Dehydrogenation of Saturated CC and BN Bonds at Cationic N-Heterocyclic Carbene Stabilized M(III) Centers (M = Rh, Ir)

Abstract

Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)(2)(H)(2)Cl [M = Rh, Ir; NHC = IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes = N,N'-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)(2)(H)(2)Cl (1) with Na[BAr(f)(4)] in fluorobenzene generates [Ir(IPr)(2)(H)(2)](+)[BAr(f)(4)](-) (4) in which the iridium center is stabilized by a pair of agostic interactions utilizing the methyl groups of the isopropyl substituents. After a prolonged reaction period C-H activation occurs, ultimately leading to the dehydrogenation of one of the carbene (i)Pr substituents and the formation of [Ir(IPr)(IPr'')(H)(2)](+)[BAr(f)(4)](-) (5), featuring the mixed NHC/alkene donor IPr'' ligand. By contrast, the related IMes complexes M(IMes)(2)(H)(2)Cl (M = Rh, Ir), which feature carbene substituents lacking beta-hydrogens, react with Na[BAr(f)(4)] in fluorobenzene to give rare examples of NaCl inclusion compounds, viz., [M(IMes)(2)(H)(2)Cl(Na)](+)[BAr(f)(4)](-) (M = Rh, 6; M = Ir, 7). Intercalation of the sodium cation between the mesityl aromatic rings of the two NHC donors has been demonstrated by crystallographic studies of 7. Synthetically, 6 and 7 represent convenient yet highly reactive sources of the putative 14-electron [M(NHC)(2)(H)(2)](+) cations, readily eliminating NaCl in the presence of potential donors. Thus 7 can be employed in the synthesis of the dinitrogen complexes [Ir(IMes)(2)(N(2))(2)](+)[BAr(f)(4)](-) (8a) and [Ir(IMes)(2)(N(2))THF](+)[BAr(f)(4)](-) (8b) (albeit with additional loss of H(2)) by stirring in toluene under a dinitrogen atmosphere and recrystallization from the appropriate solvent system. The interactions of 6 and 7 with primary, secondary, and tertiary amineboranes have also been investigated. Although reaction with the latter class of reagent simply leads to coordination of the amineborane at the metal center via two M-H-B bridges {and formation, for example, of the 18-electron species [M(IMes)(2)(H)(2)(mu-H)(2)B(H).NMe(3)](+)[BAr(f)(4)](-) (M = Rh, 9; M = Ir, 10)}, the corresponding reactions with systems containing N-H bonds proceed via dehydrogenation of the BN moiety to give complexes containing unsaturated aminoborane ligands. Thus, for example, 6 catalyzes the dehydrogenation of R(2)NH x BH(3) (R = (i)Pr, Cy) in fluorobenzene solution (100% conversion over 6 h at 2 mol % loading) to give R(2)NBH(2); the organometallic complex isolated at the end of the catalytic run in each case is shown to be [Rh(IMes)(2)(H)(2)(mu-H)(2)BNR(2)](+)[BAr(f)(4)](-) (R = (i)Pr, 11; R = Cy, 12). In contrast to isoelectronic alkene donors, the aminoborane ligand in these complexes (and in the corresponding iridium compounds 13 and 14) can be shown by crystallographic methods to bind in end-on fashion via a bis(sigma-borane) motif. Similar dehydrogenation chemistry is applicable to the primary amineborane (t)BuNH(2) x ...

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

“…The observed δ ( 11 B) chemical shift of 51 ppm is more consistent with the former as amino‐boranes bound to one metal center show chemical shifts around 40–50 ppm,12, 13, 24, 31 while bridging borylenes32 are generally observed between 90 and 100 ppm 23, 33…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…These weak interactions arise primarily from donation from the σ B-H orbital to the metal; the B-H σ* orbital is high in energy, meaning that back-donation from the metal is negligible [11,61]. Often, sigma complexes are isolated using tertiary amine-boranes, Amine-boranes can also bind to the metal centre through two B-H sigma bonds, resulting in η 2 complexes [19,64,65]. Oligomeric species such as H 3 B · NMe 2 BH 2 · NMe 2 H have also been observed to bind in this manner (Fig.…”

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

“…Other Ir-based systems have also recently been shown to effect turnover in H 3 B·NMe 2 H dehydrocoupling. [27,40] The mechanism of dehydrogenation of 2 a to form 3 has been investigated by computational methods, using a model…”

“…This species resembles a recently reported Ir III complex formed by B À H activation of a coordinated aminoborane, which features a direct Ir À B bond and a bridging Ir-H-B interaction and can be described in one limiting form as an a-agostic boryl. [27] The IrÀB interaction in INT2 is disrupted in the subsequent NÀH bond activation transition state (TS(NH), G = + + 126.3 kJ mol À1 , Ir À B = 2.12 ), in which rotation about the Ir À B bond allows transfer of H4 onto Ir. TS(NH) leads directly to the model aminoborane adduct 3'.…”