Cationic dihydrogen complexes of rhenium with isonitrile coligands

Abstract

The syntheses, characterizations, and reactivities of the following new complexes are reported: Re(CNtBu)$\rm\sb3(PR\sb3)\sb2$X(R = Cy, X = H, F, Cl, Br, I; R = Ph, X = H, Cl), (Re(CNtBu)$\rm\sb3(PR\sb3)\sb2L\rbrack\sp+$ (R = Cy, L = H$\sb2$, agostic H-C, CNtBu, N$\rm\sb2,\ O\sb2,\ PMe\sb3,\ NH\sb3,\ CO,\ C\sb2H\sb4$; R = Ph, L = H$\sb2$, agostic H-C, N$\sb2$), (Re(CNtBu)$\rm\sb3(PR\sb3)\sb2Cl\rbrack\sp+$ (R = Cy, Ph), (Re(CNtBu)$\rm\sb5(H\sb2)\rbrack\sp+.$ Thermodynamic parameters have been obtained for the novel equilibrium, Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2Cl + H\sb2\ \rightleftharpoons$ (Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2(H\sb2)\rbrack Cl.\ \sp1H$ and $\rm\sp{31}P\{\sp1H\}$ NMR studies (CD$\rm\sb2Cl\sb2)$ over the temperature range 286-316 K afford values of $\Delta H\sp\circ = -18.0\ \pm\ 0.7$ kcal/mol, $\Delta S\sp\circ = -44.0\ \pm\ 2.2$ e.u., and $\Delta G\sp\circ\sb{298} = -4.8 \pm 1.3$ kcal/mol. The complexes (Re(CNtBu)$\rm\sb3(PR\sb3)\sb2(H\sb2)\rbrack\sp+$ (R = Cy, Ph) are characterized as dihydrogen complexes. Their $J\sb{\rm HD}$ (R = Cy, 30.3 Hz; R = Ph, 30.9 Hz) and $\rm T\sb{1(min)}$ values of 8 msec (300 MHz) are consistent with H-H distances of ca. 0.80 A. Direct competition studies found the strength of the metal dihydrogen bond in (Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2(H\sb2)\rbrack\sp+$ to be stronger than those in (Re(CO)$\rm\sb3(PCy\sb3)\sb2(H\sb2)\rbrack\sp+$ and W(CO)$\rm\sb3(PCy\sb3)\sb2(H\sb2).$ (Re(CNtBu)$\rm\sb5(HD)\rbrack\sp+$ is also characterized as a dihydrogen complex based on a $J\sb{\rm HD}$ of 33.4 Hz. The agostic complex (Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2\rbrack\sp+$ is found to undergo a dynamic isonitrile rearrangement on the NMR time-scale. $\sp1$H NMR spin saturation transfer studies over a temperature range of 255-298 K afford the activation parameters: $\Delta H\sp\ddagger = 11.4 \pm 0.8$ kcal/mol $\Delta S\sp\ddagger = -17.6 \pm 1.6$ e.u., and $\Delta G\sp\ddagger\sb{298} = 16.6 \pm 1.2$ kcal/mol. The molecular structures of (Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2(H\sb2)\rbrack\sp+,$ Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2Cl,$ and (Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2Cl\rbrack\sp+$ have been determined by X-ray crystallography. Re(CNtBu)$\rm\sb3(PCy\sb3)\sb2Cl$ shows an unusually long Re-Cl bond distance of 2.596(2) A.