Phosphinoborane and Sulfidoborohydride as Chelating Ligands in Polyhydride Ruthenium Complexes: Agostic σ‐Borane versus Dihydroborate Coordination

Abstract

The chemistry of s-borane complexes remains at an early stage of development by comparison to the analogous sdihydrogen and s-silane families. [1][2][3][4][5] However, since the isolation of the first s-borane metal complex in 1996, [6] important findings dealing with B À H bond activation have been regularly disclosed. [7][8][9] The s-borane complexes are often invoked as intermediates in the formation of the corresponding hydridoboryl complexes and the microscopic reverse reductive elimination. More and more compelling evidence for s-B À H coordination is found in catalyzed borylation processes, [10] which supports the recent metathesis mechanism termed s-CAM for late-transition-metals (scomplex-assisted metathesis, which involves metal-induced dynamic rearrangements of EÀH bonds at constant oxidation state). [11] Moreover, s-borane complexes and the related samine-borane species are now found to be relevant to the catalytic dehydrogenation of amine-boranes. [12][13][14][15][16] As part of our research on B À H bond activation of less-common borane reagents, [17,18] we have decided to develop a new class of borane-attached ligands to study hydrogen-transfer reactions and compare the reactivity of true s-borane complexes with agostic species. [19,20] The synthesis of bidentate and potentially hemilabile L~BHR (L = P, S and R = amino, alkyl) borane-containing ligands was challenging as this class of compounds remains unexplored in particular by comparison to the ambiphilic borane derivatives. [21,22] Braunschweig et al. very recently reported access to a vinylborane by insertion of a borylene chromium complex into an olefinic CÀH bond. [23] The corresponding complex [Cr{HBN-(SiMe 3 ) 2 CH=CHCMe 3 }(CO) 4 ] was isolated and characterized as a formally chromium(0) olefin complex with a weak B À H coordination to the metal center. We now report the synthesis of two new boron-containing compounds, the phosphinomethyl(amino)borane Ph 2 PCH 2 BHNiPr 2 (1) and the lithium sulfido(methyl)borohydride Li[MeSCH 2 BH 2 Me] (2), as well as their coordination to the two ruthenium precursors [RuH 2 (h 2 -H 2 ) 2 (PCy 3 ) 2 ] (3) and [RuHCl(h 2 -H 2 )-(PCy 3 ) 2 ] (4). The strategy is based on the one we recently used to isolate the bis s-borane complex [RuH 2 (h 2 :h 2 -H 2 BMes)-(PCy 3 ) 2 ] upon reaction of (3) with mesitylborane or the chloro complex (4) with lithium mesitylborohydride. [17] Our study includes the isolation of the first d-agostic ruthenium complex containing an h 2 -BÀH bond that involves a trivalent boron atom and illustrates the competition between s-coordination and dihydroborate ligation.The addition of 1 to the bis(dihydrogen) complex 3 in toluene at room temperature led to the substitution of the two dihydrogen ligands by 1 (Scheme 1). The new complex [RuH 2 {h 2 -H-B(NiPr 2 )CH 2 PPh 2 }(PCy 3 ) 2 ] (5) was fully characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. The 31 P{ 1 H} NMR spectrum of 5 displays an AM 2 pattern with a triplet at d = 2.58 ppm and a doublet at ...

Ruthenium Bis(σ‐BH) Aminoborane Complexes from Dehydrogenation of Amine–Boranes: Trapping of H2BNH2

“…The trigonal planar geometry of both boron and nitrogen atoms and the BÀN distance reflect the expected multiple N sp2 À B sp2 bond character in monomeric aminoboranes owing to the p donation of the nitrogen lone pair into the vacant p atomic orbital on boron. It is worth noting that the BÀN and BÀH distances are very similar to those found in two agostic complexes [Cr{HBN(SiMe 3 ) 2 CH=CHCMe 3 }(CO) 4 ] and [RuH 2 {h 2 -HÀB(NiPr 2 )CH 2 PPh 2 }(PCy 3 ) 2 ] as reported recently by Braunschweig and our group (B À N/B À H: 1.4034(16) /1.257 , [17] and 1.404(9) /1.23 (7) , [18] respectively). All these features are in full agreement with the trapping of H 2 BÀNH 2 , [19] the first formal elementary step of dehydrogenation of ammonia-borane [Eq.…”

Ruthenium‐Catalyzed Tandem Activation of C≡N and B−H Bonds under Dihydrogen: Synthesis of BN Heterocycles

“…Compound 3 a was found to be stable at room temperature and was fully characterized by multinuclear NMR spectroscopy. A doublet at δ =37.5 ppm ( 1 J BH =115 Hz) and a broad singlet at δ =5.52 ppm in the 11 B NMR and 1 H{ 11 B} NMR spectra, respectively, reflect the presence of a B(sp 2 )HN i Pr 2 group in 3 a . The integrity of the cyano group is evidenced by a strong nitrile IR stretching frequency observed at =2219 cm −1 .…”

B–C Bond Cleavage and Ru–C Bond Formation from a Phosphinoborane: Synthesis of a Bis-σ Borane Aryl-Ruthenium Complex

“…6 The postulated agostic complexes B and/or C ( Figure 3) likely to give 3 by B−C bond cleavage and ligand rearrangement were never detected. 4,5 Calculations indicated that substitution of the two H 2 ligands in 1 by ligand 2 affords complex B in an exoergic transformation with ΔG = −2.5 kcal mol −1 . 12 This complex features an agostic B−H bond that is elongated with respect to the free ligand (1.304 Å, B; 1.208 Å, 2).…”

“…4,5 The reaction of 2 with [RuH 2 (η 2 -H 2 ) 2 (PCy 3 ) 2 ] (1) carried out in pentane, at room temperature, for 1 min, led after workup to a brown solid analyzed as an equimolar mixture of complex 3 and PCy 3 (Scheme 1). 3 was found to be unstable in solution at room temperature and had to be stored in the solid state, below −30°C .…”