Hydrido Copper Clusters
Supported by Dithiocarbamates:
Oxidative Hydride Removal and Neutron Diffraction Analysis of [Cu<sub>7</sub>(H){S<sub>2</sub>C(aza-15-crown-5)}<sub>6</sub>]
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Abstract
Reactions of Cu(I) salts with Na(S<sub>2</sub>CR) (R
= N<sup><i>n</i></sup>Pr<sub>2</sub>, NEt<sub>2</sub>, aza-15-crown-5),
and (Bu<sub>4</sub>N)(BH<sub>4</sub>) in an 8:6:1 ratio in CH<sub>3</sub>CN solution at room temperature yield the monocationic hydride-centered
octanuclear Cu<sup>I</sup> clusters, [Cu<sub>8</sub>(H){S<sub>2</sub>CR}<sub>6</sub>](PF<sub>6</sub>) (R = N<sup><i>n</i></sup>Pr<sub>2</sub>, <b>1</b><sub><b>H</b></sub>; NEt<sub>2</sub>, <b>2</b><sub><b>H</b></sub>; aza-15-crown-5, <b>3</b><sub><b>H</b></sub>). Further reactions of [Cu<sub>8</sub>(H){S<sub>2</sub>CR}<sub>6</sub>](PF<sub>6</sub>) with 1 equiv
of (Bu<sub>4</sub>N)(BH<sub>4</sub>) produced neutral heptanuclear
copper clusters, [Cu<sub>7</sub>(H){S<sub>2</sub>CR}<sub>6</sub>]
(R = N<sup><i>n</i></sup>Pr<sub>2</sub>, <b>4</b><sub><b>H</b></sub>; NEt<sub>2</sub>, <b>5</b><sub><b>H</b></sub>; aza-15-crown-5, <b>6</b><sub><b>H</b></sub>)
and clusters <b>4</b>–<b>6</b> can also be generated
from the reaction of Cu(BF<sub>4</sub>)<sub>2</sub>, Na(S<sub>2</sub>CR), and (Bu<sub>4</sub>N)(BH<sub>4</sub>) in a 7:6:8 molar ratio
in CH<sub>3</sub>CN. Reformation of cationic Cu<sup>I</sup><sub>8</sub> clusters by adding 1 equiv of Cu<sup>I</sup> salt to the neutral
Cu<sub>7</sub> clusters in solution is observed. Intriguingly, the
central hydride in [Cu<sub>8</sub>(H){S<sub>2</sub>CN<sup><i>n</i></sup>Pr<sub>2</sub>}<sub>6</sub>](PF<sub>6</sub>) can
be oxidatively removed as H<sub>2</sub> by Ce(NO<sub>3</sub>)<sub>6</sub><sup>2–</sup> to yield [Cu<sup>II</sup>(S<sub>2</sub>CN<sup><i>n</i></sup>Pr<sub>2</sub>)<sub>2</sub>] exploiting
the redox-tolerant nature of dithiocarbamates. Regeneration of hydride-centered
octanuclear copper clusters from the [Cu<sup>II</sup>(S<sub>2</sub>CN<sup><i>n</i></sup>Pr<sub>2</sub>)<sub>2</sub>] can be
achieved by reaction with Cu(I) ions and borohydride. The hydride
release and regeneration of Cu<sup>I</sup><sub>8</sub> was monitored
by UV–visible titration experiments. To our knowledge, this
is the first time that hydride encapsulated within a copper cluster
can be released as H<sub>2</sub> via chemical means. All complexes
have been fully characterized by <sup>1</sup>H NMR, FT-IR, UV–vis,
and elemental analysis, and molecular structures of <b>1</b><sub><b>H</b></sub>, <b>2</b><sub><b>H</b></sub>, and <b>6</b><sub><b>H</b></sub> were clearly established
by single-crystal X-ray diffraction. Both <b>1</b><sub><b>H</b></sub> and <b>2</b><sub><b>H</b></sub> exhibit
a tetracapped tetrahedral Cu<sub>8</sub> skeleton, which is inscribed
within a S<sub>12</sub> icosahedron constituted by six dialkyl dithiocarbamate
ligands in a tetrametallic-tetraconnective (μ<sub>2</sub>, μ<sub>2</sub>) bonding mode. The copper framework of <b>6</b><sub><b>H</b></sub> is a tricapped distorted tetrahedron in which
the four-coordinate hydride is demonstrated to occupy the central
site by single crystal neutron diffraction. Compounds <b>1</b>–<b>3</b> exhibit a yellow emission in both the solid
state and in solution under UV irradiation at 77 K, and the structureless
emission is assigned as a <sup>3</sup>metal to ligand charge transfer
(MLCT) excited state. Density functional theory (DFT) and time-dependent
density functional theory (TDDFT) calculations on model compounds
match the experimental structures and provide rationalization of their
bonding and optical properties
