Reactivity of Ruthenium Vinylidene Complexes Containing Indenyl/dppe Ligands and Unsaturated Bonds at Cδ with Trimethylsilyl Azide

This study presents a new reaction of cationic vinylidene complexes with Me3SiN3 (TMSN3), which yields N-coordinated nitrile complexes 3. Treatment of a ruthenium acetylide precursor containing indenyl and dppe ligands with a series of organic halides produced the corresponding vinylidene complexes 2 in good yield. Further reaction of 2 with TMSN3 at room temperature produced N-coordinated ruthenium nitrile complexes 3. Unlike the reaction of cyclopropenylruthenium complexes with TMSN3, which yielded different products depending on the substituent at Cγ, the vinylidene complexes containing unsaturated bonds at Cδ yielded similar N-coordinated nitrile complexes. This transformation did not seemingly occur in the reaction of ruthenium vinylidene complexes containing Cp and PPh3 ligands with TMSN3. Deprotonation of these vinylidene complexes yielded cyclopropenyl or thermodynamic furylruthenium complexes, depending on the substitute at Cγ. Subsequent reactions of the cyclopropenyl or furylruthenium complexes with TMSN3 afforded different products.


Introduction
The chemical properties of metal vinylidene complexes are valuable for many organic transformations [1][2][3][4]. Vinylidene complexes of various metals also function as strategic intermediates for the catalytic conversion of alkynes [5,6], and active substrates in a series of stoichiometric reactions [7,8]. The formation of a metal vinylidene intermediate has been used to promote various carbon-carbon bond-forming reactions, with the addition of a nucleophilic carbon center to the electrophilic vinylidene α-carbon atom. This subject has been extensively reviewed [9,10]. The optimal entry into the transition metal vinylidene complexes is with the addition of electrophiles to the electron-rich carbon of metal alkynyl complexes [11,12]. Ruthenium vinylidene complexes are well-known active species in organometallic chemistry [1,5,13,14]. The vinylidene complexes of iron with dppe ligands have also been obtained [15][16][17]. The use of the acidity of the aliphatic protons on a coordinated dppe ligand in a cationic iron vinylidene complex [18] has induced the intramolecular cyclization between the dppe and vinylidene ligand.
We believe the electron-withdrawing group at C of the vinylidene complexes might play a role in the acidity enhancement of its neighboring proton. We have recently reported some preliminary results on vinylidene complexes containing indenyl and dppe ligands [19]. This study reports the synthesis and the reactivity of ruthenium vinylidene complexes containing unsaturated double bonds at C, and the reactivity of these vinylidene complexes with TMSN 3 . This study also presents a deprotonation reaction of these vinylidene complexes, as well as the subsequent reactivity of the deprotonation products with TMSN 3 .

Preparation of Cationic Ruthenium Vinylidene Complexes 2a-e
Ruthenium acetylide complex 1 was prepared via deprotonation of the corresponding vinylidene precursor following the literature method [20]. The indenylruthenium vinylidene complexes 2a-e containing dppe ligands and various substituents at Cγ were obtained as air-stable pink solids in 96-73% yields. The ruthenium vinylidene complexes 2a-e were synthesized as shown in Scheme 1. Treatment of [Ru]-C≡C-Ph (1, [Ru]=(η 5 -C 9 H 7 )(dppe)Ru) with organic halides such as allyl iodide at room temperature produced cationic vinylidene complex 2a in 89% yield. The 31 P{ 1 H}-NMR spectrum of 2a exhibited a singlet at  76.0, indicating the chemical equivalence of the two phosphorus atoms. In the 13 C{ 1 H} NMR spectrum, the typical low field Ru-Cα resonance appeared as a triplet at δ 352.0, with a C-P coupling constant of 16.9 Hz.
Complex 2a was air-stable at room temperature. Single crystals of 2a suitable for X-ray diffraction analysis were obtained via recrystallization from CH 2 Cl 2 /ether. The ORTEP drawing of 2a with thermal ellipsoids is shown at the 30% probability level in Figure 1, with selected bond distances and angles listed in Table 1. The C(37)-C(36)-Ru(1) linkage was basically linear. The C(36)-Ru(1) bond length of 1.841(4) Å indicated a typical metal-carbon bond in the vinylidene complexes [21,22]. Ruthenium vinylidene complexes containing indenyl ligand have been reported [23][24][25][26][27]. The X-ray structure of the vinylidene indenyl-ruthenium complex [Ru{=C=C(CH 3 )(C 6 H 9 )}( 5 -C 9 H 7 )(PPh 3 ) 2 ] + with bond length Ru-C(α) 1.838(5) Å [26]. The bond angle of this complex is 176.2(4) also very close linear. , 2e, R=CCH) were prepared using the same synthesis method as for 2a. All indenylruthenium vinylidene complexes 2a-e displayed a characteristic pink color in the solid state. These complexes were characterized via NMR spectroscopy and X-ray diffraction. In the 1 H-NMR spectrum, the singlet or doublet resonances for the CH 2 group at Cβ appeared at 2.2-2.8 ppm.  Cationic vinylidene complexes are known to react with alcohols or water to yield alkoxycarbene or acyl complexes, respectively [24,[28][29][30][31]. The reaction is believed to proceed by nucleophilic attack at the vinylidene α carbon, followed by a proton shift from the oxonium ion to the β-carbon. A theoretical study of vinylidene complexes indicated localization of electron density on Cβ (HOMO) and the electron deficiency at Cα [32,33]. A study of the reaction of alcohols with ruthenium vinylidene complexes indicated that electron-withdrawing groups on the acetylide unit or on the metal facilitate nucleophilic attack on Cα [34].
The reaction of ruthenium vinylidene complex 2a with TMSN 3 yielded the N-coordinated nitrile complex 3a as a yellow powder in 83% yield. Regardless of the equivalent ligand distribution around ruthenium, the presence of two monodentate phosphine ligands, instead of the chelating diphosphine dppe, marks a difference in the reactivity of their derivatives. Unlike the previously reported reactivity of ruthenium vinylidene complexes, the ruthenium vinylidene complex containing indenyl and dppe ligands therefore demonstrate a distinctly different reactivity from that of the Cp and PPh 3 system.
Complex 3a was stable in solution and air, and soluble in polar solvents, such as CH 2 Cl 2 , acetone, and THF. Complex 3a was characterized by 1 H-, 31 P-, 13 C-NMR, as well as 2D-NMR spectroscopy. In the 31 P{ 1 H}-NMR spectrum of 3a, two doublet resonances at  83.4 and 81.4 with a P-P coupling constant of 27.2 Hz indicated the presence of a enantiotopic center in the N-coordinated nitrile ligand. In the 1 H-NMR spectrum, a triplet pattern at  3.82 with J H-H = 7.15 Hz was assigned to the proton at this enantiotopic center. The parent peak in the HRMS spectrum of 3a clearly indicated that 3a resulted by adding a nitrogen atom to 2a. Slow diffusion of the diethyl ether into a solution of 3a in dichloromethane permitted a collection of suitable single crystals for X-ray diffraction studies (Table 2). An ORTEP diagram of one of the stereoiomers 3a is shown in Figure 2, showing 30% thermal ellipsoids, and selected structural parameters are listed in Table 3. The nitrile ligand was coordinated to the metal center via the nitrogen atom. The bond lengths of Ru(1)-N(1) of 2.031(4) Å and N(1)-C (27) bond length of 1.130(7), respectively, were typical. The N(1)-C(27)-C(28) bond angle of 176.8° was close to 180°. X-ray analysis unequivocally confirmed the molecular structure (Table 3).    Conversion of a vinylidene precursor to an N-coordinated nitrile with hydrazine, an organometallic Beckmann rearrangement, has been reported in an iron system [35] (Scheme 1). In the Cp system, similar products can be obtained via the reaction of cyclopropenyl complex containing a phenyl group at C, but the N-coordinated ruthenium complex product containing N 3 − counter anion is unstable. An exchange of the N 3 − with PF 6 − stabilized the N-coordinated complex [36,37]. Ruthenium vinylidene complexes containing Cp and PPh 3 ligands have been reported. However, no reaction was observed between these vinylidene complexes with TMSN 3 [33]. In the indenyl and dppe ligands system, we previously reported that the N-coordinated ruthenium complex product containing N 3 − counter anion is stable. This product can be obtained via the reaction of a cyclopropenyl complex with TMSN 3 [19]. The reaction of the vinylidene precursor 2a with TMSN 3 may advance through the nucleophilic addition of azide anion at Cα and the electrophilic addition of a (CH 3 ) 3 Si (TMS) group at Cβ. Subsequent loss of N 2 would result in metal migration and hydrolysis of the (CH 3 ) 3 Si group, to cause N-coordinated nitrile complex 3a, accompanied with the halide anion (Scheme 1). In the Cp and two triphenylphosphine ligands system, no reaction occurs between vinylidene complexes and TMSN 3 , possibly due to the stereo effect of the vinylidene complexes. In the vinylidene complex 2a, the bond angle P(1)-Ru(1)-P(2) of 83.29 (3) is smaller than other vinylidene complexes containing two monodentated ligands [38,39]. The more sterically demanding two triphenylphosphine ligands compared to the bidentated dppe ligand may prevent nucleophilic addition at the Cα position of the vinylidene complexes.
A similar reaction occurs in THF for a number of vinylidene complexes with various unsaturated substituents at Cγ, including the ester group (compounds 2b,c), crotonate group (compound 2d), and alkynyl group (compound 2e). Reaction of vinylidene complexes 2a-e with TMSN 3 yielded similar N-coordinated nitrile complexes 3a-e is good yield (Scheme 1).
With the alkyl group at Cγ, vinylidene complex 2e reacting with TMSN 3 yielded complex 3e as the major product, and another undetermined minor product with a 5:1 ratio. One of the well-known reactions of organic alkynes with azide can afford trizoles via cycloaddition [40]. The chemistry of the 1,3-dipolar cycloaddition of azides and alkynes is widely used in applications in organic, materials, and medicinal chemistry [41][42][43][44][45][46][47][48]. Ruthenium vinylidene complex 2e containing an alkyl group at Cγ and reacting with TMSN 3 yielded an N-coordinated complex as the major product. The electron deficiency at Cα of the ruthenium vinylidene complex 2e may play a significant role in the reaction with TMSN 3 . That the vinyl group reacting with azide produced 1,2,3-triazoles via 3+2 cycloaddition has also been reported. Unlike the reactivity of cyclopropenyl complexes with TMSN 3 , different pathways operated depending on the substituent on the cyclopropenyl ring. The N-coordinated nitrile products 3a-e are similar, and were air stable in solid-state and soluble in CH 2 Cl 2 , but insoluble in ether and hexane. In this series of the N-coordinated products, complex 3c gave a good yield of 90%.

Deprotonation Reaction of the Vinylidene Complexes
The synthesis of a number of ruthenium cyclopropenyl complexes by deprotonation of readily accessible ruthenium vinylidene complexes containing a CH 2 R group bound to Cβ has been reported [49][50][51][52][53]. A cyclopropenylruthenium complex containing pentamethylcyclopentadiethyl (Cp*) and dppp ligands was synthesized as well [50]. In the iron complexes containing Cp and dppe ligands, the vinylidene complex containing an allyl group at Cγ can be synthesized. The deprotonation reaction of this complex, the relatively more acidic proton of the dppe ligand in the cationic iron vinylidene complex could direct the reaction to proceed via a different route. The metallacyclic iron complex was obtained [54]. The intramolecular cycloaddition of two C=C bond system with an allylic ligand to produce a cyclobutylidene ring has also been reported [55]. This study focuses on the deprotonation reaction of the vinylidene complex containing an allyl group or crotonate group at Cγ.
Deprotonation of the vinylidene complex 2a via n-Bu 4 NOH in acetone inducing the intramolecular cyclization reaction yielded a neutral cyclopropenyl complex 4a as a single product (Scheme 2). The reaction produces a yellow crystalline product in analytically pure form. Use of acetone or acetonitrile as a solvent produces a good yield, and use of other bases such as DBU (1,8-diazabicyclo[5,4,0] undec-7-ene) produces 4a with a comparable yield. No metallacyclic product has been observed. The 31 P-NMR spectrum of 4a displays two doublet resonances at  94.3 and 89.3 of an AX pattern with J P-P = 23.8 Hz, due to the presence of a stereogenic carbon center at the three-membered ring. On the 1 H-NMR spectrum of 4a, the methyne (methine) proton appears at  1.88.

Scheme 2. Deprotonation reaction of vinylidene complexes.
The synthesis and chemical reactivity of several neutral rutheniumcyclopropenyl complexes in which the metal bonds to one sp 2 carbon atom of the three-membered cyclopropenyl ring in the Cp system have been reported [37,[56][57][58]. These cyclopropenylruthenium complexes can be prepared via deprotonation reaction of their vinylidene precursor. When a crotonate group was at Cγ, the vinylidene complex yielded a cyclopropenyl complex after deprotonation [56].
The reaction of the vinylidene complex 2b containing an ester group at Cγ with n-Bu 4 NOH in acetone yields the furyl complex 4b as the thermodynamic product (Scheme 2). The reaction proceeds via deprotonation at Cγ, followed by an intramolecular cyclization first causing the three-membered cyclopropenyl complex 5b as the kinetic product, with a small amount of 4b within 1h. Conversion of 5b to 4b is completed within 4 h. The 31 P-NMR spectrum of 4b displays a singlet at δ = 96.7 ppm. However, the 31 P-NMR spectrum of the kinetic product 5b displays a two-doublet pattern at δ = 93.8, 88.5 ppm, with J P-P = 26.1 Hz indicating the presence of a stereogenic carbon center at the cyclopropenyl ligand. As shown in Scheme 2, the furylruthenium complex 4c was also prepared via deprotonation of the vinylidene complex 2c containing a ethyl acetate at Cγ. Similar reaction has been reported [52]. Deprotonation reaction of dinuclear vinylideneruthenium complexes containing an ester substituent at Cγ gave the dinuclear bisfuryl complexes [49]. Organic furan adds to [Ir(COD)(PMe 3 ) 3 ]Cl to yield a furyl iridium hydride complex have also been reported [59]. In the Cp and triphenylphosphine ligands system, the furyl complexes reacting with oxygen for two weeks produced the oxygen addition product [56]. In the indenyl and dppe system, complexes 4b and 4c are highly stable, and no oxygen addition reaction was observed.
In the deprotonation reaction of the vinylidene complex containing a methyl crotonate substituent at C, only the cyclopropenyl complex 4d was obtained as the stable product. However, deprotonation of the vinylidene complex 2e containing an alkynyl group at C yielded several unidentifiable decomposition products, and no cyclopropenyl complex was observed (Scheme 3). Deprotonation of vinylidene complexes containing an ester group at Cγ produced furylruthenium complexes as thermodynamic products.

Scheme 3.
Deprotonation reaction of vinylidene complex containing alkynyl group at C.

Reaction of Cyclopropenyl Complexes with TMSN 3
The reaction of the cyclopropenylruthenium complex 4a containing an allyl substituent with an excess of TMSN 3 resulted in the formation of a five-membered triazolate ring organic product 6a (Scheme 4) and [Ru]-CN.

A B C
The organic product is 6a [56]. We altered the substituent at C to the crotonate group. The result is the same as that observed for the reaction of TMSN 3 with the cyclopropenyl complex containing an allyl group (Scheme 5). In the reaction, the TMS group reacts with the unsaturated double bond at C to induce a ring-opening reaction of the three-membered ring. No reaction occurs between the unsaturated double bond at C of the vinylidene complexes and TMSN 3 . The reactivity of Cα in the vinylidene complexes causes more activity than the unsaturated bond at C. We reported the reactivity of cyclopropenylruthenium complexes containing indenyl and dppe ligands with TMSN 3 . Various substituents at the sp 3 carbon of the three-membered ring govern the reactivity of the cyclopropenyl complexes with TMSN 3 . The reaction of ruthenium cyclopropenyl complexes containing indenyl and dppe ligands with TMSN 3 may proceed via an electrophilic addition of the TMS group to the three-membered ring, followed by hydrolysis to afford vinylidene intermediates containing an azide counter anion [19]. Further nucleophilic additions of N 3 − at Cα, and an electrophilic addition of a second TMS group at Cβ followed by a loss of N 2 leads to the N-coordinated nitrile complexes [56]. This process is similar to the chemistry of other cyclopropenylruthenium complexes. When a CN group was on the cyclopropenyl ring, a [3+2] cycloaddition of the nitrile group with azide afforded the tetrazolate complex [23]. Reactions of the cyclopropenyl complexes containing phenyl and its derivatives at C yielded the stable N-coordinated complexes. In the Cp system, the reaction of cyclopropenyl complexes containing a methyl crotonate substituent or a vinyl substituent on the sp 3 carbon of the cyclopropenyl ring with TMSN 3 produced [Ru]-CN and the five-membered triazole ring. The ester group on the cyclopropenyl ring is the kinetic product, while a more stable five-membered furylruthenium product is likely the thermodynamic product. Reaction of the furylruthenium complexes with TMSN 3 yielded [Ru]-N 3 and the corresponding organic products by cleavage of the M-C bond. Unlike the published reactions of ruthenium cyclopropenyl complexes with TMSN 3 , which proceed through rupture of the three-membered-ring, the reaction of furylruthenium complexes with TMSN 3 caused cleavage of the M-C bond [56]. However, those reports observed no reaction between ruthenium vinylidene complexes and TMSN 3 .

Reaction of Furyl Complexes with TMSN 3
Upon applying an excess of TMSN 3 to 4b in THF at room temperature, the solution displayed color changes during the course of the reaction. Reaction of the furylruthenium complexes with TMSN 3 yielded [Ru]-N 3 and the corresponding organic products by opening the five-membered ring (Scheme 6). A series of successive color changes were noted during the course of the reaction: the yellow solution of 4b first turned red upon adding TMSN 3 at room temperature, and subsequently turned orange after 1 h, and deep orange after 2 h. Unlike the published reactions of furylruthenium complexes with TMSN 3 , which caused cleavage of the M-C bond, the reaction of furylruthenium complexes containing indenyl and dppe ligands with TMSN 3 were similar to the cyclopropenylruthenium complexes [56]. The X-ray diffraction analysis of the furylruthenium complex containing Cp and two PPh 3 ligands has been reported [52]. The Ru-Cα bond length in the furylruthenium complex of 2.076(7) Å indicates a single bond. We have reported the X-ray diffraction analysis of the cyclopropenylruthenium complex containing indenyl and dppe ligands [19]. The Ru-Cα bond length in this complex is 2.028(2) Å, slightly shorter than other cyclopropenyl complexes [50,52]. This length might cause the ring-opening reaction when the cyclopropenyl or furylruthenium complexes to react with TMSN 3 . The reaction of 4b with TMSN 3 leading to [Ru]-N 3 and the corresponding organic product 6b may proceed via the similar pathway of synthesis complex 3b. Followed by the N 3 − attacks at Cα, the organic fragment can be obtained.

General
All reagents were purchased from commercial sources and used without further purification. NMR spectra were obtained with a Bruker-AC 500 spectrometer at 500 MHz ( 1 H), 202 MHz ( 31 P), or 125 MHz ( 13 C). The chemical shifts are provided in parts per million from SiMe 4 ( 1 H and 13 C{ 1 H}) or 85% H 3 PO 4 ( 31 P{ 1 H}), and are reported in units of δ. Mass spectra were recorded using a LCQ Advantage (ESI) instrument. X-ray diffraction studies were conducted at the Regional Center of Analytical Instruments at the National Taiwan Normal University.
All synthetic manipulations were performed in oven-dried glassware under nitrogen using vacuum lines and standard Schlenk techniques. Solvents were dried by standard methods and distilled under nitrogen before use. THF was distilled from sodium benzophenone ketyl and CH 2 Cl 2 was distilled from CaH 2 . Methanol was distilled from Mg/I 2 . Complexes (η 5 -C 9 H 7 )(dppe)Ru-C≡C-Ph (1) [20] was prepared using the methods reported in the literature. The atom labels shown in Figure 3 were used for the 1 H and 13 C{ 1 H} spectroscopic data:

Synthesis of Furylruthenium Complex (η 5 -C 9 H 7 )(dppe)Ru-C=C(Ph)CH=C(OCH 3 )O (4b)
A sample of 2b (0.25 g, 0.32 mmol) was dissolved in acetone (10 mL) at room temperature. A methanol solution of n-Bu 4 NOH (2 mL, 2 mmol, 1M in MeOH) was added. After the mixture stirred for 4 hours, the resulting solution was concentrated to about 0.5 mL. Then CH 3 CN (5 mL) was added the yellow precipitate thus formed was filtered off and washed with CH 3 CN. The product was dried under vacuum and identified as 4b (0.21 g, 0.27 mmol) in 84% yield. 1

Reaction of 4a with TMSN 3
To a solution of 4a (0.15 g, 0.20 mmol) in THF (5 mL) was added TMSN 3 (0.1 mL, 0.76 mmol). The solution was stirred overnight at room temperature. Then the solvent was reduced under to about 2 mL, and slowly added to stirring hexane (20 mL). The orange precipitates thus formed were filtered off and wash with hexane and identified as [Ru]-CN (0.08 g, 0.12 mmol) in 60% yield. The organic product was extracted with hexane and collected by extraction with hexane and purified by chromatography, then, the solvent was removed under vacuum to give 6a (0.015 g, 0.087 mmol, 44% yield). Spectroscopic data for 6a are as follows [37]. 1