Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cationic ligands - from monodentate to pincer systems

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

Recent Advances on the Chemistry of Transition Metal Complexes with Monoanionic Bidentate Silyl Ligands

The chemistry of transition-metal (TM) complexes with monoanionic bidentate (κ2-L,Si) silyl ligands has considerably grown in recent years. This work summarizes the advances in the chemistry of TM-(κ2-L,Si) complexes (L=N-heterocycle, phosphine, N-heterocyclic carbene, thioether, ester, silylether or tetrylene). The most common synthetic method has been the oxidative addition of the Si-H bond to the metal center assisted by the coordination of L. The metal silicon bond distances in TM-(κ2-L,Si) complexes are in the range of metal-silyl bond distances. TM-(κ2-L,Si) complexes have proven to be effective catalysts for hydrosilylation and/or hydrogenation of unsaturated molecules among other processes.

Doubly Base-Stabilized Diborane(4) and Borato-Boronium Species and Their Chemistry with Chalcogens

In an effort to isolate diborane(4) derivatives, we have developed an efficient and uncatalyzed approach using [BH3·THF] and the mercaptopyridine ligand. Thermolysis of 2-mercaptopyridine, in the presence of [BH3·THF], afforded a doubly base-stabilized diborane(4) species 1, [HB(μ-C5H4NS)]2, along with the formation of its isomeric species 2, [HB(μ-C5H4NS)]2, albeit in less yield. Based on the coordination of the boron with the mercaptopyridine ligand in 2 and its spectroscopic data, compound 2 has been designated as a borato-boronium species, in which the anionic borate and cationic boronium units are covalently bonded to each other. Furthermore, we have demonstrated the oxidative insertion of chalcogen atoms (S and Se) through the B-B bond of the base-stabilized diborane(4), 1, that yielded chalcogenido-diboron species, 3(S) and 4(Se).

B-P Coupling: Metal Stabilized Phosphinoborate Complexes

In an effort to establish B-P coupling reactions without the use of phosphine-borane dehydrocoupling agent, we have developed a new synthetic methodology employing group 8 metal σ-borate complex [{κ3 -H,S,S'-BH2 L2 }Ru{κ3 -H,H,S-BH3 L}] (L=NC5 H4 S), 1. Treatment of 1 with chlorodiphenyl phosphine (PPh2 Cl) yielded 1,5-P,S chelated Ru-dihydridoborate species [PPh2 H{κ3 -H,H,S-BH(OH)L}Ru{κ2 -P,S-(Ph2 P)BH2 L}], 2. The insertion of phosphine moiety (PPh2 ) by the cleavage of 3c-2e σ(Ru… H-B) bonding interaction led to the formation of B-P bond. The κ2 -P,S chelated six-membered ring adopted a boat conformation in complex 2. The heterocycle is made of all different atoms, which is one of the rarest examples of heteroatomic ring systems. Theoretical outcomes demonstrated the electronic insight of B-P coupling and stabilization through transition metal. In order to explore an alternate route of B-P bond formation, we have further explored the reaction of 1 and Ru-bis(dihydridoborate) complex, 5 with secondary phosphine oxide (SPO). Although, thermolysis of 1 with diphenylphosphine oxide yielded analogous σ-borate complex 3, the similar reaction of 5 at room temperature led to the formation of novel phosphinous(III) acid incorporated Ru(σ-borate)(dihydridoborate) complex, 6. In a similar fashion, the reaction of 5 with phosphite ligand generated Ru(σ-borate)(dihydridoborate) complex, 7, which is analogous to 6.

Three-Center M-H-B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt

We describe new compounds of stoichiometry M(CH2NMe2BH3)3 (M = Ti, Cr, and Co), each of which contains three chelating boranatodimethylaminomethyl (BDAM) ligands. In all three compounds, the BDAM anion, which is isoelectronic and isostructural with the neopentyl group, is bound to the metal center at one end by a metal-carbon σ bond and at the other by one three-center M-H-B interaction. The crystal structures show that the d1 titanium(III) compound is trigonal prismatic (or eight-coordinate, if two longer-ranged M···H interactions with the BH3 groups are included), whereas the d3 chromium(III) compound and the d6 cobalt(III) compounds are both fac-octahedral. The Cr and Co compounds exhibit two rapid dynamic processes in solution: exchange between the Δ and Λ enantiomers and exchange of the terminal and bridging hydrogen atoms on boron. For the Co complex, the barrier for Δ/Λ exchange (ΔG⧧298 = 10.1 kcal mol-1) is significantly smaller than those seen in other octahedral cobalt(III) compounds; DFT calculations suggest that Bailar twist and dissociative pathways for Δ/Λ exchange are both possible mechanisms. The UV-vis absorption spectra of the cobalt(III) and chromium(III) species show that the ligand field splittings Δo caused by the M-H-B interactions are unexpectedly large, thus placing them high on the spectrochemical series (near ammonia and alkyl groups); their nephelauxetic effect is also large. The DFT calculations suggest that these properties of M-H-B interactions are in part a consequence of their three-center nature, which delocalizes electron density away from the metal center and reduces electron-electron repulsions.

Reactivity of triphosphinoboranes towards H3B·SMe2: access to derivatives of boraphosphacycloalkanes with diverse substituents

Triphosphinoboranes activated the B-H bond in the BH₃ molecule without any catalysts at room temperature. Hydroboration reactions led to boraphosphacyloalkanes with diverse structures. The outcomes of reactions depend on the size of the phosphanyl substituent on the boron atom of the parent triphosphinoborane, where derivatives of boraphosphacyclobutane and boraphosphacyclohexane were obtained. Furthermore, the precursor of triphosphinoboranes, namely bromodiphosphinoborane, also exhibited high reactivity towards H₃B·SMe₂, yielding bromo-substituted boraphosphacyclobutane. The obtained products were characterized by heteronuclear NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis.

Cooperative B-H activation by Cp* based κ2-N,S-chelated Ru(II) and Mo(II) complexes (Cp* = η5-C5Me5)

The chemistry of the Cp* based κ²-N,S-chelated ruthenium complex, [Cp*RuPPh₃(κ²-N,S-(NC₇H₄S₂)], 1 with different boranes has been explored. The room temperature reaction of 1 with BH₃·THF and bulky boranes, such as MesBH₂ and H₂BAr^F, led to the formation of different dihydridoborate complexes, [{κ³-S,H,H-(NBH₂R)(S₂H₄C₇)}RuCp*], 2-4 (2: R = H, 3: R = Mes, and 4: R = Ar^F; Mes = 2,4,6-trimethylphenyl, and Ar^F = 3,5-bistrifluoromethyl-benzene). In contrast, the Cp* based κ²-N,S-chelated molybdenum complex, [Cp*Mo(CO)₂{κ²-N,S-(NC₇H₄S₂)}], 5, yielded the agostic borate species, [Cp*Mo(CO)₂{κ²-S,H-(NBH₂R) (NC₇H₄S₂)}], 6 and 7 (6: R = Mes and 7: R = Ar^F) at elevated temperatures.

Cooperative B-H bond activation: Dual sites borane activation by redox active κ2-N,S-chelated complexes

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR₃{κ²-N,S-(L)}₂Ru{κ¹-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC₇H₄S₂), generated from the aerial oxidation of borate complexes, [PR₃{κ²-N,S-(L)}Ru{κ³-H,S,S′-BH₂(L)₂}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC₇H₄S₂), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal–ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a–b with BH₃·THF led to the isolation of fac-[PR₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(S₂C₇H₄)₂}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ²-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB₂S]. Analogous fac-3c, [PPh₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(SC₅H₄)₂}], can also be synthesized from the reaction of BH₃·THF with [PPh₃{κ²-N,S-(SNC₅H₄)}{κ³-H,S,S′-BH₂(SNH₄C₅)₂}Ru], cis–fac-1c. In stark contrast, when mer-2b was treated with BH₂Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ³-S,H,H-(NH₂BMes)Ru(S₂C₇H₄)}₂], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)₂–B–NCS] ruthenacycles with κ²-H–H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru–N site is more facile than the Ru–S one.

Redox-active ruthenium complexes supported by hemilabile κ²-N,S-chelated ruthenacycles undergo unusual dual site B–H bond activation through metal–ligand cooperation with free and bulky boranes.