Diorganoruthenium Complexes Incorporating Noninnocent [C₆H₂(CH₂ER₂)₂-3,5]₂^2- (E = N, P) Bis-Pincer Bridging Ligands:  Synthesis, Spectroelectrochemistry, and DFT Studies

Abstract

The dinuclear complex [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (bridging PCP-PCP = 3,3',5,5'-tetrakis(diphenylphosphinomethyl)biphenyl, [C6H2(CH2PPh2)2-3,5]22-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN = [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2' '-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN-NCN = [C6H2(CH2NMe2)2-3,5]22-) obtained by two-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ and [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN-NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP-PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal-metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong sigma-donor NMe2 groups are replaced by weak sigma-donor, pi-acceptor PPh2 groups.

[Re(CO)3Cl]‐Chelation‐Mediated Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of 2,2′‐Bipyridine

“…However, the observation of multiple IVCT bands have been reported in some M-L-M type MV compounds. [10] However, these are best described as ligand-mediated charge-transfer transitions instead of genuine IVCT bands. [9] The presence of a redox-active bridge could also be responsible for the appearance of multiple NIR transitions.…”

“…However, the observation of multiple IVCT bands have been reported in some M-L-M type MV compounds. [9,10] Multiple ligand-mediated orbital interactions in transition-metal complexes can give rise to up to five lowenergy transitions (three of IVCT origin and two of interconfigurational origin) split by low symmetry and spin-orbit coupling. [9] The presence of a redox-active bridge could also be responsible for the appearance of multiple NIR transitions.…”

“…[9] The presence of a redox-active bridge could also be responsible for the appearance of multiple NIR transitions. [10] However, these are best described as ligand-mediated charge-transfer transitions instead of genuine IVCT bands. MV systems with multiple intense NIR transitions are potential materials for optoelectronic applications, such as NIR electrochromic films.…”

Ruthenium‐to‐Platinum Interactions in η6,η1 NCN‐Pincer Arene Heterobimetallic Complexes: An Experimental and Theoretical Study

“…[5,6] However, when the bridging ligand becomes longer, metal-to-metal interactions can become more elusive, hence difficult to characterize. [7,8] Most of the time, long-distance metal-tometal interactions are transmitted through conjugated π systems like polyalkene, [9][10][11][12] polyalkyne, [13,14] or polyphenylene [15][16][17][18][19] ligands. Typically, both metal fragments can be connected to the bridging ligand in an axial, η 1 ,η 1 fashion, and interact across the organic manifold through the conjugated π system (Scheme 1, a).…”

“…Typically, both metal fragments can be connected to the bridging ligand in an axial, η 1 ,η 1 fashion, and interact across the organic manifold through the conjugated π system (Scheme 1, a). [18][19][20][21] Alternatively, both metal fragments can coordinate in an η 5 ,η 5 or η 6 ,η 6 fashion when the Ru-Pt distance diminishes, and electrochemical and 195 Pt NMR spectroscopic studies show a decreasing electron density on platinum from [5 A ] + to [2] + . Theoretical DFT calculations were undertaken, which show an increasing charge on platinum from [5 A ] + to [2] + .…”