Nitrenium ions as ligands for transition metals

Article abstract of DOI:10.1038/nchem.1068

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective ¹⁵ N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor σ-donors and good μ-acceptors.

Full text of DOI:10.1038/nchem.1068

ARTICLES
PUBLISHED ONLINE: 19 JUNE 2011 | DOI: 10.1038/NCHEM.1068
Nitrenium ions as ligands for transition metals
Yuri Tulchinsky1, Mark A. Iron2, Mark Botoshansky1 and Mark Gandelman1
*
Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived
analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a
ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared,
and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several
N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The
formation of relatively strong nitrenium–metal bonds is unambiguously confirmed, in solution by selective15 N-labelling
experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic
properties of this novel type of ligand suggest that they are poor s-donors and good p-acceptors.
rduengo’s landmark development of stable N-heterocyclic donors would bring the metal close to the central nitrogen of the
carbenes (NHCs; 1a)1 opened up new horizons in organic triazolium ion, allowing its coordination ability to be addressed.
A
and organometallic chemistry. Since then, NHCs have Pincer ligand frameworks are commonly used as a means to
received an extraordinary level of attention in homogeneous cataly- isolate elusive species and to investigate kinetically challenging
sis as free molecules2 and as ligands for transition metals3. chemical processes27–29
Numerous NHC derivatives with various molecular structures
.
have been prepared, including abnormal4–6 and acyclic carbenes7,8, Rh(I) and Ru(II) nitrenium complexes. When the Rh(I) precursor
as well as cyclic carbenes with inorganic backbones9,10. The ([Rh(C2H4)2Cl]2) was added to an equivalent amount of ligand 2a,
explosion of NHC-based chemistry has inspired researchers to sys- immediate precipitation of a new product was observed (Fig. 2b).
tematically explore isovalent analogues of NHCs within the p-block The 31P{ H} NMR spectrum of this product contains only one
1
elements, collectively referred to as main-group carbene analogues. doublet (1JP-Rh ¼ 131 Hz) at d ¼ 13.7 ppm, a downfield shift of
Silicon11,12, germanium13, boron14,15, gallium16,17, phosphorus18,19 33.8 ppm relative to the free ligand. This indicates that the two
and arsenic20,21 analogues of the Arduengo-type carbenes have phosphine arms are symmetrically coordinated to the rhodium
been prepared and shown to be excellent ligands for transition centre. To assess the coordination of the central nitrogen of the
metals. The missing link in this series of ligands is the nitrogen triazolium ring, an isotopologue of 2a selectively labelled with 15N
counterpart, the nitrenium ion, defined as a divalent cationic nitro- at the 2-position was prepared. This labelled ligand (2b) resonates
gen centre bearing a lone pair of electrons and an accessible vacant
p-orbital in its singlet state. Despite their close analogy to carbenes,
R
R
to the best of our knowledge, the application of nitrenium species as
ligands for metals is unprecedented. Here, we report that triazolium
1h, which is isoelectronic and isostructural to NHC 1a, can effi-
ciently coordinate to transition metals. It constitutes the first
example of such a species as a ligand in metal-based chemistry.
Experimental and theoretical studies of the electronic properties
of this novel type of ligand suggest that they are poor s-donors
and reasonable p-acceptors.
N
N
M
E
M
E
N
N
R
1'a–h
R
1a–h
R
R
R
N+
N
N
+
N
N
N
Results and discussion
N+
R
N
N
Nitrenium-based ligands. A closer look at nitrenium species 1h
reveals that it is essentially an alternative resonance form of a 1,3-
disubstituted 1,2,3-triazolium cation (forms A, B in Fig. 1). After
Arduengo’s synthesis of NHC, the stable and easily obtainable
1,2,3-triazolium ions received renewed interest as closely related
isovalent NHC analogues, and their significant nitrenium versus
triazolium character was highlighted in earlier work22,23. However,
in all previously reported cases, all 1,3-dialkyl triazolium species
R
R
1h
C
A
B
1a: E=C (carbene-NHC); 1b: E=Si (sillylene);
1c: E=Ge (germylene); 1d: E=B– (borylene);
1e: E=Ga– (gallium yilidene); 1f: E=P+ (phosphenium);
1g: E=As+ (arsenium); 1h: E=N+ (nitrenium)
were non-coordinative to metals24–26
To examine the coordination properties of nitrenium ions we Figure 1 | General representation of N-heterocyclic p-block carbenoids.
prepared ligands and (for synthetic protocols, see N-heterocyclic carbene 1a and its isovalent analogues (1b–h), together with
.
2
3
Supplementary Information), in which the triazolium ring was their contributing resonance forms (A–C) are presented. Species 1a–g are
incorporated within a pincer-type system with two chelating well recognized as ligands for transition metals, but the corresponding
‘arms’ (Fig. 2a). We anticipated that ligation by these examples for analogous nitrenium ions (1h) are unprecedented.
1Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel, 2Computational Chemistry Unit, Department of
Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel. e-mail: chmark@tx.technion.ac.il
*
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525

NATURE CHEMISTRY DOI: 10.1038/NCHEM.1068
ARTICLES
a
c
PR2
PR2
PPh2
PPh2
N
N
+
+
N
E
N
N
–
X–
PF6
2a: E=N
2b: E=15
3a: R=i-Pr, X=Br;
3b: R=Ph, X=PF6
N
–20
–40
–60
–80
ppm
b
d
PPh2
N
[(C2H4)2RhCl]2
2a, 2b
+
Cl
E
Rh
PPh2
N
–
PF6
4a: E=N (100%)
4b: E=15N (100%)
–20
–40
–60
–80
ppm
e
solv
R'
R'
R2P
N
(PPh3)3RuCl2
Ru
+
E
Cl
Cl
2b, 3a
N
X–
PR2
5: E=15N, R'=–(CH)2–,
R=Ph, X=PF6 (75%)
6: E=N, R'=H, R=i-Pr,
X=Br (64%)
–20
–40
–60
–80
ppm
Figure 2 | Nitrenium-based ligands, complexes and their 15N NMR spectra in solution. a, Benzotriazolium- and triazolium-based ligands bearing two
assisting chelating arms as substituents of the core ring. An isotopologue of 2a selectively labelled with 15N at the 2-position (2b) was also prepared to verify
coordination of the central nitrogen to metal precursors. b, Ligands 2 and 3a smoothly react with equimolar amounts of Rh(I) and Ru(II) precursors, resulting
in the clean formation of corresponding stable complexes 4–6 (isolated yields are given in brackets). These reactions are the first examples of nitrenium
cations as ligands for transition metals. c, 15N NMR spectrum of 2b. d, 15N NMR spectrum of 4b. e, 15N NMR spectrum of 5. The characteristic chemical
shifts and splitting patterns of signals of compounds 4b and 5 in the 15N NMR spectra unambiguously prove the coordination of nitrenium nitrogen to metal
centres in solution.
at d ¼ –23.7 ppm in 15N NMR (Fig. 2c). To our delight, when the deduced by several authors37,38. As in our case, unusually large
Rh(I) precursor was reacted with this labelled ligand, the 15N 31P–103Rh coupling constants of 316 Hz (ref. 39) and 326 Hz
NMR spectrum of the product 4b unambiguously indicated the (ref. 38) (as opposed to typical JRh-P values of 100–200 Hz) were
formation of an N–Rh bond. Coordination induced
a
observed for the only two reported phosphenium–Rh(I)-
characteristic upfield shift of 62.9 ppm and, more importantly, it chloride complexes.
appeared as a doublet of triplets due to the magnetic coupling of
The molecular structure of complex 4a was confirmed by single-
15N with 103Rh and 31P (Fig. 2d). Importantly, the observed crystal X-ray analysis (Fig. 3a). The rhodium centre has a slightly dis-
15N–103Rh coupling constant of 29 Hz obtained for 4b is much torted square planar geometry, as expected for d8 Rh(I) complexes.
larger than any other reported value for aliphatic or aromatic Notably, the N–Rh bond distance of 1.991(7) Å in 4a is somewhat
nitrogen-based ligands (10–16 Hz)30–32. Comparably large N–Rh shorter than that in the related pyridine-derived pincer complex 7
coupling constants have only been reported for sp-hybridized (N–Rh bond length ¼ 2.040(4) Å)40. This difference is even more
inorganic nitrogen ligands, such as terminal N2 or linear NOþ significant if one takes into account that the pincer arms in 7
(28–30 and 33–36 Hz, respectively)33,34. It is accepted35 that the
1
Cl
magnitude of JN-Rh is directly proportional to the s-fraction of
the hybrid nitrogen orbital that interacts with the metal. Thus, we
Ph2P
PPh2
Rh
N
can attribute the unusually large coupling constant observed for
4b to the increased s-character of the formally sp2-hybridized
nitrogen lone-pair orbital. As such a phenomenon is observed for
the central carbon of NHCs36, it may indicate that the analogous
triazolium nitrogen experiences an increase in its carbene-like
7
character on coordination to metal. Along these lines,
a
substantial s-character of the lone-pair orbital on phosphorous are one carbon unit shorter and, therefore, are expected to force the
in the analogous phosphenium cation 1f, as well as in other rhodium atom closer to the coordinating nitrogen. It is informative
N-heterocyclic phosphenium species, has been computationally also to note a slight N–N bond elongation in the coordinated species
526
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1068
ARTICLES
a
b
Cl1
Cl1
Cl2
P2
O1
N2
P1
Rh1
Ru1
N1
P2
P1
N1
N2
N3
N3
Br2
Figure 3 | Solid-state structures of 4a and 6 (hydrogen atoms are omitted for clarity). a, Selected bond lengths (Å) and angles (deg) of 4a. Bond lengths:
Rh(1)–N(1), 1.991(7); Rh(1)–Cl(1), 2.414(2); Rh(1)–P(1), 2.318(2); Rh(1)–P(2), 2.282(2); N(1)–N(2), 1.330(9); N(1)–N(3), 1.359(9). Angles: N(1)–Rh(1)–Cl(1),
177.6(2); P(1)–Rh(1)–P(2), 174.4(8); N(2)–N(1)–N(3), 103.9(6). b, Selected bond lengths (Å) and angles (deg) of 6. Bond lengths: Ru(1)–N(1), 2.040(6);
Ru(1)–Cl(1), 2.414(2); Ru(1)–Cl(2), 2.508(2); Ru(1)–P(1), 2.325(2); Ru(1)–P(2), 2.304(2); Ru(1)–O(1), 2.240(5); N(1)–N(2), 1.340(9); N(1)–N(3), 1.349(8).
Angles: N(1)–Ru(1)–Cl(1), 178.9(2); P(1)–Ru(1)–P(2), 104.1(1); Cl(1)–Ru(1)–Cl(2), 91.9(1); N(2)–N(1)–N(3), 102.2(6).
4a (average N(1)–N(2) and N(1)–N(3) bond length ¼ 1.346(1) Å) nitrenium and phosphines to the ruthenium centre (Fig. 2e). The
1
in comparison with the reported free dimethyl benzotriazolium 31P{ H} NMR spectrum has two broad signals that coalesce at elev-
cation 8 (N–N ¼ 1.313(7) Å, on average)41. The significance of ated temperatures into a single peak at d ≈ 28 ppm. This phenom-
this bond elongation is apparent if one considers the structurally enon can be rationalized by a relatively fast interconvertion of two
related benzopyrazolium cation 9 (N–N ¼ 1.364(1) Å)42. Whereas chemically non-equivalent phosphines, thus indicating their asym-
the N–N bonds in benzotriazolium have a partial multiple-bond metric manner of binding. In octahedral or trigonal bipyramidal
character due to the significant contributions from resonance struc- complexes, this is only possible if the phosphine arms of the
tures A and B (Fig. 1), the corresponding N–N bond in 9 is most ligand are in a cis-coordination relationship.
reasonably described as a single bond. As such, the N–N bond
Similar reactivity of the Ru(II) precursor was also observed with
distance in coordinated triazolium 4a is approximately halfway ligand 3a (Fig. 2b). The NMR features of the product are similar to
between those of 8 and 9.
those described for complex 5. In this case, crystals of resulting
complex 6 suitable for X-ray analysis were obtained from an N,N-
dimethylformamide (DMF) solution (Fig. 3b). The geometry
around the ruthenium atom is a slightly distorted octahedron
with the two phosphine-chelating arms occupying mutually cis-pos-
itions and a coordinated DMF solvent molecule. Similar to rhodium
system 4a, the N–N bonds in the coordinated nitrenium ligand are
somewhat elongated in comparison to free 1,3-disubstituted triazo-
lium cations (1.345(5) Å in 6 versus 1.324(7) Å or 1.313(6) Å in the
diethyl26 and dibenzyl44 triazolium salts, respectively; average values
Me
N+
O
N
Me
Me
Me
N
N
N
+
O–
8
9
Therefore, we may assume that the relative contribution of resonance of N–N bonds are given). The Ru–Cl(2) bond (2.508(2) Å), located
structure C (Fig. 1) is greater for the bound than the non-bound trans to the tertiary phosphine ligand, is significantly longer than
species, thus indicating an increase in the nitrenium character of the the Ru–Cl(1) bond, which is trans to the triazolium ring
central nitrogen upon its coordination to metal. This decrease in the (2.414(2) Å). As these bonds are related to the same system, it
p-occupation in the N–N bond of 4a could be a result of population allows us to suggest that the nitrenium ligand has a weaker trans
of the p-orbital of the carbene-like central nitrogen via efficient influence than the strongly s-donating trialkylphosphine.
p-back-donation from the rhodium centre. A similar slight lengthen-
ing of ꢀ2–3 pm of the phosphenium P–N bonds in the corresponding Electronic properties of nitrenium ligands. To study the electronic
metal (Rh, Co, Fe) complexes of type 1′f (Fig. 1) when compared properties of the new nitrenium ligands, we prepared rhodium
with the free cation 1f has been documented21,38,43. Analogous complexes with a CO ligand trans to the triazolium ring as a
N–As bond changes were also observed in N-heterocyclic arsenium diagnostic IR probe. Reactions of ligands 2 or 3b with [Rh(CO)2Cl]2
species coordinated to a cobaltate anion21.
result in the formation of new coordination compounds 10a or 10b,
Coordination of the novel nitrenium ligands is not limited to respectively, in which the nitrenium unit remains uncoordinated
rhodium. For example, reaction of ligand 2b with RuCl2(PPh3)3 (Fig. 4a). This absence of nitrenium unit coordination in solution is
results in the formation of a dark brown crystalline compound evident from the 15N NMR spectrum of the isotopically labelled
that was unambiguously identified as complex 5 by multinuclear analogue of 10a (compound 10c in the Supplementary
NMR techniques (Fig. 2b). Its 15N NMR spectrum exhibits an Information), which exhibits a singlet at –28.4 ppm, in the same
upfield-shifted (41.9 ppm relative to the free ligand) triplet due to region as the free ligand 2b. The structure of 10a was confirmed by
15N–31P coupling, which confirms the coordination of both single-crystal X-ray analysis, which clearly shows the absence of a
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527

NATURE CHEMISTRY DOI: 10.1038/NCHEM.1068
ARTICLES
O
a
O
Ph
Ph
Ph
Ph
C
C
PPh2 Ph2P
Ph
Ph
Ph
Ph
P
P
P
P
Rh
Rh
H2
N
Cl
N
[RhCl(CO)2]2
AgPF6
N
N
+
N
+
N
+
–
N
N
N
PF6
–
–
PF6
2PF6
R
R
R
R
R
R
2a, 3b
10a,b
11a (97%), 11b (93%)
2a, 10a, 11a: R=–(CH)2–
3b, 10b, 11b: R=H
c
b
O1
O1
C6
C36
Rh1
P1*
N2*
Rh1
P1
P1
P2
N3
N2
N1
Cl1
N2
N1
Figure 4 | Preparation of carbonyl complexes 10 and 11 and their solid-state structures. a, Complexes 10 and 11 were prepared to study the electronic
properties of the new nitrenium ligands. Isolated yields are given in brackets. CO ligands in 11 are located in a trans relationship to the triazolium ring as a
diagnostic IR probe. b, Solid-state structure of 10a. Selected bond lengths (Å): Rh(1)–C(38), 1.815(5); Rh(1)–P(1), 2.324(1); Rh(1)–P(2), 2.320(1); Rh(1)–Cl(1),
2.384(1); C(36)–O(1), 1.144(6); N(1)–N(3), 1.328(5); N(2)–N(3), 1.327(6). Selected angles (deg): C(36)–Rh(1)–Cl(1), 174.1(2); P(2)–Rh(1)–P(1), 172.4(5);
Rh(1)–C(36)–O(1), 176.2(5); N(1)–N(3)–N(2), 105.4(4). c, Solid-state structure of 11a (the structure presented here is for a molecule located on a C2
symmetry axis; another molecule, the structure of which is not shown, is found within the unit cell). Selected bond lengths (Å): Rh(1)–N(1), 2.146(9);
Rh(1)–C(6), 1.804(13); Rh(1)–P(1) ¼ Rh(1)–P(1)*, 2.332(2); C(1)–O(6), 1.126(14); N(1)–N(2) ¼ N(1)–N(2)*, 1.334(8); Selected angles (deg): C(36)–Rh(1)–N(1),
180.0(0); P(1)–Rh(1)–P(1)*, 176.1(1); Rh(1)–C(6)–O(1), 180.0(0); N(2)–N(1)–N(2)*, 106.6(8). In both structures, hydrogen atoms are omitted for clarity.
N–Rh bond (N–Rh distance ¼ 2.981 Å, Fig. 4b). Coordination of the Rh(I) centre in both structures. However, in contrast to 11a,
triazolium unit can, however, be induced by chloride abstraction complex 10a lacks a central Rh–N bond, and the metal unit is not
with an equimolar amount of AgPF6, leading to formation of the in the benzotriazolium plane. This demonstrates that despite the
desired products 11a,b in nearly quantitative yield (Fig. 4a).
trans-chelation by phosphine arms in this 16e complex (which
The measured CO bond vibrations of complexes 11a bring the metal into relative proximity to the triazolium nitrogen),
(y ¼ 2,024 cm21) and 11b (y ¼ 2,019 cm21) provide a direct this is insufficient to force coordination to the metal centre. Only
probe for the donor characteristics of the nitrenium ligands. It is par- when the proper conditions are created (abstraction of chloride in
ticularly interesting to compare the data for 11b with the CO frequen- this case) does the N–Rh bond form. In contrast to standard
cies of the structurally related NHC- and pyridine-based rhodium pincer ligands bearing rigid methylene arms, the flexibility of
complexes 12 (y ¼ 1,933 cm21
)
45 and 13 (y ¼ 1,980 cm21)46.
our ethylene-based donors allows both coordinated and non-
coordinated situations for the central triazolium motif.
O
O
In this regard, it is interesting to compare the Rh–N bond dis-
tances in complexes 11a and 4a. Although the steric environment
in both complexes around the metal centre is very similar, the
Rh–N bond in Rh–carbonyl complex 11a (2.151(6) Å) is substan-
tially longer than in its Rh–chloride counterpart 4a (1.991(7) Å).
This shows that the factors influencing the nitrenium–metal bond
in these systems are of an electronic (rather than steric) character.
Because CO is a strong p-acceptor (in contrast to the chloride
ligand), the lengthening of the N–Rh bond in 11a, in comparison
C
C
Ph2P
PPh2
Cl–
Ph2P
Rh
N
PPh2
Cl–
Rh
N
N
12
13
The considerably higher CO vibration frequency of 11b indicates to 4a, is possible if another p-acceptor (and poor s-donor)—the
that the nitrenium ligands are weaker s-donors than both the nitrenium ligand in this case—is located in the position trans to
NHC and pyridine. This data is consistent with the weaker trans the carbonyl. This is due to the destabilizing competitive inter-
influence of the nitrenium ring relative to phosphine in the ruthe- actions of the p systems of both ligands with the d-electrons of
nium complex 6 that was previously discussed.
Rh, namely p*–dp and pz–dp pull–pull considerations. By these
A comparison of the crystal structures of 10a and 11a is informa- same considerations, the N–N bonds in the triazolium ring of 11a
tive (Fig. 4b,c). These compounds bear the same pincer ligand (1.336(6) Å) are slightly shorter than those of compound 4a
framework, and the phosphine arms are trans-chelated to the (1.345(1) Å), but are still somewhat longer than the N–N bonds
528
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1068
ARTICLES
a
Cl
Table 1 | Selected bond distances (R, in Å) and Wiberg and
Mayer bond indices in model ligands 14a,b and the
corresponding RhCl complexes 15a,b.
H3P Rh
PH3
L
Me
Me
16a–d
Cl
Me
Me
P
P
N
Rh
N
+
P
N
+
Me
Me
HN
NH
HN
NH
HN
NH
+
N
+
N
N
L=
N
N
a
b
c
d
R
R
R
R
b
15a,b
14a,b
14a, 15a: R=H
14b, 15b: R=–(CH)2–
–6.5eV
Bond
R
Wiberg
Mayer
R
Wiberg
Mayer
14a
14b
C–C
C–N
N–N
N–CH3
1.396
1.383
1.368
1.482
15a
1.52
1.23
1.32
0.92
1.45
1.20
1.19
1.429
1.396
1.365
1.476
15b
1.24
1.13
1.33
0.92
1.26
0.95
1.22
0.81
0.86
–9.8eV
C–C
C–N
N–N
N–CH2
N-Rh
1.381
1.389
1.407
1.477
1.941
1.58
1.17
1.16
0.92
0.62
1.52
1.16
0.95
0.85
0.69
1.423
1.396
1.406
1.472
1.940
1.24
1.10
1.15
0.92
0.64
1.28
0.93
0.95
0.90
0.73
in the non-coordinated species 10a (1.327(1) Å) (the latter are very
similar to the free cation 8). These experimental data and the rela-
tively weak s-donor ability of the nitrenium cation suggest that the
primary bonding component of these new ligands comes from their
reasonable p-acceptor properties (supported by density functional
theory (DFT) calculations; vide infra). The weak s-donor and
good p-acceptor properties of the closely related phosphenium
and arsenium ligands, as well as the Lewis acid chemistry of phos-
phenium ions47, have been broadly documented. As such, the elec-
tronic characteristics of nitrenium are consistent with the properties
of other cationic NHC-analogues of group 15 elements and inverse
to those of native NHCs.
–17.8eV
Figure 5 | Electronic characteristics of nitrenium–metal bonding. a, The
nature of Rh–nitrenium bonding was examined by a CDA (see text) of
model system 16a and comparing the results to 16b–d. b, Selected molecular
orbitals and corresponding orbital energies in model complex 16a.
Top, lowest unoccupied molecular orbital (LUMO). Middle, highest
occupied molecular orbital (HOMO): Rh ꢁ N p-back-donation. Bottom,
HOMO: N ꢁ Rh s-donation. A CDA of the model nitrenium cation and
(PH3)2Rh(Cl) fragment revealed two dominant interactions: the HOMO-15,
which is the N ꢁ Rh s-donation, and the HOMO, which is the Rh ꢁ N
p-back-donation, contributing 13.9 and 14.6 kcal mol21 to the total binding
energy of 47.6 kcal mol21, respectively.
Theoretical studies of nitrenium–metal binding. The binding
properties of nitrenium ligands 2 and 3 were also studied using
DFT. The molecular geometries of two model ligands and their
Rh–Cl complexes were optimized at the DF-PBEþd/SDD(d) level
of theory, and the corresponding bond indices (Wiberg and
Mayer) and partial atomic charges were calculated (Table 1; see of the bonds are barely affected, there is some notable elongation of
Supplementary Information for full computational details). For the N–N bonds (by 0.04 Å) and a corresponding decrease in the
the calculation efficiency, the chelating diphenylphosphinoethyl bond indices, which is in full agreement with the experimental
arms of the free ligands were substituted by methyl groups, data discussed above. There is a slight increase in charge on the
resulting in model ligands 14a,b, but in the rhodium complexes central nitrogen atom of 15a,b (Supplementary Table S2), but,
only the phenyl groups were replaced by methyl groups (model otherwise, there is little change in the partial atomic charges
complexes 15a,b) to maintain the overall tridentate pincer on complexation.
framework. The optimized geometries show good correlation
The nature of metal–ligand bonding was further examined using
with the crystal structures when direct comparison is possible. a model system 16a (Fig. 5a). A charge decomposition analysis48
For instance, most deviations in bond lengths calculated for (CDA, PBE0/SDD(d) level of theory) of the nitrenium and trans-
model complex 15b relative to the corresponding experimental (PH3)2Rh(Cl) fragments revealed two dominant interactions. The
values obtained from the X-ray structure of 4a are within the HOMO-15 (Fig. 5b, bottom molecular orbital (MO)) is the N ꢁ Rh
range of 1–3%.
s-donation, and CDA analysis shows that it involves the donation
As evident from Table 1, bonding within the nitrenium ring in of 0.211 electrons and contributes 13.9 kcal mol21 to the total
14a and 14b is rather similar, except for the elongation of the binding energy of 46.7 kcal mol21. The HOMO (Fig. 5b, middle
C¼C bond in 14b due to conjugation with the rest of the phenyl MO) is the Rh ꢁ N p-back donation of 0.121 electrons contribut-
ring. As expected for a system with multiple resonance structures, ing 14.6 kcal/mol to the binding energy. This CDA analysis was
bond indices within the ring are considerably higher than observed repeated for the analogous phosphenium, NHC and pyridine
for single bonds. Although the coordination of the Rh–chloride unit ligands 16b–d (Table 2). One immediate observation is that in the
does not result in major changes in the ligand geometries, and most nitrenium and phosphenium systems there is more total M ꢁ L
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1068
ARTICLES
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Table 2 | Results of CDA for model systems 16a–d.
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membered N-heterocyclic carbenes with tunable electronic properties. J. Am.
Chem. Soc. 127, 10182–10183 (2005).
L
M ꢁ L p-donation
L ꢁ M s-donation
BE
(total electrons*)
(total electrons*)
(kcal mol21
)
16a
16b
16c
16d
0.249
0.494
0.229
0.098
0.112
46.7
60.6
75.8
45.7
0.162
0.450
0.231
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p-back-donation than s-donation. This is in stark contrast to the
other systems where the L ꢁ M s-donation is dominant.
To evaluate the relative binding strengths of the different triden-
tate species, ligand exchange reactions of the tridentate version of
14a with the corresponding phosphenium and NHC analogues
were examined (Supplementary Table S3). As with the monodentate
case, the order of binding strength is NHC . phosphenium . nitre-
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NHCs versus phosphorous- and nitrogen-based ligands.
As shown by DFT calculations, coordination of a simple mono-
dentate triazolium ion to a metal centre has a considerable binding
energy, comparable to that of pyridine. However, unlike pyridine,
coordination of the monodentate triazolium cation has yet to be
documented. This suggests that the problem of complexation of
such species is of a kinetic rather than thermodynamic character.
In our system, however, this kinetic obstacle is overcome by pre-
coordination of two chelating arms to the metal, leading to a stabi-
lizing complexation of the nitrenium unit.
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as ligands for transition-metal chemistry. This fills a missing link
in the series of both fundamentally and practically important
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stabilized nitrenium ions: experimental and theoretical study of 1,3-dimethyl
nitrenium ions to electron-rich transition metals has been demon-
strated and characterized both in the solid state and in solution.
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and DFT calculations showed that nitrenium complexation to a
metal has a considerable binding energy (47 kcal mol21). Studies
of electronic properties show that these nitrenium ligands are rela-
tively weak s-donors and reasonable p-acceptors. The simplicity of
the triazolium synthesis allows for facile and broad modification of
the steric environments of the ligands. Further investigation of the
properties of these novel systems, including expansion of the nitre-
nium ligand family and the application of their metal-based com-
plexes for catalysis, are currently under way in our laboratory.
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Received 20 December 2010; accepted 10 May 2011;
published online 19 June 2011
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Acknowledgements
The authors acknowledge financial support from the US–Israel Binational Science
Foundation (grant no. 2008391), the Israel Science Foundation (grant no. 1292/07) and the
FIRST Program of the Israel Science Foundation (grant no. 1514/07). The authors are also
grateful to G. Molev for fruitful discussions and D. Milstein for ongoing support.
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Author contributions
Y.T. designed and performed the experiments, and wrote the manuscript. M.A.I. performed
the DFT calculations. M.B. collected single-crystal X-ray crystallographic data and solved
the structures. M.G. designed and managed the project.
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Additional information
The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://www.
nature.com/reprints/. Correspondence and requests for materials should be addressed to M.G.
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