Change in Coordination of NCN Pincer Iron Complexes Containing Bis(oxazolinyl)phenyl Ligands

The coordination of bis­(oxazolinyl)­phenyl (phebox) ligands
to an
Fe center was investigated in the reaction of (phebox-R)­Fe­(CO)2Br (1a: R = Me2; 1b:
R = i-Pr) with phosphine and isocyanide compounds.
Reaction of 1 with an excess amount of PMe3 in toluene proceeded at 50 °C to give the corresponding cationic
complexes [(phebox-R)­Fe­(CO)­(PMe3)2]Br [2a: R = Me2 (79%); 2b: R = i-Pr (83%)]. The molecular structures of 2a and 2b were confirmed by X-ray diffraction analysis
that revealed the pseudo-octahedral geometry with NCN meridional coordination
of the phebox ligand. In contrast, reaction of 1 with
PMe2Ph gave the neutral phosphine complexes (η2-phebox-R)­Fe­(CO)­(PMe2Ph)2Br [3a: R = Me2 (87%); 3b: R = i-Pr (79%)], in which the phebox ligand was coordinated to Fe as an
NC bidentate ligand with the oxazoline and phenyl groups. Subsequent
reaction of the neutral phosphine complex 3a resulted
in the formation of the corresponding cationic complexes [(phebox-Me2)­Fe­(CO)­(PMe2Ph)2]Br (4) via change in coordination to the tridentate mode. The reaction
of 1 with tert-butylisocyanide CN­(t-Bu) gave a mixture of neutral isocyanide complexes (phebox-Me2)­Fe­(CO)­[CN­(t-Bu)]Br (5, 6) in 57 and 10% yields, respectively, via exchange of one of the CO ligands. Subsequent reaction of 5 with CN­(t-Bu) resulted in formation of
the cationic complex {(phebox-Me2)­Fe­[CN­(t-Bu)]3}Br (7a). Similarly, treatment of 1 with an excess amount of CN­(t-Bu) afforded {(phebox-R)­Fe­[CN­(t-Bu)]3}Br [7a: R = Me2 (83%); 7b: R = i-Pr (69%)].