PCP Pincer Iridium Chemistry – Coordination of Pyridines to [(^tBuPCP)IrH(Cl)]

Abstract

The complexes of [( tBu PCP)IrH(Cl)] {1, tBu PCP = 1,3bis[(di-tert-butylphosphanyl)methyl]benzene} with pyridine (py, 2), 4-aminopyridine (4apy, 3) and 2-aminopyridine (2apy, 4) have been synthesized and fully characterized by variable-temperature spectroscopy (NMR, IR, and UV/Vis). In each case, the reaction yields a mixture of cis-and trans-hydrido-chlorido isomers. The stereochemistry of the latter was confirmed by single-crystal X-ray analysis of 2a and 3a, which showed that the pyridine ligands coordinated opposite the aryl ring do not produce an additional twist of the pincer ligand but lead to the elongation of all bonds involving the iridium atom. The chlorido ligand becomes substantially labilized owing to the strong trans effect of the hydrido ligand, and that in 3a it is prone to inter- [a] Complex 2b: 1 H NMR (300 MHz, CD 2 Cl 2 , 260 K): δ = 10.14 (pt, 2 J N,H = 4.5 Hz, 3 J H,H = 4.9 Hz, 1 H, o-Py), 7.89 (pt, 2 J N,H = 5.3 Hz, 3 J H,H = 4.5 Hz, 1 H, o-Py), 7.59 (t, 3 J H,H = 6.7 Hz, 1 H, p-Py), 7.22 (t, 3 J H,H = 6.1 Hz, 1 H, m-Py), 6.98 (t, 3 J H,H = 6.1 Hz, 1 H, m-Py), 6.88 (d, 3 J H,H = 7.3 Hz, 2 H, m-Ph), 6.72 (t, 3 J H,H = 7.3 Hz, 1 H, p-Ph), 2.94 (pq, 2 J P,H = 16.4 Hz, 4 H, CH 2 ), 1.25 (t, 2 J H,H = 6.3 Hz, 18 H, CH 3 ), 0.92 (t, 2 J H,H = 6.3 Hz, 18 H, CH 3 ), -21.64 (pq, 2 J N,H = 18.2 Hz, 2 J P,H = 15.5 Hz, 1 H, H) ppm. 31 P{ 1 H} NMR (121 MHz, CD 2 Cl 2 , 260 K): δ = 46.1 ppm. 15 N NMR (61 MHz, CD 2 Cl 2 , 260 K): δ = -138.7 ppm. ( tBu PCP)IrH(Cl)(4apy): Yield 87 %, ratio of isomers 1:2.5. C 29 H 50 ClP 2 N 2 Ir (716.35): calcd. C 48.62, H 7.04, N 3.91; found C 49.41, H 7.25, N 3.92.

Effect of Ligands on the Lewis Acidity of the Metal and the Binding of N‐Bases to Iridium Pincer Complexes

“…The molecular electrostatic potential (MEP) minima, V S, min , are also located on the Cl ligand in complexes 1 , 4 – 10 and possess two extrema points (Figure ), which correspond to the two occupied p‐orbitals. This is in good agreement with the experimentally observed hydrogen bonding of alcohols and –NH 2 to chloride , . The introduction of COOMe and NO 2 substituents (complexes 3 , 7 , 8 ) expectedly decreases the chloride basicity (see V S, min in Table ), and the global MEP minima in these complexes are located on the substituents′ oxygen atoms.…”

“…The apical isomers a are predominant for both 2·Py and 3·Py , the isomers ratio being 24.9:1 and 23.2:1, respectively, at 210 K decreasing to 17.8 and 18.7 at 270 K (Table S1). This is quite different from 2.5:1 ratio observed for 1·Py_a / 1·Py_b or 5:1 ratio found 4·Py_a / 4·Py_b that could be the result of different steric protection of iridium centers in 1 – 4 . Another interesting point is that coalescence temperature for the resonance of isomer a is lower for 4·Py_a (220 K) than for 1·Py_a (260 K).…”

“…The resonances of 1·Py_a ‐ 3·Py_a appear as pseudoquartets at low temperatures (Figure , Figure S1), whereas for 4·Py_a it is a pseudo‐triplet (Figure ); all because of 2 J NH ≈ 2 J PH (Table ). Such a strong effect evidences the coordination of pyridine trans to the hydride ligand , . The dissociative exchange is evident from the broadening of the hydride resonances of apical isomers at room temperature (Figure , Figure ).…”

“…Recently, we have shown that the interaction of [(PCP)IrH(Cl)] ( 1 ) with pyridine yields instantaneously and quantitatively hexacoordinate complexes 1·Py_a and 1·Py_b (Scheme , Table ) . At room temperature the pyridine binding is also quantitative in the case of pyrazolate‐containing complex 4 (no signals of the starting material 4 are observed in NMR spectra), but not in the case of [(POCOP)IrH(Cl)] ( 2 ) and [{EtOC(O)‐POCOP}IrH(Cl)] ( 3 ).…”

The Role of (tBuPOCOP)Ir(I) and Iridium(III) Pincer Complexes in the Catalytic Hydrogenolysis of Silyl Triflates into Hydrosilanes

“…We expected that formation of this mixture would be influenced in the presence of a coordinating solvent. Indeed, when heated in acetonitrile, this mixture gave back the sole iridium(I) species [( tBu-POCOP)Ir(NCMe)] 9 (Eqn 6), which has already been reported 81 when reacting NaOtBu with 1 in acetonitrile.…”