Stable, Yet Highly Reactive Nonclassical Iron(II) Polyhydride Pincer Complexes: Z-Selective Dimerization and Hydroboration of Terminal Alkynes

Abstract

The synthesis, characterization, and catalytic activity of nonclassical iron(II) polyhydride complexes containing tridentate PNP pincer-type ligands is described. These compounds of the general formula [Fe(PNP)(H)(η-H)] exhibit remarkable reactivity toward terminal alkynes. They efficiently promote the catalytic dimerization of aryl acetylenes giving the corresponding conjugated 1,3-enynes in excellent yields with low catalyst loadings. When the reaction is carried out in the presence of pinacolborane, vinyl boronates are obtained. Both reactions take place under mild conditions and are highly chemo-, regio-, and stereoselective with up to 99% Z-selectivity.

Pseudo‐tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E‐Enynes

“…Nevertheless, the use of microwave radiation has permitted a remarkable improvement in the activity of a palladium catalyst, to obtain high yields after 30 min, but with harsher reactions conditions (130 °C and addition of an external base) . More recently, an iron(II) polyhydride catalyst, [Fe(PNP)(H) 2 (η 2 ‐H 2 )] (PNP=2,6‐bis{diphenylphosphanyl(methyl)amino}pyridine), was reported to promote efficiently alkyne dimerization to enynes in comparable times . In addition, the selective and very fast cross‐dimerization of alkynes to 1,3‐enynes catalyzed by titanium complexes has also been reported .…”

“…[46] More recently,a ni ron(II) polyhydride catalyst, [Fe(PNP)(H) 2 (h 2 -H 2 )] (PNP = 2,6-bis{diphenylphosphanyl(methyl)amino}pyridine), was reportedt o promote efficiently alkyne dimerization to enynes in comparable times. [47] In addition, the selectivea nd very fast cross-dimerization of alkynes to 1,3-enynes catalyzed by titaniumc omplexes has also been reported. [48] Our attempts at the cross-dimerization of alkynes to 1,3-enynes were fully unselective resulting in am ixture of the three possible E-enynes.…”

Iron Dihydride Complexes: Synthesis, Reactivity, and Catalytic Applications

“…Subsequent evacuation under high vacuum removed the coordinated MeCN from cis ‐[( R PN pyr P)FeH(MeCN) 2 ]BH 4 , providing the desired hydridoborohydride complexes (eq 12) ,. Complexes ( iPr P NR N NR P)FeH( κ 2 ‐BH 4 ) (R=H, Me) and ( iPr P NH N NH P)FeH(CO)( κ 1 ‐BH 4 ) were synthesized from the pre‐made dibromide complexes (eq 13) and the hydridobromide complex (eq 14), respectively. Similar reactions with CO‐bound complexes trans ‐( iPr PN pyr P)FeBr 2 (CO) (eq 15) and trans ‐( R PN H P)FeX 2 (CO) ( R PN H P=NH(CH 2 CH 2 PR 2 ) 2 ; R= i ‐Pr, Cy, Et; X=Cl, Br; eq 16),, led to the isolation of the corresponding hydridoborohydride complexes.…”

“…Isolation of these complexes in a pure form was not possible. The reaction of ( iPr P NR N NR P)FeBr 2 with NaAlH 4 also produced highly reactive aluminiumhydride complexes that decomposed readily (eq 20) . Hydrolysis of the in situ generated samples led to the formation of ( iPr P NR N NR P)FeH 2 (H 2 ).…”

“…The work by the Jones group indicated that the in situ generated ( iPr PN H P)FeH 2 (CO) interacted with BH 3 ⋅ THF to produce ( iPr PN H P)FeH(CO)( κ 1 ‐BH 4 ) . Kirchner's ( iPr P NMe N NMe P)FeH 2 (H 2 ) was also shown to react with BH 3 ⋅ THF and HBpin to form ( iPr P NMe N NMe P)FeH( κ 2 ‐BH 4 ) and ( iPr P NMe N NMe P)FeH( κ 2 ‐H 2 Bpin), respectively . In this case, loss of the dihydrogen ligand created a vacant coordination site for the borate ligand to adopt the κ 2 coordinate mode.…”