Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts

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Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts for ROMP of Norbornene Sudarsan VenkatRamani, Christopher D. Roland, James G. Zhang, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: This report details the synthesis and characterization of a series of Nb complexes and one tantalum complex supported by the [CF3−ONO]3− trianionic pincer-type ligand. Access to the trianionic Nb dichloride complex, [CF3− ONO]NbCl2(OEt2) (3-Et2O), allows for the synthesis of Nb-dialkyl complexes, [CF3−ONO]NbR2 (where R = benzyl (5), neopentylsilyl (6), neophyl (7)). The sterically encumbered Nb-neophyl complex (7) is thermally stable and fails to convert to the alkylidene even in the presence of donor ligands. Complex 7, however, promotes catalytic ring-opening metathesis polymerization (ROMP) of norbornene, suggesting the plausible intermediacy of a Nb-alkylidene. The corresponding Ta analogue, [CF3−ONO]Ta(CH2C(CH3)2(C6H5))2 (9), requires dramatically higher temperatures to initiate ROMP and provides poor yields of polymer. Complexes 5 and 6 also promote ROMP of norbornene. Characterization of all new complexes includes multinuclear NMR spectroscopy and combustion analysis. For complex 7, characterization also includes solid-state structure elucidation via a single crystal X-ray diffraction experiment.

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INTRODUCTION Well-documented over the past 40 years are examples of ROMP of norbornene employing well-defined organotransition metal complexes bearing an alkylidene moiety.1−15 Notable early work with group IV and V metal ions includes Grubbs’ use of “Cp2TiCH2” (Tebbe reagent),16 and Schrock’s Ta CHtBu(THF) (OR)3,17 both of which initiate living ROMP. Contemporary norbornene ROMP catalysts employing Rucarbenes3,4,18−20 and W- and Mo-alkylidenes,1,2,5,6,9,14,21−24 continue to be extensively studied. However, numerous other “recipes” involving metal halides and cocatalysts (often alkylating agents),25−31 also catalyze ROMP of norbornene to generate polynorbornene in comparable yields and activity, despite the fact that no alkylidene moiety is ever detected. The presumption is that metal alkylidenes form in situ, which rapidly initiate ROMP. Nb- and Ta-alkylidenes, which promote little or no olefin metathesis, 32−34 can initiate ROMP of strained cyclic alkenes.17,35 Extensive studies by Mashima and co-workers specifically demonstrate the efficacy of Ta and Nb-alkyl complexes as precatalysts for ROMP of norbornene.36−40 Often kinetically unstable,41−44 group V alkyl complexes, when suitably sterically protected, transform into alkylidenes45−47 and alkylidynes,48−50 and the alkyl complexes also react with aniline−and phosphine−derivatives to form imido51−53 and phosphido complexes,54,55 respectively. The alkyl complexes may also be appended to other monodentate and multidentate ligands to generate potent catalytic systems.56−61 © XXXX American Chemical Society

Trianionic pincer ligands constrain three anionic donors to the meridional plane and can stabilize metal ions in highoxidation states.62 Among the numerous trianionic pincer and pincer-type ligands known, the [CF3−ONO]3− ligand, in particular, engenders unique reactivity to metal complexes. The combination of weakly basic fluoro alkoxides and a πdonating amido donor within the [CF3−ONO]3− motif generates metal complexes with interesting structure/bonding relationships63−68 and reactivity.63,69,70 Specific to metal− carbon multiple bonds, the lone pair of electrons on the central amido donor within the [CF3−ONO]3− ligand can align with adjacent M = C or MC π-bonds, thereby enhancing their nucleophilicity via the inorganic enamine effect.63,69,70 Previously, we sought to accentuate the already highly nucleophilic character of Ta-alkylidenes71 with an inorganic enamine capable ONO pincer-type ligand to form [CF3− ONO]TaCHR complexes.64 However, the precursor dialkyl complexes [CF3−ONO]TaR2 were unusually stable, thus thwarting the synthesis of the corresponding alkylidenes. Recognizing that Nb can display dramatically different reactivity relative to Ta because the electronic structure of Nb complexes permit access to a higher density of states,72 this current work centers on attempts to access trianionic pincer niobium alkylidenes. Received: May 25, 2016

A

DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Syntheses of [CF3−ONHO]NbCl3 (2) and [CF3−ONO]NbCl2(OEt2) (3-Et2O)

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Scheme 2. Ligand Displacement: Synthesis of [CF3− ONO]NbCl2(L) (3-L)

RESULTS AND DISCUSSION Entry into niobium chemistry with the [CF3−ONO]H3 (1) proligand is straightforward. Mixing benzene solutions of the proligand and NbCl5 at room temperature and stirring for 4 h generates the dianionic pincer ligand complex, [CF3−ONHO]NbCl3 (2), with loss of 2 equivalents of HCl (Scheme 1). The acidity of the fluorinated alcohols in the [CF3−ONO]H3 proligand and the oxophilicity of niobium are responsible for the relatively easy metalation. Addition of diethyl ether to the dianionic pincer complex 2 provides access to the trianionic pincer ligand complex, [CF3−ONO]NbCl2(OEt2) (3), as its ether adduct. The 19F spectrum of 2, in CDCl3, exhibits four distinct quartets (−68.4, −69.7, −70.9, and −72.4 ppm) for the −CF3 groups on the ligand, indicating the complex is C1 symmetric in solution. The quartet at −72.4 ppm is a doublet of quartets. Previously observed in the tantalum analogue, [CF3−ONHO]TaCl3,64 this resonance pattern arises from an intraligand N− H···F coupling of the amine proton (−NH) with a −CF3 group.64 Crystallographic evidence for the N−H···F interaction was demonstrated within [CF3−ONHO]TaCl3.64 Remarkably, for both the tantalum and niobium dianionic pincer ligand complexes [CF3−ONHO]MCl3 (where M = Ta, Nb), the N− H···F interaction also persists in the solution-state. Irradiation of the −CF3 group at −68.4 ppm decouples the doublet of quartets at −72.4 ppm into a doublet (JH···F = 6.6 Hz; CDCl3), thus confirming the interaction with the amine proton. Correspondingly, in the 1H NMR spectrum of 2 the amine proton exhibits coupling to the −CF3 group and appears as a quartet (JH···F = ∼6.4 Hz in C6D6). Moreover, irradiating the −CF3 group at −72.4 ppm resolves the amine proton into a singlet. The doublet of quartets resonance pattern in the 19F NMR spectrum serves as a useful spectroscopic handle because it is a clear indication that the ONO pincer-type ligand is in the dianionic state. The diethyl-ether-induced conversion of 2 to 3-Et2O involves a vivid color change from tan to maroon. A 19F NMR spectrum of 3-Et2O in C6D6 exhibits only two sets of quartets for the four −CF3 groups on the ligand, indicating 3Et2O is C2 symmetric in solution. The 1H NMR spectrum in C6D6 also reflects C2 symmetry, with the ligand methyl groups resonating as a singlet at 1.91 ppm, rather than two singlets expected for a C1 symmetric complex. The resonances for the bound diethyl ether are shifted downfield to 3.90 ppm (−OCH2) and upfield to 1.06 ppm (CH3), which differ significantly from free diethyl ether (3.26 and 1.11 ppm in C6D6).73 L-type donors74 such as CH3CN (ACN), THF, pyridine, DME, and Ph3PO displace the diethyl ether ligand; alternatively, addition of the same donors (viz., ACN, THF, DME, Ph3PO, and pyridine) also effect the conversion of [CF3−ONHO]NbCl3 (2) to [CF3−ONO]NbCl2(L) (3-L; Scheme 2).

Adding pyridine to [CF3−ONHO]NbCl3 generates the complex salt ([CF3−ONO]NbCl3) (Pyr·H) (major), in addition to the desired [CF3−ONO]NbCl2(Pyr) (minor). The propensity to form a complex salt is true for any L-type ligand that can also serve as an external base. a

The halide ligands within the trianionic pincer complex [CF3−ONO]NbCl2(OEt2) (3-Et2O) are useful for derivatization. Access to [CF3−ONO]NbX2 (where X = OtBu, alkyl) is possible through straightforward salt-metathesis reactions, and one-electron reduction processes seem to be minimized for the substrates used in this study.75,76 All of the salt-metathesis reactions follow a similar procedure involving addition of the Xtype ligand74 (−OtBu, Grignard reagents), to a cold stirring solution of the trianionic dichloride complex (3-Et2O) (Scheme 3). n-Pentane is the preferred solvent as the salt byproducts are insoluble. Typically, a tacky oil results upon workup of reactions left to stir for 4 h. Addition of a few milliliters of acetonitrile to the tacky oil and then evaporation of all volatiles produces microcrystalline powders. Employing the above procedure, the tert-butoxide (−OtBu) and a series of alkyl complexes lacking β-hydrogens were prepared. In light of the similarity in their syntheses and their spectral features, only pertinent spectral attributes are discussed. The 1H and 19F NMR spectra of [CF3−ONO]Nb(OtBu)2 (4) in C6D6 exhibit signals attributable to a C2-symmetric complex. The −CF3 groups appear as two quartets at −71.2 ppm and −75.5 ppm. The −OtBu protons appear as a singlet at 1.18 ppm. Complex 4 is somewhat unstable in solution, as B

DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 3. Syntheses of Complexes 4−7 by Salt Metathesis

evidenced during NMR measurements, and decomposes over a few days at ambient temperature. The dibenzyl complex [CF3−ONO]Nb(CH2C6H5)2 (5) is a red microcrystalline powder. In the solution state, the complex exhibits averaged C2 symmetry. Two quartets appear at −70.4 and −74.9 ppm in the 19F spectrum in C6D6. A distinct feature within the 1H NMR spectrum (C6D6) is a singlet resonance for the methylene protons (on the benzyl moiety). The methylene protons, expected to be diastereotopic, coincidentally resonate at 2.52 ppm and therefore do not couple. The singlet resonance resolves into two doublets upon cooling a toluene-d8 solution of 5 to −55 °C. Similar coincidental overlap, and subsequent low-temperature resolution of the resonances for the methylene protons, was also observed in the tantalum complex, [CF3− ONO]Ta(CH2C6H5)2.64 The neopentylsilyl derivative, [CF3−ONO]Nb(CH2SiMe3)2 (6), is an orange microcrystalline powder. Exhibiting C2 symmetry in solution, two quartets appear at −70.6 and −75.1 ppm in the 19F spectrum in C6D6. The 1H NMR spectrum of 6 in C6D6 is unexceptional. The methylene protons, unlike the benzyl analogue, appear as two doublets centered at 2.21 and 2.14 ppm. The −SiMe3 protons integrating to 18H appear as one singlet at 0.05 ppm. Considered sterically equivalent to neopentyl, numerous complexes of Nb and Ta bearing the neophyl ligand exist.77−83 Adding a diethyl ether solution of 2-methyl-2-phenylpropylmagnesium chloride (neophylmagnesium chloride or PhCMe2CH2MgCl) to 3-Et2O generates [CF3−ONO]Nb(CH2(CMe2Ph)2) (7), as an orange microcrystalline powder. The spectral features of 7 are analogous to the other dialkyl complexes 4−6. Two quartets centered at −69.8 and −74.4 ppm in the 19F NMR spectra in C6D6 confirm the complex is C2 symmetric in solution. In the 1H NMR spectrum of 7, the methyl protons from the pincer ligand resonate as a singlet at 1.94 ppm. Within the neophyl ligand, the methylene protons are diastereotopic and appear as two doublets centered at 2.38 and 2.11 ppm with a characteristic geminal coupling constant of 11.6 Hz (2JHH). The neophyl methyl protons are also diastereotopic and appear as two singlets at 1.31 and 1.20 ppm. Conclusive evidence for the molecular structure of 7 comes from X-ray diffraction performed on single crystals obtained from the slow evaporation of a diethyl ether-pentane solution of 7 (Figure 1). The non-H atoms in 7 were refined with

Figure 1. Molecular structure of 7 with hydrogen atoms removed for clarity (only complex A of the asymmetric unit shown). Selected bond distances (Å): Nb1−O1 1.9108(14), Nb1−O2 1.8960(14), Nb1−N1 2.0312(18), Nb1−C21 2.152(2), Nb1−C31 2.181(2), N1−C7 1.423(3), N1−C17 1.426(3). Selected bond angles (deg): ∠O2− Nb1−O1 163.68(6), ∠C21−Nb1−C31 120.49(8), ∠C7−N1−C17 114.06(17), ∠C7−N1−Nb1 124.88(14), ∠C17−N1−Nb1 120.62(14), ∠C22−C21−Nb1 128.12(15), ∠C32−C31−Nb1 120.11 (15).

anisotropic thermal parameters; all of the H atoms were calculated in idealized positions and refined riding on their parent atoms. The asymmetric unit of 7 consists of two chemically equivalent but crystallographically independent Nb complexes. Both complexes (A and B) exhibit similar bond lengths and angles (Table 1). The discussion, therefore, is restricted to the crystallographic metric parameters of complex A. The molecular structure of 7 exhibits C1 symmetry in the solid state. The Nb(V) ion in 7 adopts a distorted trigonal bipyramidal geometry (τ5 = 0.72),84 with the oxygen donors from the alkoxide ligand occupying apical positions, and the neophyl ligands occupying equatorial positions. The sum of the angles around N1 of 359.56(26)° suggests it is sp2-hybridized. The short Nb−N1 bond length of 2.0312(18) Å further corroborates the anionic nature of N1 (amido form; X-type ligand74). The bond length of 2.0312(18) Å is similar to other known group V complexes bearing the [CF3−ONO]3− ligand; relevant examples include {[CF3−ONO]TaCl3} {HNEt3}64 with a Ta−Npincer bond length of 2.0222(19) Å, and [CF3− ONO]Ta(NMe2)2(HNMe2)64 with a Ta−Npincer bond length of 2.119(5) Å. The Ta−Npincer bond is considerably longer when the Npincer is in its amine form (L-type ligand74), exemplified by the complex [CF3−ONHO]TaCl3 with Ta− Npincer bond length of 2.327(4) Å. The weakly basic alkoxides on the axial sites subtend 163.68(6)° (∠O2−Nb1−O1 = 163.68(6)°), and the methylene carbons on the equatorial neophyl ligands subtend 120.49(8)° (∠C21−Nb1−C31 = 120.49(8)°). In both the neophyl ligands within 7, the ∠Nb− CH2−Ctert angles are rather skewed from expected tetrahedral geometry, with ∠C22−C21−Nb1 measuring 128.12(15)° and ∠C32−C31−Nb1 measuring 120.11(15)°. The bent arrangement seems to conform with the only other Nb−neophyl C

DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX

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Table 1. Comparison of Selected Bond Lengths (Å) and Angles (deg) of Complexes A and B in the Asymmetric Unit of 7 complex A Nb1A−O1A Nb1A−O2A Nb1A−N1A Nb1A−C21A Nb1A−C31A N1A−C7A N1A−C17A complex A ∠O2A−Nb1A−O1A ∠C21A−Nb1A−C31A ∠C7A−N1A−C17A ∠C7A−N1A−Nb1A ∠C17A−N1A−Nb1A ∠C22A−C21A−Nb1A ∠C32A−C31A−Nb1A

bond length (Å)

complex B

1.9108(14) 1.8960(14) 2.0312(18) 2.152(2) 2.181(2) 1.423(3) 1.426(3) bond angle (deg)

Nb1B−O1B Nb1B−O2B Nb1B−N1B Nb1B−C21B Nb1B−C31B N1B−C7B N1B−C17B complex B

bond length (Å) 1.9141(14) 1.9073(14) 2.0211(17) 2.145(2) 2.166(2) 1.426(3) 1.431(3) bond angle (deg)

∠O2B−Nb1B−O1B ∠C21B−Nb1B−C31B ∠C7B−N1B−C17B ∠C7B−N1B−Nb1B ∠C17B−N1B−Nb1B ∠C22B−C21B−Nb1B ∠C32B−C31B−Nb1B

163.68(6) 120.49(8) 114.06(17) 124.88(14) 120.62(14) 128.12(15) 120.11(15)

crystal structure known, [Tp*NbCl(neophyl)(NtBu)] from Gomez et al.,78 where the ∠Nb−CH2−Ctert subtends an even larger angle of 138.0(5)°. The deviation in the ∠Nb−CH2−Cter angle is likely a consequence of steric congestion. The Nb− CH2 bond distance in 7 measures 2.152(2) Å and 2.181(2) Å for Nb−C21 and Nb−C31 respectively; the same bond measures 2.232(7) Å within [Tp*NbCl(neophyl)(NtBu)]. Metal dialkyl complexes transform via α-hydrogen abstraction into their corresponding alkylidenes either through thermolysis, or in the presence of σ-donor ligands such as THF and PMe3.45,49,85−88 Considering the steric congestion around the niobium metal center in 7, the complex was heated in a sealable NMR tube with the aim of promoting α-hydrogen abstraction to afford the Nb-neophylidene. Much like the tantalum analogue supported by the [CF3−ONO]3− ligand, 7 exhibits remarkable stability; extensive heating to 100 °C for a week induces minimal decomposition. Even addition of PMe3 (and heating to 100 °C) does not induce α-hydrogen abstraction. Displaying similar stability, Ti(IV)-alkyl complexes supported by the [CF3−ONO]3− ligand fail to react with substrates (H2, CH2CH2, and CO2).66 Despite the apparent stability of 7, in situ alkylidene formation was probed through addition of norbornene (NBE) (Scheme 4). Addition of 1.5 equiv of norbornene to a C6D6 solution of 7 in a sealable NMR tube at ambient temperature shows immediate growth of resonances between 5 to 5.5 ppm in the 1H NMR spectrum. The resonances are attributable to the alkene protons of polynorbornene formed through ring-opening metathesis polymerization (ROMP). Over time (3 h), the alkene signal intensity and viscosity of

163.30(6) 117.84(8) 114.66(17) 125.28(14) 119.97(13) 128.34(15) 119.21(14)

the reaction mixture increases in the NMR tube confirming polymerization, and thus presumed intermediacy of an alkylidene initiator. Importantly, the other dialkyl complexes 5 and 6 behave analogously to 7, generating cis-selective polynorbornene. A control experiment employing just the dichloride complex 3-THF gave poor polymerization results and a polymer that was not stereoselective. For brevity, the ensuing discussion on polymer characterization and tacticity determination will be restricted to polynorbornene generated by 7. NMR tube-scale experiments with varying ratios of 7 and norbornene (1:25, 1:50, 1:100) in C6D6 at 60 °C (for 15 h) serve to elucidate ROMP activity (Table 2). Elevated Table 2. Polymerization of Norbornene by Precatalysts 5−7 with Different Monomer/Catalyst Ratios entry

precatalyst

[monomer/catalyst]0

yield (%)

% cisb

1 2 3 4 5 6 7

7 7 7 7 7 5 6

25:1 (NMR tube) 50:1 (sealed RBF)a 50:1 (NMR tube) 100:1 (sealed RBF)a 100:1 (NMR tube) 100:1 (sealed RBF)a 100:1 (sealed RBF)a

60 56 76 53 62 64 46

89 76 85 83 85 77 86

a

A toluene solution of the precatalyst was added to norbornene in toluene; [monomer]0 = 0.1 M. bDetermined by 1H NMR spectroscopy (CDCl3).

temperatures are necessary to drive the reaction to completion and to overcome complications from an increase in viscosity. Upon completion of the polymerization, pouring the contents of the NMR tube into stirring methanol precipitates the poly(norbornene). The isolated polymer exhibits 1H NMR resonances (CDCl3) that are consistent with literature reports for poly(norbornene) formed via ROMP.4 Focusing on the alkene region (5−6 ppm), the 1H NMR spectra of the poly(norbornene) produced by 7 display enriched cis (5.21 ppm) relative to trans (5.35 ppm) resonances, indicating the active catalyst produces ∼85% cis-selective poly(norbornene).4 This ∼85% cis-enrichment of the double bond in the polymer is consistent for all polymerization trials, regardless of the ratio of 7 to norbornene. 13C{1H} NMR spectroscopy can establish tacticity in poly(norbornene). The 13C{1H} NMR spectrum for

Scheme 4. Polymerization of Norbornene by 7

D

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Figure 2. Expansion of 1H NMR spectrum of partially brominated (2% conversion) ∼85% cis, syndiotactic enriched poly(norbornene); inset shows the 1′ and 6′ protons (two overlapping doublets) in the partially brominated product.

dicyclopentadiene (DCPD),24,35 another common strained cyclic alkene. The poly(norbornene) samples generated by 7 were analyzed via gel permeation chromatography (GPC). Solubility of the polymers in THF was low. The low solubility coupled with broad dispersities (see SI Figures S94 and S95 for traces) make accurate molecular weight determination difficult, if not impossible. Poor solubility may also indicate the resulting polymers are of substantial molecular weight. Nevertheless, the presence of isolable polymer provides support for in situ generation of a niobium alkylidene with activity, albeit low, for the polymerization of norbornene. Considering the reactivity of 7 with norbornene, it seemed reasonable that the corresponding Ta-analogue would also initiate ROMP. The Ta-neophyl complex [CF3−ONO]Ta(CH2C(CH3)2(C6H5))2 (9), forms upon addition of two equiv of neophylmagnesium chloride to the dichloride complex [CF3−ONO]TaCl2(THF) (8) (Scheme 5). The yellow microcrystalline complex 9 exhibits NMR resonances similar to the Nb-analogue. The methyl resonances from the pincer ligand on 9 resonate as a singlet at the identical value of 1.94 ppm in the 1H NMR spectrum, indicating an averaged C2symmetry in solution. The diastereotopic methylene protons within the neophyl ligands resonate as doublets centered at 1.91 (2JHH = 12.7 Hz) and 1.71 ppm (2JHH = 12.8 Hz). Finally, in the 19F NMR spectrum two resonances at −70.0 and −74.7 ppm attributable to the −CF3 groups are again consistent with C2-symmetry. Treating a C6D6 solution of 9 with 21 equiv of norbornene at 60 °C in an NMR tube experiment shows very little conversion to the polymer (∼10%). Increasing the temperature pro-

poly(norbornene) obtained from precatalyst 7 exhibits resonances that indicate the polymer is highly tactic,6 and within the same spectrum there exists resonances attributable to atactic poly(norbornene) (see SI).6 Although iso- and syndiotactic poly(norbornene) exhibit very similar 13C NMR chemicals shifts for their olefinic carbon atoms (133.86 for syndio- vs 133.87 ppm for isotactic, in CDCl3 at 100.61 MHz), the resonances pertaining to the aliphatic carbons (42.68 and 33.19 ppm for syndio-; 42.61 and 33.24 ppm for isotactic) are distinguishable, even in a 9:1 mixture of syndiotactic:isotactic poly(norbornene).6 The chemical shifts for the polymer generated by 7 match with those assigned for cis, syndiotactic poly(norbornene), suggesting stereoselective enrichment. Requiring additional proof of tacticity, precatalyst 7 fails to react with bis((menthyloxy)carbonyl)norbornadiene (BMCNBD), a chiral substrate commonly used to establish tacticity.23,89,90 However, post-polymerization modification of poly(norbornene) via bromination of the double bonds provides conclusive data for determining tacticity, as recently described by Schrock and co-workers.5 The brominated polymer exhibits two doublets at 3.84 ppm (J = 10.1 Hz) and 3.81 ppm (J = 10.3 Hz) (Figure 2). Consistent with reported cis, syndiotactic polynorbornene, irradiating the methine protons at 2.61 ppm results in two singlets (Figure 3).5 The intensity of resonances attributable to precatalyst 7 in the 1H NMR spectra do not change over the course of the polymerization indicating kp > ki (where p = propagation; i = initiation). Consequently, no information is available to assess the active species, and attempts to trap the presumed transient Nb-neophylidene (generated from 7) via cycloaddition with ethylene failed.23,35 Oddly, precatalyst 7 does not ring open E

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Figure 3. Expansion of 1H NMR spectrum of partially brominated (2% conversion) ∼85% cis, syndiotactic enriched poly(norbornene) with decoupling of the methine protons H(2′, 5′) at 2.61 ppm; inset shows the 1′ and 6′ protons (two resolved singlets) in the partially brominated product.

alternative explanation for the disparate rates is necessary. Wolczanski et al. noted that Nb was much faster than Ta within oxygen-atom-transfer92,93 and olefin substitution reactions.94 An increase in the density of states at the transition state (a consequence of better s/d-orbital mixing for Nb relative to Ta) was offered as the explanation.72 Considering M−carbon bond making/breaking is critical in the α-hydrogen abstraction that converts the M-dialkyl to the M-alkylidene, it is plausible that again a higher density of states for Nb facilitates the conversion and therefore explains its better ROMP activity relative to Ta.

Scheme 5. Synthesis of the Ta-Neophyl Complex, 9

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gressively to 85 °C and holding it for 18 h maximizes conversion to 50%. Further increasing the temperature does not significantly influence the amount of polymer generated. Extended exposure to elevated temperatures results in decomposition of 9. In contrast, the Nb-analogue 7 catalyzes polymerization of 100 equiv of NBE in 12−15 h at 60 °C, displaying significantly higher reactivity.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00421. Crystallographic data (CIF) 1 H, 13C{1H}, 19F, 2D NMR spectra (PDF)

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CONCLUSIONS The [CF3−ONO]H3 ligand provides easy access to Nb-halide complexes [CF3−ONHO]NbCl3 (2) and [CF3−ONO]NbCl2(OEt2) (3). The Nb-alkyl complexes 5−7 are precatalysts for the ROMP of norbornene, suggesting the intermediacy of a transient alkylidene. Poly(norbornene) produced by precatalysts 5−7 are all cis (∼75−85%) and syndiotactic enriched, presumably due to stereogenic metal control.2 In contrast to 7, the Ta-neophyl analogue 9 requires elevated temperatures to initiate polymerization and only 50% conversions are attainable. Considering the M−C bond strengths between Ta and Nb are expected to be similar,91 an

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Tel.: 352-392-9844. Fax: 352392-3255. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation CHE-1265993 (ASV). KAA acknowledges F

DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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the NSF and UF for funds to purchase the X-ray equipment CHE-0821346.

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DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00421 Organometallics XXXX, XXX, XXX−XXX