Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph2PC6H4CHNC6H4-4-Cl (PCHNArCl) complexes of iron(0), iron­(I), and iron­(II). The reaction of PCHNArCl with Fe­(bda)­(CO)3 gives Fe­(PCHNArCl)­(CO)3 (1), featuring an η2-imine. DNMR studies, its optical properties, and DFT calculations suggest that 1 racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The N-imine isomer is more stable in Fe­(PCHNArOMe)­(CO)3 (1 OMe ), which crystallized despite being the minor isomer in solution. Protonation of 1 by HBF4·Et2O gave the iminium complex [1H]­BF4. The related diphosphine complex Fe­(PCHNArCl)­(PMe3)­(CO)2 (2), which features an η2-imine, was shown to also undergo N protonation. Oxidation of 1 and 2 with FcBF4 gave the Fe­(I) compounds [1]­BF4 and [2]­BF4. The oxidation-induced change in hapticity of the imine from η2 in [1]0 to κ1 in [1]+ was verified crystallographically. Substitution of a CO ligand in 1 with PCHNArCl gave Fe­[P2(NArCl)2]­(CO)2 (3), which contains the tetradentate diamidodiphosphine ligand. This C–C coupling is reversed by chemical oxidation of 3 with FcOTf. The oxidized product of [Fe­(PCHNArCl)2(CO)2]2+ ([4]2+) was prepared independently by the reaction of [1]+, PCHNArCl, and Fc+. The C–C scission is proposed to proceed concomitantly with the reduction of Fe­(II) via an intermediate related to [2]+.

This report describes examples of combined Fe-and O-centered reactivity of Fe(P 2 O 2 )(CO) 2 (1), where P 2 O 2 is the diphosphinoglycolate (Ph 2 PC 6 H 4 CHO) 2 2−. This 18e low-spin ferrous dialkoxide undergoes substitution of CO to give the labile monosubstituted derivatives Fe(P 2 O 2 )(CO)-(L) (L = PMe 3 , pyridine, MeCN). Treatment of Fe(P 2 O 2 )-(CO) 2 with Brønsted acids results in stepwise O-protonation, affording rare examples of low-spin Fe(II) complexes containing alcohol ligands. Substitution reactions with amides (RC(O)NH 2 ) proceeds with binding of the carbonyl and formation of an intramolecular hydrogen bond between NH and the neighboring alkoxo ligand. This two-site binding was confirmed with crystallographic characterization of the thiourea-substituted derivative. Fe(P 2 O 2 )(CO) 2 reacts with Ph 2 SiH 2 to give the O-silylated hydrido complex, which is inactive for hydrosilylation. The monocarbonyl derivatives Fe(P 2 O 2 )(CO)(L) (L = NCMe, PMe 3 , acetamide) are precursors to catalysts for the hydrosilylation of benzaldehyde, acetophenone, and styrene. ■ INTRODUCTIONMonomeric alkoxoiron(II) complexes are rare. 1 The oxygen centers in such complexes are expected to be highly basic owing to electrostatic interactions between the metal and ligand, although the Fe−O bonds themselves remain strong. 2 The enhanced basicity of the alkoxide ligand in such complexes usually results in multinuclear complexes with bridging alkoxo ligands. For these reasons, studies on the reactivity of ferrous alkoxides are few.We recently described a one-pot synthesis of an iron(II) diphosphine-dialkoxide dicarbonyl complex Fe(P 2 O 2 )(CO) 2 (1; P 2 O 2 = (Ph 2 PC 6 H 4 CHO) 2 2− ). This complex arises by the coupling of 2 equiv of 2-diphenylphosphinobenzaldehyde (PCHO) 3 in the presence of iron(0) carbonyl reagents (eq 1; bda = benzylideneacetone). 4 Similar condensations had been observed using PCHO with low-valent tungsten and technetium. 5 Our iron system is attractive because the conversion is highly efficient and the reagents are inexpensive. As a dialkoxo iron(II) carbonyl, 1 is unique. In the previous report, 1 was shown to form stable adducts with a variety of Lewis acids, including BF 3 , which bind to the alkoxide sites. In this paper, we provide new perspectives on the O-centered reactivity of 1. These results demonstrate the high basicity of alkoxo ligands and an unprecedented activation mode of Si−H bonds by alkoxo-iron functionalities to afford a series of new hydrido-iron complexes. We also found that the diphosphino-dialkoxo iron species with labile ligands (e.g., MeCN, MeC(O)NH 2 ) are active catalysts for the hydrosilylation of aldehydes, ketones, and even alkenes. The spectroscopic features of all newly synthesized compounds are summarized in Table 1. ■ RESULTS Substitution of Fe(P 2 O 2 )(CO) 2 by Lewis Bases. The CO ligands of 1 are labile. In CH 2 Cl 2 solutions, 1 exchanges with 1 atm of 13 CO over the course of several hours (see the Supporting Information). The lability of the CO ligands ...