1333
Organometallics 1995, 14, 1333-1338
Organoruthenium Thermochemistry. Absolute Metal-Ligand Bond Disruption Enthalpies in the (q6-C6Me6)(CO)&u-XSystem (X = H, C1, Br, I) and Thermodynamic Influences of Ancillary Ligand Variation on the Ru-X Bond Disruption Enthalpy Lubin Luo, Chunbang Li, Michele E. Cucullu, and Steven P. Nolan* Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Received October 31, 1994@
The enthalpies of reaction of (q5-C5Me5)(C0)2Ru-H(1)with cc14, CBr4, and CH31 leading to the formation of (q5-C5Me5)(C0)2Ru-X complexes (X = C1, Br, I) have been measured by solution calorimetry in THF at 30 “C. On the basis of enthalpies of reaction and the recently reported Ru-H in ( C ~ H ~ ) R U ( C O(65 ) ~ Hf 1.0 kcal/mol), a n absolute Ru-X bond disruption enthalpy (BDE) scale can be established for ruthenium halide complexes. The absolute BDE scale for ruthenium halide complexes, (q5-C5Me5)(C0)2Ru-X , is as follows (X, kcal/mol): C1, 80.7; Br, 61.1; and I, 55.6, respectively. These ruthenium BDE values can be directly compared with other metal-based systems and lead to a clearer understanding of general BDE trends in M-X systems. In addition, a number of enthalpies of reaction were measured involving organoruthenium hydride complexes with varied ancillary ligands in order to examine the ancillary ligand effects on the relative Ru-X bond enthalpies.
Introduction The field of organometallic thermochemistry has gained recognition as one of great relevance to chemistry and cata1ysis.l Such valuable investigations have led to a better understanding of bonding and reactivity patterns in a small number organometallic In spite of the general view that such studies are fundamental t o a better understanding of organometallic systems, this area remains one where few absolute metal-ligand bond disruption enthalpy investigations have been performed in solution. We have recently reported on ligand substitution Abstract published in Advance ACS Abstracts, February 1, 1995. (1)For leading references in this area, see: (a)Nolan, S. P. Bonding Energetics of Organometallic Compounds. In Encyclopedia oflnorganic Chemistry; J. Wiley and Sons: New York, 1994.(b) Hoff, C. D. Prog. Inorg. Chem. 1992,40, 503-561. (c) Martinho SimBes, J. A.; Beauchamp, J. L. Chem. Rev. 1990,90,629-688. (d) Marks, T.J., Ed. Bonding Energetics In Organometallic Compounds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990;p 428.(e) Marks, T.J.,Ed. Metal-Ligand Bonding Energetics in Organotransitwn Metal Compounds.; Polyhedron Symposium-in-Print; Pergamon: London, 1988;p 7.(0 Skinner, H. A.; Connor J. A. In Molecular Structure and Energetics; Liebman, J . F., Greenberg, A., Eds.; VCH: New York, 1987;Vol. 2,Chapter 6.(g) Skinner, H. A,; Connor, J. A. Pure Appl. Chem. 1985,57,79-88. (h) Pearson, R. G. Chem. Rev. 1985,85,4159.(i) Mondal, J. U.; Blake, D. M. Coord. Chem. Rev. 1983,47,204(k) Pilcher, 238.fj)Mansson, M. Pure Appl. Chem. 1983,55,417-426. G.; Skinner, H. A. In The Chemistry ofthe Metal-Carbon Bond; Harley, F. R., Patai, S., Eds.; Wiley: New York, 1982;pp 43-90.(1) Connor, J. A. Top. Curr. Chem. 1977,71,71-110. (2)(a) Nolan, S. P.; Hoff, C. D.; Stoutland, P. 0.; Newman, L. J.; Buchanan, J. M.; Bergman, R. G.; Yang, G. K.; Peters, K. G. J.Am. Chem. Soc. 1987,109,3143-3145 and references therein. (b)Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. organometallics 1986,5,25292537. (3)(a) Nolan, S.P.; Porchia, M.; Marks, T. J. Organometallics 1991, 10,1450-1457.(b) Nolan, S.P.; Stern, D.; Marks, T. J . J.Am Chem. SOC. 1989,111,7844-7854. (4)(a) Nolan, S.P.; Stern, D.; Hedden, D.; Marks, T. J . In ref Id, pp 159-174. (b) Nolan, S. P.; Lopez de la Vega, R.; Mukeiee, S. L.; Gongalez, A. A.; Hoff, C. D. In ref le, pp 1491-1498. (c) Marks, T. J.;GagnB, M. R.; Nolan, S. P.; Schock, L. E.; Seyam, A. M.; Stern, D. L. Pure Appl. Chem. 1989,61,1665-1672. (d) Schock, L. E.; Marks, T. J. J . Am Chem. SOC.1988,110,7701. @
0276-7333/95/2314-1333$09.0QfQ
reactions focusing on group 8 metal-centered organometallic systemsP Cp’Ru(COD)Cl(soln)
+ 2 PR,(soln)
Cp’Ru(PR,),Cl(soln)
THF
+ COD(so1n) (1)
THF + PP(so1n) a Cp’Ru(PP)Cl(soln) + COD(so1n) (2) THF (BDA)Fe(CO),(soln) + 2PR,(soln)
Cp’Ru(COD)Cl(soln)
diaxial-(PR,)2Fe(CO),(soln)+ BDA(so1n) (3)
Cp’ = C5H5or C5Me5;BDA = PhCH=CHCOMe; PR, = tertiary phosphine; PP = chelating diphosphine These data are of utility in calculating enthalpies of ligand exchange and in estimating equilibrium constants for a number of reactions. There are however much less thermochemical data available for homolytic cleavage of the M-X bond to produce radical fragments. A series of papers on the XzMo(C5H5)z complexes has Hoff and led to generation of Mo-X bond e~timates.~-lO co-workers have investigated the related X-Mo(C0)3(5)For organoruthenium systems, see: (a)Nolan, S. P.; Martin, K. L.; Stevens, E. D.; Fagan, P. J. Organometallics 1992,11,3947-3953. (b) Luo, L.; Fagan, P. J.; Nolan, S. P. Organometallics 1993,12,43054311.(c) Luo, L.; Zhu, N.; Zhu, N.-J.; Stevens, E. D.; Nolan, S. P.; Fagan, P. J. Organometallics 1994,13,669-675. (d) Li, C.; Cucullu, M. E.; McIntyre, R. A.; Stevens, E. D.; Nolan, S. P. Organometallics 1994,13,3621-3627. (6)For organoiron systems, see: (a) Luo, L.; Nolan, S. P. Organometallics 1992,11,3483-3486. (b) Luo, L.; Nolan, S. P. Inorg. Chem. 1993,32,2410-2415. (c) Li, C.; Nolan, S. P. Organometallics 1995, 14,1327-1332. (7)Calado, J. C. G.; Dias, A. R.; Marthinho-SimBes, J. A.; Ribeiro da Silva, M. A. V. Rev. Port. Quim. 1979,21,129-131. ( 8 ) Calado, J. C. G.; Dias, A. R.; Marthinho-SimGes, J. A. J . Organomet. Chem. 1980,195,203-206.
0 1995 American Chemical Society
Luo et al.
1334 Organometallics, Vol. 14,No. 3, 1995
(C5H5) system and derived Mo-X BDE values.'l Hoff and Bergman have performed such a study for the XzIr(PMed(C5Me5)system.2a The few thermodynamic studies focusing on organoruthenium complexes have included the kinetic determination of relative bond enthalpy data for the Cp*(PMe&Ru-X system reported by Bercaw and co-workers.12 Cp*(PMe,),Ru-OH
+ HL -
Cp*(PMe,),Ru-L
+ H 2 0 (4)
Cp* = C5Me,; L = CCPh, NHPh, SH, CN, H Collman has examined the kinetics of cleavage of one of the Ru-ethyl bonds (21.7 k 1.5 kcaymol) in the (OEP)Ru(Et)z system.13 (OEP)Ru(Et),
-
(OEP)Ru(Et)'
+ Et'
(5)
Halpern and Mancuso have reported a kinetic determination of D(Ru-R) (33 kcdmol) in Cp(C0)zRuCH(CH3)C6H514 Cp(CO),Ru-CH(CH,)Ph
-
Cp(CO),Ru'
+ 'CH(CH,)Ph
(6)
and Parker and Tilset have most recently estimated absolute bond disruption enthalpies for a number of transition metal hydrides including H-Ru(CO)2Cp.l5 Cp(CO),Ru-H -,Cp(CO),Ru'
+H
(7)
We now report new thermochemical data which allow for estimation of Ru-X bond disruption enthalpy values for the X-RU(CO)Z(C~M~~) system and also report on reaction enthalpy variations as a function of ancillary ligand modifications.
Experimental Section General Considerations. All manipulations involving organoruthenium complexes were performed under inert atmospheres of argon or nitrogen using standard high vacuum or Schlenk tube techniques or in a VacuudAtmospheres glovebox containing less than 1 ppm oxygen and water. Tetrahydrofuran was stored over sodium wire, distilled from sodium benzophenone ketyl, stored over Na/K alloy, and vacuum transferred into flame-dried glassware prior to use. Infrared spectra were recorded using a Perkin-Elmer FTIR Model 2000 spectrometer in 0.1mm NaCl cells. NMR spectra were recorded using a Varian Gemini 300 MHz spectrometer. Calorimetric measurements were performed using a Calvet calorimeter (Setaram C-80) which was periodically calibrated using the TRIS reaction16 or the enthalpy of solution of KCl (9) (a)Calado, J. C. G.; Dias, A. R.; Marthinho-SimBes, J. A.; Ribeiro da Silva, M. A. V. J. Organomet. Chem. 1979,174, 77-80. (b)Calado, J. C. G.; Dias, A. R.; Minas de F'iedades, M. E.; Marthinho-SimBes J. A. Rev. Port. Quim. 1980,22, 53-58. (10) Calado, J. C. G.; Dias, A. R.; Salem, M. S.; Marthinho-SimBes, J. A. J. Chem., SOC.,Dalton Trans. 1981, 1174-1177. (11)Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. J . Organomet. Chem. 1986,315, 187-199. (12) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J.Am. Chem. SOC.1987,109, 1444-1456. (13) Collman, J. P.; McElwee-White, L.; Brothers, P. J.; Rose, E. J. Am. Chem. SOC.1966, 108, 1332-1333. (14) Mancuso, C.; Halpern, J. J. Organomet. Chem. 1992,428, C8-
c11.
(15) Tilset, M.; Parker, V. D. J. Am. Chem. SOC.1989,111, 67116717; 1990, 112, 2843. (16) Ojelund, G.;Wadso, I. Acta Chem. Scand. 1968,22,1691-1699.
in water.17 The experimental enthalpies for these two standard reactions compared very closely to literature values. This calorimeter has been previously described,18 and typical procedures are described below. The organoruthenium complexes Cp*Ru(C0)2H,lgC ~ R U ( P P ~ & HCpRu(dppe)H,20 ,~O and CpRu(PMe&H21 were synthesized according to literature procedures. Only materials of high purity as indicated by IR and NMR spectroscopies were used in the calorimetric experiments. All ligands were purchased from Strem Chemicals (Newburyport, MA) and used as received. 'H NMR Titrations. Prior to every set of calorimetric experiments involving a new reaction, an accurately weighed amount (fO.lmg) of the organoruthenium complex was placed in a Wilmad screw-capped NMR tube fitted with a septum, and THF-d8 was subsequently added. The solution was titrated with a solution of the reactant of interest by injecting the latter in aliquots through the septum with a microsyringe, followed by vigorous shaking. The reactions were monitored by lH NMFt spectroscopy, and the reactions were found to be rapid and quantitative, conditions necessary for accurate and meaningful calorimetric results. These criteria were satisfied for all organoruthenium reactions investigated.
Calorimetric Measurement for Reaction Involving Cp*(CO)&u-H (1) and CC4 The mixing vessels of the
.
Setaram C-80 were cleaned, dried in an oven maintained at 120 "C, and then taken into the glovebox. A 20-30 mg sample of recrystallized Cp*(CO)zRuH(1)was accurately weighed into the lower vessel, and it was closed and sealed with 1.5 mL of mercury. A 4 mL amount of a stock solution of CCl4 (5 mL of the phosphine ligand in 25 mL of THF) was added, and the remainder of the cell was assembled, removed from the glovebox, and inserted in the calorimeter. The reference vessel was loaded in an identical fashion with the exception that no organoruthenium complex was added to the lower vessel. After the calorimeter had reached thermal equilibrium at 30.0 "C (about 2 h), the reaction was initiated by inverting the calorimeter. At the end of the reaction (1-2 h), the vessels were removed from the calorimeter, taken into the glovebox, and opened, and the infrared cell was filled under inert atmosphere. An infrared spectrum of each product was recorded using this procedure. Conversion t o Cp*(CO)ZRuCl was found to be quantitative under these reaction conditions. The enthalpy of reaction, -36.7 f 0.6 kcaymol represents the average of five individual calorimetric determinations. This methodology represents a typical procedure involving all organometallic compounds and all reactions investigated in the present study.
Calorimetric Determination of the Enthalpy of Solution of cp*(cO)~RuH(1) in THF. In order to consider all species in solution, the enthalpy of solution of 1 had t o be directly measured. The calorimeter cells were loaded in the exact fashion as in the example described above with the exception that no ligands were introduced in the reaction cell. The measured enthalpy is 4.4f 0.3 kcaumol and represents seven separate determinations. To ensure that no decomposition had occurred during the thermal equilibration at 30 "C, a THF solution of Cp*(C0)2RuH was maintained at 30 "C for 3 h, the solvent was then removed, and the residue was examined by NMR and IR spectroscopies. Both analytical techniques clearly showed the complex to have remained intact during this time. (17)Kilday, M. V. J. Res. Natl. Bur. Stand. (US.)1960, 85,467481. ~. (18)(a) Nolan, S. P.; Hoff, C. D. J. Organomet. Chem. 1986, 290, 365-373. (b)Mukerjee, S. L.; Nolan, S. P.; Hoff, C. D.; de la Vega, R. Znorg. Chem. 1988,27, 81-85. (19) Fagan, P. J.; Mahoney, W. S.; Calabrese, J. C.; Williams, I. D. Organometallics 1990, 9, 1843-1852. (20) Bruce, M. I.; Humphrey, M. G.; Swincer, A. G.; Wallis, R. C. Aust. J. Chem. 1984,37, 1747-1755. (21) Mayer, J. M.; Calabrese, J. C. Organometallics 1984,3, 12921298. ~~
Organometallics, Vol. 14, No. 3, 1995 1335
Organoruthenium Thermochemistry
Table 1. Ru-X Bond Disruption Enthalpy Estimates for Cp*Ru(CO)zX Complexes (kcal/mol) Cp*Ru(CO),H(soln)
+ XR(so1n)
THF
X
Cp*Ru(CO),X(soln) X-R Cl-c13 Br-CBr3 I-CH3
-W" D(Ru-H)" 41.1(0.6) 44.4(0.4) 39.3(0.5)
D(X-R)b
65 65 65
D(R-H)b
70.4 56.2 56.3
95.8 96.0
105
+ HR(so1n) D(Ru-X) 80.7 61.1 55.6
a D(Ru-H) Taken from ref 15. Ancillary thermodynamic data taken from ref 22.
Table 2. Enthalpies of Reactions of Cp'Ru(L)zH with RX (kcavmol) Cp'Ru(L)H(soln)
THF + XR(so1n) 30"~
Cp'Ru(L),X(soln) entry 1 2 3 4 5 6 7
CP' CP* CP* CP* CP* CP CP CP
L
X-R"
co co co
Cl-CCl3 Br-CBr3 I-CH3 I-CH3 I-CH3 I-C& Cl-cc13
PMe3 PMe3 dppe dppe
+ HR(so1n) -e 41.1(0.6) 44.4(0.4) 39.3(0.5) 45.0(0.7) 44.1(0.4) 35.2(0.3) 40.4(0.2)
Ancillary thermodynamic data taken from ref 22.
Results A number of reactions involving halogenating agents have been used in organometallic thermochemistry to extract metal-halide BDE values and trend^.^^,^ In the present study, we have found reactions 8-10 to provide direct solution calorimetric access to enthalpies of reaction that allow for a determination of absolute ruthenium-halide BDE values. Cp*(CO),Ru-H
+ CCl,
THF
Cp*(CO),Ru-Cl+ CHC13 Cp*(CO),Ru-H
THF + CBr, 30"~
Cp*(CO),Ru-Br Cp*(CO),Ru-H
Table 3. Bond Disruption Enthalpy Estimates (kcavmol) for Cp*Ru(CO)zX and Related Systems
+ CH31 THF Cp*(CO),Ru-I
+ CHBr, + CH,
9)
(10)
Enthalpies of reaction with appropriate halogenating agents are reported in Table 1. All reactions where enthalpies of reaction values are reported were subjected to NMR titrations prior to performing the calorimetric experiments and were determined to be rapid and quantitative under calorimetric conditions. All reactions investigated lead to a unique product under the calorimetric conditions as determined by NMR spectroscopy. All reported enthalpies of reaction are solution phase values and take into account the enthalpy of solution of the appropriate ruthenium hydride complex. These enthalpies of solution have been included in halogenation enthalpies of reaction reported in Table 2.
H C1 Br I
Cp*Rucp*IrCl(C0)Ir(C0)zX" Cp2Mo(X)zb CpMo(C0)3XC (PMe3)(X)zd (PR&(X)f 65.0 80.7 61.1 55.6
66.0 72.4 60.5 58.1
60.0 72.9 57.8 49.4
74.2 90.3 16.0 63.8
60 71 53 35
This work, average uncertainties in absolute bond disruption enthalpy values are on the order of f 5 kcal/mol. For experimental errors on specific measurements, see text. bTaken from refs 7-10. 'Taken from ref 11. Taken from ref 2a. e Taken from ref 23.
Discussion The Cp*(CO)zRuH (1)complex (Cp* = q5-C5Me5)was selected as the entryway into the thermochemistry of the Cp*(CO)zRuXsystem in view of the recent thermochemical determination of the Ru-H bond disruption enthalpy (BDE) in CpRu(C0)zH. Parker and Tilset have recently determined the BDE of a number of metal-hydride species one of which is Cp(C0)zRuH where the Ru-H BDE is estimated to be worth some In this study, two 65 f 1.0 kcaVmo1 in a~etonitri1e.l~ other second row metal-hydride BDE's were also determined: Cp(C0)3Mo-H (62 kcaVmo1) and Cp*(C0)3Mo-H (61 kcaVmo1). These values are the same within experimental error. Substitution of Cp for Cp*, as ancillary ligation, does not appear to greatly affect the strength of the Ru-H bond. Furthermore, Hoff and co-workers have determined the Mo-H BDE in the Cp(C0)3Mo-H system as 66 k 5 kcaVmol in THF solution.18a It is based on these BDE trends that we assign a value of 65 kcaVmol to the Cp*(C0)2RuHbond.
-
Cp*(CO),Ru-H
D(Ru-H)
Cp*(CO),Ru'
+ H'
(11)
= 65 f 2.0 kcaVmol
Having measured enthalpies of reaction 8-10 and established a thermodynamic anchor point for this system, we could derive absolute ruthenium-halide BDE values utilizing known thermochemical data for organic compounds involved in the halogenation reactiorm2, This absolute BDE scale for Cp*(CO)zRu-X complexes (Table 1)allows for comparisons with other known M-X BDE values and for the examination or existence of any bonding trends in L,M-X (L, = ancillary ligands) systems. The solution thermochemistry of two iridium(II1) systems has been investigated. The binding of halides has been investigated by Blake and co-workersZ3for Vaska's complex (Table 3). These reactions result in the oxidative addition of halogens producing Ir(CO)(Cl)(PR&(X)2 complexes. Ir(CO)(ClXPR,),
+ X,
& = I,,
-
Ir(CO)(Cl)(PR,),(X),
(12)
Br,, Cl,, and H,
The other Ir system investigated is Cp*Ir(PMe3)(X)z, whose solution thermochemistry was investigated by (22) (a)Weast, R. C., Ed. Handbook of Chemistry and Physics, 62th ed.; CRC Press: Cleveland, OH, 1981; p F-180. (b) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: New York, 1970. (23) Yoneda, G.; Blake, D. M. Znorg. Chem. 1981, 20, 67-71 and references therein.
1336 Organometallics, Vol. 14, No. 3, 1995
55
60
Luo et al.
65
70
D(I3u-X)
75
80
85
(kcallmol)
Figure 1. Metal-X (X = H, C1, Br, I) bond disruption enthalpy data in Cp*(CO)2Ru-X vs Ir(CO)(Cl)(PR& (A)(R= 0.92; slope = 1.28) and Cp*Ir(PMed(X)z(A) (R= 0.96; slope = 0.97) systems. 75
-
70
h
65 2
9
c
? 5
60
55
50
45 55
60
65
D(Ru-X)
70
75
80
85
(kcol/mol)
Figure 2. Metal-X (X = H, C1, Br, I) bond disruption enthalpy data in Cp*(CO)2Ru-X vs CpMo(C0)S (A)(R = 0.93 ; slope = 0.76) and CpzMo(X)2 (A) (R= 0.99; slope = 0.89) systems.
Hoff and Bergman.2a Cp*Ir(PMe,)(H),
+ 2R-X
-
Cp*lr(PMe,)(X),
+ 2R-H
(13)
Rx = CCl,, CBr,, and CH31 Both these iridium systems have the metal center in a formal M3+ (d9 oxidation state rendering them isoelectronic with the present Ru2+(d6)system. The BDE correlation between the two systems is depicted in Figure 1. The point could be made, that in view of similar ancillary ligation, a better correlation should be expected for the Cp*Ir(PMe&X)2 system, and in fact a better correlation coefficient is found for this relationship (R= 0.96) versus the Vaska system (R= 0.92). In both these iridium systems, however, the average of two
BDE's is considered and a direct correlation with the ruthenium system, where a single halide bond is formed, is presented. Regardless of this fact, the relative Ru-X bond disruption enthalpy trend found follows that of previously investigated systems: M-Cl > M-H > M-Br > M-I.lC A more appropriate comparison may exist with organometallic complexes of second row transition metals since ruthenium falls in this category. "he only systems investigated are the CpMo(CO)& (Mo2+)11and Cp2MoX2 (Mo4+)'-lo systems. A graphic representation of possible correlations with the present ruthenium data can be found in Figure 2. With the similar Cp ligation, it might be expected that the two second row molybdenum systems correlate more closely to their ruthenium neighbor. In fact, correlation coefficients of 0.93 and 0.99 are calculated for the CpMo(Colaand CppMoX2 systems, respectively. Both sec-
Organometallics, Vol. 14,No. 3, 1995 1337
Organoruthenium Thermochemistry
ond row systems show good correlations with the present ruthenium data (slopes of 0.76 and 0.89, respectively). In an effort to compare enthalpies or reaction involving the cleavage of the Ru-H bond in different ligand environments, the reactivity of a number of phosphinesubstituted ruthenium hydride complexes were tested under calorimetric conditions. Only a few of the complexes tested reacted rapidly and cleanly enough to be investigated by solution calorimetry. Results are presented in Table 2. The enthalpies of reaction associated with variation in the ancillary ligation provide insights into the factors influencing the strength of the Ru-W Ru-X bonds and can be explained in terms of ligand donorlacceptor proper tie^.^^ The difference in enthalpy of reaction between reactions 14 and 15, some 5.7 kcallmol, can be attributed to THF + CH3130“~ Cp*(CO),Ru-I + CH,
Cp*(CO),Ru-H
(14)
AH = -39.3(0.5) kcal/mol Cp*(PMe,),Ru-H
+ C H 3THF 1 x
+
CP*(PM~,)~RU-I CH4 (15)
AH = -45.0(0.7) kcal/mol the change in electronic properties of the ancillary ligands. The substitution of trimethyl phosphine for CO in the Cp*Ru(L)zH system makes these species less acidic. This effect is consistent with the greater u donor, poorer x acceptor character of PMe3, relative to CO. This substitution results in a strengthening of the Ru-H bond. The strength of the Ru-H bond in reaction 15 is increased, relative to the reaction involving CO, as a result of the change in metal-hydride acidity. These trends in metal basicities on going to increased 0 donation have previously been observed.25 The relative metal basicity has been shown t o decrease on going from Cp to Cp* as ancillary ligations. This has been observed by Angelici and SowaZ6in their studies of enthalpies of protonation reaction: Cp’Ir(l,5 COD)
+ CF,S03H -
Cp’Ir(l,5 COD)H+CF,SO,- (16) Cp’ = C5Me,H,-,;
3c
= 0, 1,3-5
A difference in enthalpies of protonation of some 5 kcall mol was found to exist between the Cp and Cp* complexes. This enthalpy difference of 5 kcallmol was also apparent in our calorimetric investigations of ligand substitution reaction^:^^^^^ (24)(a) Tolman, C. A. Chem. Reu. 1977,77,313-348. (b) Manzer,
L. E.; Tolman, C. A. J. Am. Chem. Sac. 1976, 97, 1955-1986. (c) Pignolet, L. H., Ed. Homogeneous Catalysis with Metal Phosphine Complexes; Plenum: New York, 1983. (25)(a) Tolman, C. A. J.Am. Chem. Sac. 1970,92,2953-2956. (b) Walker, H. W.; Pearson, R. G.; Ford, P.C. J.Am. Chem. SOC.1083, 105,1179-1186.( c ) Collman,J. P.;Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science: Mill Valley, CA, 1987. (26)Sowa, J. R.;Angelici, R. J. J . Am. Chem. SOC.1991,113,25372544. (27)Cucullu, M.E.;Luo, L.;Nolan, S. P.;Fagan, P. J.; Jones, N. L.; Calabrese, J. C. Organometallics 1996,14,289-296.
Cp’(C0D)RuCl
THF + 2PR, 30“~
Cp’(PR,),RuCl+ COD (17) Cp’ = C5H5and C5Me5 However, as in complexes studied by Tilset and Parker, the thermochemical results illustrated by reactions 15 and 18 display no measurable M-L BDE difference on going from Cp* to Cp as ancillary ligation.15 Cp(PMe,),Ru-H
+ CH,I
THF
Cp(PMe,),Ru-I
AH = -44.1(0.4)
+ CH,
(18)
kcal/mol
It would therefore appear that Ru-X bonds are not significantly affected by a change in ancillary ligation. Alternatively, it could be argued that there is, on going from Cp* to Cp, a constant factor affecting both the strength of the Ru-H and Ru-I bonds which would result in no apparent difference in enthalpy of reaction between the two pairs of complexes. At this point, in view of a lack of absolute Ru-H BDE data as a function of ancillary ligand variation, the exact reason for the almost constant enthalpy data seen in reactions 15 and 18 cannot be unequivocally explained. There is however a significant difference in enthalpy values between reactions 18 and 19. Cp(dppe)Ru-H
+ CH,I
THF
Cp(dppe)Ru-I
AH = -35.2(0.3)
+ CH,
(19)
kcaVmol
dppe = bis(dipheny1phosphino)ethane In this case, it appears that the poorer u donation of dppe, compared to 2 equiv of PMe3, is the source of the enthalpy difference. This u donation term may be the overall factor dictating the magnitude of the Ru-H bond enthalpy in these complexes. A relative CJ donation scale can therefore be constructed for the present system and proceeds in increasing donating ability in the following order: 2PMe3 > 2CO > dppe. This relative scale has also be tested for reaction 20, which has a measured Cp(dppe)RuH + CC1,
THF
Cp(dppe)RuCl
+ CHC1,
(20)
AH = -40.4(0.2) kcal/mol enthalpy of reaction less exothermic than its Cp*(CO)zRu-X analog. On the basis of the u donation scale and arguments presented above, a less exothermic enthalpy value would have been predicted.
Conclusion The reported solution calorimetric investigation represents the first detailed thermochemical study of metal-halogen bond enthalpies in organoruthenium systems. The present study allows for the determina-
Luo et al.
1338 Organometallics, Vol. 14, No. 3, 1995 tion of the first absolute Ru-halide BDE values in the Cp*Ru(CO)& system. The absolute BDE scale parallels those of other systems investigated by solution calorimetry. Enthalpies of reactions were also measured for complexes bearing phosphine ancillary ligands. The stability scale and the magnitude in enthalpies of reaction point toward the degree of (T donation from the ancillary ligand to be an important factor in dictating the strength of the Ru-H interaction. Further studies are underway in order to clarify factors influencing metal-ligand bond disruption enthalpies.
Acknowledgment. The National Science Foundation (Grant CHE-9305492)and the Louisiana Education Quality Support Fund are gratefully acknowledged for support of this research. The Louisiana Board of Regents is also acknowledged for allocating funds allowing the purchase of the FT-IR spectrophotometer (Grant ENH-TR-41,1993-1994). We are also indebted to Johnson MattheyIAesar for a generous loan of ruthenium salts. OM940829K
