J. Am. Chem. Soc. 1994,116, 208-214
208
Binding to and Fluxional Behavior of Ir( H ) 2 X ( = C1, Br, I; R = Me, Ph)
H 2
PtBu2R)2
(X
Bryan E. Hauger, Dmitry Gusev,' and Kenneth G. Caulton' Contribution from the Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Received July 12, 1993"
Abstract: The molecules Ir(H)2X(PBuzPh)z (X = C1, Br, I) all show low-temperature 1H N M R spectra consistent with a structure with inequivalent hydrides, one of which is trans to X and one trans to an empty coordination site. T1 measurements suggest that all three molecules have very similar hydride/hydride distances. Hydride rearrangement AH*values show little dependence on the halide identity. The decoalescence behavior is not observed for the analogous compounds with P'BuzMe. While PtBu2Ph compounds show only T I and spin-saturation transfer evidence for H2 binding, the P'BuzMe compounds show (at low temperature) signals for both H2 adduct and five-coordinate species. Line shape analysis of the variable-temperature 3lP(lH] N M R spectra permits the determination of PHO, ASo, A", and AS*for H2 dissociation. These parameters have been determined for Cl, Br, and I and (for the chloride case) when deuterium replaces all hydrogens on Ir. The transition state for H2 dissociation is "tight" (based on AS*)and it is enthalpically most accessible for the best a-donor halide, C1. This same effect stabilizes the five-coordinate species most for chloride, and thus makes the activation enthalpy largest for H2 binding. The AHO values for H2 dissociation increase down the halogen group. Introduction A detailed study of the reactions between the formally unsaturated Ir(H)2X(PR3)2 and H2 is interesting for several reasons. Many q2-H2complexes have been synthesized for their theoretical significance and potential importance in catalysis.' However, few systems are amenable to kinetic analysis of q2-H2 complex formation (or destruction). M(H2)(CO)s(PCy3)2 (M = Cr, W) is the only system to date for which both kinetic and thermodynamic parameters of H2 binding have been determined.2 The formation of Ir(H)2(H2)CI(PR3)2(R = Pri, Cy, 'Bu) from Ir(H)$l(PR3)2 and H2 (eq 1) was initially described by Jensen
Here we report the observation of inequivalent hydrides in Ir(H)2X(PtBu2Ph)2 by lH N M R spectroscopy. We will connect this observation to previous theoretical studies and discuss the effect of the halide and phosphine identity on H2 binding in related systems.
Results and Discussion
Fluxional Behavior of Ir(H)zX(PtBu&. (a) Decoalescence. The variable-temperature 1H N M R spectroscopic study of Ir(H)2X(PtBu2Ph)2 reveals decoalescence behavior of the hydride signal. For example, in the case of a toluene-dg solution of Ir(H)zI(PBu2Ph)2, this signal is observed as a triplet ( 2 J p = ~ 13 1r(H)2X(PR3)2 + H2 1r(H)2(H2)x(pR3)2 (l) Hz) at 6 -32.2 at room temperature, but it splits into two broad (145 Hz) resonances* at 6 -20.1 and 4 4 . 4 as the temperature et al.3 We also observed this reaction as a part of complex is lowered to-100 OC (Figure 1). Thedecoalescencetemperature transformations of IrHClz(PR3)z (R = Pri, Cy) under hydrogen is ca. -65 OC, which provides9 a free energy of activation of 8.0 in s0lution.~~5 A detailed understanding of the thermodynamics kcal/mol for the hydride site exchange at -65 OC. Line shape and kinetics of equilibrium 1 would provide a unique opportunity analysis9 of the 1H N M R spectra recorded between -20 and -95 for a meaningful discussion of the effect of halide identity on the OC resulted in an Eyring plot (Figure 2) with AH* = 7.9 f 0.2 stability of both the unsaturated complex, Ir(H)2X(PR3)2,and kcal/mol and AS*= 0.1 f 1.2 eu. This near-zero entropy change the 18e Ir(H)2(H2)X(PRs)2. We report here such a study. is consistent with an intramolecular mechanism, and the triplet A recent neutron diffraction study of I ~ ( H ) ~ C ~ ( P ' B U ~ P ~ ) ~ structure at 25 OC demonstrates that there is no rupture of the revealed that the hydride ligands were crystallographically Ir-H or Ir-P bonds during the fluxional process. distinct.5 Previous reports3~6Jofproton NMR spectra of molecules The low-temperature limiting hydride chemical shifts are of this type have shown only equivalent hydrides. This raises the similar to those of other relevant complexes. The hydridechemical possibility of different structures in the solid state and in solution. shift of IrHC12(PtBuzPh)2, in which the hydride occupies the *Abstract published in Advance ACS Abstracts, December 1, 1993. apical position in a square-pyramidal geometry, is -48.5 ppm.6 (1) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155. The hydride chemical shift trans to chloride in Ir(H)zCl(Cyttp) Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93, 913. is -22.3 ppm.1° Therefore, we assign the 4 4 - p p m resonance to (2) (a) Gonzalez, A. A,; Hoff, C. D. Inorg. Chem. 1989, 28, 4295. (b) Millar, J. M.; Kastrup, R. V.; Melchior, M. T.; Horvath, I. T.; Hoff, C. D.; a hydride which is pseudo-trans to an empty coordination site Crabtree, R. H.J. Am. Chem.SOC.1990,112,9643.(c) Zhang, K.; Gonzalez, and the -20-ppm resonance to a hydride which is pseudo-trans A. A.; Hoff, C. B. J. Am. Chem. SOC.1989, 111, 3627. to the halide ligand. This spectroscopic assignment would also ( 3 ) (a) Mediati, M.; Tachibana, G. N.; Jensen, C. M. Inorg. Chem. 1990, 29,3. (b) Mediati, M.; Tachibana, G. N.; Jensen, C. M. Inorg. Chem. 1992, be consistent with the solid-state structure (neutron diffraction) 31, 1827. of Ir(H)zCl(P'BuzPh)2 (Figure 3) in which one of the H-Ir-Cl (4) Gusev, D. G.; Bakhmutov, V. I.; Grushin, V. V.; Vol'pin, M. E. Inorg. angles is 1 5 6 O and the other is 131O . 5 Chim. Acta 1990, 177, 115. ( 5 ) Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.; Eisenstein, 0.;Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster, W.; Koctzle, T. F.; McMullan, R. K.; OLoughlin, T. J.; Pblissier, M.; Ricci, J. S.; Sigalas, M. P.; Vymenits, A. E.J. Am. Chem. SOC.1993, 115, 7300. ( 6 ) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 20, 2069. (7) (a) Werner, H.; Wolf, J.; Hbhn, A. J. Organornet. Chem. 1985, 287, 395. (b) Goldman, A. S.; Halpern, J. J. Organomet. Chem. 1990, 382, 237.
(8) The line width was 35 Hz in CDzC12 at -100 OC and no evidence was obtained for any large (>30 Hz) proton-proton coupling constants in the low-temperature 'H NMR spectra of the Ir(H)2X(PBuzPh)z complexes. (9) Oki, M. In Applications of Dynamic NMR Spectroscopy to Organic Chemistry; VCH Publishers: Deerfield Beach, 1985, Chapter 1. (10) Yang,C.;Sml,S. M.; Kountz, D. J.;Meek, D. W.;Glaser,R. Inorg. Chim. Acta 1986, 114, 119.
OO02-7863/94/ 1516-0208$04.50/0 0 1994 American Chemical Society
H2 Binding to and Fluxional Behavior of Ir(H)2X(PfBu2R)2
k= 7300008'
J. Am. Chem. SOC.,Vol. 116, No. 1 , 1994 209
Scheme I r
l*
-22°C 100000
A
5
A
0
2500
I
I
Table 1. TI(nu)Relaxation Time of the Hydride Ligands in Ir (H)2X(PLBu2Phl2 in Toluene-ds ~~
~
.X
IllllllnlllllllllllllllTTnTrrm -20 - 3 0 -40 PPM
- 1
-20 -30 -40 PPM Figure 1. Experimental (left) and calculated (right, with rate constants) IH NMR spectra (hydrideregion only) for Ir(H)2I(PBu2Ph)z in tolueneds
-
10
" " . " " " " " " '
\
i=
,004
,006
,005
,007
1/T Figure 2. Eyring plots for the rate of hydride site exchange in Ir(H)2X(PBu2Ph)z for X = I (a, in CDzCl2; b, in toluene-ds), Br (c, in CDzCIz; . b and d d, in toluene-ds), and C1 (e, CDzC12; f, in toluene- d ~ )Lines include date derived from different concentrations of complex.
T,K
C1
Br
I
282 268 251 234 217 20 1 184
342 248 181 148 143 198 274
337 244 193 149 135 a a
353 263 204 147 a a 240
TIwas not determined because of strong broadening of the IrHz resonance(s). The experimental kinetic data for Ir(H)2I(PBuzPh)z can be rationalized in terms of the site exchange shown in Scheme1 where a ground state T-shaped structure lies 8 kcal/mol below a transition state of Y geometry. The disparity between the ground-state structure calculated (ab initio) for Ir(H)2Cl(PH3)~ and that observed for examples with extremely bulky phosphines must originate in steric repulsion between phosphine substituents and the Ir(H)2X moiety. Steric interactions of this type were identified in the T-distorted ground-state structure of Ir(H)2Cl(P'Bu2Ph)z.' (b) Is the H/H Distance Dependent on the Halide Coligand? A variable-temperature T1 study can be useful for a qualitative comparison of the H-Ir-H distances as the group X is varied in Ir(H)zX(PtBu2Ph)2. The relaxation rate, l / T l , for the metalbound protons is where the contributions are due to the dipole
1/T, = l/Tl(H ...H)
+ l/T1(H...H*)
dipole interactions between the phosphine protons and hydride ligands ( l / T l ( H...H*)) and between the hydrides themselves (1/ Tl(H ...H))." The latter contribution is quite significant in the present case. In Ir(H)2C1(PtBu2Ph)z, 1/ Tlhnis 7 s-l at 300 MHz, and from the H...H distance of 1.817 A (neutron diffraction)S the l/Tlmin(H...H) contribution of 3.6 s-1 (51% of the observed relaxation rate) can be calculated. It is reasonable to assume12 anegligibledependenceof 1/T1(H ...H*) onX andany substantial changes in 1/T1 should be due to the l / T l ( H ...H) contribution which is extremely sensitive to H...H distance variations. For example, l/T1dn(H...H) = (3.87 X 104)r1r(H H)4, where Y is the resonance frequency in MHz and r(H H) is the distance in A.ll Thus, the equation predicts a 32% increase in Tlhnif the H-Ir-H angle increases by 10" assuming the same Ir-H distances: 1.512 and 1.553 A. The experimental T I data for the hydride relaxation in Ir(H)zX(PBuzPh)z (Table 1) are very similar at comparable temperatures. The observed differences are typically within the error limits of the technique and do not exceed 14%. This observation leads to the conclusion that the distortion of the molecular geometry which is the cause of the inequivalence of the hydrides in Ir(H)2X(PBuzPh)z does not involve appreciable
...
Figure 3. Structure of IrH$l(PBuZPh)z.
An ab initio study of Ir(H)2Cl(PH3)2 provides an explanation for this di~tortion.~ Although the Y structure (Scheme 1) was calculated to be the energy minimum, distortions of the H-Ir-Cl angles do not require large amounts of energy ( q 3 . 3 kcal/mol) ifthe H-Ir-H angle remains approximately 73'. Thus, pseudoT-shaped structures are energetically accessible.
...
(1 1) Proton-phosphorus dipole4ipole interactions are weak and can be neglected. Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J . Am. Chem. SOC.1991, 113,3021. (12) A referee suggests that the constancy of the T Ivalue down the halides arises from fortuitous compensation of decreasing TI(H- -H) and T I (H- - -He). We prefer our explanation as simpler.
-
210 J. Am. Ckem. SOC.,Vol. 116, No. 1, 1994
Hauger et al.
Table 2. Spectroscopic and Hydride-Site Exchange Activation Parameters for Ir(H)2X(PtBu2Ph)2
X
c1 CI Br Br I I
low-temperature 6(IrH) a
-24.9; -23.6; -23.1; -20.1; -19.8;
-43.1' 42.4' -44.9' 44.4' 45.9'
AH*
As*
1.4 f 0.26 7.3 f 0.3' 8.0 f 0.2' 7.8 f 0.2' 7.9 f 0.26 8.7 f 0.2'
7.1 i 0.4' 4.6 f 1.5' 5.6 f 1.4b 2.2 f 0.9' 0.1 f 1.26 2.1 f 0.9c
The decoalescencewas not reached in toluene-dg; A6 = 15.2 ppm was assumed, based on comparison to Ir(H)2Br(PtBuzPh)z in toluene-d8.' In toluene-& In CD2C12.
changes in the H...H distance and, thus, the H-Ir-H angle. For the latter, the actual changes should be less than '5 based on the maximum difference in the T1 values (14%). (c) Fluxionality for a Smaller Phosphine Analog. In contrast to the above, Ir(H)2X(PtBu2Me)2 compounds show only one hydridechemicalshift down to-100 OC. At thelowest attainable temperature, the broadest line (A = 80Hz,-100 "C) wasobserved for Ir(H)zI[PBu2Me]2in toluene. High viscosity of the solvent is, perhaps, responsible for the line width rather than a fluxional process, since in less viscous CD2C.12, this resonance was sharp enough to not obscure the proton-phosphorus coupling. For a related compound Ir(H)2Cl[PiPr3]2 the hydrides were found to be equivalent at 77 K by solid-state NMR.13 Thus, the absence of the decoalescence behavior for Ir(H2)X[PtBu2Me]2 most probably reflects undistorted Y-shaped structure for this smaller phosphine. (d) Halide Dependence of the Fluxionality Barrier. The 'H NMR spectroscopic and kinetic data for the full range of halide complexes Ir(H)zX(PtBuzPh)z are summarized in Table 2. The rate of intramolecular exchange significantly accelerates in the order I < Br < C1. For example, ka/kI = 143 and ker/k1 = 12 in toluene at 200 K. Surprisingly, however, the effect is not enthalpic in origin. As it is seen from Table 2 and from the nearly-identical slopes in Figure 2, the AH*values do not depend (significantly) on X, changing only from 7.4 to 8.0 kcal/mol in toluene. We have expended the effort to look for solvent effects on the site exchange rate. There are two reasons to anticipate only small solvent effects on the activation parameters (Table 2) when comparing CHZCl2 and aromatic solvents. First, the Ir(H)2X(PtBu2R)z molecules are very crowded due to the large phosphines, leaving no site for solvent incorporation. In addition, the hydride motion occurs with very little structural reorganization of the IrX(P)2 atoms and thus the solvent sheath should be very similar throughout the rearrangement. Indeed, AH* values show no major changes on comparing the two solvents. Any solvent effect is reflected in the ratio k(toluene-ds)/k(CD2C12), which is between 2.9 and 3.8 for Ir(H)zX(PtBu2Ph)z at 200 K.14 Significantly positive AS* values of 7.7 f 0.4 and 5.6 f 1.4 eu found for the hydride exchange in Ir(H)2Cl(PBu2Ph)z and Ir(H)~Br(PtBuzPh)2,respectively, seem at first glance to be inconsistent with an intramolecular process. At the same time, they are inconsistent with a bimolecular process, where a
substantially negative AS* is expected. In general, it is unreasonable to suspect that different site exchange mechanisms can operate in Ir(H)zX(PtBuzPh)2 depending on the halide identity. Experiments with different concentrations of the complexes ( 1 3 ) Wishniewski,L.L.;Mediati, M.; Jensen, C. M.; Zilm, K. W. J. Am. Chem. SOC.1993, 1 1 5 , 1 5 3 3 . (14) (a) There is currently an inability of physical theories to account quantitatively for solvent effects on rates. A decrease in reaction rate upon increasing solvent polarity is commonly associatedl4b~cwith a transition state of lower polarity than the ground state. Ground-state stabilization by the more coordinating solvent (CDzC12 us toluene-ds) can be an alternative explanationto purely polarity-basedconsiderations.(b) Reichardt, C. Solvents ond Solvent Effects in Organic Chemistry; VCH Publishers: New York, 1988, Chapter 5. (c) Connon, K. A. ChemicolKinetics: TheStudy ofReaction Rates in Solution; VCH Publishers: New York, 1990.
I rH2
m
1 tD
9
9
N
I
I
m
P
A
h
Figure 4. Effects of saturation transfer in *HNMR spectra (free H2 and hydride regions) of Ir(H)2C1(PtBu2Ph)2 under 700 Torr of H2 in toluened8 at -5 OC.Saturation was applied a t the indicated positions or, in the case of the reference (bottom) spectrum, between the resonances. All spectra were measured and plotted under otherwise identical conditions.
(Figure 2, X = Br, I) show no concentration dependence, strongly supporting the intramolecular character of the exchange. We believe that quantum-mechanical tunneling of the hydride ligands can operate, to a varying degree, in all Ir(H)2X(PR3)2complexes, precluding correct determination of the AS* v a l u e ~ . ~ ~ J ~ H2BmdingtoIr(H)2X(PBugh)2 (X = Cl,Br,I). Theextreme steric bulk of PtBu2Ph strongly disfavors H2 binding by these molecules. For each halide, spectroscopic evidence supports equilibrium 1, but lying very far to the left. At room temperature, the lH NMR of all Ir(H)2X(FBu2Ph)2 molecules under H2 show broadened resonances of dissolved hydrogen (A = 56-62 Hz) at 6 4.5 and that of Ir(H)2 (A = 40-46 Hz) at ca. 6 -32. Above -20 OC, the TI relaxation measurements give identical TI values for dissolved hydrogen and for the hydrides. The TIvalues for the Hz signals (200-300 ms a t -10 "C) are appreciably shorter than those of free Hz in the absence of the metal complex (> 1 s). Only at temperatures below -20 OC are the relaxation times for dissolved H2 increased because of slowing the rate of equilibrium 1 on the relaxation time scale. The most convincing evidence for the existence of equilibrium 1 is obtained by a spin saturation-transfer experiment for Ir(H)2Br(PtBu2Ph)2. At +10 OC, C W decoupling at 6 4.5 effectively saturates the Ir(H)Z peak at 6 -32.7 and vice versa. This clearly shows that protons of the dihydride complex and Hz are exchanging quickly in the unobserved fluxional Ir(H)2Br(H2)(PtBu2Ph)2. Figure 4 shows this behavior for the chloride complex. I n all cases, in the temperature range 20 to -90 OC, no detectable amount of Ir( H ) ~ X ( H ~ ) ( P B Uis~ directly P ~ ) ~ observed by lH and 31P{lH) NMR. While this precludes further kinetic and thermodynamic study, the averaging and shortening of TIfor free H2 and for coordinated hydrides represents a phenomenon useful for proving the existence of a hydride/dihydrogen compound when direct ( 1 5 ) Bell, R. P. In The Tunnel Effect in Chemistry, Chapman and Hall: London and New York, 1980, Chapter 3. (16) We attempted to slow any quantum tunnelling contribution by replacement of H by deuterons. The 2H NMR spectrum of Ir(D)2CI(PBuZPh)z shows no decoalescence at -102 OC in CHzC12. Since the rwnance frequency is reduced by a factor of 6.5 compared to the protio compound,this factor overwhelms any change in site exchange rate and thus leaves the experiment inconclusive.
H2 Binding to and Fluxional Behavior of Ir(H)2X(PtBu2RJ2
J . Am. Chem. SOC.,Vol. 116, No. 1, 1994 211
k- 43000 K1
h
t
6500
&. c -
1-50
do _i, -
a
1
-2.5
A
,005
,004
T1--
50
YO
30
PPM 59
Ya
30
Figure 5. Observed (left) and calculated (right, with rate constants for
HZloss from Ir(H)2Cl(H2)(FBuzMe)2) variable-temperature 31P(lH) NMR spectra of Ir(H)2CI(PBu2Me)2 under 700 mm of HZin tolueneds. The resonanceof Ir(H)2CI(Hz)(PBuzMe)2appearsat 35.5 ppm. The asterisk indicates an inert impurity at 32 ppm. observation is not possible (e&, when the equilibrium population is low). Kinetics of H2 Binding to Ir(H)S(PBuzMe)z (X = Cl, Br, I). Below 20 O C , variable-temperature IH and 31P{1H}N M R study demonstrates appreciable HZ binding by all of these halide compounds, consistent with the smaller size of FBu2Me (Le., dominant steric effect)." A significant steric influence on the extent of H2 binding has been demonstrated previously in Ir(H)zCl(PR3)z systems.3b Rate data for loss of H2 from Ir(H)*X(H2)(PtBuzMe)2 obtained by fitting the experimental variabletemperature 3IP{IH}N M R spectra (see, for example, Figure 5 ) using the simulation program DNMR5 are displayed in an Eyring plot in Figure 6. Data obtained for the dissociation of DZfrom Ir(D)2(D2)C1(PtBu~Me)~ are also compared to those for loss of Hz from Ir(H)z(Hz)Cl(PtBuzMe)2 in an Eyring plot in Figure 6. Activation parameters obtained by linear least-squares fitting of the data are shown in Table 3. The AS*values are all positive but of small magnitude consistent with a process with only slightly increased degrees of freedom in the transition state as a result of the departing HZmolecule. The values show that the transition state is reached with only a small amount of bond breaking. Perhaps the higher entropy of D2 us Hz is reflected in a higher AS* for Ir(D)zCl(D2)(PtBu2Me)2 than for Ir( H)2C1(H2) (P'BuzMe) 2. Unfortunately, the relatively large error makes further quantitative comparisons impossible. The compounds Ir(H)2X(H2)(PtBu2Me)z are among the most unstable known dihydrogen complexes and the AH* values listed in Table 3 are at the lower limit of values found for the loss of H2. Comparable activation parameters of 8.8 f 0.1 and 12.1 f 1.0 kcal/mol were reported for [ R U ( H Z ) ( H ) ~ ( P P ~and ~ ) ~Cr(Hz)]+ (CO)3(PCy3)2, respectively.zbJ8 In the case of the kinetically more stable W( H2) (CO)3(PCy3)2 and Ru( H2) (H)2(PPh3)3, AH* values are noticeably higher: 16.9 f 2.2 and 17.9 f 0.2 kcal/ mol, respectively.2cJ4 Unfortunately, meaningful comparison of the activation entropies is practically impossible because of the large error (several cal mol-' K-I) in the AS* data available in the literature. The error, perhaps, is mainly responsible for the strongly different values reported for [Ru(Hz)(H)3(PPh3)3]+ (-1 2 4 , C ~ ( H ~ ) ( C O ) ~ ( (-2.1 P C Yeu), ~ ~W(HZ)(CO)~(PCY~)Z (+10.4 eu), and Ru(Hz)(H)2(PPh3)3 (+3 ~ u ) . ~ ~ , ~ J ~ (17) It is not possible that the PBuZPh analog binds HZpoorly due to a phenyl o-hydrogen interaction becausethe neutron diffraction structureJ shows these to be absent. Also, the 4 4 ppm hydride chemical shift is diagnostic of hydride trans to an empty coordination site. (18) Halpem, J.; Cai, L. S.;Desrosiers, P. J.; Lin, Z . R. J. Chem. Soc.. Dalton Trans. 1991, 717.
-B Figure 6.
1/T (A) Eyring plot of rate data for Hz loss from Ir(H)ZX(Hz)(P-
Bu2Me)z for chloride (a), bromide (b), and iodide (c). Lincs b and c contain data for two different concentrations of complex (identified by different symbols) and show the concentration independence of k. (E) Eyring plots for dissociation of Dz from I ~ ( D ) Z ( D ~ ) C I ( P B U(b) ~M~)~ and H2 from Ir(H)2C1(Hz)(PtBuzMe)2 (a). Table 3. Kinetic and Thermodynamic Data for Hz and D2 Loss from Ir(H)z(H2)X(PBuzMe)z(X = C1, Br, I) and
Ir(D)z(Dz)Cl(FBu2Me)2 in Toluene-d8 M m* (X 1P2) kcal/mol A S ,eu C1 41 9.4 f 0.2 2.3 f 0.9
m,
9
Br
29
10.2f 0.2
2.4 f 1.0
I
47 22
10.1 f 0.2 10.8 f 0.5
2.4 i 0.9 2.8 i 2.0
11.3 f 0.3
3.7 f 1.1
Clb
35 28
11.7 f 0 9
8.7 f 2.2"
41
kcal/mol
W,eu
6.8 f 0.2
19.2 i 0.7
7.9 f 0.9"
19.7 f 3.2"
9.3 f 0.2'
22.7 i 0.80
7.7 i 0.5
20.7 i 1.8
a Parametersweredeterminedcombiningthedata at twoconcentrations when less than five data points were measured for one concentration. Data for I~(D)zCI(DZ)(PBU~M~)Z.
As could be expected for a process involving bond rupture, the kinetic isotope effect found for the D2 loss from Ir(Dz)(D)2Cl(PtBu2Me)2 is relatively high; kH/kD are 1.8 and 6 a t 20 OC and -50 OC, for example. These values are within the range of kH/kD for W ( H Z ) ( C ~ ) ~ ( P C Y Cr(Hd(C0)5, ~)Z, and IrHCh(H2)(PCy3)2: 1.7 (25 "C), 5 (room temperature) and 6-7 (-60 to-70 "C), respectively.2c*4J9 AH* values increase in the order C1< Br < I. The dissociative transition state could be expected to show the opposite trend based purely on size of the halide, with the larger iodide20 promoting a decrease in coordination number. Therefore, the data indicate that electronic factors dominate steric considerations. We suggest a transition state in which a halide lone pair interacts (19) Church, S.P.; Grevels, F.-W.; Hermann, H.; Schaffner,K.J . Chem. SOC.,Chem. Commun. 1985, 30. (ZO)Tolman, C. A. Chem. Rev. 1977, 77, 313.
212 J. Am. Chem. Soc., Vol. 116, No. I, 1994
3 4x103
3 6x10”
3 ~3x10.~
4OXIO-~
1/T Figure 7. Equilibriumconstantsfor Hzdissociation from Ir(H)ZX(Hz)(PBuzMe)~displayed as In K vs 1/T. Lines a, c, and d are for I, Br, and C1, respectively. Line b is for Dz binding to Ir(D)zC1(PtBu2Me)z. Lines c and d include data collected at two different concentrations of complex.
Hauger et al.
In general, a great thermodynamic difference between the reactions of dihydrogen coordination or oxidative addition should not be expected. A higher enthalpy change can, perhaps, be associated with the latter reaction. The few entropy unit change found for the equilibria M(H2)Ln* M(H)2Lnclearly shows that the intramolecular part of an oxidative addition of H2 is not significantly“entropy consuming”.’ Thus, ASo should be mainly determined by the loss of the rotational freedom of H2on “adduct” formation. The “adduct” can be a stable dihydrogen complex or a transition state if further (oxidative) reaction occurs. Unfortunately, the existing literature data are too limited to argue either for or against these considerations. If for the oxidative addition of H2 to CpzTa(p-CH2)zIr(CO)(PPh3) the ASoof -23.7 f 0.6 eu is “reasonable”, the example with W I Z ( P M ~gave ~ ) ~a very large ASo of -45 f 2 eu.23924 Reactivity of H2 with IrH2F(PtBu2R)2 (R = Me). We have shown25that late transitionmetal fluorides areoften quite different from C1, Br, and I in their binding and also in their reactivity. For example, RuHF(CO)(PtBu2Me)2 reacts with H2 to eliminate H F whereas this behavior was not observed for the other halides. We were therefore interested in obtaining kinetic and thermodynamic informationon HZbinding to iridium dihydride fluorides. Unfortunately, IrH2F(PtBu2Ph)2reacts in an analogous fashion to the ruthenium fluoride mentioned above producing IrH@ Bu2Ph)z (3lP and 1H NMR evidence) and H F (through observation of glass etching), precluding further study. The common reactivity for these different compounds underscores the high Brernsted basicity of fluoride in the presumed dihydrogen complexes RuHF(H2) (CO)(PBuZMe) and Ir(H)2F(H2)(PBu2Ph)2. It is also worth highlighting the low acidity of H F under these conditions as evidenced by the lack of significant back reaction between IrHsL2 and HF.
constructively with the metal center, partially displacing the dihydrogen ligand. The AH*values contain evidence for the existence of increasing multiple-bond character between the iridium metal center and the halide ligands in the transition state with the established21 capacity for ?r-bonding (C1 > Br > I) determining the trend. Thermodynamics of H2 Loss from Ir(H)&(PtBulMe)2 (X = CI, Br, I). The a-donation invoked to explain the trends in AH* should also influence the thermodynamics of H2 binding to Ir(H)zX(PBuzMe)2 complexes. Thermodynamic data were obtained at temperatures above -30 OC due to the slow rate of H2 transport into and out of toluene in an NMR tube at temperatures below -30 OC. Data obtained for dissociation of Conclusion Ir(H2)(H)2X(PtBu2Me)2are displayed in a plot of In K us 1 / T in Figure 7. The data obtained here are best represented graphically. In Values obtained by linear least-squares fitting of the data are Figure 8 reaction coordinate profiles of all of the compounds contained in Table 3. The ASo values are large and positive, as measured are superimposed with the transition state enthalpy expected for a dissociative reaction. The magnitude of ASoalso fixed at the same energy for the purpose of comparing AH*values. compares well with previously obtained values: 23.2 f 1, 23.8 The ordering of halides is reversed in the five- and the f 2.1, and 25.6 1.7 eu for RU(H~)HCI(CO)(P~P~~)Z,~~ Mosix-coordinate sides precisely because what assists during dis(Hz) (CO) 3(P C Y ~ )and ~ , Cr( ~ ~H2) (CO) 3(P c y j ) ~ respectively. ,~~ sociation of H2 (X Ir ?r-donation) inhibits binding of H2 (or These same complexes, having been characterized by AHo of 7.7 any Lewis base). The magnitude of this effect upon H2 addition f 0.2, 6.5 f 0.2, and 7.3 f 0.1 kcal/mol, respectively, have is modest (
