Probing Electronic Effects in Tridentate Copper(I) Complexes by CIVP Spectroscopy

Ligand electronic effects play an important role in catalysis, where small changes to ligand structure can bring about large changes in catalytic activity. Therefore, accurate experimental quantification of ligand electronic properties plays a crucial role in understanding and tuning chemical reactivity. In this work, we used cryogenic ion vibrational predissociation (CIVP) spectroscopy to experimentally quantify electronic effects in terpyridine ligands, as simple model systems, by measuring CIVP spectra of their copper complexes tagged by N2 molecules. We used the N2 stretching vibration as a reporter chromophore to probe electronic effects of the investigated ligands and employed quantum chemical calculations to better understand how different substituents influence the vibrational frequencies of the stretching vibration of the chromophore. Our data show that the electronic character, as well as position and number of substituents, can affect the N≡N vibrational frequency, and that the N≡N bond serves as a sensitive probe for electronic and steric effects.

Preparation and characterization of mixed-ligand cobalt(III) complexes containing (3-aminopropyl)dimethylphosphine (pdmp). Conformation of the six-membered pdmp chelate ring

Several new cobalt(III) complexes containing (3-aminopropyl)dimethylphosphine (pdmp) have been prepared, and their molecular structures have been determined. A dichloro complex of trans(Cl,Cl)-cis(P,P)-[CoCl(2)(pdmp)(2)]PF(6) (1) was prepared from trans-[CoCl(2)(py)(4)]Cl.6H(2)O and pdmp. X-Ray crystallography confirmed the (C(2))-chair(2) conformation of two six-membered pdmp chelate rings in 1, while the analogous 1,3-bis(dimethylphosphino)propane (dmpp) complex trans-[CoCl(2)(dmpp)(2)]ClO(4) (3) exhibited the (D(2d))-twist(2) conformation. Substitution reactions of 1 for ethane-1,2-diamine (en), pentane-2,4-dionate (acac), and N,N-dimethyldithiocarbamate (dtc) gave the mixed-ligand tris(chelate)-type complexes of [Co(en)(2)(pdmp)]Cl(2)(PF(6)) (5), [Co(acac)(pdmp)(2)](PF(6))(2) (7), and [Co(dtc)(3-n)(pdmp)(n)](PF(6))(n) [n = 1 (9) or 2 (10)], respectively. The conformer of the complex cation in 5 was assigned as lel.ob.chair by X-ray analysis. In the case of the acac complex 7, both trans(P,N) (7a) and trans(N,N) (7b) isomers were isolated, and the complex cations were characterized as syn-chair(2) and anti-chair(2) conformers, respectively, with respect to the six-membered pdmp chelate rings. These conformers coincide with the most stable ones anticipated by the DFT optimum geometry calculations. In the crystal structure of trans(P,N)-[Co(dtc)(pdmp)(2)](BPh(4))(2) (10') one of the pdmp chelate rings adopted a skew-boat (twist) conformation, which reduced the intramolecular steric ring-ring interaction effectively. The DFT optimized geometries for several isomers and/or conformers of [CoCl(2)(pdmp)(2)](+) were compared.

Syntheses, Structural Determinations of [Ru(ROCS2)2(PPh3)2]+/0 Pairs, and Kinetic Analyses of Thermal Reactions Involving Transient trans-[Ru(iPrOCS2)2(PPh3)2] Species (ROCS2- = Ethyl- or Isopropyldithiocarbonate and PPh3 = Triphenylphosphine)

Controlled-potential electrochemical oxidation of cis-[Ru(ROCS2)2(PPh3)2] (R = Et, iPr) yielded corresponding Ru(III) complexes, and the crystal structures of cis-[Ru(ROCS2)2(PPh3)2] and trans-[Ru(ROCS2)2(PPh3)2](PF6) were determined. Both pairs of complexes exhibited almost identical coordination structures. The Ru-P distances in trans-[Ru(III)(ROCS2)2(PPh3)2](PF6) [2.436(3)-2.443(3) A] were significantly longer than those in cis-[Ru(II)(ROCS2)2(PPh3)2] [2.306(1)-2.315(2) A]: the smaller ionic radius of Ru(III) than that of Ru(II) stabilizes the trans conformation for the Ru(III) complex due to the steric requirement of bulky phosphine ligands while mutual trans influence by the phosphine ligands induces significant elongation of the Ru(III)-P bonds. Cyclic voltammograms of the cis-[Ru(ROCS2)2(PPh3)2] and trans-[Ru(ROCS2)2(PPh3)2]+ complexes in dichloromethane solution exhibited typical dual redox signals corresponding to the cis-[Ru(ROCS2)2(PPh3)2](+/0) (ca. +0.15 and +0.10 V vs ferrocenium/ferrocene couple for R = Et and iPr, respectively) and to trans-[Ru(ROCS2)2(PPh3)2](+/0) (-0.05 and -0.15 V vs ferrocenium/ferrocene for R = Et and iPr, respectively) couples. Analyses on the basis of the Nicholson and Shain's method revealed that the thermal disappearance rate of transient trans-[Ru(ROCS2)2(PPh3)2] was dependent on the concentration of PPh3 in the bulk: the rate constant for the intramolecular isomerization reaction of trans-[Ru(iPrOCS2)2(PPh3)2] was determined as 0.338 +/- 0.004 s(-1) at 298.3 K (deltaH* = 41.8 +/- 1.5 kJ mol(-1) and deltaS* = -114 +/- 7 J mol(-1) K(-1)), while the dissociation rate constant of coordinated PPh3 from the trans-[Ru(iPrOCS2)2(PPh3)2] species was estimated as 0.113 +/- 0.008 s(-1) at 298.3 K (deltaH* = 97.6 +/- 0.8 kJ mol(-1) and deltaS* = 64 +/- 3 J mol(-1) K(-1)), by monitoring the EC reaction (electrode reaction followed by chemical processes) at different concentrations of PPh3 in the bulk. It was found that the trans to cis isomerization reaction takes place via the partial dissociation of iPrOCS2(-) from Ru(II), contrary to the previous claim that it takes place by the twist mechanism.