Fourteen-Electron Organoiron Species: Synthesis and Characterization of High-Spin Iron (II) Alkyls and the Crystal Structure of the Benzyl Derivative Fe(CH₂C₆H₄CH₃)₂(dippe)

The four-coordinate iron phosphine complex FeBr₂(dippe) (dippe = l,2-bis(diisopropylphosphino)ethane) can be alkylated with Grignard or dialkylmagnesium reagents to give the unusual 14-electron iron(II) alkyls FeR₂(dippe) (R = CH₂SiMe₃, CH₂CMe₃, CH₂CMe₂Ph, CH₂Ph, CH₂C₆H₄Me). With bulky alkyl groups or with proper control of stoichiometry, the monoalkyl species Fe(C₆H₂Me₃)Br (dippe) and Fe-(CH₂CMe₃)Br(dippe) may be obtained. All of these four-coordinate alkyls possess magnetic moments characteristic of high-spin species (/x= 4.9 mb)and exhibit large contact shifts in their¹²H NMR spectra. An X-ray crystal structure of the p-methylbenzyl complex Fe(CH₂C₆H₄Me)2(dippe) reveals a distorted tetrahedral geometry, with C-Fe-C and P-Fe-P angles of 111.8 (2)° and 83.33 (7)°. The Fe-C distances of 2.120 (6) A show little change from low-spin iron(II) species, while the Fe-P distances of 2.462 (2) A are greatly lengthened. Despite the low electron count, there are no FeHC interactions or Fe—C contacts to benzylic positions in the p-methylbenzyl groups, and this fact may be accounted for on electronic grounds, although steric effects may also play a part. Specifically, the lack of empty d orbitals in these high-spin alkyls inhibits “agostic” interactions, despite the low electron counts. This behavior is consistent with the observation that the iron alkyls are incapable of olefin insertion processes that would lead to Ziegler-Natta catalytic activity. X-ray crystallographic data for Fe(CH₂C₆H₄Me)₂(dippe) at 25 °C: monoclinic, space group C2/c, a = 8.957 (2) A, b = 15.620 (5) A, c = 22.510 (8) A, 0 = 98.20 (2)°, V = 3117 (2) A^{34 5 6 78}, Z = 4, R = 5.2%, and R_w= 5.8% for 1234 data and 151 variables.