Synthesis and characterization of bis- and tris-carbonyl Mn(I) and Re(I) PNP pincer complexes

Abstract A series of neutral bis- and cationic tris-carbonyl complexes of the types cis-[M(κ3P,N,P-PNP)(CO)2Y] and [M(κ3P,N,P-PNP)(CO)3]+ was prepared by reacting [M(CO)5Y] (M = Mn, Re; Y = Cl or Br) with PNP pincer ligands derived from the 2,6-diaminopyridine, 2,6-dihydroxypyridine, and 2,6-lutidine scaffolds. With the most bulky ligand PNPNH-tBu, the cationic square-pyramidal 16e bis-carbonyl complex [Mn(PNPNH-tBu)(CO)2]+ was obtained. In contrast, in the case of rhenium, the 18e complex [Re(PNPNH-tBu)(CO)3]+ was formed. The dissociation of CO was studied by means of DFT calculation revealing in agreement with experimental findings that CO release from [M(κ3P,N,P-PNP)(CO)3]+ is in general endergonic, while for [Mn(κ3P,N,P-PNPNH-tBu)(CO)3]+, this process is thermodynamically favored. X-ray structures of representative complexes are provided. Graphical abstract Electronic supplementary material The online version of this article (10.1007/s00706-018-2307-7) contains supplementary material, which is available to authorized users.


Introduction
In recent years, manganese pincer complexes, where the metal centers adopt a formal oxidation state of +I, have received considerable importance in the field of homogeneous catalysis [1][2][3][4][5][6]. In comparison to manganese, rhenium pincer complexes remained comparatively unexplored until very recently but are becoming increasingly important as catalysts for hydrogenation/dehydrogenation reactions [7][8][9][10]. The most common ligand architecture is a PNP pincer system featuring an aromatic pyridine backbone with phosphine donors in the two ortho positions linked via CH 2 , O, NH, or NMe moieties. In particular, Mn(I) halo and hydride complexes of the type cis-[Mn(PNP)(CO) 2 Y] (Y = Cl, Br, H) as shown in Scheme 1 were found to be highly active catalysts in hydrogenation Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0070 6-018-2307-7) contains supplementary material, which is available to authorized users.

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reactions of carbonyl compounds, including CO 2 , as well as nitriles to yield alcohols, formate, and amines, respectively. Moreover, these types of complexes turned out to be also very active catalysts for the opposite process, i.e., dehydrogenation reactions of alcohols to obtain carbonyl compounds. These reactive intermediates are utilized for follow-up reactions such as condensation reactions in the presence of amines to yield functionalized amines, imines, or heterocycles such as pyridines, quinolines, or pyrroles [11,12].
All neutral bis-carbonyl and cationic tricarbonyl complexes, respectively, are orange and off-white air-stable compounds. Selected NMR and IR spectroscopic data are provided in Table 1. In the IR spectrum, complexes 2 and 4 exhibit the two carbonyl stretching frequencies typical for a cis-CO arrangement. Complexes 3 and 5 give rise to two or three strong absorption bands typical of a mer CO arrangement. In 13 C{ 1 H} NMR, the two or three CO ligands give rise to low field triplets in the range of 238-196 ppm. Due to the quadrupole moment of 55 Mn (I = 5/2), the resonances of the manganese compounds are not always fully resolved giving rise to rather broad signals. Also, in 31 P{ 1 H} NMR spectra, broad singlets are observed.
In addition to the spectroscopic characterization of all complexes, the molecular structures of complexes [Mn(PNP NH -iPr)(CO) 3 3 ]Br·acetone (5bBr·acetone) and [Re(PNP NH -tBu)(CO) 3 ]Br (5eBr) were determined by X-ray crystallography. Structural views are depicted in Figs. 1, 2, 3, 4, 5 with selected bond distances and angles given in the captions. Complexes 3aOTf, 3dBr, 5bBr, and 5eBr adopt a distorted octahedral geometry around the metal center. The PNP ligand is coordinated to the iron center in a typical tridentate meridional mode, with P-M-P angles between 154.1° and 164.8°. The C (CO) -M-C (CO) angles vary between 165.1 and 175.6°. The coordination geometry of Complex 3eBF 4 is a square pyramid with N(1), P(1), P(2), and C(23) defining the basal plane and C(22) defining the apex.
In the presence of a strong base such as NaH, [Mn(PNP NH -tBu)(CO) 2 ] + (3e) was readily deprotonated to afford the neutral 16e complex [Mn(PNP N -tBu)(CO) 2 ] (6) in 95% isolated yield (Scheme 4). In the 31 P{ 1 H} NMR spectrum, the now inequivalent phosphorous atoms of this complex exhibit a characteristic AB pattern with signals at 145.7 and 142.2 ppm (J PP = 84.5 Hz). The carbonyl stretches (ν CO = 1913, 1838 cm −1 ) are indicative of an increased backbonding effect relative to the cationic bis-carbonyl complex 3e (ν CO = 1936, 1865 cm −1 ). Recently, Sortais et al.   3 ] + (3e), this process is thermodynamically favored by − 25.5 kJ/mol. This may be attributed to the bulkiness of the PNP NH -tBu ligand, together with the fact that the Mn-C CO bonds are weaker than the Re-C CO bonds [24].

Conclusion
In sum, we have prepared a series of coordinatively satu- with PNP pincer ligands derived from the 2,6-diaminopyridine, 2,6-dihydroxypyridine, and 2,6-lutidine scaffolds. In the case of the most bulky ligand PNP NH -tBu, the cationic calculations. This may be attributed to the bulkiness of the PNP NH -tBu ligand, but also due to the fact that Mn-C CO bonds are generally weaker than Re-C CO bonds. Several complexes were also characterized by single crystal X-ray diffraction studies.

[[2,6-Bis[(diisopropylphosphanyl)methyl]pyridine](tricarbonyl)manganese(I)] bromide, [Mn(PNP CH2 -iPr)(CO) 3 ]Br (3dBr, C 22 H 35 BrMnNO 3 P 2 )
This complex was prepared analogously to 2c with 172 mg PNP CH2 -iPr (1d, 0.50 mmol) and 137 mg [Mn(CO) 5 Br] (0.50 mmol) as starting materials. Crystals suitable for X-ray diffraction were grown by diffusion of a n-pentane into a CH 2 Cl 2 solution of 3dBr. Yield: 265 mg (95%); 1 H NMR of [Mn(PNP CH2 -iPr)(CO) 3   and [Re(PNP NH -tBu)(CO) 3 ]Br (5eBr) (CCDC numbers 1865227-1865231) were collected at T = 100 K in a dry stream of nitrogen on a Bruker Kappa APEX II diffractometer system using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine sliced φ-and ω-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS or TWINABS [30]. The structures were solved by the dual-space approach implemented in SHELXT [31] and refined against F 2 with SHELXL [32]. Non-hydrogen atoms were refined anisotropically. The H atoms connected to C atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. The amine-Hs were located from difference Fourier maps and refined freely (3eBF 4 ) or restrained to a N-H distance of 0.87 Å (5eBr). The Mn atoms and CO ligands in 3eBF 4 were refined as disordered about two positions. Contributions of disordered solvent molecules to the intensity data were removed for 5eBr using the SQUEEZE routine of the PLATON [33] software suite. Molecular graphics were generated with the program MERCURY [34].

Computational details
Calculations were performed using the Gaussian 09 software package [35] with the B3LYP functional without symmetry constraints, the Stuttgart/Dresden ECP (SDD) basis set to describe the electrons of the manganese and rhenium atoms and a standard 6-31G** basis for all other atoms as already described previously [36].