Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN­{CH₂CH₂(PR₂)}₂ [R = isopropyl (^iPrPNP) or cyclohexyl (^CyPNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron­(II) complexes (^iPrPNP)­FeCl₂(CO) (1a) and (^CyPNP)­FeCl₂(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl₂ in the presence of CO. The iron(0) complex (^iPrPNP)­Fe­(CO)₂ (2a) was prepared through the reaction of Fe­(CO)₅ with ^iPrPNP, while irradiating with UV light. Compound 2a is unstable in CH₂Cl₂ and is oxidized to 1a via the intermediate iron­(II) complex [(^iPrPNP)­FeCl­(CO)₂]Cl (3a). The reaction of 2a with HCl generated the related complex [(^iPrPNP)­FeH­(CO)₂]Cl (4a), while the neutral iron hydrides (^iPrPNP)­FeHCl­(CO) (5a) and (^CyPNP)­FeHCl­(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of ⁿBu₄NBH₄. The related reaction between 1a and excess NaBH₄ generated the unusual η¹-HBH₃ complex (^iPrPNP)­FeH­(η¹-HBH₃)­(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η¹-HBH₃ ligand and the N–H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(^iPrPNP)­FeH­(CO)}₂(μ₂,η¹:η¹-H₂BH₂)]­[BPh₄] (7a), which features a rare example of a μ₂,η¹:η¹-H₂BH₂ ligand. Unlike all previous examples of complexes with a μ₂,η¹:η¹-H₂BH₂ ligand, there is no metal–metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.