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Potassium and Lithium Complexes with Monodeprotonated, Dearomatized PNP and PNCNHC Pincer-Type Ligands Thomas Simler,† Lydia Karmazin,‡ Corinne Bailly,‡ Pierre Braunstein,*,† and Andreas A. Danopoulos*,†,§ †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 Rue Blaise Pascal, 67081 Strasbourg Cedex, France ‡ Service de Radiocristallographie, Institut de Chimie de Strasbourg (UMR 7177 CNRS), 1 Rue Blaise Pascal, BP 296/R8 67008 Strasbourg Cedex, France § Institute for Advanced Study (USIAS), Université de Strasbourg, 67083 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: The reaction of 2,6-bis(di-tert-butylphosphinomethyl)pyridine (tBuPNtBuP) with 1 molar equiv of KCH2C6H5 or LiCH2SiMe3 gave M(tBuP*NatBuP) (M = K, Li; P* = vinylic P donor, tBuP = PtBu2, Na = anionic amido N donor) after monodeprotonation of the α-lutidinyl-CH2 and concomitant dearomatization of the heterocycle. Evidence is provided that the anion tBuP*NatBuP may exist as Z- and E-isomers, interconvertible by rotation about the Cα‑N−Cα‑P exocyclic formal double bond. Thus, the two isomers of K( tBu P*N a tBu P), i.e., [K{(Z)-( tBu P*N a tBu PκP*,κN a ,κP)}(THF)], 1-Z·(THF), and [K{(E)-( tBu P*N a tBu PκNa,κP)}(THF)], 1-E·(THF), cocrystallized from THF in a 4:1 ratio. However, in the presence of DME, the isomerically pure [K{(E)-(tBuP*NatBuP-κNa,κP)}(DME)2], 1-E·2(DME), was crystallized. The α-picolinyl-CH2 moiety in RPNCNHC (N = substituted 2picoline, RP = PCy2, R = Cy; RP = PtBu2, R = tBu; CNHC = N-heterocyclic carbene) was similarly deprotonated with concomitant dearomatization using LiN(SiMe3)2. This afforded the complexes Li(RP*NaCNHC), which were crystallized as the Z- or E-isomers, [Li{(E)-(tBuP*NaCNHC-κNa,κCNHC)}(Et2O)2], 3tBu-E·2(Et2O), [Li{(Z)-(CyP*NaCNHC-κP*,κNa,κCNHC)}(Et2O)], 3Cy-Z·(Et2O), and [Li{(Z)-(tBuP*NaCNHC-κP*,κNa,κCNHC)}(Et2O)], 3tBu-Z·(Et2O). The Z- and E-isomers reversibly interconvert in solution as shown by 31P{1H} and 7Li NMR spectroscopy. The acidity of the α-picolinyl-CH2 in RPNCNHC is higher than in the known tBu PNtBuP (ca. by 6 pKa units).

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through the “proton responsiveness” of the ligand.11 Multifunctional ligands that enable metal−ligand cooperation are also of considerable relevance to 3d-metal catalysis.4a,12 Consequently, the coordinated anionic RP*NaRP (RP* = vinylic P donor, Na = anionic amido N donor)13 has been readily generated by α-CH2P deprotonation in RPNRP complexes, this reaction resulting in dearomatization of the lutidine heterocycle.3a,11b,12 Modification of the electronic structures of the resulting complexes is also a direct outcome of deprotonation; consequently, the latter broadens the potential for enhanced versatility in reactivity studies and catalyst tuning. Despite the fast growing literature on the use of dearomatized pincers in catalysis, only a few reports describe well-defined, structurally characterized complexes with a dearomatized pincer, and they usually feature the RP*NaRP scaffold. The first report, from 1988, on a Pt(II) and a Pd(II) complex,14 was followed almost 20 years later by the structural characterization of Ir(I)15 and Ru(II) complexes by Milstein,16 which rekindled the interest in

INTRODUCTION

In the rich chemistry of pincer ligands,1 the bis(dialkylphosphinomethyl)pyridine2 (RPNRP) ligand scaffold has probably attracted the most interest (Figure 1). RPNRP complexes with 4d- and 5d-metals have especially shown high potential for catalytic applications,3 although recent efforts have been extended to 3d-congeners with the aim to develop catalysts with earth-abundant, environmentally innocuous metals.4 Representative catalytic applications of the latter with R PNRP (R = alkyl or phenyl) ligands include, for example, ethylene oligomerization (Cr),5 CO2 reduction (Fe),6 hydrogenation (Fe),7 C−H borylation (Co),8 and CO2 hydrosilylation (Co).9 Although other tridentate ligands with P and N donors (e.g., aliphatic-PNP ligands) are being used with the same overall objective,10 the results presented below concern rigid pincer ligands with pyridine-type (i.e., lutidine and picoline) backbones. In the past decade, attention has been focused on the intrinsic acidity of the α-CH2P group of coordinated RPNRP, in particular in relation to metal−ligand cooperation in catalysis © XXXX American Chemical Society

Received: January 20, 2016

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DOI: 10.1021/acs.organomet.6b00048 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Structurally characterized types of complexes with dearomatized pincer ligands originating from a pyridine-based backbone. P* refers to vinylic P donor, and CNHC* to vinylic imidazol-2-ylidene.

Scheme 1. Deprotonation of MePNMeP and Relevant Resonance Structures of MeP*NaMePa

the area; most of the currently available literature on characterized complexes appeared since 2010.17 A few related phosphaalkene systems have also been reported.18 Fully characterized metal complexes with dearomatized pyridinebased pincer ligands, formally derived from replacement of either one phosphine group of RPNRP by a N-based donor (amine,19 imine,20 pyridine,21 iminophosphorane22) or a NHC donor,23 or of both phosphine groups by two NHCs or one NHC and one amine (i.e., CNHC*NaCNHC and CNHC*NaEtN′ (CNHC* refers to vinylic imidazol-2-ylidene), respectively),24 have also recently been described (Figure 1). The majority of dearomatized picoline- or lutidine-based transition metal pincer complexes have been obtained by treatment of the neutral coordinated ligand with external strong bases.25 This approach has limited scope, requiring prior complex formation, and can suffer from competing regioselectivity of the external base attack. We thus envisaged an alternative strategy with a broader scope, consisting of the synthesis of well-defined, isolable alkali metal complexes of R P*NaRP that could serve as versatile ligand transfer reagents to numerous electrophilic metal centers. It is of relevance that the deprotonation of 2-picoline leads to anions described as either carbanions or amides (on the N of the dearomatized heterocycle).26 Attachment of additional heteroatom substituent(s) (e.g., Si,27 P28) at the α-methyl(s) of 2-picoline or 2,6lutidine, respectively, will further stabilize the carbanions obtained by deprotonation of the methylene group(s). The resonance contributions in MeP*NaMeP have been analyzed recently (with Me groups on P for simplicity) (Scheme 1).29 This reasoning can be extended to nonsymmetrical trifunctional pincer-type ligands RPNCNHC containing an NHC in place of one of the phosphine donors of RPNRP. Interestingly, in this case the effect of the NHC donor group on the acidity of the remaining α-CH2P may lead to further tuning of the “proton responsiveness” of the pincer ligand. Recently, we communicated the deprotonation of RPNCNHC (R = Cy, tBu) by KN(SiMe3)2 and showed by 2D-NOESY NMR experiments that the product K(RP*NaCNHC) existed as an equilibrium mixture of E- and Z-isomers about the Cα‑N− Cα‑P formal double bond.23 The Z-isomer of the dearomatized conjugate base coordinated to K was crystallographically characterized. Herein, we extend these studies and compare the side arm deprotonation of the f ree ligands tBuPNtBuP and R PNCNHC (R = Cy, tBu) with selected potassium and lithium bases; we also provide insight into the structure and behavior of the new monoanionic species in the solid state and in solution. Since the nature and speciation of the described complexes can

a NBO weight of the resonance structures, electronic π-populations (in red), and Wiberg bond indexes (in blue) calculated at the M06-2X/631++G(d,p) level of theory.29

differ in solution and in the solid state depending on the solvents used, we will use in the following discussion (i) the generic designations M(RP*NaRP) or M(RP*NaCNHC) (M = K, Li) if no further information on ligand denticity, regioisomerism, or metal solvation is available or (ii) more detailed designations or compound numbering bearing information about the above features if specifically justified experimentally.

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RESULTS AND DISCUSSION PNRP Monodeprotonation. Potassium Salts. Monodeprotonation of tBuPNtBuP was investigated using different bases. The reaction with KN(SiMe3)2 in ether (pKa = 26 in THF)30 afforded less than 15% conversion to K(tBuP*NatBuP), implying that the pKa for the first deprotonation of tBuPNtBuP can be estimated at ca. 28. By using the stronger benzyl potassium as base (KBn, pKa = ca. 42 in polar aprotic solvents),31 complete conversion to K(tBuP*NatBuP) occurred. This complex was obtained as an equilibrium mixture of Z- and E-isomers of the anion (Z/E 1.4:1 in THF). The nature of both isomers in solution (Scheme 2) was determined by 1H-NOESY analysis in THF-d8, following a methodology similar to that previously described for K(tBuP*NaCNHC)23 (see the SI). The identity of the species in the solid state was confirmed by X-ray crystallography, after fortuitous cocrystallization of both 1-Z and 1-E as THF adducts in the same orange-red crystals. The components in the highly disordered centrosymmetric crystal structure were successfully modeled and refined B

R


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Organometallics

Scheme 2. Synthesis of K(tBuP*NatBuP) as an Equilibrium Mixture between Z- and E-Isomers, 1-Z and 1-E, Respectively, and Enrichment to One Isomer as a Function of Solvent

with a calculated Z/E ratio of 4:1 and provided insight into their structural features (Figure 2).

More precise metrical data have been obtained for the DME solvate 1-E·2(DME) (see below). Monitoring a solution obtained by dissolving the orange-red {1-Z·(THF)/1-E·(THF)}-co-crystallized complexes in C6D6 by 1 H and 31P{1H} NMR spectroscopy revealed an almost complete conversion to the 1-Z isomer within a few hours; at equilibrium, ca. 8% of the 1-E isomer remained present. Complex 1-Z obtained in this way exhibited two singlets at δ 27.3 and 15.2 ppm in the 31P{1H} NMR spectrum (cf. δ 35.2 ppm for tBuPNtBuP) and doublets at δ 3.52 (2JPH = 7.3 Hz, 1H, CHP) and 2.43 ppm (2JPH = 4.8 Hz, 2H, CH2P) in the 1H NMR spectrum, all in support of the nonequivalence of the two pincer arms. The resonance for the ring protons in 1-Z were observed between δ 5.4 and 6.7 ppm (shielded relative to the corresponding neutral tBuPNtBuP ligand). Interestingly, recrystallization of the orange-red THF adduct described above in the presence of small amounts of dimethoxyethane (DME) led to single crystals of the complex 1-E·2(DME) suitable for X-ray diffraction analysis. Here the potassium cation is found in a distorted octahedral environment, four sites being occupied by two chelating DME ligands (Figure 3). The potassium cation is situated 1.582 Å out of the mean lutidine plane, probably for steric and electrostatic reasons. Perusal of the bond distances of the heterocycle in 1-E· 2(DME) reveals an alternation of single and double bonds, in support of dearomatization of the lutidine ring. Interestingly, the C15(H)−P2 bond (1.775 (2) Å) is slightly shorter than C6(H2)−P1 (1.856 (2) Å), while the P2−CtBu distances are marginally longer (by ca. 0.01 Å) than the P1−CtBu, confirming charge stabilization by negative hyperconjugation onto the phosphine moiety.28 DFT calculations on the model system MeP*NaMeP closely reproduce this trend, with calculated C(H)−P and C(H2)−P bond distances of 1.790 and 1.865 Å, respectively, for the optimized structures.29 The C α‑P lone pair→σ* P−CMe interaction has been estimated to be 73 kJ·mol−1. Lithium Salts. In order to study a possible influence of the countercation on the solution speciation of tBuP*NatBuP, the synthesis of Li(tBuP*NatBuP) was investigated. NMR monitoring of the deprotonation of tBuPNtBuP with LiCH2SiMe3 in C6D6 (Scheme 3) revealed almost exclusive formation of the “pincer”-like isomer 2-Z. Alternatively, 2-Z was conveniently prepared by reaction of the doubly deprotonated Li2(tBuP*NatBuP*)29 with 1 molar equiv of tBuPNtBuP (Scheme

Figure 2. Representation of the components 1-Z·(THF) (top) and 1E·(THF) (bottom) of the centrosymmetric structure of [K(tBuP*NatBuP)·THF]2; see text for details.

In the structure of 1-Z·(THF), the K+ is coordinated by a Zconfigured tBuP*NatBuP-κP*,κNa,κP anionic pincer and one THF molecule. The molecules of 1-Z·(THF) form dimers consisting of two subunits mutually interacting through K+ and the backbone Na (K1−N1′ 3.171(5) Å), Cα‑N (K1−C5′ 3.277(6) Å), and CHα‑P (K1−C15′ 3.383(6) Å) sites, which possess high π-electron density (Scheme 1).29 Thus, the K+ is found in a distorted seven-coordinate environment. In contrast, in the dimeric structure of 1-E·(THF), the tBuP* group is dangling and the K+ is coordinated by the amido Na, the P atom of the CH2P arm (i.e., κNa,κP), and one THF molecule (Figure 2). The K−C, K−Na, and K−P/K−P* bond distances are in line with electrostatic interactions and not unusual.32 However, a detailed analysis of the metrical data cannot be performed owing to the high level of disorder in the structure. C


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Figure 4. 31 P{ 1 H} (left) and 7 Li (right) NMR spectra of Li(tBuP*NatBuP) in a concentrated C6D6 solution. The assignments shown support the presence of 2-Z (ca. 94%) and 2-E (ca. 6%) at equilibrium.

Figure 3. Structure of 1-E·2(DME) with thermal ellipsoids at 30% probability except the atoms of the tBu and DME moieties, which are depicted as isotropic spheres. For clarity H atoms have been omitted except α-CHP (C15) and α-CH2P (C6). Selected bond distances (Å) and angles [deg]: K1−N1 2.828(2), K1−P1 3.333(1), N1−C1 1.344(3), N1−C5 1.386(3), C1−C2 1.377(4), C2−C3 1.396(4), C3−C4 1.354(4), C4−C5 1.438(3), C5−C15 1.390(3), C1−C6 1.515(3); C1−C6−P1 114.1(2), C5−C15−P2 124.3(2).

amount of the E isomer decreasing with increasing temperatures. However, no coalescence of the signals was observed, precluding any easy estimation of the isomerization energy barrier in C6D6. When a sample of 2-Z in C6D6 was taken to dryness and the solvent replaced with THF-d8, another major species was observed, resulting from decoordination of the CH2P arm (31P{1H} and 7Li NMR monitoring), since the corresponding 1 J(PCH2−Li) coupling was lost and 1J(PCH−Li) was retained. The minor species gave two singlets in the 31P{1H} NMR, consistent with decoordination of both P donors (see the SI). PNCNHC Monodeprotonation. Replacement of one Pdonor arm of RPNRP by an NHC donor, leading to the nonsymmetrical ligand RPNCNHC,23 should influence the acidity of the remaining α-CH2P protons. Access to the RPNCNHC free carbene and K(RP*NaCNHC) was recently achieved by the selective deprotonation of the imidazolium, phosphonium salt precursor using 2 or 3 equiv of KN(SiMe3)2, respectively (Scheme 4).23 Herein, we report on the isolation and characterization of the corresponding deprotonation products Li(RP*NaCNHC) (R = Cy, tBu) and compare them with the related M(tBuP*NatBuP) (M = K, Li) systems. The reaction of the nonsymmetrical imidazolium proligand to RPNCNHC, (RPNCim)Br (R = Cy, tBu), with 2 molar equiv of LiN(SiMe3)2 or LiCH2SiMe3 in ether afforded Li(RP*NaCNHC), after the successive abstraction of two protons, one at the azolium NCHN and one at the α-CH2P positions (Scheme 5). The 1H NMR spectrum of Li(RP*NaCNHC) in C6D6 confirmed side arm deprotonation, based on a doublet integrating for one proton assignable to the α-CHP (δ 3.61, 2JPH = 6.7 Hz for Li( Cy P*N aC NHC) and δ 3.83, 2J PH = 6.9 Hz, for Li(tBuP*NaCNHC)) and signals due to the dearomatized heterocycle (in the range δ 5.3−6.7 ppm, upfield compared to the aromatic pyridine ring). An experimental estimate of the α-CH2P acidity in Cy PNCNHC was attempted by carrying out the deprotonation of (CyPNCim)Br with 2 molar equiv of two different bases (Scheme 4) and monitoring the speciation in solution by 1H and 31P{1H} NMR spectroscopic methods. Thus, by using potassium carbazolide (pKa(carbazole) = 19.9 in DMSO),36 the Cy PNCNHC free carbene was formed without any trace of K(CyP*NaCNHC), even when a moderate excess of the base was used. In contrast, with potassium fluorenide (pKa(fluorene) = 22.6 in DMSO, 22.9 in THF),36,37 deprotonation of (CyPNCim) Br led to complete conversion to K(CyP*NaCNHC). Thus, the

Scheme 3. Synthesis of Li(tBuP*NatBuP) as the 2-Z Isomer with Trace Amounts of 2-E

3). In concentrated C6D6 solutions, traces (ca. 6%) of the second isomer, 2-E, could be detected (vide inf ra). In C6D6, such species most probably exist as tight ion-pairs.33 It is noteworthy that, for the related (2,6-bis((dimethylphosphino)methyl)phenyl)lithium featuring a central monoanionic C− donor, a dimeric structure was observed in the solid state that involves a bridging mode of this C− donor.34 In the 31P{1H} NMR spectrum of 2-Z in C6D6 (Figure 4, left), the two main 1:1:1:1 quartets, centered at δ 24.0 (1JPLi = 69 Hz) and 13.5 ppm (1JPLi = 87 Hz), can be assigned to the CH2P and CHP* arms, respectively. The multiplicity of the signals arises from the coordination of both P donors to the lithium cation (7Li, I = 3/2, 92%). Accordingly, a doublet of doublets was observed in the 7Li NMR spectrum (Figure 4, right) with the same coupling constants. The assignment of these spectroscopic signatures to the 2-Z isomer was confirmed by 1H-NOESY analysis (see the SI). In concentrated C6D6 solutions, traces (ca. 6%) of the 2-E isomer were evidenced on the basis of the following spectroscopic features: a doublet in the 7Li NMR spectrum, revealing coordination of one P donor only; accordingly, in the 31P{1H} NMR spectrum, the coordinated CH2P gave rise to a weak 1:1:1:1 quartet, and the dangling P* to a singlet at δ 13.2 ppm, overlapping with the signals due to 2-Z. Variable-temperature NMR experiments in C6D6 resulted in a slight variation of the ratio between 2-Z and 2-E, the relative D


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Organometallics Scheme 4. Successive Deprotonations of the Pro-ligands (RPNCim)Br·n(HBr) (R = Cy, tBu; n = 0, 1)a

a The stepwise deprotonations and the isolation of the free carbenes RPNCNHC (R = Cy, tBu) and the corresponding K(RP*NaCNHC) salts, using KN(SiMe3)2 as base, have been reported in previous communications (DiPP = 2,6-diisopropylphenyl).23,35

Scheme 5. Synthesis of the Different Isomers of Li(RP*NaCNHC) (R = Cy, tBu)a

a

Complexes underlined have been structurally characterized.

pKa corresponding to the monodeprotonation of α-CH2P in Cy PNCNHC could be estimated to be around 21−22 in THF, ca. 6 pKa units lower than for the α-CH2P in tBuPNtBuP (vide supra). This difference may be attributed to polarization and electron delocalization effects involving the NHC substituent. The existence of K(RP*NaCNHC) in solution as Z- and Eisomers in equilibrium, the extent of which was solvent dependent,23 raised the question of similar behavior with Li(RP*NaCNHC) and possible dependence of the speciation and the position of the equilibrium on the nature of the alkali metal (K vs Li). Reaction of 2 molar equiv of organolithium or lithium silylamide bases with (CyPNCim)·Br led to the selective formation of 3R-Z and 3R-E subject to the reaction conditions (as in Scheme 5 and the Experimental Section). The proposed nature of the species shown in Scheme 5 was confirmed by Xray crystallography in the solid state and NMR spectroscopy in solution. Crystallization by slow diffusion of pentane in a Et2O solution of Li(tBuP*NaCNHC) at −40 °C led to the isolation of 3tBu-E·2(Et2O), featuring a Li center chelated by the anionic ligand in a κNa,κCNHC bonding mode and further coordinated by two molecules of Et2O (Figure 5). The tBuP* arm, in E configuration, is dangling, and the plane of the picoline is slightly tilted (8.0°) with respect to that of the NHC ring (see Table 1 for metrical data). Crystallization of Li(RP*NaCNHC) by cooling a pentane solution in the presence of a few drops of Et2O led to 3R-Z· (Et2O), R = Cy, tBu (Figure 6). In both cases, the ligand is “pincer”-like (i.e., κP*,κNa,κCNHC), with the exocyclic double

Figure 5. Structure of 3tBu-E·2(Et2O) with thermal ellipsoids at 30% probability (the C atoms of the tBu and DiPP groups are depicted as spheres). H atoms are omitted except the α-CH (C21).

bond in Z configuration. The lacuna created by the anionic ligand is filled by the Li+ cation in electrostatic interactions, the coordination sphere of which is completed by one molecule of ether. In all three structures of Li(RP*NaCNHC) (Figures 5 and 6), alternating single and double bonds within the heterocycle (Table 1) point toward dearomatization of the picoline, notably with Cα‑N−Cα‑P separations consistent with double bonds (C20−C21 1.377(3)−1.385(3) Å) and concomitant sp2 hybridization of Cα‑P (C20−C21−P angle of 121.4(1)− 123.7(2)°). In all cases, the Li+ cation is in a distorted E


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Organometallics

P{1H} NMR spectrum (in C6D6, δ 14.5 (R = tBu) and −14.5 ppm (R = Cy) with 1JPLi = 69−70 Hz); similarly, in the 7Li NMR spectrum a doublet at δ 3.4−3.5 ppm was observed with the same coupling constant. Conversely, in 3tBu-E, the 1JPLi coupling was absent, and a singlet was observed in both the 31 1 P{ H} and the 7Li NMR spectra (in THF-d8, δ 12.6 and 1.54 ppm in the 31P and 7Li NMR spectra, respectively). From the spectral appearance of the individual isomers, the evolution of 3tBu-E to 3tBu-Z in pentane monitored by 31P{1H} NMR, depicted in Figure 7, is evident: the singlet corresponding to the dangling P in the E-isomer is slowly replaced by a 1:1:1:1 quartet (at δ 14.4 ppm, 1JPLi = 62 Hz in pentane) (Figure 7A). The reverse behavior occurred upon evaporation of the pentane solvent and redissolution in THF (Figure 7B). In addition, in the 7Li NMR spectrum (Figure 7C), the doublet (δ 1.37 ppm in THF for the Z-isomer) slowly disappeared at the expense of a singlet (δ 1.54 ppm in THF for the E-isomer). Therefore, the solvent polarity has a distinct influence on the relative Z-/Eisomer stability. Noteworthy, a significant variation of the 1JPLi coupling constant was observed for 3tBu-Z in different solvents (ca. 30 Hz in THF), probably due to modifications of the Li+ coordination sphere. Remarkably, addition of 12-crown-4 (ca. 2.4 equiv) to a C6D6 solution of Li(CyP*NaCNHC) did not result in decomplexation of the Li+ and/or subsequent isomerization of the CyP*NaCNHC moiety. However, both these processes were observable when 12-crown-4 was added to a THF-d 8 solution of Li(CyP*NaCNHC); this behavior is consistent with the organolithium species forming solvent-separated ions in THF solution and close contact ion-pairs in C6D633,38 facilitating in the former case the (partial) decomplexation of Li+ from Li(CyP*NaCNHC) in the presence of 12-crown-4. The major species inferred spectroscopically in the latter case is an Eisomer, with no identifiable interaction between the Li+ and the anion (i.e., interaction only with the Na or formation of a loose or solvent-separated ion-pair38). However, evaporation of the THF-d8 and redissolution of the residue in C6D6 reverses the coordination mode back to the Z-(κP*,κNa,κCNHC) mode, illustrating the influence of solvent polarity on formation and nature of organolithium species and emphasizing the higher stability of this tridentate coordination mode with Li+ in a nonpolar solvent.23 31

Table 1. Selected Bond Distances (Å) and Angles (deg) for 3Cy-Z·(Et2O), 3tBu-Z·(Et2O), and 3tBu-E·2(Et2O) R

P*NaCNHC

3Cy-Z·(Et2O)

3tBu-Z·(Et2O)

3tBu-E·2(Et2O)

Li1−P1 Li1−N3 Li1−C1 N3−C16 N3−C20 C16−C17 C17−C18 C18−C19 C19−C20 C20−C21 C20−C21−P1

2.680(4) 2.018(5) 2.156(5) 1.337(3) 1.390(3) 1.367(3) 1.415(4) 1.353(4) 1.437(4) 1.377(3) 123.7(2)

2.653(3) 2.012(3) 2.158(3) 1.329(2) 1.383(2) 1.364(2) 1.406(3) 1.346(3) 1.443(2) 1.382(2) 121.4(1)

2.020(4) 2.139(4) 1.337(3) 1.403(3) 1.372(3) 1.422(3) 1.351(3) 1.446(3) 1.385(3) 123.4(2)

tetrahedral environment, probably arising from a minimization of the steric repulsion. The monomeric structures of 3R-Z·(Et2O) are to be contrasted with those of the K+ analogues, which were reported as dimers in the solid state.23 The anionic ligand backbone in 3R-Z·(Et2O) is virtually planar. The plane of the dearomatized picoline and that of the NHC form angles of 5.9−8.7°; this is to be contrasted with the K complex [K{(Z)-(CyP*NaCNHCκP*,κNa,κCNHC}(Et2O)],23 in which they form angles of ca. 35−36°; these features could be ascribed to the relative size of the alkali metal ions and the nature of metal−ligand interactions. The Li+ cation is situated 0.543 and 0.651 Å from the plane defined by the RP*NaCNHC-κP*,κNa,κCNHC donors for R = Cy and tBu, respectively (cf. 1.592 and 1.674 Å for K+ in [K{(Z)-(CyP*NaCNHC-κP*,κN,κCNHC}(Et2O)]23). The metrical data in 3Cy-Z·(Et2O) and 3tBu-Z·(Et2O) are very similar (Table 1), but Li1−P is slightly longer in the former (2.680(4) Å vs 2.653(3) Å). In 3tBu, Li1−C1 is slightly shorter in the E- than in the Z-isomer (2.139(4) Å vs 2.158(3) Å, respectively). Interconversion between the isomers 3R-Z and 3R-E occurred in solution and could be conveniently monitored by the evolution of the magnitude of 1JPLi in their 31P{1H} and 7Li NMR spectra (Figure 7). In nonpolar solvents (e.g., toluene, C6D6, or pentane), 3R-E obtained as described above underwent slow isomerization to 3R-Z by rotation about the exocyclic Cα‑N−Cα‑P bond; the conversion was complete after a few days at room temperature. Specific spectral features assignable to the two isomers are easily accountable: in 3R-Z, the P donor coordinated to the lithium cation (7Li, I = 3/2, 92%) led to a 1:1:1:1 quartet in the

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CONCLUSION In conclusion, we have shown the viability and the diversity of K and Li reagents featuring dearomatized lutidine and picoline skeletons monodeprotonated at the α-C, and two P or one P

Figure 6. Structures of 3Cy-Z·(Et2O) (left) and 3tBu-Z·(Et2O) (right) with thermal ellipsoids at 30% probability (the C atoms of the Cy, tBu, and DiPP groups are depicted as spheres). H atoms are omitted except the α-CH (C21). F


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Organometallics

Figure 7. Evolution of the 31P{1H} (A, B) and 7Li (C) NMR spectra of Li(tBuP*NaCNHC) upon dissolution in pentane (left): 3tBu-E converting into 3tBu-Z; in THF (middle and right): 3tBu-Z converting into 3tBu-E; after 20 min (a), 1 day (b), and 4−5 days (c). literature procedures. Solid LiCH2 SiMe 3 was obtained from commercial solutions (Aldrich) by evaporation of the pentane under vacuum. For the synthesis of potassium carbazolide and fluorenide, see the SI. All other chemicals were obtained from commercial sources and used without further purification. NMR spectra were recorded on Bruker spectrometers (AVANCE I−300 MHz, AVANCE III−400 MHz, AVANCE III−600 MHz, or AVANCE I−500 MHz equipped with a cryogenic probe). Downfield shifts are reported in ppm as positive and referenced using signals of the residual protio solvent (1H), the solvent (13C), or externally (31P, 7Li). For 31P and 7Li NMR spectra recorded in nondeuterated solvents, the signal was “locked” on the THF-d8 present in a sealed capillary insert. All NMR spectra were measured at 298 K, unless otherwise specified. The multiplicity of the signals is indicated as s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet, and br = broad. Assignments (Figure 8)

and one NHC donor, respectively. These reagents are easily accessible by proper choice of the organic base, in line with the increased acidity of the nonsymmetrical RPNCNHC compared to the symmetrical RPNRP pincer-type ligand. This may point to tuning handles in the design of proton-responsive ligands, while maintaining a strong σ-donor environment. In the related tBu PNEtN′ ligand (EtN′ = NEt2) it has been shown computationally that more stable dearomatization structures were obtained for the α-CHPR2 vs the α-CHNR2 isomer, presumably due to inductive reasons.39 The facile formation and isolation of the E- and Z-isomers described here, which has not been alluded to in previous studies, is anticipated to be common with other dearomatized pincer-type ligands and illustrates that tridentate behavior may not always occur in coordination chemistry, in contrast to common belief. Furthermore, the interconversion of isomers under subtle variation of reaction conditions and the nature of the cation underlines a ligand characteristic that may occasionally have implications in the synthesis of transition metal complexes, using the alkali metal complexes described here as transfer reagents. Such interconversion may also support dynamic hemilability40 of one arm under catalytic conditions, in particular with metals featuring weaker interactions with the donor group and/or weaker ligand fields (e.g., 3d, nd10, electropositive transition metals).

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Figure 8. Atom numbering for the assignment of the NMR resonances in the dearomatized complexes. were determined on the basis of either unambiguous chemical shifts, coupling patterns (see the SI), and 13C-DEPT experiments or 2D correlations (1H−1H COSY, 1H−13C HSQC, 1H−13C HMBC). For the 1H−1H NOESY analysis of K(tBuP*NatBuP) and Li(tBuP*NatBuP), see the SI. Synthetic Procedure for K(tBuP*NatBuP). To a solution of tBuPNtBuP (0.175 g, 0.44 mmol) in THF (10 mL) precooled at −78 °C was added a cold solution of KBn (0.058 g, 0.44 mmol) in THF (5 mL). The resulting red solution was allowed to reach rt and stirred for 1 h. The deprotonation, monitored by 31P{1H} NMR spectroscopy, was confirmed by the disappearance of the peak at δ 35.2 and by the presence of four peaks (corresponding to the Z- and E-isomers, vide supra) in the range δ 10−30. If needed, additional KBn (5 mg, 0.04 mmol) was added until completion of the reaction. The solution was evaporated to dryness in vacuo, leading to quantitative isolation of [K(tBuP*NatBuP)·(THF)n] as a pink-red, extremely air-sensitive solid. A variable amount (0 ≤ n ≤ 1) of residual coordinated THF was

EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were performed under a dry argon atmosphere using standard Schlenk techniques or in an MBraun glovebox containing an atmosphere of N2. THF and Et2O were dried by refluxing over sodium/benzophenone ketyl and distilled under an argon atmosphere prior use. Pentane and toluene were dried by passing through columns of activated alumina and subsequently purged with argon. The solvents used for the synthesis of the Li and K salts were stored, after drying, over K mirror in the glovebox until use. C6D6 and THF-d8 were distilled over KH and were degassed by freeze−pump−thaw cycles. Benzyl potassium (KBn),41 2,6-bis(di-tert-butylphosphinomethyl)pyridine (tBuPNtBuP),2a Li2(tBuP*NatBuP*),29 (RPNCim)Br,35 and K(RP*NaCNHC)23 (R = Cy, tBu) were prepared according to the G


Article

Organometallics

Synthetic Procedure for Li(CyP*NaCNHC). To a suspension of ( PNCim)Br (0.10 g, 0.17 mmol) in Et2O (10 mL) precooled at −78 °C was added a solution of LiN(SiMe3)2 (0.060 g, 0.36 mmol) in Et2O (5 mL). The resulting suspension was allowed to reach rt and was stirred for 1 h, giving a pink-red solution. Removal of the volatiles under reduced pressure, extraction of the resulting solid with toluene (10 mL), filtration, and evaporation of the solvent gave Li(CyP*NaCNHC) as a red solid. Total conversion was confirmed by 31 1 P{ H} NMR spectroscopy, and the product was obtained in almost quantitative yields. Single crystals of 3Cy-Z·(Et2O) suitable for X-ray diffraction studies were obtained by cooling at −40 °C a pentane solution of the complex in the presence of a few drops of Et2O. Satisfactory elemental analysis data could not be obtained, due to the extreme air sensitivity of the complex and the variable amount of solvent incorporated in the isolated products. The purity of the complex can be assessed from its NMR spectra (see the SI). NMR data of 3Cy-Z·(Et2O) in C6D6: 1H NMR (400.13 MHz, C6D6): δ 7.19 (t, 3JHH = 7.7 Hz, 1H, p-CHDiPP), 7.05 (d, 3JHH = 7.7 Hz, 2H, m-CHDiPP), 6.95 (d, 3JHH = 1.8 Hz, 1H, CHimid), 6.67 (ddd, 3JHH = 8.7, 6.8 Hz, 5JPH = 1.6 Hz, 1H, CHpyr H4), 6.44 (d, 3JHH = 8.7 Hz, 1H, CHpyr H3), 6.31 (d, 3JHH = 1.8 Hz, 1H, CHimid), 5.32 (d, 3JHH = 6.8 Hz, 1H, CHpyr H5), 3.61 (d, 2JPH = 6.7 Hz, 1H, CHP), 3.25 (q, 3JHH = 7.0 Hz, 4H, CH2 residual Et2O), 2.53 (sept, 3JHH = 6.9 Hz, 2H, CH(CH3)2), 2.13−2.04 (m, 2H, Cy), 1.78−1.61 (m, 8H, Cy), 1.61− 1.50 (m, 2H, Cy), 1.42−1.11 (m, 10H, Cy), 1.09 (t, 3JHH = 7.0 Hz, 6H, CH3 residual Et2O), 1.06 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 1.01 (d, 3 JHH = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR (125.77 MHz, C6D6): δ 198.8 (br m, CNHC), 167.9 (d, 2JPC = 21.2 Hz, NCpyr), 151.0 (d, 4JPC = 1.8 Hz NCpyr), 146.0 (o-CDiPP), 137.7 (NCDiPP), 133.6 (d, 4JPC = 1.8 Hz, CHpyr C4), 129.6 (p-CHDiPP), 124.0 (m-CHDiPP), 122.4 (CHimid), 117.3 (d, 3JPC = 8.8 Hz, CHpyr C3), 116.1 (CHimid), 83.7 (CHpyr C5), 66.1 (CH2 residual Et2O), 63.2 (d, 1JPC = 23.6 Hz, CHP), 34.3 (d, JPC = 5.5 Hz, Cy), 31.2 (d, JPC = 13.7 Hz, Cy), 29.1 (d, JPC = 3.5 Hz, Cy), 28.3 (CH(CH3)2), 27.90 (d, JPC = 13.2 Hz, Cy), 27.86 (d, JPC = 7.0 Hz, Cy), 27.2 (Cy), 24.4 (CH(CH3)2), 24.3 (CH(CH3)2), 15.3 (CH3 residual Et2O). 31P{1H} NMR (161.98 MHz, C6D6): δ −14.6 (1:1:1:1 quartet, 1JPLi = 69 Hz). 7Li NMR (155.50 MHz, C6D6): δ 3.4 (d, 1JPLi = 69 Hz). NMR data of 3Cy-E (major species) in THF-d8 after addition of 12crown-4 (ca. 2.4 equiv): 1H NMR (400.13 MHz, C6D6): δ 7.61 (d, 3 JHH = 1.8 Hz, 1H, CHimid), 7.37 (dd, 3JHH = 8.3, 7.2 Hz, 1H, pCHDiPP), 7.26 (2 overlapping d, 3JHH ≈ 8.3, 7.2 Hz, 2H, m-CHDiPP), 7.06 (d, 3JHH = 1.7 Hz, 1H, CHimid), 6.73 (ddd, 3JHH = 9.0 Hz, 4JPH = 3.1 Hz, 4JHH = 0.6 Hz, 1H, CHpyr H3), 6.26 (ddd, 3JHH = 9.0, 6.6 Hz, 5 JHH = 0.9 Hz, 1H, CHpyr H4), 5.26 (br d, 3JHH = 6.6 Hz, 1H, CHpyr H5), 3.58 (s, 12-c-4), 3.15 (dd, 2JPH = 8.3 Hz, 5JHH = 0.9 Hz, 1H, CHP), 2.68 (sept, 3JHH = 6.9 Hz, 2H, CH(CH3)2), 1.95−1.56 (m, 12H, Cy), 1.38−1.15 (m, 10H, Cy), 1.15 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 1.09 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2). 31P{1H} NMR (161.98 MHz, C6D6): δ −20.0 (s). 7Li NMR (155.50 MHz, C6D6): δ 4.5 (s). Synthetic Procedure for Li(tBuP*NaCNHC). To a suspension of tBu ( PNCim)Br (0.22 g, 0.40 mmol) in Et2O (10 mL) precooled at −78 °C was added a solution of LiCH2SiMe3 (0.079 g, 0.84 mmol) in Et2O (5 mL). The resulting solution was allowed to reach rt and was stirred for 1 h, giving a pink-red solution. Removal of the volatiles under reduced pressure, extraction of the resulting solid with pentane (20 mL), filtration, and evaporation of the solvent gave Li(tBuP*NaCNHC) as a red solid in almost quantitative yield. Single crystals of 3tBu -Z· (Et2O) suitable for X-ray diffraction studies were obtained by cooling at −40 °C a pentane solution of the complex in the presence of a few drops of Et2O. Selective crystallization of 3tBu-E·2(Et2O) was carried out by slow diffusion of pentane in an Et2O solution of the complex at −40 °C. Satisfactory elemental analysis data could not be obtained, due to the extreme air sensitivity of the complex and the variable amount of solvent incorporated in the isolated products. The purity of the complex can be assessed from its NMR spectra (see the SI). NMR data of 3tBu-Z·(Et2O) in C6D6: 1H NMR (400.13 MHz, C6D6): δ 7.16 (t, 3JHH = 7.8 Hz, 1H, p-CHDiPP), 7.03 (d, 3JHH = 7.8 Hz,

detected in the 1H NMR spectrum depending on the drying time under reduced pressure. Recrystallization from a THF/pentane solution at −40 °C gave pink-red crystals of cocrystallized {1-Z· (THF)/1-E·(THF)} in a Z/E ratio of ca. 4:1. Single crystals of 1-E· 2(DME) were obtained by recrystallization at −40 °C of an Et2O solution of [K(tBuP*NatBuP)·(THF)n] in the presence of a few drops of DME. Satisfactory elemental analysis data could not be obtained, due to the extreme air sensitivity of the complex and the variable amount of solvent incorporated in the isolated products. The purity of the complex can be assessed from its NMR spectra (see the SI). 1H NMR (400.13 MHz, C6D6): δ 6.66 (ddt, 3JHH = 8.8, 6.5 Hz, 5JPH = 0.8 Hz, 1H, CHpyr H4), 6.37 (br d, 3JHH = 8.8 Hz, 1H, CHpyr H3), 5.46 (d, 3JHH = 6.5 Hz, 1H, CHpyr H5), 3.58−3.54 (m, 1.7H, OCH2 THF), 3.52 (d, 2 JPH = 7.3 Hz, 1H, CHP), 2.43 (d, 2JPH = 4.8 Hz, 2H, CH2P), 1.45− 1.39 (m, 1.7H, CH2 THF), 1.36 (d, 3JPH = 10.8 Hz, 18H, C(CH3)3), 1.06 (d, 3JPH = 11.0 Hz, 18H, C(CH3)3). 13C{1H} NMR (125.77 MHz, C6D6): δ 168.5 (dd, 2JPC = 19.0 Hz, 4JPC = 0.6 Hz, Cpyr C2/C6), 157.1 (d, 2JPC = 7.5 Hz, Cpyr C6/C2), 132.3 (d, 4JPC = 2.1 Hz, CHpyr C4), 116.1 (dd, 3JPC = 7.6 Hz, 5JPC = 1.6 Hz, CHpyr C3), 100.1 (d, 3JPC = 4.0 Hz, CHpyr C5), 67.8 (OCH2 THF), 60.7 (d, 1JPC = 1.6 Hz, CHP), 33.7 (d, 1JPC = 14.3 Hz, C(CH3)3), 32.3 (d, 1JPC = 15.6 Hz, CH2P), 31.7 (d, 1JPC = 17.3 Hz, C(CH3)3), 30.9 (d, 2JPC = 12.8 Hz, C(CH3)3), 30.0 (d, 2JPC = 12.3 Hz, C(CH3)3), 25.8 (CH2 THF) (assignment of 13 C{1H} NMR signals was confirmed by 13C DEPT and 1H−13C HSQC). 31P{1H} NMR (161.98 MHz, C6D6): δ 27.3, 15.2 + traces at δ 30.4, 14.8 (ca. 8% of the integration). In THF-d8, an equilibrium mixture between the Z- and E-isomers of [K(tBuP*NatBuP)·(THF)n] (0 < n < 1) was observed with a Z/E ratio of ca. 1.4:1 (confirmed by NOESY analysis; see the SI). Z-isomer (major): 1H NMR (400.13 MHz, THF-d8): δ 6.09 (ddd, 3JHH = 8.7, 6.5 Hz, 5JPH = 0.9 Hz, 1H, CHpyr H4), 5.68 (d, 3JHH = 8.7 Hz, 1H, CHpyr H3), 5.08 (d, 3JHH = 6.5 Hz, 1H, CHpyr H5), 3.03 (d, 2JPH = 7.7 Hz, 1H, CHP), 2.44 (d, 2JPH = 3.6 Hz, 2H, CH2P), 1.15 (d, 3JPH = 10.7 Hz, 18H, C(CH3)3), 1.08 (d, 3JPH = 10.5 Hz, 18H, C(CH3)3). 31P{1H} NMR (161.98 MHz, THF-d8): δ 24.5, 11.0. E-isomer (minor): 1H NMR (400.13 MHz, THF-d8): δ 6.65 (dd, 3JHH = 8.9 Hz, 4JPH = 3.2 Hz, 1H, CHpyr H3), 6.21 (dd, 3JHH = 8.9, 6.5 Hz, 1H, CHpyr H4), 5.14 (d, 3JHH = 6.5 Hz, 1H, CHpyr H5), 2.98 (d, 2JPH = 8.7 Hz, 1H, CHP), 2.44 (d, 2JPH = 3.6 Hz, 2H, CH2P), 1.15 (d, 3JPH = 10.6 Hz, 18H, C(CH3)3), 1.05 (d, 3JPH = 10.3 Hz, 18H, C(CH3)3). 31P{1H} NMR (161.98 MHz, THF-d8): δ 26.8, 13.0. Synthetic Procedure for Li(tBuP*NatBuP). To a solution of tBuPNtBuP (0.22 g, 0.56 mmol) in C6D6, precooled at −78 °C, was added LiCH2SiMe3 (0.053 g, 0.56 mmol). The resulting orange solution was allowed to reach rt and stirred for 1 h. Evaporation of the volatiles under vacuum afforded 2-Z as a pink-red solid in almost quantitative yield. Alternatively, to a solution of Li2(tBuP*NatBuP*) (0.015 g, 0.037 mmol) in C6D6 was added tBuPNtBuP (0.015 g, 0.037 mmol). Due to the extreme air sensitivity of 2-Z, satisfactory elemental analysis data could not be obtained, but the purity of the complex can be assessed from its NMR spectra (see the SI). 1H NMR (400.13 MHz, C6D6): δ 6.64 (ddd, 3JHH = 8.7, 6.4 Hz, 5JPH = 1.4 Hz, 1H, CHpyr H4), 6.38 (d, 3 JHH = 8.7 Hz, 1H, CHpyr H3), 5.42 (d, 3JHH = 6.4 Hz, 1H, CHpyr H5), 3.64 (d, 2JPH = 7.3 Hz, 1H, CHP), 2.45 (d, 2JPH = 5.7 Hz, 2H, CH2P), 1.30 (d, 3JPH = 12.3 Hz, 18H, C(CH3)3), 0.86 (d, 3JPH = 12.1 Hz, 18H, C(CH3)3). 13C{1H} NMR (125.77 MHz, C6D6): δ 170.2 (dd, 2JPC = 22.2 Hz, 4JPC = 1.6 Hz, Cpyr C2), 155.5 (d, 2JPC = 5.2 Hz, 4JPC = 1.7 Hz, Cpyr C6), 133.6 (dd, 4JPC = 2.4, 1.0 Hz, CHpyr C4), 116.3 (dd, 3JPC = 10.8 Hz, 5JPC = 1.2 Hz, CHpyr C3), 99.2 (d, 3JPC = 4.1 Hz, CHpyr C5), 60.4 (d, 1JPC = 26.8 Hz, CHP), 32.7 (d, 1JPC = 3.9 Hz, C(CH3)3), 31.4 (d, 1JPC = 1.4 Hz, CH2P), 31.1 (d, 1JPC = 5.5 Hz, C(CH3)3), 30.1 (d, 2 JPC = 10.6 Hz, C(CH3)3), 29.4 (d, 2JPC = 10.0 Hz, C(CH3)3) (assignment confirmed by 13C DEPT). 31P{1H} NMR (161.98 MHz, C6D6): δ 24.0 (1:1:1:1 quartet, 1JPLi = 69 Hz, integrating for 1P, CH2P), 13.5 (1:1:1:1 quartet, 1JPLi = 87 Hz, integrating for 1P, (Z)CHP) + signals integrating for ca. 0.06P at δ 21.1 (1:1:1:1 quartet, 1JPLi ≈ 60 Hz, CH2P) and 13.2 (s, (E)-CHP). 7Li NMR (155.50 MHz, C6D6): δ 4.23 (dd, 1JPLi = 87, 69 Hz, P-Li-P), 1.43 (d, 1JPLi = 63 Hz, ca. 6% of the integration, P-Li).

Cy

H


Article

Organometallics 2H, m-CHDiPP), 6.95 (d, 3JHH = 1.8 Hz, 1H, CHimid), 6.68 (ddd, 3JHH = 8.8, 6.8 Hz, 5JPH = 1.7 Hz, 1H, CHpyr H4), 6.44 (dd, 3JHH = 8.8 Hz, 1H, CHpyr H3), 6.31 (d, 3JHH = 1.8 Hz, 1H, CHimid), 5.32 (d, 3JHH = 6.8 Hz, 1H, CHpyr H5), 3.82 (d, 2JPH = 6.9 Hz, 1H, CHP), 3.25 (q, 3JHH = 7.0 Hz, 6H, CH2 residual Et2O), 2.53 (sept, 3JHH = 6.9 Hz, 2H, CH(CH3)2), 1.21 (d, 3JPH = 11.8 Hz, 18H, C(CH3)3), 1.10 (t, 3JHH = 7.0 Hz, 9H, CH3 residual Et2O), 1.05 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 1.00 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR (125.77 MHz, C6D6): δ 198.6 (1:1:1:1 quartet, 1JCLi = 32.0 Hz, CNHC), 167.2 (d, 2JPC = 21.2 Hz, NCpyr), 151.0 (d, 4JPC = 2.5 Hz NCpyr), 146.0 (o-CDiPP), 137.6 (NCDiPP), 133.6 (d, 4JPC = 2.7 Hz, CHpyr C4), 129.6 (p-CHDiPP), 124.0 (m-CHDiPP), 122.4 (CHimid), 117.3 (d, 3JPC = 9.1 Hz, CHpyr C3), 116.1 (CHimid), 83.7 (CHpyr C5), 66.0 (CH2 residual Et2O), 64.9 (d, 1JPC = 23.4 Hz, CHP), 32.6 (d, 1JPC = 0.7 Hz, C(CH3)3), 30.1 (d, 2JPC = 11.2 Hz, C(CH3)3), 28.3 (CH(CH3)2), 24.5 (CH(CH3)2), 24.2 (CH(CH3)2), 15.4 (CH3 residual Et2O). 31P{1H} NMR (161.98 MHz, C6D6): δ 14.5 (1:1:1:1 quartet, 1JPLi = 70 Hz). 7Li NMR (155.50 MHz, C6D6): δ 3.5 (d, 1JPLi = 70 Hz). NMR data of 3tBu-E·2(Et2O) in C6D6 (E/Z mixture in a ratio 1:0.23 due to partial E → Z isomerization): 1H NMR (400.13 MHz, C6D6): δ 7.61 (dd, 3JHH = 9.0 Hz, 4JPH = 3.5 Hz, 1H, CHpyr H3), 7.11 (t, 3JHH = 7.8 Hz, 1H, p-CHDiPP), 6.97 (dd, 3JHH = 7.8 Hz, 4JHH = 1.3 Hz, 1H, mCHDiPP), 6.94 (d, 3JHH = 1.7 Hz, 1H, CHimid), 6.91 (dd, 3JHH = 7.8 Hz, 4 JHH = 1.3 Hz, 1H, m-CHDiPP), 6.67 (dd, 3JHH = 9.0, 6.9 Hz, 1H, CHpyr H4), 6.25 (d, 3JHH = 1.7 Hz, 1H, CHimid), 5.42 (d, 3JHH = 6.9 Hz, 1H, CHpyr H5), 3.54 (d, 2JPH = 7.8 Hz, 1H, CHP), 3.26 (q, 3JHH = 7.0 Hz, 5.3H, CH2 residual Et2O), 3.02 (sept, 3JHH = 6.9 Hz, 1H, CH(CH3)2), 1.94 (sept, 3JHH = 6.9 Hz, 1H, CH(CH3)2), 1.30−0.97 (m, 30H, C(CH3)3 + CH(CH3)2), 1.11 (t, 3JHH = 7.0 Hz, 8.0H, CH3 residual Et2O). 31P{1H} NMR (161.98 MHz, C6D6): δ 13.8 (s). 7Li NMR (155.50 MHz, C6D6): δ 4.9 to −1.5 (br s). X-ray Crystallography. General Methods. Suitable crystals for the X-ray analysis of all compounds were obtained as described above. Summary of the crystal data and data collection and refinement for compounds are given in Table S1 (see the SI). The crystals were mounted on a glass fiber with grease, from Fomblin vacuum oil. Data sets were collected at 173(2) K on a Bruker APEX-II CCD Duo diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.710 73 Å). Specific comments for each data set are given below. The cell parameters were determined (APEX2 software)42 from reflections taken from three sets of 12 frames, each at 10 s exposure. The structures were solved by direct methods using the program SHELXS-2013.43 The refinement and all further calculations were carried out using SHELXL-2013.43b The H atoms were introduced into the geometrically calculated positions (SHELXL-2013 procedures) and refined riding on the corresponding parent atoms. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. The following specific comments apply to the models of the structures: In the structure of {1-Z·(THF)/1-E·(THF)}, the atoms C2, C3, C4, C5, N1, C15, P2, C16, C17, C18, C19, C20, C21, C22, C23, and K1 are disordered over two positions with an occupancy ratio of 0.8/ 0.2. The atoms C8, C9, C10, O1, C24, C25, and C26 are disordered over two positions with an occupancy ratio of 0.5/0.5. A SQUEEZE44 procedure was applied, and the residual electron density was assigned to one-half disordered molecule of pentane. The R factors are relatively high (R1 = 10.5%, wR2 = 30.6%) due to the high level of disorder. In the structure of 3tBu-Z·(Et2O), a SQUEEZE44 procedure was applied and the residual electron density was assigned to one-half disordered molecule of Et2O.

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NMR spectra, assignment of the 1H NMR resonances, NOESY structural analyses, and crystallographic summary table (PDF) Crystallographic data for {1-Z·(THF)/1-E·(THF)}, 1-E· 2DME, 3tBu-E·2(Et2O), 3Cy-Z·(Et2O), and 3tBu-Z·(Et2): CCDC 1446242−1446246 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The USIAS, CNRS, Unistra, Région Alsace, and Communauté Urbaine de Strasbourg are acknowledged for the award of fellowships and a Gutenberg Excellence Chair (2010−11) to A.A.D. We thank the CNRS and the MESR (Paris) for funding and for a Ph.D. grant to T.S. We are grateful to Dr. Gilles Frison (LCM, CNRS, Ecole Polytechnique, Université ParisSaclay) for the DFT calculations.

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REFERENCES

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