Comparative Study of Rhodium and Iridium Porphyrin Diaminocarbene and N‑Heterocyclic Carbene Complexes

Iridium meso-tetratolylporphyrinato (TTP) mono-
and bis-diaminocarbene complexes, [Ir­(TTP)­[C­(NHBn)­(NHR)]2–x(CNBn)x]­BF4, where R = Bn, n-Bu and x = 1, 0, were synthesized by nucleophilic addition of amines
to the bis-isocyanide complex [Ir­(TTP)­(CNBn)2]­BF4. Rhodium and iridium porphyrinato N-heterocyclic carbene
(NHC) complexes M­(TTP)­CH3(NHC), where NHC = 1,3-diethylimidazolylidene
(deim) or 1-(n-butyl)-3-methylimidazolylidene (bmim),
were prepared by the addition of the free NHC to M­(TTP)­CH3. The NHC complexes displayed two dynamic processes by variable-temperature
NMR: meso-aryl–porphyrin C–C bond rotation
and NHC exchange. meso-Aryl–porphyrin C–C
bond rotation was exhibited by both rhodium and iridium complexes
at temperatures ranging between 239 and 325 K. Coalescence data for
four different complexes revealed ΔG⧧ROT values of 59 ± 2 to 63 ±
1 kJ·mol–1. These relatively low rotation barriers
may result from ruffling distortions in the porphyrin core, which
were observed in the molecular structures of the rhodium and iridium
bmim complexes. Examination of NHC exchange with rhodium complexes
by NMR line-shape analyses revealed rate constants of 3.72 ±
0.04 to 32 ± 6 s–1 for deim displacement by
bmim (forward reaction) and 2.7 ± 0.4 to 18 ± 2 s–1 for bmim displacement by deim (reverse reaction) at temperatures
between 282 and 295 K, corresponding to ΔGf⧧ of 65.2 ± 0.6 kJ·mol–1 and ΔGr⧧ of
66.2 ± 0.5 kJ·mol–1, respectively. Rates
of NHC exchange with iridium were far slower, with first-order dissociation
rate constants of (1.75 ± 0.04) × 10–4 s–1 for the forward reaction and (1.2 ± 0.1)
× 10–4 s–1 for the reverse
reaction at 297.1 K. These rate constants correspond to ΔG⧧ values of 94.2 ± 0.6 and 95.2
± 0.2 kJ·mol–1 for the forward and reverse
reactions, respectively. Equilibrium constants for the exchange reactions
were 1.6 ± 0.2 with rhodium and 1.56 ± 0.04 with iridium,
favoring the bmim complex in both cases, and the log­(K) values for NHC binding to M­(TTP)­CH3 were 4.5 ±
0.3 (M = Rh) and 5.4 ± 0.5 (M = Ir), as determined by spectrophotometric
titrations at 23 °C. The molecular structures also featured unusually
long metal–Ccarbene bonds for the bmim complexes
(Rh–CNHC: 2.255(3) Å and Ir–CNHC: 2.194(4) Å).