Synthesis and characterization of TADDOL-based chiral group six PNP pincer tricarbonyl complexes.

ABSTRACT: The new chiral PNP pincer ligand N 2,N 6-bis((3aR, 8aR)-2,2-dimethyl-4,4,8,8-tetraphenyltetrahydro[1,3]dioxolo[4,5-e][1,3,2]dioxaphosphepin-6-yl)pyridine-2,6-diamine (PNP-TADDOL) was synthesized in 80% isolated yield. Complexes of the type [M(PNP-TADDOL)(CO)3] (M = Cr, Mo, and W) were prepared via a solvothermal approach. This methodology constitutes a fast, simple, and practical synthetic method to obtain complexes of that type in high isolated yields. The X-ray structure of the molybdenum complex is presented.

Synthesis and characterization of cationic dicarbonyl Fe(II) PNP pincer complexes.

ABSTRACT: In the present work, we have prepared a series of octahedral Fe(II) complexes of the type trans-[Fe(PNP)(CO)2Cl]+-PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine. These complexes are formed irrespective of the size of the substituents at the phosphorus sites and whether cis-[Fe(PNP)(Cl2)(CO)] or trans-[Fe(PNP)(Cl2)(CO)] are reacted with CO in the presence of 1 equiv of silver salts. X-ray structures of representative complexes are presented. Based on simple bonding considerations the selective formation of trans-dicarbonyl Fe(II) complexes is unexpected. In fact, DFT calculations confirm that trans-dicarbonyl complexes are indeed thermodynamically disfavored over the respective cis-dicarbonyl compounds, but are favored for kinetic reasons.

Synthesis and reactivity of BINEPINE-based chiral Fe(II) PNP pincer complexes.

ABSTRACT: A new asymmetric chiral PNP ligand based on the 2,6-diaminopyridine scaffold featuring a R-BINEPINE moiety was prepared. Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv of PNP-iPr,BIN at room temperature afforded the coordinatively unsaturated paramagnetic complexes [Fe(PNP-iPr,BIN)X2]. The structure of [Fe(PNP-iPr,BIN)Cl2] is described. Both complexes react readily with the strong π-acceptor ligand CO in solution to afford selectively the diamagnetic complexes trans-[Fe(PNP-iPr,BIN)(CO)X2] in quantitative yield. Due the lability of the CO ligand, these complexes are only stable under a CO atmosphere and isolation in pure form was not possible. The preparation of the carbonyl hydride complex [Fe(PNP-iPr,BIN)(H)(CO)Br] was achieved albeit in low yields via a one pot procedure by treatment of [Fe(PNP-iPr,BINEP)Br2] with CO and subsequent reaction with Na[HBEt3]. This complex was obtained as an inseparable mixture of two diastereomers in a ca. 1:1 ratio and was tested as catalyst for the hydrogenation of ketones. The catalyst showed acceptable activity under mild conditions (5 bar H2, room temperature) with yields up to >99 % within 18 h.

Twinning of three Fe-PNP pincer complexes interpreted according to order-disorder (OD) theory.

The systematic twinning of three 2,6-diaminopyridine-based Fe-PNP complexes is interpreted using order-disorder (OD) theory. The monoclinic [Fe(0)(PNP(Et)-(i)Pr)(CO)2] [P112(1)/b, Z' = 4] possesses pseudo-orthorhombic metrics and crystallizes as a reflection twin by pseudo-merohedry with the twin plane (100). The structure is made up of layers with idealized p2(1)a(b) symmetry. The a glide planes of adjacent layers do not overlap, leading to OD polytypism. trans-[Fe(II)(PNP-Et)Br2(CO)] [P2(1)/n, Z' = 1] is systematically twinned via twofold rotation about [001]. It is made up of OD layers with idealized p2(1)2(1)(2) symmetry. OD polytypism is caused by the twofold rotation axes of adjacent layers which do not overlap. [Fe(II)(κ(2)P,N-PNP-(i)Pr,TAD)Cl2]·THF [P1, Z^{\prime} = 2] is systematically twinned via a twofold rotation about [010]. It is made up of layers with idealized p121(1) symmetry. OD polytypism is caused by screw rotations relating adjacent layers with an intrinsic translation along a fourth of a primitive lattice vector. In all three structures the twin individuals are a polytype with a maximum degree of order (MDO) and at the twin interface is located a fragment of the second MDO polytype.

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.

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.

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

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)2Cl]+ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)2]+ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H2 to give cis-[Fe(PNP-iPr)(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)2Cl]+. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNPMe-iPr)(CO)2Cl]+ was prepared, showing no reactions with Zn and H2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)2H]+ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)2]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)2Cl]+ or [Fe(PNP-iPr)Cl2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)2] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)2H]+. X-ray structures of both cis-[Fe(PNP-iPr)(CO)2H]+ and [Fe(PNP-iPr)(CO)2] are presented.

Synthesis and Characterization of Hydrido Carbonyl Molybdenum and Tungsten PNP Pincer Complexes.

In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(µ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

2-[(Diisopropyl-thio-phosphor-yl)amino]-pyridinium tetra-fluoro-borate.

The title compound, C(11)H(20)N(2)PS(+)·BF(4) (-), is a salt of 2-(diisopropyl-thio-phospho-ryl-amino)-pyridine, a chelating bidentate ligand that furnishes an S atom as a soft donor and a pyridine N atom as a hard atom for transition-metal complexation. The title salt crystallizes with two formula units in the asymmetric unit. The two independent cations are protonated at the pyridine N atoms and have the S atoms syn-oriented to them so as to form bent intra-molecular N-H⋯S hydrogen bonds, one of which one is bifurcated by involving also an N-H⋯F inter-action. The phospho-ryl-amino NH groups form near linear hydrogen bonds to proximal tetra-fluoro-borate anions. Five weak C-H⋯F and three weak C-H⋯S inter-actions link the constituents into a three-dimensional framework. As a result of the crystal packing, the two cations differ notably in conformation, as can be seen from the S-P-N-C torsion angles of -18.7 (1)° in the first and -35.1 (1)° in the second cation.