Different Reductive Reactivities of SmCpx2(THF)n (Cpx = C5Me5 and

Mar 31, 2017 - The reduction of tetraphenyldiphosphine with two differently substituted samarocenes(II) proceeds via different pathways. With [SmCp*2(...
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Different Reductive Reactivities of SmCpx2(THF)n (Cpx = C5Me5 and C5H3tBu2) Samarocenes toward P2Ph4: THF Ring-Opening and LigandExchange Pathways Nikolay A. Pushkarevsky,*,†,‡ Igor Yu. Ilyin,† Pavel A. Petrov,†,‡ Denis G. Samsonenko,†,‡ Maxim R. Ryzhikov,†,‡ Peter W. Roesky,§ and Sergey N. Konchenko†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentieva Avenue 3, 630090 Novosibirsk, Russia Novosibirsk State University, Department of Natural Sciences, Pirogova Street 2, 630090 Novosibirsk, Russia § Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, 76131 Karlsruhe, Germany ‡

S Supporting Information *

ABSTRACT: The reduction of tetraphenyldiphosphine with two differently substituted samarocenes(II) proceeds via different pathways. With [SmCp*2(THF)2] (Cp* = η5C5Me5), the reaction had been known to result in the THF ring-opening product, [SmCp*2(O(CH2)4PPh2)], 3, owing to the instability of phosphido complex [SmCp*2(PPh2)] in the presence of THF. Complex 3 crystallizes from apolar solvents as dimeric or polymeric polymorph with butoxo-phosphine bridging ligands in both cases. In contrast, the phosphide [SmCp″2(PPh2)] (Cp′′ = η5-1,3-C5H3tBu2), 5, is not prone to ring-opening owing to insufficient space in the Sm coordination sphere for a THF ligand. Product 5 is inevitably accompanied by homoleptic complex [SmCp″3] 6 and dinuclear mixed-valent complex [SmIIICp″2(μ-PPh2)2SmIICp″] 7 as the further products of redox transformations and ligand exchange. The formation of 5−7 is rationalized by a sequence of initial coordination of one or two {SmIICp″2} fragments by P atoms and reductive elimination of PPh2· or Cp″· radicals. Further reaction with another equivalent of [SmCp″2] results in the trapping the radicals and formation of all three products.

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according to the first two approaches may be complicated by the formation of byproducts, which could be -ate complexes with the alkali metal cations bound by the base or arene ligands or leaving conjugate acid (amine, alkyne, etc.). Potentially, the reductive approach (c) can result in cleaner reaction without any byproducts, which was in fact demonstrated on the examples of reductive dimerization of phosphaalkyne tBuCP,7 white phosphorus,8 and several polyphosphide and polyarsenide d-metal complexes.9 The most evident restriction of this approach is the limited number of accessible complexes of Ln2+, which are very sensitive to oxidation and thus difficult to handle.10 Moreover, the redox behavior of Ln2+ cations is greatly influenced by the ligand environment and by steric crowding in the complex.11 By using very bulky ligands, the stabilization of oxidation state 2+ is facilitated.12 In this work, we discuss the reaction of two differently substituted samarocenes, [SmCp*2(THF)2] and [SmCp″2(THF)] (Cp* = η5-C5Me5, Cp′′ = η5-C5H3tBu2), with tetraphenyldiphosphine P2Ph4 (Scheme 1) to determine the influence of electronic and steric factors as well as the

INTRODUCTION Substituted bis(cyclopentadienide) complexes of lanthanides(II) (lanthanocenes) have been introduced by Andersen and Zalkin,1 Watson,2 and Evans and Atwood3 as the first soluble organolanthanide complexes in the 2+ oxidation state. Very soon these compounds gained much attention as versatile reagents for synthesis of organolanthanide complexes and catalysts. In the majority of relevant studies, lanthanide complexes with N- or O-donor ligands were synthesized and investigated,4 but in recent years, very interesting examples of lanthanide complexes with heavier group 15 based ligands were published, demonstrating that such soft ligands are quite applicable to this chemistry and often result in novel structures and properties.5 There are three common approaches to lanthanide (Ln) complexes with (formally) anionic ligands containing heavier group 15 element donor atoms:6 (a) salt methathesis reactions with the formation of alkali metal halogenides, (b) protonolysis reactions with the participation of phosphine or arsine as the proton donor, and (c) reactions based on reduction by Ln(II) precursors. The latter type of reaction exploits the high reduction potential of divalent lanthanides to reduce neutral ligands or their precursors to the anionic form. Syntheses © XXXX American Chemical Society

Received: January 10, 2017

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Organometallics Scheme 1. (a) Reduction by [SmCp*(THF)2] and (b) Reduction by [SmCp″2(THF)n] (n = 0 and 1)

reaction media on the formation of phosphide complexes of samarocenes.

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RESULTS AND DISCUSSION Reaction of [SmCp*2(THF)2] with P2Ph4: Structures of THF Ring-Opening Product. In 1996, Evans and co-workers published their study of reactions of [SmCp*2(THF)x] with EPh3 and E2Ph4 (E = P, As, Sb, and Bi; x = 0 and 2) which showed the reductive cleavage of E−C bonds (for EPh3 and E2Ph4, E = Sb and Bi) and E−E bonds (for all E2Ph4), leading to the formation of either [SmCp*2EPh2] (for P and As) or complex mixtures for heavier pnicogenide reagents.13 This reductive behavior of samarocenes(II) was shown to be analogous to that of alkali metals, which is known to result in the same products of E−E or E−C bond cleavage. An interesting feature of the phosphide and arsenide complexes is the instability toward ring-opening of coordinated THF molecule, which is caused by the intramolecular nucleophilic attack of the EPh2− on it. As a result, the formation of butoxide complexes [SmCp*2(O(CH2)4EPh2)] is observed (Scheme 1a). After repeating the reaction of [SmCp*2(THF)2] (1) with P2Ph4, we were able to crystallize this product (E = P) from pentane in the form of coordination polymer [SmCp*2(μO(CH2)4PPh2)]∞ (3a). In this complex, the bidentate phosphine-butoxide acts as a bridging ligand between two Sm centers. This is distinctive from the known structure of analogous arsine complex, which contains an uncoordinated arsine moiety and a THF molecule bound to the Sm atom.13 Our product does not contain an additional THF ligand (Figure 1), while the same complex reported by Evans contained one THF molecule per complex unit, as determined by elemental analysis and NMR. Supposedly, the THF ligand is lost during recrystallization from apolar solutions; its leaving could be facilitated by coordination of free phosphide group. Interestingly, while keeping the pentane mother liquor after separation of complex 3a at +4 °C for several weeks, a bunch of yellow-brown crystals of the dimeric complex [SmCp*2(μO(CH2)4PPh2)]2 (3b, Figure 2) were formed. The coordination mode of the phosphine-butoxide ligand in 3b is the same as that in 3a, but the structure is not polymeric. Instead, two units form a cycle. Upon recrystallization of 3a, the cold saturated mother liquor was decanted, which then yielded sufficient amounts (ca. 30% of the total yield) of 3b upon

Figure 1. Two units of the polymer chain of 3a (ORTEP representation of 50% probability level); hydrogen atoms are omitted. Methyl groups are clipped; only backbone is shown for second unit. Selected interatomic distances [Å] and angles [°]: Sm1−P1 3.1246(13), Sm1−O1 2.105(4), Sm1−Cp1 2.500(17), Sm1−Cp2 2.465(17), O1−Sm1−P1 83.92(12), Cp1−Sm1−Cp2 131.78(18). Cpi is the centroid of the corresponding C5-ring; primed atoms are symmetry equivalents.

standing. Thus, 3a can be considered a kinetic product, while 3b is a thermodynamically more stable isomer. The crystal structure of 3a consists of zigzag polymeric chains, formed owing to the μ:κ2O,P-coordination of the O(CH2)4PPh2− anion, which extend roughly along the a crystallographic axis. This coordination mode of the O(CH2)4ER2− (E = pnictide) leading to polymer structures is to date unique among not only for the rare earth elements but also for the transition metal complexes. The above-mentioned [SmCp*2(THF){O(CH2)4AsPh2}] complex features terminal (via the O atom) coordination of the alkoxide.13 In dimeric complex [LuCp2{μ-O(CH2)4PPh2}]2, the phosphoalkoxide is coordinated in μ,κ 2 O bridging fashion. 1 4 Complex [(AlCH2SiMe3)2{μ-O(CH2)4PPh2Cr(CO)5}2]2 is also worth mentioning as it represents the μ3:κ2O,κP- fashion of coordination, where the O atom bridges two hard ions Al3+ and the soft PPh2 terminus is coordinated to the soft Cr0 center.15 Besides the phosphido complexes, THF ring-opening in the coordination sphere of lanthanide complexes was described for several cases initiated by other nucleophilic B

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to unsolvated SmCp″2 is necessary for the samarocene to react with the P2Ph4, and the released THF molecules suppress further dissociation. When the reaction is carried out in hexane, the starting compounds completely disappear only after several repeated evaporation and addition of new portions of the solvent. The proposition of dissociative route of this reaction is further supported by the fact that this reaction does not proceed in THF or ether, where the samarocene cannot be desolvated, even at elevated temperatures: The starting compounds can be fully recovered from these reaction mixtures. In the case where [SmCp″2(Et2O)] is used instead of [SmCp″2(THF)], the reduction of diphosphine proceeds somewhat faster. The dissociation step can be eliminated by using base-free [SmCp″2]. We have found that the reaction of P2Ph4 with this complex runs to completion but still proceeds slowly in hexane (ca. 1 h at room temperature, ca. 10 min at 60 °C). For comparison, in the case of base-free [SmCp*2] this reaction occurs immediately.13 Irrespective of the starting complex taken, the phosphide [SmCp″2(PPh2)] (5) is formed in the reaction. Compound 5 was crystallized from concentrated solutions as green-brown dichroic plate-like crystals. In addition, noticeable amounts of byproducts are always formed: yellow block-shaped crystals of [SmCp″3] (6) and dark-brown rhombus-shaped plates of [SmCp″2(μ-PPh2)2SmCp″] (7, Scheme 1b). The formation of complexes 6 and 7 is not influenced by the choice of starting Sm complex, reaction time, or temperature, so we could not suppress their formation. By changing the solvent to toluene, the reactions of base-free [SmCp″2] with P2Ph4 at 60 °C proceed visually faster. After the compounds are allowed to react under these conditions for 2 days, compound 7 is practically absent in the reaction mixture, implying that some decomposition of this product may occur. Mononuclear products 5 and 6 are highly soluble in alkanes and tend to crystallize together (see Figure S1), which precluded their separation. We could only manually separate some large crystals of the two complexes obtained by slow cooling of concentrated hexane or pentane solutions. Complex 7 is less soluble in alkanes and crystallizes first upon storing the cold solutions. Single crystals of all three products 5−7 suitable for X-ray diffraction were picked from crystallized reaction mixtures. For comparison purposes, [SmCp″3] was independently synthesized by a reaction of SmI3 with 3 equiv of KCp′′ in Et2O. The synthesis results in a pure product, but the yield does not exceed 50% even after prolonged heating of the reaction mixture (2 weeks). The analogous reaction of SmCl3 with 3 equiv of KCp′′ only proceeds to doubly substituted compound [Cp″2SmCl] (see Supporting Information for details). It is noteworthy that the silylated congener of 6, Sm(C5H3(SiMe3)2)3, has been obtained from SmCl3 and K(1,3(SiMe3)2C5H3) with almost quantitative yield.21 Crystal Structures of Complexes 5−7. The crystal structure of 5 consists of mononuclear complexes [Cp″2SmPPh2], which retain the main structural features of bent metallocenes (Figure 3). The Sm−C distances vary within the range of 2.6475−2.7265 Å with the average value of 2.69(3) Å. The Sm−P distance equals to 2.7957(5) Å. The Sm−P distances in the related bis(pentamethylcyclopentadienyl)(2,3,4,5-tetramethylphospholyl)samarium [Cp*2Sm(PC4Me4)], reported by Nief and Ricard, is slightly longer than those in 5 (2.856(1) and 2.891(1) Å for two crystallographically independent molecules).22 However, the Sm−P distance is substantially shorter than the Sm−As distance (avg 2.970 Å) in [Cp*2SmAsPh2], possessing the similar coordination environ-

Figure 2. Molecular structure of 3b (ORTEP representation of 50% probability level); hydrogen atoms are omitted. Methyl groups are clipped. Selected interatomic distances [Å] and angles [°]: Sm1−P1 3.0458(8), Sm1−O1′ 2.1128(17), Sm1−Cp1 2.4917(9), Sm1−Cp2 2.4792(7), O1′−Sm1−P1 81.61(6), Cp1−Sm1−Cp2 134.275(12). Cpi is the centroid of the corresponding C5-ring; primed atoms are symmetry equivalents.

ligands, such as H−,16 Cp*−,17 and cyclopentaphosphido ligand entering the Ln coordination sphere.18 The Sm atoms in both 3a and 3b are eight-coordinated. The Cp* ring centroids and O and P atoms form approximately a tetrahedral arrangement around the central atom. The coordination core in the centrosymmetric dimer 3b is fairly similar to that in 3a. The angles between Cpcentr−Sm−Cpcentr and O−Sm−P planes equal 89.4 and 89.8° in 3a and 3b, correspondingly. The value of the Sm−O−C angle in both 3a (155.8(4)°) and 3b (162.97(15)°) is smaller than that in analogous complex [SmCp* 2 (THF)(O(CH 2 ) 4 AsPh 2 )] (175.4(4)°).13 This may be due to the steric hindrance caused by the coordination of the bulky PPh2 moiety of the butoxide ligand (see ref 19). As a result, the Sm−P distances with neutral phosphine ligand in 3a (3.1246(13) Å) and 3b (3.0458(8) Å) are noticeably longer than those in seven-coordinated complex 5 with terminal PPh2− (2.7957(5) Å) and in eight-coordinated complex 7 with bridging PPh 2 − ligands (2.9662(9)− 2.9910(10), vide inf ra). Reaction of [SmCp″2] with P2Ph4. The use of differently substituted Cpx ligands in samarocenes results in differences in steric crowding and hence changes in solvatation degree and reductive reactivity of the complexes. Although Cp′′ ligands were shown to take a slightly smaller cone angle than Cp* (116.2 vs 122.4° for (η7-C7H7)ZrCpx complexes),20 the two Cp′′ ligands in SmCp″2 leave space for only one additional ethereal ligand L (L = THF or Et2O) as compared with two of such ligands in [SmCp*2L2] because the tBu groups protrude further away from the Cp planes and occupy a larger part of the sphere around the Sm atom. Owing to such shielding, the [SmCp″2L] complexes should be less reactive in the reduction processes as compared with their Cp* analogs. Indeed, the reaction of [SmCp″2(THF)] (4) with P2Ph4 proceeds noticeably more slowly than that of [SmCp*2(THF)2] (1) with the same diphosphine. When carried out in hexane or toluene, the reaction is not finished after several hours at room temperature, and even after heating to 60 °C for 30 min, the solution contains unreacted samarocene. Supposedly, initial dissociation C

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Sm−C distances correspond to the least sterically crowded carbon atoms in the 4 and 5 positions of the cyclopentadienyl cycle. The average Sm−C distance (2.80 Å) is slightly longer than that in [Sm(1,3-(Me3Si)2C5H3)3] (2.76 Å), which is in agreement with the larger steric cone angle of the Cp′′ ligand.23 The mean Sm−Cpcentr distance in 6 is 2.53 Å. The centroids of the three Cp′′ ligands define an almost regular triangle with the angles Cp centr −Sm−Cp centr 119.72(6), 120.00(6), and 120.29(6)°. The tert-butyl substituents define a regular prism, and the dihedral angle between two triangles of the quaternary C atoms is only 0.6° compared to 0° for a perfect trigonal prism. The crystal structure of dimeric complex 7 is somewhat less ordinary and contains two crystallographically independent molecules of the complex with close geometries (Figure 5, see

Figure 3. Molecular structure of 5 (ORTEP representation of 50% probability level); hydrogen atoms are omitted. Methyl groups are clipped. Selected interatomic distances [Å] and angles [°]: Sm1−P1 2.7957(5), Sm1−Cp1 2.408(17), Sm1−Cp2 2.405(17), Cp1−Sm1− Cp2 128.51(6), Cp1−Sm1−P1 114.88(6), Cp2−Sm1−P1 116.34(5). Cpi is the centroid of the corresponding C5-ring.

ment.13 However, short (2.901−3.117 Å) Sm−C contacts between the Sm atom and the phenyl ring of the AsPh2 moiety were observed in the structure of [Cp*2SmAsPh2]. On the contrary, no such contacts were found in the structure of 5, while the PPh2 group is coordinated in symmetrical fashion. The Sm atom lies about 0.07 Å above the plane defined by the centers of two cyclopentadienyl rings and the P atom. The angle Cipso−P−Cipso equals 102.68(9)°; the angles of Cpcentr− Sm−P are roughly equal (116.34(5) and 114.88(6)°). The sum of the angles at the P atom is 357.1°. Two phenyl rings are tilted from this plane by 23.9 and 69.2°. The crystal structure of 6 is isomorphous with those published for its analogues [CeCp″3],23 [YbCp″3],24 and [ThHCp″3]25 (Figure 4). On the contrary, 6 is not isostructural

Figure 5. Molecular structure of 7 (ORTEP representation of 50% probability level); hydrogen atoms are omitted. Methyl groups are clipped; one of two independent molecules is shown. Selected interatomic distances [Å] and angles [°] (corresponding values for the second molecule are given in square brackets): Sm1−P1 2.9662(9) [2.9910(10)], Sm1−P2 2.9880(10) [2.9770(10)], Sm2−P1 3.0897(12) [3.0551(18)], Sm2−P2 3.1247(11) [3.1497(17)], Sm2′−P1 3.161(2) [3.234(3)], Sm2′−P2 3.027(2) [3.084(2)], Sm1−Cp1 2.496(3) [2.481(4)], Sm1−Cp2 2.467(3) [2.476(4)], Sm2−Cp3 2.491(5) [2.484(10)], Sm2′−Cp3′ 2.485(15) [2.498(17)], Sm2−C21 2.907(4) [2.854(4)], Sm2−C26 2.818(4) [2.769(5)], Sm2′−C31 2.736(5) [2.845(4)], Sm2′−C32 2.542(5) [2.715(5)], Cp1−Sm1−Cp2 138.96(12) [139.32(12)], P1−Sm1−P2 73.28(3) [75.99(3)], P1−Sm2−P2 69.75(3) [72.59(4)], P1−Sm2′−P2 70.06(4) [71.06(5)]. Cpi is the centroid of the corresponding C5ring; second positions of Sm2 and Cp3 are denoted as Sm2′ and Cp3′.

Figure 4. Molecular structure of 6 (ORTEP representation of 50% probability level); hydrogen atoms are omitted. Methyl groups are clipped. Selected interatomic distances [Å] and angles [°]: Sm1−Cp1 2.526(2), Sm1−Cp2 2.541(2), Sm1−Cp3 2.527(2), Cp1−Sm1−Cp2 120.29(6), Cp1−Sm1−Cp3 120.00(6), Cp2−Sm1−Cp3 119.72(6). Cpi is the centroid of the corresponding C5-ring.

caption for distances and angles). Each of these contains two PPh2− anions and two Sm atoms forming a distorted square. One Sm atom is further coordinated by two Cp′′ ligands, and the other is coordinated by one Cp′′ ligand. Evidently, they are Sm 3+ and Sm 2+ ions (Sm1 and Sm2 on Figure 5), correspondingly. The presence of two different ions correlates with the Sm−P bond lengths. The SmIII−P distances (2.9662(9)−2.9910(10) Å) are substantially longer that those in 5. This is due to both the larger coordination number and the bridging nature of the PPh2− anions in 7. In turn, the SmII− P distances (3.027(2)−3.234(3) Å) are even longer due to a larger radius of Sm2+ ion as compared to that of Sm3+.26 Furthermore, the {SmIICp″} moiety in each molecule is split over two positions with the occupancies ratio of 0.74:0.26 for

to its trimethylsilyl substituted analog [Sm(1,3(Me3Si)2C5H3)3],21 although the latter crystallizes in the same monoclinic crystal system. The Sm−C bond lengths in 6 vary within the wide range of 2.685(2)−2.914(2) Å. The carbon atoms of the cyclopentadienyl rings located between the tert-butyl substituents, i.e., 2 position of the cyclopentadienyl cycle, are the most distant from the metal atom. The shortest D

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Organometallics Scheme 2

Scheme 3

Another possibility is the initial coordination of two samarocene units to the both P atoms of the diphosphine (Scheme 3). Intermediate [(SmCp″2)2P2Ph4] formed during this route can be unstable due to the steric crowding of the ligands; compared to 7, it possesses one more Cp′′ ligand on a Sm atom, for which there is insufficient space in the Sm coordination sphere. Hence, this intermediate can undergo either reductive dissociation of the P−P bond to form two molecules of 5 or elimination of one Cp′′ radical to form 7 (the radical is captured by another molecule of [SmCp″2] to form 6). This route does not involve an electron transfer between two Sm atoms and thus more likely corresponds to this reaction. However, it cannot be excluded that with more reactive and less sterically encumbered samarocenes the reaction may follow the first route (Scheme 2), while the stage of liberation of Cpx radical can be less pronounced depending on the Sm−Cpx bond strength. Note that the first route requires only two molecules to be combined for the reaction, while the second route (Scheme 3) requires association of three molecules (two samarocene and one diphosphide), which could be a kinetic reason for a slower rate of the latter processes. This can explain the slower reaction of P2Ph4 with SmCp″2, as compared with a faster reaction with [SmCp*2], where no products analogous to 6 or 7 were observed.13 The above-mentioned absence of product 7 in the case where the reaction is carried out in toluene at elevated temperatures can be explained by successive decomposition of 7 to 5. The leaving SmII phosphide species may remain as oligomeric complexes [SmCp″PPh2]n. We also note that ligand redistribution under any reaction conditions cannot be excluded, analogous to that reported for Sm carbene complexes SmCp′2Cl(C(NiPr)2(CMe)2) (Cp′ = η5-C5H4tBu).28 This process should result in [SmCp″3] and more phosphorus-rich complexes, e.g., bis(diphenylphosphido) species [SmCp″(PPh2)2]. Although the latter were not observed in our case, this can be explained by their instability with respect to the reduction process leading to Sm2+ species. It

one molecule and 0.60:0.40 for the other. In both positions, the coordinatively unsaturated divalent Sm atom has short contacts with one of the four phenyl rings, while one of two Ph cycles of different PPh2 groups has contacts with a given position of Sm2+ ions. These contacts make two Ph rings η2-coordinated to a Sm2+ ion. The same η2-type of coordination is known for arsenido complex [SmCp*2(AsPh2)].13 Other types of Ph rings’ coordination to Ln2+ and Ln3+ ions (Ln = Sm and Yb) are known for complexes with BPh4− anions.27 In one of the two independent molecules, the SmII−P bond supported by the coordination of Ph ring is significantly shorter than the unsupported one; the largest difference between adjacent SmII−P bond lengths for all four independent Sm2+ positions is 0.15 Å. Reaction Sequences. The structures and the reaction behavior of the compounds can give some clue on the possible routes of their formation. Products 5−7 are sufficiently stable under reaction conditions and do not further react into other species, which is seen by a nearly unchanged composition after different reaction times. Hence, none of these compounds can be considered as an intermediate to others. The reduction of diphosphine must be preceded by coordination of a P to a coordinatively unsaturated Sm atom. There is a possibility that the mononuclear intermediate thus formed, [SmCp″2P2Ph4], undergoes an intramolecular redox process, and a radical is released, either ·PPh2 or ·Cp′′ (Scheme 2). These radicals can be further captured by another equivalent of [SmCp″2] to form complexes 5 and 6, correspondingly. Complex [SmIIICp″(PPh2)2] formed simultaneously with a Cp′′ radical can also combine with [SmCp″2] to give a complex with the formula [Sm2Cp″3(PPh2)2]. This complex is the same in composition to 7, but the Sm atoms have inverse oxidation states in the reacting species, i.e., 2+ in SmCp″2 and 3+ in [SmCp″(PPh2)2] while in 7 the oxidation states are 3+ in {SmCp″2} and 2+ in {SmCp″} moieties. Thus, an intramolecular electron transfer between the Sm atoms is necessary for this route to form 7. E

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Figure 6. Electron localization function (ELF) isosurface (gray) at 0.93 value and ELF map for (a) SmCp2(PPh2) and (b) SmCp2(PPh2)(THF) complexes.

is well-established that PPh2− and PtBu2− anions are capable of reduction of Sm3+, Eu3+, and Yb3+ into the corresponding Ln2+ phosphido complexes.29 As a result, a mixture of 5, 6, and 7 could represent the equilibrium state, which could further be proven by observing the behavior of mixtures of separated compounds at different ratios and reaction conditions. Electronic Structure Analysis. As proposed above, phosphido complex 5 cannot be synthesized in THF from the divalent samarocene because of the steric blocking of Sm reaction site by THF ligands. Once synthesized in hexane, this complex is stable in THF solutions and does not show any reactivity that can be attributed to THF ring-opening or ligand disproportionation. Since phosphide 5 is stable in THF by itself while analogous complex 2 with coordinated THF is prone to breaking of the Sm−P bond, it could be questioned how the presence of an O-donor ligand in the coordination sphere changes the stability of Sm−P bond. In order to analyze the difference in bonding patterns, we performed quantum chemical calculations of model complexes with unsubstituted cyclopentadienyl ligands without and with a coordinated THF molecule, [SmCp2(PPh2)] and [SmCp2(PPh2)(THF)]. Due to the high spin state for the most stable form (Sz = 5/2) of the investigated complexes, we calculated the spin density and integrated it over QTAIM basins. Resulting spin density is primarily localized on lanthanide with 5.45 and 5.35 unpaired electrons on the Sm atom for [SmCp 2 (PPh 2 )] and [SmCp2(PPh2) (THF)] complexes, respectively (Figure S3). The analysis of bonding patterns was done with electron localization function (ELF) and QTAIM topological methods. The analysis of ELF function shows that the PPh2 and SmCp2 fragments are linked by ionic interactions with the absence of disynaptic attractor on the Sm−P line (Figure 6) with some exchange between the Sm and P atoms.30 QTAIM bond critical point (bcp) on the Sm−P line is characterized by total energy density H ≈ 0 and ratio of absolute value of potential energy density to kinetic energy density |V|/G ≈ 1;31 it could be classified as an ionic bond and thus confirms results of the ELF analysis (Figure S4 and Table S2). Two monosynaptic V(P) valence attractors related with two lone pairs of the PPh2− anion are directed toward the Sm atom. Four valence attractors (2 × V(C, P), 2 × V(P)) form a tetrahedral environment around the P atom and apparently correspond to four sp3 hybrid orbitals (Figure 6a). The population of two V(P) basins are equal (1.79e) in [SmCp2(PPh2)]. The addition of THF

leads to distortion of the lone pairs of the P atom (Figure 6b), and the population of V(P) basins becomes different (1.63e and 2.04e). Consequently, the Sm−P distance becomes longer in [SmCp2(PPh2)(THF)] as compared to that of the base-free complex (Tables S3 and S4). The energy of Sm−P bond was calculated by the formula ESm−P = EComplex − ESmfrag − EPPh2 (Table S5). As a result, the Sm−P bond in [SmCp2(PPh2)(THF)] is 8.52 kcal/mol weaker than that in [SmCp2(PPh2)]. The weakness of the Sm−P bond in the complex with coordinated THF could explain increased nucleophilic activity of the phosphide anion and consequent opening of the THF ring followed by formation of coordination polymer. Note that the THF ligand in [SmCp2(PPh2)(THF)] is almost identical to free THF by geometrical, energetic, and topological parameters. Thus, in addition to bringing the reaction sites closer upon coordination of THF, electronic effects resulting from the coordination of a THF molecule to the Sm center weaken the Sm−P bond in the phosphido complex. Considering the unsubstituted Cp and Ph rings of the model molecules, the steric effects do not play an important role, although it could be supposed that increase of the steric bulk of the ligands could lead to increased separation of Sm and P center, resulting in even larger nucleophilic activity of the phosphido anion. For more detailed understanding of the THF ring-opening process, the calculation of transition states could be needed.

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CONCLUSION Stable lanthanide diphenylphosphido complexes can be obtained by reduction of corresponding diphosphides by lowvalent Ln2+ species, in our case, substituted samarocenes(II). With this example and earlier contributions by other groups, we observed several factors which influence the process of oneelectron reduction with successive binding of the reduced phosphido species to a lanthanide center. Such reactions are often solvent-dependent; coordinative solvents can block the reaction site so that the reaction will not proceed or that it is be slowed down. In other cases, coordinative solvents can influence the product formation, e.g., if the solvent causes dissociation of coordinatively bound dimeric (oligomeric) products.9b Steric hindrance of the Ln reaction center plays a very important role, as the spectator ligands can influence the accessibility of the central atom or even the reactivity of the reduced phosphido species that enter the Ln coordination sphere.18 Finally, the reduction of diphosphides by Sm2+ F

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Organometallics

CH2), 3.49 (s, 2H, CH2), 3.33 (br s, Δν1/2 = 18 Hz, 2H, CH2), 3.18 (br s, Δν1/2 = 18 Hz, 2H, CH2), 2.08 (br s, Δν1/2 = 10 Hz, 30H, Me). After addition of a small amount of THF, the spectrum changes to that reported for the complex with coordinated THF molecule and free PPh2 group, [SmCp*2(THF)(O(CH2)4PPh2)].13 IR νmax (cm−1) 3060m, 2959vs, 2903s, 2865s, 1963w, 1891w, 1816w, 1653w, 1592w, 1478m, 1462m, 1437m, 1390w, 1362s, 1252m, 1228w sh, 1200w, 1157s, 1131s, 1069w, 1041m, 1021m, 999w, 932m br, 896w, 806s, 746s, 726m, 696s, 666w, 558s. MS m/z (rel. intensity, ion) [calculated for C36H48OPSm+ 679.256]: 560.1 (50, MO+ − Cp*), 544.1 (100, M+ − Cp*), 438.1 (18, SmOCp*2+), 422.1 (18, SmCp*2+), 287.0 (52, SmCp*+), 136.1 (8, HCp*+), 121.1 (18, HCp*+ − CH3), 106.1 (13, HCp*+ − 2CH3). Anal. of 3b, found: C, 63.45; H, 7.0; Sm, 22.5; P, 4.5%. Calcd for C36H48OPSm: C, 63.76; H, 7.13; Sm, 22.2; P, 4.57%. Reactions of [SmCp″2(THF)] with P2Ph4. No reaction was noticed when [SmCp″2(THF)] (0.105 g, 0.182 mmol) and P2Ph4 (0.034 g, 0.092 mmol) were mixed in 10 mL of THF in a closed evacuated vessel at 80 °C for 6 h. The mixture reserved the initial color, and the starting compounds crystallized upon concentrating and cooling of the solution. When the same quantities of reagents were stirred in 10 mL of hexane in a closed evacuated vessel, the reaction proceeded very slowly at room temperature and somewhat faster upon heating. After 30 min of reaction at 50 °C, concentration and crystallization resulted in a mixture of 5 (green-brown dichroic platelike crystals, main product) and small amounts of 6 (yellow blocks) and 7 (dark-brown rhombus-shaped crystals, Figure S1); approximately half of the starting samarocene remained unreacted. Reaction of [SmCp″2(Et2O)] with P2Ph4. To a mixture of [SmCp″2(Et2O)] (0.102 g, 0.176 mmol) and P2Ph4 (0.033 g, 0.089 mmol) was added 10 mL of hexane. The solution was stirred in a closed vessel at 70 °C for 3 h and then evaporated to dryness in vacuum. The solids were redissolved in 10 mL of hexane, and the solution was held for another 3 h at 70 °C. The resulting solution was evaporated to dryness in vacuum. Slow extraction of the solid with pentane in a sealed right-angle-bent ampule resulted in separate crystallization of dark-brown crystals of [SmCp″2(μ-PPh2)2SmCp″] (7) in the middle part of the ampule, and a mixture of brown crystals of [SmCp″2PPh2] (5) and yellow ones of [SmCp″3] (6) in the bottom part. Complex 5 is visually the predominant product; the combined yield of 5 and 6 is 0.096 g. The crystals of pure 7 were separated (yield ca. 15 mg) and used for analyses, IR, and mass spectra. For NMR, a mixture of 5, 6, and 7 was used; the known signals of 5 and 6 were subtracted from the spectrum (see Suporting Information). 1H NMR: 19.82, 16.89, 16.06, 15.82 (all br s with Δν1/2 = 18 Hz, C−H of Cp′′ rings), 10.10 (br d, Δν1/2 = 12 Hz, p-Ph), 7.38 and 7.02 (both m, oand m-Ph), −3.10 and −3.69 (both s, tBu). IR νmax (cm−1) 3069m, 2962vs, 2905m, 2868m, 1958w, 1887w, 1814w, 1579m, 1480w sh, 1464m, 1438m, 1392w, 1361s, 1302w, 1252s, 1203m, 1168m, 1133w, 1055w, 1027w, 934m, 892m, 807s, 732s, 698s, 587m br. MS m/z (rel. intensity, ion) [calculated for C63H83P2Sm2+ 1202.421]: 848.3 (10, Sm2(PPh2)2Cp″+), 612.3 (20, SmPPhCp″2+ − 2H), 584.2 (24, SmPPhCp″2+ − 2CH3), 546.2 (20, SmPPhCp″2+ − C5H8), 531.2 (20, SmPPhCp″2+−C5H8−CH3), 506.2 (100, SmCp″2+), 370.1 (38, P2Ph4+), 329.0 (80, SmCp″+), 313.0 (45, SmCp″+−CH4), 299.0 (12, SmCp″+ − 2CH3), 186.0 (80, HPPh2+), 162.1 (23, Cp″+ − CH3), 147.1 (35, HCp″+ − 2CH3), 108.0 (86, PPh+). Anal. for C63H83P2Sm2, found: C, 62.8; H, 6.9%. Calcd: C, 62.90; H, 6.95%. Reaction of Desolvated [SmCp″2] with P2Ph4. To a mixture of [SmCp″2] (0.152 g, 0.300 mmol) and P2Ph4 (0.0560 g, 0.151 mmol) was condensed 10 mL of hexane. The solution turned brown after mixing at ambient temperature for 1 h; it was further stirred at 60 °C for 2 days to ensure completeness of the reaction. The brown solution was evaporated to dryness. The oily crystalline mixture was redissolved in pentane, and the solution was filtered and slowly evaporated in a closed ampule to yield a mixture of crystals of compounds 5, 6, and 7. The crystalline mixture was washed with 2 mL of cold pentane to remove oily impurities; the crystals were dried in vacuum. Complex 5 was manually separated as green-brown large crystalline aggregates, contaminated with minor amounts of 6. Visually homogeneous and

complexes may lead to complex reaction pathways since the corresponding phosphido anions are rather strong reductants themselves and can, on the contrary, reduce some Ln3+ cations to Ln2+ state (for Sm, Eu, and Yb).29 Hence, in order to apply Ln2+ complexes for reduction of P-based species, stronger reductants may be more useful, like the lately described “unusual” lanthanate anions [LnCpx3]− (Ln = all lanthanides).10b−d,32 In contrast, more preferable for reductive chemistry could be phosphorus compounds whose reduced forms are more stable due to delocalization (in case of heteroconjugated systems), intramolecular bond formation, or coordination to a transition metal, similar to the known examples of reduction of transition metal polyphosphides.9a,e

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EXPERIMENTAL SECTION

All operations were carried out in evacuated vessels or ampules. The compounds were handled in an argon glovebox. Starting compounds SmI2 and [SmCp*2(THF)2],3b [SmCp″2(THF)],33 and 313 were synthesized according to published procedures; other reagents were obtained from commercial sources and used as received. Solvents were distilled in inert atmospheres over common drying agents, stored with the addition of Na−K alloy prior to the use, and transferred in vacuum. 1 H NMR spectra (δ, ppm) in C6D6 solutions were recorded by means of a Bruker Avance 500 MHz NMR spectrometers at room temperature. Chemical shifts are referenced to the solvent resonance (1H: 7.16) and are reported relative to SiMe4. IR spectra were recorded in KBr pellets by means of a FT-801 Fourier spectrometer (Simex). Elemental analysis for C and H was carried out by means of a Vario Micro Cube analyzer (Elementar). For Sm and P analyses, the samples were dissolved in hot concentrated HNO3, the solutions were diluted and analyzed by means of a iCAP-6500 optical emission spectrometer (Thermo Scientific). Mass spectra were measured on a DFS ThermoFisher spectrometer with EI ionization (70 eV, source temperature 150 °C, positive region). Synthesis of [SmCp″2(Et2O)] and [SmCp″2]. The procedure was similar to that for published complexes [YbCp″2]34 and [SmCp*2].35 To the mixture of base-free SmI2 (0.612 g, 1.51 mmol) and KCp′′ (0.659 g, 3.05 mmol) was condensed ca. 20 mL of Et2O. The vessel was closed, and the mixture was stirred for 1 day with periodical activation in an ultrasonic bath. The dark gray-brown solution was filtered, and the filtrate was slowly evaporated to dryness. The solid product was recrystallized from Et2O to yield dark brown-green elongated crystals of [SmCp″2(Et2O)]. Yield 0.569 g (65%). 1H NMR: 18.10 (br m, Δν1/2 = 108 Hz, 4H, 4- and 5-C−H), 12.1 (br s, Δν1/2 = 280 Hz, 36H, tBu), −0.06 (m, 6H, OCH2Me), −3.76 (m, 4H, OCH2Me), −12.8 (br s, Δν1/2 = 250 Hz, 2H, 2C−H). Anal. for C30H52OSm, found: C, 62.0; H, 8.4; Sm, 26.2%. Calcd: C, 62.22; H, 9.05; Sm, 25.96%. The solvent-free complex was made according to the published procedure of repeated refluxing in toluene with slow evaporation of the solvent mixture (3 times) to yield dark-green crystalline powder, which was recrystallized from pentane.13 Yield of [SmCp″2] 52%. 1H NMR: 22.55 (br s, Δν1/2 = 510 Hz, 4H, 4C−H and 5C−H), 10.35 (br s, Δν1/2 = 132 Hz, 36H, tBu), −12.84 (br s, Δν1/2 = 92 Hz, 2H, 2C−H). Anal. for C26H42Sm, found: C, 61.6; H, 8.5; Sm, 29.5%. Calcd: C, 61.84; H, 8.38; Sm, 29.78%. Synthesis and Crystallization of [SmCp*2(μ-O(CH2)4PPh2)] (3). Similar to the procedure of Evans et al.,13 the reaction was carried out by stirring of a mixture of P2Ph4 (0.079 g, 0.21 mmol) and [SmCp*2(THF)2] (0.241 g, 0.426 mmol) in 10 mL of toluene at 80 °C; after 7 days, the yellow-brown solution was evaporated to dryness, the solids repeatedly extracted with 10 mL of pentane in a sealed rightangle-bent ampule, the solution cooled to 4 °C, and the precipitated yellow crystals of 3a separated (yield ca. 0.025 g, 9%). The mother liquor was held at 4 °C for 2 weeks, during which time large orange crystals of 3b formed (yield 0.096 g, 33%). Both types of crystals were used for X-ray diffraction. 1H NMR: 6.73 (br m, Δν1/2 = 15 Hz, 2H, Ph p-H), 6.67 and 6.48 (both br m, Δν1/2 = 17 and 32 Hz, 4H and 4H, correspondingly; Ph o-H and m-H), 5.55 (br s, Δν1/2 = 25 Hz, 2H, G

DOI: 10.1021/acs.organomet.7b00014 Organometallics XXXX, XXX, XXX−XXX

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Organometallics free of impurities crystals were used for characterization. Compound 5: yield 0.114 g, 55%. 1H NMR: 17.3 (br s, Δν1/2 = 500 Hz, 2H, 2C−H), 14.27 (br s, Δν1/2 = 46 Hz, 4H, 4C−H and 5C−H), 9.16 (d, J = 7 Hz, 4H, o-Ph), 7.69 (t, J = 7 Hz, 2H, p-Ph), 7.43 (t, J = 7 Hz, 4H, m-Ph), 2.34 (br s, Δν1/2 = 182 Hz, 36H, tBu). IR νmax (cm−1) 3067m, 2966s, 2907m, 2870m, 1958w, 1885w, 1817w, 1580m, 1480w sh, 1464s, 1438m, 1393w, 1362s, 1299w, 1253m, 1204m, 1168m, 1132m, 1057m, 1026m, 1001w, 929s, 892m, 805s, 748s, 731m sh, 697s, 682w sh, 665w, 644s, 558w. MS m/z (rel. intensity, ion) [calculated for C38H52PSm+ 691.293]: 650.2 (5, M+ − C3H7), 586.2 (13, M+ − Ph − 2CH2), 546.1 (22, M+ − Ph − C5H8), 531.1 (70, M+ − Ph − C5H8 − CH3), 508.2 (100, SmCp″2+), 370.1 (90, P2Ph4+), 329.1 (80, SmCp″+), 313.0 (60, SmCp″+ − CH4), 299.0 (12, SmCp″+ − 2CH3), 185.1 (80, PPh2+), 147.1 (27, HCp″+ − 2CH3), 108.0 (73, PPh+), 57.1 (15, Bu+). Anal. for C38H52PSm, found: C, 66.5; H, 7.8%. Calcd: C, 66.13; H, 7.59%. Yellow crystals of 6 were manually separated and used for X-ray diffraction and IR; their IR spectrum is identical with that of separately synthesized [SmCp″3]. Compounds 5 and 6 remain unchanged after dissolution of the mixture in THF and successive evaporation of the solvent. Synthesis of [SmCp″3] (6). A mixture of SmI3 (0.110 g, 0.207 mmol) and KCp′′ (0.141 g, 0.652 mmol) in 10 mL of diethyl ether was heated in a sealed evacuated vessel for 14 days at 80 °C. The solution was cooled, filtered, and evaporated to dryness. The crystalline solid was extracted with 10 mL of pentane; the orange solution was filtered from unreacted KCp′′. Slow evaporation of the filtrate yielded 6 in the form of orange elongated rectangular crystals, which were washed with 2 mL of pentane, dried in vacuum, and separated from the minor amounts of KCp′′ which sticks to the walls as small colorless crystals. Yield 0.060 g (42%). 1H NMR: δ 21.14 (br s, Δν1/2 = 15 Hz, 3H, 2-C−H), 18.64 (br s, Δν1/2 = 21 Hz, 6H, 4C−H and 5C−H), −1.57 (s, 54H, tBu). IR νmax (cm−1) 3088w, 2964vs, 2905s, 2870s, 1617w, 1464s, 1390w, 1360s, 1299w, 1253s, 1203m, 1168m, 1057w, 1024w, 930m, 817s br, 743s, 677w sh, 662m, 610w. MS m/z (rel. intensity, ion) [calculated for C39H63Sm+ 683.405]: 506.2 (100, SmCp″2+), 329.0 (76, SmCp″+), 313.1 (16, SmCp″+ − CH4), 297.0 (5, SmCp″+ − 2CH4), 147.1 (12, HCp″+ − 2CH3), 121.0 (12, HCp″+ − Bu), 107.1 (11, HCp″+ − Bu − Me), 92.1 (8, HCp″+ − Bu − 2Me), 57.1 (8, Bu+). Anal. for C39H63Sm, found: C, 68.4; H, 9.2; Sm, 22.3%. Calcd: C, 68.65; H, 9.31; Sm, 22.0%. Single Crystal X-ray Diffraction. Diffraction data for the single crystal of 3a were collected at 200 K on a Bruker Duo diffractometer with an APEX II CCD detector (graphite monochromator, λ(Mo Kα) = 0.71073 Å, ϕ-scanning). Frame data were acquired with SMART software. Cell constants were obtained from the complete data set; frame data were integrated using SAINT. Absorption correction was made using the program SADABS.36 Diffraction data for the single crystals of 3b and 5−7 were collected at 130 K on an automatic Agilent Xcalibur diffractometer equipped with a two-coordinate AtlasS2 detector (graphite monochromator, λ(Mo Kα) = 0.71073 Å, ω-scanning). Data processing, absorption correction, and determination of unit cell parameters were carried out using the CrysAlisPro software package.37 The structures were solved by the direct methods and refined by least-squares methods against F2 using the SHELX2014 program suite.38 Hydrogen atoms were calculated on idealized positions and refined with a riding model. Details of data collection, solution and refinement are given in Table S1. Complete crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (CCDC 1515326−1515330 for compounds 3a, 3b, and 5−7, respectively) and may be obtained free of charge from the CCDC, 12 Union Road, Cambridge, CB2 1EZ, United Kingdom (fax, + 44-1223-336033; http://www.ccdc.cam.ac.uk/conts/retrieving. html). Computational Details. A geometry optimization of Sm complexes was performed within Gaussian 09 suite with B3LYP39 density functional and Stuttgart/Dresden basis set RSCSeg40 for Sm atom and LanL2DZ basis set for the rest of the atoms in the complex.41 In order to simplify calculations, two Cp′′ groups were replaced by two cyclopentadienyl (Cp) groups. The 1/2, 3/2, 5/2, and 7/2 spin states were calculated for both complexes (Tables S2 and S3).

Optimized structures are characterized by no imaginary frequencies at B3LYP/LanL2DZ level of theory. The most stable is the Sz = 5/2 state for both [SmCp2(PPh2)] and [SmCp2(PPh2)(THF)] complexes, the geometries of which were used for subsequent calculations. The singlepoint calculations with geometry optimized at B3LYP/LanL2DZ level of theory were performed with all-electron TZP42 basis set and B3LYP39c density functional in ADF201643 program suite. The scalar relativistic effects were taken into account with zeroth order regular approximation (ZORA) approach.44 The ELF,45 electron density, and spin density were calculated and analyzed in DGrid 4.646 with 0.0265 Å (0.05 au) mesh step.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00014. Additional IR and mass spectra data, details of synthesis and quantum chemical calculations, images and description of crystalline products behavior, and full details of X-ray crystallographic studies (PDF) Crystallographic information file for compounds 3a, 3b, 5−7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nikolay A. Pushkarevsky: 0000-0001-8668-6362 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Russian Science Foundation (project No. 14-23-00013) for the financial support. REFERENCES

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DOI: 10.1021/acs.organomet.7b00014 Organometallics XXXX, XXX, XXX−XXX