Probing for Four-Coordinate Zerovalent Iron in a π-Acidic Ligand Field: A Functional Source of FeL₄ Enabled by Labile Dinitrogen Binding

Abstract

It has long been recognized that the first intermediate in chemical reactions mediated by Fe­(CO)5 is Fe­(CO)4. However, the extreme instability of this unsaturated, high-spin species has hampered detailed structural and stoichiometric reactivity studies. The previously reported heteroleptic complex Fe­(N2)­(CO)2(CNArTripp2)2 (ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3), which is isolobal to Fe­(CO)5, is shown here to possess a labile dinitrogen ligand that allows it to perform reactions analogous to Fe­(CO)4. Specifically, Fe­(N2)­(CO)2(CNArTripp2)2 oxidatively adds dihydrogen, white phosphorus, and the Si–H bonds of triethylsilane and phenylsilane readily at room temperature and also binds tetrahydrofuran in the absence of N2. Accordingly, Fe­(N2)­(CO)2(CNArTripp2)2 effectively serves as a masked form of Fe­(CO)4, allowing for well-defined reactivity and structurally characterizable reaction products.

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

“…The mixed isocyano/carbonyl metalates K 2 [Fe(CO) 125 were also reported by the Figueroa group. 74,149 In a multistep synthetic protocol, pentacarbonyliron(0) is photolytically transformed, in the presence of the more sterically encumbered ligand CNAr Tripp2 (2.2 equiv), to the iron(0) complex Fe(CO) 3 (CNAr Tripp2 ) 2 (64), which is subsequently oxidized by molecular iodine (1.0 equiv) to afford [Fe(I) 2 (CO) 2 (CNAr Tripp2 ) 2 ] (65). The latter complex reacted with KC 8 (4.5 equiv) to yield the formally Fe(−II) mixed isocyano/carbonyl complex 62 (Scheme 6).…”

“…As with [Fe(CNXyl) 4 ] 2− (50), ferrate 62 can be oxidized (with iodine) in the presence of N 2 to yield an analogue of 61, namely Fe(N 2 )(CO) 2 (CNAr Tripp2 ) 2 (69, Scheme 7). 74,149 Compared with 61, which exhibited poor thermal stability, 69 is more stable at room temperature in benzene, n-pentane, and Et 2 O solution under an N 2 atmosphere. Such enhanced stability is a consequence of the greater steric protection of the bulkier isocyano ligands CNAr Tripp2 in combination with the additional electronic stabilization offered by the two CO ligands.…”

“…Such enhanced stability is a consequence of the greater steric protection of the bulkier isocyano ligands CNAr Tripp2 in combination with the additional electronic stabilization offered by the two CO ligands. 149 The authors found that the dinitrogen ligand has a low degree of activation, as reflected by the short N−N distance determined via X-ray diffraction (d NN = 1.1059(41) Å) and by the high-energy FT-IR stretching band (ν N≡N = 2194 cm −1 ; see Table 2 below). This lack of activation was attributed to the presence of the strongly π-acidic CO and isocyanide ligands.…”

“…Synthesis of Mixed Carbonyl/Isocyano Fe(−II) Metalates by Figueroa and Co-workers 74,149,150 Scheme 7. Reactivity of the Mixed Complexes K 2 [Fe(CO) 2 (CNAr Tripp2 ) 2 ] (62) and K 2 [Fe(CO) 2 (CNAr Dipp2 ) 2 ] (63) 74,149,150 (75, BTMSA = bis-trimethylsilylacetylene) confirmed that both complexes are isostructural, suggesting a close electronic analogy. Nonetheless, these complexes exhibit disparate reactivity: while 73 reacts with diene nucleophiles (1,3butadiene or 1,3-cyclohexadiene) to afford diphospha-Diels− Alder adducts, complex 75 failed to react with these substrates.…”

“…Although 72 is stable in solution for several Scheme 6. Synthesis of Mixed Carbonyl/Isocyano Fe(−II) Metalates by Figueroa and Co-workers 74,149,150 Scheme 7. Reactivity of the Mixed Complexes K 2 [Fe(CO) 2 (CNAr Tripp2 ) 2 ] (62) and K 2 [Fe(CO) 2 (CNAr Dipp2 ) 2 ] (63) 74,149,150 (75, BTMSA = bis-trimethylsilylacetylene) confirmed that both complexes are isostructural, suggesting a close electronic analogy.…”