Complexes between the 1,4-disubstituted 1,4-diaza-1,3-butadiene chelate ligands RNCHCHNR (R-DAB; R = alkyl, aryl) and the organoplatinum fragments PtMe2, PtMe4, and PtMes2 (Mes = mesityl) were prepared and characterized with respect to their electronic structures. All compounds are distinguished by low-energy charge-transfer transitions to low-lying π* orbitals of the R-DAB ligands, either from metal d orbitals (PtII) or from metal−carbon σ bond combinations (PtIV). These spectral assignments are supported by DFT calculations on model complexes between HNCHCHNH and PtMe2 or PtMe4. The calculations also reproduce the structural results for the complex between CyNCHCHNCy and PtMe4, which exhibits significantly longer Pt−C bonds to the axial methyl groups. The distinct solvatochromism of the long-wavelength transitions is described, as are the UV/vis spectroelectrochemical results for reversible reduction to PtIV(R-DAB•-) or PtII(R-DAB•-) species (no evidence for a PtI state). In contrast, the oxidation is electrochemically irreversible except for the dimesitylplatinum compounds. The electrochemical potentials of corresponding PtMe2 and PtMe4 compounds are very similar, demonstrating that the binding of two additional methyl carbanions compensates for the effect of the higher metal oxidation state. While the organoplatinum(II) species are emissive, the tetramethylplatinum(IV) complexes are photoreactive and undergo metal-to-ligand methyl transfer reactionsin agreement with the structurally confirmed weaker bonding to the axial methyl groups.

Chelate complexes between the 1,4-diazabuta-1,3-diene ligands RN᎐ ᎐ CH᎐CH᎐ ᎐ NR = R᎐DAB (R = alkyl, aryl) and the organoplatinum fragments PtMe 2 , PtMe 4 and PtMes 2 (Mes = mesityl) can be reversibly reduced to paramagnetic compounds, formulated as platinum(II) or (IV) species bound by radical anion ligands (R-DAB Ϫ ). EPR studies in fluid and frozen solution support this assignment; however, the metal contribution to the singly occupied MO is higher for the paramagnetic Pt II species than for the Pt IV systems. Comparison with related radical complexes of the main group and transition element series reveals that even the organoplatinum(IV) compounds exhibit a relatively high degree of ligand-to-metal spin transfer as evident from small 1 H(CH) and large 14 N and 195 Pt EPR hyperfine coupling constants. The tetramethylplatinum(IV) complexes are photoreactive; a platinum-containing primary dissociation product from platinum᎐carbon σ bond homolysis has been detected by EPR spectroscopy using Bu t -NO as a spin trap reagent during irradiation. Group transfer reactivity has also been noted for radical anions where the conversion [(R-DAB)PtMe 2 ] Ϫ → [(R-DAB)PtMe 4 ] Ϫ could be monitored by EPR spectroscopy.

In spite of their very similar cyclic voltammograms, absorption spectra, and solvatochromic behavior, the two 1,4-diazabutadiene title complexes exhibit markedly different photoreactivities and underlying electronic structures, as evident from absorption and EPR spectra of the persistent anion radical forms. The lowest excited state of the nonphotoreac- 1905.7(4) x lo6 pm3; Z = 4) has a lowlying photoreactive LLCT (ligand-to-ligand charge-transfer, ( T~, -~

Symmetrically dinuclear complexes between the bis-bidentate bridging ligands µ-L (µ-L ) 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(1-phenyliminoethyl)pyrazine (bpip)) and the organoplatinum fragments PtMes 2 (Mes ) mesityl), PtMe 2 , or PtMe 4 were synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal d orbitals (Pt II ) or metal-carbon σ bond combinations (Pt IV ) to low-lying π* orbitals of the π acceptor ligands are responsible for long-wavelength absorption maxima λ max (CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [Pt IV ] 2 (µ-L •-) and [Pt II ] 2 (µ-L •-) species, however, the latter exhibit a significant metal contribution according to a Pt II /Pt I formulation. Cyclic voltammetry reveals that the remarkable system [Mes 2 Pt(µ-bptz)PtMes 2 ] n forms an enormously stabilized radical anion (n ) 1-) with ∆E 1/2 ) 1250 V and K c ) 10 21.2 and a Pt III /Pt II mixed-valent state (n ) 1+) with ∆E 1/2 ) 80 mV and K c ) 23. This small K c value is attributed to the predominantly d σ orbital character of the redox orbitals on the Pt(II) centers. Experimental SectionMaterials and Procedures. The ligands bptz 8 and bpip 9 and the platinum precursor complexes (DMSO)2PtMes2, 10 [Pt2-Me8(µ-SMe2)2], 11 and [Pt2Me4(µ-SMe2)2] 12 were obtained following literature procedures. All preparations and physical measurements were carried out in dried solvents under an argon atmosphere, using Schlenk techniques. Furthermore, the tetramethylplatinum(IV) compounds had to be prepared and studied in the absence of intense light.Bis(dimesitylplatinum) Complexes (µ-L)[PtMes2]2: 1 (L ) bptz) and 4 (L ) bpip). In a typical reaction, 295 mg (0.5 mmol) of dimesitylbis(dimethylsulfoxido)platinum(II) was suspended together with 0.25 mmol of the bridging ligand in 70 mL of toluene and heated under reflux for 5 days. The sulfoxide vibration ν(SdO) at 1130 cm -1 of the platinum precursor complex 13 had by then disappeared. At the end of the reaction, the temperature was lowered within another day to accomplish slow precipitation of the products. The solids were collected on a microporous frit and washed with diethyl ether. We thus obtained a poorly soluble blue-black powder for the bptz complex 1 in a 224 mg (81%) yield.

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