Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

Coordination complexes of precious metals with the d 6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis.Until now, d 6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive.While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d 6 metal complex reported so far.The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines.With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d 6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination.Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d 6 metals.
Upon photo-irradiation of a suitable metal complex, the promotion of an electron from the metal to a coordinated ligand can generate a metal-to-ligand charge transfer (MLCT) excited state with diverse applications in photophysics and photochemistry 1 .In many noble metal complexes, MLCT excited states luminesce and have lifetimes of several tens of nanoseconds or longer, which forms the basis for their use in lighting applications and photocatalysis [2][3][4] .Octahedral Ru(II), Os(II) and Ir(III) complexes with π-conjugated ligands are prototypical examples with a low-spin d 6 configuration 5,6 (Fig. 1a), in which three degenerate d-orbitals are all occupied with one electron pair, and two degenerate vacant d-orbitals are energetically above the lowest empty ligand π* orbital (Fig. 1d).In complexes with such an electronic structure, emissive and redox-active MLCT states can then emerge.
First-row transition metals experience weaker ligand fields than second-and third-row transition metals 7 , and the lowest unoccupied orbitals of 3d 6 complexes become metal-based (Fig. 1e), which typically causes ultrafast MLCT deactivation by metal-centred (MC) states 8,9 .MLCT lifetimes in Fe(II) complexes only recently reached the pico-and nanosecond timescale [10][11][12][13] , and currently only a handful of 3d 6 metal complexes show MLCT photoluminescence in solution at 20-25 °C14-19 , where the highest reported quantum yield is 0.09% (ref.16).This situation is very different from the d 10 electron configuration of semiprecious Cu(I) 20 , for which luminescent charge transfer excited states are more readily obtainable 21,22 , because there are no low-lying MC states when all d-orbitals are filled 23 .Owing to their privileged 3d 10 electron configuration, Cu(I) complexes and their photophysical properties are therefore not directly comparable to 3d 6  Mn(I), the MLCT luminescence and the excited-state lifetimes remained inferior to noble metal compounds 16,18 .Two complementary molecular design principles now yield the first 3d 6 complexes (Fig. 1c) with photophysical and photochemical behaviour competitive with precious metal-based analogues.The electronic structures of these Cr(0) complexes (Fig. 1f) resemble those of well-known noble metal analogues (Fig. 1g,i), more than those of Fe(II) polypyridines (Fig. 1h).
The new complexes [Cr(L Mes ) 3 ] and [Cr(L Pyr ) 3 ] were obtained in 78% and 47% yields, respectively, by reacting the previously unknown ligands L Mes and L Pyr with CrCl 3 (THF) 3 in the presence of Na/Hg in dry and de-aerated tetrahydrofuran (THF) at room temperature.Nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, combustion analysis and infra-red spectroscopy establish the identity and purity of the complexes.The key characteristics of isocyanide complexes including the 13 C NMR resonances of the coordinating carbon atoms, as well as C≡N stretches in infra-red spectroscopy, are readily detectable Ir(III) complexes with competitive photophysical and photochemical properties have been unknown 27,28 .
In this Article, we report two Cr(0) complexes with MLCT excited-state lifetimes close to 50 ns and photoluminescence quantum yields competitive with benchmark Os(II) polypyridines.These photophysical properties permit MLCT-based photoredox catalysis analogous to that known from many precious d 6 metal complexes.

Molecular design, synthesis and characterization
Non-radiative MLCT deactivation in d 6 complexes decelerates in strong ligand fields, because the MC states are shifted to higher energies 29 .Isocyanide ligands create strong ligand fields 30 , which provide W(0) complexes with promising photophysics and photochemistry 31,32 .We developed isocyanide chelate ligands that provided brightly emissive Mo(0) complexes 33 , but with the first-row transition metals Cr(0) and  (Supplementary Figs. 15, 31, 72 and 81).In the X-ray crystal structure of [Cr(L Mes ) 3 ] (Fig. 2a), the six mesityl substituents ortho to the isocyanide groups wedge in between the m-terphenyl ligand backbones, and thus impart cooperative rigidity to the overall complex while simultaneously protecting the Cr(0) atom.The absence of EXSY peaks between the two ortho methyl groups of the mesityl in the NOESY and ROESY NMR spectra (Supplementary Figs.18 and 19) as well as the distinctly different NOE patterns (Supplementary Fig. 18) for the methyl pointing towards the chromium, compared with the outward-oriented methyl group, clearly demonstrate that this rigidity is also maintained in solution.Solid [Cr(L Mes ) 3 ] and [Cr(L Pyr ) 3 ] can be stored under air for several weeks without undergoing noticeable degradation, and an initially de-aerated solution of [Cr(L Mes ) 3 ] showed only 3% of decomposition over 15 days of exposure to air (Supplementary Fig. 33).The single crystal used for X-ray diffraction was grown in an NMR tube that was open to air.Both complexes remained intact for several days in de-aerated toluene-d 8 at 115 °C (Supplementary Figs.32 and 88).A single set of sharp 1 H NMR resonances indicates that the three ligands in [Cr(L Mes ) 3 ] are symmetry related at 298 K, whereas for [Cr(L Pyr ) 3 ] analogous behaviour is only observed at 378 K, due to hindered rotation of the tert-butyl groups at lower temperatures (Supplementary Fig. 75).Thus, while the pyrene substituents on the backbone of L Pyr rotate freely above 318 K as shown by variable temperature NMR (Supplementary Fig. 76) and NOE contacts between the terphenyl protons to both sides of the pyrene substituent at 378 K (Supplementary Fig. 84), the coalescence pattern of the tert-butyl resonances suggests that the structural rigidity of [Cr(L Pyr ) 3 ] and the steric protection of the metal centre are mainly due to inter-ligand contacts caused by the tert-butyl groups.

Electrochemistry and photophysics
Oxidation of Cr(0) to Cr(I) occurs reversibly near −0.7 V versus Fc + /Fc in both complexes (Fig. 2b), and, along with oxidation of Cr(I) to Cr(II) at higher potentials, is typical for hexakis(arylisocyanide) complexes of Cr(0) (ref.34).A reversible wave at −2.50 V versus Fc + /Fc observed for [Cr(L Pyr ) 3 ] is attributable to reduction of the pyrene substituents, whereas reduction of the m-terphenyl backbones of the diisocyanide ligands is outside the electrochemical window of suitable electrolytes 15,17 .
The free L Mes and L Pyr ligands absorb only ultraviolet light, but [Cr(L Mes ) 3 ] and [Cr(L Pyr ) 3 ] feature MLCT bands covering large parts of the visible absorption spectrum (Fig. 3a,b).The increased π-conjugation network of the pyrene-decorated ligand causes a 100 nm red shift of the MLCT absorption band maximum of [Cr(L Pyr ) 3 ] compared with [Cr(L Mes ) 3 ].Upon photo-excitation, both complexes show broad and unstructured luminescence.Between cyclohexane and THF, the luminescence band maxima shift from 695 nm to 745 nm in [Cr(L Mes ) 3 ] and from 713 nm to 840 nm in [Cr(L Pyr ) 3 ], because the emissive MLCT state is energetically more stabilized in high polarity solvents 35 .Thus, the luminescence of the two Cr(0) complexes occurs in the same spectral range as the MLCT emission of [Os(bpy) 3 ] 2+ (Table 1).Differences in solubility between the charge-neutral Cr(0) compounds and the dicationic Os(II) complex preclude direct comparison in the same solvent, yet the energies of the emissive MLCT excited states of [Cr(L Mes ) 3 ] and [Cr(L Pyr ) 3 ] in cyclohexane and of [Os(bpy) 3 ] 2+ in acetonitrile are evidently similar.The comparison of photophysical properties between these three compounds is therefore more meaningful than comparison with [Ru(bpy) 3 ] 2+ .
The MLCT lifetimes and luminescence quantum yields of [Cr(L Mes ) 3 ] and [Cr(L Pyr ) 3 ] exceed those of previously reported 3d 6 complexes by at least an order of magnitude [8][9][10][11][12][13][14][15][16]19,27 . Asidefrom the rigid interlocked molecular structures discussed above, the extended π-conjugation network of the new diisocyanide ligands contributes to this behaviour.In the ultraviolet (UV)-visible (Vis) transient absorption spectrum of [Cr(L Pyr ) 3 ] (Fig. 3d), the negative signal around 340 nm coincides with the lowest pyrene-localized 1 π-π* transition in the ground state, indicating that the photoactive MLCT state has admixed pyrene character.Thus, the excited electron of the emissive MLCT state appears to be strongly delocalized, in line with the strong emission solvatochromism.Such delocalization causes weaker distortion of the MLCT excited state relative to the ground state (ΔQ e in Fig. 1i) 36 , making non-radiative relaxation less dominant, somewhat reminiscent of the even much more weakly distorted spin-flip excited states of Cr(III) (d 3 ) compounds 25,27,37 .Compared with these spin-flip MC states of d 3 complexes, long-lived and strongly emissive MLCT excited states in first-row d 6 complexes are far more difficult to obtain 38 .

Article
https://doi.org/10.1038/s41557-023-01297-9 commercial reductant capable of regenerating Cr(I) to Cr(0) after initial photo-induced electron transfer to the individual substrates (Fig. 4a).Similar hydrodehalogenation reactions with organic or precious metal-based photocatalysts typically require up to two blue or green photons per turnover [45][46][47] , whereas [Cr(L Mes ) 3 ] drives the reactions with red light, keeping photodegradation at an acceptable level (Supplementary Fig. 100).Photocatalysis with isoelectronic Fe(II) complexes usually occurs from MC states 48,49 , and there is only one single report involving a dark (non-luminescent) MLCT state, which, however, relies on consecutive ligand-to-metal charge transfer (LMCT) and MLCT excitation of an Fe(III)/Fe(II) system 50 .Aryl iodides, bromides and even an activated chloride with reduction potentials between −2.4 V and −2.7 V versus Fc + / Fc are reductively dehalogenated by MLCT-excited [Cr(L Mes ) 3 ] (Fig. 4c), providing the proof of concept for demanding photoreductions under red illumination that are not accomplishable in the same fashion with typical noble metal-based d 6 complexes (Fig. 4b) 4,45,46 .TDAE is a commercial reductant, but it is more expensive than other commonly used tertiary amine donors; hence, it seemed interesting to explore overall redox-neutral base-promoted homolytic aromatic substitution (BHAS) reactions, for which no electron donor at all needs to be added 33,44 .We chose 1-(2-iodobenzyl)-pyrrole as a substrate enabling an intramolecular variant of the BHAS reaction.Using [Cr(L Mes ) 3 ] as a photosensitizer, TMP (2,2,6,6-tetramethylpiperidine) as a base, and red light (Fig. 4d), the anticipated C-C coupled product formed in 38% yield at a catalyst loading of 10 mol%.The lower yield of the BHAS reaction and the need for higher catalyst loadings with respect to the hydrodehalogenations in Fig. 4c could have several reasons, including the following.First, the driving force for the reductive dehalogenation step in this specific substrate is only roughly 0.04 V. Second, the driving force for the regeneration of Cr(I) to Cr(0) in the catalytic cycle by the tricylic radical (Fig. 4d) is not known, but is probably in competition with nucleophilic attack of iodide anions at Cr(I), thereby leading to degradation of the sensitizer 33 .Nonetheless, the BHAS reaction in Fig. 4d provides an important proof of concept for overall redox-neutral reactions involving a thermodynamically demanding reduction step.[Ru(bpy) 3 ] 2+ , [Ir(ppy) 3 ] and the vast majority of their precious metal-based congeners are unable to catalyse comparable BHAS reactions 43 , because they lack sufficient reducing power in their MLCT excited states.After thousands of publications exploiting the MLCT excited states of precious 4d 6 and 5d 6 metal complexes for photoredox catalysis, the hydrodehalogenation and BHAS reactions demonstrated herein represent the first examples in which a luminescent MLCT excited state of a 3d 6 metal complex has been used for photoredox catalysis.
After decades of research targeting 3d 6 complexes emitting from the same type of MLCT excited state with competitive photophysical properties as hundreds of precious 4d 6 and 5d 6 metal complexes, only one single Fe(II) complex has been reported to emit from a 3 MLCT excited state.This Fe(II) complex has an MLCT lifetime of 1 ns and a luminescence quantum yield close to the detection limit 19 .Two Mn(I) complexes exhibited 3 MLCT lifetimes around 1 ns and luminescence quantum yields below 0.1% (refs.18,38), and two Cr(0) complexes had slightly longer 3 MLCT lifetimes (2-6 ns) and equally modest luminescence quantum yields (0.001-0.09%) 15,16 .Evidently, these previously reported MLCT-based 3d 6 luminophores possess very short MLCT lifetimes and poor luminescence quantum yields.With the Cr(0) complexes reported herein, the MLCT phosphorescence lifetimes and quantum yields of 3d 6 complexes finally become competitive with 4d 6 or 5d 6 compounds based on precious metals, and photocatalysis based on luminescent MLCT excited states is now possible using first-row d 6 metal complexes.These findings complement recent key advances with LMCT excited states in complexes based on other abundant transition metals, in particular 3d 5 Fe(III) LMCT luminophores with fluorescence lifetimes up to 2.0 ns and quantum yields up to 2.0% (refs.26,51), in which the direction of charge transfer is opposite.Access to both LMCT and MLCT excited states with mutually complementary charge transfer directionalities is important to target the complete spectrum of photophysical and photochemical applications of first-row transition metal complexes 52 .