Calix[4]pyrrolato Stannate(II): A Tetraamido Tin(II) Dianion and Strong Metal-Centered σ-Donor

Abstract

Anionic, metal-centered nucleophiles are emerging compounds with unique reactivities. Here, we describe the isolation and full characterization of the first tetraamido tin(II) dianion, its behavior as ligand towards transition metals, and its reactivity as a tin-centered nucleophile. Experimental values such as the Tolman electronic parameter (TEP) and computations attest tin-located σ-donor ability exceeding that of carbenes or electron-rich phosphines. Against transition metals, the stannate(II) can act as η¹ - or η⁵ -type ligand. With aldehydes, it reacts by hydride substitution to give valuable acyl stannates. The reductive dehalogenation of iodobenzene indicates facile redox pathways mediated by halogen bond interaction. Calix[4]pyrrolato stannate(II) represents the first example of this macrocyclic ligand in low-valent p-block element chemistry.

Keywords: Calix[4]Pyrrole; Dianions; Low-Valent; Tin; σ-Donor.

Figure 1

Solvation‐corrected HOMO energy levels for the NHC (A), CAAC (B), the N‐heterocyclic stannylene (NHSn) (C), the amidinato stannylene (ADSn) (D), the anionic stannate(II) [E]⁻ , the dianionic stannates(II) [F]²⁻ , and [G]²⁻ , boryl H and the aluminyl anion [J]⁻ , and [2]²⁻ (B97M‐D3(BJ)/def2‐TZVPP CPCM(THF)//B97M‐D3(BJ)/def2‐TZVPP). For C and [2]²⁻ the HOMO and HOMO−1 are ligand‐centered and the energies of their tin electron lone pairs centered HOMO−2 are given in parentheses.

Figure 2

A) Synthesis of the dianionic calix[4]pyrrolato stannate(II) as the lithium salt [Li₂(thf) _x ][2] and B) subsequent salt exchange reactions with PPh₄Cl and NBu₄Cl forming the phosphonium salt [(PPh₄)₂(oDFB)_0.5][2] and the ammonium salt [(NBu₄)₂][2]. C) Solid‐state molecular structure of [(NBu₄)₂][2]. Displacement ellipsoids are drawn at 50 % probability level. Hydrogens and NBu₄ ⁺ counter cations are omitted for clarity. Selected bond lengths [pm] and angels [°]: Sn1−N1 232.27(16), Sn1−N2 230.61(15), Sn1−N3 230.73(15), Sn1−N4 232.11(15), Sn1−N₄‐plane 95.8, N1−Sn1−N4 133.23(5), N2−Sn1−N3 129.10(5), cis−N−Sn−N between 79.64(5) and 80.85(5). D) Occupied frontier molecular orbitals calculated at the B97M‐D3(BJ)/def2‐TZVPP CPCM(THF)//B97M‐D3(BJ)/def2‐TZVPP level of theory. E) Effects of Lewis base coordination (i) and distortion into the calix[4]pyrrole coordination geometry (ii) on the energy of the lone pair (lp) containing orbital at tin(II) for the hypothetical bis(pyrrolato)stannylene K, tetrakis(pyrrolato)stannate(II) [L]²⁻ , and [2]²⁻ .

Figure 3

Synthesis of A) the lithium stanna(II) selenido complex [Li₂(thf) _x ][2‐Se], the η ¹‐coordinated metal carbonyl complexes [(PPh₄)₂][W(η ¹‐2)(CO)₅] (B) and [(NBu₄)₂][Ni(η ¹‐2)(CO)₃] (C) and the η ⁵‐coordinated ruthenium(II) sandwich complex Ru(η ⁵‐2)(p‐cymene) (D).

Figure 4

Solid‐state molecular structure of A) the η ¹‐complex [(PPh₄)₂][W(η ¹‐2)(CO)₅] and B) the η ⁵‐complex Ru(η ⁵‐2)(p‐cymene). Displacement ellipsoids are drawn at 50 % probability level. Hydrogens, counter cations, solvent molecules, and meso‐ethyl residues in B) are omitted for clarity. Selected bond lengths [pm] and angles [°] for [(PPh₄)₂][W(η ¹‐2)(CO)₅]: Sn1−N1 222.3(2), Sn1−N3 225.7(2), Sn1−N4 224.6(2), Sn1−N5 222.5(2), W1−Sn1 283.44(6), W1−C_ax. 196.9(3), W1−C_eq. between 202.0(3) and 204.4(3), Sn1−N₄‐plane 83.2, N1−Sn1−N4 133.41(9), N3−Sn1−N4 139.25(9), cis−N−Sn−N between 81.54(8) and 82.53(8); and Ru(η ⁵‐2)(p‐cymene): Sn1−N1 219.80(11), Sn1−N2 232.01(11), Sn1−N3 233.01(12), Sn1−N4 259.95(12), Sn1−N₄‐plane 103.3, Ru1‐pyrrole 183.7, Ru1‐cymene 171.8, N1−Sn1−N4 126.45(4), N2−Sn1−N3 128.10(4), cis−N−Sn−N between 74.76(4) and 83.13(4).

Figure 5

Reactivity of A) [Li₂(thf) _x ][2] and B) [(NBu₄)₂][2] with electrophiles and D) reaction of [(NBu₄)₂][2] with iodobenzene. Yields where determined by NMR spectroscopy. Oxidation potentials were determined by cyclic voltammetry of [(NBu₄)₂][2] (10⁻³ M) and Bu₄NPF₆ (0.01 M) in 1,2‐difluorobenzene at a scan rate of 0.05 V s⁻¹. C) Solid state molecular structure of [Li(thf) _x ][3‐COPh]. Displacement ellipsoids are drawn at 50 % probability level. Hydrogens and toluene solvent molecules are omitted for clarity. Selected bond lengths [pm] and angles [°]: Sn1−N1 208.11(10), Sn1−N2 210.25(10), Sn1−N3 211.02(12), Sn1−N4 211.31(11), Sn1−N₄‐plane 49.4, Sn1−C9 220.94(13), C9−O3 123.20(14), N1−Sn1−N4 152.56(4), N2−Sn1−N3 153.09(4), cis−N−Sn−N between 84.37(4) and 89.43(4).

Figure 6

Comparison between the TEP values of [2]²⁻ and other literature‐known carbene and phosphine ligands. Absolute values and references are given in Table S9.