Synthesis and characterization of heterobimetallic complexes and their application on conversion of carbon dioxide to organic compounds

Abstract

The heterobimetallic complexes [(n5-C5R5)Ru (CO)(u-dppm)M (CO)2(n5-C5H5)](M = Mo,W.; R=H , CH3)(1-4) are prepared by metathetical reactions between (n5-C5R5)Ru(dppm)Cl and Na+[(n5-C5H5)M(CO)3]-.IR spectroscopic and X-ray structural studies show that each of these complexes contains a semi-bridging carbonyl ligand. The low activity of these complexes in catalytic CO2 hydrogenation to formic acid can be attributed to the non-facile rapture of the metal-metal bond to yield the active metal dihydride species.The metal-metal bonds can be protonated to form the cationic complexes , which contain strong Ru-H-M bridges but no metal-metal bonding interaction . The bridging hydrogen atom is weakly acidic since it can be removed by a strong base, and it protonates BPh4- to give BPh3 and benzene. The protonated species also reacts with the hydridic hydrogen of Et3SiH to yield H2; the hydridicity of the hydrogen of Et3SiH is enbanced by attachment of a chloride anion to Si to form the adduct Cl-...SiEt3-H. The bridging hydrogen, however, cannot be removed by hydride scavengers such as Ph3C+OTf and Me3Si+OTf. The sluggishness of the catalytic formic acid decomposition by l-4 is attributable to the stability of the protonated bimetallic intermediate [(n5-C5R5)Ru (CO)(u-dppm)(u- H)M (CO)2(n5-C5H5)]+ HCOO- formed during the catalysis. The second part of the research concerns the synthesis of the heterobimetallic complexes [(n5-C5R5)Ru (CO)(u-dppm)Mn(CO)4] (7) and {[n5-C5(CH3)5]Ru(u- CO)2(u-dppm)Mn(CO)3} (8) and studies of their reactivities. These complexes were prepared via a novel synthetic method by reacting (n5-C5R5)Ru(dppm)H (R = H,CH3), a relatively hydridic metal hydride species , with HMn(CO)5 , a relatively protonic metal hydride species. X-ray structure study shows the existence of a semibridging carbonyl ligand in 7. Complex 8 consists of two bridging carbonyl ligands, along with the metal-metal bond. Both 7 and 8 can also be prepared by metathetical reaction between (n5-C5R5)Ru (dppm)Cl with LiMn(CO)5. Our work served as an example of the synthetic method concerning the use of two different meta1hydride species. Relaxation time T1(min) measurement and NOE study show that there is only weak dihvdrogen-bonding interaction between the two metal hydride species (n5-C5R5)Ru (dppm)H and HMn(CO)5 at room temperature. However, H/D exchange between (n5-C5R5)Ru (dppm)D and HMn(CO)5 was observed, implicating the existence of dihydrogen-bonding interaction between the species (n5-C5R5)Ru (dppm)H and HMn(CO)5, and the intermediacy of an n2-dihydrogen species. The catalytic activity in CO2 / epoxide coupling reactions of 7 and 8 were studied. It was found that both 7 and 8 catalyzed the coupling reaction of CO2 with a series of oxiranes to afford the corresponding cyclic carbonates in good yields. It was 1earned that both the steric and electronic properties of the oxiranes influenced the rate of the reaction. Two possible pathways for the 7-catalyzed CO2 / oxirane coupling reaction were proposed. The reactivities of the monometallic fragments of 7 and 8 had also been studied and compared with that of 7 and 8. Only the species Li+Mn(CO)4(PPh3)-(9)showed catalytic activity towards the coupling reaction, which is lower than that of 7 and 8. It is believed that the proximity of the metal centers in 7 and 8 is one of the reasons for their showing high catalytic activity; the two metal centers of the complexes act synergistically in the catalytic reaction: the manganese center reacts with CO2 to generate the metallocarboxylate anion, and the oxirane is activated by coordination to the ruthenium center via the oxygen donor atom. The species [(n5-C5H4)PPh2] Ru(PPh3)2H (10) which contains a Cp-tethered phosphino group was pepared by refluxing (n5-C5H5)Ru(PPh3)2Cl and KPPh2 in THF. A symmetric bimetallic complex {[(u-n5-C5H4) PPh2]Ru(PPh3)Cl}2 (11) was obtained by refluxing a CDCl3 solution of 10. The X-ray structure of 11 showed that there is no Ru-Ru bond,and the (C5H4PPh2) ligands are arranged in transoid configuration. The reaction of TpRu(PPh3)(CH3CN)Cl with 1.1 equiv of pyrazole resulted in the production of TpRu (PPh3)(pzH)Cl (12) On the other hand, a dimeric species [TpRu(PPh3)(u-pz)]2 (13) was formed when TpRu(PPh3)(CH3CN)H was reacted with 1.1 equiv of pyrazole. This synthesis is believed to be promoted by hydrogen-bonding interaction between the N-H of the coordinated pyrazole and the ruthenium hydride species.