We have prepared and characterized a series of unprecedented group 6–group 11, N2-bridged, heterobimetallic ML4(η1-N2)(μ-η1:η1-N2)Au(NHC)+ complexes (M = Mo, W, L2 = diphosphine) by treatment of ...trans-ML4(N2)2 with a cationic gold(I) complex Au(NHC)+. The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M–N–N–Au motif. To verify how far the analogy goes, we computed the electronic structures of W(depe)2(η1-N2)(μ-η1:η1-N2)AuNHC+ (10 W +) and W(depe)2(η1-N2)(μ-η1:η1-N2)B(C6F5)3 (11 W ). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N2 with a d orbital of the group 6 metal overlaps in 10 W + with an empty sd hybrid orbital at gold, whereas in 11 W with an sp3 hybrid orbital at boron. The bent N–N–LA arrangement maximizes these interactions, providing a similar level of N2 “push–pull” activation in the two compounds. In the gold case, the HOMO–2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N2–Au(NHC)+ bond.
The first example of a formal 1,3‐B−H bond addition across the M−N≡N unit of an end‐on dinitrogen complex has been achieved. The use of Piers’ borane HB(C6F5)2 was essential to observe this ...reactivity and it plays a triple role in this transformation: 1) electrophilic N2‐borylation agent, 2) Lewis acid in a frustrated Lewis pair‐type B−H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid‐state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2‐bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N2 occurred without hydride transfer.
And the B goes on. Application of frustrated Lewis pair (FLP)‐type reactions in dinitrogen coordination chemistry has led to the achievement of 1,3‐B−H bond addition across the M−N≡N unit of a N2 complex. A chelating, strongly σ‐donating phosphine ligand is necessary to observe the title reaction. The use of HB(C6F5)2 is essential as it plays a triple role: N2‐borylation agent, Lewis acid in a FLP‐type B−H bond activation, and hydride shuttle.
The regio- and stereoselective addition of germanium and zinc across the C–C triple bond of nitrogen-, sulfur-, oxygen-, and phosphorus-substituted terminal and internal alkynes is achieved by ...reaction with a combination of R3GeH and Et2Zn. Diagnostic experiments support a radical-chain mechanism and the β-zincated vinylgermanes that show exceptional stability are characterized by NMR spectroscopy and X-ray crystallography. The unique feature of this new radical germylzincation reaction is that the C(sp2)–Zn bond formed remains available for subsequent in situ Cu(I)- or Pd(0)-mediated C–C or C–heteroatom bond formation with retention of the double bond geometry. These protocols offer modular access to elaborated tri- and tetrasubstituted vinylgermanes decorated with heteroatom substituents β to germanium that are useful for the preparation of stereodefined alkenes.
A Lewis superacidic bis(borane) C6F4{B(C6F5)2}2 was reacted with tungsten N2-complexes W(N2)2(R2PCH2CH2PR2)2 (R = Ph or Et), affording zwitterionic boryldiazenido W(ii) complexes ...trans-W(L)(R2PCH2CH2PR2)2(N2{B(C6F5)2(C6F4B(C6F5)3}) (L = ø, N2 or THF). These compounds feature only one N–B linkage of the covalent type, as a result of intramolecular boron-to-boron C6F5 transfer. Complex trans-W(THF)(Et2PCH2CH2PEt2)2(N2{B(C6F5)2C6F4B(C6F5)3}) (5) was shown to split H2, leading to a seven-coordinate complex W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)2}2C6F4) (7). Interestingly, hydride storage at the metal triggers backward C6F5 transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N2 complex W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)3} (10) differing only in the Lewis acid (LA), namely B(C6F5)3, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N2 activation up to the diazene-diide (N22−) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N2.
Although the metal‐based catalyzed cyanomethylation of aldehydes is well‐developed, a similar approach to ketones with acetonitrile derivatives remains a challenge. Thanks to Tolman type complexes, ...NiII‐complexes with a pyridine(dicarboxamide) pincer ligand, an alternative metal‐catalyzed (1–5 mol% catalyst) cyanomethylation of an array of isatins and activated ketones is reported at room temperature. High isolated yields (up to 99%) were obtained not only with the poorly acidic acetonitrile but also with more challenging substituted alkyl cyanide nucleophiles. The in situ generated putative ion paired catalysts NiCR2CN−Cat+ likely benefit from the cation part (Cs+ or n‐Bu4N+) to facilitate their formation and catalytic activity.
Two different dinitrogen‐derived molybdenum nitrido complexes varying by their geometry, ligand spheres and oxidations states were shown to engage their N ligand in dative bonding with the strong ...Lewis acid B(C6F5)3. The stable adducts were assessed for frustrated Lewis pair‐type heterolytic E−H bond splitting of hydrosilanes (E=Si) and HB(C6F5)2. Whereas Si−H bond activation was achieved, HB(C6F5)2 was shown to substitute B(C6F5)3 in a quantitative or equilibrated fashion, depending on the nature of the nitrido complex. No B−H bond splitting was observed. Thermodynamics of these reactions, computed by DFT, are in agreement with the experimental outcomes.
Dinitrogen‐derived nitrides have been explored as Lewis base component for frustrated Lewis pair type heterolytic bond splitting with B(C6F5)3 as the Lewis acid. d0 and d2 molybdenum nitrides both form stable adducts with B(C6F5)3. If Si−H bond splitting could be achieved, B−H bond activation of HB(C6F5)2 was not observed, this borane entering in competition with B(C6F5)3 to pair with the nitride without any further reactivity.
The hydroboration of CO2 into bis(boryl)acetal (BBA) compounds is an important transformation, since it enabled to selectively reduce CO2 by 4e‐ and to subsequently use the BBA compounds as C1 and Cn ...sources. However, the influence of the nature of the boryl moieties on the reactivity of BBA compounds has not been evaluated so far. In the present study, four BBA compounds – including two new ones – were reacted with 2,6‐diisopropylaniline to afford the expected imine. Significant differences in the rate of the reaction from minutes to weeks have been observed depending on the BBA used, showing the importance of the nature of the boryl moieties. Theoretical investigations enabled to propose a mechanism involving the addition of the aniline to the BBA as the rate‐determining step and to determine that the steric hindrance of the BBA compounds is the main factor driving the rate of this condensation reaction.
Four bis(boryl)acetal compounds – including two new ones – were synthesized under identical conditions. Mechanistic investigations proved that their condensation toward 2,6‐diisopropylaniline is very sensitive to the steric hindrance of the boryl moieties.
A Lewis superacidic bis(borane) C
6
F
4
{B(C
6
F
5
)
2
}
2
was reacted with tungsten N
2
-complexes W(N
2
)
2
(R
2
PCH
2
CH
2
PR
2
)
2
(R = Ph or Et), affording zwitterionic boryldiazenido W(
ii
) ...complexes
trans
-W(L)(R
2
PCH
2
CH
2
PR
2
)
2
(N
2
{B(C
6
F
5
)
2
(C
6
F
4
B(C
6
F
5
)
3
}) (L = ø, N
2
or THF). These compounds feature only one N-B linkage of the covalent type, as a result of intramolecular boron-to-boron C
6
F
5
transfer. Complex
trans
-W(THF)(Et
2
PCH
2
CH
2
PEt
2
)
2
(N
2
{B(C
6
F
5
)
2
C
6
F
4
B(C
6
F
5
)
3
}) (
5
) was shown to split H
2
, leading to a seven-coordinate complex W(H)
2
(Et
2
PCH
2
CH
2
PEt
2
)
2
(N
2
{B(C
6
F
5
)
2
}
2
C
6
F
4
) (
7
). Interestingly, hydride storage at the metal triggers backward C
6
F
5
transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N
2
complex W(H)
2
(Et
2
PCH
2
CH
2
PEt
2
)
2
(N
2
{B(C
6
F
5
)
3
} (
10
) differing only in the Lewis acid (LA), namely B(C
6
F
5
)
3
, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N
2
activation up to the diazene-diide (N
2
2−
) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N
2
.
The first two-fold Lewis acid adduct of a terminal N
2
ligand was prepared by employing a bis(borane). The influence of double coordination is benchmarked against the adduct of a related, non-chelating Lewis acid.
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