Correlation of CrOCr .pi.-bonding strength with .pi.-donor effectiveness of bridging and nonbridging ligands in (tris(2-pyridylmethyl)amine)chromium(III) dimers

Abstract

The preparations of two [Cr(tmpa)L]202+ dimers in which the pseudohalide ligand is a weaker -donor than NCS* are reported (tmpa = tris(2-pyridylmethyl)amine): [Cr(tmpa)(NCSe)]20(C104)2 and [{Cr(tmpa)(NCS)}2(0)-Pd(CA)](C104)2-2CH3CN; CA* 12" = 2,5-dioxo-3,6-dichloro-l,4-benzoquinone. The NCS" ligands are constrained to lie on the same side of the CrOCr bond axis by chelation of the Pd(II) center in the latter complex, contrary to the anti orientation of the parent dimer. [Cr(tmpa)(NCSe)]2OH3+ (pAfa = 1.48) and [{Cr(tmpa)(NCS)}2-(OH)Pd(CA)]3"1" (pXa = 1.15) are more acidic than [Cr(tmpa)(NCS)]2OH3+ by factors of 3.7 and 7.9, respectively.NMR spectra demonstrate that all [Cr(tmpa)L]202+ complexes reported to date have symmetric chelation by the tmpa ligands in which both apical N atoms are cis to the oxo bridge. A linear correlation of Cr(III,IV/III,III)half-wave reduction potential with p£a(Cr^-OH)Cr) pertains for both singly-and doubly-bridged (tmpa)CrOCr-(tmpa) dimers, with the exception of [Cr(tmpa)(CN)]202+. On this basis, it is concluded that the b2g HOMO energy increases with -donor effectiveness of both bridging and nonbridging ligands, in the following order: m-C6HsC02= µ-HCO2" < m-CH3C02" = NCS" < NCO" = Cl" < N3" « µ-F" < µ-". The trend toward larger pKa of Cr^-OH)Cr conjugate acids with increasing substituent Lewis basicity shows that CrOCr -bonding strength is highly sensitive to the competition between µ-2" and µ-' or L" groups for donation to the metal. In this way, both the reduced oxo group basicities and the diamagnetism of the NCSe* and NCS{Pd(CA)/2}" derivatives may be understood.

A simple methodology for constructing ferromagnetically coupled Cr(iii) compounds

“…The most relevant interactions are summarized in Table 2 and Fig. S10-S11 2 ] bridging unit, in contrast to that seen in 1-8. In the extended structure the closest inter-dimer interactions are between the terminally bonded Cl ions and the Me groups of the Me-deaH ligands at a (C-H⋯Cl) distance of ∼2.9 Å ( Table 2 and Fig.…”

“…One potentially pertinent structural difference between compounds 1-8 lies in the nature of the substituent present in the carboxylate group. We therefore investigated the effect of introducing halide atoms to the carboxylates in a model complex of formula [Cr 2 (Me-deaH) 2 (O 2 CX)Cl 2 ]Cl where X = F, Cl, Br and I (Fig. S30 †).…”

“…[1][2][3] Indeed, although more than 40 years have passed since the publication of the first ferromagnetically coupled complex in 1976, namely Na 4 [Cr(mal) 2 7 and the most recent example a di-hydroxo bridged species of formula (Ph 4 P) 4 [Cr(NCS) 4 OH] 2 published in 2014. 8 Larger Cr (III) clusters displaying ferromagnetic exchange are even more rare, with only two examples known.…”

“…The compound [Cr 2 (Me-deaH) 2 Cl 4 ] (9) was synthesised in order to study the effect of removing/adding the carboxylate bridge on the observed magnetic behaviour.…”

“…This perhaps suggests that exploitation of the orbital counter-complementarity effect may be a promising route toward the development of polymetallic Cr(III) species displaying ferromagnetic exchange interactions. 12 To this end we recently began a project to examine the magnetic exchange, and uncover the magneto-structural relationship, between the metal ions in [Cr III (OR) 2 …”