Effect of CO on the Oxidative Addition of Arene CH Bonds by Cationic Rhodium Complexes

Electron Density and Molecular Orbital Analyses of the Nature of Bonding in the η3-CCH Agostic Rhodium Complexes Preceding the C-C and C-H Bond Cleavages

In our recent work, we revisited C-H and C-C bond activation in rhodium (I) complexes of pincer ligands PCP, PCN, PCO, POCOP, and SCS. Our findings indicated that an η3-Csp2Csp3H agostic intermediate acts as a common precursor to both C-C and C-H bond activation in these systems. We explore the electronic structure and bonding nature of these precleavage complexes using electron density and molecular orbital analyses. Using NBO, IBO, and ESI-3D methods, the bonding in the η3-CCH agostic moiety is depicted by two three-center agostic bonds: Rh-Csp2-Csp3 and Rh-Csp3-H, with all three atoms datively bound to Rh(I). IBO analysis specifically highlights the involvement of three orbitals (CC→Rh and CH→Rh σ donation, plus Rh→CCH π backdonation) in both C-C and C-H bond cleavages. NCIPLOT and QTAIM analyses highlight anagostic (Rh-H) or β-agostic (Rh-Csp2-H) interactions and the absence of Rh-Csp3 interactions. QTAIM molecular graphs suggest bond path instability under dynamic conditions due to the nearness of line and ring critical points. Several low-frequency and low-force vibrational modes interconvert various bonding patterns, reinforcing the dynamic η3-CCH agostic nature. The kinetic preference for C-H bond breaking is attributed to the smaller reduced mass of C-H vibrations compared to C-C vibrations.

From Coordination to Noncoordination: Syntheses and Substitution Lability Studies of Titanium Triflato Complexes

A new concept for obtaining cationic complexes with triflate counteranions from coordinating triflato ligands was developed. Various routes are leading to titanium(IV) and titanium(III) triflato complexes efficiently. The reactions of pentafulvene titanium complexes with either triflic acid or silver triflate give the corresponding titanium(IV) triflato complexes in excellent yields. Hydrolysis of the titanium(IV) bistriflato complexes leads to cationic aqua complexes via displacement of the triflato ligand, which consequently acts as a noncoordinating anion. A functionalized titanium(IV) monotriflato complex was synthesized by insertion of a nitrile into the Ti-C bond and the triflato ligand was displaced by an NHC. While the titanium(IV) complexes are mostly inert toward substrates, the donor-free titanium(III) triflato complex is a strong Lewis acid and forms various adducts with monodentate Lewis bases. The titanium(III) complex was oxidized by reaction with TEMPO, resulting in a diamagnetic titanium(IV) complex. The reaction with bidentate ligands results in cationic titanium(III) complexes due to displacement of the triflato ligand by the bidentate ligands. Treatment with acetone leads to an aldol reaction of two acetone molecules and the formation of a cationic diacetone alcohol complex.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn₂(CO)₁₀] with the ligands (P(C-Br)P^CH₂-iPr) (1a) and (P(C-Br)P^O-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCP^CH₂-iPr)(CO)₃] (2a) and [Mn(-PCP^O-iPr)(CO)₃] (2b) were obtained. Protonation of 2a with HBF₄·Et₂O led to the formation of [Mn(κ³P,CH,P-P(CH)P^CH₂-iPr)(CO)₃]BF₄ (3) featuring an η²-C_aryl-H agostic bond. The solvothermal reaction of 1a with [Fe₂(CO)₉] afforded the Fe(II) PCP pincer complex [Fe(PCP^CH₂-iPr)(CO)₂Br] (4). Treatment of 4 with Li[HBEt₃] afforded the Fe(I) complex [Fe(PCP^CH₂-iPr)(CO)₂] (5a). When using the sterically more demanding ligands (P(C-Br)P^CH₂-tBu) (1c) and (P(C-Br)P^O-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn₂(CO)₁₀], the reaction with [Fe₂(CO)₉] and [Fe(CO)₅] led to the formation of the Fe(I) complexes [Fe(PCP^CH₂-tBu)(CO)₂] (5b) and [Fe(PCP^O-tBu)(CO)₂] (5c). X-ray structures of representative complexes are provided.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of M^I(CO) and M^IIICl₂(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Secondary Oxide Phosphines to Promote Tandem Acyl-Alkyl Coupling/Hydrogen Transfer to Afford (Hydroxyalkyl)rhodium Complexes. Theoretical and Experimental Studies

Acyl(σ-norbornenyl)rhodium(III) dimer [Rh(μ-Cl)(C₉H₆NCO)(C₇H₉)L]₂ (1) (C₇H₉ = σ-norbornenyl; L = 4-picoline, isoquinoline) reacts with diphenylphosphine oxide (SPO) to undergo a one-pot reaction involving (i) cleavage of the chloride bridges and coordination of the phosphine, (ii) C-C bond coupling between acyl and norbornenyl in a 18e species, and (iii) ligand-assisted outer-sphere O(P)-to-O(C) hydrogen transfer, to afford mononuclear 16e species [RhCl{(C₉H₆NC(O)C₇H₉)(Ph₂PO)H}(L)] (2) containing a quinolinyl-(norbornenylhydroxyalkyl) fragment hydrogen-bonded to a κ¹- P-phosphinite ligand. Pentacoordinated 2, which adopt a distorted trigonal bipyramidal structure, are kinetic reaction products that transform into the thermodynamic favored isomers 3. Structures 3 contain an unusual weak η¹-C anagostic interaction involving the rhodium atom and one carbon atom of the olefinic C-H bond of the norbornenyl substituent in the chelating quinolinyl-hydroxyalkyl moiety. Their structure can be described as pseudoctahedral, through a 5 + 1 coordination, with the anagostic interaction in a trans disposition with respect to the phosphorus atom of the phosphinite ligand. Complexes were characterized in solution by NMR spectroscopy and electrospray ionization mass spectrometry. Complex [RhCl{(C₉H₆NC(O)C₇H₉)(Ph₂PO)H}(4-picoline)] (3a) was also identified by X-ray diffraction. Density functional theory calculations confirm the proposed structures by a plausible set of mechanisms that accounts for the 1 (monomer) → 2 → 3 transformation. Lowest-energy pathways involve reductive elimination of quinolinylnorbornenylketone, still coordinated in the rhodium(I) species thus formed, followed by O-to-O hydrogen transfer from κ¹- P-SPO to the sp³ hybridized carbonyl group (formal alkoxide) avoiding the otherwise expected classical release of ketone. Theoretical ¹³C NMR studies also confirm the experimental spectral data for the considered structures.

A Cobalt(I) Pincer Complex with an η(2) -C(aryl)-H Agostic Bond: Facile C-H Bond Cleavage through Deprotonation, Radical Abstraction, and Oxidative Addition

The synthesis and reactivity of a Co(I) pincer complex [Co(ϰ(3) P,CH,P-P(CH)P(NMe) -iPr)(CO)2](+) featuring an η(2)-C(aryl)-H agostic bond is described. This complex was obtained by protonation of the Co(I) complex [Co(PCP(NMe) -iPr)(CO)2]. The Co(III) hydride complex [Co(PCP(NMe) -iPr)(CNtBu)2(H)](+) was obtained upon protonation of [Co(PCP(NMe) -iPr)(CNtBu)2]. Three ways to cleave the agostic C-H bond are presented. First, owing to the acidity of the agostic proton, treatment with pyridine results in facile deprotonation (C-H bond cleavage) and reformation of [Co(PCP(NMe) -iPr)(CO)2]. Second, C-H bond cleavage is achieved upon exposure of [Co(ϰ(3)P,CH,P-P(CH)P(NMe) -iPr)(CO)2](+) to oxygen or TEMPO to yield the paramagnetic Co(II) PCP complex [Co(PCP(NMe) -iPr)(CO)2](+). Finally, replacement of one CO ligand in [Co(ϰ(3) P,CH,P-P(CH)P(NMe) -iPr)(CO)2](+) by CNtBu promotes the rapid oxidative addition of the agostic η(2) -C(aryl)-H bond to give two isomeric hydride complexes of the type [Co(PCP(NMe) -iPr)(CNtBu)(CO)(H)](+).

Characterization of a palladium dihydrogen complex

The preparation and isolation of the first palladium dihydrogen complex is described. NMR spectroscopy reveals a very short H-H bond length, but the hydrogen molecule is activated toward heterolytic cleavage. An X-ray crystal structure suggests that proton transfer to the (tBu) PCP (κ(3)-2,6-((t)Bu2PCH2)2C6H3) pincer ligand is possible. The basicity of the ipso-carbon atom of the pincer ligand was investigated in a related complex.

Formation of a C-C double bond from two aliphatic carbons. Multiple C-H activations in an iridium pincer complex

The search for novel, atom-economic methods for the formation of C-C bonds is of crucial importance in synthetic chemistry. Especially attractive are reactions where C-C bonds are formed through C-H activation, but the coupling of unactivated, alkane-type Csp3 -H bonds remains an unsolved challenge. Here, we report iridium-mediated intramolecular coupling reactions involving up to four unactivated Csp3 -H bonds to give carbon-carbon double bonds under the extrusion of dihydrogen. The reaction described herein is completely reversible and the direction can be controlled by altering the reaction conditions. With a hydrogen acceptor present a C-C double bond is formed, while reacting under dihydrogen pressure leads to the reverse process, with some of the steps representing net Csp3 -Csp3 bond cleavage. Mechanistic investigations revealed a conceptually-novel overall reactivity pattern where insertion or deinsertion of an Ir carbene moiety, formed via double C-H activation, into an Ir-C bond is responsible for the key C-C bond formation and cleavage steps.

Xantphos-type complexes of group 9: rhodium versus iridium

Treatment of the dimer [Rh(μ-Cl)(C8H14)2]2 (1a) with 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene [xant(P(i)Pr2)2] leads to the d(8) square-planar complex RhCl{xant(P(i)Pr2)2} (2), whereas reaction of the iridium counterpart [Ir(μ-Cl)(C8H14)2]2 (1b) gives the d(6) octahedral compound IrHCl{xant(P(i)Pr2)[(i)PrPCH(Me)CH2]} (3) as a result of the intramolecular C-H bond activation of one of the isopropyl substituents of the phosphine. Stirring 2 and 3 in 0.5 N KOH solutions of 2-propanol gives rise to the formation of hydrides RhH{xant(P(i)Pr2)2} (4) and IrH3{xant(P(i)Pr2)2} (5), respectively. In n-octane at 60 °C, complex 2 is stable. However, compound 3 activates the alkane to give the cis-dihydride IrH2Cl{xant(P(i)Pr2)2} (6) and a mixture of 3- and 4-octene. Complex 6 can be also obtained by the reaction of 3 with H2. Under the same conditions, 2 affords the rhodium analogue RhH2Cl{xant(P(i)Pr2)2} (7). Compounds 2-4 react with triflic acid (HOTf) to give RhHCl(OTf){xant(P(i)Pr2)2} (8), IrHCl(OTf){xant(P(i)Pr2)2} (9), and RhH2(OTf){xant(P(i)Pr2)2} (10), respectively. The related iridium derivative IrH2(OTf){xant(P(i)Pr2)2} (11) has also been prepared by the reaction of 6 with Tl(OTf). Complexes 2, 6, and 9 have been characterized by X-ray diffraction analysis. The {xant(P(i)Pr2)2}M skeleton is T-shaped with the metal center situated in the common vertex.