Group 7 carbonyl complexes of a PNN-heteroscorpionate ligand

Abstract

A series of rhenium and manganese carbonyl complexes of a heteroscorpionate ligand with an atypical N₂P-donor set has been prepared to better understand their electronic and CO releasing properties. Thus, the ligand, pz₂TTP, with an a,a-bis(pyrazol-1-yl)tolyl group decorated with an ortho-situated di(p-tolyl)phosphanyl reacts with carbonyl group 17 reagents to give [fac-(κ²NP-pz₂TTP)Re(CO)₃Br], 1, and [fac-(κ³N₂P-pz₂TTP)M(CO)₃](OTf = O₃SCF₃), 2-M (M = Re, Mn), if care is taken during the preparation of the manganeses derivative. When heated in CH₃CN, 2-Mn slowly transforms to [fac,cis-(κ³N₂P-pz₂TTP)Mn(CO)₂(NCCH₃)](OTf), 3-Mn. In contrast, the corresponding 3-Re can only be prepared from 2-Re using Me₃NO; pure 3-Mn can also be prepared by this method. Experimental and density functional calculations at the M06L/Def2-TZVP/PCM(CH₃CN) level show that the replacement of a carbonyl with an acetonitrile solvent decreases the oxidation potential by around 0.8 V per carbonyl released, making decarbonylated species potent reductants. At the same time, the electronic spectrum broadens and undergoes a red-shift, making dicarbonyl complexes more susceptible to photo-initiated decarbonylation reactions than tricarbonyls. When 2-Mn or 3-Mn are irradiated in with 390 nm LED light in aerated solutions, [trans-Mn(pz₂TTP = O)₂](OTf)₂, 4, along with insoluble manganese oxides are rapidly formed.