Yb(II) Triple-Decker Complex with the μ-Bridging Naphthalene

Jul 12, 2016 - Institute of Organometallic Chemistry of Russian Academy of Sciences, ... LPCNO, Avenue de Rangueil 135, 31077 Toulouse, France...
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Yb(II) Triple-Decker Complex with the μ‑Bridging Naphthalene Dianion [CpBn5Yb(DME)]2(μ‑η4:η4‑C10H8). Oxidative Substitution of [C10H8]2− by 1,4-Diphenylbuta-1,3-diene and P4 and Protonolysis of the Yb−C10H8 Bond by PhPH2 Alexander N. Selikhov,† Tatyana V. Mahrova,† Anton V. Cherkasov,† Georgy K. Fukin,† Evgueni Kirillov,‡ Carlos Alvarez Lamsfus,§ Laurent Maron,§ and Alexander A. Trifonov*,†,∥ †

Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, GSP-445, 630950 Nizhny Novgorod, Russia Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Rennes, France § Université de Toulouse, INSA, UPS, CNRS-UMR5215, LPCNO, Avenue de Rangueil 135, 31077 Toulouse, France ∥ Institute of Organoelement compounds of Russian Academy of Sciences, Vavilova str. 28, 119334 Moscow, Russia ‡

S Supporting Information *

ABSTRACT: Two synthetic approaches to the new threedecker Yb(II) complex [CpBn5Yb(DME)]2[μ-C10H8] (1) were successfully employed: the reaction of [CpBn5Yb(DME)(μ-I)]2 (2) with 2 molar equiv of [C10H8]−·K in DME and the reaction of [YbI(DME)2]2[μ-C10H8] (3) with CpBn5K in a 1:2 molar ratio in DME. Complex 1 was proved to be a Yb(II) binuclear triple-decker complex containing a dianionic naphthalene ligand bridging two CpBn5Yb(DME) fragments in a μ-η4:η4 fashion. An oxidative substitution of (C10H8)2− by trans-(1E,3E)-1,4-diphenylbuta-1,3-diene afforded the threedecker Yb(II) complex [Cp B n 5 Yb(DME)] 2 (μ-η 4 :η 4 -PhCHCHCHCHPh) (4) with a dianionic μ-η4:η4-bridging diphenylbutadiene ligand and naphthalene. The reaction of 1 with excess P4 also occurs with oxidation of (C10H8)2−, whereas Yb remains divalent. The reaction results in the formation of the trinuclear Yb(II) complex with a μ-bridging P73− ligand [CpBn5Yb(DME)]3(P7) (5). Protonation of the Yb−C10H8 bond in 1 with PhPH2 (1:2 molar ratio) afforded the dimeric phosphido complex [CpBn5Yb(THF)(μ2-PHPh)]2 (6) in 64% yield, while an attempt to obtain a phosphinidene Yb(II) species by reacting equimolar amounts of 1 and PhPH2 in DME resulted in the isolation of the metallocene complex CpBn52Yb(DME) (7).

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INTRODUCTION Enormous progress has been achieved in organolanthanide chemistry during past three decades: a series of previously unknown types of compounds containing nonconventional ligands for lanthanide chemistry such as an olefin,1 internal acetylene,2 neutral and anioinic arenes,3 and dinitrogen4 have been synthesized and characterized. Arene ligands have attracted particular interest due to their inherent set of properties. Arenes are stable due to their aromaticity. Accessibility of the π and π* orbitals of arenes and their electron affinity and ability to convert into radical anions or dianions by accepting one or two electrons allow them to act as either neutral or anionic ligands. The diversity of πcoordination modes of arenes (featuring diverse hapticity modes from η2 to η6) make these complexes promising objects for investigation of the nature of f-metal−ligand bonding. Since the appearance of the first report on their synthesis,5 the naphthalene derivatives of lanthanides have attracted consid© 2016 American Chemical Society

erable attention and they have become a well-studied class of lanthanide−arene complexes.3 The high electropositivity of lanthanide6 metals significantly predetermines the ionic character of the metal−ligand bonding in their naphthalene complexes. Most of these complexes reported to date constitute Ln(III) derivatives, while Ln(II) complexes remain scarce, and also little is known about their reactivity. The combination in one molecule two strongly reductive centers (Yb(II) ion and the dianion (C10H8)2−) together with a highly reactive Yb− (C10H8)2− fragment can bestow exceptionally rich reactivity patterns. Herein we report on a new synthetic approach to a triple-decker Yb(II) complex containing μ-bridging (C10H8)2−, its solid-state structure, and the reactions of oxidative substitution of a naphthalene ligand. The protonation reactions of Yb−C10H8 with PhPH2 will also be described. Received: May 30, 2016 Published: July 12, 2016 2401

DOI: 10.1021/acs.organomet.6b00428 Organometallics 2016, 35, 2401−2409

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RESULTS AND DISCUSSION Synthesis and Structure of the Complex [CpBn5Yb(DME)]2(μ-η4:η4-C10H8). With the aim of preparing the mixedligand three-decker Yb(II) complex containing a μ-bridging dianionic naphthalene ligand [CpBn5Yb(DME)]2[μ-C10H8] (1), two different synthetic approaches were employed (Scheme 1).

shows only two sets of resonances for the C10H8 moiety (at 3.35 and 5.89 ppm). This suggests that the bridging naphthalene unit is symmetrically bound in 1. The CpBn5 ligands give only one resonance in the 1H NMR spectrum for the benzyl protons (δ 3.80 ppm), which reflects the absence of constraints of the free rotation of the pentabenzylcyclopentadienyl ligands around the metal−Cp ring and of the benzyl groups around Cp−CH2Ph bonds. Single crystals of 1 suitable for an X-ray diffraction study were obtained by slow concentration of a DME solution at ambient temperature. Complex 1 crystallizes in the form of a solvate, 1·2DME. The asymmetric unit in 1 contains two crystallographically independent molecules; each of them lies on an inversion center. Geometrical parameters of this molecules are close to each other. The molecular structure of complex 1 is shown in Figure 1, and the structure refinement

Scheme 1

The first consists of substitution of an iodo ligand with [C10H8]2− in the reaction of the dimeric half-sandwich iodo complex [CpBn5Yb(DME)(μ-I)]2 (2) 7 with 2 equiv of [C10H8]−·K in DME (Scheme 1A). It should be noted that most of the binuclear lanthanide complexes containing a [C10H8]2− ligand reported to date were synthesized by using this approach.3 However, in this case complex 1 was isolated in rather low yield (21%). In order to increase the yield of 1, we developed a new synthetic method based on a salt metathesis reaction of complex [YbI(DME)2]2[μ-C10H8] (3)8 with CpBn5K in a 1:2 molar ratio in DME (Scheme 1B). Surprisingly, this reaction readily occurs in DME solution at ambient temperature and allows for the synthesis and isolation of 1 in 81% yield. No products corresponding to the ligand redistribution reaction (CpBn5)2Yb7 and (C10H8)Yb(THF)25 were found in the reaction mixture. Addition of a solution of CpBn5K in DME to a suspension of [YbI(DME)2]2(μ-C10H8)8 in DME at ambient temperature resulted in an immediate change of color from pale violet to dark lilac and precipitation of a white powder of KI as well as the concomitant formation of deep purple crystals of the complex [CpBn5Yb(DME)]2(μ-η4:η4-C10H8) (1). Extraction of the reaction products with DME and subsequent recrystallization allowed for the isolation of 1 in an analytically pure state. Hereby we demonstrated that terminal iodo ligands in complex 3 can be replaced by a CpBn5 anion while the YbII(μ-C10H8)YbII fragment remains intact under the salt metathesis reaction conditions. This synthetic approach can be extended to the synthesis of new heteroleptic complexes containing a μbridging naphthalene ligand. Complex 1 is extremely air- and moisture-sensitive. Moreover, it turned out to be thermally unstable: heating a solution of 1 in benzene-d6 or THF-d8 at 60 °C for 1 day resulted in complete decomposition of the compound. CpBn52Yb was isolated from the reaction mixtures, while the second expected product, the complex (C10H8)Yb(THF)2, was not detected in the reaction mixture, most likely due to its thermal instability and decomposition under the reaction conditions. Complex 1 is highly soluble in THF, sparingly soluble in toluene and DME, and completely insoluble in hexane. According to the 1H and 13C{H} NMR spectroscopic data complex 1 is diamagnetic, indicating a divalent state of both ytterbium ions. The 1H NMR spectrum of 1 was found to be similar to those of previously published related complexes of Y9 and Sc10 with a μ-bridging dianionic naphthalene ligand and

Figure 1. Molecular structure of complex 1. Thermal ellipsoids are drawn at the 30% probability level. Ph groups of CpBn5 ligands, CH3 and CH2CH2 fragments of DME, and hydrogen atoms are omitted for clarity. Selected bond distances (Å): Yb−C(Cp) 2.73, Yb(1A)−C(41) 2.687(2), Yb(1A)−C(42) 2.681(2), Yb(1A)−C(43) 2.695(2), Yb(1A)−C(44) 2.720(2), Yb(1A)−C(45) 3.030(2), Yb(1A)−O(1A) 2.472(1), Yb(1A)−O(2A) 2.454(2), C(43)−C(42) 1.372(3), Yb(1A)−C(Bn) 3.90.

data are summarized in Table S1 in the Supporting Information. In the solid state complex 1 adopts a centrosymmetric structure with a binuclear three-decker motif containing a dianionic naphthalene ligand μ-bridging two CpBn5Yb(DME) fragments. It is reminiscent of structures containing Ln(μ-C10H8)Ln moiety, which were previously described for a series of both Ln(III)9−11 and Ln(II)12 complexes. Each ytterbium center in 1 is coordinated by an η4-naphthalene ligand, an η5-cyclopentadienyl ring, and one DME molecule. The two CpBn5Yb(DME) fragments are located above and below the two six-membered rings, respectively, of the bridging naphthalene ligand. The bond lengths between YbII cations and four carbon atoms have rather similar values (Yb−C(41) 2.687(2) Å, Yb−C(42) 2.681(2) Å, Yb−C(43) 2.695(2) Å, Yb−C(44) 2.720(2) Å), while the distances with the two remaining carbons of the six-membered rings (3.030(2) and 3.045(2) Å) are noticeably longer. This is indicative of an η4-bonding mode of the naphthalene ligand in 1. In addition, the naphthalene ligand in 1 adopts a geometry typical of a dianionic state9−13 and is distorted from planarity. For instance, only the central fragment composed of the six carbon atoms (C(41), C(45A), C(44A), C(41A), C(45), C(44)) is planar, while atoms C(42), C(43) and C(42A), C(43A) are displaced in opposite directions with respect to this plane. The value of 2402

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Organometallics the dihedral angles between these two planes is 164.4(2)°. The bonding situation within the naphthalene ligand is substantially different from that of the free C10H8 molecule.14 The C(42)−C(43) and C(42A)−C(43A) bonds in the naphthalene fragment are noticeably shorter in comparison to other bonds, and their length (1.372(3) Å) is indicative of double-bond character.15 The C−C bonds of the central planar fragment have similar values (1.421(2), 1.427(2), and 1.460(3) Å), thus reflecting negative charge delocalization in this eightelectron six-center π system. In complex 1 four benzyl groups of each CpBn5 ligand are directed away from the metal atom, while one phenyl ring is turned toward Yb(II) ions, thus resulting in short Ln−C distances (3.90(2) Å). The mean Yb− C(Cp) bond length in 1 (of 2.73(2) Å) is slightly longer in comparison to that in the parent half-sandwich complex 2 (2.68(2) Å);7 however it is comparable to that in the sandwich complex CpBn52Yb (2.72(2) Å).7 The Yb−O bond lengths (2.472(2) and 2.454(2) Å) fall into the range of those normally observed for derivatives of divalent ytterbium.16 Complex 1 is the first example of a three-decker homometallic Ln(II) complex containing a μ-bridging dianionic naphthalene ligand.3a Due to the presence of two reductive centers (Yb(II) and C10H82−) this compound presents an intriguing object for redox reactivity studies, whereas the high lability of the YbII−C10H82− bond makes it a promising precursor for synthesis of new classes of mixed-ligand Yb(II) species (for examples of the synthetic applications of Ln− naphthalene complexes see refs 3a and 17−19). The bonding situation in complex 1 was examined using computational methods. The methodology proposed by Maron and Eisenstein in 2000,20 which has proven its reliability,21 has been used (see Computational Details in the Supporting Information). Especially, the oxidation state of each ytterbium centers was set to 2 by using f-in-core RECPs. The optimized geometry fits nicely with the experimental geometry, and the main distances are well reproduced. Among other results, the Yb−Cp distances are 2.80 Å (vs 2.73 Å experimentally) and the Yb−O(DME) distances are 2.59 Å (vs 2.46 Å experimentally). In the same way, the bonding of the naphthalene ligand is also found to be η4 with four distances around 2.80 Å and two around 3.10 Å. It should be kept in mind that, as expected with f-in-core, the bond lengths are slightly longer, as there is no treatment of the core polarization as shown by Castro et al.22 Therefore, the naphthalene ligand has been formally doubly reduced and the two Yb atoms have undergone a single-electron transfer (SET) each. This complex is reminiscent of those reported by Arnold et al.,23 Diaconescu et al.,24 and Mazzanti et al.25 in uranium chemistry. In those studies, the final demonstration of the doubly reduced character of the sandwich ligand was the nature of the HOMO of the system, which was of δ type due to the population of the π* orbital of the arene ligand that overlaps with an f−d hybrid orbital of the metal. Therefore, following the same strategy, the HOMO of complex 1 was scrutinized. Interestingly enough, although the f orbitals were not explicitly treated, a δ-type orbital was found to be the HOMO of the system. Explicit treatment of the 4f electrons leads to locating the HOMO also as a δ-type orbital, involving both d and f orbitals on each Yb in a 87%:13% ratio (5d−4f) (Figure 2). Therefore, one can conclude that the naphthalene ligand is doubly reduced in complex 1 and that the 5d orbitals are the most important in ensuring the bonding.

Figure 2. δ orbital (HOMO) of complex 1.

Synthesis and Structure of the Complex [CpBn5YbII(DME)]2(μ-η4:η4-PhCHCHCHCHPh). Recently it was reported that the dianionic naphthalene ligand in the related LnIII(μ-C10H8)LnIII complexes can be oxidatively replaced by stilbene or anthracene (L) to afford the corresponding LnIII(μ-L2−)LnIII species.19 Though little is still known about lanthanide complexes with olefin and diene ligands,3 this prompted us to explore the reactions of 1 with various unsaturated molecules such as stilbene, tolane, styrene, and trans-(1E,3E)-1,4-diphenylbuta-1,3-diene (DPB). The reactions were carried out in DME or THF solvent at ambient temperature. The color change of all reaction mixtures as well as liberation of free C10H8, which was detected by GC analysis, clearly indicated that the reactions took place in each case. However, we succeeded in isolating the reaction product in a crystalline state only in the case of the reaction with DPB. The reaction of a dark purple solution of 1 in THF with a solution of DPB in DME occurs with a color change to cherry red (Scheme 2). As it possesses noticeably higher electron affinity, Scheme 2

DPB (DPB 1.47 eV26 vs C10H8 0.152 eV27) easily oxidizes the C10H82− ligand to free naphthalene, which was found as a byproduct in the mother liquor by GC. Red crystals of the complex [CpBn5YbII(DME)]2(μ-η4:η4-PhCHCHCHCHPh) (4) were obtained in 90% yield by slow condensation of hexane into its DME solution at room temperature. Complex 4 is highly air- and moisture-sensitive; however, unlike 1 it is rather thermally robust both in the solid state and in solution. Heating solutions of 4 in toluene-d8 or THF-d8 at 60 °C for several hours does not lead to complex decomposition or to a ligand redistribution reaction. Complex 4 is readily soluble in THF and DME, less soluble in toluene, and insoluble in hexane. 1H NMR spectroscopy gives evidence for diamagnetism of complex 4, thus indicating the divalent 2403

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lengths within the butadiene fragment of 4 are noticeably different: the bond C(47)−C(45) (1.265(8) Å) is the shortest, while C(45)−C(46) is the longest (1.44(2) Å). The length of the C(46)−C(47A) bond has an intermediate value (1.358(8) Å). An analogous bonding situation within a DPB ligand was formerly documented for a La complex.29 The mean Yb− Cpcenter distance in 4 (2.41(2) Å) falls into the interval typical for Yb(II) sandwich [(Me3Si)2C5H3]2Yb(THF) (2.39 Å)30 and half-sandwich [(C5Me5)YbI(THF)2]2 (2.41 Å)31 complexes. The Yb−Cpcenter and Yb−O (2.447(3) and 2.463(3) Å) bond distances are both indicative of the divalent state of ytterbium (for a comparison of Yb−Cpcenter and Yb−O bond lengths in Yb(II) complexes see refs 31 and 32). Studies on the Reactivity of Complexes with a Dianionic Naphthalene Ligand. Complexes 1 and 4 appeared to be inactive in isoprene polymerization (THF or toluene solutions, 20 °C, [isoprene]/[Ln] = 100). The controlled activation and functionalization of white phosphorus (P4) promoted by coordination to transition or main-group metals is currently the focus of interest as an ecological and less hazardous alternative to classic methods of synthesis of organophosphorus derivatives.33 Divalent samarium proved to be a good candidate which allows for controlled reduction and dimerization of the P4 molecule and formation of a tetranuclear Sm(III) complex with a μ-bridging P8 4− tetraanion.34 At the same time Diaconescu and co-workers demonstrated that rare-earth(III) complexes containing a μbridging C10H8 dianion readily reduce P4 to P73− or P84− anions depending on the rare-earth metal and the reaction conditions.11,35 Moreover, coordination to Sc or Y allows for the further transformation of P73− to P7(SiMe3)3.35 Taking into account that complex 2 contains at the same time two different types of reducing centers (Ln(II) and C10H82−), this compound can offer a varied number of electrons for the P4 reduction. The reaction of 1 with an excess of white phosphorus was carried out in toluene at room temperature. Addition of P4 to a deep purple solution of 1 results in immediate color change to reddish brown. Removal of all volatiles, including the remaining P4, under vacuum and subsequent recrystallization of the solid residue from toluene afforded the new trinuclear Yb(II) complex [CpBn5Yb(DME)]3(P7) (5) containing a P7 trianion in 48% yield (Scheme 3).

state of Yb ions as well as the dianionic state of the DPB ligand. In the room-temperature 1H NMR spectrum of complex 4 the signals from the protons of the CHCHCHCH fragment are shifted to high field in comparison to their position in the spectrum of free DPB and appear as two multiplets at δ 2.57 and 2.32 ppm. Similarly to 1, the CH2 protons of the benzyl groups give rise to a singlet at 3.86 ppm. Single crystals of complex 4 suitable for an X-ray study were obtained by slow concentration of DME solution at room temperature. The molecular structure of complex 4 is shown in Figure 3. The structure refinement data are given in Table S1 in

Figure 3. Molecular structure of complex 4. Thermal ellipsoids are drawn at the 30% probability level. Ph groups of CpBn5 ligands, CH3 and CH2CH2 fragments of DME, and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and bond angles (deg): Yb(1)− Cpcenter 2.41, Yb(1)−C(45) 2.726(6), Yb(1)−C(46) 2.745(6), Yb(1)−C(47) 2.789(6), Yb(1)−C(47A) 2.821(4), C(47)−C(45) 1.265(8), C(45)−C(46) 1.442(10), C(46)−C(47A) 1.357(9), Yb(1)−O(1) 2.447(3), Yb(1)−O(2) 2.463(3); C(48)−C(47)− C(45) 127.1(5), C(47)−C(45)−C(46) 129.0(6), C(45)−C(46)− C(47A) 125.5(6).

the Supporting Information. The X-ray diffraction study revealed that 4 is a binuclear three-decker complex containing a diphenylbutadiene ligand μ-η4:η4-bridging two CpBn5Yb(DME) moieties. It is noteworthy that trans-(1E,3E)diphenyl-1,3-butadiene upon coordination with the two Yb centers changes its geometry and adopts the cis-(1E,3E) conformation. The Yb ions are located above and below the planar PhCHCHCHCHPh ligand (the maximum deviation of atoms from the CHCHCHCH plane is 0.13 Å) and are η5 coordinated by a CpBn5 ligand and one DME molecule. The central butadiene moiety is μ-bridging and coordinates the both Yb(II) ions in an η4 fashion. The bond distances between Yb ions and the internal carbons C(45) (2.632(6) and 2.726(6) Å) and C(46) (2.616(6) and 2.745(6) Å) are shorter in comparison to those with the C(47) and C(47A) (2.821(4) and 2.789(6) Å) atoms. The same coordination mode of the dianionic DPB ligand was described for related Li28 and La29 complexes. In contrast with the related DPB complex of Gd30 incorporating a distorted diphenylbutadiene ligand, in 4 the DPB ligand adopts a planar structure. Thus, the C−C bond

Scheme 3

Reddish brown crystals of 5 are extremely air- and moisturesensitive. The compound is readily soluble in toluene and DME but is insoluble in hexane. Monocrystalline samples of 5 suitable for X-ray diffraction studies were obtained by slow concentration of a toluene solution at ambient temperature. The complex crystallizes as a solvate, 5·3C7H8. The 1H NMR spectrum of 5 contains the expected set of sharp signals corresponding to the CpBn5 and DME ligands at their normal positions, giving evidence for the diamagnetism of 2404

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Organometallics 5 and consequently for the divalent state of ytterbium ions.4 In the room-temperature 31P{1H} spectrum of 5 in toluene-d8 only a broadened singlet at δ −174.2 ppm was observed. Variation of the temperature of recording the 31P{1H} spectra in a large region (−60 to +60 °C) did not result in improvement of the signal resolution. An X-ray analysis revealed that complex 5 adopts a trinuclear structure in the solid state with the Zintl-type polyphosphide anion P73− μ-bridging three CpBn5Yb(DME) moieties (Figure 4). Similar structures featuring a central Ln3P7 core were

under these conditions and the starting materials were recovered from the reaction mixtures in quantitative yields. The reactions of rare-earth complexes containing highly reactive LnII−η4-C10H82− bonds with E−H acids were successfully used as a synthetic approach to new classes of hardly accessible Ln(II) complexes.17,18,5,38 Despite the considerable attention that phosphido39 and phosphinidene40 lanthanide complexes have attracted during past two decades, they still remain scarce. Currently organolanthanide phosphides have gained special significance due to the recently discovered catalytic activity of the related amido and alkyl complexes in intermolecular olefin hydrophosphination reactions in which phosphides41 are supposed to be catalytically active species while phosphinidene complexes proved to be promising reagents for PAryl group transfer.40a,b To investigate the reactivity of 1 in protonolysis reactions and to develop a new synthetic approach to mixed-ligand Ln(II) phosphido and phosphinidene complexes, the reactions of 1 with PhPH2 in 1:2 and 1:1 molar ratios were carried out. The reaction of 1 with 2 equiv of PhPH2 in THF at 20 °C leads to an immediate color change from deep purple to red (Scheme 4). Evaporation of the volatiles under vacuum and subsequent Scheme 4

Figure 4. Molecular structure of complex 5. Thermal ellipsoids are drawn at the 30% probability level. Ph groups of CpBn5 ligands, CH3 and CH2CH2 fragments of DME, and hydrogen atoms are omitted for clarity. Selected bond distances (Å): Yb(1A)−Cpcentr 2.42, Yb(1B)− Cpcentr 2.40, Yb(1C)−Cpcentr 2.42, P(1)−P(2) 2.195(3), P(1)−P(4) 2.197(3), P(1)−P(6) 2.201(3), P(2)−P(3) 2.180(3), P(3)−P(5) 2.245(3), P(3)−P(7) 2.246(3), P(4)−P(5) 2.179(3), P(5)−P(7) 2.258(3), P(6)−P(7) 2.182(3), P(1)−Yb(1A) 3.513(2), P(2)− Yb(1A) 2.935(2), P(2)−Yb(1C) 2.962(2), P(4)−Yb(1A) 2.942(2), P(4)−Yb(1B) 2.953(2), P(5)−Yb(1B) 3.457(2), P(6)−Yb(1B) 2.893(2), P(6)−Yb(1C) 2.910(2), P(7)−Yb(1B) 3.410(2), P(7)− Yb(1C) 3.523(2).

recrystallization of the reaction product from a THF/hexane mixture (1/3) afforded the complex [CpBn5Yb(THF)(μ2PHPh)]2 (6) in 64% yield. Complex 6 is highly air and moisture sensitive. It is readily soluble in THF, DME, and toluene and insoluble in hexane. NMR spectroscopy indicates that 6 is diamagnetic, thus corroborating the divalent state of ytterbium. The 1H and 13 C{1H} spectra of 6 (C6D6, 293 K) contain the expected sets of signals from the CpBn5 ligand, THF molecule, and phenyl group of the phosphido ligand. The PH protons appear in the 1 H spectrum as a doublet at δ 3.81 ppm (JPH = 199.0 Hz). The 31 1 P{ H} NMR spectrum of 6 presents a broad singlet at −87.3 ppm, reflecting the equivalence of both phosphido groups. Variation of the temperature of recording the 31P{1H} spectra over a large region (−60 to +60 °C) did not improve the signal resolution. Red monocrystalline samples of 6 suitable for X-ray analysis were obtained by slow condensation of hexane into a THF solution at room temperature. The molecular structure of complex 6 is shown in Figure 5. The structure refinement data are given in Table S1 in the Supporting Information. The X-ray crystallography study reveals that the complex crystallizes as a solvate, 6·2THF (Figure 5). Complex 6 adopts a centrosymmetric dimeric structure with two phosphido ligands μ-bridging two CpBn5Yb(THF) moieties. Within the rhombic Yb(1)− P(1)−Yb(1A)−P(1A) fragment the Yb−P bonds have similar lengths (2.9174(9) and 2.956(2) Å) and are close to those reported for six-coordinate Yb(II) phosphido complexes:

recently reported for Ln(III) (Ln = Sc, Y, Lu) species.35,11 Each Yb2+ ion in 5 is coordinated by two phosphorus atoms of the middle P3 rim. The P−P bond distances in 5 fall into a rather narrow interval (2.179(3)−2.258(3) Å), thus reflecting negative charge delocalization within the P73− cage. The P−P bond lengths are nearly identical with those previously reported for the related Ln3P7 (Ln = Sc, Y, Lu).35 The average Yb−P bond length in 5 of 2.93 Å is comparable to the appropriate distances in Yb(II) phosphido complexes: (Ph2P)2Yb(THF)4 (2.99 Å),36 [(2,4,6-Me3C6H2)2P]2Yb(THF)4 (2.93 Å).37 The average Yb−Cpcenter distances (2.40−2.42 Å) and the lengths of Yb−O coordination bonds (2.430(6)−2.498(6) Å) are both consistent with the divalent state of ytterbium ions. In an attempt to functionalize the P73− core (for an example of functionalization see ref 35), the reactions of 5 with iPrN CNiPr and PhSiH3 were carried out in toluene at 50 °C (24 h) in a 1:3 molar ratio. However, no reactions were detected 2405

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Organometallics

benzene-d6 at 20 °C, contains a set of signals that correspond to those of DME-free CpBn52Yb and noncoordinated DME molecule, indicating that dissociation of DME from 7 readily occurs in solutions of noncoordinating solvents, most likely due to the intramolecular stabilizing interactions of Yb(II) centers with Ph rings of the pendant benzyl substituents. Recently we reported on our unsuccessful trials to synthesize Lewis base complexes of CpBn52Ln (Ln = Sm, Eu, Yb)7 by reacting them with excess THF, PMe3, and TMEDA in toluene. These observations are in line with the results of our study of the solution behavior of 7 and are indicative of a rather strong interaction of Yb(II) with Ph rings of the pendant benzyl substituents. Complex 7 was characterized by an X-ray study (Figure 6), which proved that it has a bent-metallocene structure. The Figure 5. Molecular structure of the complex [CpBn5Yb(THF)(μ2PHPh)]2 (6). Thermal ellipsoids are drawn at the 30% probability level. Ph groups of CpBn5 ligands, CH2 groups of THF, and hydrogen atoms except PhPH are omitted for clarity. Selected bond distances (Å) and bond angles (deg): Yb(1)−Cpcenter 2.38, Yb(1)−O(1S) 2.38(2), Yb(1)−P(1) 2.9174(9), Yb(1)−P(1A) 2.956(2), Yb(1)− Yb(1A) 4.82, Yb(1)−C(8) 2.919(2); P(1)−Yb(1)−P(1A) 69.70(3), Yb(1)−P(1)−Yb(1A) 110.30(3).

[(2,4,6-Me 3 C 6 H 2 ) 2 P] 2 Yb(THF) 4 (2.925(2) Å), 37 Yb[(PtBu2)2Li(THF)]2 (2.948(1)−2.985(1) Å).42 The Yb(1)− P(1)−Yb(1A)−P(1A) fragment is planar with a value of the P(1)−Yb(1)−P(1A) angle of 69.7(1)°. The Yb−Cpcentr (2.38 Å) and Yb−O (2.38(2) Å) bond distances are consistent with the divalent state of ytterbium.30,31,16 To the best of our knowledge, Ln(II) phosphinidene complexes still remain unknown.40b−d In an attempt to synthesize Yb(II) phosphinidene complexes, the reaction of equimolar amounts of 1 and PhPH2 was carried out in toluene at 20 °C (Scheme 5). Unlike the reaction of 1 with excess Figure 6. Molecular structure of complex 7. Thermal ellipsoids are drawn at the 30% probability level. Ph rings and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and bond angles (deg): Yb(1)−O(1) 2.546(2), Yb(1)−O(2) 2.450(2), Yb(1)−C(Cp) 2.75, Yb(1)−CpCentr 2.47, Yb(1)−O(1) 2.546(2), Yb(1)−O(2) 2.450(2), Yb(1)−C(69) 3.987(3); CpCentr−Yb(1)−CpCentr 140.3.

Scheme 5

structure refinement data are given in Table S1 in the Supporting Information. The complex crystallizes as a solvate with a DME half-molecule, 7·0.5DME. The Yb(II) ion in 7 is η5-coordinated by two CpBn5 ligands and one chelating DME molecule. The average value of Yb−C(Cp) bond lengths (2.75(3) Å) and the Yb−Cpcentr distances (2.47(3) Å) in 7 are in good agreement with the values previously reported for eight-coordinate ytterbocenes,43a−c and they are expectedly somewhat longer in comparison to those of the related sixcoordinate complex CpBn52Yb (2.72(2) and 2.43(2) Å respectively).7 The Yb−O(DME) bond lengths (2.450(2)− 2.546(2) Å) in 7 fall into the interval typical for eightcoordinate ytterbocenes. 43a,b,44a,b It is noteworthy that coordination of a DME molecule to the Yb(II) center does not greatly influence the value of the CpCentr−Yb−CpCentr bond angle. In complex 7 the CpCentr−Yb−CpCentr bond angle (140.3°) is just slightly narrower in comparison to that in the base-free analogue CpBn52Yb (143.0°).7 On the other hand, unlike the complex CpBn52Yb featuring short Yb−C contacts with two Ph rings of both CpBn52Yb ligands (2.952(2), 3.197(2) Å), for complex 7 due to the presence of a coordinated DME

PhPH2 described above, the reaction mixture became dark brown. Recrystallization of the reaction product from a DME/ hexane mixture led to the formation of dark blue crystals of the unexpected metallocene complex Cp Bn5 2 Yb(DME) (7) (Scheme 5), which was isolated in 32% yield. Complex 7 is most likely the product of disproportionation of the phosphinidene complex [{CpBn5Yb(DME)}2(μ-PPh)] formed at the intermediate reaction stage. Unfortunately, all our attempts to isolate the second expected product [PhPYb(DME)x]n were unsuccessful. Complex 7 was characterized by 1H and 13C spectroscopy, Xray crystallography, and microanalysis. Complex 7 is readily soluble in THF, DME, and toluene but insoluble in hexane. Recrystallization of 7 from toluene at room temperature resulted in the base-free complex CpBn52Yb7 in nearly quantitative yield. The 1H NMR spectrum of 7, recorded in 2406

DOI: 10.1021/acs.organomet.6b00428 Organometallics 2016, 35, 2401−2409

Article

Organometallics

internally to the residual solvent resonances and are reported relative to TMS. All coupling constants (J) are given in Hz. IR spectra were recorded as Nujol mulls with a Bruker Vertex 70 instrument. Lanthanide metal analyses were carried out by complexometric titration.47 The C, H, N elemental analyses were performed in the microanalytical laboratory of the G. A. Razuvaev Institute of Organometallic Chemistry. Synthesis of [CpBn5Yb(DME)]2[μ-C10H8] (1). Method A. A solution of C10H8K in DME (15 mL) obtained in situ from C10H8 (0.212 g, 1.66 mmol) and K (0.064 g, 1.66 mmol) was added to a solution of 2 (1.50 g, 1.66 mmol) in DME (20 mL) at room temperature, and the mixture was stirred for 1 h. The solution was separated, and the solid residue was extracted three times with DME (3 × 10 mL). Slow concentration of the combined DME extracts and cooling of the solution to −30 °C afforded 0.35 g of deep purple crystals of 1 (21%). Method B. A solution of CpBn5K in DME (30 mL) obtained in situ from CpBn5H (1.640 g, 3.17 mmol) and KH (0.165 g, 4.10 mmol) was added to a solution of 3 (1.808 g, 1.58 mmol) in DME (20 mL) at room temperature, and the mixture was stirred overnight. The solution was separated, and the solid residue was extracted twice with DME (2 × 20 mL). Slow concentration of the combined DME extracts afforded 2.93 g of deep purple crystals of 1 (81%). Anal. Calcd for C106H118O8Yb2 (1866.08): C, 68.22; H, 6.36; Yb, 18.54. Found: C, 67.88; H, 6.07; Yb, 18.75. 1H NMR (d8-THF, 400 MHz, 293 K): δ 3.28 (s, 12H, OMe, DME), 3.35 (br s, 4H, CH, naphthalene), 3.44 (s, 8H, CH2, DME), 3.80 (s, 20H, CH2Ph), 5.89 (br s, 4H, CH, naphthalene), 6.88−7.11 (complex m, 50H, C6H5). 13C{1H} NMR (d8-THF, 100 MHz, 293 K): δ 29.4 (s, C or CH C10H8), 158.4, 120.3, and 95.0 (C and CH on naphthalene), 156.8, 122.0, 99.5, and 71.0 (C or CH on naphthalene fragment), 33.0 (s, CH2Ph), 57.9 (s, OCH3, DME), 71.7 (s, CH2, DME), 112.7 (s, Cp), 123.8 (s, C6H5), 124.3 (s, C or CH, C10H8), 127.5 (s, C6H5), 127.7 (s, C or CH C10H8), 128.1 (s, C6H5), 133.8 (s, C or CH C10H8), 146.3 (s, ipso-C C6H5). IR (KBr, Nujol, cm−1): υ̃ 1601 (s), 1577 (s), 1493 (s), 1337 (w), 1321 (w), 1279 (m), 1166 (m), 1150 (m), 1072 (s), 1029 (s), 968 (s), 913 (w), 875 (m), 821 (w), 695 (s), 636 (m), 614 (w), 580 (s), 538 (w), 507 (s), 476 (s). Synthesis of [CpBn5YbII(DME)]2(μ-PhCHCHCHCHPh) (4). A solution of DPB (0.064 g, 0.30 mmol) in DME (1 mL) was added to a solution of 1 (0.55 g, 0.30 mmol) in DME (20 mL) at ambient temperature, and the reaction mixture was stirred overnight. Slow condensation of hexane into the reaction mixture afforded 0.52 g (90%) of red crystals of 4. In the solution C10H8 (0.03 g, 78%) was found by GC. Anal. Calcd for C118H118O4Yb2 (1946.20): C, 72.82; H, 6.10; Yb, 17.77. Found: C, 72.50; H, 6.00; Yb, 17.99. 1H NMR (C6D6, 400 MHz, 25 °C): δ 2.29−2.35 (m, 2H, H45/H46), 2.55−2.59 (m, 2H, H47/H47A), 3.12 (s, 12H, OMe, DME), 3.33 (s, 8H, CH2, DME), 3.86 (s, 20H, CH2Ph), 6.87 (d, 3JHH = 7.3, 20H, o-C6H5, CH2Ph), 7.01 (complex m, 40H, m,p-C6H5, CH2Ph; C6H5, DPB). 13 C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 32.5 (s, CH2Ph), 34.9 (s, C45/C46), 35.8 (s, C47/C47A), 58.7 (s, OCH3, DME), 72.2 (s, CH2, DME), 117.6 (s, Cp), 125.5 (s, p-C6H5, CH2Ph), 126.0, 128.2, 129.0 (s, o,m,p-C6H5, DPB), 127.6, 128.7 (s, o,m-C6H5, CH2Ph), 140.0 (s, ipso-C6H5, DPB), 141.8 (s, ipso-C6H5, CH2Ph). IR (KBr, cm−1): υ̃ 1600 (m), 1579 (s), 1553 (w), 1490 (s), 1320 (m), 1288 (m), 1241 (w), 1198 (s), 1175 (s), 1154 (w), 1104 (s), 1060 (s), 1027 (m), 982 (s), 860 (s), 578 (s), 470 (s). Synthesis of [CpBn5Yb(DME)]3(P7) (5). A solution of P4 (0.045 g, 1.5 mmol) in toluene (5 mL) was added to a solution of 1 (0.350 g, 0.21 mmol) in toluene (20 mL). The reaction mixture changed color from deep purple to reddish brown in 10 min. The volatiles were removed under vacuum, and the solid residue was recrystallized from toluene by slow concentration at room temperature. Red crystals of 5 (0.17 g) were isolated in 48% yield. Anal. Calcd for C153H159O6P7Yb3 (2829.70): C, 64.88; H, 5.61; Yb, 18.34. Found: C, 64.80; H, 5.53; Yb, 18.50. 1H NMR (C6D6, 400 MHz, 25 °C): δ 3.12 (s, 18H, OMe, DME), 3.33 (s, 12H, CH2, DME), 4.33 (s, 30H, CH2Ph), 6.90−7.10 (complex m, 75H, o,m,p-C6H5). 13C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 34.1 (s, CH2Ph), 58.7 (s, OCH3, DME), 72.1 (s, CH2, DME),

molecule only one short Yb−C contact (3.987(3) Å) was detected. In order to synthesize a half-sandwich Yb(II) hydrido complex, reactions of 1 with PhSiH3 (1:2 molar ratio) and H2 (excess) were carried out in toluene at room temperature. However, the reactions did not take place and in 1 week complex 1 was recovered from the reaction mixtures nearly quantitatively.

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CONCLUSIONS Hereby we have demonstrated for the first time that the iodo ligand in [YbI(DME)2]2[μ-C10H8] can be readily substituted by a CpBn5 anion using a salt metathesis reaction, preserving the Yb2(μ-C10H8) moiety. The complex [CpBn5Yb(DME)]2[μC10H8] (1) is the first homobimetallic Ln(II) triple decker with a dianionic naphthalene ligand bridging two [CpBn5Yb(DME)] fragments in a μ-η4:η4 fashion. Due to the presence of two reductive centers and a highly reactive Yb−C10H8 bond, complex 1 has a rich reactivity in oxidative substitution of the C10H8 ligand as well as in protonolysis reactions of the Yb− C10H8 bond. The reaction of 1 with trans-(1E,3E)-diphenylbuta-1,3-diene occurs with oxidation of the dianionic (C10H8)2− ligand to C 1 0 H 8 and formation of the complex [CpBn5YbII(DME)]2(μ-η4:η4-PhCHCHCHCHPh) (4) featuring a dianionic diphenylbutadiene ligand in a cis-(1E,3E) conformation. Oxidation of the (C10H8)2− ligand was also observed in the reaction of 1 with P4, which afforded the trinuclear complex [CpBn5Yb(DME)]3(P7) (5) containing a P73− anion. Interestingly, despite the presence of excess P4 the Yb(II) centers retain their oxidation state. Protonolysis of 1 with 2 equiv of PhPH2 led to cleavage of the Yb−C10H8 bond and formation of the dimeric Yb(II) phosphido complex [CpBn5Yb(THF)(μ2-PHPh)]2. The reaction of 1 with PhPH2 aimed at the synthesis of a phosphinidene complex unexpectedly afforded the Yb(II) metallocene complex CpBn52Yb(DME), which is most likely the product of disproportionation of the phosphinidene complex [{CpBn5Yb(DME)}2(μ-PPh)] formed at the intermediate reaction stage. The study of the solution behavior of 7 in noncoordinating aromatic solvents gave further evidence for the considerable strength of intramolecular YbII−Ph interactions, which is obviously comparable with that of YbII−Lewis base interactions. Complex 1 proved to be inert toward H2 and PhSiH3. Currently we are continuing our studies on the synthesis of mixed-ligand Yb(II) complexes with a dianionic (C10H8)2− ligand and evaluation of their catalytic activity in the transformation of unsaturated substrates.

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EXPERIMENTAL SECTION

All experiments were performed in evacuated tubes by using standard Schlenk or glovebox techniques, with rigorous exclusion of traces of moisture and air. After being dried over KOH, THF and DME were purified by distillation from sodium/benzophenone ketyl. Hexane and toluene were dried by distillation from sodium/triglyme and benzophenone ketyl prior to use. C6D6 was dried with sodium/ benzophenone ketyl and condensed under vacuum into NMR tubes prior to use. Naphthalene and trans-(1E,3E)-diphenylbuta-1,3-diene were used after sublimation and recrystallization. PhPH2 was purchased from Synor Ltd., dried with CaH2, and condensed under vacuum. YbI2(THF)245 and pentabenzylcyclopentadiene CpBn5H46 were prepared according to literature procedures. NMR spectra were recorded with Bruker DPX 200 and Bruker Avance DRX 400 spectrometers in CDCl3 or C6D6 at 25 °C, unless stated otherwise. Chemical shifts for 1H and 13C NMR spectra were referenced 2407

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117.4 (s, Cp), 124.9, 127.6, 128.9 (s, o,m,p-C6H5, CH2Ph)), 142.8 (s, ipso-C6H5, CH2Ph). 31P NMR (C7D8, 100 MΓ, 25 °C): δ −174.2 (s, P73−). IR (KBr, cm−1): υ̃ 1945 (m), 1875 (m), 1803 (m), 1652 (s), 1600 (s), 1492 (s), 1330 (w), 1292 (w), 1245 (w), 1191 (s), 1154 (m), 1107 (s), 1062 (s), 1027 (s), 978 (m), 860 (s), 830 (m), 790 (w), 620 (w), 604 (w), 578 (m), 519 (w). Synthesis of [CpBn5Yb(THF)(μ2-PHPh)]2 (6). PhPH2 (0.06 g, 0.55 mmol) was added to a solution of 1 (0.500 g, 0.26 mmol) in THF (5 mL) at room temperature, and the reaction mixture was stirred for 2 h. The color of the reaction mixture changed from deep purple to reddish brown. The volatiles were removed under vacuum, and the solid residue was dried for 1 h at room temperature. The solid residue was redissolved in THF (3 mL), and slow condensation of hexane afforded 0.32 g (64%) of dark burgundy crystals of 6. Anal. Calcd for C100H98P2O2Yb2 (1738.08): C, 69.10; H, 5.67; Yb, 19.90. Found: C, 69.40; H, 5.78; Yb, 19.61. 1H NMR (C6D6, 400 MHz, 25 °C): δ 1.41 (br s, 8H, β-CH2-THF), 3.54 (br s, 8H, α-CH2-THF), 3.81 (d, 1JPH= 199.0 Hz, 2H, PhPH), 3.87 (s, 20H, CH2Ph), 6.88 (d, 3JHH = 7.3 Hz, 20H, o-C6H5, CH2Ph), 7.00 (complex m, 36H, m,p-C6H5, CH2Ph; o,pC6H5, PhPH), 7.23 (m, 4H, C6H5, PhPH). 13C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 25.5 (s, β-CH2-THF), 32.6 (s, CH2Ph), 67.6 (s, αCH2-THF), 117.7 (s, Cp), 125.1, 128.2, 128.3 (s, o,m,p-C6H5, PhPH), 125.5 (s, p-C6H5, CH2Ph), 127.6, 128.7 (s, o,m-C6H5, CH2Ph), 134.7 (d, 1JCP= 15.5 Hz, ipso-C, PhPH), 141.9 (s, ipso-C6H5, CH2Ph). 31P NMR (C6D6, 100 MΓ, 25 °C): δ −87.3 (br, s, PHPh). IR (KBr, cm−1): υ̃ 2248 (s, HPPh), 1938 (w), 1877 (w), 1800 (w), 1598 (s), 1576 (m), 1295 (w), 1259 (w), 1212 (w), 1175 (m), 1151 (m), 1100 (w), 1069 (s), 1024 (s), 977 (m), 907 (m), 874 (m), 583 (m). Reaction of 1 with PhPH2 in a 1:1 Molar Ratio. PhPH2 (0.025 g, 0.23 mmol) was added to a solution of 1 (0.401 g, 0.24 mmol) in toluene (20 mL), and the reaction mixture was stirred at room temperature for 1 h. The volatiles were removed under vacuum, and the oily residue was dissolved in DME (5 mL). Hexane (5 mL) was slowly added, and the resulting solution was cooled to −20 °C and was kept at that temperature for 2 days. Blue crystals of 7 (0.23 g) were isolated in 73% yield. Anal. Calcd for C84H80O2Yb (1294.5): C, 77.93; H, 6.22; Yb, 13.36. Found: C, 77.80; H, 6.00; Yb, 13.54. 1H NMR (C6D6, 400 MHz, 25 °C): δ 3.09 (s, 6H, OMe, DME), 3.36 (s, 4H, CH2, DME), 3.86 (s, 20H, CH2Ph), 6.87 (d, 3JHH = 7.3 Hz, 20H, oC6H5), 6.97 (t, 3JHH= 7.28 Hz, 10H, p-C6H5), 7.06 (t, 3JHH= 7.44, 20H, m-C6H5). 13C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 32.5 (s, CH2Ph), 58.5 (s, OMe, DME), 72.0 (s, CH2, DME), 117.6 (s, Cp), 125.5 (s, p-C6H5), 127.9, 128.7 (s, o,m-C6H5), 141.8 (s, ipso-C6H5). IR (KBr, Nujol, cm−1) υ̃ 1955 (w), 1870 (w), 1811 (w), 1600 (s), 1494 (s), 1330 (w), 1283 (w), 1243 (w), 1189 (m), 1154 (w), 1111 (s), 1062 (s), 1027 (s), 982 (w), 900 (w), 860 (s), 616 (w), 583 (m). X-ray Crystallography. The X-ray data for 1 and 4−7 were collected on Bruker Smart Apex (1, 5, 7), Agilent Xcalibur E (4), and Bruker D8 Quest (6) diffractometers (graphite monochromated, Mo Kα radiation, ω-scan technique, λ = 0.71073 Å, T = 100 K). The structures were solved by direct (1) and dual-space (4−7) methods48 and were refined by full-matrix least squares on F2 for all data using SHELX48 and CrysAlis PRO (4)49 and SADABS (1, 5−7)50 were used to perform area-detector scaling and absorption corrections. All nonhydrogen atoms were found from Fourier syntheses of electron density and were refined anisotropically. All hydrogen atoms in 1 and 4−7 (except PH hydrogen atoms of phosphido ligands in 6) were placed in calculated positions and were refined in the “riding” model with Uiso(H) = 1.2Ueq of their parent atoms (Uiso(H) = 1.5Ueq for CH3 groups). The hydrogen atoms of the phosphido ligands in 6 were found from Fourier syntheses of electron density and refined isotropically (in the “riding” model also). Crystallographic data and structural refinement details are given in Table S1 in the Supporting Information. CCDC 1471704 (1), 1471707 (4), 1471708 (5), 1471705 (6), and 1041706 (7) contain supplementary crystallographic data for this paper. These data are provided free of charge by the Cambridge Crystallographic Data Centre.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00428. Crystallographic data (CIF) Spectroscopic data, details of the crystal structure study, and computational details (PDF) computed Cartesian coordinates of all of the molecules reported in this study. (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*A.A.T.: fax, 007 831 4627497; tel, 007 831 4623532; e-mail, [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Russian Foundation for Basic Research (Projects 14-0300527 and 16-33-50231) is acknowledged for the financial support of this work. We thank Dr. Konstantin Lyssenko for Xray measurement of complex 1. L.M. thanks the Agence Nationale de la Recherche (ANR), the Humboldt foundation, and the Chinese academy of science for funding. CalMip is also acknowledged for a generous grant of computing time.

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REFERENCES

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Article

Organometallics

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DOI: 10.1021/acs.organomet.6b00428 Organometallics 2016, 35, 2401−2409