Communication
Organometallic consequences of a redox reaction: Terminal trimethylsilylmethylidene titanium complexes prepared by a one-electron oxidation step

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Abstract

One-electron oxidation of the titanium(III) bis-trimethylsilylmethyl complex (nacnac)Ti(CH2SiMe3)2 (1) (nacnac = [ArNC(Me)]2CH, Ar = 2,6-iPr2C6H3), readily prepared from (nacnac)TiCl2(THF) and 2 equiv. of LiCH2SiMe3 in Et2O, with AgOTf results in formation of the five-coordinate and terminal titanium alkylidene complex (nacnac)Tidouble bondCHSiMe3(OTf)(THF) (2)-THF concurrent with extrusion of tetramethylsilane and precipitation of silver metal. Complex 2-THF eliminates THF slowly under dynamic vacuum to generate the four-coordinate alkylidene 2 along with some decomposition products. Alternatively, the four-coordinate and non-solvento alkylidene complex, 2, can be prepared from 1 and AgOTf in pentane. Complex 2 undergoes cross-metathesis transformation to afford [ArNC(Me)CHC(Me)double bondCHSiMe3]Tidouble bondNAr(OTf) (3) as the major product after 34 h at room temperature. Complexes 1, 2, 2-THF, and 3 have been fully characterized spectroscopically, and single crystal X-ray diffraction analysis for 1 and 2-THF are presented.

Graphical abstract

Four and five-coordinate trimethylsilylmethylidene complexes can be readily assembled via a one-electron oxidatively induced α-hydrogen abstraction step. Shown is the crystal structure of a rare example of terminal trimethylsilylmethylidene complex of titanium. This complex was prepared by a general one-electron oxidation reaction of the corresponding bis-alkyl Ti(III) precursor.

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Introduction

High-oxidation state transition metal complexes of groups such as 5 [1], [2], [3], [4], [5], [6], [7], 6 [8], [9], [10], [11], [12], [13], [2], [14] and 7 [2] share a common ground in stabilizing the terminal trimethylsilylmethylidene ligand (M = CHSiMe3) [2], [15], [16]. However, the number of terminal trimethylsilylmethylidene metal complexes plummets dramatically when the metal in question represents the lighter congener for each group (e.g., Ti, V, Cr) [2], [15], [16], [17], [18]. In contrast, group 4 transition metal compounds bearing the terminal trimethylsilylmethylidene ligand are even more scant, with the only reported examples (isolable) being the titanium and zirconium complexes (PNP)Tidouble bondCHSiMe3(X) (PNP = N[2-P(CHMe2)2-4-methylphenyl]2) [19], [20], [21], [22]; X = OTf [23], CH2SiMe3 [17]) and [C5H3(SiMe2PiPr2)2]Zrdouble bondCHSiMe3(Cl) [24], [25], respectively. In most cases, the M = CHSiMe3 functionality in question must be trapped or generated in situ, to avoid decomposition pathways from occurring [26], [27], [28], [29]. In rare cases however, dimers arise from bridging of the trimethylsilylmethylidene unit [30], [31]. Hence, the relatively few examples of group 4 alkylidene complexes bearing the trimethylsilyl group is surprising since the sterically demanding alkyl reagent LiCH2SiMe3 is both commercially available and affordable much more so than the corresponding salts LiCH2tBu [32], LiCH2CMe2Ph [33], and LiCH2Ph [34]. Likewise, nucleophilic charge at the α-alkylidene carbon should be expected to be lower given the inherent electron withdrawing effect of trimethylsilyl group and sigma-hyperconjugation.

In this manuscript we report a facile synthetic route to low-coordinate and terminal titanium complexes possessing the terminal trimethylsilylmethylidene motif, namely the compounds (nacnac)Tidouble bondCHSiMe3(OTf)(THF) and its non-solvento, four-coordinate counterpart (nacnac)Tidouble bondCHSiMe3(OTf) (nacnac = [ArNC(Me)]2CH, Ar = 2,6-iPr2C6H3) [35]. Our strategy involves a one-electron oxidation reaction of a Ti(III) bis-alkyl precursor (nacnac)Ti(CH2SiMe3)2 to promote the α-hydrogen abstraction step.

The only examples of a titanium complexes bearing the terminal trimethylsilylmethylidene unit were recently reported by us, and involved the compounds (PNP)Tidouble bondCHSiMe3(X) (X = OTf [23], CH2SiMe3 [17]). Rothwell and co-workers have reported bridging trimethylsilylmethylidene complexes of titanium [30], [31]. In their case, dimerization presumably occurs due to the unsaturated nature of the putative, three-coordinate Ti(IV) complex, in combination with the unhindered composition of the supporting ancillary ligand [30], [31]. In the case of terminal and stable titanium trimethylsilylmethylidene complexes, spectroscopic discrepancies arise when these ligands are compared to the C-based neopentylidene analogues. When measured up against (PNP)Tidouble bondCHtBu(CH2tBu), the alkylidene carbon resonance for (PNP)Tidouble bondCHSiMe3(CH2SiMe3) (310.5 ppm) shifts further downfield in the 13C{1H} NMR spectrum (Δ = 50.5 ppm) [17]. This parameter reflects the electronic effect imposed by the trimethylsilyl unit on the α-C. In addition, the JC–H of 99 Hz (vs. 86 Hz for (PNP)Tidouble bondCHtBu(CH2tBu)) lends support for the trimethylsilylmethylidene complex having a weaker α-hydrogen agostic interaction taking place with the Ti(IV) center. This implies then that the Ti(IV) center is more electron rich when supported by a trimethylsilylmethylidene ligand as opposed to the neopentylidene motif (the role of steric effects might also be contributing to this parameter and we cannot discard this possibility). Therefore, the above spectroscopic features arguably imply that the Tidouble bondCHSiMe3 derivative is less electron hungry than the Tidouble bondCHtBu, which is a dichotomy considering the relative few examples reported for the former system.

Our approach into incorporating the trimethylsilylmethylidene ligand onto Ti(IV) involved a similar protocol for the synthesis of (nacnac)Tidouble bondCHtBu(OTf) [36]. Accordingly, complex (nacnac)TiCl2(THF) [36] can be readily alkylated with LiCH2SiMe3 to afford green-blue crystals of (nacnac)Ti(CH2SiMe3)2 (1) in 73% isolated yields upon work-up of the reaction mixture (Scheme 1). Complex 1 displays a magnetic moment of 1.91 μB thus consistent with a d1 electronic configuration. In order to establish an accurate connectivity for 1, we collected single crystal X-ray diffraction data. Consequently, complex 1 crystallizes in the Monoclinic space group P2(1)/n, and as indicated in Fig. 1, the molecule structure adopts a pseudo tetrahedral geometry whereby the β-diketiminate ligand occupies the remaining two coordination sites. The Ti–C distances (2.104(3) and 2.139(3) Å) are within range to similar bis-alkyl titanium complexes reported by us [36], [37], [38] and Budzelaar and co-workers [39]. Although all hydrogens in the molecule were located and refined isotropically, their location does not suggest an α-hydrogen agostic interaction taking place with the Ti(III) center. The Ti center in 1 deviates 0.601 Å out of the imaginary NCCCN plane. Each flanking aryl group bound to the α-nitrogens is oriented perpendicular to the N–Ti–N plane, placing the sterically hulking isopropyl groups up and down relative to the TiNCCCN framework. Consequently, this feature maximizes steric repulsion with the trimethylsilyl group, hence making these ligands orient opposite of each other and in one direction (propeller-like fashion). A selected list of metrical parameters for the molecular structure of 1 are displayed with Fig. 1.

In the absence of oxidants or moisture, compound 1 is remarkably stable. However, exposure of 1 to an oxidant such as AgOTf in THF evinces an immediate color change from blue-green to red-brown concurrent with formation of Ag0 mirror. Filtration of the metal, and subsequent work-up of the reaction mixture results in the isolation of brown blocks of the trimethylsilylmethylidene complex (nacnac)Tidouble bondCHSiMe3(OTf)(THF) (2)-THF (Scheme 1). The 1H NMR spectrum of 2-THF reveals a highly downfield α-alkylidene hydrogen resonance at 12.30 ppm as well as a complex retaining an equivalent of THF. In addition, the 13C NMR spectrum clearly exposes a highly deshielded alkylidene carbon resonance at 319.4 ppm with a JC–H component of 106 Hz. This latter feature suggests moderate α-hydrogen agostic interaction taking place with the metal center and presents 2-THF to be less electrophilic at Ti(IV), when compared to (nacnac)Tidouble bondCHtBu(OTf) (JC–H = 96 Hz) [38], [36]. Formation of a OTf complex in 2-THF is also evident from the 19F NMR spectrum (singlet resonance at −77.8 ppm). We speculated whether complex 2-THF possessed a smaller α-hydrogen agostic interaction due to higher coordination number at Ti(IV) via binding of THF to the metal center. With this hypothesis in mind, complex 2-THF was subjected to dynamic vacuum over several hours, with only partial elimination of THF (20% conversion to a new product after 24 h), which we attribute to the low-coordinate species (nacnac)Tidouble bondCHSiMe3(OTf) (2) (Scheme 1). Solutions of 1 also decompose at room temperature and in the absence of dynamic vacuum. However, solutions of 2 were marred with content of starting material, in addition to two new decomposition products (vide infra). Independently, complex 2 can be generated cleanly, via one-electron oxidation of 1 with AgOTf in pentane (76% isolated yield, Scheme 1). 1H, 13C, and 19F NMR spectra are essentially analogous to its solvated counterpart, 2-THF. Despite this similarity, the 1H NMR spectrum shows a slight deviation in the chemical shift for the alkylidene hydrogen (12.39 ppm), while the 13C NMR spectrum displays a further downfield chemical shift at 320.1 ppm, congruently with a stronger Ti⋯H α-hydrogen agostic interaction (when compared to 2-THF) taking place with the four-coordinate metal center (JC–H = 98 Hz) [38], [36]. This feature implies that coordination of THF is not a necessity for the stability of 2, and that binding of the Lewis base to the Ti(IV) center reduces the α-hydrogen agostic interaction. As anticipated, addition of THF to 2 rapidly generates 2-THF quantitatively (Scheme 1).

As observed with 2-THF, complex 2 also decomposes in solution at room temperature over 34 h. We have determined one of the compounds resulting from solution decomposition of 2 and found it to be the cross-metathesis product [ArNC(Me)CHC(Me)double bondCHSiMe3]Tidouble bondNAr(OTf) (3) along with another species (which we have not identified) in a 3:1 ratio, respectively (Scheme 1). Based on 1H NMR spectra, the latter product does not appear to be the intramolecular double C–H metallated product: an analogous byproduct formed from the decomposition of (nacnac)Tidouble bondCHtBu(OTf), namely the complex Ti(OTf){[2,6-(CMe2)(CHMe2)C6H3]NC(Me)}2CH [38], [36]. We do however, observe SiMe4 in the mixture, but have not been able to separate the second metal-based product. Separation of 3 was achieved by extracting the reaction mixture with hexane, filtering, and crystallizing the filtrate at −35 °C. Formation of compound 3 is supported by a combination of 1H, 13C, and 19F NMR spectra as well as comparison of its spectral shifts with the previously reported analogue [ArNC(Me)CHC(Me)double bondCHtBu]Tidouble bondNAr(OTf) [38], [36]. Our earlier reports revealed that complex (nacnac)Tidouble bondCHtBu(OTf) had an estimated t1/2 of 45 min at 57 °C and that complete decomposition typically occurred over ∼2 h [38], [36]. In comparison, compound 2 transforms at room temperature over a period of 34 h and decomposes completely within 70 min at 60 °C. Therefore, it appears that the titanium trimethylsilylmethylidene is thermally less stable than the neopentylidene derivative, but not significantly. This implies that complex 2, despite having a more delocalized charge at the alkylidene carbon, might display enhanced reactivity given the more exposed nature of the alkylidene unit. The latter property could facilitate faster intramolecular cross-metathesis of the alkylidene with the nacnac imine residue more than with the bulkier neopentylidene ligand.

Single crystal X-ray analysis of 2-THF clearly exposes a rare example of a five-coordinate titanium complex bearing a terminal trimethylsilylmethylidene functionality (Tidouble bondC, 1.863(3) Å, Fig. 2). Binding of a THF ligand is also evident (Ti–O, 2.155(2) Å) thus resulting in a pseudo square pyramid geometry at titanium, whereby the alkylidene moiety occupies the axial position. The Ti atom sits ∼0.448 Å above the mean plane defined by the β-diketiminate nitrogens, and the THF and OTf oxygen atoms. Distortion about the trimethylsilylmethylidene unit is clearly evident from the obtuse Ti–C–Si angle of 156.4(2)°, which is also diagnostic of 2-THF enjoying from an α-hydrogen agostic interaction. Coordination of THF renders the system C1 symmetric in the solid state structure. However, the solution NMR (1H and 13C, vide supra) spectrum of 2-THF is consistent with this system having Cs symmetry thus implying that a rapid fluxional process by which THF dissociates in solution, might be operational. Other salient parameters associated with the solid state structure of 2-THF are shown with Fig. 2.

The two alkylidene systems presented here represent a new class of titanium trimethylsilylmethylidene complexes that can be readily assembled by a facile one-electron oxidatively induced α-hydrogen abstraction reaction [15], [16], [40]. Depending on the nature of the solvent (THF vs. pentane), higher coordination numbers about the metal center can be conveniently suppressed. Consequently, the lower coordination number results in a more electron deficient metal center, and an enhanced interaction of the metal with the α-hydrogen on the alkylidene ligand. Given the unsaturated nature and oxophilicity of the titanium presented here, compounds such 2 and 2-THF might be excellent scaffolds for olefin metathesis or Wittig-like transformations [28], and we are currently exploring their reaction potential.

Section snippets

General considerations

Unless otherwise stated, all operations were performed in a M. Braun Lab Master double-dry box under an atmosphere of purified nitrogen or using high vacuum standard Schlenk techniques under an argon atmosphere. Anhydrous n-hexane, pentane, toluene, and benzene were purchased from Aldrich in sure-sealed reservoirs (18 L) and dried by passage through two columns of activated alumina and a Q-5 column [41]. Diethylether was dried by passage through a column of activated alumina [41]. THF was

Acknowledgements

We thank Indiana University-Bloomington, the Camille and Henry Dreyfus Foundation, the Alfred P. Sloan Foundation (fellowship award to D.J.M.), and the National Science Foundation (CHE-0348941, PECASE award) for financial support of this research.

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