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 ((PNP)-P-iPr) or cyclohexyl ((PNP)-P-Cy)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes ((PNP)-P-iPr)FeCl2(CO) (la) and ((PNP)-P-Cy)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex ((PNP)-P-iPr)Fe(CO)(2) (2a) was prepared through the reaction of Fe(CO)(5) with (PNP)-P-iPr, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to la via the intermediate iron(II) complex [( (PNP)-P-iPr)FeCl(CO)(2)]Cl (3a). The reaction of 2a with HCI generated the related complex E((PNP)-P-iPr)FeH(CO)(2)]Cl (4a), while the neutral iron hydrides ((PNP)-P-iPr)FeHCl(CO) (5a) and ((PNP)-P-Cy)FeHCl-(CO) (5b) were synthesized through the reaction of la or 1b with 1 equiv of (Bu4NBH4)-Bu-n. The related reaction between la and excess NaBH4 generated the unusual eta(1)-HBH3 complex ((PNP)-P-iPr)FeH(eta(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the eta(1)-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 [{((PNP)-P-iPr)FeH(CO)}(2)(mu(2),eta(1):eta(1)-H2BH2)][BPh4] (7a), which features a rare example of a mu(2),eta(1):lambda(1)-H2BH2 ligand. Unlike all previous examples of complexes with a mu(2),eta(1):lambda(1)-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 la, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.