Reversible Addition of CO to Coordinatively Unsaturated High-Spin Iron(II) Complexes

Phosphinoquinoline supported CoII, NiII, and FeII complexes: divergent behaviour upon reduction

The reduction of [CoLBr2], a CoII complex supported by a diisopropylphosphinoquinoline (L) ligand, induced a ligand coupling giving access to a (PNNP) supported CoII complex which was isolated in 70% yield. This complex was formed using a minimum of 2 equivalents of a reductant (either Mn or KC8). The fate of [CoLBr2] in the presence of 1 equivalent of a reductant was more difficult to study; nevertheless, a CoI complex was characterised in the solid state. In order to determine whether this ligand coupling could occur with other 3d metals, L supported FeII and NiII complexes were synthesised. While no compound could be identified upon reduction of [FeLBr2], both [NiLBr2] and [NiL2Br](Br) led to the reduction at the metal center allowing the isolation of an original Ni0 trimer in a satisfactory yield. This study shows the different behaviours of these 3d metal complexes in the presence of a reductant.

A 1,2,3-triazole-derived pincer-type mesoionic carbene complex of iron(II): carbonyl elimination and hydrosilylation of aromatic aldehydes via the concerted reaction with hydrosilane and a base

Iron complexes bearing 1,2,3-triazol-5-ylidene were synthesized and applied to the reaction with hydrosilane and homogeneous catalytic hydrosilylation of aromatic ketones and aldehydes. Addition of a free carbene to a solution of Fe(CO)₄Br₂ yielded an octahedral, diamagnetic and cationic iron(II) complex [Fe(1,2,3-triazolylidene)(CO)₂Br]⁺. Pyrolysis of the dicarbonyl complex eliminated the two CO ligands to form a paramagnetic four-coordinate complex. A theoretical study using DFT calculations indicated that the spin state changed from singlet to quintet during ligand elimination. Investigations of the successful hydrosilylation of acetophenone and benzaldehyde derivatives using MIC-iron(II) bromide suggested the importance of the base for efficient conversion in the catalytic process. The bromide-to-hydride exchange reaction, transmetallation, of MIC-iron(II) bromide in the presence of KO^tBu and HSi(OEt)₃ which could occur in the initial process of hydrosilylation was proposed, and supported by a theoretical study.

Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Group 8 complexes of π-extended N-heterocyclic ligands present some unexpected results. Benzannulation shifts the lowest energy excitation to the blue by impacting the character of the transition rather than affecting frontier orbital energies.

Iron catalyzed oxidation of benzylic alcohols to benzoic acids

The bidentate N,O-ligands phenol-pyrazole (HL1), naphthol-pyrazole (HL2) and the commercially available ligand 5-methylphenol-benzotriazole (HL3) were used for the synthesis of novel iron(iii) complexes. The mononuclear iron complexes [FeCl(L1)2] (1), [FeCl(L2)2] (2) and [FeCl(L3)2] (3) are stable to air and moisture, both in the solid state as well as in solution, while the dinuclear, μ-oxido bridged complex [{Fe(L1)2}2(μ-O)] (1a) is air sensitive. All four complexes 1, 2, 3 and 1a were investigated for their catalytic activity in the direct one-pot oxidation of primary alcohols to carbonic acids with 30% aq. hydrogen peroxide (H2O2) as the oxidation agent. The activity in oxidation reactions of the isolated, mononuclear complexes 1-3 was further compared to their in situ prepared analogues IS1-3. Experimentally obtained results indicate a tendency of higher activity for the oxidation of primary alcohols for the in situ prepared complexes. In conclusion, the oxidation of aromatic primary alcohols to carboxylic acids using isolated iron(iii) complexes and in situ generated complexes in the presence of H2O2 results in good to high yields. The reaction is straight-forward, clean and generates water as the only by-product.

Synthesis, coordination behavior and structural features of chiral iron(ii) PNP diferrocene complexes

New chiral PNP and PNHP ferrocene ligands and the corresponding Fe(ii) complexes of PNP ligands with the general formula [Fe(PNP)X₂] (X = Cl, Br) were synthesized.

Synthesis and reactivity of TADDOL-based chiral Fe(II) PNP pincer complexes-solution equilibria between κ(2)P,N- and κ(3)P,N,P-bound PNP pincer ligands

Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv. of the asymmetric chiral PNP pincer ligands PNP-R,TAD (R = iPr, tBu) with an R,R-TADDOL (TAD) moiety afforded complexes of the general formula [Fe(PNP)X2]. In the solid state these complexes adopt a tetrahedral geometry with the PNP ligand coordinated in κ(2)P,N-fashion, as shown by X-ray crystallography and Mössbauer spectroscopy. Magnetization studies led to a magnetic moment very close to 4.9μB reflecting the expected four unpaired d-electrons (quintet ground state). In solution there are equilibria between [Fe(κ(3)P,N,P-PNP-R,TAD)X2] and [Fe(κ(2)P,N-PNP-R,TAD)X2] complexes, i.e., the PNP-R,TAD ligand is hemilabile. At -50 °C these equilibria are slow and signals of the non-coordinated P-TAD arm of the κ(2)P,N-PNP-R,TAD ligand can be detected by (31)P{(1)H} NMR spectroscopy. Addition of BH3 to a solution of [Fe(PNP-iPr,TAD)Cl2] leads to selective boronation of the pendant P-TAD arm shifting the equilibrium towards the four-coordinate complex [Fe(κ(2)P,N-PNP-iPr,TAD(BH3))Cl2]. DFT calculations corroborate the existence of equilibria between four- and five-coordinated complexes. Addition of CO to [Fe(PNP-iPr,TAD)X2] in solution yields the diamagnetic octahedral complexes trans-[Fe(κ(3)P,N,P-PNP-iPr,TAD)(CO)X2], which react further with Ag(+) salts in the presence of CO to give the cationic complexes trans-[Fe(κ(3)P,N,P-PNP-iPr,TAD)(CO)2X](+). CO addition most likely takes place at the five coordinate complex [Fe(κ(3)P,N,P-PNP-iPr,TAD)X2]. The mechanism for the CO addition was also investigated by DFT and the most favorable path obtained corresponds to the rearrangement of the pincer ligand first from a κ(2)P,N- to a κ(3)P,N,P-coordination mode followed by CO coordination to [Fe(κ(3)P,N,P-PNP-iPr,TAD)X2]. Complexes bearing tBu substituents do not react with CO. Moreover, in the solid state none of the tetrahedral complexes are able to bind CO.

Synthesis, crystal structures and magnetic behaviour of four coordination compounds constructed with a phosphinic amide-TEMPO radical and [M(hfac)2] (M = Cu(II), Co(II) and Mn(II))

In the present work we describe the synthesis, crystal structures and magnetic properties of four coordination compounds obtained by assembling a new phosphinic amide containing the TEMPO moiety, 1-piperidinyloxy-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl radical (dppnTEMPO), with [M(hfac)2] building blocks (M = Cu(II), Co(II), Mn(II)). The crystal structures of the coordination compounds revealed the usefulness of the functionalized radical to provide discrete or extended architectures. In the copper compound () the ligand is coordinated through the oxygen atom of the NP[double bond, length as m-dash]O linkage to the metal, which exists in a square pyramidal or octahedral geometry. For the cobalt and manganese complexes (), both the phosphinic amide and the nitroxide oxygen atoms are involved in the coordination to the metal leading to one dimensional systems. In the cobalt complex () an interesting spin topology in the zig-zag chain was obtained due to the oxygen atom of the phosphinic amide group being μ2 coordinated to two cobalt(ii) ions. The magnetic behaviour of the coordination compounds shows overall antiferromagnetic interactions involving the metal ion and the organic radical. DFT calculations were performed in order to assign the main path for the magnetic interactions.

Six-coordinate high-spin iron(ii) complexes with bidentate PN ligands based on 2-aminopyridine - new Fe(ii) spin crossover systems

Several new octahedral iron(ii) complexes of the type [Fe(PN(R)-Ph)2X2] (X = Cl, Br; R = H, Me) containing bidentate PN(R)-Ph (R = H, Me) (1a,b) ligands based on 2-aminopyridine were prepared. (57)Fe Mössbauer spectroscopy and magnetization studies confirmed in all cases their high spin nature at room temperature with magnetic moments very close to 4.9μB reflecting the expected four unpaired d-electrons in all these compounds. While in the case of the PN(H)-Ph ligand an S = 2 to S = 0 spin crossover was observed at low temperatures, complexes with the N-methylated analog PN(Me)-Ph retain an S = 2 spin state also at low temperatures. Thus, [Fe(PN(H)-Ph)2X2] (2a,3a) and [Fe(PN(Me)-Ph)2X2] (2b,3b) adopt different geometries. In the first case a cis-Cl,P,N-arrangement seems to be most likely, as supported by various experimental data derived from (57)Fe Mössbauer spectroscopy, SQUID magnetometry, UV/Vis, Raman, and ESI-MS as well as DFT and TDDFT calculations, while in the case of the PN(Me)-Ph ligand a trans-Cl,P,N-configuration is adopted. The latter is also confirmed by X-ray crystallography. In contrast to [Fe(PN(Me)-Ph)2X2] (2b,3b), [Fe(PN(H)-Ph)2X2] (2a,3a) is labile and undergoes rearrangement reactions. In CH3OH, the diamagnetic dicationic complex [Fe(PN(H)-Ph)3](2+) (5) is formed via the intermediacy of cis-P,N-[Fe(κ(2)-P,N-PN(H)-Ph)2(κ(1)-P-PN(H)-Ph)(X)](+) (4a,b) where one PN ligand is coordinated in a κ(1)-P-fashion. In CH3CN the diamagnetic dicationic complex cis-N,P,N-[Fe(PN(H)-Ph)2(CH3CN)2](2+) (6) is formed as a major isomer where the two halide ligands are replaced by CH3CN.

Synthetic, structural, NMR and catalytic studies of phosphinic amide-phosphoryl chalcogenides (chalcogen = O, S, Se) as mixed-donor bidentate ligands in zinc chemistry

Synthesis and catalytic activity of the title compounds are reported.