Fluxionality and bonding in formamidine and formamidinate complexes of PtII and PdII containing the 2,6-bis[(dimethylamino)methyl]phenyl ligand: a proton nuclear magnetic resonance study with X-ray crystal structures of [Pt{C6H3(Ch2NMe2)2-2,6}{p-MeC6H4NCHN(H)C6H4Me-p}][CF3SO3] and [Pt{C6H3(CH2NMe2)2-2,6}(p-MeC6H4NCHNC6H4Me-p)]

Abstract

The N¹,N²-di-p-tolylformamidine ligand [p-McC₆H₄NCHN (H)C₆H₄Me-p, Hdptf] reacts smoothly with the ionic species [M{C₆H₃(CH₂NMe₂)₂-2,6}(H₂O)]X (M = Pd or Pt, X = BF₄^– or CF₃SO₃^–) to generate [M{C₆H₃(CH₂NMe₂)2-6}(Hdptf)]X [M = Pd (1) or Pt (2)]. The ¹H n.m.r. data (250-MHz, CD₂Cl₂) for (2)(room temperature) and (1)(203 K) are consistent with a rigid monodentate bonding mode of neutral Hdpft; for the palladium species (1) Hdptf dissociation becomes an important process at higher temperatures. The structure of (2b)(M = Pt; X = CF₃SO₃^–) was revealed by a single-crystal X-ray diffraction study. Crystals of this complex are triclinic, space group P1, with unit-cell dimensions a= 11.912(3), b= 14.372(6), c= 9.449(5)Å, α= 92.04(4), β= 95.28(3), γ= 72.40(3)°, and Z= 2. Structure solution and refinement by standard methods gave R= 0.046 for 5 421 independent reflections. Complex (2b) is the first structurally characterized organometallic species containing a neutral formamidine and it possesses a monodentate imine-N² bonding mode of neutral Hdptf to an approximately square planar Pt^II centre [Pt–N (3)= 2.16(1)Å]. The cationic species (1) and (2) react with NaOH to afford, in high yield, neutral [M{C₆H₃(CH₂NMe₂)₂-2,6}(dptf)][M = Pd (3) or Pt (4); dptf =p-McC₆H₄NCHNC₆H₄Me-p] for which low-temperature ¹H n.m.r. studies indicate the uncommon σ-N¹ monodentate bonding mode of the formamidinate anion. However, at room temperature in CD₂Cl₂ both (3) and (4) are subject to an intramolecular fluxional process that would be consistent with the existence of a chelate bonded formamidinate intermediate. A single-crystal X-ray study of (4) has been carried out. Crystals of this complex are monoclinic, space group P2₁/_n, with unit-cell dimensions a= 17.28(2), b= 15.148(6), c= 9.62(2)Å, = 101.1(2)° and Z= 4. In a manner analogous to that used for (2b) the structure of (4) was solved to R= 0.024 for 4 211 independent reflections. The virtually flat N¹,N²-di-p-tolylformamidinate ion is σ-N¹ monodentate bonded [Pt-N(3)= 2.1 32(6)Å] and is orientated almost perpendicular to the co-ordination plane of this approximately square-planar Pt^II species.