Copper Complexes with Diazoolefin Ligands and their Photochemical Conversion into Alkenylidene Complexes

In Crystallo Wolff Rearrangement of a Metalated Diazoester: Structural Confirmation of the Singlet Carbene Wolff-Intermediate

The Wolff rearrangement (WR) is widely used for the synthesis of ketenes from diazoketones and -esters. Stepwise WR reactions are proposed to proceed through transient carbonylcarbene (R-C-C(O)-R') intermediates, which so far have evaded structural characterization. Here, a Wolff metallocarbene (PtII-C-C(O)-OEt) is reported as a fleeting intermediate in the photoinitiated fragmentation of a diazoester ligand. Frozen solution and crystal matrix isolation experiments enabled the spectroscopic, magnetic, crystallographic, and computational characterization of this highly reactive species. All methods confirmed a singlet ground state for the WR metallocarbene, which is stabilized by π interactions with the carboxyl substituent, thus complementing computational and transient spectroscopy studies for classic organic WR reactions.

P-P Coupling with and without Terminal Metal-Phosphorus Intermediates

Terminal metal-phosphorus (M-P) complexes are of significant contemporary interest as potential platforms for P-atom transfer (PAT) chemistry. Decarbonylation of metal-phosphaethynolate (M-PCO) complexes has emerged as a general synthetic approach to terminal M-P complexes. M-P complexes that are stabilized by strong M-P multiple bonds are kinetically persistent and isolable. In the absence of strong M-P stabilization, the formation of diphosphorus-bridged complexes (i.e., M-P-P-M species) is often interpreted as evidence for the intermediacy of reactive, unobserved M-P species. Here, we demonstrate that while diphosphorus complexes can arise from reactive M-P species, P-P coupling can also proceed directly from M-PCO species without the intermediacy of M-P complexes. Photochemical decarbonylations of a pincer-supported Ni (II)-PCO complex at 77 K afford a spectroscopically observed terminal Ni-P complex, which is best described as a triplet, Ni(II)-metallophosphinidene with two unpaired electrons localized on the atomic phosphorus ligand. Thermal annealing of this transient Ni-P complex results in rapid dimerization to afford the corresponding P22--bridged dinickel complex. Unexpectedly, the same P22--bridged dinickel complex can also be accessed via a thermally promoted process in the absence of light. The analysis of reaction kinetics, isotope-labeling studies, and computational results indicate that the thermal P-P coupling process proceeds via a noncanonical mechanism that avoids terminal M-P intermediates. Together, these results represent the first observation of P-P coupling from characterized terminal M-P species and demonstrate that terminal M-P intermediates are not required to obtain P-P coupling products. These observations provide critical mechanistic understanding of the activation modes relevant to P-atom transfer.

An Isolable Base-Stabilized Diazosilenyl Cation

In contrast to the emerging chemistry of stable diazoalkenes, there are not yet any studies devoted to heavier silicon analogues, diazosilenes. Here, we report the synthesis of a base-stabilized diazosilenyl cation 2 by the reaction of base-stabilized C-phosphonio-silyne 1 with N2O. This silicon analog of diazoalkenes 2 exhibits a remarkable stability thanks to the coordination of phosphine and phosphine oxide ligands at the cationic silicon center. In addition, DFT calculations predict that, due to a particular stabilizing effect of the electropositive silicon atom, for diazosilenes (R2Si=C=N2), the presence of π-donor substituents is not essential to prevent N2 dissociation, contrary to carbon analogues. Interestingly, diazosilenyl cation 2 tends to isomerize into (silylene)(phosphonio)diazomethane 7 via a 1,2-phosphine ligand migration and thus exhibits dual reactivity as a diazoalkene and as a diazo-substituted silylene.

Solid/Gas In Crystallo Reactivity of an Ir(I) Methylidene Complex

In crystallo stabilization of known, but solution unstable, methylidene complex [Ir(tBu-PONOP)(=CH2)][BArF 4] allows single-crystal to single-crystal solid/gas reactivity associated with the {Ir=CH2} group to be studied. Addition of H2 results in [Ir(tBu-PONOP)(H)2][BArF 4]; exposure to CO forms iridium(I) carbonyl [Ir(tBu-PONOP)(CO)][BArF 4], and reaction with NH3 gas results in the formation of methylamine complex [(tBu-PONOP)Ir(NH2Me)][BArF 4] via an aminocarbene intermediate. Periodic density functional theory and electronic structure analyses confirm the Ir=CH2 bond character but with a very low barrier to rotation around the Ir=CH2 bond. Calculations show that addition of NH3 to the electrophilic alkylidene carbon gives an initial ammonium ylid intermediate. Stepwise N-H and C-H transfers then form the aminocarbene intermediate as a kinetic product from which two successive C-H couplings lead to the more stable methylamine product.

An Acceptor-Substituted N-Heterocyclic ortho-Quinodimethane: Pushing the Boundaries of Polarization in Donor-Acceptor-Substituted Polyenes

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted ortho-quinodimethane (oQDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted ortho-N-heterocyclic quinodimethane (oNHQ). We have examined the extent of polarization of the conjugated π-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations. The bond lengths in the phenyl linker do not exhibit the alternation typical of oQDMs. In addition, the 13C and 15N NMR shifts suggest significant charge separation, an interpretation supported by the diatropic ring current determined by NICSZZ(r) computations, which is characteristic of aromatic compounds. DFT calculations show that polarization is an electronic effect that is amplified by steric influences. More strikingly, the oxidation and reduction potentials of the push-pull substituted oQDM are virtually identical to those of authenticated anionic and cationic derivatives. The results therefore indicate that an aromatic zwitterionic structure represents the electronic structure more accurately than a neutral quinoidal Lewis structure, which indicates that the acceptor-substituted oNHQ is a rare example of an organic zwitterion in which the centers of charge are in conjugation. The ambiphilic reactivity of the acceptor-substituted oNHQ, which is evidenced by the dehydrogenation of ammonia borane and the addition of phenylacetylene via heterolytic C-H bond cleavage, further supports its notation as an organic zwitterion and is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted oNHQ can be considered to be an intramolecular carbogenic FLP in terms of its reactivity.

Triplet carbenes with transition-metal substituents

The extraordinary advances in carbene (R1-C-R2) chemistry have been fuelled by strategies to stabilize the electronic singlet state via π interactions. In contrast, the lack of similarly efficient approaches to obtain authentic triplet carbenes with appreciable lifetimes beyond cryogenic temperatures hampers their exploitation in synthesis and catalysis. Transition-metal substitution represents a potential strategy, but metallocarbenes (M-C-R) usually represent high-lying excited electronic configurations of the well-established carbyne complexes (M≡C-R). Here we report the synthesis and characterization of triplet metallocarbenes (M-C-SiMe3, M = PdII, PtII) that are persistent beyond cryogenic conditions, and their selective reactivity towards carbene C-H insertion and carbonylation. Bond analysis reveals significant stabilization by spin-polarized push-pull interactions along both π-bonding planes, which fundamentally differs from bonding in push-pull singlet carbenes. This bonding model, thus, expands key strategies for stabilizing the open-shell carbene electromers and closes a conceptual gap towards carbyne complexes.

A Lead(II) Substituted Triplet Carbene

Reaction of the pincer-type ligand L3 supported complex [L3PbBr][BArF24] (1) with Li[(C(═N2)TMS)] furnishes [L3Pb(C(═N2)TMS)][BArF24] (2). Diazo-compound 2 eliminates dinitrogen upon irradiation affording formal plumba-alkyne 3, which persists in cold fluoroarene solutions. Variable temperature UV/Vis and NMR spectroscopies in combination with quantum-chemical calculations identify 3 as a metal-substituted triplet carbene. In-crystallo irradiation of [L3Pb(C(═N2)TMS)(tol)][BArF24] (2·tol) provides a snapshot of intermolecular C-H bond insertion with toluene (4).

Cycloadditions of Diazoalkenes with P4 and tBuCP: Access to Diazaphospholes

Diazoalkenes readily react with tert-butylphosphaalkyne (tBuCP) and white phosphorus (P4) to afford novel phosphorus heterocycles, 3H-1,2,4-diazamonophospholes and 1,2,3,4-diazadiphospholes. Both species represent rare examples of neutral heterophospholes. The mechanism of formation and the electronic structures of these formal (3+2) cycloaddition products were analyzed computationally. The new phospholes form structurally diverse coordination compounds with transition metal and main group elements. Given the growing number of stable diazoalkenes, this work offers a straightforward route to neutral aza(di-)phospholes as a new ligand class.

Metal-Mediated Synthesis of a Mixed Arduengo-Fischer Carbodicarbene Ligand

Carbodicarbenes are strong C-donor ligands, which have found numerous applications in organometallic and main group element chemistry. Herein, we report a structurally distinct carbodicarbene ligand, which is formed by dinitrogenative coupling of a Fischer carbene complex with an N-heterocyclic diazoolefin. The resulting carbonyl complex serves as a stable source for the mixed Arduengo-Fischer carbodicarbene ligand. Facile ligand transfer reactions were demonstrated to occur with gold(I), copper(I), palladium(II), and rhodium(I) complexes.

Nitrous oxide as diazo transfer reagent

Nitrous oxide, commonly known as "laughing gas", is formed as a by-product in several industrial processes. It is also readily available by thermal decomposition of ammonium nitrate. Traditionally, the chemical valorization of N2O is achieved via oxidation chemistry, where N2O acts as a selective oxygen atom transfer reagent. Recent results have shown that N2O can also function as an efficient diazo transfer reagent. Synthetically useful methods for synthesizing triazenes, N-heterocycles, and azo- or diazo compounds were developed. This review article summarizes significant advancements in this emerging field.

Photoinitiated Single-Crystal to Single-Crystal Redox Transformations of Titanium-Oxo Clusters

Titanium-oxo clusters can undergo photochemical reactions under UV light, resulting in the reduction of the titanium-oxo core and oxidation of surface ligands. This is an important step in photocatalytic processes in light-absorbing Ti/O-based clusters, metal-organic frameworks, and (nano)material surfaces; however, studying the direct outcome of this photochemical process is challenging due to the fragility of the immediate photoproducts. In this report, titanium-oxo clusters [TiO(OiPr)(L)]n (n = 4, L = O2PPh2, or n = 6, L = O2CCH2tBu) undergo a two-electron photoredox reaction in the single-crystal state via an irreversible single-crystal to single-crystal (SC-SC) transformation initiated by a UV laser. The process is monitored by single crystal X-ray diffraction revealing the photoreduction of the cluster with coproduction of an (oxidized) acetone ligand, which is retained in the structure as a ligand to Ti(3+). The results demonstrate that photochemistry of inorganic molecules can be studied in the single crystal phase, allowing characterization of photoproducts which are unstable in the solution phase.

Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]2 to give the corresponding [Pd(η3-allyl)Cl(mNHO)] coordination complexes. 13C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH3-mNHO adduct by reacting the triazolium salts with AuCl(SMe2) and BH3·THF, respectively. The BH3-mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.

Diazoalkenes: From an Elusive Intermediate to a Stable Substance Class in Organic Chemistry

Over decades diazoalkenes (R2 C=C=N2 ) were postulated as reactive intermediates in organic chemistry even though their direct spectroscopic detection proved very challenging. In the 1970/80ies several groups probed their existence mainly indirectly by trapping experiments or directly by matrix-isolation studies. In 2021, our group and the Severin group reported independently the synthesis and characterization of the first room-temperature stable diazoalkenes, which initiated a rapidly expanding research field. Up to now four different classes of N-heterocyclic substituted room-temperature stable diazoalkenes have been reported. Their properties and unique reactivity, such as N2 /CO exchange or utilization as vinylidene precursors in organic and transition metal chemistry are presented. This review summarizes the early discoveries of diazoalkenes from their initial postulation as transient, elusive species up to the recent findings of the room-temperature stable derivatives.

Synthesis and Reactivity of an Anionic Diazoolefin

Bent (hetero)allenes such as carbodicarbenes and carbodiphosphoranes can act as neutral C-donor ligands, and diverse applications in coordination chemistry have been reported. N-Heterocyclic diazoolefins are heterocumulenes, which can function in a similar fashion as L-type ligands. Herein, we describe the synthesis and the reactivity of an anionic diazoolefin. This compound displays distinct reactivity compared to neutral diazoolefins, as evidenced by the preparation of diazo compounds via protonation, alkylation, or silylation. The anionic diazoolefin can be employed as an ambidentate, X-type ligand in salt metathesis reactions with metal halide complexes. Extrusion of dinitrogen was observed in a reaction with PCl(NiPr2 )2 , resulting in a stable phosphinocarbene.