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workup. No intermediates were observed when the reaction
was monitored by NMR spectroscopy (1H, 31P; C6D6).
Aromatic azides also react with 1 in toluene to yield purple
solutions of the corresponding imido complexes [Rh-
(PhBP3)(NR)] [R = 2,6-iPr2C6H3 (3), C6F5 (4)]. The complex
3 was isolated as a purple microcrystalline solid, while 4 was
characterized in situ.
atom. Moreover, their hydrodynamic radii (rH), evaluated by
DOSY experiments, exhibit similar values (6.84, 6.69, and
6.36 ꢀ, for 2, 3, and 4, respectively), thus showing that they
are mononuclear in solution and excludes plausible mono-
mer/dimer equilibria. Furthermore, these radii are close to the
average molecular ones calculated by DFT methods (7.07 and
6.74 ꢀ, for 2a and 4a, respectively).
For 2, we were fortunate to obtain reliable X-ray
diffraction data of a single crystal,[15] and the structure
revealed a rhodium atom confined to an almost perfect
tetrahedral with an overall C3v symmetry resulting from its
bonding to three equivalent phosphorus atoms of the tripodal
PhBP3. The fourth site is represented by the imido ligand,
having a Rh-Nimido bond distance of 1.780(2) ꢀ, which is
shorter by about 0.3 ꢀ when compared to similar species
having bridging amido or imido ligands,[10,13,16] but similar to
that observed for [IrCp*NR].[7] The short distance together
with an almost linear Rh-Nimido-CAd angle [177.5(2)8] are
Complexes 2–4 are thermally stable, but highly sensitive
to moisture, with 2 being the least sensitive. Their solutions in
C6D6 are immediately hydrolyzed by traces of water, thus
giving the free amine and uncharacterized beige solids which
are insoluble in common organic solvents. Therefore, the
imido nitrogen atom is a Brønsted base which abstracts
protons from water. In this sense, 2 reacts cleanly with protic
acids such as HBF4·Et2O (2 molar equivalents) in acetonitrile
to give the amine H2NAd and [Rh(PhBP3)(NCMe)3](BF4)2.[14]
No other compounds, except the starting complex, were
observed in this reaction when using a 1:1 molar ratio.
To gain insight into the electronic structures of 2–4 and the
=
indicative of such ligands having a Rh N multiple bond.
À
Although the adamantyl group was found to be disordered in
two positions, for the purpose of clarity we display only one as
shown in Figure 1.
characteristics of the Rh N bond, DFT calculations [b3lyp,
LanL2Tz(f)/6-311G(d)/6-31G(d,p)] were carried out on the
complexes [Rh(PhBP3)(NR)] [R = Ad (2a), C6F5(4a)] and on
a model compound of 3 (3a), in which the iPr substituents on
the imido aryl were replaced by hydrogen. The results of the
calculation for 2a gave geometrical parameters which are in
good agreement with the experimentally determined struc-
ture of 2 (see Figure S4 and Table S1 in the Supporting
Information). For the aryl imido complexes 3a and 4a, the
symmetry of the calculated structure moves away from C3 by
distortion of the metal coordination sphere. An off-axis
distortion in which the Rh–N vector is bent away from the B–
C vector gives rise to a core which is slightly distorted towards
a trigonal pyramid. In addition, the Rh-N-C angles (166.738
and 170.688, respectively) are more bent than those for the
alkylimido 2, which is typical for arylimido complexes.[4b,17]
The ground-state frontier molecular orbital (MO) diagram
for 2a is shown in Figure 2. The LUMO and LUMO + 1
correspond to two antibonding combinations of the metal-
based orbitals 2ea and 2es (of parentage dxz and dyz) with the px
and py orbitals of the imido nitrogen atom. The respective
bonding combinations, HOMO-39 and HOMO-40, are found
to be lower in energy, so that they correspond to two Rh-N p-
bonds. The s component mainly arises from the overlap
between the filled dz2 orbital (1a1) with a filled sp orbital of
the nitrogen atom, as observed in HOMO-2 and HOMO-63.
Repetitive attempts to grow single crystals of 3 or 4
systematically gave very small and geminated microcrystals,
thus preventing crystallographic structure determination.
Nonetheless, 2–4 display a doublet (JP, Rh ꢁ 110 Hz) in the
31P{1H} NMR spectra, which is distinctive of species with high
symmetry, as shown by the structure of 2. All of them possess
identical connectivity, which is clearly evident by the cou-
plings of the phosphorus nuclei with carbon, proton, or
fluorine nuclei of the groups R bonded to the imido nitrogen
À
From this perspective, the Rh N bond would lack the
s component, and hence be more in accord with a double
bond. Nonetheless, the topology of the dz2 Àbased MOs
clearly suggests further mixing with the empty low-lying 2a1
orbital of the [Rh(PhBP3)]2+ fragment (see inset in Figure 2).
This symmetry-allowed mixing stabilizes the dz2 orbital, thus
mitigating the destabilizing effect of populating the Rh–N s*
interaction. Moreover, the measured metrical parameters
Figure 1. Top: Structure (ORTEP at 50% level) of complex [Rh-
(PhBP3)NAd] (2; only the Cipso atoms of the phenyl groups from PPh2
are shown for clarity). Selected bond distances [ꢄ] angles [8]: Rh–P1
2.2629(7), Rh–P2 2.2839(7), Rh–P3 2.2591(7), Rh–N 1.780(2), N–C46
1.431(3); P1-Rh-N 127.83(8), P2-Rh-N 127.51(7), P3-Rh-N 126.39(8),
axis(B,Rh)-N 179.59(9), Rh-N-C46 177.5(2). Bottom: Space-filling rep-
resentation of 2.
À
imply that the Rh N bond should actually be considered to be
close to a triple bond according to the bond distance (1.80 ꢀ)
À
computed for a Rh N triple bond using the method put forth
by Pauling.[18] On the whole, the bonding in this imido
complex seems to be similar to that of pseudo-tetrahedral
cobalt(III) imido complexes with tripodal ligands.[4b,5a–f]
2
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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