Protonolysis of an α-Hydroxyl Ligand for the Generation of a PCcarbeneP Pincer Complex and Subsequent Reactivity Studies

A Metallabicycle From Thiocarbonyl-Cyclopropenium Coupling

Addition of triphenylcyclopropenium bromide to the thiocarbonyl complex [RhCl(CS)(PPh3 )2 ] affords novel bicyclic metalla-3-mercapto-thiapyrylliums [Rh(κ2 -C,S-C5 S2 Ph3 )(PPh3 )2 X2 ] (X=Cl, Br) - heterocycles with no metal-free isolobal precedent. Halide abstraction with silver triflate (AgOTf) in acetonitrile affords the salt [Rh(κ2 -C,S-C5 S2 Ph3 )(NCMe)2 (PPh3 )2 {Ag(OH2 )2 }{Ag(OTf)3 }]-OTf which in turn reacts with sodium chloride to return [Rh(κ2 -C,S-C5 S2 Ph3 )(PPh3 )2 Cl2 ].

Tuning Ruthenium Carbene Complexes for Selective P-H Activation through Metal-Ligand Cooperation

The use of iminophosphoryl‐tethered ruthenium carbene complexes to activate secondary phosphine P−H bonds is reported. Complexes of type [(p‐cymene)‐RuC(SO₂Ph)(PPh₂NR)] (with R = SiMe₃ or 4‐C₆H₄−NO₂) were found to exhibit different reactivities depending on the electronics of the applied phosphine and the substituent at the iminophosphoryl moiety. Hence, the electron‐rich silyl‐substituted complex undergoes cyclometallation or shift of the imine moiety after cooperative activation of the P−H bond across the M=C linkage, depending on the electronics of the applied phosphine. Deuteration experiments and computational studies proved that cyclometallation is initiated by the activation process at the M=C bond and triggered by the high electron density at the metal in the phosphido intermediates. Consistently, replacement of the trimethylsilyl (TMS) group by the electron‐withdrawing 4‐nitrophenyl substituent allowed the selective cooperative P−H activation to form stable activation products.

Access to and Reactivity of Fe0 , Fe-I , FeI , and FeII PCcarbene P Pincer Complexes

Despite their promising metal-ligand cooperative reactivity, PC_carbene P pincer ligands are rarely reported for first-row transition-metal centres. Using a dehydration methodology, we report access to an Fe⁰ PC_carbene P pincer complex (1) that proceeds via an isolated α-hydroxylalkyl hydrido complex (3). Reversible carbonyl migration to the carbene position in 1 is found to allow coordination chemistry and E-H bond addition (E=H, B, Cl) across the iron-carbene linkage, representing a unique mechanism for metal-ligand cooperativity. The PC_carbene P pincer ligand is also found to stabilize formal Fe^II , Fe^I , and Fe^-I oxidation states, as demonstrated with synthesis and characterization of the complexes [11-X][BAr^F ₂₀ ] (X=Br, I), 12, and K[13]. Compound K[13] is found to be highly reactive, and abstracts hydrogen from a range of aliphatic C-H sources. Computational analysis by DFT suggests that the formal Fe^I and Fe^-I complexes contain significant carbene radical character. The ability of the PC_carbene P ligand scaffold to partake in metal-ligand cooperativity and to support a range of iron oxidation states renders it as potentially useful in many catalytic applications.

Decarbonylative ether dissection by iridium pincer complexes

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C-C bond, one C-O bond, and two C-H bonds, along with formation of two new C-H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups. Intermolecular decarbonylation of cyclic and linear ethers was observed when diethylaminophenylphosphinite iridium(i) dinitrogen or norbornene complexes were employed. Mechanistic studies reveal the nature of key intermediates along a pathway involving initial iridium(i)-mediated double C-H bond activation.

H/D exchange under mild conditions in arenes and unactivated alkanes with C6D6 and D2O using rigid, electron-rich iridium PCP pincer complexes

The synthesis and characterization of an iridium polyhydride complex (Ir-H4) supported by an electron-rich PCP framework is described. This complex readily loses molecular hydrogen allowing for rapid room temperature hydrogen isotope exchange (HIE) at the hydridic positions and the α-C-H site of the ligand with deuterated solvents such as benzene-d6, toluene-d8 and THF-d8. The removal of 1-2 equivalents of molecular H2 forms unsaturated iridium carbene trihydride (Ir-H3) or monohydride (Ir-H) compounds that are able to create further unsaturation by reversibly transferring a hydride to the ligand carbene carbon. These species are highly active hydrogen isotope exchange (HIE) catalysts using C6D6 or D2O as deuterium sources for the deuteration of a variety of substrates. By modifying conditions to influence the Ir-Hn speciation, deuteration levels can range from near exhaustive to selective only for sterically accessible sites. Preparative level deuterations of select substrates were performed allowing for procurement of >95% deuterated compounds in excellent isolated yields; the catalyst can be regenerated by treatment of residues with H2 and is still active for further reactions.

Dimetalla-heterocyclic carbenes: the interconversion of chalcocarbonyl and carbido ligands

Different classes of dirhodium μ-carbido complexes cleave CS₂ to afford mono- and bi-nuclear CS complexes, the CSe analogues of which are also described.

Ln(ii) and Ca(ii) NCsp3N pincer type diarylmethanido complexes – promising catalysts for C–C and C–E (E = Si, P, N, S) bond formation

The first examples of Ln(ii) (Ln = Yb, Sm) and Ca [NC_sp3N] pincer type diarylmethanido complexes were synthesized and successfully used as efficient and selective precatalyst for intermolecular C–C and C–E bond formation.

Halogenation of A-frame μ-carbido complexes: a diamagnetic rhodium(ii) carbido complex

Chlorination of the new μ-carbido [Rh₂(μ-C)Cl₂(μ-dppf)₂] (dppf = 1,1′-bis(diphenylphosphino)ferrocene) affords the dirhodium(ii) complex [Rh₂(μ-C)Cl₄(μ-dppf)₂] the carbido bridge of which can only be adequately described by delocalised bonding.

(Diphenylphosphino)alkylaldehyde affords hydride- or alkyl-[(diphenylphosphino)alkylacyl]rhodium(iii) or (diphenylphosphino)alkylester complexes: theoretical and experimental diastereoselectivity

The reaction of [RhCl(COD)]2 (COD = 1,5-cyclooctadiene) with racemic PPh2(CH(Ph)CH2CHO) and pyridine (py) led to the oxidative addition of the aldehyde, and a single geometric isomer of [RhHCl(PPh2(CH(Ph)CH2CO))(py)2] (1), with hydride trans to chloride, was obtained as a mixture of two diastereomers in a 95 : 5 ratio; this was in agreement with density functional theory (DFT) calculations. In a chloroform solution, the exchange of hydride by chloride yielded [RhCl2(PPh2(CH(Ph)CH2CO))(py)2] (2) as a mixture of a kinetically preferred species, trans-py-2a, and two diastereomers, cis-Cl-2b' and cis-Cl-2b, with cis pyridines and a chloride trans to acyl; as predicted by the DFT calculations, the latter was the major species. Complex 1 reacted with racemic PPh2(CH(Ph)CH2CHO) or PPh2(o-C6H4CHO) to afford [RhHCl(PPh2(CH(Ph)CH2CO))(κ1-PPh2(CH(Ph)CH2CHO))(py)] (3) or [RhHCl(PPh2(o-C6H4CO))(κ1-PPh2(CH(Ph)CH2CHO))(py)] (4), respectively, both with a dangling alkylaldehyde. Diastereomeric mixtures with the ratios 3a/3a' = 80 : 20 and 4a/4a' = 50 : 50 were obtained. Complex 4 reacted with N-donors to afford cationic [RhH(NN)(PPh2(o-C6H4CO))(κ1-PPh2(CH(Ph)CH2CHO))]BPh4 (NN = 1,10-phenanthroline, 5; 2,2'-bipyridine, 6) or with 8-aminoquinoline (aqui) or 2-(aminomethyl)pyridine to yield imination products with terdentate ligands: [RhH(PPh2(o-C6H4CO))(κ3-PNN)]BF4 (PNN = PPh2(CH(Ph)CH2CNC9H6N), 7 and PPh2(CH(Ph)CH2CNCH2C5H4N), 8, respectively. Compounds 5-8 were obtained as equimolar a/a' mixtures of diastereomers. Moreover, 5a and 5a' could be separated. [RhCl(NBD)]2 reacted with racemic PPh2(CH(Ph)CH2CHO) and N-donors to provide nortricyclyl (Ntyl) derivatives [RhCl(NN)(Ntyl)(PPh2CH(Ph)CH2CO)] (NN = phen, 9 and bipy, 10) as an a/a' = 75 : 25 mixture of diastereomers. By reacting [RhCl(NBD)]2 with PPh2(CH(Ph)CH2CHO) and quinoline-8-carbaldehyde in methanol, the phosphino-ester complex [RhCl(Ntyl)(C9H6NCO)(κ2-PPh2CH(Ph)CH2CO(OCH3)] 11 was obtained. The initial equimolar mixture of two diastereomers readily transformed into a single diastereomer, which was found to be thermodynamically most stable by the DFT calculations. Furthermore, single crystal X-ray diffraction analysis of cis-Cl-2b, 5a, 7a, 10a and 11 is reported.

Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine-Benzophenone Ligand in Alkyne Cyclotrimerization

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2'-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [( p-tolL1)Ni(BPI)] ( p-tolL1 = 2,2'-bis(di(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C=O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Direct oxide transfer from an η2-keto ligand to generate a cobalt PCcarbeneP(O) pincer complex

We report the direct carbonyl cleavage in a κ³-P′,(η²-C,O),P′′ ligand by a monomeric cobalt centre through metal–ligand cooperativity.

Synthesis and reactivity of a PCcarbeneP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N)

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand.

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand. Complex 1 shares bonding similarities with cobalt PBP and PNP pincer complexes where the donor atom engages in π-bonding with the cobalt centre, and thus completes the PXP (X = B, C, N) pincer ligand series for cobalt (for X donors that partake in M–L π-bonding). As compared to PBP and PNP pincer complexes, which are known to be good hydride and proton acceptors (respectively), complex 1 is found to be an effective hydrogen atom acceptor. Complex 1 partakes in cooperative ligand reactivity, engaging in several small molecule activations with styrene, bromine, carbon disulphide, phenyl acetylene, acetonitrile, hydrogen, benzaldehyde and water (through microreversibility). The mechanism for the formation of complex 1 is studied through the isolation and computational analysis of key intermediates. The formation of 1 is found to avoid C–H activation of the proligand, and instead proceeds through a combination of O–H activation, hydrogen atom transfer, β-hydride elimination and hydrogen activation processes.

Redox-state dependent activation of silanes and ammonia with reverse polarity (PCcarbeneP)Ni complexes: electrophilic vs. nucleophilic carbenes

A rigidified PC_alkylP ligand allowed for the synthesis and characterization of cationic and radical PC_CarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Facile generation of iridium PCcarbeneP pincer complexes via water elimination from an alcohol proligand

We report the facile generation of Ir PC_carbeneP pincer systems.