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.
Despite the accessibility of numerous transition metal polyphosphido complexes through transition‐metal‐mediated activation of white phosphorus, the targeted functionalization of Pn ligands to obtain ...functional monophosphorus species remains challenging. In this study, we introduce a new 3+1 fragmentation procedure for cyclo‐P4 ligands, leading to the discovery of acylcyanophosphanides and ‐phosphines. Treatment of the complex K(18c‐6)(Ar*BIAN)Co(η4‐P4) (K(18c‐6)3, 18c‐6=18crown‐6, Ar*=2,6‐dibenzhydryl‐4‐isopropylphenyl, BIAN=1,2‐bis(arylimino)acenaphthene diimine) with acyl chlorides results in the formation of acylated tetraphosphido complexes (Ar*BIAN)Co(η4‐P4C(O)R) (R=tBu, Cy, 1‐Ad, Ph; 4 a–d). Subsequent reactions of 4 a–d with cyanide salts yield acylated cyanophosphanides RC(O)PCN− (9 a–d−) and the cyclo‐P3 cobaltate anion (Ar*BIAN)Co(η3‐P3)(CN)− (8−). Further reactions of 4 a–d with trimethylsilyl cyanide (Me3SiCN) and isocyanides provide insight into a plausible mechanism of this 3+1 fragmentation reaction, as these reagents partially displace the P4C(O)R ligand from the cobalt center. Several potential intermediates of the 3+1 fragmentation were characterized. Additionally, the introduction of a second acyl substituent was achieved by treating K(18c‐6)9b with CyC(O)Cl, resulting in the first bis(acyl)monocyanophosphine (CyC(O))2PCN (10).
White phosphorus (P4) can be selectively activated and functionalized via low‐valent cobalt complexes. Subsequent treatment of the acylated P4 ligands with cyanide induces a rare 3+1 fragmentation and yields cyanophosphanides (P1) and triphosphido cobaltate (P3). Additional experiments with neutral cyanide or isocyanide provide insight into a plausible fragmentation mechanism and allow the isolation of potential intermediates.
Although numerous polyphosphido complexes have been accessed through the transition-metal-mediated activation and functionalization of white phosphorus (P
), the selective functionalization of the ...resulting polyphosphorus ligands in these compounds remains underdeveloped. In this study, we explore the reactions between cyclotetraphosphido cobalt complexes and heterocumulenes, leading to functionalized P
ligands. Specifically, the reaction of carbon disulfide (CS
) with K(18c-6)(Ar*BIAN)Co(η
-P
) (K(18c-6)
, 18c-6 = 18crown-6) affords the adduct K(18c-6)(Ar*BIAN)Co(η
:η
-P
CS
) (K(18c-6)
), in which CS
is attached to a single phosphorus atom (Ar* = 2,6-dibenzhydryl-4-isopropylphenyl, BIAN = 1,2-bis(arylimino)acenaphthene diimine). In contrast, the insertion of bis(trimethylsilyl)sulfur diimide S(NSiMe
)
into a P-P bond of K(18c-6)
yields K(18c-6)(Ar*BIAN)Co(η
:η
-P
SN
(SiMe
)
) (K(18c-6)
). This salt further reacts with Me
SiCl to form (Ar*BIAN)Co(η
:η
-P
SN
(SiMe
)
(
), featuring a rare azatetraphosphole ligand. Moreover, treatment of the previously reported complex (Ar*BIAN)Co(η
:η
-P
C(O)
Bu) (
) with isothiocyanates results in P-C bond insertion, yielding (Ar*BIAN)Co(η
:η
-P
C(S)N(R)C(O)
Bu) (
,
; R = Cy, Ph).
We show that countercations exert a remarkable influence on the ability of anionic cobaltate salts to catalyze challenging alkene hydrogenations. An evaluation of the catalytic properties of ...CatCo(η4‐cod)2 (Cat=K (1), Na (2), Li (3), (Depnacnac)Mg (4), and N(nBu)4 (5); cod=1,5‐cyclooctadiene, Depnacnac={2,6‐Et2C6H3NC(CH3)}2CH)) demonstrated that the lithium salt 3 and magnesium salt 4 drastically outperform the other catalysts. Complex 4 was the most active catalyst, which readily promotes the hydrogenation of highly congested alkenes under mild conditions. A plausible catalytic mechanism is proposed based on density functional theory (DFT) investigations. Furthermore, combined molecular dynamics (MD) simulation and DFT studies were used to examine the turnover‐limiting migratory insertion step. The results of these studies suggest an active co‐catalytic role of the counterion in the hydrogenation reaction through the coordination to cobalt hydride intermediates.
The counterion makes a difference in alkene hydrogenations promoted by low‐valence cobaltate salts. The best performing catalyst cleanly hydrogenated various alkenes (including tri‐ and tetra‐substituted) under mild conditions. Mechanistic studies point toward a homotopic catalyst and show how counterion coordination to hydride intermediates modulates the barrier for the turnover‐limiting migratory insertion.
σ‐Bond metathesis reactions between (6‐Dipp)CuOtBu (6‐Dipp=:C({Dipp}NCH2)2CH2, Dipp=2,6‐iPr2−C6H3) and three diboranes gave access to three new copper(I) boryl complexes (6‐Dipp)CuBcat, ...(6‐Dipp)CuBneop, and (6‐Dipp)CuBhex (cat=1,2‐O2C6H4; neop=(OCH2)2C(CH3)2; hex=OC(CH3)HCH2C(CH3)2O). Whilst (6‐Dipp)CuBcat and (6‐Dipp)CuBneop formed rapidly in toluene, access to (6‐Dipp)CuBhex required heating to 60 °C for days. The complexes were characterised by single‐crystal X‐ray crystallography which showed in all three cases that the systems were monomers and distorted‐linear at the copper atom. The stability of (6‐Dipp)CuBneop was found to be comparable to that of (IPr*)Cu‐Bneop (IPr*=1,3‐bis(2,6‐(diphenylmethyl)‐4‐methylphenyl)imidazol‐2‐ylidene); it persisted in solution for days with no sign of decomposition. (6‐Dipp)CuBhex is a rare crystallographically characterised example of a complex containing a boryl anion supported by the hexylene glycolato ligand.
A ring‐expanded NHC with flanking di‐iso‐propylphenyl groups has been shown to support 3 copper(I) boryl systems. This allows the structural characterisation of (6‐Dipp)CuBcat, (6‐Dipp)CuBneop, and (6‐Dipp)CuBhex (cat=1,2‐O2C6H4; neop=(OCH2)2CMe2; hex=OC(CH3)HCH2C(CH3)2O). All three compounds are reasonably stable, readily isolable sources of boryl nucleophiles, and (6‐Dipp)CuBhex is a rare example of a boryl anion bearing the hexylene glycol ester.
The reaction of phosphorus(III) esters with pinacolborane generates phosphines via the action of an NHC-copper(I) catalyst. This gives access, within minutes, to 12 P–H bonded species, including ...secondary and primary phosphines as well as PH3, in excellent conversions. These phosphines can be subsequently applied in the copper(I)-catalyzed hydrophosphination of heterocumulenes in a telescoped, one-pot fashion. This approach yielded 12 phosphaureas, 3 phosphaguanidines, and 2 phosphathioureas in moderate to excellent yields without the need to handle toxic, pyrophoric, or gaseous P–H bond containing compounds. The crystal structures of two of the phosphaureas, PhP(C(O)NHPh)2 and P(C(O)NHPh)3, are presented.
Reaction of bis(pinacolato)diboron with (6-Dipp)CuO
t
Bu generates a ring-expanded N-heterocyclic carbene supported copper(
i
) boryl, (6-Dipp)CuBpin. This compound showed remarkable stability and ...was characterised by NMR spectroscopy and X-ray crystallography. (6-Dipp)CuBpin readily dechalcogenated a range of heterocumulenes such as CO
2
, isocyanates and isothiocyanates to yield (6-Dipp)CuXBpin (X = O, S). In the case of CO
2
catalytic reduction to CO is viable in the presence of excess bis(pinacolato)diboron. In contrast, in the case of iso(thio)cyanates, the isocyanide byproduct of dechalcogenation reacted with (6-Dipp)CuBpin to generate a copper(
i
) borylimidinate, (6-Dipp)CuC(&z.dbd;NR)Bpin, which went on to react with heterocumulenes. This off-cycle reactivity gives selective access to a range of novel boron-containing heterocycles bonded to copper, but precludes catalytic reactivity.
A copper(
i
) boryl supported by a ring-expanded N-heterocyclic carbene has been synthesised and its reactivity towards heterocumulenes has been investigated.
A range of N-heterocyclic carbene-supported copper diphenylphosphides (NHC = IPr, 6-Dipp, SIMes and 6-Mes) were synthesised. These include the first reports of ring-expanded NHC-copper(
i
) ...phosphides. The compounds were characterised by NMR spectroscopy and X-ray crystallography. Reaction of (6-Dipp)CuPPh
2
with isocyanates, isothiocyanates and carbon disulfide results in the insertion of the heterocumulene into the Cu-P bond. The NHC-copper phosphides were found to be the most selective catalysts yet reported for the hydrophosphination of isocyanates. They provide access to a broad range of phosphinocarboxamides in excellent conversion and good yield.
The first copper(
i
) phosphides supported by ring-expanded N-heterocyclic carbenes have been synthesised and react readily with heterocumulenes. These copper(
i
) phosphides are highly active and selective in the hydrophosphination of isocyanates.
Frustrated Lewis pairs (FLPs) have evolved from a revolutionary concept to widely applied catalysts. We recently reported the ring-expanded N-heterocyclic carbene supported copper(I) ...boryliminomethanide, (6-Dipp)CuC(=NtBu)Bpin and noted it reacted with heterocumulenes in a fashion reminiscent of FLPs. We thus set out to explore its reactivity with a range of other substrates known to react with FLPs. This was undertaken by a series of synthetic studies using NMR spectroscopy, mass spectrometry, IR spectroscopy, and single crystal X-ray crystallography. (6-Dipp)CuC(=NtBu)Bpin was investigated for its reactivity towards water, hydrogen, and phenylacetylene. Its solution stability was also explored. Upon heating, (6-Dipp)CuC(=NtBu)Bpin decomposed to (6-Dipp)CuCN, which was characterised by SC-XRD and NMR spectroscopy, and pinBtBu. Although no reaction was observed with hydrogen, (6-Dipp)CuC(=NtBu)Bpin reacted with water to form (6-Dipp)CuC(=N(H)tBu)B(OH)pin, which was structurally characterised. In contrast to its FLP-reminiscent heterolytic cleavage reactivity towards water, (6-Dipp)CuC(=NtBu)Bpin acted as a Brønsted base towards phenyl acetylene generating (6-Dipp)CuCCPh, which was characterised by SC-XRD, IR, and NMR spectroscopy, and HC(=NtBu)Bpin
The (IPr)CuO
t
Bu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in most cases. Overreduction, which hampers almost all ...isocyanate hydroborations, was restricted to electron poor aryl isocyanates (4-NC-C
6
H
4
NCO, 4-F
3
C-C
6
H
4
NCO, 3-O
2
N-C
6
H
4
NCO). Computational analysis showed stability of
(IPr)CuH
2
, which was proposed to be the catalyst resting state, drives selectivity, suggesting an approach to prevent overreduction in future work. In the case of iPrNCO, formation of this species renders overreduction kinetically inaccessible. For 4-NC-C
6
H
4
NCO, however, the barrier height for the first step of over-reduction is much lower, even relative to
(IPr)CuH
2
, resulting in unselective reduction.
The (IPr)CuO
t
Bu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in the case of electron rich isocyanates. A computational study provided insight into this selectivity.
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