Amido Analogues of Nonbent Lanthanide (II) and ... - ACS Publications

Jan 29, 2015 - Nizhny Novgorod State University, Gagarina 23, 603950, Nizhny Novgorod, Russia. §. Institute of Organoelement Compounds of Russian ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/Organometallics

Amido Analogues of Nonbent Lanthanide (II) and Calcium Metallocenes. Heterolytic Cleavage of π‑Bond Ln−Carbazolyl Ligand Promoted by Lewis Base Coordination Alexander N. Selikhov,†,‡ Anton V. Cherkasov,†,‡ Georgy K. Fukin,†,‡ Alexander A. Trifonov,*,†,‡,§ Iker del Rosal,∥ and Laurent Maron*,∥ †

Institute of Organometallic Chemistry of Russian Academy of Sciences, 49 Tropinina str., GSP-445, 630950, Nizhny Novgorod, Russia ‡ Nizhny Novgorod State University, Gagarina 23, 603950, Nizhny Novgorod, Russia § Institute of Organoelement Compounds of Russian Academy of Scienes, Vavilova str. 28, 119334, Moscow, Russia ∥ Université de Toulouse, INSA, UPS, CNRS-UMR5215, LPCNO, Avenue de Rangueil 135, 31077 Toulouse, France S Supporting Information *

ABSTRACT: Introduction of four tBu groups into a carbazol-yl framework leads to switching of the metal−ligand bonding in the Ln(II) and Ca complexes from σ to π. Complexes [(tBu4Carb)2Ln] (Ln = Sm, Eu, Yb, Ca) are amido analogues of metallocenes, which adopt the sandwich structures with parallel disposition of the aromatic ligands and strong contribution of η3-mode into η5 metal−ligand bonding. The DFT calculations demonstrated that the geometry is due to steric effects (presence of the bulky tBu groups) as well as the maximization of the overlap between the Sm 4f orbital and the π-type nitrogen lone pair of the carbazol-yl ligand. Coordination of DME to the metal centers in [(tBu4Carb)2M] (M = Sm, Yb) results in the heterolytic dissociation of the metal−ligand π-bond and the formation of ionic complexes [tBu4Carb−]2[Ln2+(DME)n].

■

INTRODUCTION Unlike d-transition metals, the base-free Ln(II) (Ln = Sm, Eu, Yb) and heavy alkaline earth metals (M = Ca, Sr, Ba) bis(cyclopentadienides) (C5H5−nRn)2M (R = Me, iPr, tBu, SiMe3; n = 2−5) feature a distinctly “bent” geometry (Chart 1).1 The nonlinear structures were detected in both the crystal1

CpCentr bond angles in base-free Ln(II) metallocenes reveals scattering of these values within a rather broad region depending on steric demand of the cyclopentadienyl ligands.8 Indeed an increase of the ligand bulkiness results in the enlargement of the CpCentr−Ln−CpCentr angles. Reaffirming this trend, application of bulky Cp ligands ([C5(4-nBu-C6H4)5],9a,b [C5(iPr)5],9c and [C5Ph5]9d) allowed for the synthesis of nonbent lanthanoidocenes featuring a CpCentr−Ln−CpCentr angle of 180°. Formerly, it was shown that introduction of bulky tert-butyl substituents into a carbazolyl platform influences dramatically its coordination mode to the main group metals.10 In order to get a deeper insight into the effect of the ligand electronic structure and steric demand on the structures of Ln(II) and Ca metallocenes and to attempt their linearization, a bulky 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligand10 (tBu4Carb−) was employed. Another goal we pursued in the present work was the synthesis of low coordinate Ln(II) speciesprecursors for coordination with nonconventional for organolanthanide chemistry ligands (SiH4, CO, N2, H2, olefins, and internal acetylenes). Herein, we report on the synthesis, characterization, electronic structure, and reactivity of complexes [(tBu4Carb)2M] (Ln = Sm, Eu, Yb, and Ca).

Chart 1

and the gas phase,2 and several models have been developed for rationalization of bending phenomenon, including a molecular orbital model,3 an electrostatic (polarized-ion) model,4 and a model based on van der Waals attractive forces.5 It is widely assumed that the 4f electrons do not contribute to metal−ligand interactions6 and the bonding is essentially ionic.7 This makes steric properties of ligands, coordination, and steric saturation of the metal sphere crucial for complex structure, stability, and reactivity. The analysis of CpCentr−Ln− © 2015 American Chemical Society

Received: May 29, 2014 Published: January 29, 2015 555

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics Scheme 1. Synthesis of Complexes 1−6

■

RESULTS AND DISCUSSION The reactions of Ln(II) iodides [MI2(THF)3] (M = Sm, Eu) with 2 equiv of tBu4CarbK10c were carried out in THF at 20 °C (1 h), whereupon the solvent was changed to toluene and the reaction mixtures were heated at 70 °C for 1 h. These reactions afforded bis(carbazol-yl) complexes [(tBu4Carb)2M] (M = Sm (1), Eu (2)), which were isolated in 79% and 56% yields, respectively (Scheme 1). Surprisingly [MI2(THF)2] (M = Yb, Ca) under the similar conditions do not react with [tBu4CarbK] and the starting reagents were quantitatively recovered from the reaction mixtures. However, we succeeded to obtain the Yb- and Ca-containing analogues [(tBu4Carb)2M] (M = Yb (3), Ca (4)) by reacting [(C10H8)M(THF)2]11 with a 2-fold molar excess of tBu4CarbH (THF, 20 °C, 48 h; 60 °C, 72 h) (Scheme 1). This synthetic approach allowed for preparation of complexes 3 and 4 in 53% and 70% yields, respectively. As evidenced by 1H and 13C NMR spectroscopy, complex 3 is diamagnetic, in line with the divalent oxidation state of the ytterbium atom. The 1H and 13C NMR spectra of diamagnetic complexes 3 and 4 contain the expected sets of signals due to carbazol-yl ligands and indicate the absence of coordinated Lewis bases. The values of effective magnetic moments measured for 1 (3.8 μB, 293 K) and 2 (7.6 μB, 293 K) are consistent with the divalent oxidation state of Ln ions.12 The crystal structures of complexes 1−4 were established by the X-ray diffraction studies. Complexes 1, 2, and 4 crystallize as solvates [(tBu4Carb)2M]·0.5C7H8. Asymmetric unit cells of 1, 2, and 4 contain two crystallographically independent molecules. Since both molecules have similar geometric parameters, just one of them will be discussed. The X-ray study of complex 2 established the order of connectivity of the atoms in the molecule; however, the poor quality of the crystals does not allow us to discuss the geometric parameters of the molecule. Complex 1 also crystallizes without solvent molecules of toluene (1x). The geometrical parameters of 1 and 1x are similar. Therefore, complex 1x is not discussed in the paper. The molecular structures of 1, 3, and 4 are depicted in Figure 1; the crystal and structural refinement data are summarized in Table 1. According to the X-ray analysis, in complexes 1−4, the Ln(II) ions are coordinated by two tBu4Carb− ligands, wherein the metals are disposed in the inversion centers. Despite high oxophylicity of Ln(II) and Ca centers, their low coordination numbers and use of THF as the reaction solvent 1−4 do not contain coordinated THF molecules. Complexes 1−4 adopt rather unusual structures for Ln(II) cyclopentadienyl and heterocyclopentadienyl complexes13 with parallel disposition of tBu4Carb− ligands (the values of LCentr−Ln−LCentr bond angles

Figure 1. ORTEP of complexes [(tBu4Carb)2M] (M = Sm (1), Yb (3), Ca (4)) with ellipsoids set at 30% probability; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]. 1: Sm(1)−center (N(1),C(1),C(6),C(7),C(12)) 2.614(1), Sm(1)−N(1) 2.541(1), Sm(1)−C(1) 2.762(2), Sm(1)−C(6) 3.139(2), Sm(1)− C(7) 3.134(2), Sm(1)−C(12) 2.762(2). 3: Yb(1)−center (N(1),C(1),C(6),C(7),C(12)) 2.506(2), Yb(1)−N(1) 2.439(4), Yb(1)−C(1) 2.623(2), Yb(1)−C(6) 3.052(3), Yb(1)−C(7) 3.039(3), Yb(1)−C(12) 2.670(3), N(1)−C(1) 1.407(3). 4: Ca(1)− center (N(1),C(1),C(6),C(7),C(12)) 2.505(1), Ca(1)−N(1) 2.373(2), Ca(1)−C(1) 2.637(3), Ca(1)−C(6) 3.080(3), Ca(1)− C(7) 3.079(3), Ca(1)−C(12) 2.656(3).

are 180°). The tBu4Carb− ligands are rotated 180° away from each other (Figure 1), apparently due to steric repulsion between bulky tBu-groups. Unlike the lanthanide complexes of unsubstituted (Carb)2Ln(THF)414 or tetrasubstituted pincer [1,8-(Ph2PNAr)2-3,6-dimethylcarbazol-9-yl]15a and [1,8(PPh2)2-3,6-(tBu)2-carbazol-9-yl]15bligands featuring metal− ligand σ-interaction through the amide N-donor, in 1−4, the tBu4Carb− ligands are π-coordinated to the M(II) ions through the central five-membered ring. However, it should be noted that, unlike cyclopentadienyl, 2,5-di-tert-butylpyrrolyl13 and phospholyl13 complexes in 1−4, the Ln atoms are not disposed above the centroids of five-membered rings but are shifted toward the azaallylic fragments. The Ln−CpCentr−N bond angles are noticeably narrower than 90° (1: 72.9; 3: 72.7°; 4: 81.5). The Ln−N bond lengths in 1 (2.5412(13) Å) and 3 (2.409(5) Å) are similar to those in the previously described six-coordinated bis(carbazol-9-yl) Ln(II) complexes ([Carb2Sm(THF)4]: 2.547(3) and 2.583(3) Å;14b [Carb2Yb(DME)(THF)2]: 2.45(2), 2.43(3) Å14c). The Ca−N bond lengths in 4 (2.373(2) Å) fall into the region previously 556

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics Table 1. Main Crystallographic Data and Structure Refinement Details for Complexes 1, 1x, 3−5 empirical formula formula weight T, [K] wavelength, [Å] crystal system space group a, [Å] b, [Å] c, [Å] α, [deg] β, [deg] γ, [deg] volume, [Å3] Z ρcalcd, [g/cm3] absorption coefficient, [mm−1] F(000) crystal size, [mm] θ range for data collection, [deg] index ranges

reflections collected independent reflections Rint completeness to θ, [%] goodness-of-fit on F2 final R indices (I > 2σ(I)) R indices (all data) largest diff. peak and hole, [e/Å3]

1

1x

3

4

5

C63H88N2Sm 1023.7 100(2) 0.71073 triclinic P1̅ 12.56118(11) 14.70392(15) 15.93046(15) 71.7518(9) 81.2580(8) 80.6670(8) 2741.25(4) 2 1.240 1.110 1084 0.40 × 0.40 × 0.20 3.03−27.00 −16 ≤ h ≤ 16 −18 ≤ k ≤ 18 −20 ≤ l ≤ 20 44316 11901 0.0365 99.5 1.032 R1 = 0.0297, wR2 = 0.0709 R1 = 0.0429, wR2 = 0.0755 1.567 and −0.528

C56H80N2Sm 931.57 100(2) 0.71073 orthorhombic Pbca 18.3094(8) 12.7495(6) 21.0149(9) 90 90 90 4905.6(4) 4 1.261 1.234 1968 0.35 × 0.31 × 0.11 1.94−26.00 −22 ≤ h ≤ 22 −15 ≤ k ≤ 15 −25 ≤ l ≤ 25 39525 4781 0.0219 99.2 1.037 R1 = 0.0330 wR2 = 0.0968 R1 = 0.0410 wR2 = 0.1026 0.634 and −1.386

C56H80N2Yb 954.26 100(2) 0.71073 monoclinic P2(1)/n 12.6667(7) 18.5268(11) 12.6715(7) 90 119.1440(10) 90 2597.2(3) 2 1.22 1.835 1000 0.18 × 0.08 × 0.05 1.10−28.00 −16 ≤ h ≤ 16 −24 ≤ k ≤ 24 −16 ≤ l ≤ 16 25919 6244 0.0489 99.5 1.008 R1 = 0.0397, wR2 = 0.0906 R1 = 0.0692, wR2 = 0.1009 1.238 and −1.233

C63H88CaN2 913.43 100(2) 0.71073 triclinic P1̅ 12.4695(19) 14.556(2) 15.983(2) 71.503(3) 81.849(3) 80.914(3) 2703.7(7) 2 1.122 0.156 1000 0.30 × 0.17 × 0.11 1.35−25.24 −14 ≤ h ≤ 14 −17 ≤ k ≤ 17 −19 ≤ l ≤ 19 21450 9651 0.1040 98.8 0.824 R1 = 0.0628 wR2 = 0.1104 R1 = 0.1737 wR2 = 0.1302 0.737 and −0.312

C84H144N2O11Sm 1508.36 100(2) 0.71073 trigonal P3(2)21 21.2928(6) 21.2928(6) 16.5110(5) 90 90 120 6482.9(3) 3 1.159 0.734 2436 0.49 × 0.21 × 0.16 1.66−26.00 −26 ≤ h ≤ 26 −26 ≤ k ≤ 26 −20 ≤ l ≤ 20 55889 8426 0.0363 98.7 1.063 R1 = 0.0352, wR2 = 0.0909 R1 = 0.0372, wR2 = 0.0926 1.369 and −0.496

tBu4Carb− in 1−4 can be described as rather η5 with a considerable tilt toward η3-mode. In order to gain a better insight into metal−ligand bonding in complexes 1−4, DFT (B3PW91) calculations were carried out for 1. The method (B3PW91/Sm(RECP+g)//6-31G** other atoms) proved to be appropriate to describe the geometry of the complex 1 (Figure 2) with a maximum deviation of 0.02 Å on the bond lengths and 1.5° on the bond angles (Table 2; for the atom numbering, see the Supporting Information, Figure S5). It is noteworthy that, at this level of calculation, the aforementioned described geometry of the complex 1 is characterized by one small imaginary frequency (−6 cm−1), corresponding to the slight rotation of one of the carbazol-yl

reported for five- and six-coordinate calcium amides.16 The bond distances from the metal ions to the carbons neighboring to the nitrogen (1: 2.762(2), 2.762(2) Å; 3: 2.623(2), 2.670(3) Å; 4: 2.637(3), 2.656(3) Å) are much shorter compared to the distances to two other carbon atoms (1: 3.134(2), 3.139(2) Å; 3: 3.039(3), 3.052(3) Å; 4: 3.080(3), 3.079(3) Å). The bond distances from the metal ion to the carbon atoms of azaallylic fragments in 1, 2, and 4 have values close to those in [(C5Me5)2M] (Sm−Caverage = 2.79 Å;17 Yb−Caverage = 2.66; 2.67 Å;1e Ca−Caverage = 2.64 Å1g). However, the bond distances between the Ln atoms and the “remote” carbons atoms fall out the range of values characteristic for [(C5Me5)2Ln] complexes and exceed even the appropriate Ln−C bond distances in the related bis(fluorenyl) Ln(II) complexes18 featuring η3 ring slippage. However, these distances can be compared to the Ln− C bond lengths in complexes [η5-(Me3Si-fluorenyl)-η6-(Me3Sifluorene-AlMe3)Yb] (Yb−C (η6-arene) 2.793−3.031 Å, Yb− center (η6-arene) 2.531 Å) and [η6-bis(Me3Si-fluorene-AlR3)Sm] (Sm−C (η6-arene) 3.004−3.170 Å, Sm center (η6-arene) 2.754−2.773 Å).19 η5-coordination of related 2,5-di-tertbutylpyrrolyl ligands documented for Ln(III) complexes20 excludes the possibility of an explanation of unsymmetrical disposition of the Ln(II) atoms relative to five-membered rings of carbazolyl ligands in 1 and 2 due to the difference of electronegativities of C and N atoms. Thereby, complexes 1−4 present the first examples of amido analogues of nonbent Ln(II) and Ca metallocenes. The bonding interaction M-

Figure 2. Optimized structure of complex 1. 557

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics

Table 2. Comparison between the Experimental and DFT Optimized Geometries of Complex 1 for Selected Metric Parameters exp

DFT

exp

DFT

1.408 1.393 1.423 1.386 2.541 2.541

1.406 1.386 1.422 1.390 2.574 2.574

−0.9 −1.3

−0.1 0.3

distances (Å) N1−C1 N1−C4 C1−C2 C1−C5 C3−C4 C4−C9

1.392 1.390 1.423 1.425 1.423 1.433

1.383 1.383 1.432 1.428 1.432 1.428

N1−Sm−N2

180

180

C10−C3−C2−C6 C10−C4−C1−C6

2.0 0.0

0.3 0.2

C2−C8 C5−C6 C6−C7 C7−C8 N1−Sm N2−Sm angles (deg)

dihedral angle (deg) C11−C3−C2−C7 C11−C4−C1−C7

ligands that is staggered with the other carbazol-yl ligand in the experimental structure. Thus, two true minima were located in which both ligands are eclipsed (Figure 3) in order to avoid the

Figure 4. Optimized structure of less bulky (Me4Carb)2Sm and (Carb)2Sm.

cases, three complexes were considered, namely, a first compound in which the nitrogen atoms are trans to each other (SmMe4trans, Figure 4a and SmH4trans, Figure 4d); in the second one, the nitrogen atoms are in a cis conformation (SmMe4cis, Figure 4b and SmH4cis, Figure 4e), and finally a left (or right) conformation (SmMe4, Figure 4c and SmH4, Figure 4f). For the former, the two carbazol-yl ligands are parallel, leading to a linear-type complex, whereas the complexes are bent for the latter two, decreasing the π interaction. Computationally, for (Me4 Carb)2Sm, there is a slight preference for the trans conformation of the nitrogen atoms (but the two other conformations are lying only 1.0 kcal·mol−1 higher in energy). Contrarily, the cis conformation is the most stable structure for (Carb)2Sm. Thus, the presence of bulky substituents on the carbazol-yl ligands is important to account for the geometry of complex 1. It should be noted that, without THF ligands, pure σ coordination (η1 coordinated carbazol-yl ligand) was never obtained. However, the coordination of one THF molecule, (Carb)2Sm(THF), induces an η3-to-η2 hapticity change (see the Supporting Information, Figure S6). Then, successive addition of a second, third, and fourth THF molecule leads to a pure σ coordination of both carbazol-yl ligands (see Figure S6), as experimentally observed for (Carb)2Sm(THF)4.14b In order to describe the bonding situation in complex 1, the molecular orbitals were analyzed. A previous study, by Maynadié et al. on uranium complexes,21 described an unusual

Figure 3. Energetic profile corresponding to the small rotation of one of the carbazol-yl ligands.

nitrogen atoms not to be in trans position to each other. The small activation barrier for this rotation clearly indicates that, at room temperature, the system should oscillate between the two eclipsed structures so that only the averaged structure that corresponds to the staggered one could be detected, in line with the experimental observation. This rotation is mainly due to the high steric hindrance around the metal center because of the tBu groups. For a deeper analysis of the interaction between Sm and both carbazol-yl ligands, a natural bond order (NBO) analysis was performed. The second-order perturbation NBO analysis confirms the η3 hapticity of both ligands. Indeed, this analysis indicates a donation from the nitrogen lone pair (12 kcal· mol−1) and from the N−C (14 kcal·mol−1) and C−C (15 kcal· mol−1) bonding π orbitals toward empty p and d orbitals of samarium. In order to identify the origin of the nonbent structure of 1, the steric influence of the tBu groups on this particular structure was first examined by computing three rotamers in which the tBu groups were replaced by methyl fragments (Me4Carb)2Sm and then by hydrogen atoms (Carb)2Sm (Figure 4), starting from a π coordinated geometry. In both 558

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics linear structure of an uranocene complex that was involving hybrid f/d orbitals of the metal and π orbitals on the cyclopentadienyl ligand. For complex 1, the most important orbital contribution to the bonding between the carbazol-yl ligands and the samarium center involves the nitrogen lone pairs of both ligands and a mixture of the samarium f orbitals (Figure 5), which leads to the formation of linear orbitals. The

Figure 6. ORTEP of cationic and anionic parts of complex 5 with ellipsoids set at 30% probability; hydrogen atoms are omitted for clarity. The range of Sm−O distances is 2.591(2)−2.636(2) [Å].

nitrogen atoms of carbazol-yl ligands in the crystal is 6.57 Å. The Sm−O distances fall into the range of 2.591(2)−2.636(2) [Å]. It is worthy to note that the N−C bond lengths (1.367(3), 1.377(3) Å) in separated tBu4carb− anions are slightly shorter compared to those in 1 (1.385(2)−1.390(2) Å). Unfortunately, all the attempts to obtain single crystals of ytterbium complex 6 suitable for X-ray analysis failed; the NMR and microanalysis data are consistent with the formula [tBu4carb−]2[Yb2+(DME)3]. We assume that complex 6, similarly to 5, is an ionic compound. Reversible dissociation of η5 Cp−Ln(II) bonds was formerly documented when a bis(pentaphenylcyclopentadienyl) ytterbium(II) complex was treated with THF; however, in noncoordinating solvents, the metallocene structure of [(C5Ph5)2Yb] was restored.9d Unlike [(C5Ph5)2Yb], complexes 1 and 2 do not react with THF, whereas their reactions with DME are irreversible. Mechanistically, the formation of 5 can be rationalized by a stepwise addition pathway of four DME. The incoming first DME coordinates to the samarium center through its oxygen atoms in an equatorial position between both carbazol-yl ligands (see the Supporting Information, Figure S7a). This coordination is exothermic by −5.9 kcal·mol−1 and kinetically accessible (ΔrH⧧ = 9.9 kcal·mol−1) with respect to the separated reactants. The second-order perturbation NBO analysis confirms the presence of a stabilizing interaction between the oxygen atoms and the metal center by the donation from the oxygen lone pairs (30 kcal·mol−1) toward empty p and d orbitals of samarium. This coordination induces a hapticity change of one carbazol-yl ligand from η3 to pure σ, as evidenced by the increase of the Sm−C1 and Sm−C4 distances with respect to 1 (3.236 and 3.280 Å vs 2.705 and 2.713 Å, respectively). At the same time, the second carbazol-yl ligand adopts an η5 coordination to the metal center. The distances between the five-membered ring and the samarium are between 2.858 and 2.922 Å. As for the first step, the coordination of a second DME is an exothermic reaction by −7.6 kcal·mol−1. This coordination leads to the cleavage of the σ-bond between Sm and the carbazol-yl ligand (Sm−N distance is equal to 6.174 Å) as well as a hapticity change of the second carbazol-yl ligand η5 to η3 (see the Supporting Information, Figure S7b), with a principal Sm−N distance of 2.636 Å, secondary Sm−C distances of 2.772 and 2.795 Å, and two Sm− C distances of 2.939 and 2.942 Å. Finally, as experimentally observed, the coordination of four DME molecules leads to heterolytic cleavage of the π-bonds between Sm and carbazol-yl ligands and to the formation of ionic complexes (see the Supporting Information, Figure S7c). This reaction is also an exothermic reaction by −11.3 kcal·mol−1 with respect to the separated reactants. In good agreement with the aforemen-

Figure 5. Bonded high occupied molecular orbitals of Sm(a).

formation of a parallel configuration is mainly due to the maximization of the overlap between the linear molecular f orbital of the samarium center and the π-nitrogen lone pairs of both carbazol-yl ligands. This is reminiscent of the situation reported by Labouille et al. for phospholyl samarium complexes.22 Regarding the HOMO and LUMO orbitals (Figure 5), as expected, the HOMO is a pure f orbital, whereas the LUMO is an antibonding d−π(carbazylyl) interaction. Coordination unsaturation of the metal centers in 1−4 is suggestive of a tendency to undergo complexation; therefore, the reactions with a series of molecules were carried out. The reactions of complexes 1, 3, and 4 with CO, H2, N2, SiH4, PMe3, ethylene, 2-hexyne, PhCCPh, styrene, butadiene, and THF (toluene, r.t., 1 atm) were attempted. Surprisingly 1, 3, and 4 turned out to be inert toward these reactants. Obviously, such an inertness of these componds originates from shielding the metal centers by bulky tBu-groups. However, the treatment of 1 and 3 with an excess of DME affords complexes containing coordinated base molecules [tBu 4 carb − ] 2 [Ln 2+ (DME) n ] (Scheme 1, Ln = Sm, n = 4 (5), Ln = Yb, n = 3 (6)). After recrystallization from THF, complexes 5 and 6 were isolated in 87% and 90% yields, respectively. The crystal structure of complex 5 was established by the Xray diffraction study, which revealed that it is an ionic compound consisting of the Sm(II) cation coordinated by four DME molecules and not bonded tBu4carb− anions (Figure 6). The formation of 5 is a result of the heterolytic cleavage of π-bonds between Sm(II) and carbazol-yl ligands promoted by the coordination of DME molecules to the metal center. Complex 5 crystallizes as a solvate [tBu4carb−]2[Sm(DME)4]+· 3THF. The molecular structure of 5 is depicted in Figure 6; the crystal and structural refinement data are summarized in Table 1. The shortest distance between the cation of Sm2+ and the 559

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics

instrument (300.13 and 75.47 MHz for 1H and 13C, respectively). Chemical shifts are reported in ppm (δ) relative to TMS, referenced to the chemical shifts of residual solvent resonances (1H and 13C), and coupling constants are given in Hz. IR spectra were recorded as Nujol mulls or KBr plates on FSM 1201 and Bruker-Vertex 70 instruments. Magnetic susceptibility data were collected by using a Quantum Design MPMS-XL SQUID magnetometer. The N, C, H elemental analyses were carried out in the microanalytical laboratory of the IOMC by means of a Carlo Erba Model 1106 elemental analyzer with an accepted tolerance of 0.4 unit on carbon (C), hydrogen (H), and nitrogen (N). Lanthanide metal analysis was carried out by complexonometric titration.27 Synthesis of [(tBu4carb)2Sm] (1). A solution of [tBu4CarbK] (2.14g, 5 mmol) in THF (15 mL) was added to a dark-blue solution of [SmI2(THF)3] (1.55 g, 2.5 mmol) in THF (30 mL). The reaction mixture was stirred at 30 °C for 1 h, and THF was removed in vacuum. The solid residue was redissolved in toluene (40 mL) and was stirred at 70 °C for 1 h. The reaction mixture was filtered from KI and slowly concentrated. The mother liquor was decanted, and the darkpurple crystals were dried in vacuum at 22 °C for 1 h. 1.87 g of 1 was isolated (yield 79%). 1H NMR (200 MHz, C6D6, 293 K): δ = −2.32 (4H, s, carbazolyl CH), 3.89 (36H, s, tBu), 4.75 (36H, s, tBu), 11.56 (4H, s, carbozolyl CH). IR (KBr, Nujol): ν = 1302 (m), 1281 (m), 1246 (s), 1216 (m), 1198 (w), 1119 (w), 1084 (m), 1024 (w), 965 (w), 907 (m), 865 (s), 841 (s), 775 (m), 759 (w), 670 (w), 651 (s), 584 (m), 541 (w), 511 (m) cm−1. Anal. Calcd for C56H80N2Sm (931.6 g·mol−1): C, 72.20; H, 8.66; N, 3.01; Sm, 16.14. Found: C, 71.85; H, 8.18; N, 2.82; Sm, 16.00. Synthesis of (tBu4carb)2Eu (2). A solution of tBu4carbK (2.15 g, 5.0 mmol) in THF (20 mL) was added to a yellow solution of EuI2(THF)3 (1.56 g, 2.5 mmol) in THF (30 mL). The reaction mixture was stirred at 30 °C for 1 h, and THF was removed in vacuum. The solid residue was dissolved in toluene (40 mL) and was stirred at 100 °C for 7 h. The reaction mixture was filtered from KI and slowly concentrated. The mother liquor was decanted, and the crystals were dried in vacuum at 22 °C for 1 h. 1.30 g of bright yellow rhombic crystals of compound 2 were isolated (yield 56%). IR (KBr, Nujol): ν = 1302 (m), 1280 (m), 1243 (s), 1215 (m), 1196 (w), 1116 (w), 1087 (m), 1024 (w), 966 (w), 905 (m), 866 (s), 840 (s), 776 (m), 756 (w), 671 (w), 650 (s), 585 (m), 540 (w), 510 (m) cm−1; Anal. Calcd for C56H80N2Eu (933.55 g·mol−1): C, 72.07; H, 8.64; N, 3.00; Eu, 16.28; found: C, 71.86; H, 8.25; N, 2.77; Eu, 16.52. Synthesis of [(tBu4carb)2Yb] (3). A solution of tBu4CarbH (1.23 g, 3.14 mmol) in THF (20 mL) was added to a suspension of [(C10H8)Yb(THF)2] (0.70 g, 1.57 mmol) in THF (20 mL) at room temperature. The reaction mixture was stirred at room temperature for 2 days and 3 days at 60 °C. The volatiles were removed in vacuum; the solid residue was dried in vacuum for 1 h. The resulting dark-brown solid was dissolved in toluene, and the solution was filtered. Slow concentration of the toluene solution at room temperature afforded bright orange crystals of 3 in 53% yield (0.80 g). 1H NMR (400 MHz, C6D6, 293 K): δ = 1.39 (s, 36H, tBu), 1.48 (s, 36H, tBu), 7.42 (d, 4JHH = 1.6 Hz, 4H, carbazolyl CH), 7.91 (d, 4JHH = 1.6 Hz, 4H, carbozolyl CH). 13C{1H} NMR (100 MHz, pyridine-d5, 293 K): 30.2 (CCH3), 32.0 (CCH3), 34.6 (CCH3), 34.8 (CCH3), 114.6 (C carbazolyl CH), 120.3 (C carbazolyl CH), 124.5 (C carbazolyl), 129.2 (C carbazolyl), 132.1 (C carbazolyl), 142.3 (C carbazolyl). IR (KBr, Nujol): ν = 1302 (m), 1280 (m), 1244 (s), 1216 (m), 1188 (w), 1120 (w), 1084 (m), 1020 (w), 970 (w), 900 (m), 865 (s), 841 (s), 777 (m), 757 (w), 670 (w), 651 (s), 580 (m), 541 (w), 510 (m) cm−1. Anal. Calcd for C56H80N2Yb (954.3 g·mol−1): C, 70.48; H, 8.45; N, 2.94; Yb, 18.13. Found: C, 70.06; H, 8.13; N, 2.59; Yb, 17.91. Synthesis of (tBu4carb)2Ca (4). A suspension of [C10H8Ca(THF)2] (0.70 g, 2.24 mmol) in THF (10 mL) was added to a solution of tBu4CarbH (1.76g, 4.5 mmol) in THF (20 mL) at room temperature. The reaction mixture was stirred at room temperature for 2 days and for 3 days at 60 °C. The volatiles were removed in vacuum; the solid residue was dried in vacuum for 1 h. A pale pink solid was recrystallized from toluene by slow concentration of the solution. The white rhombic crystals of 4 were washed with cold hexane and dried

tioned experimental data, the Sm−O distances are similar for the four DME ligands, between 2.621 and 2.700 Å, whereas the distances between the Sm center and the nitrogen atoms of the carbazol-yl ligands are equal to 6.262 and 6.370 Å. In order to find out the effect of the metal oxidation state on the coordination mode of the tBu4carb ligand, the oxidation reactions of 1 with various oxidants were investigated. The reactions were carried out in toluent at ambient temperature in 1:1 (CuI, CuCl, Ph3SnCl, bipy) or 1:0.5 (PhCH2SSCH2Ph, [Me2NC(S)S]2) molar ratios. All reactions resulted in the appearance of a deep blue color indicative of the formation of a tBu4carb• radical, which was detected in the reaction mixtures by EPR spectroscopy (gi = 2.0032, aN = 6.85G, aH1 = aH2 = 0.95G; for the EPR spectra, see the Supporting Information, Figure 21).23 Unfortunately, all of our attempts of isolation of the samarium-containing oxidation products were unsuccessful.

■

CONCLUSIONS Thus, it was demonstrated that introduction of four tert-butyl groups into 1,3,6,8-positions of carbazol-9-yl ligands allows switching the M(II)−ligand bonding mode from σ to π and results in the synthesis of amido analogues of Ln(II) and Ca metallocenes featuring a parallel orientation of tBu4carb− ligands. The metal−ligand interaction in 1−4 can be described as η5 with a considerable tilt toward η3-mode. The DFT calculations were carried out to understand the key factors that govern the geometry of complex 1. The nonbent geometry is thus partially due to steric effects as the presence of the bulky tBu groups on the two carbazol-yl ligands prevents the bending. Moreover, the nonbent structure of complex 1 is found to allow the maximization of the overlap between the Sm 4f orbital and the π-type nitrogen lone pair of the carbazol-yl ligand. Despite coordination unsaturation of the metal centers, complexes 1, 3, and 5 proved to be chemically inert toward small molecules (H2, N2, CO, SiH4), various Lewis bases (both hard and soft), olefin, diene, and acetylene ligands. The comparison of chemical properties of 1 and (C5Me5)2Sm24 indicates the decrease of reactivity when replacing C5Me5− by a tBu4carb− ligand. Such a drop of reactivity can originate either from shielding the metal centers by bulky tBu groups or from less electron donating properties of the tBu4carb− ligand. On the other hand, 1 and 3 readily react with DME, undergoing heteroleptic dissociation of Ln−tBu4carb π-bonds promoted by the coordination of DME molecules to the metal centers. The reactions result in ionic [tBu4carb−]2[Ln2+(DME)n] complexes.

■

EXPERIMENTAL SECTION

General Considerations and Materials Characterization. All air- and/or moisture-sensitive reactions were performed under an inert atmosphere in flame-dried flasks using standard Schlenk-type techniques or in a drybox filled with nitrogen. THF and DME were purified by distillation from sodium/benzophenone ketyl, after drying over KOH. Toluene was purified by distillation from sodium/triglyme benzophenone. Benzene-d6 was dried over sodium/benzophenone ketyl and condensed in vacuo over activated 4 Å molecular sieves prior to use. Carbazole, anhydrous AlCl3, and 2-chloro-2-methylpropane were purchased from Aldrich and were used without further purification. LnI2(THF)3,25 C10H8Yb(THF)2,11 1,3,6,8-tetra(tertbutyl)carbazole,26 and tBu4carbK10c were prepared according to literature procedures. All the other reagents and solvents were used as purchased from commercial suppliers. 1H and 13C{1H} NMR spectra were obtained on either a Bruker Avance DRX-400 (400.13 and 100.62 MHz, respectively) or a Bruker Avance 300 MHz 560

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics for 1 h in vacuum. Compound 4 was isolated in 70% yield (1.26 g). 1H NMR (400 MHz, C6D6, 293 K): δ = 1.41 (36H, s, tBu), 1.48 (36H, s, tBu), 7.43 (4H, s, carbazolyl CH), 7.94 (4H, s, carbozolyl CH). 13 C{1H} NMR (100 MHz, C6D6, 293 K): 29.5 (CH3 tBu), 31.9 (CH3 tBu), 34.7 (C tBu), 35.4 (C tBu), 114.6 (C carbazolyl CH), 120.2 (C carbazolyl CH), 125.3 (C carbazolyl), 138.8 (C carbazolyl), 141.4 (C carbazolyl), 144.5 (C carbazolyl). IR (KBr, Nujol): ν = 1300 (m), 1281 (m), 1244 (s), 1215 (m), 1187 (w), 1120 (w), 1085 (m), 1021 (w), 970 (w), 900 (m), 864 (s), 842 (s), 776 (m), 755 (w), 670 (w), 650 (s), 582 (m), 541 (w), 511 (m) cm−1. Anal. Calcd for C56H80N2Ca (820.59 g·mol−1): C, 81.89; H, 9.82; N, 3.41; Ca, 4.88. Found: C, 81.40; H, 9.55; N, 3.25; Ca, 4.99. Synthesis of [tBu4Carb−]2[Sm2+(DME)4] (5). 1 (0.50 g, 0.53 mmol) was dissolved in THF (15 mL) at room temperature, and an excess of DME was added. The reaction mixture changed color from brown to green. Slow concentration of the mother liquor at room temperature afforded blue-green crystals of 5. The mother liquor was decanted, and the crystals were dried in vacuum at 22 °C for 1 h. 0.60 g of 5 was isolated (87%). IR (KBr, Nujol): ν = 1291 (m), 1248 (w), 1218 (w), 1187 (w), 1111 (w), 1057 (s), 850 (s), 777 (w), 648 (m) cm−1. Anal. Calcd for C72H120N2O8Sm (1292,1 g·mol−1): C, 66.93; H, 9.36; N, 2.17; Sm, 11.64. Found: C, 66.47; H, 9.01; N, 2.00; Sm, 11.80. Synthesis of [tBu4Carb−]2[Yb2+(DME)3] (6). 3 (0.20 g, 0.21 mmol) was dissolved in THF (5 mL) at room temperature, and an excess of DME was added. The reaction mixture changed color from brown to pale yellow. Slow concentration of the reaction mixture at room temperature afforded yellow crystals of 6. The mother liquor was decanted, and the crystals were dried in vacuum at 22 °C for 1 h. 0.23 g of 6 was isolated (90%). 1H NMR (400 MHz, pyridine-d5, 293 K): δ = 1.46 (s, 36H, tBu), 1.56 (s, 36H, tBu), 3.22 (s, 18H, CH3, DME), 3.45 (s, 12H, CH2, DME), 7.66 (s, 4H, carbazolyl CH), 8.14 (s, 4H, carbozolyl CH). 13C{1H} NMR (100 MHz, pyridine-d5, 293 K): 30.2 (CCH3), 31.9 (CCH3), 34.6 (CCH3), 34.8 (CCH3), 58.4 (CH3, DME), 71.8 (CH2, DME), 114.6 (carbazolyl CH), 120.3 (carbazolyl CH), 124.5 (carbazolyl C), 129.2 (carbazolyl C), 132.1 (carbazolyl C), 142.3 (carbazolyl C). IR (KBr, Nujol): ν = 1292 (m), 1245 (w), 1220 (w), 1186 (w), 1114 (w), 1060 (s), 851 (s), 776 (w), 650 (m) cm−1. Anal. Calcd for C68H110N2O6Yb (1224.66 g·mol−1): C, 66.69; H, 9.05; N, 2.29; Yb, 14.13. Found: C, 66.20, H, 8.84; N, 2.07; Yb, 14.54. X-Ray Crystallography. The X-ray data for 1, 1x, and 3−5 were collected on Agilent Xcalibur E (1) and Bruker Smart Apex (1x, 3−5) diffractometers (graphite-monochromated, MoKα radiation, ω-scan technique, λ = 0.71073 Å, T = 100(2) K). The structures were solved by direct methods and were refined on F2 using SHELXTL28 and CrysAlis Pro29 packages. All non-hydrogen atoms were found from Fourier syntheses of electron density and were refined anisotropically. All hydrogen atoms were placed in calculated positions and were refined in the riding model. Metal atoms in 1, 1x, 3, and 4 lie in the inversion center; the Sm atom in 5 lies in the 2-fold rotation axis. Complex 3 is disordered on two positions (0.5:0.5) in the inversion center, and disordered molecules have a similar geometry. SADABS30 and ABSPACK29 were used to perform area-detector scaling and absorption corrections. CCDC-978585 (1), 1030720 (1x), 1030689 (2), 978586 (3), 1030719 (4), and 978587 (5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via ccdc.cam.ac.uk/products/csd/request. Computational Details. All DFT calculations were performed with Gaussian 09.31 Calculations were carried out at the DFT level of theory using the hybrid functional B3PW9132 and M06.33 Geometry optimizations were achieved without any symmetry restriction. Calculations of vibrational frequencies were systematically done in order to characterize the nature of stationary points. Stuttgart effective core potentials and their associated basis set were used for Samarium.34 The basis sets were augmented by a set of polarization functions (ζf = 1.0). Hydrogen, carbon, and nitrogen atoms were treated with 6-31G(d.p) double-ζ basis sets.35 The electron density and partial charge distribution were examined in terms of localized electron-pair bonding units by using the NBO program.36 Through this method, the input atomic orbital basis set is transformed via

natural atomic orbitals (NAOs) and natural hybrid orbitals (NHOs) into natural bond orbitals (NBOs), which correspond to the localized one-center (“lone pair”) and two-center (“bond”) elements of the Lewis structure. All possible interactions between “filled” (donor) Lewis-type NBOs and “empty” (acceptor) non-Lewis NBOs together with their energetic quantification (stabilization energy) were obtained by a second-order perturbation theory analysis of the Fock matrix. Only a stabilization energy higher than 10 kcal·mol−1 was considered.

■

ASSOCIATED CONTENT

S Supporting Information *

Figures giving NMR and IR spectra, CIF files giving crystallographic data, and tables giving computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: (+7)8314633532. E-mail: [email protected] (A.A.T.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research is supported by the Russian Foundation for Basic Research, Grant 14-03-00527, and by the grant (the agreement of August 27, 2013, No. 02.B.49.21.0003, between The Ministry of Education and Science of the Russian Federation and Lobachevsky State University of Nizhni Novgorod).

■

REFERENCES

(1) (a) Long, N. Metallocenes; Blackwell Sientific Publications: Oxford, U.K., 1997. For Ln(II), see: (b) Apostolidis, C.; Deacon, G. B.; Dornberger, E.; Edelmann, F. T.; Kanellakopulos, B.; MacKinnon, P.; Stalke, D. Chem. Commun. 1997, 1047−1048. (c) Hitchcock, P. B.; Howard, J. A. K.; Lappert, M. F.; Prashar, S. J. Organomet. Chem. 1992, 437, 177−189. (d) Visseaux, M.; Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Richard, P.; Weber, P. New J. Chem. 2000, 24, 939−942. (e) Schultz, M.; Burns, C. J.; Schwartz, D. J.; Andersen, R. A. Organometallics 2000, 19, 781−789. (f) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285−1291. For heavy alkaline earth metals, see: (g) Williams, R. A.; Hanusa, T. H.; Huffman, J. C. Organometallics 1990, 9, 1128−1134. (h) Harvey, M. J.; Quisenberry, K. T.; Hanusa, T. P.; Young, V. G. Eur. J. Inorg. Chem. 2003, 3383− 3390. (i) Williams, R. A.; Tesh, K. F.; Hanusa, T. P. J. Am. Chem. Soc. 1991, 113, 4843−4851. (2) Andersen, R. A.; Blom, R.; Boncella, J. M.; Burns, C. J.; Volden, H. V. Acta Chem. Scand. 1987, A 41, 24−35. (3) Green, J. C.; Hohl, D.; Rösch, N. Organometallics 1987, 6, 712− 720. (4) (a) Guido, M.; Gigli, M. J. Chem. Phys. 1976, 65, 1397−1402. (b) DeKock, R. L.; Peterson, M. A.; Timmer, L. K.; Baerends, E. J.; Vernooijs, P. Polyhedron 1990, 9, 1919−1934. (c) Kaup, M.; Schleyer, P. V. R.; Dolg, M.; Stoll, H. J. Am. Chem. Soc. 1992, 114, 8202−8208. (5) (a) Timofeeva, T. V.; Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1995, 117, 7452−7459. (b) Hollis, T. K.; Burdett, J. K.; Bosnich, B. Organometallics 1993, 12, 3385−3386. (6) (a) Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140− 7143. (b) Freedman, D.; Melman, J. H.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1998, 37, 4162−4163. (7) (a) Krinsky, J. L.; Minasian, S. G.; Arnold, J. Inorg. Chem. 2011, 50, 345−357. (b) Minasian, S. G.; Krinsky, J. L.; Arnold, J. Chem. Eur. J. 2011, 17, 12234−12245. (8) For comparison, see: (a) (C5H5)2Yb, Cp−Ln−Cp = 117.6°.1b (b) (Me4C5H)2Yb, Cp−Ln−Cp = 132.0°; (C5Me5)2Yb, Cp−Ln−Cp = 146.0°, 145.0°; [1,3-(Me3C)2C5H3]2Yb, Cp−Ln−Cp = 147.0°.1e (c) [1,2,4-(Me3C)3C5H3]2Yb, Cp−Ln−Cp = 166.0°. Weber, F.; 561

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562

Article

Organometallics Sitzmann, H.; Schultz, M.; Sofield, C. D.; Andersen, R. A. Organometallics 2002, 21, 3139−3146. (9) (a) Ruspic, C.; Moss, J. R.; Schürmann, M.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 2121−2126. (b) Harder, S.; Naglav, D.; Ruspic, C.; Wickleder, C.; Adlung, M.; Hermes, W.; Eul, M.; Pöttgen, R.; Rego, D. B.; Poineau, F.; Czerwinski, K. R.; Herber, R. H.; Nowik, I. Chem.Eur. J. 2013, 19, 12272−12280. (c) Sitzmann, H.; Dezember, T.; Schmitt, O.; Weber, F.; Wolmershäuser, G. Z. Anorg. Allg. Chem. 2000, 626, 2241−2244. (d) Deacon, G. B.; C. M. Forsyth, C. M.; Jaroschik, F.; Junk, P. C.; Kay, D. L.; Maschmeyer, T.; Masters, A. F.; Wang, J.; Field, L. D. Organometallics 2008, 27, 4772−4778. (10) (a) Kuhn, N.; Schulten, M.; Boese, R.; Bläser, D. J. Organomet. Chem. 1991, 421, 1−8. (b) Blake, A. J.; Lewis, W.; McMaster, J.; Moorhouse, R. S.; Moxey, G. J.; Kays, D. L. Dalton Trans. 2011, 40, 1641−1645. (c) Mansaray, H. B.; Kelly, M.; Vidovic, D.; Aldridge, S. Chem.Eur. J. 2011, 5381−5386. (d) Moorhouse, R. S.; Moxey, G. J.; Ortu, F.; Reade, T. J.; Lewis, W.; Bake, A. J.; Kays, D. L. Inorg. Chem. 2013, 52, 2678−2683. (11) Bochkarev, M. N.; Trifonov, A. A.; Fedorova, E. A.; Emelyanova, N. S.; Basalgina, T. A.; Kalinina, G. S.; Razuvaev, G. A. J. Organomet. Chem. 1989, 372, 217−224. (12) Evans, W. J.; Hozbor, M. A. J. Organomet. Chem. 1987, 326, 299−306. (13) Nief, F. Eur. J. Inorg. Chem. 2001, 891−904. (14) (a) Müller-Buschbaum, K.; Quitmann, C. C. Z. Anorg. Allg. Chem. 2003, 629, 1610−1616. (b) Evans, W. J.; Rabe, G. W.; Ziller, J. W. Organometallics 1994, 13, 1641−1645. (c) Abrahams, C. T.; Deacon, G. B.; Gatehouse, B. M.; Ward, G. N. Acta Crystallogr. 1994, C50, 504−507. (d) Deacon, G. B.; Forsyth, C. M.; Gatehouse, B. M. Aust. J. Chem. 1990, 43, 795−806. (15) (a) Johnson, K. R. D.; Hayes, P. G. Organometallics 2011, 30, 58−67. (b) Wang, L.; Cui, D.; Hou, Z.; Li, W.; Li, Y. Organometallics 2011, 30, 760−767. (16) For Ca−N bond distances, see: (a) Jenter, J.; Köppe, R.; Roesky, P. W. Organometallics 2011, 30, 1404−1413. (b) Gärtner, M.; Görls, H.; Westerhausen, M. Organometallics 2007, 26, 1077−1083. (c) Buffet, J.-C.; Davin, J. P.; Spaniol, T. P.; Okuda, J. New J. Chem. 2011, 35, 2253−2257. (17) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106, 4270−4272. (18) (a) Trifonov, A. A.; Kirillov, E. N.; Dechert, S.; Schumann, H.; Bochkarev, M. N. Eur. J. Inorg. Chem. 2001, 2509−2514. (b) Evans, W. J.; Gummersheimer, T. S.; Boyle, T. J.; Ziller, J. W. Organometallics 1994, 13, 1281−1284. (19) Nakamura, H.; Nakayama, Y.; Kanehisa, N.; Yasuda, Y.; Kai, Y.; Maruo, T. Organometallics 2000, 19, 5392−5399. (20) (a) Schumann, H.; Rosenthal, E. C. E.; Winterfeld, J.; KociokKöhn, G. J. Organomet. Chem. 1995, 495, C12−C14. (b) Schumann, H.; Winterfeld, J.; Hemling, H.; Kuhn, N. Chem. Ber. 1993, 126, 2657−2659. (21) Maynadié, J.; Barros, N.; Berthet, J.-C.; Thuéry, P.; Maron, L.; Ephritikhine, M. Angew. Chem., Int. Ed. 2007, 46, 2010−2012. (22) Labouille, S.; Nief, F.; Le Goff, X.-F.; Maron, L.; Kindra, D. R.; Houghton, H. L.; Ziller, J. W.; Evans, W. J. Organometallics 2012, 31, 5196−5203. (23) For parameters of tBu4carb• radical, see: Neugebauer, F. A.; Fischer, H. Angew. Chem. 1971, 83, 756. (24) Evans, W. J. Coord. Chem. Rev. 2000, 206−207, 261−283. (25) Girard, P.; Namy, J.-L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, 2693−2698. (26) Mudadu, M. S.; Singh, A. N.; Thummel, R. P. J. Org. Chem. 2008, 73, 6513−6520. (27) Lyle, S. J.; Rahman, M. M. Talanta 1953, 10, 1177−1182. (28) Sheldrick, G. M. SHELXTL v.6.12: Structure Determination Software Suite; Bruker AXS: Madison, WI, 2000. (29) CrysAlis Pro; Agilent Technologies Ltd: Yarnton, England, 2011. (30) Sheldrick, G. M. SADABS v.2.01. Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998.

(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, M.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, N.; Honda, Y.; Kitao, O.; Nakai, O.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, J.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian. Inc.: Wallingford, CT, 2009. (32) (a) Perdew, J. P. Unified Theory of Exchange and Correlation Beyond the Local Density Approximation. In Electronic Structure of Solids; Ziesche, J. P., Eschrig, H., Eds.; Akademie: Berlin, 1991. (b) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533− 16539. (c) Burke, C.; Perdew, J. P.; Wang, Y. Derivation of a Generalized Gradient Approximation: The PW91 Density Functional. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Mas, M. P., Eds.; Plenum: New York, 1998. (d) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671−6687. (e) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1998, 57, 14999−15033. (f) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1993, 48, 4978−4978. (33) Zhao, M.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (34) (a) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730−1734. (b) Cao, X.; Dolg, M. J. Mol. Struct.:THEOCHEM 2002, 581, 139−147. (35) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213−222. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (36) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066− 4074. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926.

562

DOI: 10.1021/om500572y Organometallics 2015, 34, 555−562