Organometallics
Article
lithium as a base. L7-H contained two isomers in a ca. 1:1
ratio. In the 31P{1H} NMR spectrum in C6D6 at room
temperature, four 31P NMR resonances were observed. 31P
NMR resonances at δ 148.7 and 147.9 were assigned to P−O of
two isomers, respectively, and 31P NMR resonances at δ 61.2
and 60.6 were assigned to P−N of two isomers, respectively. The
1H NMR spectrum also showed two sets of resonances that
correspond to two isomers. The nature of the two isomers was
not determined, but we tentatively assume that these are
rotamers arising from restricted rotation associated with the N−
P and/or C−O−P bonds. L15-H was synthesized from 3-
hydroxybenzyl alcohol. 3-Hydroxybenzyl alcohol 1 was
chlorinated with thionyl chloride and pyridine to provide 3-
hydroxybenzyl chloride 2. 2 was treated with n-butyllithium and
ClPtBu2 to provide compound 3. 3 was then treated with sodium
tert-butylthiolate to provide L15-H (Scheme 2).
Scheme 1. (A) Catalytic C−H Borylation using HBpin or B2
pin2, (B) (POCOP)Ir-Catalyzed Arene C−H Borylation, and
(C) Proposed Reaction Mechanism of (POCOP)Ir-
Catalyzed Arene C−H Borylation
Synthesis of Ir and Rh Pincer Complexes. All of the
pincer complexes in this study are shown in Figure 2. The pincer
complexes of Rh, (L3)Rh(H)(Cl),52 (L16)Rh(H2),53 and
(L18)Rh(H)(Cl),51 were prepared as previously described.
Literature procedures were also used to obtain Ir complexes with
central donors other than aryl, (L16)Ir(H)(Cl),54 (L17)Ir-
(COE)50 and (L18)Ir(H)4,55 and (L19)Ir(H)(Cl).43
Installation of the PCP ligands into the coordination sphere of
Ir is most conveniently accomplished via reaction of the ligand
precursor with [(COD)IrCl]2 (COD = 1,5-cyclooctadiene) or
[(COE)2IrCl]2. The insertion of a metal center into the central
aromatic C−H bond and the loss of the olefin placeholder
ligand, COD and COE, can ideally lead to the five-coordinate
Ir(III) complex (L)Ir(H)(Cl) (Scheme 3, method A). However,
method A does not always provide for the clean formation of the
desired product; therefore, different strategies were sampled to
access the desired (L)Ir complexes.
Reacting [(COD)IrCl]2 or [(COE)2IrCl]2 with pyridine
prior to the addition of the ligand precursor results in the
formation of a pyridine adduct, (L)Ir(H)(Cl)(py) (Scheme 3,
method B). The pyridine ligand can then be abstracted with
boron trifluoride diethyl etherate (BF3·OEt2) to give the desired
five-coordinate complex (L)Ir(H)(Cl). This approach has been
reported to access clean (POCOP)Ir and (POCOP)Rh
complexes.42,48,56 Alternatively, [(COD)M(OAc)]2 (M = Ir,
Rh) was also used in the metalation of the pincer ligands to yield
the products (L)Ir(H)(OAc).30,48 The κ2-acetate ligand
provides protection to the sixth coordination site of the Ir
center (Scheme 3, method C). Another approach (Scheme 3,
method D) was described by Waterman and co-workers, who
reported that the reaction of the ligand precursor L1-H with
[(COE)2IrCl]2 under an atmosphere of H2 gave a high yield of
(L1)Ir(H)(Cl).57
Methods A−D were then screened for the metalation of a
ligand on the 10−100 μmol scale (Table 1). Ir complexes
(L3)Ir(H)(Cl),42 (L4)Ir(H)(Cl), (L5)Ir(H)(Cl),44 (L7)Ir-
(H)(Cl), (L16)Ir(H)(Cl),54 and (L19)Ir(H)(Cl)43 were
successfully formed via method A. Using method B, (L8)Ir-
(H)(Cl)(py) and (L13)Ir(H)(Cl)(py) were successfully
formed. (L15)Ir(H)(OAc) was formed cleanly by using method
C. Method D was used to yield (L4)Ir(H)(Cl) and (L11)Ir-
(H)(Cl). Unfortunately, we were not able to optimize the
syntheses or isolation of the desired Ir products from the
reactions using L6-H, L9-H, L10-H, L12-H, and L14-H.
Scheme 4 shows the syntheses of new Ir complexes (top) and
the improved syntheses of (L11)Ir(H)(Cl) and (L11)Rh(H)-
(Cl) (bottom) on a preparative scale. (L11)Ir(H)(Cl) and
In addition, we tested some analogous pincer complexes of Rh.
The present report documents the results of our study.
RESULTS AND DISCUSSION
■
Synthesis of Pincer Ligands. We set out to examine a
variety of ligands with a central aryl group, as well as ligands that
possess other central anionic donors based on B, N, or Si (L3-
H−L19-H, Figure 1). Ligands L3−L14 have phosphines as L-
type arm donors and an aryl site as an X-type central donor
(PCP ligands). The series of ligands was selected to explore
steric and electronic variations in the PCP pincer structure.
Ligand L15 was chosen to compare a sulfur donor to a
phosphorus donor and also to examine the steric effect of three
tBu groups about the metal as opposed to four Pr or four Bu
groups. Ligands L16−L19 were chosen to test the viability of
different X-type central donors: amido, boryl, and silyl.
i
t
L3-H,42 L5-H,44 L6-H,41 L8-H,45 L9-H,45 L10-H,46 L11-
H,47 L12-H,47 L13-H,48 L14-H,48 L16-H,49 L17-H,50 L18-
Ph,51 and L19-H43 have been previously reported and were
synthesized as described in the literature. L4-H was prepared
from 1,3-resorcinol, chlorodicyclohexylphosphine, and triethyl-
amine as a base. L7-H was prepared analogously from 2-
hydroxycarbazole, chlorodiisopropylphosphine, and n-butyl-
1005
Organometallics 2021, 40, 1004−1013