Post‐Assembly Reactivity of N‐Aryl Iminoboronates: Reversible Radical Coupling and Unusual B−N Dynamic Covalent Chemistry

Abstract Post‐assembly reaction of a dynamic covalent iminoboronate system following addition of Cp2Co resulted in the formation of a series of new reductively coupled dianionic dimers via C−C bond formation. The dimers formed as a mixture of BN‐containing isomeric products: diastereomers rac 5 and meso 5, with coupled five‐membered rings, and enantiomeric rac 6, with a fused six‐membered ring bicyclic system from C−C bond formation and rearrangement of the B−N bonds. Each isomer was identified using 1H NMR spectroscopy in combination with single crystal X‐ray structure determination. Interestingly, interconversion between the coupled five‐membered rings (rac 5) and fused bicyclic systems (rac 6) was found to occur through an unprecedented breaking and reforming of the B−N covalent bond. Further, the coupled products could be converted quantitatively back to their iminoboronate precursors with addition of the electron abstractor Ph3C+.


Materials and Methods
Reagents and solvents were purchased from commercial suppliers and used without further purification, unless otherwise specified. 2-formylphenylboronic acid was purchased from Arcos. Pyrocatechol, toluidine, tetrachlorocatechol monohydrate, bis(cyclopentadienyl) cobalt, bis(pentamethylcyclopentadienyl) cobalt, tritylium tetrafluoroborate, 2,2,6,6tetramethylpiperidinyloxy were purchased from Sigma Aldrich. The solvents were purchased from Sigma Aldrich; prior to use, deuterated acetonitrile was distilled over calcium hydride and deuterated DMSO was dried over calcium hydride, filtered and stored over molecular sieves.
Due to the water and air sensitivity of the reductively coupled products, all manipulations were carried out in a glovebox under a nitrogen atmosphere using dry solvent.

NMR Spectroscopy
NMR spectra were recorded on a Bruker Avance DRX-400, Bruker Avance 500 BB ATM, Bruker DRX-500, or Bruker Avance 500 Cryo spectrometers. Chemical shifts for 1 H, 13 C, 19 F and 11 B spectra are expressed in parts per million (ppm) and coupling constants (J) are reported in Hertz (Hz). 1 H and 13 C were referenced to the solvent residual peak and 11 B was referenced to BF3·Et2O at 0.0 ppm. All measurements were carried out at 298 K unless reported otherwise. The following abbreviations are used to describe signal multiplicity for 1 H, 13 C and 11 B NMR spectra: s: singlet, d: doublet, t: triplet, m: multiplet, b: broad.
Each isomer of the reductively coupled products was fully characterised in solution, where possible, using 1 H NMR spectroscopy. Mixtures of two isomers were characterised in cases where one of the products could not be obtained as a single isomer by crystallisation (e.g. rac5-2a-d, rac6-2e, meso5-2b) or due to fast interconversion between isomers (e.g. rac6-2d and rac5-2d). In some instances, unambiguous assignment of all signals in these mixtures was not possible due to the number of overlapping signals. The broadness of the 13 C NMR signal for the carbon directly attached to the boron atom (Ca) often precluded assignment of this signal.

Mass Spectrometry
The mass spectra of the iminoboronates were acquired on a Jeol AccuTOF mass spectrometer. It was not possible to obtain mass spectra of the reductively coupled dimers in all cases due to their air and water sensitivity as well as fragmentation under mass spectrometry conditions. Where reported, nanospray ionisation (NSI) mass spectra provided by the EPSRC National MS Service Centre at Swansea were acquired on a Thermofisher LTQ Orbitrap XL.

X-Ray Crystallography
Data were with collected using a Bruker D8 VENTURE equipped with high-brilliance IμS Cu-Kα radiation (1.54178 Å), with ω and ψ scans at 180(2) K or at Beamline I19 of Diamond Light Source 1 employing silicon double crystal monochromated synchrotron radiation (0.6889 Å) with ω scans at 100(2) K. Data integration and reduction were undertaken with SAINT and XPREP. 2 Subsequent computations were carried out using the WinGX-32 graphical user interface. 3 Multiscan empirical absorption corrections were applied to the data using SADABS. 2 Structures were solved by direct methods using SHELXT 4 or charge-flipping using SUPERFLIP 5 then refined and extended with SHELXL. 6 In general, non-hydrogen atoms with occupancies greater than 0.5 were refined anisotropically. Some disordered solvent molecules were refined with isotropic thermal parameters. Carbon-bound hydrogen atoms were included in idealised positions and refined using a riding model. Disorder was modelled using standard crystallographic methods including constraints, restraints and rigid bodies where necessary. Crystallographic data have been deposited with the CCDC (1844532-1844541).

rac5-2a and meso5-2a Mixture in CD3CN
In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1a (5.00 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.
Reaction progress was monitored by NMR spectroscopy. A loss of the imine signal and the appearance of the two methine signals 5 and 5' around 5.5 ppm were observed in the 1 H NMR spectrum ( Figure S13). The rac5-2a (protons 1-7) and meso5-2a (protons 1'-7') products were subsequently identified and characterised through a combination of X-ray crystallography and NMR spectroscopy (Sections 3.2-3.5). Full assignment of the signals for each isomer was not carried out due to the overlapping signals. The 19 F NMR spectrum showed the loss of the fluorine signal for 1a at -111.17 ppm and the appearance of several new fluorine-containing species between -130 ppm and -140 ppm ( Figure  S14). The two major species were subsequently identified as the rac5-2a and meso5-2a products. The 11 B NMR spectrum showed broad peaks around 14 ppm, consistent with the formation of a tetrahedrally coordinated boron complex ( Figure S15).

rac5-2a, rac6-2a and meso5-2a Mixture in DMSO-d6
In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of DMSO-d6. This solution was then agitated with 1a (5.03 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.
rac6-2a (Section 3.3) and meso5-2a (Section 3.4) were subsequently characterised by dissolving isolated crystals in DMSO-d6, allowing identification of these isomers in the reaction mixture (Figures S16, S17). Furthermore, the redissolved rac6-2a crystals were observed to interconvert to rac5-2a and thus, all three products were characterised in DMSO-d6. Due to the similarity of the chemical shifts of the coupled products in DMSO-d6 and CD3CN, the two products of the reaction in CD3CN were identified as meso5-2a and rac5-2a ( Figure S18).

meso5-2a
In a nitrogen atmosphere glove box, Cp2Co (6.00 mg, 0.032 mmol) was dissolved in 0.5 mL of CH3CN. This solution was then agitated with 1a (5.00 mg, 0.017 mmol) until the solid had fully dissolved and left to crystallise for several days at room temperature. The solution was decanted and the remaining crystals were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6.

rac6-2a
1a (11.62 mg, 0.37 mmol) was treated with Cp2Co (6.93 mg, 0.37 mmol) in CD3CN (1 mL) in a nitrogen filled glove box at room temperature. Crystals grown from the unperturbed reaction mixture were isolated by decanting the solution. They were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6.
While the 1 H NMR spectrum of the redissolved rac6-2a crystals showed one species initially ( Figure S24), a new set of signals grew over time corresponding to the rac5-2a isomer (see Section 3.5). Due to this interconversion to the rac5-2a isomer, it was not possible to record 13 C, 19 F and 2D NMR spectra of the rac6-2a isomer alone. The 13 C chemical shifts reported below were assigned from this mixture.

rac5-2a
Characterisation data for the rac5-2a isomer could be inferred from the NMR data of the equilibrated rac5-2a and rac6-2a mixture from Section 3.4. Due to the number of overlapping signals in the 1 H NMR spectrum of the mixture, the 1 H NMR chemical shifts are not reported below but Figure S30 shows the assignments. The 13 C NMR chemical shifts reported below were assigned from the 13 C NMR and HSQC/HMBC spectra ( Figures S25, S29) and the 19 F NMR chemical shift was assigned from the spectrum in Figure S26.

rac5-2b and meso5-2b Mixture in CD3CN
In a nitrogen atmosphere glove box, Cp2Co (3.02 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1b (5.00 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.
As meso5-2b was not observed to crystallise from the reaction mixture, it was characterised in solution as a mixture with rac5-2b. The signals in the 1 H NMR spectrum could be assigned to the two isomers due to good dispersion of the signals. Signals marked with ' are attributed to meso5-2b. The high-resolution mass spectral data reported below was obtained from an analogous reaction using the reductant potassium graphite instead of cobaltocene, as it was not possible to observe 2b in the mass spectrum from the reduction with cobaltocene.      Figure S36. Overlay of HSQC (blue) and HMBC (red) NMR spectra of the reaction mixture from the reductive coupling of 1b. Protons/carbons without prime labels correspond to rac5-2b and protons/carbons with prime labels correspond to meso5-2b.

rac6-2b
In a nitrogen atmosphere glove box, Cp2Co (3.01 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1b (4.98 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube. Crystals grown from the unperturbed reaction mixture were isolated by decanting the solution. They were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6.
While the 1 H NMR spectrum of the redissolved rac6-2b crystals showed one species initially ( Figure S39), a new set of signals grew over time corresponding to the rac5-2b isomer (see Section 4.4, Figure S44). Due to this interconversion to the rac5-2b isomer, it was not possible to record 13 C, 19 F and 2D NMR spectra of the rac6-2b isomer alone. The 13 C chemical shifts reported below were assigned from this mixture.     Figure S43. Overlay of HSQC (blue) and HMBC (red) NMR spectra (DMSO-d6) of rac6-2b and rac5-2b after partial interconversion.

rac5-2b
Characterisation data for the rac5-2b isomer could be inferred from the NMR data of the equilibrated rac5-2b and rac6-2b mixture from Section 4.3. Due to the number of overlapping signals in the 1 H NMR spectrum of the mixture, the 1 H NMR chemical shifts are not reported below but Figure S44 shows the assignments. The 13 C NMR chemical shifts reported below were assigned from the 13 C NMR and HSQC/HMBC spectra ( Figures S40, S43)

rac5-2c and meso5-2c Mixture in CD3CN
In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1c (6.5 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.

meso5-2c
After standing unperturbed at room temperature for 7 days, the 2c mixture (Section 5.1) contained only the meso5 isomer as crystallization of rac6-2c removed nearly all of the rac species (rac5-2c and rac6-2c) from solution. Due to the small quantity of meso5-2c in solution and overlapping signals for protons 1-4, unambiguous assignment of carbons b-f and h by 2D techniques (HSQC and HMBC, Figure S49) was not possible ( Figure S47).

rac6-2c
Crystals grown from the unperturbed reaction mixture in Section 5.1 were isolated by decanting the solution. They were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6.
The 1 H and COSY NMR spectra of the redissolved crystals were recorded before interconversion to rac5-2c ( Figures S51, S52), but partial interconversion had occurred during the recording of the 13 C, NOESY, HSQC and HMBC spectra ( Figures S53-S54). The 13 C chemical shifts reported below were assigned from this mixture.      of rac6-2c and rac5-2c after partial interconversion. Key assignments are shown.

rac5-2c
Characterisation data for the rac5-2c isomer could be inferred from the NMR data of the equilibrated rac5-2c and rac6-2c mixture in Section 5.3. However, only the 13 C NMR shifts are reported below due to overlapping signals with rac6-2c in the 1 H NMR spectrum. Figure S55 shows the assignment of both isomers in the 1 H NMR spectrum

Solution Characterisation of the Methoxyaniline-Pyrocatechol Reductively
Coupled Dimer (2d) 6. 1 rac5-2d and meso5-2d Mixture in CD3CN In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1d (5.21 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.
As meso5-2d was not observed to crystallise from the reaction mixture, it was characterised in solution as a mixture with rac5-2d. Signals marked with ' are attributed to the mesodiastereomer. Unlike the reductive couplings of 1a-1c, small quantities of rac6-2d were also observed in the reaction mixture in CD3CN before it crystallised from the reaction mixture ( Figure  S56). The spectrum of the reaction mixture was not observed to change significantly over 6 h ( Figure S57). As a complex mixture of three products, complete assignment of the 13 C NMR data was not possible ( Figure S58). The signals corresponding to rac5-2d were fully assigned (with the exception of carbon a since this signal is too broad) and assignments of meso5-2d are reported below as signals marked with '.     Small quantities of rac6-2d (green signals) was also observed. Figure S61. Overlay of HSQC (blue) and HMBC (red) NMR spectra of the reaction mixture from the reductive coupling of 1d. Labels without primes correspond to rac5-2d and labels with primes correspond to meso5-2d. 2 rac5-2d, rac6-2d and meso5-2d Mixture in DMSO-d6 In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1d (5.22 mg, 0.016 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube. Figure S62. 1 H NMR spectrum (400 MHz, DMSO-d6) of the reaction mixture from the reductive coupling of 1d. As a complex mixture of three products, full assignment was not carried out but key protons for rac5-2d (red), meso5-2d (red') and rac6-2d (green) were assigned.

rac6-2d and rac5-2d
Crystals grown from the unperturbed reaction mixture in Section 6.1 were isolated by decanting the solution. They were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6.
X-ray analysis of crystals from an identical reaction were found to be rac6-2d (Section 8.5), however, the NMR spectrum of the crystals immediately after they were dissolved in DMSO-d6 showed the presence of both rac6-2d and rac5-2d ( Figure S63). This is attributed to the fast interconversion between the two isomers. Therefore, it was not possible to characterise rac6-2d before interconversion to rac5-2d and signals marked with ' are attributed to rac5-2d signals. The multiplets at 6.33 ppm and 6.41 ppm could not be accurately integrated since the sample was a mixture of two species and the catechol signals were overlapping in both the 1 H and 13 C NMR spectra.

rac5-2e and meso5-2e Mixture in CD3CN
In a nitrogen atmosphere glove box, Cp2Co (2.52 mg, 0.013 mmol) was dissolved in 0.5 mL of CD3CN. This solution was then agitated with 1e (6.00 mg, 0.013 mmol) until the solid was fully dissolved and transferred to a J. Young NMR tube.
As meso5-2e was not observed to crystallise from the reaction mixture, it was characterised in solution as a mixture with rac5-2e. Signals marked with ' are attributed to the mesodiastereomer.      Figure S74. Overlay of HSQC (blue) and HMBC (red) NMR spectra of the reaction mixture from the reductive coupling of 1e. Labels without primes correspond to rac5-2e and labels with primes correspond to meso5-2e.

rac5-2e
In a nitrogen atmosphere glove box, Cp2Co (3.00 mg, 0.016 mmol) was dissolved in 0.5 mL of CH3CN. This solution was then agitated with 1e (7.15 mg, 0.016 mmol) until the solid was fully dissolved. Crystals grown from the unperturbed reaction mixture were isolated by decanting the solution. They were washed twice with CH3CN (ca. 1 mL) and redissolved in 0.5 mL DMSO-d6. The 13 C NMR spectrum was recorded after partial interconversion to rac6-2e. Due to the overlapping signals and lack of protons on the tetrachlorocatechol unit, full assignment of the mixture was not possible. However, the signals of rac5-2e (with the exception of two catechol carbons) were assigned using HSQC and HMBC NMR data acquired before the interconversion and are reported below ( Figure S78).   Figure S78. Overlay of HSQC (blue) and HMBC (red) NMR spectra (DMSO-d6) of rac5-2e crystals. Partial interconversion to rac6-2e had occurred when the 13 C NMR spectrum (used here as a projection) was recorded. Figure S79. 13 C NMR spectrum (126 MHz, DMSO-d6) of rac6-2e and rac5-2e after partial interconversion (unmarked and peaks marked with * could not be unambiguously assigned to specific carbon atoms on each isomer).

rac6-2e
It was not possible to fully characterise the rac6-2e isomer from the mixture with rac5-2e due to the number of overlapping signals and the observance of a small amount of decomposition after 1 day at room temperature ( Figure S80). Several signals could be assigned in the 1 H ( Figure S80) and 13 C NMR ( Figure S79) spectra based on the HSQC/HMBC NMR data in Figure S81.  The crystals immediately lost solvent after removal from the mother liquor and rapid handling prior to flash cooling in the cryostream was required to collect data. Despite these measures and the use of a high intensity laboratory source few reflections at greater than 0.9 Å resolution were observed. Nevertheless, the quality of the data is far more than sufficient to establish the connectivity of the structure. Reflecting the less than ideal diffraction, all of the solvent molecules within the crystal lattice were disordered and modelled over two or three locations. As a consequence of this disorder there are a few short contacts involving hydrogen atoms of low occupancy acetonitrile molecules. The catechol unit was modelled as disordered over two locations with similarity restraints (SAME) employed to the ensure similar bond lengths and angles between the two parts. One cyclopentadienyl ring of the Cp2Co + counterion was also modelled as disordered over two locations. Crystals were obtained from the unperturbed reaction of 1c (6.46 mg, 0.016 mmol) and Cp2Co (3.09 mg, 0.016 mmol) in 0.5 mL of CD3CN in a J Young NMR tube.
The catechol unit was modelled as disordered over two locations with similarity restraints (SAME) employed to the ensure similar bond lengths and angles between the two parts. The methyl groups of the tert-butyl substituent, one cyclopentadienyl ring of the Cp2Co + counterion and two acetonitrile solvent molecules were also modelled as disordered over two locations.

rac5-2e
Three different polymorphs of rac5-2e as the Cp2Co + salt were obtained. The structure of the Cp*2Co + analogue was also determined. Data for all four structures is given below.   Crystals were obtained from the unperturbed reaction of 1e (7.12 mg, 0.016 mmol) and Cp2Co (12.02 mg, 0.063 mmol) in 0.5 mL of CD3CN in a J Young NMR tube.
One complete Cp2Co + counterion and one cyclopentadienyl ring of the other Cp2Co + were modelled as disordered over two locations. The structure was refined as a racemic twin with the Flack parameter 7 refining to 0.402 (6). Crystals were obtained from the unperturbed reaction of 1e (5.14 mg, 0.011 mmol) and Cp*2Co (3.65 mg, 0.011 mmol) in 0.5 mL of CD3CN in a J Young NMR tube.
The catechol unit, one Cp*2Co + counterion and one acetonitrile solvent molecule were modelled as disordered over two locations. As a consequence of this disorder there is one short contact involving a hydrogen atom of the disordered Cp*2Co + counterion which was not accurately located.

Reductive Coupling of 1c in CD3CN
In Section 5.2 meso5-2c was characterised in solution following the crystallisation of rac6-2c from the reaction mixture. The NMR spectra of the reaction mixture over a period of 7 days show that the peak for rac5-2c decreases as it is converted to rac6-2c, which crystallises from the reaction mixture ( Figure S91). Figure S91. Time-course NMR spectra of the reductive coupling of 1c in CD3CN showing the disappearance of the rac5-2c over time due to its interconversion to the rac6-2c product, which subsequently crystallised from the reaction mixture.

Reductive Couplings in DMSO-d6
In order to investigate the formation of the rac6-2 isomer in solution, the reductive coupling was monitored by NMR spectroscopy over time in DMSO-d6. Iminoboronates 1a and 1d were chosen as the reactants since they contained the most electron-deficient and electron-rich anilines, respectively. In both cases, meso5-2 and rac5-2 formed initially as the kinetic products but over time rac5-2 converted to rac6-2, the thermodynamic product ( Figures S92, S93).   rac5-2d, rac6-2d and meso5-2d products.

NMR Studies of Interconversion between the meso5-2 and rac5/6 Isomers
The redissolved meso5-2a crystals in Section 3.3 were left to equilibrate at room temperature for 6 days and no interconversion between meso5-2a and the rac5/6 isomers was observed during this time ( Figure S94). In addition, there was no evidence for the formation of meso6-2a. In order to probe if heat is required for these interconversions to take place, the sample was heated for 1 day at 90 °C but only decomposition was observed, particularly with prolonged heating for 4 days. Figure S94. Time-course NMR spectra of redissolved meso5-2a crystals in DMSO-d6 upon equilibration at room temperature and heating.
The redissolved rac6-2c crystals were heated for several hours at 90 °C for 2 h. During this time interconversion to rac5-2c took place and the two species were stable to heating, however, no interconversion to meso5-2c was observed ( Figure S95). Increasing the temperature to 130 °C led to decomposition of the sample. Figure S95. Time-course NMR spectra of redissolved rac6-2c crystals in DMSO-d6 upon heating.

Time-Course NMR Studies for Rac5/Rac6 Equilibration
Crystals of rac6-2a-d were obtained (Section 8) and redissolved in DMSO-d6 in order to monitor the equilibration between the rac6-2 and rac5-2 isomers over time (Scheme S2, Figures S96-100). Due to difficulties redissolving the crystals and the relatively fast equilibration times, quantitative studies were not possible. However, the studies showed qualitatively that the substituent on the aniline influenced the rate of interconversion; the rate was fastest for rac6-2d with the most electron-rich substituent and consequently, the initial spectra were a mixture of the two isomers, whereas spectra were predominantly rac6-2a-c with less electron-rich substituents.

2d
Crystals obtained from two different reactions were redissolved in DMSO-d6 and the initial spectrum in both cases was found to contain both rac6-2d and rac5-2d ( Figures S99 and S100). The appearance of both isomers following immediate dissolution and measurement of the NMR spectrum (15 min, Figure S99) is attributed to the fast interconversion between the two isomers.

Comparison of Chemical Shifts
In all cases, the redissolved rac6-2a-c and rac5-2e crystals equilibrated with a second species over time due to B-N bond rearrangements. It was not possible to crystallise this second species from DMSO-d6 but the second species could be identified based on the methine chemical shift; a comparison of the equilibrated mixtures revealed that the methine chemical shift of the rac6-2 and rac5-2 isomers did not vary significantly with substitution of the catechol and amine subcomponents ( Figure S102).

Reversible Radical Coupling with Ph3CBF4
While interconversion between the meso5-2 and rac5/6-2 was not observed under the tested conditions (Section 9.3), the reaction of the reductively coupled product mixture with the tritylium cation was investigated. The reaction mixtures of 2b and 2e were chosen to study as their 1 H NMR spectra in CD3CN were well-defined with dispersed signals and competing crystallisation was slow compared to the timeframe of the experiment.
Scheme S4. Reductive coupling of iminoboronates 1b and 1e and subsequent tritylium-induced oxidative decoupling of dimers 2b and 2e. 11.1 2b 2b was obtained from the reaction of 1b (5.03 mg, 0.016 mmol) and Cp2Co (3.02 mg, 0.016 mmol) in 0.5 mL of CD3CN in a J Young NMR tube. Ph3CBF4 (5.80 mg, 0.018 mmol) was added to this solution of 2b in CD3CN in a J Young NMR tube under an inert atmosphere at room temperature. After 10 minutes the reaction had gone to completion as indicated by the 1 H NMR spectrum ( Figure S103). Figure S103. 1 H NMR spectra (400 MHz, CD3CN) of the reaction of 2b with Ph3CBF4. a) spectrum of 1b, b) spectrum of 2b, c) reaction mixture containing 2b (red), trityl dimer (yellow), and Cp2CoBF4 (blue).

2e
2e was obtained from the reaction of 1e (5.00 mg, 0.011 mmol) and Cp2Co (2.10 mg, 0.011 mmol) in 0.5 mL of CD3CN in a J Young NMR tube. Ph3CBF4 (4.03 mg, 0.012 mmol) was added to this solution of 2e in CD3CN in a J Young NMR tube under an inert atmosphere at room temperature. After 10 minutes the reaction had gone to completion as indicated by the 1 H NMR spectrum ( Figure S104). 12 Reaction of 2e with TEMPO 2e was obtained from the reaction of 1e (5.10 mg, 0.011 mmol) and Cp2Co (2.10 mg, 0.011 mmol) in 0.5 mL of CD3CN in a J Young NMR tube. TEMPO (1.74 mg, 0.011 mmol) was added to this solution of 2e in CD3CN in a J Young NMR tube under an inert atmosphere, and the reaction was heated at 70 °C for 3 days.
The reaction of 2e with TEMPO was investigated to further probe the reaction of the reductively coupled products with radicals (Scheme S5). The 1 H NMR spectrum of the reaction mixture showed a new species (blue triangles) with a signal above 9 ppm had formed, in addition to the unreacted 2e (red triangles, Figure S105). Crystals obtained from the reaction revealed the product was 3 by X-ray crystallography ( Figure S106).  Crystals of 3 were obtained upon cooling to room temperature the reaction mixture described above.