An Acidity Scale of Tetrafluoroborate Salts of Phosphonium and Iron Hydride Compounds in [D₂]Dichloromethane

Abstract

Equilibrium constants (K) for reactions between acids and the conjugate base forms of a number of phosphonium salts, [HPR3][BF4], and iron hydrides, [Fe(CO)3H(PR3)2][BF4], in CD(2)Cl(2) have been determined by means of 31P and 1H NMR spectroscopy at 20 degrees C. The anchor compound chosen for pK(CD(2)Cl(2)) determinations was [HPCy3][BF4] with a pK(CD(2)Cl(2)) value of 9.7, as assigned by literature convention (Cy: cyclohexyl). A continuous scale of pK(CD(2)Cl(2)) values covering the range from 9.7 to -3 was created and correlated with the DeltaH values reported by Angelici and co-workers and literature pK(a) values. The pK(CD(2)Cl(2)) values for 15 other hydride or dihydrogen complexes of the iron group elements and of diethyl ether were also placed on this scale. The crystal structures of [Fe(CO)3H(PCy(2)Ph)2][BF4] and [Fe(CO)3(PCy(2)Ph)2] revealed that the trans-oriented, bulky, unsymmetrical phosphane ligands distort the equatorial plane of the complexes. The acidity of iron carbonyl hydrides is an important feature of the reactions of iron hydrogenase enzymes.

Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal–Ligand Cooperation

“…Since Fe(0) complexes are known to undergo facile protonation at the metal center to afford Fe(II) hydride complexes, 21 protonation of 7 should constitute an alternative synthetic route to obtain complex 5 . As already expected from the shape and energy of the metal-centered HOMO (Figure 4), addition of HBF 4 to a CH 2 Cl 2 solution of 7 resulted in an immediate color change from red to pale yellow consistent with protonation at the iron center to generate cis -[Fe(PNP- i Pr)(CO) 2 H] + ( 5 ) in quantitative yield (Scheme 5), as established by NMR spectroscopy.…”

Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications

“…The p K a CD 2 Cl 2 values of the ammonium compounds [H 2 ′] + and [H 2 ] + were measured as 3.3 and 3.2, respectively, by titration of 2 ′ and 2 with [HPMe 2 Ph]BF 4 (p K a CD 2 Cl 2 = 5.7) 37. We could not directly determine the corresponding p K a of [H 2 ′] 2+ because [ 2 ′] + undergoes quasi-disproportionation upon treatment with acids, even in solution at −78 °C.…”

Aza- and Oxadithiolates Are Probable Proton Relays in Functional Models for the [FeFe]-Hydrogenases

“…The strong acid [H(Et 2 O) 2 ]BAr F 4 protonated 1, 2, and 3 quickly at −90 °C, but the billion fold weaker acid [HPMe 2 Ph]BF 4 (p K CD 2 Cl 2 = 5.7) protonated only 2 (−90 °C), not 1 or 3 16. The p K a of [2H] + is bracketed by the finding that 2 is not protonated by [HPBu 3 ]BF 4 .…”

“…Using [HPMe 2 Ph]BF 4 (p K CD 2 Cl 2 = 5.7), however, [ 2 ( t -H)]BF 4 is catalytically active, but [ 1 ( t -H)]BF 4 and [ 3 ( t -H)]BF 4 are not. The p K CD 2 Cl 2 of [HPMe 2 Ph]BF 4 has been estimated to correspond to an aqueous p K a of 6.8 16. Catalysis by the amine [ 2 ( t -H)]BF 4 with strong acids is complicated because protonation occurs at both the amine and terminally at Fe 7,10,18.…”