Synthesis and Structure of Six-Coordinate Iron Borohydride
Complexes Supported by PNP Ligands
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Abstract
The preparation of a number of iron
complexes supported by ligands
of the type HN{CH<sub>2</sub>CH<sub>2</sub>(PR<sub>2</sub>)}<sub>2</sub> [R = isopropyl (<sup><sup>i</sup>Pr</sup>PNP) or cyclohexyl (<sup>Cy</sup>PNP)] is reported. This is the first time this important
bifunctional ligand has been coordinated to iron. The iron(II) complexes
(<sup><sup>i</sup>Pr</sup>PNP)FeCl<sub>2</sub>(CO) (<b>1a</b>) and (<sup>Cy</sup>PNP)FeCl<sub>2</sub>(CO) (<b>1b</b>) were
synthesized through the reaction of the appropriate free ligand and
FeCl<sub>2</sub> in the presence of CO. The iron(0) complex (<sup><sup>i</sup>Pr</sup>PNP)Fe(CO)<sub>2</sub> (<b>2a</b>) was
prepared through the reaction of Fe(CO)<sub>5</sub> with <sup><sup>i</sup>Pr</sup>PNP, while irradiating with UV light. Compound <b>2a</b> is unstable in CH<sub>2</sub>Cl<sub>2</sub> and is oxidized
to <b>1a</b> via the intermediate iron(II) complex [(<sup><sup>i</sup>Pr</sup>PNP)FeCl(CO)<sub>2</sub>]Cl (<b>3a</b>). The
reaction of <b>2a</b> with HCl generated the related complex
[(<sup><sup>i</sup>Pr</sup>PNP)FeH(CO)<sub>2</sub>]Cl (<b>4a</b>), while the neutral iron hydrides (<sup><sup>i</sup>Pr</sup>PNP)FeHCl(CO)
(<b>5a</b>) and (<sup>Cy</sup>PNP)FeHCl(CO) (<b>5b</b>) were synthesized through the reaction of <b>1a</b> or <b>1b</b> with 1 equiv of <sup>n</sup>Bu<sub>4</sub>NBH<sub>4</sub>. The related reaction between <b>1a</b> and excess NaBH<sub>4</sub> generated the unusual η<sup>1</sup>-HBH<sub>3</sub> complex (<sup><sup>i</sup>Pr</sup>PNP)FeH(η<sup>1</sup>-HBH<sub>3</sub>)(CO) (<b>6a</b>). This complex features a bifurcated
intramolecular dihydrogen bond between two of the hydrogen atoms associated
with the η<sup>1</sup>-HBH<sub>3</sub> ligand and the N–H
proton of the pincer ligand, as well as intermolecular dihydrogen
bonding. The protonation of <b>6a</b> with 2,6-lutidinium tetraphenylborate
resulted in the formation of the dimeric complex [{(<sup><sup>i</sup>Pr</sup>PNP)FeH(CO)}<sub>2</sub>(μ<sub>2</sub>,η<sup>1</sup>:η<sup>1</sup>-H<sub>2</sub>BH<sub>2</sub>)][BPh<sub>4</sub>] (<b>7a</b>), which features a rare example of a μ<sub>2</sub>,η<sup>1</sup>:η<sup>1</sup>-H<sub>2</sub>BH<sub>2</sub> ligand. Unlike all previous examples of complexes with a
μ<sub>2</sub>,η<sup>1</sup>:η<sup>1</sup>-H<sub>2</sub>BH<sub>2</sub> ligand, there is no metal–metal bond
and additional bridging ligand supporting the borohydride ligand in <b>7a</b>; however, it is proposed that two dihydrogen-bonding interactions
stabilize the complex. Complexes <b>1a</b>, <b>2a</b>, <b>3a</b>, <b>4a</b>, <b>5a</b>, <b>6a</b>, and <b>7a</b> were characterized by X-ray crystallography
