Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

B-substituted group 1 phosphides: synthesis and reactivity

1-Boryl-8-phosphinonaphthalenes 1-BCy2-8-PCl2-C10H6 (1) and 1-BCy2-8-PPhCl-C10H6 (2) were prepared and used as starting materials for the synthesis of B-substituted phosphides. The reduction of 1 and 2 by Mg provided neutral compounds [1-BCy-8-PCy-C10H6]2 (3) and [1-BCy2-8-PPh-C10H6]2 (4). Compound 3 represents the dimer of phosphinoborane 1-BCy-8-PCy-C10H6 while complex 4 is a rare example of a discrete B ← P coordinated diphosphine. The reduction of 2 by Na or K in THF yielded B-substituted group 1 phosphides [Na(THF)3]+[1-BCy2-8-PPh-C10H6]- (5) and {[K(THF)2]+[1-BCy2-8-PPh-C10H6]-}∞ (6), which structurally resembled bulky group 1 phosphides. Complex 5 showed easy activation of elemental chalcogens E (E = O, S, Se) to give B-substituted chalcogenophosphinites {[Na(THF)2]+[1-BCy2-8-P(E)Ph-C10H6]}2 (E = O (7), S (8), Se (9)) as the products of chalcogen insertion into the P-Na bond. Importantly no oxidation to dichalcogenophosphinates was observed. Compound 5 is tolerant of the CO polar bonds in organic substrates and the reactions of 5 with 2,3-butanedione or an acyl chloride provided {[Na(THF)2]+[1-BCy2-8-P{CHC(O)C(Me)O}Ph-C10H6]-}2 (10) and [1-BCy2-8-P{C(O)tBu}Ph-C10H6] (11). Finally, B-coordinated phosphatetrylenes [1-BCy2-8-P(SnL)Ph-C10H6] (12) and [1-BCy2-8-P(PbL)Ph-C10H6] (13) (L is {2,6-(Me2NCH2)C6H3}-) were also prepared by substitution reactions of 5.

Exploring the Reactivity of B‐Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH₂ and tBuPH₂ react with 9‐iodo‐m‐carborane yielding B9‐connected secondary carboranylphosphines 1,7‐H₂C₂B₁₀H₉‐9‐PHR (R=2,4,6‐Me₃C₆H₂ (Mes; 1  a), tBu (1  b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1  a, b resulted in the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BF(C₆F₅)₂ (2  a, b) through nucleophilic para substitution of a C₆F₅ ring followed by fluoride transfer to boron. Further reaction with Me₂SiHCl prompted a H−F exchange yielding the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BH(C₆F₅)₂ (3  a, b). The reaction of 2  a, b with one equivalent of R'MgBr (R’=Me, Ph) gave the extremely water‐sensitive frustrated Lewis pairs 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)B(C₆F₅)₂ (4  a, b). Hydrolysis of the B−C₆F₄ bond in 4  a, b gave the first tertiary B‐carboranyl phosphines with three distinct substituents, 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄H) (5  a, b). Deprotonation of the zwitterionic compounds 2  a, b and 3  a, b formed anionic phosphines [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BX(C₆F₅)₂]⁻[DMSOH]⁺ (R=Mes, X=F (6  a), R=tBu, X=F (6  b); R=Mes, X=H (7  a), R=tBu, X=H (7  b)). Reaction of 2  a, b with an excess of Grignard reagents resulted in the addition of R’ at the boron atom yielding the anions [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BR’(C₆F₅)₂]⁻ (R=Mes, R’=Me (8  a), R=tBu, R’=Me (8  b); R=Mes, R’=Ph (9  a), R=tBu, R’=Ph (9  b)) with [MgBr(Et₂O)_n]⁺ as counterion. The ability of the zwitterionic compounds 3  a, b to hydrogenate imines as well as the Brønsted acidity of 3  a were investigated.

Synthesis of a carborane-substituted bis(phosphanido) cobaltate(i), ligand substitution, and unusual P4 fragmentation

Oxidative addition of the P-P single bond of an ortho-carborane-derived 1,2-diphosphetane (1,2-C2(PMes)2B10H10) (Mes = 2,4,6-Me3C6H2) to cobalt(-i) and nickel(0) sources affords the first heteroleptic complexes of a carborane-bridged bis(phosphanido) ligand. The complexes also incorporate labile ligands suitable for further functionalisation. Thus, the cobalt(i) complex [K([18]crown-6)][Co{1,2-(PMes)2C2B10H10}(cod)] (cod = 1,5-cyclooctadiene) bearing a labile cyclooctadiene ligand undergoes facile ligand exchange reactions with isonitriles and tert-butyl phosphaalkyne with retention of the bis(phosphanido) ligand. However, in the reaction with one equivalent of P4, the electron-rich bis(phosphanido) moiety abstracts a single phosphorus atom with formation of a new P3 chain, while the remaining three P atoms derived from P4 form an η3-coordinating cyclo-P3 ligand. In contrast, when the same reaction is performed with two equivalents of the cobalt(i) complex, a dinuclear product is formed which features an unusual P4 chain in its molecular structure.

Accessing the First nido-Carborane-Substituted Diphosphetane: A Ligand and Synthon for nido-Carboranylphosphanes

Deboronation of a carborane-substituted diphosphetane 2 in toluene yielded the first nido-carboranyldiphosphetane 1. The P-P bond in 1 can be broken via dismutation reactions with diaryl dichalcogenides yielding nido-carboranyl bis-phosphanes that were not accessible via established synthetic protocols. Additionally, transition metal complexes of 1 could be isolated including one coordination polymer. Notably, when the deboronation of 2 is carried out in ethanol, unprecedented nido-carborane-substituted secondary bis-phosphane monoxides (3, 4) are obtained. These compounds are interesting starting materials for further reactivity studies due to their P-H bonds. Experimental findings are supported by DFT calculations including the calculation of reaction mechanisms and NMR spectroscopic parameters.

Molecular doping: accessing the first carborane-substituted 1,2,3-triphospholanide via insertion of P− into a P−P bond

Insertion of a P⁻ anion into a P–P bond yielding the first carborane-substituted 1,2,3-triphospholanide 1 was achieved by treating a carborane-substitued 1,2-diphosphetane with sodium phosphaethynolate.

Exploiting the Ring Strain of Diphosphetanes: A Synthetic and Computational Approach towards 1,2,5-Selenadiphospholanes

A carboranyl-based meso-1,2,5-selenadiphospholane diselenide was synthesised starting from a strained carborane-substituted 1,2-diphosphetane and subsequently reduced to an unprecedented carboranyl-based meso-1,2,5-selenadiphospholane. The electronic structure and the bonding situation for both compounds were investigated by density functional theory (DFT), Natural Bond Orbital (NBO) analyses, Fractional Occupation Density (FOD) analysis, Complete Active Space Self Consistent Field (CASSCF) calculations and time-dependent DFT (TDDFT) calculations. Ring-opening reactions of meso-1,2,5-selenadiphospholane with nucleophiles and electrophiles are reported together with calculated reaction mechanisms (DFT level). Isolated compounds were characterised by NMR and IR spectroscopy, high-resolution mass spectrometry, elemental analysis and single-crystal X-ray diffraction.

12-Vertex Zwitterionic Bis-phosphonium-nido-carborates through Ring-Opening Reactions of 1,2-Diphosphetanes

Carborane-substituted 1,2-diphosphetanes (Ia,b) react with elemental lithium in THF with cleavage of the P-P bond to give a deep red solution from which, in the case of Ia, red crystals of a lithiated intermediate, [{1-Li(THF)PtBu-6-PtBu-4,1,6-closo-Li(THF)C₂ B₁₀ H₁₀ }{Li(THF)₃ }]₂ ⋅2 THF (2 a), are obtained. The compound is dimeric, C₂ -symmetric and contains six lithium and four phosphorus atoms. Two lithium atoms cap the six-membered C₂ B₄ faces, resulting in two 13-vertex closo-clusters (according to Wade's rules) with docosahedral geometry. The addition of methyl iodide resulted in the formation of zwitterionic bis-phosphonium-nido-carborates 7,10-bis(tert-butyldimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 a) and 7,10-bis(N,N-diisopropylaminodimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 b) in moderate to good yields. Compounds 1 a and 1 b exhibit short C_cluster -P bonds and large C_cluster ⋅⋅⋅C_cluster distances in the solid state. Further insight into the ring opening and reduction potential of the alkyl halide was obtained from methylation reactions of different 1,2-bis-phosphinocarboranes. The reaction of rac-/meso-1,2-bis(tert-butylmethylphosphino)-1,2-dicarba-closo-dodecaborane(12) (3 a) with two equivalents of methyl iodide also resulted in the formation of 1 a (as shown by NMR spectroscopy), whereas the reaction of 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane(12) with methyl triflate afforded the phosphonium salt 1-methyl-diphenylphosphonium-2-diphenylphosphino-1,2-dicarba-closo-dodecaborane(12) triflate (4) without reduction of the cluster.

Various types of non-covalent interactions contributing towards crystal packing of halogenated diphospha-dicarbaborane with an open pentagonal belt

We have synthesized and crystalized 3-Cl-10-I-nido-7,8,9,11-P₂C₂B₇H₇. Quantum chemical calculations have demonstrated that the obtained crystal structure is stabilized by hydrogen, dihydrogen and pnictogen bonds.

C2-Symmetric P,N Ligands Derived from Carborane-Based Diphosphetanes: Synthesis and Coordination Chemistry

Racemic carborane-based bisphosphanes were obtained by dismutation reactions between a carborane-based diphosphetane and diaryl dichalogenides. NMR spectroscopic and theoretical studies revealed a two-step mechanism explaining the high stereoselectivity of these reactions. The coordination chemistry of the multidentate P,N ligands 6c and 6d in copper(I) and silver(I) complexes was studied. While 6d acted exclusively as tetradentate ligand, 6c showed either tridentate or tetradentate coordination depending on the metal and the counterion.