Pt(ii)-coordinated tricomponent self-assemblies of tetrapyridyl porphyrin and dicarboxylate ligands: are they 3D prisms or 2D bow-ties?

Abstract

Thermodynamically favored simultaneous coordination of Pt(ii) corners with aza- and carboxylate ligands yields tricomponent coordination complexes with sophisticated structures and functions, which require careful structural characterization to paint accurate depiction of their structure-function relationships. Previous reports claimed that heteroleptic coordination of cis-(Et₃P)₂Pt^II with tetrapyridyl porphyrins (M'TPP, M' = Zn or H₂) and dicarboxylate ligands (XDC) yielded 3D tetragonal prisms containing two horizontal M'TPP faces and four vertical XDC pillars connected by eight Pt(ii) corners, even though such structures were not supported by their ¹H NMR data. Through extensive X-ray crystallographic and NMR studies, herein, we demonstrate that self-assembly of cis-(Et₃P)₂Pt^II, M'TPP, and four different XDC linkers having varied lengths and rigidities actually yields bow-tie (⋈)-shaped 2D [{cis-(Et₃P)₂Pt}₄(M'TPP) (XDC)₂]⁴⁺ complexes featuring a M'TPP core and two parallel XDC linkers connected by four heteroleptic Pt^II corners instead of 3D prisms. This happened because (i) irrespective of their length (∼7-11 Å) and rigidity, the XDC linkers intramolecularly bridged two adjacent pyridyl-N atoms of a M'TPP core via Pt^II corners instead of connecting two cofacial M'TPP ligands and (ii) bow-tie complexes are entropically favored over prisms. The electron-rich ZnTPP core of a representative bow-tie complex selectively formed a charge-transfer complex with highly π-acidic 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-heaxacarbonitrile but not with a π-donor such as pyrene. Thus, this work not only produced novel M'TPP-based bow-tie complexes and demonstrated their selective π-acid recognition capability, but also underscored the importance of proper structural characterization of supramolecular assemblies to ensure accurate depiction of their structure-property relationships.

Scheme 1. Pt(ii)-driven self-assembly of M′TPP ligands (M′ = Zn or H₂) and four different XDC linkers (HDC, BPDC, BDC, and NDC) yielded novel bow-tie complexes [{cis-(Et₃P)₂Pt}₄(M′TPP)(XDC)₂]·4(TfO) (BT1–BT4 and BT1′–BT4′). No tetragonal prism was formed irrespective of the length and rigidity of the XDC linkers.

Fig. 1. The chemical structures of bow-tie complexes.

Fig. 2. Single-crystal structures of BT2, BT2′, BT3, BT3′ and BT4 bow-tie complexes. Atom legends: green: Pt, cyan: Zn, pink: P, red: O, blue: N, and grey: C. The H-atoms and TfO⁻ anions were omitted for clarity.

Fig. 3. The optimized structures of BT1, BT1′, BT3, and BT3′ complexes calculated by the PM6 method.

Fig. 4. Partial ³¹P NMR spectra (122 MHz, acetone-d₆) of the cis-(Et₃P)₂Pt(TfO)₂ and BT1–BT4 complexes.

Fig. 5. Partial ¹H NMR spectra (500 MHz) of (a) ZnTPP, (b) BT1, (c) BT2, (d) BT3, and (e) BT4. The enclosed H_c′ pyrrole protons (highlighted in red) located inside the isosceles triangles of bow-tie structures were shielded by adjacent XDC linkers, whereas the exposed H_c′′ pyrrole protons (highlighted in blue) were not.

Fig. 6. Partial ¹H–¹H ROESY NMR spectra (500 MHz, acetone-d₆) of (a) BT1, (b) BT2, (c) BT3, and (d) BT4 show that the enclosed H_c′ pyrrole protons of these bow-tie complexes located inside the isosceles triangles are through-space coupled with the protons of adjacent XDC linkers but the exposed H_c′′ pyrrole protons are not coupled with the distant XDC protons.

Fig. 7. The ¹H NMR titration data (500 MHz, 3 : 7 CD₂Cl₂/CD₃NO₂) of BT4 with (a) HATHCN and (b) pyrene show gradual up-field shift, i.e., shielding of H_c′ and H_c′′n (pyrrole) signals of the ZnTPP core by the former but no such change with the latter.

Fig. 8. The UV-vis spectra of BT4 (in CH₂Cl₂) in the presence of (a) HATHCN and (b) pyrene. Insets: the amplified 475–1000 nm regions show the appearance of the CT band with HATHCN but not with pyrene.