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Deprotonation of C-Alkyl Groups of Cationic Triruthenium Clusters Containing Cyclometalated C-Alkylpyrazinium Ligands: Experimental and Computational Studies

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Deprotonation of C-Alkyl Groups of Cationic Triruthenium Clusters Containing Cyclometalated C-Alkylpyrazinium Ligands: Experimental and Computational Studies __________________________________________________FULL PAPER DOI: 10.1002/chem.201204250 Deprotonation of C-Alkyl Groups of Cationic Triruthenium Clusters Containing Cyclometalated C-Alkylpyrazinium Ligands: Experimental and Computational Studies Javier A. Cabeza,*[a] José M. Fernández-Colinas,[a] Pablo García-Álvarez,[a] Enrique Pérez- Carreño,[b] Vanessa Pruneda,[a] and Juan F. Van der Maelen[b] Abstract: The C-alkyl groups of cationic in all cases, the deprotonated C–H bond is decacarbonyl complexes and the way these triruthenium cluster complexes of the type that having the smallest electron density at ligands coordinate to the metal atoms in the 2 1 2 + [Ru3(µ-H)(µ-κ N ,C -L)(CO)10] (HL the bond critical point, the greatest nonacarbonyl products. The mechanisms of represents a generic C-alkyl-N- Laplacian of the electron density at the bond these decacarbonylation processes have methylpyrazium) have been deprotonated to critical point, and the greatest total energy been investigated by DFT methods, which give kinetic products that contain density ratio at the bond critical point have rationalized the structures observed for unprecedented C-alkylidene derivatives and (QTAIM calculations). The kinetic the final products and have shed light on the maintain the original edge-bridged decacarbonyl products evolve, under different kinetic and thermodynamic decacarbonyl structure. When the starting appropriate reaction conditions that depend stabilities of the reaction intermediates, complexes contain various C-alkyl groups, upon the position of the C-alkylidene group explaining the reaction conditions the selectivity of these deprotonation in the heterocyclic ring, toward face-capped experimentally required by each reactions is related to the atomic charges of nonacarbonyl derivatives (thermodynamic transformation. the alkyl H atoms (as suggested by DFT- products). The position of the C-alkylidene NBO calculations). Three additional group in the heterocyclic ring determines Keywords: C–H deprotonation · electronic properties of the C-alkyl C–H the distribution of single and double bonds cationic ligands · N-ligands · bonds have also been found to correlate within the ligand ring and this strongly pyrazinium ligands · C–H activation with the experimental regioselectivity, since, affects the stability of the neutral contain an N-substituted N’-coordinated two-N heterocycle, and are well represented in the literature,[5,6] particularly for N- Introduction methylpyrazinium, which is a very strong π-acceptor ligand (Figure 1, B). A few reports dealing with their one-electron- Metal complexes containing cationic ligands derived from reduction to mononuclear radical derivatives have been aromatic six-membered-ring N-heterocycles can be classified in published,[6] but ligand deprotonation processes have not been two broad groups according to the atom through which the ligand hitherto reported. is attached to the metal: a) those having a C-metalated “inium” ligand and b) those containing an N-metalated “inium” ligand. [1–3] [4] The first group comprises mono- and trinuclear complexes R R that generally derive from N-substituted pyridines[1,2,4] or other N N L M L M L M N N R one-N[1,2,4] or two-N[3] heterocycles, but their ligands are best n n n described as neutral N-heterocyclic carbenes (Figure 1, A). The (A) (B) complexes of the second group are mostly mononuclear, they Figure 1. “Carbene” and “inium” resonance structures of a C-metalated pyridinium (A) and an N-metalated pyrazinium (B) in cationic metal complexes. [a] Prof. J. A. Cabeza, Dr. J. M. Fernández-Colinas, Dr. P. García- Álvarez, Dr. V. Pruneda Complexes that can concurrently be ascribed to both groups (a Departamento de Química Orgánica e Inorgánica-IUQOEM and b), i.e., those derived from C- and N-metalated “inium” Universidad de Oviedo-CSIC, E-33071 Oviedo (Spain) Fax: (+) 34-985103446 cations, were unknown before the recent description of the 2 + E-mail: [email protected] cationic triruthenium derivatives [Ru3(µ-H)(µ-κ N,C-L)(CO)10] , [7] [7] [b] Dr. E. Pérez-Carreño, Dr. J. F. Van der Maelen HL = N-methylquinoxalinium, N-methylpyrazinium, N- [8] [9] Departamento de Química Física y Analítica methylpyrimidinium, 1,2-dimethylpyrimidinium, and 1- Universidad de Oviedo, E-33071 Oviedo (Spain) methyl-1,5-naphthyridinium.[10] The cationic ligands of these Supporting information for this article is available on the WWW under triruthenium clusters are readily attacked by anionic nucleophiles http://dx.doi.org/10.1002/chem.201204250. at selected C atoms of their ligand rings (some examples are or from the author. 1 [7–9] R5 depicted in Scheme 1), and are also prone to undergo one- 4 electron-reduction processes that lead to dimeric hexanuclear 5 N R6 R3 + [Ru (CO) ] [8–10] 6 3 3 12 products. These nucleophilic attacks and reduction reactions 1 N 2 are orbital-controlled rather than charge-controlled processes and OTf lead to neutral complexes with unsaturated but nonaromatic N- R5 R5 Me N N heterocyclic ligands that in some cases are N-heterocyclic R6 R3 R6 R3 carbenes (Scheme 1). N N Ru Ru MeOTf Ru Ru Ru Ru Me Me H H Me 3 5 6 3 5 6 N N R R R R R R N 1a H Me H 1b H Me H N N N 2a H Me Me 2b H Me Me Ru Ru Ru Ru Ru Ru 3a Me Me H 3b Me Me H 4a Me Me Me 4b Me Me Me Ru Ru Ru 5a H Et H 5b H Et H H H H H H H Scheme 2. Synthesis of compounds 1b–5b. The labeling scheme used for the heterocyclic ring atoms and alkyl substituents is also shown. Me H H H Me H Me N H N N C-Alkyl deprotonation of the cationic cluster precursors: The N H N N reactions of compounds 1b, 2b, 4b, and 5b with K[N(SiMe3)2] Ru Ru Ru Ru Ru proceeded quickly in THF at room temperature to give the Ru Ru 5 Ru H Ru respective C -alkylidene nonacarbonyl derivatives 1c, 2c, 4c, and H H 5c (Scheme 3), as major components of reaction mixtures that = CO also contained small amounts of the N-demethylated clusters 1a, 2a, 4a, or 5a, respectively. All these reaction products were Scheme 1. Nucleophilic addition of a hydride to cationic triruthenium clusters derived from N-methylquinoxalinium (left),[7] N-methylpyrazinium (center),[7] and satisfactorily separated by chromatographic techniques and were N-methylpyrimidinium (right).[8] characterized by spectroscopic and analytical methods, and, in the case of compounds 2c and 4c (Figure 2), also by X-ray diffraction. This paper reports that the alkyl groups of triruthenium clusters containing C-alkylpyrazimium-derived ligands can be selectively deprotonated to give neutral products that contain OTf R5 Me novel C-alkylidenepyrazine-derived ligands.[11] Theoretical N R3 R5 R6 R6 R3 studies (DFT-NBO atomic charges and QTAIM analysis of the 1b H Me H N 2b H Me Me electron density) have been used to rationalize the regioselectivity Ru 3b Me Me H of deprotonation reactions of starting materials containing various Ru Ru 4b Me Me Me C-alkyl groups on different positions of the pyrazine ring. DFT H 5b H Et H analysis of potential energy surfaces has been used to shed light K[N(SiMe ) ] o on the mechanisms of observed decarbonylation processes, 3 2 20 C H including that of an unexpected and very interesting Me R7 Me Me R5 transformation of a decacarbonyl complex having a methylidene N N H N 3 group on the C carbon atom of the pyrazine ring into a derivative R6 R3 R6 R3 5 N N H N that formally has that methylidene group on the C carbon atom of Ru Ru Ru Ru + the pyrazine ring. or Ru Ru Ru Ru Ru H H H (3c, 100%) C-Alkyl deprotonation is unprecedented for C- R3 R6 R7 % R3 R5 R6 % alkylpyrazinium metal complexes. Although it has been observed 1c H H H 50 1a H Me H 20 for metal-free C-alkylpyrazinium cations,[12,13] the corresponding 2c H Me H 95 2a H Me Me 5 4c Me Me H 70 4a Me Me Me 20 deprotonated products, which are useful intermediates in 5c H H Me 30 5a H Et H 20 heterocyclic syntheses,[12] are not stable enough to be isolated. Scheme 3. Reactions of compounds 1b–5b with K[N(SiMe3)2]. The yields given were estimated by 1H NMR integration of the spectra of aliquots taken from the Results and Discussion crude reaction mixtures after consumption of the starting cationic cluster. Synthesis of the cationic cluster precursors: The cationic C- All the C5-alkylidene derivatives have a common IR ν(CO) alkylated triruthenium clusters used in this work (compounds 1b– pattern and a hydride NMR resonance at ca. –14.0 ppm, 5b, Scheme 2) were prepared from [Ru3(CO)12], the confirming that they all have an analogous molecular structure. corresponding C-alkylpyrazine, and methyl triflate, following the Their +FAB mass spectra are also in agreement with their synthetic procedure previously used to prepare [Ru3(µ-H)(µ- nonacarbonyl structures. The stereochemistry of the ethylidene 2 [7] 1 κ N,C-L)(CO)10]OTf (HL = N-methylquinoxalinium). The group of compound 5c was established by NOE H NMR, which experimental details of these reactions and the spectroscopic and clearly indicated that the N–Me group is closer to the C=CH other analytical data of their products are given in the Supporting proton than to the C=CMe protons, which, in turn, are in the Information. vicinity of the C6–H ring proton (Figure S4 of Supporting Information). 2 whose ν(CO) absorptions are nearly identical to those of 3a.
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