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)(2)Br (1a: R = Me-2; 1b: R = i-Pr) with phosphine and isocyanide compounds. Reaction of 1 with an excess amount of PMe3 in toluene proceeded at 50 degrees C to give the corresponding cationic complexes [(phebox-R)Fe(CO)(PMe3)(2)]Br [2a: R = Me-2 (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 (eta(2)-phebox-R)Fe(CO)(PMe2Ph)(2)Br [3a: R = Me-2 (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-Me-2)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-Me-2)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-Me-2)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 = Me-2 (83%); 7b: R = i-Pr (69%)].