Design and synthesis of tridentate facially chelating ligands of the [2.n.1]-(2,6)-pyridinophane family

Syntheses are reported for tripyridine macrocycles 2 and 3 and some of their alkyl derivatives. The macrocycles are designed to stabilize to various extents coordinated d(8) metal precursors and d(6) alkane oxidative addition products (Pt(IV)), therefore allowing favorable kinetics and thermodynamics of (e.g., Pt(II)) the cleavage of substrate H-C(sp(3)) bonds. Both the Chichibabin protocol and oxidative coupling of carbanions by copper(I) iodide were used for the macrocyclization step. Crystal structures of singly and doubly protonated 2 establish atom connectivity in the macrocycle, and reveal structural features which are obscured in solution NMR by rapid proton migration.     

N.1.1]-(2,6)-Pyridinophanes: a new ligand type imposing unusual metal coordination geometries

A series of new ligands with three pyridines linked into a macrocycle by various CH(2), CMe(2), and CH(2)CH(2) groups at all sites ortho to the pyridine nitrogen have been synthesized and attached to PdCl(2) or PtMe(2). The ligands bind to them through only two nitrogens, and the third pyridine is constrained in close proximity to the planar complex with a filled d(z)2 orbital. Rapid reversible migration of PdCl(2) or PtMe(2) to the unused pyridine nitrogen is observed at 20 degrees C in the case of the CH(2)-bridged macrocycle and does not occur in the case of the CMe(2)-bridged analogue, and the mechanism of this fluxionality has been established by NMR and computational techniques.     

Four-coordinate,planar RuII. A triplet state as a response to a 14-valence electron configuration

The ligand (tBu2PCH2SiMe2)2N1- (PNP) in [PNP]RuCl leads to an intermediate spin ground state, S = 1, which has been characterized by NMR and X-ray diffraction as having a planar structure. This spin state is attributed in part to N --> Ru pi donation. DFT calculations confirm that the singlet state lies higher in energy and is nonplanar. The molecule is converted to a diamagnetic product by addition of 2 mol of PhCN. The half-filled orbitals of the S = 1 state are suggested to be the reason agostic interactions do not compensate for the 14-valence electron count.     

Reactivity of RuCl(2)(CO)(P(t)()Bu(2)Me)(2) toward H(2) and Brønsted Acids: Aggregation Triggered by Protonation and Phosphine Loss

Reaction of H(2) with RuCl(2)(CO)L(2) (L = P(t)()Bu(2)Me) in benzene forms RuHCl(CO)L(2) and HCl. The latter reacts with RuCl(2)(CO)L(2) to give [LH][Ru(2)Cl(5)(CO)(2)L(2)] and [LH]Cl. The Ru(2)Cl(5)(CO)(2)L(2)(-) ion is detected (NMR) as several isomers, and is shown by X-ray diffraction to have a face-shared bioctahedral structure: LCl(OC)Ru(&mgr;-Cl)(3)Ru(CO)ClL(-). The loss of phosphine from Ru(II) is triggered by electrophilic attack, but not directly on P or on the Ru-P bond. It is shown (low-temperature NMR studies) that HCl reacts with RuHCl(CO)L(2) to give initially RuCl(2)(H(2))(CO)L(2), in which H(2) is trans to Cl. From this study, and also direct observation of the reaction of HCl with RuCl(2)(CO)L(2) to produce Ru(2)Cl(5)(CO)(2)L(2)(-), the Br?nsted basicity of chloride in RuCl(2)(CO)L(2) is established. This accounts for its reaction with PhC(2)H and NEt(3) to give Ru(C(2)Ph)Cl(CO)L(2). Crystallographic data (-173 degrees C) for [P(t)()Bu(2)MeH][Ru(2)Cl(5)(CO)(2)(P(t)()Bu(2)Me)(2)]: a = 16.418(2)?, b = 12.578(2)?, c = 20.044(3)?, beta = 103.38(1) degrees with Z = 4 in space group P2(1)/a.     

Exceptionally facile CO addition to a saturated ruthenium complex

Synthesis and characterization of Cp*Ru[eta3-HC(PPh2NPh)2], 1, reveals it to have a "piano stool" structure with the ligand bound to Ru(II) via two N and the unique, sp3 hybridized carbon. While the analogous (cymene) Ru[eta3-HC(PPh2NPh)2]+ does not react with CO, under the same conditions, 1 adds one CO rapidly (25 degrees C, 1 atm CO). Characterization, including an X-ray structure determination, shows that CO has displaced one chelate ligand nitrogen, which then hangs off the molecule, free of Ru. DFT calculations reveal a possible mechanism via a remarkably low energy (+9.3 kcal/mol) intermediate, pendant N, but with one phenyl on phosphorus stabilizing Ru via donation from a C(ipso)=C(ortho) bond. DFT calculations show that the electronic energy change for binding CO is over 20 kcal/mol less favorable for cymene than for C5Me5- as ligand; the reactivity difference is thus thermodynamic in origin.     

Influence of the [2.1.1]-(2,6)-pyridinophane macrocycle ring size constraint on the structure and reactivity of copper complexes

The macrocycle [2.1.1]-(2,6)-pyridinophane (L) binds to CuCl to give a monomeric molecule with tridentate binding of the ligand but in a distorted tetrahedral "3 + 1" geometry, where one nitrogen forms a longer (by 0.12 A) bond to Cu. In dichloromethane solvent this pyridine donor undergoes facile site exchange with a second pyridine in the macrocycle, to give time-averaged mirror symmetry. Both experimental and density functional theory studies of the product of chloride abstraction, using NaBAr(F)(4) in CH(2)Cl(2), show that the Cu(+) binds in a trigonal pyramidal, not planar, arrangement in LCu(+). This illustrates the ability of macrocyclic ligand constraint to impose an electronically unfavorable geometry on 3-coordinate Cu(I). LCuBAr(F)(4) and a triflate analogue LCu(I)(OTf) readily react with oxygen in dichloromethane to produce, in the latter case, a hydroxo-bridged dimer [LCu(II)(micro-OH)](2)(OTf)(2), of the intact (unoxidized) ligand L. Since the analogous LCuCl does not react as fast with O(2) in CH(2)Cl(2), outer-sphere electron transfer is concluded to be ineffective for oxidation of cuprous ion here.     

Influence of chelate substituents on the structure and spin state of unsaturated [N(SiMe2CH2PtBu2)2]Ru-X

Density functional theory calculations on the conformational preferences in the two fused five-membered rings of anionic N(SiR2CH2PR'2)2 chelated to RuX+ are compared to several experimental structures (X=halide). The calculations consider the structures of both singlet and triplet states and reveal that both the four tBu groups and the crowded juncture (N(SiMe2)2) of the two rings must be included computationally to understand the observed structures. Computational experiments with different substituents R and R' show the reality of N-->Ru pi donation. The cases where X=H and CH3 are also studied.