REACTION MECHANISMS, KINETICS, SPECTROSCOPY, ELECTROCHEMICAL AND THEORETICAL STUDIES OF IRON CARBONYL ORGANOMETALLIC RADICALS.

摘要

Little is known about the odd electron chemistry of iron, the transition element with the most fully developed organometallic chemistry. Choosing seventeen electron (17e) Fe(CO)(,3)L(,2)('+) complexes as model compounds and using cyclic voltammetry along with double potential step chronocoulometry as analytical tools, we found that these metal radicals undergo substitution by Lewis bases via an associative pathway. The substituted complex then participates in an outer sphere electron transfer reaction to yield ion(II) and iron(O) products.;In order to understand how reactivity and substitution lability are affected at dimeric iron radicals, a series of 33e binuclear phosphido-bridged complexes of the type Fe(,2)(CO)(,7)((mu)-PPh(,2)) were examined. Electrochemical studies show these species are 10('5) times more reactive than analogous diamagnetic 34e dimers toward CO substitution.;There has been some discussion whether organometallic radicals of the type Fe(CO)(,3)L(,2)+ adopt a ground state square pyramidal or trigonal bipyramidal geometry since these species are nonpersistent. Isotopic labeling studies in conjunction with FTIR spectroscopy show these species possess a trigonal bipyramidal ground state geometry.;Molecular orbital calculations (SCF-X(alpha)-DV Method) predict a ground state geometry assignment of trigonal bipyramid as well. Theoretical studies show that a square pyramidal structure cannot account for the observed electronic transitions. Similar calculations on isoelectronic Mn(CO)(,5), a structure believed to possess a square pyramidal ground state geometry, show that the electronic spectra cannot be unambiguously interpreted in terms of a square pyramidal structure and that both square pyramidal and trigonal bipyramidal structures may exist in equilibrium.;It was shown that Fe(CO)(,3)L(,2)+ complexes undergo substitution at a rate 10('9) times faster than their neutral eighteen electron analogues. This tremendous rate acceleration of substitution at seventeen electron metal radicals can be used to promote carbonyl insertion reactions, a key step in industrially important processes such as hydroformylation. Insertions of this type at odd electron iron occur a billion fold faster than for analogous even electron iron complexes. Electrochemical studies establish that the insertion reaction proceeds at 17e iron via nucleophilic attack by Lewis base to produce a hypervalent 19e species which then inserts to form the metal formyl product.