Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of S-donors in ligand-assisted catalysis using first-row metals has not been broadly investigated. Herein is described a combined experimental and computational mechanistic study of the dihydroboration of nitriles with pinacolborane (HBpin) catalyzed by the Mn(i) complex, Mn(κ3-SMeNS)(CO)3, that features thioether, imine, and thiolate donors. Mechanistic studies revealed that catalysis requires the presence of UV light to enter and remain in the catalytic cycle and evidence is presented for loss of two CO ligands. Stoichiometric reactions showed that HBpin reduces the imine N 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 C of the ligand backbone in the absence of nitrile, forming an inactive off-cycle by-product. DFT calculations showed that the bifunctional thiolate donor, coordinative flexibility of the SMeNS ligand, and access to an open-shell intermediate are all crucuial to accessing low-energy intermediates during catalysis.


VI. Crystallographic Details
Crystals of 1 were mounted on thin glass fibers using cyanoacrylate glue and cooled to 200 ± 2 K during data collection. The data were collected on a Bruker single-crystal diffractometer equipped with a sealed Mo tube source (wavelength 0.71073 Å) and APEX II CCD detector.
The raw data collection and processing were performed with the Bruker APEX II software package. 5 Semi-empirical absorption correction based on equivalent reflections was applied. 6 Systematic absences and unit cell parameters were consistent with monoclinic P21/n (#14) for 1. The structure was solved by intrinsic phasing and refined with a full-matrix least-squares procedure based on F 2 , using SHELXL. 7 All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms bonded to carbon atoms were placed in idealized positions.

Refinement details for Mn(κ 3 -S Me NS)(CO)3 (1).
The structure was refined without additional restraints / constraints. No disorder was present.

VII. DFT Calculations
All calculations were carried out using DFT 8 as implemented in the Jaguar 9.1 suite 9 of ab initio quantum chemistry programs. Geometry optimizations were performed with the B3LYP functional including Grimme's D3 dispersion correction. [10][11][12][13][14][15] The 6-31G** basis set was used for main group atoms and Mn was represented using the Los Alamos LACVP basis set that includes relativistic effective core potentials. The energies of optimized structures were reevaluated by additional single-point calculations on each optimized geometry using Dunning's correlation consistent triple-ζ basis set cc-pVTZ(-f) 16  Two undesirable intermediates 9 and 10 are depicted in Figure S29. In accordance with the catalytic incapability of the off-cycle intermediate 9, HBpin insertion to imine gives rise to a highly exergonic pathway. The intermediate 9 will be a thermodynamic sink considering further hydride transfer steps. Additional CO can bind to 9 resulting in 9`, while the two energetically close intermediates are unable to participate in the catalytic cycle. Manganese hydride species, 10 is 23.8 kcal/mol higher in energy than 2. Coordination of nitrile followed by a Bpin moiety transfer step will furnish a more unreasonable pathway than the proposed mechanism in terms of energy. We envision that the instability of the Manganese hydride species is attributed to the SNS ligand scaffold generating a weak ligand field. The catalytic pathway initiates with removing two COs which are strong field ligands. Regarding the Mn(I) center supported by the soft SNS ligand, the hydride moiety which contains dense electrons is placed in a deleterious environment. Figure S30. Energy profile including all spin states. Figure S31. Energy profile for the hydride transfer in the presence and absence of coordinating THF. The oxygen atom of HBpin can bind to the Mn center in lieu of THF as depicted in 4-TS``. In the outer sphere mechanism traversing 4-TS, the π-accepting CO ligand should be in the trans position to the transient amido moiety. In compliance with the formation of a sterically hindered Mn-S-B-O metallacycle in 4-TS``, a conformer featuring a distinct position of the CO ligand trans to the thiolate moiety, 4-TS``` is favored by 5.8 kcal/mol. Due to the strong trans effect of the CO ligand, the thiolate moiety departs from the Mn center resulting in 3.6 Å of Mn-S distance. Relieved structural hindrance is reflected in the lower barrier of 35.8 kcal/mol, while the proposed pathway traversing 4-TS having barrier of 31.1 kcal/mol is still favored. The amido species generated during hydride transfer in 4-TS``` is not stabilized by an electron-withdrawing ligand. Moreover, the Mn(I) species adopts a trigonal bipyramidal geometry giving rise to the 16-electron species as Mn-S bond cleaves, while 4-TS affords 18-electron species comprising the stabilized amido moiety.