Formation of a σ ‑ alkane Complex and a Molecular Rearrangement in the Solid-State: [Rh(Cyp 2 PCH 2 CH 2 PCyp 2 )( η 2 : η 2 ‑ C 7 H 12 )][BAr F4 ]

: Addition of H 2 to the precursor [Rh(Cyp 2 PCH 2 CH 2 PCyp 2 )( η 2 : η 2 C 7 H 8 )][BAr F4 ] gives the σ -alkane complex [Rh(Cyp 2 PCH 2 CH 2 PCyp 2 )( η 2 : η 2 C 7 H 12 )][BAr F4 ] by a single-crystal to single-crystal reaction, as characterized by X-ray crystallography, SSNMR spectroscopy, and periodic DFT. An unexpected rearrangement of the {Rh(L 2 )} + fragment is revealed.

C omplexes in which an alkane acts as a ligand to a metal center using its C−H bonding electrons, so-called σalkane complexes, are of considerable interest due to their role as intermediates in C−H activation processes and the challenges involved with their synthesis and characterization. 1 As C−H bonds are strong and non-nucleophilic, alkanes are very poor ligands, and direct observation of σ-alkane complexes has generally been limited to very low temperature in situ solution spectroscopic techniques. 2 We have recently reported that they may be prepared by solid/gas single-crystal to singlecrystal transformations (Scheme 1), 3 enabling structural characterization by single crystal X-ray crystallography. Addition of H 2 to the diene precursor [Rh(R 2 PCH 2 CH 2 PR 2 )-(η 2 :η 2 -NBD)][BAr F 4 ] (NBD = norbornadiene, Ar F = 3,5-(CF 3 Figure 1A) demonstrated crystallographically imposed C 2 rotational symmetry with Rh1 and NBD bridge methylene (C7) occupying special positions. The Cyp groups are disordered, while the long-range structure was found to be modulated (Supporting Information), 8 which was modeled with a q vector of (0,0.3433,0). 9  . Crystallinity is retained in this process, and the resulting structure determined by single-crystal X-ray diffraction (100 K, R(2σ) = 7.0%) clearly shows a saturated NBA fragment interacting with the Rh center through two 3c-2e Rh···H−C interactions, in which the relevant hydrogen atoms were located but refined using a riding model ( Figure 2). Although there are no significant changes to the arrangement of the [BAr F 4 ] − anions, this transformation proceeds with loss of the crystallographically imposed C 2 rotational symmetry: 10 Space group changes have been noted in other single-crystal transformations. 5,11 The alkane ligand binds to the {Rh(L 2 )} + fragment through two C− H σ-interactions, resulting in a Rh(I) square-planar, d 8 metal center coordination motif. The Rh−C1/C2 distances (2.388(5) and 2.392(5) Å, respectively) are the same, within error, as for The changes upon hydrogenation were also probed by the optimization of the extended solid-state structure with periodic DFT calculations at the PBE-D3 level. These provided excellent agreement both for the molecular geometries of the Rh cations (see the caption of Figure 2 for [BAr F 4 ] and the Supporting Information for [BAr F 4 ]) and for the extended structure (see Figure S15 in the Supporting Information). Moreover, the calculations provide further insight into the Rh···alkane interaction, with short average computed Rh···H11/H21 distances (1.89 Å) and elongated C1−H11 and C2−H21 distances (1.15 Å). In contrast, the C1−H12 and C2−H22 bonds exhibit standard distances of 1.10 Å, consistent with an absence of any interaction with Rh in that case. The Rh···H−C σ-interaction is confirmed by a quantum theory of atoms in molecules (QTAIM) study that identifies Rh···H11 and Rh···H21 bond paths with reduced electron densities for the C1−H11 and C2−H21 bond critical points in comparison to the C1−H12 and C2−H22 bonds. 9 Consideration of the relationship between the cation and the proximal capping [BAr F 4 ] − anion (Figure 1) shows that the NBA ligand in [BAr F 4 ] adopts an orientation very similar to that of the NBD ligand in [BAr F 4 ],i n particular the orientation of the C7-methylene protons that are still directed toward the aryl rings. In contrast the {Rh(L 2 )} + fragment has undergone a ca. 90°twist with respect to the precursor as defined by, for example, the interplane angle C38,C54,Rh1/Rh1,P1,P2 = 88.12(12)° (Figure 1). We have previously noted 12 that the octahedral cavity described by the anions accommodates a variety of structural changes associated at the metal cation. Clearly it can also accommodate significant movement of the {Rh(L 2 )} + fragment. Whether this occurs in concert with hydrogenation, or immediately afterward, is opaque to experiment. This situation is different from that observed in the transformations that afforded [2-NBA][BAr F 4 ], in which it is the organic fragment that responds by moving rather than {Rh(L 2 )} + . 5 Isomerization, or dynamic processes, in single-crystal organometallics have been reported. 13 Computationally the alternative form of [3-NBA][BAr F 4 ] was probed by rotating one complete cation within the unit cell and reoptimizing the structure (Scheme 2). This gives a local "parallel" structure analogous to that seen within [2-NBA]-[BAr F 4 ] and equivalent to the structure that would be formed if, upon hydrogenation, the NBD ligand in [BAr F 4 ] underwent rotation rather than the {Rh(L) 2 } + cation. For [BAr F 4 ] the parallel structure is ca. 14 kcal/mol above the optimized "orthogonal" form. The parallel structure is therefore not viable thermodynamically, but it may be kinetically accessible, 14 kcal/mol representing a lower limit to the   , consistent with at least two crystallographically independent 31 P environments. Such a downfield shift and increase in the J RhP value are consistent with the formation of a σ-alkane complex. 4,5,14 The 13 C{ 1 H} SSNMR spectrum lacks signals between δ 100 and 50 due to NBD. 9 We, 5,14 and others, 15 have previously used HETCOR SSNMR experiments to indirectly detect 1 H NMR signals in σ complexes, which in combination with computed chemical shifts has led to assignment of signals arising from σ-Rh···H−C interactions in σ-alkane complexes. The projected 1 H NMR spectrum from the 13 C/ 1 H FSLG-HETCOR SSNMR experiment conducted on freshly prepared [3-NBA][BAr F 4 ] showed a broad high-field signal centered at ca. δ − 1.46 which correlates to a partially obscured 13 C signal at δ 25. 9 These chemical shifts are comparable to those reported for the σ-Rh···H−C interactions in Assignment of these NMR data on the basis of the computed 13 C and 1 H chemical shifts for [BAr F 4 ] derived from the full extended solid-state structure (GIPAW method, PBE functional) is shown in Figure 3. Those for C1 and C2 (δ calc ca. 27) are in good agreement with experiment, while the associated H11 and H21 exhibit appreciable shielding (coincident at δ calc −4.65) but are shifted more upfield than those in experiment (δ −1.46). The C7 position (δ calc 40.4) matches experiment, and the ring current effect that shields the C7 hydrogens is also captured. The different values for H71 and H72 (δ calc −1.35 and 0.04) reflect the asymmetry of the environment in the static structure. The observation experimentally of single resonances for H71/72 (δ −0.16) and H11/ 21 may therefore reflect a fluxional process occurring on the NMR time scale at 298 K that renders these sets of protons equivalent. Cooling to 158 K resulted in a broadened 31 P{ 1 H} SSNMR spectrum and a HETCOR in which signals due to the NBA ligand were too broad to be observed, consistent with a low-energy (∼7 kcal/mol) fluxional process occurring in the solid-state. 16 SSNMR calculations also reproduce well the change in J RhP upon hydrogenation. 9 Powdered crystalline   9 Decomposition is effectively halted at 273 K.
Although we currently do not have a definitive explanation for the factors that control relative stabilities and the movements of NBA versus the metal fragment in the solid state, it may be noteworthy that the PCyp 2 groups offer a more curved exterior surface in comparison with the rigid/tall PCy 2 that could encourage {Rh(L 2 )} + movement for the former in the relatively high symmetry cavity of [BAr F 4 ] − anions ( Figure  4). The delineation of such electronic and steric influences will be important in the continued development of the coordination chemistry and reactivity of σ-alkane complexes in the solidstate.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00645.
Experimental and characterization details, including NMR spectroscopic data, X-ray crystallographic data, and computational details (PDF) X-ray crystallographic data (CIF)

Notes
The authors declare no competing financial interest.