SO2-binding properties of cationic η6,η1-NCN-pincer areneruthenium platinum complexes: spectroscopic and theoretical studies

Abstract

The SO₂-binding properties of a series of η⁶,η¹-NCN-pincer ruthenium platinum complexes (NCN = 2,6-bis[(dimethylamino)methyl]phenyl anion) have been studied by both UV-visible spectroscopy and theoretical calculations. When an electron-withdrawing [Ru(C₅R₅)]⁺ fragment (R = H or Me) is η⁶-coordinated to the phenyl ring of the NCN-pincer platinum fragment (cf. [2]⁺ and [3]⁺, see Scheme 1), the characteristic orange coloration (pointing to η¹- SO₂ binding to Pt) of a solution of the parent NCN-pincer platinum complex 1 in dichloromethane upon SO₂-bubbling is not observed. However, when the ruthenium center is η⁶-coordinated to a phenyl substituent linked in para-position to the carbon-to-platinum bond, i.e. complex [4]⁺, the SO₂-binding property of the NCN-platinum center seems to be retained, as bubbling SO₂ into a solution of the latter complex produces the characteristic orange color. We performed theoretical calculations at the MP2 level of approximation and TD-DFT studies, which enabled us to interpret the absence of color change in the case of [2]⁺ as an absence of coordination of SO₂ to platinum. We analyze this absence or weaker SO₂-coordination in dichloromethane to be a consequence of the relative electron-poorness of the platinum center in the respective η⁶-ruthenium coordinated NCN-pincer platinum complexes, that leads to a lower binding energy and an elongated calculated Pt–S bond distance. We also discuss the effects of electrostatic interactions in these cationic systems, which also seems to play a destabilizing role for complex [2(SO₂)]⁺.