Stimulation of Redox‐Induced Electron Transfer by Interligand Hydrogen Bonding in a Cobalt Complex with Redox‐Active Guanidine Ligand

Abstract Octahedrally coordinated cobalt(II) complexes with a redox‐active bisguanidine ligand and acac co‐ligands were synthesized and their redox chemistry analysed in detail. The N−H functions in a bisguanidine ligand with partially alkylated guanidino groups form N−H⋅⋅⋅O hydrogen bonds with the acac co‐ligands, thereby massively influencing the redox chemistry. For all complexes, the first one‐electron oxidation is metal‐centred, leading to CoIII complexes with neutral bisguanidine ligand units. Further one‐electron oxidation is ligand‐centred in the case of Co–bisguanidine complexes with fully alkylated guanidino groups, giving CoIII complexes with radical monocationic bisguanidine ligands. On the other hand, the hydrogen‐bond strengthening upon oxidation of the Co–bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (CoIII→CoII) and two‐electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox‐induced electron transfer by interligand hydrogen bonding.


Introduction
Cobalt complexes with redox-active ligands have been the subject of intense research over the last decades, [1][2][3][4][5][6] Especially,m ononuclear complexes with one or two dioxolenetype ligands and dinuclear complexes with abridging tetraoxolene ligand were thoroughly studied, in part due to the huge potential of complexes with redox-active ligands for catalysis. [7][8][9][10][11][12][13] Intramolecular electron transfer in such complexes could be triggered by various stimulants.I n1 980, the first complex showing temperature-dependent equilibrium between two redox isomers in solution was synthesized, converting alow-spin Co III complex preferred at low temperature into ah igh-spin Co II complex favoured at higher temperature. [14] Thet erm valence tautomerism (VT) was coined for equilibria between redox isomers.In1993, the first complex showing VT in the solid state was reported, [15] and several other examples followed, [16] with various conversion temperatures,n arrow or wide temperature regions for interconversion, and small or large thermal hysteresis. [17,18] It proved possible to trigger intramolecular electron transfer (IET), converting aCo III into ahigher-energetic Co II complex by light at low temperature (similar to the light-induced excited spin state trapping (LIESST) effect discovered by Gütlich et al. [19] )a nd also to stimulate relaxation back to the Co III complex by light (similar to reverse-LIESST). [20][21][22][23][24][25] These studies gave useful information about the kinetics of IET,and highlighted lattice-softening effects induced by co-crystallized solvent molecules. [25] Moreover,p ressure-induced VT was observed, [26] and chain polymers were synthesized in which IET triggered by light could lead to measurable changes in the crystal length. [27] It has also been shown that VT could initiate macroscopic crystal-melt phase transitions [28] or change of polarization. [29] Miller et al. studied the redox-chemistry of dinuclear cobalt complexes with bridging tetraoxolene ligands,p roviding first examples for redox-induced electron transfer (RIET). [30][31][32] In aR IET process,aredox reaction is coupled with intramolecular electron transfer, leading seemingly paradoxically to metal reduction in an overall oxidation or to metal oxidation in an overall reduction of am etal complex.
Although these extensive studies on Co-oxolene complexes greatly expanded our knowledge about the fundamentals of IET processes and disclosed av ariety of different potential applications,there is still ahigh demand for studies in this field regarding both the fundamental understanding and the development of applications.F urther redox-active ligand classes have to be included into this research theme.In the last years,our group established redox-active guanidines, comprising guanidino-functionalized aromatics (GFAs), as an ew class of versatile redox-active ligands [33][34][35] and intensively studied IET in copper complexes with GFAligands.We showed that IET could be triggered thermally (thermal equilibrium between two redox isomers (VT)), [36][37][38][39] by redox reactions (redox-induced electron transfer (RIET)), [38] by coligand addition [40,41] or substitution, [42] and by metal coordination to as econdary coordination sphere. [43] Moreover, copper complexes with redox-active guanidine ligands were applied in catalytic aerobic phenol homo-and cross-coupling reactions, [44] achieving asignificant improvement of reactivity and selectivity.

Results and Discussion
Thet hree neutral Co II complexes [Co(acac) 2 (L1)], [Co(acac) 2 (L2)],a nd [Co(acac) 2 (L3)] (see Lewis structures of the complexes with L1 and L3 in Scheme 1) were synthesized in 93-98 %y ield by reaction of one of the guanidine ligands L1-L3 with Co(acac) 2 in CH 2 Cl 2 solution, and crystallized from hot n-hexane solutions.Indifference to the complexes with L1 and L2, two N À H···O interligand hydrogen bonds between an NÀHgroup of L3 and an Oatom of the acac co-ligands are established in [Co(acac) 2 (L3)] (see Lewis structure in Scheme 1). One-electron oxidation was carried out with the ferrocenium (Fc + )s alt Fc(PF 6 ), and two electron oxidation either also with Fc(PF 6 )o rw ith the stronger oxidant NO(SbF 6 ). Thec omplexes were isolated and fully characterized in all three redox states.The first oneelectron oxidation is metal-centred (Co II !Co III )f or all complexes.F urther one-electron oxidation of the complexes with L1 or L2 is ligand-centred, giving Co III complexes with radical monocationic ligand L1C + or L2C + .Onthe other hand, oxidation of the monocationic complex [Co(acac) 2 (L3)] + to the dication [Co(acac) 2 (L3)] 2+ is coupled with IET (RIET) leading to cobalt reduction (Co III !Co II ). RIET is triggered by the increase of the interligand NÀH···O hydrogen-bond strength upon two-electron oxidation of the ligand. Hence detailed analysis clearly shows that the dication [Co(acac) 2 -(L3)] 2+ is ah igh-spin Co II complex with dicationic guanidine ligand. In principle,afurther one-electron oxidation is imaginable,l eading to at ricationic Co III complex with dicationic guanidine ligand. Indeed, cyclic voltammetry measurements indicate the possibility of reversible threeelectron oxidation (see below). However, it was not possible to isolate these ultimately oxidized complexes in pure form in experiments that relied on NO(SbF 6 )a so xidizing reagent. According to the experiments,p artial degradation leads to cleavage of the bond between the metal and the dicationic guanidine in significant amount. In the following,t he electronic structures of the neutral, as well as the singly and doubly oxidized complexes will be evaluated in detail. The discussion concentrates on the comparison between complexes of the ligands L1 and L3. Thee xperimental results obtained for the complexes with L2 showed similar behaviour as the complexes of L1.
Cyclic voltammetry. Fort he free ligands L1 and L2, two reversible and potentially separated one-electron processes are detected with cyclic voltammetry in CH 2 Cl 2 solution, [38] located at E 1/2 = À0.25 V(E ox = À0.19 V) and À0.11 V(E ox = À0.05 V) for L1C + /L1 0 and L1 2+ /L1C + ,r espectively,a nd at E 1/2 = À0.46 V( E ox = À0.37 V) and À0.38 V( E ox = À0.27 V) for L2C + /L2 0 and L2 2+ /L2C + ,respectively.For free L3, the redox events are not reversible (SI, Figure S17);t wo oxidation waves appear, at E ox = À0.41 Va nd + 0.05 V, tentatively assigned to oxidation of L3 to ah ydrogen-bonded dimer (L3 2+ )L3 (in equilibrium with small amounts of L3C + )a nd to L3 2+ ,respectively.T hree waves are visible in the direction of reduction. Awave at E red = À0.11 Vb elongs to one-electron reduction of the dication (L3) 2+ .Asmaller wave at À0.51 V and alarger one at À0.79 Vare assigned to reduction of free (L3)C + and of the (L3 2+ )L3 hydrogen-bonded dimer, respectively.For tetrakisguanidino-benzenes with partially alkylated guanidino groups,s uch aggregates were already synthesized in high yield and structurally characterized. [45,46] Hence,t he CV data of free L3 already point to the importance of hydrogen bonding.  In the cyclic voltammogram recorded for [Co(acac) 2 (L1)] ( Figure 2), three one-electron redox processes are visible,a t E 1/2 = À0.56 Vf or the redox couple [Co(acac) 2 2+ .T he potential for the first one-electron oxidation is almost equal for [Co(acac) 2 -(L1)] (E 1/2 = À0.56 V) and [Co(acac) 2 (L2)] (E 1/2 = À0.54 V), and significantly lower than the potentials required to oxidize the free ligands L1 (E 1/2 = À0.25 Vfor L1C + /L1 0 )orL2(E 1/2 = À0.46 Vf or L2C + /L2 0 ). These results clearly indicate that the first one-electron oxidation is metal-centred (Co II !Co III ), as expressed by the Lewis structures in Scheme 1a.Itshould be noted that at fast scan rates (more than 50 mV s À1 )as mall feature is visible on the high-potential side of the first oxidation wave for all three complexes,v anishing at slower scan rates.The disappearance of this feature at slow scan rates and the similar behaviour of all three compounds argue against the presence of ac ompound with different composition in the solutions.T he change from high-spin Co II to lowspin Co III brings about massive structural and electronic changes,and therefore it might be possible that the presence of an intermediate complex (e.g.aCo III complex with different spin multiplicity that relaxes to the low-spin Co III ground state) causes the additional weak features at high scan rates.
The E 1/2 values measured for the second oxidation process, À0.03 Vf or [Co(acac) 2 (L1)] 2+ /[Co(acac) 2 (L1)] + and À0.21 V for [Co(acac) 2 (L2)] 2+ /[Co(acac) 2 (L2)] + ,a re both higher than the potentials for one-electron oxidation of the free ligands. Such an increase could easily be explained by the bonding to the Lewis-acidic metal. Moreover,t he difference in the E 1/2 values of 0.18 Visalmost equal to the difference of 0.21 Vfor the free ligands.O nt hese grounds,t he second one-electron oxidation is assigned to al igand-centred oxidation process, leading to Co III complexes with radical monocationic guanidine ligands (Scheme 1a). Thethird oxidation process should then produce Co III complexes with dicationic guanidine ligand. Thep otentials for one-and two-electron oxidation of the complexed ligands (being the second and third oxidation steps of the complex) are clearly potentially separated (DE 1/2 = 0.44 Vfor L1 and 0.53 Vfor L2), comparing with DE 1/2 = 0.14 and 0.08 Vf or the free ligands L1 and L2, respectively.
Thec yclic voltammogram of [Co(acac) 2 (L3)] is also included in Figure 2. Here,a lso three redox waves appear, at 2+ ,r espectively.T he potential of the first one-electron process is close to those measured for the analogue complexes with L1 and L2, and lower than the potential measured for the free ligand L3;t herefore it is assigned to am etal-centred oxidation (Co II !Co III ).
Thes econd and third one-electron redox events then belong to ligand-centred oxidation. We will see that the second redox event is accompanied by aRIET process (from Co III to Co II ,see Scheme 1). Thepotential difference between the oxidation and reduction wave of the redox pair [Co(acac) 2 (L3)] 2+ /[Co(acac) 2 (L3)] + is large (0.22 V), possibly due to the significant structural changes induced by the RIET process.F rom the synthesis of stable salts of the monocation [Co(acac) 2 L3] + and dication [Co(acac) 2 L3] 2+ by chemical oxidation of the neutral compound (see below) an inherent instability of the redox states could be excluded. TheC V measurements suggest the third one-electron redox process, leading to aCo III complex with dicationic guanidine ligand, to be reversible.However,itwas not possible to synthesize salts of these complexes in pure form.
EPR spectroscopy. EPR spectra of the three neutral complexes,m easured at 6K in frozen CH 2 Cl 2 solution, are displayed in Figure 3. Thesignals in the EPR spectra vanish at room temperature due signal broadening by fast relaxation processes;a ll data clearly show that Co II complexes with neutral guanidine ligands prevail at all temperatures.T he gvalues of g ? = 6.99 and g k = 2.34 give an effective gvalue of  With 72 G, the Avalue is slightly smaller, presumably due to the structural peculiarities produced by the two hydrogen bonds (see structure below).
On the other hand, no sharp signal appears in the roomtemperature EPR spectrum of [Co(acac) 2 (L3)](PF 6 ) 2 ,c onfirming the absence of an organic radical and the formulation as aC o II complex with dicationic ligand unit L3 2+ .I n similarity to the neutral complexes,t he absence of clear signals could be rationalized by signal broadening due to fast relaxation at room temperature.I nterestingly,aweak and broad signal that might be assignable to an organic radical appeared in the 6K EPR spectrum, together with weak, broad signals in the region 550-3000 G( SI, Figure S26). In principal, we could not completely exclude that these signals arise from the presence of aminor impurity,but their absence in the room temperature spectra seems to be not in accordance with this explanation. Another possibility is ac hange in the electronic structure of the [Co(acac) 2 (L3)] 2+ complex at very low temperature,that is also motivated by the calculated small energy difference between two redox isomers (see discussion below).
UV/Vis spectroscopy. In the UV/Vis spectra of the free ligands in CH 3 CN solution, bands at 334 and 300 nm belong to L1, bands at 336 and 277 nm to L2, [38] and strong bands around 340 and 234 nm as well as as mall shoulder around 265 nm to L3 (see SI, Figure S4). Bands around 450 and 300 nm were observed for the dications L1 2+ and L2 2+ . [38] Then, the free radical monocationic ligand L1C + exhibits ab and at 370 nm with al ong tail extending into the visible region. Aband at ca. 370 nm (with ashoulder around 385 m) is also characteristic for free L2C + ,together with abroad band in the vis region (absorption maxima at 675/733 nm).
In the UV/Vis spectra of the neutral cobalt complexes, strong absorptions appear in the UV region, but only an unstructured, weak, and extremely broad absorption (400-500 nm) in the visible region, being responsible for the red colour of the compounds.The visible regions in the spectra of the monocations [Co(acac) 2 (L1)] + ,[ Co(acac) 2 (L2)] + ,a nd [Co(acac) 2 (L3)] + are also free of strong bands,a rguing for metal-centred oxidation (Co II !Co III ), in line with the results from cyclic voltammetry and EPR spectroscopy.T he spectra recorded for the dications [Co(acac) 2 (L1)] 2+ and [Co(acac) 2 -(L2)] 2+ obtained upon two-electron oxidation contain aband at 364 nm and abroad absorption in the visible region with an absorption maximum at ca. 554 nm for [Co(acac) 2 (L1)] 2+ and 503 nm for [Co(acac) 2 (L2)] 2+ ,i ndicating the presence of aC o III complex with radical monocationic ligand. On the other hand, bands at 295 and 450 nm in the spectrum of [Co(acac) 2 (L3)] 2+ in CH 2 Cl 2 (being close to the absorptions detected for similar oxidized, dicationic bisguanidines [38] ) clearly indicate the presence of the dicationic ligand, L3 2+ , implying the presence of aCo II complex. In summary,the UV/ Viss pectra are in line with cyclic voltammetry,E PR, and Lewis structures in Scheme 1.
Amuch higher c T value of ca. 2.5 cm 3 Kmol À1 was found for [Co(acac) 2 (L3)](PF 6 ) 2 at 300 K. This large value fully supports the formulation as ahigh-spin Co II complex (S = 3/2) with dicationic ligand, L3 2+ and an unquenched orbital contribution. Due to significant spin-orbit coupling ( 4 T 1g ground term in O h symmetry), it is higher than the spin-only value for high-spin Co II (S = 3/2) of 1.876 cm 3 Kmol À1 predicted by the Curie law.T he value decreases almost linearly with decreasing temperature until ca. 50 K ( Figure 7), where it drops down to reach avalue of 1.1 cm 3 Kmol À1 at 2K.T he sharp decrease at low temperature [51] is characteristic for octahedral high-spin Co II complexes. [52,53] Thed egeneracyo f the 4 T g ground state is removed by spin-orbit coupling and the distortion of the crystal field due to the asymmetric coordination of two different ligands.A ccording to the Boltzmann distribution, the population of the resulting doublet ground state increases with decreasing temperature,l eading to reduction of the c T value at low temperature. [52] Additionally, ap ossible VT process favouring the Co III redox isomer with L3C + ligand unit at very low temperature might lead to afurther decrease of the c T value.The latter is supported by the appearance of aw eak, broad signal attributable to an organic radical in the low temperature (6 K) EPR spectrum of af rozen CH 2 Cl 2 solution (SI, Figure S26), and by the calculated small energy difference between two redox isomers (see below). However,i tis not possible to reach af inal conclusion on this point.
Quantum-chemical calculations. Finally,B 3LYP/def2-TZVP calculations were carried out, the solvent effect being included by single-point calculations with the conductor-like screening model (COSMO). Thec alculated structures are in pleasing agreement with the experimentally derived structures in the solid state (see SI for details). In Figure 8the spin density distribution is plotted for [Co(acac) 2 Table 3). Thel ow-spin Co III redox isomer with radical monocationic ligand L1C + (S = 1/2) is preferred by DE = 85 kJ mol À1 at e r = 37.5 with respect to this intermediate-spin Co III redox isomer.
In the calculated Co II redox isomer of [Co(acac) 2 (L3)] 2+ (S = 3/2) with dicationic ligand, L3 2+ ,t he NÀH···O hydrogen bond lengths measure 1.824 ,b eing significantly shorter than in the calculated Co III redox isomer with radical monocationic L3C + unit (2.061 ). Additionally,the calculated   . Thed ecrease of the hydrogen-bond lengths signals as tronger interligand interaction, leading to astabilization of the Co II redox isomer by the two strong interligand hydrogen bonds.C onsequently,t he calculations predict the otherwise highly unfavourable quartet spin state (being high-energy states for complexes of L1 and L2) to be even slightly preferred over the doublet state (by 2kJmol À1 )a te r = 37.5 (CH 3 CN solution);w ithout solvent effect the doublet state (Co III redox isomer) is slightly preferred, but by not more than 10 kJ mol À1 . TheGibbs free energy for ahypothetical ligand exchange reaction was calculated to further highlight the impact of hydrogen bonding (Figure 9). Forthe reaction of [Co(acac) 2 -(L1)] 2+ in its S = 3/2 state with free L3 to give [Co(acac) 2 -(L3)] 2+ in the S = 3/2 ground state and free L1, a DG value (at 298 K) of À76 kJ mol À1 was obtained, underlining the massive stabilization of the S = 3/2 state (Co II redox isomer) with respect to the S = 1/2 state (Co III redox isomer) for the complex with L3 by hydrogen-bonding.

Conclusion
Thef irst mononuclear,o ctahedrally coordinated cobalt complexes with redox-active bisguanidine ligands were synthesized and their redox chemistry was studied. Three different redox-active bisguanidines were used in this study,two of them with fully alkylated and one with partially alkylated guanidino groups.I nterligand N À H···O hydrogen bonds between two guanidino N À Hf unctions and oxygen atoms of the acetylacetonate (acac) co-ligands are established in the Co-bisguanidine complex with partially alkylated guanidino groups.T he increase of the hydrogen-bond strength upon oxidation of this guanidine ligand triggers ar edox-induced electron transfer (RIET) that is absent in the complexes with the guanidine ligands having fully alkylated guanidino groups. Hence,o ne-electron oxidation of the monocationic Co III complex with the neutral guanidine ligand leads to metal reduction and two-electron oxidation of the guanidine ligand, resulting in aC o II complex with dicationic guanidine ligand unit. Ther esults of this study demonstrate the possibility to alter the outcome of redox processes and to initiate intramolecular electron transfer (IET) by introducing intramolecular hydrogen-bond interactions.T hereby,they pave the way to asophisticated control of redox and IET processes.Nature extensively uses the weakening or strengthening of hydrogen bonding up to the point of proton transfer to trigger electron transfer, [54] for example,i navariety of redox enzymes. Therefore,t he development of model reactions in which electron transfer is initiated by hydrogen bonding leads to the realization of new biomimetic redox reaction motifs.The first example for aRIET enabled by interligand hydrogen bonding discloses new opportunities for the systematic study of intramolecular electron transfer, leading eventually to new reactivity patterns for use in synthetic chemistry.