Several hydride Mn(I) and Re(I) PNP pincer complexes were applied as catalysts for the homogeneous chemoselective hydrogenation of aldehydes. Among these, [Mn(PNP-iPr)(CO)2(H)] was found to be one of the most efficient base metal catalysts for this process and represents a rare example which permits the selective hydrogenation of aldehydes in the presence of ketones and other reducible functionalities, such as C=C double bonds, esters, or nitriles. The reaction proceeds at room temperature under base-free conditions with catalyst loadings between 0.1 and 0.05 mol% and a hydrogen pressure of 50 bar (reaching TONs of up to 2000). A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed. Analogous isoelectronic and isostructural Re(I) complexes were only poorly active.

In the current investigation, the reaction of [Cr(CO)6] with the ligand precursor PO(C-Br)OP-tBu (1) was investigated. When a suspension of [Cr(CO)6] and 1 in toluene was transferred into a sealed microwave glass vial and stirred for 3 h at 180 o C the square-planar Cr(II) complex [Cr(POCOP-tBu)Br] (2) was obtained. Treatment of 2 with 1 equiv of Li[HBEt3] in THF led to the formation of the borohydride complex [Cr(POCOP-tBu)(κ 2-BH4)] (3). Exposure of a toluene solution of 3 to NO gas (1 bar) at room temperature affords the Cr(I) complex [Cr(POCOP-tBu)(NO)(κ 2-BH4)] (4). Alternatively, 4 was also obtained by reacting [Cr(POCOP-tBu)(NO)(Br)] (5) with Li[HBEt3]. Based on magnetic and EPR measurements as well as DFT calculations, compounds 4 and 5 adopt a low-spin d 5 configuration and feature a nearly linear bound NO ligand suggesting Cr I NO + rather than Cr II NO • character. The reaction of 2 with 1 equiv of LiCH2SiMe3 in toluene afforded the square planar alkyl complex [Cr(POCOP-tBu)(CH2SiMe3] (6) in 57% yield. This compound is catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 0.5 mol%. X-ray structures of all complexes are presented.

In the current investigation, the reaction of Fe(CO) with the ligand precursor 2-chloro-N,N-bis(diisopropylphosphanyl)-N,N-diethylbenzene-1,3-diamine (P(C-Cl)P- iPr) (1) was investigated. When a suspension of Fe(CO) and 1 in CHCN was transferred in a sealed microwave glass vial and stirred for 18 h at 110 °C the complex [Fe(PCP- iPr)(CO)Cl] (2) was obtained. In an attempt to prepare the hydride Fe(II) complex [Fe(PCP- iPr)(CO)H] (3), 2 was reacted with 1 equiv of Li[HBEt] in THF. Instead of ligand substitution, this complex underwent a one electron reduction which led to the formation of the low-spin d Fe(I) complex [Fe(PCP- iPr)(CO)] (4). Exposure of a benzene solution of 4 to NO gas (1 bar) at room temperature affords the diamagnetic complex [Fe(PCP- iPr)(CO)(NO)] (5). This is the first iron PCP nitrosyl complex. Protonation of 5 with HBF·EtO affords the cationic Fe(0) complex [Fe(κ P,CH,P-P(CH)P- iPr)(CO)(NO)]BF (6) which features an η-C-H agostic bond. Even with relatively weak bases such as NEt the agostic C-H bond can be deprotonated with reformation of the starting material 5. Therefore, protonation of 5 is completely reversible.

In the present study, Cr(III) complexes of the type trans-[Cr(PCP NEt -iPr)(solvent)Cl 2 ] (solvent = CH 3 CN, THF), the Cr(0) complex [Cr(κ 3 P,CH,P-P(CH)P NEt -iPr)-(CO) 3 ] which features an η 2 -C aryl −H agostic bond as well as seven coordinate cationic chloro carbonyl Mo(II) and W(II) complexes of the type [M(PCP NEt -iPr)(CO) 3 Cl] featuring PCP pincer ligands based on a 1,3-diaminobenzene scaffold were prepared and characterized. The seven coordinate chloro tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. Another interesting aspect is a rapid and reversible addition of one CO ligand across the metal−C ipso bond. The mechanism of the dynamic process of the chloro carbonyl complexes was investigated by means of DFT calculations. The Mo(II) and W(II) tricarbonyl complexes could be reduced to the respective anionic Mo(0) and W(0) complexes [Mo(PCP NEt -iPr)(CO) 3 ] − and [Mo(PCP NEt -iPr)(CO) 3 ] − . These air sensitive compounds are readily protonated by MeOH to form agostic and hydride species of the types [Mo(κ 3 P,CH,P-P(CH)P NEt -iPr)(CO) 3 ] and [W(PCP NEt -iPr)(CO) 3 H]. Structures of representative complexes were determined by X-ray single crystal analyses.

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn2(CO)10] with the ligands (P(C-Br)PCH2-iPr) (1a) and (P(C-Br)PO-iPr) (1b), Mn(I)...