Selectivity of Rh⋅⋅⋅H−C Binding in a σ‐Alkane Complex Controlled by the Secondary Microenvironment in the Solid State

Abstract Single‐crystal to single‐crystal solid‐state molecular organometallic (SMOM) techniques are used for the synthesis and structural characterization of the σ‐alkane complex [Rh(tBu2PCH2CH2CH2PtBu2)(η2,η2‐C7H12)][BArF 4] (ArF=3,5‐(CF3)2C6H3), in which the alkane (norbornane) binds through two exo‐C−H⋅⋅⋅Rh interactions. In contrast, the bis‐cyclohexyl phosphine analogue shows endo‐alkane binding. A comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, traces this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ‐alkane complex is also a catalyst for solid/gas 1‐butene isomerization, and catalyst resting states are identified for this.

Such remarkable reactivity and stability in the solid state results from the periodic arrangement of [BAr F 4 ] À anions. These form an encapsulating, approximately octahedral (O h )m icroenvironmenta round the cationic s-complex ( Figure 1D), [11,14] which providess tabilizing noncovalent interactions and substrate-accessible hydrophobic channels. [17] We now report that the regioselectivity of alkane binding can be controlled by subtle changes to this periodic microenvironment, which are encoded by the cationic precursor metal-ligand fragment. This parallels selectivity in enzymest hat is promoted using primary and secondary coordination sphere control,a ss upported by the tertiary structure.

Results and Discussion
Synthesis and structural analysis of the s-alkanec omplex  Figure 2B), as we have commented upon previously. [13] In [tBu-NBD][BAr F 4 ],t he cation and anion-aryl groups are slightly tilted (Figure 2A), but in opposite directions, which combine to direct the NBD methylene bridge towardasingle anion-aryl face. The drivers for this change may come from differencesi nl igand peripheries [18] and the more compact RhÀC NBD distances in [tBu-NBD][BAr F 4 ]. Whereast he pocket volume defined by removal of the NBD does not change significantly between the Cy and tBu variants, the shape does ( Figure 2B). These structural differences carry over into the resulting s-alkane complex, [tBu-exo-NBA] [BAr F 4 ]. Addition of H 2 (1 atm, 10 min, optimized) to orange single crystalso f[tBu-NBD][BAr F 4 ] resulted in an SC-SC transformation and the formation of dark red [tBu-exo-NBA][BAr F 4 ].A crystal was transferred rapidly to ap recooled diffractometer at 150 Kf or analysis. Figure 3s hows the solid-state structure of the cation (R = 7.5 %). Longer reaction times result in loss of crystallinity and the formation of hydride species, [20] which we have not characterized further.T he bond lengths in the hydrocarbon ligand indicate that CÀCs ingle bonds have formed upon addition of H 2 ,w ith the NBA binding through two h 2 ,h 2 Rh···HÀC [14] 3c-2e interactions at the Rh I center.T he metrical data are very similar to those of [Cy-endo-NBA][BAr F 4 ], [14] despite the slightly increased PÀRhÀPb ite angle [96.39(7)8 versus 93.91(2)8]a nd decreased buried volume [%V bur = 60.4 versus 57.9, [21] Figure S36 The major difference between the tBu and Cy variantsi st he regioselectivity of alkane binding: exo-CÀHb inding for tBu and endo for Cy.A sb oth retain an % O h motif fort he arrangement of the [BAr F 4 ] À anions around the cation in the lattice,i tf alls to more subtle differences in the microenvironment,a se ncoded in the tertiary periodic structure of the NBD precursors, to in-   fluence the regioselectivity of alkane binding. Figure 4A shows the cation, proximal anion, and cage motif for [tBu-exo-NBA] [BAr F 4 ],w hich highlight the similarity with the NBD precursor (i.e.,F igure 2), in particular the orientation of the NBA ligand, with an ethylene bridge( C6/C7) directed toward a single aryl face.
The differences between the interactions of the NBA ligands in [tBu-exo-NBA][BAr F 4 ] and [Cy-endo-NBA][BAr F 4 ] with the local microenvironment are furtherh ighlighted in the noncovalent interaction (NCI) plots calculated for the proximal ion pairs ( Figure 5). In both cases, broad areas of green, weakly stabilizing, dispersive interactions are seen between the NBA and two aryl groups of the neighboring [BAr F 4 ] À anion. This feature is rather symmetric for [Cy-endo-NBA][BAr F 4 ],a nd involves the C 3 HÀC 4 H 2 ÀC 5 Hbridge of the NBA ligand ( Figure 5A). In contrast, the exo-NBA ligand in [tBu-exo-NBA][BAr F 4 ] interacts primarily along C 7 H 2 ÀC 6 H 2 ÀC 5 H( Figure 5B). In both cases, more localized disc-like features reflect the presence of stabilizing nonclassical CÀH d + ···F dÀ ÀCh ydrogen bonds. Underscoring the importance of this local anion environment, the exo-regioselectivity for NBA binding is also observed in [Rh(dcpe)-exo- To quantify the impact of the microenvironment and primary coordination sphere (i.e.,t he chelating ligand)o nt he regioselectivity of alkane coordination, we performed periodic-DFT cal- .T he latter structures were generatedb yarock/pivot motion [23] of one NBA ligand within the unit cell to generate the alternative NBA binding mode. Figure 6s hows the computed free energy profiles for this process. In each case, the crystallographically observed structure is computed to be more stable:b y3 . 5  , and in both systems TS rock is the higher transition state along the rearrangement profile.
In addition to these periodic-DFT calculations, molecular calculations were also run on the isolated [tBu-NBA] + + and [Cy-NBA] + + cations.I nt his case, both systems showedapreference for the exo isomer,b y1 .0 and 1.5kcal mol À1 ,r espectively. Therefore, in the absence of the solid-state microenvironment, there is as mall intrinsic preference for the exo bindingm ode, irrespectiveofthe nature of the chelatingl igand. [24] This preference is further enhanced in the solid state for [tBu-exo-NBA] [BAr F 4 ] but is strongly disfavored in [Cy-endo-NBA][BAr F 4 ],u nderliningt he defining role that the microenvironment can have in the selectivity of alkane binding.  The selectivity of alkane binding wasf urther probed using Hirshfelds urfaces and fingerprint plots generated with Crystal Explorer (Figure 7). [25][26][27] The Hirshfeld surfacei dentifies short contactsb etween atoms on the central probe entity (here the different[(R 2 P(CH 2 ) 3 PR 2 )Rh(NBA)] + cations)and the surrounding environment (here the neighboring octahedron of [BAr F 4 ] À anions): red identifiess hort contacts (lesst han the sum of the van der Waals radii); white indicates contactst hat are close to the sum of the van der Waals radii;a nd blue depictst hose contactst hat are longert han the sum of the van der Waals radii. Note that as hort contact may be either stabilizing or destabilizing, depending on the pair of atoms involved.F or example,s tabilizing nonclassicalC ÀH d + ···F dÀ ÀCh ydrogen bonds and destabilizing short H···H or H···C contacts will all appear as red features.I ns uch instances the fingerprint plot allows identificationo ft he atoms involved in ap articular short contact.
The fingerprint plot for [Cy-endo-NBA][BAr F 4 ] ( Figure 7A)i s typical of theses ystems with the feature terminating around (1.0, 1.0) corresponding to H···H contacts, that at (0.95, 1.2) attributed to H···F contacts, and the broad feature around (1.  Figure 7B)e xhibitsm any more red short contacts, and the fingerprintp lot suggests some shorter H···F contacts are present. However,t he major change is the broad new feature (circled in Figure 7B), which corresponds to destabilizing CÀH NBA ···C aryl short contacts. This is reflected in the presence of the broad red features in the Hirshfeld surface, situated below the aryl groups of a[ BAr F 4 ] À anion.T hese significant differences between the endo-a nd exo-bound forms are consistent with the    Figure 7C,D), as might be expected given the smaller energy differenceb etween the two (3.5 kcal mol À1 ). The latter indicates somes horter H···F contactsb ut also more destabilizing H···C aryl contacts, as evidencedb yt he filling in of the "bay" between the H···F and H···C "peninsulas",a sh ighlighted in Figure 7D [28] )a nd transfer to the spectrometer,t wo environments are still observed in the 31 P{ 1 H} SSNMRs pectrum, but they are downfield shifted [d % 63.8] and show increased coupling constants[ J(RhP) % 200 Hz],F igure 8. The 13 C{ 1 H} SSNMR spectrumi ss ilent in the alkene region (110-40ppm, Figure S12, Supporting Information). These spectroscopic data signal s-alkane complex formation. [12][13][14] [tBu-exo-NBA][BAr F 4 ] evolvesw ith time in the solid state at 298 K. After 24 h, new signals have developed at slightly higher andl ower chemical shifts. There is no furtherc hange after seven days at 298 K. Simulating this data (Figure 8a nd Figures S10a nd S11, Supporting Information) allows these new signals to be modelled as the outer lines of two doublets [d = 62.0, 65.8 ppm; J(RhP) % 210 Hz],i n4 0% relative proportion to the initial species. This ratio does not change in the 158 K 31 P{ 1 H} SSNMR spectrum,i ndicating that ad ynamic equilibrium with ar elativelyl ow barrier is not operational. We see no evi-dence for coordinationo ft he [BAr F 4 ] À counterion [13] or dehydrogenation [12] of NBA to form norbornene (solution trapping experiments only recover NBA). Although the relatively sharp SSNMR spectrum suggestst he local order is retained around the cation, [29] inspection of the X-ray diffraction pattern reveals ap hasec hange has occurred so that it is now strongly modulated, showing multiple satellites around the main Bragg reflections ( Figure S14, Supporting Information). [30] We have not been able to model this successfully,a nd therefore, it is not clear whether it is caused by anion and/or cation reorientation. However,spectroscopic data, trappinge xperiments, and catalysis (vide infra) suggestthat it is astructuralrather than achemical change, and a s-alkane complex is retained. Periodict oi ncommensurately modulated phase changes in single crystals have been reported previously, [31] and we have noted both modulated structures [18] and phase changes [20] in SMOM systems. As [Cy-endo-NBA][BAr F 4 ] is not reportedt ou ndergo a phase change, [14] we suggest that the subtle changes in the microenvironmentr esult in a metastable system for [tBu-exo-NBA][BAr F 4 ].T his may be related to the more kinetically and thermodynamically accessible alkane ligand reorganization in the latter (e.g.,F igure 6), but as the precise structure of the modulated phase is currently not known we are reluctant to comment further.

Addition of 1-butene:Solid/gas butene isomerization
Additiono f1 -butene to either immediatelyp repared, or aged under Ar,s amples of [tBu-exo-NBA][BAr F 4 ] (Scheme 2) resulted in significant loss of diffraction quality,s oareliable structural solution was not possible, althoughB ragg diffraction was still observed. [32] 31 P{ 1 H} SSNMR spectroscopy showedt he complete consumption of the s-alkane complex and the formationo f three products ( Figure S26, Supporting Information). [33] Vacuum transfer of CD 2 Cl 2 onto the crystalline material and analysisa t 183 Ku sing solution 31  equilibrium. DFT calculations on the molecular cations in CH 2 Cl 2 solvent reveal that these three species all lie within 0.8 kcal mol À1 of one another,c onsistent with the equilibrium mixture observed ( Figure S50, Supporting Information). All three complexes have inequivalent 31 Pe nvironments that show coupling with 103 Rh. Diagnostic signals in the high-field region of the 1 HNMR spectrum are attributed to agostic Rh···HÀCi nteractions, and aR h ÀHg roup [d = À29.04 ppm, J(RhH) = 32 Hz] in the allylh ydride, 3.W hereas similar butene complexes to 1 and 2 are formed from solid/gas reactions with Rh I s-alkane complexesh aving R 2 P(CH 2 ) 2 PR 2 ligands (R = Cy, [36] tBu [20] ), tautomeric Rh III allyl hydrides that come from CÀHo xidative cleavage have only been identified indirectly by mechanistic and DFT studies as higher-energy intermediates in alkene double-bond isomerizationp rocesses for these [36] and related MOF systems. [37] Here, we suggest the larger PÀRhÀPl igand bite-angle makes the Rh III oxidation state more accessible, [38] and this key intermediate can now be observed. Warming this mixture to room temperaturer esulted in decomposition to a variety of unidentified complexes. This, again, [36] demonstrates the utility of the SMOM technique in stabilizing reactive species not accessible using solution methods. The observation of 1, 2,a nd 3 suggestst hat crystalline [tBuexo-NBA][BAr F 4 ] would be as olid/gas butene isomerization catalyst. This is the case, and using finely crushed material under batch conditions at 298 K, at hermodynamic mixture of 1-butene (5 %) and2 -butenes (95 %) is established (see Figure 9). Freshlyp repareda nd modulated materials show the same temporal profile. However,c atalysis is rather slow (TOF 90 % = 35 h À1 )c ompared with [Rh(dtbpe)(NBA)][BAr F 4 ] (80 h À1 ) [20] and [Rh(dcpe)(NBA)][BAr F 4 ]( 3000 h À1 ), [36] despite having the same % O h arrangemento fa nions, and compared with single-crystalline Rh-MOF systems( 2000 h À1 ). [9] Thep recursor complex [tBu-NBD][BAr F 4 ] is inactive, demonstratingt he requirementf or ar elatively weakly bound alkane ligand for catalysis in the solid state.

Conclusions
We show here that subtle differences in the microenvironment that supports s-alkane complex formation in SMOM systems controlst he regioselectivity of alkane bonding, whereas changes at the primary active site result in different catalytic activities ando bserved restings tates for butene isomerization. What is initially surprising, but perhaps more obvious with hindsight, is that these microenvironment effects seem to dominatef or the cyclohexyl system,s witching the intrinsic regioselectivity for exo-NBA binding seen in the isolated cation, whereas fortert-butyl the microenvironment is such that regioselectivity is unaffected. Whether this increased influence leads to the remarkabler elative stabilities observed for aw ide range of s-alkane complexes of the cyclohexyl system [11][12][13][14]36] remains to be determined. These observations not only reinforce the analogy between single-crystalline SMOM systems and metalloenzymes,b ut also suggestt hat the Cy-functionalized phosphine SMOMs ystems are the current best candidates for exploring chemical space with regard to new s-alkane complexes. The influence of the microenvironment in controlling both stability and reactivity in s-alkane complexes is thus of particulari nterest,e specially with regard to simple but challenging transformations such as acceptorless alkane dehydrogenation. [12]