Alkali-Metal-Mediated Manganation: A Method for Directly Attaching Manganese(II) Centers to Aromatic Frameworks

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.

Contrasting Synergistic Heterobimetallic (Na-Mg) and Homometallic (Na or Mg) Bases in Metallation Reactions of Dialkylphenylphosphines and Dialkylanilines: Lateral versus Ring Selectivities

A series of dialkylphenylphosphines and their analogous aniline substrates have been metallated with the synergistic mixed-metal base [(TMEDA)Na(TMP)(CH₂ SiMe₃ )Mg(TMP)] 1. Different metallation regioselectivities for the substrates were observed, with predominately lateral or meta-magnesiated products isolated from solution. Three novel heterobimetallic complexes [(TMEDA)Na(TMP)(CH₂ PCH₃ Ph)Mg(TMP)] 2, [(TMEDA)Na(TMP)(m-C₆ H₄ PiPr₂ )Mg(TMP)] 3 and [(TMEDA)Na(TMP)(m-C₆ H₄ NEt₂ )Mg(TMP)]  4 and two homometallic complexes [{(TMEDA)Na(EtNC₆ H₅ )}₂ ] 5 and [(TMEDA)Na₂ (TMP)(C₆ H₅ PEt)]₂  6 derived from homometallic metallation have been crystallographically characterised. Complex 6 is an unprecedented sodium-amide, sodium-phosphide hybrid with a rare (NaNNaP)₂ ladder motif. These products reveal contrasting heterobimetallic deprotonation with homometallic induced ethene elimination reactivity. Solution studies of metallation mixtures and electrophilic iodine quenching reactions confirmed the metallation sites. In an attempt to rationalise the regioselectivity of the magnesiation reactions the C-H acidities of the six substrates were determined in THF solution using DFT calculations employing the M06-2X functional and cc-pVTZ Dunning's basis set.

Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C-H Functionalizations

Merging two topical themes in main-group chemistry, namely, cooperative bimetallics and frustrated-Lewis-pair (FLP) activity, this Forum Article focuses on the cooperativity-induced outcomes observed when the tris(alkyl)gallium compound GaR₃ (R = CH₂SiMe₃) is paired with the lithium amide LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) or the sterically hindered N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (I^tBu). When some previously published work are drawn together with new results, unique tandem reactivities are presented that are driven by the steric mismatch between the individual reagents of these multicomponent reagents. Thus, the LiTMP/GaR₃ combination, which on its own fails to form a cocomplex, functions as a highly regioselective base (LiTMP)/trap (GaR₃) partnership for the metalation of N-heterocycles such as diazines, 1,3-benzoazoles, and 2-picolines in a trans-metal-trapping (TMT) process that stabilizes the emerging sensitive carbanions. Taking advantage of related steric incompatibility, a novel monometallic FLP system pairing GaR₃ with I^tBu has been developed for the activation of carbonyl compounds (via C═O insertion) and other molecules with acidic hydrogen atoms such as phenol and phenylacetylene. Shedding new light on how these non-cocomplexing partnerships operate and showcasing the potential of gallium reagents to engage in metalation reactions or FLP activations, areas where the use of this group 13 metal is scant, this Forum Article aims to stimulate more interest and activity toward the advancement of organogallium chemistry.

Metal exchange in lithiocuprates: implications for our understanding of structure and reactivity

New reagents have been sought for directed ortho cupration in which the use of cyanide reagents is eliminated. CuOCN reacts with excess TMPLi (TMP = 2,2,6,6-tetramethylpiperidide) in the presence of limited donor solvent to give crystals that are best represented as (TMP)2Cu0.1Li0.9(OCN)Li2(THF) 8, whereby both Lipshutz-type lithiocuprate (TMP)2Cu(OCN)Li2(THF) 8a and trinuclear (TMP)2(OCN)Li3(THF) 8b are expressed. Treatment of a hydrocarbon solution of TMP2CuLi 9a with LiOCN and THF gives pure 8a. Meanwhile, formation of 8b is systematized by reacting (TMPH2)OCN 10 with TMPH and nBuLi to give (TMP)2(OCN)Li3(THF)211. Important to the attribution of lower/higher order bonding in lithiocuprate chemistry is the observation that in crystalline 8, amide-bridging Cu and Li demonstrate clear preferences for di- and tricoordination, respectively. A large excess of Lewis base gives an 8-membered metallacycle that retains metal disorder and analyses as (TMP)2Cu1.35Li0.659 in the solid state. NMR spectroscopy identifies 9 as a mixture of (TMP)2CuLi 9a and other copper-rich species. Crystals from which the structure of 8 was obtained dissolve to yield evidence for 8b coexisting in solution with in situ-generated 9a, 11 and a kinetic variant on 9a ( i-9a), that is best viewed as an agglomerate of TMPLi and TMPCu. Moving to the use of DALi (DA = diisopropylamide), (DA)2Cu0.09Li0.91(Br)Li2(TMEDA)212 (TMEDA = N,N,N',N'-tetremethylethylenediamine) is isolated, wherein (DA)2Cu(Br)Li2(TMEDA)212a exhibits lower-order Cu coordination. The preparation of (DA)2Li(Br)Li2(TMEDA)212b was systematized using (DAH2)Br, DAH and nBuLi. Lastly, metal disorder is avoided in the 2 : 1 lithium amide : Lipshutz-type monomer adduct (DA)4Cu(OCN)Li4(TMEDA)213.

Structural and Magnetic Diversity in Alkali-Metal Manganate Chemistry: Evaluating Donor and Alkali-Metal Effects in Co-complexation Processes

By exploring co-complexation reactions between the manganese alkyl Mn(CH2SiMe3)2 and the heavier alkali-metal alkyls M(CH2SiMe3) (M=Na, K) in a benzene/hexane solvent mixture and in some cases adding Lewis donors (bidentate TMEDA, 1,4-dioxane, and 1,4-diazabicyclo[2,2,2] octane (DABCO)) has produced a new family of alkali-metal tris(alkyl) manganates. The influences that the alkali metal and the donor solvent impose on the structures and magnetic properties of these ates have been assessed by a combination of X-ray, SQUID magnetization measurements, and EPR spectroscopy. These studies uncover a diverse structural chemistry ranging from discrete monomers [(TMEDA)2 MMn(CH2SiMe3)3] (M=Na, 3; M=K, 4) to dimers [{KMn(CH2SiMe3)3 ⋅C6 H6}2] (2) and [{NaMn(CH2SiMe3)3}2 (dioxane)7] (5); and to more complex supramolecular networks [{NaMn(CH2SiMe3)3}∞] (1) and [{Na2Mn2 (CH2SiMe3)6 (DABCO)2}∞] (7)). Interestingly, the identity of the alkali metal exerts a significant effect in the reactions of 1 and 2 with 1,4-dioxane, as 1 produces coordination adduct 5, while 2 forms heteroleptic [{(dioxane)6K2Mn2 (CH2SiMe3)4(O(CH2)2OCH=CH2)2}∞] (6) containing two alkoxide-vinyl anions resulting from α-metalation and ring opening of dioxane. Compounds 6 and 7, containing two spin carriers, exhibit antiferromagnetic coupling of their S=5/2 moments with varying intensity depending on the nature of the exchange pathways.

Extending motifs in lithiocuprate chemistry: unexpected structural diversity in thiocyanate complexes

Lithio(thiocyanato)cuprates have been developed. These reveal planar, boat-shaped and chair-shaped Lipshutz-type dimers in the solid state, while Lipshutz-type and Gilman structures are seen in solution.

Towards the Synthesis of Guanidinate- and Amidinate-Bridged Dimers of Mn and Ni

Reactions of Cp2M (Cp = cyclopentadienyl, M = Mn, Ni) with lithium amidinates and guanidinates are reported. The highly oxophilic nature of Mn leads to the isolation of the interstitial oxide Mn4O(MeN···CH···NMe)6 (4) in preference to the intended paddle-wheel homodimer Mn2(MeN···CH···NMe)4 when employing the sterically uncongested amidinate [MeN···CH···NMe]– ligand. In contrast, an analogous reaction using Cp2Ni yielded Ni2(MeN···CH···NMe)4 (5). The use of monoprotic guanidinate ligands also gave contrasting results for Mn and Ni. In the first case, the highly unusual spirocycle Mn{μ-NC(NMe2)2}4Li2·3THF (6) was produced in low yield. For M = Ni, use of the [hpp]– (1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) ligand gives results comparable with the synthesis of 5, with Ni2(hpp)4 (7) isolated. In contrast to recent data obtained using Cp2Cr, the guanidinate ligands do not sequester coformed CpLi. Density functional theory analysis corroborates the view that the intermetal distance in each of the reported dinickel paddle-wheel complexes (2.4846(8) and 2.3753(5) Å in 5 and 7 respectively) is defined by the geometric parameters of the bidentate ligands and that intermetal bonding is not present.

New avenues in the directed deprotometallation of aromatics: recent advances in directed cupration

Advances in directed aromatic deprotometallation are reported in the context of recent developments in our understanding of lithium cuprates.

Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

The synthesis, structure and reactivity of donor-free sodium and potassium magnesiates are assessed culminating in the unique 1,4-dimetallation of naphthalene.

Recent advances in direct C-H arylation: Methodology, selectivity and mechanism in oxazole series

Catalytic direct (hetero)arylation of (hetero)arenes is an attractive alternative to traditional Kumada, Stille, Negishi and Suzuki–Miyaura cross-coupling reactions, notably as it avoids the prior preparation and isolation of (hetero)arylmetals. Developments of this methodology in the oxazole series are reviewed in this article. Methodologies, selectivity, mechanism and future aspects are presented.

Main group multiple C-H/N-H bond activation of a diamine and isolation of a molecular dilithium zincate hydride: experimental and DFT evidence for alkali metal-zinc synergistic effects

The surprising transformation of the saturated diamine (iPr)NHCH(2)CH(2)NH(iPr) to the unsaturated diazaethene [(iPr)NCH═CHN(iPr)](2-) via the synergic mixture nBuM, (tBu)(2)Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N',N'-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2) (3), (TMEDA)Li[(iPr)NCH(2)CH(2)CH(2)N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (6), and {(TMEDA)Na[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (11), characterized by single-crystal X-ray diffraction, are discussed in relation to their role in the formation of (TMEDA)M[(iPr)NCH═CHN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li](2)[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2)3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li-H bond of 7 in solution was confirmed by (7)Li-(1)H HSQC experiments. Also, the (7)Li NMR spectrum of 7 in C(6)D(6) solution allowed for the rare observation of a scalar (1)J(Li-H) coupling constant of 13.3 Hz. Possible mechanisms for the transformation from diamine to diazaethene, a process involving the formal breakage of four bonds, have been determined computationally using density functional theory. The dominant mechanism, starting from (TMEDA)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu) (4), involves the formation of a hydride intermediate and leads directly to the observed diazaethene product. In addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a (TMEDA)LiH ligand, also provides a secondary mechanism for the formation of the diazaethene. The two reaction pathways (i.e., starting from 4 or 7) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation.

Synthesis, electronic structure, and reactivity of strained nickel-, palladium-, and platinum-bridged [1]ferrocenophanes

The group 10 bis(phosphine)metalla[1]ferrocenophanes, [{Fe(η(5)-C(5)H(4))(2)}M(Pn-Bu(3))(2)] [M = Ni (4a), Pd (4b), and Pt (4c)], have been prepared by the reaction of Li(2)[Fe(η(5)-C(5)H(4))(2)]·tmeda (5, tmeda = N,N,N',N'-tetramethylethylenediamine) with trans-[MCl(2)(Pn-Bu(3))(2)] [M = Ni (trans-6a) and Pd (trans-6b)] and cis-[PtCl(2)(Pn-Bu(3))(2)] (cis-6c), respectively. Single crystal X-ray diffraction revealed highly tilted, strained structures as characterized by α angles of 28.4° (4a), 24.5° (4b), and 25.2° (4c) and a distorted square planar environment for the group 10 metal center. UV/visible spectroscopy and cyclic voltammetry indicated that all three compounds had smaller HOMO-LUMO gaps and were more electron-rich in nature than ferrocene and other comparable [1]ferrocenophanes. DFT calculations suggested that these differences were principally due to the electron-releasing nature of the M(Pn-Bu(3))(2) metal-ligand fragments. Attempts to induce thermal or anionic ring-opening polymerization of 4a-c were unsuccessful and were complicated by, for example, competing ligand dissociation processes or unfavorable chain propagation. In contrast, these species all reacted rapidly with acids effecting clean extrusion of the bis(phosphine)metal fragment. Carbon monoxide inserted cleanly into one of the palladium-carbon bonds of 4b to afford the ring-expanded, acylated product [{Fe(η(5)-C(5)H(4))(η(5)-C(5)H(4))(CO)}Pd(Pn-Bu(3))(2)] (10). The nickel analogue 4a, however, afforded [Ni(CO)(2)(Pn-Bu(3))(2)] whereas the platinum-bridged complex 4c was inert. Remarkably, all compounds 4a-c were readily oxidized by elemental sulfur to afford the [5,5']bicyclopentadienylidene (pentafulvalene) complexes [{η(4):η(0)-C(5)H(4)(C(5)H(4))}M(Pn-Bu(3))(2)] [M = Ni (11a)] and [(η(2)-C(10)H(8))M(Pn-Bu(3))(2)] [M = Pd (11b) and Pt (11c)] by a formal 4-electron oxidation of the carbocyclic ligands. Compounds 11b and 11c represent the first examples of [5,5']bicyclopentadienylidene as a neutral η(2)-ligand. The relative energies of η(2)-coordination with respect to that of η(4):η(0) bonding were investigated for 11a-c by DFT calculations.

Avant-garde metalating agents: structural basis of alkali-metal-mediated metalation

Metalation, one of the most useful and widely used synthetic methodologies, transforms a relatively inert carbon-hydrogen bond to a more labile carbon-metal bond. Until recently, most organometallic reagents that facilitate this process have combined strongly electropositive metals, such as lithium, with organic reagents to form highly polar and, by implication, highly reactive carbon-metal bonds. For example, the alkyllithium reagents and bulky lithium amides that are commonly employed for this purpose can suffer from low functional group tolerance. Lithio-products of these reactions generally have low kinetic stabilities. More recently, several groups around the world have pioneered alternative metalation reagents, complex metalators, which can be interpreted as composite molecules or mixtures made up of two or more distinct compound types. Several examples include magnesiate complexes, Lochmann-Schlosser superbases, Kondo and Uchiyama's 2,2,6,6-tetramethylpiperidide (TMP)-zincate complexes, and Knochel's turbo-Grignard and related salt-supported reagents. This Account describes our rational development of novel complex metalators based on existing structural templates and designed to execute alkali-metal-mediated metalations (AMMMs). By changing the nonalkali metal in these structures, we have produced tailor-made dianionic-dicationic structures such as [(TMEDA).Na(mu-TMP)(mu-(n)Bu)Mg(TMP)], [(TMEDA).Na(mu-TMP)(mu-(t)Bu)Zn((t)Bu)], and [(TMEDA).Li(mu-TMP)Mn(CH(2)SiMe(3))(2)] (TMEDA = N,N,N',N'-tetramethylethylenediamine). These compounds can perform unprecedented magnesiations, zincations, or manganations on aromatic substrates that are generally inert toward conventional Mg, Zn, or Mn(II) reagents. Although the alkali metal is an essential component of these new complex metalators, interestingly, the less electropositive, less polar nonalkali metal [Mg, Zn, or Mn(II)] actually carries out the deprotonation. We view this unique behavior as a mixed-metal synergic effect: intramolecular communication through metal-ligand-metal bridges directs special regioselectivities or polydeprotonations. We demonstrate structurally defined alkali-metal-mediated magnesiations (AMMMg), zincations (AMMZn), and manganations [AMMMn(II)] of representative aromatic substrates (including benzene, toluene, anisole, and ferrocene). In addition, we present remarkable meta-orientated metalations of toluene and N,N-dimethylaniline. We also review 2-fold metalations of arenes, in which an arenediide guest is encapsulated within a 12-atom polymetallic cationic (NaNNaNMgN)(2) host ring to form inverse crown structures. Furthermore, using X-ray crystallography of a turbo-Grignard reagent, we establish a link between our complex metalators and turbo-Grignard reagents. Armed with this accruing knowledge of complex metalators, we think rapid progress in "low polarity metalation" should now be possible. The greatest remaining challenge is to develop methodologies that shift these processes from stoichiometric reactions into more economical catalytic ones.

Direct C-H metalation with chromium(II) and iron(II): transition-metal host/benzenediide guest magnetic inverse-crown complexes

Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low-polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.