Exploiting chemical cooperativity in main-group bimetallic catalysis

Hydrophosphanylation of Alkynes via Magnesium Complexes: Evidence for Ligand Dependency in Structure-Activity Relationships

Pursuing practical, straightforward, and sustainable methods for forming carbon-phosphorus bonds is crucial in academia and industry. In this study, we showed that bis(diiminate)-based magnesium complexes [L(Mg-nBu)2] (nBu = n-butyl) could effectively catalyze the hydrophosphanylation of alkynes, resulting in monophosphanylated vinyledene- and 1,2-diphosphanylated alkanes in a stepwise manner. This transformation showcases an excellent atom economy, broad functional group tolerance, and gram-scale synthesis for organophosphorus compounds. Through controlled experiments and with the support of DFT calculations, we elucidated the reaction mechanism, identifying the active catalytic species and revealing a stepwise hydrophosphanylation process of alkynes. Although complex Mg-1 showed its potential in this transformation, complexes Mg-2 and Mg-3, having ethyl and phenyl spacers, produced a lower yield of hydrophosphanylated products, indicating the role of ligand (spacer) in this catalytic reaction. Further, the activity of Mg-1 was compared with a monomeric magnesium complex, Mg-4, and it was found that the performance of the Mg-4 in alkyne hydrophosphanylation is quite lower than the results obtained by using Mg-1. This work demonstrated that a dimeric magnesium complex with a suitable spacer can enhance the catalytic activity manyfolds in the hydrophosphanylation of alkynes.

Room-Temperature Intermolecular Hydroamination of Vinylarenes Catalyzed by Alkali-Metal Ferrate Complexes

Alkene hydroamination of multiple bonds represents a valuable and atom-economical approach to accessing amines, using simple and widely available starting materials. This reaction requires a metal catalyst, and despite the success of noble transition metals, s-block, or f-block elements, iron organometallic complexes have found limited applications. Partnering iron with an alkali metal and switching on bimetallic cooperativity, we report the synthesis and characterization of a series of highly reactive alkali-metal alkyl ferrate complexes, which can deprotonate amines and activate them toward the catalytic hydroamination of vinylarenes. An alkali-metal effect has been observed, with the sodium analogue being the best for an efficient hydroamination of different styrene derivatives and amines. Stoichiometric studies on the reaction of the sodium tris(alkyl) ferrate complex with 3 mol equiv of piperidine evidenced the ability of the three alkyl groups on Fe to undergo amine metalation, furnishing a novel tris(amido) sodium ferrate which is postulated as a key intermediate in these catalytic transformations. The enhanced reactivity of these alkali-metal ferrates contrasts sharply with that of the Fe(II) bis(alkyl) precursor which is completely inert toward alkene hydroamination.

Multielectron Reduction of Nitrosoarene via Aluminylene-Silylene Cooperation

The cooperative effects of main-group elements pave the way for novel chemical transformations. However, the potential of bimetallic complexes featuring the most abundant aluminum and silicon elements remains largely unexplored. In this study, we present the synthesis and characterization of bis(silylene)-stabilized aluminylene 2. The cooperation between aluminylene and silylene allows for the facile cleavage of the N-O bond in nitrosoarenes, producing an aluminum imide complex 4 and tetracyclic oxazasilaalanes 5 and 6, and also promotes the dearomatization of 2-methylquinoline, yielding a silylalane 7. In addition, 2 is an effective precatalyst for the reductive coupling of nitrosoarenes to azoxyarenes. These results outline an approach for orchestrating aluminum and silicon cooperation to facilitate chemical bond activation.

Accessing Homo- and Heterobimetallic Complexes with a Dianionic Pentadentate Ligand

The tetrapyrazolylpyridyl diborate (B2Pz4Py) ligand provides a suitable platform for the isolation of heterobimetallic main-group element compounds as well as homotetrametallic copper complexes. The heterobimetallic tin(II)-lithium(I) (1) and tin(II)-thallium(I) (2) complexes have been synthesized, isolated, and fully characterized including single-crystal X-ray diffraction analysis. When reacted with copper(I) sources, complex 2 grants access to a homotetrametallic copper(I) complex (4). Upon subsequent oxidation, 4 gives rise to the bimetallic copper(II) complex 5, in which the two copper(II) centers are connected via a bridging bromido ligand (CuII-μ-Br-CuII).

Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

Synthesis and reactivity of alkali metal aluminates bearing bis(organoamido)phosphane ligand

In this study, we report a group of alkali metal aluminates bearing bis(organoamido)phosphane ligand. The starting complex {[PhP(NtBu)2]AlMe2}Li·OEt2 (1) was prepared by stepwise deprotonation of the parent PhP(NHtBu)2 by nBuLi and AlMe3. Further derivatization of aluminate 1 was performed by the virtual substitution of lithium -{[PhP(NtBu)2]AlMe2}K (2), methyl substituents - {[PhP(NtBu)2]AlH2}Li·THF (3), modification of steric bulk and induction effects on the phosphorus atom - {[tBuP(N-2,6-iPr2C6H3)2]AlMe2}Li·(OEt2)2 (4), and phosphorus atom oxidation state {[Ph(Y)P(NtBu)2]AlMe2}Li (Y = O (5), S (6), Se (7), Te (8)). The structure causing non-covalent interactions in 1-4 were evaluated with the help of theoretical calculations and topological analysis ranging from π-electron system-metal to agostic interactions of various types. The further reactions of 1 with various nucleophiles were found to be a versatile tool for the preparation of iminophosphonamides via the formation of P-E bond (E = Si, Ge, Sn, Pb, P, and C) and followed by P(III) → P(V) tautomeric shift.

Polarized Au(I)/Rh(I) bimetallic pairs cooperatively trigger ligand non-innocence and bond activation

The combination of molecular metallic fragments of contrasting Lewis character offers many possibilities for cooperative bond activation and for the disclosure of unusual reactivity. Here we provide a systematic investigation on the partnership of Lewis basic Rh(I) compounds of type [(η5-L)Rh(PR3)2] (η5-L = (C5Me5)- or (C9H7)-) with highly congested Lewis acidic Au(I) species. For the cyclopentadienyl Rh(I) compounds, we demonstrate the non-innocent role of the typically robust (C5Me5)- ligand through migration of a hydride to the Rh site and provide evidence for the direct implication of the gold fragment in this unusual bimetallic ligand activation event. This process competes with the formation of dinuclear Lewis adducts defined by a dative Rh → Au bond, with selectivity being under kinetic control and tunable by modifying the stereoelectronic and chelating properties of the phosphine ligands bound to the two metals. We provide a thorough computational study on the unusual Cp* non-innocent behavior and the divergent bimetallic pathways observed. The cooperative FLP-type reactivity of all bimetallic pairs has been investigated and computationally examined for the case of N-H bond activation in ammonia.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Hydrocarbon Soluble Alkali-Metal-Aluminium Hydride Surrog[ATES]

A series of group 1 hydrocarbon-soluble donor free aluminates [AM(t BuDHP)(TMP)Al(i Bu)2 ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-t BuC5 H5 N)AM)] containing a surrogate hydride (sp3 C-H) with [(i Bu)2 Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li(t BuDHP)(TMP)Al(i Bu)2 ] and [(TMEDA)Na(t BuDHP)(TMP)Al(i Bu)2 ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog[ATES].

Heavy Alkali Metal Manganate Complexes: Synthesis, Structures and Solvent-Induced Dissociation Effects

Rare examples of heavier alkali metal manganates [{(AM)Mn(CH2 SiMe3 )(N'Ar )2 }∞ ] (AM=K, Rb, or Cs) [N'Ar =N(SiMe3 )(Dipp), where Dipp=2,6-iPr2 -C6 H3 ] have been synthesised with the Rb and Cs examples crystallographically characterised. These heaviest manganates crystallise as polymeric zig-zag chains propagated by AM⋅⋅⋅π-arene interactions. Key to their preparation is to avoid Lewis base donor solvents. In contrast, using multidentate nitrogen donors encourages ligand scrambling leading to redistribution of these bimetallic manganate compounds into their corresponding homometallic species as witnessed for the complete Li - Cs series. Adding to the few known crystallographically characterised unsolvated and solvated rubidium and caesium s-block metal amides, six new derivatives ([{AM(N'Ar )}∞ ], [{AM(N'Ar )⋅TMEDA}∞ ], and [{AM(N'Ar )⋅PMDETA}∞ ] where AM=Rb or Cs) have been structurally authenticated. Utilising monodentate diethyl ether as a donor, it was also possible to isolate and crystallographically characterise sodium manganate [(Et2 O)2 Na(n Bu)Mn[(N'Ar )2 ], a monomeric, dinuclear structure prevented from aggregating by two blocking ether ligands bound to sodium.

Chemistry of the Bis(imine)-Based Tetradentate Ligand Stabilized Group 14 E(II) Cations (E=Ge and Sn)

The ambiphilic Ge(II) and Sn(II) cationic species have been reported to be isolated through kinetic or thermodynamic stabilizations. Nonetheless, steric congestion or excessive coordination of donor atoms to the cationic center concurrently disfavors its prompt reactivity. Our research in this field revolves around the utilization of structurally non-rigid bis(imine) based tetradentate supporting ligands for the stabilization of Ge(II) and Sn(II) cationic species. Such E(II) cationic systems have been advantaged due to inherent flexibility present at the ligand backbone allowing disposal of E(II) orbitals through geometric rearrangements for further reactivity. The bifunctionality present in the ligand enables the first examples of Ge(II) bis-monocations. Furthermore, the redox-active nature of the ligand encourages participation in chemical transformations. In this personal account we have provided a detailed discussion of our published work in this direction in the last five years.