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 N ₂P-donor set has been prepared to better understand their electronic and CO releasing properties. Thus, the ligand, pz ₂TTP, 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-(κ ²NP-pz ₂TTP)Re(CO) ₃Br], 1, and [ fac-(κ ³N ₂P-pz ₂TTP)M(CO) ₃](OTf = O ₃SCF ₃), 2-M (M = Re, Mn), if care is taken during the preparation of the manganeses derivative. When heated in CH ₃CN, 2-Mn slowly transforms to [ fac, cis-(κ ³N ₂P-pz ₂TTP)Mn(CO) ₂(NCCH ₃)](OTf), 3-Mn. In contrast, the corresponding 3-Re can only be prepared from 2-Re using Me ₃NO; pure 3-Mn can also be prepared by this method. Experimental and density functional calculations at the M06L/Def2-TZVP/PCM(CH ₃CN) 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(pz ₂TTP = O) ₂](OTf) ₂, 4, along with insoluble manganese oxides are rapidly formed.

Abstract

A PNN heteroscorpionate ligand facilitates CO-release in univalent tricarbonyl group 7 complexes.