Preparation, characterization, and luminescence properties of a 58-electron linear Pt-4 cluster, [Pt-4(dmb)(4)(PPh3)(2])(2+) (dmb = 1,8-diisocyano-p-menthane), and its diphosphine polymers

The title compounds [Pt-4(dmb)(4)(PPh3)(2)]Cl-2 (1) and {[Pt-4(dmb)(4)(diph os)]Cl-2}(n) (diphos = dppb (2), dppp (3), dpph (4)) have been prepared in good yields from the reaction of Pt-2(dba)(3). CHCl3 with 2 equiv of dmb an d 1 equiv of PPh3 for 1 (dba = dibenzylideneacetone) and from the reactions of Pt-2(dba)(3). CHCl3 with 2 equiv of dmb and 0.5 equiv of diphos for 2-4 . The structure for 1 consists of a quasi-linear Pt4L22+ species (L = PPh3; d(PtPt) = 2.666(2), 2.655(2), 2.641(2) Angstrom), where the dmb ligands br idge the Pt atoms forming a catenate. From Raman spectroscopy, the two nu(P tPt) active modes for 1 are observed at 162 and 84 cm(-1) (F(PtPt) = 2.36 m dyn Angstrom(-1)). For 2-4, the diphos ligands induce the formation of amor phous polymeric materials (X-ray powder diffraction patterns) with MW rangi ng from 84 000 to 307 000 according to viscometry. EHMO calculations predic t that the HOMO and LUMO are the two d sigma* orbitals arising from four in teracting Pt atoms via the d(x2-y2), d(z2), s, and p(x) M atomic orbitals. These are mixed with the d delta and CNR(pi*) MO's. From the examination of the position, absorptivity, and fwhm (full width at half maximum) of the s trongly allowed low-energy UV-vis band, a d sigma* --> assignment is made ( lambda(max) = 405 nm, epsilon = 35 800 M-1 cm(-1); EtOH for 1). The four co mpounds are luminescent at 77 K in EtOH, where lambda(emi) are 750, 736, 75 0, and 755 nm and tau(e) are 2.71, 4.78, 5.15, and 5.17 ns for 1-4, respect ively. On the basis of the Stokes shifts (10 000-12 000 cm(-1)) and the lon g emission lifetimes, a phosphorescence d sigma* --> d sigma* assignment is made for the observed emissions. Crystal data for 1: crystal system tricli nic; space group P1; a = 12.624(4) Angstrom; b = 14.24(2) Angstrom; c = 27. 312(3) Angstrom; alpha = 92.35(3)degrees; beta = 91.655(15)degrees; gamma = 90.28(5)degrees; V = 4903(7) Angstrom(3); Z = 2; D-calc = 1.528 g cm(-3); R-1 = 0.0738; wR(2) = 0.2097; S = 1.018.