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.