Fixation and spontaneous dehydrogenation of methanol on a triruthenium–iridium framework: synthesis and structure of the cluster anion [HRu 3 Ir(CO) 12 (OMe)] 2 Susanne Haak,† Antonia Neels, Helen Stoeckli-Evans, Georg Süss-Fink* and Christophe M. Thomas Institut de Chimie, Université de Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland. E-Mail: GEORG.SUESS-FINK@ICH.UNINE.CH Received (in Basel, Switzerland) 7th July 1999, Accepted 19th August 1999 The anionic mixed-metal cluster [Ru 3 Ir(CO) 13 ] 2 1, found to be catalytically active in the carbonylation of methanol, reacts with methanol at 70 °C to give, with O–H activation of the substrate, the cluster anion [HRu 3 Ir(CO) 12 (OMe)] 2 2, which upon prolonged reaction loses formaldehyde to give the cluster anion [H 2 Ru 3 Ir(CO) 12 ] 2 3; both anions 2 and 3 crystallise together as the double-salt [N(PPh 3 ) 2 ] 2 [HRu 3 Ir- (CO) 12 (OMe)][H 2 Ru 3 Ir(CO) 12 ] the single-crystal X-ray structure analysis of which reveals a butterfly Ru 3 Ir skeleton for 2 and a tetrahedral Ru 3 Ir skeleton for 3. Mixed-metal clusters have received steadily increasing atten- tion over the last three decades owing to their inherent catalytic potential. 1 The presence of different metals in the same complex may have synergistic effects in the catalytic activity. 2 Recently we reported a series of Ru 3 Ir and Os 3 Ir mixed-metal clusters which show reasonable catalytic activity in the carbonylation of methanol to give acetic acid. 3,4 The cluster anions [Ru 3 Ir(CO) 13 ] 2 1 and [Os 3 Ir(CO) 13 ] 2 4, used as catalyst precursors in this process, are however unstable under the reaction conditions, which has been confirmed by the isolation of the fragmentation products Ir 4 (CO) 12 and [N(PPh 3 ) 2 ][M(CO) 3 I 3 ] (M = Ru, Os). 4 In this context we were interested in the behaviour of these anionic clusters towards reagents participating in the catalytic process, especially in the reaction with methanol. The anion [Ru 3 Ir(CO) 13 ] 2 1 (as its [N(PPh 3 ) 2 ] + salt) reacts with methanol at 70 °C within 2 h to give the anionic cluster [HRu 3 Ir(CO) 12 (OMe)] 2 2 [eqn. (1)], as indicated by a colour change of the solution from red to orange. The formation of 2 involves O–H activation of the methanol molecule, which is split into m-H and m-OMe ligands, replacing a carbonyl group of the starting compound. Anion 2 can be isolated as the [N(PPh 3 ) 2 ] + salt by extraction with diethyl ether after evapora- tion of the reaction solution to dryness, washing of the residue with hexane–dichloromethane (5+1) and recrystallisation from ethanol–pentane (1+3) at 218 °C.‡ [Ru 3 Ir(CO) 13 ] 2 + MeOH ? [HRu 3 Ir(CO) 12 (OMe)] 2 + CO (1) 1 2 Upon prolonged reaction (48 h) in methanol, especially at higher temperature (90 °C), anion 2 is transformed into the anion [H 2 Ru 3 Ir(CO) 12 ] 2 3 with liberation of formaldehyde [eqn. (2)]. The formation of formaldehyde is evidenced by the 1 H NMR spectrum of the reaction mixture, which shows the signal for HCHO at d 9.75 as well as a signal at d 5.31 attributed to the formaldehyde trimer trioxane (by comparison with an authentic sample). [HRu 3 Ir(CO) 12 (OMe)] 2 ? [H 2 Ru 3 Ir(CO) 12 ] 2 + HCHO (2) 2 3 If the thermal reaction of 1 with methanol at 90 °C is stopped half-way (after 12 h), both anions 2 and 3 are present in the solution, and the double-salt [N(PPh 3 ) 2 ] 2 [HRu 3 Ir(CO) 12 (O- Me)][H 2 Ru 3 Ir(CO) 12 ] can be isolated by crystallisation of the reaction residues from a 4+1 mixture of diethyl ether and hexane. A suitable crystal§ of this double-salt revealed the molecular structure of both anions 2 and 3 (Fig. 1 and 2), disordered over the same position. Three of the four metal positions are occupied by metal atoms of both anions 2 and 3, whereas one ruthenium atom splits into Ru(1) for anion 2 and Ru(1a) for anion 3, the refinement of the occupancy factors giving a 1+1 ratio. In contrast to anion 2, anion 3 is already known, 3 its structure in the double-salt being very similar to that we found in the salt [N(PPh 3 ) 2 ][H 2 Ru 3 Ir(CO) 12 ]. 3 In anion 2, the dihedral angle Ru(1)–Ir(1)–Ru(3)–Ru(2), that is to say the spreading of the wing tips is relatively small (82.9°) compared to those in other chalcogen- or halogen-bridged clusters, such as HRu 3 Co(CO) 12 (m-SMe 2 ) (96.1°) 5 or HRu 3 Ir- (CO) 12 (m-Cl) (89.3°). 6 A similar dihedral angle as in 2 has been observed in the hydroxy-bridged osmium cluster cation [H 4 Os 4 - (CO) 12 (m-OH)] + (82.3°). 7 As a result of the small dihedral angle, the distance between the wing tip metal atoms Ru(1) and Ru(2) is only 3.3536(2) Å. The hydride-bridged edge Ru(1)– Ru(3), at 3.095(5) Å, is significantly longer than all the other metal–metal bonds. The wing tips are bridged asymmetrically by the methoxy group, the Ru–O bond distances being 2.02(2) and 2.16 (2) Å, respectively. This is well in line with the bond lengths found in other methoxy-bridged clusters, such as HOs 3 Pt 2 (CO) 10 (m 5 -C)(m-OMe)(PCy 3 ) 2 8 or Os 3 (CO) 10 (m- † Present address: Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF. Fig. 1 Molecular structure of [HRu 3 Ir(CO) 12 (OMe)] 2 2 showing the atom numbering scheme. Selected bond distances (Å) and angles (°): Ir(1)–Ru(1) 2.854(2), Ir(1)–Ru(2) 2.758(1), Ir(1)–Ru(3) 2.737(1), Ru(1)–Ru(3) 3.095(5), Ru(2)–Ru(3) 2.797(1), Ru(1)–H(1) 1.47(1), Ru(3)–H(1) 1.77(1), Ru(1)–O(13) 2.02(2), Ru(2)–O(13) 2.16(2), O(13)–C(13) 1.39(3), Ru(1)– O(13)–C(13) 119.3(16), Ru(2)–O(13)–C(13) 125.7(16). Published in Chemical Communications, 1959-1960, 1999 which should be used for any reference to this work 1