Hydrogen Bonding Behavior of Amide-Functionalized α‑Diimine Palladium Complexes

A class
of (N,N′-diaryl-α-diimine)­Pd
complexes bearing amide substituents on the N-aryl rings is described.
Hydrogen bonding interactions involving the amide groups influence
the structures, isomer distributions, and ligand coordination behavior
of these compounds. The amide-functionalized α-diimine ligands
(2,6-iPr2-Ph)­NCMeCMeN­(2-C­(O)­NMe2-6-iPr-Ph) (4a), (2,6-iPr2-Ph)­NCMeCMeN­(2,6-(C­(O)­NMe2)2-Ph) (4b), and (2-C­(O)­NMe2-6-iPr-Ph)­NCMeCMeN­(2-C­(O)­NMe2-6-iPr-Ph) (4c) were prepared by condensation
reactions of 2,3-butanedione and the appropriate anilines. The attempted
preparation of (2,6-iPr2-Ph)­NCMeCMeN­(2-C­(O)­NHMe-6-iPr-Ph) (4d) yielded the corresponding 1,2-dihydroquinazolinone
derivative 4d′ formed by nucleophilic attack of
the amide nitrogen at the proximal imine carbon. 4a and 4b react with (cod)­PdMeCl to yield square planar (α-diimine)­PdMeCl
complexes 5a,a′ and 5b,b′, respectively, which exist as two isomers
that differ in the orientation (trans/cis) of the Pd–Me ligand
and the amide-substituted arylimine unit. 4c reacts with
(MeCN)2PdCl2 and (cod)­PdMeCl to yield (4c)­PdCl2 (6c-anti,syn) and (4c)­PdMeCl (5c-anti,syn), which exhibit anti/syn isomerism due
to hindered rotation of the Caryl–N bonds. In the
solid state, the amide oxygen atoms in 6c-anti and 5c-syn engage in hydrogen bonding with cocrystallized CH2Cl2 solvent molecules. 4d′ reacts
with (MeCN)2PdCl2 via ring-opening metalation
to afford the α-diimine complex (4d)­PdCl2 (6d). Transmetalation of 6d with SnMe4 yields (4d)­PdMeCl (5d,d′) as a mixture of trans and cis isomers. The reaction of 5d,d′ with AgOAc yields (4d)­PdMe­(OAc) (7d) as a single isomer in which the Pd–Me
group is trans to the amide-functionalized arylimine unit. 5d, 6d, and 7d exhibit intramolecular N–H···Cl
and N–H···O hydrogen bonding interactions involving
the amide NH units. The reactions of 5a,a′, 5c-anti, and 5d,d′ with AgSbF6 in the presence
of pyrazole yield the corresponding (α-diimine)­PdMe­(pz)+SbF6– salts (8a,c,d; pz = pyrazole), which exhibit an intramolecular
hydrogen bond between the amide oxygen and the pyrazole NH unit. 8a,c,d undergo partial dissociation
of pyrazole in CD3CN solution to generate the corresponding
CD3CN complexes 9a,c,d. The non-hydrogen-bonded complex {(2,6-iPr2-Ph)­NCMeCMeN­(2,6-iPr2-Ph)}­PdMe­(pz)+SbF6– (8e) and its
pyrazole dissociation product {(2,6-iPr2-Ph)­NCMeCMeN­(2,6-iPr2-Ph)}­PdMe­(CD3CN)+SbF6– (9e) were generated in a
similar fashion. The pyrazole dissociation constants, Keq = [(α-diimine)­PdMe­(CD3CN)+] × [pz] × [(α-diimine)­PdMe­(pz)+]−1, vary in the order 8e > 8d > 8a > 8c, span more than 2 orders
of
magnitude, and reflect the enhancement of pyrazole binding in 8a,c,d by amide–pyrazole
hydrogen bonding. The intramolecular hydrogen bonding in 8c strengthens pyrazole binding by a factor of ca. 120 (i.e., ΔΔG = 2.8(1) kcal mol–1) relative to the
case of 8e.