7355. (13) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75, 173−194. Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441−450. (14) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431−1441. (15) Nief, F.; Mathey, F. Synlett 1991, 10, 745−746. (16) Evans, W. J.; Grate, J. W.; Choi, H. W.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1985, 107, 941−946. (17) For problems of incomplete combustion involving P-containing lanthanide complexes, see e.g.: Tardif, O.; Hou, Z.; Nishiura, M.; Koizumi, T.; Wakatsuki, Y. Organometallics 2001, 20, 4565−4573. (18) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 3552−3558. (19) Maunder, G. H.; Sella, A. Polyhedron 1998, 17, 63−68. (20) Douglas, T.; Theopold, K. H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1367−1368. (21) Fedushkin, I. L.; Nevodchikov, V. I.; Bochkarev, M. N.; Dechert, S.; Schumann, H. Russ. Chem. Bull. 2003, 52, 154−159. (22) Schultz, M.; Burns, C. J.; Schwartz, D. J.; Andersen, R. A. Organometallics 2000, 19, 781−789. (23) De Proft, F.; Martin, J. M. L.; Geerlings, P. Chem. Phys. Lett. 1996, 250, 393−401.

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CONCLUSION In this paper, we have shown that, despite their similarity, the πligands Cp* and Tmp significantly alter the reductive properties of samarium(II): while Tmp2Sm in pyridine produced only the simple adduct Tmp2Sm(pyridine)2 (1), in which samarium stays at the +II oxidation state, Cp*2Sm in the presence of pyridine gave a product of pyridine dimerization in the samarium(III) complex [Cp*2 Sm(C 5H5N)]2[μ-(NC5H5− C5H5N)] (2). However, interaction of Tmp2Sm with acridine, a stronger π-acceptor, did lead to oxidation of samarium and the formation of [Tmp2Sm]2[μ-(NC13H9−C13H9N)], in which acridine has been dimerized. DFT calculations allowed us to propose a pathway involving a single electron transfer step as the key step of the overall dimerization mechanism. The results are in good agreement with the experimental data and reproduce well the difference of reactivity observed between Cp*2Sm and Tmp2Sm. Hence, the results of this study give new insights into the electronic structure of SmII complexes bearing five-membered-ring η5bonded ligands.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data (CIF), absolute energies and coordinates of the DFT-optimized structures, and a summary of CASSCF calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; laurent.maron@ irsamc.ups-tlse.fr; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Institut Universitaire de France, CALMIP and CINES for a grant of computing time, and CNRS, Ecole Polytechnique, UPS, and the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-FG03-86ER13514) for financial support of this work.

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REFERENCES

(1) Kagan, H. B. Tetrahedron 2003, 59, 10351−10372. (2) Bombieri, G.; Paolucci, G. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr.; Eyring, L., Eds.; 1998; Vol. 25, pp 265−413. 5202

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(24) See for example: Gholivand, K.; Mahzouni, H. R.; Esrafili, M. D. Theor. Chem. Acc. 2010, 127, 539−550. Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140−7143. (25) Bruce, Ê . D. V.; Rocha, W. R. Organometallics 2004, 23, 5308− 5313. (26) Yahia, A.; Kramer, M. U.; Okuda, J.; Maron, L. J. Organomet. Chem. 2010, 695, 2789−2793.

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