Photochemical Reactivity of Transition Metal Hydrides

An overview of the photochemistry of transition metal hydrides is given by Geoffrey, Perutz and Procacci.^[246,247] Among the most frequent photochemical reactions of metal hydride complexes is the reductive elimination of H2 by polyhydrides. Since this reaction gives rise to a low-valent metal center, photoinduced H2 liberation followed by substrate activation has found application in research on C-H activation. Following the initial report on photochemical benzene activation by [WH2Cp2] (Cp = cyclopentadienyl), the quantum yield for H2 loss was determined as ≥0.01±0.002 and [WCp2] could be identified as intermediate by photolysis in Ar and N2 matrices.^[248–250] Homolytic metal hydrogen bond cleavage does not take place as labeling experiments suggest a concerted pathway for reductive elimination.^[249] Similarly, the formation of square-planar [Ru(dmpe)2] from cis-[RuH2(dmpe)2] (dmpe = bis(dimethylphosphino)ethane) by photolysis proceeds intramolecularly and subsequent substrate activation is reported.^[251,252] In case of related cis-[RuH2(CO)(PPh3)3], transient IR and UV-vis spectroscopy show formation of the H2 elimination product within t = 6 ps.^[253,254] Time-dependent density functional theory (TD-DFT) was performed on model systems [RuH2(PH3)4] and [RuH2(CO)(PH3)3] showing that H2

elimination results from population of the dissociative S1 excited state.^[255] In both cases, excitation results in transfer of electron density from metal centered d orbitals to a combination of metal centered 4d_x2-y², 4d_z2

as well as 1g of the H2 fragment, resulting in bonding H-H and antibonding metal-H2 interactions.

Photochemical substitution reactions of adjacent ligands in metal hydride complexes are known, however in most cases do not directly involve the hydride ligand. Jones has shown that the phosphine ligand in [ReH2(PPh3)2Cp] can be substituted photochemically and in the presence of alkanes and deuterated benzene as solvent H/D scrambling of the alkane is observed.^[256,257] Notably, the hydride ligands are not involved in the scrambling process, which contradicts the initially assigned mechanism based on phosphine dissociation and subsequent coordination of the low coordinate rhenium by the substrate. Kinetic investigation suggests photoinduced stepwise reductive transfer of both hydride ligands to the Cp ligand giving a ³-allyl Re^I 14e^ intermediate which undergoes oxidative addition of the substrates.^[258]

Turning to metal monohydride complexes, homolytic metal hydrogen bond dissociation is described repeatedly and is of synthetic use as [ReCp*2] is prepared by irradiation of [ReHCp*2] using a Hg arc lamp.^[259] Photochemical reactivity of mixed metal carbonyl hydride complexes can result in competing CO and H loss by irradiation at the same wavelength as observed for [CoH(CO)4].^[260] Experimental work on photolysis of [MnH(CO)5] in an Ar matrix allows for the IR spectroscopic detection of [MnH(CO)4] as product of decarbonylation and [Mn(CO)5] from H atom loss.^[261,262] Analysis by EPR spectroscopy further

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reveals the formation of H radicals and their carbonylation to formyl radicals.^[263] While in the Ar matrix, decarbonylation is the major reaction, loss of CO is reversible in a CO matrix and results in selective formation of [MnH(CO)4].^[246] The optical transitions of high oscillator strength are attributed to MLCT (d→*CO) transitions to the carbonyl ligand and ligand centered (MnH→*MnH) transitions which lead to CO loss and Mn-H bond homolysis, respectively.^[264,265]

As in case of photochemical H2 loss by metal dihydrides, photoinduced intramolecular reductive elimination on metal monohydrides is reported for C-H and O-H bond formation.^[266][267] While photoinduced reductive elimination of alkanes from [ZrH(R)(Cp*)2] (R = Et, Pr, ⁱBu) can also be performed thermally, [IrH(OEt)PPh3Cp*] undergoes thermal -H elimination and photochemical alcohol formation.

Photoinduced reductive elimination of trans-[PtH(R)(PPh3)2] (R = CH2CN, (CH2)2CN, (CH2)3CN) is shown to proceed via initial photochemical isomerization to cis-[PtH(R)(PPh3)2] followed by thermal concerted alkane elimination.^[268]

Photoexcitation can have a strong effect on the redox properties and the acidity of a compound. Accordingly, population of an excited state influences the driving force for proton, hydrogen atom or hydride transfer of a hydride ligand. The excited state thermodynamics of PCET on a hydride complex are probably best examined for [IrH(L)Cp*]X (L = bpy, phen: 1,10-phenanthroline), initially reported by Ziessel.^[269]

Following early reports on photochemical WGS catalysis and formic acid dehydrogenation, photo(electro-)chemical proton reduction from different acids including water was investigated more recently by Miller.^[270–275] Aside from photocatalytic application, [IrH(L)Cp*]X or its derivatives catalyze a great number of chemical reactions including electrochemical CO2 reduction to formate, thermal (transfer-)hydrogenation of CO2 to formic acid, formic acid disproportionation to methanol, water oxidation and hydrogenation of carboxylic acids among others.^[276–281] Early mechanistic studies on the photochemical WGS catalysis suggest protonation of the hydride and subsequent H2 liberation as photochemical step of the reaction since no formation of H2 is observed in the dark.^[282] Photochemical H2 liberation can be rationalized based on enhanced hydricity of the triplet excited state (= 80 ns at 293 K) as predicted by an excited state cube scheme of [IrH(bpy)Cp*]X.^[283,284] The low excited state hydricity is a result of photoacidic behavior and a low excited state reduction potential. Accordingly, [IrH(bpy)Cp*]X acts as both photochemical proton and hydride donor.^[284,285] Detailed mechanistic work by Miller recently showed that H2 formation proceeds via excited state self-quenching giving Ir^II/Ir^IV hydrides followed by thermal bimolecular H2 formation rather than exited state hydride donation.^[275] This mechanistic understanding reveals [IrH(bpy)Cp*]X as a rare example of a monohydride complex which liberates H2 in a bimolecular reaction upon photoexcitation. Photochemical insertion of CO2 into metal hydride bonds giving the formate

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complex is reported for several compounds, however excited state thermodynamics or mechanistic investigations are not available in the literature.[211,286,287]

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2.2 Outline

The reduction of carbon dioxide to carbon monoxide is of high relevance for liquid fuel generation since it is a common intermediate in production of hydrocarbons. While several approaches to this transformation exist, including thermal rWGS catalysis and (photo-)electrochemically driven reactions, photochemical rWGS catalysis on a single molecular catalyst is not reported in the literature. To perform such reactivity, the abnormal CO2 insertion from a transition metal hydride to the hydroxycarbonyl can be regarded as the key step since this selectivity determining step results in carbon monoxide formation instead of formate production. Molecular nickel complexes play an outstanding role in CO selective CO2 reduction chemistry as exemplified by [NiFe] CODH and Ni based electrocatalysts. Further, nickel hydride complexes show comparably high hydricity which results in low driving force for unwanted normal CO2 insertion to the formate.

Scheme 33: Photochemical substrate activation by Ni^II hydride 12 and follow-up chemistry yielding hydrogenation products.

Starting from Ni^II hydride [NiH(^tBuP=N=P)] (12), the thermal and photochemical reactivity with CO2 is to be investigated. While normal CO2 insertion to the Ni^II formate is expected in the dark, photochemical excitation might result in a different selectivity for PCET of the hydride ligand. Mechanistic investigation by kinetic measurements and analysis of the photophysical processes by transient spectroscopy will be performed to examine the influence of photoexcitation. Monitoring the excited state metal hydride stretching vibration by UV-pump-IR-probe spectroscopy is of high interest and unprecedented in the literature. In case of successful photochemical activation of CO2, the scope of photochemical reactivity will be broadened to other substrates. Based on the results of photochemical substrate activation, investigation of the follow-up reactivity will be performed to determine the suitability for catalytic application (Scheme 33). The

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importance of the ^tBuP=N=P pincer ligand and the generality of the observed photochemical reactivity of 12 will be evaluated by comparison of the results to related pincer based Ni^II hydride complexes.

Metal azide complexes show a rich photochemistry including nitride/nitrene formation by N2 loss and homo-/heterolytic metal nitrogen bond cleavage. Nickel nitrene complexes have been postulated based on photochemical reactivity of nickel azides, however experimental proof of these elusive intermediates has not been presented. The ^tBuP=N=P pincer ligand has repetitively shown outstanding properties in stabilization of highly reactive nitrido/nitrene complexes of late transition metal complexes.^[288,289] Starting from Ni^II bromide 3 a Ni^II azide will be synthesized and its photochemical reactivity will be investigated.

Transient spectroscopy will be performed to identify the photophysical and -chemical processes involved.

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