Communication
metallic fragment in the singlet state is retained in the com-
plex and the HOMO–LUMO splitting in 2a (3.49 eV) is even
bigger. Moreover, NBO analysis of 2a gives a bond order of
1.99 for the C46ꢁC47 bond, which is compatible with a strong
donation from alkyne to the rhodium center, and an almost
zero natural charge (ꢁ0.059) was calculated for the metal. Con-
sequently, complex 2 is quite well represented as a RhI com-
plex with the alkyne h2-coordinated to the metal through
a double (s and p) bond.
For comparative purposes, the optimized geometry for 2a
was calculated for the triplet state. It was found to be higher
in energy than the singlet one (by 24.8 kcalmolꢁ1). Although
still pseudotetrahedral, the coordination geometry and the dis-
position of the alkyne are quite different from those deter-
mined by X-ray diffraction. In particular, the alkyne is rotated
and bound to the metal by one single carbon atom because
one of the RhꢁC distances, 2.344 ꢃ, is clearly nonbonding (see
the Supporting Information). In consequence, the pseudotetra-
hedral geometry of the alkyne complex is a result of the d-or-
bital splitting of the RhP3 fragment as a closed-shell species
generated by a strong-field ligand that leaves two empty
metal orbitals able to accept four electrons from the alkyne.
A survey on the literature revealed that h2-alkyne coordina-
tion to rhodium is dominated by a two-electron donicity stabi-
lizing both trigonal bipyramid (TBPY) and square-planar (SP)
complexes. From the few examples that have been crystallo-
graphically characterized, electronically saturated TBPY com-
plexes are derived from 16 electron metal fragments such as
RhCl(PMe3)3,[18] RhCpPiPr3,[19] or RhTp(L),[20] (Tp=tris(pyrazolyl-
borate)) while T-shaped 14 electron fragments such as Rh(X)-
(PiPr3)2 (X=Cl, I)[21] and Rh(acac)(olefin)[22] (acac=acetylaceto-
nate) bind alkynes to yield 16 electron SP compounds. Func-
tionalized alkynes like thioether alkynylborates,[23] and P(CꢀC)P
pincer-type ligands[24] also bind rhodium centers as two-elec-
tron donors. Consequently, complex 2 represents the first au-
thenticate rhodium complex with the alkyne donating more
than two electrons, thus resulting in the unusual coordination
environment for rhodium.
Figure 4. 31P{1H} NMR spectrum of complex [Rh(PhBP3)(PhCꢀCH)] (2) in
CD2Cl2 at ꢁ808C (top). Total energy (in hartree) versus scan coordinate for
the rotation of the alkyne around the Rh–Ct axis in the model [Rh{MeB-
(CH2PMe2)3}(HCꢀCPh)] (2a; bottom).
scribed as pseudotetrahedral, as found for 2, and undergoing
a fast rotation around the Rh–Ct axis.
The deep colour of 2 is mainly due to spin-allowed transi-
tions to the LUMO, which result in bands in the UV/Vis spec-
trum at about 530 and 380 nm (see the Supporting Informa-
tion, Figures S12 and S13) according to time-dependent DFT
(TD-DFT) calculations.
Complex 2 was found to be a reactive complex despite
being electronically saturated (18 electron), a fact that could
be related to the low-lying LUMO (p*, Figure 3 right), which is
pointing toward a possible vacant site. Thus, complex 2 easily
reacts with hydrogen under atmospheric pressure to give [{Rh-
(PhBP3)(H)(m-H)}2][8a] and ethylbenzene (quantitative by NMR
analysis). Moreover, addition of a two-electron donor such as
PMe3 triggers a CꢁH activation reaction to give the alkynylhy-
drido rhodium(III) complex, [Rh(PhBP3)(CꢀCPh)(H)PMe3] (3,
Scheme 1), which was isolated as an off-white solid.
Complex 2 maintains in solution the symmetric structure
Relevant signals correspond to the hydride ligand (d=
ꢁ8.74 ppm, dddt) and to the acetylide carbon atoms (d=111.5
(ꢀCPh), 109.5 ppm (RhCꢀ)) in the 1H and 1H,13C-HMBC NMR
spectra, respectively. The expected ABCMX spin system (M=
1
found in the solid state. The signals in the H and 13C{1H} NMR
spectra coming from the HCꢀC proton and the CꢀC carbon
atoms are largely low-field shifted (d=10.04, and d=164.9,
151.8 ppm, respectively), as found typically for alkynes behav-
ing as four-electron donors (see the Supporting Information,
Figures S5 and S6).[15] The three phosphorus nuclei from the
PhBP3 ligand remain equivalent even at ꢁ708C. This fluxional
behavior has to be attributed to a free rotation of the alkyne
around the Rh–Ct axis. Indeed, calculation of this motion in
the 0–1208 range in 108 steps leads to a very low activation
barrier of 0.9 kcalmolꢁ1 (Figure 4). This low-energy barrier is as-
sociated with an almost continuous overlap of the orbitals in-
volved in the metal–alkyne bond all along the move, thus
avoiding a bond cleavage. The easy rotation of the alkyne cor-
roborates our above description of the complex and excludes
an expectedly more rigid metallacyclopropene–RhIII resonant
structure.[25] Most probably, the original complexes reported by
Bianchini,[10] having similar spectroscopic data, are better de-
Scheme 1. Synthesis of complexes 3 and 4.
Chem. Eur. J. 2014, 20, 2732 – 2736
2734
ꢄ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim