Inductive modulation of tris(phosphinomethyl)phenylborate donation at group VI metals via borate phenyl substituent modification

Abstract

A series of zerovalent group VI metal complexes of tris(diisopropylphosphinomethyl)phenylborate ([PhB(CH2PiPr2)3]-, PhBPiPr3), including [PPN][M(CO)3(PhBPiPr3)] (M = Cr, Mo, W) and the first bimetallics in which PhBPiPr3 serves as a bridging ligand via binding M(CO)3 units at the three phosphorus atoms and the borate phenyl substituent, have been synthesized and fully characterized. Two new tris(phosphinomethyl)borates featuring 3,5-dimethylphenyl and 3,5-bis(trifluoromethyl)phenyl borate substituents were prepared as crystallographically characterized thallium salts, and metallated giving their inaugural transition metal complexes [PPN][M(CO)3(((3,5-Me)C6H3)BPPh3)] and [PPN][M(CO)3(((3,5-CF3)C6H3)BPPh3)]. A comparative ν(CO) infrared spectroscopic analysis and examination of half wave potentials assessed by cyclic voltammetry supports a ligand donor ranking of Tp > PhBPiPr3 ≥ Cp > PhBPPh3 > triphos. For these anionic complexes, in which a lower electrostatic contribution to zerovalent metal-PhBPR3 binding is likely operative relative to that present in the zwitterionic complexes most commonly prepared with tris(phosphinomethyl)borates, PhBPR3 ligands do not function as strongly donating scorpionates. Nevertheless, PhBPPh3 is a substantially stronger donor than triphos towards zerovalent M(CO)3; the half wave potentials of [Et4N][M(CO)3(PhBPPh3)] are ∼340 mV lower than those of M(CO)3(triphos). The potentials of the ((3,5-Me)C6H3)BPPh3 group VI metal tricarbonyl anions are more negative than those of the corresponding ((3,5-CF3)C6H3)BPPh3 group VI metal tricarbonyl anions by ∼50 mV, suggesting a modest, yet rational, tuning of PhBPPh3 donation via inductive modulation of the borate anion charge.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

“…77 This result is consistent with the enhanced donation previously observed for phosphines with anionic borates. 68,83,86,[90][91][92]95,96,100,101,[109][110][111][112] In addition to metal carbonyl adducts, phosphine selenide compounds have also been used to quantify the donor strength and basicity of phosphines via their P-Se coupling constants (JP-Se). 113,114 The use of NMR coupling constants is advantageous due to greater instrumental resolution and sensitivity in the coupling value as compared to vibrational spectroscopy.…”

“…65,66 The efficacy of both methods in rationalizing reactivity trends supports that both electrostatic and inductive interactions are active, and methods to distinguish between them are still being pursued to this day. [67][68][69] A more thorough understanding of the ARTICLE Journal Name 2 | J. Name., 2012, 00, [1][2][3] This journal is © The Royal Society of Chemistry 20xx…”

“…However, the origin of this increase (electrostatic or inductive) has remained elusive, and has largely been considered inductive by default. 68,69,[94][95][96][98][99][100][101] The ambiguity regarding through space effects of covalently bound distal anions limits the rational design of ligand scaffolds that leverage electric fields to enhance reactivity. For instance, a recent computational study predicts accelerated oxidative addition (OA) reactivity at a PMe3 supported Pd complex in the presence of either an externally applied electric field or a correctly positioned chloride ion.…”

“…65,66 The efficacy of both methods in rationalizing reactivity trends supports that both electrostatic and inductive interactions are active, and methods to distinguish between them are still being pursued to this day. [67][68][69] A more thorough understanding of the inductive and electrostatic factors inuencing the electronic structure and reactivity of distally charged ligands and complexes would be instructive, particularly as leveraging through-space interactions can serve as a strategy to break free-energy relationships. [70][71][72][73][74][75][76] Phosphines are ideal scaffolds for quantifying the inuence of electrostatics as these ligands feature prominently in catalysis and have well dened parameters for rationalizing reactivity trends, such as the Tolman Electronic Parameter (TEP) and cone angle.…”

Metal Dependent Formation of Imidazolidine or Hemiaminal Ether Complexes from Multicomponent One‐pot Reactions

“…Triazacyclononane (tacn), cyclohexanetriamine (chta), trithiacyclononane (ttcn), etc., are known as crown ligands. The HC(CH 2 PPh 2 ) 3 (tpddmm), tris(pyrazolyl)methane (tpm), tris(pyridyl)methane (tpym), tris(pyrazolyl)borate (tpb), tris(imidazol‐2‐ylidene)borate (timyb), tris(aminoethyl)alkane (taea), tris(diphenyl‐phosphinomethyl)ethane (tppe), tris(phosphinomethyl)borate (tpmb), tris(imidazolyl)phosphine (tip), tris(diphenylphosphineoxide)methane (tpmo), etc., are scorpionate ligands, whereas diethylenetriamine (dien), dipicolylamine (dpa), koneramine, are linear ribbon type ligands . The crowns and scorpionates strictly coordinate to a metal ion facially ( fac ) whereas the ribbon type ligands are known to undergo both facial or meridional ( mer ) coordination modes according to the size of the metal ions .…”