Article,

Strong metal-metal coupling in mixed-valent intermediates Cl(L)Ru(m-tppz)Ru(L)Cl+, L = b-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

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Dalton Transactions, 41 (43): 13429--13440 (2012)
DOI: 10.1039/c2dt31763j

Abstract

Five diruthenium(II) complexes Cl(L)Ru(m-tppz)Ru(L)Cl (1-5; tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) contg. differently substituted b-diketonato derivs. (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochem. (UV-visible-NIR, EPR). X-ray structural characterization revealed anti (1, 2, 5) or syn (3) configuration as well as nonplanarity of the bis-tridentate tppz bridge and strong dp(RuII) $\rightarrow$ p*(pyrazine, tppz) back-bonding. The widely sepd. 1-electron oxidn. steps, RuIIRuII/RuIIRuIII and RuIIRuIII/RuIIIRuIII, result in large comproportionation consts. (Kc) of $\geq$1010 for the mixed-valent intermediates. The syn-configurated 3n exhibits a particularly high Kc of 1012 for n = 1+, accompanied by d. functional theory (DFT)-calcd. min. Ru-N bond lengths for this RuIIRuIII intermediate. The electrogenerated mixed-valent states 1+-5+ exhibit anisotropic EPR spectra at 110 K with av. values \textlessg\textgreater of 2.304-2.234 and g anisotropies Dg = g1-g3 of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1+-5+ in the NIR region at 1660 nm-1750 nm (e $\approx$ 2700 dm3 mol-1 cm-1, Dn1/2 $\approx$ 1800 cm-1). DFT calcns. of 1+ and 3+ yield comparable Mulliken spin densities of $\sim$0.60 for the metal ions, corresponding to valence-delocalized situations (Ru2.5)2. Rather large spin densities of $\sim$-0.4 were calcd. for the tppz bridges in 1+ and 3+. The calcd. electronic interaction values (VAB) for 1+-5+ are $\sim$3000 cm-1, comparable to that for the Creutz-Taube ion at 3185 cm-1. The DFT calcns. predict that the RuIIIRuIII forms in 12+-52+ prefer a triplet (S = 1) ground state with DE (S = 0 - S = 1) $\sim$5000 cm-1. One-electron redn. takes place at the tppz bridge which results in Cl(L)RuII(m-tppz.bul.-)RuII(L)Cl- (1.bul.--3.bul.-, 5.bul.-) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4n is distinguished by lability of the Ru-Cl bonds. on SciFinder(R)

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