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Kinetic Studies on the Reaction of (η<sup>5</sup>-MeCp)Mn(CO)<sub>2</sub>(THF) with PR<sub>3</sub> (R
Kinetic Studies on the Reaction of (η5-MeCp)Mn(CO)2(THF) with PR3 (R
Journal of the Korean Chemical Society. 2008. Oct, 52(5): 476-481
Copyright © 2008, The Korean Chemical Society
  • Received : September 12, 2008
  • Published : October 20, 2008
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Sang Hyuck Kim
Min Yoo
Sung Kuk Park
Do Hyung Lee
Dong Shin Yun
Jin Ho Kim
Yong Kwang Park

Abstract
The solventometal carbonyl complex, (η 5 -MeCp)Mn(CO) 2 (THF) was found to undergo a dissociative ligand substitution reaction with PR 3 (R=Me, OEt, C 6 H 5 ) in THF at rather low temperatures (0℃ up to 20℃) under the pseudo-first-order reaction conditions (usually 10-fold excess of PR 3 with respect to the solventometal carbonyl complex concentrations) where the coordinating solvent, THF is displaced by PR 3 . This substitution reaction follows overall first order dependence on [(η 5 -MeCp)Mn(CO) 2 (THF)]; therefore, the positive entropy change of activation (△S = 2.3 e.u. for PMe 3 ; △S =11.64 e.u. for P(OEt) 3 ; △S =10.52 e.u. for P(C 6 H 5 ) 3 ), together with the similar enthalpy change of activations( △H =20.52 (kcal/mol) for PMe 3 ; △H =23.49 (kcal/mol) for P(OEt) 3 ; △H =23.10 (kcal/mol) for P(C 6 H 5 ) 3 ) is likely to ensure the possible intermediate, (η 5 -MeCp)Mn(CO) 2 , which on reaction with PR 3 , transforms to the product, (η 5 -MeCp)Mn(CO) 2 (PR 3 ). However, in case the reaction temperature is raised up to 30℃, this reaction is no longer dependent solely on the solventometal carbonyl complex concentration. The reaction rate is assumed to be more or less related to the PR 3 concentration. Therefore, in this case, there is some possibility of a mechanistic reaction change from the dissociative character toward the associative one.
Keywords
INTRODUCTION
Solventometal carbonyl complexes seem to have their possible role in homogeneous catalysis. 1 - 3 Metal carbonyls of 18-electrons with η 5 - C 5 H 5 react by an associative process via a pair of electrons localized on the ligand(Cp ring slippage) in the ligand substitution reaction. 4
However, in our substitution reaction of (η 5 - MeCp)Mn(CO) 2 (THF) with PR 3 , the Mn-O bond (of Mn-THF) is relatively weak compared with other metal-ligand bond so that which process (Mn-O bond cleavage vs MeCp(η 5 →η 3 ) ring slippage is more facile one, will determine the reaction pathway whether it is involved in associative ligand substitution or dissociative ligand substitution.
The displacement of coordinated THF from square planar platinum(II) complex shows that the enthalpies of the reaction are -26.4 kcal/mol for P(OEt) 3 and -26.2 kcal/mol for PMe 3 5 (Eq. 1).
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Another mechanistic study on the substitution reaction of M(CO) 5 (THF)(M=Cr,Mo,W) with PR 3 (at least 10-fold excess of PR 3 ) shows an associative process indicating the overall 2nd order kinetics and the activation parameters(△H = 40~80 kJ/ mol and △S =8~-122e.u.); however, there seems to be a gradual mechanistic changeover from a dissociative character toward an associative one along the series of Cr, Mo, and W. 3
EXPERIMENTAL
All operations were carried out under a nitrogen atmosphere by using standard Schlenk techniques or an Ar-filled glove box. Rigorous exclusion of trace moisture and oxygen degassed as described below. Infrared spectra were recorded on a Jasco FT-IR 460 spectrophotometer using 0.10-mm sealed CaF 2 or KBr solution cells. 1 H NMR spectra were previously obtained from Varian Gemini-200FT spectro meter in solutions of CDCl 3 , DMSO-d 6 , and CD 3 OD. 7 The GC-MASS spectral data were previously collected from the GC-MASS(Hewlett Packard Model GC 5890). 7 Photochemical reactions were performed by using a 450 watt Hg vapor lamp(Hanovia). THF were distilled under nitrogen from purple sodium/benzophenone ketyl. Hexane was stirred over concentrated H 2 SO 4 overnight, washed with aqueous NaHCO 3 and distilled from Na 0 /benzophenone ketyl. All other reagents purchased were reagent grade and used without further purification.
Preparation of (η5-MeCp)Mn(CO)2(THF).6
The preparation procedure of (η 5 -MeCp)Mn(CO) 2 -(THF) is similar to the method in the literature. A solution of (η 5 -MeCp)Mn(CO) 3 (0.05 g, 0.2 mmol) in freshly distilled THF(50 ml) is photoylzed for 10min with nitrogen purge. A slow, steady flow of N 2 through the solution during photolysis assists in removing CO and prevents the reverse reaction.
5-MeCp)Mn(CO)2(THF) . υ(CO)IR(THF): 1923(s), 1847(s) cm -1
Identification of (η5-MeCp)Mn(CO)2(PR3) (R =Me, OEt, C6H5) . After each kinetic experiment was over, the respective reaction mixture solution was dried under vacuum to have the oily product. This reaction mixture was then on column chromatograph using hexane/THF mixed solvent system. Products such as (η 5 -MeCp)Mn(CO) 2 PR 3 (R=Me,OEt,C 6 H 5 ) were identified by the respective (CO) spectra as compared with those of the authentic compounds previously isolated and characterized in our laboratory( 1 ). 7
Solution IR spectral data for (η5-MeCp)Mn(CO)2PR3(R=Me, OEt, C6H5) in THF
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Solution IR spectral data for (η5-MeCp)Mn(CO)2PR3 (R=Me, OEt, C6H5) in THF
Kinetic Measurements . The kinetic experiments were carried out in a dried 10 ml vial securely fitted with a rubber septum. In a typical kinetic experiment, 0.05mmol of (η 5 -MeCp)Mn(CO) 2 (THF) freshly prepared from the photochemical reaction of (η 5 -MeCp)Mn(CO) 3 in THF at ambient temperature under N 2 , was loaded into a 10ml vial. The respective THF solution of triethylphosphite, triphenylphosphine, or trimethylphosphine of known concentration was added by a syringe. This solution was then stirred and placed in a water bath equipped with a constant-temperature controller(±0.1℃). Samples for IR spectral analysis were withdrawn periodically with a syringe and placed in a sealed 0.1 mm pathlength KBr or CaF 2 infrared solution cell. Rates of reaction were observed by following the disappearance in absorbance of the reactant’s υ(CO)IR which does not overlap with the other carbonyl bands: (η 5 -MeCp)Mn(CO) 2 (THF) at 1845 cm -1 . Under the psendo-first-order reaction conditions, rate constants were calculated using a linear least-squares program for the first order rate plots of [A t -A ] vs time, where A t is the absorbance at time t and A is the absorbance at time infinity.
RESULTS AND DISCUSSION
The solventometal carbonyl complex, (η 5 -MeCp)Mn(CO) 2 (THF), which was prepared from the photochemical reaction of (η 5 -MeCp)Mn(CO) 3 in THF at UV irradiation, 6 on reaction with PR 3 (R=Me, OEt,C 6 H 5 ) yielded the corresponding (η 5 -MeCp) Mn(CO) 2 (PR 3 )(Eq. 2).
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It is widely accepted that the neutral metal carbonyl complex with Cp ligand is likely to undergo an associative ligand substitution with PR 3 in case this complex is electronically saturated due to the Cp(η 5 →η 3 ) ring slippage. 4 , 8 In the substitution reaction of M(CO) 5 (THF)(M=Cr, Mo, W) with PR 3 shows mainly an associative ligand substitution process. 3 However, in this case, as compared with the variations of metal and the PR 3 ligand, there appeared a wide range of △S (-122e.u.~+8e.u.), which may demonstrate some mechanistic change from associative character to a dissociative one to some degree. Here, the substitution reaction of (η 5 -MeCp)Mn(CO) 2 (THF) with PR 3 shows the overall first order dependence with respect to the concentration of (η 5 -MeCp)Mn(CO) 2 (THF). The reaction rate was observed to have no linear relationship with the PR 3 concentration ( 2 ).
The relationship between PR3(R=Me, OEt, C6H5)concentrations and the rates(kobs) of the reaction of (η5-MeCp)Mn(CO)2(THF) with PR3in THF at 10℃
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[(η5-MeCp)Mn(CO)2(THF)]=5.0 mM
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Pseudo-first-order plot for the reaction of (η5-MeCp)Mn(CO)2(THF) with PR3(R=Me, OEt, C6H5)(10-fold excess) in THF at 10℃.
However, in case of PMe 3 , there may be a minor associative process to some degree in competition with the major dissociative one ( 2 ).
The activation parameters were calculated from the Eyring Plot ( 2 ).
During the reaction, as υ(CO)IR(THF)(1920, 1845 cm -1 ) of (η 5 -MeCp)Mn(CO) 2 (THF) shrinks, the corresponding (η 5 -MeCp)Mn(CO) 2 (PR 3 )(R=Me, 1915, 1856; R=OEt, 1935, 1870; R=C 6 H 5 , 1927, 1863) gradually grows; therefore, the absorbance change vs time of the (CO)IR of (η 5 -MeCp)Mn(CO) 2 (THF) at 1845cm -1 was monitored for the kinetic purpose ( 3 ). The reaction rate was also observed to be enhanced with temperature rise (0℃-20℃) ( 3 ).
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Eyring plot for the reaction of (η5-MeCp)Mn(CO)2(THF) with PR3(R=Me, OEt, C6H5)(10-fold excess) in THF.
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The (CO)IR spectral variations during the reaction of (η5-MeCp)Mn(CO)2(THF) with P(OEt)3(10-fold excess) in THF at 10℃.
Temperature dependence on the rate of the reaction of (η5-MeCp)Mn(CO)2(THF) with PR3(R=Me, OEt, C6H5) in THF
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[(η5-MeCp)Mn(CO)2(THF)]=5.0 mM; [PR3]=50 mM
The activation parameters(△H =20.52 kcal/mol, △S =2.3e.u. for PMe 3 ; △H =23.49 kcal/mol, △S =11.64e.u. for P(OEt) 3 ; △H =23.10 kcal/mol, △S =10.52e.u. for P(C 6 H 5 ) 3 ) together with the reaction rate being of overall the first order with respect to [(η 5 -MeCp)Mn(CO) 2 (THF)] strongly suggest a dissociative character at the rate-determining step, which may involve the initial cleavage of Mn-THF bond, followed by the attack of PR 3 , leading to the corresponding product, (η 5 -MeCp)Mn(CO) 2 (PR 3 ) ( 4 ).
Activation parameters from the reaction of (5-MeCp)Mn(CO)2(THF) with PR3(R=Me,OEt,C6H5) in THF
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[(η5-MeCp)Mn(CO)2(THF)]=5.0 mM; [PR3]=50 mM
Mechanistic Considerations. The respective υ(CO)IR for the (η 5 -MeCp)Mn(CO) 2 (PR 3 )(R=Me, 1915, 1849; R=OEt, 1935, 1870; R=C 6 H 5 , 1927, 1863) ( 1 ) suggests that the relative -donation ability of ligand, PR 3 is as follows: 9
  • PMe3> P(C6H5)3 > P(OEt)3
At the elevated temperature (30℃), it looks rather surprising that the k obs is more or less dependent upon the PR 3 concentrations.
The relative σ-donating ability of PR 3 seems to work definitely on the reaction rate at the elevated reaction temperature (30℃) toward the associative process. 4 shows that the reaction with the most nucleophilic PMe 3 proceeds much faster than the one with less nucleophilic P(C 6 H 5 ) 3 , followed by the one with least nucleophile, P(OEt) 3 of the three. This change in reaction mechanism from the dissociative character toward associative one is assumed to be even clearer with far excess of [PR 3 ] at the elevated temperature. Similar mechanistic changes in ligand substitution are observed elsewhere in the literatures. 3 , 10 It seems reasonable that more nucleophilic PMe 3 with high kinetic energy(at elevated temp.) is more likely to directly attack the Mn metal center without waiting for the THF to depart from the Mn metal complex. However, the steric hindrance(ligard cone angle) of PR 3 (106 o for P(OEt) 3 ; 118 o for PMe 3 ; 142 o for P(C 6 H 5 ) 3 ) does not seem to be inversely proportional to the rate ( 4 and 5 ).
The relationship between steric factor of PR3and the reaction rates in THF
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The relationship between steric factor of PR3 and the reaction rates in THF
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The relationship between kobs and [PR3] in THF at 30℃.
This reaction situation(associative ligand substitution) is assumed to be very similar to S N 2 reaction where strong nucleophile readily attacks C δ+ - X prior to C-X bond cleavage, particularly at high concentration of the nucleophile and high reaction temperature as well. The entropy change of activation for PMe 3 (△S =2.3 e.u. ) being rather smaller than those of the other ligand(△S =11.64 e.u. for P(OEt) 3 ; △S =10.52 e.u. for P(C 6 H 5 ) 3 ) may also support the possibility of the reaction pathway change(major dissociative process in competition with minor associative one for reaction involving PMe 3 ). In this case, 1 is the major possible process, concurrently with 2 (minor one).
At reasonably low reaction temperature (0℃~20℃), this ligand substitution on the whole follows the dissociative process( 1 ), which is evidenced by the activation parameters and the overall first order kinetic data; however, at elevated temperatures (≥30℃) where the kinetic energy is merely enough for the reactive PR 3 (R=Me, OEt, C 6 H 5 ), chances are high for the associative ligand substitution to occur( 2A , or 2B ). However, during the above-mentioned reaction no intermediate (η 5 -MeCp)Mn(CO) 2 , or (η 3 -MeCp) Mn(CO) 2 (THF)PR 3 could be observed, based on the υ(CO)IR spectral changes vs reaction time ( 3 ). Therefore, at elevated reaction temperature, the associative interchange(I a ) involving the plausible transition state( 2B ) can also be proposed. More in-depth kinetic study on this reaction at elevated temperature should be done in near future.
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Acknowledgements
This research was supported by the Research Grant from Kangwon National University in 2006. The Author (Y. K. Park) greatly appreciates those generous donations(metal carbonyls, ligand and so on) from Professor J. J. Ko, Department of Chemistry, Korea University.
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