Synthesis And Organometallic Properties Of Osmium Benzylidyne Complexes

Transition metal carbyne complexes have been the focus of considerable fundamental research on structure,bonding and reactivity,especially their relation to alkyne metathesis reactions.However,the catalysts currently used in alkyne metathesis are still limited to early transition metal(especially Mo and W)carbyne complexes.Whilst no examples of alkyne metathesis-active late transition metal carbyne complexes have been reported,there have been a number of ruthenium and osmium carbyne complexes reported in literature,which have shown rich reactivity and growing application in organometallic synthesis and organic synthesis.In particular,many interesting and novel reactivity patterns for osmium complexes have been demonstrated in recent years.In view of the the great success of ruthenium carbene complexes in alkene metathesis and the advantages of ruthenium-based catalysts,the structurally related ruthenium carbyne complexes and their heavier osmium counterparts may possess similar activity for alkyne metathesis.We thus decided to synthesize osmium benzylidyne complexes with structures closely realated to the Grubbs-type ruthenium carbene derivatives RuCl2(=CHPh)(PCy3)2 for organometallic property and reactivity studies,particularly on reactivity towards alkyne and alkene substrates.In this dissertation,a series of osmium benzylidyne complexes have been synthesized from the reactions of phosphorus yilde with simple osmium starting material and the organometallic properties the carbyne complexes obtained have been investigated.of some of the alkenylcarbyne complexes have also been investigated.This dissertation consists of following four parts.In chapter 1,the research progress on osmium carbyne complexes,including synthetic routes and reactivities are briefly reviewed.In addition,the previous research work of our group in this field are introduced and the research objectives of this thesis are also presented.Chapter 2 describes the preparation of osmium aryl carbyne complexes starting from a series of phosphorus benzyl ylides with different substituent at the para position of the phenyl ring.Reactions of OSCl2(PPh3)3 with Ph3P=CH(p-C6H4R)(R=Me,OMe,F)(2-2)under heating condition produce the hydride benzylidyne complexes OsHCl2(=C-p-C6H4R)(PPh3)2(2-3).Preliminary investigation on the influence of the substituents at the para position of the benzyl ylides on the reaction rate shows that the ylide with electron donating group reacts faster than that with electron-withdrawing group.This result is consistent with the mechanism for the formation of the hydride carbyne that the reaction is initiated by the nucleophilic attack of the benzyl yilde at the osmium center.Treatment of 2-3 with H2O2 in the presence of NaCl afford the trichlorocarbyne complexes OsCl3(=CPhR)(PPh3)2(2-4).Phosphine ligand substitution of the carbyne complexes OsHCl2(=C-p-C6H4R)(PPh3)2(2-3)and OsCl3(≡ C-p-C6H4R)(PPh3)2(2-4)with PCy3 affords OsHCl2(=C-p-C6H4R)(PCy3)2(2-5)and OsCl3(≡ C-p-C6H4R)(PCy3)2(2-6),respectively.Chapter 3 is about the organometallic properties of the osmium benzylidyne complexes.Reactions of the osmium hydride benzylidyne OsHCl2(≡C-p-C6H4R)(PPh3)2(2-3)with bidentate phosphine ligand such as dppf and dppe produces the osmium carbene complexes OsCl2(≡CH-p-C6H4R)(dppf)(3-1)and[OsCl(≡CH-p-C6H4R)(dppe)2]Cl(3-2).Treatment of OsCl3(≡C-p-C6H4R)(PCy3)2(2-6)with a ether solution of HBF4 give the cationic 16-e five-coordinate products[OsCl2(≡C-p-C6H4R)(PCy3)2](BF4)(3-3)derived from the electrophilic abstraction of the chloride ligand.Reaction of hydride carbyne OsHCl2(≡C-p-C6H4R)(PCy3)2(2-5)with AgOTf in the presence of benzonitrile affords cationic momo benzonitrile-coordinated product[OsHCl(≡CH-p-C6H4R)(PhCN)(PCy3)2](BF4)(3-4).When an excess of AgOTf was used in the reaction,further abstraction of another chloride ligand occured,which then resulted in the migration of the hydride ligand to the carbyne carbon to give the dicationic carbene complex[Os(=CH-p-C6H4R)(PhCN)3(PCy3)2](BF4)2(3-5).Treatment of the trichloro benzylidyne OsCl3(≡C-p-C6H4R)(PCy3)2(2-6)with sulfur lead to oxidation of one of the PCy3 ligand followed by dimerization of the metal fragments to give the dichloro-bridged binuclear osmium benzylidyne[OsCl3(≡C-p-C6H4R)(PCy3)]2(3-6),which can be readily dissociated upon react with water to yield the aqua omplex OsCl3(≡C-p-C6H4R)(PCy3)(H2O)(3-7).The reactivity of some of these carbyne complexes towards alkynes and olefins have also been investigated preliminaryly.Whilst low reactivity of these carbyne complexes towards terminal alkynes and neither alkyne metathesis activity has been shown in our current study,the osmium hydride benzylidyne OsHCl2(≡CAr)(PR3)2(R = Ph,Cy)are good catalyst precursors,especially in the presence of copper salt as the phosphine scavenger,for the ROMP of NBE and NBD,and the RCM of 2,2’-diallyl malonate derivatives.Treatment of OsHCl2(≡C-p-C6H4R)(PCy3)2(2-5)with GeCl2 lead to the formation of the GeCl3 coordinated product OsHCl(GeCl3)(≡C-p-C6H4R)(PCy3)2(3-8)due to the nucleophilic attack of the highly electrophilic GeCl2 by the choloride.Moreover,the chlorine ligand substitution of the hydride osmium benzylidyne OsHCl2(≡C-p-C6H4R)(PCy3)2(2-5)with LiBr or LiI lead to the formation of corresponding bromo and iodo complexes OsHBr2(≡C-p-C6H4R)(PCy3)2(3-9)and OsHI2(≡C-p-C6H4R)(PCy3)2(3-11),respectively.Treatment of the bromo complex OsHBr2(≡C-p-C6H4Me)(PCy3)2(3-9a)with t-BuOK produced an air-sensitive product,which is tentatively identified to be the four-coordinate 16-e benzylidyne complex OsBr(≡C-p-C6H4Me)(PCy3)2(3-10),in which the formal oxidation state of the metal center can be viewed as Os(Ⅳ).Chapter 4 summarizes the main contents of this dissertation.