Variable-temperature carbon-13 nuclear magnetic resonance spectra of phosphine-substituted dodecarbonyl-triangulo-triosmium

Abstract

13C N.m.r. spectra of the compounds [Os,(CO),,-,(PEt,),](n = 1 or 2) indicate that carbonyl exchange occurs selectively along one edge of the Os, triangle at low temperatures. The process involves only six of the 12n carbonyl groups in [Os,(CO),,-,(PEt3),](n = 1 or 2). and a mechanism involving a double carbonyl bridge is proposed.Laboratory, Lensfield Road, Cambridge CB2 1 EW A CARBON-^^ n.m.r. study of isotopically enriched [Os,(CO),,] from ca. 70-180 "C showed that rapid intramolecular scrambling of the equatorial and axial carbonyl groups 0ccurs.l At least three mechanisms may be proposed for this process, two involving the formation of CO bridges and the third a polytopal rearrangement about individual osmium atoms. It is not possible to differentiate between the mechanisms on the evidence a t present available. We have now undertaken a variabletemperature 13C n.m.r. study of some phosphine-substituted trinuclear osmium carbonyls in order to obtain further information about the mechanisms of carbonyl exchange in cluster compounds, although it must be borne in mind that the introduction of the phosphine may give rise to a different mechanism from that occurring in the parent carbonyls. There is evidence that the introduction of tertiary phosphines into polynuclear carbonyl clusters induces the formation of CO bridges2 I t therefore seemed reasonable to suppose that phosphinesubstituted derivatives [0s,(C0)1~-~(PEt,),] would more readily undergo CO scrambling by a CO migration process.However, in addition, and more interestingly, we find that for the clusters [Os3(CO)1z-,(PEt,),] (n = 1 or 2) two distinct fluxional processes occur at different temperatures. This has been found to be the case. RESULTS AND DISCUSSIONAt -60 "C the 13C n.m.r. spectrum of the trisubstituted species [OS,(CO),(PE~,),] (in toluene) showed two 13C carbonyl peaks at 6 199.4 and 186.4 p.p.m. (downfield from SiMe,) with an intensity ratio of ca. 2 : 1. The lower-field peak was split, but this is not clearly resolved.

Synthesis, structure, and nonrigidity of Os4(CO)15, Os4(CO)13(PMe3)[P(OMe)3], and Os4(CO)14(CNBut)

“…This type of CO exchange has rarely been proposed for trinuclear group-8 carbony1 clusters (43). Axial-equatorial, merry-go-round CO exchange appears more common (41,44,45). The suggested mechanism for the low-energy carbonyl exchange in 1 is, however, consistent to that proposed for 0sj(CO),,(PMe3) (6) and for (OC),C~[OS(CO)~(PM~~)],.…”

“…This is shown in Chart 4. The lowest energy carbonyl exchange in Os,(CO) , ,(PEt,) (45) and Os,(CO), , [P(OMe),] (4 1) has been interpreted as proceeding by a similar mechanism (i.e., in that plane that contains both axial and equatorial carbonyls and is also approximately orthogonal to the 0s-P bond). In 2 it is proposed there are two such merry-go-round exchanges, both of which involve the axial and equatorial carbonyls on Os(1).…”

Crystal and molecular structure of an unusual unsymmetrical isomer of 1,2-Os3(CO)10{P(C6H5)[(η-C5H4)Fe(η-C5H5)]2}2

“…1 ,~-OS,(CO),~(PE~,), is reported to exist only as the unsymmetrical isomer l c (15). These results suggest that electronic effects also play a part in determining the molecular geometry.…”

W(η2-PhCCPh)3(η1-Ph2PCCPPh2) as a ligand to prepare homo- and hetero-nuclear cluster complexes

“…Thus, the major isomer of 4 likely has the configuration 1,2-B, which contains nonequivalent phosphorus nuclei, and the minor isomer has the configuration 1,2-C with equivalent phosphorus nuclei. The isomerism of phosphine-substituted triosmium clusters is well documented in the literature [11][12][13][14].…”