Organometallics 1995, 14, 3636-3640
3636
Carbonyl Fluxionality in the nido Cluster Ru~(CO)~(cL~-CO)(cL~-Nph). NMR Evidence and Mechanism for the Exchange of the Triply-Bridging CO with the Terminal CO Groups D. Wang,laIbH. Shen,la~b~c M. G . Richmond,*Jbic and M. Schwartz*Jb Department of Chemistry and Center for Organometallic Research and Education, University of North Texas, Denton, Texas 76203-5068 Received March 6, 199~5~ Variable-temperature 13C NMR studies on Ru3(CO)9013-CO)(,u3-NPh)reveal the presence of two distinct CO exchange processes that involve localized equatoriallaxial carbonyl scrambling a t each ruthenium vertex and exchange of the equatorial and triply-bridging CO groups. The kinetics and activation parameters for these exchange processes have been determined by both band shape analysis and 2D-EXSY experiments. The latter NMR method unequivocally rules out a sequence involving the direct exchange of the axial and triplybridging CO groups. The somewhat faster of the two processes (AG* = 54.9 (0.03)kJ/mol, AH* = 41.7 (1.4)kJmol, AS* = -50(6) J/mol K) serves to equilibrate the equatoriallaxial CO groups at each Ru(CO)3 center, while the scrambling of the equatorialltriply-bridging CO groups displays comparable activation parameters (AG* = 58.1(0.14)kJ/mol, AH*= 45.4 (3.4)kJ/mol, AS* = -49(14) J/mol K). Plausible mechanisms invoking a turnstile rotation p#O interconversions are presented and discussed relative to each CO and ,u3-C0 exchange step.
-
Introduction The fluxional behavior of ancillary CO groups about polynuclear metal clusters has been extensively studied over the last two decades by variable-temperature 13C NMR spectroscopy.2 Such studies have provided valuable insight into the relationship between the observed solid-state structure (X-ray) and the solution structure adopted by a given cluster. The observed ligand fluxionality has been proposed to function as a model for the movement of chemisorbed species on metallic surf a c e ~ .The ~ complete scrambling of terminal carbonyls about a cluster polyhedron generally proceeds via a series of terminal-to-bridge CO sequences, of which the one-for-one, two-center, and merry-go-round exchange processes represent the best known mechanism^.^ Our research groups have been interested in the NMR behavior of a wide variety of organometallic compounds, especially with regard to bonding considerations between a metal centeds) and different ancillary ligands, Abstract published in Advance ACS Abstracts, June 15, 1995. (1)( a ) Robert A. Welch Predoctoral Fellow. (b) Department of Chemistry, University of North Texas. (c) Center for Organometallic Research and Education, University of North Texas. (2) (a1 Band, E.; Muetterties, E. L. Chem. Rev. 1978, 78, 639. ( b ) Evans, J. Adu. Organomet. Chem. 1977, 16, 319. (c) Aime, S.;Milone, L. Prog. Nucl. Magn. Reson. Spectrosc. 1977, 11, 183. (d) Cotton, F. A,; Hanson, B. E. In Rearrangements in Ground and Excited States; de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 2, Chapter 12. (3) (a1 Johnson, B. F. G.; Benfield, R. E. In Transition Metal Clusters; Johnson, B. F. G., Ed.; Wiley: New York, 1980; Chapter 7. (bl Johnson, B. F. G.; Rodgers, A. In The Chemistry of Metal Cluster Complexes; Shriver, D. F., Kaesz, H. D., Adams, R. D., Eds.; VCH Publishers: New York, 1990; Chapter 6. ( 4 ) ( a )Evans, J.; Johnson, B. F. G.; Lewis, J.; Matheson, T. W.; Norton, J. R. J . Chem. SOC.,Dalton Trans. 1978, 626. (b) Cotton, F. A.; Jamerson, J. D. J . A m . Chem. SOC.1976, 98, 5396. (c) Stuntz, G. F.; Shapley, J . R. J . Am. Chem. SOC.1977, 99, 607. (d) Washington, J.;Takats, J. Organometallics 1990,9,925. (e) Cotton, F. A,; Hanson, B. E.; Jamerson, J. D. J . Am. Chem. SOC.1977,99,6588. (DAdams, R. D.; Cotton, F. A. J . Am. Chem. Sot. 1973, 95, 6589.
0276-7333/95/2314-3636$09.00/0
as deduced by NMR spin-lattice ( T I )measurements and quadrupole coupling constants (QCC's). For example, exploration of the dynamics of internal rotation of capping benzylidyne and phenylphosphinidene ligands in tri- and tetranuclear clusters has yielded useful information concerning the presence of ~t overlap between the phenyl groups of these ligands and the cluster f r a m e ~ o r k .Recently, ~ we examined the rotation rates of the phenyl ligand in the imido-capped cluster RUB(CO)g(p3-CO)b-NPh)due t o its isolobal relationship to the phenylphosphinidene fragment,6 and we observed that all of the carbonyl groups undergo rapid exchange at most temperatures. Accordingly, we next conducted a more in-depth investigation into the exchange pathways operative for the carbonyl groups in Ru3(C0)9@3CO)+-NPh). The molecular structure of this cluster, with its three types of CO groups, is shown below.
a'
triply-bridging
CO
(5) ( a ) Schwartz, M.; Richmond, M. G.; Chen, A. F. T.; Martin, G. E.; Kochi, J. K. Inorg. Chem. 1988,27,4698. (b) Wang, S.P.; Chen, A. F. T.; Richmond, M. G.; Schwartz, M. J . Organomet. Chem. 1989,371, 81. (c) Yuan, P.; Richmond, M. G.; Schwartz, M. Inorg. Chem. 1991, 30, 588; 679. (d) Yuan, P.; Don, M.-J.; Richmond, M. G.; Schwartz, M. Inorg. Chem. 1991, 30, 3704. (6) Wang, D.;Shen, H.; Richmond, M. G.; Schwartz, M. Inorg. Chim Acta 1995, in press.
0 1995 American Chemical Society
Carbonyl Fluxionality in Ru~(CO)~~~-CO)(~~-NP~) 400
-
Table 1. Temperature Dependence of Carbonyl Exchange Ratesay* T
300 -
.-d
C
4
a
200
-
100
-
.0 VI
= 9 f
-
kea
281 270 257 250 236 223
A
3
Organometallics, Vol. 14,No.8, 1995 3631
AG*,c (kJ/mol) AH* (kJ/mol)
224(3) 140(2) 51(2) 23(3) 8.3(
