Organometallics 1996, 14, 698-702
698
Reaction of Os2(CO)s(/r-q1,q1-C2H~) with (q5-C5H5)Rh(C0)PR3(R = Me, Ph): Characterization and Dynamic Processes in Isomeric Os2Rh(CO)s(lt5-C5H5)PMe3 Jason Cooket and Josef Takats" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Received August 12, 1994@
Thermal reaction of Os2(CO)s~-y1,y1-CzH4) with (v5-C5H5)Rh(CO)PR3(R = Me, Ph) yields a n array of trimetallic products. The known clusters, Os2Rh(C0)9(v5-C5H5)and Osg(CO)11PR3, are recovered in each case while, for R = Me, a third product formulated as Os2Rh(CO)s(v5-C5H5)PMe3(1)is also isolated. Compound 1 exists in solution as two interconvertible isomers la and lb,each of which exhibits distinctly different modes of carbonyl scrambling processes due to the position of the PMe3 ligand on one of the Os centers. Global carbonyl scrambling and isomer interconversion occur a t high temperature via a restricted trigonal twist mechanism at the phosphine-substituted osmium center. Line shape analysis of the 13C NMR spectra yields activation energies of AG*239 = 11.5 f 0.4 kcal-mo1-l for pairwise exchange in the (0C)rOs-Rh plane of la and AG*253 = 13.0 f 0.4 kcal*mol-l for merry-goround CO migration in the Os-Os plane of lb. Line shape simulation of the 'H NMR spectra provides an activation energy of AG*303= 15.1f 0.4 kcal-molp1for the isomer interconversion process.
Introduction One of the remarkable features of transition metal carbonyl clusters and their derivatives is the wide range of carbonyl and other ligand migration processes exhibited by these mo1ecules.l In a recent report from our laboratory, Washington and Takats described painvise carbonyl exchange in the heterotrinuclear clusters OszRh(C0)9(q5-C5R5)( R = H, Me).2 For OszRh(C0)g(q5C5H5),carbonyl scrambling was observed only across the Os-Rh centers, while in Os2Rh(C0)9(q5-C5Me5),global carbonyl migration was observed at high temperature. Subsequently, Riesen et al. have reported a similar study for the analogous Os21r(C0)dq5-C5R5)( R = H, Me) c~mplexes,~ the results of which essentially confirmed those established by Washington and Takats. A fashionable method of providing support for the mechanism of carbonyl ligand migration in transition metal clusters has been the introduction of some form of built-in restriction to the system which physically makes it impossible for scrambling to occur via the proposed mechanism. Lack of carbonyl migration in the hindered system is then taken as evidence for the validity of the proposed mechanism in the unencumbered ~ o m p l e x .It~ is well-known that one method of preventing painvise exchange of carbonyl ligands between metal centers is to introduce a ligand that is incapable of bridging the two metals. Phosphines and NSERC Undergraduate Research Awardee. Abstract published in Advance ACS Abstracts, December 1,1994. (1)(a) Evans, J . Adu. Organomet. Chem. 1977,16, 319. (b) Band, E.; Muetterties, E. L. Chem. Rev. 1978,78,639.(c) Johnson, B.F. G.; Benfield, R. E. In Transition Metal Clusters; Johnson, B. F. G., Ed.; Wiley: Chichester, England, 1980;p 471. (d) Mann, B. E. Comprehensive Organometallic Chemistry;Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1982;Vol. 3,p 89. (2) Washington, J.; Takats, J . Organometallics 1990,9, 925. (3)Riesen, A.; Einstein, F. W. B.; Ma, A. K.; Pomeroy, R. IC;Shipley, J. A. Organometallics 1991,10, 3629. (4)(a) Cotton, F.A.; Hunter, D. L.; Lahuerta, P.; White, A. J. Znorg. Chem. 1976,15, 557. (b) Cotton, F. A.; Kruczynski, L.; White, A. J. Inorg. Chem. 1974,13, 1402. +
@
0276-7333/95/2314-0698$09.00/0
Figure 1. Carbonyl scrambling in OszRh(CO)~(q5-C5R~) ( R = H, Me).2 phosphites have been used extensively in such studies of trinuclear clusters of ~ s m i u m .With ~ this in mind, and following the methodology established by Washington and Takats,2 the reaction between OS~(CO)~@q1,+C2H4) and (q5-C5H5)Rh(CO)PR3(R = Me, Ph) was investigated with a view to introducing a phosphine ligand into the OszRh(C0)g(q5-C5H5)framework. The successful incorporation of a phosphine ligand at the group nine metal would effectively block carbonyl migration in the Os-Rh planes, and the absence of fluxionality would thus lend further credence to the pairwise exchange mechanisms proposed to explain the dynamic NMR features of the parent nonacarbonyl clusters. Conversely, phosphine substitution at an osmium center would open possibilities for isomer formation and diverse carbonyl scrambling processes.
Experimental Section General Procedures. All manipulations were performed under a static atmosphere of purified nitrogen or argon using standard Schlenk techniques. Solvents were dried by refluxing under nitrogen with the appropriate drying agent and were distilled just prior to use. O S ~ ( C O ) ~ C ~ - ~ ~ ,13CO-enriched ~ ~ - C Z H ~ )O,S ~ Z(CO)S(LL-~~,~~C2H4),2and (r,f-C5H5)Rh(CO)PR3(R = Me,7aPh7b)were prepared by published procedures. (5)(a) Alex, A. F.; Pomeroy, R. K. Organometallics 1987,6 , 2437. (b) Deeming, A. J. Adu. Organomet. Chem. 1986,26,1, and references therein.
0 1995 American Chemical Society
Isomeric Os$h(CO)s(y5-Cf15)PMe3 Infrared spectra were recorded on a Bomem MB-100 FT-IR spectrometer. NMR spectra were obtained on Bruker WM360 ('H) and WP-400 (13C, 31P) spectrometers. 'H and 13C NMR chemical shifts (6) were internally referenced to solvent and are reported in ppm relative to tetamethylsilane (TMS) while 31PNMR chemical shifts were externally referenced to 85% H3P04. Chemical shifts and isomer ratios (for 1) were found to be both temperature and solvent dependent. NMR samples were prepared under nitrogen. The sample tubes were either sealed with a serum stopper or flame sealed under vacuum. Electron impact mass spectra were recorded on an AEI MS-12 spectrometer operating at 70 eV. Elemental analyses were performed by the Microanalytical Laboratory of this department. Reaction of Os2(CO)s@-C&IJwith (C&)Rh(CO)PMes. OSZ(CO)&L-CZH~) (53.7mg, 0.0849mmol) and (CsHs)Rh(CO)PMe3 (23.0mg, 0.0845 mmol) were placed in a three-necked 100 mL flask equipped with a reflux condenser. Degassed hexane (25mL) was added, and the stirred solution was heated to ca. 50 "C in a silicon oil bath overnight (ca. 16 h). The solvent was removed in UQCUO, leaving a deep red-brown residue. The residue was extracted with 2 x 1 mL CHzClz and loaded, under argon, onto a 20 cm x 4 cm silica gel column packed in hexane. The column was eluted with 3:l hexane1 CHZClz. Three mobile bands (red, yellow, deep red) separated cleanly; an immobile yellow-brown band remained at the top of the column. The solvent was removed in uacuo from each fraction, and the respective residues were recrystallized from pentane at -80 "C. In order of recovery from the column, the air-stable solids were as follows: red-brown crystals of OSZRh(C0)9(C5H5)2(11.6mg, 17%), yellow crystals of Os3(CO)llPMe38 (19.2mg, 36%), and deep red crystals of OszRh(C0)e(C5H5)PMe3 (1) (29.8 mg, 41%). Anal. Calcd for C16H14OsPRhOsz: C, 22.65;H, 1.66. Found: C, 22.89;H, 1.58. IR (pentane, YCO): 2083 m, 2079 sh, 2024 s, 2007 s, 1995 vs, 1977 m, 1964 m, 1953 mw, 1938 vw cm-l. lH NMR (360 MHz): (CDC13; -20 "C) 6 5.48 (s, C&, lb), 5.40(s, C a s , la), 1.90 (d, 'JP-H= 10.2Hz, P(CH3)3, la), 1.84 (d, VP-H= 10.2 Hz, P(CH3)3, lb), 1a:lb = 3.0:l.O;(toluene-&; +80 "C) 6 5.18 (9, C&5), 1.26(d, 2 J p - ~= 10.2 Hz, P(CH313);(toluene-ds; -20 "C) 6 5.16 (s, C a s , lb), 5.00 (9, C a s , la), 1.03 (d, 'JP-H= 10.2 Hz, P(CH3)3, la), 1.02(d, 2 J p - ~= 10.2Hz, P(CH3)3, lb), la: lb 2.1:l.O.31P{H}NMR (162MHz): (CDCl3; +50 "C) 6 -49 (br);(CDC13; -20 "C)6 -46.43 (d, %Tn-p = 5.5 Hz, lb), -48.16 (s, la), 1a:lb= 3.0:l.O; (toluene-&; -20 "C) 6 -48.28 (d, zJn-~ = 5.3Hz, lb), -49.98 (s, la), 1a:lb = 2.1:l.O.13CNMR (100.6 MHz, toluene-ds, CO region only): (+80 "C) 6 188.2(br); (-53 "C) 198.8(br d, V a - c = 37 Hz, 2C,la), 198.6(s, 2C, lb), 194.1 (br d, IJn-c = 37 Hz, 2C, lb), 189.6(s, 2C, la), 188.3(br, 2C, la), 184.5(br, 2C, lb), 178.5(s, lC,lb),175.3 (9, lC,la), 175.1 (s, lC,la), 174.2(s, lC,lb). MS (70eV, 180 "C, mle): M+ nCO, n = 0-8. Reaction of Os2(CO)*@-Cd&) with (C&)Rh(CO)PPhs. OSZ(CO)&L-CZH~) (32.0mg, 0.0506 mmol) and (CsHs)Rh(CO)PPh, (23.5mg, 0.0513mmol) were dissolved in 25 mL benzene in a three-necked 100 mL flask equipped with a reflux condenser. Reaction conditions and workup procedures were analogous to that for the reaction with (CsHs)Rh(CO)PMea except that the column was eluted with 4:l hexaneICHzCl2. In order of recovery from the column, the air-stable solids were red-brown crystals of O S ~ R ~ ( C O ) ~ ( (21.0 C ~ Hmg, ~ ) ~52%) and P P ~mg, ~ ~ 28%). yellow crystals of O S ~ ( C O ) I ~(10.6 Variable-TemperatureNMR Studies of Complex 1. Temperature measurements were made with a Bruker (6) Burke, M. R.; Seils, F.;Takats, J. Organometallics 1994, 13, 1445. (7) (a) Feser, R.; Werner, H. J. Organomet. Chem. 1982,233, 193. (b) Hart-Davis, A. J.; Graham, W. A. G.Inorg. Chem. 1970, 9, 2658. Dalton Trans. 1973, (8) Deeming, A. J.;Underhill, M. J.Chem. SOC., 2727. (9)Bradford, C. W.; van Bronswijk, W.; Clark, R. J. H.; Nyholm, R. S. J. Chem. SOC. A 1970, 17, 2889.
Organometallics, Vol. 14,No. 2, 1995 699 B-VT1000 temperature control unit using a Cu-constantan thermocouple; the temperature at the NMR spectrometer probe is believed to be accurate to fl K. Rate constants for the exchange processes were determined by visual comparison of computer-simulated and observed spectra (Carbonyl migration (13C NMR) in la: k = 2.0f 0.2 s-l, 203 K 8.0 f 0.5 s-l, 213 K, 18 f 1 s-l, 220 K,46 f 2 s-l, 227 K,80 f 5 s-l, 233 K, 150 f 10 s-l, 239 K,310 f 20 s-l, 246 K. Carbonyl migration (13C NMR) in lb: k = 1.2 f 0.2s-l, 227 K 2.6 f 0.2 s-l, 233 K,6.0 f 0.5 s-l, 239 K, 14 f 1 s-l, 246 K; 30 f 2 s-l, 253 K. Isomer interconversion ('H NMR): K = 1.0f 0.1 s-l, 253 K, 2.0f 0.2s-l, 263 K,6.5 f 0.5s-l, 273 K 18 f 1 s-l, 283 K, 42 f 2 s-l, 293 K,80 f 5 s-l, 303 K,180 f 10 s-l, 313 K,375 f 25 s-l, 323 K, 800 f 50 s-l, 333 K, 1500 f 100 s-l, 343 K, 2500 f 200 s-l, 353 K). The activation parameters for the dynamic processes were obtained by a least-squares linear regression fit to the Eyring equation (Carbonyl migration in la: A P = 11.1 f 0.2 kcabmol-l, AS* = -1.5 f 0.8 eu. Carbonyl migration in lb: A P = 13.8 f 0.2 kcal-mol-', AS* = 3.1 f 0.9 eu. Isomer interconversion: A . F = 13.7 f 0.2 kcal-mol-', AS* = -4.5 f 0.6 eu). The free energies of activation were calculated from AG* = AP - TAS* and are reported at the coalescence temperatures (Carbonyl migration in la: AG*239 = 11.5 f 0.4kcal-mol-'. Carbonyl migration in lb: AG*263 = 13.0f 0.4 kcal-mol-l. Isomer interconversion: hG*303 = 15.1 f 0.4 kcal-mol-l). Alternatively, the free energies of activation can be calculated from AG* = -RT, ln[(hk)(k~T,)-l](Carbonyl migration in l a at 239 K, k = 150 f 10 s-l; AG*239 = 11.5 f 0.3 kcal-mol-l. Carbonyl migration in l b at 253 K, k = 30 f 2 s-l; AG*263 = 13.0f 0.3 kcabmol-l. Isomer interconversion at 303 K, k = 80 f 5 s-l; AG*303= 15.1 f 0.3kcal*mol-l).l0 Computer simulation and calculation of activation parameters were carried out with programs written by Professor R. E. D. McClung of this department. A progressive change in the equilibrium composition of the isomerization process was taken into account while the variable-temperature lH N M R spectra were being simulated. Line shapes were matched to the rate of exchange, and the isomer ratio was adjusted to fit the peak heights. A roughly linear correlation of isomer ratio to temperature was found. For the carbonyl migration processes in l a and lb, the simulation was based on a model that assumed that the fast process in each isomer was in essence proceeding at a n infinitely rapid rate; i.e., the limiting s p e c t m exhibited signals characteristic of the averaged state for the fast process. The simulation was then focused upon the signals of la and l b which were only affected by the slow carbonyl exchange process in each case (la: 6 175.3,175.1.lb: 6 178.5,174.2).
Results and Discussion Thermal reaction of O S P ( C O ) B O L - ~ ~ , ~ ;with ~~-C (r5ZH~) C5H,)Rh(CO)PMe3 affords three trimetallic products. In
Os,(CO),Cu-ql,l;ll-C,H,)
+ hexane
(q5-C5H5)Rh(CO)PMe35o oc,16 ; OS,R~(CO)~(~~-C,H,) (17%)
Os3(CO)llPMe3(36%)
+
+
Os,Rh(CO),(C,H,)PMe,
(1)(41%)
the analogous reaction with (r5-C5H5)Rh(CO)PPh3,only the known clusters OszRh(C0)9(r5-C5H5)and Os3(CO)llPPh3 are formed. The OszRh(C0)9(q5-C5H5)and 0 5 3 (CO)11PR3complexes are readily identified by comparison of their characteristic IR spectra with those previously reported in the l i t e r a t ~ r e . ~The ,~,~ product dis(10) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press: London, 1982; p 96.
700 Organometallics, Vol. 14, No. 2, 1995
*J,.,
-d I
-48
Cooke and Takats
= 5.3 Hz
l l b l L w I
I
I
-49
-50
S (ppm)
Figure 2. 162 MHz 31PNMR spectrum of 1 at -20 "C in toluene-ds and schematic diagrams of the corresponding instantaneous solution structures of the isomeric forms la and lb. tribution, coupled with the known greater lability of the triphenylphosphine ligand,ll suggests that the formation of the OszRh clusters proceeds by phosphine, rather than carbonyl loss from (r5-C5H5)Rh(CO)PR3,12 Characterization of 1. The mass spectrum of 1 shows the molecular ion followed by sequential loss of eight carbonyl ligands; this, together with elemental analysis, suggests the formulation of 1 as OszRh(CO)8(C5H5)PMe3. The NMR spectra are temperature dependent; full discussion of the rearrangement processes responsible for this are deferred for later discussion. The lH NMR spectrum at -20 "C in toluene-ds shows two sets of signals for C5H5 and PMe3 in a 2.1:l.O(la: lb) ratio, establishing the presence of two isomers in solution. The 31P{H} NMR spectrum (Figure 2) also shows two signals at 6 -49.98 (s, la) and -48.28 (d, 2 J R h - p = 5.3 Hz, lb) in the same 2.1:1 ratio. The absence of large one-bond 103Rh-31Pcoupling constants immediately suggests that the PMe3 ligand is bonded to osmium and not to rhodium in both isomers. Typically, ' J R h - p exceeds 100 Hz (cf. ' J R h - p = 186.0 Hz in (C5H5)Rh(C0)PMe3).l3The two-bond Rh-P splitting of 5.3 Hz in the minor isomer lb indicates a trans relationship between rhodium and phosphorus, while in the major isomer la, the lack of observable Rh-P splitting suggests that the coupled nuclei are related by a nonlinear P-Os-Rh angle.l* Equatorial phosphine substitution at osmium is clearly the only possible geometry that satisfies the trans relationship required for lb and is suggested for l a also. This is in accord
I
200
.
.
.
.
.
1 b
.
.
S(ppm)
.
. 1'
.
.
. li0
Figure 3. Variable-temperature 13C NMR spectra of 1 at 100.6 MHz in toluene-ds. For designation of the carbonyl ligands, see Scheme 1.
with the observation that phosphine ligands tend t o occupy equatorial sites in trinuclear clusters of osmium5 and is corroborated by the 13CNMR spectra as none of the signals shows trans P-C coupling, which would be expected if an axial phosphine were present (vide infra). Carbonyl Migration in l a and lb. The variabletemperature 13CNMR spectra of 1 are shown in Figure 3. The line shape changes are reversible and are consistent with the intramolecular nature of carbonyl exchange processes. It is also clear from the figure that the low-temperature limiting spectrum has not been achieved. Indeed, the instantaneous structure of each isomer has C1 symmetry and should give rise to eight distinct carbonyl signals (ie., 16 signals in total). Focusing at the -53 "C spectrum, one observes two similar signal patterns which can be reliably assigned to isomers la and lb by integration. Each isomer exhibits three signals of intensity two ( l a at 6 198.8(d, 'JRh-C = 37 HZ), 189.6,188.3;lb at 6 198.6 (d, ' J R h - c = 37 Hz), 194.1,184.5)and two signals of intensity one ( l a at 6 175.3,175.1;lb a t 6 178.5,174.2),accounting for eight carbonyls per isomer. The line shape changes observed below -20 "C can be rationalized by invoking the familiar pairwise carbonyl exchange mechanism,15 which accounts for the C~H~) fluxional nature of the parent O S ~ R ~ ( C O ) ~ (clus(11)Janowicz, A. H.; Bryndza, H. E.; Bergman, R. G. J . Am. Chem. SOC.1981,103, 1516. ter.2 In transition metal clusters, this process requires (12) Substitution via phosphine dissociation has been documented that the carbonyls involved be roughly coplanar and that in (C5H5)Co(PPh3)211and accounts for the reactivity of (C5H&o(PMe3)2 the coplanar ligands must all be mobile. In the case of (Leonard, K.; Werner, H. Angew. Chem.,Int. E d . Engl. 1977,16,649) and formation of (C5HdRh(CO)PMe3 from ( C E H ~ ) R ~ ( P M ~At ~ ) ~ . ~ 'O S ~ R ~ ( C O ) ~ ( C ~ H the ~ )phosphine P M ~ ~ , ligands serve to present, we are unaware of any similar results for (C5H5)Rh(CO)PR3 block certain routes to exchange below -20 "C as they complexes. (13) Bitterwolf, T.E.Inorg. Chim. Acta 1986,122, 175. (14)Verkade, J . G.; Quin, L. D. 31PNMR in Stereochemical Analysis; VCH: Deerfield Beach, FL, 1987;p 226.
(15) (a) Adams, R. D.; Cotton, F. A. J . Am. Chem. SOC.1973,95, 6589. (b) Cotton, F. A,; Hunter, D. L. Inorg. Chim. Acta 1974,11,L9.
Organometallics, Vol. 14,No. 2, 1995 701
la
lb
Figure 4. Allowed planes for pairwise CO exchange in la and lb (Cp = 175-C5H5). Scheme 1. Pairwise CO Exchange Mechanism and Assignment of Signals as They Appear in the lSC NMR Spectrum at -53 "C (a) Isomer la.
0
CP.
a= b=fj e=f C
@) Isomer I b
.CD
W
s=w u=*
Y4'
remain fixed in their respective equatorial sites. Rgure 4 details the available planes for carbonyl migration in each isomer by assuming that the phosphine ligands remain static. Note that in isomer lb all carbonyls are allowed to scramble while in l a the carbonyl labeled d cannot be involved in a pairwise exchange process. This is significant because a single carbonyl signal at 6 175.3 remains sharp at and below -20 "C while the other signals broaden. This sharp signal is thus assigned to the unique, nonparticipating carbonyl d of la. Beginning with the major isomer la, the assignment of the signals at -53 "C is based on a model where rapid exchange is occurring in the OSA- Rh plane with no exchange in the OSB-R~plane (see Scheme la). This proposal is consistent with previous observations that substitution by a a-donor group in trinuclear clusters of osmium facilitates the carbonyl scrambling process in the plane adjacent to, but not blocked by, the electrondonating ligand.5 The signal at 6 175.1 is assigned t o c, the other nonfluxional equatorial carbonyl under the exchange model proposed in Scheme la. Unlike d, signal c broadens with increasing temperature, indicating that its carbonyl lies within an exchange plane that is accessible at higher temperature. The sharp singlet of intensity two at 6 189.6 is attributed to the axial carbonyls e and f , which are rendered equivalent by rapid exchange in the OsA-Rh plane. As was the case for c , this signal also broadens with increasing temperature. The two-carbonyl doublet at 6 198.8 ( l J ~ - c = 37 Hz) is assigned t o carbonyls a and g because they exchange between Rh and an axial position on Os*, time-averagingthe expected one-bond rhodium-tenninal carbonyl coupling of 77 Hz2 to 37 Hz. The broad twocarbonyl singlet at 6 188.3 is assigned t o carbonyls b and h as these ligands alternate between an axial and
an equatorial position on OSA. The presence of the cyclopentadienyl ligand at rhodium restricts the exchange to a back-and-forth movement that averages a and g and b and h but does not involve exchange between the two pairs. Between -53 and -20 "C, all signals of l a except d broaden and coalesce into the baseline. This is consistent with the onset of rapid exchange in the OsB-Rh plane as this ultimately leads to the averaging of all signals except d. Below -53 "C, the alg and blh signals also broaden, indicating a decrease in the rate of exchange across the OSAand Rh centers. For the minor isomer lb, assignment of the signals at -53 "C is based on the exchange model given in Scheme lb, which also shows the averaged signals. At this temperature, carbonyl migration is already fast in the OsD-Rh plane with no exchange taking place in the OSC-OSDplane.2 The two nonfluxional equatorial CO signals IJ and t are separated by 4.3 ppm. This is attributed to the fact that the carbonyls are bonded to two different osmium centers, one of which is phosphine substituted. In view of the generally observed downfield shift upon phosphine substitution, the signal at 6 178.5 is assigned to carbonyl t. Assignment of the remaining signals follows the discussion presented for la. The signals corresponding to slw and to ulx are not visible at -70 "C but rise from the baseline at -53 "C and sharpen further by -40 "C. This is clearly consistent with a slower rate of exchange in the OsD-Rh plane at -70 "C followed by a progressively faster exchange rate as the temperature is raised. Above -53 "C, the signals assigned to t, u, and ylz all broaden and this heralds the onset of exchange in the OSC-OSD plane. In this case, eventually all the carbonyls of l b can exchange with one another as the two allowed exchange planes intersect at OSD. Note that the exchange in the OscOSDplane is a full merry-go-round process. The line shape changes in the 13CNMR spectra reflect the normally observed effect that phosphine substitution at an osmium center has on the energetics of pairwise carbonyl exchange, namely, that pairwise exchange is more facile in a plane that includes a phosphinesubstituted metal.5 Thus, exchange in the OsA-Rh plane of la is more facile than that of the O S B - R ~plane because OSAis phosphine substituted while OSBis not. Similarly, exchange in the OsA-Rh plane of l a is more facile than that of the OsD-Rh plane in lb. This is evident because, at -70 and -53 "C, the signals corresponding to exchanging carbonyls in l b are much broader than those of la. The onset of carbonyl migration in the OSC-OSD plane of lb, which occurs above ca. -53 "C, can be compared to the parent OszRh(C0)g(C5H5) cluster, which showed no evidence of carbonyl exchange between the two osmium centers at 0 "C.2 Again, phosphine substitution at Osc greatly facilitates this process in lb. This enhanced trend in fluxionality also explains why the low-temperature limiting spectrum could not be achieved for 1. The near-limiting temperature for OSZRh(CO)g(CsHs)was -115 "C and is expected to be lower for either isomer of 1. Indeed, at the lowest recorded temperature (-100 "C, not shown in Figure 3), the only differences when compared to the -70 "C spectrum were the complete coalescence of signals blh and SIW and some renewed broadening of the alg and ylz resonances.
L v -
702 Organometallics, Vol. 14, No. 2, 1995
Cooke and Takats
b
P.'
0
u
4
8
I bC 'Me
coo
Figure 6. Steric influence of PMe3 on the migration of the C5H5 group that accompanies pairwise carbonyl exchange in the OsB-Rh plane of isomer la. A quantitative study of the slower carbonyl exchange processes in each isomer by line shape simulation yielded interesting results. The free energy of activation for pairwise CO exchange in the OSC-OSDplane of lb (13.0 f 0.4 kcal-mol-l) is similar to that for the analogous process in OszRh(CO)g(C5Me5)(12.7 f 0.3 kcal*mol-1).2 This is not unexpected as the function of both the PMe3 and C5Me5 moieties is electron donation, which decreases the barrier to exchange in the Os-Os plane in each case relative to the ca. 17 kcabmol-l value for O S ~ ( C O ) I ~ . ~ ~ A more interesting result is that the activation energy for pairwise exchange in the OsB-Rh plane of l a (11.5 f 0.4 kcal*mol-l) is 3 kcal.mo1-l higher than for the similar process in the parent cluster, Os2Rh(CO)g(C5H5) (8.4 f 0.4 kcalmol-1).2 Qualitatively, signal broadening is already observed at -115 "C ( k = 20 s-l) in OsaRh( C O ) ~ ( C S Hwhile ~ ) ~ comparable broadening ( k = 18 f 1 s-l) in l a is only seen at -53 "C. This finding was certainly unexpected, as one would predict a decrease in activation energy for the exchange because of electron donation by PMe3 t o OSB. The absence of electronic rational made us search for possible steric arguments. As shown in Figure 5, the bridged intermediate required for the pairwise CO exchange brings the cyclopentadienyl ring through the plane of the metals such that the ring centroid is found approximately along an extension of the OsB-Rh bond vector. The phosphine ligand is directly adjacent to the ring as it passes through this position, and it is possible that the steric interference exerted upon the cyclopentadienyl ring by the methyl groups of the phosphine ligand is sufficient to hinder its passage through the plane of the metals. Such a situation would clearly result in an increased barrier t o carbonyl migration when compared with the unsubstituted analog. Isomer Interconversion between l a and lb. The reversible temperature-dependent line shape changes in the lH and 31P{H}NMR spectra clearly indicate that isomers la and lb interconvert above -20 "C. The most likely mechanism for the isomer interconversion is a restricted trigonal twist (turnstile process) at the substituted osmium center, which brings about the desired (16) (a) Foster, A.; Johnson, B. F. G.; Lewis, J.; Matheson, T. W.; Robinson, B. H.; Jackson, W. G. J. Chem. Soc., Chem. Commun. 1974, 1042. (b) Aime, S.; Gambino, 0.; Milone, L.; Sappa, E.; Rosenberg, E. Inorg. Chim. Acta 1976, 15, 53.
la
lb
Figure 6. Restricted trigonal twist at the PMes-substituted Os center, which initiates participation of CO d in the carbonyl scrambling process. transformation in a nondissociative manner.5a Line shape simulation of the variable-temperature lH NMR spectra provides a free energy of activation of 15.1 f 0.4 kcalnmol-l for the isomer interconversion, which is similar t o the activation energies determined for the analogous turnstile processes in Os3(CO)l0[P(OMe)312 (13.8 ~ ) ~f I~ (15.0 f 0.4 kcabmol-l) and O S ~ ( C O ) ~ [ P ( O M 0.4 k~al-mol-~).~" The effect of the turnstile a t the substituted osmium centers is to exchange the previously invariant carbonyl d of l a with an equatorial site (v) in l b (Figure 6). Consequently, signal d broadens above -20 "C in the 13C NMR spectra (Figure 3). Although a high-temperature limiting 13C NMR spectrum is not achieved, the broad signal observed in the +80 "C spectrum is an indication of averaging of all the carbonyl signals in accord with global carbonyl scrambling and rapid isomer interconversion.
Conclusions Thermal reaction of Os2(CO)sOl-)71,y1-C2H4)with (v5C5H5)Rh(CO)PMe3 gave cluster 1, which exists as a mixture of two interconverting isomers. Isomer interconversion a t ambient temperature occurs via a restricted turnstile mechanism at the phosphine-substituted osmium center. In addition, each of the isomers exhibit carbonyl fluxionality at low temperature, which can be accounted for by the familiar pairwise carbonyl exchange mechanism. The 13CNMR spectra additionally provide a clear illustration of the downfield shift of carbonyl resonances with phosphine ~ubstitution.~ In particular, the signals attributed to the exchanging carbonyls in l a are downfield of their corresponding signals in lb because the exchange plane in the former includes a phosphine-substituted osmium center.
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for funding and for an Undergraduate Summer Research Award to J.C. Financial support from University of Alberta is also gratefully acknowledged. The expert assistance of Dr. John Washington and Dr. Wenyi Fu in recording the variable-temperature N M R spectra presented in this study is sincerely appreciated. We thank Professor R. E. D. McClung for useful discussion and help with the simulation of the lH and 13C NMR spectra. OM940647Q
