A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0) complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical

Using the potentially tridentate N,N′-bis(N-heterocyclic silylene)pyridine [SiNSi] pincer-type ligand, 2,6-N,N′-diethyl-bis[N,N′-di-tert-butyl(phenylamidinato)silylene] diaminopyridine, led to the first isolable bis(silylene)pyridine-stabilized manganese(0) complex, {κ3-[SiNSi]Mn(dmpe)} 4 (dmpe = (Me2P)2C2H4), which represents an isolobal 17 VE analogue of the elusive Mn(CO)5 radical. The compound is accessible through the reductive dehalogenation of the corresponding dihalido (SiNSi)Mn(ii) complexes 1 (Cl) and 2 (Br) with potassium graphite. Exposing 4 towards the stronger π-acceptor ligands CO and 2,6-dimethylphenyl isocyanide afforded the related Mn(0) complexes κ2-[SiNSi]Mn(CO)3 (5) and κ3-[SiNSi]Mn(CNXylyl)2(κ1-dmpe) (6), respectively. Remarkably, the stabilization of Mn(0) in the coordination sphere of the [SiNSi] ligand favors the d7 low-spin electronic configuration, as suggested by EPR spectroscopy, SQUID measurements and DFT calculations. The suitability of 4 acting as a superior pre-catalyst in regioselective hydroboration of quinolines has also been demonstrated.


General considerations
All experiments and manipulations were carried out using standard Schlenk techniques or in an MBraun inert atmosphere glovebox under dry oxygen free nitrogen atmosphere. Hexane, Et2O, toluene and THF were dried by standard methods. Benzene-d6 and THF-d8 were stirred over a sonicated potassium mirror for a period of 24 h and recondensed into a Schlenk tube containing activated 4 Å mol sieves. The starting material 2,6-N,N'-diethyl-bis[N,N'-di-tertbutyl(phenylamindinato)silylene] amidopyridine [SiNSi] was prepared according to literature procedure. [1] Potassium graphite was prepared by reacting potassium with previously dried graphite in a 1:8 ratio at 160 °C for 2 h under dried nitrogen. MnCl2, MnBr2 were purchased from Acros Organics. 1,2-Bis(dimethylphosphino)ethane was purchased from abcr GmbH.
2,6-Dimethylphenyl isocyanide was bought from Alfa Aesar. NMR spectra were recorded on a Bruker AV 400 or 500 Spectrometer. The 1 H and 13  constants. [2] For simulation and analysis of the data, the program "JulX2", written by Dr.

Optimization of reaction conditions
In a glove box, suitable catalyst (1-6) (5.0 mol%, 5.0 µmol) and reducing agent (H-BR2, 0.2 mmol ) was added to a solution containing quinoline (0.1 mmol) in 0.45 mL C6D6 in a J. Young type NMR tube. After taken out of the glovebox and heated at specified temperature for specified time. Reaction progress was monitored by 1 H-NMR spectroscopy.

Preparative scale reaction
In a N2 filled glovebox, a 25 mL schlenk tube was charged with quinoline (118 µL, 1.0 mmol), HBpin (2.0 equiv, 2.0 mmol), and 5 mol% of compound 4 (4.4 mg, 0.05 mmol). To this 1.0 mL of C6D6 was added. The flask was sealed and taken outside. The reaction mixture was heated at 50 °C for 24 hours. After cooling the flask to room temperature, the volatiles were removed.
The residue was dissolved in hexane and product was obtained upon recrystallization.

HBpin using [SiNSi]Mn(dmpe) (4) as a Precatalyst
In a glove box, 5 mol% [SiNSi]Mn(dmpe) (4.4 mg, 5.0 µmol) was added to a solution containing N-heteroarene (0.1 mmol) in 0.5 mL C6D6 in a J. Young type NMR tube. Then, pinacolborane HBpin (0.2-0.4 mmol) was added to the resulting mixture. After taken out of the glovebox and heated at 50 °C for specified time, it was measured by NMR spectroscopy. The NMR yields were calculated using mesitylene as an internal standard.

General procedure for the synthesis of DBpin
DBpin was synthesized according to literature procedure. [

NMR study of reaction of 4 with pinacolborane (HBpin)
In a N2 filled glove box, 20 mg 4 (0.02 mmol) and 11.5 µL of HBpin (0.08 mmol), were dissolved in 0.5 mL C6D6 in a J. Young NMR tube. The NMR tube was taken out and heated at 50 °C.
the reaction progress was monitored by 1 H-NMR. After 6 h, formation of a new diamagnetic species was indicated in 1 H NMR spectrum with a signal at -9.6 ppm ( Figure S32). 1 H-31 P HMQC NMR spectrum exhibited a correlation NMR signal corresponding to the hydridic signal ( Figure S28).

Kinetic isotope effect:
Two NMR-samples were prepared, each containing 5 mol% of 4 (4.4 mg, 5.0 µmol) in 0.5 mL C6D6 containing mesitylene as internal standard (0.05 mmol, 6.9 µL), quinoline (0.1 mmol, 12.8 µL) and 2.0 equivalents of HBpin or DBpin. 1 H-NMR spectra were obtained at regular intervals approx. 1 h and the NMR tubes were shaken after each measurement. The concentration of quinoline was plotted against time and the data points were fitted with a linear function (R 2 = 0.99/0.98).   Hydrogen atoms are omitted for clarity.

Computational details
The DFT calculations were performed with Gaussian 16 (Revision A.03) program. 9 Geometry optimizations and frequency calculations were conducted at the PBE0 10 -D3BJ 11 /Def2-SVP 12 ∼ma-TZVP 13-14 level of theory in the gas phase. The ma-TZVP is the abbreviation of def2-TZVP with minimal augmentation, proposed by Truhlar and co-workers. All the principal interacting orbital (PIO) [15][16] and principal interacting spin orbitals (PISO) 17 analyses were performed by NBO 7.0 program 18 at the same level based on the optimized structure. All the orbitals were plotted with the help of Multiwfn 19 and VMD programs. 20 We can't locate the structure of Mn(CO)5 in the quartet and sextet states by a full optimization as one of the CO ligands will dissociate during the optimization. The relative electronic energy of compound Table S12 is obtained by a partial optimization with imaginary frequencies by fixing the Mn-C bonds at 1.836 and 1.837 Å in the quartet and sextet states, respectively, which are the average of Mn-C bonds in compounds 5 in the quartet and sextet states.               interacting spin molecular orbital). The PBI is used to quantify the strength of the interaction.

Mn(CO)5 in
The total PBI value of Mn-C2 is 0.70, with the contribution of 0.36 from α system and 0.34 form β system. The isosurfaces with 0.080 au isovalue are plotted for the PISO pairs. Here 1 st , 2 nd and 3 rd mean the first, second and third PISO pairs, respectively.