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.
Tri‐organyl and tricoordinate N‐heterocyclic carbene (NHC) Zn–NHC alkyl cations (nNHC)2Zn‐Me+ (nNHC=C2‐bonded‐IMes/‐IDipp; 3+ and 4+; IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene, ...IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were first synthesized and structurally characterized by ionization of the corresponding neutral precursors (nNHC)ZnMe2 with Ph3CB(C6F5)4 in the presence of one equivalent of free NHC. Whereas cation (nIMes)2Zn‐Me+ (3+) is stable, its sterically congested analogue (nIDipp)2Zn‐Me+ (4+) readily undergoes an nNHC‐to‐aNHC isomerization in the presence of THF or IDipp to afford the more thermodynamically stable (aIDipp)(nIDipp)Zn‐Me+ (aIDipp=C4‐bonded IDipp, 5+), reflecting the adaptable‐to‐sterics coordination chemistry of these cations for improved stability. Cations 3+–5+ are the first Zn cations of the type Zn(C)(C′)(C′′)+ (C, C′, C′′=σ‐donor carbyl ligand). Kinetic studies combined with DFT calculations agree with an nNHC‐to‐aNHC process proceeding through the initial deprotonation of 4+ (at a Zn‐bonded C4‐IDipp moiety) by IDipp. Unlike 3+ and 4+, the rearranged cation 5+ reacts with CO2 through insertion into the Zn–Me bond yielding the corresponding Zn(κ2‐OAc)+ cation 6+. Both cations 5+ and 6+ were successfully used in CO2 hydrosilylation catalysis for silylformate formation.
Set in its ways: Tri‐organyl ZnII cations, (NHC)2Zn‐Me+, were synthesized and structurally characterized to unveil a coordination chemistry strongly dependent on sterics. Severe steric hindrance may induce a normal‐to‐abnormal N‐heterocyclic carbene (NHC) isomerization. Cation (aIDipp)(nIDipp)Zn‐Me+ (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) inserts CO2 into its Zn–alkyl bond at room temperature and catalyzes CO2 hydrosilylation (see scheme).
The present account reviews the most recent noteworthy developments on the synthesis, structure and catalytic applications of Zn‐NHC species, a class of complexes that have attracted attention over ...the past five to ten years due to their enhanced robustness and hydrolytic stability versus classical Zn organometallics. In particular, thanks to NHC stabilization, access to unprecedented Zn species were recently achieved, including two‐coordinate Zn(II) organocations and thermally stable molecularly well‐defined Zn hydride species, opening the way to effective Zn‐mediated hydro‐silylation/‐boration catalysis of various unsaturated substrates under mild conditions. The potential of NHC−Zn species for the stabilization of unprecedented Zn species and use in various catalytic applications is only emerging and the vast array of readily available NHC structures should promote future developments of the field.
The most recent noteworthy developments on Zn‐NHC species are reviewed. In particular, thanks to NHC stabilization, two‐coordinate Zn(II) organocations and molecularly well‐defined Zn hydride species are now accessible.The fundamental reactivity of Zn‐NHC adducts and their use in various catalytic applications are also discussed.
Low-coordinate Al(III) and Ga(III) alkyl cations stabilized by N-heterocyclic carbene ligands were characterized as strong Lewis acidic species and successfully exploited as selective as alkyne, ...benzaldehyde and CO2 hydrosilylation catalysts.
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•Low-coordinate carbene-stabilized Al(III) and Ga(III) alkyl cations are characterized.•They stand as strong Lewis acids as estimated from the Gutmann-Beckett method.•They mediate the trans-hydrosilylation of various alkynes.•Carbonyl substrates could also be catalytically reduced using such cationic electrophiles.
Cationic Al(III) and Ga(III) species supported by N-heterocyclic carbene (NHC) ligands, (IDipp)AlMe2(PhBr)+ (1+, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and (IDipp)GaMe2+ (2+), were prepared and structurally characterized as B(C6F5)4− salts via ionization of the corresponding neutral precursors (IDipp)MMe3 (M = Al, Ga) with Ph3CB(C6F5)4 in PhBr at room temperature. Both 1B(C6F5)4 and 2B(C6F5)4 salt were isolated in high yield and their solid state structures established through X-ray crystallographic studies. Cations 1+ and 2+, which are rare examples of structurally characterized tris-organyl Al(III) and Ga(III) cations, stand as potent Lewis acids as experimentally estimated through the Gutmann-Beckett method. These cations were further exploited in hydrosilylation catalysis of alkynes, benzaldehyde and CO2 using HSiEt3 as an hydrosilane source. Hydrosilylation of 1-hexyne, 4-phenylbutyne and phenylacetylene led to the formation of the corresponding Z-selective products 3–5, respectively, while benzaldehyde was converted to PhCH2OSiEt3 (6). Cations 1+ and 2+ also slowly catalyze CO2 hydrosilylation with the selective formation of the methanol-equivalent MeOSiEt3.
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.
A Lewis superacidic bis(borane) C
F
{B(C
F
)
}
was reacted with tungsten N
-complexes W(N
)
(R
PCH
CH
PR
)
(R = Ph or Et), affording zwitterionic boryldiazenido W(ii) complexes
-W(L)(R
PCH
CH
PR
)
...(N
{B(C
F
)
(C
F
B(C
F
)
}) (L = ø, N
or THF). These compounds feature only one N-B linkage of the covalent type, as a result of intramolecular boron-to-boron C
F
transfer. Complex
-W(THF)(Et
PCH
CH
PEt
)
(N
{B(C
F
)
C
F
B(C
F
)
}) (5) was shown to split H
, leading to a seven-coordinate complex W(H)
(Et
PCH
CH
PEt
)
(N
{B(C
F
)
}
C
F
) (7). Interestingly, hydride storage at the metal triggers backward C
F
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
complex W(H)
(Et
PCH
CH
PEt
)
(N
{B(C
F
)
} (10) differing only in the Lewis acid (LA), namely B(C
F
)
, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N
activation up to the diazene-diide (N
) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N
.
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(C
F
)
was essential to observe this reactivity ...and it plays a triple role in this transformation: 1) electrophilic N
-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 N
occurred without hydride transfer.
Abstract
We herein report on the synthesis, structure, and use in alkyne hydroboration catalysis of (IPrCl)Zn−R
+
and (I
t
Bu)Zn−R
+
cations bearing IPrCl ...(IPrCl=1,3‐bis2,6‐bis(1‐methylethyl)phenyl‐4,5‐dichloro‐1,3‐dihydro‐imidazol‐2‐ylidene) and the sterically demanding I
t
Bu carbene (I
t
Bu=1,3‐bis(1,1‐dimethylethyl)−1,3‐dihydro‐imidazol‐2‐ylidene). Ionization of neutral precursors (IPrCl)ZnR
2
(
1 a
, R=Et;
1 b
, R=Me) and (I
t
Bu)ZnEt
2
(
2
) with one equivalent of Ph
3
CB(C
6
F
5
)
4
led to with robust and stable two‐coordinate Zn
II
cations (IPrCl)Zn−R
+
(
3 a
, R=Et;
3 b
, R=Me) and (I
t
Bu)Zn−Et
+
(
4
), respectively, all isolated as B(C
6
F
5
)
4
−
salts. Further derivatization of alkyl cations
3 b
and
4
by reaction with one equivalent of B(C
6
F
5
)
3
afforded cations (IPrCl)Zn−C
6
F
5
+
(
5
) and (I
t
Bu)Zn−C
6
F
5
+
(
6
) as B(C
6
F
5
)
4
−
salts, with cation
6
displaying a limited stability in solution. The molecular structures of cations
3 b
,
4
and
5
were confirmed through X‐ray diffraction studies. Among stable cations, Fluoride ion affinity (FIA) estimations agree with cation
5
being the most Lewis acidic in thus far reported (IPrCl)Zn−R
+
cations. In the presence of pinacol borane and 1‐octyne, cations
3 a–b
,
5
and (IPrCl)Zn−C
6
F
5
+
(5 mol%) slowly catalyze the selective
cis
‐hydroboration of 1‐octyne to the vinylborane product
A
. Cation
5
also mediates 2‐hexyne hydroboration to afford a mixture of hydroboration products
B
and
C
. In the case of hydroboration catalysis mediated by cations
5
and (IPrCl)Zn−C
6
F
5
+
, experimental data and preliminary DFT calculations are consistent a Lewis‐acid‐type catalysis.
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(C
F
)
. The stable adducts were assessed for frustrated Lewis pair-type heterolytic E-H bond splitting of hydrosilanes (E=Si) and HB(C
F
)
. Whereas Si-H bond activation was achieved, HB(C
F
)
was shown to substitute B(C
F
)
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.
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