Mononuclear NHC−Pd−π-Allyl Complexes Containing Weakly Coordinating Ligands

Attempts are described to synthesize ionic 14e [(π-allyl)Pd(NHC)]Y complexes (NHC = C(N(tBu)CH)2, C(N(C6H3-2,6-iPr2)CH)2). Reaction of {(η3-2-RC3H4)Pd(μ-Cl)}2 (R = H, Me) with AgOTf yields
polymeric, helical {(η3-C3H5)Pd(μ-OTf)}n (5a) and dimeric {(η3-2-MeC3H4)Pd(μ-OTf)}2 (5b). Treatment
of (η3-2-RC3H4)Pd(NHC)Cl (R = H, Me) (6a−d) with LiMe or (tmeda)MgMe2 affords the Pd−methyl
complexes (η3-2-RC3H4)Pd(NHC)CH3 (7a−d). Adduct formation of 5a,b with NHC, metathesis of 6a−d
with AgOTf, and protolysis of 7c,d with HOTf yield the triflate complexes (η3-2-RC3H4)Pd(NHC)(OTf)
(8a−d), whereas protolysis of 7a,b with acids (HOTf, HBF4) results in complex degradation. From the
reactions of 6a,c with AgBF4 in THF, and AgPF6 and AgAlRF4 (RF = OC(CF3)3) in THF and CH2Cl2,
respectively, the ionic THF-solvates [(η3-C3H5)Pd{C(N(tBu)CH)2}(THF)]Y (Y = BF4, 9a(BF4); PF6,
9a(PF6); Al{OC(CF3)3}4, 9a(AlRF4)) and [(η3-C3H5)Pd{C(N(C6H3-2,6-iPr2)CH)2}(THF)]Y (Y = BF4,
9c(BF4); PF6, 9c(PF6)) and the CH2Cl2-solvates [(η3-C3H5)Pd{C(N(tBu)CH)2}(CH2Cl2)]AlRF4 (11a(AlRF4))
and [(η3-C3H5)Pd{C(N(C6H3-2,6-iPr2)CH)2}(CH2Cl2)]PF6·2CH2Cl2 (11c(PF6)) have been isolated. In
contrast, reaction of 6a,c with AgBF4 in toluene or CH2Cl2 yields the neutral solvent-free BF4 adducts
(η3-C3H5)Pd{C(N(tBu)CH)2}(BF4) (12a) and (η3-C3H5)Pd{C(N(C6H3-2,6-iPr2)CH)2}(BF4) (12c), consistent
with the marked nucleophilicity of the BF4 anion. Drying the CH2Cl2-solvates 11a,c(Y) (Y = PF6, AlRF4)
at ambient temperature under vacuum affords the PF6 and AlRF4 adducts (η3-C3H5)Pd{C(N(tBu)CH)2}(PF6) (13a), (η3-C3H5)Pd{C(N(C6H3-2,6-iPr2)CH)2}(PF6) (13c), (η3-C3H5)Pd{C(N(tBu)CH)2}(AlRF4) (14a),
and (η3-C3H5)Pd{C(N(C6H3-2,6-iPr2)CH)2}(AlRF4) (14c). Reaction of 8−14 with water produces the
(nonisolated) water adducts [(η3-C3H5)Pd(NHC)(H2O)]Y (10a−d(Y)). The molecular structures of 5a,
9a(BF4), 11c(PF6), and 12a have been determined. The various anions and solvate ligands are arranged
in their order of donor strength/nucleophilicity toward the [(π-allyl)Pd(NHC)]+ moiety.