Electronic and steric effects of substituents in 1,3-diphenylprop-2-yn-1-one during its reaction with Ru3(CO)12

Thermal reaction of Ru3(CO)12 with alkynyl ketones PhC^CC(O)R (R]Ph (1); 2-Cl-Ph (2); 4-NO2-Ph (3); 2NH2-Ph (4); and 2-CH3COO-Ph (5)) proceeds in toluene with the formation of Ru3(CO)9(m-CO)(h triruthenium) derivatives (1a–5a), Ru(CO)3(h -ruthenole) derivatives (1b–4b, 1c–5c and 4d) and cyclotrimerization products (1e–2e and 1f–3f). Compounds 1a–5a were isolated from the reaction of Ru3(CO)12 with one equivalent of 1–5, respectively. Ruthenoles 1b–3b and 1c–3c were collected by adding 1–3 to the corresponding 1a–3a in a molar ratio of 1 : 1. Cyclotrimerization products 1e–2e and 1f–3f were obtained when 1–3 were added to their corresponding 1b–2b and 1c–3c, respectively. 4b, 4c and 4d were afforded during the reaction of 4 with 4a, but only 5c was collected during the reaction of 5 with 5a. All compounds were characterized by NMR, FT-IR, and MS-ESI and most of them were structurally confirmed by single crystal X-ray diffraction. The results suggest that electronic and steric effects of the substituents in the phenyl ring of 1,3-diphenylprop-2-yn-1-one play important roles in regulating the reaction pathways. An electron-withdrawing group is beneficial to the formation of b, c and further formation of e and f; an electron-donating group favors the production of ruthenoles b, c and d, but disfavors the formation of e and f; a substituent with large steric-hindrance prefers only the formation of c.


Introduction
Ru 3 (CO) 12 has attracted the great interest of researchers in inorganic, organic and catalytic chemistry due to its abundant reactivity and unique catalytic activity. 1 It has been highly effective in the activation and conversion of chemical bonds in the construction of diverse C-X (X ¼ C, N, O, Si, etc.) bonds, 2 and widely used in reactions such as hydroamination, 3 hydro-esterication, 4 silylation, 5 cycloaddition, 6 transfer hydrogenation, 7 carbonylation 8 and hydrohydroxyalkylation. 9 In order to understand the mechanisms of Ru 3 (CO) 12 activated C-C and C-H bonds, reactions of Ru 3 (CO) 12 with NHCs, arenes, alkenes and alkynes were extensively investigated. 10 Some carbonyl ruthenium compounds formed via Ru 3 (CO) 12 and unsaturated hydrocarbons have been used in catalytic systems. 11 In these reactions, coordination atoms play a very important role in the activation of neighboring chemical bonds and construction of compound skeletons, 12 such as the interaction between oxygen atoms and ruthenium atoms in the activation of open-cage fullerenes with ruthenium carbonyl clusters. 13 These conclusions provide more possibilities for Ru 3 (CO) 12 to become a potential catalyst for the conversion of alkyne compounds.
The reactions of Ru 3 (CO) 12 with alkynes have been studied for many years. A variety of coordination modes were reported. 14 There are usually relevant to many catalytic processes involving polynuclear species and unsaturated organic molecules. 15 In 2005, compound (m-H) 2 Ru 3 (CO) 9 {m 3
Alkynyl ketones are important structural units with unsaturated functional groups. They are oen employed as key templates in modern chemical synthesis. 20 The C^C bonds of ynones are good p-acceptors, 21 whilst the adjacent carbonyl groups can coordinate as 2e s donors. 22 For understanding the reactive activity of 1,3-ynones with Ru 3 (CO) 12 , thermolytic reaction of Ru 3 (CO) 12 with 1-(4-methoxyphenyl)-3-phenylprop-2yn-1-one have been investigated systematically in our group, and a series of Ru 2 -Ru 4 framework clusters including a triruthenium and a tetraruthenium intermediates and three ruthenoles were isolated. 23 Aer a detailed analysis of the results, we proposed that the electron-donating methoxy group in the phenyl ring of the 1-(4-methoxyphenyl)-2-propyn-1-one may play an important role in the transformation reaction. As a continuing work in the chemistry of Ru 3 (CO) 12 with alkynyl ketones, we investigated the electronic and steric effects of the substituents in the phenyl ring of 1,3-diphenylprop-2-yn-1-one on the structures of the intermediates and nal products during its thermolytic reactions with Ru 3 (CO) 12 .
In this paper, we examined in detail the reaction process of Ru 3 (CO) 12 with 1,3-diphenylprop-2-yn-1-one derivatives (1-5) containing in its phenyl ring, for example, no substituent (1), electron-withdrawing groups (2-3), an electron-donating group (4) and a sterically hindered group (5). The coordination and coupling of the alkynyl ketone molecules formed a series of intermediate ruthenium clusters and cyclotrimerization products. Through a detailed examination of the reaction processes, we revealed that substituents in the phenyl rings of the acetylenic ketones are vital for the reaction process and that the formation of products has been affected comprehensively by the steric hindrance and/or electronic properties of the substituents.

Syntheses and characterization
Reactions of Ru 3 (CO) 12 and the 1,3-ynones were performed in toluene at 90 C under nitrogen atmosphere and monitored consistently by TLC technique. It was found that color of each reaction solution gradually changed from orange to red-brown and the non-reacted Ru 3 (CO) 12 precipitated aer cooling. All isolated compounds were completely characterized by FT-IR, NMR, ESI mass spectrometry and most of them were additionally characterized by single crystal X-ray diffraction. The reaction process is illustrated in Scheme 1 according to the experimental results and TLC technique as well as the structural characterization analyses.

Synthesis of Ru 3 (CO) 9 (m-CO)(h 4 -triruthenium) derivatives 1a-5a
When a toluene solution of an 1,3-ynone (1-5) (0.03 mmol) and Ru 3 (CO) 12 (0.01 mmol) was stirred at 90 C for 30 min under nitrogen atmosphere, Ru 3 (CO) 9 (m-CO)(h 4 -triruthenium) derivatives (1a-5a) were formed due to binding of the C^C bonds of 1-5 and the Ru metal skeletons. Each of the new clusters has a similar structural framework according to the characterization results and the crystal structure of 5a is shown in Fig. 1 was taken as an example. The IR spectrum of 5a was similar to those observed in the compounds M 3 (CO) 9 (m-CO)L (M ¼ Ru, Os). 23,24 The absorption bands peaked in the range of 2017-2104 cm À1 and at 1878 cm À1 were assigned to the terminal and bridging CO groups, respectively. The dramatic red shi of the stretch vibration of the C^C bond in 5a from 2191 cm À1 to ca. 1446 cm À1 reveals further that the coordination interaction between the carbon atoms of the C^C bond and the Ru central metals is very strong. The chemical shis of the C^C bond moved downwards to 150.8 and 148.2 ppm, conrming again the strong interaction between the C^C bond and the Ru atoms. The chemical shis of the carbonyl carbon atoms directly with Ru atoms cannot be distinguished, indicating that in solution there are uxional for the terminal and bridging CO on the 13 C{1H} NMR time scale.
The compound 5a consists of a triangular arrangement of the ruthenium atoms, in which the Ru-Ru bond distances are in the range of 2.712(2)-2.845(2) A. The coordination of the alkynyl ketone (5) with Ru3 is in a h 2 mode, but with Ru1 and Ru2 is in a h 1 mode. The C10-C11 bond length is 1.396(3) A, located between the typical C-C single bond and double bond, indicative again of the strong coordination strength of the Ru-C bonds. 25 The distances of Ru1-C19 and Ru3-C19 are 1.939(3) and 2.873(2) A, respectively. The angle of Ru1-C19-O5 is 169.2(2) . In addition, there is a carbonyl exists between Ru1 and Ru3 atoms.
Synthesis of Ru(CO) 3 (h 4 -ruthenole) derivatives 1b-4b, 1c-5c and 4d When the reaction time of the above reaction was prolonged from 0.5 h to 1 h, Ru(CO) 3 c (1c-5c) and d (4d) were separated by ash chromatography. The structural characterizations showed that structures of the new ruthenoles are of diverse coupling combinations of the C^C units in the 1,3-ynones, such as modes b (head to tail coupling), c (head to head coupling) and d (tail to tail coupling). 23 Each ruthenole contains a metallacyclopentadienyl framework, which is alike in structure to the carbonyl complexes [M(CO)(h 4metallole)] (M ¼ Fe, Ru, Os) formed by combination of alkynes with group 8 metals. 23,26 The molecular structure of 1b, as an example, was depicted in Fig. 2. The structure of 1b showed an eclipsed conformation of the carbonyls on the two ruthenium centers, with the dihedral angle of R1-Ru2-C35 plane and R1-Ru2-C32 plane being 2.1(1) . The special arrangement geometrically prevents an apical carbonyl to be close enough to the ring metal Ru1 for a CO bridging conformation. Hence, all carbonyls are terminal with almost parallel Ru-C-O angle.
Compared the structures of b and c, each of the crystal structures of ruthenoles c (taking 2c$1/2C 6 H 14 shown in Fig. 3 as an example) appeared a routine staggered conformation of the carbonyls. The dihedral angles between Ru1-Ru2-C35 plane and Ru1-Ru2-C31 plane , Ru1-Ru2-C32 plane are 58.4(2) and 41.0(2) , respectively. The distances of Ru1-C35 is 2.742 (5) A. The staggered orientation in 2c$1/2C 6 H 14 allows the C35 close to the Ru1 metal for a weak interaction, which formed a semibridging carbonyl across the Ru-Ru bond. The IR absorption at 1944 cm À1 also conrms its presence.

Synthesis of the cyclotrimerization products e (1e-2e) and f (1f-3f)
When Ru 3 (CO) 12 (0.1 mmol) reacted with 3 equivalent of an 1,3ynone (1-3) (0.3 mmol) in toluene (90 C) for 2 h under nitrogen atmosphere, two types of cyclotrimerization products e (1e-2e) and f (1f-3f) were formed. The structures of 1e and 3f$CH 2 Cl 2 are illustrated in Fig. 5, taking as examples. The reaction of Ru 3 (CO) 12 with 4 or 5 afforded, however, no cyclotrimerization products in the same reaction conditions and even extended the reaction time. M. Kawatsura also isolated the alkyne cyclotrimerization products using Ru 3 (CO) 12 as catalyst. 11c Step-wise conversion from ynones to nal products To clarify the formation process of the products at each step, the reaction of Ru 3 (CO) 12 with ynones 1-5 were studied in detail and monitored by TLC technique. It was observed that the triruthenium clusters 1a-5a are unstable, decomposing slowly in toluene. The mixing of 1a-5a with Ru 3 (CO) 12 in toluene did not give any new ruthenium clusters. By adding the corresponding 1,3-ynones 1-3 in the clusters 1a-3a in toluene at 90 C, ruthenoles b (1b-3b) and c (1c-3c) were detected. If 4 was added in 4a in toluene, three ruthenoles 4b, 4c and 4d were found. However, the addition of 5 in 5a in toluene only 5c was monitored. Compared with our previous work, 23 no corresponding tetraruthenium clusters were detected in the reaction, however. It is possible that the corresponding tetraruthenium clusters is extremely unstable in the reaction system and cannot be found. The formation of the diverse ruthenoles through reaction of the triruthenium clusters a with the corresponding 1,3-ynones justify that the triruthenium cluster a is the key intermediates in the generation of ruthenoles.
Then reactions of the ruthenoles with the corresponding ynones were studied to explore how the cyclotrimerization products e and f were formed. It was found that e (1e-2e) was detected when ruthenoles 1c and 2c were mixed with 1 and 2, respectively, in toluene at 90 C. The cyclotrimerization products 1f-3f were found in the reaction of ruthenoles 1b-3b with the corresponding 1-3, respectively. Therefore, we believed that ruthenoles are intermediates during the formation of the cyclotrimerization products.   The electronic effect of substituents on the acetylene cyclotrimerization has been reported by T. Takahashi. 27 He found that acetylenes with electron-withdrawing groups (CN, CONMe 2 or COPh) could be used for cycloaddition reaction, and unactivated alkynes such as 4-octyne did not give the desired benzene derivatives even at higher temperature. 27 According to the explanation of T. Takahashi, 1,3-diphenylprop-2-yn-1-one is an electron-withdrawing group activated alkyne, so it is feasible that the reaction of 1 with the corresponding ruthenole can afford the cyclotrimerization products 1e and 1f. Therefore, it is easier that 2e, 2f and 3f can be produced in the similar reactions. Compound 4 is a derivative of 1 with a strong electrondonating group in the phenyl ring, which weakens the reactivity of 4 in the cyclotrimerization reaction. However, an electron-donating group in the phenyl ring of 1 favors the formation of ruthenole d, which can explain the formation of 4d and a similar compound we reported previously. 23 An orthoacetyl group in the phenyl ring of 5 has certain electronwithdrawing ability, but its larger steric hindrance results in the forming of 5c as the unique ruthenole in the reaction of 5a with 5, and this strong steric effect further prevents the formation of cyclotrimerization products.

Materials and equipment
All reactions and manipulations were performed using standard Schlenk line techniques under dry nitrogen. Ru 3 (CO) 12 and the 1,3-ynones were synthesized according to the literature procedure. 28 Solvents were puried, dried and distilled under nitrogen atmosphere prior to use. FT-IR spectra were recorded on a Bruker Tensor 27 Fourier-transform spectrometer. 1 H and 13 C{1H} NMR spectra were performed on a Bruker Avance 400 MHz spectrometer unless indicated. ESI was recorded on a Thermo DecaMax (LC-MS) mass spectrometer with an ion-trap mass detector. While high-resolution mass spectra were recorded in ESI mode on a Waters UPLC-Q-TOF mass spectrometer. The structural measurements of single crystals were carried out with a Bruker SMART APEX-II CCD detector.