Solid-state molecular organometallic catalysis (SMOM-cat): synthetic routes, unique structural motifs, mobility in the solid-state and very active gas/solid isomerization catalysts.

The non-oxidative catalytic dehydrogenation of light alkanes via C–H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C–H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh­(Cy2PCH2CH2PCy2)­(η:η-(H3C)­CH­(CH3)2]­[BArF 4] and [Rh­(Cy2PCH2CH2PCy2)­(η:η-C6H12)]­[BArF 4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C–H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson–Mehl–Avrami–Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [k H/k D = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C–H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C–H bond elongation in both rate-limiting transition states and suggest that the large k H/k D for the second dehydrogenation results from a pre-equilibrium involving C–H oxidative cleavage and a subsequent rate-limiting β-H transfer step.

A systematic study of the catalyst structure and overall charge for the dehydropolymerization of HB·NMeH to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR groups are selected from Et, Pr, andBu. The most efficient systems are based upon {Rh(Xantphos-Pr)}, i.e., [Rh(κ-P,O,P-Xantphos-Pr)(H)(η-HB·NMe)][BAr], 6, and Rh(κ-P,O,P-Xantphos-Pr)H, 11. While H evolution kinetics show both are fast catalysts (ToF ≈ 1500 h) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (M = 28 000 g mol, Đ = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (M = 9000 g mol, Đ = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ-P,O,P-Xantphos-Pr)}B] as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.

Solid/gas single-crystal to single-crystal (SC−SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy 2 P(CH 2 ) n PCy 2 )(L)]-[BAr F 4 ] (n = 3, 4) and [RhH(Cy 2 P(CH 2 ) 2 (CH)-(CH 2 ) 2 PCy 2 )(L)][BAr F 4 ] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by singlecrystal X-ray diffraction, have cations exhibiting Rh•••H−C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η 2 ,η 2 Rh•••H−C (e.g., [Rh(Cy 2 P(CH 2 ) n PCy 2 )(η 2 η 2 -NBA)][BAr F 4 ], n = 3 and 4), through to more weakly bound η 1 Rh•••H−C in which C−H activation of the chelate backbone has also occurred (e.g., [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(η 1 -COA)]-[BAr F 4 ]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )][COA⊂BAr F 4 ], in which the metal center instead forms two intramolecular agostic η 1 Rh•••H−C interactions with the phosphine cyclohexyl groups. CH 2 Cl 2 adducts formed by displacement of the η 1 -alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(κ 1 -ClCH 2 Cl)][BAr F 4 ], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh•••alkane interactions along a C−H activation trajectory. These are negligible in [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )][COA⊂BAr F 4 ]; in [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(η 1 -COA)][BAr F 4 ], σ C−H → Rh σ-donation is supported by Rh → σ* C−H "pregostic" donation, and in [Rh(Cy 2 P(CH 2 ) n PCy 2 )(η 2 η 2 -NBA)][BAr F 4 ] (n = 2−4), σ-donation dominates, supported by classical Rh(dπ) → σ* C−H π-back-donation. Dispersive interactions with the [BAr F 4 ] − anions and Cy substituents further stabilize the alkanes within the binding pocket.

[Rh(κ 2 -PP-DPEphos){η 2 η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ] acts as an effective precatalyst for the dehydropolymerization of H 3 B·NMeH 2 to form N -methylpolyaminoborane (H 2 BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H 2 : with M n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H 2 evolution studies (1,2-F 2 C 6 H 4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh] TOTAL . Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh 2 (κ 2 -PP-DPEphos) 2 (μ-H)(μ-H 2 BN=HMe)][BAr F 4 ], followed by the crystallographically characterized amidodiboryl complex [Rh 2 ( cis -κ 2 -PP-DPEphos) 2 (σ,μ-(H 2 B) 2 NHMe)][BAr F 4 ]. Adding ∼2 equiv of NMeH 2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh] TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M n = 40 000 g/mol). Speciation studies suggest that NMeH 2 acts to form the  precatalysts [Rh(κ 2 -DPEphos)(NMeH 2 ) 2 ][BAr F 4 ] and [Rh(κ 2 -DPEphos)(H) 2 (NMeH 2 ) 2 ][BAr F 4 ], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ 2 -PP-DPEphos){η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ], which has been doped with amine. This promoting effect of added amine in situ is shown to be genera...