Acyl Migration in d 6 Iron Complexes

The η 2 ‐acyl complex [Fe(η 2 ‐EtCO)I (CO)(PEt 3 ) 2 ] ( 1a ) reacts with phenylacetylene in refluxing Et 2 O to form a mixture of products including [Fe(C(Ph)CHC(O)Et}I(CO)(PEt 3 ) 2 ] ( 3 ), [Fe{C(Ph)CHC(O)Et}1(CO) 2 (PEt 3 ) 2 ] ( 4 ), and [Fe{C(CHPh)C(O)Et)I(CO)(PEt 3 ) 2 ] ( 5 ). Compound 3 contains a ‘metallafurane’ unit, while 5 consists of a 4‐member metallacycle. Compound 5 was characterized by X‐ray crystallography. In contrast to 1a , the η 2 ‐acyl complex [Fe{η 2 ‐(i‐Pr)CO}I(CO)(PEt 3 ) 2 ] ( 1b ) reacts with phenylacetylene under similar conditions to produce [Fe{η 2 ‐C(O)C(Ph)CH(i‐Pr)}I(CO)(PEt 3 ) 2 ( 7 ) which was characterized spectroscopically. Modified extended Hückel theory calculations were made on the model compounds [Fe(η 2 ‐CHO)(CO) 2 (PH 3 ) 2 ] + ( 14 ), [Fe(CHO)(HCCH)(CO) 2 (PH 3 ) 2 ] + ( 15 ), [Fe(CCH 2 )(CHO)(CO) 2 (PH 3 ) 2 ] + ( 16 ), [Fe{C(CH 2 )CHO}(CO) 2 (PH 3 ) 2 ] + ( 17 ), and {Fe(CHCHCHO)(CO) 2 (PH 3 ) 2 ] + ( 18 ). The calculations show that in the transformation 15 → 18 , there exists a transition state lying 83.6 kJ/mol above the optimized geometry of 15 in which the formyl unit is rotated by 80° with respect to the Fe‐acetylene plane. Conversion of 15 to 17 involves a [1,2]‐H shift and an acyl‐acetylene coupling reaction which probably occur synchronously.