Spectroscopy and Chemical Bonding in Transition Metal Complexes

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

The photoredox activity of well-known Ru^II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr^III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr^III complex is very similar to that for a Ru^II polypyridine, and thereby validate the concept of estimating the redox potential of d³ spin-flip excited states in analogous manner as for the MLCT states of d⁶ compounds. Building on this insight, we use our Cr^III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d⁶ valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)₂](PF₆), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L^CNC)₂](PF₆) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L^CNC)₂](PF₆), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

The substitution of three water molecules around trivalent chromium in CrBr₃·6H₂O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br₃] complexes. Stepwise treatments with Ag(CF₃SO₃) and KCN under microwave irradiations provide mer-[Cr(L)(CN)₃] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN₃C₃] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr^III. Room-temperature ligand-centered UV-excitation at 18 170 cm⁻¹ (λ_exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(²E → ⁴A₂) phosphorescence in the red to near infrared domain 13 150–12 650 cm⁻¹ (760 ≤ λ_em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)₃] in frozen acetonitrile.

The heteroleptic mer-[Cr(L)(CN)₃] (L = tpy, ddpd, dqp) complexes with their C_2v-symmetrical [CrC₃N₃] luminescent chromophores represent the missing links between pseudo-octahedral [CrN₆] and [CrC₆] units found in their well-known homoleptic parents.

Luminescent First-Row Transition Metal Complexes

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

A Near-Infrared-II Emissive Chromium(III) Complex

The combination of π-donating amido with π-accepting pyridine coordination units in a tridentate chelate ligand causes a strong nephelauxetic effect in a homoleptic CrIII complex, which shifts its luminescence to the NIR-II spectral range. Previously explored CrIII polypyridine complexes typically emit between 727 and 778 nm (in the red to NIR-I spectral region), and ligand design strategies have so far concentrated on optimizing the ligand field strength. The present work takes a fundamentally different approach and focusses on increasing metal-ligand bond covalence to shift the ruby-like 2 E emission of CrIII to 1067 nm at 77 K.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

Diisocyanide ligands with a m-terphenyl backbone provide access to Mo0 complexes exhibiting the same type of metal-to-ligand charge transfer (MLCT) luminescence as the well-known class of isoelectronic RuII polypyridines. The luminescence quantum yields and lifetimes of the homoleptic tris(diisocyanide) Mo0 complexes depend strongly on whether methyl- or tert-butyl substituents are placed in α-position to the isocyanide groups. The bulkier tert-butyl substituents lead to a molecular structure in which the three individual diisocyanides ligated to one Mo0 center are interlocked more strongly into one another than the ligands with the sterically less demanding methyl substituents. This rigidification limits the distortion of the complex in the emissive excited-state, causing a decrease of the nonradiative relaxation rate by one order of magnitude. Compared to RuII polypyridines, the molecular distortions in the luminescent 3MLCT state relative to the electronic ground state seem to be smaller in the Mo0 complexes, presumably due to delocalization of the MLCT-excited electron over greater portions of the ligands. Temperature-dependent studies indicate that thermally activated nonradiative relaxation via metal-centered excited states is more significant in these homoleptic Mo0 tris(diisocyanide) complexes than in [Ru(2,2′-bipyridine)3]2+.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: III. [Fe(tpen)]Cl2 in acetonitrile

In order to achieve an in-depth understanding of the role played by the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(ii) complexes, an accurate description of the solvated complexes in the two spin states is required. To this end, we are applying state-of-the-art ab initio molecular dynamics (AIMD) simulations to the study of the structural and vibrational properties of iron(ii) polypyridyl complexes. Two aqueous LS complexes were investigated in this framework, namely, [Fe(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830; Lawson Daku, Phys. Chem. Chem. Phys., 2018, 20, 6236] and [Fe(tpy)₂]²⁺ (tpy = 2,2′:6′,2′′-ter-pyridine) [Lawson Daku, Phys. Chem. Chem. Phys., 2019, 21, 650]. For aqueous [Fe(bpy)₃]²⁺, combining the results of forefront wide-angle X-ray scattering experiments with those of the AIMD simulations allowed the visualization of the interlaced coordination and solvation spheres of the photoinduced HS state [Khakhulin et al., Phys. Chem. Chem. Phys., 2019, 21, 9277]. In this paper, we report the extension of our AIMD studies to the spin-crossover complex [Fe(tpen)]²⁺ (tpen = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine) in acetonitrile (ACN). The determined LS and HS solution structures of the complex are in excellent agreement with the experimental results obtained by high-resolution transient X-ray absorption spectroscopy [Zhang et al., ACS Omega, 2019, 4, 6375]. The first solvation shell of [Fe(tpen)]²⁺ consists of ACN molecules located in the grooves defined by the chelating coordination motif of the tpen ligand. Upon the LS → HS change of states, the solvation number of the complex is found to increase from ≈9.2 to ≈11.9 and an inner solvation shell is formed. This inner solvation shell originates from the occupancy by about one ACN molecule of the internal cavity which results from the arrangement of the 4 pyridine rings of the tpen ligand, and which becomes accessible to the solvent molecules in the HS state only thanks to the structural changes undergone by the complex. The presence of this inner solvation shell for the solvated HS complex probably plays a key role in the spin-state dependent reactivity of [Fe(tpen)]²⁺ in liquid solutions.

AIMD study of the SCO [Fe(tpen)]²⁺ complex in acetonitrile: radial distribution functions and running coordination numbers characterizing its solvation structure in the HS state.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: II. aqueous [Fe(tpy)2]Cl2

LS and HS Fe–O radial distribution functions and running coordination numbers for aqueous [Fe(tpy)₂]Cl₂: in both spin states, the first hydration shell of [Fe(tpy)₂]²⁺ consists in a chain of ∼15 hydrogen-bonded water molecules wrapped around the ligands.

Photoactive Complexes with Earth-Abundant Metals

In this invited Perspective, recent developments and possible future directions of research on photoactive coordination compounds made from nonprecious transition metal elements will be discussed. The focus is on conceptually new, structurally well-characterized complexes with excited-state lifetimes between 10 ps and 1 ms in fluid solution for possible applications in photosensitizing, light-harvesting, luminescence and catalysis. The key metal elements considered herein are Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W and Ce in various oxidation states equipped with diverse ligands, giving access to long-lived excited states via a range of fundamentally different types of electronic transitions. Research performed in this area over the past five years demonstrated that a much broader spectrum of metal complexes than what was long considered relevant exhibits useful photophysics and photochemistry.

Luminescent complexes made from chelating isocyanide ligands and earth-abundant metals

In this invited frontier article, recently discovered d⁶ and d¹⁰ complexes with long-lived metal-to-ligand charge transfer (MLCT) excited states are highlighted. Chelating diisocyanide ligands give access to emissive Mo(0) and Cr(0) complexes with d⁶ electron configuration exhibiting photophysical properties similar to those of Ru(ii) polypyridines or cyclometalated Ir(iii) complexes. With Ni(0), these ligands yield luminescent tetrahedral d¹⁰ complexes similar to isoelectronic Cu(i) bis(diimine) compounds.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: aqueous [Fe(bpy)3]Cl2, a case study

LS and HS IR spectra of aqueous [Fe(bpy)₃]²⁺ and corresponding HS–LS difference IR spectrum as obtained from state-of-the-art ab initio molecular dynamics simulations applied to the determination of the structural and vibrational properties of the solvated complex.

A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2'-Bipyridine)32

A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CN^tBuAr₃NC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr(CN^tBuAr₃NC)₃ complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr(CN^tBuAr₃NC)₃ is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet-triplet annihilation upconversion is observed. With an excited-state oxidation potential of -2.43 V vs Fc⁺/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements.