Dan A. Smith

Metal Hydrides Form Halogen Bonds: Measurement of Energetics of Binding

The formation of halogen bonds from iodopentafluorobenzene and 1-iodoperfluorohexane to a series of bis(η5-cyclopentadienyl)metal hydrides (Cp2TaH3, 1; Cp2MH2, M = Mo, 2, M = W, 3; Cp2ReH, 4; Cp2Ta(H)CO, 5; Cp = η5-cyclopentadienyl) is demonstrated by 1H NMR spectroscopy. Interaction enthalpies and entropies for complex 1 with C6F5I and C6F13I are reported (ΔH° = −10.9 ± 0.4 and −11.8 ± 0.3 kJ/mol; ΔS° = −38 ± 2 and −34 ± 2 J/(mol·K), respectively) and found to be stronger than those for 1 with the hydrogen-bond donor indole (ΔH° = −7.3 ± 0.1 kJ/mol, ΔS° = −24 ± 1 J/(mol·K)). For the more reactive complexes 2–5, measurements are limited to determination of their low-temperature (212 K) association constants with C6F5I as 2.9 ± 0.2, 2.5 ± 0.1, <1.5, and 12.5 ± 0.3 M–1, respectively.

Exploiting non-innocent ligands to prepare masked palladium(0) complexes.

The last decade has shown an upsurge of interest in “noninnocent ligands” (NILs), a term originally coined by Jørgensen to describe metal scaffolds that are subject to redox changes at a site remote from the metal they support. Such systems are well-established in biochemistry, being at the heart of many metalloproteins, and are now found throughout coordination chemistry due to their diverse spectroscopic, magnetic, and electrochemical properties. This versatility has extended the application of NILs to organometallic chemistry, where they have been used to enhance coordinated substrate reactivity. More recently, the term “NIL” has been expanded to include non-redox-active systems, where the metal s coordination sphere is perturbed through variation in ligand coordination mode and/or structure, which occurs in “response” to external stimuli. These changes have been demonstrated to affect not only the rates of co-ligand dissociation, but also their intrinsic reactivity. 14] An unusual class of “responsive” NIL are the pyridyl-Nphosphinoimines 1, with the “open” bis(diisopropylamino) derivative 1a existing in tautomeric equilibrium with a “closed” anellated iminophosphorane form 1a’ (Scheme 1). As a result, 1a has been shown to behave as either a k-PN or a k-N ligand depending on the nature of the partner metal, and to exhibit unusual redox behavior. 16] In contrast, the diphenyl analogue 1b behaves only as a bidentate k-PNPy ligand and exhibits no tautomerism. Despite the importance of palladium catalysis in a variety of organic transformations, there have been very few studies of Pd–NIL complexes. 19, 20] This combination could help surmount the contradictory electronic and steric needs of the oxidative addition and reductive elimination steps key to many Pd-centered catalytic cycles through changes in NIL binding. To this end, we report here a study of the behavior of compounds 1 in the coordination sphere of palladium and the preparation of low-coordinate Pd complexes. Since it is well-established that bulky ligands such as PtBu3 can promote the formation of unsaturated Pd species that show enhanced reactivity in Pd-catalyzed cross-coupling reactions, the PtBu2-substituted derivative 1c (Scheme 2)

The Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors

The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability β (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (-23.5 ± 0.3 kJ mol(-1)) interlocks our study with Laurences scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of pπ-dπ bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis.