Revisiting Isomerism in Square Planar Palladium NHC Complexes with NacAc Chelators.

Quantifying the donor ability of bidentate ligands in inorganic chemistry is not straightforward, and the Huynh electronic parameter for chelators (HEP2) presents a rare solution to the task. Aiming to extend the ligand scope in this work, the soundness and practicality of HEP2 was further scrutinized with seven stereoelectronically diverse ß-ketiminato ("NacAc") chelators using palladium complexes of the type [PdBr(iPr2-bimy)(ArNacAc)] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene). Notably, the unsymmetrical nature of this κ2-N,O ligand family allowed for an intriguing exploration into isomerism in the square planar PdII products, which depends on the N-aryl ortho- and para-substituents based on experimental and theoretical findings. The HEP2 values of both isomeric forms correlate well with the widely used σp Hammett constants, which reinforces the reliability of this modern methodology.

Diverse Post-Metathesis Reactivities of Mixed Amido-Isothiocyanato Molybdenum Complexes.

Attempts to prepare mixed isothiocyanato-bis(imido) MoVI complexes led to the discovery of post-metathesis rearrangements toward three distinct products (1-3), which feature the NCS-derived chelators [N(NMe2)CS]2- (L1 in dinuclear 1 and 2) and [N(SiMe3)(NMe2)CS]- (L2 in mononuclear 3). Notably, the preparation of bidentate ligand L1 and its coordination chemistry are unprecedented. Together with computational studies, it is proposed that the putative "mono-substituted" intermediate [Mo(NtBu)2(NMe2)(NCS)] serves as the common starting point for the observed molecular transformations. Construction of the [Mo(NtBu)2(NCS)2] core was ultimately possible in the presence of additional stabilizing donors (THF or PMe3), which yielded the complexes [Mo(NtBu)2(NCS)2(THF)2] (4) and [Mo(NtBu)2(NCS)2(PMe3)2] (5).

Controlled Access to Four- and Six-Membered Palladacycles via Modifying Donor Abilities of ß-Ketiminato Ligands ("NacAcs").

The synthesis of Pd complexes of the type [PdBr(iPr2-bimy)(NacAc)] (NacAc = ß-ketiminate, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was attempted, in a continuing effort to quantify donor abilities of chelating ß-ketiminate ligands using the Huynh electronic parameter for bidentate donors (HEP2). Subtle variation of N-substituents on the NacAc backbone was discovered to induce a drastic change in the preferred chelating mode, in that the commonly encountered κ2-N,O-six-membered palladacycles were observed with R = Me and Et, while the unusual κ2-C,N-four-membered palladacycles were isolated with R = iPr, Cy, and tBu. Computational studies subsequently corroborated these findings, in the form of an overall exergonic six-to-four-membered ring contraction process and a lower associated activation energy for the three more electron-donating alkyl moieties. This trend in the established energy profiles can be attributed to a reduced HOMO-LUMO gap in the corresponding optimized structures of the six-membered ring complexes.

An Acid-Free Anionic Oxoborane Isoelectronic with Carbonyl: Facile Access and Transfer of a Terminal B═O Double Bond.

We disclose the synthesis and structural characterization of the first acid-free anionic oxoborane, [K(2.2.2-crypt)][(HCDippN)2BO] (1) (Dipp = 2,6- iPr2C6H3), which is isoelectronic with classical carbonyl compounds. 1 can readily be accessed from its borinic acid by a simple deprotonation/sequestration sequence. Crystallographic and density functional theory (DFT) analyses support the presence of a polarized terminal B═O double bond. Subsequent π bond metathesis converts the B═O bond to a heavier B═S containing system, affording the first anionic thioxoborane [K(2.2.2-crypt)][(HCDippN)2BS] (2), isoelectronic with thiocarbonyls. Facile B═O bond cleavage can also be achieved to access B-H and B-Cl bonds and, via a remarkable oxide (O2-) ion abstraction, to generate a borenium cation [(HCDippN)2B(NC5H5)][OTf] (4). By extension, 1 can act as an oxide transfer agent to organic substrates, a synthetic role traditionally associated with transition-metal compounds. Hence we show that B-O linkages, which are often considered to be thermodynamic sinks, can be activated under mild conditions toward bond cleavage and transfer, by exploiting the higher reactivity inherent in the B═O double bond.

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners.

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)2 BO]- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)2 BO]- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO.

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2- .

A ß-Diketiminate-Stabilized Sila-Acyl Chloride: Systematic Access to Base-Stabilized Silicon Analogues of Classical Carbonyl Compounds.

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel ß-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the SiIV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding SiII systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry.

Highly Electron-Rich ß-Diketiminato Systems: Synthesis and Coordination Chemistry of Amino-Functionalized "N-nacnac" Ligands.

The synthesis of a class of electron-rich amino-functionalized ß-diketiminato (N-nacnac) ligands is reported, with two synthetic methodologies having been developed for systems bearing backbone NMe2 or NEt2 groups and a range of N-bound aryl substituents. In contrast to their (Nacnac)H counterparts, the structures of the protio-ligands feature the bis(imine) tautomer and a backbone CH2 group. Direct metalation with lithium, magnesium, or aluminium alkyls allows access to the respective metal complexes through deprotonation of the methylene function; in each case X-ray structures are consistent with a delocalized imino-amide ligand description. Transmetalation using lithium N-nacnac complexes is then exploited to access p- and f-block metal complexes, which allow for like-for-like benchmarking of the N-nacnac ligand family against their more familiar Nacnac counterparts. In the case of SnII , the degree of electronic perturbation effected by introduction of the backbone NR2 groups appears to be constrained by the inability of the amino group to achieve effective conjugation with the N2 C3 heterocycle. More obvious divergence from established structural norms is observed for complexes of the harder YbII ion, with azaallyl/imino and even azaallyl/NMe2 coordination modes being demonstrated by X-ray crystallography.

Gauging the donor strength of iron(0) complexes via their N-heterocyclic carbene gold(I) adducts.

We isolate and characterize the gold(I)-iron(0) adducts [(iPr2-bimy)Au-Fe(CO)3(PMe3)2][BArF4] and [Au-{Fe(CO)3(PMe3)2}2][BArF4] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, BArF4 = tetrakis(pentafluorophenyl)borate). DFT analysis reveals that the gold-iron interaction in [(iPr2-bimy)Au-Fe(CO)3(PMe3)2][BArF4] is predominantly a σ-donation from iron to gold. We further extend this class of compounds to include [(iPr2-bimy)Au-Fe(CO)3(PR3)2][BArF4] (PR3 = PPh3, PCy3, PCyPh2, PMePh2, PMe2Ph, P(4-C6H4F)3) and [(iPr2-bimy)Au-Fe(CO)4(PPh3)][BArF4] and correlate the iPr2-bimy carbenic 13C NMR signal with the relative donor strength of the iron(0) ligand. This approach allows for a fast and simple approach to gauge relative donor strength of Fe(0) donors.

A flat carborane with multiple aromaticity beyond Wade-Mingos' rules.

It is widely known that the skeletal structure of clusters reflects the number of skeletal bonding electron pairs involved, which is called the polyhedral skeletal electron pair theory (PSEPT) or Wade and Mingos rules. While recent computational studies propose that the increase of skeletal electrons of polyhedral clusters leads to the flat structure beyond the PSEPT, little experimental evidence has been demonstrated. Herein, we report the synthesis of a C2B4R4 carborane 2 featuring a flat ribbon-like structure. The C2B4 core of 2 bearing 16 skeletal electrons in the singlet-ground state defies both the [4n + 2] Hückel's rule and Baird's rule. Nevertheless, the delocalization of those electrons simultaneously induces two independent π- and two independent σ-aromatic ring currents, rendering quadruple aromaticity.

Probing the non-innocent nature of an amino-functionalised ß-diketiminate ligand in silylene/iminosilane systems.

Electron-rich ß-diketiminate ligands, featuring amino groups at the backbone ß positions ("N-nacnac" ligands) have been employed in the synthesis of a range of silylene (SiII) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe3)2, P(SiMe3)2 and Si(SiMe3)3). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl SiIV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X, such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe3)2, but not for X = H, N(SiMe3)2 and Si(SiMe3)3; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups, since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the SiII centre at the ß-carbon position of the N-nacnac ligand backbone.

N-H cleavage vs. Werner complex formation: reactivity of cationic group 14 tetrelenes towards amines.

ß-Diketiminate ligands featuring backbone NMe2 groups have been exploited to access a series of two-coordinate cations of the type [(N-nacnac)E]+ (E = Si, Ge, Sn), whose reactivity towards N-H bonds has been investigated. While the heavier group 14 systems react via simple adduct formation, N-H oxidative addition occurs for E = Si consistent with differences in EII/EIV redox potentials. The structurally characterized Ge/Sn adducts can be viewed as models for the corresponding (transient) Si systems [(N-nacnac)Si·(NH2R)]+ (R = H, tBu) - which are potential intermediates in the formation of [(N-nacnac)Si(H)(NHR)]+via a proton-shuttling mechanism.