# Taming 2,2'-biimidazole ligands in trivalent chromium complexes

Julien Chong, Amina Benchohra, Céline Besnard, Laure Guénée, Arnulf

Rosspeintner, Carlos M Cruz, Juan-Ramón Jiménez, Claude Piguet

► To cite this version:

Julien Chong, Amina Benchohra, Céline Besnard, Laure Guénée, Arnulf Rosspeintner, et al. Tam-
ing 2,2'-biimidazole ligands in trivalent chromium complexes. Dalton Transactions, 2024, 53 (38),
pp.15801 - 15814. 10.1039/d4dt01608d. hal-05412672

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# Dalton
Transactions

An international journal of inorganic chemistry

rsc.li/dalton

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ISSN 1477-9226

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PAPER

Claude Piguet et al.

Taming 2,2'-biimidazole ligands in trivalent chromium complexes
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Cite this: *Dalton Trans.*, 2024, **53**,
15801

Received 3rd June 2024,

Accepted 4th July 2024

DOI: 10.1039/d4dt01608d

rsc.li/dalton

# Taming 2,2'-biimidazole ligands in trivalent chromium complexes†

Julien Chong,ª Amina Benchohra, ID a,b Céline Besnard, ID c Laure Guénée, c Arnulf Rosspeintner, ID d Carlos M. Cruz, ID e Juan-Ramón Jiménez ID f and Claude Piguet ID,ª a

Complete or partial replacement of well-known five-membered chelating 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2'-biimidazole ($H_2biim$) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex $[Cr(H_2biim)_3]^{3+}$ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral $[CrN_6]$ units are found to be intermolecularly connected via peripheral N-H···X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) $Cr^{III}$-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2'-biimidazole ligands to a single unit in the challenging heteroleptic $[Cr(phen)_2(H_xbiim)]^{(1+x)^+}$ ($x = 2-0$) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these $[CrN_6]$ chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2'-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising 'complex-as-ligand' strategies.

## Introduction

The lack of first-order orbital momentum in pseudo-octahedral trivalent chromium complexes makes isotropic and spin-only $[CrX_6]$ units ideal partners for programming, tuning and rationalizing magnetic coupling with neighbouring paramagnetic d-block¹⁻⁵ and f-block⁶⁻¹⁰ cations. In this context, many of the chromium-containing heterometallic assemblies exploit the

kinetically inert $[Cr(CN)_6]^{3-}$ synthon, which behaves as a 'complex-as-ligand' connecting cationic metals *via* cyanide bridges in solid state materials.¹⁻¹⁰ The realization that, beyond magnetic properties, both lanthanide-based light downshifting¹¹,¹² and light upconversion¹³,¹⁴ can be boosted by close Cr(III) sensitizers/emitters resulted in some active search for novel synthetic strategies, leading to Cr³⁺-Ln³³¹ molecular pairs (Ln(III) is a trivalent lanthanide cation) beyond serendipitous co-crystallization processes.¹⁵ One approach involves self-assembly processes with segmental multisite ligands where labile CrII precursors are selectively recognized by didentate binding units while Ln³³¹ is caught by adjacent tridentate sites. The subsequent CrII to Cr³³¹ oxidation provides kinetically inert heterometallic triple-stranded Cr³³¹-Ln³³¹ and Cr³³¹-Ln³³¹-Cr³³¹ helicates with remarkable photophysical properties,¹⁶,¹⁷ among which is the programming of the first molecular-based energy transfer light-upconversion (ETU) process.¹³

A more versatile synthetic strategy for incorporating open-shell $[CrX_6]$ chromophores as tuneable and operable sensitizers in multimetallic (supra)molecular architectures involves extending the 'complex-as-ligand' strategy, originally used for introducing $[Cr(CN)_6]^{3-}$ into multimetallic coordination polymers.¹⁻¹⁰,¹⁸ Taking advantage of kinetically-controlled ligand exchange processes around inert Cr(III), a few heterolep-

<figure><img src="image_5.png" /></figure>

<footnote>ªDepartment of Inorganic and Analytical Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland.</footnote>

<footnote>E-mail: Claude.Piguet@unige.ch</footnote>

<footnote>ᵇLaboratoire CEMCA UMR, CNRS 6521, UFR Sciences and Techniques, 6, avenue Victor Le Gorgeu, 29238 Brest Cedex 3, France</footnote>

<footnote>ᶜLaboratory of Crystallography, University of Geneva, 24 quai E. Ansermet, CH-1211 Geneva 4, Switzerland</footnote>

<footnote>ᵈDepartment of Physical Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland</footnote>

<footnote>ᵉDepartment of Organic Chemistry, Unidad de Excelencia de Química (UEQ), University of Granada, Avda. Fuente Nueva s/n, 18071 Granada, Spain</footnote>

<footnote>fDepartamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada and Unidad de Excelencia en Química (UEQ), Avda. Fuente Nueva s/n, 18071 Granada, Spain</footnote>

<footnote>†Electronic supplementary information (ESI) available. CCDC 2355642-2355650. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt01608d</footnote>
<figure><img src="image_6.png" /><figcaption>**Scheme 1** Kinetically-inert heteroleptic six-coordinate Cr¹¹¹ complexes used as complex-as-ligand when preparing multimetallic assemblies.<sup>12,18–21</sup></figcaption></figure>

tic six-coordinate complexes could be prepared, in which an oxalate (Scheme 1a),¹² a phenyl-carboxylate (Scheme 1b),¹⁹ a difluoride (Scheme 1c)²⁰ or an extended ethyne-bis(benzimidazole)pyridine (Scheme 1d)²¹ acted as a bridging unit between the Cr(III)-based complex-as-ligand unit and some adjacent d-block or f-block partners. However, the molecular aspects of their association processes in solution remain elusive and only solid-state crystal data support the physicochemical analyses. Considering the recent recognition that strong-field [CrN₆]

chromophores are ideal for maximizing phosphorescence quantum yields, emission lifetimes and sensitization in ‘molecular rubies’,²²⁻²⁴ there is clearly a need for the design of novel [CrN₆] analogues working as complex-as-ligand, but using more accessible and reliable bridging units.

With this in mind, the 2,2'-biimidazole ligand (H₂biim, Scheme 2a)²⁵ is famous for working as a versatile bridging ligand after its binding to a metallic cation (Scheme 2b). In its protonated form, it can either form hydrogen bonds for

<figure><img src="image_7.png" /><figcaption>**Scheme 2** a) Successive acid-base equilibria of fully protonated 2,2'-biimidazole (H₄biim²⁺) with pKₐ values measured in DMF: H₂O = 7:3 (I = 0.1 M).²⁵ b) Different coordination modes encountered in the literature for H₂biim (A²⁶,²⁷ and B²⁸), Hbiim⁻ (C and D)³⁰ and biim²⁻ (E-I).³¹⁻³⁴</figcaption></figure>
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sensing anions (mode A in Scheme 2b)²⁶,²⁷ or connect other cations in a linear way (mode B in Scheme 2b).²⁸ More often,²⁹ multimetallic assemblies are obtained after stepwise deprotonation of the bound 2,2'-biimidazole ligand to give bridging Hbiim⁻ (modes C and D in Scheme 2b)³⁰ and biim²⁻ scaffolds (modes E-I in Scheme 2b).³¹⁻³⁴ Surprisingly, the studies dealing with the complexation of inert Cr³³ centers by potentially bridging 2,2'-biimidazole are undervalued, probably due to the synthetic difficulties associated with the synthesis of these poorly soluble and pH-sensitive complexes.³⁵⁻³⁸ To the best of our knowledge, only the molecular structures of [Cr(H₂biim)₃]³⁺,³⁶ [Cr(H₂biim)₂(Hbiim)]²⁺ (ref. 37) and [Cr(Hbiim)₃]³⁶ have been characterized in the solid state following poorly reproducible and serendipitous crystallization from intricate mixtures of ligands and metals in the presence of various amounts of counter-anions and/or solvent molecules. A recent synthetic improvement, which used anhydrous THF under microwave heating, gave [Cr(H₂biim)₃](NO₃)₃ in 94% yield (Fig. 1).³⁵

Beyond the detailed descriptions of (i) sophisticated hydrogen-bonding networks in which the latter homoleptic complexes are a part and (ii) some expected axial flattening produced by didentate five-membered chelating 2,2'-biimidazole bound to Cr³³,³⁶⁻³⁸ no effort has been focused on the thermodynamic deprotonation processes and the associated control of the photophysical properties. Moreover, to the best of our knowledge, no attempt to prepare heteroleptic [L₂Cr(H₂biim)]³⁺ has been made, while related systems with inert 4d and 5d metal ions have been designed regularly to access the two crucial pKₐ values for their use as complex-as-ligand (Table S1 in the ESI†).²⁵,³⁹⁻⁴⁷ In order to provide new perspectives for exploiting 2,2'-biimidazole as a bridging ligand between photo-physically-active Cr³³ and promising open-shell lanthanides, we

<figure><img src="image_9.png" /><figcaption>**Fig. 1** Synthesis of [Cr(H₂biim)₃]³⁺ (ref. 35) and [Cr(Hbiim)₃].³⁶ The molecular structures of the complexes are those found in the crystal structures of [Cr(H₂biim)₃](NO₃)₃ (CCDC-603707) and [Cr(Hbiim)₃]·C₆H₆·2H₂O (CCDC-603730).³⁶ Color code: C = grey, N = blue, H = white, and Cr = orange.</figcaption></figure>

report here on the molecular structures and photophysical properties of the accessible and isolable homoleptic complexes [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ in solution and in the solid state. The second part proposes the synthesis of the unprecedented heteroleptic [(phen)₂Cr(H₂biim)]³⁺ complex with its detailed acid-base and photophysical properties, which become accessible in the solid state and in solution.

## Results and discussion

### Preparation and structures of the homoleptic [Cr(Me₂biim)₃]³⁺, [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

2,2'-Biimidazole (H₂biim) was synthesized from ammonium acetate and glyoxal under aqueous conditions with moderate yield.⁴⁸ Its methyl derivative 1,1'-dimethyl-2,2'-bi-1H-imidazole (Me₂biim) could be obtained by deprotonation, followed by methylation with methyl iodide (Fig. 2, top).⁴⁹

The original literature synthesis of [Cr(H₂biim)₃](NO₃)₃ involved the reaction of CrCl₃·3THF with [Ag(H₂biim)]NO₃ in MeOH³⁶ or in THF³⁵ (Fig. 1) because the formation of highly insoluble AgCl drove the reaction to completion. To simplify the procedure, the intermediate [Ag(H₂biim)]NO₃ was not isolated in this work, but was formed *in situ* by mixing AgNO₃

<figure><img src="image_10.png" /><figcaption>**Fig. 2** Synthesis of ligands 2,2'-biimidazole (H₂biim) and 1,1'-dimethyl-2,2'-bi-1H-imidazole (Me₂biim) and their homoleptic [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃], [Cr(Hbiim)₂(biim)]⁻ and [Cr(Me₂biim)₃]³⁺ complexes. The molecular structures of the metallic complexes are those found in the associated crystal structures. Color code: C = grey, N = blue, H = white, and Cr = orange. The counter-ions and hydrogen atoms (for Me₂biim ligands) are omitted for clarity.</figcaption></figure>
and H₂biim in MeOH. Then CrCl₃·3THF was added into the resulting solution of [Ag(H₂biim)]NO₃. The purification method was identical to that used in ref. 35 and [Cr(H₂biim)₃](NO₃)₃ was isolated in good yield (Fig. 2). Recrystallization by vapor diffusion of Et₂O into a methanolic solution provided crystals suitable for X-ray diffraction, the structural resolution of which at 120 K confirmed the crystal structure previously reported for [Cr(H₂biim)₃](NO₃)₃ at 290 K (Fig. 2, Tables S2–S3 and Fig. S1†).³⁶ The synthesis of [Cr(Hbiim)₃] was first described by Gruia *et al.*,³⁶ where they used a stoichiometric amount of NaOMe to deprotonate [Cr(H₂biim)₃]³⁺ in MeOH. However, the complete insolubility of the formed neutral [Cr(Hbiim)₃] complex did not allow any reproducible recrystallization techniques. To overcome this problem, a methanolic solution of [Cr(H₂biim)₃]³⁺ was treated in this work with vapor diffusion of an excess of volatile triethylamine, the limited pKₐ of which (10.74)⁵⁰ prevented any double deprotonation of the bound Hbim ligand and finally gave crystals of [Cr(Hbiim)₃] with good yield (61%) and in a reproducible way (Fig. 2, Tables S4–S6 and Fig. S2†). Further deprotonation of [Cr(Hbiim)₃] could not be obtained by Gruia *et al.*,³⁶ but some partial reports of analogs of [Cr(biim)₃]³⁻, i.e. Ba₁.₅[Co(biim)₃]⁴¹ and K₃[Ru(biim)₃]⁵¹ that were obtained using harsh basic conditions (aqueous NaOH 2 M/BaCl₂ and tBuOK 1.2 M in MeOH, respectively) have been noted. Consequently, [Cr(Hbiim)₃] was dissolved in an excess of aqueous NaOH (0.5 M) until a clear yellow solution was formed. The addition of a concentrated solution of PPh₄Cl immediately resulted in the formation of an orange precipitate. Recrystallization from MeOH by vapor diffusion of tBuOMe provided block-shaped crystals of [Cr(Hbiim)₂(biim)]PPh₄(CH₃OH) suitable for X-ray diffraction analysis in a good yield (66%) (Fig. 2, Tables S7–S9 and Fig. S3†). A detailed geometrical analysis of these homoleptic complexes (Appendix 2 in the ESI†) concludes that the [Cr(Hₓbiim)₃]ⁿ⁺ units display standard pseudo-octahedral arrangements of the six bound nitrogen donor atoms, with a compression along the pseudo-C₃ axis due to the 79.6–80.4° ligand bite angles (Table A2-1 in Appendix 2†), which is characteristic of five-membered chelating polyaromatic ligands²³ as reported for related [Cr(bipy)₃]³⁺ (bipy = 2,2'-bipyridine, 79.1°)⁵² and [Cr(phen)₃]³⁺ (phen = 1,10-phenanthroline, 81.0°) complexes.⁵³ The Cr–N distances in [Cr(Hₓbiim)₃]ⁿ⁺ do not vary drastically (2.028–2.037 Å, compared with averages of 2.042 Å for [Cr(bipy)₃]³⁺ and 2.051 Å for [Cr(phen)₃]³⁺) and do not show obvious correlations with the degree of deprotonation of the bound ligand. Due to the chelation of semi-rigid didentate polyaromatic ligands, some standard trigonal distortions characterize all these complexes (Table A2-1 in Appendix 2†).

The main difference between [Cr(bipy)₃]³⁺ and [Cr(phen)₃]³⁺ on one hand, and the family of [Cr(Hₓbiim)₃]ⁿ⁺ complexes, is associated with the bound didentate 2,2'-biimidazole ligands which may act as N–H hydrogen-bond donors when they are protonated and as N⁻ hydrogen-bond acceptors when they are deprotonated. For the fully protonated [Cr(H₂biim)₃](NO₃)₃ complex, the weak N–H···O hydrogen bonds observed between

the bound ligand and the nitrate counter-anion (Fig. A2-2†) are responsible for the circa 400 cm⁻¹ decreases of the N–H stretching frequency in the vibrational spectrum with respect to the free ligand (ν(NH) ≈ 3000 cm⁻¹ in Fig. S14 and S18-top, Table S29†). The formation of strong intermolecular N–H···N hydrogen bonds observed in the deprotonated complexes [Cr(Hbiim)₃] (Fig. A2-4†) and [Cr(Hbiim)₂(biim)]PPh₄ (Fig. A2-6†) further weakens the N–H bond force constants and stepwise shifts the associated stretching frequency toward ν(NH) ≈ 2400 cm⁻¹ (Fig. S15 and S18 center, Table S30†) and ν(NH) ≈ 2300 cm⁻¹ (Fig. S16 and S18 bottom, Table S31†), respectively. In this context, the replacement of hydrogen atoms with methyl groups to give [Cr(Me₂biim)₃](CF₃SO₃)₃ via the Kane-Maguire synthetic strategy (Fig. 2) maintains the pseudo-octahedral structure of the [CrN₆] core (Tables S16–S17 and Fig. S7†),⁵⁴ but it limits intermolecular hydrogen bonds in the crystal structure (Fig. A2-10 in Appendix 2†).

The spectroscopic properties of the ligands H₂biim and Me₂biim and the chromium complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ could be recorded in methanol or in acetonitrile. Unfortunately, the neutral complex [Cr(Hbiim)₃] is not soluble enough in common organic solvents to perform reliable measurements in solution. The electrospray ionization mass spectrometry (ESI-MS) spectra (Fig. S19–S38 and Tables S33–S35†) show complicated mixtures in the gas phase containing various amounts of intact 1 : 3 complexes with variable degrees of deprotonation ([Cr(Hₓbiim)₃]ⁿ⁺) together with (i) partial ligand dissociation to give 1 : 2 complexes ([Cr(Hₓbiim)₂]ⁿ⁺ + H₂biim) and (ii) metal reduction into Cr(II)-based systems. All peaks could be identified from high-resolution MS spectra with a special emphasis on the detection of the ‘full’ deprotonated [Cr(Hbiim)₂(biim)]⁻ anion (negative mode) for [Cr(Hbiim)₂(biim)]PPh₄ (Fig. S25 and Table S34†). It is concluded that these deprotonatable complexes suffer from the ionization process and exist as fragmented mixtures in the gas phase.

## Photophysical properties of the homoleptic [Cr(Me₂biim)₃]³⁺, [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

The absorption (Fig. 3 and 4a) and emission (Fig. 4b and S39†) spectra of the ligands H₂biim and Me₂biim and the complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ could be recorded in MeOH. Unfortunately, the neutral complex [Cr(Hbiim)₃] is not soluble enough to perform spectroscopy except for solid-state investigations (Fig. S40†). The UV parts of the absorption spectra are dominated by intraligand-centered π* ← π transitions (ILCT), which are split and red-shifted upon coordination to Cr³⁺ in the associated complexes. Additional LMCT (Ligand-to-Metal Charge Transfer) transitions are known to also contribute within the 330–400 nm range for these [CrN₆] chromophores (Fig. 3).²⁹,⁵⁵⁻⁵⁷ The spin-allowed, but parity forbidden ligand-field Cr(⁴T₂ ← ⁴A₂) transition (intensity 10 < ε < 100 M⁻¹ cm⁻¹, highlighted in Fig. 3b) can be detected as a minor contribution to the low-energy tail of the charge transfer bands.⁵⁸⁻⁶³ Careful spectral deconvolutions are required (Fig. S41–S43 and Tables
<figure><img src="image_11.png" /><figcaption>Fig. 3 UV-visible absorption spectra of (a) non-coordinated ligands H₂biim (purple) and Me₂biim (dark blue) and (b) complexes [Cr(H₂biim)₃(NO₃)₃] (orange), [Cr(Hbiim)₂(biim)]PPh₄ (green) and [Cr(Me₂biim)₃(CF₃SO₃)₃] (red) in MeOH at 293 K.</figcaption></figure>

S36–S39†) to safely assign E(⁴T₂), i.e., the energy of the Cr(⁴T₂) excited level with respect to the ground state E(⁴A₂) = 0 (Table 1, column 2).

Interestingly, for pseudo-octahedral d³ complexes, E(⁴T₂) provides a straightforward estimation of the ligand-field splitting (eqn (1)),⁶⁴,⁶⁵ which covers a narrow 19 956 ≤ Δ ≤ 21 026 cm⁻¹ range for [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₂(biim)]⁻ and [Cr(Me₂biim)₃]³⁺ in solution (Table 1).

$$ \Delta = E(^{4}\text{T}_{2}) - E(^{4}\text{A}_{2}) \quad (1) $$

As established for the programming of spin crossover processes while tuning the ligand-field strength,⁶⁶ the replacement of didentate 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen), made of two connected 6-membered heterocyclic rings, with 2,2'-biimidazole (H₂biim), made of two connected five-membered heterocyclic rings, is accompanied by an increase of the trigonal distortion in their pseudo-octahedral complexes. This is measured by a stepwise increase in the θ angular distor-

<figure><img src="image_12.png" /><figcaption>Fig. 4 NIR (a) absorption spectra of [Cr(H₂biim)₃](NO₃)₃ (water) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (acetonitrile) and (b) emission (λexc = 330 nm) spectra of complexes [Cr(H₂biim)₃](NO₃)₃ (orange), [Cr(Hbiim)₂(biim)]PPh₄ (green) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (red) in MeOH at 293 K.</figcaption></figure>

tion (θ[Cr(phen)₃]³⁺ = 47.5° < θ[Cr(bpy)₃]³⁺ = 63.6° < θ[Cr(Me₂biim)₃]³⁺ = 80.4° computed with eqn (A2-2) and gathered in Table A2-1, see Appendix 2†), which results in a concomitant stepwise reduction of the ligand-field strength Δ[Cr(bpy)₃]³⁺ ≈ Δ[Cr(phen)₃]³⁺ ≈ 22 500 cm⁻¹ > Δ[Cr(Me₂biim)₃]³⁺ ≈ 20 500 cm⁻¹ > Δ[Cr(tpy)₂]³⁺ = 18 750 cm⁻¹ (Table 1).

The interelectronic repulsion is estimated using the Racah parameters B (eqn (2)) and C (eqn (3)), which requires the energy of the lowest doublet levels E(²E) and E(²T₁) to be accessible (Fig. 5).⁶⁴,⁶⁵

$$ B = \sqrt{\frac{[E(2T_1) - E(2E)] \cdot \Delta}{26}} \quad (2) $$

$$ C = \frac{E(2E) - 9B + 50B^2/\Delta}{3} \quad (3) $$

Assuming an Oₕ symmetry, the ground state absorption bands Cr(²E ← ⁴A₂) and Cr(²T₁ ← ⁴A₂) have very weak intensities (0.05 < ε < 1 M⁻¹ cm⁻¹) due to the breaking of both parity
<table><caption>**Table 1** Energy of intrashell d-d transitions (in MeOH solution for E($^{4}$T$_{2}$) and in the solid state for E($^{2}$T$_{1}$) and E($^{2}$E)), ligand-field strength Δ (eqn (1)) and Racah parameters B (eqn (2)) and C (eqn (3)) for [Cr(H$_{2}$biim)$_{3}$](NO$_{3}$)$_{3}$, [Cr(Hbiim)$_{2}$(biim)]PPh$_{4}$, [Cr(Me$_{2}$biim)$_{3}$](CF$_{3}$SO$_{3}$)$_{3}$, and [Cr(phen)$_{2}$(H$_{2}$biim)]$^{3+}$ and for closely related pseudo-octahedral [Cr$^{III}$N$_{6}$] chromophores<sup>a</sup></caption><thead><tr><th>Complex</th><th>E(<sup>4</sup>T<sub>2</sub>)/cm<sup>-1</sup></th><th>E(<sup>2</sup>T<sub>1</sub>)/cm<sup>-1</sup></th><th>E(<sup>2</sup>E)/cm<sup>-1</sup></th><th>Δ/cm<sup>-1</sup></th><th>B/cm<sup>-1</sup></th><th>C/cm<sup>-1</sup></th><th>E(<sup>4</sup>T<sub>2</sub>) - E(<sup>2</sup>E)/cm<sup>-1</sup></th><th>Δ/B</th><th>C/B</th><th>Ref.</th></tr></thead><tbody><tr><td>[Cr(H<sub>2</sub>biim)<sub>3</sub>]<sup>3+</sup></td><td>20 268</td><td>14 376</td><td>13 717</td><td>20 268</td><td>717</td><td>2845</td><td>6551</td><td>28.3</td><td>4.0</td><td>This work</td></tr><tr><td>[Cr(Hbiim)<sub>3</sub>]</td><td>—</td><td>14 263</td><td>13 670</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>This work</td></tr><tr><td>[Cr(Hbiim)<sub>2</sub>(biim)]<sup>−</sup></td><td>19 956</td><td>14 096</td><td>13 482</td><td>19 956</td><td>686</td><td>2828</td><td>6474</td><td>29.1</td><td>4.1</td><td>This work</td></tr><tr><td>[Cr(Me<sub>2</sub>biim)<sub>3</sub>]<sup>3+</sup></td><td>21 026</td><td>14 164</td><td>13 538</td><td>21 026</td><td>712</td><td>2779</td><td>7488</td><td>29.6</td><td>3.9</td><td>This work</td></tr><tr><td>[Cr(phen)<sub>2</sub>(H<sub>2</sub>biim)]<sup>3+</sup></td><td>23 202</td><td>14 381</td><td>13 723</td><td>23 202</td><td>766</td><td>2697</td><td>9479</td><td>30.3</td><td>3.5</td><td>This work</td></tr><tr><td>[Cr(bpy)<sub>3</sub>]<sup>3+</sup></td><td>23 400</td><td>14 450</td><td>13 800</td><td>23 400</td><td>765</td><td>2730</td><td>9600</td><td>30.6</td><td>3.6</td><td>60</td></tr><tr><td>[Cr(phen)<sub>3</sub>]<sup>3+</sup></td><td>22 075</td><td>14 451</td><td>13 736</td><td>22 075</td><td>779</td><td>2700</td><td>8339</td><td>28.3</td><td>3.5</td><td>53</td></tr><tr><td>[Cr(tpy)<sub>2</sub>]<sup>3+</sup></td><td>18 750</td><td>13 584</td><td>12 953</td><td>18 750</td><td>790</td><td>2512</td><td>5797</td><td>23.7</td><td>3.2</td><td>61</td></tr><tr><td>[Cr(ddpd)<sub>2</sub>]<sup>3+</sup></td><td>22 990</td><td>13 550</td><td>12 903</td><td>22 990</td><td>756</td><td>2419</td><td>10 087</td><td>30.4</td><td>3.2</td><td>23</td></tr><tr><td>[Cr(dqp)<sub>2</sub>]<sup>3+</sup></td><td>24 937</td><td>13 864</td><td>13 405</td><td>24 937</td><td>656</td><td>2791</td><td>11 532</td><td>38.0</td><td>4.3</td><td>24</td></tr><tr><td>[Cr(dpc)<sub>2</sub>]<sup>3+</sup></td><td>19 200</td><td>—</td><td>9370</td><td>19 200</td><td>470</td><td>1880</td><td>9830</td><td>40.9</td><td>4.0</td><td>62</td></tr><tr><td>[Cr(CN)<sub>6</sub>]<sup>3+</sup></td><td>26 600</td><td>—</td><td>12 400</td><td>26 600</td><td>480</td><td>2800</td><td>14 200</td><td>55.4</td><td>5.8</td><td>63</td></tr><tr><td>[Cr(bik)<sub>3</sub>]<sup>3+</sup></td><td>23 094</td><td>14 771</td><td>14 044</td><td>23 094</td><td>804</td><td>2737</td><td>9050</td><td>28.7</td><td>3.4</td><td>57</td></tr><tr><td>[Cr(bim)<sub>3</sub>]<sup>3+</sup></td><td>21 008</td><td>14 859</td><td>14 104</td><td>21 008</td><td>781</td><td>2842</td><td>6904</td><td>26.9</td><td>3.6</td><td>57</td></tr><tr><td>[Cr(bie)<sub>3</sub>]<sup>3+</sup></td><td>20 747</td><td>14 837</td><td>14 124</td><td>20 747</td><td>754</td><td>2902</td><td>6623</td><td>27.5</td><td>3.8</td><td>57</td></tr></tbody></table>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 5** Jablonski diagram of homoleptic [Cr(H$_{2}$biim)$_{3}$]$^{3+}$ or [Cr(Me$_{2}$biim)$_{3}$]$^{3+}$ complexes showing the antenna effect upon UV excitation at 330 nm. The modeling of the energies of the Cr levels is taken from ref. 64.</figcaption></figure>

and spin conservation rules. These transitions could be detected in the NIR domain of the solid-state absorption spectra of the four investigated complexes (Fig. S40†), while related solution data could be recorded only for the most soluble [Cr(H$_{2}$biim)$_{3}$](NO$_{3}$)$_{3}$ (c ≥ 10$^{-2}$ M in water) and [Cr(Me$_{2}$biim)$_{3}$](CF$_{3}$SO$_{3}$)$_{3}$ (c ≥ 10$^{-2}$ M in acetonitrile) complexes (Fig. 4a). Introducing E($^{4}$T$_{2}$), E($^{2}$E) and E($^{2}$T$_{1}$) into eqn (1)–(3) provides the Racah parameters B and C collected in Table 1. Interestingly, 686 ≤ B ≤ 717 cm$^{-1}$

found for [Cr(H$_{2}$biim)$_{3}$](NO$_{3}$)$_{3}$, [Cr(Hbiim)$_{2}$(biim)]PPh$_{4}$ and [Cr(Me$_{2}$biim)$_{3}$](CF$_{3}$SO$_{3}$)$_{3}$ points to a global increase of the nephelauxetic effect when didentate 2,2'-bipyridine or 1,10-phenanthroline type ligands bound to Cr$^{3+}$ (765 ≤ B ≤ 790 cm$^{-1}$) are replaced with 2,2'-biimidazole type ligands.

For absorption spectra recorded in solution (Fig. 4a), it is possible to calculate the radiative rate constant of the emissive levels $^{2}$E and $^{2}$T$_{1}$ using the Strickler–Berg eqn (4) $^{67,68}$, which is derived from Einstein's relationship for spontaneous emission (Table 2, column 2 and Table S40†).$^{69}$

$$k_{\text{rad}} = 2303 \times \frac{8\pi c n^2 \bar{\nu} g_{\text{GS}}}{N_A g_{\text{ES}}} \int \epsilon(\bar{\nu}) d\bar{\nu} \quad (4)$$

Here c is the speed of light in vacuum (cm s$^{-1}$), n is the refractive index of the solvent, N$_{A}$ is the Avogadro number (mol$^{-1}$), g$_{GS}$ is the degeneracy of the ground state (g($^{4}$A$_{2}$) = 4), g$_{ES}$ is the degeneracy of the excited state (g($^{2}$T$_{1}$) = 6 and g($^{2}$E) = 4), $\bar{\nu}$ is the barycenter of the transition in the wavenumber (cm$^{-1}$) and $\int \epsilon(\bar{\nu})d\bar{\nu}$ is the area under the absorption spectrum of each transition (M$^{-1}$ cm$^{-2}$, Fig. S44 and S45†). The radiative rate constants of the emissive Cr($^{2}$E) excited level estimated for [Cr(H$_{2}$biim)$_{3}$]$^{3+}$ ($k_{\text{rad}}$ = 75(4) s$^{-1}$) and [Cr(Me$_{2}$biim)$_{3}$] ($k_{\text{rad}}$ = 73(4) s$^{-1}$) are smaller than those reported for [Cr(bpy)$_{3}$]$^{3+}$ ($k_{\text{rad}}$ = 182 s$^{-1}$) and [Cr(phen)$_{3}$]$^{3+}$ ($k_{\text{rad}}$ = 319 s$^{-1}$, Table 2),$^{58,69-73}$ but fall within the expected range for pseudo-octahedral [CrN$_{6}$] chromophores (Table S40†).$^{70-77}$ Upon room-temperature ligand-based excitation at 330 nm in solution, the complexes [Cr(H$_{2}$biim)$_{3}$](NO$_{3}$)$_{3}$, [Cr(Hbiim)$_{2}$(biim)]PPh$_{4}$ and [Cr(Me$_{2}$biim)$_{3}$](CF$_{3}$SO$_{3}$)$_{3}$ show the expected down-
<figure><img src="image_16.png" /></figure>

<table><caption>**Table 2** Experimental Cr(²E) lifetimes (τ) and radiative (k<sub>rad</sub>) and non-radiative (k<sub>non-rad</sub>) relaxation rate constants and intrinsic quantum yields (φ<sub>complex</sub> = k<sub>rad</sub>/(k<sub>rad</sub> + k<sub>non-rad</sub>)) of chromium complexes</caption><thead><tr><th>Complex</th><th>k<sub>rad</sub>/s<sup>-1</sup></th><th>293 K</th><th>Ar</th><th>μs</th><th>k<sup>293K</sup>air</th><th>b/10<sup>3</sup></th><th>s<sup>-1</sup></th><th>φ<sup>intrinic</sup></th><th>293 K, air</th><th>τ<sup>77</sup> K</th><th>μs</th><th>k<sup>77K</sup><sub>non-rad</sub>/s<sup>-1</sup></th><th>φ<sup>intrinic</sup></th><th>77 K</th><th>Ref.</th></tr><tr><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th>complex</th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>[Cr(H<sub>2</sub>biim)<sub>3</sub>]<sup>3+</sup></td><td>75(4)</td><td>0.47(2)<sup>d</sup></td><td>2120(106)</td><td>3(1) × 10<sup>-5</sup></td><td>0.44(2)<sup>d</sup></td><td>2260(113)</td><td>3(1) × 10<sup>-5</sup></td><td>2950(150)<sup>e</sup></td><td>263(13)</td><td>2.0(6) × 10<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>This work</td><td></td></tr><tr><td>[Cr(Hbiim)<sub>2</sub>][biim]<sup>−</sup></td><td>—</td><td>0.178(9)<sup>d</sup></td><td>—</td><td>—</td><td>0.141(7)<sup>d</sup></td><td>—</td><td>—</td><td>2780(140)<sup>e</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>This work</td><td></td></tr><tr><td>[Cr(Me<sub>2</sub>biim)<sub>3</sub>]<sup>3+</sup></td><td>73(4)</td><td>4.28(4)<sup>d</sup></td><td>234(12)</td><td>3(1) × 10<sup>-4</sup></td><td>3.31(9)<sup>d</sup></td><td>302(1)</td><td>2.4(8) × 10<sup>-4</sup></td><td>3005(150)<sup>e</sup></td><td>260(13)</td><td>2.2(7) × 10<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>This work</td><td></td></tr><tr><td>[Cr(phen)<sub>2</sub>(H<sub>2</sub>biim)]<sup>3+</sup></td><td>65(3)</td><td>38(1)<sup>d</sup></td><td>26(7)</td><td>2.5(8) × 10<sup>-3</sup></td><td>14(1)<sup>d</sup></td><td>71(7)</td><td>9(3) × 10<sup>-4</sup></td><td>2940(140)<sup>g</sup></td><td>—</td><td>1.9(6) × 10<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>This work</td><td></td></tr><tr><td>[Cr(bpy)<sub>3</sub>]<sup>3+</sup></td><td>182</td><td>74<sup>h</sup></td><td>13.3</td><td>1.4(4) × 10<sup>-2</sup></td><td>52<sup>h</sup></td><td>19</td><td>9(3) × 10<sup>-3</sup></td><td>5000<sup>j</sup></td><td>18</td><td>9(3) × 10<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>54, 58 and 70-72</td><td></td></tr><tr><td></td><td></td><td>39<sup>d</sup></td><td>25.5</td><td>7(2) × 10<sup>-3</sup></td><td>29<sup>d</sup></td><td>34</td><td>5(2) × 10<sup>-3</sup></td><td>6500<sup>j</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>≈1</td><td></td></tr><tr><td>[Cr(phen)<sub>3</sub>]<sup>3+</sup></td><td>319</td><td>356<sup>h</sup></td><td>2.49</td><td>1.1(4) × 10<sup>-1</sup></td><td>74<sup>h</sup></td><td>13</td><td>2.4(7) × 10<sup>-2</sup></td><td>2100<sup>g</sup></td><td>157</td><td>6.7(2) × 10<sup>-1</sup></td><td>53, 54, 58 and 70-73</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td></td><td></td><td>34<sup>d</sup></td><td>29.1</td><td>1.1(3) × 10<sup>-2</sup></td><td>22<sup>d</sup></td><td>45</td><td>7(2) × 10<sup>-3</sup></td><td>5300<sup>j</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>≈1</td><td></td></tr><tr><td>[Cr(phen)<sub>3</sub>]<sup>3+</sup></td><td>—</td><td>224<sup>h</sup></td><td>4.15</td><td>7(2) × 10<sup>-2</sup></td><td>37<sup>f</sup></td><td>27</td><td>1.2(3) × 10<sup>-2</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(tpy)<sub>2</sub>]<sup>3+</sup></td><td>—</td><td>270<sup>k</sup></td><td>3.38</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(dqP)<sub>2</sub>]<sup>3+</sup></td><td>30</td><td>1270<sup>h</sup></td><td>0.76</td><td>4(1) × 10<sup>-2</sup></td><td>83<sup>h</sup></td><td>12</td><td>2.5(8) × 10<sup>-3</sup></td><td>670<sup>g</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>61, 74 and 75</td><td></td></tr><tr><td></td><td></td><td>2140<sup>f</sup></td><td>0.44</td><td>6(2) × 10<sup>-2</sup></td><td>31<sup>f</sup></td><td>32</td><td>9(3) × 10<sup>-4</sup></td><td>3070<sup>j</sup></td><td>296</td><td>9(3) × 10<sup>-2</sup></td><td>24</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(CN)<sub>6</sub>]<sup>3-</sup></td><td>97</td><td>—</td><td>—</td><td>—</td><td>0.12<sup>f</sup></td><td>—</td><td>1.2(4) × 10<sup>-5</sup></td><td>3950<sup>l</sup></td><td>296</td><td>9(3) × 10<sup>-2</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(bik)<sub>3</sub>]<sup>3+</sup></td><td>31</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>4(1) × 10<sup>-3</sup></td><td>8220<sup>g</sup></td><td>156</td><td>4(1) × 10<sup>-1</sup></td><td>72</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(bim)<sub>3</sub>]<sup>3+</sup></td><td>37</td><td>209<sup>f</sup></td><td>4.75</td><td>7(2) × 10<sup>-3</sup></td><td>128<sup>g</sup></td><td>7.8</td><td>2.7(8) × 10<sup>-3</sup></td><td>6750<sup>g</sup></td><td>91</td><td>2.6(8) × 10<sup>-1</sup></td><td>57</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td>[Cr(bie)<sub>3</sub>]</td><td>60</td><td>131<sup>f</sup></td><td>7.60</td><td>5(2) × 10<sup>-3</sup></td><td>73<sup>f</sup></td><td>14</td><td>5(2) × 10<sup>-4</sup></td><td>5250<sup>g</sup></td><td>111</td><td>2.5(8) × 10<sup>-1</sup></td><td>57</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr><tr><td></td><td></td><td>143</td><td>—</td><td>4(1) × 10<sup>-4</sup></td><td>8<sup>f</sup></td><td>125</td><td>—</td><td>—</td><td>130</td><td>3(1) × 10<sup>-1</sup></td><td>57</td><td>—</td><td>—</td><td>—</td><td>—</td><td></td></tr></tbody></table>

<sup>a</sup>Recorded in degassed solution at room temperature. <sup>b</sup>Recorded in air-equilibrated solution at room temperature. <sup>c</sup>Recorded in a frozen solvent mixture at 77 K. <sup>d</sup>In MeOH. <sup>e</sup>In MeOH/H<sub>2</sub>O. <sup>f</sup>In CH<sub>3</sub>CN. <sup>g</sup>In CH<sub>3</sub>CN/C<sub>2</sub>H<sub>5</sub>CN. <sup>h</sup>In H<sub>2</sub>O. <sup>i</sup>In H<sub>2</sub>O/DMSO. <sup>j</sup>In aq. HClO<sub>4</sub>/MeOH. <sup>k</sup>In aq. HCl.

shifted NIR spin-flip Cr(²E → ⁴A₂) phosphorescence at 730–740 nm together with weak shoulders corresponding to Cr(²T₁ → ⁴A₂) within the 700–710 nm domain (Fig. 4b and 5). The latter dual emission disappears at 77 K (Fig. S39†) as a result of the depopulation of the high-energy Cr(²T₁) level, and a single band Cr(²E → ⁴A₂) contributes to phosphorescence. Very similar results are observed in the solid state upon 330 nm excitation for the four complexes [Cr(H₂biim)](NO₃)₃, [Cr(Hbiim)₃], [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)](CF₃SO₃)₃ (Fig. S40b†), which ultimately demonstrate (i) the expected negligible Stokes shifts affecting the spin-flip Cr(²T₁, ²E ↔ ⁴A₂) transitions (Fig. S46†) and (ii) the efficient sensitization of the spin-flip phosphorescence by all the accessible ligand-based excited states (Fig. S47†).

Upon pulsed laser excitation at 355 nm at 77 K in frozen MeOH/H₂O solutions, the characteristic lifetime of the emissive Cr(²E) levels for [Cr(H₂biim)₃](NO₃)₃ (τ<sup>77 K</sup> = 3.0(1) ms), [Cr(Hbiim)₂(biim)]PPh₄ (τ<sup>77 K</sup> = 2.7(1) ms) and [Cr(Me₂biim)](CF₃SO₃)₃ (τ<sup>77 K</sup> = 3.0(1) ms) tends toward the radiative lifetime τ<sub>rad</sub> = 13.4(7) ms, in agreement with minor non-radiative vibrational quenching constants at this temperature for rigid triple-helical units, as similarly reported for [Cr(bpy)₃]<sup>3+</sup> and [Cr(phen)₃]<sup>3+</sup> (Table 2 column 8). At room temperature, the Cr(²E) lifetimes drastically drop below the microsecond range for [Cr(H₂biim)](NO₃)₃ and [Cr(Hbiim)₂(biim)]PPh₄, which possess high-energy N-H oscillators, while the decrease of the lifetime for the methylated analogue [Cr(Me₂biim)](CF₃SO₃)₃ (τ<sup>293 K, Ar</sup> = 4.28(4) µs) is less dramatic and mirrors those reported for [Cr(bpy)₃]<sup>3+</sup> and [Cr(phen)₃]<sup>3+</sup> (Table 2 and Fig. S48–S56†). Consequently, the non-radiative rate constant k<sub>non-rad</sub><sup>293K,Ar</sup> = 1/τ<sup>293 K, Ar</sup> - k<sub>rad</sub> = 2.1(1) × 10<sup>6</sup> s<sup>-1</sup> measured for [Cr(H₂biim)₃](NO₃)₃ in MeOH corresponds to the largest vibrational quenching process along the series of [Cr(N<sup>+</sup>N)₃]<sup>3+</sup> chromophores collected in Table 2. Finally, the presence of ³O₂ in solution has only a minor effect on the lifetime, indicating that quenching by oxygen is not a major contributor to energy relaxation in these systems (τ<sup>293 K, air</sup> in Table 2, column 6).

## Acid-base properties of homoleptic [Cr(H₂biim)₃]<sup>3+</sup>, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

In order to extract the acid-base thermodynamic constants connecting [Cr(H₂biim)₃]<sup>3+</sup> with its successive deprotonated forms [Cr(H₂biim)₂(Hbiim)]<sup>2+</sup> (Kₐ₁), [Cr(H₂biim)(Hbiim)₂]<sup>+</sup> (Kₐ₂), [Cr(Hbiim)₃] (Kₐ₃) and [Cr(Hbiim)₂(biim)]⁻ (Kₐ₄), we performed two successive pH-metric titrations of aqueous solutions of [Cr(H₂biim)₃](NO₃)₃ using NaOH as the titrant. An initial addition of 0.2 equivalents of HNO₃ ensured that the complex exists initially in its fully protonated form [Cr(H₂biim)₃]<sup>3+</sup> (Fig. 6). The pH curve profile is different from the standard pH-metric titration expected for a weak polyacid with a strong base due to the concomitant precipitation of insoluble [Cr(Hbiim)₃] (Fig. 6). It is therefore not possible to extract the searched pKₐ values and one can only estimate qualitatively pKₐ₁, pKₐ₂ < 7. Moreover, the preservation of [Cr(Hbiim)₃] as a precipitate in the presence of a large excess
<figure><img src="image_17.png" /><figcaption>**Fig. 6** Reproducible acid-base titrations (blue trace for titration 1 and red trace for titration 2) of a solution of $[Cr(H_2biim)_3](NO_3)_3$ + 0.2 eq. $HNO_3$ with NaOH (water, 293 K) highlighting the formation of an insoluble yellow precipitate.</figcaption></figure>

of base indicates its negligible deprotonation for pH ≤ 12 in water and $pK_{a4} > 12$.

## Preparation and structures of the heteroleptic $[Cr(phen)_2(H_xbiim)]^{(1+x)^+}$ ($x = 2-0$) complexes

In order to limit the number of bound deprotonatable ligands and the concomitant formation of insoluble neutral complexes, we followed the Kane-Maguire synthetic strategy for preparing inert heteroleptic $[Cr(phen)_2(H_2biim)]^{3+}$ (Fig. 7).⁵⁴ Taking advantage of the *trans* influence,⁷⁸ the complexation of 2.0 eq. of 1,10-phenanthroline (phen) to $CrCl_3$ yields almost quantitatively $cis-[Cr(phen)_2Cl_2]^+$ under reducing conditions for catalyzing $Cr(m)/Cr(u)$ exchange processes and ligand-exchange dynamics.⁵³ The replacement of inert Cr-Cl bonds with labile Cr-OSO₂CF₃ bonds⁷⁹⁻⁸¹ was performed under soft conditions by using Ag(O₃SCF₃) instead of an excess of triflic acid.²⁴ The addition of $H_2biim$ finally provided $[Cr(phen)_2(H_2biim)](CF_3SO_3)_3$ in moderate yield (37%, Fig. 7). Subsequent deprotonation with aqueous NaOH under stoichiometric conditions gave $[Cr(phen)_2(Hbiim)](CF_3SO_3)_2$, whereas the use of an excess of base yielded the doubly deprotonated complex $[Cr(phen)_2(biim)](CF_3SO_3)$. X-ray quality monocrystals could be grown by slow diffusion of diethyl ether into solutions of the complexes either in acetonitrile to give $[Cr(phen)_2(H_2biim)](CF_3SO_3)_3(H_2O)_{0.25}$ (Fig. 7 and Tables S18-S20†) or in methanol to provide $[Cr(phen)_2(biim)](CF_3SO_3)(CH_3OH)_{1.5}$ (Fig. 7 and Tables S24-S26†). X-ray quality prisms of $[Cr(phen)_2(Hbiim)](CF_3SO_3)_2(H_2O)_{0.5}$ (Fig. 7 and Tables S21-S23†) were isolated from a cooled (4 °C) aqueous solution.

The molecular structures of the three $[Cr(phen)_2(H_xbiim)]^{(1+x)^+}$ cations ($x = 2-0$) display pseudo-octahedral $[CrN_6]$ chromo-

<figure><img src="image_18.png" /><figcaption>**Fig. 7** Synthesis of the heteroleptic $cis-[Cr(phen)_2Cl_2]^{+}$ (CCDC-1865022),⁵³ $[Cr(phen)_2(H_2biim)]^{3+}$, $[Cr(phen)_2(Hbiim)]^{2+}$ and $[Cr(phen)_2(biim)]^{+}$ complexes. The molecular structures of the metallic complexes are those found in the associated crystal structures. Color code: C = grey, N = blue, H = white, and Cr = orange. The counter-ions and hydrogen atoms (for 1,10-phenanthroline ligands) are omitted for clarity.</figcaption></figure>

phores with bond lengths and trigonal distortions typical of chromium complexes bound by three didentate 5-membered chelate polyaromatic ligands (Appendix 3†), similar to the discussion in the previous section for the homoleptic analogues $[Cr(H_2biim)_3]^{3+}$, $[Cr(Hbiim)_3]$ and $[Cr(Hbiim)_2(biim)]^-$ (see Appendix 2†). For the heteroleptic complexes, one notices that the Cr-N bond lengths are shorter for the bound biimidazole ligands compared to those of the bound phen ligands ($d_{Cr-N(biim)} < d_{Cr-N(phen)}$, Table 3). The contraction of the $d_{Cr-N(biim)}$ bond lengths upon stepwise deprotonation can be assigned to the increased basicity of the bound biimidazole ligand. The compensating longer $d_{Cr-N(phen)}$ bond lengths result from the reduced charge borne by the central chromium metal. In terms of intermolecular hydrogen bonds, the bound protonated $H_2biim$ ligand in $[Cr(phen)_2(H_2biim)](CF_3SO_3)_3(H_2O)_{0.25}$ acts as a NH donor for acceptor oxygen atoms of triflate counter-anions and interstitial water molecules. For the mono-deprotonated bound $Hbiim^-$ ligand in $[Cr(phen)_2(Hbiim)](CF_3SO_3)_2(H_2O)_{0.5}$, intermolecular hydrogen bonds between two adjacent complexes through N-H···N bonds are observed (Fig. A3-3†), as previously described for $[Cr(Hbiim)_3]$ (Fig. A2-4†). Finally, the totally deprotonated bound $biim^{2-}$ ligand in $[Cr(phen)_2(biim)](CF_3SO_3)(CH_3OH)_{1.5}$ is not involved in hydrogen bonding.
<table><caption>**Table 3** Comparison of the average Cr–N bond lengths (d̃$_{Cr-N}$)ª of complexes [Cr(phen)₂(H₂biim)](CF₃SO₃)₃(H₂O)₀.₂₅, [Cr(phen)₂(Hbiim)](CF₃SO₃)₂(H₂O)₀.₅ and [Cr(phen)₂(biim)](CF₃SO₃)(CH₃OH)₁.₅ in their crystal structures</caption><thead><tr><th>Complex</th><th>d̃<sub>Cr-N</sub>/Å</th><th>d̃<sub>Cr-N(phen)</sub>/Å</th><th>d̃<sub>Cr-N(biim)</sub>/Å</th></tr></thead><tbody><tr><td>[Cr(phen)₂(H₂biim)](CF₃SO₃)₃</td><td>2.04(2)</td><td>2.054(3)</td><td>2.024(7)</td></tr><tr><td>[Cr(phen)₂(Hbiim)](CF₃SO₃)₂</td><td>2.05(2)</td><td>2.062(6)</td><td>2.018(0)</td></tr><tr><td>[Cr(phen)₂(biim)]CF₃SO₃</td><td>2.05(4)</td><td>2.07(1)</td><td>1.997(5)</td></tr></tbody></table>

ª The standard deviations refer to deviations from the computed averages.

In the IR spectra of the heteroleptic complexes, the O-H stretching vibrations of co-crystallized water or methanol molecules involved in hydrogen bonding (3500 ≤ νOH ≤ 2500 cm⁻¹) hinder a straightforward interpretation of N-H stretching bands associated with the bound Hₓbiim ligands (x = 2-0; Fig. S57†). On the other hand, and as previously mentioned for the related homoleptic complexes, the ESI-MS spectra recorded in acetonitrile do not vary significantly with the degree of protonation of the bound 2,2'-biimidazole ligands for the different [Cr(phen)₂(Hₓbiim)]⁽¹⁺ₓ⁾ cations (x = 2-0, Fig. S58†). High-resolution ESI-MS analyses confirm the formation of [Cr(phen)₂(Hbiim)]²⁺ as the major gas-phase cation, regardless of the degree of protonation of the bound 2,2'-biimidazole ligand in the selected complex (Table S41 and Fig. S58-62†).

## Acid-base properties of the heteroleptic [Cr(phen)₂(H₂biim)]³⁺ complex

The successive deprotonation of bound 2,2'-biimidazole in [Cr(phen)₂(H₂biim)]³⁺ has been quantitatively studied by pH metric titrations in water at fixed ionic strength (0.1 M KNO₃, Fig. 8a). The two successive pH jumps (5 ≤ pH ≤ 8 and 9 ≤ pH ≤ 11) are accompanied by concomitant abrupt changes in colors from yellow ([Cr(phen)₂(H₂biim)]³⁺) to dark orange ([Cr(phen)₂(Hbiim)]²⁺) and finally to dark brown ([Cr(phen)₂(biim)]⁺, Fig. 8a). The associated occupancy factors θ⁺[Cr(phen)₂(biim)] experimentally obtained with eqn (5) (red circles in Fig. 8b) display a two-step binding isotherm typical of the anti-cooperative successive fixation of two protons according to equilibria (6)–(7) and modeled with eqn (8). The best fits for the two successive deprotonation steps of [Cr(phen)₂(H₂biim)]³⁺ (black trace in Fig. 8b) correspond to pKₐ₁ = 4.67(3) and pKₐ₂ = 8.59(11), which are approximately eight orders of magnitude more acidic than those measured for free 2,2'-biimidazole (pKₐ₁ = 12.31 and pKₐ₂ = 16.33),²⁵

$$ \theta^{H^+}_{[Cr(phen)_2biim]} = \frac{|H^+|_{\text{bound}}}{2|[Cr(phen)_2biim]|_{\text{tot}}} = \frac{2|[Cr(phen)_2biim]|_{\text{tot}} - (V_{\text{NaOH}} \cdot c_{\text{NaOH}}/V_{\text{tot}}) - |H^+|}{2|[Cr(phen)_2biim]|_{\text{tot}}} \quad (5) $$

$$ \begin{gathered} [Cr(phen)_2(biim)]^+ + H^+ \rightarrow [Cr(phen)_2(Hbiim)]^{2+} \quad (6) \\ \log(\beta_{1,1}^{\text{Crbiim,H}}) = -pK_{a2} \end{gathered} $$

<figure><img src="image_19.png" /><figcaption>**Fig. 8** (a) Titration of 51 mg (52 µmol) of [Cr(phen)₂(H₂biim)](CF₃SO₃)₃ (10 mL aqueous KNO₃ 0.1 M, c = 5.2 mmol L⁻¹) with NaOH 0.1 N highlighting the color changes and (b) associated binding isotherm depicted as plots of experimental (red circles, eqn (5)) and fitted (dashed black trace, eqn (8)) occupancy factors as a function of log(|H⁺|).</figcaption></figure>

$$ \begin{gathered} [Cr(phen)_2(biim)]^+ + 2H^+ \rightarrow [Cr(phen)_2(H_2biim)]^{3+} \\ \log(\beta_{1,2}^{\text{Crbiim,H}}) = -(pK_{a1} + pK_{a2}) \end{gathered} \quad (7) $$

$$ \theta^{H^+}_{[Cr(phen)_2biim]} = \frac{\beta_{1,1}^{\text{Crbiim,H}}|H^+| + 2\beta_{1,2}^{\text{Crbiim,H}}|H^+|^2}{2(1 + \beta_{1,1}^{\text{Crbiim,H}}|H^+| + \beta_{1,2}^{\text{Crbiim,H}}|H^+|^2)} \quad (8) $$

Compared with [CoIII(en)₂(H₂biim)]³⁺ (pKₐ₁ = 5.5 and pKₐ₂ = 9.9; ionic radius = 0.545 Å),⁴¹ the one order of magnitude lower pKₐ measured for [CrIII(phen)₂(H₂biim)]³⁺ (ionic radius = 0.615 Å) suggests that the {CrIII(phen)₂}³⁺ scaffold (with respect to {CoIII(en)₂}³⁺) better stabilizes the negative charges brought by the bound deprotonated 2,2'-biimidazole ligand. Consequently, upon successive deprotonation, one can reasonably predict the appearance of low energy (phen)π* ← (Hₓbiim)π (x = 2-0) intramolecular ligand-to-ligand charge transfer (LLCT) bands in the absorption spectra of [CrIII(phen)₂(Hₓbiim)]⁽¹⁺ₓ⁾ upon deprotonation. These tran-

<figure><img src="image_20.png" /></figure>
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>Fig. 9 Absorption spectra of (a) [Cr(phen)₂(H₂biim)]³⁺ (orange trace), [Cr(phen)₂(Hbiim)]²⁺ (red trace) and [Cr(phen)₂(biim)]⁺ (black trace) in CH₃CN (c ≈ 10⁻⁵ mol L⁻¹) and (b) [Cr(phen)₂(H₂biim)]³⁺ in CH₃CN (c ≈ 10⁻² mol L⁻¹). LMCT = ligand to metal charge transfer and LLCT = ligand to ligand charge transfer.</figcaption></figure>

sitions are confirmed by TD-DFT calculations (Fig. A4-2 to A4-10 and Tables A4-6 to A4-8 in Appendix 4†) and indeed observed in solution (Fig. 9).

Finally, taking advantage of the pH-dependence of the absorption spectra, the determination of the pKₐ values in water at ionic strength close to zero (I ≈ 0) could be carried out with the help of spectrophotometry to give pKₐ₁ = 3.9(1) and pKₐ₂ = 7.8(1) (Fig. S63†), which are close to those determined at I = 0.1 M (KNO₃).

## Photophysical properties of the heteroleptic [Cr(phen)₂(Hₓbiim)]⁽¹⁺⁾⁺ (x = 2–0) complexes

The three heteroleptic complexes exhibit similar and intense absorption bands in the UV range (below 350 nm, Fig. 9a), which can be assigned to intraligand π-π* transitions (ILCT) completed by variable amounts of interligand (LLCT), ligand to-metal (LMCT) and metal-to-ligand (MLCT) charge transfer bands (see Appendix 4† for TD-DFT calculations). The spectra differ in the visible domain (350–750 nm, highlighted in

Fig. 9a) mainly due to a shift of the interligand (phen)π* ← (Hₓbiim)π (x = 0–2, LLCT) transitions toward lower energies upon successive deprotonations (Fig. S64 and Fig. A4-4, A4-7 and A4-9†). This shift is responsible for the color change accompanying the stepwise deprotonation of [Cr(phen)₂(H₂biim)]³⁺ (Fig. 8a). According to theoretical TD-DFT calculations (Table 1, entries 1–3) and CASSCF(7,12)/FIC-NEVPT2 (Table A4-2†) and in line with the trend observed in the homoleptic complexes, the ligand-field strength Δ, remniscent of Cr(⁴T₂ ← ⁴A₂) transitions in octahedral geometry, also decreases with stepwise deprotonation: [Cr(phen)₂(H₂biim)]³⁺ (412.9 nm, Δ ≈ 24 219 cm⁻¹) > [Cr(phen)₂(Hbiim)]²⁺ (434 nm, Δ ≈ 23 042 cm⁻¹) > [Cr(phen)₂(biim)]⁺ (457.8 nm, Δ ≈ 21 844 cm⁻¹). This reduction in the ligand-field strength is accompanied by (i) a decrease of the computed total spin density at the chromium center (Fig. A4-1†) which reflects the reduced total positive charge borne by this atom and (ii) a negligible change in the estimated Racah parameters B and C (Table A4-1†). One can thus predict that the stronger interactions with the deprotonated 2,2-biimidazole ligand (d̄ₚₘₐₙₐₘ) is becoming shorter) in [Cr(phen)₂(Hₓbiim)]⁽¹⁺⁾⁺ (x = 2–0) are more than compensated by the removal of the bound 1,10-phenanthroline ligands (d̄ₚₘₐₙₐₙ) is becoming longer) as exemplified in the molecular structures in the crystalline state (push-pull effect, Table 3).

The NIR absorption spectrum of [Cr(phen)₂(H₂biim)]³⁺ (Fig. 9b) exhibits two well-resolved absorption bands that are assigned to the spin-flip transitions Cr(²E ← ⁴A₂) and Cr(²T₁ ← ⁴A₂) assuming Oₕ symmetry and with ε = 0.27 and 0.19 M⁻¹ cm⁻¹, respectively. For the deprotonated derivatives [Cr(phen)₂(Hbiim)]²⁺ and [Cr(phen)₂(biim)]⁺, the larger residual interligand charge transfer bands mask these weak forbidden spin-flip transitions (Fig. S65†). Consequently, Racah parameters B = 766 cm⁻¹ and C = 2697 cm⁻¹ (eqn (2) and (3), Table 1, entry 4) together with the radiative rate constant of kₚₐ = 65(3) s⁻¹ (eqn (4), and Table 2, column 1), typical of [CrN₆] chromophores, could be estimated only for [Cr(phen)₂(H₂biim)]³⁺.

At room temperature in CH₃CN, only [Cr(phen)₂(H₂biim)]³⁺ is emissive and shows the typical dual Cr(²T₁ → ⁴A₂) and Cr(²E → ⁴A₂) emission observed for many Cr(III) complexes (Fig. 10a). In addition to [Cr(phen)₂(H₂biim)]³⁺, the NIR emission of the complex [Cr(phen)₂(Hbiim)]²⁺ can be detected at low temperature in frozen solvent mixtures (CH₃CN/C₂H₅CN 6 : 4 at 77 K, Fig. 10b), whereas [Cr(phen)₂(biim)]⁺ remains non-emissive. Compared with the emission band of [Cr(phen)₂(H₂biim)]³⁺ (λₘₐₓ = 732 nm; ν = 13 661 cm⁻¹), the first deprotonated analog [Cr(phen)₂(Hbiim)]²⁺ shows a red-shifted Cr(²E → ⁴A₂) transition at λₘₐₓ = 750 nm; ν = 13 333 cm⁻¹ (Fig. 10b). Additionally, the spectrum of [Cr(phen)₂(Hbiim)]²⁺ contains a band foot at 733 nm originating from a small amount of [Cr(phen)₂(H₂biim)]³⁺, which is inevitably present in solution due to the proton-transfer equilibrium (9):

$$2[\text{Cr}(\text{phen})_2(\text{Hbiim})]^{2+} \rightleftharpoons [\text{Cr}(\text{phen})_2(\text{H}_2\text{biim})]^{3+} + [\text{Cr}(\text{phen})_2(\text{biim})]^{+} \quad K_{\text{exch}} = \frac{K_{\text{a2}}}{K_{\text{a1}}} \quad (9)$$
<figure><img src="image_23.png" /><figcaption>**Fig. 10** Emission spectra ($\lambda_{exc} = 350$ nm) of (a) $[Cr(phen)_2(H_2biim)]^{3+}$ at 293 K in CH$_3$CN and (b) $[Cr(phen)_2(H_2biim)]^{3+}$ (orange trace) and $[Cr(phen)_2(Hbiim)]^{2+}$ (red trace) at 77 K in frozen CH$_3$CN/C$_2$H$_5$CN (6 : 4).</figcaption></figure>

Introducing $K_{a1}$ and $K_{a2}$ gives $K_{exch} = 1.2(2) \times 10^{-4}$, from which the ratio of the equilibrium concentration $\frac{\left| [Cr(phen)_2(Hbiim)] \right|}{\left| [Cr(phen)_2(H_2biim)] \right|} = \sqrt{K_{exch}} = 1.1(1) \times 10^{-2}$ implies contamination of $[Cr(phen)_2(Hbiim)]^{2+}$ by circa 1% with the more emissive (protonated) $[Cr(phen)_2(H_2biim)]^{3+}$ complex (red trace in Fig. 10b).

Excited state lifetimes for the NIR emission arising from the $Cr(^2E)$ excited state upon excitation at 355 nm were recorded in solution at room temperature and at 77 K for $[Cr(phen)_2(H_2biim)]^{3+}$, and only at 77 K for $[Cr(phen)_2(Hbiim)]^{2+}$ (Fig. S67-S70†). In frozen solutions at 77 K, $[Cr(phen)_2(H_2biim)]^{3+}$ displays a mono-exponential decay of 2.94 ms, which is typical of Cr(III)-polyimine complexes (Table 2). The emission decay curve of $[Cr(phen)_2(Hbiim)]^{2+}$ could not be fit with a mono-exponential function. Since the complex exists as a 98 : 1 : 1 mixture of $[Cr(phen)_2(Hbiim)]^{2+}$, $[Cr(phen)_2(H_2biim)]^{3+}$ and $[Cr(phen)_2(biim)]^{+}$ (eqn (9)), one

expects multi-exponential decays weighted by the mole fractions and the quantum yields of each contributor. A rough bi-exponential fit (Fig. S70†) is compatible with the experimental decay curves showing a long component (1.77(9) ms), which is reminiscent of $[Cr(phen)_2(H_2biim)]^{3+}$, and a short contribution (723(30) µs) which is tentatively assigned to $[Cr(phen)_2(Hbiim)]^{2+}$, for which the smaller energy gap between the emissive doublet state level $Cr(^2E)$ and the silent LLCT band probably boosts the efficiency of non-radiative decay (Fig. 11 and Appendix 4†). At room temperature, only the protonated complex $[Cr(phen)_2(H_2biim)]^{3+}$ is emissive and it exhibits a mono-exponential emission decay of 38 µs in deaerated CH$_3$CN. This excited state lifetime is orders of magnitude longer than those of the three isolated homoleptic parent complexes $[Cr(H_xbiim)_3]^{n+}$ (Table 2) due to the replacement of two 2,2'-biimidazole ligands with two 1,10-phenanthroline units, which are devoid of high-energy N-H stretching vibrations. The lifetime of $[Cr(phen)_2(H_2biim)]^{3+}$ is in the same range as that of $[Cr(bpy)_3]^{3+}$; however, it is one order of magnitude shorter than that of $[Cr(phen)_3]^{3+}$ (Table 2). The emission lifetime is reduced to 14 µs in aerated solution because of some additional quenching via energy transfer to the $^3O_2$ molecules present in solution.

Finally, the global luminescence quantum yield of the complex $[Cr(phen)_2(H_2biim)]^{3+}$ upon ligand-based excitation at $\lambda_{exc} = 450$ nm was determined experimentally using the relative method (Fig. S71†). We found $\phi_{complex}^{global} = 2.8(3) \times 10^{-3}$ in deaer-

<figure><img src="image_24.png" /><figcaption>**Fig. 11** Jablonski diagram of heteroleptic $[Cr^{III}(phen)_2(H_xbiim)]^{(1+x)^+}$ complexes showing the antenna effect upon UV excitation at 330 nm, the existence of low-energy ligand-to-ligand charge transfer excited levels and their potential unfavorable effect on the global quantum yields via back energy transfer (BET) processes.</figcaption></figure>
ated acetonitrile and $\phi_{\text{complex}}^{\text{global}} = 9.5(9) \times 10^{-4}$ in the presence of dioxygen at room temperature. One further notes that the intrinsic Cr(III)-centered quantum yields calculated with the help of the emission lifetimes $\tau_{\text{tot}}(2\text{E})$ and $k_{\text{rad}}(2\text{E})$ collected at room temperature (Table 2 and eqn (10)) amount to $\phi_{\text{complex}}^{\text{intrinsic}} = 2.5 \times 10^{-3}$ in deaerated acetonitrile and $\phi_{\text{complex}}^{\text{intrinsic}} = 9.1 \times 10^{-4}$ in aerated acetonitrile.

$$ \phi_{\text{complex}}^{\text{intrinsic}} = \frac{k_{\text{rad}}}{k_{\text{rad}} + k_{\text{non-rad}}} = \frac{\tau_{\text{tot}}}{\tau_{\text{rad}}} \quad (10) $$

Since (i) $\phi_{\text{complex}}^{\text{global}} = \eta_{\text{sens}}\phi_{\text{complex}}^{\text{intrinsic}}$ and (ii) $\phi_{\text{complex}}^{\text{global}} \approx \phi_{\text{complex}}^{\text{intrinsic}}$, one concludes that the ligand-to-metal sensitization process is close to being quantitative ($\eta_{\text{sens}} \approx 100\%$). These values are in the same range as those reported for $[\text{Cr(bpy)}_3]^{3+}$ ($\phi_{\text{complex}}^{\text{global, noair}} = 1.7 \times 10^{-3}$ in deaerated acetonitrile and $\phi_{\text{complex}}^{\text{global, air}} = 8.9 \times 10^{-4}$ in aerated water),$^{70,71,79,80}$ but four times smaller than that of $[\text{Cr(phen)}_3]^{3+}$ ($\phi_{\text{complex}}^{\text{global, noair}} = 1.2 \times 10^{-2}$ in deaerated water + 1 M HCl),$^{53,81}$ which makes $[\text{Cr(phen)}_2(\text{H}_2\text{biim})]^{3+}$ a moderate emitter for a Cr(III) complex.$^{72}$

## Conclusions

In line with the only minor interest attracted by the homoleptic $[\text{Cr(H}_x\text{biim)}_3]^{n+}$ during the past few decades,$^{35-38}$ we confirm here that their low solubility in any common solvent upon stepwise deprotonation, together with their poorly attracting photo-physical properties (a short phosphorescence lifetime and negligible intrinsic quantum yield), makes them poorly adapted to be involved in the complex-as-ligand strategy for programming the assemblies of polymetallic optically-active Cr-based complexes. The solution to the problem arises from the successful synthesis of the soluble heteroleptic $[\text{Cr(phen)}_2(\text{H}_x\text{biim})]^{(1+x)}$ ($x = 2-0$) complexes via a modified Kane-Maguire strategy. The bound H$_2$biim ligand can be stepwise deprotonated in solution at pH compatible for further complexation processes with d-block or f-block cations. The deprotonation processes are accompanied by characteristic color changes resulting from the appearance of low-energy intramolecular interligand (phen)π* $\leftarrow (\text{H}_x\text{biim})\pi$ ($x = 2-0$, LLCT) ligand-to-ligand charge transfer transitions confirmed by theoretical TD-DFT calculations. The latter reversible process makes the putative heterometallic dyads $[\text{Cr(phen)}_2(\text{biim})-\text{M}^{2+}]^{(z+1)+}$ ($\text{M}^{2+}$ is an open-shell d- or f-block cation) reminiscent of protonated $[\text{Cr(phen)}_2(\text{H}_2\text{biim})]^{3+}$ in terms of photophysical properties, which paves the way for their use as sensitizers in luminescent polymetallic assemblies.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

Financial support from the Swiss National Science Foundation is gratefully acknowledged (grant 200020_207313).

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