Photochemical H2 activation by an Zn–Fe heterometallic: a mechanistic investigation

Addition of H2 to a Zn–Fe complex was observed to occur under photochemical conditions (390 or 428 nm LED) and leads to the formation of a heterometallic dihydride complex. The reaction does not occur under thermal conditions and DFT calculations suggest this is an endergonic, light driven process. Through a combined experimental and computational approach, the plausible mechanisms for H2 activation were investigated. Inhibition experiments, double-label cross-over experiments, radical trapping experiments, EPR spectroscopy and DFT calculations were used to gain insight into this system. The combined data are consistent with two plausible mechanisms, the first involving ligand dissociation followed by oxidative addition of H2 at the Fe centre, the second involving homolytic fragmentation of the Zn–Fe heterometallic and formation of radical intermediates.

NMR scale reaction of 1a with [CpFe(CO)2]2 in the presence of H2: 1a (10 mg, 0.02 mmol) and [CpFe(CO)2]2 (3.5 mg, 0.01 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by H2 atmosphere (1 bar).Then, the tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 8 hours and the transformation was monitored by 1 H NMR spectroscopy (Figure S1) leading to a 0.2:0.8mixture of 2a and 3a (97% NMR yield).NMR scale reaction of 2a with H2: 2a (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by H2 atmosphere (1 bar).Then, the tube was exposed to light (LED Kessil lamp, 428 nm-40W) for a total of 6 hours and the transformation was monitored by 1 H NMR leading to a mixture of 3a, [CpFe(CO) 2 ] 2 and 1a in 52%, 10% and 15% NMR yields, respectively.When the tube was exposed to a different light source (LED Kessil lamp 390 nm-40W) for 6 hours, the NMR yields were: 55%, 26% and 20 % for 3a, [CpFe(CO) 2 ] 2 and 1a respectively. 9nthesis of 3a: 2a (50 mg, 0.08 mmol) was dissolved in C6H6 (5 mL) and the yellow solution was transferred into an ampoule.The ampoule was freeze-pump thawed and the atmosphere was changed to dihydrogen (1 bar).The mixture was stirred for 6 hours in the presence of blue LED light (390 nm-40 W).After removing all the solvent under vacuum, diethyl ether (4 mL) was added forming an orange solution which was filtered forming an orange/yellow solid after removal of the solvent (21 mg, 0.04 mmol, 45% yield).NMR scale reaction of 2c with H2: 2c (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by H2 atmosphere (1 bar).Then, the tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 6 hours and the transformation was monitored by 1 H NMR leading to formation of 3c in 43% NMR yield.The yellow solution in benzene was transferred to a vial inside the glovebox and the solvent was removed under vacuum, the residue was washed with cold npentane (1 mL) leading to a yellow solid (4 mg, 0.006 mmol, 36 % yield).NMR scale reaction of 2e with H 2 : 2e (10 mg, 0.016 mmol) were dissolved in benzene-d 6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by H2 atmosphere (1 bar).Then, the tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 6 hours and the transformation was monitored by 1 H NMR leading to formation of 3e in 40% NMR yield.

Cross-over experiments:
A) NMR scale reaction of 2a and 2c with H2: 2a (10 mg, 0.016 mmol) and 2c (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by H2 atmosphere (1 bar).Then, the tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 4 hours and the transformation was monitored by 1 H NMR leading to a 1:1:1:1 mixture of 3a:3c:3d:3e in 30% NMR yield.B) NMR scale reaction of 3a and 3c: 3a (4 mg, 0.006 mmol) and 3c (4 mg, 0.006 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 4 hours and the transformation was monitored by 1 H NMR leading to the formation of 3d and 3e in 21 and 22% NMR yields.

Figure S2
. 1 H NMR spectra, hydride region fragment.From top to bottom: mixture of the four hydrides obtained via experiment A; independently synthesised hydrides species 3a, 3c and 3e.C) NMR scale reaction of 2a and 2c in the absence of H2: 2a (10 mg, 0.016 mmol) and 2c (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 4 hours and the transformation was monitored by 1 H NMR leading to the formation of adducts 2d and 2e in 15% and 15% NMR yield, respectively.Iron dimers Fp, Fp'2 and Fp−Fp' were observed as by-products in 3, 3 and 6% NMR yields, respectively.

Other experiments:
NMR scale reaction of 2a and syngas CO:H2 (1:1 mixture): 2a (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freezepump thawed and the N2 atmosphere was replaced by the syngas atmosphere (1 bar).The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 24 hours and the transformation was monitored by 1 H NMR. Product 3a was not observed.
NMR scale reaction of 2a and 13 CO: 2a (10 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube (2 parallel tubes).Both tubes were then freeze-pump thawed and the N2 atmosphere was replaced by the 13 CO atmosphere (1 bar).Tube a was left at room temperature over 20 hours, while tube b was exposed to light (LED Kessil lamp, 390 nm-40W) for 2 hours and the transformation was monitored by 13 C NMR.The incorporation of 13 CO in the adduct was 40% higher in the photochemical reaction compared to the thermal conditions.
NMR scale reaction of 2a and D2: 2a (10 mg, 0.016 mmol) were dissolved in benzene (0.5 mL) and transferred to a borosilicate J. Young NMR tube with a ferrocene capillary in tol-d8.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by the D2 atmosphere (1 bar).The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for 6 hours and the transformation was monitored by 2 H NMR and 1 H NMR experiments.A 92% incorporation of deuterium was obtained for the final product 3a-d.
Solvent screening: a) 2a (10 mg, 0.016 mmol) were dissolved in fluorobenzene (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by the H2 atmosphere (1 bar).The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for 6 hours and the transformation was monitored by 1 H NMR experiments leading to the formation of 3a in a 47% NMR yield.
b) 2a (10 mg, 0.016 mmol) were dissolved in THF (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was then freeze-pump thawed and the N2 atmosphere was replaced by the H2 atmosphere (1 bar).The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for 6 hours and the transformation was monitored by 1 H NMR experiments leading to the formation of 3a in a 43% NMR yield.

EPR experiments
Equipment details: The EPR experiments were performed in the PEPR-Centre for Pulse EPR Spectroscopy at Imperial College London in a benchtop CW X-band spectrometer (Magnettech ESR5000) at room temperature in benzene-d6.The EPR experiments were recorded in 5-10 minutes time after the exposure to the LED light, whereas the 1 H NMR were recorded within 1 hour at room temperature after the exposure time.
NMR scale reaction of 2a and 1 equivalent of benzophenone in the absence of H2: 2a (10 mg, 0.016 mmol) and benzophenone (3 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 6 hours and the transformation was monitored by 1 H NMR and EPR experiments (Figure 4b, main text).
NMR scale reaction of 2a and 1 equivalent of TEMPO in the absence of H2: 2a (10 mg, 0.016 mmol) and TEMPO (2.5 mg, 0.016 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 6 hours and the transformation was monitored by 1   NMR scale reaction of [ Mes (BDI)Zn]2 and 2 equivalents of benzophenone: [ Mes (BDI)Zn]2 (10 mg, 0.012 mmol) and benzophenone (4 mg, 0.024 mmol) were dissolved in benzene-d6 (0.5 mL) and transferred to a borosilicate J. Young NMR tube.The tube was exposed to light (LED Kessil lamp, 390 nm-40W) for a total of 6 hours and the transformation was monitored by 1 H NMR (46% NMR conversion) and EPR experiments (Figure S18).Table S1.Crystal data, data collection and refinement parameters for the structures 2a + 3a and 2b.Data were collected using a Xcalibur PX Ultra A diffractometer, and the structures were solved and refined using the OLEX2 and SHELX-2019 program systems.98% Completeness to 0.84 Å resolution.
-X-ray structure of complex 2b: the crystal structure of 2b has one independent molecule in the asymmetric unit, crystallizing in the orthorhombic space group Pbca.All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model.
-X-ray structure of complexes 2a and 3a: two independent molecules were found in this crystal structure.One molecule of compound 2a and 3a crystallize together in the triclinic space group P-1.All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model, except the two bridging hydrides of compound 3a which have been determined from the Fourier map and refined freely.Carbon atoms of one of the Cp rings (C3, C4, C5, C7, C8) were found to be slightly disordered and were modelled by using simu and delu restraints.

DFT calculations
Density Functional Theory (DFT) calculations were run with Gaussian 09 (Revision D.01) 10 using the BP86 11 functional and an ultrafine integrations grid (keyword int=ultrafine).Empirical dispersion was not included in the optimization as it was found to over-estimate binding energies. 12Metal atoms (Fe, Zn) were described with Stuttgart SDDAll RECPs and associated basis sets, while a hybrid basis set was used for the other atoms: 6-31g**(C, H)/ 6-311+g*(N, O). 13 The basis set for metal hydrides was expanded by adding diffuse and polarisation functions.The graphical user interface used to visualise the various properties was GaussView 6.0. 14Single point corrections were performed using a higher basis set (def2TZVP) for C, H, N and O atoms and included solvent corrections (benzene, epsilon = 2.2706) which were applied using the polarized continuum model (PCM) to free energies. 15atural Bond Orbital analysis was carried out in NBO 6.0. 16QTAIM calculations were performed using the AIMAll sotware. 17ETS-NOCV calculations were performed for complex 3a using Orca 4.2.1 quantum chemistry software package. 18Optimised geometries of complex 3a from the Gaussian 09 calculations detailed above were used.Single-point calculations were performed using the wb97x functional on the relevant fragments.The def2-tzvpp basis set was used for all atoms.Graphical surface representations shown below were plotted using Avogadro 1.2.0.TD-DFT calculations 19 were performed on complex 2a using the same functional and basis set employed for the geometry optimizations to predict UV-VIS spectroscopy data (the two more intense transitions are shown in Table S4).Table S3.Structural parameters of 3a (experimental vs computational data).a fsr = d(Zn-Fe)/(r(Zn) + r(Fe)); r = Pauling radii.20 TD-DFT calculations on complex 2a: the computed UV-vis spectrum agrees with the experimental UVvis absorption spectra (maximum absorption at 350 nm).The two more intense transitions (328 and 316 nm) in the computed spectrum correspond to a combination of several transitions (Table S4) between orbitals that are highly delocalized across the structure.Some of them involve transitions from orbitals containing metal-metal bond character (160, 161) to orbitals with metal-metal antibonding character (167) or centred in the Fe fragment (166) or BDI ligand (165).Orbitals 160 to 167 (HOMO-4 to LUMO+2) are represented in Figure S20.

Figure S3 1 H
Figure S3 1 H NMR spectra.From top to bottom: equimolar mixture of 2a and 2c after 4 h exposure to LED lamp (experiment C); independently synthesised adducts 2a, 2c and 2e.

Figure S4. 1 H
Figure S4. 1 H NMR spectrum fragment of equimolar mixture of 2a and 2c after 4 h exposure to LED lamp (experiment C).

Figure S17 .
Figure S17.CW X-band EPR spectrum recorded in benzene solution at ambient temperature; microwave frequency 9.44769 GHz; field modulation amplitude, 0.25 mT.

Figure S18 .
Figure S18.CW X-band EPR spectrum recorded in benzene solution at ambient temperature; microwave frequency 9.44728 GHz; field modulation amplitude, 0.25 mT.

Figure S19 .
Figure S19.Crystal structure of 2b.Thermal ellipsoids are shown at a 50 % probability level.Hydrogens atoms have been hidden for clarity.

Table S4 .
TD-DFT calculations.Excited states 13 and 18 are the two more intense transitions in the calculated UV-vis spectrum.

Table S5 .
ETS -NOCV data (in kcal/mol) computed for complex 3a fragmentation.Selected deformation density plots.Charge flow from red to blue.