1057
Inorg. Chem. 1987, 26, 1057-1060
Contribution from the J. Tuzo Wilson Laboratories, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6
Oxidation and Substitution Kinetics of Rez(CO)$P( C6H1
3)2
Anthony J. Poe,* Clifford N. Sampson, and Chandra V. Sekhar Received May 13, 1986
The kinetics of reactions of ax,ax-Re2(C0)8(PCy,j2 (Cy = C6Hll)with CI6H3,I,P(OPh),, and P(OEt), in decalin have been studied. Reaction with C16HB,Ileads to ~is-Re(C0)~(PCy,)1 by three paths. The major one accounts for ca. 93% of the total reaction rate, and the data are consistent with rate-determining dissociation of PCy,. Rate-determining CO dissociation can account for another 5%, but there seems to be an additional, but very minor, path accounting for ca. 2% of the reaction. The values of AH* and AS* corresponding to PCy, dissociation are close to 30 kcal mol-' and -3 cal K-l molT1,respectively. It is suggested that, as the PCy, ligand leaves, the remaining R ~ ~ ( C O ) B ( P Cmoiety ~ , ) adjusts its bonding so as to maintain 18-electron configurations for both Re atoms and that this accounts for the low values of both activation parameters. In spite of the bulk of the PCy3 substituents, a rather high lower limit, AH* 2 41 kcal mol-', is estimated for homolysis of the Re-Re bond. quencies, assignable to Re2(C0)9(PCy,), were evident. Introduction Kinetic Measurements. Amounts of complex sufficient to provide Kinetic evidence and crmover experiments suggest very strongly concentrations generally of ca. 2 X lo4 M were dissolved in decalin (10 that the metal-metal-bonded complexes U ~ , U ~ - M ~ ~ ((L C O ) ~mL) L ~in Schlenk tubes. After the tubes were sealed with rubber septum = P-n-Bu,, PPh,, P(C6Hl1)3, etc.) can undergo rate-determining caps, the solutions were degassed by several freeze-pump-thaw cycles homolysis in substitution and oxidation reactions.' For L = PPh,, and the atmosphere above the solutions was established by introduction of the appropriate gas. The reaction tubes were placed in an oil bath and at least, the homolysis process can be induced by some of the the reacting solutions maintained at a constant temperature (f0.2 "C) oxidizing or substituting reagents as well as occur spontaneously. that was measured with an iron-constantan thermocouple connected to There is a pronounced decrease in the values of AH* with ina digital multimeter. The other reactants were introduced, either as the creasing Tolman cone anglesZof the substituents, and this appears pure species or in solution as convenient, by means of syringes fitted with to be due to release of steric strain on formation of transition states stainless steel needles and the solutions thoroughly shaken. Reacting that must have considerably lengthened Mn-Mn bonds.'^^ On solutions were well-shielded from laboratory light. Samples were withthe other hand, neither MnZ(CO),,, nor ReZ(CO),,, reacts via drawn by syringe at noted times, cooled in small phials in ice, and stored rate-determining spontaneous homolysis: and their reaction in the dark and their IR spectra measured when convenient. Slight mechanisms are still under investigation. Even ax,ax-Rezpositive pressures of the gas above the reacting solutions were maintained (CO)s(PPh3)z,with its relatiyely large substituents, does not appear at all times. Reactions were monitored for ca. 3 half-lives by following the decreasing absorbance of the band at 1952 cm-I, and values of A , to undergo homolysis.4b We have studied some reactions of were measured after 7-8 half-lives. Excellent linear plots of In (A, - A,) ax,~x-Re~(C0)~(PC (Cy y ~= ) ~C6Hll) in order to see whether were obtained. the much larger cone angle of PCy, is sufficient to bring about rate-determining homolysis, but the results reported below suggest Results and Discussion that ligand dissociative mechanisms predominate. Reaction Products. Reaction of the bisaxial complex Rez(CO)s(PCy3)z with 1-iodohexadecane (RI) proceeds smoothly at Experimental Section elevated temperatures to form a product that shows IR bands at The ligands PCy, (Strem Chemicals, Inc.) and P(OPh), (BDH) were 2087 m, 2004 m, 1992 vs, and 1937 s cm-I. The same product used as received, and P(OEt), (Eastman Kodak) was distilled over sois obtained from the very fast reaction of the complex with 12. Very dium. l-Iodohexadecane (RI, ICN Pharmaceuticals) was distilled at similar spectra are also shown by the products of reactions of reduced pressure. Decalin (Aldrich) was dried over molecular sieves and Mnz(C0)8(PCy,)z with 12' and of M ~ I ~ ( C O (L ) ~ =L PPh3, ~ Pstored under argon. It was shown to be free of detectable tetralin by UV-vis spectroscopic measurement^.^ Re2(CO)lo(Strem Chemicals, n-Bu,, PCy,) with 1,1,2,2-tetra~hloroethane.~ These reactions Inc.) was used as received. Argon was obtained from Canox Ltd., CO have all been concluded to form cis-Mn(CO),(L)X, and it seems from Union Carbide, and analyzed CO-N2 mixtures from Matheson of that this type of product is generally formed by reactions of Canada, Ltd. Infrared spectra were measured with a Perkin-Elmer 298 M2(CO)sLzwith halogens or halocarbons. The initial reaction spectrophotometer, and UV-vis spectra were measured with a Cary 210 of Re2(C0)8L2with RI can therefore be described stoichiometspectrophotometer. rically as in eq 1. The alkyl radicals presumably react further The known complex6 ~ X , U X - R ~ ~ ( C O ) ~ ( was P C ~prepared ,), in high yield as follows. Re2(CO)lo(400 mg, 0.61 mmol) and PCy, (1.3 g, 5 Re2(CO)s(PCy,)z + 2RI 2ci~-Re(CO)~(PCy,)1 + 2R' (1) mmol) were dissolved in decalin (20 mL), and the solution was degassed by several freeze-pump-thaw cycles in a Schlenk tube sealed with a to form C16H3zand C16H34. c i ~ - R e ( C 0 ) ~ ( P C y , )appears 1 to rubber septum cap. The solution was heated to 185 'C under partial undergo a further slow reaction to form ( ~ L - I ) ~ R ~ ~ ( C O ) , ( P C ~ , ) ~ vacuum for 40 min, and a white precipitate was deposited on cooling. as evidenced by the appearance of bands a t 2016 s, 1925 s, and This was separated by filtration, washed several times with dry cyclo1909 m cm-'. hexane, recrystallized from a dichloromethane-methanol mixture, and Reaction with P(OPh), proceeds cleanly to form a product with dried under vacuum. The product was characterized spectroscopically. a strong band at 1976 cm-'. This is intermediate between, and It showed a maximum at 299 nm in decalin (e 1.84 X lo4 M-' cm-' ) and well-separated from, the bands at 1952 and 1984 cm-' shown6 CO-stretching bands in decalin at 1976 (sh) vw and 1952 s cm-' (e 3.1 1 X lo4 M-I cm-') as compared with bands at 295 nm and 1997 w and respectively by Re2(CO)s(PCy,)2 and ReZ(CO),(P(OPh),),. I t 1951 s cm-' in acetonitrile.6 No CO-stretching bands at higher fretherefore seems reasonable to assign it to the mixed-ligand complex ~X,~~-R~~(CO)~(PC~~)(P(OP~)~). Similar intermediate bands have been ascribed to the mixed-ligand complexes Mn2(CO)s(1) P&, A. J.; Sekhar, C. V. J . Am. Chem. SOC.1985, 107, 4874-4883. (P-n-Bu,)(PPh,) and Mn2(CO),(PPh3)(PCy,), which are formed (2) Tolman, C. A. Chem. Reu. 1977, 77, 313-348. during crossover experiments with unmixed c o m p l e x e ~ . ' ~Mixed ~ (3) Jackson, R. A.; P&, A. J. Inorg. Chem. 1979, 18, 3331-3333. (4) (a) Schmidt, S. P.; Trogler, W. C.; Basolo, F. Inorg. Chem. 1982, 21, 1698-1699. (b) Stolzenberg, A. M.; Muetterties, E. L. J . Am. Chem. SOC.1983, 105, 822-827. (c) Coville, N. J.; Stolzenberg, A. M.; (7) Kramer, G.; Patterson, J.; Poe, A. J.; Ng, L. Inorg. Chem. 1980, 19, 1161-1 169. Patterson, J. MSc. Thesis, University of Toronto, 1978. Muetterties, E. L. J. Am. Chem. SOC.1983, 105, 2499-2500. (8) Jackson, R. A,; Poe, A. J. Inorg. Chem. 1978, 17, 997-1003. ( 5 ) Weast, R. C., Ed. Handbook of Chemistry and Physics, 53rd ed.; Chemical Rubber Co.: Cleveland, OH, 1972. (9) Poe, A. J.; Sekhar, C. V. J . Chem. SOC.,Chem. Commun. 1983, 566-567. (6) Cox, D. J.; Davis, R. J . Organomet. Chem. 1980, 186, 347-352.
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0020-1669/87/1326-lO57$01 SO/O
0 1987 American Chemical Society
1058 Inorganic Chemistry, Vol. 26, No. 7, I987
Po&et al.
Table 1. Rate Constants for the Reaction of Re2(CO)8(PCy,)2uin the Absence of Added PCv? in Decalin T , OC i04koM, s - ~ 160.0 15.9, 15.6, 15.5, 16.5 5.97, 6.25, 6.24, 6.10, 6.06, 5.60 150.0 140.2 2.86, 2.36, 2.65 129.8 1.02, 1.01, 1.04 119.3 0.381, 0.398, 0.385 150.0 5.83? 5.77,b5.90,c 5.74c
AH* = 29.9 i 0.4 kcal mol-'; AS* = -3.2 i 0.9 cal K-' mol-' u(kobd)d
= f6.2%
4l
?h Y: 3
a Unless indicated otherwise, reaction is with RI where [RI] varies M. The initial concentration of the over the range (5-600) X complex is ca. 2 X lo4 M. bReaction with 0.01 M P(OEt),. CReaction with 0.1 M P(OPh),. dProbable error for an individual measurements of kabsd.
Table 11. Dependence of kobdon [PCy,] and [RI] for the Reaction of Re2(CO)R(PCy,),with RI at 150 OC in Decalin 1O2[RI1, 1O2[PCy3l, [PCy,I/ lOSkOM, 10Skald: 100A,b M S-I % lRIl S-1 M 1.oo 0.10 0.10 50.4 48.7 3.4 1 .oo 1 .oo
1.oo 1 .oo
1.oo 1.00 1 .oo 1.00
1.00 1.00 1.00 5.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00
'
0.25 0.30 0.40 1 .oo 2.00 5.00 10.00 20.00 20.00 20.00 20.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
0.25 0.30 0.40 1.00 2.00 5.00 10.00 20.00 20.00 20.00 20.00 0.40 0.20 0.10 0.067 0.050 0.040 0.033 0.029
33.3 30.7 26.2c 15.4 12.5 7.78 5.19 4.90 4.76 2.02d 1.97' 25.5 36.7 46.1 56.3 57.2 61.4 61.3 63.9
33.5 30.5 26.1 15.7 10.3 6.94 5.74 5.13 5.13
-0.7 0.5 0.4 -1.5 21.5 12.1 -9.6 -4.4 -7.2
26.1 37.3 48.7 54.6 58.1 60.5 62.2 63.4
-2.3 -1.6 -5.3 +3.2 -1.5 1.6 -1.4 0.7
= 1471 + 'Values calculated according to l/(kaslcd- 4.5 X 7900[PCy3]/[RI], where 1471 and 7900 s are the gradient and intercept, respectively, found from the linear least-squares analysis of the data in Figure 1. b A = (kobsd- kcalcd)/kcalcd.
