Metal-only Lewis pairs between group 10 metals and Tl(<scp>i</scp>) or Ag(<scp>i</scp>): insights into the electronic consequences of Z-type ligand binding

Abstract

Complexes bearing electron rich transition metal centers, especially those displaying coordinative unsaturation, are well-suited to form reverse-dative σ-interactions with Lewis acids. Herein we demonstrate the generality of zerovalent, group 10 <i>m</i>-terphenyl isocyanide complexes to form reverse-dative σ-interactions to Tl(i) and Ag(i) centers. Structural and spectroscopic investigations of these metal-only Lewis pairs (MOLPs) has allowed insight into the electronic consequences of Lewis-acid ligation within the primary coordination sphere of a transition metal center. Treatment of the bis-isocyanide complex, Pt(CNAr^Dipp2)₂ (Ar^Dipp2 = 2,6-(2,6-(i-Pr)₂C₆H₃)₂C₆H₃) with TlOTf (OTf = [O₃SCF₃]^-) yields the Pt/Tl MOLP [TlPt(CNAr^Dipp2)₂]OTf (<b>1</b>). ¹H NMR and IR spectroscopic studies on <b>1</b>, and its Pd congener [TlPd(CNAr^Dipp2)₂]OTf (<b>2</b>), demonstrate that the M → Tl interaction is labile in solution. However, treatment of complexes <b>1</b> and <b>2</b> with Na[BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) produces [TlPt(CNAr^Dipp2)₂]BAr^F₄ (<b>3</b>) and [TlPd(CNAr^Dipp2)₂]BAr^F₄ (<b>4</b>), in which Tl(i) binding is shown to be static by IR spectroscopy and, in the case of <b>3</b>, ¹⁹⁵Pt NMR spectroscopy as well. This result provides strong evidence that the M → Tl linkages can be attributed primarily to σ-donation from the group 10 metal to Tl, as loss of ionic stabilization of Tl by the triflate anion is compensated for by increasing the degree of M → Tl σ-donation. In addition, X-ray Absorption Near-Edge Spectroscopy (XANES) on the Pd/Tl and Ni/Tl MOLPs, [TlPd(CNAr^Dipp2)₂]OTf (<b>2</b>) and [TlNi(CNAr^Mes2)₃]OTf, respectively, is used to illustrate that the formation of a reverse-dative σ-interaction with Tl(i) does not alter the spectroscopic oxidation state of the group 10 metal. Also reported is the ability of M(CNAr^Dipp2)₂ (M = Pt, Pd) to form MOLPs with Ag(i), yielding the complexes [AgM(CNAr^Dipp2)₂]OTf (<b>5</b>, M = Pt; <b>6</b>, M = Pd). As was determined for the Tl-containing MOLPs <b>1-4</b>, it is shown that the spectroscopic oxidation states of the group 10 metal in <b>5</b> and <b>6</b> are essentially unchanged compared to the zerovalent precursors M(CNAr^Dipp2)₂. However, in the case of <b>5</b> and <b>6</b>, the formation of a dative M → Ag σ-bonding interaction facilitates the binding of Lewis bases to the group 10 metal <i>trans</i> to Ag, illustrating the potential of acceptor fragments to open up new coordination sites on transition metal complexes without formal, two-electron oxidation.