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.