Transmetalation of Chromocene by Lithium-Amide, -Phosphide, and -Arsenide Nucleophiles

Abstract

The pnictogen-centered nucleophiles LiE(SiMe3)2 (E = N, P, or As) substitute a cyclopentadienide ligand of chromocene (Cp2Cr), with elimination of lithium cyclopentadienide, to give the series of pnictogen-bridged compounds [(μ:η(2):η(5)-Cp)Cr{μ-N(SiMe3)2}2Li] (1) and [(η(5)-Cp)Cr{μ-E(SiMe3)2}]2, with E = P (2) or E = As (3). Whereas 1 is a heterobimetallic coordination polymer, 2 and 3 are homometallic dimers, with the differences being due to a structure-directing influence of the hard or soft character of the bridging group 15 atoms. For compound 1, the experimental magnetic susceptibility data were accurately reproduced by a single-ion model based on high-spin chromium(II) (S = 2), which gave a g-value of 1.93 and an axial zero-field splitting parameter of D = -1.83 cm(-1). Determinations of phosphorus- and arsenic-mediated magnetic exchange coupling constants, J, are rare: in the dimers 2 and 3, variable-temperature magnetic susceptibility measurements identified strong antiferromagnetic exchange between the chromium(II) centers, which was modeled using the spin Hamiltonian H = -2J(S(CrA)·S(CrB)), and produced large coupling constants of J = -166 cm(-1) for 2 and -77.5 cm(-1) for 3.

Paramagnetic Chromium(II) Complexes and Chromium(IV) Nitrides with Bulky Alkylcyclopentadienyl Ligands

“…Also interesting for comparison are the coordination polymer [(µ,η 2 :η 5 ‐C 5 H 5 )Cr{µ‐N(SiMe 3 ) 2 } 2 Li] ∞ with the lithium ion coordinated by two bridging silylamide ligands and two carbon atoms of the bridging cyclopentadienyl ligand . This material was obtained from chromocene and lithium bis(trimethylsilyl)amide and displays Cr–C distances between 235.9(5) and 240.1(4) pm very similar to those of 7b (Figure ).…”

“…This is no surprise, because we compare a bridging situation between chromium and lithium with the terminal Cr–N situation of 7b . Magnetic susceptibility data for the coordination polymer at temperatures between 2 and 300 K are characteristic for a high spin Cr II central atom . Interestingly, a reaction of the chromium(III) starting material [(C 5 H 5 )CrCl 2 (THF)] with lithium bis(trimethylsilyl)amide resulted in one substitution step with Cr–N bond formation and one hydrogen chloride elimination step with Cr–C bond formation yielding the dinuclear complex [(C 5 H 5 )Cr(µ‐CH 2 SiMe 2 –NSiMe 3 )] 2 with a tricyclic Cr 2 C 2 N 2 Si 2 core containing two bridging alkyl groups between the chromium atoms.…”

Divalent Transition Metal Silylamide Ate Complexes

“…The chromium(II) amide ate complexes (thf) 4 Na 2 Cr[NPh 2 ] 4 , (thf) 2 Li 2 Cr[NEt 2 ] 4 , and (py) 2 Li 2 Cr[NEt 2 ] 4 gave effective magnetic moments of 4.63, 4.91, and 4.83 μ B , respectively 37. The molar magnetic susceptibility of polymeric {Li[μ‐N(SiMe 3 ) 2 Cr(μ:η 2 :η 5 ‐Cp)]} ∞ was determined to be χ M T = 2.80 cm 3 K/mol at 300 K, which is equivalent to a μ eff value of 4.73 μ B 38. A series of three‐coordinate chromium(II) complexes of type (nacnac)CrR {R = CH 2 t Bu, CH 3 , CH 2 CH 3 , SiHMes 2 , O(2,6‐ i Pr 2 C 6 H 3 ), N(CH 3 ) 2 ; nacnac = [ArNC( t Bu)] 2 CH, Ar = 2,6‐ i Pr 2 C 6 H 3 } has been investigated by the Evans method, which revealed magnetic moments ranging from 4.69 to 5.09 μ B 39.…”

Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

“…Indeed, there is a general paucity of studies addressing the magnetic properties of compounds containing P-and As-donor ligands of the type [R x E] yÀ , where x ¼ 2 and y ¼ 1, x ¼ 1 and y ¼ 2, or x ¼ 0 and y ¼ 3. 5 We recently reported studies of the P-and As-mediated exchange coupling in the phosphide-and arsenide-bridged dimers [(h 5 -Cp)M{m-E(SiMe 3 ) 2 }] 2 , with M ¼ Cr(II) 6 or Mn(II), 7 and determined the exchange coupling constants via phenomenological spin Hamiltonians. In the case of the chromium(II) dimers, the antiferromagnetic super-exchange coupling between the high-spin S ¼ 2 ions was found to be strong and also pnictogen-dependent, with exchange coupling constants of J ¼ À166 cm À1 for E ¼ P and J ¼ À77.5 cm À1 for E ¼ As (2J formalism).…”

“…9 Chromocene itself has a low-spin, S ¼ 1 conguration, 10 but substituting a [Cp] À ligand by [(Me 3 Si) 2 E] À to give [CpCr{m-E(SiMe 3 ) 2 }] 2 (E ¼ P or As) evidently weakens the ligand eld sufficiently to give high-spin S ¼ 2 chromium(II). 6 In an attempt to induce thermal spin crossover in [CpCr{m-E(SiMe 3 ) 2 }] 2 by forcing the ground state of the metal to switch from S ¼ 2 to S ¼ 1 with the aid of a stronger ligand eld, we have targeted the analogous pentamethyl-cyclopentadienyl (Cp*) dimers [(Cp*)Cr{m-E(SiMe 3 ) 2 }] 2 .…”