Borinium Cations as σ-B−H Ligands in Osmium Complexes

From trihydroborates to bisborylenes: a route to dinuclear bisborylene complexes

A new route for the synthesis of dinuclear bisborylene complexes was described. A series of novel diruthenium bisborylenes were prepared through unprecedented triple B-H oxidative addition of trihydroborates with concomitant hydrogen liberation. Conversion of trihydroborates to bisborylenes involved the formation of tris(σ-B-H) borate as the crucial intermediate stage.

Synthesis and structures of anionic rhenium polyhydride complexes of boron-hydride ligands and their application in catalysis

The rhenium complex, [K(DME)(18-c-6)][ReH₄(Bpin)(η²-HBpin)(κ²-H₂Bpin)] 1, comprising hydride and boron ligands only, has been synthesized by exhaustive deoxygenation of the commercially available perrhenate anion (ReO₄ ^-) with pinacol borane (HBpin). The structure of 1 was analysed by X-ray crystallography, NMR spectroscopy, and DFT calculations. While no hydrides were located in the X-ray crystal structure, it revealed a trigonal arrangement of pinacol boron ligands. Variable-temperature NMR spectroscopy supported the presence of seven hydride ligands but further insight was hindered by the fluxionality of both hydride and boron ligands at low temperature. Further evaluation of the structure by Ab Initio Random Structure Searching (AIRSS) identified the presence of hydride, boryl, σ-borane, and dihydroborate ligands. This complex, either isolated or prepared in situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple operating procedures. It also acts as a reagent for the stoichiometric C-H borylation of toluene, displaying high meta regioselectivity in the borylated products. Reaction of 1 with 9-BBN resulted in HBpin substitution to form the new anionic tetra(dihydroborate) complex [K(DME)(18-c-6)][Re(κ²-H-9-BBN)₄] 4 for which the hydride positions were clearly identified by X-ray crystallography. The method used to generate these isolable yet reactive boron-hydride complexes is direct and straightforward and has potential utility for the exploitation of other metal oxo compounds in operationally simple catalytic reactions.

Striking transformations of the hydroborylene ligand in a HB:→NiII complex with isocyanides and CO

The first reactivity of the hydroborylene ligand is described, giving access to a remarkable range of unprecedented boron-centered species.

For the first time, the reactivity of the metal- and N-heterocyclic carbene-supported monovalent hydroborylene is reported. Isocyanides react with the hydroborylene Ni^II complex [{cat(^TMSL)Si}(Cl)Ni←:BH(NHC)₂] 1 (cat = ortho-C₆H₄O₂; ^TMSL = N(SiMe₃)(Dipp); Dipp = 2,6-Prⁱ₂C₆H₃; NHC = :C[(Prⁱ)NC(Me)]₂) to form the hydride-bridged hydroborylene-Ni^II complexes 2. The reaction of 1 with isoelectronic CO, however, is reversible and furnishes the related unprecedented hydride- and CO-bridged hydroborylene Ni^II complex 2-CO, which undergoes isomerisation through silyl/NHC exchange at ambient temperature to afford the corresponding hydro(silyl)boryl Ni^II complex 3. Markedly, 2 readily and quantitatively react with one further molar equiv. of isocyanide to give, under borylene liberation and H/Cl ligand exchange, boraketiminium species, which represent cationic B^I complexes. These latter compounds are highly reactive in solution, and can undergo quantitative transformation into previously unknown cyanoborenium cations.

Dihydrobis(methimazolyl)borato complexes of ruthenium and osmium

The reaction of Na[H₂B(mt)₂] (mt = 2-mercapto, 3-methylimidazol-1-yl, methimazolyl) with [Ru(X)Cl(CO)(PPh₃)_n] (n= 3 X = H;n= 2 BO₂C₆H₄, SiCl₃, SiMe₃) affords the complexes [Ru(X)(CO)(PPh₃){κ²-H,S,S′-H₂B(mt)₂}].

An Entry to Stable Mixed Phosphine-Osmium-NHC Polyhydrides

An entry to mixed phosphine-osmium-NHC polyhydride complexes is described, starting from the five-coordinate hydrido-alkylidyne compounds [OsHCl(≡CPh)(IPr)(PR3)]OTf (IPr = 1,3-bis(2,6-disopropylphenyl)imidazolylidene; OTf = CF3SO3; PR3 = P(i)Pr3 (1), PPh3 (2)). The experimental procedure involves the borylation of the Os-C triple bond of 1 and 2 with NaBH4 and the subsequent alcoholysis of the borylation products OsH2Cl(η(2)-H-BCH2Ph)(IPr)(PR3) (PR3 = P(i)Pr3 (3), PPh3 (4)) or OsH2(η(2):η(2):H2BCH2Ph)(IPr)(P(i)Pr3) (5). Stirring of 3 in 2-propanol affords the five coordinate chloride-trihydride OsH3Cl(IPr)(P(i)Pr3)2 (6), which reacts with NaBH4 to give OsH3(κ(2)-H2BH2)(IPr)(P(i)Pr3) (7). This trihydride-tetrahydrideborate derivative and its PPh3 counterpart OsH3(κ(2)-H2BH2)(IPr)(PPh3) (8) can be also obtained in a one-pot procedure, starting from 1 and 2 and using methanol at -60 °C instead of 2-propanol as alcoholysis agent. The bonding situation in 7 and 8, analyzed by DFT calculations using AIM and NBO methods, resembles that found in B2H6 and contrasts with the bonding situation in the bis-σ-borane derivative 5. Stirring of 7 and 8 in 2-propanol leads to the corresponding d(2)-hexahydride derivatives OsH6(IPr)(PR3) (PR3 = P(i)Pr3 (9), PPh3 (10)), which reduce the C≡N triple bond of benzonitrile and promote the subsequent chelate-assisted ortho-CH bond activation of the resulting phenylmethanimine, to form the trihydride compounds OsH3{κ(2)-N,C-(NH═CH-C6H4)}(IPr)(PR3)2 (PR3 = P(i)Pr3 (11), PPh3 (12)), containing a stabilized orthometalated aldimine.

A homobimetallic complex of chromium(0) with a σ-borane component

The synthesis, characterization, and reactivity of a chromium(0) complex bearing an amine-borane moiety (η(6)-C(6)H(5)CH(2)NMe(2)·BH(3))Cr(CO)(3) (2) is reported. Photolysis of complex 2 results in the elimination of a CO ligand followed by the formation of an intramolecular σ-borane complex (η(1)-(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H))Cr(CO)(2) (3). This species was characterized in solution by NMR spectroscopy. Reaction of complex 2 with photochemically generated (OC)(5)Cr(THF) affords a novel homobimetallic σ-borane complex (OC)(3)Cr(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H-Cr(CO)(5)) (4), wherein one of the BH moieties is bound to the chromium center in an η(1)-fashion. The σ-borane complex 4 was isolated in moderate to good yield (72%). The BH(3) fragment in the complexes 3 and 4 are highly dynamic involving exchange of the BH hydrogen bound to the metal with the terminal BH hydrogen atoms. The dynamics has been studied using variable-temperature NMR spectroscopy. Complexes 2 and 4 have been characterized by X-ray crystallography.

Probing mesitylborane and mesitylborate ligation within the coordination sphere of Cp*Ru(P(i)Pr3)+: a combined synthetic, X-ray crystallographic, and computational study

The reaction of Cp*Ru(P(i)Pr(3))Cl (1) with MesBH(2) (Mes = 2,4,6-trimethylphenyl) afforded the mesitylborate complex Cp*Ru(P(i)Pr(3))(BH(2)MesCl) (2, 66%). Exposure of 2 to the chloride abstracting agent LiB(C(6)F(5))(4)·2.5OEt(2) provided [Cp*Ru(P(i)Pr(3))(BH(2)Mes)](+)B(C(6)F(5))(4)(-) (3, 54%), which features an unusual η(2)-B-H monoborane ligand. The related borate complex Cp*Ru(P(i)Pr(3))(BH(3)Mes) (5, 65%) was prepared from 1 and LiH(3)BMes. Attempts to effect the insertion of unsaturated organic substrates into the B-H bonds of 3 were unsuccessful, and efforts to dehydrohalogenate 2 using KO(t)Bu instead afforded the mesitylborate complex Cp*(P(i)Pr(3))Ru(BH(2)MesOH) (6, 48%). Treatment of 1 with benzyl potassium generated an intermediate hydridoruthenium complex (7) resulting from dehydrogenation of a P(i)Pr fragment, which in turn was observed to react with MesBH(2) to afford the mesitylborate complex Cp*(P((i)Pr)(2)(CH(3)CCH(2)))Ru(BH(3)Mes) (8, 47%). Crystallographic characterization data are provided for 2, 3, 5, 6, and 8. A combined X-ray crystallographic and density functional theory (DFT) investigation of 3 and 5, using Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analysis, revealed that 3 and 5 are best described as donor-acceptor complexes between a Cp*(P(i)Pr(3))Ru(+) fragment and a bis(η(2)-B-H) coordinating mesitylborane(borate) ligand. Significant σ-donation from the B-H bonds into the Ru(II) center exists as evidenced by the NBO populations, bond orders, and AIM delocalization indices. In the case of 3, the vacant p orbital on boron is stabilized by Ru→B π back-donation as well as by resonance with the mesityl group.