Tuning the formal potential of ferrocyanide over a 2.1 V range
We report the synthesis and characterization of homoleptic borane adducts of hexacyanoferrate(II). Borane coordination blueshifts d-d transitions and CN IR and Raman frequencies. Control over redox properties is established with respect to borane Lewis acidity, reflected in peak anodic potential shifts per borane of +250 mV for BPh 3 and +350 mV for B(C 6 F 5 ) 3 . Electron transfer from [Fe(CN-B(C 6 F 5 ) 3 ) 6 ] 4À to photogenerated [Ru(2,2 0 -bipyridine) 3 ] 3+ is very rapid, consistent with voltammetry data. Coordination by Lewis acids provides an avenue for selective modification of the electronic structures and electrochemical properties of cyanometalates.
Introduction<!>Results and discussion<!>Conclusions
<p>Duward Shriver pioneered the study of borane adducts of cyanometalates. 1a-d Although he established from analysis of vibrational spectroscopic data that borane coordination (CN-BR 3 ) enhanced M(d 6 )-CN p backbonding, surprisingly, to the best of our knowledge, no structures of d 6 low-spin borane adducts have been hitherto reported. 2 Indeed, only three homoleptic borane adducts have been crystallographically characterized, (TEA) 3 [Cr(NC-BPh 3 ) 6 ] (TEA ¼ tetraethylammonium, BPh 3 ¼ triphenylborane), (CPh 3 ) 2 [Ni(CN-B(C 6 F 5 ) 3 ) 4 ] and (CPh 3 ) 2 [Pd(CN-B(C 6 F 5 ) 3 ) 4 ] (CPh 3 + ¼ trityl cation, B(C 6 F 5 ) 3 ¼ tris(pentauorophenyl)borane). 3,4 Also surprising is that very little is known about the electrochemistry of coordinatively-saturated borane adducts of hexacyanometalates, with the majority of previous work focused on the solvent dependence of cyanometalate redox potentials. 1d, [5][6][7][8] We show herein that the formal potentials of these boronated adducts span an unusually wide range (over 2.1 V), providing a way to use as oxidants and reductants in energy storage devices.</p><!><p>Tetrabutylammonium (TBA) and TEA hexacyanoferrate(II) compounds (1a, 1b) were prepared using a modied literature procedure.  13 C{ 1 H} NMR, UV-vis, solid-state IR, and voltammetry conrmed the purity of 1, with a single, reversible redox couple with a formal potential of À1.25 V vs. Fc +/0 in MeCN.</p><p>The borane adducts of 1 were synthesized by combining six equivalents of borane with one equivalent of 1 dissolved in dichloromethane (DCM) in a nitrogen-lled glove box. Coordination of borane generated (TEA) 4 [Fe(CN-BPh 3 ) 6 ] (2) and (PPN) 4 [Fe(CN-B(C 6 F 5 ) 3 ) 6 ] (3). Each complex was puried and subsequently analyzed by 11 B and 13 C NMR spectroscopy, X-ray crystallography, UV-vis spectroscopy, elemental analysis, and voltammetry.</p><p>Full borane coordination for all species was conrmed by analysis of X-ray crystallographic data (see ESI †), with representative structures depicted in Fig. 1. The average M-CN (1.91 Å) and C-N (1.17 Å) bond lengths for 2 compared to 1, 1.93 Å and 1.17 Å (Fig. S11 †), respectively, are consistent with competing s and p interactions, where s donation from nitrogen to boron weakens the M-CN bond, while p backbonding strengthens it. Thus, the contraction of the M-CN bond in 2 is negligibly small. The average M-C-N (174.6 ) and C-N-B (172.3 ) bond angles in complex 2 are not perfectly linear, likely due to the effects of steric clash among the aryl groups. The average M-C-N (176.9 ) and C-N-B (173.9 ) bond angles for complex 3 are similarly perturbed. There are slight contractions in average M-CN (1.90 Å) and C-N (1.15 Å) bond lengths for 3 compared to those in complex 2.</p><p>In the 13 C{ 1 H} NMR spectra, the cyanide carbon exhibits only one peak, indicating that boranes are bound to all six cyanides The solid-state IR and Raman spectra of 1 and its various adducts are shown in Fig. 2. As expected, the CN stretching frequency increases as the Lewis acidity of the borane or the oxidation state of the metal increases. Increased Lewis acidity causes increased stretching frequency, a result of lowering the absolute energies of the cyanide-based p-bonding orbitals. 9 The lowest energy absorption band in the UV-vis spectra (Fig. 3) of the adducts is attributable to a spin-allowed dd transition ( 1 A 1g / 1 T 1g ). 10 This band, 323 nm in water, redshis to 357 nm in MeCN. Upon coordination of BPh 3 and B(C 6 F 5 ) 3 , 2 and 3 exhibit blueshied absorbance maxima, indicating that increased backbonding outweighs decreased s bonding in the octahedral ligand eld. 10 For 2 and 3, bands below 270 nm are attributable to borane p to p* transitions (Fig. S16-S18 †).</p><p>Cyclic voltammograms of pure 1b in DCM solution and one with two equivalents of BPh 3 added are shown in Fig. 4A. Both CVs were corrected for the non-faradaic charging current and integrated to ensure that the anodic charge passed was equal for both, which suggests that the only redox reaction taking place is the one of interest (Fe III/II ). By adding two equivalents of BPh 3 , seven anodic peaks are visible, corresponding to a distribution of all possible numbers of boranes coordinated to hexacyanoferrate(II). Addition of a greater number of equivalents of borane increases the peak current for the coordinativelysaturated species (Fig. S7 †). The seven anodic peaks corresponding to different coordination numbers are clearly seen in differential pulse voltammetry (Fig. 4B). The differential current responses for the two-and three-coordinate species are broader, likely due to the existence of isomers being oxidized at slightly different potentials, which decreases the intensity and broadens the observed differential wave for these adducts. 11 As expected, the addition of borane results in a more positive peak anodic potential, likely due to lowering the absolute energies of the metal-based orbitals. The trend in peak anodic potential shi (from cyclic voltammetry) per borane added to hexacyanoferrate(II) is linear (Fig. 4C), suggesting an electron withdrawing effect that is solely dependent on the Lewis acidity of the borane, with little to no attenuating effects as more boranes are added to the secondary coordination sphere. 12 Assuming the peak anodic potential correlates with the formal potential for each borane species, the linear behavior is consistent with,</p><p>where the change in formal potential upon coordination of an additional borane, E n À E nÀ1 , is proportional to the ratio of binding constants of BPh 3 to the Fe(II) and Fe(III) states of the complex, K II n /K III n . 12 The approximate E 1/2 value for 2 is 0.32 V vs. Fc +/0 .</p><p>Although full coordination of borane in complex 2 was conrmed by both X-ray crystallography and 13 C NMR, the voltammetry of 2 in both DCM and MeCN is not electrochemically reversible (Fig. 4D).</p><p>Cyclic voltammetry suggests that the complex undergoes electron transfer followed by a borane dissociation, with the metalloproduct undergoing reduction and subsequent reoxidation (EC mechanism). 13 The proposed mechanism is supported by a peak current ratio that is less than one for the sixcoordinate species and by the presence of cathodic waves that correspond to reduction of the four-, ve-, and six-coordinate species (Fig. 4A and D). 13 We observe a 28 mV shi in peak anodic potential with a ten-fold increase in scan rate, consistent with the theoretical value of 30 mV for a purely kinetic EC mechanism. 13 This mechanism also is supported by the appearance of anodic waves for the four-and ve-coordinate species as a result of sweeping through multiple cycles at fast scan rates and by the decrease in anodic current for the sixcoordinate species aer the rst scan (Fig. S8 †). The peak cathodic current does not increase with increasing scan rate, consistent with very rapid BPh 3 dissociation. 13 Additionally, 2 was oxidized with dibenzo-1,4-dioxin radical cation in MeCN, and 1 H NMR in CD 3 CN conrmed the presence of Ph 3 B-NCMe (Fig. S9 †).</p><p>In contrast to 2, complex 3 displays a single, electrochemically reversible redox event with a formal potential of 0.85 V vs. Fc +/0 (Fig. 5A), corresponding to a 2.1 V anodic shi in the Fe III/II couple, which is a 350 mV anodic shi per B(C 6 F 5 ) 3 added to hexacyanoferrate(II). Similar to 2, a linear potential shi per borane added to hexacyanoferrate(II) was observed for 3 (Fig. S10 †). Borane adducts of Fe(phen) 2 (CN) 2 in dichloromethane showed an approximately 300 mV increase in peak anodic potential per borane, with BBr 3 producing the largest shi. 1d Cyanorhenate(I) complexes, Re(R 2 phen)(CO) 3 [CN-B(C 6 F 5 ) 3 ], where R ¼ H, Me, displayed a $320 mV shi in peak anodic potential (E p,a ) in acetonitrile compared to the parent. 14 Cyanoosmate(II) complexes, [Os(4,4 0 -R 2 (bpy)) 2 (CNBL 3 ) 2 ], where R ¼ H, Me, showed $420 and $290 mV anodic shis per borane for L ¼ (C 6 F 5 ) 3 B and BPh 3 in MeCN, respectively. 15 As the Fe III/II peak anodic potential shis are very near those observed for Re(I) and Os(II) complexes, there is minimal dependence on metal identity or oxidation state.</p><p>The peak current, i p , of an electrochemically reversible, diffusion-controlled voltammogram is dened by, 16  where n is the number of electrons, F is Faraday's constant, D 0 is the diffusion coefficient (cm 2 s À1 ), n is the scan rate (V s À1 ), A is the surface area of the electrode (A ¼ 0.0707 cm 2 ) and C * 0 is the bulk concentration of the redox-active species (mol cm À3 ). The diffusion coefficient for 3 in MeCN, obtained from a Randles-Sevcik plot of peak current vs. n 1/2 , was found to be 4.9 Â 10 À6 cm 2 s À1 , which is similar to values for potassium ferrocyanide in aqueous electrolyte. 17,18 Oxidation of 3 by ash-quench-generated [Ru(2,2 0bipyridine) 3 ] 3+ was very rapid. From a linear t of electron transfer rate versus concentration (Fig. S15 †), the apparent second order rate constant, k ex , was found to be 8.4 Â 10 8 M À1 s À1 . Using an ion-pair preequilibrium model, K 0 , the ion-pair association constant, was found to be 0.19 M À1 , with k et estimated to be 4.5 Â 10 9 s À1 . 19 We conclude that electron tunneling from Fe II to Ru III in the ion-paired precursor complex is not inhibited by the wall of 90 uorine atoms in the boronated cyanide complex.</p><!><p>We have demonstrated that boranes can tune the formal potentials of reversible redox couples. This nding means that researchers can selectively alter the formal potentials of cyanide-based inorganic complexes, providing opportunities for creating new quenchers, oxidants, and single-electron transfer reagents. Importantly, the uorinated "cage" surrounding the iron center of 3 does not hinder outer-sphere electron transfer, indicating that boronated cyanide complexes will likely be useful as electrolytes in non-aqueous redox ow batteries. 20</p>