Structural Changes in the Carbon Sphere of a Dirhodium Complex Induced by Redox or Deprotonation Reactions

Abstract A carbon‐rich molecule is synthesized, which mainly contains conjugated sp2 and sp hybridized carbon centers. Alkenyl and alkynyl binding sites are arranged such that this compound serves as ligand to a binuclear metal unit with a RhI─RhI bond. Furthermore, CH units are placed in proximity to the metal centers. The dicationic complex [Rh2(bipy)2{Ph2Ptrop(C≡CCy)2}]2+(OTf−)2 allows to study possible responses of the carbon‐framework to redox reactions as well as deprotonation reactions. All products are, whenever possible, characterized by X‐ray diffraction (XRD) methods, NMR and EPR spectroscopy as well as electrochemical methods. It is shown that the carbon skeleton of the ligand framework undergoes C─C bond rearrangement reactions of remarkable diversity. In combination with DFT (density functional theory) studies, these results allow to gain insight into the electronic structure changes caused by metal sites in a carbon‐rich environment, which may be of relevance for the properties of metal particles on carbon support materials when they are exposed to hydrogen, electrons, or protons.


Introduction
The investigation of redox induced changes of the properties and reactivity of transition metal complexes belongs to one of the core disciplines of organometallic chemistry [1][2][3][4][5][6] and finds applications ranging from synthetic chemistry to the development of molecular machines. [7,8]Classic and especially well DOI: 10.1002/advs.[15] Relevant to this work are also redox promoted C─C coupling reactions of metal vinylidene or alkynyl complexes. [5,6,16][22] How does the carbon material respond to chemical processes occurring at the metal centers?An answer to this question is difficult to give.A recent DFT (density functional theory) study investigated possible interactions between small rhodium clusters, (Rh) n , and a carbon support (Figure 1a). [23]It was found that the presence of the metal modifies the hybridization at the carbon centers and, indeed, the ring size which (Rh) n is coordinated to plays a role for the efficiency of in H 2 activation.
We recently synthesized ligands containing a central 5Hdibenzo[a,d]cyclohepten-5-yl (trop) unit to which a diphenylphosphanyl group and two alkynyl units are attached (Ph 2 Ptrop (C≡CR)2 , R = TMS, Ph; TMS = SiMe 3 ). [24,25]The overall concave structure of this compound and its electron accepting properties provided by the conjugated R─C≡C─C═C trop ─C≡C─R unit allows to bind two low-valent metal centers in close proximity.When the homobimetallic low-valent dirhodium complex [Rh 2 {Ph 2 Ptrop (C≡CS)2 }(L) 2 (dppm)] 2+ (OTf − ) 2 (L = MeCN, dppm = diphenylphosphinomethane) was exposed to dihydrogen gas, reversible addition of H 2 to the Rh 2 unit was observed.A compound could be identified, which closely resembles the ones proposed by DFT for small Rh clusters units deposited on carbon (Figure 1b).On the other hand, the complex [Rh 2 {Ph 2 Ptrop (C≡CS)2 }(OTf) 2 ] reacted irreversibly with two equivalents of H 2 hydrogen, which were transferred to carbon centers within the ligand skeleton.
In this study, we prepared a slightly modified Ph 2 Ptrop (C≡CR)2type ligand with R = Cy (cyclohexyl), which contains CH groups in proximity to a dinuclear Rh 2 core (Figure 1c).Significant C─C bond rearrangements of the extended conjugated carbon ligand framework were observed upon chemical redox and deprotonation reactions.The electron transfer processes were investigated with electrochemical studies and DFT calculations.
In the dinuclear complexes [2], [3], and [4] 2+ the Rh 1 ─Rh 2 unit is tightly embedded in the ligand framework and especially Rh 1 interacts strongly with the olefinic C1═C2 unit of the ligand.This interaction is reflected in short distances between Rh 1 and the centroid (ct1) of the C1═C2 (C═C trop ) bond (average 1.923 Å).The C1═C2 bond distance is elongated from 1.366 Å in 1 to ≈1.503 Å (average) in complexes [2]-[4] 2+ .Coordination of the alkynyl units C3≡C4/C6≡C7 (average 1.203 Å) to Rh 2 likewise leads to an elongation of the C≡C bonds (average 1.243 Å) but to a much lesser extent than for C1═C2.All bimetallic complexes show Rh─Rh distances in between 2.63 and 2.80 Å (see Table 1) that are in the range of single bonds [26][27][28][29] and have been previously observed in related dinuclear rhodium trop phosphane complexes. [24,25]The nature of the metal-metal interaction can be explained in a simplified way as follows: Not taking the Rh─Rh interaction into account, the coordination sphere around Rh 1 corresponds to a trigonal pyramid.This is an unusual and rare coordination sphere for a 16-valence electron metal complex, which renders the metal center Lewis-acidic (vide infra). [30]On the contrary, Rh 2 resides in a 16-valence electron square-planar environment (again not considering the Rh─Rh interaction), which makes the metal center Lewis-basic.This is true for all complexes [2]-[4] 2+ and consequently, the Rh─Rh bonds in all complexes can be viewed as a dative Rh→Rh donor-acceptor interaction.This assumption is fully supported by DFT calculations, where a molecular orbital was found representing this donor-acceptor interaction (see Figure S57, Supporting Information).QTAIM (quantum theory of atoms in molecules) was used to locate a bond critical point (BCP) between Rh 1 and Rh 2 (for a contour map of the Laplacian of the electron density of complex [4] 2+ see Figure S56, Supporting Information) and the electron localization function (ELF) shows a value of 0.208, which indicates a rather weak, but observable Rh─Rh bond.
In order to support this assumption on the nature of the Rh─Rh bond, we prepared the mononuclear rhodium complex [Rh(bipy)(Ph 2 Ptrop (C≡CCy)2 )] + (OTf − ), [6] + (OTf − ), by first reacting the labile precursor complex [Rh 2 (μ 2 -Cl) 2 (H 2 C═CH 2 ) 4 ] with 1.This gives the dinuclear complex [5] as product, which is subsequently split to the desired mononuclear complex [6] + (OTf − ) upon addition of KOTf and bipy (Figure 2b).Single crystals were grown and subjected to an XRD analysis.The solid-state structure of the complex cation [6] + is shown in Figure 4b.Indeed, the coordination sphere around the Rh center in [6] + is best described as a slightly distorted trigonal pyramid (deviation of the Rh center from the best plane through C1,C2,N1,N2 = 0.252(2) Å).In comparison to the dinuclear dication [4] 2+ , the Rh 1 ─C distances in [6] + are similar (Rh─ct (C1-C2) 1.936 Å, C1═C2 1.504 Å).Noteworthy is the long interaction of 2.435(3) Å between Rh 1 and one of the carbon centers, C3, of one of the alkynyl units reflecting the Lewis-acidity of the 16 e Rh I center in a trigonal pyramidal environment as mentioned above.] and correspond well to literature values. [24,31,32]We could also determine the chemical shift for Rh The cyclic voltammogram (CV) of the mononuclear rhodium complex [6] + shows a reversible reduction at E ½ = −1.63V versus Fc/Fc + (ferrocene/ferrocenium) (Figure 3a,b), which is more negative than the ones in related trop phosphane rhodium(I) complexes (E ½ = −1.00 and −1.19 V) [33] and trop amine complexes (E ½ = −1.27 to −1.46 V). [34] A second, irreversible reduction wave is observed at E p = −2.06V, which exhibits a triangular peak shape resulting from an adsorption phenomenon of the generated species (see Figure S54, Supporting Information).The mono reduced neutral complex [Rh(bipy)(Ph 2 Ptrop (C≡CCy)2 )] • , [6] • , was generated in situ with CoCp* 2 (decamethylcobaltocene) as chemical reductant and detected by EPR spectroscopy, shown in Figure 3c.With respect to [6] + , DFT calculations show a slight change of the structure of [6] • , which now shows a nearly perfect trigonal pyramid with a deviation of Rh from the trigonal plane 0.32 Å.The spin density of the unpaired electron is mostly located on the bipy ligand, only 22% are based on the rhodium center.This finding is in line with previous data of formally d 9 -Rh°c omplexes with trop-type ligands and distorted tetrahedral structures.Those likewise show relatively low spin densities of 24%-36% on the metal center and are best described as delocalized organometallic radicals. [35]Although, small structural changes may lead to electromeric structures in which >80% of the spin density is located on the metal center. [36,37]Remarkably, when the reduction is performed on larger scale with the intention to isolate [6] • , reaction between two such radicals is observed.C─C bond formation between two -alkyne-C7 atoms gives exclusively the dimer [6] 2 (see Figure 3a and a plot of the solid state structure in Figure 4d).Comparable dimerization reactions have been detected in redox reactions of metal alkynyl complexes before. [5,6,16]he CV of the dinuclear complex  6) Å] is shortened while no significant differences in bond lengths were observed for the C═C trop and C≡C bonds within the accuracy of the measurement (see Table 1).The radical cation [4] •+ was further characterized by EPR spectroscopy.A rhombic g-tensor with a small anisotropy and an isotropic g-value (2.0170 vs 2.0023 of the free electron) indicates that the unpaired electron is mainly localized on the ligand scaffold with little spin density on the dinuclear Rh─Rh unit (Figure 3f).This is confirmed by DFT calculations, which show that the spin density is mainly located on the bipy ligand coordinated to Rh 2 (bipy: 0.52 e, Rh 1 : 0.06, Rh 2 : 0.09 e) (see Figure S60, Supporting Information).
The use of two equivalents of a strong reductant leads to the formation of a poorly soluble, green complex [7] ( max = 696 nm).The solid-state structure was determined by X-ray crystallography, which surprisingly did not appear to be the one of the fully reduced complex [4].Instead, the structure of isolated complex  rearrangement, the dinuclear Rh─Rh unit (Rh 1 ─Rh 2 2.7109(6) Å) is now bound to a curved eleven-membered carbon-cycle with benzo groups attached in 9,10 and 12,13 position and two cyclohexyl substituents in 4,7 positions.To the best of our knowledge, a comparable ring expansion has not been reported before.
All carbon centers are sp/sp 2 -hybridized, apart from the sp 3hybridized carbon center C11, which carries the PPh 2 group at the bow of the eleven-membered ring.A conjugated delocalized -electron system is formed in which the C─C distances vary between 1.315 Å (average C2-C3/C1-C6) and 1.470 Å (average C1-C9/C2-C13).The Rh 1 center is bound to the phosphorus atom [2.2017(15) Å], Rh 2 [Rh 1 ─Rh 2 = 2.7109( 6) Å], the two carbon centers C1 [2.073( 7) Å] and C2 [2.076( 6) Å] and one bipy ligand.The other Rh 2 center is coordinated to the second bipy ligand, Rh 1 , and to the C6-C7-C4-C3 unit at the stern of the C 11 ring (Rh 2 -ct1 1.7332(4) Å; ct1 = centroid of C6-C7-C4-C3).Formally, the ─C1 = C6 = C7(Cy)-C4(Cy) = C3 = C2─ part of the C 11 ring can therefore be described as dianionic bis(allenyl) to which Rh 1 is bound via two Rh─C ─bonds while Rh 2 is coordinated in a  4fashion.This renders the C 11 ring an eight-electron donor ligand in total.Following the arguments brought to the fore in the description of [4] 2+ (vide supra), four electrons of the C 11 ring are donated to Rh 1 , which together with the phosphanyl group (2 e) and the bipy ligand (4 e) reaches a valence electron configuration of 18. Rh 2 retains its 16 valence electron count as discussed above.This description is supported by calculation of IBOs (intrinsic bond orbitals), which indeed show that the ligand carries two negative charges and both rhodium centers remain in the formal oxidation state of +1.A QTAIM calculation shows a bond critical point is located between Rh 1 and Rh 2 , indicating a metal-metal single bond.The contour map of the Laplacian of the electron density of complex [7] is shown in Figure S56 (Supporting Information).The electron density at the BCP in [7] (0.048) shows a slightly higher value compared to dication [4] 2+ (0.042) indicating a stronger interaction between the rhodium centers in the latter, which is in line with the shorter Rh─Rh distance observed in [7].Furthermore, analysis of the energy parameters at the BCP indicates that the Rh─Rh interaction in both species is best described as a weak dative metal-metal interaction.
The conversion of the neutral fully reduced complex [4] to the rearranged product A CV of the rearranged complex [7] was recorded and shows the expected two reduction events (Figure 5b): Starting from the open-circuit-potential (OCP) at −1.59 V, the complex can be oxidized at E½ = −1.37 and −1.01 V (both considered quasireversible).Note that the potentials are shifted to less negative potentials when compared to the reductions of A bulk electrolysis experiment was performed in order to investigate whether the rearranged complex [7] can be prepared electrochemically.The electrosynthesis was set up in a divided cell with a Pt mesh working electrode and Zn-wire counter electrode as sacrificial anode with a Ag/Ag + reference electrode in the working compartment.During electrolysis, a dark green solid formed on the cathode.This solid product was separated from the reaction mixture and dissolved in pyridine.Indeed, NMR spectroscopic characterization revealed formation of complex [7], which is hence an alternative and simple way of its preparation.
Finally, we investigated the possibility to deprotonate the dication [4] 2+ , which contains a propargylic proton at each of the cyclohexyl groups.This would be an alternative way to convert the dicationic complex stepwise into a neutral one.[40][41][42] These isomerization reactions are related to a certain extent to the rearrangement we observed upon reduction of [4] 2+ .We reacted [4] 2+ with one equivalent of NaOtBu (sodium tert-butoxide), which caused a color change from red to deep green ( max = 630 nm) and cleanly led to the formation of a new product [8] + (Figure 6a).This compound was crystallized, and its structure analyzed by XRD methods (the solid-state structure is depicted in Figure 6b).Indeed, the CH unit of the cyclohexyl substituent in -position to the alkynyl group was deprotonated and an allenyl unit, R 2 C═C═C─R is formed [C5 = C4 1.339(13) Å; C4 = C3 1.298(15) Å], which coordinates with its anionic terminus to Rh 2 [C3-Rh 2 2.035(11) Å].The second, previously coordinated, carbon center C4 of the for-mer alkynyl unit is now at a distance of 2.856(10) Å from Rh 2 .Related structural motifs have been observed in mono-and dinuclear rhodium complexes, [43][44][45][46] or in Ru and Os clusters, where however the allenyl fragment additionally coordinates side-on to a second metal center. [47,48]ith respect to The redox properties of [8] + were investigated with CV.Again, two reversible redox waves were observed at E ½ = −1.89and −2.28 V (Figure 6c).The notable shift to more negative potentials with respect to [4] 2+ is attributed to the reduced positive charge in [8] + and conversion of a -electron withdrawing alkynyl unit into a -electron donating allenyl unit.DFT calculations indicate that the reduction takes place at the bipy ligands.Additionally, a reversible oxidation event was observed at −0.50 V, which is shifted ≈1 V to more negative potentials when compared to the oxidation of [6] + or [4] 2+ (see also Table 1).None of these electrochemically generated species have been found to be stable enough to allow for isolation.
A second equivalent of KOtBu was added to

Conclusion
The molecules Ph 2 Ptrop (C≡CR)2 contain an extended conjugated carbon framework composed mainly of alkenyl (sp 2 ) and alkynyl (sp) carbon centers as well as some CH (sp 3 ) units.The special design of these compounds allows to bind low-valent metal centers in close proximity such that a Rh I Rh I dimeric unit [24] can be tightly fixed on this "carbon-bed".The metal-metal bond in these complexes is best described as a donor-acceptor bond, where -neglecting the M─M bond -one d 8 -metal center in a trigonal pyramidal environment serves as Lewis-acid [24] and the other, in a square planar environment, as donor.Based on the presented experiments in combination with previous work, an amazing diversity of reactivity becomes evident, which seem to be genuine to the low-valent dinuclear Rh I Rh I complexes (while the mono-nuclear complexes show different reactivity): i) Upon reaction with molecular hydrogen, the alkynyl unit of the Ph 2 Ptrop (C≡CR)2 ligand may be converted into a carbene donor unit. [24]ii) Consecutive reduction by two electrons converts the dication [Rh 2 (bipy) 2 {Ph 2 Ptrop (C≡CCy)2 }] 2+ is shifted mainly to one of the redox noninnocent bipy ligands [49][50][51] bound to Rh 2 (in [4] •+ this bipy ligand carries a 0.52 e charge, in [4] ≈1 e).In course of the slightly exergonic rearrangement process, this charge is depleted from the bipy ligand, which becomes almost neutral, and localized in the C 11 ring.The Rh─Rh bond is relatively little affected by these changes but, interestingly, becomes shorter in the reduced complexes indicating a strengthening of the Rh─Rh bond in the latter.Overall, the Rh─Rh bond is best viewed as a very weak Rh 2 -donor→Rh 1 -acceptor bond, which is significantly longer than a typical Rh─Rh single bond as observed in rhodium(II) acetate, [Rh 2 (O 2 CMe) 4 ]:2.29 Å. [52] Oxidation of

Figure 1 .
Figure 1.a) DFT studies of H 2 splitting on a heterogenous small rhodium cluster and carbon support.b) H 2 activation by dinuclear, molecular dirhodium complexes.c) Compound investigated in this work.

[ 7 ]
shows a rearrangement of the carbon ligand framework (see Figure 4c): Cleavage of the C═C trop double bond of the central seven membered ring occurred, where the C1─C2 distance increases from 1.512(6) to 2.564(9) Å.This is accompanied by the formation of a new C─C bond between the C4 and C7 carbon atoms of the former alkyne units (the C4-C7 distance shrinks from 3.224(6) Å in [4] 2+ to 1.468(8) Å in [7]).As a result of this

[ 7 ]
was calculated by DFT methods (Figure 5a).Complex [7] is 3.2 kcal mol −1 more stable than neutral [4].The rearrangement proceeds smoothly via opening of the C1═C2 bond and concomitant closure of the C4-C7 distance via one activated complex TS at the transition state, which is 29.2 kcal mol −1 higher in energy than [4].These results are in agreement with the fact that two reversible redox waves are observed in the CV of [4] 2+ indicating that [4] is sufficiently long-lived on the time scale of the CV experiment, while it rearranges to [7] upon attempts of isolation.The 31 P NMR chemical shifts of [4] and [7] were calculated by DFT methods ( 31 P = 54 and 7 ppm, respectively).The latter is closer to the experimentally observed chemical shift of  31 P = 18.9 ppm and we therefore assume that [7] is the predominant species in solution.

Figure 5 .
Figure 5. a) Minimum energy path for the interconversion between the native form of double reduced complex [4] to the rearranged complex [7] via the transition state TS.Calculated structures, bipy ligands and H atoms omitted for clarity.b) Redox waves at E ½ = -1.37 and E ½ = -1.01V (vs Fc/Fc + ) of [7] in THF, OCP and scan direction are shown by an arrow.Conditions: 1 mm analyte, 100 mm [nBu 4 N]PF 6 electrolyte, WE: Pt, CE: Pt on TiO x , RE: Ag/Ag + , 100 mV s −1 scan rate.

[ 4 ]
2+ (E½ = −1.35 and −1.61 V) indicating that [7] •+ and [7] 2+ likely retain the overall structure of [7] although follow-up reactions might occur after the second oxidation on the time scale of the CV experiment.The formation of side products was verified by 31 P NMR spectroscopy (see Figure S42, Supporting Information), which shows that indeed [4] 2+ is partially regenerated upon chemical oxidation of [7].

[ 8 ]
+ in an attempt to obtain neutral [9], in which both propargylic CH units in the cyclohexyl substituents are deprotonated.Time dependent NMR studies showed formation of [9], although the compound started to decompose (or rearrange) into an unidentified species after 1-2 h.The presence of [9] was confirmed by in situ NMR spectroscopy and complemented by DFT calculations of geometry and chemical shifts.The experimental value of  31 P = 61.3ppm
; and Figure S43, Supporting Information).As expected, the proton resonances of the cyclohexyl group indicate C s symmetry for compound [4] 2+ .In the mono allenyl complex [8] + the two cyclohexyl units become asymmetric, which is reflected in a complex 1 H NMR spectrum.In [9] only one set of 1 H resonances is detected again reflecting the C s symmetry.Moreover, the 1 H resonances of the propargylic CH nuclei have disappeared (for details of the NMR spectra see Figures S37 and S44, Supporting Information).Likewise, the 13 C NMR spectra of [9] indicate the absence of the alkyne groups and presence of a C s symmetric compound where the signals of the  1 -C coordinated allenylide units appear at  13 C = 114.6 (C═C═C cy ), 184.4 (C═C═C cy ), and 89.6 ppm (C═C═C cy ).These values only slightly differ from related compound [8] + (see Table 1 ).The chemical shift for Rh 1 in [9] ( 103 Rh 1 = −7606 ppm) is found at more negative values compared to complexes [4] 2+ and [8] + .The resonance of the Rh 2 nucleus follows the same trend from  103 Rh 2 = −743 ppm in [4] 2+ to −5373 ppm in [8] + to −6183 ppm in complex [9].Although an experimental structure determination is not possible, DFT calculations give some insight into a possible structure of complex [9], which contains a dianionic conjugated bisallenyl fragment.This structural motif consists of two allenyl fragments with C3 and C6 as anionic 2e-sdonors.Both are conjugated via the double bond of the tropmoiety (see formula of [9] in Figure 6a; Figure S64, Supporting Information).

[ 4 ]
2+ to neutral [4] (as indicated by reversible redox waves in the cyclic voltammogram), but [4] subsequently rearranges in an unprecedented and concerted manner into complex [7], which contains dibenzocyclo(undecatriene) as central part of the ligand.According to DFT calculations, this curved eleven-membered carbon cycle carries a two-fold negative charge.EPR spectra and DFT calculations indicate that in the reduction process of [4] 2+ most of the electron density in [4] •+ and [4]

[ 7 ]
partly re-establishes the central dibenzocycloheptenyl unit and regenerates [4] 2+ to which two alkynyl substituents are bound, indicating that this rearrangement process is at least partially reversible.iii) Stepwise deprotonation of the CH units in the cyclohexyl substituent in proximity to the Rh I 2 (bipy) 2 core of [4] 2+ affect again the alkynyl units, which rearrange one after the other into allenyl units to give complexes [8] + and [9].Although the reactions reported here have certainly limited model character with respect to metal particles on carbon support materials, they may nevertheless indicate what transformations and reactions are possible when such materials are exposed to hydrogen, redox processes, or deprotonation reactions with bases.