Unrealized concepts of masked alkylidenes in (PNP)FeXY systems and alternative approaches to LnXmFe(IV)=CHR

Treatment of PNP ligands with iron(II) halides yielded pseudo tetrahedral {(Ph2PCH2)(2)NR}FeX2 (R '' = Bu-t, 1a-X; Me, 1b-X) and {((Bu2PCH2)-Bu-i)(2)NR ''}FeX2 (R '' = Bu-t, 2a-X; Me, 2b-X) complexes, which could be mono- and dialkylated to afford various {(Ph2PCH2)(2)NR ''}FeCl(R) (R '' = Bu-t, 3a-R; Me, 3b-R), {((Bu2PCH2)-Bu-i)(2)NR ''}FeCl (R) (R '' = Bu-t, 4a-R; Me, 4b-R), {(Ph2PCH2)(2)(NBu)-Bu-t}Fe((neo)Pe)(2) (5a-(neo)Pe) and {((Bu2PCH2)-Bu-i)(2)(NBu)-Bu-t}Fe(1-nor)(2) (6a-nor). All of the complexes were high spin (S = 2), and structural studies of {(Ph2PCH2)(2)NMe}FeCl2 (1b-Cl), {((Bu2PCH2)-Bu-i)(2)(NBu)-Bu-t}FeCl2 (2a-Cl), and {((Bu2PCH2)-Bu-i)(2)(NBu)-Bu-t}FeCl(1-nor) (4a-nor), revealed chair conformations for the PNP ligands, which were calculated to have modest barriers to boat configurations via twist-boats as calculational minima. Attempts to convert these complexes to zwitterionic NCRR'Fe-containing species as masked alkylidenes failed to materialize. Alternative approaches involving structurally characterized diamagnetic {((Bu2PCH2)-Bu-i)(2)(NBu)-Bu-t}(CO)(2)FeCl(CO(neo)Pe) (8), the carbonylation product of 4a-neoPe, and low-valent {(Ph2PCH2CH2)(2)(NBu)-Bu-t}Fe((H2C=CHSiMe2)(2)O) (9) also failed. An analysis of orbital energies and overlap is given for alkylidenes. Covalence is identified as a crucial feature necessary for the alkylidene to metalacyclobutane transformation in metathesis. (C) 2020 Elsevier Ltd. All rights reserved.