Template Synthesis of Iron(II) Complexes Containing Tridentate P−N−S, P−N−P, P−N−N, and Tetradentate P−N−N−P Ligands

Abstract

A series of mer-tridentate iron(II) complexes bearing P-N-S (3), P-N-P (4), and P-N-N (5) ligands have been prepared via the metal template effect in one pot involving air-stable phosphonium dimers [cyclo-(-PPh(2)CH(2)C(OH)H-)(2)](Br)(2) (1) and [cyclo-(-PCy(2)CH(2)C(OH)H-)(2)](Br)(2) (2), KOtBu, [Fe(H(2)O)(6)][BF(4)](2) and 2-aminothiolphenol (for 3), 2-(diphenylphosphino)ethylamine (for 4), and 2-(aminomethyl)pyridine (for 5). The new phosphonium dimer 2 was prepared via an S(N)2 reaction of PCy(2)H with BrCH(2)CH(OEt)(2). The complexes Fe{PR(2)CH(2)CH=N(2-C(6)H(4))S}(2)FeBr(2) (3a, R = Ph; 3b, R = Cy) are paramagnetic, and X-ray diffraction studies revealed that they are bimetallic, in which the S atoms of the bis-tridentate (PNS)(2)Fe unit bridge to a FeBr(2) fragment. Complexes [Fe(PR(2)CH(2)CH=NC(2)H(4)PPh(2))(NCMe)(3)]X(2) (4a, R = Ph; 4b, R = Cy; X(2) = FeBr(4) or (BF(4))(2)) form when 1 equiv of iron is reacted with PPh(2)CH(2)CH(2)NH(2) and 0.5 equiv of the appropriate phosphonium dimer. The evidence for P-N-P coordination is the large (2)J(PP) coupling constant in the (31)P {(1)H} NMR spectrum for the trans phosphorus nuclei. If 0.5 equiv of [Fe(H(2)O)(6)][BF(4)](2) were added in the synthesis, the complex trans-[Fe(NCMe)(2)(Ph(2)PC(2)H(4)NH(2))(2)][FeBr(4)] (4c) formed, and this has been characterized by X-ray diffraction. Complexes [Fe{PR(2)CH(2)CH=NCH(2)(2-C(5)H(4)N)}(2)](BPh(4))(2) (5) are bis-tridentate iron(II) complexes with pyridyl donors trans to the phosphine donors. Interestingly, addition of the diamines ethylenediamine, (1R,2R)-(-)-1,2-diaminocyclohexane, (1R,2R)-(-)-1,2-diphenylethylenediamine, or o-phenylenediamine, in the template synthesis with 2 led directly to tetradentate P-N-N-P iron(II) complexes trans-[Fe(NCMe)(2)(PCy(2)CH(2)CH=N-Q-N=CHCH(2)PCy(2)](BPh(4))(2) (Q = CH(2)CH(2), 6a; Q = (1R,2R)-cyclo-C(6)H(10), 6b; Q = (1R,2R)-CHPhCHPh, 6c; Q = C(6)H(4), 6d). In contrast, similar reactions under the same conditions with dimer 1 led to complexes mer-[Fe(P-N-N)(2)](2+) as reported previously. Complexes 6a and 6b have been characterized by X-ray diffraction and exhibited large P-Fe-P bond angles of 112.92(2) and 111.96(4) degrees, respectively.

Asymmetric catalysis using iron complexes – ‘Ruthenium Lite’?

“…It was not clear, in the case of asymmetric transfer hydrogenation, 15 whether the C=N bonds in the ligands were reduced to single bonds in the same way that they are speculated to be in the pressure hydrogenation reactions described earlier, with the subsequent mechanistic implications. The reaction is however a very practical one, with the iron catalysts exhibiting higher 20 TOF values than have been measured for the more established ruthenium-based transfer hydrogenation catalysts. The iron catalysts are also tolerant to a number of functional groups in the substrate.…”

“…The Haber process for ammonia production, 20 dating back to 1909, depends on an iron catalyst, 12 and many enzymes, for example hydrogenases, contain iron at their active sites. 13 Compared to other transition metals, iron is significantly less developed as a homogeneous catalyst for organic reactions, particularly asymmetric processes.…”

“…An alternative, and highly effective method, which involved the iron-templated complex formation through the in situ condensation of the chiral diamine component with the precursor phosphinoaldehyde dimer. 18,19,20 Of the complexes tested, 1 proved to be an effective in the 60 asymmetric hydrogenation of ketones (Scheme 1). At a relatively low loading of ca 0.45 mol% (S/C 225), which is typically used for many Ru(II)-based asymmetric catalytic systems, acetophenone was reduced in 40% conversion and 27% ee after 18h at 50 o C (25 atm H 2 ).…”

“…19 This complex could be assembled using an efficient metal-templated process in which the components formed the complex following their combination in a one pot process (Scheme 4). 18,20 The process 25 greatly facilitates the synthesis of the complexes and is a method that has not to date been successfully applied to the equivalent ruthenium complexes. Complex 20 proved to be an excellent catalyst for ketone 40 reduction using isopropanol as the reducing agent, not just with respect to activity but also enantioselectivity ( Figure 8).…”

“…In many cases, full conversions were observed. 20 A range of complexes containing ligands with P=N bonds, of which 43 and 44 are representative examples, and representing an interesting variation on the traditional 'PNNP' tetradentate ligand were introduced by Le Floch et al 49 Although not asymmetric, their modular nature and derivation 25 from 1,2-diamines opens possibilities for future asymmetric versions. Complex 43, formed with an Fe(II) salt, was characterised by X-ray crystallography and bears some resemblance to the Morris systems described earlier.…”