Multinuclear solid-state nuclear magnetic resonance studies on transition- metal clusters containing hydrides * Taro Eguchi,"Brian T. Heaton,b Rachel Harding,b Kei Miyagi," Giuliano Longoni,' Jens Nahring,b Nobuo Nakamura,' Hirokazu Nakayama," Tapani A. Pakkanen,d Jouni Pursiainen and Anthony K. Smithb a Dclpurtment of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Depurtrnont qf Chemistry, University of Liverpool, Liverpool L69 3BX, UK Dipurtimento di Chimica Fisica ed Inorganica, Viale del Risorgimento 4, 40136, Bologna, Italy Dclpurtment of'chemistry, University of Joensuu, FIN-80101, Joensuu 10, Finland Depurtrnc'nt of Chemistry, University of Oulu, FIN-90570, Oulu, Finland Solid-state I H and D NMR measurements have been made for all transition-metal carbonyl clusters containing interstitial hydrides/deuterides previously characterised by neutron diffraction. There is a close agreement between the values of 6( 'H/D) in solution and the solid state except for [Co,H(CO), 5] ~. The values of 6( 'H) for interstitial hydrides are in the range 6 + 18.5 to -26.8; the shift to high field is shown to be due to an increasing displacement of H from the centre of the metal octahedral cavity, consistent with surface tensor harmonic theory. Whereas migration of H readily occurs in both [Rh13Hx(C0)24](5-X)- (x = 2 or 3) and [Ru,Rh,H,(CO),,] in solution, solid-state 'H NMR measurements on [Rh13Hx(C0)24](5-X'- (x = 2 or 3) showed that there is no evidence for such migration in the solid state and for [Ru,Rh,H,(CO),,] oscillation of H about the metal-metal edge(s) occurs rather than migration to different edges as found in solution. - There are now many examples of hydride transition-metal carbonyl clusters and the hydrogen site occupancy is difficult to predict. Examples of terminal, edge-, face-bridging and interstitial hydrides are known and unambiguous location comes only from neutron diffraction or solution NMR studies particularly when the metal has a nuclear spin, e.g. lo3Rh. Characterisation through multinuclear NMR measurements is particularly useful when the hydride clusters are only stable at low temperature preventing isolation of X-ray-quality crystals [e.g. terminal hydride [Rh,H(CO), 5] -,' edge-bridge hydride [ Rh,HC( CO) J - ,, face-bridge hydride [Rh,HC(CO) 5]- ' ) and for distinguishing between similar metals in heterometallic clusters v.,q. [Ru,Rh,H,(CO),,] (see below). However, hydrogen migration easily occurs in solution and, although solution NMR studies have been useful for elucidating the migratory mechanism(s), it is sometimes difficult to obtain static structures in solution because it is impossible to retain solubility at low temperatures and some migrations are of extremely low energy. This paper reports solid-state NMR studies on: (1) all the transition-metal carbonyl clusters presently known to contain interstitial hydrides in order better to understand the apparently wide variation of the hydride chemical shift in solution and the mobility of the hydride; (2) [Rhl3HX- (CO),,]'" (x = 2 or 3) which previously has been found to undergo facile interstitial hydrogen migration;' and (3) [Ru,Rh,H,(CO),,] which has been shown through solution NMR studies to undergo first hydrogen movement between the occupied and unoccupied Ru-Rh edges before exchange of the incquivalent hydrides associated with the Ru-Rh and Ru-Ru edges? Results and Discussion 1. Interstitial hydrides The direct location of hydrogen in transition-metal hydride clusters relies heavily on neutron diffraction measurements. Claims for the presence of interstitial hydrides using other methods are fraught with difficulties as exemplified by the recent low-temperature neutron diffraction study of [N(PPh,),],[Os, OH4(C0)24] which shows the four hydrides to be on the surface of the metal framework in p and p3 sites whereas earlier work suggested them to be all located in the interstitial sites.' The only transition-metal carbonyl clusters which have been characterised by neutron diffraction and shown to have the hydride inside metal cavities, which are either octahedral or distorted octahedral, are shown in Table 1 together with the solution value of 6(' H) for the hydride resonance. The chemical shift values in solution span a wide range and are essentially independent of cation and solvent for a given cluster. Tensor surface harmonic (TSH) theory has been used to account for the solution low-field chemical shifts of the Ru, and Co, clusters,' but we now show that there is a significant difference between solution- and solid-state values of 6('H) for [Co,H(CO), 5] -. Otherwise there is little variation in the values of the solution- and solid-state chemical shifts of the interstitial hydrides (Table 1). For [CO,(H/D)(CO), 5] - the ' H MAS (magic angle spinning) NMR spectrum at room temperature (see Fig. 1) clearly shows a resonance at 6 + 1 for the interstitial hydride which disappears on deuteriation; the resonance at 6 0 in both Fig. l(a) and l(b) is due to Pr'O(H/D), which was used for recrystallisation, and there is a broad resonance at 6 7 due to the cation. The resonances at 6 0 and 1 [Fig. l(u)] exhibit only a small chemical shift anisotropy and comparison of their relative intensities suggests there is only a trace of residual Pr'OH present as solvent of crystallisation which proved impossible to remove under high vacuum [ 1 0-3 mmHg (ca. 0.133 Pa) for 6 h]. The chemical shift value for the methyl group in neat Pr'OH is 6 1.1 and the slight difference between 6(' H) in solution and the * Basis of the presentation given at Dalton Discussion No. I, 3rd-5th January 1996, University of Southampton, UK. J. Chem. SOC., Dalton Trans., 1996, Pages 625430 625