A Rhodium–Pentane Sigma‐Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques

Abstract The pentane σ‐complex [Rh{Cy2P(CH2CH2)PCy2}(η2:η2‐C5H12)][BArF 4] is synthesized by a solid/gas single‐crystal to single‐crystal transformation by addition of H2 to a precursor 1,3‐pentadiene complex. Characterization by low temperature single‐crystal X‐ray diffraction (150 K) and SSNMR spectroscopy (158 K) reveals coordination through two Rh⋅⋅⋅H−C interactions in the 2,4‐positions of the linear alkane. Periodic DFT calculations and molecular dynamics on the structure in the solid state provide insight into the experimentally observed Rh⋅⋅⋅H−C interaction, the extended environment in the crystal lattice and a temperature‐dependent pentane rearrangement implicated by the SSNMR data.


Abstract:
The pentane s-complex [Rh{Cy 2 P-(CH 2 CH 2 )PCy 2 }(h 2 :h 2 -C 5 H 12 )][BAr F 4 ]i ss ynthesized by as olid/gas single-crystal to single-crystal transformation by addition of H 2 to ap recursor 1,3-pentadiene complex. Characterization by lowt emperature single-crystal X-ray diffraction (150 K) and SSNMR spectroscopy(158 K) reveals coordination through two Rh···H À Ci nteractions in the 2,4positions of the linear alkane.P eriodic DFT calculations and molecular dynamics on the structure in the solid state provide insight into the experimentally observed Rh···HÀCinteraction, the extended environment in the crystal lattice and at emperature-dependent pentane rearrangement implicated by the SSNMR data.
The synthesis of s-alkane complexes, [1] in which an alkane interacts with am etal center through 3-center-2-electron M···HÀCb onds, [2] is of significant interest in terms of the development of new synthetic methodologies for C À H activation processes,e specially the controlled functionalization of fossil-derived hydrocarbons. [3,4] Owing to the strong, non-polar,C ÀHb ond, and steric interactions from proximal alkyl groups,alkanes are poor ligands, [5] meaning that solvent or other Lewis bases can compete for metal coordination. Direct observation of alkane complexes in solution thus requires low temperatures (typically 130-190 K) and spectroscopic techniques,s uch as time-resolved infrared spectroscopy (TRIR) [6] or in situ NMR spectroscopy, [7] and is often (although not exclusively [8] )based on reversible ligand photoejection. When this relative instability is coupled with the less than 100 %efficiency often associated with the generation of s-alkane complexes,t he production of single-crystalline material suitable for detailed structural characterization is very challenging.Nevertheless,initially serendipitous,singlecrystal X-ray diffraction studies have shown alkane CÀH bonds in close approach with metal centers (Fe 2+ , [9] U 3+ , [10] K + [11] ), in which host-guest interactions with alkane ligands are suggested to play an important role.W erecently reported an alternative approach for the targeted generation of a salkane complex by solid/gas single-crystal to single-crystal transformations [12] that Thef unctionalization by C À Ha ctivation (via intermediate s-complexes) of simple,light, but low-value hydrocarbons, such as butane and pentane,isimportant for their conversion into valuable products that can then enter the chemical feedstock chain. [15] Although s-complexes of light alkanes have been characterized in situ by low temperature NMR and TRIR techniques, [16] published examples that have been structurally characterized are highly disordered (heptane-Fe-porphyrin), [9] or characterized as being mainly electrostatic in nature (K + /alkane). [11] Powder neutron diffraction studies at 4-8 Ks how the binding of ethane or propane with an Fe center in am etal-organic framework. [17] We now demonstrate that awell-defined s-alkane complex of alinear light alkane (pentane) at alate transition metal (rhodium) can be generated by as imple solid/gas reaction, and structurally characterized in the solid state using single-crystal X-ray diffraction, solid-state NMR (SSNMR), and computational techniques (Scheme 2). This provides detailed structural metrics for the coordination of al inear, light, alkane at am etal center well known for promoting C À Ha ctivation. [4] To produce the target Rh-pentane complex, the pentadiene precursor [Rh{Cy 2 P(CH 2 CH 2 )PCy 2 }(h 2 :h 2 - ) was synthesized. [18] As ingle-crystal Xray diffraction study ( Figure 1A)showed disorder in both the Cy groups and the pentadiene fragment (P-1, Z = 2, V = 3244(1) 3 , R(2s) = 7.2 %), with the latter showing two orientations (approximately 1:1ratio) of the alkene fragment, discriminated by the methyl group pointing in opposite directions (that is,t owards P1 or P2). Nevertheless,C ÀC/C= Ca lternations in bond lengths of the diene can be clearly discriminated, while an orientation is adopted as expected for coordination through the p-face (angle between the planes defined by Rh1-P1-P2 and C6 to C10 being 102.38 8). A 31 P{ 1 H} SSNMR spectrum of [2][BAr F 4 ]s hows four environments (298 K, J(RhP) % 145 Hz;S upporting Information, Figure S1), reflecting the lack of crystallographic symmetry in the molecule and that the two orientations of the diene do not interchange in the solid state on the SSNMR timescale (that is,s tatic disorder). Likewise,t he 13 C{ 1 H} SSNMR spectrum shows 7alkene resonances between d = 104 and 60 ppm, with the lowest field signal assigned to a1+ 1coincidence.Nohigh field signals,w hich would signal Rh···HÀCo rC ÀH···aryl interactions, [13,19] were observed in the 1 HNMR projection from af requency-switched Lee-Goldburg (FSLG) SSNMR experiment ( Figure S2).
Periodic DFT calculations [18] employing the PBE-D3 functional on the extended solid-state structure of [3][BAr F 4 ] Scheme 2. Synthesis of complex [3][BAr F ]b yasolid/gas single-crystal to single-crystaltransformation, and its onward reactivity. reproduce the metrics associated with the heavy atoms extremely well (see Figure 3A for an overlay with the unit cell of the experimental structure). Thec omputed structure also permits am ore detailed discussion of the rhodiumpentane interaction (Figure 2), and shows an average computed Rh···H21/Rh···H41 distance of 2.02 , significantly shorter than Rh···H22/H42 (2.31 );inaddition the C2-H21/ C4-H41 distance is 1.14 , somewhat longer than C2-H22/C4-H42 (1.11 ). These data suggest ab is-h 2 -HC binding mode similar to that observed for [1][BAr F 4 ]. This was supported by an Atoms-in-Molecules (AIM) study of the computed electron density of the [3] + cation in which bond critical points (BCPs) were only located between Rh and each of H21 and H41 (1 ave = 0.047 au), with no BCPs between Rh and either H22 or H42. Moreover,reduced electron densities are associated with the C2-H21/C4-H41 BCPs (1 ave = 0.245 au), compared to the C2-H22/C4-H42 BCPs (1 ave = 0.265 au). These computed geometric and AIM data also suggest the rhodium-pentane interaction in [3][BAr F 4 ]i ss omewhat weaker than the Rh-NBAi nteraction in [1b][BAr F 4 ], and this was confirmed by an NBO analysis in which the overall degree of C À H!Rh s-donation and Rh!C À H p-back donation is approximately halved in the pentane complex (Table S5).
Previously,w eh ave shown that the intrinsic alkane binding energy computed in isolated [1a] + ,[1b] + ,and related molecular cations bears little correlation to the observed longevity of the alkane complex in the solid state. [13] The extended environment is therefore crucial in stabilizing these s-alkane complexes,and some components of this are evident in the non-covalent interaction (NCI) plot ( Figure 3B). This highlights broad regions of green (weakly stabilizing) van der Waals interactions that enfold the pentane moiety between two aryl arms of the [BAr F 4 ] À anion. In addition, the green disks located along several of the CÀF···HÀCv ectors are indicative of weak hydrogen bonding,the cumulative effect of which will also be to enhance the solid-state stability.
As with the solid-state structure,t he low-temperature (223 K) 31 P{ 1 H} SSNMR spectrum of [3][BAr F 4 ]a gain presents as impler analysis than [2][BAr F 4 ], as the static disorder in the pentadiene complex is no longer present ( Figure S4). Thep rincipal component is observed as two sets of doublets centered at d = 107. 6 [13,14,22] At 223 Kt he spectra remain unchanged overnight, but on warming to 298 K( 12 hrs), 4 grows in at the expense of [3][BAr F 4 ], demonstrating that the pentane complex is thermally unstable ( Figure S8). This instability can also be followed by single-crystal X-ray diffraction, which shows arelatively rapid (15 minutes at 298 K) loss of high-angle data on warming ( Figure S13)   has the same {RhL 2 } + fragment) does not decompose in the solid state.
The 13 C{ 1 H} SSNMR (223 K, Figure S4) spectrum shows the disappearance of signals in the 100-60 ppm region, consistent with the hydrogenation of the diene.W eh ave previously shown the utility of 1 H/ 13 CF SLG HETCOR SSNMR experiments combined with computed chemical shifts to identify resonances owing to the sÀRh···HÀC interactions and ring-current-affected chemical shifts in [1b]-[BAr F 4 ], [13] following the use of similar techniques to identify agostic Ru···H À Si interactions in the solid state. [19] Calculations using the periodic GIPAW methodology indicate that the sÀRh···HÀCi nteractions (C2/4) correspond to d( 1 H) = À1.6/À2.5 ppm, and correlate to d( 13 C) = 7.3/9.9 ppm, respectively.T he central methylene group (C3) experiences ar ing current shift from proximal [BAr F 4 ]a ryl rings: d( 1 H) = À2.4/ À0.9 ppm, d( 13 C) = 40.1 ppm (Scheme 3). At 223 K, the 13 C-{ 1 H} NMR spectrum shows somewhat broad signals in the aliphatic region and no clear high-field correlations observed by FSLG HETCOR SSNMR experiments.C ooling to 158 K resolves the 13 C{ 1 H} NMR spectrum more clearly,s ot hat sharper signals and strong correlations are now observed that match well with the computational model, and show the expected relative differences in chemical shifts ( Figure S5-7). On temperature cycling between 223 Ka nd 158 K, there are no significant changes in the 31 P{ 1 H} NMR spectra. Overall, these observations perhaps suggest afluxional process may be occurring in the solid state at 223 Kthat is slowed at 158 K, so that the key C À Hc orrelations and 13 Cs ignals that signal pentane coordination can be observed at this low temperature (Table S1). If [3][BAr F 4 ]isleft at 298 Kfor 7days to form [4], the resulting 1 H/ 13 CF SLG HETCOR spectrum shows the absence of the correlations at d( 13 C) = 10 and 39 ppm that are assigned to the bound pentane at 158 K.
To probe this fluxional process,w ep erformed ab initio molecular dynamics on the structure of [3][BAr F 4 ]inthe solid state,w here the metadynamics approach was applied to accelerate the exploration of conformational space.T wo collective variables were selected to probe different coordination modes of the pentane ligand ( Figure 3C): CV1 discriminates between structures I and III (the 2,4-and 1,3isomers,r espectively), while CV2 allows for further conformational flexibility,a ccessing structures such as II (a 1,4isomer). Metadynamic runs (! 120 ps) at 75, 150, and 300 K highlight changes in the amplitude of the structural fluctua-tions and allow the free energy surface (FES) to be plotted at each temperature.A ll three runs show variations in the coordination of pentane to Rh, with the initial 2,4-isomer (CV1 %À1.8;C V2 % 180.08 8)a nd the 1,3-isomer (CV1 % 1.8; CV2 % 180.08 8)b eing stable states throughout;a t1 50 K, the 2,4-isomer is more stable by approximately 4kJmol À1 .T he transition region linking these states corresponds to avariety of more flexible configurations in which pentane coordinates through one or two C À Hb onds,s uggesting facile interconversion with barriers of 15-25 kJ mol À1 .T he projection of the FES onto the subspace of the two CVs shows that increasing the temperature changes the relative stability of I and III,the trend favoring the 2,4-isomer at 75 K, while the less rigid 1,3isomer dominates at 300 K( Figure S22). Similarly,the central region of the surface becomes more accessible at higher temperatures,reflecting astabilization of the transition region owing to larger structural disorder.O verall, these results are consistent with afluxional process between I and III at higher temperatures (223 K), which slows and shifts toward the 2,4isomer upon cooling to 158 K.
It is interesting to contrast the molecular dynamics FES with the outcome of static periodic calculations in which different pentane coordination modes are optimized at one of the Rh centers in the unit cell (Table S6). Thec omputed electronic energies now indicate the 2,4-isomer (I)l ies 35 kJ mol À1 below the 1,3-isomer (III). This reiterates the role of entropy in stabilizing the more conformationally flexible 1,3-isomer,and how the TDS term makes this species more accessible at higher temperatures.
This proposed fluxional process is closely related to the chain-walking events that have directly [8b,16c,e] and indirectly [23] been shown to occur in transient, or only stable at low temperature,transition-metal alkane s-complexes in solution (Scheme 4). With the pentane complex [3][BAr F 4 ], similar processes could well be occurring in the solid state,connecting terminal (M···H 3 C) and internal (M···H 2 C) s-binding motifs, and the estimated barriers to this (15-25 kJ mol À1 )are similar to experimentally measured values derived from solutionmeasurements at very low temperature. [8b] Afuture challenge in this area is to explore if related complexes and processes are also possible for s-complexes with different alkane chain lengths,a nd whether these can exploited in the selective activation of C À Hbonds.