Stable BH3 adducts to rhodium amide bonds

Abstract

Rh(I) diolefin amides, [Rh(trop 2 N)(L)] (trop 2 N = bis(5H-dibenzo[a,d]cyclohepten-5-yl)amide), form the corresponding hydrido amine species, [RhH(trop 2 NH)(L)], by reaction with Me 2 HN-BH 3 (DMAB). Both amide and amine complexes are active dehydrocoupling catalysts, forming the monomer [Me 2 N=BH 2 ], the linear [Me 2 NHBH 2 NMe 2 BH 3 ] and the cyclic dimer [Me 2 BNH 2 ] 2. Good catalytic activity was observed especially for complexes which contain a metal hydride unit, Rh-H, in a co-planar cis-arrangement with respect to the N-H unit and for those bearing an N-heterocyclic carbene ligand (IMe) in transposition to the active basic site of the ligand. Four-membered Rh-N-B-H metallacycles [Rh{(-H)BH 2 }(Ntrop 2)(L)] (L = PPh 3 , IMe) were isolated by direct reaction of the amide complex with BH 3 (THF). These stable species are not active in the dehydrogenation of DMAB. Their isolation and lack of reactivity gives some indication for a possible catalyst deactivation. This observation is consistent with a mechanism in which the unprotected amine or amide ligand is essential for N-H and B-H bond cleavage.

Ru(II) Complexes with a Chemical and Redox-Active S2N2 Ligand: Structures, Electrochemistry, and Metal–Ligand Cooperativity

“…The terminal B−H resonances were not observed in the 1 H NMR spectrum of 5 even when the solutions were cooled to −90 °C. Difficulties observing the terminal B−H 1 H NMR resonances have been reported previously in other MLC-bound BH 3 complexes, 41,43 but the infrared (IR) spectrum of 5 confirmed their presence.…”

“…Compounds with MLC-bound BH 3 generally fall into one of two categories according to their IR spectra: (1) those with two absorptions assigned to terminal B–H stretching (symmetric and asymmetric) in the B–H stretching region (∼2200–2500 cm –1 ) and one M–H–B stretch at lower wavenumbers (∼1700–2000 cm –1 ) , and (2) those with three absorptions assigned to one symmetric and two asymmetric B–H stretches (i.e., no M–H–B stretch at significantly lower wavenumbers) …”

“…30 Addition of NaBH 4 to 2 in THF resulted in a color change from yellow to orange and eventually green upon workup and extraction with Et 2 O. Crystallization yielded dark green prisms from concentrated Et 2 O/pentane solutions at −25 °C that revealed that the second nitrogen in 2 had indeed been deprotonated as desired, but the reaction unexpectedly led to the isolation of N-capped, borane-bridged hydride Ru[(μ-H)BH 2 ]( Me SNNS Me )(PPh 3 ) (5) (Figure 5). Langer, Gade, and Trincado and Gruẗzmacher separately reported this type of MLC BH 3 binding across tricoordinate N-centered ligands with Fe, 41 Ru, 42 and Rh, 43 respectively. MLC binding of BH 3 has also been observed across a Ru−thiolate bond involving a tetradentate S 2 N 2 ligand with an ethylenediamine backbone and thiolate groups instead of thioethers.…”

“…Compounds with MLC-bound BH 3 generally fall into one of two categories according to their IR spectra: (1) those with two absorptions assigned to terminal B−H stretching (symmetric and asymmetric) in the B−H stretching region (∼2200−2500 cm −1 ) and one M−H−B stretch at lower wavenumbers (∼1700−2000 cm −1 ) 42,44 and (2) those with three absorptions assigned to one symmetric and two asymmetric B−H stretches (i.e., no M−H−B stretch at significantly lower wavenumbers). 43 These two categories can been further classified as those in which there is a strong M−H interaction (case 1) or, as described by Gruẗzmacher and co-workers, 43 those in which the M−H interaction is weak (case 2). The IR data collected for 5 were consistent with the latter situation (case 2): three B−H stretches were observed at 2338, 2404, and 2430 cm −1 in the solid state (Figure S26).…”

“…It is important to note that the M−H−B hydride resonances, as reported above for 5, are still observed in the 1 H NMR spectra despite weak M−H bonding. 43 Reactivity of Ru( Me SNNS Me )(PPh 3 ) (3) with CO and Acids. The reversible binding of MeCN to 3 led us to investigate how increasing the field strength of the neutral donor ligand affected its affinity for the open coordination site.…”