Toward Frameworks with Multiple Aligned and Interactive Fe(CO)3 Rotators: Syntheses and Structures of Diiron Complexes Linked by Two trans-Diaxial ,-Diphosphine Ligands Ar2P(CH2)nPAr2.
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Reactions of (4-benzylideneacetone)Fe(CO)3 and the ,-diphosphines Ar2P(CH2)nPAr2 afford the trigonal bipyramidal diiron tetraphosphorus complexes trans,trans-(CO)3Fe[Ar2P(CH2)nPAr2]2Fe(CO)3 (n/Ar = 3/Ph 3, 4/Ph 4a, 4/p-tol 4b; 56-19%). Crystal structures establish essentially parallel P-Fe-P axes, iron-iron distances of 5.894(9)-5.782(1) (3) and 6.403(1)-6.466(1) (4a,b), and van der Waals radii of 4.45 for the Fe(CO)3 rotators, the planes of which are offset by 0.029-1.665 . Analogous reactions of Ph2P(CH2)6PPh2 yield the square pyramidal monoiron complex trans-(CO)3Fe[Ph2P(CH2)6PPh2] (6', 31%), a rare case where a diphosphine spans trans basal positions (P-Fe-P 147.4(2)). Both 3 and 6' exhibit two CO 13C NMR signals at room temperature, indicating slow exchange on the NMR time scale, which in the former could entail Fe(CO)3/Fe(CO)3 gearing. Under analogous conditions, 4a,b exhibit one signal. Previously reported adducts of Fe(CO)3 and Ph2P(CH2)nPPh2 are surveyed (1:1, n = 1-5; 2:2, n = 5), and the IR CO band patterns and energies of all complexes analyzed with the aid of DFT calculations. The diiron complexes are preferred thermodynamically. Attention is given to limiting types of Fe(CO)3/Fe(CO)3 interactions in the diiron complexes.
