Phosphorescence and Structure of a Tetrameric Copper(I)−Amide Cluster

The colorless copper(I) cluster [CuN(Si(CH3)3)2]4, which contains a square-planar Cu4N4 core, phosphoresces in
CH2Cl2 solution (λmax, 512 nm; lifetime, 30 μs) and in the solid state at room temperature. Its electronic absorption
spectrum in CH2Cl2 consists of two intense bands at 283 and 246 nm; these transitions, as well as the
phosphorescence, are likely to involve population of MOs reflecting substantial Cu···Cu interactions. Solid [CuN(Si(CH3)3)2]4 luminesces with approximately the same spectrum as that of the CH2Cl2 solutions. At 77 K, the
solid-state luminescence red-shifts slightly (λmax, 524 nm) and narrows substantially (fwhm, 2400 cm-1; vs 3500
cm-1 at 300 K); the emission lifetime in glassy Et2O solution is 690 μs. X-ray analysis of crystals of [CuN(Si(CH3)3)2]4 at 130 and 296 K shows that, although the previously reported structure solution (in space group I2/m,
with the molecules on 2/m sites; Eur. J. Solid State Inorg. Chem. 1992, 29, 573−583) is approximately correct,
the lattice is actually primitive, P2/n, and the only crystallographically required symmetry element for the molecule
is a 2-fold axis. C24H72Cu4N4Si8:  monoclinic, space group P2/n, Z = 2. At 130 K, a = 9.285(3) Å, b = 13.393(3) Å, c = 17.752(5) Å, and β = 90.53(2)°. [At 296 K, a = 9.3773(4) Å, b = 13.5836(7) Å, c = 17.814(2) Å,
and β = 90.207(7)°.] At 130 K, the Cu and N atoms in the cluster are planar within 0.007 Å, and the Cu−N and
Cu···Cu distances are 1.917(4)−1.925(4) and 2.6770(7)−2.6937(7) Å, respectively. Despite the low volatility
of the compound, it can be used as a precursor for chemical vapor deposition (CVD) of copper metal, under H2
carrier gas, with both source and substrate at ca. 200 °C. Smaller amounts of Cu metal films are also deposited
when the substrate temperature is as low as 145 °C (in the dark) or 136−138 °C (under Pyrex-filtered Xe arc
lamp illumination). Thus, Cu CVD with this precursor shows slight photochemical enhancement.