Flexible binding of PNP pincer ligands to monomeric iron complexes.

Transition metal complexes supported by pincer ligands have many important applications. Here, the syntheses of five-coordinate PNP pincer-supported Fe complexes of the type (PNP)FeCl2 (PNP = HN{CH2CH2(PR2)}2, R = iPr ((iPr)PNP), tBu ((tBu)PNP), or cyclohexyl ((Cy)PNP)) are reported. In the solid state, ((iPr)PNP)FeCl2 was characterized in two different geometries by X-ray crystallography. In one form, the (iPr)PNP ligand binds to the Fe center in the typical meridional geometry for a pincer ligand, whereas in the other form, the (iPr)PNP ligand binds in a facial geometry. The electronic structures and geometries of all of the (PNP)FeCl2 complexes were further explored using (57)Fe Mössbauer and magnetic circular dichroism spectroscopy. These measurements show that in some cases two isomers of the (PNP)FeCl2 complexes are present in solution and conclusively demonstrate that binding of the PNP ligand is flexible, which may have implications for the reactivity of this important class of compounds.

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 (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(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 [{(((i)Pr)PNP)FeH(CO)}2(µ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a µ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a µ2,η(1):η(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 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.