Bis-ferrocenyl-pyridinediimine trinuclear mixed-valent complexes with metal-binding dependent electronic coupling: Synthesis, structures, and redox-spectroscopic characterization

Structures, and Redox-Spectroscopic Characterization

Article

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes

with Metal-Binding Dependent Electronic Coupling: Synthesis,

Cole Carter, Yosi Kratish, Titel Jurca, Yanshan Gao,* and Tobin J. Marks*

Cite This: https://dx.doi.org/10.1021/jacs.0c10015

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sı Supporting Information

ABSTRACT: A family of metal dichloride complexes having a bis-

ferrocenyl-substituted pyridinediimine ligand was systematically

synthesized ((Fc PDI)MCl , M = Mg, Zn, Fe, and Co) and

characterized crystallographically, spectroscopically, electrochemically,

and computationally. Electronic coupling between the ligand ferrocene

units is switched on upon binding to a MCl fragment, as evidenced by

both sequential oxidation of the ferrocenes in cyclic voltammetry

(

ΔE ≈ 200 mV) and by Inter-Valence Charge Transfer electronic

excitations in the near IR. Additionally, UV−vis spectra are used to

directly observe orbital mixing between the ferrocenyl units and the

imine π system since breaking of the orbital symmetry results in

allowed transitions (ϵ = 2800 M cm vs ϵ ≈ 200 M cm in free

ferrocene) as well as broadening and red-shifting of the ferrocenyl

−

−1

transitionsindicating organic character in formerly pure metal-centered transitions. DFT analysis reveals that interaction between

the ferrocenes and the MCl fragment is small and suggests that communication is mediated by better energy matching between the

ferrocene and organic π* orbitals upon coordination, allowing superexchange coupling through the LUMO. Furthermore, single

crystal diﬀraction data obtained from oxidation of one and both ferrocenes show distortions, introducing the empty d /d

orbitals

xy x2‑y2

into the secondary coordination sphere of the MCl fragment. Such structural rearrangements are infrequent in ferrocenyl mixed-

valent compounds, and implications for catalysis as well as electronic communication are discussed.

INTRODUCTION

metal centers. Although most ligands can become “non-

4,6

■

innocent” under the appropriate conditions, the pyridine-

diimine (“PDI”; Scheme 1) ligand family has become a

Traditional paradigms in understanding the reactivity of

coordination complexes focus predominantly on metal identity

and formal oxidation state. In catalysis, the metal center is

conventionally viewed as the primary site of reactivity, and this

metal-centric view is fruitful when the metal center’s properties

and its interactions with substrates can be reliably predicted

as is often the case in second- or third-row transition metals

where strong ligand ﬁelds tend to localize frontier orbitals on

prominent example in noninnocent ligand catalysis. PDI

ligands can act as electron reservoirs to store electrons or other

reducing equivalents (e.g., hydrides or alkyls at electrophilic

7c,10

sites) during catalytic cycles,

permitting ﬁrst-row metal

complexes to both emulate the 2-electron chemistry of noble

metals, as well as access highly reduced states. While the

reactivity of such metal−ligand coupled systems has been

extensively studied, challenges remain in deﬁning electron

localization/delocalization throughout the complex, and

reliably predicting which scaﬀolds will exhibit strong

delocalization. Such questions are central to the study of

mixed-valent compounds, which contain multiple metal centers

in diﬀerent oxidation states connected by π-linkers that serve

either the metal or ligand. However, there is growing

recognition that this view is not generalizable to all complexes,

particularly those of ﬁrst-row metals with highly conjugated

ligands. The combination of weak ligand ﬁelds and highly

delocalized frontier orbitals generates complexes with proper-

ties not uniquely deﬁned by metal identity and oxidation state,

as the frontier orbitals involve substantial mixing with the π-

ligand backbone. As such, these catalytic systems can be

Received: October 1, 2020

viewed as being electronically coupled between the metal and

ligandas delocalization of frontier orbitals across the metal

and ligand causes changes at one site to perturb the chemistry

at the other.

An increasing number of catalysts take advantage of such

non-innocent” ligands that are electronically coupled to

“

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Scheme 1. Generic Bis-Ferrocenyl and PDI Complexes and Guiding Design of a Bis-Ferrocenyl PDI Complex

as a conduit for delocalization. Diverse spectroscopic and

theoretical tools have been developed for quantifying and

characterizing the degree of coupling and delocalization in

mixed-valent systems such as NMR, XPS, EPR, UV/vis−NIR,

systems. Notably, iron sulfur clusters are ubiquitous in proteins

and serve as both electron buﬀers and electron transfer

conduits for intermediates in enzymatic catalysis. Given

mixed valency’s ubiquity in nature, inorganic catalysis could

beneﬁt from the detailed examination of how electronic

coupling aﬀects catalyst structure and reactivity. Under this

lens, mixed valency provides an opportunity to explore

chemical space in noninnocent ligand catalysis while capital-

izing on well-characterized electronic structures, synthetic

strategies, and established theoretical frameworks. Never-

theless, such systems are challenging to create in a reliable,

modular way, and the chemical space explored remains

10e,11c,d,12

CV, DSC, Mossbauer, and IR/Raman techniques.

Mixed-valent compounds have attracted attention from both

physical and synthetic chemists for their interesting electronic

structures and electron transfer properties, resulting in a wide

diversity of synthetic and structural data to utilize in molecular

design. The impetus for such longstanding interest is driven

by potential applications in molecular wires, nonlinear

optics, catalysis, and testing models for electron transfer.

However, the application of mixed-valency in structures

capable of acting as ligands toward other transition metals is

rare. Indeed, there is great interest in metal−ligand redox and

reactivity cooperativity and metallo-ligand mediated electronic

coupling between metals in the chemistry community. Despite

interest, the application of mixed valency toward exploring the

eﬀects of electronically coupled binuclear scaﬀolds that could

be applied to catalysis has been challenging, due to issues of

A) chemical stability in diﬀerent redox states; (B)

establishing clear design criteria for engendering strong

coupling, as similar structures often exhibit diﬀerent coupling

strengths as a result of the balance between geometric and

17b,18,19,24

insuﬃcient to deduce general principles.

Thus, we sought to construct ferrocene-based ligand

frameworks with well-understood electronic structures to

better characterize the interactions between metals and their

mechanisms of interaction. The PDI ligand structure (Scheme

1) was targeted because heteroaromatics such as pyrrole and

pyridine are known to enhance coupling between ferrocenyl

11b,19b,25

groups.

Furthermore, both ferrocenes and the central

(

metal share bonding with the imines, providing close through-

bond connectivity, and a low-lying π* network to mix the

metal orbitalsboth important for strong electronic commu-

1a,26

nication.

Note that thus far only the main group

11b,19

electronic factors that lead to coupling;

and (C) designing

coordination chemistry of ferrocene-substituted PDI ligands

scaﬀolds that are modular enough to retain coupling while

parametrizing steric and electronic properties to optimize

catalysis. Most mixed-valent catalysts capitalize on cooperative

eﬀects between metals that have diﬀerent chemistries in

diﬀerent oxidation states (e.g., using Class I mixed valent

structures with little to no delocalization between metals), or

otherwise do not well characterize the eﬀect of metal

has been reported, and the transition metal chemistry

remains unexplored. Here we report the synthesis, solid state

structural, and spectroelectrochemical characterizations of a

series of s- and d-block ferrocenyl PDI (“Fc PDI”) complexes

(Scheme 1). Intriguingly we ﬁnd the electronics of these

complexes to be largely independent of the central metal ion

identity, yet that metal binding is required for coupling

between ferrocenes. This ﬁnding is further explained by DFT

computation, showing that the frontier orbitals are primarily

7b,20

delocalization on reactivity.

However, work in direct

metal−metal interactions between ferrocene and d metals has

revealed promising electronic structures and catalytic activity.

ferrocene- and PDI- based, featuring little MCl character.

We are exploring electronically coupled metal sites as an

opportunity to engineer gated electron transfer and multi-

electron redox events, binuclear mechanisms, strong electric

ﬁeld gradients, and other properties that feature prominently in

eﬃcient biological and abiotic catalysts. Despite this open

question in inorganic chemistry, mixed valent motifs with

known Class II or III behavior exist prominently in biological

Electronic characterizations suggest that M(II) binding

stabilizes the ligand π-system which increases mixing of the

Fc moiety and imine π-orbitals, providing a conduit for

delocalization. Notably a 3-state IVCT mechanism involving

mutual coupling from the Fc units to the pyridine aryl system

in the excited state is suggested as the mode of coupling.

Furthermore, crystallography and DFT analysis performed on

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

the neutral, mixed valent (Fc/Fc ), and doubly oxidized (Fc /

Spectroelectrochemistry. Spectroelectrochemistry was per-

formed on a CH Instruments Electrochemical Workstation by bulk

electrolysis in a glovebox using a carbon foam working electrode, Pt

wire counter electrode, and Ag pseudoreference (prepared as

described above). Aliquots were taken and transferred to an oven-

dried 0.2 cm quartz cuvette sealed with a Teﬂon valve. Spectra were

taken on a Shimadzu UV-36000 plus UV/vis−NIR spectrometer

(300−3000 nm). The diﬀerence spectra were generated by

subtraction of the scan featuring the previous electrochemical species

Fc ) species indicate signiﬁcant structural changes upon

oxidation, which may limit the degree of coupling but presents

the Fe(III) ferrocenium close to the MCl center, suggesting

opportunities for catalysis.

EXPERIMENTAL SECTION

■

General Considerations. All procedures for air- and moisture-

sensitive compounds were carried out with rigorous exclusion of O

and moisture in ﬂame or oven-dried Schlenk-type glassware interfaced

to a dual-manifold Schlenk line or Ar fed high-vacuum (10 − 10

Torr) line using an oil diﬀusion pump, or in an Ar-ﬁlled MBraun

glovebox with a high capacity recirculator (<0.5 ppm of O ). Argon

(

e.g., neutral/monocation, monocation/dication). Due to solvent

evaporation over long electrolysis times, these plots are presented in

un-normalized absorbance units. Potential was incremented by 50 mV

steps until potentiometry indicated oxidation was sustained at the

working electrode and maintained until current became negligible.

−

−7

Syntheses. Fc PDI. para-Toluenesulfonic acid (∼2 mg, ∼0.01

for high-vacuum lines (Airgas, UHP grade) was puriﬁed by passage

through MnO/vermiculite and Davison 4 Å molecular sieve columns.

All solvents were degassed by freeze−pump−thaw techniques,

mmol), 2,6-diacetal pyridine (154 mg, 0.94 mmol), and 399 mg of

FcNH (1.98 mmol) were added to a ﬂame-dried Schlenk ﬂask under

inert atmosphere. A dry reﬂux condenser was attached, and the

excepting tetrahydrofuran (THF) and diethyl ether (Et O) which

atmosphere was replaced with N . Then, 10 mL of dry and degassed

were freshly distilled from sodium using benzophenone indicator

persistent purple). All solvents were stored under inert atmosphere

in a Teﬂon-sealed solvent ﬂask for syringe or vacuum transfer.

Dichloromethane (DCM) was distilled oﬀ CaH . Methanol and

ethanol were dried over 3 Å molecular sieves and ﬁltered through a

cannula (oven-dried with a glass ﬁber ﬁlter). NMR solvents were

purchased from Cambridge Isotope Laboratories (>99 atom % D).

CD Cl used for NMR characterization of complexes was distilled oﬀ

ethanol was added by syringe, and the reaction was allowed to reﬂux

for 18 h, over which time the solution turned dark red and crystalline

material precipitated from solution. The reaction mixture was cooled

to 0 °C, concentrated in vacuo, and ﬁltered by cannula. The red

crystalline product was washed with cold ethanol and dried. Isolated

yield: 292 mg, 58%. Further product can be recovered from the ﬁltrate

(

by recrystallization from DCM/pentane. H NMR (CD Cl , 500

MHz): 8.27 (d, J = 7.8 Hz, 2H, m-Py), 7.81 (t, J = 7.8 Hz, 1H, p-Py),

CaH , degassed by freeze−pump−thaw method, and vacuum

4.46 (t, J = 1.9 Hz, 4H, Cp-β), 4.26 (t, J = 1.9 Hz, 4H, Cp-γ), 4.21 (s,

transferred as needed into Teﬂon valve-sealed NMR tubes (dried in

⁰^H⁾^,²^.⁵⁶⁽^s^,⁶^H^,^C^H3⁾^. C (CD Cl , 126 MHz): 166.15(CN),

an oven overnight). Iodoferrocene and aminoferrocene were

synthesized according to literature procedures. Electrodes were

purchased from Bioanalytical Services in West Lafayette, IN.

Physical and Analytical Measurements. Solution H and ¹³C

NMR spectra were recorded on a 500 MHz Bruker Avance III HD

57.08(Py-ipso.), 136.95 (Py-p), 121.66 (Py-m), 103.98 (Cp-ipso),

9.93 (Cp), 67.29 (Cp-β), 66.16 (Cp-γ), and 16.82 (CH ). Anal.

Calcd for Fc PDI, formula C H N Fe : C, 65.81; H, 5.14; N, 7.94.

29 27

Found: C, 65.66; H, 4.99; N, 7.87.

Fc PDI)CoCl . Fc PDI (50 mg, 0.094 mmol) and 12 mg (0.094

(

system equipped with a TXO Prodigy probe. Chemical shifts for H

mmol) of anhydrous CoCl were added to a ﬂame-dried Schlenk ﬂask

and ¹³C spectra were referenced using internal solvent resonances and

are reported relative to tetramethylsilane (TMS). UV−vis measure-

ments were taken on a PerkinElmer LAMBDA 1050 double beam

spectrophotometer, using a 1 cm quartz cuvette in dry-degassed

in a glovebox. Then, 10 mL of DCM was added via syringe and the

reaction was allowed to stir at room temperature for 24 h as it turned

from red to blue. Over this time blue crystals fell from solution. The

solution was diluted with DCM to dissolve all the blue solids and

ﬁltered by cannula. The ﬁltrate was subsequently concentrated and

diluted with Et Oto precipitate the product. The product (Fc PDI)-

DCM. Fitting of the UV−vis spectra was done using Fityk software

using pure Gaussian peaks. Elemental analyses were performed in an

air free glovebox by Midwest Microlab, Indianapolis, Indiana for % C,

N, H. Crystallographically observed DCM solvates were included in %

C, H, N calculation.

CoCl was washed with Et O under inert atmosphere until the ﬁltrate

was clear and recrystallized by DCM/Et O layering at −40 °C.

Isolated yield: 42 mg of blue crystals 68%. Anal. Calcd for

Computational Methods. All quantum chemical calculations

were performed using the Gaussian 09.D01 software. Geometry

optimizations of all complexes were carried out at the B3LYP /6-

1+G(d,p) level of theory. Frequency calculations at the same level

(

Fc PDI)CoCl ·DCM (1 crystallographic DCM per unit cell),

2 2

formula C H N Fe CoCl : C, 45.43; H, 3.74; N, 5.08. Found: C,

4.92; H, 3.77; N, 5.07.

(

Fc PDI)FeCl . Fc PDI (100 mg, 0.0198 mmol) and 24 mg (0.0198

were performed to validate each structure as a minimum. The UV−

visible spectra of all complexes were calculated using TD-DFT theory

at the B3LYP/6-311++G(d,p) level of theory. The dichloromethane

solvent eﬀect was calculated by the Polarizable Continuum Model

mmol) of anhydrous FeCl were added to a ﬂame-dried Schlenk ﬂask

under inert atmosphere. Five mL of freshly distilled THF was added

by syringe, and the reaction was allowed to stir at room temperature

for 24 h. The solvent was removed under vacuum, and the crude

mixture was taken up in minimal DCM and ﬁltered by cannula. Twice

(

PCM).

Crystallography. Single crystals were mounted in Paratone oil

the volume in freshly distilled Et O was layered on-top and the

and transferred to a cold nitrogen gas stream of a Bruker Kappa APEX

CCD area detector equipped with a MoKα microsource. The crystal

was maintained at 100.0 K during data collection. The structures were

mixture was allowed to crystallize at −40 °C. The resulting blue

crystals were collected by cannula ﬁltration and dried under high

vacuum overnight to give (Fc PDI)FeCl . Isolated yield: 50 mg of

solved using Olex2 with the XT structure solution program and

blue-crystals (40%). Anal. Calcd for (Fc PDI)FeCl , formula

2 2

reﬁned using the ShelXL reﬁnement package with full-matrix least-

squares procedures.

C H N Fe N Cl : C, 52.73; H, 4.14; N, 6.16. Found: C, 53.10;

29 27 3 3 3 2

H, 4.15; N, 6.41.

(Fc PDI)ZnCl . Fc PDI (400 mg, 0.76 mmol) and 100 mg of ZnCl

Electrochemistry. Electrochemistry was performed in dry-

degassed DCM. The electrolyte, tetrabutylammonium tetrakis[3,5-

(0.73 mmol) were added to a ﬂame-dried Schlenk ﬂask under inert

atmosphere, and 5 mL of DCM was added. The reaction was allowed

to stir overnight at room temperature. The resulting blue solution was

ﬁltered by cannula, and the microcrystalline product was washed with

Et Oto give (Fc PDI)ZnCl , which was recrystallized by DCM/Et O

bis(triﬂuoromethyl)phenyl]borate₃(TBA-BArF) was prepared ac-

cording to literature procedures. CV was performed in the dark

under a dynamic blanket of N (100 mM TBA-BArF, 1 mM analyte)

using Pt working and counter electrodes, and an Ag wire

pseudoreference with a Cp* Fe internal standard of known

layering at −40 °C. Isolated yield: 260 mg of blue crystals (52%). H

NMR (CD Cl , 500 MHz): 8.29 (pseudodd, J = 8.3, 7.6 Hz, 1H, py-

concentration. The working electrode was polished with alumina

paste, while the counter electrode was ﬂame polished and the silver

wire was polished with ﬁne sandpaper.

p), 8.13 (d, J = 8.0 Hz, 2H, py-m), 5.13 (t, J = 2.0 Hz, 4H, Cp-β), 4.51

(t, J = 2.0 Hz, 4H, Cp-γ), 4.29 (s, 10H, Cp), 2.82 (s, 6H, CH₃).

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

(

CD Cl , 126 MHz): 157.44 (CN), 151.44 (Py-ipso), 143.68 (Py-

condensed with diacetylpyridine under reﬂux in dry degassed

ethanol with a catalytic amount of p-TsOH to aﬀord the air-

stable product as a microcrystalline solid. The color of the

product ranges from bright red to maroon depending on

purity, crystallinity, and crystal polymorph (cations and acids

p), 124.08 (Py-m), 97.15 (Cp-ipso), 70.82 (Cp), 70.07 (Cp-β), 69.50

Cp-γ), 18.88 (CH ). Anal. Calcd for (Fc PDI)ZnCl ·DCM (1

(

crystallographic DCM per unit cell), formula C H N Fe ZnCl : C,

8.01; H, 3.89; N, 5.60. Found: C, 48.28; H, 3.78; N, 5.82.

Fc PDI)MgCl . Fc PDI (212 mg, 0.4 mmol) and 38 mg (0.4

(

can be coordinated and impart purple coloration). Fc PDI was

isolated in 58% yield as red/purple microcrystals by ﬁltration

and washing with cold ethanol. If required, Fc

recrystallized by DCM/pentane layering.

Next, the Co, Fe, Zn, and Mg complexes of the Fc PDI

ligand were prepared by direct metalation with corresponding

MCl precursors, in decent to good isolated yields (Scheme 2).

Note that in our hands, we experienced high levels of static

electricity which can result in somewhat diminished isolated

yields on small scales. The rate of metalation varies with

mmol) of anhydrous MgCl were added to a ﬂame-dried Schlenk ﬂask

under inert atmosphere. Then, 5 mL of anhydrous DCM and 5 mL of

anhydrous MeOH were added by syringe, and the solution was

allowed to stir for 24 h at room temperature. The solvent was

evaporated under vacuum and the blue/purple solids were redissolved

in DCM, then ﬁltered by cannula. The solution was concentrated

under vacuum, and the complex was recrystallized overnight by

layering with Et O at −40 °C. The highly moisture-sensitive crystals

were dried under high vacuum overnight to give (Fc PDI)MgCl .

Isolated yield: 211 mg of blue/purple plate-like crystals (84%). H

NMR (CD Cl , 500 MHz): 8.29 (pseudodd, J = 8.3, 7.7 Hz, 1H, py-

PDI can be

solvent for each MCl speciespresumably due to solubility

p), 8.07 (d, J = 8.1 Hz, 2H, Py-m), 5.11 (t, J = 2.0 Hz, 4H, Cp-γ), 4.52

(

t, J = 2.0 Hz, 4H, Cp-β), 4.30 (s, 10H, Cp), 2.85 (s, 6H, CH₃).

CD Cl , 126 MHz): 161.31 (CN), 153.08 (py-ipso), 143.94 (Py-

considerations. ZnCl and CoCl react readily with 1.0 equiv of

(

the ligand overnight in DCM aﬀording blue-purple crystalline

complexes. For FeCl₂and MgCl , the metalation is

prohibitively slow in DCM and requires THF or MeOH for

metalation to occur. All complexes were puriﬁed by trituration

with Et O or recrystallization from layered DCM/Et O solvent

p), 123.99 (Py-m), 97.72 (Cp-ipso), 70.82 (Cp), 69.89 (Cp-γ), 69.43

Cp-β), 19.10 (CH ). Anal. Calcd for (Fc PDI)MgCl ·0.5DCM,

(

formula C₃₀_.₅H N Fe MgCl C, 53.74; H, 4.72; N, 5.97. Found: C,

3.42; H, 4.55; N, 6.22.

to yield dark blue/purple solids. The complexes exhibit varied

RESULTS

■

stabilities under ambient conditions, as the (Fc PDI)CoCl₂

Synthesis. The Fc PDI ligand synthesis was modiﬁed from

and (Fc PDI)ZnCl complexes are reasonably resistant to

8,29

literature

to streamline the procedure (Scheme 2).

small amounts of air and moisture for days, while solutions of

Fc PDI)FeCl turn brown and precipitate overnight, and

(

Scheme 2. Synthesis of the Fc PDI Ligand and

Fc PDI)MCl Complexes

2 2

(Fc PDI)MgCl₂turns red immediately on exposure to

(

moisture.

CV experiments indicate that oxidation occurs at similar

potential to ferrocene, rendering the slightly more oxidizing

actylferrocenium and Ag salts as suitable oxidants to generate

the mixed valent species. Initial attempts to oxidize with AgPF₆

were hindered by high insolubility of the product PF salt,

preventing further characterization. However, careful layering

of a DCM solution of (Fc PDI)CoCl on top of a DCM

solution of AgPF inside an NMR tube slowed the reaction

enough to allow precipitation to occur in the form of single

crystals suitable for diﬀraction. Attempts with other complexes

yielded crystalline material not suitable for single crystal X-ray

−

diﬀraction studies. Using BArF salts to improve solubility

presented unpredicted complications, as multiple attempted

oxidations of (Fc PDI)FeCl with 1 equiv of [AcFc][BArF] in

DCM resulted in a black solution, which was dried and washed

with pentane to remove acetylferrocene byproduct, and used

directly for crystallization. Interestingly the dication,

Ferrocene can be selectively monolithiated using t-BuLi and

t-BuOK, which was trapped with iodine to aﬀord iodoferro-

cene (FcI) as a black oily crude that may crystallize on

standing if suﬃciently pure. After puriﬁcation through a silica

plug and removal of unreacted ferrocene by sublimation (100

[(Fc PDI)FeCl ][BArF] , rather than the expected mono-

2 2 2

cation, crystallizes from the solution as X-ray suitable crystals

(Figure 1F). We believe that a small equilibrium amount of the

dication is present due to intermolecular electron transfer

(disproportionation constant: K ≈ 0.001 based on 200 mV

separation between ﬁrst and second oxidationssee electro-

chemistry below). The dication likely preferentially crystallizes

in the nonpolar DCM solvent, permitting isolation in a similar

C, 0.1 Torr), pure product was obtained as dark orange

needles in 80% isolated yield on scales up to 15 g. Amination

of FcI with aqueous ammonia using a Fe O /CuI catalyst

system was carried out following a literature procedure, while

way to that of the crystallization of MgMe through the

the amino ferrocene (FcNH ) was puriﬁed by sublimation (90

Schlenk equilibrium of Grignards. We emphasize that the

mixed-valent species are stable in solution, indicated by CV

and spectroelectrochemistry.

C, 0.1 Torr) instead of chromatography. Note that FcNH₂

gradually decomposes in air to an NMR indistinguishable

CDCl ) brown solid (presumably the N-oxide) at ambient

(

NMR Spectroscopy. While (Fc PDI)FeCl₂and

temperature, and therefore should be stored in the freezer

under inert atmosphere or in a glovebox. Puriﬁcation can be

readily achieved by sublimation or passage through an alumina

plug with hexanes/EtOAc elution (1:1). FcNH₂was

(Fc PDI)CoCl₂are paramagnetic and did not provide

informative NMR spectra, the comparison of the free ligand

with the corresponding diamagnetic complexes (Fc PDI)-

MgCl and (Fc PDI)ZnCl aﬀords interesting insight into the

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 1. Single crystal X-ray structures for the free ligand, (Fc PDI)MCl (M = Co, Fe, Zn, Mg), and oxidized (Fc PDI)MCl (M = Fe, Co)

complexes. Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity. For structure F and G,

BArF and PF anions are omitted, respectively.

change in magnetic and electronic environment upon

coordination. The para-pyridyl protons (relative to N) of the

Mg and Zn complexes appear as apparent doublets of doublets

rather than triplets, due to second order eﬀects. The Cp

protons shift substantially downﬁeld upon metalation, from δ

Solid State Structures. Single crystals of (Fc PDI)MCl

2 2

(M = Fe, Co, and Zn) complexes suitable for X-ray diﬀraction

studies were grown from DCM/pentane, while diﬀraction

quality crystals of the Mg complex were grown from DCM/

Et O. [(Fc PDI)CoCl ][PF ] was crystallized by layering

.46 to 5.13 ppm for β-Cp protons, and δ 4.26 to 4.51 ppm for

DCM solutions of AgPF and (Fc PDI)CoCl , while crystals

6 2 2

γ-Cp protons distal to the imine (Table 1; Scheme 2). Notably,

of [(Fc PDI)FeCl ][BArF] were grown from DCM/pentane

2 2 2

layering of the crude reaction product prepared by treatment

Table 1. H NMR Chemical Shift Data (ppm) for Selected

of (Fc PDI)FeCl with 1 equiv of [AcFc][BArF] in DCM.

Resonances of Fc PDI, (Fc PDI)ZnCl , and (Fc PDI)MgCl

Reﬁnement statistics are summarized in the Supporting

Information (SI, S1 and S2 ). Crystal structures are shown in

Figure 1 and key structural characteristics are compared in

Table 2. All structures have eclipsed ferrocene conformations.

The complexes all exhibit slight deviations from Cp-PDI-Cp

coplanarity, with torsions between the Cp-imine planes on the

in CD Cl at 25 °C

Fc PDI

(Fc PDI)ZnCl

(Fc PDI)MgCl₂

Py-p

7.81

8.27

4.46

4.26

8.29

8.13

5.13

4.51

8.29

8.07

5.11

4.52

Py-m

Cp-β

Cp-γ

order of 7°−30°, which may reﬂect crystal packing force-

11a,13c,25b,45

The Zn and Co structures also exhibit a

distortion of the PDI backbone, manifested as a puckering of

the imine carbons above and below the ligand plane.

Interestingly, in contrast to other PDI complexes that generally

have coordination geometries approaching square pyrami-

asymmetry in ferrocene Cp magnetic environment (e.g.,

diﬀerent resonance frequencies for diﬀerent Cp protons) has

been previously assigned to diﬀerential occupation of the d_x_y/

44b

dal,

all structures in this study are distorted trigonal

bipyramidal. The ligand bond lengths in all the neutral

complexes are essentially equivalent (within the given ESD for

each bond) and consistent with a neutral ligand bound to a

d_x₂_‑_y₂and d /d orbitals caused by donation into or

xz yz

21a,h,43

withdrawal from the Fc center.

The pyridyl region also

exhibits substantial changes, but interestingly with opposite

shift trends for meta and para protons (where meta and para

are relative to N). Upon coordination, the pyridyl meta

protons shift upﬁeld from δ 8.29 to 8.13 ppm, while the para

protons shift downﬁeld from δ 7.81 to 8.29 ppm (Table 1).

Diﬀerent shielding eﬀects on the meta and para protons might

be explained given that the pyridine based frontier orbitals are

largely bonding with respect to the ipso and meta carbons, but

M(II) fragment.

conformation in the solid state, likely due to steric factors.

The mixed valent compound [(Fc PDI)CoCl ][PF ] shows a

The free ligand adopts an “open”

break in symmetry, as the oxidized ferrocene swings to be

nearly orthogonal (83°) to the backbone plane. The site of

oxidation is assigned by slight elongation of the Fe−Cp

distance, and high numbers of close cation−anion distances on

inspecting the extended crystal packing which features a sheet

10b,d,44

antibonding with respect to the meta and para carbons.

That is, the diﬀerent magnetic response at each position may

reﬂect diﬀerent bonding character at each position. However,

the crystallographic data (vide infra) suggest that rotational

of alternating ferrocenium cations and PF

Furthermore, the imine connected to this ferrocenium

contracts slightly, and the CoCl center drifts away from the

anions (see SI ).

freedom about the C_i_p_s_o−C

bond produces an “open”

ferrocenium arm. These changes on oxidation are unfavorable

for strong coupling, as diﬀerences in coordination geometry

between metal sites leads to large reorganizational energies,

imine

conformer (Figure 1A) and a “closed” conformer (Figure 1B−

G). These conformational changes may also impact the

observed chemical shifts, though symmetry (e.g., relative

orbital overlaps should be conserved) suggests that orbital

interactions and therefore electronic communication should

not change between conformers.

11c

and higher barriers to electron transfer.

We point out that stronger ion pairing is known to stabilize

localized cations and weaken coupling, motivating crystal-

−

lization with BArF anions. While the monocationic BArF salt

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Figure 2 shows that the free ligand displays a concerted two-

Article

Table 2. Selected Bond Distances (Å) and Bond Angles

small for ferrocenes separated by multiple bonds, electrostatic

and resonance eﬀects typically dominate the observed ΔE₁_/₂.

Furthermore, because electrostatic eﬀects are dependent on

geometry, solvent, and electrolyte; an isostructural series of

compounds enables more direct evaluation of delocalization.

Thus, the separation between ligand oxidations can be

correlated as a function of metalation (e.g., with FeCl₂,

CoCl , ZnCl , or MgCl ) with the delocalization of charge

(

deg) for Fc PDI Ligand and (Fc PDI)MCl Complexes (M

Zn, Co, Fe, and Mg)

ligand, Fc PDI

C −N₂

1.285(2)

Cp torsion

22.7°

8.1°

(

Fc PDI)MgCl

N −Mg

2.0907(2)

2.2826(2)

2.2518(2)

1.2909(1)

(

N −Mg−N

Cp torsion

148.19°

13.4°

13.9°

11a,b,19b

between the two ferrocene groups.

N −Mg

With these considerations in mind, CV was performed in

DCM because coordinating solvents promote ECE mecha-

N −Mg

C −N₂

Cl−Mg−Cl

124.95°

nisms in PDI complexes, while TBA-BArF electrolyte was

Fc PDI)ZnCl

employed both to improve the electrochemistry in this

N −Zn

2.044(3)

2.304(3)

2.255(3)

1.289(4)

(

N −Zn−N

Cp torsion

150.00°

17.63°

14.91°

124.28°

nonpolar solvent and also because the PF salts of these

N −Zn

complexes are insoluble, precipitating onto the electrode.

N −Zn

C −N₂

Cl−Zn−Cl

Fc PDI)FeCl

N −Fe

2.067(2)

N −Fe−N

149.1°

N −Fe

2.227(2)

Cp torsion

18.8°

13.5°

.262(2)

C −N₂

1.289(3)

Cl−Fe−Cl

125.7°

(

Fc PDI)CoCl

2 2

N −Co

2.003(2)

2.222(2)

2.199(2)

1.294(3)

N −Co−N

Cp torsion

152.98°

29.49°

8.98°

N −Co

C −N₂

Cl −Co−Cl

127.40°

[

(Fc PDI)CoCl ][PF ]

2 2 6

N −Co

2.01247(8)

2.37077(8)

2.17412(8)

1.28654(5)

N −Co−N

Cp torsion

150.94°

4.18°

95.32°

118.37°

N −Co

C −N₂

Cl −Co−Cl

.27531(5)

[

(Fc PDI)FeCl ][BArF]

Figure 2. Cyclic voltammograms of Fc PDI, (Fc PDI)FeCl ,

N −Fe

2.07669(4)

2.29128(5)

2.28597(5)

2.29128(5)

N −Fe−N

148.45°

65.82°

76.07°

130.25°

(Fc PDI)CoCl , (Fc PDI)ZnCl , and (Fc PDI)MgCl (bottom to

2 2 2 2 2 2

N −Fe

Cp torsion

top); scan rate: 100 mV/s; 1 mM analyte, 100 mM TBA-BArF in dry

degassed DCM under N at 25° in the dark. CVs are referenced to

Cp* Fe internal standard at 0.0 mV. Dotted lines at 564 and 810

N −Fe

C −N₂

Cl −Fe−Cl

mV correspond to Fc PDI and (Fc PDI)ZnCl oxidations for

.28597(5)

comparison. Reduction waves not presented for Fc PDI and

“

Cp Torsion” is measured between planes deﬁned by the Cp ring

and the Cp −N and C N bonds.

(

Fc PDI)MgCl due to rapid decomposition and electrode fouling.

2 2

electron wave, suggesting weak to no coupling. Most

−

complexes show sequential 1 e waves separated by ∼200

could not be crystallized, the doubly oxidized structure,

(Fc PDI)FeCl ][BArF] , shows both ferrocenes canted

mV (Table 3). Notably (Fc PDI)FeCl exhibits a larger ΔE

1/2

[

nearly orthogonal (85°) but with a more symmetric

Table 3. Cyclic Voltammetry Data for (Fc PDI)MCl (M =

coordination environment about the FeCl center.

Co, Fe, Mg, Zn), Values in mV vs Cp* Fe

Electrochemistry. Due to the highly reversible and stable

Fc/Fc redox couple, electrochemistry is an informative tool in

₁st E₁_/₂

₂nd E₁_/₂

compound

ΔE₁_/₂

11a−d

evaluating electronic communication between Fc groups.

Fc PDI

564

617

584

641

616

For symmetric, uncoupled Fc sites, a single concerted

oxidation wave is expected. However, if oxidation of one Fc

perturbs the electronic environment of the second, i.e., the

metal centers are coupled, then the second oxidation occurs at

increased potential, resulting in sequential waves. There are

multiple mechanisms that can give rise to split redox

(

Fc PDI)CoCl₂

820

867

835

810

203

264

194

Fc PDI)FeCl₂

Fc PDI)MgCl₂

Fc PDI)ZnCl₂

1a,c,12b

waves,

and the observed ΔE is a net sum of the

by 60 mV. The ﬁrst oxidation is approximately isoenergetic

with free Fc in these compounds (E = 590 mV vs Cp* Fe

following: ΔE = ΔE

+ ΔE + ΔE

obs

resonance

electrostatic

statistical

1/2

ΔE

+ ΔE

. Due to the diﬃculty in delineating

exchange

for Fc), which suggests that the Fc-based occupied orbitals

are not substantially perturbed. Upon complexation, the

oxidation shifts slightly to higher potentialan additional

inductive

individual contributions to ΔE, it is challenging to

quantitatively assess delocalization by CV. However, since

12b

the exchange and statistical terms are typically small (∼36 mV

20−80 mV higher potential than Fc PDI, reﬂecting Fc

at 25 °C) and inductive eﬀects are often assumed to be

bonding orbital stabilization. Reductions and oxidations are

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

assigned as primarily being ligand-based, since Zn(I) and

Zn(III) species are implausible, and the free ligand

decomposes at −1.5 V. Note that no additional redox couples

are observed within the DCM solvent window of −1.5 to 1.5

V. We propose that when a central metal is coordinated by

observation of note is the substantial increase in ferrocenyl

Article

−1

absorptivity (∼4000 M cm ). The energy of this ferrocenyl

band changes between ligand formation and metalation,

however. While the FcNH ferrocenyl band is centered at

442 nmtypical for a Fc derivative, Fc PDI exhibits a 35 nm

redshift to 478 nm. The (Fc PDI)MCl complexes further

Fc PDI, the reduced ligand is stabilized in solution. Note that

redshift ∼125 nm to ∼600 nm. The Zn and Mg complexes

display the ferrocenyl band at slightly higher energy (ca. 10

nm) than the Fe and Co complexes (Figure 3). It is interesting

that all four (Fc PDI)MCl (Co, Fe, Mg, Zn) metal complexes

the (Fc PDI)FeCl₂compound exhibits the largest ΔE,

suggesting it for spectroelectrochemical investigation (vide

infra).

UV−vis Spectra. Because CV data cannot unambiguously

have similar absorption in the range of 600 nm which is not

strongly aﬀected by the metal. This suggests that the orbitals

responsible for this transition are not localized on the central

metal center and are probably mainly ligand centered orbitals.

Note also the changes in bandwidth from FcNH , to Fc PDI,

assign the reason for underlying sequential oxidation, redox

12b

splitting can be misleading in assessing electronic coupling.

Therefore, it is more prudent to combine electrochemical data

with optical spectroscopy, through both traditional and

spectroelectrochemistry experiments. The visible electronic

to the (Fc PDI)MCl compounds which is another indication

transitions in ferrocene are best considered d-d transitions,

of delocalization in the donor/acceptor orbitals. To more

concretely analyze these changes, the experimental spectra

were ﬁt to gaussians using the known electronic structure of

so by exploiting the well understood electronic spectrum of

ferrocene, the Fc d-orbital mixing can be tracked as a

perturbation of the relevant d-d transitions. For consistency,

we refer to the visible region band as “the ferrocenyl band” to

remain agnostic as to its character, i.e., d-d, charge transfer

50b

ferrocene. The ferrocenyl band

(442 nm in FcNH ) is

known to contain two transitions which can be successfully

ﬁt to two Gaussian peaks (Figures S9 −S14; Table 4). The full

(

CT), or overlapping d-d and CT transitions. Furthermore, we

diﬀerentiate between the ferrocenyl iron and the organic

pyridine parts of the ligand when describing “metal” versus

Table 4. Summary of Experimental UV−vis Band

Parameters and Fitting of the Ferrocenyl Band in Figure 3

“

ligand” centered processes. The UV−visible spectra of the

to Two Gaussians

Fc PDI ligand and its (Fc PDI)MCl complexes were

UV−vis bands

absorptivity

Gaussian ﬁtting

recorded in DCM, using amino ferrocene (FcNH ) for

comparison.

molecule

FcNH₂

absorptivity fwhm

From the UV−vis data shown in Figure 3, the ﬁrst

442

478

616

605

590

207

2782

3120

2517

2601

4179

468

422

458

131

142

Fc PDI

1791

1406

1950

1650

1660

1210

1320

1520

2200

102

100

174

150

160

170

148

160

146

577

576

552

568

(

Fc PDI)CoCl₂

Fc PDI)FeCl₂

Fc PDI)MgCl₂

Fc PDI)ZnCl₂

fwhm: full-width-at-half-maximum.

width at half-maximum (fwhm) of each ferrocenyl transition

increases from 70 to 80 nm for FcNH to 100−102 nm for

Fc PDI, and to ∼160−175 nm for the (Fc PDI)MCl

complexes (Table 3).

To probe the optical spectroscopic assignments further,

DFT and time-dependent DFT calculations were carried out.

Geometry optimizations for the structures of (Fc PDI)MCl₂

Figure 3. UV−vis spectra of FcNH , Fc PDI, and (Fc PDI)MCl (M

(Co, Fe, Mg, Zn) were carried out at the B3LYP/6-31+G(d,p)

level of theory and are in excellent agreement with the X-ray

structure geometries. B3LYP was applied here based on its

Co, Fe, Zn, Mg) in dry, degassed DCM. Inset: enlarged spectrum of

FcNH₂.

19a,52

success in analogous mixed-valent complexes

and the

transition intensity in Fc PDI. For simple ferrocene deriva-

excellent agreement with our experimental and computed

geometries as well as our experimental and computed UV/

visible spectra. According to the DFT calculations, all four

complexes have nearly identical frontier molecular orbitals

(Figure 4) in which the HOMO is localized in the d-orbitals of

tives, the absorptivity of this symmetry-forbidden transition is

−

−1 50b,51

∼

100−200 M cm .

The ferrocenyl band in FcNH₂

shows an absorptivity of ϵ ≈ 200, which increases to ϵ ≈ 2800

M cm , or 1400 per Fc center, for Fc PDI. This absorptivity

−

−1

range is consistent with a symmetry allowed transition, but

the two ferrocene substituents (d /d_x₂_‑_y₂) with minor

likely not a full charge transfer which would typically have ϵ >

contribution from nitrogen and central metal orbitals. The

LUMO, in contrast, is mainly delocalized over the pyridine and

imine groups with some contribution from the ferrocene d_x_z/

d_y_zorbitals. In other studies, small levels of delocalization have

−

−1 51

000−10 000 M cm . Interestingly, the absorptivity of the

band does not change signiﬁcantly upon metalation, except for

the (Fc PDI)ZnCl₂complex which shows a greater

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 4. DFT-computed frontier molecular orbitals of the indicated

(

Fc PDI)MCl complexes (M = Co, Fe, Zn, Mg).

2 2

been seen even in cases of direct metal−metal interaction from

21d

donation of ferrocene into high valent metals. In addition,

the calculated UV−vis spectra are in good agreement with the

experimental values; for more details see the SI. The observed

absorption of these complexes in the range of 600 nm is

attributed, according to the calculations, to the HOMO →

LUMO transition. Thus, the DFT calculations support the

empirical ﬁndings for orbital combinations and bonding.

Spectroelectrochemistry. To reinforce assignments from

the UV−vis experiments and DFT, spectroelectrochemistry

measurements were employed. This technique was per-

formed by measuring the UV−vis spectrum of aliquots taken

during bulk electrolysis, and thus detecting electronic structure

changes resulting from oxidation. Spectroelectrochemistry

was performed in a quartz Schlenk cuvette with 100 mM TBA-

BArF electrolyte, using carbon foam working and Pt counter

electrodes, with an Ag wire pseudo reference. The sharp

spectroscopic features near 2000 nm are due to quartz

absorptions and solvent vibrational overtones, as demonstrated

Figure 5. UV−vis-NIR spectra of (A) Fc PDI (black) [Fc PDI]

by solvent blanks (see SI Figure S15). Upon oxidation, Fc PDI

(blue) and change upon oxidation (red dashed); (B) (Fc PDI)FeCl

2 2

−

changes from red to blue/green via a 2 e eventat constant

(

black), [(Fc PDI)FeCl ] (blue), and (C) [(Fc PDI)FeCl ]

2 2 2 2

voltage, 2 equiv of charge are transferred. This correlates with a

decrease in the 478 nm ferrocenyl band (Figure 5A), reﬂecting

removal of the electron in the Fc-based donor orbital in this

transition. The growth of a broad band at low energy centered

around 776 nm (Figure 5A) is also observed, which contrasts

(blue), [(Fc PDI)FeCl ] (green) and change upon oxidation (red

2 2

dashed) under spectroelectrochemical conditions.

NIR that best assigns to a Inter-Valence charge transfer

(IVCT) between the ferrocene and ferrocenium groups. This

assignment is conﬁrmed by further oxidation by 1 e- to the

with the free Fc LMCT band, which is comparatively narrow

50b

and centered at 625 nm.

Given that (Fc PDI)FeCl displays the largest ΔE in the CV

[(Fc

PDI)FeCl

]

state, forming a yellow/green solution

lacking IVCT features.

experiments, we decided to explore the spectroelectrochemical

properties of this compound. Immediately evident is that the

−

DISCUSSION

complex becomes black upon oxidation in a 1 e event. There

■

is a slight decrease in its corresponding ferrocenyl band (at 605

nm) as well as substantial broadening. A new band system at

Metalation-Induced Electronic Coupling. After metal-

ation, electronic coupling between the ferrocenes is observed,

as evidenced by the oxidation of one ferrocene perturbing the

oxidation potential of the second via cyclic voltammetry. While

∼

850 nm appears, presumably of ferrocenium LMCT

character. Interestingly, a very broad transition appears in the

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 6. Qualitative MO diagram for proposed electronic interactions between the ferrocene and organic π system. The d₅_dligand ﬁeld symmetry

labels are used for the ferrocene fragment.

there does seem to be an increase in rigidity of the ligand

scaﬀold upon metalation, rigidity does not explain the lack of

XRD. Although one expects that a coplanar conformation

would induce optimal coupling, ferrocenyl groups exhibiting

coupling in low valent main group Fc PDI complexes reported

substantial torsions in the solid state can still engage in

27b

11b,d,25b,45a

by the Ragogna group. Furthermore, we observe increased

redox stability, demonstrated through highly reversible

oxidation events. In marked contrast, the main group analogs

electronic coupling in solution.

This suggests that as

long as coplanar geometries are readily sampled at room

temperature, then coupling remains intact, even if it is not the

sole conformation. We also conclude that changes in

conformation (“open” vs “closed”) have little bearing on the

occupied bonding orbitals, as the ligand bond distances do not

signiﬁcantly vary in any of the compounds studied. This

suggests that conformational changes do not appreciably alter

the bonding characteristics, which is to be expected based on

symmetrysimilar orbital overlaps in both conformations. No

change in bond lengths suggests that for the neutral species,

spectral changes originate largely from orbital interactions

occurring in the unoccupied manifold of the MO diagram,

which cannot be probed by XRD. This is also evidenced by a

6a,b

reported previously

yielded CV data suggestive of

decomposition during redox events. These combined observa-

tions lead us to conclude that the electronic manifold is

diﬀerent in the transition metal series versus the main group

series. This conclusion is further supported by the small ΔE in

the more metallic Sn compound, which is absent in the P, Se,

S, Te, and Ge complexes, indicating that transition metal

chemistry of this ligand is fundamentally diﬀerent from the

nonmetallic main group analogues. If diﬀerent conformers gave

rise to diﬀerent electrochemistry, we would expect an

admixture of signals or an ECE mechanism in the rapid

exchanging free ligandwhich is not observed. It seems that

more acidic heteroatoms in the binding pocket are necessary

for this coupling to occur, suggesting that the eﬀect is not due

to conformational changes. Conformation changes would

explain the data if interaction between the ferrocenes were

predominantly electrostatic (e.g., changes in interferrocene

distance altering the electrostatic interaction between them).

Rather, it appears that the coupling is driven by coordination

more extreme reduction potential for Fc PDI compared to the

(Fc PDI)MCl complexes, suggesting a stabilization of the π*

system upon metalation (E_r_e_d≈ −1.5 V for Fc PDI, E

≈

red

−0.9 to −1.2 V for (Fc PDI)MCl ).

Interestingly, the conformation of the oxidized ferrocene

changes, canting inward toward the chloride ligands. The

distance between chloride and ferrocenyl iron is ca. 4.2 Å,

suggesting an electrostatic attraction rather than formal

coordination. This has the added eﬀect of decreasing the

intermetallic distance, which leads to uncertainty in extracting

an electronic coupling constant using Hush theory, which

employs the internuclear distance as a parameter. This

distortion also shines light on a complication infrequently

discussed in mixed valent studiesthat changes in charge

distribution upon oxidation can promote rearrangements that

may explain why some highly conjugated and promising

scaﬀolds fail to engender strong coupling. In (Fc PDI)FeCl ,

of Fc PDI to a transition metal. Observation of d-element-

driven coupling is not unprecedented, as dithiolato ligands

undergo “π-reorganization” upon coordination to transition

metals. This reorganization modiﬁes the electronic structure of

the dithiolate ligand to promote Proton Coupled Electron

Transfer (PCET) chemistry.

Furthermore, the reported lack of coupling in low-valent

main group Fc PDI complexes provides a clue to the

mechanism of coupling. Less acidic P(I), Te(II), Se(II), and

S(II) centers do not promote interaction between ferrocenes,

while more acidic M(II) ions are likely to stabilize the electron-

this distortion brings the formally Fe(III) center closer to the

chlorides in the MCl fragment and likely stabilizes a localized

rich Fc PDI ligand system, enhancing the energy match

Fe(III) center. Thus, the structure change likely limits the

ultimate amount of charge delocalization. This would be akin

to ion pair eﬀects known to have strong impacts on degree of

delocalization, as probed as a function of supporting electrolyte

in CV experiments. We focus in this work on suggesting

how this conformer presents interesting prospects for reactivity

as opposed to how to optimize toward increased delocaliza-

tion. This structure change, bringing the cationic Fe(III) into

between metal and organic π* orbitals. Lowering the energy of

the LUMO would better facilitate a 3-state mechanism for

electron transfer, resulting in coupling between metals. This

argument is supported by the present crystallographic, CV,

UV−vis, and spectroelectrochemistry data. Depiction of the

proposed coupling process and the molecular orbital level is

then given in Figure 6 and explained below.

11a

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

the secondary coordination sphere of the MCl fragment

possible to modify the ancillary chloride ligands (e.g., with

larger ligands such as bromide) to allow better energy

matching so that the MX₂fragment participates in the

HOMO or LUMO to involve it inside the superexchange

mechanism and engender coupling through all three metals.

Under this understanding, the calculated H_a_bis better

considered an eﬀective coupling constant, since a two state

model is applied. This approximation is invalid in the case of

suggests that substrates bound to the central M(II) center can

interactcovalently or electrostaticallywith the Fe(III) in a

cooperative way. Ferrocenium is a useful mild oxidant that

given the close distance to ligands on the central metal may

undergo facile oxidation of reaction intermediates. Further-

more, a cationic Fe(III) can generate local electric ﬁeld

gradients and polarize substrates, ferrocenium LMCT states

have been shown to be useful in photocatalysis, and it is

strong coupling, which does not appear present.

known to act as a Lewis acid catalyst. Addition of Lewis acids

in the secondary coordination sphere is an attractive strategy in

UV/vis−DFT. The orbital interactions can be further

understood on the basis of the intensities, energies, and line

shapes of the ferrocenyl transitions. Since transitions occur

from occupied to unoccupied orbitals, we can infer character-

istics of the higher energy orbitals unobservable by X-ray

diﬀraction. In observing a large increase in absorptivity for the

bond activation. Furthermore, the acceptor orbital in charge

transfer transitions for the neutral complexes (see theory

section) features a similar ligand centered LUMO that is one

of the occupied frontier orbitals in highly reduced PDI

structures.

Based on these interesting excited states, we

ferrocenyl band in the Fc PDI ligand relative to ferrocene, it is

suggest that an internal electron donor could function as a

photoreductant in lieu of aggressive reductants such as

logical to conclude that the symmetry forbidden transition has

become symmetry-allowed. Similar results are seen in Fc-Ru

and Fc-Pd/Pt compounds where mixing of Fc orbitals with

other ligand or metal orbitals breaks inversion symmetry and

NaEt BH or Na(Hg). Electronic coupling in excited states is

an area of continued interest and exploration and the easily

accessible charge transfer in this system yields only a partial

hole (see theory section) at ferrocene, which may reveal

interesting properties

21c,h,55b

leads to absorptivity increases.

We infer that mixing of

Fc and imine orbitals can explain a breaking of symmetry, but

cannot posit a priori if the organic character is in the HOMO

or LUMO. Given that the HOMO energy of the present

ferrocenes is not signiﬁcantly perturbed (see Electrochemistry

section), and bond distances do not change substantially (see

Crystallography), the mixing likely involves the Fc LUMO, Fe

Spectroelectrochemistry. In accord with the CV experi-

ments, free Fc PDI is not stable in its mixed valent state and

cannot be observed by spectroelectrochemistryconstant

potential electrolysis continues until 2e⁻are removed from

−

the molecule. If electrolysis is halted after 1e is removed, then

centered d /d orbitals. Inspection of the DFT results (Figure

xz yz

all species coexist in equilibrium due to disproportionation.

4) supports this assertion, as the HOMO is largely Fc d_x_y/

(

Fc PDI)FeCl is stable in its mixed valent form. Pausing

d_x₂₋_y₂while the LUMO displays d /d character on the

xz yz

electrolysis after oxidation by one electron reveals a broad low

energy feature assignable as an IVCT. Fitting IVCT bands and

application of Marcus−Hush theory allows for the electronic

coupling of two metal sites to be extracted based on the

electron transfer properties. Thus,

ferrocene units and a large amount of organic PDI character.

To further understand the DFT results, which suggests

mixing of Fc d /d orbitals with the imine π* instead of π, we

xz yz

consider the eﬀects of each interaction. Donation from the

imine π into the unoccupied d /d orbitals would result in a

xz yz

bonding/antibonding interaction, raising LUMO levels,

changing imine bond strengths, and placing more electron

density in the ferrocene system. Instead, the electrochemistry

supports a largely unperturbed ferrocene HOMO, and the

spectroscopy supports a stabilized ligand LUMO (red-shifted

absorption). Rather, mixing between the Fc unoccupied dxz/dyz

and imine π* orbitals such that the excited state assumes ligand

character and becomes more delocalized (Figure 6) is

consistent with the experimental data and DFT results. Similar

observations and conclusions have been made that binding of

PDI macrocycles to Lewis acids stabilizes the PDI π* orbitals

and leads to better energy matching between metal and organic

−

^Δ^ν1/2)¹^/²

.06 × 10 (ν ε

max max

H =

^rab

where H is the Electronic coupling constant, while ν_m_a_x, ε_m_a_x

Δν , and r are the energy of the IVCT band, the extinction

coeﬃcient, band fwhm, and distance between metal sites a and

b, respectively. Application of this theory gives an electronic

−

coupling of 348 cm , taking r to be the crystallographically

determined distance between ferrocenes in the neutral (all

Fe(II)) structure (8.1 Å) and ϵ calculated as an upper bound

based on the initial electrolysis concentrations (e.g., ignoring

solvent evaporation). This is a modest degree of electronic

coupling, indicating Class II behavior, but represents a rare

example of a bis-ferrocenyl complex acting as a ligand to

another metal fragment that is in a known catalytically active

structure (e.g., PDI catalysts). While CV strongly suggests

IVCT occurs between ferrocenes (as the second ferrocene is

the most easily oxidizable group) on closer inspection, it is

interesting that ET occurs between the more disparate metals.

This could be explained by a 3-state electron transfer

orbitals, causing greater delocalization. We can thus describe

a metal to mixed metal−ligand charge transfer (“MMMLCT”)

in analogy with mixed metal−ligand to ligand CT

54a,62

(“MMLLCT”) excitations.

As shown in Figure 6, this

assignment explains the present observations of an unper-

turbed HOMO and a red-shifted HOMO−LUMO transition.

The red-shift of the ferrocenyl band in Fc PDI is relatively

small, suggesting the degree of mixing between metal and

organic orbitals is too small to produce coupling that can be

observed by CV. Nevertheless, the increase in intensity allows

us to conﬁdently assign a break in symmetry and demonstrate

orbital interaction. Fitting the spectrum to Gaussians reveals

two transitions within the 478 nm band, which accounts for

both d-d transitions seen in the similar region of free Fc.

Furthermore, a weak tail is also observed, which is intuitively

assigned to the spin-forbidden counterpart transition, as

mechanism

through mutual coupling of each Fc to the

ligand π system, providing a mode for electronic coupling

through unoccupied orbitals using a superexchange mediated

electron transfer mechanism. This would also explain the lack

of decreased intensity for the ferrocenyl band, as 3 state

coupling results in an increase in the intensity and broadening

of absorption features on the unoxidized metal. It also appears

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c10015.

Article

observed in ferrocene.

Partial permission (symmetry-

pounds typically exhibit high physicochemical sensitivities to

substitution and modiﬁcation. This aﬀords a unique reactivity

possibility that the coupling triggered by metal ion binding will

be seen with a variety of ions, increasing the versatility of this

ligand system. Insensitivity of the interaction toward metal

allowed, spin forbidden) causes the intensity to increase

−

−1

from 7 M cm (seen in ferrocene), to ∼50−100 M cm .

We are unable to model the pure Fc d-d transitions at 442 nm,

meaning we are only able to account for all the ligand ﬁeld

transitions by assigning this as a metal to mixed metal/ligand

CT rather than a new MLCT to the PDI backbone in addition

to the original Fc d-d transitions. This is also in accord with

this transition being weaker than expected for a full MLCT (ϵ

identity in the Fc PDI system is further rationalized on similar

ionic radii and identical charge numbers, leading to similar

electrostatic stabilizations. This suggests that ﬁne-tuning of the

coupling interaction is possible by substituting metal ions with

diﬀerent charge densities. Note however that the larger ΔE

1400 per metal center, rather than ϵ > 5000−10 000). This

band must be a shift of the Fc d-d transition rather than a new

MLCT in addition to the normal Fc transitions. It is

uncommon to observe shifts larger than ca. 10−15 nm in

simple ferrocenyl compounds, consistent with the donor and

acceptor orbitals in ferrocene being metal-centered and

for (Fc PDI)FeCl vs the other complexes suggests there are

2 2

further eﬀects to be studied, and that our optical, electro-

chemical, and crystallographic characterizations do not

uniquely deﬁne these eﬀects.

50a

insensitive to ligand perturbation. In Fc PDI, a change in

CONCLUSIONS

■

the ligand ﬁeld appears to take place since it exhibits a much

larger shift (36 nm). This is reminiscent of observations by

Diaconescu and co-workers, where arylamino substituted

ferrocenes show more ligand character in the LUMO, whereas

the HOMO is generally still metal-centered, lowering the

Structural, electrochemical, spectroscopic, and computational

evidence argues that metal binding to Fc PDI triggers mixing

of (Fc PDI)MCl (M = Fe, Co, Mg, Zn) ferrocenyl Fc d-

orbitals with the PDI π* network, engendering coupling

between the two Fc sites. Curiously, this coupling appears

dependent on metal ion binding but does not not disappear

when diﬀerent s- or d-block metals are employed

demonstrating the coupling is generalizable and worth

exploring further. This is an interesting contrast to p-block

analogues which do not exhibit coupling in most cases. CV

displays a ∼200 mV separation between oxidation of each

ferrocene for the Fe, Co, Zn, and Mg complexes, but no

separation is seen in the free ligand. Minimal π interaction

HOMO−LUMO gap.

To ﬁnd support for the present assignments in the literature,

UV/vis spectra of several mixed-valent Fc compounds were

also considered. However, since Fc mixed-valent studies

typically focus on the NIR region and the related MMCT

transitions arising from electronic coupling, it is challenging to

directly compare the present results to known Class II and

Class III mixed valent compounds. Note however that some Fc

conjugated systems have absorbances in the 500−600 nm

range, however it is unclear if these represent Fc mixing,

MLCT, or π/π* transitions.

cases where low energy CT bands are observed, they do not

correlate in a consistent way with the degree of coupling. Cases

with coupling and no spectral shifts are observed,

cases with spectral shifts and no coupling.

between the ligand and MCl fragment is evident in the crystal

and DFT computed structures, which show no variation in

back-bonding into imine π* orbitals between the free ligand

and the complexes. UV−vis spectroscopy shows mixing of Fc d

orbitals with ligand π orbitals, with the extent of mixing

dependent on metal binding but not appreciably on metal

identity. Upon oxidation, broad and low energy ferrocenium

transitions appearindicative of highly delocalized mixed

metal−ligand orbitals. Importantly we also observe a very

broad IVCT in the NIR, which clearly indicates coupling

between the ferrocenes. We assign this as a Class II compound

4a,63

As a further complication, in

45b,64

as are

Central Metal Not Directly Engaging in Coupling

Pathway. The imine bond lengths do not change signiﬁcantly

upon metalation, suggesting that π bonding between the

organic π* system and MCl d orbitals is insigniﬁcantas

10a,66

−1

seen in other neutral ﬁrst row PDI complexes.

The DFT

based on the weak (<5000 M cm ) and broad (>2000

−

analysis further supports this picture as negligible π-bonding is

observed in the frontier orbitals. Elongation of the imine bond

is frequently used to estimate the degree of electron density

residing in PDI π/π* networks. The unchanged imine bond

distances argue that only small amounts of electron density are

transferred from the MCl fragment onto the ligand or from

the ligand π system into the MCl fragment. For this reason,

the Fc delocalization onto the MCl fragment cannot be

cm ) IVCT feature. We propose that when Fc PDI binds to a

Lewis acidic MCl fragment, coupling is promoted by a better

energy match between Fc and organic π orbitals. Crystallog-

raphy suggests that in the mixed valent state the ferrocenium

10a

pivots into the MCl coordination sphere, potentially opening

doors for cooperative catalysis. Interestingly, this system

displays orbital mixing between PDI and Fc orbitals, but

most delocalization is seen in unoccupied orbitals. We propose

that second and third row transition metals that are known to

directly implicated. Small contributions from the MCl₂

fragment are seen in the DFT orbitals, and small spectral/

electrochemical changes are seen, suggesting it is a non-

engage in stronger back bonding into PDI ligands may place

electron density in the orbitals engaged in the delocalized

orbitals, leading to stronger coupling and possibly new

coordination chemistry.

negligible but weak locus of bonding. Thus, the MCl fragment

can be thought of as triggering coupling although it does not

participate substantially in the delocalization pathway. Since π-

bonding does not seem to be important, it is proposed that an

electrostatic interaction triggers coupling. Similar interactions

are proposed in ferrocenyl polypyridyl complexes, where

coordination to Lewis acids creates intense charge transfer

ASSOCIATED CONTENT

sı Supporting Information

■

The Supporting Information is available free of charge at

24f,67

transitions from ferrocene and is useful for ion sensing.

Moreover, the present observation of similar electronic

properties across a series of metals with diverse coordination

chemistries is highly unexpected since mixed-valent com-

UV−vis characterizations, DFT optimized geometries

and energies, and calculated UV−vis for these complexes

(PDF)

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

L.; Malacria, M.; Mouries

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AUTHOR INFORMATION

Corresponding Authors

■

(8) (a) Flisak, Z.; Sun, W.-H. Progression of Diiminopyridines:

Yanshan Gao − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208 −3113, United States;

orcid.org/0000-0003-0329-705X; Email: yanshan@

northwestern.edu

Tobin J. Marks − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208 −3113, United States;

orcid.org/0000-0001-8771-0141; Email: t −marks@

northwestern.edu

From Single Application to Catalytic Versatility. ACS Catal. 2015, 5

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Authors

Cole Carter − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208−3113, United States

Yosi Kratish − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208−3113, United States

Titel Jurca − Department of Chemistry, Northwestern University,

Evanston, Illinois 60208−3113, United States; orcid.org/

(

876.

9) Sandoval, J. J.; Alvarez, E.; Palma, P.; Rodríguez-Delgado, A.;

Cam

pora, J. Neutral Bis(imino)-1,4-dihydropyridinate and Cationic

Bis(imino)pyridine σ -Alkylzinc(II) Complexes as Hydride Exchange

Systems: Classic Organometallic Chemistry Meets Ligand-Centered,

Biomimetic Reactivity. Organometallics 2018, 37 (11), 1734 −1744.

000-0003-3656-912X

(10) (a) Romelt, C.; Weyhermuller, T.; Wieghardt, K. Structural

Characteristics of Redox-Active Pyridine-1,6-Diimine Complexes:

Electronic Structures and Ligand Oxidation Levels. Coord. Chem.

Rev. 2019, 380, 287−317. (b) Tondreau, A. M.; Milsmann, C.;

Patrick, A. D.; Hoyt, H. M.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J.

Synthesis and Electronic Structure of Cationic, Neutral, and Anionic

Bis(imino)pyridine Iron Alkyl Complexes: Evaluation of Redox

Activity in Single-Component Ethylene Polymerization Catalysts. J.

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Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c10015

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Financial support was provided by the National Science

Foundation through Grant No. CHE-1856619 (C.C.). This

work used the IMSERC facility at Northwestern University,

which has received support from the NSF (CHE-1048773 and

CHE-9871268); Soft and Hybrid Nanotechnology Exper-

imental (SHyNE) Resource (NSF NNCI-1542205); the State

of Illinois and International Institute for Nanotechnology. We

also thank Katia Kratish for graphic design of the Table of

Contents ﬁgure, as well as Prof. Y. Apeloig of the Technion

Israel Institute of Technology for providing computational

resources. We also thank Dr. Boris Tumanskii of the Technion

Israel Institute of Technology for informative and helpful

discussions. Dr. N. LaPorte of Northwestern U. assisted with

spectroelectrochemical and NIR experiments.

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