Toward Frameworks with Multiple Aligned and Interactive Fe(CO)3 Rotators: Syntheses and Structures of Diiron Complexes Linked by Two trans-Diaxial α,ω-Diphosphine Ligands Ar2P(CH2)nPAr2

Reactions of (η4-benzylideneacetone)­Fe­(CO)3 and the α,ω-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) Å (3) and 6.403(1)–6.466(1) Å (4a,b), and van der Waals radii of 4.45 Å for the Fe­(CO)3 rotators, the planes of which are offset by 0.029–1.665 Å. 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 νCO 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.