Chapter I: Introduction: An Overview of Relevant Techniques and Concepts
1.4 Lewis Acid-Base Interactions in Organometallic Chemistry: History and
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important conclusion demonstrated from this work was the blueshift in the CN stretching frequency observed in infrared spectra. Soon after, in 1963, Shriver extended the concept of BF3 coordination to cyanometallates with Fe(phen)2(CN)2. With the compounds originally studied, Shriver demonstratedby UV-Vis reflectance, absorption by mass, and IR spectroscopy the stoichiometry and characteristics of borane-appended cyanometallates. One particularly salient point made in this publication is the discussion of the charge-transfer spectrum of Fe(phen)2(CN)2, which displays a dramatic color change as a result of adduct formation with BF3 that is consistent with a significant blueshift in the metal-to- ligand charge transfer spectra of the complex.41 Extending these studies even further, in 1966, Shriver and Posner demonstrated the variable shift in IR frequencies and charge-transfer bands in borane adducts of Fe(phen)2(CN)2 depending on the Lewis acidity of the borane.42 This study reported the first example of the effects of neutral Lewis acids on the formal potential of transition metal complexes, with BBr3 exerting a more dramatic shift on the formal potential relative to BF3, demonstrating the stronger Lewis acidity of BBr3.
The Gutmann Donor and Acceptor model for solvents, first delineated in the 1970s, rationalizes the relative Lewis acid and Lewis base strengths of solvents towards dissolved substrates. Acceptor number values are measured by finding the 31P-NMR shift of triethylphosphine oxide (Et3PO with SbCl5 in 1,2- dichloroethane is arbitrarily set to AN = 100), where solvents with greater electrophilicity will inductively withdraw a greater amount of electron density through oxygen.43
In the 1970s, Gutmann published detailed studies on the formal potential of both hexacyanoferrate(III/II) and hexacyanomanganate(IV/III/II) in a variety of protic and aprotic solvents.44,45,46 His studies demonstrated that the formal potentials for both hexacyanoferrate and
11 hexacyanomanganate could be tuned over a 1.9 V range simply through judicious choice of solvent, as shown in Table 1.3. An approximately linear correlation is found for the formal potential of the Fe(III/II) redox couple and the acceptor number of the solvent, with small variations due to dielectric constant and solvation effects. These studies laid the groundwork for a more definitive understanding of the effects of both dielectric constant and donor-acceptor interactions on the ground-state electronic properties of cyanometallates and provided a unifying theory for the effects of donor-acceptor interactions of solvents on the reactivity of small molecules. The Gutmann donor-acceptor has since been verified by a variety of solvent-dependence studies for both chemical and electrochemical reactions.
One interesting comment made in the original publications by Gutmann mentioned the inability to boronate [Fe(CN)6]3− with BF3, despite the success that Shriver experienced with [Fe(CN)6]4−.
Table 1.3. Formal potentials for (TBA)3[Fe(CN)6] in aprotic and protic solvents with varying dielectric constants and acceptor numbers. All voltammetry was acquired with 0.1 M TBAClO4 as the electrolyte. Table adapted from References 46,44,47,48
Solvent Formal Potential (V vs. Fc+/0)
Dielectric Constant
Acceptor Number
N-Methylpyrrolidinone (NMP) −1.75 32.0 13.3
N,N-Dimethylformamide −1.72 36.1 16.0
Acetonitrile −1.54 38.0 18.9
Dimethylsulfoxide −1.51 45.0 19.3
N,N-Dimethylthioformamide −1.48 51.2 18.8
1,2-Dichloroethane −1.43 10.1 16.7
Propylene carbonate −1.32 69.0 18.3
Nitromethane −1.26 35.9 20.5
Ethanol −0.88 24.3 37.9
Methanol −0.74 32.6 41.5
Acetic Acid 0.15 6.2 52.9
In the 1980s, Woodcock and Shriver extended the concept of Lewis acid coordination to heteroleptic cyanometallates (CpFe(CO)2CN and Fe(phen)2(CN)2) through solvation in an acidic molten salts.49 These studies also preface the use of ionic liquids for electrochemical and energy-related applications.
In the AlCl3 melt, Fe(phen)2(CN)2) demonstrates dramatic color changes and blueshifted MLCT transition maxima and CN stretching frequencies.
Following the studies by Shriver, little research was reported on the incorporation of boranes with cyanometallates. In 2001, Bochmann et al. reported the use of tris(pentafluorophenyl)borane as a Lewis acid with tetracyanonickelate and tetracyanopallidate to generate non-coordinating anions for dramatically improved metallocene polymerization catalysts.50 These studies demonstrate the greatly improved activity of polymerization catalysts as a result of stabilizing the negative charge of the counterion.
Beyond non-coordinating anions and fundamental Lewis acid coordination and its effect on spectroscopic properties, other studies have demonstrated that boranes can induce the structural rearrangement of cyanometallate species to generate the thermodynamically favored product. In 2005, Dunbar et al. demonstrated that coordination of BPh3 to [Cr(CN)6]3− at elevated temperatures results in isomerization of the cyanometallate complex to a nitrile complex, suggesting the variable ligand field strength of cyanide.
Since 2014, multiple studies by Ko et al. have demonstrated the utility of Lewis acid coordination on the modification of formal potentials and luminescence spectroscopy for heteroleptic cyanometallates to generate longer-lived phosphorescent species and modified electronic structures. In 2014, Ko et al.
13 characterized derivatives of Re(phen)(CO)3(CN) appended with B(C6F5)3, which exhibited improved quantum yields and electrochemical properties as a result of boronation.51 These reports were followed up by characterization of Os(bpy)2(CN)2 and its derivatives with BPh3 and B(C6F5)3.52 These complexes also extended the concept of blueshifting MLCTs and increasing lifetimes in heteroleptic cyanometallate complexes through boronation. Other studies have shown this effect in heteroleptic Ir(III) and Cu(I) diimine complexes, demonstrating the universality of modification of cyanometallates using boranes.53,54
Despite multiple studies demonstrating borane coordination to cyanometallates, the field is underdeveloped, and previous research provides significant fodder for the utilization of boranes to modify the ground- and excited-state electronic properties of both homoleptic and heteroleptic cyanometallates.